Quinary and senary non-stoichiometric double perovskites such as Ba2Ca0.66Nb1.34−xFexO6−δ (BCNF) have been utilized for gas sensing, solid oxide fuel cells and thermochemical CO2 reduction. Herein, we examined their potential as narrow bandgap semiconductors for use in solar energy harvesting. A cobalt co-doped BCNF, Ba2Ca0.66Nb0.68Fe0.33Co0.33O6−δ (BCNFCo), exhibited an optical absorption edge at ∼800 nm, p-type conduction and a distinct photoresponse up to 640 nm while demonstrating high thermochemical stability. A nanocomposite of BCNFCo and g-C3N4 (CN) was prepared via a facile solvent-assisted exfoliation/blending approach using dichlorobenzene and glycerol at a moderate temperature. The exfoliation of g-C3N4 followed by wrapping on perovskite established an effective heterojunction between the materials for charge separation. The conjugated 2D sheets of CN enabled better charge migration resulting in increased photoelectrochemical performance. A blend composed of 40 wt% perovskites and CN performed optimally, whilst achieving a photocurrent density as high as 1.5 mA cm−2 for sunlight-driven water-splitting with a Faradaic efficiency as high as ∼88%.
Polymeric carbon nitride-based photocatalysts for photoreforming of biomass d...Pawan Kumar
Photoreforming of biomass to value-added chemicals and fuels is a chemical approach to extract photosynthetically-trapped energy in complex biomolecules which otherwise disintegrate naturally in the environment. Designing precise photocatalytic materials that can selectively break the sturdy, nature-designed biomass with multiplex chemical composition/bonding and inaccessible sites is central to deploying this technology. Polymeric carbon nitride (CN) comprised of a 2D network of condensed heptazine/triazine (C6N7/C3N3) core has shown great promise for photoreforming of biomass derivatives due to intriguing physicochemical and optical properties. This review comprehensively summarizes the state-of-the-art applications of CN-based photocatalysts for the conversion of lignocellulosic biomass derivatives. Various chemical and structural modifications in CN structure such as doping, surface functionalization, hybridization entailing to higher selectivity and conversion have been discussed aiming at providing valuable guidance for future CN-based materials design.
Photocatalytic Mechanism Control and Study of Carrier Dynamics in CdS@C3N5 Co...Pawan Kumar
We present a potential solution to the problem of extraction of photogenerated holes from CdS nanocrystals and nanowires. The nanosheet form of C3N5 is a low-band-gap (Eg = 2.03 eV), azo-linked graphenic carbon nitride framework formed by the polymerization of melem hydrazine (MHP). C3N5 nanosheets were either wrapped around CdS nanorods (NRs) following the synthesis of pristine chalcogenide or intercalated among them by an in situ synthesis protocol to form two kinds of heterostructures, CdS-MHP and CdS-MHPINS, respectively. CdS-MHP improved the photocatalytic degradation rate of 4-nitrophenol by nearly an order of magnitude in comparison to bare CdS NRs. CdS-MHP also enhanced the sunlight-driven photocatalytic activity of bare CdS NWs for the decolorization of rhodamine B (RhB) by a remarkable 300% through the improved extraction and utilization of photogenerated holes due to surface passivation. More interestingly, CdS-MHP provided reaction pathway control over RhB degradation. In the absence of scavengers, CdS-MHP degraded RhB through the N-deethylation pathway. When either hole scavenger or electron scavenger was added to the RhB solution, the photocatalytic activity of CdS-MHP remained mostly unchanged, while the degradation mechanism shifted to the chromophore cleavage (cycloreversion) pathway. We investigated the optoelectronic properties of CdS-C3N5 heterojunctions using density functional theory (DFT) simulations, finite difference time domain (FDTD) simulations, time-resolved terahertz spectroscopy (TRTS), and photoconductivity measurements. TRTS indicated high carrier mobilities >450 cm2 V–1 s–1 and carrier relaxation times >60 ps for CdS-MHP, while CdS-MHPINS exhibited much lower mobilities <150 cm2 V–1 s–1 and short carrier relaxation times <20 ps. Hysteresis in the photoconductive J–V characteristics of CdS NWs disappeared in CdS-MHP, confirming surface passivation. Dispersion-corrected DFT simulations indicated a delocalized HOMO and a LUMO localized on C3N5 in CdS-MHP. C3N5, with its extended π-conjugation and low band gap, can function as a shuttle to extract carriers and excitons in nanostructured heterojunctions, and enhance performance in optoelectronic devices. Our results demonstrate how carrier dynamics in core–shell heterostructures can be manipulated to achieve control over the reaction mechanism in photocatalysis.
Polymeric carbon nitride-based photocatalysts for photoreforming of biomass d...Pawan Kumar
Photoreforming of biomass to value-added chemicals and fuels is a chemical approach to extract photosynthetically-trapped energy in complex biomolecules which otherwise disintegrate naturally in the environment. Designing precise photocatalytic materials that can selectively break the sturdy, nature-designed biomass with multiplex chemical composition/bonding and inaccessible sites is central to deploying this technology. Polymeric carbon nitride (CN) comprised of a 2D network of condensed heptazine/triazine (C6N7/C3N3) core has shown great promise for photoreforming of biomass derivatives due to intriguing physicochemical and optical properties. This review comprehensively summarizes the state-of-the-art applications of CN-based photocatalysts for the conversion of lignocellulosic biomass derivatives. Various chemical and structural modifications in CN structure such as doping, surface functionalization, hybridization entailing to higher selectivity and conversion have been discussed aiming at providing valuable guidance for future CN-based materials design.
Remarkable self-organization and unusual conductivity behavior in cellulose n...Pawan Kumar
Aqueous suspensions of cellulose nanocrystals were blended with Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) [PEDOT:PSS], and cast into thin films. The morphology, structure and electrical properties of the resulting nanocomposite thin films were thoroughly characterized. We found that the CNC–PEDOT:PSS blends self-organize into a layered vertical stack with a pitch of 100–200 nm while retaining a continuous percolation network for PEDOT. Atomic force microscopy, dynamic light scattering and multi-angle light scattering measurements confirmed the wrapping of polymer chains around the rod-like CNCs. The blended films exhibited improved molecular ordering of the PEDOT chains with concomitant improvement in the carrier mobility. The remarkable self-organization and enhanced structural order enabled the CNC–PEDOT:PSS blends to exhibit a high conductivity typical of PEDOT:PSS even when the content of the insulating CNCs in the nanocomposite was as high as 50 wt%.
Synthesis and Characterization of Zinc Phthalocyanine-Cellulose Nanocrystal (...Pawan Kumar
We report highly fluorescent cellulose nanocrystals (CNCs) formed by conjugating a carboxylated zinc phthalocyanine (ZnPc) to two different types of CNCs. The conjugated nanocrystals (henceforth called ZnPc@CNCs) were bright green in color and exhibited absorption and emission maxima at ∼690 and ∼715 nm, respectively. The esterification protocol employed to covalently bind carboxylated ZnPc to surface hydroxyl group rich CNCs was expected to result in a monolayer of ZnPc on the surface of the CNCs. However, dynamic light scattering (DLS) studies indicated a large increase in the hydrodynamic radius of CNCs following conjugation to ZnPc, which suggests the binding of multiple ZnPc molecular layers on the CNC surface. This binding could be through co-facial π-stacking of ZnPc, where ZnPc metallophthalocyanine rings are horizontal to the CNC surface. The other possible binding mode would give rise to conjugated systems where ZnPc metallophthalocyanine rings are oriented vertically on the CNC surface. Density functional theory based calculations showed stable geometry following the conjugation protocol that involved covalently attached ester bond formation. The conjugates demonstrated superior performance for potential sensing applications through higher photoluminescence quenching capabilities compared to pristine ZnPc.
Vapor Deposition of Semiconducting Phosphorus Allotropes into TiO2 Nanotube A...Pawan Kumar
Recent evidence of exponential environmental degradation will demand a drastic shift in research and development toward exploiting alternative energy resources such as solar energy. Here, we report the successful low-cost and easily accessible synthesis of hybrid semiconductor@TiO2 nanotube photocatalysts. In order to realize its maximum potential in harvesting photons in the visible-light range, TiO2 nanotubes have been loaded with earth-abundant, low-band-gap fibrous red and black phosphorus (P). Scanning electron microscopy– and scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron microscopy, and UV–vis measurements have been performed, substantiating the deposition of fibrous red and black P on top and inside the cavities of 100-μm-long electrochemically fabricated nanotubes. The nanotubular …
Effect of morphology on the photoelectrochemical performance of nanostructure...Pawan Kumar
Cu2O is a promising earth-abundant semiconductor photocathode for sunlight-driven water splitting. Characterization results are presented to show how the photocurrent density (Jph), onset potential (Eonset), band edges, carrier density (NA), and interfacial charge transfer resistance (Rct) are affected by the morphology and method used to deposit Cu2O on a copper foil. Mesoscopic and planar morphologies exhibit large differences in the values of NA and Rct. However, these differences are not observed to translate to other photocatalytic properties of Cu2O. Mesoscopic and planar morphologies exhibit similar bandgap (e.g.) and flat band potential (Efb) values of 1.93 ± 0.04 eV and 0.48 ± 0.06 eV respectively. Eonset of 0.48 ± 0.04 eV obtained for these systems is close to the Efb indicating negligible water reduction overpotential. Electrochemically deposited planar Cu2O provides the highest photocurrent density of 5.0 mA cm−2 at 0 V vs reversible hydrogen electrode (RHE) of all the morphologies studied. The photocurrent densities observed in this study are among the highest reported values for bare Cu2O photocathodes.
Flexible and Ultrasoft Inorganic 1D Semiconductor and Heterostructure Systems...Pawan Kumar
Low dimensionality and high flexibility are key demands for flexible electronic semiconductor devices. SnIP, the first atomic-scale double helical semiconductor combines structural anisotropy and robustness with exceptional electronic properties. The benefit of the double helix, combined with a diverse structure on the nanoscale, ranging from strong covalent bonding to weak van der Waals interactions, and the large structure and property anisotropy offer substantial potential for applications in energy conversion and water splitting. It represents the next logical step in downscaling the inorganic semiconductors from classical 3D systems, via 2D semiconductors like MXenes or transition metal dichalcogenides, to the first downsizeable, polymer-like atomic-scale 1D semiconductor SnIP. SnIP shows intriguing mechanical properties featuring a bulk modulus three times lower than any IV, III-V, or II-VI semiconductor. In situ bending tests substantiate that pure SnIP fibers can be bent without an effect on their bonding properties. Organic and inorganic hybrids are prepared illustrating that SnIP is a candidate to fabricate flexible 1D composites for energy conversion and water splitting applications. SnIP@C3N4 hybrid forms an unusual soft material core–shell topology with graphenic carbon nitride wrapping around SnIP. A 1D van der Waals heterostructure is formed capable of performing effective water splitting.
Polymeric carbon nitride-based photocatalysts for photoreforming of biomass d...Pawan Kumar
Photoreforming of biomass to value-added chemicals and fuels is a chemical approach to extract photosynthetically-trapped energy in complex biomolecules which otherwise disintegrate naturally in the environment. Designing precise photocatalytic materials that can selectively break the sturdy, nature-designed biomass with multiplex chemical composition/bonding and inaccessible sites is central to deploying this technology. Polymeric carbon nitride (CN) comprised of a 2D network of condensed heptazine/triazine (C6N7/C3N3) core has shown great promise for photoreforming of biomass derivatives due to intriguing physicochemical and optical properties. This review comprehensively summarizes the state-of-the-art applications of CN-based photocatalysts for the conversion of lignocellulosic biomass derivatives. Various chemical and structural modifications in CN structure such as doping, surface functionalization, hybridization entailing to higher selectivity and conversion have been discussed aiming at providing valuable guidance for future CN-based materials design.
Photocatalytic Mechanism Control and Study of Carrier Dynamics in CdS@C3N5 Co...Pawan Kumar
We present a potential solution to the problem of extraction of photogenerated holes from CdS nanocrystals and nanowires. The nanosheet form of C3N5 is a low-band-gap (Eg = 2.03 eV), azo-linked graphenic carbon nitride framework formed by the polymerization of melem hydrazine (MHP). C3N5 nanosheets were either wrapped around CdS nanorods (NRs) following the synthesis of pristine chalcogenide or intercalated among them by an in situ synthesis protocol to form two kinds of heterostructures, CdS-MHP and CdS-MHPINS, respectively. CdS-MHP improved the photocatalytic degradation rate of 4-nitrophenol by nearly an order of magnitude in comparison to bare CdS NRs. CdS-MHP also enhanced the sunlight-driven photocatalytic activity of bare CdS NWs for the decolorization of rhodamine B (RhB) by a remarkable 300% through the improved extraction and utilization of photogenerated holes due to surface passivation. More interestingly, CdS-MHP provided reaction pathway control over RhB degradation. In the absence of scavengers, CdS-MHP degraded RhB through the N-deethylation pathway. When either hole scavenger or electron scavenger was added to the RhB solution, the photocatalytic activity of CdS-MHP remained mostly unchanged, while the degradation mechanism shifted to the chromophore cleavage (cycloreversion) pathway. We investigated the optoelectronic properties of CdS-C3N5 heterojunctions using density functional theory (DFT) simulations, finite difference time domain (FDTD) simulations, time-resolved terahertz spectroscopy (TRTS), and photoconductivity measurements. TRTS indicated high carrier mobilities >450 cm2 V–1 s–1 and carrier relaxation times >60 ps for CdS-MHP, while CdS-MHPINS exhibited much lower mobilities <150 cm2 V–1 s–1 and short carrier relaxation times <20 ps. Hysteresis in the photoconductive J–V characteristics of CdS NWs disappeared in CdS-MHP, confirming surface passivation. Dispersion-corrected DFT simulations indicated a delocalized HOMO and a LUMO localized on C3N5 in CdS-MHP. C3N5, with its extended π-conjugation and low band gap, can function as a shuttle to extract carriers and excitons in nanostructured heterojunctions, and enhance performance in optoelectronic devices. Our results demonstrate how carrier dynamics in core–shell heterostructures can be manipulated to achieve control over the reaction mechanism in photocatalysis.
Polymeric carbon nitride-based photocatalysts for photoreforming of biomass d...Pawan Kumar
Photoreforming of biomass to value-added chemicals and fuels is a chemical approach to extract photosynthetically-trapped energy in complex biomolecules which otherwise disintegrate naturally in the environment. Designing precise photocatalytic materials that can selectively break the sturdy, nature-designed biomass with multiplex chemical composition/bonding and inaccessible sites is central to deploying this technology. Polymeric carbon nitride (CN) comprised of a 2D network of condensed heptazine/triazine (C6N7/C3N3) core has shown great promise for photoreforming of biomass derivatives due to intriguing physicochemical and optical properties. This review comprehensively summarizes the state-of-the-art applications of CN-based photocatalysts for the conversion of lignocellulosic biomass derivatives. Various chemical and structural modifications in CN structure such as doping, surface functionalization, hybridization entailing to higher selectivity and conversion have been discussed aiming at providing valuable guidance for future CN-based materials design.
Remarkable self-organization and unusual conductivity behavior in cellulose n...Pawan Kumar
Aqueous suspensions of cellulose nanocrystals were blended with Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) [PEDOT:PSS], and cast into thin films. The morphology, structure and electrical properties of the resulting nanocomposite thin films were thoroughly characterized. We found that the CNC–PEDOT:PSS blends self-organize into a layered vertical stack with a pitch of 100–200 nm while retaining a continuous percolation network for PEDOT. Atomic force microscopy, dynamic light scattering and multi-angle light scattering measurements confirmed the wrapping of polymer chains around the rod-like CNCs. The blended films exhibited improved molecular ordering of the PEDOT chains with concomitant improvement in the carrier mobility. The remarkable self-organization and enhanced structural order enabled the CNC–PEDOT:PSS blends to exhibit a high conductivity typical of PEDOT:PSS even when the content of the insulating CNCs in the nanocomposite was as high as 50 wt%.
Synthesis and Characterization of Zinc Phthalocyanine-Cellulose Nanocrystal (...Pawan Kumar
We report highly fluorescent cellulose nanocrystals (CNCs) formed by conjugating a carboxylated zinc phthalocyanine (ZnPc) to two different types of CNCs. The conjugated nanocrystals (henceforth called ZnPc@CNCs) were bright green in color and exhibited absorption and emission maxima at ∼690 and ∼715 nm, respectively. The esterification protocol employed to covalently bind carboxylated ZnPc to surface hydroxyl group rich CNCs was expected to result in a monolayer of ZnPc on the surface of the CNCs. However, dynamic light scattering (DLS) studies indicated a large increase in the hydrodynamic radius of CNCs following conjugation to ZnPc, which suggests the binding of multiple ZnPc molecular layers on the CNC surface. This binding could be through co-facial π-stacking of ZnPc, where ZnPc metallophthalocyanine rings are horizontal to the CNC surface. The other possible binding mode would give rise to conjugated systems where ZnPc metallophthalocyanine rings are oriented vertically on the CNC surface. Density functional theory based calculations showed stable geometry following the conjugation protocol that involved covalently attached ester bond formation. The conjugates demonstrated superior performance for potential sensing applications through higher photoluminescence quenching capabilities compared to pristine ZnPc.
Vapor Deposition of Semiconducting Phosphorus Allotropes into TiO2 Nanotube A...Pawan Kumar
Recent evidence of exponential environmental degradation will demand a drastic shift in research and development toward exploiting alternative energy resources such as solar energy. Here, we report the successful low-cost and easily accessible synthesis of hybrid semiconductor@TiO2 nanotube photocatalysts. In order to realize its maximum potential in harvesting photons in the visible-light range, TiO2 nanotubes have been loaded with earth-abundant, low-band-gap fibrous red and black phosphorus (P). Scanning electron microscopy– and scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron microscopy, and UV–vis measurements have been performed, substantiating the deposition of fibrous red and black P on top and inside the cavities of 100-μm-long electrochemically fabricated nanotubes. The nanotubular …
Effect of morphology on the photoelectrochemical performance of nanostructure...Pawan Kumar
Cu2O is a promising earth-abundant semiconductor photocathode for sunlight-driven water splitting. Characterization results are presented to show how the photocurrent density (Jph), onset potential (Eonset), band edges, carrier density (NA), and interfacial charge transfer resistance (Rct) are affected by the morphology and method used to deposit Cu2O on a copper foil. Mesoscopic and planar morphologies exhibit large differences in the values of NA and Rct. However, these differences are not observed to translate to other photocatalytic properties of Cu2O. Mesoscopic and planar morphologies exhibit similar bandgap (e.g.) and flat band potential (Efb) values of 1.93 ± 0.04 eV and 0.48 ± 0.06 eV respectively. Eonset of 0.48 ± 0.04 eV obtained for these systems is close to the Efb indicating negligible water reduction overpotential. Electrochemically deposited planar Cu2O provides the highest photocurrent density of 5.0 mA cm−2 at 0 V vs reversible hydrogen electrode (RHE) of all the morphologies studied. The photocurrent densities observed in this study are among the highest reported values for bare Cu2O photocathodes.
Flexible and Ultrasoft Inorganic 1D Semiconductor and Heterostructure Systems...Pawan Kumar
Low dimensionality and high flexibility are key demands for flexible electronic semiconductor devices. SnIP, the first atomic-scale double helical semiconductor combines structural anisotropy and robustness with exceptional electronic properties. The benefit of the double helix, combined with a diverse structure on the nanoscale, ranging from strong covalent bonding to weak van der Waals interactions, and the large structure and property anisotropy offer substantial potential for applications in energy conversion and water splitting. It represents the next logical step in downscaling the inorganic semiconductors from classical 3D systems, via 2D semiconductors like MXenes or transition metal dichalcogenides, to the first downsizeable, polymer-like atomic-scale 1D semiconductor SnIP. SnIP shows intriguing mechanical properties featuring a bulk modulus three times lower than any IV, III-V, or II-VI semiconductor. In situ bending tests substantiate that pure SnIP fibers can be bent without an effect on their bonding properties. Organic and inorganic hybrids are prepared illustrating that SnIP is a candidate to fabricate flexible 1D composites for energy conversion and water splitting applications. SnIP@C3N4 hybrid forms an unusual soft material core–shell topology with graphenic carbon nitride wrapping around SnIP. A 1D van der Waals heterostructure is formed capable of performing effective water splitting.
TiO2-HfN Radial Nano-Heterojunction: A Hot Carrier Photoanode for Sunlight-Dr...Pawan Kumar
The lack of active, stable, earth-abundant, and visible-light absorbing materials to replace
plasmonic noble metals is a critical obstacle for researchers in developing highly efficient and costeffective photocatalytic systems. Herein, a core–shell nanotube catalyst was fabricated consisting of
atomic layer deposited HfN shell and anodic TiO2 support layer with full-visible regime photoactivity
for photoelectrochemical water splitting. The HfN active layer has two unique characteristics: (1) a
large bandgap between optical and acoustic phonon modes (2) and no electronic bandgap, which
allows a large population of long life-time hot carriers, which are used to enhance the photoelectrochemical performance. The photocurrent density (≈2.5 mA·cm−2 at 1 V vs. Ag/AgCl) obtained in
this study under AM 1.5G 1 Sun illumination is unprecedented, as it is superior to most existing
plasmonic noble metal-decorated catalysts and surprisingly indicates a photocurrent response that
extends to 730 nm. The result demonstrates the far-reaching application potential of replacing active
HER/HOR noble metals such as Au, Ag, Pt, Pd, etc. with low-cost plasmonic ceramics.
Vapor Deposition of Semiconducting Phosphorus Allotropes into TiO2 Nanotube A...Pawan Kumar
Recent evidence of exponential environmental degradation will demand a drastic shift in research and development toward
exploiting alternative energy resources such as solar energy. Here, we
report the successful low-cost and easily accessible synthesis of hybrid
semiconductor@TiO2 nanotube photocatalysts. In order to realize its
maximum potential in harvesting photons in the visible-light range, TiO2
nanotubes have been loaded with earth-abundant, low-band-gap fibrous
red and black phosphorus (P). Scanning electron microscopy− and
scanning transmission electron microscopy−energy-dispersive X-ray
spectroscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron microscopy, and UV−vis measurements have been performed,
substantiating the deposition of fibrous red and black P on top and
inside the cavities of 100-μm-long electrochemically fabricated nanotubes. The nanotubular morphology of titania and a vapor-transport technique are utilized to form heterojunctions of P and
TiO2. Compared to pristine anatase 3.2 eV TiO2 nanotubes, the creation of heterojunctions in the hybrid material resulted in
1.5−2.1 eV photoelectrocatalysts. An enhanced photoelectrochemical water-splitting performance under visible light compared
with the individual components resulted for the P@TiO2 hybrids. This feature is due to synergistically improved charge
separation in the heterojunction and more effective visible-light absorption. The electronic band structure and charge-carrier
dynamics are investigated in detail using ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy to elucidate
the charge-separation mechanism. A Fermi-level alignment in P@TiO2 heterojunctions leads to a more reductive flat-band
potential and a deeper valence band compared to pristine P and thus facilitates a better water-splitting performance. Our results
demonstrate effective conversion efficiencies for the nanostructured hybrids, which may enable future applications in
optoelectronic applications such as photodetectors, photovoltaics, photoelectrochemical catalysts, and sensors.
Heterostructured nanocomposite tin phthalocyanine@mesoporous ceria (SnPc@CeO2...Pawan Kumar
Heterostructured tin phthalocyanine supported to mesoporous ceria was synthesized and used a
photocatalyst for CO2 reduction under visible light. The photoreduction CO2 activities of the
heterostructures were investigated in the presence of triethylamine as sacrificial agent. The developed
photocatalyst exhibited high catalytic activity for photoreduction of CO2 and after 24 hours of visible
light irradiation 2342 mmol g1 cat of methanol (fMeOH ¼ 0.0223 or 2.23%) and 840 mmol g1 cat of CO
(fCO ¼ 0.0026 or 0.26%) were obtained as the major reaction products. The methanol formation rate
(RMeOH) and CO formation rate (RCO) was found to be 97.5 mmol h1 g1 cat and 35.0 mmol h1 g1 cat
respectively. While under the identical experimental conditions mesoporous ceria (meso-CeO2) gave
only 316 mmol g1 cat of methanol (fMeOH ¼ 0.003 or 0.30%) and 126 mmol g1 cat CO (fCO ¼ 0.0004
or 0.04%) with product formation rate RMeOH ¼ 13.2 mmol h1 g1 cat and RCO ¼ 5.3 mmol h1 g1 cat.
Furthermore, the recovered catalyst showed consistent catalytic activity for at least five runs without any
significant loss in product yields
Vapor growth of binary and ternary phosphorus-based semiconductors into TiO 2...Pawan Kumar
We report successful synthesis of low band gap inorganic polyphosphide and TiO2 heterostructures with the aid of short-way transport reactions. Binary and ternary polyphosphides (NaP7, SnIP, and (CuI)3P12) were successfully reacted and deposited into electrochemically fabricated TiO2 nanotubes. Employing vapor phase reaction deposition, the cavities of 100 μm long TiO2 nanotubes were infiltrated; approximately 50% of the nanotube arrays were estimated to be infiltrated in the case of NaP7. Intensive characterization of the hybrid materials with techniques including SEM, FIB, HR-TEM, Raman spectroscopy, XRD, and XPS proved the successful vapor phase deposition and synthesis of the substances on and inside the nanotubes. The polyphosphide@TiO2 hybrids exhibited superior water splitting performance compared to pristine materials and were found to be more active at higher wavelengths. SnIP …
Sunlight-driven water-splitting using two dimensional carbon based semiconduc...Pawan Kumar
The overwhelming challenge of depleting fossil fuels and anthropogenic carbon emissions has driven research
into alternative clean sources of energy. To achieve the goal of a carbon neutral economy, the harvesting of
sunlight by using photocatalysts to split water into hydrogen and oxygen is an expedient approach to fulfill
the energy demand in a sustainable way along with reducing the emission of greenhouse gases. Even though
the past few decades have witnessed intensive research into inorganic semiconductor photocatalysts, their
quantum efficiencies for hydrogen production from visible photons remain too low for the large scale
deployment of this technology. Visible light absorption and efficient charge separation are two key necessary
conditions for achieving the scalable production of hydrogen from water. Two-dimensional carbon based
nanoscale materials such as graphene oxide, reduced graphene oxide, carbon nitride, modified 2D carbon
frameworks and their composites have emerged as potential photocatalysts due to their astonishing
properties such as superior charge transport, tunable energy levels and bandgaps, visible light absorption,
high surface area, easy processability, quantum confinement effects, and high photocatalytic quantum yields.
The feasibility of structural and chemical modification to optimize visible light absorption and charge
separation makes carbonaceous semiconductors promising candidates to convert solar energy into chemical
energy. In the present review, we have summarized the recent advances in 2D carbonaceous photocatalysts
with respect to physicochemical and photochemical tuning for solar light mediated hydrogen evolution
Photo-assisted oxidation of thiols to disulfides using cobalt ‘‘Nanorust’’ un...Pawan Kumar
Heterogeneous ‘‘Nanorust’’ containing cobalt oxide has been developed for the visible light assisted
oxidation of thiols to disulfides using molecular oxygen as an oxidant under alkaline free conditions and
therefore more environmentally friendly. Pyrolysis of heterogenized tetrasulfonated cobalt(II) phthalocyanine
(CoPcS) supported on mesoporous ceria (CeO2) transforms it into a novel heterogeneous ‘‘Nanorust’’
containing CoOx-C,N@CeO2 which exhibited higher catalytic activity than the homogeneous CoPcS as well
as the ceria immobilized CoPcS catalyst. Importantly, these catalysts could easily be recovered and recycled
for several runs, which makes the process greener and cost-effective.
Vapor growth of binary and ternary phosphorusbased semiconductors into TiO2 n...Pawan Kumar
We report successful synthesis of low band gap inorganic polyphosphide and TiO2 heterostructures with
the aid of short-way transport reactions. Binary and ternary polyphosphides (NaP7, SnIP, and (CuI)3P12)
were successfully reacted and deposited into electrochemically fabricated TiO2 nanotubes. Employing
vapor phase reaction deposition, the cavities of 100 mm long TiO2 nanotubes were infiltrated;
approximately 50% of the nanotube arrays were estimated to be infiltrated in the case of NaP7. Intensive
characterization of the hybrid materials with techniques including SEM, FIB, HR-TEM, Raman
spectroscopy, XRD, and XPS proved the successful vapor phase deposition and synthesis of the
substances on and inside the nanotubes. The polyphosphide@TiO2 hybrids exhibited superior water
splitting performance compared to pristine materials and were found to be more active at higher
wavelengths. SnIP@TiO2 emerged to be the most active among the polyphosphide@TiO2 materials. The
improved photocatalytic performance might be due to Fermi level re-alignment and a lower charge
transfer resistance which facilitated better charge separation from inorganic phosphides to TiO2.
Photocatalytic Mechanism Control and Study of Carrier Dynamics in CdS@C3N5 Co...Pawan Kumar
We present a potential solution to the problem of extraction of photogenerated holes from CdS nanocrystals and nanowires. The nanosheet form of C3N5 is a low-band-gap (Eg = 2.03 eV), azo-linked graphenic carbon nitride framework formed by the polymerization of melem hydrazine (MHP). C3N5 nanosheets were either wrapped around CdS nanorods (NRs) following the synthesis of pristine chalcogenide or intercalated among them by an in situ synthesis protocol to form two kinds of heterostructures, CdS-MHP and CdS-MHPINS, respectively. CdS-MHP improved the photocatalytic degradation rate of 4-nitrophenol by nearly an order of magnitude in comparison to bare CdS NRs. CdS-MHP also enhanced the sunlight-driven photocatalytic activity of bare CdS NWs for the decolorization of rhodamine B (RhB) by a remarkable 300% through the improved extraction and utilization of photogenerated holes due to surface passivation. More interestingly, CdS-MHP provided reaction pathway control over RhB degradation. In the absence of scavengers, CdS-MHP degraded RhB through the N-deethylation pathway. When either hole scavenger or electron scavenger was added to the RhB solution, the photocatalytic activity of CdS-MHP remained mostly unchanged, while the degradation mechanism shifted to the chromophore cleavage (cycloreversion) pathway. We investigated the optoelectronic properties of CdS-C3N5 heterojunctions using density functional theory (DFT) simulations, finite difference time domain (FDTD) simulations, time-resolved terahertz spectroscopy (TRTS), and photoconductivity measurements. TRTS indicated high carrier mobilities >450 cm2 V–1 s–1 and carrier relaxation times >60 ps for CdS-MHP, while CdS-MHPINS exhibited much lower mobilities <150 cm2 V–1 s–1 and short carrier relaxation times <20 ps. Hysteresis in the photoconductive J–V characteristics of CdS NWs disappeared in CdS-MHP, confirming surface passivation. Dispersion-corrected DFT simulations indicated a delocalized HOMO and a LUMO localized on C3N5 in CdS-MHP. C3N5, with its extended π-conjugation and low band gap, can function as a shuttle to extract carriers and excitons in nanostructured heterojunctions, and enhance performance in optoelectronic devices. Our results demonstrate how carrier dynamics in core–shell heterostructures can be manipulated to achieve control over the reaction mechanism in photocatalysis.
Boosting Photocatalytic Activity Using Carbon Nitride Based 2D/2D van der Waa...Pawan Kumar
The surging demand for energy and staggering pollutants in the environment have geared the scientific community to explore sustainable pathways that are economically feasible and environmentally compelling. In this context, harnessing solar energy using semiconductor materials to generate charge pairs to drive photoredox reactions has been envisioned as a futuristic approach. Numerous inorganic crystals with promising nanoregime properties investigated in the past decade have yet to demonstrate practical application due to limited photon absorption and sluggish charge separation kinetics. Two-dimensional semiconductors with tunable optical and electronic properties and quasi-resistance-free lateral charge transfer mechanisms have shown great promise in photocatalysis. Polymeric graphitic carbon nitride (g-C3N4) is among the most promising candidates due to fine-tuned band edges and the feasibility of optimizing the optical properties via materials genomics. Constructing a two-dimensional (2D)/2D van der Waals (vdW) heterojunction by allies of 2D carbon nitride sheets and other 2D semiconductors has demonstrated enhanced charge separation with improved visible photon absorption, and the performance is not restricted by the lattice matching of constituting materials. With the advent of new 2D semiconductors over the recent past, the 2D/2D heterojunction assemblies are gaining momentum to design high performance photocatalysts for numerous applications. This review aims to highlight recent advancements and key understanding in carbon nitride based 2D/2D heterojunctions and their applications in photocatalysis, including small molecules activation, conversion, and degradations. We conclude with a forward-looking perspective discussing the key challenges and opportunity areas for future research.
Nanostructured composite materials for CO2 activationPawan Kumar
The increasing energy crisis and the worsening global climate caused by the excessive
utilization of the fossil fuel have boosted tremendous research about CO2 capture, storage and
utilization. Among these approaches, utilization of carbon dioxide to produce valuable chemicals
is preferred than dumping it. Particularly, utilization of CO2 as feedstock for the photocatalytic
conversion into valuable products is a viable approach for harvesting solar radiation as an energy
source and to mitigate increasing CO2 concentration. Artificial photosynthesis by using
nanostructured materials as photocatalyst has immense potential to convert carbon dioxide into
renewable fuels such as methanol/CO etc. The present chapter focuses on the synthesis, characterization and application of various nanostructured materials for CO2 activation including
photoreduction of CO2 to valuable products.
Nanophotonic enhancement and improved electron extraction in perovskite solar...Pawan Kumar
While vertically oriented metal oxide nanowires have been intensely researched for use as electron transport layers (ETLs) in halide perovskite solar cells (HPSCs), horizontal nanowires (oriented roughly parallel to the substrate) have received much less attention despite their higher photonic strength due to overlapping electric and magnetic dipolar Mie resonance modes. Herein, we demonstrate the fabrication of an assembly of horizontally aligned TiO2 nanorods (HATNRs) on FTO substrates via a facile hydrothermal route. The HATNRs are employed as the ETL to achieve 15.03% power conversion efficiency (PCE) in HPSCs which is higher than the PCE of compact TiO2 based devices (10.12%) by a factor of nearly 1.5. A mixed halide, mixed cation organometal perovskite FA0.83MA0.17Pb(Br0.17I0.83)3 with optimized composition is used as the active layer. The excellent refractive index matching between the perovskite and TiO2, coupled with strong Mie scattering in the nanorod geometry results in broadband near-zero backscattering and high forward scattering, upon coating of HATNRs with perovskite. The maximum suppression of backscattering is found at ∼600 nm. The HATNRs ETL also improves the extraction of electrons from the perovskite layer and results in superior blocking of carrier recombination at the perovskite layer/FTO interface.
Nanophotonic enhancement and improved electron extraction in perovskite solar...Pawan Kumar
While vertically oriented metal oxide nanowires have been intensely researched for use as electron transport layers (ETLs) in halide perovskite solar cells (HPSCs), horizontal nanowires (oriented roughly parallel to the substrate) have received much less attention despite their higher photonic strength due to overlapping electric and magnetic dipolar Mie resonance modes. Herein, we demonstrate the fabrication of an assembly of horizontally aligned TiO2 nanorods (HATNRs) on FTO substrates via a facile hydrothermal route. The HATNRs are employed as the ETL to achieve 15.03% power conversion efficiency (PCE) in HPSCs which is higher than the PCE of compact TiO2 based devices (10.12%) by a factor of nearly 1.5. A mixed halide, mixed cation organometal perovskite FA0.83MA0.17Pb(Br0.17I0.83)3 with optimized composition is used as the active layer. The excellent refractive index matching between the …
Water-splitting photoelectrodes consisting of heterojunctions of carbon nitri...Pawan Kumar
Quinary and senary non-stoichiometric double perovskites such as Ba2Ca0.66Nb1.34-xFexO6-δ (BCNF) have been utilized for gas sensing, solid oxide fuel cells and thermochemical CO2 reduction. Herein, we examined their potential as narrow bandgap semiconductors for use in solar energy harvesting. A cobalt co-doped BCNF, Ba2Ca0.66Nb0.68Fe0.33Co0.33O6-δ (BCNFCo), exhibited an optical absorption edge at ~ 800 nm, p-type conduction and a distinct photoresponse upto 640 nm while demonstrating high thermochemical stability. A nanocomposite of BCNFCo and g-C3N4 (CN) was prepared via a facile solvent assisted exfoliation/blending approach using dichlorobenzene and glycerol at a moderate temperature. The exfoliation of g-C3N4 followed by wrapping on perovskite established an effective heterojunction between the materials for charge separation. The conjugated 2D sheets of CN enabled better charge migration resulting in increased photoelectrochemical performance. A blend composed of 40 wt% perovskite and CN performed optimally, whilst achieving a photocurrent density as high as 1.5 mA cm-2 for sunlight-driven water-splitting with a Faradaic efficiency as high as ~ 88%.
Heterojunctions of halogen-doped carbon nitride nanosheets and BiOI for sunli...Pawan Kumar
A fluorine-doped, chlorine-intercalated carbon nitride (CNF-Cl) photocatalyst has been synthesized for simultaneous improvements in light harvesting capability along with suppression
of charge recombination in bulk g-C3N4. The formation of heterojunctions of these CNF-Cl nanosheets with low bandgap, earth abundant bismuth oxyiodide (BiOI) was achieved, and the
synthesized heterojunctions were tested as active photoanodes in photoelectrochemical water splitting experiments. BiOI/CNF-Cl heterojunctions exhibited extended light harvesting with a
band-edge of 680 nm and generated photocurrent densities approaching 1.3 mA cm−2 under AM1.5 G one sun illumination. Scanning Kelvin probe force microscopy under optical bias
showed a surface potential of 207 mV for the 50% BiOI/CNF-Cl nanocomposite, while pristine CNF-Cl and BiOI had surface photopotential values of 83 mV and 98 mV, respectively, which in turn, provided direct evidence of superior charge separation in the heterojunction blends. Enhanced charge carrier separation and improved light harvesting capability in BiOI/CNF-Cl hybrids were found to be the dominant factors in increased photocurrent, compared to the pristine constituent materials.
TiO2-HfN Radial Nano-Heterojunction: A Hot Carrier Photoanode for Sunlight-Dr...Pawan Kumar
The lack of active, stable, earth-abundant, and visible-light absorbing materials to replace
plasmonic noble metals is a critical obstacle for researchers in developing highly efficient and costeffective photocatalytic systems. Herein, a core–shell nanotube catalyst was fabricated consisting of
atomic layer deposited HfN shell and anodic TiO2 support layer with full-visible regime photoactivity
for photoelectrochemical water splitting. The HfN active layer has two unique characteristics: (1) a
large bandgap between optical and acoustic phonon modes (2) and no electronic bandgap, which
allows a large population of long life-time hot carriers, which are used to enhance the photoelectrochemical performance. The photocurrent density (≈2.5 mA·cm−2 at 1 V vs. Ag/AgCl) obtained in
this study under AM 1.5G 1 Sun illumination is unprecedented, as it is superior to most existing
plasmonic noble metal-decorated catalysts and surprisingly indicates a photocurrent response that
extends to 730 nm. The result demonstrates the far-reaching application potential of replacing active
HER/HOR noble metals such as Au, Ag, Pt, Pd, etc. with low-cost plasmonic ceramics.
Vapor Deposition of Semiconducting Phosphorus Allotropes into TiO2 Nanotube A...Pawan Kumar
Recent evidence of exponential environmental degradation will demand a drastic shift in research and development toward
exploiting alternative energy resources such as solar energy. Here, we
report the successful low-cost and easily accessible synthesis of hybrid
semiconductor@TiO2 nanotube photocatalysts. In order to realize its
maximum potential in harvesting photons in the visible-light range, TiO2
nanotubes have been loaded with earth-abundant, low-band-gap fibrous
red and black phosphorus (P). Scanning electron microscopy− and
scanning transmission electron microscopy−energy-dispersive X-ray
spectroscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron microscopy, and UV−vis measurements have been performed,
substantiating the deposition of fibrous red and black P on top and
inside the cavities of 100-μm-long electrochemically fabricated nanotubes. The nanotubular morphology of titania and a vapor-transport technique are utilized to form heterojunctions of P and
TiO2. Compared to pristine anatase 3.2 eV TiO2 nanotubes, the creation of heterojunctions in the hybrid material resulted in
1.5−2.1 eV photoelectrocatalysts. An enhanced photoelectrochemical water-splitting performance under visible light compared
with the individual components resulted for the P@TiO2 hybrids. This feature is due to synergistically improved charge
separation in the heterojunction and more effective visible-light absorption. The electronic band structure and charge-carrier
dynamics are investigated in detail using ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy to elucidate
the charge-separation mechanism. A Fermi-level alignment in P@TiO2 heterojunctions leads to a more reductive flat-band
potential and a deeper valence band compared to pristine P and thus facilitates a better water-splitting performance. Our results
demonstrate effective conversion efficiencies for the nanostructured hybrids, which may enable future applications in
optoelectronic applications such as photodetectors, photovoltaics, photoelectrochemical catalysts, and sensors.
Heterostructured nanocomposite tin phthalocyanine@mesoporous ceria (SnPc@CeO2...Pawan Kumar
Heterostructured tin phthalocyanine supported to mesoporous ceria was synthesized and used a
photocatalyst for CO2 reduction under visible light. The photoreduction CO2 activities of the
heterostructures were investigated in the presence of triethylamine as sacrificial agent. The developed
photocatalyst exhibited high catalytic activity for photoreduction of CO2 and after 24 hours of visible
light irradiation 2342 mmol g1 cat of methanol (fMeOH ¼ 0.0223 or 2.23%) and 840 mmol g1 cat of CO
(fCO ¼ 0.0026 or 0.26%) were obtained as the major reaction products. The methanol formation rate
(RMeOH) and CO formation rate (RCO) was found to be 97.5 mmol h1 g1 cat and 35.0 mmol h1 g1 cat
respectively. While under the identical experimental conditions mesoporous ceria (meso-CeO2) gave
only 316 mmol g1 cat of methanol (fMeOH ¼ 0.003 or 0.30%) and 126 mmol g1 cat CO (fCO ¼ 0.0004
or 0.04%) with product formation rate RMeOH ¼ 13.2 mmol h1 g1 cat and RCO ¼ 5.3 mmol h1 g1 cat.
Furthermore, the recovered catalyst showed consistent catalytic activity for at least five runs without any
significant loss in product yields
Vapor growth of binary and ternary phosphorus-based semiconductors into TiO 2...Pawan Kumar
We report successful synthesis of low band gap inorganic polyphosphide and TiO2 heterostructures with the aid of short-way transport reactions. Binary and ternary polyphosphides (NaP7, SnIP, and (CuI)3P12) were successfully reacted and deposited into electrochemically fabricated TiO2 nanotubes. Employing vapor phase reaction deposition, the cavities of 100 μm long TiO2 nanotubes were infiltrated; approximately 50% of the nanotube arrays were estimated to be infiltrated in the case of NaP7. Intensive characterization of the hybrid materials with techniques including SEM, FIB, HR-TEM, Raman spectroscopy, XRD, and XPS proved the successful vapor phase deposition and synthesis of the substances on and inside the nanotubes. The polyphosphide@TiO2 hybrids exhibited superior water splitting performance compared to pristine materials and were found to be more active at higher wavelengths. SnIP …
Sunlight-driven water-splitting using two dimensional carbon based semiconduc...Pawan Kumar
The overwhelming challenge of depleting fossil fuels and anthropogenic carbon emissions has driven research
into alternative clean sources of energy. To achieve the goal of a carbon neutral economy, the harvesting of
sunlight by using photocatalysts to split water into hydrogen and oxygen is an expedient approach to fulfill
the energy demand in a sustainable way along with reducing the emission of greenhouse gases. Even though
the past few decades have witnessed intensive research into inorganic semiconductor photocatalysts, their
quantum efficiencies for hydrogen production from visible photons remain too low for the large scale
deployment of this technology. Visible light absorption and efficient charge separation are two key necessary
conditions for achieving the scalable production of hydrogen from water. Two-dimensional carbon based
nanoscale materials such as graphene oxide, reduced graphene oxide, carbon nitride, modified 2D carbon
frameworks and their composites have emerged as potential photocatalysts due to their astonishing
properties such as superior charge transport, tunable energy levels and bandgaps, visible light absorption,
high surface area, easy processability, quantum confinement effects, and high photocatalytic quantum yields.
The feasibility of structural and chemical modification to optimize visible light absorption and charge
separation makes carbonaceous semiconductors promising candidates to convert solar energy into chemical
energy. In the present review, we have summarized the recent advances in 2D carbonaceous photocatalysts
with respect to physicochemical and photochemical tuning for solar light mediated hydrogen evolution
Photo-assisted oxidation of thiols to disulfides using cobalt ‘‘Nanorust’’ un...Pawan Kumar
Heterogeneous ‘‘Nanorust’’ containing cobalt oxide has been developed for the visible light assisted
oxidation of thiols to disulfides using molecular oxygen as an oxidant under alkaline free conditions and
therefore more environmentally friendly. Pyrolysis of heterogenized tetrasulfonated cobalt(II) phthalocyanine
(CoPcS) supported on mesoporous ceria (CeO2) transforms it into a novel heterogeneous ‘‘Nanorust’’
containing CoOx-C,N@CeO2 which exhibited higher catalytic activity than the homogeneous CoPcS as well
as the ceria immobilized CoPcS catalyst. Importantly, these catalysts could easily be recovered and recycled
for several runs, which makes the process greener and cost-effective.
Vapor growth of binary and ternary phosphorusbased semiconductors into TiO2 n...Pawan Kumar
We report successful synthesis of low band gap inorganic polyphosphide and TiO2 heterostructures with
the aid of short-way transport reactions. Binary and ternary polyphosphides (NaP7, SnIP, and (CuI)3P12)
were successfully reacted and deposited into electrochemically fabricated TiO2 nanotubes. Employing
vapor phase reaction deposition, the cavities of 100 mm long TiO2 nanotubes were infiltrated;
approximately 50% of the nanotube arrays were estimated to be infiltrated in the case of NaP7. Intensive
characterization of the hybrid materials with techniques including SEM, FIB, HR-TEM, Raman
spectroscopy, XRD, and XPS proved the successful vapor phase deposition and synthesis of the
substances on and inside the nanotubes. The polyphosphide@TiO2 hybrids exhibited superior water
splitting performance compared to pristine materials and were found to be more active at higher
wavelengths. SnIP@TiO2 emerged to be the most active among the polyphosphide@TiO2 materials. The
improved photocatalytic performance might be due to Fermi level re-alignment and a lower charge
transfer resistance which facilitated better charge separation from inorganic phosphides to TiO2.
Photocatalytic Mechanism Control and Study of Carrier Dynamics in CdS@C3N5 Co...Pawan Kumar
We present a potential solution to the problem of extraction of photogenerated holes from CdS nanocrystals and nanowires. The nanosheet form of C3N5 is a low-band-gap (Eg = 2.03 eV), azo-linked graphenic carbon nitride framework formed by the polymerization of melem hydrazine (MHP). C3N5 nanosheets were either wrapped around CdS nanorods (NRs) following the synthesis of pristine chalcogenide or intercalated among them by an in situ synthesis protocol to form two kinds of heterostructures, CdS-MHP and CdS-MHPINS, respectively. CdS-MHP improved the photocatalytic degradation rate of 4-nitrophenol by nearly an order of magnitude in comparison to bare CdS NRs. CdS-MHP also enhanced the sunlight-driven photocatalytic activity of bare CdS NWs for the decolorization of rhodamine B (RhB) by a remarkable 300% through the improved extraction and utilization of photogenerated holes due to surface passivation. More interestingly, CdS-MHP provided reaction pathway control over RhB degradation. In the absence of scavengers, CdS-MHP degraded RhB through the N-deethylation pathway. When either hole scavenger or electron scavenger was added to the RhB solution, the photocatalytic activity of CdS-MHP remained mostly unchanged, while the degradation mechanism shifted to the chromophore cleavage (cycloreversion) pathway. We investigated the optoelectronic properties of CdS-C3N5 heterojunctions using density functional theory (DFT) simulations, finite difference time domain (FDTD) simulations, time-resolved terahertz spectroscopy (TRTS), and photoconductivity measurements. TRTS indicated high carrier mobilities >450 cm2 V–1 s–1 and carrier relaxation times >60 ps for CdS-MHP, while CdS-MHPINS exhibited much lower mobilities <150 cm2 V–1 s–1 and short carrier relaxation times <20 ps. Hysteresis in the photoconductive J–V characteristics of CdS NWs disappeared in CdS-MHP, confirming surface passivation. Dispersion-corrected DFT simulations indicated a delocalized HOMO and a LUMO localized on C3N5 in CdS-MHP. C3N5, with its extended π-conjugation and low band gap, can function as a shuttle to extract carriers and excitons in nanostructured heterojunctions, and enhance performance in optoelectronic devices. Our results demonstrate how carrier dynamics in core–shell heterostructures can be manipulated to achieve control over the reaction mechanism in photocatalysis.
Boosting Photocatalytic Activity Using Carbon Nitride Based 2D/2D van der Waa...Pawan Kumar
The surging demand for energy and staggering pollutants in the environment have geared the scientific community to explore sustainable pathways that are economically feasible and environmentally compelling. In this context, harnessing solar energy using semiconductor materials to generate charge pairs to drive photoredox reactions has been envisioned as a futuristic approach. Numerous inorganic crystals with promising nanoregime properties investigated in the past decade have yet to demonstrate practical application due to limited photon absorption and sluggish charge separation kinetics. Two-dimensional semiconductors with tunable optical and electronic properties and quasi-resistance-free lateral charge transfer mechanisms have shown great promise in photocatalysis. Polymeric graphitic carbon nitride (g-C3N4) is among the most promising candidates due to fine-tuned band edges and the feasibility of optimizing the optical properties via materials genomics. Constructing a two-dimensional (2D)/2D van der Waals (vdW) heterojunction by allies of 2D carbon nitride sheets and other 2D semiconductors has demonstrated enhanced charge separation with improved visible photon absorption, and the performance is not restricted by the lattice matching of constituting materials. With the advent of new 2D semiconductors over the recent past, the 2D/2D heterojunction assemblies are gaining momentum to design high performance photocatalysts for numerous applications. This review aims to highlight recent advancements and key understanding in carbon nitride based 2D/2D heterojunctions and their applications in photocatalysis, including small molecules activation, conversion, and degradations. We conclude with a forward-looking perspective discussing the key challenges and opportunity areas for future research.
Nanostructured composite materials for CO2 activationPawan Kumar
The increasing energy crisis and the worsening global climate caused by the excessive
utilization of the fossil fuel have boosted tremendous research about CO2 capture, storage and
utilization. Among these approaches, utilization of carbon dioxide to produce valuable chemicals
is preferred than dumping it. Particularly, utilization of CO2 as feedstock for the photocatalytic
conversion into valuable products is a viable approach for harvesting solar radiation as an energy
source and to mitigate increasing CO2 concentration. Artificial photosynthesis by using
nanostructured materials as photocatalyst has immense potential to convert carbon dioxide into
renewable fuels such as methanol/CO etc. The present chapter focuses on the synthesis, characterization and application of various nanostructured materials for CO2 activation including
photoreduction of CO2 to valuable products.
Nanophotonic enhancement and improved electron extraction in perovskite solar...Pawan Kumar
While vertically oriented metal oxide nanowires have been intensely researched for use as electron transport layers (ETLs) in halide perovskite solar cells (HPSCs), horizontal nanowires (oriented roughly parallel to the substrate) have received much less attention despite their higher photonic strength due to overlapping electric and magnetic dipolar Mie resonance modes. Herein, we demonstrate the fabrication of an assembly of horizontally aligned TiO2 nanorods (HATNRs) on FTO substrates via a facile hydrothermal route. The HATNRs are employed as the ETL to achieve 15.03% power conversion efficiency (PCE) in HPSCs which is higher than the PCE of compact TiO2 based devices (10.12%) by a factor of nearly 1.5. A mixed halide, mixed cation organometal perovskite FA0.83MA0.17Pb(Br0.17I0.83)3 with optimized composition is used as the active layer. The excellent refractive index matching between the perovskite and TiO2, coupled with strong Mie scattering in the nanorod geometry results in broadband near-zero backscattering and high forward scattering, upon coating of HATNRs with perovskite. The maximum suppression of backscattering is found at ∼600 nm. The HATNRs ETL also improves the extraction of electrons from the perovskite layer and results in superior blocking of carrier recombination at the perovskite layer/FTO interface.
Nanophotonic enhancement and improved electron extraction in perovskite solar...Pawan Kumar
While vertically oriented metal oxide nanowires have been intensely researched for use as electron transport layers (ETLs) in halide perovskite solar cells (HPSCs), horizontal nanowires (oriented roughly parallel to the substrate) have received much less attention despite their higher photonic strength due to overlapping electric and magnetic dipolar Mie resonance modes. Herein, we demonstrate the fabrication of an assembly of horizontally aligned TiO2 nanorods (HATNRs) on FTO substrates via a facile hydrothermal route. The HATNRs are employed as the ETL to achieve 15.03% power conversion efficiency (PCE) in HPSCs which is higher than the PCE of compact TiO2 based devices (10.12%) by a factor of nearly 1.5. A mixed halide, mixed cation organometal perovskite FA0.83MA0.17Pb(Br0.17I0.83)3 with optimized composition is used as the active layer. The excellent refractive index matching between the …
Water-splitting photoelectrodes consisting of heterojunctions of carbon nitri...Pawan Kumar
Quinary and senary non-stoichiometric double perovskites such as Ba2Ca0.66Nb1.34-xFexO6-δ (BCNF) have been utilized for gas sensing, solid oxide fuel cells and thermochemical CO2 reduction. Herein, we examined their potential as narrow bandgap semiconductors for use in solar energy harvesting. A cobalt co-doped BCNF, Ba2Ca0.66Nb0.68Fe0.33Co0.33O6-δ (BCNFCo), exhibited an optical absorption edge at ~ 800 nm, p-type conduction and a distinct photoresponse upto 640 nm while demonstrating high thermochemical stability. A nanocomposite of BCNFCo and g-C3N4 (CN) was prepared via a facile solvent assisted exfoliation/blending approach using dichlorobenzene and glycerol at a moderate temperature. The exfoliation of g-C3N4 followed by wrapping on perovskite established an effective heterojunction between the materials for charge separation. The conjugated 2D sheets of CN enabled better charge migration resulting in increased photoelectrochemical performance. A blend composed of 40 wt% perovskite and CN performed optimally, whilst achieving a photocurrent density as high as 1.5 mA cm-2 for sunlight-driven water-splitting with a Faradaic efficiency as high as ~ 88%.
Heterojunctions of halogen-doped carbon nitride nanosheets and BiOI for sunli...Pawan Kumar
A fluorine-doped, chlorine-intercalated carbon nitride (CNF-Cl) photocatalyst has been synthesized for simultaneous improvements in light harvesting capability along with suppression
of charge recombination in bulk g-C3N4. The formation of heterojunctions of these CNF-Cl nanosheets with low bandgap, earth abundant bismuth oxyiodide (BiOI) was achieved, and the
synthesized heterojunctions were tested as active photoanodes in photoelectrochemical water splitting experiments. BiOI/CNF-Cl heterojunctions exhibited extended light harvesting with a
band-edge of 680 nm and generated photocurrent densities approaching 1.3 mA cm−2 under AM1.5 G one sun illumination. Scanning Kelvin probe force microscopy under optical bias
showed a surface potential of 207 mV for the 50% BiOI/CNF-Cl nanocomposite, while pristine CNF-Cl and BiOI had surface photopotential values of 83 mV and 98 mV, respectively, which in turn, provided direct evidence of superior charge separation in the heterojunction blends. Enhanced charge carrier separation and improved light harvesting capability in BiOI/CNF-Cl hybrids were found to be the dominant factors in increased photocurrent, compared to the pristine constituent materials.
Heterojunctions of halogen-doped carbon nitride nanosheets and BiOI for sunli...Pawan Kumar
A fluorine-doped, chlorine-intercalated carbon nitride (CNF-Cl) photocatalyst has been synthesized for simultaneous improvements in light harvesting capability along with suppression of charge recombination in bulk gC 3 N 4. The formation of heterojunctions of these CNF-Cl nanosheets with low bandgap, earth abundant bismuth oxyiodide (BiOI) was achieved, and the synthesized heterojunctions were tested as active photoanodes in photoelectrochemical water splitting experiments. BiOI/CNF-Cl heterojunctions exhibited extended light harvesting with a band-edge of 680 nm and generated photocurrent densities approaching 1.3 mA cm− 2 under AM1. 5 G one sun illumination. Scanning Kelvin probe force microscopy under optical bias showed a surface potential of 207 mV for the 50% BiOI/CNF-Cl nanocomposite, while pristine CNF-Cl and BiOI had surface photopotential values of 83 mV and 98 mV …
Air- and water-stable halide perovskite nanocrystals protected with nearly-mo...Pawan Kumar
Halide perovskites are exciting candidates for broad-spectrum photocatalysts but have the problem of ambient stability. Protective shells of oxides and polymers around halide perovskite nano- and micro-crystals provide a measure of chemical and photochemical resilience but the photocatalytic performance of perovskites is compromised due to low electron mobility in amorphous oxide or polymer shells and rapid charge carrier recombination on the surface. Herein an in situ surface passivation and stabilization of CsPbBr3 nanocrystals was achieved using monolayered graphenic carbon nitride (CNM). Extensive characterization of carbon nitride protected CsPbBr3 nanocrystals (CNMBr) indicated spherical CsPbBr3 nanoparticles encased in a few nm thick g-C3N4 sheets facilitating better charge separation via percolation/tunneling of charges on conductive 2D nanosheets. The CNMBr core-shell nanocrystals demonstrated enhanced photoelectrochemical water splitting performance and photocurrent reaching up to 1.55 mA cm−2. The CNMBr catalyst was successfully deployed for CO2 photoreduction giving carbon monoxide and methane as the reaction products.
tA highly efficient, recyclable and magnetically separable core-shell structured CuZnO@Fe3O4microspherewrapped with reduced graphene oxide (rGO@CuZnO@Fe3O4) photocatalyst has been developed and usedfor the photoreduction of carbon dioxide with water to produce methanol under visible light irradiation.Owing to the synergistic effect of the components and to the presence of a thin Fe2O3layer on Fe3O4,rGO@CuZnO@Fe3O44 exhibited higher catalytic activity as compared to the other possible combinationssuch as CuZnO@Fe3O42 and GO@CuZnO@Fe3O43 microspheres. The yield of methanol in case of using2 and 3 as photocatalyst was found to be 858 and 1749 mol g−1cat, respectively. However, the yieldwas increased to 2656 mol g−1cat when rGO@CuZnO@Fe3O44 was used as photocatalyst under sim-ilar experimental conditions. This superior photocatalytic activity of 4 was assumed to be due to therestoration of the sp2hybridized aromatic system in rGO, which facilitated the movement of electronsand resulted in better charge separation. The synthesized heterogeneous photocatalyst could readily berecovered by external magnet and successfully reused for six subsequent cycles without significant loss in the product yield.
Radial Nano-Heterojunctions Consisting of CdS Nanorods Wrapped by 2D CN:PDI P...Pawan Kumar
Solar energy harvesting using semiconductor photocatalysis offers an enticing solution to two of the biggest societal challenges, energy scarcity and environmental pollution. After decades of effort, no photocatalyst exists which can simultaneously meet the demand for excellent absorption, high quantum efficiency and photochemical resilience/durability. While CdS is an excellent photocatalyst for hydrogen evolution, pollutant degradation and organic synthesis, photocorrosion of CdS leads to the deactivation of the catalyst. Surface passivation of CdS with 2D graphitic carbon nitrides (CN) such as g-C3N4 and C3N5 has been shown to mitigate the photocorrosion problem but the poor oxidizing power of photogenerated holes in CN limits the utility of this approach for photooxidation reactions. We report the synthesis of exfoliated 2D nanosheets of a modified carbon nitride constituted of tris-s-triazine (C6N7) linked pyromellitic dianhydride polydiimide (CN:PDI) with a deep oxidative highest occupied molecular orbital (HOMO) position, which ensures sufficient oxidizing power for photogenerated holes in CN. The heterojunction formed by the wrapping of mono-/few layered CN:PDI on CdS nanorods (CdS/CN:PDI) was determined to be an excellent photocatalyst for oxidation reactions including photoelectrochemical water splitting, dye decolorization and the photocatalytic conversion of benzyl alcohol to benzaldehyde. Extensive structural characterization using HR-TEM, Raman, XPS, etc., confirmed wrapping of few-layered CN:PDI on CdS nanorods. The increased photoactivity in CdS/CN:PDI catalyst was ascribed to facile electron transfer from CdS to CN:PDI in comparison to CdS/g-C3N4, leading to an increased electron density on the surface of the photocatalyst to drive chemical reactions.
Metal-organic hybrid: Photoreduction of CO2 using graphitic carbon nitride su...Pawan Kumar
A novel heteroleptic iridium complex supported on graphitic carbon nitride was synthesized and used for photoreduction of carbon dioxide under visible light irradiation. The methanol yield obtained after 24 h irradiation was 9934 μmol g−1cat (TON 1241 with respect to Ir) by using triethylamine (TEA) as a sacrificial donor, which was significantly higher as compared to the semiconductor carbon nitride 145 μmol g−1cat under identical conditions. The presence of triethylamine was found to be vital for the higher methanol yield. After the reaction, the photocatalyst could easily be recovered and reused for subsequent six runs without significant loss in photo activity.
Metal-organic hybrid: Photoreduction of CO2 using graphitic carbon nitride su...Pawan Kumar
A novel heteroleptic iridium complex supported on graphitic carbon nitride was synthesized and used
for photoreduction of carbon dioxide under visible light irradiation. The methanol yield obtained after
24 h irradiation was 9934 mmol g1cat (TON 1241 with respect to Ir) by using triethylamine (TEA) as a
sacrificial donor, which was significantly higher as compared to the semiconductor carbon nitride
145 mmol g1cat under identical conditions. The presence of triethylamine was found to be vital for the
higher methanol yield. After the reaction, the photocatalyst could easily be recovered and reused for
subsequent six runs without significant loss in photo activity.
Metal-organic hybrid: Photoreduction of CO2 using graphitic carbon nitride su...Pawan Kumar
A novel heteroleptic iridium complex supported on graphitic carbon nitride was synthesized and used
for photoreduction of carbon dioxide under visible light irradiation. The methanol yield obtained after
24 h irradiation was 9934 mmol g1cat (TON 1241 with respect to Ir) by using triethylamine (TEA) as a
sacrificial donor, which was significantly higher as compared to the semiconductor carbon nitride
145 mmol g1cat under identical conditions. The presence of triethylamine was found to be vital for the
higher methanol yield. After the reaction, the photocatalyst could easily be recovered and reused for
subsequent six runs without significant loss in photo activity.
Enhanced charge separation in g-C3N4–BiOI heterostructures for visible light ...Pawan Kumar
Heterojunctions of the low bandgap semiconductor bismuth oxyiodide (BiOI) with bulk multilayered
graphitic carbon nitride (g-C3N4) and few layered graphitic carbon nitride sheets (g-C3N4-S) are
synthesized and investigated as an active photoanode material for sunlight driven water splitting. HRTEM and elemental mapping reveals formation of a unique heterostructure between BiOI platelets and
the carbon nitride (g-C3N4 and g-C3N4-S) network that consisted of dendritic BiOI nanoplates
surrounded by g-C3N4 sheets. The presence of BiOI in g-C3N4-S/BiOI and g-C3N4-S/BiOI
nanocomposites extends the visible light absorption profile from 500 nm up to 650 nm. Due to excellent
charge separation in g-C3N4/BiOI and g-C3N4-S/BiOI, evident from quenching of the carbon nitride
photoluminescence (PL) and a decrease in the PL lifetime, a significant increase in photoelectrochemical
performance is observed for both types of g-C3N4–BiOI heterojunctions. In comparison to
heterojunctions of bulk g-C3N4 with BiOI, the nanocomposite consisting of few layered sheets of gC3N4 and BiOI exhibits higher photocurrent density due to lower recombination in few layered sheets. A
synergistic trap passivation and charge separation is found to occur in the g-C3N4-S/BiOI
nanocomposite heterostructure which results in a higher photocurrent and a lower charge transfer
resistance.
C3N5: A Low Bandgap Semiconductor Containing an Azo-Linked Carbon Nitride Fra...Pawan Kumar
Modification of carbon nitride based polymeric 2D materials for tailoring their optical, electronic and chemical properties for various applications has gained significant interest. The present report demonstrates the synthesis of a novel modified carbon nitride framework with a remarkable 3:5 C:N stoichiometry (C3N5) and an electronic bandgap of 1.76 eV, by thermal deammoniation of the melem hydrazine precursor. Characterization revealed that in the C3N5 polymer, two s-heptazine units are bridged together with azo linkage, which constitutes an entirely new and different bonding fashion from g-C3N4 where three heptazine units are linked together with tertiary nitrogen. Extended conjugation due to overlap of azo nitrogens and increased electron density on heptazine nucleus due to the aromatic π network of heptazine units lead to an upward shift of the valence band maximum resulting in bandgap reduction
C3N5: A Low Bandgap Semiconductor Containing an Azo-Linked Carbon Nitride Fra...Pawan Kumar
Modification of carbon nitride based polymeric
2D materials for tailoring their optical, electronic and
chemical properties for various applications has gained
significant interest. The present report demonstrates the
synthesis of a novel modified carbon nitride framework with a
remarkable 3:5 C:N stoichiometry (C3N5) and an electronic
bandgap of 1.76 eV, by thermal deammoniation of the melem
hydrazine precursor. Characterization revealed that in the
C3N5 polymer, two s-heptazine units are bridged together with
azo linkage, which constitutes an entirely new and different
bonding fashion from g-C3N4 where three heptazine units are
linked together with tertiary nitrogen. Extended conjugation
due to overlap of azo nitrogens and increased electron density
on heptazine nucleus due to the aromatic π network of heptazine units lead to an upward shift of the valence band maximum
resulting in bandgap reduction down to 1.76 eV. XRD, He-ion imaging, HR-TEM, EELS, PL, fluorescence lifetime imaging,
Raman, FTIR, TGA, KPFM, XPS, NMR and EPR clearly show that the properties of C3N5 are distinct from pristine carbon
nitride (g-C3N4). When used as an electron transport layer (ETL) in MAPbBr3 based halide perovskite solar cells, C3N5
outperformed g-C3N4, in particular generating an open circuit photovoltage as high as 1.3 V, while C3N5 blended with
MAxFA1−xPb(I0.85Br0.15)3 perovskite active layer achieved a photoconversion efficiency (PCE) up to 16.7%. C3N5 was also
shown to be an effective visible light sensitizer for TiO2 photoanodes in photoelectrochemical water splitting. Because of its
electron-rich character, the C3N5 material displayed instantaneous adsorption of methylene blue from aqueous solution reaching
complete equilibrium within 10 min, which is significantly faster than pristine g-C3N4 and other carbon based materials. C3N5
coupled with plasmonic silver nanocubes promotes plasmon-exciton coinduced surface catalytic reactions reaching completion
at much low laser intensity (1.0 mW) than g-C3N4, which showed sluggish performance even at high laser power (10.0 mW).
The relatively narrow bandgap and 2D structure of C3N5 make it an interesting air-stable and temperature-resistant
semiconductor for optoelectronic applications while its electron-rich character and intrasheet cavity make it an attractive
supramolecular adsorbent for environmental applications.
Reduced graphene oxide–CuO nanocomposites for photocatalyticconversion of CO2...Pawan Kumar
Reduced graphene oxide (rGO)–copper oxide nanocomposites are prepared by covalent grafting of CuOnanorods on the rGO skeleton. Chemical and structural features of rGO–CuO nanocomposites are probedby FTIR, XPS, XRD and HRTEM analyses. Photocatalytic potential of rGO–CuO nanocomposites is exploredfor reduction of CO2into the methanol under the visible light irradiation. The breadth of CuO nanorods andthe oxidation state of Cu in the rGO–CuO/Cu2O nanocomposites are systematically varied to investigatetheir photocatalytic activities. The pristine CuO nanorods exhibited very low photocatalytic activity owingto fast recombination of charge carriers and yielded 175 mol g−1methanol, whereas rGO–Cu2O andrGO–CuO exhibited significantly improved photocatalytic activities and yielded five (862 mol g−1) andseven (1228 mol g−1) folds methanol, respectively. The superior photocatalytic activity of CuO in therGO–CuO nanocomposites was attributed to slow recombination of charge carriers and efficient transferof photo-generated electrons through the rGO skeleton. This study further excludes the use of scavengingdonor.
Reduced graphene oxide–CuO nanocomposites for photocatalyticconversion of CO2...Pawan Kumar
tReduced graphene oxide (rGO)–copper oxide nanocomposites are prepared by covalent grafting of CuOnanorods on the rGO skeleton. Chemical and structural features of rGO–CuO nanocomposites are probedby FTIR, XPS, XRD and HRTEM analyses. Photocatalytic potential of rGO–CuO nanocomposites is exploredfor reduction of CO2into the methanol under the visible light irradiation. The breadth of CuO nanorods andthe oxidation state of Cu in the rGO–CuO/Cu2O nanocomposites are systematically varied to investigatetheir photocatalytic activities. The pristine CuO nanorods exhibited very low photocatalytic activity owingto fast recombination of charge carriers and yielded 175 mol g−1methanol, whereas rGO–Cu2O andrGO–CuO exhibited significantly improved photocatalytic activities and yielded five (862 mol g−1) andseven (1228 mol g−1) folds methanol, respectively. The superior photocatalytic activity of CuO in therGO–CuO nanocomposites was attributed to slow recombination of charge carriers and efficient transferof photo-generated electrons through the rGO skeleton. This study further excludes the use of scavengingdonor.
Nitrogen-Enriched Carbon Nanobubbles and Nanospheres for Applications in Ener...Devika Laishram
Multifunctional carbon nanomaterials have attracted remarkable consideration for use in various energy
conversion and storage devices because of their ultrahigh specific
surface area, unique morphology, and excellent electrochemical
properties. Herein, we report the synthesis of highly uniform and
ordered nitrogen-enriched carbon nanospheres (CS) and nanobubbles (CNB) by a modified Stöber reaction using resorcinol and
formaldehyde in the presence of ethylenediamine as a nitrogen
source. A comparative study of the prepared CS and CNB
nanomaterials is presented here with potential use in a wide variety
of applications involving large surface area and electrical
conductivity. As counter electrode materials in solar cells, CNB and CS showed enhanced photoelectrochemical activity for
catalytically reducing I3
− to I− and improved capacitive behavior with a low charge transfer resistance and remarkable power
conversion efficiency (PCE) of 10.40% with improved Jsc (20.20 mA/cm2
) and Voc (0.73 V). The enhanced performance of the
fabricated photoelectrochemical cell is due to the excellent point contact and good conductivity that offered better charge
transportation of electrons with minimum recombination. The enhanced adsorption upon increasing the pressure without an
apparent saturation level signified the large CO2 adsorption with 2 mmol/g for the CS. Additionally, the rectangular-shaped CV
curve indicated the double-layer capacitive behavior, good electrochemical reversibility, and high-power characteristics, prerequisites
for supercapacitor application. This study probes the practical possibility of nitrogen-enriched carbon nanostructures as a
multifunctional material for prospective applications.
Arrays of TiO2 nanorods embedded with fluorine doped carbon nitride quantum d...Pawan Kumar
Graphenic semiconductors such as carbon nitride are attracting increasing attention as photocatalysts due to their chemical stability, visible light absorption and excellent electronic properties. The photocatalytic activity of nanostructured TiO2 catalysts is constrained by the wide bandgap and concomitant low visible light responsivity of TiO2. In this context we present the formation of new fluorine doped carbon nitride quantum dots (CNFQDs) by solid state reaction and the subsequent examination of their heterojunctions with TiO2 for photoelectrochemical water splitting. Arrays of rutile phase TiO2 nanorods embedded with CNFQDs were synthesized by a simple in situ hydrothermal approach and the resulting nanomaterials were found to exhibit strong visible light absorption. The energetics at the heterojunction were favorable for efficient electron transfer from CNFQDs to TiO2 under visible light irradiation and …
Arrays of TiO2 nanorods embedded with fluorine doped carbon nitride quantum d...Pawan Kumar
Graphenic semiconductors such as carbon nitride are attracting increasing attention as photocatalysts due to their chemical stability, visible light absorption and excellent electronic properties. The photocatalytic activity of nanostructured TiO2 catalysts is constrained by the wide bandgap and concomitant low visible light responsivity of TiO2. In this context we present the formation of new fluorine doped carbon nitride quantum dots (CNFQDs) by solid state reaction and the subsequent examination of their heterojunctions with TiO2 for photoelectrochemical water splitting. Arrays of rutile phase TiO2 nanorods embedded with CNFQDs were synthesized by a simple in situ hydrothermal approach and the resulting nanomaterials were found to exhibit strong visible light absorption. The energetics at the heterojunction were favorable for efficient electron transfer from CNFQDs to TiO2 under visible light irradiation and transfer of holes to the aqueous electrolyte. CNFQD-sensitized TiO2 nanorods exhibited a strong photoelectrochemical response up to 500 nm. Reuse experiments confirmed robustness and long term stability of the sample without exhausting the catalytic performance. The present work demonstrates a new pathway to sensitize TiO2 to visible photons by the in situ formation of embedded heterojunctions with fluorine doped carbon nitride quantum dots
Noble Metal Free, Visible Light Driven Photocatalysis Using TiO2 Nanotube Arr...Pawan Kumar
Bulk g‐C3N4 is an earth‐abundant, easily synthesizable, and exceptionally stable photocatalyst with an electronic bandgap of 2.7 eV. Herein, the concepts of P‐doping and size quantization are combined to synthesize highly fluorescent P‐doped carbon nitride quantum dots (CNPQDs) with a bandgap of 2.1 eV. CNPQDs are hosted on anatase‐phase and rutile‐phase TiO2 nanotube array scaffolds, and examined as photoanodes for sunlight‐driven water‐splitting and as photocatalysts for surface catalytic reactions. Square‐shaped rutile phase TiO2 nanotube arrays (STNAs) decorated with CNPQDs (CNPQD‐STNA) generate 2.54 mA cm−2 photocurrent under AM1.5 G simulated sunlight. A champion hydrogen evolution rate of 22 µmol h−1 corresponds to a Faradaic efficiency of 93.2%. In conjunction with Ag nanoparticles (NPs), the CNPQD‐STNA hybrid is also found to be an excellent plexcitonic photocatalyst for the visible light‐driven transformation of 4‐nitrobenzenethiol (4‐NBT) to dimercaptoazobenzene (DMAB), producing reaction completion at a laser power of 1 mW (532 nm) while Ag NP/TNA and Ag NP/STNA photocatalysts cannot complete this transformation even at 10 mW laser power. The results point the way forward for photochemically robust, noble metal free, visible light harvesting photoacatalysts based on nanostructured heterojunctions of graphenic frameworks with TiO2.
Enhanced charge separation in gC 3 N 4–BiOI heterostructures for visible ligh...Pawan Kumar
Heterojunctions of the low bandgap semiconductor bismuth oxyiodide (BiOI) with bulk multilayered graphitic carbon nitride (g-C3N4) and few layered graphitic carbon nitride sheets (g-C3N4-S) are synthesized and investigated as an active photoanode material for sunlight driven water splitting. HR-TEM and elemental mapping reveals formation of a unique heterostructure between BiOI platelets and the carbon nitride (g-C3N4 and g-C3N4-S) network that consisted of dendritic BiOI nanoplates surrounded by g-C3N4 sheets. The presence of BiOI in g-C3N4-S/BiOI and g-C3N4-S/BiOI nanocomposites extends the visible light absorption profile from 500 nm up to 650 nm. Due to excellent charge separation in g-C3N4/BiOI and g-C3N4-S/BiOI, evident from quenching of the carbon nitride photoluminescence (PL) and a decrease in the PL lifetime, a significant increase in photoelectrochemical performance is observed …
Similar to Water-splitting photoelectrodes consisting of heterojunctions of carbon nitride with a p-type low bandgap double perovskite oxide (20)
Isolated Iridium Sites on Potassium-Doped Carbon-nitride wrapped Tellurium Na...Pawan Kumar
Many industrial processes such transesterification of fatty acid for biodiesel production, soap manufacturing and biosynthesis of ethanol generate glycerol as a major by-product that can be used to produce commodity chemicals. Photocatalytic transformation of glycerol is an enticing approach that can exclude the need of harsh oxidants and extraneous thermal energy. However, the product yield and selectivity remain poor due to low absorption and unsymmetrical site distribution on the catalyst surface. Herein, tellurium (Te) nanorods/nanosheets (TeNRs/NSs) wrapped potassium-doped carbon nitride (KCN) van der Waal (vdW) heterojunction (TeKCN) is designed to enhance charge separation and visible-NIR absorption. The iridium (Ir) single atom sites decoration on the TeKCN core-shell structure (TeKCNIr) promotes selective oxidation of glycerol to glyceraldehyde with a conversion of 45.6% and selectivity of 61.6% under AM1.5G irradiation. The catalytic selectivity can reach up to 88% under 450 nm monochromatic light. X-ray absorption spectroscopy (XAS) demonstrates the presence of undercoordinated IrN2O2 sites which improved catalytic selectivity for glycol oxidation. Band energies and computational calculations reveal faile charge transfer in the TeKCNIr heterostructure. EPR and scavenger tests discern that superoxide (O2•−) and hydroxyl (•OH) radicals are prime components driving glycerol oxidation.
Isolated Iridium Sites on Potassium-Doped Carbon-nitride wrapped Tellurium Na...Pawan Kumar
Many industrial processes such transesterification of fatty acid for biodiesel production, soap manufacturing and biosynthesis of ethanol generate glycerol as a major by-product that can be used to produce commodity chemicals. Photocatalytic transformation of glycerol is an enticing approach that can exclude the need of harsh oxidants and extraneous thermal energy. However, the product yield and selectivity remain poor due to low absorption and unsymmetrical site distribution on the catalyst surface. Herein, tellurium (Te) nanorods/nanosheets (TeNRs/NSs) wrapped potassium-doped carbon nitride (KCN) van der Waal (vdW) heterojunction (TeKCN) is designed to enhance charge separation and visible-NIR absorption. The iridium (Ir) single atom sites decoration on the TeKCN core-shell structure (TeKCNIr) promotes selective oxidation of glycerol to glyceraldehyde with a conversion of 45.6% and selectivity of 61.6% under AM1.5G irradiation. The catalytic selectivity can reach up to 88% under 450 nm monochromatic light. X-ray absorption spectroscopy (XAS) demonstrates the presence of undercoordinated IrN2O2 sites which improved catalytic selectivity for glycol oxidation. Band energies and computational calculations reveal faile charge transfer in the TeKCNIr heterostructure. EPR and scavenger tests discern that superoxide (O2•−) and hydroxyl (•OH) radicals are prime components driving glycerol oxidation.
Isolated Iridium Sites on Potassium-Doped Carbon-nitride wrapped Tellurium Na...Pawan Kumar
Many industrial processes such transesterification of fatty acid for biodiesel production, soap manufacturing and biosynthesis of ethanol generate glycerol as a major by-product that can be used to produce commodity chemicals. Photocatalytic transformation of glycerol is an enticing approach that can exclude the need of harsh oxidants and extraneous thermal energy. However, the product yield and selectivity remain poor due to low absorption and unsymmetrical site distribution on the catalyst surface. Herein, tellurium (Te) nanorods/nanosheets (TeNRs/NSs) wrapped potassium-doped carbon nitride (KCN) van der Waal (vdW) heterojunction (TeKCN) is designed to enhance charge separation and visible-NIR absorption. The iridium (Ir) single atom sites decoration on the TeKCN core-shell structure (TeKCNIr) promotes selective oxidation of glycerol to glyceraldehyde with a conversion of 45.6% and selectivity of 61.6% under AM1.5G irradiation. The catalytic selectivity can reach up to 88% under 450 nm monochromatic light. X-ray absorption spectroscopy (XAS) demonstrates the presence of undercoordinated IrN2O2 sites which improved catalytic selectivity for glycol oxidation. Band energies and computational calculations reveal faile charge transfer in the TeKCNIr heterostructure. EPR and scavenger tests discern that superoxide (O2•−) and hydroxyl (•OH) radicals are prime components driving glycerol oxidation.
Solar-Driven Cellulose Photorefining into Arabinose over Oxygen-Doped Carbon ...Pawan Kumar
Biomass photorefining is a promising strategy to address the energy crisis and transition toward carbon carbon-neutral society. Here, we demonstrate the feasibility of direct cellulose photorefining into arabinose by a rationally designed oxygen-doped polymeric carbon nitride, which generates favorable oxidative species (e.g., O2–, •OH) for selective oxidative reactions at neutral conditions. In addition, we also illustrate the mechanism of the photocatalytic cellulose to arabinose conversion by density functional theory calculations. The oxygen insertion derived from oxidative radicals at the C1 position of glucose within cellulose leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in the subsequent gluconic acid formation. The following decarboxylation process of gluconic acid via C1–C2 α-scissions, triggered by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This work not only offers a mechanistic understanding of cellulose photorefining to arabinose but also sets up an example for illuminating the path toward direct cellulose photorefining into value-added bioproducts under mild conditions.
Solar-Driven Cellulose Photorefining into Arabinose over Oxygen-Doped Carbon ...Pawan Kumar
Biomass photorefining is a promising strategy to address the energy crisis and transition toward carbon carbon-neutral society. Here, we demonstrate the feasibility of direct cellulose photorefining into arabinose by a rationally designed oxygen-doped polymeric carbon nitride, which generates favorable oxidative species (e.g., O2–, •OH) for selective oxidative reactions at neutral conditions. In addition, we also illustrate the mechanism of the photocatalytic cellulose to arabinose conversion by density functional theory calculations. The oxygen insertion derived from oxidative radicals at the C1 position of glucose within cellulose leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in the subsequent gluconic acid formation. The following decarboxylation process of gluconic acid via C1–C2 α-scissions, triggered by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This work not only offers a mechanistic understanding of cellulose photorefining to arabinose but also sets up an example for illuminating the path toward direct cellulose photorefining into value-added bioproducts under mild conditions.
Solar-Driven Cellulose Photorefining into Arabinose over Oxygen-Doped Carbon ...Pawan Kumar
Biomass photorefining is a promising strategy to address the energy crisis and transition toward carbon carbon-neutral society. Here, we demonstrate the feasibility of direct cellulose photorefining into arabinose by a rationally designed oxygen-doped polymeric carbon nitride, which generates favorable oxidative species (e.g., O2–, •OH) for selective oxidative reactions at neutral conditions. In addition, we also illustrate the mechanism of the photocatalytic cellulose to arabinose conversion by density functional theory calculations. The oxygen insertion derived from oxidative radicals at the C1 position of glucose within cellulose leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in the subsequent gluconic acid formation. The following decarboxylation process of gluconic acid via C1–C2 α-scissions, triggered by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This work not only offers a mechanistic understanding of cellulose photorefining to arabinose but also sets up an example for illuminating the path toward direct cellulose photorefining into value-added bioproducts under mild conditions.
Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single...Pawan Kumar
Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer–Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.
Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single...Pawan Kumar
Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer–Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.
Selective Cellobiose Photoreforming for Simultaneous Gluconic Acid and Syngas...Pawan Kumar
Here, we demonstrate the selective cellobiose (building block of cellulose) photoreforming for gluconic acid and syngas co-production in acidic conditions by rationally designing a bifunctional polymeric carbon nitride (CN) with potassium/sulfur co-dopant. This heteroatomic doped CN photocatalyst possesses enhanced visible light absorption, higher charge separation efficiency than pristine CN. Under acidic conditions, cellobiose is not only more efficiently hydrolyzed into glucose but also promotes the syngas and gluconic acid production. Density functional theory (DFT) calculations reveal the favorable generation of •O2− during the photocatalytic reaction, which is essential for gluconic acid production. Consequently, the fine-designed photocatalyst presents excellent cellobiose conversion (>80%) and gluconic acid selectivity (>70%) together with the co-production of syngas (~56 μmol g-1 h-1) under light illumination. The current work demonstrates the feasibility of biomass photoreforming with value-added chemicals and syngas co-production under mild condition.
Selective Cellobiose Photoreforming for Simultaneous Gluconic Acid and Syngas...Pawan Kumar
Here, we demonstrate the selective cellobiose (building block of cellulose) photoreforming for gluconic acid and syngas co-production in acidic conditions by rationally designing a bifunctional polymeric carbon nitride (CN) with potassium/sulfur co-dopant. This heteroatomic doped CN photocatalyst possesses enhanced visible light absorption, higher charge separation efficiency than pristine CN. Under acidic conditions, cellobiose is not only more efficiently hydrolyzed into glucose but also promotes the syngas and gluconic acid production. Density functional theory (DFT) calculations reveal the favorable generation of •O2− during the photocatalytic reaction, which is essential for gluconic acid production. Consequently, the fine-designed photocatalyst presents excellent cellobiose conversion (>80%) and gluconic acid selectivity (>70%) together with the co-production of syngas (~56 μmol g-1 h-1) under light illumination. The current work demonstrates the feasibility of biomass photoreforming with value-added chemicals and syngas co-production under mild condition.
Selective Cellobiose Photoreforming for Simultaneous Gluconic Acid and Syngas...Pawan Kumar
Here, we demonstrate the selective cellobiose (building block of cellulose) photoreforming for gluconic acid and syngas co-production in acidic conditions by rationally designing a bifunctional polymeric carbon nitride (CN) with potassium/sulfur co-dopant. This heteroatomic doped CN photocatalyst possesses enhanced visible light absorption, higher charge separation efficiency than pristine CN. Under acidic conditions, cellobiose is not only more efficiently hydrolyzed into glucose but also promotes the syngas and gluconic acid production. Density functional theory (DFT) calculations reveal the favorable generation of •O2− during the photocatalytic reaction, which is essential for gluconic acid production. Consequently, the fine-designed photocatalyst presents excellent cellobiose conversion (>80%) and gluconic acid selectivity (>70%) together with the co-production of syngas (~56 μmol g-1 h-1) under light illumination. The current work demonstrates the feasibility of biomass photoreforming with value-added chemicals and syngas co-production under mild condition.
Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single...Pawan Kumar
Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer–Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.
Recent advancements in tuning the electronic structures of transitional metal...Pawan Kumar
The smooth transition from finite non-renewables to renewable energy conversion technologies will require efficient electrocatalysts which can harness intermittent energies to store in the form of chemical bonds. The oxygen evolution reaction (OER) impedes the widespread usage of water electrolyzers to convert H2O into H2 and persists as a bottleneck, including other energy conversion devices with sluggish four H+/e− kinetics. In this context, designing highly active and stable catalysts capable of driving a lower overpotential in the OER to produce continuous hydrogen (H2) is a primary demanded. This chapter discussed the mechanism of the OER in conventional adsorbate oxygen and lattice oxygen participation in transition metal oxides (TMOs). Further, the influences of surface engineering, doping, and defects in the TMOs and understanding the electronic structure to screen electrodes towards the structure–activity relationship are highlighted. Specifically, the adsorption strength of O 2p is understood in detail as its binding ability over the surface of TMOs can be correlated directly to the OER activity. The iterative development of TMOs in terms of understanding electronic structural attributes is essential for the commercial deployment of energy conversion technologies. The comprehensive outlook of this chapter investigates thoroughly how TMOs can be used as significant materials for the OER in the near future.
Hole transport materials (HTMs) have a significant impact on the effectiveness of organic electronic devices; therefore, we present a molecular architecture of pyrazino[2,3-g]quinoxaline (PQ10)-based room-temperature organic liquid crystalline semiconductor (OLCS) as an alternative HTM. The PQ10 compound exhibits three different rectangular columnar (Colr) phases offering an impressive hole mobility of 8.8 × 10−3 cm2V−1s−1 which is found to be dexterous than most of existing polymeric hole transport materials. The charge transport mechanism is governed by the hole polarons hopping through H-aggregates of the PQ10 molecules and the hole mobility remains nearly constant throughout the mesophase range, but it decreases with increasing applied electric field. The current-voltage characteristics of the PQ10 have also been investigated in all three Colr phases and explained via the Poole-Frenkel conduction mechanism. The dielectric spectroscopy has been eventually carried out to understand the nature of dielectric permittivity and conductivity as a function of temperature and a correlation is established between the molecular architecture of the Colr phases and aforementioned physical properties. Solar cell simulation has been additionally performed to demonstrate that the PQ10 material can be a better choice as HTM for organic electronics and photovoltaic applications.
Multifunctional carbon nitride nanoarchitectures for catalysisPawan Kumar
Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.
Production of Renewable Fuels by the Photocatalytic Reduction of CO2 using Ma...Pawan Kumar
The photo-reductive performance of natural ilmenite was boosted and the production of renewable fuels from the reduction of CO2 was enhanced by doping the natural mineral with magnesium. The doping was achieved by high energy ball milling in the presence of MgO and Mg(NO3)2. The photo-reduction of CO2 in aqueous solution led to the evolution of H2, CH4, C2H4, and C2H6, and the insertion of Mg in the structure of ilmenite enabled increases of up to 1245% in the fuel production yield, reaching total production of 210.9 µmol h-1 gcat-1. Displacements of the conduction band to more negative potentials were evidenced for the samples doped with magnesium. Indirect effects such as increases in the valence band maximum, and the introduction of intermediate energy levels were also evidenced through the measurement of the crystallite size and the determination of the band structure of the materials. Mott-Schottky analyses of the samples showed the n-type nature of the semiconductor materials and enabled the estimation of the density of charge carriers, which strongly influenced the photocatalytic performance. The strong potential of the application of natural ilmenite in gas phase artificial photosynthesis was proved by the evaluation of CO2 reduction in gas conditions, which allowed the enhancement in the selectivity and significantly increased the production of CH4 as compared to aqueous solution, reaching an important yield of CH4 of 16.1 µmol h-1 gcat-1.
Nanoengineered Au-Carbon Nitride Interfaces Enhance PhotoCatalytic Pure Water...Pawan Kumar
Photocatalytic pure water splitting using solar energy is one of the promising routes to produce sustainable green hydrogen (H2). Tuning the interfacial active site density at catalytic heterojunctions and better light management are imperative to steer the structure-activity correlations to enhance the photo-efficiency of nanocomposite photocatalysts. Herein, we report the decoration of nitrogen defects-rich carbon nitride CN(T) with metallic Au nanostructures of different morphologies and sizes to investigate their influence on the photocatalytic hydrogen evolution reactions (HER). The CN(T)-7-NP nano-heterostructure comprises Au nanoparticles (NPs) of ~7 nm and thiourea-derived defective CN exhibits an excellent H2 production rate of 76.8 µmol g–1 h–1 from pure water under simulated AM 1.5 solar irradiation. In contrast to large-size Au nanorods, the high activity of CN(T)-7-NP was attributed to their strong localized surface plasmon resonance (LSPR) mediated visible absorption and interfacial charge separation. The surface ligands used to control Au nanostructures morphology were found to play a major role in the stabilization of NPs and improve interfacial charge transport between Au NPs and CN(T). First-principles calculations revealed that defects in CN and Au-CN interfacial sites in these nanocomposites facilitate the separation of e-/h+ pairs after light excitation and provide lower energy barrier pathways for H2 production by photocatalytic water splitting.
Nanoengineered Au-Carbon Nitride Interfaces Enhance Photo-Catalytic Pure Wate...Pawan Kumar
Photocatalytic pure water splitting using solar energy is one of the promising routes to produce sustainable green hydrogen (H2). Tuning the interfacial active site density at catalytic heterojunctions and better light management are imperative to steer the structure-activity correlations to enhance the photo-efficiency of nanocomposite photocatalysts. Herein, we report the decoration of nitrogen defects-rich carbon nitride CN(T) with metallic Au nanostructures of different morphologies and sizes to investigate their influence on the photocatalytic hydrogen evolution reactions (HER). The CN(T)-7-NP nano-heterostructure comprises Au nanoparticles (NPs) of ~7 nm and thiourea-derived defective CN exhibits an excellent H2 production rate of 76.8 µmol g–1 h–1 from pure water under simulated AM 1.5 solar irradiation. In contrast to large-size Au nanorods, the high activity of CN(T)-7-NP was attributed to their strong localized surface plasmon resonance (LSPR) mediated visible absorption and interfacial charge separation. The surface ligands used to control Au nanostructures morphology were found to play a major role in the stabilization of NPs and improve interfacial charge transport between Au NPs and CN(T). First-principles calculations revealed that defects in CN and Au-CN interfacial sites in these nanocomposites facilitate the separation of e-/h+ pairs after light excitation and provide lower energy barrier pathways for H2 production by photocatalytic water splitting.
Cooperative Copper Single Atom Catalyst in Two-dimensional Carbon Nitride for...Pawan Kumar
Renewable electricity powered carbon dioxide (CO2) reduction (eCO2R) to high-value fuels like methane (CH4) holds the potential to close the carbon cycle at meaningful scales. However, this kinetically staggered 8-electron multistep reduction still suffers from inadequate catalytic efficiency and current density. Atomic Cu-structures can boost eCO2R-to-CH4 selectivity due to enhanced intermediate binding energies (BEs) resulting from favorably shifted d-band centers. Herein, we exploit two-dimensional carbon nitride (CN) matrices, viz. Na-polyheptazine (PHI) and Li-polytriazine imides (PTI), to host Cu-N2 type single atom sites with high density (∼1.5 at%), via a facile metal ion exchange process. Optimized Cu loading in nanocrystalline Cu-PTI maximizes eCO2R-to-CH4 performance with Faradaic efficiency (FECH4) of ≈68% and a high partial current density of 348 mA cm−2 at a low potential of -0.84 V versus RHE, surpassing the state-of-the-art catalysts. Multi-Cu substituted N-appended nanopores in the CN frameworks yield thermodynamically stable quasi-dual/triple sites with large interatomic distances dictated by the pore dimensions. First-principles calculations elucidate the relative Cu-CN cooperative effects between the two matrices and how the Cu-Cu distance and local environment dictate the adsorbate BEs, density of states, and CO2-to-CH4 energy profile landscape. The 9N pores in Cu-PTI yield cooperative Cu-Cu sites that synergistically enhance the kinetics of the rate-limiting steps in the eCO2R-to-CH4 pathway.
Bioinspired multimetal electrocatalyst for selective methane oxidationPawan Kumar
Selective partial electrooxidation of methane (CH4) to liquid oxygenates has been a long-sought goal. However, the high activation energy of C–H bonds and competing oxygen evolution reaction limit product selectivity and reaction rates. Inspired by iron (IV)-oxo containing metalloenzymes’ functionality to activate the C–H bond, here we report on the design of a copper-iron-nickel catalyst for selective oxidation of CH4 to formate via a peroxide-assisted pathway. Each catalyst serves a specific role which is confirmed via electrochemical, in situ, and theoretical studies. A combination of electrochemical and in situ spectroelectrochemical studies revealed that H2O2 oxidation on nickel led to the formation of active oxygen species which trigger the formation of iron (IV) at low voltages. Density functional theory analysis helped reveal the role of iron (IV)-oxo species in reducing the activation energy barrier for CH4 deprotonation and the critical role of copper to suppress overoxidation. Our multimetal catalyst exhibits a formate faradaic efficiency of 42% at an applied potential of 0.9 V versus a reversible hydrogen electrode.
Salas, V. (2024) "John of St. Thomas (Poinsot) on the Science of Sacred Theol...Studia Poinsotiana
I Introduction
II Subalternation and Theology
III Theology and Dogmatic Declarations
IV The Mixed Principles of Theology
V Virtual Revelation: The Unity of Theology
VI Theology as a Natural Science
VII Theology’s Certitude
VIII Conclusion
Notes
Bibliography
All the contents are fully attributable to the author, Doctor Victor Salas. Should you wish to get this text republished, get in touch with the author or the editorial committee of the Studia Poinsotiana. Insofar as possible, we will be happy to broker your contact.
What is greenhouse gasses and how many gasses are there to affect the Earth.moosaasad1975
What are greenhouse gasses how they affect the earth and its environment what is the future of the environment and earth how the weather and the climate effects.
Deep Behavioral Phenotyping in Systems Neuroscience for Functional Atlasing a...Ana Luísa Pinho
Functional Magnetic Resonance Imaging (fMRI) provides means to characterize brain activations in response to behavior. However, cognitive neuroscience has been limited to group-level effects referring to the performance of specific tasks. To obtain the functional profile of elementary cognitive mechanisms, the combination of brain responses to many tasks is required. Yet, to date, both structural atlases and parcellation-based activations do not fully account for cognitive function and still present several limitations. Further, they do not adapt overall to individual characteristics. In this talk, I will give an account of deep-behavioral phenotyping strategies, namely data-driven methods in large task-fMRI datasets, to optimize functional brain-data collection and improve inference of effects-of-interest related to mental processes. Key to this approach is the employment of fast multi-functional paradigms rich on features that can be well parametrized and, consequently, facilitate the creation of psycho-physiological constructs to be modelled with imaging data. Particular emphasis will be given to music stimuli when studying high-order cognitive mechanisms, due to their ecological nature and quality to enable complex behavior compounded by discrete entities. I will also discuss how deep-behavioral phenotyping and individualized models applied to neuroimaging data can better account for the subject-specific organization of domain-general cognitive systems in the human brain. Finally, the accumulation of functional brain signatures brings the possibility to clarify relationships among tasks and create a univocal link between brain systems and mental functions through: (1) the development of ontologies proposing an organization of cognitive processes; and (2) brain-network taxonomies describing functional specialization. To this end, tools to improve commensurability in cognitive science are necessary, such as public repositories, ontology-based platforms and automated meta-analysis tools. I will thus discuss some brain-atlasing resources currently under development, and their applicability in cognitive as well as clinical neuroscience.
Seminar of U.V. Spectroscopy by SAMIR PANDASAMIR PANDA
Spectroscopy is a branch of science dealing the study of interaction of electromagnetic radiation with matter.
Ultraviolet-visible spectroscopy refers to absorption spectroscopy or reflect spectroscopy in the UV-VIS spectral region.
Ultraviolet-visible spectroscopy is an analytical method that can measure the amount of light received by the analyte.
Toxic effects of heavy metals : Lead and Arsenicsanjana502982
Heavy metals are naturally occuring metallic chemical elements that have relatively high density, and are toxic at even low concentrations. All toxic metals are termed as heavy metals irrespective of their atomic mass and density, eg. arsenic, lead, mercury, cadmium, thallium, chromium, etc.
(May 29th, 2024) Advancements in Intravital Microscopy- Insights for Preclini...Scintica Instrumentation
Intravital microscopy (IVM) is a powerful tool utilized to study cellular behavior over time and space in vivo. Much of our understanding of cell biology has been accomplished using various in vitro and ex vivo methods; however, these studies do not necessarily reflect the natural dynamics of biological processes. Unlike traditional cell culture or fixed tissue imaging, IVM allows for the ultra-fast high-resolution imaging of cellular processes over time and space and were studied in its natural environment. Real-time visualization of biological processes in the context of an intact organism helps maintain physiological relevance and provide insights into the progression of disease, response to treatments or developmental processes.
In this webinar we give an overview of advanced applications of the IVM system in preclinical research. IVIM technology is a provider of all-in-one intravital microscopy systems and solutions optimized for in vivo imaging of live animal models at sub-micron resolution. The system’s unique features and user-friendly software enables researchers to probe fast dynamic biological processes such as immune cell tracking, cell-cell interaction as well as vascularization and tumor metastasis with exceptional detail. This webinar will also give an overview of IVM being utilized in drug development, offering a view into the intricate interaction between drugs/nanoparticles and tissues in vivo and allows for the evaluation of therapeutic intervention in a variety of tissues and organs. This interdisciplinary collaboration continues to drive the advancements of novel therapeutic strategies.
Slide 1: Title Slide
Extrachromosomal Inheritance
Slide 2: Introduction to Extrachromosomal Inheritance
Definition: Extrachromosomal inheritance refers to the transmission of genetic material that is not found within the nucleus.
Key Components: Involves genes located in mitochondria, chloroplasts, and plasmids.
Slide 3: Mitochondrial Inheritance
Mitochondria: Organelles responsible for energy production.
Mitochondrial DNA (mtDNA): Circular DNA molecule found in mitochondria.
Inheritance Pattern: Maternally inherited, meaning it is passed from mothers to all their offspring.
Diseases: Examples include Leber’s hereditary optic neuropathy (LHON) and mitochondrial myopathy.
Slide 4: Chloroplast Inheritance
Chloroplasts: Organelles responsible for photosynthesis in plants.
Chloroplast DNA (cpDNA): Circular DNA molecule found in chloroplasts.
Inheritance Pattern: Often maternally inherited in most plants, but can vary in some species.
Examples: Variegation in plants, where leaf color patterns are determined by chloroplast DNA.
Slide 5: Plasmid Inheritance
Plasmids: Small, circular DNA molecules found in bacteria and some eukaryotes.
Features: Can carry antibiotic resistance genes and can be transferred between cells through processes like conjugation.
Significance: Important in biotechnology for gene cloning and genetic engineering.
Slide 6: Mechanisms of Extrachromosomal Inheritance
Non-Mendelian Patterns: Do not follow Mendel’s laws of inheritance.
Cytoplasmic Segregation: During cell division, organelles like mitochondria and chloroplasts are randomly distributed to daughter cells.
Heteroplasmy: Presence of more than one type of organellar genome within a cell, leading to variation in expression.
Slide 7: Examples of Extrachromosomal Inheritance
Four O’clock Plant (Mirabilis jalapa): Shows variegated leaves due to different cpDNA in leaf cells.
Petite Mutants in Yeast: Result from mutations in mitochondrial DNA affecting respiration.
Slide 8: Importance of Extrachromosomal Inheritance
Evolution: Provides insight into the evolution of eukaryotic cells.
Medicine: Understanding mitochondrial inheritance helps in diagnosing and treating mitochondrial diseases.
Agriculture: Chloroplast inheritance can be used in plant breeding and genetic modification.
Slide 9: Recent Research and Advances
Gene Editing: Techniques like CRISPR-Cas9 are being used to edit mitochondrial and chloroplast DNA.
Therapies: Development of mitochondrial replacement therapy (MRT) for preventing mitochondrial diseases.
Slide 10: Conclusion
Summary: Extrachromosomal inheritance involves the transmission of genetic material outside the nucleus and plays a crucial role in genetics, medicine, and biotechnology.
Future Directions: Continued research and technological advancements hold promise for new treatments and applications.
Slide 11: Questions and Discussion
Invite Audience: Open the floor for any questions or further discussion on the topic.
THE IMPORTANCE OF MARTIAN ATMOSPHERE SAMPLE RETURN.Sérgio Sacani
The return of a sample of near-surface atmosphere from Mars would facilitate answers to several first-order science questions surrounding the formation and evolution of the planet. One of the important aspects of terrestrial planet formation in general is the role that primary atmospheres played in influencing the chemistry and structure of the planets and their antecedents. Studies of the martian atmosphere can be used to investigate the role of a primary atmosphere in its history. Atmosphere samples would also inform our understanding of the near-surface chemistry of the planet, and ultimately the prospects for life. High-precision isotopic analyses of constituent gases are needed to address these questions, requiring that the analyses are made on returned samples rather than in situ.
The ability to recreate computational results with minimal effort and actionable metrics provides a solid foundation for scientific research and software development. When people can replicate an analysis at the touch of a button using open-source software, open data, and methods to assess and compare proposals, it significantly eases verification of results, engagement with a diverse range of contributors, and progress. However, we have yet to fully achieve this; there are still many sociotechnical frictions.
Inspired by David Donoho's vision, this talk aims to revisit the three crucial pillars of frictionless reproducibility (data sharing, code sharing, and competitive challenges) with the perspective of deep software variability.
Our observation is that multiple layers — hardware, operating systems, third-party libraries, software versions, input data, compile-time options, and parameters — are subject to variability that exacerbates frictions but is also essential for achieving robust, generalizable results and fostering innovation. I will first review the literature, providing evidence of how the complex variability interactions across these layers affect qualitative and quantitative software properties, thereby complicating the reproduction and replication of scientific studies in various fields.
I will then present some software engineering and AI techniques that can support the strategic exploration of variability spaces. These include the use of abstractions and models (e.g., feature models), sampling strategies (e.g., uniform, random), cost-effective measurements (e.g., incremental build of software configurations), and dimensionality reduction methods (e.g., transfer learning, feature selection, software debloating).
I will finally argue that deep variability is both the problem and solution of frictionless reproducibility, calling the software science community to develop new methods and tools to manage variability and foster reproducibility in software systems.
Exposé invité Journées Nationales du GDR GPL 2024
Deep Software Variability and Frictionless Reproducibility
Water-splitting photoelectrodes consisting of heterojunctions of carbon nitride with a p-type low bandgap double perovskite oxide
1. Nanotechnology
PAPER
Water-splitting photoelectrodes consisting of
heterojunctions of carbon nitride with a p-type low
bandgap double perovskite oxide
To cite this article: Pawan Kumar et al 2021 Nanotechnology 32 485407
View the article online for updates and enhancements.
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3. nano-spikes etc), surface area enhancement, light trapping and
heterojunction formation [13]. Unfortunately, none of the sys-
tems can consistently approach desirable photoconversion effi-
ciencies in excess of 10% in conjunction with durability and low
cost. The key impediments to high performance are limited
visible light absorption, charge carrier recombination losses and
high overpotentials for redox reaction steps.
Recently, halide perovskites have gained attention for
solar energy harvesting due to their intense optical absorption
and long carrier diffusion lengths (lower recombination).
Halide perovskites, which constitute a topic of intense sci-
entific and commercial interest for photovoltaic devices,
suffer the problem of severe ambient instability due to air and
moisture. Recently, many efforts have been paid to make
perovskite stable i.e. some halide-based perovskites encap-
sulated/passivated with inorganic, organic and graphenic
materials have been reported to exhibit excellent photo-
catalytic performance in photovoltaics, CO2 reduction, dye
degradation and water splitting [14–21]. However, for long-
term usage, stability still remains a significant consideration.
Further, the usage of toxic lead-based perovskites is highly
undesirable from environmental and health viewpoints.
The family of oxide perovskites (ABO3) has received
significant research attention as highly stable and durable
potential substitutes for halide perovskite. However, the vast
majority of oxide perovskites studied for photocatalytic appli-
cations, such as SrTiO3, CaxTiyO3, CoTiO3, LaMnO3,
LaCoO3, LaNiO3, PbZrO3, NaNbO3, KNbO3, SrNbO3,
NaTaO3, CaSnO3, CaVO3, SrVO3, etc have wide band gaps
exceeding 3.0 eV due to which they harvest a small portion of
the solar spectrum [22]. BiFeO3 has a moderate bandgap of
2.2 eV. There has been far less work on low bandgap oxide
perovskites (Eg<2 eV) for photocatalysis. Members of a
subclass of the oxide perovskite family called double per-
ovskites with a general formula A2B′B″O6 are intriguing can-
didate materials due to their earth abundance, structural, optical
(small direct bandgap), thermal stability and magnetic proper-
ties [23]. Double perovskite materials have just begun to be
explored for photovoltaics, CO2 reforming, fuel cells and
electrocatalysis [22, 24–31]. Barium and niobium-based double
perovskite oxides are found to work as a bifunctional catalyst
for overall water splitting in acidic and basic media [31–34].
Ba2Bi1.4Nb0.6O6 [35, 36], SrNb0.1Co0.7Fe0.2O3–δ perovskite
nanorods (SNCF‐NRs) [37] and layered NdBaMn2O5.5 [38]
showed excellent hydrogen evolution reaction and oxygen
evolution reaction performance under visible light.
Only a handful of reports are available on the use of
double perovskite oxides in photocatalytic/photoelec-
trocatalytic applications due to high polaron binding energies
(which limits the open circuit voltage that can be generated
under solar illumination) and short carrier diffusion length/
low mobility which reduces the recombination lifetime and
fill-factor of photoelectrochemical devices [39]. Substitution
with different metal/nonmetal atoms can significantly influ-
ence the electronic and optoelectronic properties to optimize
photocatalytic performance [40, 41].
Another approach to lower the charge carrier recombi-
nation is to form heterojunctions with other semiconductors
or high mobility materials, for instance, graphenic networks.
Recently, graphitic carbon nitride (g-C3N4, CN), a polymeric
semiconductor composed of tri-s-triazine (heptazine, C6N7)
units linked together with tertiary N atoms forming 2D sheets
structure has shown great promise for various applications
including catalysis (base free oxidation, reduction and cou-
pling reactions etc) and photocatalysis (water splitting, CO2
reduction, photo-organic transformation) due to its remark-
able physical, chemical and optoelectronic properties [42–44].
In contrast with graphene which is a zero bandgap semi-
conductor, the conjugated network of alternative aromatic
carbons and nitrogen gives rise to a bandgap of 2.7 eV (ECB, –
1.1 eV and EVB, +1.6 eV) in g-C3N4 [45]. Additionally,
graphitic carbon nitride can be synthesized with cheap
nitrogenous precursors such as urea, dicyandiamide, mela-
mine and thiourea constituting an additional advantage over
graphene which requires chemical vapor deposition or harsh
chemical synthesis from graphite and therefore difficult to
scale up in synthesis to the industrial scale [46]. Due to the
presence of a finite bandgap and a conjugated network, car-
bon nitride (CN) performs a dual function via assisting in
charge separation by forming a heterojunction and enabling
fast percolation of charge carriers in the CN sheets. The
bandgap of 2.7 eV is associated with blue light absorption and
CN’s band structure can be manipulated to harvest more
visible light through doping (P, F, S etc) [47, 48], chemical
structure modification [49–51] and heterojunction formation
[48, 52–54]. Even though carbon nitride is a promising
photocatalyst, its activity in bulk form is limited due to
intersheet recombination of photogenerated charge carriers in
stacked sheets. On the other hand, the transformation of bulk
CN into one or two atom-thick sheets increases its bandgap
due to the quantum confinement effect restricting its absorp-
tion to the UV region [55–57]. Additionally, the transfor-
mation of CN into monolayer sheets enables intersheets
hydrogen bonding which promotes charge localization and
acts as trap sites, quenching photogenerated charge. Previous
experimental and theoretical studies on graphenic 2D mate-
rials suggest a profound quantum confinement effect, which is
observed only if the number of CN sheets becomes lower than
4. Hence the best way to preserve crystallinity (ordered
stacking) to maintain a low bandgap and lower the intersheets
charge recombination is to keep the numbers of sheets higher
than 4–6 [58–63]. Solvent-assisted exfoliation using a polar
solvent is a facile route to get few-layered carbon nitride
sheets while maintaining crystallinity [64]. Interestingly,
carbon nitride (due to the 2D structure and flexibility of
sheets) can form a heterojunction with most of the semi-
conductor materials via weak van der Waals (vdW) interac-
tion thus removing the constraint of epitaxial lattice matching
in conventional inorganic semiconductor heterojunctions.
Indeed, numerous carbon nitride-based 1D/2D and 2D/2D
van der Waals heterojunctions such as BiOI/CN, MoS2/CN,
LDHs/CN, phosphorene/CN, etc have been synthesized
recently for various applications including photocatalysis
[65–67]. Further, our previous studies demonstrated that
heterojunctions of inorganic semiconductor materials (SnIP as
1D semiconductor [68] and BiOI as layered semiconductor
2
Nanotechnology 32 (2021) 485407 P Kumar et al
4. [69]) with few-layered CN show improved charge
separation. Perovskite oxides and CN-based vdW hetero-
junction has shown great promise due to their excellent
photophysical properties, photocorrosion resistance and better
interfacial charge separation [70, 71]. Inspired by these
observations, we report a nanocomposite of double per-
ovskite, Ba2Ca0.66Nb0.68Fe0.33Co0.33O6−δ (BCNFCo), and
few-layered CN synthesized via a simple solvent assisted
exfoliation accompanied by mild heating. The CN sheets
wrapped around the Ba2Ca0.66Nb0.68Fe0.33Co0.33O6−δ in the
composite which facilitated better charge separation resulting
in improved performance in photoelectrochemical water
splitting reaching a maximum current density of 1.5 mA cm−2
in 40% BCNFCo/CN composite.
2. Results and discussion
2.1. Structure and composition of BCNFCo-CN hybrids
The condensation polymerization of dicyandiamide at high
temperature (550 °C) forms a tertiary nitrogen-linked tris-s-
triazine network leading to the formation of graphitic car-
bon nitride (CN). Solvent-mediated exfoliation of graphitic
materials is a simple and powerful technique to prepare few-
layered CN sheets. We chose o-dichlorobenzene (ODCB) as
the exfoliating agent given its nearly optimal interfacial
tension (∼37 mJ m−2
) as well as its aromaticity and
resulting π–π stacking interactions with CN sheets [72]. We
further note that ODCB is effective in exfoliating unfunc-
tionalized graphene to yield stable, non-aggregating dis-
persions [73]. Furthermore, binary solvent mixtures
containing glycerol have been shown to stabilize aggregates
and assist in exfoliating graphitic materials [74, 75]. The
Ba2Ca0.66Nb0.68Fe0.33Co0.33O6−δ (BCNFCo) double per-
ovskite oxide was prepared using a conventional solid-state
method by physically mixing and sintering (1350 °C/24 h)
stoichiometric amount of BaCO3, CaCO3, Nb2O5, Fe2O3,
and Co3O4 [76]. The sonication and stirring of BCNFCo
with CN at 80 °C in o-dichlorobenzene/glycerol facilitated
exfoliation and improved the dispersion of CN affording
BCNFCo/CN nanocomposite (figure 1). The carbon nitride
interacts with BCNFCo via weak van der Waals (vdW)
interaction leading to the formation of CN wrapped
BCNFCo core–shell type structure.
The fine morphological attributes of CN and BCNFCo/CN
samples were investigated with high-resolution transmission
electron microscopy (HR-TEM) (figure 2). The HR-TEM ima-
ges of CN at low magnification show a nanoporous, crumpled
sheets morphology (figure 2(a)). The HR-TEM at high magni-
fication (50 nm scale bar) clearly shows graphene-like carbon
nitride sheets (figure 2(b)). The nanoporous structure of syn-
thesized heptazine-based carbon nitride was clearly visible in the
HR-TEM image of CN samples at a 5 nm scale bar (figure 2(c)).
The FFT of the TEM image in figure 2(c) shows a broad dif-
fraction ring corroborating the amorphous nature of carbon
nitride complying with the reported literature [77]. Additionally,
electron energy loss spectroscopy (EELS) which provides
information on the nature of C and N bonding was performed
on CN samples to examine the C K-edge and N K-edge (figure
S1 (available online at stacks.iop.org/NANO/32/485407/
mmedia)). Two major peaks due to the contribution of C
K-edge and N K-edge loss were observed in the EELS spectrum
of CN. The C K-edge was composed of two peak components
located at 283.4 and 293.8 eV assigned to 1s-π*
and 1s-σ*
electronic transition of sp2
hybridized carbons trigonally coor-
dinated with nitrogens in heptazine motif (figure S1) [78–80].
The intense π*
C K-edge signal demonstrates conjugated π
orbital overlap in the heptazine ring system. Similarly, the two
peak components in the N K-edge’s energy loss located at 393.6
and 401.6 eV, were attributed 1s-π*
and 1s-σ*
electronic trans-
ition of sp2
hybridized nitrogens in heptazine ring and tertiary
bridging N verifying successful synthesis of N-rich carbon
nitride framework (figure S1) [81]. The HR-TEM images of
BCNFCo/CN demonstrate dense BCNFCo wrapped and
sandwiched between carbon nitride sheets in the solid-state
matrix (figure 2(d)). The high magnification image at the 5 nm
scale bar shows the amorphous CN containing crystalline
BCNFCo (figure 2(e)). To confirm that the dense region was
composed of BCNFCo, we performed FFT on the selected area
demonstrating sharp spots indicating crystalline structure
while the less dense region did not display any diffraction
spots suggesting these regions were constituted of amorphous
CN (inset of figure 2(e)). Further, iFFT on the selected
area shows a d-spacing of 0.27 nm ascribed to BCNFCo
(figure 2(f)). Another high magnification HR-TEM image at
5 nm scale bar also shows crystalline BCNFCo surrounded
with a zone of amorphous carbon nitride revealing wrapping of
CN sheets around crystalline BCNFCo (figure 2(g)). The
magnified region of crystalline lattice and iFFT of the image
Figure 1. Synthesis protocol for formation of Ba2Ca0.66Nb0.68Fe0.33Co0.33O6−δ/g-C3N4 (BCNFCo/CN) nanocomposite.
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Nanotechnology 32 (2021) 485407 P Kumar et al
5. shows lattice fringes with 0.27 nm interplanar d-spacing
assigned to Ba2Ca0.66Nb0.68Fe0.33Co0.33O6−δ double perovskite
(figure 2(h)).
To discern the presence of CN around the BCNFCo
perovskite in BCNFCo/CN nanocomposite, EDX elemental
mapping was performed in STEM mode (figures 3 and S2).
The bright-field image of BCNFCo/CN shows dense
BCNFCo perovskite surrounded with less dense CN sheets
(figures 3(a) and S2(a)). The EDX elemental maps of
BCNFCo/CN show the presence of all the perovskite con-
stituent elements Ba, Ca, Co, Fe, Nb and O concentrated in
the dense region, while carbon and nitrogen constituting CN
were distributed all around the composite, which confirms the
presence of BCNFCo cemented in CN sheets (figures 3(b)–(i)
and S2(b)–(k)). Further, the composite images prepared by
the permutation of different elemental combinations of
BCNFCo and CN show that the elements present in BCNFCo
were centered in the dense region while C and N were
overlapped with the dense region and also present in the less
dense region suggesting CN was wrapped around BCNFCo
(figures 3(j)–(l) and S2(j)–(o)). The observance of relatively
low intensities of C and N in the dense region was because of
the low atomic weight of C and N compared to heavy Ba and
Ca resulting in low electron counts in the STEM detector.
Additionally, the RGB composite of the images made by
selecting Ba and Ca as representative elements of BCNFCo
while C and N as representative of CN followed by line scan
show homogeneous distribution of C and N element all
around the heterostructure validating the presence of CN
enwrapped BCNFCo (figure S3).
The surface/sub-surface chemical composition and
binding energies of CN, BCNFCo and BCNFCo/CN were
determined using XPS (figures S4–S6). The XPS elemental
survey scan of CN, BCNFCo and BCNFCo/CN exhibited
featured peaks (CN: C1s, N1s and O1s; W6: Ba3d, Ca2p,
Nb3d, Fe2p and O1s and BCNFCo/CN: Ba3d, Ca2p, Nb3d,
Fe2p, C1s, N1s and O1s) along with sub-core level peaks (i.e.
OKLL, CKLL, BaMNN, Ba2p etc) confirming the presence
of all constituent elements in pristine and composite materials
(figures S4–6(a)). The core level high-resolution XPS spec-
trum of CN in the C1s region can be deconvoluted into three
peak components centered at BE≈284.8, 286.2 and
288.1 eV (figure S4(b)). The XPS peak located at 284.8 eV
originated from sp3
hybridized adventitious and turbostratic
carbons, while the peaks at 286.2 and 288.1 eV were assigned
to sp2
hybridized C–(N)3 and N=C–N aromatic carbons of
heptazine unit in carbon nitride, respectively [82–84]. The
deconvoluted HR-XPS of CN in the N1s region exhibits four
peaks located at 398.6, 399.7, 400.9 and 404.4 eV (figure
S4(c)). The XPS peaks at 398.6 and 399.7 eV were assigned
to secondary C=N–C and tertiary N–(C)3 nitrogens present in
the heptazine unit of carbon nitride ring systems while
another peak at 400.9 eV appeared due to the contribution of
terminal primary nitrogens (–NH2) [82, 85–87]. Additionally,
a small peak at 404.4 eV originated due to π–π*
transition in
the conjugated aromatic system. HR-XPS in the O1s region of
Figure 2. TEM images of CN (a) low magnification image at 0.5 μm scale bar and (b) moderate magnification image at 100 nm scale bar
showing nanosheets structure (c) HR-TEM image at 10 nm scale bar showing nanoporous structure; inset showing corresponding FFT image
(d) HR-TEM of BCNFCo/CN nanohybrid at 5 nm scale bar 50 nm scale bar showing CN wrapped around BCNFCo. (e) HR-TEM images of
BCNFCo/CN at 5 nm scale bar demonstrating BCNFCo entrapped in CN matrix, Inset 1 and 2 are FFT of the selected region showing
amorphous CN and crystalline BCNFCo domains (f) enlarged region showing close contact between CN and BCNFCo and iFFT showing
lattice fringes overlapping with crystal plane of BCNFCo and corresponding d-spacing (g) high magnification image at 5 nm scale bar
showing lattice fringes of double perovskite oxide and wrapping of CN around the lattice; inset FFT of entire images and (f) magnified view
of the selected area showing interplanar d-spacing and corresponding overlapped iFFT showing 0.27 nm d-spacing.
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Nanotechnology 32 (2021) 485407 P Kumar et al
6. CN was deconvoluted into three peaks component centered at
531.7, 533.5 and 534.8 eV corresponding to C=O, N=C–O
and –OH oxygens of adventitious oxygen and oxidized
terminal carbons (figure S4(d)) [88].
The HR-XPS spectra of Ba2Ca0.66Nb0.68Fe0.33
Co0.33O6−δ (BCNFCo) in Ba3d region displayed two main
peak components located at 779.4 and 794.8 eV originating
from Ba3d5/2 and Ba3d3/2 orbital splitting of Ba2+
in
BCNFCo lattice (figure S5(b)) [89, 90]. The Co2p peak could
not be observed due to the coinciding binding energy of Ba3d
and the lower concentration of Co. The two peak components
in HR-XPS of BCNFCo in the Ca2p region at BE≈346.8
and 350.2 eV were assigned to Ca2p3/2 and Ca2p1/2 peak
components of Ca2+
state (figure S5(c)) [29]. The two intense
peak components in Nb3d region at 205.9 and 208.7 eV were
assigned to Nb3d5/2 and Nb3d3/2 components of Nb present
in 5+oxidation state (figure S5(d)) [91, 92]. Fe2p region of
BCNFCo displayed two main peaks due to Fe2p3/2 and
Fe2p1/2 orbital splitting (figure S5(e)). The Fe2p3/2 peak was
deconvoluted into two components at BE value 710.5 and
713.4 eV assigned to Fe present in Fe3+
and Fe2+
state
[85, 93–95]. Further, observance of satellite peak at BE value
718.0 eV confirmed the presence of iron in the mixed-valence
state. The deconvoluted HR-XPS spectra of BCNFCo in the
O1s region displayed three peaks at 528.4, 529.2 and
530.9 eV assigned to oxygen atoms present in the lattice
structure, C=O and –OH of adsorbed oxygens respectively
[38, 96]. The relatively intense C=O peak might be due to the
residual solvent used for the film formation. In BCNFCo/CN
composite, all the peaks corresponding to CN and BCNFCo
were observed at almost identical binding energy values
suggesting weak van der Waals interaction between CN and
BCNFCo and undermining the presence of any strong che-
mical interactions (figure S6).
The nature of chemical functional groups present in the
materials was determined using Fourier transform infrared
(FTIR) spectroscopy (figure 4(a)). The FTIR spectrum of
BCNFCo showed intense IR absorption bands at 533, 1035,
1568, 2840 and 3219 cm−1
. The absorption band at ca.
533 cm−1
was assigned to the characteristic metal-oxygen
stretch (M-O) while another peak at 3219 cm−1
originated
due to O–H stretching (νO–H) vibration. Other IR peaks at
1035, 1568 and 2840 cm−1
appeared due to the presence of
the residual solvent used for the formation of the film. The
Figure 3. STEM EDX elemental mapping of 40% BCNFCo/CN nanocomposite (a) bright field (BF) image of mapped area, and EDX
elemental mapping for (b) Ba (red), (c) Ca (blue), (d) Co (green), (e) Fe (cyan), (f) Nb (magenta), (g) O (pink), (h) C (yellow), (i) N (violet);
(j) composite image for Ba, Ca, Co, Fe, Nb, O (k) composite of C, N and (l) composite of Ba, Ca, Co, Fe, Nb, O, C and N.
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Nanotechnology 32 (2021) 485407 P Kumar et al
7. FTIR spectrum of CN exhibited an IR absorption band at
3127 cm−1
due to residual –NH2 and –OH (νN–H, νO–H)
stretch [97]. Characteristic IR absorption bands at 1126–1522
cm−2
and 799 cm−1
were arisen due to triazine (C3N3) ring
stretch and bending vibration (figure 4(a)) [98–100]. In the
40% BCNFCo/CN composite all characteristic peaks of CN
were observed and no shift in the peak position was found,
verifying that the intact CN sheets interacted with BNCFCo
through weak vdW interactions.
Figure 4(b) displays the Raman spectra of CN, BCNFCo
and 40% BCNFCo/CN collected using a 632 nm laser as the
excitation source. The Raman spectrum of BCNFCo exhib-
ited an intense peak at 780 cm−1
. The Raman spectrum of CN
shows an intense peak at 1453 cm−1
that originated due to the
cumulative D+G bands of the graphitic carbon nitride
structure [69, 101–103]. The D band arises due to out-of-
plane vibration of sp3
hybridized C and N in graphitic carbon
nitride sheets while the G band appears due to the in-plane
vibration of sp2
hybridized C–N’s [104–106]. Due to exten-
sive stacking in bulk carbon nitride, some vibrations were
restricted and a merged D+G band was observed. In the
Raman spectrum of 40% BCNFCo/CN nanocomposite, two
Raman peaks at 1353 and 1570 cm−1
assigned to D and G
bands of the graphitic structure were clearly observed
[105, 107]. Usually, in the carbon nitride, E2g modes (G band)
arise from the sp2
bonded C–N are Raman active and
observed at 1581 cm−1
. Another Raman-active D band cen-
tered at ∼1360 cm−1
originates from defects state in carbon
nitride structures. The sp2
linked heptazine units of carbon
nitride structure are not completely planar due to variation of
the bond angle in N rich structure [108, 109]. The non-
coherent out-of-plane vibration of these C=N’s in N-linked
heptazine units gives rise to the D band. The relative inten-
sities of the D band in carbon nitride-based structure is highly
dependent on the amount of nitrogen which influences the
number of out-of-plane vibrations in the CN network [110].
As CN is composed of alternate C and N’s present in 3:4
stoichiometry, the equal intensity of the D and G band can be
expected. Compared to bulk CN, the well-separated D and G
band in BCNFCo/CN composite can be explained as follows:
the synthesis of BCNFCo/CN composite proceeds via exfo-
liation of bulk carbon nitride in glycerol/dichlorobenzene
under continuous heat and stirring resulting in the formation
of few-layered sheets. Due to the breaking of stacked struc-
ture and the formation of few-layered structures on the surface
of BCNFCo, the restricted out-of-plane vibration becomes
free to show well-separated D and G bands [111, 112].
The crystallinity of CN, BCNFCo and their composites
was determined using x-ray diffraction (XRD) (figure 4(c)).
The crystal structure of the investigated phase Ba2
Ca0.67Nb0.67Fe0.33Co0.33O6−δ exhibited a 1:1 ordered cubic
double perovskite structure (JCPDS# 49-0425). For refine-
ment, an ideal phase double-perovskite (figure S8),
Ba3CaNb2O9, with space group Fm-3m (No. 225) was used.
Table S1 lists the atomic position and occupancy values
obtained from Rietveld refinement analysis. Furthermore, the
lattice constant of BCNFCo was determined to be a=8.3739
(9) Å; where the absence of any secondary phases was also
confirmed by reliability factors (χ2
=1.21; Rp =12.71%). In
an ideal Fm-3m space group, cubic double-perovskite
involves an octahedral arrangement (figure S8) of alternative
layers of oxygens around Ca(Nb) and Nb atoms, which shares
the crystallographic positions 4a and 4b, respectively. The Ba
atoms are located in spaces between these octahedra. For each
Ba atom, there are twelve oxygens at close distances—so that,
the Ba–O sublattice involves BaO12 units with dodecahedral
geometry. In the case of BCNFCo, the transition metal (Fe,
Co) doping seems like a (proper) replacement of Nb cations at
Figure 4. (a) FTIR spectra of BCNFCo, CN and 40% BCNFCo/CN, (b) Raman spectra of BCNFCo, CN and 40% BCNFCo/CN (c) powder
XRD (P-XRD) diffraction pattern of CN, BCNFCo and their composites with different wt%; Color: CN (black), 10% BCNFCo/CN (pink),
20% BCNFCo/CN (green), 30% BCNFCo/CN (blue), 40% BCNFCo/CN (red), 50% BCNFCo/CN (yellow), BCNFCo (violet).
6
Nanotechnology 32 (2021) 485407 P Kumar et al
8. the B-site of the double perovskite—subsequently forming
oxygen vacancies. The B-site substitution on similar per-
ovskite-type oxides has previously been reported by Than-
gadurai et al group—showcasing the materials for solid oxide
fuel cells [113] and CO2 gas sensor [114] applications.
Likewise, co-authors of this work have previously demon-
strated BCNF to be a p-type semiconductor in the bulk using
ambient-dependent AC and DC impedance measure-
ments [114].
As shown in figure 4(c), the XRD pattern of bulk g-C3N4
displayed two distinct peaks at 2θ values 27.1° and 13.0°
indexed to the 002 and 100 planes of graphitic carbon nitride
[15, 115]. The 002 reflection with a 0.32 nm interplanar d-
spacing originated due to interplanar stacking of carbon
nitride sheets while in-plane packing of heptazine units in
carbon nitride sheets was responsible for the 100 reflection
with 0.68 nm spacing [116]. The characteristic 002 and 100
peaks of carbon nitride were present in all the BCNFCo/CN
composites indicating the preservation of the stacked structure
of CN in the composite. Furthermore, specific peaks of
BCNFCo were also observed in the BCNFCo/CN composite
materials demonstrating the intact crystalline structure of
BCNFCo.
Thermogravimetric analysis (TGA) of CN and best per-
forming 40% BCNFCo/CN was performed to elucidate the
thermal stability of materials (figure S10). The TGA ther-
mogram of CN demonstrated a very small weight loss (∼5%)
in the 50 °C–550 °C temperature range due to loss of surface
adsorbed water/organic molecules and polymerization of
residual uncondensed moieties in CN framework releasing
NH3. Another sharp weight loss (∼92%–95%) in the temp-
erature range of 600 °C–750 °C was ascribed to degradation
of heptazine (C6N7) moieties. The 40% BCNFCo/CN het-
erostructure composite demonstrated three weight loss
regions in the TGA thermogram. The first less prominent
weight loss (∼3%–4%) in the region of 50 °C–190 °C was
observed due to the removal of adsorbed water and o-
dichlorobenzene. The second minor weight loss (∼4%–5%)
in the temperature range of 250 °C–425 °C was assigned to
the degradation of intercalated glycerol and loss of NH3
during condensation of unreacted heptazine units. The third
major weight loss of approximately ∼38% was observed in
the temperature range of 550 °C–700 °C due to the degrada-
tion of carbon nitride tris-s-triazine moieties. Subsequently,
no weight loss was observed which suggests high-temperature
stability of remaining BCNFCo perovskite oxide crystals.
2.2. Optical and photoluminescence spectra of BCNFCo-CN
hybrids
The optical absorption of materials was evaluated from dif-
fuse reflectance UV–vis (DR-UV–vis) spectra (figure 5(a)).
The DR-UV–vis spectrum of CN exhibited the characteristic
absorption band in the 200–420 region and an extended band
tail up to 460 nm arising from the band-to-band transition
between the valence band composed of nitrogen 2p orbitals
and conduction band composed of the carbon atom 2p orbi-
tals. The weak peak at 330 nm in the UV–vis spectrum of CN
was assigned to π→π*
transition while another sharp peak
around 390 nm was assigned to n→π*
transition in carbon
nitride framework [117–120]. The UV–vis spectrum of
BCNFCo perovskite oxide shows a broad absorption range
extending all the way down to the near-infrared region. After
the formation of heterojunction, the absorption profile of the
composite was extended toward longer wavelengths as per the
percentage of blended perovskite due to contribution from
perovskite absorption. Additionally, the effective optical
bandgaps of CN, BCNFCo and BCNFCo/CN composites
were calculated using a Tauc plot. A plot between (αhν)1/2
versus hν followed by extrapolation of the linear tangent to
abscissa provide band gap value of material; where α is
absorption coefficient, h is plank constant and ν is the light
frequency (figure S7). The values of optical bandgap obtained
from the Tauc plot for CN and as-prepared BCNFCo were
found to be 2.58 and 0.69; whereas the bandgap values for
BCNFCo/CN composites with BCNFCo content varying
from 10%, 20%, 30%, 40% and 50% were 1.02, 1.00, 0.88,
0.88 and 0.70 eV, respectively. The obtained bandgap for CN
was in good agreement with previously reported literature
[121, 122].
Photoluminescence (PL) spectra were acquired at an
excitation wavelength of 365 nm (figure 5(b)). CN displayed
a broad intense PL peak centered at 451 nm suggesting a
populated charge recombination process [123–125]. Bulk
carbon nitride composed of 2D tris-s-triazine units’ network
possesses several uncondensed NH/NH2 moieties leading to
intersheets and intrasheets hydrogen bonding. These moieties
also work as radiative recombination centers giving rise to an
intense PL peak. In multilayer carbon nitride, intersheets
charge recombination remains prevalent resulting in a
decrease in photocatalytic performance. After the formation
of composite with BCNFCo, the PL peak intensity of
BCNFCo/CN composite was not quenched. Interestingly, the
observed small gradual decrease in the emission intensity of
CN as wt% of BCNFCo was increased, is attributable to the
lower mass fraction of CN in the composites and interfacial
radiative recombination in the vdW heterostructure. dis-
sociated at the CN/BCNFCo heterojunction. This inference
has the major implication that the photoelectrochemical per-
formance of CN/BCNFCo heterojunction nanocomposite
photoanodes (discussed in the next section) derives mostly
from the separation of photogenerated charge carriers in the
BCNFCo.
2.3. Photoelectrochemical water splitting performance of
BCNFCo-CN hybrids
The pristine and composite materials were tested as photo-
anodes for photoelectrochemical (PEC) water splitting
(figure 6). The PEC water splitting experiments were carried
out in a three-electrode configuration with FTO deposited
bare/hybrid materials as photoanode (working electrode)
while Pt and Ag/AgCl electrode were used as photocathode
(counter electrode) and reference electrodes respectively. For
the measurement, all three electrodes were immersed in a
0.1 M Na2SO4 electrolyte followed by irradiation of the
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Nanotechnology 32 (2021) 485407 P Kumar et al
9. photoanode by AM1.5 G simulated sunlight (one sun inten-
sity). The photoresponse of materials was measured using
linear sweep voltammetry by sweeping the applied bias from
−1.0 to +1.0 V versus Ag/AgCl. All the samples demon-
strated a negligible current density under dark conditions
(figure 6(a)). However, under AM1.5 G irradiation all the
samples displayed a photoresponse. Pristine CN and
BCNFCo exhibited low photocurrent densities of 0.45 and
0.40 mA cm−2
at +0.6 V versus Ag/AgCl. After the forma-
tion of heterojunction between BCNFCo and CN, the pho-
tocurrent density was remarkably improved. The 40%
BCNFCo/CN sample displayed the highest photocurrent
density (1.51 mA cm−2
) under AM1.5 G irradiation—a nearly
four-fold increase in performance over the pristine material
photoanodes (figures 6 and 7). Under identical conditions, the
photocurrent densities for 10% BCNFCo /CN, 20%
BCNFCo /CN, 30% BCNFCo/CN and 50% BCNFCo/CN
were measured to be 0.61, 0.67, 0.99 and 0.73 mA cm−2
respectively. A similar pattern ensued when a 420 nm
cut-off filter was used to exclude short wavelength photons
(figures S13–18).
The photocurrent response of material during light On–
Off cycles was measured as the function of time (i–t curve) at
0.6 V applied bias to clearly show that the current originated
from illumination rather than a redox effect (figures 6(b),
7(a)–(c)). The test duration of 180 s for the On–Off cycling is
long enough to show the effects (if any) of short-term che-
mical or photochemical degradation. No such degradation
was observed which demonstrates the photochemical stability
of the heterojunction nanocomposite. As in the LSV mea-
surement, the photocurrent response of 40% BCNFCo/CN
sample increased linearly as a function of voltage, which
seems like the ohmic current. To verify the originated current
was not ohmic in nature LSV during the On–Off cycle was
collected. As clear from figure 7(a), the photocurrent reaches
almost zero during dark suggesting superimposing with dark
current suggests the observed current was purely originated
from photoelectrochemical water splitting. The steady pho-
tocurrent response of all the samples throughout measurement
and presence of peak and trough in photocurrent response
during light On–Off validates the photoresponse of the
materials. The observance of spikes in the transient photo-
current measurement followed by the attainment of constant
current during the light-on cycle was due to the generation of
a large number of electron–hole pairs during light illumina-
tion which are accumulated at the surface on opposite sides of
the semiconductor-electrolyte interface. Photogenerated
electrons in BCNFCo are injected into CN while photo-
generated holes in BCNFCo are supplied to hydroxyl ions at
the interface. Due to the slow transport of electrons in the CN,
interfacial recombination of a portion of accumulated elec-
trons and holes occurs, reducing the photocurrent to a steady-
state value given by the flat portion of the pulses in the linear
sweep voltammograms. Note that the recombination process
has a time constant of several seconds. In contrast, the initial
charge separation process directed by the space charge
capacitance has a time-constant of ∼1 μs as shown in table
S11. Figures 7(c) and (d) show appreciable visible light
harvesting by the 40% BCNFCo/CN nanocomposite with a
distinguishable photoresponse even at 640 nm. This shows
the potential of optimized double perovskite oxide systems
based on BCNFCo to overcome the issue of poor visible light
Figure 5. (a) DR-UV–vis spectra and (b) steady-state PL (ssPL) spectra of CN, 10% BCNFCo/CN, 20% BCNFCo/CN, 30% BCNFCo/CN,
40% BCNFCo/CN, 50% BCNFCo/CN, BCNFCo acquired at 360 nm excitation; Color: CN (black), 10% BCNFCo/CN (pink), 20%
BCNFCo/CN (green), 30% BCNFCo/CN (blue), 40% BCNFCo/CN (red), 50% BCNFCo/CN (yellow), BCNFCo (violet).
8
Nanotechnology 32 (2021) 485407 P Kumar et al
10. harvesting by conventional photochemically and thermally
stable photoanodes such as TiO2, SrTiO3, NaNbO3, etc.
To discern the true origin of photocurrent from photo-
electrochemical water splitting and overrule the possibility of
any side reactions/photocorrosion or electrolysis, the evolved
gaseous reaction product was collected on Pt cathode in H-cell
(Please ESI for experimental detail). The gaseous product was
analyzed in a gas chromatograph equipped with a pulse dis-
charge detector (GC-PDD). The rates of hydrogen evolution
for BCNFCo, CN and 40% BCNFCo/CN under AM1.5 G one
sun illumination were found to be 0.57, 0.84 and 1.15 μmol
h−1
respectively which was consistent with the observed trend
in photocurrent (figures 6, 7 and S13–S18). The increased
hydrogen evolution rate for 40% BCNFCo/CN indicates the
synergistic enhancement in photoactivity. Furthermore, the
Faradaic efficiency, which is a true measure of efficiency in
actual operating conditions was calculated for the photoelec-
trochemical water splitting using BCNFCo, CN and 40%
BCNFCo/CN. Faradaic efficiency (FE%) is a ratio of observed
hydrogen in experimental condition to theoretically evolved H2
Figure 6. (a) Linear sweep voltammograms of CN, 10% BCNFCo/CN, 20% BCNFCo/CN, 30% BCNFCo/CN, 40% BCNFCo/CN, 50%
BCNFCo/CN, BCNFCo samples showing change in photocurrent density versus applied potential (J–V ), under dark and simulated solar
AM1.5 G light irradiation (100 mW cm−2
) (b) photocurrent versus time (i–t) plot showing response during light On–Off cycles at +0.6 V
applied bias, under AM1.5 G light irradiation (100 mW cm−2
); all the measurements were performed in 0.1 M Na2SO4 solution at a scan rate
of 0.1 mV s−1
and (c) calculated Faradaic efficiencies for hydrogen evolution using BCNFCo, CN and 40% BCNFCo/CN photocatalyst.
9
Nanotechnology 32 (2021) 485407 P Kumar et al
11. was calculated using the following expression.
⎡
⎣
⎢
⎤
⎦
⎥
( )
( )
( )
·
=
Faradaic efficiency %
Experimental gas evolution measured H
Theoretical H gas evolution based on photocurrent
100.
2
2
The calculated Faradaic efficiencies for the BCNFCo,
CN and BCNFCo/CN photoanode-based photoelec-
trochemical H-cells were found to be 64.32%, 83.38% and
87.72% respectively (figure 6(c)). In comparison to the pris-
tine materials, the increased FE% for the BCNFCo/CN
demonstrates better charge separation in the heterojunction.
Additionally, the high stability of the material toward pho-
tocorrosion was inferred from the excellent FE for the
BCNFCo. It is well-established that in photoelectrochemical
oxidation/reduction, the additional current generated from the
non-photophysical processes derives from the material’s
corrosion and side reaction. These reactions promote the
oxidation of photoanode which compromises the photo-
stability of the materials. The high Faradaic efficiency sug-
gests only a tiny fraction of BCNFCo photoanode is
potentially corroded during the measurement which suggests
the hybridization of BCNFCo not only improves the photo-
electrochemical performance but also increases its resilience.
Figure 7. (a) Linear sweep voltammogram of 40% BCNFCo/CN showing photocurrent density versus applied potential (J–V ),
photoresponse during light On–Off cycle under dark, solar simulated AM1.5 G light irradiation without filter (100 mW cm−2
) and AM1.5 G
light irradiation with 420 nm cut-off filter, (b) photocurrent versus time (i–t) plot of 40% BCNFCo/CN showing response during light On–
Off cycle at +0.6 V applied bias, under solar simulated AM1.5 G light irradiation without filter (100 mW cm−2
) and AM1.5 G light
irradiation with 420 nm cut-off filter, (c) photocurrent response versus time of 40% BCNFCo/CN during light On–Off cycle at +0.6 V
applied bias, under 420, 460, 520, 580, 640 and 840 nm LEDs (100 mW cm−2
) (d) enlarged photocurrent versus time graph showing
photoresponse of 40% BCNFCo/CN under 580, 640 and 840 nm LEDs (100 mW cm−2
) all the measurement were performed in 0.1 M
Na2SO4 solution at a scan rate of 0.1 mV s−1
.
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Nanotechnology 32 (2021) 485407 P Kumar et al
12. The band alignment at the heterojunction was determined
using XPS valence band (XPS-VB) spectra (figure 8). All
energy band positions in the ensuing discussion are listed
versus the vacuum level (Evac). From the XPS VB spectra, the
valence band positions for CN, BCNFCo and BCNFCo/CN
were found to be 2.04, 0.68 and 0.80 eV respectively below
the Fermi level [126]. The shifting of valence band position in
BCNFCo/CN indicating the establishment of a type-II het-
erojunction. Interestingly, the calculated valence band posi-
tions for CN, BCNFCo and 40% BCNFCo/CN were found to
be close to the optical band gap value of materials calculated
from the Tauc plot and demonstrate that the Fermi level was
close to the conduction band thus displaying the n-type
character. Further, the Mott–Schottky plot obtained from
impedance potential measurement also demonstrate n-type
characteristics of materials and calculated flat band positions
(Vfb) of CN, 10% BCNFCo/CN, 20% BCNFCo/CN, 30%
BCNFCo/CN, 40% BCNFCo/CN, 50% BCNFCo/CN were
found to be −0.37, −0.45, −0.33, −0.32, −0.48 and
−0.54 V versus Ag/AgCl respectively. The n-type conduc-
tion of BCNFCo/CN hybrids determined from the Mott–
Schottky plot (figure 8(d)) indicates the Fermi level to be
close to the conduction band. Hence the obtained flat band
potential (Vfb) can be assigned to the conduction band posi-
tion. As the BCNFCo content in the composite is increased,
the conduction band minimum (ECB) gets upshifted versus the
vacuum level (Evac). The upshifting of ECB in the composite
can be explained based on the formation of a p–n type
Figure 8. XPS valence band (XPS-VB) spectra showing valence band position with respect to Fermi level of (a) CN, (b) BCNFCo and (c)
40% BCNFCo/CN and (d) Mott–Schottky plots showing flat and potential of CN (black), 10% BCNFCo/CN (pink), 20% BCNFCo/CN
(green), 30% BCNFCo/CN (blue), 40% BCNFCo/CN (red), 50% BCNFCo/CN (yellow).
11
Nanotechnology 32 (2021) 485407 P Kumar et al
13. heterojunction between p-type BCNFCo and n-type CN. As
we know the formation of heterojunction involves Fermi level
alignment. Before contact with CN, the p-type BCNFCo was
electron-deficient; however, after contact with CN, the elec-
trons from the CN were transferred to the BCNFCo com-
pensating the positive hole leading to Fermi level
equilibration. During Fermi level alignment, the CB of
BCNFCo gets upshifted due to the accumulation of excessive
electrons from CN and a built-in electric field is generated
which facilitates the transfer of electrons from BCNFCo to
CN in the depletion layer (figure 9). The translation of the flat
band potential to the RHE (NHE) scale which is usually used
for the expression of redox potential gives the following
values of Vfb for the various BCNFCo/CN composites:
−0.17, −0.253, −0.13, −0.12, −0.28 and −0.34 V versus
NHE at pH-0. The more negative CB compared to the
reduction potential of hydrogen (H+
/H2; 0.00 V versus NHE
at pH-0) suggests all the composites can catalyze the process
of hydrogen evolution.
PL spectra (figure 5(b)) show that the bound exciton in
CN is not dissociated at the BCNFCo/CN interface. There-
fore, the dramatic improvement of photoelectrochemical
performance in the 40% BCNFCo/CN photoanodes does not
derive from improved separation and collection of photo-
generated excitons in CN. Instead, the increased photocurrent
density (Jph) derives from better separation and utilization of
electron–hole pairs generated in BCNFCo. Individually, bare
CN and bare BCNFCo photoanodes generated maximum Jph
values of 0.4 and 0.55 mA cm−2
respectively under AM1.5 G
illumination at an applied bias of 0.6 V versus Ag/AgCl
(figures 6(a) and (b)). Under identical conditions, the 40%
BCNFCo/CN photoanode generates ∼1.5 mA cm−2
, which
is higher than the sum of the maximum photocurrent densities
generated by the bare CN and bare BCNFCo photoanodes. In
figures 7(c) and (d), it is clear that the 40% BCNFCo/CN
hybrids show a photoresponse all the way down to 640 nm,
where the CN does not even absorb. Therefore, the increased
photoelectrochemical performance derives from better
separation of carrier-pairs generated in BCNFCo.
The better charge separation and migration in BCNFCo/
CN nanocomposite were further confirmed by EIS measure-
ment which demonstrates a smaller arc of the semicircle in the
Nyquist plot for composite materials (lower charge transfer
resistance) (figure S12). The pristine BCNFCo and CN
samples exhibit rather a high charge transfer resistance (RCT)
values of 219.5 Ω and 264.7 Ω respectively (table S2) due to
the absence of a heterojunction to facilitate charge separation.
The 10% BCNFCo/CN sample exhibits the highest RCT
value (pink curve in figure S12(a)) due to an inadequate
amount of the p-type component of the heterojunction
resulting in more dead-ends for holes. As the amount of
BCNFCo is increased in the nanocomposites, there exists a
sufficient amount of both the n- and p-type components of the
heterojunction to ensure charge separation and also deliver
holes to the electrolyte species. Therefore, we see the RCT
value dropping sharply for the 20% BCNFCo/CN, 30%
BCNFCo/CN and 40% BCNFCo/CN samples with the
lowest charge transfer resistance of 39.84 Ω seen for the 30%
BCNFCo/CN sample. However, charge transport limitations
become increasingly important as the amount of BCNFCo is
further increased. The poor carrier transport in the BCNFCo
phase increases the series resistance (RS) of the sample and
concomitantly increases the charge transfer resistance. Thus
we see the 40% BCNFCo/CN and 50% BCNFCo/CN
samples exhibiting both a higher RS and RCT than the 20%
BCNFCo/CN and 20% BCNFCo/CN samples. At the same
time, BCNFCo is the primary photon absorber with a band-
edge that extends to well beyond 700 nm as shown in
figure 5(a). As the amount of BCNFCo is increased, the
number of generated electron–hole pairs is also increased
albeit with a higher fraction recombining due to charge
transport limitations (also accounting for the larger arc for the
40% BCNFCo/CN sample in figure S12(b)). As a con-
sequence of the trade-off between light absorption and charge
separation, the 40% BCNFCo/CN blend exhibits the best
photoelectrochemical performance as shown in figure 6.
To explain the improved photocatalytic performance, a
plausible mechanism was proposed based on optical band
Figure 9. A plausible mechanism of Fermi level alignment and formation of p–n heterojunction between BCNFCo and CN.
12
Nanotechnology 32 (2021) 485407 P Kumar et al
14. gap/electronic band positions and existing literature. For self-
sustained water splitting under solar light, the bandgap of
materials should be higher than 1.23 eV with a conduction
band position more negative than 0.00 V versus NHE at pH-0
(water reduction potential, H+
/H2) while the position of the
valence band should be more positive than +1.23 V versus
NHE at pH-0 (water oxidation potential, H2O/O2) [127]. The
calculated optical band gap value of BCNFCo was found to
be 0.69 eV, demonstrating its inability to perform water
splitting by itself. Further, the poorly oxidative valence band
(0.68 eV below the Fermi level) also prevents BCNFCo to
work as a stand-alone photocatalyst for water splitting which
explains the observation of a small photocurrent in PEC. On
the other hand, CN has a bandgap of 2.58 eV with CB and VB
positions of −0.37 and +2.27 V respectively, calculated from
Mott–Schottky measurement and optical band gap, suggest-
ing CN has the potential to facilitate water splitting reaction.
However, the absorption of CN is limited to the blue region of
the solar spectrum and prevalent inter-sheet charge recombi-
nation decrease the PEC performance. The formation of a
nanocomposite between BCNFCo and CN significantly
increased the photocatalytic performance for 40% BCNFCo/
CN materials. The boosted photocatalytic performance can be
explained due to the formation of p–n type heterojunction and
better charge separation in BCNFCo/CN nanocomposite as
evident from the lower RCT of 40%BCNFCo/CN than pris-
tine CN and BCNFCo (figure S12) [128–131]. The p-type
BCNFCo interacts with n-type CN leading to charge transfer
from CN to BCNFCo during the Fermi level alignment and
forms p–n heterojunctions shown in figure 9. This Fermi level
alignment upraised the CB position of BCNFCo. In
BCNFCo/CN heterojunction, due to the presence of charge
gradient originated from the accumulation of negative charge
on BCNFCo and the positive charge on CN interface, an in-
built electric field is generated across the depletion layer. Due
to the establishment of p–n heterojunction and the presence of
an in-built electric field the photogenerated electrons are
transferred to the conduction band of CN while holes move in
the opposite direction. In photoelectrochemical conditions,
BCNFCo after absorption of visible light transfers photo-
generated electrons to the conduction band of CN from where
the electrons are transported to the Pt cathode through the
external circuit to reduce protons in the electrolyte to
hydrogen while holes left in the VB of BCNFCo are sca-
venged by Na2SO4.
3. Conclusion
This is the first report on the use of double perovskites as
narrow bandgap semiconductors for light harvesting appli-
cations. A nanocomposite of double perovskite oxide with
Ba2Ca0.66Nb0.68Fe0.33Co0.33O6−δ composition and g-C3N4
was prepared by a facile mixing/sonication approach in
DCB/glycerol solution. The extensive mixing leads to inter-
calation of solvents in between the sheets which afford few-
layered carbon nitride sheets wrapped around the perovskite
material. The materials characterization with various tools
confirms the formation of nanocomposite between two
materials. HR-TEM and Raman validate the presence of
carbon nitride in a few-layered sheet structure which provides
a charge transport pathway to facilitate better charge separa-
tion in the composite materials. Among various wt% com-
positions of composite 40%BCNFCo/CN nanocomposite
displayed the highest photocurrent density (1.5 mA cm−2
)
under solar simulated AM1.5 G irradiation. A particularly
promising result is the observation of a distinct photoresponse
at 640 nm in most of the blends, which is attributable only to
absorption by the double perovskite since CN does not absorb
at this wavelength. This result indicates electron–hole pairs
generated in the double perovskite are separated and perform
useful work.
Acknowledgments
We would like to thank the Natural Sciences and Engineering
Research Council of Canada (NSERC), the National Research
Council Canada (NRC), Future Energy Systems (FES) for
direct and indirect (equipment use) financial support. DL
thanks DST-SERB/UoA for providing research fellowship.
National Research Council—National Institute for Nano-
technology (NRC-NINT) and the University of Alberta
Nanofab facilities are acknowledged for support in the ana-
lysis of samples. Dr Kai Cui is kindly acknowledged for HR-
TEM and EDX elemental mapping of the samples. The
authors are thankful to Sheng Zeng and Ehsan Vahidzadeh for
helping with GC and TGA analysis respectively.
Data availability statement
The data generated and/or analyzed during the current study
are not publicly available for legal/ethical reasons but are
available from the corresponding author on reasonable
request.
ORCID iDs
Pawan Kumar https:/
/orcid.org/0000-0003-2804-9298
Karthik Shankar https:/
/orcid.org/0000-0001-7347-3333
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