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.
A ruthenium trinuclear polyazine complex was synthesized and subsequently immobilized through
complexation to a graphene oxide support containing phenanthroline ligands (GO-phen). The developed
photocatalyst was used for the photocatalytic reduction of CO2 to methanol, using a 20 watt white cold
LED flood light, in a dimethyl formamide–water mixture containing triethylamine as a reductive
quencher. After 48 h illumination, the yield of methanol was found to be 3977.57 5.60 mmol gcat
1.
The developed photocatalyst exhibited a higher photocatalytic activity than graphene oxide, which
provided a yield of 2201.40 8.76 mmol gcat
1. After the reaction, the catalyst was easily recovered and
reused for four subsequent runs without a significant loss of catalytic activity and no leaching of the
metal/ligand was detected during the reaction.
Synthesis of flower-like magnetite nanoassembly: Application in the efficient...Pawan Kumar
A facile approach for the synthesis of magnetite microspheres with flower-like morphology is reported that proceeds via the reduction of iron (III) oxide under hydrogen atmosphere. The ensuing magnetic catalyst is well characterized by XRD, FE-SEM, TEM, N2 adsorption-desorption isotherm and Mössbauer spectroscopy and explored for a simple yetbut efficient transfer hydrogenation reduction of a variety of nitroarenes to respective anilines in good to excellent yields (up to 98%) employing hydrazine hydrate. . The catalyst could be easily separated at the end of reaction using an external magnet and can be recycled up to 10 times without any loss in catalytic activity.
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.
A ruthenium trinuclear polyazine complex was synthesized and subsequently immobilized through
complexation to a graphene oxide support containing phenanthroline ligands (GO-phen). The developed
photocatalyst was used for the photocatalytic reduction of CO2 to methanol, using a 20 watt white cold
LED flood light, in a dimethyl formamide–water mixture containing triethylamine as a reductive
quencher. After 48 h illumination, the yield of methanol was found to be 3977.57 5.60 mmol gcat
1.
The developed photocatalyst exhibited a higher photocatalytic activity than graphene oxide, which
provided a yield of 2201.40 8.76 mmol gcat
1. After the reaction, the catalyst was easily recovered and
reused for four subsequent runs without a significant loss of catalytic activity and no leaching of the
metal/ligand was detected during the reaction.
Synthesis of flower-like magnetite nanoassembly: Application in the efficient...Pawan Kumar
A facile approach for the synthesis of magnetite microspheres with flower-like morphology is reported that proceeds via the reduction of iron (III) oxide under hydrogen atmosphere. The ensuing magnetic catalyst is well characterized by XRD, FE-SEM, TEM, N2 adsorption-desorption isotherm and Mössbauer spectroscopy and explored for a simple yetbut efficient transfer hydrogenation reduction of a variety of nitroarenes to respective anilines in good to excellent yields (up to 98%) employing hydrazine hydrate. . The catalyst could be easily separated at the end of reaction using an external magnet and can be recycled up to 10 times without any loss in catalytic activity.
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.
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
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.
Consistently High Voc Values in p-i-n Type Perovskite Solar Cells Using Ni3+-...Pawan Kumar
Leading edge p-i-n type halide perovskite solar cells (PSCs) severely underperform n-i-p PSCs. p-i-n type PSCs that use PEDOT:PSS hole transport layers (HTLs) struggle to generate open-circuit photovoltage values higher than 1 V. NiO HTLs have shown greater promise in achieving high Voc values albeit inconsistently. In this report, a NiO nanomesh with Ni3+ defect grown by the hydrothermal method was used to obtain PSCs with Voc values that consistently exceeded 1.10 V (champion Voc = 1.14 V). A champion device photoconversion efficiency of 17.75% was observed. Density functional theory modeling was used to understand the interfacial properties of the NiO/perovskite interface. The PCE of PSCs constructed using the Ni3+-doped NiO nanomesh HTL was ∼34% higher than that of conventional compact NiO-based perovskite solar cells. A suite of characterization techniques such as transmission electron microscopy, field emission scanning electron microscopy, intensity-modulated photocurrent spectroscopy, intensity-modulated photovoltage spectroscopy, time-resolved photoluminescence, steady-state photoluminescence, and Kelvin probe force microscopy provided evidence of better film quality, enhanced charge transfer, and suppressed charge recombination in PSCs based on hydrothermally grown NiO nanostructures.
Synthesis of flower-like magnetite nanoassembly: Application in the efficient...Pawan Kumar
A facile approach for the synthesis of magnetite microspheres with flower-like morphology is reported
that proceeds via the reduction of iron(III) oxide under a hydrogen atmosphere. The ensuing magnetic
catalyst is well characterized by XRD, FE-SEM, TEM, N2 adsorption-desorption isotherm, and
Mössbauer spectroscopy and explored for a simple yet efficient transfer hydrogenation reduction of a
variety of nitroarenes to respective anilines in good to excellent yields (up to 98%) employing hydrazine
hydrate. The catalyst could be easily separated at the end of a reaction using an external magnet and
can be recycled up to 10 times without any loss in catalytic activity.
Visible light assisted reduction of nitrobenzenes using Fe(bpy)3+2/rGOnanocom...Pawan Kumar
Visible-light-induced photocatalytic reduction of aromatic nitrobenzenes to the corresponding anilinesat room temperature using reduced graphene oxide (rGO) immobilized iron(II) bipyridine complex asphotocatalyst is described. The rGO-immobilized iron catalyst exhibited superior catalytic activity thanhomogeneous iron(II) bipyridine complex and much higher than metal free rGO photocatalysts. Theheterogeneous photocatalyst was found to be robust and could easily be recovered and reused for severalruns without any significant loss in photocatalytic activity.
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.
Mixed-Valence Single-Atom Catalyst Derived from Functionalized GraphenePawan Kumar
Single-atom catalysts (SACs) aim at bridging the gap between homogeneous and heterogeneous catalysis. The challenge is the development of materials with ligands enabling coordination of metal atoms in different valence states, and preventing leaching or nanoparticle formation. Graphene functionalized with nitrile groups (cyanographene) is herein employed for the robust coordination of Cu(II) ions, which are partially reduced to Cu(I) due to graphene-induced charge transfer. Inspired by nature's selection of Cu(I) in enzymes for oxygen activation, this 2D mixed-valence SAC performs flawlessly in two O2-mediated reactions: the oxidative coupling of amines and the oxidation of benzylic CH bonds toward high-value pharmaceutical synthons. High conversions (up to 98%), selectivities (up to 99%), and recyclability are attained with very low metal loadings in the reaction. The synergistic effect of Cu(II) and Cu(I) is the essential part in the reaction mechanism. The developed strategy opens the door to a broad portfolio of other SACs via their coordination to various functional groups of graphene, as demonstrated by successful entrapment of FeIII/FeII single atoms to carboxy-graphene.
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 intra sheet cavity make it an attractive supramolecular adsorbent for environmental applications.
Bicrystalline Titania Photocatalyst for Reduction of CO2 to Solar FuelsA'Lester Allen
Degussa P25, a mixture of anatase and rutile crystal structures, is the most commonly used precursor to form the photoactive layer in solar cells; however, the photocatalytic activity of rutile is inferior to brookite. This presentation discusses the enhancement in photocatalytic activity of an antase brookite mixture.
Asymmetric Multipole Plasmon-Mediated Catalysis Shifts the Product Selectivit...Pawan Kumar
Cu/TiO2 is a well-known photocatalyst for the photocatalytic transformation of CO2 into methane. The formation of C2+ products such as ethane and ethanol rather than methane is more interesting due to their higher energy density and economic value, but the formation of C–C bonds is currently a major challenge in CO2 photoreduction. In this context, we report the dominant formation of a C2 product, namely, ethane, from the gas-phase photoreduction of CO2 using TiO2 nanotube arrays (TNTAs) decorated with large-sized (80–200 nm) Ag and Cu nanoparticles without the use of a sacrificial agent or hole scavenger. Isotope-labeled mass spectrometry was used to verify the origin and identity of the reaction products. Under 2 h AM1.5G 1-sun illumination, the total rate of hydrocarbon production (methane + ethane) was highest for AgCu-TNTA with a total CxH2x+2 rate of 23.88 μmol g–1 h–1. Under identical conditions, the CxH2x+2 production rates for Ag-TNTA and Cu-TNTA were 6.54 and 1.39 μmol g–1 h–1, respectively. The ethane selectivity was the highest for AgCu-TNTA with 60.7%, while the ethane selectivity was found to be 15.9 and 10% for the Ag-TNTA and Cu-TNTA, respectively. Adjacent adsorption sites in our photocatalyst develop an asymmetric charge distribution due to quadrupole resonances in large metal nanoparticles and multipole resonances in Ag–Cu heterodimers. Such an asymmetric charge distribution decreases adsorbate–adsorbate repulsion and facilitates C–C coupling of reaction intermediates, which otherwise occurs poorly in TNTAs decorated with small metal nanoparticles.
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 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%.
Electron transfer between methyl viologen radicals and graphene oxidekamatlab
Methyl viologen radicals are capable of transferring electrons to graphene oxide and partially restore the sp2 network. The reduced graphene oxide serves as a scaffold to anchor Ag nanoparticles. The growth of these silver nanoparticles is dictated by the ability of RGO to store and shuttle electrons. The RGO/Ag nanocomposites discussed in the present work offer new opportunities to design next generation photocatalysts.
Visit our website, KamatLab.com, for the latest news, publications, and research from our group.
Vapor growth of binary and ternary phosphorus-based semiconductors into TiO2 ...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@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.
Hot hole transfer from Ag nanoparticles to multiferroic YMn2O5 nanowires enab...Pawan Kumar
Plasmonic hot carriers with a nonthermal distribution of kinetic energies have opened up new avenues in photovoltaics, photodetection and photocatalysis. While several articles have reported ultrafast hot electron injection from coinage metals into n-type semiconductors across Schottky barriers and efficient subsequent utilization of injected hot electrons, reports of hot hole harvesting are comparatively rare due to the difficulty in forming Schottky junctions between p-type semiconductors and high work function metals. In this communication, we report the fabrication, characterization and theoretical calculations of a novel integrated multiferroic-plasmonic system comprising YMn2O5 nanowires decorated on their surface with Ag nanoparticles (NPs). A Schottky barrier for holes exists at the YMn2O5-Ag hetero-interface and hot holes were injected from Ag across this barrier. The synthesized hybrid along with bare Ag NPs were tested for Raman surface photocatalytic reduction of 4-NBT (4-nitrobenzenethiol) to DMAB (p, p′-dimercaptoazobenzene) where the composite demonstrated superior activity compared to the bare metal. Ultraviolet photoelectron spectroscopy (UPS) revealed a significantly reduced work function of the composite compared to the pristine Ag, indicative of more energetic hot electrons on the surface of the composite required for efficient photoreduction. Density functional theory (DFT)-based calculations revealed localization of molecular orbitals supportive of a possible hole transfer from YMn2O5 to Ag and a reorganization of electronic states beneficial for plasmon-induced charge carrier enhancement. DFT results also indicated a purely electronic contribution to the ferroelectric polarization of YMn2O5 over and above the ionic contribution, which originated from the magnetic polarization of O 2p states.
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.
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.
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.
Optical Control of Selectivity of High Rate CO2 Photoreduction Via Interband-...Pawan Kumar
Photonic crystals consisting of TiO2 nanotube arrays (PMTiNTs) with periodically modulated diameters were fabricated using a precise charge-controlled pulsed anodization technique. The PMTiNTs were decorated with gold nanoparticles (Au NPs) to form plasmonic photonic crystal photocatalysts (Au-PMTiNTs). A systematic study of CO2 photoreduction performance on as-prepared samples was conducted using different wavelengths and illumination sequences. A remarkable selectivity of the mechanism of CO2 photoreduction could be engineered by merely varying the spectral composition of the illumination sequence. Under AM1.5 G simulated sunlight (pathway#1), the Au-PMTiNTs produced methane (302 µmol h-1) from CO2 with high selectivity (89.3%). When also illuminated by a UV-poor white lamp (pathway#2), the Au-PMTiNTs produced formaldehyde (420 µmol h-1) and carbon monoxide (323 µmol h-1) with almost no methane evolved. We confirmed the photoreduction results by 13C isotope labeling experiments using GC-MS. These results point to optical control of the selectivity of high-rate CO2 photoreduction through selection of one of two different mechanistic pathways. Pathway#1 implicates electron-hole pairs generated through interband transitions in TiO2 and Au as the primary active species responsible for reducing CO2 to methane. Pathway#2 involves excitation of both TiO2 and surface plasmons in Au. Hot electrons produced by plasmon damping and photogenerated holes in TiO2 proceed to reduce CO2 to HCHO and CO through a plasmonic Z-scheme.
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
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.
Consistently High Voc Values in p-i-n Type Perovskite Solar Cells Using Ni3+-...Pawan Kumar
Leading edge p-i-n type halide perovskite solar cells (PSCs) severely underperform n-i-p PSCs. p-i-n type PSCs that use PEDOT:PSS hole transport layers (HTLs) struggle to generate open-circuit photovoltage values higher than 1 V. NiO HTLs have shown greater promise in achieving high Voc values albeit inconsistently. In this report, a NiO nanomesh with Ni3+ defect grown by the hydrothermal method was used to obtain PSCs with Voc values that consistently exceeded 1.10 V (champion Voc = 1.14 V). A champion device photoconversion efficiency of 17.75% was observed. Density functional theory modeling was used to understand the interfacial properties of the NiO/perovskite interface. The PCE of PSCs constructed using the Ni3+-doped NiO nanomesh HTL was ∼34% higher than that of conventional compact NiO-based perovskite solar cells. A suite of characterization techniques such as transmission electron microscopy, field emission scanning electron microscopy, intensity-modulated photocurrent spectroscopy, intensity-modulated photovoltage spectroscopy, time-resolved photoluminescence, steady-state photoluminescence, and Kelvin probe force microscopy provided evidence of better film quality, enhanced charge transfer, and suppressed charge recombination in PSCs based on hydrothermally grown NiO nanostructures.
Synthesis of flower-like magnetite nanoassembly: Application in the efficient...Pawan Kumar
A facile approach for the synthesis of magnetite microspheres with flower-like morphology is reported
that proceeds via the reduction of iron(III) oxide under a hydrogen atmosphere. The ensuing magnetic
catalyst is well characterized by XRD, FE-SEM, TEM, N2 adsorption-desorption isotherm, and
Mössbauer spectroscopy and explored for a simple yet efficient transfer hydrogenation reduction of a
variety of nitroarenes to respective anilines in good to excellent yields (up to 98%) employing hydrazine
hydrate. The catalyst could be easily separated at the end of a reaction using an external magnet and
can be recycled up to 10 times without any loss in catalytic activity.
Visible light assisted reduction of nitrobenzenes using Fe(bpy)3+2/rGOnanocom...Pawan Kumar
Visible-light-induced photocatalytic reduction of aromatic nitrobenzenes to the corresponding anilinesat room temperature using reduced graphene oxide (rGO) immobilized iron(II) bipyridine complex asphotocatalyst is described. The rGO-immobilized iron catalyst exhibited superior catalytic activity thanhomogeneous iron(II) bipyridine complex and much higher than metal free rGO photocatalysts. Theheterogeneous photocatalyst was found to be robust and could easily be recovered and reused for severalruns without any significant loss in photocatalytic activity.
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.
Mixed-Valence Single-Atom Catalyst Derived from Functionalized GraphenePawan Kumar
Single-atom catalysts (SACs) aim at bridging the gap between homogeneous and heterogeneous catalysis. The challenge is the development of materials with ligands enabling coordination of metal atoms in different valence states, and preventing leaching or nanoparticle formation. Graphene functionalized with nitrile groups (cyanographene) is herein employed for the robust coordination of Cu(II) ions, which are partially reduced to Cu(I) due to graphene-induced charge transfer. Inspired by nature's selection of Cu(I) in enzymes for oxygen activation, this 2D mixed-valence SAC performs flawlessly in two O2-mediated reactions: the oxidative coupling of amines and the oxidation of benzylic CH bonds toward high-value pharmaceutical synthons. High conversions (up to 98%), selectivities (up to 99%), and recyclability are attained with very low metal loadings in the reaction. The synergistic effect of Cu(II) and Cu(I) is the essential part in the reaction mechanism. The developed strategy opens the door to a broad portfolio of other SACs via their coordination to various functional groups of graphene, as demonstrated by successful entrapment of FeIII/FeII single atoms to carboxy-graphene.
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 intra sheet cavity make it an attractive supramolecular adsorbent for environmental applications.
Bicrystalline Titania Photocatalyst for Reduction of CO2 to Solar FuelsA'Lester Allen
Degussa P25, a mixture of anatase and rutile crystal structures, is the most commonly used precursor to form the photoactive layer in solar cells; however, the photocatalytic activity of rutile is inferior to brookite. This presentation discusses the enhancement in photocatalytic activity of an antase brookite mixture.
Asymmetric Multipole Plasmon-Mediated Catalysis Shifts the Product Selectivit...Pawan Kumar
Cu/TiO2 is a well-known photocatalyst for the photocatalytic transformation of CO2 into methane. The formation of C2+ products such as ethane and ethanol rather than methane is more interesting due to their higher energy density and economic value, but the formation of C–C bonds is currently a major challenge in CO2 photoreduction. In this context, we report the dominant formation of a C2 product, namely, ethane, from the gas-phase photoreduction of CO2 using TiO2 nanotube arrays (TNTAs) decorated with large-sized (80–200 nm) Ag and Cu nanoparticles without the use of a sacrificial agent or hole scavenger. Isotope-labeled mass spectrometry was used to verify the origin and identity of the reaction products. Under 2 h AM1.5G 1-sun illumination, the total rate of hydrocarbon production (methane + ethane) was highest for AgCu-TNTA with a total CxH2x+2 rate of 23.88 μmol g–1 h–1. Under identical conditions, the CxH2x+2 production rates for Ag-TNTA and Cu-TNTA were 6.54 and 1.39 μmol g–1 h–1, respectively. The ethane selectivity was the highest for AgCu-TNTA with 60.7%, while the ethane selectivity was found to be 15.9 and 10% for the Ag-TNTA and Cu-TNTA, respectively. Adjacent adsorption sites in our photocatalyst develop an asymmetric charge distribution due to quadrupole resonances in large metal nanoparticles and multipole resonances in Ag–Cu heterodimers. Such an asymmetric charge distribution decreases adsorbate–adsorbate repulsion and facilitates C–C coupling of reaction intermediates, which otherwise occurs poorly in TNTAs decorated with small metal nanoparticles.
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 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%.
Electron transfer between methyl viologen radicals and graphene oxidekamatlab
Methyl viologen radicals are capable of transferring electrons to graphene oxide and partially restore the sp2 network. The reduced graphene oxide serves as a scaffold to anchor Ag nanoparticles. The growth of these silver nanoparticles is dictated by the ability of RGO to store and shuttle electrons. The RGO/Ag nanocomposites discussed in the present work offer new opportunities to design next generation photocatalysts.
Visit our website, KamatLab.com, for the latest news, publications, and research from our group.
Vapor growth of binary and ternary phosphorus-based semiconductors into TiO2 ...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@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.
Hot hole transfer from Ag nanoparticles to multiferroic YMn2O5 nanowires enab...Pawan Kumar
Plasmonic hot carriers with a nonthermal distribution of kinetic energies have opened up new avenues in photovoltaics, photodetection and photocatalysis. While several articles have reported ultrafast hot electron injection from coinage metals into n-type semiconductors across Schottky barriers and efficient subsequent utilization of injected hot electrons, reports of hot hole harvesting are comparatively rare due to the difficulty in forming Schottky junctions between p-type semiconductors and high work function metals. In this communication, we report the fabrication, characterization and theoretical calculations of a novel integrated multiferroic-plasmonic system comprising YMn2O5 nanowires decorated on their surface with Ag nanoparticles (NPs). A Schottky barrier for holes exists at the YMn2O5-Ag hetero-interface and hot holes were injected from Ag across this barrier. The synthesized hybrid along with bare Ag NPs were tested for Raman surface photocatalytic reduction of 4-NBT (4-nitrobenzenethiol) to DMAB (p, p′-dimercaptoazobenzene) where the composite demonstrated superior activity compared to the bare metal. Ultraviolet photoelectron spectroscopy (UPS) revealed a significantly reduced work function of the composite compared to the pristine Ag, indicative of more energetic hot electrons on the surface of the composite required for efficient photoreduction. Density functional theory (DFT)-based calculations revealed localization of molecular orbitals supportive of a possible hole transfer from YMn2O5 to Ag and a reorganization of electronic states beneficial for plasmon-induced charge carrier enhancement. DFT results also indicated a purely electronic contribution to the ferroelectric polarization of YMn2O5 over and above the ionic contribution, which originated from the magnetic polarization of O 2p states.
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.
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.
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.
Optical Control of Selectivity of High Rate CO2 Photoreduction Via Interband-...Pawan Kumar
Photonic crystals consisting of TiO2 nanotube arrays (PMTiNTs) with periodically modulated diameters were fabricated using a precise charge-controlled pulsed anodization technique. The PMTiNTs were decorated with gold nanoparticles (Au NPs) to form plasmonic photonic crystal photocatalysts (Au-PMTiNTs). A systematic study of CO2 photoreduction performance on as-prepared samples was conducted using different wavelengths and illumination sequences. A remarkable selectivity of the mechanism of CO2 photoreduction could be engineered by merely varying the spectral composition of the illumination sequence. Under AM1.5 G simulated sunlight (pathway#1), the Au-PMTiNTs produced methane (302 µmol h-1) from CO2 with high selectivity (89.3%). When also illuminated by a UV-poor white lamp (pathway#2), the Au-PMTiNTs produced formaldehyde (420 µmol h-1) and carbon monoxide (323 µmol h-1) with almost no methane evolved. We confirmed the photoreduction results by 13C isotope labeling experiments using GC-MS. These results point to optical control of the selectivity of high-rate CO2 photoreduction through selection of one of two different mechanistic pathways. Pathway#1 implicates electron-hole pairs generated through interband transitions in TiO2 and Au as the primary active species responsible for reducing CO2 to methane. Pathway#2 involves excitation of both TiO2 and surface plasmons in Au. Hot electrons produced by plasmon damping and photogenerated holes in TiO2 proceed to reduce CO2 to HCHO and CO through a plasmonic Z-scheme.
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 …
Carbon-cuprous oxide composite nanoparticles
were chemically deposited on surface of thin glass tubes of spent
energy saving lamps for solar heat collection. Carbon was
obtained from fly ash of heavy oil incomplete combustion in
electric power stations. Impurities in the carbon were removed by
leaching with mineral acids. The mineral free-carbon was then
wet ground to have a submicron size. After filtration, it was
reacted with concentrated sulfuric/fuming nitric acid mixture on
cold for 3-4 days. Potassium chlorate was then added drop wise on
hot conditions to a carbon slurry followed by filtration.
Nanocarbon sample was mixed with 5% by weight PVA to help
adhesion to the glass surface. Carbon so deposited was doped with
copper nitrate solution. After dryness, the carbon/copper nitrate
film was dipped in hydrazine hydrate to form cuprous oxide -
carbon composite, It was then roasted at 380-400 °C A heat
collector testing assembly was constructed of 5 glass coils
connected in series with a total surface area of 1250 cm2
. Heat
collection was estimated by water flowing in the glass coils that
are coated with the carbon/copper film,. Parameters affecting the
solar collection efficiency such as time of exposure and mass flow
rate of the water were studied. Results revealed that the prepared
glass coil has proven successful energy collector for solar heat.
Synthesis of flower-like magnetite nanoassembly: Application in the efficient...Pawan Kumar
A facile approach for the synthesis of magnetite microspheres with flower-like morphology is reported
that proceeds via the reduction of iron(III) oxide under a hydrogen atmosphere. The ensuing magnetic
catalyst is well characterized by XRD, FE-SEM, TEM, N2 adsorption-desorption isotherm, and
Mössbauer spectroscopy and explored for a simple yet efficient transfer hydrogenation reduction of a
variety of nitroarenes to respective anilines in good to excellent yields (up to 98%) employing hydrazine
hydrate. The catalyst could be easily separated at the end of a reaction using an external magnet and
can be recycled up to 10 times without any loss in catalytic activity.
Direct Synthesis Carbon/Metal Oxide Composites for Electrochemical Capacitors...drboon
This paper deals with the study of the carbon/metal oxide composites synthesis for electrochemical capacitor electrode material. Transition metal salts, such as FeCl3 and TiCl3 act as activator in the synthesis of activated carbon from gelam wood sawdust (Melaleuca cajuputi Powell) which also have the functions as substrates for the composites. The surface functionalities of activated carbons were modified using oxidative treatments. The changes on crystallography and surface functionalities were analyzed based on XRD and FTIR data. The electrical conductivities and electrochemical properties were determined using kelvin and cyclic voltammetry methods, respectively. FTIR analyses showed that the activation and oxidation treatments affected their surface functionalities. The XRD analyses showed that oxidative treatments also affected carbons crystallite. The electrical conductivities and electrochemical properties were influenced by their crystallite and surface functionalities. The shape of the cyclic voltammograms varied according to the changes on the surface functionalities and on the metals loading. TEM analyses indicated the existence of nanoparticles metal oxides in the carbon samples.
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.
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 …
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.
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.
Electrochemical study of anatase TiO2 in aqueous sodium-ion electrolytesRatnakaram Venkata Nadh
In this paper, a basic electro-analytical study on the behavior of anatase TiO2 in aqueous NaOH has been presented using cyclic voltammetry technique (CV). The study has explored the possibility of using TiO2 as anode material for ARSBs in presence of 5 M NaOH aqueous electrolyte. CV profiles show that anatase TiO2 exhibits reversible sodium ion insertion/de-insertion reactions. CV studies of TiO2 anode in aqueous sodium electrolytes at different scan rate shows that the Na+ ion insertion reaction at the electrode is diffusion controlled with a resistive behavior. Proton insertion from aqueous sodium electrolytes into TiO2 cannot be ruled out. To confirm the ion inserted and de-inserted, CV studies are done at different concentration of NaOH and it is found that at lower concentrations of NaOH, proton insertion process competes with Na+ ion insertion process and as the concentration increases, the Na+ ion insertion process becomes the predominant electrode reaction making it suitable anode materials for aqueous sodium batteries in 5 M NaOH.
Edge-confined under-coordinated Cu atoms on Ru nanosheets enable efficient CH...Pawan Kumar
In this issue of Chem Catalysis, Jinchang Fan et al. demonstrate that Cu atoms confined in ultrathin 2D metallic Ru nanosheets (Ru11Cu NSs) favor bi-coordinated adsorption of O∗ species to reduce the activation energy barrier for a facile C–H bond cleavage to C1 oxygenates (CH3OOH and CH3OH) with 99% selectivity.
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.
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.
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.
Comparing Evolved Extractive Text Summary Scores of Bidirectional Encoder Rep...University of Maribor
Slides from:
11th International Conference on Electrical, Electronics and Computer Engineering (IcETRAN), Niš, 3-6 June 2024
Track: Artificial Intelligence
https://www.etran.rs/2024/en/home-english/
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.
Nutraceutical market, scope and growth: Herbal drug technologyLokesh Patil
As consumer awareness of health and wellness rises, the nutraceutical market—which includes goods like functional meals, drinks, and dietary supplements that provide health advantages beyond basic nutrition—is growing significantly. As healthcare expenses rise, the population ages, and people want natural and preventative health solutions more and more, this industry is increasing quickly. Further driving market expansion are product formulation innovations and the use of cutting-edge technology for customized nutrition. With its worldwide reach, the nutraceutical industry is expected to keep growing and provide significant chances for research and investment in a number of categories, including vitamins, minerals, probiotics, and herbal supplements.
Professional air quality monitoring systems provide immediate, on-site data for analysis, compliance, and decision-making.
Monitor common gases, weather parameters, particulates.
Richard's entangled aventures in wonderlandRichard Gill
Since the loophole-free Bell experiments of 2020 and the Nobel prizes in physics of 2022, critics of Bell's work have retreated to the fortress of super-determinism. Now, super-determinism is a derogatory word - it just means "determinism". Palmer, Hance and Hossenfelder argue that quantum mechanics and determinism are not incompatible, using a sophisticated mathematical construction based on a subtle thinning of allowed states and measurements in quantum mechanics, such that what is left appears to make Bell's argument fail, without altering the empirical predictions of quantum mechanics. I think however that it is a smoke screen, and the slogan "lost in math" comes to my mind. I will discuss some other recent disproofs of Bell's theorem using the language of causality based on causal graphs. Causal thinking is also central to law and justice. I will mention surprising connections to my work on serial killer nurse cases, in particular the Dutch case of Lucia de Berk and the current UK case of Lucy Letby.
2. Catalysts 2021, 11, 1374 2 of 13
nanoparticles were used for electrocatalytic oxygen evolution, and the highest performance
was 10 mA·cm−2 at an overpotential of 358 mV [10]. Another similar work was presented
by Yang et al. reporting electrocatalytic hydrogen evolution using nitrogen-plasma-treated
hafnium oxyhydroxide [11]. Based on the findings from pioneer studies, HfN is worth
investigating as a photocatalyst for solar energy conversion due to the following reasons:
(1) high thermal and chemical resistance, and photochemical stability under harsh con-
ditions; (2) long hot carrier lifetime; (3) plasmon resonance in the Vis-NIR regime; and
(4) cost-efficient alternative to noble metals. Despite the aforementioned properties that
are particularly beneficial to photocatalytic energy conversion, no one has reported the
successful utilization of HfN for any kind of photocatalytic reactions. With this aim in
mind, a photoanode consisting of TiO2(core)–HfN(shell) nanotube arrays (HfN-TNT) was
deployed in photoelectrochemical water splitting and achieved a champion photocurrent of
2.48 mA·cm−2 under 1 Sun illumination (AM 1.5G) at an applied bias of +0.6 V vs. Ag/AgCl
reference electrode in 1 M KOH solution. A photocurrent response of 2.39 mA·cm−2 during
light on–off cycling was measured under identical conditions. To the best of our knowledge,
this performance is superior to most plasmonic noble metal-decorated TiO2-based catalysts.
The TiO2 nanotube array (core) was grown by electrochemical anodization of a Ti film
sputtered at room temperature on a non-native fluorine-doped tin oxide (FTO)-coated glass
substrate. The conformal HfN shell layer was formed by plasma-enhanced atomic layer
deposition, which is a facile technique to form conductive transition metal nitride coatings
of good structural and stoichiometric quality [12,13]. The catalyst is economically efficient
and suitable for large-scale production due to the scalability and existing industrial usage
of electrochemical anodization and atomic layer deposition.
Wide bandgap (WBG) metal oxides have been demonstrated to be photochemically sta-
ble, high-performance photocatalysts for artificial photosynthesis [14–17]. One-dimensional
WBG architectures such as vertically aligned arrays of nanotubes and/or nanorods are
established to be highly optimal from the point of view of solar energy harvesting and
reduction of carrier recombination losses, since this architecture orthogonalizes the com-
peting processes of light harvesting and charge separation [18,19]. However, the low
visible light-harvesting ability of catalytically-active metal oxides such as TiO2, Nb2O5,
SrTiO3, KNbO3, etc. has motivated the search for visible light-harvesting heterojunction
formers/co-catalysts that sensitize the photocatalyst to visible photons where the bulk
of the solar energy resides. One strategy consists of forming core@shell heterojunctions
wherein WBG nanotubes or nanorods (“core”) are coated with a thin absorbing layer
of a lower bandgap semiconductor (“shell”) [20,21]. While core–shell heterojunctions
have shown considerable promise in improving the performance of photoelectrochemi-
cal water splitting [22–24], photoanodes constituted exclusively of semiconductors suffer
thermodynamic losses for supra-bandgap photons. On the other hand, photoanodes
consisting of plasmonic metal–semiconductor heterojunctions can reduce thermalization
losses by efficiently harvesting hot carriers while simultaneously sensitizing WBG semi-
conductors to visible photons [25,26]. Currently, plasmonic coinage metal (Au, Ag, Cu)
nanoparticles [27–31] and 2D material nanosheets (g-C3N4, MoS2, etc.) [15,32–34] are the
most commonly used, photochemically stable co-catalysts that are used to decorate nanos-
tructured metal oxides and achieve the desired photosensitization. Coinage metals are very
expensive and suffer from ultrafast hot carrier relaxation processes. Two-dimensional (2D)
materials have low absorption coefficients and frequently require sacrificial agents for opti-
mal charge transfer. We demonstrate here that plasmonic transition metal nitride ceramics,
specifically HfN, form a third class of visible-light absorbing compounds which can be used
to form heterojunctions with metal oxides that result in photocatalysts/photoelectrodes
that combine intrinsic photochemical stability, low cost, and high performance without the
need for a sacrificial agent or carrier scavenger.
3. Catalysts 2021, 11, 1374 3 of 13
2. Results and Discussion
2.1. Photoelectrochemical Performance
The photoelectrochemical J-V characteristics of HfN-TNT (shown in Figure 1a,b)
were collected in the dark and under AM 1.5G 1 sun illumination without and with a
UV filter in 1 M KOH solution. A baseline was measured without any illumination, in
which we observed almost zero photocurrent density. The highest photocurrents (shown
in Figure 1a) are 2.48 mA·cm−2 under 1 Sun illumination (AM 1.5G) and 1.8 mA·cm−2
after adding a UV filter (420 nm) at an applied bias of +0.6 V vs. Ag/AgCl. The data
collected under light on–off mode is shown in Figure 1b, in which the photocurrents are
2.39 mA·cm−2 and 1.74 mA·cm−2 without and with a UV filter, respectively. To understand
the spectral composition of the generated photocurrent, a set of near-monochromatic
LEDs were used to illuminate the sample (Figure 1c,d) for testing that were conducted
in 0.1 M KOH electrolyte and, unless otherwise stated, the photocurrents were at a bias
of +0.6 V versus Ag/AgCl. We observed a photocurrent response that extended all the
way to 730 nm wavelength illumination, which is quite unusual in the field of sunlight-
driven water-splitting. The 730 nm photons have an energy of 1.7 eV, which is merely
0.47 eV above the minimum energy required for water electrolysis. It is important to
understand how the performance of the HfN-TNT photoanodes stands in comparison to
the plasmonic noble metal sensitized metal oxide photoanodes they seek to replace. Table
1 is a performance summary of the photoelectrochemical performance under visible light
illumination of TiO2-based photoanodes that derived an enhancement of photoactivity in
the visible regime after decoration by nanoparticles or coatings of plasmonic noble metals,
namely Ag and Au. Table 1 shows that the harvesting of visible light using hot carriers by
previously reported noble metal-TiO2 nanostructured plasmonic heterojunctions is able
to generate no more than 224 µA·cm−2 in a photocurrent. Such a poor performance is
not entirely surprising, since it is well-understood that the ultrafast timescale (10 ps)
of electron–electron scattering and electron–phonon scattering render extremely hard to
drive chemical reactions using hot carriers before their thermal equilibration. In contrast,
our unoptimized HfN-TNT photoanode is able to generate 1.8 mA·cm−2 under identical
conditions, as shown in Table 1, indicating a potentially superior hot carrier harvesting
ability.
Table 1. A summary of photocurrent densities observed with plasmonic noble metal-decorated TiO2-based photoanodes for
PEC water splitting under visible light illumination.
Sample
Photocurrent
(µA/cm2)
Light Intensity
(mW/cm2)
Light Spectrum
Applied
Bias
Electrolyte Reference
HfN-TNT 1800 100 visible light (420 nm) 0.6 V 1 M KOH This work
Ag/N-TiO2 26 120 visible light (420 nm) 0 V 0.5 M Na2SO4 [35]
AgNPs/TiO2 NWs 47 100 visible light (420 nm) 0.4 V 0.1 M Na2SO4 [36]
In situ
AgNPs/TNTs
40 80 visible light (420 nm) 0.3 V 0.1 M Na2SO4 [37]
Ag/N-TiO2 0.5 500 400–900 nm 0.3 V 1 M KOH [38]
Au/RGO/H-TNTs 224 100 visible light (400 nm) 0.2 V 1 M KOH [39]
LE-Au/TNTs 202 100 visible light (400 nm) 0.2 V 1 M KOH [40]
AuNPs/TiO2 23 7000 532 nm, 633 nm 0 V 1 M KOH [41]
Au embedded TiO2 3 visible light (420 nm) 0.2 V
1 M
KOH + 25%
MeOH
[42]
AuNPs/TiO2 BNRs 125 100 visible light (420 nm) 0.5 V 1 M KOH [43]
AuNPs/TiO2 NWs 11 73.3 visible light (430 nm) 0 V 1 M KOH [44]
AuNPs/TiO2 PhC 150 100 visible light (420 nm) 0.2 V 1 M KOH [45]
4. Catalysts 2021, 11, 1374 4 of 13
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AuNPs/TiO2 PhC 150 100 visible light (420 nm) 0.2 V 1 M KOH [45]
Figure 1. Photocurrent density plots: (a) Photocurrent measured by linear sweep voltammetry un‐
der AM 1.5G illumination; (b) Photocurrent response during light on–off cycles, measured by linear
sweep voltammetry under AM 1.5G illumination with and without UV cut‐off filter (420 nm); (c)
and (d) Amperometric I‐t curves showing photocurrent response under the illumination of near‐
monochromatic LEDs above at 0.3 V vs. Ag/AgCl.
2.2. Physicochemical Characterization
The morphologies were examined before and after HfN coating using a Zeiss Sigma
field emission scanning electron microscope (FESEM). The outer diameter of nanotubes is
80–100 nm, while the inner diameter is 30–50 nm (Figure 2a). After HfN atomic layer dep‐
osition (ALD), the mouths of nanotubes are almost closed, and the space between the
nanotubes is filled. The high‐resolution transmission microscopy (HRTEM) image (Figure
2c) shows the tubular structure with double layers. The TiO2 core is slightly brighter than
the HfN shell, which is darker. Figure 2d shows the lattice fringes at the interface of the
core and shell with 0.358 nm and 0.256 nm interplanar d‐spacing, corresponding to ana‐
tase TiO2 (101) and HfN (111), respectively.
Figure 1. Photocurrent density plots: (a) Photocurrent measured by linear sweep voltammetry under
AM 1.5G illumination; (b) Photocurrent response during light on–off cycles, measured by linear
sweep voltammetry under AM 1.5G illumination with and without UV cut-off filter (420 nm);
(c,d) Amperometric I-t curves showing photocurrent response under the illumination of near-
monochromatic LEDs above at 0.3 V vs. Ag/AgCl.
2.2. Physicochemical Characterization
The morphologies were examined before and after HfN coating using a Zeiss Sigma
field emission scanning electron microscope (FESEM). The outer diameter of nanotubes
is 80–100 nm, while the inner diameter is 30–50 nm (Figure 2a). After HfN atomic layer
deposition (ALD), the mouths of nanotubes are almost closed, and the space between
the nanotubes is filled. The high-resolution transmission microscopy (HRTEM) image
(Figure 2c) shows the tubular structure with double layers. The TiO2 core is slightly brighter
than the HfN shell, which is darker. Figure 2d shows the lattice fringes at the interface of
the core and shell with 0.358 nm and 0.256 nm interplanar d-spacing, corresponding to
anatase TiO2 (101) and HfN (111), respectively.
In the Hf 4f XPS spectrum (Figure 3a), the splitting of a spin orbit results in clear
doublet peaks, which are Hf 4f5/2 and Hf 4f7/2. Thus, the peaks at 16.9 eV and 18.6 eV can
be assigned to Hf 4f7/2-N and Hf 4f5/2-N bonds, respectively. Similarly, N1s (Figure 3b) also
exhibited two major peaks at binding energies of 399.7 eV and 402.7 eV, which originated
from HfN and HfON bonds, respectively [46]. The HfON is formed due to the surface
oxidation in air environment. Since the HfN film was 20 nm thick and conformally coated
throughout the sample, there is no trace of Ti 2p signal, which is too deep for detection by
the high-resolution XPS measurement.
5. Catalysts 2021, 11, 1374 5 of 13
Catalysts 2021, 11, x FOR PEER REVIEW 5 of 14
Figure 2. FESEM images for sample before and after ALD HfN coating, (a) and (b) are top view
images before and after HfN coating; (c) HRTEM cross‐section view image demonstrates the two
layers constituting the core–shell nanotube morphology; (d) HRTEM image at the HfN and TiO2
interface showing anatase TiO2 (101) lattice fringes and HfN (111) lattice fringes.
In the Hf 4f XPS spectrum (Figure 3a), the splitting of a spin orbit results in clear
doublet peaks, which are Hf 4f5/2 and Hf 4f7/2. Thus, the peaks at 16.9 eV and 18.6 eV can
be assigned to Hf 4f7/2 ‐N and Hf 4f5/2 ‐N bonds, respectively. Similarly, N1s (Figure 3b)
also exhibited two major peaks at binding energies of 399.7 eV and 402.7 eV, which origi‐
nated from HfN and HfON bonds, respectively [46]. The HfON is formed due to the sur‐
face oxidation in air environment. Since the HfN film was 20 nm thick and conformally
coated throughout the sample, there is no trace of Ti 2p signal, which is too deep for de‐
tection by the high‐resolution XPS measurement.
Figure 2. FESEM images for sample before and after ALD HfN coating, (a,b) are top view images
before and after HfN coating; (c) HRTEM cross-section view image demonstrates the two layers
constituting the core–shell nanotube morphology; (d) HRTEM image at the HfN and TiO2 interface
showing anatase TiO2 (101) lattice fringes and HfN (111) lattice fringes.
The value of work function was calculated using the equation: WF = 21.2 − Ecut-off in
which Ecut-off is the energy of emitted secondary electrons and 21.2 eV is according to the
He laser UV light source. The secondary electron cut-off energy of this material is 18.4 eV
(Figure 3c). According to the expression given before, the work function with respect to
the vacuum level is 2.8 eV, which is up shifted by 1.5 eV from the normal WF value of
4.3 eV for anatase TiO2 [24]. Additionally, the valence band maximum was calculated to be
5.21 eV (Figure 3d). Based on upward shifting and WF position, the material is electron
degenerated after HfN coating due to its metallic property.
The phonon dispersion in HfN makes it an outstanding candidate for hot carrier-
mediated photocatalysis. The constituent Hf and N atoms have a great difference in
mass resulting in a large phonon gap between the optical and acoustic modes. In the
Raman spectrum of the HfN-TNT sample (Figure 4a), the dominant four peaks are at
144 cm−1 (Eg), 399 cm−1 (B1g), 515 cm−1 (A1g and B1g), and 639 cm−1 (Eg) from anatase
TiO2. Further investigation focused on HfN phonon dispersion. Although there are some
peaks that do not stand out from TiO2 peaks, the signature Raman peaks for HfN phonon
distribution are still noticeable. The peak representing the HfN transverse acoustic phonon
mode is submerged in the TiO2 Eg band at 140 cm−1. Nevertheless, another signature
peak for longitudinal acoustic phonons in the first-order acoustic band is observable at
199 cm−1 in the spectrum. The first-order optical phonon band is well resolved at 540 cm−1
and energetically distant from the first-order acoustic band. Other peaks, at 250 cm−1,
350 cm−1, 420 cm−1, 800 cm−1, and 1100 cm−1, can be assigned to second-order transverse
acoustic mode, the sum of transverse and longitudinal acoustic modes, the difference
6. Catalysts 2021, 11, 1374 6 of 13
between optical and acoustic modes, the sum of optical and acoustic modes, and second-
order optical mode, respectively. It is worth noting that the first-order scattering peaks are
not prominent in the Raman spectrum, which is due to the suppression of the first-order
Raman effect by stoichiometric HfN with a low concentration of defects [47]. The source
of first-order Raman scattering is usually attributed to N vacancies i.e., stoichiometric
defects. The X-ray diffractogram (XRD) (Figure S1 in supporting information) indicates
that HfN has a cubic fcc crystal structure, and the dominant (111) diffraction peak is a
strong indication of stoichiometric HfN [48].
Catalysts 2021, 11, x FOR PEER REVIEW 6 of 14
Figure 3. (a,b) Deconvoluted XPS spectra for Hf 4f and N 1s, respectively; (c,d) work function and
valence band maximum with respect to vacuum level from ultraviolet photoelectron spectroscopy.
The value of work function was calculated using the equation: WF = 21.2‐Ecut‐off in
which Ecut‐off is the energy of emitted secondary electrons and 21.2 eV is according to the
He laser UV light source. The secondary electron cut‐off energy of this material is 18.4 eV
(Figure 3c). According to the expression given before, the work function with respect to
the vacuum level is 2.8 eV, which is up shifted by 1.5 eV from the normal WF value of 4.3
eV for anatase TiO2 [24]. Additionally, the valence band maximum was calculated to be
5.21 eV (Figure 3d). Based on upward shifting and WF position, the material is electron
degenerated after HfN coating due to its metallic property.
The phonon dispersion in HfN makes it an outstanding candidate for hot carrier‐
mediated photocatalysis. The constituent Hf and N atoms have a great difference in mass
resulting in a large phonon gap between the optical and acoustic modes. In the Raman
spectrum of the HfN‐TNT sample (Figure 4a), the dominant four peaks are at 144 cm−1
(Eg), 399 cm−1 (B1g), 515 cm−1 (A1g and B1g), and 639 cm−1 (Eg) from anatase TiO2. Further
investigation focused on HfN phonon dispersion. Although there are some peaks that do
not stand out from TiO2 peaks, the signature Raman peaks for HfN phonon distribution
are still noticeable. The peak representing the HfN transverse acoustic phonon mode is
submerged in the TiO2 Eg band at 140 cm−1. Nevertheless, another signature peak for lon‐
gitudinal acoustic phonons in the first‐order acoustic band is observable at 199 cm−1 in the
spectrum. The first‐order optical phonon band is well resolved at 540 cm−1 and energeti‐
cally distant from the first‐order acoustic band. Other peaks, at 250 cm−1, 350 cm−1, 420
cm−1, 800 cm−1, and 1100 cm−1, can be assigned to second‐order transverse acoustic mode,
the sum of transverse and longitudinal acoustic modes, the difference between optical and
acoustic modes, the sum of optical and acoustic modes, and second‐order optical mode,
Figure 3. (a,b) Deconvoluted XPS spectra for Hf 4f and N 1s, respectively; (c,d) work function and
valence band maximum with respect to vacuum level from ultraviolet photoelectron spectroscopy.
Klemens decay is the primary pathway for the loss of energy of optical phonons and
is attenuated by a large phononic bandgap [49]. According to the Raman spectrum we
presented in Figure 4a, the first-order acoustic and the first-order optical phonons were
located at 140 cm−1 and 540 cm−1 respectively, meaning that the large energy difference
between them prevents Klemens decay. This is a critical factor, because when the decay
of optical phonons is suppressed, a phonon bottleneck is created and hot carriers tend to
have a longer lifetime. HfN is a conductive ceramic with no electronic bandgap, which is
a perfect characteristic for a hot carrier absorber [9]. The combination of these properties
explains why we observed a small difference in photocurrent with and without a UV
filter, obtaining a photocurrent response 1.74 mA·cm−2 and 2.39 mA·cm−2, respectively,
at an applied bias of +0.6 V during on–off cycles. Incident photons stimulate plasmon
oscillations on the sample surface, which then decay into hot electron–hole pairs through
Landau damping. Due to the presence of applied bias and relatively longer hot carrier
lifetimes in HfN, the high-energy holes were pushed to the anode–electrolyte interface
prior to thermalization and oxidized the OH− ions in the water-based electrolyte to form
O2. From a thermodynamic point of view, holes with energies higher than 1.23 eV are
capable of splitting water. According to the AM 1.5 G solar spectral irradiance data, the
percentage of photon flux (vs the cumulative flux of photons in sunlight) up to 750 nm
(practical cut-off wavelength) is 31% with UV filter and 36% without UV filter, which
7. Catalysts 2021, 11, 1374 7 of 13
is in close agreement with the ratio of photocurrent we obtained with and without UV
filter [50]. The conformal coating of HfN modified the optical properties of TiO2 nanotubes
(Figure 4b). The band edge shifted from 385 nm (anatase TiO2) to 625 nm (HfN-TNT),
thus enhancing visible light absorption. There also appeared a localized surface plasmon
resonance (LSPR) peak centered at 720 nm with a Q factor equal to 0.95. Close to the
LSPR peak, the maximum local electric field enhancement is observed at the HfN-TiO2
interface (Figure S3 in Supporting Information). The peaks after 800 nm are due to the
interference fringes that are well-known for TiO2 nanotube arrays (Figure S4 in Supporting
Information). It is worth noting that in the infrared regime, it displayed strongly increasing
absorption, which indicated metal-like property due to high free carrier concentration.
Catalysts 2021, 11, x FOR PEER REVIEW 7 of 14
fcc crystal structure, and the dominant (111) diffraction peak is a strong indication of stoi‐
chiometric HfN [48].
Figure 4. (a) Raman spectrum of HfN@TNT, TA stands for transverse acoustic mode, LA stands for
longitudinal acoustic mode, O represents optical mode, and (b) UV‐Vis absorption spectrum of
HfN‐TNT, and the inset is a zoom‐in view of the localized surface plasmon resonance peak and its
calculated quality factor.
Klemens decay is the primary pathway for the loss of energy of optical phonons and
is attenuated by a large phononic bandgap [49]. According to the Raman spectrum we
presented in Figure 4a, the first‐order acoustic and the first‐order optical phonons were
located at 140 cm−1 and 540 cm−1 respectively, meaning that the large energy difference
between them prevents Klemens decay. This is a critical factor, because when the decay
of optical phonons is suppressed, a phonon bottleneck is created and hot carriers tend to
have a longer lifetime. HfN is a conductive ceramic with no electronic bandgap, which is
a perfect characteristic for a hot carrier absorber [9]. The combination of these properties
explains why we observed a small difference in photocurrent with and without a UV filter,
obtaining a photocurrent response 1.74 mA∙cm−2 and 2.39 mA∙cm−2, respectively, at an ap‐
plied bias of +0.6 V during on–off cycles. Incident photons stimulate plasmon oscillations
on the sample surface, which then decay into hot electron–hole pairs through Landau
Figure 4. (a) Raman spectrum of HfN@TNT, TA stands for transverse acoustic mode, LA stands
for longitudinal acoustic mode, O represents optical mode, and (b) UV-Vis absorption spectrum of
HfN-TNT, and the inset is a zoom-in view of the localized surface plasmon resonance peak and its
calculated quality factor.
2.3. Quantum Computation Results
The HOMO–LUMO plots (Figure 5) demonstrated that charge transfer excitation is
prominent on HfN due to the fact that occupied and unoccupied regions are primarily
composed of HfN orbitals, which is also supported by the projected density of states (PDOS)
plot (Figure 6). Some tiny penetration of LUMO was observed in the close geometries,
specifically in the case of TiO2 (101) and the Hf-terminated HfN (111) system. These results
indicate that the HfN planes are chemically more active compared to TiO2. Figure 6a
showed the PDOS of selected atoms in the HfN-TiO2 system, where TiO2 and HfN slabs
8. Catalysts 2021, 11, 1374 8 of 13
are kept far apart to ensure there is no interaction between them. As expected, the plot
showed TiO2 to be a wide bandgap n-type semiconductor whose valence and conduction
bands are mostly composed of O-2p and Ti-3d states [51]. These figures also showed the
electronic properties of HfN to be metallic with overlapped valence and conduction bands
that are mostly composed of Hf d-states, which is consistent with earlier findings [8]. In the
composite system, hot electron generation upon plasmon dephasing (Landau damping)
within HfN is expected to be dominated by an intra-band transition involving the Hf
d-orbital. These hot electrons can be injected into the TiO2 d-orbital from HfN in an indirect
plasmon-induced charge transfer excitation route.
The HOMO–LUMO plots (Figure 5) demonstrated that charge transfer excitation is
prominent on HfN due to the fact that occupied and unoccupied regions are primarily
composed of HfN orbitals, which is also supported by the projected density of states
(PDOS) plot (Figure 6). Some tiny penetration of LUMO was observed in the close geom‐
etries, specifically in the case of TiO2 (101) and the Hf‐terminated HfN (111) system. These
results indicate that the HfN planes are chemically more active compared to TiO2. Figure
6a showed the PDOS of selected atoms in the HfN‐TiO2 system, where TiO2 and HfN slabs
are kept far apart to ensure there is no interaction between them. As expected, the plot
showed TiO2 to be a wide bandgap n‐type semiconductor whose valence and conduction
bands are mostly composed of O‐2p and Ti‐3d states [51]. These figures also showed the
electronic properties of HfN to be metallic with overlapped valence and conduction bands
that are mostly composed of Hf d‐states, which is consistent with earlier findings [8]. In
the composite system, hot electron generation upon plasmon dephasing (Landau damp‐
ing) within HfN is expected to be dominated by an intra‐band transition involving the Hf
d‐orbital. These hot electrons can be injected into the TiO2 d‐orbital from HfN in an indi‐
rect plasmon‐induced charge transfer excitation route.
Figure 5. DFT optimized structures showing spatial positions of highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) for TiO2 (101) and HfN (111) are located
at far (a) and close (b) configurations.
Catalysts 2021, 11, x FOR PEER REVIEW 9 of 14
Figure 5. DFT optimized structures showing spatial positions of highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) for TiO2 (101) and HfN (111) are located
at far (a) and close (b) configurations.
Note that according to the density of states analysis, the plasmon‐induced chemical
interface damping (CID) is not expected to play any significant role in the heterostruc‐
tures. In the CID mechanism, plasmon dephasing occurs at the plasmonic metal–semicon‐
ductor interfacial region, where dephasing and the subsequent generation of hot electrons
occur directly in the newly formed interfacial hybridized orbitals [52,53]. According to
Figure 6b, almost no indication of the formation of such preferred hybridized orbitals in
the vicinity of TiO2 conduction band edge is evident due to a large discrepancy in the DOS
intensity and associated orbital types. The overlap between relevant HfN orbitals with
TiO2 acceptor orbitals (in the conduction band region) was found to be inadequate for a
CID‐type mechanism to be dominant.
Figure 6. Projected density of states (PDOS) of selected atoms for HfN@TiO2 heterostructures in far
and close configurations. TiO2 (101) and HfN (111) with Hf‐edge planes are located at far (a) and
close (b) configurations.
3. Materials and Methods
3.1. TiO2 Nanotube Array Synthesis
The fluorine‐doped tin oxide (FTO)‐coated glasses were first washed by soap water
and then ultrasonicated in deionized water, acetone, and methanol sequentially for 10 min
each. After being dried under a nitrogen stream, the top 3 mm of each substrate was cov‐
ered with Kapton tape, which was done to keep an FTO‐exposed area for contact in later
experiments. Then, the substrates were loaded into a direct current magnetron sputtering
system. The sputtering chamber was first evacuated to 10−6 Torr and later filled with argon
to achieve a working pressure of 1 mTorr. A Ti target with 99.99% purity was used to
deposit a 500 nm thick, smooth Ti film on the substrates at room temperature while ben‐
Figure 6. Projected density of states (PDOS) of selected atoms for HfN@TiO2 heterostructures in far
and close configurations. TiO2 (101) and HfN (111) with Hf-edge planes are located at far (a) and
close (b) configurations.
Note that according to the density of states analysis, the plasmon-induced chemical
interface damping (CID) is not expected to play any significant role in the heterostructures.
In the CID mechanism, plasmon dephasing occurs at the plasmonic metal–semiconductor
interfacial region, where dephasing and the subsequent generation of hot electrons occur
directly in the newly formed interfacial hybridized orbitals [52,53]. According to Figure 6b,
almost no indication of the formation of such preferred hybridized orbitals in the vicinity
9. Catalysts 2021, 11, 1374 9 of 13
of TiO2 conduction band edge is evident due to a large discrepancy in the DOS intensity
and associated orbital types. The overlap between relevant HfN orbitals with TiO2 accep-
tor orbitals (in the conduction band region) was found to be inadequate for a CID-type
mechanism to be dominant.
3. Materials and Methods
3.1. TiO2 Nanotube Array Synthesis
The fluorine-doped tin oxide (FTO)-coated glasses were first washed by soap water
and then ultrasonicated in deionized water, acetone, and methanol sequentially for 10 min
each. After being dried under a nitrogen stream, the top 3 mm of each substrate was
covered with Kapton tape, which was done to keep an FTO-exposed area for contact
in later experiments. Then, the substrates were loaded into a direct current magnetron
sputtering system. The sputtering chamber was first evacuated to 10−6 Torr and later
filled with argon to achieve a working pressure of 1 mTorr. A Ti target with 99.99% purity
was used to deposit a 500 nm thick, smooth Ti film on the substrates at room temperature
while benefiting from the atomic peening mechanism [54]. After deposition, the substrates
were cut into 2.5 cm × 3 cm pieces with edges protected by Kapton tape to limited high
currents at edges and corners during the electrochemical anodization process. The samples
were anodized at 40 V in a mixed ethylene glycol-based electrolyte containing 0.3wt %
NH4F and 4 v% deionized water. The electrochemical anodization was conducted in a
two-electrode cell using the as-prepared sample as the working electrode and a 6 mm
diameter graphite rod as the counter electrode with a 3 cm distance between the anode and
cathode. The anodization process took around 10 min until the current began to rise and
the substrate turned semi-translucent from metallic dark. After the anodization completed,
the samples were rinsed with methanol and dried under nitrogen flow. To remove the
debris that formed on the top of the TiO2 nanotubes, a dry etching process was applied on
synthesized samples using an Oxford PlasmaPro NGP80 Reactive Ion Etcher with SF6 as
the working gas at 20 mTorr and a forward power of 250 W for 200 s, and it was followed
by O2 plasma at 150 mTorr and a forward power of 225 W for 10 min. As-prepared samples
were annealed in a three-zone tube furnace (STF55666C-1, Thermo Scientific Lindberg/Blue
M), in which the temperature increased to 450 in 4 h and the dwell time was another 4 h.
3.2. ALD Deposition of Hafnium Nitride
HfN films were grown using a plasma-enhanced atomic layer deposition (PE-ALD)
technique in a continuous flow ALD system (ALD150-LX, Kurt J. Lesker) at 1.01 Torr reactor
pressure using remote inductively coupled plasma (13.56 MHz ICP, 0.6 kW, 60 sccm FG
with 100 sccm Ar carrier). Tetrakis(dimethylamino)hafnium (TDMAHf) and forming gas
(FG: 5% H2 + 95% N2) were the Hf-precursor and N-source, respectively. Process conditions
for self-limiting HfN PEALD were characterized with in situ spectroscopic ellipsometry
measurements (M2000DI, J. A. Woollam). Using growth-per-cycle (GPC) determined on a
planar Si (111) substrate for PEALD cycle: 0.1 s TDMAHf pulse, 12 s post-precursor purge,
9 s FG plasma exposure, and 5 s post-plasma purge, 20 nm thick HfN film was grown on
TiO2 nanotube array structures in 300 cycles.
3.3. Materials Characterization
The optical spectra of the TiO2@HfN nanotube arrays were measured in diffuse reflec-
tion mode using a UV-Vis-NIR spectrophotometer with an integrating sphere (Perkin Elmer
Lambda 1050). The surface chemical composition was investigated by X-ray photoelectron
spectroscopy (XPS) using a Kratos AXIS Ultra instrument, and the survey scan is shown as
Figure S2 in the supporting information. To obtain the electronic structure information and
band edge energy, the work function spectrum and valence band spectrum were acquired
using ultraviolet photoelectron spectroscopy (UPS) using a He laser UV source.
10. Catalysts 2021, 11, 1374 10 of 13
3.4. Photoelectrochemical Measurements
The measurement of photoelectrochemical water splitting was performed in a three-
electrode system consisting of the as-prepared sample photoanode, Pt cathode, and
Ag/AgCl reference electrode, in KOH electrolyte. A Newport Oriel solar simulator with
Class A output was used to generate simulated solar light (AM 1.5G), and the power density
upon the sample surface was 100 mW/cm2. In order to measure the photocurrent response
under visible light illumination, the simulated sun light (AM 1.5G) was filtered by a UV
cut-off filter (λ 420 nm). The photocurrent was obtained under linear sweep voltammetry
mode, and the sweeping voltage was from −0.8 to +1.0 V versus Ag/AgCl. To investigate
the photocurrent response of the sample at discrete wavelengths, near-monochromatic
light LEDs were used to illuminate the sample at the power density of 10 mW/cm2. The
photocurrent response was collected at a constant potential of 0.3 V vs. Ag/AgCl.
3.5. DFT Modeling
Structures of HfN-TNT composite systems for DFT calculations were built based
on our collected X-ray diffractogram (XRD) data (shown in Figure S1 in the Supporting
Information). The dominant TiO2 anatase plane (101) was used to build composite systems
with HfN (111) and (200) planes. While HfN (200) planes have both Hf and N atoms,
HfN (111) planes have either Hf or N terminated atoms. Thus, we have considered three
planes of HfN with the (10) plane of anatase TiO2. All these three composite systems were
constructed in both distant and proximate configurations. An OpenMx 3.9.2 (Open source
package for Material eXplorer) package was used for density functional theory (DFT)-based
quantum chemical calculations [55], where norm-conserving pseudopotentials [56] and
pseudo-atomic localized basis functions [55] are implemented. The computational protocol
involved two steps: geometry optimization followed by electronic property calculations
using generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE)
exchange-correlation functional [57]. Spin-polarization with periodic boundary conditions
was employed for all the calculations. Hubbard U-corrections (DFT+U) were considered in
our DFT model for considering the computational errors associated with on-site Coulomb
interactions. The Hubbard U-value for Ti 3d orbitals was considered to be 3.3 eV [58]. This
Hubbard U-correction provides an additional on-site Coulomb interaction to compensate
for the inaccurate description of self-interaction particularly in a partially occupied TiO2
3d state [59]. In this approach, the additional functions force the electrons of a particular
orbital (for TiO2 it is 3d orbital) to be more localized, thus ensuring the reproduction of
experimental bandgaps [59]. The energy cut-off value was chosen to be 220 eV, while the
threshold for convergence criterion for the self-consistent loop was set to be as small as
10−5. The Gaussian broadening method, with a broadening parameter of 0.08 eV, was used
for the projected density of states (PDOS) plots. The highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) were constructed using VMD
(visual molecular dynamics) visualization software.
4. Conclusions
In this study, we reported a method for the fabrication of core–shell nanotube arrays
consisting of a 20 nm ALD HfN shell and an anodic TiO2 nanotube support layer. To
the best of our knowledge, it is the first report using HfN for photoelectrochemical water
splitting. We observed excellent full visible regime photoactivity up to 730 nm for water
splitting, and based on our literature survey, the photocurrent density is superior to any
plasmonic noble metal-enhanced TiO2 based photoanodes. We consider the finding to be
significant, as it demonstrated the far-reaching application potential of replacing active
HER/HOR noble metals such as Au, Ag, Pt, Pd, etc. with lower-cost transition metal nitride
ceramics. Moreover, the experimental results evidence two unique characteristics allowing
a large population of long lifetime hot carriers, namely, a large bandgap between optical
and acoustic phonon modes and the absence of an electronic bandgap, which particularly
benefit hot carrier-based optoelectronic devices. It opens the window to a wide range of
11. Catalysts 2021, 11, 1374 11 of 13
applications that enhance their performance using plasmonic noble metals including but
not limited to photocatalysis, photosynthesis, dye degradation, and hot carrier solar cells.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10
.3390/catal11111374/s1, Additional information on materials characterization and electromagnetic
simulations, Figure S1: X-ray diffractogram of HfN-TNT sample illustrating material components by
separating signals, Figure S2: XPS survey scan of HfN@TNT sample, Figure S3: Results of FDTD
simulations of HfN-TNT showing electric field intensities for (a) xy plane and (b) xz plane at the
resonant wavelength of 650 nm, Figure S4: Optical extinction spectrum of a bare TiO2 nanotube array,
and Details related to the estimation of H2 generation yields.
Author Contributions: Conceptualization, S.Z., K.C.C. and K.S.; methodology, S.Z., T.M., E.V.,
P.K. K.M.M.A. and A.E.K.; validation, S.Z., T.M., S.R. and R.K.; formal analysis, S.Z., A.P.M. and
P.K.; investigation, S.Z., T.M., S.R., K.M.M.A. and A.E.K.; resources, S.G. and K.S.; data curation,
S.Z.; writing—original draft preparation, S.Z.; writing—review and editing, R.K., K.C.C. and K.S.;
visualization, S.Z. and A.P.M.; supervision, S.G., K.S. and K.C.C.; project administration, S.G. and
K.S.; funding acquisition, S.G. and K.S. All authors have read and agreed to the published version of
the manuscript.
Funding: This research was funded by Future Energy Systems (FES) Canada First Research Excellence
Fund, project T12-P02, Natural Sciences and Engineering Research Council of Canada (NSERC),
grant number CREATE-463990-2015, and the National Research Council Canada (NRC)-University
of Alberta NanoInitiative, project A1-014009. The APC was funded by Future Energy Systems.
Data Availability Statement: The data supporting the results and analyses presented in the paper
are available upon reasonable request.
Acknowledgments: The authors utilized user-fee based characterization facilities at the University of
Alberta nanoFAB. Kai Cui is acknowledged for collecting HRTEM data at the NRC Nanotechnology
Research Centre. R.K. and A.P.M. thank NSERC for scholarship support. An Alberta Excellence
Graduate Scholarship to S.Z. is acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.
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