This document summarizes the synthesis and characterization of halogen-doped carbon nitride nanosheets and BiOI heterojunctions for use in photoelectrochemical water splitting. Specifically:
1) Fluorine-doped, chlorine-intercalated carbon nitride (CNF-Cl) nanosheets were synthesized using thermal annealing to improve light absorption and charge separation.
2) CNF-Cl nanosheets were combined with bismuth oxyiodide (BiOI) nanoplates via a hydrothermal method to form heterojunctions.
3) Characterization using TEM, XRD, Raman and EDS mapping confirmed the formation of CNF-Cl
Sunlight-driven water-splitting using two dimensional carbon based semiconduc...Pawan Kumar
The overwhelming challenge of depleting fossil fuels and anthropogenic carbon emissions has driven research
into alternative clean sources of energy. To achieve the goal of a carbon neutral economy, the harvesting of
sunlight by using photocatalysts to split water into hydrogen and oxygen is an expedient approach to fulfill
the energy demand in a sustainable way along with reducing the emission of greenhouse gases. Even though
the past few decades have witnessed intensive research into inorganic semiconductor photocatalysts, their
quantum efficiencies for hydrogen production from visible photons remain too low for the large scale
deployment of this technology. Visible light absorption and efficient charge separation are two key necessary
conditions for achieving the scalable production of hydrogen from water. Two-dimensional carbon based
nanoscale materials such as graphene oxide, reduced graphene oxide, carbon nitride, modified 2D carbon
frameworks and their composites have emerged as potential photocatalysts due to their astonishing
properties such as superior charge transport, tunable energy levels and bandgaps, visible light absorption,
high surface area, easy processability, quantum confinement effects, and high photocatalytic quantum yields.
The feasibility of structural and chemical modification to optimize visible light absorption and charge
separation makes carbonaceous semiconductors promising candidates to convert solar energy into chemical
energy. In the present review, we have summarized the recent advances in 2D carbonaceous photocatalysts
with respect to physicochemical and photochemical tuning for solar light mediated hydrogen evolution
Single-atom catalysts for biomass-derived drop-in chemicalsPawan Kumar
Conversion of biomass to fuel and drop-in chemicals is envisaged to solve the problem of depleting fossil fuel reserves while leveling-off the staggering CO2 concentration. By-passing the natural carbon cycle via the transformation of abundant lignocellulosic biomass into chemicals does not add any extra CO2 to the environment and the net CO2 concentration remains the same. The paradigm shifts from fossil fuel-based chemicals to biomass-derived products will rely on efficient and cost-effective catalysts that can compete with cheap and readily available fossil fuels. Existing transition and noble metal-based nanoparticle catalysts either in the supported or unsupported form are crippling due to poor activity/selectivity, deactivation of catalytically active sites, and the complex composition, recalcitrant nature, and high moisture content of biomass. Single-atom catalysts (SACs) possessing single-atom centers decorated on support have shown great promise in biomass conversion due to their unique geometric configuration, electronic properties, and ensemble effect. In contrast to traditional catalytic systems, SACs encompass the advantages of both heterogeneous and homogeneous catalysts with improved performance and easy recyclability. Because of the availability of each metal center for the reaction and unique geometrical configuration, SACs have displayed exceptional catalytic activity and selectivity (~95% in most cases). In addition, the SACs show increased thermal and chemical stability due to the stabilization of the metal center on the support. The present chapter highlights the various aspects of SACs for efficient and selective biomass conversion into drop-in chemicals.
Polymeric carbon nitride-based photocatalysts for photoreforming of biomass d...Pawan Kumar
Photoreforming of biomass to value-added chemicals and fuels is a chemical approach to extract photosynthetically-trapped energy in complex biomolecules which otherwise disintegrate naturally in the environment. Designing precise photocatalytic materials that can selectively break the sturdy, nature-designed biomass with multiplex chemical composition/bonding and inaccessible sites is central to deploying this technology. Polymeric carbon nitride (CN) comprised of a 2D network of condensed heptazine/triazine (C6N7/C3N3) core has shown great promise for photoreforming of biomass derivatives due to intriguing physicochemical and optical properties. This review comprehensively summarizes the state-of-the-art applications of CN-based photocatalysts for the conversion of lignocellulosic biomass derivatives. Various chemical and structural modifications in CN structure such as doping, surface functionalization, hybridization entailing to higher selectivity and conversion have been discussed aiming at providing valuable guidance for future CN-based materials design.
Photocatalytic Mechanism Control and Study of Carrier Dynamics in CdS@C3N5 Co...Pawan Kumar
We present a potential solution to the problem of extraction of photogenerated holes from CdS nanocrystals and nanowires. The nanosheet form of C3N5 is a low-band-gap (Eg = 2.03 eV), azo-linked graphenic carbon nitride framework formed by the polymerization of melem hydrazine (MHP). C3N5 nanosheets were either wrapped around CdS nanorods (NRs) following the synthesis of pristine chalcogenide or intercalated among them by an in situ synthesis protocol to form two kinds of heterostructures, CdS-MHP and CdS-MHPINS, respectively. CdS-MHP improved the photocatalytic degradation rate of 4-nitrophenol by nearly an order of magnitude in comparison to bare CdS NRs. CdS-MHP also enhanced the sunlight-driven photocatalytic activity of bare CdS NWs for the decolorization of rhodamine B (RhB) by a remarkable 300% through the improved extraction and utilization of photogenerated holes due to surface passivation. More interestingly, CdS-MHP provided reaction pathway control over RhB degradation. In the absence of scavengers, CdS-MHP degraded RhB through the N-deethylation pathway. When either hole scavenger or electron scavenger was added to the RhB solution, the photocatalytic activity of CdS-MHP remained mostly unchanged, while the degradation mechanism shifted to the chromophore cleavage (cycloreversion) pathway. We investigated the optoelectronic properties of CdS-C3N5 heterojunctions using density functional theory (DFT) simulations, finite difference time domain (FDTD) simulations, time-resolved terahertz spectroscopy (TRTS), and photoconductivity measurements. TRTS indicated high carrier mobilities >450 cm2 V–1 s–1 and carrier relaxation times >60 ps for CdS-MHP, while CdS-MHPINS exhibited much lower mobilities <150 cm2 V–1 s–1 and short carrier relaxation times <20 ps. Hysteresis in the photoconductive J–V characteristics of CdS NWs disappeared in CdS-MHP, confirming surface passivation. Dispersion-corrected DFT simulations indicated a delocalized HOMO and a LUMO localized on C3N5 in CdS-MHP. C3N5, with its extended π-conjugation and low band gap, can function as a shuttle to extract carriers and excitons in nanostructured heterojunctions, and enhance performance in optoelectronic devices. Our results demonstrate how carrier dynamics in core–shell heterostructures can be manipulated to achieve control over the reaction mechanism in photocatalysis.
This document describes the synthesis and characterization of a core-shell structured reduced graphene oxide wrapped magnetically separable rGO@CuZnO@Fe3O4 microspheres photocatalyst and its use for the photoreduction of carbon dioxide to methanol under visible light irradiation. The photocatalyst takes advantage of the high photocatalytic efficiency of zinc oxide, the high surface area and charge carrier mobility of reduced graphene oxide, and the magnetic properties of an iron oxide core. Experimental results showed the rGO@CuZnO@Fe3O4 photocatalyst had higher catalytic activity than other possible combinations, with a methanol yield of 2656 μmol/gcat under visible light, and could be readily recovered and
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.
Water-splitting photoelectrodes consisting of heterojunctions of carbon nitri...Pawan Kumar
Quinary and senary non-stoichiometric double perovskites such as Ba2Ca0.66Nb1.34-xFexO6-δ (BCNF) have been utilized for gas sensing, solid oxide fuel cells and thermochemical CO2 reduction. Herein, we examined their potential as narrow bandgap semiconductors for use in solar energy harvesting. A cobalt co-doped BCNF, Ba2Ca0.66Nb0.68Fe0.33Co0.33O6-δ (BCNFCo), exhibited an optical absorption edge at ~ 800 nm, p-type conduction and a distinct photoresponse upto 640 nm while demonstrating high thermochemical stability. A nanocomposite of BCNFCo and g-C3N4 (CN) was prepared via a facile solvent assisted exfoliation/blending approach using dichlorobenzene and glycerol at a moderate temperature. The exfoliation of g-C3N4 followed by wrapping on perovskite established an effective heterojunction between the materials for charge separation. The conjugated 2D sheets of CN enabled better charge migration resulting in increased photoelectrochemical performance. A blend composed of 40 wt% perovskite and CN performed optimally, whilst achieving a photocurrent density as high as 1.5 mA cm-2 for sunlight-driven water-splitting with a Faradaic efficiency as high as ~ 88%.
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.
Sunlight-driven water-splitting using two dimensional carbon based semiconduc...Pawan Kumar
The overwhelming challenge of depleting fossil fuels and anthropogenic carbon emissions has driven research
into alternative clean sources of energy. To achieve the goal of a carbon neutral economy, the harvesting of
sunlight by using photocatalysts to split water into hydrogen and oxygen is an expedient approach to fulfill
the energy demand in a sustainable way along with reducing the emission of greenhouse gases. Even though
the past few decades have witnessed intensive research into inorganic semiconductor photocatalysts, their
quantum efficiencies for hydrogen production from visible photons remain too low for the large scale
deployment of this technology. Visible light absorption and efficient charge separation are two key necessary
conditions for achieving the scalable production of hydrogen from water. Two-dimensional carbon based
nanoscale materials such as graphene oxide, reduced graphene oxide, carbon nitride, modified 2D carbon
frameworks and their composites have emerged as potential photocatalysts due to their astonishing
properties such as superior charge transport, tunable energy levels and bandgaps, visible light absorption,
high surface area, easy processability, quantum confinement effects, and high photocatalytic quantum yields.
The feasibility of structural and chemical modification to optimize visible light absorption and charge
separation makes carbonaceous semiconductors promising candidates to convert solar energy into chemical
energy. In the present review, we have summarized the recent advances in 2D carbonaceous photocatalysts
with respect to physicochemical and photochemical tuning for solar light mediated hydrogen evolution
Single-atom catalysts for biomass-derived drop-in chemicalsPawan Kumar
Conversion of biomass to fuel and drop-in chemicals is envisaged to solve the problem of depleting fossil fuel reserves while leveling-off the staggering CO2 concentration. By-passing the natural carbon cycle via the transformation of abundant lignocellulosic biomass into chemicals does not add any extra CO2 to the environment and the net CO2 concentration remains the same. The paradigm shifts from fossil fuel-based chemicals to biomass-derived products will rely on efficient and cost-effective catalysts that can compete with cheap and readily available fossil fuels. Existing transition and noble metal-based nanoparticle catalysts either in the supported or unsupported form are crippling due to poor activity/selectivity, deactivation of catalytically active sites, and the complex composition, recalcitrant nature, and high moisture content of biomass. Single-atom catalysts (SACs) possessing single-atom centers decorated on support have shown great promise in biomass conversion due to their unique geometric configuration, electronic properties, and ensemble effect. In contrast to traditional catalytic systems, SACs encompass the advantages of both heterogeneous and homogeneous catalysts with improved performance and easy recyclability. Because of the availability of each metal center for the reaction and unique geometrical configuration, SACs have displayed exceptional catalytic activity and selectivity (~95% in most cases). In addition, the SACs show increased thermal and chemical stability due to the stabilization of the metal center on the support. The present chapter highlights the various aspects of SACs for efficient and selective biomass conversion into drop-in chemicals.
Polymeric carbon nitride-based photocatalysts for photoreforming of biomass d...Pawan Kumar
Photoreforming of biomass to value-added chemicals and fuels is a chemical approach to extract photosynthetically-trapped energy in complex biomolecules which otherwise disintegrate naturally in the environment. Designing precise photocatalytic materials that can selectively break the sturdy, nature-designed biomass with multiplex chemical composition/bonding and inaccessible sites is central to deploying this technology. Polymeric carbon nitride (CN) comprised of a 2D network of condensed heptazine/triazine (C6N7/C3N3) core has shown great promise for photoreforming of biomass derivatives due to intriguing physicochemical and optical properties. This review comprehensively summarizes the state-of-the-art applications of CN-based photocatalysts for the conversion of lignocellulosic biomass derivatives. Various chemical and structural modifications in CN structure such as doping, surface functionalization, hybridization entailing to higher selectivity and conversion have been discussed aiming at providing valuable guidance for future CN-based materials design.
Photocatalytic Mechanism Control and Study of Carrier Dynamics in CdS@C3N5 Co...Pawan Kumar
We present a potential solution to the problem of extraction of photogenerated holes from CdS nanocrystals and nanowires. The nanosheet form of C3N5 is a low-band-gap (Eg = 2.03 eV), azo-linked graphenic carbon nitride framework formed by the polymerization of melem hydrazine (MHP). C3N5 nanosheets were either wrapped around CdS nanorods (NRs) following the synthesis of pristine chalcogenide or intercalated among them by an in situ synthesis protocol to form two kinds of heterostructures, CdS-MHP and CdS-MHPINS, respectively. CdS-MHP improved the photocatalytic degradation rate of 4-nitrophenol by nearly an order of magnitude in comparison to bare CdS NRs. CdS-MHP also enhanced the sunlight-driven photocatalytic activity of bare CdS NWs for the decolorization of rhodamine B (RhB) by a remarkable 300% through the improved extraction and utilization of photogenerated holes due to surface passivation. More interestingly, CdS-MHP provided reaction pathway control over RhB degradation. In the absence of scavengers, CdS-MHP degraded RhB through the N-deethylation pathway. When either hole scavenger or electron scavenger was added to the RhB solution, the photocatalytic activity of CdS-MHP remained mostly unchanged, while the degradation mechanism shifted to the chromophore cleavage (cycloreversion) pathway. We investigated the optoelectronic properties of CdS-C3N5 heterojunctions using density functional theory (DFT) simulations, finite difference time domain (FDTD) simulations, time-resolved terahertz spectroscopy (TRTS), and photoconductivity measurements. TRTS indicated high carrier mobilities >450 cm2 V–1 s–1 and carrier relaxation times >60 ps for CdS-MHP, while CdS-MHPINS exhibited much lower mobilities <150 cm2 V–1 s–1 and short carrier relaxation times <20 ps. Hysteresis in the photoconductive J–V characteristics of CdS NWs disappeared in CdS-MHP, confirming surface passivation. Dispersion-corrected DFT simulations indicated a delocalized HOMO and a LUMO localized on C3N5 in CdS-MHP. C3N5, with its extended π-conjugation and low band gap, can function as a shuttle to extract carriers and excitons in nanostructured heterojunctions, and enhance performance in optoelectronic devices. Our results demonstrate how carrier dynamics in core–shell heterostructures can be manipulated to achieve control over the reaction mechanism in photocatalysis.
This document describes the synthesis and characterization of a core-shell structured reduced graphene oxide wrapped magnetically separable rGO@CuZnO@Fe3O4 microspheres photocatalyst and its use for the photoreduction of carbon dioxide to methanol under visible light irradiation. The photocatalyst takes advantage of the high photocatalytic efficiency of zinc oxide, the high surface area and charge carrier mobility of reduced graphene oxide, and the magnetic properties of an iron oxide core. Experimental results showed the rGO@CuZnO@Fe3O4 photocatalyst had higher catalytic activity than other possible combinations, with a methanol yield of 2656 μmol/gcat under visible light, and could be readily recovered and
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.
Water-splitting photoelectrodes consisting of heterojunctions of carbon nitri...Pawan Kumar
Quinary and senary non-stoichiometric double perovskites such as Ba2Ca0.66Nb1.34-xFexO6-δ (BCNF) have been utilized for gas sensing, solid oxide fuel cells and thermochemical CO2 reduction. Herein, we examined their potential as narrow bandgap semiconductors for use in solar energy harvesting. A cobalt co-doped BCNF, Ba2Ca0.66Nb0.68Fe0.33Co0.33O6-δ (BCNFCo), exhibited an optical absorption edge at ~ 800 nm, p-type conduction and a distinct photoresponse upto 640 nm while demonstrating high thermochemical stability. A nanocomposite of BCNFCo and g-C3N4 (CN) was prepared via a facile solvent assisted exfoliation/blending approach using dichlorobenzene and glycerol at a moderate temperature. The exfoliation of g-C3N4 followed by wrapping on perovskite established an effective heterojunction between the materials for charge separation. The conjugated 2D sheets of CN enabled better charge migration resulting in increased photoelectrochemical performance. A blend composed of 40 wt% perovskite and CN performed optimally, whilst achieving a photocurrent density as high as 1.5 mA cm-2 for sunlight-driven water-splitting with a Faradaic efficiency as high as ~ 88%.
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.
The most difficult goal in the next few decades is the replacement of conventional petro-based fuels with more sustainable fuels that can be used in the existing infrastructure. By the use of Renewable energy or nuclear energy, CO2 and H2O can be recycled into liquid hydrocarbon fuels (the reverse of fuel combustion). Capture of CO2 from the atmosphere will form a close carbon-neutral fuel cycle loop. This article also reviews the aspects regarding thermodynamics involved, involved mechanisms and possible technological pathways for recycling CO2 into fuels using renewable energy. These pathways can be broken into three staged- CO2 capture from atmosphere, H2O and CO2 dissociation, and fuel synthesis.
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.
A closed loop ammonium salt system for recovery of high-purity lead tetroxide...Ary Assuncao
This document describes a closed-loop hydrometallurgical process for recovering high-purity lead tetroxide from spent lead-acid battery paste. The process involves leaching the paste with a mixed solution of ammonium acetate, acetic acid, and hydrogen peroxide. The leachate is then reacted with ammonium carbonate to precipitate lead carbonate. Impurities are removed during leaching and precipitation. The regenerated leachate is recycled for the next leaching. Lead carbonate is calcined to produce lead tetroxide with low impurity levels meeting industry standards. This process allows for reagent recirculation and production of a high value lead recovery product.
Hexamolybdenum clusters supported on graphene oxide: Visible-light induced ph...Pawan Kumar
This document summarizes a study on immobilizing hexamolybdenum clusters on graphene oxide nanosheets for photocatalytic reduction of carbon dioxide into methanol. Specifically, Cs2Mo6Bri8Bra6 and (TBA)2Mo6Bri8Bra6 clusters were immobilized on graphene oxide through replacement of apical bromide ions with oxygen functional groups on the graphene oxide surface. The developed GO-Cs2Mo6Bri8Brax and GO-(TBA)2Mo6Bri8Brax composites were then used as heterogeneous photocatalysts for CO2 reduction under visible light, producing methanol yields of 1644 and 1294 μmol g−1cat, respectively
Hexamolybdenum clusters supported on graphene oxide: Visible-light induced ph...Pawan Kumar
Hexamolybdenum (Mo6) cluster-based compounds namely Cs2Mo6Bri
8Bra6
and
(TBA)2Mo6Bri
8Bra
6 (TBA = tetrabutylammonium) were immobilized on graphene oxide (GO)
nanosheets by taking advantage of the high lability of the apical bromide ions with
oxygen-functionalities of GO nanosheets. The loading of Mo6 clusters on GO nanosheets
was probed by Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron
spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and elemental
mapping analyses. The developed GO-Cs2Mo6Bri
8Bra
x and GO-(TBA)2Mo6Bri
8Bra
x
composites were then used as heterogeneous photocatalysts for the reduction of CO2 under
visible light irradiation. After 24 h visible light illumination, the yield of methanol was
found to be 1644 and 1294 lmol g1 cat for GO-Cs2Mo6Bri
8Bra
x and GO-(TBA)2Mo6Bri
8Bra
x,
respectively. The quantum yields of methanol by using GO-Cs2Mo6Bri
8Bra
x and
GO-(TBA)2Mo6Bri
8Bra
x as catalysts with reference to Mo6 cluster units presented in 0.1 g
amount of catalyst were found to be 0.015 and 0.011, respectively. The role of immobilized
Mo6 clusters-based compounds on GO nanosheets is discussed to understand the
photocatalytic mechanism of CO2 reduction into methanol.
Nanostructured composite materials for CO2 activationPawan Kumar
This document discusses nanostructured composite materials for CO2 activation, specifically for the photocatalytic reduction of CO2 to valuable products. It provides background on the increasing energy crisis and climate change caused by fossil fuel use. It then summarizes the basic principles and challenges of using semiconductor photocatalysts for CO2 reduction, including appropriate band gap positions and preventing electron-hole recombination. The document discusses various approaches to overcoming these challenges, such as forming heterojunction composites and using co-catalysts to facilitate charge separation and transfer.
2014 Journal of Power Sources 247 (2014) 572-578Alexis B. B
The document summarizes research on the durability of a nickelealuminum layered double hydroxide/carbon (NieAl LDH/C) composite as a cathode material in nickel metal hydride batteries. The composite was prepared using a liquid phase deposition method and optimized to contain 19.2 mol% aluminum. Electrochemical testing was conducted over 869 charge/discharge cycles at two different current regimes: 5 mA for 300 cycles in half-cell conditions and 5.8 mA for 569 cycles in battery regime. The composite exhibited good lifespan and stability, maintaining a capacity retention above 380 mAh/g. Cyclic voltammetry and X-ray diffraction analysis showed that the a-Ni
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.
Electrochemical carbon dioxide reduction is a promising approach to addressing rising CO2 levels and providing an alternative fuel source. Noble metal catalysts have been used, but their cost is prohibitive for large-scale applications. The document discusses various catalyst materials studied for CO2 reduction, including copper and gold nanoparticles. Key factors that influence the selectivity and efficiency of products like carbon monoxide and formate include the choice of electrocatalyst, electrolyte, particle size and morphology, and presence of grain boundaries or edge sites. Oxidized nanoparticles and high pressures can also improve performance. Further optimization of catalyst properties is needed to develop scalable and economical CO2 reduction technologies.
Coproduction of hydrogen and lactic acid from glucose photocatalysis on band-...Pawan Kumar
Photocatalytic transformation of biomass into value-added chemicals coupled with co-production of hydrogen provides an explicit route to trap sunlight into the chemical bonds. Here, we demonstrate a rational design of Zn1-xCdxS solid solution homojunction photocatalyst with a pseudo-periodic cubic zinc blende (ZB) and hexagonal wurtzite (WZ) structure for efficient glucose conversion to simultaneously produce hydrogen and lactic acid. The optimized Zn0.6Cd0.4S catalyst consists of a twinning superlattice, has a tuned bandgap, and displays excellent efficiency with respect to hydrogen generation (690 ± 27.6 μmol·h−1·gcat.−1), glucose conversion (~90%), and lactic acid selectivity (~87%) without any co-catalyst under visible light irradiation. The periodic WZ/ZB phase in twinning superlattice facilitates better charge separation, while superoxide radical (⋅O2-) and photogenerated holes drive the glucose transformation and water oxidation reactions, respectively. This work demonstrates that rational photocatalyst design could realize an efficient and concomitant production of hydrogen and value-added chemicals from glucose photocatalysis.
High rate CO2 photoreduction using flame annealed TiO2 nanotubesPawan Kumar
The photocatalytic reduction of CO2 into light hydrocarbons using sunlight and water is a challenging reaction involving eight electron transfer steps; nevertheless, it has great potential to address the problem of rising anthropogenic carbon emissions and enable the use of fossil fuels in a sustainable way. Several decades after its first use, TiO2 remains one of the best performing and most durable photocatalysts for CO2 reduction albeit with a poor visible light absorption capacity. We have used flame annealing to improve the response of TiO2 to visible photons and engineered a nanotubular morphology with square-shaped cross-sections in flame-annealed nanotubes. An enhanced CH4 yield was achieved in the photoreduction of CO2 using flame annealed TiO2 nanotubes, and isotope labeled experiments confirmed the reaction products to originate from the CO2 reactant. Flame-annealed TiO2 nanotubes formed in aqueous electrolyte (FANT-aq) yielded 156.5 μmol gcatalyst–1.hr–1 of CH4, which is in the top tier of reported performance values achieved using TiO2 as a stand-alone photocatalyst. This performance resulted because appreciable amounts of CH4 were generated under visible light illumination as well. TiO2 nanotubes exhibited CO2 photoreduction activity up to a wavelength of 620 nm with visible light driven photocatalytic activity peaking at 450 nm for flame annealed TiO2 nanotubes. Isotope labelling studies, using GC–MS and gas-phase FTIR, indicated photoreduction of 13CO2 to 13CH4. The detection of 13CO in the product mixture, and the absence of HCHO and HCOOH provides strong support for the photoreduction proceeding along a carbene pathway. The enhanced CO2 photoreduction performance of FANT-aq is attributed to increased visible light absorption, square morphology, and the presence of rutile as the only crystalline phase with (110) as the dominant plane.
High rate CO2 photoreduction using flame annealed TiO2 nanotubesPawan Kumar
The photocatalytic reduction of CO2 into light hydrocarbons using sunlight and water is a challenging reaction involving eight electron transfer steps; nevertheless, it has great potential to address the problem of rising anthropogenic carbon emissions and enable the use of fossil fuels in a sustainable way. Several decades after its first use, TiO2 remains one of the best performing and most durable photocatalysts for CO2 reduction albeit with a poor visible light absorption capacity. We have used flame annealing to improve the response of TiO2 to visible photons and engineered a nanotubular morphology with square-shaped cross-sections in flame-annealed nanotubes. An enhanced CH4 yield was achieved in the photoreduction of CO2 using flame annealed TiO2 nanotubes, and isotope labeled experiments confirmed the reaction products to originate from the CO2 reactant. Flame-annealed TiO2 nanotubes
The document discusses photocatalytic conversion of carbon dioxide into fuels and chemicals. It describes how semiconductor-based photocatalysts like TiO2 can be used to drive the reduction of CO2 into products like methanol using solar energy. Challenges include the large band gap of most semiconductors, which limits them to using only UV light. The document explores using metal complexes immobilized on photoactive supports as an alternative, as they have visible light activity and can be tuned to favor specific products. Specific examples discussed include cobalt phthalocyanine and tin phthalocyanine immobilized on graphene oxide and mesoporous ceria, respectively, as well as heteroleptic ruthenium complexes immobilized on graphene oxide
N-doped graphene quantum dots (NGQDs) catalyze the efficient electrochemical reduction of carbon dioxide into multi-carbon hydrocarbons and oxygenates such as ethylene, ethanol, and n-propanol. The NGQDs achieve high total faradaic efficiencies of up to 90% for carbon dioxide reduction, with selectivities for ethylene and ethanol conversions reaching 45%. Control experiments confirm the NGQDs are responsible for catalyzing the reaction. Compared to undoped graphene quantum dots, the NGQDs have higher activity and selectivity for producing valuable fuel and chemical products from carbon dioxide due to the presence of pyridinic nitrogen defects introduced during synthesis.
Carbon Dioxide to Chemicals and Fuels Course Material.
National Centre for Catalysis Research (NCCR, IIT Madras), considered for the first on-line course the topic of Carbon dioxide to Chemicals and Fuels. NCCR has learnt many such lessons which are necessary for the researchers to understand and also have a complete comprehension of the limitations.
Carbon Nitride Grafted Cobalt Complex (Co@npg-C3N4) for Visible LightAssiste...Pawan Kumar
1) A cobalt complex was covalently grafted to nanoporous graphitic carbon nitride (npg-C3N4) via a click reaction to create a heterogeneous photocatalyst called Co@npg-C3N4.
2) Under visible light irradiation at room temperature, Co@npg-C3N4 efficiently catalyzed the direct esterification of aldehydes without the need for an external base.
3) Characterization of Co@npg-C3N4 showed the cobalt complex was successfully immobilized via click chemistry, providing a robust photocatalyst that could be easily recovered and reused without significant loss of activity.
2021 influence of basic carbon additives on the electrochemical performance ...Ary Assuncao
This study investigates the effect of carbon surface basicity on the electrochemical performance and dynamic charge acceptance of lead-carbon batteries. Five activated carbons with different pH values ranging from 9.5 to 11.1 were prepared by ammonia and hydrogen gas treatments. Cyclic voltammetry showed that the hydrogen evolution reaction activity increased with higher carbon surface basicity. Testing of lead-carbon electrodes found a correlation between carbon pH and dynamic charge acceptance, with higher pH carbons showing improved charge currents and final dynamic charge acceptance. The carbon content also affected charge currents during simulated microcycles, demonstrating that surface chemistry and amount of carbon additive both influence the electrochemical properties and performance of lead-carbon batteries.
Photocatalytic reduction of carbon dioxide issues and prospects bentham scie...Hariprasad Narayanan
The conversion of carbon dioxide into worthwhile chemicals through photocatalysis has been a matter of attraction for the last four decades among the scientific community. However, the conversion rate has not yet been achieved to the desired efficiency due to the inevitable barriers associated with the process making it as a Holy Grail. This presentation deals with the identification and critical evaluation of the hurdles that pulls back the photocatalytic processes on track and the recent advances in the scientific field that pertain to the photocatalytic conversion of carbon dioxide in the near future.
Visible light assisted hydrogen generation from complete decomposition of hyd...Pawan Kumar
Hydrogen is considered to be an ideal energy carrier, which produces only water when combined with
oxygen and thus has no detrimental effect on the environment. While the catalytic decomposition of
hydrous hydrazine for the production of hydrogen is well explored, little is known about its photocatalytic
decomposition. The present paper describes a highly efficient photochemical methodology for the production
of hydrogen through the decomposition of aqueous hydrazine using titanium dioxide nanoparticles
modified with a Rh(I) coordinated catechol phosphane ligand (TiO2–Rh) as a photocatalyst under visible
light irradiation. After 12 h of visible light irradiation, the hydrogen yield was 413 μmol g−1 cat with a hydrogen
evolution rate of 34.4 μmol g−1 cat h−1. Unmodified TiO2 nanoparticles offered a hydrogen yield of
83 μmol g−1 cat and a hydrogen evolution rate of only 6.9 μmol g−1 cat h−1. The developed photocatalyst
was robust under the experimental conditions and could be efficiently reused for five subsequent runs
without any significant change in its activity. The higher stability of the photocatalyst is attributed to the
covalent attachment of the Rh complex, whereas the higher activity is believed to be due to the synergistic
mechanism that resulted in better electron transfer from the Rh complex to the conduction band of TiO2
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%.
Water-splitting photoelectrodes consisting of heterojunctions of carbon nitri...Devika Laishram
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%.
Proton‐functionalized graphitic carbon nitride for efficient metal‐free disin...Journal Papers
This document summarizes a study on using proton-functionalized graphitic carbon nitride (P-CN) as a metal-free photocatalyst for disinfecting Escherichia coli (E. coli) bacteria under low-power light irradiation. P-CN demonstrated 100% disinfection of E. coli within 4 hours under 23W light bulb irradiation. Testing with scavengers showed that hydroxyl radicals (·OH) played a major role in the disinfection process, while superoxide (·O2-) played a minimal role. P-CN exhibited higher disinfection efficiency compared to unmodified graphitic carbon nitride, due to the introduction of protons enhancing conductivity and charge transfer in P-CN. This
The most difficult goal in the next few decades is the replacement of conventional petro-based fuels with more sustainable fuels that can be used in the existing infrastructure. By the use of Renewable energy or nuclear energy, CO2 and H2O can be recycled into liquid hydrocarbon fuels (the reverse of fuel combustion). Capture of CO2 from the atmosphere will form a close carbon-neutral fuel cycle loop. This article also reviews the aspects regarding thermodynamics involved, involved mechanisms and possible technological pathways for recycling CO2 into fuels using renewable energy. These pathways can be broken into three staged- CO2 capture from atmosphere, H2O and CO2 dissociation, and fuel synthesis.
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.
A closed loop ammonium salt system for recovery of high-purity lead tetroxide...Ary Assuncao
This document describes a closed-loop hydrometallurgical process for recovering high-purity lead tetroxide from spent lead-acid battery paste. The process involves leaching the paste with a mixed solution of ammonium acetate, acetic acid, and hydrogen peroxide. The leachate is then reacted with ammonium carbonate to precipitate lead carbonate. Impurities are removed during leaching and precipitation. The regenerated leachate is recycled for the next leaching. Lead carbonate is calcined to produce lead tetroxide with low impurity levels meeting industry standards. This process allows for reagent recirculation and production of a high value lead recovery product.
Hexamolybdenum clusters supported on graphene oxide: Visible-light induced ph...Pawan Kumar
This document summarizes a study on immobilizing hexamolybdenum clusters on graphene oxide nanosheets for photocatalytic reduction of carbon dioxide into methanol. Specifically, Cs2Mo6Bri8Bra6 and (TBA)2Mo6Bri8Bra6 clusters were immobilized on graphene oxide through replacement of apical bromide ions with oxygen functional groups on the graphene oxide surface. The developed GO-Cs2Mo6Bri8Brax and GO-(TBA)2Mo6Bri8Brax composites were then used as heterogeneous photocatalysts for CO2 reduction under visible light, producing methanol yields of 1644 and 1294 μmol g−1cat, respectively
Hexamolybdenum clusters supported on graphene oxide: Visible-light induced ph...Pawan Kumar
Hexamolybdenum (Mo6) cluster-based compounds namely Cs2Mo6Bri
8Bra6
and
(TBA)2Mo6Bri
8Bra
6 (TBA = tetrabutylammonium) were immobilized on graphene oxide (GO)
nanosheets by taking advantage of the high lability of the apical bromide ions with
oxygen-functionalities of GO nanosheets. The loading of Mo6 clusters on GO nanosheets
was probed by Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron
spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and elemental
mapping analyses. The developed GO-Cs2Mo6Bri
8Bra
x and GO-(TBA)2Mo6Bri
8Bra
x
composites were then used as heterogeneous photocatalysts for the reduction of CO2 under
visible light irradiation. After 24 h visible light illumination, the yield of methanol was
found to be 1644 and 1294 lmol g1 cat for GO-Cs2Mo6Bri
8Bra
x and GO-(TBA)2Mo6Bri
8Bra
x,
respectively. The quantum yields of methanol by using GO-Cs2Mo6Bri
8Bra
x and
GO-(TBA)2Mo6Bri
8Bra
x as catalysts with reference to Mo6 cluster units presented in 0.1 g
amount of catalyst were found to be 0.015 and 0.011, respectively. The role of immobilized
Mo6 clusters-based compounds on GO nanosheets is discussed to understand the
photocatalytic mechanism of CO2 reduction into methanol.
Nanostructured composite materials for CO2 activationPawan Kumar
This document discusses nanostructured composite materials for CO2 activation, specifically for the photocatalytic reduction of CO2 to valuable products. It provides background on the increasing energy crisis and climate change caused by fossil fuel use. It then summarizes the basic principles and challenges of using semiconductor photocatalysts for CO2 reduction, including appropriate band gap positions and preventing electron-hole recombination. The document discusses various approaches to overcoming these challenges, such as forming heterojunction composites and using co-catalysts to facilitate charge separation and transfer.
2014 Journal of Power Sources 247 (2014) 572-578Alexis B. B
The document summarizes research on the durability of a nickelealuminum layered double hydroxide/carbon (NieAl LDH/C) composite as a cathode material in nickel metal hydride batteries. The composite was prepared using a liquid phase deposition method and optimized to contain 19.2 mol% aluminum. Electrochemical testing was conducted over 869 charge/discharge cycles at two different current regimes: 5 mA for 300 cycles in half-cell conditions and 5.8 mA for 569 cycles in battery regime. The composite exhibited good lifespan and stability, maintaining a capacity retention above 380 mAh/g. Cyclic voltammetry and X-ray diffraction analysis showed that the a-Ni
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.
Electrochemical carbon dioxide reduction is a promising approach to addressing rising CO2 levels and providing an alternative fuel source. Noble metal catalysts have been used, but their cost is prohibitive for large-scale applications. The document discusses various catalyst materials studied for CO2 reduction, including copper and gold nanoparticles. Key factors that influence the selectivity and efficiency of products like carbon monoxide and formate include the choice of electrocatalyst, electrolyte, particle size and morphology, and presence of grain boundaries or edge sites. Oxidized nanoparticles and high pressures can also improve performance. Further optimization of catalyst properties is needed to develop scalable and economical CO2 reduction technologies.
Coproduction of hydrogen and lactic acid from glucose photocatalysis on band-...Pawan Kumar
Photocatalytic transformation of biomass into value-added chemicals coupled with co-production of hydrogen provides an explicit route to trap sunlight into the chemical bonds. Here, we demonstrate a rational design of Zn1-xCdxS solid solution homojunction photocatalyst with a pseudo-periodic cubic zinc blende (ZB) and hexagonal wurtzite (WZ) structure for efficient glucose conversion to simultaneously produce hydrogen and lactic acid. The optimized Zn0.6Cd0.4S catalyst consists of a twinning superlattice, has a tuned bandgap, and displays excellent efficiency with respect to hydrogen generation (690 ± 27.6 μmol·h−1·gcat.−1), glucose conversion (~90%), and lactic acid selectivity (~87%) without any co-catalyst under visible light irradiation. The periodic WZ/ZB phase in twinning superlattice facilitates better charge separation, while superoxide radical (⋅O2-) and photogenerated holes drive the glucose transformation and water oxidation reactions, respectively. This work demonstrates that rational photocatalyst design could realize an efficient and concomitant production of hydrogen and value-added chemicals from glucose photocatalysis.
High rate CO2 photoreduction using flame annealed TiO2 nanotubesPawan Kumar
The photocatalytic reduction of CO2 into light hydrocarbons using sunlight and water is a challenging reaction involving eight electron transfer steps; nevertheless, it has great potential to address the problem of rising anthropogenic carbon emissions and enable the use of fossil fuels in a sustainable way. Several decades after its first use, TiO2 remains one of the best performing and most durable photocatalysts for CO2 reduction albeit with a poor visible light absorption capacity. We have used flame annealing to improve the response of TiO2 to visible photons and engineered a nanotubular morphology with square-shaped cross-sections in flame-annealed nanotubes. An enhanced CH4 yield was achieved in the photoreduction of CO2 using flame annealed TiO2 nanotubes, and isotope labeled experiments confirmed the reaction products to originate from the CO2 reactant. Flame-annealed TiO2 nanotubes formed in aqueous electrolyte (FANT-aq) yielded 156.5 μmol gcatalyst–1.hr–1 of CH4, which is in the top tier of reported performance values achieved using TiO2 as a stand-alone photocatalyst. This performance resulted because appreciable amounts of CH4 were generated under visible light illumination as well. TiO2 nanotubes exhibited CO2 photoreduction activity up to a wavelength of 620 nm with visible light driven photocatalytic activity peaking at 450 nm for flame annealed TiO2 nanotubes. Isotope labelling studies, using GC–MS and gas-phase FTIR, indicated photoreduction of 13CO2 to 13CH4. The detection of 13CO in the product mixture, and the absence of HCHO and HCOOH provides strong support for the photoreduction proceeding along a carbene pathway. The enhanced CO2 photoreduction performance of FANT-aq is attributed to increased visible light absorption, square morphology, and the presence of rutile as the only crystalline phase with (110) as the dominant plane.
High rate CO2 photoreduction using flame annealed TiO2 nanotubesPawan Kumar
The photocatalytic reduction of CO2 into light hydrocarbons using sunlight and water is a challenging reaction involving eight electron transfer steps; nevertheless, it has great potential to address the problem of rising anthropogenic carbon emissions and enable the use of fossil fuels in a sustainable way. Several decades after its first use, TiO2 remains one of the best performing and most durable photocatalysts for CO2 reduction albeit with a poor visible light absorption capacity. We have used flame annealing to improve the response of TiO2 to visible photons and engineered a nanotubular morphology with square-shaped cross-sections in flame-annealed nanotubes. An enhanced CH4 yield was achieved in the photoreduction of CO2 using flame annealed TiO2 nanotubes, and isotope labeled experiments confirmed the reaction products to originate from the CO2 reactant. Flame-annealed TiO2 nanotubes
The document discusses photocatalytic conversion of carbon dioxide into fuels and chemicals. It describes how semiconductor-based photocatalysts like TiO2 can be used to drive the reduction of CO2 into products like methanol using solar energy. Challenges include the large band gap of most semiconductors, which limits them to using only UV light. The document explores using metal complexes immobilized on photoactive supports as an alternative, as they have visible light activity and can be tuned to favor specific products. Specific examples discussed include cobalt phthalocyanine and tin phthalocyanine immobilized on graphene oxide and mesoporous ceria, respectively, as well as heteroleptic ruthenium complexes immobilized on graphene oxide
N-doped graphene quantum dots (NGQDs) catalyze the efficient electrochemical reduction of carbon dioxide into multi-carbon hydrocarbons and oxygenates such as ethylene, ethanol, and n-propanol. The NGQDs achieve high total faradaic efficiencies of up to 90% for carbon dioxide reduction, with selectivities for ethylene and ethanol conversions reaching 45%. Control experiments confirm the NGQDs are responsible for catalyzing the reaction. Compared to undoped graphene quantum dots, the NGQDs have higher activity and selectivity for producing valuable fuel and chemical products from carbon dioxide due to the presence of pyridinic nitrogen defects introduced during synthesis.
Carbon Dioxide to Chemicals and Fuels Course Material.
National Centre for Catalysis Research (NCCR, IIT Madras), considered for the first on-line course the topic of Carbon dioxide to Chemicals and Fuels. NCCR has learnt many such lessons which are necessary for the researchers to understand and also have a complete comprehension of the limitations.
Carbon Nitride Grafted Cobalt Complex (Co@npg-C3N4) for Visible LightAssiste...Pawan Kumar
1) A cobalt complex was covalently grafted to nanoporous graphitic carbon nitride (npg-C3N4) via a click reaction to create a heterogeneous photocatalyst called Co@npg-C3N4.
2) Under visible light irradiation at room temperature, Co@npg-C3N4 efficiently catalyzed the direct esterification of aldehydes without the need for an external base.
3) Characterization of Co@npg-C3N4 showed the cobalt complex was successfully immobilized via click chemistry, providing a robust photocatalyst that could be easily recovered and reused without significant loss of activity.
2021 influence of basic carbon additives on the electrochemical performance ...Ary Assuncao
This study investigates the effect of carbon surface basicity on the electrochemical performance and dynamic charge acceptance of lead-carbon batteries. Five activated carbons with different pH values ranging from 9.5 to 11.1 were prepared by ammonia and hydrogen gas treatments. Cyclic voltammetry showed that the hydrogen evolution reaction activity increased with higher carbon surface basicity. Testing of lead-carbon electrodes found a correlation between carbon pH and dynamic charge acceptance, with higher pH carbons showing improved charge currents and final dynamic charge acceptance. The carbon content also affected charge currents during simulated microcycles, demonstrating that surface chemistry and amount of carbon additive both influence the electrochemical properties and performance of lead-carbon batteries.
Photocatalytic reduction of carbon dioxide issues and prospects bentham scie...Hariprasad Narayanan
The conversion of carbon dioxide into worthwhile chemicals through photocatalysis has been a matter of attraction for the last four decades among the scientific community. However, the conversion rate has not yet been achieved to the desired efficiency due to the inevitable barriers associated with the process making it as a Holy Grail. This presentation deals with the identification and critical evaluation of the hurdles that pulls back the photocatalytic processes on track and the recent advances in the scientific field that pertain to the photocatalytic conversion of carbon dioxide in the near future.
Visible light assisted hydrogen generation from complete decomposition of hyd...Pawan Kumar
Hydrogen is considered to be an ideal energy carrier, which produces only water when combined with
oxygen and thus has no detrimental effect on the environment. While the catalytic decomposition of
hydrous hydrazine for the production of hydrogen is well explored, little is known about its photocatalytic
decomposition. The present paper describes a highly efficient photochemical methodology for the production
of hydrogen through the decomposition of aqueous hydrazine using titanium dioxide nanoparticles
modified with a Rh(I) coordinated catechol phosphane ligand (TiO2–Rh) as a photocatalyst under visible
light irradiation. After 12 h of visible light irradiation, the hydrogen yield was 413 μmol g−1 cat with a hydrogen
evolution rate of 34.4 μmol g−1 cat h−1. Unmodified TiO2 nanoparticles offered a hydrogen yield of
83 μmol g−1 cat and a hydrogen evolution rate of only 6.9 μmol g−1 cat h−1. The developed photocatalyst
was robust under the experimental conditions and could be efficiently reused for five subsequent runs
without any significant change in its activity. The higher stability of the photocatalyst is attributed to the
covalent attachment of the Rh complex, whereas the higher activity is believed to be due to the synergistic
mechanism that resulted in better electron transfer from the Rh complex to the conduction band of TiO2
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%.
Water-splitting photoelectrodes consisting of heterojunctions of carbon nitri...Devika Laishram
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%.
Proton‐functionalized graphitic carbon nitride for efficient metal‐free disin...Journal Papers
This document summarizes a study on using proton-functionalized graphitic carbon nitride (P-CN) as a metal-free photocatalyst for disinfecting Escherichia coli (E. coli) bacteria under low-power light irradiation. P-CN demonstrated 100% disinfection of E. coli within 4 hours under 23W light bulb irradiation. Testing with scavengers showed that hydroxyl radicals (·OH) played a major role in the disinfection process, while superoxide (·O2-) played a minimal role. P-CN exhibited higher disinfection efficiency compared to unmodified graphitic carbon nitride, due to the introduction of protons enhancing conductivity and charge transfer in P-CN. This
Enhanced charge separation in gC 3 N 4–BiOI heterostructures for visible ligh...Pawan Kumar
Heterojunctions of the low bandgap semiconductor bismuth oxyiodide (BiOI) with bulk multilayered graphitic carbon nitride (g-C3N4) and few layered graphitic carbon nitride sheets (g-C3N4-S) are synthesized and investigated as an active photoanode material for sunlight driven water splitting. HR-TEM and elemental mapping reveals formation of a unique heterostructure between BiOI platelets and the carbon nitride (g-C3N4 and g-C3N4-S) network that consisted of dendritic BiOI nanoplates surrounded by g-C3N4 sheets. The presence of BiOI in g-C3N4-S/BiOI and g-C3N4-S/BiOI nanocomposites extends the visible light absorption profile from 500 nm up to 650 nm. Due to excellent charge separation in g-C3N4/BiOI and g-C3N4-S/BiOI, evident from quenching of the carbon nitride photoluminescence (PL) and a decrease in the PL lifetime, a significant increase in photoelectrochemical performance is observed …
Enhanced charge separation in g-C3N4–BiOI heterostructures for visible light ...Pawan Kumar
Heterojunctions of the low bandgap semiconductor bismuth oxyiodide (BiOI) with bulk multilayered
graphitic carbon nitride (g-C3N4) and few layered graphitic carbon nitride sheets (g-C3N4-S) are
synthesized and investigated as an active photoanode material for sunlight driven water splitting. HRTEM
and elemental mapping reveals formation of a unique heterostructure between BiOI platelets and
the carbon nitride (g-C3N4 and g-C3N4-S) network that consisted of dendritic BiOI nanoplates
surrounded by g-C3N4 sheets. The presence of BiOI in g-C3N4-S/BiOI and g-C3N4-S/BiOI
nanocomposites extends the visible light absorption profile from 500 nm up to 650 nm. Due to excellent
charge separation in g-C3N4/BiOI and g-C3N4-S/BiOI, evident from quenching of the carbon nitride
photoluminescence (PL) and a decrease in the PL lifetime, a significant increase in photoelectrochemical
performance is observed for both types of g-C3N4–BiOI heterojunctions. In comparison to
heterojunctions of bulk g-C3N4 with BiOI, the nanocomposite consisting of few layered sheets of g-
C3N4 and BiOI exhibits higher photocurrent density due to lower recombination in few layered sheets. A
synergistic trap passivation and charge separation is found to occur in the g-C3N4-S/BiOI
nanocomposite heterostructure which results in a higher photocurrent and a lower charge transfer
resistance.
Enhanced charge separation in g-C3N4–BiOI heterostructures for visible light ...Pawan Kumar
Heterojunctions of the low bandgap semiconductor bismuth oxyiodide (BiOI) with bulk multilayered
graphitic carbon nitride (g-C3N4) and few layered graphitic carbon nitride sheets (g-C3N4-S) are
synthesized and investigated as an active photoanode material for sunlight driven water splitting. HRTEM and elemental mapping reveals formation of a unique heterostructure between BiOI platelets and
the carbon nitride (g-C3N4 and g-C3N4-S) network that consisted of dendritic BiOI nanoplates
surrounded by g-C3N4 sheets. The presence of BiOI in g-C3N4-S/BiOI and g-C3N4-S/BiOI
nanocomposites extends the visible light absorption profile from 500 nm up to 650 nm. Due to excellent
charge separation in g-C3N4/BiOI and g-C3N4-S/BiOI, evident from quenching of the carbon nitride
photoluminescence (PL) and a decrease in the PL lifetime, a significant increase in photoelectrochemical
performance is observed for both types of g-C3N4–BiOI heterojunctions. In comparison to
heterojunctions of bulk g-C3N4 with BiOI, the nanocomposite consisting of few layered sheets of gC3N4 and BiOI exhibits higher photocurrent density due to lower recombination in few layered sheets. A
synergistic trap passivation and charge separation is found to occur in the g-C3N4-S/BiOI
nanocomposite heterostructure which results in a higher photocurrent and a lower charge transfer
resistance.
Air- and water-stable halide perovskite nanocrystals protected with nearly-mo...Pawan Kumar
Halide perovskites are exciting candidates for broad-spectrum photocatalysts but have the problem of ambient stability. Protective shells of oxides and polymers around halide perovskite nano- and micro-crystals provide a measure of chemical and photochemical resilience but the photocatalytic performance of perovskites is compromised due to low electron mobility in amorphous oxide or polymer shells and rapid charge carrier recombination on the surface. Herein an in situ surface passivation and stabilization of CsPbBr3 nanocrystals was achieved using monolayered graphenic carbon nitride (CNM). Extensive characterization of carbon nitride protected CsPbBr3 nanocrystals (CNMBr) indicated spherical CsPbBr3 nanoparticles encased in a few nm thick g-C3N4 sheets facilitating better charge separation via percolation/tunneling of charges on conductive 2D nanosheets. The CNMBr core-shell nanocrystals demonstrated enhanced photoelectrochemical water splitting performance and photocurrent reaching up to 1.55 mA cm−2. The CNMBr catalyst was successfully deployed for CO2 photoreduction giving carbon monoxide and methane as the reaction products.
In Situ Photo-Fenton-Like Tandem Reaction for Selective Gluconic Acid Product...Pawan Kumar
Biomass photorefining to selectively produce value-added bioproducts is an emerging alternative biomass valorization approach to alleviate energy crisis and achieve carbon neutrality. Here, we demonstrate an efficient and selective glucose photo-oxidation to gluconic acid via a rationally designed dual-functional carbon nitride photocatalyst that not only allows H2O2 production via 2e– oxygen reduction reaction (2e-ORR) but also realizes in situ photo-Fenton-like reaction. As a result, the essential oxidative species (•O2– and •OH) for glucose oxidation into gluconic acid are generated that achieves >60% glucose conversion and >60% of gluconic acid selectivity within 4 h. Density functional theory calculations demonstrate the superior performance of the photocatalyst for •O2– and H2O2 generation. Further experimental results reveal that the moderate concentration of H2O2 produced by 2e-ORR reaction plays a vital role in regulatinge gluconic acid selectivity. This work demonstrates a good example to realize selective biomass photorefining through tandem reaction of ORR and in situ photo-Fenton-like process, which could have profound impact on artificial photoenzyme systems involving moderate H2O2 modulation.
Sunlight-driven water-splitting using two-dimensional carbon based semiconduc...Pawan Kumar
The overwhelming challenge of depleting fossil fuels and anthropogenic carbon emissions has driven research into alternative clean sources of energy. To achieve the goal of a carbon neutral economy, the harvesting of sunlight by using photocatalysts to split water into hydrogen and oxygen is an expedient approach to fulfill the energy demand in a sustainable way along with reducing the emission of greenhouse gases. Even though the past few decades have witnessed intensive research into inorganic semiconductor photocatalysts, their quantum efficiencies for hydrogen production from visible photons remain too low for the large scale deployment of this technology. Visible light absorption and efficient charge separation are two key necessary conditions for achieving the scalable production of hydrogen from water. Two-dimensional carbon based nanoscale materials such as graphene oxide, reduced …
Sunlight-driven water-splitting using twodimensional carbon based semiconductorsPawan Kumar
The overwhelming challenge of depleting fossil fuels and anthropogenic carbon emissions has driven research
into alternative clean sources of energy. To achieve the goal of a carbon neutral economy, the harvesting of
sunlight by using photocatalysts to split water into hydrogen and oxygen is an expedient approach to fulfill
the energy demand in a sustainable way along with reducing the emission of greenhouse gases. Even though
the past few decades have witnessed intensive research into inorganic semiconductor photocatalysts, their
quantum efficiencies for hydrogen production from visible photons remain too low for the large scale
deployment of this technology. Visible light absorption and efficient charge separation are two key necessary
conditions for achieving the scalable production of hydrogen from water. Two-dimensional carbon based
nanoscale materials such as graphene oxide, reduced graphene oxide, carbon nitride, modified 2D carbon
frameworks and their composites have emerged as potential photocatalysts due to their astonishing
properties such as superior charge transport, tunable energy levels and bandgaps, visible light absorption,
high surface area, easy processability, quantum confinement effects, and high photocatalytic quantum yields.
The feasibility of structural and chemical modification to optimize visible light absorption and charge
separation makes carbonaceous semiconductors promising candidates to convert solar energy into chemical
energy. In the present review, we have summarized the recent advances in 2D carbonaceous photocatalysts
with respect to physicochemical and photochemical tuning for solar light mediated hydrogen evolution.
Metal-organic hybrid: Photoreduction of CO2 using graphitic carbon nitride su...Pawan Kumar
A novel heteroleptic iridium complex supported on graphitic carbon nitride was synthesized and used for photoreduction of carbon dioxide under visible light irradiation. The methanol yield obtained after 24 h irradiation was 9934 μmol g−1cat (TON 1241 with respect to Ir) by using triethylamine (TEA) as a sacrificial donor, which was significantly higher as compared to the semiconductor carbon nitride 145 μmol g−1cat under identical conditions. The presence of triethylamine was found to be vital for the higher methanol yield. After the reaction, the photocatalyst could easily be recovered and reused for subsequent six runs without significant loss in photo activity.
Metal-organic hybrid: Photoreduction of CO2 using graphitic carbon nitride su...Pawan Kumar
A novel heteroleptic iridium complex supported on graphitic carbon nitride was synthesized and used
for photoreduction of carbon dioxide under visible light irradiation. The methanol yield obtained after
24 h irradiation was 9934 mmol g1cat (TON 1241 with respect to Ir) by using triethylamine (TEA) as a
sacrificial donor, which was significantly higher as compared to the semiconductor carbon nitride
145 mmol g1cat under identical conditions. The presence of triethylamine was found to be vital for the
higher methanol yield. After the reaction, the photocatalyst could easily be recovered and reused for
subsequent six runs without significant loss in photo activity.
Metal-organic hybrid: Photoreduction of CO2 using graphitic carbon nitride su...Pawan Kumar
A novel heteroleptic iridium complex supported on graphitic carbon nitride was synthesized and used
for photoreduction of carbon dioxide under visible light irradiation. The methanol yield obtained after
24 h irradiation was 9934 mmol g1cat (TON 1241 with respect to Ir) by using triethylamine (TEA) as a
sacrificial donor, which was significantly higher as compared to the semiconductor carbon nitride
145 mmol g1cat under identical conditions. The presence of triethylamine was found to be vital for the
higher methanol yield. After the reaction, the photocatalyst could easily be recovered and reused for
subsequent six runs without significant loss in photo activity.
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.
The document describes a study that used direct coal liquefaction residue (DCLR) as a microwave absorbent to improve the upgrading of lignite via microwave-assisted pyrolysis. DCLR was added in amounts ranging from 0-20% during the microwave pyrolysis of lignite. Analysis showed that DCLR accelerated the heating rate and facilitated changes to the lignite's chemical and physical properties. These changes improved the slurryability and increased the maximum solid concentration of lignite water slurry from 41.73% to 65.42% with the addition of 12% DCLR. The approach provides an effective way to utilize DCLR waste while upgrading lignite to make it more suitable for transportation and utilization.
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.
HYDROGEN GENERATION FROM WASTE WATER BY USING SOLAR ENERGY | J4RV3I11004Journal For Research
Objective of this paper is to produce hydrogen which is an ideal fuel for the next generation because it is abundantly available in nature, energy efficient and clean. Wide varieties of technologies are available to produce hydrogen but only few of them are considered environmental friendly. Solar water splitting via photo catalytic reaction is one of them which have attracted tremendous attention. In this paper we are working on hydrogen production via solar splitting. Photo catalytic water splitting is one of the promising technologies to produce pure and clean hydrogen. Since it is reasonable having low process cost and has a small reactor, it can be made for house hold application and hence has a huge market potential. Generation of hydrogen under visible irradiation is the main area of work. Based on the literature reported here, visible irradiation can be achieved by doping of TiO2 with metal or non-metal. We have used Fe doping to increase the efficiency. The result indicates that Fe doped sieves produce more hydrogen than the normal TiO2 coated sieve and the efficiency can be increased if we increase the number of doped sieves and surface area.
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.
On the Current Status of the Mechanistic Aspects of Photocatalytic Reduction ...Hariprasad Narayanan
Photocatalytic reduction of carbon dioxide, one of the pathways involved in the carbon dioxide conversion process, has been receiving significant attention from the scientific community in the last four decades. Nevertheless, the mechanism of carbon dioxide reduction is still unclear and the information available is not sufficient for developing it into large scale applications, possibly because of the invariable hurdles associated with the reduction process. The reductive photocatalytic conversion of CO2 involves all the redox reactions occurring at the interface of the semiconductor such as water splitting, hydrogen evolution, oxygen evolution, photo-oxidation reactions and reactions of radical intermediates. The overall product yield is highly dependent on the extent of these competing reactions. Herein, we discuss our perceptions and current status of the interface reactions and their involvement in the fundamental mechanistic aspects of the photocatalytic conversion of CO2.
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.
Similar to Heterojunctions of halogen-doped carbon nitride nanosheets and BiOI for sunlightdriven water-splitting (20)
Isolated Iridium Sites on Potassium-Doped Carbon-nitride wrapped Tellurium Na...Pawan Kumar
Many industrial processes such transesterification of fatty acid for biodiesel production, soap manufacturing and biosynthesis of ethanol generate glycerol as a major by-product that can be used to produce commodity chemicals. Photocatalytic transformation of glycerol is an enticing approach that can exclude the need of harsh oxidants and extraneous thermal energy. However, the product yield and selectivity remain poor due to low absorption and unsymmetrical site distribution on the catalyst surface. Herein, tellurium (Te) nanorods/nanosheets (TeNRs/NSs) wrapped potassium-doped carbon nitride (KCN) van der Waal (vdW) heterojunction (TeKCN) is designed to enhance charge separation and visible-NIR absorption. The iridium (Ir) single atom sites decoration on the TeKCN core-shell structure (TeKCNIr) promotes selective oxidation of glycerol to glyceraldehyde with a conversion of 45.6% and selectivity of 61.6% under AM1.5G irradiation. The catalytic selectivity can reach up to 88% under 450 nm monochromatic light. X-ray absorption spectroscopy (XAS) demonstrates the presence of undercoordinated IrN2O2 sites which improved catalytic selectivity for glycol oxidation. Band energies and computational calculations reveal faile charge transfer in the TeKCNIr heterostructure. EPR and scavenger tests discern that superoxide (O2•−) and hydroxyl (•OH) radicals are prime components driving glycerol oxidation.
Isolated Iridium Sites on Potassium-Doped Carbon-nitride wrapped Tellurium Na...Pawan Kumar
Many industrial processes such transesterification of fatty acid for biodiesel production, soap manufacturing and biosynthesis of ethanol generate glycerol as a major by-product that can be used to produce commodity chemicals. Photocatalytic transformation of glycerol is an enticing approach that can exclude the need of harsh oxidants and extraneous thermal energy. However, the product yield and selectivity remain poor due to low absorption and unsymmetrical site distribution on the catalyst surface. Herein, tellurium (Te) nanorods/nanosheets (TeNRs/NSs) wrapped potassium-doped carbon nitride (KCN) van der Waal (vdW) heterojunction (TeKCN) is designed to enhance charge separation and visible-NIR absorption. The iridium (Ir) single atom sites decoration on the TeKCN core-shell structure (TeKCNIr) promotes selective oxidation of glycerol to glyceraldehyde with a conversion of 45.6% and selectivity of 61.6% under AM1.5G irradiation. The catalytic selectivity can reach up to 88% under 450 nm monochromatic light. X-ray absorption spectroscopy (XAS) demonstrates the presence of undercoordinated IrN2O2 sites which improved catalytic selectivity for glycol oxidation. Band energies and computational calculations reveal faile charge transfer in the TeKCNIr heterostructure. EPR and scavenger tests discern that superoxide (O2•−) and hydroxyl (•OH) radicals are prime components driving glycerol oxidation.
Isolated Iridium Sites on Potassium-Doped Carbon-nitride wrapped Tellurium Na...Pawan Kumar
Many industrial processes such transesterification of fatty acid for biodiesel production, soap manufacturing and biosynthesis of ethanol generate glycerol as a major by-product that can be used to produce commodity chemicals. Photocatalytic transformation of glycerol is an enticing approach that can exclude the need of harsh oxidants and extraneous thermal energy. However, the product yield and selectivity remain poor due to low absorption and unsymmetrical site distribution on the catalyst surface. Herein, tellurium (Te) nanorods/nanosheets (TeNRs/NSs) wrapped potassium-doped carbon nitride (KCN) van der Waal (vdW) heterojunction (TeKCN) is designed to enhance charge separation and visible-NIR absorption. The iridium (Ir) single atom sites decoration on the TeKCN core-shell structure (TeKCNIr) promotes selective oxidation of glycerol to glyceraldehyde with a conversion of 45.6% and selectivity of 61.6% under AM1.5G irradiation. The catalytic selectivity can reach up to 88% under 450 nm monochromatic light. X-ray absorption spectroscopy (XAS) demonstrates the presence of undercoordinated IrN2O2 sites which improved catalytic selectivity for glycol oxidation. Band energies and computational calculations reveal faile charge transfer in the TeKCNIr heterostructure. EPR and scavenger tests discern that superoxide (O2•−) and hydroxyl (•OH) radicals are prime components driving glycerol oxidation.
Solar-Driven Cellulose Photorefining into Arabinose over Oxygen-Doped Carbon ...Pawan Kumar
Biomass photorefining is a promising strategy to address the energy crisis and transition toward carbon carbon-neutral society. Here, we demonstrate the feasibility of direct cellulose photorefining into arabinose by a rationally designed oxygen-doped polymeric carbon nitride, which generates favorable oxidative species (e.g., O2–, •OH) for selective oxidative reactions at neutral conditions. In addition, we also illustrate the mechanism of the photocatalytic cellulose to arabinose conversion by density functional theory calculations. The oxygen insertion derived from oxidative radicals at the C1 position of glucose within cellulose leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in the subsequent gluconic acid formation. The following decarboxylation process of gluconic acid via C1–C2 α-scissions, triggered by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This work not only offers a mechanistic understanding of cellulose photorefining to arabinose but also sets up an example for illuminating the path toward direct cellulose photorefining into value-added bioproducts under mild conditions.
Solar-Driven Cellulose Photorefining into Arabinose over Oxygen-Doped Carbon ...Pawan Kumar
Biomass photorefining is a promising strategy to address the energy crisis and transition toward carbon carbon-neutral society. Here, we demonstrate the feasibility of direct cellulose photorefining into arabinose by a rationally designed oxygen-doped polymeric carbon nitride, which generates favorable oxidative species (e.g., O2–, •OH) for selective oxidative reactions at neutral conditions. In addition, we also illustrate the mechanism of the photocatalytic cellulose to arabinose conversion by density functional theory calculations. The oxygen insertion derived from oxidative radicals at the C1 position of glucose within cellulose leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in the subsequent gluconic acid formation. The following decarboxylation process of gluconic acid via C1–C2 α-scissions, triggered by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This work not only offers a mechanistic understanding of cellulose photorefining to arabinose but also sets up an example for illuminating the path toward direct cellulose photorefining into value-added bioproducts under mild conditions.
Solar-Driven Cellulose Photorefining into Arabinose over Oxygen-Doped Carbon ...Pawan Kumar
Biomass photorefining is a promising strategy to address the energy crisis and transition toward carbon carbon-neutral society. Here, we demonstrate the feasibility of direct cellulose photorefining into arabinose by a rationally designed oxygen-doped polymeric carbon nitride, which generates favorable oxidative species (e.g., O2–, •OH) for selective oxidative reactions at neutral conditions. In addition, we also illustrate the mechanism of the photocatalytic cellulose to arabinose conversion by density functional theory calculations. The oxygen insertion derived from oxidative radicals at the C1 position of glucose within cellulose leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in the subsequent gluconic acid formation. The following decarboxylation process of gluconic acid via C1–C2 α-scissions, triggered by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This work not only offers a mechanistic understanding of cellulose photorefining to arabinose but also sets up an example for illuminating the path toward direct cellulose photorefining into value-added bioproducts under mild conditions.
Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single...Pawan Kumar
Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer–Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.
Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single...Pawan Kumar
Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer–Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.
Selective Cellobiose Photoreforming for Simultaneous Gluconic Acid and Syngas...Pawan Kumar
Here, we demonstrate the selective cellobiose (building block of cellulose) photoreforming for gluconic acid and syngas co-production in acidic conditions by rationally designing a bifunctional polymeric carbon nitride (CN) with potassium/sulfur co-dopant. This heteroatomic doped CN photocatalyst possesses enhanced visible light absorption, higher charge separation efficiency than pristine CN. Under acidic conditions, cellobiose is not only more efficiently hydrolyzed into glucose but also promotes the syngas and gluconic acid production. Density functional theory (DFT) calculations reveal the favorable generation of •O2− during the photocatalytic reaction, which is essential for gluconic acid production. Consequently, the fine-designed photocatalyst presents excellent cellobiose conversion (>80%) and gluconic acid selectivity (>70%) together with the co-production of syngas (~56 μmol g-1 h-1) under light illumination. The current work demonstrates the feasibility of biomass photoreforming with value-added chemicals and syngas co-production under mild condition.
Selective Cellobiose Photoreforming for Simultaneous Gluconic Acid and Syngas...Pawan Kumar
Here, we demonstrate the selective cellobiose (building block of cellulose) photoreforming for gluconic acid and syngas co-production in acidic conditions by rationally designing a bifunctional polymeric carbon nitride (CN) with potassium/sulfur co-dopant. This heteroatomic doped CN photocatalyst possesses enhanced visible light absorption, higher charge separation efficiency than pristine CN. Under acidic conditions, cellobiose is not only more efficiently hydrolyzed into glucose but also promotes the syngas and gluconic acid production. Density functional theory (DFT) calculations reveal the favorable generation of •O2− during the photocatalytic reaction, which is essential for gluconic acid production. Consequently, the fine-designed photocatalyst presents excellent cellobiose conversion (>80%) and gluconic acid selectivity (>70%) together with the co-production of syngas (~56 μmol g-1 h-1) under light illumination. The current work demonstrates the feasibility of biomass photoreforming with value-added chemicals and syngas co-production under mild condition.
Selective Cellobiose Photoreforming for Simultaneous Gluconic Acid and Syngas...Pawan Kumar
Here, we demonstrate the selective cellobiose (building block of cellulose) photoreforming for gluconic acid and syngas co-production in acidic conditions by rationally designing a bifunctional polymeric carbon nitride (CN) with potassium/sulfur co-dopant. This heteroatomic doped CN photocatalyst possesses enhanced visible light absorption, higher charge separation efficiency than pristine CN. Under acidic conditions, cellobiose is not only more efficiently hydrolyzed into glucose but also promotes the syngas and gluconic acid production. Density functional theory (DFT) calculations reveal the favorable generation of •O2− during the photocatalytic reaction, which is essential for gluconic acid production. Consequently, the fine-designed photocatalyst presents excellent cellobiose conversion (>80%) and gluconic acid selectivity (>70%) together with the co-production of syngas (~56 μmol g-1 h-1) under light illumination. The current work demonstrates the feasibility of biomass photoreforming with value-added chemicals and syngas co-production under mild condition.
Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single...Pawan Kumar
Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer–Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.
Recent advancements in tuning the electronic structures of transitional metal...Pawan Kumar
The smooth transition from finite non-renewables to renewable energy conversion technologies will require efficient electrocatalysts which can harness intermittent energies to store in the form of chemical bonds. The oxygen evolution reaction (OER) impedes the widespread usage of water electrolyzers to convert H2O into H2 and persists as a bottleneck, including other energy conversion devices with sluggish four H+/e− kinetics. In this context, designing highly active and stable catalysts capable of driving a lower overpotential in the OER to produce continuous hydrogen (H2) is a primary demanded. This chapter discussed the mechanism of the OER in conventional adsorbate oxygen and lattice oxygen participation in transition metal oxides (TMOs). Further, the influences of surface engineering, doping, and defects in the TMOs and understanding the electronic structure to screen electrodes towards the structure–activity relationship are highlighted. Specifically, the adsorption strength of O 2p is understood in detail as its binding ability over the surface of TMOs can be correlated directly to the OER activity. The iterative development of TMOs in terms of understanding electronic structural attributes is essential for the commercial deployment of energy conversion technologies. The comprehensive outlook of this chapter investigates thoroughly how TMOs can be used as significant materials for the OER in the near future.
Hole transport materials (HTMs) have a significant impact on the effectiveness of organic electronic devices; therefore, we present a molecular architecture of pyrazino[2,3-g]quinoxaline (PQ10)-based room-temperature organic liquid crystalline semiconductor (OLCS) as an alternative HTM. The PQ10 compound exhibits three different rectangular columnar (Colr) phases offering an impressive hole mobility of 8.8 × 10−3 cm2V−1s−1 which is found to be dexterous than most of existing polymeric hole transport materials. The charge transport mechanism is governed by the hole polarons hopping through H-aggregates of the PQ10 molecules and the hole mobility remains nearly constant throughout the mesophase range, but it decreases with increasing applied electric field. The current-voltage characteristics of the PQ10 have also been investigated in all three Colr phases and explained via the Poole-Frenkel conduction mechanism. The dielectric spectroscopy has been eventually carried out to understand the nature of dielectric permittivity and conductivity as a function of temperature and a correlation is established between the molecular architecture of the Colr phases and aforementioned physical properties. Solar cell simulation has been additionally performed to demonstrate that the PQ10 material can be a better choice as HTM for organic electronics and photovoltaic applications.
Multifunctional carbon nitride nanoarchitectures for catalysisPawan Kumar
Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.
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
This document summarizes a study that investigated copper single atom catalysts supported on two-dimensional carbon nitride materials for enhancing the electrochemical reduction of carbon dioxide to methane. Specifically, copper ions were incorporated into the nanoporous structures of poly(heptazine imide) and poly(triazine imide) using a room temperature ion exchange process. This allowed for high loading densities of isolated copper sites. The proximity of copper atoms within the nanopores was found to enable cooperative catalysis that boosted the selectivity and efficiency of the multi-electron conversion of CO2 to CH4. Density functional theory calculations helped explain how the copper-copper distance and coordination environment modulated the binding of reaction intermediates. Optimized copper loading in the
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.
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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.
ESR spectroscopy in liquid food and beverages.pptxPRIYANKA PATEL
With increasing population, people need to rely on packaged food stuffs. Packaging of food materials requires the preservation of food. There are various methods for the treatment of food to preserve them and irradiation treatment of food is one of them. It is the most common and the most harmless method for the food preservation as it does not alter the necessary micronutrients of food materials. Although irradiated food doesn’t cause any harm to the human health but still the quality assessment of food is required to provide consumers with necessary information about the food. ESR spectroscopy is the most sophisticated way to investigate the quality of the food and the free radicals induced during the processing of the food. ESR spin trapping technique is useful for the detection of highly unstable radicals in the food. The antioxidant capability of liquid food and beverages in mainly performed by spin trapping technique.
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The binding of cosmological structures by massless topological defectsSérgio Sacani
Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field
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Current Ms word generated power point presentation covers major details about the micronuclei test. It's significance and assays to conduct it. It is used to detect the micronuclei formation inside the cells of nearly every multicellular organism. It's formation takes place during chromosomal sepration at metaphase.
When I was asked to give a companion lecture in support of ‘The Philosophy of Science’ (https://shorturl.at/4pUXz) I decided not to walk through the detail of the many methodologies in order of use. Instead, I chose to employ a long standing, and ongoing, scientific development as an exemplar. And so, I chose the ever evolving story of Thermodynamics as a scientific investigation at its best.
Conducted over a period of >200 years, Thermodynamics R&D, and application, benefitted from the highest levels of professionalism, collaboration, and technical thoroughness. New layers of application, methodology, and practice were made possible by the progressive advance of technology. In turn, this has seen measurement and modelling accuracy continually improved at a micro and macro level.
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Abstract: Equivariant neural networks are neural networks that incorporate symmetries. The nonlinear activation functions in these networks result in interesting nonlinear equivariant maps between simple representations, and motivate the key player of this talk: piecewise linear representation theory.
Disclaimer: No one is perfect, so please mind that there might be mistakes and typos.
dtubbenhauer@gmail.com
Corrected slides: dtubbenhauer.com/talks.html
Immersive Learning That Works: Research Grounding and Paths ForwardLeonel Morgado
We will metaverse into the essence of immersive learning, into its three dimensions and conceptual models. This approach encompasses elements from teaching methodologies to social involvement, through organizational concerns and technologies. Challenging the perception of learning as knowledge transfer, we introduce a 'Uses, Practices & Strategies' model operationalized by the 'Immersive Learning Brain' and ‘Immersion Cube’ frameworks. This approach offers a comprehensive guide through the intricacies of immersive educational experiences and spotlighting research frontiers, along the immersion dimensions of system, narrative, and agency. Our discourse extends to stakeholders beyond the academic sphere, addressing the interests of technologists, instructional designers, and policymakers. We span various contexts, from formal education to organizational transformation to the new horizon of an AI-pervasive society. This keynote aims to unite the iLRN community in a collaborative journey towards a future where immersive learning research and practice coalesce, paving the way for innovative educational research and practice landscapes.
Heterojunctions of halogen-doped carbon nitride nanosheets and BiOI for sunlightdriven water-splitting
1. Nanotechnology
PAPER
Heterojunctions of halogen-doped carbon nitride nanosheets and BiOI
for sunlight-driven water-splitting
To cite this article: Kazi M Alam et al 2020 Nanotechnology 31 084001
View the article online for updates and enhancements.
This content was downloaded from IP address 129.128.216.34 on 22/11/2019 at 14:16
3. ZnO, BiVO4, and complex composition materials i.e. La- and
Rh-codoped SrTiO3 (SrTiO3:La, Rh) have been explored as
photoanodes for water splitting. However, no photoelec-
trocatalyst has achieved a satisfactory performance for the
deployment of PEC technology at an industrial scale [3, 4].
Graphitic carbon nitride (g-C3N4), a metal-free two-
dimensional conjugated semiconducting polymer, composed
of tris-s-triazine (C6N7) units linked together with tertiary
nitrogen is currently being intensely studied for the realization
of efficient artificial photosynthetic systems [2, 5–14]. g-C3N4
and related graphenic frameworks are particularly promising
for PECs because of the following reasons: (i) They are
constituted of earth abundant elements (carbon and nitrogen)
(ii) They have relatively simple, scalable and inexpensive
methods of synthesis (iii) g-C3N4 has a moderate bandgap
(2.7 eV) with suitable band-edge positions (Ecb: −1.1 V and
Evb: +1.6 V versus NHE at pH-0) for water-splitting and (iv)
They possesses good thermal stability (stable up to 600 °C),
photochemical stability (non-photocorrosive) and chemical
stability (resistant to strong acids and bases) [2, 6, 7, 9, 15].
Despite these attractive features, g-C3N4 suffers from fast
charge carrier recombination and narrow visible light
absorption limited to the blue region of the solar spectrum
[14]. Many attempts have been made to improve the light
absorption capability, such as surface area modification,
doping with P, N, F, I, Cl, B etc and incorporation of N-rich
units in cross-linked heptazine framework [16–21]. The
synthesis of F-doped carbon nitride using ammonium fluoride
(NH4F) as a dopant material revealed that F atoms are bonded
to C atoms in the corners and bays through transformation of
sp2
C to sp3
C [22]. This shifts the highest occupied mole-
cular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) positions and results in a reduced bandgap
and enhanced photocatalytic performance [22]. Bulk g-C3N4
possesses hydrogen bonding between graphitic sheets, which
remains a potential source of localized interlayer charge
recombination, detrimental to photocatalytic activity. There-
fore, few-layered or single-layered crystalline sheets (with
periodic heptazine units) are highly desirable to realize better
charge separation through the suppression of inter-layer
charge recombination [23–25]. Several attempts have been
made to transform bulk g-C3N4 into few-layered or single-
layered nanosheets namely, solvent exfoliation, oxidation,
varying precursors, etc [26–29]. In this regard, using sheets-
forming-precursor is most attractive as it precludes hazardous
chemicals involved in chemical exfoliation [30, 31]. Lu et al
demonstrated the use of ammonium chloride (NH4Cl) with
dicyandiamide; the carbon nitride precursor released NH3 and
HCl gases during thermal annealing which blew dicyandia-
mide derived polymers into numerous large bubbles, which
behaved as gas templates and yielded few-layered g-C3N4
nanosheets [30]. In another report Liu et al showed that
NH4Cl not only assisted in the transformation into sheets but
excess NH4Cl also facilitated chlorine intercalation in
between g-C3N4 sheets which acted as a charge transport
gallery to reduce the recombination rate and improved the
porous structure [19]. Motivated by these findings, we have
demonstrated the synthesis of few-layered fluorine-doped and
chlorine-intercalated carbon nitride nanosheets (CNF-Cl)
using dicyandiamide as a precursor for carbon nitride skeleton
while NH4F and NH4Cl were used as sources for F and Cl
(sheets determining agent), respectively.
Recently, earth abundant ternary compound semi-
conductors from the bismuth oxyhalide (BiOX, X=Cl, Br, I)
family have received significant attention for their high pho-
tocatalytic activities owing to small bandgaps (1.7–1.9 eV)
and suitable nanomorphology. Narrow bandgap BiOI pos-
sesses a layered crystal structure in which positive [Bi2O2]2+
layers are interleaved with negatively charged iodide slabs
[32–36]. This creates an internal built-in electric field which
facilitates the separation and transport of photogenerated
electron–hole pairs [37–41]. Another attractive feature of the
BiOI band structure lies in its dispersive nature, which
enables excitation of electrons through multiple pathways
[40]. Despite the above mentioned attributes, BiOI remains a
poor photocatalyst due to many unwanted features such as
low conductivity, fast charge recombination, etc [38, 40]. The
key scientific challenge therefore is to achieve a much higher
photocatalytic and/or photoelectrochemical performance
using BIOI-based platforms. Recently, certain research
groups have reported the photocatalytic performances of
BiOI-based type-II heterostructures [42–49]. In this work, we
have attempted to suppress interlayer charge recombination of
bulk carbon nitride by two efficient, facile and cost-effective
steps, namely (1) formation of few-layered fluorine-doped
and chlorine-intercalated nanosheets (CNF-Cl) and (2) in situ
growth of heterojunctions of CNF-Cl with BiOI. The resulting
BiOI/CNF-Cl heterojunctions showed remarkable improve-
ment in the measured photocurrent densities under AM1.5 G
one sun illumination. BiOI/CNF-Cl demonstrated excellent
photoelectrochemical H2 evolution rate (19.71 μmol h−1
) and
Faradaic efficiency (82.83%) in comparison to pristine BiOI
and CNFCl. The superior performance of the nanocomposite
catalysts is attributed to the enhanced charge separation and
improved light harvesting capability.
2. Results and discussion
2.1. Synthesis, morphological, structural and compositional
analysis
The synthesis of few-layered F doped Cl-intercalated carbon
nitride (CNF-Cl) sheets was pursued by thermal annealing of
dicyandiamide, NH4F and NH4Cl at 550 °C through a slight
modification of previous reported methods (figure 1)
[19, 22, 30]. In this synthesis protocol dicyandiamide serves
as the source of carbon and nitrogen. High temperature con-
densation polymerization of dicyandiamide yields heptazine
(C6N7) units containing g-C3N4 framework via melamine,
melon, melem, intermediates and evolution of ammonia.
NH4F facilitates fluorine doping of heptazine moieties
because of degradation of NH4F during thermal annealing and
released F atom make bonds to heptazine ring carbons. The
incorporation of electron withdrawing F atoms in g-C3N4
scaffold transforms some sp2
hybridized C atoms in to sp3
C
2
Nanotechnology 31 (2020) 084001 K M Alam et al
4. which partially distorted in-plane symmetry of conjugated
network resulting in the shifting of HOMO and LUMO
positions (band edge positions) and a reduction of the band-
gap [22]. NH4Cl plays a dual role in the synthesis- (1)
facilitating the formation of few-layered sheets due to release
of NH3 and HCl gases at elevated temperature that blow
dicyandiamide-derived polymers into numerous large bub-
bles, yielding F doped g-C3N4 nanosheets [30] and (2)
chlorine intercalation in between g-C3N4 layers. The chlorine
intercalation in between few-layered F-doped carbon nitride
sheets provides interlayer galleries for better charge migration
along with uplifting of the conduction band and a narrower
bandgap [19]. The detailed experimental protocols and phy-
sicochemical characterization methodologies i.e. Fourier
transform infrared spectroscopy, electrochemical impedance
spectroscopy (EIS) and Mott–Schottky plots, efficiency
calculations (ABPE, IPCE, APCE and Faradaic efficiency)
and reuse experiment results are provided in supporting
information.
The fine morphological attributes of BiOI/CNFCl com-
posite to discern the presence of CNF-Cl sheets and dendritic
BiOI nanoplates were determined using high-resolution
transmission electron microscopy (HR-TEM). The HR-TEM
image of BiOI/CNF-Cl clearly shows dense and more crys-
talline domains correspond to BiOI while less dense domains
indicate CNF-Cl in the heterostructure (figures 2(a)–(c)).
High magnification HRTEM image displayed lattice fringes
of CNF-Cl and BiOI with interplanar d-spacing of 0.32 and
0.28 nm assigned to (002) plane of CNF-Cl and (110) plane
of BiOI, respectively (figures 2(b), (c)) [45, 50]. The obtained
d-spacings were in excellent agreement with XRD results
(figure 3(a)). The presence of (002) plane specific to graphitic
structure suggests CNF-Cl sheets were stacked together in the
BiOI/CNF-Cl nanocomposites. The overlapping of these
materials onto each other is unambiguously evident in
figure 2(b), where both the crystal lattices are identified in the
same region. Figure 2(e) shows the FESEM image of BiOI/
CNF-Cl composite, where both stacked layers comprising the
two materials are visible, in agreement with the TEM images.
A closer observation reveals the hydrothermally synthesized
dendritic BiOI nanoplates scaffolded with CNF-Cl frame-
work. Elemental mapping BiOI/CNF-Cl heterojunction in
scanning transmission electron microscopy mode clearly
showed even distribution of the Bi, I and O in dense region
and C, N and F in both less dense and dense regions
(figures 2(f)–(l), respectively). Due to the partial overlap of Bi
and Cl peaks, pristine CNF-Cl was also mapped which clearly
demonstrates the presence of Cl in nanosheets [51].
Figures 2(m)–(q) show the constituent elements of CNF-Cl,
C, N, F and Cl, respectively.
The XRD spectra of CNF-Cl exhibited a broad peak
centered at ∼27.1° corresponding to the (002) plane which
indicates interlayer stacking of conjugated aromatic sheets,
with a 0.33 nm interlayer d spacing, while the other small
peak at 13.1° corresponding to the (100) reflection represents
in-plane repetition of tri-s-triazine unit, consistent with pre-
viously reported data for g-C3N4 (figure 3(a)) [13, 52]. This d
spacing was slightly higher than reported for pristine g-C3N4
(0.32 nm), which might be due to the increased repulsion
between sheets caused by out-of-plane F atoms [53]. The
XRD plot for pristine BiOI demonstrated all the peaks asso-
ciated with tetragonal phase of BiOI which was in good
agreement with reported literature [43, 54]. As expected, the
XRD peak intensities of BiOI decrease, while these intensities
increase for CNF-Cl with the increase of CNF-Cl wt% in the
BiOI/CNF-Cl composites. The appearance of both (002) and
Figure 1. Synthetic outline for the preparation of F doped Cl-intercalated g-C3N4 (CNF-Cl) and BiOI/CNF-Cl nanostructured heterojunction.
3
Nanotechnology 31 (2020) 084001 K M Alam et al
5. (100) peaks of CNF-Cl in the composite heterostructures,
confirms the presence of CNF-Cl with preserved graphitic
carbon nitride framework.
Figure 3(b) shows the Raman spectra for pristine CNF-
Cl, BiOI and BiOI/CNF-Cl the heterostructures. Two char-
acteristic peaks in between 1300 and 1600 cm−1
assigned to
D and G bands of graphene-based materials were clearly
visible in the Raman spectra of the pristine CNF-Cl and their
composites with BiOI. The D band is associated with the out
of plane vibrations of sp3
carbon atoms, representative of
defects and disorder in the systems, and the G band is
assigned to E2g
phonons at Γ point, which originates from the
in-plane vibrations of sp2
carbon atoms [55, 56]. Unlike
graphene or graphene oxide based materials, carbon nitride
based materials do not exhibit sharp D and G bands due to
plenty of defects and short range ordered structure. The
relative intensities of the Raman peaks corresponding to the D
and G bands, provide an indication of defects and order in
carbon nitride materials. In pristine bulk g-C3N4, ID/IG=
0.36, which increases to 0.82 in fluorinated g-C3N4 due to
increased disorder [57]. In both pristine CNF-Cl and the
CNF-Cl containing composites prepared by us, the ratio
ID/IG∼0.74, close to that of fluorinated g-C3N4. The almost
unchanged intensity ratio (ID/IG) of these bands, in the BiOI/
CNF-Cl nanocomposites, compared to the pristine CNF-Cl is
indicative of unperturbed crystallinity, order and in-plane sp2
domain size in the nanocomposites, consistent with the XRD
results. The relatively smaller peak around 714 cm−1
, which
has been attributed to the out of plane C–C vibrations [58],
is present in the spectrum of both pristine CNF-Cl and
Figure 2. (a) HRTEM image of 50% BiOI/CNF-Cl; (b) and (c) selected magnified regions of (a) showing crystal planes of CNF-Cl and BiOI;
(d) bright field STEM image of 50% BiOI/CNF-Cl; (e) FESEM image of 50% BiOI/CNF-Cl film, (f) and (m) bright field STEM images of
50% BiOI/CNF-Cl and pristine CNF-Cl, respectively; (g)–(l) EDX elemental mapping of 50% BiOI/CNF-Cl under STEM mode for C (g),
N (h), F (i), Bi (j), I (k) and O (l); (n)–(q) EDX elemental mapping of pristine CNF-Cl under STEM mode for C (n), N (o), F (p), and Cl (q).
Figure 3. (a) X-ray diffractograms of pristine BiOI, BiOI/CNF-Cl composites and pristine CNF-Cl and (b) Raman spectra (λexc=785 nm)
of pristine CNF-Cl, pristine BiOI and BiOI/CNF-Cl heterostructures.
4
Nanotechnology 31 (2020) 084001 K M Alam et al
6. BiOI/CNF-Cl heterostructures. The sharp Raman peaks at the
lower wavenumbers are vibrational modes of BiOI. The peak
at 148 cm−1
is assigned to the Eg
internal Bi–I stretching
mode [59]. As expected, the relative intensities of the D and
G bands of CNF-Cl decreased in BiOI/CNF-Cl composite as
the wt% of BiOI increased.
2.2. Photophysical properties
Prior to the photocatalytic testing experiments, we studied the
photophysical properties of conventional bulk g-C3N4, as-
prepared CNF-Cl, pristine BiOI, and BiOI/CNF-Cl nano-
composites using UV–vis spectroscopy (collected in diffuse
reflectance mode), steady state photoluminescence (SSPL)
spectroscopy and time resolved photoluminescence (TRPL).
Figure 4a shows the UV–vis absorption spectra of these
materials. CNF-Cl shows an absorption band ∼270 nm due to
π→π*
transition that originates from the two-coordinated
nitrogen vacancy in the conjugated ring, while another band
around 380 nm with band tail extended up to 500 nm, is
believed to originate from the n→π*
transition, associated
with N lone pair (LP) electrons in triazine/heptazine rings
[60, 61]. The fluorine doping and chlorine intercalation in
CNF-Cl improves visible harvesting over conventional
g-C3N4 by generating a ca. 50 nm redshift in the optical
absorption. Despite the red-shifted absorption, the bandgap of
pristine CNF-Cl is still relatively large, while the composite
heterojunction with a small bandgap material, such as BiOI
improves the light harvesting capability to a significant extent.
Pristine BiOI exhibits a band-edge at ∼680 nm. Two
clear band edges, corresponding to CNF-Cl and BiOI, can
be seen from the plots of nanocomposite heterostructures
(figure 4(a)). Further, bandgaps of the as-prepared materials
were estimated using Tauc plots (figure 4(b)) through extra-
polation of the linear regions of graphs between (αhν)1/2
versus hν on abscissa where α is the absorption coefficient, h
is Planck’s constant and ν is the frequency of light. The
effective bandgaps of g-C3N4, pristine CNF-Cl, pristine BiOI,
10% BiOI/CNF-Cl, 25% BiOI/CNF-Cl, 50% BiOI/CNF-Cl
and 75% BiOI/CNF-Cl were found to be 2.46, 2.05 eV,
1.88 eV, 1.86 eV, 1.88 eV, 1.81 eV and 1.88 eV, respectively.
Figure 4. (a) UV–vis absorption spectra collected in diffuse reflectance spectroscopy (DRS) mode and (b) Tauc plots for the determination of
the effective optical bandgaps of pristine g-C3N4, pristine CNF-Cl, pristine BiOI and BiOI/CNF-Cl heterostructures. (c) Steady state
photoluminescence spectra (λexc=360 nm) and (d) time resolved photoluminescence spectra (λexc=405 nm) for conventional g-C3N4,
pristine CNF-Cl and BiOI/ CNF-Cl heterostructures.
5
Nanotechnology 31 (2020) 084001 K M Alam et al
7. The observed bandgap of g-C3N4 was slightly lower than
reported value for bulk g-C3N4 (2.7 eV) which might be due
to the use of dicyandiamide precursor instead of melamine
and programmed annealing which also affects the degree of
polymerization and stacking pattern modifying the bandgap
[62–65]. The reduction in the bandgap of fluorinated carbon
nitride, compared to g-C3N4 was attributed to the successful
doping of F into the C−N matrix, in contrast to counterion-
bounded F [22]. The pristine BiOI and BiOI/CNF-Cl com-
posites demonstrate almost similar bandgap (∼1.88 eV) sug-
gesting major absorption was dominated by BiOI. These
bandgap values clearly indicate the superior light absorption
capabilities of the nanocomposite heterostructures, compared
to pristine CNF-Cl.
SSPL spectra of the investigated pristine and composite
materials are presented in figure 4(c). CNF-Cl shows an
intense and broad SSPL peak centered at ∼500 nm, indicative
of a fast band-to-band radiative recombination process of
photogenerated electrons and holes. The emission peak of
CNF-Cl was red-shifted by ca. 50 nm from the emission peak
of bulk g-C3N4, and is similar to the absorption shift seen in
figure 4(c). The ∼50 nm redshifts in both the absorption and
emission spectra clearly demonstrate the distinct optoelec-
tronic properties of CNF-Cl in comparison to g-C3N4. The
SSPL signals for BiOI/CNF-Cl heterostructures were quen-
ched significantly compared to the pristine CNF-Cl, sug-
gesting efficient charge transfer between BiOI and CNF-Cl.
These results were similar to the previously reported works of
nanocomposite systems comprising BiOI and g-C3N4 [44, 45].
Notice that a significant blue-shift can be observed in the
emission spectra of the nanocomposites, relative to pristine
CNF-Cl. While no conclusive reason has been identified, elec-
tronic interactions between CNF-Cl and BiOI might be
responsible for this shift through affecting the relative strength of
the π→π*
and n→π*
transitions in the composite hetero-
structures. Time resolved photoluminescence (TRPL) spectra
were collected (figure 4(d)), and fitted to a tri-exponential decay
function of the form:
= + +t t t- - -I t A A Ae e e , 1t t t
1
1
2
2
3
3( ) ( )/ / /
where A1, A2 and A3 represent normalized percentage of
each decay components andτ1, τ2 andτ3 are the lifetimes of
each decay component, respectively. The presence of three
lifetime decay components in most of the fitted curves, is in
accord with previously reported TRPL data for carbon
nitride-based materials [66, 67]. Table 1 shows the lifetime
decay constants, average lifetime and curve-fitting parameters
for conventional g-C3N4, pristine CNF-Cl and BiOI/CNF-Cl
nanocomposites. The longest lifetime component in the emis-
sion decay g-C3N4 (14.83 ns in table 1) was extended to
61.73 ns in CNF-Cl together with a strong enhancement of its
relative intensity, which provides definitive evidence for the
suppression of non-radiative recombination pathways due to
chlorine intercalation-mediated increased separation between
adjacent stacked sheets. The PL decay data taken together with
the UV–vis and steady state PL spectra provides further evi-
dence for the distinct optoelectronic properties of CNF-Cl in
comparison to bulk g-C3N4.
The carbon nitride framework is constituted of tri-s-
triazine (heptazine; C6N7) units where sp2
hybridized carbon
and nitrogen overlap together to give σ and π bonding and σ*
and π*
antibonding molecular orbitals. Nonbonded LP elec-
trons on the secondary nitrogens (C–N=C) also contribute to
the delocalized π system which promotes formation of an
intermediate energy LP-π hybridized π molecular orbital (LP-π
hybridized MO) [67–71]. The observed first lifetime decay
component for g-C3N4 and CNF-Cl was attributed to direct
band-to-band radiative recombination (antibonding σ*
→ LP-π
hybridized MO transition and antibonding π*
→LP-π hybri-
dized MO) while the second decay component arises due to
nonradiative relaxation of σ*
electron to π*
orbital followed by
radiative recombination with LP-π hybridized MO (σ*
→π*
(non-radiative) and π*
→ LP-π hybridized MO (radiative)
[72–74]. The value of the first lifetime and its relative
contribution for g-C3N4 was found to be 0.715 ns and 0.765,
respectively, while for CNF-Cl, this value was determined to be
2.117 ns and 0.78 ns, respectively. The increased first lifetime of
CNF-Cl was attributed to the addition of F atom in heptazine
motif breaking the symmetry and transforming some sp2
hybridized carbons to sp3
carbon which increases the population
of relatively long lived σ*
→LP-π hybridized MO transitions
[22]. The second decay lifetime of CNF-Cl increased to 12.96 ns
in comparison to g-C3N4 (3.097 ns) which suggests that the
introduction of more electronegative F bonded to sp3
C in
heptazine motif promotes long lived σ*
→LP-π hybridized MO
transition via σ*
→π*
non-radiative relaxation mechanism.
Further, decrease in relative contribution (0.194) of the second
lifetime of CNF-Cl compared to g-C3N4 (0.321) supports the
theory that creation of sp3
defects via fluorine atom doping
reduces the contribution of direct π*
→ LP-π MO recombination
due to the reduced conjugation [75, 76]. Interestingly, the third
lifetime specific to inter/intra-sheet radiative recombination for
Table 1. Summary of fitted parameters obtained from time resolved photoluminescence plots shown in figure 6(b) for conventional g-C3N4,
pristine CNF-Cl and BiOI/CNF-Cl composites.
Photocatalyst τ1 (ns)/A1 τ2 (ns)/A2 τ3 (ns)/A3 Average lifetime τ (ns) Reduced χ2
Adj. R2
g-C3N4 0.715/0.765 3.097/0.321 14.827/0.059 8.84 1.91×10−5
0.999
CNF-Cl 2.117/0.780 12.960/0.194 61.726/0.023 23.06 2.90×10−7
0.999
10% BiOI/CNF-Cl 0.072/0.467 3.880/0.398 20.832/0.129 14.53 2.67×10−6
0.999
25% BiOI/CNF-Cl 0.153/0.441 2.867/0.441 16.626/0.114 10.89 7.75×10−7
0.999
50% BiOI/CNF-Cl 1.829/0.776 12.960/0.137 8.02 1.46×10−6
0.999
75% BiOI/CNF-Cl 1.934/0.801 12.900/0.150 8.02 5.40×10−7
0.999
6
Nanotechnology 31 (2020) 084001 K M Alam et al
8. CNF-Cl was found to be 61.726 ns which was much higher than
bulk g-C3N4 (14.827 ns). The increased third lifetime suggest
better inter-sheets charge migration due to Cl-intercalation which
provide interlayer galleries [19]. Further formation of few-
layered sheets also facilitate charge separation and reduce
recombination resulting in an increased lifetime [66, 67, 77, 78].
The average lifetime (τavg) is a good metric to quantita-
tively evaluate the effectiveness of electron–hole separation in
the photocatalysts and was determined using the following
equation:
t t t t
t t t
= + +
´ + +
A A A
A A A . 2
avg 1 1
2
2 2
2
3 3
2
1 1 2 2 3 3
( )
( ) ( )
/
The average lifetimes of CNF-Cl, 10% BiOI/CNF-Cl, 25%
BiOI/CNF-Cl, 50% BiOI/CNF-Cl, 75% BiOI/CNF-Cl were
found to be 23.06, 14.53, 10.89, 8.02 and 8.02 ns, respec-
tively. The lifetimes of both the longest and second-longest
PL decay components of CNF-Cl monotonically decreased in
the blends with BiOI with the effect saturating at a blend
concentration of 50% and higher. Average lifetimes are sig-
nificantly reduced for the nanocomposites compared to the
pristine CNF-Cl, consistent with the quenching of SSPL
intensity discussed earlier in this section. Moreover, the SSPL
quenching follows same trend with the TRPL lifetime decay,
with the maximum quenching and decay occur for the 50%
BiOI/CNF-Cl and 75% BiOI/CNF-Cl heterojunctions. The
obtained steady state and transient photoluminescence
spectroscopy results suggest better charge separation in the
nanocomposites compared to the constituent pristine
materials.
2.3. Photoelectrochemical activity test for water splitting
PEC tests were carried out using three electrode system
(photocatalyst deposited FTO as anode, Pt cathode and Ag/
AgCl reference electrode and Na2SO4). From figure 5(a), it
can be seen that the obtained photocurrent density for pristine
CNF-Cl was a mere 0.28 mA cm−2
at 1.23 V versus RHE
(water oxidation potential) which suggests fast intersheet
Figure 5. (a) Photocurrent of pristine CNF-Cl, pristine BiOI and BiOI/CNF-Cl composite catalysts under AM1.5 G one sun illumination;
(b)–(d) light on–off experiment for pristine CNF-Cl, pristine BiOI and 50% BiOI/CNF-Cl under AM1.5 G one sun illumination, 425 nm
(54.15 mW cm−2
) and 505 nm (40.48 mW cm−2
) LEDs, respectively.
7
Nanotechnology 31 (2020) 084001 K M Alam et al
9. recombination of photogenerated electron hole pairs. Further,
due to a bandgap of 2.05 eV, the CNF-Cl absorbs only the
high energy fraction of the visible spectrum to generate
electron hole pairs. Similarly, the obtained value of photo-
current density for pristine BiOI was found to be a mere
0.40 mA cm−2
. However, after formation of heterojunction
with CNF-Cl, the photoresponse of the resulting BiOI/CNF-
Cl hybrid increased dramatically figure 8(a)). Among various
wt% of BiOI (10% BiOI/CNF-Cl, 25% BiOI/CNF-Cl, 50%
BiOI/CNF-Cl and 75% BiOI/CNF-Cl) the nanocomposite
composed of 50% BiOI and 50% CNF-Cl displayed the
highest photocurrent density (1.28 mA cm−2
). The photo-
current densities for 10% BiOI/CNF-Cl, 25% BiOI/CNF-Cl
and 75% BiOI/CNF-Cl heterostructures were measured to be
0.34, 0.95 and 1.01 mA cm−2
, respectively. The improved
PEC performance of BiOI/CNF-Cl hybrid structure clearly
demonstrates improved charge separation due to formation of
heterojunction and better charge transport on the surface of
conjugated carbon nitride sheets. To validate the true origin of
photocurrent and instantaneous photoresponse of materials,
photocurrent was measured during light On–Off cycles which
clearly displayed rise and drop in photocurrent in On–Off
cycles (figure 5(b)).
To discern the PEC performance of the synthesized
materials at visible wavelengths, the photocurrent response of
materials during On–Off cycles was measured using mono-
chromatic 425 nm and 505 nm wavelength LEDs having a
power density of 54.15 and 40.48 mW cm−2
, respectively, at
the surface of photoanode (figures 5(c) and (d)). In compar-
ison to pristine CNF-Cl and BiOI, the photocurrent response
of heterostructured BiOI/CNF-Cl was much higher which
demonstrates synergistic charge separation due to the for-
mation of a heterojunction. The photocurrent densities for
pristine CNF-Cl, BiOI and heterostructure BiOI/CNF-Cl
under 425 nm monochromatic irradiation were obtained to be
0.08, 0.12 and 0.24 mA cm−2
, respectively (figure 8(c)).
Under 505 nm irradiation the photocurrent densities for CNF-
Cl, BiOI and heterostructure BiOI/CNF-Cl were found to be
0.04, 0.06 and 0.18 mA cm−2
, respectively (figure 5(d)).
505 nm photons are primarily absorbed by BiOI (see UV–vis
spectra in figure 4(a)). Under illumination by the 505 nm
LED, the pristine BiOI photoanode exhibits a slow rise in
photocurrent during each On cycle (red curve in figure 5(d))
which is characteristic of the presence of a large number of
carrier traps; the photocurrent rises as traps are filled by
photogenerated carriers. On the other hand, the BiOI/CNF-Cl
photoanode displayed intense spikes instantaneously during
light on cycles which decreased and reached a steady state.
The spike in photocurrent is due to the instantaneous move-
ment of a large number of photogenerated charge carriers
because of fast charge separation in the BiOI/CNF-Cl het-
erojunction. We attribute the near-exponential decrease in
current to reach steady state during each on cycle to trap
mediated recombination in BiOI. The EIS Nyquist plot
revealed a smaller semicircle arc for the 50% BiOI/CNF-Cl
in comparison to pristine films and other blends evidencing a
lower charge transfer resistance at semiconductor-electrolyte
interface for this composition. Further, the calculated
recombination lifetime values were shorter for BiOI/CNFCl
heterojunctions suggesting lower resistance for hole transfer
to electrolyte and better charge separation within the hetero-
structures (see section 4 in the supplementary information,
figures S2–S4 and table S1, available online at stacks.iop.org/
NANO/31/084001/mmedia). The positive slope in Mott–
Schottky plot (C−2
−V curves) shows n-type conduction of
the materials and the calculated flat band positions VFB( ) of
CNF-Cl, BiOI, 25% BiOI/CNF-Cl, and 50% BiOI/CNF-Cl
were calculated to be −0.461, −0.418, −0.521 and −0.675
versus NHE scale at pH-0 (see supplementary data; section
4.0, figures S2 and S4).
The light harvesting performance and efficiency of sub-
sequent transformation into electrical energy were calculated
from the diagnostic efficiencies under applied bias and irra-
diation conditions (figure 6) [79, 80]. The applied bias pho-
ton-to-current efficiencies (ABPEs) of pristine CNF-Cl and
BiOI were found to be 0.08% and 0.11%, respectively (figure
S5 in the supplementary data), while ABPE% of 10% BiOI/
CNF-Cl, 25% BiOI/CNF-Cl, 50% BiOI/CNF-Cl and 75%
BiOI/CNF-Cl heterostructures were calculated to be 0.09%,
0.31%, 0.71%, and 0.34%, respectively. The maximum
Figure 6. (a) IPCE% and APCE% of CNF-Cl, BiOI and BiOI/CNF-Cl heterojunction. (b) Graph of actually observed H2 evolution rate
determined using GC, theoretically calculated evolution rates from photocurrent and corresponding Faradaic efficiencies.
8
Nanotechnology 31 (2020) 084001 K M Alam et al
10. ABPE% was found for 50% BiOI/CNF-Cl which was 8.75
times higher than pristine CNF-Cl and 6.45 times higher than
pristine BiOI, respectively. The incident photon-to-current
efficiency (IPCE) or external quantum efficiencies (EQEs) of
CNF-Cl, BiOI and 50% BiOI/CNF-Cl were calculated to be
0.43%, 0.64% and 1.29% at 425 nm and 0.24%, 0.36% and
1.09% for 505 nm irradiation (figure 5(a)). Furthermore,
absorbed photon-to-current efficiency (APCE) or internal
quantum efficiency for CNF-Cl, BiOI and BiOI/CNF-Cl
under 425 nm irradiation was calculated to be 0.53, 0.73 and
1.61 while under 505 nm these values was found 0.49%,
0.42% and 1.54%, respectively (figure 6(a)). The similarity of
the EQE and APCE values merely confirms almost complete
light harvesting at 425 and 505 nm wavelengths due to the
strong visible absorption of the BiOI/CNF-Cl composites.
The EQE values measured under 425 and 505 nm LED illu-
mination are low despite excellent photon harvesting
(figure 4(a)), superior charge separation (figures 4(c), (d)
and S2–S4) and enhanced recombination suppression
(section 2.5) in the BiOI/CNF-Cl heterojunctions. We attri-
bute the low PEC quantum yields to the large exciting binding
energy (Eb) of 0.20–0.35 eV (∼10 times thermal energy at
room temperature) in heptazine motif carbon nitrides [81, 82].
As previously mentioned, fluorination of carbon nitride
introduces structural distortions that reduce the conjugation
length and further increase Eb [83]. The large values of Eb
mean that only a small fraction of the photogenerated excitons
are dissociated into free carriers.
2.4. Photoelectrochemical H2 evolution measurement
A three-electrode H-cell containing the deposited materials as
the photoanode, Pt cathode and Ag/AgCl reference electrode
was used for the measurement of evolved H2 (figure S6). The
evolved hydrogen at the Pt counter electrode was analyzed
using a gas chromatograph equipped with a pulse discharge
ionization detector (figure S7). The observed H2 evolution rates
using CNF-Cl, BiOI and BiOI/CNF-Cl as a photoelec-
trocatalyst were found to be 0.71, 4.35 and 19.71 μmol h−1
,
respectively. The theoretical H2 evolution rates, based on
photocurrent densities for CNF-Cl, BiOI and BiOI/CNF-Cl
were calculated to be 5.22, 7.46 and 23.80 μmol h−1
, respec-
tively. From these values, the Faradaic efficiencies for CNF-Cl,
BiOI and BiOI/CNF-Cl were calculated to be 13.51, 58.29 and
82.83 (percent), respectively (figure 6(b) and table 2). Water
splitting occurs at higher applied bias which also promotes side
reaction. In such conditions, due to lower reduction potential of
SO4
2−
ions (+0.172 V versus NHE at pH-0) of Na2SO4, it
competes with proton reduction resulting in poor Faradaic
efficiency. PEC performance and subsequent characterization
of reused BiOI/CNF-Cl showed almost identical photoactivity
and structural features as freshly prepared materials, indicating
demonstrate resilience and durability of the materials (see
supplementary data; section 5.0 figures S8–S10).
2.5. Photo-Kelvin probe force microscopy (KPFM)
KPFM has been used for simultaneous structural and elec-
tronic property mapping of photovoltaic and photocatalytic
materials [84, 85]. Herein, we performed KPFM to image
carrier photogeneration (figure 7). Surface potential, which is
the difference between contact potentials of the tip and
semiconductor surface, was determined both in the dark and
under illumination (425 nm LED). The difference between
these two surface potentials can be termed as the surface
photopotential. Upon illumination, the quasi-Fermi level of a
n-type semiconductor rises (w.r.t Evac) and thus the difference
between the tip work function and semiconductor surface
work function increases, compared to the difference in dark
condition.
By observing the surface photopotential of different
semiconductors, we can quantitatively analyze the photo-
generated charge carriers and study the comparative photo-
response. KPFM data was acquired for pristine CNF-Cl,
pristine BiOI and 50% BiOI/CNF-Cl. Surface topographic
images, surface potential maps and surface potential dis-
tributions for all the photocatalysts are shown in figure 7.
Dark and illumination conditions are clearly visible in the
surface potential mapping and distribution plots. The com-
parative analysis shows that the nanocomposite photocatalyst
50% BiOI/CNF-Cl has the largest surface photopotential of
207 mV, while pristine CNF-Cl and BiOI have surface pho-
topotential values of 83 mV and 98 mV, respectively. These
KPFM data indicates the highest charge carrier (electron)
separation occurs in BiOI/CNF-Cl heterostructure compared
to the pristine materials. The order of the negative shift of
surface potentials for these three materials nicely correlates
with their obtained photocurrents ((figure 8). Therefore, the
KPFM data corroborates the view of enhanced charge
separation in the heterostructured photocatalyst, discussed
earlier.
2.6. Photoresponse enhancement mechanism
To explain the improved photocatalytic performance a plau-
sible mechanism was proposed based on electronic band
structure and existing literature (figure 8) [46, 86–88]. The
process of overall water splitting includes two half reactions
(1) hydrogen evolution reaction at the cathode by reduction of
protons to hydrogen and (2) oxygen evolution reaction by
oxidation of water to oxygen at anode. To achieve efficient
Table 2. Photoelectrochemical H2 evolution rate determined
experimentally using GC, theoretically calculated evolution rates
from photocurrent, and corresponding Faradaic efficiencies.
Sample
Experimentally
evolved H2
(μmol h−1
cm−2
)
Theoretically
calculated H2
(μmol h−1
cm−2
)
Faradaic
efficiency
(%)
CNF-Cl 0.71 5.22 13.51
BiOI 4.35 7.46 58.29
BiOI/CNF-Cl 19.71 23.80 82.83
9
Nanotechnology 31 (2020) 084001 K M Alam et al
11. water splitting, the conduction band (ECB) position of the
semiconductor should be more negative than 0.00 eV versus
NHE (reduction potential of proton, H+
/H2) and valence
band (EVB) should be more positive than +1.23 eV versus
NHE (oxidation potential of water (H2O/O2). These
requirements suggest that the bandgap of materials should be
higher than 1.23 eV [14, 83, 89]. However, wide bandgap
compromises visible light absorption and results in a poor
light harvesting efficiency. The optical bandgap of CNF-Cl
was found to be 2.05 eV and the ECB and EVB positions were
calculated to be −0.461 and +1.589 V versus NHE. For
BiOI, the bandgap value was 1.82 eV and respective con-
duction (ECB) and valence (EVB) band positions was found to
be −0.418 and +1.402 V versus NHE. The bandgaps and
Figure 7. (a)–(c) Surface topographic AFM images of pristine CNF-Cl, pristine BiOI and 50% BiOI/CNF-Cl, respectively; (d)–(f) KPFM
surface potential mappings under light (LED 425 nm) (yellow region) and in the dark (purple region) of the samples corresponding to (a)–(c);
(g), (i) and (k) AFM tip scan in dark and (h), (j) and (l) AFM tip scan under light (425 nm LED) for the samples corresponding to (a)–(c);
(m)–(o) surface potential distribution in the dark and under illumination condition (LED 425 nm) for pristine CNF-Cl, pristine BiOI and 50%
BiOI/CNF-Cl, respectively.
Figure 8. Proposed mechanism of charge separation in BiOI/CNF-Cl composite photoanodes.
10
Nanotechnology 31 (2020) 084001 K M Alam et al
12. band edge positions of CNF-Cl and BiOI demonstrate that
both materials individually fulfill the requirement of water
splitting. However, observed photocurrents for pristine CNF-
Cl and BiOI were extremely low which can be explained by
fast band-to-band charge recombination in BiOI and inter-
layer charge recombination in CNF-Cl. After the formation of
heterojunction between BiOI and CNF-Cl, the PEC perfor-
mance of BiOI/CNF-Cl was dramatically enhanced. The
improved PEC performance of BiOI/CNF-Cl nanostructures
can be explained on the basis of Fermi level alignment which
leads to formation of a Type II (staggered) n–n heterojunction
between BiOI and CNF-Cl [90–92]. CNF-Cl displayed strong
n-type characteristics in Mott Schottky measurement which
implies its Fermi level lies just below the conduction band.
BiOI displayed weak n-type character implying an inter-
mediate position of Fermi level between ECB and EVB. Het-
erostructure formation of BiOI with CNF-Cl aligns the Fermi
levels in the two materials, and is accompanied by a built-in
field at the interface between the two semiconductors [93].
During Fermi level alignment, the conduction band of BiOI
bends downward while conduction band of CNF-Cl experi-
ences upward band bending as depicted in figure 8 [94].
3. Conclusion
We have demonstrated the synthesis of few-layered fluorine-
doped and chlorine-intercalated carbon nitride nanosheets
(CNF-Cl) with a reduced bandgap. CNF-Cl also exhibits a
red-shifted emission spectrum and substantially longer PL
lifetime components in comparison to g-C3N4. The likely
origin of these lifetime components in the emission decays of
g-C3N4 and CNF-Cl were explained. These newly synthe-
sized F-doped and Cl-intercalated carbon nitride nanosheets
were used to develop heterostructured photocatalysts with
earth abundant BiOI. BiOI was found to exhibit weak n-type
conduction while CNF-Cl was found to be a strongly n-type
semiconductor. A broad swathe of characterization data
including steady state and time-resolved PL spectra, Mott–
Schottky plots. KPFM data and photoelectrochemical J–V
curves are consistent with enhanced charge separation in
BiOI/CNF-Cl hybrids due to the formation of a type-II n–n
heterojunction. The enhanced charge separation and extended
light harvesting (band-edge of 680 nm) results in significantly
increased photoelectrochemical performance compared to the
stand-alone constituent materials with photocurrent densities
as high as 1.28 mA cm−2
observed under AM1.5 G one sun
illumination. The energy conversion efficiency metric known
as ABPE% for the top performing heterojunction (50% BiOI/
CNF-Cl) was 8.75 times higher than pristine CNF-Cl and
6.45 times higher than pristine BiOI. Reusability tests and
characterization of used photoanodes demonstrated the
required robustness and are indicative of long-term opera-
tional stability. Furthermore, unusual electronic interactions
between BiOI and CNF-Cl were observed that manifested as a
significant blue-shift in the steady-state emission spectra of
the BiOI/CNF-Cl heterostructures but with little or no effect
in the absorption spectra.
Acknowledgments
The authors thank NSERC, NRC, CMC Microsystems,
Future Energy Systems and CFI for direct and indirect
(equipment use) financial support. UKT acknowledges
scholarship support from Alberta Innovates. The conduct of
this work used several routine characterization tools at the
University of Alberta Nanofab (a fee-based facility). Pro-
fessor Thomas Thundat is kindly acknowledged for allowing
the use of the KPFM facility in his lab and Professor Alki-
viathes Meldrum is thanked for allowing the use of the time-
resolved PL facility in his lab.
ORCID iDs
Pawan Kumar https://orcid.org/0000-0003-2804-9298
Karthik Shankar https://orcid.org/0000-0001-7347-3333
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