Cu/TiO2 is a well-known photocatalyst for the photocatalytic transformation of CO2 into methane. The formation of C2+ products such as ethane and ethanol rather than methane is more interesting due to their higher energy density and economic value, but the formation of C–C bonds is currently a major challenge in CO2 photoreduction. In this context, we report the dominant formation of a C2 product, namely, ethane, from the gas-phase photoreduction of CO2 using TiO2 nanotube arrays (TNTAs) decorated with large-sized (80–200 nm) Ag and Cu nanoparticles without the use of a sacrificial agent or hole scavenger. Isotope-labeled mass spectrometry was used to verify the origin and identity of the reaction products. Under 2 h AM1.5G 1-sun illumination, the total rate of hydrocarbon production (methane + ethane) was highest for AgCu-TNTA with a total CxH2x+2 rate of 23.88 μmol g–1 h–1. Under identical conditions, the CxH2x+2 production rates for Ag-TNTA and Cu-TNTA were 6.54 and 1.39 μmol g–1 h–1, respectively. The ethane selectivity was the highest for AgCu-TNTA with 60.7%, while the ethane selectivity was found to be 15.9 and 10% for the Ag-TNTA and Cu-TNTA, respectively. Adjacent adsorption sites in our photocatalyst develop an asymmetric charge distribution due to quadrupole resonances in large metal nanoparticles and multipole resonances in Ag–Cu heterodimers. Such an asymmetric charge distribution decreases adsorbate–adsorbate repulsion and facilitates C–C coupling of reaction intermediates, which otherwise occurs poorly in TNTAs decorated with small metal nanoparticles.
The threat of global warming is high due to the extensive use of fossil fuels.Using non-renewable resources is a viable solution. Sunlight can be converted in two ways - into electrical energy and into chemical energy. Water splitting and CO2 are two important methods which can be used in solar cells.
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%.
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.
Renewable Fuels by Photocatalytic Reduction of carbondioxide (CO2); (Artifici...SAAD ARIF
This presentation contains the enhancement of photocatalytic Titania (TiO2) by Graphene, their synthesis method by solution mixing or in-situ growth and also the application for carbondioxide (CO2) reduction for renewable fuel using solar energy.
Bicrystalline Titania Photocatalyst for Reduction of CO2 to Solar FuelsA'Lester Allen
Degussa P25, a mixture of anatase and rutile crystal structures, is the most commonly used precursor to form the photoactive layer in solar cells; however, the photocatalytic activity of rutile is inferior to brookite. This presentation discusses the enhancement in photocatalytic activity of an antase brookite mixture.
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 threat of global warming is high due to the extensive use of fossil fuels.Using non-renewable resources is a viable solution. Sunlight can be converted in two ways - into electrical energy and into chemical energy. Water splitting and CO2 are two important methods which can be used in solar cells.
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%.
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.
Renewable Fuels by Photocatalytic Reduction of carbondioxide (CO2); (Artifici...SAAD ARIF
This presentation contains the enhancement of photocatalytic Titania (TiO2) by Graphene, their synthesis method by solution mixing or in-situ growth and also the application for carbondioxide (CO2) reduction for renewable fuel using solar energy.
Bicrystalline Titania Photocatalyst for Reduction of CO2 to Solar FuelsA'Lester Allen
Degussa P25, a mixture of anatase and rutile crystal structures, is the most commonly used precursor to form the photoactive layer in solar cells; however, the photocatalytic activity of rutile is inferior to brookite. This presentation discusses the enhancement in photocatalytic activity of an antase brookite mixture.
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
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.
TiO2-HfN Radial Nano-Heterojunction: A Hot Carrier Photoanode for Sunlight-Dr...Pawan Kumar
The lack of active, stable, earth-abundant, and visible-light absorbing materials to replace
plasmonic noble metals is a critical obstacle for researchers in developing highly efficient and costeffective photocatalytic systems. Herein, a core–shell nanotube catalyst was fabricated consisting of
atomic layer deposited HfN shell and anodic TiO2 support layer with full-visible regime photoactivity
for photoelectrochemical water splitting. The HfN active layer has two unique characteristics: (1) a
large bandgap between optical and acoustic phonon modes (2) and no electronic bandgap, which
allows a large population of long life-time hot carriers, which are used to enhance the photoelectrochemical performance. The photocurrent density (≈2.5 mA·cm−2 at 1 V vs. Ag/AgCl) obtained in
this study under AM 1.5G 1 Sun illumination is unprecedented, as it is superior to most existing
plasmonic noble metal-decorated catalysts and surprisingly indicates a photocurrent response that
extends to 730 nm. The result demonstrates the far-reaching application potential of replacing active
HER/HOR noble metals such as Au, Ag, Pt, Pd, etc. with low-cost plasmonic ceramics.
Noble Metal Free, Visible Light Driven Photocatalysis Using TiO2 Nanotube Arr...Pawan Kumar
Bulk g‐C3N4 is an earth‐abundant, easily synthesizable, and exceptionally stable photocatalyst with an electronic bandgap of 2.7 eV. Herein, the concepts of P‐doping and size quantization are combined to synthesize highly fluorescent P‐doped carbon nitride quantum dots (CNPQDs) with a bandgap of 2.1 eV. CNPQDs are hosted on anatase‐phase and rutile‐phase TiO2 nanotube array scaffolds, and examined as photoanodes for sunlight‐driven water‐splitting and as photocatalysts for surface catalytic reactions. Square‐shaped rutile phase TiO2 nanotube arrays (STNAs) decorated with CNPQDs (CNPQD‐STNA) generate 2.54 mA cm−2 photocurrent under AM1.5 G simulated sunlight. A champion hydrogen evolution rate of 22 µmol h−1 corresponds to a Faradaic efficiency of 93.2%. In conjunction with Ag nanoparticles (NPs), the CNPQD‐STNA hybrid is also found to be an excellent plexcitonic photocatalyst …
Noble Metal Free, Visible Light Driven Photocatalysis Using TiO2 Nanotube Arr...Pawan Kumar
Bulk g‐C3N4 is an earth‐abundant, easily synthesizable, and exceptionally stable photocatalyst with an electronic bandgap of 2.7 eV. Herein, the concepts of P‐doping and size quantization are combined to synthesize highly fluorescent P‐doped carbon nitride quantum dots (CNPQDs) with a bandgap of 2.1 eV. CNPQDs are hosted on anatase‐phase and rutile‐phase TiO2 nanotube array scaffolds, and examined as photoanodes for sunlight‐driven water‐splitting and as photocatalysts for surface catalytic reactions. Square‐shaped rutile phase TiO2 nanotube arrays (STNAs) decorated with CNPQDs (CNPQD‐STNA) generate 2.54 mA cm−2 photocurrent under AM1.5 G simulated sunlight. A champion hydrogen evolution rate of 22 µmol h−1 corresponds to a Faradaic efficiency of 93.2%. In conjunction with Ag nanoparticles (NPs), the CNPQD‐STNA hybrid is also found to be an excellent plexcitonic photocatalyst for the visible light‐driven transformation of 4‐nitrobenzenethiol (4‐NBT) to dimercaptoazobenzene (DMAB), producing reaction completion at a laser power of 1 mW (532 nm) while Ag NP/TNA and Ag NP/STNA photocatalysts cannot complete this transformation even at 10 mW laser power. The results point the way forward for photochemically robust, noble metal free, visible light harvesting photoacatalysts based on nanostructured heterojunctions of graphenic frameworks with TiO2.
Heterostructured nanocomposite tin phthalocyanine@mesoporous ceria (SnPc@CeO2...Pawan Kumar
Heterostructured tin phthalocyanine supported to mesoporous ceria was synthesized and used a
photocatalyst for CO2 reduction under visible light. The photoreduction CO2 activities of the
heterostructures were investigated in the presence of triethylamine as sacrificial agent. The developed
photocatalyst exhibited high catalytic activity for photoreduction of CO2 and after 24 hours of visible
light irradiation 2342 mmol g1 cat of methanol (fMeOH ¼ 0.0223 or 2.23%) and 840 mmol g1 cat of CO
(fCO ¼ 0.0026 or 0.26%) were obtained as the major reaction products. The methanol formation rate
(RMeOH) and CO formation rate (RCO) was found to be 97.5 mmol h1 g1 cat and 35.0 mmol h1 g1 cat
respectively. While under the identical experimental conditions mesoporous ceria (meso-CeO2) gave
only 316 mmol g1 cat of methanol (fMeOH ¼ 0.003 or 0.30%) and 126 mmol g1 cat CO (fCO ¼ 0.0004
or 0.04%) with product formation rate RMeOH ¼ 13.2 mmol h1 g1 cat and RCO ¼ 5.3 mmol h1 g1 cat.
Furthermore, the recovered catalyst showed consistent catalytic activity for at least five runs without any
significant loss in product yields
Maiyalagan,Performance of carbon nanofiber supported pd ni catalysts for elec...kutty79
Carbon nanofibers (CNF) supported Pd–Ni nanoparticles have been prepared by chemical reduction
with NaBH4 as a reducing agent. The Pd–Ni/CNF catalysts were characterized by X-ray diffraction
(XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical
voltammetry analysis. TEM showed that the Pd–Ni particles were quite uniformly distributed on the
surface of the carbon nanofiber with an average particle size of 4.0 nm. The electro-catalytic activity of
the Pd–Ni/CNF for oxidation of ethanol was examined by cyclic voltammetry (CV). The onset potential
was 200mV lower and the peak current density four times higher for ethanol oxidation for Pd–Ni/CNF
compared to that for Pd/C. The effect of an increase in temperature from 20 to 60 ◦C had a great effect on
increasing the ethanol oxidation activity
Photo-assisted oxidation of thiols to disulfides using cobalt ‘‘Nanorust’’ un...Pawan Kumar
Heterogeneous ‘‘Nanorust’’ containing cobalt oxide has been developed for the visible light assisted
oxidation of thiols to disulfides using molecular oxygen as an oxidant under alkaline free conditions and
therefore more environmentally friendly. Pyrolysis of heterogenized tetrasulfonated cobalt(II) phthalocyanine
(CoPcS) supported on mesoporous ceria (CeO2) transforms it into a novel heterogeneous ‘‘Nanorust’’
containing CoOx-C,N@CeO2 which exhibited higher catalytic activity than the homogeneous CoPcS as well
as the ceria immobilized CoPcS catalyst. Importantly, these catalysts could easily be recovered and recycled
for several runs, which makes the process greener and cost-effective.
Flexible and Ultrasoft Inorganic 1D Semiconductor and Heterostructure Systems...Pawan Kumar
Low dimensionality and high flexibility are key demands for flexible electronic semiconductor devices. SnIP, the first atomic-scale double helical semiconductor combines structural anisotropy and robustness with exceptional electronic properties. The benefit of the double helix, combined with a diverse structure on the nanoscale, ranging from strong covalent bonding to weak van der Waals interactions, and the large structure and property anisotropy offer substantial potential for applications in energy conversion and water splitting. It represents the next logical step in downscaling the inorganic semiconductors from classical 3D systems, via 2D semiconductors like MXenes or transition metal dichalcogenides, to the first downsizeable, polymer-like atomic-scale 1D semiconductor SnIP. SnIP shows intriguing mechanical properties featuring a bulk modulus three times lower than any IV, III-V, or II-VI semiconductor. In situ bending tests substantiate that pure SnIP fibers can be bent without an effect on their bonding properties. Organic and inorganic hybrids are prepared illustrating that SnIP is a candidate to fabricate flexible 1D composites for energy conversion and water splitting applications. SnIP@C3N4 hybrid forms an unusual soft material core–shell topology with graphenic carbon nitride wrapping around SnIP. A 1D van der Waals heterostructure is formed capable of performing effective water splitting.
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.
Nanostructured composite materials for CO2 activationPawan Kumar
The increasing energy crisis and the worsening global climate caused by the excessive
utilization of the fossil fuel have boosted tremendous research about CO2 capture, storage and
utilization. Among these approaches, utilization of carbon dioxide to produce valuable chemicals
is preferred than dumping it. Particularly, utilization of CO2 as feedstock for the photocatalytic
conversion into valuable products is a viable approach for harvesting solar radiation as an energy
source and to mitigate increasing CO2 concentration. Artificial photosynthesis by using
nanostructured materials as photocatalyst has immense potential to convert carbon dioxide into
renewable fuels such as methanol/CO etc. The present chapter focuses on the synthesis, characterization and application of various nanostructured materials for CO2 activation including
photoreduction of CO2 to valuable products.
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.
Arrays of TiO2 nanorods embedded with fluorine doped carbon nitride quantum d...Pawan Kumar
Graphenic semiconductors such as carbon nitride are attracting increasing attention as photocatalysts due to their chemical stability, visible light absorption and excellent electronic properties. The photocatalytic activity of nanostructured TiO2 catalysts is constrained by the wide bandgap and concomitant low visible light responsivity of TiO2. In this context we present the formation of new fluorine doped carbon nitride quantum dots (CNFQDs) by solid state reaction and the subsequent examination of their heterojunctions with TiO2 for photoelectrochemical water splitting. Arrays of rutile phase TiO2 nanorods embedded with CNFQDs were synthesized by a simple in situ hydrothermal approach and the resulting nanomaterials were found to exhibit strong visible light absorption. The energetics at the heterojunction were favorable for efficient electron transfer from CNFQDs to TiO2 under visible light irradiation and …
Arrays of TiO2 nanorods embedded with fluorine doped carbon nitride quantum d...Pawan Kumar
Graphenic semiconductors such as carbon nitride are attracting increasing attention as photocatalysts due to their chemical stability, visible light absorption and excellent electronic properties. The photocatalytic activity of nanostructured TiO2 catalysts is constrained by the wide bandgap and concomitant low visible light responsivity of TiO2. In this context we present the formation of new fluorine doped carbon nitride quantum dots (CNFQDs) by solid state reaction and the subsequent examination of their heterojunctions with TiO2 for photoelectrochemical water splitting. Arrays of rutile phase TiO2 nanorods embedded with CNFQDs were synthesized by a simple in situ hydrothermal approach and the resulting nanomaterials were found to exhibit strong visible light absorption. The energetics at the heterojunction were favorable for efficient electron transfer from CNFQDs to TiO2 under visible light irradiation and transfer of holes to the aqueous electrolyte. CNFQD-sensitized TiO2 nanorods exhibited a strong photoelectrochemical response up to 500 nm. Reuse experiments confirmed robustness and long term stability of the sample without exhausting the catalytic performance. The present work demonstrates a new pathway to sensitize TiO2 to visible photons by the in situ formation of embedded heterojunctions with fluorine doped carbon nitride quantum dots
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.
TiO2-HfN Radial Nano-Heterojunction: A Hot Carrier Photoanode for Sunlight-Dr...Pawan Kumar
The lack of active, stable, earth-abundant, and visible-light absorbing materials to replace
plasmonic noble metals is a critical obstacle for researchers in developing highly efficient and costeffective photocatalytic systems. Herein, a core–shell nanotube catalyst was fabricated consisting of
atomic layer deposited HfN shell and anodic TiO2 support layer with full-visible regime photoactivity
for photoelectrochemical water splitting. The HfN active layer has two unique characteristics: (1) a
large bandgap between optical and acoustic phonon modes (2) and no electronic bandgap, which
allows a large population of long life-time hot carriers, which are used to enhance the photoelectrochemical performance. The photocurrent density (≈2.5 mA·cm−2 at 1 V vs. Ag/AgCl) obtained in
this study under AM 1.5G 1 Sun illumination is unprecedented, as it is superior to most existing
plasmonic noble metal-decorated catalysts and surprisingly indicates a photocurrent response that
extends to 730 nm. The result demonstrates the far-reaching application potential of replacing active
HER/HOR noble metals such as Au, Ag, Pt, Pd, etc. with low-cost plasmonic ceramics.
Noble Metal Free, Visible Light Driven Photocatalysis Using TiO2 Nanotube Arr...Pawan Kumar
Bulk g‐C3N4 is an earth‐abundant, easily synthesizable, and exceptionally stable photocatalyst with an electronic bandgap of 2.7 eV. Herein, the concepts of P‐doping and size quantization are combined to synthesize highly fluorescent P‐doped carbon nitride quantum dots (CNPQDs) with a bandgap of 2.1 eV. CNPQDs are hosted on anatase‐phase and rutile‐phase TiO2 nanotube array scaffolds, and examined as photoanodes for sunlight‐driven water‐splitting and as photocatalysts for surface catalytic reactions. Square‐shaped rutile phase TiO2 nanotube arrays (STNAs) decorated with CNPQDs (CNPQD‐STNA) generate 2.54 mA cm−2 photocurrent under AM1.5 G simulated sunlight. A champion hydrogen evolution rate of 22 µmol h−1 corresponds to a Faradaic efficiency of 93.2%. In conjunction with Ag nanoparticles (NPs), the CNPQD‐STNA hybrid is also found to be an excellent plexcitonic photocatalyst …
Noble Metal Free, Visible Light Driven Photocatalysis Using TiO2 Nanotube Arr...Pawan Kumar
Bulk g‐C3N4 is an earth‐abundant, easily synthesizable, and exceptionally stable photocatalyst with an electronic bandgap of 2.7 eV. Herein, the concepts of P‐doping and size quantization are combined to synthesize highly fluorescent P‐doped carbon nitride quantum dots (CNPQDs) with a bandgap of 2.1 eV. CNPQDs are hosted on anatase‐phase and rutile‐phase TiO2 nanotube array scaffolds, and examined as photoanodes for sunlight‐driven water‐splitting and as photocatalysts for surface catalytic reactions. Square‐shaped rutile phase TiO2 nanotube arrays (STNAs) decorated with CNPQDs (CNPQD‐STNA) generate 2.54 mA cm−2 photocurrent under AM1.5 G simulated sunlight. A champion hydrogen evolution rate of 22 µmol h−1 corresponds to a Faradaic efficiency of 93.2%. In conjunction with Ag nanoparticles (NPs), the CNPQD‐STNA hybrid is also found to be an excellent plexcitonic photocatalyst for the visible light‐driven transformation of 4‐nitrobenzenethiol (4‐NBT) to dimercaptoazobenzene (DMAB), producing reaction completion at a laser power of 1 mW (532 nm) while Ag NP/TNA and Ag NP/STNA photocatalysts cannot complete this transformation even at 10 mW laser power. The results point the way forward for photochemically robust, noble metal free, visible light harvesting photoacatalysts based on nanostructured heterojunctions of graphenic frameworks with TiO2.
Heterostructured nanocomposite tin phthalocyanine@mesoporous ceria (SnPc@CeO2...Pawan Kumar
Heterostructured tin phthalocyanine supported to mesoporous ceria was synthesized and used a
photocatalyst for CO2 reduction under visible light. The photoreduction CO2 activities of the
heterostructures were investigated in the presence of triethylamine as sacrificial agent. The developed
photocatalyst exhibited high catalytic activity for photoreduction of CO2 and after 24 hours of visible
light irradiation 2342 mmol g1 cat of methanol (fMeOH ¼ 0.0223 or 2.23%) and 840 mmol g1 cat of CO
(fCO ¼ 0.0026 or 0.26%) were obtained as the major reaction products. The methanol formation rate
(RMeOH) and CO formation rate (RCO) was found to be 97.5 mmol h1 g1 cat and 35.0 mmol h1 g1 cat
respectively. While under the identical experimental conditions mesoporous ceria (meso-CeO2) gave
only 316 mmol g1 cat of methanol (fMeOH ¼ 0.003 or 0.30%) and 126 mmol g1 cat CO (fCO ¼ 0.0004
or 0.04%) with product formation rate RMeOH ¼ 13.2 mmol h1 g1 cat and RCO ¼ 5.3 mmol h1 g1 cat.
Furthermore, the recovered catalyst showed consistent catalytic activity for at least five runs without any
significant loss in product yields
Maiyalagan,Performance of carbon nanofiber supported pd ni catalysts for elec...kutty79
Carbon nanofibers (CNF) supported Pd–Ni nanoparticles have been prepared by chemical reduction
with NaBH4 as a reducing agent. The Pd–Ni/CNF catalysts were characterized by X-ray diffraction
(XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical
voltammetry analysis. TEM showed that the Pd–Ni particles were quite uniformly distributed on the
surface of the carbon nanofiber with an average particle size of 4.0 nm. The electro-catalytic activity of
the Pd–Ni/CNF for oxidation of ethanol was examined by cyclic voltammetry (CV). The onset potential
was 200mV lower and the peak current density four times higher for ethanol oxidation for Pd–Ni/CNF
compared to that for Pd/C. The effect of an increase in temperature from 20 to 60 ◦C had a great effect on
increasing the ethanol oxidation activity
Photo-assisted oxidation of thiols to disulfides using cobalt ‘‘Nanorust’’ un...Pawan Kumar
Heterogeneous ‘‘Nanorust’’ containing cobalt oxide has been developed for the visible light assisted
oxidation of thiols to disulfides using molecular oxygen as an oxidant under alkaline free conditions and
therefore more environmentally friendly. Pyrolysis of heterogenized tetrasulfonated cobalt(II) phthalocyanine
(CoPcS) supported on mesoporous ceria (CeO2) transforms it into a novel heterogeneous ‘‘Nanorust’’
containing CoOx-C,N@CeO2 which exhibited higher catalytic activity than the homogeneous CoPcS as well
as the ceria immobilized CoPcS catalyst. Importantly, these catalysts could easily be recovered and recycled
for several runs, which makes the process greener and cost-effective.
Flexible and Ultrasoft Inorganic 1D Semiconductor and Heterostructure Systems...Pawan Kumar
Low dimensionality and high flexibility are key demands for flexible electronic semiconductor devices. SnIP, the first atomic-scale double helical semiconductor combines structural anisotropy and robustness with exceptional electronic properties. The benefit of the double helix, combined with a diverse structure on the nanoscale, ranging from strong covalent bonding to weak van der Waals interactions, and the large structure and property anisotropy offer substantial potential for applications in energy conversion and water splitting. It represents the next logical step in downscaling the inorganic semiconductors from classical 3D systems, via 2D semiconductors like MXenes or transition metal dichalcogenides, to the first downsizeable, polymer-like atomic-scale 1D semiconductor SnIP. SnIP shows intriguing mechanical properties featuring a bulk modulus three times lower than any IV, III-V, or II-VI semiconductor. In situ bending tests substantiate that pure SnIP fibers can be bent without an effect on their bonding properties. Organic and inorganic hybrids are prepared illustrating that SnIP is a candidate to fabricate flexible 1D composites for energy conversion and water splitting applications. SnIP@C3N4 hybrid forms an unusual soft material core–shell topology with graphenic carbon nitride wrapping around SnIP. A 1D van der Waals heterostructure is formed capable of performing effective water splitting.
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.
Nanostructured composite materials for CO2 activationPawan Kumar
The increasing energy crisis and the worsening global climate caused by the excessive
utilization of the fossil fuel have boosted tremendous research about CO2 capture, storage and
utilization. Among these approaches, utilization of carbon dioxide to produce valuable chemicals
is preferred than dumping it. Particularly, utilization of CO2 as feedstock for the photocatalytic
conversion into valuable products is a viable approach for harvesting solar radiation as an energy
source and to mitigate increasing CO2 concentration. Artificial photosynthesis by using
nanostructured materials as photocatalyst has immense potential to convert carbon dioxide into
renewable fuels such as methanol/CO etc. The present chapter focuses on the synthesis, characterization and application of various nanostructured materials for CO2 activation including
photoreduction of CO2 to valuable products.
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.
Arrays of TiO2 nanorods embedded with fluorine doped carbon nitride quantum d...Pawan Kumar
Graphenic semiconductors such as carbon nitride are attracting increasing attention as photocatalysts due to their chemical stability, visible light absorption and excellent electronic properties. The photocatalytic activity of nanostructured TiO2 catalysts is constrained by the wide bandgap and concomitant low visible light responsivity of TiO2. In this context we present the formation of new fluorine doped carbon nitride quantum dots (CNFQDs) by solid state reaction and the subsequent examination of their heterojunctions with TiO2 for photoelectrochemical water splitting. Arrays of rutile phase TiO2 nanorods embedded with CNFQDs were synthesized by a simple in situ hydrothermal approach and the resulting nanomaterials were found to exhibit strong visible light absorption. The energetics at the heterojunction were favorable for efficient electron transfer from CNFQDs to TiO2 under visible light irradiation and …
Arrays of TiO2 nanorods embedded with fluorine doped carbon nitride quantum d...Pawan Kumar
Graphenic semiconductors such as carbon nitride are attracting increasing attention as photocatalysts due to their chemical stability, visible light absorption and excellent electronic properties. The photocatalytic activity of nanostructured TiO2 catalysts is constrained by the wide bandgap and concomitant low visible light responsivity of TiO2. In this context we present the formation of new fluorine doped carbon nitride quantum dots (CNFQDs) by solid state reaction and the subsequent examination of their heterojunctions with TiO2 for photoelectrochemical water splitting. Arrays of rutile phase TiO2 nanorods embedded with CNFQDs were synthesized by a simple in situ hydrothermal approach and the resulting nanomaterials were found to exhibit strong visible light absorption. The energetics at the heterojunction were favorable for efficient electron transfer from CNFQDs to TiO2 under visible light irradiation and transfer of holes to the aqueous electrolyte. CNFQD-sensitized TiO2 nanorods exhibited a strong photoelectrochemical response up to 500 nm. Reuse experiments confirmed robustness and long term stability of the sample without exhausting the catalytic performance. The present work demonstrates a new pathway to sensitize TiO2 to visible photons by the in situ formation of embedded heterojunctions with fluorine doped carbon nitride quantum dots
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.
Vapor Deposition of Semiconducting Phosphorus Allotropes into TiO2 Nanotube A...Pawan Kumar
Recent evidence of exponential environmental degradation will demand a drastic shift in research and development toward exploiting alternative energy resources such as solar energy. Here, we report the successful low-cost and easily accessible synthesis of hybrid semiconductor@TiO2 nanotube photocatalysts. In order to realize its maximum potential in harvesting photons in the visible-light range, TiO2 nanotubes have been loaded with earth-abundant, low-band-gap fibrous red and black phosphorus (P). Scanning electron microscopy– and scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron microscopy, and UV–vis measurements have been performed, substantiating the deposition of fibrous red and black P on top and inside the cavities of 100-μm-long electrochemically fabricated nanotubes. The nanotubular …
Effect of morphology on the photoelectrochemical performance of nanostructure...Pawan Kumar
Cu2O is a promising earth-abundant semiconductor photocathode for sunlight-driven water splitting. Characterization results are presented to show how the photocurrent density (Jph), onset potential (Eonset), band edges, carrier density (NA), and interfacial charge transfer resistance (Rct) are affected by the morphology and method used to deposit Cu2O on a copper foil. Mesoscopic and planar morphologies exhibit large differences in the values of NA and Rct. However, these differences are not observed to translate to other photocatalytic properties of Cu2O. Mesoscopic and planar morphologies exhibit similar bandgap (e.g.) and flat band potential (Efb) values of 1.93 ± 0.04 eV and 0.48 ± 0.06 eV respectively. Eonset of 0.48 ± 0.04 eV obtained for these systems is close to the Efb indicating negligible water reduction overpotential. Electrochemically deposited planar Cu2O provides the highest photocurrent density of 5.0 mA cm−2 at 0 V vs reversible hydrogen electrode (RHE) of all the morphologies studied. The photocurrent densities observed in this study are among the highest reported values for bare Cu2O photocathodes.
This presentation describes about recent progress in bringing down the cost of Hydrogen fuel cells. Around 3 papers were summarised and all of them belong to a timespan of 2012-2013.
tA highly efficient, recyclable and magnetically separable core-shell structured CuZnO@Fe3O4microspherewrapped with reduced graphene oxide (rGO@CuZnO@Fe3O4) photocatalyst has been developed and usedfor the photoreduction of carbon dioxide with water to produce methanol under visible light irradiation.Owing to the synergistic effect of the components and to the presence of a thin Fe2O3layer on Fe3O4,rGO@CuZnO@Fe3O44 exhibited higher catalytic activity as compared to the other possible combinationssuch as CuZnO@Fe3O42 and GO@CuZnO@Fe3O43 microspheres. The yield of methanol in case of using2 and 3 as photocatalyst was found to be 858 and 1749 mol g−1cat, respectively. However, the yieldwas increased to 2656 mol g−1cat when rGO@CuZnO@Fe3O44 was used as photocatalyst under sim-ilar experimental conditions. This superior photocatalytic activity of 4 was assumed to be due to therestoration of the sp2hybridized aromatic system in rGO, which facilitated the movement of electronsand resulted in better charge separation. The synthesized heterogeneous photocatalyst could readily berecovered by external magnet and successfully reused for six subsequent cycles without significant loss in the product yield.
A perovskite solar cell is a type of solar cell which includes a perovskite structured compound, most commonly a hybrid organic-inorganic lead or tin halide-based material, as the light-harvesting active layer.
Vapor growth of binary and ternary phosphorus-based semiconductors into TiO 2...Pawan Kumar
We report successful synthesis of low band gap inorganic polyphosphide and TiO2 heterostructures with the aid of short-way transport reactions. Binary and ternary polyphosphides (NaP7, SnIP, and (CuI)3P12) were successfully reacted and deposited into electrochemically fabricated TiO2 nanotubes. Employing vapor phase reaction deposition, the cavities of 100 μm long TiO2 nanotubes were infiltrated; approximately 50% of the nanotube arrays were estimated to be infiltrated in the case of NaP7. Intensive characterization of the hybrid materials with techniques including SEM, FIB, HR-TEM, Raman spectroscopy, XRD, and XPS proved the successful vapor phase deposition and synthesis of the substances on and inside the nanotubes. The polyphosphide@TiO2 hybrids exhibited superior water splitting performance compared to pristine materials and were found to be more active at higher wavelengths. SnIP …
Mixed‐Valence Single‐Atom Catalyst Derived from Functionalized GraphenePawan Kumar
Single‐atom catalysts (SACs) aim at bridging the gap between homogeneous and heterogeneous catalysis. The challenge is the development of materials with ligands enabling coordination of metal atoms in different valence states, and preventing leaching or nanoparticle formation. Graphene functionalized with nitrile groups (cyanographene) is herein employed for the robust coordination of Cu(II) ions, which are partially reduced to Cu(I) due to graphene‐induced charge transfer. Inspired by nature's selection of Cu(I) in enzymes for oxygen activation, this 2D mixed‐valence SAC performs flawlessly in two O2‐mediated reactions: the oxidative coupling of amines and the oxidation of benzylic CH bonds toward high‐value pharmaceutical synthons. High conversions (up to 98%), selectivities (up to 99%), and recyclability are attained with very low metal loadings in the reaction. The synergistic effect of Cu(II) and Cu …
Isolated Iridium Sites on Potassium-Doped Carbon-nitride wrapped Tellurium Na...Pawan Kumar
Many industrial processes such transesterification of fatty acid for biodiesel production, soap manufacturing and biosynthesis of ethanol generate glycerol as a major by-product that can be used to produce commodity chemicals. Photocatalytic transformation of glycerol is an enticing approach that can exclude the need of harsh oxidants and extraneous thermal energy. However, the product yield and selectivity remain poor due to low absorption and unsymmetrical site distribution on the catalyst surface. Herein, tellurium (Te) nanorods/nanosheets (TeNRs/NSs) wrapped potassium-doped carbon nitride (KCN) van der Waal (vdW) heterojunction (TeKCN) is designed to enhance charge separation and visible-NIR absorption. The iridium (Ir) single atom sites decoration on the TeKCN core-shell structure (TeKCNIr) promotes selective oxidation of glycerol to glyceraldehyde with a conversion of 45.6% and selectivity of 61.6% under AM1.5G irradiation. The catalytic selectivity can reach up to 88% under 450 nm monochromatic light. X-ray absorption spectroscopy (XAS) demonstrates the presence of undercoordinated IrN2O2 sites which improved catalytic selectivity for glycol oxidation. Band energies and computational calculations reveal faile charge transfer in the TeKCNIr heterostructure. EPR and scavenger tests discern that superoxide (O2•−) and hydroxyl (•OH) radicals are prime components driving glycerol oxidation.
Isolated Iridium Sites on Potassium-Doped Carbon-nitride wrapped Tellurium Na...Pawan Kumar
Many industrial processes such transesterification of fatty acid for biodiesel production, soap manufacturing and biosynthesis of ethanol generate glycerol as a major by-product that can be used to produce commodity chemicals. Photocatalytic transformation of glycerol is an enticing approach that can exclude the need of harsh oxidants and extraneous thermal energy. However, the product yield and selectivity remain poor due to low absorption and unsymmetrical site distribution on the catalyst surface. Herein, tellurium (Te) nanorods/nanosheets (TeNRs/NSs) wrapped potassium-doped carbon nitride (KCN) van der Waal (vdW) heterojunction (TeKCN) is designed to enhance charge separation and visible-NIR absorption. The iridium (Ir) single atom sites decoration on the TeKCN core-shell structure (TeKCNIr) promotes selective oxidation of glycerol to glyceraldehyde with a conversion of 45.6% and selectivity of 61.6% under AM1.5G irradiation. The catalytic selectivity can reach up to 88% under 450 nm monochromatic light. X-ray absorption spectroscopy (XAS) demonstrates the presence of undercoordinated IrN2O2 sites which improved catalytic selectivity for glycol oxidation. Band energies and computational calculations reveal faile charge transfer in the TeKCNIr heterostructure. EPR and scavenger tests discern that superoxide (O2•−) and hydroxyl (•OH) radicals are prime components driving glycerol oxidation.
Isolated Iridium Sites on Potassium-Doped Carbon-nitride wrapped Tellurium Na...Pawan Kumar
Many industrial processes such transesterification of fatty acid for biodiesel production, soap manufacturing and biosynthesis of ethanol generate glycerol as a major by-product that can be used to produce commodity chemicals. Photocatalytic transformation of glycerol is an enticing approach that can exclude the need of harsh oxidants and extraneous thermal energy. However, the product yield and selectivity remain poor due to low absorption and unsymmetrical site distribution on the catalyst surface. Herein, tellurium (Te) nanorods/nanosheets (TeNRs/NSs) wrapped potassium-doped carbon nitride (KCN) van der Waal (vdW) heterojunction (TeKCN) is designed to enhance charge separation and visible-NIR absorption. The iridium (Ir) single atom sites decoration on the TeKCN core-shell structure (TeKCNIr) promotes selective oxidation of glycerol to glyceraldehyde with a conversion of 45.6% and selectivity of 61.6% under AM1.5G irradiation. The catalytic selectivity can reach up to 88% under 450 nm monochromatic light. X-ray absorption spectroscopy (XAS) demonstrates the presence of undercoordinated IrN2O2 sites which improved catalytic selectivity for glycol oxidation. Band energies and computational calculations reveal faile charge transfer in the TeKCNIr heterostructure. EPR and scavenger tests discern that superoxide (O2•−) and hydroxyl (•OH) radicals are prime components driving glycerol oxidation.
Solar-Driven Cellulose Photorefining into Arabinose over Oxygen-Doped Carbon ...Pawan Kumar
Biomass photorefining is a promising strategy to address the energy crisis and transition toward carbon carbon-neutral society. Here, we demonstrate the feasibility of direct cellulose photorefining into arabinose by a rationally designed oxygen-doped polymeric carbon nitride, which generates favorable oxidative species (e.g., O2–, •OH) for selective oxidative reactions at neutral conditions. In addition, we also illustrate the mechanism of the photocatalytic cellulose to arabinose conversion by density functional theory calculations. The oxygen insertion derived from oxidative radicals at the C1 position of glucose within cellulose leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in the subsequent gluconic acid formation. The following decarboxylation process of gluconic acid via C1–C2 α-scissions, triggered by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This work not only offers a mechanistic understanding of cellulose photorefining to arabinose but also sets up an example for illuminating the path toward direct cellulose photorefining into value-added bioproducts under mild conditions.
Solar-Driven Cellulose Photorefining into Arabinose over Oxygen-Doped Carbon ...Pawan Kumar
Biomass photorefining is a promising strategy to address the energy crisis and transition toward carbon carbon-neutral society. Here, we demonstrate the feasibility of direct cellulose photorefining into arabinose by a rationally designed oxygen-doped polymeric carbon nitride, which generates favorable oxidative species (e.g., O2–, •OH) for selective oxidative reactions at neutral conditions. In addition, we also illustrate the mechanism of the photocatalytic cellulose to arabinose conversion by density functional theory calculations. The oxygen insertion derived from oxidative radicals at the C1 position of glucose within cellulose leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in the subsequent gluconic acid formation. The following decarboxylation process of gluconic acid via C1–C2 α-scissions, triggered by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This work not only offers a mechanistic understanding of cellulose photorefining to arabinose but also sets up an example for illuminating the path toward direct cellulose photorefining into value-added bioproducts under mild conditions.
Solar-Driven Cellulose Photorefining into Arabinose over Oxygen-Doped Carbon ...Pawan Kumar
Biomass photorefining is a promising strategy to address the energy crisis and transition toward carbon carbon-neutral society. Here, we demonstrate the feasibility of direct cellulose photorefining into arabinose by a rationally designed oxygen-doped polymeric carbon nitride, which generates favorable oxidative species (e.g., O2–, •OH) for selective oxidative reactions at neutral conditions. In addition, we also illustrate the mechanism of the photocatalytic cellulose to arabinose conversion by density functional theory calculations. The oxygen insertion derived from oxidative radicals at the C1 position of glucose within cellulose leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in the subsequent gluconic acid formation. The following decarboxylation process of gluconic acid via C1–C2 α-scissions, triggered by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This work not only offers a mechanistic understanding of cellulose photorefining to arabinose but also sets up an example for illuminating the path toward direct cellulose photorefining into value-added bioproducts under mild conditions.
Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single...Pawan Kumar
Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer–Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.
Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single...Pawan Kumar
Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer–Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.
Selective Cellobiose Photoreforming for Simultaneous Gluconic Acid and Syngas...Pawan Kumar
Here, we demonstrate the selective cellobiose (building block of cellulose) photoreforming for gluconic acid and syngas co-production in acidic conditions by rationally designing a bifunctional polymeric carbon nitride (CN) with potassium/sulfur co-dopant. This heteroatomic doped CN photocatalyst possesses enhanced visible light absorption, higher charge separation efficiency than pristine CN. Under acidic conditions, cellobiose is not only more efficiently hydrolyzed into glucose but also promotes the syngas and gluconic acid production. Density functional theory (DFT) calculations reveal the favorable generation of •O2− during the photocatalytic reaction, which is essential for gluconic acid production. Consequently, the fine-designed photocatalyst presents excellent cellobiose conversion (>80%) and gluconic acid selectivity (>70%) together with the co-production of syngas (~56 μmol g-1 h-1) under light illumination. The current work demonstrates the feasibility of biomass photoreforming with value-added chemicals and syngas co-production under mild condition.
Selective Cellobiose Photoreforming for Simultaneous Gluconic Acid and Syngas...Pawan Kumar
Here, we demonstrate the selective cellobiose (building block of cellulose) photoreforming for gluconic acid and syngas co-production in acidic conditions by rationally designing a bifunctional polymeric carbon nitride (CN) with potassium/sulfur co-dopant. This heteroatomic doped CN photocatalyst possesses enhanced visible light absorption, higher charge separation efficiency than pristine CN. Under acidic conditions, cellobiose is not only more efficiently hydrolyzed into glucose but also promotes the syngas and gluconic acid production. Density functional theory (DFT) calculations reveal the favorable generation of •O2− during the photocatalytic reaction, which is essential for gluconic acid production. Consequently, the fine-designed photocatalyst presents excellent cellobiose conversion (>80%) and gluconic acid selectivity (>70%) together with the co-production of syngas (~56 μmol g-1 h-1) under light illumination. The current work demonstrates the feasibility of biomass photoreforming with value-added chemicals and syngas co-production under mild condition.
Selective Cellobiose Photoreforming for Simultaneous Gluconic Acid and Syngas...Pawan Kumar
Here, we demonstrate the selective cellobiose (building block of cellulose) photoreforming for gluconic acid and syngas co-production in acidic conditions by rationally designing a bifunctional polymeric carbon nitride (CN) with potassium/sulfur co-dopant. This heteroatomic doped CN photocatalyst possesses enhanced visible light absorption, higher charge separation efficiency than pristine CN. Under acidic conditions, cellobiose is not only more efficiently hydrolyzed into glucose but also promotes the syngas and gluconic acid production. Density functional theory (DFT) calculations reveal the favorable generation of •O2− during the photocatalytic reaction, which is essential for gluconic acid production. Consequently, the fine-designed photocatalyst presents excellent cellobiose conversion (>80%) and gluconic acid selectivity (>70%) together with the co-production of syngas (~56 μmol g-1 h-1) under light illumination. The current work demonstrates the feasibility of biomass photoreforming with value-added chemicals and syngas co-production under mild condition.
Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single...Pawan Kumar
Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer–Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.
Recent advancements in tuning the electronic structures of transitional metal...Pawan Kumar
The smooth transition from finite non-renewables to renewable energy conversion technologies will require efficient electrocatalysts which can harness intermittent energies to store in the form of chemical bonds. The oxygen evolution reaction (OER) impedes the widespread usage of water electrolyzers to convert H2O into H2 and persists as a bottleneck, including other energy conversion devices with sluggish four H+/e− kinetics. In this context, designing highly active and stable catalysts capable of driving a lower overpotential in the OER to produce continuous hydrogen (H2) is a primary demanded. This chapter discussed the mechanism of the OER in conventional adsorbate oxygen and lattice oxygen participation in transition metal oxides (TMOs). Further, the influences of surface engineering, doping, and defects in the TMOs and understanding the electronic structure to screen electrodes towards the structure–activity relationship are highlighted. Specifically, the adsorption strength of O 2p is understood in detail as its binding ability over the surface of TMOs can be correlated directly to the OER activity. The iterative development of TMOs in terms of understanding electronic structural attributes is essential for the commercial deployment of energy conversion technologies. The comprehensive outlook of this chapter investigates thoroughly how TMOs can be used as significant materials for the OER in the near future.
Hole transport materials (HTMs) have a significant impact on the effectiveness of organic electronic devices; therefore, we present a molecular architecture of pyrazino[2,3-g]quinoxaline (PQ10)-based room-temperature organic liquid crystalline semiconductor (OLCS) as an alternative HTM. The PQ10 compound exhibits three different rectangular columnar (Colr) phases offering an impressive hole mobility of 8.8 × 10−3 cm2V−1s−1 which is found to be dexterous than most of existing polymeric hole transport materials. The charge transport mechanism is governed by the hole polarons hopping through H-aggregates of the PQ10 molecules and the hole mobility remains nearly constant throughout the mesophase range, but it decreases with increasing applied electric field. The current-voltage characteristics of the PQ10 have also been investigated in all three Colr phases and explained via the Poole-Frenkel conduction mechanism. The dielectric spectroscopy has been eventually carried out to understand the nature of dielectric permittivity and conductivity as a function of temperature and a correlation is established between the molecular architecture of the Colr phases and aforementioned physical properties. Solar cell simulation has been additionally performed to demonstrate that the PQ10 material can be a better choice as HTM for organic electronics and photovoltaic applications.
Multifunctional carbon nitride nanoarchitectures for catalysisPawan Kumar
Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.
Production of Renewable Fuels by the Photocatalytic Reduction of CO2 using Ma...Pawan Kumar
The photo-reductive performance of natural ilmenite was boosted and the production of renewable fuels from the reduction of CO2 was enhanced by doping the natural mineral with magnesium. The doping was achieved by high energy ball milling in the presence of MgO and Mg(NO3)2. The photo-reduction of CO2 in aqueous solution led to the evolution of H2, CH4, C2H4, and C2H6, and the insertion of Mg in the structure of ilmenite enabled increases of up to 1245% in the fuel production yield, reaching total production of 210.9 µmol h-1 gcat-1. Displacements of the conduction band to more negative potentials were evidenced for the samples doped with magnesium. Indirect effects such as increases in the valence band maximum, and the introduction of intermediate energy levels were also evidenced through the measurement of the crystallite size and the determination of the band structure of the materials. Mott-Schottky analyses of the samples showed the n-type nature of the semiconductor materials and enabled the estimation of the density of charge carriers, which strongly influenced the photocatalytic performance. The strong potential of the application of natural ilmenite in gas phase artificial photosynthesis was proved by the evaluation of CO2 reduction in gas conditions, which allowed the enhancement in the selectivity and significantly increased the production of CH4 as compared to aqueous solution, reaching an important yield of CH4 of 16.1 µmol h-1 gcat-1.
Nanoengineered Au-Carbon Nitride Interfaces Enhance PhotoCatalytic Pure Water...Pawan Kumar
Photocatalytic pure water splitting using solar energy is one of the promising routes to produce sustainable green hydrogen (H2). Tuning the interfacial active site density at catalytic heterojunctions and better light management are imperative to steer the structure-activity correlations to enhance the photo-efficiency of nanocomposite photocatalysts. Herein, we report the decoration of nitrogen defects-rich carbon nitride CN(T) with metallic Au nanostructures of different morphologies and sizes to investigate their influence on the photocatalytic hydrogen evolution reactions (HER). The CN(T)-7-NP nano-heterostructure comprises Au nanoparticles (NPs) of ~7 nm and thiourea-derived defective CN exhibits an excellent H2 production rate of 76.8 µmol g–1 h–1 from pure water under simulated AM 1.5 solar irradiation. In contrast to large-size Au nanorods, the high activity of CN(T)-7-NP was attributed to their strong localized surface plasmon resonance (LSPR) mediated visible absorption and interfacial charge separation. The surface ligands used to control Au nanostructures morphology were found to play a major role in the stabilization of NPs and improve interfacial charge transport between Au NPs and CN(T). First-principles calculations revealed that defects in CN and Au-CN interfacial sites in these nanocomposites facilitate the separation of e-/h+ pairs after light excitation and provide lower energy barrier pathways for H2 production by photocatalytic water splitting.
Nanoengineered Au-Carbon Nitride Interfaces Enhance Photo-Catalytic Pure Wate...Pawan Kumar
Photocatalytic pure water splitting using solar energy is one of the promising routes to produce sustainable green hydrogen (H2). Tuning the interfacial active site density at catalytic heterojunctions and better light management are imperative to steer the structure-activity correlations to enhance the photo-efficiency of nanocomposite photocatalysts. Herein, we report the decoration of nitrogen defects-rich carbon nitride CN(T) with metallic Au nanostructures of different morphologies and sizes to investigate their influence on the photocatalytic hydrogen evolution reactions (HER). The CN(T)-7-NP nano-heterostructure comprises Au nanoparticles (NPs) of ~7 nm and thiourea-derived defective CN exhibits an excellent H2 production rate of 76.8 µmol g–1 h–1 from pure water under simulated AM 1.5 solar irradiation. In contrast to large-size Au nanorods, the high activity of CN(T)-7-NP was attributed to their strong localized surface plasmon resonance (LSPR) mediated visible absorption and interfacial charge separation. The surface ligands used to control Au nanostructures morphology were found to play a major role in the stabilization of NPs and improve interfacial charge transport between Au NPs and CN(T). First-principles calculations revealed that defects in CN and Au-CN interfacial sites in these nanocomposites facilitate the separation of e-/h+ pairs after light excitation and provide lower energy barrier pathways for H2 production by photocatalytic water splitting.
Cooperative Copper Single Atom Catalyst in Two-dimensional Carbon Nitride for...Pawan Kumar
Renewable electricity powered carbon dioxide (CO2) reduction (eCO2R) to high-value fuels like methane (CH4) holds the potential to close the carbon cycle at meaningful scales. However, this kinetically staggered 8-electron multistep reduction still suffers from inadequate catalytic efficiency and current density. Atomic Cu-structures can boost eCO2R-to-CH4 selectivity due to enhanced intermediate binding energies (BEs) resulting from favorably shifted d-band centers. Herein, we exploit two-dimensional carbon nitride (CN) matrices, viz. Na-polyheptazine (PHI) and Li-polytriazine imides (PTI), to host Cu-N2 type single atom sites with high density (∼1.5 at%), via a facile metal ion exchange process. Optimized Cu loading in nanocrystalline Cu-PTI maximizes eCO2R-to-CH4 performance with Faradaic efficiency (FECH4) of ≈68% and a high partial current density of 348 mA cm−2 at a low potential of -0.84 V versus RHE, surpassing the state-of-the-art catalysts. Multi-Cu substituted N-appended nanopores in the CN frameworks yield thermodynamically stable quasi-dual/triple sites with large interatomic distances dictated by the pore dimensions. First-principles calculations elucidate the relative Cu-CN cooperative effects between the two matrices and how the Cu-Cu distance and local environment dictate the adsorbate BEs, density of states, and CO2-to-CH4 energy profile landscape. The 9N pores in Cu-PTI yield cooperative Cu-Cu sites that synergistically enhance the kinetics of the rate-limiting steps in the eCO2R-to-CH4 pathway.
Bioinspired multimetal electrocatalyst for selective methane oxidationPawan Kumar
Selective partial electrooxidation of methane (CH4) to liquid oxygenates has been a long-sought goal. However, the high activation energy of C–H bonds and competing oxygen evolution reaction limit product selectivity and reaction rates. Inspired by iron (IV)-oxo containing metalloenzymes’ functionality to activate the C–H bond, here we report on the design of a copper-iron-nickel catalyst for selective oxidation of CH4 to formate via a peroxide-assisted pathway. Each catalyst serves a specific role which is confirmed via electrochemical, in situ, and theoretical studies. A combination of electrochemical and in situ spectroelectrochemical studies revealed that H2O2 oxidation on nickel led to the formation of active oxygen species which trigger the formation of iron (IV) at low voltages. Density functional theory analysis helped reveal the role of iron (IV)-oxo species in reducing the activation energy barrier for CH4 deprotonation and the critical role of copper to suppress overoxidation. Our multimetal catalyst exhibits a formate faradaic efficiency of 42% at an applied potential of 0.9 V versus a reversible hydrogen electrode.
ANAMOLOUS SECONDARY GROWTH IN DICOT ROOTS.pptxRASHMI M G
Abnormal or anomalous secondary growth in plants. It defines secondary growth as an increase in plant girth due to vascular cambium or cork cambium. Anomalous secondary growth does not follow the normal pattern of a single vascular cambium producing xylem internally and phloem externally.
Richard's aventures in two entangled wonderlandsRichard Gill
Since the loophole-free Bell experiments of 2020 and the Nobel prizes in physics of 2022, critics of Bell's work have retreated to the fortress of super-determinism. Now, super-determinism is a derogatory word - it just means "determinism". Palmer, Hance and Hossenfelder argue that quantum mechanics and determinism are not incompatible, using a sophisticated mathematical construction based on a subtle thinning of allowed states and measurements in quantum mechanics, such that what is left appears to make Bell's argument fail, without altering the empirical predictions of quantum mechanics. I think however that it is a smoke screen, and the slogan "lost in math" comes to my mind. I will discuss some other recent disproofs of Bell's theorem using the language of causality based on causal graphs. Causal thinking is also central to law and justice. I will mention surprising connections to my work on serial killer nurse cases, in particular the Dutch case of Lucia de Berk and the current UK case of Lucy Letby.
Seminar of U.V. Spectroscopy by SAMIR PANDASAMIR PANDA
Spectroscopy is a branch of science dealing the study of interaction of electromagnetic radiation with matter.
Ultraviolet-visible spectroscopy refers to absorption spectroscopy or reflect spectroscopy in the UV-VIS spectral region.
Ultraviolet-visible spectroscopy is an analytical method that can measure the amount of light received by the analyte.
What is greenhouse gasses and how many gasses are there to affect the Earth.moosaasad1975
What are greenhouse gasses how they affect the earth and its environment what is the future of the environment and earth how the weather and the climate effects.
This presentation explores a brief idea about the structural and functional attributes of nucleotides, the structure and function of genetic materials along with the impact of UV rays and pH upon them.
Travis Hills' Endeavors in Minnesota: Fostering Environmental and Economic Pr...Travis Hills MN
Travis Hills of Minnesota developed a method to convert waste into high-value dry fertilizer, significantly enriching soil quality. By providing farmers with a valuable resource derived from waste, Travis Hills helps enhance farm profitability while promoting environmental stewardship. Travis Hills' sustainable practices lead to cost savings and increased revenue for farmers by improving resource efficiency and reducing waste.
Comparing Evolved Extractive Text Summary Scores of Bidirectional Encoder Rep...University of Maribor
Slides from:
11th International Conference on Electrical, Electronics and Computer Engineering (IcETRAN), Niš, 3-6 June 2024
Track: Artificial Intelligence
https://www.etran.rs/2024/en/home-english/
Observation of Io’s Resurfacing via Plume Deposition Using Ground-based Adapt...Sérgio Sacani
Since volcanic activity was first discovered on Io from Voyager images in 1979, changes
on Io’s surface have been monitored from both spacecraft and ground-based telescopes.
Here, we present the highest spatial resolution images of Io ever obtained from a groundbased telescope. These images, acquired by the SHARK-VIS instrument on the Large
Binocular Telescope, show evidence of a major resurfacing event on Io’s trailing hemisphere. When compared to the most recent spacecraft images, the SHARK-VIS images
show that a plume deposit from a powerful eruption at Pillan Patera has covered part
of the long-lived Pele plume deposit. Although this type of resurfacing event may be common on Io, few have been detected due to the rarity of spacecraft visits and the previously low spatial resolution available from Earth-based telescopes. The SHARK-VIS instrument ushers in a new era of high resolution imaging of Io’s surface using adaptive
optics at visible wavelengths.
Remote Sensing and Computational, Evolutionary, Supercomputing, and Intellige...University of Maribor
Slides from talk:
Aleš Zamuda: Remote Sensing and Computational, Evolutionary, Supercomputing, and Intelligent Systems.
11th International Conference on Electrical, Electronics and Computer Engineering (IcETRAN), Niš, 3-6 June 2024
Inter-Society Networking Panel GRSS/MTT-S/CIS Panel Session: Promoting Connection and Cooperation
https://www.etran.rs/2024/en/home-english/
Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic ...Sérgio Sacani
We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
imaging field observed with JWST. We make use of the ancillary Hubble optical images (5 filters
spanning 0.4−0.9µm) and novel JWST images with 14 filters spanning 0.8−5µm, including 7 mediumband filters, and reaching total exposure times of up to 46 hours per filter. We combine all our data
at > 2.3µm to construct an ultradeep image, reaching as deep as ≈ 31.4 AB mag in the stack and
30.3-31.0 AB mag (5σ, r = 0.1” circular aperture) in individual filters. We measure photometric
redshifts and use robust selection criteria to identify a sample of eight galaxy candidates at redshifts
z = 11.5 − 15. These objects show compact half-light radii of R1/2 ∼ 50 − 200pc, stellar masses of
M⋆ ∼ 107−108M⊙, and star-formation rates of SFR ∼ 0.1−1 M⊙ yr−1
. Our search finds no candidates
at 15 < z < 20, placing upper limits at these redshifts. We develop a forward modeling approach to
infer the properties of the evolving luminosity function without binning in redshift or luminosity that
marginalizes over the photometric redshift uncertainty of our candidate galaxies and incorporates the
impact of non-detections. We find a z = 12 luminosity function in good agreement with prior results,
and that the luminosity function normalization and UV luminosity density decline by a factor of ∼ 2.5
from z = 12 to z = 14. We discuss the possible implications of our results in the context of theoretical
models for evolution of the dark matter halo mass function.
2. through the formation of a heterojunction interface, providing
new sites for adsorption of reactant molecules on the surface of
the photocatalyst, etc. Transition metals are known for
improving the hydrogenation reaction rather than C−C
coupling, due to which decoration of semiconductor photo-
catalysts with transition-metal nanoparticles favor the
production of methane compared to C2+ hydrocarbons.7
Carbon dioxide anion radical (CO2
•−
), electronically excited
state of carbon monoxide (CO*), formyl radical (•
CHO), and
carboxyl radical (COOH) figure prominently as reaction
intermediates in multiple pathways proposed for CO2
photoreduction.11,12
Interadsorbate steric and dipole repulsion
forces hinder the adsorption and C−C coupling of these
reaction intermediates on adjacent reaction sites of metal
atoms.13
The interplay between long-range attractive forces
and short-range Pauli repulsion and repulsive dipole−dipole
coupling is well studied for CO monolayers of various
coverages on Cu(100), Cu(110), and Cu(111) surfaces.14−16
Nanoparticles of Ag, Au, Cu, and Al exhibit strong localized
surface plasmon resonances at visible wavelengths, which
consist of coherent and collective charge density oscillations of
conduction-band electrons. Nanoparticles smaller than ∼50
nm primarily exhibit a symmetric dipole resonance wherein
optical excitation displaces the center of negative charge with
respect to the center of positive charge due to the polarization
of the electron cloud by the incident field.17
The combination
of adsorbate−adsorbate repulsion between reaction intermedi-
ates and a symmetric distribution of surface charge due to
dipolar localized surface plasmon resonance (LSPR) excitation
makes C−C coupling more unlikely on such metal nano-
particle surfaces. This may be because vapor-phase CO2
photoreduction on TiO2 nanotubes surface-decorated by <10
nm-sized nanoparticles of Cu, Pt, Ru, Au, Pd, AuPd, ZnPd, Ag,
etc. has primarily been reported to result in the dominant
Figure 1. TEM analysis of the AgCu-TNTA sample. (a, b) Cross-sectional view, (c, d) HR-TEM analysis of AgCu nanoparticles and corresponding
d-spacings, and (e) elemental mapping of the AgCu-TNTA sample.
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3. formation (>95% of all product molecules formed) of C1
products such as methane, carbon monoxide, methanol, and
formaldehyde, with C2 products such as ethane and ethylene
being formed as minor byproducts (<5% of all products).18−25
In metal particles larger than 50 nm, the quasi-static
approximation is no longer valid and the quadrupole plasmon
mode becomes significant, which produces an antisymmetric
distribution of charge density oscillations on the surface of the
metal nanoparticle when excited.26,27
In even larger nano-
particles and aggregates of dissimilar nanoparticles, multipole
plasmon excitations with rapidly alternating surface charge
density oscillations dominate.28
Multipolar plasmon resonance
modes and symmetry breaking in heterodimers29,30
afford the
possibility of closely lying reaction sites having opposite
charge, thus enabling a reduction in the dipole repulsion of
adsorbed C1 reaction intermediates and facilitating their C−C
coupling to form C2 products.
There are a few reports on the production of C2+
hydrocarbons (more specifically ethane) in the CO2 photo-
reduction process. Recently, Sorcar et al. reported production
of ethane with 28% selectivity at a rate of 77 μmol g−1
over a
Pt-sensitized graphene-wrapped titania photocatalyst. Fu et al.2
reported C−C coupling reaction on Au NPs with almost 40%
selectivity for ethane production.3
Our previous work in
Angew. Chem. Int. Ed. on a Cu−Pt nanoparticle-supported
titania nanotube array photocatalyst exhibited a maximum 14%
selectivity toward ethane formation.4
In all of the reported
literature so far, the cocatalyst typically consists of one or more
precious metals such as Pt, Au, Ru, and Pd.18,21,31
Herein, we
successfully synthesized large-sized, less expensive Ag, Cu, and
AgCu nanoparticles and successfully hosted these nano-
particles on a TiO2 nanotube array (TNTA) scaffold. The
results from our experiments demonstrate that the synergetic
effect of large (>80 nm) Ag and Cu nanoparticles as cocatalysts
in CO2 photoreduction increases both the efficiency and
selectivity of CO2 photoreduction; a C2 product, namely,
ethane, is generated with an unprecedented selectivity as high
as 60%. The key conceptual innovation consists of the use of
hybrid quadrupolar and multipolar plasmon resonances in
large plasmonic nanoparticles and their aggregates to achieve
an asymmetric photoexcited charge distribution on the
photocatalyst surface.
2. RESULTS AND DISCUSSION
2.1. Structure and Chemical Composition of the
Samples. Electrochemical anodization was used to synthesize
TiO2 nanotube arrays (TNTAs),32,33
and photodeposition
from Ag(acac) and Cu(acac)2 solutions34,35
was used to form
monometallic and bimetallic nanoparticles (NPs) decorating
and infiltrating the TNTAs (see the Materials and Methods
section in the Supporting Information). The fabrication
process is schematically depicted in Figure S1 in the
Supporting Information. The morphologies of TNTAs were
investigated using high-resolution transmission electron
microscopy (HR-TEM) and field emission scanning electron
microscopy (FESEM). The cross-sectional images of the
TNTAs (Figures 1a,b and S2 in the Supporting Information)
clearly show an array of hollow, vertically oriented nanotubes
with an average length of ∼6 μm and an average outer
diameter of 110 nm. The top view of the AgCu-TNTA sample
(Figures 1a and 2b) shows 80−200 nm polydispersed
nanoparticles of Ag and Cu agglomerated on the top of the
TiO2 nanotube arrays. From Figure 1b, it is evident that in
addition to the aggregates of larger Ag and Cu nanoparticles on
top of the nanotubes, there are smaller nanoparticles with
diameters 25−80 nm comparable to the inner diameter of the
Figure 2. (a, b) Top-view FESEM images of the TNTA sample before (a) and after (b) photodeposition of metal naoparticles; (b, c) X-ray
diffraction (XRD) patterns of TNTA, Cu-TNTA, Ag-TNTA, and AgCu-TNT samples; and (d) Raman spectra of TNTA, Cu-TNTA, Ag-TNTA,
and AgCu-TNTA samples.
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4. nanotubes, which penetrate along the length of the TNTAs.
Previous studies reported that bulk Cu and Ag exhibit lattice d-
spacings of 0.208 and 0.240 nm, respectively, and the alloy of
AgCu exhibits a d-spacing in the range 0.210 < d < 0.240 nm.36
d-spacings of 0.350 and 0.240 nm were determined from
selected area electron diffraction (SAED) patterns collected
from higher-magnification HR-TEM images (Figure 1c,d) and
correspond to the (101) plane of anatase TiO2 and FCC
Ag(111). In some cases, a d-spacing of 0.22 nm was observed,
indicative of the formation of AgCu bimetallic alloy through
galvanic replacement.36
The elemental map analysis of the area
of interest in the AgCu-TNTA sample is shown in Figure 1e,
which further confirms infiltration of the TiO2 nanotubes with
both Ag and Cu nanoparticles. Figure 2a shows the pristine
TiO2 nanotubes before photodeposition, while Figure 2b
confirms the TEM micrograph and shows a layer of densely
aggregated metal nanoparticles on top of the TNTAs.
The X-ray diffractograms of TNTAs, Ag-TNTAs, and AgCu-
TNTAs are shown in Figure 2c. The bare TNTA sample
exhibited reflections corresponding to the pure anatase phase
(JCPDS No. 21-1272). This is in agreement with prior reports
that show the dominance of the anatase phase when anodic
TiO2 nanotubes are annealed at temperatures lower than 550
°C. The temperature of 450 °C used in this work to crystallize
the nanotubes (see the Materials and Methods section in the
Supporting Information) is not high enough to induce the
anatase-to-rutile phase transition. In addition to anatase peaks,
other peaks originating from pure Ti could be detected in the
XRD pattern of TNTA samples, which correspond to the
portion of the titanium foil substrate that was not anodized.
For the Ag-TNTA and AgCu-TNTA samples, additional peaks
located at 2θ values of 44.08 and 64.3° are present in the
diffractograms, which were indexed to face-centered cubic
silver (JCPDS No. 04-0783). No peaks related to Cu could be
detected in the XRD patterns of AgCu-TNTA and Cu-TNTA
samples, which indicates that the Cu nanoparticles were highly
scattered, and the amount of Cu was extremely small
compared to Ag, Ti, and anatase TiO2.4
Consequently, it is
reasonable to assume that the aggregated layer of large metal
NPs seen in Figure 2b consists of a mixture of monometallic
Ag and bimetallic AgCu NPs. However, the UV−vis spectra
(discussed in Section 2.2) strongly suggest bimetallic AgCu
NPs.
The Raman spectra of the TNTA, Cu-TNTA, Ag-TNTA,
and AgCu-TNTA samples (Figure 2d) exhibit one intense and
three less intense peaks, which confirm the phonon modes in
tetragonal anatase TiO2. The prominent peak at 145 cm−1
is
due to the Eg mode, while less intense peaks at 395, 516, and
640 cm−1
correspond to the B1g, A2g, and Eg modes.35
An
expected shift and broadening for the main Eg modes in the
Raman spectra of Cu-TNTA, Ag-TNTA, and AgCu-TNTA has
been observed, which is related to the changes induced in the
structure of TNTAs by the presence of Ag and Cu
nanoparticles.35
The chemical composition of the catalyst
surface and subsurface was determined using X-ray photo-
electron microscopy (XPS) (Figure 3). The signature peaks of
Ti, O, Ag, and Cu are present in the XPS elemental survey scan
(Figure 3a), confirming the successful photodeposition of Cu
and Ag on the surface of the TNTAs. The deconvoluted core-
level high-resolution XPS (HR-XPS) spectra of TNTA and
AgCu-TNTA samples in the Ti 2p region exhibited two
symmetric peaks located at binding energies (BE) of 458.96
Figure 3. (a) XPS elemental survey scan of compact TiO2 nanotubes arrays (black) and AgCu-TNTA (red) and core-level HR-XPS spectra of
TNTA and AgCu-TNTA in the (b) Ti 2p region, (c) O 1s region (d) Cu 2p region, and (e) Ag 3d region.
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5. and 464.84 eV, assigned, respectively, to the Ti 2p3/2 and Ti
2p1/2 peak components of Ti4+
ions present in the TiO2 crystal
lattice (Figure 3b).37−39
The obtained peak splitting between
Ti 2p3/2 and Ti 2p1/2 peak components was found to be 5.88
eV, suggesting O2−
coordinated with Ti4+
in the TiO2 lattice
structure.40,41
The HR-XPS of TNTAs in the O 1s region can
be deconvoluted into two chemically shifted components
centered at BEs of 530.33 and 532.17 eV. The strong peak
component at BE ∼ 530.33 eV originated from the Ti-bonded
oxygens present in the crystal lattice (Ti−O−Ti), while the
weak shoulder peak at 532.17 eV was attributed to surface-
bounded −OH and nonlattice adventitious oxygens (Figure
3c).42−44
The binding energy of the O 1s peak components in
AgCu-TNTA remains almost identical to TNTA, demonstrat-
ing that decoration of Ag and AgCu nanoparticles on TiO2
nanotubes does not induce chemical changes on the surface.
Two well-resolved Cu 2p peaks for AgCu-TNTA at BEs of
932.63 and 952.65 eV (Figure 3d) originated due to Cu 2p3/2
and Cu 2p1/2 peak components of metallic Cu(0),
respectively.45,46
The absence of any shoulder or satellite
peak indicates the absence of any Cu1+
or Cu2+
oxides. The Ag
3d spectra (Figure 3e) of AgCu-TNTA showed two peaks at
368.08 and 374.13 eV with 6.08 eV peak splitting corroborated
to Ag(0) and the absence of shoulder peaks verified phase-pure
metallic silver.47,48
Integration and subsequent comparison of
the peak areas of the Ag 3d and Cu 2p peaks followed by
normalization to their respective photoelectron yields
indicated a Ag:Cu ratio of 43.4:1 (see Figure S11 in the
Figure 4. (a) Schematic illustration of Ag@Cu core−shell NP homodimers on top of adjacent TiO2 nanotubes. (b) Simulated optical absorption of
TNTAs decorated with Ag@Cu homodimer NPs with a core diameter of 100 nm and different shell thicknesses. (c, d) Electric field profile for a
TNTA/100 nm Ag core−5 nm Cu shell homodimer with a 3 nm edge-to-edge spacing in the substrate (xy) plane (c) and xz plane (d). The
incident plane wave is polarized in the x-direction and propagates along the z-direction.
Figure 5. (a) DRS spectra of TNTA, Ag-TNTA, and AgCu-TNTA samples. (b) Comparison between simulated absorption cross section (obtained
via FDTD modeling of small metal NPs) of AgCu-TNTA sample and its DRS spectrum. While the peak positions in the experimental and
simulated spectra show correspondence, the measured amplitude of the extinction is much higher than expected from FDTD modeling.
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6. Supporting Information), which confirms the small quantity of
Cu relative to Ag in the AgCu-TNTA samples.
2.2. Plasmon Excitation and Optical Properties. The
galvanic displacement of Ag atoms by Cu atoms on the surface
of silver nanoparticles mentioned earlier results in Ag@Cu
core−shell-type nanoparticles. We used finite-difference time
domain (FDTD) electromagnetic simulations to study the
interaction of light with TiO2 nanotubes decorated with a
single layer of Ag@Cu NP homodimers perched on top of the
pore openings. The simulated optical properties of the dimers
were used to obtain more general insight into the behavior of
the aggregated nanoparticles on the surface of TNTAs seen in
Figure 2b. The diameter of the silver core was kept constant at
100 nm, and the thickness of the copper shell was varied from
5 to 20 nm. The results of these simulations are shown in
Figure 4. Due to the strong interband damping-mediated
absorption of Cu, TNTAs decorated with Ag@Cu homo-
dimers exhibit significant visible light absorption. The strong
damping due to the presence of Cu also ensures that the
highest electric field intensities are found not in-between the
two particles constituting the dimer but rather at Ag−TiO2 and
Ag−air interfaces. The above analysis ignores the effect of
smaller Ag@Cu NPs located deeper in the pores of the TiO2
nanotubes.
The optical properties of the TNTA, Cu-TNTA, Ag-TNTA,
and AgCu-TNTA samples were investigated by diffuse-
reflectance UV−vis spectroscopy (DRS), as well as finite-
difference time domain (FDTD) electromagnetic simulations.
The DRS spectra of TNTA sample (black line in Figure 5a)
exhibits a band edge (dashed line in Figure 5a) around 380
nm, which is in accordance with its band gap (3.2 eV).49
The
TNTA samples also exhibit a broad absorption feature peaking
around 700 nm, which arises due to defects (Ti3+
states, Ti2+
states, O vacancies) as well as absorption by the Ti substrate.
In comparison to the bare TNTA sample, the band edge(s) for
the Ag-TNTA and AgCu-TNTA samples were blue-shifted to
lower wavelengths. This blue shift is due to the Mie scattering
of ultraviolet photons by metal nanoparticles.50
On the other
hand, the Cu-TNTA samples do not exhibit this blueshift due
to the Mie scattering attenuated by strong interband damping
in Cu. In Figure 5, the optical spectra of Ag-TNTA and AgCu-
TNTA samples do not resemble each other at all in spite of the
similarity in the XRD and Raman spectra. For the Ag-TNTA
sample, there is a broad, featureless localized surface plasmon
resonance (LSPR) peak centered at 450 nm, which agrees with
the polydispersed nature of Ag NPs fabricated in this study.
The AgCu-TNTA sample exhibits two LSPR peaks at 425 and
525 nm. The 425 nm peak is due to the hybrid quadrupolar
resonance of larger Ag NPs of size >75 nm27,30
that is likely
red-shifted by 50 nm from the corresponding resonance of Ag
nanoislands due to the higher permittivity of the TiO2
nanotube array scaffold.51
The 525 nm peak corresponds to
the dipolar resonance of AgCu NPs in the vicinity of TiO2 and
weakened in amplitude by the interband damping of Cu.27
The
optical spectra indicate the AgCu-TNTA samples to be less
polydispersed than the Ag-TNTA samples, which we attribute
to the nucleation of Cu on Ag NPs and the etching of already
agglomerated particles by galvanic replacement. The much
lower scattering amplitude for AgCu NPs (green curve in
Figure 4) due to damping provides persuasive evidence for the
dominant presence of bimetallic AgCu nanoparticles over
monometallic Ag NPs. The galvanic replacement by Cu2+
ions
reduces the size of the Ag nanoparticles and also makes either
an alloy hybrid of the two materials or a core−shell structure
consisting of an Ag core and a Cu shell.36
Considering that
further elevated annealing treatments were not used, the core−
Figure 6. (a, c) Electric field profile for a Ag−Cu heterodimer with a 3 nm edge-to-edge spacing in the substrate (xy) plane and corresponding
schematic illustration showing the surface charge distribution. (b, d) Electric field profile in the xz plane and schematic illustration of simulated
geometry showing large (120 nm diameter) spherical Ag and Cu nanoparticles perched on top of the nanotubes covering their mouth as in Figure
1a. The incident plane wave is polarized in the x-direction and propagates along the z-direction.
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7. shell scenario is more likely. The simulated absorption spectra
of large Ag@Cu core−shell NP homodimers as a function of
Cu shell thickness shown in Figure 4 and discussed in the
previous paragraph have some similarities and differences with
the experimentally measured spectrum (green curve in Figure
5a). Unlike the simulated homodimers, the experimental
spectra do not exhibit any peaks at red and near-infrared peaks.
However, the experimental spectra do show a significant albeit
decreasing longer-wavelength absorption similar to the
homodimers. The experimental spectrum shows two broad
peaks at visible wavelengths similar to the homodimer
simulated spectra, but the peak positions do not match.
FDTD simulations of smaller nanoparticles (intended to
capture the interactions of smaller bimetallic nanoparticles
found deeper in the nanotube pore) captured the key peak
locations (∼425 and 525 nm) in the optical spectra of AgCu-
TNTA samples, but could not capture the large amplitude of
the extinction at visible wavelengths seen in Figure 4b because
the simulation geometry of well-separated metal nanoparticles
infiltrated into TNTAs (Figure S3 in the Supporting
Information) did not capture the multipolar resonances
expected in the layer of aggregated large nanoparticles on
top of the nanotubes (Figures 1a and 2b).
The simulated spectra of TNTAs decorated with Ag@Cu
core−shell homodimers (Figure 4b) and small, separated Ag
and Cu NPs (Figure 5b) do not fully explain the huge
differences observed between the CO2 photoreduction
performance of TNTAs decorated with Ag NPs, Cu NPs,
and AgCu NPs, as will be discussed in the next section. If Ag
NPs deep in the nanotube pores are the species dominating
light−matter interactions (since chemical interactions indicate
the overall amount of Cu to be quite small), then the
electromagnetics of Ag-TNTA samples and AgCu-TNTA
samples would be similar. If the Cu shells coating large Ag
particles are responsible for the catalytic activity and light−
matter interactions, then the CO2 photoreduction performance
of AgCu-TNTA and Cu-TNTA samples ought to be similar
(but they are not). Therefore, we next considered the
possibility of heterodimers consisting of Ag and Cu nano-
particles (which also approximates the situation of Ag and
Ag@Cu nanoparticles). To further investigate the behavior of
the aggregated layer of large metal NPs on top of the TNTAs,
we performed FDTD simulations of close-lying Ag−Cu
heterodimers (Figure 6).
Although the heterodimers expected in our samples are
closely lying Ag NPs and Cu-coated Ag NPs, Ag−Cu
heterodimers constitute a more general case that captures
the largest range of possibilities based on the structure and
composition data discussed in Section 2.1. We also simulated
the behavior of isolated Ag, Cu NPs, as well as Ag−Ag and
Cu−Cu homodimers (Figures S4 and S5 in the Supporting
Information). At very few spots at the right edge of the Ag NP
directly opposite the Cu NP on top of TiO2, an absolute local
field intensity enhancement factor as high as 90 was obtained
due to the well-known electrodynamic coupling of plasmon
modes in closely lying metal nanoparticles.52
Absolute local
field intensity enhancement factors of 15 and higher were
obtained at a number of spots all along the Ag−TiO2 interface
due to the low loss of Ag and the additional field concentration
effect at the high-index interface with TiO2 nanotubes. The
electric field profiles were scaled using a jet colormap from
dark blue (low field amplitude) to dark red (large field
amplitude), respectively. While the scale factor does not
change the intensity values, the purpose of scaling is to
Figure 7. (a) Reaction products under irradiation with AM1.5G 1-sun simulated sunlight. (b) Comparison of ethane selectivities using Cu-TNTA,
Ag-TNTA, and AgCu-TNTA photocatalysts. (c) Reusability test of AgCu-TNTA sample for CO2 photoreduction reaction under standard
condition.
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8. normalize the colors in the monitor so that field intensity
colors that are otherwise too small to see can be clearly seen
for comparison. In Figure 6a, the relatively low field intensity at
Cu−TiO2 and Cu−air interfaces occurs due to the large
interband damping in Cu. In Figure 6b, regions of highest
electric field intensity are to the right of the Ag particle
member of the heterodimer, closer to its interface with the Cu
particle, which can be explained by a multipolar asymmetric
surface charge distribution as in Figure 6c. Both the
homodimer and heterodimer simulations confirmed a high
density of hotspots in the Ag-TNTA and AgCu-TNTA
samples, where the local field intensity was enhanced by a
factor of at least 4.
2.3. CO2 Photoreduction Performance. The photo-
catalytic activities of the Cu-TNTA, Ag-TNTA, and AgCu-
TNTA samples were investigated under identical conditions
involving 2 h of illumination by AM1.5G simulated sunlight at
50 °C. Immediately following illumination, the reaction
products were analyzed by gas chromatography (Figures S6−
S8 in the Supporting Information). The amounts of reaction
products are reported in this work as micromoles of evolute
per gram of photocatalyst per duration of the test (μmol g−1
h−1
). The results of the photoreduction tests are shown in
Figure 7a whose inset shows the total hydrocarbon production
by the samples. The main reaction product was ethane (C2H6)
for AgCu-TNTA samples, while methane was the main
product for the Ag-TNTA and Cu-TNTA samples. The total
rate of hydrocarbon production (methane + ethane) of 23.88
μmol g−1
h−1
was highest for AgCu-TNTA samples. The
CxH2x+2 production rates for the Ag-TNTA and Cu-TNTA
samples were 6.54 and 1.8 μmol g−1
h−1
, respectively. The
ethane selectivity for these three samples is shown in Figure
7b. The selectivity was highest for the AgCu-TNT sample
(60.7%), while the ethane selectivities were calculated to be
15.9 and 10% for Ag-TNTA and Cu-TNTA, respectively. The
highest selectivity of ethane for the AgCu-TNTA sample and
the fact that ethane was the primary product of CO2
photoreduction for this sample indicate that the synergistic
effect between the Cu and Ag cocatalysts was responsible for
the dominant production of ethane.
A comparison was conducted between the photocatalytic
activity of AgCu-TNTAs reported in this work and some of the
previously reported studies on TiO2 photocatalysts decorated
with monometallic Ag or Cu nanoparticles (Table S2 in the
Supporting Information). It is evident from this table that the
AgCu-TNTA sample reported here outperforms most of the
previously reported photocatalysts. Figure 7c exhibits the
reusability of the best performing photocatalysts (AgCu-
TNTA) during three consecutive runs, and the AgCu-TNTA
photocatalyst still maintained a high level of photocatalytic
activity during the second and third runs with a slight decrease
in performance. The shows that the higher selectivity toward
ethane than to methane can be explained by looking into the
carbene reaction pathway, which is widely agreed to be the
mechanistic route for CO2 photoreduction on nanoporous/
nanotubular TiO2 surfaces.49,53,54
Depending on the CO2
adsorption mode and the amount of energy provided by the
charge carriers, CO2 reduction can go through two different
reaction pathwaysthe carbene pathway, in which the main
product is methane and/or ethane, and the formaldehyde
pathway, in which formaldehyde itself is one of the main
reaction products/intermediates.55−57
The carbene pathway can be written as follows
l
m
o
o
o
o
o
n
o
o
o
o
o
|
}
o
o
o
o
o
~
o
o
o
o
o
CO CO CO OH CO
C OH CH CH
CH
CH
C H
2
e
2
e H e
e H e H e H
2
e H
3
e H
4
CH
2 6
3
→ ⎯ →
⎯⎯⎯⎯⎯ + →
⎯ →
⎯⎯⎯⎯⎯ + ⎯ →
⎯⎯⎯⎯⎯ ⎯ →
⎯⎯⎯⎯⎯
⎯ →
⎯⎯⎯⎯⎯
⎯ →
⎯⎯⎯⎯⎯
⎯ →
⎯⎯⎯⎯
− −
• + − − −
•
+ − + • +
*
+
*
•
+
− +
− + − + − +
− +
− +
*
•
The overall reaction for methane and ethane can be written
as
CO 8H 8e CH 2H O
2 4 2
+ + → +
+ −
2CO 14H 14e C H 4H O
2 2 6 2
+ + → +
+ −
The photocatalytic conversion of CO2 to hydrocarbons
through the carbene pathway is a complex reaction with several
intermediates. The methyl radical (•
CH3*) plays an important
role in the selectivity of the reaction. It requires 14 electrons to
produce ethane from CO2, while it requires eight electrons to
produce methane, thus making the ethane production
significantly more complicated. A photocatalyst, which can
stabilize methyl radical and provide a lot of electron−hole
pairs, is more successful in the production of ethane. If a •
CH3*
reacts with a proton, the product is methane. On the other
hand, ethane can be formed by dimerization of two •
CH3*
radicals. It has been suggested that the Cu+
sites on a
photocatalyst can stabilize the •
CH3* radicals and make the
formation of ethane more favorable.50,58,59
Interestingly, our
results show that ethane can be produced with higher
selectivity through CO2 photoreduction by Ag-TNTA samples
compared to Cu-TNTA samples, which provides empirical
evidence that Ag also enables the production of ethane by
stabilizing the methyl radical. Since the selectivity is higher
(Figure 7b), we deduced that the Ag cocatalyst can stabilize
methyl radicals even better than Cu. We note here that Ag
cocatalyst is known to lower CH4 production and increase C2+
selectivity in the Fischer−Tropsch reaction.60
The highest
selectivity of ethane for AgCu-TNTA samples proves that the
synergistic effect between the Ag and Cu in AgCu bimetallic
cocatalyst can boost methyl radical stabilization and pave the
way for the dominant production of ethane in the CO2
photoreduction process. Furthermore, the dipole repulsive
forces between adsorbed reaction intermediates, particularly
CO2
•−
and CO*, are weakened by the asymmetric surface
charge distribution explained in Section 2.2, which in turn
promotes C−C coupling.
Ultraviolet photoelectron spectra (UPS) were obtained to
determine the energy-level alignment at the metal−semi-
conductor interface in the heterojunction photocatalyst
(Figure S9a,b in the Supporting Information). The resulting
determination of the interfacial band structure confirmed the
presence of a Schottky junction in AgCu-TNTA with a
concomitant depletion region and built-in potential exceeding
1.2 V (Figure S9c,d in the Supporting Information). Since the
number of electron transfer steps required for ethane
formation is more than double that required for methane
generation, carrier recombination losses are known to cripple
ethane production and shift the product selectivity toward C1
products.7
The strong built-in electric field at the interface
reduces recombination losses through facilitating the separa-
tion of photogenerated charge carriers.35,61
This, in turn,
enables the high photocatalytic activity observed in this work
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7255
9. and preserves the selectivity for ethane formation (i.e., C2
product). Hot electrons produced in the metal NP by plasmon
decay are injected over the Schottky barrier into TiO2, where
they possess unusually long lifetimes due to the long dielectric
relaxation time and low hole concentration of TiO2.17,62,63
The
resulting positively charged metal NP (due to the residual
hole) is a powerful stabilizer of the methyl radical.64
The Cu−
CH3
+
metal-ion stabilized methyl radical has a measured
lifetime of at least 50 μs, and metal−carbon bond dissociation
energies of 288 and 177 kJ mol−1
have been calculated for Cu−
CH3 and Ag−CH3, respectively.64,65
Hot spots with a high
local electric field have been suggested to favor the formation
of C2+ products in CO2RR.8
The plasmonic architecture of
AgCu-TNTA samples enables such hot spots, where CO2
molecules can be polarized by intense local electric fields66
(as
pointed our earlier in our discussion of the optical simulations
and spectra), and we surmise that these hot spots favor C2+
products in CO2 photoreduction as well.
To confirm that the reaction products stem from CO2 and
not from any source of carbon contamination, a series of sanity
tests were conducted. The sanity tests performed included
conducting the reaction in a reactor filled with nitrogen gas
(instead of CO2), conducting the reaction in the dark (with
and without photocatalyst), conducting the experiment
without heating the reactor, illuminating the reactor filled
with only water and CO2 (no photocatalyst), and conducting
the reaction using a titanium foil instead of the photocatalyst.
In all of these sanity tests, no product could be detected by the
gas chromatograph, which provides direct evidence that the
simultaneous presence of photocatalyst, water vapor, heat, and
light (solar simulator) is essential for driving the chemical
reaction. In addition, we performed a 13
CO2 isotope labeling
test, in which instead of introducing the regular 12
CO2 gas into
the reactor, 13
CO2 alone was introduced into the reactor while
keeping all other reaction conditions identical. The gaseous
products of the isotope labeling test were performed by gas
chromatography−mass spectrometry (GC−MS), and the
resulting ion chromatogram is shown in Figure S10 in the
Supporting Information. The isotope labeling test confirmed
the presence of 13
CH4 at an e/m value of 17 and 13
C2H6 at an
e/m value of 32.
3. CONCLUSIONS
The next frontier in CO2 photoreduction research is improving
product selectivity. The research focus is shifting from larger
yield numbers to higher-purity and more valuable products. A
whole gamut of chemicals including C1 products (CO, CH4,
and CH3OH) and heavier C2 hydrocarbons (C2H6, C2H5OH)
can be produced during the CO2 photoreduction process.
Heavier hydrocarbons are preferred because of their higher
energy density and higher economic value. Obtaining heavier
hydrocarbons is extremely difficult due to lower yields
(recombination and other losses in more numerous electron
transfer steps), poorer selectivity (more intermediates and
byproducts), interadsorbate repulsion inhibiting C−C cou-
pling, and less kinetic and thermodynamic preference. Herein,
we presented a method toward the preferential formation of C2
products during CO2 photoreduction using TiO2 nanotube
arrays (TNTAs) decorated with large-sized (80−200 nm)
photodeposited AgCu nanoparticles (NPs). Large, close-lying
plasmonic NPs exhibit an asymmetric surface charge
distribution following photoexcitation due to antisymmetric
quadrupole resonances and hybrid multipole plasmonic modes
with electric fields that vary rapidly in space. AgCu-TNTA
exhibited higher absolute product yields (14.5 μmol g−1
h−1
of
ethane and 9.38 μmol g−1
h−1
of methane) and higher
selectivity (60.7% selectivity toward ethane production) for C2
product compared to TNTAs decorated with monometallic Ag
or Cu NPs. We attribute the superior performance to the (i)
improved C−C coupling due to lowered dipole repulsion
between adsorbed reaction intermediates, (ii) Schottky barrier-
mediated long-lived charge separation following excitation and
plasmon decay, (iii) local electric field enhancement at hot
spots, and (iv) enhanced stabilization of methyl radicals by
hole-rich AgCu NPs. To date, the best selectivity for ethane
reported in the literature was achieved through expensive
cocatalysts such as Pt and Au, with the highest reported
selectivity of ∼40% using Au NP cocatalysts. In this report, we
utilized a relatively cheap cocatalyst consisting of AgCu NPs
with an unprecedented selectivity of 60% toward ethane. Our
results show that bimetallic cocatalysts made of relatively cheap
materials have the potential to replace the typical expensive
cocatalysts used in the literature and even outperform them in
selectivity.
■ ASSOCIATED CONTENT
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsami.0c21067.
Materials and methods; schematic illustration of
fabrication process; additional FDTD simulated electric
field profiles; gas chromatograms of CO2 photo-
reduction experiments collected using a gas chromato-
graph equipped with a pulse discharge detector; ion
chromatograms of isotope-labeled CO2 photoreduction
experiments collected using a gas chromatography−mass
spectrometry system (GC−MS); elemental chemical
maps of Cu-TNTA sample; and ultraviolet photon
spectra and band alignment at AgCu-TNTA interface
(PDF)
■ AUTHOR INFORMATION
Corresponding Authors
Ehsan Vahidzadeh − Department of Electrical and Computer
Engineering, University of Alberta, Edmonton, AB T6G 1H9,
Canada; Email: vahidzad@ualberta.ca
Karthik Shankar − Department of Electrical and Computer
Engineering, University of Alberta, Edmonton, AB T6G 1H9,
Canada; orcid.org/0000-0001-7347-3333;
Email: kshankar@ualberta.ca
Authors
Sheng Zeng − Department of Electrical and Computer
Engineering, University of Alberta, Edmonton, AB T6G 1H9,
Canada
Ajay P. Manuel − Department of Electrical and Computer
Engineering, University of Alberta, Edmonton, AB T6G 1H9,
Canada
Saralyn Riddell − Department of Electrical and Computer
Engineering, University of Alberta, Edmonton, AB T6G 1H9,
Canada
Pawan Kumar − Department of Electrical and Computer
Engineering, University of Alberta, Edmonton, AB T6G 1H9,
Canada; orcid.org/0000-0003-2804-9298
ACS Applied Materials & Interfaces www.acsami.org Research Article
https://dx.doi.org/10.1021/acsami.0c21067
ACS Appl. Mater. Interfaces 2021, 13, 7248−7258
7256
10. Kazi M. Alam − Department of Electrical and Computer
Engineering, University of Alberta, Edmonton, AB T6G 1H9,
Canada; National Research Council Nanotechnology
Research Centre, Edmonton, AB T6G 2M9, Canada;
orcid.org/0000-0001-5075-5928
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsami.0c21067
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
All authors thank the Natural Sciences and Engineering
Research Council of Canada (NSERC), the National Research
Council Canada (NRC), and Future Energy Systems CFREF
for direct and indirect (equipment use) financial support.
Scholarship support to APM from the NSERC CGSD program
and to EV from the MITACS Globalink program is
acknowledged. The authors acknowledge use of the deposition
and characterization facilities at the University of Alberta
nanoFAB.
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