The lack of active, stable, earth-abundant, and visible-light absorbing materials to replace plas... more 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 cost-effective 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 and (2) 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 n...
The harvesting of hot carriers produced by plasmon decay to generate electricity or drive a chemi... more The harvesting of hot carriers produced by plasmon decay to generate electricity or drive a chemical reaction enables the reduction of the thermalization losses associated with supra-band gap photons in semiconductor photoelectrochemical (PEC) cells. Through the broadband harvesting of light, hot-carrier PEC devices also produce a sensitizing effect in heterojunctions with wide-band gap metal oxide semiconductors possessing good photostability and catalytic activity but poor absorption of visible wavelength photons. There are several reports of hot electrons in Au injected over the Schottky barrier into crystalline TiO2 and subsequently utilized to drive a chemical reaction but very few reports of hot hole harvesting. In this work, we demonstrate the efficient harvesting of hot holes in Au nanoparticles (Au NPs) covered with a thin layer of amorphous TiO2 (a-TiO2). Under AM1.5G 1 sun illumination, photoanodes consisting of a single layer of ∼50 nm diameter Au NPs coated with a 10 nm shell of a-TiO2 (Au@a-TiO2) generated 2.5 mA cm-2 of photocurrent in 1 M KOH under 0.6 V external bias, rising to 3.7 mA cm-2 in the presence of a hole scavenger (methanol). The quantum yield for hot-carrier-mediated photocurrent generation was estimated to be close to unity for high-energy photons (λ < 420 nm). Au@a-TiO2 photoelectrodes produced a small positive photocurrent of 0.1 mA cm-2 even at a bias of -0.6 V indicating extraction of hot holes even at a strong negative bias. These results together with density functional theory modeling and scanning Kelvin probe force microscope data indicate fast injection of hot holes from Au NPs into a-TiO2 and light harvesting performed near-exclusively by Au NPs. For comparison, Au NPs coated with a 10 nm shell of Al2O3 (Au@Al2O3) generated 0.02 mA cm-2 of photocurrent in 1 M KOH under 0.6 V external bias. These results underscore the critical role played by a-TiO2 in the extraction of holes in Au@a-TiO2 photoanodes, which is not replicated by an ordinary dielectric shell. It is also demonstrated here that an ultrathin photoanode (<100 nm in maximum thickness) can efficiently drive sunlight-driven water splitting.
Cu/TiO2 is a well-known photocatalyst for the photocatalytic transformation of CO2 into methane. ... more 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.
Polyethersulfone (PES) is a polymeric permeable material used in ultrafiltration (UF) membranes d... more Polyethersulfone (PES) is a polymeric permeable material used in ultrafiltration (UF) membranes due to its high thermomechanical and chemical stability. The hydrophobic nature of PES membranes renders them prone to fouling and restricts the practical applications of PES in the fabrication of water treatment membranes. The present study demonstrates a non-solvent-induced phase separation (NIPS) approach to modifying PES membranes with different concentrations of discrete TiO2 nanotubes (TNTs). Zeta potential and contact angle measurements showed enhanced hydrophilicity and surface negative charge in TNTs/PES nanocomposite membranes compared to unmodified PES membranes. To discern the antifouling and permeation properties of the TNTs/PES membranes, steam assisted gravity drainage (SAGD) wastewater obtained from the Athabasca oil sands of Alberta was used. The TiO2 modified polymer nanocomposite membranes resulted in a higher organic matter rejection and water flux than the unmodified ...
Polyphosphide–TiO2 hybrid materials, like SnIP@TiO2, are used as photocatalysts for PEC-water-oxi... more Polyphosphide–TiO2 hybrid materials, like SnIP@TiO2, are used as photocatalysts for PEC-water-oxidation: SnIP a double helix semiconductor reacted as nanofibers onto and into TiO2 nanotube arrays. Due to synergetic effects an enhanced water splitting performance was found.
Recent evidence of exponential environmental degradation will demand a drastic shift in research ... more 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...
Heterojunctions of the low bandgap semiconductor bismuth oxyiodide (BiOI) with bulk multilayered ... more Heterojunctions of the low bandgap semiconductor bismuth oxyiodide (BiOI) with bulk multilayered graphitic carbon nitride (g-C3N4) and few layered graphitic carbon nitride sheets (g-C3N4-S) are synthesized and investigated as an active photoanode material for sunlight driven water splitting.
The availability of robust superhydrophobic materials with the ability to withstand harsh environ... more The availability of robust superhydrophobic materials with the ability to withstand harsh environments are in high demand for many applications. In this study, we have presented a simple method to fabricate superhydrophobic materials from TiO₂ nanotube arrays (TNTAs) and investigated the resilience of the materials when they are subjected to harsh conditions such as intense cavitation upon ultrasonication, corrosion in saline water, water-jet impact, and abrasion. The TNTAs were prepared by anodization of Ti foil in buffered aqueous electrolyte containing fluoride ions. The hydrophilic TNTAs were functionalized with octadecylphosphonic acid (ODPA) or 1H, 1H', 2H, 2H'-perfluorodecyl phosphonic acid (PFDPA) to form a self-assembled monolayer on the TNTA surface to produce superhydrophobic ODPA@TNTA or PFDPA@TNTA surfaces. The superhydrophobic ODPA@TNTA and PFDPA@TNTA have contact angles of 156.0° ± 1.5° and 168° ± 1.5°, and contact angle hysteresis of 3.0° and 0.8°, respective...
The lack of active, stable, earth-abundant, and visible-light absorbing materials to replace plas... more 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 cost-effective 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 and (2) 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 n...
The harvesting of hot carriers produced by plasmon decay to generate electricity or drive a chemi... more The harvesting of hot carriers produced by plasmon decay to generate electricity or drive a chemical reaction enables the reduction of the thermalization losses associated with supra-band gap photons in semiconductor photoelectrochemical (PEC) cells. Through the broadband harvesting of light, hot-carrier PEC devices also produce a sensitizing effect in heterojunctions with wide-band gap metal oxide semiconductors possessing good photostability and catalytic activity but poor absorption of visible wavelength photons. There are several reports of hot electrons in Au injected over the Schottky barrier into crystalline TiO2 and subsequently utilized to drive a chemical reaction but very few reports of hot hole harvesting. In this work, we demonstrate the efficient harvesting of hot holes in Au nanoparticles (Au NPs) covered with a thin layer of amorphous TiO2 (a-TiO2). Under AM1.5G 1 sun illumination, photoanodes consisting of a single layer of ∼50 nm diameter Au NPs coated with a 10 nm shell of a-TiO2 (Au@a-TiO2) generated 2.5 mA cm-2 of photocurrent in 1 M KOH under 0.6 V external bias, rising to 3.7 mA cm-2 in the presence of a hole scavenger (methanol). The quantum yield for hot-carrier-mediated photocurrent generation was estimated to be close to unity for high-energy photons (λ < 420 nm). Au@a-TiO2 photoelectrodes produced a small positive photocurrent of 0.1 mA cm-2 even at a bias of -0.6 V indicating extraction of hot holes even at a strong negative bias. These results together with density functional theory modeling and scanning Kelvin probe force microscope data indicate fast injection of hot holes from Au NPs into a-TiO2 and light harvesting performed near-exclusively by Au NPs. For comparison, Au NPs coated with a 10 nm shell of Al2O3 (Au@Al2O3) generated 0.02 mA cm-2 of photocurrent in 1 M KOH under 0.6 V external bias. These results underscore the critical role played by a-TiO2 in the extraction of holes in Au@a-TiO2 photoanodes, which is not replicated by an ordinary dielectric shell. It is also demonstrated here that an ultrathin photoanode (<100 nm in maximum thickness) can efficiently drive sunlight-driven water splitting.
Cu/TiO2 is a well-known photocatalyst for the photocatalytic transformation of CO2 into methane. ... more 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.
Polyethersulfone (PES) is a polymeric permeable material used in ultrafiltration (UF) membranes d... more Polyethersulfone (PES) is a polymeric permeable material used in ultrafiltration (UF) membranes due to its high thermomechanical and chemical stability. The hydrophobic nature of PES membranes renders them prone to fouling and restricts the practical applications of PES in the fabrication of water treatment membranes. The present study demonstrates a non-solvent-induced phase separation (NIPS) approach to modifying PES membranes with different concentrations of discrete TiO2 nanotubes (TNTs). Zeta potential and contact angle measurements showed enhanced hydrophilicity and surface negative charge in TNTs/PES nanocomposite membranes compared to unmodified PES membranes. To discern the antifouling and permeation properties of the TNTs/PES membranes, steam assisted gravity drainage (SAGD) wastewater obtained from the Athabasca oil sands of Alberta was used. The TiO2 modified polymer nanocomposite membranes resulted in a higher organic matter rejection and water flux than the unmodified ...
Polyphosphide–TiO2 hybrid materials, like SnIP@TiO2, are used as photocatalysts for PEC-water-oxi... more Polyphosphide–TiO2 hybrid materials, like SnIP@TiO2, are used as photocatalysts for PEC-water-oxidation: SnIP a double helix semiconductor reacted as nanofibers onto and into TiO2 nanotube arrays. Due to synergetic effects an enhanced water splitting performance was found.
Recent evidence of exponential environmental degradation will demand a drastic shift in research ... more 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...
Heterojunctions of the low bandgap semiconductor bismuth oxyiodide (BiOI) with bulk multilayered ... more Heterojunctions of the low bandgap semiconductor bismuth oxyiodide (BiOI) with bulk multilayered graphitic carbon nitride (g-C3N4) and few layered graphitic carbon nitride sheets (g-C3N4-S) are synthesized and investigated as an active photoanode material for sunlight driven water splitting.
The availability of robust superhydrophobic materials with the ability to withstand harsh environ... more The availability of robust superhydrophobic materials with the ability to withstand harsh environments are in high demand for many applications. In this study, we have presented a simple method to fabricate superhydrophobic materials from TiO₂ nanotube arrays (TNTAs) and investigated the resilience of the materials when they are subjected to harsh conditions such as intense cavitation upon ultrasonication, corrosion in saline water, water-jet impact, and abrasion. The TNTAs were prepared by anodization of Ti foil in buffered aqueous electrolyte containing fluoride ions. The hydrophilic TNTAs were functionalized with octadecylphosphonic acid (ODPA) or 1H, 1H', 2H, 2H'-perfluorodecyl phosphonic acid (PFDPA) to form a self-assembled monolayer on the TNTA surface to produce superhydrophobic ODPA@TNTA or PFDPA@TNTA surfaces. The superhydrophobic ODPA@TNTA and PFDPA@TNTA have contact angles of 156.0° ± 1.5° and 168° ± 1.5°, and contact angle hysteresis of 3.0° and 0.8°, respective...
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Papers by Karthik Shankar