Co-reporter:Susann Neubert, Dariusz Mitoraj, Stephen A. Shevlin, Petra Pulisova, Manuel Heimann, Yonghua Du, Gregory K. L. Goh, Michał Pacia, Krzysztof Kruczała, Stuart Turner, Wojciech Macyk, Zheng Xiao Guo, Rosalie K. Hocking and Radim Beranek
Journal of Materials Chemistry A 2016 vol. 4(Issue 8) pp:3127-3138
Publication Date(Web):30 Dec 2015
DOI:10.1039/C5TA07036H
Highly active photocatalysts were obtained by impregnation of nanocrystalline rutile TiO2 powders with small amounts of Cu(II) and Fe(III) ions, resulting in the enhancement of initial rates of photocatalytic degradation of 4-chlorophenol in water by factors of 7 and 4, compared to pristine rutile, respectively. Detailed structural analysis by EPR and X-ray absorption spectroscopy (EXAFS) revealed that Cu(II) and Fe(III) are present as single species on the rutile surface. The mechanism of the photoactivity enhancement was elucidated by a combination of DFT calculations and detailed experimental mechanistic studies including photoluminescence measurements, photocatalytic experiments using scavengers, OH radical detection, and photopotential transient measurements. The results demonstrate that the single Cu(II) and Fe(III) ions act as effective cocatalytic sites, enhancing the charge separation, catalyzing “dark” redox reactions at the interface, thus improving the normally very low quantum yields of UV light-activated TiO2 photocatalysts. The exact mechanism of the photoactivity enhancement differs depending on the nature of the cocatalyst. Cu(II)-decorated samples exhibit fast transfer of photogenerated electrons to Cu(II/I) sites, followed by enhanced catalysis of dioxygen reduction, resulting in improved charge separation and higher photocatalytic degradation rates. At Fe(III)-modified rutile the rate of dioxygen reduction is not improved and the photocatalytic enhancement is attributed to higher production of highly oxidizing hydroxyl radicals produced by alternative oxygen reduction pathways opened by the presence of catalytic Fe(III/II) sites. Importantly, it was demonstrated that excessive heat treatment (at 450 °C) of photocatalysts leads to loss of activity due to migration of Cu(II) and Fe(III) ions from TiO2 surface to the bulk, accompanied by formation of oxygen vacancies. The demonstrated variety of mechanisms of photoactivity enhancement at single site catalyst-modified photocatalysts holds promise for developing further tailored photocatalysts for various applications.
Co-reporter:Susann Neubert;Dr. Ayyappan Ramakrishnan;Dr. Jennifer Strunk;Haoyu Shi;Dr. Bastian Mei;Lidong Wang;Michal Bledowski;Dr. Dmitrii A. Guschin;Max Kauer;Dr. Yuemin Wang;Dr. Martin Muhler ;Dr. Radim Beranek
ChemPlusChem 2014 Volume 79( Issue 1) pp:163-170
Publication Date(Web):
DOI:10.1002/cplu.201300277
Abstract
Surface-modified TiO2 photocatalysts were synthesized by a photosynthetic route involving visible-light-induced (λ>455 nm) activation of benzene and toluene at the surface of TiO2 leading to the formation of carbonaceous polymeric deposits. IR spectroscopic and photoelectrochemical experiments showed that the mechanism of the photosynthetic reactions involves intra-bandgap surface states at TiO2 related to surface OH groups interacting with adsorbed aromatic molecules. The photosynthesized surface-modified TiO2 materials exhibited enhanced activity, relative to pristine TiO2, in photocatalytic degradation (and complete mineralization) of 4-chlorophenol. The improvement was pronounced particularly under visible-light (λ>455 nm) irradiation with the relative initial photodegradation rate enhanced by a factor of four. The surface-modified photocatalysts exhibited good stability under the operating conditions, and the optimum carbon content was approximately 0.5 wt %. Mechanistic studies showed that the enhanced visible-light photodegradation of 4-chlorophenol is due to modified surface-adsorption properties that facilitate formation of a surface complex between titania and 4-chlorophenol, rather than due to any sensitizing effect of the carbonaceous deposits. The study highlights the importance of considering the interaction between pollutant molecules and the photocatalyst surface in heterogeneous photocatalysis, and possibly opens up a route for photosynthesis of further surface-modified photocatalysts with tuned surface properties.
Co-reporter:Michal Bledowski ; Lidong Wang ; Susann Neubert ; Dariusz Mitoraj
The Journal of Physical Chemistry C 2014 Volume 118(Issue 33) pp:18951-18961
Publication Date(Web):August 4, 2014
DOI:10.1021/jp506434a
The efficient coupling between light-harvesting absorbers and cocatalysts allowing for chemical transformation along multielectron pathways is of fundamental importance for the development of solar-fuel-producing photochemical systems. Herein we demonstrate that IrOx nanoparticles acting as efficient cocatalyst for water oxidation can be photoelectrochemically deposited from hexahydroxoiridate solutions into the porous structure of TiO2-PH (polyheptazine, “graphitic carbon nitride”) hybrid photoanodes for water photooxidation. As compared to photoanodes loaded with IrOx by the conventional colloidal deposition method, hybrid photoanodes with photodeposited IrOx exhibit significantly enhanced dioxygen evolution under long-term irradiation with visible light (λ > 420 nm). Photocurrent transient measurements show that the undesired accumulation of holes in the TiO2-PH absorber is significantly reduced due to improved coupling between the absorber and the photodeposited cocatalyst. This decreases significantly the recombination rate, leads to more efficient dioxygen evolution, and improves the stability against photocorrosion. Photocurrent measurements under potentiodynamic conditions revealed that at low bias potentials (<0.6 V vs RHE) the photoconversion efficiency of hybrid photoanodes is limited by fast primary back electron transfer and by reduction of Ir(IV) to Ir(III). The performance and stability of hybrid photoanodes are also found to be drastically influenced by the solution chemistry (electrolyte composition and pH). The highest photoconversion efficiency was observed in sulfate-based electrolytes at pH ∼6.
Co-reporter:Jan M. Macak, Tomas Kohoutek, Lidong Wang and Radim Beranek
Nanoscale 2013 vol. 5(Issue 20) pp:9541-9545
Publication Date(Web):29 Aug 2013
DOI:10.1039/C3NR03014H
Fast and robust infiltration of anodic TiO2 nanotube layers with a model chalcogenide As3S7 glass via spin-coating is reported for the first time. Effective sensitization leads to a significant visible light photocurrent response. This easy and cheap infiltration method can be extended for deposition of other absorbers into nanotubular layers.
Co-reporter:Bastian Mei, Harriet Byford, Michal Bledowski, Lidong Wang, Jennifer Strunk, Martin Muhler, Radim Beranek
Solar Energy Materials and Solar Cells 2013 Volume 117() pp:48-53
Publication Date(Web):October 2013
DOI:10.1016/j.solmat.2013.05.024
•Photoelectrodes based on Nb-doped TiO2 prepared by spray drying were investigated.•Enhanced photocurrents in visible light-active TiO2-polyheptazine hybrids.•The optimum Nb content was 0.1 at%.•The enhancement is due to enhanced mobility of electrons in Nb-doped TiO2 samples.•Effect of doping particularly at longer wavelengths and lower bias potentials.Nb-doped TiO2 in pure anatase form prepared by spray drying exhibits enhanced photoelectrochemical performance both in its bare form (under UV irradiation) and when used as an electron collector in TiO2-polyheptazine hybrid photoanodes for water photooxidation under visible (λ>420 nm) light. The optimum Nb-doping concentration was 0.1 at%, and the enhancement of photocurrents was found to be chiefly due to enhanced mobility of electrons in Nb-doped TiO2. Accordingly, the beneficial effect of Nb doping on photocurrent generation in hybrid photoanodes was pronounced particularly at longer irradiation wavelengths and lower bias potentials.
Co-reporter:Ayyappan Ramakrishnan, Susann Neubert, Bastian Mei, Jennifer Strunk, Lidong Wang, Michal Bledowski, Martin Muhler and Radim Beranek
Chemical Communications 2012 vol. 48(Issue 68) pp:8556-8558
Publication Date(Web):10 Jul 2012
DOI:10.1039/C2CC34243J
Benzene can be activated by visible light (λ > 455 nm) in the presence of TiO2, which leads to formation of carbonaceous polymeric deposits on the titania surface. These photosynthesized surface-modified materials exhibit enhanced photoactivity in degradation of phenolic compounds, particularly under visible light irradiation.
Co-reporter:Michal Bledowski;Lidong Wang;Ayyappan Ramakrishnan;Angélique Bétard;Dr. Oleksiy V. Khavryuchenko; Radim Beranek
ChemPhysChem 2012 Volume 13( Issue 12) pp:3018-3024
Publication Date(Web):
DOI:10.1002/cphc.201200071
Abstract
A cobalt oxide-based oxygen-evolving cocatalyst (Co-Pi) is photodeposited by visible-light irradiation onto nanocrystalline TiO2–polyheptazine (TiO2–PH) hybrid photoelectrodes in a phosphate buffer. The Co-Pi cocatalyst couples effectively to photoholes generated in the surface polyheptazine layer of the TiO2–PH photoanode, as evidenced by complete photooxidation of water to oxygen under visible-light (λ>420 nm) irradiation at moderate bias potentials. In addition, the presence of the cocatalyst also reduces significantly the recombination of photogenerated charges, particularly at low bias potentials, which is ascribed to better photooxidation kinetics resulting in lower accumulation of holes. This suggests that further improvements of photoconversion efficiency can be achieved if more effective catalytic sites for water oxidation are introduced to the surface structure of the hybrid photoanodes.
Co-reporter:Michal Bledowski, Lidong Wang, Ayyappan Ramakrishnan, Oleksiy V. Khavryuchenko, Volodymyr D. Khavryuchenko, P. Carlo Ricci, Jennifer Strunk, Till Cremer, Claudia Kolbeck and Radim Beranek
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 48) pp:21511-21519
Publication Date(Web):07 Nov 2011
DOI:10.1039/C1CP22861G
We investigated photoelectrodes based on TiO2–polyheptazine hybrid materials. Since both TiO2 and polyheptazine are extremely chemically stable, these materials are highly promising candidates for fabrication of photoanodes for water photooxidation. The properties of the hybrids were experimentally determined by a careful analysis of optical absorption spectra, luminescence properties and photoelectrochemical measurements, and corroborated by quantum chemical calculations. We provide for the first time clear experimental evidence for the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO2 (acceptor), which is responsible for a significant red shift of absorption and photocurrent response of the hybrid as compared to both of the single components. The direct optical charge transfer from the HOMO of polyheptazine to the conduction band edge of TiO2 gives rise to an absorption band centered at 2.3 eV (540 nm). The estimated potential of photogenerated holes (+1.7 V vs.NHE, pH 7) allows for photooxidation of water (+0.82 V vs.NHE, pH 7) as evidenced by visible light-driven (λ > 420 nm) evolution of dioxygen on hybrid electrodes modified with IrO2 nanoparticles as a co-catalyst. The quantum-chemical simulations demonstrate that the TiO2–polyheptazine interface is a complex and flexible system energetically favorable for proton-transfer processes required for water oxidation. Apart from water splitting, this type of hybrid materials may also find further applications in a broader research area of solar energy conversion and photo-responsive devices.
Co-reporter:Bastian Mei, Harriet Byford, Michal Bledowski, Lidong Wang, Jennifer Strunk, Martin Muhler, Radim Beranek
Solar Energy Materials and Solar Cells (October 2013) Volume 117() pp:48-53
Publication Date(Web):1 October 2013
DOI:10.1016/j.solmat.2013.05.024
•Photoelectrodes based on Nb-doped TiO2 prepared by spray drying were investigated.•Enhanced photocurrents in visible light-active TiO2-polyheptazine hybrids.•The optimum Nb content was 0.1 at%.•The enhancement is due to enhanced mobility of electrons in Nb-doped TiO2 samples.•Effect of doping particularly at longer wavelengths and lower bias potentials.Nb-doped TiO2 in pure anatase form prepared by spray drying exhibits enhanced photoelectrochemical performance both in its bare form (under UV irradiation) and when used as an electron collector in TiO2-polyheptazine hybrid photoanodes for water photooxidation under visible (λ>420 nm) light. The optimum Nb-doping concentration was 0.1 at%, and the enhancement of photocurrents was found to be chiefly due to enhanced mobility of electrons in Nb-doped TiO2. Accordingly, the beneficial effect of Nb doping on photocurrent generation in hybrid photoanodes was pronounced particularly at longer irradiation wavelengths and lower bias potentials.Download full-size image
Co-reporter:Michal Bledowski, Lidong Wang, Ayyappan Ramakrishnan, Oleksiy V. Khavryuchenko, Volodymyr D. Khavryuchenko, P. Carlo Ricci, Jennifer Strunk, Till Cremer, Claudia Kolbeck and Radim Beranek
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 48) pp:NaN21519-21519
Publication Date(Web):2011/11/07
DOI:10.1039/C1CP22861G
We investigated photoelectrodes based on TiO2–polyheptazine hybrid materials. Since both TiO2 and polyheptazine are extremely chemically stable, these materials are highly promising candidates for fabrication of photoanodes for water photooxidation. The properties of the hybrids were experimentally determined by a careful analysis of optical absorption spectra, luminescence properties and photoelectrochemical measurements, and corroborated by quantum chemical calculations. We provide for the first time clear experimental evidence for the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO2 (acceptor), which is responsible for a significant red shift of absorption and photocurrent response of the hybrid as compared to both of the single components. The direct optical charge transfer from the HOMO of polyheptazine to the conduction band edge of TiO2 gives rise to an absorption band centered at 2.3 eV (540 nm). The estimated potential of photogenerated holes (+1.7 V vs.NHE, pH 7) allows for photooxidation of water (+0.82 V vs.NHE, pH 7) as evidenced by visible light-driven (λ > 420 nm) evolution of dioxygen on hybrid electrodes modified with IrO2 nanoparticles as a co-catalyst. The quantum-chemical simulations demonstrate that the TiO2–polyheptazine interface is a complex and flexible system energetically favorable for proton-transfer processes required for water oxidation. Apart from water splitting, this type of hybrid materials may also find further applications in a broader research area of solar energy conversion and photo-responsive devices.
Co-reporter:Susann Neubert, Dariusz Mitoraj, Stephen A. Shevlin, Petra Pulisova, Manuel Heimann, Yonghua Du, Gregory K. L. Goh, Michał Pacia, Krzysztof Kruczała, Stuart Turner, Wojciech Macyk, Zheng Xiao Guo, Rosalie K. Hocking and Radim Beranek
Journal of Materials Chemistry A 2016 - vol. 4(Issue 8) pp:NaN3138-3138
Publication Date(Web):2015/12/30
DOI:10.1039/C5TA07036H
Highly active photocatalysts were obtained by impregnation of nanocrystalline rutile TiO2 powders with small amounts of Cu(II) and Fe(III) ions, resulting in the enhancement of initial rates of photocatalytic degradation of 4-chlorophenol in water by factors of 7 and 4, compared to pristine rutile, respectively. Detailed structural analysis by EPR and X-ray absorption spectroscopy (EXAFS) revealed that Cu(II) and Fe(III) are present as single species on the rutile surface. The mechanism of the photoactivity enhancement was elucidated by a combination of DFT calculations and detailed experimental mechanistic studies including photoluminescence measurements, photocatalytic experiments using scavengers, OH radical detection, and photopotential transient measurements. The results demonstrate that the single Cu(II) and Fe(III) ions act as effective cocatalytic sites, enhancing the charge separation, catalyzing “dark” redox reactions at the interface, thus improving the normally very low quantum yields of UV light-activated TiO2 photocatalysts. The exact mechanism of the photoactivity enhancement differs depending on the nature of the cocatalyst. Cu(II)-decorated samples exhibit fast transfer of photogenerated electrons to Cu(II/I) sites, followed by enhanced catalysis of dioxygen reduction, resulting in improved charge separation and higher photocatalytic degradation rates. At Fe(III)-modified rutile the rate of dioxygen reduction is not improved and the photocatalytic enhancement is attributed to higher production of highly oxidizing hydroxyl radicals produced by alternative oxygen reduction pathways opened by the presence of catalytic Fe(III/II) sites. Importantly, it was demonstrated that excessive heat treatment (at 450 °C) of photocatalysts leads to loss of activity due to migration of Cu(II) and Fe(III) ions from TiO2 surface to the bulk, accompanied by formation of oxygen vacancies. The demonstrated variety of mechanisms of photoactivity enhancement at single site catalyst-modified photocatalysts holds promise for developing further tailored photocatalysts for various applications.
Co-reporter:Ayyappan Ramakrishnan, Susann Neubert, Bastian Mei, Jennifer Strunk, Lidong Wang, Michal Bledowski, Martin Muhler and Radim Beranek
Chemical Communications 2012 - vol. 48(Issue 68) pp:NaN8558-8558
Publication Date(Web):2012/07/10
DOI:10.1039/C2CC34243J
Benzene can be activated by visible light (λ > 455 nm) in the presence of TiO2, which leads to formation of carbonaceous polymeric deposits on the titania surface. These photosynthesized surface-modified materials exhibit enhanced photoactivity in degradation of phenolic compounds, particularly under visible light irradiation.
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