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We report on the photocatalytic activity of ZnO layers deposited by atomic layer deposition on a porous anodic aluminum oxide substrate with hexagonal pore symmetry and varied pore dimensions. ZnO/Al2O3 composites were prepared with pore diameters in the range 93–134 nm and interpore distance in the range 185–286 nm, and their photocatalytic activity was measured for gas-phase photocatalytic oxidation of acetaldehyde at varying UV illumination intensities (0.08–3.94 mW cm−2). The results show that substrates with narrower pore diameters (<115 nm, in the case of this study) have a detrimental effect on the photocatalyst performance, despite their higher effective surface. The results are explained on the basis of limited mass transfer inside the porous structure and can be used as a guideline in the purposeful design of photocatalysts with a nanoporous or nanotubular structure.
Bozhidar Stefanov; Blagoy Blagoev; Lars Österlund; Boriana Tzaneva; George Angelov. Effects of Anodic Aluminum Oxide Substrate Pore Geometry on the Gas-Phase Photocatalytic Activity of ZnO/Al2O3 Composites Prepared by Atomic Layer Deposition. Symmetry 2021, 13, 1456 .
AMA StyleBozhidar Stefanov, Blagoy Blagoev, Lars Österlund, Boriana Tzaneva, George Angelov. Effects of Anodic Aluminum Oxide Substrate Pore Geometry on the Gas-Phase Photocatalytic Activity of ZnO/Al2O3 Composites Prepared by Atomic Layer Deposition. Symmetry. 2021; 13 (8):1456.
Chicago/Turabian StyleBozhidar Stefanov; Blagoy Blagoev; Lars Österlund; Boriana Tzaneva; George Angelov. 2021. "Effects of Anodic Aluminum Oxide Substrate Pore Geometry on the Gas-Phase Photocatalytic Activity of ZnO/Al2O3 Composites Prepared by Atomic Layer Deposition." Symmetry 13, no. 8: 1456.
There has been a substantial research effort worldwide to develop non-noble metal catalysts in electrolysers for H2 production from renewable energy sources. Pt catalysts are found to display the highest hydrogen evolution reaction (HER) activity under typical experimental conditions with relatively low acidity and over-potentials. However, it is noted that catalytic activity is highly dependent on acidity and applied potential used. In real practice of a high workload electrolyser high acidity and large negative potentials are required to optimize the HER activity. We hereby report that inexpensive silver catalysts, particularly the cubic form of silver nanoparticles, can clearly exhibit superior HER activity over Pt with a different rate determining step in an electrolyser when such conditions are reached. This is attributed to the weaker Ag-H bond at the surface than Pt-H which is more favorable for H recombination to form H2. It is thus believed that this study provides new insights into designing economical and highly efficient catalysts that can replace the expensive noble metal analogues in a working electrolyser.
Jiaying Mo; Bozhidar I. Stefanov; Thomas H. M. Lau; Tianyi Chen; Simson Wu; Zhiqiang Wang; Xue-Qing Gong; Ian Wilkinson; Günter Schmid; Shik Chi Edman Tsang. Superior Performance of Ag over Pt for Hydrogen Evolution Reaction in Water Electrolysis under High Overpotentials. ACS Applied Energy Materials 2019, 2, 1221 -1228.
AMA StyleJiaying Mo, Bozhidar I. Stefanov, Thomas H. M. Lau, Tianyi Chen, Simson Wu, Zhiqiang Wang, Xue-Qing Gong, Ian Wilkinson, Günter Schmid, Shik Chi Edman Tsang. Superior Performance of Ag over Pt for Hydrogen Evolution Reaction in Water Electrolysis under High Overpotentials. ACS Applied Energy Materials. 2019; 2 (2):1221-1228.
Chicago/Turabian StyleJiaying Mo; Bozhidar I. Stefanov; Thomas H. M. Lau; Tianyi Chen; Simson Wu; Zhiqiang Wang; Xue-Qing Gong; Ian Wilkinson; Günter Schmid; Shik Chi Edman Tsang. 2019. "Superior Performance of Ag over Pt for Hydrogen Evolution Reaction in Water Electrolysis under High Overpotentials." ACS Applied Energy Materials 2, no. 2: 1221-1228.
We demonstrate that spectral selective photocatalytic multilayer films can be tailored such that they can harness the full solar spectrum for enhanced photocatalytic gas-phase oxidation of acetaldehyde. Thin films of anatase TiO2 were deposited on a thin solar absorber TiAlN film to fabricate bilayer TiO2/TiAlN films by dc magnetron sputtering on aluminium substrates. The structural and optical properties of the films were characterized by X-ray diffraction and Raman spectroscopy. The reaction rate and quantum yield for acetaldehyde removal was measured and an almost tenfold enhancement of the quantum yield was observed for the TiO2/TiAlN films compared with the single TiO2 film, on par with enhancements achieved with new heterojunction photocatalysts. The results were interpreted by a temperature-induced change of the reaction kinetics. Absorption of simulated solar light illumination resulted in a temperature increase of the TIAlN film that was estimated to be at most 126 K. We show that a concomitant temperature increase of the top layer TiO2 by 100 K shifts the water gas-surface equilibrium from multilayer to submonolayer coverage. We propose that this is the main reason for the observed enhancement of the photocatalytic activity, whereby gas phase molecules may come in direct contact with free surface sites instead of having to diffuse through a thin water film. The implications of the results for judicious control of temperature and relative humidity for efficient gas-phase photocatalysis and exploitation of selective solar absorbing films are discussed.
Lars Österlund; Andreas Mattsson; Martin Brischetto; Joel Johansson Byberg; Bozhidar I. Stefanov; Yu-Xia Ji; Gunnar A. Niklasson. Spectral Selective Solar Light Enhanced Photocatalysis: TiO2/TiAlN Bilayer Films. Topics in Catalysis 2018, 61, 1607 -1614.
AMA StyleLars Österlund, Andreas Mattsson, Martin Brischetto, Joel Johansson Byberg, Bozhidar I. Stefanov, Yu-Xia Ji, Gunnar A. Niklasson. Spectral Selective Solar Light Enhanced Photocatalysis: TiO2/TiAlN Bilayer Films. Topics in Catalysis. 2018; 61 (15-17):1607-1614.
Chicago/Turabian StyleLars Österlund; Andreas Mattsson; Martin Brischetto; Joel Johansson Byberg; Bozhidar I. Stefanov; Yu-Xia Ji; Gunnar A. Niklasson. 2018. "Spectral Selective Solar Light Enhanced Photocatalysis: TiO2/TiAlN Bilayer Films." Topics in Catalysis 61, no. 15-17: 1607-1614.
Ni doping induces modifications and considerable changes in the optical, electrical and magnetic properties of ZnO films. In this work, the influence is discussed of Ni-doping (two nickel concentrations) and annealing temperature (ranging from 300 ° C to 800 °C) on the structural and optical properties of sol-gel derived ZnO:Ni films. Uniform and smooth films were obtained by spin-coating on quartz and Si substrates. The ZnO:Ni films were crystallized in wurtzite phase with no impurity phases found for annealing temperatures up to 600 °C. The size of the crystallites was strongly affected by the Ni content and the heat treatment. Furthermore, the Ni doping improved the optical transparency of the sol-gel films, while the AFM studies showed that the film morphology and the roughness were influenced by the nickel doping.
T Ivanova; A Harizanova; T Koutzarova; B Vertruyen; B Stefanov. Structural and morphological properties of sol-gel ZnO:Ni films. Journal of Physics: Conference Series 2018, 992, 012044 .
AMA StyleT Ivanova, A Harizanova, T Koutzarova, B Vertruyen, B Stefanov. Structural and morphological properties of sol-gel ZnO:Ni films. Journal of Physics: Conference Series. 2018; 992 (1):012044.
Chicago/Turabian StyleT Ivanova; A Harizanova; T Koutzarova; B Vertruyen; B Stefanov. 2018. "Structural and morphological properties of sol-gel ZnO:Ni films." Journal of Physics: Conference Series 992, no. 1: 012044.
T. Ivanova; A. Harizanova; T. Koutzarova; B. Vertruyen; B. Stefanov. Structural and morphological characterization of sol-gel ZnO:Ga films: Effect of annealing temperatures. Thin Solid Films 2018, 646, 132 -142.
AMA StyleT. Ivanova, A. Harizanova, T. Koutzarova, B. Vertruyen, B. Stefanov. Structural and morphological characterization of sol-gel ZnO:Ga films: Effect of annealing temperatures. Thin Solid Films. 2018; 646 ():132-142.
Chicago/Turabian StyleT. Ivanova; A. Harizanova; T. Koutzarova; B. Vertruyen; B. Stefanov. 2018. "Structural and morphological characterization of sol-gel ZnO:Ga films: Effect of annealing temperatures." Thin Solid Films 646, no. : 132-142.
This work presents morphological, structural and optical studies of ZnO and ZnO:In nanostructured thin films depending on In doping (four different concentrations). XRD study of 600°C annealed ZnO:In films reveals that crystallization strongly depends on indium concentration. The films are crystallized in wurtzite structure and only for the films with highest In addition, two crystal phases are detected wurtzite ZnO and cubic In 2 O 3 . The AFM investigation reveals that the lowest Root Mean Squared Roughness(RMS) is revealed for ZnO:In 0.5 film (15.96 nm) and the roughness increases up to 64.52 nm for ZnO:In 1. Columnar type structures can be observed in the AFM micrographs of the other two films - ZnO:In 2 and ZnO:In 3, as the columns vary in height and size. The effect of the indium doping into ZnO reveals changing of optical transmittance compared to ZnO film. The optical band gap of ZnO:In films, annealed at 600°C is in the range of 3.06-3.27 eV.
Tatyana Ivanova; Antoaneta Harizanova; Tatyana Koutzarova; Benedicte Vertruyen; Bozhidar Stefanov. Morphological study of Sol-Gel derived ZnO:In thin films. 2017 40th International Spring Seminar on Electronics Technology (ISSE) 2017, 1 -6.
AMA StyleTatyana Ivanova, Antoaneta Harizanova, Tatyana Koutzarova, Benedicte Vertruyen, Bozhidar Stefanov. Morphological study of Sol-Gel derived ZnO:In thin films. 2017 40th International Spring Seminar on Electronics Technology (ISSE). 2017; ():1-6.
Chicago/Turabian StyleTatyana Ivanova; Antoaneta Harizanova; Tatyana Koutzarova; Benedicte Vertruyen; Bozhidar Stefanov. 2017. "Morphological study of Sol-Gel derived ZnO:In thin films." 2017 40th International Spring Seminar on Electronics Technology (ISSE) , no. : 1-6.
We present a systematic in situ study of the environmental reaction conditions on the photocatalytic activity of sputter deposited anatase TiO2 films with controlled amounts of preferential 〈0 0 1〉 orientation. In particular, the effects of relative humidity (RH) and substrate temperature (Tsurf) are investigated. It is found that {0 0 1} facets, which are present in higher abundance on highly oriented samples, exhibit an order of magnitude higher reactivity for gas-phase photocatalytic oxidation of the indoor air pollutant acetaldehyde (CH3CHO) than {1 0 1} facets do, and a functional dependence of the reaction rate on facet orientation is determined. It is proposed that water adsorbed on the film contributes to two counteracting effects on the photocatalytic activity: (i) It provides hole acceptors to complete the photo-induced redox cycle and subsequent OH radical formation for pollutant degradation, and (ii) it creates a diffusion barrier between the catalyst interface and pollutant molecules adsorbed in the water layer. As a consequence, increasing Tsurf at high RH has the beneficial effect of removing excess water and reducing the diffusion barrier, thereby improving the photocatalytic activity. A comparison is also made with a commercial anatase TiO2 film, with less developed surface crystallinity and random facet distribution, where the improvement is even more pronounced. Films with a higher degree of orientation exhibit much more stable performance over a range of operating conditions, which suggests that it is possible to tune the effects of water and exposed facet orientation to achieve optimum activity and make TiO2 films amenable to a larger (RH, Tsurf) parameter space for practical applications.
B.I. Stefanov; G.A. Niklasson; C.G. Granqvist; L. Österlund. Gas-phase photocatalytic activity of sputter-deposited anatase TiO2 films: Effect of 〈0 0 1〉 preferential orientation, surface temperature and humidity. Journal of Catalysis 2016, 335, 187 -196.
AMA StyleB.I. Stefanov, G.A. Niklasson, C.G. Granqvist, L. Österlund. Gas-phase photocatalytic activity of sputter-deposited anatase TiO2 films: Effect of 〈0 0 1〉 preferential orientation, surface temperature and humidity. Journal of Catalysis. 2016; 335 ():187-196.
Chicago/Turabian StyleB.I. Stefanov; G.A. Niklasson; C.G. Granqvist; L. Österlund. 2016. "Gas-phase photocatalytic activity of sputter-deposited anatase TiO2 films: Effect of 〈0 0 1〉 preferential orientation, surface temperature and humidity." Journal of Catalysis 335, no. : 187-196.
We demonstrate a quantitative relationship between exposed (001) crystal surfaces and photocatalytic activity of nanocrystalline films prepared by DC magnetron sputtering that follow a square dependence.
Bozhidar I. Stefanov; Gunnar A. Niklasson; Claes G. Granqvist; Lars Österlund. Quantitative relation between photocatalytic activity and degree of 〈001〉 orientation for anatase TiO2 thin films. Journal of Materials Chemistry A 2015, 3, 17369 -17375.
AMA StyleBozhidar I. Stefanov, Gunnar A. Niklasson, Claes G. Granqvist, Lars Österlund. Quantitative relation between photocatalytic activity and degree of 〈001〉 orientation for anatase TiO2 thin films. Journal of Materials Chemistry A. 2015; 3 (33):17369-17375.
Chicago/Turabian StyleBozhidar I. Stefanov; Gunnar A. Niklasson; Claes G. Granqvist; Lars Österlund. 2015. "Quantitative relation between photocatalytic activity and degree of 〈001〉 orientation for anatase TiO2 thin films." Journal of Materials Chemistry A 3, no. 33: 17369-17375.
Bozhidar Stefanov; C G Granqvist; Lars Österlund. Fine control of the amount of preferential orientation in DC magnetron sputtered nanocrystalline TiO2films. Journal of Physics: Conference Series 2014, 559, 012011 .
AMA StyleBozhidar Stefanov, C G Granqvist, Lars Österlund. Fine control of the amount of preferential orientation in DC magnetron sputtered nanocrystalline TiO2films. Journal of Physics: Conference Series. 2014; 559 ():012011.
Chicago/Turabian StyleBozhidar Stefanov; C G Granqvist; Lars Österlund. 2014. "Fine control of the amount of preferential orientation in DC magnetron sputtered nanocrystalline TiO2films." Journal of Physics: Conference Series 559, no. : 012011.
We present a demonstration of online monitoring of gas-phase photocatalytic reactions. A cotton cloth impregnated with commercial titanium dioxide nanoparticles is used as a photocatalytic filter to clean air contaminated with a model pollutant. A fan forces air through the filter while it is irradiated by UV diodes. The concentration of the air pollutant is measured online by an inexpensive, commercially available semiconductor air quality sensor. The structural parts of the reactor were 3D printed in polylactide bioplastic. We provide all schematics, 3D printed model parts, hardware, firmware, and computer code of the reactor and control units. The device can be used for interactive learning of both gas phase photocatalysis and gas sensing, as well as in student laboratory classes for measuring air pollutants and their photodegradation. The experimental setup can also form the basis for a project work for chemical engineering university students, and it can be employed as a building block for development of other gas phase chemical reaction demonstrations.
Bozhidar Stefanov; Delphine Lebrun; Andreas Mattsson; Claes G. Granqvist; Lars Österlund. Demonstrating Online Monitoring of Air Pollutant Photodegradation in a 3D Printed Gas-Phase Photocatalysis Reactor. Journal of Chemical Education 2014, 92, 678 -682.
AMA StyleBozhidar Stefanov, Delphine Lebrun, Andreas Mattsson, Claes G. Granqvist, Lars Österlund. Demonstrating Online Monitoring of Air Pollutant Photodegradation in a 3D Printed Gas-Phase Photocatalysis Reactor. Journal of Chemical Education. 2014; 92 (4):678-682.
Chicago/Turabian StyleBozhidar Stefanov; Delphine Lebrun; Andreas Mattsson; Claes G. Granqvist; Lars Österlund. 2014. "Demonstrating Online Monitoring of Air Pollutant Photodegradation in a 3D Printed Gas-Phase Photocatalysis Reactor." Journal of Chemical Education 92, no. 4: 678-682.
Anatase TiO2 thin films were deposited by DC reactive magnetron sputtering on glass substrates at 20 mTorr pressure in a flow of an Ar and O2 gas mixture. The O2 partial pressure (PO2) was varied from 0.65 mTorr to 1.3 mTorr to obtain two sets of films with different stoichiometry. The structure and morphology of the films were characterized by secondary electron microscopy, atomic force microscopy, and grazing-angle X-ray diffraction complemented by Rietveld refinement. The as-deposited films were amorphous. Post-annealing in air for 1 h at 500 °C resulted in polycrystalline anatase film structures with mean grain size of 24.2 nm (PO2 = 0.65 mTorr) and 22.1 nm (PO2 = 1.3 mTorr), respectively. The films sputtered at higher O2 pressure showed a preferential orientation in the direction, which was associated with particle surfaces exposing highly reactive {001} facets. Films sputtered at lower O2 pressure exhibited no, or very little, preferential grain orientation, and were associated with random distribution of particles exposing mainly the thermodynamically favorable {101} surfaces. Photocatalytic degradation measurements using methylene blue dye showed that oriented films exhibited approximately 30% higher reactivity. The measured intensity dependence of the degradation rate revealed that the UV-independent rate constant was 64% higher for the oriented film compared to randomly oriented films. The reaction order was also found to be higher for films compared to randomly oriented films, suggesting that the oriented film exposes more reactive surface sites.
Bozhidar Stefanov; L. Österlund. Tuning the Photocatalytic Activity of Anatase TiO2 Thin Films by Modifying the Preferred Grain Orientation with Reactive DC Magnetron Sputtering. Coatings 2014, 4, 587 -601.
AMA StyleBozhidar Stefanov, L. Österlund. Tuning the Photocatalytic Activity of Anatase TiO2 Thin Films by Modifying the Preferred Grain Orientation with Reactive DC Magnetron Sputtering. Coatings. 2014; 4 (3):587-601.
Chicago/Turabian StyleBozhidar Stefanov; L. Österlund. 2014. "Tuning the Photocatalytic Activity of Anatase TiO2 Thin Films by Modifying the Preferred Grain Orientation with Reactive DC Magnetron Sputtering." Coatings 4, no. 3: 587-601.
Thin films of W–Ti oxide were prepared by reactive DC magnetron sputtering and were characterized by Rutherford backscattering spectrometry, X-ray diffraction, scanning electron microscopy and atomic force microscopy. The electrochromic properties were studied by cyclic voltammetry in an electrolyte of lithium perchlorate in propylene carbonate and by optical transmittance measurements. The addition of Ti significantly promoted the amorphous nature of the films and stabilized their electrochemical cycling performance and dynamic range for electrochromism.
M.A. Arvizu; C.A. Triana; Bozhidar Stefanov; C.G. Granqvist; Gunnar Niklasson. Electrochromism in sputter-deposited W–Ti oxide films: Durability enhancement due to Ti. Solar Energy Materials and Solar Cells 2014, 125, 184 -189.
AMA StyleM.A. Arvizu, C.A. Triana, Bozhidar Stefanov, C.G. Granqvist, Gunnar Niklasson. Electrochromism in sputter-deposited W–Ti oxide films: Durability enhancement due to Ti. Solar Energy Materials and Solar Cells. 2014; 125 ():184-189.
Chicago/Turabian StyleM.A. Arvizu; C.A. Triana; Bozhidar Stefanov; C.G. Granqvist; Gunnar Niklasson. 2014. "Electrochromism in sputter-deposited W–Ti oxide films: Durability enhancement due to Ti." Solar Energy Materials and Solar Cells 125, no. : 184-189.
Conversion of acetaldehyde to crotonaldehyde on anatase TiO2 films was studied by in situ Fourier transform infrared (FTIR) spectroscopy and by density functional theory (DFT) calculations. In situ FTIR showed that acetaldehyde adsorption is accompanied by the appearance of a hitherto non-assigned absorption band at 1643 cm−1, which is shown to be due to acetaldehyde dimers. The results were supported by DFT calculations. Vibrational frequencies calculated within a partially relaxed cluster model for molecular acetaldehyde and its dimer, and for the corresponding adsorbed species on the anatase (1 0 1) surface, were in good agreement with experimental results. A kinetic model was constructed based on the combined FTIR and DFT results, and was shown to explain the essential features of the acetaldehyde condensation reaction.
B.I. Stefanov; Z. Topalian; C.G. Granqvist; L. Österlund. Acetaldehyde adsorption and condensation on anatase TiO2: Influence of acetaldehyde dimerization. Journal of Molecular Catalysis A: Chemical 2013, 381, 77 -88.
AMA StyleB.I. Stefanov, Z. Topalian, C.G. Granqvist, L. Österlund. Acetaldehyde adsorption and condensation on anatase TiO2: Influence of acetaldehyde dimerization. Journal of Molecular Catalysis A: Chemical. 2013; 381 ():77-88.
Chicago/Turabian StyleB.I. Stefanov; Z. Topalian; C.G. Granqvist; L. Österlund. 2013. "Acetaldehyde adsorption and condensation on anatase TiO2: Influence of acetaldehyde dimerization." Journal of Molecular Catalysis A: Chemical 381, no. : 77-88.
Adsorption and photocatalytic oxidation of acetaldehyde have been investigated on TiO2 and sulfate-modified TiO2 films (denoted SO4TiO2). In situ Fourier transform infrared spectroscopy was used to study surface reactions as a function of time and number of experimental cycles. Spectral analysis and micro-kinetic modeling show that crotonaldehyde formation occurs spontaneously on TiO2 but is impeded on SO4TiO2, where instead acetaldehyde desorption is significant. Photo-oxidation yields significant amounts of formate on TiO2 and was identified as the rate-determining step and associated with site blocking. Significantly smaller amounts of formate were observed on SO4TiO2, which is due to the acidity of this surface resulting in weaker bonding of aldehyde and carboxylate intermediate species. Our results are of considerable interest for applications to photocatalytic air purification and to surfaces with controlled wettability.
Z. Topalian; Bozhidar Stefanov; C.G. Granqvist; L. Österlund. Adsorption and photo-oxidation of acetaldehyde on TiO2 and sulfate-modified TiO2: Studies by in situ FTIR spectroscopy and micro-kinetic modeling. Journal of Catalysis 2013, 307, 265 -274.
AMA StyleZ. Topalian, Bozhidar Stefanov, C.G. Granqvist, L. Österlund. Adsorption and photo-oxidation of acetaldehyde on TiO2 and sulfate-modified TiO2: Studies by in situ FTIR spectroscopy and micro-kinetic modeling. Journal of Catalysis. 2013; 307 ():265-274.
Chicago/Turabian StyleZ. Topalian; Bozhidar Stefanov; C.G. Granqvist; L. Österlund. 2013. "Adsorption and photo-oxidation of acetaldehyde on TiO2 and sulfate-modified TiO2: Studies by in situ FTIR spectroscopy and micro-kinetic modeling." Journal of Catalysis 307, no. : 265-274.
Bozhidar I. Stefanov; Nina V. Kaneva; Gianluca Li Puma; Ceco D. Dushkin. Novel integrated reactor for evaluation of activity of supported photocatalytic thin films: Case of methylene blue degradation on TiO2 and nickel modified TiO2 under UV and visible light. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2011, 382, 219 -225.
AMA StyleBozhidar I. Stefanov, Nina V. Kaneva, Gianluca Li Puma, Ceco D. Dushkin. Novel integrated reactor for evaluation of activity of supported photocatalytic thin films: Case of methylene blue degradation on TiO2 and nickel modified TiO2 under UV and visible light. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2011; 382 (1-3):219-225.
Chicago/Turabian StyleBozhidar I. Stefanov; Nina V. Kaneva; Gianluca Li Puma; Ceco D. Dushkin. 2011. "Novel integrated reactor for evaluation of activity of supported photocatalytic thin films: Case of methylene blue degradation on TiO2 and nickel modified TiO2 under UV and visible light." Colloids and Surfaces A: Physicochemical and Engineering Aspects 382, no. 1-3: 219-225.