Multilayer WO3/BiVO4 Photoanodes for Solar-Driven Water Splitting Prepared by RF-Plasma Sputtering

A series of WO3, BiVO4 and WO3/BiVO4 heterojunction coatings were deposited on fluorine-doped tin oxide (FTO), by means of reactive radio frequency (RF) plasma (co)sputtering, and tested as photoanodes for water splitting under simulatedAM1.5 G solar light in a three-electrode photoelectrochemical (PEC) cell in a 0.5 M NaSO4 electrolyte solution. The PEC performance and time stability of the heterojunction increases with an increase of the WO3 innermost layer up to 1000 nm. A two-step calcination treatment (600 \ub0C after WO3 deposition followed by 400 \ub0C after BiVO4 deposition) led to a most performing photoanode under back-side irradiation, generating a photocurrent density of 1.7 mA cm-2 at 1.4 V vs. SCE (i.e., two-fold and five-fold higher than that generated by individual WO3 and BiVO4 photoanodes, respectively). The incident photon to current efficiency (IPCE) measurements reveal the presence of two activity regions over the heterojunction with respect to WO3 alone: The PEC efficiency increases due to improved charge carrier separation above 450 nm (i.e., below the WO3 excitation energy), while it decreases below 450 nm (i.e., when both semiconductors are excited) due to electron\u2013hole recombination at the interface of the two semiconductors

Effect of titanium dioxide crystalline structure on the photocatalytic production of hydrogen

The effect of the crystalline phase of TiO 2 (anatase, rutile and brookite) on its photocatalytic activity in hydrogen production from methanol-water vapours has been investigated by testing a series of both home-made and commercial TiO 2 photocatalysts, either bare or surface-modified by deposition of a fixed amount, i.e. 1 wt%, of platinum as co-catalyst. For all of the TiO 2 samples the rate of hydrogen production increased by one order of magnitude upon Pt deposition, because of the ability of Pt to enhance the separation of photoproduced electron-hole pairs. Under irradiation in the 350-450 nm wavelength range, brookite and anatase showed similar photoactivities, both superior to that of rutile. By contrast, rutile, possessing a narrower band gap, was active also under visible light (λ > 400 nm), whereas no hydrogen evolution was observed with anatase and brookite under such conditions. Surface area proved to be a key parameter, strongly influencing photoactivity. However, as the particle size became ultra-small, the semiconductor absorption edge was blue-shifted because of size quantisation effects, with a consequent decrease in hydrogen production rate due to the smaller portion of incident photons absorbed by the photocatalyst. © The Royal Society of Chemistry and Owner Societies 2011

Effective Visible Light Exploitation by Copper Molybdo-tungstate Photoanodes

The need for stable oxide-based semiconductors with a narrow band gap, able to maximize the exploitation of the visible light portion of the solar spectrum, is a challenging issue for photoelectrocatalytic (PEC) applications. In the present work, CuW1 12xMoxO4 (Eg = 2.0 eV for x = 0.5), which exhibits a significantly reduced optical band gap Eg compared with isostructural CuWO4 (Eg = 2.3 eV), was investigated as a photoactive material for the preparation of photoanodes. CuW0.5Mo0.5O4 electrodes with different thicknesses (80 12530 nm), prepared by a simple solution-based process in the form of multilayer films, effectively exhibit visible light photoactivity up to 650 nm (i.e., extended compared with CuWO4 photoanodes prepared by the same way). Furthermore, the systematic investigation on the effects on photoactivity of the CuW0.5Mo0.5O4 layer thickness evidenced that long-wavelength photons can better be exploited by thicker electrodes. PEC measurements in the presence of NaNO2, acting as a suitable hole scavenger ensuring enhanced photocurrent generation compared with that of water oxidation while minimizing dark currents, allowed us to elucidate the role that molybdenum incorporation plays on the charge separation efficiency in the bulk and on the charge injection efficiency at the photoanode surface. The adopted Mo for W substitution increases the visible light photoactivity of copper tungstate toward improved exploitation and storage of visible light into chemical energy via photoelectrocatalysis

Hydrogenation of ZnFe2O4 Flat Films: Effects of the Pre-Annealing Temperature on the Photoanodes Efficiency for Water Oxidation

The effects induced by post-synthesis hydrogenation on ZnFe2O4 flat films in terms of photoelectrochemical (PEC) performance of photoanodes for water oxidation have been deeply investigated as a function of the pre-annealing temperature of the materials. The structure and morphology of the films greatly affect the efficacy of the post synthesis treatment. In fact, highly compact films are obtained upon pre-annealing at high temperatures, and this limits the exposure of the material bulk to the reductive H2 atmosphere, making the treatment largely ineffective. On the other hand, a mild hydrogen treatment greatly enhances the separation of photoproduced charges in films pre-annealed at lower temperatures, as a result of the introduction of oxygen vacancies with n-type character. A comparison between present results and those obtained with ZnFe2O4 nanorods clearly demonstrates that specific structural and/or surface properties, together with the initial film morphology, differently affect the overall contribution of post-synthesis hydrogenation on the efficiency of zinc ferrite-based photoanodes

Bismuth vanadate photoanodes for water splitting deposited by radio frequency plasma reactive co-sputtering

Photoactive bismuth vanadate (BiVO4) thin coatings were deposited on fluorine-doped tin oxide glass by plasma reactive sputtering from Bi2O3 and vanadium (V) radio frequency (RF) powered targets. The films were characterized by x-ray diffraction, scanning electron microscopy, energy dispersion spectroscopy, and UV-vis spectroscopy. The effects that the power density supplied to the Bi2O3 target, the post-annealing treatment, and the film thickness have on the structural features and on the photoelectrochemical (PEC) performances of the so obtained BiVO4 film-based photoelectrodes were investigated. Their PEC performance in water splitting was evaluated in a three-electrode cell by both incident photon to current efficiency (IPCE) and linear sweep voltammetry measurements under AM 1.5 G simulated solar light irradiation. A monoclinic phase of BiVO4, which is more photoactive than the tetragonal BiVO4 phase, was obtained by optimizing the power density supplied to the Bi2O3 target, i.e., by tuning the Bi:V:O atomic ratio. The best PEC performance was obtained for a stoichiometric 1:1 Bi:V atomic ratio, attained with 20 W power supplied at the Bi2O3 target and 300 W power supplied at the vanadium target, and an optimal 200 nm thickness of the BiVO4 film, with a 0.65 mA/cm2 photocurrent density attained at 1.23 V vs. standard calomel electrode, under simulated solar light. These results show the suitability of plasma reactive sputtering with two RF powered electrodes for the deposition of BiVO4 photoanodes for water splitting

Complex Power Distribution Network Investigation Using SPICE Based Extraction from First Principle Formulations

The modeling and the analysis of the power distribution networks (PDN) within multi-layer printed circuit board is crucial for the investigation of the performance of PCB systems. Carrying out such analyses in SPICE based tools has the advantage of being faster than the corresponding full-wave modeling and it allows obtaining both frequency and time domain results

TiO2-based materials for photocatalytic hydrogen production

Hydrogen, the cleanest and most promising energy vector, can be produced by solar into chemical energy conversion, either by the photocatalytic direct splitting of water into H2 and O2, or, more efficiently, in the presence of sacrificial reagents, e.g., in the so-called photoreforming of organics. Efficient photocatalytic materials should not only be able to exploit solar radiation to produce electron-hole pairs, but also ensure enough charge separation to allow electron transfer reactions, leading to solar energy driven thermodynamically up-hill processes. Recent achievements of our research group in the development and testing of innovative TiO2-based photocatalytic materials are presented here, together with an overview on the mechanistic aspects of water photosplitting and photoreforming of organics. Photocatalytic materials were either i) obtained by surface modification of commercial photocatalysts, or produced ii) in powder form by different techniques, including traditional sol gel synthesis, aiming at engineering their electronic structure, and flame spray pyrolysis starting from organic solutions of the precursors, or iii) in integrated form, to produce photoelectrodes within devices, by radio frequency magnetron sputtering or by electrochemical growth of nanotube architectures, or photocatalytic membranes, by supersonic cluster beam deposition

In situ attenuated total reflection infrared spectroscopy study of the photocatalytic steam reforming of methanol on Pt/TiO2

The effect of Pt deposition on TiO2 and of Pt particle size on the photocatalytic steam reforming of methanol was studied by in situ attenuated total reflectance infrared spectroscopy (ATR-IR). Two 0.5 wt.% Pt/TiO2 samples were investigated, one possessing Pt nanoparticles of ca. 4 nm mean size, the other Pt clusters of ca. 1.3 nm mean size showing significantly different photoactivity in terms of both hydrogen production rate and selectivity to CO, CO2 and all other by-products. The presence of Pt nanoparticles strongly affected both the adsorption/desorption and the reactivity properties of the TiO2 surface. Moreover, the variation of the IR spectrum background upon UV\u2013vis irradiation proved that the photopromoted electrons can be trapped by the Pt particles with the consequent increase of electron-hole separation. Reducing the Pt size from nanoparticles to clusters increased the rate of methanol and water absorption and hindered the detrimental formation of irreversibly adsorbed CO on Pt. All of these aspects contribute to increase the photocatalytic performance of Pt cluster-decorated TiO2 with respect to Pt nanoparticles containing TiO2. Finally, prolonged exposure of all samples to methanol/water vapour in the dark led to the formation of unreactive formate which persisted also under UV\u2013vis irradiation. By contrast, this spectator species did not form when the sample was exposed to methanol/water vapour under UV\u2013vis irradiation

Innovative photoelectrocatalytic water remediation system for ammonia abatement

Ammonia, produced by human and animal activities, contributes to water and soil pollution because it is toxic for aquatic flora and fauna, and responsible for eutrophication. In this work, the photoelectrocatalytic (PEC) oxidation of ammonia is investigated employing a stainless-steel PEC reactor, consisting of a central UV Hg-vapor lamp surrounded by a metallic Ti mesh coated with a photoactive TiO2 film, directly grown by Plasma Electrolytic Oxidation (PEO). The so prepared TiO2 film is characterized by XRD, SEM, UV–vis DRS and IPCE. The PEC reactor operates at 4 V potential drop between the TiO2 coated mesh (photoanode) and the body of the reactor (cathode). The effect of the operating parameters (recirculation flowrate and air bubbling) and type of electrolyte solution (KCl or K2SO4) on the PEC performance are investigated in terms of ammonia conversion and selectivity to nitrite, nitrate and molecular nitrogen. Full ammonia conversion (XNH3) with a selectivity to molecular nitrogen up to 67 % are attained after 12 h in 5 mM KCl electrolyte solution. Nitrite is produced within the first 6 h irradiation and then fully converted into nitrate. By contrast, only a slight XNH3 (ca. 10 %) is observed in K2SO4 electrolyte solution. These results suggest that chlorine has a crucial role in the ammonia PEC oxidation process: photo-generated holes on the photoanode surface can oxidize Cl− to Cl• (electro-induced process), which is a reactive radical able to oxidase ammonia

Fabrication of Pt/Ti/TiO2 Photoelectrodes by RF-Magnetron Sputtering for Separate Hydrogen and Oxygen Production

Evolution of pure hydrogen and oxygen by photocatalytic water splitting was attained from the opposite sides of a composite Pt/Ti/TiO2 photoelectrode. The TiO2 films were prepared by radio frequency (RF)-Magnetron Sputtering at different deposition time ranging from 1 up to 8 h and then characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet-visible-near infrared (UV-vis-NIR) diffuse reflectance spectroscopy. The photocatalytic activity was evaluated by incident photon to current efficiency (IPCE) measurements and by photocatalytic water splitting measurements in a two-compartment cell. The highest H2 production rate was attained with the photoelectrode prepared by 6 h-long TiO2 deposition thanks to its high content in the rutile polymorph, which is active under visible light. By contrast, the photoactivity dropped for longer deposition time, because of the increased probability of electron-hole recombination due to the longer electron transfer path