Search

Photoelectrochemical (PEC) water splitting converts solar light and water into oxygen and energy-rich hydrogen. WO

In this study, the effects of photo-electrocatalysis (PEC) were evaluated as an innovative application of conventional photocatalysis (PC) to remediate water in a recirculating system for rainbow trout (Oncorhynchus mykiss) culture, in relation to fish welfare and health, with a multidisciplinary approach. Three tanks were employed, equipped with conventional biological filters as a control system, and three tanks equipped with the PEC purification system. The concentrations of ammonia, nitrite and nitrate ions in water were monitored, and the fish’s oxidative damage and stress response were evaluated in parallel. The water of the PEC-treated experimental group showed lower ammonia (TAN) and nitrite concentrations and higher nitrate concentration, possibly deriving from TAN oxidation through PEC, also leading to gaseous N2. Histological analysis did not reveal any pathological alteration in the gills and liver of both groups. The superoxide dismutase (sod1), glutathione reductase (GR), glutathione peroxidase (GPx1), and Tumor necrosis factor (TNFα) gene expressions were significantly higher in the control group than in the PEC-treated group, while the Heat shock protein 70 (Hsp70) expression did not show any difference in the two groups. These results indicate that the use of PEC filters has a positive effect on water quality, compared to the use of conventional biological filters, inducing a high level of welfare in O. mykiss.

The interplay between high film crystallinity and n-type doping in enhancing the performance of ZnFe2O4 thin film photoanodes has been revealed. Maximum benefit was achieved for the ca. 300 nm-thick photoactive layer with superior photon harvesting.

The Surfaces Special Issue entitled “Surface Aspects of Semiconductor Photochemistry” is mainly devoted to the 7th International Conference on Semiconductor Photochemistry (SP7), which was held on 11–14 September 2019 in Milano, Italy, in the beautiful Renaissance “Ca’ Granda” main building of the University Milan [...]

Porous crystalline Ni-doped TiO2 films were produced using DC plasma electrolytic oxidation in refrigerated H2SO4 aqueous solutions containing NiSO4. The crystalline phase structure consisted of a mixture of anatase and rutile, ranging from ~30 to ~80 wt % rutile. The oxide films obtained at low NiSO4 concentration showed the highest photocurrent values under monochromatic irradiation in the UV-vis range, outperforming pure TiO2. By increasing NiSO4 concentration above a threshold value, the photoelectrochemical activity of the films decreased below that of undoped TiO2. Similar results were obtained using cyclic voltammetry upon polychromatic UV-vis irradiation. Glow discharge optical emission spectrometry (GD-OES) analysis evidenced a sulfur signal peaking at the TiO2/Ti interface. XPS spectra revealed that oxidized Ni2+, S4+ and S6+ ions were included in the oxide films. In agreement with photocurrent measurements, photoluminescence (PL) spectra confirmed that less intense PL emission, i.e., a lower electron-hole recombination rate, was observed for Ni-doped samples, though overdoping was detrimental.

BiVO4 is an efficient and stable visible active photoanode material. However, due to its unfavorable conduction band position which falls below the H2 reduction potential, it fails to carry out the complete water splitting reaction. On the other hand, larger band gap TiO2 is able to photocatalytically split water, thanks to its negative conduction band energy. Aiming at verifying the possibility of sensitizing TiO2 with BiVO4 and employing the so obtained composite material in photocatalytic water splitting under visible light, we prepared and photoelectrochemically characterized TiO2/BiVO4 heterojunction electrodes. The photocatalytic reduction of methyl viologen, an electron acceptor probe with a reduction potential close to that of protons, was used to evaluate the reducing ability of the photoactive materials under visible light. An apparently counterintuitive electron transfer from photoexcited BiVO4 to the TiO2 conduction band occurs in the TiO2/BiVO4 heterojunction, resulting in TiO2 sensitization and production of highly reductive electrons, which appears to be favored by the band alignment occurring at the heterojunction.

CuWO4 is a ternary metal oxide semiconductor with promising properties for photoelectrochemical (PEC) water splitting and solar light conversion, due to its quite low band gap (2.3 eV) and high stability in an alkaline environment. Aiming at understanding the origin of the relatively low PEC efficiency attained with CuWO4 photoanodes, we here investigate transparent CuWO4 electrodes prepared by a simple solution-based method through the combination of femtosecond transient absorption spectroscopy with electrochemical, PEC, and photochromic characterizations. The very fast recombination dynamics of the charge carriers photogenerated in CuWO4, which is the reason for its low efficiency, is discussed in relation with its PEC performance and with the recently calculated band structure of this material, also in comparison with the behavior of other semiconductor oxides employed in PEC applications, in particular Fe2O3.

Energy is the most important issue of the 21st century. Due to severe environmental problems, mainly related to air pollution of urban areas and green house effects leading to global warming, fossil fuels need to be replaced by renewable energy sources. Beside the wind energy, which is progressively more and more exploited, solar energy represents a clean, abundant, diffuse and economical energy which can be converted into heat, electricity or in the form of so called solar fuels, provided special materials are developed able to efficiently absorb solar light and produce charge separation. This is the prerequisite for obtaining electricity from sunlight and solar fuels, e.g. hydrogen from water photosplitting or carbon containing species from carbon dioxide photoinduced reduction, which can be stored and used on need. Converting solar energy into chemical energy is thus a big challenge for chemistry.

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 simulated AM 1.5 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 &deg, C after WO3 deposition followed by 400 &deg, C after BiVO4 deposition) led to a most performing photoanode under back-side irradiation, generating a photocurrent density of 1.7 mA cm&minus, 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&ndash, hole recombination at the interface of the two semiconductors.

The dynamics of photopromoted electrons in BiVO4, WO3, and WO3/BiVO4 heterojunction electrodes has been directly probed by transient absorption (TA) midinfrared (mid-IR) spectroscopy in the picosecond to microsecond time range. By comparison of the dynamics recorded with the two individual oxides at 2050 cm–1 with that of the heterojunction system after excitation at different wavelengths, electron-transfer processes between selectively excited BiVO4 and WO3 have been directly tracked for the first time. These results support the charge carrier interactions which were previously hypothesized by probing the BiVO4 hole dynamics through TA spectroscopy in the visible range. Nanosecond mid-IR TA experiments confirmed that charge carrier separation occurs in WO3/BiVO4 electrodes under visible-light excitation, persisting up to the microsecond time scale.

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–vis 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–vis irradiation. By contrast, this spectator species did not form when the sample was exposed to methanol/water vapour under UV–vis irradiation.

In the present work, two H2 evolution photocatalysts were prepared by employing two different oxides, TiO2 and zirconium titanate (ZrTiO4), as the support of various copper phases. For both the supports the same Cu loading (0.5% w/w) was adopted, but two different impregnation procedures have been followed, leading to different forms of Cu in the final composite material that are: (i) Cu(II) species dispersed on the oxide surface and (ii) Cu2O particles dispersed on the oxide surface. The present paper based on the parallel use of photocatalytic test and spectroscopic analysis performed in catalytic conditions illustrates the evolution of photocatalytic systems occurring during the H2 evolution reaction tests, pointing out that the as-prepared materials represent a pre-catalyst and they are modified during irradiation leading to the real working systems different from the starting ones. The herein presented spectroscopic analysis aims to contribute to the living debate on the oxidation state of copper in mixed Cu/oxide materials and on its role in hydrogen evolution under photocatalytic conditions.

Photoelectrodes with the WO3/BiVO4 heterojunction architecture are among the most efficient semiconductor oxide-based photoanodes for water oxidation. However, because of the different experimental conditions employed in their photoelectrochemical characterization (in particular the nature and concentration of the contact electrolyte and the pH of the solution), a direct and unambiguous comparison of their performance obtained in different laboratories is often quite difficult. In this work, we systematically tested identical WO3/BiVO4 photoanodes in contact with solutions of the most commonly used electrolytes, i.e. Na2SO4 and the potassium phosphate buffer (KPi), at various electrolyte concentration and pH. Furthermore, WO3/BiVO4 photoanodes protected with a Ni/Fe oxyhydroxide oxygen evolution catalyst were also tested under identical conditions. In contact with KPi solutions, both unprotected and protected electrodes produce higher photocurrent than in contact with Na2SO4 solutions, but they proved to ...

Aiming at clarifying the interplay on TiO2 photoactivity between particle morphology and surface fluorination, the photocatalytic performance of anatase nanocrystals, characterized by a pseudo-spherical shape or a nanosheet structure, is investigated in both a reduction and an oxidation reaction, either in the absence or in the presence of added fluoride anions. Cr(VI) photocatalytic reduction is strongly favored by a large exposure of anatase {001} facets; however, surface fluorination leads in this case to a morphology-independent photoactivity decrease, due to the decreased adsorption of the reaction substrate. More interestingly, a beneficial synergistic effect between the platelet-like anatase morphology and TiO2 surface fluorination is clearly outlined in Rhodamine B photocatalytic degradation, possibly resulting from the intrinsic ability of fluorinated {001} anatase facets of boosting •OH radical mediated oxidation paths, due to their larger amount of surface –OH groups, as revealed using Fourier-transform infrared spectroscopy.

The role of Mo6+ doping on the photoelectrochemical (PEC) performance of BiVO4 photoanodes was investigated both in the presence and in the absence of sulfite as hole scavenger. Optically transparent, flat BiVO4 photoanodes containing different amounts of Mo6+ dopant were synthesized by spin coating. An increase of Mo6+ dopant amount was found to both improve the electron transport in the BiVO4 bulk by increasing its conductivity, as unequivocally ascertained when employing a Ni/Fe oxyhydroxide co-catalyst, and facilitate the charge transfer at the electrode/electrolyte interface in water oxidation, in the absence of hole scavenger. On the other hand, increasing amounts of the Mo6+ dopant in BiVO4 induced an unexpected decrease in PEC performance per unit surface area in sulfite oxidation, resulting from enhanced interfacial charge transfer resistance, as demonstrated by electrochemical impedance spectroscopy. First evidence is thus provided of a different behaviour observed upon Mo6+ doping of BiVO4 depending on the nature of the involved electron donor species, together with an intriguing multifaceted role played by Mo6+ doping in enhancing the PEC performance of modified BiVO4 electrodes.

A series of TiO2–based photocatalysts obtained by deposition of Pt nanoparticles (NPs) has been tested in the photocatalytic decomposition of aqueous ammonia under UVA irradiation. Two main deposition routes were employed, i.e. (i) the deposition of surfactant-stabilized preformed metal NPs and (ii) a modified version of the well-known deposition-precipitation technique, employing urea as precipitating agent. The effects that the deposition route and the amount of deposited metal have on both ammonia conversion (XNH3) and selectivity (SY) towards the different N-containing products (Y = N2, NO2−, NO3−) have been investigated systematically, in relation to the morphological distribution of NPs on TiO2, as evidenced by HR-TEM analysis. The combination of Pt (0.8 wt.%) with a relatively low amount of Ru NPs (0.1 wt.%) as co-catalysts, both deposited on TiO2 under optimized conditions, results in an exceptional stabilization of nitrite ions (SNO2− ∼ 70%), from which N2, the most desired ammonia oxidation product, might subsequently be obtained catalytically.

Photocatalytic CO 2 reduction is a useful way to convert solar energy into fuels, mimicking artificial photosynthesis. The gas phase photocatalytic reduction of CO 2 was investigated with a series of TiO 2 -based photocatalysts, either bare or modified by Cu(II) grafting and/or by Pt nanoparticles deposition and the results obtained in this reaction were compared with those obtained in the photocatalytic production of hydrogen by photosteam reforming of methanol-water mixtures employing the same photocatalysts series. The rates of the two reactions largely depend on the type of photocatalyst and exhibit an almost parallel behaviour, both being higher with photocatalysts containing noble metal nanoparticles. However, the yields in CH 4 , CO and other products obtained from photocatalytic CO 2 reduction still remain very low and often decline during the runs. Furthermore, methane was found to be produced under irradiation when the photocatalysts were contacted with a gas phase containing water vapour and no carbon dioxide. This points to a major role of carbon-containing impurities on the photocatalyst surface, which may act both as hole scavengers and as carbon source. Two step photocatalytic tests, i.e. a cleaning step in the presence of water vapour followed by purging of the gas phase and a second step after CO 2 addition, allow one to discern between the carbon-containing gaseous products originated from carbon impurities and those produced in the presence of CO 2 . In this second step, the presence of copper grafted on the photocatalyst surface favours CO evolution with respect to fully reduced CH 4 .

A novel Ni-based 2,2′-bisdipyrrinato complex bearing peripheral benzoic acid groups (Ni-bisdpmCOOH) has been synthesized and characterized. Electrochemical investigations revealed a high energy LUMO level that, combined with its absorption properties extending up to the NIR region, is compatible with its use as a sensitizer in thermodynamically uphill photocatalytic reactions, such as proton and carbon dioxide photoreduction. A sensitized photo-anode was prepared by adsorbing Ni-bisdpmCOOH onto a TiO2 film, which showed an incident photon to current efficiency, measured at 0.62 V vs. RHE, extending up to the visible light region. This demonstrates that electronically excited Ni-bisdpmCOOH is effectively able to inject electrons into the conduction band of TiO2, thus opening the way for effective TiO2 sensitization or MOF implementation of the molecule itself for energetic applications.

The PEC performance of WO

[Excerpt] This special issue of Catalysis Today is a compilation of full articles from selected presentations made during the 4th Photocatalytic and Superhydrophilic Surfaces Workshop, PSS2017, which took place in Manchester, UK on the 7th/8th December 2017. There is increasing worldwide interest in (predominantly) titania-based photocatalytic and superhydrophilic surfaces for applications including self-cleaning windows and surfaces, water treatment, anti fogging lenses, antimicrobial surfaces, solar fuels and air purification. In applications such as these, titania requires UV light for activation. Consequently, new visible light activated materials are also subject to considerable industrial and academic R&D. However, there are a multitude of production and testing techniques currently in use, often resulting in conflicting results, hampering greater understanding of the photocatalytic process and the further development of new products. PSS2017 aimed to address issues relating to the production, development and testing of photocatalytic materials by bringing together product end users with academics, industrialists and related professionals from across this field. Subject areas represented included chemical and surface engineering, physics, physical chemistry, catalysis and materials science, clearly indicating the multidisciplinary nature of the field. [...]

Differently shape-controlled anatase TiO2 materials were tested as photocatalysts under both simulated and natural solar irradiation. Their photocatalytic activity in 2-propanol and propene partial oxidation and in the complete mineralization of acetaldehyde appears to increase with decreasing the shape control of the TiO2 material. This insight seems to be related to the residual presence of the templating species (fluoride anions) employed during the preparation of the shape-controlled TiO2 materials. In fact, the calcination of the powders, leading to levelling of the fluoride ions content, but also to a remarkable surface area decrease, gave rise to an increase of photocatalytic activity per unit surface area of the materials. In these photocatalytic materials two opposing effects may concur in determining their photoactivity, i.e. the possibly beneficial effect of an increased extent of exposed {001} facets and the detrimental effect resulting from a larger amount of residual fluoride ions employed as capping agents during their synthesis.

The WO3/BiVO4 heterojunction is a promising photoanode architecture for water splitting applications. Here, the photoinduced charge carrier dynamics occurring in this system in operando photoelectrochemical conditions, i.e., under an applied anodic potential, are studied through femtosecond transient absorption spectroscopy to unveil the effects of an applied bias on the early charge carrier dynamics following WO3/BiVO4 excitation. Electrochromic measurements on BiVO4 suggest the presence of intraband gap (IBG) states in this oxide, which play an important role in the charge carrier dynamics in the heterojunction. The differences observed in WO3/BiVO4 with respect to individual BiVO4 electrodes are rationalized in terms of the electron equilibration between the two oxides at the heterojunction, dominated by the WO3/BiVO4 interfacial electric field arising from their band energy offset, and the bias-dependent alteration of the IBG states, both determining the rate of hole transfer and accumulation at the BiVO4 surface.

Improving the performance of mostly employed anatase TiO 2 photocatalysts by properly controlling their crystal shape represents a big challenge to improve their efficiency in photocatalytic applications. After the synthesis, reported in 2008, of anatase microcrystals enriched in high-energy {001} facets, many efforts have been made aimed at tuning the crystal morphology of anatase, by means of either fluorine-mediated or more environmentally friendly methods, producing a deviation from its regular crystal growth. In this relatively new field of investigation, controversial opinions emerged concerning the role of each type of facet and its relative amount in relation to photoefficiency optimization. This review addresses this topic by presenting a critical survey of selected literature reports. After a brief introduction on the main synthetic strategies adopted to obtain shape-controlled anatase photocatalysts, the attention is focused on the methods employed for their comprehensive characterization, including the identification and quantification of exposed facets and the assessment of their influence on bulk and surface properties relevant to photoactivity. Potential interferences, derived from synthetic routes and possibly affecting the conclusions of facet-dependent photoactivity investigations, are also discussed. Key examples of test reactions actually demonstrating how both the type and/or the amount of specific facets influence photocatalytic activity are finally reported, aiming at providing rational bases for the design of better performing shape-controlled anatase photocatalysts.

The photocatalytic performance of a series of fluorinated TiO 2 samples (F for O molar substitution ranging from 0 to 1.5 at.%) synthesized by flame spray pyrolysis (FSP) in single step is examined in relation to the results of several complementary characterization techniques, including BET, XRD, XPS and UV–vis analyses. While the rate of photocatalytic hydroxyl radicals production increased when the photocatalyst contained small amounts of fluorine, the rate of formic acid mineralization was maximum with flame-made pure TiO 2 , showing an activity even better than that of P25 TiO 2 in experiments performed in the presence of the same photocatalyst amount. However, the reaction rate decreased with increasing the nominal fluorine content of the materials, as expected for reactions occurring by direct interaction of the substrate with photoproduced holes. On the other hand, incident photon to current efficiency measurements evidenced that the presence of fluorine in the photocatalyst is beneficial, 0.29 at.% effective F for O substitution in TiO 2 leading to the best performing photocatalyst powder, in line with previous results obtained in photocatalytic hydrogen production from methanol steam reforming with similarly FSP-prepared TiO 2 -based materials also containing platinum nanoparticles.

Aiming at producing photocatalysts with minimized photoproduced electron – hole pairs recombination, a series of titanium – tungsten mixed oxides has been prepared, by coupling TiO2 with different amounts of WO3, according to an alkaline-catalyzed sol-gel method followed by an incipient wetting procedure. The photocatalysts were characterized by surface and bulk techniques and tested in both an oxidation and a reduction photocatalytic reaction, i.e. in formic acid (FA) mineralization and in Cr(VI) reduction. Tungsten was mainly present as hexagonal WO3 on the TiO2 surface, though a fraction of W migrates into the TiO2 lattice substituting Ti atoms, as evidenced by XRPD analysis. Different photoactivity scales were found in the two test reactions, both occurring in the same pH range under similar substrate-photocatalyst electrostatic interactions. In fact, photoexcited electrons transferred from the conduction band (CB) of TiO2 to the CB of coupled WO3, being energetically unable to reduce O2 molecules, easily recombine with photoproduced holes, with a consequent photoactivity decrease in FA photo-mineralization with increasing W/Ti ratio. On the contrary, coupling TiO2 with small WO3 amounts (0.2–1.0 mol%) is beneficial in the removal of Cr2O72− anions. These species, being characterized by a redox potential more positive than the CB edge of WO3, may efficiently accept the electrons trapped in WO3 domains, converting into less toxic Cr(III) species. Thus, WO3 surface domains effectively promote photoproduced charge separation by efficiently trapping CB electrons; increased photocatalytic efficiency depends on the redox potential of the electron acceptor species directly (or indirectly) involved in the photocatalytic process.

A catalyst series consisting in platinum nanoparticles photodeposited on pure titania and on W/Ti mixed oxides, these latter prepared by the sol-gel method, were tested in the hydrogenation of 4-nitrostyrene. A remarkable increase in the reaction rate occurred when the catalyst support contained tungsten, with a parallel boosting in the selective reduction of the nitro group. With the selective W-containing catalysts, the reaction proceeded at constant rate (zero order rate law), while the tungsten-free catalyst showed a rate-dependence on the 4-nitrostyrene concentration (positive order reaction). The presence of tungsten in the support is beneficial not only because a higher surface area is obtained, thanks to the stabilization of anatase owing to the presence of tungsten, but also because it allows the photodeposition of smaller, better dispersed platinum particles, on which the adsorption of the aromatic part of 4-nitrostyrene is less favored. Tungsten not only substitutes titanium in the titania lattice, as revealed by HAAF-STEM analysis, but it is also present as WOx species partly covering the Pt nanoparticles photodeposited on the mixed oxide support, as revealed by an in depth distribution XPS analysis. This accounts for the progressively lower performance observed with increasing tungsten content in the catalysts, the highest conversion and selective hydrogenation of the 4-nitrostyrene nitro group having been achieved on the catalyst with a 1% W/Ti molar ratio.

Nanocrystalline materials are widely employed as photocatalysts for water splitting applications. In particular, cadmium selenide quantum dots (QDs) attracted growing interest for their superior photon to H2 conversion efficiency and their stability under irradiation. A direct insight is here presented on the effects that CdSe QDs size has on their photoactivity in hydrogen evolution from Na2SO3—containing aqueous solutions under visible light irradiation. Four highly monodisperse CdSe quantum dots with different nanocrystal diameters were synthesized and, after exchange of the capping ligand, employed as photocatalyst in H2 production in the absence of any co-catalyst. Their size-dependent photocatalytic activity is discussed in relation to their absorption properties, their conduction band energy and the recombination probability of the photogenerated charges inside the nanocrystals. High quantum efficiency in H2 generation results from a balance among these three QDs features, their activity scale being not exclusively dominated by the photocatalyst conduction band position.

Nanoparticulate titania was synthesized by flame spray pyrolysis and was used to construct photoanodes for photoelectrochemical cells and quantum dot sensitized solar cells. Powders obtained by flame spray pyrolysis were composed of smaller nanoparticles and had higher specific surface areas than common commercial types of titania and this was carried over to the structure of the photoanodes and reflected on the photoelectrocatalytic and solar cell behavior of the photoanodes. The highest specific surface area and the smallest nanoparticle size produced in this work were 249 m2 g−1 and 7 nm, respectively. CdS-sensitized photoanodes were affected by the amount of the deposited sensitizer, which was the largest in the case of the powder with the highest specific surface area. When, however, the photoanodes were loaded with a relatively large amount of CdSe sensitizer, the role of the latter increased and the differences between the different forms of titania diminished.

Preferential growth of anatase crystallites along different directions is known to deeply affect their photocatalytic properties, especially with respect to the exposure of the reactive {001} facets. Its extent can be easily quantified through simple geometric calculations, on the basis of crystal sizes extracted for specific directions by means of X-Ray Diffraction data analysis. Nevertheless, the actual results depend on the method employed for such a quantification. Here we report on a comparative morphological investigation, performed by employing the Scherrer equation and the line profile from Rietveld refinements, on shape-controlled anatase photocatalysts produced by employing HF as capping agent. Compared to the Rietveld-based method, the use of the Scherrer equation produces a systematic underestimation of crystallite dimensions, especially concerning the [100] direction, which in turn causes the percentage of exposed {001} crystal facets to be underestimated. Neglecting instrumental-related effects may further reduce the estimate.

In the framework of a large systematic investigation on the effects of doping TiO 2 with first raw p -block elements of the Period Table, we discuss here the results obtained with a series of TiO 2 photocatalysts doped or co-doped with boron and fluorine, prepared by sol–gel synthesis and calcined at 500 °C or at 700 °C. The photocatalytic activity of such materials in the oxidative decomposition of formic acid is related to their surface and structural properties, determined by BET, XRPD and Uv–vis absorption analyses. The photoactivity of singly doped or codoped TiO 2 full anatase materials calcined at 500 °C appears to increase with their specific surface area and to be not related to electronic structure modifications due to the presence of the dopants. The photoactivity increase observed upon B-doping TiO 2 , which leads to B 2 O 3 surface segregation, may thus simply result from their increased surface area, the presence of B 2 O 3 having negligible effects on photoactivity. Highly crystalline F-doped or BF-codoped TiO 2 materials calcined at 700 °C exhibit the highest photoactivity, essentially due to the retarded anatase into rutile transformation intrinsic of fluorine-doped materials.

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 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 to the Bi2O3 target and 300 W power supplied to 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 versus 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.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 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 atomi...

The dynamics of photopromoted electrons in BiVO

In the present work, two H

The time-resolved photoluminescence (PL) in the nanosecond time scale of TiO2 materials undoped or codoped with nitrogen and fluorine (N,F-doping) and modified by noble metals (NM, i.e. Au or Pt) nanoparticles (NPs) photodeposition has been systematically investigated in relation to their photocatalytic activity in hydrogen production. The main aim of the study is to elucidate the origin of the NM-dependent synergistic effects in photoactivity produced by N,F-doping of TiO2 and NM NPs deposition on the oxide surface. While TiO2 doping with fluorine and nitrogen introduces new stabilized luminescent defective trap states below the conduction band revealed by long-living PL components, the presence of NM NPs on the TiO2 surface produces a PL intensity suppression, which is more relevant for Au rather than for Pt-NPs containing materials. Time-resolved PL analysis indicates that the electron transfer occurring at the TiO2/metal interface is affected by both the defective structure of anatase N,F-doped TiO2 and the type of NM (Au or Pt). In particular, Au rather than Pt NPs appear to strongly interact with the charge carriers trapped at surface defect sites, gold NPs being expected to preferentially grow on such sites during photodeposition. Furthermore, plasmonic gold NPs excitation upon PL light absorption is evidenced by the PL spectral shape variation observed only in the case of Au/TiO2. Thus, the larger synergistic effect on photocatalytic hydrogen production observed upon Au NPs photodeposition on N,F-doped TiO2 results from the opening of a new efficient electron transfer path from luminescent defective trap states on doped TiO2 to Au NPs, where proton reduction occurs.

The effects of the TiO 2 crystal structure and its surface modification with Pt nanoparticles on the photocatalytic oxidation of NH 3 in the aqueous phase were studied with home-made TiO 2 photocatalyst powders synthesized by a thermo-hydrolysis approach that allowed a fine control of the crystallographic phase composition. Ammonia conversion and the selectivity toward mildly oxidized N 2 and highly oxidized nitrite and nitrate anions were monitored during the runs. Pure brookite powders modified by Pt nanoparticles, deposited either from a colloidal suspension or by deposition–precipitation, were found to be the most efficient photocatalysts in NH 3 degradation, with a desired good selectivity toward innocuous N 2 . The photo-activity in NH 3 oxidation was correlated to the crystallographic features of the investigated TiO 2 -based materials, also taking into account the effects induced on the reaction paths by the presence of Pt nanoparticles on TiO 2 .

Bismuth vanadate (BiVO4) with a band gap of ∼2.4 eV has emerged as one of the visible photocatalysts that can absorb light below 520 nm. The electron/hole pairs that are generated following BiVO4 band gap excitation are effective for water splitting, especially when BiVO4 is combined with other metal oxides such as WO3. We report a solution processed method for designing transparent WO3/BiVO4 heterojunction electrodes and observe a synergistic effect on the photoelectrochemical activity of WO3/BiVO4, with the combined system performing dramatically better than either individual component. Using ultrafast transient absorption spectroscopy, we elucidated the electronic interaction between WO3 and excited BiVO4. Moreover, the photocatalytic reduction of thionine by WO3/BiVO4 as well as by each individual oxide component is used to track electron injection processes and determine the energetics of the studied systems. In the composite WO3/BiVO4 film a shifted quasi-Fermi level results, due to electronic equil...

TiO2 and Ti-W mixed oxide photocatalysts, with W/Ti molar ratios in the 0–5% range, were prepared through a simple sol-gel method, followed by annealing at 500 or 700 °C, and their photoactivity was tested in the photo-oxidation of formic acid in the aqueous phase under ambient aerobic conditions. XRPD analysis evidenced that in the presence of tungsten the anatase phase was stable even after calcination at 700 °C, with a progressively larger surface area and smaller particle dimensions with increasing tungsten content. Tungsten can both enter the titania lattice, as demonstrated by HAADF-STEM analysis, and also segregate as amorphous WO3 on the photocatalysts surface, as suggested by XPS analysis. The best performing Ti/W oxide photocatalyst is that containing 1.0 mol% W/Ti, mainly due to the tungsten-induced stabilization effect of the anatase phase, whereas electron transfer from TiO2 to WO3, though compatible with the here performed EPR measurements, appears to have no beneficial effect in the investigated reaction, likely due to the low energy level of the conduction band of WO3, from which electrons cannot efficiently transfer to adsorbed dioxygen.

A series of Pt/Cu/TiO2 photocatalysts, showing very high performance in photocatalytic hydrogen production from methanol/water vapour mixtures, were prepared under mild conditions by Cu(II) grafting on commercial P25 TiO2, with nominal Cu/TiO2 ratios ranging from 0.05 to 0.5 wt.%, followed by 0.5 wt.% Pt nanoparticles deposition by the deposition-precipitation method in the presence of urea. The structural features of the so obtained materials were fully characterized by X-ray absorption spectroscopy, which provided information on the oxidation state of the two metals and on the metal-metal and metalTiO(2) interactions, and by EPR analysis, which evidenced electron transfer phenomena involving copper under irradiation. The photocatalysts showed a volcano-shaped photoactivity trend in hydrogen production with increasing nominal Cu content, the maximum rate of H2 evolution (27.2 mmol h-1 gcat-1) being attained with the photocatalyst containing 0.1 wt.% of copper. In this sample CuO nanoclusters appear to be intimately coordinated with surface Ti atoms in a surface structure that partially stabilizes pre grafted copper in metallic form, possibly acting as an electron-transfer bridge at the interface between CuO nanoclusters and TiO2. Synergistic effects in H-2 photocatalytic production are clearly induced by the co-presence of grafted Cu nanoclusters and Pt nanoparticles on the TiO2 surface, with the copper oxidation state switching under UV-vis irradiation, facilitating electron transfer to adsorbed protons. (C) 2017 Elsevier B.V. All rights reserved.

The effect of Cu or Cu-Pt nanoparticles in TiO2 photocatalysts prepared by flame spray pyrolysis in one step was investigated in hydrogen production from methanol photo-steam reforming. Two series of titanium dioxide photocatalysts were prepared, containing either (i) Cu nanoparticles (0.05–0.5 wt%) or (ii) both Cu (0 to 0.5 wt%) and Pt (0.5 wt%) nanoparticles. In addition, three photocatalysts obtained either by grafting copper and/or by depositing platinum by wet methods on flame-made TiO2 were also investigated. High hydrogen production rates were attained with copper-containing photocatalysts, though their photoactivity decreased with increasing Cu loading, whereas the photocatalysts containing both Cu and Pt nanoparticles exhibit a bell-shaped photoactivity trend with increasing copper content, the highest hydrogen production rate being attained with the photocatalyst containing 0.05 wt% Cu.