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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.

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 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.

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.

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 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...

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.

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...

We report that 1-hydropyridinyl radicals (1-PyH^•) photogenerated in solution react with dissolved CO_2 en route to its 2e^– reduction into carboxylic acids. The 254 nm excitation of pyridine (Py) in deaerated 2-PrOH/H_2O mixtures saturated with 1 atm of CO_2 yields a suite of products, among which we identified Na(HCOO)_2^– (m/z^– = 113), C_5H_6NCOO– (m/z^– = 124), and C_5H_(10)O_2NCOO– (m/z^– = 160) species by electrospray ionization mass spectrometry. These products demonstrably contain carboxylate functionalities that split CO_2 neutrals via collisionally induced dissociation. We infer that 1-PyH^• [from (1) ^3Py^* + 2-PrOH → 1-PyH^• + ^•PrOH] adds to CO_2, in competition with radical–radical reactions, leading to intermediates that are in turn reduced by ^•PrOH into the observed species. The formation of carboxylates in this system, which is shown to require CO_2, Py, 2-PrOH, and actinic radiation, amounts to the homogeneous 2e^– reduction of CO_2 by 2-PrOH initiated by Py^*. We evaluate a rate constant (2) k_2(1-PyH^• + CO_2 → •Py-1-COOH) ≈ O (10) M^(–1) s^(–1) and an activation energy E_2 ≥ 9 kcal mol^(–1) that are compatible with thermochemical estimates for this reaction.

The photocatalytic production of hydrogen from methanol steam reforming was investigated over a series of fluorinated Pt/TiO 2 samples (F for O nominal molar substitution ranging from 5 to 15 at%) synthesized by flame spray pyrolysis in single step. The so obtained photocatalysts were characterized by BET, XRD, XPS, UV–vis absorption analyses and in situ X-ray absorption spectroscopy (XAS) at the Pt L3-edge. XPS analysis confirmed the presence of both surface and bulk fluorine. XAS revealed that Pt, mostly in oxidized form in the as prepared samples, readily reduced to the metallic form under gas-phase photocatalytic reaction conditions. Photocatalytic hydrogen production tests showed that 5 at% F for O substitution in TiO 2 led to an increase in the H 2 and CO 2 production rates, whereas above this value the H 2 production rate decreased linearly with increasing the nominal F loading in the photocatalyst. A significant increase of selectivity to CO 2 , from 35% to 45%, was obtained over the best performing fluorinated photocatalyst with respect to the flame-made F-free one. Thus, TiO 2 fluorination favors the indirect methanol photo-oxidation mechanism by increasing surface hydroxyl radical formation with a consequent increase of selectivity toward CO 2 production. On the other hand, excessive fluorination introduces structural defects, acting as electron–hole recombination centers, and may hinder interface electron transfer, because of the F-induced surface electronegativity. Both effects concur to decrease photo-activity.

Electrochemical anodization of low-concentration (0.02-0.2 at% Au) TiAu alloys in a fluoride electrolyte leads to self-organized TiO2 nanotubes that show a controllable, regular in situ decoration with elemental Au nanoclusters of ≈5 nm in diameter. The degree of self-decoration can be adjusted by the Au concentration in the alloy and the anodization time. Such Au particle decorated tubes show a high activity for photocatalytic H2 production from ethanol solutions.

Peas in a pod: A highly aligned Au(np)@TiO2 photocatalyst was formed by self-organizing anodization of a Ti substrate followed by dewetting of a gold thin film. This leads to exactly one Au nanoparticle (np) per TiO2 nanocavity. Such arrays are highly efficient photocatalysts for hydrogen generation from ethanol.

Aiming at producing TiO2-based photocatalytic materials with reduced charge carriers recombination, WO3–TiO2 mixed oxides were synthesized by a sol–gel method employing either an inorganic salt, Na2WO4, or an organic alkoxide, W(OC2H5)6, as tungsten precursor, with different W/Ti ratios. The so-obtained materials were characterized by XRPD, BET, UV–vis reflectance, XPS and EDX analyses and their photoactivity was tested under UV–visible irradiation in both the mineralization of formic acid in aqueous suspension and the gas phase oxidation of acetaldehyde. Both photoactivity results and photocurrent measurements point to a superior performance of photocatalysts obtained from the organic precursor with an optimal tungsten content (3%). The formation of an intimately mixed oxide, as revealed by XRPD analysis, results in photoactivity higher than that of pure TiO2, and also of benchmark P25 TiO2, consequent to a better charge separation due to the migration of photoproduced holes from WO3 domains to TiO2 and of photopromoted electrons in the opposite direction. The persistence of pure anatase phase in W-containing photocatalysts also after calcination at 700 °C and their higher surface area with respect to pure TiO2 also contribute in increasing the photocatalytic activity of the WO3–TiO2 mixed oxides.

TiO2 powders modified through the deposition of surfactant-stabilized preformed metal (Pt, Pd, Au and Ag) nanoparticles (NPs) were tested as photocatalysts for batch ammonia photo-oxidation in aqueous suspension, under UVA irradiation. NH3 conversion and the selectivity toward mildly oxidized N2 and highly oxidized nitrite and nitrate anions were monitored during the runs, in order to study the effects of noble metal NPs deposition on the reaction mechanism. This involves parallel and consecutive paths, also depending on the type of metal deposited on TiO2, as also highlighted by the nitrite photo-oxidation tests performed in the present study. Metal NPs acted as electron traps ensuring better separation of the photoproduced charge couples and consequently higher rates of hole-mediated oxidation reactions. Apart from Au/TiO2, the metal-modified powders exhibited higher photoactivity with respect to that of naked titania, with Ag/TiO2 being the best performing photocatalyst in terms of ammonia conversion. On the other hand, Pd NPs on TiO2 ensured a considerable increase of the selectivity toward N2 and may thus be considered the best co-catalysts of TiO2 for environmentally friendly photocatalytic NH3 abatement.

A critical overview is presented on the role that first row p-block elements boron, carbon, nitrogen and fluorine, employed as dopants of TiO2, have in improving the capability of this photocatalyst in harvesting solar light for photocatalytic applications. The peculiar physicochemical properties of doped TiO2 materials are described in terms of the results of both theoretical calculations and photocatalytic efficiency tests, in relation to their bulk and surface features. The limitations of doping titania with non metal elements are outlined and a few recent examples of very promising co-doping effects are discussed.

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.

A bilayer WO3 photoelectrode was obtained by radio frequency (RF) plasma sputtering in a reactive 40% O-2/Ar atmosphere by depositing two successive WO3 coatings on a tungsten foil at two different total gas pressures (3 Pa and 1.7 Pa, respectively), followed by calcination at 600 degrees C. Two monolayer samples deposited at each of the two pressures and a bilayer sample deposited at inverted pressures were also prepared. Their photoelectrocatalytic (PEC) activity was evaluated by both Incident Photon-to-Current Efficiency (IPCE) measurements and separate evolution of H-2 and O-2 by water splitting in a two-compartment PEC cell. SEM analysis revealed that the photoanodes have a nanostructured porous double layer surmounting a columnar basement (Staffa-like morphology, after the name of the Scottish island). Mott-Schottky analysis showed that the single layer deposited at 3 Pa has a conduction flat band potential 0.1 V more positive than that deposited at 1.7 Pa. The equivalent n-n heterojunction at the interface of the double-layer creates a built-in electric field that facilitates the photopromoted electron transfer toward the lower lying conduction band material, while the columnar innermost layer introduces percolation paths for efficient electron transport toward the conductive tungsten foil. Both phenomena contribute to decrease the interfacial charge transfer resistance (R-ct) and lead up to a ca. 30% increase in the PEC performance compared to the monolayer and the inverted bilayer coatings and to a 93% faradaic efficiency, which is among the highest reported so far for WO3 photoanodes. Upon methanol addition an outstanding 4-fold photocurrent density increase up to 6.3 mA cm(-2) was attained over the bilayer WO3 photoanode, much larger than the usually observed current doubling effect.

Crystal shape control on a series of anatase photocatalysts was achieved by varying the amount of HF employed as a capping agent in their hydrothermal synthesis. A systematic comparison between their physicochemical properties, determined by several complementary surface and bulk techniques before and after thermal treatment at 500 °C, allowed one to discern the influence of the relative amount of exposed {001} crystal facets among a series of effects simultaneously affecting their oxidative photocatalytic activity. The results of both formic acid and terephthalic acid photo-oxidation test reactions point to the primary role played by calcination in making {001} facets effectively photoactive. Annealing not only removes most of the residual fluorine capping agent from the photocatalyst surface, thus favoring substrate adsorption, but also produces morphological modifications to a crystal packing that makes accessible a larger portion of surface {001} facets due to the unpiling of platelike crystals. The photocatalyst bearing the highest amount of exposed {001} facets (60%) shows the highest photoactivity in both the direct and the (•)OH-radical-mediated photocatalytic test reaction.

The photocatalytic abatement of aqueous ammonia, the major nitrogen-containing compound in livestock manure, was investigated employing (i) a batch reactor equipped with an immersion UVA lamp, containing aqueous suspensions of different commercial TiO 2 powders and (ii) a 40 L prototype plug flow photo-reactor, suitable for the photocatalytic treatment of real swine and bovine manure, consisting of 9 vertical elements connected in series, internally coated with a TiO 2 film, each one centrally housing a UVA lamp. A preliminary systematic investigation performed with the first photo-reactor evidenced that ammonia conversion and the selectivity toward the main oxidation products (N 2 , NO 2 − and NO 3 − ) depend on the amount of dissolved O 2 , on pH and on the photocatalyst's amount and structural features. Extremely promising results, in terms of both ammonia conversion (up to the 88%) and selectivity to nitrogen gaseous species (99%), were achieved when the prototype photo-reactor was fed with suitably pre-treated manure. The TiO 2 -based photocatalytic treatment can thus be considered an effective remediation technology for the abatement of nitrogen-containing pollutants in surface and groundwater, originating from large-scale animal husbandries.

Commercial TiO(2) samples with different phase composition and surface area were tested as photocatalysts in the photoinduced reduction of Cr(VI) in aqueous suspensions at pH 3.7 under UV-visible light irradiation. This reaction was also coupled with the simultaneous photocatalytic oxidation of the pollutant azo dye Acid Orange 8 (AO8) and of formic acid, acting as hole scavengers. The co-presence of oxidizable and reducible species ensured better separation of photogenerated charge carriers, resulting in a higher rate of both organics' oxidation and Cr(VI) reduction, especially in the case of high surface area anatase TiO(2), having the strongest affinity for Cr(VI) and AO8, as demonstrated by competitive adsorption tests. The effects on Cr(VI) photocatalytic reduction of gold nanoparticles photodeposited on TiO(2) and of the Au loading were also investigated, aiming at ascertaining if this noble metal plays a role in the electron transfer processes involved in Cr(VI) reduction.

The photocatalytic reforming of methanol was investigated kinetically under steady conditions as a function of the methanol-to-water partial pressure ratio in the gas mixture fed to the photoreactor. Similar results were obtained with two TiO2-based photocatalysts, one containing platinum nanoparticles and prepared by flame spray pyrolysis and the other prepared by the deposition of preformed Au nanoparticles on P25 TiO2. Methanol oxidation proceeds on the photocatalyst surface up to CO2 through the formation of formaldehyde and formic acid as intermediate species. The steady-state formaldehyde, formic acid, and carbon dioxide production rates, plotted vs. the methanol molar fraction in the aqueous solution generating the gaseous reaction mixture, were successfully fitted on the basis of a reaction scheme, in which each elementary oxidation step occurs through either an indirect ·OH radical-mediated path, or a hole-mediated direct path, or a water-assisted path, when oxidation occurs on titania surface sites far from noble metal nanoparticles. H2O/D2O isotopic-exchange experiments allow a clear distinction between the direct and the indirect oxidation paths and fully support the proposed reaction scheme.

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.

The photocatalytic production of hydrogen from aqueous systems is reviewed, stressing the very promising features of the process as an environmentally friendly, perfectly renewable way to produce hydrogen, the ideal fuel for the future. Starting with a brief historical background, the most recent achievements in the field on the basis of both relevant patents and published literature, are discussed here, with particular emphasis on the development of innovative materials able to capture a larger portion of the solar spectrum with respect to traditional photocatalytic materials, and on the different setups and devices which have been developed and tested.

The photocatalytic production of hydrogen by methanol steam reforming was studied over a series of pristine or noble metal (Ag, Au, Au–Ag alloy and Pt) – modified TiO2 photocatalysts, synthesised by flame spray pyrolysis or by the deposition of preformed noble metal nanoparticles on TiO2. A closed recirculation apparatus was employed, with the photocatalyst bed continuously fed with methanol/water vapours. Methanol underwent oxidation up to CO2 through the formation of formaldehyde and formic acid. Carbon monoxide, methane, methyl formate, acetaldehyde and dimethyl ether were identified as side products. Hydrogen evolved at constant rate, which significantly increased upon noble metal addition, Pt being the most effective co-catalyst, followed by gold and silver, according to their work function values. A systematic investigation into the effects of the inlet gas composition gave valuable information on the prevailing reaction paths and on the conditions of process optimisation, also in terms of distribution of CH3OH oxidation products.

Anatase–rutile and anatase–brookite–rutile composite nanocrystals were synthesized by a controlled sol–gel reaction followed by hydrothermal growth and by mechanically mixing pure crystalline phase precursors, respectively. Their physicochemical and photocatalytic properties were investigated in comparison with those of several home-made and commercial pure anatase, rutile and mixed phase TiO 2 samples. In particular, the phase composition and the crystallite size were determined by X-ray diffraction measurements. The photocatalytic degradation of the azo dye Acid Red 1 (AR1) and of formic acid (FA) in aqueous suspension were employed as test reactions. Hydrogen peroxide evolution, the main reductive process occurring in parallel, was also monitored during the runs. The investigated photocatalysts exhibit different photoactivity scales towards the two organic substrates, depending on the prevailing photodegradation path. High surface area anatase samples were particularly active in FA degradation, principally occurring through direct interaction with photoproduced valence band holes, whereas rutile samples, even possessing high surface areas, but exhibiting poor wettability, were scarcely active. A good linear correlation was found between H 2 O 2 evolution and the rate of AR1 photodegradation on the investigated series of photocatalysts, whereas no H 2 O 2 was detected during FA mineralization, apart in the case of a low surface area anatase sample, whose peculiar photocatalytic properties were ascribed to the presence of Ti 3+ ions in its structure.

The photocatalytic activity of thin, nanostructured films of titanium dioxide, synthesized by supersonic cluster beam deposition (SCBD) from the gas phase, has been investigated employing the photodegradation of salicylic acid as test reaction. Because of the low deposition energy, the so-deposited highly porous TiO2 films are composed of nanoparticles maintaining their original properties in the film, which can be fully controlled by tuning the deposition and post-deposition treatment conditions. A systematic investigation on the evolution of light absorption properties and photoactivity of the films in relation to their morphology, determined by AFM analysis, and phase composition, determined by Raman spectroscopy, has been performed. The absorption and photocatalytic activity of the nanostructured films in the visible region could be enhanced either through post-deposition annealing treatment in ammonia containing atmosphere or employing mild oxidation conditions, followed by annealing in N2 at 600 °C.

TiO2 and 1% Au/TiO2 powders, synthesised by flame spray pyrolysis and possessing high specific surface area (106 m2 g−1) and anatase content (ca. 90%), were tested as photocatalysts in hydrogen production from methanol photoreforming, employing a closed recirculation apparatus. The irradiated photoreactor consisted either in a quartz vessel containing an aqueous suspension of the photocatalyst, or in a newly set-up Plexiglas cell, containing the same amount of catalyst immobilised on quartz grains, which was continuously fed with methanol/water vapours. The gas-phase composition during irradiation was analysed by gas chromatography and quadrupolar mass spectrometry, which allowed the identification of formaldehyde as the only intermediate species. The photocatalytic activity of the flame-made materials was higher than that of commercial Degussa P25 TiO2 and of 1% Au/P25 obtained via deposition of preformed gold nanoparticles on P25. In particular, a 30 times higher photocatalytic hydrogen production was obtained upon gold addition to TiO2. Furthermore, a 30% higher reaction rate was attained with the vapour phase reactor, i.e. in the absence of liquid-phase mass transfer rate limitations, ensuring the production of up to 10.2 mmol of H 2 h - 1 g cat - 1 , with an apparent photon efficiency of 6.3%.

A set of titanium dioxide and gold-modified titanium dioxide samples were prepared by flame spray pyrolysis (FP) and characterized by BET, XRD, HRTEM and UV–vis reflectance analysis. Their photocatalytic activity in hydrogen production in water suspension, either from water photosplitting or from methanol photoreforming, was tested in an expressly set up, closed recirculation laboratory scale photoreactor and compared to those of commercial TiO2 samples, including Degussa P25, pure rutile and pure anatase. The rate of hydrogen evolution ( r H 2 ) increased with increasing the anatase content, pure anatase being the most active photocatalyst. Surface area and crystallinity, both key properties of photocatalysts, can be tuned up by properly setting FP operation parameters, including the selection of the organic solvent/fuel. In particular, FP-made TiO2 prepared from a xylene solution showed more active than P25. Finally, r H 2 increased by one order of magnitude in water photosplitting and by ca. 30 times in methanol photoreforming upon 1% gold addition on TiO2.

The rate of 1,4-dichlorobenzene (1,4-DCB) degradation and mineralization in the aqueous phase was investigated either under direct photolysis or photocatalysis in the presence of commercial or sol-gel synthesized TiO2, or under sonolysis at 20 kHz with different power inputs. Two lamps, both emitting in the 340-400 nm wavelength range with different energy, were employed as irradiation sources. Photocatalysis ensured faster removal of 1,4-DCB with respect to sonolysis and direct photolysis. The highest degradation and mineralization rate was attained with the combined use of photocatalysis and sonolysis, i.e. under sonophotocatalytic conditions. The efficiency of the employed advanced oxidation techniques in 1,4-DCB degradation is discussed also in relation to their energy consumption, which might be decisive for their practical application.

The effects of TiO

We introduce the use of pure molten ortho-phosphoric acid (o-H3PO4) as an electrolyte for selforganizing electrochemistry. This electrolyte allows for the formation of self-organized oxide architectures (one-dimensional nanotubes, nanochannels, nanopores) on metals such as tungsten that up to now were regarded as very difficult to grow self-ordered anodic oxide structures. In this work, we show particularly the fabrication of thick, vertically aligned tungsten oxide nanochannel layers, with pore diameter of ca. 10 nm, and illustrate their potential use in some typical applications.

The photocatalytic degradation of formic acid (FA) and benzoic acid (BA), chosen as model organic molecules with acidic properties, was investigated in TiO2 and ZnO water suspensions under different experimental conditions. Hydrogen peroxide evolution, formed through a reductive pathway started by conduction band electrons, was also simultaneously monitored during the degradation runs. The effect of different initial amounts of substrates and the dependence of the reaction rate on the initial pH of the TiO2 suspensions was interpreted under the light of a pseudo-steady state Langmuir–Hinshelwood rate form and of the electrostatic interactions occurring at the water–semiconductor interface. ZnO appeared a more effective photocatalyst than TiO2 for BA, but not for FA degradation. A much higher amount of hydrogen peroxide was detected in ZnO irradiated suspensions, both in the presence and in the absence of the substrates, mainly because of its lower photocatalytic decomposition rate on such oxide. The rate of hydrogen peroxide evolution during the photocatalytic oxidation of BA on TiO2 could be related to the rate of the oxidation process, while H2O2 could not be detected during the photocatalytic degradation of FA on this oxide, mainly because of the reduced shielding ability of this substrate.

The mechanism of atrazine degradation in aqueous phase was investigated under sonolysis at 20 kHz, ozonation, photolysis at 254 nm and photocatalysis in the presence of TiO2, employed either separately or in combination. Ozonation and photocatalysis induced atrazine de-alkylation, followed by slower de-chlorination, while direct photolysis at 254 nm produced very efficient de-chlorination. Simultaneous sonolysis had beneficial effects on ozonation and photocatalysis, especially by increasing the rate of photocatalytic de-alkylation, and no effect on the unimolecular photolytic de-chlorination of atrazine. Although complete atrazine mineralization could not be attained, because of the stability of the s-triazine ring toward oxidation, atrazine degradation and overall detoxification, as related to the disappearance of chlorinated by-products, proceeded at the highest rate when photolysis at 254 nm was combined with ozonation.

The effect of TiO2 surface fluorination on the hydrogen peroxide evolution occurring in photocatalytic runs was investigated employing the azo dye Acid Red 1 (AR1) and two model organic molecules with acidic properties, i.e. formic acid (FA) and benzoic acid (BA), as substrates of oxidative degradation. While AR1 and BA photocatalytic degradation on fluorinated titanium dioxide (F–TiO2) was markedly faster than on unmodified TiO2, because of enhanced hydroxyl radical formation, H2O2 concentration during the photodegradation of both substrates on F–TiO2 was lower, possibly because of the reduced rate of interfacial electron transfer. By contrast, FA underwent slower photocatalytic degradation on F–TiO2, but, at the same time, hydrogen peroxide concentration was relatively high, while no H2O2 could be detected during FA photodegradation on unmodified TiO2. Photocatalytic runs in the presence of the nitrate anion, able to react with the CO2˙− species produced from FA oxidation, but not with conduction band electrons, demonstrated that CO2˙− plays a relevant role in H2O2 formation during FA degradation on F–TiO2. In fact, surface fluoride, having a shielding effect at the semiconductor–water interface, not only inhibits the photocatalytic decomposition of H2O2, but also favours CO2˙− desorption and reaction with dissolved O2, generating H2O2. By contrast, CO2˙− mainly gives electron transfer to the conduction band of naked TiO2 and surface reduction of the photocatalytically produced H2O2.

Summary: Surface modification induced on poly(ethylene terephthalate) (PET) fibers by cold SF6 plasma treatment has been investigated systematically as a function of plasma device parameters. The observed wettability modifications of fibers plasma-treated under different operating conditions were correlated to their dyeability modifications and to the changes in surface chemical composition, determined by X-ray Photoelectron Spectroscopy (XPS), and topography, investigated by atomic force microscopy (AFM). Optical emission spectra from the SF6 plasma at different pressures gave information on its content of fluorine atoms. A striking transition was observed between the increased hydrophilicity and high dyeability, imparted by plasma treatment at low pressure (