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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 .

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.

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.