2 Graphical abstract HIGHLIGHTS La and Ce doping greatly improves the photoresponse of BiVO4 photoanodes for PEC water oxidation.  Ce and La doping triggers a significantly shift of the flat band potential to more negative values.  Surface modification of the pristine and doped BiVO4 photoanodes with Au nanoparticles further enhances the photocurrent.  Gold nanoparticles act solely as co-catalytic centers without a contribution from visible sensitization. ABSTRACTCurrently, one of the most attractive and desirable ways to solve the energy challenge is harvesting energy directly from the sunlight through the so-called artificial photosynthesis. Among the ternary oxides based on earth-abundant metals, bismuth vanadate has recently emerged as a promising photoanode. Herein, BiVO4 thin film photoanodes have been successfully synthesized by a modified metal-organic precursor decomposition method, followed by an annealing treatment. In an attempt to improve the photocatalytic properties of this semiconductor material for photoelectrochemical water oxidation, the electrodes have been modified (i) by doping with La and Ce (by modifying the composition of the BiVO4 precursor solution with the desired concentration of the doping element), and (ii) by surface modification with Au nanoparticles potentiostatically electrodeposited. La and Ce doping at concentrations of 1 and 2 at% in the BiVO4 precursor solution, respectively, enhance significantly the photoelectrocatalytic performance of BiVO4 without introducing important changes in either the material structure or the electrode morphology, according to XRD and SEM characterization. In addition, surface modification of the electrodes with Au nanoparticles further enhances the photocurrent as such metallic nanoparticles act as co-catalysts, promoting charge transfer at the semiconductor/solution interface. The combination of these two complementary ways of modifying the electrodes has resulted in a significant increase in the photoresponse, facilitating their potential application in artificial photosynthesis devices.

HIGHLIGHTS  Nickel hydroxide behaves as an efficient co-catalyst for bismuth vanadate and iron vanadate photoanodes for water oxidation.  Chemical bath deposition of nickel hydroxide allows for a fine control of the co-catalyst loading on the photoanode surface.  Co-catalyst loading is critical for its performance, being optimal values of the order of only one monolayer.  Electrochemical methods can be used to estimate the co-catalyst loading provided that it experiences a reversible redox behavior.

Among the different strategies that are being developed to solve the current energy challenge, harvesting energy directly from sunlight through a tandem photoelectrochemical cell (water splitting) is most attractive. Its implementation requires the development of stable and efficient photocathodes, NdFeO 3 being a suitable candidate among ternary oxides. In this study, transparent NdFeO 3 thin-film photocathodes have been successfully prepared by a citric acid-based sol−gel procedure, followed by thermal treatment in air at 640°C. These electrodes show photocurrents for both the hydrogen evolution and oxygen reduction reactions. Doping with Mg 2+ and Zn 2+ has been observed to significantly enhance the photoelectrocatalytic performance of NdFeO 3 toward oxygen reduction. Magnesium is slightly more efficient as a dopant than Zn, leading to a multiplication of the photocurrent by a factor of 4−5 for a doping level of 5 at % (with respect to iron atoms). This same trend is observed for hydrogen evolution. The beneficial effect of doping is primarily attributed to an increase in the density and a change in the nature of the majority charge carriers. DFT calculations help to rationalize the behavior of NdFeO 3 by pointing to the importance of nanostructuring and doping. All in all, NdFeO 3 has the potential to be used as a photocathode in photoelectrochemical applications, although efforts should be directed to limit surface recombination.

Nanostructuring, oxygen vacancy engineering and surface passivation set a new record performance LaFeO3 photoanode for water oxidation.