Investigating the behavior of various cocatalysts on LaTaON₂ photoanode for visible light water splitting

We performed a comparative study on the photoelectrochemical performance of LaTaON₂ loaded with NiO_x, Ni_0.7Fe_0.3O_x, CoO_x and IrO_x as cocatalysts. Ni-based oxides lead to the highest improvement on the photoelectrochemical performance, while CoO_x and IrO_x also enhance the performance though to a lower extent, but they simultaneously introduce more pseudocapacitive current thus resulting in an inefficient utilization of the photo-generated holes. Repetitive voltage cycling between 1.0 V_RHE and 1.6 V_RHE transforms the NiO_x and Ni_0.7Fe_0.3O_x into oxyhydroxides known to possess higher catalytic activities. However, these oxyhydroxides lead to lower photoelectrochemical performance compared to the as-loaded oxides, most probably due to the decay of the passivation centers at the photoelectrode-cocatalyst interface. High catalytic activities cannot be achieved without sufficient passivation of surface recombination states. Despite that the photoelectrochemical performance of LaTaON₂ can be improved by cocatalysts, the maximum achievable photocurrent density is still not comparable to that reported for other oxynitride compounds. Our study suggests that poor electronic conductivity or severe bulk recombination of the photo-generated electron-hole pairs are the main limiting factors for the photon-to-current conversion efficiency in LaTaON₂ photoanodes.