The direct photo-electrochemical conversion of water and CO2 into chemical fuels represents an exciting new pathway for the combined conversion and storage of solar energy. One of the main challenges in this field is to find semiconductors that are efficient, chemically stable, and easy to synthesize. I will show recent progress on two oxide-based semiconductors, BiVO4 and CuBi2O4, to illustrate some of the challenges in the design and optimization of these novel materials. Of particular interest is the interaction between semiconducting photoelectrodes and co-catalysts, such as cobalt phosphate. Although the latter are commonly assumed to enhance the reaction kinetics, we found compelling evidence that their main role on BiVO4 is to passivate surface states. In the second part of the talk I will show initial results on the surface chemistry of BiVO4 studied under operando conditions with near-ambient pressure XPS, and on in-line XPS studies of ALD-grown TiO2 protection layers.