We continue to attack the three major barriers hindering the realization of high-efficiency devices for solar-driven water splitting using an integrated experimental and theoretical approach that offers fundamental insights into the underlying photoelectrolysis processes occurring in bandgap-narrowed semiconductor and catalyst components. First, we are developing viable catalysts for the difficult four-electron water oxidation process by exploring the catalytic activity and mechanisms of two promising systems: transition-metal hydrous oxides immobilized on nanoparticle substrates, and dinuclear transition-metal complexes with quinonoid ligands. Second, we are tuning band energies to optimize the light-harvesting and charge-separation abilities of known photostable semiconductors in order to improve the existing materials through a better understanding of their structural and electronic properties, and in addition, we are characterizing new classes of photoactive semiconductors. Finally, we are exploring the interfacial water-decomposition reactions that occur at bare and catalyst-functionalized semiconductor surfaces using carriers generated by dark- and photo-currents with the goal of maximizing semiconductor-catalyst-substrate charge transport.