Co-catalysts play an important role in photocatalytic water splitting. The co-catalyst is generally deposited in the form of nanoparticles on the catalyst surface, and is believed to provide water oxidation and reduction sites. However, the minimum size of a co-catalyst that can function as a reaction site and the detailed local environment of the photocatalytic reaction centers are not yet fully understood. Here, we show that even isolated single-atom Rh dopants in two-dimensional titanium oxide crystals can effectively act as co-catalysts for the water-splitting reaction. At an optimal doping concentration, the hydrogen production rate is increased substantially in comparison to that found with the undoped crystals. We also present first-principles simulations based on density functional theory to provide insights into the atomic-scale mechanism by which the isolated Rh dopants induce changes to the dissociation reaction energy landscape. These results provide new insights for better understanding the role of the co-catalyst in the photocatalytic reaction.

Understanding and rationally tailoring defect-mediated optical absorption of nondilute oxide solid solutions represents a complex challenge. In this work, we investigate compositions in the SrTiO3–SrFeO2.5 solid solution, departing from the simpler dilute Fe-substituted SrTiO3 case. Through ex situ and in situ optical absorption measurements of mixed conducting thin films prepared by pulsed laser deposition and through density functional theory simulations, we demonstrate understanding and rational tailoring of the optical absorption behavior. Experimentally, broad subgap absorption peaks, centered around 2.1 and 2.8–3 eV, increase in intensity with increasing Fe and/or O concentrations and decrease with increasing La donor dopant concentration. Consistent with these observations, the absorption is found to be proportional to the hole concentration and to the Fe concentration under fully oxidized conditions. This behavior is similar to the dilute case; however, the solid solution electronic structure and optical absorption behavior cannot be represented simply by Fermi level shifts in a rigid-band model. Simulations confirm these trends and identify transitions responsible for absorption as occurring from within the valence band to empty states at the hybrid (O 2p/Fe 3d) valence band maximum and to empty states at the hybrid (Ti/Fe 3d) conduction band minimum. Adding oxygen or iron, or removing La, increases the density of empty states at the top of the valence band and bottom of the conduction band, increasing the intensity of these transitions. This approach for studying solid solution behavior can be extended to other systems in the future, and the fundamental understanding of the origins of absorption also enables its in situ use as a quantitative probe of thin film point defect concentrations and kinetics.

The hydrogen evolution reaction using semiconductor photocatalysts has been significantly improved by cocatalyst loading. However, there are still many speculations regarding the actual role of the cocatalyst. Now a photocatalytic hydrogen evolution reaction pathway is reported on a cocatalyst site using TiO nanosheets doped with Rh at Ti sites as one-atom cocatalysts. A hydride species adsorbed on the one-atom Rh dopant cocatalyst site was confirmed experimentally as the intermediate state for hydrogen evolution, which was consistent with the results of density functional theory (DFT) calculations. In this system, the role of the cocatalyst in photocatalytic hydrogen evolution is related to the withdrawal of photo-excited electrons and stabilization of the hydride intermediate species; the presence of oxygen vacancies induced by Rh facilitate the withdrawal of electrons and stabilization of the hydride.

We present a systematic assessment of the structural properties, the electronic density of states, the charge densities, and the phase stabilities of AgInSe2 and AuInSe2 using screened exchange hybrid density functional theory, and compare their properties to those of CuInSe2. For AgInSe2, hybrid density functional theory properly captures several experimentally measured properties, including the increase in the band gap and the change in the direction of the lattice distortion parameter u in comparison to CuInSe2. While the electronic properties of AuInSe2 have not yet been experimentally characterized, we predict it to be a small gap (≈ 0.15 eV) semiconductor. We also present the phase stability of AgInSe2 and AuInSe2 according to screened-exchange density functional theory, and compare the results to predictions from conventional density functional theory, results tabulated from several online materials data repositories, and experiment (when available). In comparison to conventional density functional theory, the hybrid functional predicts phase stabilities of AgInSe2 in better agreement with experiment: discrepancies in the calculated formation enthalpies are reduced by approximately a factor of three, from ≈ 0.20 eV/atom to ≈ 0.07 eV/atom, similar to the improvement observed for CuInSe2. We further predict that AuInSe2 is not a stable phase, and can only be present under non-equilibrium conditions.