A building block approach to photochemical water-splitting catalysts based on layered niobate nanosheets

We present a modular approach to the synthesis of nanostructured catalysts for photochemical splitting of water into hydrogen and oxygen. The catalysts are built from exfoliated, semiconducting niobate nanosheets derived from the layered perovskite HCa2Nb3O10. The latter is a catalyst for photochemical evolution of hydrogen from water under UV irradiation. After chemical modification with 3-aminopropyltrimethoxysilane (APS), IrO2 or Pt particles can be attached to the nanosheets to produce various two-component nanostructures that were fully characterized with transmission electron microscopy and ultraviolet and infrared spectroscopy. Cyclic voltammetry was used to determine the onset potentials for O-2 and H-2 evolution. At pH = 14, the observed values are in the range +0.61 to +1.24 V (NHE, water oxidation) and -1.36 to -1.62 V (NHE, water reduction). Under UV irradiation, all catalysts evolve hydrogen from water without any sign of deactivation for 5 h. The highest quantum efficiency of 3.49% is observed for a structure with Pt directly grown onto the nanosheets. No O-2 is evolved, which we attribute to the adsorption of O-2 to the catalyst surface. For Pt-[HCa2Nb3O10], this process starts to shut down H-2 evolution after 9 h of constant irradiation, but the activity can be restored to > 60% by evacuating the catalyst dispersion and purging it with Ar. Catalysts assembled from preformed citrate-coated Pt nanoparticles are slightly less active for H, evolution and so are catalysts that use the linker aminoethyl-aminoundecanetrimethoxysilane (AEAUS) instead of APS. The activity of IrO2-APS-[Ca2Nb3O10] is lowest among two component catalysts, near the activities of the pure or APS-modified nanosheets. On the basis of XPS data, IrO2 in this catalyst undergoes photochemical reduction to Ir(0) upon UV irradiation.