Improved Niobate Nanoscroll Photocatalysts for Partial Water Splitting

Layered K4Nb6O12 is a known UV-light-driven photocatalyst for overall water splitting, with a band gap of 3.5 eV. Following ion exchange and exfoliation with tetrabutylammonium hydroxide, the layered material separates into nanosheets that coil into 1.0 +/- 0.5 mu m long and 10 +/- 5 nm wide nanoscrolls to reduce their surface energy. Pt and IrOx (x = 1.5-2) nanoparticles were photochemically deposited onto the surface of the nanoscrolls to produce two- and three-component photocatalysts. Under UV irradiation, the nanostructures produced H-2 from pure water and aqueous methanol, with turnover numbers ranging from 2.3 and 18.5 over a 5 h period. The activity of the catalysts for H-2 evolution can be directly correlated with the varying overpotentials for water reduction (210-325 mV). From water, no oxygen is evolved. Instead, the formation of surface-bound peroxides in a 1:1 stoichiometry with H-2 is observed. Slow photochemical oxygen evolution can be achieved with the sacrificial electron acceptor AgNO3, and under an electrochemical bias. The electrochemical water oxidation overpotentials are ca. 600 mV across the series of scrolls. From the photo onset potential the conduction band edge for the unmodified scrolls is estimated as -0.75 V at pH 7. Deposition of a co-catalyst is found to depress this value by 58 mV (IrOx), 148 mV (Pt/IrOx), and 242 mV (Pt). However, because water oxidation remains rate-limiting, this does not affect the overall performance of the catalysts.