Aluminum enhances photochemical charge separation in strontium titanate nanocrystal photocatalysts for overall water splitting

Abstract

Strontium titanate (SrTiO₃) is a well-known photocatalyst for overall water splitting (OWS) under ultra-violet irradiation. Recently, it was shown that Al-doped SrTiO₃ microparticles prepared by flux-mediated solid-state reaction catalyze OWS with an apparent quantum efficiency of 30% at 360 nm, the highest value reported so far for strontium titanate. However, the roles of Al³⁺ and of the particle morphology in controlling the activity are not clear. Here we report a new synthetic route to well defined nanocrystals of Al:SrTiO₃ with controllable Al-concentration (3–8 atom%) using TiO₂, Sr(OH)₂ and Al(NO₃)₃ as starting materials. X-Ray diffraction and transmission electron microscopy confirm the presence of perovskite-type cubic nanoparticles with size of 60 nm. After loading with a Rh_2−yCr_yO₃ cocatalyst, nano-Al:SrTiO₃ catalyzes OWS under UV/vis illumination from a Xe lamp. The activity shows a volcano-like dependence on Al concentration, confirming that Al³⁺ content is a key factor for the activity. Nanocrystals doped with 7.2 atom% Al show the activity of 53 μmol H₂ per hour under 240 mW cm⁻² UV illumination and an AQE of 0.06% at 375 nm. Based on surface photovoltage spectroscopy (SPS), aluminum enhances photochemical charge separation in the Al:SrTiO₃ nanocrystals and reduces electron and hole trapping. However, under UV/vis illumination, the Al:SrTiO₃ nanocrystals are found to be 40 times less active than Al:SrTiO₃ microparticles made by flux reaction in SrCl₂. The lower activity of the nanoparticles is due to optical shielding from the Rh_2−yCr_yO₃ co-catalyst.