CrO_x-Mediated Performance Enhancement of Ni/NiO-Mg:SrTiO₃in Photocatalytic Water Splitting

By photodeposition of CrO_xon SrTiO₃-based semiconductors doped with aliovalent Mg(II) and functionalized with Ni/NiO_xcatalytic nanoparticles (economically significantly more viable than commonly used Rh catalysts), an increase in apparent quantum yield (AQYs) from ∼10 to 26% in overall water splitting was obtained. More importantly, deposition of CrO_xalso significantly enhances the stability of Ni/NiO nanoparticles in the production of hydrogen, allowing sustained operation, even in intermittent cycles of illumination.In situelemental analysis of the water constituents during or after photocatalysis by inductively coupled plasma mass spectrometry/optical emission spectrometry shows that after CrO_xdeposition, dissolution of Ni ions from Ni/NiO_x-Mg:SrTiO₃is significantly suppressed, in agreement with the stabilizing effect observed, when both Mg dopant and CrO_xare present. State-of-the-art electron microscopy and energy-dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) analyses demonstrate that upon preparation, CrO_xis photodeposited in the vicinity of several, but not all, Ni/NiO_xparticles. This implies the formation of a Ni-Cr mixed metal oxide, which is highly effective in water reduction. Inhomogeneities in the interfacial contact, evident from differences in contact angles between Ni/NiO_xparticles and the Mg:SrTiO₃semiconductor, likely affect the probability of reduction of Cr(VI) species during synthesis by photodeposition, explaining the observed inhomogeneity in the spatial CrO_xdistribution. Furthermore, by comparison with undoped SrTiO₃, Mg-doping appears essential to provide such favorable interfacial contact and to establish the beneficial effect of CrO_x. This study suggests that the performance of semiconductors can be significantly improved if inhomogeneities in interfacial contact between semiconductors and highly effective catalytic nanoparticles can be resolved by (surface) doping and improved synthesis protocols.