CrO_x-Mediated Performance Enhancement of Ni/NiO-Mg:SrTiO₃ in Photocatalytic Water Splitting

By photodeposition of CrO_x on SrTiO₃-based semiconductors doped with aliovalent Mg(II) and functionalized with Ni/NiO_x catalytic 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_x also significantly enhances the stability of Ni/NiO nanoparticles in the production of hydrogen, allowing sustained operation, even in intermittent cycles of illumination. In situ elemental analysis of the water constituents during or after photocatalysis by inductively coupled plasma mass spectrometry/optical emission spectrometry shows that after CrO_x deposition, dissolution of Ni ions from Ni/NiO_x-Mg:SrTiO₃ is significantly suppressed, in agreement with the stabilizing effect observed, when both Mg dopant and CrO_x are 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_x is photodeposited in the vicinity of several, but not all, Ni/NiO_x particles. 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_x particles and the Mg:SrTiO₃ semiconductor, likely affect the probability of reduction of Cr(VI) species during synthesis by photodeposition, explaining the observed inhomogeneity in the spatial CrO_x distribution. 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.