Overall photocatalytic water splitting with NiOx–SrTiO3 – a revised mechanism

Abstract

NiO x (0 < x < 1) modified SrTiO 3 (STO) is one of the best studied photocatalysts for overall water splitting under UV light. The established mechanism for this and many other NiO x containing catalysts assumes water oxidation to occur at the early transition metal oxide and water reduction at NiO x . Here we show that NiO x -STO is more likely a three component Ni-STO-NiO catalyst, in which STO absorbs the light, Ni reduces protons, and NiO oxidizes water. This interpretation is based on systematic H 2 /O 2 evolution tests of appropriately varied catalyst compositions using oxidized, chemically and photochemically added nickel and NiO nanoparticle cocatalysts. Surface photovoltage (SPV) measurements reveal that Ni(0) serves as an electron trap (site for water reduction) and that NiO serves as a hole trap (site for water oxidation). Electrochemical measurements show that the overpotential for water oxidation correlates with the NiO content, whereas the water reduction overpotential depends on the Ni content. Photodeposition experiments with NiCl 2 and H 2 PtCl 6 on NiO-STO show that electrons are available on the STO surface, not on the NiO particles. Based on photoelectrochemistry, both NiO and Ni particles suppress the Fermi level in STO, but the effect of this shift on catalytic activity is not clear. Overall, the results suggest a revised role of NiO in NiO x -STO and in many other nickel-containing water splitting systems, including NiO x -La : KTaO 3 , and many layered perovskites.

Photocatalytic Water-Splitting Reaction from Catalytic and Kinetic Perspectives

“…Surface voltage spectroscopy, the photodeposition of Pt nanoparticles, and (photo) electrochemical measurements suggested that the nickel species contributed to both HER and OER. Ni nanoparticles serve as electron traps and lower the proton reduction overpotential, whereas NiO nanoparticles serve as hole traps and lower the water oxidation overpotential [43]. This view corresponds well with the conclusions of recent works on the coloading of cocatalysts for hydrogen evolution and oxygen evolution, which have found that coloading appropriate amounts of oxygen evolution catalysts can improve the overall water-splitting rate [9,31].…”

“…It was suggested that the Ni/NiO core/shell produced the higher photocatalytic activity because the Ni metal between NiO and SrTiO 3 facilitated electron transfer between the photocatalyst and cocatalyst. In contrast, it was recently reported that nickel species prepared using similar procedures on SrTiO 3 consisted of a mixture of Ni and NiO nanoparticles [43]. Surface voltage spectroscopy, the photodeposition of Pt nanoparticles, and (photo) electrochemical measurements suggested that the nickel species contributed to both HER and OER.…”

From natural to artificial photosynthesis

“…Nanoclusters of Ni, Co and their alloys with molybdenum were demonstrated to be potential electrocatalysts for HER [116,117]. Interfacing NiMo alloy nanoparticles with Si microarrays resulted in an effective photocathode that produced impressive photocurrents in the region of 15 mA cm 22 at 0 V versus reversible hydrogen electrode (RHE) powered by one Sun illumination [118].…”

The effect of crystallinity on photocatalytic performance of Co3O4 water-splitting cocatalysts

“…11 In the case of OER, a clear dominant cocatalyst has yet to be established. Several inorganic cocatalysts, such as IrOx, 12 CoOx, 10 Co3O4, 13 NiOx, 9,13 and RuO2, 14 were found to enhance the photocatalytic reaction.…”