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Overall water splitting, evolving hydrogen and oxygen in a 2:1 stoichiometric ratio, using particulate photocatalysts is a potential means of achieving scalable and economically viable solar hydrogen production. To obtain high solar energy conversion efficiency, the quantum efficiency of the photocatalytic reaction must be increased over a wide range of wavelengths and semiconductors with narrow bandgaps need to be designed. However, the quantum efficiency associated with overall water splitting using existing photocatalysts is typically lower than ten per cent1,2. Thus, whether a particulate photocatalyst can enable a quantum efficiency of 100 per cent for the greatly endergonic water-splitting reaction remains an open question. Here we demonstrate overall water splitting at an external quantum efficiency of up to 96 per cent at wavelengths between 350 and 360 nanometres, which is equivalent to an internal quantum efficiency of almost unity, using a modified aluminium-doped strontium titanate (SrTiO3:Al) photocatalyst3'4. By selectively photodepositing the cocatalysts Rh/Cr2O3 (ref. 5) and CoOOH (refs. 3'6) for the hydrogen and oxygen evolution reactions, respectively, on different crystal facets of the semiconductor particles using anisotropic charge transport, the hydrogen and oxygen evolution reactions could be promoted separately. This enabled multiple consecutive forward charge transfers without backward charge transfer, reaching the upper limit of quantum efficiency for overall water splitting. Our work demonstrates the feasibility of overall water splitting free from charge recombination losses and introduces an ideal cocatalyst/photocatalyst structure for efficient water splitting.
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Demonstrating photocatalytic overall water splitting at an internal quantum efficiency (IQE) of 100% is an important challenge in the study of photocatalysis. Overall water splitting is a greatly uphill (Gibbs energy of +237 kJ mol-1) reaction consisting of multiple electron transfer processes. To achieve a 100% IQE, the first requirement is that all photoexcited carriers must migrate to surface reaction sites before bulk recombination. In addition, two-electron injection for the hydrogen evolution reaction (HER) and four-hole injection for the oxygen evolution reaction (OER) must proceed consecutively without any backward charge transfer. However, because there are many opportunities for backward electron transfer, overall water splitting with an external quantum efficiency (EQE) greater than 50% has rarely been demonstrated, even when using ultraviolet-responsive photocatalysts3'7-10. Thus, it is important to establish whether a...