Solar/Photocatalytic water splitting for the formation of hydrogen and oxygen gas is regarded as one of the most promising techniques for the development of a hydrogen-sustained economy. For efficient water splitting, photocatalytic systems need to be developed which i) are able to absorb visible light, ii) have proper band positions allowing proton reduction and water oxidation, iii) have a low overpotential, and iv) repress electron/hole recombination. To tackle the latter two, cocatalytic nanoparticles can be loaded on semiconductor materials: the overpotential of the photocatalyst will be lowered, and the cocatalytic nanoparticles will act as charge carrier beacons, improving charge carrier separation and thus repressing electron/hole recombination.^1,2 For proton reduction, noble metals (e.g. Pt, Pd) can be employed as cocatalytic nanoparticles, whereas metal oxides (e.g. RuO₂, IrO₂) are very suitable cocatalysts for water oxidation. 

Photodeposition is often considered to be a convenient, effective and green technique to deposit cocatalytic nanoparticles on semiconductor materials. However, on a fundamental level, photodeposition is not as simple as it seems. This is reflected in a limited amount of studies demonstrating that physicochemical properties of as-deposited nanoparticles, such as the oxidation state, size and dispersion, are dependent on e.g. the employment of a sacrificial reagent or adjustment of the pH.^3,4 However, this dependence on the reaction conditions also makes photodeposition a very promising technique to engineer cocatalytic nanoparticles with desired physicochemical properties. 

In this work, we investigated the influence of the sacrificial reagent methanol in the photodeposition of platinum (Pt) on tungsten oxide (WO₃), which is a suitable water oxidation catalyst. We demonstrate that the employment of methanol has drastic consequences for i) the amount of Pt deposited, ii) the oxidation state of as-deposited Pt, and iii) the size and dispersion of the Pt particles. Furthermore, we demonstrate that small, metallic Pt particles can be engineered by employing photodeposition in the right conditions, followed by hydrogenation. The mechanisms behind the photodeposition process are thoroughly discussed, as well as the consequences of these findings in solar water splitting. 

References

(1) Ran, J.; Zhang, J.; Yu, J.; Jaroniec, M.; Qiao, S. Z. Chemical Society Reviews 2014, 43, 7787.
(2) Maeda, K. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2011, 12, 237.
(3) Sungbom, C.; Kawai, M.; Tanaka, K. Bulletin of the Chemical Society of Japan 1984, 57, 871.
(4) Zhang, F.; Chen, J.; Zhang, X.; Gao, W.; Jin, R.; Guan, N.; Li, Y. Langmuir 2004, 20, 9329