Plasmon-enhanced photoelectrochemical water splitting with size-controllable gold nanodot arrays

Size-controllable Au nanodot arrays (50, 63, and 83 nm dot size) with a narrow size distribution (±5%) were prepared by a direct contact printing method on an indium tin oxide (ITO) substrate. Titania was added to the Au nanodots using TiO₂ sols of 2-3 nm in size. This created a precisely controlled Au nanodot with 110 nm of TiO₂ overcoats. Using these precisely controlled nanodot arrays, the effects of Au nanodot size and TiO₂ overcoats were investigated for photoelectrochemical water splitting using a three-electrode system with a fiber-optic visible light source. From UV-vis measurement, the localized surface plasmon resonance (LSPR) peak energy (E_LSPR) increased and the LSPR line width (γ) decreased with decreasing Au nanodot size. The generated plasmonic enhancement for the photoelectrochemical water splitting reaction increased with decreasing Au particle size. The measured plasmonic enhancement for light on/off experiments was 25 times for the 50 nm Au size and 10 times for the 83 nm Au nanodot size. The activity of each catalyst increased by a factor of 6 when TiO₂ was added to the Au nanodots for all the samples. The activity of the catalyst was proportional to the quality factor (defined as Q = E_LSPR/γ) of the plasmonic metal nanostructure. The enhanced water splitting performance with the decreased Au nanodot size is probably due to more generated charge carriers (electron/hole pair) by local field enhancement as the quality factor increases.