A large-area fabrication of moth-eye patterned Au/TiO₂ gap-plasmon structure and its application to plasmonic solar water splitting

TiO₂ is promising candidate for solar water splitting material. To enhance the insufficient absorption range of TiO₂, TiO₂ decorated with Au nanoparticles(AuNPs) structure have been studied intensively. This plasmonic approach can extend absorption range to visible light, but its incident photon-to-current conversion efficiency is still insufficient. Therefore, it is important to amplify surface plasmon resonance, obtained by using AuNPs. At the same time, for application to next-generation energy device, the way to improve this plasmonic effect needs to be available to large area fabrication, high reproducibility, low cost and so on. In this study, we successfully fabricated a three-dimensional (3D) moth-eye AuNP/TiO₂/Au hierarchical structure for water splitting via direct printing method and deposition process. The proposed structure can effectively intensify the light–matter interaction owing to two mechanisms: Photonic mode light trapping attributed by moth-eye structure and enhanced surface plasmon resonance by gap-plasmon structure. Moth-eye structure, densely packed subwavelength-nanocone array, was easily fabricated by direct printing method. Using this moth-eye structure as a template, we effectively fabricated 3D moth-eye patterned gap-plasmon structure on 5 × 5 cm². Compared with the two-dimensional (2D) AuNP/TiO₂/Au absorber, the 3D moth-eye type absorber has higher absorption in entire 300–800 nm range. In accordance with this result, the 3D moth-eye absorber provides a photocurrent density of approximately 52.82 μA cm⁻², which is approximately 2.3 times higher than that of the flat 2D TiO₂/Au thin film (22.96 μA cm⁻²). Notably, it exhibits extraordinary enhancement of the photocurrent density—from 1.5 to 22.51 μA cm⁻²—in the visible range (≥420 nm).