Nanotransfer printed plasmonic heterostructure for solar water splitting

Abstract

A search for a new paradigm is underway to replace conventional photochemical systems by directly converting solar energy to chemical fuels, more specifically by photocatalytic water splitting. So far, conventional systems - namely oxide semiconductors and organic dyes - have displayed problems, including low light harvesting, instability, and charge recombination, thus limiting the overall light-to-fuel efficiency. Given the issues regarding the two representative water splitting systems, a solution to the aforementioned problems can be plasmonic metal nanostructures. Unlike oxide semiconductors, plasmonic metal nanostructures encompass a broad and strong absorption of visible light based on localized surface plasmon resonance (LSPR). The stability of plamonic metals is also exceptionally high compared to semiconductors or dyes. Most importantly, the metal nanostructures can generate “hot carriers” via plasmon decay which are energetic enough to overcome the Schottky barrier formed between a metal and a semiconductor. With this a plasmon-induced charge separation process can be achieved. Here, we investigate solar water splitting using a plasmonic gold (Au)-semiconductor ($TiO_2$) heterostructure prepared via the block copolymer (BCP)-based nanotransfer printing technique. The transfer printing process allows ordered Au nanopatterns to be easily incorporated onto a $TiO_2$-deposited FTO substrate, thereby forming an $Au-TiO_2$ junction through which a direct hot carrier transfer can occur. The technique also allows us to control the geometry of the Au pattern namely nanowires and nanorods, allowing for tunable absorption of light. Given this advantage, the photocurrent densities of samples comprising Au nanowires (AuNWs) and Au nanorods (AuNRs) are measured for comparison. Electrochemical analysis is made using cyclic voltammetry to visualize the effect of illumination of the plasmonic heterostructure on the onset potential for water oxidation. In doing so, it has been demonstrated that heterostructures comprising Au nanorods are more effective light harvesters than the Au nanowire counterpart.