Plasmonic enhanced photoelectrochemical water splitting by hematite-based nanocomposites

Abstract

As a promising solar energy harvesting technology, artificial solar water splitting enables direct solar-to-hydrogen conversion and thus exhibits huge potential in the field of alternative energy source. Hematite is one of the most common photoactive semiconductors and can be utilized as photoanodes in photoelectrochemical water splitting systems. Despite being non-toxic, cost-efficient and earth-abundant, pristine hematite has poor light harvesting efficiency. Various strategies to improve the efficiency are essential for hematite photoanodes. Plasmonic enhancement that employs plasmonic nanostructures for enhanced electromagnetic field is one of the promising approaches to boost the solar water splitting performance of hematite. This PhD project focused on the implementation of plasmonic enhancement in hematite photoanodes, together with other optimization strategies such as nanostructuring, surface modification and upconversion. Modelling work was carried out as a theoretical framework to guide plasmonic nanostructures’ design. In particular, FDTD modelling for plasmonic nanodisk arrays, nanotriangle arrays and nanohole arrays were carried out to identify their plasmonic resonance wavelength as well as the spatial distribution of the enhanced field. Plasmonic nanohole arrays were chosen as the best candidates to be utilized in solar water splitting systems due to the significantly enhanced field originated from surface plasmon polaritons. Various novel plasmonic nanohole arrays including dual-layer nanohole arrays and non-noble, aluminum-based nanohole arrays are fabricated and applied to hematite photoanodes. Improved photocurrent density can be observed due to plasmonic enhancement. Furthermore, plasmonic enhanced triplet-triplet annihilation upconversion nanoparticles that can convert low-energy photons to high-energy photons were utilized to overcome the limitation of hematite's bandgap. The concepts of plasmonic enhanced upconversion and upconversion induced solar water splitting were proven by observing below-bandgap enhancement in incident photon-to-current efficiency measurements. This work not only demonstrated the feasibility of implementing plasmonic enhancement in hematite photoanodes but also paved the way towards employing plasmonic nanostructures in other solar water splitting devices.