Photoelectrochemical water splitting to generate hydrogen uses the renewable sources to meet the daily increasing energy demand. Lots of metal oxides have been investigated as photoanode in a photoelectrochemical cell, where hydrogen generation occurs on the metal counter electrode. Despite great efforts, the solar-to-hydrogen conversion efficiency is still not fully up to expectations. On the other side, p-type semiconductors can be employed to generate photoelectrons that directly reduce water on the photocathode to hydrogen. Copper based metal oxides, including binary and ternary oxides, represent a promising class of p-type semiconductors due to their low cost and abundance in the earth. From the perspective of photoelectrochemical energy applications, they typically have large photocurrent, higher conduction band level for large hydrogen evolution driving force, and very positive onset potentials as well. However, the fatal issues that copper-based metal oxides suffer are the stability and efficiency in the aqueous electrolyte solution. In our presentation, we will show our recent efforts in both experiment and theory to manipulate copper-based metal oxides from the perspectives of morphology, geometry and electronic band structure by passivating the surface, engineering electronic band structures, and optimizing hydrogen evolution co-catalysts with the aim to achieve the long-term photostability and improve the solar-to-hydrogen conversion efficiency. The resultant nanostructures could be used as photocathode in photoelectrochemical cell for solar hydrogen generation.
Efficient solar energy conversion in photovoltaics and solar to chemical conversion is hindered by large band gaps and poor absorption in thin films. The easily tunable absorption and scattering cross section of localized surface plasmon resonance (LSPR) make it an ideal solution to capturing lost light. For above band edge light, scattering and light trapping can be used to increase absorption in thin semiconductor films, improving photoconversion without sacrificing recombination times. Below the band edge, plasmonic hot electrons can transfer to the semiconductor directly or resonant energy transfer can non-radiatively induce charge separation, allowing photoconversion where the semiconductor cannot absorb. In this brief review, we explore the mechanisms and efficiency of light recovery in plasmonics. Surface plasmon polaritons are used to increase light trapping in semiconductor nanowires using a metal nanohole array. Metal-semiconductor nanostructures with varying energy alignment, insulating barrier thickness, and spectral overlap are systematically varied to differentiate hot electron and resonant energy transfer. Transient absorption spectroscopy and action spectrum analysis are applied to track plasmonic charge creation and transfer, linking short and long time scale behavior. Guidelines are given for achieving optimal plasmonic light capturing and enhancement across the solar spectrum.