ABSTRACT
As a clean and inexhaustible energy source, solar energy is the most important alternative to non-renewable fossil fuels to solve the problems of an energy shortage, global warming, and environmental pollution. The key issue is how to efficiently harvest and utilize solar energy. Photoelectrochemical (PEC) solar energy conversion is regarded as one of the most promising technologies for solar energy conversion and application. However, there are some challenges of employing this material for PEC water splitting application: (a) conduction band (CB) potential is not high enough to drive water reduction without bias, (b) a relatively low absorption coefficient because of indirect bandgap and (c) poor majority carrier conductivity and short hole diffusion length (2–4 nm). Metal oxides carved into hierarchical nanostructures are thought to be promising for improving photo-electrochemical performance by enhancing charge separation and transport. ZnO is studied as one of the most relevant optoelectronic materials, due to its unique optical (like large direct bandgap of 3.37 eV) and electrical properties (high exciton room temperature binding energy 60 meV, high electron mobility, etc.). However, the efficiency of the ZnO based photoanode has limited success because of its high recombination rate of e−h+ (electron-hole) pairs and poor catalytic 252activity. Over the years, several research efforts have been devoted to explore the effective methods to address those drawbacks. Such attempts have been employed through various approaches, such as doping, hetero-structure, dye sensitization, quantum dot sensitization, and co-catalysts modification. Out of these, nowadays, impurity-doped ZnO has been given more and more attention because the impurity element directly and simply enhances conductivity, carrier concentration, and optoelectronic performance of ZnO .
