Natural dyes as TIO2 sensitizers with membranes for photoelectrochemical water splitting: An overview

The development of titanium dioxide-based semiconductors has been widely studied due to the efficiency of this material in photoelectrochemical water splitting. However, the large band gap of titanium dioxide can only absorb UV light, thus reducing its performance in photoelectrochemical applications. Moreover, the recombination of electron-hole pairs also affects the efficiency of phototelectrochemical reactions. Thus, many efforts have been made to enhance the performance of titanium dioxide, and considerable attention has been focused on dye sensitizers, particularly natural dyes, due to their environmental friendliness and low cost. In addition, dye sensitizers exhibit fast electron injection and slow backward reactions. However, natural dye sensitizers are unstable in solution, thus requiring a protective layer, such as a conductive polymer layer. This paper presents an overview of common pigments found in natural dyes, extraction methods, the general efficiency of natural dyes, the types of natural dyes used in water splitting and membranes used as protective layers.