Another actively pursued research area is to extend the
excitation spectrum of TiO2 to include visible light. The general
strategy is doping metal impurities, dye sensitizers, narrow
band-gap semiconductors, or anions into nano-TiO2 to
form hybrid nanoparticles or nanocomposites (Fujishima
et al., 2008; Ni et al., 2007). Metals and anions create impurity
energy levels or narrow the band gap; upon visible light
excitation, dye sensitizers and narrow band-gap semiconductors
inject electrons into TiO2 to initiate the catalytic
reactions.