Narrow band gap semiconductors with appropriate band structure can play a similar photosensitizing role as organic dyes when they are coupled with TiO2 [84-86]. If the optical absorbance of narrow band gap semiconductors can gradually be tuned to absorb in the visible region by controlling the particle size, the overall photoactivity of the hybrid semiconductor system can be greatly expanded . Figure 12.10 illustrates the charge transfer mechanism of a TiO2 and CdSe coupled system. Band gap excitation of CdSe quantum dots leads to charge separation, which is followed by the accumulation of separated electrons and holes in the conduction and valence bands of CdSe quantum dots, respectively. The electrons in the conduction band of CdSe can transfer to the conduction band of TiO2, which is driven by the relative energy difference between the conduction band edge. Besides improving in photoresponse, the charge separation in the TiO2 and CdSe system can efficiently retard the recombination of photogenerated charge carriers by injecting electrons into the lower-energy conduction band of the large band gap semiconductor (TiO2). This quantum-dot sensitization can be adapted to other narrow band gap semiconductors, but the objective of physically separating electron-hole pairs and facilitating reduction and oxidation reactions on different semiconductors remains the same.