Photonic crystals (PCs) have promising characteristic to raise the light harvest of materials. PCs with one photonic band gap (PBG) were usually used to enhance the light harvest of materials. In this research work, we used PCs with dual PBGs to increase the light harvest. SiO.sub.2 PCs with dual PBGs were constructed through depositing a SiO.sub.2 PCs layer on another SiO.sub.2 PCs layer by vertical deposition method. Then the bi-layer structured SiO.sub.2 PCs were composited with nanocrystalline TiO.sub.2 film which was sensitized by CdS quantum dots (QDs) (Q-T), to produce photocatalyst for photodegradation of gaseous acetaldehyde. By adjusting the PBGs of the bi-layer SiO.sub.2 PCs, namely one PBG matching with the absorption edge of TiO.sub.2 and another PBG matching with the absorbing range of CdS QDs, the photocatalytic activity of the composite film under white light irradiation was increased to 2.11 times of that of the control photocatalyst (the Q-T film coated on a disordered porous SiO.sub.2 film). When Q-T film was coupled with a monolayer SiO.sub.2 PCs, which has single PBG centered at TiO.sub.2's electronic band gap (EBG) of 380 nm, its photocatalytic activity was increased by 1.34 times. When coupled with a monolayer SiO.sub.2 PC with the PBG centering at the EBG of CdS QDs (465 nm), the photo-catalytic activity of the Q-T film was increased by 1.54 times. Because the SiO.sub.2 PCs with dual-PBGs can simultaneously enhance the light harvest in ultraviolet and visible light region, the Q-T film coupled with dual-PBGs SiO.sub.2 PCs showed the highest activity.