Photonic crystals have attracted extensive interest for the potential applications in manipulating light by nontraditional ways based on photonic band structure concepts. In this paper, 3D inverse-opal TiO 2 photonic crystals (TiO 2 -PCs) with designed photonic band gaps are prepared. It is worth noting that when the photonic band gaps of the TiO 2 -PCs are matched with the absorption peaks of the dyes (methyl orange, rhodamine B, and methylene blue), the photocatalytic activity of the corresponding sample is improved under simulated solar light (320 nm < λ < 800 nm) and visible light (420 nm < λ < 800 nm) irradiation. The enhancement could be attributed to the intensified dye sensitization as a result of slow photon effect on the edges of the photonic band gaps. Furthermore, the TiO 2 -PCs exhibit much higher photocatalytic activity and stability than TiO 2 nanoparticle film. It is believed that the presence of inverse opal structure plays an essential role in affecting the dye sensitization and photoreactivity, which could provide valuable information on the design of photocatalysts and set the foundation for the future environmental and energy technologies.

This review article provides a comprehensive review of recent progress in photonic nanostructures for efficient solar energy conversion.

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In the paper, the synthesis of ZnO/TiO 2 nanocomposites with different main parts (TiO 2 or ZnO) is studied. When TiO 2 is the main part of the ZnO/TiO 2 heterojunction photocatalysts (ZnO/TiO 2 ), the photocatalytic activity is decreased rapidly with the increase of the amount of ZnO. The reason may be attributed to the relative p−n junction (p-ZnO/ n-TiO 2 ) produced between ZnO and TiO 2 . The migration directions of the electrons and holes in the relative p−n junction are opposite to the transfer directions of the photogenerated electrons and holes in the valence band (VB) and conduction band (CB). However, when ZnO is the primary part of the heterojunction photocatalysts (TiO 2 /ZnO), the photocatalytic activity of the samples increases with the increase of the TiO 2 amount up to 5% (95% ZnO/TiO 2 ). The reason may be that the migration directions of the electrons and holes in the relative p−n junction (p-TiO 2 /n-ZnO) are the same as the transfer directions of the photoexcited electrons and holes in VB and CB between the two semiconductors. It is proposed that the conductivity of the heterojunction photocatalyst will be changed with the difference of content for the two semiconductors, which in turn affects the migration directions of the electrons and holes in the heterojunctions and their photocatalytic activity.