If a photon energy, hv, irradiating TiO2 exceeds its BG, absorption of the photon by TiO2 excites an e" from the valance band to the conduction band while an electron vacancy (i.e., electron hole or simply hole), h+ is left in the valance band (Reaction 2) [3,9]. High energy from short wavelength light (UV or near-UV photons) is required for the excitation due to the high BG energy (EG) of TiO2 (below UV region at 387 nm corresponding to 3.2 eV for the anatase crystal phase of TiO2). Most of electron and hole pairs generated either simultaneously recombine in the bulk or migrate to the TiO2 surface and then recombine, releasing useless thermal energy (Reaction 3) [3,9]. This recombination reaction is detrimental to the process and limits the activity of the catalyst .
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