First Principle Evaluation of Photocatalytic Suitability for TiO2-Based Nanotubes

Abstract

Water splitting under the influence of solar light on semiconducting electrodes Immersed in aqueous electrolyte is a potentially clean and renewable source for hydrogen fuel production. Its efficiency depends on relative position of the band gap edges (the visible light interval between infrared and ultraviolet (UV) ranges of electromagnetic spectrum corresponds to gap widths 1.5-2.8 eV) accompanied by a proper band alignment relative to both reduction (H+/H-2) and oxidation (O-2/H2O) potentials (-4.44 eV and -5.67 eV on energy scale for vacuum, respectively) which must be positioned inside the band gap. Its width for TiO2 anatase-structured bulk is experimentally found to be 3.2 eV, which corresponds to photocatalytic activity under UV light possessing only similar to 1% efficiency of sunlight energy conversion. Noticeable growth of this efficiency can be achieved by by adjusting the band gap edges for titania bulk through nanoscale transformation of its morphology to anatase-type nanotubes (NTs) (formed by folding of (001) or (101) nanothin TiO2 sheets consisting of 9 or 6 atomic layers and possessing either (n, 0) or (-n, n) chiralities, respectively) accompanied by partial substitution of pristine atoms by C-O, Fe-Ti, N-O and SO single dopants as well as N-O+S-O codopants. In the latter case, the band gap can be reduced down to 2.2 eV while the efficiency is achieved up to similar to 15%. The energy differences between the edges of band gap (VB and CB), the highest occupied and lowest unoccupied impurity levels inside the band gap (HOIL and LUIL, respectively) induced in doped NTs, while preserving the proper disposition of these levels relatively to the redox potentials, so that epsilon(VB) < epsilon(HOIL) < epsilon(LUIL)< epsilon(CB), thus reducing the photon energy required for dissociation of H2O molecule. In this chapter, we analyze applicability of large-scale first principle calculations on the doped single-wall titania NTs of different morphologies with the aim of establishment of their suitability for photocatalytic water splitting.