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 /H 2 O) 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 TiO 2 anatase-structured bulk is experimentally found to be 3.2 eV, which corresponds to photocatalytic activity under UV light possessing only ~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 TiO 2 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 S O 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 ~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 ε VB < ε HOIL < ε O 2 / H 2 O < ε H + /H 2 < ε LUIL < ε CB , thus reducing the photon energy required for dissociation of H 2 O 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.

CdSe/TiO2NTs Heterojunction-Based Nonenzymatic Photoelectrochemical Sensor for Glucose Detection

“…Denser k-point meshes were used for density of states (DOS) calculations as 7 × 7 × 1 for the CdSe/TiO 2 NTs heterojunction. In the CdSe/TiO 2 system, the TiO 2 nanotube wall was simulated with a 2 × 4 supercell of triple-layer anatase TiO 2 (101) . The convergence criteria of the residual Hellmann–Feynman force and energy during structure optimization were set to 0.01 eV Å –1 and 10 –5 eV, respectively.…”

First principles modeling of 3d-metal doped three-layer fluorite-structured TiO2 (4,4) nanotube to be used for photocatalytic hydrogen production

“…At the same time, a more effective way to improve the photocatalytic activity of titania was found to be the reduction of its dimensionality from 3D crystalline bulk to TiO 2 2D nanothin sheets or to 1D nanotubes. These materials possess, in addition, large surface areas and unique shapes with few interfacial grain boundaries, which promote charge transport and electron-hole pair separation [16]. Nano-tubular TiO 2 can be prepared by the hydrothermal method, via a solegel process, or when the carbon nanotubes (CNTs) being used as templates for titania growth, and a few others [17e23].…”

“…With the goal in mind to obtain unbiased information and to estimate the importance of theoretical simulations, we have applied two different ab initio techniques, namely, the method of the linear combination of atomic orbitals (LCAO) with Gaussiantype orbitals [33,34] and the method of linearized augmented cylindrical waves (LACW) [35,36]. Previously, we studied the structural stability and the electronic band structure for pristine singleand multi-layer fluorite as well as anatase-type TiO 2 nanotubes with first-principle quantum chemistry [16,26,27,37,38]. The photocatalytic suitability of C, Fe, N, and S doped (e.g., NþS co-doped) anatase-structured TiO 2 NTs with either the [001] or the [101] orientation of the chiral vectors was evaluated by us theoretically too [16,39e42].…”