Polymorph Engineering of TiO₂: Demonstrating How Absolute Reference Potentials Are Determined by Local Coordination

Abstract

We report that the valence and conduction band energies of TiO 2 can be tuned over a 4 eV range by varying the local coordination environments of Ti and O. We examine the electronic structure of eight known polymorphs and align their ionization potential and electron affinity relative to an absolute energy reference, using an accurate multi-scale quantum-chemical approach. For applications in photocatalysis, we identify the optimal combination of phases to enhance activity in the visible spectrum. The results provide a coherent explanation for a wide range of phenomena, including the performance of TiO 2 as an anode material for Li-ion batteries, allow us to pinpoint hollandite TiO 2 as a new candidate transparent conducting oxide, and serve as a guide to improving the efficiency of photoelectrochemical water splitting through polymorph engineering of TiO 2 .

Water splitting and the band edge positions of TiO2

“…As can be seen in Table 1, the free surface shifts the CBM (and E F ) on an absolute scale from the bulk values (A: À4.94 eV; R: À4.73 eV, w.r. to vacuum [31]), even further below the H + /H 2 redox potential. Obviously, however, one has to consider the effect of water, which we simulate (as described above) by 1 ML molecular water above the surface (Fig.…”

“…: Fermi-level pinning, as explained above, would preclude water splitting -as observed.) In a recent theoretical paper, the CBM positions of various TiO 2 modifications were investigated in the bulk, using a high level hybrid meta-GGA (generalized gradient approximation) functional in density functional theory (DFT) [31]. The values À4.73 eV for R and À4.94 eV for A were obtained, noting that surface termination and morphology might change that.…”

Deep vs shallow nature of oxygen vacancies and consequentn-type carrier concentrations in transparent conducting oxides

“…A more detailed account of the approach can be found in Refs. [96,[99][100][101]. We note that the method has previously been applied successfully to treat defects and band alignments in wide gap materials as well as systems of reduced dimensionality [23,27,[102][103][104].…”

Modelling materials for solar fuel synthesis by artificial photosynthesis; predicting the optical, electronic and redox properties of photocatalysts

“…Vertical potentials, based on approach I or II, are routinely reported in the literature [33,[89][90][91][92][93][94][95][96][97][98][99]. See the work of Stevanovic et al [89] for a nice illustration for the case of GW vertical potentials for the surfaces of typical crystalline photocatalysts, which also clearly illustrates that potentials are surface rather than bulk properties (see figure 3).…”