Two-Dimensional Titania: Structures and Properties Predicted by First Principle Calculation

Abstract

Material innovations, especially in classic semiconductor materials such as titania, are expected and more and more accelerated by ab initio design for their wide applications. For better realization in laboratory, feasible stability of structures in ambient temperature and pressure should be prior to the structure−property relationship study. However, there is discrepancy between experiments and ab initio calculations about how the freestanding two-dimensional TiO 2 exists. Herein, by the structural search method with the first principles evolution algorithm, the stable freestanding monolayers (MLs) were found to be composed by octahedral, hexahedral, or pentahedral Ti−O block (o-ML, h-ML, or p-ML) with nonplanar structure, where o-ML is right the lepidocrocite type found experimentally. Meanwhile, novel structure-dependent properties mainly due to quantum-confine effects were investigated and the abnormal optical absorption, better properties of insulation, and redox in photocatalysis were found for these titania sheets.

Recent Advancements in the Understanding of the Surface Chemistry in TiO2 Photocatalysis

“…Unfortunately, no further details on properties such as band-gap values and chemical composition were reported. Hence, these properties were studied theoretically for the thinnest possible nanoblankets containing just two layers of Ti atoms of rectangular [35] and square shapes [36]. It turned out that the quantum-size effect was present in the square-shaped nanosheets but was obscured in the nanoribbons due to their smaller thickness.…”

Bandgaps in free‐standing monolayer TiO2: Ab initio diffusion quantum Monte Carlo study

“…A quasi‐particle bandgap of 5.30 eV was reported by the G 0 W 0 + BSE method [21]. Besides, a comparatively large GW gap (~5.97 eV) has already been obtained by Wang et al [17], approximately twice as large as the DFT bandgaps [16–18]. According to the literature, the obtained GW results strongly depended on the type of approximations of the used GW method.…”

“…The materials in devices must have an appropriate electronic bandgap for applications. The successful synthesis of free‐standing 2D lepidocrocite‐phase (α‐phase) titania nanosheets [14, 15] and the theoretical predictions of other 2D‐TiO 2 (such as β‐phase) [16] provide a new opportunity to replace bulked TiO 2 as next‐generation promising candidates for solar‐driven photocatalysis.…”

“…In the case of the α‐phase 2D‐TiO 2 , many researchers investigated the electronic structure using first‐principles calculations and the obtained bandgaps are 3.01 eV by Wang et al [16], 3.09, and 4.22 eV by Wang et al [17] and Sato et al reported a bandgap of 3.15 eV [18]. As a standard electronic structure method, however, Kohn–Sham density functional theory (DFT) for electronic structure calculations suffers from the well‐known bandgap problem, systematically underestimating the bandgaps of most materials, particularly for the transition‐metal oxides materials, such as TiO 2 , due to so‐called exchange‐correlation effects [19, 20].…”

“…In addition to the α‐phase of TiO 2 film, Wang et al [16] theoretically predicted the presence of the β‐phase TiO 2 by the ab initio structure prediction method. Based on PBE and hybrid functional (HSE06) calculations [16], the bandgaps for the β‐phase TiO 2 were 2.64 and 3.84 eV, respectively. It is worth mentioning that, finally, both the α‐ and β‐phases TiO 2 have direct bandgaps in all the reports mentioned above.…”