Effects of defects on the electronic and optical properties of TiO ₂ nanosheet

Abstract

Hydrogen energy is one of the most potential clean energy. It could be obtained via photocatalytic water splitting.The key point on photocatalytic hydrogen production is to improve its efficiency. Owing to the distinctive physical and chemical properties, 2D materials show excellent activity in photocatalytic reactions [1][2][3][4][5][6]. To date, a large number of 2D materials have been successfully synthesized [1,4,[7][8][9][10][11][12][13][14][15], such as graphitic carbon nitride (g-C 3 N 4 ) and graphene [1,4,6,[16][17][18][19]. As a typical metal oxide, titanium dioxide (TiO 2 ) has been widely applied in the fields like heterogeneous photocatalysis, solar cells and biomedicine because of its excellent features of high stability, nontoxicity and low cost [20][21][22][23][24]. Besides its 3D crystals, including the rutile and anatase phases, 2D TiO 2 nanosheets have attracted much attention in recent years [15,[25][26][27][28].Currently, two kinds of TiO 2 nanosheets have been prepared in experiment, i.e., the anatase-type TiO 2 (A-TiO 2 ) nanosheet [13,14] and the lepidocrocite-type TiO 2 (L-TiO 2 ) nanosheet [11,12]. Early theoretical work proposes that the stablest polymorph is the L-TiO 2 one [29]. Researchers have carried out a series of studies on the L-TiO 2 nanosheet [30][31][32][33][34][35][36][37][38][39]. Wang et al find that strains could result in a significant change in the band gap and optical absorption of the L-TiO 2 nanosheet, improving its photocatalytic performance [34]. Zhou et al find that the L-TiO 2 nanosheet shows a direct band gap character and significant anisotropy behaviors in carrier mobility and optical response [38]. This study also demonstrates that the L-TiO 2 nanosheet can photocatalyze the hydrogen reduction reaction [38]. In addition, the L-TiO 2 nanosheet has been applied in dye-sensitized solar cells and lithium-sulfur batteries for electrochemical energy storage systems [40][41][42]. Experimentally, the L-TiO 2 nanosheet is obtained from some kinds of lepidocrocite-type parent materials like K 0.

Reaction Pathway of Photocatalytic Water Splitting on Two-Dimensional TiO2 Nanosheets

“…For a better understanding of the structure of the L-TiO 2 nanosheet, we also show the side view of its unit cell (Figure b) and the top view of the L-TiO 2 nanosheet (Figure c). A 30 Å vacuum is included above the nanosheet surface, whose convergence has been well tested in our previous work …”

“…A 30 Å vacuum is included above the nanosheet surface, whose convergence has been well tested in our previous work. 43 Ground-state configurations are optimized by the spinpolarized DFT + U approach with the Perdew−Burke− Ernzerhof (PBE) exchange−correlation functional employing the plane-wave-based Vienna Ab initio Simulation Package (VASP). 44,45 The PBE + U method is commonly used in computing bulk TiO 2 to remedy the error of the pure PBE functional in describing the electronic interactions within TiO 2 .…”

Reactivity of anatase (001) surface from first-principles many-body Green's function theory

“…Atom-centered Gaussian orbitals with the decay constants (in atomic unit) for Ti atoms, O atoms, and pseudohydrogen atoms were the same as those in our recent studies on TiO 2 . 31,32,40 In the calculation of the (1 Â 4) supercell, we went one further step beyond the G 0 W 0 level to evaluate more accurate quasiparticle energies and wave functions by diagonalizing the full G 0 W 0 Hamiltonian, including both the diagonal and off-diagonal elements. Electronic screening was calculated by employing the randomphase approximation and the plasmon-pole model proposed by von der Linden and Horsch.…”

Polaron-Induced Deep Defect Levels in Brookite TiO2: A Many-Body Green’s Function Theory Study

“…Origin of the deep defect states is very disputable. Various experimental techniques, including photoelectron spectroscopy, scanning tunneling microscopy, transmission Fourier-transform infrared spectroscopy, resonant photoelectron diffraction, electron paramagnetic resonance measurement, have been employed to explore the nature of deep defect states. − Massive first-principles calculations at the level of density functional theory (DFT), especially DFT + U and hybrid DFT using B3LYP and HSE functionals to describe the exchange–correlation interactions, provide very valuable information to help us understand the electronic properties of defects. ,,− With the development of theoretical approaches and computer power, GW method within the many-body Green’s function theory, which is considered to be superior to DFT in electronic structure calculations, is being applied to study defective TiO 2 recently. − According to the results from DFT + U and hybrid DFT, presently it is commonly thought that the deep defect states in rutile and anatase arise from small polarons induced by the excess electrons from oxygen vacancies (Ov) or hydroxyl groups. However, according to their experiments, Wendt et al ascribed the deep defect state in rutile to Ti interstitials (Ti-ints); Brandão et al proposed that Ov in rutile can only lead to shallow defect states near the CBM .…”

“…It is still an open question which values for these two parameters are suitable to describe the defected TiO 2 . The GW method is a parameter-free first-principles approach superior to DFT and has shown its power in predicting the electronic structures of both perfect and defected systems with extremely high accuracy. − In our previous work, we have used it to examine the electronic properties of defects and dopants in the anatase and rutile TiO 2 , successfully interpreting the experimental phenomena. ,− In this article, we will explain the origin of shallow and deep defect states in brookite as measured in experiments, discuss the discrepancy between brookite, anatase, and rutile in the behavior of polarons induced by defects, and compare as well the performance of GW method with that of DFT + U in the calculation on defected brookite.…”