Article pubs.acs.org/JPCC
The Role of Atomic Vacancy on Water Dissociation over Titanium Dioxide Nanosheet: A Density Functional Theory Study Chenghua Sun,*,†,‡ Ting Liao,† Gao Qing (Max) Lu,‡ and Sean C. Smith*,§ †
Centre for Computational Molecular Science, Australia Institute for Bioengineering and Nanotechnology, The University of Queensland, Qld 4072, Australia ‡ ARC Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Qld 4072, Australia § Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6494, United States S Supporting Information *
ABSTRACT: The interaction of water with titanium dioxide (TiO2) nanosheets has been studied under the framework of density functional theory plus Hubbard model. Particularly, the effect of an oxygen vacancy and a titanium vacancy on the dissociation of water has been investigated. It is found that molecular adsorption is favored on perfect TiO2 nanosheets and a titanium vacancy, while over an oxygen vacancy water prefers to dissociate spontaneously due to the strong bonding between water and unsaturated titanium and oxygen. With the formation of water−water hydrogen bonds, dissociated water can be further stabilized. The role of titanium and oxygen vacancies is further discussed from the viewpoint of electronic structure.
1. INTRODUCTION Atomic vacancies in solids, particularly in transition metal oxides, play a key role in a variety of surface reactions and have been extensively investigated for many years.1−5 Generally, vacancies lead to low coordination of local atoms and thus are more active than perfect bulk structures and surfaces. A typical example is water dissociation on titanium dioxide (TiO2) surfaces. On perfect rutile TiO2(110), water prefers to adsorb molecularly; however, water can spontaneously dissociate over oxygen vacancies (OVs).6−15 Therefore, vacancies are of great significance in understanding the surface chemistry of solids. For photocatalytic water-splitting over photocatalysts, the importance of vacancies basically lies in two aspects: (i) vacancies often give rise to local states above the intrinsic valence bands, which directly affects the sunlight harvest and the electron−hole separation and recombination;16−19 (ii) water dissociation directly leads to the formation of surface hydroxyl (OH), which can be transferred as •OH radicals and promote the production of hydrogen.20 In the recent years, lepidocrocite-type titanium dioxides (TiO2) layers, namely, TiO2 nanosheets, have been synthesized21−24 and extensively studied as photocatalysts for watersplitting applications.25−30 A typical advantage of such nanosheets is that most atoms are on the surface and exposed to external environment, which may increase the chance of photoinduced charge carriers to react with adsorbed species. For instance, nitrogen and iodine doping can be realized easily,28,30 which reduces band gaps to suit the need of visible light harvest. As of now, however, the interaction between water and TiO2 nanosheets is not clear, especially the role of © 2012 American Chemical Society
atomic vacancies on water adsorption and dissociation, especially, titanium vacancies (TiVs), which are widely observed in TiO2 nanosheets21−24 but rarely found in asprepared rutile and anatase TiO2. On the basis of an analogy to manganese dioxide (MnO2) nanosheets, cation vacancies can dramatically reduce the band gap and thus may improve its photocatalytic performance.31 Through electron irradiation, OVs can also be generated, as reported in our early work.32,33 Similarly, TiV- and OV-derived band gap narrowing has also been predicted in ultrathin TiO2 nanosheets (not lepidocrocitetype).34 Therefore, it is desirable to understand how water interacts with TiO2 nanosheets. In this article, the role of TiVs and OVs in lepidocrocite-type TiO2 nanosheets will be discussed with a focus on water dissociation, together with an investigation of the hydrogen bonds (HBs) in the H2O/TiO2 interface.
2. COMPUTATIONAL METHODS In this work, spin-polarized density functional theory (DFT) calculations were carried out using the Perdew−Burke− Ernzerhof functional35 and projected augmented wave (PAW) pseudopotentials,36,37 as embedded in the Vienna ab initio simulation package (VASP).38 The reciprocal space is spanned with a plane-wave basis with a kinetic energy cutoff of 480 eV. The k-space is sampled by the γ point due to the large size of the supercells. Such a setting was based on our tests, because Received: September 16, 2011 Revised: December 7, 2011 Published: January 5, 2012 2477
dx.doi.org/10.1021/jp208951p | J. Phys. Chem. C 2012, 116, 2477−2482
The Journal of Physical Chemistry C
Article
Figure 1. Top view of TiO2 nanosheet models. (a) Perfect 3 × 3 × 1 supercell and 3 × 3 × 1 supercell with (b) one OV and (c) one TiV. Ti and O are indicated by gray and red spheres. Only the first (oxygen) and the second (titanium) atom layers are shown as balls and the rest as sticks.
larger cutoff energies (600 eV) and k-grid settings (3 × 3 × 1) only lead to a small difference (
