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Letter
Role of Surface Stress on the Reactivity of Anatase TiO2(001) Yongliang Shi, Huijuan Sun, Wissam A. Saidi, Manh Cuong Nguyen, Caizhuang Wang, Kaiming Ho, Jinlong Yang, and Jin Zhao J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00181 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017
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Role of Surface Stress on the Reactivity of Anatase TiO2(001) Yongliang Shi,1 Huijuan Sun,1,2 Wissam A. Saidi,3 Manh Cuong Nguyen,4 Caizhuang Wang, Kaiming Ho,4 Jinlong Yang1,6 and Jin Zhao1,5,6* 1
ICQD/Hefei National Laboratory for Physical Sciences at Microscale, and Key Laboratory
of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
2
3
Department of Physics, Qingdao University, Qingdao, 266071, China
Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States
4
5
Ames Laboratory—US DOE and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, United States
Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh PA 15260, United States
6
Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
ABSTRACT In contrast to theoretical predictions in which anatase TiO2(001) and its (1×4) reconstructed surfaces are highly reactive, recent experimental results show this surface to be inert except for the defect sites. In this report, based on a systematic study of anatase TiO2(001)-(1×4) surface using first-principles calculations, the tensile stress is shown to play a crucial role on the surface reactivity. The predicted high reactivity based on add-molecule model is due to the large surface tensile-stress, which can be easily suppressed by a stress-release mechanism. We show that various surface defects can induce stress-release concomitantly with surface passivation. Thus, the synthesis of anatase(001) surface with few defects is essential to improve the reactivity, which can be achieved for example via H2O adsorption. Our study provides a uniform interpretation of
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controversial experimental observations and theoretical predictions on anatase TiO2(001) surface, and further proposes new insights on the origin of surface reactivity. TOC
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[email protected] TiO2 is an intensely studied semiconductor with important applications in photocatalysis and solar energy conversion.1-5 The surface stability and reactivity is believed to be critical for the high energy conversion efficiency.4 This explains why there have been extensive efforts to identify TiO2 surfaces with high reactivity.4-10 Of the commonly investigated crystalline phases of TiO2, anatase is conventionally considered to be more reactive than rutile.1-2, 4-5 The (001) and its (1×4) reconstructed anatase surfaces are believed to have high catalytic reactivity for water and other small molecules.11-16 Unlike the most studied rutile(110) and anatase(101) surfaces, where the H2O dissociation needs to overcome an energy barrier, it is argued that H2O can dissociate directly without energy barrier on anatase (001) and its (1×4) reconstructed surface.12, 14, 16 These predictions are based on the add-molecule model (ADM), which is obtained from unreconstructed anatase (001) by adding rows of TiO2 molecules on the surface.12, 14, 16 Motivated by such exciting theoretical predictions, there have been many efforts to synthesis and investigate anatase TiO2(001) surface.6-10, 17-23 However, the experimental results were not consistent with the theoretical predictions.24-28 For example, a comparison between the activity of anatase(001) and rutile(110) surfaces showed that their photochemical rate constants are nearly equal.24 Further, a clean anatase(001) surface exhibited a lower reactivity than the anatase (101) surface in photocatalytic reactions.25-26 Furthermore, recent atom-resolved STM measurements showed that the anatase(001)-(1×4) reconstructed surface does not activate H2O adsorption except at intrinsic Ti3+ defects.27 This work challenged the well-accepted ADM structure by proposing the add-oxygen model (AOM), which is constructed by adding oxygen ad-atoms to ADM. Yet recent theoretical calculations showed that AOM structure is not the most stable structure within a large range of temperature and oxygen pressure.28 Therefore, clear insights to understand the deviation between the theoretical predictions and experimental observations are still missing. In this letter, based on a systematic study on anatase TiO2(001)-(1×4) reconstructed surface using first principles density functional theory (DFT) calculations, we show that the surface tensile stress plays an important role on the surface reactivity for the deprotonation of small molecules.
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The predicted high reactivity based on ADM model is due to the large surface tensile stress. Different kinds of surface defects can release the tensile stress and concomitantly passivate the surface. To keep the high reactivity, it is essential to synthesize the ADM anatase(001)-(1×4) surface with few defects. We argue that by introducing H2O during the synthesis, the ADM structure without defects can be easily stabilized. This study provides a uniform picture to understand the controversial experimental observation and theoretical prediction on anatase TiO2(001) surface. It also proposes new insights to understand and improve its surface reactivity. Our calculations are based on DFT as implemented in the Vienna Ab initio simulation package (VASP)29-31, in conjunction with a global search method based on adaptive genetic algorithm (AGA) 32-34 to obtain the low-energy structures. The electron-nuclear interactions are described using projector augmented wave (PAW) method.35 We use the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional in all calculations.36 A slab model is used with a vacuum of 10 Å, in order to avoid the interlayer interaction. We used a 4×8 supercell with 4 TiO2 layers to describe the anatase (001)-(1×4) reconstructed surface. With such a large supercell, we only use Γ point to sample the Brillouin zone. For each structure, the bottom two TiO2 layers are kept fixed at bulk positions, while as the positions of other atoms are allowed to relax until the force on each atom is less than 0.02 eV/Å. We use a DFT optimized constant a0 = 3.821 Å for most of the calculations. In addition, up to 8% compressive strain is applied to understand the relationship between the surface stress, Ti-O bond length and surface reactivity. A larger lattice constants a0 =3.905 Å is also used to investigate effects due to epitaxial strain. It is known that DFT suffers from the infamous self-interaction error. We tested our results using DFT+U method with U=4.1 eV and we found that our conclusions are not affected. All the DFT+U calculations are included into the Supporting Information. The stress tensor is calculated using a tightly converged electron wavefunction obtained using a convergence of 10-9 eV on the energy in the self-consistent electron step, and an energy cutoff of 500 eV. The structures of different surface compositions, including oxygen vacancy (OV), titanium interstitial (Tiini), and oxygen adatoms are obtained using an adaptive genetic algorithm (AGA). This approach combines an adaptive atomistic force field approach with DFT to find the structures. More details are given in ref.32-34 A slab model which contains the substrate and the reconstructed surface is used in the AGA simulations. The substrate contains (4×4) surface supercell, which includes 32 TiO2 units, and two O-Ti-O layers. During the AGA simulation, the atoms in the substrate are fixed. On top of the substrate, we use 20 TiO2 units to simulate the reconstructed part of the stoichiometric surface. Different surface compositions are modeled by adding or removing Ti and O atoms. The initial pool of structures of the reconstructed surface is generated by randomly creating the surface atoms within 5 Å above the substrate. A region of vacuum with 10 Å is used to avoid the interaction between the adjacent slabs. More details can be found in the Supporting Information. The stress on the bulk-truncated anatase TiO2(001) surface has been reported in ref.15 Similar with their results, we find that the two Ti-O bonds connected to twofold-coordinated bridging oxygen (Ob) with lengths of 2.19 and 1.79 Å [Fig. 1a] are significantly different, while as these are equivalent in the bulk with a length of 1.94 Å. As shown in Fig. 1b, by adding a row of TiO2, the ADM (1×4) reconstructed model reduces the Ti-O bond lengths along [100] direction to a range within 1.82 to 1.87 Å. However, if we examine the ridge of the ADM structure along [010] direction as shown in Fig. 1c, we find two inequivalent Ti-O bonds with bond lengths of 1.86 and
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2.12 Å, which suggests that there is still surface tensile on the ridge along this direction. The diagonal elements of stress tensor can be calculated to understand how the surface stress changes. As shown in Table I, the stress tensor is zero for the optimized bulk structure, while as the stress is negative (indicating tensile stress) for the unreconstructed bulk-truncated anatase (001) surface along [100] and [010] directions. The ADM reconstruction partly releases the tensile stress along the [100] direction. However, the tensile stress is increased in the [010] direction by adding the TiO2 ridge. The stress induced by the ridge can be calculated by gridge=gADM - gbulk-truncated.
gbulk [100] [010]
0.0
gbulk-truncated
gADM
gridge
-29.7
-1.2
28.5
-35.6
-52.2
-16.6
Table I. The diagonal elements of stress tensor along [100] and [010] directions for bulk-truncated surface (gbulk-truncated) and ADM (gADM) structure. The stress contributed by the adding ridge (gridge) is also listed. All values are in kbar. g>0 (g0 (g
