Structure and Reactivity of Anatase TiO2(001)-(1Ã4) Surface

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Structure and Reactivity of Anatase TiO(001)-(1×4) Surface Huijuan Sun, Wen-Cai Lu, and Jin Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02777 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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The Journal of Physical Chemistry

Structure and Reactivity of Anatase TiO2(001)-(1×4) Surface Huijuan Sun1*, Wencai Lu1 and Jin Zhao2,3,4* 1

College of Physics and Laboratory of Fiber Materials and Modern Textile, the Growing Base for State Key Laboratory, Qingdao University, Qingdao, 266071, China

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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 3

Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh PA 15260, United States 4

Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

ABSTRACT TiO2 anatase (001) surface which usually exhibits (1×4) surface reconstruction has attracted lots of research interests for its potentially high photocatalytic activity. The atomic structure of the reconstruction and defects of this surface play an important role on its reactivity. Besides the well-

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known add-molecule model (ADM) reconstruction the add-oxygen model (AOM) for anatase (001)-(1×4) reconstructed surface by adding one oxygen atom to each Ti4c atom was proposed. In this work we investigate the geometric and electronic structures as well as the H2O and O2 adsorption behavior on anatase (001) surface with AOM systematically. The different defect structures including oxygen vacancy (OV), Ti interstitial (Tiini) and TiO2 vacancy (TiO2)V are also studied. Our calculations show that oxidization makes the AOM surface inert to molecular adsorption. The TiO2 vacancy with Ti interstitial, (TiO2)V-Tiini, is found to be the only reactive site for water and oxygen molecules adsorbing on the TiO2 anatase (001)-(1×4) reconstructed surface. The investigations based on AOM can explain the atomic-resolved STM results appropriately. Our systematic study of the AOM reconstruction of anatase (001) provides new insights into the understanding of the structure and reactivity on this surface.


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INTRODUCTION TiO2 surface has drawn enormous attention due to its wide applications in photocatalysis, photovoltaics, and photoelectrocatalysis.1-4 The anatase polymorph of TiO2 has been studied intensively since it is the major component of TiO2 nanoparticles and displays higher photocatalytic activity than rutile TiO2.2, 4 Anatase nanoparticles typically expose (101) and (001) surfaces.2, 4 The (001) surface has been considered to be the most reactive surface by several theoretical calculations, 5-10 and lots of efforts have been devoted to synthesis and investigate the structure and reactivity of anatase TiO2 (001) surface.11-24 It is known that TiO2 anatase (001) surface undergoes a (1×4) reconstruction, but it is not easy to figure out the atomic structure of the reconstruction and defects on this surface. Lazzeri et al. proposed the add-molecule model (ADM) by adding rows of TiO2 on the reconstructed surface. Based on this model, there will be TiO2 ridges on the surface with two coordinated O (O2C) and four coordinated Ti (Ti4C) on the surface. Small molecules like H2O and CH3OH will dissociate directly on the ridge of ADM without energy barrier, suggesting a high surface reactivity.9 Motivated by such exciting theoretical predictions, there have many efforts to synthesis TiO2 with high portion of (001) surface.11-15 However, recently there have been lots of experimental results which are not consistent with the theoretical predictions based on ADM.25-29 The main disagreement between ADM model and the experimental results is that ADM predicts that on anatase(001) surface, the H2O and CH3OH can dissociate without energy barrier, suggesting that this surface is more reactive than anatase(101) and rutile(110) surfaces. For example, a comparison between the activity of anatase(001) and rutile(110) surfaces, showed that their photochemical rate constants are nearly equal.25 Further, another experimental work shows that a clean anatase(001) surface exhibited a lower reactivity than the anatase (101) surface in photocatalytic reactions.26 Recently, Wang et al. performed atomresolved STM and TPD measurements on anatase(001)-(1×4) reconstructed surface28, 30. First, they showed that there are two types of defects on this surface, showing dark and bright characters. And these “dark” and “bright” defects can convert to each other. Second, they found that the surface shows an “inert” property according to the adsorption of H2O and O2. These molecules does not


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even adsorb on the surface except at “bright” defects. After H2O or O2 adsorption, the “bright” defect will turns into a dim spot. All these work challenges the well-accepted ADM structure and the understanding of the structure of this surface at the atomic scale is essential. Some efforts have been already made trying to understand the contradiction between the ADM model and the experimental results.16, 28, 30, 31 New surface reconstruction models were proposed including the add-oxygen model (AOM), which is constructed by adding one oxygen adatoms to each Ti4C atom on the ADM ridge, and the modified ADM (M-ADM) structure.28, 31 Recently Shi et al. used global search method based on adaptive genetic algorithm to investigate this surface and proposed defect models to explain the “inert” property of this surface and the “dark” and “bright” defects as well as their conversion based on ADM models.22 Yet their results can not explain the “bright to dim” conversion induced by H2O and O2 adsorption on the “bright” defect. Thus the fully understanding of this surface structure is still not achieved. In this report, we investigate the atomic and electronic structures of AOM surface systematically based on first principles calculations. The different defects including the oxygen vacancy (OV), Ti interstitial (Tiini), TiO2 vacancy (TiO2)V and TiO2 vacancy with Ti interstitial [(TiO2)V-Tiini] are investigated. STM images are simulated and compared with the experimental results based on the first principles calculations. First, AOM can explain the “inert” property of the anatase(001)-(1×4) surface. Second, the “bright” and “dark” spots observed in the STM measurements can be explained by the (TiO2)V and [(TiO2)V-Tiini] defects. Their conversion can also be understood. Third, the “bright-to-dim” transition induced by H2O and O2 adsorption can be interpreted by AOM with [(TiO2)V-Tiini] defect. The stability of AOM and (TiO2)V has been tested using molecular dynamics. The systematic study of AOM and the defects provides important insights into the comprehensive understanding of the structure and reactivity of TiO2 anatase(001)(1×4) surface. This paper is organized as follows. The details of the calculation method are provided in METHODOLOGY part. In the part of RESULTS AND DISCUSSION, we show the results of geometric and electronic structures of clean AOM surface, OV, Tiini, (TiO2)V and (TiO2)V-Tiini. The adsorption of H2O and O2 on different defects are also studied. The results are compared with the STM measurements with atomic scale resolution.


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METHODOLOGY Our calculations are performed using a Vienna ab initio simulation package (VASP).32-34 The electron-nuclear interactions are described using projector-augmented wave (PAW) methods.35 All calculations are performed spin polarized. The energy cutoff for the plane-wave basis set is 400.0 eV. For bulk anatase TiO2, an orthorhombic unit cell with experimental parameters a=b=3.78 Å, c=9.5 Å, α=β=γ= 90.00° is adopted36. A slab model that contains four TiO2 tri-layers is used to describe the anatase TiO2 (001)-(1×4) reconstructed surface. During the geometrical optimization, the bottom TiO2 tri-layer is kept ﬁxed. A 1×4 supercell of 3.78×15.10×30 Å3 is used to simulate the clean reconstructed surface and a 6×4 supercell of 22.66×15.10×30 Å3 is used for the simulation of the defects. The criteria of convergence for the electronic structure is set to be 10-5 eV. For the 1×4 supercell, which contains 52 atoms for AOM, the geometries are relaxed with all forces on each atom ≲ 0.005 eV/Å. For the 6×4 supercell, which contains more than 300 atoms, we used the total energy difference as convergence criterion, i.e., the geometry relaxation stops when the energy difference between two iterations is smaller than 10-4 eV. The exchange correlation functional is described with the HSE functional for the clean surface with 1×4 supercell.37 PBE functional is used for the defect systems with 6×4 supercell.38 In order to verify the PBE results, we choose one of the defect systems to test using HSE functional in the Supporting Information. A vacuum of 17 Å is used together with dipole correction39 to avoid the interlayer interactions. A 2×8×1 k-point sampled grid using the Monkhorst–Pack scheme40 is applied to the clean (1×4) reconstructed surface. For the simulation of defects on this surface, reciprocal space sampling is restricted to the Γ point. 1

The formation energy for ADM to AOM is calculated by 𝐸𝑓𝑜𝑟𝑚 = 𝐸𝐴𝑂𝑀 – (𝐸𝐴𝐷𝑀 + 2 𝐸𝑂2 ), in which 𝐸𝐴𝑂𝑀 and 𝐸𝐴𝐷𝑀 represent the energies of the AOM and ADM per 1×4 reconstructed unit cell. Because DFT has problems in describing the triplet states of O2, 𝐸𝑂2 is computed from the 1

reaction 2 O2 + H2 →H2O and the experimental dissociation energy of 2.45 eV is used41. The formation energies for OV and Tiini defects are calculated by 𝐸𝑓𝑜𝑟𝑚 = 𝐸𝑡𝑜𝑡𝑎𝑙 – (𝐸𝐴𝑂𝑀 ± n𝐸𝑂/𝑇𝑖 ), 1

where 𝐸𝑡𝑜𝑡𝑎𝑙 , 𝐸𝐴𝑂𝑀 , 𝐸𝑂 and 𝐸𝑇𝑖 represent the energies of the defects model, AOM, 2 𝐸𝑂2 , and


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bulk Titanium. The formula for the formation energy of (TiO2)V is calculated as 𝐸𝑓𝑜𝑟𝑚 = 𝐸(𝑇𝑖𝑂2)𝑉 − (𝐸𝐴𝑂𝑀 − 𝐸(𝑇𝑖𝑂2)𝑢𝑛𝑖𝑡 ), and for the formation energy of (TiO2)V-Tiini is calculated as 𝐸𝑓𝑜𝑟𝑚 = 𝐸(𝑇𝑖𝑂2)𝑉−𝑇𝑖𝑖𝑛𝑖 − (𝐸(𝑇𝑖𝑂2 )𝑉 + 2𝐸𝑇𝑖 ), in which 𝐸(𝑇𝑖𝑂2 )𝑉 , 𝐸(𝑇𝑖𝑂2)𝑢𝑛𝑖𝑡 , 𝐸(𝑇𝑖𝑂2)𝑉−𝑇𝑖𝑖𝑛𝑖 and 𝐸𝑇𝑖 represent the energies of (TiO2)V, one TiO2 unit of bulk anatase TiO2, (TiO2)V-Tiini and bulk Titanium. The adsorption energies of different molecules are recalculated according to 𝐸𝑎𝑑𝑠 = 𝐸𝑡𝑜𝑡𝑎𝑙 − (𝐸𝐴𝑂𝑀 + 𝐸𝑚𝑜𝑙 ) , where 𝐸𝑡𝑜𝑡𝑎𝑙 , 𝐸𝐴𝑂𝑀 and 𝐸𝑚𝑜𝑙 represent the energies of the whole adsorption system, AOM and the free molecule, respectively. A negative formation energy or adsorption energy indicates a stable configuration. Molecular dynamics simulations are performed to confirmed the stability of AOM model and (TiO2)V defect. Canonical ab initio molecular dynamics trajectories for 1 ps are generated with a time step of 1 fs at 80 and 300 K respectively. 80 K is the temperature at which the surface were grown and measured in the STM experiments.28 A 2×4 supercell are adopted in molecular dynamics simulations. Bader charge analysis is employed to investigate the charge transfer among atoms.42-44 The simulated STM images are generated using the Tersoff and Hamann method45 in constant current mode with a positive bias of 2.0 eV, namely from the conduction band minimum (CBM) to CBM+2.0 eV, which is similar to the parameters used in experiments.

RESULTS AND DISCUSSION Clean AOM surface The structure of clean TiO2 anatase (001)-(1×4) surface based on AOM is shown in Fig. 1a. The one additional O atom to each surface cell can make two Ti-O bonds, changing Ti4C to sixcoordinated Ti (Ti6C). We find the ad-oxygen atom has four symmetrically equivalent sites around the ridge Ti atom, as seen in Figs. 1b and c, and the rotation barriers are 0.35 and 0.04 eV by PBE functional. During the STM measurement, using the bias voltages of 1.5-2.0 V, the ad-oxygen atom can switch easily among these equivalent sites, resulting in a symmetric STM image, as shown in Fig. 1a. The electronic structures of AOM and the spatial orbital distribution of valance band maximum (VBM) and CBM orbitals are shown in Fig. 1d. The VBM is contributed by the ad-oxygen atom, bridging O atom, and ridge Ti atom, while the CBM is contributed by the bulk


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Ti 3d orbital. The CBM exhibits as a separate peak in the plotting, which is similar to the CBM of the ADM structure.46, 47 The charge transfer among Ti and O atoms is studied by bader charge analysis. The charge gained by ad-oxygen atom and bridging O atom are 0.43 and 0.55 e, which are about half the value of bulk O atom (0.97 e). There is no much difference in charge transferred from Tiridge and the bulk Ti atoms, of which the values are 1.89 e and 1.93 e. The Ob-Oad bond length is 1.46 Å, which confirms that Ob-Oad forms a peroxide like structure. This surface is spinunpolarized, resulting in a zero magnetic moment.

Figure 1. (a) Side and top views of AOM structure and simulated STM image averaged by the four equivalent sites at a bias of +2.0 V. The unit of bond lengths is in Å. (b, c) Structural models showing the equivalent positions for the ad-oxygen atom, and the simulated image obtained only from the ad-oxygen atom at position as indicated. (d) Total and partial DOS of AOM structure using the HSE06 functional, together with the spatial orbital distributions of VBM and CBM. Energies are given relative to CBM.

Molecular dynamics simulations show that during the 1 ps at 80 K, AOM stably exists with ad-oxygen atom and bridging O atom slightly rotating around the equilibrium positions. Such simulation suggests that AOM is stable at 80 K. Side and top views of snapshots from the molecular dynamics trajectories are shown in Fig. 2a. Further, we increase the temperature to 300 K. At this temperature the ad-oxygen atoms rotate among several equivalent positions, as shown


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in Fig. 2b, which is in agree with the energy barrier calculations. Yet the basic structure of AOM still keeps even at 300 K, which proved that AOM could stably exist under experimental temperatures.

Figure 2. The snapshots of the molecular dynamics of AOM model at 80 K (a) and 300 K (b) respectively.

Oxygen vacancy We start from the most common intrinsic defect on oxide surfaces: OV. Two types of OVs are considered, single O atom vacancy (OV1) and double O atoms vacancy (OV2), as shown in Figs. 3a and b. OV1 is formed by missing one ad-oxygen atom from the surface, with the formation energy of -0.63 eV. OV2 is generated by removing one ad-oxygen atom together with the bridging O atom bonding to it, which is analogous to the oxygen vacancy in the ADM structure. The formation energy of OV2 is 4.81 eV, indicating the less stability. In addition, the formation energy of OV on ADM surface is 2.9 eV, while the formation energy of OV on unreconstructed anatase (001) surface is 1.08 eV.


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Usually the loss of oxygen atom from stoichiometric TiO2 would gain two electrons to the system and give rise to defect levels, thus OV usually demonstrates as a bright spot in STM images at positive bias voltage.2, 48, 49 However, given that the AOM structure is oxygen rich, removing one O atom (OV1 defect) would not cause much effect on the electronic structure, as shown in Fig. 3c, where TiOV refers to the two Ti atoms bonding to the missing O atom. By bader charge analysis, TiOV atoms have almost the same charge with the other Ti atoms, while the Ob atom at the vacancy site traps the charge which should be trapped by both of the ordinary Ob and Oad atoms, as listed in Fig. 3e. Thus, removing OV1 atom changes the Ob-Oad peroxide structure to a Ob2- site. From the simulated STM image in Fig. 3g, one can see that OV1 defect does not show distinct character in the STM image. Nevertheless, for OV2, defect states contributed by TiOV (three coordinated) atoms arise, resulting in a bright spot in the simulated STM image, as can be seen in Figs. 3d and h. The two TiOV atoms in OV2 loses 0.29 e and 0.16 e less charge than the other ridge Ti atoms owing to the lose of the two oxygen atoms, as listed in Fig. 3f. The magnetic moment for OV1 is zero. OV2 has a spin-polarized configuration, and the magnetic moment is 1.00 μB, which mainly originated from the two TiOV atoms (0.76 μB in total).

Figure 3. (a, b) Side views of two different OV structures. (c, d) Total and partial DOS of the TiOV atoms. The inset shows the spatial orbital distribution of the defect states induced by OV. (e, f) Charge transfer analysis using bader analysis. (g, h) Simulated STM images of different OV structures for a bias of +2.0 V. Energies are given relative to CBM.


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Ti interstitial Tiini is also an intrinsic defect commonly existed in TiO250. Two different Tiini structures labeled as Tiini1 and Tiini2 are studied, as shown in Figs. 4a and b. In Tiini1, the interstitial Ti atom (Tiini atom) locates in the cavity under the ridge Ti atom, while in Tiini2, interstitial Ti atom locates under the bridging O atom. Bonding of Tiini2 atom with the bridging O atom above would weaken the bond between this bridging O atom and the ad-oxygen atom, making the ad-oxygen atom dangling. The formation energy for Tiini1 and Tiini2 are -1.2 and -4.4 eV respectively. Negative formation energies of Tiini structures indicate forming Ti interstitial is exothermic. The interstitial Ti atom in Tiini1 bonds with four neighboring O atoms while the interstitial Ti atom in Tiini2 bonds with five neighboring O atoms. More Ti-O bonds in Tiini2 together with the relaxation of the dangling ad-oxygen atom would stabilize interstitial Ti atom and reduce the energy.

Figure 4. (a, b) Side views of two different Tiini structures. (c, d) Total and partial DOS of the Tiini atoms. The inset shows the spatial orbital distribution of the defect states induced by Tiini. (e, f) Charge transfer analysis using bader analysis. (g, h) Simulated STM images of different Tiini structures for a bias of +2.0 V. Energies are given relative to CBM.


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The electronic structures of Tiini defects are demonstrated in Figs. 4c and d. The Tiini atom will introduce four electrons to the system. The gap states originated from the Tiini 3d orbitals occur for both Tiini structures. The interstitial Ti induced excess electrons are mostly localized around the Tiini positions. In Tiini1, the interstitial atom and one neighbouring Ti atom (Tin) lose less charge (1.64 and 1.80 e respectively) than the bulk Ti atoms (1.93 e), as shown in Fig. 4e. Bader charges of Ti atoms in Tiini2 do not show much difference with the non-defective TiO2 (Fig. 4f). The addition of the Tiini2 atom would reduce both the dangling O atom and the neighboring Ob atom, and thus fewer Ti4+ ions are reduced than if the interstitial were in an unreconstructed surface. The simulated STM images are displayed in Figs. 4g and h, both of which show very weak bright spot character. The magnetic moments of the two Tiini structures are 0.02 and 0.12 μB, respectively.

TiO2 vacancy A large number of dark and bright spots are observed for the defects in the STM images of anatase TiO2 (001)-(1×4) surface.28 In addition, the bright spot defect can be changed to dark spot defect by applying a voltage pulse of 3.7 V or higher using the STM tip.28 Obviously, the dark spot can not be explained by Tiini or OVs. It is even more difficult to explain the dark-bright switching behavior. Based on the results of STM, X-ray spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and first-principles calculations, another two kinds of defects are proposed to explain the dark and bright spots measured by STM.28 First, the dark spot can be explained by (TiO2)V by removing a TiO2 unit from the ridge in AOM structure, as shown in Fig. 5a. The formation energy of this structure is 4.46 eV and the simulated STM image (Fig. 5g) is consistent with the experiment. The (TiO2)V might be formed by Ar+ sputtering and annealing processes in experiments. Based on this (TiO2)V, the bright spot defect can be explained by intercalating two Ti atoms into the dark spot defect, namely (TiO2)VTiini, as shown in Fig. 5b. The formation energy of this bright spot defect is -5.92 eV. Due to the contribution of the intercalated titanium atoms, this defect is shown as a bright spot in the simulated STM image (Fig. 5h) under a 2.0 V bias voltage. Molecular dynamic simulations of (TiO2)V defect are performed at 80 and 300 K to prove the stability of (TiO2)V. From the snapshots and the relative


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free energies shown in Fig. 6, we can see that the (TiO2)V defect exists stably with the dangling O atoms rotating around the neighboring Ti atoms.

Figure 5. (a, b) Side and top views of (TiO2)V and (TiO2)V-Tiini defects. (c, d) Total and partial DOS of Odangling and Tiini. The inset shows the spatial orbital distribution of the defect states. (e, f) Charge transfer analysis using bader analysis. (g, h) Simulated STM images for a bias of +2.0 V. Energies are given relative to CBM.

To further understand the electronic structure of the (TiO2)V and (TiO2)V-Tiini defects, the total and partial DOS of selected atoms are plotted in Figs. 5c and d. For (TiO2)V, the gap states contributed by the dangling O (Odangling) atoms are located in the band gap. The Ti atom in the dark spot defect has a character of Ti4+, which agrees with the XPS spectra of the as-prepared TiO2 anatase(001) surface.28 The bader charges of atoms around defects are demonstrated in Figs. 5e and f. The spin-polarized configuration leads to a magnetic moment of 0.45 μB. For (TiO2)V-Tiini defect, the intercalated Ti pair contribute gap states. There are two defect states locate at 0.6 and 0.8 eV below the CBM, which agrees qualitatively with the defect states observed 0.9 eV below VBM in UPS. The intercalated Ti pair presents Ti3+ states, which is also in agreement with the UPS measurements.28


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Figure 6. The snapshots of the molecular dynamics and relative free energies of (TiO2)V defect at 80 K (a) and 300 K (b) respectively.

Switchable behavior between (TiO2)V and (TiO2)V-Tiini In the STM experiments,28 it was found that three kinds of transformations could happen to the bright spot defect when applying a 3.7 V or higher voltage pulse by the STM tip. (I) The bright spot turns directly to a dark spot, which has the same depth as the intrinsic dark spot. Furthermore, at the same time when a voltage pulse made the bright spot a dark spot, sometimes a nearby dark spot could become a bright one. (II) The bright spot could turns to a dark spot in company with a pair of smaller bright spots. This process is reversible. (III) The bright spot becomes a shouldered dark spot under the voltage pulse, which is also reversible. The (TiO2)V and (TiO2)V-Tiini models can explain all of these processes, as shown in Fig. 7. In process I, under the voltage pulse, the intercalated Ti4c pair removed from the bright spot can make the bright spot become a dark spot. The formation energy of removing each Ti4c is 2.96 eV, which is comparable with the energy imposed by the STM tip. If the intercalated Ti4c pair is driven to a nearby dark spot, this dark spot


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could become a bright spot. If the intercalated Ti4c pair falls on the ridge, it would become a paired Tiini defect (process II) showing paired small bright spots (Fig. 7c). The adsorption energy for each Ti4c atom fallen on the ridge is -3.65 eV, which is an exothermic process. This process is apparently reversible by driving the intercalated Ti4c pair back to the bright spot. In process III, the STM tip moves the intercalated Ti4c pair inside of the ridge, resulting in a shouldered dark spot (Fig. 7d). When the intercalated Ti4c pair is driven out from the ridge, the shouldered dark spot turns back to a bright spot. The calculated formation energy of the shouldered dark spot from a bright spot is 2.50eV. It can be seen that our models are consistent well with the experimental results.

Figure 7. Switchable behavior between the (TiO2)V and (TiO2)V-Tiini defects. (a) Side view of (TiO2)V-Tiini defect. (b-d) Side and top views, together with the simulated STM images for a bias of +2.0 V of (TiO2)V (b) and two kinds of paired Tiini2 (c-d).


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H2O and O2 Adsorption behavior The STM measurements of H2O and O2 adsorbed on this surface shows that both H2O and O2 can only adsorb on the bright spot defect. In addition, such adsorption can turn the bright spot into a dim spot. Here we investigate the H2O and O2 adsorption on clean and defective AOM surface, as shown in Fig. 8. Our calculation shows that H2O would adsorb on AOM surface molecularly (Fig. 8a), with O atom from H2O bonding with the ridge Ti atom and one H atom from H2O pointing to the ad-oxygen atom forming a hydrogen bond. The adsorption energy is -0.45 eV, which is probably not large enough to stand up to the bias voltage of 2.0 V. So there is a large possibility that H2O would be desorbed from the surface during the STM measurements. We notice that the adsorption energy on AOM surface is much smaller than that on ADM surface (-1.82 eV).9 This is because the oxidization increases the coordination number of Ti atoms on the surface, making the surface inert, which is consistent with the experimental results.28 If there is any chance H2O is not desorbed from the surface, the surface will have a bright spot character in STM images (Fig. 8a). For O2, on clean AOM surface, the adsorption energy is 0.69 eV, suggesting the surface is completely inert for O2.

Figure 8. Structures and simulated STM images for H2O adsorbed on AOM (a), OV1 (b) ,OV2 (c), Tiini1 (d), Tiini2 (e), (TiO2)V-Tiini (f) and O2 adsorbed on (TiO2)V-Tiini (g).


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The adsorption energies of H2O and O2 on different defect structures are shown in Table 1. H2O on clean (001) and ADM are also considered for reference. It is shown that H2O is stable on (TiO2)V-Tiini. In addition, after H2O adsorption, the intercalated titanium atoms are saturated and the bright spot STM character turns into the dim spot. This is a unique phenomenon different from all the other defects which agrees well with the experimental results. Besides, H2O does not adsorb on the (TiO2)V defect which shows a dark spot character, which also agrees with the STM measurements. Although H2O could also adsorb on OV2, after the adsorption the STM image of the bright spot is still bright, which goes against the experimental results, suggesting that OV2 is not the bright spot defect observed in the experiments. Similar with H2O, when O2 adsorbed on the anatase TiO2(001) surface, it can easily adsorb on (TiO2)V-Tiini defect. Such an adsorption will saturate the intercalated titanium atoms and turn the bright spot into a dim character, suggesting the (TiO2)V-Tiini defect can also explain the O2 adsorption behavior observed by STM measurements. Table 1. Adsorption energies of H2O and O2 on different defect structures. molecule

H2O

a

substrate

Eads(eV)

Clean (001)

-1.25a

ADM

-1.82b

AOM

-0.45

OV1

-0.38

OV2

-0.99

Tiini1

-0.29

Tiini2

-0.53

molecule

H2O

O2

substrate

Eads(eV)

(TiO2)V

0.34

(TiO2)V-Tiini

-0.96

AOM

0.69

(TiO2)V

0.53

(TiO2)V-Tiini

-1.73

Reference 8. b Reference 9.

DISCUSSION The formation energy of oxidizing ADM to AOM is 0.24 eV by HSE functional and 0.40 eV by PBE functional, suggesting that AOM is less stable. Nevertheless, considering that different growth condition can produce different surface structure, AOM is still possible to exist in oxygen rich environment. The energy barrier of forming ADM+O2 from AOM calculated by HSE


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functional is 2.03 eV (details can be found in the Supporting Information). Thus a systematic understanding of AOM surface is important for a comprehensive understanding of this surface. Besides the STM measurements by Wang et al,28 there is also an experimental work supporting our AOM structure reported by S. Pillai et al.51 They found that oxygen rich anatase nanoparticles could stably exist up to 900 ℃ in the form of peroxo-titania complex. Our AOM structure has the peroxide like structure and is a kind of oxygen rich anatase TiO2, so it is probably the true appearance of anatase(001)-(1×4) surface in experiments. Recently Shi et al. used global search method based on adaptive genetic algorithm to investigate this surface and they explained the inert behavior of this surface to be induced by the stress release on ADM structure.21 The stress release can be induced by defects, oxidization and molecule adsorption. In this sense, AOM structure is one of the strategies to release the surface stress by adding oxygen atoms to the ridge of ADM. This stress release can be understood by the bond length change. By adding oxygen atoms on the ridge of ADM, the Ti-O bond length decreases from 2.12 Å in ADM to 1.99 and 1.91 Å in AOM concomitantly with the Ti-O-Ti bond angle reduces from 147°to 136°. Stress change contributed by the ridge of ADM and AOM is 8.7 kbar21, indicating a surface tensile release in AOM. In our report we show that the (TiO2)V and (TiO2)VTiini defect models can be used to explain all the STM measurements, including the dark and bright spots character and their conversion, as well as the H2O and O2 adsorption. The models proposed in this report and in Shi et al.’s paper provide different possibilities. Considering the complex of this surface, convincing the correct structure models still require further theoretical and experimental investigations.

CONCLUSION To summarize, we investigate the structure and reactivity of AOM reconstruction structure for TiO2 anatase(001)-(1×4) surface systematically. Properties of different surface defects and the adsorption behavior of H2O and O2 on this surface are investigated by first-principles calculations. We find that (TiO2)V and (TiO2)V-Tiini defect models can be used to explain all the STM measurements, including the dark and bright spots character and their conversion, as well as the H2O and O2 adsorption. The proposed AOM reconstruction model and the (TiO2)V and (TiO2)V-


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Tiini model provide important insights into a comprehensive understanding the structure and reactivity of TiO2 anatase(001)-(1×4) surface.

ASSOCIATED CONTENT Supporting Information Comparison of the spatial orbital distributions calculated by PBE and HSE06 functional; Energy barrier and reaction pathway of forming ADM+O2 from AOM.

AUTHOR INFORMATION Corresponding Author *

Email: [email protected].

*

Email: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT H.J. S. acknowledges the support of the Project funded by China Postdoctoral Science Foundation (No. 2016M590614), Natural Science Foundation of Shandong Province (No.


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ZR2017BA021), the Qingdao Postdoctoral Application Research Project (No. 2016019). J. Z. acknowledges the support of National Natural Science Foundation of China (NSFC) (Nos. 21421063, 11620101003), National Key Foundation of China, Department of Sci. & Technol. (Nos. 2016YFA0200600 and 2016YFA0200604), the Fundamental Research Funds for the Central Universities of China WK3510000005, the support of National Science Foundation (Nos. CHE1213189 and CHE-1565704). W.C. L. acknowledges the support of NSFC (No. 21773132). Calculations were performed at Environmental Molecular Sciences Laboratory at the PNNL, a user facility sponsored by the DOE Office of Biological and Environmental Research, and Supercomputing Center at USTC.

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Structure and Reactivity of Anatase TiO2(001)-(1Ã4) Surface

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Structure and Reactivity of Anatase TiO2(001)-(1Ã4) Surface