In Situ STEM Determination of the Atomic Structure and

Feb 28, 2017 - The widely studied anatase TiO2 (001) surface usually shows a (1 × 4) reconstruction, which may directly influence its physical and ch...
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In Situ STEM Determination of the Atomic Structure and Reconstruction Mechanism of the TiO2 (001) (1 × 4) Surface Wentao Yuan,† Hanglong Wu,† Hengbo Li,† Zhongxu Dai,‡ Ze Zhang,† Chenghua Sun,*,§ and Yong Wang*,† †

Center of Electron Microscopy and State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, College of Materials and Chemical Engineering, China Three Gorges University, Yichang 443002, China § Department of Chemistry and Biotechnology, Faculty of Science, Engineering & Technology, Swinburne University of Technology, Hawthorn, VIC 3122, Australia S Supporting Information *

ABSTRACT: The widely studied anatase TiO2 (001) surface usually shows a (1 × 4) reconstruction, which may directly influence its physical and chemical properties. Although various atomic models are proposed, the debates regarding the models and the formation mechanism of such reconstruction remain until now due to the lack of direct experimental evidence at the atomic level. Herein, we report the atomic-scale determination of the atomic structure and the reconstruction mechanism of the TiO2 (001) (1 × 4) surface by in situ spherical aberration corrected scanning transmission electron microscopy (STEM) at elevated temperature. The atomic features of the reconstructed surface are unambiguously identified in our experiments, providing a solid evidence to verify the ad-molecule model, which was predicted by the calculations 15 years ago. Furthermore, the mysterious reconstruction route is revealed by our real time STEM images, which involves a new metaphase of the (001) surface. These results are expected to help resolve current dispute concerning the reconstruction models and understand the true performances of the anatase TiO2 (001) surface.

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bulk truncated-(001) is released by periodically replacing the O bridging rows with TiO3 rows.21 Based on the STM results in oxygen, on the other hand, Wang et al. proposed an add oxygen model (AOM) of the (1 × 4) reconstruction, through adding coordination oxygen adatoms to the ridge Ti4c atoms of an ADM model.19 Xia et al. also proposed a modified ADM model consisting of two basic atomic building blocks.28 All of these atomic models only show tiny difference in structural features and the critical differences depend on the exact positions of surface Ti and O rows along the [010] direction, which is quite difficult to determine their exact positions just through STM images. Some recent advances show that the (scanning) transmission electron microscope ((S)TEM) is also able to provide surface crucial information on oxide nanocrystals.30−34 Although HRTEM images of the reconstructed (001) surface match well with the simulated image of the ADM model,26 we could not exactly determine the positions of O columns and Ti ridge columns limited by the TEM resolution. Consequently, no consensus regarding the atomic structure of reconstructed

s a popular research system in surface science, TiO2 has attracted tremendous attention and a lot of fundamental studies have been done,1−6 especially in its majority surface of (101).5−10 Recently, inspired by the prediction that the (001) surface has a higher photocatalytic activity than its majority (101) surface based on the bulk truncated-(001) surface,11−14 substantial effort has been invested in the exploration of anatase TiO2 nanocrystals with (001) surface.15−21 However, some recent experimental results do not meet the early expectations and even show distinctly different conclusions.22−25 A critical factor is that the bulk truncated-(001) surface usually undergoes a (1 × 4) reconstruction due to its high surface energy,21,26,27 which may result in significantly different performances of the (001) surface. Thus, to clarify the behaviors of the anatase TiO2 (001) surface, determining the atomic structure and investigating the reconstruction mechanism become particularly important. To understand the (1 × 4) reconstruction, several atomic models19−21,28,29 have been proposed in early studies based on the top view images obtained by scanning tunneling microscopy (STM) or noncontact atomic force microscopy (NC-AFM). For example, Selloni et al. proposed the admolecule model (ADM) according to theoretical calculations 15 years ago. They claimed that the large surface stress of the © 2017 American Chemical Society

Received: January 21, 2017 Revised: February 28, 2017 Published: February 28, 2017 3189

DOI: 10.1021/acs.chemmater.7b00284 Chem. Mater. 2017, 29, 3189−3194

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Chemistry of Materials (1 × 4) surface has been reached due to the lack of direct experimental evidence. Herein, using an aberration corrected STEM and a heating holder system, we performed an in situ atomic-scale observation of the reconstructed TiO2 (001) surface from the side view. Through both high-angle annular dark-field (HAADF) and bright field (BF) STEM images, for the first time, the Ti and O atomic columns in feature positions of the reconstruction are identified at the atomic-scale in vacuum. Our in situ observation also reveals a reconstruction route, in which a new metastable structure of (001) surface is observed. Our experiments were performed in an FEI Titan scanning transmission electron microscope with a spherical aberration corrector (200 kV). We used the hydrothermal synthesized anatase TiO2 nanorods as the sample,35 the tips of which are exposed by the (001) surfaces. The samples were dispersed on a SiNx-based heating-chip and then loaded onto the TEM by a double tilt heating holder (Wildfire D6, DENS solutions). As the oxide samples are highly sensitive to the electron beam (ebeam) irradiation,26,36−38 a low-dose beam current (∼0.6 nA) is applied during the imaging process. The bulk-truncated (001) surface of anatase TiO2 is exposed by 5-fold coordinated Ti (Ti5c) atoms, 3-fold coordinated O atoms (O3c), and 2-fold coordinated O (O2c) atoms, referring the atomic models in Figure 1a,f. The Ti5c atoms are linked by

1b), which also corresponds to the Ti−O atomic columns. In addition, in the BF image, the small gray dots are also observed between two neighboring Ti−O atomic columns along [010] direction. By comparing the image with the atomic structure, these gray dots can be attributed to the O3c atomic columns, and one typical surface O3c column is marked by the red arrow in Figure 1c. All these features match the bulk-trucated (1 × 1)(001) surface well, which is further confirmed by the simulated HAADF (Figure 1d) and BF STEM (Figure 1e) images. It should be noted that the bulk-trucated (001) surface here is not very clean, and we can see some amorphous species on it, which is suspected of stabilizing the (001) surface. This amorphous layer is also observed on the (001) surface in our previous work.26 The previous calculations suggest that the corrugation of the (1 × 1)-(001) surface increases slightly from 0.82 to 0.95 Å upon relaxation.14 Although, this tiny relaxation is hardly to be identified in our STEM images. Due to its high surface energy (0.90 J/m2), the bulk-truncated (001) surface is not stable and prefers to form a (1 × 4) reconstruction at elevated temperature.4,27 Based on its STM images, several atomic models have been proposed, and among them the ADM model is the most accepted one (as shown in Figure 2a,f), which is

Figure 2. Atomic structure of the (1 × 4) reconstructed (001) surface of the anatase TiO2 nanocrystal. The images are acquired at ∼750 °C and the TEM column gas pressure is ∼10−5 Pa. (a) The atomic structural models of the (1 × 4) reconstructed (001) surface, viewing along [010] (up) and [100] (down) direction. (Ti, blue; O, red). Panels (b) and (c) are the atomic resolution HAADF and the corresponding BF images of the (1 × 4) reconstructed (001) surface, viewed along the [010] axis. Panels (d) and (e) are the simulated HAADF and BF STEM images of the (1 × 4) reconstructed (001) surface. (f) The 3-dimentional structure of the ad-molecule (ADM) model.

Figure 1. Atomic structure of the anatase TiO2 bulk-truncated (001) surface. The images are acquired at room temperature, and the TEM column gas pressure is ∼10−5 Pa. (a) The atomic structural models of the bulk-truncated (001) surface, viewing along [010] (up) and [100] (down) direction. (Ti, blue; O, red). Panels (b) and (c) are the atomic resolution HAADF and the corresponding BF images of the bulktruncated (001) surface, viewed along the [010] axis. Panels (d) and (e) are the simulated HAADF and BF STEM images of the bulktruncated (001) surface. (f) The 3-dimentional atomic model of the bulk-truncated (001) surface.

the bridging O2c atoms along the [010] direction, and the O3c atoms are bonded to the Ti5c atoms along the [100] direction. Before the heating experiments, we recorded the HAADF and BF STEM images of the as-synthesized (001) surface simultaneously (Figure 1b,c) at room temperature. As the contrast of HAADF STEM image is proportional to ∼ Z1.6−2.0 (Z indicates atomic number),39 the bright dots in the HAADF image (marked by the green arrow in Figure 1b) represent the Ti−O atomic columns according to the atomic model (Figure 1a), and the contrast of the oxygen columns in HAADF image is too faint to identify. While in the BF image (Figure 1c), the black dots (marked by the green arrow) locate on the same positions of the bright dots in HAADF STEM image (Figure

supported by the DFT calculations. However, up to now, it is still not completely confirmed and also challenged by the two modified ADM models raised recently.19,28 As shown in Figure 2a,f, the ADM model is constructed by periodically replacing the bridging oxygen rows in the (1 × 1)-(001) surface (Figure 1a) with TiO3 species rows, along the [010] direction (parallel to the surface). Comparing the atomic structures of all these models,19−21,26,28,29 the most significant features of these atomic models are different in the arrangements of Ti and O rows in surface and subsurface layers along the [010] direction (refer to Figure S1), which is quite difficult to distinguish just through STM images from a top view. Ideally, atomic-scale images of the reconstructed surface can be collected from the 3190

DOI: 10.1021/acs.chemmater.7b00284 Chem. Mater. 2017, 29, 3189−3194

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Chemistry of Materials side view, which offers the exact positions of surface Ti and O atomic columns and provide unparalleled evidence for finally determining this reconstruction structure. The HAADF and BF images of the reconstructed (001) surface are successfully collected at ∼750 °C, as shown in Figure 2b,c, respectively. As clearly shown in the HAADF image (Figure 2b), three extra bright dots locate on the outmost atomic layer, which slightly protrude from the (001) surface (marked by the green arrow), and the nearby bright dots (Ti−O columns) of the outmost layer get closer. The contrasts of the protruding bright dots look quite similar to the Ti−O columns of the (1 × 1)-(001) surface, which suggests these protruding dots also indicate the Ti−O columns. These Ti−O columns are also displayed as the black dots in the BF image (marked by the green arrow in Figure 2c). More importantly, two gray dots in the BF image are visible (marked by the red arrows in Figure 2c) between the protruding black dot (Ti-containing or Ti−O column) and the Ti−O columns on its both sides. Since the contrasts of these gray dots are too faint to be detected in the HAADF image and are much weaker than that of Ti−O columns in the BF image, it suggests these gray dots represent the O columns. These atomic-level features with exact positional information on both Ti and O columns provide the most convincing evidence to the ADM model, which was proposed 15 years ago.21 Such results are also supported by the simulated HAADF image (Figure 2d) and BFSTEM image based on the ADM model (Figures 2e and S1) which match our experimental results well. With the different distances between the TiO3 rows (n times the interplanar spacing of the (100) facet), the ADM reconstructed surface exhibits the nd-configurations (n = 3, 4, 5, ...). The structure of nd-configuration is also confirmed in our experiments, as shown in Figure 2b. The next key issue is to understand the underlying mechanism of the reconstruction. During our in situ experiments, the sample was gradually heated to ∼750 °C in vacuum (gas pressure: ∼10−5 Pa). To minimize the e-beam irradiation damage of the sample, no electron beam irradiation is involved during the heating process. The HAADF and BF images of the (001) surface (Figure 3a,d) are recorded after keeping the temperature at ∼750 °C for ∼30 min, which shows the same period with the TiO2 bulk structure. The amorphous layer covered on the surface is disappeared, caused by the thermal evaporation or electron beam irradiation. To figure out the definite surface structure we obtained (Figure 3a,d), we compared the features of the HAADF and BF images with

two representative atomic models of the anatase TiO2 (001) surface (the bulk-truncated (001) surface and the (1 × 4) reconstructed (001) surface, as shown in Figure S2). Surprisingly, however, neither of them can match the observed surface (Figure 3a,d), from both [010] and [100] directions (refer to Figures 1, 2 and S2). It is easier to exclude the possibility of the (1 × 4) reconstructed (001) surface. Viewing the ADM model from the [010] direction (Figure 2a-up), it shows a different period (4×) with the observed surface in Figure 3a,d, so we can directly rule it out. By observing the (1 × 4) reconstructed (001) from the [100] direction (Figure 2adown), the outmost Ti columns are right above the Ti−O columns in the secondary, which indicates the bright dots in the outmost layer should be right above the bright dots in the secondary in HAADF image. This is not consistent with the surface structure of what we see here (Figure 3a). Another possible structure is the bulk-truncated (001) surface. Looking at the bulk-truncated (001) surface from the [100] direction (as shown in Figure 1a-down), the outmost-layer Ti−O columns are right above the Ti−O columns of the secondary layer, so we can expect the outmost bright dots should be on top of the secondary bright dots in the HAADF image (refer to the corresponding simulated HAADF STEM image), which is not accorded with Figure 3a. On the other hand, viewing along the [010] direction (refer to Figure 1b) of the bulk-truncated (001) surface, it matches the as-observed surface better, while some additional dots with weak contrasts (one marked by the red arrow in Figure 3a) appear above the outmost Ti−O column (one marked by the green arrow in Figure 3a) layer in the HAADF image. This difference looks more obvious in the BF image (Figure 3d), that a layer of gray dots (one marked by the red arrow in Figure 3d) locates above the layer of black dots (Ti−O columns), which are not seen in the BF image of the bulk-truncated (001) surface along the [010] direction (Figure 1c). According to the above discussion, it is obvious that the observed surface in Figure 3a,d is neither the bulk-truncated (001) surface nor the (1 × 4) reconstructed (001) surface. Based on our DFT calculations results, we believe the observed surface could be regarded as some additional TiOx species arranged regularly on the bulk-truncated (001) surface (refer to Figure 4, here we call it as “(001)-HT” structure). Its structure and formation details will be discussed later. In the following minutes, the (001)-HT surface does not remain stable and then transforms into the ADM reconstructed surface, as shown in the in situ images in Figure 3. After 166 s, the (001)-HT surface has been partially reconstructed, as

Figure 3. In situ STEM images show the evolution of surface structures. The images are acquired at ∼750 °C and the TEM column gas pressure is ∼10−5 Pa. The atomic resolution HAADF STEM images (a−c) and the corresponding BF images (d−f) show the (1 × 1)-(001) surface (a, d) partially reconstructed at 166 s (b, e) and then fully reconstructed after 222 s (c, f). The yellow arrows indicate the same positions at the different time. The four yellow arrows in (a−f) indicate the same positions, respectively.

Figure 4. Formation of the (001)-HT surface. The upper images of (a−d) are the atomic models of (a) the perfect bulk-truncated (001) surface, (b) the oxygen-loss (001) surface, (c) the perfect (001)-HT surface, and (d) the (001)-HT surface with atoms loss. The middle images and the nether images are the simulated HAADF-STEM and BF STEM images of the corresponding atomic models, respectively. 3191

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Chemistry of Materials

beam under the TEM mode, which will lead to the layer-bylayer dissolution of the TiO2 crystals. On the other hand, to some extent, the e-beam also facilitates the reconstruction, cleaning the surface and providing energy to overcome the barrier. Stimulated by the tiny dose of e-beam irradiation, the metastable (001)-HT surface are triggered to reconstruct to the energetically preferred ADM structure. The scanning of the focused e-beam first induced the dissolution of the outmost atomic layer (TiOx species), which results in the bulk-truncated (001) surface shows up somewhere. These exposed bulktruncated surface sites are suffering from significantly large surface stress and cannot keep stable for a long time, so the additional TiO2 molecule rows are dragged into the surface forming the ADM reconstruction island (as shown in the right part of Figure 3b,e). The atoms of the additional TiO2 molecule rows come from the dissolution of the outmost TiOx species or surface Ti−O columns according to the comparison of in situ images in Figure 3. The mixed configurations could not maintain longer and the ADM reconstruction islands formed one by one along the [100] direction, which is different from the situation in oxygen that the ADM reconstruction islands appear almost at the same time. Finally, the (001)-HT surface completely transforms into the ADM reconstructed surface (Figure 3c,f). It should be noted that there is no other species are introduced in the experiments, and thus the mass transport during the reconstruction is within the TiO2 crystal itself. Under the elevated temperature (750 °C) and the e-beam irradiation, the Ti and O atoms can easily migrate and be stabilized at the most favorable sites. In conclusion, we performed an in situ spherical aberration corrected STEM study of (1 × 4) reconstructed (001) surface of anatase TiO2. For the first time, combining both the HAADF and BF images of the reconstructed (001) surface with atomicscale resolution, we unambiguously determined the atomic positions of the Ti and O columns from the [010] direction, which gives direct evidence for the ADM atomic model proposed 15 years ago. Based on our straightforward in situ images, we unveiled a pathway for the (1 × 4) reconstruction, in which a (001)-HT termination of the anatase (001) surface is revealed. Such structural information serves as a crucial basis to understand the true behaviors and performances of the anatase TiO2 (001) surface, enlightening further surface studies of oxide nanocrystals by aberration corrected STEM at the atomic-scale.

shown in Figure 3b,e. We can see on the right side of the surface, the outmost TiOx species (marked by the red arrows in Figure 3a,d) layer and even some Ti−O columns of the second layer (marked by the green arrows in Figure 3a,d) disappear, leaving two protruding Ti−O columns on the right side (marked by the two yellow arrows on the right side in Figure 3b,e). The arrangement in the right part of the surface layer somehow becomes a little bit disordered. In particular, the surface Ti−O columns between those two protruding Ti−O columns get closer, which are separated by a distance of three times as the interplanar spacing of the (100) facet. The features of the partially reconstructed surface above is well consistent with the ADM reconstructed (001) surface, and it is further verified by its BF image (Figure 3e). In the meantime, the contrast of the left part of the outmost gray dots layer in Figure 3e (TiOx species) also becomes weaker, indicating the surface dissolution occurring at the initial stage of the reconstruction. After 222 s, the (001)-HT dominated surface is completely transformed into the ADM reconstructed surface, with 3d and 4d configurations as shown in Figure 3c,f. To reveal why and how the (001)-HT structure formed in our experiments, theoretical calculations were performed under the scheme of density functional theory with an additional Hubbard-like term to treat strong on-site Coulomb interaction (DFT+U).40−43 More computational details are listed in the Supporting Information. Starting from a perfect (001) structure (see Figure 4a), we investigated oxygen loss under e-beam, which has been observed by several groups36,38,44,45 and results in the formation of three-coordinated titanium (Ti3c), as shown in Figure 4b. Such Ti3c atoms are highly reactive and may further shift by half unit cell, generating four-coordinated titanium (Ti4c) and two-coordinated oxygen (O2c), as shown in Figure 4c. According to early studies, O2c has been frequently observed in various TiO2 surfaces while Ti3c is rarely observed,46 indicating that the above surface shift may be energetically favorable. This hypothesis has been supported by three pieces of evidence: (i) the shift can result in an energy decrease by 4.77 eV (for the model employed in our calculation) once half of the surface oxygen atoms are lost based on our DFT calculations; (ii) the movement of surface rows by one unit cell has been reported in our early work,26 during which partially transformed states have been observed, indicating surface shifting can happen under our experimental conditions; and (iii) the dynamic evolution of surface layers and the formation of the reconstructed shown in Figure 3 clearly support the loss of surface atoms and row shift. To validate the (001)-HT model proposed in Figure 4c, HAADF and BF STEM images are further simulated as listed under each structure, and the result for Figure 4c agrees well with the experimental images (Figure 3a,d), except the contrast, which is due to the concentration of surface atoms. Decreasing the concentration of surface atoms, as shown in Figure 4d, such simulation deviation from experimental images can be corrected. In our previous work, we presented an in situ observation of reconstruction process in oxygen environment (oxygen pressure: 5 × 10−2 Pa) at 750 °C.26 Without the protection of the oxygen environment, the surface structure is very sensitive to electron beam irradiation and will be damaged rapidly.26 If we scan the sample several times under the STEM mode, not only the fine atomic structure of the surface will be destroyed or dissolved, but also some Ti clusters will precipitate on the surface. It is slightly different from the effect of electron

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EXPERIMENTAL SECTION

Preparation of the Anatase TiO2 Nanorods. The nanorods of anatase TiO2 were synthesized through a two-step hydrothermal method.35 (1) Synthesis of Na-titanates: TiO2 nanoparticles (1 g, Degussa P25) and NaOH (40 mL, 99.8 wt %, Sinopharm Chemical Reagent Co., Ltd.) were mixed and stirred for 10 min at ambient conditions. The solution was transferred to a Teflon-lined stainless steel autoclave (50 mL) and heated at 120 °C. After 24 h, it was cooled in air. Then we collected white products (Na-titanates) by centrifugation (10 000 r/min, 10 min) and washed it by deionized water for several times. (2) Synthesis of anatase nanorods with minority facets: The Na-titanates (1 g) and deionized water (40 mL) were mixed, stirred for 10 min, and then transferred to a Teflon-lined stainless steel autoclave (50 mL). After heating at 200 °C for 24 h, the autoclave was cooled naturally in air. We collected precipitates by centrifugation (10 000 r/min, 10 min) and then washed it by deionized water and ethanol for several times. We finally got the products after drying it in vacuum (60 °C and 12 h). In Situ Experiments and Image Simulation. The in situ HAADF-STEM and BF-STEM experiments were performed in a FEI 3192

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Chemistry of Materials Titan G2 80-200 ChemiSTEM at 200 kV, which was equipped with an aberration corrector and provided us a sufficiently high spatial resolution (∼0.8 Å). The convergence angle of the probe was ∼21 mrad and the screen beam current is about 0.6 nA. Before the in situ experiments, all of the low-order aberrations have been adjusted to an acceptable level (2-fold astigmatism A1 < 2 nm, Cs < 0.5 μm, 3-fold astigmatism A2 and coma B2 are less than 20 nm). The inner and outer collection semiangles of the HAADF detector are ∼42 and ∼288 mrad, while the outer collection semiangle of the BF detector is ∼20 mrad. During our experiments, the DENS solutions double tilt heating holder (Wildfire D6) was used to heat the sample to 750 °C by a rate of ∼0.5 °C/s. The simulations of the HAADF and BF-STEM images were performed by using the QSTEM software through multislice algorithm. The parameters for the simulation were set based on our experimental conditions.

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(8) Gong, X. Q.; Selloni, A.; Batzill, M.; Diebold, U. Steps on anatase TiO2 (101). Nat. Mater. 2006, 5, 665−670. (9) Scheiber, P.; Fidler, M.; Dulub, O.; Schmid, M.; Diebold, U.; Hou, W.; Aschauer, U.; Selloni, A. Sub)Surface mobility of oxygen vacancies at the TiO2 anatase (101) surface. Phys. Rev. Lett. 2012, 109, 136103. (10) Li, Y. F.; Selloni, A. Theoretical study of interfacial electron transfer from reduced anatase TiO2 (101) to adsorbed O2. J. Am. Chem. Soc. 2013, 135, 9195−9199. (11) Gong, X. Q.; Selloni, A. Reactivity of anatase TiO 2 nanoparticles: The role of the minority (001) surface. J. Phys. Chem. B 2005, 109, 19560−19562. (12) Lazzeri, M.; Vittadini, A.; Selloni, A. Structure and energetics of stoichiometric TiO2 anatase surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 155409. (13) Vittadini, A.; Casarin, M.; Selloni, A. Chemistry of and on TiO2anatase surfaces by DFT calculations: a partial review. Theor. Chem. Acc. 2007, 117, 663−671. (14) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gratzel, M. Structure and energetics of water adsorbed at TiO2 anatase (101) and (001) surfaces. Phys. Rev. Lett. 1998, 81, 2954−2957. (15) Xiong, F.; Yu, Y. Y.; Wu, Z.; Sun, G.; Ding, L.; Jin, Y.; Gong, X. Q.; Huang, W. Methanol conversion into dimethyl ether on the anatase TiO2 (001) surface. Angew. Chem., Int. Ed. 2016, 55, 623−628. (16) Tanner, R. E.; Sasahara, A.; Liang, Y.; Altman, E. I.; Onishi, H. Formic acid adsorption on anatase TiO2 (001)-(1 × 4) thin films studied by NC-AFM and STM. J. Phys. Chem. B 2002, 106, 8211− 8222. (17) Selcuk, S.; Selloni, A. Facet-dependent trapping and dynamics of excess electrons at anatase TiO2 surfaces and aqueous interfaces. Nat. Mater. 2016, 15, 1107−12. (18) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 single crystals with a large percentage of reactive facets. Nature 2008, 453, 638−641. (19) Wang, Y.; Sun, H. J.; Tan, S. J.; Feng, H.; Cheng, Z. W.; Zhao, J.; Zhao, A. D.; Wang, B.; Luo, Y.; Yang, J. L.; Hou, J. G. Role of point defects on the reactivity of reconstructed anatase titanium dioxide (001) surface. Nat. Commun. 2013, 4, 2214. (20) Liang, Y.; Gan, S. P.; Chambers, S. A.; Altman, E. I. Surface structure of anatase TiO2 (001): Reconstruction, atomic steps, and domains. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 235402. (21) Lazzeri, M.; Selloni, A. Stress-driven reconstruction of an oxide surface: The anatase TiO2 (001)-(1 × 4) surface. Phys. Rev. Lett. 2001, 87, 266105. (22) Wu, Q.; Liu, M.; Wu, Z. J.; Li, Y. L.; Piao, L. Y. Is photooxidation activity of {001} facets truly lower than that of {101} facets for anatase TiO2 crystals? J. Phys. Chem. C 2012, 116, 26800− 26804. (23) Pan, J.; Liu, G.; Lu, G. M.; Cheng, H. M. On the true photoreactivity order of {001}, {010}, and {101} facets of anatase TiO2 crystals. Angew. Chem., Int. Ed. 2011, 50, 2133−2137. (24) Tachikawa, T.; Yamashita, S.; Majima, T. Evidence for crystalface-dependent TiO2 photocatalysis from single-molecule imaging and kinetic analysis. J. Am. Chem. Soc. 2011, 133, 7197−7204. (25) Zhao, Y. B.; Ma, W. H.; Li, Y.; Ji, H. W.; Chen, C. C.; Zhu, H. Y.; Zhao, J. C. The surface-structure sensitivity of dioxygen activation in the anatase-photocatalyzed oxidation reaction. Angew. Chem., Int. Ed. 2012, 51, 3188−3192. (26) Yuan, W. T.; Wang, Y.; Li, H. B.; Wu, H. L.; Zhang, Z.; Selloni, A.; Sun, C. H. Real-time observation of reconstruction dynamics on TiO2 (001) surface under oxygen via an environmental transmission electron microscope. Nano Lett. 2016, 16, 132−137. (27) Selcuk, S.; Selloni, A. Surface structure and reactivity of anatase TiO2 crystals with dominant {001} facets. J. Phys. Chem. C 2013, 117, 6358−6362. (28) Xia, Y.; Zhu, K.; Kaspar, T. C.; Du, Y.; Birmingham, B.; Park, K. T.; Zhang, Z. Atomic Structure of the Anatase TiO2(001) Surface. J. Phys. Chem. Lett. 2013, 4, 2958−2963.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00284. DFT calculations, the atomic models, and the simulated STEM images of the (001) surface (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yong Wang: 0000-0002-9893-8296 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of National Natural Science Foundation of China (51390474, 91645103, 11234011, and 11327901) and the Ministry of Education of China (IRT13037). C.S. thanks the financial support from the Australian Research Council through Discover Project (DP130100268) and Future Fellowship (FT130100076). We also thank the National Computational Infrastructure (NCI), which is supported by the Australian Government, for providing the computational resources.

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REFERENCES

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DOI: 10.1021/acs.chemmater.7b00284 Chem. Mater. 2017, 29, 3189−3194

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DOI: 10.1021/acs.chemmater.7b00284 Chem. Mater. 2017, 29, 3189−3194