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Cite This: Chem. Mater. 2018, 30, 288−295

Unveiling the Atomic Structures of the Minority Surfaces of TiO2 Nanocrystals Wentao Yuan,†,⊥ Jun Meng,‡,∥,⊥ Beien Zhu,‡ Yi Gao,*,‡ 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 ‡ Division of Interfacial Water and Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China § Department of Chemistry and Biotechnology, Faculty of Science, Engineering & Technology, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia ∥ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Determining the atomic structures of minority surfaces of TiO2 is of critical importance in terms of accurately interpreting their demonstrated properties. Unlike well-studied majority surfaces [such as anatase TiO2 (101)], the structures of the more attractive minority surfaces and edges are poorly known, which hampers the further understanding of their unique behaviors. Herein, through the aberration-corrected scanning transmission electron microscope, the atomic structures of the five minority surfaces [(100), (001), (102), (103), and (301)] and edges between six facets are experimentally determined. Several unique configurations are unveiled on the (301) and (102) surfaces. Intriguingly, the defective (103) and (102) surfaces are identified as distinct structures, completely different from the early predictions of stoichiometric surfaces, which are further confirmed by first-principles calculations. With the calculations based on density functional theory, the intrinsic electronic properties of the minority surfaces are also revealed. This work provides new information on minority surfaces and edges, which contributes to advancing our knowledge in surfaces and better understanding their versatile performances.

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exhibit fantastic properties.15−17 In particular, the steps and edges on the rutile TiO2 (110) surface are proven to be the active sites for the dissociation of water and methanol;18 the edges on the anatase (101) surface can trap the excess electrons, which results in preferred adsorption of O2.19 However, the atomic structures of those minority surfaces and edges are mainly based on theoretical calculations, and little experimental information is known because of their instabilities.5−7,20 As a consequence, to determine the atomic structures of minority surfaces experimentally is highly demanded. As an emerging technique for surface science, Cs-corrected transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are demonstrated to be able to provide the crucial structural information on oxide surfaces.21−34 These early studies, however, are mostly confined

urface structure can fundamentally affect the physical and chemical properties of metal oxides, which have promising applications in the fields of catalysis, environment, and electronic devices.1−5 It is therefore of crucial importance to explore the atomic structure of oxide surfaces and fully understand the structure−property relationship, especially for the minority surfaces, which are expected to possess superior properties.3−8 Taking titanium dioxide (TiO2), the most studied metal oxide system in surface science,4−6 as an example, most of its functional applications generally rely on the interaction between molecules/ions and its surface facets and defects.4−7 When exposed with different facets, TiO2 nanocrystals show distinctly different performances in photocatalysis,9 dye-sensitized solar cell (DSSC),10 lithium battery,11 and water splitting for hydrogen.12 Some minority facets even exhibit new physical or chemical phenomena;5−7,13 for instance, the dissociative adsorption of water can occur on anatase minority {001} surface but not on its thermodynamically stable {101} surface.14 In addition to the minority facets, steps and edges are usually believed to be the active sites for reaction and © 2017 American Chemical Society

Received: October 30, 2017 Revised: November 28, 2017 Published: December 8, 2017 288

DOI: 10.1021/acs.chemmater.7b04541 Chem. Mater. 2018, 30, 288−295

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Chemistry of Materials

minimization of the irradiation damage, a series of steps have been taken to reduce the total exposure dose on the sample (less than 100 e/Å2). In particular, the target surface is shifted to the illuminating region just before imaging. It should be noted that, among those surfaces, the atomic terminations of the three high-index surfaces [(103), (102), and (301)] are experimentally investigated for the first time. Showing higher photodegradation efficiency than the common (101) surface, 39 the (103) facet has raised concerns recently.6,20,39 The atomic-level HAADF-STEM image of the (103) surface is shown in Figure 2a. Since the image contrast of

in majority surfaces. The structures of TiO2 minority surfaces, such as (100), (102), (103), and (301), and edge configurations between different minority surfaces are poorly known. In this Article, through Cs-corrected STEM, we revealed the atomic structures of five minority surfaces [i.e., (100), (001), (102), (103), and (301)] of the wet-chemicalsynthesized anatase TiO 2 nanocrystal. The five edges configurations between these minority surfaces are also unveiled for the first time. Combined with the DFT calculations, the intrinsic electronic structures of the minority surfaces and edges are discussed. The tetragonal faceted-nanorods of anatase TiO2 were synthesized by a recently reported two-step hydrothermal reaction.35 The high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging (Z-contrast) of the surfaces was performed in an FEI Titan G2 80−200 TEM/STEM instrument with a spherical aberration corrector operated at 200 kV. During the experiments, the TEM column was kept in a high vacuum condition (∼10−5 Pa). A typical HAADF-STEM image of the nanorod is shown in Figure 1a, and it has a length of ∼650 nm and a width of ∼65

Figure 2. Atomic structures of the (103) surface. (a) HAADF-STEM image of the (103) surface. Two proposed atomic models of the (103) surface, (b) (103)f and (c) (103)s (Ti, gray; O, red). (d) Enlarged HAADF-STEM image of the (103) surface. (e) Atomic model of the (103)s surface with oxygen vacancy.

HAADF-STEM is roughly proportional to Z1.6−2.0 (Z: atomic number),40 the bright dots in Figure 2a indicate the overlapped Ti−O atomic columns, and the darker contrast represents the oxygen atomic columns (refer to the insets of Figure 2a). The (103) surface in our experiments shows a highly uniform sawtooth structure, which is exposed by (100) and (001) microfacets. Two possible stoichiometric terminations have been proposed,41,42 a “faceted” (103)f and a “smooth” (103)s (see the atomic structure in Figure 2b,c), while both of them have not been experimentally confirmed yet. The significant difference between these two surfaces is the coordination number of surface Ti atoms. The (103)f surface is exposed by both 5-fold coordinated Ti (Ti5c) and 6-fold coordinated Ti (Ti6c) atoms, but the (103)s surface is exposed by Ti4c and Ti6c atoms. Such a difference in the surface termination may cause the significant difference in the performances. The (103)f surface is predicted to have the higher activity for the water decomposition as compared to the (100) surface, while the activity of (103)s is lower than that of the (100) surface;43 in DSSC, the electron injection efficiency of (103)f is even larger than that of (103)s by several orders of magnitude.44 Comparing the positions of Ti−O columns of the STEM image (Figure 2d) with the atomic models of (103)f (Figure 2b) and (103)s (Figure 2c), it is easy to derive that the image contrast observed here is consistent with the (103)s because the observed (103) surface is terminated by Ti column pairs, which is different from the (103)f surface exposed by individual Ti5c columns. This result is different from theoretical predictions,41,42 because the (103)f termination is more acceptable for its lower surface energy (surface energies: 0.83 J m−2 for (103)f, 0.93 J m−2 for (103)s). To explore the reason why the higher-energy (103)s structure appears in the experiments, the

Figure 1. HAADF-STEM images of the anatase TiO2 nanorod. (a) Typical morphology of the TiO2 nanorod. (b) Exposed facets of the nanorod-tip.

nm. Viewing along the [010] direction, the sidewall of the nanorod is exposed by a large area of {100} surfaces, and the axis direction of the nanorod is along the [001] direction. The tips of the nanorod show curved features, which in fact consist of many tiny facets as observed at a higher magnification (Figure 1b). The nanorod-tip is constructed by consecutively connected (101), (100), (301), (102), and (103) facets, which is also confirmed by high-resolution HAADF-STEM images. For the guarantee of high-quality HAADF-STEM imaging, all of the low-order aberrations (up to C5) have been turned to an acceptable level (Cs < 0.5 μm, 2-fold astigmatism A1 < 2 nm, 3fold astigmatism A2 < 20 nm, and coma B2 < 20 nm). As a radiation-sensitive material, the TiO2 crystal can be easily damaged under e-beam irradiation in TEM.29,36−38 The lattice oxygen atoms, especially the surface oxygen atoms, can be removed by e-beam irradiation via the Knotek−Feibelman mechanism.37,38 The damage could be more severe under the STEM mode because of the higher electron density. For 289


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(marked by the white arrow in Figure 3b) compared to the Aconfiguration. According to the positions of Ti columns of Aand B-configurations, we built the stoichiometric (102)A and (102)B surfaces (refer to Figure 3c,d). Our DFT calculations show that the stoichiometric (102)B surface is more stable than (102)A by 0.009 eV/Å2. Similarly, the relative stability of defective (102)A and (102)B surfaces is also reversed. The (102)A surface with VOs is more stable than the (102)B structure with VOs by 0.007 eV/Å2. It is well-explained that the (102)A surface observed in experiment is a defective structure, and the (102)B surface should be a defect-free structure from the point of view of energy. The optimized surface structures of (102)A with VOs are shown in Figure 3e. Such mixed configurations could increase the configurational entropy of the (102) surface as well, leading to the decreasing of the total free energy. Another high-index surface is the (301) minority surface, which is shown in Figure 4a and has an orderly stepped structure containing the (101) terrace and (001) step sites. In an ideal case (as shown in Figure 4a), the stepped (301) surface is obtained by succession of a (101) terrace with three titanium atom pairs (indicated as 3d configuration, the “d” indicates the interplanar spacing of the (002) surface.), which are separated by a (001) step. At the same time, other configurations with the terraces consisting of n (n = 2, 4, or even 5) titanium atomic pairs are also observed (Figure 4b), which is indicated by “nd configuration”. Thus, the (301) surface prefers to form a mixed termination with nd configurations (n = 2, 3, 4, and 5), dominated by the 3d configuration. Given that the coordination of surface atoms is same for different nd configurations, the formation of a mixed configuration should not be driven by surface energies, but due to the increment of mixing entropy at the elevated temperature.49,50 Such entropy-driven surface variation has been reported experimentally in the GaAs surface. Aside from the three high-index minority surfaces, the majority (101) surface and other two low-index surfaces of anatase TiO2 are also studied. With the lowest surface energy (0.44 J/m2),41,42 the thermodynamically stable (101) surface is terminated by the bulk-truncated structure (Figure 5a), which is exposed by both 6-fold and 5-fold coordinated Ti atoms (Ti6c and Ti5c), with 2- and 3-fold oxygen atoms (O2c and O3c) located on the ridge of the sawtooth-like structure (refer to the enlarged HAADF-STEM image in Figure 5d and the corresponding atomic model in Figure 5e). According to the STEM image, we find that the distance between the outmost Ti5c atoms and Ti6c atoms beneath the Ti5c atoms (marked by

surface defects have been taken into consideration, considering that the presence of oxygen vacancies (VOs) in prepared TiO2 is pretty common.45,46 Here, we studied the relative stability of defective (103)s and (103)f surfaces with VOs through the density functional theory (DFT) calculations combined with the Perdew−Burke−Ernzerhof (PBE) exchange-correlation function. The preference position of VO is tested in the (1 × 1) slabs containing 10 TiO2 layers, since the VO might be stabilized at the surface or subsurface position (refer to Figure S1).47 Surface energies of perfect surface γp and defective surface γd are calculated. γp is obtained by optimizing slab models containing increasing TiO2 layers, which could eliminate the quantum-size effects.48 The surface energies of defective surfaces are calculated by the following expression 1 (EnTiO2 − 2O − EnTiO2 + EO2) γd = γp + (1) 2A where EnTiO2−2O is the total energy of the slab model with an oxygen vacancy on both terminations. EnTiO2 is the total energy of the slab model with a perfect surface. EO2 is the energy of a gas phase oxygen molecule. (Computational details are described in the SI.) Table 1 gives the surface energies for Table 1. Calculated Surface Energies of Perfect Surfaces γp and Defective Surfaces γd Surface energy (eV/Å2)

(103)s

(103)f

(102)A

(102)B

γp γd

0.058 0.193

0.051 0.206

0.052 0.124

0.043 0.131

perfect and defective surfaces; we found that the relative stability of (103)s and (103)f surfaces could be altered by the presence of VOs.7 As the stoichiometric surface structures, (103)f is more stable than (103)s by 0.007 eV/Å2. While considering the defective surfaces with VOs, (103)s is more stable than (103)f by 0.013 eV/Å2. The relative stability is reversed, indicating that the observed (103)s structure should be a defective surface containing oxygen vacancies. The optimized (103)s structure with VOs is presented in Figure 2e. Different from the (103) surface exposed by a unique configuration, the (102) surface (Figure 3a) is terminated by the mixed configurations, labeled as A- and B-configurations in Figure 3b. The sawtooth A-configurational surface (Figure 3b, left) is terminated by (101) and (001) microfacets, which are exposed by Ti column pairs. While on the B-configuration (Figure 3b, right), there is an additional single Ti column

Figure 3. Atomic structures of the (102) surface. (a) HAADF-STEM image of the (102) surface. (b) Enlarged HAADF-STEM image of the (102) surface. Two different configurations of the (102) surface, indicated by (c) (102)A and (d) (102)B (Ti, gray; O, red). (e) Atomic model of the (102)A surface with oxygen vacancy. 290


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Figure 4. Atomic structures of the anatase TiO2 (301) surface. (a) Typical HAADF-STEM image of the (301) surface, with 3d configuration. (b) HAADF-STEM image of the (301) surface with mixed nd configurations. The “d” indicates the interplanar spacing of the (002) surface. (c) Atomic structures of the (103) surface with 3d configurations (Ti, gray; O, red). (d) Enlarged HAADF-STEM image of the (301) surface.

Figure 5. Atomic structures of the low-indexed surfaces of anatase TiO2. (a−c) HAADF-STEM images of the (101), (100), and (001) surfaces at the atomic scale, respectively. (d, f, h) Images of the (101), (100), and (001) surfaces in higher magnification, respectively. (e, g, i) Corresponding atomic structural models of (101), (100), and (001), respectively (Ti, gray; O, red).

the yellow rectangle in Figure 5d) is slightly shorter (∼0.2 Å) than that of the bulk structure (marked by the white rectangle in Figure 5d), which is consistent with the previous prediction7,14,41 that the Ti5c atoms move inward by ∼0.2 Å upon relaxation, respectively. Aside from surface relaxation, the terrace step is a pretty common defect on the (101) surface, and a typical one is shown in Figure 5a, marked by a yellow arrow. Notably, the contrast of the outmost terrace gradually becomes darker from its left to right. Given that (101) terraces invariably exhibit a trapezoidal shape,15,51 we can expect a trapezoidal (101) terrace located on the outmost layer, which results in a gradient of thickness (see the atomic model in Figure S3). Compared to the (101) surface, the (100) surface (with a higher surface energy of 0.53 J/m2) is less investigated as the (100) epitaxial film is rarely achieved. The as-synthesized nanorods provide us a large area of bulk-truncated (100) surface with atomic flatness (see Figure 5b,f,g), which does not appear on the equilibrium shape of anatase TiO2 based on the Wulff construction.41,42 The Ti−Ti distance of Ti5c pairs in the outmost layer (marked by a yellow rectangle in Figure 5f) becomes 2.47 Å, which is slightly larger (∼4.7%) than that of the bulk structure (2.36 Å, marked by a white rectangle), agreeing well with previous calculations that Ti−Ti distance of the outmost layer is 4% larger than that of the bulk structure.41

As for the (101) surface, we observed some terrace steps on the (100) surface (see Figure 5b) as well. However, different from the (101) terrace with a gradient contrast, the (100) terrace shows uniform contrast, suggesting that the terrace has a sharp step-side along the [010] direction (as shown in Figure S4). The existence of (101) and (100) terrace steps also provides evidence for the layer-wise growth of the TiO2 nanorods along both the (101) and (100) surface.28 Aside from the terrace step, very few defects are found in the (100) surfaces, confirming its high stability. The (001) surface is the most widely studied minority surface of anatase TiO2,5−7,52,53 which has a high surface energy (0.90 J/m2)41,42 and thus usually undergoes a (1 × 4) reconstruction at high temperature.5,29,54 The HAADF-STEM image of the (001) surface is captured from a facet within 10 nm in projection length (Figures 5c). Because of its poor stability, defects can be easily found in the (001) surface. As the bulk-truncated (001) surface has been well-characterized in our previous work,30 here we focus on the defective (001) surface. As shown in Figure 5h, in the outmost atomic layer, the distances between the Ti−O columns along the [100] direction (denoted by Ti−Ti distances) are not uniform. Compared to those in the bulk structure (distance between two Ti−O columns in bulk structure: 3.79 Å), some Ti−Ti distances become larger, and some become shorter, marked by the white 291


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Figure 6. Atomic structures of the edges between the different surfaces. (a−e) HAADF-STEM images of the edges of (100)−(301), (301)−(101), (101)−(102), (102)−(103), and (103)−(001), respectively. The yellow arrows in parts a−e indicate the edge sites. (f−j) Atomic structures of the edges in parts a−e (Ti, gray; O, red).

arrows in Figure 5h. It was predicted that the oxygen vacancy in the (001) surface will lead to a larger Ti−Ti distance (∼4.75− 5.05 Å);55−57 however, the expanded Ti−Ti distance observed here is ∼4.20 Å, which is slightly less than that of the predication based on the hypothesis of oxygen vacancy. Thus, it is implied that the observed defect is not the surface oxygen vacancy. In addition, if we carefully observe the region between the two Ti−O columns (the location of bridge oxygen of the bulk-truncated (001) surface) with larger distance (marked by yellow arrow in Figure 5h), we can find a faint white spot located at the middle of two bright Ti−O columns, which is also not consistent with oxygen vacancy. Oppositely, this faint spot suggests some extra interstitial Ti atoms embedded in the outmost atomic layer, for it is lighter than the contrast of surface oxygen. As demonstrated by previous DFT calculations and in situ TEM experiments,29,54 considerable surface stress exists on the bulk-truncated (001) surface and the outmost atomic layer of the (1 × 1)−(001) surface tends to be more compact. Thus, the appearance of these titanium interstitial atoms may significantly release the high stress of the (001) surface. However, it should be noted that we cannot completely rule out the possibility that the defects with interstitial Ti atoms are generated by e-beam radiation. Nevertheless, such a structure is stable during the experiments and quite easily found on the (001) surface. Given that bulk-truncated (001) surface is not stable, forming interstitial defects may be an effective way to release the surface stress (or energy) under the external perturbations. In addition to the investigation of many minority surfaces in great detail, from our experiments, the atomic structures of the edges between these minority surfaces can be generated at the same time, which is hard to obtain by other surface-analyzing techniques. The atomic structures of the edges between (100)− (301), (301)−(101), (101)−(102), (102)−(103), and (103)− (001) have been determined in our experiments (see Figure

6a−e). It is found that titanium atoms at the (100)−(301) edge and (301)−(101) edge are 5-fold coordinated, while the titanium atoms at (101)−(102) edge, (102)−(103) edge, and (103)−(001) edge are 4-fold coordinated (Figure 6f−j), suggesting their high reactivity. Such definite atomic structural information provides critical factors to explore the real chemical reactivity and electronic properties of nanocrystals. Also, the atomic structures of the edges have a significant influence on the thermal stability of the TiO2 nanocrystal.58 On the basis of our first-hand experimental data on the detailed TiO2 surface structures, the properties of chemically reactive TiO2 nanocrystals could be well-estimated. Through the Hubbard-corrected DFT calculations, the intrinsic electronic properties of the observed surfaces and edges are thoroughly investigated, including the pure (101), (100), (001), (301) surface, the defective (102)A and (103)s surface, and five edges. As shown in Figure 7, the local densities of states (LDOS) of surface Ti atoms and O atoms of individual surfaces are analyzed. The valence band edge is dominated by O 2p orbitals, and the conduction band edge is formed by Ti 3d orbitals. For defect-free surfaces, the locations of Fermi energy levels and the conduction band edges are similar. Other than stoichiometric structures, the (102)A and (103)s surfaces are in fact defective and exposed by oxygen vacancies according to the discussion above. Generally, the presence of VOs could change the catalysis properties.59−61 Compared with that of defect-free surfaces, the Fermi energy of defective (103)s is much higher because of the contribution of occupied states from Ti 3d orbitals. The higher Fermi energy level indicates the higher probability to donate electrons.4,43,62 For the defective (102)A surface, the Fermi energy is also raised greatly, showing that the surface is capable with reducibility. Optimized edge structures are presented in Figure S5 in which (103)s and (102)A surfaces are defective surface structures discussed above. The densities of state for edge structures are presented in Figure S6. For 292


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completely different from the predicted stoichiometric structures, which highlights the importance of the experimental study of nanocrystal surfaces. Our work demonstrates that HAADF-STEM technique observation does not rely on highquality single crystals; actually, it can be employed to study minority facets and complex surface structures (edges, steps, etc.) of oxide nanocrystals, which can advance our knowledge of crystals, which is especially valuable for those hard-to-obtain surface structures, like minority surfaces and edges between them.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04541. Materials and methods, DFT calculations, slab models, total energy vs number of TiO2 layers, HAADF-STEM images, atomic models, densities of states, spin density maps, and formation energies (PDF)

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Figure 7. Local density of states of surface Ti and O atoms of (101), (100), (001), (301), (102)B, defective (102)A, and defective (103)s surfaces. Energy is aligned by vacuum energy level.

AUTHOR INFORMATION

Corresponding Authors

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

defect-free edge structures (100)−(301) and (301)−(101), the Fermi energy levels are near the valence band edge, suggesting the lower change of catalytic activities compared with individual surfaces. With the presence of VOs, the Fermi energies of (103)s−(001), (101)−(102)A, and (102)A−(103)s edges increase near the conduction band edges, leading to high reducibility. With the deficiency of O atoms, there is excess charge localized on the position of surface Ti atoms forming surface polarons which could facilitate charge transfer to catalyzed species.62 (Spin density is presented in Figure S7.) According to the above discussion, our experiments show that the surface structures of TiO2 may be different from the bulk-truncated and even theory-predicted ones, which unambiguously highlights the importance of experimentally investigating the surface atomic structure of nanocrystals. Such a direct, efficient comparison of features of minority facets is hardly done by the conventional surface analytical techniques, especially for those minority facets, which are only observed in nanocrystals. Moreover, compared to the wellstudied single-crystal surfaces, nanocrystal surfaces could be more complex. Employing different synthesizing approaches, the surfaces with the same index may show the different terminations,23 and sometimes, even in the same crystal, various configurations may appear in the same surface.25 Thus, a case-by-case study of nanocrystal surfaces is of course needed, which demands an efficient, easy-to-interpret technique to explore the surface of nanocrystals. In this regard, our work serves as a decent example for exploring minority surfaces and edges of advanced functional nanocrystals at the atomic level. In conclusion, the atomic structures of five minority surfaces and five edges of anatase TiO2 are obtained by Cs-corrected HAADF-STEM. For the first time, the atomic structures of the (102), (103), and (301) facets, and the edges between these minority surfaces are experimentally determined. Interestingly, it is revealed that the (301) and (102) surfaces are terminated by several unique configurations. More interestingly, the (103) and (102) surfaces show distinct terminations with VO defects,

ORCID

Beien Zhu: 0000-0002-0126-0854 Yi Gao: 0000-0001-6015-5694 Yong Wang: 0000-0002-9893-8296 Author Contributions ⊥

W.Y. and J.M. contributed equally.

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, 11327901, 21773287, 11604357, 11574340). C.S. acknowledges the financial support from ARC Discover Project (DP130100268) and Future Fellowship (FT130100076). The computational resources utilized in this research were provided by Shanghai Supercomputer Center, National Supercomputing Center in Tianjin and Shenzhen, and Special Program for Applied Research on Super Computation of the NSFCGuangdong Joint Fund (the second phase) under Grant No. U1501501. We also acknowledge the financial support of Natural Science Foundation of Shanghai (16ZR1443200).

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