An Environmental Transmission Electron Microscopy Study of the

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

An Environmental Transmission Electron Microscopy Study of the Stability of TiO (1×4) Reconstructed (001) Surface 2

Ke Fang, Guanxing Li, Yang Ou, Wentao Yuan, Hangsheng Yang, Ze Zhang, and Yong Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04590 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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The Journal of Physical Chemistry

An Environmental Transmission Electron Microscopy Study of the Stability of TiO2 (1×4) Reconstructed (001) Surface

Ke Fang,‡ Guanxing Li,‡ Yang Ou, Wentao Yuan,* Hangsheng Yang, Ze Zhang, Yong Wang*

Center of Electron Microscopy and State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

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ABSTRACT: The anatase TiO2 nanocrystals with dominant (001) facets attracted tremendous attention in the last decade. However, the reported intrinsic property of the (001) surface is still a cause of controversy. A crucial reason is that the (001) surface usually undergoes a (1×4) reconstruction, which may result in a remarkable difference in property. Herein, we performed an in situ environmental transmission electron microscopy (ETEM) study regarding the formation and stability of the (1×4)-(001) surface of TiO2 nanocrystals. The systematic ETEM studies confirmed that the (1×4)-(001) surface could be generated at elevated temperature (above 300 °C) when the surface contaminants were removed, and the formed reconstruction could survive in different conditions, which indicate the surface reconstruction should be taken into account in related property research. In addition, an oriented layer-by-layer beam damage process on (001) surface is confirmed, and optimal imaging conditions were also investigated, which would help to identify the intrinsic structure of TiO2 (001) surface.


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1. INTRODUCTION As one of the most investigated metal oxides, titanium dioxide (TiO2) has extensive application in photocatalysis, lithium ion batteries, and dye-sensitized solar cells, etc.1-4 Since all these applications involve the molecules/ions-TiO2 surface interaction, great efforts have been devoted to studying the surfaces of TiO2.4-6 Normally, the surface of anatase TiO2 is mainly exposed by its thermodynamically stable (101) facet, which has a low surface energy (0.44 J/m2).7 Compared with the (101) surface, the minority (001) surface (surface energy: 0.90 J/m2) is predicated to have a better property, which attracted great attention in the past decade.8-10 However, distinct conclusions derived from the existing experimental results regarding the property of the (001) surface.11-13 A crucial but often overlooked issue is that the characterization of detailed surface structures of TiO2 nanocrystals is absent in most of these relevant studies. In particular, the high energy (001) surface usually undergoes a (1×4) reconstruction to reduce its surface energy,14-15 which may result in a remarkable difference in performance from the bulk-truncated TiO2 (001) surface to the reconstructed one. So, to clarify the intrinsic property of TiO2 (001) surface, it is of great importance to study the structure of (001) surface at the atomic level.15-18 The (1×4) reconstruction of TiO2 (001) surface was initially reported on the micron-sized TiO2 epitaxial thin films by conventional surface analytical techniques.16, 19-22 To illustrate the reconstructed structure, several atomic models have been proposed16,

19-22

, including MF,

AMR, ADM, AOM and ARM. Among them, the most accepted model is ADM model, which was proposed via theoretical calculation and confirmed by recent in situ TEM techniques on chemically synthesized TiO2 nanocrystals.17-18, 22 Although considerable progress has been 3 ACS Paragon Plus Environment


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made in the study of TiO2 (1×4)-(001) surfaces,23-28 several fundamental issues remain to be resolved. For example, different environmental conditions, such as vacuum, atmosphere and solution29 probably influenced the surface structure predominantly, and the reconstructing conditions and the stability of (1×4)-(001) surface under different environments are still ambiguous. Considering post-treatments of TiO2 nanocrystals are commonly involved in the studies,15-18 a comprehensive study of the formation and the stability of (1×4)-(001) surface under various gaseous and heating environments is demanded. In addition, it is still a challenge to characterize the intrinsic structure of the TiO2 (001) surface, which is quite vulnerable under electron beam irradiation. Herein, we performed an in situ TEM study of TiO2 (001) surface under various gaseous and heating environments. Beam effects were systematically explored, which could help to characterize the intrinsic surface structure of the TiO2 (001) surface.

2. METHODS AND EXPERIMENTS For the synthesis of anatase TiO2 nanosheets, hydrofluoric was used as the capping agent through hydrothermal route. The detailed synthetic process is as follows: 10 ml of tetrabutyl titanate was used as the precursor, mixed with 50 wt % hydrofluoric (1.8 ml), and then the mixture was transferred into a Teflon reaction autoclave (50 ml). The vessel was kept at 200 °C for 20 hours. To remove the fluorine absorbed on (001) surfaces, the sample was washed by NaOH solution (1mol/l) 3 times, and the PH of the solution was tuned to neutral by following deionized water washing. Then the sample was dried in a vacuum oven at 60 °C for 12 h. A typical TiO2 nanosheet has a length of ~25 nm and a thickness of ~5 nm.


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A DENSsolution (DH30) heating holder and a protochips (Aduro100) heating holder were used in the heating experiments. The synthesized TiO2 nanosheets dissolved in ethanol solution were dispersed on a heating chip and loaded on TEM heating holder. The in situ observations were performed in an environmental transmission electron microscopy (ETEM, Hitachi H9500) and a conventional TEM (FEI Tecnai G2 F20).

3. RESULTS AND DISCUSSION 3.1 Formation of the (1×4) Reconstruction. The bulk-truncated (1×1)-(001) surface is exposed by 5-fold coordinated Ti (Ti5c) atoms, 3-fold coordinated O (O3c) atoms and 2-fold coordinated O (O2c) atoms,30 as shown in Figure 1a. Due to the high surface energy of (1×1)(001) surface, it usually reconstructs to a (1×4)-(001) surface at high temperature. The ADM model is the most accepted atomic model for this (1×4)-(001) surface (Figure 1a), which was initially proposed by density functional theory (DFT) calculations and confirmed by recent ETEM and Cs-corrected STEM observations17-18. Previous studies show that the formation of ADM structure could significantly relieve large surface stress and reduce surface energy of (001) surface22, 31. As-synthesized nanosheets are usually covered by amorphous contamination species (Figures 1b and S1), which could stabilize the (001) surface. As previous studies indicate, the (001) surface would reconstruct under heating treatment in oxygen environment.17, 20 A typical reconstruction process in oxygen atmosphere was shown in Figures 1b-1e along the TiO2 [100] axis. After the samples were directly heated to 500 °C in oxygen (5×10-2 Pa), the surface amorphous species were gradually disappeared over several seconds, which is consistent with recent findings.17 After the amorphous species was removed, 5 ACS Paragon Plus Environment


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a thin top layer with weaker contrast was detected on the surface, which could be attributed to formation of TiOx.17 Driven by the tendency to reduce the number of dangling bonds,17 TiOx randomly adsorbed on the surface formed ordered rows with 3d and 4d patterns (Figure 1d). The TiOx rows with 3d configuration are not stable enough compared with 4d patterns, which gradually converted to 4d rows completely during following observation (Figure 1e). In vacuum, the reconstruction also underwent similar procedures in oxygen, while the step of removing surface contaminants spends more time without oxygen. During our observations, in most cases reconstructions underwent a surface cleaning process, and the (001) surfaces covered by contaminants showed the bulk-truncated structure. Therefore, a clean surface is necessary to induce the reconstruction. After removing the surface contaminants, the huge surface stress of (001) surface is released, and the free surface tends to reconstruct to reduce the surface energy. The next question is: does a clean (001) surface necessarily reconstruct to a (1×4) structure? In our experiments, most reconstruction processes occur at elevated temperatures (usually higher than 300 °C). Without heating, the great mass of bulk-truncated structure would decompose gradually without notable reconstruction under electron irradiation (Figure S2). So enough energy provided by environment is also a necessary condition for the reconstruction, while elevated temperature is a suitable and convenient condition. According to these experiments, two essential conditions are required in the formation of the (1×4) reconstructed (001) surface of chemically synthesized TiO2 nanocrystals, clean surface and enough energy for atomic reconstruction, as illustrated in Figure 1f.


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Figure 1. (a) The simulated figure of unreconstructed (1×1) and reconstructed (1×4) structure. (b-e) ETEM images show the reconstruction process of TiO2 (001) surface in oxygen environment (oxygen pressure: 5×10-2 Pa; temperature: 500 °C). The scale bar indicates 2 nm. (f) A scheme shows the reconstruction process. (g) Reconstruction time as a function of temperature in oxygen environment (oxygen pressure: 5×10-2 Pa). The reconstruction time represents the time cost in the structural evolution from an as-synthesized (001) surface covered by organic species, to a completely reconstructed (1×4)-(001) surface, e.g. from (b) to (e). (h) Diagram of formation conditions of (1×4) reconstruction as a function of temperature and oxygen pressure.



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To explore the role played in the formation of the (1×4)-(001) surface, the in situ experiments were performed at different temperatures. Keeping the oxygen pressure and electron beam dose as constant, the reconstruction is more easily formed at higher temperature. As shown in Figure 1g, a typical reconstruction formation process spent ~ 718 s at 300 °C, from an as-synthesized (1×1)-(001) covered with amorphous species to a completely reconstructed (1×4)-(001) surface, while the typical reconstructions time at 600 °C is less than 150 s. The higher temperature promotes the formation of the (1×4)-(001) surface, which is mainly due to two reasons: (1) oxidation removal of surface contaminants is quick at higher temperature; (2) the higher temperature promotes the diffusion of surface atoms, which makes them more easily migrate to the thermodynamically stable sites. Based on the above understanding regarding the formation of (1×4)-(001) surface, we carried out more experiments to achieve a scope of formation conditions for the (1×4)-(001) surface (Figure 1h). Our results showed that the (1×4) reconstruction could be formed from vacuum to an O2 pressure of 10-2 Pa at a temperature window of 300-800 °C, which suggests that it is necessary to check the surface structure of (001)-exposed TiO2 nanocrystals if annealing experiments involved. We also note that the experiments were performed in a limited range, and the situations in other gas environments or higher pressure remain to be checked. 3.2 Stability of (1×4) Reconstructed (001). The next question is whether the (1×4) reconstructed (001) surface could survive in different environments. Considering the catalytic reactions usually involve heating and atmosphere environments, we designed the experiments to illustrate its stability under different oxygen pressures and temperatures. Initially, the (1×4) reconstructed (001) surfaces were prepared under a typical reconstruction condition (e.g. 8 ACS Paragon Plus Environment

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oxygen pressure: 5×10-2 Pa; temperature: 500 °C). We changed the oxygen pressure to check the stability of the (1×4)-(001) surface. Keeping the temperature at 500 °C, the reconstruction could be observed under vacuum (5×10-5 Pa) and different oxygen pressures (5×10-3 Pa, 2.5×10-2 Pa and 5×10-2 Pa), as shown in Figure 2a-d. Then, the reconstructed (001) surface was studied in a series of temperatures under both oxygen (Figure 2e-2h, 5×10-2 Pa) and vacuum conditions (Figures 2i-2l, 5×10-5 Pa). The (1×4)-(001) surface also kept stable at the temperature window from 300 to 800 °C, as shown in Figure 2e-l. In addition, our experiments show that the reconstructed surface could even survive atmospheric pressure condition (refer to Fig. S3). These results suggest that once the (1×4)-(001) surface formed, it could be survived under more severe environments. We should note that our experiments were performed under limited conditions within a small pressure/temperature range, the situation in other conditions is still needed to be explored in further studies. Although various defects, such as surface oxygen vacancies, Ti interstitial atoms and TiO2 vacancies,16, 23-24, 32 on TiO2 (001) surface were reported by the DFT calculations, the high stability of (1×4)-(001) surface is mainly contributed by the reconstruction instead of defects, due to the very low surface energy of (1×4)-(001) surface (0.51 J/m2).



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Figure 2. In situ TEM images show the stability of (1×4) reconstructed (001) surface under various conditions: in different gas environments at 500 °C (a-d); in different temperatures under oxygen (5×10-2 Pa, e-h) and vacuum (5×10-5 Pa, i-l) environments. The scale bars in (a), (e) and (i) indicate 2 nm.

3.3 Electron Beam Irradiation on (1×4)-(001) Surface. Tremendous effort has been devoted to deducing the intrinsic performance of TiO2 (001) surface.4, 7 However, it is known that a big challenge is that TiO2 could be easily destroyed by e-beam irradiation during TEM characterization. Compared to the bulk structure, TiO2 surfaces are more sensitive to the ebeam and usually undergo structural damage under TEM observation. Thus, it is of significant importance to understand the mechanism of e-beam irradiation damage of the (001) surface and to develop an effective method to characterize its intrinsic structure during TEM observation. Figures 3a-3d show a typical destructive process induced by electron beam irradiation at 600 °C. Under the e-beam irradiation (dose: 140 A cm-2), the TiO2 nanocrystal is severely damaged and decomposed rapidly, the thickness of TiO2 nanosheet shown in Figures 10 ACS Paragon Plus Environment

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2a-3d decreased from 4.3 nm to 2.6 nm in 192 s, corresponding to the decomposition of 7 layers of (004) facet. The (001) surface was found to be decomposed layer by layer, and the height of each layer is ~ 0.24 nm, which is equal to the interplanar spacing of the (004) facet. During the decomposition, a cycle involves the alternate disappearance of two typical surface configurations, an atomically flat surface (Figure 3e) and then the (1×4)-(001) surface (Figure 3h) as shown in Figures 3e-3l. Considering the (1×4)-(001) surface is formed through periodically replacing the surface bridging oxygen rows along [010] direction by TiO3 rows,22 after disappearance of a reconstructed (004) surface layer (Figures 3e-3h), the subsurface layer exposed and its reconstruction will lead to the formation of new surface reconstructed TiO3 rows 90o differing from the former one, according with the TiO2 intrinsic crystal structure. So, we could identify the atomically flat surface (Figure 3e) is a (4×1)-(001) surface, in which the TiO3 rows are along the [100] direction. By carefully analyzing the in situ images, we found the decomposition proceeded along a preferential orientation, in which the surface reconstruction was involved. It is obvious that the (4×1)-(001) surface (Figure 3e) layer (layer 1 in Figure 3o) decomposed from right to left (Figures 3e-3h) and the TiO3 rows in subsurface (layer 2) appeared one by one along [-100] direction, refer to the intensity profiles of the top layer (Figure 3m); While during the decomposition of the (1×4)-(001) surface (Figures 3i-3l), the TiO3 rows almost disappeared simultaneously, as confirmed by the intensity profiles as well (Figure 3m). The distinct decomposition behaviors of the two layers suggest it is a well-defined process, which always



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proceeds along the direction parallel to TiO3 rows. Such process is illustrated in Figures 3n and 3o.

Figure 3. (a-d) ETEM images show an electron beam irradiation induced surface decomposition of TiO2 (1×4)-(001) surface (temperature: 600 °C; gas pressure: 10-5 Pa; electron beam dose: 140 A cm-2). (e-f) Enlarged ETEM images show layer by layer decomposition process. (m) Intensity profiles along the outmost layers in (e-l). The peaks of TiO3 rows are marked by black arrows. (n-o) Atomic models and schematic diagram show the layer by layer decomposition process, respectively.

Since the TiO2 (1×4)-(001) surface is so vulnerable under beam irradiation, it’s essential to eliminate the electron beam effect during the structural characterization. The irradiation damage under different electron current density (φ) was studied to derive the intrinsic surface structure. Figures 4a-4i show the processes of irradiation damage of reconstructed surface under different electron current density (φ) in vacuum. When the electron current density was 12 ACS Paragon Plus Environment

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140 A cm-2, the (1×4)-(001) surface was not stable and damaged soon (Figure 4b). With the increasing of electron irradiation time, the bulk structure of TiO2 was also decomposed (Figure 4c). Under a lower electron current density (100 A cm-2), the beam irradiation damage was relatively reduced (Figures 4d-4f). When the electron current density was decreased to 30 A cm-2, the (1×4)-(001) surface could survive several hundred seconds, without significant electron irradiation damage (Figures 4g-4i). To qualitatively illustrate electron beam effect, we roughly divide it into three cases according to beam damage degree (“intrinsic surface”, “surface damaged” and “bulk-structure damaged”). (1) The “intrinsic surface” indicates there is no notable damage on the surface, and the surface structure maintains intrinsic (1×4)-(001) structure; (2) The “surface damaged” indicates the outmost atomic layer of (1×4)-(001) is damaged, but there is no notable damage on subsurface layers; (3) The “bulk-structure damaged” indicates than not only the outmost atomic layer were damaged, but also the subsurface layers show notable damage. Thus, two structural evolution diagrams were obtained as a function of electron current density and temperature, as shown in Figures 4j and 4k, respectively. According to our results, the intrinsic structure of TiO2 (1×4)-(001) surface could be achieved in the green region shown in Figures 4j and 4k, which would help to investigate the intrinsic structural evolution under different environments.



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Figure 4. ETEM images show the structural evolution of the (1×4)-(001) TiO2 surface under the electron beam irradiation with different dose (a-c: 140 A cm-2, d-f: 100 A cm-2, g-i: 30 A cm-2). The images acquired in vacuum (5×10-5 Pa) at 600 °C. (j) Structure evolution diagram of the TiO2 (1×4)-(001) surface as a function of electron current density and dose (5×10-5 Pa, 600 °C). (k) Structure evolution diagram of the TiO2 (1×4)-(001) surface as a function of electron current density and temperature (5×10-5 Pa). The dots with same shape in each vertical column are acquired in the same sample.

CONCLUSIONS In this article, we performed an ETEM study of the formation and decomposition of the (1×4)reconstructed TiO2 (001) surface under various environments. According to the ETEM experiments, clean surface and elevated temperature are the two essential factors for the


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formation of the reconstruction. The stability of the (1×4)-reconstructed surfaces were systematically investigated, which should be taken into account in property research. In addition, the dynamic process of e-beam irradiation damage on the (001) surface is also studied at the atomic scale, which shows a preferential orientation. The optimal imaging conditions were confirmed, which could be used to identify the intrinsic surface structure of TiO2 (001) surface. We believe this work provides critical experimental evidences for the stability of TiO2 (001) surface at an atomic scale, which would benefit elucidating the intrinsic property of TiO2 (001) surface.



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (W.Y.). *E-mail: [email protected] (Y.W.). Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the support of National Natural Science Foundation of China (91645103, 51801182, 51390474, and 51872260) and China Postdoctoral Science Foundation (2018M642407, 2019T120502).


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TOC


An Environmental Transmission Electron Microscopy Study of the

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