Atomistic Observation of Structural Evolution during Magnesium Oxide

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Atomistic Observation of Structural Evolution during Magnesium Oxide Growth Fan Cao, He Zheng, Shuangfeng Jia, Huihui Liu, Lei Li, Boyun Chen, Xi Liu, Shujing Wu, Huaping Sheng, Ru Xing, Dongshan Zhao, and Jianbo Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08833 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 15, 2016

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The Journal of Physical Chemistry

Atomistic Observation of Structural Evolution during Magnesium Oxide Growth Fan Cao†, He Zheng†, Shuangfeng Jia*,†, Huihui Liu†, Lei Li†, Boyun Chen†,‡, Xi Liu†,§, Shujing Wu†, Huaping Sheng†, Ru Xing┴, Dongshan Zhao†, and Jianbo Wang*,† †

School of Physics and Technology, Center for Electron Microscopy, MOE Key Laboratory of

Artificial Micro- and Nano-structures, and Institute for Advanced Studies, Wuhan University, Wuhan 430072, China ‡

Shuiguohu Senior Middle School, Wuhan 430071, China

§

Middle School Attached to Huazhong University of Science and Technology, Wuhan 430074,

China ┴

The Department of Physics Science and Technology, Baotou Normal College, Baotou 014030,

China

ABSTRACT

Defects, such as dislocations, grain boundaries and surfaces, play vital roles in the properties of nanocrystalline metal oxides. Thus, it is crucial to understand the structural evolution during the growth of metal oxides. In this work, atomistic observation of the structural dynamics in MgO grains during magnesium oxidation is realized by the in situ high resolution transmission electron microscopy. There are three major equivalent orientation relationships between MgO

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grains and Mg. Interestingly, our experiments reveal the grain rotation and grain boundary (GB) migration, resulting in the change of the orientation in MgO grains and the morphology of GBs. The dislocation-activity which may mediate the GB migration and/or grain rotation is captured in MgO with high lattice strain/stress. Our results provide the atomic-scale structural evolution of the growth of passivating oxide layer on the Mg surface, which not only facilitate the development of metal oxide growth method but also provide a possible anti-corrosion strategy.

INTRODUCTION Metal oxides find extensive applications in various fields,1,2 such as supercapacitors, solar and fuel cells, catalysts, and gas sensors, owing to their unique properties. In the past decades, remarkable attention was paid to the nanocrystalline metal oxides, where defects (e.g., dislocations, grain boundaries and surfaces) serve as the key factors to determine the properties of nanocrystalline materials.3-6 As a result, understanding the growth process, especially the evolution of nano-grains and interfaces during the oxide growth, is of great significance for the utilization of metal oxides. Meanwhile, metal oxidation has been applied for the synthesis of nanocrystalline metal oxides,1 due to its simplicity and low-cost. However, the full exploration of oxide growth processes during oxidation has been rarely reported. On the other hand, metal oxidation, which leads to the serious corrosion problems, has been an issue of interests for a long time. The growth of surface oxide layer during metal oxidation can also serve as a passivating and protective layer against further oxidation,7,8 since either metal or oxygen ions should diffuse across the oxide to sustain the oxidation.9-12 A general strategy to protect the underlying metal requires the growth of continuous oxide layer,8 while the discontinuities in the oxide phase, such as grain boundaries (GBs) and dislocations are


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preferential pathways for the diffusion of metal and oxygen ions.7,12-14 Towards this end, it is desperately needed to gain a deep insight into the structural evolution during the oxide layer growth. Herein we report in situ atomic-scale exploration of the growth of bulk MgO by magnesium oxidation. GB migration during the competitive growth of two adjacent MgO grains and grain rotation to satisfy the preferred orientation relationship (OR) were revealed in situ. Furthermore, the evolution of dislocations which may account for the above behaviors (GB migration and grain rotation) was directly visualized in MgO lattice with extremely high strain. EXPERIMENTAL SECTION Sample preparation: The original sample used here was pure hexagonal-close-packed magnesium with lattice parameters: aMg = bMg = 3.21 Å, cMg = 5.21 Å. Firstly, very thin Mg films were obtained by traditional mechanical grinding and ion polishing. Afterwards, the Mg films were annealed in air atmosphere at 400 oC for 10 hours. Thus, MgO films covered the surface of Mg matrix. Characterization: The in situ high resolution transmission electron microscopy (HRTEM) observations were performed inside a JEOL JEM-2010 FEF (UHR) electron microscope (with a field emission gun and an Omega-type in-column energy filter system) operated at an accelerating voltage of 200 kV. RESULTS AND DISCUSSION Firstly, pure hexagonal-structure Mg film was oxidized in air at 400 oC for 10 hours and cubicstructure MgO phases nucleated on its surface (Figure 1a, green substrate and pink grids stand


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for the Mg and MgO, respectively; only the upper surface is shown for simplicity). It is noted that there are three dominant MgO domains related by 120° rotation along the [0001]H,15-19 corresponding to three equivalent ORs between Mg and MgO (see also S1, S2 and S3 in Supporting

Information

(SI)),

OR-I: [0001]H //[001]CI

and

(1010)H //(110)CI ,

OR-II:

[0001]H //[001]CII and (1100) H //(110)CII , and OR-III: [0001]H //[001]CIII and (0110) H //(110)CIII (the subscripts H and C represent the hexagonal and the cubic structure of Mg and MgO, while CI, CII and CIII stand for the three variants of MgO, respectively). To be specific, Figure 1b shows the HRTEM image along the [0001]H zone axis. The inset in Figure 1b is the corresponding fast Fourier-transformed (FFT) pattern, indicating the common OR-I (see also S3d in SI) between Mg and MgO, in accordance with the OR where Mg is oxidized in ambient condition.20 Additionally, the overlap between the Mg substrate and MgO crystals is manifested by the occurrence of Moiré fringes (see also in S4 in SI) possessing approximate spacing of dm = 1.89 nm. The Moiré fringes result from the double diffraction based on (1210) H and (220)CI reflections (pointed out with a white arrowhead in the inset), with a theoretical value dm = 1.99 nm (close to 1.89 nm in Figure 1b) calculated by the formula d m = d H dC / d H − dC .21 Furthermore, the lattice mismatch between Mg and MgO of current OR is pretty small (0.16%) as predicted by a near coincidence site lattice (CSL) model (see details in S5 in SI).21,22 To better investigate the structural characteristics of MgO phases, focused electron beam (ebeam) was applied to drill nanopores in the sample (Figure 1c). As a result, the Mg substrate within the irradiation zone can be easily removed22-25 while some of the MgO phases remain on the sample surfaces (Fig. 1c). The HRTEM image in Figure 1d clearly shows the existence of


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three orientation domains in MgO, which are represented as grain “I” (colored red), “II” (colored green), and “III” (colored blue), corresponding to the three equivalent ORs. Figure 2 shows the enlarged view of the centered area in Figure 1d. The different MgO variants (grain I, II, and III) as well as the GB between grain I and III (pointed out by an arrow) are presented. It is noted that all the three variants are enclosed by the {200}C planes of MgO with the lowest surface energy, consistent with the previous report.22,23,26 Afterwards, further oxidation led to the growth of the MgO layers and two dominant growth features were found: (1) The MgO preferred the double-layer (indicated with arrowheads in Figure 2c) rather than the single-layer growth mode reported previously, which focuses on the initial oxidation process in pure Mg;22 (2) The growth process involves the orientation adjustment of the three MgO variants. For example, as the oxidation continued, grain I (at the lower right part) grew upward remarkably, indicated by the change of the orientation of grain III to that of grain I and the GB migration towards grain III (as shown in Figure 2b-d) (see details in Movie 1 in SI). Additionally, according to the previous report, the oxygen inside the microscope column could facilitate the oxidation of Mg and thus the MgO growth.22,27,28 The change of the orientation should be related with the grain rotation and/or the GB migration, both of which were captured, as shown with FFT-filtered images in Figure 3 (Movie 2 in SI). For example, the original angle between (1010)H in Mg matrix and (200)C planes in MgO (depicted with two black lines in Figure 3a) was approximately 15°, corresponding to ORII: [0001]H //[001]CII and (1100) H //(110)CII . Additionally, the d-spacings of (200)C and (1010)H are measured and marked in figure 3. It is noticed that the original d-spacing of (200)C is larger


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than the theoretical value of 0.21 nm, which can be attributed to the formation of a metastable oxygen-deficiency MgOx (0 < x < 1) phase22 due to the e-beam irradiation. However, accompanied with the growth process, the angle increased to 20°, 30°, 40° and 45° (the right side grain of Figure 3b-e, respectively) and stabilized at a value of 45° (Figure 3e), indicating the rotation of MgO crystals (named as grain “R”). Finally, the new orientation of the MgO phase is equivalent with OR-I: [0001]H //[001]CI and (1010)H //(110)CI (Figure 3e). In the meantime, another MgO grain (named as grain “L”) showed up at the left side in Figure 3d and the GB between grain R and L moved towards left during the growth, as indicated by arrowheads in Figure 3d and e (see details in S6 in SI). The GB migration is supposed to be motivated by the energy reduction of the system, i.e., the boundary area between MgO grains (R and L) and the vacuum reduced.29,30 As depicted with dashed black lines in S6, accompanied with the GB migration, the initial abrupt surface of MgO grains becomes smoother to reduce the surface energy.29,30 To gain a better understanding of the above phenomena, a schematic illustration according to Figure 3e is shown in Figure 3f, whereas the GB between the two MgO grains is constructed, based on the (410)/[001] tilt GB model.31,32 Moreover, the atomic model of MgO growth is shown in Figure 4 for a better understanding of the grain rotation and GB migration processes. Figure 4a-c exhibit the grain rotation from ORII to OR-I, while in Figure 4d-f, grain I and III nucleated at different positions (Figure 4d) and coalesced based on the competitive growth (Figure 4e) and the GB migration (Figure 4e and f). It is widely accepted that both GB migration and grain rotation are likely to be mediated by dislocation activities,33-35 which frequently occurred during recrystallization and high temperature deformation of polycrystalline materials.29,33,36,37 However, they were rarely


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reported at the atomic-scale during the growth of oxides. Interestingly, the generation and movement of dislocations during the GB migration shown in Figure 3d and e were directly captured (Figure 5a-d). The insets in Figure 5 are the enlarged and FFT-filtered views corresponding to the black-boxed regions. It is obvious that a dislocation (as designated by T) generated (Figure 5a), propagated towards left (Figure 5b and c), and finally moved out of this area (see also in Movie 3 in SI). The Burgers vector is 1/2, having an angle of 45° with respect to the projecting direction. Accompanied with the dislocation activity, the GB between the grain L and R migrated from the right to the left side. The nucleation and movement of dislocations may result from the high lattice strain/stress around the GB.38-42 The large lattice strain of MgO grains in Figure 3e was carefully examined. The inter-planar angles of the {200}C planes are measured (Figure 5e), which gradually changed from the normal value 90° at the left side of grain L to 80° around the GB, and finally back to 90° again at the right side of grain R (Figure 5f), implying the large lattice shear strain at the GB. Additionally, in Figure 5g, from the topside layer (1st layer) to the 6th layer, the average lattice distances are measured at three different regions: the GB, grain R and L, as indicated with red, blue and green lines. Subsequently, the strains of the lattice in various positions are obtained (Figure 5h) according to the formula ε = (d − D ) / D ,43 where d represents the measured lattice distance and D denotes the {200}C lattice distance of the undeformed MgO (see details in S7 in SI). In general, they shifted from compressive to tensile strain gradually as the layers went downward from the topside layer near the vacuum, and in particular, both the compressive and tensile strains around the GB (colored red) were much higher. Such internal stress may act as the driving force for the movement of dislocations.


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It is well-accepted that the coalescence of misoriented oxide islands during the growth process can reduce the density of GBs, thus hinder further oxidation.11,44 Nonetheless, the detailed structural evolutions of oxide islands especially how they “interact” with each other are missing. The atomic-scale processes regarding the grain rotation and dislocation-mediated GB migration mentioned above should provide new insight into the oxide growth mechanisms. Moreover, it is worth noting that the e-beam can neither induce significant damages (e.g., dislocations) in MgO,45,46 nor heat the sample remarkably.23 Therefore, the results are characteristic to the MgO growth during magnesium oxidation in ambient condition, which can possibly be extended to other metal oxides. In addition, it was reported that the growth of MgO nanorod occurred at the places where the metal nanoparticles (e.g., Au) are spread on the pure MgO surfaces.47 However, no metal nanoparticles is used in current experiment and thus the MgO growth is motivated by the oxidation process. CONCLUSIONS In summary, the competitive growth of oxide grains and the “interactions” among them are observed during the oxidation of Mg. There are three equivalent ORs between Mg and MgO. The atomic-scale grain rotation and dislocation-mediated GB migration were revealed. This work offers the first direct observation of the real-time structural evolution in oxide grains/islands with different orientations and thus enhances the basic understanding of the fundamental mechanisms in metal oxidation and its applications such as metal oxide growth.


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ASSOCIATED CONTENT

Supporting Information M1-M3, Three videos showing the oxidation processes; S1-S3, the orientation relationship between Mg and MgO; S4, a lower magnification TEM image showing the Moiré fringes; S5, calculation of the lattice mismatch; S6, the interface between vacuum and MgO; S7, measurement of the reference lattice distance. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors *(S. Jia) Email: [email protected]. Tel.: +86-27-6875-2481, ext. 8147. *(J. Wang) Email: [email protected]. Tel.: +86-27-6875-2462 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. F. Cao and H. Zheng contributed equally to this paper. ACKNOWLEDGMENTS This work was supported by the 973 Program (2011CB933300), the National Natural Science Foundation of China (51671148, 51271134, J1210061, 11674251, 51501132, 51601132), the Hubei Provincial Natural Science Foundation of China (2016CFB446, 2016CFB155), the Fundamental Research Funds for the Central Universities, and the CERS-1-26 (CERS-China Equipment and Education Resources System) and the China Postdoctoral Science Foundation (2014T70734). REFERENCES


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2002, 374, 732-735. (47) Nasibulin, A. G.; Sun, L.; Hämäläinen, S.; Shandakov, S. D.; Banhart, F.; Kauppinen, E. I. In Situ TEM Observation of MgO Nanorod Growth. Cryst. Growth Des. 2010, 10, 414-417.


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Figure Captions.

Figure 1. a,c) Schematic illustrations of drilling nanopore by focused e-beam on the substrate. b,d) Corresponding HRTEM images indicating the existence of MgO on Mg film.

Figure 2. a-d) Time-lapsed images showing the dynamic structural evolution of MgO grains during the oxidation process.

Figure 3. a-e) Sequential images showing the grain rotation and GB migration processes of MgO crystals. f) Schematic illustration of the ORs between Mg matrix and MgO grains according to Figure 3e.

Figure 4. Atomic models showing the growth of MgO. a-c) Grain rotation from OR-II to OR-I. d-f) Competitive growth between grain I and III.

Figure 5. a-d) The dislocation activity during the GB migration in MgO. e.g) Measurement of the inter-planar angles and the strains in the MgO lattice. f.h) The measured results corresponding to e and g, respectively.


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Figure 1

Figure 2


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Figure 3

Figure 4


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TOC GRAPHIC


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Atomistic Observation of Structural Evolution during Magnesium Oxide

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