In Situ Atomic-Scale Observation of the Two ... - ACS Publications

May 1, 2017 - Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States. §. Department of Che...
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In Situ Atomic-Scale Observation of the Two-Dimensional Co(OH)2 Transition at Atmospheric Pressure Xiaochen Shen,†,⊥ Sheng Dai,‡,§,⊥ Changlin Zhang,† Shuyi Zhang,‡,§ Stephen M. Sharkey,† George W. Graham,‡,§ Xiaoqing Pan,*,§,∥ and Zhenmeng Peng*,† †

Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, Ohio 44325, United States Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States § Department of Chemical Engineering and Materials Science, University of California-Irvine, Irvine, California 92697, United States ∥ Department of Physics and Astronomy, University of California-Irvine, Irvine, California 92697, United States ‡

S Supporting Information *

ABSTRACT: Two-dimensional (2D) materials have been recognized as one of the promising materials for various applications due to their unique characteristics. However, the formation and transformation mechanisms behind the 2D structure still largely remain unknown, which is significant for synthesis controlling and property tuning. In this scope, in situ microscopy characterization of two-dimensional (2D) materials under atmospheric reaction conditions offers a powerful tool for understanding their structural evolution at the atomic scale, which, however, has been seldom reported. Here, taking the 2D CoO as the model material which is the promising electro-/ photoelectro-catalyst, we report real-time visualization of the structural transition of 2D Co(OH)2 nanosheets to CoO using in situ electron microscopy. Three intermediate phases, including one pseudo-Co(OH)2 phase, one transition phase, and one pseudo-CoO phase, are identified and characterized during the transition process. The detailed transition pathways and mechanisms are discussed based on the combined in situ STEM and FTIR data. The transition starts with the rapid dehydration process followed by two rearrangement periods and one relaxation process, respectively. The complete transition process is as follows: Co(OH)2 → (dehydration) → Co(OH)2,p → (rearrangement) → transition phase → (rearrangement) → CoOp → (relaxation) → CoO.

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INTRODUCTION Two-dimensional (2D) materials have attracted tremendous attention owing to their various potential applications, for instance, in batteries, supercapacitors, sensors, and catalysts.1−4 A large number of 2D materials, including graphene and transition metal dichalcogenides/hydroxides/oxides, have been synthesized and shown to exhibit unique properties.5−7 Taking CoO material, for example, it has been found that 2D CoO is an outstanding catalyst for electrocatalytic/photoelectrocatalytic and battery-related applications.8−12 Cui et al. studied the CoO nanosheets and found a low overpotential of 284 mV at 10 mA cm−2 for water oxidation.13 Wang et al. synthesized the highly ordered ultrathin CoO nanosheet arrays for lithium-ion batteries and obtained excellent cyclability and rate capability.14 In the scope of these reports, the 2D structure of CoO plays an important role for the good performance. To form the 2D structure, the synthesis of 2D materials usually involves the strategy of controllable synthesis. Thermal decomposition of layer-structured Co(OH) 2 nanosheets has offered one important synthesis approach for obtaining the 2D CoO structure. However, there are many open questions regarding the experimental process, in particular how the structural transition occurs at an atomic scale and why the 2D © 2017 American Chemical Society

morphology can be retained, given the dramatic difference between CoO and Co(OH)2 phases. Through studying these questions, the mechanisms behind the 2D structure formation can be revealed, and thus the strategies of constructing 2D structure and tuning properties can be guided. In situ electron microscopy has been developed in recent years for the characterization of structural transformation under a reactive environment and has proven to be a powerful tool to study any condition-induced changes.15−21 For instance, Zheng et al. studied the interfacial dynamics during Au-PbS core−shell nanostructure formation, and Nijhuis et al. investigated the growth process of Au−Ag core−shell nanoparticles in the liquid TEM cell using real-time imaging;22,23 Helveg et al. imaged the oscillatory behavior of Pt nanoparticles during CO oxidation, and Liu et al. visualized the redox dynamics of a single Ag/AgCl heterogeneous nanocatalyst at the atomic scale using the gas TEM cell.24,25 Nevertheless, there are still few in situ electron microscopy studies on 2D materials that have just started being reported recently,26−28 which is primarily because of the Received: March 29, 2017 Revised: April 28, 2017 Published: May 1, 2017 4572

DOI: 10.1021/acs.chemmater.7b01291 Chem. Mater. 2017, 29, 4572−4579

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In Situ FTIR Study. In situ diffuse reflectance infrared Fourier transform (DRIFT) study was conducted on a Thermo Scientific Nicolet 6700 spectrometer with a DRIFT system and Praying Mantis High Temperature Reaction Chamber (Praying Mantis, Harrick Scientific Products, Inc.). Simply, the as-prepared Co(OH)2 sample was loaded in the sample chamber with two KBr windows for the IR light passing through as will be elaborated later in Figure 3B. The FTIR spectra were recorded under argon protection at 1 atm with a heating rate of 2 °C/min. Characterization. The X-ray diffraction (XRD) patterns were collected on a Bruker AXS Dimension D8 X-ray diffractometer operating at 40 kV and 35 mA (Cu Kα, λ = 0.154184 nm). The morphologies of these samples were acquired using a JEOL JEM-1230 transmission electron microscopy (TEM) with the accelerating voltage of 120 kV. The high-resolution TEM (HRTEM) images were taken using a FEI Tecnai G2 F20 microscope operating at 200 kV.

technical difficulty in imaging the atomically thin structures under reaction conditions. Hersam et al. reported the in situ thermal decomposition of exfoliated 2D black phosphorus,29 Taheri et al. studied the oxidation of 2D Ti3C2 to form the carbon-supported TiO2,30 and Wang et al. observed the growth mechanisms of two-dimensional MoS2 flakes on in situ electron microscopy.31 In this regard, observing the dynamic atomicscale transition from Co(OH)2 to CoO is a challenging task, particularly identifying intermediate species at an atmospheric reaction condition, but it has the potential of offering atomiclevel visualization of the transition process, which should provide more insight into understanding 2D structure transition mechanisms and their connection to material properties. Hence, in this paper we conducted in situ electron microscopy studies on the transition of Co(OH)2 nanosheets into CoO, in combination with in situ Fourier transform infrared spectroscopy (FTIR) experiments. The whole transition process was investigated in real time using highresolution high-angle annular dark field (HAADF) and bright field (BF) STEM imaging at an atmospheric condition. Three intermediate phases and four transition stages were identified during the transition process, with the initial O lattice atom restructuring caused by dehydration, Co lattice atom rearrangement induced by O restructuring, and lattice relaxation toward CoO generation occurring as a result.

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RESULTS AND DISCUSSION The as-prepared Co(OH)2 nanosheets show typical characteristics of a layered hydroxide structure, evidenced by the X-ray diffraction (XRD) pattern (Figure 1A).34,35 All the diffraction

EXPERIMENTAL SECTION

Preparation of Co(OH)2 Nanosheets. The Co(OH)2 nanosheets were prepared by a one-step precipitation method. In a typical procedure, 0.58 g of Co(NO3)2·6H2O was first dissolved in 10 mL of DI water, and the solution was purged with high purity Ar (99.9999%) for 10 min to remove any soluble oxygen. Under vigorous stirring and argon protection, 2 mL of 1 M NaOH solution was dropwise added into the solution. The reaction was kept for 10 min and then collected by centrifuging at 4500 rpm for 5 min. The sample was washed with water several times to remove any residuals before drying. Preparation of ex Situ CoO Nanosheets. The as-prepared Co(OH)2 nanosheets were used as a precursor to synthesize the ex situ CoO nanosheets. In brief, the as-prepared Co(OH)2 nanosheets were placed in a tube reactor and purged with high purity Ar for at least 15 min before annealing. Then under argon protection, the sample was heated to 300 °C with the heating rate of 5 °C/min and maintained for 1 h. For comparison, CoO nanosheets were also prepared under different conditions (i.e., temperature, heating rate, and heating time). The samples were denoted as CoO-M/N@X, where M (°C) is the heating temperature, N (min) is the heating time, and X (°C/min) is the heating rate. In Situ STEM Study. In situ scanning transmission electron microscopy (STEM) study of Co(OH)2 nanosheets converting to CoO nanosheets was performed on a JEOL 3100-R05 microscope with double Cs correctors equipped with the Protochips Atmosphere system that had already been reported before.32,33 In a typical procedure, the as-prepared Co(OH)2 sample was first dispersed in anhydrous methanol, and the suspension was deposited directly onto a thermal E-chip, which is equipped with a thin ceramic heating membrane controlled by the Protochips Atmosphere system. A second E-chip window was then placed on top of the thermal chip in the holder, creating a thin gas cavity sealed from the high vacuum of the TEM column. The 2D Co(OH)2 sample was situated between two SiN membranes, each 30−50 nm in thickness, with a 5 μm gap in between. The cross section view of the assembled gas cell will be shown later in Figure 3A. The operation accelerating voltage was 300 kV with a small probe current of 20 pA, and the heating temperature for the gas cell was 300 °C with high purity nitrogen (99.9995%) protection at 1 atm (760 Torr).

Figure 1. (A) XRD patterns of as prepared Co(OH)2 and CoO nanosheets, (B) crystal structure of Co(OH)2, and (C) crystal structure of CoO.

peaks are assignable to a hexagonal crystal phase (JCPDS No. 074-1057, space group type: P3̅m1, a = b = 0.317 nm, c = 0.464 nm). The main peaks at around 19.3°, 32.7°, 38.2°, 51.6°, 58.2°, and 61.8° are ascribed to (0001), (101̅0), (011̅1), (011̅2), (112̅0), and (112̅1) planes, respectively (Table S1). The Co(OH)2 layers stack along the [0001] orientation as illustrated in Scheme S1. The transmission electron microscopy (TEM) image of Co(OH)2 confirms the nanosheet morphology (Figure 2A). Figure 2B shows the HAADF-STEM image and displays a clear hexagonal symmetry of the Co atom columns in the [0001] direction, which could be verified by the FFT pattern (Figure 2B, inset). The in situ Co(OH)2 transformation experiments were conducted on a double Cs-corrected JEOL 3100-R05 microscope equipped with the Protochips Atmosphere system. Figure 3A shows the scheme of this gas cell setup. Prior to the in situ experiment, the electron beam effect was studied and ruled out for the structure change. Detailed discussion can be seen in the Supporting Information. Then the sample chip loaded with Co(OH)2 nanosheets was rapidly heated to 300 °C, and both the DF and BF STEM images were recorded simultaneously in a time series. Figure 4A shows one representative image acquired after temperature stabilization, which contains three areas with distinct phases. The contrast between the three phases was improved for display purpose by applying a mask on the amorphous region followed by inverse fast Fourier 4573

DOI: 10.1021/acs.chemmater.7b01291 Chem. Mater. 2017, 29, 4572−4579

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Co(OH)2,p phase under the in situ condition already reflects the transition from Co(OH)2, with only slight position changes in Co lattice atoms but dramatic structural changes in the H and O atoms. Figure 4A2 shows the FFT pattern of the HAADF image of the transition phase area. Comparing with Figure 4A1, the bright dots corresponding to β1 and γ1 disappear and only dots in α2 direction remain. The change in the FFT pattern indicates the Co atom skeletal structure transforms. We conjecture that, during this transition process, the Co atoms move and rearrange in both the horizontal and the vertical directions (i.e., x and y directions), which can be substantiated from the real-time in situ microscopy images shown in Figure 5. The FFT pattern of the CoOp phase is shown in Figure 4A3. Besides the dots in the α3 direction, new bright dots in the δ3 direction emerge, showing an orthogonal pattern, which is consistent with the CoO sample as in Figure S2. The two directions of the orthogonal pattern are verified to be the [220] and [11̅ 1]̅ .40−42 There are still some discrepancies between the measured lattice d-spacing and that of CoO phase, which is likely due to the phase not being fully relaxed, and we thus denote this phase as CoOp. In our experiment, the CoOp phase continued to transform toward the final CoO phase with increasing reaction time, but the lattice orientations remained (Figure S3). Thus, the overall transition is from [0001] orientated Co(OH)2 nanosheets to [1̅12] orientated CoO nanosheets. Figure 4B3−B5 shows the false colored IFFT images of Figure 4A. We filtered Figure 4B3 into two IFFT images along different orientations by applying masks (i.e., αi direction and directions other than αi) to identify the dislocations. The αi filter generates the horizontal lattice fringes (Figure 4B4). We can see that the lattice fringes of the transition phase area are different from those of the Co(OH)2,p and CoOp phase areas (indicated by the dashed circle). The edge dislocations at the Co(OH)2,p/transition and transition/CoOp boundaries reveal big strains when the structure is transforming from one to another. Especially the strain from Co(OH)2,p to the transition phase indicates a divergence in the atom arrays that will be further discussed in Figure 5. The nonuniform lattice fringes across the intermediate transition phase indicates the transition phase is unstable, and the atoms in this phase move vigorously. The non-αi masked lattice fringes (Figure 4B5) show smooth changes from the Co(OH)2,p to the transition phase and then to the CoOp, suggesting that the lattice atoms undergo gradual rearrangement in directions. The variant lattice changes in different directions reveal that the transformation from Co(OH)2,p to CoOp is anisotropic. To visualize the lattice change during the Co(OH)2-to-CoO transition process, we plotted the lattice d-spacing profile along the αi direction (Figure 4C). It is clear that the d-spacing of the (1̅100) lattice plane in the pristine Co(OH)2 increases significantly when transforming to the Co(OH)2,p phase. In the following transition to the intermediate transition phase, the (1̅100) lattice plane splits, causing a new lattice plane to emerge, and thus the d-spacing decreases greatly. Afterward, the d-spacing of this new lattice plane gradually decreases, eventually forming the (220) lattice plane in CoO. Sequential in situ STEM images were taken in real time for understanding the mechanism of the structural transition process (Figure 5). We focused on the Co(OH)2,p/transition phase boundary region and conducted continuous observation for identifying the structural changes. As shown in Figure 5A1− A4, the first image of the boundary region is set as the time zero

Figure 2. TEM image of as-prepared Co(OH)2 nanosheets (A), the HAADF-STEM image (B) with the FFT pattern (inset), the corresponding bright field (BF) image (C), and the top view model of the Co(OH)2 (0001) surface (D).

Figure 3. Schematic cross section view of the assembled gas cell with sample loaded for in situ STEM (A) and in situ FTIR (B).

transform (IFFT) (Figure 4B1). The phase in the left region of the image shows a hexagonal structure and can be ascribed to Co(OH)2,p, wherein the notation p stands for a pseudo Co(OH)2 phase. The horizontal fringes in the middle represent a transition phase where the atoms are rearranging significantly. The phase in the right shows an orthogonal structure and is indexed to the CoOp phase (pseudo CoO phase) in the [1̅12] direction. The three phases can be further confirmed by detailed FFT analyses (Figure 4A1−A3). The FFT pattern of the lattices on the left shows a similar symmetry as that of the Co(OH)2 (Figure 4A1). More careful characterization finds that there are slight variations in the lattice d-spacing values, with about 0.330, 0.280, and 0.293 nm along three directions (i.e., α1: [1̅100], β1: [011̅0], γ1: [101̅0]) versus 0.271 nm for Co(OH)2. The data indicate that the Co(OH)2,p phase is not exactly identical to the pristine Co(OH)2 phase. Because the dark field (DF) imaging can only detect Co atoms and the lighter H and O elements are hardly visible under the mode,36−39 the data suggest the Co atom skeletal structure is largely retained in the Co(OH)2,p phase, with small stretches in the lattice. The bright field (BF) image of the same region was also taken for comparative analyses (Figure 4B2). It can be seen clearly that the Co(OH)2,p BF image in Figure 4B2 is quite different from that of pristine Co(OH)2 shown in Figure 2C. The Co(OH)2,p possesses a tetragonal pattern rather than a hexagonal pattern for Co(OH)2. Since the Co lattice atoms still largely retain their original structure, the significant change is due to the H and O atoms. In other words, the observed 4574

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Figure 4. In situ HAADF-STEM image of Co(OH)2 transition (A) and the corresponding FFT patterns of three different phase areas: (A1) Co(OH)2,p phase area, (A2) transition phase area, and (A3) CoOp phase area. IFFT image of the Co(OH)2 transition (B1) and the corresponding bright field image (B2). The false colored IFFT images with different filters for identifying dislocations: (B3) all direction filter, and (B4) filter of αi direction, (B5) filter of the other directions except αi. d-spacing correlation in αi direction (C).

3B). The sample was heated from room temperature to 400 °C which made it completely transformed from pristine Co(OH)2 to final CoO. The pristine Co(OH)2 nanosheets show a sharp peak at about 3637 cm−1, which is related to the −OH stretching vibration of free Co−OH groups on interlayer surfaces (Figure 6A,B shows the 3D and 2D spectra, respectively).43,44 There is no significant change in the peak intensity when the temperature increases until about 210 °C (sample surface: ∼175 °C), indicating the Co(OH)2 structure remains under this temperature. This peak disappears abruptly, however, with a further increase in the temperature, indicating the interlayer −OH groups are removed from the system very rapidly as shown in Figure 6C. We ascribe this process to the dehydration of the Co(OH)2 sample. The dehydration takes place in a very short time, which could explain why we failed to capture this process under in situ microscopy. From the 2D spectra series (Figure 6B), the peak position of the interlayer −OH group shows a continuous shift toward a lower wavenumber when the temperature is below 210 °C. This red shift indicates an increase in the bond length of the interlayer −OH group and a decrease in the bond energy.

structure, which possesses a Co(OH)2,p phase in the left part and a transition phase in the right. Along with the reaction time, the boundary (marked by the oblique dashed line) gradually moves toward the left (indicated by the vertical dashed line). This means the Co(OH)2,p phase slowly transformed to the intermediate transition phase. Additionally, it can be noticed that each individual horizontal lattice plane in the Co(OH)2,p phase diverges at the boundary and splits into two planes in the transition phase. This split could be ascribed to Co lattice atom rearrangement along the [1̅100] (i.e., vertical) direction. Considering the d-spacing shows significant change, and consequently a big stress exists in the [1̅100] direction during the Co(OH)2-to-Co(OH)2,p conversion; it is reasonable to conjecture that the splitting of atom arrays during the Co(OH)2,p-to-transition phase conversion is caused by the strain, balanced by filling up the lattice space, which leads to the formation of the (220) lattice plane of CoO. The Co(OH)2-to-Co(OH)2,p transition, which seems to be occurring rapidly and was not observed under our current in situ microscopy experiment conditions, was investigated using in situ FTIR (the in situ cell setup scheme is shown in Figure 4575

DOI: 10.1021/acs.chemmater.7b01291 Chem. Mater. 2017, 29, 4572−4579

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Figure 5. Real time in situ microscopy image of the Co(OH)2,p transition: (A1−D1) HAADF-STEM images, (A2−D2) the corresponding IFFT images, (A3−D3) false colored IFFT images, and (A4−D4) bright field images.

STEM observations (Figure 4C), also facilitates the release of produced H2O molecules. On the basis of the in situ electron microscopy and FTIR results, a model for the 2D Co(OH)2-to-CoO transition is illustrated in Scheme 1. The whole process includes five phases at different transition stages, including three intermediate phases (Co(OH)2,p, the transition phase, and CoOp) besides the pristine Co(OH)2 phase and the final CoO phase. The process undergoes four sequential steps, including dehydration during the Co(OH)2-to-Co(OH)2,p transition wherein the Co lattice remains largely unchanged and the O lattice atoms restructure dramatically, Co lattice splitting and rearrangement during Co(OH)2,p-to-transition phase conversion, lattice atoms rearrangement during transition phase-to-CoOp conversion, and lattice relaxation during CoOp-to-CoO transition. It is noted that although the nanosheet morphology was largely maintained during the Co(OH)2-to-CoO transition, nanosized pores were generated on the resultant CoO nanosheets.45 The pore generation was attributed to the dramatic structural and lattice parameter changes during the transition process, which cause significant volume shrinkage and thus void formation to release the lattice strain. The pores seemed to grow in size with an increase in the temperature (Figures S5 and S6). Interestingly, most of the pores were faceted, showing an angle of ∼120° at the corners and corresponding dihedral angle between (1̅1̅1) and (31̅1̅) planes. The facet formation could be associated with surface energy minimization during the pore generation and growth, which

Figure 6. In situ FTIR spectra of Co(OH)2 transition: 3D spectra view (A) and 2D spectra view (B). The interlayer −OH group peak evolution with temperature increasing: (C) peak absorbance variance and (D) peak position shift. (The temperature is measured from the thermocouple, and the corresponding sample surface temperature profile can be found in Figure S4).

Figure 6D shows the peak shifting from 3637 to 3630 cm−1 which identifies a 7 cm−1 wavenumber decrease. It is likely the Co(OH)2-to-Co(OH)2,p transition occurs when the continuous increase in the bond length reaches a threshold, causing the bond breakage. Besides, the lattice stretch, as evidenced by the 4576

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Scheme 1. Schematic of Transformation from Co(OH)2 to CoO

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01291. Discussions of the electron beam effect, additional TEM images, HAADF-STEM and FFT pattern, XRD data summary, and other figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Z.P.) E-mail: [email protected]. *(X.P.) E-mail: [email protected]. ORCID

Sheng Dai: 0000-0001-5787-0179 Changlin Zhang: 0000-0002-1207-4264 Xiaoqing Pan: 0000-0002-0965-8568 Zhenmeng Peng: 0000-0003-1230-6800 Author Contributions ⊥

(X.S. and S.D.) These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the University of Akron (Z.P.). The in situ microscopy characterization was performed at the Michigan Center for Materials Characterization at the University of Michigan. The HRTEM test was taken at the (cryo)TEM facility at the Liquid Crystal Institute, Kent State University, supported by the Ohio Research Scholars Program Research Cluster on Surfaces in Advanced Materials. The authors thank Dr. Gao for technical support with the TEM experiments.

however needs more detailed studies and is beyond the scope of this work.

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CONCLUSIONS In summary, we combined the in situ electron microscopy and in situ FTIR techniques, which have rarely been used together,46,47 to conduct studies on the Co(OH)2-to-CoO transition in 2D nanosheets. The STEM images taken under reaction conditions showed clear evidence of intermediate phases generated at different transition stages. The real time imaging technique provided detailed information about the structural evolution at phase boundaries. By combining the in situ electron microscopy and the in situ FTIR spectroscopy experiments, three intermediate phases and four transition stages during the transition were revealed. The first stage is the rapid dehydration process which involves the transition from pristine Co(OH)2 to Co(OH)2,p phase. The second stage is the conversion of Co(OH)2,p phase to the transition phase, which is followed by the further transition to the CoOp phase. The last stage is the relaxation of the CoOp phase to the final CoO phase. The complete transition process follows the sequence of pristine Co(OH)2 → (dehydration) → Co(OH)2,p → (rearrangement) → transition phase → (rearrangement) → CoOp → (relaxation) → final CoO. The obtained knowledge of the 2D Co(OH)2-to-CoO transition could help to guide 2D CoO nanomaterials preparation and understand their unique physiochemical properties. The in situ techniques demonstrated in this study can also be extended to many other material transition systems, which offer a novel opportunity to get insightful understanding of 2D materials and develop ideal materials with excellent performance.

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DOI: 10.1021/acs.chemmater.7b01291 Chem. Mater. 2017, 29, 4572−4579

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DOI: 10.1021/acs.chemmater.7b01291 Chem. Mater. 2017, 29, 4572−4579