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C: Physical Processes in Nanomaterials and Nanostructures

A Novel Route from a Wurtzite to a Rock-Salt Structure in CoO Nanocrystals: In Situ Transmission Electron Microscopy Study Kyu Yeon Jang, Sang Jung Ahn, Ji-Hwan Kwon, Ki Min Nam, and Young Heon Kim J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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The Journal of Physical Chemistry

A Novel Route from a Wurtzite to a Rock-Salt Structure in CoO Nanocrystals: In Situ Transmission Electron Microscopy Study Kyu Yeon Jang1, 2, Sang Jung Ahn1, 2, Ji-Hwan Kwon1, Ki Min Nam3* and Young Heon Kim4*

1Korea

Research Institute of Standard and Science, 267 Gajeong-ro, Yuseonggu, Daejeon 34113, Republic of Korea

2Korea

University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea

3Department

of Chemistry, Pusan National University, Busan 46241, Republic of Korea,

1

ACS Paragon Plus Environment

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4Graduate

School of Analytical Science and Technology (GRAST), Chungnam

National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea

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ABSTRACT

The phase transformation from a metastable wurtzite (WZ) CoO to stable rocksalt (RS) CoO nanocrystals via an intermediate spinel (SP) Co3O4 was examined based on in situ transmission electron microscopy (TEM). Specifically, the atomic evolution during the phase transformation was monitored using high-resolution TEM images of individual nanocrystals. A two-step phase transformation was identified during the heating experiment: rapid oxidation of WZ CoO to SP Co3O4 and reduction of SP Co3O4 to RS CoO. At a low temperature range from 200 C to 280 C, the oxidation reaction, which was the first phase transformation from WZ CoO to SP Co3O4, occurred spontaneously via remnant oxygen in the TEM column. On the other hand, the reduction reaction, which was the second phase transformation from SP Co3O4 to RS CoO at high temperatures over 280 C, occurred under a low oxygen partial pressure. The phase transformation phenomena

were

analyzed

from

thermodynamical

viewpoints. 3


and

microstructural


4


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1. Introduction Transition metal oxides are an important group of materials because they form a wide variety of structures and have numerous applications.1–3 In particular, cobalt oxides have attracted great attention because of their chemical stability and magnetic properties for potential in applications such as catalysts, magnetic data storage devices, lithium-ion battery materials, and solid-state sensors.4–8 In general, cobalt oxide typically crystallizes in two stable phases: cubic rock-salt CoO (RS CoO, space group Fm3m) with octahedral Co2+ ions and spinel Co3O4 (SP Co3O4, space group Fd-3m), in which Co2+ and Co3+ ions are tetrahedrally and octahedrally coordinated, respectively.9 In general, the relative stability of different crystallographic phases and possible phase transformations are of great interest in the field of materials chemistry. A non-native phase that is thermodynamically less stable is rarely observed in bulk forms. However, in nanomaterials, the surface energy exceeds the crystal formation energy, which stabilizes an unstable phase of the nanomaterial and

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dominates its physicochemical properties.10 For example, non-native cobalt oxide phases corresponding to cubic zinc blende (space group F43m) and hexagonal wurtzite (WZ, space group P63mc) structures have been observed in the nanometer range due to the lowering of the energy required for lattice distortion to the non-native phase on a nanometer scale.11 Recently, several research groups reported a pure phase of hexagonal WZ CoO by decomposition of Co(acac)2 and Co-oleate complexes.12–14 Comparison of the physical and chemical properties of the WZ CoO phase to those of the stable phases is a very interesting issue. For practical applications, the stability of WZ CoO has been investigated in detail. Liu et al. demonstrated the structural stability of the WZ CoO phase in terms of the WZ-to-RS structural transformation during hydrothermal coarsening of WZ CoO nanoparticles.15 Jiang et al. reported that the WZ CoO phase transformed into the RS CoO phase at 6.9 GPa, which they observed with in-situ high-pressure synchrotron radiation X-ray diffraction (XRD) measurements.16 Our group also studied the phase transition of WZ CoO

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to RS CoO and simultaneous oxidation to SP Co3O4. 9 Notably, the spontaneous phase transition of WZ CoO to Co(OH)2 suggests the possibility of efficient chemical bonding between Co(OH)2 and a substrate without binder materials, which would offer an extra degree of freedom in the design of conventional electrodes.5 In general, the reversible oxidation-reduction behavior of cobalt oxides is an important property for use in electrochemical water splitting and CO2 reduction reactions. Since cobalt oxide rapidly renews itself during catalytic reactions, the development of new analytical methods is needed to elucidate the oxidationreduction mechanism in an oxygen-deficient environment.17,18 Herein, we demonstrate the phase transformation from WZ CoO to RS CoO nanocrystals via an SP Co3O4 phase based on an in situ transmission electron microscopy (TEM) study. Specifically, high-resolution TEM images were used to monitor the atomic evolution during the phase transformation at individual nanocrystals. A two-step phase transformation was identified during heating

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experiments: rapid oxidation of WZ CoO to SP Co3O4 and reduction of SP Co3O4 to RS CoO. The results contribute to a thorough understanding of multiphase metal oxides, which can open a promising avenue for the discovery of entirely new physicochemical properties.

2. Experimental Materials. Co(acac)3 (99.99+%) and benzylamine (99%) were purchased from Sigma-Aldrich, and ethanol (99.5%) was purchased from Daejung Chemicals (Korea). Preparation of hexagonal WZ CoO nanocrystals. The CoO nanocrystals were synthesized via standard Schlenk line under an argon atmosphere. A slurry of Co(acac)3 (0.20 g, 0.56 mmol) and benzylamine (12.0 g, 112.0 mmol, 200 equiv) in a 100 mL flask connected to a bubbler was submerged in a preheated oil bath at 190 °C along with vigorous stirring. The reaction mixture was maintained at this temperature for 2 h, and the resulting green reaction mixture was cooled to room temperature. The suspension was centrifuged and the supernatant was removed. 8


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The residue was washed with ethanol (10 mL, 3 times) to provide green WZ CoO nanocrystals. The resulting nanocrystals were hexagonal pyramids with an average side edge length of 30 ± 3 nm and a basal edge length of 19 ± 3 nm. TEM heating stage. The WZ CoO nanocrystals were first dispersed in ethanol, and the suspension was dropped onto an observation area composed of a silicon nitride

membrane

on

a

microelectromechanical

system.

The

microelectromechanical system consisted of an electrode, a heating element (silicon carbide), a membrane window (silicon nitride), and a main silicon body. The ethanol was allowed to fully evaporate at room temperature. The morphologies of the WZ CoO nanocrystals were observed using TEM. In situ TEM heating experiments were carried out to observe the phase transition of the WZ CoO using a Fusion heating stage from Protochips Inc. (Raleigh, North Carolina). The initial shape, morphology, composition, and phase of the samples were observed before the heating experiments. After observation of initial conditions, the nanocrystals were heated from room temperature up to 400 °C at

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a heating rate of 10 °C/min. During heating, the beam was turned off to reduce damage to the sample. Changes in the WZ CoO crystals were observed and obtained as TEM images. All TEM experiments were carried out using a FEI Tecnai G2 F30 instrument at 300 kV. Ex situ phase transition of CoO. Hexagonal WZ CoO powders were loaded in an alumina boat in a box furnace and were annealed at 200 °C, 300 °C, and 400 °C for 20 min under an atmospheric pressure of N2, which caused the phase transformation from WZ to RS.

3. Results Thermal decomposition of Co(acac)3 in benzylamine under an inert atmosphere yielded WZ CoO nanocrystals.9 The XRD analysis indicated that the crystal structure of the as-prepared nanocrystals was a hexagonal WZ phase (Supporting

Information

Fig.

S1).

Furthermore,

the

WZ

CoO

phase

transformation was studied by thermal treatment at variable temperatures from 25 to 400 °C under N2. The progress of the phase transformation was monitored 10


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by XRD (SI Fig. S1), which indicated the conversion of WZ to RS CoO. However, it was difficult to understand the exact phase transformation process of the CoO nanocrystals using only this method. For real-time monitoring of the phase transformation of the CoO nanocrystals, an in situ TEM study was performed under continuous heating conditions. The morphological and microstructural properties of the prepared WZ CoO nanocrystals were carefully analyzed before the in situ heating experiments. Hexagonal pyramidal-shaped nanocrystals were observed on the TEM grid (Fig. 1 and Fig. S2(a), SI). A short base below the hexagonal pyramid appeared in a few nanocrystals (Fig. 1(a)). Observation of the shape of the nanocrystal bases more clearly indicated a hexagonal pyramidal shape. The average dimensions of the nanocrystals were 19 ± 3 nm basal edge length and 30 ± 3 nm side edge length. The HRTEM image in Fig. 1(d) of the atomic structure of the basal plane shows that the nanocrystal is surrounded by six facet planes. Although slight black-and-white contrast was observed in the HRTEM images, continuous lattice 11



planes were detected throughout the nanocrystals (Fig. 1(b) and 1(e)). Thus, we concluded that the individual nanocrystals were single crystalline in nature. Analyses of the HRTEM images and their fast Fourier transformations (FFTs) indicated that the nanocrystals were present in the WZ phase. The FFT diffractogram shown in Fig. 1(c) was taken from the HRTEM image in Fig. 1(b) and indexed along the [2110] direction of the WZ structure. The {0001} interplanar spacing in Fig. 1(b) was measured to be 5.24 Å, which is consistent with the caxis lattice constant of the CoO WZ structure (a = 3.21 Å, c = 5.24 Å).19 A streak along the [1011] direction was detected in the diffractogram in Fig. 1(c), which was caused by the side facet of the hexagonal pyramidal shape. This microstructural property of the nanocrystal was also proved by the atomic structure of the basal plane. The nearest six spots in the FFT diffractogram of the HRTEM image in Fig. 1(f) indicated a six-fold symmetry of the atomic arrangement with an interplanar spacing of 2.79 Å, which matched that of the {101 0} planes. By combining the analysis results of HRTEM images and

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diffractograms, it was confirmed that the side facets of the WZ CoO are the {101 1} planes. Thus, we concluded that the nanocrystals had a hexagonal pyramidal shape surrounded by {1011} planes as side facets and a (0001) basal plane. Furthermore, the short base had {1010} planes as side facets when observed under the pyramidal shape.

Figure 1. HRTEM images (a and d) and magnified HRTEM images (b and e) of as-prepared WZ CoO nanocrystals. Cross-sectional (a and b) and plan-view 13



images (d and e) of a hexagonal pyramid with a base. (c and f) FFT diffractograms of HRTEM images (a) and (d), respectively. The diffractograms in (c) and (d) were indexed along the [2110] and [0001] directions of the WZ structure of CoO, respectively.

The morphological and microstructural evolutions of the nanocrystals were observed as the temperature was increased. When the temperature reached 150 C, morphological modifications appeared on the corner and side edge of the nanocrystal, indicated by the dotted circles in Fig. 2(b). The contrast and continuity at the corner and side facet gradually changed. Upon further heating to 200 °C, the modified area expanded, and Moiré fringes were detected near the corner and side edges, indicated by the dotted circles and dashed square in Fig. 2(c). The contrast change began near the side edges and corners and expanded with increasing temperature. The uniformity of the contrast in the nanocrystal changed completely when the heating temperature reached 280 C (Fig. 2(d)).

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Minor surface and contrast changes were also observed at temperatures above 280 C (Figs. 2(e) and 2(f)). The bottom column in Fig. 2 shows the FFT diffractograms from the selected areas in the HRTEM images taken at room temperature, 280 C, and 400 C, indicated by red dotted squares in Figs. 2(a), 2(d), and 2(f), respectively. The room temperature diffractogram of the asprepared CoO nanocrystal was the same as that in Fig. 1(c), which indicated that the nanocrystal was crystallized to the WZ phase. The diffractogram taken at 280 C was completely different from that of the as-prepared WZ nanocrystal. Four diffraction spots, indicated by the dotted rectangle in Fig. 2(d), were observed at the same distance from the 000 spot as the nearest spot, and the angles between the spots were approximately 70 and 110. Hence, it was concluded that this pattern originated from a cubic lattice. Zinc-blende, SP, and RS phases with cubic lattices have been reported in cobalt oxides. To determine the phases that emerged at 280 C, the interplanar spacing reflected to the nearest spots in the diffractogram was calculated by considering a reciprocal space, which was close

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to 5.23 Å. From these analyses, the newly formed cobalt oxide phase in the WZ phase area was found to be Co3O4 in an SP phase with a stoichiometric change.

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Figure 2. HRTEM images and corresponding FFT results of the as-prepared WZ CoO nanocrystal during the heating experiment. (a) Room temperature, (b) 150 °C, (c) 200 °C, (d) 280 °C, (e) 340 °C, and (f) 400 °C. The FFT results in (g), (h), and (i) were taken from the dotted red squares in (a), (d), and (f) and indexed along the [2110] direction of the WZ structure of CoO, the [011] direction of the SP structure of Co3O4, and the [011] of the RS structure of CoO, respectively.

However, the SP Co3O4 phase transformed to another new phase upon increased heating temperatures. The spots from the {111} planes of the SP phase disappeared in the diffractogram in Fig. 2(h), and new strong spots located about twice as far as the (111) spots of the SP phase appeared at 400 C. The angle and distance relationships between the spots in the diffractogram in Fig. 2(i) are similar to those in Fig. 2(h), although the absolute distances are approximately doubled in Fig. 2(i). Therefore, it was deduced that the phase contributing to the diffraction pattern had a cubic lattice, with {111} interplanar spacings possibly

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close to one-half that of the SP {111} planes. The analysis results indicated that the as-prepared WZ CoO nanocrystal was finally transformed to an RS phase via the SP phase because of its cubic lattice and lattice parameter, ars = 4.26 Å, of half that of the SP phase, asp = 8.14 Å. Because the CoO nanocrystal experienced two consecutive phase transformations, the triangle shape changed slightly with temperature. Specifically, the corners became rounded, and the narrow base ultimately disappeared (SI Fig. S2). To better understand the evolution of the atomic structure during the phase transformation, detailed analyses using HRTEM images were conducted on the single nanocrystal. The changes in atomic structure originated at the corners of the CoO nanocrystal (Fig. 3(a) and 3(d)). The new atomic structure emerged at the bottom-left and top-right corners of the nanocrystal, indicated by dashed red squares. The periodicity of the lattice planes in the newly emerged phase along the WZ [0001] direction was much larger than that of the WZ area. In the large

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rhombus lattice in the inset of Fig. 3(b), the distance between the two sides was close to the {001} interplanar spacing of the WZ CoO (Figs. 3(b) and 3(e)).

Figure 3. HRTEM images (a and d) and magnified HR-TEM images (b and e) of the CoO nanocrystal showing the atomic structure during the phase transformation from WZ to SP at (a and b) 220 °C and (d and e) 260 °C. (b and e) Atomic structures in squares R1 and R2 in (a) and (d), respectively. (c and f) FFT diffractograms from HRTEM images (a) and (d), respectively. The two

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diffractograms in (c) and (f) were indexed along the same zone axis, the [011] direction of the SP CoO structure.

The atomic arrangements were continuous across the interface between the new phase and the WZ phase, indicated by the dashed line in Fig. 3(b), although it is impossible to clearly determine the atomic-scale interface. In addition, the observation of many Moiré fringes in the CoO nanocrystal indicated that it experienced atomic modifications on the bottom and/or top surfaces during the phase transformation (Figs. 3(a) and 3(d)). The diffractogram from the new phase in Fig. 3(c), taken from the dashed square, R1, in Fig. 3(a), was the same as that in Fig. 2(h). Thus, it was deduced that the phase transformation from WZ to SP began at approximately 220 C, a relatively low temperature compared to those reported by other research groups.15,16 The spots in the diffractogram in Fig. 3(e) taken from the dotted square, R2, in Fig. 3(d) had the same relationship as those in Fig. 3(c), although the diffraction spot from the {111} planes in Fig. 3(c) moved

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to a new position, indicated by the blue dotted circle. The {111} planes may function as mirror planes in R1 and R2. The analyses of Figs. 2 and 3 thus identified orientational relationships of [2110]wz//[011]sp and (0001)wz//(111)sp between the WZ and SP phases. After the appearance of the SP phase at approximately 200 C, the SP phase area expanded with increasing temperature up to 280 C. After the first phase transformation of WZ to SP, a second phase transformation process began with increasing temperature. Another novel phase emerged in the bottom-right corner, near the region R4 in Fig 4(a), when the heating temperature exceeded 280 C. The diffractograms from regions R3 and R4 in Fig. 4(a) at 320 C agreed well with those in Figs. 2(h) and 2(i), demonstrating that the phases in regions R3 and R4 were SP and RS, respectively. However, the SP phase (region R3) transformed to the RS phase when the temperature reached 400 C. The diffractogram from region R3 changed to the RS phase pattern, whereas the diffractogram from region R4 remained as the RS phase pattern during the

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heating process (Figs. 4(d), 4(e), and 4(f)). Finally, the whole area of the CoO nanocrystal was covered with the same RS phase. A lattice plane with a 2.43 Å interplanar spacing was observed throughout the crystal, which corresponds to the {111} interplanar spacing of the RS phase. This indicated that the SP phase formed by the first transformation in the temperature range of 200–280 C experienced a second transformation at temperatures above 280 C. Furthermore, orientational relationships of [110]sp//[110]rs and (111)sp//(111)rs were identified between the SP and RS phases by the HRTEM image and FFT diffractogram analyses (Figs. 2 and 4). Although the CoO nanocrystal experienced two consecutive phase transformations, the nanocrystal was still a single crystal, as continuous lattice planes were observed throughout the crystal.

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Figure 4. HRTEM images of the CoO nanocrystal showing the atomic structure during the phase transformation from SP to RS at (a) 320 °C and (d) 400 °C. (b, c, e, and f) FFT diffractograms from HRTEM images (a) and (d). The FFT results in (b and e) and (c and f) were taken from the red dotted squares R3 and R4 in (a) and (d), respectively. The diffractogram in (b) was indexed along the [011] direction of the SP structure of Co3O4 and (c, e, and f) along the [011] of the RS structure of CoO.

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4. Discussion It is possible to calculate lattice mismatches by considering the atomic arrangement at an interface. Because the orientational relationships between the WZ and SP phases was [2110]wz//[011]sp and (0001)wz//(111)sp, the effective lattice mismatch along the [2110] direction of the WZ and the [011] of the SP phases, f1, was calculated by considering the effective interplanar spacings of the WZ {2110} planes and the SP {044} planes as:

f1 = (dsp440-dwz2-1-10)/dwz2-1-10 = (1.44-1.61)/1.61 = -0.1056,

where dsp440 and dwz2-1-10 are the interplanar spacings of the (220) and (1010) planes of the SP and WZ phases, respectively. The effective lattice mismatch along the WZ [0001] direction and the SP [111] direction, f2, was calculated in the same fashion as f1:

f2 = (dsp111-dwz0001)/dwz0001 = (4.70-5.24)/5.24 = -0.1031,

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where dsp111 and dwz0001 are the interplanar spacings of the (111) and (0001) planes of the SP and WZ phases, respectively. The same logic can be used to calculate the lattice mismatches at the SP/RS interface. Because the interplanar spacings of the RS {111} and {220} planes (2.46 Å and 1.51 Å, respectively) were close to those of the SP {222} and {440} planes (2.35 Å and 1.44 Å, respectively), the lattice mismatch was approximated as 0.05. From the analyses of the lattice mismatches, it was deduced that the formation of the SP phase arose from the presence of tensile strain in the WZ phase, whereas that of the RS phase occured under the presence of compressive stress in the SP phase. The phase transformations upon increased temperatures in our experimental results involved redox chemical reactions as follows:

Oxidation reaction: 6 CoO(wz) + O2 → 2 Co3O4(sp)

(1)

Reduction reaction: 2 Co3O4(sp) → 6 CoO(rs) + O2

(2)

Total reaction: CoO(wz) → CoO(rs)

(3) 25



The spontaneity of reactions (1) and (2) depended on the reaction conditions of temperature, pressure, and environmental gas in our experiments because the heating was conducted in a TEM column. From the analysis of experimental results, we deduced that the remnant gas in the column played a key role in deciding the direction of the reactions. At the low temperature range from 200 to 280 C, the reaction was determined by the remnant oxygen, whereas the reduction reaction (oxygen removal) was dominant at high temperatures over 280 C under a low oxygen partial pressure. Reaction (1), the oxidation of CoO to Co3O4 related to the WZ-to-SP phase transformation, occurred spontaneously in the TEM experiment at a low temperature range. The Gibbs energy of the reaction is negative, calculated as -102,958 J/mol at 600 C by considering the Gibbs energy suggested by Chase et al.20 In addition, because the as-prepared CoO nanocrystals had a metastable WZ phase, the cubic RS CoO was energetically more stable than the hexagonal WZ CoO by 0.27 eV per pair, and

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oxidation is more favorable if there is a source of oxygen. We believe that the remnant oxygen in the TEM column was the source of the oxidation reaction. Since the content of oxygen in the atmosphere was approximately 21%, the oxygen partial pressure during the heating experiments was close to 10-11 ~ 1012

bar. On the other hand, at high temperatures over 280 C, reaction (2), the

reduction of Co3O4 to CoO related to the SP-to-RS phase transformation, occurred. The TEM column in our experiment was kept at a vacuum of approximately 10-10 bar. It has been reported that CoO is more stable than Co3O4 at low oxygen partial pressures.21 The emerging temperature of the RS phase in the area of the SP phase, approximately 280 C in our experiments, was lower than the temperature at which CoO was reported to be more stable than Co3O4, over approximately 441 C at an oxygen partial pressure of approximately 10-12 bar.21 The surface structures arising from the reduced size of the CoO nanocrystals may have led to this reduction from Co3O4 to CoO at a relatively low temperature under a low oxygen partial pressure.

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The oxygen sublattice in the WZ phase had a hexagonal-close-packed structure, whereas those in the other two other phases, SP and RS, had a facecentered cubic (fcc) closest-packed structure. As shown in Fig. 5, the fcc sublattice of oxygen in the SP and RS phases can be achieved by the movement of oxygen atoms on the in-plane of the {0001} planes, which is the basal plane of the WZ phase. The oxygen atoms on sites b and c in Fig. 5 moved to [-1/3, 1/3, 0, 0], and those on site a moved to [1/3, -1/3, 0, 0] on the {0001} planes for the fcc sublattice. Through these oxygen atom movements, it was also possible to achieve the atomic arrangement observed along the [111] direction in the fcc oxygen sublattice of the SP and RS phases from the hcp oxygen sublattice of the WZ phase. Rapid movement of oxygen atoms to other positions has previously been reported in cobalt oxide systems.9,22 The similarity of the anion sublattices in both SP Co3O4 and RS CoO has also been shown, where the oxygen sublattices in the two phases had fcc close-packed structures in which the O2- – O2- nearest-neighbor distances matched to within 5%.23,24 Based on the close

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anion structure, the epitaxial formation of SP Co3O4 on CoO (100) and the phase transformation from RS CoO to SP Co3O4 in nanocrystalline materials have been reported.9,23

Figure 5. Atomic structures of the WZ CoO, SP Co3O4, and RS CoO. Arrows in (b) and (c) indicate the in-plane shift of the oxygen atoms on the (222) and the (111) planes of the SP and RS structures, respectively, from on the (0001) planes of the WZ structure (The atomic structure was constructed using the program developed by Momma and Izumi.25) 29



Although the transformation direction in our experiments from SP Co3O4 to RS CoO was opposite that of transformations reported in the literature, we determined that the orientational relationship between SP Co3O4 and RS CoO was well-conserved during the phase transformation due to the close oxygen sublattices. Moreover, the existence of the SP phase with tetrahedral and octahedral sites (one-eighth and seven-eighths, respectively) as an intermediate state was favorable for the phase transformation from WZ with pure tetrahedral sites to RS with pure octahedral sites.

5. Conclusion The phase transformation from WZ CoO to RS CoO nanocrystals via an intermediate SP Co3O4 phase state was elucidated based on in situ TEM. The phase evolution process is a specific and unique phenomenon. The overall outline and single crystalline property of the nanocrystal were maintained during in situ heating experiments despite the surface modifications and slight volume 30


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shrinkage. This observation differed substantially from the results of ex-situ experiments, and the phase transformation processes were free of polycrystalline grains or nucleation-growth steps. A two-step phase transformation was identified during the heating experiment: rapid oxidation of WZ CoO to SP Co3O4 and reduction of SP Co3O4 to RS CoO. The orientational relationships of [211 0]wz//[011]sp//[011]rs and (0001)wz//(111)sp//(111)rs were conserved throughout the phase transformation process. This study suggests a possible route for the control of phase transformations of a single nanocrystal. Condition dependency caused by small variations in temperature, pressure, and environmental remnant gas must be considered to manipulate the microstructural properties of metastable nanocrystals.

ASSOCIATED CONTENT

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Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxx/xxxx

XRD patterns of thermally treated CoO nanoparticles; bright-field TEM images of CoO nanoparticles during heating experiment.

AUTHOR INFORMATION

Corresponding Author

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

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

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This work was supported by research fund of Chungnam National University. This work was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017R1E1A1A01074224).

ABBREVIATIONS TEM, transmission electron microscopy; HRTEM, high-resolution transmission electron microscopy; RS, rock salt; SP, spinel; WZ, wurtzite; XRD, X-ray diffraction.

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A Novel Route from a Wurtzite to a Rock-Salt ... - ACS Publications

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