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Real-time observation of phase transition from layered to spinel phase under electron beam irradiation

Journal of Analytical Science and Technology volume 14, Article number: 31 (2023) Cite this article

Abstract

Transition metal hydroxides (TM(OH)2) have drawn significant attention for their potential applications in the environmental and energy field. In this study, we investigated the phase transition of cobalt hydroxide (Co(OH)2) under electron beam irradiation using in situ high-resolution transmission electron microscopy. Both the α- and β-Co(OH)2 phases underwent a phase transition to a spinel Co3O4 phase, forming nanograins with a porous morphology. However, the α-Co(OH)2 showed a faster phase transition and dramatic volume shrinkage during the phase transition. Our results provide a detailed explanation of the mechanism behind the deterioration of the layered structure and the emergence of defects during the phase transition.

Introduction

Transition metal hydroxides (TM(OH)2) have been extensively studied for various applications in the environmental and energy fields, including for water splitting, catalysts, supercapacitors and batteries (Jia et al. 2017; Kim et al. 2018; Ma et al. 2016; Ye et al. 2018) . Recently, thanks to the great interest in lithium-ion batteries, TM(OH)2, used as precursors in cathode active materials, has attracted wide attention (Amou et al. 2021; Park et al. 2018; Qian et al. 2020) . Among the various types of TM(OH)2, cobalt hydroxides (Co(OH)2) have two distinctive phases: α- and β-Co(OH)2 phase (Jayashree and Kamath 1999; Liu et al. 2005; Ma et al. 2006). β-Co(OH)2 adopts the structure of the mineral brucite, in which the hexagonal packing of hydroxyl ions coordinated with divalent Co(II) cations occupies alternate rows of octahedral sites. On the other hand, α-Co(OH)2 is an isomorph-layered double hydroxide with a hydrotalcite-like structure consisting of positively charged brucite-like layers and negatively charged anions in the hydrated interlayer regions. The hydrotalcite-like structure is generally represented as \({\left[{M}_{1-x}^{II}{M}_{x}^{III}{\left(\mathrm{OH}\right)}_{2}\right]}^{x+}{({A}^{n-})}_{x/n}\cdot m{H}_{2}O\) where \({M}_{1-x}^{II}\) are divalent cations, \({M}_{x}^{III}\) are trivalent cations, \({A}^{n-}\) are interlayer anions with negative charge n, and m is the number of water molecules in the interlayer region. Since the anions and water molecules are intercalated in the interlayer region, α-Co(OH)2 has a larger interlayer spacing (7–8 Å) than β-Co(OH)2 (4.6 Å) (Liu et al. 2005).

Electron irradiation offers the most basic form of in situ experiment in the transmission electron microscope (TEM) system. The accelerated electrons (typically 200–300 kV) trigger structural changes in local areas, which can be continuously monitored in real time. The electron irradiation typically causes crystallization/amorphization or phase changes from one crystalline phase to another, depending on the experimental conditions and the materials (Ishimaru et al. 2003; Jiang et al. 2019; Parajuli et al. 2020; Yao et al. 2014). These electron beam effects have typically been treated as electron beam damage, to be mostly avoided; however, sometimes the artificial phase transitions induced by electron beam have been studied to better understand the complex mechanism of the phase change in a real environment.

For example, Jing et al. observed direct crystallization and amorphization in GeSb2Te4 thin films under electron beam irradiation, where the melting process was absent. The authors claimed that the nonthermal amorphization contributed to the knock-on collision effect of the electron beam (Jiang et al. 2019). In addition, the phase transition induced by the electron beam irradiation provided crucial information about the structural changes in the actual electrochemical reactions. Parajuli et al. investigated the electron beam-induced spinel to defective rock-salt phase transition in MgCrMnO4 and found that the emergence of oxygen vacancies reduced the difference in free energy of the two phases, overcoming the kinetic barrier and facilitating cation migration, which eventually led to a defective rock-salt phase transition (Parajuli et al. 2020).

While the α- and β-Co(OH)2 are not typically used as electrode materials in lithium-ion batteries, they have a similar layered structure to materials that are commonly used as cathodes, such as LiCoO2 and LiNi1-x-yCoxMnyO2 (NCM). The phase changes and defect formations that occur in the reaction or degradation process of these cathode materials can also be observed in Co(OH)2. In this study, we directly observed the phase transition from a layered structure to a spinel structure in Co(OH)2 under electron beam irradiation. Both the α- and β-Co(OH)2 transformed into nanograin spinel Co3O4 during the electron beam irradiation. The transition from the layered phase to the spinel phase was accompanied by enormous structural changes, including significant volume shrinkage and many defects.

Experimental methods

Synthesis of α-Co(OH)2 and β-Co(OH)2 particles

The α-Co(OH)2 and β-Co(OH)2 particles were synthesized under ambient conditions according to a previous work (Park et al. 2022b). For α-Co(OH)2, cobalt chloride hexahydrate (10 mM, CoCl2·6H2O, Sigma-Aldrich), sodium chloride (50 mM, NaCl, Oriental Chemical Industry), hexamethylenetetramine (60 mM, C6H12N4, 99.0%, Sigma-Aldrich) were dissolved in 150 mL of a 1:9 mixture of ethanol and deionized water. The solution was heated in an oil bath at 90 ℃ for one hour, resulting in a suspension with green particles. Then, the suspended product was centrifuged at 3500 rpm for 15 min and washed several times with deionized water and anhydrous ethanol. The final green particles were fully dried at room temperature. The synthesis procedure of β-Co(OH)2 was the same as that of α-Co(OH)2; however, NaCl was not included in the initial solution. For β-Co(OH)2, pink powders were obtained.

Electron microscopy

Scanning electron microscope (SEM) images were taken using a ZEISS SIGMA 300 VP. The morphology of α- and β-Co(OH)2 particles was studied through scanning electron microscopy (SEM). Transmission electron microscopy (TEM) experiments were performed on a FEI Tecnai F30 operated at the accelerated voltage of 300 kV. The dose rate for the electron beam irradiation was 2 × 104 e−/Å2⋅s, and the sample drift during the irradiation was corrected by manual operation. The Co(OH)2 powders were well dispersed in ethanol and dropped onto a lacey carbon grid for the TEM observation. The cross-sectional TEM specimens were prepared by focused ion beam (FIB), following the typical TEM specimen preparation method for 2D materials using a Quanta 3D FEG (Vilá et al. 2016; Yang et al. 2019). A carbon and Pt layer were deposited to protect the Co(OH)2 sample during the FIB sampling and milling process. The damage caused by the Ga ion beam of 30 kV was removed with a low voltage of 5 kV.

Results and discussion

Figure 1a, b shows the SEM images of α-Co(OH)2 and β-Co(OH)2, respectively. In the α-Co(OH)2 sample, two shapes can be observed, hexagonal microplates and the flower-shaped α-Co(OH)2. The hexagonal microplate contains a typical 2D structure, while the flower-shaped sample is formed by the aggregation of several hexagonal microplates, resulting in a 3D structure. The lateral size of the α-Co(OH)2 varies from 1 to 6 µm, depending on the shapes; the flower-shaped α-Co(OH)2 tends to have a smaller size compared to the hexagonal microplates (Li et al. 2021). However, both the microplates and flower-shaped structure have the same crystal structure of α-Co(OH)2 as observed by X-ray diffraction in Fig. 1c. On the other hand, β-Co(OH)2 has a rather uniform shape and size. The β-Co(OH)2 sample shows hexagonal microplates, and the average lateral size is 6 µm. The X-ray diffraction (XRD) patterns in Fig. 1c show that the XRD results of α-Co(OH)2 and β-Co(OH)2 agree well with the previous reported data, and both phases were synthesized with high quality without any other phases (Liu et al. 2005). The interlayer spacings for α-Co(OH)2 have been reported to vary depending on the different types of intercalated anions and their concentrations (Hu et al. 2009; Ismail et al. 1995; Rajamathi et al. 2000; Tian et al. 2011). The interlayer spacing in our α-Co(OH)2 was determined in the XRD to be as 8.03 Å. The schematics of the atomic structures of α- and β-Co(OH)2 are displayed in Fig. 1d.

Fig. 1
figure 1

SEM images of a α-Co(OH)2 and b β-Co(OH)2 microplates, c XRD patterns of α- and β-Co(OH)2. d Schematic of atomic structure of α-Co(OH)2 and β-Co(OH)2

Full size image

Figures 2 and 3 show a time series of high-resolution TEM images and the corresponding fast Fourier transform (FFT) patterns of α-Co(OH)2 and β-Co(OH)2 under electron beam irradiation, respectively. The TEM data were captured continuously, and specific data were then selected. The high-resolution images show some changes in contrast over time. The differences are more clearly observed in FFT patterns, as shown in Fig. 2a2–d2 and Fig. 3a2–d2. Initially, the FFT pattern shows the typical diffractograms of α-Co(OH)2 and β-Co(OH)2 observed from the [001] zone axis, respectively. However, in the case of α-Co(OH)2, there are some hazy intensities near the diffraction peaks, forming elongated peaks along the outer direction as shown in Fig. 2a2 ({110} peaks indicated by the cyan arrow). These extended signals indicate that some regions where the electron beam was irradiated already transformed into cobalt oxyhydroxide (CoOOH) phase during the first image acquisition time (~ 0.13 s). It is worth noting that CoOOH contains the same octahedral cobalt framework as Co(OH)2 but with a slightly smaller lattice parameter a. Consequently, the peaks of CoOOH appear near those of Co(OH)2, but in the outer region of the diffraction space (Deliens and Goethals 1973). Hydrogen atoms can be easily kicked out by electron beam irradiation, resulting in a rapid phase transition into CoOOH. In contrast, β-Co(OH)2 in Fig. 3a2 shows sharp peaks at the first image acquisition time, suggesting that β-Co(OH)2 is more resistant to electron beam irradiation compared to α-Co(OH)2. After some electron beam irradiation, both α-Co(OH)2 and β-Co(OH)2 show a CoOOH phase transition, as shown in Figs. 2b2 and 3b2.

Fig. 2
figure 2

Time series of high-resolution TEM images (a1–d1) and corresponding fast Fourier transform patterns (a2–d2) of α-Co(OH)2 during the e-beam irradiation. The data were selected for 0 s, 30 s, 120 s and 300 s. Scale bar is 10 nm. FFT data are rotated and magnified for a clear observation

Full size image
Fig. 3
figure 3

Time series of high-resolution TEM images (a1–d1) and corresponding fast Fourier transform patterns (a2–d2) of β-Co(OH)2 during the electron beam irradiation. The data were selected for 0 s, 30 s, 120 s and 300 s. Scale bar is 10 nm. FFT data are rotated and magnified for a clear observation

Full size image

After 30 s of electron beam irradiation, both the α-Co(OH)2 and β-Co(OH)2 show drastic changes. The structural changes could be clearly observed in the FFT patterns: New peaks from the spinel Co3O4 phase emerge, forming a new hexagon pattern which is rotated by 30° from the initial {100} peaks of the Co(OH)2 (Figs. 2b2 and 3b2). Although both the α-Co(OH)2 and β-Co(OH)2 show the same phase transition to the spinel Co3O4, the detailed phase evolution is different. In α-Co(OH)2, the newly emerged spinel Co3O4 peaks appear strong after 30 s of electron beam irradiation, as displayed in Fig. 2b2. However, in β-Co(OH)2, the same spinel Co3O4 peaks emerge with relatively weak intensity, as shown in Fig. 3b2, suggesting the fact that the phase transition from Co(OH)2 to spinel occurred faster in α-Co(OH)2 than in β-Co(OH)2. Indeed, the electron beam interacted more sensitively with α-Co(OH)2, causing a rapid decrease in its crystallinity and inducing a faster phase transition upon exposure to the electron beam. Further electron beam irradiation resulted in the diminishing of Co(OH)2 and CoOOH phases; eventually, the spinel Co3O4 phase dominated most of the irradiated area in both α-Co(OH)2 than in β-Co(OH)2, as shown in Figs. 2c2 and 3c2. However, some CoOOH peaks remained after 300 s of irradiation, as shown in Figs. 2d2 and 3d2 (indicated by cyan arrow).

To examine the phase transition of Co(OH)2 in more detail, cross-sectional samples are investigated. Figure 4 displays cross-sectional HR-TEM images of the Co(OH)2 under electron beam irradiation. As the electron beam irradiated the sample, the layered structure began to show some local degradation, in which the atomic structure appeared to deviate from the typical layered structure, showing some disordered phases, as in Fig. 4b. The disordered regions were approximately a few nanometers in size. In addition, the layers broke into smaller grains and some of the layers were tilted or bent depending on the location. Further electron beam irradiation resulted in roughening of the surface and the overall surface moved downward. The total thickness of α-Co(OH)2 shrank significantly after the electron beam irradiation (Additional file 1), which might be due to the evaporation of water molecules between the layers. The electron beam irradiation eventually led to the formation of spinel phases, as displayed in Fig. 4e, f. The d-spacing values are consistent with the spinel Co3O4 phase. The spinel phase exhibited a polycrystalline FFT comprising of nanograins, in contrast to the single-crystalline appearance observed in the top view FFT (Figs. 2 and 3). The inconsistency between the results from the cross section and the plane-view sample could be attributed to the tilting of the nano-sized grains of spinel during the electron beam irradiation. Distinguishing the tilted nanograins in a specific direction only from the top-view diffraction pattern is difficult, as the tilted diffraction overlaps with other patterns. For example, if the spinel nanograins were tilted with respect to the [112]spinel (zone axis for cross-sectional sample), the rotation of nanograins would not be observable from the [111]spinel since the FFT pattern of the tilted nanograins overlapped those of the original spinel phase. Thus the FFT pattern for the [111]spinel zone axis showed a single-crystalline feature in spite of the rotation of the nanograins (see Additional file 1 for details). The size of the nanograins is expected to be ~ 5 nm based on the length of the discontinuity observed in Fig. 4c. It appears that an orientational relationship exists between Co(OH)2 and spinel Co3O4 in the FFT pattern observed from the top view, such as [001]Co(OH)2 || [111]Co3O4 and (110)Co(OH)2 || (\(\overline{1 }\)10)Co3O4. However, as the spinel Co3O4 nanograins form during the phase transition, the tilting of these nanograins becomes evident when observed in the cross-sectional view. Consequently, the previously observed orientational relationship in the top view is no longer valid.

Fig. 4
figure 4

High resolution of cross-sectional TEM images of α-Co(OH)2 after electron beam irradiation for a 0 s, b 15 s, c 30 s, d 60 s and e 80 s. f Total FFT pattern of cross section of α-Co(OH)2. Scale bar is 5 nm

Full size image

After a long time of electron beam irradiation during which the phase change no longer occurred, the final structure was further investigated. Figure 5 shows the high-resolution TEM and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image of α-Co(OH)2 after electron beam irradiation. Both TEM and HAADF-STEM images showed contrast variation with granular features, indicating that the converted spinel Co3O4 contained a porous morphology. The size of the contrast variation varied but was comparable to the size of nanograins shown in Fig. 4c. It is worth noting that β-Co(OH)2 also showed similar contrast variation, indicating that the morphological change did not involve the extraction of water molecules. The porous morphology is a common feature shared by both α-Co(OH)2 and β-Co(OH)2.

Fig. 5
figure 5

a High-resolution TEM image of α-Co(OH)2 and b HAADF-STEM image of α-Co(OH)2 after the phase transition to spinel Co3O4 was complete

Full size image

The phase changes that occur when Co(OH)2 is exposed to electron beam irradiation are similar to the thermal decomposition that occurs during the synthesis of nickel-rich cathode materials with LiOH (Park et al. 2022a). According to Park et al. (2022a), the formation of core–shell-shaped intermediate microstructures depends on the availability of lithium: When the transition metal precursor and lithium precursor can easily come into contact, well-formed nickel-rich-layered structures form at the interface (the lithiation process). However, when there is no lithium precursor present, the thermal decomposition of the TM(OH)2 leads to the formation of porous structures in the particle core. In contrast to the actual synthesis process of layered cathode materials, electron beam irradiation in TEM does not involve the presence of lithium or heat (at least insignificant). However, the phase evolution that occurs under the electron beam irradiation is similar to the decomposition of TM(OH)2, and the structural defects that form, such as nanopores and cracks, are similar to the intragranular nanopores and intergranular voids that form during the synthesis process.

The phase transition in layered transition metal oxides often goes beyond the formation of the spinel phase. In particular, the layered transition metal oxides used as cathode materials in Li-ion batteries containing nickel can readily change into a rock-salt phase (Hua et al. 2020; Wang et al. 2021). However, a rock-salt phase was not observed in the phase transition of Co(OH)2 when examined using high-resolution TEM or electron diffraction. Both rock-salt cobalt monoxide (CoO, Fm\(\overline{3 }\)m) and spinel Co3O4 (Fd\(\overline{3 }\)m) are thermodynamically stable phases, and it has been reported that the transition between CoO and spinel Co3O4 is reversible (Kong et al. 2019). The rock-salt phase can be oxidized to the spinel phase by annealing at 240 °C under atmospheric pressure of air (Nam et al. 2010), while the reduction process from spinel to rock-salt phase requires a higher temperature of 400 °C under vacuum or very low oxygen partial pressure conditions (Chen et al. 2021; Jang et al. 2019). In the current study, the spinel phase does not undergo any further changes when subjected to a higher e-beam dose or a longer irradiation time. We believe that electron beam irradiation alone is not sufficient to trigger the reduction process from the spinel to the rock-salt phase and that the annealing is necessary for the phase transition to rock salt to occur.

Conclusions

The phase transition of Co(OH)2 from a layered to a spinel structure, induced by electron beam irradiation, was investigated with TEM. The morphology and structural changes were monitored in real time. Both α-Co(OH)2 and β-Co(OH)2 underwent a similar phase transition from the layered to the spinel Co3O4 structure under the electron beam irradiation. The transformed spinel Co3O4 consisted of nanograins with porous morphology. However, α-Co(OH)2 interacted more sensitively with the e-beam, inducing a faster phase transition and causing a significant volume shrinkage due to the extraction of water molecules. Our findings reveal a comprehensive process which breaks a layered structure and forms nanograins and defects during a phase transition, providing important insight into the phase transition between layered and spinel structures.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Acknowledgements

This research was supported by characterization platform for advanced materials funded by Korea Research Institute of Standards and Science (KRISS–2023–GP2023-0014). J.H.J. was supported by the Technology Innovation Program (20010542) funded by the MOTIE, Korea.

Funding

This work was supported by research fund of Korea Research Institute of Standards and Science.

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Authors and Affiliations

  1. Korea Research Institute of Standard and Science, Daejeon, 34113, Republic of Korea

    Jong Hyeok Seo, Yong-Il Kim & Ji-Hwan Kwon

  2. Korea University of Science and Technology, Daejeon, 34113, Republic of Korea

    Jong Hyeok Seo & Ji-Hwan Kwon

  3. Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan, 46241, Republic of Korea

    Joon Yong Park & Ki Min Nam

  4. Electron Microscopy Research Center, Korea Basic Science Institute, Daejeon, 34133, Republic of Korea

    Jae Hyuck Jang

  5. Graduate School of Analytical Science and Technoglogy, Chungnam National University, Daejeon, 34134, Republic of Korea

    Jae Hyuck Jang

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Contributions

J.H.K. conceived the project and designed the experiments; the samples were grown by K.M.N. and J.Y.P.; X-ray diffraction experiments have been carried out by Y.K.; TEM and SEM experiments have been carried out by J.H.S. and J.H.J.; data analysis has been done by J.H.K. and J.H.S.; the manuscript has been written by J.H.K. and J.H.S., collecting feedback from all the authors.

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Correspondence to Ji-Hwan Kwon.

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Seo, J.H., Park, J.Y., Kim, YI. et al. Real-time observation of phase transition from layered to spinel phase under electron beam irradiation. J Anal Sci Technol 14, 31 (2023). https://doi.org/10.1186/s40543-023-00395-0

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  • DOI: https://doi.org/10.1186/s40543-023-00395-0

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