Electron Beams Transform Amorphous Cobalt Hydroxide Nanosheets

The study demonstrated that continuous electron-beam irradiation can induce the crystallization and phase transition of amorphous cobalt hydroxide nanosheets into crystalline cobalt oxides. The team observed that under specific irradiation conditions, the initially amorphous nanosheets partially converted into cobalt oxides, with the crystallized regions expanding as the duration of irradiation increased.

The investigation focused on cobalt hydroxide nanosheets synthesized through a specific aqueous nutrient solution method. The synthesis involved mixing cobalt nitrate hexahydrate and hexamethylenetetramine, followed by the addition of sodium hexadecyl sulfate at the water-air interface. The resulting sheets were incubated in a convection oven at temperatures of 60 degrees Celsius and 80 degrees Celsius for 180 minutes each. The researchers selected nanosheets synthesized at 80 degrees Celsius for the primary electron-beam experiments due to their higher synthesis reproducibility and consistent thickness.

Before exposure to the electron beam, the synthesized nanosheets were characterized using scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The SEM images revealed that the nanosheets formed continuous sheets through the accumulation of triangular islands, particularly at the 60-degree Celsius synthesis temperature. At 80 degrees Celsius, the sheets exhibited a more continuous arrangement of approximately round structures. X-ray photoelectron spectroscopy of the O1s spectra showed that the chemical bonding states consisted mainly of cobalt hydroxide and cobalt oxide. The intensity of the hydroxyl peak was the highest, and the C = O ratio appeared to be relatively high, which was attributed to higher residue of the precursor used in synthesizing the nanosheets owing to the relatively high reaction temperature.

The core of the study involved analyzing the crystal structure and phase transitions using a transmission electron microscope operated at an accelerating voltage of 300 kilovolts. The specimens were placed on Quantifoil TEM grids for imaging. The electron beam dose rate was maintained at 4 times 10 to the power of 4 electrons per square angstrom per second.

High-resolution TEM imaging revealed that the pristine synthesized nanosheets were indeed amorphous, as evidenced by the absence of precision spots or ring patterns in the digital diffractograms. However, after continuous irradiation with the electron beam, a partially crystallized region began to appear. The accompanying digital diffractogram showed distinct spot and ring patterns, confirming the emergence of crystalline order. The researchers noted that the longer the electron-beam irradiation period, the higher the crystallinity of the material.

Further inspection of the crystallized regions using successive HR-TEM images revealed distinct phases within the partial crystallized area. The digital diffractograms of these regions suggested phase transitions from amorphous to crystalline structures caused by the electron beam. The synthesized nanosheet, originally composed of amorphous cobalt hydroxide, transformed into cobalt oxides. The study identified cobalt monoxide (CoO) as a primary product of this transformation. CoO is known to exhibit a cubic phase, specifically a rock salt structure. The crystal structures observed in the top and bottom regions of the HR-TEM images were attributed to CoO, with specific lattice planes identified. The white line in the images indicated the (111) plane, while the yellow line indicated the (200) plane, providing structural confirmation of the cubic phase.

The researchers observed that the cubic phase formed at the bottom of the imaged area was oriented to the (200) plane, as demonstrated by successive images of the same region. An enlarged image of the d-spacing showed values of 0.24 nanometers and 0.21 nanometers, which are characteristic of the CoO phase. These measurements provided quantitative evidence of the lattice structure of the newly formed cobalt oxide. The expansion of the crystallized region over time indicated a dynamic process where the phase boundary moved as more of the amorphous material converted to the crystalline oxide.

The study also highlighted the dominance of the cubic phase in the transitioned material. The areas marked with colored dashed lines in the HR-TEM images indicated several regions of cubic cobalt oxides. Because the transitioned cubic phase was nanosized, the digital diffractograms showed an increase in the number of spots due to the different orientations of these nanocrystals within the random area.

The findings suggest that electron-beam irradiation can be used to accelerate and realize phase transitions in cobalt oxides with various stoichiometric compositions. The study demonstrated that the transition to cubic cobalt oxides was dominant under the 300 kilovolt accelerating voltage used in the experiment. This behavior is consistent with the known stability of cubic cobalt oxides, such as Co3O4 and CoO, which are widely used in batteries, gas sensing, and catalysis.

The research contributes to the broader understanding of how electron beams interact with amorphous metal hydroxides. The observed transitional behavior suggests that amorphous cobalt hydroxide nanosheets are susceptible to structural changes under electron irradiation.

The study was conducted by a team including Minjeong Lee, Hye Seong Jang, Gayoung Yoon, and Gyeong Hee Ryu. The research was published in Applied Microscopy in March 2025, appearing as Volume 55, article number 2. The work is available as open access. The transmission electron microscopy analyses were conducted at the Center for Research Facilities, Gyeongsang National University.

Cobalt oxides and hydroxides have attracted significant research interest due to their exceptional chemical and physical properties, which make them promising candidates for various applications. These materials are widely used in batteries, gas sensing, and catalytic processes, where their structural stability and reactivity play crucial roles. Among the different phases, cobalt hydroxide nanosheets exhibit excellent activity and durability when in an amorphous state, a characteristic that distinguishes them from their crystalline counterparts. Additionally, these nanosheets possess good electrocatalytic properties, further enhancing their utility in energy-related technologies. The study builds on previous reports of real-time phase transitions in oxidizing and reducing atmospheres, extending the understanding of how external energy sources, such as electron beams, can drive these transformations on an atomic scale.

The aqueous nutrient solution contained 2 mM cobalt nitrate hexahydrate and 2 mM hexamethylenetetramine, with a calculated amount of a chloroform solution of sodium hexadecyl sulfate added at the water–air interface. The reaction was carried out at two different temperatures, 60 degrees Celsius and 80 degrees Celsius, for 180 minutes each, resulting in variations in the nanosheet structure. The higher temperature synthesis produced thicker sheets with higher reproducibility, which were selected for the primary electron-beam experiments.

The use of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) provided additional insights into the composition and distribution of elements within the nanosheets. The EDS mapping confirmed a uniform distribution of cobalt and oxygen throughout the nanosheet, indicating a homogeneous chemical composition prior to irradiation.

Subject of Research: Electron-beam induced phase transition of amorphous cobalt hydroxide nanosheets to crystalline cobalt oxides

Article Title: Phase transition of amorphous cobalt hydroxide to crystalline cobalt oxides by electron-beam irradiation

Article References: Lee, M., Jang, H. S., Yoon, G., & Ryu, G. H. (2025). Phase transition of amorphous cobalt hydroxide to crystalline cobalt oxides by electron-beam irradiation. Applied Microscopy, 55(1), Article 2. https://doi.org/10.1186/s42649-025-00107-5

Image Credits: AI Generated

DOI: 10.1186/s42649-025-00107-5

Keywords: cobalt hydroxide, electron beam, phase transition, nanosheets, crystallization, TEM, materials science, cobalt oxide, Phase, transition, amorphous, cobalt