Recently, the research team led by Pan Liu from the School of Materials Science and Engineering at Shanghai Jiao Tong University, in collaboration with partners, published a research paper in the international top-tier journal Matter titled “Atomic-scale dynamics of a bulk-diffusion-driven nonconservative phase transformation in vapor-phase dealloying.” This study reports, for the first time, the real-time observation of a bulk-diffusion-driven nonconservative phase transformation process in vapor-phase dealloying at the atomic scale. This challenges the conventional surface diffusion theory that has persisted for decades and opens up new pathways for the design of functional nanoporous materials.
Article link: https://doi.org/10.1016/j.matt.2025.102601
The first author is Dr. Xinyao Wang from Shanghai Jiao Tong University. The co-corresponding authors are Assoc. Prof. Pan Liu from Shanghai Jiao Tong University, Prof. Chen Mingwei from Southern University of Science and Technology, and Prof. Yi Gao from Shanghai Advanced Research Institute, Chinese Academy of Sciences. The research work also received support from collaborators, including Dr. Lei Ying from Shanghai Institute of Applied Physics, Assoc. Prof. Qing Chen from Hong Kong University of Science and Technology, Prof. Jiuhui Han from Southern University of Science and Technology, Prof. Fuqiang Huang from Shanghai Jiao Tong University, and Prof. Yuqiao Zeng from Southeast University. The project was funded by the National Natural Science Foundation of China, the Shanghai Natural Science Foundation, and others.
Main Research Content:
Dealloying is a classic method for preparing nanoporous metals through selective dissolution and is widely used in fields such as catalysis and sensing. For a long time, it was commonly believed in the academic community that dealloyed structures form solely via self-assembly driven by surface diffusion. This theory has struggled to explain the intermediate phase evolution and morphology development associated with solid-state phase transformations observed in experiments. Due to the limitations of traditional characterization techniques in achieving atomic-scale in-situ observation, key experimental evidence for understanding the solid-state phase transformation mechanisms in dealloying has been lacking.
The research team innovatively combined vapor-phase dealloying with in-situ heating techniques in a transmission electron microscope (TEM). Using γ-CoZn alloy as the precursor, they achieved real-time atomic-scale observation of the phase transformation dynamics during the dealloying process under conditions of 400 °C and 10-5 Pa.
The study shows that initial Zn sublimation leaves a substantial number of vacancies in the γ-CoZn precursor, inducing the formation of a metastable vacancy-rich structure (γ-CoZn-vac). Its lattice framework is similar to the parent γ-CoZn phase, but its long-range order characteristics are significantly reduced. Simulated HRTEM images also confirmed that the increase in vacancy concentration leads to a gradual weakening of the superlattice diffraction intensity.
Through mutual corroboration of HAADF-STEM characterization and DFT calculations, the bulk diffusion pathway of Zn atoms during vapor-phase dealloying was established. Specifically, Zn1 atoms in the γ-CoZn precursor preferentially form vacancies, and the Zn2→Zn2 pathway (with an energy barrier of only ~0.29 eV) dominates the migration of Zn atoms from the bulk to the surface.
In-situ HRTEM observations further indicated that as Zn atoms continuously sublimate, the γ-CoZn-vac transition structure undergoes lattice rearrangement due to the increasing concentration of Zn vacancies, transforming into the α-Co phase (the final dealloying product). This transformation manifests as a synergistic process of atomic column merging and lattice relaxation. This reconstruction tends to occur on crystal planes where the change in interplanar spacing between the parent and product phases is minimal, thereby minimizing the energy barrier.
The atomic-scale analysis of the dealloying-induced solid-state phase transformation reported in this paper provides kinetic insights into broader nonconservative solid-state phase transformation processes (such as the Kirkendall effect, solid-state ion transport, etc.), emphasizing the dominant role of vacancy-driven bulk diffusion. The identification of the vacancy-rich transition structure inspires new strategies for materials synthesis, namely driving phase transformations by controlling vacancy concentration (i.e., “vacancy engineering”). More importantly, the dealloying-induced solid-state phase transformation mechanism revealed in this study can also offer a reverse perspective for understanding the thermodynamics and kinetics of solid-state alloying processes (such as the formation of intermetallic compounds).
Figure 1 Schematic diagram of the in-situ vapor-phase dealloying experimental setup within the (S)TEM.
Figure 2 Transition structure (γ-CoZn-vac) formed from the γ-CoZn precursor during vapor-phase dealloying.
Figure 3 STEM characterization showing the evolution from the γ-CoZn precursor to the γ-CoZn-vac transition structure.
Figure 4 In-situ HRTEM characterization of the phase transformation process regulated by bulk diffusion and atomic column merging at 400 °C.