Daewon Lee1,2,Sophia Betzler1,2,Mark Asta1,2,Haimei Zheng1,2
Lawrence Berkeley National Laboratory1,University of California2
Daewon Lee1,2,Sophia Betzler1,2,Mark Asta1,2,Haimei Zheng1,2
Lawrence Berkeley National Laboratory1,University of California2
Hydrogen-induced phase transformations in palladium (Pd) is a topic that has attracted significant interest for many years. The palladium hydride (PdH<i><sub>x</sub></i>) system consists of a hydrogen-poor α phase and a hydrogen-rich β phase. The α to β transformation involves the large Pd lattice expansion and thus causes specific interfacial structures to accommodate the lattice mismatch strain. Although the bulk PdH<i><sub>x</sub></i> phase transformations are well-understood, the phase transformations of Pd nanocrystals have recently started to be explained due to advances in in-situ characterization techniques. In-situ environmental transmission electron microscopy (ETEM) and X-ray imaging demonstrated the corner nucleation and propagation of the β phase during absorption of Pd nanocrystals. However, there have been no direct atomic-scale insights into the phase boundaries during the dynamic hydrogen-induced phase transformations in Pd nanocrystals.<br/><br/>In this work, we use the in-situ liquid cell TEM to visualize the hydrogen-induced phase transformations in Pd nanocrystals at the atomic level. We synthesize Pd nanocubes and load them with an aqueous solution into a liquid cell. H<sub>2</sub> gas is generated by the electron-beam radiolysis of water in liquid cells. We image the hydrogen-induced phase transformations in individual Pd nanocubes at the atomic level. During the H<sub>2</sub> absorption, a unique interface is formed. The interface boundary is broadened, and there is a lattice tilt within the boundary. No dislocation is observed. However, during the H<sub>2</sub> desorption, the atomically sharp interface is found with dislocations identified at the interface. We also systematically study the particle size effects on hydrogen-induced phase transformations in Pd nanocubes. Variations in the interfaces suggesting differences in lattice strain relaxation mechanisms are revealed. This research expands our fundamental understanding of the solute-induced phase transformations, and assists the future design of advanced materials for hydrogen storage or other applications.<br/><br/><b>Acknowledgement</b>: This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (BES), Materials Sciences and Engineering Division under Contract No. DE-AC02-05-CH11231 within the in-situ TEM program (KC22ZH). Work at the Molecular Foundry of Lawrence Berkeley National Laboratory (LBNL) was supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. S.B.B. acknowledges financial support from the Alexander-von-Humbold Association. D.L. acknowledges the Kwanjeong Study Abroad Scholarship from the KEF (Kwanjeong Educational Foundation) (KEF-2019).