(376p) First-Principles Study on Unexpectedly Large Lattice Expansion of PdHx Nanoparticles Generated in Situ in TEM Graphene Liquid Cell

Palladium hydride (PdH_x) nanoparticles(NP) are synthesized in situ in graphene liquid cell under transmission electron microscopy (TEM). During the synthesis, radiolysis of water by the electron beam from TEM produces hydrogen radicals that interact with Pd ions to form PdH_x. In the course of PdH_x NP growth, unexpectedly large lattice expansion is observed, which greatly deviates from what is previously studied from bulk PdH_x. Here, we demonstrate the reason for the unexpected lattice expansion using first-principles density functional theory (DFT) calculations. Firstly, thermodynamic stabilities of cuboctaheral and icosahedral PdH_x NP at different compositions with various hydrogen positions are studied via Energy Convex Hull diagram. Then, hydrogen behaviors on (111) and (100) Pd surfaces are analyzed by calculating adsorption energies in different conditions. Lastly, Nudged Elastic Band (NEB) calculation is utilized to investigate the activation barriers for hydrogen insertions through particular directions. Our results strongly suggest that metastability of PdH_x NP structures and asymmetric hydrogen adsorption are the main keys responsible for the lattice expansion.