H 2 adsorption mechanisms on Ti-Zr-V-Cu non-evaporable getter: surface orientation effect

Abstract Maintaining ultra-high vacuum (UHV) conditions is essential for the reliable operation of modern particle accelerators. Non-evaporable getter (NEG) films have been widely used in accelerator vacuum chambers due to their effective hydrogen pumping capability. However, limitations such as relatively high activation temperature, high secondary electron yield (SEY), and high surface resistivity have motivated the development of modified NEG films, such as Ti-Zr-V-Cu and Ti-Zr-V-Hf-Cu. Nevertheless, despite these developments, no density functional theory (DFT) studies have so far explored the interaction of such modified NEG films with active gases at the atomic scale. In this study, first-principles DFT calculations were performed on the Ti-Zr-V-Cu (100) and (001) surfaces to explore hydrogen adsorption mechanisms. Results show that the overall adsorption energies were found to be more negative on the (001) surface than on the (100) surface, indicating a stronger interaction with hydrogen and suggesting that surface orientation plays a key role in adsorption efficiency. Additionally, electronic structure analyses including Mulliken, Hirshfeld, electron density difference (EDD), and partial density of states (PDOS) indicate that dissociative adsorption occurs at bridge and hollow sites on both surfaces with strong orbital interactions, while molecular adsorption is retained at top sites. While Cu-containing sites generally show weaker hydrogen adsorption than Cu-free ones, several mixed-metal configurations involving Cu still demonstrate sufficient adsorption energies, suggesting that the impact of Cu is moderate and site-specific rather than broadly detrimental.