Decoupling interface and thickness effects on hydrogen absorption in V/MgO: experiments and DFT

Abstract

We report combined experimental and first principles investigations of hydrogen absorption in epitaxial vanadium films on MgO(001) with nominal thicknesses of 10 nm and 50 nm. In - situ optical transmission and four - probe resistance isotherms show that the 50 nm film reproduces bulk like behavior with a clear first order alpha-beta hydride transition, the formation enthalpy and entropy gradually decrease with increasing hydrogen concentration. The 10 nm film, by contrast, displays continuous uptake without plateaus, with formation enthalpies H that are relatively close in magnitude to the 50 nm film (both exhibiting exothermic behavior in the range of approximately 0.5 to 0.3 eV/H), but with a more negative entropy change S (larger S) indicating reduced configurational freedom for hydrogen in the ultrathin limit; the critical temperature for phase coexistence is suppressed below 400 K. Density functional theory calculations on MgO V superlattices (Vn/(MgO)n, n = 3,5,7) reveal pronounced V 3d and O 2p hybridization and interfacial charge redistribution that weaken hydrogen binding near the interface and recover toward bulk values with increasing V thickness. These results indicate that interfacial electronic structure, in addition to finite size energetics, governs hydride stability in ultrathin V films and that layer - thickness and interface engineering can tune reversible hydrogen uptake.