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

TL;DR

This work addresses how interfacial electronic structure and finite-size confinement separately influence hydrogen uptake and hydride phase behavior in ultrathin vanadium films on MgO. By combining in-situ optical transmission and four-probe resistance measurements on 10 nm and 50 nm V/MgO(001) with DFT on $V_n/(MgO)_n$ superlattices, it decouples interface effects from thickness energetics. The key findings show that 50 nm films display bulk-like $\alpha$--$\beta$ transitions with plateaus, while 10 nm films exhibit continuous uptake with similar $\Delta H$ but more negative $\Delta S$ and suppressed phase coexistence; DFT reveals pronounced interfacial $V$-$3d$ and $O$-$2p$ hybridization and a thickness-driven evolution toward bulk-like electronic structure, which lowers hydrogen binding near the interface. The results highlight interfacial engineering and layer thickness as viable routes to tune reversible hydrogen uptake in V-based systems, with implications for designing stable, tunable hydrogen storage and transport media.

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.

Figures (5)

• Figure 1: Thermodynamic characterization of hydrogen absorption in epitaxial V films on MgO(001): 10 nm (top row) and 50 nm (bottom row). (a) Normalized resistance change $\Delta R/R_0$ versus hydrogen concentration [H/V]; plateaus in the 50 nm film mark a first-order $\alpha$--$\beta$ transition, while the 10 nm film shows continuous uptake. (b) Hydride formation enthalpy $\Delta H$ from van't Hoff analysis. The $\Delta H$ values for the 10 nm and 50 nm films are relatively close in magnitude, both exhibiting exothermic behavior in the range of approximately $-0.5$ to $-0.3$ eV/H, suggesting modest thickness dependence of the binding energetics. (c) Entropy change $\Delta S$ as a function of concentration: the 10 nm film displays a more negative $\Delta S$ (larger $|\Delta S|$) than the 50 nm film, indicating restricted configurational freedom or less degrees of freedom for hydrogen in the ultrathin limit. (d) Calculated phase boundaries from $(\partial\mu/\partial c)_T=0$. The critical temperature for $\alpha$--$\beta$ coexistence exceeds $400\,$K for the 50 nm film but is suppressed below $400\,$K for the 10 nm film across the measured range.
• Figure 2: Primitive cell samples of MgO-V superlattice structure used in DFT calculations.
• Figure 3: Projected density of states (PDOS) for V $3d$ and $4s$ orbitals at interface (top panels) and center (bottom panels) sites, and V $3p$ and $3s$ orbitals in V$_3$(MgO)$_3$. The Fermi level is set at 0 eV.
• Figure 4: Partial density of states of V $3d$, $4s$, $3p$ and $3s$ orbitals at interface and center sites in V$_5$(MgO)$_5$. The Fermi level is set at 0 eV.
• Figure 5: Partial density of states of V $3d$, $4s$, $3p$ and $3s$ orbitals at interface and center sites in V$_7$(MgO)$_7$. The Fermi level is set at 0 eV.