Hydrogen Absorption in Epitaxial V Films

• The paper demonstrates that hydrogen absorption in epitaxial vanadium films is controlled by the interplay of film thickness, interface chemistry, and finite size effects.
• In situ optical transmission, resistance measurements, and DFT modeling elucidate the mechanisms of hydride formation, phase coexistence, and altered electronic structure.
• Results highlight how interface-induced electronic reconstruction and strain modulate hydrogen uptake, enabling tailored hydride transitions for storage, sensing, and catalysis.

Hydrogen absorption in epitaxial vanadium films is governed by the interplay of crystallographic structure, film thickness, interface chemistry, and finite size effects. These films, typically realized as V/MgO(001) heterostructures or superlattices with transition-metal spacers, serve as both model systems and technological platforms for hydrogen storage, sensing, and catalysis. Experimental and theoretical investigations—combining in situ optical transmission, electrical resistance isotherms, channeling NRA, neutron reflectivity, and DFT—have resolved the mechanisms of hydride formation, phase coexistence, and the critical roles of interface-induced electronic reconstruction and layer thickness.

1. Sample Preparation and Film Structure

Epitaxial vanadium films are routinely grown on single-crystalline MgO(001) substrates using DC magnetron sputtering under ultrahigh vacuum conditions (base pressure <5×10⁻⁸ Torr), yielding nominal thicknesses of 10 nm to 50 nm for single-layer films (Zhang et al., 9 Nov 2025), or repeating units of 14 ML vanadium interleaved with 2 ML Fe or Cr for superlattice structures (Fe/V or Cr/V) (Komander et al., 11 Nov 2024, Droulias et al., 2018). All films are capped with Pd (5–7 nm) to catalyze H₂ dissociation and suppress oxidation; this layer does not contribute a measurable hydrogen signal in absorption experiments. Structural characterization via x-ray reflectivity and low-angle x-ray diffraction confirms epitaxial V(001) order and coherent layering, with thickness determination accurate to ≲5%. Vanadium films are typically clamped in-plane to the MgO lattice (a_MgO/√2=2.98 Å), and superlattices exhibit initial V tetragonality c/a≈1.01 under coherent strain (Komander et al., 11 Nov 2024).

2. Experimental Probes of Hydrogen Uptake

Hydrogen uptake is evaluated through in situ optical transmission and resistivity, calibrated against known H/V ratios. Optical transmission (typically using 625–639 nm LED sources and lock-in detection) is converted to hydrogen concentration c (H/V) via the Beer–Lambert law:

$I/I_0 = \exp(-A\,c)$

where $A$ is derived from bulk VHx calibrations (Zhang et al., 9 Nov 2025). For superlattices, the total transmitted intensity scales as $T_\text{tot} = T_\text{layer}^N$ where $N$ is the number of V layers (Droulias et al., 2018). Four-probe resistance is measured simultaneously, reported as normalized changes:

$\Delta R / R_0 = [R(c, T) - R(0, T)] / R_0$

highlighting electronic modifications upon hydrogenation (Zhang et al., 9 Nov 2025). Absolute concentration calibration in Fe/V or Cr/V superlattices uses N-NRA via 15N resonance for direct H quantification (Komander et al., 11 Nov 2024). Equilibrium is ascertained by holding the hydrogen pressure at each temperature for ≥5 min until probe signals stabilize.

3. Thermodynamic and Phase Behavior

3.1 Bulk-like α–β Hydride Transition

In 50 nm V/MgO films and bulk-analogous superlattices, pressure–concentration (p–c) isotherms at T = 300–400 K show clear plateaus (Δc ≈ 0.12–0.20 H/V), characteristic of first-order α–β phase coexistence (Zhang et al., 9 Nov 2025). ΔR/R₀ displays step-like jumps at the plateau pressures, confirming hydride transitions. Corresponding Van’t Hoff plots,

$$\ln P_{H_2} = \frac{\Delta H(x)}{RT} - \frac{\Delta S(x)}{R}$$

(where $x =$ H/V), yield formation enthalpies in the range –0.47 to –0.34 eV/H and entropy changes –100 to –95 J mol⁻¹ K⁻¹ (Zhang et al., 9 Nov 2025).

3.2 Ultrathin Film Regime

In 10 nm films, p–c isotherms are monotonic without plateaus down to T ≈ 300 K; ΔR/R₀ also varies continuously, indicating absence of phase separation. Extracted formation enthalpies remain comparable to thicker films (–0.50 to –0.33 eV/H), but entropy drops further to –120 to –110 J mol⁻¹ K⁻¹ (Zhang et al., 9 Nov 2025). The greater magnitude of –ΔS suggests reduced configurational freedom for hydrogen in the ultrathin limit. Critically, the α–β coexistence temperature is suppressed below ≈400 K, and the system manifests as a continuous solid solution.

3.3 Interface and Superlattice Effects

Fe/V and Cr/V superlattices (with consistent V thickness, strain, and cap) reveal pronounced differences in hydrogen solubility and phase behavior. Cr/V superlattices exhibit higher hydrogen uptake—c≈0.18 H/V at 1 bar and T=200 °C (versus 0.12 H/V for Fe/V), with pseudo-plateaus at lower equilibrium pressures (Komander et al., 11 Nov 2024). Critical concentrations at the α–β boundary stand at 0.078 H/V (Fe/V, 160 °C) and 0.133 H/V (Cr/V, 196 °C), compared to ∼0.15 H/V for bulk V. Solubility is limited by hydrogen-depleted regions—depletion thickness δ{Fe} ≈ 4–5 Å for Fe/V (∼2–3 ML), δ{Cr} ≈ 2–3 Å for Cr/V (∼1–2 ML) (Komander et al., 11 Nov 2024). No measurable hysteresis (<±0.01 H/V) is observed in either system.

Table: Hydrogen Solubility and Phase Parameters (Komander et al., 11 Nov 2024)

Sample	T (°C)	c at 1 bar H/V	Critical Concentration (H/V)	Depletion Thickness (Å)
Fe/V	200	0.12	0.078 (160 °C)	≈4–5
Cr/V	200	0.18	0.133 (196 °C)	≈2–3
Bulk V	206	∼0.20	∼0.15	—

4. Site Occupancy, Vibrational Dynamics, and Optical Effects

Hydrogen in these epitaxial V films occupies octahedral z sites exclusively across 0.05 ≤ c ≤ 0.15 H/V, verified by angular channeling N-NRA scans yielding deep minima as expected for this site symmetry (Komander et al., 11 Nov 2024). Fractional occupancy θz is essentially unity (±0.05). Vibrational amplitude for hydrogen in O_z sites, determined by simulation fits, is u{rms} ≈ 0.20–0.25 Å at all concentrations and for both Fe/V and Cr/V systems.

Optical transmission changes upon hydrogenation are dominated by volume expansion, not concentration per se (Droulias et al., 2018). DFT calculations yield volumetric expansion coefficients of k_T=0.064 for tetrahedral (Fe/V-like) and k_O=0.042 for octahedral (Cr/V-like) sites. The transmission follows:

$$T(x) = \exp\left[-\frac{4\pi d k_0}{\lambda (1+kx)}\right]$$

and variations collapse onto a universal curve when plotted versus volumetric expansion ε_v, rather than c or site occupancy. In Fe/V and Cr/V films, expansion at c=0.15 yields ε_v ≃ 0.0096 (0.96%) and ε_v ≃ 0.0063 (0.63%), respectively.

5. Interface Electronic Structure and DFT Insights

Density functional theory (DFT, PBEsol, Quantum ESPRESSO) applied to Vₙ/(MgO)ₙ superlattices (n=3,5,7) reveals pronounced interfacial electronic reconstruction (Zhang et al., 9 Nov 2025). Projected density of states (PDOS) shows that V atoms at the MgO interface undergo strong 3d–O 2p hybridization (–5 to –2 eV), with narrowed 3d features near E_F. Central layers restore PDOS profiles akin to bulk vanadium—broader 3d bands crossing the Fermi level. Bader charge analysis confirms modest V → O charge transfer at the interface. Hydrogen binding energies, calculated for symmetry-allowed sites, demonstrate weakened interfacial binding compared to bulk-like central layers:

• Interfacial sites: ≈ –0.30 eV/H (V₃/(MgO)₃), –0.32 eV/H (n=5,7),
• Central bulk-like sites: ≈ –0.50 eV/H, and average binding energy approaches –0.48 eV/H as n increases (Zhang et al., 9 Nov 2025).

Enthalpy of mixing at intermetallic interfaces also discriminates Fe/V and Cr/V superlattices: ΔH_mix ≈ –7 meV/atom for V–Fe and ≈ –2 meV/atom for V–Cr, indicating stronger local alloying and hydrogen site depletion near Fe (Komander et al., 11 Nov 2024).

6. Mechanistic Interpretation and Phase Engineering

The emergence and suppression of the α–β hydride transition in epitaxial V films arise from two principal effects:

• Finite-size energetics: In ultrathin films, the energetic cost of creating α–β interfaces across the entire thickness penalizes macro-scale phase separation, pushing the transition to lower T or eliminating it entirely; hence, continuous uptake for t_V = 10 nm (Zhang et al., 9 Nov 2025).
• Interfacial electronic reconstruction: V 3d–O 2p hybridization at the MgO interface weakens local hydrogen binding, broadens the ensemble of chemical environments, and reduces configurational entropy (Zhang et al., 9 Nov 2025). This effect, stronger at Fe/V interfaces than at Cr/V, manifests as the “inverse spillover” phenomenon: hydrogen-depleted near Fe, less so near Cr (Komander et al., 11 Nov 2024).

A plausible implication is that the quantum of entropy loss and binding weakening at interfaces can be tuned to suppress or restore hydride formation. Strategies include increasing vanadium layer thickness, inserting inert spacer layers, or chemical substrate modifications (e.g., metallic buffers) to mitigate V–O hybridization (Zhang et al., 9 Nov 2025). Conversely, to prevent hydride-induced embrittlement or leverage continuous uptake, ultrathin V films and strong V–O interfaces are optimal.

7. Applications, Limitations, and Outlook

Hydrogen absorption in epitaxial vanadium films, dictated by a balance of bulk-forming enthalpy (ΔH ≈ –0.5 to –0.3 eV/H) and interface-driven entropy penalties, enables precise engineering of hydride transitions and storage capacities. Cr/V superlattices approach bulk vanadium solubility (up to 90%), while Fe/V recover only 50–70% (Komander et al., 11 Nov 2024). Identical site occupancy and vibrational amplitude across systems confirm that interface depletion zones—not changes in local H site symmetry—are determinant (Komander et al., 11 Nov 2024). The universal relation between optical transmission and volume expansion enables robust hydrogen sensing, provided site occupancy remains constant (Droulias et al., 2018).

For catalysis, interface-controlled hydrogen depletion can steer H atoms to active sites or prevent hydride formation at sensitive regions. For storage, maximizing uptake and reversibility is achieved by minimizing depletion regions and restoring bulk-like energetics. The clear connection between thickness, electronic structure, interface chemistry, and hydride behavior in vanadium films provides a foundation for rational design and device scaling.

Further advances will require atomic-scale mapping of interface hybridization, direct quantification of local entropy contributions, and exploration of spacers or substrates beyond MgO, Fe, and Cr. These strategies are essential for tuning hydride stability, cyclic uptake, and functional integration in nanostructured hydrogen-sensitive devices.