Stability of Highly Hydrogenated Monolayer Graphene in Ultra-High Vacuum and in Air

TL;DR

This study evaluates the environmental stability of hydrogenated monolayer graphene using XPS C1s and EELS under ultra-high vacuum (UHV) and ambient air. The hydrogenation state is tracked via the $sp^3$ fraction, quantified by the area ratio $\frac{sp^3}{sp^2+sp^3}$, revealing remarkable stability in UHV (sp$^3$ ≈ 61–65%) over four months, but rapid oxidation in air with carbon-oxide formation. Re-exposure to atomic hydrogen can partially reverse oxidation, as evidenced by a restored CH-stretch in EELS and reduced oxide signals; the in-air oxidation follows a saturating exponential with a time constant of $\tau = 2.8 \pm 1.2$ h. Implications include the suitability of hydrogen (or tritium) storage in graphene only under vacuum, with a feasible recovery pathway via hydrogen exposure, and a preliminary radiolysis assessment suggesting minimal risk for tritiated graphene pending dedicated experiments.

Abstract

The stability of hydrogenated monolayer graphene was investigated via X-ray photoemission spectroscopy (XPS) for two different environmental conditions: ultra-high vacuum (UHV) and ambient pressure. The study is carried out by measuring the C 1s line shape evolution for two hydrogenated samples one kept in the UHV chamber and the other progressively exposed to air. In particular, the $sp^3$ relative intensity in the C 1s core-level spectrum, represented by the area ratio $\frac{sp^3}{sp^2+sp^3}$, was used as a marker for the hydrogenation-level. After four months in UHV, it resulted almost unchanged within the experimental uncertainty. Thus, a long-term stability of hydrogenated monolayer graphene was found, that indicates this material as a good candidate for hydrogen (or tritium) storage as long as it is kept in vacuum. On the other hand, the C 1s spectrum of the sample exposed to air shows a significant oxidation. A rapid growth up to saturation of the carbon oxides was observed with a time constant $τ$ = 2.8 $\pm$ 1.2 hours. Finally, the re-exposure of the oxidised sample to atomic hydrogen was found to be an effective method for the recovery of hydrogenated graphene. The CH stretching mode was measured via electron energy loss spectroscopy as direct footprint of hydrogenated graphene recovery.

Figures (6)

• Figure 1: Schematic summary of the experimental steps performed on samples A and B. The shaded blue-grey blocks represent the steps discussed in this work, each with a reference to its corresponding figure. The withe blocks refer to experimental steps not reported in this work, the details of which can be found in Apponi2024HYDRO.
• Figure 2: Fit analysis of the C 1s core-level spectra: (a) sample A after hydrogen exposure (left) and after 4 months in UHV (right); (b) sample B after hydrogen exposure (left) and after 11 months in air (right). The black dots represent the experimental data, the best fit curve is shown with red solid line, the shadowed curves are the $sp^2$ (blue), $sp^3$ (yellow), C-O-C (green) and O-C=O (violet) components and finally the dashed grey line is the integral background.
• Figure 3: C 1s core level spectrum of non-hydrogenated sample A before and after air-exposure, along with a fit analysis. Representation follows same criteria used in Figure \ref{['fig:Figure2']} except for $\pi$-plasmon component shown in grey and C-OH component in red.
• Figure 4: (a) Fit analysis of the C 1s core level for each step: 11 months in air, 250 $^\circ$C annealing, first (40 kL) and second (80 kL) re-hydrogenation. Color coding follows same criteria used in Figure \ref{['fig:Figure2']}. (b) O 1s core-level spectra of sample B measured after hydrogenation (grey), 11 months in air (purple), 250 $^\circ$C annealing (violet), first (cyan) and second (light blue) step of re-hydrogenation.
• Figure 5: Vibrational EELS spectra measured on sample B before the hydrogenation (black), after the hydrogenation (blue) and after the re-hydrogenation (light blue). The dashed line guides the eye to the peak associated to the CH-stretching at 350 meV.