Structure and stability of hydrogenated carbon atom vacancies in graphene

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Adsorption of hydrogen atoms to a carbon atom vacancy in graphene is investigated by means of periodic first principles calculations, up to the fully hydrogenated state where six H atoms chemically bind to the vacancy. Addition of a single H atom is highly exothermic and barrierless, and binding energies remain substantial for further hydrogenation, with a preference towards structures with the least number of geminal pairs. Thermodynamic analysis shows that defective graphene is extremely sensitive to hydrogenation, with the triply hydrogenated anti- structure prevailing at room temperature and for a wide range of H₂ partial pressures, from 1 bar down to <10⁻²⁰ bar. This structure has one unpaired electron and provides a spin-half local magnetic moment contribution to graphene paramagnetism. Comparison of our results with recent TEM, STM and μ-SR experiments suggest that carbon atom vacancies may actually be hydrogenated to various degrees under varying conditions.