Revealing the Multibonding State between Hydrogen and Graphene-Supported Ti Clusters

Abstract

Hydrogen adsorption on graphene-supported metal clusters has brought much controversy due to the complex nature of the bonding between hydrogen and metal clusters. The bond types of hydrogen and graphenesupported Ti clusters are experimentally and theoretically investigated. Transmission electron microscopy shows that Ti clusters of nanometer size are formed on graphene. Thermal desorption spectroscopy captures three hydrogen desorption peaks from hydrogenated graphene-supported Ti clusters. First-principles calculations also found three types of interaction: two types of bonds with different partial ionic character and physisorption. The physical origin for this rests on the charge state of the Ti clusters: when Ti clusters are neutral, H 2 is dissociated, and H forms bonds with the Ti cluster. On the contrary, H 2 is adsorbed in molecular form on positively charged Ti clusters, resulting in physisorption. Thus, this work clarifies the bonding mechanisms of hydrogen on graphene-supported Ti clusters. ■ INTRODUCTIONThe unique nature of metal clusters and 2D materials has advanced the field of materials science; however, a fundamental understanding behind scientific phenomena related to such materials often leads to further questions. 1,2 Utilizing metal clusters and 2D materials results in functional materials that possess outstanding physical and chemical properties. 3−5 2D materials like graphene are found to be good substrates for supporting metal clusters due to their chemical stability and high surface to volume ratio. 6−8 Graphene-supported metal clusters show high reactivity, which leads toward applications in the fields of catalysis and energy storage. 9−11The chemistry of metal clusters on graphene involves complex bonding between metal clusters and gas molecules when gases such as hydrogen are introduced, resulting in difficulty of experimental measurements and analysis. In particular, hydrogen adsorption on graphene-supported metal clusters often leads to controversial arguments over how hydrogen is adsorbed, whether the hydrogen adsorption takes place on the metal clusters or on defect sites of graphene, whether spillover effects occur, and what types of bonds between hydrogen and metal are formed. Experimental studies report multiple hydrogen desorption peaks, and the hydrogen desorption properties are difficult to reproduce. 7,8,12−14 First-principles calculations and experimental measurements are performed to achieve a fundamental understanding of the interactions between hydrogen and graphene-supported metal clusters. In particular, Ti clusters when combined with graphene are predicted to be a particularly suitable material for hydrogen adsorption with range of 3.6 to 7.8 wt % hydrogen uptake. 15−19 Therefore, in this paper, the interactions between hydrogen and graphene-supported Ti clusters are theoretically and experimentally explored. ■ MATERIALS AND METHODSSiC and Cu substrates are implemented to support graphene because such substrates are reported to preserve the electronic structure of g...

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