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Hydrogen spillover and storage for single-site metal catalysts, including single-atom catalysts (SACs) and single nanocluster catalysts, have been elucidated for various supports but remain poorly understood for inert carbon supports. Here, we use synchrotron radiation-based methods to investigate the role of single-site Ti catalysts on graphene for hydrogen spillover and storage. Our insitu angle-resolved photoemission spectra results demonstrate a bandgap opening and the X-ray absorption spectra reveal the formation of C−H bonds, both indicating the partial graphene hydrogenation. With increasing Ti deposition and H2 exposure, the Ti atoms tend to aggregate to form nanocluster catalysts and yield 13.5% sp 3 -hybridized carbon atoms corresponding to a hydrogen-storage capacity of 1.11 wt% (excluding the weight of the Ti nanoclusters [1]). Our results demonstrate how a simple spillover process at Ti SACs can lead to covalent hydrogen bonding on graphene, thereby providing a strategy for a rational design of carbon-supported single-site catalysts.

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Ferroelectricity in two-dimensional (2D) van der Waals (vdW) layers has revolutionized our understanding of the vdW layered coupling, and has been introduced in the domains of moiré superlattice patterns generated by interlayer twisting or sliding. In the smallest limit of thickness, untwisted and epitaxial vdW stacking layers exhibiting ferroelectricity would clearly serve as building blocks for realizing 2D devices with nonvolatile and reconfigurable functionalities. In this study, we grew ferroelectric hexagonal boron nitride (h-BN) films on single-crystal graphene synthesized on a SiC (0001) substrate using nitrogen plasma-assisted molecular beam epitaxy (PA-MBE). Systematic angle-resolved photoemission spectroscopy (ARPES) studies and first-principles calculations revealed that the epitaxial mono-, bi-, and tri-layer h-BN films exhibit layer-number-dependent µ-band dispersions due to an AB stacking sequence on a Bernal-stacked graphene substrate. Furthermore, our piezoelectric force microscopy (PFM) confirmed the coexistence of robust moiré and sliding ferroelectricity at the well-aligned h-BN/graphene heterojunction and in multilayered h-BN films, respectively. In principle, as-developed epitaxially ferroelectric h-BN is limited only by the size of the crystalline graphene substrate, thereby providing a versatile and scalable 2D ferroelectric platform with promising exotic physics and vdW device applications down to a few atomic layers.