0428同步年報-2021-全

Chemical Science 027 Fig. 1 : (a) FT-EXAFS spectra of np-MoS 2 compared with Lnp-MoS 2 and P-MoS 2 . Corresponding FT-EXAFS fitted curves also shown in (a). (b,c) Schematic of the atomic structure of Ru/Lnp-MoS 2 and Ru/np-MoS 2 derived from (a). [Reproduced from Ref. 1] Rational Strain Engineering of Single-Atom Ruthenium on Nanoporous MoS 2 for Highly Efficient Hydrogen Evolution Rational strain can enhance the accumulation of OH - and H 2 O in the sulfur vacancies resulting in an increased efficient of hydrogen evolution reaction. S ingle-atom catalysts (SAC) were considered as ideal hydrogen evolution reaction (HER) electrocatalysts to achieve high catalytic activity and decreasing the metal loading because of their maximized efficiency of atom use, well defined single-atom dispersion and unique coordination environments. Nevertheless, the catalytic activity of SAC at the state of the art still has much scope for improvement with an aim to maximize the catalytic activity, especially for multistep reactions. This limitation arises from the simplicity of the single-atom sites that are generally capable of efficiently catalyzing one step of a reaction rather than the entire reaction sequence. The sulfur vacancie (SV) around a Ru single atom in MoS 2 - supported Ru catalysts are generally considered to be the active site for HER, but a deep exploration of a synergetic effect between SV and Ru atoms has not been achieved, especially under realistic reaction conditions. Yongwen Tan (Hunan University, China) and his team thus hope to utilize strain engineering to amplify the synergistic interaction between SV and Ru atoms and to understand mechanistically the synergistic effect, thus maximizing the catalytic activity of SAC. Collaborating with Ting-Shan Chan (NSRRC), Tan’s team recorded Ru and Mo K-edge X-ray absorption near edge structure (XANES) spectra and Fourier transforms of extended X-ray absorption fine structure (FT- EXAFS) spectra at TLS 01C1 , which is helpful to deduce the coordination environment of active sites. Collectively, S K- and Mo L 3 -edge XANES spectra were recorded at TLS 16A1 . To validate the role of bending strain in boosting the intrinsic activity, control samples of plane MoS 2 (P-MoS 2 ), nanoporous MoS 2 (np-MoS 2 ) and nanoporous MoS 2 with a larger ligament (Lnp-MoS 2 ) were prepared. Ideally, the strain in P-MoS 2 is negligible, whereas Lnp-MoS 2 possesses less strain than np-MoS 2 . As shown in Fig. 1(a) , np-MoS 2 exhibits the largest high- R shift of Mo-Mo signals among these catalysts. The strain in these catalysts originated from the nanotube-shaped ligament, thus formatting the atomically curved MoS 2 ( Figs. 1(b) and 1(c) ). On an atomic scale the resultant bending strain can be approximately replaced by a tensile strain. A ligament with a smaller diameter ( D 2 < D 1 ) hence possesses the most strained surface-atom arrangement. This change can be detected with the Mo-Mo radial distance as an indicator, as confirmed by the aforementioned FT-EXAFS results. The absorption edge of the Ru K-edge XANES spectrum for Ru/np-MoS 2 under an open-circuit condition shows a positive shift, indicating an increased Ru oxidation state ( Fig. 2(a) , see next page), which likely results from binding of H 2 O and OH - , leading to the delocalization of an electron. When cathodic potentials were applied, a negative shift of the absorption edge occurred, indicating the recovery of Ru in a low oxidation state after water dissociation occurred. Corresponding FT-EXAFS spectra for Ru/np-MoS 2 at varied applied potentials appear in Fig. 2(b) . The main feature obtained under an open-circuit condition displays a low- R shift, which is ascribed to the contribution of a