0428同步年報-2021-全

028 ACTIVITY REPORT 2021 Fig. 2 : (a) Operando Ru K-edge XANES spectra of Ru/np-MoS 2 recorded at varied applied voltages; the inset shows the fitted oxidation states from (a). (b) Corresponding FT-EXAFS spectra from (a). (c) Operando Mo K-edge XANES spectra of np-MoS 2 recorded at varied applied voltages; the inset shows the corresponding first derivatives of the XANES spectra. (d) Corresponding FT-EXAFS spectra from (c). (e) Operando Mo K-edge XANES spectra of Ru/np-MoS 2 recorded at varied applied voltages; the inset shows the corresponding first derivatives of the XANES spectra. (f) Corresponding FT-EXAFS spectra from (e). [Reproduced from Ref. 1] Ru-O bond that overlaps a Ru-S bond. During electrochemical H 2 O reduction, this signal shows a high- R shift by 0.07 Å. This effect indicates the distortion of the coordination environment for Ru atoms, resulting from a redistribution of electrons in Ru atoms between S ligands and the Ru-O bond under alkaline HER. The operando X-ray absorption spectra (XAS) measurements of np-MoS 2 and Ru/npMoS 2 at Mo K-edge were conducted to reveal the nature of MoS 2 basal planes before and after the introduction of Ru atoms. Figure 2(c) shows operando XANES spectra of np-MoS 2 at the Mo K-edge. There is a negative-shift of a rising edge under an open-circuit condition compared with that under a condition ex situ , indicating a decreased Mo oxidation state. The location of Mo sites (central sublayer) notably hinders the H 2 O adsorption and dissociation because of a steric effect. An altered Mo oxidation state might thus result from an interaction between S atoms (outermost sublayer) and the electrolyte. When cathodic potentials were applied, the rising edge of np-MoS 2 was still located at the small-energy side relative to the condition ex situ . Correspondingly, FT-EXAFS spectra of np-MoS 2 remain substantially unchanged ( Fig. 2(d) ), indicating that Mo atoms in basal planes of np-MoS 2 are inert. Operando XANES spectra of Ru/np-MoS 2 at the Mo K-edge are presented in Fig. 2(e) . The rising edge of Ru/np-MoS 2 displays a positive shift under an open-circuit condition relative to that under a condition ex situ , meaning an increased Mo oxidation state. This effect is more obviously indicated by the first derivatives of the XANES spectra (inset of Fig. 2(e) ). Different from np-MoS 2 , the Mo sites in Ru/ np-MoS 2 are exposed because of the formation of SV. This change thus results likely from the binding of H 2 O and OH - . When cathodic potentials were applied, the rising edge of Ru/np-MoS 2 further shifted to higher energy, which implies a further increase of the Mo oxidation state in Ru/np-MoS 2 during the HER. Combining the prediction of density function theory, H 2 O and OH - are abundantly adsorbed on Mo sites without subsequent dissociation, which balances the reduction trend of the cathodic voltage resulting in a further increase of the oxidation state. The corresponding FT-EXAFS spectra of Ru/np-MoS 2 are presented in Fig. 2(f) . Under an open-circuit condition, the rise of a signal for H 2 O and OH - adsorption was detected, further supporting the