On the origin of photoluminescence of noble metal NCs, there are always hot debates: metal-centered quantum-size confinement effect VS ligand-centered surface state mechanism. Herein, we provided solid evidence that structural water molecules (SWs) confined in the nanocavity formed by surface-protective-ligand packing on the metal NCs are the real luminescent emitters of Au-Ag bimetal NCs. The Ag cation mediated Au-Ag bimetal NCs exhibit the unique pH-dependent dual-emission characteristic with larger Stokes shift up to 200 nm, which could be used as potential ratiometric nanosensors for pH detection. Our results provide a completely new insight on the understanding of the origin of photoluminescence of metal NCs, which elucidates the abnormal PL emission phenomena, including solvent effect, pH-dependent behavior, surface ligand effect, multiple emitter centers, and large-Stoke’s shift.

Despite the fundamental and practical significance of the hydrogen evolution reaction (HER), the reaction kinetics at the molecular level are not well-understood, especially in basic media. Here, with ZIF-67-derived Co-based carbon frameworks (Co/NCs) as model catalysts, we systematically investigated the effects of different reaction parameters on the HER kinetics and discovered that the HER activity was directly dependent not on the type of nitrogen in the carbon framework but on the relative content of surface hydroxyl and water (OH – /H 2 O) adsorbed on Co active sites embedded in carbon frameworks. When the ratio of the OH – /H 2 O was close to 1:1, the Co/NC nanocatalyst showed the best reaction performance under the condition of high-pH electrolytes, e.g., an overpotential of only 232 mV at a current density of 10 mA cm –2 in the 1 M KOH electrolyte. We unambiguously identified that the structural water molecules (SWs) in the form of hydrous hydroxyl complexes absorbed on metal centers {OH ad ·H 2 O@M + } were catalytic active sites for the enhanced HER, where M + could be transition or alkaline metal cations. Different from the traditional hydrogen bonding of water, the hydroxyl (hydroxide) groups and water molecules in the SWs were mainly bonded together via the spatial interaction between the p orbitals of O atoms, exhibiting features of a delocalized π-bond with a metastable state. These newly formed surface bonds or transitory states could be new weak interactions that synergistically promote both interfacial electron transfer and the activation of water (dissociation of O–H bonds) at the electrode surface, i.e., the formation of activated H adducts (H*). The capture of new surface states not only explains pH-, cation-, and transition-metal-dependent hydrogen evolution kinetics but also provides completely new insights into the understanding of other electrocatalytic reductions involving other small molecules, including CO 2 , CO, and N 2 .

Metal nanoclusters (NCs) with intrinsic dual-emission are not only significant for fundamental research but also for accurate ratiometric sensing and imaging. Using 1-dodecanethiol (DT) as protected-cum-reduced ligand, the core-shell structured Au-Ag bimetallic NCs with well-resolved dual-emission bands centered at 440 nm and at 630 nm were successfully synthesized by a facile one-pot approach. A combined characterizations of X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM) and optical spectrum revealed that the Au-Ag bimetallic NCs had a core-shell structure with a metallic Au(0) core coated by a shell of Au(I)-and Ag(I)-thiolate motifs. Very interestingly, the dual-emissive Au-Ag NCs exhibited unique ratiometric pH-dependent emissions, which could be used an ideal pH ratiometric nanosensors. Most importantly, our observed Ag + mediated pH-dependent dual-emissive behavior suggests that the metal NCs core is not real emitter center, while structural water molecules (SWs) confined at the nanoscale interface of metal core are true luminous centers, which answers that the optical properties of metal NCs are very sensitive to the delicate change of surrounding microenvironment of metal NCs, including the dosing of Ag + , pH and packing mode of surface ligands. Our discovery not only opens up new possibilities for the design of ultra-small ratiometric pH nanoindicators, but also provides new insights on the origin of photoluminescent (PL) emission of metal NCs. the physicochemical properties of metal NCs [33,34] . Several strategies have been developed to the synthesis of bimetallic Ag-Au NCs, such as one-pot co-reduction method [35][36][37] (spontaneous reduction of as-mixed Ag and Au precursors through balancing the redox potentials of metal pairs by thiol ligand [38,39] ), classical galvanic replacement reaction approach [40] (involves the spontaneous reduction of a noble-metal cationic by a less noble metal in solution driven by the difference in redox potentials), abnormal anti-galvanic replacement reaction approach [41,42] (inverse process for galvanic replacement reaction recently observed for the synthesis of thiolate-protected Ag-Au NCs) and addition reaction [43][44][45] (a hydride-mediated controlled growth process), etc. And intriguingly, as-formed heteroatom doped metal NCs generally exhibited dramatically enhancement of luminescent and catalytic performance [30,[46][47][48][49][50][51][52] , which are expected to have synergistic effects in their physicochemical properties compared with their mono-metallic analogues [53] . Nevertheless, the origin of PL or the nature of emitter of heteroatom doped metal NCs remain unclear and even controversial, which limits the rational design of metal nanoclusters with improved and tailored optical and catalytic properties.Luminescent metal NCs with dual-wavelength emission have been exploited as ratiometric nanoprobes for sensing and imaging [54] , rather than absolute intensity-dependent signal readout of single-emissive metal NCs, ratio...

Despite the fundamental and practical significance of the hydrogen evolution reaction (HER), the reaction kinetics at the molecular level is not clear, particularly in basic media. Here, with ZIF-67 derived Co-based carbon frameworks (Co/NC) as a model catalyst, we systematically investigated the effects of different reaction parameters on the HER kinetics, and surprise found that HER activity was not directly dependent on the type of nitrogen in the carbon framework, but on the relative content of surface hydroxyl and water (OH-/H2O) adsorbed on Co active sites embedded in carbon frameworks. When the ratio of the OH-/H2O was close to 1:1, Co/NC nanocatalyst showed the best reaction performance under the condition of high-pH electrolytes, e.g., with an overpotential of only 232 mV at a current density of 10 mA cm-2 in 1 M KOH electrolyte. We unambiguously identified that, the structural water molecules (SWs) in the form of hydrous hydroxyl complex absorbed on metal centers {OH-∙H2O@M+} were catalytic active sites for enhanced HER, where M+ could be transition and/or alkaline metal cations. Different from the traditional hydrogen bonding of water, the hydroxyl (hydroxide) groups and water molecules in SWs were mainly transiently bonded together through the spatial interaction between the p orbitals of O atoms, showing characteristics of delocalized π bond with dynamic feature. These new formed surface bonds or transient states could be a new weak interaction, which can act as an alternative channel for concerted electron and proton transfer at electrode surface. The capturing of new surface states not only answers pH-, cation- and transition metal- dependent hydrogen evolution kinetics, but also provides completely new insights into the understanding of other electrocatalytic reduction involved by other small moleculesm, including CO2, CO and N2 etc.

Metal nanoclusters (NCs) with intrinsic dual-emission are not only significant for fundamental research but also for accurate ratiometric sensing and imaging. Using 1-dodecanethiol (DT) as protected-cum-reduced ligand, the core-shell structured Au-Ag bimetallic NCs with well-resolved dual-emission bands centered at 440 nm and at 630 nm were successfully synthesized by a facile one-pot approach.A combined characterizations of X-ray photoelectron spectroscopy(XPS), transmission rlectron microscope (TEM) and optical spectrum revealed that the Au-Ag bimetallic NCs had a core-shell structure with a metallic Au(0) core coated by a shell of Au(I)- and Ag(I)-thiolate motifs, and very interestingly, the dual-emissive Au-Ag NCs exhibited unique ratiometric pH-dependent emissions, which could be used an ideal pH ratiometric nanosensors. Most importantly, our observed Ag+ mediated pH-dependent dual-emissive behavior suggests that the metal NCs core is not real emitter center, while structural water molecules (SWs) confined at the nanoscale interface of metal core are true luminous centers, which answers that the optical properties of metal NCs are very sensitive to the delicate change of surrounding microenvironment of metal NCs, including the dosing of Ag+, pH and packing mode of surface ligands. Our discovery not only opens up new possibilities for the design of ultrasmall ratiometric pH nanoindicators, but also provides new insights on the origin of photoluminscent (PL) emission of metal NCs.