Boosting the oxygen reduction activity of silver nanoclusters via selective exposure of solvent-coordinated sites

Abstract

Metal nanoclusters (MNCs) have gained extensive research interest in catalysis. However, addressing the spatial resistance induced by surface ligands and the high aggregation tendency of MNCs is a challenge during the catalytic process. In this work, we present a rational strategy to expose the metal sites occupied by weak coordination, thereby activating the catalytic centers. In particular, Ag12 nanoclusters containing six acetonitrile auxiliary ligands were prepared (AgNCs) and solvent molecules were removed via mild photo-reduction. Moreover, graphene oxide (GO) was introduced as a support to stabilize the AgNCs for further control of the geometrical structure while modulating the coordination and electronic environment. According to density functional theory (DFT) calculations, compared with the bulk AgNCs, the free Ag atoms without CH₃CN enhanced the interactions with the intermediates of *OOH, pointing to promising catalytic ability for oxygen reduction reactions (ORRs). The activated AgNCs@GO catalyst exhibited a half-wave potential of 915 mV vs. RHE and a low Tafel slope of 45 mV dec⁻¹ for the ORR. In this work, a rational method was developed to boost the catalytic activity of MNCs towards electrochemical scenarios, demonstrating the promising potential of MNC-based materials for electrocatalysis.

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Introduction

Metal nanoclusters (MNCs) protected by ligands have been explored with well-defined electronic structures, precise geometrical configurations and various properties, holding great theoretical significance and application value to understand the mechanism of complex catalytic reactions from the atomic level.^1–4 Theoretically, MNCs could provide ideal platforms to unravel the correlation between structure and performance for investigating the catalytic mechanism.^5–8 Due to the high atomic utilization ratio and the special synergistic effect among internal metal atoms, MNCs are expected to tune the adsorption energy and reduce the reaction energy barrier in catalytic processes, towards high activity and selectivity.^9–12 Recently, metal nanoclusters have attracted considerable attention in the field of the oxygen reduction reaction (ORR). For example, Wu and co-workers designed a reduced thiolated Pd nanocluster with adequate charge states and space structures, which exhibited excellent catalytic activity for the ORR.¹³ Lately, Zhu et al. also investigated the application value of M₁Ag₂₁ nanoclusters loaded on activated carbon for the ORR in alkaline solutions.¹⁴ Although many efforts have been made, the catalytic properties of metal nanoclusters are far from being developed.

An inherent barrier of MNCs for catalysis is the presence of modified ligands, which are necessary for stabilizing metal nanoclusters but hinder the access of the reactants to the active center and the sluggish transfer of charge carriers, severely inhibiting the display of their catalytic activity.^15–17 To address this situation, selectively removing the ligands seems to be an effective way to expose the active sites and reduce the steric hindrance for enhanced catalytic ability.^18–20 In the past few years, pyrolysis has been widely adopted to eliminate ligands and activate MNCs as catalysts.^21–23 Nevertheless, the thermodynamic instability makes the nanoclusters prone to sintering and agglomeration during the high-temperature treatment, resulting in larger particles and reconfiguration of the metal core. To this point, it could be assumed that metal nanoclusters stabilized by weak coordination bonds hold the possibility to preserve the open sites available for catalysis.^24–26 For example, Zhang et al. reported a copper-based boron imidazolate cage with high efficiency for the photocatalytic reduction of CO₂, in which each Cu center was linked with a water molecule through a weak coordination bond to form an unsaturated coordinated open metal site.²⁷ Since solvent molecules involved in coordination tend to be terminal ligands that play no role in structural junctions, their removal/adsorption or exchange would not damage the skeleton structure or the arrangement pattern of fragments, which indicates that the catalytic activity of nanoclusters are able to be improved with the core structure of the nanoclusters unchanged.

Hence, we involved acetonitrile (CH₃CN) as a coordinated molecule and presented atomically precise nanoclusters Ag₁₂(SCH₂C₁₀H₇)₆(CF₃CO₂)₆(CH₃CN)₆ (termed AgNCs), which contained six isolated unsaturated coordinated Ag sites.²⁸ Benefiting from the weak coordination of the solvent ligand, these Ag atoms could be exposed as free active sites under mild photo-reduction. Moreover, considering that catalysis always occurs under complex external conditions involving the transfer of substrates and charge carriers, we further introduced graphene oxide (GO) as a support to avoid the aggregation and enhance the charge transfer ability. Notably, during the photo-reduction procedure, graphene oxide could be reduced, which not only stabilized the AgNCs for precision control in the geometrical structure but also modulated the coordination and electronic environment.

Expectedly, the activated AgNCs stabilized by graphene oxide (termed AgNCs@GO) demonstrated conspicuously promoted catalytic performance. The activated AgNCs@GO catalyst with free Ag atoms demonstrated excellent oxygen reduction reaction (ORR) performance with a half-wave potential of 915 mV (vs. RHE) and a lower Tafel slope of 45 mV dec⁻¹, which even outperforms the activity of commercial 20 wt% Pt/C in an alkaline medium. According to density functional theory (DFT) calculations, these free Ag atoms without CH₃CN can be easily occupied by the intermediates of *OOH (O₂ → *OOH, which was proved to be the potential-determining step) compared with the bulk AgNCs, pointing to promising catalytic ability for the oxygen reduction reaction. Moreover, the release space generated by the detachment of CH₃CN would allow the deformation of adsorbed CF₃COO⁻, which further freed the covered Ag atoms and excited the activity for ORR. In this work, a rational method was developed to boost the catalytic activity of MNCs towards electrochemical scenarios not only providing a successful showcase to overcome the challenges in exploring MNC-based catalysts but also paving the way to construct model materials with atomic accuracy for instructive studies.

Results and discussion

The atomically precise nanocluster Ag₁₂(SCH₂C₁₀H₇)₆(CF₃CO₂)₆(CH₃CN)₆ (named AgNCs) was synthesized according to a previously reported work. The entire cluster comprised an Ag12 kernel protected by six thiolate ligands (2-yl-methanethiol) and six trifluoroacetic acid and stabilized by six acetonitrile auxiliary ligands (Fig. 1a and Fig. S1†). The phase purity of AgNCs was jointly validated by powder X-ray diffraction (PXRD) patterns, the UV–vis spectrum and solid-state diffuse reflectance spectrum (Fig. 1b and Fig. S2 and S3†), while the elemental status of C, S, F, N, O and Ag was confirmed by X-ray photoelectron spectroscopy (XPS) (Fig. S4†). The high-resolution Ag 3d spectra of AgNCs exhibited obvious Ag 3d_5/2 and Ag 3d_3/2 signals located at 368.6 eV and 374.6 eV, respectively (Fig. 1c). Typically, the acetonitrile auxiliary ligands could only produce weak coordination forces. According to gas chromatography-mass spectrometry (TG-MS), acetonitrile was indeed first removed easily (Fig. 1d). Moreover, we estimated the effects of photo-reduction procedure on AgNCs. As expected, compared to the original AgNCs, the crystalline structure of photo-treated AgNCs was well reserved since similar PXRD patterns and FT-IR spectra were obtained (Fig. S5†), indicating that the separation of acetonitrile auxiliary ligands hardly affected the well-defined AgNCs. However, the XPS binding energy of Ag 3d for photo-treated AgNCs is negatively shifted from that of Ag 3d for the original Ag NCs, which indicates that the loss of acetonitrile auxiliary ligands has caused a change in the coordination environment and surface states of Ag.

Fig. 1 (a) Crystal structure of AgNCs. (Colour labels: green, Ag; yellow, S; gray, C; red, O; blue, N; turquoise, F; white, H. All hydrogen atoms are omitted for clarity.) (b) XRD patterns of AgNCs, AgNCs@GO and AgNCs with photo-treatment. (c) High-resolution Ag 3d XPS profiles of AgNCs and AgNCs with photo-treatment. (d) TG-MS curve of AgNCs. (e) XANES spectra of AgNCs, AgNCs with photo-treatment, Ag foil, and Ag₂O recorded at the Ag K-edge. (f) Enlarged view of the boxed area in (e). (g) EXAFS spectra of AgNCs, AgNCs with photo-treatment, Ag foil, and Ag₂O.

To investigate the electronic structure information of the AgNCs, the synchrotron-based X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) spectra were recorded. As shown in Fig. 1e, the normalized Ag K-edge XANES spectra of photo-treated AgNCs exhibit a similar main peak position and shape as the original AgNCs. However, the main peak of photo-treated AgNCs shifted to a lower energy direction over that of original AgNCs, implying the reduced oxidation state of Ag caused by the separation of acetonitrile auxiliary ligands (Fig. 1f). This is further indicated by the corresponding R spacing curves of the Ag K-edge EXAFS spectra, as depicted in Fig. 1g, the attenuated peaks of both Ag–O and Ag–Ag coordination are presented for photo-treated AgNCs, and these results suggest that the loss of acetonitrile molecules results in a decrease in the coordination numbers of Ag and a slight alteration in the structural symmetry of the original AgNCs. The above-mentioned results indicate that the detachment of the acetonitrile auxiliary ligands does not affect the overall compositional framework of the silver clusters, except for the silver coordination microenvironment.

The as-obtained AgNCs were subsequently stabilized on graphene oxide by photodeposition (detailed in the ESI†). From the transmission electron microscopic (TEM) images, it could be found that GO has a two-dimensional morphology (Fig. S6†), offering ideal platforms as supports. When AgNCs were involved, the two-dimensional morphology was well reserved in AgNCs@GO, as revealed by both high-resolution transmissions electron microscopic (HR-TEM) and scanning electron microscopic (SEM) images (Fig. 2a and Fig. S7†). Moreover, it could be observed that AgNCs were uniformly distributed on GO with an average diameter of 2 nm without any aggregation or overgrowth (Fig. 2b and c). The lattice spacings were determined to be 0.24 nm, corresponding to the (111) plane of AgNCs. Furthermore, the associated energy-dispersive X-ray spectroscopic (EDS) elemental mapping manifested the homogeneous distribution of C, S, F, O and Ag derived from the ligands of AgNCs over AgNCs@GO (Fig. 2d and S8†). The content of Ag in the AgNCs@GO catalyst was measured to be 1.9 wt% by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Compared with the original AgNCs (Fig. S9†), the morphology of well-defined clusters was also maintained after photodeposition onto supports. Thus, the AgNCs were stabilized onto a graphene matrix with the structure and morphology well maintained, providing platforms for the following assessment of the catalytic performance.

Fig. 2 (a)–(c) HR-TEM images of AgNCs@GO. (d) Elemental mappings of AgNCs@GO.

The ORR performance of stabilized and activated AgNCs@GO was evaluated in an O₂-saturated 0.1 M KOH solution using a typical three-electrode electrochemical cell with the rotating disk electrode (RDE). All potentials mentioned in this work were referenced to the reversible hydrogen electrode (RHE). As shown in Fig. 3a, the cyclic voltammogram (CV) of AgNCs@GO demonstrated a distinct oxygen reduction peak in the O₂-saturated electrolyte compared to that in the N₂-saturated electrolyte. The linear sweep voltammetry (LSV) measurements were carried out to further explore the ORR activity of AgNCs@GO. As controlled sample, pure GO demonstrated very limited activity for the ORR (Fig. S10†). Impressively, the as-obtained AgNCs@GO catalyst possessed prominent electrocatalytic activity with a remarkable half-wave potential (E_1/2) of 0.915 V (vs. RHE) and a diffusion-limited current density of −5.76 mA cm⁻² (Fig. 3b), which were better than those of the commercial Pt/C catalyst (E_1/2 = 0.85 V vs. RHE). Moreover, the pristine AgNCs (0.71 V), bulk Ag metal (0.76 V) and Ag nanoparticles (0.62 V) were also investigated for further proving the high activity of AgNCs@GO. These results suggested the superior catalytic behavior of catalysts derived from metal nanoclusters (Table S1†). The Tafel slope of AgNCs@GO calculated from the LSV curve was only 45 mV dec⁻¹ (Fig. S11†), suggesting the favorable kinetics of AgNCs@GO for O₂ adsorption/activation during the ORR. After changing the rotating speed from 400 to 2500 rpm, it could be observed that the limited diffusion current densities increased with the rotation speed while the onset potentials remaining constant, revealing that the AgNCs@GO catalysts generated a kinetics-controlled process for the ORR (Fig. 3c). These phenomena confirmed that once the active sites were exposed, AgNCs with free Ag atoms could intensely boost the electrocatalytic activity. Moreover, electrochemical impedance spectroscopy (EIS) measurements further proved the efficient synergetic effects between the activated AgNCs and the graphene matrix, which could afford the faster electron transfer from the catalyst surface to the reactants, thus exhibiting low impedance (Fig. S12†).

Fig. 3 (a) CV curves of AgNCs@GO in N₂- and O₂-saturated 0.1 M KOH. (b) LSV curves of AgNCs@GO in O₂-saturated 0.1 M KOH at a rotating speed of 1600 rpm. (c) ORR polarization curves of AgNCs@GO at different rotating speeds (inset: corresponding K–L plots of AgNCs@GO). (d) Peroxide yield and electron transfer numbers at different potentials of AgNCs@GO.

Furthermore, we comprehensively evaluated the catalytic performance of AgNCs@GO during the ORR. Linear and near-parallel Koutecky–Levich (K–L) plots were obtained at different applied potentials (inset Fig. 3c), based on which the electron transfer number (n) was calculated to be 3.82–4. Consistently, rotating ring-disk electrode (RRDE) measurements were also carried out (Fig. 3d), in which AgNCs@GO demonstrated a low H₂O₂ yield with an average electron transfer number of 3.93. Thus, after activation via the exposure of solvent–ligand-occupied sites, AgNCs@GO could lead to an efficient four-electron reduction pathway for the reduction of oxygen.^29,30 Moreover, the stability of AgNCs@GO was also improved once stabilized. During the chronoamperometric I–t test at a reduced potential of 0.9 V (vs. RHE), the current density remained 86.5% after 13 000 s of continuous operation (Fig. S13†). As confirmed by TEM, the morphology of AgNCs@GO, especially the AgNCs, was well reserved without obvious aggregation after the stability test (Fig. S14†). In addition, the elemental status of Ag was maintained in the XPS spectra after electrocatalysis (Fig. S15†), implying that the activated and stabilized AgNCs@GO catalyst was quite reliable as an electrocatalyst.

The density functional theory (DFT) calculation was employed to explore the origin of activity on AgNCs and reveal the ORR mechanism at the atomic level. The computational hydrogen electrode (CHE) model was used to estimate the ORR activity. In the AgNCs, all of the Ag atoms were covered by the groups of the ligands. According to the free energy curves of the Ag atoms (Fig. 4a), the AgNCs demonstrated low activity due to the little downhill energy of O₂ → *OOH (potential determining step, PDS). Such low activity was primarily caused by the weak adsorption of intermediates, especially for *OOH which was close to zero. Generally, the co-adsorption of *OOH and CH₃CN on the same single Ag atom resulted in competitive adsorption and weakened the adsorption of *OOH.

Fig. 4 (a) Gibbs free energy diagrams for the ORR on AgNCs. (b) The pathways for AgNCs are summarized at U = 0 V, 0.54 V, and 1.23 V, respectively. (c) Partial density of state (PDOS) for Ag 3d in AgNCs. (d) Pourbaix diagram of AgNCs.

To this point, the Ag atoms, which used to be occupied by CH₃CN via weak coordination forces, were ideal candidates for active centers. Once exposed, these Ag could offer adequate space for ORR intermediate adsorption. From the free energy curves (Fig. 4a), it could be observed that the free adsorption energy of the intermediate was obviously strengthened. The *OOH level was below that of AgNCs, suggesting that the free Ag atoms could strengthen the adsorption and improve the ORR activity. The computed free-energy diagrams for the ORR on the AgNCs surface are shown in Fig. 4b, each ORR step was thermodynamically downslope at zero electrode potential (U = 0 V), implying all these elementary steps were exothermic. At U = 1.23 V (equilibrium potential), several endothermic steps were observed, suggesting that O₂ → *OOH was the limiting step. The theoretical limiting potential of AgNCs with a CH₃CN defect was 0.54 V, implying the superior catalytic activity towards the ORR. Such phenomenon was also proved by the change in the Ag-4d orbital (Fig. 4c, PDOS). As expected, the 4d band of the free Ag atom was more toward the Fermi level, which increased the adsorption strength of the ORR intermediate. Overall, based on this discussion, it could be inferred that the free Ag sites were crucial for the ORR catalysis. Interestingly, during the calculation, we found that the adsorption of intermediates, especially the *OOH and *O, might lift out the Ag atoms and lead to the deformation of the kernel of the clusters (Fig. S16†). The release space generated by the detachment of CH₃CN would allow the deformation of adsorbed CF₃COO⁻, which further freed the covered Ag atoms and excited the activity for the ORR. The increase in such deformation was able to store more energy, making *OOH → *O less exothermic and become the PDS. Furthermore, the stored energy would be released at the reaction of *O → *OH and *OH → H₂O (Fig. 5). These phenomena indicated that suitable deformation of metal nanoclusters could be another factor for the ORR activity. The Pourbaix Diagram was also employed to analyse the stable adsorption of intermediates on AgNCs (Fig. 4d). For bare AgNCs, the relationship of free adsorption energy of intermediate and the U_SHE demonstrated that the lowest area was determined by the clean surface and *O at U_SHE < 1.23 V (Fig. S17†). Moreover, the line for *O was below that of *OH, indicating that the ORR cannot fully occur. Nevertheless, for the as-obtained AgNCs without CH₃CN (Fig. S18†), the line for surface was below all the intermediates at broad U_SHE, indicating that all the intermediates on the catalyst allowed the complete reaction to occur. Based on this discussion, the free Ag site and the deformation of AgNC clusters are the two main factors for the ORR activity.

Fig. 5 Proposed reaction mechanism.

Conclusions

In summary, through rational design, we prepared an Ag12 kernel containing six acetonitrile auxiliary ligands. These solvent ligands with weak coordination forces could be easily removed, activating highly efficient sites for electrochemical catalysis. The experiments and DFT calculations jointly revealed that not only can the free Ag atoms without CH₃CN be easily occupied by the intermediates of *OOH, but they also offer adequate space for the deformation of residual ligands, pointing to promising catalytic ability for oxygen reduction reactions. Compared with the bulk AgNCs, the as-expected catalyst AgNCs@GO exhibited efficient and reliable catalytic activity during the ORR. Hence, our work developed a new strategy for boosting the catalytic activity of MNCs towards electrochemical scenarios. Even though the development of metal nanoclusters as electrochemical catalysts is still in its infant stage, it is foreseeable that increasingly more NC-based catalysts with high activity and selectivity will be explored.

Data availability

All relevant data are within the manuscript and its additional files.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No.92061201, 21825106, 22375184), China Postdoctoral Science Foundation (2022TQ0288) and Zhengzhou University. This work was also supported by the Zhongyuan Thousand Talents (Zhongyuan Scholars) Program of Henan Province (234000510007).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01335b

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