All-alkynyl-protected coinage metal nanoclusters: from synthesis to electrocatalytic CO₂ reduction applications

Abstract

Atomically precise metal nanoclusters have been attracting considerable research interests in the last two decades, thanks to their quantum confinement effect and molecule-like properties. Such properties are significantly affected, if not dominated, by the interfacial metal–ligand bonding motifs. Recently, ligand engineering has been applied to alkynyl molecules, as they can bind to metal atoms with σ and/or π bonds, yielding versatile interfacial bonding motifs hence drastically different properties and functionalities can be realized, as compared to the most commonly employed thiolate ligands. This review first describes the unique advantages of alkynyl-protected metal nanoclusters, with elaboration on the comparison of the interfacial coordination mode and the optical features between thiolate/alkynyl protection. Following that, the recent progress regarding the synthetic strategy is discussed, with an emphasis on the direction reduction method and the synchronous nucleation and passivation strategy. Next, alkynyl-protected metal nanoclusters for the electrochemical CO₂ reduction reaction (eCO₂RR) are mainly discussed, with some explicit examples to elucidate the metal core effect and surface ligand effect as well as to explain the structure–performance relationship. At last, the challenges and perspectives from synthesis to the eCO₂RR of alkynyl-protected metal nanoclusters are proposed. We envision this review to stimulate more research efforts to be dedicated to developing effective synthetic strategies and advancing the catalytic mechanistic understanding of alkynyl-protected metal nanoclusters toward the eCO₂RR and beyond.

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1. Introduction

1.1. Why study alkynyl-protected metal nanoclusters?

Coinage metal (Au, Ag, Cu, etc.) nanoclusters with an ultrasmall size (usually below 3.0 nm) hold a unique position in the field of nanomaterials, as they possess significantly different optical properties and electronic and microscopic structural features because of the relatively large nanoparticles or nanocrystals thanks to their strong quantum confinement effect.^1–3 Such an effect of the metal nanoclusters also leads to their application as antibacterial agents^12,13 and versatile applications in various fields, such as photo/electrocatalysis,^4,5 environmental remediation,^6,7 sensing,^8,9 bioimaging or biolabeling,^10,11 and so on. More importantly, metal nanoclusters can be chemically synthesized with molecular purity, that is, atomic precision. Atomically precise metal nanoclusters have a well-defined size, a crystallographically resolved structure, and a definitive coordination environment at the atomic level, hence they play a critical role in the fundamental nanoscience study, because they can serve as a model to correlate the structure–property/performance relationship. In most cases, metal nanoclusters hold a spherical core@shell configuration,^14–16 where a few or tens of metal atoms are arranged in a certain pattern in the core and the ligand molecules are capped onto the metal core as the protective shell.^17–19

The surface ligands coordinate with a certain portion of core metal atoms to form some interfacial bonding motifs, which affect if not dominate the physicochemical properties of metal nanoclusters. Currently, thiolate molecules,^20–22 phosphine,^17,23 halogen,^24,25N-heterocyclic carbenes,^26,27 and other organic compounds^28,29 have been widely employed to stabilize the metal core to form a large quantity of molecular metal nanoclusters. Recently, alkynyl ligands have been continuously gaining increasing research attention thanks to their characteristic binding patterns with surface metal atoms.^30–33 For instance, compared with the most extensively employed thiolate molecules, alkynyl molecules can form σ and/or π bonding with Au or Ag atoms,^34,35 yielding more diverse interfacial bonding motifs, which endow alkynyl-protected coinage metal nanoclusters with drastic physicochemical properties and functionalities.

As illustrated in Fig. 1, this review first emphasizes the uniqueness of alkynyl ligands over other ligands for preparing metal nanoclusters, with a particular focus on surface coordination motifs, optical properties, and catalytic performance. Following that, the controllable synthesis of molecular alkynyl-protected metal nanoclusters is discussed, mainly elaborating on the direct reduction (DR) method and the synchronous nucleation and passivation (SNP) strategy. Final and the major part is regarding atomically precise alkynyl-protected metal nanoclusters for the electrochemical CO₂ reduction reaction (eCO₂RR), involving the significance and mechanism of eCO₂RR and some explicit examples for elucidating the structure–performance relationship.

Fig. 1 The main content in this review regarding all-alkynyl-protected metal nanoclusters.

1.2. The comparison of alkynyl ligand vs. thiolate ligand protected nanoclusters

1.2.1. The comparison of coordination modes. The coordination modes of an alkynyl molecule on a metal surface are more variable due to the C≡C bond, as the terminal C atom can attach to the metal core by forming a σ bond, while the C≡C bond can coordinate the metal atom in the form of π bonding. For example, the interfacial binding motifs of alkynyl-protected Au nanoclusters can be divided into the five essential types:³⁴ μ₁-η¹; μ₂-η¹, η¹; μ₃-η¹, η¹, η¹; μ₂-η¹, η² and linear PhC≡C–Au–C≡CPh (Fig. 2a), much more diverse and complex than the thiolate-protected Au nanoclusters, which has only roughly two types of binding motifs,³⁶ that is, linear RS–Au–SR, and lengthened RS–Au–SR–Au–SR (Fig. 2c). For Ag nanoclusters, there are more diverse approaches for alkynyl molecules to coordinate with Ag atoms.^32,37 As illustrated in Fig. 2b, in motifs C, D, and E, the –C≡C–R group can bind to Ag than Au at least one or even two more metal atoms. Such a much more diverse binding behaviour from alkynyl molecules than thiolate ligands might be favourable for catalysis.

Fig. 2 Fundamental coordination modes of alkynyl ligands on (a) Au atoms and (b) Ag atoms. (c) Essential bonding modes of thiolate ligands on metal atoms. Comparison of the geometric configurations of (d) V-shaped staples and (e) surface motifs on [Au₂₅(C≡CAr)₁₈]⁻ and [Au₂₅(SR)₁₈]⁻. Copyright 2018, American Chemical Society and Wiley,^34,38 and Copyright 2020, Wiley.³⁶

The structure comparison of [Au₂₅(C≡CAr)₁₈]⁻ and [Au₂₅(SR)₁₈]⁻ is an explicit example.³⁸ Both clusters have Au₁₃ icosahedra capped by six Au₂L₃ units with similar arrangements. Even if the six V-shaped [Au₂(C≡CAr)₃] staples in [Au₂₅(C≡CAr)₁₈]⁻ are similar to [Au₂(SR)₃] in [Au₂₅(SR)₁₈]⁻, the connection to the Au₁₃ kernel is quite different. Three Au atoms in V-shaped staples (labelled in blue, Fig. 2d) of [Au₂₅(C≡CAr)₁₈]⁻ are twisted about 60°, referring to that in [Au₂₅(SR)₁₈]⁻ (labelled in green, Fig. 2d), resulting in the two Au atoms in [Au₂(C≡CAr)₃] capping the adjacent triangular pattern, while two Au atoms in [Au₂(SR)₃] are capped by the uniformly distributed triangles. Moreover, it also leads to [Au₂₅(C≡CAr)₁₈]⁻ being present as a racemate. In addition, the angle between the terminal C≡C to Au atom is about 180°, while the angle of C–S–Au is less than 106° (Fig. 2e). This case exemplifies that the alkynyl ligand significantly affects the metal core configuration of the coinage metal nanoclusters.

1.2.2. The comparison of optical properties. As the ligand interacts with a significant amount of metal core atoms of the nanocluster, it plays a critical role in modulating the electronic structure of the metal nanoclusters. For instance, the optical absorbance feature of alkynyl-protected metal nanoclusters is (drastically) different from other ligand capped nanoclusters. As shown in Fig. 3a, both alkynyl-protected and thiolate protected Au₂₅ nanoclusters have a broad peak at ∼700 nm, and all the other peaks from the alkynyl-protected Au₂₅ molecules are red-shifted about 50 nm compared to thiolate protected Au₂₅ nanoclusters.³⁸Fig. 3b compares the absorbance spectra between the alkynyl and thiolate protected Au₃₆ nanoclusters. Au₃₆(SR)₂₄ displays multiple absorbance peaks, and in stark contrast, a significant red shift in the wavelength region beyond 500 nm is observed for Au₃₆(PhC≡C)₂₄ and the intensity of the characteristic peaks in the low wavelength is strongly reduced even disappears to some extent.³⁹

Fig. 3 The comparison of optical properties of (a) [Au₂₅(PhC≡CAr)₁₈]⁻, [Au₂₅(SR)₁₈]⁻and (b) Au₃₆(PhC≡C)₂₄, Au₃₆(SR)₂₄. Copyright 2017 and 2018, Wiley.^38,39

In addition to the optical absorption feature, alkynyl molecules can also affect the luminescence properties of the metal nanoclusters, e.g. the photoluminescent behaviour and the corresponding quantum yield. For example, Konishi et al. reported that, despite the photoluminescence spectra of [Au₁₃(dppe)₅(C≡CPh)₂](PF₆)₃ is similar to that of [Au₁₃(dppe)₅Cl₂](PF₆)₃, the quantum yield of the former one (0.16) is significantly higher than that of the latter one (0.11).⁴⁰ The Wang group discovered that, Au₂₂(^tBuC≡C)₁₈ has a strong luminescence in the solid state with a quantum yield of 15%, and interestingly, it displays thermochromic luminescence.⁴¹ By using the chiral alkynyl group, the Zang group fabricated a new enantiomeric pair of superatomic Ag₁₇ nanoclusters, and both molecules emit near-infrared light with a quantum yield of 8.0% under ambient conditions as well as NIR circularly polarized luminescence thanks to the chirality of the excited states.⁴² The emission colour of Ag₅₁(^tBuC≡C)₃₂ reported by Lu and Xie groups can change from blue to red by changing the solvent polarity from the less polar dichloromethane to the more polar methanol, exhibiting a strong solvatochromic effect.⁴³

1.2.3. The comparison of catalytic performance. As the electronic structure and physiochemical properties of metal nanoclusters can be affected by the surface ligand, engineering the surface ligand can fine-tune the catalytic performance of the metal nanoclusters. Specifically, once the alkynyl molecules are attached onto the metal core to form a large –C≡C–M (M = metal) conjugated system, it favours the electron shuttling and hence promotes the catalytic performance for certain reactions. For instance, Wan et al. reported that, in alkyne semi-hydrogenation catalysis, the conversion rate of [Au₃₈(L)₂₀(Ph₃P)₄]²⁺ (L = alkynyl ligands) as catalyst reached up to 97%, while that of thiolate stabilized Au₃₈ with the same core structure was less than 2%.⁴⁴ Such a ligand effect also occurs in the hydrogen evolution reaction (HER). The Tsukuda group found that, the onset potential of hydrogen evolution for Au₂₅(C≡CAr^F)₁₈ was about 70 mV positive than that of Au₂₅(SC₂H₄Ph)₁₈ under the same conditions, indicating that alkynyl ligands significantly enhanced the HER activity.⁴⁵

2. Controllable synthesis

Since the seminal Brust method was reported for preparing monodisperse nanoclusters in 1994, various strategies have been developed for the controllable synthesis of metal nanoclusters with molecular purity. For alkynyl-protected metal nanoclusters, the applicable synthetic methods include a direct reduction (DR) of the precursor,^39,46 synchronous nucleation and passivation (SNP),^37,47,48 ligand exchange,^40,49,50 one-pot method^41,51,52 and so on (Fig. 4). With these methods on hand, plus the advancement of crystallographic techniques, a great deal of all-alkynyl-protected Au, Ag, and AuAg alloy nanoclusters with crystal structures have been recorded. This review will mainly discuss the DR method and SNP strategy.

Fig. 4 Controllable synthesis strategies for all-alkynyl-protected metal nanoclusters.

2.1. Direct reduction of the precursor

Direct reduction (DR) of the precursor is the most facile and straightforward method to fabricate metal nanoclusters, but manipulating the reaction kinetics to obtain a monodisperse product is quite challenging. Currently, the DR approach is the most widely employed strategy to acquire alkynyl-protected metal nanoclusters. Specifically, metal salts (e.g. Me₂SAuCl,^30,53,54 Ag₂O^31,55) were first reacted with the alkynyl ligand to generate the Au(I)/Ag(I)-alkynyl complexes (the precursor), then the reducing agent (e.g. NaBH₄^43,56) was added to reduce the precursor to form metal nanoclusters. In some cases, a mixture of cluster molecules is acquired, hence the post-synthetic isolation and purification are necessary. So far, Au₄₂,³⁰ Au₉₉,⁵³ Ag₄₇,³¹ Ag₁₁₂,³² and Au₃₄Ag₂₈⁵⁷ have been chemically prepared using this method.

It is worth noting that the morphology and aggregation state of the precursor can be critical for forming the final product. Our group disclosed that, by introducing ethanol in the preparation of Au–PA (PA = phenylacetylene) precursor, the amorphous precursor can self-assemble into flower-like macromolecules⁵⁴ (Fig. 5, denoted as (Au–PA)_f). Consequently, compared with the amorphous precursor, the well-defined (Au–PA)_f precursor can lead to the formation of Au₁₄₄(PA)₆₀ with higher yield and monodispersity. Such a phenomenon is reminiscent of the high-yield synthesis of Au₂₅(SR)₁₈, in which the monodisperse Au–SR precursor was obtained by controlling the reaction kinetics.⁵⁸

Fig. 5 SEM images of Au–PA precursors at (a) 5 μm, (b) 500 nm and (Au–PA)_f precursors at (c) 5 μm, (d) 500 nm. (e) XRD patterns of the amorphous Au–PA precursor generated without ethanol and the flower-like (Au–PA)_f precursor generated with the presence of ethanol. Copyright 2020, Royal Society of Chemistry.⁵⁴

2.2. Synchronous nucleation and passivation strategy

To form a nanocluster molecule, it undergoes nuclei growth and surface passivation process which compete with each other. For the DR approach, it might be difficult to manipulate these two processes to improve the yield of the target cluster. For instance, an excess amount of reducing agent may lead to excessive reducibility to form large nanoparticles spontaneously, while a rapid reduction rate may induce the surface passivation occurring inhomogeneously hence increasing the polydispersity of the product. From that, regulating the reaction rate and manipulating the reducing process might be critical for enhancing the yield of molecular metal nanoclusters.

In 2020, our group developed a synchronous nucleation and passivation strategy to fabricate Au₃₆(PA)₂₄ and Au₂₂(PA)₁₈ nanoclusters with a high yield of 17.1% and 70.1%, respectively.⁴⁷ In this method, the ligand and reducing agent were added simultaneously, and more importantly, NaBH₃CN instead of NaBH₄ was employed as the reductant, and the reaction was also conducted at a low temperature. By doing that, the cluster nuclei growth and surface ligand passivation can be more easily manipulated to obtain molecular metal nanoclusters. As illustrated in Fig. 6, the equivalent ratio of NaBH₃CN-to-Au is critical for yielding different products. When NaBH₃CN is much more excessive (eq ≫ 3), polydisperse Au nanoclusters were acquired; when NaBH₃CN was decreased to a certain low amount (eq = 1/2, 1/3), Au₂₂(PA)₁₈ as the reaction intermediate formed, but it can transform into Au₃₆(PA)₂₄ overnight eventually. In this work, we also proposed the structure evolution from Au₂₂(PA)₁₈ to Au₃₆(PA)₂₄, and the calculated stoichiometric ratio of Au₂₂(PA)₁₈-to-Au₃₆(PA)₂₄ was verified by the designed experiment as well. Such synchronous nucleation and passivation strategy is of great universalities to prepare alkynyl-protected Ag, AuAg alloy nanoclusters, and in later studies, other molecules such as Ag₃₂,³⁷ and Au₉Ag₉⁴⁸ clusters were synthesized by this method with high yields.

Fig. 6 Synthetic routes for Au₃₆(PA)₂₄ with the different amounts of reducing agents in the synchronous nucleation and passivation strategy. Copyright 2020, Springer.⁴⁷

2.3. Other ingenious synthesis strategies

Besides the above two approaches, there are some other ingenious strategies to synthesize homoleptic alkynyl-protected metal nanoclusters. One method is the ligand exchange reaction, in which another ligand is introduced into a solution of the parent cluster with high polydispersity or molecular purity to yield new cluster molecules.⁵⁹ Ligand exchange can transform the cluster size, morphology, composition, and structural arrangement and also introduce new chemical/physical properties.^49,60,61 Pioneering work using this method to prepare all-alkynyl-protected metal clusters was done by Tsukuda et al. In 2011, by reacting polyvinylpyrrolidone stabilized Au nanoclusters with a series of alkynyl molecules, several magic clusters such as Au₅₄(PA)₂₆, Au₉₄(PA)₃₈, and Au₁₁₀(PA)₄₀ can be identified in the mass spectra of the final mixture product.⁶² In 2019, the same group reported a quasi ligand exchange reaction, in which hydride-doped [HM@Au₈(PPh₃)₈]⁺ (M = Pd, Pt) reacting with Au(I)-alkynyl oligomer to yield [MAu₂₄(C ≡ CAr^F)₁₈]²⁻ clusters.⁶³ Interestingly, Hosier et al. revealed that the simple thiolate-to-acetylide ligand exchange is enthalpically unfavorable, while metathesis reactions between these ligands are enthalpically favorable.⁶⁴ However, the direct reaction between phenylacetylene and thiolate Au clusters can not proceed, and the acetylide-for-thiolate ligand exchange is only facile when using either a gold(I)–phenylacetylide complex or lithium phenylacetylide as an incoming ligand to thiolate protected Au clusters. From this point, using ligand exchange to prepare alkynyl-protected metal nanoclusters has been restricted to some extent.

Another strategy is the one-pot synthetic approach, which is quite straightforward and of easy operability. The most significant feature of this method is that all the reagents (the metal salt, the ligand, the reductant, etc.) were added together to react in one system, similar to SNP but with less kinetic control and easier to operate. For instance, the Tsukuda group fabricated a series of homoleptic alkynyl-protected Au₂₂(C≡CR)₁₈ cluster molecules protected with four different ligands by reacting (Me₂S)AuCl, the ligand, and triethylamine together.⁵¹ Interestingly, triethylamine not only serves as a base, but also is the reducing agent, and its low reducing capability left (Me₂S)AuCl not completely reduced, hence the yields of all the clusters were less than 8%. In another study, the Wang group synthesized a pair of Au₂₃(C≡C^tBu)₁₅ clusters (Au₂₃-1 and Au₂₃-2), which are the first isomers observed in all-alkynyl-protected metal nanoclusters.⁵² Specifically, Au₂₃-1 was prepared by reduction of Me₂SAuCl and HC≡C^tBu/Et₃N with NaBH₄, but Au₂₃-2 has to be acquired with a similar synthetic procedure in the presence of tetraphenylphosphonium chloride or tetrabutylammonium chloride. The yield of Au₂₃-1 and Au₂₃-2 was 31% and 10%, respectively. It shows that, by optimizing the synthetic parameters, the one-pot method can obtain molecular metal nanoclusters with high yields, but more cases await to be explored.

3. Alkynyl-protected metal nanoclusters for eCO₂RR

3.1. The importance of eCO₂RR

Since the last century, the overuse of fossil fuels and rapid industrialization has caused serious greenhouse effects globally.⁶⁵ CO₂ is the major greenhouse gas, whose emission must be reduced to a certain minimum level to meet the framework of the Paris Agreement. Capturing, storing, and utilizing CO₂ is one of the effective means to mitigate the great CO₂ emission pressure.⁶⁶ Among that, electrochemical CO₂ reduction reaction (eCO₂RR) has been attracting increasing research attention recently, as it can convert CO₂ into valuable chemicals. More importantly, eCO₂RR can be conducted under mild conditions with high efficiency, and take advantage of renewable electricity.^67,68 However, the C=O bond is difficult to be broken because it has large bond dissociation energy of 750 kJ mol⁻¹. Meanwhile, as it has multiple proton-coupled electron transfer steps with comparable potentials, plus the side reaction of hydrogen evolution, it is extremely challenging to control the product selectivity.⁶⁵ The presence of electrocatalysts can lower the energy barrier of eCO₂RR, but most of the electrocatalysts still suffer from high overpotential, low current density, and poor product selectivity.⁶⁹ Therefore, developing electrocatalysts with high efficiency, high activity and selectivity is imperative.

3.2. The mechanism of eCO₂RR

As multiple protons/electrons transfer are involved in eCO₂RR, the reaction pathway and mechanism of eCO₂RR are rather complicated. Based on the electron-transfer number, the reduction products can be classified into C₁ molecules such as CO, HCO₂H, CH₃OH, and C₂₊ molecules such as CH₃CH₂OH, CH₃CHO, C₂H₆, C₂H₄, etc. Regardless of the product generated, eCO₂RR follows the basic process: 1. Adsorption of CO₂ molecules on the catalyst surface; 2. Proton coupled electron transfer reactions of the intermediates; 3. Recombination of molecules and desorption from the surface.⁷⁰ As molecular Au/Ag nanoclusters mainly produce CO products, we here focus on 2e⁻ reaction only. An important concept regarding the reaction process is the rate-determining step (RDS), which is defined as the step with the largest energy barrier in the process. The overall reaction rate is governed by the RDS.

The general mechanism of the 2e⁻ reaction is presented in Fig. 7. Generally, the *COOH formation is the key step for producing CO, and there are two pathways to form *COOH. In one way, CO₂ grasps an electron to transform into *CO₂⁻ and then obtains a proton to produce *COOH, while in another way, CO₂ directly combines with a pair of proton and electron (H⁺/e⁻) on the catalyst surface to form *COOH. CO and H₂O are the products once *COOH reacts with a pair of H⁺/e⁻. From *CO, CH₃OH and CH₄ can also be yielded, depending on the number of electron pairs. The formation of HCO₂H also has two possible pathways. On the one hand, the protonation of *CO₂⁻ generates *OCHO first and *OCHO gets a pair of H⁺/e⁻ to form HCO₂H. On the other hand, CO₂ interacts with H which is pre-adsorbed on the catalyst surface by a pair of H⁺/e⁻ to form *HCOO, and then HCO₂H is produced through reacting with a pair of H⁺/e⁻.⁶⁵ In addition, density functional theory (DFT) calculations help to visualize the reaction path of eCO₂RR and the required energy to produce various intermediates, specific examples will be discussed in detail below.

Fig. 7 Schematic diagram of the 2e⁻ reaction path in eCO₂RR. Copyright 2021, Elsevier.⁶⁵

3.3. Alkynyl-protected metal nanoclusters for eCO₂RR

Metal nanoclusters especially Au and Au-alloy nanoclusters have been widely employed as eCO₂RR catalysts, thanks to the unique advantages that are not available from nanoparticle counterparts. First of all, the molecule-like transition of metal nanoclusters can facilitate the electron transfer to the electrode and reactant hence decreasing the overpotential; secondly, metal nanocluster has tunable and versatile chemical functionalities, e.g. by tuning the size, surface ligand, charge state, doping another metal, and structure engineering, the redox potential can be manipulated to enhance the catalytic performance. Finally and most importantly, metal nanoclusters have a well-defined molecular structure with atomic precision, which is quite favourable for theoretically simulating the experimental findings to unravel the reaction mechanism and disclose the catalytic reaction center, hence establishing the structure–performance relationship eventually.⁶⁷

So far, Au nanoclusters capped by all kinds of ligands (e.g. thiolate,^71,72 phosphine,²⁷ hydrides,⁷³ NHC,⁷⁴ and alkynyl^75,76) especially thiolate molecules have been studied for eCO₂RR. Pioneering work has been conducted on thiolate-stabilized molecular Au nanoclusters by Jin groups,^77–80 and multiple groups have also made significant contributions.^81–84 Other ligands and mixed ligands protected Au nanoclusters also exhibited impressive performance toward eCO₂RR.⁷³ For instance, the Wang group reported that the mixed ligand capped [Au₅₅(p-MBT)₂₄(Ph₃P)₆](SbF₆)₃ (Au₅₅) can reach the highest FE_CO of 94.1% at −0.6 V with the maximal CO production rate of 50.34 μL min⁻¹ at −0.9 V. Moreover, the Au₅₅ catalyst exhibited good stability for over 4 h. The authors attributed the good catalytic properties to the ultrasmall size of the Au₅₅ cluster, the corner Au atom, and the hydrophobic nature of the ligand.⁸⁵ In another study, the same group developed all-amidinate-protected [Au₂₈(Ph-form)₁₂](OTf)₂ clusters loaded on carbon nanotubes as a catalyst, which showed the maximal FE_CO of 97.5% at −0.57 V, and can still maintain over 91% for 40 h at −0.69 V in eCO₂RR.⁸⁶

Recent high-quality review papers regarding metal nanoclusters for eCO₂RR mainly focus on the thiolate ligand, whereas a systematic summary of alkynyl-protected metal clusters for eCO₂RR is missing.^67,87,88 Here, we mainly discuss all-alkynyl-protected Au and Ag nanoclusters for eCO₂RR, with particular attention on elucidating the metal core effect and the surface ligand effect.

3.3.1. Metal core effects. Given the uniqueness of alkynyl ligands over thiolate molecules mentioned in Section 1.2, we tend to believe alkynyl-protected metal nanoclusters might be more favourable for eCO₂RR. Nevertheless, rare reports can be found on alkynyl-protected Au or Ag nanoclusters for the eCO₂RR.

In 2021, our group reported the structure analysis and eCO₂RR performance of the smallest all-alkynyl-protected [Ag₁₅(C≡C^tBu)₁₂]⁺ (abbreviated as Ag₁₅) nanocluster.⁸⁹ As shown in Fig. 8a, Ag₁₅ possesses a body-centered-cubic (bcc) structure with an Ag@Ag₈@Ag₆ metal core configuration. Interestingly, the two diagonal Ag atoms in the Ag₈ cube can connect to the two O atoms of the CO₂ molecule at two opposite sides, making the Ag₈ cube elongated and the Ag₁₅ nanocluster assembled into one-dimensional material (Fig. 8b). Interestingly, Ag₁₅ exhibited excellent catalytic activity toward eCO₂RR with CO as the main product and H₂ evolution is significantly suppressed, and the maximum FE_CO can reach ca. 95% at -0.6 V (Fig. 8c). In addition, the Ag₁₅ catalyst can maintain good stability over 10 h (Fig. 8d).

Fig. 8 Diagram of CO₂ adsorption on (a) [Ag₁₅(C≡C^tBu)₁₂]⁺ and (b) Ag₈ cube. (c) Faradaic efficiency of CO and (d) stability testing of [Ag₁₅(C≡C^tBu)₁₂]⁺. Comparison of free energy of electroreduction of CO₂ to CO process on (e) [Ag₁₅(C≡C–CH₃)₁₂]⁺ and (f) [Ag₁₅(C≡C–CH₃)₁₁]⁺. Copyright 2021, Wiley.⁸⁹

DFT calculations were further conducted to determine the catalytic site and compare the catalytic selectivity of eCO₂RR to CO vs. H₂ evolution. The methyl group was used to replace the tert-butyl group to simplify the calculations, and one ligand stripping to expose the undercoordinated metal atom serves as the active site. For both the intact [Ag₁₅(C≡C–CH₃)₁₂]⁺ cluster and one ligand stripped [Ag₁₅(C≡C–CH₃)₁₁]⁺, the formation of *COOH is the rate determining step (RDS). As shown in Fig. 8e and f, [Ag₁₅(C≡C–CH₃)₁₁]⁺ has much lower free energy (0.44 eV) to form *COOH rather than [Ag₁₅(C≡C–CH₃)₁₂]⁺ (1.19 eV). Correspondingly, [Ag₁₅(C≡C–CH₃)₁₁]⁺ has higher energy barrier for H₂ evolution than [Ag₁₅(C≡C–CH₃)₁₂]⁺. Moreover, the energy barrier for *H formation is much larger than forming *COOH for [Ag₁₅(C≡C–CH₃)₁₁]⁺, suggesting H₂ generation can be largely restricted. On the intact [Ag₁₅(C≡C–CH₃)₁₂]⁺ cluster, the staple Ag atom acts as the active site for eCO₂RR, while in the one-ligand removed [Ag₁₅(C≡C–CH₃)₁₁]⁺, the loss of one ligand causes 4 shell-Ag atoms to expose two equivalent triangular faces, in which one undercoordinated Ag atom prefers to CO₂ adsorption and reduction. This study manifests that the alkynyl-protected Ag nanocluster holds great potential for the eCO₂RR in terms of activity and stability and can advance the fundamental understanding of the eCO₂RR catalyzed by alkynyl-protected metal nanoclusters.

Doping another metal into the metal core to form bimetallic nanoclusters can alter the physiochemical properties of the clusters and lead to enhanced catalytic performance due to the synergistic catalytic effect.^90–92 The well-defined Ag@Ag₈@Ag₆ metal core structure of the Ag₁₅ nanocluster allows us to conduct such metal exchange to tune and optimize the eCO₂RR performance. In a following study, through the metal exchange, our group first prepared the [Au₇Ag₈(^tBuC≡C)₁₂]⁺ (Au₇Ag₈ in short) cluster, of which the optical feature and crystal structure agree well with the previous report from Zheng group.⁹³ Meanwhile, the [Ag₉Cu₆(^tBuC≡C)₁₂]⁺ (Ag₉Cu₆ in short) cluster was also synthesized via a one-pot strategy, and it displayed significantly different optical properties and chemical stability from Au₇Ag₈.⁹⁴ Furthermore, the [Au₂Ag₈Cu₅(^tBuC≡C)₁₂]⁺ cluster (Au₂Ag₈Cu₅ in short) was also prepared by a metal exchange approach from Ag₉Cu₆.⁹⁵

It is worth noting that, Au₇Ag₈, Ag₉Cu₆, and Au₂Ag₈Cu₅ are all M₁₅ clusters with a similar metal core configuration. As demonstrated in Fig. 9a, they all possess an M@M₈@M₆ metal core, with one metal atom in the center, an M₈ cube in the middle layer, and an M₆ octahedron in the outmost layer in the core. Interestingly, the three clusters exhibited drastically different catalytic performances toward eCO₂RR. Au₇Ag₈ presented high selectivity for CO formation (Fig. 9b), with the highest FE_CO of 98.1% at −0.49 V. For both Ag₉Cu₆ and Au₂Ag₈Cu₅, CO and formate are the main products, despite the FE_CO showed a similar volcanic shape with Au₇Ag₈ and the highest FE_CO can reach 94.1% and 95.0% at −0.49 V for Ag₉Cu₆ and Au₂Ag₈Cu₅, respectively, the FE_CO decreased rapidly when the potential goes more negatively (Fig. 9b). Fig. 9c depicts FE_formate of Ag₉Cu₆, and Au₂Ag₈Cu₅. When the potential goes negative, FE_formate of Ag₉Cu₆ kept increasing while FE_formate of Au₂Ag₈Cu₅ first increased gradually and then decreased. The maximal FE_formate value for Ag₉Cu₆ and Au₂Ag₈Cu₅ is 47.0% at −1.19 V and 28.3% at −0.99 V, respectively. In addition, the highest FE_CO+formate for Ag₉Cu₆ and Au₂Ag₈Cu₅ is ∼100.0% and ∼97.4%, suggesting both clusters were able to convert CO₂ into valuable chemicals. Meanwhile, both Au₇Ag₈ and Ag₉Cu₆ can suppress H₂ evolution, while at a higher negative potential range, HER gradually becomes dominant for Au₂Ag₈Cu₅ (Fig. 9d). Correspondingly, both Au₇Ag₈ and Ag₉Cu₆ exhibited robust stability while Au₂Ag₈Cu₅ showed slightly inferior long-term durability than the above two clusters, probably due to its metal core (Au₁@Au₁Ag₄Cu₃@Ag₄Cu₂) is slightly asymmetric.

Fig. 9 (a) Structural anatomy of three nanoclusters. (b) FE_CO of three nanoclusters and (c) FE_formate of [Ag₉Cu₆(C≡C^tBu)₁₂]⁺, [Au₂Ag₈Cu₅(C≡C^tBu)₁₂]⁺. (d) Product selectivity of three nanoclusters. Copyright 2022, Royal Society of Chemistry.⁹⁵

By using the simplified single-crystal structures to build models, DFT calculations were next conducted to unravel the reaction mechanism. The eCO₂RR and HER compete at the same metal site, specifically, Au for Au₇Ag₈, Cu for Ag₉Cu₆, and for Au₂Ag₈Cu₅, the staple Cu acts as the active site for HCOO* binding, while *COOH, *CO, and *H tend to bind with the sub-surface Au atom. It is predicted that one ligand stripping to expose the metal atom is the active center. For the intact Au₇Ag₈ cluster, the energy barrier to form *H (0.88 eV) is lower than that of forming *COOH (1.08 eV, Fig. 10a) hence not favours eCO₂RR. However, once one ligand is stripped, trans-COOH* prefers to bind to the undercoordinated Ag atom and ΔG significantly decreases to 0.47 eV, much lower than ΔG for desorption of *H (1.07 eV) hence eCO₂RR is quite favourable (Fig. 10b). For the Ag₉Cu₆ cluster, eCO₂RR and HER compete at the Cu site regardless of the structural integrity. As shown in Fig. 10c, the RDS for CO generation is forming *COOH (ΔG = 0.43 eV), and the RDS for formate generation is *HCOO-to-HCOOH (ΔG = 0.46 eV), while after stripping a ligand, the value is changed into 0.40 eV and 0.54 eV (Fig. 10d), respectively. However, for the intact Ag₉Cu₆ cluster, the energy barrier for H₂ formation is only 0.14 eV, hence HER prevails. The reaction mechanism of Au₂Ag₈Cu₅ is rather complicated, and it turns out that the removal of one alkynyl ligand bonded to two Ag atoms near the shell Au atom is more thermodynamically supported. When the Au₂Ag₈Cu₅ cluster is intact, ΔG of the RDS in *COOH generation is 0.62 eV (Fig. 10e), rather larger than the corresponding RDS energy barrier when removing one ligand (Fig. 10f), suggesting that the latter one favours CO formation. Furthermore, for forming formate, the active site shifted from the Cu site to the Au site, and the RDS changed from *HCOO-to-HCOOH (0.67 eV) to the generation of *HCOO (0.44 eV). Such shift of the active site may be responsible for the decrease of formate selectivity of Au₂Ag₈Cu₅ compared to Ag₉Cu₆.

Fig. 10 Free energy of eCO₂RR and HER on (a) and (b) intact and incomplete Au₇Ag₈, (c) and (d) Ag₉Cu₆ and (e) and (f) Au₂Ag₈Cu₅. Copyright 2022, Royal Society of Chemistry.⁹⁵

The series of the M₁₅ clusters (Ag₁₅, Au₇Ag₈, Ag₉Cu₆, Au₂Ag₈Cu₅) with the identical ligand shell offers a good example to examine the metal core effect of alkynyl-protected metal nanoclusters. For both Ag₁₅ and Au₇Ag₈, CO is formed exclusively with high selectivity. However, with the presence of Cu, formate can be produced by Ag₉Cu₆ and Au₂Ag₈Cu₅, and the two-atom-difference led to drastically different eCO₂RR behaviours. Ag₉Cu₆ has higher FE_formate than Au₂Ag₈Cu₅ at the applied potential, and at high negative potentials, the HER becomes dominant for Au₂Ag₈Cu₅. For Ag₁₅ and Au₇Ag₈, one ligand stripping exposed the Ag and Au atom as the active site to form CO, respectively, while for Ag₉Cu₆ and Au₂Ag₈Cu₅, one ligand stripping exposed the Cu atom as the active site in the eCO₂RR, but the active site was shifted to the Au atom for producing formate for Au₂Ag₈Cu₅. That is, the exposed (111)-like Ag₂Cu₂ surface and the Au₁Cu₁Ag₂ surface are responsible for the markedly different eCO₂RR properties especially the selectivity of formate.

3.3.2. Ligand effects. The surface of the metal nanoclusters is capped by organic molecules, and these ligands can affect the physicochemical properties and electronic structure of the metal nanoclusters hence play a critical role in fine-tuning the electrocatalytic performance of the metal nanoclusters. For instance, an early study by the Jin group discovered that, compared to the phenylselenol (–SePh) protected Au₂₅ cluster, the thiolate Au₂₅ (–PET) cluster exhibited higher FE_CO and mass activity of CO while the former one exhibited higher FE_H2.⁹⁶ DFT calculations revealed that the breakage of ligand carbon tail to expose S/Se atoms as the active site. Due to the higher electron density, the S site is more favourable for forming the key intermediate of *COOH.

Recently, Wang et al. reported a ligand–shell engineering of Au₂₈ clusters toward eCO₂RR, in which Au₂₈(C₂B₁₀H₁₁S)₁₂(tht)₄Cl₄ (Au₂₈–S) and [Au₂₈(C₄B₁₀H₁₁)₁₂(tht)₈]³⁺ (Au₂₈–C) with the identical Au₂₈ metal core showed different eCO₂RR performance.⁹⁷ Specifically, Au₂₈–S is co-protected by carboranethiolate, tetrahydrothiophene (tht), and chloride, while Au₂₈–C is co-protected by carboranealkynyl and tht ligands (Fig. 11a and b). Interestingly, despite the same metal kernel, the different protecting layer leads to the change in the overall electronic structure, e.g. Au₂₈–C has 13 valence electrons while Au₂₈–S has 12. In addition, compared to Au₂₈–C, the emission peak is blue-shifted at about 167 nm with the intensity increased about 5 times for Au₂₈–S. Furthermore, Au₂₈–S exhibited markedly superior eCO₂RR properties than Au₂₈–C. As illustrated in Fig. 11c, Au₂₈–S showed higher FE_CO values in all tested potentials, and it achieves the maximum FE_CO of 98.5% at −0.9 V, about 1.4 times than that of Au₂₈–C. However, both clusters exhibited satisfactory stability with a negligible change of FE_CO (Fig. 11d). In situ FTIR analysis shows that the intensity of the peaks at 1362 and 1636 cm⁻¹ attributed to the *COOH species from Au₂₈–S is stronger than that from Au₂₈–C. Moreover, the authors conducted DFT calculations to unravel the active site and elucidate the reaction mechanism. For Au₂₈–C, the Au atoms in the linear C₂B₁₀H₁₁–C≡C–Au–C≡C–C₂B₁₀H₁₁ motifs are accessible for CO₂ molecules, while removing Cl atoms to expose Au atoms are probably the active center for Au₂₈–S. Both clusters adopt the CO₂(g)–*CO₂–*COOH–*CO–CO pathway, while the formation of *COOH is the rate-determining step. Au₂₈–C has a ΔG of 0.85 eV, much smaller than that of Au₂₈–C with 1.11 eV, while high ΔG of H* adsorption indicated that HER is unfavorable on both Au₂₈–S and Au₂₈–C (Fig. 11e and f), yet Au₂₈–S is even more unlikely for HER to occur.

Fig. 11 The total structures of (a) Au₂₈(C₂B₁₀H₁₁S)₁₂(tht)₄Cl₄ and (b) [Au₂₈(C₄B₁₀H₁₁)₁₂(tht)₈]³⁺. (c) Faradaic efficiency of CO and (d) stability testing of two nanoclusters. Free energy of (e) eCO₂RR and (f) HER on Au₂₈(C₂B₁₀H₁₁S)₁₂(tht)₄Cl₄ (red) and [Au₂₈(C₄B₁₀H₁₁)₁₂(tht)₈]³⁺ (blue). Copyright 2022, Wiley.⁹⁷

In another recent study reported by our group, the surface ligand effect of Ag₃₂ nanoclusters toward eCO₂RR has been probed.³⁷ In this study, two metal clusters, a homoleptic alkynyl-protected Ag₃₂L₂₄ (L = 3,5-bis(trifluoromethylbenzene) acetylide, Ag₃₂ in short) plus thiolate and phosphine co-protected [Ag₃₂(DPPE)₅(SR)₂₄]²⁻ were prepared. Interestingly, thanks to the σ and/or π binding between Ag and the C≡C bond, Ag₃₂ presented more versatile ligand–metal binding motifs than that of [Ag₃₂(DPPE)₅(SR)₂₄]²⁻. Such structure difference leads to drastically different eCO₂RR performance. As demonstrated in Fig. 12a and b, in all the tested potentials, FE_CO of Ag₃₂ is higher than that of [Ag₃₂(DPPE)₅(SR)₂₄]²⁻. The highest FE_CO is 96.44% at −0.8 V for Ag₃₂, while in stark contrast, only a maximum of 56.67% can be achieved at −1.0 V for [Ag₃₂(DPPE)₅(SR)₂₄]²⁻. Impressively, both samples exhibited excellent rather comparable stability for prolonged operation, as no significant current decrease was observed for 15 h (Fig. 12c). DFT calculations disclosed that, one ligand stripping to form Ag₃₂L₂₃ as the active center for Ag₃₂, however, for [Ag₃₂(DPPE)₅(SR)₂₄]⁻, as the Ag–P bond strength is stronger than that of the Ag–S bond, one –SR ligand stripping to form [Ag₃₂(DPPE)₅(SR)₂₃]⁻ as the active center. As depicted in Fig. 12d, the free energy profile shows that [Ag₃₂(C≡C–CH₃)₂₃]⁺ has a smaller energy for forming *COOH (0.4 eV) rather than [Ag₃₂(P₂C₂H₆)₅(SCH₃)₂₃]⁻ (0.5 eV); meanwhile, [Ag₃₂(C≡C–CH₃)₂₃]⁺ has a larger thermodynamic barrier for H₂ formation from adsorbed H* (0.51 eV) compared to [Ag₃₂(P₂C₂H₆)₅(SCH₃)₂₃]⁻ (0.05 eV), indicating a higher CO selectivity. This study indicates that, without the presence of an easy stripping ligand (e.g. halogen), with a similar metal kernel, the alkynyl-protected metal nanocluster might have better eCO₂RR performance than thiolate counterparts.

Fig. 12 Faradaic efficiency of CO for (a) Ag₃₂(C≡CAr^F)₂₄ and (b) [Ag₃₂(DPPE)₅(SR)₂₄]²⁻ at different potentials. (c) Stability testing of two nanoclusters. (d) Comparison of ΔG of eCO₂RR on Ag₃₂(C≡CCH₃)₂₄, Ag₃₂(C≡CCH₃)₂₃, [Ag₃₂(P₂C₂H₆)₅(SCH₃)₂₄]²⁻ and [Ag₃₂(P₂C₂H₆)₅(SCH₃)₂₃]²⁻ with the HER. Copyright 2022, Springer.³⁷

3.3.3. Other factors. There are other factors such as the metal–ligand motif arrangement can also affect the eCO₂RR performance. Recently, Zhu and Pei groups reported the evolution from superatomic Au₂₄Ag₂₀ monomers into molecular-like Au₄₃Ag₃₈ dimeric nanoclusters, whereas the monomeric and dimeric nanoclusters exhibited markedly different eCO₂RR performance.⁷⁵ Specifically, Au₂₄Ag₂₀(C₁₂H₁₃)₂₄Cl₂ (abbreviated as Au₂₄Ag₂₀-1) was first obtained and it can transform into Au₄₃Ag₃₈(C₁₂H₁₃)₃₆Cl₁₂ (abbreviated as Au₄₃Ag₃₈-1), while Au₂₄Ag₂₀(C₉H₇)₂₄Cl₂ (denoted as Au₂₄Ag₂₀-2) was acquired and it was further converted into Au₄₃Ag₃₈(C₉H₇)₃₆Cl₉ (denoted as Au₄₃Ag₃₈-2). The two Au₂₄Ag₂₀ monomer has the identical Au₁₂@Ag₂₀ metal kernel, which undergoes a self-assembly pathway to form a Au₁₂@Ag₁₉–Ag–Au₁₂@Ag₁₉ kernel, and forms two dimeric Au₄₃Ag₃₈ nanoclusters eventually (Fig. 13a). As shown in Fig. 13b, the monomers had higher current density than the dimers, while the CO partial current density followed the descending order of Au₂₄Ag₂₀-1 > Au₂₄Ag₂₀-2 > Au₄₃Ag₃₈-1 > Au₄₃Ag₃₈-2 (Fig. 13c). Notably, the monomers exhibited much higher FE_CO than the dimers in the potential range from −0.4 to −0.8 V (Fig. 13d and e). The authors ascribed the different catalytic performances to the atomic-packing structures (individual-core vs. dual-core) and surface motif arrangements (parallel vs. crossed).

Fig. 13 (a) Structural anatomy of four nanoclusters. (b) LSV curves and (c) the corresponding CO partial current density of the Au₂₄Ag₂₀ and Au₄₃Ag₃₈ nanoclusters. Faradaic efficiency for CO₂RR products obtained on (d) Au₂₄Ag₂₀ and (e) Au₄₃Ag₃₈ nanoclusters. Copyright 2022, Royal Society of Chemistry.⁷⁵

4. Challenges and perspectives

In this review, we highlight the uniqueness of employing alkynyl molecules to prepare Au and Ag nanoclusters with atomic precision. Compared to thiolate molecules, the versatile binding motifs between alkynyl molecules and Au/Ag atoms facilitate the alkynyl-protected nanoclusters to possess drastically different optical properties, geometric configurations, and electronic structures. More intriguingly, significantly different catalytic performances can be achieved. The metal core effect and the surface ligand effect of the all-alkynyl-protected Au and Ag nanoclusters toward the eCO₂RR are discussed, demonstrating their great potential to modulate the performance through metal exchange or ligand shell engineering. Some conclusions and future perspectives can be drawn from the above discussions:

(1) Developing efficient synthetic methods with high yield to obtain more atomically precise alkynyl-protected metal nanoclusters is still highly desirable. Currently, in the case of Au and Ag, only Au₂₂,⁵¹ Au₂₃,⁵² Au₃₆,⁴⁷ Au₅₀,³⁰ Au₆₇,⁹⁸ Au₉₉,⁵³ Au₁₁₀⁵⁶ and Ag₁₅,⁸⁹ Ag₃₂,³⁷ Ag₅₁,⁴³ and Ag₇₄⁹⁹ clusters with crystal structures have been documented. The case for Cu,¹⁰⁰ AuCu,¹⁰¹ AgCu,⁹⁴ AuPd,⁶³ AuPt⁴⁵ and AgPt¹⁰² is quite rare, and some (e.g. AgPd) are even missing. The family members of homoleptic alkynyl-protected metal nanoclusters need to be enriched. The direct reduction method is quite straightforward, but to control the monodispersity of the precursor is critical and deserves more research efforts. A synchronous nucleation and passivation strategy has proved its applicability for preparing single metal and some bimetallic nanoclusters with molecular purity, but more cases and examples need to be explored to confirm its generality. For ligand exchange and the one-pot method, more precise reaction condition control (e.g. the choice of introducing a ligand, the temperature, the concentration of the reactant, and the post-reaction purification) is essential to achieve precise synthesis;

(2) To obtain more valuable products from the eCO₂RR, synthesizing a Cu nanocluster or a Cu-alloy nanocluster is indispensable hence holds great potential. With Au and Ag nanoclusters as catalysts, CO is formed with high selectivity, and in the presence of Cu, HCO₂H can be acquired. So far, no C₂₊ products have been documented by coinage metal nanoclusters with atomic precision, regardless of the ligand type. In the presence of a hydride ligand for Cu nanoclusters, the negatively charged hydride plays a critical role in determining the selectivity, where HCO₂H is formed via the lattice-hydride mechanism.¹⁰³ The above case of all-alkynyl-protected Cu alloy clusters (Ag₉Cu₆, Au₂Ag₈Cu₅) shows that the HCO₂H formation occurs through a rather different proton-hydride mechanism or hydride-proton mechanism. Obtaining all-alkynyl-protected Cu nanoclusters for the eCO₂RR application would be of particular interest to advance the fundamental understanding of the eCO₂RR mechanism but still remains challenging, and Cu based bimetallic or trimetallic nanoclusters are also hoped to promote the activity and selectivity as well as to improve the mechanistic understanding.

(3) Advanced in situ techniques are urgently needed to examine the eCO₂RR process for gaining more mechanistic insights. For instance, DFT calculations predict one ligand stripping to expose undercoordinated metal atoms as the active site but losing several ligands might not necessarily favour the eCO₂RR reaction for the Ag₁₅ cluster, and in situ techniques such as surface enhanced Raman scattering, infrared spectroscopy, high energy X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS),¹⁰⁴ X-ray absorption near-edge structure (XANES),¹⁰⁴ small-angle neutron scattering (SANS)¹⁰⁵etc. can provide some key structural information to confirm that. In addition, the in situ monitoring of the structural evolution of the catalyst during the eCO₂RR process may detect the metal–carbon cleavage, the adsorption configuration of CO₂ molecules on metal nanoclusters, the interaction between the cluster and the electrolyte, the real active site, the evolution of the key intermediates, and the formation process of the product more accurately.

In summary, with the advancement of synthetic methods, more molecular homoleptic alkynyl-protected metal nanoclusters with precise structures are anticipated to be discovered. The unique surface binding motifs and different physicochemical properties of the all-alkynyl-protected coinage metal nanoclusters can bring profound and valuable mechanistic insights of the eCO₂RR. We envision that increasing research efforts should be dedicated to this fundamentally interesting yet important field.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Z. T. acknowledges the financial support from Natural Science Foundation of Guangdong Province (No. 2022A1515011840).

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