New Advances in Atomically Precise Silver Nanoclusters

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New Advances in Atomically Precise Silver Nanoclusters Jie Yang, and Rongchao Jin ACS Materials Lett., Just Accepted Manuscript • DOI: 10.1021/acsmaterialslett.9b00246 • Publication Date (Web): 04 Sep 2019 Downloaded from pubs.acs.org on September 5, 2019

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ACS Materials Letters

New Advances in Atomically Precise Silver Nanoclusters Jie Yang†, ‡, * and Rongchao Jin†, * †Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ‡School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China ABSTRACT: Atomically precise noble metal nanoclusters are ultra-small particles that are typically composed of tens to hundreds of metal atoms in the core (equivalent sizes 1-3 nm). This new class of nanomaterials is unique in that they are atomically precise and possess uniform structures, high stability, and attractive properties. Built on the significant success in Au nanoclusters, Ag nanoclusters have recently received increasing attention. The majority of reported silver nanocluster sizes exhibit molecular-like properties, whereas larger ones exhibit plasmons characteristic of metallic state (as opposed to molecular state in smaller sizes). Both molecular (i.e., nonmetallic) and metallic nanoclusters hold promise in a wide range of applications. To deepen the understanding of their physical and chemical properties, precise control over size and determination of the crystal structure are the top priorities. In recent developments, dozens of silver nanoclusters with definite formulas have been prepared through various strategies, albeit the structural determination still lags behind. In this short Review, we summarize the recent progress in ligand-protected silver nanoclusters, including the size-focusing synthetic methods, new sizes, structures and properties.

Atomically precise noble metal nanoclusters (NCs) with sizes below 3 nm have emerged as a new class of materials. Such NCs exhibit unique properties.1 Their synthesis, molecular formulas, structures and properties, as well as potential applications, have been widely investigated in recent years.2 Among the NCs, Au and Ag are the two important types, and various sizes of Au and Ag NCs have been reported.1-10

Ag nanoclusters,1-7 here we focus our discussions on the recent 3 years of advances in Ag(0) NCs with different sizes and well-defined molecular formulas that have been prepared via robust synthetic strategies. In particular, we highlight some recent advances in understanding the formation mechanism of Ag NCs, together with their structures and properties. On a note, Ag(I) NCs are not included in this Review.

While intensive research on Au NCs has been done,1 Ag NCs are less explored.5 Silver and gold are in the same group and have nearly identical atomic radii (2.89 vs 2.88 A), but Ag in the zero valent state is more reactive and easier to be oxidized than Au, which makes Ag NCs difficult to prepare and investigate the properties. The determination of crystal structures of Ag NCs is still quite challenging due to the often occurred disorders and less stability than Au NCs. Nevertheless, there has been impressive progress; for example, the achievements of thiolate (SR) protected Ag44(SR)3011,12 and Ag25(SR)1813 crystal structures are an important step for understanding the structures of Ag NCs.

1. Progress in understanding the formation mechanism. Typically, Ag NCs are prepared by reduction in solution, and the “bottom-up” methods are the most extensively used, while some are “top-down” routes.4,18,19 Understanding the synthetic mechanisms has largely benefited from the recent, effective measurements, including optical spectroscopy, mass spectrometry (MS), nuclear magnetic resonance (NMR), and X-ray absorption (XAS) spectroscopy by monitoring the synthetic process.

Atomically precise metal nanoclusters possess molecular-like properties, e.g., HOMO–LUMO transitions,14 chirality,15,16 luminescence,17 catalytic reactivity,7 etc. These properties are determined by the size, composition and structure of NCs. A small change in the formula or structure often gives rise to large differences in the properties, which necessitates the atom-precise control in the synthesis of NCs. Some recent reviews have summarized the synthesis methods for preparing Au and

Wu’s group used UV-Vis absorption spectroscopy to monitor the synthesis evolution of the Ag14 and Ag46 NCs.20,21 Zhang and Xie et al.22 studied the evolution of thiolate-stabilized Ag NCs by using electrospray ionization mass spectrometry (ESI-MS), Fig. 1A, which provided a detailed understanding of the synthesis mechanism. As the sampling was slow, NaOH was added to slow down the reduction kinetics, which helped to analyze the reaction intermediates.22 The formation of Ag17(SPh-tBu)123− and Ag44(SPh-tBu)304− was found not to follow the typical “bottom-up” route in which discrete intermediates with different number of Ag atoms are produced during the process; rather, the processes involved Ag-thiolate cluster

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intermediates with tens of Ag atoms and a similar number of

Figure 1. A. Time-dependent ESI-MS spectra. The spectra show the evolution of atomically precise thiolate-stabilized Ag nanoclusters in the reaction. The rectangles highlight the evolution of the final products, Ag17(SPh-tBu)123− and Ag44(SPh-tBu)304−. The sampling started as soon as methanol was added, the reaction time in each panel: a. 0 min, b. 2 min, c. 4 min, d. 7 min, e. 10 min, f. 15 min and g. 25 min; B. Trace of phosphine ligands with 31P NMR in CDCl3. a. dppp, b. Ag+dppp+PA, c. Ag+dppp, reaction for 2h d and 12h e after adding NaBH4; C. XANES of Ag29 NCs at various stages of the synthesis. Adapted with permission from ref.22,23,25. Copyright 2018 Springer Nature and 2017,2018 American Chemical Society, respectively.

ligands, and the numbers of Ag atoms in the intermediates were found to determine the final size of Ag NCs. In recent work by Zhang’s group,23 NMR measurements were carried out to explore the function of phosphine in the synthetic evolution, Fig.1 B. For the alkynyl-protected Ag74, the bidentate phosphine did not participate the protection of this cluster, but it played a key role in the synthesis process. It was used as a Ag mediator which must be present in the synthesis, and mechanistically, it first binds with Ag (then released as oxide). This gives an insight and also a novel method for Ag NCs preparation. Pradeep et al.24 reported a new silver cluster [Ag59(2,5DCBT)32]3− which can be used as a precursor to prepare the [Ag44(2,4-DCBT/4-FTP)30]4−, [Ag25(2,4-DMBT)18]− and [Ag29(1,3-BDT)12(PPh3)4]3− by transforming Ag59 to those smaller sizes through thiol-etching. In this work, they employed UV-Vis and ESI-MS to study the details of conversion, and suggested that the transformations were caused by the recombination of the generated fragments. According to the ESI results, after adding the new ligand, the Ag59 are dissociated, giving rise to Ag-thiolate fragments like [Ag3(SR)4]−, [Ag4(SR)5]−, [Ag5(SR)6]−, etc., and then under the induction of the ligand added, they recombined and transformed to new clusters. De Groot and co-workers investigated the synthesis mechanism of Ag29(LA)123− via mass spectrometry, X-ray absorption spectroscopy (XAS), and optical spectroscopy.25 The introduction of XAS provided details of electronic structure and geometry information of the synthesis evolution, Fig.1 C. They found that the formation of Ag29 involved an etching process, in which large-sized products of Ag~100 was first generated, followed by size focusing to Ag29. The employment of UV-Vis, ESI-MS, NMR, and XAS has brought new insights in terms of understanding both the

detailed information of the formed ultra-small particles and their formation mechanisms. Such advances will help chemists to achieve a better control in the synthesis and make the routes more efficient and high-yielding. 2. Progress in obtaining new sizes and structures In addition to the mechanistic understanding, recent advances in the synthesis of Ag NCs have also led to a number of new sizes. In early work, a thiol-protected Ag NC that contains 7 silver atoms was reported by Wu et al,18 and Bakr et al achieved a distinct Ag NC in 2009 (later identified to be Ag44(SR)304-).5 Recently, the synthesis of Ag NCs has been extended to quite large sizes.26 On the smaller end, some recent silver NCs are Ag11,27 Ag14,20 Ag25,13 Ag29,28 Ag44,11,12 etc. The first breakthrough in crystal structure determination of Ag NCs was Ag44,11,12 and then the Ag2513 which shares an identical structure as that of Au25. In the last three years, some impressive progress has been obtained in silver nanocluster research. Table 1 lists those well-characterized Ag NCs in recent works. The stability and properties of NCs are influenced by a number of factors, including the metal core structure and the surface ligand,1,2 as well as the surface coordination between the core and the ligand.29,30 Thiol and phosphine are widely used to protect Ag NCs, and alkynyl has also drawn recent attention. Furthermore, two or more types of ligands have also been used to protect Ag NCs. Here, we categorize the new works of Ag NCs based on the ligands, i.e., single type of ligand protected Ag NCs,5,18 and two or multiple types of ligands protected ones.28,29 2.1 Ag NCs protected by single type of ligand In Pradeep’s work, the [Ag59(SR)32]3− synthesis24 involved 2,5-DCBT and its geometric isomer (2,4-DCBT) were both tested as ligands for synthesizing the Ag59 NCs. Fig.2 A shows the ESI-MS results. It is interesting to note that the 2,5-DCBT can form Ag59, while the 2,4-DCBT tends to form


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ACS Materials Letters Ag44 under the same synthesis conditions. Such a ligand effect involving the substituent position was earlier explored in Au NCs synthesis.31 The [Ag59(2,5-DCBT)32]3−

can also be etched to form three other sizes: Ag25, Ag29 and Ag44, and the conversion mechanism was found to be not similar as the ligand-exchange route.

Table 1. Silver Nanoclusters Protected by Various Types of Ligands.

formula

Ligand(s)

crystal structure

ref(s)

core

shell

[Ag13]5+

[Ag7{Se2P(OiPr)2}12]5-

33

]5+

]4−

33

Single ligand [Ag20{Se2P-(OiPr)2}12]

diselenophosphate

]+

[Ag21{Se2P(OEt)2}12

[Ag25(2,4-DMBT)18]−

NH4[Se2P-(OiPr)2] NH4[Se2P(OEt)2]

thiolate

[Ag44(SR)30]4−

[Ag13

[Ag8{Se2P(OEt)2}12

2,4-dimethylbenzenethiol

-

24

2,4-dichlorobenzenethiol;

-

24

4-fluorothiophenol; 4-chlorobenzenethiol [Ag46S7(2,4-DMBT)24]NO3


[Ag59(2,5-DCBT)32]3−

2,5-dichlorobenzenethiol

Ag51(tBuC≡C)32

alkynyl

Ag74(C≡CPh)44

Ag6S@Ag32

tert-butylethynide phenylacetylene

Ag(SR)3, Ag(SR)2, SR

21

-

24

Ag@Ag14

38

Ag4@Ag22@Ag48

23

Mixed ligands [Ag19(dppm)3(PhC≡C)14](SbF6)3

alkynyl,

Phenylacetylene,

phosphine/

bis(diphenylphosphino)methane

[Ag34(BTCA)3(C≡ CBut)9(tfa)4(CH3OH)3]SbF6

p-tert-butylthiacalix[4]-arene, trifluoroacetate, methanol

[Ag25(dpppe)3(MeOPhC≡C)20](SbF6)3

Phenylacetylene,

C

≡

CBut,

Ag13

Ag6(dppm)3(PhC≡C)14

50

Ag@Ag12

Ag5@BTCA

52

Ag13

Ag12(dpppe)3(MeOPhC≡C)20

50

Ag10

Ag12(dppe)4(2,5-DMBT)12Cl4

45

1,5-bis(diphenylphosphino)pentane [Ag22(dppe)4(2,5-DMBT)12Cl4]2+

thiolate,

1,2-bis(diphenylphosphino)ethane;

phosphine calixarene


Ag23(PPh3)8(SC2H4Ph)18

Phenylethanethiol, phosphine

[Ag32(dppm)5(SAdm)13Cl8]3+

bis(diphenyphosphino)methane,

48 Ag13

Ag19S13Cl8P10

46

49

1-adamantanethiol Ag33(SCH2CH2Ph)24(PPh3)4

Phenylethanethiol,triphenylphosphine

Ag13

Ag20S24P4

[Ag40(2,4-DMBT)24(PPh3)8]

2,4-dimethylbenzenethiolate, triphenylphosphine

Ag8

Ag32(SPhMe2)24(PPh3)8

[Ag45(dppm)4(S-But)16Br12]3+

bis(diphenyphosphino)methane,

Ag23

Ag22S16P8Br12

Ag14

Ag32(SPhMe2)24(PPh3)8

43,44 46

tert-butyl mercaptan [Ag46(2,5-DMBT)24(PPh3)8]2+

2,5dimethylbenzenethiolate, triphenylphosphine

Ag51(BDT)19(TPP)3

1,3-benzenedithiol, triphenylphosphine

Ag78(DPPP)6(SR)42

1,3-bis(diphenyphosphino)propane, SPhCF3;

Ag78(R/S-BDPP)6(SR)42

2,4-bis-(diphenylphosphino)pentane, SPhCF3

[Ag141X12(SAdm)40]3+

thiolate, halide

Ag146Br2(SR)80 Ag48(C≡CtBu)20(CrO4)7

+inorganic

43,44

-

42

Ag@Ag21

47

Cl, Br, I,1-adamantanethiolate

Ag71

Ag70X12(S-Adm)40

53

Br,4-isopropylbenzenethiolate

Ag51

Ag95Br2S80

54

CrO42−,tBuC≡C−

Ag23

[Ag25(C≡CtBu)20]

55



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Figure 3. Total structures of A. Ag46S7(SPhMe2)24; B. Ag74(C≡ CPh)44; and C. Ag51(tBuC ≡ C)32. Reproduced and redrawn with permission from ref. 21, 23, 38. Copyright 2018 Wiley-VCH, 2017 American Chemical Society and 2019 Royal Society of Chemistry, respectively.

Figure 2. ESI-MS spectrum: A. [Ag59(2,5-DCBT)32]3− (inset: comparison of the experimental and simulated isotope patterns confirms the assigned composition; B. [Ag46S7(SPhMe2)24] + (inset: the calculated isotopic pattern agrees well with the experimental one); C. Ag74(C ≡ CPh)44; D. Ag51(tBuC ≡ C)32 (inset: the experimental isotopic distribution pattern). Adapted with permission from refs. 24,21,23,38. Copyright 2019 Royal Society of Chemistry, 2018 Wiley-VCH and 2017 American Chemical Society, respectively.

Wu et al. have recently reported a new, 2,4dimethylbenzenethiolate-protected Ag NC: [Ag46S7(2,4DMBT)24]NO3.21 Fig.2B shows the ESI-MS spectrum and Fig.3 A is the structural analysis. It contains interstitial sulfur atoms in the crystal structure but such atoms did not cause any distortion or expansion of the lattice, which goes against with the typical theory of solid solution. This Ag46 possesses a Ag6S@Ag32 face-centered cubic structure surrounded by Ag(SR)3 motifs and asymmetric µ4-S. The introduction of sulfur atoms brings special chemical bonds and structural features; they show short Ag-Ag and Ag-S bond lengths with larger charge density. The reason for the different fluorescence between the crystallized and the amorphous forms of Ag46 was interesting, but details still require further work in the future. Liu’s group reported the first Se-donor ligand-capped, 8electron superatom [Ag20{Se2P-(OiPr)2}12] and + 32 [Ag21{Se2P(OEt)2}12] NCs, which were prepared via a ligand change reaction based on their previous work.3335They successfully determined the structures of these NCs and compared with the parent dithiophosphate-protected Ag20 and Ag21 NCs. The [Ag20{Se2P-(OiPr)2}12] shares a similar structure as that of [Ag20{S2P(OiPr)2}12], both having a Ag-centered Ag13 icosahedral core with 7 capping Ag atoms and 12 diselenophosphate ligands. While the [Ag21{Se2P(OEt)2}12]+ is also of the same 8e [Ag13]5+ core as that of [Ag21{S2P(OiPr)2}12]+, some differences are indeed present on the shell, that is, the eight capping Ag atoms form a large cube to engage the Ag13 core, which gives rise to a perfect T symmetry.

Alkynyl has been employed for the synthesis of a number of Au NCs and Au-Ag alloys.36,37 Recently, alkynylprotected Ag NCs have also been reported, and these exhibit good stability and crystallizability. Zhang23 introduced alkynyl as ligands for the synthesis of Ag74 by using both phenylacetylene and 1,3bis(diphenylphosphino)propane in the synthesis. The product was determined to be [Ag74(C≡CPh)44]2+ via ESIMS, Fig.2 C. This was indeed the first report of all-alkynylprotected Ag NC. The crystal structure exhibits a threeshell structure, Fig.3 B. The kernel is a tetrahedron of 4 silver atoms, which is surrounded by a Ag22 shell, and the remaining 48 Ag atoms are in the outermost shell. Lu and Xie38 used the tert-butylethynide ligand to protect the Ag NCs, and the molecular formula and crystal structure of Ag51(tBuC≡C)32 was obtained via ESI-MS and single crystal X-ray analysis, Fig.2 D and Fig.3 C. The structure is also arranged into three-shells, Ag@Ag14@Ag36, which is capped by 32 tBuC ≡ C− on the surface. Ag51 exhibits strong fluorescence, and the fluorescence of Ag51 is very sensitive to and dependent on the solvent polarity, which provides great potential in future sensing and imaging applications. 2.2 Ag NCs protected by two or multiple types of ligands Ag NCs can be protected with mixed ligands like thiolate, alkynyl, phosphine and halide. Through cooperation between different ligands, a better stability of NCs and new properties could be achieved, and a few crystal structures have been reported, including Ag14,39 Ag16,40 Ag29,12 Ag67,41 etc. Here we discuss some new works of Ag NCs protected by two or multiple types of ligands. Fig.4 shows the total structures of the selected Ag NCs. 2.2.1. Ag NCs co-protected by thiolate and phosphine. Thiolate and phosphine ligands have an excellent compatibility in protecting metal NCs. Pradeep et al. obtained a new Ag51(BDT)19(TPP)3 nanocluster (BDT: 1,3benzenedithiol, and TPP: triphenylphosphine).42

Au25(SR1)18+Ag51(SR2)19→Au25-xAgx(SR1)18 Scheme 1. Reaction between Ag51(BDT)19(TPP)3 and Au25(SR)18. Redrawn from refs. 42. Copyright 2017 Royal Society of Chemistry.


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Figure 4. Total structures of the selected Ag NCs protected by two or multiple types of ligands. Redrawn from refs. 43,45-50,52-55. Copyright 2017, 2018, 2019 American Chemical Society, 2017, 2019 Royal Society of Chemistry and 2018 Springer Nature, respectively.

They studied and compared the chemical reactivity of this dithiolate-protected cluster with that of the monothiolateprotected one.In addition, the reaction between Ag51 and Au25 only showed metal ion exchange, in contrast with the fact that both metal ion and ligand exchange occurred between monothiolate-protected Ag25 and Au25, Scheme 1. This gives a new insight into the reaction between nanoclusters. The Ag40 and Ag46 NCs co-protected by phosphine and thiolate have been reported by both Zhu’s and Pradeep’s groups.43,44 The two NCs share the same shell of Ag32S24P8, while the metal cores are arranged into different types. The Ag46 contains a Ag14 core with a fcc structure. In contrast, Ag40 presents a newly found loose Ag8 core with a simplecubic structure. The two reports used different synthesis methods. Zhu et al. directly synthesized the NCs in one pot, while Pradeep et al. used the ligand exchange-induced structure transformation of Ag18 into new sizes. In Zhu’s work, they successfully transformed Ag40 to Ag46 via a ligand exchange strategy. Although Ag40 and Ag46 share the same shell, they showed different optical absorption behavior, which revealed that the core structure has an important role in controlling the optical properties.

2,5-DMBT (2,5-dimethylbenzenethiol) in combination with 1,2-dppe (i.e., bis(diphenylphosphino)ethane) was also used to prepare [Ag22(dppe)4(2,5-DMBT)12Cl4]2+ in Pradeep’s recent work.45 The formula and crystal structure have been obtained. Ag22 exhibits crystallization-enhanced photoluminescence. Metal nanoclusters with chiral configurations have aroused great attention due to their chiro-optical properties and applications in optics, catalysis, etc. The reported chiral Ag NCs are not as prosperous as Au NCs, especially the structures of chiral Ag NCs are still rare. Nevertheless, two chiral Ag NCs ([Ag32(dppm)5(SAdm)13Cl8]3+ and [Ag45(dppm)4(SBut)16Br12]3+) were synthesized by Zhu et al. (dppm = bis(diphenyphosphino)methane, SAdm = 1adamantanethiol and S-But = tert-butyl mercaptan).46 The crystal structures were determined, which help to understand the chiral properties. They found that the Ag32 possesses an achiral Ag13 kernel and Ag45 also has an achiral core of 23 Ag atoms, but the cores are protected by chiral shells: Ag19S13Cl8P10 and Ag22S16P8Br12, respectively. Thus, the chirality of Ag32 and Ag45 NCs is caused by the asymmetric distribution of the ligands.



In Zheng’s work,47 a pair of optically pure enantiomers Ag78 NCs capped by chiral 2,4-bis-(diphenylphosphino) pentane and SPhCF3 were achieved by one-pot synthesis, which was inspired by analyzing the structure of racemic Ag78 NCs protected by achiral 1,3bis(diphenyphosphino)propane and -SPhCF3. This enantioselective synthesis method is desirable, as separation of a racemic mixture is generally very difficult. Such chiral nanoclusters may be promising for applications in chemistry, biology and medicine. al.48

Liu et designed a facile one-pot method to prepare chiral Ag23 protected by mixed ligands of phosphine and phenylethanethiolate. The crystal structure contains a helical fcc kernel that is composed of two twisted fcc units. They further studied the optical properties based on the structure via time-dependent density functional theory simulations. In Chen’s work,49 a new chiral Ag33(SCH2CH2Ph)24(PPh3)4 nanocluster was synthesized with the participation of a Pd regent. The structure was determined by single-crystal Xray and further studied by NMR and electronic circular dichroism (ECD) spectra. The Ag33 contains a Ag13 icosahedron core and a chiral shell of Ag20S24P4. The Pd(PPh3)4 played a crucial role in the formation of Ag33 by assisting the formation of Ag33 but not replacing silver atoms to form alloys. The mechanism needs to be further studied. 2.2.2. Ag NCs capped diphosphine/calixarene.

by

alkynyl

and

Wang et al.50 prepared two alkynyl-protected silver nanoclusters, Ag19 and Ag25, and the full formulas were determined as [Ag19(dppm)3(PhC ≡ C)14](SbF6)3 and [Ag25(dpppe)3(MeO–PhC ≡ C)20](SbF6)3 (where, dpppe is 1,5-bis(diphenylphosphino)pentane). These clusters comprise an anticuboctahedral Ag13 kernel. This is different from the thiolate-protected clusters, where the 13 metal atoms grow into icosahedral and cuboctahedral structures. The alkynyl ligand may bring new insight into nanocluster preparations. In Wang’s previous work,51 [Ag35(H2BTCA)2(BTCA)(C≡ CBut)16](SbF6)3 was reported with crystal structure (where, H4BTCA: p-tert-butylthiacalix[4]-arene), and the calixarene was confirmed that it can be used as a ligand to protect the NCs. Wang et al. carried out further study, and by an improved the synthesis method, [Ag34(BTCA)3(C ≡ CBut)9(tfa)4(CH3OH)3]SbF6 (tfa is trifluoroacetate) was prepared, which displayed a better stability.52 Ag34 has a kernel of centered icosahedron Ag@Ag12 which is capped with 21 peripheral silver atoms, and the calixarene are arranged in Ag5@BTCA as binding motifs. They pointed out that the coordination saturation plays important role in the stability of NCs. 2.2.3. Ag NCs capped by thiolate and halide. Zheng et al.53 investigated Ag NCs capped by bulky 1adamantanethiolate together with halides as ligands. Such

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NCs have higher surface reactivity than other ligandsprotected NCs due to a lower coverage of surface thiolates. It can become water-soluble by ligand-exchange, which is important for use in bio-applications. The bulky-ligandcovered surface also provides opportunities for the formation of plasmonic multiple-twinned Ag nanoparticles. The formula was determined as [Ag141X12(SAdm)40]3+ (X = Cl, Br, I) by ESI-MS, and they also studied the crystal structure. Zhu & Jin et al.54 also tackled the large-sized Ag NCs. A thiolate-protected Ag146 with molecular-like optical properties was synthesized via a one-pot method. The formula was determined to be Ag146Br2(TIBT)80 and its crystal structure shows 51 Ag atoms in the core and a Ag95Br2S80 shell capping the core. The femtosecond electron dynamics and optical absorption revealed that Ag146 NCs still possess a molecular-like nature. 2.2.4. Ag NCs capped by organic and inorganic ligands. Sun et al.55 introduced inorganic CrO42− anions mixed with alkynyl tBuC≡C− to synthesize Ag48. In this work, the crystal structure show that the inorganic oxo anions have important interactions with both the core and the shell. The involvement of both organic and inorganic protection is a new path for synthesizing Ag NCs and controlling of the formation and structure. In summary, in this review we have introduced the recent works on silver nanoclusters capped with single, two or multiple types of ligands. The employment of ESI, NMR, and X-ray spectroscopy to study the evolution of Ag clusters is quite illuminating. The mixed-ligands-capped Ag clusters are generally more stable and also easier in crystallization. Nevertheless, the attainment of new, Ag clusters protected by a single type of ligands is still critical, especially their crystal structures, for the fundamental studies, especially the relationship between the kernel and the shell. The case of protection by single type of ligands is apparently simpler and thus more effective to unravel the kernel-shell interaction mechanism. We expect that the Ag NCs will make as much progress as Au NCs in the near future. For future perspective, we outline the following aspects. New stable sizes. Although a number of Ag NCs have been obtained with determined formulas, the sizes are not sufficiently complete. New sizes of Ag NCs with predicted magic numbers remain to be synthesized, which calls for new synthetic strategies. Further work may identify more sizes of Ag NCs and reveal the formation mechanism and the relationships between size and properties. Transition of molecular to metallic state. Future research on the new sizes of Ag NCs (especially >100 atoms) will lead to insights into the electronic transition from molecular to metallic state. In the case of gold, a sharp transition from nonmetallic Au246 to metallic Au279 has been mapped out.56,57 Although a few large sizes have been reported, detailed investigation on their electronic

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ACS Materials Letters properties remain to pursue in future work. It would be of fundamental interest to compare the Ag and Au systems. Such studies will lead to fundamental understanding of the electronic structure evolution and quantum effects.58,59 Crystallization of Ag NCs. Among the reported Ag NCs, only a few of them have been crystallized. To better understand the fundamental mechanisms and properties, it is necessary to gain the detailed information of their structures. The stability and crystallizability of Ag NCs are the key points, which calls for future breakthroughs. Especially, more efforts are needed for the Ag NCs with single type of ligands (such as Agn(SR)m), as these can offer insight into the basic relationship between the kernel and shell. Optical properties. One of the advantages of Ag NCs is their attractive optical properties, which have potential in applications such as biology, medicine, diagnose, etc. However, there are still challenges in understanding the mechanism of fluorescence and how to retain the strong fluorescence from rapid attenuation caused by insufficient stability, solvent, pH, temperature, etc. The roles of ligand and the silver core, their bonding mode, size dependency, and the electronic structure are all important and should be investigated in future work.

AUTHOR INFORMATION Corresponding Author * (J.Y.) [email protected] and (R.J.) [email protected].

ACKNOWLEDGMENT J.Y. acknowledges the support by the National Natural Science Foundation of China (Grants No. 51801077).

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