Customizing the Structure, Composition, and Properties of Alloy

Article Cite This: Acc. Chem. Res. 2018, 51, 2784−2792

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Customizing the Structure, Composition, and Properties of Alloy Nanoclusters by Metal Exchange Published as part of the Accounts of Chemical Research special issue “Toward Atomic Precision in Nanoscience”. Shuxin Wang,† Qi Li,‡ Xi Kang,† and Manzhou Zhu*,† †

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Department of Chemistry and Centre for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei, Anhui 230601, China ‡ Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States CONSPECTUS: The properties of metal materials can be greatly enriched by including various elements to generate alloys. The galvanic replacement represents a classical method for the preparation of both bulk- and nanoalloy materials. The difference of the electrochemical potential between the two metals acts as the driving force for the galvanic replacement reaction. However, this classical rule partially fails at the ultrasmall size scale, for that novel chemistry emerges by the decrease of the size of materials down to less than 3 nm due to the strong quantum effect. In this Account, we discuss an emerging topic of nanochemistry, the metal exchange in atomically precise ultrasmall ( Cd > Co > Ni > Pb > Cu > Hg > Ag > Pd > Pt > Au) and such metal exchange reactions in the nanocluster range is, to a large extent, related with the electron shell closing and the structural stability of nanoclusters. In the subsequent sections, we present effective control over the number, position, and distribution of the dopants. The shape and structure of the final alloy products can be tailored by recently developed metal exchange methods. More importantly, modulation and enhancement of the properties of NCs through metal exchange are realized. For example, the largely increased quantum yield and the significantly improved catalytic activity. In addition, we shall also discuss the real-time characterization of the metal exchange reaction by the combination of UV−vis absorption spectroscopy, high resolution electrospray ionization mass spectrometry (ESI-MS), matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and single crystal X-ray diffraction (SC-XRD). By controlling the charge of the templating metal nanoclusters and the different types of metal complexes, the driving force of metal exchange has been studied, which is considered to be the thermodynamics rather than the electrochemical potential. In summary, the metal exchange reactions in the ultrasmall nanocluster range are totally different compared with the case of larger-sized metal nanoparticles. Depending on this novel method, atomically precise alloy nanoclusters can be prepared by reacting the nanocluster composed of inert metal (such as Au) with complexes of high-activity metals (e.g., Cd/Hg/Cu/Ag). We anticipate that future research on the metal exchange will contribute to the fundamental understanding of reaction behavior of metal atoms in ultrasmall nanoclusters and to the design of alloy nanoclusters with enhanced properties.

1. INTRODUCTION

vs 0.337 V versus the standard hydrogen electrode, respectively), Fe tends to be oxidized to Fe2+ accompanied by the reduction of Cu2+ to Cu. This galvanic replacement has also been used for the synthesis of alloy nanoparticles.3−5 For instance, Au/Pd/Pt hollow nanostructures could be synthesized by reacting the template (Ag nanocubes) with the Au(III)/Pd(II)/Pt(II) metal salts.

Metal replacement, as a simple method for the extraction of a more inert metal at the expense of an active metal, was reported in ancient China more than 2000 years ago, and is currently exploited widely for metal welding and alloying. The driving force of this metal replacement (also called galvanic replacement) is found to be the difference in electrochemical potential between the two metals.1,2 For example, as the reduction potential of Fe2+/Fe is more negative than the Cu2+/Cu potential (−0.440 © 2018 American Chemical Society

Received: July 3, 2018 Published: November 2, 2018 2784

DOI: 10.1021/acs.accounts.8b00327 Acc. Chem. Res. 2018, 51, 2784−2792

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Accounts of Chemical Research

3. METAL EXCHANGE WITH METAL COMPLEX

Those larger gold nanoparticles which show optical properties controlled by surface plasmon resonances (SPR). Customizing the structures and compositions of larger nanoparticles will induce changes of SPR and further affect the SPR-dependent optical (absorption) and catalytic properties. Compared with large nanoparticles, ultrasmall gold/silver nanoparticles (often called nanoclusters, Cd > Co > Ni > Pb > Cu > Hg > Ag > Pd > Pt > Au. The specific exchange with Cd(II)/Hg(II)/Cu(II)/Ag(I)-complex demonstrated that the metal exchange does not follow the classical metal activity sequence under the condition of using metal complexes as the metal source. It is worth noting that the high yield of metal-exchange reaction (e.g., ∼100% for the synthesis of the HgAu24 nanocluster), fast conversion reveals that such nanoclusters would not be decomposed in the doping process. The negative charge of homo-Au25 nanoclusters retained when being doped with Cu/Ag; however, doping Cd/Hg into the nanocluster produced the neutral [MAu24(SR)18]0. According to the shell-closing electron count for [Aux (SR) y ] q nanoclusters,31 each [M25(SR)18]q nanocluster exhibited 8 shell-closing electrons which fully fills the 1S (2e) and 1P (6e) of the delocalized “superatomic orbitals”. Therefore, the homo-Au25 and alloyed M25 nanoclusters are analogous to the noble-gas atom (i.e., Ne), hence exceptionally stable. The detailed exchange condition of Au25(SR)18− with Cu(SR)2 provides more evidence for the stability of superatom nanoclusters. Direct exchange of Au25(SR)18− with Cu(SR)2 only resulted in the decomposition of Au25 nanoclusters with no Cu x Au 25−x alloy nanoclusters produced. However, the CuxAu25−x alloy nanoclusters could be synthesized by the addition of reductant agent (NaBH4) into the reaction system. According to the shell-closing electron count, doping Cu(II) into the homogold nanocluster will change the 8e− close-shell to 7e− open-shell (without the full filling of superatomic orbitals), and thus, the [CuAu24(SR)18]0 nanocluster would rapidly decompose. The presence of NaBH4 enables the reduction of the [CuAu24(SR)18]0 to negatively charged [CuAu24(SR)18]− with 8e− close-shell, and hence, it is stable. More importantly, these results also explain why only single Cd/Hg atom can be doped into the [Au25(SR)18]− nanocluster. The reversible exchange of [Au25(SR)18]− nanoclusters with AgSR complexes further indicated the metal exchange between gold nanoclusters and metal−thiolate complexes does not follow the metal activity sequence. As shown in Figure 2,

2. EARLY WORKS In early work, Murray and co-workers reported the reaction of the [Au25(SC2H4Ph)18]− nanocluster with AgNO3, which was considered as a redox process.25 In 2012, Wu carried out an investigation on the reaction of [Au25(SC2H4Ph)18]0 with inorganic metal salts.26 Based on the experimental results, Wu updated the reactivity sequence as Fe > Ni > Pb > Au (up to ∼3 nm size) > Cu >Ag and proposed the antigalvanic reduction (AGR) between gold nanoclusters and metal ions of Cu and Ag elements. Of note, this so-called AGR process was found to be affected by solvent.27 Theoretical work has predicted the structure of the doped [MAu24(SR)18]q nanoclusters (M = Ag/Cu/Zn/Cd.).28,29 The calculation results manifest two significant trends: (1) Doping the foreign atom into the nanocluster has a strong delocalized electron shell closing (DESC) effect at ns = 8, suggesting a superatom picture for these nanoclusters; and (2) the Ag/Cd doped Au25 nanoclusters possess almost the same energy as the parent Au25. 2785

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Figure 2. Reversible metal exchange between Au25(SR)18− and AgxAu25−x(SR)18−. Adapted with permission from ref 30. Copyright 2015 American Chemical Society.

MALDI-TOF-MS spectra clearly show that the silver atoms in AgxAu25−x could be re-exchanged by AuSR complexes. Thus, the metal exchange is a concentration-driven reversible reaction. Metal exchange can also be used to synthesize nanoclusters with multimetals. By exchanging the Pt1Ag24(SR)18 nanocluster with the AuSR complexes, Kang et al. produced the Pt1AuxAg24−x(SR)18 nanocluster; the X-ray crystallographic structure indicated the gold atoms are doped into the surface of the icosahedral metal core without changing the geometric structure.32 Yang et al. applied a two-step metal exchange method for the synthesis of a trimetal M1AgxAu24−x(SR)18 nanocluster.33 In this work, the AgxAu25−x(SR)18− nanocluster was first synthesized by reaction of the homogold Au25(SR)18− nanocluster with the AgSR complex and then the obtained bimetallic nanocluster further reacted with M(SR)2 complexes (M = Cd/Hg) which finally produced M1AgxAu24−x(SR)18. Very recently, Negishi and co-workers reported the synthesis of MAg4Au20(SR)18 (M = Pd/Pt) nanoclusters by doping the MAu24(SR)18 nanoclusters with AgSR complexes.34 Structure-retained metal exchange generally requires the same ligands in the nanocluster as well as in the metal complexes. For example, lightly doped AgxAu23‑x(SR)16− maintains the structure of Au23(SR)16− nanocluster.35 In addition, metal exchange with thiolated metal complexes can also be applied to the silver nanoclusters. Due to the thermo-dynamically favorable results, the gold tends to be doped into the inner shell of the Ag44(SR)30 nanoclusters. The crystal structure of the directly synthesized [Au12Ag32(SR)30]4− nanocluster indeed shows a Au@Ag core-shell structure.13,36 Interestingly, the Xie group reported the surface metal exchange between [Ag44(p-MBA)30]4− and the Au-pMBA complex and presented an isomeric [Au12Ag32(SR)30]4− nanocluster with an inverted core-shell structure, i.e., [email protected] This thermodynamically less favorable Ag@Au structure was considered to be that the Ag20 external shell of [Ag44(pMBA)30]4− prevents the gold atoms entering into the kernel. The surface metal exchange provides a new way to precisely customize the structures of alloy nanoclusters as well as in-depth understanding of metal synergy at the atomic level. Du et al. reported the alloy AgxAu50−x nanocluster with the maximum of seven gold atoms by using Ag50 as the template to react with AuSR complexes (Figure 3).38 The X-ray crystallographic structures of Ag50 and alloy AgxAu50−x were similar, and the only difference is that the hollow Ag12 kernel has been partially

Figure 3. Partial metal exchange of the Ag12 metal core with gold in the Ag50 nanocluster. Adapted with permission from ref 38. Copyright 2017 American Chemical Society.

exchanged by gold atoms. Doping the gold atoms into the silver nanocluster largely enhanced the stability of homosilver nanoclusters. 3.2. Structure-Altered Metal Exchange

The structure of nanoclusters is highly dependent on the capped ligands. Exchanging with different types of ligands usually results in the size transformation and structure change of the nanocluster. For instance, Chen et al. reported the ligand exchange of Au18(SR) 14 with HS-Adm ligand, which produced the Au21(SAdm)15 nanocluster.39 Similar as the ligand exchange of homogold nanocluster, metal exchange reaction using the metal complexes with different ligands usually leads to the alternation of the structure. Kang et al. reported that the exchange of the Pt1Ag24(SR)18 nanocluster with the AuSR complexes produced Pt1AuxAg24−x(SR)18 nanocluster.32 As a comparison, using Au(PPh3)Br instead of AuSR complexes resulted in the Pt 2 Au 10 Ag 13 (PPh 3 ) 10 Br 7 nanocluster (Figure 4). 32 The Au(PPh3)Br complex plays three roles: (i) as the gold source that is doped into the metal core; (ii) as the phosphine ligand source that peels the outside Ag2(SR)3 shell; and (iii) as the Br− ion source that connects the two independent PtAg12 units to form the Pt2Au10Ag13(PPh3)Br7 nanoclusters. Combination of the ligand- and metal-exchange provides a new route for highly efficient synthesis of alloy nanoclusters. Unlike the [Au25(SR)18]−, metal exchange shows the strong ability to tailor the atomic structure of another nanocluster: [Au23(SR)16]− (Figure 5). Li et al. reported site-specific replacement, surface-structure reconstruction and total-structure transformation induced by Ag/Cd/Cu into the [Au23 (S- cyclohexyl)16]−.35,40,41 This series of metal exchange work starting from [Au23(SR)16]− demonstrates the fascinating 2786

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[AgxAu18−x(Dppm)6Br4]2+ bimetallic nanocluster (Figure 6).42 However, the gold atoms in other positions were unable to be substituted by Ag.

Figure 6. [Agx Au18−x (Dppm) 6Br4]2+ nanocluster obtained by targeted metal exchange of [Au18(Dppm)6Br4]2+. Adapted with permission from ref 42. Copyright 2017 American Chemical Society.

Figure 4. Shape-controlled synthesis of sphere and rodlike PtAuAg trimetallic nanoclusters by the metal exchange method. Adapted with permission from ref 32. Copyright 2016 WileyVCH.

3.4. Single-Atom Doping Using Metal Exchange Method

Single Cd/Hg atom doping into Au25(SR)18− could be achieved due to the different valence electron configuration of Cd/Hg (d10s2) compared with Au (d10s1).30,43,44 However, as the doping of silver/copper (d10s1) atoms into the homogold nanocluster will not change the number of delocalized s electrons, it is difficult to control the doping number of silver/ copper atoms into gold nanoclusters and this usually resulting a distribution of Cu/Ag dopants in the alloy nanoclusters, such as AgxAu25−x(SR)18−.30 A new strategy of hollowing-and-refilling has been devised, in which a single Ag or Cu atom should easily fill into a hole which is preformed within the gold nanocluster.45 Wang et al. added the CuCl or AgCl salt into the solution containing hollow [Au24(PPh3)10(SC2H4Ph)5Cl2]+ nanoclusters, and single Cu/ Ag doped [MAu24(PPh3)10(SC2H4Ph)5Cl2]2+ (M = Cu/Ag) was successfully produced (Figure 7).45 The thermodynamics and ligands are considered as the driven force of single atom shuttling, i.e., for the hollowing process the thermodynamics drives the central Au atom to squeeze the waist gold atom out of the nanocluster, which is further trapped by excess PPh3 in solution; then, the surface ligands (−Cl and −SR) tow the Ag+/ Cu+/Au+ into the nanocluster for the doping process. The stronger binding of AgCl compared with AgSR leads to the Ag atom doping at the apex of the nanocluster, while the similar energy of CuCl and CuSR results in that the Cu atom doping into both the waist and apex sites. Due to the different electronic structure of Cd (d10s2), Cd2+ cannot be doped into the hollow Au24 nanocluster, which further indicates that the electronic structure plays a key role in the metal exchange process. Doping heterometals from the same group of elements does not change the number of free valence electrons. However, the electronic structure (HOMO−LUMO gap as well as the charge density) of nanoclusters is actually changed. Kang et al. synthesized the AgxAu18−x(SR)14 (x = 0,1) nanoclusters by reacting the Au18(SR)14 template with the AgSR complexes. Further ligand exchange of the mixed nanocluster with HSAm produced AgAu16(SAdm)13 nanoclusters (Figure 8).46 Using ligandexchange to amplify the minor electronic structure changes from doping and then obtaining alloy nanoclusters with new size provides a new strategy to synthesize cognate single-atom-doped nanoclusters. Doping a single gold atom into homosilver nanoclusters was also achieved by the Bakr group. The PdAg24(SR)182− was used as a template to react with Au salt, and the central Pd atom in the parent nanocluster was replaced by the Au atom, which resulted in the single Au doped AuAg24(SR)18− nanocluster.47

Figure 5. “Metal exchange” story of [Au23(SR)16]−. (A) Structure tailoring of [Au23(SR)16]− through doping with various metal−thiolate complexes. (B) Molecular surgery on the [Au23(SR)16]−: two-step metal-exchange induced a transformation from the original [Au23(SR)16]− through [Au23−xAgx(SR)16]− (x ∼ 1) to the final [Au21(SR)12(P−C−P)2]+. Adapted with permission from ref 35. Copyright 2017 American Association for the Advancement of Science.

chemistry of metal exchange for manipulating the nanoclusters’ atomic structures, which greatly expands the borders of doping chemistry. 3.3. Targeted Metal Exchange

For some of the nanoclusters which are protected by biligands (e.g., phosphine and halogen), the lattice energy of silverhalogen (i.e., AgBr, 905 kJ/mol) is much lower than that of gold/ copper-halogen (i.e., AuBr, 1070 kJ/mol; CuBr, 969 kJ/mol); silver-exchange of the gold atoms that are bonded with the halogen is more energetically favorable. For example, four gold atoms which linked with Br in [Au18(Dppm)6Br4]2+ could be specifically exchanged by silver atoms, which results in a 2787

DOI: 10.1021/acs.accounts.8b00327 Acc. Chem. Res. 2018, 51, 2784−2792

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Figure 7. Doping single Ag or Cu atom into the hollow [Au24(PPh3)10(PET)5Cl2]+ nanocluster. Adapted with permission from ref 45. Copyright 2017 Nature Publishing Group.

Figure 8. Synthesis of single Ag doped AgAu16(SAdm)13 nanocluster by the combination of metal exchange and ligand-exchange strategies. Adapted with permission from ref 46. Copyright 2018 American Chemical Society.

4. INTERCLUSTER METAL EXCHANGE In recent studies, metal exchange reactions between two nanoclusters have been reported from different groups. The reader is referred to reviews in the literature.48−50

the strong sd hybridization of Au atoms due to the relativistic effect, the lowest unoccupied molecular orbital (LUMO, which contributed by Au 6s and 6p in Au25 nanocluster) will be significantly altered (Figure 9b). Such substitution gives rise to a larger energy gap. The ultrafast relaxation dynamics together with the DFT calculations indicates the boost in quantum yield is due to the high oscillator strength of these HOMO → LUMO transitions.52 Finally, highly luminescent nanoclusters are particularly useful in the labeling of cancer cells and imaging applications (Figure 9c). Recently, Kang et al. reported that metal exchange of the Pt1Ag24(SR)182− nanocluster with AuSR, which results in a trimetallic Pt@AgxAu12‑x@Ag12(SR)182− nanocluster. Pt@ AgxAu12‑x@Ag12(SR)182− shows a drastically enhanced stability, but the photoluminescence is almost quenched. Interestingly, metal exchange with AuBrPPh3 leads to the structure transformation from spherical Pt1Ag24(SR)18 to rodlike Pt 2 Au 10 Ag 13 (PPh 3 ) 10 Br 7 which comprises two PtAu 5 Ag 7 units via sharing a vertex silver atom, and the photoluminescence drastically increased about 150-fold (0.1% vs 14.7%).32 The photoluminescence of Ag25 was found to be significantly affected by Au doping. Bakr and co-workers reported the synthesis of the center-Au-doped AuAg24(SR)18− nanocluster through doping Ag25 with AuClPPh3.21 The photoluminescence of doped nanocluster enhanced about 25-fold compared with the Ag25

5. TAILORING THE PROPERTIES OF ALLOY PRODUCTS BY METAL EXCHANGE Metal exchange is an effective way to alter the physical and chemical properties of the homometal nanoclusters. More importantly, the precise control of the number and position of dopants provides an excellent chance to study the metal synergetic effect at the atomic level. 5.1. Luminescence

The rod-like [AgxAu25−x(PPh3)10(SR)5Cl2]2+ nanocluster can be synthesized by “bottom up” and “top down” metal exchange method with different x ranges (Figure 9a). The “top down” method results in less-silver-doped [AgxAu25−x(PPh)10(SR)5Cl2]2+ nanocluster (x ⩽12). On the contrary, a unique Ag13Au12 species can be found in the “bottom up” products, which shows ultrabright luminescence (QY ∼ 40.1%).51 According to the X-ray structure analysis of Ag-doped M25 (M = Au/Ag) rodlike nanoclusters, the doped Ag atoms first substitute the two Au atoms at the vertex site, and subsequently the Au atoms at the waist as well as in the center. When 13 Au atoms are exchanged by silver, strong distortion to the electronic structure occurs. Because of 2788

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Figure 9. (a) Crystal structure of homogold [Au25(PPh3)10(SR)5Cl2]2+ and [AgxAu25−x(PPh3)10(SR)5Cl2]2+ with different x range. (b) UV−vis absorption, photoexcitation (PLE), and photoluminescence properties of [AgxAu25−x(PPh3)10(SR)5Cl2]2+ (x ⩽ 13). (c−f) Confocal images of human cancer cells (7402) incubated with [AgxAu25−x(PPh3)10(SR)5Cl2]2+ (x ⩽ 13) nanocluster. Adapted with permission from ref 51. Copyright 2014 WileyVCH.

6. UNDERSTANDING THE METAL EXCHANGE IN THE NANOCLUSTER RANGE According to the principle of metal electronegativity, direct synthesis of Au−Ag alloy nanoparticles should result in alloy nanoparticles with gold in the center and silver at the shell. However, experimental determination of AgxAu25−x(SR)18− nanocluster by X-ray crystallography reveals that the silver atoms are in the metallic core instead of the shell.57 On the other hand, the relative energy of Ag doped AgAu24(SR)18 nanocluster suggests that dopant silver in the core exhibits the lowest energy relative to other positions.28,29 Therefore, the driving force of the metal exchange could be the reduction of the total energy of nanoclusters. Similar results could also be observed in Pt/Pd/Cu doped Au25 nanoclusters and other alloy nanoclusters of different sizes, such as AgxAu38‑x(PET)24, Ag3Au15, and AgAu16 nanoclusters. The most extreme cases are the Ag46Au24 and Au57Ag53 nanoclusters prepared by in situ synthesis method, which contain a silver core surrounded by gold atoms. These nanoclusters are found to be extremely stable at room temperature under the air condition.58,59 In view of metal activity that was found in nanoparticles and bulk metal, metal exchanging of Au25(SR)18− nanocluster with AgSR complexes would most likely produce the shell-doped AgxAu25−x(SR)18− nanoclusters, since exchanging the Au(I) at the shell sites with Ag(I) does not involve the oxidation−reduction reaction. Meanwhile, the resulting shell-doped AgxAu25−x(SR)18− nanocluster satisfies the most stable structure according to the previously reported alloy nanoparticles,60 that is, an inert metal core surrounded by a less inert metal shell. However, the metal exchange on Au25(SR)18− by Ag heteroatoms results in the same structure as that from direct synthesis.30 More importantly, the obtained AgxAu25−x(SR)18− nanocluster can be re-exchanged by AuSR complexes back to the homogold Au25(SR)18− nanocluster. Such results suggest that the metal exchange between homogold Au25(SR)18− nanoclusters and the Ag-doped products is reversible.30 The reversible exchange has clearly proved that the metal exchange in nanocluster scale undergoes a completely different mechanism compared with the galvanic replacement. Two-step metal exchange of [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ to [MAu24(PPh3)10(SC2H4Ph)5Cl2]2+ (M = Au/Ag/Cu)

template. Recently, Du et al. reported AuxAg50−x(Dppm)6(SR)30, which exhibited an enhanced emission in the near-infrared region (∼870 nm) compared with Ag50.38 These dramatic enhancements of photoluminescence demonstrate that the gold doping is an efficient way to enhance the photoluminescence of homosilver nanoclusters. In these cases, the retained framework in the heteroatom doping as well as the atomic precision of the doping process provide an ideal model for understanding the origin of photoluminescence at the atomic level. 5.2. Chirality

Metal exchange has also been applied in the chiral nanocluster system. Kang et al. reported the synthesis of Ag3Au15(SPhMe2)14 nanocluster by exchanging Au18(SPhMe2)14 with Ag-SPhMe2 complexes.53 The X-ray crystallographic structure reveals that the silver atoms are doped into the middle layer of the trilayered Au9 (3:3:3) metal core. The DFT calculation reported by Molina et al. showed that Ag doping of the Au18(SR)14 nanocluster disturbed the optical and chiroptical properties of the parent nanocluster, especially the optical peak at 3.49 eV.54 Kumara et al. reported the crystal structure of AgxAu38−x(SR)24 and demonstrated that the preferred doping positions in Au38 for silver atoms is the Au23 core surface.55 Of note, doping silver atoms into the Au23 core surface not only changes the CD spectra significantly, but also lowers the racemization temperature.22 Heavily Ag-doped AgxAu38−x(SR)24 nanoclusters showed racemization even at room temperature. 5.3. Catalysis

Metal exchange method allows introduction of high-activity metals (e.g., Cd) into the more inert homogold nanoclusters. The accumulated dielectric charge density of such alloy nanocluster demonstrated that the central Cd atom depleted the electronic charge density and illustrated the positive charge of the central Cd atom. Such accumulation resulted in electron transfer from Cd to Au. Accordingly, the gold atoms in the CdAu24 nanocluster are more negative in Cd1Au24 relative to Au25, and thus the CdAu24 nanocluster exhibits higher catalytic activity (with TON of 1162.9) than the homogold Au25 (TON = 635.5) in aerobic benzyl alcohol oxidation.56 2789

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Accounts of Chemical Research provides more details and enables us to gain deeper understanding of metal exchange behavior in the ultrasmall size range; that is, the metal exchange is driven by the thermodynamics, rather than the electrochemical potential (for details, please see section 3.4).45

and obtained M.S. in Macromolecular Chemistry and Physics from Fudan University in 2014.

7. LIMITATIONS OF METAL EXCHANGE AND FUTURE PERSPECTIVES The metal exchange method has shown its high flexibility in synthesis of alloy nanoclusters with atomic precision. However, this field is just starting to flourish. Currently, the metal exchange reaction is focused on the reaction of the Au/Ag nanoclusters with group 10, 11, and 12 elements or nanoclusters. Only a few cases about doping with other group metals (such as Ir) have been reported.61 Theoretically, a variety of metals (such as Zn, Cd, Hg, Ni, Pt, Pd, Fe, Mn, etc.) can be doped into gold nanoclusters, which result in the alternation of the chemical-physical properties.28,29,62 However, only a few of them have been successfully achieved. The driving force of electrochemical potential in metal exchange has been ruled out in the nanocluster size range. The superatom model can explain the stability of such doped products. Meanwhile, the thermodynamics as the driving forces for the metal exchange reaction have been observed. However, the reaction process is still not fully understood. Details of the mechanistic aspects remain a mystery, such as (i) what is the relationship between the dopant and the electronic/geometrical structures of the cluster; (ii) what is the role of ligands in the metal exchange process; and (iii) how does the heteroatom pass the metal complex shell and then substitute the gold atoms in the inner metal core? Doping specific number of heterometal atoms into specific positions of the nanocluster template is still one of the most challenging tasks in the nanofield. For alloy nanoclusters, exchange of a single metal atom may bring significant changes to properties, which greatly stimulates further research on such alloy nanoclusters. The metal exchanged products have shown significances in optics, catalysis, and other fields. We believe that future research on atomically precise alloy nanoclusters via the metal-exchange strategy will lead to a new generation of smart nanomaterials bearing intriguing functions.

Manzhou Zhu is currently the Changjiang Chair Professor of Chemistry at Anhui University. He received his Ph.D. in Chemistry from the University of Science and Technology of China (USTC) in 2000. He then conducted postdoctoral research at USTC and Carnegie Mellon University (CMU, Pittsburgh, USA). He joined the chemistry faculty of Anhui University in 2010. His current research interests include atomically precise nanoclusters, structure−property correlation of nanoclusters, and applications.

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Xi Kang is now a Ph. D candidate of Chemistry supervised by Prof. Manzhou Zhu at Anhui University. He received his B.E. in Department of Chemistry from Anhui University in 2014.

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ACKNOWLEDGMENTS We acknowledge research support by NSFC (U1532141, 21631001, 21871001, and 21803001), the Ministry of Education, the Education Department of Anhui Province, 211 Project of Anhui University.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shuxin Wang: 0000-0003-0403-3953 Manzhou Zhu: 0000-0002-3068-7160 Notes

The authors declare no competing financial interest. Biographies Shuxin Wang is a Research Professor of Chemistry at Anhui University. He received his B.S./M.S. and Ph.D. in Chemistry of Anhui University from 2004 to 2016. As a joint Ph.D student, he studied at Carnegie Mellon University under the supervision of Prof. Rongchao Jin between 2014 and 2016. After that, he joined the chemistry faculty of Anhui University in 2016. Qi Li is now a Ph.D candidate of Chemistry supervised by Prof. Rongchao Jin at Carnegie Mellon University. He received his B.E. in Polymer Materials and Engineering from Zhejiang University in 2011 2790

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