Toward the Tailoring Chemistry of Metal Nanoclusters for Enhancing

Article Cite This: Acc. Chem. Res. 2018, 51, 2764−2773

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Toward the Tailoring Chemistry of Metal Nanoclusters for Enhancing Functionalities Published as part of the Accounts of Chemical Research special issue “Toward Atomic Precision in Nanoscience”. Tatsuya Higaki, Qi Li, Meng Zhou, Shuo Zhao, Yingwei Li, Site Li, and Rongchao Jin*

Acc. Chem. Res. 2018.51:2764-2773. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/20/18. For personal use only.

Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States CONSPECTUS: Ultrasmall metal nanoparticles (often called nanoclusters) possess unique geometrical structures and novel functionalities that are not accessible in conventional nanoparticles. Recent progress in their synthesis and structural determination by X-ray crystallography has led to deep understanding of the structural evolution, structure−property correlation, and growth modes, such as the layer-by-layer growth in face-centered cubic (fcc)-type nanoclusters, linear assembly of vertex-shared icosahedral units, and other unique modes. The enriched knowledge on the correlation between the structure and the properties has rendered metal nanoclusters a new class of functional nanomaterials. Despite the significant achievements in structural determinations, mapping out the structure−property correlation is still very challenging because of the core−shell structures of nanoclusters (e.g., Aun(SR)m protected by thiolate ligands) with metal atoms partitioned between the core and the shell. In such structures, the core and the surface are entangled and cannot be separately studied because changing the core structure would inevitably change the surface (or vice versa). Thus, it is of great importance to develop the “tailoring” chemistry for structural modification of the core (or surface) while retaining the other parts, in order to achieve fundamental understanding of what part of the nanocluster structure plays what role in the functionalities. In this Account, we summarize some recent work on the strategies to control the atomic structures of metal nanoclusters for tuning their properties, such as stability, optical absorption, excited-state electron dynamics, and photoluminescence, as well as their catalytic reactivity. The development of a ligand-based strategy has permitted the synthesis of structural isomers of nanoclusters with the same size but different functionalities. Successful modification of the core (or surface) structure while maintaining the other components has led us to gain some fundamental understanding of the respective roles of the core and the surface in the nanocluster functionalities. Such “tailoring” chemistry on metal nanoclusters can provide a strong basis for functional nanomaterials consisting of nanocluster components with desired properties. Further development of the tailoring chemistry will guide materials chemists to new directions and tailor-made functional nanomaterials for specific applications.

1. INTRODUCTION The chemistry of nanoparticles has a major issue of “no two nanoparticles are the same”, even for very tightly distributed nanoparticles (e.g., 5% standard deviation). This poses major problems in studies of precise size dependences of the properties. To eliminate this issue, recent efforts have developed atomically precise nanochemistry.1−9 It has now become possible to precisely control metal nanoparticles in the 1−3 nm range (often called nanoclusters). The atomic precision and molecular purity of nanoclusters (NCs) have enabled precise determination of their atomic structures by single-crystal X-ray crystallography.1 Such progress has revealed intriguing noncrystalline structures, such as icosahedral10,11 and decahedral12,13 structures, as well as crystalline structures, including face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal close-packed (hcp) ones.14 The fcc/bcc/hcp structures were previously thought to be unstable in the nanocluster regime, and thus, the observations of bcc/ hcp gold nanoclusters are remarkable since bulk gold and regular nanoparticles exclusively adopt the fcc structure. © 2018 American Chemical Society

The ultrasmall size of metal nanoclusters gives rise to quantum confinement effects and intriguing properties that are not observed in conventional nanoparticles.1 Compared with the continuous electronic band structure in plasmonic nanoparticles, nanoclusters exhibit molecular-like discrete energy levels with nonzero gaps,1 and the optical absorption spectra of nanoclusters show multiple peaks in the UV−vis− NIR region, in contrast to the single plasmon band for conventional spherical nanoparticles. Very recently, the transition from the plasmonic state to the molecular state in gold nanoclusters has been reported to occur between Au246(SR)80 and Au279(SR)84.15 Such a sharp transition over size indeed goes against 50 years of theoretical prediction by Kubo’s theory,1 according to which the band gap (Eg) should smoothly evolve as Eg ∼ EF/n, where EF and n represent the Fermi energy and the number of free valence electrons, respectively. Below the size of Au246, the electronic structure Received: July 31, 2018 Published: October 29, 2018 2764

DOI: 10.1021/acs.accounts.8b00383 Acc. Chem. Res. 2018, 51, 2764−2773

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Accounts of Chemical Research

layer growth of fcc-Au52(SR)32 to realize different functionalities,21,22 and (3) the tailoring chemistry of the surface structure while maintaining the core geometry.23−25 Overall, the tailoring chemistry has led to critical understanding of the specific roles of the core and the surface in optical absorption, excited-state electron dynamics, and PL as well as in the catalytic reactivity.

and optical properties of nanoclusters exhibit molecular-like features dictated by the detailed geometrical structure and are thus hard to predict simply from the size.1 For example, structural isomerism has recently been discovered in metal nanoclusters.16 Given the structural importance, meticulous analysis of structure−property correlations is of great importance for understanding the universal rules in nanoclusters. To date, major efforts have been spent on determining the structures of nanoclusters and understanding the core and surface structure evolution, for example, the structural evolution pattern of fcc-type Au8n+4(TBBT)4n+8 nanoclusters (n = 3−6, TBBT = 4-tert-butylbenzenethiolate)17 and linear assembly of icosahedral units.18 Despite the recent impressive progress in structure determination and exploration of new properties, major tasks still remain. One of them is how to understand the structure− property correlations in nanoclusters (e.g., Aun(SR)m), which is still difficult because the core and the surface are integrated and cannot be separately studied and also because changing the surface structure inevitably alters the core geometry (or vice versa) as a result of core−shell bonding and the natural trend for stable structures. Therefore, modification of the core (or surface) structure while maintaining the other component has long been a major challenge. The goal of achieving a fundamental understanding of the respective roles of the core and the surface in the nanocluster functionalities raises the necessity for the development of new tailoring chemistry on metal nanoclusters. Recent progress has led to some successes in tailoring gold nanoclusters and revealing the core/surface roles in specific functionalities, for example, altering the surface of Au28 without affecting the core to understand the catalysis,23 tailoring the surfaces of Au102 and Au103 with a common Au79 core to understand the electron dynamics,24 and site-specific “surgery” of Au23 to achieve large photoluminescence (PL) enhancement.25 In this Account, we focus on the emergent tailoring chemistry of metal nanoclusters (Scheme 1), which is critical

2. TAILORING THE CORE STRUCTURE 2.1. Isomerism in Au38(PET)24

Isomerism is very popular in organic molecules. In nanoclusters, structural isomers are defined as two nanoclusters sharing the same formula but having different geometrical structures. Tian et al.16 reported the first structural isomerism in Au38(PET)24 (PET = SCH2CH2Ph) by X-ray crystallography. The new Au38 nanocluster (Au38T) is protected by the same number and type of thiolate ligands as in the Au38 nanocluster previously reported by Qian et al. (Au38Q).26 The two nanoclusters show significantly different structures. Au38Q has an icosahedral Au23 core protected by six dimeric (−S− Au−S−Au−S−) and three monomeric (−S−Au−S−) staples, and the Au23 core consists of two icosahedral Au13 units that share a Au3 facet along the common threefold rotational axis (Figure 1). The two face-fused icosahedrons are bridged by

Scheme 1. Tailoring Chemistry of Metal Nanoclusters

Figure 1. (A) Total structure of Au38Q. (B) Anatomy of the biicosahedral Au23 core of Au38Q. (C) Top and side views and (D) enantiomers of Au38Q. Color code: magenta/blue/green = gold; yellow = sulfur; gray = carbon. Reproduced with permission from ref 14. Copyright 2016 Royal Society of Chemistry.

for tuning their properties and functionalities. This achievement is built on the major progress in the synthesis and structural determination of metal nanoclusters. This Account will cover (1) the structural isomerism of Au38(SR)24 with significantly different properties of different isomers,16 (2) a ligand-based strategy enabling the achievement of new crystalline phases that are not observed in bulk gold or conventional nanoparticles, including the bcc Au3819 and hcpAu30,20 as well as the achievement of tailoring the layer-by-

three monomeric staples along the C3 axis. The top and bottom of the Au23 core are further protected by six dimeric staples, making up two triblade patterns and inducing chirality in Au38Q. On the other hand, Au38T possesses a Au23 core made up of a single icosahedral Au13 with a Au12 cap comprising tetrahedral Au4 units, and this core is further protected by 2765


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synthesis for Au38Q.26 The differences in their structures also lead to different catalytic properties. For example, Au38T offers higher activity than Au38Q in catalytic 4-nitrophenol reduction (Figure 3C).16 Furthermore, femtosecond spectroscopic analysis reveals different electron dynamics of the two isomers.27 Upon photoexcitation at 1050 nm, Au38Q shows a rapid decay with τ ∼ 1.5 ps followed by a slower relaxation in 5.0 ns (Figure 3D). The rapid process is assigned to core−shell charge transfer or electronic rearrangement within the metal core. In comparison, Au38T shows a shorter excited-state lifetime (τ ∼ 1.0 ps) for the first relaxation stage. The coherent phonon frequency is also different between Au38Q (25 cm−1) and Au 38T (27 and 60 cm −1 ). 27 Overall, this study demonstrates that careful selection of synthetic protocols enables tailoring of the geometrical structure while retaining the formula, and the isomerization impacts the nanocluster stability, optical absorption, catalytic performance, and electron dynamics.

trimeric, dimeric, and monomeric motifs as well as a bridging thiolate (Figure 2).16

2.2. Controlling the Core’s Crystalline Phase

The Au38 isomerism led to different structures and thus very different properties. Recently, a ligand-based strategy was developed to tailor the crystalline phase of Au38, which led to even more drastic changes in properties. Using 1-adamantanethiol (HS-Adm) as opposed to the PET ligand, Liu et al.19 reported a new Au38 formulated as Au38S2(S-Adm)20, whose structure was surprisingly bcc, which is not adopted by bulk gold or regular nanoparticles. bcc-Au38 shares the same number of gold atoms as Au38(PET)24, though the ligand is different. bcc-Au38 was synthesized by thermal size focusing at 90 °C. Au38S2(S-Adm)20 possesses a bcc-Au30 core (Figure 4), which is protected by four dimeric motifs and eight bridging thiolates. The trapezoidal Au5 facets are further protected by bridging sulfido ligands (μ3-S bare atoms), which is unique and not observed in other Au38 nanoclusters.

Figure 2. (A) Total structure of Au38T. (B) Anatomy of the capped icosahedral Au23 core of Au38T. (C) Step-by-step protection of the Au23 core by various motifs. Color code: yellow = sulfur; gray = carbon; other colors = gold in different positions. Adapted with permission from ref 16. Copyright 2015 Springer Nature.

The structural isomerism of Au38(PET)24 induces distinct differences in their properties. Figure 3A shows the differences in the isomers’ optical absorption spectra.16 Besides, Au38T is less thermodynamically stable and can be transformed to Au38Q by thermal treatment (Figure 3B); therefore, Au38T is synthesized via a nonthermal approach and chromatographically isolated, as opposed to the harsh thermal size-focusing

Figure 3. (A) Optical absorption spectra of Au38Q and Au38T. (B) Time course of the absorption spectrum during thermal conversion from Au38T to Au38Q. (C) Catalytic reduction of 4-nitrophenol. (D) Electronic relaxation pathways. Reproduced (A−C) with permission from ref 16 and (D) from ref 27. Copyright 2015 Springer Nature and 2017 American Chemical Society, respectively. 2766


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arrangement (Figure 6D−F) and is protected by trimeric and monomeric motifs and six bridging thiolates. This fcc-Au30 was

Figure 4. (A) Total structure of bcc-Au38S2(S-Adm)20. (B) Dimeric staples and sulfido bridges highlighted in green and by arrows, respectively. (C) The bcc-Au30 kernel with slight distortions indicated by arrows. (D) The ideal bcc-Au30 kernel. (E) Facets of the bcc-Au30 kernel. Color code: yellow = sulfur; gray = carbon; other colors = gold at different positions. Reproduced with permission from ref 14. Copyright 2016 Royal Society of Chemistry.

Figure 6. (A) Total structure of hcp-Au30(S-Adm)18. (B, C) Fourlayer hcp structure of the Au18 core. (D) Total structure of fccAu30(S-tBu)18. (E, F) Three-layer fcc structure of the Au22 core. Reproduced (A−C) with permission from ref 20 and (D−F) from ref 29. Copyright 2016 Wiley-VCH and 2016 American Chemical Society, respectively.

The bcc-Au38 nanocluster shows different properties in contrast to the bi-icosahedral Au38 because of its intriguing core geometry. For example, the optical absorption spectrum of bcc-Au38 shows two peaks at 650 and 750 nm with Eg ∼ 1.5 eV,19 in contrast to the multiple peaks for bi-icosahedral Au38 with Eg ∼ 0.9 eV.26 bcc-Au38 exhibits stronger NIR emission at 964 nm28 (Figure 5) and remarkable photocatalytic activity by

prepared by partial thermal size focusing followed by chromatographic separation. Recently, Higaki et al.20 achieved an Au30 protected by 18 adamantanethiolates. This new Au30 was synthesized by size focusing at room temperature followed by selective precipitation with dichloromethane. In contrast to the fcc core of Au30(S-tBu)18, Au30(S-Adm)18 has a Au18 core with an hcp structure protected by six dimeric motifs, three each on the top and bottom along the C3 axis (Figure 6A−C). hcp-Au30 belongs to the S6 point group and thus is achiral. The hcp-Au18 core can also be viewed as an assembly of tetrahedral Au4 units.20 The optical absorption spectrum of fcc-Au30 shows a single band at 620 nm (Figure 7), and the solution has a greenish

Figure 5. (A) Optical absorption spectrum of bcc-Au38. (B) PL spectra of bcc-Au38 under N2 and O2. Reproduced (A) with permission from ref 19 and (B) from ref 28. Copyright 2015 WileyVCH and 2017 American Chemical Society, respectively.

singlet-oxygen formation. Under irradiation at 532 nm, bccAu38 selectively catalyzed oxidation of methyl phenyl sulfone to the sulfoxide with 57% conversion, whereas bi-icosahedral Au38 showed no conversion under the same conditions.28 Higher catalytic performance of bcc-Au38 was also observed for oxidation of benzylamine to the imine, with 99% conversion under 455 nm irradiation, in contrast to the mere 20% conversion by bi-icosahedral Au38.28 These results demonstrate that the ligand-based strategy to tailor the core crystalline phase can dramatically tune the properties, such as the optical absorption, PL, and photocatalytic reactivity. The ligand-based strategy was also used to tailor the core crystalline phase of different-sized nanoclusters, such as Au30. Previous work by Dass et al.29 reported a Au30 NC protected by 18 tert-butylthiolates. The Au22 core of Au30(SR)18 shows an interpenetrating bicuboctahedron structure with an fcc

Figure 7. Optical absorption spectra of (A) hcp-Au30 and (B) fccAu30. Reproduced (A) with permission from ref 20 and (B) from ref 29. Copyright 2016 Wiley-VCH and 2016 American Chemical Society, respectively.

color. In contrast, the spectrum of hcp-Au30 shows two distinct peaks at 368 and 550 nm, and its solution has a purple color. It is worth noting that hcp-Au30 can only be dissolved in benzene.20 The peculiar solubility of hcp-Au30(S-Adm)18 remains puzzling and unusual compared with those of similar-sized nanoclusters protected by the same S-Adm ligand,19 which are easily dissolved in dichloromethane, toluene, or tetrahydrofuran. The fcc- and hcp-Au30(SR)18 2767


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Accounts of Chemical Research nanoclusters with different ligands show different crystalline phases in the core, but the two thiolate ligands share a similar tertiary carbon bonded to the sulfur, which demonstrates that the ligand-based strategy can be effective in tailoring the nanocluster structure even with slight modifications of the ligands.

bridging thiolates. Recently, another Au52(SR)32 NC based on PET (as opposed to TBBT) was reported.22 Au52(PET)32 was isolated by chromatography from a polydispersed product prepared by acid-involved reduction.22 The Au50 core of Au52(PET)32 also shows a fcc phase, but its layer-by-layer pattern is different from the Au48 core of Au52(TBBT)32. Au52(PET)32 is made up of three 5 × 5 layers along the [001] direction (Figure 8A,C), which further grows up with a layer with half-diagonal coverage on the top and another on the bottom. Thus, the resultant incomplete 5 × 5 × 5 core of fccAu50 has a steps exposing {311} facets on the top and bottom. The two Au52(SR)32 NCs show different optical absorption spectra (Figure 8E).22 For the fcc series of Au8n+4(TBBT)4n+8 (n = 3−6), femtosecond electron dynamics and PL were also studied, revealing intriguing scaling laws in the properties.22,31 Future analyses of Au52(PET)32 from these perspectives may reveal more effects of tailoring the layer-by-layer fcc core on the properties.

2.3. Layer-by-Layer Modification of the Core in Au52

The ligand-based approach is also applicable to tailor the layerby-layer growth of fcc-type nanoclusters with the same size.21,22 The fcc structure is known as a stable crystalline phase in bulk gold and conventional nanoparticles, but it had been considered to be unstable in the ultrasmall size region because of the high surface energy until the first report of highly stable Au36(SR)24 with an fcc core.30 In later research, a general growth pattern of the core structure along the [001] direction was observed in a series of fcc nanoclusters, Au8n+4(TBBT)4n+8 (n = 3−6). The variable n corresponds to the number of layers along the [001] direction, with a constant Au8(TBBT)4 spacing between successive sizes. For example, Au52(TBBT)32 (i.e., n = 6) has a Au48 core consisting of six 4 × 4 fcc layers along the [001] direction (Figure 8B,D).21 On the top (or bottom) of the columnar core (Figure 8B), a pair of monomeric motifs reside on two opposite edges in addition to two bridging thiolates on the remaining top (or bottom) Au atoms. The four sides of the core are protected exclusively by

3. TAILORING THE SURFACE STRUCTURE 3.1. Quasi-Isomerism in Au28(SR)20 for Surface Tailoring

It is critical to understand the respective roles of the core and the surface of Aun(SR)m nanoclusters in determining the properties. However, the entangled core and surface effects pose major challenges because changing the core structure (e.g., isomerism) also alters the surface, as discussed in section 2. Thus, new strategies for selective modification are needed. Recently, surface tailoring without affecting the core has been realized in Au28(SR)20 with different thiolates. Compared with the Au28(TBBT)20,32 the newly obtained Au28(CHT)20 (CHT = cyclohexanethiolate) has the same numbers of Au atoms and ligands (though different R groups)23 and hence is quasi-isomeric. Au28(CHT)20 is prepared by ligand exchange of Au28(TBBT)20 with excess CHT at 80 °C. Interestingly, the conversion is reversible, i.e., the reverse ligand exchange (i.e., reaction of Au28(CHT)20 with TBBT) restores Au28(TBBT)20 in high yield (Figure 9).23 X-ray crystallography reveals that Au28(CHT)20 and Au28(TBBT)20 share the same inter-

Figure 9. (top) Ligand-induced quasi-isomerism in Au28(CHT)20 and Au28(TBBT)20. (bottom) Structural comparison of the kernels (A, B vs C, D) and surfaces (E, F vs G, I). Color code: magenta/blue = gold; yellow = sulfur; gray = carbon; other colors = staple motifs in different positions. Reproduced from ref 23. Copyright 2016 American Chemical Society.

Figure 8. (A, B) Total structures and (C, D) fcc cores of (A, C) Au52(PET)32 and (B, D) Au52(TBBT)32. (E) Optical absorption spectra. Adapted with permission from (A, C, E) ref 22 and (B, D) ref 21. Copyright 2017 Royal Society of Chemistry and 2015 American Association for the Advancement of Science, respectively. 2768


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Figure 10. (A) Optical absorption spectra of Au28(CHT)20 (black) and Au28(TBBT)20 (red). (B, C) CO oxidation light-off curves for CeO2supported Au28(CHT)20 (black) and Au28(TBBT)20 (red) catalysts (B) pretreated with O2 at 150 °C for 1 h or (C) pretreated with O2 at 300 °C for 1 h to remove ligands. Reproduced from ref 23. Copyright 2016 American Chemical Society.

penetrated bicuboctahedral Au20 core (Figure 9A,C). However, differences are found in the surface protection patterns. The Au20 core of Au28(CHT)20 is protected by two monomeric motifs, one each on the top and bottom, and two trimeric motifs on each side as well as eight bridging thiolates (Figure 9B,E,F). On the other hand, the Au20 core of Au28(TBBT)20 is protected by four dimeric motifs, two each on the top and bottom {111} facets, as well as eight bridging thiolates at the boundaries of the {111} and {100} facets. Theoretical calculations by Jiang and co-workers revealed that the ligandinduced quasi-isomerism adopts favored structures based on interactions between the carbon tails,23 which provides insights into the ligand-based strategy. The optical absorption spectra of Au28(CHT)20 and Au28(TBBT)20 share similar profiles (Figure 10A) but with peak shifts. Their Eg values are similar (∼1.7 eV).23 These observations demonstrate that the core structure plays a dominant role in optical absorption. The catalytic activities of the two Au28 isomers provide insights into the effect of the surface structure. Au28(CHT)20 (supported on CeO2) shows higher activity in CO oxidation, with 50% conversion at ∼90 °C compared with merely 20% for catalysis by Au28(TBBT)20 (Figure 10B).23 Interestingly, after pretreatment at 300 °C for 1 h (to remove ligands), the two Au28 NCs show similar activities (Figure 10C), which is due to the disappearance of surface differences after ligand removal. Interestingly, in the reactions of cyclohexane oxidation and benzyl alcohol oxidation,33 Au28(TBBT)20 showed higher activity than Au28(CHT)20 (both supported on titania, alumina, or ferric oxide). Overall, these studies demonstrate the critical role of surface structure in catalysis. 3.2. Tailoring the Surface Structure of 58-Electron Gold Nanoclusters

Surface structure tailoring has also been achieved in large-sized NCs (e.g., >100 atoms). A novel Au103S2(S-Nap)41 (S-Nap = 2-naphthalenethiolate) has been synthesized, and the valence electron count for Au103 is 58 electrons,24 the same as that for the previous Au102(SR)44. The Au103 NC has a decahedral Au79 core that is made up of shell by shell starting from Au7 to Au39 to Au79, and the core is protected by 10 monomeric motifs, five each on the top and bottom (Figure 11A−D). The waist of the core is protected by one dimeric, six monomeric, and two trimeric motifs, making up C2 rotational symmetry in the overall structure. The decahedral Au79 core of Au103 shares the same core as Au102(p-MBA)44,12 and both are 58-electron nanoclusters. The surface structure of Au102 shows some similarities to that of Au103, such as the protecting pattern on

Figure 11. (A) Shell-by-shell illustration of the common Au79 core of Au103S2(SR)41 and Au102(SR)44. (B, E) Total structures, (C, F) overall surface staple motifs, and (D, G) staple motifs at the waist position for (B−D) Au103S2(SR)41 and (E−G) Au102(SR)44, respectively. Reproduced from ref 24. Copyright 2017 American Chemical Society.

the top and bottom (i.e., by five monomeric motifs in each case) (Figure 11A,E−G). The waist of the Au79 core of Au102 is 2769


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Accounts of Chemical Research protected by two dimeric and nine monomeric motifs, making up C2 rotational symmetry for the core-plus-staples structure. Au102 and Au103 possess the same core, and thus, they show similar features in the steady-state optical absorption spectra, confirming that the absorption spectrum is largely determined by the core geometry (Figure 12A).24 The Eg of Au103 (0.42 eV

Specifically, this molecular “surgery” enables the tailoring of specific −S(R)−Au−S(R)− surface motifs on the monocuboctahedral Au23 nanocluster (Figure 13A).25 The tailoring

Figure 12. (A) Optical absorption spectra of Au103S2(SR)41 and Au102(SR)44. (B) Transient absorption data map for Au103S2(SR)41. (C) Global fitting analysis for Au103S2(SR)41. (D) Relaxation pathways. Reproduced from ref 24. Copyright 2017 American Chemical Society.

Figure 13. (A) Schematic illustration of molecular surgery on [Au23(CHT)16]−. (B) Step-by-step surface motif tailoring from Au23 to Au21 via Au23−xAgx. Reproduced with permission from ref 25. Copyright 2017 American Association for the Advancement of Science.

as determined by optical spectroscopy or 0.38 eV as determined by electrochemical voltammetry) 24 is also comparable to the reported optical band gap of Au102 (0.45 eV).34 The similar absorption profiles and band gaps of Au102 and Au103 are consistent with the observations for the Au28 quasi-isomers, i.e., the core determines the optical absorption and the Eg value.23 To further probe the respective roles of the core and the surface in the nanocluster properties, femtosecond spectroscopic analysis was performed for Au103.24 Upon photoexcitation at 480 nm, the transient absorption decays with three processes with different lifetimes (Figure 12B,C). The fast decay (2.0 ps) is due to internal conversion from higher excited states (Sn) to the lowest excited state (S1). The slow process (420 ps) is assigned to the relaxation from the lowest excited state to the ground state (S0). The intermediate process (16.6 ps) still shows bleaching signals, indicating hot electrons in the core, so this process is assigned to relaxation from hot to cooled lowest excited states. Previous spectroscopic studies of Au102 revealed that the excited-state lifetime for the relaxation from the lowest excited state (S1 → S0) is ∼3 ns,35,36 which is significantly longer than that of Au103 (Figure 12D). These studies have revealed that although Au102 and Au103 have similar band gaps, their long-lived lifetimes differ by a factor of more than 7, indicating the major role of the surface in the excited-state dynamics. Therefore, surface structure tailoring of nanoclusters can largely modify the excited-state behavior without changing the steady-state absorption and band gap.

chemistry starts with doping of Ag into [Au23(CHT)16]− by reaction with the AgI(CHT) complex (Figure 13B). Partial replacement of Au with Ag results in Au23−xAgx (x ≈ 1). The Au23−xAgx alloy is further reacted with bis(chlorogold(I)) bis(diphenylphosphino)methane to replace the −S(R)−Au− S(R)− staples residing on the Ag-doped positions with bidentate organophosphine ligands, forming [Au21(CHT)12(Ph2P−CH2−PPh2)]2+ (Figure 13B) since Ph2P−CH2−PPh2 structurally resembles the S(R)−Au−S(R) staple, and concomitantly, Ag dopants are also replaced by Au atoms from bis(chlorogold(I)) bis(diphenylphosphino)methane. The replaced Ag atoms combine with chloride in the reactant to form the counterion [AgCl2]− of the cluster product (Figure 13B). The Au21 product shares the same core geometry and surface structure as the original Au23 except for the pair of organophosphine ligands. In regard to the effects of surface tailoring of Au23 on the properties, optical absorption and PL are illustrated.25 The absorption spectra of Au23 and Au21 are similar (Figure 14A). This observation corroborates the above conclusion that the core plays the major role. On the other hand, the PL of Au21 is ∼10 times stronger than that of Au23, although the peak positions are similar (Figure 14B).25 The similarity in the PL peak positions is consistent with the similar core geometries of Au23 and Au21. However, the surface structure difference affects the excited-state behavior significantly, resulting in the 10-fold difference in PL intensity.25 This observation implies the major role of the surface in excited-state properties, consistent with the cases of surface structure tailoring of Au103 and Au102.24

3.3. Molecular “Surgery” of Eight-Electron Single-Cuboctahedron Nanoclusters

Selective modification of surface staples without altering the core and other surface structures has recently been realized. 2770


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been reported to date. Theoretically, further elongation to infinite layer-by-layer 1D rods is predicted to result in a nonzero band gap (0.78 eV),37 which is in contrast to the face-shared or vertex-shared icosahedral assembly.38 Therefore, future experiments with n > 7 are desired. • Second, other than fcc-type or icosahedral structures, further efforts are needed to expand the libraries of hcp and bcc nanoclusters in order to study the structural evolution and correlation with properties. • Third, the tailoring chemistry of nanoclusters of other metals (e.g., Ag, bimetallic) should be developed.39−41 For example, fcc-type structures of Ag nanoclusters have been reported to show a cubic growth pattern,41 in contrast to layer-by-layer growth for Au nanoclusters. The differences between Ag and Au remain to be investigated. • Last but not least, the tailoring chemistry with different types of protecting ligands deserves to be studied in future work. For example, alkynyl-protected Au36L24 and Au44L28 (L = phenylethynyl) are reported to exhibit the same geometry as Au36(TBBT)24 and Au44(TBBT)28 but with different optical absorption.42 In addition, isostructural [Au38L20(PPh3)]2+ and [Au38(SR)20(PPh3)]2+ NCs show different optical absorption profiles and catalytic performance.43 These reports have demonstrated the potential of tailoring the properties by using different types of ligands.

Figure 14. (A) Optical absorption spectra of Au23, Au23−xAgx, and Au21. (B) Photoluminescence spectra of Au21 (solid line) and Au23 (dashed line). Reproduced with permission from ref 25. Copyright 2017 American Association for the Advancement of Science.

4. CONCLUSIONS AND PERSPECTIVES In this Account, we have summarized some recent progress in the tailoring chemistry of metal nanoclusters for enhancing functionalities. In terms of the core tailoring, the structural isomerism of Au38(PET)24 demonstrates that two nanoclusters (Au38Q and Au38T) with the same formula can have different geometries, manifested in both the core and the surface. The effects of structural isomerism are manifested in the cluster stability, optical properties, and catalytic activity. The ligand-based strategy realizes tailoring of the Au38 structure by using a bulky thiolate (adamantanethiolate), and the resulting bcc-Au38 shows different optical absorption than Au38Q as well as a larger band gap (1.5 vs 0.9 eV), enhanced photoluminescence, and higher photocatalytic activity. With the ligand-based strategy, one can also tailor the crystalline phase of Au30 (fcc vs hcp), leading to significantly different optical properties and solubility. The attainment of new crystalline phases (bcc and hcp, which do not exist in bulk gold or plasmonic nanoparticles) is remarkable and leads to significant modifications of the properties. The two Au52(SR)32 NCs with different ligands (TBBT vs PET) have also demonstrated the ligand-based strategy for tailoring the fcc-type core with different layer-by-layer geometries. In terms of the surface-structure tailoring, it has provided critical information on the respective roles of the core and the surface in the properties. The quasi-isomerism of Au28(CHT)20 and Au28(TBBT)20 reveals a reversible transformation of the two nanoclusters. The pair of Au28 NCs sharing the same Au20 core, together with the Au103S2(SR)41 and Au102(SR)44 pair sharing the same Au79 core, reveals that the core structure plays a major role in the optical absorption spectrum while the surface structure is critical for catalysis, excited-state dynamics and lifetimes, and the PL properties. Molecular “surgery” of Au23 to Au21 permits the replacement of monomeric −S−Au− S− staple motifs with bidentate organophosphine (−P−CH2− P−), which leads to 10 times stronger emission in Au21 than Au23 without affecting the peak position of the PL, consistent with the conclusion that the core determines the Eg. For future perspectives of the tailoring chemistry of metal nanoclusters, it is of great importance to develop a systematic approach to tailoring the core/surface structures in order to achieve the desired properties and enhance the functionalities such as PL and catalytic reactivity: • First, further efforts are still needed to tailor the layer-bylayer fcc or icosahedral structure in metal nanoclusters. For fcc nanoclusters, Au8n+4(TBBT)4n+8 (n = 3−6) has

Overall, the development of the tailoring chemistry of metal nanoclusters will offer a tailor-made approach to functional nanomaterials with desired properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tatsuya Higaki: 0000-0002-6759-9009 Meng Zhou: 0000-0001-5187-9084 Rongchao Jin: 0000-0002-2525-8345 Author Contributions

All of the authors have approved the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Tatsuya Higaki is a Ph.D. candidate in the Jin group and works on gold nanoclusters. Qi Li is a Ph.D. candidate and works on nanocluster photoluminescence. Meng Zhou is a postdoctoral research associate and works on ultrafast spectroscopy. Shuo Zhao is a Ph.D. candidate and works on electrocatalysis by metal nanoclusters. Yingwei Li is a graduate student and works on nanocluster structure and properties. Site Li is a graduate student and works on electrocatalysis by metal nanoclusters. 2771


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Rongchao Jin is a professor of chemistry at Carnegie Mellon University and works on metal nanoclusters and nanoparticles.

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ACKNOWLEDGMENTS R.J. is thankful for the financial support from the AFOSR (FA9550-15-1-0154) and NSF (DMR-1808675). REFERENCES

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Toward the Tailoring Chemistry of Metal Nanoclusters for Enhancing

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