Structural Evolution Patterns of FCC-Type Gold Nanoclusters

HIGAKI Tatsuya, JIN Rongchao

Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States.

Abstract: Recent progress in the research of atomically-precise metal nanoclusters has identified a series of exceptionally stable nanoclusters with specific chemical compositions. Structural determination on such “magic size” nanoclusters revealed a variety of unique structures such as decahedron, icosahedron, as well as hexagonal close packing (hcp) and body-centered cubic (bcc) packing arrangements in gold nanoclusters, which are largely different from the face-centered cubic (fcc) structure in conventional gold nanoparticles. The characteristic geometrical structures enable the nanoclusters to exhibit interesting properties, and these properties are in close correlation with their atomic structures according to the recent studies. Experimental and theoretical analyses have been applied in the structural identification aiming to clarify the universal principle in the structural evolution of nanoclusters. In this mini-review, we summarize recent studies on periodic structural evolution of fcc-based gold nanoclusters protected by thiolates. A series of nanoclusters exhibit one-dimensional growth along the [001] direction in a layer-by-layer manner from Au28(TBBT)20 to Au36(TBBT)24,Au44(TBBT)28, and to Au52(TBBT)32 (TBBT: 4-tert-butylbenzenethiolate). The optical properties of these nanoclusters also evolve periodically based on steady-state and ultrafast spectroscopy. In addition, two-dimensional growth from Au44(TBBT)28 toward both [100] and [010] directions leads to the Au92(TBBT)44 nanocluster, and the recently reported Au52(PET)32 (PET: 2-phenylethanethiol) also follows this growth pattern with partial removal of the layer. Theoretical predictions of relevant fcc nanoclusters include Au60(SCH3)36, Au68(SCH3)40, Au76(SCH3)44, etc, for the continuation of 1D growth pattern, as well as Au68(SR)36 mediating the 2D growth pattern from Au44(TBBT)28 to Au92(TBBT)44. Overall, this mini-review provides guidelines on the rules of structural evolution of fcc gold nanoclusters based on 1D, 2D and 3D growth patterns.

Key Words: Gold; Nanoclusters; Thiolate; Face-centered-cubic; Layer-by-layer growth

1 Introduction

Atomically precise metal nanoclusters have been intensively studied in the nanoscience field in terms of fundamentals and applications1–15. These studies are driven by broad interest in the unique properties and structures of nanoclusters, which are often drastically different from the case of conventional nanoparticles. For example, the crystallographic structure determination revealed that some thiolate-protected gold nanoclusters, such as Au102(SR)4416, Au25(SR)1817,18, Au38(SR)2419,and Au246(SR)8020, show decahedral or icosahedral kernel structures, whereas bulk gold and its nanoparticles adopt the face-centered-cubic (fcc) structure. Recent progress in crystallographic studies has also led to the observation of novel kernel packing (or crystalline) arrangements21–24that are not present in bulk gold or regular-sized gold nanoparticles (＞ 5 nm), such as hexagonal-close-packing (hcp)25and bodycentered-cubic (bcc)26.

According to recent reports, the discovery of new structures and properties can be guided via a ligand-based strategy because of the identified major roles of surface ligands in determining the structural patterning and close correlation between the surface structure and properties of nanoclusters25,27–30. For example, the choice of bulky thiolate ligands (i.e., 1-adamantanethiol vs tert-butylthiol) is demonstrated to be effective in controlling the kernel structure of the Au30 nanocluster under appropriate reaction conditions, resulting in totally different optical properties between the two nanoclusters25. In addition, the ligand-based strategy is proved to be effective even in large-sized nanoclusters (e.g., ＞ 100 Au atoms) by recent work of controlling the surface structures of Au103 with the same Au79Marks decahedral kernel as in Au10229. The different surface structures indeed led to distinct changes in ultrafast electron dynamics29.

Despite recent advances in structural analysis by crystallography, some questions still remain elusive. We briefly outline such questions.

First, researchers have not figured out how to predict new magic size gold nanoclusters with exceptional stability. Early theoretical efforts were focused on the superatom theory to identify magic size nanoclusters31. In this theory, the stability of certain sizes of nanoclusters is considered to originate from the shell closure of valence electrons in superatomic orbitals,which is analogous to atomic shell closure. For example,Au102(SR)44has 58 valence electrons, which is counted by subtracting the number of ligands from the number of Au atoms(i.e., 102 - 44 = 58), giving rise to complete filling of the superatomic orbitals (i.e., 1S21P61D102S21F142P61G18).However, the majority of experimentally discovered nanoclusters instead possess valence electrons that would be of open shell, so the superatom criterion is limited and cannot cover all the experimentally reported nanoclusters1,32–44.

Second, fundamental rules are still missing about structural evolution with increase in size. So far, some structural growth modes have been proposed based on experimentally-determined structures. For example, the structural elucidation of the tri-icosahedral Au37nanocluster32indicates one-dimensional structural growth with Au13icosahedral units via vertex sharing for linear assembly, and such linear superstructures exhibited interesting electron localization in studies of ultrafast electron dynamics33. There are also other growth modes, including fusion, interpenetration, shell-by-shell, layer-by-layer, and Au4tetrahedron-based vertex-sharing growth modes1. As for the surface protection of such nanoclusters, several types of protecting motifs including simple bridging thiolate (―S―),monomeric (―S―Au―S―) and dimeric (―S―Au―S―Au―S―) staple motifs up to octameric ring-like motifs have been discovered by researchers in this filed1. However, the overall relationship between the Aun(SR)mnanocluster structure and the growth rules needs more efforts to explain explicitly,especially because of the fact that the surface protection patterns of nanoclusters can only be determined by crystallography while crystallization (in particular, giant sizes20)remains huge challenges.

Rongchao Jin is currently a Professor of Chemistry at Carnegie Mellon University. He received his Ph.D. from Northwestern University in 2003.After three years of postdoctoral research at the University of Chicago,he joined the chemistry faculty of Carnegie Mellon University in 2006 and was promoted to Associate Professor in 2012 and Full Professor in 2015. His current research interests are atomically precise nanoparticles,nano-optics, and catalysis.

Herein, we highlight recent progress in structural analyses on the series of fcc-based gold nanoclusters. A periodic series of gold nanoclusters include Au28(TBBT)20, Au36(TBBT)24,Au44(TBBT)28, and Au52(TBBT)32, in which layer-by-layer growth occurs along the [001] axis (or z axis). This one-dimensional, layer-by-layer growth can be further expanded toward two-dimensional (2D) growth along [100]and/or [010] directions. This 2D growth mode (e.g., from Au44(TBBT)28to Au92(TBBT)44, both are experimental structures) has added a new dimension to map out the structural evolution patterns. Theoretical work by Pei and coworkers45has proposed a Au68(SR)36nanocluster which fits in the 2D growth. Interestingly, the recently reported Au52(PET)32by Wu and coworkers46through a ligand-based strategy shows some structural similarities to the Au68(SR)36nanocluster, whereas its formula suggests an isomerism with Au52(TBBT)32. All such results motivate us to discuss some insights into the growth patterns, and the structural periodicity provided in this review also leads to predictions of both new magic sizes and geometrical structures about fcc-type gold nanoclusters.

2 Layer-by-layer growth of fcc Au Nanocluster

2.1 Growth toward [001] direction

Periodicities in structural growth were reported in gold nanoclusters protected by 4-tert-butylbenzenethiolate(TBBT, ―S―Ph―4―tBu)34–37. The series of gold nanoclusters include Au28(TBBT)20, Au36(TBBT)24,Au44(TBBT)28, and Au52(TBBT)32, formulated as Au8n+4(TBBT)4n+8, where n = 3-6, and all of their structures have been attained by X-ray crystallography.

The kernel structures of the series of nanoclusters can be interpreted as double helical arrangement of Au4tetrahedra(Fig. 1A). The smallest nanocluster (Au28) has double helixes,each chain containing two Au4 tetrahedra, and the kernel grows up by successively adding two Au4tetrahedra to each end of the double helixes. The number of tetrahedra in each helix increases from 2 in Au28to 5 in Au52. The top and bottom surfaces of each nanocluster are protected by Au2(SR)3dimeric staples (Fig. 1B, highlighted in yellow), while the sides are protected by different type of staple motifs based on the length of coiling-ups (e.g., trimeric (―S―Au―S―Au―S―Au―S―) staples for Au28, dimeric staples for Au36, and dimeric staples besides monomeric ones for both Au44and Au52).

Fig. 1 Structural growth patterns in the Au8n+4(SR)4n+8 magic series,n = 3-6 revealed by crystallography.(A, B) Tetrahedron-based double helixes; (C, D) anisotropic growth of the fcc lattice.Yellow = S, others = Au, yellow line = S-Au-S-Au-S. Color online.Adapted with permission from Ref. 36. Copyright 2016 American Chemical Society.

Cheng et al.38previously theorized an Au4 chain model for some non-superatom nanoclusters and in their model each tetrahedron is a single unit of “superatom” with 2-electrons.Since the Au28(TBBT)20, Au36(TBBT)24, Au44(TBBT)28, and Au52(TBBT) nanoclusters have valence electrons of 8e, 12e,16e, and 20e, respectively36, the electron counts can be readily rationalized, i.e., 2e times the total number of tetrahedral units(4, 6, 8, and 10, respectively) gives rise to 8e, 12e, 16e, and 20e for the series. The anatomy with Au4tetrahedral units is further supported by the experimental observation in bond length distribution by X-ray absorption analysis; Specifically, the bonds within Au4are much shorter than those between Au4units39. The periodicity regarding the 1D growth is also observed in the physicochemical properties, including optical absorption and electron dynamics according to recent reports36,40.

The structural evolution of this fcc series of nanoclusters can be also explained by a layer-by-layer growth along the [001]direction (Fig. 1C,D). In this view, the kernels comprise a regular fcc-type box with all six surfaces being {100} facets.The top and bottom have a square shape of 0.63 nm × 0.63 nm(atomic center-to-center distance), and are protected by two monomeric staples and two bridging thiolates (Fig. 1D, Au atoms in the monomers highlighted in blue). The protecting motifs are incorporated into the general formula, Au8n+4(TBBT)4n+8, as the constant term independent of the value n, i.e., Au4S8= 2Au2S4={2(AuS2) ) + 2S } for top and bottom. The fcc-layered kernel grows up by subsequent addition of (001) layer with eight gold-atoms toward the [001] direction. The waist of the nanoclusters is protected by 4 bridging thiolates for each (001)layer increase in the z-direction. The numbers regarding the individual layer growth, 8 for Au and 4 for S, show up as thecoefficient for the value n in the general formula of Au8n+4(TBBT)4n+8. Therefore, the value n corresponds to the number of layers in each nanocluster, where Au28(TBBT)20,Au36(TBBT)24, Au44(TBBT)28, and Au52(TBBT)32consist of three, four, five, and six (001) layers of Au8, respectively, hence,n = 3-6 in Au8n+4(TBBT)4n+8.

Theoretical calculations were performed to expand the periodicities to n ＞ 6 (e.g., 7-layered Au60(SR)36, 8-layered Au68(SR)40, 9-layered Au76(SR)44, and so on, Fig. 2)41,42. The optical HOMO-LUMO gaps of these calculated structures are larger than 1.0 eV, suggesting the high thermodynamic stability of the predicted structures. Of note, a kinetically controlled synthesis of Au76(SPh-4-COOH)44was reported and the synthesized nanocluster showed intense NIR absorption at 1340 nm (ε = 3 ×105mol?-1?L?cm-1)43. Whether the Au76nanocluster adopts the layered growth or not still remains unclear, and the experimental synthesis of Au60(SR)36and Au68(SR)40is also a challenge. New synthetic methods should be devised in future work to realize the predicted sizes experimentally.

2.2 Growth toward [100] and/or [001] direction

The recently solved X-ray crystallographic structure of Au92(TBBT)44nanocluster also possessed a fcc structure44.However, the formula cannot be expressed as Au8n+4(TBBT)4n+8 with an integer n, thus its structure is out of the periodicity for layer-by-layer growth toward the [001] direction. Specifically,the Au92structure comprises six (100) layers, six (010) layers,and five (001) layers along the x-, y-, and z-axis (e.g., 6 × 6 × 5)via partial truncation of 6 Au atoms in two columns at two edges along the z-axis (Fig. 3A, right, highlighted in gray).

The regular shaped Au84kernel exhibits (001) facets with 16 surface atoms and, (100) and (010) facets with 12 surface atoms, respectively (Fig. 3B,C). The (100) and (010) facets are protected in a similar manner, where each bridging thiolate simply connects two neighboring surface gold atomsdiagonally from upper right to lower left, although the right and bottom sides of (100) facets are connected by monomeric staple motifs while the top and left for (010) facets (Fig. 3C). In the case of (001) facets, however, the two adjacent surface gold atoms are bridged by thiolate horizontally and, upper-left and lower-right ends are protected by monomeric staple motifs.

Fig. 3 Atomic structure of Au92(TBBT)44 nanocluster.(A) layer-by-layer model of the gold kernel; (B) the Au84 kernel from the viewpoint of the [110] direction (left) and [001] direction (right); (C) surface protection patterns on the {100} facets; (D) total structure from [110] and [001] directions. Color codes:yellow = S; purple = C; pink = H; other colors = Au. Arrows = bridging thiolates;braces = monomeric staple motifs. Color online.Adapted with permission from Ref. 44. Copyright 2016 American Chemical Society.

Since the Au92nanocluster is a 6 × 6 × 5 layered structure which is deviated from the series of Au8n+4(TBBT)4n+8 with 4 ×4 × n via [001] layer-by-layered evolution, two dimensional growth to [010] and/or [100] direction is theoretically proposed to organize the growth pattern in fcc-type nanoclusters comprehensively45. The structure evolution to Au92(TBBT)44arises from the Au44(TBBT)28 and is also mediated by the newly proposed Au68(SR)36nanocluster (Fig. 4). From the perspective of tetrahedral Au4double helixes, 1D growth along the crystal [001] direction can be interpreted as the 1D preferential growth of the double helical tetrahedron network in the case of Au28, Au36, Au44, and Au52. In contrast, Au3triangles as well as Au4 tetrahedra are involved in structural evolution in 2D growth pattern. As for the structural evolution from Au44to Au68, one Au4tetrahedron as well as three Au3triangles are placed toward [010] and [100] directions, respectively. On the other hand, the growth from Au68to Au92involves a similar manner but opposite direction, where one Au4tetrahedron as well as three Au3 triangles are placed toward [010] and[100]directions, respectively. The number of valence electrons for Au44, Au68, and Au92are 16e, 32e, and 48e, respectively45, so the interval is 16e regarding the increase of one Au4tetrahedron plus three Au3triangles to each of the [010] and [100]directions. Therefore, the Au4superatom electron counting exactly matches, if the Au3 triangle is also responsible for 2e.

The structural evolution from Au44to Au92via Au68can be explained in 2D layer-by-layer growth as well. From Au44to Au68, one layer is added to [010] and [100] directions so that all{100} facets can expose 12 surface gold atoms with 5 × 5 × 5 layered structure. On each facet of the Au68nanocluster, four bridging thiolates connect two adjacent surface gold atomsdiagonally. Two monomeric staple motifs further protect two adjacent sides on these facets. From Au68to Au92, one layer is added to[010] and[100] directions so that (100) and (010)facets can expose 12 surface gold atoms, finally leading to the 6 × 6 × 5 layered structure. In order to rationalize the formation of Au68, the formation energy of Au68(SR)36 was theoretically calculated to be 0.49 eV45, indicating that Au68(SR)36possesses relatively high thermodynamic stability.

Fig. 4 View of the 2D crystal facet growth patterns in the fcc nanoclusters, including Au44(SR)28, Au68(SR)36, and Au92(SR)44.Red = S, others = Au. Color online.Adapted with permission from Ref. 45. Copyright 2017 American Chemical Society.

2.3 Growth & removal in layer-by-layer manner

Very recently, a new Au52nanocluster protected by PET(2-phenylethanethiolate, SCH2CH2Ph) was synthesized and successfully characterized by X-ray crystallography46. The novel synthetic method involves HNO3, followed by chromatographic separation. Interestingly, the synthesized nanocluster has the chemical formula, Au52(PET)32, which is exactly the same numbers of Au atoms and protecting ligands as in the previously reported Au52(TBBT)32nanocluster37.However, the atomic structure of the Au52(PET)32nanocluster(Fig. 5) is quite different from that of Au52(TBBT)32, indicating a critical effect of the ligand. In terms of the layer-by-layer mode, the previous Au52(TBBT)32structure comprises a fcc-rectangular Au48kernel (e.g., 4 × 4 ×6) protected by four monomeric staple motifs, two on the top and two on the bottom(Fig. 1). On the other hand, the PET-protected Au52(PET)32nanocluster has a quasi-cubic Au50kernel (e.g., 5 × 5 × (5 - x))protected by two monomeric staple motifs on its waist (Fig. 5).

The (001) facets of the Au52(PET)32nanocluster are exclusively protected by bridging thiolates (Figure 5A). The(001) facts are exposed with {100} plane of 5 × 5 with the step of 8-surface-gold atoms. On the step of (001) facets, each bridging thiolate connects two neighboring surface gold atomsdiagonally from upper left to lower right. Other surface atoms besides the step are also protected by bridging thiolates diagonally from upper-right to lower-left, which is perpendicular to the pattern on the step. The protecting pattern in this Au52(PET)32nanocluster sheds light on the general rules for surface protection on the facets with steps. The waist of the nanocluster is protected by simple thiolates, where adjacent surface gold atoms are bridgeddiagonally from upper right to lower left, as well as one monomeric staple motif on the ends of the facets. The diagonal direction of protecting bridging thiolates remains the same in the facets without steps in this case. This feature is significantly different from the Au52(TBBT)32nanocluster; the latter is exclusively exposed with flat facets. The similarity in the formula but differences in the structures observed in the Au52(TBBT)32 and Au52(PET)32 pair can be classified as quasi-isomerism based on the ligandbased strategy via careful selection of protecting ligands and synthetic conditions as well.

Fig. 5 View of the 2D crystal facet growth pattern in the fcc Au52(PET)32 nanocluster.Yellow = S, others = Au. Color online.Redrawn from the cif file provided in Ref. 46.

Interestingly, the structure of this Au52(PET)32exhibits some similarities to the theoretically-proposed Au68(SR)36nanocluster45. The kernel of the Au68 nanocluster has an isotropic cubic structure with 5 × 5 × 5 layering. The facets along with z-direction expose 12-surface-gold atoms protected by four bridging thiolates diagonally from upper-left to lower-right, as well as two monomeric staple motifs at the top and left ends of the facets. The structure of the Au52(PET)32nanocluster can be reproduced via partial truncation of the Au68 nanocluster. The first step to the Au52(PET)32structure is removal of upper-right 6-surface atoms on (001) facets together with two monomeric staple motifs. The exposed surface is then protected by the bridging thiolate diagonally where its protecting pattern is perpendicular to the pattern on the step.The comparison between these two nanoclusters implies such a partial removal of layer as a novel operation for the structural rules about fcc-based nanoclusters, suggesting a new perspective on the layer-by-layer growth pattern on fcc-based gold nanoclusters.

3 Conclusion & outlook

Overall, this review is intended to offer a summary on the recent progress in fcc structural determination and insights,mainly on the gold nanoclusters. It is worth noting that recently Teo et al. has summarized fcc-structured, periodic Ag nanoclusters and the reader is referred to Ref. 12 for detail.

The discussed series of nanoclusters (i.e., from Au28(TBBT)20,Au36(TBBT)24, Au44(TBBT)28, to Au52(TBBT)32) exhibit periodicity in structural evolution in the form of one-dimensional growth toward [001] direction in a layer-by-layer manner. The optical properties of these nanoclusters also evolve periodically based on both steady-state and ultrafast absorption spectroscopy. On the other hand, the Au92(TBBT)44 nanocluster falls into a two-dimensional growth pattern, i.e., from Au44(TBBT)28toward [100] and [010] directions, with the intermediate size being theoretically identified (i.e., the Au68(SR)36nanocluster). The recently reported Au52(PET)32can fit in this growing pattern with partial removal of the layer from the theoretical Au68(SR)36 structure. The discussions here have provided some guides on the rules about fcc-based nanoclusters, specifically, the addition and/or removal in a layer-by-layer manner. The predicted periodic three-dimensional growth44may enable one to map out structural evolution of gold nanoclusters with fcc-based structure.

The structural periodicity in fcc-type gold nanoclusters is intriguing, and some future directions are discussed below.

First, further effort is needed in the synthesis and structural characterization via crystallography in order to prove that theoretically predicted structures can be synthesized experimentally (e.g., 7-layered Au60(SR)36, 8-layered Au68(SR)40,9-layered Au76(SR)44, and so on). The anisotropic growth requires specific synthetic conditions such as slow reduction under basic condition, and the new nanoclusters are expected to exhibit novel properties such as intense NIR absorption as in Au76(SR)4443. In terms of optical properties, the energy gap law about electron dynamics is recently reported in the form of shorter excited-state lifetimes with decreasing band gaps as the size increases47. However, the series of nanoclusters from Au28(TBBT)20, Au36(TBBT)24, Au44(TBBT)28, to Au52(TBBT)32,exhibit an increase in excited state lifetime with the size increase or bandgap decrease. Therefore, further experimental studies are required to map out the relationship between structural evolution and electron dynamics.

Second, future work may also focus on the question of whether there are similar periodicities in gold nanoclusters with hcp or bcc structures. The structural periodicity in fcc series exhibits a distinct interval in the formula (e.g., Au8TBBT4unit)as well as the periodicity in optical properties (e.g., decrease in the HOMO-LUMO gap, and increase in the longer-lived excited-state lifetime). The periodicity in the hcp or bcc based nanoclusters may be different from that of the fcc series,because packing structures normally evolve to the fcc arrangement when the size reaches to nanoparticle regime.Since the number of hcp and bcc structures is still limited, it is still a question whether hcp or bcc crystalline structures can be observed in other sizes than the reported ones25,26. In order to obtain conclusive results, further research on the crystal structure analysis should be pursued in future work. It is hoped that future work will provide more experimental progress in periodic nanoclusters and also map out more structural evolution patterns in atomically precise nanoclusters and their relationships to regular sized nanoparticles.

Acknowledgment: We thank Prof. Yong Pei for providing the theoretical structure of Au68(SCH3)36.