Cd-driven surface reconstruction and photodynamics in gold nanoclusters

With atomically precise gold nanoclusters acting as a starting unit, substituting one or more gold atoms of the nanocluster with other metals has become an effective strategy to create metal synergy for improving catalytic performances and other properties. However, so far detailed insight into how to design the gold-based nanoclusters to optimize the synergy is still lacking, as atomic-level exchange between the surface-gold (or core-gold) and the incoming heteroatoms is quite challenging without changing other parts. Here we report a Cd-driven reconstruction of Au44(DMBT)28 (DMBT = 3,5-dimethylbenzenethiol), in which four Au2(DMBT)3 staples are precisely replaced by two Au5Cd2(DMBT)12 staples to form Au38Cd4(DMBT)30 with the face-centered cubic inner core retained. With the dual modifications of the surface and electronic structure, the Au38Cd4(DMBT)30 nanocluster exhibits distinct excitonic behaviors and superior photocatalytic performances compared to the parent Au44(DMBT)28 nanocluster.


Introduction
Metal synergy is of paramount importance as the rationale to modulate the intrinsic properties of metal nanoparticles. 1,2 However, the precise synergistic interaction in an intermetallic nanoparticle has so far been elusive, due to the challenges in determining the atomic-level arrangement of the metal heteroatoms in the nanoparticle. Atomically precise metal nanoparticles (oen called nanoclusters) lead to unprecedented opportunities in signalling clear directions to exploit the cooperativity between the two metal elements within a single nanocluster. 3 Thiolate-protected gold nanoclusters, Au n (SR) m , where n is the number of gold atoms and m is the number of thiolate ligands, SR, have gained momentum over the past few years as an exciting area and have opened up new horizons in precise tailoring of the composition and structure to control the physicochemical properties. [4][5][6][7][8][9][10][11][12] The Au n (SR) m nanoclusters are typically congured with an inner gold core (or kernel) and various surface motifs, in which the motifs containing both gold and thiolate resemble staples. Both the gold core and the surface motifs can contribute to the physicochemical properties such as the optical and electronic properties, as well as catalysis. [13][14][15][16][17][18] It has been recognized that substituting one or more gold atoms in either the core or the motifs with other metals can tune the overall performances of the parent nanoclusters. [19][20][21][22][23][24][25][26] Therefore, it has become possible to access the previously inaccessible metal synergy in the bimetallic nanoclusters with atomic-precision.
Among the gold-based bimetal nanoclusters, cadmiumcontaining bimetal clusters provide synergistic strategies to adjust the electronic structures and further modulate the physicochemical properties in the clusters, since Cd has one more valence electron than Au. 21,26,27 Cd introduction usually causes surface reconstruction of gold nanoclusters. For example, Au 19 Cd 2 (SR) 16 was obtained through the substitution of two neighboring surface Au atoms with one Cd with the cuboctahedral Au 13 unchanged. 26 Au 19 Cd 3 (SR) 18 was formed by retaining the icosahedral Au 13 core but only changing the surface of Au 25 (SR) 18 . 27 However, the surface reconstruction strategy remains challenging and no examples of bimetal clusters formed without breaking the face-centered cubic (fcc) core of the parent gold clusters have been documented, which might thus impede gaining a higher understanding of how to tailor the surface structure of gold-based nanoclusters and accordingly optimize their synergy.

Results and discussion
X-ray crystallography analysis shows that the parent Au 44 (-DMBT) 28 nanocluster is composed of an Au 26 kernel, six Au 2 (SR) 3 and two Au(SR) 2 staples (Fig. 1a, c and Table S1 †). The formula of Au 44 (DMBT) 28 is further conrmed by electrospray ionization mass spectroscopy (ESI-MS, Fig. S1a †). The structural framework of Au 44 (DMBT) 28 is identical to that of the reported Au 44 (TBBT) 28 (TBBT ¼ 4-tert-butylbenzenethiol) (Fig. S2 †), 28 both of which can be assembled into the layered structures ( Fig. S3-S5 †). Notably, a signicant difference is observed in the layer's interior, where all the molecules of Au 44 (TBBT) 28 in the layer (marked with the same color, Fig. S3 †) are packed along the same direction, while Au 44 (DMBT) 28 molecules are arranged in different directions (Fig. S5 †). Such a difference may be ascribed to the different steric hindrance between TBBT and DMBT. The UV-vis-NIR spectra of the two Au 44 (SR) 28 nanoclusters show only small deviations. As shown in Fig. S6, † the prominent peak at 380 nm for Au 44 (TBBT) 28 is slightly red-shied to 388 nm for Au 44 (DMBT) 28 , and the broad peaks at 650 and 725 nm become apparent when TBBT is replaced by DMBT.
With Au 44 (DMBT) 28 as a starting unit, a Cd-doped nanocluster was further synthesized via an ion-exchange strategy. From ESI-MS data ( Fig. S1b †), the prominent peak at 6025.43 m/ z with a +2 charge is assigned to Au 38 Cd 4 (DMBT) 30 (theoretical value: 6025.48 m/z), which is further conrmed by the excellent match between experimental and calculated isotopic patterns (inset of Fig. S1b †). Single crystallography analysis reveals that Au 38 Cd 4 (DMBT) 30 contains a 26-Au-atom kernel, two Au 5 Cd 2 (-SR) 12 staples, two Au(SR) 2 staples and two bridging SR ligands, as shown in Fig. 1b, d, and Table S2. † Note that the retained kernel of Au 38 Cd 4 (DMBT) 30 experiences a slight distortion from "slender" to "stocky" in comparison with that of the parent Au 44 (DMBT) 28 (Fig. 1e-h). Further analysis shows that the Au 26 kernel in Au 38 Cd 4 (DMBT) 30 can be viewed as the assembly of tetrahedral Au 4 units in a double-helical mode, as well as that in Au 44 (DMBT) 28 (Fig. 2). Furthermore, the two nanoclusters have almost identical distances between neighboring Au 4 units, which is clearly manifested in the similar Au-Au bond lengths according to the different positions of the Au atoms (Fig. S7 †). Therefore, Au 38 Cd 4 (DMBT) 30 can be viewed as the gentle surface reconstruction without breaking the double-helical Au 26 kernel based on the parent Au 44 (DMBT) 28 . In addition, Au 38 Cd 4 (-DMBT) 30 is also patterned along different directions in the layer structure (Fig. S8 †).
To gain an in-depth insight into the Cd-induced surface reconstruction mechanism, density functional theory (DFT) calculations were performed. Starting from the Au 44 (SR) 28 cluster, as presented in Fig. S9, † four Au 2 (SR) 3 À protecting motifs of Au 44 (SR) 28  , in which the Cd(SR) 2 is quickly separated from Cd(SR) 3 À leaving a bridging SR À motif on the surface of the Au  core. Finally, the stable Au 38 Cd 4 (SR) 30 is formed with a formation energy of À12.50 eV. The proposed conversion process from Au 44 (SR) 28 to Au 38 Cd 4 (SR) 30 includes two key steps: (i) the substitution of SR[Au(SR)] 2 À by Cd(SR) 2 and (ii) the structural isomerization of surface ligands.
To investigate the electronic structure changes induced by Cd-atom surface modication, the optical adsorption spectra of the Au 44 (DMBT) 28 and Au 38 Cd 4 (DMBT) 30 nanoclusters were measured. The absorption peaks of Au 38 Cd 4 (DMBT) 30 are mainly centered at 400, 465, 550 and 678 nm (Fig. 3b), which differ from those observed in the parent nanocluster (387, 452, 635 and 725 nm; Fig. 3a). These optical features can be well reproduced by theoretical calculations (Fig. 3a, b and S10 †). The Kohn-Sham (KS) molecular orbital (MO) energy levels and atomic orbital components in each KS MO of Au 44 (SR) 28 and Au 38 Cd 4 (SR) 30 suggest that the absorption peaks mainly involve the Au(sp) / Au(sp) transitions ( Fig. 3c and d). In particular, for Au 44 (SR) 28 , the rst absorption peak centered at 734 nm originates from the highest occupied molecular orbital / the lowest unoccupied molecular orbital (HOMO / LUMO) transition, while for Au 38 Cd 4 (SR) 30 , the rst absorption peak centered at 695 nm originates from the HOMO / LUMO, HOMO / LUMO+1, HOMO / LUMO+4, HOMOÀ1 / LUMO, HOMOÀ1 / LUMO+1, and HOMOÀ1 / LUMO+5 transitions. The more complex orbital transitions in Au 38 Cd 4 (SR) 30 than in Au 44 (SR) 28 can be attributed to the dopant Cd. This behaviour can also be observed for other absorption peaks.
Moreover, femtosecond and nanosecond carrier dynamics of the two nanoclusters were measured via time-resolved transient absorption (TA) spectroscopy to decipher their potential energyrelated applications. The femtosecond-resolved TA spectra of the Au 44 (DMBT) 28 and Au 38 Cd 4 (DMBT) 30 nanoclusters are provided in Fig. 4a and b. Similarly, both Au 44 (DMBT) 28 and Au 38 Cd 4 (DMBT) 30 nanoclusters showed broad excited state absorption (ESA) signals overlapped with ground state bleaching (GSB) peaks near 675 nm. We selectively extracted the TA spectra at different delay times, combined with the dynamic traces probed at 515 and 675 nm to study the transient evolution and the relaxation dynamics ( Fig. 4c-f). A 0.6 ps process at the early stage, which is attributed to the ultrafast internal conversion from higher excited states to lower excited states, 29 was observed in the two nanoclusters ( Fig. S11 and Table S3 †). It is worth noting that the major divergence between the two nanocluster systems emerged aer a delay of 2 ps. For Au 44 (-DMBT) 28 , the TA spectra remained nearly unchanged aer 2 ps (Fig. 4c), which is consistent with the at decay kinetic traces shown in Fig. 4e. A 19 ps process obtained by exponential tting was ascribed to the structural relaxation caused by conformational changes aer pumping. [29][30][31] For Au 38 Cd 4 (DMBT) 30 , interestingly, an obvious spectral transformation was observed and the lifetime of this component was determined to be 57 ps (Table S3 †), which differs from the 19 ps structural relaxation observed in Au 44 (DMBT) 28 and might be related to the charge transfer states between the ligand and the metal core of Au 38 -Cd 4 (DMBT) 30 , 32-35 which can be manifested by the overall Hirshfeld charge of the Au 26 core in Au 38 Cd 4 (SH) 30 (0.46) and in Au 44 (SH) 28 (0.56). Of note, deduced from nanosecond-resolved TA analysis, as shown in Fig. S12, † the Au 38 Cd 4 (DMBT) 30 nanocluster exhibits a faster carrier recombination process with  30 . Kinetic traces at selected wavelengths of (e) Au 44 (DMBT) 28 and (f) Au 38 Cd 4 (DMBT) 30 . The data are plotted in a scale normalized to the amplitude of the signal probed at 515 nm at a delay of 2 ps. The gray dots in (e) and (f) are the original data, while the corresponding multi-exponential fits are plotted as colored lines. The distinguishable electronic and optical properties of the two nanoclusters would apparently impact their catalytic properties. Thus, visible light-driven degradation of methyl orange was selected to explore the photocatalysis of the two nanoclusters. From Fig. 5a and b, within 50 min, methyl orange can be completely degraded on the Au 38 Cd 4 (DMBT) 30 catalyst under visible light illumination, while on the Au 44 (DMBT) 28 catalyst it was completed in 70 min. The plots of methyl orange degradation on the catalysts versus reaction time further indicate the better photocatalytic performance of the Au 38 Cd 4 (-DMBT) 30 catalyst (Fig. 5c). Electrochemical impedance spectroscopy was performed to investigate the interfacial transfer of electrons. In Fig. 5d, the semicircular diameter of Au 38 Cd 4 (DMBT) 30 was smaller than that of Au 44 (DMBT) 28 , which implies faster electron-transfer in the Au 38 Cd 4 (DMBT) 30 system. The photocatalysis difference in the two cluster catalysts is suggested to arise from their different equilibria established between the carrier recombination and the electron transfer inuenced by metal synergy.

Conclusions
In summary, we have developed a Cd-driven surface reconstruction strategy for synthesizing a new Au 38 Cd 4 (DMBT) 30 bimetallic nanocluster with the fcc Au 26 core retained from the parent Au 44 (DMBT) 28 nanocluster. The two nanoclusters that exhibit elegant patterns of Au 4 tetrahedra show distinct differences in the electronic structures, optical properties, and photocatalytic performances. Beyond the Cd-mediated surface reconstruction case, we anticipate that this heteroatom-doping mechanism will nd applications in using gold and other metals in a series of challenging gold-based nanocluster formations and tuning of their intrinsic properties.

Conflicts of interest
There are no conicts to declare.