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X-ray Crystal Structure and Optical Properties of Au38-xCux(2,4-(CH3)2C6H3S)24 (x=0-6) Alloy Nanocluster Jinsong Chai, Ying Lv, Sha Yang, Yongbo Song, Xiaofeng Zan, Qinzhen Li, Haizhu Yu, Mingzai Wu, and Manzhou Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05074 • Publication Date (Web): 04 Sep 2017 Downloaded from http://pubs.acs.org on September 5, 2017

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The Journal of Physical Chemistry

X-ray Crystal Structure and Optical Properties of Au38−xCux(2,4-(CH3)2C6H3S)24 (x=0-6) Alloy Nanocluster Jinsong Chai,a Ying Lv,b Sha Yang, b Yongbo Song, b Xiaofeng Zan, b Qinzhen Li, b Haizhu Yu, b,* Mingzai Wu a,* and Manzhou Zhu b,* a

School of Physics and Materials Science, Anhui University, Hefei, Anhui, 230601,

China b

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

ABSTRACT: In this work, we report the synthesis and crystal structure of Au38−xCux(2,4-DMBT)24

(x=0-6,

2,4-DMBTH=2,4-dimethylbenzenethiol)

alloy

nanocluster for the first time. A variety of characterizations including ESI-MS, TGA and XPS reveal the composition as Au38−xCux(2,4-DMBT)24 (x=0-6). The single crystal structure has been determined by X-ray single crystal diffractometer. From the anatomy of structure, a bi-icosahedral Au23 core protected by six dimeric [−SR−M−SR−M−SR−] units (M=Cu/Au) and three monomeric [−SR−Au−SR−] units was presented. It is interesting that all the Cu atoms are selectively doped in the motifs of Au38−xCux(2,4-DMBT)24 nanocluster. This phenomenon is distinct from the exclusive core doping of the Ag atoms in the previously reported Au38−xAgx alloy. Both the experimental results and DFT calculations of UV-vis spectra imply that the 1

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optical property of Au38−xCux(2,4-DMBT)24 nanocluster is consistent with that of Au38(2,4-DMBT)24 nanocluster, because the Cu dopants make little contribution to the frontier orbitals of the alloy NC.

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INTRODUCTION

Alloy nanoclusters (NCs) with precise structure have emerged as a new class of nanomaterial in the past few years.1-3 Owing to their unique performance in bio-labelling,4 sensing5 and catalysis,6-7 alloy NCs have received more attention compared to their mono-metal NCs counterpart.8-12 For example, alloy NCs such as Pd1Au24,13 Pt1Au2414 and Au24Ag4615 display higher catalytic activity than Au25 NC. Meanwhile, the rod like Au25-xAgx16 and Au1Ag2417 also show stronger fluorescence than the rod like Au25 and Ag25. To this end, the elucidation of the accurate atomic structure of the alloy NCs is highly desirable to understand the structure-optical property relationship. Single crystal X-ray crystallography might provide sound information for the structure of alloy NCs, and the structures of a series of alloy NCs have been well identified in the past years.18-22 According to these studies, the metal composition and doping sites are key to tailoring the alloy NC’s properties. In spite of this, preparing alloy NCs with controllable composition/doping sites is still challenging.

The previous studies indicate that the type of foreign metal atom(s) remarkably affect the metal composition and doping sites in alloy NCs.23 In the family of M25(SR)18 NCs, the crystal structures of Pd1Au24(SR)1824 or Cd1Au24(SR)1823 reveal that the single Pd or Cd atom locates in the centroid of the M25(SR)18 cluster. In contrast, one of the outer-shell gold atom was replaced by the Hg atom in Hg1Au24(SR)18,25 as evidenced by both theoretical calculations and the experimental

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results. In Au25-xAgx(SR)18, the Ag dopants distribute not only in the icosahedral core, but also in the surface staple motifs.26 Interestingly, the Cu atoms are all doped in the motifs of Au25-xCux(SR)204-.27 The above results show that the different foreign metal atoms might be doped into different sites in the same NC. Nevertheless, in some other NCs, the Cu and Ag atoms might also occupy the same doping sites. For example, in the case of the rod-like Au25, both the top and waist sites could be partially occupied by Cu28 or Ag16 atoms. Similar results were also observed in the case of M44 NCs, in which the Au12 kernel could be protected by Ag-S-Ag motifs or Cu-S-Cu motifs in Au12Ag32(SR)3029 and Au12Cu32(SR)30,27 respectively. Therefore, the same placeholder between Ag and Cu relies on the structure of clusters. Inspired by these observations, we anticipated that Cu atoms might be doped into different sites from the Ag atoms in M38 NCs, due to its structural similarity with M25 (one icosahedra Au13 core for M25 and a bi-icosahedral Au23 core for M38).

Herein, we reported our success in the synthesis and crystal structure characterization of the bimetallic Au-Cu NC, Au38-xCux(2,4-DMBT)24 (2,4-DMBT = 2,4-dimethylbenzenethiol) for the first time. The precise atomic structure of this NC has been determined by single crystal X-ray crystallography. Similar to the structures of Au38(SR)2430 and Au38-xAgx(SR)2431 (R=C2H4Ph), Au38-xCux(2,4-DMBT)24 has a bi-icosahedral Au23 core, which was assembled by two icosahedra via sharing a common Au3 face. The Au23 core was protected by six dimeric [−SR−M−SR−M−SR−] units (M=Cu/Au) and three monomeric [−SR−Au−SR−] units. Herein, it is noteworthy that the Cu atoms occupy different locations compared with the Ag atoms 4


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in the previously reported Au38-xAgx(SR)24.31 That is, the Ag atoms in Au38-xAgx(SR)2431 are doped in the M23 core while the Cu atoms are all doped in the motifs of six dimeric [−SR−M−SR−M−SR−] units.

EXPERIMENT SECTION

All reagents and solvents were commercially available and were used as received without further purification, including tetrachloroauric(III) acid (HAuCl4•3H2O, ≥ 99.99% metals basis), copper(II) chloride (≥ 98%), methylene chloride (HPLC, ≥ 99.9%), toluene (HPLC, ≥ 99.9%), methanol (HPLC, ≥ 99.9%), sodium borohydride (≥

98%),

hydrogen

peroxide

(30%),

borane

tert-butylamine

(≥

97%),

2-phenylethanethiol(≥ 97%) and 2,4-dimethylbenzenethiol (≥ 97%). Pure water was purchased from Wahaha Co. Ltd, and the other chemicals were purchased from Aladdin (Shanghai, China). All glassware was thoroughly cleaned with aqua regia (HCl: HNO3 = 3:1, v:v), rinsed with copious pure water, and then dried in an oven prior to use.

Au38−xCux(2,4-DMBT)24 (x=0-6) NC was synthesized by the in situ two-phase ligand exchange method.32 10 mg CuCl2 was dissolved in 12 mL H2O at 55 oC. After 2 min, 500 mg glutathione (GSH) was added to the aqueous solution directly. Continuing to stir for 15 min, 1 mL HAuCl4•3H2O (0.2 g/mL) was added into the above solution. Then, a mixed solution including 220 mg borane tert-butylamine and 500 µL 2, 4-dimethylbenzenethiol (dissolved in 10 mL toluene) was added after 20 min. 4 h later, the aqueous phase changed into colorless before being removed. The 5



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organic phase was rotary evaporated at room temperature. The product was washed five times with CH3OH.

1 mL HAuCl4•3H2O (0.2 g/mL) was added in H2O and mixed with glutathione (GSH) at 55 oC. Continuing to stir for 20 minutes, a mixed solution including 220 mg borane tert-butylamine and 500 µL 2,4-dimethylbenzenethiol (dissolved in 10 mL toluene) was added. After 4 h, the aqueous phase was removed, and the organic phase was rotary evaporated at room temperature. The product was washed several times with

CH3OH

and

the

Au38(2,4-DMBT)24

nanocluster

was

gained.

The

Au38(C2H4PhS)24 cluster was gained according to the literature.30

For better comparison, the stability tests (e.g. under thermal or oxidizing/reducing environments) of Au38−xCux(2,4-DMBT)24, Au38(2,4-DMBT)24, and Au38(C2H4PhS)24 NCs were performed under the same conditions. Reducing environment: the cluster (2 mg) was dissolved in 6 mL CH2Cl2 and added to a flask, then 1 mg NaBH4 (dissolved in 200 µL ethanol) was added. Oxidizing environment: the cluster was dissolved in 6 mL CH2Cl2 and mixed with 300 µL H2O2 (30%). Thermal environment: cluster (2 mg) was dissolved in toluene at 80 °C.

Single crystal X-ray diffraction data of Au38-xCux(2,4-DMBT)24 was performed on a Bruker APEX-II CCD area detector using graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å). A block of black crystal with dimensions 0.1 x 0.08 x 0.05 mm3 was mounted onto a MiTeGen capillary with fluorolube, and the structure was solved by direct methods using ShelXT (Sheldrick, 2015). Structure was also refined 6


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by the full-matrix least-squares methods ShelXL program (Sheldrick, 2015) within the Data collection was performed under low temperature (130 K). The placement of the heteroatoms and fractional site occupancy in these alloy NCs were ascertained by the method of modifying the disorderly free variables.

Data reductions and absorption corrections were performed using the SAINT and SADABS programs, respectively. The structure was solved by direct methods and refined with full-matrix least squares on F2 using the SHELXTL software package. All non-hydrogen atoms were refined anisotropically, and all the hydrogen atoms were set in geometrically calculated positions and refined isotropically using a riding model. Free solvent molecules were highly disordered, and location and refinement of the solvent peaks were unsuccessful. The diffuse electron densities resulting from these residual solvent molecules were removed from the data set using the SQUEEZE routine of PLATON and refined further using the data generated.

UV-vis absorption spectra were obtained using an Agilent 8453 instrument, and solution samples were prepared using toluene or CH2Cl2 as the solvent. ESI-MS was recorded using a Waters Q-TOF mass spectrometer equipped with Z-spray source. Matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI TOF MS) was performed on an Applied Biosystems Voyager DE-STR MALDI-TOF equipped

with

a

nitrogen

laser.

Trans-2-[3-(4-tert-butyl-phenyl)-2-methyl-2-propenylidene] malononitrile (DCTB) was used as MALDI matrix. The source temperature was kept at 70 oC. The sample

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was directly infused into the chamber at 5 µL/min. The spray voltage was kept at 2.20 kV and the cone voltage at 60 V. The X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 high-performance electron spectrometer with monochromated Al Kα radiation as the excitation source. All binding energies were calibrated using the C (1s) carbon peak (284.8 eV). Thermal gravimetric analysis (TGA) (~3 mg sample used) was conducted in a N2 atmosphere (flow rate ~50 mL/min) on a TG/DTA 6300 analyzer (Seiko Instruments, Inc), and the heating rate was 10 oC/min.

Theory. We performed the density functional theory (DFT) calculation to investigate the mechanism of optical transition of Au38-xCux(2,4-DMBT)24. To reduce the computational cost, the 2,4-DMBT ligand is simplified with SH in calculations. The geometry of the modeling Au37Cu1(SH)24 was optimized at generalized gradient approximation (GGA) level of theory as implemented by ADF software,33 and the PBE34,35 exchange correlation energy functional was used here. The frozen core approximation was used to describe the metal atoms, and thus the 5d106s1, 3d104s1 electrons were treated as valence electrons for Au and Cu, respectively. The double zeta basis set DZP was used for S and H. Then, based on the optimized structure of Au37Cu1(SH)24, UV-vis spectra calculations were carried out with the TD-PBE method together with the TZP basis set, the lowest 150 singlet-singlet excited states were calculated. Finally, we calculated the KS-orbitals using Gaussian 09 software,36 using the PBE functional with the LANL2DZ34 basis set for Au and 6-31G(d) basis set for all other atoms. 8


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RESULTS AND DISCUSSION

The black crystal of Au38-xCux(2,4-DMBT)24 NCs were obtained via crystallization in CH2Cl2/CH3OH over 2−3 days. Single crystal X-ray diffraction analysis shows that the structure of the crystal has a trigonal space group R -3 c (Table S4).

Figure 1. The crystal structure of Au38-xCux(2,4-DMBT)24 nanocluster.

The structural details of the Au38-xCux(2,4-DMBT)24 are shown in Figure 1 and Figure 2. Similar to the structure of Au38(SR)24,30 Au38-xCux(2,4-DMBT)24 can be viewed as the combination of one bi-icosahedral Au23 core with six dimeric M2S3 staple motifs (M=Au/Cu) and three monomeric Au1S2 staple motifs. The Cu atoms access twelve doping sites (M2S3 staple motifs) in the Au38-xCux(2,4-DMBT)24 nanocluster. In these twelve positions, the Au/Cu atomic ratio is 91.7: 8.3. The average number of Cu atoms from X-ray data in the Au38-xCux(2,4-DMBT)24 nanocluster is 0.996. The details of occupancies and structural parameters for Au38-xCux(2,4-DMBT)24 are shown in Supporting Information (SI).38

To achieve a more detailed anatomy of the total structure, we start with the 9



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bi-icosahedral Au23 core (Figure 2 a), which was fused together by two icosahedral sharing a common Au3 face (Figure 2 a, pink balls). The Au-Au bond lengths in the Au23 core range from 2.76 to 3.23 Å (average: 2.89 Å), which were close to those in Au38(SR)2430 (the Au−Au distances in the Au23 core of Au38 range from 2.67 to 3.29 Å). The assembly of these two icosahedra was supported by the three bridging monomeric Au1S2 staple motifs (Figure 2 b-c, side and top views). Like those in Au38, the (R)S-Au-S(R) in Au38-xCux are all nearly linear (the bond angle is 171.38°), and the average Au-S bond length is 2.30 Å. Meanwhile, the Cu atoms could also occupy the “V”-shaped motifs (Figure 2d). The bond lengths of Au(Cu)-S in the“V”-shaped M2S3 staple motifs range from 2.27 to 2.31 Å (average: 2.29Å).

Figure 2. Anatomy of the surface structure of Au38-xCux(2,4-DMBT)24. (a) The core; (b) and (c) Side view and top view for core and monomeric Au1S2 staple motifs, respectively; (d) “V”-shaped M2S3 staple motifs; (e) The Au38-xCuxS24 framework (Color labels: green/pink = Au; blue = Cu/Au; red = S; all C and H atoms are not shown).

Electrospray ionization mass spectrometry (ESI-MS) was applied to determine the 10


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exact formula of the as-prepared nanoclusters. As shown in Figure S4, the positive mode of ESI-MS showed a series of peaks between m/z=9960 and 10800 Da. These peaks are assigned to [Au38-xCux(2,4-DMBT)24]+ (x=0-6). The matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) test was employed to reveal the possible species of Au38-xCux(2,4-DMBT)24. For comparison, the MALDI-MS test of Au38(2,4-DMBT)24 was also implemented. As shown in Figure S8 (red line), the spectrum

clearly

shows

the

fragmentation

of

Au38(2,4-DMBT)24

(i.e.

Au33(2,4-DMBT)20S). Meanwhile, the three peak fragmentations centered at 9140.160, 9006.364 and 8873.444 Da (black line) were assigned to the Au32Cu1(2,4-DMBT)20S, Au31Cu2(2,4-DMBT)20S,

and

Au30Cu3(2,4-DMBT)20S

fragments

of

Au38-xCux(2,4-DMBT)24. The TGA was also carried out to verify its formula (Figure S5). The result of the TGA shows a weight loss of 30.69 wt%, which is in good agreement with the theoretical value of Au37Cu(2,4-DMBT)24 (30.94 wt%). X-ray photoelectron spectroscopy (XPS) measurement was operated to confirm the presence of the doped metals in the NCs. The Au 4f spectrum of Au38-xCux(2,4-DMBT)24 nanocluster was shown in Figure S6 a. The Au 4f peak in Au38-xCux(2,4-DMBT)24 is 84.04 eV and close to that of Au(0) (84.0 eV), indicating that the charge state of Au atoms in Au38-xCux(2,4-DMBT)24 nanocluster are close to 0 (i.e. Au(0)). From Figure S6 b, a characteristic peak in the Cu 2p spectrum of Au38-xCux(2,4-DMBT)24 nanocluster appears at 932.80 eV.

The optical absorption spectrum of Au38-xCux(2,4-DMBT)24 nanocluster shows four major peaks centered at 420 nm, 640 nm, 790 nm and 1000 nm. There are also three 11



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weaker absorption bands centered at 380 nm, 550 nm and 470 nm (Figure 3 black line). Of note, the UV-vis absorption spectrum of Au38-xCux(2,4-DMBT)24 is almost the same as that of Au38(2,4-DMBT)24 (the formula is determined by ESI-MS, Figure S7) which is shown in Figure 3 red line, indicating that the Cu dopants in the motifs does not affect its electronic structure. This observation is in sharp contrast to that in Au38-xAgx(SR)24, where the incorporation of Ag atoms in the core result in loss or smearing out of distinct UV-vis features.37

Figure 3. (a) Optical absorption spectra of Au38-xCux(2,4-DMBT)24 (black line), Au38(2,4-DMBT)24 (red line) and theoretical optical absorption spectrum of Au37Cu1(SR)24. (b) Kohn–Sham orbital energy level diagram for Au37Cu1(SR)24.

Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were utilized to probe the influence of the Cu dopants on the electronic and optical properties of this alloy NC. As shown in Figure 3 (blue line), the experimental UV-vis spectrum has been well reproduced by theoretical calculations. That is, the modeling Au37Cu1(SR)24 exhibits a sequence of peaks centered at 380 nm, 550 nm, 640 nm, and

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others in the region of 750-1000 nm. Beyond that, Kohn–Sham orbital energy level diagram for Au37Cu1(SR)24 in Figure 3b indeed shows that the atomic orbitals of Cu dopants on motifs in Au37Cu1(SR)24 make little contribution to the frontier molecular orbitals of this alloy NC. Herein, it’s noteworthy that another typical component elucidated by ESI-MS, i.e. Au35Cu3(SR)24, was also modeled in theoretical calculations. Nevertheless, the UV-vis spectra of Au35Cu3(SR)24 significantly deviate the spectra of the theoretical Au37Cu1(SR)24 and the experimental UV-vis spectrum, indicating that the ratio of such component should be very low.

Comparing the optical spectra of the series of Au38-xMx(SR)24 (M=Au, Ag and Cu) NCs, we found that the optical properties could be tailored by different dopants. The optical absorption of the homometallic Au38 NC could be retained via the surface doping of Cu atoms, while is obviously disturbed when the Ag atoms were doped in the core.

The stability tests (i.e. under thermal/oxidizing/reducing environments) for Au38−xCux(2,4-DMBT)24, Au38(2,4-DMBT)24, and Au38(C2H4PhS)24 nanoclusters were performed to explore the influence of ligand and the copper dopants on the stability of M38 nanoclusters. Under the oxidizing environment (by mixing 300 µL H2O2 (30%) with 2 mg cluster in 6 mL CH2Cl2), all the three nanoclusters show excellent stability. As shown in Figure S9 a, the UV-vis of Au38−xCux(2,4-DMBT)24 almost unchanged after 24 h, and the similar phenomena were also found in the cases of Au38(2,4-DMBT)24 and Au38(C2H4PhS)24 nanoclusters (Figure S9 b and c). Therefore,

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the type of ligand and the Cu dopants do not affect the stability of M38 nanoclusters under oxidizing environments. Meanwhile, when reducing environment (by mixing the CH2Cl2 solvent of the cluster with 200 µL EtOH solvent of 1 mg NaBH4) was used, the UV-vis spectra of Au38−xCux(2,4-DMBT)24 were consistent after 24 h (Figure S10 a). No impurities were found in this reaction system. The UV-vis of Au38(2,4-DMBT)24 also show little change after 24 h (Figure S10 b), albeit a few precipitates were found in the reaction system. Therefore, Au38−xCux(2,4-DMBT)24 is slightly more stable than Au38(2,4-DMBT)24, indicating that the doping of Cu slightly stabilizes the M38 nanoclusters. Besides, as shown in Figure S10 c, Au38(C2H4PhS)24 was completely etched under the NaBH4 reducing environment after 5 min. Finally, the Au38−xCux(2,4-DMBT)24 and Au38(2,4-DMBT)24 are instable under 80 oC (Figure S11 a and b), while Au38(SC2H4Ph)24 shows excellent thermal-stability (Figure S11 c). Therefore, the phenylethanethiol ligand is conducive to the thermal stability while the 2,4-dimethylbenzenethiol is helpful to withstand reducing environment for M38 nanoclusters.

CONCLUSIONS

In summary, a new Au-Cu bi-metallic nanocluster, Au38−xCux(2,4-DMBT)24 (x=0-6), was gained via the in-situ ligand exchange method. The crystal structure of Au38−xCux has been confirmed through the single crystal X-ray diffraction. The structure of Au38−xCux is highly similar to those of Au38(SR)24 and Au38-xAgx(SR)24 (R=C2H4Ph). It is noteworthy that the Cu atoms were selectively doped in the motifs of

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M38(2,4-DMBT)24, in contrast to the core doping of Ag atoms. The other tests including ESI-MS, TGA, XPS were also employed to verify the composition of Au38−xCux. The UV-vis absorption spectra indicate that Au38−xCux(2,4-DMBT)24 displays analogous optical performance to Au38(2,4-DMBT)24. TD-DFT calculations were utilized to unravel that the average doping number in Au38−xCux(2,4-DMBT)24 is 1 and the Cu dopant make little contribution to the frontier molecular orbitals for this alloy NC. In this paper, the structure of Au38−xCux(2,4-DMBT)24 with Cu dopants in the motif will shed light on the structure-optical property relationships of alloy NCs.

ASSOCIATED CONTENT

Supporting Information.

Details of the occupancy data, crystal data, and supporting figures (PDF)

Crystallographic information for Au38-xCux(2,4-DMBT)24 (CIF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT 15



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We acknowledge financial support from the National Natural Science Foundation of China (21631001, 21372006, and U1532141), the Ministry of Education, the Education Department of Anhui Province, and the 211 Project of Anhui University.

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TOC Graphic:

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Alloy Nanocluster - ACS Publications - American Chemical Society

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