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Surface Single Atom Tailoring of Gold Nanoparticle Zibao Gan, Jishi Chen, Lingwen Liao, Hongwen Zhang, and Zhikun Wu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02982 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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The Journal of Physical Chemistry Letters

Surface Single Atom Tailoring of Gold Nanoparticle Zibao Gan, Jishi Chen, Lingwen Liao, Hongwen Zhang and Zhikun Wu* Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS Center for Excellence in Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, China. *Email: [email protected]

Supporting Information Placeholder ABSTRACT: Surface single atom tailoring of gold nanoparticle, that is, adding, removing or replacing one surface atom on a structure-resolved nanoparticle in a controlled manner, is very exciting yet challenging, and has not been hitherto achieved. Herein we report that introducing a single sulfur atom onto the surface of the structure-unraveled Au60S6(SCH2Ph)36 nanoparticle is first realized. Single crystal X-ray crystallography reveals that the as-obtained nanoparticle consists of one Au17 kernel protected by one Au20S3(SCH2Ph)18 and one unprecedented Au23S4(SCH2Ph)18 motif with the introduced sulfur atom included as a tetrahedral-coordinated µ4-S. The introduced sulfur leads to the changes of both internal structure and crystallographic arrangement. Unlike the case of 6HLH arrangement in Au60S6(SCH2Ph)36 crystals, the “ABAB” arrangement in Au60S7(SCH2Ph)36 crystals enhances the solid photoluminescence of amorphous Au60S7(SCH2Ph)36 and brings a slight redshift of the maximum emission. The extensive near-infrared emission bestows Au60S7(SCH2Ph)36 potential applications in some fields such as anti-counterfeiting, imaging, etc.

Controlling nanoparticles by way of controlling organic molecules has long been a dream for scientists1-6. With the development of nanochemisty, ultrasmall thiolated nanoparticles (so-called nanoclusters) can now be controlled with atomic precision7-14. Following that, precisely manupulating nanoparticles with metal doping has also come true, and over a dozen of well-defined, atomically mono-disperse alloy nanoparticles have been successfully synthesized15-32(including the very recent “molecular surgery”31-32 conducted by Jin group). However, single atom doping, which provides excellent opportunities for understanding the doping effect, is still challenging, and less than 10 single-atom-doped nanoparticles have been hitherto reported after many efforts15-17, 22, 26, 27. It is known that stable nanoparticles are always protected by surfaceprotecting ligands (including organic and inorganic species), thus tailoring the surface ligands could be a strategy to tune the properties of metal nanoparticles, and this was indeed testified by

previous efforts33-38. However, precise and subtle surface tailoring of metal nanoparticles (note: complete or almost complete ligand exchange is not considered here39, 40) did not come true until 2014, in which Xie et al reported a successful synthesis of multithiolate-protected Au25 nanoparticles with atomic precision41. On the other hand, surface single atom tailoring, that is, adding, removing or replacing one surface atom on a structure-resolved nanoparticle in a controlled manner has not been realized before, to the best of our knowledge. Note that, Dass et al. found that Au30(S-t-Bu)18 could be changed to Au30S(S-t-Bu)18 in the crystal growth process42, however, the structure of Au30(S-t-Bu)18 was unravelled and some transformation details remain uncertain (or unclear) to date; for example, where the coating sulfur atom is from is not yet reported. The scientific community is waiting to see how a single atom (eg. a sulfur atom) can be introduced onto the surface of a metal nanoparticle in a controlled and clear way and what impacts on the structures and properties of the metal nanoparticle can the single atom bestow. Such a surface single atom tailoring is very exciting yet highly challenging, which inspires our research enthusiasm. In addition, we recently synthesized a sulfur/thiolate co-protected gold nanoparticle Au60S6(SCH2Ph)36 (abbreviated as Au60S6), and revealed a novel crystallographic arrangement in its crystals43, therefore we were eagerer to investigate the possibility of surface single atom tailoring and the possible impacts of the single sulfur on the structures (especially the crystallographic arrangement) and the solid-state photoluminescence. Herein, we report our success in introducing a single sulfur atom onto the surface of Au60S6 in a controlled manner and also our findings that the introduced sulfur greatly changes both the internal and the external structures of Au60S6, as well as the solid-state photoluminescence.

Figure 1. (a) ESI-MS (positive ion mode) and (b) TGA results of the as-obtained nanoparticles (The insets correspond to the experimental and calculated isotopic patterns and the zoom-in mass spectrum at [Au60S7(SCH2Ph)36Cs2]2+).

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Figure 2. (a) Au20S3(SCH2Ph)18 motif, (b) Au20 kernel, (c) Au20S3(SCH2Ph)18 motif, (d) the overall Au60S6(SCH2Ph)36 framework, (e) Au20S3(SCH2Ph)18 motif , (f) Au17 kernel, (g) Au23S4(SCH2Ph)18 motif, (h) Au20S3(SCH2Ph)18 motif, and (h) the overall Au60S7(SCH2Ph)36 framework. Color labels: the µ4-S atoms in orange, other S atoms in yellow, the kernel gold atoms in green, staple gold atoms in magenta (To highlight the transformation from kernel to staple, the three gold atoms are shown in blue). purified nanoparticles over two weeks. The product exhibits an obvious different color in solution or PTLC plate from that of Au60S6, and can be well separated from Au60S6 by PTLC (Figure S1), although its absorption spectrum has a slight difference with that of Au60S6 (Figure S2), indicating that it is different from the well-defined Au60S6. Electrospray ionization mass spectrometry (ESI-MS) was applied to determine the precise composition of the as-obtained nanoparticle. Of note, without the addition of cesium acetate (CsOAc), no strong signal was observed in either positive or negative mode, which implies charge neutrality of the nanoparticle. To impart charges, CsOAc was added to the nanoparticle solution to form positively charged [cluster+xCs]x+ adducts in the ionization process. As shown in Figure 1a, two intense peaks at mass/charge ratio (m/z) 8371.8 and 5625.8 are observed, which can be readily assigned to [Au60S7(SCH2Ph)36Cs2]2+ (calculated m/z: 8372.3, deviation: 0.5) and [Au60S7(SCH2Ph)36Cs3]3+ (calculated m/z: 5626.2, deviation: 0.4), respectively. The isotopic comparison confirms the assignment (Figure 1 inset). Thus, it is preliminarily concluded that the as-obtained nanoparticle could be Au60S7(SCH2Ph)36, which is supported by the thermogravimetric analysis (TGA); the latter reveals a weight loss of 28.1 wt % (Figure 1b), in excellent agreement with the theoretical value (28.3 wt %) of Au60S7(SCH2Ph)36 (abbreviated as Au60S7 hereafter). The formula is unambiguously confirmed by single crystal Xray crystallography, which reveals that the as-obtained nanoparticles crystallize in a triclinic space group of P-1 and have no symmetric axis, center or plane as shown in Figure S3. The formula definition demonstrates that one single sulfur atom was successfully introduced into Au60S6, giving rise to a new product (Au60S7). The introduced sulfur should come from the decomposition of free phenylmethanethiol because, without the thiol, Au60S7 cannot be obtained in control experiment 1 (see control experiments and Figure. S4 in supporting information)45. The identification of a side-product, PhCH2SCH2Ph (see control experiments 25 and Figures. S5-9), indicates that the dimerization of phenylmethanethiol occurs during the reaction, which provides the sulfur source for the transformation of Au60S6 to Au60S7. Note that PhCH2SSCH2Ph can also be observed in GC-MS chromatograms at longer retention times and the intensity depends on the control reactions (Figure S5, 10). Next, an intriguing issue pertains to what impacts on the structure and solid photoluminescence of Au60S7 can the single atom endow. To address this, the internal structures of Au60S6 and Au60S7 nanoclusters were first dissected. As shown in Figure. 2a-

d, Au60S6 consists of a Au20 kernel protected by a pair of Au20S3(SCH2Ph)18 motifs, while the Au60S7 is composed of a Au17 kernel protected by one Au20S3(SCH2Ph)18 and one Au23S4(SCH2Ph)18 motif. Thus, both of them have the same Au20S3(SCH2Ph)18 motif (Figure 2a, e). However, Au60S7 also has the Au23S4(SCH2Ph)18 motif which might be regarded as the adduct of Au20S3(SCH2Ph)18 with tetrahedral Au3S (Figure 2c, g) and represents the largest motif found in metal nanoclusters till now, to the best of our knowledge43. Of note, the Au17 kernel in Au60S7 (Figure 2f) might be regarded as the Au20 kernel in Au60S6 losing three local gold atoms (highlighted in blue) but with other gold atoms and structural features remained. This structural comparison between Au60S7 and Au60S6 provides some clues for the possible atomic structure transformation caused by the introduction of the single sulfur atom. As shown in Figure 3, the net structure transformation can be regarded as that the foreign sulfur atom derived from the dimerization of phenylmethanethiol inserts into the centre of the Au4 tetrahedron in the kernel of Au60S6, coordinates with the four gold atoms and forms the tetrahedralcoordinated µ4-S, then the three outer gold atoms in the Au4 tetrahedron are detached from the main kernel and join the adjacent Au20S3(SCH2Ph)18 motif, forming the giant Au23S4(SCH2Ph)18 unit with the other atoms and structural features essentially remained (Figure 3). This structure difference between Au60S7 and Au60S6 is very interesting and has important implications to the structure transformation of metal nanoclusters. The fact that sulfur powder or thiourea (which can release S2- under heating) can not result in the formation of Au60S7 indicates that the position of a sulfur atom is not an isolated addition process (Figure S11a-c); phenylethanethiol can not lead to the target product indicates that the C-S bond breakage is important for the addition of one sulfur atom to Au60S6 (Figure S11d). Thus a possible pathway should include the absorption of phenylmethanethiol, C-S bond breakage, dissociation, structure recombination, etc. However, for the understanding of detailed structure transformation mechanism, it needs more experimental and theoretical effort in the future. The specific position of the introduced S atom in Au60S7 might be mainly thermodynamically controlled on basis of the following experimental results: a) Au60S7 is more thermostable than Au60S6 monitored by UV-Vis/NIR spectrometry (see Figure S12); b) Other S atom location was not found in the single crystal structure of Au60S6; c) The isolated Au60S7 in different reaction stage have the same structures indicated by the superimposed UV-Vis/NIR spectra and PTLC analysis (see Figure S13).

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Figure 3. Color labels: the µ4-S atoms in orange, other S atoms in yellow, the kernel gold atoms in green, staple gold atoms in magenta (To highlight the transformation from kernel gold to staple gold, the three gold atoms are shown in blue). The introduced single sulfur atom not only leads to the local change of internal structure of Au60S6, but also results in the essential alternation to the external structure (the crystallographic arrangement). As shown in Figure. 4b, the Au60S7 adopts the “ABAB” arrangement, obviously different from the previously reported 6H left-handed helical (6HLH) arrangement in Au60S6 crystals (Figure 4a) 43. In the previous work43, we found that a 6HLH arrangement causes the photoluminescence loss of amorphous Au60S6, while here we found that the “ABAB” arrangement increases the photoluminescence of amorphous Au60S7 by ~100%. In addition, the “ABAB” arrangement induces a slight redshift (~ 6 nm) of the maximum emission of amorphous Au60S7 (Figure 5), while the 6HLH arrangement brings a subtle blueshift (~ 4 nm) of the maximum emission of amorphous Au60S643. These results both indicate that the crystallographic arrangement influences the solid photoluminescence of gold nanoparticles, but it may have different influence mode (increasement vs decreasement; redshift vs blueshift, etc.). The near-infrared emission of solid Au60S7 reminds that this nanocluster could be applied in labeling. To illustrate this, we made a film and found that it can be clearly imaged by laser scanning confocal microscopy (Figure 6a). Moreover, the imaging was not obviously influenced after Au60S7 was mixed with commercial black ink and still stayed for one week exposing to the atmosphere environment at room temperature (Figure 6b-d), indicating that Au60S7 is well tolerant of commercial black ink and can be added into commercial black ink for anticounterfeiting purpose (security ink). Note that, the Au60S7 nanocluster emits extensively at near-infrared region in solution (Figure S14), indicating it might also have some potential applications in liquid, which is beyond the scope of this study.

Figure 5. The solid-state photoluminescence spectra of Au60S7(SCH2Ph)36 nanoclusters. Note: The measurement was performed on a laser scanning confocal Raman/photoluminescence scope (HORIBA Jobin Yvon, λex = 514 nm, Power = 0.05 mW, for the absorption spectrum of Au60S7(SCH2Ph)36, see Figure S2).

Figure 6. Laser confocal images of films made from different materials. (a) Au60S7(SCH2Ph)36; (b) the commercial black ink; (c) the mixture of Au60S7(SCH2Ph)36 and the commercial black ink; (d) the mixture of Au60S7(SCH2Ph)36 and the commercial black ink stayed for one week. Figure 4. The crystallographic arrangement of Au60S6(SCH2Ph)36 (a) and Au60S7(SCH2Ph)36 (b) nanoclusters in the unit cells of the single crystals. (Note: To highlight the arrangement, the Au atoms of the nanoclusters in each close-packed plane are labeled in different colors).

In summary, surface single atom tailoring of gold nanoparticle was for the first time fulfilled in a controlled and clear manner, and the resulting structure and photoluminescence changes have been carefully studied. The successful introduction of a single sulfur atom onto the surface of Au60S6(SCH2Ph)36 was unambiguously identified by ESI-MS, TGA and single crystal X-ray crystallography, and it was unraveled that the introduced sulfur comes from the dimerization of phenylmethanethiol. Single crystal X-ray crystallography revealed that the novel nanoparticle

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consists of a Au17 kernel protected by one Au20S3(SCH2Ph)18 motif and one Au23S4(SCH2Ph)18 giant motif with the introduced single sulfur atom included as a tetrahedral-coordinated µ4-S, distinctly different from the known structure of Au60S6(SCH2Ph)36 which consists of a Au20 kernel protected by a pair of Au20S3(SCH2Ph)18 motifs. It is worth noting that the Au23S4(SCH2Ph)18 giant motif is indeed the first observation in metal nanoparticles and is the largest motif reported thus far to the best of our knowledge. Based on the structure difference between Au60S6 and Au60S7, the net structure transformation can be regarded as that the foreign sulfur atom inserts into the centre of the Au4 tetrahedron in the kernel of Au60S6, then the three outer gold atoms in the Au4 tetrahedron are detached from the main kernel and join the adjacent Au20S3(SCH2Ph)18 motif, forming the giant Au23S4(SCH2Ph)18 unit with the other atoms and structural features essentially remained. The introduced single sulfur atom not only leads to the local change of the internal structure, but also results in the essential change of the crystallographic arrangement of nanoclusters in the macroscopic crystal. Unlike the case of 6HLH arrangement, the “ABAB” arrangement enhances the solid emission and causes a slight redshift of the maximum emission, indicating that the crystallographic arrangement variously influences the solid photoluminescence of gold nanoparticles. The extensive near-infrared emission of solid Au60S7(SCH2Ph)36 endows the novel nanoparticle a potential application in some fields such as anti-counterfeiting, which was preliminarily testified by our experiments of security ink. It is expected that our work may have some important implications for future metal nanoparticle manipulating via subtle surface tailoring, provide new views to the structure and surface of gold nanoparticles, and advance the understanding of crystallographic arrangementphotoluminescence correlation, and thus it may stimulate further exciting research in the related fields.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. Experimental details, characterization, Figures S1-14 and Tables 1-6. Xray crystallographic data of Au60S7(SCH2Ph)36 (CIF).

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We would like to thank National Natural Science Foundation of China (Nos. 51502299, 21528303, 21771186, 21222301, 21171170), National Basic Research Program of China (No. 2013CB934302), and the CAS/SAFEA International Partnership Program for Creative Research Teams for financial support.

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