Covalently Binding Atomically Designed Au9 Clusters to Chemically Modified Graphene**

Atomic-resolution transmission electron microscopy was used to identify individual Au9 clusters on a sulfur-functionalized graphene surface. The clusters were preformed in solution and covalently attached to the surface without any dispersion or aggregation. Comparison of the experimental images with simulations allowed the rotational motion, without lateral displacement, of individual clusters to be discerned, thereby demonstrating a robust covalent attachment of intact clusters to the graphene surface.

allowing direct correlation between property and structural features.
Graphene (G), as at wo-dimensional system with outstanding electronic properties and ah igh surface area, [13] offers the ideal platform for the deposition of NPs. [14] In addition, the diversity of carbon chemistry offers many routes to producing chemically modified graphene (CMG). [13,15,16] Nanoparticles have been stably attached to both Gand CMGs for ap lethora of different applications. [17] In particular, different routes for the hybridization of Au NPs with G have been studied, [18][19][20][21][22][23] but surprisingly,t he fabrication of atomically precise Au NCs supported on Gr emains unexplored. Recently,w eh ave described an easy way to chemically modify graphene with sulfur functionalities, [24] and now,b yt aking advantage of the affinity between gold and sulfur, we describe the stable attachment of preformed [Au 9 (PPh 3 ) 8 ](NO 3 )c lusters [25] to our CMG.A berration-corrected transmission electron microscopy (ac-TEM) has been employed to directly identify individual covalently attached Au 9 clusters,a nd to track their relative orientation.
Chemically modified graphene with sulfur functionalities was synthesized by treatment of graphene oxide (GO) with potassium thioacetate,f ollowed by an aqueous work-up. [24] This route is simple and scalable,a nd gives as ingle-layer material with reactive thiol groups that offer anchoring points for further functionalization;t his material is referred to as GOSH. Moreover,t he synthetic route results in ap artial reduction of the GO when the sulfur functionalities are introduced, thereby giving am ore graphene-like substance; this is particularly relevant for applications in which as emiconducting/conducting behavior is required, as the reduction of GO results in ap artial restoration of the sp 2 structure of G. [26] The[ Au 9 (PPh 3 ) 8 ](NO 3 )c luster (abbreviated as Au 9 )w as selected as the target cluster. [10,25] The D 2h -symmetric cluster is composed of nine gold atoms arranged such that one central gold atom is surrounded by the remaining eight gold atoms, each of which is coordinated by am onodentate phosphine ligand (see Figures S1 and S2 in the Supporting Information). Thea verage metal-metal distance is around 0.27 nm, which results in acluster diameter between 0.45 nm and 0.54 nm, [10] far smaller than that typically exhibited by Au NPs (particle size > 3nm). [11] Theb inding between Au 9 and GOSH was achieved by simply stirring Au 9 with ad ispersion of GOSH (Scheme 1). Ac ovalent bond is formed between sulfur and gold, which is accompanied by displacement of ap hosphine ligand. As ar esult, an eutral GOSH@Au 9 hybrid is formed.
Ac omparison of the thermogravimetric analysis (TGA) results for GOSH and GOSH@Au 9 gives the first evidence of hybrid formation ( Figure S7). In both cases,afirst weight loss appears around 130 8 8C, which corresponds to desorption of water, while the major mass loss is above 500 8 8Ca nd comes from decomposition of graphene-like sheets.The main difference appears at higher temperatures,where amass of 10.5 % remains for GOSH@Au 9 ,t hus pointing to the presence of metallic centers at al evel of order of 1atomic %. The presence of gold in the hybrid was confirmed by energydispersive X-ray (EDX) elemental analysis,w hich showed atomic gold and phosphorus contents of around 1atomic % for each element. Additional, and more accurate,c orroboration came from X-ray photoelectron spectroscopy (XPS; Figures S4-S6), in which, in addition to the signals corresponding to C, S, and Ot hat are typical of GOSH, [24] signals for Au and Pw ere found, consistent with the presence of Au 9 . [6] Moreover,t he ratio between the atomic content determined by XPS (1.6 %A u:1.3 %P )i sc onsistent with the presence of Au 9 P 7 clusters.
Acontrol experiment was performed whereby,instead of GOSH, GO was treated with Au 9 .W ith GO,n og old was detected by either EDX or by XPS ( Figure S5), thus ruling out the possibility of an interaction between Au 9 and any remaining oxygen functionalities on the GOSH, thus confirming that Au 9 attachment is through the sulfur functionalities.
Determining where the Au 9 is attached and whether the clusters remain intact requires direct imaging of the GOSH@Au 9 hybrid at atomic resolution. This was achieved by ac-TEM. Atomically thin 2D materials such as graphene or graphene oxide have been employed as supports for imaging molecular species; [27,28] at one atom thickness of carbon, they are almost transparent under the electron beam so that individual molecules can be resolved at atomic resolution, whilst their well-defined crystal lattice enables accurate calibration for quantitative measurements.T EM grids were prepared by adding one drop of aw ell-dispersed 0.08 mg mL À1 solution of GOSH@Au 9 in DMSO to alacey carbon support and allowing it to dry under ambient conditions.
An ac-TEM image of at ypical area of GOSH@Au 9 is shown in Figure 1a.A si sc haracteristic for samples derived from graphene oxide, [29] the image shows ordered regions where the graphene-like lattice is visible,r egions of higher contrast that are apparently disordered and can be attributed to oxidation debris [30,31] or other carbonaceous material adhered to the surface,d efects,a nd small holes.Asingle hexagon of spots can be seen in the inset diffraction pattern, which shows that the long-range crystalline order of graphene is retained, and that this region consists of asingle monolayer

Angewandte
Chemie of chemically modified graphene (see also Figure S8). [27] Additional features are seen, not observed on GO or GOSH. Clusters of dark spots indicative of atoms with ah igh atomic number are apparent and dispersed across the GOSH surface.T here is clear structure within the clusters and, as discussed below,t his can be used to unambiguously identify these features as Au 9 clusters.Noaggregation of the clusters was observed, with isolated clusters distributed uniformly across the sheets ( Figure S9b), thus suggesting that they are attached to the GOSH and hence unable to diffuse and coalesce.
Counting the Au 9 clusters gives an estimate of the extent of functionalization. From analysis of images such as Figure 1a,the estimated concentration of Au 9 clusters is 7 AE 2per 100 nm 2 ,which corresponds to just under 0.2 %ofthe carbon atoms in GOSH being functionalized through the sulfur to Au 9 linkage.N ote that this is only an estimate of the Au 9 cluster density,asthe image contrast of the Au 9 clusters varies considerably depending on their orientation (as discussed below), which complicates their identification. However,this functionalization density would correspond to an atomic content of 1.6 AE 0.4 %A u, which is in close agreement with the TGA, XPS,a nd EDX measurements.
More detailed analysis requires comparison between the experimental images and image simulations of the Au 9 clusters on GOSH. Using the known crystal structure of Au 9 ,atableau of multislice image simulations was constructed by rotating the molecule about two orthogonal symmetry axes ( Figure S10). ForA u 9 ,t he contrast is dominated by the gold atoms (Z = 79), which scatter the electrons to agreater extent than do the phosphorus (Z = 15) or carbon (Z = 6) atoms. However,f or accurate comparison all atoms were contained in the image simulation, including the coordination sphere of ligands and as ection of graphene lattice. [32] Figure 1b shows three different regions on the GOSH@Au 9 ,each from an ac-TEM image acquired with 0.3 se xposure.T he region in Figure 1b1ismarked by the box in Figure 1a.Acomparison with the simulations (Figure 1c)f acilitates identification of the orientation of the Au 9 clusters (Figure 1d).
Comparison between the image simulations and experimental images clearly shows that the clusters are Au 9 ,w ith aclose match for the contrast arising from the gold atoms and also subtler variations in contrast that are consistent with the ligands still being present. Further confirmation comes from the measurement of the spacings of the atomic columns within the clusters,w hich shows that the Au-Aud istances in the experimental images are consistent with those expected for Au 9 ( Figure S11). This proves that intact, undamaged, Au 9 NCs are present on the GOSH surface.
As tudy of the dynamics of these Au 9 clusters demonstrates that they are covalently bound to GOSH and not simply adsorbed to the surface.T he ac-TEM image shown in Figure 1a was taken with a0.3 sexposure;the match between the experimentally observed contrast and image simulation shows not only that Au 9 is present and intact, but also that those clusters are stationary on the GOSH surface for the period of that exposure.H owever,i nspection of subsequent images shows that the clusters are not permanently fixed and their contrast changes over time. Figure 2shows asequence of ac-TEM images of as ingle Au 9 cluster on GOSH;i mages were acquired at 0.3 si ntervals over ap eriod of more than 10 s( the full image sequence is shown in Figure S12 in the Supporting Information, with only selected images shown in Figure 2). Theimages are from the same region and, through comparison with fixed points in the larger image ( Figure S9a), show no apparent lateral displacement of the cluster relative to the underlying GOSH. Thechange in contrast is indicative of rotations of the Au 9 cluster relative to the GOSH surface, and comparison with the image simulation tableau enables each of the images shown in Figure 2t ob ei dentified as specific Au 9 orientations.From this it is apparent that the Au 9 cluster is rotating, but without lateral displacement. This is consistent with covalent attachment of Au 9 to GOSH through the ÀSÀAu bond.
Thei nhibited rotation is caused by interaction with the electron beam and is indicative of as et of metastable orientations.P rior work has observed similar electron beam induced molecular motion on graphene oxide, [32] on carbon nanotubes, [33,34] and for molecules attached to carbon nanohorns,where it was shown that lower acceleration voltages in the TEM resulted in higher frequency of molecular motion as aresult of alarger scattering cross-section. [35,36] An accelerat- Figure 2. Selected ac-TEM image frames taken from the dynamics of asingle Au 9 with their corresponding molecular models (the full ac-TEM sequence representingthe motion of asingle Au 9 cluster over ap eriod of 11.4 si sshown in Figure S12).
ing voltage of 80 kV was used here to minimize damage to the chemically modified graphene by the electron beam. At this acceleration voltage,the clusters are fixed in each orientation for timescales on the order of seconds before switching to another orientation. This indicates that each observed orientation is metastable,a nd corresponds to al ocal energy minimum. As each image shows well-defined spots rather than blurred streaks,itis also clear that the transition between orientations must be relatively rapid.
In conclusion, we have proven that atomically designed clusters can be covalently attached to chemically modified graphene by taking advantage of the affinity between gold and the sulfur functionalities present on the surface.W ehave demonstrated that the Au 9 clusters are intact and welldispersed over GOSH, and show no apparent aggregation. Dynamic ac-TEM measurements show how as ingle molecular cluster rotates as ar esult of the effect of the electron beam, but without lateral diffusion, which is indicative of as trong covalent interaction between Au 9 and GOSH. Moreover,t he results of the dynamic study suggest the presence of metastable orientations that may appear as ac onsequence of the steric demands of the ligands.
Our approach is generally applicable to the whole family of gold nanoclusters,and may be extended to other atomically designed clusters.This will allow fine-tuning of the graphenenanocluster properties (e.g.optical or charge-transfer properties), thus permitting exploration of the effect that the size and morphology of clusters have in applications ranging from biosensors or biomedicine,t oe nergy storage,o rh eterogeneous catalysis.