Experimental and theoretical evidence for unprecedented strong interactions of gold atoms with boron on boron/sulfur-doped carbon surfaces

The 16e square-planar bis-thiolato-Au(iii) complexes [AuIII(1,2-dicarba-closo-dodecarborane-1,2-dithiolato)2][NBu4] (Au-1) and [AuIII(4-methyl-1,2-benzenedithiolato)2][NBu4] (Au-2) have been synthesized and fully characterized. Au-1 and Au-2 were encapsulated in the symmetrical triblock copolymer poloxamer (Pluronic®) P123 containing blocks of poly(ethylene oxide) and poly(propylene oxide), giving micelles AuMs-1 and AuMs-2. High electron flux in scanning transmission electron microscopy (STEM) was used to generate single gold atoms and gold nanocrystals on B/S-doped graphitic surfaces, or S-doped amorphous carbon surfaces from AuMs-1 and AuMs-2, respectively. Electron energy loss spectroscopy (EELS) data suggested strong interactions of gold atoms/nanocrystals with boron in the B/S-doped graphitic matrix. Density-functional theory (DFT) calculations, also supported the experimental findings, pointing towards strong Au–B bonds, depending on the charge on the Au–(B-graphene) fragment and the presence of further defects in the graphene lattice.


Introduction
][9][10] The fabrication of nanocrystal matrices is of signicant importance for the development of nanodevices. 11,124][15] The combination of these two challenges in nanocrystal chemistry (matrix fabrication and interaction of heavy metal single atoms/nanocrystals with the fabricated matrices) is addressed in the present work.
5][16][17][18] In this work, we use scanning transmission electron microscopy (STEM), and in particular the very high electron ux from beam irradiation, to induce chemical reactions of gold(III) complexes encapsulated in polymeric micelles spread across the holes of lacey carbon sample support grids.We show that the surface produced can be either a boron-sulfur-doped graphitic surface or an amorphous carbon surface, depending on the nature of the goldcoordinated ligand.Although Au-B s bonds and 3c-2e B-Au-B bonding have been reported within a pure Au-B alloy, 19 as well as a gold boride complex with uxional Au-B bonding, 20 there appear to be no precedents for gold-boron interactions within boron/sulfur-doped graphitic matrices, as observed here by electron energy loss spectroscopy (EELS) studies.The occurrence of such Au-B interactions is supported by DFT calculations.

Preparations, STEM and EELS data
We compared the two 16e square-planar bis-thiolato-Au(III) complexes Au-1 and Au-2 that differ only in the presence of carboranyl or tolyl groups as substituents on chelated 1,2dithiolato ligands.Both gold complexes are highly hydrophobic, but become water-dispersed once encapsulated in the symmetrical triblock copolymer poloxamer (Pluronic®) P123 containing blocks of poly(ethylene oxide) and poly(propylene oxide) (Scheme 1).
We observed structural changes within the high-energy electron beam irradiation region of this grid on a JEOL ARM 200F microscope using an accelerating voltage of 80 keV.We illuminated an area roughly equal in size to a grid-bar region using a stationary electron beam dened by a 150-mm aperture with total beam current of 9.6 nA for 15 minutes.Such irradiation causes rapid C-H and other bond dissociations. 22STEM imaging showed the formation of Au-nanoclusters with sizes varying from single atoms to ca. 9 nm diameter (Fig. 1(a) and S1 ‡) on a graphitic structured surface (Fig. 1(b)) aer 5 min of beam irradiation.The measured Au(111) d-spacing was 2.35 Å, consistent with that of bulk gold (Fig. S2 ‡). 23The spots in the fast Fourier transform (FFT) for the region in Fig. 1(b) correspond to a lattice spacing of 2.18 Å, the (100)/(010) planes of graphite (Fig. 1(c)). 24The histogram of the raw image (Fig. 1(d)) shows that the annular dark-eld signal is not saturating with the imaging conditions used.The main framework consists of 6-membered C/B rings, alongside visible defects in the regular array.Fig. 1(b) shows many topological defects (Stone-Wales defects). 25,26Such defects are commonly observed in graphene samples. 25,26These increases in ring size are likely to be due to the presence of S in the lattice, which is considerably larger in size than C and B, causing expansion and irregular ring shapes scattered through the structure.
In order to conrm the simultaneous presence of C, B and S in the matrix, electron energy loss spectroscopy (EELS) was used to characterise the graphitic material.The result clearly indicates that all three elements are present (Fig. 2(a)-(d), and S3 ‡).The relatively sharp edge at ca. 194.0 eV and broader peak centred at ca. 203 eV (Fig. 2(e)) on the B K-edge indicate that all the boron atoms are sp 2 hybridized in a planar bonding conguration within the graphite-like structure. 27These features are in good agreement with the B K-edge spectrum of hexagonal boron nitride, for which a p * B¼N peak at 192.0 eV and s * B¼N peak at ∼199 eV are observed. 27Interestingly, the B K-edge signal also resembles well that reported for B 2 O 3 . 28Involvement of sp 2 hybridized carbon in the formation of the graphite-like matrices was clearly evident from the C K-edge with a sharp p* feature at ca. 286 eV (Fig. S4 ‡).Principal component analysis (PCA) was carried out to further support the simultaneous presence of C, B, and S in the graphitic matrices.The "scree plot" (Fig. S5 ‡) obtained from the PCA provides an indication of how many components contribute the most to the EELS data.
Here we have chosen 7 components (Fig. S5 ‡).The rst component is the average of the dataset.Component-2 is dominated by the C-K edge.The B-K edge is captured in component-4.Moreover, the density of B is higher in the regions rich in Au nanocrystals, as observed in the corresponding HAADF STEM image.S6 ‡).Gold content, determined by ICP-MS analysis, indicated the presence of ca.112 ± 24 molecules of Au-2 per micelle.On deposition of aqueous AuMs-2 droplets ([AuMs-2] = 1 mg ml −1 ) onto a lacey carbon-coated TEM grid followed by electron beam irradiation (80 keV) of ca.9.6 nA for 15 min on an area roughly equal in size to a grid-bar region, many clusters of gold atoms, ranging in size single atoms to ca. 2.7 nm diameter nanocrystals, were observed on the surface (Fig. 3(a)).The average Au-Au distance of 0.26 ± 0.01 nm (122 measurements, Fig. S7 ‡), is consistent with that of bulk gold. 29omplementary high-resolution transmission electron microscopy (HRTEM) studies tracking the same individual Au nanoclusters under 10 min of beam irradiation in a JEOL 2100 TEM microscope operating at 200 keV, showed highly specic surface elemental rearrangements of these clusters (Fig. 3(b)), and the formation of well faceted nanoparticles.The lattice spacing of 2.4 Å corresponding to the {111} planes is in good agreement with the literature. 23As clearly shown in movie-1 (see supporting le ‡), these clusters are of different sizes ranging from 2 to 4 nm and can be regarded as resulting from coalescence of smaller particles.Furthermore, the movie conrms the presence of twinning (Fig. 3(b), which can be attributed to the coalescence of two or more small crystals).
For elemental identication, energy-ltered TEM (EFTEM) images were acquired (Fig. 3(c) and (d)) by employing the "jump ratio" method of the Au O edge using 30 eV energy windows for pre-and post-edge images, energy slit width of 20 eV, exposure      In order to obtain insights into possible bonding of gold to the B-doped graphene, DFT modelling was performed.Graphene was modelled with the C 72 H 22 molecule shown in Fig. 4. The molecule displays C 2v symmetry consisting of four equivalent parts.Modelling was performed for the fragment marked with the black rectangle.Each carbon was replaced with a boron atom, apart from those on the rim bound to hydrogen, which are assumed to be less relevant for the system investigated in the experiment.Thus, 14 different C 71 BH 22 molecules were used for the modelling as listed in Table 1.The rst part of the calculations considered AuC 71 BH 22 models.We performed optimizations of the geometry of the isolated molecule followed by frequency calculations using the TPSS functional 30 and QZVP basis set. 31The Gaussian 16 (ref.The results of the calculations showing the obtained geometries and estimated binding energies of Au-B bonding are collected in Table 1.For comparison, the result obtained for gold bound to C27 are shown. The analysis of the calculated formation energies reveals a very high stabilization of the Au(I) systems with 531 electrons with estimated values of −480 to −530 kJ mol −1 depending on the position of the boron.Equally, the binding to carbon in the B-doped graphene fragments seems to be signicant, with a predicted binding energy of −420 kJ mol −1 .The diamagnetic 532-electron molecules exhibit the predicted binding energies in the range of −95 to ca. −170 kJ mol −1 .Taking into account that the diamagnetic 532-electrons system may be considered both as the neutral-ligand Au(0) and negative ligand Au(I) complexes, implies that binding of Au(I) is preferred for the neutral B-doped graphene rather than for the negativelycharged graphene.The triplet 532 electron molecules showed binding energies of roughly 60 kJ mol −1 lower than the singlet ones.The values for those are in a similar range as for the 533electron systems.
The geometries of the Au B-graphene adducts fall into three categories.The gold atom may be bound axially to boron, forming a bond perpendicular to the graphene plane, with a slightly pyramidal distortion of the BC 3 fragment.This geometry is the preferred one for the 533-electron molecules.Another geometry corresponds to the Au-B bond forming an angle of more than 100°with the graphene plane and additional bond/contact with the carbon bound to boron.The C-Au length varies from 2.18 Å to 2.56 Å.Finally, for B32 substituted systems, a third mode of coordination was found in which the Au-B bond is again inclined to the graphene plane, but there is no shi of Au towards a particular carbon.Rather, the gold atom is shied over the six-membered carbon ring.The three modes of coordination are show in Fig. 5.
In the next step, we performed the modelling of gold adducts of the model molecule with an Au 5 assembly bound to boron in position 30.Similar to the model involving the single Au atom, we considered four possible isomers: (i) 848-electron molecule with a doublet ground state corresponding formally to a negatively-charged C 71 BH 22 fragment bound to Au 5 (0), (ii) triplet and singlet states of 848-electron molecules corresponding formally to a neutral C 71 BH 22 fragment bound to Au 5 (0) with the   1.Interestingly, while for the 849-electron and triplet 848-electron systems the binding energies lie roughly in the ranges obtained for the single Auatom models, the respective binding energy for the 847-electron ((Au 5 ) +1 L) system is reduced to ca. 50% of that for the single gold complex, while that for diamagnetic 848-electron system is reduced at ca. 25% (for complexes involving the boron in position 30).Consequently, for the 848-electron system, the stabilisation of the singlet state with respect to the triplet state is reduced from 55 to 17 kJ mol −1 .
In each case, the Au 5 cluster is bound as a trapezium.Two types of geometries were identied for the Au 5 complexes.All systems, except for the 848-electron triplet, reveal the trapezium side parallel to the ligand plane with the bonding of one gold atom to boron and another (Au ′ ) to the atom C44, located in para position to boron belonging to the same six-membered ring.The Au-B and Au ′ -C bond distances lie in the range 2.28-2.34 and 2.53-2.59Å, respectively.For the 848 triplet species, only one gold atom of the trapezium side is coordinated to both B30 and the neighbouring C26 atoms with the respective bond lengths of 2.348 and 2.386 Å.The two geometries are shown in Fig. 6.A similar variety of bonding patterns was found for DFT Au 4 adducts of graphene models. 33.2.2.B-doped graphene with further defects.Finally, we considered the effects of further modications of the B-doped graphene.First, taking into account that the systems described in this paper contain nitrogen, we considered the systems with B30 graphene containing the nitrogen atom in position 70 (see Fig. 7, top le).This system corresponds to the reference B30-C 71 BH 22 with one more electron.On the other hand, the C 70 B 2 H 22 system in which one carbon of the B30-C 71 BH 22 is replaced with boron corresponds to one with one electron less.We chose the isomer with the remote B atoms in positions 30 and 47 (Fig. 7, top, right).The system with neighbouring borons in positions 30 and 70 binds the gold with both B atoms (see ESI ‡) and is, therefore, less relevant to compare with the system under consideration.
Finally, a defect structure with a vacancy at the position of C27 was considered.Different defects, including vacancies are known to change the electronic and structural properties of graphene. 34It seemed therefore of interest to look at the bonding properties of such a structure in comparison to a nondefected one.The structures of the defected B30C 70 BH 22 are shown in Fig. 7.Both the structure on the anion and of the

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neutral species are shown, as in this case they differ signicantly.For the neutral system, the two carbon atoms at the defect come closer to each other to form a strained vemembered ring with a corresponding C-C distance of 1.82 Å.The graphene ring stays planar.For the anionic species, the formation of the ve-membered ring is completed (with a respective C-C distance of 1.62 Å) leading to the folding of the graphene structure (see the pdb defect_ligB30_anion and defect_ligB30_neutral les in ESI ‡) corresponding to experimentally observed effects using TEM technique. 35The results for the "defected" structures were obtained using the abovementioned TPSS/QZVP method, this time with Grimme's dispersion correction. 36The results for the corresponding Au(0) L − , Au(0)L and Au(I)L systems are collected in Table 2 including the data for the reference B30-C 71 BH 22 system.It should be noted that the application of dispersion correction does not inuence the geometries obtained (the Au-B bonds do not differ by more than 0.005 Å), while all the energies obtained for the adducts formed are higher by 30 kJ mol −1 .Importantly, when comparing the different systems, the Au(0)L − , Au(0)L and Au(I)L molecules may be of different multiplicities that may make the comparison of the energy values more complex.For example, while the B30-model of the Au(0)L − system has one unpaired electron, its analogue with additional nitrogen may be either in singlet or triplet states (Fig. 7).
The results obtained for the N-doped B-30 substituted molecule allow the following conclusions.Firstly, the energy of adduct formation for Au(0)L and Au(I)L is signicantly (60-70 kJ mol −1 ) lower than those for the non-doped B-30 reference molecules with the exception of the triplet Au(0)L spin isomer of B-30 system, that reveals a formation energy higher by only 13 kJ mol −1 .The corresponding Au-B bond lengths are larger for the N-doped molecules, with the difference for the Au(0)L below 0.1 Å On the other hand for the Au(0)L − adducts the energy of formation of the doublet of the reference molecule lies between those of the singlet and triplet states of its N-doped analogue (−102, compared to −120 and −89 kJ mol −1 , respectively.).Also, the Au-B bond length for the doublet of the reference molecule lies between these for the singlet and triplet isomers of the N-doped one (2.292 vs. 2.295 and 2.238 Å).Noteworthy, however is the clear change of the geometry of the B-Au binding.In all adducts of the N-doped graphene analogue, the DFT optimisation indicates that the B-Au is skewed relative to the graphene plane, yielding N-B-Au angles of 104-106°(see Fig. 8 and pdb les Au_532_ligN_B30, Au_533_ligN_B30, Au_singlet_534_ligN_B30 and Au_triplet_534_ligN_B30 in ESI ‡).This may indicate either a repulsive interaction between the Au atom and a lone pair of electrons on nitrogen or/and the electron-rich part of the molecule.The results obtained for the 30,47-B-C 70 B 2 H 22 system with the second boron atom remote from the B30 centre seem to indicate the importance of the nitrogen low-pair repulsion effects.Although the system contains two electrons less than the reference B30 molecule the formation energies of the Au(0)L and Au(I)L systems are very similar (−529 and −527 kJ mol −1 for the reference and B-doped Au(I)L species, and −150 and −155 kJ mol −1 (singlet) for the  The results obtained for the systems with a vacancy in position 27, i.e. lacking one carbon in the graphene moiety, may be directly compared to those for the reference B30-C 71 BH 22 system.While containing six electrons less, the systems reveal the same mulitplicities for all formal charges of gold and graphene.The calculated formation energies point towards some additional stabilisation of the species with a defect in graphene for the Au(0) systems, while the defects seem not bring about signicant energetic effects compared to a reference system.The obtained structures shown in Fig. 9 display the difference between the graphene deformations for the 526/527 and 525 species (see the pdb les Au_defect_527, Au_defect_525, Aude-fect_silnglet_526and AU_defect_527_triplet).Although the isolated neutral and anionic B30-C 70 BH 22 defected graphene models reveal the different geometries (vide supra), they both form differently folded structures when bound to a gold atom.The 526/525 electron species (Au(0)L) and Au(I)L, respectively) reveal the ve-membered ring due to formation of the C46-C73 bond corresponding to the structure of the folded anionic free graphene species.On the other hand, the 527 electron form (formally Au(0)L − ) displays the ve-membered ring formed by the bond between C42 and C46 (note that C42 and C73 atoms are not equivalent for the B30 species).This effect implies that the overall charge on Au-B-doped-graphene with a defect inuences the localisation of the defect.An interesting feature is that, contrary to all Au-B species discussed before, the Au-B bond length for the triplet Au(0)L species is shorter than that for

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the singlet state.Additionally, the latter reveals a different geometry with an Au-B34 distance of 2.304 Å and C-B-Au angle of 72°.This together with the comaprable formation energies of the singlet and triplet states (−147 and −142 kJ mol −1 , respectively) may suggest that the singlet-state form may correspond to the previously mentioned possibility that the complex is a negatively-charged BC 70 BH 22 bound to Au(I), isolectronic with that of Au(0) bound to the neutral ligand.
Comparison of bond lengths with the reference B30-C 71 BH 22 indicates a pronounced difference of 0.15 Å between them, with that of the "defected" Au(0)L species revealing only a 2.151 Å Au-B bond.In summary, DFT modelling of the interactions of gold with the doped graphene suggests the possibility of formation of Au-B bonds with strengths higher than those for Au-C in the same systems, reveals the effects of doping the graphene with N-and B-defects for the geometry of Au-bonding, and shows pronounced geometrical effects brought about in the systems with a carbon vacancy.The change of the affinity for gold binding by introducing a vacancy is dependent on the overall charge of the system and is pronounced for the anionic species.

Conclusions
Recent advances in aberration-corrected transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) provide an opportunity for imaging ultradynamic systems in real-time.However, as yet, there are few reports of studies of such dynamic processes at the single-atom level.Here, we synthesized two 16e square-planar bis-thiolato-Au(III) complexes, [Au III (1,2-dicarba-closo-dodecarborane-1,2dithiolato) 2 ][NBu 4 ] (Au-1) and [Au III (4-methyl-1,2benzenedithiolato) 2 ][NBu 4 ] (Au-2).Au-1 and Au-2 differ only in the presence of carboranyl or tolyl groups as substituents on chelated 1,2-dithiolato ligands.Au-1/Au-2 was then encapsulated in the symmetrical triblock copolymer poloxamer (Plur-onic®) P123 containing blocks of poly(ethylene oxide) and poly(propylene oxide) to prepare the micelles AuMs-1 and AuMs-2.Then, we generated single gold atoms and gold nanocrystals on S-doped boronic graphitic surfaces or S-doped amorphous carbon surfaces by electron irradiation (using scanning transmission electron microscopy (STEM)) of AuMs-1 and AuMs-2.The boron doping appears to create anchoring sites for Au atoms.EELS data, along with DFT calculations, suggested the presence of unprecedented gold-boron interactions within the nanomaterial.Although Au-B bonds within pure Au-B alloys 19 or in gold boride 20 have been reported, direct Au-B interactions appear to be unknown in nanomaterials.
This strategy has potential for controlling the dynamics of metal atoms on fabricated matrices by suitable choice of metals and ligands, and might lead to next-generation nano-devices with a wide range of applications.There is also potential for extending such fabrication to novel nano-alloys.The DFT modelling reveals that the defects, particularly vacancies, may affect the binding of gold atoms to non-ideal graphenic materials containing boron.

COMMUNICATION 2 .
Results and discussion

Fig. 1
Fig. 1 (a) Gold crystal generated from AuMS-1 by electron beam irradiation.(b) Raw HAADF image with FOV of 6.8 nm showing the formation of graphitic matrices upon beam irradiation and topological defects (yellow circles) within the graphitic matrices.(c) FFT of the masked image (hanning window applied to remove edge effects), corresponding to a lattice spacing of 2.18 Å, that of the (100)/(010) planes of graphite.(d) Histogram of the raw image after applying a Gaussian blur (with 2 pixels kernel) showing that the annular darkfield signal is not saturating with the imaging conditions used.

Fig. 3
Fig. 3 (a) STEM-HAADF image showing Au-clusters with size varying from single atoms to ca. 2.7 nm diameter generated from AuMs-2.(b) HRTEM image showing specific surface elemental rearrangements of Au clusters leading to the formation of a well faceted nanocrystal.(c) and (d) EFTEM maps showing that the nanoclusters generated from AuMs-2 are primarily composed of Au.

22 . 1 )
For comparison, the values obtained for the systems with gold bound to C27 are given.The Au-C distances and C-B-Au angles are given when the Au-B coordination is not axial, i.e. the Au-B bond is not perpendicular to the graphene plane Atom bound to gold Energy of Au-adduct formation (kJ mol −Geometry of coordination (angles in deg.) and bond distances (Å) Molecule, number of electrons, spin state Geom.
32) package was used.For each B-substitution four different isomers were considered: (i) 533electron molecule with a doublet ground state corresponding formally to a negatively charged C 71 BH 22 fragment bound to Au(0) (ii) triplet and singlet states of 532-electron molecules corresponding formally to the neutral C 71 BH 22 fragment bound to Au(0) with the doped-graphene fragment and gold coupled ferromagnetically and antiferromagnetically, respectively.Note that the diamagnetic isomer may also represent the complex of negatively-charged C 71 BH 22 bound to Au(I).(iii) 531-electrons molecule with a doublet ground state, corresponding formally to the neutral AuC 71 BH 22 fragment bound to Au(0).

Fig. 5
Fig. 5 Three types of geometry for Au coordination to B-doped graphene.Top: axial.Middle: B-Au-C.Bottom: bent.

Fig. 6
Fig. 6 Different modes of Au 5 binding to B-doped graphene.Top: 848-electron system in the triplet state; bottom: 848-electron system in the singlet state.

Fig. 7
Fig. 7 Different modifications of B30C 71 BH 22 model system used for DFT modelling.The number of electrons and the multiplicity is given for the anion and neutral species.Top: left, B30-N70C 70 BNH 22 , right B30-B47C 70 B 2 H 22 .Bottom, the neutral (left) and anionic (right) forms of the defected C 70 BH 22 .Note that the reference molecule C 71 BH 22 has 453 electrons in the neutral doublet state and 454 electrons in the anionic singlet state.

1 )a
71 BH22 species with a defect in the graphene fragment using the TPSS/qzvp modelling including the dispersion correction.The values for the reference AuC 71 BH 22 are given System Energy Au-adduct formation (kJ mol −Geometry of coordination and bond distances (Å) Molecule, number of electrons, The geometry corresponding to the trigonaly pyramidal Au-BC Communication Nanoscale Advances respective Au(0)L analogues).For the Au(0)L − species again the formation energy of the reference molecule (−102 kJ mol −1 ) lies between those of the triplet and singlet states of the B-doped analogue (−94 and −150 kJ/respectively).The bond lengths do not reveal a signicant difference compared to those in the isomers of the reference B30-C 71 BH 22 species, particularly when the differences in the multiplicities of the species with the same formal charge of Au and graphene moiety are taken into account.

Fig. 9
Fig. 9 The optimised structures of the 527 electron (Au(0)L − , top left), 526 electron (Au(0)L, triplet and singlet, right, top and bottom, respectively) and 525 electron (left, bottom) species of vacancy system B30-C 70 BH 22 .The inset shows the molecules with 525-526 electrons (A) and 527 electrons (B) with the same orientation of the Au-B-C26 atoms.The optimised structure of the anionic ligand is also shown in the inset with the same alignment of B and C26 (C).

Table 1
Calculated energies of Au-B bond formation for the modelled AuC

Table 2
Calculated energies and bond distance of Au-B bond formation for the modelled AuC