New Type of Organically Capped Gold Nanoparticles Prepared by Controlled Thermolysis of Gold Complexes

Gold 2003
Mari Yamamoto,
Abstract Plenty of preparative investigations on gold nanoparticles and its surface modification have appeared, since the preparation of thiol-derivatized gold nanoparticles by the two-phase reduction of AuCl4- was reported. Those preparative methods generally involve the reduction of AuCl4- by reducing agent, NaBH4, in the presence of suitable organic stabilizers such as alkanethiol, phosphane, quarternary ammonium salts, surfactants or polymers. On the other hand, the one phase preparation of organically capped gold nanoparticles by the reduction of AuCl4- at 190 °C was recently reported, where tri-n-octylphosphine oxide and/or n-octadecylamine are used as both a reaction medium and passivating ligand, but this procedure also needs NaBH4 as reducing agent.
Our novel preparative method for gold nanoparticles is thermochemically induced reduction of gold complexes, which combines the reductive elimination of organic ligands with simultaneous attachment of organic moiety on the growing nuclei. Thermolysis is conducted by heating the powder of precursor gold complexes, as a result the decomposition can proceed in the absence of organic solvent. Organic components deriving from the ligands can function as a stabilizer and afford a new type of organically capped gold nanoparticles.
We now report the controlled thermolysis of gold(I) thiolate complexes, [R(CH3)3N][Au(SC12H25)2] and [R(CH3)3N][Au(SC6H4-p-R’)2] (R= C14H29, C12H25; R’= C8H17, CH3), under an N2 atmosphere. In spite of no use of reducing agent and solvent, reduction reaction is thermochemically induced to afford novel gold nanoparticles passivated by alkyl groups rather than by alkanethiolate ligands.1
The powder of [C14H29(CH3)3N][Au(SC12H25)2] was heated under an N2 atmosphere up to 130 °C to cause completely melting and afforded a liquid of precursor complex. Further heating up slowly to 180 °C and holding at that temperature for 5 h made precursor liquid gradually decompose to afford a mixture of gold nanoparticles and organic liquid. After cooling to room temperature, the brown gold nanoparticles were isolated, 93% yield based on Au.
A transmission electron microscope (TEM) image of gold nanoparticles prepared by thermolysis of [C14H29(CH3)3N][Au(SC12H25)2] at 180 °C for 5 h indicates a controlled growth of spherical gold particles. The particle size distributes in the range of 5 to 50 nm in diameter. Although the particle size is larger than that of thiol-derivatized gold nanoparticles (less than 5 nm) prepared by NaBH4 reduction, the aggregation of gold nuclei is smoothly regulated and the growth of core gold is limited to an average diameter of 26 nm in spite of thermal procedure.
The oxidation state of gold(0) in the nanoparticles was confirmed by X-ray photoelectron spectroscopy (XPS), showing the characteristic Au(0) binding energies of Au 4f7/2 (83.4 eV) and Au 4f5/2 (87.0 eV). Powder X-ray diffraction pattern of the gold nanoparticles showed slightly broad reflections of metallic gold in a face-centred cubic lattice. The average particle size of the metal nuclei (19 nm) was calculated by Scherrer equation using the half width of the intense (111) reflection.
The present gold nanoparticles redissolved in acetone showed the characteristic plasmon absorption centred at 538 nm in the uv-visible absorption spectra. The wavelength of this absorption maximum slightly shifts toward longer wavelength compared with those of the previous reported thiol-derivatised gold nanoparticles with smaller particle sizes (520 nm). Its peak maximum is compatible with the average particle size determined by TEM image.
Thermogravimetric analysis of the present gold nanoparticles indicates the gold content of 92.5 % and the existence of 7.5 % organic moiety. The gold content depended on the preparative conditions, but was almost regulated between 90 to 97 % gold.
There are two possibilities for the origin of organic moiety: the quaternary ammonium part or dodecanethiolate ligand. The 1H-NMR spectrum of the present gold nanoparticles showed CH3- and CH2-signals of long alkyl groups, but no signal of NCH3 and NCH2-groups. Examination of nitrogen was also conducted by elemental analysis, but nitrogen was not left in the particles. Furthermore, XPS evidenced the absence of the sulfur atom in the gold nanoparticles. Thus, these results exclude the alkylamine-capped and thiol-derivatized nanoparticles, and suggest the present gold nanoparticles are a new class of gold nanoparticles surrounded by alkyl groups.
Thermolysis of gold(I) thiolate complexes finally afforded a mixture of gold nanoparticles and organic liquid. FAB mass spectroscopy and elemental analysis of this organic liquid revealed the formation of disulfide, (C12H25S)2. Furthermore, trimethylamine was detected in the gaseous phase by GC/MS analysis after thermolysis at 180 °C for 3 h. These results support that thermolysis of gold(I) thiolate complexes causes reductive elimination of the thiolate ligand to reduce gold(I) to metallic gold(0) and to afford disulfide, accompanying the protection of gold nanoparticles with alkyl groups derived from the quarternary ammonium cation of the precursor complex.
It is noteworthy that disulfide, (C12H25S)2, plays a role of stabilizer, which was confirmed by the thermolysis of gold(I) benzenethiolate complexes, [C14H29(CH3)3N] [Au(SC6H4-p-R’)2] (R’= C8H17, CH3). Particle size distributions of gold nanoparticles prepared by the thermolysis of those precursors at 180 °C for 8 h, respectively are strongly reflected by the difference of the alkyl chains on the benzene rings. Disulfide with longer p-octyl group prevents the aggregation of gold particles and effectively regulates the growth of core nuclei compared with the methyl derivative. Thus, disulfide produced through thermolysis plays an important role of size control.
On the other hand, the ammonium cations with almost same chain lengths, [R(CH3)3N]+ (R= C14H29, C12H25), do not show a pronounced affect on the tuning of the particles size. However, the ammonium cations of [R(CH3)3N]+ (R= C14H29, C12H25) containing only one long alkyl group effectively afford gold nanoparticles compared with the ammonium cation containing two longer alkyl groups, [(C18H37)2(CH3)2N]+. The reason may be due to the decomposition pathway, because the former can easily eliminate gaseous trimethylamine and supply long alkyl group as a protecting group surrounding core gold.
In conclusion, thermolysis of gold(I) thioalte complexes, [R(CH3)3N][Au(SC12H25)2] (R= C14H29, C12H25) can regulate the growth of gold nuclei and afford novel gold nanoparticles passivated by alkyl groups. Work to develop a thermolysis approach to prepare another type of organically capped gold nanopartilces by the use of a variety of precursor gold complexes2 is also described.

1) M. Nakamoto, M. Yamamoto and M. Fukusumi, J. Chem. Soc., Chem. Commun., 1622, 2002.
2) M. Yamamoto and M. Nakamoto, Chem. Lett., in the press.
Keywords: Gold, Alkyl group, thermolysis, gold(I) complex, Ammonium salt, thiolate, nanoparticles, Gold(I)-thiolate complex
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