Monodispersed Gold Nanoparticles Capped by Myristate and PPh3 Ligands Prepared by Controlled Thermolysis of Au{OC(O)C13H27}{PPh3}

Gold 2003
Masami Nakamoto,
Abstract Much attention has been paid to the preparation and structure determination of transition metal nanoparticles, because of their widespread use in technological applications. In general, the chemical reduction of metal salts and organometallic complexes by the use of reducing agent is carried out to prepare gold nanoparticles. In some cases, a large amount of solvent and stabilizers such as surfactants, polymers and ligands are used to prevent agglomeration of particles and control the particle size.
Recently, we have shown that thermolysis of gold(I) thiolate complex, [C14H29N(CH3)3][Au(SC12H25)2], produced gold nanoparticles (average diameter 26 nm) passivated by alkyl groups deriving from the counter cation of the precursor complex.1 However, the size distribution of the particles was relatively broad, ranging from 5 to 50 nm. In order to prepare the new type of monodispersed gold nanoparticles by thermolysis procedure, it may be important how to design the most suitable precursor gold complexes which decompose at low temperature and supply the organic protecting groups to prevent the agglomeration and afford the ligand stabilized nanoparticles. On the basis of this idea, we have prepared gold(I) complex, Au{OC(O)C13H27}{PPh3}, as the precursor and conducted its controlled thermolysis with no use of solvent, reducing agent and stabilizer. As a result, we have succeeded to obtain the new type of monodispersed gold nanoparticles capped by myristate and a small amount of PPh3 ligands. The thermolysis process of gold(I) complex was also investigated in detail to reveal the role of ligands.2
Thermolysis of gold(I) complex, Au{OC(O)C13H27}{PPh3} (1), prepared by the transmetallation of Au(PPh3)Cl with Ag{OC(O)C13H27}, was conducted by the heating of the powder of complex 1 with no use of solvent under an N2 atmosphere. The complex 1 completely melted to afford the precursor liquid and then the liquid gradually decomposed at 180 ‹C for 5 h. The reactant changed to deep purple coloured oil, and finally afforded a mixture of gold nanoparticles and organic liquid. After cooling to room temperature, the brown precipitate was separated by addition of methanol and washed with methanol for 2 times. The crude product was dispersed in acetone by irradiation of ultrasonic wave and again precipitated with methanol. The product was isolated as a brown solid by removing supernatant liquid or as golden leaves by slowly evaporation from acetone solution. The gold nanoparticles were redispersed in acetone and the colloidal solution did not show any signs of aggregation over a period of two weeks.
The transmission electron microscopy (TEM) was measured by placing a drop of the acetone solution of nanoparticles onto a carbon film supported on a copper mesh grid. The particles are spherical in shape and have narrow size distribution ranging from 18 to 27 nm, and almost all particles are the same particle size with a mean diameter of 23 nm. The powder X-ray diffraction pattern and X-ray photoelectron spectrum (XPS) indicate that the core gold in the nanoparticles must be present as Au0 state. The UV-visible spectrum of gold nanoparticles in acetone shows a sharp absorption peak at 554 nm attributed to the surface plasmon resonance. Furthermore, TG/DTA analysis of gold nanoparticles indicates the nanoparticles contain 88% gold, suggesting the presence of organic moiety around the core gold. The 1H-NMR spectrum of gold nanoparticles displayed resonances associated with methyl group, methylene groups and PPh3. However, the intensity of PPh3 signals are rather weaker than those of methyl and methylene groups. IR, XPS and GC/MS also revealed that myristate and a small amount of PPh3 ligands are coordinated to the surface of core gold.
After thermolysis of 1, a mixture of gold nanoparticles and organic liquid was obtained. The organic liquid consisted of myristic acid and gold(I) complex, [Au(PPh3)2]+, which was determined by IR and 31P-NMR. These results suggest that thermolysis of 1 causes elimination of myristate ligand to reduce gold(I) to metallic gold(0), accompanying the protection of gold nanoparticles by myristate ligand. At the same time, the eliminated PPh3 reacted with precursor complex 1 to produce [Au(PPh3)2][OC(O)C13H27] which was not decomposed under this reaction condition.
In conclusion, the new type of monodispersed gold nanoparticles bearing a surface coating of myristate and a small amount of PPh3 ligands have been prepared by the controlled thermolysis of Au{OC(O)C13H27}{PPh3}.

1) M. Nakamoto, M. Yamamoto and M. Fukusumi, J. Chem. Soc., Chem. Commun., 2002, 1622.
2) M. Yamamoto and M. Nakamoto, Chem. Lett., in the press.
Keywords: thermolysis, gold(I) complex, carboxylate, monodispersibility, nanoparticles, Gold, PPh3
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