Published: September 29, 2011 r2011 American Chemical Society 13854 dx.doi.org/10.1021/la203113r | Langmuir 2011, 27, 13854–13860 ARTICLE pubs.acs.org/Langmuir Size Control in the Synthesis of 1À6 nm Gold Nanoparticles via Solvent-Controlled Nucleation Jieun Song, Dukhan Kim, and Dongil Lee* Department of Chemistry, Yonsei University, Seoul 120-749, Korea b S Supporting Information ’ INTRODUCTION Preparation of monodisperse metal nanoparticles with controlled size is of crucial importance for both fundamental science and technological applications in optics, electronics, and catalysis. 1À5 Gold nanoparticles (AuNPs) of size in the range 1À5 nm are particularly interesting because they represent a bridge between the bulk and molecular behavior. The size-dependent optical, electrochemical, and catalytic properties have been reported. 5À13 Among synthetic schemes for the preparation of AuNPs, the Brust method, 14 a two- phase synthesis, has been most extensively studied. This method, however, typically produced polydisperse particles, and thus postsynthetic treatments, such as solvent fractiona- tion and thermal annealing, were usually employed to im- prove size purity. 15À17 It has been reported that many types of semiconductor nanoparticles can be routinely prepared in nearly monodis- perse form using the organometallic and its alternative appro- aches. 18À21 Mechanism studies have revealed that the key factor for the preparation of monodisperse particles is the separation of nucleation and growth processes. 19,22À24 Si- multaneous nucleation is followed by particle growth without additional nucleation, allowing the growth histories of parti- cles to be identical and thus enabling the control of the size distribution of the ensemble of particles as a whole during growth. This synthetic strategy, however, cannot be readily applicable to the two-phase Brust synthesis in which both nucleation and growth can occur only at the interface of the two liquid phases. A number of one-phase methods have been developed for the synthesis of monodisperse metal nanoparticles. In the synthesis of gold and other noble metal nanoparticles, Jana and Peng demonstrated that nearly monodisperse particles could be obtained by adopting one-phase and one-pot approaches. 25 Strategies applied to achieve the size and size distribution control included using weak ligands such as amines and fatty acids in place of thiols and decreasing the reducing power of the reducing reagents. Stucky and co-worker further im- proved the one-phase method by employing amineÀborane complexes as reducing agents. 26 They also found that strong thiol ligands facilitate the formation of monodisperse nano- particles better than weak ligands such as phosphines and amines. However, the role of thiols and the formation mechanism remain unaddressed. This work builds upon Stucky’s procedure 26 in which AuNPs are synthesized by reducing a gold salt precursor AuPPh 3 Cl with an amineÀborane complex in the presence of thiol ligand. Mechanism studies have revealed that the presence of both phosphine and thiol ligands is required for the formation of monodisperse, large nanoparticles. It was further found that nucleation occurs simultaneously and that the concentration of nuclei can be controlled by the fraction of CHCl 3 , which determines the final size of AuNPs. This method provides a novel, facile strategy for the size-controlled preparation of Received: August 10, 2011 Revised: September 28, 2011 ABSTRACT: We report a facile synthetic route for size- controlled preparation of gold nanoparticles. Nearly monodis- perse gold nanoparticles with core diameters of 1À6 nm were obtained by reducing AuP(Phenyl) 3 Cl with tert-butylamine borane in the presence of dodecanethiol in the solvent mixture of benzene and CHCl 3 . Mechanism studies have shown that the size control is achieved by the solvent-controlled nucleation in which the nuclei concentration increases with increasing the fraction of CHCl 3 , leading to smaller particles. It was also found that, following the solvent-controlled nucleation, particle growth occurs via ligand replacement of PPh 3 on the nuclei by Au(I)thiolate generated by the digestive etching of small particles. This synthetic strategy was successfully demonstrated with other alkanethiols of different chain length with which size-controlled, monodisperse gold nanoparticles were prepared in remarkable yield without requiring any postsynthesis treatments.