Published: May 06, 2011 r2011 American Chemical Society 8179 dx.doi.org/10.1021/ja107934h | J. Am. Chem. Soc. 2011, 133, 8179–8190 ARTICLE pubs.acs.org/JACS Spectroscopic and Microscopic Investigation of Gold Nanoparticle Formation: Ligand and Temperature Effects on Rate and Particle Size Rajesh Sardar † and Jennifer S. Shumaker-Parry* Department of Chemistry, University of Utah, 315 South 1400 East, RM 2020, Salt Lake City, Utah 84112, United States b S Supporting Information ’ INTRODUCTION Applications in electronic and optical detection systems, 1,2 device development, 3À5 therapeutics, 6 and catalysis 7,8 have made gold nanoparticles (AuNPs) the focus of much nanoscience research. The optical, electronic, and catalytic properties of metal nanoparticles are correlated with the physical characteristics of the particles, such as size 9À13 and shape 14À23 as well as the local dielectric environment. 24À27 In addition to the optical and electronic properties, the chemical properties of AuNPs are strongly related to the core size of the particles, and as the size of the particles decreases, the fraction of the atoms present on the vertex and edge sites increases in comparison to the terrace sites. 28 For example, the atoms in different sites on the nano- particle surface substantially influence surface behavior including ligand place exchange reactions 29À31 as well as the electronic properties, such as the double-layer capacitance 32À41 and the anion-induced adsorption. 42À44 Because of the strong interrela- tionship, precise control of metal nanoparticle structural proper- ties, such as size, surface chemistry, and even crystalline character, is a key goal for fundamental studies to better under- stand and control the optical, electronic, chemical, and electro- chemical properties of AuNPs. Despite all of the synthetic work to produce metal nanoparticles, the extent of control of structural properties when particles are prepared in solution-based synth- esis continues to be a challenge. 45À51 The Brust two-phase synthesis and its various modifications are the most common approaches used to generate AuNPs with average diameters of 1À4 nm using NaBH 4 as a reducing agent. 52À61 In these synthetic methods strong stabilizing agents, such as alkyl or arylthiols, have been most commonly used to control the size of the nanoparticles. In these cases, the reduction usually reaches completion within a few hundred milliseconds after addition of the strong reducing agent NaBH 4 that typically is used. Other than NaBH 4 , few other borohydride-based reducing agents have been used to synthesize stable, monodisperse AuNPs with diameters of <5.0 nm. 62,63 A key aspect of producing nanopar- ticles with a high degree of control of structural characteristics, such as particle size, size dispersion, shape, and crystallinity, is characterizing the nanoparticle formation process, including the role of changes in reaction parameters. Recently, mass spectro- metry was used to investigate the growth of thiolate-protected AuNPs at various stages of particle formation. 64 Using mass spectrometry requires vigorous cleaning of the sample for every step of the analyses to remove unwanted or side products in order to achieve adequate resolution for data interpretation making this approach challenging. Another approach is to use real-time, in situ transmission electron microscopy (TEM) analysis with nanometer scale resolution, although this is quite challenging due to the fast rate of most nanoparticle formation processes. As an example, Alivisatos and co-workers used TEM to monitor the nucleation and the growth of platinum nanopar- ticles (PtNPs) in situ using a liquid cell. 65 In this case, the electron beam actually initiated the reduction reaction and was then used for imaging of the nanoparticle formation. The PtNP formation was quite rapid, making it difficult to obtain detailed information about the early nucleation and growth processes. However, the in situ TEM monitoring made it possible to at least observe the later growth stages and identify different growth mechanisms. Recognizing the challenges associated with these approaches, a much more ideal situation would be that more simple spectroscopic and microscopic methods could be used to Received: September 10, 2010 ABSTRACT: We report a spectroscopic and microscopic investigation of the synthesis of gold nanoparticles (AuNPs) with average sizes of less than 5 nm. The slow reduction and AuNP formation processes that occur by using 9-borabicyclo- [3.3.1]nonane (9-BBN) as a reducing agent enabled a time- dependent investigation based on standard UVÀvis spectros- copy and transmission electron microscopy (TEM) analyses. This is in contrast to other borohydride-based syntheses of thiolate monolayer protected AuNPs which form particles very rapidly. We investigated the formation of 1-octadecanethiol (ODT) protected AuNPs with average diameters of 1.5À4.3 nm. By studying the progression of nanoparticle formation over time, we find that the nucleation rate and the growth time, which are interlinked with the amount of ODT and the temperature, influence the size and the size dispersion of the AuNPs. High-resolution TEM (HRTEM) analyses also suggest that the nanoparticles are highly single crystalline throughout the synthesis and appear to be formed by a diffusion-controlled Ostwald-ripening growth mechanism.