Nanoparticle Size Determination by Different Methods

G. CORTHEY; EUGENIA ZELAYA; L. MOGNI; ALDO A. RUBERT; G. BENITEZ; L. GIOVANETTI; J. M. RAMALLO-LÓPEZ; F. REQUEJO; M. FONTICELLI; R.C. SALVAREZZA

Montevideo

Workshop; International School on Fundamental Crystallography; 2010

The determination of the particle size distribution is one of the first characterization steps after nanoparticles (NPs) synthesis. Transmission Electron Microscopy (TEM) has been one of the most widely used method to determine particle size. Additionally it is possible to measure the size of the NPs by Small-Angle X-ray Scattering (SAXS) or by considering the peak broadening in X-ray Diffraction (XRD) using the Scherrer equation. One of the advantages of XRD and SAXS methods over TEM is that they are average techniques and every particle contributes to the particle size distribution. However, if the particles are not single crystals, an underestimation of the size will be done if XRD is used. In SAXS the particles are measured without drying or mounting the sample. In this work, we compare the results obtained using two different types of NPs. Dodecanethiolate monolayer-protected gold NPs (AuSC12) and NPs composed of a gold metallic core surrounded by a multilayer gold(I)-thiomalate shell (Au@Au(I)-TM) were used [1]. For Au@Au(I)-TM NPs a mean diameter of (3.7 ± 1.3) nm was obtained by TEM. It is well known that this diameter corresponds to the gold metallic core. However, X-ray Photoelectron Spectroscopy (XPS) and Extended X-ray Absorption Fine Structure (EXAFS) results showed that the NPs were composed by both metallic gold (Au(0)) and oxidized gold (Au(I)) [1]. For the Au@Au(I)-TM NPs, the better curves fitting of SAXS data was obtained proposing a constant density attributed to the Au(0) core and a Gaussian electronic density distribution for a the Au(I)-TM shell. The obtained diameter for the Au(0) core was (1.02 ± 0.07) nm, much smaller than the one obtained by TEM. On the other hand, the diameter obtained by TEM for the AuSC12 NPs was (3.8 ± 0.5) nm, which is in good agreement with that observed by SAXS: (3.6 ± 0.4) nm. While in the AuSC12 particles the Au is in the metallic chemical state, two types of Au were found in the Au@Au(I)-TM NPs. When Au@Au(I)-TM NPs are observed by TEM, the oxidized gold, which is part of the Au(I)-thiolate multilayer, can be reduced by the electron bean, leading to the formation of metallic gold, increasing the apparent particle size [2,3,4]. In summary, it is shown that the size of NPs cannot be determined if the chemical nature of the material has not previously been established. In this regard, the protocols for NPs synthesis need to be validated through a comprehensive analysis before their use as standard methods. In this exhaustive characterization of synthesis products, TEM, SAXS and XRD should be viewed as complementary techniques rather that alternative regarding size determination. [1] G. Corthey, L. J. Giovanetti, J. M. Ramallo-López, E. Zelaya, A. A. Rubert, G. A. Benitez, F. G. Requejo, M.H. Fonticelli, R. C. Salvarezza. ACS Nano 4 (2010) 3413. [2] Fraser, D. B.; Kammlott, G. W. U.S. Patent US 4072768 (1978). [3] Kim, J.; Cha, S.; Shin, K.; Jho, J. Y.; Lee, J. J. Am. Chem. Soc., 127 (2005) 9962. [4] Corbierre, M. K.; Beerens, J.; Lennox, R. B. Chem. Mat., 17 (2005) 5774.