“Nanoparticle Size Determination by TEM: Drawbacks and the Necessity of the Combination with Average Techniques”

GASTÓN CORTHEY; EUGENIA ZELAYA; ALDO ALBERTO RUBERT; LISANDRO J. GIOVANETTI; JOSE MARTIN RAMALLO LOPEZ; FÉLIX GREGORIO REQUEJO; MARIANO H. FONTICELLI; ROBERTO C. SALVAREZZA

Rio de Janeiro

Congreso; International Microscopy Congress; 2010

The particle size distribution determination 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, Small-angle X-ray Scattering (SAXS) has become a commonly used technique for this purpose. One of the advantages of SAXS method over TEM is that it is an average technique and every particle contributes to the particle size distribution. Also, the particles are measured without drying or mounting the sample. However it is necessary to propose a model for the particle morphology to determine size using SAXS. While it is widely known that an electron beam can be used to produce metallic arrangement starting form metallic oxides or other compounds [1], it has only recently been used to produce metallic NPs [2,3]. In this work, we compare the results obtained using two different types of NPs which were characterized by TEM and SAXS. 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. 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)). 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. In conclusion, it was 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 standards methods. In this exhaustive characterization of synthesis products, TEM and SAXS should be viewed as complementary techniques rather that alternative regarding size determination.