Journal of Colloid and Interface Science 287 (2005) 366–369 www.elsevier.com/locate/jcis Note Platinum states in citrate sols by EXAFS Chia-Shiang Lin, Maksudur R. Khan 1 , Shawn D. Lin ∗ Department of Chemical Engineering and Material Science, Yuan Ze University, Chung-Li, Taiwan 320, Republic of China Received 12 November 2004; accepted 24 January 2005 Available online 10 March 2005 Abstract Platinum sols have been prepared by citrate reduction in the temperature range of 343–363 K. The Pt state in the solution was examined by EXAFS (extended X-ray absorption fine-structure spectroscopy). It did not show any Pt–Pt bonding, a characteristic for reduced Pt sols. EXAFS model fitting further proved the presence of Pt–O with 4 oxygen neighbors, which suggests a tetraplanar coordination configuration. The possibility of neighboring Pt sharing oxygen ligand or the formation of PtO x is rejected by EXAFS model fitting. Citrate was found to be the most likely ligand to orient its oxygen end toward a charged Pt center. Thus we have revealed that the citrate treatment at this temperature range was clearly insufficient to reduce H 2 PtCl 6(aq) . Neither an extended period of reaction time nor an excess citrate reduced the Pt precursor. It is therefore highly recommended that the citrate sols should be carefully prepared and used.  2005 Elsevier Inc. All rights reserved. Keywords: Pt sols; Citrate; EXAFS 1. Introduction Platinum nanoparticles have been prepared by many dif- ferent methods. However, for applications such as cataly- sis the nanoparticles should have an average size smaller than 5 nm and be used without complicating procedures for removing unwanted ingredients such as high-molecular- weight protecting agents. Therefore, protecting agents hav- ing smaller molecular weights and free of heteroatoms may be more suitable. The reduction of hexachloroplatinic acid in solution by citrate, proposed by Turkevich et al., is one of the most practiced methods for generating Pt colloids [1]. The overall reaction for the reduction of PtCl 2− 6 by citrate takes place as follows [2]: PtCl 2− 6 + Cit − + H 2 O → Pt 0 + CO 2 + H + + Cl − . (1) Many authors have reported the preparation of Pt sols by boiling the precursor solution in the presence of mono-, * Corresponding author. Fax: +886-3-4559373. E-mail address: sdlin@saturn.yzu.edu.tw (S.D. Lin). 1 On leave from the Shah Jalal University of Science and Technology, Bangladesh. di- or trisodium citrate which resulted in the formation of black sols with an average particle size of 3–5 nm [1,3–5]. Still some authors conducted the reaction at lower temper- atures and a yellow brown or brown sol was found with Pt particle sizes of 1.5–3 nm (from TEM) [2,6–8]. However, electron microscope is an ex situ characterization technique for Pt (and other) sols. It may be possible that the image ob- tained in the electron microscope may not be the same as the species present in the solution [6,9]. Furlong et al. [10] and Henglein [11] gave a comprehensive characterization of the citrate sols by UV-visible spectroscopy. The reduction of hexachloroplatinic acid was monitored by the disappear- ance of the strong band at 260 nm for PtCl 2− 6 . At the end of the reaction the sol showed a featureless spectrum. Though featureless, such a UV-visible spectrum was used to interpret the extent of reduction and also the Pt particle size. This can- not provide unambiguous characterization of the prepared Pt sols. In this study, we use the nondestructive technique, EXAFS, to characterize Pt morphology within these sol so- lutions. To our surprise, it was found that citrate was not able to reduce hexachloroplatinic acid at temperatures from 343 to 363 K. 0021-9797/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.01.070