Journal of Alloys and Compounds 417 (2006) 60–62 Selective oxidation of Pd and compositional reconstruction in Pd 70 Ag 30 alloy nanoparticles Kuan-Wen Wang, Shu-Ru Chung, Tsong P. Perng ∗ Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan Received 21 July 2005; accepted 9 August 2005 Available online 25 October 2005 Abstract The Pd 70 Ag 30 alloy nanoparticles were heated in air at 250–450 ◦ C. During heating, segregation of Ag to the surface driven by its lower surface energy competes with the oxidation of Pd. At 250 ◦ C, selective oxidation of Pd occurred preferentially, and the formation of large free Ag crystallites was observed. As temperature increased, oxidation of Pd became more intensive and dominating, which attracted more Pd to the surface for reaction, resulting in fewer and smaller free Ag particles on the surface. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline materials; Palladium; Silver; Oxidation Surface segregation is a common phenomenon in alloy sys- tems [1–5], where one of the components preferentially diffuses to the surface, resulting in a surface composition different from the nominal composition. Surface segregation may be driven by a number of factors, such as surface energy, affinity of gases, heat of sublimation, and strain energy [6–9]. These factors may determine the surface-rich element in the alloy system. When an alloy is annealed, the composition distribution within the alloy may be affected by the annealing atmosphere [10]. For example, it has been observed that thermal annealing in air promotes de-alloying of Cu–Au alloy. Cu is extracted by oxygen from the alloy to form Cu 2 O and Au precipitates to form metallic clusters [11,12]. The driving force of de-alloying is the stronger interaction between Cu and oxygen. On the other hand, in Ag–Au alloy system where the interaction between Ag or Au and oxygen is relatively low, there is no remarkable composi- tional reconstruction of the alloy. For Pd–Ag binary alloy nanoparticles prepared by chemical wet reduction [13], the inherent composition within the parti- cles is not uniform because of different reduction potentials of Pd and Ag during precipitation [14]. This inhomogeneous alloy may undergo further rearrangement of the elements when some kinds of driving force are provided. For example, when there is ∗ Corresponding author. Tel.: +886 3 5742634; fax: +886 3 5723857. E-mail address: tpperng@mx.nthu.edu.tw (T.P. Perng). He, H 2 or O 2 in the environment, the affinity of these gases to Pd will attract Pd to the surface [6,15]. When it is subjected to heat- ing, since the surface energy of Ag is lower (γ Ag = 930 erg/cm 2 versus γ Pd = 1500 erg/cm 2 ), Ag will migrate from the interior to the surface [14]. In this study, the synergistic effects of sur- face segregation of Ag and oxidation of Pd in the Pd 70 Ag 30 nanoparticles are examined. These two processes are induced by independent driving forces that determine the variation of surface composition in the alloy nanoparticles. The alloy nanoparticles Pd 70 Ag 30 were prepared by a chem- ical precipitation method [13]. Pd(NO 3 ) 2 and AgNO 3 were mixed at the atomic ratio of 70:30, and then reduced by formalde- hyde in a basic environment. The average size of the particles was 8 nm [12,13]. The alloy particles were pressed as pellets and heated in air at 250–450 ◦ C for 1 h. The heating rate was 10 ◦ C/min. The phase structure of the nanoparticles was exam- ined by X-ray diffraction (XRD, Rigaku) with a Cu K source. The surface morphology of the pellets was examined by field emission scanning electron microscopy (FESEM, JEOL JSM- 6500F). Electron spectroscopy for chemical analysis (ESCA, Physical Electronics PHI 1600) equipped with a spherical capac- itor analyzer was used to study the surface composition of the alloy nanoparticles. Fig. 1 shows the XRD patterns of the alloy nanoparticles after heating in air for 1 h at various temperatures. The composition shown for each pattern is the ratio of Pd to Ag of the alloy phase after heating. The values are calculated by Vigard’s law based 0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.08.065