LETTERS Thermal Stability of Supported Platinum Clusters Studied by in Situ GISAXS Randall E. Winans,* ,† Stefan Vajda, † Byeongdu Lee, ‡ Stephen J. Riley, † So 1 nke Seifert, ‡ George Y. Tikhonov, † and Nancy A. Tomczyk † Chemistry DiVision and Experimental Facility DiVision, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439 ReceiVed: September 30, 2004; In Final Form: October 19, 2004 Sintering of supported nanocatalysts often leads to the loss of the catalytic activity and selectivity. This paper reports on synchrotron X-ray studies of the thermal stability of supported platinum nanoparticles produced by cluster deposition on the naturally oxidized surface of a silicon wafer (SiO 2 /Si(111)). The temperature region of aggregation was determined by gradually heating the samples up to above 400 °C, and recording two-dimensional in situ X-ray scattering images during the heat treatment. The data analysis reveals an unexpectedly high stability of the supported particles, which preserve their original size up to about 320 °C, at which an abrupt onset of the agglomeration takes place. Introduction Sintering of supported catalytically active nanoparticles during chemical reactions at elevated temperatures often leads to the loss of the catalytic activity and selectivity of these particles. 1-3 This paper is devoted to the study of the thermal stability of deposited platinum nanoparticles by employing grazing inci- dence small-angle X-ray scattering technique (GISAXS). GISAXS can provide the same type of information as regular SAXS, but in addition, it can give depth profile information. It is ideal for in situ studies because it is very sensitive to surface species and there is less parasitic scattering resulting from the substrate compared to a direct transmission scattering experi- ment. GISAXS was first demonstrated using a lab X-ray source for studying gold films. 4 More recently the technique has become popular for studying quantum dots and clusters on and imbedded into surfaces. For instance, CdS quantum dots prepared from ion implantation of S and Cd in SiO 2 followed by annealing were characterized by GISAXS. 5 Variation of the size of the quantum dots (on the order of 4-5 nm) was determined as a function of depth. In a study of gold aggregates and carbon-platinum clusters, Naudon et al. 6 make an important point that this method samples a large area, yielding good statistics which is difficult to do with microscopy. They noted that clusters as small as 40 atoms were observed for the C-Pt clusters, and the anisotropy of the aggregates can be clearly resolved as well. Recent work of Renaud et al. 7 demonstrated the high sensitivity of the GISAXS technique to monitor in real time the growth of Pd clusters on MgO(100) and of Co on Au(111) during metal vapor deposition and this work has been extended to a quantitative study of the growth of Pd particles on MgO. 8 Also, GISAXS was used to characterize Au clusters deposited on a Si wafer, which was covered by a C sublayer to make a comparison with TEM. 9 With the clusters sitting on the surface and not embedded, it is possible to process the horizontal and vertical slices of the data using the reflected main beam as the reference and use the classical SAXS approaches for analyzing the data. 9 This is done for data taken at the critical angle of the substrate, in this case, silicon, R c ) 0.15°. Experimental Methods Cluster Production and Deposition. The experimental setup and the design of the cluster source has been described in details * Corresponding author. E-mail: rewinans@anl.gov. † Chemistry Division. ‡ Experimental Facility Division. © Copyright 2004 by the American Chemical Society VOLUME 108, NUMBER 47, NOVEMBER 25, 2004 10.1021/jp045549p CCC: $27.50 © 2004 American Chemical Society Published on Web 10/30/2004