vibrations at the liquid surface and cavitation at the gas±liquid interface [16].All the chemicals were obtained from Aldrich Chemicals. In a typical experiment, a methanolic solution of the acetate of zinc, cadmium, cobalt, or lead was prepared with a concentration of 40 g L ±1. The solution was nebulized and the spray carried into a preheated silicon carbide furnace maintained between 800 and 900 C using Ar as a carrier gas. Typical flow rates of Ar used were between 500 to 1000 standard cubic centimeter per minute (sccm). All experiments were performed using a quartz tube with an inner diameter of 25 mm. The reaction was typically carried out for 30 min. The shiny deposits obtained at the outlet (shown in Fig. 1) were used for further analysis. The yield of the nanowires was generally high (~90 %).Characterization: Powder X-ray diffraction (XRD) patterns were recorded using a Seifert XRD 3000 TT instrument. SEM images were obtained with a Leica S-440I microscope. TEM images were recorded with a JEOL JEM 3010 instrument operating at an accelerating voltage of 300 kV fitted with a Gatan CCD camera. Photoluminescence measurements were carried out at room temperature using a 325 nm excitation wavelength with a Perkin-Elmer model LS50B luminescence spectrometer. Thermogravimetric analysis of the samples was carried out on a Mettler-Toledo-TG-850 apparatus. Platinum nanoparticles are currently the subject of intense interest because of their unique catalytic properties; hence, synthesis methodologies of platinum-related catalysts have developed into an increasingly important research area at the frontier of advanced materials chemistry.[1±3] In catalysis, it is of extreme importance to maintain a high surface area for a long period of time at the catalyst's operating temperature.[4]The fabrication of nanoparticlesÐin particular, controlling their shape and ensuring a narrow size distribution of metal nanoparticlesÐhas thus become an important topic in nanotechnology.[5] A number of porous support materials, such as silica, alumina, zeolites, and MCM-41 (mobile crystalline material), have been used to deposit platinum nanoparticles by employing impregnation or ion-exchange methods.[6±8] However, such an approach has not provided a simple way to form robust and reusable catalysts for heterogeneous catalysis. The core±shell approach, whereby nanoparticles enclosed by protecting agents are grown in the channels of porous materials, is of considerable technological importance for improving the lifetime and the reusability of catalysts. [9,10] Herein, we describe the direct synthesis of spherical, mesoporous, nanosized platinum composites (Pt/PVP@MCM-41) via a core±shell approach.As shown in Scheme 1, Pt/PVP@MCM-41 composite particles were prepared by liquid-phase self-assembly. Initially, a colloidal dispersion of platinum nanoparticles was synthesized by employing poly(vinylpyrrolidone) (PVP) as the protecting agent under solvothermal conditions.[11] The Pt/PVP nanoparticles were found to be stable for longer times in ...

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