Published: November 16, 2011 r2011 American Chemical Society 104 dx.doi.org/10.1021/jp2071716 | J. Phys. Chem. C 2012, 116, 104–114 ARTICLE pubs.acs.org/JPCC Chemical Kinetics of Photoinduced Chemical Vapor Deposition: Silica Coating of Gas-Phase Nanoparticles Adam M. Boies,* ,† Steven Calder, ‡ Pulkit Agarwal, § Pingyan Lei, § and Steven L. Girshick § † Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, U.K. ‡ Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States § Department of Mechanical Engineering, University of Minnesota, 111 Church Street, Minneapolis, Minnesota 55455, United States b S Supporting Information ABSTRACT: Experimental studies of gas-phase nanoparticle coating by photoinduced chemical vapor deposition (photo- CVD) have shown that silica coatings can be produced with controllable thicknesses on di fferent nanoparticle cores for a variety of applications. This study presents a chemical reac- tion sequence for the photo-CVD process to describe the production of silica coatings from the decomposition of tetraethyl orthosilicate (TEOS). The model incorporates photochemical reactions into known thermal reaction se- quences involving gas-phase and surface reactions to describe the nanoparticle coating process. Modeled results of the photo-CVD process indicate that the dominant reactions for the production of silica coatings on the surface of the nanoparticles are the photodecomposition of TEOS and the removal of surface ethyl groups from adsorbed TEOS species. Relative concentrations of gas-phase and surface species are compared for different model configurations and system parameters. Modeled coating thicknesses agree well with experimental findings and demonstrate that coating thickness increases with increasing TEOS concentration and increased residence time within the reaction chamber. 1. INTRODUCTION The synthesis of coreshell nanoparticles has received much interest in recent years as such structures can enhance nanopar- ticle properties, such as thermal stability, 1 plasmon resonance, 2 catalytic activity 3 and surface functionality. 4 There are currently a wide variety of gas-, 5 liquid- 6 and solid-phase 7 synthesis techni- ques available to produce composite particle structures. While all approaches have inherent advantages, the production of compo- site nanoparticles by gas-phase methods allows for particles to be produced at high throughputs in inert or nonreacting environ- ments with little or no surface impurities. Photoinduced chemical vapor deposition (photo-CVD) is a gas-phase approach that allows for the production of a variety of coreshell compositions, including organic 8 and inorganic coatings 9 of nanoparticles. While the approach has been shown to work experimentally, a fundamental study of the chemical reaction sequence involved in the coating process has not been presented. Silica coatings are particularly important at both the macro- and nanoscale because of silica’s thermal stability, 10 high elec- trical resistivity, 11 and surface that is easily functionalized through ligand attachment. 12 Several research groups have examined the chemical reaction sequences involved in the thermal decomposi- tion of tetraethyl orthosilicate (TEOS) onto macroscopic surfaces for purposes of producing films relevant to the semiconductor industry. 1316 Coltrin et al. examined the thermal decomposition of TEOS to produce silica films on a series of wafers placed within a reacting flow stream. While the approach highlighted the pro- duction of silica for a specific geometry, it provided a general reaction sequence that relied on experimental 15 and theoretical 17 studies of the TEOSsilica system. The Coltrin et al. reaction sequence included four gas-phase reactions and eight surface reactions, none of which considered oxygen as a separate re- actant. The primary gas-phase reaction involved in the produc- tion process was reported as the β-hydride elimination of ethylene, which is the least endothermic gas-phase reaction (∼10 kcal/mol) for TEOS. 18 The primary surface reactions responsible for film growth were found to be the adsorption of the TEOS radical, triethoxysilanol [Si(OH)(OC 2 H 5 ) 3 ], onto the surface and then subsequent removal of the remaining ethyl groups. Modeled growth rates from Coltrin et al. provide adequate agreement with molecular beam experiments and CVD experiments performed by Kalidindi and Desu. 19 Their study concluded that at lower temperatures the gas-phase TEOS reaction is the rate-limiting reaction while at higher temperatures the ethyl removal at the surface is rate limiting. 14 Work by Romet et al. focused on the production of silica from TEOS by examining ozone related reactions. 13 Their sequence included the gas-phase reactions of TEOS with monatomic Received: July 27, 2011 Revised: November 7, 2011