Role of Metal Nanoparticles in TiO 2 /Ag Nanocomposite-Based Microheterogeneous Photocatalysis P. Davide Cozzoli, ² Elisabetta Fanizza, ² Roberto Comparelli, ² M. Lucia Curri, ‡ and Angela Agostiano* ,²,‡ Dipartimento di Chimica, UniVersita ` di Bari, Via Orabona 4, 70126 Bari, Italy, and CNR-Istituto per i Processi Chimico Fisici (IPCF), sez. Bari, Via Orabona 4, 70126 Bari, Italy Danie ` le Laub § EPFL-CIME-Centre Interdisciplinaire de Microscopie Electronique, Batiment MXC, 1015 Lausanne, Switzerland ReceiVed: December 22, 2003; In Final Form: March 9, 2004 The photocatalytic performance of anatase TiO 2 nanorod-stabilized Ag nanoparticles has been investigated during the reductive bleaching of a model dye, Uniblue A (UBA), in homogeneous organic solutions. The activity of the TiO 2 /Ag nanocomposite has been found to vary continuously during the course of photocatalysis, following a concomitant light-induced modification of the metal nanoparticle size and size distribution. The direct involvement of the metal particles in mediating electron transfer between photoexcited TiO 2 and the target UBA is explained on the basis of the size-dependent redox properties of the metal nanoparticles. The present results can be useful in the design of new composite materials with well-tailored photocatalytic properties and long-term stability. 1. Introduction Major efforts in modern material chemistry are devoted to the design and fabrication of nanostructured systems with tunable physical-chemical properties for advanced catalytic applications. Recent discoveries on the improved technological performances obtainable by coupling different nanostructured materials have encouraged research toward exploring the potential offered by noble metal/semiconductor oxide nano- composites in catalysis. 1-7 In the nanosized regime, as the oxide function can strongly deviate from its conventional role as simple support, unusual or enhanced catalytic activity 2 can be indeed detected due to specific metal-oxide electronic interactions. 3-5 Furthermore, the interest in combining noble metal nanoparticles with semiconductor oxides basically relies on the metal ability in acting both as a sink for electrons and as redox catalyst. Noble metal nanoparticles have been recognized to promote electron- transfer processes, 8-10 as demonstrated in seed-mediated metal growth mechanisms 10b,11 and metal-catalyzed reduction of organic compounds. 9a,b,10a Photoinduced charge carrier separa- tion 5-7 can be actually favored in semiconductor/metal nano- composites, as proved by the enhancement of photocatalytic 5,6 and photoelectrochemical 7 performances in such combined materials. In the field of photocatalysis, most recent studies have focused on the characterization of the nanocomposites before irradiation or just following short laser pulse excitation. 6a-c Little or no information is available on whether the initial properties of semiconductor/metal composites are retained upon long-term light exposure. 5b,c,6b,c,7c For example, because of their high photoactivity, noble metal nanoclusters can be expected to undergo size-shape transformation when subjected to intense light. 12,13 Moreover, metal cluster etching, based on oxygen- driven chemical 11a,b,15 and photochemical dissolution, 6b,15 is known to be facile, especially in the presence of strong ligands for the metal ions 11b and/or of electron scavengers. 6b,12d,e Recognition of such processes is of paramount importance, as they may originate a nanostructured material whose catalytic activity can vary during the course of photocatalysis. Accord- ingly, the size-regime dependence of catalytic reactions has been largely emphasized in the literature. 3a,b,9a,b,10,14 Further investiga- tions are undoubtedly needed in order to rationalize the performances of such composite materials, as it has been also found that during photocatalysis, a semiconductor/metal nano- junction may not necessarily facilitate electron discharge into the solution. 6b,d,e,7a In the present work, the morphological changes occurring to a TiO 2 /Ag nanocomposite material have been followed during steady-state photocatalysis in order to establish a relationship between the overall photocatalytic activity of the nanocomposite and the metal particle size upon long-term UV illumination. Recently synthesized TiO 2 nanorod-stabilized Ag nanoparticles 15 have been employed as a convenient semiconductor/metal photocatalyst to easily follow the elementary steps involved in organics reductive photodegradation. Such nanocomposite ap- pears suitable to this purpose for several reasons. First, the TiO 2 nanorod-Ag nanoparticle composite is soluble in organic media, providing a macroscopically homogeneous (i.e., without phase boundaries) photocatalyst dispersion, which however remains heterogeneous at the nanoscale. More importantly, the resulting solution is optically transparent, thus allowing the course of * To whom correspondence should be addressed. Fax: +39-080- 5442129/5442209. E-mail: csildc28@area.ba.cnr.it; agostiano@area.ba.cnr.it. ² Universita ` di Bari. ‡ CNR-Istituto per i Processi Chimico Fisici (IPCF). § EPFL-CIME-Centre Interdisciplinaire de Microscopie Electronique. 9623 J. Phys. Chem. B 2004, 108, 9623-9630 10.1021/jp0379751 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/05/2004