DOI: 10.1021/la101124q 12215 Langmuir 2010, 26(14), 12215–12224 Published on Web 06/17/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Weak Polyion Multilayer-Assisted in Situ Synthesis as a Route toward a Plasmonic Ag/TiO 2 Photocatalyst Manca Logar,* ,† Bo stjan Jan car, † Sa  so  Sturm, ‡ and Danilo Suvorov † † Advanced Materials Department and ‡ Nanostructured Materials Department, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Received March 21, 2010. Revised Manuscript Received June 1, 2010 Nanocrystalline Ag/TiO 2 composite thin films were synthesized using a two-step synthesis methodology: the in situ precipitation of Ag nanoparticles followed by an in situ sol-gel reaction of titanium iso-propoxide in a weak polyion multilayer (PEM) template formed by the layer-by-layer (LbL) self-assembly of poly(acrylic acid) (PAA) and polyallylamine (PAH). Because the PEM template is assembled from weak polyions, it contains nonionized carboxylic groups that are able to react with the inorganics, resulting in the formation of a homogeneous Ag(x)/TiO 2 -PEM precursor film, where the content of Ag is controlled by repeating the Ag loading cycle. The subsequent annealing of the precursor yields nanostructured Ag(x)/TiO 2 films with thicknesses controlled by the PEM template on the nanometer scale. Transmission electron, field-emission scanning electron, and atomic force microscopy methods were employed to evaluate the morphology and growth characteristics of the metallic and semiconductor nanocrystallites in the Ag(x)/ TiO 2 composite thin films. The as-formed Ag(x)/TiO 2 composite thin films exhibited UV-visible photoactivity monitored by the decomposition of methylene blue (MB). In the near-UV range, the expected photocatalytic behavior of TiO 2 is greatly enhanced because it is assisted by the near-field amplitudes of the localized surface plasmon resonance (LSPR) of the Ag nanoparticles in the Ag(x)/TiO 2 films. 1. Introduction Semiconductor nanoparticles and nanostructured films have recently been the subject of intense research, because of the ability of photon-induced charge separation, which provides the basis for the photocatalytic degradation properties of self-cleaning surfaces, 1 the operation of dye-sensitized solar cells, 2 and chemi- cal sensors. 3 Its excellent chemical stability, nontoxicity, and potential ability to destroy organics totally are factors that lead to TiO 2 being a promising material for conventional photo- catalysis, 4 whereas the low quantum yield and the limited photo- responding range limit its utilization and commercialization. To solve these problems, numerous strategies have been proposed. The chemical modification of TiO 2 , by doping the lattice with a transition-metal ion, has proven to be effective in the extension of the absorption threshold toward the visible regime. Metal ions in the TiO 2 act as charge-carrier traps, which effectively enhance the charge separation of electrons and holes and hence increase the quantum yield of the surface photocatalytic processes. A consi- derable increase in the visible region has been observed in Fe- and Ni-doped TiO 2 . 5,6 Besides doping, the ability to obtain control over the morphology, surface-active sites, and size of the nano- crystals may also enhance the interfacial charge-carrier transfer rates, yielding better photoreactivity for TiO 2 . Additionally, suc- cessful attempts at suppressing the electron-hole recombination rate have been made by employing two different semiconductors or semiconductor-metal composite films. Previous studies have shown that by the deposition of noble metal particles, such as Au and Pt on TiO 2 , the noble metal particles work as electron traps, thus aiding electron-hole separation. 7,8 With the noble metal- semiconductor contact, under photoexcitation the two particles undergo a charge equilibration, which then enhances the catalytic efficiency of the composite system. 9 Recently, the idea of a plasmonic photocatalyst was introduced. 10-12 Ag-nanoparticle-TiO 2 composite films have attracted much attention because of their enhanced photoactivity, which is attributed to the Ag’s size- and shape-dependent loca- lized surface plasmon resonance (LSPR) absorption of metallic silver and to the consequent expansion of the photoresponding range. 13 Because TiO 2 is a semiconductor with a band gap of 3.26 eV, near-UV irradiation can excite both electrons and holes. Because of the intense LSPR of the Ag nanoparticles in the near- UV, an enhanced electric near-field in the vicinity of the Ag nanoparticle could boost the excitation of electrons from the Ag nanoparticles to the TiO 2 and thus improve the photoactivity. 12 With this process, a large interfacial area between Ag and TiO 2 , as well as control over the composite film thickness, is needed for the enhanced photoactivity of the composite films. The layer-by-layer (LbL) self-assembly of the inorganics with charged polyelectrolytes has proven to be a facile method of inorganic-organic composite film fabrication with the ability to control the final film thickness on the nanometer scale. 14 *Corresponding author. E-mail: manca.logar@ijs.si. Tel: þ386 1477 3762. Fax: þ386 1251 9385. (1) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikumi, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (2) Cahen, D.; Graetzel, M.; Guillemoles, J. F.; Hodes, G. Dye Sensitized Solar Cells: Principles of Operation. In Electrochemistry of Nanomaterials; Hodes, G., Ed.; Wiley-VCH Verlag GmbH: Weinheim, Germany, 2001; p 201. (3) Kamat, P. V.; Huehn, R.; Nicolaescu, R. J. Phys. Chem. B 2002, 106, 788. (4) Fujishima, A.; Rao, T. N.; Tryk, D. N. J. Photochem. Photobiol., C 2000, 1, 1. (5) Li, Z.; Shen, W.; He, W.; Zu, X. J. Hazard. Mater. 2008, 155, 590. (6) Jing, D.; Zhang, Y.; Guo, L. Chem. Phys. Lett. 2005, 415, 74. (7) Chandrasekharan, N.; Kamat, P. V. J. Phys. Chem. B 2000, 104, 10851. (8) Yamakata, A.; Ishibashi, T.; Onishi, H. J. Phys. Chem. B 2002, 106, 9122. (9) Subramanian, V.; Wolf, E.; Kamat, P. V. J. Phys. Chem. B 2001, 105, 11439. (10) Hirakawa, T.; Kamat, P. V. Langmuir 2004, 20, 56455647. (11) Hirakawa, T.; Kamat, P. V. J. Am. Chem. Soc. 2005, 127, 3928. (12) Awazu, K.; Fujimaki, M.; Rockstuhl, C.; Tominaga, J.; Murakami, H.; Ohki, Y.; Yoshida, N.; Watanabe, T. J. Am. Chem. Soc. 2008, 130, 1676. (13) Hirakawa, T.; Kamat, P. V. 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