Photocatalytic growth and plasmonic properties of Ag nanoparticles on TiO 2 films
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ing nanoparticulate TiO2 films, the photocatalytic growth of Ag nanoparticles (NPs) in the AgNO3 aqueous solution has been studied in terms of reduction, nucleation, and coalescence. It was proved that Ag primary particles were formed in a growth time of ,1 s after the photocatalysis started. The growth dynamics was found to be critical for isotropic and anisotropic growth of Ag NPs, depending on the AgNO3 concentration and surface properties of TiO2 films. In the AgNO3 solutions of #300 mg/L, the isotropic growth dominates the growth dynamic behavior, producing irregularly spherical Ag NPs. In the AgNO3 solutions of $400 mg/L, the increased reduction rate promotes the formation of Ag nanoplates in the product. Ostwald ripening and oriented attachment were suggested to be the mechanisms dominating the isotropic and anisotropic growth, respectively. A photocatalytic growth model of Ag NPs was proposed by taking Ag atom and Ag1 ion diffusion into consideration. The plasmonic properties of the Ag–TiO2 films were studied in terms of extinction, surface enhanced Raman scattering, and fluorescence enhancement.
I. INTRODUCTION
TiO2 is the most promising material in the field of photocatalysis because of its superior photocatalytic activity, chemical stability, low cost, and nontoxicity. As such, its performances in photocatalytic degradation of chemical pollutants and solar energy conversion have been widely studied in the past decades, and its practical application has been seriously considered.1,2 TiO2 assembled with Ag nanoparticles (NPs) has received much attention because such a hybrid can effectively separate the charges and redshift the absorption to visible light via plasmonic resonance.3–5 For example, Awazu et al. combined TiO2 and Ag core/SiO2-shell NPs in a heterostructure and the photocatalytic activity was enhanced by a factor of 7 under near-ultraviolet (UV) irradiation.4 TiO2 assisted by the localized surface plasmon resonance (LSPR) is promising in applications such as photocatalysis,3–5 photovoltaic cells,6,7 reversible imaging,8 LSPR sensors,9 etc. Photocatalytic synthesis is a cost-effective way to fabricate Ag–TiO2 films. Using TiO2 films as the photocatalytic reduction media, the Ag1 ions in the AgNO3 solution are reduced on TiO2 films under UV irradiation, forming TiO2-catalyzed Ag NP films, which have been used as the substrates for surface enhanced Raman scattering (SERS).10,11 However, the growth dynamics of Ag NPs
still remains an open question and has attracted fundamental interest. Ag NPs synthesized by photocatalytic reduction generally exhibit irregular spherelike shapes. Recently, the photocatalytic growth of anisotropic Ag NPs on the TiO2 films in the AgNO3 ethanol solution was reported, and the vertical growth of Ag nanoplates was attributed to the surface roughness and random features of substrate.12,13 Using an annealed single-crystalline TiO2 substrate, Kazuma et al. reported the bi- and uniaxially oriented growth of Ag NPs.14 The preferential orientation and morphology of these Ag NPs were fou
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