COLLOIDAL SYNTHESIS OF Ag, Au, Pt NANOPARTICLES STABILIZED BY TiO2 NANORODS: A GENERAL STRATEGY TOWARD SEMICONDUCTOR/METAL NANOCOMPOSITES

Nanosized semiconductors and metals [1, 2] are extensively studied due to their unusual catalytic and opto-electronic properties, ultimately traceable to the large fraction of atoms residing at the surface compared to the bulk, and to confinement of charge carriers inside a small volume of material. At present, major goals in modern material chemistry are represented by the ability to design, fabricate and manipulate nanostructured systems to achieve well-tailored chemical-physical characteristics for advanced applications. The wide technological potential offered by lowdimensional inorganic solids is reasonably expected to be even more extended by combining different materials. In particular, metal/semiconductor oxide composite systems [3] are extremely attractive since they represent efficient bifunctional catalysts. In the nanosized regime, specific metal-oxide interactions can indeed be responsible for the detection of new physical properties or for enhanced catalytic activity, as the oxide can play a precise functional role, thus strongly deviating from its conventional task as simple support. The coupling with noble metals has also been demonstrated to increase the photocatalytic and photoelectrochemical responses of semiconductor oxides by reducing the fast recombination of the photogenerated charge carriers. Among the various materials, the well-known TiO2 and noble metals, like Ag, Au and Pt, can still offer unexplored opportunities for the realization of novel nanocomposite systems. TiO2 is the most studied semiconductor for environmental clean-up applications, due to its unique ability in photocatalyzing the degradation of a variety of organic contaminants in wastewater and air, to its chemical inertness and non-toxicity. Platinum is extremely attractive to be investigated at the nanoscale, due to its remarkable catalytic activity, while silver and gold show unusual sizeand shape-dependent optical properties, being promising for applications in chemical and biological sensing, based on surface-enhanced Raman scattering (SERS), localized surface plasmon resonance (LSPR) and metal enhanced fluorescence (MEF). In this work, the preparation and the characterization of novel TiO2/noble metal nanocomposite systems are reported. It is shown that organic-soluble anatase TiO2 nanorods [4] can act as effective stabilizers for segregated metal nanoparticles [5] in optically clear nonpolar solutions in the absence of specific ligands for the metals (Figure 1). Metallic nanoparticles are photocatalytically generated upon UV-illumination of deaerated TiO2 solutions containing the respective metal salt precursor. Alternatively, a two-step approach can be used, in which metal ions are first chemically reduced and subsequently irradiated. In both cases, the metal nanoparticles are obtained in different sizemorphological regimes as a function of the illumination time. A mechanism for the colloidal