Geometric Effect of Single or Double Metal-Tipped CdSe Nanorods on Photocatalytic H₂ Generation

In the present work, we focused on geometrical (singleor double-tipped) and compositional (Pt or Au) variations of active metal components in a well-defined CdSe nanorod system. These colloidal nanostructures were employed for photocatalytic hydrogen generation from water under the identical reaction conditions with visible light irradiation. The catalysts exhibited significant dependency of the catalytic activity, specifically on the catalyst geometry and the choice of the metal tips, determined by the energetic consideration of electron transfer to the metal tips and hole transfer to the sacrificial reagents on the CdSe nanorods. SECTION: Energy Conversion and Storage; Energy and Charge Transport P hydrogen generation is one of the potential reactions for the production of environmentally clean and renewable energy resources from sun light. Although TiO2 has been studied as a highly effective photocatalyst for decades, the wide bandgap of TiO2 in the ultraviolet region shifted the research interest into semiconductors that absorb visible light, such as complex metal oxides, nitrides, and sulfides. Among these materials, cadmium chalcogenides have mostly been used as essential elements in photocatalysts due to their narrow band gaps and band energies at relatively low potentials. Recently, several hybrid nanostructures of CdS and CdSe with metals have been synthesized and employed for the photosplitting of water. In contrast to previous catalytic studies, the welldefined morphology and components of the hybrid nanocatalysts provided valuable information pertaining to the photoinduced charge separation and transfer processes during the reaction. Bao et al. explored photocatalytic hydrogen production using porous CdS nanostructures loaded with Pt nanoparticles in the presence of sacrificial reagents. Banin et al. synthesized CdS nanorods decorated with noble metals and investigated the compositional effect on the photocatalytic activities. Alivisatos et al. introduced CdSe seeds as a hole acceptor on Pt-tipped CdS rods, which facilitated photocatalytic stability as well as activity. Although these systems were well characterized, each experiment was performed under distinct reaction conditions, and thus comparisons of different systems were not meaningful. In order to understand the factors that significantly affect the reaction activity, a reaction study using various catalysts under identical experimental conditions is required. In the present work, we focused on geometrical (singleor double-tipped) and compositional (Pt or Au) variations of active metal components on CdSe nanorods. These colloidal nanostructures were employed for photocatalytic hydrogen generation from water under identical reaction conditions with visible light irradiation (λ ≥ 420 nm). The catalysts exhibited significant dependency of the catalytic activity, both on the catalyst geometry and on the choice of the metal tips, as determined by the energetic consideration of hole transfer to the sacrificial reagents and electron transfer to the metal tips on the CdSe nanorods. During the preparation of the catalyst, CdSe nanorods were synthesized in the presence of trioctylphosphine oxide and tetradecylphosphonic acid according to a method in the literature. A mixture of the CdSe nanorods and Pt(II) acetylacetonate in 1,2-dicholorobenzene was injected into a hot solution of surfactants, specifically oleic acid, oleyl amine, and 1,2-hexadecanediol at 200 °C, and the reaction mixture was stirred for 4 min. After centrifugation, the particles were dispersed in toluene. The addition of the Pt precursor (0.064 mmol) yielded single Pt-tipped CdSe nanorods (1). The transmission electron microscopy (TEM) image in Figure 1a shows that most of Received: October 26, 2012 Accepted: December 5, 2012 Published: December 5, 2012 Letter