Controlled Fabrication of Colloidal Semiconductor–Metal Hybrid Heterostructures: Site Selective Metal Photo Deposition

Abstract

Reliable synthesis of semiconductor-metal heterostructures would increase their availability for fundamental studies and applications in catalytic, magnetic, and opto-electronic devices. Here, we demonstrate there are three main pathways for the formation of Pt and Pd nanoparticles on CdS and CdS 04 Se 0.6 nanorods. A thermal pathway and photochemical pathway occur when the metal precursor is heated or irradiated directly in the presence of an electron donor, leading to homogeneous nucleation and formation of freestanding metal nanoparticles. A separate photochemical pathway occurs in the presence of semiconductor nanorods, leading to exciton formation and quenching by electron trapping at surface defect sites. The localized electrons act as seeding points, leading to heterogeneous nucleation and formation of surface-bound metal nanoparticles. Careful selection of synthetic conditions allows deposition of Pt and Pd particles on CdS and CdS 0.4 Se 0.6 nanorods with a high degree of selectivity (90-95% surface-bound obtained photochemically) over the formation of freestanding metal particles (70-94% unattached under thermal conditions). In addition, metal photo deposition occurs on specific segments of CdS 0.4 Se 0.6 nanorods with compositional anisotropy by taking advantage of the band gap differential between different nanodomains. Irradiation at short wavelengths favors formation of Pd nanoparticles on the large band gap CdS-rich region of the nanorods (57% and 55% at 350 and 420 nm, respectively), while irradiation at longer wavelengths favors the formation of Pd nanoparticles on the small band gap CdSe-rich region of the nanorods (83% at 575 nm). The ability to tune the spatial composition of these and similar heterostructures will impact the ability to engineer and direct energy flows at the nanoscale. ' INTRODUCTIONSemiconductorÀmetal hybrid heterostructures are promising building blocks for applications in catalytic, magnetic, and optoelectronic devices. 1À7 The semiconductor's tunable band gap (300À4000 nm 4.1À0.3 eV), 8,9 broad and intense absorption (ε ≈ 10 5 À10 6 L mol À1 cm À1 ), 10 and long-lived exciton (up to 40 ns for CdSe, 1.8 μs for PbS) 11,12 provide unmatched light absorption and emission capabilities. Large aspect ratio semiconductors such as nanorods are of particular interest because of their ability to generate multiple excitons. 13,14 The metal can serve as an additional chromophore, fluorescence enhancer, paramagnet, or charge-collecting material where carriers localize after exciton quenching. For example, semiconductorÀmetal hybrid heterostructures have been shown to convert solar energy into potential and chemical energy. They become redox-active upon illumination and remain redox-active after being stored in the dark for several hours. 15 In addition, semiconductor and metal nanocrystals display a high degree of chemical-, photo-, and colloidal-stability (solubility) unmatched by other materials such as organic polymers and transition metal complexes. The ability to selecti...

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