Self-Assembly of Linear Arrays of Semiconductor Nanoparticles on Carbon Single-Walled Nanotubes †

Ligand-stabilized nanocrystals (NCs) were strongly bound to the nanotube surfaces by simple van der Waals forces. Linear arrays of CdSe and InP quantum dots were formed by self-assembly using the grooves in bundles of carbon single-walled nanotubes (SWNTs) as a one-dimensional template. A simple geometrical model explains the ordering in terms of the anisotropic properties of the nanotube surface. CdSe quantum rods were also observed to self-organize onto SWNTs with their long axis parallel to the nanotube axis. This approach offers a route to the formation of ordered NC/SWNT architectures that avoids problems associated with surface derivatization. Both semiconductor quantum dots (QDs) 1 and carbon singlewalled nanotubes (SWNTs) 2 possess interesting and potentially useful optical and electronic properties due to their nanoscale structures. In the case of QDs, quantum confinement in three dimensions produces a size-dependent modification of the electronic band structure, resulting in the formation of discrete electronic states. QDs exhibit unique behaviors such as efficient photoluminescence and photon up-conversion, slowed relaxation and cooling of hot carriers, enhanced lasing, and carrier multiplication via impact ionization. 3 SWNTs, however, consist of sp 2 -hybridized carbon atoms that form the walls of nanometer-wide, seamless cylinders. Past efforts to attach semiconductor nanocrystals (NCs) to nanotubes have focused on forming chemical attachments between the two different nanostructures. In this approach, defects in the nanotube lattice, i.e., any site where the sp 2 -bonded carbon network is broken, are used as sites for chemical bond formation. Such defects are typically present after acid-based purification methods or may be specifically introduced by chemical derivatization. In this paper, we report the formation of organized, onedimensional (1-D) arrays of semiconductor QDs by van der Waals (vdW) adsorption onto SWNTs. Two representative II-VI and the III-V semiconductor NCs, CdSe and InP, respectively, demonstrated linear ordering when adsorbed from nonaqueous colloidal solutions onto high-purity, low-defectdensity SWNTs. The tendency to form linear arrays was greatest when tube-tube alignment was relatively good within bundles and when the QDs were relatively large. The edge-to-edge (ee) separation distance between QDs in the 1-D arrays was ∼18 Å for both the InP and the CdSe QDs, indicating that QD-QD separation is governed by the thickness of the ligand shells, as is the case in two-and three-dimensional QD arrays

Synthesis-Dependent Oxidation State of Platinum on TiO₂ and Their Influences on the Solar Simulated Photocatalytic Hydrogen Production from Water

Platinized TiO₂ photocatalysts of different compositions of Pt⁰ and PtO₂ were prepared by modifying the synthesis procedures. The physicochemical properties of the composite materials were characterized by X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. Energy dispersive X-ray spectroscopy measurements confirmed the presence of Pt species existing as PtO₂ and/or mixtures of Pt⁰ and PtO₂. The composite material, Pt–TiO₂–2%H, contained a high amount of metallic Pt⁰ and PtO₂ in close proximity with TiO₂ that promoted an enhanced photocatalytic hydrogen evolution activity under simulated solar light irradiation. Although Pt–TiO₂–2%C and Pt–TiO₂–2%T consisted of similar compositions of PtO₂, these oxidized platinum species seem to appear further apart from TiO₂ in Pt–TiO₂–2%C than Pt–TiO₂–2%T. This caused dramatic variation in their optical behaviors such as strong fluorescence quenching and lower photocatalytic hydrogen evolution activity in the former photocatalyst. A photocatalyst prepared by the conventional photodeposition method was also prepared, characterized, and its photocatalytic activity assessed. This work provides an opportunity to understand the role of PtO₂ for photocatalytic production of hydrogen from platinized TiO₂ composites and the importance of heterojunctions in such photocatalysts for solar energy conversion