Qikui Fan

Porous Au–Ag Nanospheres with High-Density and Highly Accessible Hotspots for SERS Analysis

Colloidal plasmonic metal nanoparticles have enabled surface-enhanced Raman scattering (SERS) for a variety of analytical applications. While great efforts have been made to create hotspots for amplifying Raman signals, it remains a great challenge to ensure their high density and accessibility for improved sensitivity of the analysis. Here we report a dealloying process for the fabrication of porous Au-Ag alloy nanoparticles containing abundant inherent hotspots, which were encased in ultrathin hollow silica shells so that the need of conventional organic capping ligands for stabilization is eliminated, producing colloidal plasmonic nanoparticles with clean surface and thus high accessibility of the hotspots. As a result, these novel nanostructures show excellent SERS activity with an enhancement factor of ∼1.3 × 10(7) on a single particle basis (off-resonant condition), promising high applicability in many SERS-based analytical and biomedical applications.

Robust synthesis of ultrathin Au–Ag nanowires as a high-surface-area, synergistic substrate for constructing efficient Pt-based catalysts

Ultrathin Au (or Ag) nanowires represent an excellent substrate for atomic layer deposition of Pt to afford highly active and cost-effective catalysts due to the large surface area and possible synergistic effect. An ideal synthesis of such nanowires should avoid using strong capping agents for convenient post-synthesis treatments and should be easily scaled up and reproduced in a high yield, which remains a challenge. Here, we report a novel strategy to synthesize sub-2 nm Au–Ag alloy nanowires with high quality in N,N-dimethyl formamide (DMF), which relies on Ag modification of the nanocrystal surface and Ag–halide interactions for regulating the one-dimensional growth of the nanowires, without involving strong capping agents that are usually required in conventional syntheses. Sub-monolayer Pt atoms were successfully deposited on these ultrathin Au–Ag alloy nanowires without forming ensembles despite a high loading amount (up to 20% in terms of Pt/(Au + Ag)) due to the large surface area. The resulting Au–Ag@Pt core/shell nanowires demonstrate superior activities in the formic acid oxidation reaction (FAOR) due to the synergistic ligand effect and the absence of Pt ensembles. We believe that the novel synthesis and the demonstration of these ultrathin Au–Ag alloy nanowires as a general platform for constructing cost-effective noble metal catalysts open new opportunities in designing catalysts for a broad range of reactions.

Solid to Hollow Conversion of Silver Nanocrystals by Surface-Protected Etching

Although hollow silver nanocrystals possess unique plasmonic properties, there is a lack of robust strategies to synthesize such nanocrystals with high efficiency and controllability. To solve this problem, a new surface-protected etching strategy to convert solid Ag nanocrystals, which are widely available from conventional syntheses, into their hollow counterparts, producing a family of hollow Ag nanocrystals is reported. Hollow Ag nanospheres and nanotubes were prepared conveniently in this way. The key was the surface modification of Ag nanocrystals by a minor amount of Pt prior to a controllable etching process, which accounts for enhanced stability of the Ag surface and subsequent etching of Ag from the inner part of the nanocrystals while retaining the overall crystal morphology. These hollow Ag nanocrystals showed distinctive optical properties, as demonstrated by the enhanced optical transmittance of flexible electrodes fabricated with Ag nanotubes, compared to nanowires. These hollow Ag nanocrystals hold promise in different plasmonic and electronic applications.

Ultra-Small Platinum Nanoparticles Encapsulated in Sub-50 nm Hollow Titania Nanospheres for Low-Temperature Water–Gas Shift Reaction

Ultra-small platinum nanoparticles loaded over titania is a promising catalyst for the low-temperature water-gas shift (WGS) reaction and shows the potential to work in a mobile hydrogen fuel cell system. Their precise size engineering (<3 nm) and reliable stabilization remain challenging. To address these issues, we report a reverse-micelle synthesis approach, which affords uniform ultra-small platinum nanoparticles (tunable in ∼1.0-2.6 nm) encapsulated in hollow titania nanospheres with a shell thickness of only ∼3-5 nm and an overall diameter of only ∼32 nm. The Pt@TiO2 yolk/shell nanostructured catalysts display extraordinary stability and monotonically increasing activity with the decreasing size of the Pt nanoparticles in the WGS. The size-dependent variation in the electronic property of the Pt nanoparticles and the reducible oxide encapsulation that prevents the Pt nanoparticles from sintering are ascribed as the main reasons for the excellent catalytic performance.