Hai-Xin Lin

Synthesis, Characterization, and 3D-FDTD Simulation of Ag@SiO2 Nanoparticles for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

Au-seed Ag-growth nanoparticles of controllable diameter (50-100 nm), and having an ultrathin SiO(2) shell of controllable thickness (2-3 nm), were prepared for shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Their morphological, optical, and material properties were characterized; and their potential for use as a versatile Raman signal amplifier was investigated experimentally using pyridine as a probe molecule and theoretically by the three-dimensional finite-difference time-domain (3D-FDTD) method. We show that a SiO(2) shell as thin as 2 nm can be synthesized pinhole-free on the Ag surface of a nanoparticle, which then becomes the core. The dielectric SiO(2) shell serves to isolate the Raman-signal enhancing core and prevent it from interfering with the system under study. The SiO(2) shell also hinders oxidation of the Ag surface and nanoparticle aggregation. It significantly improves the stability and reproducibility of surface-enhanced Raman scattering (SERS) signal intensity, which is essential for SERS applications. Our 3D-FDTD simulations show that Ag-core SHINERS nanoparticles yield at least 2 orders of magnitude greater enhancement than Au-core ones when excited with green light on a smooth Ag surface, and thus add to the versatility of our SHINERS method.

Supersaturation-Dependent Surface Structure Evolution: From Ionic, Molecular to Metallic Micro/Nanocrystals

Deduced from thermodynamics and the Thomson-Gibbs equation that the surface energy of crystal face is in proportion to the supersaturation of crystal growth units during the crystal growth, we propose that the exposed crystal faces can be simply tuned by controlling the supersaturation, and higher supersaturation will result in the formation of crystallites with higher surface-energy faces. We have successfully applied it for the growth of ionic (NaCl), molecular (TBPe), and metallic (Au, Pd) micro/nanocrystals with high-surface-energy faces. The above proposed strategy can be rationally designed to synthesize micro/nanocrystals with specific crystal faces and functionality toward specific applications.

Understanding the Cubic Phase Stabilization and Crystallization Kinetics in Mixed Cations and Halides Perovskite Single Crystals

The spontaneous α-to-δ phase transition of the formamidinium-based (FA) lead halide perovskite hinders its large scale application in solar cells. Though this phase transition can be inhibited by alloying with methylammonium-based (MA) perovskite, the underlying mechanism is largely unexplored. In this Communication, we grow high-quality mixed cations and halides perovskite single crystals (FAPbI3)1-x(MAPbBr3)x to understand the principles for maintaining pure perovskite phase, which is essential to device optimization. We demonstrate that the best composition for a perfect α-phase perovskite without segregation is x = 0.1-0.15, and such a mixed perovskite exhibits carrier lifetime as long as 11.0 μs, which is over 20 times of that of FAPbI3 single crystal. Powder XRD, single crystal XRD and FT-IR results reveal that the incorporation of MA+ is critical for tuning the effective Goldschmidt tolerance factor toward the ideal value of 1 and lowering the Gibbs free energy via unit cell contraction and cation disorder. Moreover, we find that Br incorporation can effectively control the perovskite crystallization kinetics and reduce defect density to acquire high-quality single crystals with significant inhibition of δ-phase. These findings benefit the understanding of α-phase stabilization behavior, and have led to fabrication of perovskite solar cells with highest efficiency of 19.9% via solvent management.

Uniform gold spherical particles for single-particle surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) benefits from the enhanced electromagnetic field of the localized surface plasmon resonance effect of metallic (especially coinage metals) nanoparticles or nanostructures. The detection sensitivity and reproducibility of SERS measurement appear to be the two critical issues in SERS. To solve the problem associated with traditional nanoparticle aggregates and SERS substrates, we propose in this work single particle SERS. We prepared uniform gold microspheres with controllable size and surface roughness using an etching-assisted seed-mediated method. Single particle dark-field spectroscopy and SERS measurements show that particles with a larger roughness give a stronger SERS signal, but still retain a good reproducibility. This study points to the promising future of the practical application of the single particle SERS technique for trace analysis.

A comparative investigation of electrocatalysis at Pt monolayers on shape-controlled Au nanocrystals: facet effect versus strain effect

In order to explore how and to what extent differently shaped Au nanocrystals (NCs) as the cores may affect the electrocatalytic properties of the resulting Au@Pt catalysts; a monolayer (ML) of Pt was deposited on cubic, octahedral and rhombic dodecahedral Au nanocrystals by using the well-established galvanic redox replacement method. The as-obtained Au-NCs@Pt-ML catalysts exhibited strongly inhibited COad oxidation in acidic media and a significantly enhanced ethanol oxidation reaction (EOR) in alkaline media as compared to commercial Pt/C, with an Au-shape dependent activity sequence in line with that of Pt single crystals of the corresponding orientations. Strain and facet effects were found to be jointly involved in determining the overall electrocatalytic activities at the three Au-NCs@Pt-ML catalysts, with the facet effect superimposed on the more pronounced strain effect.

A cleaning method with centrifugation for removing surfactants from gold nanoparticles and its mechanism

We developed an effective cleaning method based on a centrifugal effect to remove the surfactant cetyltrimethylammonium bromide (CTAB) from the surfaces of noble metallic such as 50 nm gold nano-octahedrons. The cleaned surface can be obtained by choosing the proper centrifugal speed and times under the appropriate temperature and humidity conditions. Thermal gravimetric analysis (TGA), surface-enhanced Raman scattering (SERS) and Fourier transform infrared spectroscopy (FTIR) are conducted to systematically investigate the adsorbed CTAB layer structure on gold nano-octahedrons surface. The results indicate that CTAB molecules bind to gold surface via their long hydrophobic tail and form a (sub) monolayer on the gold surface, and the bromide ions chemisorbed at the gold surface are desorbed. The dilution effect of water can act on the transformation of CTAB on gold surface during the cleaning procedure. Additionally, a localized solution flow is induced by the polarization of the electrical double layer gold nanoparticle with high speed in a centrifugal field. Mass transfer around the gold nanoparticle is occurring, and thus to be the influences on the surface cleaning of gold nanoparticles. We also performed the cyclic voltammetry (CV) studies in 0.5 mol/L sulfuric acid solution and alkaline solution contained with 0.1 mol/L NaOH + 1.0 m mol/L Pb(NO3)2, used the gold nano-octahedrons film as electrode. The results suggest that, after the cleaning procedures with centrifugation, the surface of gold nano-octahedrons can be purified enough to be used for electrochemical studies instead of traditional gold (111) facets.