Jianfeng Huang

Ag Dendrite-Based Au/Ag Bimetallic Nanostructures with Strongly Enhanced Catalytic Activity

Dendritic Ag/Au bimetallic nanostructures have been synthesized via a galvanic replacement reaction (GRR) of Ag dendrites in a chlorauric acid (HAuCl4) solution. After short periods of time, one obtains structures with protruding flakes; these will mature into very porous structures with little Ag left over. The morphological, compositional, and crystal structural changes involved with reaction time t were analyzed by using scanning and transmission electron microscopy (SEM and TEM, respectively), energy-dispersive X-ray spectrometry (EDX), and X-ray diffraction. High-resolution TEM combined with EDX and selected area electron diffraction confirmed the replacement of Ag with Au. A proposed formation mechanism of the original Ag dendrites developing pores while growing Au flakes cover this underlying structure at longer reaction times is confirmed by exploiting surface-enhanced Raman scattering (SERS). Catalytic reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) is strongly enhanced, implying promising applications in catalysis.

Site-specific growth of Au-Pd alloy horns on Au nanorods: a platform for highly sensitive monitoring of catalytic reactions by surface enhancement Raman spectroscopy.

Surface-enhanced Raman scattering (SERS) is a highly sensitive probe for molecular detection. The aim of this study was to develop an efficient platform for investigating the kinetics of catalytic reactions with SERS. To achieve this, we synthesized a novel Au-Pd bimetallic nanostructure (HIF-AuNR@AuPd) through site-specific epitaxial growth of Au-Pd alloy horns as catalytic sites at the ends of Au nanorods. Using high-resolution electron microscopy and tomography, we successfully reconstructed the complex three-dimensional morphology of HIF-AuNR@AuPd and identified that the horns are bound with high-index {11l} (0.25 < l < 0.43) facets. With an electron beam probe, we visualized the distribution of surface plasmon over the HIF-AuNR@AuPd nanorods, finding that strong longitudinal surface plasmon resonance concentrated at the rod ends. This unique crystal morphology led to the coupling of high catalytic activity with a strong SERS effect at the rod ends, making HIF-AuNR@AuPd an excellent bifunctional platform for in situ monitoring of surface catalytic reactions. Using the hydrogenation of 4-nitrothiophenol as a model reaction, we demonstrated that its first-order reaction kinetics could be accurately determined from this platform. Moreover, we clearly identified the superior catalytic activity of the rod ends relative to that of the rod bodies, owing to the different SERS activities at the two positions. In comparison with other reported Au-Pd bimetallic nanostructures, HIF-AuNR@AuPd offered both higher catalytic activity and greater detection sensitivity.

Two-dimensional gold nanostructures with high activity for selective oxidation of carbon-hydrogen bonds

Efficient synthesis of stable two-dimensional (2D) noble metal catalysts is a challenging topic. Here we report the facile synthesis of 2D gold nanosheets via a wet chemistry method, by using layered double hydroxide as the template. Detailed characterization with electron microscopy and X-ray photoelectron spectroscopy demonstrates that the nanosheets are negatively charged and [001] oriented with thicknesses varying from single to a few atomic layers. X-ray absorption spectroscopy reveals unusually low gold–gold coordination numbers. These gold nanosheets exhibit high catalytic activity and stability in the solvent-free selective oxidation of carbon–hydrogen bonds with molecular oxygen.

Harnessing structural darkness in the visible and infrared wavelengths for a new source of light

Engineering broadband light absorbers is crucial to many applications, including energy-harvesting devices and optical interconnects. The performances of an ideal absorber are that of a black body, a dark material that absorbs radiation at all angles and polarizations. Despite advances in micrometre-thick films, the absorbers available to date are still far from an ideal black body. Here, we describe a disordered nanostructured material that shows an almost ideal black-body absorption of 98-99% between 400 and 1,400 nm that is insensitive to the angle and polarization of the incident light. The material comprises nanoparticles composed of a nanorod with a nanosphere of 30 nm diameter attached. When diluted into liquids, a small concentration of nanoparticles absorbs on average 26% more than carbon nanotubes, the darkest material available to date. By pumping a dye optical amplifier with nanosecond pulses of ∼100 mW power, we harness the structural darkness of the material and create a new type of light source, which generates monochromatic emission (∼5 nm wide) without the need for any resonance. This is achieved through the dynamics of light condensation in which all absorbed electromagnetic energy spontaneously generates single-colour energy pulses.

A porous silicon optical microcavity for sensitive bacteria detection

A porous silicon microcavity (PSM) is highly sensitive to subtle interface changes due to its high surface area, capillary condensation ability and a narrow resonance peak (∼10 nm). Based on the well-defined optical properties of a PSM, we successfully fabricated a bacteria detection chip for molecular or subcellular analysis by surface modification using undecylenic acid (UA), and the specific recognition binding of vancomycin to the D-alanyl-D-alanine of bacteria. The red shift of the PSM resonance peak showed a good linear relationship with bacteria concentration ranging from 100 to 1000 bacteria ml( - 1) at the level of relative standard deviation of 0.994 and detection limit of 20 bacteria ml( - 1). The resulting PSM sensors demonstrated high sensitivity, good reproducibility, fast response and low cost for biosensing.

Optical sensing nanostructures for porous silicon rugate filters

Porous silicon rugate filters [PSRFs] and combination PSRFs [C-PSRFs] are emerging as interesting sensing materials due to their specific nanostructures and superior optical properties. In this work, we present a systematic study of the PSRF fabrication and its nanostructure/optical characterization. Various PSRF chips were produced with resonance peaks that are adjustable from visible region to near-infrared region by simply increasing the periods of sine currents in a programmed electrochemical etching method. A regression analysis revealed a perfect linear correlation between the resonant peak wavelength and the period of etching current. By coupling the sine currents with several different periods, C-PSRFs were produced with defined multiple resonance peaks located at desired positions. A scanning electron microscope and a microfiber spectrophotometer were employed to analyze their physical structure and feature spectra, respectively. The sensing properties of C-PSRFs were investigated in an ethanol vapor, where the red shifts of the C-PSRF peaks had a good linear relationship with a certain concentration of ethanol vapor. As the concentration increased, the slope of the regression line also increased. The C-PSRF sensors indicated the high sensitivity, quick response, perfect durability, reproducibility, and versatility in other organic gas sensing.

Unravelling Thiol’s Role in Directing Asymmetric Growth of Au Nanorod–Au Nanoparticle Dimers

Asymmetric nanocrystals have practical significance in nanotechnologies but present fundamental synthetic challenges. Thiol ligands have proven effective in breaking the symmetric growth of metallic nanocrystals but their exact roles in the synthesis remain elusive. Here, we synthesized an unprecedented Au nanorod-Au nanoparticle (AuNR-AuNP) dimer structure with the assistance of a thiol ligand. On the basis of our experimental observations, we unraveled for the first time that the thiol could cause an inhomogeneous distribution of surface strains on the seed crystals as well as a modulated reduction rate of metal precursors, which jointly induced the asymmetric growth of monometallic dimers.

Experimental evidence of chiral gold nanowires with boerdijk-coxeter-bernal structure by atomic-resolution imaging

The assembly of chiral crystals from highly symmetric atoms, i.e., the origin of chirality, is of fundamental importance. Bulk crystals with chiral point groups are known, but they rarely exhibit overall chiral morphology or asymmetric surface chemistry. [1] As the size of crystal domains decrease to nanoscale, however, chiral morphology can sometimes arise (often in nanowires). [2] In contrast to the packing of chiral molecules, the atomic packing in metal is always highly symmetric and thus, more interesting when forming chiral structures. To date, there are only a few cases of chiral metal nanocrystals, all in the form of nanowires (NWs), [2] whereas most of the metallic NWs reported in the literature are achiral, as dictated by their intrinsic lattice symmetry.