Hai Jun Xu

Recyclable three-dimensional Ag nanoparticle-decorated TiO2 nanorod arrays for surface-enhanced Raman scattering

Multifunctional Ag nanoparticle-decorated TiO2 nanorod arrays were prepared by two simple processes. TiO2 nanorod arrays were first fabricated by the hydrothermal route and then Ag nanoparticles were decorated on the nanorods by the chemical reduction impregnation method. Three-dimensional Ag/TiO2 arrays were used as an active substrate for surface-enhanced Raman scattering (SERS). The results show that the detection limit for rhodamine 6G (R6G) was as low as 10(-7)M and the Raman enhancement factor was as large as 10(5). After calibrating the Raman peak intensities of R6G, it could be quantitatively detected. More importantly, the photocatalytic activity of TiO2 provides a self-cleaning capability to the SERS substrate, which can be recycled and used to degrade many Ag surface adsorbates such as R6G, methyl orange, Congo red, and methylene blue after exposure to visible light. The absorbed small molecules can all be rapidly and completely removed from the SERS substrate, which has been successfully reused four times without a decrease in accuracy or sensitivity. Our results reveal that the unique recyclable property not only paves a new way to solve the single-use problem of traditional SERS substrates but also provides more SERS platforms for multiple detections of other organic molecular species.

Ag dendritic nanostructures as ultrastable substrates for surface-enhanced Raman scattering

Dendritic silver nanostructures coated with silica nanofilm are synthesized via hydrothermal etching using silver nitrate, hydrofluoric acid, and hydrogen peroxide and controlling the reagent concentration, reaction time, and temperature. The silver dendritic nanostructures are employed as substrates for surface-enhanced Raman scattering (SERS) and exhibit high sensitivity and excellent stability. Calibration of the Raman peak intensities of rhodamine 6G and thiram allowed quantitative detection of these organic molecules. Our findings are a significant advance in developing robust SERS substrates for fast detection of trace organic contaminants.

ZnO/Si arrays decorated by Au nanoparticles for surface-enhanced Raman scattering study

Large scale and highly ordered flowerlike ZnO/Si nanostructures are successfully prepared by combining two common techniques, viz. hydrothermally etch fabrication of nanoporous Si pillar array (NSPA) and self-catalytic chemical vapor transport growth of ZnO nanowires. Au nanoparticles are decorated onto the ZnO/Si nanoflowers by the hydrothermal method. The formed Au/ZnO/NSPA array is evaluated as a surface-enhanced Raman scattering SERS-active substrate, which exhibits very high sensitivity and good stability and reproducibility. The excellent SERS enhancement is mainly attributed to the strong local electromagnetic effect which is associated with the unique flowerlike nanostructures of Au/ZnO/NSPA and the formed metal-induced gap states at the Au/ZnO interfaces. The results indicated that Au/ZnO/NSPA might be employed as a promising SERS substrate for the fast detection of low-concentration biomolecules.

Recyclable surface-enhanced Raman scattering template based on nanoporous gold film/Si nanowire arrays

Nanoporous gold film (NPGF) composed of gold nanoparticles was used to cover a large Si nanowire array (SiNWA) by simple metal-assisted chemical etching and metal reduction processes. Three-dimensional SiNWA/NPGF was employed as an active substrate for surface-enhanced Raman scattering (SERS). The results show that the detection limit for crystal violet was as low as 10−12M, and the Raman enhancement factor was as large as 107 with a relative standard deviation of less than 20%. After calibrating the Raman peak intensities of crystal violet and thiram could be quantitatively detected. More importantly, the SERS substrates are recyclable and can be used for many gold surface adsorbates such as p-aminothiophenol, crystal violet, Rhodamine 6G, and methyl orange, which can all be rapidly and completely removed from the NPGF surface. Our findings are an important advance in SERS substrates and will allow the quantitative and recyclable detection of trace organic contaminants.

Cicada wing decorated by silver nanoparticles as low-cost and active/sensitive substrates for surface-enhanced Raman scattering

A green, low-cost and highly efficient surface-enhanced Raman scattering (SERS) substrate was achieved by a chemical deposition of silver nanoparticles on a cicada wing, which has the large-scale nanosized protrusions on its surface. Employing the already-formed Ag/cicada wing as substrate for SERS detection, the detection limit for rhodamine 6G could reach 10−7M, the Raman enhancement factor of the substrate was as large as 106 and the relative standard deviation remains lower than 7%. The three-dimensional finite-difference time-domain simulation results showed that two types of inter-Ag-nanoparticle nanogaps in the formed geometry created a huge number of SERS “hot spots” where the electromagnetic field is substantially amplified and contributes to the higher SERS sensitivity. Meanwhile, the water contact angle of the SERS substrate is roughly 150°, which indicates the super-hydrophobic surface of the substrate. This feature may be conducive to the gathering of target molecules during the SERS detectio...

Enhanced field emission from ZnO nanowires grown on a silicon nanoporous pillar array

A large scale heterostructure array of ZnO nanowires/silicon nanoporous pillar array (Si-NPA) was prepared by a self-catalytic thermal evaporation and vapor-phase transport method, and an ultrahigh field emission current density of 1.55 mA cm−2 was obtained under an operating electric field of 4.0 V μm−1, with a low turn-on field of 1.65 V μm−1. The enhancement factor calculated according to the Fowler–Nordheim theory was ∼3141. The excellent field emission performance was attributed to the unique structure of ZnO/Si-NPA, especially the formation of ZnO nanowires on regular Si pillar array. Our work indicated that ZnO/Si-NPA might be an ideal candidate cathode of potential applications in flat panel displays and high brightness electron sources.