Mohammad Mehdi Shahjamali

Surfactant‐Free Sub‐2 nm Ultrathin Triangular Gold Nanoframes

Ultrathin triangular gold nanoframes are synthesized in high yield through selective gold deposition on the edges of triangular silver nanoprisms and subsequent silver etching with mild wet etchants. These ultrathin gold nanoframes are surfactant-free with tailorable ridge thickness from 1.8 to 6 nm and exhibit adjustable and distinct surface plasmon resonance bands in the visible and near-IR region. In comparison, etching of the nanoprism template by galvanic replacement can only create frame structures with much thicker ridges, which have much lower catalytic activity for 4-nitrophenol reduction than the ultrathin gold nanoframes.

Plasmon-Enhanced Hydrogen Evolution on Au-InVO4 Hybrid Microspheres

We demonstrate plasmon-enhanced hydrogen evolution from photocatalytic water reduction using Au-InVO4 hybrid microspheres. The surface plasmons of gold nanoparticles enhance the sub-band gap excitation of InVO4 and promote charge separation on its surface through plasmon-exciton coupling, thereby significantly improving their photocatalytic efficiency.

Influence of Surfactant Bilayers on the Refractive Index Sensitivity and Catalytic Properties of Anisotropic Gold Nanoparticles

Shape-controlled synthesis of gold nanoparticles generally involves the use of surfactants, typically cetyltrimethylammonium (CTAX, X = Cl(-) , Br(-)), to regulate the nucleation growth process and to obtain colloidally stable nanoparticles. The surfactants adsorb on the nanoparticle surface making further functionalization difficult and therefore limit their use in many applications. Herein, the influence of CTAX on nanoparticle sensitivity to local dielectric environment changes is reported. It is shown, both experimentally and theoretically, that the CTAX bilayer significantly reduces the refractive index (RI) sensitivity of anisotropic gold nanoparticles such as nanocubes and concave nanocubes, nanorods, and nanoprisms. The RI sensitivity can be increased by up to 40% by removing the surfactant layer from nanoparticles immobilized on a solid substrate using oxygen plasma treatment. This increase compensates for the otherwise problematic decrease in RI sensitivity caused by the substrate effect. Moreover, the removal of the surfactants both facilitates nanoparticle biofunctionalization and significantly improves their catalytic properties. The strategy presented herein is a simple yet effective universal method for enhancing the RI sensitivity of CTAX-stabilized gold nanoparticles and increasing their potential as transducers in nanoplasmonic sensors, as well as in catalytic and biomedical applications.

Ag–Ag2S Hybrid Nanoprisms: Structural versus Plasmonic Evolution

Recently, Ag-Ag2S hybrid nanostructures have attracted a great deal of attention due to their enhanced chemical and thermal stability, in addition to their morphology- and composition-dependent tunable local surface plasmon resonances. Although Ag-Ag2S nanostructures can be synthesized via sulfidation of as-prepared anisotropic Ag nanoparticles, this process is poorly understood, often leading to materials with anomalous compositions, sizes, and shapes and, consequently, optical properties. In this work, we use theory and experiment to investigate the structural and plasmonic evolution of Ag-Ag2S nanoprisms during the sulfidation of Ag precursors. The previously observed red-shifted extinction of the Ag-Ag2S hybrid nanoprism as sulfidation occurs contradicts theoretical predictions, indicating that the reaction does not just occur at the prism tips as previously speculated. Our experiments show that sulfidation can induce either blue or red shifts in the extinction of the dipole plasmon mode, depending on reaction conditions. By elucidating the correlation with the final structure and morphology of the synthesized Ag-Ag2S nanoprisms, we find that, depending on the reaction conditions, sulfidation occurs on the prism tips and/or the (111) surfaces, leading to a core(Ag)-anisotropic shell(Ag2S) prism nanostructure. Additionally, we demonstrate that the direction of the shift in the dipole plasmon is a function of the relative amounts of Ag2S at the prism tips and Ag2S shell thickness around the prism.

How Ag Nanospheres Are Transformed into AgAu Nanocages

Bimetallic hollow, porous noble metal nanoparticles are of broad interest for biomedical, optical and catalytic applications. The most straightforward method for preparing such structures involves the reaction between HAuCl4 and well-formed Ag particles, typically spheres, cubes, or triangular prisms, yet the mechanism underlying their formation is poorly understood at the atomic scale. By combining in situ nanoscopic and atomic-scale characterization techniques (XAFS, SAXS, XRF, and electron microscopy) to follow the process, we elucidate a plausible reaction pathway for the conversion of citrate-capped Ag nanospheres to AgAu nanocages; importantly, the hollowing event cannot be explained by the nanoscale Kirkendall effect, nor by Galvanic exchange alone, two processes that have been previously proposed. We propose a modification of the bulk Galvanic exchange process that takes into account considerations that can only occur with nanoscale particles. This nanoscale Galvanic exchange process explains the novel morphological and chemical changes associated with the typically observed hollowing process.

On-Tip Photo-Modulated Molecular Printing.

The concept of using cantilever-free scanning probe arrays as structures that can modulate nanoscale ink flow and composition with light is introduced and evaluated. By utilizing polymer pen arrays with an opaque gold layer surrounding the base of the transparent polymer pyramids, we show that inks with photopolymerizable or isomerizable constituents can be used in conjunction with light channelled through the pyramids to control ink viscosity or composition in a dynamic manner. This on-tip photo-modulated molecular printing provides novel chemically and mechanically controlled approaches to regulating ink transport and composition in real time and could be useful not only for rapidly adjusting feature size but also for studying processes including photoreactions and mass transport at the nanoscale, self-assembly, and cell-material interactions.

Edge gold-coated silver nanoprism [Ag@(Au nanoframe)] for H202 detection

In this paper we describe a facile method for synthesizing gold triangular nanoframe from edge gold-coated silver nanoprism [Ag@(Au nanoframe)] with controlled ridge thickness and interior hollow diameter via chemical etching of silver from the gold coated Ag nanoprism with Hydrogen peroxide. Our process involves two major steps: (1) Formation of Ag@(Au nanoframe) by deposition of gold on the edges of Ag nanoprisms in room temperature (2) Etching of Ag atoms from the gold fame via H2O2 oxidation. The optical properties of the resulting nanoframe can be easily tailored in a broad range by controlling the ridge thickness or hole diameter by amount of H2O2 added into the reaction. Due to the changes to the optical spectra during etching, the nanoframe formation can be employed to detect H2O2 in a broad range of concentration down to 1 μM.