Two-Dimensional Metal Nanostructures: From Theoretical Understanding to Experiment

This paper reviews recent studies on the preparation of two-dimensional (2D) metal nanostructures, particularly nanosheets. As metal often exists in the high-symmetry crystal phase, such as face centered cubic structures, reducing the symmetry is often needed for the formation of low-dimensional nanostructures. Recent advances in characterization and theory allow for a deeper understanding of the formation of 2D nanostructures. This Review firstly describes the relevant theoretical framework to help the experimentalists understand chemical driving forces for the synthesis of 2D metal nanostructures, followed by examples on the shape control of different metals. Recent applications of 2D metal nanostructures, including catalysis, bioimaging, plasmonics, and sensing, are discussed. We end the Review with a summary and outlook of the challenges and opportunities in the design, synthesis, and application of 2D metal nanostructures.

Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and Applications

As one unique group of two-dimensional (2D) nanomaterials, 2D metal nanomaterials have drawn increasing attention owing to their intriguing physiochemical properties and broad range of promising applications. In this Review, we briefly introduce the general synthetic strategies applied to 2D metal nanomaterials, followed by describing in detail the various synthetic methods classified in two categories, i.e. bottom-up methods and top-down methods. After introducing the unique physical and chemical properties of 2D metal nanomaterials, the potential applications of 2D metal nanomaterials in catalysis, surface enhanced Raman scattering, sensing, bioimaging, solar cells, and photothermal therapy are discussed in detail. Finally, the challenges and opportunities in this promising research area are proposed.

Synthesis of Gold Nanoparticles Using the Interface of an Emulsion Droplet

A facile and rapid method for synthesizing single crystal gold spherical or platelet (nonspherical) particles is reported. The reaction takes place at the interface of two immiscible liquids where the reducing agent decamethylferrocene (DmFc) was initially added to hexane and gold chloride (AuCl₄^-) to an aqueous phase. The reaction is spontaneous at room temperature, leading to the creation of Au nanoparticles (AuNP). A flow focusing microfluidic chip was used to create emulsion droplets, allowing the same reaction to take place within a series of microreactors. The technique allows the number of droplets, their diameter, and even the concentration of reactants in both phases to be controlled. The size and shape of the AuNP are dependent upon the concentration of the reactants and the size of the droplets. By tuning the reaction parameters, the synthesized nanoparticles vary from nanometer to micrometer sized spheres or platelets. The surfactant used to stabilize the emulsion was also shown to influence the particle shape. Finally, the addition of other nanoparticles within the droplet allows for core@shell particles to be readily formed, and we believe this could be a versatile platform for the large scale production of core@shell particles.

Gold nanosheets synthesized with red marine alga Actinotrichia fragilis as efficient electrocatalysts toward formic acid oxidation

In this study, gold nanosheets were synthesized with red marine alga (Actinotrichia fragilis) collected from Persian Gulf and used as an electrocatalyst for oxidation of formic acid.

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.

Selective detection and recovery of gold at tannin-immobilized non-conducting electrode

A tannin-immobilized glassy carbon electrode (TIGC) was prepared via electrochemical oxidation of the naturally occurring polyphenolic mimosa tannin, which generated a non-conducting polymeric film (NCPF) on the electrode surface. The fouling of the electrode surface by the electropolymerized film was evaluated by monitoring the electrode response of ferricyanide ions as a redox marker. The NCPF was permselective to HAuCl4, and the electrochemical reduction of HAuCl4 to metallic gold at the TIGC electrode was evaluated by recording the reduction current during cyclic voltammetry measurement. In the mixed electrolyte containing HAuCl4 along with FeCl3 and/or CuCl2, the NCPF remained selective toward the electrochemical reduction of HAuCl4 into the metallic state. The chemical reduction of HAuCl4 into metallic gold was also observed when the NCPF was inserted into an acidic gold solution overnight. The adsorption capacity of Au(III) on tannin-immobilized carbon fiber was 29±1.45 mg g(-1) at 60°C. In the presence of excess Cu(II) and Fe(III), tannin-immobilized NCPF proved to be an excellent candidate for the selective detection and recovery of gold through both electrochemical and chemical processes.