AC-deposition Mechanism Of Au-Pt Alloy Nanodendrites And Their Application In Catalytic Oxidation Of Formic Acid

The noble metal based nanostructures have attracted considerable attention dueto their good performance in catalysis applications. For example, Pt is one of the mostcommonly used catalysts and electrocatalysis in a wide range of catalytic applicationdue to its good chemical inertness, catalytic performance, and thermoelectricproperties. However, monometallic Pt is susceptible to deactivation or poisoningduring catalytic and electrocatalytic processes. It is well known that adding a secondmetallic component can enhance the activity, selectivity, and stability of pure metalcatalysts by the electronic effect, ensemble effect, and/or synergistic effect. Au-Ptalloy nanomaterials are of special interest for their potential applications in catalysis,especially for electrocatalysis in fuel cell as adding Au can effectively improve theactivity and stability of Pt. Among the various alloy nanomaterials, alloynanodendrites are important class of materials that are attractive due to their highsurface-area and their high degree of connectivity. These properties make dendritesuseful for a number of applications including catalysis and chemical sensing. Amongthe methods used for the preparation of alloy nanodendrites, AC-deposition method isone of the most promised methods as it can synthesis nanowires and nanodendriteswith highly tunable composition, size and morphology between microelectrodeswithout using any templates or sudstrates. More importantly, the as-preparednanodendrites modified microelectrodes can be applied without any pretreatment ormodification. To date, deposition has been demonstrated for a wide variety ofmaterials, including Au, Pt, Pd, Au-Pt, Au-Ag and Ag-Pd. However, previous workshave largely focused on manipulating experiment conditions such as temperature,additive, voltage, and frequency to control the morphology, size and composition ofthe nanostructures. Little is established about the origin of the branches and the phasestructure and their controllability, and the full control over the alloy morphology,phase structure, size and composition is currently not achieved. There is a clear needto gain fundamental insights into the mechanism underlying the control of themorphology, composition, size, and phase structure.This report describes novel findings of the growth mechanism for Au-Pt alloynanowires and nanodendrites controlled by single-frequency AC deposition method.Factors controlling the alloy growth are examined using an array of analyticaltechniques such as SEM,TEM, EDX, XRD and electrochemical measurement,providing new insights into the design and control of the morphology, composition, size, and phase structures of the alloy nanostructures. The obtained Au-Pt alloynanodendrites modified microelectrodes was applied to the electrocatalytic oxidationof formic acid, which exhibited good performance with high activity and goodstability. The main results are as follows:(1) Alloy mophology and size control: A square wave is found to be morefavorable for branching growth whereas a sine wave is more advantageous forreducing the dendrite size. Increasing the voltage or the frequency can increase thebranching degree while decrease the size of the nanodendrites.(2) Alloy composition, structure strucontrol and growth-control mechamism:Increasing the voltage and Pt/Au ion ratio or decreasing the frequency could facilitatethe depostion of Pt ion in the solution and thus increase the Pt content in the alloy.Thebranches of the alloy were generated by anisotropic successive nucleation. The trunk,the main-branch, and sub-branch of the nanodendrites were grown along <111>,<200>, and <111> directions, respectively. The reduction of the voltage and thefrequency was shown to favor the growth of1D nanowire along <111> direction,whereas the increase of the nucleation rate by increasing the voltage or frequency wasshown to enhance the growth rate along <200> direction, favoring the3D dendritegrowth.(3) Alloy ctalytic performance: Cyclic voltammetry results shows that the formicacid oxidation on the Au-Pt alloy nanodendrites modified electrodes takes place viathe dehydrogenation way. A decrease of the Pt fraction in the alloy leads to anincrease of oxidation peak current density and a negative shift of oxidation peakposition, indicating that scattered and isolated Pt atoms can enchance the catalyticactivity of Au-Pt alloy nanodendrites. Chronoamperometry results during the periodof1200s shown that for the Au-Pt alloy nanodendrites modified electrodes,stabilization in the oxidation current was observed, demonstrating a better stabilityand activity of the alloy catalyst over commercial Pt/C eletrodes.