Preparation Of Pt Alloy Nanowires Based On Electrodeposition Under AC-fields And Its Catalytic Properties For H₂O₂

Nanostructured materials have attracted considerable attention because of theirnovel optical, electrical, catalytic, and magnetic properties and their potentialapplications in nanoelectronic devices, nanosensors, catalysts, and informationstorage systems. Amongst the various materials, most researches have been focusedon noble metals such as gold （Au） and platinum （Pt） for their high electrocatalyticefficiency and selectivity, the applications of Au and Pt for catalyzing methanoloxidation, detecting human chorionic gonadotrophin and cholesterol have beenreported. The metallic alloy nanoparticles present better catalytic properties thanmonometallic counterparts by studying the structure of the metallic alloynanoparticles. The nanowires were prepared by the chemical synthesis andelectrochemical deposition, then transfer of the obtained array onto substrate and theconventional biosensors were obtained. The sensors mentioned above exhibited highelectrocatalytic properties, but an additional procedure of immobilizing thesecatalysts on the conventional electrodes was required before application toelectrochemical sensing platform. Herein, we present an enzyme-free hydrogenperoxide biosensor prepared by the electrodeposition of Pt–Au alloy nanowires ontogold microelectrodes. The Pt–Au alloy nanowires could be formed on the goldmicroelectrode surface with strongly mechanical intensity at room temperaturewithout any surfactant or stabilizer, and the Pt–Au alloy nanowires modifiedmicroelectrode could be applied without any pretreatment or modification. Theas-prepared biosensor was applied to the electrochemical detection of H₂O₂, whichexhibited good performances at a low applied potential with high sensitivity, fastresponse time, wide linear range, and good stability and reproducibility.Pt alloy nanowires were successfully obtained by different methods in this thesis.The better method could be used through the comparison of different preparationmethods. The Pt–Au alloy nanowires were characterized by a scanning electronmicroscopy （SEM）, energy-dispersion X-ray （EDX） spectrometer and X-raydiffraction （XRD）. The effects of electrolyte on the morphology and composition ofPt–Au were discussed. When the Pt–Au alloy nanowires were used for detecting theH₂O₂, the best conditions could be obtained by studying the nanowires with differentcomposition and operating voltage. The main results as follows:（1） Under the same conditions of the Alternating current （AC） voltage andfrequency, the diameter of the alloy nanowires were decided by the concentration of electrodeposition solution, the diameter of the alloy nanowires become greater withthe concentration turned greater. When the concentration and ion molar ratio of theelectrodeposition solution were stable, the higher frequency could make thenanowires smooth.（2） The better morphology of the alloy nanowires could be obtained whenaddition of a certain amount of reducing agent under the same conditions by thesingle-frequency method. But excessive reducing agent can result in the accumulationof nanoparticles into block.（3） The dielectrophoresis is responsible for the growth of alloy nanowires, andthe larger electric energy density at the end of the electrode or outstanding leads to thenucleation and growth, and then the dendritic nanowires were obtained because ofrandom arrangmeat. However, the reducing agent plays an important role of reductionwhen the single-frequency method works, then the nanoparticles were arranged inlines along the electric field lines because of the different potential energy.（4） Preparation of Pt–Au alloy nanowires with a variational-frequency method,more nanowires were obtained with the longer deposition time. The diameter of thealloy nanowires was smaller when the lower concentration of electrodepositionsolution was used under the same deposition conditions, also the composition of thealloy nanowires is unchangeable when the ion ratios were constant.（5） The alloy composition could be well-controlled by adjusting ion molar ratioof Pt/Au in the electrolyte, when the ion ratios of Pt/Au varied from1to65, the alloypossessed composition range from20to50at.%, and the content of Pt increase withthe increase of molar rations of Pt/Au.（6） The crystalline structure of Au、Pt、Pt₅₁Au₄₉、Pt₄₇Au₅₃and Pt₂₁Au₇₉nanowireswere characterized by XRD measurements, compared with the peak appears at the（111） plane in the pure Au nanowires, a peak appears at2=38.9in the Au53Pt47andan obviously shift by about0.8, which lay in between those of Au79Pt21（38.5） andAu49Pt51（39.1）. This observation indicates that the alloy nanowires were not themixtures of individual nanoparticles but bimetallic nanostructures.（7） The reduction peak current increased with the increase of H₂O₂concentration,which shows the linear correlation between the reduction peak current and theconcentration of H₂O₂, aslo, both the oxidation and reduction peak currents increasedlinearly with square root of the scan rates, suggesting that the kinetics of the overallprocess were controlled by diffusion process.（8） Response currents of H₂O₂for the Pt–Au alloy nanowires with different components modified microelectrodes were compared, the maximum response currentwas found at Au53Pt47, also, the alloy naowires exhibited better catalytic property thantheir monometallic counterparts of Au or Pt nanowires did, this high catalytic activitycould be attributed to the electronic effect between the Au and Pt atoms, which wereclosed to each other.（9） By the comparison of amperometric responses of the Pt₄₇Au₅₃alloynanowires modified microelectrode at different potentials Pt₄₇Au₅₃alloy nanowiresmodified electrode at different potential, the sensor exhibited the best electrocatalyticactivity to H₂O₂when the potential is0VSCEat room temperature. Under thispotential, the sensor showed linear response to H₂O₂at the concentrations range from20×10^-6to8.38×103mol dm^-3with a detection limit of1.5×10^-6mol dm^-3when thesignal-to-noise ratio was3, and the sensitivity was up to129.2A dm3mmol1cm^-2.（10） The relative standard deviation （RSD） for0.1mmol dm^-3H₂O₂sensing was3.6%for ten measurements for the same electrode. When the electrode was stored at4C for one week, the current response still retained94%, and in the next three weeksthe response still retained89%of the initial value, indicating the long-term stabilityof the proposed Pt–Au alloy nanowires modified electrode.