Ac Electrodeposition Routes To Synthesize Gold Nanomaterials And Their Properties Characterizations

AC electrodeposition in microzones is a new route for nanomaterials preparation developed toward Micro/nano devices integration in the past decade. Owing to the scale effect, an extremely high electric field intensity could be obtained with a small voltage imposed on the electrodes, and this result in a variation of the species transfer process and the double-layer structure. Kinds of nanostructures could be prepared through a variety of nanomaterials including isotropy materials, anisotropic materials and alloys utilizing the method. However, there still remain many phenomena to be elucidated. The main contents and conclusions are shown as follows.First, the form of the metal ions in the electrolyte was obtained and the mass transfer model of the AC electrochemical reaction was analyzed on account of the Raman characteristic and cycle volt-ampere characteristic tests. Numerical simulation of the liquid-electric coupling field has been conducted based on the mathematical model of Faradic AC electroosmosis. On this basis, the mechanism how mass transfer and crystal growth process are influenced by the AC voltage amplitude, frequency and DC bias voltage was discussed. Analytical results indicated that electrophoresis constitutes the main species flux in the AC electrochemical reactions. When the Faradic reaction occurs on the electrodes surface, the direction of the capacitive AC electroosmosis flow reverses. The electrode locates upstream of the micro-fluid. In consequence, electrochemical reaction tends to be diffusion controlled. In addition, when the AC voltage was DC biased, the thickness of the diffuse layer outside the more negative electrode gets thicker; meanwhile, the diffuse layer outside the other electrode gets thinner. The thickening of the diffuse layer is the major factor to induce nanomaterials growth in the microzones.Second, on account of the theory analysis and numerical simulation, gold nanomaterials exhibiting different morphologies were synthesized adjusting the electrochemical conditions including the AC voltage, the AC frequency, the DC biased voltage, and the electrolyte concentration. Then, electrochemical parameters were optimized based on the microscopic analysis and morphology comparison. Well-structured gold nanomaterials, whose growth position, direction and dimensionality could be controlled, was obtained via the method. Au/Ag bimetallic nanomaterials whose morphology and composition could be tuned through the AC frequency and bath composition were also synthesized.Third, the surface enhanced Raman scattering (SERS) activity of these gold nanomaterials and Au/Ag bimetallic nanomaterials was characterized and the latter proved to have a higher Raman enhancement factor. Au/Rhodamine B (RhB) composite nanomaterials were one-step fabricated by a combination of AC electrodeposition and dielectrophoresis force assembly. Further experiment illustrated that this method could improve the detection limit of RhB effectively.Finally, electrical characterizations for single crystalline gold nanomaterials obtained by AC electrodeposition were conducted. Experimental results illustrated that an ohmic contact with good electrical contact characteristics was formed between the electrodes and nanomaterials.To illustrate the innovative points of this dissertation, the existence form and motion model of the metal ions in the electrolyte were obtained through the Raman characteristic and cycle volt-ampere characteristic tests and the electrochemical mechanism was delineated. Based on the micro-fluid theory, we have build the liquid-electric coupling mathematical model for AC electrodeposition in the microzones. Then, Au/Ag bimetallic SERS substrates were prepared successfully and used for Raman detection. This method is expected to be applied in rapid fabrication of novel micro/nano sensors seeing that nanomaterials could be prepared at the predefined locations connecting to the electrodes, which make them convenient to integrate with other MEMS/NEMS devices.Through the discussion, the dissertation gives a more understandable insight into the AC electrodeposition process in the Microzones. The nanomaterials exhibiting better properties, such as optical and electrical properties could be obtained. This manuscript provides a theory and experimental basis for their applications in the area of biochemical detection and microelectronics.