Study On The Mechanism Of Anodic Electrodeposition And Photo Assisting Growth Of CeO₂Thin Film

Cerium oxides, particularly ceria (CeO2), have a wide range of applications due to their ability to store and release oxygen as well as good high-temperature ionic conductivity. For example, they have been used in automobile exhaust treatment, photocatalysis, ultraviolet absorption, polishing materials and metal protection, etc. Since the chemical composition, phase composition and microstructure of the materials can selectively obtained using electrochemical deposition method by changing electrolyte, potential, bath temperature, pH and additives, etc. Thus, the electrochemical method has its own advantages to produce the CeO2film comparing with the other preparation methods (mainly using chemical vapor deposition, sol-gel and magnetron sputtering deposition methods).So far, the thickness of the CeO2films obtained by anodic or cathodic electrodeposition technique is thin due to CeO2has a lower conductivity. Since Ce3+can be directly oxidized to soluble Ce(OH)22+, therefore, it is believed that anodic deposition is more appropriate than cathodic to fabricate pure CeO2film. In addition, it is well known that CeO2is a common semiconductor material, a cerain wavelength (energy) of photo illumination can greatly improve the conductivity, therefore, enhancing the film thickness.This dissertation systematicly investigated the processes, growth mechanisms of anodic electrodeposition and photo assisting anodic electrodeposition of nanostructured CeO2thin film, the relationship between the film color and film structure and composition, the relationship between the electrochemical potential noise and the fil, structure, the corrosion resistance of the as-deposited nanostructured CeO2thin films.The third chapter mainly explores the processes of anodic electrodeposition of nanostructured CeO2thin film under dark condition (without photo illumination).The influences of deposition potential, bath temperature, bath pH, oxygen and ethanol content on the compact, structure and corrosion resistance of CeO2film have been investigeted. The optimized deposition parameters are0.8V,50℃, pH6.2,10%V/V ethanol, natural aeration. Under dark state, ethanol content plays a significant role on deposition of CeO2films. The addition of ethanol can reduce the bath conductivity and increase the anodic polarization of the film deposition, result in a higher content of ethanol is not appropriate for anodic deposition of ceria film. In addition, the beneficial effects of cerium oxides on the corrosion behavior of stainless steel may be mainly attributed to the following synergistic factors.(a) The high affinity of ceria towards oxygen can possibly decrease the oxygen content adjacent to the film and also inhibits oxygen diffusion towards steel.(b) The corrosion of stainless steel in NaCl solution is electrochemical in nature, which involves electron exchange between the anodic and cathodic sites, the semiconductor or even the insulator characteristic of CeO2makes the electron diffusion onto the stainless steel surface be inhibited, and also makes the cathodic reduction of oxygen and protons take place at a higher overvoltage;(c) The product of cathodic reaction (OH") will generate a slight alkaline state adjacent to the electrode surface, and consequently further stabilize the cerium oxidant in the coatings,(d) The standard redox potential of the reaction between CeO2and Ce3+(CeO2(s)+4H++e"→Ce3++2H2O) is more than1.26V, which makes the deposited CeO2stable on the specimen surface in a wide potential range.The fourth chapter analyzes the composition-depended color evolution of anodic electrodeposited cerium oxide thin film. Cerium oxide thin films with various colors were deposited onto316L stainless steel using anodic electrodeposition approach. Results show that the film color evolves from golden yellow (1.5h), purple (3h), blue (5h), grey-green (7h) and turns back to yellow (12h) again, and all these colored films are stable in the ambient conditions. The film color is related to both the contents of Ce(Ⅲ) and hydroxide species in the films. The relative content of Ce(Ⅲ) increases from21.19%to27.57%with increasing the deposition time from1.5h to7 h, and then decreases to22.93%for the film deposited at12h. The content of hydroxide species shows the same variation trend as Ce(Ⅲ) with the deposition time.The fifth chapter explores the effect of ethanol contents and photo energies on photo assisting anodic electrochemical deposition of CeO2film. Results show that light illumination favors the film growth and improves their crystallinity. Shortening the incident wavelength (increase photo energy) favors the deposition of CeO2thin film and enhances the crystallinity of the film. With ethanol content increases from0%V/V to70%V/V in the depositon bath solution, the corrosion resistance of the deposited CeO2thin film increased.The sixth chapter analyzes the nucleation/growth information of anodic electrochemical deposition of CeO2film. Results show that the anodic deposition of CeO2film is limited by electrochemical process and following the two-dimensional (2D) layered growth mechanism. In addition, the relationship between EEN characteristics and CeO2structure has been eastablished. The energy of nucleation and growth of CeO2thin film deposition process have been analyzed, and the relationship between nucleation energy/growth energy and the corresponding film structure have established using glancing angle XRD.