Study On The Mechanism Of Electro-Deposition And Properties Of CeO₂ Thin Film

Cerium dioxide,or ceria(CeO2),possesses a cubic fluorite-type crystal structure,particularly versatile in accommodating oxygen deficiency.Thanks to their chemical and physical characteristics,ceria thin films have emerged as a fascinating material due to many successful technological applications such as solid oxide fuel cells,catalysts for three-way catalysis of exhaust gas from automobiles,UV absorbent,pharmacology and coatings for corrosion protection of many metals and alloys.The electrodeposition method is one of the most advantageous options because the process can be performed under environmentally friendly conditions.Furthermore,the technique allows for the control of the film composition,thickness,and morphology simply by adjusting the electrochemical parameters.This dissertation systematically investigated the influence of photo generated electrons;synergistic effect of nitrate and ethanol on photo-assisted anodic deposition of ceria thin films;structural,optical,electrical and magnetic properties of CeO2 thin films simultaneously prepared by anodic and cathodic electrodeposition;nucleation and growth mechanism of CeO2 anodic electrodeposition.The third chapter mainly explores the influence of ethanol content in the precursor solution on photo-assisted anodic deposition of ceria thin films.The results show that both the film thickness and particle size of ceria layers increase whereas both the film porosity and Ce(Ⅲ)content decrease with the augment of ethanol content from 0 to 70%;The important role of ethanol content in enhancing the anodic deposition rate of ceria film may be mostly achieved by the simultaneously photo generated electrons,which can largely enhance the electrochemical activity of ceria film,accelerates the cathodic reactions that in turn speeds up the anodic deposition of ceria films,slightly increases the surface tension of the solution and diminishes the wettability of the electrode due to charge effect,and decreases both the anodic polarization of film formation and the resistivity of bath solution.The fourth chapter investigate the influence of nitrate and its synergistic effect with ethanol on the photo-assisted anodic electrodeposition of ceria thin films.The results show that the current density,film thickness,particle size and Ce(IV)content increase with increasing nitrate concentration in water solution whereas decrease in 70%v/v ethanol-water solution.NO3-can effectively absorb photons to generate ·OH radicals so that the addition of NO3-into the water bath can promote the growth of Ce02 films.However,ethanol as a hole scavenger is mainly responsible for the consumption of photogenerated holes and ·OH radicals,and NO3-mainly for the consumption of electrons,which consequently inhibits the formation reactions of CeO2 films in ethanol-water solution.The fifth chapter focuses on the effect of negative potential on the microstructure and on the optical,electrical and magnetic properties of the CeO2 thin films deposited on the anode and cathode.The results show that CeO2 films on the anode are slightly affected by the potential,but the particle size,crystal orientation,strain,film thickness,resistivity and Ce(Ⅲ)content of the films on the cathode increases with increasing potential on the 316L substrate.For all of the samples deposited at different potentials,the coercivity field and the remnant magnetization are almost same.Two different trends are observed that the saturation magnetizations only slightly increases with potential for Ce02 films deposited on anode,whereas it initially increases and subsequently decreases for the Ce02 films deposited on cathode,which could be attributed to the synergistic effects of the crystalline quality and oxygen vacancies and the effect of oxygen vacancies is more prominent.On the other hand,redshift(Ed changed from 3.95 eV to 3.56 eV and Ei changed from 3.42 eV to 3.04 eV)occurring in the absorption spectrum of CeO2 deposited on the ITO-coated glass cathode indicates that the content of Ce(III)in the cathodic films is dependent on the adopted substrates and decreases as the applied potential is increased.The sixth chapter analyzes the initial crystallization nucleation and growth mechanism of CeO2 anodic electrodeposition.The results show that the deposition process of ceria is controlled by diffusion under our experimental conditions.The dominated nucleation and growth mechanism for CeO2 deposition transforms from two-dimensional(2D)to three-dimensional(3D)with increasing the deposition time,and 3D process starts prior to the completion of 2D layers.The poor conductivity of the preformed CeO2 film causes the high deposition overpotential and therefore refines the grain size of CeO2 particles.