The Formation Mechanism And Photocatalytic Property Of Plasma Electrolytic Oxide Films

Plasma electrolytic oxidation?PEO?is a novel technique which has found applications in surface enhancement for metals/alloys and eco-friendly synthesis for photocatalytic films.By tuning electrical parameters and electrolytic components,any elements from the matrix and electrolyte theoretically can be add into the PEO films,in forms of doping atoms or compositions,due to plasma discharges at electrolyte/oxide film interface during PEO process.The PEO is therefore gaining increasing amount of attention and being utilized to enhance metal surface and reduce pollutants.Current studies on PEO technique mainly focus on following aspects:?a?fundamental understanding of PEO growth mechanisms.i.e.,mechanisms of PEO film's structure and compositions tuning by varying electrolyte,metal matrix and applied electrical field;?b?developing techniques for metal surface enhancement and?c?developing techniques for photocatalytic degradation of environmental pollutants.In this thesis,PEO films were farbricated on pure Al,Mg,and Ti.We systematically studied their growth mechanisms using advanced electron microscopies,i.e.,Scanning Electron Microscopy and Transmission Electron Microscopy.Innovative models were proposed for describing the growth mechanism of PEO films.Meanswhile the concepts of“High Voltage Anodic Oxidation”was firstly established and verified as a novel technical strategies to farbricate the high-efficiently photocatalytic films.The thesis could be summarized as below:1)The discharge channels in outer layer,internal layer and amorphous alumina layer?AAL?of Aluminum PEO films were studied in detail.An intermediate state of AAL,featured by unusually large thickness?².7?m?and a cross-section typical of the ionic migration processes,was reported for the first time.A steady growth mechanism was proposed:discharges in PEO process are ignited in the gas-filled channels while passing through the outer layer and the internal layer,which act as an excellent conductor;it triggers a major voltage drop into the AAL,and results in the ionic migration of the innermost AAL at the field strength of¹ V/nm.2)Formation mechanisms in initial stages of PEO process were revealed.The present evidences show that PEO film growth in first three consecutive stages I-III is dominated by ionic migration behavior inside the AAL.The evolution of morphology in the PEO process was attributed to the interfacial reactions:in stage I,the double layer at oxide/aqueous solution interface acts as a barrier and hinders the anions migrating to the anode,which generates extra heat and makes the stage I unsustainable.To distinguish this stage from conventional anodic oxidation,we named the stage I in PEO process as“high voltage anodic oxidation?HVAO?”.In stage II and stage III,the gas discharge in oxide/aqueous solution provides oxygen ions for the PEO film oxidation.3)The self-repairing capability during the PEO process was reported for the first time.PEO film could recover from breakdown damage by decreasing porosity,tuning the components and improving corrosion resistance through depositing electrolyte anions into the breakdown pores.It was found that the pores were repaired by silica fillings formed by the plasma-induced deposition of anions(SiO₃^2-)in the electrolyte.The presence of trace Mg and minor K elements indicates the contribution of ionic migration and plasma collision.4)The deposition and migration of electrolytic Si in PEO films depended on the microstructure of the metal matrix oxide.In the case of Al,Si was only deposited on outmost plasma-modified layer and at the bottom of the ionic migration pore,rather than migrating into the compact AAL.In the case of Mg,the oxide layer was composed of nanocrystalline MgO particles,whose grain boundaries act as ionic channels for SiO₄^4-radicals migrating,and result in a double layer structure of Mg₂SiO₄/MgO film on the Mg matrix.5)TiO_2-x/WO_3-x PEO films were fabricated on pure Ti by HVAO.These films have better photocatalytic performance than those fabricated at plasma discharge stage.Characterized by surface analysis and electron microscopy,the HVAO films has an amorphous structure and is rich in oxygen vacancy.It has the advantages of absorbing photons from both spectra of visible light and far infrared light.The double layer at oxide film/electrolyte interface prevent the anions from migrating into the anode.The HVAO films at the initial plasma discharge of TiO_2-x/WO_3-x have the highest activation,when the the double layer is most efficient in prevention.