I. Preparation Methods of TiO2 Oxide Film
1. Atmosphere Heating Oxidation Method
Titanium oxidizes in the atmosphere. As the heating time increases, the thickness of the oxide film gradually increases, resulting in different tones that change from yellow to cyan and then to purple. The advantage of this method is that it can color titanium cheaply and in large quantities, obtaining a surface coloring film with good adhesion. However, the color variation is limited and the color range is not rich. The color uniformity and repeatability are poor, and the color is difficult to control accurately. Besides being heated and colored in an oxygen-containing atmosphere, when heated in a nitrogen atmosphere, a TiN film forms on the titanium surface, presenting a golden yellow color and having high wear resistance.
2. Anodic Oxidation Method
Voltage is applied between the titanium anode and the stainless steel or aluminum cathode in an electrolyte, and anodic oxidation occurs through an electrochemical reaction, forming a colored oxide film. The electrolytes for anodic oxidation include aqueous solutions, non-aqueous solutions, and molten salts. Usually, aqueous solutions of phosphoric acid, boric acid, and their salts are used to form thick oxide films; while molten salts and non-aqueous solutions are used to generate thinner oxide films. The most significant factor affecting the thickness of the TiO2 oxide film is the applied voltage, and its thickness is generally proportional to the applied voltage. Therefore, by changing the voltage, the thickness of the oxide film can be controlled, and thus the color of the oxide film can be controlled. This method can be used to prepare oxide films of different compositions and properties on valve metals (Zr, W, Nb, Ta, Al, etc.). The oxide films have characteristics such as density, stability, and strong adhesion, and can be used in corrosion-resistant coatings, dielectrics for capacitors, and gate oxides for transistors.
3. PVD (Physical Vapor Deposition) is a process in which liquid or solid materials are vaporized and then deposited on a substrate.
PVD techniques include sputtering, ion plating, thermal evaporation, laser evaporation, and ion implantation. PVD can be used to prepare films or multilayer films with thicknesses ranging from a few nanometers to a few micrometers. Different doping can be achieved by introducing different gases. The substrate material can affect the crystallization of TiO2 films. Some researchers have prepared anatase and rutile structured films on glass and stainless steel substrates respectively by PVD.
4. CVD (Chemical Vapor Deposition) is a process in which the substrate material reacts chemically with gas to deposit a film on the substrate.
Compared with PVD, CVD can deposit films on substrates with complex shapes, which is an advantage that PVD cannot match. By combining plasma, ions, lasers and other means, the deposition temperature of CVD can be reduced or the deposition rate can be increased. There have been many studies on the CVD preparation technology of TiO2 films, which have proved that the substrate material and deposition temperature have an important influence on the film structure. For example, as the deposition temperature increases, the grain size of TiO2 films increases.
5. The hydrothermal method involves reactions in a high-pressure autoclave under controlled temperature (about 200℃) and pressure (<10MPa) in an aqueous solution.
Some researchers have used this method to prepare TiO2 catalytic materials with different phase structures and morphologies (nanorods, nanoparticles), including three-phase structures (rutile + brookite + anatase), two-phase structures (rutile + anatase), and single-phase structures (rutile), which exhibit different catalytic activities. 6. Sol-Gel method: The Sol-Gel method uses compounds with high chemical activity as precursors, uniformly mixes the raw materials in the liquid phase, and undergoes hydrolysis and condensation reactions to finally form TiO2 with molecular or even nanostructures. This method is economical and simple, and can obtain high-purity TiO2 at near room temperature. By changing the preparation method and calcination temperature, the crystal structure of TiO2 can be adjusted to obtain TiO2 with different structures such as rutile and anatase. This method is often used in the preparation of TiO2 photocatalytic materials or TiO2 coatings with biological activity.
II. Influencing Factors of Anodized TiO2 Colored Film
1. Electrolyte
The electrolyte for anodizing can be classified into acidic electrolyte, alkaline electrolyte, and salt solution, etc. Due to the rapid dissolution of the oxide film in alkaline solution, there are relatively few studies on it. Some scholars have investigated the influence of electrolyte type on titanium oxide film and found that compared with alkaline electrolyte, the formation voltage of the oxide film in acidic electrolyte is higher. With the decrease of electrolyte concentration and temperature, the formation voltage and growth rate of the oxide film increase. With the decrease of current density and the ratio of anode to cathode area, the formation voltage of the oxide film decreases. Some scholars have anodized in alkaline electrolyte and found that the higher the concentration of the electrolyte, the greater the refractive index of the TiO2 oxide film, and it promotes the transformation of the oxide film from amorphous to crystalline state.
2. Oxidation Voltage
Common anodizing modes include constant current mode and constant voltage mode, and according to the voltage waveform, they can be further divided into DC, AC, and pulse modes, etc. Some scholars have studied the influence of potential linear scanning mode and potential step mode in acidic electrolyte and low voltage on the anodized film of pure titanium and found that in the linear scanning mode, rapid potential scanning forms amorphous structure oxide film, while slow scanning produces nanocrystals. In the step scanning mode, increasing the oxidation voltage can increase the proportion of Ti4+ in the oxide film. Anodizing at low voltage can obtain colored oxide films. While anodizing at high voltage generates electric sparks, forming a huge local electric field, thus accompanied by the crystallization or phase transformation process of the oxide, and the obtained oxide film often has better wear resistance.
3. Oxidation Time
With the increase of time, the growth trend of the oxide film thickness is generally fast at first and then slow. During the anodizing process, the growth and dissolution of the oxide film occur simultaneously. When the growth rate of the oxide film is greater than the dissolution rate, the thickness of the oxide film increases; when the growth rate of the oxide film is less than the dissolution rate, its thickness decreases. Some scholars have found that in the same electrolyte, the thickness of the oxide film of pure titanium increases with time at higher voltages, while it decreases with time at lower voltages. Some scholars have studied the long-term anodizing process of pure titanium at low voltage and found that in the formation stage of the oxide film, the thickness and crystallinity of the film increase with oxidation time, while in the incubation stage of the oxide film, the dissolution of the film accelerates and the crystallization of the oxide film slows down. Although it has been reported that oxidation time affects the crystallization process and crystallinity, it is generally believed that the crystal structure of the oxide film only depends on the applied voltage.
4. Other Factors
Before anodizing pure titanium, surface mechanical, chemical or electrochemical polishing is generally used to remove surface contaminants. After mechanical polishing, acid washing is usually carried out to remove the surface passivation film. It has been reported that the anodization process of electrochemically polished pure titanium locally undergoes oxygen evolution reaction, resulting in a thicker oxide film in that area, while the oxide film formed on the untreated rough surface is more uniform. Compared with the oxide film without polishing treatment, the oxide film obtained after polishing and anodizing has better corrosion resistance. Increasing the electrolyte temperature can increase the oxidation efficiency of the oxide film. Some scholars have found that at the same voltage, the oxide film of pure titanium is thicker and has higher crystallinity at high electrolyte temperature.