Titanium dioxide is widely used in optoelectronic materials, catalytic materials, antibacterial materials, and other fields due to its non-toxic and environmentally friendly nature, high oxidation activity, good optoelectronic properties, and simple preparation process. In the process of photocatalytic reaction, titanium dioxide, as a catalytic material, can only excite electron hole pairs and trigger charge migration and separation by obtaining energy greater than its bandgap width. Some excited charges can move to the reaction interface of photocatalytic reaction to participate in photocatalytic reaction, while others may recombine and disappear. The more effective light a semiconductor absorbs, the more excited charges it generates on its surface, and the higher its photocatalytic activity. However, ordinary titanium dioxide materials are greatly limited in their applications due to their fast electron hole recombination rate and wide photonic bandgap. TiO, The bandgap width is 3.0~3.2eV, which can only absorb energy in the ultraviolet region and cannot absorb visible light or infrared light, resulting in low solar energy utilization efficiency. In addition, the quantum efficiency of TiO is greatly limited by another defect in the material. Therefore, the current focus of research on titanium dioxide materials is on modified TiO, which exhibits quantum efficiency and visible light responsiveness. Recent studies have found that doping modification and defect introduction of titanium dioxide materials can significantly reduce the electron hole recombination rate and bandgap width, and significantly improve their optoelectronic properties. Oxygen vacancies, as a common crystal defect, can regulate the electronic structure and surface properties of titanium dioxide, thereby improving its photoelectric and catalytic properties, and have been widely studied. Due to the presence of oxygen vacancies in titanium dioxide, its electronic energy level changes, leading to a change in the absorption wavelength range of light, resulting in significant color changes on the surface of titanium dioxide. Titanium dioxide rich in oxygen vacancies presents blue, gray, and black colors based on the content of their oxidation vacancies.

Plasma treatment

At present, although the methods used for titanium dioxide modification are effective, most of them rely on high temperatures, complex steps, long processing cycles, and poor reproducibility. In recent years, plasma treatment has been increasingly used to modify photocatalysts to enhance their catalytic performance. The obvious advantages of plasma treatment include: 1) Plasma treatment is a fast and simple process; 2) Plasma treatment is a low-energy process that does not require solvents or generate chemical waste; 3) For a series of surface modifications, they can be implemented in a single processing environment. Plasma treatment can be conditionally decomposed into several basic processes: first, energetic free radicals attack the substrate surface, and then, a series of physical and chemical reactions occur when energy is transferred from the free radicals to the substrate material, thereby changing the physical and chemical properties of the material surface.

Valence State Analysis of Plasma Treatment on Surface Elements of TiO2 (XPS)

In order to further investigate the effect of plasma treatment on the surface state of samples, XPS tests were conducted on TiO2-0, TiO2-150, and TiO2-250 in this study. The change in the chemical state of the elements may correspond to the presence of vacancies. As shown in Figure 1 (a), there is no significant difference in the total spectrum of the sample before and after plasma treatment, and plasma treatment does not change the composition of the sample. The spectrum shows that the sample contains two elements, Ti and O. Subsequently, charge calibration was performed through C1s orbitals as shown in Figure 1 (b). As shown in Figure 1 (c), in the sample without plasma treatment, O1s split into two main characteristic peaks of 529.71eV and 531.30eV, respectively. After plasma treatment, the peaks of O1s in the signals of TiO2-150 and TiO2-250 samples shifted towards lower energy levels, and the corresponding characteristic peaks were 529.07eV, 530.67eV, and 529.06eV, 529.79eV, respectively. According to relevant research reports, a decrease in the distance between two peaks and a shift in the extreme energy level may be due to the presence of O vacancies. As shown in Figure 1 (d), the Ti2p3/2 and Ti2p1/2 orbitals were at 458.46eV and 464.12eV before plasma treatment. After plasma treatment, both orbitals shifted towards lower energy levels. The Ti2p3/2 and Ti2p1/2 orbitals of TiO2-150 and TiO2-250 samples moved to 457.84eV and 463.50eV, respectively. The energy level shift may be due to the formation of O defects around Ti, which caused a change in the valence state of the Ti element. The equal distance between the peaks of Ti2p3/2 and Ti2p1/2 orbitals in the three samples indicates that the Ti element itself is relatively stable before and after plasma treatment. XPS testing further showed that O defects were generated on the surface of the sample after plasma treatment.

Figure 1 XPS spectra of TiO2-0, TiO2-150, and TiO2-250: (a) full spectrum; (b)C 1 s; (c)O 1 s; (d)Ti 2p

In summary, plasma treatment is a method of using gases such as hydrogen and nitrogen to generate plasma through discharge to reduce the surface of titanium dioxide and prepare titanium dioxide rich in oxygen vacancies. This method has simple process conditions, does not produce wastewater or exhaust gas, is low-carbon and energy-saving, and meets environmental protection requirements.