## 3 Thin-Film Materials and Their Applications
### Inorganic Thin-Film Materials
#### 3.1.1 Metal Oxide Thin-Films  
The development of inorganic materials, especially their oxides, has greatly promoted social progress. Binary oxides with the advantages of simple structure, easy control of material composition, low cost, good stability, and compatible with complementary metal-oxide-semiconductor (CMOS) technology, stays as an important material system in the field of microelectronics. Binary metal oxide thin-films refer to a class of substances in the form of M\({}^{\text{I}}\)O- M\({}^{\text{II}}\)O. It has some relatively simple forms like ZnO, TiO\({}_{2}\), MgO, SnO\({}_{2}\), or MoO\({}_{3}\), while having some complex forms like Fe\({}_{2}\)O\({}_{3}\) or V\({}_{2}\)O\({}_{5}\). There is not much of a difference between these molecular formulas. But thin films made from these different materials have different functions in the environment. Taking into consideration the promising physical and chemical properties of metal oxide thin-films, they could be applied to gas sensors for waste gas treatment. Some of these can absorb CO\({}_{2}\) or NH\({}_{3}\), while some others can catalyze CO into CO\({}_{2}\). Other special usage can also be found in the application of metal oxide thin-films, as shown in Figure 2[36]. Mechanisms of waste gas treatment by the thin film can be divided into two major categories, one is the approach to reduce the gasses' electron donors. In this case, the surface complex is taken place by the solid-gas interaction, so that the chemical nature of the modifier in acid-basic or redox reactions plays a decisive role. When there are some highly efficient platinum metals catalysts (Pt, Pd Ru, Rh) in the reactions, the speed and effect of reflection will be greatly improved. The other approach is acted as oxidizing gases' electron acceptors. In this case, the surface of metal oxide thin-films in the air is covered by chemisorbed oxygen species (O\({}^{2-}\), O\({}_{2}\), O\({}^{-}\)). At the appropriate temperatures, these thin-films themselves become intermediate catalysts. They make it easier for NO gases in the air to combine with O\({}_{2}\) to produce NO\({}_{2}\), resulting in easier management. In some specific conditions, thin films could even convert O\({}_{2}\) to O\({}_{3}\), which could be used in sterilization in the environment. Figure 2: Oxide thin-films with various properties in the application of the waste-gas treatment field. Left part: Reducing gasses’ electron donors to make the waste-gas harmless through the ion-exchange membrane; Right part: Oxidizing gases’ electron acceptors to make the waste-gas harmless through oxidation catalysis. Among metal oxide thin-films, TiO\({}_{2}\) thin film, a traditional metal oxide material, processes the photocatalytic decontamination ability to deprive of requiring harsh reagents or yielding toxic byproducts, which is also one of the most efficient and environmental-friendly materials for the degradation of contaminants [83; 84; 85]. For example, [PERSON] et al. used a TiO\({}_{2}\) film (containing gold nanoparticles) as a high-efficient photocatalysis to develop water-driven Mg-based micromotors for photocatalytic decomposition of biological and chemical warfare agents [83]. Compared with TiO\({}_{2}\), ZnO thin film, as a promising metal-oxide semiconducting material, has been widely studied and investigated in many fields including transistors, sensors, energy harvester, catalysis, and resistive switching devices because of biocompatibility, chemical stability, high electron mobility, electrochemical activity, ease of synthesis, and high surface-to-volume ratio [86; 87; 88; 89; 90; 42]. [PERSON] et al. successfully synthesized hexagonal ZnO thin films using the chemical spraying pyrolysis technique, which was used for photo electrocatalytic degradation of terephthalatic acid as a model organic pollutant and detecting CO\({}_{2}\) gas with high selectivity, good response, and short recovery time periods [92].