## 3 Thin-Film Materials and Their Applications
### Inorganic Thin-Film Materials
#### 3.1.1 Metal Oxide Thin-Films
The results showed that ZnO thin film was a great potential candidate for both photocatalytic and CO\({}_{2}\) gas sensing multifunctional applications. In 2018, [PERSON] et al. proposed that Mg-ZnO Au hybrids can autonomously generate H\({}_{2}\)O\({}_{2}\) without an instrument for disinfection and organic pollutant degradations [92]. Their system adopted degradable metal (such as Mg in their work) to activate the photocatalytic reaction of transition metal oxides (such as ZnO in their work) through a galvanic corrosion process. Other types of oxide thin films are commonly used in the detection of NH\({}_{3}\) and C\({}_{2}\)H\({}_{5}\)OH. The conductivity of thin films varies when the potency of NH\({}_{3}\) differs in the environment. Some NH\({}_{3}\) detection sensors can be made from this type of thin film. Taking the SnO\({}_{2}\)-MoO\({}_{3}\) film as an example, its signal intensity varies with corresponding concentration. So, we could conclude that it is the NH\({}_{3}\) chemisorption on Lewis acid sites which changes the conductivity [36]. Despite the ability to detect NH\({}_{3}\), this type of thin films makes dehydrogenation easier which means it can lead to a higher response to C\({}_{2}\)H\({}_{5}\)OH due to a reaction with chemisorbed oxygen O\({}_{8}\)\({}^{-}\):  
\[\text{C}_{2}\text{H}_{5}\text{OH}+\text{O}_{8}^{-}\rightarrow\text{CH}_{3} \text{CHO}+\text{H}_{2}\text{O}+\text{e}^{-}, \tag{1}\]  
From this reaction, the amount of electrons is increasing, while the conductivity also changes simultaneously. This type of detection is more than just a method of gas detection because it changes the reactant. Therefore, this application is a combination of the two mechanisms above [93; 94; 95]. We can conclude that this type of oxidation reaction is usually applied to the treatment of harmful gases in air pollution. When harmful gases in the environment pass through these thin films at a specific temperature, they can be easily converted into gases that are harmless or have a low pollution coefficient. These sensors play a huge role where environmental monitoring is needed. They can provide precise supervisory in real-time under all conditions. In addition to the above gas detection, binary metal oxides are also considered to be an upcoming new semiconducting material stated in post Moore's law. In the earliest literature, binary oxides have been used in bioresorbable electronics [74] as one of the key materials to fabricate TFTs [96], mechanical energy harvesters [97], supercapacitors [15; 98], and non-volatile storage materials [99]. In 2013, [PERSON] et al. used ZnO thin-film as the active layer to fabricate water-soluble ZnO TFTs and energy harvesters/strain gauges [42]. The other components of the ZnO-based devices presented include Mg for electrodes and interconnection, MgO for the dielectrics, and films of silk fibroin for the substrate and package. They studied the dissolution properties of the ZnO-based devices in phosphate buffer saline (PBS) and deionized water (DI water). In 2014, [PERSON] et al. reported biodegradable/transient CMOS on biodegradable polymers [9]. This CMOS consists of SiO\({}_{2}\) layer for gate and interlayer dielectrics, Mg metal-layer for source, drain, gate contacts, and interconnects. More importantly, the poly (methyl methacrylate) (PMMA) and diluted polyimide (D-PI) was used as the sacrificial layer, which is for transferring the CMOS from Si substrate to a biodegradable substrate, such as poly lacticco-glycolic acid (PLGA) in that work. In 2018, [PERSON] et al. used MgO thin-film as a resistive-switching layer to fabricate physical transient resistive random access memory (RRAM) with Mg/MgO/W structure [100]. The MgO thin-films were fabricated by the solution process and spin-coating, respectively. After which, they studied the resistive switching performances of Mg/solution processed MgO/W and Mg/sputtered MgO/W devices in great details.