**3. Thin film metal oxide gas sensors**

In semiconductor gas sensor applications, advantages of thin film using are low resource waste, high surface/volume ratio, low power consumption, easy compliance with integrated circuits and easy alteration of electrical properties with changing film production parameters. Thin film technology allows the film properties to be changed by keeping the thickness parameter under considerable control. In this way, thin films are easily integrated into the device during the material production process. They can also be used as electronic circuit elements by acting as new materials when they are produced in multilayer.

Thin film metal oxides are used by the detection a lot of gas types such as Carbon-based (CO, CO2, CH4, C2H5OH, C3H8), nitrogen-based (NH3, NO, NO2), H2, H2S, ethanol, acetone, LPG and moisture.

The large number of grain boundaries in thin film polycrystalline MO's limits mobility, thus reducing carrier concentration and decreasing gas sensitivity. The presence of depletion layers in these grain boundaries is the most important factor that reduces mobility. Grain boundaries affect mobility due to their positioning to potential barriers with high intensity defect levels.

There have been a lot of ZnO thin film study to detect NO2 gas sensing that have been reported with different morphologies nanowires, nanorods [21], nanoprisms [22] and nanospheres [23] in order to enhance surface area. In 2019, Duoc et al. synthesized ZnO nanowires and nanorods with using on-chip grown via hydrothermal method at room temperature NO2 gas sensing [24]. The diameter of these structures severely affected gas sensing, indicating nanowires were more sensitive than nanorods. ZnO nanobarded fibers were synthesized by electrospinning and chemical bath deposition. These structures showed improved NO2 detection performance for gas concentrations up to 30 ppb [25].

In our previous study, nanoflower shaped n-type ZnO films synthesized by chemical bath deposition and their 0.5 ppm NO2 gas sensing was detected, showing in **Figures 3** and **4** [26]. Operating temperature was chosen at 200°C due to statical recovery kinetics were worse under this temperature. Oxygen vacancies (oxygendeficient ZnO) acted as adsorption sites, electron donor sites and nucleation centers for small metal clusters. Reaction on the ZnO film surface was given by two

equations between exposing oxidizing type NO2 molecules and oxygen species in the ZnO grain boundaries;

$$\text{2O}\_{2(\text{ads.})} + \text{2e}^- \leftrightarrow \text{2O}^- \text{(ads.)} \tag{1}$$

$$\text{NO}\_2 + \text{O}^- \rightarrow \text{NO}\_3 + \text{e}^- \tag{2}$$

With increasing annealing temperature and thereby decreased grain sizes caused an increase surface/volume ratio and NO2 gas sensing, as expected for n-type ZnO. It was interesting that very high annealing temperature (>500°C) could lead to deterioration on the substrate/deposited layer interface, as showed in **Figure 3d**.

**Figure 3.** *SEM images of (a) ZnO and annealed ZnO films at (b) 450°C, (c) 500°C and (d) 550°C (reprinted from [26]).*
