## 3 Results and Discussion  
Photodegradation Characteristics of Acetaldehyde against Heat-Treatment Temperature during Catalyst Preparation  
The optimum firing temperature was first determined among the three heat-treatment temperatures of 450, 650, and 850 \({}^{\circ}\)C. Acetaldehyde decomposition experiments were performed on three heat-treated catalysts, and the best conversion was obtained using the catalyst heat-treated at 650 \({}^{\circ}\)C. At an initial concentration of 1800 ppm, a maximum conversion rate of 57% was observed after 30 min of light source irradiation, which stabilized at 50% after 2 h of irradiation. The lowest conversion rate was achieved while testing catalysts heat-treated at 850 \({}^{\circ}\)C. The rate increased from 8% after the initial 30 min to 23% after 75 min, with a marginal conversion rate of 17-18%. The maximum conversion rate achieved using the catalyst treated at 450 \({}^{\circ}\)C was 43%, which was lower than that of the catalyst treated at 650 \({}^{\circ}\)C (Figure 4). FE-SEM analysis was performed to observe the surface morphology of the three catalysts (Figure 5). Overall, the sol-gel thin film shrank on the surface of the alumina substrate, the pores disappeared, and the surface density increased as the heat treatment temperature increased. This was not observed for the alumina substrate without TiO\({}_{2}\) surface treatment (control). However, surface cracks occurred in the heat-treated sample,  
Figure 4: Acetaldehyde photodegradation of the catalyst according to heat-treatment temperature: (a) 450 \({}^{\circ}\)C, (b) 650 \({}^{\circ}\)C, and (c) 850 \({}^{\circ}\)C (cc (sccm): standard cubic centimeter per minute (cm\({}^{3}\)/min)). and the density and number of cracks increased as the heat-treatment temperature increased. At 450 \({}^{\circ}\)C, the TiO\({}_{2}\) thin film was not uniformly coated on the surface of the alumina substrate and cracks in the thin film appeared to coexist. Increasing the temperature from 450 \({}^{\circ}\)C to 650 \({}^{\circ}\)C and 850 \({}^{\circ}\)C increased the density and number of cracks, and most cracks were observed in the catalyst heat-treated at 850 \({}^{\circ}\)C.  
The TiO\({}_{2}\) thin film tended to shrink as the heat treatment progressed, causing the pores to disappear and densify. In addition, as the heat treatment temperature increased, tensile stress was generated. Moreover, because the thermal expansion coefficient of the alumina substrate was low, it seemed that cracks occurred on the surface when the thin film was accompanied by additional shrinkage. This can be noted in a previous study that stated the effect of annealing temperature on the crystallization stage of TiO\({}_{2}\) thin films [39]. For cracks in such shrinkage, it seems necessary to consider the variables of the type of solvent in the initial synthesis or the firing temperature increment conditions. Figure 6 shows the anatase phase, coexisting rutile phase, and anatase and rutile phases of the catalysts heat-treated at 450, 650, and 850 \({}^{\circ}\)C, respectively. In particular, the coexistence of the rutile and anatase phases occurred in the catalyst at 650 \({}^{\circ}\)C, which had the highest conversion rate in our experiment. As the firing temperature increased, the rutile phase was formed and showed low photocatalytic activity. The photocatalytic activity of the TiO\({}_{2}\) thin film coated on alumina was highly dependent on the firing temperature, and the surface area of the rutile phase decreased due to the sintering and growth of the TiO\({}_{2}\) crystal phase [40]. The crystal sizes of the treated catalysts at 450, 650, and 850 \({}^{\circ}\)C were calculated to be 14, 31, and 33 nm, respectively, using the Scherrer equation. Therefore, as the sintering temperature increased, the crystal size increased. This is because the sintering temperature can increase the grain size and TiO\({}_{2}\) crystal growth can be induced as the sintering temperature rises. It can be seen that they correspond [41,42].