Mixing 0.2 M of titanium (IV) isopropoxide precursor (TTIP, 97%, Sigma Aldrich) and 64 ml of deionized water (DI) was used to prepare TiO₂ sol. The TTIP was added drop by drop (1 drop/minute). Then, 0.4 ml of hydrochloric acid (HCl, 37%, Merck) was poured into the solution. The solution was stirred for 3 hours. After that, the solution was allowed to age for 48 hours.

The glass substrate was submerged in the TiO₂ sol using a TEFINI Model DP1000 precision single dip coater equipment. The glass substrates were submerged in the solution with a dipping withdrawal speed of 0.5 mm/second and a dwell duration of 90 seconds to ensure minimal coating deposition, as previously investigated and verified in prior project studies [10]. To finish the single dipping number, the samples were dried at room temperature for 30 minutes before being dried in an oven at 110˚C for 30 minutes. This dipping step was repeated ten times to produce a homogeneous coating. The thin films were permitted throughout the oven to dry at 110˚C for 1 hour.

For the spin coating process, 90 μL of TiO₂ sol was dispensed onto the surface of a glass substrate using an Eppendorf pipette. The substrate was secured to a vacuum-assisted spin coater and spun at a fixed speed of 1500 rpm for 30 seconds. This process was repeated twice. The deposited TiO₂ thin films were then dried in an oven at 110˚C for 1 hour. For the final treatment, both dip-coated and spin-coated TiO₂ thin films were heat-treated at 200˚C, 300˚C, 400˚C, and 500˚C for 3 hours, with a heating rate of 5˚C/min.

Structural of the TiO₂ thin films was performed with X-ray diffractometer (XRD) PANalytical X’PERT PRO MPD Model PW 3060/60 within the range of 10˚ - 80˚. The cross-section morphology of TiO₂ thin films was observed by scanning electron microscope (SEM) with a ZEIS EVO 50. While EDX analysis was run with JEOL model JSM-6010PLUS/LV. The optical absorption of TiO₂ thin films was measured by Lambda 35 UV-Vis spectrometer, Perkin Elmer with wavelength ranges of 200 nm - 1100 nm. The band gap of the TiO₂ thin films was calculated with the Tauc plot method.

The photocatalytic test was carried out using an OSRAM model with 200 Watt of visible radiation function as a light source 16 cm distant from the dye solution. The temperature in the chamber was maintained at about 25˚C using a cooling fan. A TiO₂ thin film coating was submerged in 25 ml with a concentration of 1 x 10⁻⁶ M methylene blue (MB) in a beaker as part of this experiment [11]. To guarantee that the adsorption-desorption equilibrium was attained to radiation, the methylene blue solution with photocatalyst (TiO₂ thin film coating) sample was maintained in the dark for 30 min. The dyes were taken and the absorbance was measured every 1 hour.

The degradation of the MB solution was measured at 665 nm and the SHIMADZU UV-1700 UV-Vis spectrometer was used to analyze the degradation of MB derived by residual absorbance. The reduction in absorbance indicates that MB is degrading. Equation 1 was used to compute the percentage of MB deterioration. The test was replicated 3 times to increase the accuracy and reliability of the results.

D e g r a d a t i o n     ( % ) = C o − C C o = A o − A A o Equation 1

where Cₒ denotes the starting concentration after equilibrium adsorption, C denotes the MB solution’s response concentration, Aₒ denotes the initial absorbance, and A denotes the MB solution’s altered absorbance.

Figure 1shows the XRD patterns of the deposited TiO₂ thin films obtained through dip and spin coating at various heat treatment temperatures. As shown in Figure 1(a), the TiO₂ thin films fabricated through dip coating consist of a mixture of anatase and rutile. The anatase and rutile were compared to the standard JCPDS No. of 21-1271 and 75-1753, respectively. The TiO₂thin film heated at 200˚C was produced only anatase (101) at 2θ of 25˚. When heated at 300˚C, the anatase (101) and rutile (110) were recognized only at 2θ of 25˚ and 27˚, respectively. At 400˚C and 500˚C, three peaks of anatase (101), (200) and (105) were identified at 2θ of 25˚, 48˚ and 54˚ together with rutile (110) at 2θ of 27˚. The relative intensity of anatase and rutile peaks increased with increased heat treatment temperature. This higher relative intensity designated an increased degree of crystallization [12]. After heat treatment temperature was used, the crystallization process occurred during nucleation and growth procedures. In addition, the occurrence of phase transformations is influenced by the heat treatment temperature of the thermal treatment and surface defects of the materials [13].

Meanwhile, Figure 1(b) shows the XRD patterns of TiO₂ thin films fabricated through spin coating. The XRD patterns of TiO₂ thin film heated at 200˚C displayed brookite peaks (111) and (023) at 2θ of 31˚ and 66˚, respectively. The brookite peak (111) at 2θ of 31˚ with d-spacing of 2.8 Å was compared to the standard JCPDS No. of 84-1750 that had a standard of 2θ = 31.3˚ with d-spacing of 2.81 Å and different with the standard JCPDS No. 29-1360 (2θ = 30.8˚, d-spacing of 2.9 Å). At 300˚C and 400˚C, the XRD patterns showed only a single brookite peak (111) produced at 2θ of 31˚. As can be seen from the figure, the relative intensity of the brookite (111) showed a decrease with increased heat treatment temperature. Once the heat treatment temperature was raised to 500˚C, the XRD pattern became amorphous, indicating that no TiO₂ crystalline phase formed. The absence of brookite production at high temperatures of 500˚C may be due to its thermodynamically metastable nature, which prefers to exist at low temperatures [11]. Haggerty et al. [14] stated that designing a synthetic approach to a metastable phase of brookite necessitates an understanding of both equilibrium and non-equilibrium processes that may occur. Although phase diagrams of the bulk Ti-O system under standard conditions have been developed, phase selection during synthesis is not just an issue of bulk equilibrium thermodynamics, as indicated by the prevalence of metastable, non-equilibrium synthesis products such as brookite. As previously observed in comparable ionic systems, synthesis routes to various stable and metastable phases can be influenced by finite-size effects, nucleation kinetics, bulk and surface off-stoichiometry.

This is also consistent with the findings of Allen et al. [15] who found that at 110˚C, 39.7% of the brookite crystalline phase had dropped to 15.1% when the temperature was raised to 600˚C. Furthermore, at temperatures of 700˚C and higher, brookite is not detected. It seems to be possible that the absence of the TiO₂ crystalline phase is related to the lack of a solvent in the sol formulation. In the sol-gel procedure, the solvent slows the rate of hydrolysis and condensation. Thus, during the TiO₂ sol preparation, the precursor of TTIP was added at 1 drop per minute to control the hydrolysis process [10]. In addition, the use of HCl as a catalyst also controls the TiO₂ hydrolysis and nucleation rates. Herman et al. [16] observed that the desorption temperature for water is lower (−23.15˚C) compared to methanol (336.85˚C). As a result, it is predicted that in this study, where no solvent was employed compared to the previous study that used a common TiO₂ sol formulation, a faster rate of hydration would be seen, particularly at high temperatures, preventing crystallization at 500˚C.

The XRD results reveal that TiO₂ phase development is influenced by deposition methods. Dip coating produces a mixture of anatase and rutile, consistent with Yazid et al. [17] and Bakri et al. [12] while spin coating predominantly forms brookite, aligning with Komaraiah et al. [18] and Singh et al. [19]. The deposition method affects crystallization: spin coating promotes brookite growth through uniform seed layers, while dip coating relies on gravitational forces. WAXS analysis confirmed TiO₂ crystalline phases, unlike SAXS. These findings highlight the critical role of deposition techniques in tailoring TiO₂ thin film structures for targeted applications. Overall, the XRD analysis confirms that the TiO₂ phase formation is strongly influenced by the deposition method, with dip coating producing a mixed anatase-rutile structure, while spin coating predominantly forms brookite.

Figure 1. XRD patterns of the deposited TiO₂ thin films obtained through (a) dip and (b) spin coating at various heat treatment temperatures.

Figure 2 shows the cross-sectional morphology of the TiO₂ thin films deposited via dip and spin coating at various heat treatment temperatures. From the cross-sectional morphology of the TiO₂ thin films, the coating thickness can be determined. The cross-sectional morphology for the TiO₂ thin film deposited via dip coating shows that the thickness coating at 200˚C was 1005.9 nm. The thickness of the TiO₂ thin film deposited via dip coating decreased to 745.9 nm and 320.5 nm at 300˚C and 400˚C, respectively. However, when the heat treatment temperature was increased to 500˚C, the coating thickness of the TiO₂ thin film deposited via dip coating was increased to 679.2 nm. This is due to the agglomerate particle that can be observed from the surface morphology of the thin film.

Meanwhile, for spin coating, the cross-sectional morphology of the TiO₂ thin film at 200˚C shows that the coating thickness was 512.9 nm. When increased the heat treatment temperature to 300˚C, the coating thickness of the TiO₂ thin film was 618.7 nm. The thickness of the TiO₂ thin film deposited via spin coating at 400˚C and 500˚C was 436.6 nm and 350.6 nm, respectively. An increase in heat treatment temperature shows a decrease in the coating thickness of the TiO₂ thin film deposited via spin coating. The increase and vice versa of the coating thickness of the TiO₂ thin film is possible due to the pore size increasing as the heat treatment temperature increases [20]. The water permeance of thin films increases at first with increasing heat treatment temperature and then drops at a heat treatment temperature turning point.

Figure 2.Cross-sectional view of the TiO₂ thin films.

Figure 3shows FTIR spectra of TiO₂thin film deposited via dip and spin coating heated at various heat treatment temperatures. For the dip coating (Figure 3(a)), a mixture of anatase and rutile film had the transmittance bands around 400 cm⁻¹ - 800 cm⁻¹ denoted to the Ti-O and/or Ti-O-Ti bonds regardless of any heat treatment temperature [21]. The transmittance band at 1532 cm⁻¹ - 1693 cm⁻¹ shows that the anatase and rutile film was assigned to the O-H bending mode when heated at 200˚C, 300˚C and 400˚C. Increased heat treatment temperature at 500˚C shows there is no peak at 1532 cm⁻¹ - 1693 cm⁻¹ and indicates that the O-H bending mode was completely removed from the film. Sadek et al. [21] also observed the intensity of the peak-related water absorptions decreased by increasing the annealing temperature. This is due to the elimination of O-H during the heating process, possibly by hydrogen diffusion out of crystal and oxidation of Ti3+ to Ti4+, proceed concomitantly. The FTIR spectra also show the transmittance band in the range of 2500 cm⁻¹ - 3746 cm⁻¹ of diluted titanium tetraisopropoxide (TTIP) in an aqueous solution due to Ti-OH and –OH vibrations [22]. This indicated the stretching vibration of the surface hydroxyl group or adsorbed water on the surface of TiO₂ samples [23].

Figure 3.FTIR spectra of TiO₂ thin films deposited via (a) dip and (b) spin coating heated at various heat treatment temperatures.

While for the spin coating (Figure 3(b)), the transmittance band also existed at 400 cm⁻¹ - 800 cm⁻¹ was credited to Ti-O and/or Ti-O-Ti bonds regardless of deposition method and heat treatment temperature. The transmittance band at 1532 cm⁻¹ - 1693 cm⁻¹ at brookite film at 200˚C, 300˚C, and 400˚C were allocated to an O-H bending mode. At 500˚C, no transmittance band at 1532 cm⁻¹ - 1693 cm⁻¹, and it shows the O-H bending mode was removed from the brookite film during heating. The FTIR spectra also show the transmittance band in the range of 2500 cm⁻¹ - 3746 cm⁻¹ for brookite film at 200˚C where it is indicated that the dilution of TTIP in aqueous solution was due to Ti-OH and –OH vibrations. The transmittance band of 2500 cm⁻¹ - 3746 cm⁻¹ shows that the stretching vibration of the surface hydroxyl group or adsorbed water on the surface of brookite thin film coating [23].

In summary, the FTIR spectra show the TiO₂ thin film obtained via dip and spin coating contained the hydroxyl group on the surface of the thin film. The hydroxyl group on semiconductor surfaces is important in photocatalytic applications because it may collect photoinduced holes off the surface of materials, preventing electron-hole recombination and generating hydroxyl radicals with a high oxidation potential.

Figure 4shows the band gap energy of the TiO₂ thin film deposited via dip and spin coating heated at various heat treatment temperatures. The band gap energy of dip-coated TiO₂ thin films varies with heat treatment temperature due to changes in crystallite size, phase composition, and structural defects. At 200˚C, the band gap energy is 2.81 eV, decreasing to 2.61 eV at 300˚C due to improved crystallinity and the formation of rutile phase, which has a lower band gap than anatase. However, at 400˚C, the band gap increases to 2.77 eV, likely due to continued anatase-to-rutile transformation, which modifies the electronic structure. At 500˚C, the band gap drops significantly to 2.48 eV, as rutile becomes the dominant phase and crystallite growth enhances charge transport. This trend indicates that heat treatment plays a crucial role in tailoring TiO₂ optical properties, making dip-coated films at 500˚C more suitable for visible-light-driven photocatalysis due to their lower band gap energy.

Meanwhile, TiO₂ thin film via spin coating has a band gap energy of 2.72 eV at 200˚C. The band gap energy of brookite thin film coating heated at 300˚C was 2.45 eV. Next, at 400˚C, the band gap energy was calculated at 3.04 eV. When the heat treatment temperature is raised to 500˚C, the band gap energy of the brookite thin film coating is 3.62 eV. The band gap energy for the brookite thin film coatings heated at 200˚C, 300˚C and 400˚C was shown in agreement with the literature [24]. From the figure, it can be observed that the band gap energy decreased when the heat treatment temperature increased from 200˚C to 300˚C.

However, the band gap energy becomes increased when the heat treatment temperature increases from 400˚C to 500˚C. Shi et al. [25] stated that the band gap energy was affected by the thickness coating. Shi et al. also reported that increasing the TiO₂ film thickness from 2.55 nm to 20.61 nm affected the optical band gap decreased from 3.718 eV to 3.417 eV. The thickness of the brookite thin film coating heated at 200˚C was 512.9 nm and at 300˚C was 618.7 nm. This thickness of the brookite thin film coating was increased affecting the decreased of the band gap energy to 2.72 eV at 200˚C and 2.45 eV at 300˚C. Yet, brookite thin film coating heated at 400˚C and 500˚C shows a decrease in the thickness measurement which is 436.6 nm and 350.6 nm, respectively. Resulting to the band gap energy of brookite thin film coatings was 3.3.04 eV at 400˚C and 3.62 eV at 500˚C. In summary, the thickness of the film causes a shift in the optical absorbance edge and therefore a change in the band structure of the film.

Figure 4.Band gap energy of the TiO₂ thin films at different heat treatment temperatures.

Figure 5 shows the degradation efficiency of methylene blue (MB) degradation under visible light for TiO₂ thin films via dip coating and spin coating. The degradation of MB of dip-coated TiO₂ thin film shows an increase with time. Dip-coated TiO₂ thin films exhibit moderate MB degradation efficiency, primarily due to the presence of mixed anatase and rutile phases. At 200˚C, the TiO₂ thin film showed the lowest MB degradation efficiency, achieving 72.7 ± 1.81% after 4 hours. As the heat treatment temperature increased to 300˚C, 400˚C, and 500˚C, the MB degradation efficiency also increased to 83.6 ± 2.09%, 84.5 ± 2.11%, and 84.8 ± 2.12%, respectively. The anatase phase is known for its high photocatalytic activity, while rutile has a narrower band gap, allowing for better light absorption in the visible spectrum. The low MB degradation at 200˚C is attributed to the larger band gap energy, whereas at 500˚C, the band gap energy decreased to 2.48 eV, enhancing photocatalytic activity.

For spin coating, the highest MB degradation efficiency was observed in the TiO₂ thin film heated at 300˚C, achieving 97.7 ± 2.44% after 4 hours, followed by the film heated at 200˚C, which achieved 95.3 ± 2.38%. However, as the heat treatment temperature increased to 400˚C and 500˚C, the MB degradation efficiency decreased significantly to 53.0 ± 1.32% and 51.8 ± 1.29%, respectively. A similar factor contributing to the high MB degradation efficiency at 300˚C was its lower band gap energy (2.45 eV) compared to other temperatures.

Figure 5.Degradation efficiency of methylene blue degradation under visible light for TiO₂ thin films via (a) dip coating and (b) spin coating.