Effect of Calcination Temperature on Photocatalytic Activity of Synthesized TiO 2 Nanoparticles via Wet Ball Milling Sol ‐ Gel Method

: In this work, TiO 2 nanoparticles were successfully synthesized with narrow size distribution via a wet ball milling sol - gel method . The effect of calcination temperature on photocatalytic activity was observed from particle size, crystallite size, and phase transition of TiO 2 nanoparticles . Increasing calcination temperature increased particle size, crystallite size, and the crystallinity of synthesized TiO 2 . Phase transition depended on variation in calcination temperatures . A two ‐ phase mixture of anatase and brookite was obtained with lower calcination temperature whereas a three ‐ phase mixture appeared when calcination temperature was 500–600 °C. With higher temperature, the rutile phase kept increasing until it was the only phase observed at 800 °C. Anatase strongly affected the photocatalytic activity from 300 °C to 600 °C while the particle size of TiO 2 was found to have a dominant effect on the photocatalytic activity between 600 °C and 700 °C . A mixture of three phases of TiO 2 - 600 exhibited the highest methylene blue degradation with the rate constant of 9 . 46 × 10 - 2 h - 1 under ultraviolet (UV) irradiation .


Introduction
Titanium dioxide (TiO2) photocatalyst has been extensively studied and widely applied in water and air treatment [1], paint [2], protective coatings [3], and cosmetics [4]. There are many advantages of TiO2 such as chemical stability, self-cleaning, biocompatibility, and strong antibacterial qualities. TiO2 with high photocatalytic and strong antibacterial activities has attracted much attention in decades [5]. TiO2 photocatalyst has three polymorph structures, namely anatase, rutile, and brookite.
High photocatalytic activity of TiO2 under ultraviolet (UV) light is strongly depended on phase composition, crystallinity, crystallite size, and surface area [6,7]. The relationship between TiO2 phase and its photocatalytic activity has been an interest for many researchers. Although pure anatase phase of TiO2 has been preferred, there is a limitation of fast recombination of photogenerated electrons (e -) and holes (h + ). Some works have shown that the mixture of anatase-rutile [8][9][10][11], anatasebrookite [12,13] and brookite-rutile [14,15] phases of TiO2 powder in a certain ratio provided higher photocatalytic activity than the pure phase. Kangle et al. [11] found that the mixture of anatase and rutile phases with a ratio of 57:43 provides the highest hydroxyl radical (OH) formation and photocatalytic activity under UV irradiation. Kandiel et al. [12] studied methanol photo-oxidation of TiO2 powder and found that the mixture of anatase-brookite phases of TiO2 showed higher photocatalytic activity than pure anatase phase. Cao et al. [14] determined that the highest photodegradation of phenol was achieved by using TiO2 composed of 72% of brookite and 28% of rutile. Although binary phases of TiO2 provided higher photocatalytic activity than pure phase, there have been reports that ternary phases present higher photocatalytic activity than single [16] and binary phases [17]. Paola et al. [16] investigated that the mixture of three phases including 65% anatase, 28% brookite, and 7% rutile phase showed higher photocatalytic activity degradation of 4nitrophenol than pure rutile phase. Liao et al. [17] found that the mixture of 30% anatase, 28% brookite, and 42% rutile showed higher photocatalytic activity over 90% degradation of methyl orange solution in 20 min than the mixture of anatase and rutile phase. The effect of synergetic phase junction and different band alignments of each phase provides eand h + transfer at the interface of TiO2. This can enhance e --h + separation and reduce e --h + recombination leading to an enhanced photocatalytic activity under UV light. However, a relationship between synthesis condition and mixture ratio of multiple phases of TiO2 has still been a challenge for high photocatalytic activity.
There are many synthesis methods such as hydrothermal [18], microwave [19], supercritical antisolvent [20], and sol-gel [21] methods for preparation of TiO2 nanoparticles. Hydrothermal, microwave, and supercritical antisolvents provided high purity and high surface equipment for TiO2 synthesis, whereas the sol-gel method can be undertaken at atmospheric pressure and room temperature. The sol-gel method is one of the promising methods for large-scale production [22].
However, this method suffers from controlling the shape and size distribution of TiO2 nanoparticles. Generally, the smaller the particle size of TiO2 leads to the larger surface area and higher photocatalytic activity under UV light irradiation. A grinding process is required for the high surface area of TiO2 powder. Dry ball milling has been used widely to reduce the size of nanoparticles.
However, wet ball milling provides smaller particle size and narrower size distribution [23]. Jung et al. [24] found that the size of TiO2 particles was decreased and showed a uniform size distribution by using a wet ball milling process. Furthermore, phase, crystallite size, and crystallinity of TiO2 play a critical role for high photocatalytic activity which they can be governed by calcination temperature in the sol-gel method. Behnajady et al. [25] reported the effect of calcination temperature on a phase transition of TiO2 from anatase to rutile at calcination temperatures from 350 °C to 750 °C . Mutuma et al. [26] investigated that the mixture of anatase and brookite phases was found at a calcination temperature of 200 °C. The mixture of three phases was found at 600 °C and completely transfer to rutile phase at 800 °C. Moreover, crystallite size and crystallinity of TiO2 were increased when calcination temperature increased [27,28]. However, explanation to understand the crystallite size and phase transformation of TiO2 which affect photocatalytic activity is still limited.
In this work, TiO2 particles were synthesized by the wet ball milling sol-gel (WBMS) method [29].
The effect of calcination temperature on photocatalytic activity was investigated. The crystallinity, particle size, and phase transition of synthesized TiO2 have been all observed in order to provide more understanding of phase mixture for photocatalytic activity.

Preparation of TiO2 and Characterization of Synthesized TiO2 Material
The effect of calcination temperature on TiO2 synthesis with WBMS: TiO2 was synthesized which was modified from the previous report of Bahadur et al. [30]. In this work, methanol (Thermo Fisher Scientific, Waltham, MA., USA) was used as a solvent for TiO2 synthesis from previous study [29] because the smallest particle size, the narrowest size distribution and the highest photocatalytic degradation of methylene blue could be achieved when compared to ethanol and isopropanol. Commercial P25 TiO2 (89% anatase and 11% rutile) (Kanto Chemical Co., Inc., Tokyo, Japan) was used as a TiO2 reference and purchased from ACROS. A diagram of the TiO2 synthesis via the WBMS method is shown in Figure 1. The substitution reaction of 97%wt. -titanium tetraisopropoxide (TTIP) (Sigma-Aldrich Pte Ltd., Singapore) with 99. 99%wt. -methanol (MeOH) (Sigma-Aldrich Pte Ltd., Singapore) was carried out with a molar ratio of TTIP:MeOH of 1:15. Then the hydrolysis reaction of titanium alkoxide was carried out by adding deionized water (DI) with a molar ratio of TTIP:DI water of 1:4. The solution pH was adjusted to 1. 5 by adding 65%wt. concentrated HNO3 (Carlo Erba reagents S.A.S, Val-de-Reuil, France). The white colloid of TiO2 sol was observed. The TiO2 gel mixture was washed with DI water and centrifuged at 10,000 rpm for 15 min. The white jelly-like gel was dried in an oven (Memmert Gmbh & Co.KG, Schwabach, Germany) at 110 °C for 24 h to remove organic solvents. The yellow-light xerogel was dry ground with 10 mm-ball milling at 300 rpm for 20 min and consequently wet ground by 2 mm-ball milling at 500 rpm for 3 h in isopropanol. The TiO2 solution was then dried in the oven at 110 °C for 24 h. The synthesized particles were calcined at different temperatures of 300, 400, 500, 600, 650, 700, and 800 °C for 4 h with a heating rate of 5 °C /min and labeled as TiO2-300, TiO2-400, TiO2-500, TiO2-600, TiO2-650, TiO2-700, and TiO2-800, respectively.

Characterizations of TiO2
The morphology, particle size, and phase of TiO2 powder were studied using transmission electron microscope (TEM) (JEM-2100, JEOL Ltd., Tokyo, Japan, software of 2013) operated at 200 kV. X-ray diffraction (XRD) (D8 Advance, Bruker, Germany) patterns were obtained with a Bruker D8 ADVANCE equipped with Cu source (Cu Kα 0. 154 nm, 40 kV, 40 mA). The scattered radiation was detected in the angular range of 10-80° (2Ɵ) with a scan rate of 0. 02°. A commercial Total Pattern Analysis Solution software (TOPAS version 4.2, Bruker, Germany, 2011) was employed to determine the crystallite size and phase content of TiO2 powder. The absorption edge and energy band gap of TiO2 were investigated by diffuse reflectance spectroscopy (DRS) (Cary Series, Agilent Technologies, Santa Clara, CA, USA) with the wavelength range of 200-800 nm (Agilent carry 5000). The energy band gap of synthesized TiO2 with different calcination temperatures was calculated by using Tauc plot from the equation as follows where α is the extinction coefficient, h is the Plancku s constant (J•S), v is the light frequency (s -1 ), A is the absorption constant, Eg is the energy band gap (eV) and n is the value of the specific transition, n 2 for indirect band gap, and n ½ for direct band gap [31].

Photocatalytic Activity Reaction
The degradation of methylene blue (MB) (Ajax Finechem) under UV irradiation was used as a model system to evaluate the photocatalytic performance of TiO2 powder. The photocatalytic reaction was carried out in the in-house photocatalytic chamber. The initial concentration of MB (C0) was prepared at 5 ppm. 10 mg of TiO2 was mixed with 40 mL of MB solution and then stirred in the photocatalytic chamber. The mixture was kept in the dark for 1 h to ensure the saturation of MB on the surface of the catalysts. The MB solution was irradiated by 0. 25 mWcm -2 with UV light (λ 351 nm). The sample was collected every hour for 28 h. The concentration of MB in suspension was analyzed using an ultraviolet-visible (UV-Vis) spectrophotometer with the wavelength of maximum absorbance at 664 nm. The kinetic constants of reaction rate were determined according to the pseudofirst-order kinetic model as follows where t is the irradiation time (h), k is kinetic constant (h -1 ), C0 is the initial concentration (mg/L), and C is the concentration of the MB (mg/L) at certain irradiation time.

Results and Discussion
The morphology, particle size, and phase of synthesized TiO2 with different calcination temperatures were investigated by transmission electron microscopy (TEM) as shown in Figure 2. It was observed that TiO2-300, TiO2-600, and TiO2-700 showed the mixture of spherical, cubic, and hexagonal shapes while TiO2-800 provided an almost hexagonal shape with the agglomeration and aggregation of particles. The particle size of TiO2-300, TiO2-600, TiO2-700, and TiO2-800 particles were 8. 6 ± 1. 7, 18. 7 ± 3. 7, 27. 6 ± 8. 2, and 81. 6 ± 21. 6 nm, respectively. It was observed that increasing calcination temperature leads to increasing particle size and crystallite size of TiO2 particles. At low calcination temperature of 300 °C, the mixture of anatase (101) and brookite (111)  particles. The results demonstrated that there was a mixture of anatase (211) and brookite (221) phases observed for TiO2-300. When calcination temperature was increased from 300 °C to 600 °C, there were three phases composed of anatase (101), rutile (111), and brookite (113) phases. SAED patterns showed anatase (220) and rutile (210) phases for TiO2-700 and showed only the rutile (210) phase for TiO2-800. Figure 2b-d showed that increasing of calcination temperature from 600 °C to 800 °C increased the aggregation of crystallites leading to larger crystallite size and lower surface area since the anatase and brookite phases were completely transformed to the rutile phase. This indicates that crystallite size of TiO2 powder was increased at high temperature due to larger crystallite size of the rutile phase. It was reported that the most stable crystallite size of anatase and brookite phases is less than 11 nm and 11-35 nm, respectively, while the most stable crystallite size of the rutile phase is larger than 35 nm [32]. In addition, with increasing calcination temperature the crystallinity was increased. This can be attributed to the thermally promoted crystallite growth. Therefore, TEM and SAED images confirmed phase synthesis of TiO2 powder strongly depend on calcination temperature. At low calcination temperature, anatase and brookite phases were presented. Phase transition of TiO2 to rutile phase was significant when the calcination temperature was higher. were observed before calcination and after calcination from 300 °C to 500 °C. This could be attributed to high amorphous structure of TiO2. Increasing calcination temperature above 600 °C, it was found that the crystallinity of TiO2 increased due to the sharper and narrower peaks of XRD patterns. The crystallinity increased with increasing calcination temperature since higher ordering of the structure of TiO2 particles leads to the sharper and narrower X-ray peaks. Besides, the crystallite size of all phases increased with increasing calcination temperature which indicates that the crystallization of TiO2 was improved as shown in Table 1. These results from XRD also confirm and agree well with the TEM results as previously mentioned. Therefore, calcination temperature promoted phase transition, crystallite size, and crystallinity of TiO2 powder.  The weight percentage of anatase, rutile, and brookite phases of synthesized TiO2 by different calcination temperatures was estimated by TOPAS. The relationship between calcination temperature and percentage of phase content is shown in Figure 4a. When calcination temperature was increased from 300 °C to 600 °C, the anatase phase increased from 29% to 42%wt. whereas the brookite phase decreased from 71% to 28%wt. It is interesting to note that the brookite phase was observed at calcination temperature below 600 °C. It was reported that the brookite phase can be observed as a by-product in the sol-gel method in an acidic condition at low temperature [27]. These results corresponded well with other studies as shown in Table 2. There was a mixture of anatase and brookite phases when calcination temperature was increased from 300 °C to 400 °C. This result was correlated with the study of Mutuma et al. [26]. Considering calcination temperature between 600 °C and 700 °C, it was observed that the anatase phase decreased from 42% to 38%wt. whereas the rutile phase increased from 30% to 62%wt. The rutile phase was observed above 500 °C. This result was in good agreement with the reports of Mutuma et al. [26], Bamme et al. [33] and Saalinraj et al. [34]. A mixture of the anatase and the rutile phases was found between 650 °C and 700 °C which was consistent with the study of Banjuraizah et al. [35]. The anatase and brookite phases were completely converted to the rutile phase at 800 °C because the rutile phase provided thermal stability at high temperature [34].
Absorption edge of TiO2 with different calcination temperatures was investigated by DRS as shown in Figure 4b. It was observed that absorption edge of TiO2 between 320 nm and 440 nm showed red shift when calcination temperature was increased. The higher wavelength of absorption edge provides the lower energy band gap. Energy band gap of TiO2 with direct band gap structure was determined using Tauc analysis as shown in Table 1. It was observed that the mixture of three phases provided the energy band gap between 3. 16 eV and 3. 28 eV. While the mixture of anatase and rutile phases (3. 07-3. 08 eV) showed a lower energy band gap than the mixture of anatase and brookite phases (3. 10-3. 29 eV). This result agreed well with many studies in which rutile has been reported to have a band gap of about 3. 0 eV and brookite has been reported to have a band gap between 3. 0 eV and 3. 6 eV. Anatase has been reported to have a band gap about 3. 2 eV. The lowest energy band gap of TiO2-800 composed of only the rutile phase is 3. 05 eV and well correlated with the report of Da Silva et al. [20] who also found the energy band gap of the rutile phase was 3. 05 eV.

25-800 °C
Calcination temperature below 600 °C found the mixture of anatase and brookite phases. The rutile phase was found above 600 °C and only the rutile phase was found at 800 °C. [26] 300-600 °C Pure anatase phase was found between 300 °C and 400 °C. Rutile was found above 500 °C. [33] 200-800 °C Pure anatase was found between 200 °C and 400 °C The rutile phase was found above 500 °C. [34] 400-700 °C Calcination temperature below 700 °C found only pure anatase and calcination temperature of 700 °C found the mixture of anatase and rutile phases. [35] The methylene blue adsorption capability of synthesized TiO2 and P25 is shown in Figure 5a. Before UV light illumination, the solutions were kept in dark for 1 h under stirring to saturate the adsorption of MB. It was observed that the concentration of MB solutions of TiO2-600, TiO2-650, TiO2-700, TiO2-800, and P25 was reduced about 10% of initial MB solutions while TiO2-300, TiO2-400 and TiO2-500 showed negligible adsorption capability of MB under dark condition. Figure 5b presents the study of photocatalytic activity of synthesized TiO2 nanoparticles with different calcination temperatures from 300 °C to 800 °C and a comparison with commercial P25 TiO2 nanoparticles as a reference. Table 3 shows the kinetic constant of MB degradation of synthesized TiO2. The effect of calcination temperature on phase transition of synthesized TiO2 powders provided the different kinetic constants as shown in Figure 6a. It was observed that P25 with 89:11 of anatase to rutile showed the highest photocatalytic activity with the kinetic constant of 11.41 × 10 -2 h -1 . However, among synthesized TiO2, TiO2-600 provided the highest photocatalytic activity with the kinetic constant of 9.46 × 10 -2 h -1 . Increasing calcination temperature from 300 °C to 400 °C, the kinetic constant increased from 2.54 × 10 -2 to 3.87 × 10 -2 h -1 . Photocatalytic activity of TiO2 depends on many factors such as phase composition, crystal structure, crystallinity, crystallite size, and surface area [36]. TiO2-300 and TiO2-400 composed of anatase and brookite phases and there was an increase of anatase phase from 29%wt. (TiO2-300) to 33%wt. (TiO2-400). Commercial P25 showed higher photocatalytic activity than synthesized TiO2 because P25 presented a higher ratio of anatase to rutile (89:11) than that of TiO2-600 (42:30). While the particle sizes of P25 and TiO2-600 were quite similar as 19.9 ± 1.2 nm and 18. 7 ± 3. 7 nm, respectively. Our photocatalytic oxidation result is agreed well with the study of Ozawa et al. [37] who reported that photocatalytic activity of TiO2 was enhanced with an increasing of percentage weight of anatase whereas the brookite phase was decreased. The anatase phase provides the highest photocatalytic activity compared to the rutile and brookite phases because anatase phase is an indirect band gap structure while the rutile and brookite phases appear to be the direct band gap one. Indirect band gap structure shows longer lifetime of electron and hole (e --h + ) pairs separation than direct band gap. Moreover, the anatase phase shows the highest photocatalytic activity because of the smallest particle size and the lightest effective mass of e --h + pairs which provides the fastest charge transfer to surface of TiO2 leading to the lowest e --h + recombination [38].
Charge transfer in the different band alignments at the interfaces of TiO2 shows high efficiency of charge separation for high photocatalytic activity under UV light conditions. The charge transfer mechanism of different phase combinations is proposed in Figure 6b-d. The mechanism of charge transfer of the mixture of anatase and brookite phases of TiO2-300 and TiO2-400 is shown in Figure  6b. Conduction band (CB) level can be determined from the vacuum level by using electron affinity (EA). Electron affinity of the anatase phase (EAA = 4.94 eV) is higher than that of the brookite phase (EAB = 4.15 eV). Valence band (VB) can be determined by considering energy band gap (Eg) (Eg of anatase = 3.51 eV and Eg of brookite = 3.36 eV) [39] CB of brookite is higher than that of anatase where electrons can transfer from brookite to anatase whereas the hole conversely transfers from anatase to brookite. With increasing calcination temperature from 400 °C to 600 °C, the kinetic constant was increased from 3.87 × 10 -2 to 9.46 × 10 -2 h -1 . The proposed charge transfer pathway in three phases of TiO2-500 and TiO2-600 is illustrated in Figure 6c. Electron affinity of brookite, rutile, and anatase (EAB, EAR, and EAA) are 4.15, 4.73, and 4.94 eV, respectively [39] Due to the lowest energy of EAB, the CB of brookite is the highest level compared to those of rutile and anatase phases. Electrons can transfer from brookite to rutile and anatase between interfaces while holes transfer in the opposite direction. The mixture of three phases consists of higher interfaces than two phases leading to a longer lifetime of charge transfer and a reduction of charge recombination. Therefore, the mixture of three phases including anatase, rutile, and brookite showed higher photocatalytic activity than two phases of anatase and brookite [16,17,40]. TiO2-600 showed higher photocatalytic activity than TiO2-500 because TiO2-600 consisted of higher weight percentage of anatase phase (42%wt.) than TiO2-500 (33%wt.). With increasing calcination temperature from 600 °C to 700 °C, the kinetic constant was decreased from 9.46 × 10 -2 to 6.67 × 10 -2 h -1 . It is interesting to note that even though the anatase phase was increased from 42%wt. (TiO2-600) to 53%wt. (TiO2-650), the kinetic constant was decreased from 9.46 × 10 -2 to 7.73 × 10 -2 h -1 . It was noticed that the crystallite size of anatase and rutile was increased from 600 °C to 650 °C as shown in Table 1. This can lead to the lower surface area of TiO2-650 compared to that of TiO2-600. Therefore, it is the dominance of crystallite size for the decrease of photocatalytic activity between TiO2-600 and TiO2-700 over the phase. The charge transfer mechanism of the mixture of anatase and rutile phases is shown in Figure 6d. Electrons in CB transfer from rutile to anatase because EAA was higher than EAR while holes transfer from anatase to rutile [39,41,42]. Considering the kinetic constant of TiO2-300 (2.54 × 10 -2 h -1 ) and TiO2-700 (6.67 × 10 -2 h -1 ), the mixture of anatase and rutile (TiO2-700) provided higher photocatalytic activity than that of anatase and brookite (TiO2-300). It was observed that the weight percentage of anatase phase of TiO2-700 (38%wt.) was higher than that of TiO2-300 (29%wt.) while the crystallite size of TiO2-300 was smaller than that of TiO2-700. High crystallinity of the mixture of anatase and rutile phases provided higher photocatalytic activity than that of the mixture of anatase and brookite phases because it has been reported that TiO2 comprising the brookite phase of more than 25% could inhibit photocatalytic activity of TiO2 [43]. At calcination temperature of 800 °C, it shows the lowest kinetic constant of 2.19 × 10 -2 h -1 because TiO2-800 consisted of only rutile phase and had the largest crystallite size of 52 nm.
Due to the direct band gap structure of the rutile phase, there is the fast recombination of e --h + pairs as shown in Figure 6e leading to the lowest photocatalytic activity of TiO2-800. These results correlated well with the study of Zhang et al. [44] who found that photocatalytic activity of H2 evolution of the pure rutile phase was lower than that of the mixture of anatase and rutile phases.

Conclusions
The effect of calcination temperature plays an essential role on particle size, phase transition, crystallinity, and crystallite size of synthesized TiO2 via the WBMS method. Narrow size distribution of TiO2 nanoparticles could be obtained. A mixture of brookite and anatase phases was found at calcination temperature less than 400 °C. The rutile phase was found above 500 °C. The mixture of anatase, rutile, and brookite phases was obtained at calcination temperature between 500 °C and 600 °C. Anatase and brookite phases were completely transferred to the rutile phase at 800 °C. Phase composition and crystallite size are important factors for high photocatalytic activity. The high percentage weight of the anatase phase had a strong effect on the photocatalytic activity between 300 °C and 600 °C while the crystallite and particle size of TiO2 had a dominant influence on the photocatalytic activity from 600 °C to 700 °C. The highest photocatalytic activity with three phases of TiO2-600 was achieved with kinetic constant of 9. 46 × 10 -2 h -1 . The highest photocatalytic activity in this study can be applied for environmental application such as water decontamination or pollutants removal from water.