Effect of Calcination Temperature on the Photocatalytic Activity of Zn2Ti3O8 Materials for Phenol Photodegradation

Zinc titanate (Zn2Ti3O8) is a bimetal oxide material that is especially attractive as a photocatalyst. In the preparation of the Zn2Ti3O8, the calcination temperature is a crucial parameter. Hence, in the present work, we aimed to synthesize the Zn2Ti3O8 materials from zinc(II) nitrate and titanium(IV) isopropoxide as precursors by using a solgel method and followed by calcination at 700, 900, and 1100 °C to give ZT-700, ZT-900, and ZT-100 materials, respectively. The ZT materials were characterized using Fourier transform infrared (FTIR), diffuse reflectance ultraviolet-visible (DR UV-vis), and fluorescence spectroscopies. It was confirmed that the ZT materials contained O−Ti−O, Zn−O−Ti, Zn−O, Ti−O−Ti, and Ti−O functional groups as shown from their FTIR spectra. Similar fluorescence properties were only observed on the ZT-700 and ZT-900. From the bandgap energy analysis, ZT-700 and ZT-900 contained spinel and cubic Zn2Ti3O8 (spl-Zn2Ti3O8 and c-Zn2Ti3O8) crystal phases), while ZT-1100 contained c-Zn2TiO4 and TiO2 rutile crystal phases. The kinetic analysis of photocatalytic phenol degradation showed that both ZT-700 and ZT-900 materials exhibited high photocatalytic activity with the reaction rate constants of 0.0353 and 0.0355 h−1, respectively. These values were higher than that of the ZT-1100 (0.0206 h−1). This study demonstrated that calcination at 700 and 900 °C resulted in the formation of the spl-Zn2Ti3O8 and c-Zn2Ti3O8 phases, which were effective as the photocatalyst, but the formation of c-Zn2TiO4 and rutile TiO2 at calcination of 1100 °C deteriorated the photocatalytic activity. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).


Introduction
Phenol is recognized as one organic pollutant that existed in wastewater. Photocatalytic degradation of phenols has been studied by various titanates [4] and zinc titanates [5][6][7][8][9]. Particular attention is made to the zinc titanates (ZnxTiyOz, ZT)-based materials, which have been reported as good photocatalysts for either water splitting or photocatalytic degradation of aqueous organic pollutants [5][6][7][8][9]. As an example, when the Zn2Ti3O8 was combined with TiO2 in a thin film for photocatalytic hydrogen evolution, it was found that the material owing to the Zn2Ti3O8 phase yielded a remarkable photocatalytic enhancement [5]. On the other hand, the Zn2Ti3O8/carbon nitridezinc oxide materials gave a good photocatalytic activity for the degradation of methylene blue, in which the photocatalytic activity corresponded to the amount of Zn2Ti3O8 in the composite material [6]. While the application of zinc titanates seems to be promising, one shall be careful in preparing the material due to the possible formation of different crystallographic phases.
Zinc titanates as semiconductor materials can exist in three different crystallographic phases including ZnTiO3 (cubic, c, and hexagonal, h), Zn2TiO4 (cubic, c), and Zn2Ti3O8 (spinel, spl, and cubic, c) [10][11][12][13][14]. They were usually prepared by controlling the stoichiometric ratio of Zn 2+ and Ti 4+ precursors. In addition to the stoichiometric ratio, selecting the correct temperature during the synthesis would be very important since the stability of the Zn2Ti3O8 is strongly affected by temperature and the synthesis method. The stability of the c-Zn2Ti3O8 was studied and it was reported that it could be stable up to a temperature of 800 °C [10]. The c-Zn2Ti3O8 was reported to form at the low temperature of 500 °C when using zinc chloride (ZnCl2) and titanium chloride (TiCl3) as the precursors and synthesized by a simple precipitation method followed by calcination [15]. However, they reported that the Zn2Ti3O8 was transformed to ZnTiO3 when the calcination temperature was increased to 600 °C. Another group reported that calcining zinc oxide (ZnO) and titanium dioxide (TiO2) with the mol ratio of 2:3 at 750 °C was able to obtain the Zn2Ti3O8 phase [16]. Accordingly, the Zn:Ti mol ratio of 2:3 was used in the present work to obtain the Zn2Ti3O8 phase.
Since the synthesis temperatures could determine the obtained zinc titanate phase, it is very important to study the effect of calcination temperature on the properties and photocatalytic activity of the zinc titanates. To date, a comprehensive study about the calcination temperature effect on the formation of the Zn2Ti3O8, when the Zn:Ti mol ratio is fixed to 2:3 has not been addressed yet. There-fore, in the present work, we conducted the study and examined the photocatalytic activity of the formed Zn2Ti3O8 for photocatalytic degradation of phenol. The Zn2Ti3O8 material was synthesized using a sol-gel method from the reaction between titanium(IV) isopropoxide and zinc(II) nitrate under alkaline condition followed by calcination at 700, 900, and 1100 °C to give ZT-700, ZT-900, and ZT-1100 materials. The photocatalytic activity of the synthesized materials was studied for the degradation of phenol under ultraviolet (UV) irradiation and the kinetics of photocatalytic phenol degradation was investigated to determine the reaction rate constant. was dissolved in ethanol (10 mL) in a beaker glass to produce solution A, while TTIP (4.27 g, 0.015 mol, 1.5 equivalent) was dissolved in ethanol (10 mL) in another beaker glass to produce solution B. Solution A was poured into the solution B and then NaOH 1.0 M solution (2 mL) was added dropwise into the mixture. The mixture was kept stirred for 1 hour to homogenize the mixture. Afterward, the mixture was dried at 60 °C for 24 hours to evaporate the solvent and allow the sol-gel formation. The formed solgel was ground into a powder and calcined at a different temperature of 700, 900, or 1100 °C. The calcination process was conducted with a temperature increase of 700 °C h −1 to reach the desired calcination temperature and then the samples were calcined at the certain temperature for 6 hours. The prepared ZT materials at different calcination temperatures of 700, 900, and 1100 °C were abbreviated as ZT-700, ZT-900, and ZT-1100, respectively.

Characterization of ZT materials
The ZT materials were characterized using a Fourier transform infrared (FTIR, JASCO 6800) spectroscopy attached with an attenuated total reflectance (ATR) Pro One accessory (Single-Reflection ATR). The measurement was carried out in the wavenumber range of 4000-400 cm −1 to identify the functional groups present in each sample. The diffuse reflectance ultraviolet-visible (DR UV-vis) spectrum of each sample was collected on a UV-vis spectrophotometer (JASCO V-760) using an integrating sphere, ISV-922/ISN-923/ISN-901i accessory, in which the measurement was made in the range of 200-800 nm. The emission and excitation fluorescence spectra of each sample were m e a s u r e d u s i n g a f l u o r e s c e n c e spectrophotometer (JASCO FP-8500) on an FDA-808 cell holder for the solid powder sample. The fluorescence emission was recorded at excitation of 274 nm, while the fluorescence excitation was recorded at emission of 302 nm.

Photocatalytic test of ZT materials
Each of ZT-700, ZT-900, or ZT-1100 as the photocatalyst material (50 mg) was added into 50 mg.L −1 of aqueous phenol solution (50 mL). The mixture was stirred under a dark condition for 2 hours at room temperature. The mixture was then illuminated under a UV lamp (UVLS-28 EL Series, 365 nm) for 1, 3, 6, and 24 hours. After the illumination, the sample was filtered and then the phenol concentration in the filtrate was analyzed by high-performance liquid chromatography (HPLC, Shimadzu LC-20 AT) with a photodiode array (PDA) SPD-M20 A detector at 272 nm. The C18 column and acetonitrile were used as the stationary and mobile phases, respectively, and the flow rate was set to 0.8 mL.min −1 at 40 °C. The schematic experimental setup for the evaluation of photocatalytic activity is shown in Figure 1. The phenol concentration was measured using a standard calibration curve method and the degradation percentage of phenol was calculated from the ratio of remaining phenol concentration after reaction (Ct) to the one before the reaction (C0) using Equation (1).
The kinetic photocatalytic degradation of phenol over the ZT materials was further investigated using the first-order reaction model, as generally occurred in the case of photocatalytic degradation reaction [2] and shown in equation (2). The reaction rate constant (k) was obtained from the gradient of the linear plot between -ln(Ct/Co) and reaction time (t). (2)

Characterizations of ZT Materials
The ZT materials synthesized at three different calcination temperatures, i.e. 700, 900, and 1100 °C, were characterized by using an FTIR spectrometer to identify the functional groups from the vibrational modes present in these materials. The FTIR spectra of ZT-700, ZT-900, and ZT-1100 are shown in Figure 2. Figure 2(a) shows that increasing the calcination temperature from 700 to 1100 °C resulted in the disappearance of the −OH functional groups (3400 cm −1 ), which were originated from the adsorbed water molecules [16] on the ZT surface. On the other hand, all composites showed some weak absorption signals in the fingerprint region of 418 cm −1 as well as in the range of 545-549, 639-642, 696-708, and 837-990 cm −1 (see Figure 2(b)). The sharp band at 418 cm −1 could be assigned to the Ti−O bending vibration in [TiO6] 2− octahedral in ZT materials [7]. This was supported by the absorption signals present in a region of 900 cm −1 as well as the peaks observed in the range of 639-642 cm −1 , which corresponded to the O−Ti−O stretching vibration of [TiO6] 2− octahedral in ZT materials [17]. The characteristic of ZT materials was also shown by a Zn−O−Ti vibration, which was observed as a shoulder band at the region of 696-708 cm −1 [18]. On the other hand, the shoulder bands in the range of 545-549 and 501-504 cm −1 were also observed due to the presence of [ZnO4] 2− structure in the ZT materials [19,20]. Additionally, a strong signal at 873 cm −1 was observed on ZT-700, which was related to the vibration of Ti−O stretching involving non-bridging oxygen atoms. As the calcination temperature increased, the intensity of this peak on the ZT-900 and ZT-1100 was decreased and shifted to the higher wavenumber, indicating a crystal phase transformation of Zn2Ti3O8 at a higher temperature, which affected the nature of this vibrational mode.
The DR UV-vis spectra of the ZT materials synthesized at three different calcination temperatures were also recorded to study the optical properties and calculate the bandgap energy of these semiconductor materials. The DR UV-vis spectra were plotted in Kubelka-Munk function versus wavelength as shown in Figure  3. All the ZT materials obtained at different calcination temperatures showed significant absorptions in the UV region, but not in the visible region, which was in accordance with the white color of these materials as shown in Figure 4.
The ZT-700 showed absorption peaks at 214, 254, and 286 nm. When the calcination temperature was increased to 900 °C, the absorption peaks were observed at 214, 254, and 339 nm. While the absorption peaks at 214 and 254 nm were not changed as compared to those  of the ZT-700, the broadening and peak shifting was observed from 286 to 339 nm. This change indicated that some amounts of Zn2Ti3O8 were transformed to ZnTiO3, which was in agreement with the previous report [7]. For the ZT-1100, the band at 214 nm nearly vanished and the band at 254 nm was significantly decreased. Meanwhile, the absorption at 286 nm was also decreased and a new peak was observed at a higher wavelength of 309 nm. In addition, new absorption band and shoulder peak were also observed at 343 and 410 nm, respectively. All these changes could be ascribed to a major transformation of Zn2Ti3O8 into the other crystal phases (i.e. Zn2TiO4 and rutile TiO2) due to the high temperature [21,22].
The determination of bandgap energy values is important for photocatalyst application since it gives information on the minimum energy required to excite the electrons from the valence band to the conduction band of semiconductor material. The direct bandgap energy values of all the Zn2Ti3O8 materials were calculated using the Tauc method as shown in equation (3), where , h, Eg, and υ are the absorption coefficient of the materials, Planck constant (6.636×10 −34 J.s), bandgap energy (eV), and light frequency (Hz), respectively. The  value can be replaced by the value of Kubelka-Munk function, F(R) [23] as also performed in this work. (3) The bandgap energy values can be then determined by extrapolating the linear plot between the (hυ) 2 versus (hυ) to the x-axis. The Tauc plots for the bandgap energy measurement of the Zn2Ti3O8 samples are also shown as the inset in Figure 3. The bandgap energy values of each ZT material and other reported values for zinc titanates and rutile TiO2 are summarized in Table 1. According to the reported literature, the bandgap value of c-Zn2Ti3O8 would be in the range of 3.20-3.50 eV [5,6], for spl-Zn2Ti3O8 in the range of 3.56 eV or higher [8,24], for c-Zn2TiO4 would be in the range of 3.22-3.45 eV [25], while for rutile TiO2 in the range of 3.00-3.20 eV [26,27]. Therefore, it can be proposed here that the ZT-700 would

ZT-700 ZT-900 ZT-1100
have spl-Zn2Ti3O8 and c-Zn2Ti3O8, the ZT-900 would have c-Zn2Ti3O8 as a main phase, while the ZT-1100 would have c-Zn2TiO4 and rutile TiO2. The different formed phases were possibly observed as the Zn2Ti3O8 could be decomposed to ZnTiO4 and rutile TiO2 at a high temperature of 1100 °C [21,22]. The fluorescence spectra of ZT-700, ZT-900, and ZT-1100 materials are shown in Figure 5. As depicted in Figure 5(a), all the ZT materials exhibited a strong excitation peak at 274 nm when monitored at an emission wavelength of 302 nm. While a similar level of excitation intensity could be observed in the ZT-700 and ZT-900, a higher intensity was observed in the ZT-1100. Figure 5(b) shows the emission spectra of ZT-700, ZT-900, and ZT-1100 when measured using 274 nm as the excitation wavelength. All the samples gave the same main emission peak at 302 nm. A similar trend was observed here that the ZT-700 and ZT-900 gave similar emission intensity, while the ZT-1100 gave higher emission intensity than the other ZT materials.
The higher emission intensity could be related to the high electron-hole recombination, which could play important role in photocatalytic reactions. In addition to the emission peak at 302 nm, some emission peaks were also observed in the range of 350-700 nm. It was clear that the ZT-700 and ZT-900 gave similar emission peaks in this region, showing the similar characteristics between ZT-700 and ZT-900, which could be ascribed to the presence of the Zn2Ti3O8 phase in these two materials. In contrast, the ZT-1100 gave a different shape of emission spectrum, suggesting that a different phase existed on the ZT-1100. As described above, the Zn2Ti3O8 phase could be transformed into Zn2TiO4 and TiO2 rutile phase in the ZT-1100, which in turn led to exhibit different emission spectrum.

Photocatalytic Activity of ZT Materials
The performance of ZT-700, ZT-900, and ZT-1100 materials as photocatalysts was evaluated for the photocatalytic degradation of phenol.  Figure 6(a) shows the plot between phenol degradation percentage versus the reaction time. It could be observed that the ZT-700 and the ZT-900 photocatalysts showed similar photocatalytic activity to each other, which was higher than that of the ZT-1100. After 24 hours-reaction, the ZT-700 and the ZT-900 gave 57% phenol degradation, while the ZT-1100 only gave 39%. This different result would be strongly affected by the different phases that existed in the ZT materials. The ZT-700 and the ZT-900 showed a similar level of high activity as they have the Zn2Ti3O8 phase. In contrast, the ZT-1100 gave a lower photocatalytic activity due to the absence of the Zn2Ti3O8 phase. Therefore, it was demonstrated that the calcination temperature determined the type of ZT phase formed in the synthesis and the type of ZT phase led to the different photocatalytic activity. The Zn2Ti3O8 phase was revealed to give better photocatalytic activity than the Zn2TiO4 phase and rutile TiO2.
The kinetics of the photocatalytic degradation of phenol over ZT materials was investigated by using the first-order kinetic model as shown in Figure 6(b). The obtained plots were linear and have high correlation factors that were close to unity, indicating that the photocatalytic degradation of phenol over the ZT materials indeed followed the first-order reaction. The degradation rate constant on each ZT material could be determined from the slope of the plots, giving the values of 0.0353, 0.0355, and 0.0206 h −1 for the ZT-700, the ZT-900, and the ZT-1100, respectively. The ZT-700 and the ZT-900 gave a 1.7 times faster reaction rate as compared to the ZT-1100, again demonstrating the superior photocatalytic performance of the Zn2Ti3O8 phase than the Zn2TiO4 phase and rutile TiO2.
While the bandgap energy analysis of the ZT materials was used to reveal the active sites for phenol degradation, the characterization study using fluorescence spectroscopy could be used to understand the reason why the ZT-700 and ZT-900 gave similar activity but the ZT-1100 gave the lowest activity. As discussed previously, the ZT-1100 gave a higher emission intensity as compared to both ZT-700 and ZT-900 having nearly the same intensity levels. The higher emission intensity would be originated from the higher electron-hole recombination processes on the ZT-1100, which led to the lower photocatalytic activity. The presence of c-Zn2TiO4 phase and rutile TiO2 could induce the high electron-recombination process on the ZT-1100. The rutile TiO2 was reported to have a fast electron-hole recombination process [28], which seemed to cause the depletion of the photocatalytic activity in the ZT-1100. Based on the characterization and photocatalytic activity tests, it could be recommended here that the calcination temperature in the range of 700-900 °C is suitable to obtain the active Zn2-Ti3O8 phase in the ZT materials.

Conclusions
Effect of calcination temperatures (700, 900, 1100 °C) to prepare ZT materials by sol-gel method was studied by using zinc(II) nitrate and titanium(IV) isopropoxide as the precursor materials with the fixed mol ratio of 2:3. The FTIR, DR UV-vis, and fluorescence spectroscopies showed that the ZT-700 and the ZT-900 have similar properties, but the calcination temperature of 1100 °C resulted in the formation of different ZT phases. Based on the bandgap energy values, the ZT-700 and the ZT-900 would have the Zn2Ti3O8 as the main phase, while the ZT-1100 would have Zn2TiO4 and rutile TiO2. The photocatalytic degradation of phenol followed the first-order reaction with the degradation rate constants of 0.0353, 0.0355, and 0.0206 h −1 for ZT-700, ZT-900, and ZT-1100, respectively. The ZT materials having Zn2Ti3O8 crystal phase exhibited higher photocatalytic activity than the one having Zn2TiO4 and rutile TiO2 phases, which would be due to the lower electron-hole recombination on the ZT-700 and ZT-900 as compared to the ZT-1100. This study revealed that the calcination temperature of 700-900 °C shall be used to obtain the active Zn2-Ti3O8 phase.