Shift the Photocatalytic Activity of P25 TiO2 Nanoparticles toward the Visible Region upon Surface Modification with Organic Hybrid of Phosphotungstate

In this study, a visible-light-driven P25 TiO2 was prepared upon surface modification with a colored organic hybrid of phosphotungstic acid that makes titanium dioxide suitable for photocatalytic degradation of organic pollutants under visible and sunlight irradiation. Visible shifting of the photocatalytic activity of surface-modified TiO2 was examined by studying the decolorization of methylene blue (MB) and rhodamine B (RhB). The results show that colored TiO2 is, unlike bare TiO2, a good photocatalyst in the degradation of dyes under visible and sunlight irradiation. Surface-modified Al2O3 and reduced graphene oxide (RGO) with organic hybrid of phosphotungstate failed to degrade RhB under sunlight irradiation, which prove the role of TiO2 in the photochemical process.


Introduction
The major limitation of P25 TiO 2 as a photocatalyst is its poor efficiency in the visible region of the solar spectrum due to wide band gap of 3.2 eV, which typically requires exposure of ultraviolet (UV) light for photocatalytic reactions. Hence, the photocatalytic application of TiO 2 at an industrial scale has been limited because only 4-5% of the solar spectrum corresponds to UV photons. Therefore, strong efforts have been devoted to activating the TiO 2 toward the visible and solar light for application purposes [1,2]. One of the approaches for attainment of this issue is dye sensitization, i.e., the adsorption of dyes especially phthalocyanines on the TiO 2 surface [3][4][5][6][7][8]. One other is modification with polyoxometalates (POMs), but pure POMs cannot shift the photocatalytic activity of pure P25 TiO 2 toward the visible region, and it is necessary to join them to valuable metallic or nonmetallic moieties [9][10][11][12][13]. In this work and in the following of our previous works [14,15], we have utilized the combination of dye sensitization and POMs, i.e., the adsorbed MB on phosphotungstate-hexamethylenetetramine hybrid (PTA-HMT-MB) as sensitizer. To our knowledge, the coating of P25 TiO 2 with a colored organic hybrid of POMs has not been reported yet. The hybridization and immobilization onto P25 TiO 2 resolve high solubility and poor recyclability of POMs and facilitate separation because coated TiO 2 is not suspended in the solution.

Apparatus.
The stirring of solutions was performed using a Labinco magnetic stirrer model L-81. A Metrohm type 691 pH meter was used for pH measurements. UV-Vis absorption spectra were obtained using a GBC model Cintra 6 or Shimadzu 1601PC spectrophotometer. The IR spectra were conducted on a Shimadzu 8400s FTIR spectrometer. Diffuse reflectance spectra (DRS) were recorded on an Avantes spectrometer (AvaSpec-2048-TEC), using BaSO 4 as a standard. The surface area and pore volume of the composite were measured and calculated by the BET method from nitrogen adsorption-desorption isotherms at 77 K with a surface area and pore size analyzer (BELSORP-mini II, Bel, Japan). The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on a Rheometric Scientific STA 1500 thermal analyzer under the atmosphere of air. Scanning electron microscopy (SEM) and transmission

Preparation of Composites.
The TiO 2 -PTA-HMT-MB composite was prepared in two steps. In the first step, the PTA-HMT was prepared according to our previous works [15,16]. 10 mL hexamethylenetetramine aqueous solution (1.0% w/w) was mixed with a 10 mL phosphotungstic acid aqueous solution (7.5% w/w). The immediate result was a milky suspension, which was magnetically stirred at 500 rpm for 3 h at an ambient temperature. Then, the white precipitate was filtered, washed with distilled water, and dried at 100°C. In the second step, the modified P25 TiO 2 nanopowder, i.e., 2.4. Evaluation of the Photocatalytic Activity. Solar photocatalytic experiments were performed similar to our previous works [14,15]. 100 mL RhB (15 mg L -1 ) and a specific amount of the photocatalyst were transferred to a beaker, capped by cellophane, and exposed to sunlight in September 2017 between 11 am and 3 pm at the Payame Noor University, Ardakan, Yazd, Iran (GPS coordinates: 32°29 ′ N, 53°59 ′ E). The experiments were conducted without stirring during solar irradiation. The visible light experiments were carried out using a metal halide lamp (500 W, Philips). 15 mL MB solutions (30 mg L -1 ) and a predetermined amount of the photocatalyst were transferred to a 200 mL water-cooled cylindrical Pyrex vessel reactor. The distance between the reactor and the light source was 10 cm. The radiation intensity is about 900 W/m 2 [17]. The reaction was started by switching on the light source after adding the composite to the dye solutions. The reactor was placed on a magnetic stirrer and stirred continuously. Temperature of the reactor was maintained at 27°C by considering a cooling chamber around the reactor and circulating water in it. During two processes, at given time intervals, 4 mL of suspension was withdrawn, and the composite was removed and then analyzed by a UV-Vis spectrophotometer. The C/C 0 values were obtained through the maximum absorption in the whole absorption spectrum in order to plot C/C 0 vs. time curves.

Results and Discussion
3.1. Characterizations. The IR spectrum of TiO 2 -PTA-HMT-MB exhibits the characteristic bands of the HMT and MB organic moieties and of TiO 2 and PTA inorganic moieties (Figure 1(a)). The band positioned at 555 cm -1 refers to the symmetric stretching of TiO 2 . The strong absorption peaks at 1100-750 cm -1 show the presence of PW 12 O 40 3anions with the α-Keggin structure. The 887 cm -1 band is related to the W-O b -W stretching mode of PTA while the 987 cm -1 band corresponds to its W-O d scissoring mode. The peak about 1250 cm -1 can be attributed to the vibration of the CH 2 of HMT [18], and the 1597 cm -1 corresponds to the vibration of the aromatic ring of MB [19]. The band gaps of TiO 2 and TiO 2 -PTA-HMT-MB were determined from the diffuse reflectance spectra using Tauc's plots (Figure 1(b)). It is clear that there are differences in the band gap values of TiO 2 and modified TiO 2 . The band gap of TiO 2 -PTA-HMT-MB (2.94 eV) has been shifted to the visible region (422 nm) in comparison with the band gap of TiO 2 (3.2 eV, 388 nm). To determine the specific surface area, total pore volume and mean pore diameter of the TiO 2 -PTA-HMT-MB, BET analysis method, and N 2 adsorption-desorption measurement (Figure 1(c)) were used. The measured specific surface area of the composite was 28.372 m 2 /g, which is lower than that of TiO 2 (56.191 m 2 /g). Indeed, the pores are loaded by the PTA-HMT-MB hybrid. The mean pore diameter and total pore volume were 31.795 nm and 0.2255 cm 3 g -1 , respectively. Textural parameters of the P25 TiO 2 and TiO 2 -PTA-HMT-MB hybrid are compared in Table 1. According to the IUPAC classification, the mentioned mean pore diameter belongs to the mesopore groups [20]. The TG and DSC curves of the TiO 2 -PTA-HMT-MB composite are shown in Figure 1(d). The exothermic peak at 593°C is related to the decomposition of phosphotungstate to WO 3 and P 2 O 5 . The weight loss of 3.29% is ascribed to the loss of the organic   Figures 4(a)-4(c). TiO 2 -PTA-HMT-MB cannot adsorb RhB because RhB (pK a = 3:1) is in neutral form at pH 9.0 (Figure 4(d)). The photodegradation of RhB is negligible due to photolysis and in the presence of TiO 2 (0.15 g/L) (Figure 4(d)). However, it becomes noticeable in the presence of TiO 2 -PTA-HMT-MB (0.1 g/L) that is an indicative shift in the photocatalytic activity of P25 TiO 2 nanoparticles toward the visible region upon surface modification with phosphotungstate hybrid. The composites of Al 2 O 3 -PTA-HMT-MB and RGO-PTA-HMT-MB failed to degrade RhB under sunlight irradiation (Figure 4(d)), which prove the role of TiO 2 in the photochemical process. Indeed, an excited surfaceadsorbed hybrid (PTA-HMT-MB) injects a charge into the conduction band of TiO 2 , and then reactive radicals are produced and degrade RhB.

Comparison with Other P25 TiO 2 -POM Composites.
Until now, many studies have been conducted in order to modify titanium dioxide [1,21], but the majority have used the sol-gel method not P25 TiO 2 . The photocatalytic efficiency of the TiO 2 -PTA-HMT-MB composite is compared with other P25 TiO 2 -POM composites in Table 2. It should be noted that the light sources are visible light and sunlight in this work while these are UV light and simulated sunlight in other works. As it is seen, the combination of P25 TiO 2 with lone POMs such as TiW 11 Ti [22] and HP 62 W 18 O 62 [23] does not shift the photocatalytic activity to visible light. However, this purpose has been achieved by the incorporation of a third component together with POMs such as Fe [24], polyethyleneimine [25], and HMT-MB (this work). The findings show that the advantage of the present photocatalyst is the shift of the photocatalytic activity toward the visible region, and hence, it shows the photocatalytic behavior under a visible light source (metal halide lamp) in addition to sunlight.

Conclusion
In summary, P25 TiO 2 was coated with PTA-HMT-MB hybrid. The TiO 2 -PTA-HMT-MB composite indicated a narrower band gap than P25 TiO 2 . The photocatalytic performance was evaluated by the photodegradation of RhB and MB under solar and visible light, respectively. The decolorization of these dyes with pure P25 TiO 2 in the solar/visible light is negligible while the POM hybrid coating causes decolorization with TiO 2 -PTA-HMT-MB to be remarkable. This work presents the incorporation of an organic hybrid of phosphotungstate as an effective strategy for exploration of shifting the photocatalytic activity of P25 TiO 2 toward the visible region, thereby facilitating the solar light driven photocatalysis.

Data Availability
The readers can access the supporting data through contact with the corresponding author.

Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.