Abstract
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.
1. Introduction
The major limitation of P25 TiO2 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 TiO2 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 TiO2 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 TiO2 surface [3–8]. One other is modification with polyoxometalates (POMs), but pure POMs cannot shift the photocatalytic activity of pure P25 TiO2 toward the visible region, and it is necessary to join them to valuable metallic or nonmetallic moieties [9–13]. 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 TiO2 with a colored organic hybrid of POMs has not been reported yet. The hybridization and immobilization onto P25 TiO2 resolve high solubility and poor recyclability of POMs and facilitate separation because coated TiO2 is not suspended in the solution.
2. Experimental
2.1. Chemicals and Reagents
The P25 TiO2 was from PlasmaChem GmbH (Berlin, Germany) with the nominal nm particle size, and the phosphotungstic acid hydrate (H3PW12O40·xH2O, ) was from Fluka. The hexamethylenetetramine (HMT) (C6H12N4, 99.5%) was from Sina Chemical Industries Co. (Shiraz, Iran). Activated alumina (Al2O3) and reduced graphene oxide (RGO) were consigned by Ardakan Industrial Ceramics Co. (Ardakan, Iran) and Nanostructured Coatings Institute (Yazd, Iran), respectively. Other reagents were purchased from commercial sources and used without further purification.
2.2. 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 BaSO4 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 electron microscopy (TEM) for TiO2-PTA-HMT-MB were performed using FESEM TESCAN MIRA3 and TEM Philips EM 208S, respectively.
2.3. Preparation of Composites
The TiO2-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% ) was mixed with a 10 mL phosphotungstic acid aqueous solution (7.5% ). 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 TiO2 nanopowder, i.e., TiO2-PTA-HMT-MB, was obtained by adding 0.195 g PTA-HMT to 50 mL MB solution (134 mg L-1), and the pH of the mixture reached 2.5. Then, 0.195 g P25 TiO2 was added to it, and the solution was stirred for 6 h. At the end, the precipitate was filtered and dried in an oven for 3 h, at 100°C. The Al2O3-PTA-HMT-MB and RGO-PTA-HMT-MB composites were prepared similar to the above manner.
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°29N, 53°59E). 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/m2 [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 / values were obtained through the maximum absorption in the whole absorption spectrum in order to plot / vs. time curves.
3. Results and Discussion
3.1. Characterizations
The IR spectrum of TiO2-PTA-HMT-MB exhibits the characteristic bands of the HMT and MB organic moieties and of TiO2 and PTA inorganic moieties (Figure 1(a)). The band positioned at 555 cm-1 refers to the symmetric stretching of TiO2. The strong absorption peaks at 1100-750 cm-1 show the presence of PW12O403- anions with the α-Keggin structure. The 887 cm-1 band is related to the W-Ob-W stretching mode of PTA while the 987 cm-1 band corresponds to its W-Od scissoring mode. The peak about 1250 cm-1 can be attributed to the vibration of the CH2 of HMT [18], and the 1597 cm-1 corresponds to the vibration of the aromatic ring of MB [19]. The band gaps of TiO2 and TiO2-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 TiO2 and modified TiO2. The band gap of TiO2-PTA-HMT-MB (2.94 eV) has been shifted to the visible region (422 nm) in comparison with the band gap of TiO2 (3.2 eV, 388 nm). To determine the specific surface area, total pore volume and mean pore diameter of the TiO2-PTA-HMT-MB, BET analysis method, and N2 adsorption-desorption measurement (Figure 1(c)) were used. The measured specific surface area of the composite was 28.372 m2/g, which is lower than that of TiO2 (56.191 m2/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 cm3g-1, respectively. Textural parameters of the P25 TiO2 and TiO2-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 TiO2-PTA-HMT-MB composite are shown in Figure 1(d). The exothermic peak at 593°C is related to the decomposition of phosphotungstate to WO3 and P2O5. The weight loss of 3.29% is ascribed to the loss of the organic moieties of the composite, i.e., HMT-MB. Figure 2 shows the SEM images of TiO2-PTA-HMT-MB and TiO2 nanoparticles. Comparison of these images displays that uniform distribution and neat morphology of TiO2 nanoparticles are not observed in TiO2-PTA-HMT-MB nanoparticles. The TEM images in Figure 3 show that modified TiO2 nanoparticles agglomerated to larger particles with an irregular shape than P25 TiO2. The sizes of TiO2-PTA-HMT-MB nanoparticles were estimated from the TEM image in the range of 30–80 nm which are in agreement with the sizes obtained from the SEM image (Figure 2).
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3.2. Photocatalytic Degradation of RhB
The composite was applied for photodecomposition of RhB under sunlight irradiation. Schematic illustration of the photocatalytic experiment is shown in Scheme 1. UV-Vis absorption spectra of RhB (15 mg/L, 100 mL, ) during the photodegradation under sunlight without photocatalyst (photolysis), in the presence of TiO2 and modified TiO2, are shown in Figures 4(a)–4(c). TiO2-PTA-HMT-MB cannot adsorb RhB because RhB () 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 TiO2 (0.15 g/L) (Figure 4(d)). However, it becomes noticeable in the presence of TiO2-PTA-HMT-MB (0.1 g/L) that is an indicative shift in the photocatalytic activity of P25 TiO2 nanoparticles toward the visible region upon surface modification with phosphotungstate hybrid. The composites of Al2O3-PTA-HMT-MB and RGO-PTA-HMT-MB failed to degrade RhB under sunlight irradiation (Figure 4(d)), which prove the role of TiO2 in the photochemical process. Indeed, an excited surface-adsorbed hybrid (PTA-HMT-MB) injects a charge into the conduction band of TiO2, and then reactive radicals are produced and degrade RhB.
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3.3. Photocatalytic Degradation of MB
The composite was then tested as a catalyst for the photocatalytic degradation of MB under visible light. While the P25 TiO2 (0.7 g/L) shows very low photocatalytic activity toward MB degradation, the TiO2-PTA-HMT-MB is very active (Figure 5). The photodegradation without photocatalyst (photolysis) and the removal of MB due to adsorption on the composite were evaluated in order to indicate the performance of the TiO2-PTA-HMT-MB photocatalyst (Figure 5(c)).
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3.4. Comparison with Other P25 TiO2-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 TiO2. The photocatalytic efficiency of the TiO2-PTA-HMT-MB composite is compared with other P25 TiO2-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 TiO2 with lone POMs such as TiW11Ti [22] and HP62W18O62 [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.
4. Conclusion
In summary, P25 TiO2 was coated with PTA-HMT-MB hybrid. The TiO2-PTA-HMT-MB composite indicated a narrower band gap than P25 TiO2. 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 TiO2 in the solar/visible light is negligible while the POM hybrid coating causes decolorization with TiO2-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 TiO2 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.
Acknowledgments
This work was supported by Payame Noor University. The authors express their appreciation for support of this study.