Res Chem Intermed DOI 10.1007/s11164-015-2356-z
Preparation and characterization of ZnTiO3–TiO2/ pillared montmorillonite composite catalyst for enhanced photocatalytic activity Liangxing Wu1,2 • Pingxiao Wu1,2,3 • Yajie Zhu1,2 Nengwu Zhu1,2 • Zhi Dang1,2
•
Received: 29 July 2015 / Accepted: 8 November 2015 Springer Science+Business Media Dordrecht 2015
Abstract ZnTiO3–TiO2/organic pillared montmorillonite (pMt) composite catalyst was successfully prepared in this paper by immobilizing ZnTiO3–TiO2 onto pMt. The composition and texture of the prepared composite catalyst were characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, energy dispersive spectrometry, ultraviolet–visible light (UV–Vis) diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. The photocatalytic activity was tested via photocatalytic degradation of methyl blue (MB) under both visible irradiation and UV light. The results indicated that the ZnTiO3–TiO2/pMt composite catalyst had an apparent absorption at the area of visible irradiation, and exhibited a higher efficiency of photocatalytic degredation of MB under visible irradiation. This was due to the heterostructure of ZnTiO3–TiO2, and the mesoporous structure and specific surface area of the ZnTiO3–TiO2/pMt composite. In addition, the results of the radical scavenging experiments showed that the holes and superoxide radicals are responsible for the degradation of MB under visible irradiation.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2356-z) contains supplementary material, which is available to authorized users. & Pingxiao Wu
[email protected] 1
College of Environment and Energy, South China University of Technology, Guangzhou 510006, People’s Republic of China
2
The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, Guangzhou 510006, People’s Republic of China
3
The Key Laboratory of Environmental Protection and Eco-Remediation of Guangdong Regular Higher Education Institutions, Guangzhou, People’s Republic of China
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Keywords ZnTiO3–TiO2 Organic pillared montmorillonite Visible irradiation Photocatalytic degradation
Introduction Photocatalysis is a kind of advanced oxidation process which uses active free radicals generated by the optical excitation of a semiconductor to degrade pollutants. Most semiconductors are metal oxides and sulfides, such as TiO2, Cu2O, ZnO and CdS [1–4]. TiO2 is the most commonly used material in research related to semiconductors, due to its chemical stability, high catalytic activity, nontoxicity and low cost [5, 6]. However, the following significant drawbacks with regard to TiO2 photocatalysis have limited its application in this regard: (1) TiO2 can be activated only by ultraviolet (UV) light, because of its band gap (ca. E = 3.2 eV); and (2) it has a low photo quantum efficiency yield due to the fast recombination of photogenerated electron–hole pairs [7]. A number of studies have attempted to overcome these limitations, in which doping has been determined to be one of the most effective ways to improve the catalytic activity of TiO2 [8–10]. Each TiO2 crystal has different degrees of lattice defects, and is only perfect at absolute zero. However, doping can introduce foreign materials into the crystals, thus repairing the defects. The resulting crystal not only has a significantly improved band gap, but also a much lower risk of electron–hole recombination [11, 12]. Doping can be carried out using nonmetal and metal materials, with the former often being N, C, S, Cl, and B [13–17], and the latter applying a number of transition metals, such as Fe, Zn, Nb, Ag, and lanthanide elements [18–22]. In previous works, ZnTiO3 was a dopant with a perovskite-type oxide that is used as a high-temperature adsorption desulfurization and dehydrogenation catalyst in industrial processes [23]. Some studies have shown that Zn dopants can considerably enhance the photocatalytic performance of TiO2, due to the changes in the lattice defects that affect the recombination of photogenerated electron–hole pairs [24, 25]. However, ZnTiO3–TiO2 has seldom been investigated with regard to photocatalytic applications under visible light irradiation. Although the photocatalytic activity of TiO2 can be enhanced by doping, it is necessary to achieve TiO2 immobilization in order to recycle the catalyst when photocatalytic processes are carried out in the liquid phase, which has limited the use of this material. Many studies have shown that both thin film technology and carrier load can be used to achieve TiO2 immobilization [26–29]. Montmorillonite (Mt) is a good choice for use as a carrier material, due to its hydrophobicity, tiny particles, high surface area and excellent adsorption performance. TiO2 can be immobilized onto hydrophobic Mt by means of ion exchange, with the immobilized TiO2 having very high photocatalytic efficiency, since the use of clay causes contaminants to gather on the surface of the material. The settling performance of immobilized TiO2 is also enhanced significantly, which is conducive to recycling [30–33].
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In this paper, ZnTiO3–TiO2 was prepared by a sol–gel method. Moreover, it was immobilized onto the organic pillared montmorillonite (pMt) by using the cation surfactant trimethylstearylammonium bromide (TSAB) to synthesize the ZnTiO3–TiO2/pMt composite catalyst. The structure of the obtained photocatalyst and the mechanism of the enhanced photocatalytic activity were analyzed by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The photocatalytic degradation of methyl blue (MB) was chosen as a reaction model to evaluate the photocatalytic activity of the obtained photocatalyst.
Experimental Materials Analytically pure tetrabutyl titanate (C16H36O4Ti), zinc acetate (Zn(Ac)22H2O), trimethylstearylammonium bromide (TSAB, C21H46BrN), MB (C37H27N3Na2O9S3), TiO2 (AR), KI, NaF, benzoquinone (BQ, C6H4O2), HCl and anhydrous ethanol (C2H6O) were used without further purification and purchased from the Guangzhou Chemical Reagent Factory (Guangzhou, PRC). The raw Mt was from a bentonite deposit in Nanhai (Guangdong, PRC), with a cation exchange capacity (CEC) of 78.3 mmol/100 g after precipitation. Deionized water was used in all the preparation processes. Preparation of ZnTiO3–TiO2 The ZnTiO3/TiO2 was synthesized by a sol–gel method, using the following procedure. Firstly, 2.5 g of Zn(Ac)22H2O powder was slowly added into 100 mL of C2H6O solution while vigorously stirring for 0.5–1.0 h at room temperature; then, a colorless and transparent solution was formed. Secondly, 1 mL of HCl (1.2 mol/L) and 3.9 mL of tetrabutyl titanate (with a molar ratio of Zn2? and Ti4? as 1:1) were added slowly into the above solution while stirring for 1 h at room temperature, so that ZnTiO3–TiO2 sol was prepared. Third, the sol was placed in a water bath (45 C) for 12 h as ZnTiO3–TiO2 gel, and then the gel was dried in a vacuum (55 C and -0.1 Pa) for 12 h as a pale yellow precursor. Finally, the precursor was calcined for 3 h at 600 C in a muffle furnace to produce ZnTiO3–TiO2. The obtained ZnTiO3–TiO2 was then ground to a 200-mesh particle size. Preparation of ZnTiO3–TiO2/pMt The Mt was soaked in water (10 g/100 mL) for 24 h to induce appropriate swelling. The swelled Mt was then treated with TSAB (with the same CEC as Mt) and was stirred continuously for 12 h at 60 C as an organic pMt. The pMt was collected by centrifugation and subsequently washed with deionized water until bromide ions were not present in the solution.
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ZnTiO3–TiO2 was slowly added to the pMt slurry with a ZnTiO3–TiO2: pMt weight ratio of 1:1. The slurry was stirred for 12 h at room temperature and the resulting composite was separated from the suspension by centrifugation. The ZnTiO3–TiO2/pMt was dried at 90 C until a constant weight was obtained. The sample was then ground to a 200-mesh size and stored in a vacuum container for further use. Characterizations Powder XRD patterns were obtained using a Bruker D8-ADVANCE X-ray diffractometer, with Cu Ka radiation (k = 0.154 nm) at 40 kV and 30 mA. XPS utilized an Axis Ultra DLD X-ray photoelectron spectrometer and the binding energy of C 1 s was shifted to 284.6 eV as an internal reference. The Fourier transform infrared (FTIR) spectra of the samples were collected by a PerkinElmer 1725X FTIR spectrometer using KBr pellets. The Raman spectra were observed with a Horiba Jobin–Yvon–Lab RAM spectrometer, with a 532.8-nm helium laser used as the excitation light source and a detection range of 50–1200 cm-1. The microstructures of the samples’ external surfaces were observed with a Phillips CM300 transmission electron microscope. The chemical composition of the samples was characterized by EDS analysis (Bruker, Quantax EDS detector). UV–Vis diffuse spectra were recorded by a Shimadzu 2450 UV–Vis spectrophotometer, and the reference was BaSO4. Photocatalytic degradation experiments The photocatalytic activity of the samples for the degradation of MB was determined by exposing them to visible irradiation and UV light. Appropriate amounts of catalyst were added to 60 mL of MB solution (10 ppm) and vigorously stirred in the dark for 60 min to establish the adsorption equilibrium. The suspension was then irradiated with visible irradiation for 6 h (Guangzhou China, July, 10:00–16:00) or UV light (8-W UV lamp; Philips, Holland; with the maximum emission at 254 nm). During the irradiation, at intervals of 1 h, samples (5 mL) of the suspension were collected and analyzed quantitatively with a UV–Vis spectrophotometer (Shimadzu 2450, Japan). Radical scavenging of experiments In order to examine the photocatalytic reaction mechanisms responsible for the degradation of MB under irradiation, different radical scavengers (KI, BQ and NaF) and samples (the molar ratio of radical scavenger and sample as 1:1) were added to the photocatalytic reaction. The process of radical scavenging experiments is the same as photocatalytic degradation experiments.
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Results and discussion XRD analysis In general, the d001 reflection peak is the main factor used to identify the clays. Figure 1 shows the XRD patterns of Mt and pMt. At 2h = 3–60, the d001 reflection of the pMt was shifted to a lower angle, with the d001 basal spacing increasing from 1.53 to 2.07 nm, which indicated that TSAB was successfully intercalated into the interlayers of the Mt. TSAB was a linear molecule, and its chain length was 2.14 nm. The TSAB entered into the Mt interlayers and formed tilted-vertical structures in these, because its chain length was greater than the thickness of a Mt interlayer. The orientation angle between the Mt and TSAB was calculated to be 58. Figure 2 shows the powder XRD patterns of ZnTiO3–TiO2 and ZnTiO3–TiO2/ pMt. At 2h = 20–60, the reflection peaks of ZnTiO3, anatase and rutile [34, 35] were in the same positions, which indicated that the ZnTiO3–TiO2 was successfully immobilized onto the pMt. In addition, Figs. 1 and 2 show that the d001 basal spacing of the ZnTiO3–TiO2/pMt decreased from 2.07 nm to 2.01 nm, and the reflection peak intensity became weaker compared with that of the pMt, which might be because the layered structure of pMt was destroyed by ZnTiO3–TiO2, and, thus, the crystallinity of pMt was reduced. Raman and IR analysis Figure 3 shows the Raman spectra (50–1200 cm-1) of the ZnTiO3–TiO2. In Fig. 3, the O–Ti–O symmetry variable angle vibration peak for the anatase phase’s Raman
Fig. 1 XRD patterns of samples: a Mt, b pMt, c ZnTiO3–TiO2/pMt
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Fig. 2 XRD patterns of samples: a pMt, b ZnTiO3–TiO2/pMt, c ZnTiO3–TiO2
Fig. 3 Raman spectra of ZnTiO3–TiO2
Eg vibration mode was 144 cm-1 [34], which showed that the anatase TiO2 formed after being heated at 600 C for 3 h. The characteristic peak of ZnTiO3 was at 438 cm-1, which indicated that ZnTiO3 successfully composited TiO2. In addition, the peaks at 238 cm-1 and 607 cm-1 might be caused by lattice disorder of the rutile phase or multiple scattering, while the peak at 730 cm-1 might be due to the very small amounts of Zn?2 that substituted Ti?4, thus causing distortion of the TiO2 crystal lattice.
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Figure 4 shows the FTIR spectra (500–4000 cm-1) of the Mt, pMt and ZnTiO3–TiO2/pMt. Comparing the spectrum of Mt with that of pMt, it could be seen that three new peaks emerged at 2920, 2851 and 1471 cm-1 in the latter. The peak at 2920 cm-1 indicated the presence of the C–H asymmetric stretching mode, the peak at 2851 cm-1 indicated the C–H symmetric stretching vibration mode, and the peak at 1471 cm-1 was the C–H bending mode [36], which showed that TSAB was intercalated into the interlayers of the pMt. In addition, after being loaded by ZnTiO3–TiO2, the FTIR spectra of the basic skeleton of the new composites (ZnTiO3–TiO2/pMt) were similar to that of pMt, but there were two new peaks at 1371 and 3755 cm-1. The peak at 1371 cm-1 was assigned to bending vibrations of a C–H bond in the species linking the –Ti–O–Ti– structural network [37], and the peak at 3755 cm-1 corresponded to the stretching vibration of OH groups linking with titanium atoms (Ti–OH) [38], which suggested that ZnTiO3–TiO2 intercalated into the pMt interlayers. TEM and EDS analysis TEM was used to further examine the particle size, crystallinity and morphology of samples. Figure 5a, b shows representative TEM images of ZnTiO3–TiO2/pMt and ZnTiO3–TiO2. Mt originally had a layered structure and a flake configuration. After the Mt was pillared by TSAB, it not only maintained its original layered structure and flake configuration, but the layers also became connected, so that a large number of holes appeared in the interlayers of the pMt, which was conducive to improving the adsorption performance of pMt. The ZnTiO3–TiO2 particle size was very small, and easy to reunite. When ZnTiO3–TiO2 was composited with pMt, its particles were randomly dispersed on the surfaces and interlayers of the pMt flakes.
Fig. 4 FTIR spectra of samples: a Mt, b pMt, c ZnTiO3–TiO2/pMt
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Fig. 5 TEM images of a ZnTiO3–TiO2/pMt, b ZnTiO3–TiO2 and SAED patterns of c ZnTiO3–TiO2/ pMt, d ZnTiO3–TiO2
The corresponding selected area electron diffraction (SAED) patterns of ZnTiO3–TiO2/pMt and ZnTiO3–TiO2 are shown in Fig. 5c, d, respectively. It could be seen that there was a weak polymorphic ring in Fig. 5c, which indicated that the crystallinity of ZnTiO3–TiO2/pMt was reduced. However, there were three bright rings of ZnTiO3–TiO2 including an anatase phase (101), rutile phase (110) and ZnTiO3 (101) in Fig. 5d, which possessed a polycrystalline structure and was well crystalline [39]. These were in agreement with the XRD results. In addition, the results of the EDS analysis (Table 1) show that Ti and Zn were detected in the pMt, implying that the immobilization was successful. UV–Vis analysis The diffuse reflectance UV–Vis absorption spectra of TiO2, ZnTiO3–TiO2 and ZnTiO3–TiO2/pMt composites are shown in Fig. 6. The absorption edges of
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Preparation and characterization of ZnTiO3–TiO2/pillared… Table 1 The surface chemical composition of ZnTiO3–TiO2 and ZnTiO3–TiO2/pMt Elements
ZnTiO3–TiO2
ZnTiO3–TiO2/pMt
Mass ratio (wt%)
Atomic ratio (at%)
Mass ratio (wt%)
Atomic ratio (at%)
O
32.58
59.19
48.39
56.38
Ti
34.77
21.11
1.67
0.53
Zn
29.16
13.10
1.35
0.48
C
2.25
5.44
12.58
19.53
Si
0.87
0.90
23.72
15.74
Ca
0.38
0.27
2.50
1.16
Al
0
0
6.58
4.54
Fe
0
0
1.91
0.64
Mg
0
0
1.30
1.00
Fig. 6 UV–Vis diffuse reflectance spectra of samples used for the determination of band gap energy: a TiO2, b ZnTiO3–TiO2, c ZnTiO3–TiO2/pMt
ZnTiO3–TiO2 were red-shifted in comparison with those of TiO2, and the absorption wavelength rose from 380 to 420 nm. These changes meant that the absorption of the visible light was significantly enhanced, and the band gap was decreased from 3.10 to 2.91 eV. The red-shifting of ZnTiO3–TiO2 could be attributed to different factors. The first factor was due to the change in the energy band structure of TiO2 that introduced ZnTiO3. Since ZnTiO3 was at an energy level higher than such of TiO2 [25, 40], this might result in the narrowing of the band gap of the ZnTiO3–TiO2 heterostructure [41]. The second factor was attributed to the change in the crystal phase composition of TiO2 [42], which might of enhanced the scattering ability in the UV light area and the absorption of the visible light. In addition, ZnTiO3 composited with TiO2 could
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increase the surface area, leading to surface states within the band gap to effectively reduce the band gap [41–43]. In addition, the band gap of ZnTiO3–TiO2 that was composited with pMt further reduced from 2.91 to 2.81 eV, which indicates that the pMt was able to improve the visible light absorption capacity and increase the photocatalytic activity of ZnTiO3–TiO2. XPS analysis The results of the XPS analysis for pMt and the ZnTiO3–TiO2/pMt composite catalyst are presented in Fig. 7A, and it can be seen that they mainly contain C, O, Ca, Si, Ti and Zn elements. Ti and Zn came from the preparation of the composite catalyst. Figure 7B shows the Ti 2p XPS spectra of the pMt and ZnTiO3–TiO2/pMt composite catalyst, and there were two clear peaks at 458.2 and 464.3 eV for the ZnTiO3–TiO2/pMt composite catalyst, which might be attributed to the Ti 2p3/2 and Ti 2p1/2 spin–orbital splitting photoelectrons of Ti, respectively, and this agreed well with previously reported XPS data for TiO2 [44]. The binding energy distance
Fig. 7 The XPS spectra of a pMt and b ZnTiO3–TiO2/pMt: A the survey spectra, B Ti 2p, C Zn 2p, D O 1s
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between Ti 2p3/2 and Ti 2p1/2 was 6.1 eV, which means that the Ti mainly existed as 4? valences [45]. Compared to pMt, the composite catalyst had photocatalytic properties because of the presence of Ti4? ions. The XPS spectra of the Zn 2p region are given in Fig. 7C, in which there was a Zn(2p3/2,1/2) doublet peak for the ZnTiO3–TiO2/pMt composite catalyst at 1020.6 and 1043.1 eV. The binding energy distance between Zn 2p3/2 and Zn 2p1/2 was 22.5 eV, indicating that the Zn mainly existed as 2? valence. While XPS analysis could investigate the surface composition and the existing chemical states of samples, it could not really determine the internal composition. Pervious work reported that it is difficult for Zn2? ions to substitute Ti4? ions during sample preparation, because the radius of Zn2? is larger than that of Ti4? [34]. It is, therefore, possible that the Zn ions resided over the surface of the composite catalyst particle in the form of ZnO clusters or ZnTiO3. Figure 7D shows the O 1 s XPS spectra of pMt and the ZnTiO3–TiO2/pMt composite catalyst. It can be observed that the peak of O 1 s shifts to the lower binding energy (531.6 eV to 531.2 eV), as the TiO2 composite combined with the zinc compound to form new chemical bonds (Ti–OH or O–Ti–O). In addition, the full width at half maximum (FWHM) values could also be used to obtain details of the oxidation state, with higher values meaning a greater oxidation state [46]. The O 1 s peak FWHM of the ZnTiO3–TiO2/pMt composite catalyst was larger than that of pMt, which indicated there were many surface oxygen vacancies in the ZnTiO3–TiO2/pMt composite catalyst. The surface oxygen vacancies could be considered as electron traps [25], and greater numbers of these could improve the ability to capture photoinduced electrons [47], which could enhance the separation of the electron–hole pairs, resulting in the improvement of the photocatalytic activity.
Fig. 8 Degradation of MB in different photocatalysts under visible irradiation: a None, b TiO2, c pMt, d ZnTiO3–TiO2, e ZnTiO3–TiO2/pMt
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Photocatalytic degradation performance In order to test the photocatalytic degradation performance of the ZnTiO3–TiO2/ pMt composite catalyst, we chose MB as a model pollutant to evaluate the catalytic behavior of the samples. Figure 8 shows the degradation of MB over the four kinds of catalysts under visible irradiation for 6 h (Guangzhou China, July, 10:00–16:00). It was clear that the MB solution was very stable, and no decomposition could be observed in the absence of photocatalysts. The TiO2, pMt and ZnTiO3–TiO2 were able to degrade MB by 10, 88 and 82 %, respectively. However, compared to these three photocatalysts, the use of the ZnTiO3–TiO2/pMt composite catalyst could achieve greater degradation of the MB solution, reaching 97 % for 6 h. This was mainly because of the following two reasons. Firstly, the unique mesoporous structure and higher surface area of the pMt could significantly improve the adsorption capacity of the material, which would provide a more active adsorption site toward target molecules. Secondly, the incorporation of TiO2 with ZnTiO3 could facilitate the transfer of photogenerated electrons from the bulk to surface, and, thus, inhibit the recombination of electron–hole pairs under visible irradiation. Similarly, we also studied the use of the ZnTiO3–TiO2/pMt composite catalyst for the photocatalytic degradation of MB under UV light, with the results shown in Fig. 9. It is clear that the degradation rate of TiO2 was greater than that of ZnTiO3– TiO2. This was because UV light had high energy and short wavelength characteristics, which was easily absorbed by TiO2 to photogenerate electrons and holes. In contrast, after TiO2 was composited with ZnTiO3, its absorption wavelength was red-shifted (from an UV light area to a visible irradiation area), indicating TiO2 reduced the absorption of UV light. Besides, the ZnTiO3–TiO2/pMt composite catalyst demonstrated degradation performance under UV light, and this might be attributed to the adsorption ability of pMt. Of course, we could also
Fig. 9 Degradation of MB in different photocatalysts under UV light: a None, b TiO2, c pMt, d ZnTiO3– TiO2, e ZnTiO3–TiO2/pMt
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Fig. 10 Photocatalytic degradation of MB under visible irradiation with different radical scavenging species: A BQ: a BQ alone, b ZnTiO3–TiO2 alone, c ZnTiO3–TiO2 and BQ, d ZnTiO3–TiO2/pMt alone, e ZnTiO3–TiO2/pMt and BQ; B IK: a KI alone, b ZnTiO3– TiO2 alone, c ZnTiO3–TiO2 and KI, d ZnTiO3–TiO2/pMt alone, e ZnTiO3–TiO2/pMt and KI; C NaF: a NaF alone, b ZnTiO3– TiO2 alone, c ZnTiO3–TiO2 and NaF, d ZnTiO3–TiO2/pMt alone, e ZnTiO3–TiO2/pMt and NaF
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confirm the above inference based on the results of the XRD and TEM analyses. The photocatalytic experiment was divided into two parts: the adsorption and photocatalytic phases. More than 30 % of the pollutant had degraded within 60 min after the photocatalytic experiment had started, which was the adsorption phase. In fact, whether under visible irradiation or UV light, the adsorption phase played a very important role in the photocatalytic degradation experiment. Mechanism From the results of the photocatalytic degradation of MB under visible irradiation, we concluded that the ZnTiO3–TiO2/pMt composite catalyst had greater photocatalytic activity, so its degradation mechanism could be explained as follows: First, the unique mesoporous structure of ZnTiO3–TiO2/pMt composite catalyst could enhance the visible irradiation harvest efficiency. This was due to the surface area of pMt, which could chemisorb more ZnTiO3–TiO2 molecules. Besides, the mesoporous structure improved the scattering ability of visible light, causing more light to pass into the interior cavity. Second, from the UV–Vis results, we found that the absorption edges of ZnTiO3–TiO2 were red-shifted because of its heterostructure, which was propitious to enhancing the separation efficiency of photogenerated electron–hole pairs, meaning that more electrons and holes would be involved in the photocatalytic degradation reaction. Third, the higher specific surface area of the ZnTiO3–TiO2/pMt composite catalyst would provide a stronger adsorption ability toward target molecules, which enhanced the interfacial reaction process of photoreaction [47–50]. In addition, we carried out tests with various scavengers (BQ as a scavenger for superoxide ions, KI as a scavenger for holes and the surface hydroxyl groups, NaF as a scavenger for hydroxyl groups) [51]. BQ or KI as scavengers were added separately in the reaction system; the photocatalytic reaction was significantly inhibited, as shown in Fig. 10A, B. This indicated that a superoxide (hole and the surface hydroxyl groups) might play an important role in the photocatalytic degradation of MB. However, when NaF was used in the reaction, the photocatalytic degradation of MB was not significantly affected (Fig. 10C), which suggested that surface hydroxyl groups did not play a pivotal role. Consequently, both holes and superoxide radicals are responsible for the degradation of MB under visible irradiation [50].
Conclusions In this work, ZnTiO3–TiO2/pMt composite catalyst was successfully prepared. The results of a variety of characterization tests showed that after doping and immobilization, the absorption wavelength of TiO2 was red-shifted from 380 to 420 nm, the band gap narrowed from 3.10 to 2.81 eV, and the oxygen vacancies increased significantly, indicating that the photocatalytic degradation performance of the composite catalyst was significantly improved. In addition, the results of the photocatalytic degradation experiment also showed that the photocatalytic
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degradation efficiency of MB by ZnTiO3–TiO2/pMt was significantly better than that of TiO2, pMt and ZnTiO3–TiO2 under visible irradiation. Acknowledgments The authors are grateful for financial support from the National Science Foundation of China (grant nos. 41472038, 41273122, 41073058), the Science and Technology Plan of Guangdong Province, China (grant no. 2014A020216002) and the Fundamental Research Funds for the Central Universities, South China University of Technology (SCUT; grant no. 2015ZP007).
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