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Preparation of a composite photocatalyst with enhanced photocatalytic activity: Smaller TiO2 carried on SiO2 microsphere
Applied Surface Science 493 (2019) 146–156
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Preparation of a composite photocatalyst with enhanced photocatalytic activity: Smaller TiO2 carried on SiO2 microsphere
T
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Jie Wang, Sijia Sun, Hao Ding , Wanting Chen, Yu Liang a Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Xueyuan Road, Haidian District, Beijing 100083, China b School of Materials Science and Technology, Shenyang University of Chemical Technology, 11st Road, Tiexi District, Shenyang 110142, Liaoning Province, China
A R T I C LE I N FO
A B S T R A C T
Keywords: SiO2 microsphere TiO2 Photocatalysis Surface hydroxyl
In order to improve the photocatalytic activity of nano-TiO2 and achieve its recovery, TiO2/MS-SiO2 photocatalyst is prepared when TiO2 nanoparticles are distributed on the surface of amorphous SiO2 microspheres (MS-SiO2) through sol-gel method, and its composition and morphology structure are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM). Besides, the photocatalytic properties are tested and evaluated by degradation efficiency of methyl orange under ultraviolet light. The results show that TiO2 nanoparticles uniformly dispersed on the surface of MS-SiO2 and combined with MS-SiO2 strongly by Ti-O-Si bond, whereas TiO2 is anatase crystal and grain size is about 5 nm. Under ultraviolet light for 90 min, the degradation rate of TiO2/MS-SiO2 for methyl orange reached 100%, which is better than pure TiO2 and P25. In addition, photocatalytic performance remains unchanged in a cyclic experiment. Transmission and separation efficiency of nano-TiO2 photo-generated charge carriers is improved by loading on the surface of MS-SiO2 so that MS-SiO2 can enhance the photocatalytic performance of pure TiO2. Moreover, this fact is illustrated by electron paramagnetic resonance intuitively.
1. Introduction Nano titanium dioxide (TiO2) is one of the most important inorganic photocatalytic materials [1], because of its high photocatalytic activity, biological and chemical inertness, and non-toxic nature. Moreover, it is characterized by stability, cost performance and non-toxicity and it has been applied in fields such as sewage treatment [2–4], air purification [5] and sterilization [6]. Therefore, nano-TiO2 has shown a good application prospect. However, nano-TiO2 still has some problems such as low quantization efficiency [7], easiness to aggregate [8,9] and difficulty in recovery [10] in the application process, which seriously restrict its application. Eventually, it is necessary and urgent to solve the above problems. Some studies have shown that it is an effective approach to solve the above problems that nano-TiO2 is supported by minerals and other inorganic carriers to form composite photocatalyst because the existence of carrier can improve the dispersibility of nano-TiO2 particles and attach the synergistic effect of the carrier. Which can improve the photocatalytic efficiency and reduce the actual cost of TiO2. At the same time, nano-TiO2 can be recycled because of the increasing particle size
of composite photocatalyst. At present, some minerals such as diatomite [11–13], montmorillonite [14], zeolite [15–17], kaolinite [18–20] and attapulgite [21] have been used as carriers to support nano-TiO2. Sun [11,12] gets TiO2/purified diatomite composite materials that the TiO2 nanoparticles uniformly dispersed on the surface of diatoms through a modified hydrolysis-deposition method and a modified sol–gel method. Zhang [14] gets recyclable TiO2/montmorillonite (MMT)/Fe3O4 nanocomposites that anatase-TiO2 (10–20 nm in diameter) are embedded in the layers of MMT by a single step of hydrolysis. Takeuchi [16] distributes TiO2 nanoparticles on Y-zeolite applied for the photocatalytic oxidation of benzene and toluene by a simple impregnation method. Kočí [18] found that kaolinite caused a decrease of anatase TiO2 crystallite size. Li [21] synthesizes TiO2/attapulgite clay photocatalyst with high decolorizing rate by sol-gel method. Although these efforts have made positive contributions to solve the above problems in the application of nano-TiO2, there are also some obvious deficiencies. First of all, these carriers are generally prepared by ultra-fine grinding of raw materials, with irregular grain shape and uneven particle size. Furthermore, there is a lack of hydroxyl groups that can firmly bound with TiO2 on their surface, so the effect of
⁎ Corresponding author at: Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Xueyuan Road, Haidian District, Beijing 100083, China. E-mail address: [email protected] (H. Ding).
https://doi.org/10.1016/j.apsusc.2019.07.005 Received 16 February 2019; Received in revised form 27 May 2019; Accepted 1 July 2019 Available online 02 July 2019 0169-4332/ © 2019 Published by Elsevier B.V.
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Fig. 1. The flowchart for the preparation of TiO2/MS-SiO2.
orange (C14H14N3SO3Na) from Beijing Chemical Industry Group Co., Ltd. (Beijing, China) is used as a target pollution for photocatalytic degradation. Ethanol and deionized water are also used as solvents throughout the preparation process.
combination is often not ideal. Secondly, these carriers are generally more expensive, which leads to the high cost of composite photocatalysts and difficulties to use on a large scale. Therefore, the preparation of TiO2 composite photocatalysts using the carriers with regular shape, rich surface hydroxyl groups and low cost will become an important research direction. SiO2 microspheres (MS-SiO2) are obtained from the by-product, named silica fume, which is produced during the industrial production of fused zirconia. It is a spherical micro particle with many advantages such as high purity, high spherical degree, regular shape, high dispersion and low price. Besides, it is achievable to get rich surface Si-OH groups. Therefore, it is expected to become an excellent carrier material. Thus we prepared TiO2/MS-SiO2 composite photocatalyst by the sol-gel method using MS-SiO2 as the carrier and tetrabutyl titanate (TBOT) as precursor in this study, we detect the composition and morphology of TiO2/MS-SiO2 and its photocatalytic degradation performance of methyl orange under UV light. Compared with pure TiO2, we investigate the action mechanism that nano-TiO2 loaded on the surface of MS-SiO2. Furthermore, the synergistic effect of MS-SiO2 increasing the photocatalytic performance of nano-TiO2 is discussed.
2.2. Catalysts preparation 2.2.1. Depolymerization and classification of MS-SiO2 Considering the agglomeration of the raw materials, MS-SiO2 particles need to be depolymerized and dispersed before compositing with nano-TiO2. The depolymerization method is described as follows: The MS-SiO2 materials are added into the ethanol solution to form a suspension. After adding ceramic grinding balls and polyacrylic acid solution (the ratio of ball to material, 3:1), the suspension is then ground in the stirred mill for 60 min and dispersed MS-SiO2 suspension are obtained after separating grinding ball. The dispersed MS-SiO2 suspension is centrifuged at 200 rpm for 5 min to get precipitation. After drying and grinding, MS-SiO2 with large size (about 2 μm) and uniform dispersion is obtained.
2. Experimetal
2.2.2. Preparation of TiO2/MS-SiO2 Firstly, the treated MS-SiO2 is added into the ethanol solution, which is stirred to form a suspension. Secondly, sodium hydroxide solution is added to the suspension dropwise to preprocess the MS-SiO2. Meanwhile, 20 mL of tetrabutyl titanate is added into suspension to form solution A, when 15 mL of acetylacetone and 60 mL of ethanol are dissolved into 60 mL of deionized water to form solution B. Thirdly, solution B is added into solution A and the mixture are stirred by a magnetic stirrer for 18 h to obtain MS-SiO2 loading hydrated TiO2 (TiO2·nH2O/MS-SiO2). Then, TiO2·nH2O/MS-SiO2 is filtered from the solution. After dehydrated, TiO2·nH2O/MS-SiO2 is put in a SRJX-5-13 chamber electric furnace and calcined at 600 °C for 2 h. Finally, the TiO2/MS-SiO2 photocatalyst is prepared. The specific steps are shown in Fig. 1. In order to comparative analysis, pure TiO2 is prepared without MS-SiO2 in same process.
2.1. Materials MS-SiO2 raw material, a by-product which is produced during the industrial production of fused zirconia, is provided by a zirconia production enterprise in Jiaozuo (China). The main chemical constituents (mass fraction, %) of MS-SiO2 are 93.50% SiO2 and 4.93% ZrO2. SiO2 is mainly composed of amorphous phase, that are aggregated to form the aggregates with the larger particle size. After depolymerization and classification, the scale of MS-SiO2 is about 3 μm existing in a dispersed state. Tetrabutyl titanate (C16H36O4Ti) from Beijing Chemical Industry Group Co., Ltd. (Beijing, China) is used as the titanium source. Acetylacetone (C5H8O2) supplied by Xi Long Chemical Co., Ltd. (Guangzhou, China) is used as a hydrolysis control agent. Methyl 147
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2.2.3. Characterization The crystal structure of the samples is characterized by powder Xray diffraction (XRD) analysis on a D8-ADVANCE X-ray power diffractometer (Bruker, Germany) with Cu Kα radiation (λ = 0.15406 nm) under the operation conditions of 40 kV and 40 mA. The samples are scanned in the range of 2θ from 10° to 80° with a 0.02° step at a scanning speed of 4°/min. We observe the morphology of TiO2/MS-SiO2 by scanning electron microscope (SEM) (S-3500N, Japan) and transmission electron microscope (TEM) (FEI Tecnai G2 F20, Portland). The surface functional groups are examined by an infrared spectroscope (Spectrum 100, China) with KBr as the medium, and the weights of each sample and KBr are 1 and 200 mg respectively. X-ray photoelectron spectroscopy (XPS) data are collected on Thermo ESCALAB250 instrument with a monochromatized Al Kα line source (200 W). The UV–vis diffuse reflectance spectrum is tested with a spectrophotometer (Cary 5000, Varian). Besides, the free radicals generated from different samples are tested by electron paramagnetic resonance (EPR) (Bruker A300, Germany). When DMPO is selected as free radical capture agent, OH% is measured in water and O2%− is measured in methanol. The photoluminescence (PL) spectra are measured on a high-resolution multi-function imaging spectrometer (iHR 550) using laser transmitter (532 nm).
Fig. 2. XRD patterns of MS-SiO2, pure TiO2, TiO(OH)2/MS-SiO2 and TiO2/MSSiO2.
2.3. Photocatalytic studies
pattern of TiO2·nH2O/MS-SiO2 also shows broad amorphous features that MS-SiO2 combined with TiO2·nH2O remains amorphous form. The diffraction pattern for pure TiO2 has three broad peaks at 25.4°, 37.9° and 48.1° corresponding to (101), (004) and (200) of anatase TiO2 and no other peaks indicative of crystalline phase. After calcination at 600 °C, TiO2·nH2O particles all turn into anatase crystalline state. The diffraction peaks of TiO2/MS-SiO2 still shows amorphous material peak of MS-SiO2 and significant anatase diffraction peak with strength and integrated peak shape, illustrating crystal morphology of TiO2 is good. Therefore, MS-SiO2 does not interfere with TiO2·nH2O turning into anatase phase as the carrier. Since anatase TiO2 has better photocatalytic performance than other forms of TiO2 under ultraviolet light [23]. It is believed that TiO2/MSSiO2 photocatalyst will exhibit good photocatalytic performance.
The photocatalytic degradation performance of TiO2/MS-SiO2 is tested with the methyl orange as the target degradation pollutant. The system is irradiated by a mercury lamp (300 W). Then, 50 mg TiO2/MSSiO2 is added to 50 mL prepared methyl orange dilution (concentration 10 mg/L). In order to reduce the measurement error caused by sample adsorption, the dark reaction is carried out for 1 h and then the concentration of methyl orange in the solution is measured. After turning on the light source, the concentration of methyl orange in solution is measured every 15 min. The photocatalytic degradation performance of the samples is characterized and evaluated based on the change of concentration(C/C0, C0 is the initial methyl orange concentration (mg/ L), C is the concentration at irradiation time t (mg/L)). The concentration of methyl orange is measured according to the following procedure. Firstly, the solution is centrifuged and the absorbance to 464 nm light of the supernatant is measured with a Cary 5000 UV–VIS spectrophotometer (USA Varian, USA). The concentration of methyl orange in the solution is calculated according to the relationship between absorbance (x) and concentration (c). The result of experimental calibration is as Formula (1). Besides, the degradation kinetics of methyl orange is investigated by fitting the experimental data to the Langmuir-Hinshelwood model [22]. Due to the reactant concentration is low, the degradation process is according with the following pseudo first-order kinetics equation (Formula (2)). Kapp is the pseudo-firstorder rate constant (min−1).
c = 13.9334x − 0.1867 ln
C = −kapp t C0
3.2. Morphology Fig. 3 shows the scanning electron microscopy (SEM) diagram of MS-SiO2, pure TiO2, TiO2/MS-SiO2 and the distribution diagram of O, Si and Ti elements within the SEM images of TiO2/MS-SiO2 (insert). It can be seen from Fig. 3a that MS-SiO2 is composed of spherical particles (microspheres) with a diameter of about 1–3 μm and the microspheres have a smooth surface dispersed from each other. Fig. 3b presents the SEM images of pure TiO2 prepared by sol-gel method. It can be seen that the particles are about 1 μm. Compared with MS-SiO2 (Fig. 3c), TiO2/MS-SiO2 can be seen that the surface of MS-SiO2 becomes rough after interaction with TiO2, and there are small particles uniformly coated (< 100 nm), so it can be inferred that these small particles should be TiO2. As can be seen from Fig. 3d, e and f, the distribution ranges of O, Si and Ti elements are consistent with the positions occupied by MS-SiO2, and the distribution is very uniform. This reflects not only the O and Si element properties of MS-SiO2, but also the fact that Ti elements have been uniformly loaded on the surface of MS-SiO2. In addition, XRF characterization is showed in Table S2. In TiO2/MS-SiO2, the mass ratio of TiO2 is 15% in TiO2/ MS-SiO2. It's suggested that TiO2 loads on the surface of MS-SiO2 uniformly, which is consistent with SEM images. Fig. 4 is TEM images of TiO2/MS-SiO2 and pure TiO2. On the surface of MS-SiO2, TiO2 is loaded uniformly in mono-disperse (Fig. 4a) and the scale of TiO2 is < 10 nm. As shown in Fig. 4b, the lattice fringe spacing of TiO2 display interplanar space of 0.352 nm in the particle, which respectively match well with the (001) plane of anatase TiO2 consistent
(1)
(2)
3. Results and discussions 3.1. Crystalline phase Fig. 2 is XRD spectrum of MS-SiO2, TiO2·nH2O/MS-SiO2 and TiO2/ MS-SiO2 (obtained from TiO2·nH2O/MS-SiO2 by calcining at 600 °C for 2 h) and pure TiO2 (obtained from TiO2·nH2O at 600 °C by calcining for 2 h). In this figure, there is a characteristic peak of an amorphous material with a nearly symmetrical peak shape and a certain strength between 16° and 28° in spectrum of MS-SiO2. It is showed that the phase of MS-SiO2 is amorphous SiO2. Besides, the X-ray powder diffraction 148
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Fig. 3. (a, b. c) SEM images of MS-SiO2, pure TiO2 and TiO2/MS-SiO2; (d, e, f) EDS element-mapping data of O, Si and Ti of TiO2/MS-SiO2.
3.3. Interaction between MS-SiO2 and TiO2
with the results of XRD. However, pure TiO2 exists in the form of particle aggregation. Although the grain size is about 20 nm, the agglomeration increases its size from nanometer to micrometer (Fig. 4c), which is consistent with the SEM image. Therefore, it is undoubted that the activity centers reduced, so much so performance of TiO2 will drop. The above results indicate that the composite particles are prepared that anatase nano-TiO2 loaded on the surface of MS-SiO2 in the process shown as Fig. 1. In addition, due to the presence of MS-SiO2, the size of TiO2 is significantly reduced and the dispersibility is improved, compared with the pure TiO2 prepared separately. Therefore, the exposure degree of active site will be increased and the quantization effect will be strengthened. It can be inferred that the performance of TiO2/MS-SiO2 should be improved than that of pure TiO2 and the amount of TiO2 can be saved in application.
3.3.1. FT-IR spectrum Fig. 5 shows the infrared spectra (FT-IR) of MS-SiO2, pure TiO2 and TiO2/MS-SiO2. In the spectrum of pure TiO2, it can be seen absorption peaks at 500–900 cm−1 caused by O-Ti-O bond. Peaks at 477.40 cm−1 can be ascribed to the bending vibration of O-Si-O, while those at 808.99 cm−1 and 1114.32 cm−1 could be assigned to the symmetric and anti-symmetric stretching vibration in the spectrum of MS-SiO2. Besides, peaks at 3436.34 and 1628.18 cm−1 can be ascribed to the stretching vibration and bending vibration of Si-OH or absorbed water. In comparison, the above characteristic peaks appeared in the spectrum of TiO2/MS-SiO2. But the peaks of SieO and OeH shift to 476.56 and 3437.58 cm−1, respectively, which should be attributed to the Si-O-Ti bond generated by the combination of SiO2 and TiO2 through hydroxyl groups. Therefore, it is believed that MS-SiO2 combined with TiO2 firmly
Fig. 4. (a, b) TEM image of TiO2/MS-SiO2 (c) TEM image of pureTiO2. 149
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d. As can be seen from Fig. 6a, the signal of Si2p and O1s appeared on the spectra of TiO2/MS-SiO2 and MS-SiO2. Besides, Ti2p peaks also appear on the spectra of TiO2/MS-SiO2, which is consistent with the composition of the two samples. Fig. 6b shows the change of binding energy of Si2p before and after the interaction between MS-SiO2 and TiO2. After processing by CASA XPS peaking software, the binding energy peak of Si2p of MS-SiO2 is decompose into two parts, the main peak at 103.26 eV and the subsidiary peak at 104.53 eV, respectively corresponding to the Si in the SiO2 structure and the Si on the surface connecting hydroxyl groups. In contrast, in the spectrum of TiO2/MS-SiO2 spectrum, Si2p only has a peak at 103.14 eV, which reflects the Si characteristics in the SiO2 tetrahedron. It is demonstrated that the Si connected with the surface hydroxyl group disappear. Therefore, it is obviously that Si-OH on the MS-SiO2 reacts with TiO2 to form the Si-O-Ti bond, and the Si state tends to be in the tetrahedron due to Ti replacing H. As can be seen from Fig. 6c, Ti2p has two binding energies of Ti2p3/ 2 and Ti2p1/2 in TiO2/MS-SiO2, which are 458.43 eV (main peak) and 464.12 eV (subsidiary peak), assigned to Ti4+. Compared with pure TiO2, inclusion of Ti into the system shifts the Ti2p3/2 peak maxima (Fig. 6c) from 458.53 eV to 458.43 eV and is accompanied by a decrease in Ti2p1/2 peak maxima from 464.21 to 464.12 eV, demonstrating an interaction between TiO2 and MS-SiO2 so that Ti's chemical environment is changed. Therefore, it is suggested that the combination of TiO2 and MS-SiO2 has the chemical properties and through hydroxyl groups. In Fig. 6d, O1s has three kinds of binding energy of 529.63 eV, 530.08 eV and 532.07 eV in pure TiO2, respectively represents the lattice of TiO2, oxygen in surface hydroxyl and oxygen of surface adsorption water. In TiO2/MS-SiO2, the binding energy shift to 529.76,
Fig. 5. FT-IR spectrum of MS-SiO2, TiO2/MS-SiO2, pureTiO2.
through chemical interaction in TiO2/MS-SiO2. Therefore, it will be conducive to improving the stability of TiO2/MS-SiO2 so that to improve its photocatalytic performance. 3.3.2. XPS spectra Fig. 6 is the full spectrum XPS scan of TiO2/MS-SiO2 (a) and the high-resolution scan of Si, Ti and O elements (b, c and d, respectively). For comparison, pure TiO2 is prepared. Moreover, high-resolution scan of Ti and O elements of pure TiO2 is also tested, as shown in Fig. 6c and
Fig. 6. XPS of MS-SiO2, pure TiO2 and TiO2/MS-SiO2. (a) Survey scan; (b) Si 2p regions; (c) Ti 2p regions; (d) O1s regions. 150
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Fig. 7. (a, b) Removal rate and photocatalytic degradation kinetics parameters of MO under UV light irradiation for different samples.
Fig. 8. (a, b) Removal rate and photocatalytic degradation kinetics parameters of MO with different concentration under UV light irradiation for TiO2/MS-SiO2.
Fig. 9. (a, b) Removal rate and photocatalytic degradation kinetics parameters of MO under UV light irradiation for TiO2/MS-SiO2 in stability test.
increased to 32.45% in TiO2/MS-SiO2, indicating that the carrier effect of MS-SiO2 significantly increases the number of hydroxyl groups on the surface of TiO2. Since the increase of surface hydroxyl groups can enhance the
531.87, and 532.56 eV respectively. It is suggested that chemical environmental of the oxygen is changed demonstrating the TiO2 combined with SiO2 by hydroxyl. In addition, according to the peak fitting results, the oxygen of hydroxyl content in pure TiO2 is 25.75%, which is 151
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Fig. 10. (a) UV–vis diffuse reflectance spectra and (b) the plots of between (αhν)2 and Eg, (c) Mott-Schottky plot for TiO2 electrode in saturated Na2SO4 electrolyte solution (0.1 M, pH = 6.8) vs SCE. (d) Proposed electron transfer of TiO2 under UV irradiation.
Fig. 11. (a) Photogenerated currents density and (b) Nyquist impedance plots of pure TiO2 and TiO2/MS-SiO2.
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has a significantly better photocatalytic degradation effect and efficiency than that of pure TiO2. This is undoubtedly the photocatalytic performance of TiO2 is improved after loaded on MSSiO2, which is consistent with the previous analysis. According to the above results, P25 showed the fastest degradation rate than TiO2/MS-SiO2. The methyl orange solution added with P25 is exposed to light for 30 min, with C/C0 of 0.033 and degradation rate of 96.7%. However, with the extension of light duration, C/C0 no longer changed, indicating that the degradation of P25 reached saturation. Therefore, methyl orange could not be completely degraded. Obviously, the degradation rate of methyl orange by TiO2/MS-SiO2 is slightly lower than P25, but the degradation effect is stronger than P25. Therefore, based on comprehensive judgment, it can be concluded that TiO2/MS-SiO2 composite photocatalyst has better photocatalytic performance than that of P25 and pure TiO2. Fig. 8 is the result of different concentration methyl orange added with TiO2/MS-SiO2 under ultraviolet irradiation. From Fig. 8, TiO2/MSSiO2 shows good photocatalytic performance with different concentration of methyl orange solution. In addition, the degradation rate is low when initial concentration of methyl orange solution is 80 ppm (Fig. 8b), the initial concentration of 10, 20 and 40 ppm all has high degradation rate. With the extension of time, the solution of methyl orange decreases rapidly. When the initial concentration of methyl orange solution is 10–40 ppm, degradation rate achieves 95% at 60 min. Moreover, when the initial concentration is 80 ppm, the degradation rate also achieve 90% at 90 min. Obviously, whether the concentration is high or low, the degradation effect of TiO2/MS-SiO2 is significant. It is suggested that TiO2/MS-SiO2 has good photocatalytic activity. In order to investigate the photocatalytic effect more accurately, the determination of total organic carbon (TOC) is used to evaluate the photocatalytic activity. After being irradiated for 60 min under ultraviolet light, the organic carbon content of methyl orange solution added with TiO2/MS-SiO2 is 3.97% of the initial concentration, and that with P25 is 3.17% of the initial concentration. The degradation extent converted from the determination of TOC are 96.03% and 96.83%, respectively. It can be seen that MO solution added with P25 and MSSiO2/TiO2 is basically completely mineralized. It's noteworthy that the degree of complete mineralization of MO by TiO2/MS-SiO2 is the same as P25, although the mass content of TiO2 in MS-SiO2/TiO2 is only 15%. It's suggested that the photocatalytic performance of MS-SiO2/TiO2 is better than P25.
Fig. 12. Photoluminescence (PL) spectra of pure TiO2 and TiO2/MS-SiO2.
Fig. 13. Photocatalytic degradation curves of MO over TiO2/MS-SiO2 in the presence of different radical scavengers including p-Benzoquinone (0.1 mM), EDTA-2Na (0.1 mM) and isopropanol (0.1 mM) under simulated solar light irradiation.
3.4.1.2. Cycle experiment. Fig. 9 is the photocatalytic performance on of TiO2/MS-SiO2 composite photocatalyst in the cycle experiment for 1–4 times. In Fig. 9, the photocatalytic effects of recycled samples are strong and stable, that C/C0 methyl orange solution is as low as 0.05 and the degradation rate is above 95% at the fourth recovery. Compared with initial samples, degradation rate doesn't change obviously. It is indicated that TiO2/MS-SiO2 has good reusability. In addition, TiO2/MS-SiO2 is easier to recover than P25. After the degradation, TiO2/MS-SiO2 can sediment to bottom of the container by standing so that the solution become clear. It's obviously caused by the large scale and the rapid settlement speed of TiO2/MS-SiO2. Therefore, TiO2/MS-SiO2 can be separated, recovered and recycled through centrifugation and filtration. By contrast, P25 adding in the solution is not easy to be recycled by standing. The above experiments confirm that nano-TiO2 has the characteristics of recovery and recycling due to its support with MS-SiO2.
photocatalytic efficiency [24], it is believed that the photocatalytic performance of TiO2/MS-SiO2 is better than that of pure TiO2.
3.4. Photoelectric properties and mechanism 3.4.1. Photocatalytic performance 3.4.1.1. Degradation of methyl orange. Fig. 7 is the result of the degradation of methyl orange by TiO2/MS-SiO2, pure TiO2, P25 and MS-SiO2 under ultraviolet light. As can be seen from Fig. 7a, with the increase of ultraviolet light time, the C/C0 of methyl orange solution added with MS-SiO2 only decreases slightly, which is equivalent to that without catalyst. It is indicated that MS-SiO2 has no degradation effect on methyl orange and can only play the role of carrier. In comparison, the C/C0 of methyl orange solution added with TiO2/MS-SiO2, pure TiO2 and P25 decreased significantly with the increase of light time. Among them, C/C0 of the solution is 0.179–0.016 under light for 30–90 min, which degradation rate is 82.1%~98.4%. Under light for 120 min, C/C0 is 0, which the degradation rate is 100%. However, the C/C0 of solution added with pure TiO2 is 0.803–0.050 under light for 30–120 min, which degradation rate is 19.7%~95%. Fig. 7b also reflects that degradation rate (Kapp) of methyl orange by TiO2/MSSiO2 is significantly higher than pure TiO2. Obviously, TiO2/MS-SiO2
3.4.2. Mechanism in photoelectric 3.4.2.1. Proposed charge transfer mechanism. Fig. 10 shows the UV–visible absorption spectra of TiO2/MS-SiO2, MS-SiO2, pure TiO2 and P25. It can be seen that the ultraviolet absorption degree of MSSiO2 that the wavelength < 400 nm is low, while that of pure TiO2 and P25 is high. The ultraviolet absorption degree of TiO2/MS-SiO2 is close 153
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Fig. 14. Electron paramagnetic resonance of different samples. (a) OH% generated by MS-SiO2; (b) O2%− generated by MS-SiO2; (c) OH% generated by pure TiO2; (d) O2%− generated by pure TiO2; (e) OH% generated by TiO2/MSSiO2; (f) O2%− generated by TiO2/MS-SiO2.
holes (h+), the band edge positions of conduction band (CB) and valence band (VB) are measured by Mott-Schottky measurement, which can directly probe the energy positions of TiO2. From Fig. 10c, the CB edges of TiO2 are found to be −0.43 eV (vs. SCE). Combined with the band gaps, the CB and VB edges of TiO2 are −0.18 eV and +2.980 eV (vs. NHE), respectively. Accordingly, schematic diagram of transfer of photogenerated charges of TiO2 is shown in Fig. 10d. Theoretically, TiO2/MS-SiO2 can produce OH% and O2%− under ultraviolet irradiation.
to that of pure TiO2 and P25, indicating that it has a strong ultraviolet absorption behavior similar to TiO2. In addition, although the ultraviolet light absorption of SiO2 is weak, the ultraviolet reflection is strong [25]. Moreover, MS - SiO2 treated with sodium hydroxide has rough surface so that the optical scattering effect is caused. Therefore, TiO2 can be irradiated twice or even several times by ultraviolet light in TiO2/MS-SiO2. It is doubtless TiO2 can be activated stronger to produce electrons and holes, and then improve the photocatalytic performance. According to Tauc formula [26] [αhν = A(hν − Eg)n/2], the band gaps of TiO2 and TiO2/MS-SiO2 are estimated to be 3.2 and 3.16 eV, respectively (Fig. 10b). The band gap of TiO2/MS-SiO2 is smaller than that of TiO2, which is conducive to the absorption of light by the material. In order to investigate the transfer mechanism of electrons (e−) and
3.4.2.2. Photoelectric response. To further understand the enhancement of photocatalytic activity between TiO2 and TiO2/MS-SiO2, transient photocurrent response and electrochemical impedance spectroscopy (EIS) are conducted under UV–visible light irradiation. Upon illumination, the photocurrent immediately rises and the 154
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Fig. 15. A schematic view of electron-hole separation and photocatalytic mechanism of TiO2/MS-SiO2 under UV irradiation.
of free radical generation under dark conditions. For MS SiO2, no characteristic peak exists either under dark condition or under ultraviolet condition. It is suggested that OH% and O2%− do not exist in the solution, so that MS SiO2 itself has no photocatalytic activity in the process of photocatalysis and only acts as a carrier. Reversely, strong characteristic peaks of OH% and O2%− is showed when TiO2/MS-SiO2 and pure TiO2 appeared under ultraviolet light, and the peak strength radical gradually increased with the extension of illumination time. However, compared with pure TiO2, the peak strength of hydroxyl radical and superoxide radical of TiO2/MS-SiO2 is twice as strong. It is indicated that the photocatalytic performance of TiO2 can be significantly improved by supporting on the surface of MS-SiO2, consistent with the results of AC impedance test. This should be caused by increasing the dispersion of TiO2 after loaded on MS-SiO2. Besides, ultraviolet absorption of TiO2 is strengthened because the ultraviolet is reflected by MS-SiO2. In summary, the reasons for the excellent photocatalytic activity of TiO2/MS-SiO2 composite photocatalyst can be attributed to the following three aspects: (1) Nano-TiO2 is uniform loaded on the MS-SiO2 with weak agglomeration, high dispersion, so that apparent particle size of TiO2 (< 100 nm) is significantly smaller than that of the pure TiO2 (on the micron scale). Therefore, the surface area of TiO2 increases and the exposure degree of the active site increases. (2) MS-SiO2 has abundant hydroxyl groups on its surface, which enables it to firmly bond with TiO2 through the interaction of surface hydroxyl groups, resulting in the high structural stability of TiO2/MS-SiO2. (3) The strong reflection of MS-SiO2 on ultraviolet radiation leads to the increased exposure of the supported TiO2 to ultraviolet radiation, which further enhances the generation of photoelectron and hole. The mechanism of photocatalytic activity of TiO2/MS-SiO2 composite photocatalyst is thus obtained, as shown in Fig. 15.
photocurrent rapidly decreases to zero as long as the light is switched off. As presented in Fig. 11a, the pulsed photocurrent density of the TiO2/MS-SiO2 sample is higher than TiO2, suggesting more efficient separation of photoexcited electron-hole pairs happened. As shown in the case of light (UV and visible light), arc radius of TiO2/MS-SiO2 is less than that of pure TiO2, which suggests that the electrochemical system composed of TiO2/MS-SiO2 composite photocatalyst has smaller resistance by fitting the equivalent circuit. Furthermore, the space charge layer resistance of TiO2/MS-SiO2 is smaller, namely the existence of MS-SiO2 carrier strengthens the Helmholtz layer charge separation process and changes the charge distribution of depletion layer. Hence, the existence of MS-SiO2 carrier reduces the resistance of interfacial charge transfer in the electrochemical system. It is doubtless that MS-SiO2 effectively promotes transport and separation of TiO2 photo-generated carriers. Therefore, photocatalytic effect is improved. From the photoluminescence (PL) spectra in Fig. 12, the PL emission bands at around 460 nm are observed in all samples. These belong to the emission of the band gap transition of TiO2 belong to the emission of the band gap transition of TiO2. Besides, it can be seen that the emission intensity of TiO2 is stronger than that of TiO2/MS-SiO2, indicating that charge recombination of TiO2 can be better suppressed after being carried by MS-SiO2.
3.4.2.3. Active species testing. In the photocatalytic processes, a series of reactive species, such as superoxide radical (O2%−), hole (h+), and hydroxyl radical (%OH), take part in the degradation reactions of organic contaminants. To explore the reactive species involved in the degradation of organic pollutants over TiO2/MS-SiO2, a series of contrast experiments in the presence of different radical scavengers are performed. Here, p-Benzoquinone is introduced to scavenge O2%−, EDTA-2Na for h+, and isopropanol for %OH in the solution. As shown in Fig. 13, the degradation extent of MO are significantly prohibited after the addition of BQ, TBA, and EDTA-2Na, inferring that all the O2%−, % OH and h+ are the reactive species responsible for the degradation of organic pollutants and O2%− effects photocatalytic degradation properties most. Therefore, O2%− is the most important reactive species in the photocatalytic processes. Fig. 14 is the result of OH% and O2%− generated by MS SiO2, pure TiO2 and TiO2/MS-SiO2 composite photocatalyst under dark conditions and ultraviolet conditions. As shown in the Fig. 14, all samples are free
4. Conclusion In this paper, tetrabutyl titanate is used as the precursor and nanoTiO2 is loaded on silica microspheres (MS-SiO2) by sol-gel method, and TiO2/MS-SiO2 composite photocatalyst is obtained. In TiO2/MS-SiO2, TiO2 is evenly distributed on the surface of MS-SiO2 and firmly combined with MS-SiO2 by Ti-O-Si bond. Besides, TiO2 has a high dispersion, with 1–4 nm, and apparent particles is < 100 nm. TiO2/MS-SiO2 has a good photocatalytic degradation performance 155
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for methyl orange. The degradation of methyl orange can be achieved completely under ultraviolet light for 90 min. Besides, the degradation effect and rate of TiO2/MS-SiO2 are better than that of pure TiO2. Moreover, TiO2/MS-SiO2 has good recycling usability and easy recovery, which the degradation effect of methyl orange by the fourth recycling samples is as good as that of initial samples. TiO2/MS-SiO2 has a strong absorption in the ultraviolet region, and the carriers OH% and O2%− generated under ultraviolet irradiation are the mechanism to form its photocatalytic performance. Besides, the presence of MS-SiO2 promotes the generation of OH% and O2%−. And the amount of OH% and O2%− generated by TiO2/MS-SiO2 under ultraviolet light is larger than that of pure TiO2. Moreover, TiO2/MS-SiO2 has a smaller resistance of space charge layer, and the resulting ac impedance is smaller than that of pure TiO2 under the same light. The reason for the excellent photocatalytic activity of TiO2/MS-SiO2 composite photocatalyst is due to the uniform load, solid bond, good dispersion of nano-TiO2 on the surface of MS-SiO2 and the increase in the degree of ultraviolet exposure.
10.1016/j.cattod.2013.10.090. [8] A.M. Horst, A.C. Neal, R.E. Mielke, Dispersion of TiO2 nanoparticle agglomerates by Pseudomonas aeruginosa, Appl. Environ. Microb. 76 (2010) 7292, https://doi.org/ 10.1128/AEM.00324-10. [9] L. Yali, Y. Zhanfeng, Z. Shuxue, De-agglomeration and dispersion of Nano-TiO2 in an agitator bead mill, J. Disper. Sci. Technol. 27 (2006) 983–990, https://doi.org/ 10.1080/01932690600766975. [10] C. Su, Y. Tong, M. Zhang, TiO2 nanoparticles immobilized on polyacrylonitrile nanofibers mats: a flexible and recyclable photocatalyst for phenol degradation, RSC Adv. 3 (2013) 7503–7512, https://doi.org/10.1039/c3ra40210j. [11] Z. Sun, Z. Hu, Y. Yan, Effect of preparation conditions on the characteristics and photocatalytic activity of TiO2/purified diatomite composite photocatalysts, Appl. Surf. Sci. 314 (2014) 251–259, https://doi.org/10.1016/j.apsusc.2014.06.171. [12] G. Zhang, Z. Sun, Y. Duan, Synthesis of nano-TiO2/diatomite composite and its photocatalytic degradation of gaseous formaldehyde, Appl. Surf. Sci. 412 (2017) 105–112, https://doi.org/10.1016/j.apsusc.2017.03.198. [13] B. Wang, F.C. de Godoi, Z. Sun, Synthesis, characterization and activity of an immobilized photocatalyst: natural porous diatomite supported titania nanoparticles, J. Colloid. Interf. Sci. 438 (2015) 204–211, https://doi.org/10.1016/j.jcis.2014.09. 064. [14] Z. Ping, Z. Mo, L. Han, Preparation and photocatalytic performance of magnetic TiO2/montmorillonite/Fe3O4 nanocomposites, Ind. Eng. Chem. Res. 53 (2014) 8057–8061, https://doi.org/10.1021/ie5001696. [15] Ito, M, Fukahori, S, Fujiwara. Adsorptive removal and photocatalytic decomposition of sulfamethazine in secondary effluent using TiO2–zeolite composites. Environ. Sci. Pollut. R. 21(2014)834–842. doi: https://doi.org/10.1007/s11356013-1707-9. [16] M. Takeuchi, M. Hidaka, M. Anpo, Efficient removal of toluene and benzene in gas phase by the TiO2/Y-zeolite hybrid photocatalyst, J. Hazard. Mater. 237-238 (2012) 133–139, https://doi.org/10.1016/j.jhazmat.2012.08.011. [17] A. Nasonova, K.S. Kim, Effects of TiO2 coating on zeolite particles for NO and SO2 removal by dielectric barrier discharge process, Catal. Today 211 (2013) 90–95, https://doi.org/10.1016/j.cattod.2013.03.006. [18] K. Kočí, V. Matějka, P. Kovář, Z. Lacný, L. Obalová, Comparison of the pure TiO2 and kaolinite/TiO2 composite as catalyst for CO2 photocatalytic reduction, Catal. Today 161 (2011) 105–109, https://doi.org/10.1016/j.cattod.2010.08.026. [19] Y. Zhang, H. Gan, G. Zhang, A novel mixed-phase TiO2/kaolinite composites and their photocatalytic activity for degradation of organic contaminants, Chem. Eng. J. 172 (2011) 936–943, https://doi.org/10.1016/j.cej.2011.07.005. [20] C. Wang, H. Shi, P. Zhang, Synthesis and characterization of kaolinite/TiO2 nanophotocatalysts, Appl. Clay Sci. 53 (2011) 646–649, https://doi.org/10.1016/j.clay. 2011.05.017. [21] C. Zhu, X. Wang, H. Qin, Removal of gaseous carbon bisulfide using dielectric barrier discharge plasmas combined with TiO2 coated attapulgite catalyst, Chem. Eng. J. 225 (2013) 567–573, https://doi.org/10.1016/j.cej.2013.03.107. [22] Ludvík. Beránek, An examination of the Langmuir-Hinshelwood model using ion exchange catalysts, Catal. Rev. 16 (1977) 1–35, https://doi.org/10.1080/ 03602457708079633. [23] L.K. Preethi, T. Mathews, M. Nand, S.N. Jha, C.S. Gopinath, S. Dash, Band alignment and charge transfer pathway in three phase anatase-rutile-brookite TiO2, nanotubes: an efficient photocatalyst for water splitting, Appl. Catal. B Environ. 218 (2017) 9–19, https://doi.org/10.1016/j.apcatb.2017.06.033. [24] C.S. Turchi, D.F. Ollis, Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack, J. Catal. 122 (1990) 178–192, https://doi.org/10.1016/0021-9517(90)90269-P. [25] Y. Wang, E. Chen, H. Lai, Enhanced light scattering and photovoltaic performance for dye-sensitized solar cells by embedding submicron SiO2/TiO2 core/shell particles in photoanode, Ceram. Int. 39 (2013) 5407–5413, https://doi.org/10.1016/j. ceramint.2012.12.048. [26] J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amorphous germanium, Phys. Status Solidi 3 (1966) 37–46, https://doi.org/10. 1016/0025-5408(68)90023-8.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51474194) and the Fundamental Research Funds for the Central Universities of China (No. 292018302). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.07.005. References [1] L. Lei, X. Chen, Titanium dioxide nanomaterials: self-structural modifications, Chem. Rev. 114 (2014) 9890, https://doi.org/10.1021/cr400624r. [2] I.K. Konstantinou, T.A. Albanis, TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review, Appl. Catal. B Environ. 49 (2004) 1–14, https://doi.org/10.1016/j.apcatb.2003.11.010. [3] Pu S, Zhu R, Ma H, Deng. Facile in-situ design strategy to disperse TiO2 nanoparticles on graphene for the enhanced photocatalytic degradation of rhodamine 6G, Appl. Catal. B Environ. 218(2017)208–219. doi:https://doi.org/10.1016/j. apcatb.2017.06.039. [4] S.Y. Lee, S.J. Park, TiO2 photocatalyst for water treatment applications, J. Ind. Eng. Chem. 19 (2013) 1761–1769, https://doi.org/10.1016/j.jiec.2013.07.012. [5] L. Lietti, P. Forzatti, Temperature programmed desorption/reaction of ammonia over V2O5/TiO2 De-NOxing catalysts, J. Catal. 147 (2010), https://doi.org/10. 1006/jcat.1994.1135. [6] B. Jalvo, M. Faraldos, A. Bahamonde, R. Rosal, Antimicrobial and antibiofilm efficacy of self-cleaning surfaces functionalized by TiO2 photocatalytic nanoparticles against Staphylococcus aureus and Pseudomonas putida, J. Hazard. Mater. 340 (2017) 160–170, https://doi.org/10.1016/j.jhazmat.2017.07.005. [7] N. Liu, X. Chen, J. Zhang, J.W. Schwank, A review on TiO2-based nanotubes synthesized via hydrothermal method: formation mechanism, structure modification, and photocatalytic applications, Catal. Today 225 (2014) 34–51, https://doi.org/
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Preparation of a composite photocatalyst with enhanced photocatalytic activity: Smaller TiO2 carried on SiO2 microsphere
T
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Jie Wang, Sijia Sun, Hao Ding , Wanting Chen, Yu Liang a Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Xueyuan Road, Haidian District, Beijing 100083, China b School of Materials Science and Technology, Shenyang University of Chemical Technology, 11st Road, Tiexi District, Shenyang 110142, Liaoning Province, China
A R T I C LE I N FO
A B S T R A C T
Keywords: SiO2 microsphere TiO2 Photocatalysis Surface hydroxyl
In order to improve the photocatalytic activity of nano-TiO2 and achieve its recovery, TiO2/MS-SiO2 photocatalyst is prepared when TiO2 nanoparticles are distributed on the surface of amorphous SiO2 microspheres (MS-SiO2) through sol-gel method, and its composition and morphology structure are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM). Besides, the photocatalytic properties are tested and evaluated by degradation efficiency of methyl orange under ultraviolet light. The results show that TiO2 nanoparticles uniformly dispersed on the surface of MS-SiO2 and combined with MS-SiO2 strongly by Ti-O-Si bond, whereas TiO2 is anatase crystal and grain size is about 5 nm. Under ultraviolet light for 90 min, the degradation rate of TiO2/MS-SiO2 for methyl orange reached 100%, which is better than pure TiO2 and P25. In addition, photocatalytic performance remains unchanged in a cyclic experiment. Transmission and separation efficiency of nano-TiO2 photo-generated charge carriers is improved by loading on the surface of MS-SiO2 so that MS-SiO2 can enhance the photocatalytic performance of pure TiO2. Moreover, this fact is illustrated by electron paramagnetic resonance intuitively.
1. Introduction Nano titanium dioxide (TiO2) is one of the most important inorganic photocatalytic materials [1], because of its high photocatalytic activity, biological and chemical inertness, and non-toxic nature. Moreover, it is characterized by stability, cost performance and non-toxicity and it has been applied in fields such as sewage treatment [2–4], air purification [5] and sterilization [6]. Therefore, nano-TiO2 has shown a good application prospect. However, nano-TiO2 still has some problems such as low quantization efficiency [7], easiness to aggregate [8,9] and difficulty in recovery [10] in the application process, which seriously restrict its application. Eventually, it is necessary and urgent to solve the above problems. Some studies have shown that it is an effective approach to solve the above problems that nano-TiO2 is supported by minerals and other inorganic carriers to form composite photocatalyst because the existence of carrier can improve the dispersibility of nano-TiO2 particles and attach the synergistic effect of the carrier. Which can improve the photocatalytic efficiency and reduce the actual cost of TiO2. At the same time, nano-TiO2 can be recycled because of the increasing particle size
of composite photocatalyst. At present, some minerals such as diatomite [11–13], montmorillonite [14], zeolite [15–17], kaolinite [18–20] and attapulgite [21] have been used as carriers to support nano-TiO2. Sun [11,12] gets TiO2/purified diatomite composite materials that the TiO2 nanoparticles uniformly dispersed on the surface of diatoms through a modified hydrolysis-deposition method and a modified sol–gel method. Zhang [14] gets recyclable TiO2/montmorillonite (MMT)/Fe3O4 nanocomposites that anatase-TiO2 (10–20 nm in diameter) are embedded in the layers of MMT by a single step of hydrolysis. Takeuchi [16] distributes TiO2 nanoparticles on Y-zeolite applied for the photocatalytic oxidation of benzene and toluene by a simple impregnation method. Kočí [18] found that kaolinite caused a decrease of anatase TiO2 crystallite size. Li [21] synthesizes TiO2/attapulgite clay photocatalyst with high decolorizing rate by sol-gel method. Although these efforts have made positive contributions to solve the above problems in the application of nano-TiO2, there are also some obvious deficiencies. First of all, these carriers are generally prepared by ultra-fine grinding of raw materials, with irregular grain shape and uneven particle size. Furthermore, there is a lack of hydroxyl groups that can firmly bound with TiO2 on their surface, so the effect of
⁎ Corresponding author at: Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Xueyuan Road, Haidian District, Beijing 100083, China. E-mail address: [email protected] (H. Ding).
https://doi.org/10.1016/j.apsusc.2019.07.005 Received 16 February 2019; Received in revised form 27 May 2019; Accepted 1 July 2019 Available online 02 July 2019 0169-4332/ © 2019 Published by Elsevier B.V.
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Fig. 1. The flowchart for the preparation of TiO2/MS-SiO2.
orange (C14H14N3SO3Na) from Beijing Chemical Industry Group Co., Ltd. (Beijing, China) is used as a target pollution for photocatalytic degradation. Ethanol and deionized water are also used as solvents throughout the preparation process.
combination is often not ideal. Secondly, these carriers are generally more expensive, which leads to the high cost of composite photocatalysts and difficulties to use on a large scale. Therefore, the preparation of TiO2 composite photocatalysts using the carriers with regular shape, rich surface hydroxyl groups and low cost will become an important research direction. SiO2 microspheres (MS-SiO2) are obtained from the by-product, named silica fume, which is produced during the industrial production of fused zirconia. It is a spherical micro particle with many advantages such as high purity, high spherical degree, regular shape, high dispersion and low price. Besides, it is achievable to get rich surface Si-OH groups. Therefore, it is expected to become an excellent carrier material. Thus we prepared TiO2/MS-SiO2 composite photocatalyst by the sol-gel method using MS-SiO2 as the carrier and tetrabutyl titanate (TBOT) as precursor in this study, we detect the composition and morphology of TiO2/MS-SiO2 and its photocatalytic degradation performance of methyl orange under UV light. Compared with pure TiO2, we investigate the action mechanism that nano-TiO2 loaded on the surface of MS-SiO2. Furthermore, the synergistic effect of MS-SiO2 increasing the photocatalytic performance of nano-TiO2 is discussed.
2.2. Catalysts preparation 2.2.1. Depolymerization and classification of MS-SiO2 Considering the agglomeration of the raw materials, MS-SiO2 particles need to be depolymerized and dispersed before compositing with nano-TiO2. The depolymerization method is described as follows: The MS-SiO2 materials are added into the ethanol solution to form a suspension. After adding ceramic grinding balls and polyacrylic acid solution (the ratio of ball to material, 3:1), the suspension is then ground in the stirred mill for 60 min and dispersed MS-SiO2 suspension are obtained after separating grinding ball. The dispersed MS-SiO2 suspension is centrifuged at 200 rpm for 5 min to get precipitation. After drying and grinding, MS-SiO2 with large size (about 2 μm) and uniform dispersion is obtained.
2. Experimetal
2.2.2. Preparation of TiO2/MS-SiO2 Firstly, the treated MS-SiO2 is added into the ethanol solution, which is stirred to form a suspension. Secondly, sodium hydroxide solution is added to the suspension dropwise to preprocess the MS-SiO2. Meanwhile, 20 mL of tetrabutyl titanate is added into suspension to form solution A, when 15 mL of acetylacetone and 60 mL of ethanol are dissolved into 60 mL of deionized water to form solution B. Thirdly, solution B is added into solution A and the mixture are stirred by a magnetic stirrer for 18 h to obtain MS-SiO2 loading hydrated TiO2 (TiO2·nH2O/MS-SiO2). Then, TiO2·nH2O/MS-SiO2 is filtered from the solution. After dehydrated, TiO2·nH2O/MS-SiO2 is put in a SRJX-5-13 chamber electric furnace and calcined at 600 °C for 2 h. Finally, the TiO2/MS-SiO2 photocatalyst is prepared. The specific steps are shown in Fig. 1. In order to comparative analysis, pure TiO2 is prepared without MS-SiO2 in same process.
2.1. Materials MS-SiO2 raw material, a by-product which is produced during the industrial production of fused zirconia, is provided by a zirconia production enterprise in Jiaozuo (China). The main chemical constituents (mass fraction, %) of MS-SiO2 are 93.50% SiO2 and 4.93% ZrO2. SiO2 is mainly composed of amorphous phase, that are aggregated to form the aggregates with the larger particle size. After depolymerization and classification, the scale of MS-SiO2 is about 3 μm existing in a dispersed state. Tetrabutyl titanate (C16H36O4Ti) from Beijing Chemical Industry Group Co., Ltd. (Beijing, China) is used as the titanium source. Acetylacetone (C5H8O2) supplied by Xi Long Chemical Co., Ltd. (Guangzhou, China) is used as a hydrolysis control agent. Methyl 147
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2.2.3. Characterization The crystal structure of the samples is characterized by powder Xray diffraction (XRD) analysis on a D8-ADVANCE X-ray power diffractometer (Bruker, Germany) with Cu Kα radiation (λ = 0.15406 nm) under the operation conditions of 40 kV and 40 mA. The samples are scanned in the range of 2θ from 10° to 80° with a 0.02° step at a scanning speed of 4°/min. We observe the morphology of TiO2/MS-SiO2 by scanning electron microscope (SEM) (S-3500N, Japan) and transmission electron microscope (TEM) (FEI Tecnai G2 F20, Portland). The surface functional groups are examined by an infrared spectroscope (Spectrum 100, China) with KBr as the medium, and the weights of each sample and KBr are 1 and 200 mg respectively. X-ray photoelectron spectroscopy (XPS) data are collected on Thermo ESCALAB250 instrument with a monochromatized Al Kα line source (200 W). The UV–vis diffuse reflectance spectrum is tested with a spectrophotometer (Cary 5000, Varian). Besides, the free radicals generated from different samples are tested by electron paramagnetic resonance (EPR) (Bruker A300, Germany). When DMPO is selected as free radical capture agent, OH% is measured in water and O2%− is measured in methanol. The photoluminescence (PL) spectra are measured on a high-resolution multi-function imaging spectrometer (iHR 550) using laser transmitter (532 nm).
Fig. 2. XRD patterns of MS-SiO2, pure TiO2, TiO(OH)2/MS-SiO2 and TiO2/MSSiO2.
2.3. Photocatalytic studies
pattern of TiO2·nH2O/MS-SiO2 also shows broad amorphous features that MS-SiO2 combined with TiO2·nH2O remains amorphous form. The diffraction pattern for pure TiO2 has three broad peaks at 25.4°, 37.9° and 48.1° corresponding to (101), (004) and (200) of anatase TiO2 and no other peaks indicative of crystalline phase. After calcination at 600 °C, TiO2·nH2O particles all turn into anatase crystalline state. The diffraction peaks of TiO2/MS-SiO2 still shows amorphous material peak of MS-SiO2 and significant anatase diffraction peak with strength and integrated peak shape, illustrating crystal morphology of TiO2 is good. Therefore, MS-SiO2 does not interfere with TiO2·nH2O turning into anatase phase as the carrier. Since anatase TiO2 has better photocatalytic performance than other forms of TiO2 under ultraviolet light [23]. It is believed that TiO2/MSSiO2 photocatalyst will exhibit good photocatalytic performance.
The photocatalytic degradation performance of TiO2/MS-SiO2 is tested with the methyl orange as the target degradation pollutant. The system is irradiated by a mercury lamp (300 W). Then, 50 mg TiO2/MSSiO2 is added to 50 mL prepared methyl orange dilution (concentration 10 mg/L). In order to reduce the measurement error caused by sample adsorption, the dark reaction is carried out for 1 h and then the concentration of methyl orange in the solution is measured. After turning on the light source, the concentration of methyl orange in solution is measured every 15 min. The photocatalytic degradation performance of the samples is characterized and evaluated based on the change of concentration(C/C0, C0 is the initial methyl orange concentration (mg/ L), C is the concentration at irradiation time t (mg/L)). The concentration of methyl orange is measured according to the following procedure. Firstly, the solution is centrifuged and the absorbance to 464 nm light of the supernatant is measured with a Cary 5000 UV–VIS spectrophotometer (USA Varian, USA). The concentration of methyl orange in the solution is calculated according to the relationship between absorbance (x) and concentration (c). The result of experimental calibration is as Formula (1). Besides, the degradation kinetics of methyl orange is investigated by fitting the experimental data to the Langmuir-Hinshelwood model [22]. Due to the reactant concentration is low, the degradation process is according with the following pseudo first-order kinetics equation (Formula (2)). Kapp is the pseudo-firstorder rate constant (min−1).
c = 13.9334x − 0.1867 ln
C = −kapp t C0
3.2. Morphology Fig. 3 shows the scanning electron microscopy (SEM) diagram of MS-SiO2, pure TiO2, TiO2/MS-SiO2 and the distribution diagram of O, Si and Ti elements within the SEM images of TiO2/MS-SiO2 (insert). It can be seen from Fig. 3a that MS-SiO2 is composed of spherical particles (microspheres) with a diameter of about 1–3 μm and the microspheres have a smooth surface dispersed from each other. Fig. 3b presents the SEM images of pure TiO2 prepared by sol-gel method. It can be seen that the particles are about 1 μm. Compared with MS-SiO2 (Fig. 3c), TiO2/MS-SiO2 can be seen that the surface of MS-SiO2 becomes rough after interaction with TiO2, and there are small particles uniformly coated (< 100 nm), so it can be inferred that these small particles should be TiO2. As can be seen from Fig. 3d, e and f, the distribution ranges of O, Si and Ti elements are consistent with the positions occupied by MS-SiO2, and the distribution is very uniform. This reflects not only the O and Si element properties of MS-SiO2, but also the fact that Ti elements have been uniformly loaded on the surface of MS-SiO2. In addition, XRF characterization is showed in Table S2. In TiO2/MS-SiO2, the mass ratio of TiO2 is 15% in TiO2/ MS-SiO2. It's suggested that TiO2 loads on the surface of MS-SiO2 uniformly, which is consistent with SEM images. Fig. 4 is TEM images of TiO2/MS-SiO2 and pure TiO2. On the surface of MS-SiO2, TiO2 is loaded uniformly in mono-disperse (Fig. 4a) and the scale of TiO2 is < 10 nm. As shown in Fig. 4b, the lattice fringe spacing of TiO2 display interplanar space of 0.352 nm in the particle, which respectively match well with the (001) plane of anatase TiO2 consistent
(1)
(2)
3. Results and discussions 3.1. Crystalline phase Fig. 2 is XRD spectrum of MS-SiO2, TiO2·nH2O/MS-SiO2 and TiO2/ MS-SiO2 (obtained from TiO2·nH2O/MS-SiO2 by calcining at 600 °C for 2 h) and pure TiO2 (obtained from TiO2·nH2O at 600 °C by calcining for 2 h). In this figure, there is a characteristic peak of an amorphous material with a nearly symmetrical peak shape and a certain strength between 16° and 28° in spectrum of MS-SiO2. It is showed that the phase of MS-SiO2 is amorphous SiO2. Besides, the X-ray powder diffraction 148
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Fig. 3. (a, b. c) SEM images of MS-SiO2, pure TiO2 and TiO2/MS-SiO2; (d, e, f) EDS element-mapping data of O, Si and Ti of TiO2/MS-SiO2.
3.3. Interaction between MS-SiO2 and TiO2
with the results of XRD. However, pure TiO2 exists in the form of particle aggregation. Although the grain size is about 20 nm, the agglomeration increases its size from nanometer to micrometer (Fig. 4c), which is consistent with the SEM image. Therefore, it is undoubted that the activity centers reduced, so much so performance of TiO2 will drop. The above results indicate that the composite particles are prepared that anatase nano-TiO2 loaded on the surface of MS-SiO2 in the process shown as Fig. 1. In addition, due to the presence of MS-SiO2, the size of TiO2 is significantly reduced and the dispersibility is improved, compared with the pure TiO2 prepared separately. Therefore, the exposure degree of active site will be increased and the quantization effect will be strengthened. It can be inferred that the performance of TiO2/MS-SiO2 should be improved than that of pure TiO2 and the amount of TiO2 can be saved in application.
3.3.1. FT-IR spectrum Fig. 5 shows the infrared spectra (FT-IR) of MS-SiO2, pure TiO2 and TiO2/MS-SiO2. In the spectrum of pure TiO2, it can be seen absorption peaks at 500–900 cm−1 caused by O-Ti-O bond. Peaks at 477.40 cm−1 can be ascribed to the bending vibration of O-Si-O, while those at 808.99 cm−1 and 1114.32 cm−1 could be assigned to the symmetric and anti-symmetric stretching vibration in the spectrum of MS-SiO2. Besides, peaks at 3436.34 and 1628.18 cm−1 can be ascribed to the stretching vibration and bending vibration of Si-OH or absorbed water. In comparison, the above characteristic peaks appeared in the spectrum of TiO2/MS-SiO2. But the peaks of SieO and OeH shift to 476.56 and 3437.58 cm−1, respectively, which should be attributed to the Si-O-Ti bond generated by the combination of SiO2 and TiO2 through hydroxyl groups. Therefore, it is believed that MS-SiO2 combined with TiO2 firmly
Fig. 4. (a, b) TEM image of TiO2/MS-SiO2 (c) TEM image of pureTiO2. 149
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d. As can be seen from Fig. 6a, the signal of Si2p and O1s appeared on the spectra of TiO2/MS-SiO2 and MS-SiO2. Besides, Ti2p peaks also appear on the spectra of TiO2/MS-SiO2, which is consistent with the composition of the two samples. Fig. 6b shows the change of binding energy of Si2p before and after the interaction between MS-SiO2 and TiO2. After processing by CASA XPS peaking software, the binding energy peak of Si2p of MS-SiO2 is decompose into two parts, the main peak at 103.26 eV and the subsidiary peak at 104.53 eV, respectively corresponding to the Si in the SiO2 structure and the Si on the surface connecting hydroxyl groups. In contrast, in the spectrum of TiO2/MS-SiO2 spectrum, Si2p only has a peak at 103.14 eV, which reflects the Si characteristics in the SiO2 tetrahedron. It is demonstrated that the Si connected with the surface hydroxyl group disappear. Therefore, it is obviously that Si-OH on the MS-SiO2 reacts with TiO2 to form the Si-O-Ti bond, and the Si state tends to be in the tetrahedron due to Ti replacing H. As can be seen from Fig. 6c, Ti2p has two binding energies of Ti2p3/ 2 and Ti2p1/2 in TiO2/MS-SiO2, which are 458.43 eV (main peak) and 464.12 eV (subsidiary peak), assigned to Ti4+. Compared with pure TiO2, inclusion of Ti into the system shifts the Ti2p3/2 peak maxima (Fig. 6c) from 458.53 eV to 458.43 eV and is accompanied by a decrease in Ti2p1/2 peak maxima from 464.21 to 464.12 eV, demonstrating an interaction between TiO2 and MS-SiO2 so that Ti's chemical environment is changed. Therefore, it is suggested that the combination of TiO2 and MS-SiO2 has the chemical properties and through hydroxyl groups. In Fig. 6d, O1s has three kinds of binding energy of 529.63 eV, 530.08 eV and 532.07 eV in pure TiO2, respectively represents the lattice of TiO2, oxygen in surface hydroxyl and oxygen of surface adsorption water. In TiO2/MS-SiO2, the binding energy shift to 529.76,
Fig. 5. FT-IR spectrum of MS-SiO2, TiO2/MS-SiO2, pureTiO2.
through chemical interaction in TiO2/MS-SiO2. Therefore, it will be conducive to improving the stability of TiO2/MS-SiO2 so that to improve its photocatalytic performance. 3.3.2. XPS spectra Fig. 6 is the full spectrum XPS scan of TiO2/MS-SiO2 (a) and the high-resolution scan of Si, Ti and O elements (b, c and d, respectively). For comparison, pure TiO2 is prepared. Moreover, high-resolution scan of Ti and O elements of pure TiO2 is also tested, as shown in Fig. 6c and
Fig. 6. XPS of MS-SiO2, pure TiO2 and TiO2/MS-SiO2. (a) Survey scan; (b) Si 2p regions; (c) Ti 2p regions; (d) O1s regions. 150
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Fig. 7. (a, b) Removal rate and photocatalytic degradation kinetics parameters of MO under UV light irradiation for different samples.
Fig. 8. (a, b) Removal rate and photocatalytic degradation kinetics parameters of MO with different concentration under UV light irradiation for TiO2/MS-SiO2.
Fig. 9. (a, b) Removal rate and photocatalytic degradation kinetics parameters of MO under UV light irradiation for TiO2/MS-SiO2 in stability test.
increased to 32.45% in TiO2/MS-SiO2, indicating that the carrier effect of MS-SiO2 significantly increases the number of hydroxyl groups on the surface of TiO2. Since the increase of surface hydroxyl groups can enhance the
531.87, and 532.56 eV respectively. It is suggested that chemical environmental of the oxygen is changed demonstrating the TiO2 combined with SiO2 by hydroxyl. In addition, according to the peak fitting results, the oxygen of hydroxyl content in pure TiO2 is 25.75%, which is 151
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Fig. 10. (a) UV–vis diffuse reflectance spectra and (b) the plots of between (αhν)2 and Eg, (c) Mott-Schottky plot for TiO2 electrode in saturated Na2SO4 electrolyte solution (0.1 M, pH = 6.8) vs SCE. (d) Proposed electron transfer of TiO2 under UV irradiation.
Fig. 11. (a) Photogenerated currents density and (b) Nyquist impedance plots of pure TiO2 and TiO2/MS-SiO2.
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has a significantly better photocatalytic degradation effect and efficiency than that of pure TiO2. This is undoubtedly the photocatalytic performance of TiO2 is improved after loaded on MSSiO2, which is consistent with the previous analysis. According to the above results, P25 showed the fastest degradation rate than TiO2/MS-SiO2. The methyl orange solution added with P25 is exposed to light for 30 min, with C/C0 of 0.033 and degradation rate of 96.7%. However, with the extension of light duration, C/C0 no longer changed, indicating that the degradation of P25 reached saturation. Therefore, methyl orange could not be completely degraded. Obviously, the degradation rate of methyl orange by TiO2/MS-SiO2 is slightly lower than P25, but the degradation effect is stronger than P25. Therefore, based on comprehensive judgment, it can be concluded that TiO2/MS-SiO2 composite photocatalyst has better photocatalytic performance than that of P25 and pure TiO2. Fig. 8 is the result of different concentration methyl orange added with TiO2/MS-SiO2 under ultraviolet irradiation. From Fig. 8, TiO2/MSSiO2 shows good photocatalytic performance with different concentration of methyl orange solution. In addition, the degradation rate is low when initial concentration of methyl orange solution is 80 ppm (Fig. 8b), the initial concentration of 10, 20 and 40 ppm all has high degradation rate. With the extension of time, the solution of methyl orange decreases rapidly. When the initial concentration of methyl orange solution is 10–40 ppm, degradation rate achieves 95% at 60 min. Moreover, when the initial concentration is 80 ppm, the degradation rate also achieve 90% at 90 min. Obviously, whether the concentration is high or low, the degradation effect of TiO2/MS-SiO2 is significant. It is suggested that TiO2/MS-SiO2 has good photocatalytic activity. In order to investigate the photocatalytic effect more accurately, the determination of total organic carbon (TOC) is used to evaluate the photocatalytic activity. After being irradiated for 60 min under ultraviolet light, the organic carbon content of methyl orange solution added with TiO2/MS-SiO2 is 3.97% of the initial concentration, and that with P25 is 3.17% of the initial concentration. The degradation extent converted from the determination of TOC are 96.03% and 96.83%, respectively. It can be seen that MO solution added with P25 and MSSiO2/TiO2 is basically completely mineralized. It's noteworthy that the degree of complete mineralization of MO by TiO2/MS-SiO2 is the same as P25, although the mass content of TiO2 in MS-SiO2/TiO2 is only 15%. It's suggested that the photocatalytic performance of MS-SiO2/TiO2 is better than P25.
Fig. 12. Photoluminescence (PL) spectra of pure TiO2 and TiO2/MS-SiO2.
Fig. 13. Photocatalytic degradation curves of MO over TiO2/MS-SiO2 in the presence of different radical scavengers including p-Benzoquinone (0.1 mM), EDTA-2Na (0.1 mM) and isopropanol (0.1 mM) under simulated solar light irradiation.
3.4.1.2. Cycle experiment. Fig. 9 is the photocatalytic performance on of TiO2/MS-SiO2 composite photocatalyst in the cycle experiment for 1–4 times. In Fig. 9, the photocatalytic effects of recycled samples are strong and stable, that C/C0 methyl orange solution is as low as 0.05 and the degradation rate is above 95% at the fourth recovery. Compared with initial samples, degradation rate doesn't change obviously. It is indicated that TiO2/MS-SiO2 has good reusability. In addition, TiO2/MS-SiO2 is easier to recover than P25. After the degradation, TiO2/MS-SiO2 can sediment to bottom of the container by standing so that the solution become clear. It's obviously caused by the large scale and the rapid settlement speed of TiO2/MS-SiO2. Therefore, TiO2/MS-SiO2 can be separated, recovered and recycled through centrifugation and filtration. By contrast, P25 adding in the solution is not easy to be recycled by standing. The above experiments confirm that nano-TiO2 has the characteristics of recovery and recycling due to its support with MS-SiO2.
photocatalytic efficiency [24], it is believed that the photocatalytic performance of TiO2/MS-SiO2 is better than that of pure TiO2.
3.4. Photoelectric properties and mechanism 3.4.1. Photocatalytic performance 3.4.1.1. Degradation of methyl orange. Fig. 7 is the result of the degradation of methyl orange by TiO2/MS-SiO2, pure TiO2, P25 and MS-SiO2 under ultraviolet light. As can be seen from Fig. 7a, with the increase of ultraviolet light time, the C/C0 of methyl orange solution added with MS-SiO2 only decreases slightly, which is equivalent to that without catalyst. It is indicated that MS-SiO2 has no degradation effect on methyl orange and can only play the role of carrier. In comparison, the C/C0 of methyl orange solution added with TiO2/MS-SiO2, pure TiO2 and P25 decreased significantly with the increase of light time. Among them, C/C0 of the solution is 0.179–0.016 under light for 30–90 min, which degradation rate is 82.1%~98.4%. Under light for 120 min, C/C0 is 0, which the degradation rate is 100%. However, the C/C0 of solution added with pure TiO2 is 0.803–0.050 under light for 30–120 min, which degradation rate is 19.7%~95%. Fig. 7b also reflects that degradation rate (Kapp) of methyl orange by TiO2/MSSiO2 is significantly higher than pure TiO2. Obviously, TiO2/MS-SiO2
3.4.2. Mechanism in photoelectric 3.4.2.1. Proposed charge transfer mechanism. Fig. 10 shows the UV–visible absorption spectra of TiO2/MS-SiO2, MS-SiO2, pure TiO2 and P25. It can be seen that the ultraviolet absorption degree of MSSiO2 that the wavelength < 400 nm is low, while that of pure TiO2 and P25 is high. The ultraviolet absorption degree of TiO2/MS-SiO2 is close 153
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Fig. 14. Electron paramagnetic resonance of different samples. (a) OH% generated by MS-SiO2; (b) O2%− generated by MS-SiO2; (c) OH% generated by pure TiO2; (d) O2%− generated by pure TiO2; (e) OH% generated by TiO2/MSSiO2; (f) O2%− generated by TiO2/MS-SiO2.
holes (h+), the band edge positions of conduction band (CB) and valence band (VB) are measured by Mott-Schottky measurement, which can directly probe the energy positions of TiO2. From Fig. 10c, the CB edges of TiO2 are found to be −0.43 eV (vs. SCE). Combined with the band gaps, the CB and VB edges of TiO2 are −0.18 eV and +2.980 eV (vs. NHE), respectively. Accordingly, schematic diagram of transfer of photogenerated charges of TiO2 is shown in Fig. 10d. Theoretically, TiO2/MS-SiO2 can produce OH% and O2%− under ultraviolet irradiation.
to that of pure TiO2 and P25, indicating that it has a strong ultraviolet absorption behavior similar to TiO2. In addition, although the ultraviolet light absorption of SiO2 is weak, the ultraviolet reflection is strong [25]. Moreover, MS - SiO2 treated with sodium hydroxide has rough surface so that the optical scattering effect is caused. Therefore, TiO2 can be irradiated twice or even several times by ultraviolet light in TiO2/MS-SiO2. It is doubtless TiO2 can be activated stronger to produce electrons and holes, and then improve the photocatalytic performance. According to Tauc formula [26] [αhν = A(hν − Eg)n/2], the band gaps of TiO2 and TiO2/MS-SiO2 are estimated to be 3.2 and 3.16 eV, respectively (Fig. 10b). The band gap of TiO2/MS-SiO2 is smaller than that of TiO2, which is conducive to the absorption of light by the material. In order to investigate the transfer mechanism of electrons (e−) and
3.4.2.2. Photoelectric response. To further understand the enhancement of photocatalytic activity between TiO2 and TiO2/MS-SiO2, transient photocurrent response and electrochemical impedance spectroscopy (EIS) are conducted under UV–visible light irradiation. Upon illumination, the photocurrent immediately rises and the 154
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Fig. 15. A schematic view of electron-hole separation and photocatalytic mechanism of TiO2/MS-SiO2 under UV irradiation.
of free radical generation under dark conditions. For MS SiO2, no characteristic peak exists either under dark condition or under ultraviolet condition. It is suggested that OH% and O2%− do not exist in the solution, so that MS SiO2 itself has no photocatalytic activity in the process of photocatalysis and only acts as a carrier. Reversely, strong characteristic peaks of OH% and O2%− is showed when TiO2/MS-SiO2 and pure TiO2 appeared under ultraviolet light, and the peak strength radical gradually increased with the extension of illumination time. However, compared with pure TiO2, the peak strength of hydroxyl radical and superoxide radical of TiO2/MS-SiO2 is twice as strong. It is indicated that the photocatalytic performance of TiO2 can be significantly improved by supporting on the surface of MS-SiO2, consistent with the results of AC impedance test. This should be caused by increasing the dispersion of TiO2 after loaded on MS-SiO2. Besides, ultraviolet absorption of TiO2 is strengthened because the ultraviolet is reflected by MS-SiO2. In summary, the reasons for the excellent photocatalytic activity of TiO2/MS-SiO2 composite photocatalyst can be attributed to the following three aspects: (1) Nano-TiO2 is uniform loaded on the MS-SiO2 with weak agglomeration, high dispersion, so that apparent particle size of TiO2 (< 100 nm) is significantly smaller than that of the pure TiO2 (on the micron scale). Therefore, the surface area of TiO2 increases and the exposure degree of the active site increases. (2) MS-SiO2 has abundant hydroxyl groups on its surface, which enables it to firmly bond with TiO2 through the interaction of surface hydroxyl groups, resulting in the high structural stability of TiO2/MS-SiO2. (3) The strong reflection of MS-SiO2 on ultraviolet radiation leads to the increased exposure of the supported TiO2 to ultraviolet radiation, which further enhances the generation of photoelectron and hole. The mechanism of photocatalytic activity of TiO2/MS-SiO2 composite photocatalyst is thus obtained, as shown in Fig. 15.
photocurrent rapidly decreases to zero as long as the light is switched off. As presented in Fig. 11a, the pulsed photocurrent density of the TiO2/MS-SiO2 sample is higher than TiO2, suggesting more efficient separation of photoexcited electron-hole pairs happened. As shown in the case of light (UV and visible light), arc radius of TiO2/MS-SiO2 is less than that of pure TiO2, which suggests that the electrochemical system composed of TiO2/MS-SiO2 composite photocatalyst has smaller resistance by fitting the equivalent circuit. Furthermore, the space charge layer resistance of TiO2/MS-SiO2 is smaller, namely the existence of MS-SiO2 carrier strengthens the Helmholtz layer charge separation process and changes the charge distribution of depletion layer. Hence, the existence of MS-SiO2 carrier reduces the resistance of interfacial charge transfer in the electrochemical system. It is doubtless that MS-SiO2 effectively promotes transport and separation of TiO2 photo-generated carriers. Therefore, photocatalytic effect is improved. From the photoluminescence (PL) spectra in Fig. 12, the PL emission bands at around 460 nm are observed in all samples. These belong to the emission of the band gap transition of TiO2 belong to the emission of the band gap transition of TiO2. Besides, it can be seen that the emission intensity of TiO2 is stronger than that of TiO2/MS-SiO2, indicating that charge recombination of TiO2 can be better suppressed after being carried by MS-SiO2.
3.4.2.3. Active species testing. In the photocatalytic processes, a series of reactive species, such as superoxide radical (O2%−), hole (h+), and hydroxyl radical (%OH), take part in the degradation reactions of organic contaminants. To explore the reactive species involved in the degradation of organic pollutants over TiO2/MS-SiO2, a series of contrast experiments in the presence of different radical scavengers are performed. Here, p-Benzoquinone is introduced to scavenge O2%−, EDTA-2Na for h+, and isopropanol for %OH in the solution. As shown in Fig. 13, the degradation extent of MO are significantly prohibited after the addition of BQ, TBA, and EDTA-2Na, inferring that all the O2%−, % OH and h+ are the reactive species responsible for the degradation of organic pollutants and O2%− effects photocatalytic degradation properties most. Therefore, O2%− is the most important reactive species in the photocatalytic processes. Fig. 14 is the result of OH% and O2%− generated by MS SiO2, pure TiO2 and TiO2/MS-SiO2 composite photocatalyst under dark conditions and ultraviolet conditions. As shown in the Fig. 14, all samples are free
4. Conclusion In this paper, tetrabutyl titanate is used as the precursor and nanoTiO2 is loaded on silica microspheres (MS-SiO2) by sol-gel method, and TiO2/MS-SiO2 composite photocatalyst is obtained. In TiO2/MS-SiO2, TiO2 is evenly distributed on the surface of MS-SiO2 and firmly combined with MS-SiO2 by Ti-O-Si bond. Besides, TiO2 has a high dispersion, with 1–4 nm, and apparent particles is < 100 nm. TiO2/MS-SiO2 has a good photocatalytic degradation performance 155
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for methyl orange. The degradation of methyl orange can be achieved completely under ultraviolet light for 90 min. Besides, the degradation effect and rate of TiO2/MS-SiO2 are better than that of pure TiO2. Moreover, TiO2/MS-SiO2 has good recycling usability and easy recovery, which the degradation effect of methyl orange by the fourth recycling samples is as good as that of initial samples. TiO2/MS-SiO2 has a strong absorption in the ultraviolet region, and the carriers OH% and O2%− generated under ultraviolet irradiation are the mechanism to form its photocatalytic performance. Besides, the presence of MS-SiO2 promotes the generation of OH% and O2%−. And the amount of OH% and O2%− generated by TiO2/MS-SiO2 under ultraviolet light is larger than that of pure TiO2. Moreover, TiO2/MS-SiO2 has a smaller resistance of space charge layer, and the resulting ac impedance is smaller than that of pure TiO2 under the same light. The reason for the excellent photocatalytic activity of TiO2/MS-SiO2 composite photocatalyst is due to the uniform load, solid bond, good dispersion of nano-TiO2 on the surface of MS-SiO2 and the increase in the degree of ultraviolet exposure.
10.1016/j.cattod.2013.10.090. [8] A.M. Horst, A.C. Neal, R.E. Mielke, Dispersion of TiO2 nanoparticle agglomerates by Pseudomonas aeruginosa, Appl. Environ. Microb. 76 (2010) 7292, https://doi.org/ 10.1128/AEM.00324-10. [9] L. Yali, Y. Zhanfeng, Z. Shuxue, De-agglomeration and dispersion of Nano-TiO2 in an agitator bead mill, J. Disper. Sci. Technol. 27 (2006) 983–990, https://doi.org/ 10.1080/01932690600766975. [10] C. Su, Y. Tong, M. Zhang, TiO2 nanoparticles immobilized on polyacrylonitrile nanofibers mats: a flexible and recyclable photocatalyst for phenol degradation, RSC Adv. 3 (2013) 7503–7512, https://doi.org/10.1039/c3ra40210j. [11] Z. Sun, Z. Hu, Y. Yan, Effect of preparation conditions on the characteristics and photocatalytic activity of TiO2/purified diatomite composite photocatalysts, Appl. Surf. Sci. 314 (2014) 251–259, https://doi.org/10.1016/j.apsusc.2014.06.171. [12] G. Zhang, Z. Sun, Y. Duan, Synthesis of nano-TiO2/diatomite composite and its photocatalytic degradation of gaseous formaldehyde, Appl. Surf. Sci. 412 (2017) 105–112, https://doi.org/10.1016/j.apsusc.2017.03.198. [13] B. Wang, F.C. de Godoi, Z. Sun, Synthesis, characterization and activity of an immobilized photocatalyst: natural porous diatomite supported titania nanoparticles, J. Colloid. Interf. Sci. 438 (2015) 204–211, https://doi.org/10.1016/j.jcis.2014.09. 064. [14] Z. Ping, Z. Mo, L. Han, Preparation and photocatalytic performance of magnetic TiO2/montmorillonite/Fe3O4 nanocomposites, Ind. Eng. Chem. Res. 53 (2014) 8057–8061, https://doi.org/10.1021/ie5001696. [15] Ito, M, Fukahori, S, Fujiwara. Adsorptive removal and photocatalytic decomposition of sulfamethazine in secondary effluent using TiO2–zeolite composites. Environ. Sci. Pollut. R. 21(2014)834–842. doi: https://doi.org/10.1007/s11356013-1707-9. [16] M. Takeuchi, M. Hidaka, M. Anpo, Efficient removal of toluene and benzene in gas phase by the TiO2/Y-zeolite hybrid photocatalyst, J. Hazard. Mater. 237-238 (2012) 133–139, https://doi.org/10.1016/j.jhazmat.2012.08.011. [17] A. Nasonova, K.S. Kim, Effects of TiO2 coating on zeolite particles for NO and SO2 removal by dielectric barrier discharge process, Catal. Today 211 (2013) 90–95, https://doi.org/10.1016/j.cattod.2013.03.006. [18] K. Kočí, V. Matějka, P. Kovář, Z. Lacný, L. Obalová, Comparison of the pure TiO2 and kaolinite/TiO2 composite as catalyst for CO2 photocatalytic reduction, Catal. Today 161 (2011) 105–109, https://doi.org/10.1016/j.cattod.2010.08.026. [19] Y. Zhang, H. Gan, G. Zhang, A novel mixed-phase TiO2/kaolinite composites and their photocatalytic activity for degradation of organic contaminants, Chem. Eng. J. 172 (2011) 936–943, https://doi.org/10.1016/j.cej.2011.07.005. [20] C. Wang, H. Shi, P. Zhang, Synthesis and characterization of kaolinite/TiO2 nanophotocatalysts, Appl. Clay Sci. 53 (2011) 646–649, https://doi.org/10.1016/j.clay. 2011.05.017. [21] C. Zhu, X. Wang, H. Qin, Removal of gaseous carbon bisulfide using dielectric barrier discharge plasmas combined with TiO2 coated attapulgite catalyst, Chem. Eng. J. 225 (2013) 567–573, https://doi.org/10.1016/j.cej.2013.03.107. [22] Ludvík. Beránek, An examination of the Langmuir-Hinshelwood model using ion exchange catalysts, Catal. Rev. 16 (1977) 1–35, https://doi.org/10.1080/ 03602457708079633. [23] L.K. Preethi, T. Mathews, M. Nand, S.N. Jha, C.S. Gopinath, S. Dash, Band alignment and charge transfer pathway in three phase anatase-rutile-brookite TiO2, nanotubes: an efficient photocatalyst for water splitting, Appl. Catal. B Environ. 218 (2017) 9–19, https://doi.org/10.1016/j.apcatb.2017.06.033. [24] C.S. Turchi, D.F. Ollis, Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack, J. Catal. 122 (1990) 178–192, https://doi.org/10.1016/0021-9517(90)90269-P. [25] Y. Wang, E. Chen, H. Lai, Enhanced light scattering and photovoltaic performance for dye-sensitized solar cells by embedding submicron SiO2/TiO2 core/shell particles in photoanode, Ceram. Int. 39 (2013) 5407–5413, https://doi.org/10.1016/j. ceramint.2012.12.048. [26] J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amorphous germanium, Phys. Status Solidi 3 (1966) 37–46, https://doi.org/10. 1016/0025-5408(68)90023-8.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51474194) and the Fundamental Research Funds for the Central Universities of China (No. 292018302). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.07.005. References [1] L. Lei, X. Chen, Titanium dioxide nanomaterials: self-structural modifications, Chem. Rev. 114 (2014) 9890, https://doi.org/10.1021/cr400624r. [2] I.K. Konstantinou, T.A. Albanis, TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review, Appl. Catal. B Environ. 49 (2004) 1–14, https://doi.org/10.1016/j.apcatb.2003.11.010. [3] Pu S, Zhu R, Ma H, Deng. Facile in-situ design strategy to disperse TiO2 nanoparticles on graphene for the enhanced photocatalytic degradation of rhodamine 6G, Appl. Catal. B Environ. 218(2017)208–219. doi:https://doi.org/10.1016/j. apcatb.2017.06.039. [4] S.Y. Lee, S.J. Park, TiO2 photocatalyst for water treatment applications, J. Ind. Eng. Chem. 19 (2013) 1761–1769, https://doi.org/10.1016/j.jiec.2013.07.012. [5] L. Lietti, P. Forzatti, Temperature programmed desorption/reaction of ammonia over V2O5/TiO2 De-NOxing catalysts, J. Catal. 147 (2010), https://doi.org/10. 1006/jcat.1994.1135. [6] B. Jalvo, M. Faraldos, A. Bahamonde, R. Rosal, Antimicrobial and antibiofilm efficacy of self-cleaning surfaces functionalized by TiO2 photocatalytic nanoparticles against Staphylococcus aureus and Pseudomonas putida, J. Hazard. Mater. 340 (2017) 160–170, https://doi.org/10.1016/j.jhazmat.2017.07.005. [7] N. Liu, X. Chen, J. Zhang, J.W. Schwank, A review on TiO2-based nanotubes synthesized via hydrothermal method: formation mechanism, structure modification, and photocatalytic applications, Catal. Today 225 (2014) 34–51, https://doi.org/
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