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Ionic liquid assisted hydrothermal preparation of TiO2 with largely enhanced photocatalytic performance originated from effective separation of photoinduced carriers
Journal of Physics and Chemistry of Solids 139 (2020) 109323
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Ionic liquid assisted hydrothermal preparation of TiO2 with largely enhanced photocatalytic performance originated from effective separation of photoinduced carriers Jiao Huang a, Chan Qin a, Shiyun Lei a, Jianzhang Li a, Minjiao Li b, Junbo Zhong b, *, Tao Wang a, ** a
Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Environment Engineering, Sichuan University of Science and Engineering, Zigong, 643000, PR China b College of Chemical Engineering, Sichuan University of Science and Engineering, Zigong, 643000, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Photocatalysis TiO2 Ionic liquid Separation rate of photoinduced carriers
In this paper, TiO2 with enhanced photocatalytic activity was prepared with the aid of ionic liquid 1-ethyl-3methylimidazolium hexafluorophosphate ([BMIm]PF6) through a conventional hydrothermal method. The ef fects of different amounts of [BMIm]PF6 on the preparation and corresponding photocatalytic activity of TiO2 were investigated. Preparation of TiO2 was optimized by controlling the mass ratio of [BMIm]PF6 and TiO2. Photocatalytic property of TiO2 prepared was investigated using RhB as a simulated pollutant. The results show that TiO2 prepared with the aid of [BMIm]PF6 exhibits superior photocatalytic activity to the reference TiO2, when the mass ratio of [BMIm]PF6/TiO2 is 8%, TiO2 prepared has the highest separation rate of photogenerated charge pairs, leading to the highest photocatalytic activity. The enhanced photocatalytic performance of TiO2 prepared with the aid of [BMIm]PF6 is attributable to high surface hydroxyl content and enhanced separation of photoinduced charge pairs.
1. Introduction With the rapid development of industrial technology, environmental pollution is becoming more and more serious, of which water pollution bears the brunt, leading to the shortage of global water resources [1,2]. Many methods have been employed to purify the wastewater, among which photocatalytic technology has attracted much attention because of its green and non-pollution [3]. Photocatalytic reaction mainly occurs on the surface of the catalyst, therefore, study of the surface properties of photocatalyst is a hotpot to improve the photocatalytic performance [4]. At present, the studies on surface features include surface modification, morphology control, phase structure and crystal plane engineering, construction of heterojunctions and so on. Al these approaches can regulate the surface properties of photocatalysts, modulate the optical absorption characteristics, accelerate the separation of photogenerated charge pairs and reduce the recombination rate of electrons-holes pairs [5–8]. Among all these approaches, surface modification can be easily realized by chemical treatment in the process of catalyst synthesis.
In recent years, ionic liquids (ILs) have attracted considerable attention in the synthesis of nanomaterials because of their excellent properties. ILs are composed of organic cations and strongly induced anions, and their properties stem from the combined changes of anions and cations [9]. ILs have high polarity, high thermal stability as well as chemical stability, and are often used as solvent, template and additive [10]. During the process of photocatalyst synthesis, ILs can influence the formation of catalyst particles through electrostatic action, viscosity, steric hindrance and amphiphilicity, and control the morphology and phase structure of the product [11,12]. Wei et al. synthesized BiOBr microspheres with vacancy by IL [13]. In the process of dye removal, photogenerated electrons can be captured by the oxygen vacancy, consequently reducing the recombination rate of photoexcited charge pairs. Pascal voepel prepared polyphase anatase TiO2 particle hetero junctions in the presence of IL [14]. Yang and coworkers found that preparation of (BiO)2CO3 assisted by IL could increase the separation � used different rate of photogenerated electron-hole pairs [15]. Giełdon ILs as halogen sources to prepare bismuth halide [9]. The results have
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhong), [email protected] (T. Wang). https://doi.org/10.1016/j.jpcs.2019.109323 Received 13 October 2019; Received in revised form 12 December 2019; Accepted 19 December 2019 Available online 20 December 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
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Journal of Physics and Chemistry of Solids 139 (2020) 109323
2. Experimental section 2.1. Preparation of TiO2 photocatalysts [BMIm]PF6 was obtained from Lanzhou Institute of Chemical Phys ics, Chinese Academy of Sciences. All other chemical reagents in this demonstration were of analytical grade and purchased from Chengdu Kelong Chemical reagent Factory. Generally, desired [BMIm]PF6 and 10 ml of tetrabutyl titanate were dissolved in 20 ml of absolute ethanol to form a faint yellow solution, and the mass ratio of [BMIm]PF6/TiO2 formed is 2%, 4%, 6%, 8%, and 10%, respectively. Then, 40 ml of deionized water was slowly added dropwise to the faint yellow solution mentioned above, forming white precipitation. Afterward, the suspen sion system was continuously stirred for 1 h, and then was transferred into a 100 ml Teflon-lined autoclave and hydrothermally maintained at 453 K for 24 h. The white product was washed with deionized water and absolute ethanol, and then was immersed in absolute ethanol and transferred to an oven at 333 K for drying overnight. To remove the residual [BMIm]PF6, the samples were further annealed in a muffle furnace at 673 K for 2 h. The resulting samples were named as 2 wt%, 4 wt%, 6 wt%, 8 wt% and 10 wt%. TiO2 was prepared as the above recipe in the absence of [BMIm]PF6.
Fig. 1. XRD patterns of the samples. Table 1 Average grain size of catalysts. Photocatalysts
Bare TiO2
2%
4%
6%
8%
10%
crystal size (nm)
8.1
8.8
9.6
10.4
9.6
9.7
2.2. Characterization of TiO2 photocatalysts Specific surface area (BET) of the samples was measured on a 3H2000 p.m. 2 (Beishide Instrument Technology Co., LTD., Beijing). The crystal phase of TiO2 prepared was tested on a DX-2600 X-ray diffrac tometer (Dandong Fangyuan Instrument Co., Ltd., China) with a tube voltage of 35 kV and a current of 25 mA. The scanning range is 10� –80� , the scanning step width is 0.08� , and the radiation source is Cu (λ ¼ 1.5406). UH-4150 UV–Visible near-infrared spectrophotometer (Hitachi Co., Ltd., Japan) was used to determine the UV–Visible diffuse reflec tance spectrum of the catalysts. The baseline was calibrated using BaSO4 as the reference. The scanning range is 200–800 nm. X-ray photoelec tron spectroscopy (XPS) analysis was executed on an XSAM 800. The morphology of the sample was examined using a VEGA 3 SBU scanning electron microscope (TESCAN Co., Czech) with an accelerating voltage of 15 kV and a current of 5 A. Surface photovoltage spectroscopy (SPS) measurement was carried out as the method given in Ref. [24]. Nitro tetrazolium blue chloride (NBT) assessment was performed in accor dance with Ref. [25].
Fig. 2. UV–visible diffuse reflectance spectra of the samples.
well confirmed that ILs can not only relax the structure and increase the particle size of bismuth halide, but also change the bandgap of bismuth halide. These studies demonstrate that the intense application of ILs in the preparation of semiconductor photocatalysts. TiO2 is the most studied photocatalyst due to its low cost, non-toxic and other excellent properties, and is widely used in photocatalytic hydrogen production, sewage and air purification, and other fields [16–20]. Although TiO2 has relative high photocatalytic performance, its photocatalytic efficiency still cannot meet the need of intensive application, limiting by low separation rate of photogenerated carriers [21–23]. Therefore, it is crucial to promote the photocatalytic perfor mance of TiO2 by boosting the separation of photogenerated charge pairs. Based on our previous work, herein, we prepared TiO2 with the assistance of [BMIm]PF6 through a facile hydrothermal method, and the effects of [BMIm]PF6 on the preparation and photocatalytic activity of TiO2 were systematically studied. The results confirm that the presence of [BMIm]PF6 in the synthetic system can significantly boost the photogenerated charge separation rate and surface hydroxyl content, pro moting the photocatalytic activity of TiO2 prepared.
2.3. Photocatalytic test Catalytic activity of TiO2 photocatalyst was evaluated by degrada tion of rhodamine B (RhB, 10 mg/L) under simulated sunlight illumi nation. The photocatalytic test was executed in a Newbit photochemical reactor at room temperature using a 500 W Xe lamp as light source. 50 mg of TiO2 photocatalyst was dispersed into 50 ml of RhB aqueous so lution under intense magnetic stirring. During the photocatalytic pro cess, 3 ml of the suspension system was periodically sampled and centrifuged, and the supernatant was determined at 554 nm. 3. Results and discussion 3.1. Characterization of TiO2 photocatalysts XRD profiles of the photocatalysts were shown in Fig. 1. From Fig. 1, it can be seen that the diffraction peaks of all the samples are consistent with the standard crystal phase of TiO2 (JCPDS NO.29–1354). No im purity substances were detected, indicating high purity of the TiO2 samples. The 2θ at 25.360� , 38.000� and 48.232� corresponds to the (101), (004) and (200) crystal plane, respectively. After addition of [BMIm]PF6 into the synthetic system, it can be found that the charac teristic peaks of the (101) plane of the samples become sharp and the 2
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Journal of Physics and Chemistry of Solids 139 (2020) 109323
Fig. 3. Nitrogen adsorption-desorption isotherm and the corresponding pore size distribution curve of the samples (a) TiO2; (b) 8 wt%.
Fig. 5. SPS responses of the samples.
Fig. 6. Fluorescence spectra of TiO2 and ILs-TiO2.
Fig. 4. SEM of the samples (a) the reference TiO2 (b) 8 wt%,; (c) EDS of 8 wt%; (d–h) Elements mapping of the 8 wt% sample.
examined in order to investigate the optical properties of the photo catalysts. Since the UV–Visible diffuse reflectance spectroscopy of the samples overlaps each other, only the spectra of the reference TiO2 and the 8 wt% photocatalyst were presented in Fig. 2. As demonstrated in Fig. 2, the samples have obvious absorption from 300 nm to 400 nm. Two samples have similar optical properties, which substantially sug gests that adding [BMIm]PF6 into the synthetic system cannot alter the band gap of the samples. The band gap of the samples can be calculated according to the Kubelka-Munk function, as displayed in Fig. 2 (inset), the band gap of the samples is 3.35 eV. The results further exhibit that no elements originated from [BMIm]PF6 were doped in to the lattice of TiO2 or the effects of doped elements can be totally ignored. The specific surface area of TiO2 photocatalysts was tested by N2
half width tent to become small, indicating that the crystallinity of the sample is better and the crystal size is larger than the reference TiO2. According to the Scheler formula, the average crystal size of the sample was shown in Table 1. As shown in Table 1, the average crystal size of the sample prepared by ionic liquid is generally larger than that of the reference TiO2. The results firmly confirm that [BMIm]PF6 significantly influences the growth of TiO2. Commonly, small crystal size is beneficial to the photocatalytic performance. However, combined with the results of catalytic evaluation, it is evident that crystal size is not the leading factor which determines the photocatalytic properties. UV–Visible diffuse reflectance spectroscopy of the samples was 3
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Journal of Physics and Chemistry of Solids 139 (2020) 109323
Fig. 7. XPS spectra of the 8 wt% sample (a–b); XPS spectrum of O 1s (c); High resolution spectra of O 1s (d) TiO2; (e) 4 wt%; (f) 8 wt%; (g) 10 wt%.
(116.9 m2/g) is larger than that of the 8 wt% sample (93.9 m2/g), revealing that TiO2 prepared with the assistance of [BMIm]PF6 has a larger grain size than the reference TiO2, which is consistent with the results of XRD, the results here imply that [BMIm]PF6 promotes the growth of TiO2 crystal. The morphology of the photocatalysts was observed by SEM. As shown in Fig. 4, the reference TiO2 and the 8 wt% sample possess
desorption/adsorption. Fig. 3a and Fig. 3b show the N2 adsorption/ desorption isotherms and pore size distribution of the samples. Both samples show typical IV type isotherms, which manifests that both samples have a mesoporous structure [26]. The pore size distribution of the samples was exhibited in Fig. 3, the pore diameters of TiO2 and the 8% samples are in the range of 2 nm–50 nm, which further certifies the mesoporous structure of the samples. The specific surface area of TiO2 4
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Journal of Physics and Chemistry of Solids 139 (2020) 109323
Table 2 Curve-fitting results of the high resolution XPS spectra for the O1s region. Photocatalyst TiO2 4 wt% 8 wt% 10 wt%
O1s(Ti–O)
O1s (O–H)
Eb/eV
Ri/%
Eb/eV
Ri/%
528.04 527.86 528.17 528.01
85.52 69.84 67.35 67.92
529.55 528.63 528.17 528.01
15.48 30.16 32.65 32.08
Fig. 10. Absorbance of NBT in the different photocatalytic systems (irradiation time ¼ 30 min, NBT dosage ¼ 0.05 mmol/L).
low crystal defects, since crystal defects can stimulate the recombination of photoinduced carriers [29], resulting in low separation rate of pho togenerated carriers (low SPS signals). However, when the amount of ionic liquid is excessive, SPS response of the sample is weaker than the 8 wt% sample, excessive ionic liquid will accelerate the unite of the par ticle of TiO2 during the preparation process, consequently, the grain size of TiO2 prepared tends to become large, the recombination of the pho togenerated carriers tends to increase. Commonly, separation rate of photogenerated charge plays an important role in determining the photocatalytic activity; the 8 wt% sample possesses the highest sepa ration rate of photogenerated carriers, it is anticipated that the 8 wt% sample will exhibit the highest photocatalytic performance, which can be supported by the photocatalytic test. To further explore the separation rate of photogenerated charge, the fluorescence signal of the generated 2-hydroxyterephthalic acid (TAOH) was scanned. As shown in Fig. 6, both TiO2 and ILs-TiO2 have wide emission at 400–500 nm and the typical peak around 425 nm represents the characteristic fluorescence signal of TAOH, indicating �OH exists in the process of photocatalysis. High peak indicates high level of �OH, that is ILs-TiO2 produces more �OH than TiO2. At the same time, the results demonstrate that ILs-TiO2 has higher separation rate of photogenerated charge than the reference TiO2. In order to explore the chemical composition of the photocatalyst, the samples were characterized by XPS, and the results were provided in Fig. 7. Fig. 7a and b are a full spectrum of the 8 wt % sample, it is apparent that Ti, O, F and P elements were detected. Residual P and F stems from [BMIm]PF6. Fig. 7c is high resolution spectra of O 1s, it can be seen that O 1s of the 8 wt% sample (528.22 eV) has a slight shift compared to the reference TiO2 (528.03 eV), which may be due to the interaction between residual P, F and titanium dioxide. The effects of residual P and F on the separation behavior and corresponding photo catalytic performance of ILs-TiO2 need to be further revealed in the near future. Fig. 7d–g show the high resolution spectra of O 1s region. The two peaks located at 528.0 eV and 529.5 eV can be assigned to lattice oxygen (Ti–O) and adsorbed oxygen (-OH), respectively [30–33]. The surface hydroxyl content of all the samples was listed in Table 2, and the surface hydroxyl content of TiO2 prepared with the aid of [BMIm]PF6 is higher than that of the reference TiO2, and the 8 wt% sample displays the highest surface hydroxyl content. Generally, the holes can react with the surface hydroxyl group to produce �OH radicals, which further re duces the recombination of electrons and holes, high level of �O2 will be yielded in the photocatalytic system accordingly, which can be proven by the NBT experiments. As strong oxidant, high level of �OH radicals can expedite the decay of RhB, which implies that 8 wt% sample will hold the best photocatalytic activity. In order to reveal the surface chemical bond composition of the catalyst, we carried out infrared spectroscopy experiments. Seen from Fig. 8, the main peak located at 400-700 cm 1 belongs to the
Fig. 8. FT-IR spectra of TiO2 and ILs-TiO2.
Fig. 9. The degradation efficiency of rhodamine B over the 8 wt% sample after addition of the different scavengers (irradiation time ¼ 100 min, scavenger dosage ¼ 0.2 mmol/L).
irregular block, no distinct different was observed, which indicates that the shape of TiO2 cannot be altered by [BMIm]PF6. Furthermore, the grain size of the reference TiO2 (8.1 nm) is smaller than 8 wt% (9.6 nm). Large grain size corresponds to small specific surface area, which is consistent with BET results. In addition, Fig. 4c–h show the EDS spectra of the 8% sample. The element mapping image shows the uniform dis tribution of Ti, O, F, and P. Combining XRD, EDS and XPS results, it is apparent the successful preparation of TiO2. The surface photovoltage was measured to reveal the photoelectric effect of the samples. The results of the measurements were illustrated in Fig. 5, all the photocatalysts have strong response peaks in the range of 300–380 nm, and the 8 wt% photocatalyst has the strongest response peaks, implying that the 8 wt% sample holds the highest separation rate of photogenerated charge pairs according to the principle of SPS mea surement [27,28]. Compared with the reference TiO2, the TIO2 photo catalyst prepared with the aid of [BMIm]PF6 has high crystallinity and 5
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Journal of Physics and Chemistry of Solids 139 (2020) 109323
Fig. 11. (a) Decay of RhB over the different catalysts under simulate light illumination; (b) The fitting curves of ln(C0/C) vs. t over the different catalysts under simulated sunlight irradiation; (c) Degradation rate constant of RhB over the different catalysts.
characteristic peak of TiO2, which can be attributed to the stretching vibration of Ti–O–Ti. The two peaks around 3400 cm 1 and 1630 cm 1 can be ascribed to the bending and stretching vibration of the surface hydroxyl groups of the catalyst and the adsorbed water, respectively [34]. The peak at 1131 cm 1 of the ILs-TiO2 is the plane vibration of the aliphatic group [35]. To explore active free radicals produced in the photocatalytic sys tem, isopropanol (IPA), ammonium oxalate (AO) and benzoquinone (BQ) were added to the catalytic system to capture hydroxyl radicals (�OH), holes (hþ) and superoxide radicals (�O2 ), respectively. The degradation efficiency of RhB over the 8 wt% sample after addition of the scavengers was shown in Fig. 9. It is evident that the elimination efficiency of RhB remarkably decreases due to quenching effect, and the presence of BQ significantly restrains the removal of RhB, indicating that the superoxide radical is the leading active free radicals, while the hy droxyl radicals and holes take a secondary role. To further manifest the effect of adding of [BMIm]PF6 into the synthetic system on the level of superoxide radicals, NBT measurements were executed to compare the level of �O2 in different photocatalytic systems. As displayed in Fig. 10, distinct changes of NBT concentration were detected in all the photo catalytic reaction systems, especially in the 8 wt% photocatalytic reac tion system, firmly implying that �O2 was produced in all the photocatalytic reaction systems, and the 8 wt% TiO2 produces the highest level of �O2 radicals. High level of �O2 radicals can accelerate the elimination of RhB, manifesting high catalytic performance, which accords well with the results of catalytic evaluation.
compare the photocatalytic performance of the catalysts, the abatement rate constant of RhB over the samples was investigated according to the first-order kinetic model: ln(C0/C) ¼ kt, where t is reaction time, C stands for the concentration of RhB at t, and C0 represents the initial concentration of RhB, and the k is defined as the apparent reaction rate constant [36]. The results were shown in Fig. 11b. As the amount of [BMIm]PF6 increasing, the catalytic activity of TiO2 prepared gradually increases, and then begins to drop. Among all the samples, the photo catalytic activity of the 8 wt% TiO2 is the highest, which is 1.6 times of that of the reference TiO2 (Fig. 11c). Based on the above results, it is apparent that addition of [BMIm]PF6 into the synthetic system can considerably enhance the photocatalytic performance of TiO2 originated from effective separation of photoexcited carriers, and the optimum mass ratio of [BMIm]PF6/TiO2 is 8%. Paszkiewicz et al. prepared TiO2 with the aid of ionic liquids, and the degradation efficiency of 0.43 mmol/L of phenol is 89% after 80 min under UV–visible light irradiation [12], the results in this paper is different from the report in Ref. [12] due to different ILs, simulated pollutant and experimental conditions. In this demonstration, the results substantially support that [BMIm]PF6 assis ted preparation can remarkably promote the photocatalytic perfor mance of TiO2 benefited from high surface hydroxyl content and enhanced separation of photoinduced charge pairs. 4. Conclusions In this work, TiO2 was prepared by an ionic liquid assisted hydro thermal method. The presence of [BMIm]PF6 in the synthetic system can improve the surface hydroxyl content and promote the separation rate of photoinduced charge pairs. The trapping experiments confirm the dominance of superoxide free radicals in the photocatalytic decay of RhB. NBT experiments demonstrate that the presence of [BMIm]PF6 in the synthetic system expedite the formation of �O2 . All these advanta geous factors endow TiO2 prepared with the aid of [BMIm]PF6 with prior photocatalytic activity to the reference TiO2, when the mass ratio of [BMIm]PF6/TiO2 is 8%, TiO2 prepared demonstrates the highest
3.2. Photocatalytic evaluation Adsorption of RhB over the photocatalysts after 30 min is less than 10%, combined with the results of photocatalytic evaluation; the decay of RhB is mainly assigned to photocatalysis. As shown in Fig. 11a, all the TiO2 photocatalysts prepared with the assistance of [BMIm]PF6 display superior photocatalytic activity to the reference TiO2, and the 8 wt% sample exhibits the best photocatalytic activity. In order to further 6
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photocatalytic performance.
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Declaration of competing interest The authors declare no conflict of interest. Acknowledgements This project was supported financially by Opening Project of Sichuan Key Laboratory of Comprehensive Utilization of Vanadium and Tita nium Resources (2018FTSZ01, 2018FTSZ02 and 2018FTSZ15), Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (LYJ18201) and Graduate student Innovation Fund of Sichuan University of Science and Engineering (y2018054). References [1] F. Rao, G.Q. Zhu, M. Hojamberdiev, W.B. Zhang, S.P. Li, J.Z. Gao, F.C. Zhang, Y. H. Huang, Y. Huang, Uniform Zn2þ-doped BiOI microspheres assembled by ultrathin nanosheets with tunable oxygen vacancies for super-stable removal of NO, J. Phys. Chem. C 123 (2019) 16268–16280. [2] N. Belhouchet, B. Hamdi, H. Chenchouni, Y. Bessekhouad, Photocatalytic degradation of tetracycline antibiotic using new calcite/titania nanocomposites, J. Photochem. Photobiol., A 372 (2019) 196–205. [3] G.Q. Zhu, S.P. Li, J.Z. Gao, F.C. Zhang, C.L. Liu, Q.Z. Wang, M. Hojamberdiev, Constructing a 2D/2D Bi2O2CO3/Bi4O5Br2 heterostructure as a direct Z-scheme photocatalyst with enhanced photocatalytic activity for NOx removal, Appl. Surf. Sci. 493 (2019) 913–925. [4] Y.J. Si, H.H. Liu, N.T. Li, J.B. Zhong, J.Z. Li, D.M. Ma, SDBS-assisted hydrothermal treatment of TiO2 with improved photocatalytic activity, Mater. Lett. 212 (2018) 147–150. [5] M. Hojamberdiev, Z.C. Kadirova, R.V. Gonçalves, K. Yubuta, N. Matsushita, K. Teshima, M. Hasegawa, K. Okada, Reduced graphene oxide-modified Bi2WO6/ BiOI composite for the effective photocatalytic removal of organic pollutants and molecular modeling of adsorption, J. Mol. Liq. 268 (2018) 715–727. [6] F. Cao, Y.N. Wang, J.M. Wang, X. Lv, D.Y. Liu, J. Ren, J. Zhou, R.P. Deng, S. Li, G. W. Qin, Oxygen vacancy induced superior visible-light-driven photodegradation pollutant performance in BiOCl microflowers, New J. Chem. 42 (2018) 14085. [7] J. Lu, J. Wu, W.X. Xu, H.Q. Cheng, X.M. Qi, Q.W. Li, Y.N. Zhang, Y. Guan, Y. Ling, Z. Zhang, Room temperature synthesis of tetragonal BiOI photocatalyst with surface heterojunction between (001) facets and (110) facets, Mater. Lett. 219 (2018) 260–264. [8] X.Y. Chen, D.H. Kuo, D.F. Lu, Visible light response and superior dispersed S-doped TiO2 nanoparticles synthesized via ionic liquid, Adv. Powder Technol. 28 (2017) 1213–1220. [9] A. Bielicka-Giełdo� n, P. Wilczewska, A. Malankowska, K. Szczodrowski, J. Ryl, A. Zieli� nska-Jurek, E.M. Siedlecka, Morphology, surface properties and photocatalytic activity of the bismuth oxyhalides semiconductors prepared by ionic liquid assisted solvothermal method, Separ. Purif. Technol. 217 (2019) 164–173. [10] Z.D. Wei, R.S. Li, R. Wang, Enhanced visible light photocatalytic activity of BiOBr by in situ reactable ionic liquid modification for pollutant degradation, RSC Adv. 8 (2018) 7956–7962. [11] C.R. Bender, P.R.S. Salbego, K. Wust, C.A.A. Farias, T.S. Beck, G. Machado, R. A. Vaucher, M.A.P. Martins, C.P. Frizzo, Interaction of pharmaceutical ionic liquids with TiO2 in anatase and rutile phase, J. Mol. Liq. 269 (2018) 912–919. [12] M. Paszkiewicz, J. Łuczak, W. Lisowski, P. Patyk, A. Zaleska-Medynska, The ILsassisted solvothermal synthesis of TiO2 spheres: the effect of ionic liquids on morphology and photoactivity of TiO2, Appl. Catal., B 184 (2016) 223–237. [13] Z.D. Wei, R. Wang, Hierarchical BiOBr microspheres with oxygen vacancies synthesized via reactable ionic liquids for dyes removal, Chin. Chem. Lett. 27 (2016) 769–772. [14] P. Voepel, M. Weiss, B.M. Smarsly, R. Marschall, Photocatalytic activity of multiphase TiO2(B)/anatase nanoparticle heterojunctions prepared from ionic liquids, J. Photochem. Photobiol., A 366 (2018) 34–40. [15] Q. Yang, J.Z. Li, J.B. Zhong, C.Z. Cheng, Z. Xiang, J.F. Chen, Enhanced solar photocatalytic performance of (BiO)2CO3 prepared with the assistance of ionic liquid, Mater. Lett. 192 (2017) 157–160.
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Journal of Physics and Chemistry of Solids journal homepage: http://www.elsevier.com/locate/jpcs
Ionic liquid assisted hydrothermal preparation of TiO2 with largely enhanced photocatalytic performance originated from effective separation of photoinduced carriers Jiao Huang a, Chan Qin a, Shiyun Lei a, Jianzhang Li a, Minjiao Li b, Junbo Zhong b, *, Tao Wang a, ** a
Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Environment Engineering, Sichuan University of Science and Engineering, Zigong, 643000, PR China b College of Chemical Engineering, Sichuan University of Science and Engineering, Zigong, 643000, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Photocatalysis TiO2 Ionic liquid Separation rate of photoinduced carriers
In this paper, TiO2 with enhanced photocatalytic activity was prepared with the aid of ionic liquid 1-ethyl-3methylimidazolium hexafluorophosphate ([BMIm]PF6) through a conventional hydrothermal method. The ef fects of different amounts of [BMIm]PF6 on the preparation and corresponding photocatalytic activity of TiO2 were investigated. Preparation of TiO2 was optimized by controlling the mass ratio of [BMIm]PF6 and TiO2. Photocatalytic property of TiO2 prepared was investigated using RhB as a simulated pollutant. The results show that TiO2 prepared with the aid of [BMIm]PF6 exhibits superior photocatalytic activity to the reference TiO2, when the mass ratio of [BMIm]PF6/TiO2 is 8%, TiO2 prepared has the highest separation rate of photogenerated charge pairs, leading to the highest photocatalytic activity. The enhanced photocatalytic performance of TiO2 prepared with the aid of [BMIm]PF6 is attributable to high surface hydroxyl content and enhanced separation of photoinduced charge pairs.
1. Introduction With the rapid development of industrial technology, environmental pollution is becoming more and more serious, of which water pollution bears the brunt, leading to the shortage of global water resources [1,2]. Many methods have been employed to purify the wastewater, among which photocatalytic technology has attracted much attention because of its green and non-pollution [3]. Photocatalytic reaction mainly occurs on the surface of the catalyst, therefore, study of the surface properties of photocatalyst is a hotpot to improve the photocatalytic performance [4]. At present, the studies on surface features include surface modification, morphology control, phase structure and crystal plane engineering, construction of heterojunctions and so on. Al these approaches can regulate the surface properties of photocatalysts, modulate the optical absorption characteristics, accelerate the separation of photogenerated charge pairs and reduce the recombination rate of electrons-holes pairs [5–8]. Among all these approaches, surface modification can be easily realized by chemical treatment in the process of catalyst synthesis.
In recent years, ionic liquids (ILs) have attracted considerable attention in the synthesis of nanomaterials because of their excellent properties. ILs are composed of organic cations and strongly induced anions, and their properties stem from the combined changes of anions and cations [9]. ILs have high polarity, high thermal stability as well as chemical stability, and are often used as solvent, template and additive [10]. During the process of photocatalyst synthesis, ILs can influence the formation of catalyst particles through electrostatic action, viscosity, steric hindrance and amphiphilicity, and control the morphology and phase structure of the product [11,12]. Wei et al. synthesized BiOBr microspheres with vacancy by IL [13]. In the process of dye removal, photogenerated electrons can be captured by the oxygen vacancy, consequently reducing the recombination rate of photoexcited charge pairs. Pascal voepel prepared polyphase anatase TiO2 particle hetero junctions in the presence of IL [14]. Yang and coworkers found that preparation of (BiO)2CO3 assisted by IL could increase the separation � used different rate of photogenerated electron-hole pairs [15]. Giełdon ILs as halogen sources to prepare bismuth halide [9]. The results have
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhong), [email protected] (T. Wang). https://doi.org/10.1016/j.jpcs.2019.109323 Received 13 October 2019; Received in revised form 12 December 2019; Accepted 19 December 2019 Available online 20 December 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
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Journal of Physics and Chemistry of Solids 139 (2020) 109323
2. Experimental section 2.1. Preparation of TiO2 photocatalysts [BMIm]PF6 was obtained from Lanzhou Institute of Chemical Phys ics, Chinese Academy of Sciences. All other chemical reagents in this demonstration were of analytical grade and purchased from Chengdu Kelong Chemical reagent Factory. Generally, desired [BMIm]PF6 and 10 ml of tetrabutyl titanate were dissolved in 20 ml of absolute ethanol to form a faint yellow solution, and the mass ratio of [BMIm]PF6/TiO2 formed is 2%, 4%, 6%, 8%, and 10%, respectively. Then, 40 ml of deionized water was slowly added dropwise to the faint yellow solution mentioned above, forming white precipitation. Afterward, the suspen sion system was continuously stirred for 1 h, and then was transferred into a 100 ml Teflon-lined autoclave and hydrothermally maintained at 453 K for 24 h. The white product was washed with deionized water and absolute ethanol, and then was immersed in absolute ethanol and transferred to an oven at 333 K for drying overnight. To remove the residual [BMIm]PF6, the samples were further annealed in a muffle furnace at 673 K for 2 h. The resulting samples were named as 2 wt%, 4 wt%, 6 wt%, 8 wt% and 10 wt%. TiO2 was prepared as the above recipe in the absence of [BMIm]PF6.
Fig. 1. XRD patterns of the samples. Table 1 Average grain size of catalysts. Photocatalysts
Bare TiO2
2%
4%
6%
8%
10%
crystal size (nm)
8.1
8.8
9.6
10.4
9.6
9.7
2.2. Characterization of TiO2 photocatalysts Specific surface area (BET) of the samples was measured on a 3H2000 p.m. 2 (Beishide Instrument Technology Co., LTD., Beijing). The crystal phase of TiO2 prepared was tested on a DX-2600 X-ray diffrac tometer (Dandong Fangyuan Instrument Co., Ltd., China) with a tube voltage of 35 kV and a current of 25 mA. The scanning range is 10� –80� , the scanning step width is 0.08� , and the radiation source is Cu (λ ¼ 1.5406). UH-4150 UV–Visible near-infrared spectrophotometer (Hitachi Co., Ltd., Japan) was used to determine the UV–Visible diffuse reflec tance spectrum of the catalysts. The baseline was calibrated using BaSO4 as the reference. The scanning range is 200–800 nm. X-ray photoelec tron spectroscopy (XPS) analysis was executed on an XSAM 800. The morphology of the sample was examined using a VEGA 3 SBU scanning electron microscope (TESCAN Co., Czech) with an accelerating voltage of 15 kV and a current of 5 A. Surface photovoltage spectroscopy (SPS) measurement was carried out as the method given in Ref. [24]. Nitro tetrazolium blue chloride (NBT) assessment was performed in accor dance with Ref. [25].
Fig. 2. UV–visible diffuse reflectance spectra of the samples.
well confirmed that ILs can not only relax the structure and increase the particle size of bismuth halide, but also change the bandgap of bismuth halide. These studies demonstrate that the intense application of ILs in the preparation of semiconductor photocatalysts. TiO2 is the most studied photocatalyst due to its low cost, non-toxic and other excellent properties, and is widely used in photocatalytic hydrogen production, sewage and air purification, and other fields [16–20]. Although TiO2 has relative high photocatalytic performance, its photocatalytic efficiency still cannot meet the need of intensive application, limiting by low separation rate of photogenerated carriers [21–23]. Therefore, it is crucial to promote the photocatalytic perfor mance of TiO2 by boosting the separation of photogenerated charge pairs. Based on our previous work, herein, we prepared TiO2 with the assistance of [BMIm]PF6 through a facile hydrothermal method, and the effects of [BMIm]PF6 on the preparation and photocatalytic activity of TiO2 were systematically studied. The results confirm that the presence of [BMIm]PF6 in the synthetic system can significantly boost the photogenerated charge separation rate and surface hydroxyl content, pro moting the photocatalytic activity of TiO2 prepared.
2.3. Photocatalytic test Catalytic activity of TiO2 photocatalyst was evaluated by degrada tion of rhodamine B (RhB, 10 mg/L) under simulated sunlight illumi nation. The photocatalytic test was executed in a Newbit photochemical reactor at room temperature using a 500 W Xe lamp as light source. 50 mg of TiO2 photocatalyst was dispersed into 50 ml of RhB aqueous so lution under intense magnetic stirring. During the photocatalytic pro cess, 3 ml of the suspension system was periodically sampled and centrifuged, and the supernatant was determined at 554 nm. 3. Results and discussion 3.1. Characterization of TiO2 photocatalysts XRD profiles of the photocatalysts were shown in Fig. 1. From Fig. 1, it can be seen that the diffraction peaks of all the samples are consistent with the standard crystal phase of TiO2 (JCPDS NO.29–1354). No im purity substances were detected, indicating high purity of the TiO2 samples. The 2θ at 25.360� , 38.000� and 48.232� corresponds to the (101), (004) and (200) crystal plane, respectively. After addition of [BMIm]PF6 into the synthetic system, it can be found that the charac teristic peaks of the (101) plane of the samples become sharp and the 2
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Journal of Physics and Chemistry of Solids 139 (2020) 109323
Fig. 3. Nitrogen adsorption-desorption isotherm and the corresponding pore size distribution curve of the samples (a) TiO2; (b) 8 wt%.
Fig. 5. SPS responses of the samples.
Fig. 6. Fluorescence spectra of TiO2 and ILs-TiO2.
Fig. 4. SEM of the samples (a) the reference TiO2 (b) 8 wt%,; (c) EDS of 8 wt%; (d–h) Elements mapping of the 8 wt% sample.
examined in order to investigate the optical properties of the photo catalysts. Since the UV–Visible diffuse reflectance spectroscopy of the samples overlaps each other, only the spectra of the reference TiO2 and the 8 wt% photocatalyst were presented in Fig. 2. As demonstrated in Fig. 2, the samples have obvious absorption from 300 nm to 400 nm. Two samples have similar optical properties, which substantially sug gests that adding [BMIm]PF6 into the synthetic system cannot alter the band gap of the samples. The band gap of the samples can be calculated according to the Kubelka-Munk function, as displayed in Fig. 2 (inset), the band gap of the samples is 3.35 eV. The results further exhibit that no elements originated from [BMIm]PF6 were doped in to the lattice of TiO2 or the effects of doped elements can be totally ignored. The specific surface area of TiO2 photocatalysts was tested by N2
half width tent to become small, indicating that the crystallinity of the sample is better and the crystal size is larger than the reference TiO2. According to the Scheler formula, the average crystal size of the sample was shown in Table 1. As shown in Table 1, the average crystal size of the sample prepared by ionic liquid is generally larger than that of the reference TiO2. The results firmly confirm that [BMIm]PF6 significantly influences the growth of TiO2. Commonly, small crystal size is beneficial to the photocatalytic performance. However, combined with the results of catalytic evaluation, it is evident that crystal size is not the leading factor which determines the photocatalytic properties. UV–Visible diffuse reflectance spectroscopy of the samples was 3
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Journal of Physics and Chemistry of Solids 139 (2020) 109323
Fig. 7. XPS spectra of the 8 wt% sample (a–b); XPS spectrum of O 1s (c); High resolution spectra of O 1s (d) TiO2; (e) 4 wt%; (f) 8 wt%; (g) 10 wt%.
(116.9 m2/g) is larger than that of the 8 wt% sample (93.9 m2/g), revealing that TiO2 prepared with the assistance of [BMIm]PF6 has a larger grain size than the reference TiO2, which is consistent with the results of XRD, the results here imply that [BMIm]PF6 promotes the growth of TiO2 crystal. The morphology of the photocatalysts was observed by SEM. As shown in Fig. 4, the reference TiO2 and the 8 wt% sample possess
desorption/adsorption. Fig. 3a and Fig. 3b show the N2 adsorption/ desorption isotherms and pore size distribution of the samples. Both samples show typical IV type isotherms, which manifests that both samples have a mesoporous structure [26]. The pore size distribution of the samples was exhibited in Fig. 3, the pore diameters of TiO2 and the 8% samples are in the range of 2 nm–50 nm, which further certifies the mesoporous structure of the samples. The specific surface area of TiO2 4
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Journal of Physics and Chemistry of Solids 139 (2020) 109323
Table 2 Curve-fitting results of the high resolution XPS spectra for the O1s region. Photocatalyst TiO2 4 wt% 8 wt% 10 wt%
O1s(Ti–O)
O1s (O–H)
Eb/eV
Ri/%
Eb/eV
Ri/%
528.04 527.86 528.17 528.01
85.52 69.84 67.35 67.92
529.55 528.63 528.17 528.01
15.48 30.16 32.65 32.08
Fig. 10. Absorbance of NBT in the different photocatalytic systems (irradiation time ¼ 30 min, NBT dosage ¼ 0.05 mmol/L).
low crystal defects, since crystal defects can stimulate the recombination of photoinduced carriers [29], resulting in low separation rate of pho togenerated carriers (low SPS signals). However, when the amount of ionic liquid is excessive, SPS response of the sample is weaker than the 8 wt% sample, excessive ionic liquid will accelerate the unite of the par ticle of TiO2 during the preparation process, consequently, the grain size of TiO2 prepared tends to become large, the recombination of the pho togenerated carriers tends to increase. Commonly, separation rate of photogenerated charge plays an important role in determining the photocatalytic activity; the 8 wt% sample possesses the highest sepa ration rate of photogenerated carriers, it is anticipated that the 8 wt% sample will exhibit the highest photocatalytic performance, which can be supported by the photocatalytic test. To further explore the separation rate of photogenerated charge, the fluorescence signal of the generated 2-hydroxyterephthalic acid (TAOH) was scanned. As shown in Fig. 6, both TiO2 and ILs-TiO2 have wide emission at 400–500 nm and the typical peak around 425 nm represents the characteristic fluorescence signal of TAOH, indicating �OH exists in the process of photocatalysis. High peak indicates high level of �OH, that is ILs-TiO2 produces more �OH than TiO2. At the same time, the results demonstrate that ILs-TiO2 has higher separation rate of photogenerated charge than the reference TiO2. In order to explore the chemical composition of the photocatalyst, the samples were characterized by XPS, and the results were provided in Fig. 7. Fig. 7a and b are a full spectrum of the 8 wt % sample, it is apparent that Ti, O, F and P elements were detected. Residual P and F stems from [BMIm]PF6. Fig. 7c is high resolution spectra of O 1s, it can be seen that O 1s of the 8 wt% sample (528.22 eV) has a slight shift compared to the reference TiO2 (528.03 eV), which may be due to the interaction between residual P, F and titanium dioxide. The effects of residual P and F on the separation behavior and corresponding photo catalytic performance of ILs-TiO2 need to be further revealed in the near future. Fig. 7d–g show the high resolution spectra of O 1s region. The two peaks located at 528.0 eV and 529.5 eV can be assigned to lattice oxygen (Ti–O) and adsorbed oxygen (-OH), respectively [30–33]. The surface hydroxyl content of all the samples was listed in Table 2, and the surface hydroxyl content of TiO2 prepared with the aid of [BMIm]PF6 is higher than that of the reference TiO2, and the 8 wt% sample displays the highest surface hydroxyl content. Generally, the holes can react with the surface hydroxyl group to produce �OH radicals, which further re duces the recombination of electrons and holes, high level of �O2 will be yielded in the photocatalytic system accordingly, which can be proven by the NBT experiments. As strong oxidant, high level of �OH radicals can expedite the decay of RhB, which implies that 8 wt% sample will hold the best photocatalytic activity. In order to reveal the surface chemical bond composition of the catalyst, we carried out infrared spectroscopy experiments. Seen from Fig. 8, the main peak located at 400-700 cm 1 belongs to the
Fig. 8. FT-IR spectra of TiO2 and ILs-TiO2.
Fig. 9. The degradation efficiency of rhodamine B over the 8 wt% sample after addition of the different scavengers (irradiation time ¼ 100 min, scavenger dosage ¼ 0.2 mmol/L).
irregular block, no distinct different was observed, which indicates that the shape of TiO2 cannot be altered by [BMIm]PF6. Furthermore, the grain size of the reference TiO2 (8.1 nm) is smaller than 8 wt% (9.6 nm). Large grain size corresponds to small specific surface area, which is consistent with BET results. In addition, Fig. 4c–h show the EDS spectra of the 8% sample. The element mapping image shows the uniform dis tribution of Ti, O, F, and P. Combining XRD, EDS and XPS results, it is apparent the successful preparation of TiO2. The surface photovoltage was measured to reveal the photoelectric effect of the samples. The results of the measurements were illustrated in Fig. 5, all the photocatalysts have strong response peaks in the range of 300–380 nm, and the 8 wt% photocatalyst has the strongest response peaks, implying that the 8 wt% sample holds the highest separation rate of photogenerated charge pairs according to the principle of SPS mea surement [27,28]. Compared with the reference TiO2, the TIO2 photo catalyst prepared with the aid of [BMIm]PF6 has high crystallinity and 5
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Journal of Physics and Chemistry of Solids 139 (2020) 109323
Fig. 11. (a) Decay of RhB over the different catalysts under simulate light illumination; (b) The fitting curves of ln(C0/C) vs. t over the different catalysts under simulated sunlight irradiation; (c) Degradation rate constant of RhB over the different catalysts.
characteristic peak of TiO2, which can be attributed to the stretching vibration of Ti–O–Ti. The two peaks around 3400 cm 1 and 1630 cm 1 can be ascribed to the bending and stretching vibration of the surface hydroxyl groups of the catalyst and the adsorbed water, respectively [34]. The peak at 1131 cm 1 of the ILs-TiO2 is the plane vibration of the aliphatic group [35]. To explore active free radicals produced in the photocatalytic sys tem, isopropanol (IPA), ammonium oxalate (AO) and benzoquinone (BQ) were added to the catalytic system to capture hydroxyl radicals (�OH), holes (hþ) and superoxide radicals (�O2 ), respectively. The degradation efficiency of RhB over the 8 wt% sample after addition of the scavengers was shown in Fig. 9. It is evident that the elimination efficiency of RhB remarkably decreases due to quenching effect, and the presence of BQ significantly restrains the removal of RhB, indicating that the superoxide radical is the leading active free radicals, while the hy droxyl radicals and holes take a secondary role. To further manifest the effect of adding of [BMIm]PF6 into the synthetic system on the level of superoxide radicals, NBT measurements were executed to compare the level of �O2 in different photocatalytic systems. As displayed in Fig. 10, distinct changes of NBT concentration were detected in all the photo catalytic reaction systems, especially in the 8 wt% photocatalytic reac tion system, firmly implying that �O2 was produced in all the photocatalytic reaction systems, and the 8 wt% TiO2 produces the highest level of �O2 radicals. High level of �O2 radicals can accelerate the elimination of RhB, manifesting high catalytic performance, which accords well with the results of catalytic evaluation.
compare the photocatalytic performance of the catalysts, the abatement rate constant of RhB over the samples was investigated according to the first-order kinetic model: ln(C0/C) ¼ kt, where t is reaction time, C stands for the concentration of RhB at t, and C0 represents the initial concentration of RhB, and the k is defined as the apparent reaction rate constant [36]. The results were shown in Fig. 11b. As the amount of [BMIm]PF6 increasing, the catalytic activity of TiO2 prepared gradually increases, and then begins to drop. Among all the samples, the photo catalytic activity of the 8 wt% TiO2 is the highest, which is 1.6 times of that of the reference TiO2 (Fig. 11c). Based on the above results, it is apparent that addition of [BMIm]PF6 into the synthetic system can considerably enhance the photocatalytic performance of TiO2 originated from effective separation of photoexcited carriers, and the optimum mass ratio of [BMIm]PF6/TiO2 is 8%. Paszkiewicz et al. prepared TiO2 with the aid of ionic liquids, and the degradation efficiency of 0.43 mmol/L of phenol is 89% after 80 min under UV–visible light irradiation [12], the results in this paper is different from the report in Ref. [12] due to different ILs, simulated pollutant and experimental conditions. In this demonstration, the results substantially support that [BMIm]PF6 assis ted preparation can remarkably promote the photocatalytic perfor mance of TiO2 benefited from high surface hydroxyl content and enhanced separation of photoinduced charge pairs. 4. Conclusions In this work, TiO2 was prepared by an ionic liquid assisted hydro thermal method. The presence of [BMIm]PF6 in the synthetic system can improve the surface hydroxyl content and promote the separation rate of photoinduced charge pairs. The trapping experiments confirm the dominance of superoxide free radicals in the photocatalytic decay of RhB. NBT experiments demonstrate that the presence of [BMIm]PF6 in the synthetic system expedite the formation of �O2 . All these advanta geous factors endow TiO2 prepared with the aid of [BMIm]PF6 with prior photocatalytic activity to the reference TiO2, when the mass ratio of [BMIm]PF6/TiO2 is 8%, TiO2 prepared demonstrates the highest
3.2. Photocatalytic evaluation Adsorption of RhB over the photocatalysts after 30 min is less than 10%, combined with the results of photocatalytic evaluation; the decay of RhB is mainly assigned to photocatalysis. As shown in Fig. 11a, all the TiO2 photocatalysts prepared with the assistance of [BMIm]PF6 display superior photocatalytic activity to the reference TiO2, and the 8 wt% sample exhibits the best photocatalytic activity. In order to further 6
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Journal of Physics and Chemistry of Solids 139 (2020) 109323
photocatalytic performance.
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