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Photocatalytic activity of TiO2 prepared by different solvents through a solvothermal approach
Solid State Sciences 98 (2019) 106024
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Photocatalytic activity of TiO2 prepared by different solvents through a solvothermal approach Jiao Huang a, Huanhuan Liu a, Zhao Li a, Junbo Zhong a, b, *, Tao Wang a, **, Jianzhang Li a, Minjiao Li b 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 College of Chemical Engineering, Sichuan University of Science and Engineering, Zigong, 643000, PR China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: TiO2 Solvents Photocatalytic performance Solvothermal approach
In this work, TiO2 was prepared by different solvents through a solvothermal approach using tetrabutyl titanate as raw material, afterward the samples were calcined at 723 K. The samples were investigated by Brunauer -Emmett-Teller (BET) method, X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Vis diffuse reflectance (UV–Vis DRS), X-ray photoelectron spectroscopy (XPS), and surface photovoltage spectroscopy (SPS). The photocatalytic activity of the samples was investigated using rhodamine B (RhB) as a model contaminant. TiO2 prepared using ethanol as solvent has the best photocatalytic performance under irradiation of 500 W Xe lamp. Compared with the samples without calcination, all samples after calcination display significantly improved photocatalytic performance, which can be tightly attributed to the enhanced separation of photoin duced carriers and relative high surface hydroxyl content.
1. Introduction Nowadays, environmental pollution has become a major problem that threats the development of human being. Among them, water pollution is particularly serious, therefore, it is profound significant to treat wastewater for the scientific development of human society. Various technologies haven been developed to treat wastewater, espe cially the treatment of dyes in industrial wastewater, among the stra tegies, photocatalysis technology is considered to be a promising and green approach [1–6]. Semiconductor-based photocatalysis driven by sunlight can degrade organic pollutants in the sewage into CO2 and water， therefore photocatalytic technology is environmentally friendly. Development of photocatalyst with excellent performance and low cost is the core of photocatalytic technology [7,8]. Among the photocatalysts developed, TiO2 has attracted considerable attention due to its excellent performance. In fact, TiO2 has exceptional advantages, such as good chemical stability, non-toxicity, environmental friendliness and sustainability [9–15]. However, TiO2 also has some conspicuous shortcomings, for example, TiO2 has a wide band gap (about 3.2 eV),
and only ultraviolet light with a wavelength of <387 nm can be used to drive TiO2, ultraviolet light only accounts for 4–5% of sunlight [16–19]. The practical application of TiO2 is also limited by the high recombi nation rate of photogenerated electron-hole pairs. These two main bottlenecks make the photocatalytic activity of TiO2 far from satisfied to meet the practical requirements [20–22]. Usually, photocatalytic activity of TiO2 tightly depends on many physical factors, such as specific surface area, particle size, pore volume, surface hydroxyl content and crystallinity [23]. Various approaches have been employed to synthesize TiO2, such as sol-gel method, sol vothermal method, sonochemical or chemical vapor deposition [24]. TiO2 prepared display different shapes, for example, TiO2 particles, nanotubes, nanorods, microspheres, hollow microspheres or flower-like structures [25]. Solvothermal synthesis is an effective way to prepare TiO2 with unique morphology. In the solvothermal preparation process, different solvents are employed, the conditions of solvothermal reaction can be easily controlled, such as temperature, pH, reaction time, addi tives and so on, thus TiO2 with different morphologies and structures can be obtained. Moreover, solvent can affect the nucleation and growth
* Corresponding author. 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 ** Corresponding author. E-mail addresses: [email protected] (J. Zhong), [email protected] (T. Wang). https://doi.org/10.1016/j.solidstatesciences.2019.106024 Received 12 June 2019; Received in revised form 1 October 2019; Accepted 2 October 2019 Available online 2 October 2019 1293-2558/© 2019 Elsevier Masson SAS. All rights reserved.
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of the products during the solvothermal process [26]. Up to now, various theories are proposed to elaborate how the sol vent affects the product in many aspects. The polarity of the solvent will affect the acquisition of unique morphology [27], the polarity of solvent even can alter the band gap of TiO2, and strong polar solvent can induce the shift of band gap. Hu et al. found that polarity of solvent could affect the different crystal face exposure ratios, resulting in different pro portions of unsaturated Ti5c. Different proportions of unsaturated Ti5c will cause different combinations of conduction bands and Ti 3D orbitals in different band gaps [28]. Additionally, the boiling point and viscosity affect the crystallite structure of the product [29], the surface tension affects the crystal strength and morphology of the product [30], the dielectric constant can control the crystallinity of the product [31] and so on. In order to understand the effect of solvent on the preparation and photocatalytic performance of TiO2, in this study, three alcohols (ethanol, ethylene glycol and glycerol) were used as solvent to prepare TiO2 by a solvothermal method, and the products were characterized by XRD, SEM, UV–Vis DRS and so on [32,33]. The corresponding photo catalytic performance was evaluated. The results reveal that TiO2 pre pared using ethanol as solvent displays higher photocatalytic activity than TiO2 prepared using ethylene glycol and glycerol as solvent, the underlying mechanism was proposed.
Table 1 Specific surface area and bandgap of the samples. Sample (TiO2)
TiO2EA
TiO2EG
TiO2GL
EA723K
EG723K
GL723K
SBET (m2/g) Band gap (eV)
181.6 3.34
169.6 3.25
159.4 3.23
88.5 3.35
76.8 3.21
62.4 3.25
2. Experimental 2.1. Synthesis Fig. 1. XRD patterns of the samples.
All chemicals are of analytical grade. In this work, tetrabutyl titanate was used as raw material to prepare TiO2 in different solvents by a solvothermal method. Typically, 10 ml tetrabutyl titanate was dissolved in 20 ml ethanol (EA), ethylene glycol (EG) and glycerol (GL), respec tively. After stirring for 30 min, 40 ml deionized water was added into the above solution, respectively. The suspension system formed was continuously stirred for 30 min, and then was transferred into to a 100 mL a Teflon-lined autoclave, and maintained at 453 K for 24 h. After the reaction autoclave was cooled to room temperature naturally, the solid yielded was filtered, and rinsed with deionized water and absolute ethanol, respectively, and then the solid obtained was dispersed into absolute ethanol, and dried 333 K overnight. To further study the effects of calcination on the photocatalytic performance, the samples prepared were calcined in a muffle furnace at 723 K for 2 h. The samples obtained were labeled as TiO2-EA, TiO2-EG, TiO2-GL, EA-723 K, EG-723 K and GL723 K, respectively.
312 nm, and the scanning speed was 600 nm min 1. 2.3. Photocatalytic test Photocatalytic activities of the samples were measured by degrada tion of rhodamine B (RhB, 10 mg/L) under a 500 W xenon lamp (simulated sunlight). The concentration of photocatalyst is 1 g/L. The detailed operation procedure was described in Ref. [35]. The photo catalytic degradation efficiency of RhB is calculated by (C0-C)/C0 � 100%, where C0 and C is the concentration of RhB before and after the irradiation [36], respectively. 3. Results and discussion 3.1. Characterization of the samples
2.2. Characterization
The specific surface area of the sample was shown in Table 1. It is clear that before calcination, TiO2-EA holds the highest specific surface area, while TiO2-GL displays the lowest specific surface area. After calcination, the specific surface area of all the samples greatly decreases due to collapse of partial pore. After calcination, TiO2-EA still exhibits the highest specific surface area (88.5 m2/g). In the crystallization process, TiO2 cannot easily diffuse because of the high viscosity of ethylene glycol and glycerol, thus influencing the growth of the parti cles. However, in ethanol synthetic system, the crystal seeds diffuse rapidly, there is enough space for the crystal seeds to grow, leading to relative high specific surface area. Commonly, high specific surface area means more active sites, which is beneficial to the photocatalytic ac tivity [37]. Combined with the results of photocatalytic test, it is evident that high photocatalytic activity can be benefited from relative high specific surface area. However, based on the photocatalytic activity of the samples before and after calcination, it is apparent that high specific surface area is not the leading factor which determines the photo catalytic properties. The samples were characterized by X-ray diffraction (XRD) and the crystal structures of the sample were analyzed. As shown in Fig. 1, the XRD profiles of TiO2 prepared by different solvents were exhibited in
The prepared samples were subjected to X-ray powder diffraction (XRD) test on a DX-2600 X-ray diffractometer. The voltage and current were 35 kV and 25 mA, respectively. The specific surface area of the sample was measured by BET method on a SSA-4200 automatic surface analyzer. The UV–visible diffuse reflectance of the sample was per formed on a UH-4150 spectrophotometer (UH-4150, Japan) using barium sulfate as reference. The photo-generated charge separation rate of the sample was performed on a home-built apparatus (surface pho tovoltage spectroscopy, SPS). The experimental procedure was described as Ref. [34]. A VEGA 3 SBU scanning electron microscope was used to observe the morphology of the catalyst sample, the acceleration voltage during the test is 15 kV, and the emission current is 5 A. To test the level of �OH, 50 mg of photocatalyst was dispersed into 50 mL aqueous solution containing 20 mM NaOH and 6 mM terephthalic acid (TA). The solution was stirred in dark for 40 min before exposure to UV light. After illumination under a 500 W high-pressure mercury lamp for 20 min, the suspension was centrifuged, and the supernatant was sampled for analysis by recording the fluorescence signal of 2-hydroxy terephthalic acid (TAOH) generated on a fluorescence spectrometer (Cary Eclips, Agilent, USA). The wavelength of the excitation light was 2
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Fig. 2. SEM images of the samples: (a) TiO2-EA; (b) TiO2-EG; (c) TiO2-GL; (d) EA-723 K; (e) EG-723 K; (f) GL-723 K; (g–j) EDS of EA-723 K.
Fig. 3. UV–Vis diffuse reflectance spectra of the photocatalysts, the inset is bandgap of samples; (a) calcined TiO2; (b) TiO2 without calcination. 3
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respectively, indicating that preparation of TiO2 with different solvents significantly influences the bandgap. Compared to TiO2 prepared by other different solvents, TiO2-EA appears blue-shift, suggesting that TiO2-EA has a wider band gap than other samples. The decrease in the band gap is due to the strong polarity of the solvent [28]. On the other hand, wide bandgap of TiO2-EA indicates that valence band potential and conduction band potential of TiO2-EA is more positive and negative than that of other samples, consequently, the photoinduced electrons-holes possess stronger redox ability than that of other samples, expediting the degradation of RhB. In order to elaborate the separation rate of photoexcited carriers, surface photovoltage spectroscopy (SPS) measurements were per formed. After excitation by light, photoexcited carriers will be formed, generating a weak current, the current can be detected by the surface photovoltage spectrum. According to the principle of SPS, strong SPS signal corresponds to high separation rate of photogenerated carriers [40]. Usually, higher photogenerated electron separation rate results in higher photocatalytic performance [10,34,35]. The SPS results were displayed in Fig. 4, it is clear all the samples have obvious SPS responses from 300 to 380 nm, and no SPS responses in the visible region were observed, indicating that TiO2 prepared cannot be excited by visible light. Among the samples, the EA-723 K sample has the strongest SPS response, implying that the EA-723 K sample will show the highest separation rate of photogenerated carriers. However, all the samples without calcination exhibit weaker SPS than the samples after calcina tion. After calcination, all the samples have high crystallinity and small defects, manifesting high separation rate of photoinduced carriers. Furthermore, the SPS response signal of TiO2 prepared using ethanol as solvent is stronger than that of titanium dioxide prepared by other sol vents due to the presence of brookite TiO2 and high specific surface area, thereby accelerating the photogenerated charge separation rate [38]. To further investigate the separation rate of photogenerated electron-hole pairs, the sample was subjected to photoluminescence emission spectrum scanning (PL) in TAOH solution. The PL spectrum of TiO2 was shown in Fig. 5. All the samples have a broad emission at 375–500 nm, and the significant peak around 425 nm represents the characteristic fluorescent signal of TAOH, demonstrating that �OH ra dials were yielded during the photocatalytic process. High peak value corresponds to high level of �OH produced, and it is distinct that the EA723 K sample produces the highest level of �OH. As strong oxidant, high level of �OH produced will accelerate the degradation of pollutants [41]. Meanwhile, high level of �OH indicates high separation rate of photo generated electron-hole pairs, according well with the SPS results. X-ray photoelectron spectroscopy (XPS) was subjected to determine the surface chemical composition of the samples. Fig. 6a exhibits the surface survey spectra of the GL-723 K sample. Ti, O and C were detected on the surface of the GL-723 K sample. C can be attributed to adventi tious carbon on the surface of the sample [42]. The result of XPS is in
Fig. 4. SPS responses of the samples.
Fig. 1. All the samples possess characteristic peaks of anatase TiO2 (JCPDS 21–1272) [9] at 25.28� , 37.80� , 48.05� , 53.89� , 55.06� and 62.69� , corresponding to the (101), (004), (200), (105), (211), (204) crystal faces, respectively. In addition, both TiO2-EA and EA-723 K have very weak characteristic peaks at 30.8� , belonging to the (121) crystal plane of brookite (JCPDS 76–1935). Dieqing Zhang and coworkers re ported that brookite/rutile biphasic TiO2 catalysts exhibited superior photocatalytic activity in the photocatalytic oxidation of phenol in comparison to commercial P25 TiO2 [38]. The presence of brookite phase in the sample can form heterojunctions, accelerating the separa tion of photoinduced carriers, therefore displaying high photocatalytic performance. As expected, the peaks become sharp after calcination, indicating that the crystallinity of TiO2 is higher than that without baking, resulting in low specific surface area of the sample, which ac cords well with the results of SBET. The morphologies of the prepared samples were characterized by scanning electron microscopy (SEM), and the results were represented in Fig. 2. TiO2 prepared by different solvents show irregular lump-like shape, manifesting that the solvent cannot remarkably alter the morphology of TiO2 in this case. However, TiO2 prepared using the different solvents exhibits different particle size. Big particle size results in low specific surface area, which fits well with the results of BET. In addition, Ti, O and C were observed on the EDS spectrum (Fig. 2g–j), combined with results of XRD, EDS and XPS, it is apparent that TiO2 was successfully prepared. The optical absorption characteristics of the prepared samples were investigated by UV–Vis diffuse reflectance spectroscopy. As shown in Fig. 3, all the samples show significant absorption in the UV region. The band gap of all the samples was calculated according to the KubelkaMunk function [39]. As displayed in Table 1, the band gaps of TiO2-EA, TiO2-EG and TiO2-GL are 3.35 eV, 3.25 eV and 3.23 eV,
Fig. 5. PL spectral changes of TAOH in TA solution; (a) TiO2 without calcination; (b) calcined TiO2. 4
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Fig. 7. FT-IR patterns of the catalysts.
good consistent with the XRD and EDS results. Fig. 6b reveals the XPS profile of the Ti 2p of the sample. The peaks at 458.4 and 464.0 eV can be attributed to Ti 2p3/2 and Ti 2p1/2, respectively [43,44]. Fig. 6c displays an asymmetric broad peak of about 529.6 eV for O1s. The O 1s XPS spectrum can be resolved using the XPS peak fitting program. The peaks at 529.7 eV and 530.8 eV (Fig. 6d-f) correspond to lattice oxygen and surface adsorbed oxygen, respectively. Table 2 lists the surface hydroxyl content on the samples. As shown in Table 2, the surface hy droxyl content on TiO2-EA is higher than that on TiO2-EG and TiO2-GL. Similarly, EA-723 K also holds higher surface hydroxyl content than other two samples. It is interesting to find that TiO2 samples prepared after calcination show high surface hydroxyl content than TiO2 samples prepared without further calcination. Generally, high surface hydroxyl content is conducive to the photocatalytic performance, which fits well with the results of photocatalytic evaluation. The composition and structure of all samples were investigated by FT-IR experiments, and the results were shown in Fig. 7. The peaks around 500-700 cm 1 were observed in the spectra of all the samples, which is the flexural vibration of Ti-O stretching and Ti-O-Ti [9]. The peak at 1632 cm 1 is attributable to surface hydroxyl groups or the adsorption of water in the atmosphere by the catalyst. It is worth noting that there is a peak at 2974 cm 1, which is caused by the stretching vibration of residual C-H. However, after calcination, the peak located at 2974 cm 1 become weak due to the removal of residual C-H. Combined with results of XRD, EDS and XPS, it is evident that TiO2 was obtained. To investigate the active free radicals formed in the photocatalytic reaction system, different scavengers (isopropanol for �OH, ammonium oxalate for hþ and benzoquinone for �O2 ) were added to the photo catalytic systems. As shown in Fig. 8a, the photocatalytic degradation efficiency of RhB significantly decreases after the addition of BQ, indi cating that superoxide free radical (�O2 ) dominates the catalytic per formance, and hydroxyl radicals (�OH) takes a secondary role. To further compare the level of �O2 and the separation efficiency of the photoin duced carriers, NBT (nitrotetrazolium chloride)- �O2 experiments were carried out. NBT can react with �O2 to produce a blue insoluble sub stance [45], which will reduce the concentration of NBT, resulting in low absorption of NBT at 259 nm. The NBT experimental results were revealed in Fig. 8b, it is evident that the absorbance of NBT in the EA-723 K photocatalytic system is the lowest, suggesting that EA-723 K produces the highest level of �O2 , which accords well with the results of SPS. High level of �O2 will accelerate the degradation of RhB, naturally, it is anticipated that EA-723 K will exhibit higher photocatalytic activity than other samples.
Fig. 6. Survey XPS spectrum of photocatalysts: (a) GL-723 K; (b) Ti 2p; (c) O 1s; (d) high resolution spectra of O 1s TiO2-EA; (e) TiO2-EG; (f) TiO2-GL; (g) EA723 K; (h) EG-723 K; (i) GL-723 K. Table 2 Curve fitting results of high resolution XPS spectra of catalyst in O1s region. Photocatalysts TiO2-EA TiO2-EG TiO2-GL EA-723K EG-723K GL-723K
O 1s (Ti-O)
O 1s (O-H)
Eb (eV)
ri (%)
Eb (eV)
ri (%)
529.7 529.6 529.8 529.6 529.7 529.7
61.9 74.3 76.8 57.5 65.6 65.6
532.2 531.2 531.5 532.2 530.8 530.7
38.1 25.7 23.2 42.5 34.4 34.4
Note: ri (%) represents the ratio/ΣAi (Ai is the area of each peak).
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Fig. 8. (a) Degradation efficiency of RhB in the presence different scavengers (irradiation time ¼ 2 h, dosage ¼ 0.2 mmol/L); (b) Absorbance of NBT in the different photocatalytic system (irradiation time ¼ 5 min, NBT dosage ¼ 0.05 mmol/L).
Fig. 9. (a) Degradation of RhB over different catalysts under simulated sunlight irradiation; (b) Kinetic equation of RhB over different catalysts under simulated sunlight; (c) degradation rate constant of RhB over the different photocatalysts.
3.2. Photocatalytic test
among the same batch samples, respectively. The degradation efficiency of RhB over EA-723 K reaches 88.0% after 2 h reaction. The results also indicate that TiO2 after calcination has better photocatalytic activity than TiO2 without calcination. As shown in Fig. 9b, the decay of RhB well fits a first order reaction kinetics; the decay rate constants of RhB over the different samples were calculated and shown in Fig. 9c. It is apparent that decay rate constant of RhB over EA-723 K is the highest (k ¼ 0.01716 min 1), indicating that EA-723 K displays the best photo catalytic performance. Moreover, the elimination efficiency of RhB over commercial TiO2 (P25) is remarkable lower than that on EA-TiO2.
In this study, RhB was chose as model pollutant, and the catalytic properties of the six samples were evaluated by decay of RhB. After stirring in dark for 30 min to achieve the adsorption-desorption equi librium, the curve of C/C0～t was compared to evaluate the photo catalytic activity, where t is irradiation time, C is the concentration of RhB at t, and C0 is the initial concentration of RhB [36]. The curve was shown in Fig. 9a, it can be seen that before and after calcination, TiO2-EA and EA-723 K have the highest degradation ability towards RhB 6
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Fig. 10. SEM images of fresh EA-723 K (a); SEM images of fresh EA-723 K after catalytic activity (b); XRD patterns of EA-723 K before and after catalytic activity (c); FT-IR patterns of EA-723 K before and after catalytic activity (d).
Among the samples without calcination, TiO2-EA exhibits the highest photocatalytic activity; the photocatalytic performance of TiO2-EA is more than four times of that of TiO2-GL. The result is in good consistent with the results of BET, SPS and UV–Vis DRS. Using a solvent with small viscosity to prepare TiO2 can provide high photocatalytic performance by influencing the specific surface area, surface hydroxyl content and the bandgap, especially the separation rate of photoinduced carriers. These advantageous factors lead to relative high photocatalytic perfor mance of EA-723 K compared to other samples. In order to reveal the stability of the catalyst, EA-723 K before and after the activity measurement was subjected to XRD, SEM and FT-IR tests. As shown in Fig. 10, after the activity test, the morphology of EA-723 K has no distinct change, and two samples display small spher ical (Fig. 10a–b). The XRD patterns of two photocatalysts (Fig. 10c) also exhibit no obvious change, which firmly supports the stability of the photocatalyst. After the degradation of RhB, all the position of the peaks has no shift and no new peaks appear. Combined with XRD, SEM and FTIR test results, it can be concluded that the photocatalysts possess excellent stability and have great application in wastewater treatment.
photocatalytic properties of TiO2. Preparation of TiO2 using ethanol as solvent through a solvothermal approach is a promising method. Declaration of interest statement The authors declare no competing financial 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] M.G. Wang, J. Han, Y.M. Hu, R. Guo, Y. Yin, Carbon-incorporated NiO/TiO2 mesoporous shells with p-n heterojunctions for efficient visible light photocatalysis, ACS Appl. Mater. Interfaces 43 (2016) 29511–29521. [2] A. Di Paola, E. García-L� opez, G. Marcì, L. Palmisano, A survey of photocatalytic materials for environmental remediation, J. Hazard. Mater. 211–212 (2012) 3–29. [3] Y.X. Deng, M.Y. Xing, J.L. Zhang, An advanced TiO2/Fe2TiO5/Fe2O3tripleheterojunction with enhanced and stable visible-light-driven Fenton reaction for the removal of organic pollutants, Appl. Catal. B Environ. 211 (2017) 157–166. [4] D.W. Ni, H.Y. Shen, H.Q. Li, Y. Ma, T.Y. Zhai, Synthesis of high efficient Cu/TiO2 photocatalysts by grinding and their size-dependent photocatalytic hydrogen production, Appl. Surf. Sci. 409 (2017) 241–249. [5] M.M. Momeni, Dye-sensitized solar cells based on Cr-doped TiO2 nanotube photoanodes, Rare Met. Momeni, M. M. Dye-sensitized solar cells based on Crdoped TiO2 nanotube photoanodes, Rare Met. 36 (2016) 865–871. [6] M. Afshari, M. Dinari, M.M. Momeni, Ultrasonic irradiation preparation of graphitic-C3N4/polyaniline nanocomposites as counter electrodes for dyesensitized solar cells, Ultrason. Sonochem. 42 (2018) 631–639. [7] X.W. Zheng, Q. Yang, S.T. Huang, J.B. Zhong, J.Z. Li, R.H. Yang, Y.Y. Zhang, Enhanced separation efficiency of photo-induced charge pairs and sunlight-driven
4. Conclusions In summary, TiO2 was prepared by a facile solvothermal method using different solvents. The results show that TiO2 prepared using ethanol as solvent exhibits better photocatalytic activity to other sam ples. TiO2 prepared using ethanol as solvent shows relative higher spe cific surface area, surface hydroxyl content, stronger redox ability of photoinduced carriers and higher separation efficiency of photoinduced carriers than TiO2 prepared using ethylene glycol and glycerol as sol vent, which can be definitely ascribed to the different viscosity and polarity of different solvents, remarkably affecting the growth and the surface properties of TiO2. This work provides important understanding on how the different solvents influence the preparation and 7
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Contents lists available at ScienceDirect
Solid State Sciences journal homepage: http://www.elsevier.com/locate/ssscie
Photocatalytic activity of TiO2 prepared by different solvents through a solvothermal approach Jiao Huang a, Huanhuan Liu a, Zhao Li a, Junbo Zhong a, b, *, Tao Wang a, **, Jianzhang Li a, Minjiao Li b 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 College of Chemical Engineering, Sichuan University of Science and Engineering, Zigong, 643000, PR China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: TiO2 Solvents Photocatalytic performance Solvothermal approach
In this work, TiO2 was prepared by different solvents through a solvothermal approach using tetrabutyl titanate as raw material, afterward the samples were calcined at 723 K. The samples were investigated by Brunauer -Emmett-Teller (BET) method, X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Vis diffuse reflectance (UV–Vis DRS), X-ray photoelectron spectroscopy (XPS), and surface photovoltage spectroscopy (SPS). The photocatalytic activity of the samples was investigated using rhodamine B (RhB) as a model contaminant. TiO2 prepared using ethanol as solvent has the best photocatalytic performance under irradiation of 500 W Xe lamp. Compared with the samples without calcination, all samples after calcination display significantly improved photocatalytic performance, which can be tightly attributed to the enhanced separation of photoin duced carriers and relative high surface hydroxyl content.
1. Introduction Nowadays, environmental pollution has become a major problem that threats the development of human being. Among them, water pollution is particularly serious, therefore, it is profound significant to treat wastewater for the scientific development of human society. Various technologies haven been developed to treat wastewater, espe cially the treatment of dyes in industrial wastewater, among the stra tegies, photocatalysis technology is considered to be a promising and green approach [1–6]. Semiconductor-based photocatalysis driven by sunlight can degrade organic pollutants in the sewage into CO2 and water， therefore photocatalytic technology is environmentally friendly. Development of photocatalyst with excellent performance and low cost is the core of photocatalytic technology [7,8]. Among the photocatalysts developed, TiO2 has attracted considerable attention due to its excellent performance. In fact, TiO2 has exceptional advantages, such as good chemical stability, non-toxicity, environmental friendliness and sustainability [9–15]. However, TiO2 also has some conspicuous shortcomings, for example, TiO2 has a wide band gap (about 3.2 eV),
and only ultraviolet light with a wavelength of <387 nm can be used to drive TiO2, ultraviolet light only accounts for 4–5% of sunlight [16–19]. The practical application of TiO2 is also limited by the high recombi nation rate of photogenerated electron-hole pairs. These two main bottlenecks make the photocatalytic activity of TiO2 far from satisfied to meet the practical requirements [20–22]. Usually, photocatalytic activity of TiO2 tightly depends on many physical factors, such as specific surface area, particle size, pore volume, surface hydroxyl content and crystallinity [23]. Various approaches have been employed to synthesize TiO2, such as sol-gel method, sol vothermal method, sonochemical or chemical vapor deposition [24]. TiO2 prepared display different shapes, for example, TiO2 particles, nanotubes, nanorods, microspheres, hollow microspheres or flower-like structures [25]. Solvothermal synthesis is an effective way to prepare TiO2 with unique morphology. In the solvothermal preparation process, different solvents are employed, the conditions of solvothermal reaction can be easily controlled, such as temperature, pH, reaction time, addi tives and so on, thus TiO2 with different morphologies and structures can be obtained. Moreover, solvent can affect the nucleation and growth
* Corresponding author. 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 ** Corresponding author. E-mail addresses: [email protected] (J. Zhong), [email protected] (T. Wang). https://doi.org/10.1016/j.solidstatesciences.2019.106024 Received 12 June 2019; Received in revised form 1 October 2019; Accepted 2 October 2019 Available online 2 October 2019 1293-2558/© 2019 Elsevier Masson SAS. All rights reserved.
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of the products during the solvothermal process [26]. Up to now, various theories are proposed to elaborate how the sol vent affects the product in many aspects. The polarity of the solvent will affect the acquisition of unique morphology [27], the polarity of solvent even can alter the band gap of TiO2, and strong polar solvent can induce the shift of band gap. Hu et al. found that polarity of solvent could affect the different crystal face exposure ratios, resulting in different pro portions of unsaturated Ti5c. Different proportions of unsaturated Ti5c will cause different combinations of conduction bands and Ti 3D orbitals in different band gaps [28]. Additionally, the boiling point and viscosity affect the crystallite structure of the product [29], the surface tension affects the crystal strength and morphology of the product [30], the dielectric constant can control the crystallinity of the product [31] and so on. In order to understand the effect of solvent on the preparation and photocatalytic performance of TiO2, in this study, three alcohols (ethanol, ethylene glycol and glycerol) were used as solvent to prepare TiO2 by a solvothermal method, and the products were characterized by XRD, SEM, UV–Vis DRS and so on [32,33]. The corresponding photo catalytic performance was evaluated. The results reveal that TiO2 pre pared using ethanol as solvent displays higher photocatalytic activity than TiO2 prepared using ethylene glycol and glycerol as solvent, the underlying mechanism was proposed.
Table 1 Specific surface area and bandgap of the samples. Sample (TiO2)
TiO2EA
TiO2EG
TiO2GL
EA723K
EG723K
GL723K
SBET (m2/g) Band gap (eV)
181.6 3.34
169.6 3.25
159.4 3.23
88.5 3.35
76.8 3.21
62.4 3.25
2. Experimental 2.1. Synthesis Fig. 1. XRD patterns of the samples.
All chemicals are of analytical grade. In this work, tetrabutyl titanate was used as raw material to prepare TiO2 in different solvents by a solvothermal method. Typically, 10 ml tetrabutyl titanate was dissolved in 20 ml ethanol (EA), ethylene glycol (EG) and glycerol (GL), respec tively. After stirring for 30 min, 40 ml deionized water was added into the above solution, respectively. The suspension system formed was continuously stirred for 30 min, and then was transferred into to a 100 mL a Teflon-lined autoclave, and maintained at 453 K for 24 h. After the reaction autoclave was cooled to room temperature naturally, the solid yielded was filtered, and rinsed with deionized water and absolute ethanol, respectively, and then the solid obtained was dispersed into absolute ethanol, and dried 333 K overnight. To further study the effects of calcination on the photocatalytic performance, the samples prepared were calcined in a muffle furnace at 723 K for 2 h. The samples obtained were labeled as TiO2-EA, TiO2-EG, TiO2-GL, EA-723 K, EG-723 K and GL723 K, respectively.
312 nm, and the scanning speed was 600 nm min 1. 2.3. Photocatalytic test Photocatalytic activities of the samples were measured by degrada tion of rhodamine B (RhB, 10 mg/L) under a 500 W xenon lamp (simulated sunlight). The concentration of photocatalyst is 1 g/L. The detailed operation procedure was described in Ref. [35]. The photo catalytic degradation efficiency of RhB is calculated by (C0-C)/C0 � 100%, where C0 and C is the concentration of RhB before and after the irradiation [36], respectively. 3. Results and discussion 3.1. Characterization of the samples
2.2. Characterization
The specific surface area of the sample was shown in Table 1. It is clear that before calcination, TiO2-EA holds the highest specific surface area, while TiO2-GL displays the lowest specific surface area. After calcination, the specific surface area of all the samples greatly decreases due to collapse of partial pore. After calcination, TiO2-EA still exhibits the highest specific surface area (88.5 m2/g). In the crystallization process, TiO2 cannot easily diffuse because of the high viscosity of ethylene glycol and glycerol, thus influencing the growth of the parti cles. However, in ethanol synthetic system, the crystal seeds diffuse rapidly, there is enough space for the crystal seeds to grow, leading to relative high specific surface area. Commonly, high specific surface area means more active sites, which is beneficial to the photocatalytic ac tivity [37]. Combined with the results of photocatalytic test, it is evident that high photocatalytic activity can be benefited from relative high specific surface area. However, based on the photocatalytic activity of the samples before and after calcination, it is apparent that high specific surface area is not the leading factor which determines the photo catalytic properties. The samples were characterized by X-ray diffraction (XRD) and the crystal structures of the sample were analyzed. As shown in Fig. 1, the XRD profiles of TiO2 prepared by different solvents were exhibited in
The prepared samples were subjected to X-ray powder diffraction (XRD) test on a DX-2600 X-ray diffractometer. The voltage and current were 35 kV and 25 mA, respectively. The specific surface area of the sample was measured by BET method on a SSA-4200 automatic surface analyzer. The UV–visible diffuse reflectance of the sample was per formed on a UH-4150 spectrophotometer (UH-4150, Japan) using barium sulfate as reference. The photo-generated charge separation rate of the sample was performed on a home-built apparatus (surface pho tovoltage spectroscopy, SPS). The experimental procedure was described as Ref. [34]. A VEGA 3 SBU scanning electron microscope was used to observe the morphology of the catalyst sample, the acceleration voltage during the test is 15 kV, and the emission current is 5 A. To test the level of �OH, 50 mg of photocatalyst was dispersed into 50 mL aqueous solution containing 20 mM NaOH and 6 mM terephthalic acid (TA). The solution was stirred in dark for 40 min before exposure to UV light. After illumination under a 500 W high-pressure mercury lamp for 20 min, the suspension was centrifuged, and the supernatant was sampled for analysis by recording the fluorescence signal of 2-hydroxy terephthalic acid (TAOH) generated on a fluorescence spectrometer (Cary Eclips, Agilent, USA). The wavelength of the excitation light was 2
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Fig. 2. SEM images of the samples: (a) TiO2-EA; (b) TiO2-EG; (c) TiO2-GL; (d) EA-723 K; (e) EG-723 K; (f) GL-723 K; (g–j) EDS of EA-723 K.
Fig. 3. UV–Vis diffuse reflectance spectra of the photocatalysts, the inset is bandgap of samples; (a) calcined TiO2; (b) TiO2 without calcination. 3
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respectively, indicating that preparation of TiO2 with different solvents significantly influences the bandgap. Compared to TiO2 prepared by other different solvents, TiO2-EA appears blue-shift, suggesting that TiO2-EA has a wider band gap than other samples. The decrease in the band gap is due to the strong polarity of the solvent [28]. On the other hand, wide bandgap of TiO2-EA indicates that valence band potential and conduction band potential of TiO2-EA is more positive and negative than that of other samples, consequently, the photoinduced electrons-holes possess stronger redox ability than that of other samples, expediting the degradation of RhB. In order to elaborate the separation rate of photoexcited carriers, surface photovoltage spectroscopy (SPS) measurements were per formed. After excitation by light, photoexcited carriers will be formed, generating a weak current, the current can be detected by the surface photovoltage spectrum. According to the principle of SPS, strong SPS signal corresponds to high separation rate of photogenerated carriers [40]. Usually, higher photogenerated electron separation rate results in higher photocatalytic performance [10,34,35]. The SPS results were displayed in Fig. 4, it is clear all the samples have obvious SPS responses from 300 to 380 nm, and no SPS responses in the visible region were observed, indicating that TiO2 prepared cannot be excited by visible light. Among the samples, the EA-723 K sample has the strongest SPS response, implying that the EA-723 K sample will show the highest separation rate of photogenerated carriers. However, all the samples without calcination exhibit weaker SPS than the samples after calcina tion. After calcination, all the samples have high crystallinity and small defects, manifesting high separation rate of photoinduced carriers. Furthermore, the SPS response signal of TiO2 prepared using ethanol as solvent is stronger than that of titanium dioxide prepared by other sol vents due to the presence of brookite TiO2 and high specific surface area, thereby accelerating the photogenerated charge separation rate [38]. To further investigate the separation rate of photogenerated electron-hole pairs, the sample was subjected to photoluminescence emission spectrum scanning (PL) in TAOH solution. The PL spectrum of TiO2 was shown in Fig. 5. All the samples have a broad emission at 375–500 nm, and the significant peak around 425 nm represents the characteristic fluorescent signal of TAOH, demonstrating that �OH ra dials were yielded during the photocatalytic process. High peak value corresponds to high level of �OH produced, and it is distinct that the EA723 K sample produces the highest level of �OH. As strong oxidant, high level of �OH produced will accelerate the degradation of pollutants [41]. Meanwhile, high level of �OH indicates high separation rate of photo generated electron-hole pairs, according well with the SPS results. X-ray photoelectron spectroscopy (XPS) was subjected to determine the surface chemical composition of the samples. Fig. 6a exhibits the surface survey spectra of the GL-723 K sample. Ti, O and C were detected on the surface of the GL-723 K sample. C can be attributed to adventi tious carbon on the surface of the sample [42]. The result of XPS is in
Fig. 4. SPS responses of the samples.
Fig. 1. All the samples possess characteristic peaks of anatase TiO2 (JCPDS 21–1272) [9] at 25.28� , 37.80� , 48.05� , 53.89� , 55.06� and 62.69� , corresponding to the (101), (004), (200), (105), (211), (204) crystal faces, respectively. In addition, both TiO2-EA and EA-723 K have very weak characteristic peaks at 30.8� , belonging to the (121) crystal plane of brookite (JCPDS 76–1935). Dieqing Zhang and coworkers re ported that brookite/rutile biphasic TiO2 catalysts exhibited superior photocatalytic activity in the photocatalytic oxidation of phenol in comparison to commercial P25 TiO2 [38]. The presence of brookite phase in the sample can form heterojunctions, accelerating the separa tion of photoinduced carriers, therefore displaying high photocatalytic performance. As expected, the peaks become sharp after calcination, indicating that the crystallinity of TiO2 is higher than that without baking, resulting in low specific surface area of the sample, which ac cords well with the results of SBET. The morphologies of the prepared samples were characterized by scanning electron microscopy (SEM), and the results were represented in Fig. 2. TiO2 prepared by different solvents show irregular lump-like shape, manifesting that the solvent cannot remarkably alter the morphology of TiO2 in this case. However, TiO2 prepared using the different solvents exhibits different particle size. Big particle size results in low specific surface area, which fits well with the results of BET. In addition, Ti, O and C were observed on the EDS spectrum (Fig. 2g–j), combined with results of XRD, EDS and XPS, it is apparent that TiO2 was successfully prepared. The optical absorption characteristics of the prepared samples were investigated by UV–Vis diffuse reflectance spectroscopy. As shown in Fig. 3, all the samples show significant absorption in the UV region. The band gap of all the samples was calculated according to the KubelkaMunk function [39]. As displayed in Table 1, the band gaps of TiO2-EA, TiO2-EG and TiO2-GL are 3.35 eV, 3.25 eV and 3.23 eV,
Fig. 5. PL spectral changes of TAOH in TA solution; (a) TiO2 without calcination; (b) calcined TiO2. 4
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Fig. 7. FT-IR patterns of the catalysts.
good consistent with the XRD and EDS results. Fig. 6b reveals the XPS profile of the Ti 2p of the sample. The peaks at 458.4 and 464.0 eV can be attributed to Ti 2p3/2 and Ti 2p1/2, respectively [43,44]. Fig. 6c displays an asymmetric broad peak of about 529.6 eV for O1s. The O 1s XPS spectrum can be resolved using the XPS peak fitting program. The peaks at 529.7 eV and 530.8 eV (Fig. 6d-f) correspond to lattice oxygen and surface adsorbed oxygen, respectively. Table 2 lists the surface hydroxyl content on the samples. As shown in Table 2, the surface hy droxyl content on TiO2-EA is higher than that on TiO2-EG and TiO2-GL. Similarly, EA-723 K also holds higher surface hydroxyl content than other two samples. It is interesting to find that TiO2 samples prepared after calcination show high surface hydroxyl content than TiO2 samples prepared without further calcination. Generally, high surface hydroxyl content is conducive to the photocatalytic performance, which fits well with the results of photocatalytic evaluation. The composition and structure of all samples were investigated by FT-IR experiments, and the results were shown in Fig. 7. The peaks around 500-700 cm 1 were observed in the spectra of all the samples, which is the flexural vibration of Ti-O stretching and Ti-O-Ti [9]. The peak at 1632 cm 1 is attributable to surface hydroxyl groups or the adsorption of water in the atmosphere by the catalyst. It is worth noting that there is a peak at 2974 cm 1, which is caused by the stretching vibration of residual C-H. However, after calcination, the peak located at 2974 cm 1 become weak due to the removal of residual C-H. Combined with results of XRD, EDS and XPS, it is evident that TiO2 was obtained. To investigate the active free radicals formed in the photocatalytic reaction system, different scavengers (isopropanol for �OH, ammonium oxalate for hþ and benzoquinone for �O2 ) were added to the photo catalytic systems. As shown in Fig. 8a, the photocatalytic degradation efficiency of RhB significantly decreases after the addition of BQ, indi cating that superoxide free radical (�O2 ) dominates the catalytic per formance, and hydroxyl radicals (�OH) takes a secondary role. To further compare the level of �O2 and the separation efficiency of the photoin duced carriers, NBT (nitrotetrazolium chloride)- �O2 experiments were carried out. NBT can react with �O2 to produce a blue insoluble sub stance [45], which will reduce the concentration of NBT, resulting in low absorption of NBT at 259 nm. The NBT experimental results were revealed in Fig. 8b, it is evident that the absorbance of NBT in the EA-723 K photocatalytic system is the lowest, suggesting that EA-723 K produces the highest level of �O2 , which accords well with the results of SPS. High level of �O2 will accelerate the degradation of RhB, naturally, it is anticipated that EA-723 K will exhibit higher photocatalytic activity than other samples.
Fig. 6. Survey XPS spectrum of photocatalysts: (a) GL-723 K; (b) Ti 2p; (c) O 1s; (d) high resolution spectra of O 1s TiO2-EA; (e) TiO2-EG; (f) TiO2-GL; (g) EA723 K; (h) EG-723 K; (i) GL-723 K. Table 2 Curve fitting results of high resolution XPS spectra of catalyst in O1s region. Photocatalysts TiO2-EA TiO2-EG TiO2-GL EA-723K EG-723K GL-723K
O 1s (Ti-O)
O 1s (O-H)
Eb (eV)
ri (%)
Eb (eV)
ri (%)
529.7 529.6 529.8 529.6 529.7 529.7
61.9 74.3 76.8 57.5 65.6 65.6
532.2 531.2 531.5 532.2 530.8 530.7
38.1 25.7 23.2 42.5 34.4 34.4
Note: ri (%) represents the ratio/ΣAi (Ai is the area of each peak).
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Fig. 8. (a) Degradation efficiency of RhB in the presence different scavengers (irradiation time ¼ 2 h, dosage ¼ 0.2 mmol/L); (b) Absorbance of NBT in the different photocatalytic system (irradiation time ¼ 5 min, NBT dosage ¼ 0.05 mmol/L).
Fig. 9. (a) Degradation of RhB over different catalysts under simulated sunlight irradiation; (b) Kinetic equation of RhB over different catalysts under simulated sunlight; (c) degradation rate constant of RhB over the different photocatalysts.
3.2. Photocatalytic test
among the same batch samples, respectively. The degradation efficiency of RhB over EA-723 K reaches 88.0% after 2 h reaction. The results also indicate that TiO2 after calcination has better photocatalytic activity than TiO2 without calcination. As shown in Fig. 9b, the decay of RhB well fits a first order reaction kinetics; the decay rate constants of RhB over the different samples were calculated and shown in Fig. 9c. It is apparent that decay rate constant of RhB over EA-723 K is the highest (k ¼ 0.01716 min 1), indicating that EA-723 K displays the best photo catalytic performance. Moreover, the elimination efficiency of RhB over commercial TiO2 (P25) is remarkable lower than that on EA-TiO2.
In this study, RhB was chose as model pollutant, and the catalytic properties of the six samples were evaluated by decay of RhB. After stirring in dark for 30 min to achieve the adsorption-desorption equi librium, the curve of C/C0～t was compared to evaluate the photo catalytic activity, where t is irradiation time, C is the concentration of RhB at t, and C0 is the initial concentration of RhB [36]. The curve was shown in Fig. 9a, it can be seen that before and after calcination, TiO2-EA and EA-723 K have the highest degradation ability towards RhB 6
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Fig. 10. SEM images of fresh EA-723 K (a); SEM images of fresh EA-723 K after catalytic activity (b); XRD patterns of EA-723 K before and after catalytic activity (c); FT-IR patterns of EA-723 K before and after catalytic activity (d).
Among the samples without calcination, TiO2-EA exhibits the highest photocatalytic activity; the photocatalytic performance of TiO2-EA is more than four times of that of TiO2-GL. The result is in good consistent with the results of BET, SPS and UV–Vis DRS. Using a solvent with small viscosity to prepare TiO2 can provide high photocatalytic performance by influencing the specific surface area, surface hydroxyl content and the bandgap, especially the separation rate of photoinduced carriers. These advantageous factors lead to relative high photocatalytic perfor mance of EA-723 K compared to other samples. In order to reveal the stability of the catalyst, EA-723 K before and after the activity measurement was subjected to XRD, SEM and FT-IR tests. As shown in Fig. 10, after the activity test, the morphology of EA-723 K has no distinct change, and two samples display small spher ical (Fig. 10a–b). The XRD patterns of two photocatalysts (Fig. 10c) also exhibit no obvious change, which firmly supports the stability of the photocatalyst. After the degradation of RhB, all the position of the peaks has no shift and no new peaks appear. Combined with XRD, SEM and FTIR test results, it can be concluded that the photocatalysts possess excellent stability and have great application in wastewater treatment.
photocatalytic properties of TiO2. Preparation of TiO2 using ethanol as solvent through a solvothermal approach is a promising method. Declaration of interest statement The authors declare no competing financial 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] M.G. Wang, J. Han, Y.M. Hu, R. Guo, Y. Yin, Carbon-incorporated NiO/TiO2 mesoporous shells with p-n heterojunctions for efficient visible light photocatalysis, ACS Appl. Mater. Interfaces 43 (2016) 29511–29521. [2] A. Di Paola, E. García-L� opez, G. Marcì, L. Palmisano, A survey of photocatalytic materials for environmental remediation, J. Hazard. Mater. 211–212 (2012) 3–29. [3] Y.X. Deng, M.Y. Xing, J.L. Zhang, An advanced TiO2/Fe2TiO5/Fe2O3tripleheterojunction with enhanced and stable visible-light-driven Fenton reaction for the removal of organic pollutants, Appl. Catal. B Environ. 211 (2017) 157–166. [4] D.W. Ni, H.Y. Shen, H.Q. Li, Y. Ma, T.Y. Zhai, Synthesis of high efficient Cu/TiO2 photocatalysts by grinding and their size-dependent photocatalytic hydrogen production, Appl. Surf. Sci. 409 (2017) 241–249. [5] M.M. Momeni, Dye-sensitized solar cells based on Cr-doped TiO2 nanotube photoanodes, Rare Met. Momeni, M. M. Dye-sensitized solar cells based on Crdoped TiO2 nanotube photoanodes, Rare Met. 36 (2016) 865–871. [6] M. Afshari, M. Dinari, M.M. Momeni, Ultrasonic irradiation preparation of graphitic-C3N4/polyaniline nanocomposites as counter electrodes for dyesensitized solar cells, Ultrason. Sonochem. 42 (2018) 631–639. [7] X.W. Zheng, Q. Yang, S.T. Huang, J.B. Zhong, J.Z. Li, R.H. Yang, Y.Y. Zhang, Enhanced separation efficiency of photo-induced charge pairs and sunlight-driven
4. Conclusions In summary, TiO2 was prepared by a facile solvothermal method using different solvents. The results show that TiO2 prepared using ethanol as solvent exhibits better photocatalytic activity to other sam ples. TiO2 prepared using ethanol as solvent shows relative higher spe cific surface area, surface hydroxyl content, stronger redox ability of photoinduced carriers and higher separation efficiency of photoinduced carriers than TiO2 prepared using ethylene glycol and glycerol as sol vent, which can be definitely ascribed to the different viscosity and polarity of different solvents, remarkably affecting the growth and the surface properties of TiO2. This work provides important understanding on how the different solvents influence the preparation and 7
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