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Preparation of S-doped TiO2-three dimensional graphene aerogels as a highly efficient photocatalyst
Synthetic Metals 231 (2017) 51–57
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Preparation of S-doped TiO2-three dimensional graphene aerogels as a highly efficient photocatalyst
MARK
Zhongping Chena, Jianfeng Mab, Ke Yangb, Sheng Fengb, Wensheng Tanc, Yongxin Taoa, ⁎ Huihui Maoa, Yong Konga, a b c
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China School of Environmental & Safety Engineering, Changzhou University, Changzhou 213164, China Changzhou Key Laboratory of Large Plastic Parts Intelligence Manufacturing, Changzhou College of Information Technology, Changzhou 213164, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanocomposites S-doped TiO2 Three dimensional graphene aerogels Photocatalyst
Sulfur-doped titanium dioxide (S-TiO2) was hydrothermally synthesized using thiourea as the sulfur source, and then it was successfully integrated to the three-dimensional graphene aerogels (3DGA). The obtained nanocomposites of S-TiO2 and 3DGA (S-TiO2-3DGA) were characterized by XRD, SEM, TEM, XPS and UV–vis diffuse reflectance spectroscopy. When the nanocomposites were applied for the photodegradation of methyl orange (MO), the photocatalytic activity of TiO2 was significantly improved by doping of S and integration with 3DGA, which might be due to the narrowed bandgap of S-TiO2 and the good conductive ability and 3D structure of 3DGA. More importantly, the hydrophobic property of 3DGA makes it easy to separate the S-TiO2-3DGA from the water phase, resulting in excellent recyclability and high stability of the photocatalyst.
1. Introduction The past decades have witnessed the ever-growing emission of dye wastewater from various industries such as printing, textiles, cosmetics and food [1], and the discharged natural or synthetic dyes have become a major threat to environment and public health owing to their toxicity and non-biodegradability [2–4]. To address this problem, various methods such as activated carbon adsorption and oxidation, fungus breakdown and sludge coagulation have been proposed for the treatment of polluted aquatic environment [5–7]. Compared with other methods, photocatalysis has attracted increasing attention due to its ability of complete mineralization of organic pollutants [8–10]. As a promising semiconductor, titanium dioxide (TiO2) has attracted great interest in photocatalysis including hydrogen production [11], air detoxification [12] and water purification [13] owing to its low cost, high stability and nontoxicity [14]. However, there are two drawbacks associated with the practical use of TiO2 as a photocatalyst. First, it is difficult to separate the TiO2 nanoparticles from the suspensions [15]. Second, TiO2 can be activated only by ultraviolet light owing to the wide band gap (3.2 eV) [16,17]. The former severely limits the recycling of the TiO2 catalyst; the latter decreases the overall efficiency of TiO2 under natural sunlight irradiation, since UV only accounts for circa 4% of the incoming solar energy on the surface of Earth [18]. In addition, the high recombination rate of
⁎
Corresponding author. E-mail address: [email protected] (Y. Kong).
http://dx.doi.org/10.1016/j.synthmet.2017.06.020 Received 14 April 2017; Received in revised form 24 June 2017; Accepted 26 June 2017 0379-6779/ © 2017 Elsevier B.V. All rights reserved.
photogenerated electron-hole pairs is also a bottle-neck for TiO2-based photocatalysis. To address these issues, non-metal doping into TiO2 for narrowing the band gap energy (Eg) has been proposed as promising alternatives for TiO2 photocatalyst [19]. As a doped element for TiO2, S has received particular attention due to its satisfactory photocatalytic activity, structural stability, and band gap manipulation ability [20–22], and the mixing of S 3p state with O 2p state has been reported to contribute to an increased width of the valence band, forming the defect state in the band gap [20]. Up to now, the doping of S has been demonstrated to be effective in the degradation of azo dyes and the photocatalytic hydrogen evolution from water splitting [23]. In recent years, graphene-based materials have attracted considerable attention and achieved various practical applications as an ideal support owing to the excellent electrical conductivity, large surface area, extraordinary mechanical flexibility, hydrophobic property and chemical stability of graphene [24–29], and these unique properties of graphene make it a promising candidate for separating electron-hole pairs and being separated from the water phase easily. Now, particular attention has been paid to the coupling of graphene with some semiconductors, which has shown a significant improvement of the photoelectrochemical catalytic ability for the superior charge transport properties, the intense light absorption and the unique flexible sheetlike structure of the graphene component [30–32]. In this study, S-doped TiO2 (S-TiO2) and three-dimensional
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and heated at 180 °C for 3 h under autogenous pressure, and then allowed to cool to room temperature naturally. The obtained solids were collected and washed with ultrapure water and absolute ethanol several times and then dried in a vacuum oven at 60 °C for 8 h. The resultant products, S-TiO2, were ground into powder prior to use. As a reference, un-doped TiO2 was also synthesized using the same procedure without the addition of thiourea.
graphene aerogels (3DGA) are successfully integrated, and the obtained nanocomposites of S-TiO2 and 3DGA (S-TiO2-3DGA) are well studied as a UV and visible-light catalyst for the photodegradation of methyl orange (MO), a representative aqueous persistent organic pollutant [33]. Noted that the photocatalytic activity of nanometer-sized TiO2 powder could be greatly enhanced compared with the commercial TiO2 powder due to the larger specific surface area and better photocatalytic properties of the nanometer-sized TiO2 [34], and thus nanometer-sized TiO2 is synthesized in this work by using polyethylene glycols (PEG) as the templates, which can assist in controlling crystal growth and preventing the excessive growth of crystal particle and agglomeration [35,36]. The as-prepared S-TiO2-3DGA photocatalyst possesses great promise for UV and visible-light driven destruction of MO. Especially, the composite photocatalyst exhibits high recyclability and stability, indicating that the photocatalyst could be a promising and potential candidate for recyclable photodegradation of organic contaminants.
2.3. Synthesis of S-TiO2-3DGA
2. Experimental
According to the method reported by Marcano et al. [37], graphene oxide (GO) was prepared using natural graphite as the starting material. Then, 0.75 g of S-TiO2 was added into 70 mL of homogeneous GO aqueous dispersion (1.43 g L−1) under magnetic stirring. After 30 min of ultrasonic dispersion, the solution was transferred into a Teflon-lined autoclave and heated at 180 °C for 12 h. The obtained graphene hydrogels were rinsed thoroughly with ultrapure water and freeze-dried at −52 °C for 10 h to form the S-TiO2-3DGA.
2.1. Reagents and apparatus
2.4. Photocatalytic activity measurements
Natural graphite flakes (99.95%, 325 mesh) was purchased from Qingdao Graphite Co., Ltd. (Qingdao, China). Thiourea (CH4N2S, 99.0%) was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China). Polyethylene glycol (600) (PEG (600), 99.5%) and titanium tetrachloride (TiCl4, 98.0%) were received from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals are of analytical grade and used as received without further purification. All aqueous solutions were prepared using ultrapure water (MilliQ, Millipore). X-ray diffraction (XRD) analysis was carried out on a Rigaku D/max 2500PC X-ray diffractometer. The morphologies of the catalyst were characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) on a Supra55 field-emission scanning electron microscope (Zeiss, Germany) and a JEM 2100 transmission electron microscope (JEOL, Japan), respectively. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, USA), and the UV–vis diffuse reflectance spectra (UV–vis DRS) were recorded on a UV-2700 UV–vis spectrophotometer (Shimadzu, Japan).
0.05 g of S-TiO2-3DGA or S-TiO2 photocatalyst was added into a MO solution (500 mL, 6 mg L−1). The mixture was then irradiated by a 11 W UV lamp or a 300 W Xe lamp without stirring. During the photodegradation process of MO, 5 mL solution was taken out every 30 min and centrifuged to separate the photocatalyst particles, and the concentrations of the remnant MO were monitored by UV–vis spectroscopy at the wavelength of 465 nm. 2.5. Recyclability and stability testing Finally, the recyclability and stability of the S-TiO2-3DGA photocatalyst were evaluated. The suspension was filtered at the end of each cycle, and then the filtrate was analyzed and discarded. The dose of the S-TiO2-3DGA photocatalyst was 0.05 g, and the volumes of all the MO solutions were 500 mL (6 mg L−1). 3. Results and discussion 3.1. Catalyst characterization
2.2. Synthesis of S-TiO2
3.1.1. Density and mechanical strength of S-TiO2-3DGA The photographs of the as-prepared 3DGA and S-TiO2-3DGA are shown in Fig. 1A. The densities of 3DGA and S-TiO2-3DGA are calculated to be 6.7 and 24.8 mg cm−3, respectively, and therefore the two samples can easily rest on a green leaf due to their ultra-light densities (Fig. 1B and C). According to the method proposed by Wan et al. [28],
In a typical procedure, 2.06 g of thiourea and 0.1 mL of PEG (600) were added into 26.4 mL of TiCl4 aqueous solution (1.36 M). After stirring for 30 min, this mixture was transferred into a Teflon-lined autoclave with a capacity of 100 mL. Next, the autoclave was sealed
Fig. 1. (A) Photographs of 3DGA and S-TiO2-3DGA. Ultra-light 3DGA (B) and S-TiO2-3DGA (C) resting on a leaf. The mechanical property of 3DGA (D) and STiO2-3DGA (E).
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disappears in the spectrum of S-TiO2-3DGA, suggesting that GO is successfully reduced to graphene after the hydrothermal treatment [40]. 3.1.3. Morphologies of the S-TiO2-3DGA photocatalyst The FESEM image of the S-TiO2-3DGA catalyst is shown in Fig. 3A and B. The interconnected 3D network structure is observed on the graphene aerogels (GA, dehydrated graphene hydrogels) support, since GA synthesized through the feasible self-assembly hydrothermal method has been recognized as a novel class of 3D porous graphene architectures [41–43]. The 3D structure of graphene is beneficial for the transmitting of photo-generated electrons between the graphene layers. Meanwhile, the hierarchically porous structure of 3DGA can provide an ideal support for the deposition of the nanoparticles of S-TiO2 photocatalyst. In addition, it is found that the 3DGA support is almost completely covered by the S-TiO2 nanoparticles, and the anchoring of STiO2 particles on the 3DGA is expected since it is generally known that the monomeric titanyl ions (TiO2+) could be easily adsorbed on the negative surface of GO due to the electrostatic interactions [44]. During the following hydrothermal and freeze-drying treatment, GO is reduced to 3DGA and the S-TiO2 nanoparticles are then anchored to the surface of the 3DGA. The typical TEM image of the S-TiO2-3DGA photocatalyst reveals the existence of well-dispersed S-TiO2 nanoparticles with uniform size of 16–30 nm on the 3DGA support (Fig. 3C). The sample is further examined by lattice fringe analysis of high-resolution TEM (HRTEM). As can be seen from Fig. 3D, it shows a clear lattice fringe of 0.189 nm, matching well with the (200) plane of rutile phase TiO2. It is also observed that the S-TiO2 nanoparticles in the photocatalyst are distributed over the whole surface of the 3DGA support, which agrees well with the FESEM observations.
Fig. 2. XRD patterns of GO (a), un-doped TiO2 (b), S-TiO2 (c) and S-TiO2-3DGA (d).
the mechanical strength of 3DGA and S-TiO2-3DGA is tested, and it shows that the two samples can still maintain their macroscopic bulky shapes without any deformation when a 50 g counterweight is loaded (Fig. 1D and E).
3.1.2. XRD analysis The XRD patterns of GO, un-doped TiO2, S-TiO2 and S-TiO2-3DGA were recorded to investigate the crystalline phase transition during the synthesis of the photocatalyst. As shown in Fig. 2, the peaks located at 25.3°, 37.8°, 48.1°, 54.1°, 55.2°, 62.7°, 69.1°, 70.2° and 75.3° are attributed to the diffractions of the (101), (004), (200), (105), (211), (204), (116), (220) and (215) crystal planes, respectively, of anatase TiO2 (JCPDS Card No. 21-1272) [19]. As there are no diffraction peaks due to the rutile phase of TiO2 in the spectra of un-doped TiO2 and STiO2, it could be concluded that the doping of S does not change the crystal phase of TiO2 and the as-synthesized S-TiO2 is in a purely anatase structure [38]. No apparent peaks for graphene are observed in the spectrum of S-TiO2-3DGA, which might be ascribed to the fact that the main characteristic peak of graphene (about 25°) has a relatively low intensity and severely overlaps with the strong peak of anatase TiO2 at 25.3° [39]. In addition, the main characteristic peak of GO (about 9.8°)
3.1.4. XPS analysis of the S-TiO2-3DGA photocatalyst XPS spectra are recorded to investigate the exact chemical composition of the S-TiO2-3DGA photocatalyst. The full XPS spectra are shown in Fig. 4A, which clearly indicate the existence of O 1s, Ti 2p, C 1s and S 2p core levels. Specifically, the high-resolution O 1s spectra can be fitted into three peaks at 529.6, 530.4 and 531.6 eV (Fig. 4B), corresponding to TieOeTi TieOeS and SeOeS, respectively [45]. The deconvoluted Ti 2p spectra of the S-TiO2-3DGA display two peaks located Fig. 3. FESEM (A,B), TEM (C), and HRTEM (D) images of S-TiO2-3DGA.
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Fig. 4. Full XPS (A), high resolution O 1s (B), Ti 2p (C), C 1s (D), and S 2p (E) spectra of S-TiO2-3DGA.
suggesting the C element is not doped into the crystalline lattice. It is noteworthy that the presence of S is confirmed by the peak at 168.9 eV (Fig. 4A), and this peak can be further de-convoluted into two peaks at 168.4 and 169.7 eV (Fig. 4E), which can be assigned to S 2p3/2 and S 2p1/2, respectively. The S 2p3/2 peak at lower binding energy is about twice the intensity of the S 2p1/2 peak at higher binding energy, which agrees well with the previous report by Periyat et al. [50]. Generally, the peak at ∼168.9 eV is assigned to the S6+ cation [51,52], and thus the S element doped into the lattice of the S-TiO2-3DGA might exist in the form of S6+. The semiquantitative analysis of the as-prepared STiO2-3DGA by XPS is listed in Table 1. The S content in the S-TiO23DGA photocatalyst is determined to be 0.80 at% (about 1.84 at% in the S-TiO2), indicating the successful doping of S element to the TiO2
Table 1 Ti, O, S, and C contents of the as-synthesized S-TiO2-3DGA determined by XPS. Element
Ti
O
S
C
XPS (at%)
12.48
30.27
0.80
56.45
at 458.7 and 464.4 eV (Fig. 4C), which can be attributed to the binding energies of Ti 2p3/2 and Ti 2p1/2 (the typical values of TiO2), respectively [39,46,47]. The high-resolution C 1s spectra show only one configuration of C at 284.8 eV (Fig. 4D), which can be typically illustrated as CeC bonds [48]. However, the formation of CeTi bonds is not observed because of the missing of the peak at ∼282 eV [49], 54
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TiO2 and un-doped TiO2, a more intense absorption is observed for the S-TiO2-3DGA (432.7 nm, curve c), which might be attributed to the broad and featureless absorption of the 3DGA component in the same region [28]. The value of Eg can be obtained according to the equation Eg = 1240/λ, where λ is the wavelength of the absorption edge (nm) [53,54], and the Eg values of S-TiO2, un-doped TiO2 and S-TiO2-3DGA are calculated to be 2.93, 3.14 and 2.87 eV, respectively. Here, the bandgap narrowing on the S-TiO2 catalyst might be attributed to the introduction of S element to the lattice of TiO2, because the formation of doping states can reduce the electron transition energy from the valence band to the conduction band and thus result in a red-shift of the absorption edge [38]. The interactions between S-TiO2 and 3DGA might be achieved through the combination of the oxygen containing functional groups of the two components. As a result, the discreteness of the levels in the vicinity of the Fermi level is enhanced and the band gap is reduced.
Fig. 5. UV–vis diffuse reflectance absorption of the as-prepared S-TiO2 (a), un-doped TiO2 (b) and S-TiO2-3DGA (c).
3.3. Photocatalytic activity of the S-TiO2-3DGA photocatalyst To evaluate the photocatalytic performances of the S-TiO2-3DGA and other catalysts, the degradation of MO is performed under the irradiation of UV or visible light using 3DGA, un-doped TiO2, S-TiO2 and S-TiO2-3DGA as the photocatalyst. The results under UV irradiation are shown in Fig. 6A. After 1.5 h of UV irradiation, there is negligible degradation of MO when no catalyst is used. Around 53.3% and 83.9% of MO is degraded in 1.5 h in the presence of TiO2 or S-TiO2. It is exciting to find that the S-TiO2-3DGA catalyst exhibits considerably high photocatalytic activity, and the MO pollutants are almost completely eliminated within 1.5 h. Interestingly, the S-TiO2-3DGA catalyst still exhibits good photocatalytic performance under visible light irradiation. As shown in Fig. 6B, the un-doped TiO2 has nearly no activity, however, the photocatalytic is significantly improved when the S-TiO23DGA is used as the photocatalyst. After visible light irradiation for 6 h, around 92.8% of MO is eliminated with the S-TiO2-3DGA photocatalyst, which is much higher than that with the S-TiO2 catalyst (about 73.2%). The promoting effect of S-doping on the photocatalytic performance might be ascribed to the following factors. As shown in Fig. 7A, the redshift in the bandgap transition of the S-TiO2 can narrow the bandgap and consequently decrease the value of Eg, producing a second absorption edge in the visible region [55]. On the other hand, after being modified with the S dopant, the electron-deficient oxygen could serve as an electron trap to effectively inhibit the recombination between photo-generated electrons and holes [45]. The 3DGA in the photocatalyst also plays a crucial role in the enhancement of the photocatalytic activity. Under the irradiation of UV or visible light, the photogenerated electrons transfer from the S-TiO2 nanoparticles to the 3DGA, and effective separation of these electrons and holes could be easily achieved due to the unique 3D structure of 3DGA. The photo-generated electrons can further react with O2 to generate superoxide radical anions (O2%−) and hydroxyl radicals (OH%), which are responsible for the observed high catalytic performance of the S-TiO2-3DGA photocatalyst [24,56]. The significant effect of the 3DGA can be illustrated in Fig. 7B.
Fig. 6. Photocatalytic activity measurements of different photocatalysts under UV irradiation (A) and visible light irradiation (B).
nanoparticles [38]. On the other hand, the result also reveals that the atomic ratio of O to Ti is similar to the theoretical stoichiometric atomic ratio (2:1), suggesting the formation of TiO2.
3.4. Evaluation of the recyclability and stability of the S-TiO2-3DGA photocatalyst The recyclability and stability of the S-TiO2-3DGA under UV irradiation are of great importance for the practical applications of the photocatalyst in water purification. Noted that when the S-TiO2 is used as the photocatalyst for MO degradation, the separation of the S-TiO2 catalyst and water phase must be carried out by high speed centrifugation (10,000 rpm) for a rather long time. On the contrary, owing to the outstanding hydrophobic property of 3DGA, the S-TiO2-3DGA catalyst can be easily separated from the reaction systems via a simple filtration operation without appreciable loss of the solid photocatalyst, and thus excellent recyclability of the catalyst can be achieved. The
3.2. UV–vis DRS of S-TiO2 The UV–vis DRS are recorded to analyze the optical properties and the bandgap energies (Eg) of S-TiO2, un-doped TiO2 and S-TiO2-3DGA, and Fig. 5 shows the results from the diffuse reflectance measurements of the three samples. The UV–vis absorption band wavelength of S-TiO2 shifts to the visible light range and the optical band edge shows a remarkable red-shift (423.6 nm, curve a) compared with the un-doped TiO2 (394.9 nm, curve b), suggesting that a second absorption edge in the visible region exists for the prepared S-TiO2 [45]. Compared with S55
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Fig. 7. Schematic illustration showing the promoting effect of S-doping (A) and 3DGA (B). (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
the proposed photocatalyst are also investigated in this study, which might be ascribed to the narrowed bandgap of S-TiO2 and the good conductive ability and 3D structure of 3DGA. More importantly, the outstanding hydrophobic property of 3DGA endows the catalyst with excellent recyclability as well as high stability. We believe that the high-efficient photocatalyst could be a promising candidate in future wastewater treatment and have a prosperous commercial application prospect. Acknowledgements
Fig. 8. Repeated photocatalytic degradation of MO solutions (6 mg L under UV irradiation.
The authors are grateful to the financial supports from National Natural Science Foundation of China (41371446), Changzhou high-tech Key Laboratory (CM20153001), Natural Science Foundation of Jiangsu Province (BK20140264), Advanced Catalysis and Green Manufacturing Collaborative Innovation Center (ACGM2016-06-27) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
−1
) by S-TiO2-3DGA
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Fig. 9. TEM image of S-TiO2-3DGA after 20 cycles.
degradation rate of MO as catalyzed by the S-TiO2-3DGA is shown in Fig. 8. As can be seen, MO is degraded completely in the first 13 runs, and the removal efficiency can still maintain as high as 96.3% in the 20th run. The excellent recyclability of the S-TiO2-3DGA photocatalyst could be attributed to its highly stable structure. Fig. 9 shows the TEM image of the S-TiO2-3DGA after 20 cycles. As can be seen, it maintains a well-dispersed S-TiO2 distribution on the 3DGA support after 20 cycles, which is quite similar to that of the pristine S-TiO2-3DGA. This result indicates that the proposed S-TiO2-3DGA could be a high-efficient and highly stable photocatalyst for the degradation of MO dye.
4. Conclusions In summary, S-TiO2-3DGA nanocomposites are prepared via a facile hydrothermal method followed by freeze-drying treatment. Compared with 3DGA, un-doped TiO2 and S-TiO2, the as-synthesized S-TiO2-3DGA catalyst exhibits fairly high photocatalytic activity under the irradiation of UV and visible light. The photodegradation mechanisms of MO by 56
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Preparation of S-doped TiO2-three dimensional graphene aerogels as a highly efficient photocatalyst
MARK
Zhongping Chena, Jianfeng Mab, Ke Yangb, Sheng Fengb, Wensheng Tanc, Yongxin Taoa, ⁎ Huihui Maoa, Yong Konga, a b c
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China School of Environmental & Safety Engineering, Changzhou University, Changzhou 213164, China Changzhou Key Laboratory of Large Plastic Parts Intelligence Manufacturing, Changzhou College of Information Technology, Changzhou 213164, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanocomposites S-doped TiO2 Three dimensional graphene aerogels Photocatalyst
Sulfur-doped titanium dioxide (S-TiO2) was hydrothermally synthesized using thiourea as the sulfur source, and then it was successfully integrated to the three-dimensional graphene aerogels (3DGA). The obtained nanocomposites of S-TiO2 and 3DGA (S-TiO2-3DGA) were characterized by XRD, SEM, TEM, XPS and UV–vis diffuse reflectance spectroscopy. When the nanocomposites were applied for the photodegradation of methyl orange (MO), the photocatalytic activity of TiO2 was significantly improved by doping of S and integration with 3DGA, which might be due to the narrowed bandgap of S-TiO2 and the good conductive ability and 3D structure of 3DGA. More importantly, the hydrophobic property of 3DGA makes it easy to separate the S-TiO2-3DGA from the water phase, resulting in excellent recyclability and high stability of the photocatalyst.
1. Introduction The past decades have witnessed the ever-growing emission of dye wastewater from various industries such as printing, textiles, cosmetics and food [1], and the discharged natural or synthetic dyes have become a major threat to environment and public health owing to their toxicity and non-biodegradability [2–4]. To address this problem, various methods such as activated carbon adsorption and oxidation, fungus breakdown and sludge coagulation have been proposed for the treatment of polluted aquatic environment [5–7]. Compared with other methods, photocatalysis has attracted increasing attention due to its ability of complete mineralization of organic pollutants [8–10]. As a promising semiconductor, titanium dioxide (TiO2) has attracted great interest in photocatalysis including hydrogen production [11], air detoxification [12] and water purification [13] owing to its low cost, high stability and nontoxicity [14]. However, there are two drawbacks associated with the practical use of TiO2 as a photocatalyst. First, it is difficult to separate the TiO2 nanoparticles from the suspensions [15]. Second, TiO2 can be activated only by ultraviolet light owing to the wide band gap (3.2 eV) [16,17]. The former severely limits the recycling of the TiO2 catalyst; the latter decreases the overall efficiency of TiO2 under natural sunlight irradiation, since UV only accounts for circa 4% of the incoming solar energy on the surface of Earth [18]. In addition, the high recombination rate of
⁎
Corresponding author. E-mail address: [email protected] (Y. Kong).
http://dx.doi.org/10.1016/j.synthmet.2017.06.020 Received 14 April 2017; Received in revised form 24 June 2017; Accepted 26 June 2017 0379-6779/ © 2017 Elsevier B.V. All rights reserved.
photogenerated electron-hole pairs is also a bottle-neck for TiO2-based photocatalysis. To address these issues, non-metal doping into TiO2 for narrowing the band gap energy (Eg) has been proposed as promising alternatives for TiO2 photocatalyst [19]. As a doped element for TiO2, S has received particular attention due to its satisfactory photocatalytic activity, structural stability, and band gap manipulation ability [20–22], and the mixing of S 3p state with O 2p state has been reported to contribute to an increased width of the valence band, forming the defect state in the band gap [20]. Up to now, the doping of S has been demonstrated to be effective in the degradation of azo dyes and the photocatalytic hydrogen evolution from water splitting [23]. In recent years, graphene-based materials have attracted considerable attention and achieved various practical applications as an ideal support owing to the excellent electrical conductivity, large surface area, extraordinary mechanical flexibility, hydrophobic property and chemical stability of graphene [24–29], and these unique properties of graphene make it a promising candidate for separating electron-hole pairs and being separated from the water phase easily. Now, particular attention has been paid to the coupling of graphene with some semiconductors, which has shown a significant improvement of the photoelectrochemical catalytic ability for the superior charge transport properties, the intense light absorption and the unique flexible sheetlike structure of the graphene component [30–32]. In this study, S-doped TiO2 (S-TiO2) and three-dimensional
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and heated at 180 °C for 3 h under autogenous pressure, and then allowed to cool to room temperature naturally. The obtained solids were collected and washed with ultrapure water and absolute ethanol several times and then dried in a vacuum oven at 60 °C for 8 h. The resultant products, S-TiO2, were ground into powder prior to use. As a reference, un-doped TiO2 was also synthesized using the same procedure without the addition of thiourea.
graphene aerogels (3DGA) are successfully integrated, and the obtained nanocomposites of S-TiO2 and 3DGA (S-TiO2-3DGA) are well studied as a UV and visible-light catalyst for the photodegradation of methyl orange (MO), a representative aqueous persistent organic pollutant [33]. Noted that the photocatalytic activity of nanometer-sized TiO2 powder could be greatly enhanced compared with the commercial TiO2 powder due to the larger specific surface area and better photocatalytic properties of the nanometer-sized TiO2 [34], and thus nanometer-sized TiO2 is synthesized in this work by using polyethylene glycols (PEG) as the templates, which can assist in controlling crystal growth and preventing the excessive growth of crystal particle and agglomeration [35,36]. The as-prepared S-TiO2-3DGA photocatalyst possesses great promise for UV and visible-light driven destruction of MO. Especially, the composite photocatalyst exhibits high recyclability and stability, indicating that the photocatalyst could be a promising and potential candidate for recyclable photodegradation of organic contaminants.
2.3. Synthesis of S-TiO2-3DGA
2. Experimental
According to the method reported by Marcano et al. [37], graphene oxide (GO) was prepared using natural graphite as the starting material. Then, 0.75 g of S-TiO2 was added into 70 mL of homogeneous GO aqueous dispersion (1.43 g L−1) under magnetic stirring. After 30 min of ultrasonic dispersion, the solution was transferred into a Teflon-lined autoclave and heated at 180 °C for 12 h. The obtained graphene hydrogels were rinsed thoroughly with ultrapure water and freeze-dried at −52 °C for 10 h to form the S-TiO2-3DGA.
2.1. Reagents and apparatus
2.4. Photocatalytic activity measurements
Natural graphite flakes (99.95%, 325 mesh) was purchased from Qingdao Graphite Co., Ltd. (Qingdao, China). Thiourea (CH4N2S, 99.0%) was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China). Polyethylene glycol (600) (PEG (600), 99.5%) and titanium tetrachloride (TiCl4, 98.0%) were received from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals are of analytical grade and used as received without further purification. All aqueous solutions were prepared using ultrapure water (MilliQ, Millipore). X-ray diffraction (XRD) analysis was carried out on a Rigaku D/max 2500PC X-ray diffractometer. The morphologies of the catalyst were characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) on a Supra55 field-emission scanning electron microscope (Zeiss, Germany) and a JEM 2100 transmission electron microscope (JEOL, Japan), respectively. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, USA), and the UV–vis diffuse reflectance spectra (UV–vis DRS) were recorded on a UV-2700 UV–vis spectrophotometer (Shimadzu, Japan).
0.05 g of S-TiO2-3DGA or S-TiO2 photocatalyst was added into a MO solution (500 mL, 6 mg L−1). The mixture was then irradiated by a 11 W UV lamp or a 300 W Xe lamp without stirring. During the photodegradation process of MO, 5 mL solution was taken out every 30 min and centrifuged to separate the photocatalyst particles, and the concentrations of the remnant MO were monitored by UV–vis spectroscopy at the wavelength of 465 nm. 2.5. Recyclability and stability testing Finally, the recyclability and stability of the S-TiO2-3DGA photocatalyst were evaluated. The suspension was filtered at the end of each cycle, and then the filtrate was analyzed and discarded. The dose of the S-TiO2-3DGA photocatalyst was 0.05 g, and the volumes of all the MO solutions were 500 mL (6 mg L−1). 3. Results and discussion 3.1. Catalyst characterization
2.2. Synthesis of S-TiO2
3.1.1. Density and mechanical strength of S-TiO2-3DGA The photographs of the as-prepared 3DGA and S-TiO2-3DGA are shown in Fig. 1A. The densities of 3DGA and S-TiO2-3DGA are calculated to be 6.7 and 24.8 mg cm−3, respectively, and therefore the two samples can easily rest on a green leaf due to their ultra-light densities (Fig. 1B and C). According to the method proposed by Wan et al. [28],
In a typical procedure, 2.06 g of thiourea and 0.1 mL of PEG (600) were added into 26.4 mL of TiCl4 aqueous solution (1.36 M). After stirring for 30 min, this mixture was transferred into a Teflon-lined autoclave with a capacity of 100 mL. Next, the autoclave was sealed
Fig. 1. (A) Photographs of 3DGA and S-TiO2-3DGA. Ultra-light 3DGA (B) and S-TiO2-3DGA (C) resting on a leaf. The mechanical property of 3DGA (D) and STiO2-3DGA (E).
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disappears in the spectrum of S-TiO2-3DGA, suggesting that GO is successfully reduced to graphene after the hydrothermal treatment [40]. 3.1.3. Morphologies of the S-TiO2-3DGA photocatalyst The FESEM image of the S-TiO2-3DGA catalyst is shown in Fig. 3A and B. The interconnected 3D network structure is observed on the graphene aerogels (GA, dehydrated graphene hydrogels) support, since GA synthesized through the feasible self-assembly hydrothermal method has been recognized as a novel class of 3D porous graphene architectures [41–43]. The 3D structure of graphene is beneficial for the transmitting of photo-generated electrons between the graphene layers. Meanwhile, the hierarchically porous structure of 3DGA can provide an ideal support for the deposition of the nanoparticles of S-TiO2 photocatalyst. In addition, it is found that the 3DGA support is almost completely covered by the S-TiO2 nanoparticles, and the anchoring of STiO2 particles on the 3DGA is expected since it is generally known that the monomeric titanyl ions (TiO2+) could be easily adsorbed on the negative surface of GO due to the electrostatic interactions [44]. During the following hydrothermal and freeze-drying treatment, GO is reduced to 3DGA and the S-TiO2 nanoparticles are then anchored to the surface of the 3DGA. The typical TEM image of the S-TiO2-3DGA photocatalyst reveals the existence of well-dispersed S-TiO2 nanoparticles with uniform size of 16–30 nm on the 3DGA support (Fig. 3C). The sample is further examined by lattice fringe analysis of high-resolution TEM (HRTEM). As can be seen from Fig. 3D, it shows a clear lattice fringe of 0.189 nm, matching well with the (200) plane of rutile phase TiO2. It is also observed that the S-TiO2 nanoparticles in the photocatalyst are distributed over the whole surface of the 3DGA support, which agrees well with the FESEM observations.
Fig. 2. XRD patterns of GO (a), un-doped TiO2 (b), S-TiO2 (c) and S-TiO2-3DGA (d).
the mechanical strength of 3DGA and S-TiO2-3DGA is tested, and it shows that the two samples can still maintain their macroscopic bulky shapes without any deformation when a 50 g counterweight is loaded (Fig. 1D and E).
3.1.2. XRD analysis The XRD patterns of GO, un-doped TiO2, S-TiO2 and S-TiO2-3DGA were recorded to investigate the crystalline phase transition during the synthesis of the photocatalyst. As shown in Fig. 2, the peaks located at 25.3°, 37.8°, 48.1°, 54.1°, 55.2°, 62.7°, 69.1°, 70.2° and 75.3° are attributed to the diffractions of the (101), (004), (200), (105), (211), (204), (116), (220) and (215) crystal planes, respectively, of anatase TiO2 (JCPDS Card No. 21-1272) [19]. As there are no diffraction peaks due to the rutile phase of TiO2 in the spectra of un-doped TiO2 and STiO2, it could be concluded that the doping of S does not change the crystal phase of TiO2 and the as-synthesized S-TiO2 is in a purely anatase structure [38]. No apparent peaks for graphene are observed in the spectrum of S-TiO2-3DGA, which might be ascribed to the fact that the main characteristic peak of graphene (about 25°) has a relatively low intensity and severely overlaps with the strong peak of anatase TiO2 at 25.3° [39]. In addition, the main characteristic peak of GO (about 9.8°)
3.1.4. XPS analysis of the S-TiO2-3DGA photocatalyst XPS spectra are recorded to investigate the exact chemical composition of the S-TiO2-3DGA photocatalyst. The full XPS spectra are shown in Fig. 4A, which clearly indicate the existence of O 1s, Ti 2p, C 1s and S 2p core levels. Specifically, the high-resolution O 1s spectra can be fitted into three peaks at 529.6, 530.4 and 531.6 eV (Fig. 4B), corresponding to TieOeTi TieOeS and SeOeS, respectively [45]. The deconvoluted Ti 2p spectra of the S-TiO2-3DGA display two peaks located Fig. 3. FESEM (A,B), TEM (C), and HRTEM (D) images of S-TiO2-3DGA.
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Fig. 4. Full XPS (A), high resolution O 1s (B), Ti 2p (C), C 1s (D), and S 2p (E) spectra of S-TiO2-3DGA.
suggesting the C element is not doped into the crystalline lattice. It is noteworthy that the presence of S is confirmed by the peak at 168.9 eV (Fig. 4A), and this peak can be further de-convoluted into two peaks at 168.4 and 169.7 eV (Fig. 4E), which can be assigned to S 2p3/2 and S 2p1/2, respectively. The S 2p3/2 peak at lower binding energy is about twice the intensity of the S 2p1/2 peak at higher binding energy, which agrees well with the previous report by Periyat et al. [50]. Generally, the peak at ∼168.9 eV is assigned to the S6+ cation [51,52], and thus the S element doped into the lattice of the S-TiO2-3DGA might exist in the form of S6+. The semiquantitative analysis of the as-prepared STiO2-3DGA by XPS is listed in Table 1. The S content in the S-TiO23DGA photocatalyst is determined to be 0.80 at% (about 1.84 at% in the S-TiO2), indicating the successful doping of S element to the TiO2
Table 1 Ti, O, S, and C contents of the as-synthesized S-TiO2-3DGA determined by XPS. Element
Ti
O
S
C
XPS (at%)
12.48
30.27
0.80
56.45
at 458.7 and 464.4 eV (Fig. 4C), which can be attributed to the binding energies of Ti 2p3/2 and Ti 2p1/2 (the typical values of TiO2), respectively [39,46,47]. The high-resolution C 1s spectra show only one configuration of C at 284.8 eV (Fig. 4D), which can be typically illustrated as CeC bonds [48]. However, the formation of CeTi bonds is not observed because of the missing of the peak at ∼282 eV [49], 54
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TiO2 and un-doped TiO2, a more intense absorption is observed for the S-TiO2-3DGA (432.7 nm, curve c), which might be attributed to the broad and featureless absorption of the 3DGA component in the same region [28]. The value of Eg can be obtained according to the equation Eg = 1240/λ, where λ is the wavelength of the absorption edge (nm) [53,54], and the Eg values of S-TiO2, un-doped TiO2 and S-TiO2-3DGA are calculated to be 2.93, 3.14 and 2.87 eV, respectively. Here, the bandgap narrowing on the S-TiO2 catalyst might be attributed to the introduction of S element to the lattice of TiO2, because the formation of doping states can reduce the electron transition energy from the valence band to the conduction band and thus result in a red-shift of the absorption edge [38]. The interactions between S-TiO2 and 3DGA might be achieved through the combination of the oxygen containing functional groups of the two components. As a result, the discreteness of the levels in the vicinity of the Fermi level is enhanced and the band gap is reduced.
Fig. 5. UV–vis diffuse reflectance absorption of the as-prepared S-TiO2 (a), un-doped TiO2 (b) and S-TiO2-3DGA (c).
3.3. Photocatalytic activity of the S-TiO2-3DGA photocatalyst To evaluate the photocatalytic performances of the S-TiO2-3DGA and other catalysts, the degradation of MO is performed under the irradiation of UV or visible light using 3DGA, un-doped TiO2, S-TiO2 and S-TiO2-3DGA as the photocatalyst. The results under UV irradiation are shown in Fig. 6A. After 1.5 h of UV irradiation, there is negligible degradation of MO when no catalyst is used. Around 53.3% and 83.9% of MO is degraded in 1.5 h in the presence of TiO2 or S-TiO2. It is exciting to find that the S-TiO2-3DGA catalyst exhibits considerably high photocatalytic activity, and the MO pollutants are almost completely eliminated within 1.5 h. Interestingly, the S-TiO2-3DGA catalyst still exhibits good photocatalytic performance under visible light irradiation. As shown in Fig. 6B, the un-doped TiO2 has nearly no activity, however, the photocatalytic is significantly improved when the S-TiO23DGA is used as the photocatalyst. After visible light irradiation for 6 h, around 92.8% of MO is eliminated with the S-TiO2-3DGA photocatalyst, which is much higher than that with the S-TiO2 catalyst (about 73.2%). The promoting effect of S-doping on the photocatalytic performance might be ascribed to the following factors. As shown in Fig. 7A, the redshift in the bandgap transition of the S-TiO2 can narrow the bandgap and consequently decrease the value of Eg, producing a second absorption edge in the visible region [55]. On the other hand, after being modified with the S dopant, the electron-deficient oxygen could serve as an electron trap to effectively inhibit the recombination between photo-generated electrons and holes [45]. The 3DGA in the photocatalyst also plays a crucial role in the enhancement of the photocatalytic activity. Under the irradiation of UV or visible light, the photogenerated electrons transfer from the S-TiO2 nanoparticles to the 3DGA, and effective separation of these electrons and holes could be easily achieved due to the unique 3D structure of 3DGA. The photo-generated electrons can further react with O2 to generate superoxide radical anions (O2%−) and hydroxyl radicals (OH%), which are responsible for the observed high catalytic performance of the S-TiO2-3DGA photocatalyst [24,56]. The significant effect of the 3DGA can be illustrated in Fig. 7B.
Fig. 6. Photocatalytic activity measurements of different photocatalysts under UV irradiation (A) and visible light irradiation (B).
nanoparticles [38]. On the other hand, the result also reveals that the atomic ratio of O to Ti is similar to the theoretical stoichiometric atomic ratio (2:1), suggesting the formation of TiO2.
3.4. Evaluation of the recyclability and stability of the S-TiO2-3DGA photocatalyst The recyclability and stability of the S-TiO2-3DGA under UV irradiation are of great importance for the practical applications of the photocatalyst in water purification. Noted that when the S-TiO2 is used as the photocatalyst for MO degradation, the separation of the S-TiO2 catalyst and water phase must be carried out by high speed centrifugation (10,000 rpm) for a rather long time. On the contrary, owing to the outstanding hydrophobic property of 3DGA, the S-TiO2-3DGA catalyst can be easily separated from the reaction systems via a simple filtration operation without appreciable loss of the solid photocatalyst, and thus excellent recyclability of the catalyst can be achieved. The
3.2. UV–vis DRS of S-TiO2 The UV–vis DRS are recorded to analyze the optical properties and the bandgap energies (Eg) of S-TiO2, un-doped TiO2 and S-TiO2-3DGA, and Fig. 5 shows the results from the diffuse reflectance measurements of the three samples. The UV–vis absorption band wavelength of S-TiO2 shifts to the visible light range and the optical band edge shows a remarkable red-shift (423.6 nm, curve a) compared with the un-doped TiO2 (394.9 nm, curve b), suggesting that a second absorption edge in the visible region exists for the prepared S-TiO2 [45]. Compared with S55
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Fig. 7. Schematic illustration showing the promoting effect of S-doping (A) and 3DGA (B). (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
the proposed photocatalyst are also investigated in this study, which might be ascribed to the narrowed bandgap of S-TiO2 and the good conductive ability and 3D structure of 3DGA. More importantly, the outstanding hydrophobic property of 3DGA endows the catalyst with excellent recyclability as well as high stability. We believe that the high-efficient photocatalyst could be a promising candidate in future wastewater treatment and have a prosperous commercial application prospect. Acknowledgements
Fig. 8. Repeated photocatalytic degradation of MO solutions (6 mg L under UV irradiation.
The authors are grateful to the financial supports from National Natural Science Foundation of China (41371446), Changzhou high-tech Key Laboratory (CM20153001), Natural Science Foundation of Jiangsu Province (BK20140264), Advanced Catalysis and Green Manufacturing Collaborative Innovation Center (ACGM2016-06-27) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
−1
) by S-TiO2-3DGA
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Fig. 9. TEM image of S-TiO2-3DGA after 20 cycles.
degradation rate of MO as catalyzed by the S-TiO2-3DGA is shown in Fig. 8. As can be seen, MO is degraded completely in the first 13 runs, and the removal efficiency can still maintain as high as 96.3% in the 20th run. The excellent recyclability of the S-TiO2-3DGA photocatalyst could be attributed to its highly stable structure. Fig. 9 shows the TEM image of the S-TiO2-3DGA after 20 cycles. As can be seen, it maintains a well-dispersed S-TiO2 distribution on the 3DGA support after 20 cycles, which is quite similar to that of the pristine S-TiO2-3DGA. This result indicates that the proposed S-TiO2-3DGA could be a high-efficient and highly stable photocatalyst for the degradation of MO dye.
4. Conclusions In summary, S-TiO2-3DGA nanocomposites are prepared via a facile hydrothermal method followed by freeze-drying treatment. Compared with 3DGA, un-doped TiO2 and S-TiO2, the as-synthesized S-TiO2-3DGA catalyst exhibits fairly high photocatalytic activity under the irradiation of UV and visible light. The photodegradation mechanisms of MO by 56
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