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TiO2 nanoparticles supported on PMMA nanofibers for photocatalytic degradation of methyl orange
Accepted Manuscript TiO2 nanoparticles supported on PMMA nanofibers for photocatalytic degradation of methyl orange Yang Li, Huijie Zhao, Mujie Yang PII: DOI: Reference:
S0021-9797(17)30993-1 http://dx.doi.org/10.1016/j.jcis.2017.08.076 YJCIS 22723
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
19 May 2017 14 August 2017 22 August 2017
Please cite this article as: Y. Li, H. Zhao, M. Yang, TiO2 nanoparticles supported on PMMA nanofibers for photocatalytic degradation of methyl orange, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/ 10.1016/j.jcis.2017.08.076
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TiO2 nanoparticles supported on PMMA nanofibers for photocatalytic degradation of methyl orange
Yang Li*, Huijie Zhao, Mujie Yang MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
E-mail: [email protected] (Yang Li); [email protected] (Huijie Zhao); [email protected] (Mujie Yang)
*Corresponding author. Address: Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 China E-mail: [email protected] (Yang Li). Tel.: +86-571-87952444 Fax: +86-571-87952444
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TiO2 nanoparticles supported on PMMA nanofibers for photocatalytic degradation of methyl orange
Yang Li*, Huijie Zhao, Mujie Yang MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
*Corresponding author. Address: Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 China E-mail: [email protected] (Yang Li). Tel.: +86-571-87952444 Fax: +86-571-87952444
2
Abstract In this paper, a photocatalyst composed of TiO 2 nanoparticles supported on the nanofibers of poly(methyl methacrylate) (PMMA) was successfully prepared by hydrothermally treating the electrospun PMMA nanofibers containing titanium n-butoxide precursor at 135oC for 8 h. As-prepared composite was characterized by field-emission scanning electron microscopy, X-ray diffraction pattern, thermal gravimetric analysis and Brunauer–Emmett–Teller (BET) surface area measurements. It is revealed that high content (42%) of tetragonal anatase TiO2 nanoparticles are uniformly loaded on the PMMA nanofibers to constitute the composite (TiO2 @PMMA) photocatalyst with BET surface area of 21.4 m2 g-1. The photocatalytic activity of TiO2@PMMA towards the degradation of methyl orange (MO), a model pollutant, has been investigated. It is observed that 0.1 g of the composite could degrade 100 mL of MO (10 mg/L) completely within 50 min under UV illumination, exhibiting a high catalytic activity. Moreover, the composite could be easily separated from the reaction system by filtration, and maintains high photocatalytic activity in five consecutive cycles of the degradation of MO, suggesting its potentials in recycling use. The work provides a new approach for the development of novel supported photocatalysts with high catalytic activity and good reusability. Keywords: TiO2 ; PMMA; nanocomposite; electrospinning; hydrothermal synthesis; photocatalyst; support; methyl orange; photocatalytic degradation.
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1. Introduction Photocatalytic oxidation technology is a popular method for the degradation of the contaminants in waste water, which exhibits the advantages of low costs, high efficiency and complete degradation of pollutants in a short time in comparison to the traditional technologies for the treatment of waste water [1-3]. In the process of photocatalytic
degradation
of
the
contaminants
in
waste
water,
some
semiconductors such as TiO2, ZnO, CdS, WO3, SnO2 and Fe2O3 could directly convert absorbed light energy into chemical energy, and make the difficult reactions proceed under mild conditions [4-10]. Consequently, their applications as the photocatalysts for the treatment of waste water received much attention worldwide. Currently, TiO2 represents one of the most commonly used semiconductor photocatalysts, which is mainly due to its prominent features of non-toxicity, high catalytic activity, strong oxidation ability and good stability [4, 11-14]. The TiO2 photocatalyst can produce photo-induced holes (·OH and other highly reactive oxygen species), which can degrade the toxic organic pollutants rapidly and even completely. Meanwhile, the photocatalyst of TiO2 can generate photo-induced electrons with strong reducibility for the reduction of toxic heavy metals and refractory organic halogenated pollutants [15-16]. However, the TiO2 photocatalyst powders are easy to agglomerate and lose activity. Moreover, their recycling for repeated use becomes difficult. These shortages seriously limit the wide applications of TiO2 in the photocatalysis process [17-18]. A straightforward method to solve the problem is to prepare TiO2 film on different substrates. 4
Nonetheless, the formation of dense TiO2 film leads to great decrease in the specific surface area, which results in weakened photocatalytic activity [19]. Recently, it was reported that the fixation of nanostructured TiO 2 on suitable carriers could be a good solution to the problem [20-22]. Shen et al. synthesized a three-dimensional carbon felt (CF) supported TiO2 monoliths for photocatalytic degradation of methyl orange (MO). As-prepared TiO2 @CF monoliths exhibited large surface area and much better wettability to promote the degradation of MO [23]. Guesha et al. prepared the zeolite Y supported TiO2 by selectively modifying the surface cation of zeolite Y, and the composite of TiO2 with treated zeolite could degrade 92% of MO (10 mg/L) within 90 min [24]. The supported TiO2 is easy to be collected for repeated use. Moreover, it maintains high specific surface area, and thus achieves good photocatalytic activity. Therefore, fabrication of supported TiO2 has become a hot topic for the development of advanced TiO2 photocatalyst. Electrospinning (ES) is a newly developed method for the preparation of nanostructured
inorganic
semiconductors,
including
TiO2.
Typically,
the
electrospun polymer nanofibers containing the corresponding salt precursors could be easily converted to nanostructured semiconductors by the calcinations [25-26]. Another well-known method for the preparation of nanostructured semiconductors is the hydrothermal synthesis. Under the environment of high temperature and high pressure during the hydrothermal treatment, the salt precursors are readily evolved into semiconductor nanomaterials [27].
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In this work, we combined the two methods to prepare the composite of TiO 2 nanoparticles supported on poly(methyl methacrylate) (PMMA) nanofibers (TiO2 @PMMA). Specifically, PMMA nanofibers containing the precursor of titanium n-butoxide (TBT) was obtained by ES. In contrast to the traditional method of calcinating the nanofibers for the removal of the polymer matrix and the conversion of titanium salt precursor to TiO2, the nanofibers underwent hydrothermal treatment for the evolution of TBT to TiO2 nanoparticles supported on the PMMA nanofibers. In this way, the fixation of TiO 2 nanoparticles on PMMA nanofibers was facilely realized under mild conditions. Such TiO2/PMMA nanocomposite nanofibers (TiO2@PMMA) could exhibit large specific surface area, which is beneficial for the adsorption of the contaminants in waste water. Moreover, the uniformly distributed TiO2 nanoparticles provide a large number of reaction sites for the degradation of the contaminants. Herein, MO is used as a pollutant model, and the photocatalytic degradation of MO with as-prepared TiO2 @PMMA under UV illumination has been examined. The TiO2 @PMMA catalyst is featured with good photocatalytic activity, which could be well maintained during cyclic usage, and ease of collection for repeated use. 2. Experimental 2.1 Materials Poly (methyl methacrylate) (PMMA) (MW: 350 000) was purchased from Alfa Aesar. Titanium (IV) n-butoxide (TBT) was obtained from J&K Chemical
6
Technology. Ethanol (EtOH), glacial acetic acid (HAc), dichloromethane (DCM) and methyl orange（MO）were all supplied by Sinopharm Chemical Regent Co., Ltd. All the chemicals were of analytical grade and used without further purification. 2.2 Preparation of TiO2@PMMA TiO2@PMMA was fabricated by modifying the literature method of ES combined with hydrothermal synthesis [28]. In a typical procedure, 0.32 g of PMMA was dissolved in a mixture of ethanol (1 mL) and dichloromethane (4 mL) by vigorous stirring to get the solution A. Meanwhile, 1 mL of TBT was blended with the mixture of ethanol (1 mL) and HAc (1 mL) to obtain the solution B. The mixture of A and B was then used as the ES solution, which was loaded into a plastic syringe with a pinhead with an internal diameter of 0.9 mm. The pinhead was connected to a high voltage supply (DW-P303-1ACF0, Tianjin Dongwen High Voltage Power Supply Plant). The operating voltage applied for ES was ~9 kV, and the flow rate of the ES solution was controlled at 1 mL/h by a syringe pump (WZ-50C6, Smith Medical Instrument (Zhejiang) Co., Ltd.). A grounded aluminum foil was situated 15 cm from the tip of the pinhead. The electrospun nanofibers were collected onto the Al foil for 3-7 h to form a nanofiber mat. Afterwards, the nanofiber mat was peeled from the aluminum foil and transferred into a Teflon-lined stainless-steel autoclave with moderate amount of deionized water, and maintained at 135oC for 8 h. The autoclave naturally cooled down to room temperature, and the nanofibers were taken out and dried in vacuum oven for 10 h to obtain TiO2 @PMMA.
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2.3 Measurements The morphologies of the electrospun PMMA nanofibers and TiO2@PMMA before and after hydrothermal treatment were investigated using an FEI scanning electron microscope (FE-SEM, s-4800, Hitachi, accelerating voltage of 3 kV). X-ray diffraction (XRD) patterns were recorded on a PAN analytical X'Pert PRO using CuKα (λ = 0.15418 nm) radiation with a 2θ scanning range of 20–80°. Thermal gravimetric analysis (TGA) was carried out in air with a DSCQ200 (TA Instruments) to
examine
the
actual
weight
percentage
of
TiO2 in
TiO2 @PMMA.
Brunauer-Emmett-Teller (BET) surface area of TiO2@PMMA was measured with an ASIC-2 gas adsorption analyzer accelerated surface area porosimetry system (AUTOSORB-IQ2-MP, nitrogen as the absorbate, operation temperature: -196oC). 2.4 Investigations on the photocatalytic activity The photocatalytic activity of as-prepared TiO2@PMMA was evaluated by the degradation of a pollutant model of MO using a home-made set-up (Fig. 1). The apparatus consists of a quartz beaker (150 mL) equipped with a 32 W high-pressure mercury lamp (λmax = 254 nm, Shanghai Jiping Special Lighting Co., Ltd., China) as the light source and a magnetic stirrer (stirring speed = 300 r/min). The details of the experimental procedures are shown as follows: A certain amount of TiO2@PMMA was added into in 100 mL solution of MO with different concentrations at 30oC. Before illumination, the mixture was magnetically stirred in the dark for 60 min to reach the adsorption-desorption 8
equilibrium between TiO2 @PMMA and MO solution. During the photocatalytic reaction, 5 mL of suspension was taken out of the mixture at a time interval of 5 min and
separated
from
the
catalyst
nanofibers
on
a
centrifuge
(TG-16W
Changsha Xiangzhi Centrifuge Instrument Co., Ltd.) at 10000 rpm for 5 min. The concentration of the remaining MO was determined by analyzing the absorbance of the supernatant at λmax= 464 nm (the calibration curve is illustrated in Fig. S1 in the supporting information) with a UV-Vis spectrophotometer (UV-1800 Shimadzu Instrument Co., Ltd). For the reusability test, the supernatant was carefully decanted after the completion of the degradation reaction, and fresh MO solution (10 mg/mL, 100 mL) was added to the mixture. Subsequently, the photocatalytic reaction proceeded at 30oC with constant magnetic stirring under UV illumination for 60 min. The operation was repeated for five times. Fig. 1 3. Results and discussion 3.1. Preparation and characterization of the photocatalyst of TiO2@PMMA. In this work, the TiO2 nanoparticles supported on PMMA nanofibers were prepared by a new method of hydrothermally treating the nanofibers of PMMA containing TBT. The method avoids the calcination at high temperature, and thus the polymer nanofibers are not destroyed. Moreover, the hydrothermal treatment leads to the conversion of TBT to TiO2 nanoparticles loaded on the PMMA nanofibers under mild conditions. Apparently, the work put forward a simple and effective approach for 9
the construction of nanostructured inorganic semiconductors supported on the polymer matrix. Fig. 2 presents the SEM micrographs of electrospun nanofibers of PMMA and PMMA containing TBT before and after the hydrothermal treatment. It is seen that non-uniform and crooked PMMA nanofibers are obtained during the ES process (Fig. 2a-b). Moreover, the nanofibers seem to be fused to form a porous stuff with rugged surface after the hydrothermal treatment (Fig. 2c-d). By contrast, the electrospun nanofibers of PMMA containing TBT reveal smooth surface and larger and more uniform diameter when compared with the PMMA nanofibers (Fig. 2e-f). Furthermore, the outline of the nanofibers could be roughly maintained after the hydrothermal treatment. However, the fibers present rough surface, which is composed of small TiO2 nanoparticles as seen in the SEM picture of higher magnification (Fig. 2g-h). Obviously, the TBT salt precursor plays an important role in retaining the nanostructure of the electrospun PMMA nanofibers and forming TiO2 nanoparticles under hydrothermal treatment. As-prepared TiO2@PMMA is composed of nanoparticles uniformly loaded on the PMMA nanofibers, and possesses high specific surface area. The special morphology of the nanocomposite not only enhances the adsorption of the pollutant, but also provides more reaction sites for the degradation of the pollutants under UV radiation. Fig. 2 Fig. 3 displays the N2 adsorption-desorption curve of TiO2 @PMMA, which 10
exhibits a Type II isotherm. The BET surface area of the TiO2@PMMA is calculated to be 21.4 m2 g-1. The high surface area is in agreement with the SEM observation, and is expected to promote the photocatalytic activity of the composite, as will be discussed below. Fig. 4 depicts the XRD patterns of electrospun PMMA containing TBT both before and after the hydrothermal treatment. It is observed that only a broad diffraction peak at 2θ = 29.3° is observed in the spectrum of the electrospun nanofibers (Fig. 4a), which can be ascribed to PMMA [29]. In contrast, diffraction peaks at 2θ = 26.4°, 38.3°, 48.7°, 63.4°, 76.5° and 83° are clearly identified in the patterns of the hydrothermally treated nanofibers (Fig. 4b), corresponding to (101), (112), (200), (204), (215) and (224) crystal planes of tetragonal anatase TiO2 (JCPDS 65-5714). It is expected that the nanocomposite with high crystallinity of anatase could exhibit higher photocatalytic activity, as reported in literatures [3, 30]. Fig. 3 Fig. 4 The content of inorganic TiO2 nanoparticles in the TiO2 @PMMA has been identified from the TGA curve of the nanocomposite as shown in Fig. 5. The thermal analysis was carried out with a heating rate of 5oC/min from room temperature to 1000oC under air atmosphere. The slight weight loss of the nanocomposite before 230oC is attributed to the release of the physically absorbed water [31]. According to the literature, the decomposition of PMMA starts at about 160oC. By contrast, TiO2@PMMA does not reveal obvious decomposition till about 230 oC. It is proposed 11
that the presence of TiO2 restricts the thermal motion of PMMA molecular chains and leads to improved thermal stability of the composite [32].The TGA curve of TiO2@PMMA shows a platform when the temperature is increased to 500oC, and the remainder could be reasonably ascribed to TiO2 nanoparticles. Therefore, the content of TiO2 nanoparticles in TiO2@PMMA is found to be as high as ~42%, which is comparable with other TiO2 supported composite photocatalysts [33].
The high
percentage of the loaded photoactive TiO2 nanoparticles on the polymer matrix might enhance the photocatalytic activity of the resulting composite, which is another advantage of the fabrication method. Fig. 5 3.2. Photocatalytic activity of TiO2@PMMA The photocatalytic activity of TiO2 @PMMA has been investigated by monitoring the decrease in the concentration of MO (10 mg/L, 100 mL) in the presence of the composite photocatalyst (0.1 g) under UV illumination at 30 oC, and the results are displayed in Fig. 6. Specifically, the process consists of two parts: dark adsorption and photodegradation. As clearly seen in Fig. S2 (please see the supporting information), in the dark adsorption process (100 min), the concentration of MO in the solution first decreases sharply, then reaches a stable value (67.1% of the initial concentration at 60 min). By contrast, the electrospun PMMA nanofibers were fused to form a piece of rugged and porous stuff after the hydrothermal treatment as shown in Fig. 2c-d. 12
Apparently, the specific surface area of the treated nanofibers has been seriously decreased, resulting in little adsorption of MO (the concentration of MO in solution maintains at about 99.0% of the initial value after dark adsorption for 60 min) (Fig. 6). It is known that the adsorption of pollutants onto TiO2 is an important step prior to the photocatalytic degradation. Consequently, the relatively high adsorption of MO onto the catalyst, which could be ascribed to the high specific surface area of the nanostructured composite photocatalyst, should be beneficial for the photodegradation of MO. Fig. 6 In the subsequent photodegradation process, the concentration of MO decreases to 9.1% of the initial value within a short period of 50 min, indicating the high photocatalytic activity of the TiO2 @PMMA catalyst. It is proposed that the photodegradation of pollutants by TiO2 depends on the generation of conduction-band electrons (e-) and valence-band holes (h+) when irradiated with UV-light (Eq. (1)). The highly oxidative photo-induced holes (E- = 2.8 V) can directly oxidize organic pollutants or react with adsorbed water and hydroxyl anions to form hydroxyl radicals, which then attack dye pollutants (Eq. (2-3)). Meanwhile, the e- can also react with proper electron acceptors, such as O2, to yield oxidative radicals as described by Eq. (4). In this work, the TiO2 nanoparticles are uniformly dispersed onto the PMMA nanofibers in the as-prepared composite catalyst, thus conquering the problem of agglomeration of nanoparticles and leading to high specific surface area and increased reaction sites. Therefore, both the adsorption and photochemical reactions are 13
accelerated in the composite catalyst, which is responsible for its high photocatalytic activity in the degradation of MO. TiO2 + hν e-+h+ (TiO2)
(1)
h+ + OH-ads HO.
(2)
h+ + H2Oads HO.+ H+
(3)
e- + O2
O2.-
(4)
The effect of the concentration of MO (5, 10, 20 mg/L) on the photocatalytic activity of TiO2@PMMA has been demonstrated in Fig S2 (please see the supporting information) and Fig. 7. It is seen that both dark adsorption and photodegradation processes are affected by the concentration of MO. The decrease efficiency (η) of MO by TiO2@PMMA is expressed by Eq. 5. η=（C0-C）/C0*100%
(5)
Where C0 represents the initial concentration of MO, and C is the concentration of MO during the adsorption or photo-degradation process [34]. After the 60 min dark adsorption, η is 45.8%, 32.9% and 22.7% for MO solutions of 5 mg/L, 10 mg/L and 20 mg/L, respectively. In the following photodegradation process, the kinetics of the photocatalytic degradation is described using the pseudo-first-order model (Eq.6), and the results are displayed in Fig.7b. dC/dt= -ka C, or ln(C/C0) = -ka t
(6)
where C0 is the initial concentration of MO; C is the concentration of MO at time 14
t; and ka (min-1) is the apparent pseudo-first-order decay rate constant, which can be obtained by plotting ln(C/C0) vs. t as shown in Fig. 7b [35]. It can be seen that MO degradation by the photocatalyst under UV illumination proceeds quickly. Even when the concentration of MO is as high as 20 mg/L, the degradation is almost complete within 90 min, indicating the prominent advantage of high photocatalytic activity of TiO2@PMMA. Furthermore, it is found that higher concentration of MO leads to lower photodegradation reaction rate (smaller ka). Specifically, the degradation of MO of 5 mg/L and 10 mg/L is almost complete within 30 min and 50 min, respectively. Fig. 7 The effect of the dosage of TiO2 @PMMA on the photocatalytic degradation of MO is demonstrated in Fig. 8. After dark adsorption of 60 min (the dark adsorption of the catalyst with different dosages is displayed in Fig. S3 in the supporting information), η resulting from adsorption is 7.9%, 24.1%, 32.9% and 56.4% for catalyst dosage of 0.025 g, 0.05 g, 0.1 g and 0.2 g, respectively. By contrast, under UV illumination, the photocatalytic degradation of MO is nearly completed within 90 min, except for the case of 0.025 g of the photocatalyst. As shown above, the content of TiO2 is ~42% in the composite catalyst. Thus it can be concluded that, with only very small amount of the active component of TiO2 (0.021 g), the photodegradation of MO (10 mg/L, 100 mL) could be almost finished within 80 min using TiO 2 @PMMA, suggesting its high photocatalytic activity. Fig. 8
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As is shown above, our method of in-situ conversion of titanium salt to TiO2 nanoparticles loaded on surface of PMMA nanofibers not only successfully prevents the agglomeration of nanoparticles, but also maintains high specific surface area of the resulting composite. Therefore, MO is efficiently adsorbed by the photocatalyst, and fully reacts with active TiO2 nanoparticles on the surface of the PMMA matrix under UV illumination to result in its high photocatalytic activity. Table 1 presents the results of the photocatalytic degradation of MO with TiO2 based photocatalysts reported recently and our TiO2@PMMA [36-38]. It is seen clearly that as-prepared TiO2 @PMMA could effectively degrade MO (100 mL, 10 mg/mL) within 50 min ( of 90.5%) with 100 mg of the composite (TiO2 ~ 42 mg), suggesting that its photocatalytic efficiency is more or less of the same level of or even higher than the literature work. It is proposed that the good photocatalytic activity of our catalyst is related to (1) the special micro/nanostructures established in the composite by the three-dimensionally packed PMMA nanofibers and the TiO2 nanoparticles decorated on the fibers, which facilitates the adsorption of the pollutants and allows their easy access to the photoactive TiO2; and (2) high content of TiO2 nanoparticles in the composite photocatalyst, which provides plenty of reaction sites for the degradation of adsorbed pollutant molecules. Table 1 To investigate the recycling ability of the composite, we measured its catalytic activity during five cycles of photodegradation of MO. At the end of each
16
photodegradation cycle, the composite was allowed to precipitate at the bottom of the beaker, and the colorless supernatant was decanted from the beaker before fresh MO (10 mg/L, 100 mL) was added for a new cycle lasting for 60 min. As depicted in Fig. 9, the composite did not show any significant loss of activity during the five consecutive photodegradation cycles. Specifically, the catalyst reveals of 94.4% at the end of the 5th photodegradation cycle, which proves that TiO2@PMMA could well maintain its good photocatalytic activity for repeated use. Moreover, Fig. S4 (please see the supporting information) clearly shows that TiO2 @PMMA prepared in this work could be effectively dispersed in MO solution by magnetic stirring, which could promote the adsorption of the pollutants and their contact with the photocatalysts, and thus enhances the photocatalytic activity. In addition, the composite could completely precipitate to the bottom within 1 min when the stirring is terminated, suggesting easy separation from the pollutant solution for recycling use, which is a highly desirable advantage for practical applications. The above experimental results indicate that our composite of TiO2 nanoparticles supported on the PMMA nanofibers could effectively conquer the problems encountered by nano-sized TiO2 photocatalyst, and shows great potentials for applications in future. Fig. 9 The mechanism of photocatalytic degradation of MO by the composite was briefly illustrated in Fig. 10. The special morphology characteristics of the composite
17
of TiO2 nanoparticles supported on PMMA nanofibers leads to high specific surface area, thus greatly facilitates the adsorption of MO molecules in the solution onto the composite during the dark adsorption process. In the following photocatalytic degradation process, under UV illumination, a high concentration of conduction-band electrons (e-) and valence-band holes (h+) is generated in high density of TiO2 nanoparticles uniformly dispersed on PMMA nanofibers. The highly oxidative photo-induced holes and e- can directly or indirectly react with adsorbed MO molecules for their full degradation (In accordance with Eqs. 1-4). Moreover, the investigations on the photocatalytic degradation products of methyl orange and similar dyes demonstrate that the degradation intermediates are not more toxic than MO [39-41]. Consequently, the photodegradation of MO by TiO2 catalyst could be a safe and efficient approach for the treatment of MO. Fig. 10 4. Conclusions The composite of TiO2 nanoparticles supported on the PMMA nanofibers has been successfully prepared by hydrothermally treating the electrospun PMMA nanofibers containing the titanium salt precursor. The simple method avoids the calcinations of polymer nanofiber template, which is usually required for the preparation of inorganic nanomaterials via electrospinning, and shows environmental friendliness. The composite photocatalyst is featured with high specific surface area, and relatively high content of TiO2, which is beneficial for the enhancement of its 18
photocatalytic activity. Under UV illumination, the composite could completely degrade the aqueous solution of methyl orange in a short time, exhibiting high photocatalytic activity. Moreover, the composite could be easily separated from the reaction system by decantation or filtration for recycling, and maintains high catalytic activity during the recycle use in the degradation of MO. Therefore, as-prepared TiO2@PMMA conquers the shortcoming of easy agglomeration of nano-sized TiO2 catalyst, and proves its potential applications as an efficient and recyclable photocatalyst for the degradation of pollutants. The work provides a new approach for the development of advanced photocatalysts based on nanostructured inorganic semiconductors.
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177(26): 2679-2682. [30] Haque F Z, Nandanwar R, Singh P. Evaluating photodegradation properties of anatase and rutile TiO2 nanoparticles for organic compounds. Optik-International Journal for Light and Electron Optics, 2017, 128: 191-200. [31] Zou H, Song M, Yi F, et al. Simulated-sunlight-activated photocatalysis of Methyl Orange using carbon and lanthanum co-doped Bi2O3-TiO2 composite. Journal of Alloys and Compounds, 2016, 680: 54-59. [32] Banerjee M, Sain S, Mukhopadhyay A, et al. Surface treatment of cellulose fibers with methylmethacrylate for
enhanced properties of in situ polymerized
PMMA/cellulose composites. Journal of Applied Polymer Science, 2014, 131:39808. [33] Wickramaratne N P, Jaroniec M. Ordered mesoporous carbon–titania composites and their enhanced photocatalytic properties. Journal of Colloid and Interface Science, 2015, 449: 297-303. [34] Koohestani H, Sadrnezhaad S K. Photocatalytic degradation of methyl orange and cyanide by using TiO2/CuO composite. Desalination and Water Treatment, 2016, 57(46): 22029-22038. [35] Masoud M, Nourbakhsh A, Hassanzadeh-Tabrizi S A. Influence of modified CNT-Ag nanocomposite addition on photocatalytic degradation of methyl orange by mesoporous TiO2. Inorganic and Nano-Metal Chemistry, 2017, 47(8): 1168-1174. [36] Guo N, Liang Y, Lan S, et al. Microscale hierarchical three-dimensional flowerlike TiO2/PANI composite: synthesis, characterization, and its remarkable photocatalytic activity on organic dyes under UV-Light and sunlight Irradiation. J. 24
Phys. Chem. C, 2014, 118, 18343-18355. [37] Mortazavi-Derazkola S, Salavati-Niasari M, Mazhari M-P, et al. Magnetically separable Fe3O4@SiO2 @TiO2 nanostructures supported by neodymium (III): fabrication and enhanced photocatalytic activity for degradation of organic pollution. J. Mater. Sci.: Mater. Electron., 2017: 1-11. [38] Jafari H,
Afshar S,
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TiO2–SiO2 nanohybrids immobilized on cement-based materials for dye degradation. Res. Chem. Intermed, 2016, 42: 2963-2978. [39] Baiocchia C, Brussinoa M C, Pramauroa E, Prevot A B, et al. Characterization of methyl orange and its photocatalytic degradation products by HPLC/UV–VIS diode array and atmospheric pressure ionization quadrupole ion trap mass spectrometry. International Journal of Mass Spectrometry, 2002, 214:247-256. [40] He Y H, Grieser F, Ashokkumar M. The mechanism of sonophotocatalytic degradation of methyl orange and its products in aqueous solutions. Ultrasonics Sonochemistry, 2011, 18: 974-980. [41] Lachheb H, Puzenat E, Houas A, et al. Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania. Applied Catalysis B: Environmental, 2002, 39:75-90.
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Figure captions Fig. 1 Schematic diagram of the experimental set-up for the photocatalytic reaction. Fig. 2 SEM micrographs of electrospun nanofibers of PMMA (a - b) before and (c - d) after the hydrothermal treatment, and PMMA containing TBT precursor (e - f) before and (g - h) after the hydrothermal treatment. Fig. 3 Isothermal adsorption/desorption curve of TiO2@PMMA (inset: calculation of BET surface area). Fig. 4 XRD patterns of (a) the electrospun nanofibers and (b) TiO2@PMMA. Fig. 5 TGA plot of TiO2@PMMA. Fig. 6 The photocatalytic properties of PMMA and TiO2 @PMMA (0.1 g) in degradation of MO (10 mg/L) under UV illumination. Fig. 7 (a) Effect of initial concentration of MO on its photodegradation and (b) kinetics of photodegradation of MO (catalyst = 0.1 g). Fig. 8 (a) Effect of the amount of catalyst on the degradation of MO and (b) kinetics of MO degradation ([MO] = 10 mg/L). Fig. 9 Photodegradation of MO by TiO2 @PMMA during five consecutive cycles ([MO] = 10 mg/L). Fig. 10 Proposed mechanism of photocatalytic degradation of MO by TiO2 @PMMA under UV illumination. 26
Fig-1 single column Click here to download high resolution image
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Table 1 Photodegradation of MO by TiO2 based catalysts Dosage of Catalyst
Degradation Dosage of MO
Catalyst / mg TiO2@PMMA
time / (min)
100
100 mL, 10 mg/mL
50
100
50 mL, 15 mg/mL
90
50
20 mL, 80 mg/mL
140
30
50 mL, 6.5 mg/mL
90
20
50 mL, 10 mg/mL
200
(This work) CNT-Ag/ TiO2 [35] TiO2/PANI
[36]
Fe3O4@SiO2 @ TiO2 @Nd [37] TiO2/ SiO2 [38]
32
Graphical Abstract
33
S0021-9797(17)30993-1 http://dx.doi.org/10.1016/j.jcis.2017.08.076 YJCIS 22723
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
19 May 2017 14 August 2017 22 August 2017
Please cite this article as: Y. Li, H. Zhao, M. Yang, TiO2 nanoparticles supported on PMMA nanofibers for photocatalytic degradation of methyl orange, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/ 10.1016/j.jcis.2017.08.076
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TiO2 nanoparticles supported on PMMA nanofibers for photocatalytic degradation of methyl orange
Yang Li*, Huijie Zhao, Mujie Yang MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
E-mail: [email protected] (Yang Li); [email protected] (Huijie Zhao); [email protected] (Mujie Yang)
*Corresponding author. Address: Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 China E-mail: [email protected] (Yang Li). Tel.: +86-571-87952444 Fax: +86-571-87952444
1
TiO2 nanoparticles supported on PMMA nanofibers for photocatalytic degradation of methyl orange
Yang Li*, Huijie Zhao, Mujie Yang MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
*Corresponding author. Address: Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 China E-mail: [email protected] (Yang Li). Tel.: +86-571-87952444 Fax: +86-571-87952444
2
Abstract In this paper, a photocatalyst composed of TiO 2 nanoparticles supported on the nanofibers of poly(methyl methacrylate) (PMMA) was successfully prepared by hydrothermally treating the electrospun PMMA nanofibers containing titanium n-butoxide precursor at 135oC for 8 h. As-prepared composite was characterized by field-emission scanning electron microscopy, X-ray diffraction pattern, thermal gravimetric analysis and Brunauer–Emmett–Teller (BET) surface area measurements. It is revealed that high content (42%) of tetragonal anatase TiO2 nanoparticles are uniformly loaded on the PMMA nanofibers to constitute the composite (TiO2 @PMMA) photocatalyst with BET surface area of 21.4 m2 g-1. The photocatalytic activity of TiO2@PMMA towards the degradation of methyl orange (MO), a model pollutant, has been investigated. It is observed that 0.1 g of the composite could degrade 100 mL of MO (10 mg/L) completely within 50 min under UV illumination, exhibiting a high catalytic activity. Moreover, the composite could be easily separated from the reaction system by filtration, and maintains high photocatalytic activity in five consecutive cycles of the degradation of MO, suggesting its potentials in recycling use. The work provides a new approach for the development of novel supported photocatalysts with high catalytic activity and good reusability. Keywords: TiO2 ; PMMA; nanocomposite; electrospinning; hydrothermal synthesis; photocatalyst; support; methyl orange; photocatalytic degradation.
3
1. Introduction Photocatalytic oxidation technology is a popular method for the degradation of the contaminants in waste water, which exhibits the advantages of low costs, high efficiency and complete degradation of pollutants in a short time in comparison to the traditional technologies for the treatment of waste water [1-3]. In the process of photocatalytic
degradation
of
the
contaminants
in
waste
water,
some
semiconductors such as TiO2, ZnO, CdS, WO3, SnO2 and Fe2O3 could directly convert absorbed light energy into chemical energy, and make the difficult reactions proceed under mild conditions [4-10]. Consequently, their applications as the photocatalysts for the treatment of waste water received much attention worldwide. Currently, TiO2 represents one of the most commonly used semiconductor photocatalysts, which is mainly due to its prominent features of non-toxicity, high catalytic activity, strong oxidation ability and good stability [4, 11-14]. The TiO2 photocatalyst can produce photo-induced holes (·OH and other highly reactive oxygen species), which can degrade the toxic organic pollutants rapidly and even completely. Meanwhile, the photocatalyst of TiO2 can generate photo-induced electrons with strong reducibility for the reduction of toxic heavy metals and refractory organic halogenated pollutants [15-16]. However, the TiO2 photocatalyst powders are easy to agglomerate and lose activity. Moreover, their recycling for repeated use becomes difficult. These shortages seriously limit the wide applications of TiO2 in the photocatalysis process [17-18]. A straightforward method to solve the problem is to prepare TiO2 film on different substrates. 4
Nonetheless, the formation of dense TiO2 film leads to great decrease in the specific surface area, which results in weakened photocatalytic activity [19]. Recently, it was reported that the fixation of nanostructured TiO 2 on suitable carriers could be a good solution to the problem [20-22]. Shen et al. synthesized a three-dimensional carbon felt (CF) supported TiO2 monoliths for photocatalytic degradation of methyl orange (MO). As-prepared TiO2 @CF monoliths exhibited large surface area and much better wettability to promote the degradation of MO [23]. Guesha et al. prepared the zeolite Y supported TiO2 by selectively modifying the surface cation of zeolite Y, and the composite of TiO2 with treated zeolite could degrade 92% of MO (10 mg/L) within 90 min [24]. The supported TiO2 is easy to be collected for repeated use. Moreover, it maintains high specific surface area, and thus achieves good photocatalytic activity. Therefore, fabrication of supported TiO2 has become a hot topic for the development of advanced TiO2 photocatalyst. Electrospinning (ES) is a newly developed method for the preparation of nanostructured
inorganic
semiconductors,
including
TiO2.
Typically,
the
electrospun polymer nanofibers containing the corresponding salt precursors could be easily converted to nanostructured semiconductors by the calcinations [25-26]. Another well-known method for the preparation of nanostructured semiconductors is the hydrothermal synthesis. Under the environment of high temperature and high pressure during the hydrothermal treatment, the salt precursors are readily evolved into semiconductor nanomaterials [27].
5
In this work, we combined the two methods to prepare the composite of TiO 2 nanoparticles supported on poly(methyl methacrylate) (PMMA) nanofibers (TiO2 @PMMA). Specifically, PMMA nanofibers containing the precursor of titanium n-butoxide (TBT) was obtained by ES. In contrast to the traditional method of calcinating the nanofibers for the removal of the polymer matrix and the conversion of titanium salt precursor to TiO2, the nanofibers underwent hydrothermal treatment for the evolution of TBT to TiO2 nanoparticles supported on the PMMA nanofibers. In this way, the fixation of TiO 2 nanoparticles on PMMA nanofibers was facilely realized under mild conditions. Such TiO2/PMMA nanocomposite nanofibers (TiO2@PMMA) could exhibit large specific surface area, which is beneficial for the adsorption of the contaminants in waste water. Moreover, the uniformly distributed TiO2 nanoparticles provide a large number of reaction sites for the degradation of the contaminants. Herein, MO is used as a pollutant model, and the photocatalytic degradation of MO with as-prepared TiO2 @PMMA under UV illumination has been examined. The TiO2 @PMMA catalyst is featured with good photocatalytic activity, which could be well maintained during cyclic usage, and ease of collection for repeated use. 2. Experimental 2.1 Materials Poly (methyl methacrylate) (PMMA) (MW: 350 000) was purchased from Alfa Aesar. Titanium (IV) n-butoxide (TBT) was obtained from J&K Chemical
6
Technology. Ethanol (EtOH), glacial acetic acid (HAc), dichloromethane (DCM) and methyl orange（MO）were all supplied by Sinopharm Chemical Regent Co., Ltd. All the chemicals were of analytical grade and used without further purification. 2.2 Preparation of TiO2@PMMA TiO2@PMMA was fabricated by modifying the literature method of ES combined with hydrothermal synthesis [28]. In a typical procedure, 0.32 g of PMMA was dissolved in a mixture of ethanol (1 mL) and dichloromethane (4 mL) by vigorous stirring to get the solution A. Meanwhile, 1 mL of TBT was blended with the mixture of ethanol (1 mL) and HAc (1 mL) to obtain the solution B. The mixture of A and B was then used as the ES solution, which was loaded into a plastic syringe with a pinhead with an internal diameter of 0.9 mm. The pinhead was connected to a high voltage supply (DW-P303-1ACF0, Tianjin Dongwen High Voltage Power Supply Plant). The operating voltage applied for ES was ~9 kV, and the flow rate of the ES solution was controlled at 1 mL/h by a syringe pump (WZ-50C6, Smith Medical Instrument (Zhejiang) Co., Ltd.). A grounded aluminum foil was situated 15 cm from the tip of the pinhead. The electrospun nanofibers were collected onto the Al foil for 3-7 h to form a nanofiber mat. Afterwards, the nanofiber mat was peeled from the aluminum foil and transferred into a Teflon-lined stainless-steel autoclave with moderate amount of deionized water, and maintained at 135oC for 8 h. The autoclave naturally cooled down to room temperature, and the nanofibers were taken out and dried in vacuum oven for 10 h to obtain TiO2 @PMMA.
7
2.3 Measurements The morphologies of the electrospun PMMA nanofibers and TiO2@PMMA before and after hydrothermal treatment were investigated using an FEI scanning electron microscope (FE-SEM, s-4800, Hitachi, accelerating voltage of 3 kV). X-ray diffraction (XRD) patterns were recorded on a PAN analytical X'Pert PRO using CuKα (λ = 0.15418 nm) radiation with a 2θ scanning range of 20–80°. Thermal gravimetric analysis (TGA) was carried out in air with a DSCQ200 (TA Instruments) to
examine
the
actual
weight
percentage
of
TiO2 in
TiO2 @PMMA.
Brunauer-Emmett-Teller (BET) surface area of TiO2@PMMA was measured with an ASIC-2 gas adsorption analyzer accelerated surface area porosimetry system (AUTOSORB-IQ2-MP, nitrogen as the absorbate, operation temperature: -196oC). 2.4 Investigations on the photocatalytic activity The photocatalytic activity of as-prepared TiO2@PMMA was evaluated by the degradation of a pollutant model of MO using a home-made set-up (Fig. 1). The apparatus consists of a quartz beaker (150 mL) equipped with a 32 W high-pressure mercury lamp (λmax = 254 nm, Shanghai Jiping Special Lighting Co., Ltd., China) as the light source and a magnetic stirrer (stirring speed = 300 r/min). The details of the experimental procedures are shown as follows: A certain amount of TiO2@PMMA was added into in 100 mL solution of MO with different concentrations at 30oC. Before illumination, the mixture was magnetically stirred in the dark for 60 min to reach the adsorption-desorption 8
equilibrium between TiO2 @PMMA and MO solution. During the photocatalytic reaction, 5 mL of suspension was taken out of the mixture at a time interval of 5 min and
separated
from
the
catalyst
nanofibers
on
a
centrifuge
(TG-16W
Changsha Xiangzhi Centrifuge Instrument Co., Ltd.) at 10000 rpm for 5 min. The concentration of the remaining MO was determined by analyzing the absorbance of the supernatant at λmax= 464 nm (the calibration curve is illustrated in Fig. S1 in the supporting information) with a UV-Vis spectrophotometer (UV-1800 Shimadzu Instrument Co., Ltd). For the reusability test, the supernatant was carefully decanted after the completion of the degradation reaction, and fresh MO solution (10 mg/mL, 100 mL) was added to the mixture. Subsequently, the photocatalytic reaction proceeded at 30oC with constant magnetic stirring under UV illumination for 60 min. The operation was repeated for five times. Fig. 1 3. Results and discussion 3.1. Preparation and characterization of the photocatalyst of TiO2@PMMA. In this work, the TiO2 nanoparticles supported on PMMA nanofibers were prepared by a new method of hydrothermally treating the nanofibers of PMMA containing TBT. The method avoids the calcination at high temperature, and thus the polymer nanofibers are not destroyed. Moreover, the hydrothermal treatment leads to the conversion of TBT to TiO2 nanoparticles loaded on the PMMA nanofibers under mild conditions. Apparently, the work put forward a simple and effective approach for 9
the construction of nanostructured inorganic semiconductors supported on the polymer matrix. Fig. 2 presents the SEM micrographs of electrospun nanofibers of PMMA and PMMA containing TBT before and after the hydrothermal treatment. It is seen that non-uniform and crooked PMMA nanofibers are obtained during the ES process (Fig. 2a-b). Moreover, the nanofibers seem to be fused to form a porous stuff with rugged surface after the hydrothermal treatment (Fig. 2c-d). By contrast, the electrospun nanofibers of PMMA containing TBT reveal smooth surface and larger and more uniform diameter when compared with the PMMA nanofibers (Fig. 2e-f). Furthermore, the outline of the nanofibers could be roughly maintained after the hydrothermal treatment. However, the fibers present rough surface, which is composed of small TiO2 nanoparticles as seen in the SEM picture of higher magnification (Fig. 2g-h). Obviously, the TBT salt precursor plays an important role in retaining the nanostructure of the electrospun PMMA nanofibers and forming TiO2 nanoparticles under hydrothermal treatment. As-prepared TiO2@PMMA is composed of nanoparticles uniformly loaded on the PMMA nanofibers, and possesses high specific surface area. The special morphology of the nanocomposite not only enhances the adsorption of the pollutant, but also provides more reaction sites for the degradation of the pollutants under UV radiation. Fig. 2 Fig. 3 displays the N2 adsorption-desorption curve of TiO2 @PMMA, which 10
exhibits a Type II isotherm. The BET surface area of the TiO2@PMMA is calculated to be 21.4 m2 g-1. The high surface area is in agreement with the SEM observation, and is expected to promote the photocatalytic activity of the composite, as will be discussed below. Fig. 4 depicts the XRD patterns of electrospun PMMA containing TBT both before and after the hydrothermal treatment. It is observed that only a broad diffraction peak at 2θ = 29.3° is observed in the spectrum of the electrospun nanofibers (Fig. 4a), which can be ascribed to PMMA [29]. In contrast, diffraction peaks at 2θ = 26.4°, 38.3°, 48.7°, 63.4°, 76.5° and 83° are clearly identified in the patterns of the hydrothermally treated nanofibers (Fig. 4b), corresponding to (101), (112), (200), (204), (215) and (224) crystal planes of tetragonal anatase TiO2 (JCPDS 65-5714). It is expected that the nanocomposite with high crystallinity of anatase could exhibit higher photocatalytic activity, as reported in literatures [3, 30]. Fig. 3 Fig. 4 The content of inorganic TiO2 nanoparticles in the TiO2 @PMMA has been identified from the TGA curve of the nanocomposite as shown in Fig. 5. The thermal analysis was carried out with a heating rate of 5oC/min from room temperature to 1000oC under air atmosphere. The slight weight loss of the nanocomposite before 230oC is attributed to the release of the physically absorbed water [31]. According to the literature, the decomposition of PMMA starts at about 160oC. By contrast, TiO2@PMMA does not reveal obvious decomposition till about 230 oC. It is proposed 11
that the presence of TiO2 restricts the thermal motion of PMMA molecular chains and leads to improved thermal stability of the composite [32].The TGA curve of TiO2@PMMA shows a platform when the temperature is increased to 500oC, and the remainder could be reasonably ascribed to TiO2 nanoparticles. Therefore, the content of TiO2 nanoparticles in TiO2@PMMA is found to be as high as ~42%, which is comparable with other TiO2 supported composite photocatalysts [33].
The high
percentage of the loaded photoactive TiO2 nanoparticles on the polymer matrix might enhance the photocatalytic activity of the resulting composite, which is another advantage of the fabrication method. Fig. 5 3.2. Photocatalytic activity of TiO2@PMMA The photocatalytic activity of TiO2 @PMMA has been investigated by monitoring the decrease in the concentration of MO (10 mg/L, 100 mL) in the presence of the composite photocatalyst (0.1 g) under UV illumination at 30 oC, and the results are displayed in Fig. 6. Specifically, the process consists of two parts: dark adsorption and photodegradation. As clearly seen in Fig. S2 (please see the supporting information), in the dark adsorption process (100 min), the concentration of MO in the solution first decreases sharply, then reaches a stable value (67.1% of the initial concentration at 60 min). By contrast, the electrospun PMMA nanofibers were fused to form a piece of rugged and porous stuff after the hydrothermal treatment as shown in Fig. 2c-d. 12
Apparently, the specific surface area of the treated nanofibers has been seriously decreased, resulting in little adsorption of MO (the concentration of MO in solution maintains at about 99.0% of the initial value after dark adsorption for 60 min) (Fig. 6). It is known that the adsorption of pollutants onto TiO2 is an important step prior to the photocatalytic degradation. Consequently, the relatively high adsorption of MO onto the catalyst, which could be ascribed to the high specific surface area of the nanostructured composite photocatalyst, should be beneficial for the photodegradation of MO. Fig. 6 In the subsequent photodegradation process, the concentration of MO decreases to 9.1% of the initial value within a short period of 50 min, indicating the high photocatalytic activity of the TiO2 @PMMA catalyst. It is proposed that the photodegradation of pollutants by TiO2 depends on the generation of conduction-band electrons (e-) and valence-band holes (h+) when irradiated with UV-light (Eq. (1)). The highly oxidative photo-induced holes (E- = 2.8 V) can directly oxidize organic pollutants or react with adsorbed water and hydroxyl anions to form hydroxyl radicals, which then attack dye pollutants (Eq. (2-3)). Meanwhile, the e- can also react with proper electron acceptors, such as O2, to yield oxidative radicals as described by Eq. (4). In this work, the TiO2 nanoparticles are uniformly dispersed onto the PMMA nanofibers in the as-prepared composite catalyst, thus conquering the problem of agglomeration of nanoparticles and leading to high specific surface area and increased reaction sites. Therefore, both the adsorption and photochemical reactions are 13
accelerated in the composite catalyst, which is responsible for its high photocatalytic activity in the degradation of MO. TiO2 + hν e-+h+ (TiO2)
(1)
h+ + OH-ads HO.
(2)
h+ + H2Oads HO.+ H+
(3)
e- + O2
O2.-
(4)
The effect of the concentration of MO (5, 10, 20 mg/L) on the photocatalytic activity of TiO2@PMMA has been demonstrated in Fig S2 (please see the supporting information) and Fig. 7. It is seen that both dark adsorption and photodegradation processes are affected by the concentration of MO. The decrease efficiency (η) of MO by TiO2@PMMA is expressed by Eq. 5. η=（C0-C）/C0*100%
(5)
Where C0 represents the initial concentration of MO, and C is the concentration of MO during the adsorption or photo-degradation process [34]. After the 60 min dark adsorption, η is 45.8%, 32.9% and 22.7% for MO solutions of 5 mg/L, 10 mg/L and 20 mg/L, respectively. In the following photodegradation process, the kinetics of the photocatalytic degradation is described using the pseudo-first-order model (Eq.6), and the results are displayed in Fig.7b. dC/dt= -ka C, or ln(C/C0) = -ka t
(6)
where C0 is the initial concentration of MO; C is the concentration of MO at time 14
t; and ka (min-1) is the apparent pseudo-first-order decay rate constant, which can be obtained by plotting ln(C/C0) vs. t as shown in Fig. 7b [35]. It can be seen that MO degradation by the photocatalyst under UV illumination proceeds quickly. Even when the concentration of MO is as high as 20 mg/L, the degradation is almost complete within 90 min, indicating the prominent advantage of high photocatalytic activity of TiO2@PMMA. Furthermore, it is found that higher concentration of MO leads to lower photodegradation reaction rate (smaller ka). Specifically, the degradation of MO of 5 mg/L and 10 mg/L is almost complete within 30 min and 50 min, respectively. Fig. 7 The effect of the dosage of TiO2 @PMMA on the photocatalytic degradation of MO is demonstrated in Fig. 8. After dark adsorption of 60 min (the dark adsorption of the catalyst with different dosages is displayed in Fig. S3 in the supporting information), η resulting from adsorption is 7.9%, 24.1%, 32.9% and 56.4% for catalyst dosage of 0.025 g, 0.05 g, 0.1 g and 0.2 g, respectively. By contrast, under UV illumination, the photocatalytic degradation of MO is nearly completed within 90 min, except for the case of 0.025 g of the photocatalyst. As shown above, the content of TiO2 is ~42% in the composite catalyst. Thus it can be concluded that, with only very small amount of the active component of TiO2 (0.021 g), the photodegradation of MO (10 mg/L, 100 mL) could be almost finished within 80 min using TiO 2 @PMMA, suggesting its high photocatalytic activity. Fig. 8
15
As is shown above, our method of in-situ conversion of titanium salt to TiO2 nanoparticles loaded on surface of PMMA nanofibers not only successfully prevents the agglomeration of nanoparticles, but also maintains high specific surface area of the resulting composite. Therefore, MO is efficiently adsorbed by the photocatalyst, and fully reacts with active TiO2 nanoparticles on the surface of the PMMA matrix under UV illumination to result in its high photocatalytic activity. Table 1 presents the results of the photocatalytic degradation of MO with TiO2 based photocatalysts reported recently and our TiO2@PMMA [36-38]. It is seen clearly that as-prepared TiO2 @PMMA could effectively degrade MO (100 mL, 10 mg/mL) within 50 min ( of 90.5%) with 100 mg of the composite (TiO2 ~ 42 mg), suggesting that its photocatalytic efficiency is more or less of the same level of or even higher than the literature work. It is proposed that the good photocatalytic activity of our catalyst is related to (1) the special micro/nanostructures established in the composite by the three-dimensionally packed PMMA nanofibers and the TiO2 nanoparticles decorated on the fibers, which facilitates the adsorption of the pollutants and allows their easy access to the photoactive TiO2; and (2) high content of TiO2 nanoparticles in the composite photocatalyst, which provides plenty of reaction sites for the degradation of adsorbed pollutant molecules. Table 1 To investigate the recycling ability of the composite, we measured its catalytic activity during five cycles of photodegradation of MO. At the end of each
16
photodegradation cycle, the composite was allowed to precipitate at the bottom of the beaker, and the colorless supernatant was decanted from the beaker before fresh MO (10 mg/L, 100 mL) was added for a new cycle lasting for 60 min. As depicted in Fig. 9, the composite did not show any significant loss of activity during the five consecutive photodegradation cycles. Specifically, the catalyst reveals of 94.4% at the end of the 5th photodegradation cycle, which proves that TiO2@PMMA could well maintain its good photocatalytic activity for repeated use. Moreover, Fig. S4 (please see the supporting information) clearly shows that TiO2 @PMMA prepared in this work could be effectively dispersed in MO solution by magnetic stirring, which could promote the adsorption of the pollutants and their contact with the photocatalysts, and thus enhances the photocatalytic activity. In addition, the composite could completely precipitate to the bottom within 1 min when the stirring is terminated, suggesting easy separation from the pollutant solution for recycling use, which is a highly desirable advantage for practical applications. The above experimental results indicate that our composite of TiO2 nanoparticles supported on the PMMA nanofibers could effectively conquer the problems encountered by nano-sized TiO2 photocatalyst, and shows great potentials for applications in future. Fig. 9 The mechanism of photocatalytic degradation of MO by the composite was briefly illustrated in Fig. 10. The special morphology characteristics of the composite
17
of TiO2 nanoparticles supported on PMMA nanofibers leads to high specific surface area, thus greatly facilitates the adsorption of MO molecules in the solution onto the composite during the dark adsorption process. In the following photocatalytic degradation process, under UV illumination, a high concentration of conduction-band electrons (e-) and valence-band holes (h+) is generated in high density of TiO2 nanoparticles uniformly dispersed on PMMA nanofibers. The highly oxidative photo-induced holes and e- can directly or indirectly react with adsorbed MO molecules for their full degradation (In accordance with Eqs. 1-4). Moreover, the investigations on the photocatalytic degradation products of methyl orange and similar dyes demonstrate that the degradation intermediates are not more toxic than MO [39-41]. Consequently, the photodegradation of MO by TiO2 catalyst could be a safe and efficient approach for the treatment of MO. Fig. 10 4. Conclusions The composite of TiO2 nanoparticles supported on the PMMA nanofibers has been successfully prepared by hydrothermally treating the electrospun PMMA nanofibers containing the titanium salt precursor. The simple method avoids the calcinations of polymer nanofiber template, which is usually required for the preparation of inorganic nanomaterials via electrospinning, and shows environmental friendliness. The composite photocatalyst is featured with high specific surface area, and relatively high content of TiO2, which is beneficial for the enhancement of its 18
photocatalytic activity. Under UV illumination, the composite could completely degrade the aqueous solution of methyl orange in a short time, exhibiting high photocatalytic activity. Moreover, the composite could be easily separated from the reaction system by decantation or filtration for recycling, and maintains high catalytic activity during the recycle use in the degradation of MO. Therefore, as-prepared TiO2@PMMA conquers the shortcoming of easy agglomeration of nano-sized TiO2 catalyst, and proves its potential applications as an efficient and recyclable photocatalyst for the degradation of pollutants. The work provides a new approach for the development of advanced photocatalysts based on nanostructured inorganic semiconductors.
19
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Figure captions Fig. 1 Schematic diagram of the experimental set-up for the photocatalytic reaction. Fig. 2 SEM micrographs of electrospun nanofibers of PMMA (a - b) before and (c - d) after the hydrothermal treatment, and PMMA containing TBT precursor (e - f) before and (g - h) after the hydrothermal treatment. Fig. 3 Isothermal adsorption/desorption curve of TiO2@PMMA (inset: calculation of BET surface area). Fig. 4 XRD patterns of (a) the electrospun nanofibers and (b) TiO2@PMMA. Fig. 5 TGA plot of TiO2@PMMA. Fig. 6 The photocatalytic properties of PMMA and TiO2 @PMMA (0.1 g) in degradation of MO (10 mg/L) under UV illumination. Fig. 7 (a) Effect of initial concentration of MO on its photodegradation and (b) kinetics of photodegradation of MO (catalyst = 0.1 g). Fig. 8 (a) Effect of the amount of catalyst on the degradation of MO and (b) kinetics of MO degradation ([MO] = 10 mg/L). Fig. 9 Photodegradation of MO by TiO2 @PMMA during five consecutive cycles ([MO] = 10 mg/L). Fig. 10 Proposed mechanism of photocatalytic degradation of MO by TiO2 @PMMA under UV illumination. 26
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Table 1 Photodegradation of MO by TiO2 based catalysts Dosage of Catalyst
Degradation Dosage of MO
Catalyst / mg TiO2@PMMA
time / (min)
100
100 mL, 10 mg/mL
50
100
50 mL, 15 mg/mL
90
50
20 mL, 80 mg/mL
140
30
50 mL, 6.5 mg/mL
90
20
50 mL, 10 mg/mL
200
(This work) CNT-Ag/ TiO2 [35] TiO2/PANI
[36]
Fe3O4@SiO2 @ TiO2 @Nd [37] TiO2/ SiO2 [38]
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Graphical Abstract
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