3D MoS2@TiO2@poly(methyl methacrylate) nanocomposite with enhanced photocatalytic activity

Journal of Colloid and Interface Science 557 (2019) 709–721

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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

3D MoS2@TiO2@poly(methyl methacrylate) nanocomposite with enhanced photocatalytic activity Yang Li ⇑, Zhao Wang, Hujie Zhao, Xiaojun Huang, Mujie Yang MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 18 August 2019 Revised 19 September 2019 Accepted 20 September 2019 Available online 21 September 2019 Keywords: TiO2 MoS2 Nanocomposite Photocatalyst Heterostructure Electrospinning Hydrothermal synthesis Freeze-drying Methyl orange Photocatalytic degradation

a b s t r a c t Formation of heterostructure and nanostructure is promising approach to improving the photocatalytic activity of TiO2 based photocatalysts. In this work, MoS2@TiO2@poly(methyl methacrylate) (PMMA) was prepared by freeze drying of hydrothermally treated electrospun PMMA nanofibers containing titanium n-butoxide and MoS2 nanosheets. As-prepared nanocomposite revealed a 3D nanofiber network structure with high specific surface (83.6 m2/g), in which MoS2 nanosheets loaded with TiO2 nanoparticles were distributed on the surface of PMMA fibers. MoS2@TiO2@PMMA showed better photocatalytic performance than MoS2@PMMA, TiO2@PMMA, MoS2@TiO2 and MoS2@P25@PMMA towards the photodegradation of methyl orange (MO) under UV illumination. Typically, MoS2@TiO2@PMMA (100 mg) could degrade MO (10 mg/L, 100 mL) completely in 40 min under UV irradiation, revealing good photocatalytic activity. Moreover, the nanocomposite could be facilely recovered by filtration, and maintain almost the same photocatalytic activity after cycling tests for ten times, indicating excellent stability and recyclability. The enhanced photocatalytic performance of the nanocomposite might relate to the heterostructure between MoS2 and TiO2 to suppress the recombination of photo-induced charge, and the 3D hierarchical nanoweb structure to afford large specific surface area and high density of active sites. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction ⇑ Corresponding author. E-mail addresses: [email protected] (Y. Li), [email protected] (Z. Wang), [email protected] (H. Zhao), [email protected] (X. Huang), [email protected] (M. Yang). https://doi.org/10.1016/j.jcis.2019.09.074 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

Ever increasing water, air and soil pollution worldwide as a result of intensified industrial and agricultural production has posed great threat to human health [1]. To take as an example,

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about 15% of the total amount of azo dyes, which have AN@NA bond in their molecular structures, were lost during the dyeing process [2–4]. To tackle the problem of serious environmental pollution, semiconductor photocatalysts have been extensively studied for the photodegradation of the toxic pollutants [5–14]. Among them, TiO2 based photocatalysts stand out due to the prominent advantages such as low cost, good chemical stability, and friendliness to the environment [15]. However, the TiO2 catalysts generally exhibit good photocatalytic activity in the near ultraviolet (UV) due to its large bandgap energy (3.2 eV), and fail to make use of the abundant visible light energy [16]. Moreover, serious recombination of photo-induced charge carriers could markedly decrease the catalytic efficiency. Apparently, these shortcomings greatly hinder the practical applications of the TiO2 photocatalysts. Various approaches have been developed to improve the photocatalytic activity of the semiconductor photocatalysts [8–14,17,18]. It was reported that the formation of suitable heterostructure with other semiconductors [19,13,14], carbon materials [20] or noble metals [21] could effectively reduce the recombination of the charge carriers and thus substantially increase the photocatalytic activity of the resultant composites. In the past few years, 2 D nanomaterials such as graphene, graphene oxide and molybdenum disulfide (MoS2), have been used to promote the photocatalytic performance of semiconductor photocatalysts [13,14]. In particular, MoS2 nanosheets attracted much attention due to its unique optical, electrical and catalytic properties, which is related to its ultra-thin twodimensional graphene-like nanostructure and very large specific surface area [23–28]. MoS2@TiO2 nanocomposites have been reported to exhibit improved photocatalytic properties [29–31]. However, the prepared composites are generally difficult to be recovered from the reaction system for recycle use, which might not only increase the cost but also lead to secondary pollution. Very recently, we prepared a composite photocatalyst by loading TiO2 nanoparticle on the poly(methyl methacrylate) (PMMA) nanofiber matrix, which showed relatively high specific surface area and good photocatalytic activity. Moreover, the composite could be easily recycled and maintain high photocatalytic performance during repeated use. Apparently, the work provides a new approach for the construction of high performance TiO2 based photocatalysts [32]. In this work, we reported a nanocomposite of MoS2@TiO2@PMMA with 3D hierarchical structure, which was prepared via the hydrothermal treatment of the electrospun PMMA nanofibers containing MoS2 nanosheets and the salt of titanium n-butoxide and subsequent freeze drying. The ternary nanocomposite MoS2@TiO2@PMMA revealed quite large specific surface area (83.6 m2/g), providing numerous reaction sites for the improvement of the photocatalytic activity. Moreover, the formation of heterostructure between MoS2 nanosheets and in-situ synthesized TiO2 nanoparticles could suppress the recombination of photo-induced charge carriers and substantially enhance the photocatalytic activity. Consequently, MoS2@TiO2@PMMA could effectively photodegrade methyl orange (MO), a model pollutant, under UV irradiation, and exhibit enhanced photocatalytic degradation activity than the corresponding binary composites, i.e., TiO2@PMMA, MoS2@PMMA and MoS2@TiO2, and physically mixed ternary composite of MoS2@P25@PMMA (P25 is a well-known TiO2 based photocatalyst with high photocatalytic activity). Furthermore, the nanocomposite demonstrated quite stable performance (unchanged high photocatalytic degradation activity even after the experiments of ten cycles) and could be facilely collected for repeated use, indicating its potentials as a high performance photocatalyst.

2. Experimental 2.1. Reagents The chemicals used in this work were of analytical reagent grade and used without further purification unless noted otherwise. Poly(methyl methacrylate) (PMMA) (MW: 350,000) and hexaammonium heptamolybdate tetrahydrate (HHT) were purchased from Alfa Aesar and Aladdin, respectively. P25 was supplied by Shanghai Jianghu Titanium Chemical Manufacture Co., Ltd. Titanium (IV) n-butoxide (TBT) was obtained from J&K Chemical Technology. KI, n-butanol (n-BA), ethanol (EtOH), ethylene glycol, thiourea, glacial acetic acid (HAc), dichloromethane (DCM), and methyl orange (MO) were all supplied by Sinopharm Chemical Regent Co., Ltd. Benzoquinone (BQN) was purchased from shanghai Mackin biochemical Co., Ltd. 2.2. Preparation of MoS2 nanosheets and MoS2@TiO2@PMMA MoS2 nanosheets were prepared via a modified literature method [33]. In a generic procedure, 0.7 g of HHT and 1.4 g of thiourea were dissolved in 21 mL of deionized water under vigorous stirring. The resulting solution was then transferred into a Teflon-lined stainless steel autoclave and sealed tightly, heated at 220 °C for 8 h and naturally cooled down to room temperature. Afterwards, the obtained black precipitates were collected by filtration, and washed with deionized water and ethanol for several times in sequence, and finally dried under vacuum at 60 °C for 12 h. For the preparation of the nanocomposite of MoS2@TiO2@PMMA, typically, 0.32 g of PMMA was dissolved in a mixture of ethanol (1.5 mL), dichloromethane (4 mL), ethylene glycol (0.5 mL), HAc (1 mL) and TBT (1 mL). Then, different amount of as-prepared MoS2 nanosheets (1, 3, 5, 10, 50 mg) was dispersed in the mixture via ultrasonic treatment for 4 h to obtain a homogeneous solution used for the following electrospinning (ES) process. The ES solution 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 11 kV, and the flow rate of the ES solution was controlled at 2 mL/h by a syringe pump (WZ-50C6, Smith Medical Instrument (Zhejiang) Co., Ltd.). Aluminum foil was grounded and situated 15 cm from the tip of the pinhead. The electrospun nanofibers were deposited onto the aluminum foil to form a nanofiber mat in 3.5 h. Afterwards, the collected nanofiber mat was peeled from the aluminum foil and transferred into a Teflon-lined stainlesssteel autoclave with moderate amount of deionized water, and maintained at 135 °C for 8 h. Subsequently, the autoclave was naturally cooled to room temperature, and the fiber mat was taken out and washed several times with ethanol and deionized water and freeze-dried for 12 h to obtain MoS2@TiO2@PMMA nanocomposite using a freeze dryer (FD-1A-50, BIOCOOL). For comparison, MoS2@PMMA and MoS2@P25@PMMA were obtained by ES from the PMMA solutions containing MoS2 and the physical mixture of MoS2 and P25, respectively. TiO2@PMMA was prepared as described in our previous work [32]. MoS2@TiO2 was prepared by soaking MoS2@TiO2@PMMA in dichloromethane to dissolve the PMMA nanofibers. 2.3. Measurements The morphologies of the samples were analyzed with field emission scanning electron microscopy (FE-SEM, Hitachi S-4800 and FE-SEM, Hitachi SU-8010) and transmission electron

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microscopy (TEM, HT-7700 and JEM 2010). X-ray diffraction (XRD) patterns were collected on a PAN analytical X’Pert PRO using CuKa radiation (k = 0.15418 nm). Brunauer-Emmett-Teller (BET) specific surface area was measured with an ASIC-2 gas adsorption analyzer accelerated surface area porosimetry system (AUTOSORBIQ2-MP, nitrogen as the absorbate, operation temperature:
196 °C). Raman spectroscopy was recorded on a Raman spectrometer (INVIA-REFLEX) using a He-Ne laser (k = 532 nm). X-ray photoelectron spectra (XPS) were measured using an Escalab 250Xi. 2.4. Investigations on photocatalytic activity and active species The photocatalytic performance of the nanocomposites was examined by measuring their photodegradation of MO under UV light illumination with a home-made set-up described in our previous work [32]. A 32 W high-pressure mercury lamp (kmax = 254 nm, Shanghai Jiping Special Lighting Co., Ltd., China), which was placed 8 cm above the liquid surface, was used as the light source in the experiment. In a typical process, 100 mg of MoS2@TiO2@PMMA was added into 100 mL of the solution of MO (10 mg/L) in a quartz beaker with constant magnetic stirring (stirring speed = 300 r/min) at 30 °C. Prior to the UV irradiation, the mixture was magnetically stirred in the dark for 60 min to establish adsorption/desorption equilibrium between the dye and the composite. During the photocatalytic reaction, 5 mL of the suspension was taken out of the mixture at a time interval of 10 min, and separated from the composite photocatalyst on a centrifuge (TG-16 W Changsha Xiangzhi Centrifuge Instrument Co., Ltd.) at 10000 rpm for 5 min. The concentration of the remaining MO was determined from the analysis of the absorbance of the supernatant at kmax = 464 nm using a UV–Vis spectrophotometer (UV-1800 Shimadzu Instrument Co., Ltd). The degradation percentage g of MO was calculated according to Eq. (1).

g¼

co
c  100% co

ð1Þ

where Co and C are initial and instantaneous concentrations of MO (mg/L), respectively. To evaluate the reusability of the nanocomposite, the supernatant was carefully decanted after the completion of each degradation reaction under UV illumination, and fresh MO solution

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(10 mg/mL, 100 mL) was added into the mixture. The photocatalytic degradation proceeded under UV illumination for 50 min and g was determined at the end of the reaction. The cycle was repeated for 10 times. For comparison, the photocatalytic performance of MoS2@TiO2@PMMA under visible light was evaluated by using a 350 W Xe arc lamp (Shanghai Jiguang Special Lighting Appliance Factory) with 420 nm cutoff filter as the light source. The possible active species in the photodegradation of MO under UV illumination such as holes (h+) and reactive oxygen species (ROS) including superoxide radical (O–2) and hydroxyl radicals (OH) are explored by using the scavengers of KI, p-benzoquinone (BQN), and n-butanol (n-BA). The experiments were similar to the photodegradation of MO under UV irradiation using MoS2@TiO2@PMMA with addition of 0.1 mmol of the scavengers. 3. Results and discussion 3.1. Preparation and characterization of MoS2@TiO2@PMMA The overall preparation procedure of MoS2@TiO2@PMMA is illustrated in Scheme 1. MoS2 nanosheets and TBT were first loaded on the PMMA nanofibers by ES. The following hydrothermal treatment of the nanofibers at 135 °C for 8 h resulted in the conversion of TBT to TiO2 nanoparticles which were decorated on the surface of the PMMA nanofiber. Finally, the wet nanofiber webs were treated by freeze-drying, a method well-known for obtaining 3 D porous network featured with high specific surface area [34]. The simple approach could lead to effective loading of the photocatalytic TiO2 nanoparticles and MoS2 nanosheets on the PMMA nanofiber network for the facile dispersion and recovery of the photocatalyst. Meanwhile, the ternary nanocomposite could maintain porous 3D nanostructure to provide high density of active sites and thus improve the photocatalytic activity. Moreover, the heterostructure formed between intimately contacted MoS2 nanosheets and in-situ synthesized TiO2 nanoparticles is expected to promote the interfacial charge transfer and separation to suppress the recombination of the photo-induced electrons and holes. Consequently, the photocatalytic activity could be enhanced effectively.

Scheme 1. Preparation of MoS2@TiO2@PMMA with 3D hierarchical structure.

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Fig. 1 presents the X-ray powder diffraction (XRD) patterns of as-prepared MoS2 nanoflowers, PMMA nanofibers, TiO2@PMMA and MoS2@TiO2@MMA. The patterns of electrospun PMMA nanofibers do not show obvious diffraction peaks. By contrast, the diffraction peaks at 2h = 25.8°, 37.8°, 48.2°, 55.0°, 62.7°, and 70.3° are clearly identified in the patterns of the hydrothermally treated nanofibers containing TBT (TiO2@PMMA), which corresponds to the (1 0 1), (0 0 4), (2 0 0), (2 1 1), (2 0 4) and (2 2 0) crystal planes of the anatase phase of TiO2 (JCPDS card no. 65-5714, Fig. 1c) [35]. Meanwhile, as-prepared MoS2 nanoflowers show diffraction peaks at 2h = 14.4°, 33.5°, 39.5°, 49.8° and 58.3°, which can be assigned to the (0 0 2), (1 0 1), (1 0 3), (1 0 5) and (1 1 0) crystal planes of the hexagonal phase of MoS2 (JCPDS card no. 65-1951, Fig. 1b) [36]. In the pattern of MoS2@TiO2@PMMA, the major diffraction peaks attributed to MoS2 nanoflowers and TiO2 are identified, suggesting that the composite prepared according to Scheme 1 comprises hexagonal phase of MoS2 and anatase phase of TiO2. We measured the XRD patterns of recycled MoS2@TiO2@PMMA after the photodegradation tests under UV illumination for ten times (Fig. 1d). Apparently, the major characteristic diffraction peaks at 14.4° and 25.8°, which correspond to (0 0 2) crystal plane of MoS2 and (1 0 1) crystal plane of TiO2, respectively, could be easily identified,

suggesting good stability of the nanocomposite during repeated use. The bonding state of MoS2@TiO2@PMMA was further explored by XPS, and the Ti 2p, O 1s, Mo 3d and S 2p spectra of the composite are displayed in Fig. 2. The binding energies of Ti 2p3/2 and Ti 2p1/2 are 458.2 and 463.9 eV (Fig. 3a) [37], respectively, assigned to the Ti4+ oxidation state. Moreover, O 1s is observed at 532.3 eV (Fig. 2b) [38], while the binding energy of Mo 3d3/2, Mo 3d5/2S 2p1/2 and S 2p3/2 peaks are located at 232, 228.6, 162 and 162.8 eV, respectively (Fig. 3c and d). Zhou et al. reported that the binding energies of pure layered MoS2 nanosheets should be located at 232.5, 229.3, 163.3 and 162.3 eV [29]. Apparently, the binding energies of Mo 3d5/2, Mo 3d3/2, S 2p1/2 and S 2p3/2 in MoS2@TiO2@PMMA shift to lower energy states. We proposed that the shift of the peaks might relate to the electronic interactions between TiO2 and MoS2 [39]. The MoS2 nanosheets were prepared via a hydrothermal approach. Fig. 3 illustrates the Raman spectra of MoS2 nanosheets and MoS2@TiO2@PMMA. The phonon vibrational modes at 379 cm
1 and 405 cm
1 in the Raman spectroscopy of MoS2 nanosheets correspond to the representative mode of E12g and A1g, respectively. A blue shift of 5 and 3 cm
1 for A1g and E12g modes

Fig. 1. XRD patterns of MoS2, PMMA, TiO2@PMMA and MoS2@TiO2@PMMA.

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Fig. 2. XPS spectra of MoS2@TiO2@PMMA: (a) Ti 2p, (b) O 1s, (c) Mo 3d, and (d) S 2p.

Fig. 3. Raman spectra of (a) MoS2 and (b) MoS2@TiO2@PMMA.

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might be attributed to the increased layer thickness [40]. By contrast, in the Raman spectroscopy of MoS2@TiO2@PMMA, the phonon vibrational modes at 147 cm
1, 380 cm
1 and 405 cm
1 are clearly identified, ascribing to the representative mode of Eg of TiO2, E12g and A1g of MoS2, respectively [41]. Fig. 4 presents the SEM images of MoS2, electrospun PMMA nanofibers, MoS2@PMMA, TiO2@PMMA, MoS2@P25@PMMA and MoS2@TiO2@PMMA. It is seen that MoS2 obtained via a hydrothermal process is in the shape of nanoflowers, which are composed of dozens of nanosheets with thickness of ~20 nm as determined from the image of higher magnification (Fig. 4a and b). On the other hand, the electrospun PMMA fibers exhibit smooth surface with diameters of ~400 nm. For the composite of MoS2@PMMA, the MoS2 nanosheets twisted around the PMMA nanofibers, constituting a coarse surface with enlarged diameter compared to PMMA nanofiber alone (Fig. 4d). The addition of P25, a well-known TiO2 nanoparticle photocatalyst, resulted in even rugged surface of the composite of MoS2@P25@PMMA, with P25 nanoparticle aggregates clearly observed on the surface (Fig. 4e). By contrast, the hydrothermally treated and freeze-dried TiO2@PMMA presents porous rough surface with obvious decorations of TiO2 nanoparticles (Fig. 4f and g). In the micrograph of the composite of MoS2@TiO2@PMMA, TiO2 nanoparticles are dispersed on the surface of PMMA nanofibers, while no twisted MoS2 nanosheets around the nanofibers are identified. However, the 3D network structure is well maintained upon the freeze-drying, which might lead to increased specific surface area (Fig. 4h and i). Moreover, the energy dispersive X-ray spectrometry (EDS) of MoS2@TiO2@PMMA (Fig. 5) demonstrated that the composite is composed of C, O, Ti, Mo and S, and both MoS2 and TiO2 are uniformly dispersed on the surface of PMMA nanofibers. The microscopic morphology and structure of MoS2, TiO2 and MoS2@TiO2 have been further characterized via TEM. Fig. 6a and b disclose that the MoS2 nanoflowers break into discrete nanosheets via ultrasonic treatment before the addition into the ES solution. Fig. 6c reveals the TEM image of the sample obtained by dissolution of TiO2@PMMA in dichloromethane. It is seen that the hydrothermally prepared TiO2 are in the form of well-

dispersed nanoparticles with average diameters of 5–10 nm. By contrast, the TEM image of MoS2@TiO2 (Fig. 6d), which was obtained by removing the PMMA nanofibers of MoS2@TiO2@PMMA with dichloromethane, shows that only discrete MoS2 nanosheets could be observed, while TiO2 nanoparticles are not clearly identified, suggesting that the TiO2 nanoparticles might have been uniformly enwrapped by the MoS2 nanosheets. Furthermore, the high resolution transmission electron microscopy (HR-TEM) image of MoS2@TiO2@PMMA is shown in Fig. 6e. The fringes with the lattice spacing of 0.6 nm and 0.35 nm are observed, corresponding to (0 0 2) plane of MoS2 nanosheets and (1 0 1) plane of TiO2 nanoparticles, respectively. Obviously, the HR-TEM observation confirms the existence of nanostructured MoS2 and TiO2 with high crystallinity in the nanocomposite, which agrees with the results of XRD characterizations. Fig. 7 presents the N2 adsorption-desorption curves of MoS2@TiO2@PMMA. Apparently, it shows type IV isotherm, similar to other binary or ternary composites (Figures not shown). The specific surface area of MoS2@P25@PMMA, which was prepared from the electrospinning of the PMMA solution containing the physical mixture of P25 and MoS2 nanosheets in absence of freeze-drying treatment, is calculated to be 27.9 m2/g. By contrast, the hydrothermally prepared TiO2@PMMA nanofiber network presents a much higher specific surface area of 69.8 m2/g after freeze-drying treatment. Moreover, MoS2@TiO2@PMMA reveals the highest specific surface area of 83.6 m2/g. Obviously, the introduction of MoS2 nanosheets could constitute a rough surface and increase the specific surface area of the resulting composite. Moreover, the freeze-drying treatment is found to substantially enlarge the specific surface area of the composite (the composite without freeze-drying treatment exhibits a specific surface area of only 21.4 m2/g [32]), which is expected to benefit the enhancement of photodegradation efficiency. The above characterizations clearly demonstrate that we have obtained MoS2@TiO2@PMMA in which MoS2 nanosheets and insitu synthesized TiO2 nanoparticles were dispersed on PMMA nanofiber surface. The formation of heterostructure between MoS2 and TiO2 is expected to promote the charge separation and

Fig. 4. SEM images of (a, b) MoS2, (c) PMMA, (d) MoS2@PMMA, (e) MoS2@P25@PMMA, (f, g) TiO2@PMMA and (h, i) MoS2@TiO2@PMMA.

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Fig. 5. SEM image and EDS mapping results of MoS2@TiO2@PMMA.

weaken the recombination of photo-induced electrons and holes, and thus promote the photocatalytic efficacy. Moreover, the successful loading of the active nanostructured photocatalyst component on the nanofiber support to establish a nanoweb structure is expected to not only provide large specific surface area and high density of reactive site for the photocatalysis, but also facilitate the dispersion and recycling of the photocatalysts, and thus enhance the photocatalytic performance. 3.2. Photocatalytic performance Fig. 8 exhibits the UV-vis spectra of MO solution, a model compound for the dyes, at different reaction time of the photodegradation with MoS2@TiO2@PMMA under UV illumination. The maximum absorbance of MO occurs at 464 nm, which derives from a conjugated structure formed by the azo bond (AN@NA) under the strong influence of the electron-donating dimethylamino group, and the absorbance at 270 nm is ascribed to the p ? p* transition related to aromatic rings [42]. It is seen that the bands at 464 and 270 nm become weaker with the elongation of the reaction time, and disappear in 50 min (the color of the MO solution changes from yellow to colorless), which indicates the cleavage of the azo bond and complete degradation of MO. The absorption at 464 nm is employed for the evaluation of the concentration of MO in the solution during the photodegradation process in the following investigations. Fig. 9 shows the results of the photodegradation of MO under UV illumination with six samples, i.e., PMMA nanofibers, MoS2@PMMA, TiO2@PMMA, MoS2@TiO2, MoS2@P25@PMMA and MoS2@TiO2@PMMA. In 60 min, the adsorption-desorption equilibrium is achieved in the dark for all the samples. In the dark adsorption process, the decrease in the concentration of MO follows the order of MoS2@TiO2@PMMA (19.6%) > TiO2@PMMA (14.7%) > MoS2@P25@PMMA (10.7%), which is in agreement with the order of their specific surface area, indicating that higher specific surface area provides more space for the adsorption of MO for subsequent photodegradation under the UV illumination. By contrast, little MO has been adsorbed onto MoS2@PMMA and PMMA nanofibers. In view of the importance of adsorption in the photodegradation process, such a difference in the dark adsorption process is supposed to lead to varied photodegradation performance. After 50 min of UV illumination, the photodegradation efficiency g reaches 95.7%, 83.8% and 77.5% for MoS2@TiO2@PMMA, TiO2@PMMA and MoS2@-

P25@PMMA, respectively. Nonetheless, PMMA nanofibers display no photocatalytic activity, and only a slight portion of MO has been degraded with MoS2@PMMA (g of 21.4%). Apparently, MoS2@TiO2@PMMA exhibits higher photocatalytic activity than TiO2@PMMA, which is proposed to result from the inclusion of MoS2 nanosheets. In MoS2@TiO2@PMMA, MoS2 nanosheets are decorated by in-situ synthesized TiO2 nanoparticles to form a heterostructure, which promotes the separation of photogenerated charge carriers and suppress the recombination of photo-induced holes and electrons. Therefore, the charge concentration is increased, and the highly active photo-induced holes and electrons can directly or indirectly react with adsorbed MO molecules for their complete degradation in accordance with Eqs. (1)–(4), leading to more efficient photodegradation of MO.

TiO2 þ hv ! e
þ h h

þ

þ

ðTiO2 Þ

þ OH
! HO

þ

h þ H2 O ! HO  þ H e
þ O2 ! O2


ð1Þ ð2Þ

þ

ð3Þ ð4Þ

In order to check the hypothesis, we examined the photocatalytic performance of physically mixed MoS2@P25@TiO2. P25 is a well-known photocatalyst with high activity, but the physical mixture of P25 nanoparticles and MoS2 nanosheets loaded on PMMA nanofibers showed substantially lower photocatalytic activity than MoS2@TiO2@PMMA. Apparently, the physical blending of P25 with MoS2 could not lead to formation of effective heterostructure to weaken the recombination of photo-induced charges. Instead, the composite exhibited smaller specific surface area, which might relate to the agglomeration of P25 particles, and thus lower photocatalytic efficiency is observed. MoS2@TiO2 is obtained by removing the PMMA nanofiber support from the ternary nanocomposite of MoS2@TiO2@PMMA via dissolution in dichloromethane. It was found that smaller amount of MO was adsorbed on the binary composite (12.3%) with respect to MoS2@TiO2@PMMA during the dark adsorption process, which might be ascribed to the lower specific surface area of MoS2@TiO2 in the absence of PMMA nanofiber support. Moreover, the photodegradation efficiency g of MoS2@TiO2 is 80.4% after UV illumination of 50 min, which is markedly lower than that of

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Fig. 6. TEM images of (a, b) MoS2 nanosheets, (c) TiO2 and (d) MoS2@TiO2; (e) HR-TEM image of MoS2@TiO2@PMMA.

MoS2@TiO2@PMMA (95.7%). It is proposed that the appreciable decrease in the photocatalytic activity of MoS2@TiO2 with respect to MoS2@TiO2@PMMA might relate to the decreased specific sur-

face area and lower density of reactive sites due to the absence of 3D hierarchical network structure and the agglomeration of the MoS2@TiO2 particulates.

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Fig. 7. (a) Nitrogen adsorption–desorption isotherms of MoS2@TiO2@PMMA; Calculation of BET surface area of (b) TiO2@PMMA, (c) MoS2@P25@PMMA and (d) MoS2@TiO2@PMMA.

Fig. 8. UV-vis absorption spectra of MO solution at different reaction time of the photocatalytic degradation of MO with MoS2@TiO2@ PMMA under UV irradiation.

Fig. 9. Photocatalytic degradation of MO using PMMA nanofibers and various composites under UV irradiation ([MO] = 10 mg/L).

It is reported that the composite of MoS2 and TiO2 could catalyze the photodegradation of dyes even under visible light [22]. We tested the photodegradation of MO with MoS2@TiO2@PMMA under visible light and the results are presented in Fig. 10. It is seen

that little degradation of MO is observed when the solution is illuminated with visible light. Apparently, our nanocomposite does not exhibit photocatalytic activity under visible light and needs further improvement. Therefore, the following investigations on

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Fig. 10. Photocatalytic degradation of MO using MoS2@TiO2@PMMA under visible light ([MO] = 10 mg/L).

the photocatalytic performance of MoS2@TiO2@PMMA are carried out only in the presence of UV illumination. Fig. 11 exhibits the effect of initial MO concentration on the photocatalytic degradation efficiency of MoS2@TiO2@PMMA. Apparently, the photodegradation efficiency decreases with an increase in the initial MO concentration. The nanocomposite could realize degradation percentages as high as 93.6% and 96.6% with initial MO concentrations of 1 and 5 mg/L by UV illumination for 20 min and 30 min, respectively. At higher initial concentration of MO of 20 mg/mL, the degradation percentage declines to 55.1% in 50 min. According to previous studies [43], the influence of the initial dye concentration on the photocatalytic decolorization could be described by the Langmuir-Hinshelwood kinetic model shown as follows:

  dC C ¼
kt ¼
kC; ln dt Co where Co is the initial concentration of MO; C is the concentration of MO at time t; and k is the apparent pseudo-first-order decay rate constant. The linear fit between
ln(C/Co) and irradiation time t under different initial MO concentrations could be used to describe

the pseudo-first-order kinetics, and the results are shown in Fig. 11b. The good linearity of the sensing curves indicates that the proposed kinetic model is in good agreement with the experimental data. Apparently, the initial dye concentration has a fundamental effect on the decolorization rate, and the rate constant decreases with the increase in the initial concentration of the dye. Fig. 12 demonstrates the effect of the dosage of MoS2@TiO2@PMMA on the photocatalytic degradation activity of MO. After UV irradiation for 50 min, the degradation efficiency is 60.8%, 78.4%, 95.7%, 94.3%, and 96.3% for the nanocomposite dosage of 0.01, 0.05, 0.1, 0.2 and 0.5 g, respectively. It is seen that complete degradation of MO is achieved within 50 min, and the increase in the dosage does not significantly improve the photocatalytic activity when the applied catalyst is above 0.1 g. As the catalyst dosage increases from 0.01 g to 0.5 g, k increases first, and reaches a maximum of 0.0536 min
1 at a dosage of 0.1 g. Obviously, the increment in the dosage increases the number of active sites on the photocatalyst, and the number of dye molecules and absorbed photons is elevated accordingly, leading to improved photodegradation efficacy. However, k decreases at even higher photocatalyst dosage, which might relate to the blocking of light penetration by the excessive amount of photocatalyst [44]. As discussed above, the inclusion of MoS2 is crucial for the enhanced photocatalytic activity of MoS2@TiO2@PMMA. Different dosage of added MoS2 (1 to 50 mg) was employed in the preparation of the nanocomposite. The reaction rate constant was calculated from the linear fitting of the corresponding degradation curve, and the results are illustrated in Fig. 13. The degradation percentage of MO increases when MoS2 dosage is elevated from 1 mg to 5 mg, which might relate to increased active site and heterostructure. However, further increase of the MoS2 amount could not effectively promote the photodegradation efficiency of MO. It is proposed that excess dosage of MoS2 might induce the aggregation of the nanosheets on the surface of the PMMA nanofiber, and deteriorate the heterostructure with TiO2 nanoparticles. Moreover, such an aggregation might lower the specific surface area and reduce the exposure to UV light, resulting in weakened photocatalytic activity. To examine the photocatalytic stability of MoS2@TiO2@PMMA, we repeated the experiment of its photodegradation of MO under UV irradiation for ten times. At the end of each cycle, the colorless supernatant was decanted from the beaker before fresh MO (10 mg/L, 100 mL) was added for a new cycle lasting for 50 min. The photodegradation efficiency remained constant till the last

Fig. 11. (a) Effect of initial concentration of MO on the photodegradation and (b) the kinetics of photodegradation (catalyst = 0.1 g).

Y. Li et al. / Journal of Colloid and Interface Science 557 (2019) 709–721

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Fig. 12. (a) Effect of the amount of catalyst on photodegradation of MO and (b) the kinetics of the photodegradation ([MO] = 10 mg/L, 100 mL).

Fig. 13. Effect of the amount of MoS2 on photodegradation rate constants of MO ([MO] = 10 mg/L, 100 mL; catalyst = 100 mg).

cycle (g of 95.6%), suggesting its excellent stability of photocatalytic activity (Fig. 14). Furthermore, the nanocomposite looks like a piece of sponge, and it could precipitate at the bottom of the beaker quickly when the stirring stops (Fig. 15). Apparently, it can be easily separated from the reaction system by filtration. Therefore, the photocatalyst of MoS2@TiO2@PMMA shows very good recyclability. Table 1 summarizes the results of the photocatalytic degradation of MO with TiO2 based photocatalysts reported recently and our MoS2@TiO2@PMMA [45–48]. It is seen clearly that 100 mg of as-prepared MoS2@TiO2@PMMA could completely degrade MO (100 mL, 10 mg/mL) within 50 min (g of 95.7%), suggesting that it could display photocatalytic performance more or less of the same level as the literature work. Moreover, MoS2@TiO2@PMMA is featured with very good recyclability, which has even outperformed some of the reported TiO2 based photocatalysts. As is known, the active species, including hole (h+) and ROS of superoxide radical (O–2), hydroxyl radical (OH), etc., play a crucial role in the photocatalytic process [8–10]. We tried to investigate the active species in the photodegradation of MO with MoS2@TiO2@PMMA by recording the UV-vis spectra of the photodegradation system under UV illumination with the addition of KI, n-butanol (n-BA) and benzoquinone (BQN), which are the

Fig. 14. Photocatalytic degradation of MO by MoS2@TiO2@PMMA under UV irradiation in repeated experiments ([MO] = 10 mg/L, 100 mL; catalyst = 100 mg).

Fig. 15. The picture of MoS2@TiO2@PMMA in water (a) with and (b) without stirring.

scavengers of hole, OH radical and O–2, respectively, and the results are presented in Fig. 16. It is seen that during the photodegradation process of MO with MoS2@TiO2@PMMA under UV illumination, the intensity of the

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Y. Li et al. / Journal of Colloid and Interface Science 557 (2019) 709–721

Table 1 Photodegradation of MO by TiO2 based photocatalysts under UV irradiation. Catalyst

Dosage of catalyst/mg

Photodegradation rate

Dosage of dyes

Cycling stability

TiO2 modified mesoporous carbon [45] Fe3O4@mesoporous SiO2@TiO2 [46] TiO2@carbon nanofiber [47] Porous SiO2@TiO2 [48] MoS2@TiO2@PMMA (This work)

140 50 1000 30 100

100% in 180 min 93.5% in 120 min 59.4% in 210 min 96.5% in 100 min 95.7% in 50 min

50 mL, 10 mg/L 40 mL, 10 mg/L 100 mL, 10 mg/L 50 mL, 10 mg/L 100 mL, 10 mg/L

/

Fig. 16. UV-vis absorption spectra of MO solution during the photodegradation of MO with MoS2@TiO2@ PMMA under UV irradiation. 0 and 1 refer to the spectra obtained after the photodegradation of MO for 0 and 25 min. Other curves refer to the spectra obtained after the photodegradation of MO for 25 min in the presence of different scavengers.

characteristic absorption peak of MO at 464 nm decreased markedly after the reaction of 25 min, suggesting a fast reaction process. In the presence of BQN, the photodegradation of MO is completely prohibited as indicated by unchanged intensity of the absorption peak in the spectrum obtained after UV illumination for 25 min. By contrast, the addition of n-BA does not have any effect on the photodegradation process, and the intensity of the absorption peak in the spectrum is the same as the one in the absence of the scavenger. Therefore, it is inferred that O–2 is the dominant active species in the photocatalytic process, while OH is not the active species. Moreover, it is seen that the addition of KI effectively promotes the photodegradation of MO as shown by larger decrease in

g of ~90% in 5 cycles /

g of ~95.6% in 10 cycles ~

the intensity of the absorption peak in the spectrum with respect to the pristine system. It is proposed that KI (a quencher of h+) effectively scavenges the photoinduced holes, and thus prevents the recombination of photogenerated holes and electrons. Consequently, the increased concentration of electrons leads to accelerated photodegradation of MO. Apparently, h+ itself does not act as an active species in the photocatalytic process. Based on the experimental results, we proposed a possible photocatalytic process of TiO2 and MoS2@TiO2 heterostructure as illustrated in Fig. 17. Under UV illumination, high density of TiO2 nanoparticles dispersed on PMMA surface behave as the photoactive site to generate conduction band (CB) electrons (e-) and valence band (VB) holes (h+). The photoinduced electrons reduces O2 to superoxide radical (O–2), which acts as the major active species during the photocatalytic process. Since the photogenerated electrons and holes tend to recombine, the TiO2 nanoparticles supported on PMMA nanofibers (TiO2@PMMA) exhibit low photocatalytic activity. On the other hand, a heterostructure is formed between the intimately contacted MoS2 nanosheets and TiO2 nanoparticles obtained by in-situ hydrothermal synthesis. The band gap of MoS2 nanosheets is close to 1.9 eV [22], and both CB and VB positions of MoS2 are higher than those of TiO2, leading to the matching of energy band between MoS2 and TiO2. In consequence, under UV irradiation, the photo-excited electrons from the CB of MoS2 nanosheets could transfer to CB of TiO2, while the holes from the VB of TiO2 nanoparticles are transferred to VB of MoS2. In this way, the charge separation is promoted thanks to the formation of MoS2@TiO2 heterostructure. Apparently, the high interfacial charge transfer and separation ability could effectively suppress the recombination of electron-hole pairs, and result in enhanced photocatalytic activity of the nanocomposite of MoS2@TiO2@PMMA. Furthermore, the freeze-dried MoS2@TiO2@PMMA nanoweb exhibits a large specific surface area and much increased active sites for the photocatalytic reaction, which also contributes to the improvement of its photocatalytic activity.

Fig. 17. Schematic illustration of photocatalytic process of TiO2 and MoS2@TiO2 heterostructure under UV irradiation.

Y. Li et al. / Journal of Colloid and Interface Science 557 (2019) 709–721

4. Conclusions MoS2@TiO2@PMMA was synthesized by the combination of electrospinning, hydrothermal synthesis and freeze-drying. The resulting composite with 3D hierarchical structure and high specific surface area (83.6 m2/g) could completely degrade MO in a short time under UV irradiation, revealing enhanced photocatalytic activity which is comparable to or even higher than literature work [45–48]. Moreover, the nanocomposite is featured with excellent stability and recyclability, which has been rarely reported for the TiO2 based photocatalysts. The enhanced photocatalytic performance of the nanocomposite might relate to the hindrance of the recombination of photo-induced charges due to the in-situ formation of MoS2@TiO2 heterostructure with matched energy band. Furthermore, the large specific surface area and high density of reactive sites resulting from the 3D hierarchical nanoweb structure of MoS2@TiO2@PMMA is beneficial for the improvement in the photocatalytic performance. The work provides references for the development of advanced TiO2 based photocatalysts. Declaration of Competing Interest None. References [1] R.P. Schwarzenbach, T. Egli, T.B. Hofstetter, U.V. Gunten, B. Wehrli, Annu. Rev. Environ. Resour. 35 (2010) 109–136. [2] H. Park, W. Choi, J. Photoch. Photobio. A. 159 (2003) 241–247. [3] U.G. Akpan, B.H. Hameed, J. Hazard. Mater. 170 (2009) 520–529. [4] M. Makita, A. Harata, Chem. Eng. Process. 47 (2008) 859–863. [5] L.Q. Jing, Y.H. Qu, B.Q. Wang, S.D. Li, B.J. Jiang, L.B. Yang, W. Fu, H.G. Fu, J.Z. Sun, Sol. Energy. Mat. Sol. C 90 (2006) 1773–1787. [6] J.S. Lee, J. Jang, J. Ind. Eng. Chem. 20 (2014) 363–371. [7] F. Opoku, K.K. Govender, C.G.C.E.V. Sittert, P.P. Govender, Adv. Sustainable Syst. (2017) 1700006. [8] H.G. Huang, S.C. Tu, C. Zeng, T.R. Zhang, A.H. Reshak, Y.H. Zhang, Angew. Chem. Int. Ed. 56 (2017) 11860–11864. [9] H.G. Huang, X.W. Li, J.J. Wang, F. Dong, P.K. Chu, T.R. Zhang, Y.H. Zhang, ACS Catal. 5 (2015) 4094–4103. [10] H.G. Huang, K. Xiao, Y. He, T.R. Zhang, F. Dong, X. Du, Y.H. Zhang, Appl. Catal. BEnviron. 199 (2016) 75–86. [11] F. Chen, H.G. Huang, L. Guo, Y.H. Zhang, T.Y. Ma, Angew. Chem. Int. Ed. 58 (2019) 10061–10073. [12] G.W. Huang, S.C. Tu, X. Du, Y.H. Zhang, J. Colloid Interf. Sci. 509 (2018) 113– 122. [13] C.S. Chen, X.Y. Liu, H. Long, F. Ding, Q.C. Liu, X.A. Chen, Vacuum 164 (2019) 66– 71.

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