C Fibers@WSe2 Nanoplates CoreâShell Composite: Highly Efficient

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C fibers@WSe2 nanoplates core-shell composite: highly efficient solar-driven photocatalyst Hong Li, Zhijian Peng, Jingwen Qian, Meng Wang, Chengbiao Wang, and Xiuli Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10376 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 13, 2017

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ACS Applied Materials & Interfaces

C fibers@WSe2 nanoplates core-shell composite: highly efficient solar-driven photocatalyst Hong Li,

†,‡,§

‡

Zhijian Peng,*, Jingwen Qian,

†,‡,§

Meng Wang,

†,‡,§

†

Chengbiao Wang, and Xiuli Fu*,§

†

School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China

‡

School of Science, China University of Geosciences, Beijing 100083, PR China

§

State Key Laboratory of Information Photonics and Optical Communications, and School of Science, Beijing

University of Posts and Telecommunications, Beijing 100876, P. R. China

ABSTRACT: Recently, WSe2 as a typical transition metal dichalcogenide compound has attracted extensive attention due to its potential applications in electronic and optoelectronic devices. However, WSe2 alone cannot be directly used as photocatalyst, due to its inferior performance possibly caused by the strong recombination of photogenerated electron-hole pairs. Here a novel C fibers@WSe2 nanoplates core-shell composite (NPCSC) was successfully synthesized via facile, one-step thermal evaporation, in which numerous WSe2 thin nanoplates were in-situ, densely and even vertically grown on the surface of the C fibers. Such composite presents highly solar-driven photocatalytic activity and stability for the degradation of various organic aqueous dyes including methylene blue and rhodamine B, and highly harmful gas like toluene, showing the great potential for environmental remediation by degrading toxic industrial chemicals using sunlight. Under simulated sunlight irradiation, comparing with commercially available WSe2 powder, the as-synthesized C fibers@WSe2 NPCSC presents significantly enhanced reaction rate constants by a factor of approximately 15, 9 and 3 for the degradation of aqueous methylene blue, aqueous rhodamine B, and gaseous toluene, respectively, due to the effective separation of photogenerated electron-hole pairs promoted by the rapid transfer of photogenerated electrons through C fibers. Moreover, this one-step thermal evaporation is an easy-handling, environmentally friendly and low-cost synthesis method, which is suitable for large-scale production. KEYWORDS: WSe2 nanoplates; C fiber; Composite; In-situ synthesis; Solar-driven photocatalysis; Mechanism 1

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1. INTRODUCTION Nanostructured layered transition metal dichalcogenide (TMD) compounds as a class of newly emerging two-dimensional nanomaterials have attracted extensive attention in both fundamental research and novel engineering applications.1-4. Their special graphene-like structure consists of metal atoms sandwiched between two layers of chalcogen atoms with strong covalent bonding and stacks with weak van der Waals interaction between the neighboring sandwiched layers,4,5 endowing them with unique mechanical, thermal, electrical, optical and photoelectrical properties.6-8 Therefore, they have great potential in the applications of electronics, optoelectronics, catalysis and energy storage, such as field-effect transistors,9,10 light-emitting devices,9,11 photodetectors,9,12 batteries,13 super capacitors,14 solar cells,15 photocatalysts,16,17 and electrocatalysts.18,19 As a typical TMD semiconductor, WSe2 has a relatively small band gap (roughly 1.65 eV for monolayer WSe2 and 1.2 eV for bulk WSe2) and a high absorption coefficient for the visible to infrared light,6 implying that it should be an excellent photoharvester especially for sunlight. However, WSe2 alone has not been directly used as photocatalyst due to its inferior performance, as done by its other MX2 TMD counterparts (M=W, Mo, Tc, etc; X=S, Se, Te).16,17 Jiang theoretically indicated that MX2 TMDs cannot be directly used as the photocatalysts for overall photosplitting of water because their VBM and CBM energies do not match the redox potentials of water oxidation and reduction.20 And more scientists considered that the main reason for their inferior photocatalytic activity might be ascribed to the strong recombination of photogenerated electron-hole pairs, which is a widely existed phenomenon among semiconductor photocatalysts.21-24 To reduce the recombination rate of photogenerated electron-hole pairs of semiconductor photocatalysts and thus improve their photocatalytic efficiency, constructing composites of nanoscaled semiconductor photocatalysts with other catalysts or materials of good conductivity is an efficient way,17 because in such case the specific area of the photocatalysts could be significantly enhanced and the coupling interface can provide channels for electrons 2


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transfer.21-24 In particular, carbon materials (such as graphene, carbon fibers and carbon nanotubes) have high electrical conductivity, no toxicity and usually low-cost, so the composites of carbon materials and WSe2 nanostructures might be promising for photocatalytic applications. In the past few years, several groups have reported their works on carbon/WSe2 nanostructures composites. For example, in 2008, Pol et al.25 synthesized a composite of WSe2 nanoparticles with carbon nanotubes using Se and W(CO)6 as the precursors and carbon nanotubes as the matrix through a single-step solid-state reaction. In 2011, Cao et al.26 employed maleic anhydride-grafted ethylene-octene copolymer as carbon source, and W and Se powder mixture as W and Se precursors to fabricate carbon-encapsulated WSe2 nanorods via one-pot high-temperature solid-state reaction. In 2013, Wang et al.27 prepared WSe2 nanofilms on C fibers by firstly depositing W nanofilms onto C fibers through DC magnetron sputtering and then selenizing the films, indicating that the product was a good electrocatalyst for hydrogen evolution. In 2015, Guo et al.28 reported an atomically thin WSe2/graphene nanosheets through an in-situ solvothermal method for highly efficient oxygen reduction reaction electrocatalysts by employing WCl6, Se powder and exfoliated graphene oxide as W, Se and graphene precursors, respectively. Later, Zou et al.19,29 successively fabricated three-dimensional dendritic WSe2 on carbon nanofiber mats and WSe2 nanoflakes on carbon nanofibers via employing a tungsten contained PAN precursor by chemical vapor deposition for electrocatalytic hydrogen evolution. Among this method, the W tungsten contained PAN precursor was prepared by firstly adding (NH4)6H2W12O40 powder into freshly prepared PAN-dimethyl formamide solution to form a tungsten containing solution and then electrospunning into tungsten containing PAN nanofiber and even mats. In 2016, Zhang et al.30 synthesized nanostructured WSe2/C composites through a ball-milled powder mixture of elemental W, Se and carbon black by using solid-state reaction for the anode materials of sodium-ion batteries. However, the above-mentioned synthesis method to prepare the composites of carbon materials and WSe2 nanostructures all have intrinsic limitations: (i) the products prepared by solid-state 3



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reaction or solvothermal synthesis have low purity, because the by-products are difficult to remove; (ii) the deposition of WSe2 or W-contained precursors onto C fibers by vapor deposition is difficult to control and always of low productivity, and the preparation methods with such processes are usually complicated of multi-steps and sometimes dangerous of toxic precursors; and (iii) the WSe2 nanostructures in the composites prepared by solvothermal synthesis or low-temperature vapor deposition are of low crystallinity due to their low formation temperature. Therefore, finding a facile, low-cost method to prepare the composites of carbon materials and WSe2 nanostructures in large scale has been still desired. More importantly, no report concerns with the photocatalytic activity of WSe2/carbon composites. In this paper, a novel C fibers@WSe2 nanoplates core-shell composite (NPCSC) was successfully prepared via facile, one-step thermal evaporation, in which numerous WSe2 thin nanoplates were in-situ, densely and vertically grown on the surface of C fibers. And the photocatalytic performance of the obtained composite was evaluated by decomposing various organic aqueous dyes including methylene blue (MB) and rhodamine B (RhB), and highly harmful gaseous toluene. It was revealed that due to the excellent electrical conduction of C fibers, the separation of photogenerated electron-hole pairs during photocatalysis was effectively improved, thus dramatically enhancing the solar-driven photocatalytic activity of WSe2 for the degradation of these organic environmental pollutants under simulated sunlight irradiation (SSI). Comparing with commercially available WSe2 powder, the as-synthesized C fibers@WSe2 NPCSC presents significantly enhanced reaction rate constants by a factor of approximately 15, 9 and 3 for the degradation of aqueous MB, aqueous RhB, and gaseous toluene, respectively. Our study shows the great potential of the present C fibers@WSe2 NPCSC for environmental remediation by degrading toxic industrial chemicals using sunlight. Moreover, this one-step thermal evaporation is an easy-handling, environmentally friendly and low-cost synthesis method, which is suitable for large-scale production of WSe2/carbon composites and the similar. 4


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2. EXPERIMENTAL SECTION 2.1 Samples preparation. The reported C fibers@WSe2 NPCSC was synthesized via a novel, one-step thermal evaporation method in a horizontal quartz tube furnace with two temperature zones (namely here high-temperature zone, HT zone, and low-temperature zone, LT zone) working with separate heating element.31 After a series of experiments, an optimum process could be reached (see Figure S1 and S2 in Supporting Information). Typically, 2 g of commercially available reagent grade WO3 nano-powder was first dispersed in 5 ml absolute ethanol to form a W precursor suspension. Then, 0.15 g of preoxidized polyacrylonitrile (PAN) fibers with a length of roughly 3 cm was soaked in the freshly prepared W suspension for 10 min. After soaking, the PAN fibers were taken out from the suspension and dried quickly, obtaining a PAN@WO3 composite. After drying, 0.45 g of the obtained PAN@WO3 composite was loaded on a quartz plate, setting in the middle of the HT zone in the furnace, while an alumina boat with 2 g of reagent grade Se powder was located at the center of the LT zone on the upstream of carrier gas flow in the furnace. Before heating, the furnace was evacuated and flushed repeatedly with Ar gas for 3 times. Then, the HT zone was heated from room temperature to 400 ºC in 20 min, soaking at 400 ºC for 10 min. Subsequently, the HT zone was heated from 400 to 1000 ºC in 35 min and held at 1000 ºC for 1 h; meanwhile, the LT zone was heated from room temperature to 700 ºC in 35 min and kept at 700 ºC for 1 h. Throughout the whole heating process, Ar gas was introduced into the furnace at a constant rate of 100 sccm. After heating, the furnace was cooled naturally to room temperature. Finally, a large amount of dark-blue fiber-like products was collected from the quartz plate. For comparison, pure C fibers were also prepared by pyrolyzing preoxidized PAN fibers alone at 1000 °C for 1 h under pure Ar gas flow in the above tube furnace (see Figure S3). And a commercially available WSe2 powder (Aladdin, 99.95 wt.%) was used directly in the tests without any purification (see Figure S4). 2.2 Materials characterization. The phase composition of the samples was identified by X-ray diffraction 5



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(XRD, D/max-RB, Japan; Cu Kα radiation, λ=1.54185 Å) in a continuous scanning mode at a speed 8 °/min. The elaborate chemical structure was examined by Raman spectroscopy (Horiba Jobin Yvon LabRAM HR800, Japan; Nd-YAG laser, λex=532 nm). The morphology was observed by optical microscope (OM, Sony A580, Japan), field emission scanning electron microscope (FE-SEM, Quanta FEG-650, America), and transmission electron microscope (TEM, FEI Tecnai G2 F30, America) and high-resolution TEM (HRTEM) with selected area electron diffraction (SAED). The chemical composition was measured by an energy dispersive X-ray (EDX) spectroscope attached to the FE-SEM. The UV-vis absorption spectra were recorded on a Cary 5000 UV-vis spectrometer (Varian, Salt Lake City, Utah, America). The photoluminescence (PL) spectra were recorded at room temperature on a fluorescence spectrometer (Fluorolog-Tau-3, America) with a laser excitation wavelength of 532 nm. In addition, the specific surface area was measured by the Brunauer–Emmett–Teller (BET) N2 adsorption method using a Micromeritics ASAP2020 surface analyzer (see Figure S5). 2.3 Photoelectrochemical measurements. Electrochemical impedance spectroscopy (EIS) was performed on a CHI660E electrochemical workstation (Chenhua Instrument, Shanghai, China) through a two-electrode method when the working electrode was biased at a constant value of -0.2 V under sweeping a frequency from 1 MHz to 1 Hz. The transient photocurrent response was also examined through a two-electrode method by a Keithley 2410 source meter under the irradiation of a 300 W Xe lamp at room temperature while a voltage of 1 V was applied across the FTO-FTO electrodes. For both measurements, the working electrodes were prepared on the basis of a fluorine-doped tin oxide (FTO) glass, which was separated by a non-conducting gap into two conducting areas as the electrodes. For the C fibers@WSe2 NPCSC, 10 mg of the as-prepared products was laid across the gap, and pasted with conducting tapes onto the two FTO electrodes. For the commercially available WSe2 powder, 10 mg of the powder was dispersed in 0.5 ml ethanol and then dropped onto the surface of the gap between the two FTO electrodes. After that, all the samples were dried at room temperature for 1 h. Resultantly, a 6


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thin film of the C fibers@WSe2 NPCSC or the commercially available WSe2 powder was formed, covering the gap between the two FTO electrodes. The obtained FTO-FTO electrodes were directly used for the measurements. Mott-Schottky plots were recorded by the CHI660E electrochemical workstation in a standard three-electrode system at a frequency of 1 kHz with 0.2 M K2SO4 aqueous solution as the electrolyte, a wholly conducting FTO glass covered with the samples as the working electrode, an Ag/AgCl electrode (saturated KCl) as the reference electrode, and a Pt plate as the counter electrode, respectively. During the preparation of the working electrode from the C fibers@WSe2 NPCSC, 5 mg of the as-prepared products were cut into short fibers with a length of 1 cm, which, subsequently, were neatly arranged into a square of 1 cm2 and pasted on the FTO glass (1 cm × 2 cm) by using 10 µl of Nafion solution (0.3 wt.%). For the preparation of the working electrode from the commercially available WSe2 powder, 5 mg of the powder was firstly ultrasonically dispersed in a mixture solution from 200 µl ethanol and 10 µl of Nafion solution (0.3 wt.%). Then, the obtained suspension was dropped onto the FTO glass (1 cm × 2cm) into a square of 1 cm2. Finally, all the samples were dried at room temperature for 1 h. 2.4 Photocatalysis tests. The photocatalytic activity of the as-synthesized C fibers@WSe2 NPCSC was evaluated by decomposing aqueous MB, aqueous RhB, and gaseous toluene under SSI, respectively. For the photodegradation of MB or RhB, typically, 100 mg of the as-synthesized photocatalyst was immersed into 100 mL of the aqueous solution with a concentration of 5 mg/L in a 150 mL beaker. Subsequently, the beaker was placed in the dark for 60 min to reach the adsorption-desorption equilibrium (see Figure S6). Then the beaker was exposed to the SSI provided by a 500 W Xe lamp. For the measurement, throughout the whole testing, 2-3 ml supernatant samples were extracted regularly from the beaker at a reaction interval of 10 min and centrifuged to remove the dispersed photocatalyst in it. After centrifuging, the resultant clear solution was analyzed by an UV-vis spectroscope (SP-752). The absorbance, which is correspondent to the concentration of the dyes, was evaluated as 7



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a function of the irradiation time. After the measurement, the clear solution with the sediment from centrifuging was immediately poured back into the beaker so as to keep the photocatalytic reaction in almost the same state. In this case, for comparison, under the same condition the photocatalysis tests were also performed in the absence of any photocatalyst, over the pure C fibers (see Figure S3), and over the commercially available WSe2 powder (see Figure S4), respectively. About the photocatalytic degradation of toluene over the as-synthesized C fibers@WSe2 NPCSC, the tests were carried out in a glass reactor under SSI provided by a 500 W Xe lamp, in comparison with those over the commercially available WSe2 powder. Typically, the initial concentration of toluene gas was 22 mg/m3, and the applied amount of the catalysts was 45 mg. Prior to irradiation, the reactor was placed in the dark for 80 min, letting the reaction system reach the adsorption-desorption equilibrium (see Figure S7). After that, the Xe lamp was switched on, and the concentration of toluene was measured by a gas chromatograph (GC 2014, SHIMADZU, Japan) equipped with a flame ionization detector at an interval of 20 min. 2.5 Active species trapping experiments. To investigate the active species generated in the photocatalysis, by taking the photodegradation of MB and RhB as the examples, free radicals trapping experiments were performed by employing ethylenediaminetetraacetic acid disodium salt (EDTA), benzoquinone (BQ) and tertbutyl alcohol (TBA) as the scavengers of holes (h+), superoxide radicals (•O2−) and hydroxyl radicals (•OH), respectively.32 During the investigation, 1 mM of EDTA, BQ and TBA were added into the photocatalytic reaction system, respectively, while all the other experimental parameters were kept as those of the above-described photocatalytic tests.

3. RESULTS AND DISCUSSION 3.1 Composition and microstructure. The morphology and microstructures of the as-synthesized products are displayed in Figure 1. As can be seen from the OM photograph (Figure 1a) and low-magnification SEM 8


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Figure 1 Morphology and microstructures of the as-synthesized products. (a) Typical OM photograph. (b) Typical low-magnification SEM image. (c) Typical high-magnification SEM image, where the inset shows the EDX spectrum on the nanoplates. (d) Typical SEM image of a locally naked fiber, in which the inset exhibits the EDX spectrum on the fiber. (e) Low-magnification TEM and (f) HRTEM images of the WSe2 nanoplates grown on the as-synthesized products, where the inset displays the corresponding SAED pattern. image (Figure 1b), a large amount of fiber-like products could be prepared by the proposed facile, one-step thermal evaporation method. From the high-magnification SEM image (see Figure 1c), a remarkable feature can 9



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be observed on the sample that numerous nanoplates are growing densely and even vertically on the fiber to form a core-shell composite structure. The obtained composite structures have a diameter of 6~10 µm, in which the diameter of the fiber (core) is in the range of 5~7 µm and the nanoplates with a uniform thickness (shell) are lined up orderly on the surface of the fiber. According to the corresponding EDX spectra, the nanoplates consists of mainly W and Se atoms with very little of C atoms (see the inset in Figure 1c), and the fiber is composed of only C atoms (see Figure 1d). These results imply that the outer shell nanoplates are of tungsten selenide, and the inner core fiber is of elemental carbon. It should be noted that, due to the resolution limitation of the SEM EDX spectroscope, a small amount of C atoms was also detected on the tungsten selenide nanoplates, which might be stemming from the inner core carbon fiber where they are growing. Figure 1e shows typical TEM image of the WSe2 nanoplates peeled off from the as-synthesized products by ultrasonic oscillation in absolute ethanol. It can be seen that, the WSe2 nanoplates are of very thin thickness, having a lateral size from dozens of nanometers to several micrometers. By combination of the SEM observation, it was found that the morphology of the WSe2 nanoplates was dominated by hexagons with some irregular shapes. Moreover, the well-resolved periodic lattice fringe in the HRTEM image (Figure 1f) indicates that the spacing of the observed lattice plane is roughly 0.285 nm, which can be assigned to the (100) crystal plane of hexagonal WSe2. This result reveals that the WSe2 nanoplates in the as-synthesized products are well crystalline, which was further confirmed by their corresponding SAED patterns as displayed in the inset in Figure 1f. Figure 2 shows the XRD pattern and Raman spectrum of the as-synthesized products in comparison with those of commercially available WSe2 powder and pure carbon fibers prepared by heating PAN fibers alone under similar conditions. In the XRD pattern as shown in Figure 2a, all of the diffraction peaks of the as-synthesized products can be assigned to hexagonal WSe2 (JCPDS card no. 38-1388), which are also consistent with those of commercially available WSe2 powder, indicating that WSe2 nanoplates were successfully 10


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synthesized under the present conditions. Moreover, comparing with the XRD pattern of pure carbon fibers, a similar amorphous hump could be observed in the present products, suggesting that elemental carbon exists in them; but the hump shifts somewhat to a lower 2θ value, possibly owing to the micro-crystallization of partial carbon atoms in the sample. In the Raman spectrum as shown in Figure 2b, the two peaks of the E12g mode at 249 cm-1 and the A1g mode at 252 cm-1 for WSe2 33 are very much neighboring with each other, merging into just one visible peak for the products, which is also in agreement with that of the commercially available WSe2 powder. In addition, the two broad peaks located at 1358.33 and 1599.40 cm-1 could be assigned to the D and G bands of the core carbon fibers, respectively, which is similar with those of pure carbon fibers. All the above-presented results confirm that the as-synthesized products are C fibers@WSe2 NPCSC.

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Figure 2 XRD pattern (a) and Raman spectrum (b) of the as-synthesized products. For comparison, those of the commercially available WSe2 powder and pure carbon fibers prepared by heating PAN fibers alone under similar conditions were also presented. 3.2 Growth mechanism. According to our specific preparation conditions and the pyrolysis process of preoxidized PAN, a possible formation mechanism called in-situ “symplastic growth” can be used to explain the growth of the present C fibers@WSe2 NPCSC (see Figure 3). First, the preoxidized PAN fibers@WO3 nanoparticles composite was formed after the soaking of preoxidized PAN fibers in WO3 nanoparticles suspension. Second, during the heating period, with increasing temperature, the applied PAN fibers were 11



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gradually decomposed, releasing reductive NH3, CH4, HCN and CO gases;34 meanwhile, the Se vapor transported by Ar flow from the LT zone and the released reducing gases reacted with the WO3 nanoparticles on the surface of preoxidized PAN fibers, forming oxygen-deficient WO3-X nanocrystals (also see Figure S1 and S2). Third, the PAN fibers were further decomposed and carbonized under Ar gas, forming C fibers, while more reducing CO might be produced via the dissociation of PAN and the reaction between the freshly formed WO3-X and C fibers. At the same time, with more CO releasing continuously and the sustaining transportation of more Se vapor from the LT zone, the WO3-X nanocrystals were reduced more sufficiently and then selenized, finally forming WSe2 nanoplates. With the reaction proceeded, C fibers@WSe2 NPCSC was obtained.

Figure 3 Schematic of the formation mechanism of the as-synthesized C fibers@WSe2 NPCSC. The preoxidized PAN fibers play an important role in the preparation of the present C fibers@WSe2 NPCSC, which is correlated with the pyrolysis mechanism of preoxidized PAN fibers in inert atmosphere.34 The preoxidized PAN fibers have high carbonization temperature (above 900 °C) in inert atmosphere, and at such temperatures, the reducing gases, specially CO, could be produced continuously during the dissociation of PAN and the reaction between the freshly formed WO3-X and C fibers. Both the high temperature and reducing atmosphere are beneficial for the selenation of WO3. As a result, the C fibers and WSe2 nanoplates could be generated simultaneously at the same temperature (1000 °C). Moreover, the freshly generated C fibers may play a role in the nucleation site for the earliest growth of WSe2 nanoplates, while they would act as the substrate to 12


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support the WO3 nanoparticles over their surface for the subsequent construction of WSe2 nanoplates. And this loading mode is facilitating for the formation of the present core-shell composite structure. 3.3 Photocatalytic activity. The UV-vis absorption spectrum and corresponding plot of (αhν)2 versus photon energy of the C fibers@WSe2 NPCSC are presented in Figure 4. For comparison, those of the commercially available WSe2 powder are also displayed in this figure. As is seen from the UV-vis absorption spectra (Figure 4a), both samples exhibit an absorption edge at about 900 nm. From their corresponding plots of (αhν)2 versus photon energy (Figure 4b), it can be seen that the band gaps of the C fibers@WSe2 NPCSC and commercially available WSe2 powder are almost identical, presenting a value of 1.43 and 1.39 eV, respectively. The slight discrepancy between their band gaps may be caused by the different thicknesses of the WSe2 nanoplates in them, and implies that in the present C fibers@WSe2 NPCSC, WSe2 nanoplates are just standing on the surface of C fibers without any doping effect. Moreover, the narrow band gap of the C fibers@WSe2 NPCSC suggests that it could absorb a wide range of light from the near-infrared to visible region, as done by other pure WSe2 materials.

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Figure 4 UV-vis absorption spectrum (a) and corresponding plot of (αhν)2 versus photon energy (b) of the as-synthesized C fibers@WSe2 NPCSC in comparison with those of the commercially available WSe2 powder. The photocatalytic activity of the as-synthesized C fibers@WSe2 NPCSC was first evaluated by the decolorization effect of MB solution in the dark and under SSI. For comparison, the decolorization effects in the absence of photocatalyst, over the pure C fibers and over the commercially available WSe2 powder are also 13



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investigated. The decolorization effects of MB under the designed, various conditions are illustrated in Figure 5a as a function of irradiation time. As is seen from this figure (also see Figure S6a), in the absence of photocatalyst, MB aqueous solution had a very weak photodegradation effect under SSI for 70 min, indicating that MB is somewhat self-sensitized under light. When the pure C fibers (see details in Figure S3) were applied, it would lead to an adsorption of 7% MB in the dark for 60 min but also a very weak photodegradation effect under SSI for 70 min as observed from the test in the absence of photocatalyst, revealing that the pure C fibers have no catalytic activity for the photodegradation of MB. When the commercially bought WSe2 powder (consisting mainly of WSe2 nanoplates, see Figure S4) was used, under the same condition it would result in a little higher adsorption on MB (12%), but no significantly improved photodegredation effect could be observed, compared with the results over the pure C fibers and in the absence of photocatalyst. However, when the as-synthesized C fibers@WSe2 NPCSC was utilized as the catalyst, it would present a much stronger adsorption in the dark and a greatly enhanced photocatalytic degradation effect under SSI on MB. The adsorption in the dark on MB over the C fibers@WSe2 NPCSC was about 30%, and after SSI, the photocatalytic degradation on MB over the present C fibers@WSe2 NPCSC could reach 48.83%, while those in the absence of photocatalyst, over the pure C fibers and over the commercially available WSe2 powder were only 3.00%, 3.64% and 6.31%, respectively. In a word, the as-synthesized C fibers@WSe2 NPCSC can present a significantly high, applicable decolorization effect on aqueous MB under SSI, which pure WSe2 materials have never reached. To further investigate the photocatalystic activity of the present C fibers@WSe2 NPCSC, the reaction rate of the photocatalytic degradation on MB has been described with the pseudo-first-order reaction kinetics -In(C/C0)=kt,35 where k is the reaction rate constant of photodegradation on MB, and C and C0 are the initial and instant concentrations of MB as a function of the irradiation time t, as shown in Figure 5b. As is seen from Figure 5b, the reaction rate constants of the photodegradation reactions on MB in the absence of photocatalyst and over 14


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the pure C fibers under SSI were both very small (with a tiny value of 0.0005 and 0.0007 min-1, respectively), while that over the commercially available WSe2 powder under SSI was also an almost negligibly small value of 0.0011 min-1 for a semiconductor photocatalyst. However, when the as-synthesized C fibers@WSe2 NPCSC was utilized as the catalyst, the reaction rate constant was dramatically enhanced up to 0.0162 min-1, which is comparable with many of the already reported solar-driven semiconductors photocatalysts like N-doped TiO2, WO3, WS2 and MoS2,36-40 and about 15 times higher than that over the commercially available WSe2 powder.

(a)

(b)

(c)

(d)

Figure 5 (a) Decolorization effects on aqueous MB in the dark and under SSI over the as-synthesized C fibers@WSe2 NPCSC, and (b) the corresponding plot of -ln(C/C0) vs. time for the photocatalytic degradation. For comparison, those in the absence of photocatalyst, over the pure C fibers and over the commercially available WSe2 powder are also presented. (c) Recycling test on the decolorization of MB under SSI over the C fibers@WSe2 NPCSC. (d) Stability test on the decolorization of MB in the dark and under SSI over the C fibers@WSe2 NPCSC stored for 1 month in comparison with that over the fresh catalyst. 15



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Moreover, to accommodate for practical application, the recycling performance and storage stability of the C fibers@WSe2 NPCSC were also evaluated for the degradation on MB under SSI. As is seen from Figure 5c, after three cycles, the C fibers@WSe2 NPCSC still exhibits excellent photocatalytic performance on the degradation of MB under SSI, presenting very little reduced photocatalytic activity, indicating that such catalyst has a relatively high stability during photocatalytic application. The very little reduction in photocatalytic activity with the increasing cycling time might be resulted from the exfoliation and loss of a few WSe2 nanoplates from the C fibers@WSe2 NPCSC during the repeated washing and drying after each cycle of photocatalytic test. What’s more, the SEM image of the C fibers@WSe2 NPCSC catalyst after three cycles of MB photodegradation (see Figure S8a) reveals that the microstructure of the catalyst also has a good stability, presenting almost no change, implying that the present C fibers@WSe2 NPCSC catalyst would be easy to recycle. In addition, as is illustrated in Figure 5d, after being stored for 1 month, the photocatalytic activity of the C fibers@WSe2 NPCSC on the degradation of MB under SSI almost has no change in comparison with that of the freshly prepared catalyst. The totally decolorized MB by the catalyst stored for 1 month was 76.42%, almost equaling to that by the fresh one (77.02%). This result further reveals the excellent structural stability of the C fibers@WSe2 NPCSC photocatalyst for a long period of storage. The photocatalytic performance of the C fibers@WSe2 NPCSC was further confirmed by the decolorization of RhB under the same conditions. The results are displayed in Figure 6. It can be seen from this figure (also see Figure S6b) that, being different from aqueous MB, RhB aqueous solution is stable under SSI, presenting almost no photodegradation effect by only 1.42% of the RhB degraded (see Figure 6a) and with a reaction rate constant of 0.0001 min-1 (Figure 6b) under SSI for 70 min in the absence of photocatalyst. Over the pure C fibers (see Figure S3), an adsorption of 3.51% RhB in the dark for 60 min, and a negligible photodegradation effect by 3.51% of the RhB degraded (see Figure 6a) and with a reaction rate constant of 0.0006 min-1 (see Figure 6b) under SSI 16


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(a)

(b)

(c)

(d)

Figure 6 (a) Decolorization effects on aqueous RhB in the dark and under SSI over the as-synthesized C fibers@WSe2 NPCSC, and (b) the corresponding plot of -ln(C/C0) vs. time for the photocatalytic degradation. For comparison, those in the absence of photocatalyst, over the pure C fibers and over the commercially available WSe2 powder are also presented. (c) Recycling test on the decolorization of RhB under SSI over the C fibers@WSe2 NPCSC. (d) Stability test on the decolorization of RhB in the dark and under SSI over the C fibers@WSe2 NPCSC stored for 1 month in comparison with that over the fresh catalyst. for 70 min could be observed, revealing that the pure C fibers have no catalytic activity for the photodegradation of RhB, which is similar with those tests on MB. When the commercially available WSe2 powder (see Figure S4) was used, under the same condition, an absorption of 9.67% RhB in the dark, and a photodegradation effect by 6.54% of the RhB degraded (see Figure 6a) and with a reaction rate constant of 0.0010 min-1 (see Figure 6b) under SSI could be observed, also exhibiting an almost negligibly small degradation effect on RhB for a semiconductor photocatalyst. However, over the as-synthesized C fibers@WSe2 NPCSC, under the same condition, the 17



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adsorption in the dark on RhB was 40.95%, and after SSI, the photocatalytic degradation effect on RhB could reach 38.77%. And the corresponding photodegradation reaction rate constant was 0.0091 min-1, which is about 9 times higher than that over the commercially available WSe2 powder. Therefore, it can be concluded that the photocatalytic activity of WSe2 nanoplates was greatly enhanced after composting with C in the form of the present the C fibers@WSe2 NPCSC. In addition, as observed in the photocatalytic tests on MB, after recycling for three times (see Figure 6c), the C fibers@WSe2 NPCSC still exhibited excellent photocatalytic performance on the degradation of RhB under SSI, presenting very little reduced photocatalytic activity, indicating that such catalyst has a relatively high stability and easy recycling performance during photocatalytic application. And this recycling stability was also confirmed by the SEM image of the C fibers@WSe2 NPCSC catalyst after being applied in the photodegradation of aqueous RhB under SSI for recycling use (see Figure S8b). and the storage stability could be verified by the fact that, after storing for quite long time (here 1 month, see Figure 6d), the C fibers@WSe2 NPCSC would present an almost equal photocatalytic decolorization ability on RhB (79.72%) with that over that stored for 1 month (79.02%, see Figure 6d). In a word, the as-synthesized C fibers@WSe2 NPCSC can also present a significantly high, applicable decolorization effect on aqueous RhB under SSI. To extend the application of the as-synthesized C fibers@WSe2 NPCSC, its photocatalytic activity on the degradation of gaseous toluene in the dark and under SSI was also investigated in comparison with that of the commercially available WSe2 powder. As is seen from Figure 7a (also see Figure S7), after stewing in the dark for 80 min to reach the adsorption-desorption equilibrium, the removal efficiency on toluene by the as-synthesized C fibers@WSe2 NPCSC was 40.45%, which was much higher than that by the commercially available WSe2 powder (9.65%), due to its high surface area (see Figure S5). Then, after SSI for 160 min, the C fibers@WSe2 NPCSC could further degrade 32.95% of toluene, much higher than that degraded by the commercially available WSe2 powder (about 19.39%). And the reaction rate constant of the photodegradation reaction on gaseous toluene under 18


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SSI over the as-synthesized C fibers@WSe2 NPCSC was 0.0046 min-1, which is approximately 3 times higher than that over the commercially available WSe2 powder (0.0014 min-1), as shown in Figure 7b. In addition, the SEM image of the C fibers@WSe2 NPCSC catalyst after being applied in the photodegradation of gaseous toluene under SSI reveals that its microstructure was also stable for such reaction (see Figure S9). All these results indicate that the C fibers@WSe2 NPCSC is an excellent photocatalyt for the degradation of harmful organic gases like toluene.

(b)

(a)

Figure 7 (a) Removal effects on gaseous toluene in the dark and under SSI over the as-synthesized C fibers@WSe2 NPCSC. (b) The corresponding plot of -ln(C/C0) vs. time for the photocatalytic degradation. For comparison, those over the commercially available WSe2 powder are also presented. In summary, the present C fibers@WSe2 NPCSC would be a highly efficient solar-driven photocatalyst for practical application, showing the great potential for environmental remediation by degrading toxic industrial chemicals in both waste water and air using sunlight. 3.4 Photocatalytic mechanism. The charge separation was the most complex and key factor that essentially determines the photocatalytic efficiency of semiconductors photocatalysts.41 To probe into the transfer rate of the charge carriers in the present C fibers@WSe2 NPCSC, its EIS Nyquist plot, Mott-Schottky plot, transient photocurrent response curve and PL spectrum were measured. The results are shown in Figure 8. For comparison, those of the commercially available WSe2 powder or pure C fibers are also presented. 19



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As can be seen from Figure 8a, the arc radius of the semicircle in the high frequency region in the EIS Nyquist plot of the C fibers@WSe2 NPCSC is much smaller than that of the commercially available WSe2 powder, confirming that the present C fibers@WSe2 NPCSC has very much lower resistance and higher charge carriers transfer rate than the latter one. The highly conductive C fiber in the C fibers@WSe2 NPCSC should be responsible for this phenomenon, because it can facilitate the electron transfer promptly from the WSe2 nanoplates, thus decreasing the resistance of the sample and increasing the charge carriers transfer rate through it,42 which was further confirmed by Mott-Schottky measurement. As is seen from Figure 8b, both the present C fibers@WSe2 NPCSC and the commercially available WSe2 powder are of n-type conductivity because the slopes of their Mott-Schottky plots are positive. And more importantly, the present C fibers@WSe2 NPCSC presents a substantially smaller slope than that of the commercially available WSe2 powder, suggesting a faster charge transfer in the present C fibers@WSe2 NPCSC.43,44 Furthermore, the transient photocurrent-time curves (see Figure 8c) indicate that although C fibers have no contribution to the production of photogenerated electrons, the C fibers@WSe2 NPCSC can still present a much higher photocurrent than the commercially available WSe2 powder under the identical light irradiation, revealing that there is a more efficient charge carrier separation in the present C fibers@WSe2 NPCSC. This phenomenon could be attributed to the easy transfer of the photogenerated electrons as revealed by the EIS and Mott-Schottky measurements. Besides, as shown in Figure 8d, the PL emission intensity of the C fibers@WSe2 NPCSC was significantly decreased in comparison with that of the commercially available WSe2 powder. Because the PL emission intensity is correlated to the recombination of the excited electrons and holes, the weaker PL emission intensity of the C fibers@WSe2 NPCSC indicates its lower efficiency of electrons and holes recombination, further confirming the rapid transfer and separation of photo-generated carriers in the as-synthesized C fibers@WSe2 NPCSC. And all the above results are completely consistent with each other. 20


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(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 8 EIS Nyquist plot (a), Mott-Schottky plot (b), transient photocurrent-time curve (c), and PL spectrum (d) of the as-synthesized C fibers@WSe2 NPCSC. For comparison, those of the commercially available WSe2 powder 21



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or pure C fibers are also presented. The photocatalytic degradation processes on MB (e) and RhB (f) with the addition of different radical scavengers over the C fibers@WSe2 NPCSC under SSI. (g) Schematic of the photodegradation mechanism of organic pollutants under SSI over the as-synthesized C fibers@WSe2 NPCSC catalyst. To detect the active species during the photocatalytic reactions over the present C fibers@WSe2 NPCSC catalyst, by taking MB and RhB as the examples of degraded target, BQ, EDTA and TBA were also added into the photocatalytic systems, which would serve as the quencher of •O2−, h+ and •OH, respectively.32 The photocatalytic degradation processes on MB and RhB over the C fibers@WSe2 NPCSC under SSI after the addition of different radical scavengers are illustrated in Figure 8e and 8f, respectively. Obviously, when BQ was added, the photocatalytic activities of the C fibers@WSe2 NPCSC for the photodegradation of MB and RhB were reduced slightly, suggesting that •O2− might behave as the active species in the reaction systems during the photodegradation of MB and RhB, although the contribution of •O2− was relatively small. When TBA was added, the photocatalytic degradation effects on MB and RhB over the present catalyst had a marked decrease, revealing that •OH was also the active species in the reaction systems, presenting a substantial contribution to the photodegradation of MB and RhB. Moreover, with the addition of EDTA, the photocatalytic activities of the C fibers@WSe2 NPCSC were dramatically depressed, indicating that h+ was main active species in the reaction systems, which plays a dominant role for the photodegradation degradation of MB and RhB. Hence, it can be concluded that •O2−, •OH and h+ are all the active species in the photodegradation reactions of MB and RhB over the C fibers@WSe2 NPCSC, among which h+ is the dominant one. On the basis of the aforementioned results, the photocatalytic reaction mechanism for the degradation of organic pollutants under SSI over the present C fibers@WSe2 NPCSC catalyst can be proposed in Figure 8g. Under SSI, the electrons in the valence band (VB) of the semiconducting WSe2 nanoplates were excited, which 22


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would leap into the conduction band (CB). Correspondingly, holes (h+) emerged in the VB. Because the C fibers core has good electrical conduction, the photogenerated electrons on the CB of the present WSe2 nanoplates shell could be transferred rapidly to the C fibers core in the C fibers@WSe2 NPCSC, thus inhibiting the recombination of photogenerated electron-hole pairs. The photogenerated electrons could reduce the adsorbed O2 to yield •O2− radicals via the single electron reduction of molecule oxygen,45 while the holes in the VB of the WSe2 nanoplates could oxidize the surface adsorbed water or hydroxyl group to produce •OH radicals. The photogenerated •O2− and •OH radicals, together with the holes, would all contribute to the photocatalytic degradation of MB. Because the C fibers in the C fibers@WSe2 NPCSC promoted the effective separation of photogenerated electron-hole pairs, and thus improved the utilization of carriers to produce more active species, the photocatalytic efficiency of C fibers@WSe2 NPCSC on the degradation of organic pollutants such as aqueous MB, aqueous RhB and gaseous toluene applied in this study could be greatly enhanced. As for the better decolorization effects on aqueous MB and RhB, and the higher removal efficiencies on gaseous toluene in the dark over the present C fibers@WSe2 NPCSC than those over the commercially available WSe2 powder (also see Figure S6 and S7), it might be attributed to that the WSe2 nanoplates in the present catalyst are more dispersive and much thinner than those in the commercially available WSe2 powder (see Figures 1 and S1), presenting a larger contact area, and higher specific surface area due to the large amount of mesopores in it (see Figure S5). Therefore, the C fibers@WSe2 NPCSC possesses a better adsorption on the organic pollutants, such as aqueous MB, aqueous RhB and gaseous toluene.

4. CONCLUSIONS A novel C fibers@WSe2 NPCSC was successfully prepared via facile, one-step thermal evaporation, in which numerous WSe2 thin nanoplates were in-situ, densely and even vertically growing on the surface of the C fibers. Such composite exhibits significantly high solar-driven photocatalytic activity for the degradation of aqueous MB, 23



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aqueous RhB and gaseous toluene, being comparable with many of the already reported solar-driven semiconductors photocatalysts like N-doped TiO2, WO3, WS2 and MoS2. Comparing with the commercially available pure WSe2 powder, the as-synthesized C fibers@WSe2 NPCSC photocatalyst presented an increased reaction rate constant for the degradation of aqueous MB, aqueous RhB and gaseous toluene under SSI by a factor of approximately 15, 9 and 3, respectively, showing the great potential for environmental remediation by degrading toxic industrial chemicals in waste water and air using sunlight. The highly photocatalytic activity can be attributed to the efficient separation of photogenerated electron-hole pairs caused by the rapid transfer of photogenerated electrons through the C fibers. Moreover, the composite catalyst also presents high recycling performance and storage stability for the photodegradation of organic pollutants. In addition, this one-step thermal evaporation is an easy-handling, environmentally friendly and low-cost synthesis method, which is suitable for large-scale production. With all these merits, the present C fibers@WSe2 NPCSC would be a promising, highly efficient solar-driven photocatalyst for practical application.

ASSOCIATED CONTENT S Supporting Information ○

The Supporting Information is available free of charge on the ACS Publications website at DOI:

10.1021/acsami.xxxxxxx. Additional results, including data on the commercially available WSe2 powder, the pure C fibers, various

intermediate products and the C fibers@WSe2 NPCSC catalyst after being used (PDF).

AUTHOR INFORMATION Corresponding author †

Telephone: 86-10-82320255. Fax: 86-10-82322624. E-mail: [email protected] (Z Peng).

‡

Telephone: 86-10-62282242. Fax: 86-10-62282242. E-mail: [email protected] (X Fu). 24


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Author Contributions H.L. performed the experiments with technical support from the co-authors and obtained data representation. J.Q. and M.W. designed and carried out the active species trapping experiments. C.W. designed the experiments on the transient photocurrent response. Z.P. and X.F. supervised the whole work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support for this work from the National Natural Science Foundation of China (grant nos. 11674035, 11274052 and 61274015), the Fundamental Research Funds for the Central Universities, and Fund of State Key

Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications) is gratefully acknowledged.

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Table of Contents

C fibers@WSe2 nanoplates core-shell composite: highly efficient solar-driven photocatalyst Hong Li,

†,‡,§

‡

Zhijian Peng,*, Jingwen Qian,

†,‡,§

Meng Wang,

†,‡,§

†

Chengbiao Wang, and Xiuli Fu*,§

A novel C fibers@WSe2 nanoplates core-shell composite synthesized via facile, one-step thermal evaporation for highly efficient solar-driven photocatalyst

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C Fibers@WSe2 Nanoplates CoreâShell Composite: Highly Efficient

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