Nb2CTx MXene hybrid nanosheets with enhanced visible-light-driven photocatalytic activity for organic pollutants degradation

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Bi2WO6/Nb2CTx MXene hybrid nanosheets with enhanced visible-lightdriven photocatalytic activity for organic pollutants degradation ⁎

Ce Cuia, Ronghui Guoa, , Hongyan Xiaoa, Erhui Rena, Qingshuang Songa, Cheng Xianga, Xiaoxu Laia, Jianwu Lana, Shouxiang Jiangb a b

College of Light Industry, Textile and Food Engineering, Sichuan University, Chengdu 610065, China Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Nb2CTx MXene Bi2WO6/Nb2CTx hybrid nanosheets Photodegradation

The 2D/2D hybrid photocatalyst of ultrathin Bi2WO6/Nb2CTx hybrid nanosheets was synthesized via a facile hydrothermal process. The photocatalytic performance of Bi2WO6/Nb2CTx hybrid nanosheets for the degradation of rhodamine B (RhB), methylene blue (MB) and tetracycline hydrochloride (TC-HCl) were investigated. The results showed that the Nb2CTx nanosheets greatly improved the separation efficiency of photogenerated carriers and the photocatalytic activity of Bi2WO6. Compared with Bi2WO6, Bi2WO6/Nb2CTx hybrid nanosheets photocatalysts exhibited excellent photodegradation efficiency for RhB (99.8%), MB (92.7%) and TC-HCl (83.1%), respectively. The photodegradation rate constants of Bi2WO6 coupled with 2 wt% Nb2CTx for RhB and MB were 0.072 min−1 and 0.0285 min−1, which were 2.8 times and 2 times as high as that of pristine Bi2WO6, respectively. In addition, the photodegradation rate constant of Bi2WO6 coupled with 2 wt% Nb2CTx for TC-HCl reaches 0.0171 min−1. This study provided an efficient photocatalyst with 2D/2D structures and demonstrated that 2D Nb2CTx is a promising co-catalyst to improve the photodegradation performance of photocatalysts.

1. Introduction Currently, water pollution caused by organic pollutants is one of the major environmental challenges and has attracted increasing worldwide attention [1–4]. Especially, dyes and antibiotics as representative organic pollutants are the main source of water pollution. Dyes are generally derived from industrial wastewater from textile, pulp and paper, cosmetic, and food processing industries [2,5,6]. In addition, antibiotic residues are discharged into the aquatic environment from various sources such as the hospital sewage, pharmaceutical industry, and human and livestock excretion [7–10]. Improper discharge of wastewater containing these organic pollutants poses a serious threat to ecosystems and human health [11,12]. There are several techniques to remove organic pollutants from water such as adsorption, biodegradation and photocatalytic degradation. However, absorption cannot degrade the organic pollutants and is prone to secondary pollution. In addition, biodegradation is ineffective for most antibiotics organic pollutants in water environment. Currently, photocatalytic degradation based on semiconductor materials (TiO2, g-C3N4, Ag3PO4 and Bi2WO6) is a promising and green technology to treat the organic contaminants in water [13–17].

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Among the most promising photocatalysts, Bi2WO6 is one of the simplest aurivillius oxides and has recently gained widespread attention due to its low cost, excellent photo and chemical stability, nontoxicity and preeminent photoactivity [18–20]. Bi2WO6 has been used for hydrogen evolution and the photocatalytic degradation of organic contaminant under visible light irradiation [21,22]. However, the photocatalytic activity of Bi2WO6 is limited because of the low separation rate of photogenerated electron-hole pairs [23–25]. In order to improve the photocatalytic activity of Bi2WO6, several strategies for facilitating charge-carrier separation in Bi2WO6 have been proposed such as changing the morphology, doping precious metal, surface modification and compounding with other semiconductor or co-catalysts [6,26,27,28,29]. However, some methods limit their wide application and large-scale manufacturing due to high cost and harsh preparation conditions, such as the application of precious metals and carbon-based materials. Recently, transition-metal carbides and carbonitrides (MXenes) have been widely investigated since 2D Ti3C2Tx was reported in 2011 firstly [30]. MXenes have attracted intense attention in the field of transparent conductors [31], field effect transistors [32], supercapacitors [33], Li-ion batteries [34], electromagnetic interference

Corresponding author. E-mail address: [email protected] (R. Guo).

https://doi.org/10.1016/j.apsusc.2019.144595 Received 15 July 2019; Received in revised form 11 October 2019; Accepted 2 November 2019 Available online 05 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Ce Cui, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144595

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shielders [35,36], adsorption [37], photothermal conversion [38], hybrid nanocomposites [39] and ceramic biomaterials [40] because of their excellent electrical conductivity and special 2D structure. Recently, MXenes have been applied in photocatalysts field by serving as co-catalyts due to their excellent electrical conductivity and exposed metal sites [41–45]. Meanwhile, it is easy to construct a tight interaction between MXenes and various photocatalysts due to the large number of hydrophilic functional groups (eH and eO) on the surface of MXenes [16,41]. The advantages of 2D MXenes promote the transfer of charge carriers from the photocatalysts to the MXene and form a Schottky barrier on the photocatalysts/MXene interface, which is conducive to inhibit the recombination of electrons-holes [42,46,47]. Additionally, charge accumulation in MXenes cause a negative shift and alignment of the Fermi level thereby improving photocatalytic performance [48]. 2D niobium carbide (Nb2CTx), a novel MXene possesses lower Fermi level than Ti3C2Tx, was served as an excellent co-catalyst. Recently, Nb2CTx was predicated to be a promising co-catalyst of photocatalysts [42,46]. Ultrathin 2D nanosheet structure of photocatalysts facilitates the transfer of photogenerated carriers from their interior to the surface, which leads to high photocatalytic activity. In addition, the ultrathin 2D nanosheets can capture much ultraviolet–visible light due to large fraction of uncoordinated surface atoms [23]. However, there is no literature on 2D/2D structure of Bi2WO6/Nb2CTx construction for the photocatalytic performance. In this work, ultrathin 2D/2D hybrid Bi2WO6/Nb2CTx catalysts were synthesized by growth of Bi2WO6 on the surface of the Nb2CTx nanosheets. The photocatalytic activity of the Bi2WO6/Nb2CTx hybrid nanosheets was evaluated by the degradation of organic contaminants under visible light irradiation.

Fig. 1. Schematic Nb2CTxnanosheets.

illustration

of

the

synthetic

process

of

Bi2WO6/

labeled as BN-0.5, BN-2, BN-5 and BN-10, respectively. For comparison, the pristine Bi2WO6 nanosheets were prepared. The detail preparation procedures of Nb2CTx and Bi2WO6/Nb2CTx are illustrated in Fig. 1. 2.3. Characterization The morphologies of the Nb2CTx, Bi2WO6 and Bi2WO6/Nb2CTx hybrid nanosheets were observed by a field emission scanning electron microscopy (SEM) (JSM-5900LV) and transmittance electron microscopy (TEM) (Tecnai G-F20). The XRD spectra were tested on an X-ray diffractometer using Cu Kα radiation at 40 kV and 40 mA (λ = 1.54 Å). X-ray photoelectron spectroscopy XPS (Thermo Scientific Escalab 250Xi) measurements were used to determine the chemical composition of the Nb2CTx, Bi2WO6 and Bi2WO6/Nb2CTx hybrid nanosheets. Raman spectra were obtained by Andor SR-500i. The specific surface areas and the nitrogen adsorption/desorption isotherms were obtained on a Gemini VII 2390 nitrogen adsorption apparatus. The lateral size and zeta potential of Nb2CTx were measured using dynamic light scattering (DLS) on the colloidal solution to confirm the particle size (Zetasizer Nano ZSP (ZEN5600)). UV–vis diffuse reflectance spectra (DRS) were carried out on a UV–visible spectrophotometer (UV-2700, Shimadzu, Japan). The photoluminescence (PL) spectra and time-resolved photoluminescence (TR-PL) decay spectra with the excitation wavelength of 325 nm were obtained on a FLS1000 fluorescence lifetime spectrophotometer (Edinburgh Instruments, UK).

2. Experimental 2.1. Materials and reagents Bismuth nitrate penthydrate (Bi(NO3)3·5H2O), sodium tungstate dihydrate (Na2WO4·2H2O), hexadecyltrimethylammonium bromide (CTAB), tetramethylammonium hydroxide (TMAOH), hydrofluoricacid (HF, content ≥ 40.0%), methylene blue (MB), rhodamine B (RhB), tetracycline hydrochloride (TC-HCl), EDTA-Na2, benzoquinone and isopropanol were purchased from Chengdu Kelong chemical reagent factory Co., Ltd., China. Nb2AlC (200 M) was purchased from Forsman Scientific (Beijing) Co., Ltd. All the reagents were of analytical grade and used without further purification. 2.2. Synthesis of Nb2CTx nanosheets and Bi2WO6/Nb2CTx hybrid nanosheets 1 g Nb2AlC was added into 60 mL HF solution (content ≥ 40.0%) and stirred at 55 °C for 90 h. The prepared solids were washed several times with deionized water for until neutral and then dried. The obtained sample was re-dispersed in 10 mL TMAOH (26%) at room temperature for 3 days. The suspension was then centrifuged and washed in deionized water to remove the residual TMAOH. After that, the obtained solid was re-dispersed in deionized water and sonicated for 1 h. After ultrasonication, the dispersion of Nb2CTx nanosheets was obtained by low-speed centrifugation. For preparation of Bi2WO6/Nb2CTx hybrid nanosheets, 1 mmol Na2WO4·2H2O and 0.05 g CTAB were dissolved in 80 mL deionized water and stirred until a clear solution. The mixed solution of 2 mmol Bi (NO3)3·5H2O and Nb2CTx nanosheets with different mass ratio was then added to the above solution. The suspension was magnetically stirred for 1 h to complete the precipitation reaction. The suspension was then transferred into a 200 mL Teflon-lined autoclave and kept at 120 °C for 24 h. The obtained products were thoroughly washed with deionized water and ethanol, and dried in vacuum oven at 60 °C. The obtained samples with 0.5 wt%, 2 wt%, 5 wt% and 10 wt% of Nb2CTx were

2.4. Photoelectrochemical measurements Transient photocurrent responses and elecrtrochemical impedance spectroscopy (EIS) of Bi2WO6 and Bi2WO6/Nb2CTx hybrid nanosheets was performed with an electrochemical workstation (CHI 660E Instruments Inc., Shanghai) in a standard three-electrode system. The samples coated on ITO conductive glass Ag/AgCl and platinum foil were served as the working electrode, the reference electrode and the counter electrode, respectively. 0.2 mol·L−1 Na2SO4 aqueous solution was served as the electrolyte solution. The working electrodes were prepared by using Nafion as the adhesive. 2.5. Photocatalytic degradation measurement The photodegradation of RhB, MB and TC-HCl in the presence of asprepared Bi2WO6 and Bi2WO6/Nb2CTx hybrid nanosheets were 2

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Fig. 2. (a) SEM image of multilayer Nb2CTx, (b) TEM image of single-layer Nb2CTx, SEM images of (c) Bi2WO6 and (d) BN-2 and (e) TEM image of BN-2 and (f) the enlarged area of the red circle.

Fig. 2a shows SEM image of HF-etched Nb2CTx from Nb2AlC bulk solid. The accordion-like multilayer structure is consistent with the representative MXene morphology. TEM image reveals ultrathin flakes of exfoliated Nb2CTx nanosheets after TMAOH intercalation (Fig. 2b), which exhibit the typical sheet-like morphology. In addition, the particle size distribution analysis and zeta potential analysis of Nb2CTx nanosheets are illustrated in Fig. S1. The results indicate that the size of Nb2CTx nanosheets was changed from 100 nm to 700 nm and Nb2CTx shows negative potential (−20.4 mV, pH = 7). Therefore, the Bi3+ can be easily absorbed to the surface of Nb2CTx nanosheets, which provided the basis for the fabrication of Bi2WO6/Nb2CTx hybrid materials. Pristine Bi2WO6 exhibits irregular thickness as shown in Fig. 2c. In contrast, the Bi2WO6/Nb2CTx shows a flower-like structure with obviously

performed in a glass vessel under visible-light irradiation (λ > 420 nm) using a 500 W Xe lamp (GXZ500, China). 50 mg of the photocatalyst was added to 100 mL organic contaminant (15 mg·L−1) solution. The dispersion was stirred in the dark for 20 min before photocatalytic reaction. 3 mL of the suspension was collected at given time intervals and separated by centrifugation. The residual concentration was analyzed by measuring the maximum absorbance using a 4802 doublebeam UV–vis spectrophotometer. 3. Results and discussion The morphologies of the Nb2CTx, Bi2WO6 and Bi2WO6/Nb2CTx hybrid nanosheets were investigated by SEM and HRTEM techniques. 3

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Bi2WO6 and Nb2CTx are clearly observed in the XPS spectrum of NB-2. In addition, the contents of Nb, Bi and W elements are 23.09%, 10.37% and 0.33%, respectively. The ratio of Bi to W is close to 2:1, indicating that the surface layer of BN-2 is mainly Bi2WO6. The result shows that the Bi2WO6 is grown on the surface of the Nb2CTx. The results are consistent with the characterization of XRD and SEM. The high-resolution C 1s spectra of Nb2CTx, Bi2WO6 and BN-2 are presented in Fig. 4b. The peak of Bi2WO6 located at 286.0 eV corresponds to C-O, which is derived from organic contaminant adsorbed on the surface of the sample [41]. Four peaks at 288.0, 286.2, 284.8 and 281.4 eV can be deconvoluted into the C 1s pattern of Nb2CTx, which are ascribed to OeC]O, CeO, CeC and CeNb bonds, respectively [50]. Compared with the C 1s spectrum of Nb2CTx, the peaks at 286.1 and 281.2 correspond to CeO and CeNb bonds after the combination of Nb2CTx with Bi2WO6. It can be found the binding energies of CeNb and CeO in NB-2 exhibit negative shift of 0.2 eV. Compared with the Bi 4f spectrum of Bi2WO6 as shown in Fig. 4c, the Bi 4f spectrum of BN-2 shows an opposite trend. The binding energies of Bi 4f5/2 and Bi 4f7/2 in Bi2WO6 are located at 164.4 and 159.1 eV, respectively. However, the binding energy of Bi 4f of BN-2 shifts to higher positions (164.5 and 159.2 eV). Compared with Bi2WO6, the spectra of W 4f in BN-2 (Fig. 4d) exhibit similar positive shift. The results indicate that the interaction between Nb2CTx and Bi2WO6. The Nb 3d region of Nb2CTx could be deconvoluted into three pairs of Gaussian-Lorentzian curves as shown in Fig. 4e. Each pair of 3d3/2 and 3d5/2 components is assigned to Nb-C, Nb (V) and Nb-CxOy [42,50]. Compared with the pristine Nb2CTx, the binding energies of Nb 3d in BN-2 exhibit a positive shift as shown in Fig. 4e. The results indicate the electrons are transferred from Bi2WO6 to Nb2CTx surface in the BN-2 sample and a strong interfacial interaction between Nb2CTx and Bi2WO6 was established [41]. The Raman spectra of the Bi2WO6 and BN-2 were further conducted to investigate the structure of Bi2WO6 and BN-2 as shows in Fig. 4f. The peaks at 305 cm−1, 418 cm−1, 720 cm−1, 796 cm−1 and 822 cm−1 are typical Raman peaks of pristine Bi2WO6 [24]. Obviously, three additional peaks at 575, 1462 and 1524 cm−1 corresponding to Nb2CTx can be seen from the Raman spectrum of BN-2 [42]. The results confirm the presence of Nb2CTx in Bi2WO6/Nb2CTx hybrids and strong interaction between Nb2CTx and Bi2WO6. The specific surface areas and pore size distributions of the pristine Bi2WO6 and BN-2 were tested using nitrogen adsorption-desorption analysis, and the results are illustrated in Fig. 5. The nitrogen adsorption-desorption isotherms of both the samples belong to Type IV isotherms, signifying the existence of mesopores and macropores [23]. BN2 possesses smaller pore diameter than that of Bi2WO6 as shown in the inset. The BET surface area of BN-2 is higher (42.9 m2·g−1) than that of pristine Bi2WO6 (32.7 m2·g−1) because the introduction of Nb2CTx reduced the thickness of Bi2WO6 and many small pores were formed among the Bi2WO6 nanosheets. A high BET surface area and porous structure could expose more active sites and absorbed more reactant

uniform and thin nanosheets (Fig. 2d and Fig. S2). The results indicates that Bi2WO6 nanosheets have been tightly associated with 2D Nb2CTx nanosheets, and the thickness of Bi2WO6 nanosheets is restrained by the growth limiting effect of Nb2CTx. The special flower-like structure leads to an increase of the specific surface area of Bi2WO6/Nb2CTx hybrid materials, thereby promoting more active sites to be exposed to visible light. The element mappings of BN-2 are shown in Fig. S3. The colorful images represent the distribution of C, O, Nb, Bi and W elements, and all the elements are uniformly distributed. The interface between Bi2WO6 and Nb2CTx could be obviously observed from TEM image (Fig. 2e) and the corresponding high-resolution transmission electron microscopy (HRTEM) image is shown in Fig. 2f. The inter-planar spacings of 0.273 and 0.271 nm are clearly observed and correspond to (2 0 0) and (0 0 2) crystallographic planes of Bi2WO6 [25]. In addition, the lattice spacing of 0.271 nm is assigned to (0 4 2) crystal plane of Nb2CTx [49]. The results indicate that the 2D Bi2WO6 has been successfully coupled with 2D Nb2CTx nanosheets and thinner nanosheets of Bi2WO6/Nb2CTx than pristine Bi2WO6 inhibit the recombination of electrons and holes due to the short migration distance. The crystal structure of as-prepared Nb2CTx, Bi2WO6 and Bi2WO6/ Nb2CTx hybrids were investigated by X-ray diffraction (XRD) and the results are shown in Figs. 3a and 3b. The Nb2CTx peak intensities originated from the parent Nb2AlC bulk decrease after HF treatment and TMAOH intercalation as shown in Fig. 3a. For Nb2CTx nanosheets, the peak of Nb2AlC at ≈39° disappeared because of the exfoliation of aluminum [50,51]. In addition, the (0 0 2) peak of Nb2CTx nanosheets was broadened and downshifted remarkably toward a lower 2θ angle of ≈7.7° corresponding to a c lattice parameter [52]. The low-angle (0 0 2) peak of XRD pattern of Nb2CTx is typical for MXene, which implies that Nb2AlC has been completely converted to Nb2CTx. Fig. 3b shows the XRD patterns of pristine Bi2WO6 and Bi2WO6/Nb2CTx hybrid nanosheets with various content of Nb2CTx. The distinct diffraction peaks of all the samples located at 28.3°, 32.8°, 47.1°, 56.0° and 58.5° correspond to the (1 3 1), (2 0 0), (2 6 0), (3 3 1) and (2 6 2) crystal planes of orthorhombic Bi2WO6, respectively [23]. Compared with pristine Bi2WO6, intensities of all the diffraction peaks of Bi2WO6/ Nb2CTx hybrid nanosheets were obviously broadened and decreased with the rise of content of Nb2CTx. This phenomenon can be explained by the fact that the growth limiting effect of Nb2CTx restrains the growth of Bi2WO6 grains. The Bi2WO6/Nb2CTx hybrid nanosheets are thinner than pristine Bi2WO6, which is in accordance with the results of SEM and TEM. No obvious diffraction peaks of Nb2CTx can be observed due to the low content of Nb2CTx in hybrids. The chemical states of the as-prepared samples were tested by X-ray photoelectron spectroscopy (XPS). Fig. 4a shows the XPS survey spectra of Nb2CTx, Bi2WO6 and BN-2. The result of the survey XPS spectrum of Nb2CTx shows that the Nb2CTx is mainly composed of Nb, C and O elements, which demonstrates that the Al element has been thoroughly etched from Nb2AlC bulk. The elements (Bi, W, O, C and Nb) related to

Fig. 3. XRD patterns of (a) Nb2AlC and Nb2CTx, and (b) Bi2WO6 and Bi2WO6/Nb2CTx hybrid materials. 4

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Fig. 4. (a) XPS spectra of Bi2WO6, Nb2CTx and BN-2; (b) high-resolution XPS spectra of C 1s of Bi2WO6, Nb2CTx and BN-2; High-resolution XPS spectra of Bi 4f (c) and W 4f (d) of Bi2WO6 and BN-2; (e) high-resolution XPS spectra of Nb 3d of Nb2CTx and BN-2; (f) Raman spectra of Bi2WO6 and BN-2.

molecules, resulting in higher photocatalytic performance [23]. The light absorption of Nb2CTx, Bi2WO6 and BN-2 was analyzed by UV–vis diffuse reflectance spectroscopy (DRS). Fig. 6a shows that the pristine Bi2WO6 and BN-2 exhibits significant absorption at wavelengths less than 450 nm. The absorption intensity of BN-2 increases in the wavelength range of 420-800 nm due to the full-spectrum absorption of Nb2CTx. Although enhanced photoabsorption in the range cannot induce the charge carriers in Bi2WO6, the presence of Nb2CTx can promote local surface catalytic reactions of the catalyst due to the absorption of light of Nb2CTx [50]. Furthermore, the band gap of pristine Bi2WO6 is determined from transformed DRS to be 2.94 eV (Fig. 6b). Compared with bulk Bi2WO6 (2.7 eV) [53], the increased band gap of as-prepared Bi2WO6 is ascribed to quantum confinement effect [24,54,55]. In order to investigate the separation and transfer behavior of photoinduced carriers, photoluminscence (PL) and timeresolved photoluminescence (TR-PL) decay spectra of Bi2WO6 and BN-2 were tested. PL emission intensity of the BN-2 sample is lower than that of pristine Bi2WO6 as shown in Fig. 6c, which suggests that BN-2 has lower recombination rates of photogenerated charge carriers [35]. The decay curves were fitted by a biexponential function as shown in

Fig. 5. N2 adsorption–desorption isotherms and pore size distributions of Bi2WO6 and BN-2 in the inset.

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Fig. 6. (a) UV–vis diffuse reflectance spectra (DRS) of Nb2CTx, Bi2WO6 and BN-2, (b) band gap of Bi2WO6, (c) PL spectra and (d) TR-PL decay spectra of Bi2WO6 and BN-2.

interfaces [37]. The results suggest that the introduction of Nb2CTx nanosheet can remarkably improve the carrier separation efficiency of Bi2WO6. In order to evaluate the photocatalytic performance of the Bi2WO6 and Bi2WO6/Nb2CTx hybrid nanosheets, the photodegradation of RhB and MB were investigated. The adsorption analysis of Bi2WO6 and Bi2WO6/Nb2CTx hybrid nanosheets for dyes was carried out in the dark before the photodegradation process. It can be seen that the adsorption efficiencies of Bi2WO6, BN-0.5, BN-2, BN-5 and BN-10 are 47.2%, 54.5%, 64.5%, 61.2%, and 58.3% for RhB (Fig. 8a) and 41.9%, 68.5%, 68.9%, 71.4% and 63.8% for MB (Fig. 8d), respectively. The results indicate that Bi2WO6/Nb2CTx hybrid nanosheets exhibit adsorption capacity for RhB and MB. The photodegradation of Bi2WO6 and Bi2WO6/Nb2CTx hybrid nanosheets for RhB and MB are presented in Figs. 8b and 8e, respectively. The blank experiment without the photocatalysts proves the RhB and MB can be hardly degraded, indicating that the direct photolysis of RhB and MB under visible light irradiation can be neglected. The photodegradation efficiencies of RhB and MB are

Fig. 6d. The lifetime of BN-2 (154.76 ns) is much longer than that of the pristine Bi2WO6 (135.14 ns), which indicates that the separation efficiency of the photogenerated electron-hole pairs is improved. The photocatalytic activity of Bi2WO6/Nb2CTx hybrid material can be enhanced by inhibiting the recombination of charge carriers effectively. The separation and transfer behaviors of photogenerated charge carriers were also investigated by the photoelectrochemical properties. The transient photocurrent responses of pristine Bi2WO6 and BN-2 with loading 2 wt% Nb2CTx were measured and the results are illustrated in Fig. 7a. As expected, the photocurrent response of the BN-2 is significantly higher than that of pristine Bi2WO6, indicating an enhanced separation of carriers Bi2WO6/Nb2CTx hybrid material. In addition, electrochemical impedance spectroscopy (EIS) of Bi2WO6 and BN-2 were carried out to investigate the behavior of electron separation and transfer. The result of EIS Nyquist plot shows that the semicircle radius of BN-2 is smaller than the pristine Bi2WO6 (Fig. 7b), indicating that the charge transfer resistance is lower, thereby there are more effectively separating photoelectron-hole pairs and transferring of charge between

Fig. 7. (a) Transient photocurrent and (b) EIS Nyquist plots of Bi2WO6 and BN-2. 6

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Fig. 8. (a, d) The absorption of RhB and MB in the presence of Bi2WO6, BN-0.5, BN-2, BN-5 and BN-10 in the dark, respectively, (b, e) photodegradation of RhB and MB in the presence of Bi2WO6, BN-0.5, BN-2, BN-5 and BN-10 under visible light irradiation, respectively, (c, f) the corresponding first-order kinetic plots of RhB and MB degradation, respectively.

visible light irradiation. The blank experiment without the photocatalysts shows that the concentration of TC-HCl has negligible change and the photolysis of TC-HCl can be neglected during the visible light irradiation. The Bi2WO6/Nb2CTx hybrid materials exhibit better photocatalytic degradation performance than pristine Bi2WO6 for TC-HCl. The photodegradation rates of Bi2WO6, BN-0.5, BN-2, BN-5 and BN-10 were 74.5%, 78.1%, 83.1%, 81.7% and 78.9% under the visible-light irradiation for 120 min, respectively. The photodegradation efficiency for TC-HCl increases with the rise of the amount of Nb2CTx. The photodegradation efficiency reached maximum in the presence of BN-2 photocatalyst. Photodegradation efficiency decreases when the amount of Nb2CTx is further increased because the excessive Nb2CTx covers the active sites of Bi2WO6. In addition, the corresponding absorption spectra of TC-HCl after irradiation for 120 min are shown in Fig. S5. Compared with the initial TC-HCl solution the intensity of absorption peaks of TC-HCl solution decreases obviously after photodegradation, indicating the TC-HCl molecular structure has been broken up. The kinetic curves for photodegradation of TC-HCl are presented in Fig. 9c. The rate constant (k) value of pristine Bi2WO6 and BN-2 are 0.0116 min−1 and 0.0171 min−1, respectively. The enhanced photodegradation efficiency of BN-2 can be explained by the fact that Bi2WO6/Nb2CTx hybrid material can provide more abundant active sites and adsorb more reactant molecules due to its high surface area and the effective separation of photogenerated electron-hole pairs. Some typical photocatalyst composites for RhB, MB or TC-HCl degradation reported in recent years are summarized in Table 1. Compared with these typical photocatalysts, the Bi2WO6/Nb2CTx hybrid nanosheets in this work show higher photocatalytic performance for RhB, MB or TC-HCl in consideration of the shorter photodegradation time, relatively low concentration of photocatalyst and high rate constant. To evaluate the main active oxygen species of BN-2 hybrid materials in the photocatalytic process, radicals trapping experiments were performed. Three scavengers, ethylenediamine tetra acetic acid disodium salt (EDTA-Na2, 0.01 mol·L−1), p-benzoquinone (BQ, 0.002 mol·L−1) and isopropanol (IPA, 0.01 mol·L−1) were added in the photodegradation of RhB as quenchers of h+, %O2− radical and %OH radical,

90.1% and 71.1% in the presence of pristine Bi2WO6 for 90 min, respectively (Figs. 8b and 8e). Obviously, the photocatalytic activity of the Bi2WO6/Nb2CTx hybrid materials is significantly enhanced after the Nb2CTx nanosheets are introduced. The photocatalytic activity of Bi2WO6/Nb2CTx hybrid materials is increased with the rise of content of Nb2CTx nanosheets and sample BN-2 shows the highest photocatalytic efficiency with 99.8% and 92.7% for RhB and MB, respectively. As the amount of Nb2CTx nanosheets increases, the Bi2WO6/Nb2CTx hybrid materials form large contact interface, which enhances separation and transfer of photoinduced charge carriers from Bi2WO6 to Nb2CTx and leads to high photodegradation efficiency. Although the modification of Nb2CTx to Bi2WO6 facilitates the charge transfer from Bi2WO6 to Nb2CTx, but the photodegradation efficiency of Bi2WO6/Nb2CTx hybrid nanosheets materials decreases when the Nb2CTx nanosheets content is higher than 2 wt%. The result indicates that optimization of the ratio of Nb2CTx and Bi2WO6 could enhance the photodegradation efficiency of Bi2WO6/Nb2CTx hybrid materials. The corresponding absorption spectra of Bi2WO6 and Bi2WO6/Nb2CTx hybrid materials for RhB and MB under visible-light irradiation for 90 min are illustrated in Figs. S4a and S4b, respectively. To further clarify the photocatalytic activity of Bi2WO6/Nb2CTx hybrid materials, the photocatalytic degradation kinetics of RhB or MB under visible light irradiation were investigated. Figs. 8c and 8f show the pseudo-first-order fitting curves of photodegradation of RhB and MB in the presence of Bi2WO6 and Bi2WO6/ Nb2CTx hybrid materials, respectively. Fig. 8c shows that BN-2 has the rate constant (k) of 0.072 min−1, which is 2.8 times as high as pristine Bi2WO6 (0.026 min−1). Fig. 8f shows that the rate constant (k) for BN-2 is 2 times as high as pristine Bi2WO6. The results demonstrate that the Nb2CTx plays an important role in improving the photocatalytic performance of Bi2WO6 under the visible light irradiation. A typical organic pollutant tetracycline hydrochloride (TC-HCl) was also employed to investigate the photocatalytic activity of as-prepared catalysts. The absorption of TC-HCl reaches absorption equilibrium for 20 min as shown in Fig. 9a. It can be seen that TC-HCl with 4.8%, 14.4%, 20.8%, 18.8% and 8.6% contents are adsorbed by Bi2WO6, BN0.5, BN-2, BN-5 and BN-10, respectively. Fig. 9b shows the photocatalytic activities of Bi2WO6, BN-0.5, BN-2, BN-5 and BN-10 under 7

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Fig. 9. (a) The absorption of TC-HCl in the presence of Bi2WO6, BN-0.5, BN-2, BN-5 and BN-10 in the dark, (b) photodegradation of TC-HCl in the presence of Bi2WO6, BN-0.5, BN-2, BN-5 and BN-10 under visible light irradiation, (c) the corresponding first-order kinetic plots of TC-HCl degradation.

4. Conclusion

respectively. The photocatalytic degradation of RhB was obviously inhibited after the EDTA-Na2 was added as shown Fig. 10. The result reveals that the h+ plays a significant role in the photocatalysis process. The photodegradation efficiency decreased obviously after the %O2− and %OH scavengers were added, respectively, which implied that the % OH and %O2− are also the main active species. The free radicals trapping experiments indicate that all of h+, %OH and %O2− participate in the degradation of RhB. The enhanced photocatalytic mechanism of Bi2WO6/Nb2CTx hybrid nanosheets is illustrated in Fig. 11. Bi2WO6 is excited to produce electrons (e−) and holes (h+) under visible light irradiation, and the photogenerated electrons and holes are then accumulated on the conduction band (CB) and valance band (VB) of Bi2WO6, respectively [42]. Since the CB potential is more negative than the Fermi energy levels (Ef) of Nb2CTx at the interface of Bi2WO6 and Nb2CTx, electrons flow from Bi2WO6 to Nb2CTx to align the Fermi energy levels. RhB can be degraded by %O2− radicals, which are formed by the combination of electrons with O2 adsorbed on the surfaces of Nb2CTx, and %OH was produced via the reduction of %O2−.The holes (h+), generated from VB band of Bi2WO6 further oxidize H2O to form %OH radicals, and %OH radicals degrade RhB. In addition, h+ can directly oxidize RhB. As electrons accumulate in Nb2CTx and positive charges accumulate in Bi2WO6, a space charge layer is formed, and the CB and VB of Bi2WO6 bent “upward”. A Schottky junction is formed at the interface of Bi2WO6 due to the presence of the space charge layer, and the interface prevents electrons from being transferred back to Bi2WO6 [47]. Therefore, the Schottky junction can be acted as an electron trap to efficiently capture the photoinduced electrons and extend the lifetime of electrons-hole recombination in Bi2WO6, and this significantly improves the photocatalytic activity of Bi2WO6.

Bi2WO6/Nb2CTx hybrid nanosheet photocatalysts were successfully synthesized by a facile hydrothermal process. The structures and morphologies of Bi2WO6/Nb2CTx suggest that the intimate interaction was constructed between Bi2WO6 and Nb2CTx. The photogenerated electrons can be transferred from the conduction band of Bi2WO6 to Nb2CTx and Nb2CTx can be regarded as an electron capture trap. Furthermore, a Schottky junction is formed at the Bi2WO6/Nb2CTx interface, which suppresses the recombination of photogenerated electron-hole pairs. The Bi2WO6/Nb2CTx hybrid nanosheets with higher special surface area than pristine Bi2WO6 exhibit remarkably enhanced photocatalytic performance for RhB, MB and TC-HCl degradation under the visible light irradiation. In addition, the photodegradation rate constant of BN-2 for RhB and MB is 2.8 times and 2 times as high as pristine Bi2WO6, respectively.

Declaration of Competing Interest The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements This work was financially supported by The National Natural Science Foundation of China and The Civil Aviation Administration of China (No. U1833118).

Table 1 Comparation of the photocatalytic performance of photocatalyst composites. photocatalyst

Dye

Photodegradation rate

Concentration of photocatalyst (g·L−1)

Catalytic rate constant (min−1)

Ref.

g-C3N4/Bi2WO6 Pt-decorated g-C3N4/Bi2WO6 CQDs/Bi2WO6 Fe3O4/TiO2/g-C3N4 Ti-doped g-C3N4 BiFeO3-Bi2WO6 C- Bi2WO6 Bi2WO6/MCNOs Bi2WO6/BiOBr Fe3O4/Bi2WO6 CuInS2/Bi2WO6 mes-Fe3O4/g-C3N4 γ-Fe2O3/g-C3N4 mes-Sn3O4/g-C3N4 Bi2WO6/Nb2CTx

RhB 10 mg·L−1 RhB 10 mg·L−1 RhB 10 mg·L−1 RhB 20 mg·L−1 RhB 9.58 mg·L−1 MB 6.39 mg·L−1 MB 3.2 mg·L−1 MB 10 mg·L−1 MB 10 mg·L−1 MB 10 mg·L−1 TC-HCl 10 mg·L−1 TC-HCl 10 mg·L−1 TC-HCl 10 mg·L−1 TC-HCl 10 mg·L−1 RhB 15 mg·L−1 MB 15 mg·L−1 TC-HCl 15 mg·L−1

80% in 120 min 96% in 70 min 98% in 120 min 96.4% in 80 min 99% in 100 min 54% in 75 min 98% in180 min 49% in 120 min 62% in 90 min 83% in 240 min 92.4% in 120 min 79.9% in 120 min 73.8% in 120 min 72.2% in 120 min 99.8% in 90 min 92.7% in 90 min 83.1% in 120 min

1 0.5 0.5 1 0.2 1 0.4 1 0.2 2 0.3 0.5 0.5 0.5 0.5 0.5 0.5

— 0.0670 0.0316 0.0441 0.0461 0.01083 0.018 — — — 0.0176 0.0096 0.0134 0.0108 0.072 0.0285 0.0171

[56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [9] [67] [11] This work

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Fig. 10. Photodegradation efficiency of RhB in the presence of BN-2 by addition of 0.01 mol·L−1 EDTA-Na2, 0.002 mol·L−1 BQ and 0.01 mol·L−1 IPA.

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Fig. 11. The mechanism of photodegradation of Bi2WO6/Nb2CTx hybrid nanosheets photocatalysts.

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Appendix A. Supplementary material

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.144595.

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