Ultrathin Bi2WO6 nanosheets loaded g-C3N4 quantum dots: A direct Z-scheme photocatalyst with enhanced photocatalytic activity towards degradation of organic pollutants under wide spectrum light irradiation

Journal of Colloid and Interface Science 539 (2019) 654–664

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Ultrathin Bi2WO6 nanosheets loaded g-C3N4 quantum dots: A direct Z-scheme photocatalyst with enhanced photocatalytic activity towards degradation of organic pollutants under wide spectrum light irradiation Mingjuan Zhang a,b, Yi Zhang a,b,⇑, Lin Tang a,b,⇑, Guangming Zeng a,b,⇑, Jiajia Wang a,b, Yuan Zhu a,b, Chengyang Feng a,b, Yaocheng Deng c, Wenze He a,b a

College of Environmental Science and Engineering, Hunan University, Changsha 410082, China Key Laboratory of Environmental Biology and Pollution Control, Hunan University, Ministry of Education, Changsha 410082, China c College of Resources and Environment, Hunan Agricultural University, Changsha 410128, China b

g r a p h i c a l a b s t r a c t Z-scheme mechanism of CNQDs/BWO under wide spectrum light irradiation and HR-TEM images of CNQDs, CNQDs/BWO.

a r t i c l e

i n f o

Article history: Received 12 November 2018 Revised 29 December 2018 Accepted 31 December 2018 Available online 2 January 2019 Keywords: Bi2WO6 g-C3N4 quantum dots Z-scheme mechanism Visible light Near infrared light

a b s t r a c t A novel ultrathin Bi2WO6 nanosheets loaded g-C3N4 quantum dots (CNQDs/BWO) photocatalyst was successfully fabricated, and used to catalyze two representative organic pollutants, rhodamine B (RhB) and tetracycline (TC) under wide spectrum light irradiation. The degradation experiments showed that CNQDs/BWO exhibited enhanced photocatalytic activities towards degradation of organic pollutants. Under visible light irradiation, the 5% CNQDs/BWO exhibited the best degradation efficiency with 87% and 92.51% removal of TC and RhB within 60 min, respectively. And under near-infrared (NIR) light, the 5% CNQDs/BWO still showed the best performance, its degradation efficiency to TC were
2 times than pure BWO. The upconversion behaviors of CNQDs might contribute to the enhanced photocatalysis. According to similar degradation trend, it is inferred that the catalytic mechanism in NIR light is consistent with that in visible light. The enhanced photocatalytic activity of CNQDs/BWO under wide spectrum light irradiation can be ascribed to a Z-scheme mechanism based on the calculated the lowest unoccupied molecular orbital (LUMO) of CNQDs and CB position of BWO, the free radical quenching experiment, and ESR characterization results. The composites have prominent light absorption, high stability and excellent photocatalysis efficiency, which would be used as a promising strategy for organic pollutants degradation. Ó 2019 Elsevier Inc. All rights reserved.

⇑ Corresponding authors at: College of Environmental Science and Engineering, Hunan University, Changsha 410082, China. E-mail addresses: [email protected] (Y. Zhang), [email protected] (L. Tang), [email protected] (G. Zeng). https://doi.org/10.1016/j.jcis.2018.12.112 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

M. Zhang et al. / Journal of Colloid and Interface Science 539 (2019) 654–664

1. Introduction Every year, a large number of organic pollutants enter the environment, e.g. discharge of wastewater containing chemical dyes in industrial production, antibiotic-containing excreta from animal husbandry and medical life [1,2]. Chemical dyes are relatively common organic pollutants, e.g. RhB. Azo dyes RhB is widely existed in the wastewater from textiles, cosmetics, and printing, which has been investigated in a great deal of researches due to its toxicity and non-biodegradation [3]. Besides, antibiotics have been a major concern in recent years, e.g. TC. TC is one of the most commonly used antibiotics in treatment of bacterial infection in animals and humans [4]. Owing to abuse and low water solubility, TC has been excessively accumulated in the aquatic environment [5]. The organic pollutants seriously harm environmental ecology and human health. To date, various methods have been explored to removal organic pollutants, including adsorption [6], biodegradation [7], photocatalysis [8], ultrasonic [9]. Among these, photocatalysis has been considered as an effective method for wastewater treatment due to its superb oxidation ability to degradation organic pollutants and mineralize most toxic compounds completely [3,10]. Plenty of photocatalysts with excellent photocatalytic activity have been developed for degradation of organic pollutants, such as TiO2 [11], NaTaO3 [12]. However, these photocatalysts can only absorb UV light (4% the solar spectrum). Indeed, visible light (43% the solar spectrum) and NIR light are also available. Certainly, the development of efficient photocatalysts utilizing visible light and NIR light is of great significance in application [13]. In general, the absorption property of photocatalysts is closely related to their physical and chemical structure, including shape, size of materials, and surface functional groups. So far, as one of the most attractive catalyst materials, single-layer or fewlayers two-dimensional (2D) materials, such as g-C3N4, Bi2WO6, boron nitride (BN), are attracted considerable interests in catalysis field. Especially, developing ultrathin structure photocatalyst is a quite practical and efficient strategy. The ultrathin thickness allows the photo-generated charge carriers to easily transfer from the inside to the surface to participated in redox reactions, which significantly improves the efficiency of photocatalysis [14]. Bi2WO6 as a visible light photocatalytic materials has a band gap energy of around 2.7 eV possessing a corner-sharing structure of WO6 octahedron sandwiched between (Bi2O2)2+ layers [15]. The visible-light absorption band of Bi2WO6 is attributed to the band transition from the hybrid orbitals of Bi6s and O2p to the W5d orbitals [16]. The morphology control plays an important role in the photocatalytic activity of Bi2WO6 [17]. Recently, a series of different morphologies of Bi2WO6 have been prepared, e.g. microsphere [18], snow-like microcrystalline [19], nanosheet [20], nanoplate [21] and nanoflake [22]. Among them, the nanosheet structure attracted our interest. The ultrathin Bi2WO6 nanosheets with excellent photocatalytic performance prepared by hydrothermal reaction have been proved [20]. Compared with nanoplate [21] and nanoflakes [23], due to ultrathin thickness, the Bi2WO6 nanosheets can result in a decreased recombination rate to enhance photocatalytic performance, according to the diffusion time formula of t = d2/k2D (k, D and d represent constant, diffusion coefficient of electron-hole pairs and the particle size, respectively). The ultrathin thickness can abbreviate the time for the photo-generated charge carriers to transfer from the inside to the surface, which facilitate the separation of electrons and holes, thereby improving the photocatalytic efficiency. However, it also has the drawbacks of the narrow visible light absorption and easily recombination of electrons and holes. Alternatively, the formation of composites is beneficial to expanding the absorption range of light and the

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separation of electrons and holes. Yang et al. prepared 3D-3D porous Bi2WO6/graphene hydrogel composites [10], Ge and coworkers synthesized QDs sensitized CdS-Bi2WO6 [24]. All of the obtained composites exhibited enhanced photocatalytic performance compared to pure Bi2WO6. It has been reported that the band gap of Bi2WO6 and g-C3N4 can be well matched [25]. CNQDs are made from g-C3N4, if Bi2WO6 and CNQDs were combined, what would be the result? Quantum dots (QDs), as new class of 0D nanomaterials with sizes less than 10 nm, has presented excellent photocatalytic performance, e.g. graphene QDs [26], carbon QDs [27] and CdS QDs [28]. Inspired by the graphene QDs, the g-C3N4 QDs were fabricated from bulk g-C3N4 directly by a thermal-chemical etching process [29]. The CNQDs exhibits strong upconversion behavior, i.e. the CNQDs can convert NIR light to visible light. When CNQDs were combined with a semiconductor that absorbs visible light, the CNQDs can absorb NIR light and emit high energy shorter wavelength visible light under NIR light irradiation, then the composites would utilize visible light to exhibit catalytic performance [30,31]. The ultrathin Bi2WO6 nanosheets as a visible light photocatalytic materials exhibited prominent photocatalytic performance due to the ultrathin thickness facilitate the carriers transfer from the inside to the surface. When CNQDs were combined with Bi2WO6, owing to the strong upconversion behavior of CNQDs, the composites would exhibit catalytic performance even under wide spectrum light irradiation. The obtained photocatalytic material could improve the photocatalytic effect significantly and are more suitable for practical application. Herein, we prepared the ultrathin Bi2WO6 nanosheets loaded CNQDs hybrid material by the reflux method to catalyze two representative organic pollutants, RhB and TC. A series of characterization methods were used to analyze the structures, morphologies and optical properties of the prepared samples. The degradation experiments of TC and RhB were conducted under visible light and NIR light to evaluate the photocatalytic properties. The results showed that CNQDs/BWO could have a significant catalytic degradation to organic pollutants. The composites have prominent light absorption, high stability and excellent degradation efficiency, which might be contribute to its potential application for organic pollutant degradation. 2. Experimental section 2.1. Chemicals Sodium tungstate dihydrate (Na2WO42H2O), bismuth nitrate (Bi(NO3)35H2O), ethanol (CH3CH2OH), cetyltrimethylammonium bromide (CTAB), melamine (C3N3(NH2)3), sulfuric acid (H2SO4), nitric acid (HNO3), tetracycline (TC), rhodamine B (RhB) were purchased from Sinopharm Chemical Regent Co. Ltd (Shanghai, China). All of the reagents were analytical grade and used without further purification. Deionized water was used throughout the whole experiment. 2.2. Synthesis of the g-C3N4 nanosheets The g-C3N4 nanosheets were obtained by two-step calcination process. Typically, melamine was put into an alumina crucible with a cover and then heated to 550 °C at a rate of 2.3 °C/min in a muffle furnace and maintained at this temperature for 4 h. The bulk C3N4 was obtained after the yellow agglomerates cooling down to room temperature and grounded. Then, the obtained bulk C3N4 through secondary calcination just changing the holding time to 2 h, and the g-C3N4 nanosheets were prepared.

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2.3. Synthesis of the g-C3N4 quantum dots The g-C3N4 quantum dots were fabricated by a modified method based on previous work [29]. In general, 50 mg of as prepared g-C3N4 nanosheets were added in the mixed solution of concentrated H2SO4 (10 mL) and HNO3 (30 mL) and ultrasonication until a clear solution was formed. Then, the obtained solution was diluted with deionized water and filtered through a 0.45 lm microporous membrane to remove the acids. Finally, the obtained filter residue were dispersed in 16 mL DI water and hydrothermal reaction in 200 °C for 10 h, after cooling down to room temperature, the solution were filtered through a 0.22 lm microporous membrane and followed by freeze-drying to obtain the CNQDs solid. 2.4. Synthesis of the CNQDs/BWO composites The BWO were synthesized via hydrothermal reaction [32]. The composites were synthesized by reflux, and the schematic diagram was shown in scheme 1. Typically, 1 g of as prepared BWO and a certain amount of CNQDs were added in 100 mL deionized water reflux at 80 °C for 6 h. The different mass ratio of CNQDs/BWO at 1 wt%, 3 wt%, 5 wt%, and 7 wt% were signed as 1% CNQDs/BWO, 3% CNQDs/BWO, 5% CNQNs/BWO and 7% CNQNs/BWO, respectively.

USA) with an integrating sphere attachment, using BaSO4 as the reference. The photoluminescence (PL) spectroscopy was obtained by an F-7000 fluorescence spectrometer. The photoelectrochemical experiments were performed on a CHI 760D electrochemical workstation in a three-electrode cell. A Pt electrode and an Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. FTO electrodes deposited with samples were used as the working electrode. 0.2 M Na2SO4 solution was used as supporting electrolyte [33]. A 300 W Xe lamp with different wavelength cutoff filters was used as light source. 2.6. Photocatalytic tests The photocatalytic activity of the prepared photocatalysts was tested by the degradation of organic pollutants under visible light and NIR light of a 300 W Xe lamp with different wavelength cutoff filters. TC and RhB was selected as the target pollutants with concentration of 20 mg/L, and added to the catalysts at a solid-toliquid ratio of 1.0 g/L (50 mg/50 mL). Before the photocatalytic procedure, the mixed solutions were stirred in the dark for 30 min to achieve adsorption-desorption equilibrium, then a 300 W Xe lamp (k > 420 nm) was lighted on. The reaction solution was collected at interval time of 10 min and then filtered through a 0.45 lm membrane filters and analyzed by UV–vis spectrophotometer.

2.5. Characterization

3. Results and discussion

A Rigaku-SmartLab 3 kW-Rigaku X-ray powder diffractometer with the Cu-Ka irradiation source was employed to record X-ray powder diffraction (XRD) patterns in the range from 5° to 80° at a scan rate of 8°/min. Transmission electron microscopy images were obtained by a F20 S-TWIN transmission electron microscopy (Tecnai G2, FEI Co) with an accelerating voltage of 200 kV. Atomic force microscopic (AFM) measurements were conducted to determine the thickness of the BWO nanosheets. In an AFM experiment, BWO was added in ethanol solution followed by ultrasonication for 20 min and then the dispersion was diluted in ethanol. A drop of the above diluted dispersion was deposited on a new cleaved mica surface and dried in air for AFM measurements. X-ray photoelectron spectroscopy (XPS) measurement was performed on an ESCALAB 250Xi system with a monochromatic Al-Ka X-ray source operated at 30 kV. The specific surface areas of the prepared samples were characterized by a Nova 2200e analytical system at 77 K. Total organic carbon (TOC) was analyzed using a Shimadzu TOCVCPH analyzer. The UV–vis diffused reflectance spectra (UV–vis DRS) were performed on a UV–vis spectrophotometer (Cary 300,

3.1. Characterization As shown in Fig. 1, the crystal structure of different samples was analyzed by XRD. The bulk C3N4 has two diffraction peaks at 27.8° and 12.9° [34]. The peak at 27.8°, corresponding to a distance of 0.321 nm, is the interlayer stacking of the conjugated aromatic system of graphitic carbon nitride and can be indexed as (0 0 2) crystal planes [35]. And the peak at 12.9°, corresponding to a distance of 0.687 nm, is indexed as (1 0 0) and can be associated with an inplane structural packing motif [35]. As a more intense peak suggests there are more regular repetitions between graphitic layers [34], for the g-C3N4 nanosheets, the intensity of the 27.8° peaks is weaker than that of bulk C3N4. Therefore, it is inferred that the bulk C3N4 was stripped to thin layer g-C3N4 nanosheets by secondary calcination, which is in favor of shortening the moving time of electrons from the inside to the surface. The crystal structure of the prepared BWO was found to be a orthorhombic form [36]. The intensity ratio of the (1 3 1) peak to the (2 0 0) peak of BWO is about 1.4, obviously smaller than the standard value of 5. This

Scheme 1. The schematic diagram of synthesis of the CNQDs/BWO composites.

M. Zhang et al. / Journal of Colloid and Interface Science 539 (2019) 654–664

Fig. 1. XRD patterns of the prepared samples.

result suggests that these crystals have special anisotropic growth along the (2 0 0) or (0 2 0) direction [8] attributed to their unique sheet-shaped morphologies. Furthermore, the diffraction peaks almost have no change among 1% CNQDs/BWO, 3% CNQDs/BWO, 5% CNQNs BWO, 7% CNQNs BWO, and pure BWO, which may be explained by the (0 0 2) peak of g-C3N4 overlaps with the (1 3 1) peak of BWO, or the low doping concentration and small size of CNQDs. In addition, no found impurity peaks showed that the high purity phases formed during the preparation process. To analyze the microstructures and morphology of the prepared materials, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and atomic force

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microscopic (AFM) were used. As shown in Fig. 2A, the obtained BWO was a surface smooth nanosheets with the size ranging from 30 to 50 nm, and the nanosheets were transparent. The thickness of BWO nanosheets were further determined by atomic force microscopic (AFM) measurements, the atomic force microscopic images and corresponding height profiles were showed in Fig. 3 (a) and (b). They demonstrated that the thickness of BWO nanosheets is 3.957 nm, which is in accordance with the thickness of the ultrathin BWO nanosheets [20,37], indicating that the ultrathin BWO nanosheets were synthesized. The ultrathin thickness can reduce the time for the photo-generated charge carriers to transfer from the inside to the surface, which facilitate the separation of electrons and holes, thereby improving the photocatalytic efficiency. Moreover, the HRTEM image demonstrates the lattice fringes with inter-planar spacing of 0.272 nm, which correspond to the (0 0 2) planes of orthorhombic BWO [38] (Fig. 2B). Meanwhile, the TEM image (Fig. 2C) and the diameter distribution (Fig. 2D) of the as-prepared CNQDs showed that the CNQDs were uniformly distributed with diameters in the range of 1–6 nm. Compared the TEM image of the 5% CNQDs/BWO (Fig. 2E) with that of BWO, a number of CNQDs attached to the surface of BWO, the similar sheets morphology were obtained when CNQDs were introduced into BWO, indicating the basic morphology of BWO did not changed. To further show the synthetic composite, the HRTEM was carried out. In Fig. 2F, there is the lattice spacing of 0.336 nm corresponding to the (0 0 2) plane of hexagonal g-C3N4, which is consistent with the previous literature [39]. To clarify the surface chemical composition and chemical status of the BWO and 5% CNQDs/BWO, XPS analysis was conducted. The XPS survey spectra of the BWO and CNQDs/BWO were shown in Fig. 4, which indicate the existence of Bi, W, O, N, and C elements. Binding energies are assigned to Bi 4f, W 4f, O 1s, C 1s, W 4d, Bi 4p, Bi 4d and N 1s (Fig. 4A). Fig. 4B shows the XPS spectrum in the W 4f region with the binding energy at 35.5 eV and 37.6 eV for W 4f7/2 and W 4f5/2, respectively, indicating that the existence of W was

Fig. 2. TEM images of (A, C, E) BWO, CNQDs and 5% CNQDs/BWO, HR-TEM images of (B, F) BWO and 5% CNQDs/BWO, (D) Diameter distribution of the CNQDs.

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Fig. 3. (a) AFM image, and (b) cross-section profile of ultrathin BWO nanosheets.

B

Survey

5% CNQDs/BWO

5% CNQDs/BWO

W 4f

Intensity/a.u.

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C 1s W 4d

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O 1s

Bi 4d

Bi 4p3

O KLL

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Bi 4f

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BWO

BWO

1200

1000

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600

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0

40

38

5% CNQDs/BWO

D

BWO

168

32

O 1s

5% CNQDs/BWO

BWO 164

160

156

Binding Energy/ ev C 1s

5% CNQDs/BWO

292

F

Intensity/a.u.

CNQDs

294

534

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286

284

532

530

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526

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E

34

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Bi 4f

Intensity/a.u.

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36

Binding Energy/ ev

Binding Energy/ ev

282

280

Binding Energy/ ev

412

N 1s

5% CNQDs/BWO

CNQDs

408

404

400

396

392

Binding Energy/ ev

Fig. 4. XPS spectra of BWO and 5% CNQDs/BWO: (A) Survey spectrum, (B) W 4f spectrum, (C) Bi 4f spectrum, (D) O1s spectrum, (E) C 1s spectrum, (F) N 1s spectrum.

in the chemical state of W6+ [40]. The high resolution XPS spectra of Bi 4f in BWO and CNQDs/BWO are shown in Fig. 4C. Two strong peaks at 159.3 eV and 164.6 eV are assigned to the binding energies of Bi 4f7/2 and Bi 4f5/2, respectively, implying the existence of Bi3+ in the crystal structure [41]. Compared with BWO, the Bi 4f

peak in CNQDs/BWO displays a slight shift toward lower binding energies, which may be due to the interaction between CNQDs and BWO. The O 1s spectra at 530 eV are assigned to the BiO bonds. The peak signals of O 1s shift to higher binding energy after combining with CNQDs. The peak signals of W 4f also shift to

M. Zhang et al. / Journal of Colloid and Interface Science 539 (2019) 654–664

higher binding energy after combining with CNQDs. The high resolution XPS spectra of C 1s are shown in Fig. 4E. The peaks located at 284.9 eV and 285.8 eV correspond to the N–sp2C and N–sp3C bonds, respectively. Another two peaks at 286.2 eV and 289.0 eV are assigned to the CAO and C@O bonds, respectively [29]. The N 1s spectra can be deconvoluted into four component peaks (Fig. 4F). Three peaks at 399.6 eV (CANAC), 400.2 eV (N(C)3), and 401.1 eV(NAH), which are the typical g-C3N4 XPS N 1s spectrum [42]. The peak at 407.0 eV is assigned to –NO2 that is derived from the oxidation of amino groups. The above results indicated the successful fabrication of CNQDs/BWO materials. From the point of view of specific surface area and the corresponding pore size distribution to analyze the materials, nitrogen adsorption–desorption isotherms were also performed. The specific surface area was calculated from the linear part of the multipoint plot [43] of BWO and 5% CNQDs/BWO. The pore sizes of the synthesized photocatalysts were calculated by BJH methods. As shown in Fig. 5, the isotherms of BWO and 5% CNQDs/BWO were a classical type IV, indicating mesoporous feature (2– 50 nm) of the prepared samples. The pore size distribution seen in the inset of Fig. 5 further confirms the mesoporous structure of the proposed materials. Meanwhile, a H3-type hysteresis loop in the relative pressure of 0.9–1.0 indicates the larger pores were formed between secondary particles [44]. The specific surface area of the BWO and 5% CNQDs/BWO were calculated to be 47.961 m2/g and 63.293 m2/g (Table S1), respectively. Obviously, the higher specific surface area could provide sufficient active sites and adsorbed more pollutant molecules on the surface of photocatalysts, which would improve the degradation efficiency of pollutants. 3.2. Photoelectric properties of prepared photocatalysts The absorbance properties of the obtained samples were characterized using UV–vis diffuse reflectance spectrum (DRS) and the results were shown in Fig. 6A. BWO was less responsive to visible light in view of the shorter than 450 nm absorption edge. In contrast, the CNQDs exhibited broad absorption in the visible light region. Intuitively, the curves of the CNQDs/BWO showed significant red-shift towards the longer absorption edges. It indicates that the range of responsive light has expanded, i.e. the composites could utilize longer wavelength visible light. The band gap energies of semiconductors can be estimated by the Kubelka-Munk equation

120

80 60

3 Pore volume(cm /g)

3 -1 Vollume absorbed (cm g )

100

0.0036 5% CNQDs/BWO BWO

0.0030 0.0024 0.0018 0.0012 0.0006 0.0000 0

20

40

60

80 100 120

Pore radium(nm)

40 20 0 0.0

5% CNQDs/BWO BWO

0.2

0.4

0.6

0.8

1.0

Relative PressureP/P0 Fig. 5. N2 adsorption-desorption isotherm of BWO and 5% CNQDs/BWO.

ahv ¼ Aðhv  Eg Þn=2

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ð1Þ

where a represents absorption coefficient, h is the Planck’s constant, is the light frequency, Eg is the forbidden bandwidth, A is a constant (usually1), n is decided by the optical transition type of typical semiconductor. The value of n for CNQDs and BWO were determined to be4. In light of this, the bandgaps of the materials was estimated from the inflection point between the ‘‘tangent at the

v

inflection point on ðahvÞ ¼ f ðhvÞ and the horizontal line” [45]. As shown in Fig. 6B, the bandgaps of the CNQDs and BWO were estimated to be 2:60 eV and 2:70 eV, respectively, which were close to the known values in previous reports [39,46]. The potentials of the valence band (VB) and the conduction band (CB) of a semiconductor were calculated according to the empirical equations 1=2

EVB ¼ X  E0 þ 0:5Eg

ð2Þ

ECB ¼ EVB  Eg

ð3Þ

where EVB represents the valence band edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the constituent atoms, E0 is the energy of free electrons on the hydrogen scale (
4:5eV vs NHE), and Eg is the band gap energy of the semiconductor. The calculated ECB of BWO and LUMO of CNQDs were estimated to be 0:51eV and 1:16 eV, respectively. The calculated EVB of BWO and the highest occupied molecular orbital (HOMO) of g-C3N4 were estimated to be 3:21eV and 1:44 eV, respectively. Photoluminescence (PL) emission spectra originating from the recombination of free charge carriers are useful to reveal the migration, transfer and separation of photogenerated charge carriers [47]. All samples exhibited emission peak around 430 nm and 475 nm with a certain peak shift in PL tests as shown in Fig. 6C. The emission peak can be attributed to the radiative recombination of charge carriers. And the shift of the peak position is consistent with variation of band gap energy of these samples [48]. A reduced emission peak and red shift appeared in the 5% CNQDs/BWO, suggesting the relatively lower recombination rates of photogenerated charge carriers, and the decreased band gap energy. Moreover, the transient photocurrent density of the obtained BWO and CNQDs/BWO electrodes under visible light irradiation was compared in Fig. 6D. All the electrodes generated a rapid photocurrent response with light on. And then, it instantaneously declined to zero as the light was cutoff. Obviously, the materials are photo-sensitive. The photocurrent of CNQDs/BWO was enhanced to be
4 times under visible light irradiation than that of pure BWO. It demonstrated that BWO have higher recombination rates of photo-generated electrons and holes pairs than the CNQDs/BWO, i.e. the CNQDs/BWO had an efficient charge separation. Thereafter, the introduction of CNQDs into BWO had a significant effect on the electronic properties. In addition, the photocatalytic reactions could be regarded as an electrochemical process [49]. Fig. 6E showed the electrochemical impedance spectroscopy (EIS) of BWO and CNQDs/BWO to demonstrate the charge transfer. A small arc radius suggests a higher efficiency of charge transfer. A marked decrease in the arc radius of the Nyquist plot for CNQDs/BWO, indicating a low resistance for interfacial charge transfer from catalysts to reaction molecules [50]. Besides, the upconversion properties of CNQDs were measured with excitation wavelengths from 700 to 850 nm. In Fig. 6F, the resulting upconversion PL signal changes with the corresponding excitation wavelength. Upon the NIR light irradiation, CNQDs emits visible light around 430–570 nm, and the strongest emitting light appeared in 445 nm under 750 nm excitation wavelength. Considering the nonconstant energy between the excitation and the emission light in the upconversion process, the multiphoton active

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A

CNQDs BWO

B

CNQDs

2.0

BWO

1% CNQDs/BWO

1.0

CNQDs/BWO

3% CNQDs/BWO

0.8

1.5

(Ahv)1/2

Absorbance(a.u.)

1.2

5% CNQDs/BWO 7% CNQDs/BWO

0.6

1.0

0.4

0.5 0.2 0.0 300

400

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600

700

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0.0 1.5

2.0

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10

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450

500

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D

BWO 5% CNQDs/BWO

400

3.0

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Light onLight off

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BWO 1%CNQDs/BWO 3%CNQDs/BWO 5%CNQDs/BWO 7%CNQDs/BWO

4 2 0 40

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6 BWO 1% CNQDs/BWO 3% CNQDs/BWO 5% CNQDs/BWO 7% CNQDs/BWO

01

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180

700 nm 725 nm 750 nm 775 nm 800 nm 825 nm 850 nm

4

5

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-Z''/(105ohm)

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Fig. 6. UV–vis DRS spectra (A), Estimated band gap (B), Photoluminescence spectra (C), Photocurrent response density (D), Electrochemical impedance spectra (E) of BWO and CNQDs/BWO, Up-converted photoluminescence spectra of CNQDs with excitation wavelengths from 700 to 850 nm (F).

process might be responsible for the upconversion properties of CNQDs [29,31,51]. Obviously, the upconversion of CNQDs is more beneficial to the conversion and utilization of light irradiation. 3.3. Photocatalytic activity To investigate the photocatalytic activity of the proposed CNQDs/BWO against organic pollutants, two representative organic compounds (RhB and TC) are used for catalytic analysis. Considering CNQDs have a significant effect in accelerating photocatalytic reactions but no intrinsic activity [29], it can infer that the photocatalytic effect of pure CNQDs toward TC and RhB could be negligible. Before irradiation, a dark reaction was conducted to reaching the adsorption-desorption equilibrium. As shown in Fig. 7, the composites exhibits excellent catalytic degradation to RhB and TC under visible light irradiation within 60 min, especially, the 5% CNQDs/BWO showed the highest degradation efficiency. The RhB was degraded faster, and the degradation efficiency reached 92.51% by the 5% CNQDs/BWO within 50 min

(Fig. 7A). It might be attributed to the combined of the dyesensitization effect and photocatalysis considering the colored RhB dye could absorb visible light [52]. To RhB, the kinetic behaviors of the photocatalysts was investigated by the first-order simplification of Langmuir-Hinshelwood ðL  HÞ kinetics, which is fit well for photocatalysis at low initial pollutant concentrations [3]. According to

lnðC=C 0 Þ ¼ kt

ð4Þ

where C 0 and C (mg/L) is the initial concentration (mg/L) and the remaining concentration at interval time oft, k is the apparent first-order rate constant (min1). The obtained k of the different samples was shown in Fig. 7B. The 5% CNQDs/BWO exhibited the highest rate constant of 0:051 min1, which could be explained by the enhanced separation efficiency of photo-generated electron-hole pairs and the enlarged surface area [53]. For the more difficult to degrade TC, the CNQDs/BWO photocatalysts still maintained good degradability. Only slight of TC was degraded under visible light irradiation in the absence of photocat-

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A

1.0

Light off Light on

B

λ>420 nm

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RhB BWO 1% CNQDs/BWO 3% CNQDs/BWO 5% CNQDs/BWO 7% CNQDs/BWO

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34

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Photocatalysts

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TC BWO 1% CNQDs/BWO 3% CNQDs/BWO 5% CNQDs/BWO 7% CNQDs/BWO -20

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1-without catalysts 0.035 2-BWO 3-1% CNQDs/BWO 0.030 4-3% CNQDs/BWO 0.025 5-5% CNQDs/BWO 6-7% CNQDs/BWO 0.020

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D

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TC BWO 1% CNQDs/BWO 3% CNQDs/BWO 5% CNQDs/BWO 7% CNQDs/BWO

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1-without catalysts 0.08 2-BWO 0.07 3-1% CNQDs/BWO 4-3% CNQDs/BWO 0.06 5-5% CNQDs/BWO 0.05 6-7% CNQDs/BWO

1-without catalysts 0.008 2-BWO 3-1% CNQDs/BWO 4-3% CNQDs/BWO 0.006 5-5% CNQDs/BWO 6-7% CNQDs/BWO

λ>700nm

0.004

0.002

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60

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2

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Fig. 7. Photocatalytic degradation of RhB under visible light irradiation of as-prepared photocatalysts (A), Corresponding reaction kinetic curves (B), Photocatalytic degradation of TC under visible light and near-infrared light irradiation of as-prepared photocatalysts (C, E), Corresponding reaction kinetic curves (D) and (F).

In addition, the mineralization ability of the photocatalyst towards TC and RhB was measured by total organic carbon (TOC). Compared to pure BWO, the 5% CNQDs/BWO exhibited an

100

TOC removal(%)

alyst, indicating that the photolysis of TC was negligible. Although 62.71% of TC could be degraded by the pure BWO in 60 min, the degradation efficiency was further improved by the proposed composites. Consistent with the degradation of RhB, the 5% CNQDs/ BWO also had highest degradation efficiency (Fig. 7C) with maximum rate constant. (Fig. 7D). Obviously, proper doping of CNQDs could improve light conversion efficiency, otherwise the lower light harvesting would reduce the generation of electron-hole pairs [54]. To further evaluate the photocatalytic properties of the composites, the degradation experiment of TC under NIR light was carried out. Although the photocatalytic activity of the photocatalysts decreased rapidly under NIR light irradiation due to the long wavelength and the relatively weak energy of NIR light, the composites still presented similar catalytic degradation trends as that under visible light. The 5% CNQDs/BWO still showed the best performance, its degradation efficiency to TC were
2 times than pure BWO (Fig. 7E and 7F). The upconversion behaviors of CNQDs might contribute to the enhanced photocatalysis, as mentioned in Fig. 6F, which can convert NIR light to visible light for TC degradation by the composites.

BWO 5% CNQDs/BWO

80 60 40 20 0

TC

RhB

Fig. 8. The TOC removal rate of TC and RhB by BWO and 5% CNQDs/BWO.

M. Zhang et al. / Journal of Colloid and Interface Science 539 (2019) 654–664

enhanced TOC removal efficiency of 68.79% and 81.54% to TC and RhB, respectively (Fig. 8). This result further confirmed that the CNQDs/BWO composites were an efficient photocatalyst for the degradation of organic pollutants. For the application, the stability of the BWO and CNQDs/BWO was studied by recycled 4 times. After the degradation experiments, the BWO and CNQDs/BWO was recovered via a simple filtration followed by washing with ethanol and deionized water several times and dying in air at 60 °C. As shown in Fig. S1, after four recycles of the photocatalytic degradation TC under visible light irradiation, the photocatalytic activity of the BWO and CNQDs/BWO slightly decreased, which might be attributed to the loss of photocatalyst during the filtration and washing step [8]. Anyway, it can be seen that the BWO and CNQDs/BWO had high stability. 3.4. Photocatalytic mechanism Essentially, photocatalytic degradation of organic pollutants occurs through the redox reaction between free radicals, holes and pollutants [55]. For judging the free radicals and holes produced during photocatalytic degradation of TC by the 5% CNQDs/ BWO, the trapping experiments were conducted by adding triethanolamine (a quencher of h+) [56], 1,4-benzoquinone (a  quencher of O 2 ) [57] and isopropanol (a quencher of OH) [58], respectively. As shown in Fig. S2, when adding the three scavengers, of the degradation efficiency of the 5% CNQDs/BWO respectively had about 49%, 36%, and 27% loss, indicating that h+, O 2 and  OH all played important roles in the photocatalytic reaction system. To further clarify the distribution of free radicals produced in the CNQDs/BWO photocatalytic reaction system, electron spin

(a)

BWO-DMPO superoxide radical

Intensity (a.u.)

Light on

Dark

317.8

318.0

resonance (ESR) spin-trap technique was used. The typical charac teristic peaks of DMPO-O 2 and DMPO- OH were respectively dis played in Fig. 9. It means that the O and OH radicals presented 2 in the CNQDs/BWO reaction systems [59]. And compared with  BWO, the characteristic peaks of DMPO-O 2 and DMPO- OH of  CNQDs/BWO increased significantly, i.e. more O 2 and OH radicals were produced. According to the above results, a Z-scheme photocatalytic reaction mechanism is shown in Scheme 2. Under visible light irradiation, both the BWO and CNQDs can be excited and produce the photo-generated electrons and holes, because the band gap energy of BWO and CNQDs is 2.70 eV, 2.60 eV, respectively. In terms of the  number of free radicals, no matter the O 2 or OH signal of 5% CNQDs/BWO composites were significantly enhanced relative to the monomer materials. For the O 2 , the electrons on the CB of BWO theoretically cannot reduce O2 to O 2 due to the BWO ECB is 0.51 eV positive than that of O2/O 2 (0.33 eV vs. NHE) [60]. However, the O 2 signal appeared in Fig. 9a of the ESR results, which might attribute to the electrons accumulation on the CB of BWO to reach the O2/O 2 potential [20]. Even so, the CNQDs/BWO composites would utilize more negative LUMO potential of CNQDs, which could easily help to produce more O 2 for the catalysis. Whilst, the holes in the VB of BWO (EVB = 3.21 eV) can oxide OH or H2O to OH (OH/OH (2.40 eV vs. NHE) [61] and H2O/OH (2.72 eV vs. NHE) [62]). The CNQDs/BWO composites nevertheless obtained stronger OH signals. This result implied that the VB potential of the composites was maintained, and the recombination of photo-generated carriers was decreased, which is facilitated to promote the catalysis as well. Obviously, the typical doubletransfer mechanism in heterojunction structure could not explain the more negative LUMO potential and retained VB potential in

318.2

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CNQDs/BWO-DMPO superoxide radical

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Magnetic Field(mT) BWO-DMPO hydroxyl radical

(b)

Light on

Intensity (a.u.)

662

Dark

318.0

318.2

318.4

Magnetic Field(mT)

 Fig. 9. DMPO spin-trapping ESR spectra with 5% CNQDs/BWO sample in methanol dispersion (for DMPO-O 2 ) and in aqueous dispersion (for DMPO- OH) under visible light irradiation.

M. Zhang et al. / Journal of Colloid and Interface Science 539 (2019) 654–664

663

RhB within 60 min under visible light, respectively. And its degradation efficiency to TC was
2 times than pure BWO in 60 min under NIR light. The upconversion behavior of CNQDs might contribute to the enhanced photocatalysis. The catalytic mechanism in visible light and NIR light is considered to be the same, which was deduced from their similar degradation trend. The enhanced photocatalytic activity of CNQDs/BWO under wide spectrum light irradiation can be ascribed to a Z-scheme charge transfer mechanism, by which not only the separation and transfer rates of photo-generated charges were improved but the strong redox ability were retained. Acknowledgments

Scheme 2. Z-scheme mechanism of CNQDs/BWO under wide spectrum light irradiation.

CNQDs/BWO composites. Therefore, the transfer process of photogenerated carriers could be explained by a Z-scheme mechanism. It means that the electrons from the CB of BWO could be transfer to the HOMO of CNQDs, and the electrons and holes retained in LUMO of CNQDs and VB of BWO, respectively. Thus, the CNQDs/BWO composites could obtain stronger photocatalytic capacity by utilizing more negative LUMO potential of CNQDs and maintained VB potential of BWO. In addition, the reduced emission peak in PL characterization results, the enlarged photocurrent and low impedance in the Nyquist plot of CNQDs/BWO also could reflect the recombination of photo-generated charges was decreased, i.e. more holes and free radicals would serve for the degradation of pollutants. In this way, it tends to conform to the Z-scheme mechanism, which can accelerate the separation and transfer of photogenerated charges to retain the strong redox ability. Namely, the holes in the VB of the BWO of the composites not only could attack organic pollutants directly, but also oxidize OH or H2O to OH for the reaction with TC. Simultaneously, the electrons in the LUMO of the CNQDs could react with the dissolved oxygen to form O 2, which lead to the degradation of TC. Because of the synergistic + effect of OH, O 2 and h , the photocatalytic efficiency of CNQDs/ BWO was improved significantly. Besides, under NIR light irradiation, although the degradation efficiency of organic pollutants was decreased, the ratio of degradation efficiency between the CNQDs/BWO and BWO has a similar result as in visible light. Therefore, it is inferred that the catalytic mechanism in NIR light is consistent with that in visible light. In a word, the introduction of CNQDs to construct the CNQDs/ BWO composites could significantly improve the photocatalytic performance under visible light and NIR light, and then enhance its degradation efficiency against organic pollutants. It follows that the strategy of the composite construction for degradation of organic pollutants is positive and promising.

This study was financially supported by the National Natural Science Foundation of China (51508175, 51521006, 51709285) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R17), Science and Technology Plan Project of Hunan Province, China (2015SK2001), Natural Science Foundation of Hunan Province, China (2018JJ2046), China Postdoctoral Science Foundation (2016M600490, 2017T100462). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2018.12.112. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

4. Conclusions In this work, a novel ultrathin Bi2WO6 nanosheets loaded CNQDs (CNQDs/BWO) photocatalyst was successfully fabricated and to catalyze RhB and TC under wide spectrum light irradiation. The degradation experiments showed that the CNQDs/BWO exhibited enhanced photocatalytic activities toward degradation of organic pollutants. The 5% CNQDs/BWO exhibited the best photocatalytic performance. It could degrade 87% and 92.51% TC and

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