BiVO4 Z-Scheme

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RGO-Promoted All-Solid-State g-C3N4/BiVO4 Z-Scheme Heterostructure with Enhanced Photocatalytic Activity toward the Degradation of Antibiotics Deli Jiang, Peng Xiao, Leqiang Shao, Di Li, and Min Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01840 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 20, 2017

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Industrial & Engineering Chemistry Research

RGO-Promoted All-Solid-State g-C3N4/BiVO4 Z-Scheme Heterostructure with Enhanced Photocatalytic Activity toward the Degradation of Antibiotics Deli Jianga, Peng Xiaoa, Leqiang Shaoa, Di Lib,*, and Min Chena,* †a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China b

Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China

Corresponding Author *E-mail: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: Novel all-solid-state RGO/g-C3N4/BiVO4 Z-scheme heterostructure was

prepared

using

a

facile

hydrothermal

method.

The

as-obtained

RGO/g-C3N4/BiVO4 Z-scheme heterostructure exhibited enhanced photocatalytic activities toward the degradation of tetracycline hydrochloride under visible light irradiation, which was about 1.13, 1.16 and 1.41 times higher than those of binary RGO/g-C3N4, g-C3N4/BiVO4, and RGO/BiVO4, respectively. The enhanced photocatalytic performance could be ascribed to the synergistic effect between the Z-scheme charge-carrier separation and RGO electron mediator, in which RGO serves as a promoter for accelerating charge separation at the heterojunction, thus further leading to the improved photocatalytic activity. It is anticipated that construction of RGO/g-C3N4/BiVO4 Z-scheme heterostructure is an effective strategy to develop high-performance photocatalysts for the degradation of tetracycline hydrochloride. Keywords: Photocatalysis; Z-Scheme; Heterostructure; Antibiotics; Mechanism

2


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1. INTRODUCTION Recently, the widespread use of antibiotics has drawn increasing attention due to the possible damages to human beings, agriculture, and planting.1-8 Among them, tetracycline hydrochloride (TC) are especially focused owing to their heavily use in human and veterinary medicines.9-12 The residues of TC left in aqueous environment can cause serious problems to human health.13-15 To effectively remove these antibiotics,

several

treatment

strategies

have

been

developed,

including

absorption,16,17 electrolysis,18 photocatalysis,19 microbial decomposition,20 and membranes separation.21 Among these methods, the heterogeneously photocatalytic process with semiconductor is an environmentally friendly and easily handled method for the removal of antibiotics and other organic pollutant from water.22-28 However, the activities of most photocatalysts are still low due to some draw-backs such as the limited fast recombination of photoinduced carriers, utilization of visible light, and the lack of reactive sites.29 Therefore, it is highly desirable to develop highly efficient visible-light-driven photocatalysts for the remove of antibiotics from water. Graphitic carbon nitride (g-C3N4, denoted CN) has received great attention in photocatalysis field because of its narrow band gap, good thermal-chemical stability, non-toxicity and low cost.30,31 However, the practical application of individual CN is restricted by the low charge separation efficiency, which results in low photocatalytic activity.32 To address this issue, various technologies such as loading with noble metals,33-35 forming a heterojunction,36-38 and modifying structural morphology39-40 have been employed. Recently, construction of artificial Z-scheme photocatalytic system is an alternative method, because it not only can separate the photo-excited electrons and holes, but also can maintain the excellent redox capacity of the component semiconductor.42-47 Up to now, some directed or electron-mediator-induced 3



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CN-based Z-scheme systems have been constructed and used in pollutant degradation and water splitting.48-50 Compared with the noble metal nanoparticles, graphene has been used as an excellent and cost-effective redox mediator to improve the charge transfer efficiency between two or more semiconductors.51-54 The monoclinic bismuth vanadate (denoted BVO), with narrow bandgap (about 2.4 eV), excellent oxidation ability and chemical stability, has been considered as one of the most interesting photocatalysts.55 Notably, BVO has matched band edges with those of CN.56,57 If the RGO was used as the electron-mediator, a novel solid-state Z-scheme photocatalytic system based on the CN and BVO would be constructed with enhanced photocatalytic activity. So far, there are few reports on the construction of CN-based Z-scheme ternary heterostructures for the photodegradation of TC. Based on the above considerations, herein, we reported the construction of novel RGO/g-C3N4/BiVO4 (donated as RGO/CN/BVO) ternary heterostructure by using a facile hydrothermal method. The results demonstrated that the as-obtained RGO/CN/BVO ternary heterostructure exhibited the enhanced photocatalytic activities toward the degradation of TC under visible light irradiation. The underlying reason for the enhanced photocatalytic acidity was investigated. On the basis of trapping experiments and the energy band structures analysis, a possible Z-scheme charge transfer mechanism for the enhanced photocatalytic activity toward the degradation of TC was also discussed. This work indicates that construction of Z-scheme RGO/CN/BVO ternary hybrid is an effective strategy to develop high-performance photocatalysts for the degradation of antibiotics. 2. EXPERIMENTAL SECTION 2.1. Preparation of CN Nanosheets, RGO, and CN/RGO Composite. CN nanosheets were synthesized by the polymerization of urea as reported in our 4


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previous work.58 The RGO was prepared according to a modified Hummers’ method.59 The CN/RGO binary composite was synthesized by hydrothermal method. 5 mg of RGO and 100 mg of CN powder were dispersed in 100 mL of water and was sonicated for 1 h. The dispersion was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. The obtained powders were collected and washed with deionized water and ethanol for several times. 2.2. Synthesis of Bare BVO and RGO/CN/BVO Ternary Nanocomposite. The pure BVO was fabricated according to a previously reported method.60 The RGO/CN/BVO ternary nanocomposite was synthesized by a facile hydrothermal method. Typically, 1 mmol of Bi(NO3)3·5H2O and 1 mmol of NH4VO3 were firstly dissolved into 20 mL of HNO3 solution (2 M) and 20 mL of NH3·H2O solution (2 M), respectively. Then, 0.2 g of sodium dodecyl sulfonate (SDS) dissolved in water was added to the above Bi(NO3)3 solution. Then, the Bi(NO3)3 solution was added to the NH4VO3 solution dropwise under continuous stirring for 30 min. Then the pH of the mixed solution was adjusted to 7 with NH3·H2O solution and then the mixture was kept stirring for 2 h. Subsequently, 5 mg of RGO and 100 mg of CN powder were added to the above dispersion with the aid of ultrasonic dispersion for 1 h. Finally, the as-obtained dispersion was transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h. The obtained precipitates were collected and washed with deionized water and ethanol for several times. In addition, RGO/BVO and CN/BVO were fabricated according to the similar way as the CN/RGO/BVO composite except that no CN or RGO were added, respectively. 2.3. Characterization. The phase compositions and crystal structures of the samples were examined by X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer) with Cu-Kα 5



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radiation (λ=1.5406 Å). FT-IR spectrum was performed on a Nicolet FT-IR spectrophotometer (Nexus 470, Thermo Electron Corporation) using KBr disks at room temperature. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) characterizations were carried out by FEI JEM-2100 and FEI Tecnai G2 F20 operating at 200 kV. Surface analysis of the samples was carried out by X-ray photoelectron spectroscopy (XPS) using a ESCA PHI500 spectrometer. UV–vis diffuse reflectance spectra (DRS) of the samples were determined by a Shimadzu UV-2450 spectrophotometer equipped with spherical diffuse reflectance accessory. The surface areas of the samples were measured by a TriStar II 3020-BET/BJH Surface Area. The photoluminescence (PL) spectra of the samples were determined by a Varian Cary Eclipse spectrometer with an excitation wavelength of 325 nm. The photocurrent response was measured with an electrochemical analyzer (CHI 660B). The electron spin resonance (ESR) analysis was conducted on a Bruker model ESR JES-FA200 spectrometer. 2.4. Photocatalytic Activity. The photocatalytic activities of the samples were evaluated by the degradation of TC aqueous solution (35 mg/L) under irradiation of a 500 W tungsten light lamp. 40 mg of photocatalyst was added into 40 mL of the above TC solution. Before irradiation, the suspension was magnetically stirred for 0.5 h in the dark to establish the absorption-desorption equilibrium between the samples and the TC molecule. After illumination, 3 mL of the sample solution were taken for every 30 min and separated by centrifugation. The concentration of TC during the degradation was monitored by a UV-vis spectrophotometer (Cary 8454, Agilent). 3. RESULTS AND DISCUSSION 3.1 Structural and Morphological Analyses. 6


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Figure 1a displays the XRD patterns of the prepared samples. For the bare RGO sample, a broad diffraction peak at around 24.3° was observed, which can be ascribed to the (002) reflection of RGO sheets. The diffraction peaks at 2θ value of 13.1° and 27.4° can be assigned to (100) and (002) crystal planes of carbon nitride, relating to the inter-layer packing and the interplanar stacking of the aromatic systems, respectively.61 For the bare BVO sample, all the diffraction peaks can be ascribed to the monoclinic phase of BiVO4 (JCPDS 83-1699). For the binary CN/RGO and BVO/CN samples, no diffraction peaks assigned to RGO were found which might be owing to the weak diffraction intensities of RGO or the low content of RGO in the composites. For the binary CN/BVO and RGO/CN/BVO ternary nanocomposite, the diffraction peaks assigned to BVO can be observed. To further confirm the existence of CN in the ternary nanocomposite, FTIR characterization was performed. As shown in Figure 1b, the peaks located at 1255, 1323, 1420, 1570, and 1630cm−1 can be ascribed to the typical stretching vibration modes of the CN heterocycles.37 The peak near 810 cm−1 is assignable to the characteristic breathing mode of triazine units.37 These main characteristic peaks can be observed in the CN-base binary samples and the RGO/CN/BVO ternary nanocomposite, indicating the existence of CN in these

(103) (112) (004) (200) (020) (211) (015) (-213) (024) (220) (031) (215) (132) (224)

samples.

(002) (101) (011)

(a)

(b) Transmittance (%)

CN/BVO RGO/BVO BVO (002)

RGO/CN

20

CN/BVO RGO/CN

CN

CN

(002)

(100)

10

BVO RGO/CN/BVO

RGO/CN/BVO

Intensity (a.u.)

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RGO

30 40 50 2Theta (degree)

60

70

1630

80

3500

3000

1255 810 1570 1420 1323

2500 2000 1500 -1 Wavenumber (cm )

Figure 1. (a) XRD patterns and (b) FTIR spectra of the as-prepared samples. 7


1000

500


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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Further information about the microscopic structure and morphology of the as-prepared samples were characterized by TEM and HRTEM analyses. As shown in Figure 2a, the CN sample shows layered structure with obvious pores, which is the typical feature of CN synthesized by the polymerization of urea. Figure 2b exhibits the TEM image of pure BVO sample has the irregular morphology with particle size ranging from 1-2 µm. As shown in Figure 2c, it can be obviously found that the BVO microparticles were combined with CN nanosheets, forming the binary hybrid. Figure 2d shows that the RGO, CN, and BVO coexist in the nanocomposite, confirming the formation of RGO/CN/BVO ternary nanocomposite. This result was further verified by the high-resolution TEM image, which shows the presence of the interfacial contact between the RGO nanosheets and CN or BVO. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and EDX elemental scanning analysis were performed to further identify the composition and element distribution of ternary nanocomposite. As displayed in Figure 2f, the C and N elements homogeneously co-exist in the RGO/CN/BVO ternary composite, and the Bi, V, and O elements shows almost the same shape, further indicating the formation of BVO in the sample.

8


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Figure 2. TEM images of (a) CN, (b) BVO, (c) CN/BVO (d) RGO/CN/BVO (e) HRTEM image of the RGO/CN/BVO, (f) HAADF-STEM image and the corresponding STEM-EDX elemental mapping images of the RGO/CN/BVO. To further elucidate the surface chemical states of the RGO/CN/BVO ternary nanocomposite, XPS analysis was performed. The full scan survey XPS spectrum of the RGO/CN/BVO clearly demonstrates that the elements C, N, Bi, V and O can be prominently observed at their respective standard binding energies (Figure 3a). As displayed in Figure 3b, the binding energies located at approximately 284.8, 285.8, 288.2, and 289.2 eV can be attributed to sp2 C–C bonds of graphitic carbon, C–OH bonds, C–N–C bonds, and O–C=O configurations, respectively.62,63 For the N 1s spectrum (Figure 3c), the dominant peaks at 397.9 eV, 400.0 eV, and 401.0 eV are associated with sp2-hybridized in triazine rings (C–N=C), tertiary nitrogen N–(C)3 groups, and N–H groups, respectively.64 The weak peak at about 404.9 eV can be 9



ascribed to the oxidized nitrogen (N–‘‘O’’).65,66 Figure 3d shows the XPS spectra of Bi 4f, which features well-defined peaks at 158.8 eV and 164.1 eV, originating from the Bi 4f7/2 and Bi 4f5/2 species, respectively. As for V, The V 2p peaks located at 516.3 and 524.0 eV could be ascribed to V 2p3/2 and V 2p1/2, respectively (Figure 3e). Meanwhile, the main peaks located at 529.4 and 531.0 eV are ascribed to the O 1s binding energy (Figure 3f). Evidently, the above XRD, TEM and XPS results provide strong evidence that the synthesized nanocomposites contain RGO, CN, and BVO

Bi 4f (e)

158.8 eV 164.1 eV

158

282

160 162 164 166 Binding Energy (eV)

168

Intensity (a.u.)

Intensity (a.u.)

400 600 800 1000 1200 Binding Energy (eV)

285.8 eV

288.2 eV 289.2 eV 284 286 288 290 Binding Energy (eV)

292

524.0 eV


401.0 eV 404.9 eV 396

526

N 1s

400.0 eV

V 2p (f)

516.3 eV

516

397.9 eV


406

O 1s

529.4 eV

Intensity (a.u.)

(d)

200

C 1s (c)

284.8 eV

Intensity (a.u.)

0

(b)

N 1s V 2p O 1s

C 1s

Intensity (a.u.)

(a)

Bi 4f

without any other impurities.

Intensity (a.u.)

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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531.0 eV

528

530 532 534 Binding Energy (eV)

Figure 3. XPS spectra of the RGO/CN/BVO: (a) survey spectra, (b) C 1s, (c) N 1s, (d) Bi 4f, (e) V 2p and (f) O 1s. 3.2 Optical Absorption Properties. The optical properties of the as-prepared samples were determined by the UV-Vis DRS technique. Figure 4a displays the DRS of as-prepared CN, BVO, RGO/CN, RGO/BVO, CN/BVO and RGO/CN/BVO. The BVO exhibited steep band gap absorption edge which is located at around 540 nm and the absorption edge of CN is located at about 450 nm. With comparison to bare CN and BVO, the RGO/CN, RGO/BVO and RGO/CN/BVO exhibit the enhanced visible light absorption, which 10


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could be ascribed to the intrinsic absorption of RGO.67 Meanwhile, the digital picture of the samples showing the color change was illustrated in inset of Figure 4a. The results demonstrate that the enhanced adsorption in the visible light range of the RGO/CN/BVO ternary nanocomposite could result in the formation of more electron-hole pairs and favorably enhanced photocatalytic activity. The band gap energy of bare CN and BVO samples can be determined by the following formula: αhυ = A(hυ ‒ Eg)n/2, where α, υ, A, and Eg represent the absorption coefficient, light frequency, proportionality constant and bandgap, respectively. As shown in Figure 4b, for CN, the value of n is 4 for the indirect transition and the Eg of CN was determined to be 2.7 eV according to a plot of (αhυ)1/2 versus energy (hυ). As for BVO, the Eg of BVO was determined to be around 2.4 eV from a plot of (αhυ)2 versus energy (hυ). These Eg results of CN and BVO are very approach to values previously reported.38,68-70 The above results suggest that the CN and BVO can respond to visible light and the RGO/CN/BVO ternary nanocomposite can efficiently utilize visible-light, possibly resulting in the formation of more electron-hole pairs.

200

300

400 500 600 Wavelength (nm)

700

800

2.0

3

g-C3N4

BiVO4

1.5

(eV)

1/2

4

1/2

CN RGO/CN BVO CN/BVO RGO/BVO RGO/CN/BVO

2

BVO CN/BVO RGO/BVO RGO/CN/BVO

2.5

2

RGO/CN

********************* ********* ***** ****** ****************** ******************* ****** *** ******** ****** ****************************** ********************************************** *** *** **************************************************************** * * ** * ** * * *** *** ** ************** * * * ** * * * * *** * ** * ** *** ************* ********* ***** * * **** ******** ** * *************************** *************************************** *** * * * * * * ************* **** * * * * * ** * ***** ** *** ** ****** * * * * * * * * * * * * ***** **************** *** ** *** ************************ * * * * * * ***** ************* ************************************ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *** *************** *** **** ** ************ *** *** *** ** * *************** ** ***************************** ************************************** ********** **** ***** * * * * * * * * * * * * * * * * * * * * * ****** *** * ** **** ** ****** ************** *** *** * *** *********** ****** * *** **** **** ******** ******* ** **************************************** *** **** * **** *** ******** * **** ********* ********* *** ******** *** *********** *** * ** *********** *** ************ ** ************** **** ********************************** ***** ******************************************************************************************************************************* ***** ***** ******* *** ************ * * * * * ****** ************** ** ******** ******************************************************************************************************************************************************************************************************************************************************************************************************* ******** ************** ************************************************************************************************************* *** ******************************************************************************************************************************************************************************************************************

αhv (eV)

CN

5

2 1 0 2.0

1.0

2.4 eV

2.7 eV

2.5

3.0 hv

αhv

(b)

(a) Absorbance (a.u.)

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5

3.5

0.0 4.0

Figure 4. (a) UV-vis diffuse reflectance spectra of different samples, (b) Plot of

(αhυ)1/2 vs. hυ for the band gap energy of CN and plot of (αhυ)2 vs. hυ for the band gap energy of BVO. 3.3. Photocatalytic Activity. 11



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The photocatalytic performances of the as-synthesized samples for the degradation of TC (35 mg/L) were examined under visible light irradiation. Figure 5a displays the variation of the TC concentration (C/C0) with irradiation time over different as-prepared samples. Before the illumination, the adsorption/desorption equilibrium of the TC on the photocatalyst powders after magnetically stirred for 30 min in the dark was established. It can be found that the photolysis degradation of TC in the absence of photocatalyst is negligible. The photocatalytic activities of RGO/CN/BVO were higher than that of other samples. After 150 min irradiation, the TC degradation rates over BVO, RGO/BVO, CN, RGO/CN, CN/BVO and RGO/CN/BVO were 45.8%, 50.4%, 59.5%, 64.0%, 62.0% and 72.5%, respectively. The reaction kinetics of TC degradation over the different samples was then studied. As shown in Figure 5c, it can be seen that the kapp for TC degradation over RGO/CN/BVO can reach 0.00841 min−1, which is obviously higher than those of single-component BVO (0.00390 min−1) and CN (0.00588 min−1), as well as binary RGO/BVO (0.00454 min−1), BVO/CN (0.00628 min−1), and RGO/CN (0.00663 min−1). Figure 5d shows the temporal evolution of the absorption spectra of TC in the presence of RGO/CN/BVO sample. It can be found that the main absorption peak intensity of TC molecules located at about 357 nm decreased gradually with extension of the reaction time. The above results confirm that RGO/CN/BVO ternary nanocomposite shows superior photocatalytic activity towards TC.

12


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Figure 5. (a) Photocatalytic degradation of TC as a function of irradiation time, (b and c) kinetic curves and reaction rate constant for TC degradation over the as-prepared photocatalysts, (d) UV-vis curves for the photodegradation of TC under visible light irradiation over the RGO/CN/BVO ternary nanocomposite. The stability is an important factor for the practical application of the photocatalyst. The stability and recyclability of the RGO/CN/BVO ternary nanocomposite was performed. As shown in Figure 6a, the activity of the RGO/CN/BVO photocatalyst decreased slightly after four cycles, indicating that the RGO/CN/BVO nanocomposite is stable during the photocatalytic degradation of TC. The stability of the RGO/CN/BVO was further confirmed by the XRD pattern. As shown in Figure 6b, the XRD pattern of the recycled sample is unchanged as compared to the sample before photoreaction. These results suggest the high stability of RGO/CN/BVO ternary nanocomposite, which is very important for the practical 13



application in environmental remediation. Figure 6c shows the recovery rate of RGO/CN/BVO ternary nanocomposite in various cycles. It can be found that more than 71% recovery rate can be achieved, indicating the good stability of the RGO/CN/BVO photocatalyst. (a) 1.0

1st

3rd

2nd

(b)

4th

Before After

0.9 Intensity (a.u.)

0.8

C/C0

0.7 0.6 0.5 0.4 0.3 0.2 0

100

200

300 400 Time (min) (c) 80

Recovery Rate (%)

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500

600

10

20

30

40 50 2θ (degree)

60

70

80

60

40

20

0 1

2

3

4

Cycles

Figure 6. (a) The repeated photocatalytic experiments for RGO/CN/BVO ternary heterostructure under visible light irradiation. (b) XRD patterns of RGO/CN/BVO before and after 4th run cycle photocatalytic experiments. (c) Recovery rate of the as-prepared RGO/CN/BVO sample in various cycles. Table 1 BET Surface Area and Kinetic Rate Constant of the As-prepared Samples. Surface area Kinetic rate constant (K) Samples

(m2g-1)

(min-1)

CN

30.73

0.00588

BVO

0.19

0.0039

CN/BVO

1.88

0.00628

RGO/BVO

1.67

0.0454 14


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CN/RGO

30.94

0.00663

RGO/CN/BVO

1.08

0.00841

3.4. Charge Transfer Properties. In general, the photocatalytic activity is mainly governed by light-absorption ability, surface properties (including adsorption property, specific surface area, etc.), and photogenerated charge-separation efficiency. The specific surface areas data of the as-prepared samples calculated by BET methods are summarized in Table 1. This result indicates that the BET surface area is not the crucial factor that determines the photocatalytic activity of the as-prepared samples. The separation and recombination processes of photoinduced charge carriers of the as-prepared samples are examined by PL emission spectra. Generally, the weaker PL emission intensity indicates the better separation property of the photoinduced carriers, which could lead to high photocatalytic activity. As shown in Figure 7a, the main emission peaks centered at about 532 nm and 450 nm for bare BVO and CN sample, respectively. When the RGO or CN was introduced, one can observe that the emission intensities of the binary and ternary composites are obviously decreased. Of note, the RGO/CN/BVO ternary nanocomposite shows the lowest PL intensity, suggesting that the introduction of RGO could contribute to the effective charge separation and transfer, resulting in the enhanced photocatalytic activity toward the TC degradation. To get further insight into the carrier separation efficiency in the RGO/CN/BVO system, the photocurrent-time measurement was performed (Figure 7b). It can be found that the photocurrent of the RGO/CN/BVO ternary nanocomposite was around 3-fold and 2-fold higher than that of bare BVO and CN samples, respectively, which is accordance with the photocatalytic activity results. These results further confirm that the formation of RGO/CN/BVO ternary nanocomposite is beneficial for the 15



separation and transfer of photoinduced charge carriers, thereby improving the photocatalytic efficiency. (a)

(b) 0.6

CN RGO/CN

Intensity (a.u.)

on

RGO/CN/BVO RGO/CN CN/BVO CN RGO/BVO BVO

off

0.5

440

480 520 560 Wavelength (nm)

Photocurrent (µA)

Intensity (a.u.)

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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600

BVO RGO/BVO CN/BVO RGO/CN/BVO

0.4 0.3 0.2 0.1 0.0

500

520

540 560 Wavelength (nm)

580

600

100

120

160 180 200 220 Irradiation Time (s)

240

260

280

Figure 7. (a) Photoluminescence (PL) and (b) Time-based photocurrent response plots of BVO, CN, RGO/BVO, CN/RGO, CN/BVO, and RGO/CN/BVO samples. 3.5. Possible Photocatalytic Mechanism. To determine the main active species in the photocatalysis process over RGO/CN/BVO nanocomposite, the tert-butanol (T-BuOH), ammonium oxalate (AO) and 1,4-benzoquinone (BQ) were used as scavengers for the quench of hydroxyl radical (•OH), holes (h+) and superoxide radical (•O2−), respectively. As illustrated in Figure 8, with the addition of AO and T-BuOH, the TC degradation efficiency decreased slightly, which indicates that the h+ and •OH is not the main reactive specie. In the presence of BQ, the photocatalytic degradation rate of TC was greatly inhibited, suggesting that the •O2− play the important role in the photodegradation of TC. To further verify this, a control experiment in the presence of N2 atmosphere was conducted. The result indicates that the photodegradation of TC decreased largely, confirming that O2 could perform as efficient electron traps, leading to formation of •O2−. Thus, these results explicitly indicate that the •O2− played the significant role during the photocatalytic degradation of TC.

16


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Figure 8. Trapping experiment of active species during the photocatalytic degradation of TC over RGO/CN/BVO ternary heterostructure under visible light irradiation. To further clarify the photocatalytic mechanism, the ESR spectra of DMPO spin-trapped adducts was used to detect the presence of •O2− and •OH radicals. As shown in Figure 9, no ESR signal corresponding to the DMPO-O2•− adduct can be found for RGO/CN/BVO ternary nanocomposite in the dark, indicating that the no detectable •O2− species was formed in the photocatalytic reaction. However, when turning on the light, strong ESR signal ascribed to the •O2− can be found, revealing the formation of •O2− species in the photocatalytic reaction. Similarly, the intensity of characteristic quadruple peaks of the DMPO‒•OH adducts under the visible light irradiation is enhanced as compared to the case in the dark. This result indicates the formation of •OH in the photocatalytic process. The above ESR results are in accordance with the above active species trapping experiment (Figure 8).

17



(a)

-

DMPO-O2

(b)

DMPO-OH Light on

Light on

Intensity (a.u.)

Intensity (a.u.)

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Dark

Dark

3460

3480

3500 3520 Magnetic Field (G)

3540

3560 3460

3480

3500 3520 Magnetic Field (G)

3540

3560

Figure 9. DMPO spin-trapping ESR spectra for RGO/CN/BVO: (a) in methanol dispersion for DMPO–•O2– and (b) in aqueous dispersion for DMPO–•OH. In general, II-type heterojunction and Z-scheme are the two main charge transfer mechanism to explain the photocatalytic activity of the composite systems. From the II-type heterojunction point, since the CB potential of CN is more negative than that of BVO, so the electrons on the CB of CN would migrate to the BVO, and the holes in the VB of BVO would simultaneously migrate to the VB of CN. However, as the VB potential of CN (+1.59 eV vs NHE) is more negative than the potential required to the formation of •OH radicals (+1.99 eV vs NHE), the holes on the VB of CN cannot produce •OH radicals. Similarly, since the CB potential of BVO (-1.12 eV) is more positive than the O2/•O2− potential (-0.046 V vs NHE), the electron on the CB of BVO cannot reduce oxygen molecules into •O2−. Therefore, this II-type heterojunction charge transfer mechanism was ruled out. Therefore, a plausible Z-scheme charge transfer mechanism over the RGO/CN/BVO ternary nanocomposite is proposed. As shown in Figure 10, both CN and BVO could be excited and produced electron and hole under visible-light irradiation. The electrons in the CB of BVO would transfer to the VB of CN and recombine with the holes in the VB of CN. This process would be accelerated with the assistance of RGO, which would result in the improved photocatalytic performance as compared to binary CN/BVO composite. The 18


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accumulated electrons in CB of CN and holes in VB of BVO could capture the dissolved O2 and H2O or −OH and formed the oxidizing species •O2− and •OH radicals, respectively, which are the main oxidative species for the degradation of TC under visible light irradiation. Consequently, we can conclude that the enhanced photocatalytic performance of the RGO/CN/BVO can be mainly ascribed to the efficient separation of charge carriers by following the RGO-promoted Z-scheme mechanism.

Figure 10. Possible photocatalytic mechanism scheme of the RGO/CN/BVO. 4. CONCLUSION In

summary,

novel

all-solid-state

Z-scheme

RGO/CN/BVO

ternary

heterostructure was fabricated by a facile hydrothermal method. The RGO/CN/BVO ternary heterostructure exhibited the superior photocatalytic activity for the degradation of TC under visible light irradiation. The experiment results show that the RGO/CN/BVO degradation rate is about 1.13, 1.16 and 1.41 times higher than those of binary RGO/CN, CN/BVO, and RGO/BVO, respectively. It is proved that the enhanced photocatalytic activity could be mainly due to a RGO-promoted Z-scheme transfer of the photoinduced charge carriers. In addition, the as-prepared RGO/CN/BVO nanocomposite showed good stability in the photocatalytic degradation process of TC. This study indicates that the as-prepared RGO/CN/BVO 19



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ternary nanocomposite is a promising photocatalyst for the degradation of organic pollutant. ACKNOWLEDGMENTS This work was supported by the financial supports of National Nature Science Foundation of China (No. 21606111, 21406091, and 21576121), Natural Science Foundation of Jiangsu Province (BK20140530 and BK20150482), China Postdoctoral Science

Foundation

(2015M570409),

and

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(70) Chen, F.; Yang, Q.; Li, X. M.; Zeng, G. M.; Wang, D. B.; Niu, C. G.; Zhao. J. W.; An, H. X.; Xie, T.; Deng, Y. Hierarchical Assembly of Graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme Photocatalyst: An Efficient, Sustainable and Heterogeneous

Catalyst

with

Enhanced

Visible-light

Photoactivity

towards

Tetracycline Degradation Under Visible Light Irradiation. Appl. Catal. B: Environ. 2017, 200, 330-342.

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BiVO4 Z-Scheme

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