MWCNTs Nanocomposites with

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Materials and Interfaces 3

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One-pot fabrication of g-CN/MWCNTs nanocomposites with superior visible-light photocatalytic performance Fei Ding, Zhanfeng Zhao, Dong Yang, Xuyang Zhao, Yao Chen, and Zhongyi Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05293 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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Industrial & Engineering Chemistry Research

One-pot fabrication of g-C3N4/MWCNTs nanocomposites with superior visible-light photocatalytic performance Fei Ding a, d‡, Zhanfeng Zhao a, d‡, Dong Yang b, c, Xuyang Zhao a, d, Yao Chen a, d and Zhongyi Jiang a, d* a Key Laboratory for Green Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China b Key Laboratory of Systems Bioengineering of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 30072, China c School of Environmental Science and Engineering, Tianjin University, 300072 Tianjin, China d Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China  Corresponding author. Tel: 86-22-27406646. Fax: 86-22-23500086. E-mail address: [email protected] ‡ These authors contributed equally.

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Abstract

Visible-light photocatalysis has drawn tremendous attention in the field of chemical industry owing to its considerable significance in pollution abatement and energy supply. Herein, graphitic carbon nitride/multiwalled carbon nanotubes (g-C3N4/MWCNTs) nanocomposites are fabricated using the melamine-cyanuric acid supramolecular assembly as precursor via a facile one-pot method. On one hand, the stable covalent connection between g-C3N4 and MWCNTs is built during the self-assembling process of the supramolecular precursor, which intensifies catalytic and adsorption ability. On the other hand, MWCNTs confer both active sites and radially single-track electron transfer path, which is conducive to restrain the combination of photogenerated carriers. The g-C3N4/MWCNTs nanocomposite with the optimal MWCNTs content can completely degrade rhodamine B (RhB) solution within 1 h, and the degradation rate is improved ~6.6 times compared with pure g-C3N4. This facile strategy for fabricating efficient and stable visible-light-driven photocatalysts may hold great promise in large-scale water treatment.

Keywords: g-C3N4；MWCNTs；nanocomposites; one-pot fabrication; photocatalysis.


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1. Introduction Environmental crisis has become a global challenge with the rapid growth of population and modern industry. In recent years, a number of researchers have paid tremendous efforts in utilizing clean energy, especially solar energy to replace fossil energy

1-7.

Particularly, the

visible-light photocatalysis, which can implement the solar-energy conversion using the sunlight as the only driving force, has been regarded as a feasible solution to the environmental crisis 2. Up to now, a variety of photocatalysts have been explored, such as metallic, non-metallic and sensitizer-modified semiconductors. These photocatalysts exhibit noteworthy catalytic property, however, most of them lack the ability to directly utilize visible light. Therefore, owing to the visible-light responsive ability, g-C3N4 has drawn enormous attention since its first application in H2 evolution in 2009 8. Moreover, on account of the narrow band gap, stable physicochemical property and earth-abundant raw materials, g-C3N4 quickly becomes a hotspot in the photocatalysis field. At present, there has derived many kinds of precursors to prepare g-C3N4, such as single raw material (melamine, urea, cyanamide) and two kinds of raw materials (melamine-cyanuric acid). Among these precursors, the melamine-cyanuric acid supramolecule, which is composed of up to three hydrogen bonds 9, is expected to create tunable nano- or microstructures depending on the synthetic conditions

10.

Furthermore, this kind of

supramolecular assembly possesses a pre-organized hydrogen-bond network, which may reduce the sublimation of the N-rich molecules during polycondensation at elevated temperatures. However, the pure g-C3N4 is hampered by some intrinsic drawbacks, for instance, the agglomeration of bulk phase, insufficient exposure of surface-active sites, poor electrical conductivity and fast recombination rate of photo-generated carriers strategies, such as energy band engineering

13-15

and morphology control

11-12. 16-17,

Thus, several were applied to

modify g-C3N4. Among the strategies, constructing nanocomposites has been recognized as an


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effective method. The nanocomposites can be synthesized through hybridizing g-C3N4 with other materials, including non-metallic

18-25

and metallic materials

26-39.

In recent years, non-metallic

materials, such as CNTs, ceramic, graphdiyne and graphene, have been broadly utilized for the superior electronic properties and large specific surface area 40-43. Among these materials, CNTs are very popular because of their high tensile strength, unidirectional conductivity and excellent capacity of accepting and reserving electrons. Apart from these advantages, 1D CNTs with effective electron transfer channels can also steer the electron transport more efficiently, which thereby has the potential to facilitate the visible-light utilization and the photo-generated carriers separation

44.

So far, several attempts on constructing g-C3N4/CNTs nanocomposites as

photocatalysts have been reported. Ge and his coworkers prepared g-C3N4/MWCNTs nanocomposite via a direct heating method using cyanamide as precursor 19. Liu et al. and Chen et al. also used cyanamide as precursor to synthesize g-C3N4/N-CNTs and g-C3N4/CNTs by hard template method and direct heat-treatment method, respectively

23, 45.

Though the photocatalytic

activity has been intensified through these efforts, the electrostatic repulsion between g-C3N4 and CNTs with same surface electronegativity restrained their combination. Herein, the melamine-cyanuric acid supramolecule was screened to prepare g-C3N4/MWCNTs nanocomposites by a facile one-pot method. MWCNTs are wrapped inside and the –NH2 groups of g-C3N4 can react with carboxyl (-COOH) groups of MWCNTs, which would make the interfaces more compact in the process of supramolecular self-assembly. In the nanocomposites, g-C3N4 acts as a base semiconducting photocatalyst while MWCNTs act as electron mediator for the transfer of photo-generated carriers. Therefore, the as-prepared g-C3N4/MWCNTs nanocomposites may exhibit improved photo-utilization efficiency with the addition of MWCNTs. RhB was chosen as the substrate due to its wide applications, strong carcinogenicity


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and neurotoxicity, in particular, its popularity as a model dye. Moreover, the feasible photocatalytic mechanism is proposed, and the roles of g-C3N4 and MWCNTs are investigated and discussed in detail.

2. Experimental section 2.1 Chemicals The commercially supplied MWCNTs were purchased from Nanjing XFNANO Materials Tech Co. Ltd., and the parameters were as follows: outer diameter (OD) < 8 nm, length of 10-30 μm, purity > 95% and –COOH content was about 3.86 wt.%. Other chemicals were analytical grade and bought from Tianjin Guangfu Technology Development Co. Ltd. All chemicals were used without further purification. 2.2 Synthesis of g-C3N4/MWCNTs nanocomposites G-C3N4/MWCNTs nanocomposites were prepared as follow. First, melamine (6 mmol) and cyanuric acid (6 mmol) were added as precursors to 160 mL acetonitrile solution, and then a certain amount of MWCNTs were added. This obtained reaction solution was ultrasonicated for 30 min and then stirred for another 12 hours. The evenly dispersed mixture was then transferred to the Teflon flask. The precipitate was separated and dried overnight after maintaining the temperature at 180 ℃ for 12 h, and then calcined for 2 h at 520 ℃ in a porcelain crucible (5 ℃ min-1). The solid obtained after grinding are g-C3N4/MWCNTs nanocomposite photocatalysts, denoted by g-C3N4/MWCNTs-X (X=1, 2, 3, 4, 6, 8, the weight percentage of MWCNTs in the sample). For a comparative evaluation, pure g-C3N4 was synthesized analogously in absence of MWCNTs. 2.3 Characterization


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The samples were characterized by transmission electron microscopy (TEM ， JEM-2100F, Japan), X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, UK), X-ray powder diffraction (XRD, D8-Focus, Germany), Fourier transform infrared spectroscopy (FTIR, ALPHA, Germany), UV-vis diffuse reflection spectroscopy (DRS, Hitachi U-3010, Japan) and Photoluminescence spectra (PL, FLS980, UK). The Brunauer-Emmett-Teller measurement (BET, Tristar-3000, USA) and the Barrett-Joyner-Halenda (BJH) were employed to characterize specific surface area and pore size distribution. 2.4 Photocatalytic measurements In the degradation experiment, the RhB solution (30 mL, 10 mg L-1) with 30 mg photocatalyst was firstly stirred for 1 h without light. After that, the quartz reactor was illuminated by a Xeon lamp (300 W, ＞420 nm). The degradation measurements were repeated three times to insure the reproducibility. The fast digestion-spectrophotometer method was used to determine the chemical oxygen demand (COD), and a digester (DRB200, Hach, USA) was applied. The radical trapping experiment was performed by adding different trapping agents into the RhB solution. Three radical scavengers including benzoquinone (1 mmol L-1), methanol (1:15 in volume) and disodium ethylenediaminetetraacetate (EDTA-2Na, 10 mmol L-1) were used to capture the specific reactive species superoxide anion radical (·O2-), hydroxyl radical (·OH) and photoinduced hole (h+), respectively.

3. Results and discussion 3.1 Structure and morphology The successful synthesis of g-C3N4/MWCNTs nanocomposites is evidenced by the XRD patterns. As shown in Figure 1, the characteristic diffraction peaks of g-C3N4 can be observed at


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27.5o and 13.1o: the peak at 27.5o is assigned to the stacking of aromatic segments and the peak at 13.1o reveals the existence of in-planar structure ordering of triazine units 8. The XRD peak of MWCNTs at 26.5o can be attributed to its (002) crystal plane, indicating that MWCNTs have a high crystallinity

46.

However, the characteristic peaks of MWCNTs seem to be absent in g-

C3N4/MWCNTs-2 nanocomposites, which is ascribed to their low content and high dispersion in the samples. The FTIR spectra confirm the interaction between g-C3N4 and MWCNTs in Figure 2. For pure g-C3N4, the absorption peak at 810 cm-1 is assigned to the bending vibration of triazine rings 25, and the strong absorption bands located at 1200-1700 cm-1 are attributed to the characteristic skeletal modes of C-N heterocycles 47. Moreover, the broad absorption bands at 3000-3450 cm-1 are ascribed to the N-H stretching vibration 48. The roughly similar peaks of pure g-C3N4 and gC3N4/MWCNTs-2 further prove that the introduction of MWCNTs would not damage the structural integrity of g-C3N4. On the contrary, the stronger FTIR mode of g-C3N4/MWCNTs-2 nanocomposite confirms that the addition of MWCNTs can benefit for the enhancement of exposed surface functional groups. The present of functionalized MWCNTs can be proved by the existence of -OH (3400 cm-1), -COOH (1710 cm-1) and C=O (1650 cm-1) functional groups 23, 49, as well as the stretching vibration peak of C=C at ca. 1570 cm-1

50.

However, due to the small

percentage of MWCNTs in g-C3N4/MWCNTs nanocomposite and the peak coverage of C-N triazine heterocycles, it is difficult to distinguish the peaks belonging to the functionalized MWCNTs from the FTIR spectrum.


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Figure 1. XRD patterns of pure g-C3N4, MWCNTs and g-C3N4/MWCNTs nanocomposites.

Figure 2. FTIR spectra of pure g-C3N4, MWCNTs and g-C3N4/MWCNTs-2. The surface morphology images were detected by SEM and TEM. The MWCNTs are well dispersed with the external diameter less than 8nm and length more than 10 μm (Figure 3a) and pure g-C3N4 is aggregated by irregular particles (Figure 3b and d). After the introduction of a small amount of MWCNTs, the morphology of g-C3N4/MWCNTs-2 is affected and exhibits regular structure. It can be seen in Figure 3c that MWCNTs are enclosed by g-C3N4, implying


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the in-site generation of g-C3N4 on MWCNTs. The dispersion of MWCNTs in gC3N4/MWCNTs-2 nanocomposite can be found in TEM images (Figure 3e), and a uniform dyadic structure forms due to the amidation reaction between g-C3N4 and MWCNTs. These images confirm that the MWCNTs and g-C3N4 have been effectively combined together.

Figure 3. SEM images of (a) MWCNTs, (b) pure g-C3N4 and (c) g-C3N4/MWCNTs-2; TEM images of (d) pure g-C3N4 and (e) g-C3N4/MWCNTs-2.

The interaction between the compositions in photocatalyst was revealed by XPS analysis. Figure 4a shows that there are only C, N, and O element present on the surface of the sample and no impurity exists by taking g-C3N4/MWCNTs-2 as a representative sample. The high-resolution


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C 1s XPS spectrum (Figure 4b) includes three peaks after peak fitting: 284.6 eV, 286.3 eV and 288.1 eV. These peaks can be assigned to adventitious graphitic carbon and C-C bond from MWCNTs (284.6 eV)

25,

the amide groups (-CONH-) during the covalent combination of

MWCNTs with g-C3N4 (286.3 eV)51, and the carbon atoms bonded to three nitrogen atoms in the g-C3N4 lattice (288.1 eV)

52.

The high-resolution N 1s curve (Figure 4c) exhibits chemically

different N species with binding energies of 398.6 eV, 399.8 eV and 400.9 eV, indicating the coexistence of some distinguishable nitrogen environments. The peak at 398.6 eV is formed due to N atoms sp2-hybridized to two carbon atoms (C=N-C), which confirms the presence of graphitelike sp2-bonded g-C3N4. The peaks formed at 399.8 eV and 400.9 eV correspond to the tertiary nitrogen (N-(C)3)

53

and amide groups (-CONH-)

51,

respectively, which can confirm the

formation of interaction between MWCNTs and g-C3N4.


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Figure 4. (a) XPS survey spectrum of g-C3N4/MWCNTs-2 nanocomposite photocatalyst ； High-resolution XPS spectra of g-C3N4/MWCNTs-2: (b) C 1s and (c) N 1s.


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The results of specific surface area and pore size distribution are shown in Figure 5. The typeIV isotherms with an H3 hysteresis loop reflect that mesopore structure exists in as-prepared gC3N4/MWCNTs-2 nanocomposites 54. The calculated BET surface area of g-C3N4/MWCNTs-2 and pure g-C3N4 is 82.6 m2 g-1 and 50.3 m2 g-1, respectively, both of which are larger than those of previous reports in literatures 54-55. These results confirm that the facile one-pot method using supramolecule to produce g-C3N4-based composite is in favor of enlarging the specific surface area, thereby favoring for the enhanced photocatalytic performance. The mainly pore diameter of g-C3N4/MWCNTs-2 is 3.5 nm, which is corresponding to the mesopores of g-C3N4 (3.1 nm, the inset of Figure 5). The larger pore diameter values appear at about 20 nm can be assigned to the similar pore size of MWCNTs, which can be intuitively observed in Figure 3a and e. According to above results, it can be inferred that the porosity of the nanocomposites retained adequately after introducing a small amount of MWCNTs.

Figure 5. N2 adsorption-desorption isotherms and BJH pore size distribution plot (inset) of pure g-C3N4 and g-C3N4/MWCNTs-2.


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3.2 Optical properties DRS analysis was performed to evaluate the optical absorption of photocatalyst samples. In Figure 6a, the visible light absorption of nanocomposites increases significantly and an obvious redshift can be observed as the concentration of MWCNTs increases. The band gap of photocatalyst samples can be estimated according to the tangent intercept method 57. Therefore, through the plots of (αhν)1/2 versus hν in Figure 6b, it can be estimated that the band gaps of gC3N4/MWCNTs nanocomposites are narrowed compared with pure g-C3N4 (2.80 eV). Moreover, with increasing the MWCNTs content, the absorption intensity increases obviously at the meantime, where the values from g-C3N4/MWCNTs-1 to g-C3N4/MWCNTs-8 are 2.73 eV, 2.72 eV, 2.70 eV, 2.65 eV, 2.57 eV and 2.50 eV, respectively. From these results, it can be confirmed that the introduction of MWCNTs into photocatalysts can increase light absorption performance and photocatalytic activity.


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Figure 6. (a) UV-vis diffuse reflectance spectra of pure g-C3N4 and g-C3N4/MWCNTs nanocomposites and (b) their corresponding plots of (αhν)1/2 versus hν.

Typically, the separation efficiency of photogenerated carriers can be qualitatively characterized by PL emission measurement, in which lower PL intensity corresponds to better separation of photogenerated carriers. As illustrated in Figure 7, the main emission peak is centered at 460 nm because of the band-band PL phenomenon with the energy of light approximately equal to the band gap energy of g-C3N4

25.

The lower emission peak of g-

C3N4/MWCNTs-2 is attributed to the electron transfer channel of MWCNTs. When irradiated by visible light, these electron transfer channels can fast transfer the photogenerated electron to react with reactive radical and dye molecule. These results show that incorporating MWCNTs


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can efficaciously restrain the combination of photogenerated carriers, which also benefits for promoting the photocatalytic activity.

Figure 7. PL spectra of pure g-C3N4 and g-C3N4/MWCNTs-2.

3.3 Photocatalytic performance In general, RhB degradation experiments would be interfered by incomplete adsorption of the catalytic substrate. Therefore, the dye solution that contained photocatalysts was stirred in dark for 1 h, and adsorption isothermal curve of RhB solution (Figure S1) is plotted to guarantee adsorption equilibrium. The adsorption quantity is calculated through the concentrationadsorption standard curve of RhB solution (Figure S2). The degradation curves in Figure 8a show that pure g-C3N4 can completely degrade a constant concentration of RhB within 4 h with light illumination. While most of g-C3N4/MWCNTs photocatalysts, except g-C3N4/MWCNTs-6 and g-C3N4/MWCNTs-8, show improved degradation performance with the same conditions. As increasing the content of MWCNTs, the degradation rate of g-C3N4/MWCNTs nanocomposites first elevates then descends. At the optimal condition, the g-C3N4/MWCNTs-2 nanocomposite exhibits the fastest degradation rate of 100% within 1 h. This excellent degradation efficiency is


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benefited from the combination of MWCNTs with g-C3N4 in the nanocomposite, i.e. MWCNTs are embedded into g-C3N4/MWCNTs nanocomposites as an electronic transmission network, which can effectively improve the electron transfer capability. Moreover, the optimal content of MWCNTs in g-C3N4/MWCNTs photocatalyst can facilitate the RhB adsorption on their surface. High MWCNTs content may occupy the surface-active sites of photocatalyst, and the decreased surface exposure accounts for the reduced photocatalytic activity of high MWCNTs content samples. Low content of g-C3N4 may cause the interfacial charge transfer relatively recede, and meanwhile the absorption ability for visible light declines because the MWCNTs agglomerate to smother the surface of photocatalyst. The kinetic curves of g-C3N4/MWCNTs nanocomposites are plotted in Figure 8b. All curves fit well with the pseudo-first order reaction model: -ln (C/C0) = kt, in which k represents the apparent reaction rate constant (h-1) that can reflect the photocatalytic activity explicitly, C and C0 are the instantaneous and original concentration of RhB (mg L-1) respectively. The photocatalytic activities shown in Figure 8a conform to the order of k values. The g-C3N4/MWCNTs-2 nanocomposite has the largest k value of 0.066 min-1, indicating the degradation rate is improved ~6.6 times compared with pure g-C3N4.


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Figure 8. (a) Photocatalytic activities of pure g-C3N4 and g-C3N4/MWCNTs photocatalysts and (b) their kinetic curves.

The recyclability of g-C3N4/MWCNTs photocatalysts is another important property that needed to be explored. Figure 9 shows that the g-C3N4/MWCNTs-2 nanocomposite was used as the representative sample for the RhB degradation cycles. After four-cycle degradation process, the sample shows the negligible loss of photocatalytic activity, which reveals the prominent recyclability

of

g-C3N4/MWCNTs

nanocomposites.

Moreover,

the

g-C3N4/MWCNTs

photocatalysts exhibit superior photocatalytic activity compared with most of photocatalysts with


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similar composition for dye degradation (Table 1), even though the evaluation system and reaction condition are different.

Figure 9. Cycle runs of g-C3N4/MWCNTs-2 photocatalysts for the RhB degradation.

Table 1. Comparison of g-C3N4/MWCNTs nanocomposite photocatalyst with representative gC3N4-based photocatalysts for the dye degradation under visible-light irradiation. Ccatalyst

Cdye

(mg mL-1)

(mg L-1)

Photocatalysts

Degradation efficiency

Light source

g-C3N4/ MWCNTs

1h, 100%

300 W (λ ＞ 420 1.0 nm)

RhB, 10

this work

N-CNT/ g-C3N4

1h, 95%

300 W (λ ＞ 400 0.5 nm)

RhB, 10

45

CNT/white C3N4

3h, 100%

300 W (λ ＞ 400 1.0 nm)

MB, 10

24

g-C3N4/BiVO4

5h, 85%

500 W (λ ＞ 420 1.0 nm)

RhB, 4.8

56

g-C3N4/GO aerogels

4h, 92%

500 W (λ ＞ 420 1.0 nm)

MO*, 20

57

g-C3N4/rGO

1.25h, 100%

1000 W (λ ＞ 400 1.6 nm)

RhB, 5

20

ref.


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*MO, methyl orange.

3.4 Photocatalytic mechanism In general, many kinds of reactive radical species may be involved during photocatalytic degradation, thus for exploring the photocatalytic mechanism, it is necessary to utilize radical scavengers to shield the corresponding reactive radical species. In the radical trapping experiment, methanol, benzoquinone and EDTA-2Na were respectively added to remove three major active free radicals: ·OH, ·O2-, and h+. A slight decrease of degradation efficiency can be observed in Figure 10a after adding methanol and EDTA-2Na, which indicates ·OH and h+ play minor roles in g-C3N4/MWCNTs photocatalytic system. Whereas the addition of benzoquinone molecules can affect the degradation efficiency prominently, which almost completely eliminate the photocatalytic activity. These results confirm that ·O2- is the major active substance in course of dye degradation, while ·OH and h+ perform a minor effect. The UV-vis spectra of RhB at different reaction times (Figure 10b) were conducive to the analysis of the degradation mechanism of RhB. With the irradiation duration increasing, the absorption at 553 nm attenuates quickly and there is a blue shift of the maximum absorption from 553 nm to 495 nm, which indicates the N-deethylation process of dye molecule. Moreover, the decrease of characteristic absorption peak of aromatic rings (from 230 nm to 270 nm) further confirms the thorough RhB degradation58, and this process is recognized as the destruction of conjugated structure. Thus, it can be concluded that the RhB degradation contains two competing processes: N-deethylation and destructing conjugated structure. Furthermore, chemical oxygen demand (COD) analysis was carried out to detect the organics concentration in the solution. As shown in Figure S3, the organics are removed gradually with the illumination time increased, especially the low COD


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value of the final degradation product stabilizes at 5 mg L-1 at 1 h, which further confirms the RhB mineralization.

Figure 10. (a) Effect of different scavengers on the RhB degradation in the presence of gC3N4/MWCNTs-2 photocatalyst and (b) UV-vis spectra of RhB solution at different reaction times.

Figure 11. Schematic illustration of the photocatalytic mechanism for the RhB degradation over g-C3N4/MWCNTs nanocomposites.


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Thus, the photocatalytic mechanism for degrading RhB by g-C3N4/MWCNTs nanocomposite illuminated by visible light can be preliminarily obtained according to the above results. As shown in Figure 11, the MWCNTs that are added in the catalytic system builds up fast electron transport channels, which enable g-C3N4 to in-situ grow on MWCNTs and therefore improve the electron storage capacity of photocatalysts. The effective utilization of electrons and the prolonged lifetime of photogenerated electrons greatly enhance the visible-light catalytic performance of photocatalysts. Figure 11 shows the possible photocatalytic mechanism, in which g-C3N4 first receives the photon energy to generate photogenerated carriers (Equation (1)). And then arisen from the lower Fermi level, the photogenerated electrons of g-C3N4 tend to immigrate to the surface and react with O2 to produce ·O2-, then the radicals of ·O2- can further react with H+ and e- to form ·OH (Equation (2)~(8)) 59. However, the holes which are still entrapped in the VB of g-C3N4 cannot oxidize the adsorbed H2O to ·OH because the VB potential of g-C3N4 (+1.55 eV) is lower than that of OH-/ ·OH (+1.99 eV). ·O2-, h+ and ·OH all possess the ability to degrade RhB （Equation (6)~(8)）. The photocatalytic process can be described as follow: g ― C3N4/MWCNTs + hv→e ― + h +

(1)

e ― + O2→·O2―

(2)

·O2― + H + →·OOH

(3)

·OOH + H + + e ― →H2O2

(4)

H2O2 + e ― →·OH + OH ―

(5)

·O2― +RhB→Degradation products

(6)

h + +RhB→Degradation products

(7)

·OH + RhB→Degradation products

(8)


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4. Conclusions The g-C3N4/MWCNTs nanocomposites were synthesized using melamine-cyanuric acid supramolecular assembly as precursor via one-pot method. G-C3N4 acts as basal semiconducting photocatalyst in the nanocomposites while MWCNTs render both extra active sites and electron transfer channels. The addition of MWCNTs can dramatically elevate the electron conduction and the separation efficiency of photogenerated carriers. When MWCNTs content reaches to 2 wt.%, the g-C3N4/MWCNTs nanocomposite can completely degrade the RhB solution within only 1 h. Thus, this study may offer an original strategy for preparing g-C3N4-based composite photocatalysts, which possess superior photocatalytic activity for water treatment.

Acknowledgement The authors thank the financial support from the National Natural Science Fund of China (21621004， 91534126), National Science Fund for Distinguished Young Scholars (21125627), Tianjin

Research

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of

Application

Foundation

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Advanced

Technology

(18JCYBJC21000), Program of Introducing Talents of Discipline to Universities (B06006). Supporting information Adsorption isothermal curve of g-C3N4/MWCNTs-2 for the RhB solution, concentrationadsorption standard curve of RhB solution, and COD for RhB solution. References （1）Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev. 2009, 38 (1), 253-278. （ 2 ） George, C.; Ammann, M.; D'Anna, B.; Donaldson, D. J.; Nizkorodov, S. A. Heterogeneous photochemistry in the atmosphere. Chem Rev. 2015, 115 (10), 4218-4258. （ 3 ） Chen, x.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-based Photocatalytic Hydrogen


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MWCNTs Nanocomposites with

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