Non-metal photocatalyst nitrogen-doped carbon nanotubes modified mpg-C3N4: facile synthesis and the enhanced visible-light photocatalytic activity

Accepted Manuscript Non-metal photocatalyst nitrogen-doped carbon nanotubes modified mpgC3N4: facile synthesis and the enhanced visible-light photocatalytic activity Jinyuan Liu, Yanhua Song, Hui Xu, Xingwang Zhu, Jiabiao Lian, Yuanguo Xu, Yan Zhao, Liying Huang, Haiyan Ji, Huaming Li PII: DOI: Reference:

S0021-9797(17)30010-3 http://dx.doi.org/10.1016/j.jcis.2017.01.010 YJCIS 21916

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

25 October 2016 2 January 2017 4 January 2017

Please cite this article as: J. Liu, Y. Song, H. Xu, X. Zhu, J. Lian, Y. Xu, Y. Zhao, L. Huang, H. Ji, H. Li, Non-metal photocatalyst nitrogen-doped carbon nanotubes modified mpg-C3N4: facile synthesis and the enhanced visible-light photocatalytic activity, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis. 2017.01.010

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Non-metal photocatalyst nitrogen-doped carbon nanotubes modified mpg-C3N4:

facile

synthesis

and

the

enhanced

visible-light photocatalytic activity Jinyuan Liua, Yanhua Songb, Hui Xua*, Xingwang Zhua, Jiabiao Liana, Yuanguo Xua, Yan Zhaoa, Liying Huanga, Haiyan Jia, Huaming Lia* a. School of the Environment and Safety Engineering, Institute for Energy Research, Jiangsu University, Zhenjiang 212013, P. R. China. b. School of Environmental and Chemical, Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, P. R. China *Corresponding author: Tel.:+86-0511-88791108; Fax: +86-0511-88791108; E-mail address: [email protected], [email protected]

Abstract Nitrogen-doped carbon nanotubes (N-CNT) is a promising metal-free candidate and electronic acceptor. It has been employed to modify mesoporous carbon nitride (mpg-C3N4) for photocatalytic degradation of organic dye and antibiotics under visible-light irradiation. Herein, we report a facile synthesis strategy involving polymerization of cyanamide as the precursor in the presence of N-CNT via thermal polycondensation. The morphology and structure of as-prepared N-CNT/mpg-C3N4 were analyzed by scanning electron microscopy, transmission electron microscopy, fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The N-CNT/mpg-C3N4 exhibited increased photocatalytic activity for rhodamine B (RhB), methyl orange (MO) and tetracycline (TC) degradation compared with the pure one under visible-light irradiation, which is mainly due to the efficiently separation of 1

photogenerated electron-hole pairs for the introduction of N-CNT as electronic acceptor. The photocatalytic reaction can fit the first order kinetics. Additionally, superoxide radical (O2•−) was regarded as main reactive species participating in the photodegradation reaction process. Furthermore, the reason for enhancing photocatalytic activity of N-CNT/mpg-C3N4 is mainly attributed to synergistic effects between mpg-C3N4 as main ingredient and N-CNT as electron acceptor.

Keywords: Photocatalytic activity, N-CNT, mesoporous carbon nitride

1. Introduction With increasing awareness of environmental protection and energy issues, efficient utilization of solar-energy has attracted much attention [1, 2]. The semiconductor photocatalysts, as a major building block of the modern catalytic chemistry via solar energy, are being extensively concerned for pollutant elimination and energy conversion [3]. In 1972, Fujishima and Honda opened up a new field on water splitting by using TiO2 as photocatalyst [4]. Nevertheless, the application of traditional TiO2 was restricted due to several limitations, including wide band-gap (3.2 eV) and low quantum efficiency, which would result in poor visible-light absorption and low utilization ratio of sunlight. Therefore, much attention has been paid to find visible-light-driven materials with excellent activity[5-8]. Polymeric graphitic carbon nitride (g-C3N4), as one of π-conjugated candidate materials with a suitable band gap of 2.7 eV, has been typically used as photocatalyst, due to its many advantages, including visible-light response, easy availability, 2

nontoxicity and long-term thermodynamic stability [9, 10]. Meanwhile, g-C3N4 is currently being considered for many applications, such as water splitting [11], CO2 reduction [12], bioimaging [13], electricity generation [14] and solving environmental problems. However, the previous reports overwhelmingly focused on conventional g-C3N4 with underperform bulk structure restricted photocatalytic activity for the low visible-light harvesting capability [15-17]. Some new advances on g-C3N4 based photocatalysts can be found. Firstly, Among the approaches reported emphasis is given to synthesize the modified g-C3N4 photocatalysts, including doping with nonmetal, metals species and secondary semiconductors, like K [18], Br [19], I [20], Au [21], Pt [22], Pd [23] and so on. Secondly, the other approach is the development of various carbon nitride nanostructures with optimized physicochemistry and optical properties [24-28]. In particular, much attention has been paid to mesoporous structure because of their enlarged specific area. Mesoporous graphitic carbon nitride (mpg-C3N4) has high surface area providing more active sites and increasing absorption capability, so it can increase the original g-C3N4 performance [29-31]. Unfortunately, the low quantum yield is still another factor limiting the practical application. To overcome these limitations, coupling with other co-catalyst have became an efficient approach to get the high activity [32-40]. Among the co-catalysts, carbonaceous nanomaterials have attracted great interest as electron acceptor and metal-free material [41-46]. Among the various carbon materials, Nitrogen-doped carbon nanotubes (N-CNT) has been demonstrated to be beneficial for the separation of electrons and holes-pairs during the photocatalytic 3

reaction [47, 48]. Due to the incorporation of nitrogen atom into the graphitic structure of CNTs, N-CNT has good electronic conductivity and excellent rate of electron transfer [49-52]. Besides, the surface of N-CNT has more defective structure compared with un-doped CNTs, resulting in enhanced photocatalytic activity. Until now, there are still no reports regarding the combination of mpg-C3N4 and N-CNT for photocatalytic degradation of pollutants. Herein, we incorporated N-CNT into the structure of mpg-C3N4 to form N-CNT/mpg-C3N4 composites by means of a facile one-step hard template method. N-CNT is introduced into mpg-C3N4 multilayer structure, which supplied more active sites and more opportunities to absorb visible-light (Scheme 1). The results indicated that the enhancement of photocatalytic activity for N-CNT/mpg-C3N4 was attributed to the synergistic effect between N-CNT and mpg-C3N4, the improved visible light absorption edge and facilitate electron transfer ability. The kinetic analysis indicated that the reaction kinetics is corresponded to the first order kinetics. Specifically, the effect of N-CNT content was carefully investigated and discussed in detail. A possible mechanism was proposed.

2. Experimental Section 2.1. Material. All chemicals were analytical grade and used without further purification.

2.2. Syntheses of the N-CNT/mpg-C3N4. The N-CNT/mpg-C3N4-X hybrid powers were synthesized via a facile one-step hard template method. In a typical synthesis, firstly, a measured amount of cyanamide 4

and 100 mg N-CNT were added into 50 mL beaker followed by sonication for 30 min, and then slowly dropped wise adding HS-40 silica colloidal solution. After magnetic stirring for 4 h, the mixing solution formed a black viscous gel at room temperature. Secondly, the gel was dried in an oven at 60℃ before being calcined at 823 K with a rate of 2.3℃ min-1 under N2 atmosphere. Finally, the resulting dark-yellow solid was grinded for 30 min using a pestle and treated with a 4 M NH4HF2 solution for 24 h to remove the silica template and then powers were harvested by filtration and washed with deionized water and ethanol for three times. The added contents of cyanamide in N-CNT/mpg-C3N4 composites were 3 g, 5 g, 10 g, 15 g and 30 g, which were marked N-CNT/mpg-C3N4-3,

N-CNT/mpg-C3N4-5,

N-CNT/mpg-C3N4-10,

N-CNT/mpg-C3N4-15 and N-CNT/mpg-C3N4-30, respectively. For comparison, CNT/mpg-C3N4-15 composite was prepared using CNT as electronic acceptor to replace N-CNT by the similar experiment route. The pristine mpg-C3N4 was also prepared using similar route without the adding of N-CNT.

2.3. Characterization of as-prepared sample The crystalline structures of N-CNT/mpg-C3N4 were recorded by X-ray diffraction (XRD) using Shimadzu XRD-6000 with Cu Kα radiation in the range of 2θ from 10˚ to 80˚. X-ray photoelectron spectroscopy (XPS) analysis was performed on VG MultiLab 2000 system with a monochromatic Mg-Kα line source (20 kV). Brunauer-Emmett-Teller (BET) surface area was performed using N2 adsorption and desorption at 77K on TriStar II 3020 system. The morphologies of as-prepared samples were characterized using transmission electron microscopy (TEM) (JEOL 5

JEM-2010 microscope) at an accelerating voltage of 200 kV. The scanning electron microscopy (SEM) measurements of composites were carried out on a JEOL JSM-7001F energy-dispersive X-ray spectroscope. Diffuse reflection spectra (DRS) of UV-vis (Shimadzu UV-3600 plus spectrophotometer) were used to record the light absorption properties of the N-CNT/mpg-C3N4-X using BaSO4 as a reference. Fourier transforms infrared spectra (FT-IR) spectroscopy of composition were analysed on Nexus 470 using KBr as standard material. The photoluminescence (PL) spectroscopy were monitored on a Varian Cary Eclipse spectroscopy. Electron spin resonance (ESR) spectra were conducted on a Bruker model ESR JES-FA200 spectrometer. The electrochemical impedance spectroscopy (EIS) and the photocurrent were conducted with Chenhua CHI 660B Instrument in conventional three electrodes system.

2.4. Photocatalytic activity The photocatalytic activity of the N-CNT/mpg-C3N4-X was investigated by the photodegradation of 10 mg/L RhB, 10 mg/L MO and 20 mg/L TC as target pollutants. In a typical run, 25 mg, 25 mg and 50 mg N-CNT/mpg-C3N4-X hybrid composites were respectively added in the Pyrex photocatalytic reactors for the photodegradation of 50 mL RhB, 50 mL MO and 50 mL TC under a 300 W Xe arc lamp with an appropriate UV cutoff filter (λ> 400 nm), respectively. The temperature was maintained at 30℃ using a flow cooling water system to avoid thermal catalysis. Prior to irradiation, the solution was magnetically stirred for 30 min to reach adsorption-desorption equilibrium on the materials surface. During light irradiation time, at certain time intervals, 3 mL solution was sampled and centrifuged to remove 6

the particulates for following analysis. The concentration variation of the target pollutants were measured by an UV-vis spectrophotometer at maximum absorption wavelength (553 nm, 463 nm and 356 nm for RhB, MO and TC, respectively).

2.5. Photoelectrochemical measurements Photocurrent measurements were performed on a CHI660B electrochemical workstation in a conventional three-electrode system, using Pt wire as counter electrode, Ag/AgCl electrode as the reference electrode. The working electrodes were prepared as follow: 5 mg as-prepared sample was dispersed in 1 mL ethylene glycol (EG) to produce dispersing agent. The suspension was then spread on a 3 cm2 ITO glass substrate with an active area of about 0.5 cm2 and dried to form mpg-C3N4 and N-CNT/mpg-C3N4-X with a different amount loading modified ITO electrodes. A 500 W Xe lamp was utilized as the light source on the photocurrent measurements. The electrolyte was 0.1 M Na2SO4 aqueous solution. Electrochemical impedance spectra (EIS) were measured in 0.1 M KCl solution containing 5 mM Fe(CN)63-/ Fe(CN)64-.

3. Results and discussion 3.1 Enhancement of photocatalytic activity Figure 1a shows the photo-degradation efficiency of MO in the presence of pure mpg-C3N4 and N-CNT/mpg-C3N4 composites. The pure mpg-C3N4 exhibited poor activity. However, after the introduction of N-CNT, the photocatalytic efficiency of the composites was greatly enhanced. It was found that the highest photocatalytic degradation efficiency was up to 88% in the presence of the N-CNT/mpg-C3N4-15 sample. However, it was significantly decreased for N-CNT/mpg-C3N4-30 material. 7

The results indicated that the content was an important factor to the photocatalytic activity. It should be noted that N-CNT/mpg-C3N4-30 with high N-CNT contents would limit visible light absorption, resulting in poor photocatalytic activity. The kinetics fit of MO degradation by N-CNT/mpg-C3N4-X (where X stands for cyanamide amounts) are shown in Figure 1b. A pseudo-first-order reaction model was applied to describe the degradation rate. As shown in Figure 1b, the photocatalytic degradation rate of N-CNT/mpg-C3 N4-15 is the highest, which is about 4.9 times and 65.2 times higher than that of pure mpg-C3N4 and N-CNT samples (Figure S1). The photocatalytic performance of as-prepared N-CNT/mpg-C3N4-X was also evaluated for the photo-degradation of RhB dye under visible-light irradiation, as shown in Figure 1c. The adsorption analysis data in the dark was performed (Figure S2), suggesting that the adsorption-desorption equilibrium between photocatalysts and pollutants was achieved within 30 min. Compared with pure N-CNT and mpg-C3N4, the photocatalytic degradation efficiency of RhB by N-CNT/mpg-C3N4-15 was remarkably enhanced to 95%. Tetracycline (TC) was also chosen to evaluate the photocatalytic activity of N-CNT/mpg-C3N4-15 composites. After 4 h irradiation, no degradation of TC was observed in the direct photolysis. The photo-degradation efficiency of pure mpg-C3N4 and N-CNT/mpg-C3N4 was 53.2% and 67.1% (Figure 1d), respectively. In order to further prove that the doping of nitrogen atom plays an important role in enhancement of the photocatalytic activity, the test for CNT/mpg-C3N4 under the same experimental conditions was carried out. The result shows that the photocatalytic performance of N-CNT-doped composites 8

was superior to CNT-based composites, indicating that the significant improvement could be attributed to the introduction of N-CNT (Figure S3-S5). The reusability and stability of as-prepared photocatalysts are vital for the practical application. The stability experiment (Figure 2) of N-CNT/mpg-C3N4-15 materials was performed by degrading representative RhB repeatedly for five times. The photocatalytic activity had slight decrease. The reason of the decreased activity may be attributed to the loss of the photocatalyst during the recycling process. The sample after recycle experiment was detected by XRD measurement (Figure S6). The results indicated that the morphology structure of N-CNT/mpg-C3N4-15 did not significantly change after five times cycles. Thus, the N-CNT modified mesoporous materials could be re-used in practical pollution treatment.

3.2 Structure and morphology characterization of composites A series of as-prepared N-CNT/mpg-C3N4-X composite were studied by XRD analysis to explore the phase structures. In Figure 3, the XRD patterns of N-CNT/mpg-C3N4-X and pure mpg-C3N4 are observed. The results indicate that the pure mpg-C3N4 and N-CNT/mpg-C3N4 composites have featured two distinctive peaks at 13.1o and 27.3o. The weak-intensity peak at 13.1o, which associated with in-plane repeats tri-s-triazine units, is indexed as (100). The high-intensity peak at 27.3o can be indexed as (002) corresponding to a characteristic graphitic stacking [53]. After the modification of N-CNT, no distinct variations can be found between mpg-C3N4 and N-CNT/mpg-C3N4-X, mainly due to the low quantity of N-CNT. The surface chemical composition and chemical states of N-CNT/mpg-C3N4 9

samples were further evaluated by XPS measurement. The XPS survey spectra shows the presence of elemental C 1s, N 1s, and O 1s region for pure mpg-C3N4, N-CNT and N-CNT/mpg-C3N4-15 samples (in Figure 4a). From the high-resolution C1s XPS spectra (Figure 4b), the two peaks at ~284.6 and ~288.2 eV are ascribed to typical sp2 C atoms from C-C bond of N-CNTs and N2-C=N from mpg-C3N4, respectively. From Figure 4c, it can be found that the N1s is not different between mpg-C3N4 and N-CNT/mpg-C3N4-15 composite. It is worth noticing that the deconvolution of N 1s spectrum of N-CNTs shows two nitrogen species (~398.7 and ~401.3 eV in the inset of Figure 4c) which can be attributed to pyridinic-N species and graphitic-N, respectively. The small amount of O1s can be ascribed to the chemisorbed oxygen (Figure 4d). All above results show that the composites are prepared successfully. Figure 5 shows that the microstructure and morphology of pure mpg-C3N4, pristine N-CNT and N-CNT/mpg-C3N4 with different mass ratio of N-CNT. As shown in Figure 5a-5b, a stable characteristic porous morphology and typical multilayer structure is observed for mpg-C3N4 after the removal of the silica templates. Figure 5c displays the TEM image of the pristine N-CNT. It possesses a smooth nanotube wall and maximum diameter of 15 nm. It can be observed that the skeleton of N-CNT nanotube wall being wrapped by a large number of mpg-C3N4 (Figure 5d-5e). This result is mainly due to the intimate interactions between the N-CNT and mpg-C3N4 by one-step hard template method. This structure is benefit to the photo-generated electrons transfer to the surface of mpg-C3N4. However, when the precursor content of mpg-C3N4 is further increased to 30 g (Figure 5f), the outer wall of N-CNT is 10

covered by a large amount of mpg-C3N4, which may hinder the separation of photogenerated electron and hole pairs, so the photocatalytic activity of N-CNT/mpg-C3N4-30 is lower. The SEM images of all the samples are shown in Figure S7. Figure

S8

shows

the

FT-IR

spectra

of

pristine

mpg-C3N4

and

N-CNT/mpg-C3N4-X materials. For the pure mpg-C3N4, one obvious peak located at 809 cm-1, is corresponded to the characteristic breathing mode of C6N7 heterocycles units. Additionally, typical stretching modes of CN heterocycles of the pure mpg-C3N4 appear at 1200-1650 cm-1, while C-N peaks dominate at 1251 cm-1, 1323 cm-1, 1419 cm-1, 1562 cm-1, and C=N at 1639 cm-1. In addition, the broadened band near 3400 cm-1 mainly belongs to N-H and O-H stretching vibration. The stretching vibration peak at 1570 cm-1 is ascribed to the C=C of the inherent structure of N-CNT, which overlaps with the C-N triazine heterocycles of mpg-C3N4 [32]. Figure 6a shows N2 adsorption-desorption isotherms for different materials, which exhibit a type IV with a H1 hysteresis loop according to the IUPAC classification [54], reflecting the presence of a mesoporous nature of pure mpg-C3N4 and N-CNT/mpg-C3N4. The pore-size distribution of composites was also estimated using the Barrett-Joy-ner-Halenda (BJH) methods. As shown in Figure 4b, pore-size distribution of composites is very broad, indicating the existence of different sizes mesoporous structure. Additionally, it can be clearly found that the introduction of small amount N-CNT slightly affect surface areas and pore structure of N-CNT/mpg-C3N4 (Figure 6a). The corresponding data of BET, BJH and pore 11

diameter are summarized in Table 1. In usually, the decreasement of BET would result in a decrease in photocatalytic performance. However, for N-CNT/mpg-C3N4 composites, the photocatalytic activity did not decrease, which revealing the specific area was not the main factor to responsible for the enhanced photocatalytic activity of N-CNT/mpg-C3N4. To

further

investigate

optical

properties

of

pure

mpg-C3N4

and

N-CNT/mpg-C3N4, the photoluminescence spectra of the samples was analyzed. In Figure 7a, the remarkable PL quench of N-CNT/mpg-C3N4 is shown after the introduction of N-CNT, implying that the recombination rate of photo-excited electrons and holes pairs is greatly restrained in N-CNT/mpg-C3N4 system. The results are attributed to the influence of N-CNT, suggesting that nitrogen atom is inject into internal structure of carbon nanotubes to replace some of the carbon element, and thus further accelerate the separation efficiency between electrons and holes pairs. The PL analysis confirmed that N-CNT as an electron acceptor for mpg-C3N4 inhibiting the recombination of photo-excited electron-hole pairs. The

DRS

of

the

as-prepared

pure

mpg-C3N4,

pure

N-CNT

and

N-CNT/mpg-C3N4 composites are shown in Figure 7b. The band gap of the mpg-C3N4 is estimated to be 2.73 eV, which corresponds to the ~460 nm absorption edge of the pure mpg-C3N4. As shown in Figure 7b, mpg-C3N4 exhibits no absorption in the visible region of 460-800 nm. In addition, the optical absorption of pure N-CNT is strong in the range of 200-800 nm. After incorporation of N-CNT, the

12

N-CNT/mpg-C3N4 exhibit significant increase in visible-light absorption region, whose intensity enhance with the loading content of mpg-C3N4. Figure 8a shows the photocurrent of the pure mpg-C3N4, CNT/mpg-C3N4 and N-CNT/mpg-C3N4 materials. As shown in Figure 8a, the N-CNT/mpg-C3N4-15 composite has higher photocurrent response than that of mpg-C3N4 and CNT/mpg-C3N4. It indicates that the N-CNT/mpg-C3N4 composite has a higher separation rate of photo-excited electrons and holes under the irradiation of visible light. To further analyze separation efficiency of photo-generated electron-hole pairs, the electrochemical impedance spectroscopy (EIS) measurement was employed (Figure 8b). It can be seen that N-CNT/mpg-C3N4 exhibit smaller imperdance arc radius

than

the

pure

mpg-C3N4

and

CNT/mpg-C3N4,

indicating

that

N-CNT/mpg-C3N4 has the better electrical conductivity, which would accelerate the migration of photo-generated electrons.

3.3 Structure-activity relationships and enhanced photo-activity The results indicated that N-CNT is crucial for the photocatalytic activity. (i) N-CNT is used as an efficient charge collector and transporter in this system. (ii) The presence of N-CNT can increase the visible light absorption of the composites. (iii) The π-π interaction between N-CNT and mpg-C3 N4 affects the band structure and the semiconductor properties. It is noted that N-CNT not only demonstrates great improvement of electronic conductivity after modification of nitrogen (the nitrogen loading content of N-CNT is 2.73%), but also inherits the merits of CNT such as exceptional physic-chemical property. Therefore, modification of nitrogen can 13

efficiently induce charge delocalization and tune the work function of CNTs, and then the electron transfer capability of N-CNTs can be further promoted, because of tailor the electronic property and thus accelerate the separation efficiency between electrons and holes pairs [50]. So far, it has been demonstrated that the presence of nitrogen can be a key strategy that improves the photocatalytic performance of materials [49, 55]. It could be also found that the loading mass ratio of N-CNT is important for the photocatalytic activity. With the increasing mpg-C3N4 content (cyanmide aqueous solution from 3 g to 10 g), the photo-degradation efficiency of dyes was improved. Further increasing the precursor content of mpg-C3N4 (15 g), the photo-degradation efficiency reaches to the maximum constant under visible-light irradiation. However, for N-CNT/mpg-C3N4-30 sample, a significant decreasing photocatalytic activity was observed in Figure 1a and 1c. Too much mpg-C3N4 content might limit visible light absorption of composites, which is mainly due to the skeleton of N-CNT nanotube wall being wrapped by a large number of mpg-C3N4. Therefore, N-CNT and mpg-C3N4 have a synergistic effect in N-CNT/mpg-C3N4 system.

3.4 Possible photocatalytic mechanism To reveal the main active species, trapping experiment was applied using tertiary butanol, disodium ethylenediamine tetraacetate (EDTA-2Na) and N2 as hybroxyl radical (•OH), hole (h+) and superoxide radical (O2•−), respectively. The photo-degradation of RhB exhibited no obvious inhibition when t-BuOH was added, indicating that •OH is not the main reactive species in N-CNT/mpg-C3N4 system [56]. When EDTA-2Na was added as h+ scavenger, the photo-degradation process was also 14

not obviously affected, which indicated that holes did not take part in the photocatalytic reaction. Purging N2 into the RhB solution could make an anaerobic environment, which would prohibit the formation of superoxide radical (O2•−). It can be clearly observed in Figure 9a that O2•− plays an important role in the photocatalytic process [57]. The electron spin resonance (ESR) measurement was further carried out to reveal the main active species. The main active group was tested during the photocatalytic reaction process. When the light was on, DMPO–O2•− radical species was detected in the methanol solution dispersion of N-CNT/mpg-C3N4 hybrids. However, no characteristic signals was detected for DMPO–O2•− radical species in the dark (Figure 9b), indicating that O2•− was generated and plays a crucial role in this system under visible light. In addition, no signals peaks of •OH were observed in visible light irradiation (Figure 9c), implying that the •OH was not main active species. Therefore, the ESR analysis and free radicals trapping experiment indicated that O2•− is main active species. According to the above results, the possible mechanism schematic of N-CNT/mpg-C3N4 was proposed in Figure 10. When mpg-C3N4 was irradiated by visible-light, the electron can be excited from valence band (VB) to conduction band (CB), and a fraction of photo-generated electron and hole would recombine rapidly. Due to the introduction of N-CNT, the photo-generated electron on the CB of mpg-C3N4 tend to transfer to N-CNT (O2•− can be formed), and N-CNT as electronic transfer channels can play a role in efficiently separation of photo-generated electron-hole pairs. Therefore, the recombination rate between photo-generated 15

electron and holes was greatly inhibited and thus lead to enhanced photocatalytic activity.

4. Conclusions The N-CNT/mpg-C3N4 hybrid composites were successfully synthesized via a facile one-step hard template method. The N-CNT/mpg-C3N4 hybrids have universality for the degradation of varieties organic dye (RhB and MO) and antibiotics (TC) under visible light irradiation. The enhanced photocatalytic efficiency was attributed to the introduction of N-CNT. With in-depth study, N-CNT act as a promising metal-free candidate and electronic acceptor for mpg-C3N4 could effectively inhibit the recombination of photo-generated electron-holes pairs and thus enhance visible light photocatalytic activity. In addition, it was demonstrated that O2•is the main active piece in the photo-degradation process, which was confirmed by trapping experiments and ESR analysis. The N-CNT/mpg-C3N4 hybrids composites may offer a promising photocatalyst candidate for degradation of pollutants in the future.

Acknowledgements The authors genuinely appreciate the financial support of this work by the National Nature Science Foundation of China (21476097), six talent peaks project in Jiangsu Province (2014-JNHB-014).

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Figure Captions Scheme 1. Design plans of N-CNT modified mpg-C3N4. Figure 1. photocatalyitc degradation of MO (a), their kinetic fit of MO in the presence of mpg-C3N4 and N-CNT/mpg-C3N4 with different loading amounts under visible light irradiation(b), RhB (c) and TC (d) in the presence of mpg-C3N4 and N-CNT/mpg-C3N4 with different N-CNT loading contents under visible light irradiation. Figure 2. Cycle times for the photodegradation of RhB in the presence of N-CNT/mpg-C3N4-15 under visible light irradiation. Figure 3. XRD patterns of mpg-C3N4, N-CNT and N-CNT/mpg-C3N4 with different amounts of N-CNT. Figure 4. XPS spectra of pure mpg-C3N4, pure N-CNT and N-CNT/mpg-C3N4 hybridcomposites. (a) Survey of sample, (b) C 1s, (c) N 1s and (d) O 1s. Figure 5. Photographs of mpg-C3N4 (a, b) and N-CNT (c); image of 20

N-CNT/mpg-C3N4-15 (d, e); image of N-CNT/mpg-C3N4-30 (f) Figure 6. N2 adsorption-desorption isotherms (a) and pore-size distribution curves b) of pure mpg-C3N4, N-CNT and N-CNT/mpg-C3N4 . Table 1. Structure parameter of mpg-C3N4, N-CNT and N-CNT/mpg-C3N4. Figure 7. Photoluminescence spectra of pristine mpg-C3N4 and N-CNT/mpg-C3N4-X (a) and UV-visible diffuse reflectance spectra of N-CNT, mpg-C3N4 and N-CNT/mpg-C3N4 with different amount loading of mpg-C3N4 (b). Figure 8. (a) Transient photocurrent responses under visible light irradiation, (b) electrochemical impedance spectroscopy Nynquist plots of pure mpg-C3N4 and N-CNT/mpg-C3N4 materials. Figure 9. Trapping experiment of main active species during the photocatalytic degradation of RhB over N-CNT/mpg-C3N4-15 under visible light irradiation (a), ESR spectra of radical adducts trapped by DMPO: (b) DMPO–O2•−radical species of N-CNT/mpg-C3N4-15 was detected in methanol and (c) DMPO–•OH was used to detect for N-CNT/mpg-C3N4-15 materials aqueous dispersion. Figure 10. Schmatic diagram illustrating the separation transfer of photogenerated charges of N-CNTmodified mpg-C3N4 under visible-light irradiation and the possible reaction mechanism.

Scheme 1. Design plans of N-CNT modified mpg-C3N4.

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Figure 1. photocatalyitc degradation of MO (a), their kinetic fit of MO in the presence of mpg-C3N4 and N-CNT/mpg-C3N4 with different loading amounts under visible light irradiation(b), RhB (c) and TC (d) in the presence of mpg-C3N4 and N-CNT/mpg-C3N4 with different N-CNT loading contents under visible light irradiation.

22

Figure 2. Cycle times for the photodegradation of RhB in the presence of N-CNT/mpg-C3N4-15 under visible light irradiation.

23

Intensity (a. u.)

mpg-C3N4 N-CNT/mpg-C3N4-3 N-CNT/mpg-C3N4-5 N-CNT/mpg-C3N4-10 N-CNT/mpg-C3N4-15 N-CNT/mpg-C3N4-30 N-CNT

10

20

30 40 50 60 2 Theta (degree)

70

80

Figure 3. XRD patterns of mpg-C3N4, N-CNT and N-CNT/mpg-C3N4 with different amount of N-CNT.

Figure 4. XPS spectra of pure mpg-C3N4, pure N-CNT and N-CNT/mpg-C3N4 hybrid composites. (a) Survey of sample, (b) C 1s, (c) N 1s and (d) O 1s.

24

Figure 5. Photographs of mpg-C3N4 (a, b) and N-CNT (c); image of N-CNT/mpg-C3N4-15 (d, e); image of N-CNT/mpg-C3N4-30 (f).

25

Figure 6. N2 adsorption-desorption isotherms (a) and pore-size distribution curves (b) of pure mpg-C3N4, N-CNT and N-CNT/mpg-C3N4 .

26

Figure 7. Photoluminescence spectra of pristine mpg-C3N4 and N-CNT/mpg-C3N4-X (a) and UV-visible diffuse reflectance spectra of N-CNT, mpg-C3N4 and N-CNT/mpg-C3N4 with different amount loading of mpg-C3N4 (b).

27

Figure 8. (a) Transient photocurrent responses under visible light irradiation, (b) electrochemical impedance spectroscopy Nynquist plots of pure mpg-C3N4 and N-CNT/mpg-C3N4 materials.

28

Figure 9. Trapping experiment of main active species during the photocatalytic degradation of RhB over N-CNT/mpg-C3N4-15 under visible light irradiation (a), ESR spectra of radical adducts trapped by DMPO: (b) DMPO–O2•- radical species of N-CNT/mpg-C3N4-15 was detected in methanol and (c) DMPO–•OH was used to detect for N-CNT/mpg-C3N4-15 materials aqueous dispersion.

29

Figure 10. Schmatic diagram illustrating the separation transfer of photogenerated charges of N-CNT modified mpg-C3N4 under visible-light irradiation and the possible reaction mechanism.

30

Table 1. Structure parameter of mpg-C3N4, N-CNT and N-CNT/mpg-C3N4.

Non-metal photocatalyst Nitrogen-doped carbon nanotubes modified mpg-C3N4: Facile synthesis and the enhanced visible-light photocatalytic activity Jinyuan Liua, Yanhua Songb, Hui Xua*, Xingwang Zhua, Jiabiao Liana, Yuanguo Xua, Yan Zhaoa, Liying Huanga, Haiyan Jia, Huaming Lia* c. School of the Environment and Safety Engineering, Institute for Energy Research, Jiangsu University, Zhenjiang 212013, P. R. China. d. School of Environmental and Chemical, Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, P. R. China *Corresponding author: Tel.:+86-0511-88791108; Fax: +86-0511-88791108; E-mail address: [email protected], [email protected]

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We report a facile synthesis strategy involving polymerization of cyanamide as the precursor in the presence of N-CNT via thermal polycondensation. The N-CNT/mpg-C3N4 exhibited enhanced photocatalytic activity for Rhodamine B (RhB), Methyl orange (MO) and Tetracycline (TC) degradation compared with the pure one under visible-light irradiation, which is mainly due to the efficiently separation of photogenerated electron-hole pairs for the introduction of N-CNT as electronic transfer channels. The photocatalytic reaction can fit as first order kinetics. Additionally, superoxide radical (O2 • −) was regarded as main reactive species participate in the photodegradation reaction process. Furthermore, the proposed mechanism for enhancing photocatalytic activity of N-CNT/mpg-C3N4 is attributed to synergistic effects between mpg-C3N4 as main ingredient and N-CNT as transfer media. 32

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