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PAPER

Cite this: Dalton Trans., 2013, 42, 7604

Received 30th November 2012,Accepted 27th February 2013

DOI: 10.1039/c3dt32871f

www.rsc.org/dalton

The CNT modified white C3N4 composite photocatalystwith enhanced visible-light response photoactivity†

Yuanguo Xu,a,b Hui Xu,a Lei Wang,a Jia Yan,b Huaming Li,*b Yanhua Song,b

Liying Huangb and Guobin Caib

A novel, multi-walled carbon nanotubes (CNT) modified white C3N4 composite (CNT/white C3N4) with

enhanced visible-light-response photoactivity was prepared. The white C3N4 and CNT combined together

and formed the CNT/white C3N4 composite due to electrostatically-driven self-assembly with the hydro-

thermal method. The as-prepared white C3N4 and CNT/white C3N4 composite photocatalyst were charac-

terized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared

spectroscopy (FT-IR), UV-vis absorption spectra, X-ray photoelectron spectroscopy (XPS) and photolumine-

scence spectroscopy (PL). The photoelectrochemical i–t curves were tested using several on–off cycles

of light irradiation. The photoactivity of the catalysts was evaluated by degrading methylene blue (MB)

dye solution. The results showed that the photoactivity for the degradation of MB solution was in the fol-

lowing order: CNT/white C3N4 composite > C3N4 > the white C3N4. The photoactivity of the CNT/white

C3N4 composite was 66.5% and 34.5% higher than that of the white C3N4 sample and that of the C3N4

at 1.5 h, respectively. The degradation rate of the CNT/white C3N4 photocatalyst was almost 8.1 times as

high as that of the white C3N4. The results indicated that CNT played an important role, which led to the

efficient separation of the photo-generated charge carriers. The reason why the photoactivity of the

CNT/white C3N4 was much higher than that of C3N4 and the white C3N4 was discussed. A possible mech-

anism of CNT on the enhancement of composites’ visible light performance was also proposed.

Introduction

The global crisis of energy and environment has attracted lotsof attention for decades. Among the various solutions, semi-conductor photocatalysis is a promising technique for environ-mental protection and solar energy conversion.1 During thepast 30 years, various materials have been developed as photo-catalysts for hydrogen production from water, removing pollu-tants from the gas phase and degrading the organic pollutantsin the solution under visible light irradiation.2–5 However,almost all the developed visible-light-responsive photocatalystsare composed of metal-based inorganic solids.

Graphitic carbon nitride (C3N4) is a metal-free photocatalystwith a band gap of ca. 2.7 eV. Since the first record of thepreparation of a polymeric CN, melon, was reported byLiebig in 1834,6 numerous methods have been developed for

the synthesis of C3N4.7 C3N4 has sufficient absorption and

shows photocatalytic activity for water to be reduced into H2 orfor water to be oxidated into O2 under visible light irradiation.

8

Besides, it is inexpensive, environmentally benign, and can beeasily prepared from several precursors, such as cyanamide,dicyandiamide, urea, and melamine.7 However, its high recom-bination rate of the photo-generated charge carriers alsoseriously affects the photocatalytic efficiency.9,10 To overcomethis problem and improve its photocatalytic ability, variousmethods have been developed. For one thing, templatemethods have been used for the synthesis of nano-porousC3N4 with various structures and controllable pore size.7,11–14

For the other thing, the doping of C3N4 with a non-metalelement like boron, fluorine, phosphor, graphene orsulfur,15–21 and doping with metal species such as Fe,22

Ag,23,24 Co,25 or coupling with TaON,26 Bi2WO6,27 ZnO,28

AgX,29 CdS30 etc. were also carried out. The results show thatthe hybridization or modification is an effective way toincrease the photocatalytic ability of the pure C3N4.

Recently, the carbon nanotubes (CNT) has received muchinterest due to its special structures and uniqueconductivity.31–33 In many literatures, CNT has been used invarious composites such as BiOI/MWCNT,34 CNT/ZnS,35 CNT/TiO2,

36–38 CNT/ZnO,39 Ag/CNT,40 Cu/CNT41 etc. The doped†Electronic supplementary information (ESI) available. See DOI:10.1039/c3dt32871f

aSchool of the Environment, Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013,

P. R. ChinabSchool of Chemistry and Chemical Engineering, Jiangsu University, 301 Xuefu Road,

Zhenjiang, 212013, P. R. China. E-mail: lihm@ujs.edu.cn; Fax: +86-0511-88791108;

Tel: +86-0511-88791108

7604 | Dalton Trans., 2013, 42, 7604–7613 This journal is © The Royal Society of Chemistry 2013

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CNT could transfer the photogenerated electrons efficiently,which can enhance the composites’ photocatalytic activity forthe degradation of the organic pollutants or water splitting,and can improve their other properties.

Based on the above analysis, if CNT and C3N4 are effectivelycombined together, CNT may act as an acceptor of the photo-generated electrons from C3N4. The introduction of CNT willnot only enhance the light absorption capacity of the compo-site photocatalyst (CNT is a light-harvesting material42), butalso accelerate the separation of the photo-generated electron–hole pairs (CNT is a good conductive material). Thus, CNT andC3N4 can be effectively combined into one single object, whichwill be a promising functional material. Ge et al.43 and Surya-wanshi et al.44 have reported CNT/C3N4 prepared by heatingcyanamide and CNT dispersion in aqueous solution, and itshowed an enhanced water splitting ability than the pureC3N4. However, it is difficult to control the sublimationamount of the cyanamide in the heating process. And the dis-persion of CNT and the combination of tightness betweeneach other is not so good, which may affect the photoactivity.

In the present work, we design a synthesis of the metal-freephotocatalyst CNT coupled with C3N4 composite with the fol-lowing aims: (1) to enhance visible-light-response photoactiv-ity, (2) to get a controllable way, (3) to investigate the possiblecombination process between CNT and C3N4, (4) to discuss therole of CNT in the composite, especially to find out the poss-ible mechanism of CNT on the enhancement of composites’visible light performance. The hydrothermal method is expectedto be a good method to prepare the catalyst with control.However, CNT and C3N4 both have negative polarity,45 so theywill not be effectively combined together due to electrostaticrepulsion in the solution. Therefore, in theory, it is not feasibleto prepare CNT/C3N4 in the solution. If we want to obtain thehybrid material in the solution with CNT well-dispersed onthe surface of C3N4, the following requirements should beachieved: (1) C3N4 should be modified into positive polarity(and then the two materials can be combined together effec-tively in the solution due to electrostatic attraction), (2) sincethe original C3N4 is not well dispersed in the solution, it isexpected that the modified C3N4 can be easily dispersed in thesolution, which will help CNT to be dispersed on its surface. Itis exciting that the two problems are settled by protonatingC3N4 (with HCl) into white C3N4–H

+Cl−. The white C3N4–H+Cl−

not only has positive polarity, but also dispersed muchbetter than the original C3N4 in the solution.45 Therefore, itis an ideal alternative to prepare the CNT modified whiteC3N4–H

+Cl− composite with CNT well dispersed on thesurface, for which we report a feasible hydrothermal method.

We have fully characterized the structure and properties ofthe metal-free CNT modified white C3N4–H

+Cl− (we denote itas white C3N4) composite. The photoactivity of the photo-catalyst was evaluated by degrading methylene blue (MB) dyesolution. The reason why the photoactivity of the CNT/whiteC3N4 was much higher than that of C3N4 and the white C3N4

was discussed. A possible mechanism of CNT on the enhance-ment of composites’ visible light performance was also

proposed. Furthermore, this work could provide new insightsinto the fabrication of CNT (or other materials) modifiedbinary composites as high performance photocatalysts andtheir application in environmental protection.

2. Experimental section2.1. Chemicals

All chemicals were reagent grade and were used withoutfurther purification. CNT (MWCNT-COOH, purity > 95%) waspurchased from Chengdu Organic Chemicals Co. Ltd, ChineseAcademy of Sciences. 1-Hexadecyl-3-methylimidazolium chlor-ide ([C16mim]Cl ionic liquids) was purchased from ShanghaiChengjie Chemical Co. Ltd. C3N4 was synthesized by directlyheating dicyandiamide. Typically, 2 g of dicyandiamidepowder was put into an alumina crucible with a cover, thenheated at a rate of 2.9 K min−1 to reach a temperature of350 °C, and then tempered at this temperature for another 2 hin a flowing-nitrogen atmosphere. It was followed by that theproduct was heated at a rate of 3.3 K min−1 to reach a temp-erature of 550 °C, and then tempered at this temperature foranother 2 h in a flowing-nitrogen atmosphere. The resultingpowder was ground in an agate mortar and used in subsequentstudies.

2.2. The synthesis of the white C3N4

The preparation of the white C3N4 is similar to that reported inthe literature.45 In a typical process, 2 g C3N4 was added intoHCl aqueous (12 mol L−1) solution and stirred for 12 h. Thenthe white C3N4–H

+Cl− (denoted as white C3N4) was washedinto neutral with distilled water and ethanol, before dried at50 °C for 8 h.

2.3. The fabrication of CNT/white C3N4

5 mg CNT was dispersed in solution A (200 mL of certainconcentration 1-hexadecyl-3-methylimidazolium chloride([C16mim]Cl) solution) and stirred for 24 h to ensure CNT canget a good dispersion (the role of the [C16mim]Cl ionic liquidsis for dispersing CNT, without the addition of the [C16mim]Clionic liquids, CNT cannot be dispersed well in the solution).0.1 g of the white C3N4 was dispersed in 19 mL solution A andstirred for 1 h in a 25 mL Teflon hydrothermal reactor, then1 mL CNT dispersion solution was added into the aboveaqueous solution, and the resulting solution was stirred for1 h. At last, the 25 mL Teflon hydrothermal reactor was sealedand kept at 160 °C for 4 h. The resulting powder was washedby water and ethanol before being dried at 50 °C for 8 h.

The reference photocatalyst CNT/C3N4 was also prepared.The as-prepared 0.1 g C3N4 was added into 19 mL solution Aand stirred for 1 h in the 25 mL Teflon hydrothermal reactor,and then 1 mL CNT dispersion solution was added into theabove solution. At last, the mixture was sealed and kept at160 °C for 4 h. The resulting powder was washed by water andethanol before being dried at 50 °C for 8 h. The referencephotocatalyst CNT/TiO2 (P25) was prepared with a similarmethod.

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2.4. Characterization

Transmission electron microscopy (TEM) were taken with aJEOL-JEM-2010 (JEOL, Japan) operating at 200 kV. The crystalphase was analyzed by X-ray diffraction (XRD) analysis using aBruker D8 diffractometer with Cu-Kα radiation (λ = 1.5418 Å) inthe 2θ range of 10°–80°. UV-vis absorption spectra of the sampleswere obtained on a UV-vis spectrophotometer (UV-2450,Shimadzu Corporation, Japan). The UV-vis absorption spectra(in the diffuse reflectance spectra mode) were measured in solidstate, and BaSO4 powder was used as the substrate. Fouriertransform infrared (FT-IR) spectra (KBr pellets) were recordedon Nicolet Model Nexus 470 FT-IR equipment. X-ray photo-electron spectroscopy (XPS) was measured on a PHI5300 witha monochromatic Mg Kα source to explore the elements on thesurface. The Z-potential was tested on the Malvern ZetasizerNano-S90 (England) and the suspensions of each material inaqueous solution media (0.1 mg mL−1) were prepared andmeasured separately. Photoluminescence (PL) spectra of thecatalyst were measured on the QuantaMaster and TimeMasterSpectrofluorometer, QuantaMaster™ 40 (Photon TechnologyInternational, Inc.) with excitation wavelength of 315 nm.The photocurrents were measured with an electrochemicalanalyzer (CHI660D, CHI Shanghai, Inc.)

2.5. Photocatalytic experiments

The application of the CNT/white C3N4 composite in degrad-ing organic dye MB was investigated under visible-lightirradiation at room temperature. In a typical procedure, a0.0500 g amount of the photocatalyst was suspended in 50 mLMB solution (10 mg L−1). Before the lamp (300 W Xe arc lampequipped with an ultraviolet filter to provide visible light withλ ≥ 400 nm) was turned on to start irradiation, the solutionwas stirred for 0.5 h in the dark to ensure the adsorption/desorption equilibrium between the photocatalyst and the dye.The solution was sampled at 0.5 h intervals and was cen-trifuged, and the above liquid was then monitored by UV-visspectroscopy at 664 nm.

2.6. Photoelectrochemical measurements

Photocurrent measurements were performed on an electro-chemical analyzer (CHI660D, CHI Shanghai, Inc.) in a standardthree-electrode configuration with a Pt wire as the counter elec-trode and Ag/AgCl (in saturated KCl) as a reference electrode.Irradiation proceeded with a Xe arc lamp. Na2SO4 (0.1 M)aqueous solution was used as the electrolyte. The working elec-trodes were prepared as follows: the 5 mg ground sample wasmixed with 1 mL solution to make slurry. The slurry was thendispersed onto an ITO glass electrode with a 1 cm × 1 cm areaand these electrolytes were dried at 50 °C for 8 h.

3. Results and discussion3.1 The proposed formation process of CNT/white C3N4

In order to investigate the formation process of the compo-sites, the Z-potential of the materials was tested, and the

results are shown in Fig. 1. A Z-potential of −20.4 mVsuggests that C3N4 possessed negative polarity on its surfacein the solution. The Z-potential of CNT is −24.0 mV, whichindicates CNT had plenty of –COO− on its surface. In orderto adsorb the negative polarity CNT–COO−, C3N4 (Scheme 1A,yellow powder) was protonated by HCl to obtain the whiteC3N4–H

+Cl− sample (denoted as white C3N4, Scheme 1B,white powder). The white C3N4 dispersed much better thanthe pure C3N4 in the solution (as shown in Fig. S1†), and ithad positive polarity (+3.3 mV) which is also beneficialto the fabrication of well-dispersed CNT hybrid materials.CNT and the white C3N4 can be combined together easilyvia electrostatic attraction. The decrease of the Z-potentialfrom +3.3 mV (white C3N4) to +1.6 mV (CNT/white C3N4)indicates that CNT was successfully deposited on thesurface of the white C3N4. Because the addition amount ofCNT was very small, part of the H+ still existed on its surface,which kept the material in positive polarity as a whole. Theformation process of the CNT/C3N4 and CNT/white C3N4

composites is briefly illustrated in Scheme 1a and b,respectively.

In order to see the difference between the samples preparedby these two methods clearly, the CNT content in the compo-site samples was raised to 0.3 wt%. And the photographs ofthese two composite samples (CNT/C3N4 and CNT/white C3N4)after being washed by water and ethanol are displayed inphotographs C and D in Scheme 1. It is obvious that whileCNT/white C3N4 is uniform in color, the CNT/C3N4 samplewith yellow powder in the bottom and black CNT covers onthe upper layer. This confirms that CNT and the white C3N4

can be combined effectively, while CNT and the original C3N4

separated as two individual materials.

Fig. 1 Z-potential of the C3N4 (−20.4 mV), CNT (−24.0 mV), the white C3N4

(+3.3 mV) and CNT/white C3N4 (+1.6 mV).

Paper Dalton Transactions

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3.2 TEM and XRD

The morphology of the white C3N4 and CNT/white C3N4 com-posite samples was investigated by TEM. The TEM images ofthe white C3N4 are shown in Fig. 2a and b. It can be seen that

the size of the white C3N4 is mainly in the range of 0.5 μm–

2 μm. The TEM images of the CNT/white C3N4 compositesample are shown in Fig. 2c and d. It is clear that the tube-likeCNT gets well-dispersed in the CNT/white C3N4 compositestructures (as shown in the arrows), which is different from

Scheme 1 The proposed formation process of CNT/C3N4 (a), and CNT/white C3N4 (b).

Fig. 2 TEM images of the composite samples: pure white C3N4 (a, b), CNT/white C3N4 (c, d).

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the pure white C3N4 (as shown in Fig. 2b). In order to see CNTmore clearly, the CNT/white C3N4 with higher CNT contentwas investigated and the images are shown in Fig. S2.† FromFig. S2a,† it can be seen that CNT is combined on the surfaceof the white C3N4. Fig. S2b† is the HRTEM image of CNT/whiteC3N4, which shows that CNT and the white C3N4 have beeneffectively combined together. The crystal phase of thematerials is determined by XRD, and the results are shown inFig. 3. The diffraction peak at 26.4° can be attributed to thegraphite-like structure (002) of the CNT (Fig. 3a).46,47 The pureC3N4 has two distinct diffraction peaks at 13.0° and 27.2°(Fig. 3b), which correspond to the graphitic materials as the(100) and (002) diffraction planes in JCPDS 87-1526. These twodiffraction peaks are in good agreement with the graphiticcarbon nitride C3N4.

8,48 From Fig. 3c and d, it is clear that thepeak at 13.0° disappears in the white C3N4 and CNT/whiteC3N4 samples and the main peak at 27.2° shifts to a higherdegree, which further proves that C3N4 has been successfullyprotonated.45 However, no obvious diffraction peaks of CNTare observed in the CNT/white C3N4 composite sample, whichis due to the trace amount of the loaded CNT.

3.3 UV-vis absorption spectra

The UV-vis absorption spectra of the samples (measured underthe diffuse reflection mode) are shown in Fig. 4. It can be seenthat the pure CNT can strongly absorb the light in the range of200 nm to 800 nm. The absorption edge of C3N4 is about450 nm and the absorption edge shifts to about 410 nm forthe white C3N4 sample. The blue-shift of the absorption edgeand the decreasing absorption intensity of the white C3N4

material are in agreement with the color change from yellow towhite (see Scheme 1, photo A and B). The UV-vis results showthat the absorption of the white C3N4 extends to about650 nm, and the CNT/white C3N4 sample shows absorption inthe whole visible region. So, they can absorb the visible light,and have visible light photoactivity. Compared with the whiteC3N4, the CNT/white C3N4 sample showed the increasing

absorption intensity in the range of 410 nm–800 nm, whichindicates that CNT/white C3N4 can absorb more energy of thevisible-light and may lead to the enhanced photoactivity. TheUV-vis absorption of CNT/white C3N4 with higher CNT contentwere tested, and the results show that the absorption intensityof the CNT/white C3N4 enhanced obviously (in the range of410 nm–800 nm) with the increasing CNT content (the graphis not shown here). Yu et al.42 have reported the similar result.A further observation found that the CNT/white C3N4 exhibitsalmost the same absorption edge as the white C3N4, whichsuggests that the CNT is not incorporated to the lattice of thewhite C3N4, and it is only immobilized on the surface of thewhite C3N4.

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3.4 FT-IR

Fig. 5 shows the FT-IR spectra of [C16mim]Cl ionic liquids,C3N4, CNT/C3N4, the white C3N4 and CNT/white C3N4 samples.

Fig. 4 UV-vis absorption spectra of the CNT (a), C3N4 (b), the white C3N4

(c) and CNT/white C3N4 (d).

Fig. 5 The FT-IR spectra of C3N4 (a), CNT/C3N4 (b), white C3N4 (c), CNT/whiteC3N4 (d) and [C16mim]Cl (e).

Fig. 3 XRD patterns of CNT (a), original C3N4 (b), the white C3N4 (c), CNT/white C3N4 (d).

Paper Dalton Transactions

7608 | Dalton Trans., 2013, 42, 7604–7613 This journal is © The Royal Society of Chemistry 2013

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The C3N4 (Fig. 5a) sample has absorption peaks at 808 cm−1,1248 cm−1, 1323 cm−1, 1414 cm−1, 1456 cm−1, 1566 cm−1

and 1635 cm−1, which is in good agreement with theoverall C3N4 features in the reported literatures.49,50 Theabsorption peak ascribed to [C16mim]Cl ionic liquids is notshown in the CNT/white C3N4 sample. The result indicatesthat no [C16mim]Cl ionic liquids was on the surface of theCNT/white C3N4 after it was washed. It is clear that the peaksat 1248 cm−1 and 1323 cm−1 for the white C3N4 sample appearto shift, which may be ascribed to fact that the C–N of C3N4

has a little change after it was protonated by aqueous HCl. Thepeak at 1635 cm−1 shifts to 1616 cm−1, the peak at 1456 cm−1

shifts to 1487 cm−1 and the peak at 1566 cm−1 becomes un-obvious for the white C3N4. The changes of these peaks indi-cate the successful protonation of C3N4.

45 These three peaksare attributed to the CvN stretching in the C3N4, which indi-cates that the protonated H+ has affected the CvN of theC3N4. The H+ ions modified on C3N4 should connect on the Nsites. It is found that the absorption peak at 1248 cm−1 showsa further shift in the CNT/white C3N4 sample. The red shift ofthe peak indicates that the bond strength of C–N is weakened,suggesting that the conjugated system of the white C3N4 isstretched and the interaction between the white C3N4 and CNThas already appeared.28 And the interaction between the whiteC3N4 and CNT may benefit the electron transfer and henceenhance the photocatalytic activity of the composite material.

3.5 XPS

XPS was used to investigate the valance states and chemicalenvironment of constituent elements on the surface of thesamples. The survey XPS spectrum (Fig. 6a) shows that themain elements on the surface of the products are C, N andadsorbed O. The C 1s peak (Fig. 6b) of the samples can bedeconvoluted into two peaks, which indicate that two differentchemical environments of carbon existing in these samples.All the samples showed a peak at about 284.6 eV, which isassigned to surface adventitious carbon or the sp2 C–C bonds.51

The peak at 287.6 eV is identified as sp2-bonded carbon inN-containing aromatic rings (N–CvN).52–54 It is clear that thepeak at 287.6 eV shifts to about 288.0 eV, which indicates thatthe protonation has affected the N–CvN bond in the C3N4.The C 1s peak of CNT/white C3N4 showed a similar resultto the white C3N4, which may be ascribed to the fact that asmall amount of CNT does not change the C 1s peak obviously.Besides, the XPS peaks for CNT are concealed by that of C3N4;a similar result has been reported by Yu.54 As shown in Fig. 6c,the predominant peaks at a binding energy of 398.5 eV may beattributed to nitrogen atoms in C–N–C groups.55 There is noobvious shift in the N 1s spectra. Furthermore, the Cl elementwas detected in the white C3N4 (as shown in Fig. 6d. The Clelement in the white C3N4 sample is about 0.3 at%). However,the Cl element was not detected in the CNT/white C3N4 cata-lyst. The reduction of Cl element in the CNT/white C3N4

sample may be ascribed to the reason that the Cl− on thesurface of the white C3N4 sample was ion-exchanged byCNT-COO− in the hydrothermal process.45

Fig. 6 XPS survey spectrum of catalyst (a), high-resolution XPS spectra of C 1s(b) and N 1s (c) for the catalyst and high-resolution XPS spectra of Cl of thewhite C3N4 (d).

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3.6 The photocatalytic activity

The photocatalytic activity of the samples was evaluated by thedegradation of MB solution under visible light irradiation, andMB photolysis (without photocatalyst) is also shown in Fig. 7a.It is clear that the photolysis of MB under visible lightirradiation was very slow. When the catalysts were applied, theconcentrations of MB decreased quickly under visible light

irradiation (the photoactivity of the pure CNT was also testedby degrading the MB and methyl orange (MO) dye after theCNT and the dye reached adsorption/desorption equilibrium,the result shows that the pure CNT has no photoactivity). It isclear that C3N4 and CNT/white C3N4 show higher photocatalyticability than the reference photocatalyst CNT/TiO2. The CNT/white C3N4 composite show the highest photocatalytic activity.The photocatalytic ability of CNT/white C3N4 is 66.5%, 49.2%and 34.5% higher than that of the white C3N4, CNT/TiO2 andC3N4 at 1.5 h, respectively. The high photoactivity of the CNT/white C3N4 can be ascribed to the chemically bonded inter-facial contact between CNT and the white C3N4.

42 However, itis notable that CNT/C3N4 does not obviously enhance thephotocatalytic activity of C3N4. It is probably due to the reasonthat CNT/C3N4 sample is not able to create effective interfacialcontact between CNT and C3N4, and the doping CNT cannotefficiently transfer the interfacial electrons and separate theelectron–hole pairs. The degradation of the other organic dyes((MO) and rhodamine B (RhB) 10 mg L−1) were also done withthe same CNT/white C3N4 quantities, and the degradationefficiency was 89.7% and 85.4% at 3 h for MO and RhB, respect-ively. This result further confirms that CNT/white C3N4 can alsodegrade other organic pollutants like MO and RhB dyes.

It is also interesting that the white C3N4 showed muchlower photoactivity than the C3N4 sample, which can beexplained by the following two reasons. Firstly, the white colorof the white C3N4 sample leads to it absorbs less visible-light(as shown in Fig. 4), which may affect the photocatalyticactivity. Secondly, the white C3N4 possesses the positivepolarity, and it would not tend to adsorb the MB dye (positivepolarity) in the solution due to electronic repulsion. Theadsorption ability of the catalysts is shown in Fig. S3.† It isclear that the C3N4 adsorption ability exceeds that of the whiteC3N4 by 12.2%. Because the photocatalytic degradationprocess occurred on the surface of the sample, the catalystadsorbing more organic pollutants tends to have higherphotoactivity.56–58

Based on the above analysis, the reason why the photocata-lytic ability can be enhanced could be summarized as follows:firstly, CNT/white C3N4 enhanced the absorption of visiblelight in the visible region than the white C3N4 (as shown inFig. 4, in the range of 410 nm–800 nm). In general, the catalystabsorbs more light, which means that it can utilize moreenergy of the light and tends to possess the higher photocata-lytic ability.59,60 Secondly, CNT showed great adsorptivity ofdye due to strong physical adsorption and electronic adsorp-tion of –COO− on the surface. As can be seen in Fig. S3,† CNT/white C3N4 exhibits the highest adsorption ability. That thecatalyst adsorbs more dye pollutant is beneficial to the dye oxi-dization, and leads to the increasing photocatalytic ability.Thirdly, the most important factor is that CNT/white C3N4 haseffective interfacial contact between CNT and the white C3N4.The coupled CNT can efficiently facilitate the interfacial elec-tron transfer between the white C3N4 and CNT due to its goodconductivity. Therefore, the addition of CNT can significantlyenhance the photoactivity of the CNT/white C3N4 composite.

Fig. 7 The photocatalytic degradation of MB dye (a), the kinetics of MB degra-dation using various photocatalysts (b) and recycling in the repeated MB degra-dation experiments with CNT/white C3N4 (c) under visible light irradiation.

Paper Dalton Transactions

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Similar results have been reported in other CNT coupledmaterials.42

Fig. 7b shows the linear relationship of ln(C0/C) versus time.And Table 1 displays the corresponding calculated kinetic con-stant (k) and regression coefficients (R2). The correspondingdegradation rate constants (k) were estimated to be 0.068 h−1,0.201 h−1, 0.662 h−1, 0.687 h−1 and 1.636 h−1 for MB photo-lysis, the white C3N4, CNT/C3N4, C3N4 and CNT/white C3N4,respectively. The photocatalytic activity order was CNT/whiteC3N4 > C3N4 > CNT/C3N4 > the white C3N4 > MB photolysis,which is well-consistent with the k in the above. The degra-dation rate of the CNT/white C3N4 photocatalyst is almost 8.1times as high as that of the white C3N4, and 2.4 times as highas that of C3N4. It is clear that the successful CNT depositionis beneficial to the increasing degradation rate.

Furthermore, the photocatalytic stability of CNT/white C3N4

was investigated through the repeated MB degradation experi-ments. And the result is shown in Fig. 7c. It is clear that theMB dye can be completely bleached after each decompositionexperiment, and CNT/white C3N4 is stable enough during therepeated experiments without significant loss of photocatalyticactivity. Therefore, CNT/white C3N4 can be used as an effectivephotocatalyst for organic compounds degradation with goodstability.

3.7 Possible mechanism of the photocatalytic reactionprocess

The CNT/white C3N4 composite sample shows significantlyenhanced photocatalytic activity in the degradation of MB dye.Many published works have confirmed that C3N4 can beexcited under the visible-light irradiation.61–63 Wang et al.64

reported that C3N4 could degrade the colorless pollutant4-chlorophenol under visible-light irradiation, which couldexclude the dye-sensitization. Based on the above results,analysis and the opinion of the published works, a possiblephotocatalytic mechanism of CNT/white C3N4 on the enhance-ment of visible light performance is proposed. The proposedmechanism of CNT/white C3N4 can be seen in Fig. 8. Firstly,the electrons are promoted from the VB to the CB of the whiteC3N4 under visible light, leaving the holes behind. Secondly,the photo-excited electrons are effectively collected by CNT(eqn (1)), so the recombination process of the electron–holepairs is hindered, and charge separation as well as stabilizationis achieved. Thirdly, the O2 adsorbed on the surface of catalystcan be reduced to active species (eqn (2) and (3)).55 The holesand the generated ˙OH can react with the organic dye and

generate degradation products (eqn (4)). The possible photo-catalytic decomposition reactions are proposed as follows:

CNT=white C3N4 þ hν ! CNT ðe�Þ=white C3N4 ðhþÞ ð1Þ

CNT ðe�Þ þ O2 ! CNTþ ˙O2� ð2Þ

˙O2� þ 2CNT ðe�Þ þ 2Hþ ! ˙OHþ OH� ð3Þ

white C3N4 ðhþÞ or ˙OHþMB ! degradation products ð4ÞPL emission is a useful technique to investigate the separ-

ation efficiency of the photo-generated charge carriers in asemiconductor because the recombination of excited electronsand holes gives rise to the PL emission signal.58 In order tounderstand why the addition of CNT leads to the enhancedphotoactivity, PL spectra of the white C3N4 and CNT/whiteC3N4 photocatalyst samples (excited by 315 nm) are tested toinvestigate their photo-generated charge carrier separationability. As shown in Fig. 9, the main emission peak is centeredat about 430 nm for the white C3N4,

45 and the emission peakof CNT/white C3N4 appears a little shift compared to the whiteC3N4 (428 nm), which indicates the interaction between CNTand the white C3N4.

28 Besides, the emission peak intensity ofthe CNT/white C3N4 composite photocatalysts significantly

Table 1 Kinetic constants and regression coefficients of MB degradation undervisible light irradiation

Samples Kinetic constant (k, h−1) R2

MB photolysis 0.068 0.9975White C3N4 0.201 0.9932CNT/C3N4 0.662 0.9916C3N4 0.687 0.9762CNT/white C3N4 1.636 0.9921

Fig. 9 The PL spectra of the white C3N4 (a), and CNT/white C3N4 (b).

Fig. 8 Proposed photocatalytic mechanism of the CNT/white C3N4.

Dalton Transactions Paper

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decreases, which suggests that the CNT/white C3N4 compositehas much lower recombination rate of photo-generated chargecarriers than the white C3N4 sample. The efficient transfer ofthe electron indicates that the coupled CNT is very effective inelectron–hole pair separation, which leads to the enhancingphotoactivity of the photocatalyst.

In order to supply further evidence to support that CNTplayed an important role in the photocatalytic reaction, thetransient photocurrent responses of C3N4, the white C3N4 andCNT/white C3N4 were tested for four on–off cycles ofirradiation. The result is shown in Fig. 10. It is clear that CNT/white C3N4 has the highest photocurrent density. The photo-current result confirms the lower rate of recombination andmore efficient separation of photogenerated electron–holepairs for the CNT/white C3N4 composite sample at the inter-face between CNT and the white C3N4. It indicates that CNThas been effectively combined with the white C3N4, whichleads to the efficient separation of the photogeneratedelectron–hole pairs due to CNT’s good conductivity. The abovecharacteristics of CNT/white C3N4 ultimately result in its highphotoactivity.

Generally, according to the photocurrent result, the photo-catalytic ability order of the catalysts should be as follows:CNT/white C3N4 > white C3N4 > C3N4. However, the photocata-lytic ability of the white C3N4 is much lower than that of C3N4

(about 42.8% at 3 h). This is probably for the reason that thewhite C3N4 (positive polarity) is not likely to adsorb the MBdye (positive polarity, the adsorption ability is shown inFig. S3†), which leads to the result that the photogeneratedhole cannot contact with the organic dye efficiently andcannot react with it effectively. Besides, the PL spectra resultreveals that the recombination rate of the photogenerate elec-tron–hole of the white C3N4 is very high, which can alsodecrease the photoactivity.

Based on the above analysis, the ultimate degradationability of the catalyst was determined by the polarity of the

catalyst and the dye, optical and photoelectrochemical pro-perties of the catalyst.

4. Conclusions

In conclusion, CNT/white C3N4 composite has been preparedby protonating C3N4 into white C3N4 first, and then with ahydrothermal method in the presence of CNT. The CNT/whiteC3N4 composite shows significantly enhanced photocatalyticactivity than the white C3N4, which is associated with theenhanced photo-responsive ability, great adsorptivity of dyesand high electron–hole separation efficiency (which is due tothe synergetic interactions between CNT and the white C3N4).The present study provides a new strategy to prepare CNTmodified white C3N4, and it can also be extended to the syn-thesis of various CNT modified hybridized materials and otherbinary compounds with exceptional photocatalytic activity orother properties.

Acknowledgements

The authors genuinely appreciate the financial support of thiswork from the National Nature Science Foundation of China(21007021, 21076099, 21177050 and 21206060), NaturalScience Foundation of Jiangsu Province (BK2012717), SocietyDevelopment Fund of Zhenjiang (SH2011011 and SH2012020)and Doctoral Innovation Fund of Jiangsu (CXLX12-0666).

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Dalton Transactions Paper

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