DOI: 10.1002/cctc.201200277

Ternary Polymer Composite of Graphene, Carbon Nitride,and Poly(3-hexylthiophene): an Efficient PhotocatalystSandeep Gawande[a] and Sanjay R. Thakare*[b]

Introduction

In the past few decades, photocatalytic organic dye degrada-tion from waste water with semiconducting catalysts has beenan active research topic owing to the global energy crisis andenvironmental pollution. Photocatalytic degradation of organicdyes such as methylene blue (MB) by using TiO2 is gaininggrowing research interest for water purification. However, theneed for UV light to activate the photocatalysts greatly limitsthe technology in practical applications because of the lowcontent of UV light in the solar spectrum.[1–6] As a result, re-search efforts have been made to try and exploit new photoca-talysts that are photocatalytically active under visible light irra-diation.[4–6] On the other hand, modified semiconductors (SC)can be sensitized by organic dyes under visible light irradiationin which the dye rather than the SC is excited.[7–10] This is fol-lowed by an electron transfer from excited dye to conductionband of the SC. Then, the electron is trapped by surface-ad-sorbed oxygen to form various reactive oxygen species (ROSs)that are responsible for mineralization of the dye.[7, 8] This pro-cess is superior to that of UV-activated photocatalysis becausemore sunlight can be used. However, the modified photocata-lysts do have limitations such as low degradation efficiency,stability, and so on.

Graphene, a two-dimensional carbon material with uniquemechanic and electronic properties, offers a good opportunityto prepare composite materials for photocatalytic applica-tions.[11–14] Graphene is generally prepared by chemical oxida-tion of graphite in which graphite is converted into exfoliatedsheets of graphene oxide (GO). The subsequent reduction ofGO has been proven as being useful for the preparation of re-duced GO (RGO) semiconductor composites; these includeRGO-TiO2 and RGO-ZnO.[15, 16] RGO, which is referred to as gra-phene, has the unique property of accepting excited electronsfrom a semiconductor source; this can be useful for increasingthe time of de-excitation.

Recently, a defective graphitic carbon nitride (g-C3N4) poly-mer semiconductor with an optical band gap of 2.7 eV has at-tracted increasing interest owing to its performance in the pro-

duction of hydrogen or oxygen from water under visible lightirradiation in the presence of a sacrificial donor or acceptor.[17]

The g-C3N4 photocatalyst is considered to be stable under lightirradiation in aqueous solution, as well as in acidic (HCl, pH 0)or basic (NaOH, pH 14) solutions owing to the strong covalentbonds between the carbon and nitrogen atoms. However, theabsorbance wavelength of g-C3N4 is shorter than 460 nmowing to its large band gap of 2.7 eV. The optical absorptionof carbon nitride semiconductor materials can be extendedinto the visible region up to approximately l= 750 nm bysimple copolymerization with organic monomers such as bar-bituric acid, but the photocatalytic activity is low. Therefore, itis important to further improve the photocatalytic perfor-mance of g-C3N4 to obtain a more effective catalyst. For this, itis necessary to increase the separation efficiency of photogen-erated electron–hole pairs during photocatalysis. One methodto overcome this problem is to load noble metals, metaloxides or metal sulfides onto the surface of the catalyst as co-catalysts. Another method is to form a composite photocata-lyst between two kinds of semiconductor powders. However,previous research has been limited to inorganic compositesand inorganic polymer composites. For example, a compositephotocatalyst of g-C3N4 and TaON with a visible light responsewas prepared by a milling-heat treatment method and wasused for photodegradation of rhodamine-B.[18] Poly(3-hexylthio-phene) (P3HT) is a polymer semiconductor with a band gap of1.9–2.1 eV, higher charge carrier mobility, dissolubility, process-

[a] S. GawandeNanotechnology Lab, Department of ChemistryScience College, Congress nagar, Nagpur-440012 (India)Fax: (+ 91) 0712-2440955

[b] Dr. S. R. ThakareDepartment of ChemistryGovt. Institute of ScienceNagpur-440 001 (India)

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cctc.201200277.

Herein we report on the synthesis of a ternary polymer compo-site of graphene, carbon nitride, and poly(3-hexylthiophene)(G-g-C3N4-P3HT) by a solvothermal method. The prepared poly-mer composite catalysts are characterized by TEM, XRD, FTIRspectroscopy, and photoluminescence techniques for theirmorphology, structure and photocatalytic efficiency. Graphene-

loaded polymer composites of carbon nitride (g-C3N4) andpoly(3-hexylthiophene) act as efficient photocatalysts for wastewater treatment. Methylene blue is used as the model pollu-tant and the rate of its photocatalytic degradation with G-g-C3N4-P3HT is three times higher than that achieved with a g-C3N4-P3HT composite.

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ability, and long-term stability.[19] P3HT has already been widelyused in P3HT:PCBM(phenyl-C61-butyric acid methyl ester) bulkhetero-junctions for photocurrent generation.[20]

To the best of our knowledge, the use of organic–organicpolymer composites as photocatalysts in the degradation oforganic dyes is scarce. Herein, we report an enhanced MB deg-radation through graphene-loaded ternary polymer compositephotocatalyst of g-C3N4-P3HT for photocatalytic degradation ofMB in water. Our experimental results indicate that the MBdegradation rate of a graphene-g-carbon nitride-poly(3-hex-ylthiophene) (G-g-C3N4-P3HT) ternary polymer composite is ap-proximately three times higher than that achieved by a g-C3N4-P3HT composite.

Results and Discussion

The structure and morphology of the polymer composite witha 20 % P3HT concentration was investigated by XRD and TEM.The XRD spectra of GO and the G-g-C3N4-P3HT polymeric com-posite are shown in Figure 1. The characteristic peak (0 0 1) of

GO shown at 2 q�10.278 in Figure 1 (a), does not appear inthe XRD spectrum of the G-g-C3N4-P3HT composite (Fig-ure 1 (b)). The peaks at 2 q�5.18 and 13.58 correspond to the(1 0 0) and (2 0 0) planes, and the peak at 22.18 corresponds tothe p–p stacking of P3HT (the interchain distance of face toface packing of the thiophene rings). The XRD peak at 2 q

�27.38 corresponds to the (0 0 2) plane of g-C3N4. An addition-al blunt peak at 27.40 that corresponds to the interlayer gra-phene interaction is shown in Figure 1 (b). The disappearanceof the (0 0 1) peak of GO indicates the complete conversion ofGO into graphene (Figure 2 (b)). All of these peaks are consis-tent with the previous records and JCPDS data of g-C3N4, P3HTand graphene, which indicates the formation of a polymericcomposite.

The TEM image shows that the photocatalyst has an irregu-lar geometry (Figure 2). This morphology is maintained in the

nanocomposites, but a very homogeneous dispersion of gra-phene over the g-C3N4-P3HT (and vice versa) can be clearly ob-served. The IR spectra of GO and G-g-C3N4-P3HT are shown inFigure 3. The representative absorption peaks of GO, which in-clude n= 3362 cm�1 (O�H stretching vibration), 1713 cm�1 (C=

O stretching vibration of COOH groups), 1143 cm�1 (tertiary C�OH groups stretching vibration), and 1028 cm�1 (C�O vibra-

tions), decreased dramatically in intensity or even disappearedafter combination with g-C3N4-P3HT, which indicates the reduc-tion of GO to graphene during the evaporation step of compo-site formation. The photoluminescence (PL) spectra of thepolymer composite catalysts under incident light with a wave-length of 259 nm are shown in Figure 4. The peaks at l= 453and 593 nm could be related to the electron–hole recombina-tion of g-C3N4 and P3HT, respectively. Significant PL quenchingwas observed in the polymer composite catalysts. The PL ofthe polymer composite catalysts that contained 30 mg GO wasquenched by nearly 90 % relative to that of g-C3N4. The PL in-tensity of g-C3N4-P3HT decreased with an increase in theamount of GO. Two new PL emission peaks at l= 375 and740 nm were observed in the PL spectra of the G-g-C3N4-P3HTpolymer composite catalysts, which correspond to energygaps of 3.3 and 1.66–1.72 eV, respectively.

The intensity of the new PL emission peaks increases withan increase in the amount of P3HT, and achieves a maximum ifthe amount of P3HT is approximately 20 wt %. It was reportedthat the conduction band (CB) and valence band (VB) edge po-

Figure 1. XRD spectra of a) GO-g-C3N4-P3HT and b) G-g-C3N4-P3HT polymercomposites. *: P3HT, + : GO, ~: C3N4, andN : graphene.

Figure 3. IR spectra of a) the G-g-C3N4-P3HT polymer composite and b) GO.

Figure 2. TEM image of the G-g-C3N4-P3HT polymer composite.

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S. R. Thakare and S. Gawande

tentials of g-C3N4 are �1.12 and 1.57 eV, respectively. The CBand VB edge potentials of P3HT are �1.5 eV and 0.6 eV, respec-tively.

The photon energy of 3.3 eV (l= 375 nm) is in agreementwith the value of 3.1 eV for the difference between the CB ofP3HT and the VB of g-C3N4, and the value of 1.7 eV (l=

740 nm) for the difference between the CB of g-C3N4 and theVB of P3HT. Therefore, it is reasonable to suggest that the newPL emission peaks at l= 375 and 740 nm can be attributed tothe recombination of the excited electron of P3HT with thehole of g-C3N4, and the recombination of the excited electronof g-C3N4 with the hole of P3HT, respectively. The significant PLquenching by the addition of graphene to the polymer com-posite catalysts indicates the efficient charge transfer thatoccurs between g-C3N4-P3HT and graphene.

The percentage efficiency of MB degradation with g-C3N4-P3HT, G-g-C3N4-P3HT, G-g-C3N4 and G-P3HT is shown in Figure 5.In the absence of graphene co-catalysts, a much lower amountof MB degradation was detected for the g-C3N4-P3HT compo-site, which indicates a lower photocatalytic activity for the g-C3N4-P3HT composite under visible light irradiation. The g-C3N4-P3HT composite, with graphene co-catalysts, shows three timesenhanced activity for MB degradation under visible light. Thecourse of MB degradation was studied by UV/Vis spectroscopy(Figure 6).

We studied the rate of MB degradation by changing param-eters such as the concentration of co-catalyst, the concentra-tion of P3HT, and the concentration of the polymeric composite

g-C3N4-P3HT. We found that g-C3N4-P3HT composite catalystswith a loading of 30 mg graphene, and 100 mg L�1 C3N4-P3HTthat contained 20 % P3HT showed the highest photocatalyticactivity. The degradation efficiency of the polymeric composite,with varying concentrations of graphene and varying concen-trations of P3HT, respectively is shown in Figure 7 (a–b). Therate of MB degradation increases with an increase in the

Figure 4. Photoluminescence spectra of the G-g-C3N4-P3HT polymer compo-site under excitation (l = 259 nm) at 298 K. Wavelength ranges: a) 200–800and b) 680–800 nm.

Figure 5. Efficiency of MB degradation on g-C3N4-P3HT polymer compositeswithout co-catalyst loading and those loaded with 30 mg graphene as a co-catalyst under visible light (l>400 nm, catalyst mass = 0.1 g).

Figure 6. UV/Vis spectra that show the degradation of MB in the presence ofthe G-g-C3N4-P3HT polymer composite photocatalyst (catalyst mass = 0.1 g,MB concentration = 2 � 10�5

m).

Figure 7. Rate of MB degradation on g-C3N4-P3HT polymer composites withdifferent amounts of a) graphene and b) P3HT under visible light(l>400 nm).

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Ternary Polymer Composite as an Efficient Photocatalyst

amount of g-C3N4-P3HT, and achieves a maximum if theamount of g-C3N4-P3HT is approximately 100 mg L�1. The esti-mated MB degradation efficiency is found to be over 90 %. Theactivity of the g-C3N4-P3HT catalyst with graphene is threetimes higher than that of the g-C3N4-P3HT catalyst without gra-phene. However, the rate of MB degradation decreases with anincrease in the P3HT content if the amount of P3HT is higherthan 20 %. Although, it was observed in the PL spectra thatthe higher the content of P3HT, the clearer the quenching andthe greater the number of new peaks. Furthermore, we havealso observed that the polymer composites with a high con-tent of hydrophobic P3HT tend to agglomerate together, andthat they float on the surface of the solution during the photo-catalytic reaction. This could be responsible for lowering theabsorption of light and therefore the rate of MB degradation.Introduction of graphene to the g-C3N4-P3HT composite sup-presses this phenomenon, it also decreases the amount ofelectron–hole recombination. From the above observation,a possible reaction scheme can be proposed (Figure 8). Fur-thermore, it is observed that the ternary composite photocata-

lyst does not undergo any photocorrosion or photodegrada-tion during the reaction. This was confirmed by using XRD onthe ternary composite material before and after the photocata-lytic reaction (Supporting Information, Figure S4).

Conclusions

The present results introduce a polymer composite for the en-hanced photocatalytic degradation of methylene blue in water,under visible light. The rate of methylene blue degradation in-creases up to three times if 30 mg graphene and 20 % poly(3-hexylthiophene) (P3HT) is added into 80 % carbon nitride (g-C3N4), compared with g-C3N4-P3HT. The efficient charge transferbetween graphene and g-C3N4-P3HT is seemingly responsiblefor the enhanced photocatalytic activity of the polymer com-posite catalyst. Although the photocatalytic efficiency is lowerthan that of CdS-based photocatalysts, the use of the polymercomposite photocatalysts can avoid the toxicity of cadmiumand the photocatalytic activity can be further promoted bychanging the co-catalyst.

Experimental Section

All chemicals used in our experiments were reagent grade andused without further purification. The morphology and structure ofthe products were determined by TEM (Philips-CM200 TEM) andXRD (XPERT-PRO) with CuKa radiation.

Synthesis of GO

The preparation of GO was performed by a modified Hummersmethod.[21, 22] In a typical synthesis, graphite powder (2.0 g) was putinto cold (�5 8C) concentrated H2SO4 (100 mL). Then, KMnO4

(8.0 g) was added gradually under stirring and the temperature ofthe mixture was kept below 10 8C by cooling. The reaction mixturewas kept at a temperature below 10 8C for 2 h. Then, the mixturewas stirred at 35 8C for 1 h, and subsequently diluted with deion-ized (DI) water (100 mL). As addition of water to concentrated sul-furic acid releases a large amount of heat, the addition was per-formed in an ice bath to keep the temperature below 100 8C. Afteradding all of the DI water (100 mL), the mixture was stirred for 1 h,and was then further diluted to approximately 300 mL with DIwater. After that, H2O2 (30 %, 20 mL) was added to the mixture toreduce the residual KMnO4. The mixture released a large amount

of bubbles and the color of the mixture changed tobright yellow. Then, the mixture was filtered and washedwith aq. HCl (5 %, 400 mL) to remove the metal ions.This was followed by an addition of DI water (500 mL) toremove the acid. The resulting solid was dried at 60 8Cfor 24 h.

Synthesis of the G-g-C3N4-P3HT polymeric compo-site

The ternary polymer composite catalyst of GO-g-C3N4-P3HT was prepared by impregnating GO and g-C3N4 witha chloroform solution of P3HT overnight. Then, thechloroform was evaporated by using a water bath. Thereduction of GO to graphene was performed by a photo-lytic reduction with ethanol as the solvent; this finallygave the G-g-C3N4-P3HT composite material.[23]

Evaluation of photocatalytic activity

The relative photocatalytic activities of photocatalysts [i.e. , the effi-ciency of MB degradation on g-C3N4-P3HT polymer compositeswithout co-catalyst and those loaded with 30 mg graphene as co-catalyst under visible light (l>400 nm), catalyst mass = 0.1 g] wereevaluated by the photodegradation of MB. The selected photocata-lyst was dispersed in the aqueous MB solution (1 � 10�5

m) to ach-ieve a concentration of 1 mg mL�1 (the amount of the overall pho-tocatalyst was the same in every photodegradation experiment).Firstly, the mixed suspension was stirred in the dark for 1 h toreach the adsorption–desorption equilibrium of the MB dye. A Phi-lips lamp (40 W/230 V) was placed 10 cm away from the reactionvessel, which was used to provide a full-spectrum emission withoutany filter to simulate sunlight. The photocatalytic reaction wasstarted by turning on the Philips lamp. An aliquot (4 mL) was ex-tracted at various irradiation times and centrifuged to remove thephotocatalyst. The concentration of residual MB in the upper clearlayer was determined by recording the maximum absorbance ofMB at l= 663 nm with a UV/Vis spectrophotometer.

Figure 8. A proposed mechanism of visible light-induced MB degradation on G-g-C3N4-P3HT polymer composite photocatalysts.

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Additional original TEM images of the G-g-C3N4-P3HT composite,mineralization data, and XRD patterns of the precursor samplesand the ternary composite after the photocatalytic reaction areprovided in the Supporting Information.

Acknowledgements

The authors gratefully acknowledge UGC, New Delhi(No.F-39-696/2010(SR) and F-14-11/2008(Inno./ASIST) for financial assis-tance to perform this work through the Major Research Projectand Innovative Programme.

Keywords: graphene · luminescence · nanostructures ·polymers · semiconductors

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Ternary Polymer Composite as an Efficient Photocatalyst

FULL PAPERS

S. Gawande, S. R. Thakare*

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Ternary Polymer Composite ofGraphene, Carbon Nitride, and Poly(3-hexylthiophene): an EfficientPhotocatalyst

A bolt into the blue: A ternary polymercomposite of graphene, carbon nitride,and poly(3-hexylthiophene) (G-g-C3N4-P3HT) is synthesized and proves an ef-fective photocatalyst for the degrada-tion of methylene blue under visiblelight in water. The G-g-C3N4-P3HT com-posite photocatalyzes the rate of degra-dation of this model pollutant at a ratethree times higher than that with a g-C3N4-P3HT composite and could be ap-plied to the treatment of waste water.

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