Chinese Chemical Letters

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1 Chinese Chemicl Letters 25 (2014) Contents lists ville t ScienceDirect Chinese Chemicl Letters journl homepge: Originl rticle Etching grphitic cron nitride y cid for enhnced photoctlytic ctivity towrd degrdtion of 4-nitrophenol Si-Zhn Wu, Ci-Hong Chen, Wei-De Zhng * School of Chemistry nd Chemicl Engineering, South Chin University of Technology, Gungzhou , Chin ARTICLE INFO ABSTRACT Article history: Received 7 Ferury 2014 Received in revised form 18 Mrch 2014 Accepted 3 April 2014 Aville online 14 My 2014 Keywords: Photoctlyst 4-Nitrophenol Etching g-c 3 N 4 Grphitic cron nitride (g-c 3 N 4 ) with high photoctlytic ctivity towrd degrdtion of 4-nitrophenol under visile light irrdition ws prepred y HCl etching followed y mmoni neutrliztion. The structure, morphology, surfce re, nd photoctlytic properties of the prepred smples were studied. After tretment, the size of the g-c 3 N 4 decresed from severl micrometers to severl hundred nnometers, nd the specific re of the g-c 3 N 4 incresed from 11.5 m 2 /g to 115 m 2 /g. Menwhile, the photoctlytic ctivity of g-c 3 N 4 ws significntly improved fter tretment towrd degrdtion of 4- nitrophenol under visile light irrdition. The degrdtion rte constnt of the smll prticle g-c 3 N 4 is 5.7 times of tht of ulk g-c 3 N 4, which mkes it promising visile light photoctlyst for future pplictions for wter tretment nd environmentl remedition. ß 2014 Wei-De Zhng. Pulished y Elsevier B.V. on ehlf of Chinese Chemicl Society. All rights reserved. 1. Introduction In the pst decdes, the development of humn society hs cused tremendous environmentl prolems. Tons of orgnic pollutnts were discrded into the lkes nd rivers everydy which resulted in serious wter contmintion. The polluted wter is hrmful for humn eings nd other living things. Among those orgnic pollutnts, the romtic compound, 4-nitrophenol, from the rw mteril to mnufcture drugs, pesticides, nd dyes, is one of the primry orgnic pollutnts in wter. This chemicl my cuse lood disorder [1,2]. It is difficult to remove the 4- nitrophenol y trditionl iologicl nd dsorption methods due to its high stility in wter. Fortuntely, Fujishim nd Hond first reported the photoctlyst TiO 2 for splitting wter to generte hydrogen gs in 1972 [3]. This novel photoctlytic technology ecomes promising wy to degrde orgnic pollutnts, which hs ttrcted intense reserch interest ll over the world in the pst decdes due to its environmentlly friendly nd economicl dvntges [4,5]. Although TiO 2 shows high photoctlytic ctivity for the degrdtion of mny pollutnts, it cn only respond to the UV light irrdition due to its wide ndgp (3.2 ev). Unfortuntely, the solr light contins only out 4% UV light. In order to improve * Corresponding uthor. E-mil ddress: zhngwd@scut.edu.cn (W.-D. Zhng). the utiliztion of the solr light, to explore efficient visile light driven photoctlysts is gretly demnded. Mny visile light driven photoctlysts hve een reported in recent yers [6 12]. Among these photoctlysts, the grphitic cron nitride (g-c 3 N 4 ), metl free visile light driven photoctlyst, hs ttrcted intense interest due to its unique properties such s high stility, nontoxicity, esy modifiction nd outstnding electricl property [12,13]. Efforts hve een devoted on using g-c 3 N 4 s photoctlyst for degrdtion of pollutnts nd production of hydrogen [12 20]. However, the photoctlytic ctivity of the sprepred g-c 3 N 4 is low. In order to improve the photoctlytic ctivity of g-c 3 N 4, ttempts hve een mde on incresing its surfce re including using templte to prepre porous g-c 3 N 4, or treted g-c 3 N 4 y lkli or cid [14 17]. By using silic microlls s templtes, the specific surfce re of the g-c 3 N 4 reched to 373 m 2 /g [14]. However, hydrofluoric cid must e used in this method to remove the templtes. This process is not environment friendly nd costly. Other pproches, like HCl or lkli tretment, cn improve the specific surfce re of the g- C 3 N 4 to severl tens squre meters per grm [16,17]. In this contriution, we re reporting simple method of prepring high specific surfce re g-c 3 N 4 without ny templte. After etching, the surfce re of the g-c 3 N 4 incresed from 11.5 m 2 /g to 115 m 2 /g, nd the size of the g-c 3 N 4 prticle lso decresed significntly. It shows excellent photoctlytic ctivity towrd degrdtion of 4-nitrophenol compred with the pristine g-c 3 N 4 under visile light irrdition /ß 2014 Wei-De Zhng. Pulished y Elsevier B.V. on ehlf of Chinese Chemicl Society. All rights reserved.

2 1248 S.-Z. Wu et l. / Chinese Chemicl Letters 25 (2014) [(Fig._1)TD$FIG] 2. Experimentl Melmine (C 3 H 6 N 6 ) ws purchsed from Tinjin Kemiou Chemicl Co., Ltd. NH 3 H 2 O (25%) nd HCl (37%) were purchsed from Gungzhou Chemicl Co., Ltd. All regents used in this study re nlyticl grde nd used without further purifiction. The g- C 3 N 4 ws synthesized y heting melmine to 550 8C for 2 h in muffle. The s-prepred g-c 3 N 4 ws ground to powder using n gte mortr. g-c 3 N 4 powder (0.95 g) ws dded into 40 ml HCl solution ( mol/l) under mgnetic stirring for 0.5 h. Then, the mixture ws trnslted to Teflon-lined utoclve (50 ml) nd heted t 150 8C for 5 h. After eing cooled down to the room temperture nturlly, the product ws collected y filtrtion followedyfurtherdispersedin40mlmmonisolution(2.0mol/l), nd stirred for 0.5 h. After tht, the product ws wshed with distilled wter nd ethnol for severl times, nd then dried t 80 8C for 10 h. Finlly, the product ws heted t 400 8C for1h in ir to remove mmoni. The otined product ws nmed s g-c 3 N 4 -T. Phse nd structurl of the smples were chrcterized y X- ry diffrctometer (XRD, D8 Focus X-ry diffrctometer, Bruker, Germny) using Cu K (l = nm) s rdition source. The Fourier trnsform infrred spectr (FTIR) of the products were recorded on IR Affiniy-1 FTIR spectrometer. The morphology of the smples ws oserved using field emission scnning electron microscope (FESEM, JSM-6330F, JEOL, Jpn). The specific surfce res were mesured t 77 K using 3H-2000PSI instrument nd estimted y Brunuer Emmett Teller method. The UV vis diffuse reflectnce spectroscopy (UV vis DRS) ws conducted on UV-2550 spectrophotometer using BSO 4 s reference. The photoctlytic rection smples were nlyzed y high performnce liquid chromtogrphy (HPLC, DIONEX, TCC-100) with ultimte 3000 vrile wvelength detector. A C18 column (Ecosil EC5-3237, 250 mm 4.6 mm) ws used to seprte the degrdtion products. The photoctlytic ctivity of the smples ws evluted y degrdtion of 4-nitrophenol under irrdition of visile light using 500 W Xe lmp (l: nm) s light source using sodium nitrite solution (5.0 g/l) s the UV light filter. For the photoctlytic degrdtion rection, 0.40 g of the prepred photoctlyst ws dispersed in 400 ml 4-nitrophenol solution (8 mg L -1, ph 4). A smll ir pump (2.5 W) ws used to low ir into the 4-nitrophenol solution. Before irrdition, the suspensions were stirred for 0.5 h in drk in order to rech n dsorption desorption equilirium etween the 4-nitrophenol nd photoctlyst. Then, the solution ws exposed to Xe lump irrdition under mgnetic stirring. At every 1 h intervl, 7 ml solution ws extrcted from the rector. The concentrtion of 4-nitrophenol ws [(Fig._2)TD$FIG] Fig. 1. FESEM imges of (A) g-c 3 N 4 nd (B) g-c 3 N 4 -T. determined y mesuring the sorption t l = 317 nm. The solution smples were lso nlyzed y HPLC fter filtered with 0.22 mm cellulose memrne filter. The moile phse of methnol:wter (60:40, v/v), flow rte of ml/min nd detection t the wvelength of 224 nm were used. 3. Results nd discussion The FTIR spectrum of g-c 3 N 4 -T is lmost the sme s tht of the s-prepred g-c 3 N 4, indicting tht they re the sme structure (Fig. S1 in Supporting informtion). The typicl IR chrcteristic peks of grphitic cron nitride cn e found in oth smples, nd no other impurity pek ws found in oth smples. The pek in the region 801 cm 1 cn e ttriuted to the trizine units, which re the units for forming grphitic cron nitride. Menwhile, severl other strong peks rnge from 1200 cm 1 to 1650 cm 1 cn e scried to the typicl stretching mode of CN heterocycles in g-c 3 N 4. A rod pek rnges from 2800 cm 1 to 3400 cm 1 cn e ssigned to the N H stretching virtion mode [20]. FESEM ws used to oserve the morphology of g-c 3 N 4 -T nd g- C 3 N 4. Fig. 1A shows the pristine g-c 3 N 4 smple. The size of g-c 3 N 4 is out severl micrometers. After tretment, g-c 3 N 4 -T ws corroded y HCl to severl hundred nnometers, s shown in Fig. 1B. The specific surfce re of the smples, n importnt fctor ffecting the ctivity of photoctlyst, ws lso exmined. Fig. 2A shows the nitrogen dsorption desorption isotherms of g-c 3 N 4 nd g-c 3 N 4 -T, oth of the isotherms re of type IV (BDDT Clssifiction), suggesting the presence of mesopores in ll smples [24]. The result revels tht the specific surfce re of g-c 3 N 4 -T increses to 115 m 2 /g fter tretment, which is 10 times of tht of the pure g-c 3 N 4 (11.5 m 2 /g). Fig. 2B is the pore size distriution curve, which revels tht the pore size of g-c 3 N 4 -T incresed ecuse of the cid etching. The incresed surfce re of g-c 3 N 4 -T cn e ttriuted to the decresing size of the prticles, Fig. 2. (A) The nitrogen dsorption/desorption isotherms of () g-c 3 N 4 nd () g-c 3 N 4 -T. (B) The pore-size distriution of () g-c 3 N 4 nd () g-c 3 N 4 -T.

3 [(Fig._3)TD$FIG] S.-Z. Wu et l. / Chinese Chemicl Letters 25 (2014) (A) (B) As. (.u.) (F(R)E) 1/ ev 2.75 ev Wvelength (nm) Eg (ev) Fig. 3. (A) UV vis DRS spectr, nd (B) the trnsformed Kuelk Munk function vs. the light sorption of () g-c 3 N 4 nd () g-c 3 N 4 -T. which is dvntgeous to improve the ctivity of the ctlysts. Ctlysts with lrger specific surfce re cn enhnce the dsorption of rectnts, provide more ctive sites, nd depress the recomintion of the generted electron hole pirs [14,15,25]. In ddition to specific surfce re, opticl sorption property is nother fctor ffecting the photoctlytic ctivity of the ctlysts. Fig. 3A displys the opticl sorption of g-c 3 N 4 nd g-c 3 N 4 -T. The mximum sorption wvelength of g-c 3 N 4 -T ppers slightly lue-shift compred to tht of g-c 3 N 4, which cn e scried to the smller size of g-c 3 N 4 -T [15 17]. The ndgps of the smples cn e clculted y plotting [F(R)E] 0.5 ginst the energy of excittion source [21], which were shown in Fig. 3B. The ndgp of g-c 3 N 4 -T is 2.75 ev, which is lrger thn tht of the pristine g-c 3 N 4 (2.68 ev). The lrger ndgp of g-c 3 N 4 - T enhnces the oxidtion potentil of the photogenerted holes nd/or the reduction property of the photogenerted electrons, thus improves its photoctlytic ctivity. From the XRD ptterns of g-c 3 N 4 nd g-c 3 N 4 -T, we cn clerly find tht two chrcteristic peks, g-c 3 N 4 (1 0 0) plne round nd (0 0 2) plne round 27.48, were ll detected in oth smples (Fig. S2 in Supporting informtion). No ny other impurity phse ws detected, which indicted tht fter tretment, the structure of g-c 3 N 4 ws not significntly chnged. The weker pek round is ttriuted to the in-plnr structurl pcking motif of g-c 3 N 4, which revels the distnce of the nitride pores is nm. The ltter pek round corresponds to the (0 0 2) plne of g-c 3 N 4 [12]. The intensity of (1 0 0) plne round in [(Fig._4)TD$FIG] smple g-c 3 N 4 -T is slightly lower thn tht of the g-c 3 N 4 smple. This cn e scried to the HCl cid t high temperture my slightly destroy the structure of g-c 3 N 4 nd cuse the pek weken [17]. The (0 0 2) plne pek of g-c 3 N 4 -T lso shifts from to 27.88, corresponding to the decrese in the interplnr stcking distnce from nm to nm. This phenomenon could e interpreted s the tretment process cn improve the inter lyer stcking order [22]. In ddition, impurity ppered during this process, this my e scried to the g-c 3 N 4 corroded y the HCl solution t high temperture nd the impurity ws produced. Interestingly, the impurity disppered in smple g-c 3 N 4 -T fter mmoni tretment. The mmoni not only cn remove the impurity, ut lso cn neutrlize the excess HCl cid in the g-c 3 N 4, nd the followed heting tretment lso cn remove the residul H + nd the excess mmoni which dsored on the surfce of g-c 3 N 4. The most elievle mechnism of the smll g-c 3 N 4 -T formtion is shown in Fig. S3 in Supporting informtion. The zet potentil of smples ws mesured. Firstly, the zet potentil of g-c 3 N 4 chnged from 34.5 to mv fter hydrolysis in mol/l HCl, corresponding to the chnge of g-c 3 N 4 to protonted g-c 3 N 4. After mmoni tretment, the zet potentil of smple decresed from to mv. This my e scried to tht the excess HCl cid on the surfce of smple ws neutrlized y the mmoni solution. Finlly, the zet potentil of g-c 3 N 4 -T chnged to 33.6 mv fter nneling, which illustrtes tht the H + in the g-c 3 N 4 -T ws lmost completely removed. 0.9 (A) c (B) C/C In drk Ln(C 0 /C) Visile light Time (h) Time (h) Fig. 4. (A) Photoctlytic degrdtion of 4-nitrophenol over () g-c 3 N 4, () g-c 3 N 4 -T nd (c) photolysis. (B) The plots of the first-order degrdtion rte over g-c 3 N 4 (), nd g- C 3 N 4 -T ().

4 1250 S.-Z. Wu et l. / Chinese Chemicl Letters 25 (2014) The visile light photocttlytic ctivity of the smples ws evluted y decomposing the representtive hzrdous pollutnt 4-nitrophenol under the irrdition of 500 W Xe lmp, using sodium nitrite solution (5 g/l) s the UV light filter liquor. The result is shown in Fig. 4A. After irrdition for 6 h, the degrdtion rte of 4-nitrophenol is 89.3%, 32.1% nd 5.6% over g-c 3 N 4 -T, g- C 3 N 4 nd without photoctlyst, respectively. The result clerly revels tht the 4-nitrophenol solution is very stle under visile light irrdition, nd the photoctlytic ctivity of g-c 3 N 4 -T is higher thn tht of the s-prepred g-c 3 N 4. In order to further investigte the rection kinetics of degrdtion of 4-nitrophenol, first-order kinetic model ln(c/c 0 )=kt ws used, where k is the kinetic rte constnt [23]. As indicted in Fig. 4B, the degrdtion of 4-nitrophenol over oth the pristine nd etching g-c 3 N 4 mtches to the first-order kinetics eqution ( dc/dt = kc). The kinetic rte constnts over g-c 3 N 4 -T nd pristine g-c 3 N 4 re nd 63 h 1, respectively. The kinetic rte constnt oviously revels tht the photoctlytic ctivity of g-c 3 N 4 -T is much higher thn tht of pristine g-c 3 N 4, the former is out 5.7 times of tht of the ltter. The higher photoctlytic ctivity of g-c 3 N 4 -T cn e prtly scried to the incresing specific surfce re. However, the kinetic rte constnt is not proportion to the incresed surfce re, since the surfce re of g-c 3 N 4 -T is out 10 times of tht of the pristine g-c 3 N 4. This my e scried to when the size of g- C 3 N 4 -T decresed, its specific surfce re incresed, nd the ndgp lso incresed. Fig. 5 shows the HPLC profile of 4-nitrophenol queous solution, g-c 3 N 4 -T dispersed in wter, nd 4-nitrophenol degrdtion smple ginst rection time over g-c 3 N 4 -T photoctlyst. The sorption pek of 4-nitrophenol decresed grdully t 6.42 min retention time upon the increse of irrdition time, nd severl peks ppered t round 2.3, 2.7, 3.2 nd 5.8 min, which my e scried to the solvent nd g-c 3 N 4 -T photoctlyst. After irrdition for 7 h under visile light, 4-nitrophenol ws lmost completely decomposed. No ny other peks corresponding to smll molecules decomposed from 4-nitrophenol were detected. The HPLC nlysis revels tht 4-nitrophenol ws degrded grdully to H 2 O nd CO 2 on g-c 3 N 4 -T photoctlyst under visile light. The stility of the ctlyst is nother fctor ffecting the ctlyst for prcticl ppliction. The stility of the g-c 3 N 4 -T ws lso investigted in this study (Fig. S4 in Supporting informtion). The result shows tht the photoctlytic ctivity of g-c 3 N 4 -T did [(Fig._5)TD$FIG] 4-nitrophenol Retention time (min) 4-nitrophenol solution 0 h degrdtion 1 h degrdtion 3 h degrdtion 5 h degrdtion 7 h degrdtion g-c 3 N 4 -T in wter Fig. 5. HPLC profiles of 4-nitrophenol queous solution, g-c 3 N 4 -T dispersed in wter, nd 4-nitrophenol photoctlytic degrdtion smple ginst rection time over g-c 3 N 4 -T photoctlyst. not significntly chnge fter three cycles, which revels the very high stility of g-c 3 N 4 -T in the photoctlytic rection. 4. Conclusion In conclusion, we hve developed simple method to decrese the prticle size of g-c 3 N 4 nd increse its surfce re. After etching y HCl in hydrotherml process nd then neutrlized y mmoni, the specific surfce re of g-c 3 N 4 -T is significntly incresed to 115 m 2 /g, which is 10 times of tht of the s-prepred g-c 3 N 4 (11.5 m 2 /g). It indictes tht the ggregted g-c 3 N 4 ws corroded y HCl cid to smller prticles in such process. The enlrged specific surfce re gretly enhnces the photoctlytic ctivity towrd the degrdtion of 4-nitrophenol under visile light irrdition. The kinetic rte constnt over g-c 3 N 4 -T is out 5.7 times of tht over the s-prepred g-c 3 N 4. This study provides fesile pproch to process g-c 3 N 4 with high photoctlytic ctivity, which is eneficil for its potentil pplictions in wstewter tretment nd environmentl remedition in the future. Acknowledgment The uthors thnk the Gungdong Nturl Science Foundtion (No. S ) for finncil support. Appendix A. Supplementry dt Supplementry dt ssocited with this rticle cn e found, in the online version, t References [1] Y.M. Grushko, Toxic Orgnic Compounds in Industril Wstewter: A Hndook, 2nd ed., Khimi, Leningrd, Russin, 1982, pp [2] Agency for Toxic Sustnces nd Disese Registry U.S. Pulic Helth Service, Toxicologicl Profile for Nitrophenols: 2-Nitrophenol 4-Nitrophenol, 1992, p. 3. [3] A. Fujishim, K. Hond, Electrochemicl photolysis of wter t semiconductor electrode, Nture 238 (1972) [4] Y. Wng, Solr photoctlytic degrdtion of eight commercil dyes in TiO 2 suspension, Wter Res. 34 (2000) [5] K. Kr, R. Chudhry, R.L. 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