<→DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> Etching graphitic carbon nitride by acid for enhanced photocatalytic activity toward degradation of 4-nitrophenol <→---------------------start--------------------->
  Chinese Chemical Letters  2014, Vol.25 Issue (09):1247-1251   PDF    
Etching graphitic carbon nitride by acid for enhanced photocatalytic activity toward degradation of 4-nitrophenol
Si-Zhan Wu, Cai-Hong Chen, Wei-De Zhang     
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
Abstract: Graphitic carbon nitride (g-C3N4) with high photocatalytic activity toward degradation of 4-nitrophenol under visible light irradiation was prepared by HCl etching followed by ammonia neutralization. The structure, morphology, surface area, and photocatalytic properties of the prepared samples were studied. After treatment, the size of the γ-C3N4 decreased from several micrometers to several hundred nanometers, and the specific area of the γ-C3N4 increased from 11.5 m2/g to 115 m2/g. Meanwhile, the photocatalytic activity of γ-C3N4 was significantly improved after treatment toward degradation of 4-nitrophenol under visible light irradiation. The degradation rate constant of the small particle γ-C3N4 is 5.7 times of that of bulk γ-C3N4, which makes it a promising visible light photocatalyst for future applications for water treatment and environmental remediation.
Key words: Photocatalyst     4-Nitrophenol     Etching     g-C3N4    
1. Introduction

In the past decades,the development of human society has caused tremendous environmental problems. Tons of organic pollutants were discarded into the lakes and rivers everyday which resulted in serious water contamination. The polluted water is harmful for human beings and other living things. Among those organic pollutants,the aromatic compound,4-nitrophenol,from the raw material to manufacture drugs,pesticides,and dyes,is one of the primary organic pollutants in water. This chemical may cause a blood disorder [1, 2]. It is difficult to remove the 4-nitrophenol by traditional biological and adsorption methods due to its high stability in water. Fortunately,Fujishima and Honda first reported the photocatalyst TiO2 for splitting water to generate hydrogen gas in 1972 [3]. This novel photocatalytic technology becomes a promising way to degrade organic pollutants,which has attracted intense research interest all over the world in the past decades due to its environmentally friendly and economical advantages [4, 5].

Although TiO2 shows high photocatalytic activity for the degradation of many pollutants,it can only respond to the UV light irradiation due to its wide bandgap (3.2 eV). Unfortunately, the solar light contains only about 4% UV light. In order to improve the utilization of the solar light,to explore efficient visible light driven photocatalysts is greatly demanded. Many visible light driven photocatalysts have been reported in recent years [6, 7, 8, 9, 10, 11, 12]. Among these photocatalysts,the graphitic carbon nitride (g-C3N4), a metal free visible light driven photocatalyst,has attracted intense interest due to its unique properties such as high stability,nontoxicity,easy modification and outstanding electrical property [12, 13]. Efforts have been devoted on using g-C3N4 as a photocatalyst for degradation of pollutants and production of hydrogen [12, 13, 14, 15, 16, 17, 18, 19, 20]. However,the photocatalytic activity of the asprepared g-C3N4 is low. In order to improve the photocatalytic activity of g-C3N4,attempts have been made on increasing its surface area including using a template to prepare porous g-C3N4, or treated g-C3N4 by alkali or acid [14, 15, 16, 17]. By using silica microballs as templates,the specific surface area of the g-C3N4 reached to 373 m2/g [14]. However,hydrofluoric acid must be used in this method to remove the templates. This process is not environment friendly and costly. Other approaches,like HCl or alkali treatment,can improve the specific surface area of the gC3N4to several tens square meters per gram [16, 17].

In this contribution,we are reporting a simple method of preparing high specific surface area g-C3N4without any template. After etching,the surface area of the g-C3N4 increased from 11.5 m2/g to 115 m2/g,and the size of the g-C3N4 particle also decreased significantly. It shows excellent photocatalytic activity toward degradation of 4-nitrophenol compared with the pristine g-C3N4under visible light irradiation. 2. Experimental

Melamine (C3H6N6) was purchased from Tianjin Kemiou Chemical Co.,Ltd. CH3·H2O (25%) and HCl (37%) were purchased from Guangzhou Chemical Co.,Ltd. All reagents used in this study are analytical grade and used without further purification. The g-C3N4was synthesized by heating melamine to 550°C for 2 h in a muffle. The as-prepared g-C3N4was ground to powder using an agate mortar. g-C3N4powder (0.95 g) was added into 40 mL HCl solution (1.0 mol/L) under magnetic stirring for 0.5 h. Then,the mixture was translated to a Teflon-lined autoclave (50 mL) and heated at 150°C for 5 h. After being cooled down to the room temperature naturally,the product was collected by filtration followedbyfurtherdispersedin40mLammoniasolution(2.0mol/L), and stirred for 0.5 h. After that,the product was washed with distilled water and ethanol for several times,and then dried at 80°C for 10 h. Finally,the product was heated at 400°Cfor1h in air to remove ammonia. The obtained product was named as g-C3N4-T.

Phase and structural of the samples were characterized by Xray diffractometer (XRD,D8 Focus X-ray diffractometer,Bruker, Germany) using Cu Ka(l= 0.154184 nm) as a radiation source. The Fourier transform infrared spectra (FTIR) of the products were recorded on IR Affiniy-1 FTIR spectrometer. The morphology of the samples was observed using a field emission scanning electron microscope (FESEM,JSM-6330F,JEOL,Japan). The specific surface areas were measured at 77 K using a 3H-2000PSI instrument and estimated by Brunauer-Emmett-Teller method. The UV-vis diffuse reflectance spectroscopy (UV-vis DRS) was conducted on a UV-2550 spectrophotometer using BaSO4 as a reference. The photocatalytic reaction samples were analyzed by high performance liquid chromatography (HPLC,DIONEX,TCC-100) with ultimate 3000 variable wavelength detector. A C18 column (Ecosil EC5-3237,250 mm×4.6 mm) was used to separate the degradation products.

The photocatalytic activity of the samples was evaluated by degradation of 4-nitrophenol under irradiation of visible light using a 500 W Xe lamp (l: 300-800 nm) as light source using sodium nitrite solution (5.0 g/L) as the UV light filter. For the photocatalytic degradation reaction,0.40 g of the prepared photocatalyst was dispersed in 400 mL 4-nitrophenol solution (8 mg L-1,pH 4). A small air pump (2.5 W) was used to blow air into the 4-nitrophenol solution. Before irradiation,the suspensions were stirred for 0.5 h in dark in order to reach an adsorption- desorption equilibrium between the 4-nitrophenol and photocatalyst. Then,the solution was exposed to Xe lump irradiation under magnetic stirring. At every 1 h interval,7 mL solution was extracted from the reactor. The concentration of 4-nitrophenol was determined by measuring the absorption at l= 317 nm. The solution samples were also analyzed by HPLC after filtered with a 0.22mm cellulose membrane filter. The mobile phase of methanol:water (60:40,v/v),flow rate of 1.0 mL/min and detection at the wavelength of 224 nm were used. 3. Results and discussion

The FTIR spectrum of g-C3N4-T is almost the same as that of the as-prepared g-C3N4,indicating that they are the same structure (Fig. S1 in Supporting information). The typical IR characteristic peaks of graphitic carbon nitride can be found in both samples, and no other impurity peak was found in both samples. The peak in the region 801 cm-1 can be attributed to the triazine units, which are the units for forming graphitic carbon nitride. Meanwhile,several other strong peaks range from 1200 cm-1 to 1650 cm-1 can be ascribed to the typical stretching mode of CN heterocycles in g-C3N4. A broad peak ranges from 2800 cm-1 to 3400 cm-1 can be assigned to the N-H stretching vibration mode [20].

FESEM was used to observe the morphology of g-C3N4-T and gC3N4. Fig. 1A shows the pristine g-C3N4 sample. The size of g-C3N4 is about several micrometers. After treatment,g-C3N4-T was corroded by HCl to several hundred nanometers,as shown in Fig. 1B. The specific surface area of the samples,an important factor affecting the activity of a photocatalyst,was also examined. Fig. 2A shows the nitrogen adsorption-desorption isotherms of g-C3N4 and g-C3N4-T,both of the isotherms are of type IV (BDDT Classification),suggesting the presence of mesopores in all samples [24]. The result reveals that the specific surface area of g-C3N4-T increases to 115 m2/g after treatment,which is 10 times of that of the pure g-C3N4 (11.5 m2/g). Fig. 2B is the pore size distribution curve,which reveals that the pore size of g-C3N4-T increased because of the acid etching. The increased surface area of g-C3N4-T can be attributed to the decreasing size of the particles,which is advantageous to improve the activity of the catalysts. Catalysts with larger specific surface area can enhance the adsorption of reactants,provide more active sites,and depress the recombination of the generated electron-hole pairs [14, 15, 25].

In addition to specific surface area,optical absorption property is another factor affecting the photocatalytic activity of the catalysts. Fig. 3A displays the optical absorption of g-C3N4 and g-C3N4-T. The maximum absorption wavelength of g-C3N4-T appears slightly blue-shift compared to that of g-C3N4,which can be ascribed to the smaller size of g-C3N4-T [15, 16, 17]. The bandgaps of the samples can be calculated by plotting [F(R)E] 0.5 against the energy of excitation source [21],which were shown in Fig. 3B. The bandgap of g-C3N4-T is 2.75 eV,which is larger than that of the pristine g-C3N4(2.68 eV). The larger bandgap of g-C3N4-T enhances the oxidation potential of the photogenerated holes and/or the reduction property of the photogenerated electrons, thus improves its photocatalytic activity.

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Fig. 1. FESEM images of (A) g-C3N4and (B) g-C3N4-T.

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Fig. 2. (A) The nitrogen adsorption/desorption isotherms of (a) g-C3N4and (b) g-C3N4-T. (B) The pore-size distribution of (a) g-C3N4and (b) g-C3N4-T.

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Fig. 3. (A) UV-vis DRS spectra,and (B) the transformed Kubelka-Munk functionvs. the light absorption of (a) g-C3N4and (b) g-C3N4-T.

From the XRD patterns of g-C3N4and g-C3N4-T,we can clearly find that two characteristic peaks,g-C3N4 (1 0 0) plane around 13.08 and (0 0 2) plane around 27.48,were all detected in both samples (Fig. S2 in Supporting information). No any other impurity phase was detected,which indicated that after treatment,the structure of g-C3N4was not significantly changed. The weaker peak around 13.08is attributed to the in-planar structural packing motif of g-C3N4,which reveals the distance of the nitride pores is 0.676 nm. The latter peak around 27.48corresponds to the (0 0 2) plane of g-C3N4[12]. The intensity of (1 0 0) plane around 13.08in sample g-C3N4-T is slightly lower than that of the g-C3N4sample.This can be ascribed to the HCl acid at high temperature may slightly destroy the structure of g-C3N4and cause the peak weaken [17]. The (0 0 2) plane peak of g-C3N4-T also shifts from 27.58to 27.88,corresponding to the decrease in the interplanar stacking distance from 0.324 nm to 0.321 nm. This phenomenon could be interpreted as the treatment process can improve the inter layer stacking order [22].

In addition,impurity appeared during this process,this may be ascribed to the g-C3N4 corroded by the HCl solution at high temperature and the impurity was produced. Interestingly,the impurity disappeared in sample g-C3N4-T after ammonia treatment. The ammonia not only can remove the impurity,but also can neutralize the excess HCl acid in the g-C3N4,and the followed heating treatment also can remove the residual H+ and the excess ammonia which adsorbed on the surface of g-C3N4. The most believable mechanism of the small g-C3N4-T formation is shown in Fig. S3 in Supporting information. The zeta potential of samples was measured. Firstly,the zeta potential of g-C3N4changed from -34.5 to +39.1 mV after hydrolysis in 1.0 mol/L HCl,corresponding to the change of g-C3N4 to protonated g-C3N4. After ammonia treatment,the zeta potential of sample decreased from +39.1 to +29.4 mV. This may be ascribed to that the excess HCl acid on the surface of sample was neutralized by the ammonia solution. Finally,the zeta potential of g-C3N4-T changed to-33.6 mV after annealing,which illustrates that the H+ in the g-C3N4-T was almost completely removed.

The visible light photocatatlytic activity of the samples was evaluated by decomposing the representative hazardous pollutant 4-nitrophenol under the irradiation of 500 W Xe lamp,using sodium nitrite solution (5 g/L) as the UV light filter liquor. The result is shown in Fig. 4A. After irradiation for 6 h,the degradation rate of 4-nitrophenol is 89.3%,32.1% and 5.6% over g-C3N4-T,gC3N4and without photocatalyst,respectively. The result clearly reveals that the 4-nitrophenol solution is very stable under visible light irradiation,and the photocatalytic activity of g-C3N4-T is higher than that of the as-prepared g-C3N4. In order to further investigate the reaction kinetics of degradation of 4-nitrophenol,a first-order kinetic model-ln(c/c0)=ktwas used,wherekis the kinetic rate constant [23]. As indicated in Fig. 4B,the degradation of 4-nitrophenol over both the pristine and etching g-C3N4 matches to the first-order kinetics equation (-dc/dt=kc). The kinetic rate constants over g-C3N4-T and pristine g-C3N4are 0.357 and 0.063 h-1,respectively. The kinetic rate constant obviously reveals that the photocatalytic activity of g-C3N4-T is much higher than that of pristine g-C3N4,the former is about 5.7 times of that of the latter. The higher photocatalytic activity of g-C3N4-T can be partly ascribed to the increasing specific surface area. However,the kinetic rate constant is not proportion to the increased surface area,since the surface area of g-C3N4-T is about 10 times of that of the pristine g-C3N4. This may be ascribed to when the size of gC3N4-T decreased,its specific surface area increased,and the bandgap also increased.

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Fig. 4. (A) Photocatalytic degradation of 4-nitrophenol over (a) g-C3N4,(b) g-C3N4-T and (c) photolysis. (B) The plots of the first-order degradation rate over g-C3N4(a),and gC3N4-T (b).

Fig. 5 shows the HPLC profile of 4-nitrophenol aqueous solution, g-C3N4-T dispersed in water,and 4-nitrophenol degradation sample against reaction time over g-C3N4-T photocatalyst. The absorption peak of 4-nitrophenol decreased gradually at 6.42 min retention time upon the increase of irradiation time,and several peaks appeared at around 2.3,2.7,3.2 and 5.8 min,which may be ascribed to the solvent and g-C3N4-T photocatalyst. After irradiation for 7 h under visible light,4-nitrophenol was almost completely decomposed. No any other peaks corresponding to small molecules decomposed from 4-nitrophenol were detected. The HPLC analysis reveals that 4-nitrophenol was degraded gradually to H2O and CO2 on g-C3N4-T photocatalyst under visible light.

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Fig. 5. HPLC profiles of 4-nitrophenol aqueous solution,g-C3N4-T dispersed in water,and 4-nitrophenol photocatalytic degradation sample against reaction time over g-C3N4-T photocatalyst.

The stability of the catalyst is another factor affecting the catalyst for practical application. The stability of the g-C3N4-T was also investigated in this study (Fig. S4 in Supporting information). The result shows that the photocatalytic activity of g-C3N4-T did not significantly change after three cycles,which reveals the very high stability of g-C3N4-T in the photocatalytic reaction. 4. Conclusion

In conclusion,we have developed a simple method to decrease the particle size of g-C3N4 and increase its surface area. After etching by HCl in a hydrothermal process and then neutralized by ammonia,the specific surface area of g-C3N4-T is significantly increased to 115 m2/g,which is 10 times of that of the as-prepared g-C3N4(11.5 m2/g). It indicates that the aggregated g-C3N4was corroded by HCl acid to smaller particles in such a process. The enlarged specific surface area greatly enhances the photocatalytic activity toward the degradation of 4-nitrophenol under visible light irradiation. The kinetic rate constant over g-C3N4-T is about 5.7 times of that over the as-prepared g-C3N4. This study provides a feasible approach to process g-C3N4 with high photocatalytic activity,which is beneficial for its potential applications in wastewater treatment and environmental remediation in the future. Acknowledgment

The authors thank the Guangdong Natural Science Foundation (No. S2012010008383) for financial support. Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2014.05.017.

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