Photodegradation Performance of g-C3N4 Fabricated by Directly Heating Melamine

https://www.360docs.net/doc/6e1430933.html,/Langmuir ?2009American Chemical Society

Photodegradation Performance of g-C 3N 4Fabricated by Directly Heating

Melamine

S.C.Yan,?,§Z.S.Li,*,?,?,§and Z.G.Zou ?,?,§

?

Eco-Materials and Renewable Energy Research Center (ERERC ),Department of Physics and ?Department of Materials Science and Engineering and §National Laboratory of Solid State Microstructures,Nanjing

University,Nanjing 210093,P.R.China

Received March 17,2009.Revised Manuscript Received July 3,2009

The g-C 3N 4photocatalyst was synthesized by directly heating the low-cost melamine.The methyl orange dye (MO)was selected as a photodegrading goal to evaluate the photocatalytic activity of as-prepared g-C 3N 4.The comparison experiments indicate that the photocatalytic activity of g-C 3N 4can be largely improved by the Ag loading.The strong acid radical ion (SO 42-or NO 3-)can promote the degrading rate of MO for g-C 3N 4photocatalysis system.The MO degradation over the g-C 3N 4is mainly attributed to the photoreduction process induced by the photogenerated electrons.Our results clearly indicate that the metal-free g-C 3N 4has good performance in photodegradation of organic pollutant.

Introduction

In recent years,there has been increasing interest in the study of photocatalysis technology application in water/air purification,1hydrogen production from splitting water,2self-cleaning coat-ings,3and high-efficiency solar cells.4This is related to serious environmental pollution and the global energy shortage.So far,various oxide,sulfide,and oxynitride semiconductor photocata-lysts have been developed to drive the photocatalytic reaction.5However,one of the main goals in materials science fields is to find photocatalytic materials with high quantum efficiency.For water purification,an optical material is needed that has a band gap that absorbs visible light,strong oxidative ability,and high stability in a complex water solution system.Recently,Wang et al.6reported that a metal-free photocatalyst,graphite-like carbon nitride (g-C 3N 4),has the photocatalytic performance for hydro-gen or oxygen production from water splitting under visible light irradiation.Very recently,they developed the g-C 3N 4metal-including compounds to degrade organic dyes.7The functional organic -metal hybrid material exhibited modified electronic properties.However,the authors did not report the comparison results for degrading organic dyes over the bare and metal-ion-modified g-C 3N 4.The g-C 3N 4semiconductor is recognized to be the most stable allotrope at ambient conditions.Unlike the photocatalysts of sulfide and oxynitride semiconductor,the g-C 3N 4photocatalyst is stable under light irradiation in water solution as well as in acid (HCl,pH=0)or base (NaOH,pH=14)solutions due to the strong covalent bonds between carbon and

nitride atoms.The high stability and moderate band gap imply that the metal-free g-C 3N 4has numerous potential applications in the photocatalysis fields.

Several precursors,such as cyanamide,dicyandiamide,and melamine,have been used to obtain the g-C 3N 4solids.8The former two kinds of precursors are virulent and expensive in comparison to the latter one.So far,several research groups have reported that the g-C 3N 4can be fabricated by heat treatment of melamine in the low-vacuum system 9,10or under high pressure.11Depending on reaction conditions,g-C 3N 4with different degrees of condensation and properties was obtained.However,from an industrial applications viewpoint,the g-C 3N 4material obtained under ambient pressure was expected due to the low cost and facile operation.

In the present paper,the g-C 3N 4photocatalyst powder was obtained by directly heating the melamine in a semiclosed system with two-step heat treatment.The melamine was first heated at 500°C (heating rate:20°C /min)for 2h,and the further deammonation treatment was set at 520,550,and 580°C for 2h,respectively.For comparison,a reference sample was prepared by polymerization of cyanamide at 550°C for 4h.6The photode-gradation behavior of methyl orange (MO)over the as-prepared photocatalyst was studied.The sample of treated melamine at 520°C for 2h exhibits high photodegradation activity.Our studies probably imply that the g-C 3N 4has the potential to treat industrial wastewater due to its good photodegradation perfor-mance and environmental adaptability.On the basis of our comparison experiments,a possible MO photodegradation me-chanism for the g-C 3N 4photocatalysis system was proposed.

Experimental Section

The photocatalyst of g-C 3N 4was prepared by directly heating melamine in the semiclosed system to prevent sublimation of

*Corresponding author.E-mail:zsli@https://www.360docs.net/doc/6e1430933.html, (Z.S.Li).Postal address:NO.22,Hankou Road,Nanjing,Jiangsu 210093,P.R.China.Phone number:86-25-83686630.Fax number:86-25-83686632.

(1)Fujishima,A.;Rao,T.N.;Tryk,D.A.J.Photochem.Photobiol.C 20001,1–21.

(2)Yamasita,D.;Takata,T.;Hara,M.;Kondo,J.N.;Domen,K.Solid State Ionics 2004,172,591–595.

(3)Zhang,X.T.;Sato,O.;Taguchi,M.;Einaga,Y.;Murakami,T.;Fujishima,A.

Chem.Mater.2005,17,696–700.(4)O 0Regan,B.;Gr €a tzel,M.Nature 1991,353,737–740.(5)Osterloh,F.E.Chem.Mater.2008,20(1),35–54.

(6)Wang,X.C.;Maeda,K.;Thomas,A.;Takanabe,K.;Xin,G.;Carlsson J.M.;Domen,K.;Antonietti,M.Nat.Mater.2009,8,76–80.

(7)Wang,X.C.;Chen,X.F.;Thomas,A.;Fu,X.Z.;Antonietti,M.Adv.Mater.2009,21,1–4.

(8)Thomas,A.;Fischer,A.;Goettmann,F.;Antonietti,M.;M €u

ller,J.;Schl €o gl,R.;Carlsson,J.M.J.Mater.Chem.2008,18,4893–4908.(9)Komatsu,T.J.Mater.Chem.2001,11,799–801.

(10)Zhao,Y.C.;Yu,D.L.;Yanagisawa,O.;Matsugi,K.;Tian,Y.J.Diamond Relat.Mater.2005,14,1700–1704.

(11)Ma,H.A.;Jia,X.P.;Chen,L.X.;Zhu,P.W.;Guo,W.L.;Guo,X.B.;Wang,Y.D.;Li,S.Q.;Zou,G.T.;Zhang,G.;Bex,P.J.Phys.:Condens.Matter 2002,14,11269–11273.

Article Yan et al.

melamine.10g of melamine powder was put into an alumina crucible with a cover,then heated to 500°C in a muffle furnace for 2h at a heating rate of 20°C/min;the further deammonation treatment was performed at 500,520,550,and 580°C for 2h,respectively.The samples were characterized by X-ray diffraction (XRD)for phase identification on the Rigaku RINT2000dif-fractometer.The specific surface area was determined with the Brunauer -Emmett -Teller (BET)equation at 77K by using an adsorption apparatus (Micromeritics TriStar,USA).UV -vis diffuse reflection spectra were measured using a UV -vis spectro-photometer (Varian CARY 100,USA)and converted from reflection to absorbance by the Kubelka -Munk method.Photo-catalytic activity of g-C 3N 4for methyl orange (MO)photode-gradation was evaluated in a Pyrex reactor.0.3g of g-C 3N 4was dispersed in MO aqueous solution (100mL,0.4mg L -1).The light irradiation system contains a 300W Xe lamp with cutoff filter L42for visible light and a water filter to eliminate the temperature effect.Measurement of apparent quantum efficiency for degrad-ing MO over g-C 3N 4was performed by using a monochromatic filter (420nm).The intensity of the corresponding incident light is 9.764μW/cm 2.

Results and Discussion

In order to understand the phase transformation during heat-ing of melamine,the thermal analysis was carried out by using the thermogravimetric -differential scanning calorimetry analysis (TG-DSC).The detected range of temperature is from room temperature to 1000°C at a heating rate of 10°C /min.An alumina crucible with a cover was used during thermal analysis to prevent sublimation of melamine.The DSC and TG thermo-grams for melamine (see Figure 1a)clearly show that in the semiclosed system several phase transformations are observed.The strongest endothermal peak appears in the temperature range 297-390°C,and the weight of the sample decreased rapidly.It indicates that the sublimation and thermal condensation of melamine occurred in this temperature range at the same time.The inset of Figure 1a shows that two weak endothermal peaks appeared at 545and 630°C,which correspond to the further deammonation process and decomposition of material,respec-tively.Two exothermic reaction peaks are attributed to the disappearance of this material via generation of nitrogen and cyano fragments,exhibiting at 660and 750°C,respectively.The thermal stability and phase transformation of the g-C 3N 4pre-pared at 520°C were detected in an open system,as shown in Figure 1b.It can be seen that the as-prepared g-C 3N 4becomes unstable when the heat temperature is above 600°C,and heating to 750°C results in no residue of the material being observable.The DSC curve shows that decomposition of the as-prepared g-C 3N 4occurred first at 600°C,with the decomposition products

burnt immediately.No deammonation process was observed during heating of g-C 3N https://www.360docs.net/doc/6e1430933.html,pared with the DSC curve of g-C 3N 4,we can confirm that the endothermal peak at 630°C during heating of melamine should be attributed to the decom-position of g-C 3N 4formed in the pyrolysis of melamine,which is 30°C higher than the starting decomposition temperature of as-prepared g-C 3N 4(600°C).This indicates that g-C 3N 4is more stable in the semiclosed ammonia atmosphere than in the open system.On the other hand,two combustion processes were found during heating melamine;however,just one combustion process was observed for heating the g-C 3N 4.The aforementioned evidence can be attributed to the fact that the deammonation process of melamine resulted in an ammonia atmosphere in the semiclosed system,which can improve the stability of g-C 3N 4and restrain combustion of products.

The XRD patterns for all the samples are shown in Figure 2.We can see that two peaks are found in all the samples.It is widely accepted to date that the g-C 3N 4is based on tri-s -triazine building blocks.8The strongest peak at 27.41°is a characteristic interlayer stacking peak of aromatic systems,indexed for graphitic materials as the (002)peak.The calculated interplanar distance of aromatic units (d =0.325nm)is significantly smaller than that of the crystalline g-C 3N 4(d =0.34nm).12The dense structure can be attributed to the localization of the electrons and stronger

binding

Figure 1.TG-DSC thermograms for heating the melamine (a)and the g-C 3N 4(b)obtained by heating melamine at 520°

C.Figure 2.XRD patterns for g-C 3N 4samples obtained by heating

different precursors at various temperatures.

(12)Bojdys,M.J.;M €u

ller,J.O.;Antonietti,M.;Thomas,A.Chem.;Eur.J.2008,14,8177–

8182.

Yan et al.Article

between the layers.The small angle peak at 13.08°,corresponding to interplanar distance of 0.676nm,is indexed as (100),which is associated with interlayer stacking.The distance is slightly below the size of one tris-s -triazine unit (ca.0.73nm),which presumably is attributed to the presence of small tilt angularity in the structure.

The Fourier transform infrared (FT-IR)spectra of two kinds of samples are shown in Figure 3.Several strong bands in the 1200-1650cm -1region were found,which correspond to the typical stretching modes of CN heterocycles.13Additionally,the characteristic breathing mode of the triazine units at 801cm -1was observed.It should be noted that the broad bands at around 3000cm -1are indicative of NH stretching vibration modes.12This indicates that the amino functions still existed in the products by directly heating the melamine or cyanamide.The C/N ratios for all samples were determined by elemental analysis on the elemental analyzer (vario EL II,Elementar,Germany).The experimental error in weighing was (0.001mg.For heating melamine at 500,520,550,and 580°C,the C/N ratio of the product is 0.721,0.735,0.737,and 0.742,respectively.It indicates that the C/N ratios increase with increasing condensation tem-perature.However,the C/N ratios for all cases of heating melamine are lower than 0.75for the ideal crystal g-C 3N 4.The results are in agreement with the FT-IR,implying that the amino groups originated from the incomplete condensation of the as-prepared g-C 3N 4.Indeed,as reported,the residual hydrogen atoms bind to the edges of the graphene-like C -N sheet in the form of C-NH 2and 2C-NH bonds.14On the other hand,as we know from DSC observation,the as-prepared g-C 3N 4is unstable above 600°C.The existence of amino groups and the low thermal stability of g-C 3N 4indicate that it is very difficult to lower the hydrogen content by directly heating melamine at ambient pressure.

The optical properties of the samples by heating melamine were investigated by UV -vis diffuse reflectance spectroscopy,and the results are shown in Figure 4.The spectrum of the sample of cyanamide treated at 550°C for 4h is also shown for comparison.We can see that absorption edges of the samples obtained from heating melamine shift remarkably to longer wavelengths with increasing heating temperature.The decrease in band gaps of the samples is from 2.8to 2.75eV when the heat treatment tempera-ture is increased from 500to 580°C.It is worth noting that the

main absorption of the sample heated at 520°C,which exhibits a band gap of ca.2.75eV,is close to that of the samples heated at 550and 580°C.The spectra of the samples heated at higher temperature,520°C,show a weak absorption tail,especially for the sample of cyanamide heated at 550°C,for which the absorption tail may reach 550nm.This probably is attributed to the structure defects formed in samples treated at the high temperatures,which improve the optical absorption of materials.The photocatalytic activities of all the samples obtained by pyrolysis of melamine were evaluated for methyl orange (MO)photodegradation under visible light (λ>420nm)irradiation,as shown in Figure 5a.The photodegradation of MO over the commercial nitrogen-doped-TiO 2(TPS201,Sumitomo Corp.Japan)(denoted as N-TiO 2)and the sample obtained by heating cyanamide was also given here for comparison.It is well-known that,as a typical organic contaminant,MO is stable under UV -vis irradiation if there is no photocatalyst involved.15,16The absorption spectrum of the homogeneous MO solution without catalyst submitted to illumination with UV -vis light irradiation from a 300W xenon lamp for 5h exhibits almost no difference from the original absorption.We can know that the MO is stable in our experimental conditions.The N-TiO 2possesses a specific surface area (64.8m 2/g)more than eight times higher than that (an average value,ca.8m 2/g)of g-C 3N 4samples;however,it is obvious that photocatalytic activities for all the g-C 3N 4samples are much higher than those of the N-TiO 2.The optical absorption spectrum of N-TiO 2was shown in Figure 4.We can know that the main absorption edge of N-TiO 2is at 390nm,corresponding to the essential light absorption of TiO 2.17The weak absorption tail in the wavelength range 390-480nm originated from the substitutional doping of N,because its p states contribute to narrowing of the band gap by mixing with O 2p states.18The weak light absorption of N-TiO 2means poor visible light response.As a result,N-TiO 2has a low activity for photodegrading MO dye.The g-C 3N 4sample obtained from heating melamine at 520°C (denoted as M520)shows the highest activity,which is the same as that of the sample obtained

from

Figure 3.FT-IR spectra for g-C 3N 4fabricated from different

precursors.

Figure 4.UV -vis absorption spectra for commercial N-doped-TiO 2and g-C 3N 4samples obtained by heating the different pre-cursors at different temperatures.

(13)Li,X.F.;Zhang,J.;Shen,L.H.;Ma,Y.M.;Lei,W.W.;Cui,Q.L.;Zou G.T.Appl.Phys.

A:Mater.Sci.Proc.2009,94,387–392.

(14)Zhao,Y.C.;Liu,Z.;Chu,W.G.;Song,L.;Zhang,Z.X.;Yu,D.L.;Tian,Y.J.;Xie,S.S.;Sun,L.F.Adv.Mater.2008,20,1777–1781.

(15)Wang,Y.Y.;Zhou,G.W.;Li,T.D.;Qiao,W.T.;Li,https://www.360docs.net/doc/6e1430933.html,mun.2009,10,412–415.

(16)Gao,F.;Chen,X.Y.;Yin,K.B.;Dong,S.;Ren,Z.F.;Yuan,F.;Yu,T.;Zou,Z.G.;Liu,J.M.Adv.Mater.2007,19,2889–2892.

(17)Rajeshwar,K.;de Tacconi,N.R.;Chenthamarakshan,C.R.Chem.Mater.2001,13(9),2765–2782.

(18)Asahi,R.;Morikawa,T.;Ohwaki,T.;Aoki,K.;Taga,Y.Science 2001,293,269–271.

Article Yan et al.

heating cyanamide at 550°C (denoted as C550);the dye of MO is completely degraded after 5h visible light irradiation.However,the pathway of MO photodegradation is different for the M520and C550samples (see Figure S1in the Supporting Information).No light absorption peak shift was observed during degradation of MO over M520sample,indicating that the M520sample can directly mineralize MO without intermediate products.For the C550sample,the main absorption peak of MO gradually shifted to the shorter wavelengths with increasing light irradiation times,implying that the intermediate products were formed during the photodegradation reaction process.In other words,this means that the photooxidation ability of M520is better than that of C550sample.In the UV -visible absorption spectrum of C550,the obvious absorption tail,which resulted from poor thermal stability of g-C 3N 4,suggests that the high-temperature heat treatment at 550°C leads to the decreased photooxidation ability for C550.For the samples of melamine heated at 500,550,and 580°C,only 89%,78%,and 69%MO,respectively,was photo-degraded in the same irradiation time.Indeed,compared with the M520sample,the sample treated at 500°C has a low C/N ratio of 0.721;this means that the slightly decreased photocatalytic activity is attributed to the uncompleted condensation.However,there is a big drop in photocatalytic activity for the samples treated at temperatures above 520°C.Apparently,increasing the heat temperature will decrease the photooxidation ability of g-C 3N 4.The C/N ratio,i.e.,the degree of condensation,is

inconsistent with the structural integrality.To obtain a sample with high C/N ratio,a high heat treatment temperature is needed to lower the content of hydrogen in products,which leads to decreased structural integrality for the final product.Indeed,in combination with the elemental analysis and thermal analysis results discussed above,it is obvious that in the case of g-C 3N 4preparation by pyrolysis of melamine the degrees of condensation for the final product increase with increasing heat temperature,but the thermal stability of g-C 3N 4decreases.This probably implies that,in our case of g-C 3N 4preparation,520°C is an appropriate temperature for obtaining the g-C 3N 4with good crystal structure and moderate degree of condensation,and therefore,the M520sample exhibits high activity in degrading MO.Furthermore,it is worth pointing out that a one-time increase in the rate of MO photodegradation was achieved over the M520sample under full arc light irradiation;the dye of MO can be degraded completely after 2.5h UV -vis light irradiation.Some active species,such as the hydroxyl radicals (3OH),the superoxide (O 23or HOO 3),and the holes,are formed during the photodegradation reaction induced by light irradiation.The 3OH in aqueous solutions,as the primary oxidant,is generated via the direct hole oxidation 19or photogenerated electron induced multi-step reduction of O 2(O 2te f O 23,O 23te t2H tf H 2O 2,H 2O 2te f 3OH tOH -).20Generally,it is confirmed that the generation of superoxide is associated with the photogenerated electron induced direct reduction of O 2(O 2te f O 23).In addition,the photogenerated hole can directly react with organic compounds if the semiconductor photocatalyst has moderate redox potential.For these active species,hydroxyl radical reac-tions are nonselective and will virtually react with almost all the organic compounds by either H-atom abstraction,direct electron transfer,or insertion.

In order to investigate the possible photodegradation mechan-ism of MO over g-C 3N 4semiconductor,several comparison experiments were performed,and the results were shown in Figure https://www.360docs.net/doc/6e1430933.html,pared with photodegradation of the pure MO /g-C 3N 4solution,we can know that no change in the rate of MO photodegradation is observed when methanol (10vol %)was added as a sacrificial hole acceptor.This implies that the photo-generated hole is not an effective active species during degrading MO over g-C 3N 4;this reaction is not attributed to the direct hole oxidation.Indeed,as reported by Wang et al.,6the oxidation level for water splitting is located slightly above the top of the valence band of g-C 3N 4,which would permit transfer of holes,but with a low driving force.This suggests that the low driving force is not beneficial for the hole reactions in aqueous solution system.However,there is a big increase in rate of MO photodegradation when Ag particles (5wt %)were loaded onto the g-C 3N 4surface using the photodeposition method;21the MO can be degraded completely after 1h visible light irradiation,which is 5times faster than that over the bare g-C 3N 4.A further investigation,the effect of O 2on photodegradation of MO,was preformed in argon atmosphere.The reaction setup was vacuum-treated several times in order to eliminate O 2,and then,high-purity argon gas (purity,99.999%)was followed into the reaction setup for obtaining ambient pressure.After 5h visible light irradiation,only 43%MO was photodegraded in the absence of O 2(see Figure S2in Supporting Information).It means that O 2is a main factor for MO photodegradation over the g-C 3N 4photocatalyst,which affects the formation of superoxide via direct reduction of O 2

Figure 5.Photodegradation of methyl orange:(a)over the com-mercial N-doped-TiO 2and the g-C 3N 4obtained by heating the different precursors at different temperatures;(b)in the different catalytic systems based on the g-C 3N 4prepared by heating mela-mine at 520°C.

(19)Yoon,S.H.;

Lee,J.Environ.Sci.Technol.2005,39,9695–9701.

(20)Liu,G.G.;Li,X.Z.;Zhao,J.C.;Horikoshi,S.;Hidaka,H.J.Mol.Catal.A:Chem.2000,153,221–229.

(21)Tada,H.;Ishida,T.;Takao,A.;Ito,https://www.360docs.net/doc/6e1430933.html,ngmuir 2004,20,7898–7900.

Yan et al.Article

and hydroxyl radicals via multistep reduction of O2.A valuable

fact is that,if the strong acid radical ion,such as SO42-(0.01M)

or NO3-(0.01M),was introduced into the aqueous MO/g-C3N4

solutions,the MO degradation is significantly accelerated,and

the reaction was completed after80min visible light irradiation.

Clearly,the improved reaction activity is due to the introduction

of a strong acid radical ion,which increases the Htconcentration

and therefore accelerates the reaction of O23tet2Htf H2O2, further accelerating the formation of hydroxyl radicals via multi-

step reduction of O2.The experimental fact suggests that g-C3N4

photocatalyst possesses good environmental adaptability,which

can directly apply to industrial wastewater treatment to degrade

organic pollutants.On the basis of the aforementioned evidence,

we are inclined to believe that for the MO photodegradation over

g-C3N4the photocatalytic activity has mainly resulted from active

species which originated from photogenerated electron induced

reduction of O2.For practical application of the photocatalyst,

the stability during photoreaction was a crucial factor.Our

stability test indicates that no decrease in photocatalytic activity

was observed in the MO photodegradation reaction that was

repeated three times(see Figure S5in Supporting Information).

The XRD pattern of the as-prepared g-C3N4is similar to that

of the sample after reaction(see Figure S6,in Supporting

Information),meaning that the g-C3N4is stable in the photo-

reaction.The apparent quantum efficiency is a criterion to

evaluate photocatalytic activity of a given material.For MO

degradation,taking into account a single electron process,22the

apparent quantum efficiency(denoted as E Q)for degrading MO

was defined by E Q=N nup/N nip,where N nup and N nip represent

the number of used photons and the number of incident photons,

respectively.It is assumed that all incident photons are absorbed

by the photocatalyst.The calculated value of E Q at420nm is 1.5%,implying that the low-cost g-C3N4has potential in water purification due to its good photodegradation performance and environmental applicability.

Summary

In summary,we have successfully fabricated the g-C3N4 photocatalyst by directly heating melamine.The stable MO dye was selected as a degrading goal to evaluate the photocatalytic activity of g-C3N4.Our results clearly indicate that the metal-free g-C3N4has good performance in the photooxidation of organic pollutant.For the typical MO dye photodegradation,the photo-catalytic avtivity of g-C3N4can be improved significantly when Ag is used as a cocatalyst.This means that g-C3N4is a promising material possessing a good potential in photocatalytic application fields if some techniques,such as loading cocatalyst and doping, are used to improve its photocatalytic activity.Our comparison studies showed that the photodegradation activity of MO over g-C3N4is mainly attributed to the generation of active species induced by photogenerated electrons.Moreover,the g-C3N4 photocatalyst showed good photocatalytic activity in the presence of a strong acid radical ion;this further indicates that it is feasible to apply the g-C3N4with low cost and facile synthesis to treat industrial wastewater containing organic pollutants. Acknowledgment.This work is supported by the National Natural Science Foundation of China(no.20528302),the Na-tional Basic Research Program of China(973program, 2007CB613301and2007CB613305).

Supporting Information Available:UV-visible spectro-scopic changes for MO degradation,XRD and SEM for Ag-modified g-C3N4,and the stability test for g-C3N4.This material is available free of charge via the Internet at http:// https://www.360docs.net/doc/6e1430933.html,.

(22)Bandara,J.;Morrison,C.;Kiwi,j.;Pulgarin,C.;Peringer,P.J.Photochem. Photobiol.A1996,99,57–66.