g-C3N4共轭纳米结构-光氧化还原

Facile in situ synthesis of 2D porous g-C 3N 4and g-C 3N 4/P25(N)heterojunction with enhanced quantum effect for efficient photocatalyticapplicationMingye Ding a ,Wei Wang b ,⇑,Yingjie Zhou b ,Chunhua Lu c ,Yaru Ni c ,Zhongzi Xu caCollege of Materials &Environmental Engineering,Hangzhou Dianzi University,Hangzhou 310018,PR ChinabSchool of Physics and Optoelectronic Engineering,Nanjing University of Information Science &Technology,Nanjing 210044,PR China cCollege of Materials Science and Engineering,Nanjing Tech University,Nanjing 210009,PR Chinaa r t i c l e i n f o Article history:Received 9January 2015Received in revised form 6February 2015Accepted 14February 2015Available online 21February 2015Keywords:Carbon nitride Photocatalysis Quantum effect Heterojunction TiO 2a b s t r a c tThe major challenge of employing photocatalysis for environment protection is to develop high efficient,low cost,and stable semiconductor photocatalysts.In the present study,an in situ annealing strategy is employed for large scale synthesis of two-dimensional (2D)porous graphitic carbon nitride (g-C 3N 4)and efficient g-C 3N 4/P25(N)(N doped P25)heterojunction with enhanced quantum effect.The P25not only serves as the template for g-C 3N 4polymerization,but is also modified by the N species to enhance the visible light pared to the normal bulk g-C 3N 4,the 2D porous g-C 3N 4with enhanced quan-tum effect is found to be more efficient in improving the specific surface area and the electron–hole pair’s separation,even its light absorption edge is blue-shifted.Photocatalytic degradation of Rhodamine B (RhB)and phenol indicates the 2D g-C 3N 4and g-C 3N 4/P25(N)are very efficient and stable under the xenon lamp irradiation.It is also found that the original mass ratio of urea,which is the precursor for g-C 3N 4synthesis and P25modification,to P25also plays a significant effect on the photocatalytic activity.The optimized photocatalyst (mass ratio of P25to urea is 1:8)can decompose total RhB aqueous solution (10mg/L,100ml)in 25min.Based on systematic characterizations and discussions,a possible photocat-alytic mechanism for the excellent photocatalytic performance is proposed.Ó2015Elsevier B.V.All rights reserved.1.IntroductionInorganic semiconductor photocatalysts have been studied widely in the field of environment protection,water splitting,and new fuels synthesis [1–6].Nano-sized photocatalysts often possess unique physical and chemical properties compared to their bulk counterparts.These properties,which are often determined by size,shape,crystallinity,and constitution,often play significant influence on their photocatalytic activity [7].Great research effects have been devoted to preparing nano-sized photocatalysts with defined compositions,morphologies,shapes and fine inner struc-tures.Among these factors,morphologies and shapes have great influence on separating the electrons and holes.For instance,the zero-dimensional (0D)quantum dots (e.g.carbon/CdS/CdSe quan-tum dots)are efficient in enhancing the light absorption and elec-tron–hole pair separation [8,9].One-dimensional (1D)photocatalyst,such as TiO 2and ZnO rods,can promote the fast and efficient electron transportation along the axial direction,hence high performance for solar energy application can be obtained [10,11].Semiconductors with 2D structures are often favorable for heterophotocatalysis because of the shortened per-pendicular distance for photo-generated carriers to migrate from the bulk to the surface.More electrons and holes will participate into the photocatalytic reaction because of the reduced bulk recombination [12–15].In the last several years,g-C 3N 4has been one of the most stud-ied semiconductor photocatalysts because of it is visible light absorption,environment stability,and graphene-like 2D structure,etc.[16–19].A lot of precursors and methods have been applied and various kinds of nano-structured g-C 3N 4have emerged.How-ever,the numerous layers and the relative fast electron–hole pair recombination restrict its photocatalytic activity.Various strate-gies have been applied to overcome this shortcoming [20].For instance,employing thermal treatment and sonication to exfoliate the bulk g-C 3N 4to fewer layers is a very promising idea [21–23].Synthesizing well-structured g-C 3N 4nanospheres by employing porous SiO 2as the template is also reported recently [7].With/10.1016/j.jallcom.2015.02.1110925-8388/Ó2015Elsevier B.V.All rights reserved.⇑Corresponding author.Tel.:+862583587252;fax:+862583587220.E-mail address:wwnjut@ (W.Wang).the help of these methods,the as-prepared few-layered g-C3N4tru-ly exhibits higher photocatalytic performances than the bulk g-C3N4.However,the productivity of these technologies is very low.More efficient methods are needed on this topic.Noble metal deposition,anion atoms(e.g.,O and S)doping,graphene/carbon nanotubes modification are all widely investigated methods to fur-ther improve the photocatalytic activities,but the resulted efficien-cy has not been largely improved[20,24–29].Coupling with semiconductor photocatalyst with a larger band gap to form effi-cient heterojunction is also a promising method,since it can always improve electron–hole separation efficiency and light absorption[30–32].Unfortunately,most of the heterojunction sys-tems are often prepared by physical mixing the two photocatalysts. This strategy often suffers from the drawbacks of uneven mixing and loose contact,thus the performance of the heterojunction can-not be utilized sufficiently.Moreover,practical application of photocatalysts to solve envi-ronment problems requires low cost,large scale production,and environment stable properties.Most of the photocatalysts,such as AgPO4,ZnO,CdS,often suffer serious photocorrosion during the reaction process[33–35].Noble metals and heavy metals are also harmful for the soil and water.To data,TiO2is considered as the most promising candidate for photocatalytic application. Among the industrialized products,Degussa P25is the one with the highest photocatalytic activity because of its unique structure in efficiently separating the electrons and holes.However,the large band gap and the relative high cost restrain its large scale applica-tion.Various methods have been employed to extend the TiO2light absorption edge to the visible region.Among these methods,N doping is the earliest reported and efficient method.However,a green and high efficient method in preparing N doped TiO2is still needed.Furthermore,only this technology cannot make up the above-mentioned shortcomings.In the present study,an in situ and facile annealing strategy is proposed to synthesize2D porous g-C3N4and g-C3N4/P25(N) heterojunction photocatalysts with enhanced quantum effect.Urea is chosen as the precursor both for g-C3N4synthesis and P25 modification.This strategy is a good idea in making sufficient usage of the excellent properties of both g-C3N4and P25.System-atic analysis has evidenced that the2D porous g-C3N4with enhanced quantum effect is much superior to the bulk g-C3N4. Moreover,the heterojunction formed by the2D porous g-C3N4 and P25(N)shows high ability in absorbing the visible light and separating the electrons and holes,resulting in a high and stable photocatalytic activity for phenol and RhB degradation under the xenon lamp irradiation.This study may give a new sight on large scale preparation of efficient g-C3N4-based photocatalysts by inte-grating various processes and factors within one facile but efficient method.2.Experimental2.1.Preparation of bulk g-C3N4Bulk g-C3N4was synthesized with a facial annealing method by employing urea as the precursor.15g of urea was added into a crucible with a cover at room tem-perature,then the crucible was placed into an Muffle furnace and heated at520°C for4h with a temperature ramping rate of25°C/min.The yellow colored sample, named g-C3N4(B),was washed with deionized water to remove the adsorbed impu-rities.At last,g-C3N4(B)was dried and grinded.2.2.Preparation of2D porous g-C3N4and g-C3N4/P25(N)heterojunction photocatalystsStoichiometer amounts of urea(25g)and P25(4.2g)(mass ratio of P25to urea is1:6)were grinded sufficiently in a pot mill with the addition of5ml ethanol.The resulted homogeneous composite contains P25and urea was heated at80°C to eva-porate the ethanol,then the composite was grinded and heated in the Muffle fur-nace similar to that of preparing g-C3N4(B).Thefinal product constituted by P25(N)and2D porous g-C3N4was named g-C3N4(S)–P25(N6)(Scheme1).At last,g-C3N4(S)–P25(N6)was washed with hydrofluoric acid and deionized water to remove the P25(N).The obtained pure2D porous g-C3N4with enhanced quantum effect was named g-C3N4(S).In situ composites with different mass ratios of P25to urea(1:4,1:8,1:10)were also used as the precursors to investigate its effect on the formed heterojunction and the photocatalytic activity.The resulted samples were named g-C3N4(S)–P25(N4),g-C3N4(S)–P25(N8),and g-C3N4(S)–P25(N10).Heterojunction photocatalysts constituted by P25and g-C3N4(B)/g-C3N4(S) were also prepared.In a typical procedure,0.1g of g-C3N4(B)or g-C3N4(S)was mixed with0.3g P25in20ml deionized water under sonication.After centrifuga-tion,the products,named g-C3N4(B)–P25and g-C3N4(S)–P25,were dried and grinded.2.3.CharacterizationPowder X-ray Diffraction(XRD)analysis were taken using an ARL X’TRA diffrac-tometer,a voltage of40kV,at20mA,using a Cu source with K a=0.15406nm.UV–vis diffuse reflectance spectra were measured using a Shimadzu3101spectropho-tometer,using a standard barium sulfate powder as a reference.The reflection mea-surements were then converted to absorption spectra using the Kubelka-Mulk transformation.Fourier Transform Infrared Spectroscopy(FTIR)spectra were col-lected on a Vector-22spectrometer in the wavenumber range of4000–400cmÀ1. Transmission Electron Microscopy(TEM)and Scanning Electron Microscopy (SEM)measurements were conducted using a JEOL JEM-2010and JEOL S-4800, respectively.X-ray Photoelectron Spectra(XPS)were collected on a PHI5000Ver-saprobe system with monochromatic Al K a X-rays.Nitrogen adsorption–desorption isotherms were collected at77K using mMK-tristar3000Surface Area and Porosity Analyzer.Photoluminescence(PL)emission spectra were collected on a FL3-221fluorescence spectrophotometer(excitation wavelength:300nm).2.4.Photocatalytic activity and hydroxide radical(ÅOH)testsPhotocatalytic reactions were carried out in a top-irradiation reaction vessel embraced with the circulating cooling water.In a typical test procedure,50mg of each photocatalyst was added into100ml of the RhB or phenol aqueous solution (10mg/L).Homogeneous suspension was obtained by sonication,and then kept in the dark for1h to establish the adsorption–desorption equilibrium with magnet-ic stirring.At last,the suspension was irradiated by a300W xenon lamp to test the photocatalytic activities.The absorption spectrum of RhB or phenol was analyzed on the UV–vis spectrophotometer every5min after remove the photocatalyst to estimate the degradation efficiency.GeneratedÅOH was analyzed according to our reported procedures,except the RhB or phenol was replaced by terephthalic acid and NaOH[36].3.Results and discussion3.1.Characterization of the photocatalystsFig.1a shows the typical XRD spectra of P25and g-C3N4(S)–P25(N6).All the diffraction peaks are ascribed to P25,and no other diffraction peaks ascribing to g-C3N4or urea are observed.This may because of the complete decomposition of urea and the rela-tive low diffraction intensity of the well-dispersed g-C3N4(S).The intensities of both the diffraction signals are almost the same,indi-cating the aggregation and crystal growth are not serious for P25. According to the Bragg equation,the anatase(001)diffraction peak and rutile(110)diffraction peak of P25shifts to the low angle direction suggests the crystal lattice is expanded,which is the direct evidence of doping P25with N species(Fig.1b)[37].In Fig.1c,the presence of g-C3N4and P25(N)in g-C3N4(S)–P25(N6) is further evidenced by the FTIR analysis.Obvious Ti A O A Ti vibra-tion band shift around500cmÀ1is observed in g-C3N4(S)–P25(N6), indicating impurity atoms(such as N)have been introduced.The breathing vibration at810cmÀ1for triazine unites and the skeletal vibration bands in the range of1200–1600cmÀ1for aromatic CN heterocycles are all the feature bands of g-C3N4,suggesting the successful synthesis of g-C3N4(S)even with the presence of P25 [7,38].With the above analysis,it can be observed that annealing P25with urea is an effective method in synthesizing g-C3N4(S) and modifying P25with N species to extend the light absorption (Fig.1d).As illustrated in Fig.2a,g-C3N4(S)–P25(N6)possesses a con-cave–convex surface morphology,and the g-C3N4(S)is coveredM.Ding et al./Journal of Alloys and Compounds635(2015)34–4035closely on the P25(N)surface.More clear structure is further shown in the TEM image(Fig.2b).It is clear that g-C3N4(S)in g-C3N4(S)–P25(N6)is almost transparent,indicating its few-layered structure.In addition,all the P25(N)is located on g-C3N4(S),which is agreed with Fig.1a.This tight structure is very helpful for improving the heterojunction performance.After removing P25(N)with the hydrofluoric acid,the porous-structured2D g-C3N4(S)was obtained(Fig.2c and d).Large2D plane and clear nanopores with a diameter of ca.30nm,which is agreed with the particle size of P25(Fig.2d inset),in g-C3N4(S)is clearly observed.This structure evidences that the P25(N)particles are truly embedded in g-C3N4(S).It should be clarified that even g-C3N4(S)–P25(N6)does not has the ideal layer upon layer structure,the2D structure of g-C3N4(S)is maintained.Moreover,the hetero-junction is formed all over the P25(N)surface,which is positive for improving the performance.For comparison,g-C3N4(B)was also prepared with the same procedure(Fig.2e).The different mor-phologies of g-C3N4(S)and g-C3N4(B)further evidenced that P25 is an efficient promoter in synthesizing the2D porous g-C3N4(S) with fewer layers.After mixing P25with g-C3N4(B),the resulted g-C3N4(B)–P25has a rough and uneven surface(Fig.2f).Moreover, the P25and g-C3N4(B)are not contacted tightly,which may reduce the heterojunction efficiency.The obtained C/N molar ratio of g-C3N4(S)was ca.0.718from the XPS analysis(Fig.S1),which is close to the theoretic value of fine-structured g-C3N4,suggesting the successful synthesis ofScheme1.Schematic structure of as-prepared2D porous g-C3N4(S)and g-C3N4(S)–P25(N)with enhanced quantum(b)enlarged anatase{101}and rutile{110}diffractions peaks,and(d)UV–vis absorption spectra of P25and g-C3N4(S)–P25(N6); N4(S)–P25(N6).polymeric structure.The C1s XPS spectrum of g-C3N4(S)is present-ed in Fig.S2,the C signals centered at284.6eV and287.9eV are assigned to the sp2C A C bonds of graphitic carbon and the sp2-hy-bridized carbon in N A C@N,respectively,further evidencing the fine g-C3N4framework.Structure differences between g-C3N4(S) and g-C3N4(B)werefirstly investigated by XRD analysis.In Fig.2a,two diffraction peaks belonging to g-C3N4of both the sam-ples centering at13.0°and27.4°are observed,suggesting g-C3N4(S)is stable in the hydrofluoric acid aqueous solution.The much weaker diffraction signal of g-C3N4(S)than that of g-C3N4(S)can be ascribed to the size-dependent properties,since there are fewer layers in g-C3N4(S)and the correlation length of interlayer periodicity of tri-s-triazine building blocks was reduced significantly[7,39].Obviously,the existence of P25can prevent the polymerization process of g-C3N4,hence the few-layered2D por-ous g-C3N4(S)was synthesized.The optical properties of g-C3N4(S)were significantly altered compared to that of g-C3N4(B).In Fig.3b,an obvious blue shift of the absorption edge is observed for g-C3N4(S),and the estimated band gap is increased from2.72eV of g-C3N4(B)to2.85eV of g-C3N4(S)(Fig.3b,inset).Combining with the XRD analysis,we can ascribe these changes to the strong quantum effect,which is com-monly happened when the particle size reduces to several nanometers.For g-C3N4(S),the reduced thickness is the reason for this change,which will give it many excellent properties.More-over,an enhanced light absorption in the visible light region is also observed for g-C3N4(S).The improved mesoporous structure and increased defect sites caused by the layer reduction may be the pri-mary reasons for this variation[40].Accordingly,the specific sur-face area and pore size distribution are also changed.In Fig.3c,a type-IV isotherm with a pronounced H1-type hysteresis loop in the relative pressure of0.4–0.9is clearly observed,convincing there are mesopores located in the polymeric frameworks of g-C3N4(S)[41].The calculated specific surface area and pore size are82m2/g and13nm,respectively,while the corresponding val-ues of g-C3N4(B)is8m2/g and35nm.Fig.3d shows the comparison of N1s XPS spectra.Both g-C3N4(B)and g-C3N4(S)present obvious peaks centering at 398.4eV and400.0eV,which are ascribed to the sp2-hybridized nitrogen in C A N@C and tertiary nitrogen N A(C)3groups[7].Com-bining with the C1s analysis,the existence of both the signals con-vinces the formation of tri-s-triazine which is the key structure of g-C3N4.Moreover,g-C3N4(S)has another obvious peak centered at 401.0eV,corresponding to the N species in C A N A H x.It may result from the enlarged surface area and more exposed C A N A H x groups. However,the N species located in the P25(N)lattice cannot be ana-lyzed because of the relative low content and signal superposition.In the PL emission spectra(Fig.4a),a structure-induced proper-ty change was clearly observed.The PL emission spectra shapes of both g-C3N4(B)and g-C3N4(S)are similar to each other.However, the much lower intensity of g-C3N4(S)proves it can separate the photo-generated electrons and holes more efficiently.This positive effect may be originated from the unique structure of g-C3N4(S), since the few-layered2D sheets can significantly shorten the diffu-sion length for carriers migrate to the surface and promote electron transformation on the plane.Hybrid semiconductorphotocatalysts TEM images of as-prepared photocatalysts.(a,b)g-C3N4(S)–P25(N6);(c,d)g-C3N4(S)and P25(inset);(e)g-C3N4(B);to form heterojunction is helpful for separating photo-generated electrons and holes.The PL intensity of g-C3N4(S)–P25(N6) quenches quickly,indicating that the charge recombination hap-pening in the heterojunction photocatalyst is greatly suppressed, hence,a better photocatalytic performance can be expected.The lowest PL emission intensity of P25is originated from its intrinsic structure(large band gap),which cannot directly prove it has the highest photocatalytic activity.We also investigate theÅOH gen-eration conditions of the as-prepared photocatalysts by employing terephthalic acid as the probe molecule.The typical PL emission spectrum centered at425nm of the resulted solution is shown in Fig.4b.After irradiated the suspensions for the same time,the col-lected PL intensity order is:g-C3N4(S)–P25(N6)>P25> g-C3N4(S)>g-C3N4(B).The result is agreed with that presented in Fig.4a and further evidences the obtained conclusions,since the ÅOH is formed by the reaction by electrons/holes with H2O and A OH,etc.[36].3.2.Photocatalytic tests and mechanismsThe photocatalytic activities of all the photocatalysts were evaluated by RhB(Fig.5a)and phenol(Fig.5b)degradation under the xenon lamp irradiation.Interestingly,g-C3N4(S)is superior to g-C3N4(B)in photocatalysis,and a much enhanced photocatalytic activity is obtained on g-C3N4(S)–P25(N6).The degradation effi-ciency differences between RhB and phenol may because of their different molecule structure,since the organic dyes is often easy to be decomposed.In order to further illustrate the superiorCharacterizations of g-C3N4(B)and g-C3N4(S).(a)XRD patterns;(b)UV–vis absorption spectra and estimated band gaps(inset);(c)N2 corresponding Barrett–Joyner–Halenda pore-size distribution(inset);(d)N1s XPS spectra.emission spectra under the excitation of300nm light and(b)PL emission spectra of2-hydroxyterephthalic acid to analyze the generated N4(S)and g-C3N4(S)–P25(N6).structure of g-C3N4(S),Fig.6shows the RhB degradation efficien-with the presence of g-C3N4(S)–P25and g-C3N4(B)–P25.Obvi-the photocatalytic activity of g-C3N4(S)–P25is higher thang-C3N4(B)–P25.The higher specific surface area and porous structure may give a more efficient charge transfer behavior between g-C3N4(S)and P25.All these results illustrate: 2D porous g-C3N4(S)with fewer layers and enhanced effect is an efficient candidate for photocatalytic application.situ coupling g-C3N4(S)with P25(N)with a facile one-step annealing strategy is very important in forming efficient hetero-junction photocatalyst.Further analysis was applied to investigate theratios of g-C3N4(S)to P25(N)played on the photocatalyticIn the present study,mass ratios of g-C3N4(S)to P25(N)ed by changing the mass ratios of P25to urea.It shouldout that N doping degree of P25will surely be changed,is hard to exactly measure this factor by the XPSP25(N)was employed in all the photocatalyststhe N doped P25.As shown in Fig.7a,the photocatalyticof g-C3N4(S)–P25(N4),g-C3N4(S)–P25(N6),and g-Care increased gradually,while g-C3N4(S)–P25(10)tocatalytic activity than g-C3N4(S)–P25(N8).Hence,weight ratio of urea to P25is8in the present experimentFig.7b shows that the photocatalytic activity of g-Cis very stable after four cycles repetition.This is easybecause P25and g-C3N4are both very stable semiconductors nature environment.Corresponding photocatalytic mechanisms wereshown in Fig.8a.The distance for the excited carriersto the surface of the few-layered2D porous g-C34reduced compared to that of g-C3N4(B),hence more photo-generat-ed carriers can take part in the photocatalytic reactionpose the pollutants.Even g-C3N4(B)can be excitedRhB and(b)phenol degradation efficiency curves with P25,g-C3N4(B),g-C3N4(S),and g-C3N4(S)–P25(N6)under the xenon lampRhB degradation efficiency curves with g-C3N4(B)–P25and g-C3N4(S)–P25the xenon lamp irradiation.degradation efficiency curves with g-C3N4(S)–P25(N4),g-C3N4(S)–P25(N6),g-C3N4(S)–P25(N8),and g-C3N4(S)–P25(N10)under the xenon photocatalytic tests with the presence of g-C3N4(S)–P25(N8).relative valence band and conduction band positions allow the electrons generated on g-C 3N 4(S)migrate to the conduction band of P25(N),and the holes generated on the P25(N)migrate to the valence band of g-C 3N 4(S).Moreover,g-C 3N 4(S)was directly cov-ered on P25(N)surface in the present study,hence a very strong surface interaction is existed and an efficient heterojunction is formed.The N doped impurity level located near the valence band of P25(N)also allows the absorption of visible light for photocat-alytic reaction.As a result,the facile annealing strategy to in situ hybrid g-C 3N 4(S)with P25(N)is helpful to form an effective UV–vis-driven heterojunction photocatalyst.4.Conclusions2D porous g-C 3N 4(S)and g-C 3N 4(S)/P25(N)heterojunction with enhanced quantum effect were in situ prepared by a facile but effi-cient annealing strategy.Structure analysis indicates that P25(N)is an efficient promoter in helping to form the few-layered 2D porous g-C 3N 4(S)which possesses superior properties than the bulk g-C 3N 4(B),even its band gap is enlarged because of the quantum effect.Tight contact between g-C 3N 4(S)and P25(N)is more favor-able for improving the heterojunction performance,and endow them high visible light absorption and electron–hole pair separa-tion efficiency under the xenon lamp irradiation.As a result,g-C 3N 4(S)and g-C 3N 4(S)–P25(N)possess excellent and stable photo-catalytic activities in decomposing phenol and RhB.The properties of g-C 3N 4(S)–P25(N)is optimized by adjusting the mass ratio of P25to urea,and the optimized value is 1:8.Such a facile and in situ annealing strategy may help to provide new insights for the design and preparation of high efficient semiconductor photocatalysts for pollutants degradation.AcknowledgementThis work was supported by the Startup Foundation for Intro-ducing Talent of NUIST (S8113101001).Appendix A.Supplementary materialSupplementary data associated with this article can be found,in the online version,at /10.1016/j.jallcom.2015.02.111.References[1]J.Xing,W.Q.Fang,H.J.Zhao,Chem.–Asian 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