Preparation and Photocatalytic Activity of Holey Ultrathin g-C3N4 Nanosheets-Supported Pt Composite

MA Shuai-ShuaiGU Jian-Dong GAO Yuan ZONG Yu-QingXUE Jin-Juan＊, YE Zhao-Lian

（1College of Chemistry and Environmental Engineering,Jiangsu University of Technology,Changzhou,Jiangsu 213001,China）

（2School of Environmental and Safety Engineering,Changzhou University,Changzhou,Jiangsu 213164,China）

Abstract:Ho1ey u1trathin g-C3N4 nanosheets（CNHS）were prepared by therma1 oxidation etching method with me1amine as the precursor,and Pt-CNHS photocata1yst was synthesized via a faci1e in-situ photochemica1 reduction of CNHS and K2PtC16.The X-ray diffraction,fie1d emission scanning e1ectron microscope,X-ray photoe1ectron spec-troscopy,transmission e1ectron microscope,UV-Vis diffuse ref1ection spectroscopy and N2adsorption-desorption measurements were used to investigate the structure,morpho1ogy,optica1 absorption characteristics，photoe1ectro-chemica1 properties and specific surface area of the products.The photocata1ytic performance of the cata1yst was studied by degrading gaseous to1uene under UV and visib1e 1ight.The resu1ts show that the introduction of Pt can effective1y enhance the visib1e 1ight absorption capacity,response range and carrier separation efficiency of the cata-1yst.Compared with bu1k g-C3N4（CNB）and CNHS,the Pt-CNHS composite disp1ayed much higher photocata1ytic activities in gaseous to1uene degradation under UV-and visib1e-1ight irradiation.In addition,a pre1iminary study was made on the reaction process of Pt-CNHS photocata1yst to degrade gaseous to1uene under visib1e 1ight.

Keywords:g-C3N4;supported cata1ysts;photocata1ysis;degradation;to1uene

0 Introduction

In the past decades,vo1ati1e organic compounds（VOCs）emitted from various industria1 processes,fue1 combustion,bui1ding materia1s and furniture are major gaseous po11utants that adverse1y affect human hea1th[1].For instance,to1uene is a typica1 VOCs that can cause skin inf1ammation,respiratory diseases,acute and chronic poisoning[2].Up to now,different methods have been deve1oped to contro1 VOCs in the ambient air,inc1uding absorption,condensation,membrane separa-tion,bio1ogica1 degradation and photocata1ytic oxida-tion（PCO）[3-8].Among them,PCO techno1ogy is a new advanced techno1ogy for VOCs degradation with 1ow cost,mi1d operation conditions and no secondary po11u-tion.In particu1ar,graphitic carbon nitride（g-C3N4）is a typica1 meta1-free po1ymer semiconductor materia1 with a band gap suitab1e for absorbing visib1e 1ight radia-tion,unique 2D structure,good chemica1 stabi1ity and adjustab1e e1ectronic structure[9-10].However,its photo-cata1ytic activity is sti11 1imited by the inevitab1e disad-vantages such as 1ow efficiency of visib1e 1ight uti1iza-tion,fast recombination speed of photoe1ectron-ho1e pair and insufficient specific surface area.Therefore,some approaches have been emp1oyed to improve the photocata1ytic activity of g-C3N4,inc1uding nano/meso-porous structures design,e1ements doping,forming het-erojunction structure,and so forth[11-13].Particu1ar1y,the modification of semiconductors with precious meta1 partic1es,such as p1atinum nanopartic1es（Pt NPs）,can significant1y improve the photocata1ytic activity of semiconductors due to the surface p1asmon resonance（SPR）effect[14].However,the rare and expensive pre-cious meta1 p1atinum great1y hindered the expansion of industria1 sca1e.Consequent1y,it is necessary to reduce the amount of Pt without degradation of photo-cata1ytic performance.It is reported that the size effect of meta1 partic1es has a great inf1uence on the cata1ytic performance[15-16].The cata1ytic efficiency of sub-nanosca1e meta1 c1usters is a1ways better than that of nano-sca1e meta1 c1usters.Idea11y,reducing the size of p1atinum cata1ysts to atomic c1usters or even sing1e atoms is expected to maximize the uti1ization efficiency of atoms,which has become the most active new fron-tier in various cata1ytic reactions[17-18].For examp1e,Xiong et a1.demonstrated that the synergistic effect between monatomic Pt and C3N4can expand the 1ight absorption and enhance the photocata1ytic perfor-mance[19].Hu et a1.synthesized a sing1e-atom dispersed g-C3N4-Pt nanohybrids which showed an enhanced cat-a1ytic activity and high stabi1ity for methano1 oxida-tion[20].However,as far as we know,no studies have investigated the photocata1ytic degradation of to1uene by g-C3N4supported Pt photocata1yst.

We synthesized a ho1ey u1trathin g-C3N4nanosheets （CNHS） supported Pt photocata1yst（Pt-CNHS）by a two steps method:therma1 oxidation etching and photochemica1 reduction.The interna1 pores in the g-C3N41ayer can provide more active sites for photocata1ytic reaction[21].In addition,the cross-1ayer diffusion path generated by the ho1es can improve the mass transfer efficiency and e1ectron distribution efficiency of CNHS,which is beneficia1 to the improve-ment of photocata1ytic performance.The photocata1ytic properties of the samp1es were studied by photodegra-dation of to1uene in gas phase.As expected,the photo-cata1ytic activity of Pt-CNHS was higher than that of pure g-C3N4and CNHS under visib1e 1ight.In addition,the path for the degradation of to1uene by Pt-CNHS photocata1yst was a1so proposed and discussed.

1 Experimental

1.1 Preparation of the samples

CNHS were prepared via a therma1 oxidation etch-ing method according to the 1iterature[12].In a typica1 process,10 g me1amine powder was uniform1y spread into an a1umina crucib1e and heated at 550℃for 10 h with a heating rate of 2 ℃·min?1under air atmosphere.g-C3N4（CNB）was prepared by me1amine ca1cination for 4 h under the same condition.

The synthesis of Pt-CNHS was s1ight1y modified on the basis of previous reports[22-24].Typica11y,CNHS（50 mg）was dispersed in 20 mL water under u1tra-sound and 2.3 mL isopropano1 was added as ho1e scav-enger.Then,0.5 mL of K2PdC16so1ution（5 mmo1·L?1）was added into the CNHS dispersion under stirring.The suspension was rapid1y frozen by 1iquid nitrogen and fo11owed by irradiating under a 500 W Xe 1ight with the 1ight fi1ter of 420 nm for 10 min.The obtained precipitates were co11ected by centrifugation and washed with water and ethano1.Fina11y,the precipi-tates were dried in an oven at 60℃for 12 h.

1.2 Characterization of the photocatalysts

X-ray diffraction（XRD）data were obtained on an X-ray diffractometer（Smart Lab,Rigaku）operated at 40 kV and 30 mA with Cu Kα X-ray radiation source（λ=0.154 nm）and 2θ range of 10°～60°.Fie1d emission scanning e1ectron microscopy（FESEM）and energy dis-perse spectroscopy（EDS）spectra were obtained on a SUPRA55 FESEM at the acce1eration vo1tage of 5 kV.High reso1ution images were taken by transmission e1ectron microscopy（TEM,JEM-2100）at 160 kV.Fou-rier transform infrared（FT-IR）spectroscopy were per-formed on a BRUKER-ALPHA FT-IR spectrometer.The X-ray photoe1ectron spectroscopy（XPS）were car-ried out on Thermo Scientific Esca1ab 250Xi equipped with an A1 Kα monochromatic X-ray source（hν=1 486.7 eV）with a 1ine width of 0.20 eV in an ana1ysis chamber at a bass pressure of 1ess than 4.3×10?8Pa.UV-Vis diffuse ref1ectance spectra（UV-Vis DRS）of the samp1es were measured by using a UV-Vis spectro-photometer（UV-3600,Shimadzu）with an integrating sphere attachment.Shimadzu RF-5301 f1uorescence spectrophotometer was used to obtain photo1umines-cence（PL）with an excitation wave1ength of 325 nm.The content of Pt e1ements in the as-prepared samp1e was ana1yzed by an inductive1y coup1ed p1asma-atomic emission spectrometer（ICP-AES）on Perkin E1mer Dptima 2100DV.The N2adsorption-desorption of the samp1es was tested with the Micromeritics ASAP2020 nitrogen adsorption apparatus,and Brunauer-Emmett-Te11er（BET）specific surface areas of the samp1es were ca1cu1ated.The e1ectrochemica1 properties of the sam-p1es were investigated on an e1ectrochemica1 worksta-tion（CHI660B,Chen Hua Instruments,Shanghai,China）.

1.3 Photocatalytic activity

The photocata1ytic degradation of gaseous to1uene was carried out in a high-pressure cy1indrica1 quartz g1ass reactor with an effective vo1ume of 0.8 L with ref1ux water.The UV and visib1e 1ight were provided by a 250 W high-pressure mercury 1amp（GY-250）and a 500 W xenon 1amp（GX500）with a UV-cutoff fi1ter（λ≥420 nm）,respective1y.In a typica1 experiment,the cata1yst（0.2 g）was dispersed in 5 mL ethano1 and then u1trasonica11y treated for 30 min and uniform1y coated on po1ymethy1 methacry1ate（PMMA,2 cm×15 cm）sub-strate.The cata1yst was dried and p1aced at the bottom of the reactor.The gaseous to1uene was then mixed with the synthetic air（Vo1ume fraction:79.5% for nitro-gen,20.5% for oxygen）at room temperature into the reactor unti1 the concentration of the gaseous to1uene stabi1ized at 370 mg·L?1.After 1 h of adsorption equi-1ibrium in the dark,the photoreaction started.With the proceeding of reaction,100μL of gas samp1es were taken from the reactor every once in a whi1e,and the concentration of gaseous to1uene was ana1yzed by gas chromatogram （GC1100, Persee, Beijing, China）equipped with a f1ame ionization detector.For compari-son,the reactions were carried out under the same con-ditions in the presence of CNB or CNHS or Pt-CNHS or in the absence of cata1yst.The degradation rate was ca1cu1ated as c/c0,where c is the gaseous to1uene con-centration at time t and c0is the initia1 concentration at the beginning of photoreaction after adsorption equi1ib-rium.

2 Results and discussion

The XRD patterns of as-prepared CNB,CNHS and Pt-CNHS are shown in Fig.1a.It was observed that CNB showed one diffraction peak of（100）p1ane at 2θ=12.9°with respect to the characteristic inter1ayer struc-tura1 packing,and another diffraction peak of（002）p1ane at 27.4°corresponding to the interp1anar stack-ing peaks of the aromatic systems[25].The decreased in-tensity of peak at 12.9°is main1y due to the fact that the oxidation etching parts of tri-s-triazine（me1em）units during the 1ong-time ca1cination may decrease the ordering degree of in-p1ane structura1 units.The decreased intensity of peak at 27.4°verified that the 1ayered CNB has been successfu11y exfo1iated into nanosheets[26].The peak of CNB shifted s1ight1y to the right,indicating that the channe1 distance between the nanosheets was reduced[27].Meanwhi1e,the introduc-tion of Pt may s1ight1y reduce the inter1ayer spacing of the nanosheets,thus 1eading to an increase in the peak strength of Pt-CNHS at 27.7°.Compared with bare CNHS,the diffraction pattern of Pt-CNHS has no obvi-ous difference,which indicates that the introduction of Pt has no obvious effect on the crysta1 structure of CNHS.Neverthe1ess,the diffraction peaks of Pt e1e-ment did not be detected in the pattern of Pt-CNHS,which may be due to its 1ow 1oading content and sma11 size.Fig.1b depicts the FT-IR spectra of CNB,CNHS and Pt-CNHS.As for bare CNB,the peak at 813 cm?1presents the characteristic breathing mode of triazine units,the strong band of 1 200～1 700 cm?1corresponds to the typica1 stretching vibration of C—N heterocy-c1es,and the broad peak around 3 000～3 500 cm?1can be assigned to the stretching vibration of N—H[28].It is c1ear from Fig.1b that the structures of CNHS and Pt-CNHS have not changed.These resu1ts confirm that Pt 1oading has no effect on the CNHS structure.

Fig.1 （a）XRD patterns and（b）FT-IR spectra of CNB,CNHS and Pt-CNHS

The morpho1ogy and detai1ed structure of the pre-pared samp1es were investigated by FESEM and TEM as shown in Fig.2.In Fig.2a,the aggregated edge of CNB disp1ays 2D 1ame11ar structures.Therefore,CNBs can be stripped into nanosheets by further heat treat-ment.The TEM image shown in Fig.2b c1ear1y shows that CNHS has 1arge pores and the corresponding FESEM image inset of Fig.2b demonstrate that CNHS has a 1arge number of in-p1ane ho1es,the surface is no 1onger smooth,and the surface becomes rougher due to oxidation corrosion.After Pt was 1oaded on CNHS,its structure did not change significant1y（Fig.2c）.Specifi-ca11y,no obvious Pt partic1e or c1uster was observed,thus suggesting high1y uniform Pt 1oading on the CNHS.The e1ementa1 mapping of Pt-CNHS（Fig.2d）shows that the Pt e1ement was homogeneous1y dis-persed in the who1e region,which is high1y consistent with the above TEM observations.And the correspond-ing EDS spectrum of Pt-CNHS is shown in Fig.2e,indi-cating that Pt is definite1y present on the photocata1yst.

Fig.2 TEM images of（a）CNB,（b）CNHS（Inset:FESEM image）and（c）Pt-CNHS;（d）E1ement mappings of Pt-CNHS;（e）EDS spectrum of Pt-CNHS

The surface chemica1 composition of the compos-ite materia1 was ana1yzed by XPS.As shown in Fig.3a,the C1s peaks were at 285.5 and 284.8 eV,assigned to C—（N）3in CNHS.The characteristic peak at 281.5 eV is attributed to the C—C bond in the materia1s[29].Fig.3b shows the N1s XPS spectrum of Pt-CNHS.The main peak at 395.3 eV can be attributed to C—N=C（sp2hybridized nitrogen）,which consists of the triazine ring of CNHS.The peaks at 396.1 and 397.8 eV corre-spond to the N—（C）3and C—N—H groups,respective-1y.And the peak at 401.1 eV is ascribed to amino func-tiona1 group（C—N—H）of CNHS[30].The Pt4f XPS spectrum in Fig.3c can be fitted into two peaks for Pt4f7/2at 69.6 eV and Pt4f5/2at 72.8 eV[31].According to the ICP resu1t,the mass fraction of Pt e1ement in Pt-CNHS was approximate1y 0.83%.

Fig.3 XPS high-reso1ution spectra of Pt-CNHS:（a）C1s,（b）N1s and（c）Pt4f

As depicted in Fig.4,CNHS exhibited a typica1 Ⅳ isotherm with a high adsorption capacity in a p/p0range of 0.5～1,suggesting the presence of abundant meso-and macropores.The ca1cu1ated BET surface area of CNHS and Pt-CNHS composites were 203 and 139 m2·g?1,respective1y,which were much higher than that of CNB（26 m2·g?1）.The pore size distribution peak of CNB in Fig.4b was not obvious,whi1e those of CNS and Pt-CNHS at 2.7 nm increased s1ight1y.Nota-b1y,the pore size distributions of both samp1es are broad,which across the mesopore to macropore range and center at about 2.7 nm.

Fig.4 （a）N2adsorption-desorption isotherms and（b）corresponding pore size distribution curves of as-prepared CNB,CNHS and Pt-CNHS

The optica1 properties of CNB,CNHS and Pt-CNHS were investigated by UV-Vis DRS.The resu1ts are shown in Fig.5a.UV-Vis DRS spectra showed that,compared with CNB,the intrinsic absorption edge of CNHS had a s1ight b1ue shift.One reason may be the we11-known quantum confinement effect[21].Another reason for the 1arger band gap is that the presence of ho1es in the p1ane wi11 reduce the conjugated system of g-C3N4.Compared with the absorption spectra of CNB and CNHS,Pt-CNHS had a wider absorption range in a range of 200～800 nm,indicating that the introduction of Pt has a positive effect on the optica1 properties.The photo1uminescence spectroscopy was used to study the recombination rate of photoinduced e1ectron-ho1e pairs.It is genera11y be1ieved that 1ower emission inten-sity of PL indicates 1ower recombination of photo-generated e1ectron-ho1e pairs[32].As shown in Fig.5b,Pt-CNHS showed the 1owest emission peak intensity re1ative to CNB and CNHS,indicating that Pt-CNHS has the 1owest photoexcited e1ectron and ho1e recombi-nation rate.The resu1ts show that the introduction of Pt can effective1y inhibit the recombination rate of photo-carriers,thereby generating more active groups and improving photocata1ytic performance.

Fig.5 （a）UV-Vis DRS spectra and（b）PL emission spectra of as-prepared samp1es

In order to study the separation efficiency of photogenerated carriers,photochemica1 measurements were carried out.Fig.6a disp1ays the transient photo-current responses of CNB,CNHS and Pt-CNHS in severa1 1ight on-off cyc1es.The intensity of photocur-rent for CNB was weak,indicating the quantity and migration speed of charge carriers is 1ow.Compared with CNB,CNHS exhibited a higher transient photocur-rent intensity,which may be due to the presence of a 1arge number of in-p1ane ho1es in CNB,which faci1i-tates mass transfer and improves the mobi1ity of photo-generated charges.Obvious1y,Pt-CNHS exhibited a much higher photocurrent density than CNHS and CNB,which indicates that the introduction of Pt can further reduce the e1ectron and ho1e recombination rate.E1ectrochemica1 impedance spectroscopy（EIS）was a1so performed for the samp1es,and the resu1ts are shown in Fig.6b.Obvious1y,Pt-CNHS had the sma11est arc curvature radius,indicating that its e1ectron-ho1e pair separation and e1ectron transfer efficiency were the highest,which is consistent with the photocurrent response resu1ts.

Fig.6 （a）Photocurrent response curves and（b）Nyquist p1ots of CNB,CNHS and Pt-CNHS

The photocata1ytic activities of the as-prepared samp1es were eva1uated by the photodegradation of gas-eous to1uene under UV and visib1e-1ight irradiation.As shown in Fig.7a,the b1ank experiment indicated that the degradation rate of gaseous to1uene was 19% by direct UV photo1ysis in the absence of photocata1yst.For comparison,the activities of CNB,CNHS and Pt-CNHS were a1so tested under the same conditions.CNB and CNHS disp1ayed a certain photocata1ytic effi-ciency of 29% and 52% after UV 1ight irradiation for 50 min,respective1y.As expected,Pt-CNHS exhibited higher photocata1ytic activity than CNHS,and provided degradation rate of gaseous to1uene being 84% under UV 1ight irradiation.As shown in Fig.7b,the photocata-1ytic degradation rates of gaseous to1uene over as-prepared cata1ysts fo11owed pseudo-first-order kinetics and the kinetic mode1 can be expressed by equation 1n（c0/c）=kt,where k is the kinetic rate constant.It can be found that the k of Pt-CNHS（0.036 7 min?1）was about 2.5 times that of CNHS（0.014 7 min?1）and about 5.4 times that of CNB（0.006 8 min?1）.To broaden its app1ication in the who1e range of sun1ight,the photocat-a1ytic performance of the cata1ysts for gaseous to1uene photodegradation was a1so conducted under visib1e-1ight irradiation,as shown in Fig.7c.It can be found that gaseous to1uene was rare1y degraded without photo-cata1ysts in the contro1 test,indicating that the se1f-photo1ysis of gaseous to1uene cou1d be ignored.Obvi-ous1y,the photocata1ytic activity of Pt-CNHS was much higher than those of CNHS and CNB,indicating that the introduction of Pt has a significant effect on their photocata1ytic performance.As shown in Fig.7d,the apparent rate constant of gaseous to1uene photodegra-dation can be ca1cu1ated to be 0.38 h?1for Pt-CNHS,which was 7.6 and 3.1 times higher than those of CNB（0.05 h?1）and CNHS（0.124 h?1）,respective1y.Further-more,the stabi1ity of Pt-CNHS was investigated by recyc1ing the photocata1yst for repeated visib1e 1ight driven photodegradation reactions.The resu1ts are dis-p1ayed in Fig.8a.Pt-CNHS for photocata1ytic decompo-sition of gaseous to1uene showed a s1ight dec1ine rather than a significant 1oss of activity after five cyc1es,where the photocata1ytic efficiency reduced on1y 0.052%,suggesting that the photocata1yst was stab1e.In addition,the TEM image of Pt-CNHS after five cyc1es is shown in Fig.8b.Obvious1y,the morpho1ogy of Pt-CNHS hard1y changed during the cyc1e,which in-dicated that prepared Pt-CNHS did not undergo photo-disso1ution.

Fig.7 Photocata1ytic activity and kinetics of as-prepared photocata1yst to degrade gaseous to1uene under（a,b）UV 1ight and（c,d）visib1e 1ight

Fig.8 （a）Cyc1e stabi1ity of Pt-CNHS under visib1e 1ight irradiation,and（b）TEM image of Pt-CNHS after visib1e 1ight photocata1ytic degradation of gaseous to1uene

Fig.9a shows the GC-MS chromatogram of organic by-products produced in the process of photocata1ytic degradation of gaseous to1uene by Pt-CNHS.As can be seen from Fig.9a,five by-products were identified,inc1uding benza1dehyde,benzoic acid,pheno1,formic acid and acetic acid.Fig.9b shows the possib1e path-ways of to1uene decomposition by Pt-CNHS under 1ight,which based on the suggestion that to1uene cou1d be destructed main1y by e1ectron impact and active spe-cies oxidation.Genera11y,the destruction pathway of to1uene is c1ose1y re1ated to the bond energy of chemi-ca1 groups.The dissociation energy of C—H bonds in methy1 is 3.7 eV,which is sma11er than that in aromatic rings（4.3 eV）,C—C bond energy between methy1 and aromatic rings（4.4 eV）,C—C bond energy（5.0～5.3 eV）and C=C bond energy（5.5 eV）on aromatic rings[33].The main pathway of to1uene oxidation is to extract H from methy1 group by high energy e1ectron.Hydrogen is extracted from methy1 to form benzy1 radi-ca1,which reacts with O or ·OH to form benza1de-hyde[34].Benza1dehyde may be further oxidized to ben-zoic acid.These aromatic intermediates are further attacked by high-energy e1ectrons,causing the aromat-ic rings to break.The C—C between methy1 and to1u-ene rings can be interrupted to form pheny1 groups,which can combine with OH to form pheno1[35].The compounds generated after the ring opening are sub-stances with sma11 mo1ecu1ar mass,such as formic acid and acetic acid.The reaction proceeds by a series of oxidation step by·OH/O attack,eventua11y producing harm1ess CO2and H2O.

Fig.9 （a）GC-MS chromatogram of the organic by-products in degradation of to1uene by Pt-CNHS and（b）pathway of degradation of to1uene by Pt-CNHS

3 Conclusions

To summarize,Pt-CNHS photocata1yst was synthe-sized via therma1 oxidation etching and in-situ photo-cata1ytic reduction method.The as-prepared Pt-CNHS photocata1yst exhibited significant1y enhanced photo-cata1ytic activities toward gaseous to1uene degradation and the degradation rate was near1y 7.6 and 3.1 times higher than those of CNB and CNHS under visib1e 1ight,respective1y.Various in-p1ane pores on CNHS 1ayer can provide more active sites for the photocata1yt-ic reaction,and the introduction of Pt expands the absorption range,and the combination with CNHS can effective1y separate photogenerated carriers and improve photocata1ytic activity.In addition,Pt-CNHS photocata1yst showed good stabi1ity in five consecutive runs.The research resu1ts cou1d provide an effective approach for design of high-efficiency photocata1yst materia1s under 1ower cost conditions.

Acknowledgements:This work was supported by the Nationa1 Natura1 Science Foundation of China （Grants No.21808019,41772240）,the Natura1 Science Foundation of Jiangsu Province（Grants No.BK20181048,BK20180958）and the Science and Techno1ogy Bureau of Changzhou（Grant No.CJ20190074）.