Modified g-C3N4 derived from ionic liquid and urea for promoting visible-light photodegradation of organic pollutants

Hongbing Song,Lei Liu,Bingxiao Feng,Haozhong Wang,Meng Xiao,Hengjun Gai,Yubao Tang,Xiaofei Qu,Tingting Huang,*

1 State Key Laboratory Base for Eco-Chemical Engineering in College of Chemical Engineering,Qingdao University of Science and Technology,Qingdao 266042,China

2 College of Chemistry and Molecular Engineering,Qingdao University of Science and Technology,Qingdao 266042,China

3 College of Materials Science and Engineering,Qingdao University of Science and Technology,Qingdao 266042,China

Keywords:Graphite carbon nitride Ionic liquid Photocatalysts Element doping Organic pollutants

ABSTRACT In this work,modified g-C3N4 was fabricated successfully by calcination of ionic liquid(IL)and urea.The addition of IL changed the polymerization mode of urea,induced the self-assembly of urea molecules,modified the morphological structure of the tightly packed g-C3N4,and extended the electron conjugation system.When using 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) as a modifier,the heteroatom Cl could be inserted into the g-C3N4 to optimize the electronic structure.The results of characterizations indicate that the unique structure of modified g-C3N4 has an expanded electron delocalization range,introduces an interlayer charge transmission channel,promotes the charge transmission,reduces the band gap,enhances the absorption of visible light,and inhibits electron-hole recombination.Modified g-C3N4 showed excellent photocatalytic performance for the degradation of rhodamine B and tetracycline.Furthermore,the effect of different anions in 1-butyl-3-methylimidazolium salts ([Bmim]Cl,[Bmim]Br,[Bmim][BF4],and [Bmim][PF6]) on the structure and function of g-C3N4 are discussed.

1.Introduction

Organic pollutants in industrial wastewater have severe toxicity and carcinogenicity and are difficult to degrade naturally [1,2].They can pollute clean water sources and cause serious ecological problems[3,4].Photocatalytic technology can realize the complete mineralization of organic pollutants under the light and is considered to be one of the most promising technologies for the treatment of organic pollutants in wastewater [5,6].In the past few decades,titanium dioxide (TiO2) as the traditional semiconductor photocatalyst has been widely studied by researchers[7].Although some achievements have been made,its high activity only in UV light and high charge recombination rate seriously restricts its application for organic wastewater treatment [8].Therefore,finding a suitable photocatalyst is the key to solving environmental problems.

Graphite carbon nitride (g-C3N4) has suitable energy band positions and a graphite-like layered stacking structure,thereby providing good response to visible light in photocatalytic reactions[9,10].However,the high charge recombination rate caused by the difficulty of electron transport and serious agglomeration restricts the development of g-C3N4in the field of environmental treatment[11,12].To overcome these shortcomings,the modification methods of g-C3N4have been reported continuously,such as constructing heterojunction [13,14],sensitization of dye [15],changing morphology [16],and element doping [17].Among all the methods,element doping is considered to be a way to optimize the electronic structure and band structure,which is conducive to the enhancement of photocatalytic activity [18,19].The B-doped g-C3N4prepared by Luet al.[20] can efficiently reduce,mainly because B doping reduces the band gap of g-C3N4and enhances the visible light absorption intensity.Therefore,it is important to find a suitable dopant to improve the photocatalytic activity of g-C3N4.

As an environmentally benign solvent,ionic liquids (ILs) have been widely utilized in catalytic reactions,organic synthesis,and the preparation of photocatalysts [21-23] due to their unique structure and physic-chemical properties.Zhaoet al.[24]prepared P-doped g-C3N4with 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6])and urea,which promoted charge separation,inhibited carrier recombination,and promoted the degradation of organic pollutants.However,the previous literature did not make a detailed analysis of g-C3N4modified by ILs but only attributed to the results of anion element doping [25-27].Yanget al.[28] reported that the modification of g-C3N4by 1-ethyl-3-methylimidazole iodide([Emim]I)is beneficial to the optimization of sp2planar heterocycles,which is due to the co-doping of C and I.Therefore,we infer that IL organic cations are rich in C and N elements,which can also enter the C3N4plane structure and adjust the in-plane electronic structure.Although the planar electronic structure is optimized,the charge transfer resistance between conjugate planes is still large.Therefore,it is necessary to introduce a stable charge bridge between the layers of carbon nitride to overcome the resistance and strengthen the charge transfer between the layers.Liuet al.[29]studies showed that the Cl atom was intercalated into the carbon nitride layers derived from melamine and ammonium chloride and formed a stable interlayer charge channel through covalent bonds,which was conducive to the regulation of the band gap structure.

Using ILs as modifiers can induce the orderly arrangement of the carbon nitride precursors to form g-C3N4with a regular morphology and structure.For the IL of 1-butyl-3-methylimidazolium chloride ([Bmim]Cl),its organic cation can regulate the in-plane electronic structure,and chloride ion as the Cl source can be inserted into the g-C3N4layer to enhance the inter-layer charge fluidity.Hence,the preparation of the modified g-C3N4by calcining [Bmim]Cl and urea can endow its fully optimized electronic structure,which contributes to improving its photocatalytic performance.The effects of ILs on the chemical structure,electronic structure,band gap structure,and charge transfer of carbon nitride were investigated systematically.The results show that using IL modified g-C3N4can promote the expansion of the in-plane conjugation system of carbon nitride,increase the number of π electrons,build the interlayer bridge to promote the charge transfer,reduce the band gap and change the morphology.The degradation experiments of rhodamine B(RhB)and tetracycline(TC)were carried out by adjusting the amount of[Bmim]Cl added to obtain the optimal doping amount.At the same time,the effects of the same doping amount of [Bmim]Br,[Bmim][BF4],[Bmim][PF6] on the structure and photocatalytic properties of g-C3N4were evaluated.Finally,the mechanism research and cycle test of the catalyst have proved the high efficiency,stability,and universality of the catalyst in the degradation of organic matters.

2.Experimental

2.1.Materials

Urea,1-chlorobutane,n-butyl bromide and sodium fluoroborate were obtained from Shanghai Aladdin Biochemical Technology Co.,Ltd.1-methylimidazole and sodium hexafluorophosphate were purchased from Shanghai Macklin Biochemical Co.,Ltd.All chemical reagents were not further purified.

2.2.Preparation of samples

2.2.1.Preparation of ILs

According to the previous literature reports [30,31],the preparation and purification methods of ILs have been developed,and specific details have been added in the supplementary information.

2.2.2.Preparation of the modified carbon nitride

20 g urea was mixed with [Bmim]Cl,[Bmim]Br,[Bmim][BF4],and [Bmim][PF6],respectively,and dissolved in 50 ml aqueous ethanol(Vethanol:Vwater=1:1).The mixture was placed in an oil bath at 60 °C and stirred until dry to obtain a white solid.The white solid is placed in a covered ceramic crucible.A light brown solid was obtained by heating at 3°C·min-1to 520°C for 2 h in a muffle furnace.The samples were washed with water and ethanol for three times,and dried in a vacuum drying oven at 50 °C for 12 h to obtain g-C3N4doped with different ILs.The different molar ratios of[Bmim]Cl in g-C3N4are 2.6%,7.8%and 13.0%,respectively,based on the fact that 20 g urea can calcine 1 g g-C3N4.

According to the above calcination procedure,the monomer g-C3N4was obtained and labeled as CN.The g-C3N4modified using 7.8% IL anions were labeled as 7.8% CN-Cl,7.8% CN-Br,7.8% CNBF4,and 7.8% CN-PF6,respectively.

2.3.Characterization

The physicochemical properties of the samples were characterized by the following test methods.Fourier transform infrared(FT-IR) spectra were illustrated by the Bruker Vertex 70 infrared spectrometer.XRD was derived from an X-ray diffractometer (DMax 2500/PC,Japanese Physics).Scanning electron microscopy(SEM) was obtained from the Hitachi S4800.The XPS spectrum was obtained by Thermo Kalpha X-ray Photoelectron spectrometer.BET was tested by Micromeritics ASAP 2020.The optical properties of the samples were characterized by the following test methods.The UV-vis diffuse reflectance spectrum (DRS) was recorded on a Shimadzu UV3600 spectrophotometer.The photoluminescence spectra were obtained from the FLS920 photoluminescence spectrometer.The electron paramagnetic resonance (EPR)was recorded in a Bruker A300 EPR spectrometer.

2.4.Photocatalytic activity evaluation

The degradation experiments of RhB and TC were used to evaluate the photocatalytic performance of the samples.1 g·L-1sample was dispersed in a quartz reaction tube filled with 10 mg·L-1RhB aqueous solution.After stirring in a dark place for 30 min,1.5 ml samples were placed in a 2 ml centrifuge tube.The 500 W xenon lamp(with 420 nm cut-off filter)was turned on for photodegradation reaction,and samples were taken every 15 min.The centrifuge tube was centrifuged at 10,000 r·min-1for 2 min to separate the supernatant.The supernatant was extracted and detected at 554 nm on a UV-Vis spectrophotometer.Similarly,5 mg·L-1TC was used for photocatalytic evaluation in the same way as the above steps,and the maximum absorbance peak was 357 nm.The pH values of 10 mg·L-1RhB and 5 mg·L-1TC solution were 6.52 and 6.29,respectively.

3.Results and Discussion

3.1.Surface structure analysis of catalyst

The XRD patterns of CN,7.8% CN-Cl,7.8% CN-Br,7.8% CN-BF4,and 7.8% CN-PF6are shown in Fig.1.The appearance of a CN peak at 13.6° (100) indicates the appearance of an in-plane graphite structure,and the (002) peak of graphite interlayer stacking appears at 27.3°[32].After g-C3N4is modified using ILs,its diffraction peaks is basically unchanged.This result shows that introducing ILs cannot damage the inherent crystal structure of g-C3N4.Cl and Br are easily introduced into lamellae and form an electron transport channel,which increases the distance between lamellae,and results in the shift of (002) peak to a low angle [29] and indicates that Cl and Br are doped between layers.In 7.8% CN-BF4and 7.8% CN-PF6,the (002) peak has a slight shift to a low angle,increasing the interlayer spacing of the graphite flakes.This shift may be related to the lattice distortion caused by the substitution of carbon nitride graphite structural atoms by anionic atoms [33].Therefore,some elements in the IL were introduced into the lattice of g-C3N4,but the original crystal structure was not destroyed.

Fig.1. XRD patterns of CN,7.8% CN-Cl,7.8% CN-Br,7.8% CN-BF4 and 7.8% CN-PF6 (a);and (002) partial enlarged spectra of crystal planes (b).

Fig.2 shows the changes in the CN infrared spectrum after modification with ILs.The absorption peak of CN at 809 cm-1is related to the breathing vibration of the g-C3N4triazine ring [34].The absorption region of 1200-1700 cm-1indicates the existence of the unique carbon-nitrogen heterocycle of g-C3N4,which may be related to a graphite-like conjugated structure [35].The wide absorption peak at 3000-3650 cm-1indicates the existence of hydrogen bonds,which is related to uncondensed amino groups and adsorbed water on the surface [36].The infrared spectra of 7.8% CN-Cl is similar to that of CN,and no obvious shift occurs,indicating that an appropriate amount of [Bmim]Cl has no apparent effect on the structure of carbon nitride.The spectra of 7.8%CN-Br,7.8% CN-BF4,and 7.8% CN-PF6show no change,indicating that the infrared spectra is insufficient to show changes in the structure of carbon nitride modified by 1-butyl-3-methylimidazolium salts.Changes in the chemical state of carbon nitride were investigated through XPS analysis.

Fig.2. FT-IR spectra of CN,7.8% CN-Cl,7.8% CN-Br,7.8% CN-BF4 and 7.8% CN-PF6.

Change in g-C3N4chemical element state after doping with ILs can be investigated through XPS.Fig.S1(a)shows the survey spectra of CN and 7.8% CN-Cl,which is composed of C,N,O,and Cl.Fig.3 illustrates the splitting peaks of C and N elements for CN and 7.8% CN-Cl.The C element of CN can be divided into three peaks at 284.8,286.2,and 288.5 eV,attributing to C—C,C—O,and N—C＝N (sp2hybrid carbon),respectively [37,38].Terminal amino nitrogen C—N—H,tertiary nitrogen N—(C)3and sp2hybrid nitrogen C—N＝C correspond to the three splitting peaks at 401.6,400.4,and 399.0 eV,respectively [39].7.8% CN-Cl shows similar results to those of CN,but the chemical binding energy and molar content of the chemical bonds are slightly shifted,as shown in Tables S1 and S2.This phenomenon shows that the successful doping with IL changes the original chemical environment of CN without destroying the chemical structure of CN.In the XPS spectra of O 1s (Fig.S2),the single peak of O 1s shifts from 532.4 to 532.2 eV,and this shift can be attributed to C—O,which is related to the adsorbed CO2or the oxygen-containing intermediate produced during urea pyrolysis [40].The peaks at 201.0 and 199.4 eV can be observed in the Cl 2p XPS pattern(Fig.S1(b)),which correspond to Cl 2p1/2 and Cl 2p3/2[41].Combined with the theoretical analysis results of the previous report,this result further shows that doping Cl into the CN graphite sheets results in the formation of electron transmission bridges [29].

Tables S1 and S2 show the peaks of C and N in carbon nitride doped with various ILs.After doping ILs,the binding energies and relative contents of C—C,N—C＝N,C—N—H,N—(C)3,and C—N＝C are changed compared with the original CN.[Bmim]Br,[Bmim][BF4] and [Bmim][PF6] can enter the structure of CN,changing the chemical environment of CN without destroying the original structure.XPS analysis was conducted for Br in 7.8% CN-Br;F and B in 7.8% CN-BF4;and P and F in 7.8% CN-PF6,as shown in Fig.S1.Br 3d can be divided into two peaks (66.3 and 69.6 eV),which correspond to Br-C and residual Br-on the catalyst surface,respectively [25].Br doped into the CN graphite sheets acts as an electron transmission bridge.This finding is consistent with a literature report [26].The peaks at 688.5 and 685.7 eV,corresponding to C—F and BF-4,were observed in the F 1s spectra of 7.8%CN-BF4.B 1s can be fitted to a single peak of 191.7 eV,indicating the existence of a B—N bond[42].Cuiet al.[27]used XPS Ar±etching technique to confirm that F atoms cannot enter the CN skeleton and can only form chemical bonds on the CN matrix surface.7.8% CN-PF6was analyzed through XPS.The doping molar content of P is 0.2%,the doping molar content of F is 0%,and the contents of other anion elements are shown in Table S3.The two peaks of binding energy at 134.1 and 133.2 eV can be fitted by P 2p,corresponding to the P 2p3/2 and P 2p1/2 of the N-P bond,formed by substituting graphite carbon [43,44].

Fig.3. XPS spectra of C (a),N (b) elements of CN 7.8% CN-Cl.

By calculating the ratio of C/N molar content and the ratio of the split peak content of C and N elements,we can further comprehensively analyze the process of doping ILs to CN,as shown in Tables 1 and 2.The C/N ratio of 7.8%CN-Cl decreases compared with that of CN,indicating the introduction of nitrogen atoms into the CN crystal structure.The decrease in C—N—H/C—N＝C ratio after IL doping indicates that the conjugated structure of a graphite-like structure is enlarged,and the degree of the polymerization of terminal amino groups increases.The increase in the C—N＝C/N—(C)3ratio indicates that decrease in bridging N is beneficial to the extension of a conjugated system and expansion of electron delocalization range.The regularity of 7.8% CN-Br and 7.8% CN-PF6is similar to that of 7.8%CN-Cl.However,the ratio of N—(C)3decreases sharply in 7.8% CN-PF6,possibly due to the substitution of P by C.The C—N—H ratio of 7.8% CN-PF6increases,indicating a low degree of polymerization,which may result in poor photocatalytic performance [28].In 7.8% CN-BF4,the C—C/N—C＝N ratio increases,and the decrease in C—N＝C/N—(C)3ratio indicates that the atoms on the graphite ring are partially replaced.The increase of C/N ratio indicates that the number of C atoms decreases and the C atoms in the CN graphite ring are partially replaced by B atoms.According to the previous reports [45,46],in the mixed calcination of IL and urea,the imidazole ring of the IL has a large π bond similar to abenzene ring,and the sp2 hybrid structure is conducive to the formation of a graphitization structure,thus promoting the expansion of the carbon nitride graphite structure.IL alkane chains are easily broken into gases,and anionic elements are doped into the carbon nitride structure.

Table 1 C,N,O molar content and C/N molar ratio of CN and ILs loaded g-C3N4

Table 2 Sample XPS spectra analysis

Therefore,the introduction of ILs can expand the conjugated structure of carbon nitride and is conducive to the wide charge distribution in the graphite layer.7.8% CN-Cl not only has the advantages of IL doping but also forms an electron transport channel between layers.This channel facilitates the separation of electron-hole pairs,reduces the charge transfer resistance,and has good photocatalytic performance.

The morphology of CN doped with [Bmim]Cl was analyzed by SEM.Fig.4(a)shows that CN is a layered disordered accumulation,and interlayer accumulation is relatively dense.Fig.4(b) shows that the CN surface is relatively smooth and has no obvious flaky layered structure.During the mixing process of[Bmim]Cl and urea,an IL induces the self-assembly of urea and changes the aggregation state of urea [47].Therefore,the spatial aggregation of 7.8%CN-Cl decreases,as shown in Fig.4(c).Fig.4(d) shows that 7.8%CN-Cl has a certain cavity structure,and the layered structure is more obvious,which improves photocatalytic activity.

The distribution of elements on the surface of the catalyst was investigated through element mapping.Fig.S3 shows that C,N,O,and Cl elements are present on the surface of the 7.8%CN-Cl catalyst,and they are evenly distributed.

The pore channel and specific surface area of the samples modified with ILs were evaluated with the BET method.The N2adsorption-desorption curves and pore distribution diagrams of the samples at -196 °C are shown in Fig.5(a) and (b).Fig.5(a) shows that CN and CN modified by ILs are type IV isotherms with hysteresis loops,which are typical mesoporous materials.Fig.5(b) shows that the pore size distribution of CN is mainly at 2-5 and 15-30 nm,which corroborates the existence of mesoporous in this material.After being doped with ILs,7.8% CN-Cl,7.8% CN-Br,7.8%CN-BF4,and 7.8% CN-PF6show the distribution of pores at 2-5 nm,and the pore distribution at 15-30 nm is weakened.In addition to the reduction of specific surface area and pore volume shown in Table S4,the possible reasons are the increase in the degree of polymerization of CN after IL doping and the increase in residual carbon content generated by the incomplete decomposition of IL.Although the specific surface area and pore volume of 7.8%CN-Cl decreases to a small extent,the introduction of IL led to the optimization of the electronic structure of carbon nitride and the ordered arrangement of carbon nitride morphology,which led to its stronger catalytic activity.

Fig.4. SEM images of CN ((a) and (b)),7.8% CN-Cl ((c) and (d)).

Fig.5. CN,7.8% CN-Cl,7.8% CN-Br,7.8% CN-BF4,and 7.8% CN-PF6 N2 adsorption-desorption curves (a) and pore diameter distribution diagrams (b).

3.2.Analysis of optical properties of the catalyst

Fig.6. UV-vis DRS spectra (a) and Kubelka-Munk plot (b) of CN,7.8% CN-Cl,7.8% CN-Br,7.8% CN-BF4,and 7.8% CN-PF6.

The absorption of light is an important index for evaluating photocatalysts.Therefore,the UV-vis DRS spectra of the samples were used in evaluating the light absorption,as shown in Fig.6(a).CN shows strong absorption in the ultraviolet region,and the cut-off absorption band of visible light is 462 nm.The doping of[Bmim]Cl enhances the optical absorption of CN and broadens the absorption region of CN in visible light.The absorption of light benefits from electronic transitions.Therefore,the expansion of the π-electron conjugate system of carbon nitride strengthens the absorption of light and results in a red shift in the absorption band[48].In addition,the intercalation of Cl reduces band gap and facilitates light absorption [29,49].The light absorption capacities of 7.8% CN-Br,7.8% CN-BF4,and 7.8% CN-PF6are slightly stronger than the absorption capacity of 7.8% CN-Cl.The reasons are the modulation of the electronic structure of carbon nitride by other anions and the incomplete decomposition of ILs [47].The band gap can be evaluated with the Kubelka-Munk method,which is illlustrated in Fig.6(b) [50].The band gaps of CN,7.8% CN-Cl,7.8%CN-Br,7.8% CN-BF4,and 7.8% CN-PF6are 2.68,2.48,2.43,2.37,and 2.36 eV,respectively.Through[Bmim]Cl doping,the electronic structure of CN is optimized,and the band gap of CN is reduced.These effects are beneficial to the absorption of visible light and promote the excitation of electrons.

Electron-hole recombination was investigated through PL emission spectra.Fig.7 shows that CN has a higher carrier recombination rate under excitation by 384 nm light.Through the modification of [Bmim]Cl,the structure of CN graphite ring expands,the size of the conjugated system increases,and the fluorescence emission peak shifts to a long wave,all consistent with the results of XPS,UV-Vis,and DRS [51].The bridging effect of Cl can promote effective charge transport,strengthen the mobility of electrons between layers,inhibit the recombination of carriers,promote the formation of more active species,and accelerate the photocatalytic degradation of organic pollutants.

Fig.7. PL spectra of CN,7.8% CN-Cl.

Although the amplification of the electronic conjugate system has demonstrated in previous studies,electronic excitation after amplification needs to be further confirmed through EPR.In Fig.8,all samples have a Lorentz line with a g value of 2.0034 and a magnetic field between 3460-3560.This represents the unpaired electrons of graphitic carbon in the semiconductor carbon nitride plane [52].In the dark,the intensity of 7.8% CN-Cl is much higher than that of CN,indicating that doping with ILs expands the graphite ring system and increases unpaired electrons in the delocalized state.Under visible light irradiation(λ＞420 nm),the intensity of 7.8%CN-Cl increases,indicating good electron excitation and transfer ability,which is attributed to the expansion of the in-plane aromatic ring system and the electron transport of Cl atoms between layers.

3.3.Analysis of catalyst activity

3.3.1.Degradation of organic pollutants for screening [Bmim]Cl optimal doping amount

Fig.8. EPR spectra of CN,7.8% CN-Cl.

By adjusting the doping concentration of [Bmim]Cl in g-C3N4,the photocatalytic degradation of organic dye (RhB) and drug(TC) aqueous solutions were carried out for the determination of the optimal doping concentration,as shown in Fig.9(a) and 9(c).Based on the pseudo first-order kinetics,the first-order kinetic constants of RhB and TC degradation are calculated through fitting,as shown in Fig.9(b) and 9(d) [53].With RhB as the target pollutant,after 30 min adsorption-desorption equilibrium,the effect of CN degradation on RhB is not obvious within 90 min,and the firstorder rate constant is 0.00994 min-1.Compared with the degradation efficiency of CN,the degradation efficiency of 2.6 %CN-Cl improves,and the first-order rate constant is 0.01271 min-1.When the addition of[Bmim]Cl is 2.6%,the conjugated structure of CN is optimized.The intercalation of Cl promotes charge transport between layers and reduces the resistance of charge transfer,thereby improving the photocatalytic degradation performance of 2.6%CN-Cl.7.8%CN-Cl has the best N,Cl doping amount,and fully optimized carbon nitride electronic structure,and RhB is basically degraded within 90 min.The first-order kinetic constant of 7.8%CN-Cl is 0.02888 min-1,which is 2.9 times that of CN.A proper amount of doping with[Bmim]Cl can significantly improve photocatalytic degradation performance.However,the degradation activity of 13%CN-Cl is obviously inhibited,and the corresponding first-order kinetic constant is 0.01825 min-1.Because the excessively doped ILs cannot be completely decomposed,it is easy to cause carbon deposition and block the pore structure of CN.When TC is the target pollutant,the order of the catalysts by degradation activity is:7.8% CN-Cl (0.01481 min-1) ＞ 2.6 %CN-Cl(0.01123 min-1) ＞ 13 %CN-Cl (0.00867 min-1) ＞ CN(0.00838 min-1).The degrading activity sequence of TC and RhB is the same.Therefore,by modifying CN with [Bmim]Cl,the effective degradation of organic pollutants can be achieved.

3.3.2.Degradation of organic pollutants to investigate the effects of different kinds of anions doping

For exploring changes in the photocatalytic performance of different anionic ILs with the same molar amount on carbon nitride,the degradation of RhB and TC by the catalyst was explored,as shown in Fig.10(a) and 10(c).Fig.10(b) and 10(d) show the pseudo-first-order kinetic calculations of the photodegradation process of RhB and TC by the catalysts.When RhB is an organic pollutant,CN has a low photocatalytic activity due to the lack of a conjugated system and high carrier recombination rate.After modification with[Bmim]Cl and[Bmim]Br,the photocatalytic performance of 7.8% CN-Cl and 7.8% CN-Br improves,and the corresponding first-order kinetic constants are 0.02888,and 0.01032 min-1,respectively.The reason is the expansion of the CN graphite ring structure,extension of the conjugate system,and intercalation of Cl and Br,which accelerate the flow of electrons between layers.However,7.8%CN-BF4and 7.8%CN-PF6show poor photocatalytic performance,with first-order rate constants of 0.01547 and 0.00763 min-1,respectively.Although the electronic structures of 7.8% CN-BF4and 7.8% CN-PF6are optimized by element substitution,the lack of charge transfer channels between layers limits the separation of electrons to a certain extent and promotes recombination of carriers.Each type of anionic IL has its own optimal doping amount.7.8% CN-PF6may be added in excess,introducing intermediate impurity levels and accelerating the recombination of electron-hole pairs [51].In the degradation of TC,the effects of doping with different anionic ILs are similar to those of RhB degradation.The first-order kinetic constants of CN,7.8% CN-Cl,7.8% CN-Br,7.8% CN-BF4,and 7.8% CN-PF6degrade TC are 0.00838,0.01481,0.02102,0.00839,and 0.00718 min-1.

Fig.9. CN,2.6% CN-Cl,7.8% CN-Cl,13% CN-Cl degradation RhB curves (a) and first-order kinetic curves (b);Degradation TC curves (c) and first-order kinetic curves (d).

Fig.10. CN,7.8% CN-Cl,7.8% CN-Br,7.8% CN-BF4,7.8% CN-PF6 degraded RhB curves (a) and first-order kinetic curves (b);Degradation TC curves (c) and first-order kinetic curves (d).

3.3.3.Stability analysis of catalyst

To test the stability of modified CN,7.8%CN-Cl was used in five cycles of 10 mg·L-1RhB degradation.Fig.11(a) shows that after five consecutive degradations,the photocatalytic degradation activity of 7.8% CN-Cl is still good,maintained at 82.7%.The final degradation rate of the fifth cycle is only 9.8%,different from the first cycle,due to the loss of catalyst and blockage of some pores.Therefore,we conclude that the catalyst is relatively stable and can be used in continuous experiments.To confirm the structural stability of the catalyst,we detected the chemical structure of CN through infrared spectroscopy,as shown in Fig.11(b).Fig.11(b)shows that the infrared spectra of the freshly prepared 7.8% CNCl and 7.8%CN-Cl after five cycles basically have no change.Therefore,after five cycles,the structure of the catalyst does not change and still has good photocatalytic performance.

3.3.4.Degradation mechanism analysis of catalyst

To explore the mechanism of 7.8%CN-Cl degradation of organic pollutants,we added active species trapping agents BQ,EDTA-2Na,and IPA and then explored their effects on RhB and TC degradation.Fig.12(a) shows that RhB is sufficiently degraded in 7.8% CN-Cl without any capture agent.After the addition of BQ,the ability of the catalyst to degrade RhB is greatly inhibited,and degradation activity is basically lost.Therefore,plays a vital role in the photocatalytic oxidation of organic pollutants.After the addition of EDTA-2Na,the degradation of 7.8% CN-Cl is inhibited to a certain extent,indicating that the degradation of RhB is closely related to h+.The addition of the ·OH capture agent IPA does not change the photocatalytic degradation of RhB to a large extent,but the activity is slightly inhibited.The same result was observed in the degradation of TC by 7.8%CN-Cl after the addition of active species scavengers,as shown in Fig.12(b).In summary,the main active species involved in 7.8% CN-Cl degradation of organic pollutants areand h+.

Fig.11. 7.8% CN-Cl stability test (a) and post-test catalyst FT-IR spectra (b).

Fig.12. 7.8% CN-Cl degradation of RhB (a) and TC (b) active species removal experiment.

Fig.13. 7.8% CN-Cl degradation mechanism of organic pollutants.

The possible mechanism of 7.8% CN-Cl degradation of organic pollutants is show in Fig.13.Under visible light illumination,the catalyst surface produces electron-h+pairs (Eq.(1)),electrons on the catalyst surface are excited to transition from the valence band to the conduction band.The valence band leaves oxidizing h+,which oxidizes and degrades organic pollutants.The electrons on the conduction band combine with O2in the environment to form activespecies with strong oxidation (Eq.(2)),which further strengthens the process of the photocatalytic degradation of organic pollutants.Therefore,h+anddegrade organic pollutants and eventually mineralize into CO2and H2O(Eq.(3)).According to previous reports,the valence band position of g-C3N4is approximately 1.4 eV,and the redox potentials of OH-/·OH,and H2O/·OH are 2.4,and 2.72 eV,respectively [28,54].Therefore,the oxidation of the g-C3N4valence band is inadequate to generate·OH,and·OH does not participate in the degradation of organic pollutants.The theoretical inference and the experimental results are consistent.The strong photocatalytic activity of 7.8% CN-Cl is mainly due to the developed conjugated system and the intercalation structure of Cl.This excellent structure enhances electron delocalization,optimizes the electronic structure,increases electron mobility between layers,facilitates visible light absorption,and inhibits the recombination of charges.The possible photocatalytic degradation mechanism of 7.8%CN-Cl is described by the following formula:

4.Conclusions

In summary,g-C3N4was successfully modified by[Bmim]Cl and showed excellent photocatalytic activity for the degradation of dyes and drugs.The degradation experiment and characterization analysis showed that the modified g-C3N4had an expanded electron conjugation system,strong charge separation,strong visible light absorption,low electron-hole composite rate,small band gap,orderly stacking structure,and strong photocatalytic performance.The reasons were the co-doping of N and Cl,optimized electronic structure,and the inducing effect of IL on the morphology.In addition,we comparatively analyzed the effects of doping with [Bmim]Cl,[Bmim]Br,[Bmim][BF4],and [Bmim][PF6] on the structure and photocatalytic activity of g-C3N4.Finally,we carried out mechanism analysis of the modified g-C3N4degradation of organic pollutants.This study proposes a new path for the ILs modification of g-C3N4.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the support provided by the National Natural Science Foundation of China (21878164,21978143),Shandong Provincial Key Research and Development Program (2019GGX102029),‘‘QingChuang Science and Technology Plan”Project of Colleges and Universities in Shandong Province (2020KJC005),and State Key Laboratory of Materials-Oriented Chemical Engineering-Open Fund (KL19-08).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.06.021.