Facile ultrasonic-assisted synthesis of micro–nanosheet structure Bi4Ti3O12/g-C3N4 composites with enhanced photocatalytic activity on organic pollutants☆

Huihui Gan *,Futao Yi2,Huining Zhang Yongxing Qian Huixia Jin Kefeng Zhang

1 School of Civil Engineering and Architecture,Ningbo Institute of Technology,Zhejiang University,Ningbo 315100,China

2 College of Environmental Science and Engineering,Guilin University of Technology,Guilin 541004,China

Keywords:Rhodamine B Photocatalyst Ultrasonic assisted synthesis Bi4Ti3O12 g-C3N4

A B S T R A C T The Bi4Ti3O12/g-C3N4 composites with microsheet and nanosheet structure were prepared through facile ultrasonic-assisted method.The SEM and TEM results suggested that the nanosheets g-C3N4 were stacked on the surface of regular Bi4Ti3O12 sheets.Comparing with pure Bi4Ti3O12 and g-C3N4,the Bi4Ti3O12/g-C3N4 composites showed significant enhancement in photocatalytic efficiency for the degradation of RhB in solution.With the mass ratio of g-C3N4 increasing to 10 wt%,the Bi4Ti3O12/g-C3N4-10%presented the best photocatalytic activity.Its photocatalysis reaction constant was approximately 2 times higher than the single component Bi4Ti3O12 or g-C3N4.Meanwhile,good stability and durability for the Bi4Ti3O12/g-C3N4-10%were confirmed by the recycling experiment and FT-IR analysis.The possible mechanism for the improvements was the matched band positions and the effective separation of photo-excited electrons(e-)and holes(h+).Furthermore,based on the results of active species trapping,photo-generated holes(h+)and superoxide radical(?O2-)could be the main radicals in reaction.

1.Introduction

Much concern was gained about persistent organic pollutants in water because of their carcinogenicity and toxicity in low concentration[1,2].Rhodamine B(RhB)is widely used as dyestuff in textiles,which aroused widespread concern inevitably in ecological risk[3,4].Photocatalysis has been regarded as a potential approach for the treatment of environmental pollutants,due to its eco-friendliness and absence of any disinfection by-products(DBPs)[5-9].In recent years,bismuth-based materials have been extensively studied in photocatalysis[10].As a new type of semiconductor photocatalyst,bismuth titanate is a promising photocatalyst,owing to the layered structure[11,12].According to the previous reports,bismuth titanate was found in four types,namely Bi4Ti3O12,Bi12TiO20,Bi20TiO32and Bi2Ti2O7,respectively[13,14].Among them,Bi4Ti3O12has obtained much attention and been employed as an efficient photocatalyst for decontamination[15,16].In general,the morphological structure of semiconductors is an important factor for their photocatalytic properties[10].Hou et al.synthesized Bi4Ti3O12nanofibers by electrochemical methods,and they found that it has better photocatalytic degradation properties[17].Chen et al.synthesized Bi4Ti3O12nanosheet with{001}surface exposure by sol–gel hydrothermal method for the first time,which also possessed excellent photocatalytic efficiency,and they also found that the nanosheet structure could provide more active sites to improve the reaction[10].Besides morphology controls,many attempts have been made to modify Bi4Ti3O12due to its large band gap(3.0 eV)and limited quantum efficiency,such as doping[18-20]and introduction of other components to form a heterojunction[21,22].Dopants,however,might introduce the second energy level and form a recombination center,and suppress the photocatalytic activity[23,24].Heterojunction construction could be an effective way in modifying photocatalyst and promoting the separation of holes and electrons.Recently,Bi4Ti3O12/Bi2Ti2O7[33]and CeO2/Bi4Ti3O12[24]have been fabricated and their better photocatalytic performance than pure Bi4Ti3O12was reported.

Graphitic carbon nitride(g-C3N4)with a stacking layer structure and large specific surface,is a familiar,non-toxic,and earth-abundant semiconductor[25,26].In addition,g-C3N4possesses good thermal stability due to the basic structure tri-s-triazine ring[27].Furthermore,as the previous reports,g-C3N4is the p-type semiconductor[28]and bismuth titanate is the n-type semiconductor[28].Construction of a p–n heterojunction structure could be an effective method to improve photocatalytic activity[22,29].In view of that,g-C3N4could be a potential component to form the heterostructure with bismuth titanate for improving the photocatalytic activity.Cheng et al.synthesized Bi20TiO32/g-C3N4with 2D/2D structures,whose activity for degrading RhB was approximately 10 times than pure Bi20TiO32[6].Nan et al.reported g-C3N4/Bi12TiO20nanosheet and its efficiency in photocatalysis was revealed to be approximately 3 times than pure Bi12TiO20[8].

In this work,the Bi12Ti3O4/g-C3N4composites with sheet structure were synthesized by mild ultrasonic assisted method.The photooxidation property of Bi12Ti3O4/g-C3N4composites was evaluated by degradation of Rhodamine B under visible irradiation.The microsheet Bi12Ti3O4and nanosheet g-C3N4were shown by SEM and TEM.Moreover,main active species in RhB photocatalytic degradation over Bi12Ti3O4/g-C3N4composites were ascertained by trapping experiment.Furthermore,the separation mechanism for photo-excited electrons(e-)and holes(h+)in the degradation process was also discussed via photoluminescence spectrum.

2.Methods and Materials

2.1.Preparation of photocatalysts

All the reagents were of analytical grade and used without further purification.The Bi4Ti3O12was synthesized by the molten-salt method[30].Typically,the Bi2O3and TiO2powders were mixed with NaCl and KCl according to the mole ratio of[Na]:[K]:[Bi]:[Ti]=50:50:4:3.Then,the mixture was grounded thoroughly for 20 min and heated at 800°C for 2 h in air.The obtained precipitates were washed with deionized water for several times and dried at 80°C.

The g-C3N4was prepared by heating the melamine at 550°C for 2 h in N2atmosphere and then ultrasonicating in deionized water for 1 h as liquid phase exfoliation[31].The prepared g-C3N4was washed,centrifuged and dried at 60°C in air for 12 h.

The Bi4Ti3O12/g-C3N4composite photocatalyst was synthesized by adding 0.1 g of prepared Bi4Ti3O12powder and a certain amount of g-C3N4to 50 ml deionized water and then ultrasonicating in water for 1 h at 25°C,and then the suspension was continuously stirred at room temperature for 48 h and finally evaporated to obtain the Bi4Ti3O12/g-C3N4powders.With the mass ratio of g-C3N4to Bi4Ti3O12from 5 wt%,10 wt%to 15 wt%,the prepared Bi4Ti3O12/g-C3N4samples were denoted as Bi4Ti3O12/CN-5%,Bi4Ti3O12/CN-10%and Bi4Ti3O12/CN-15%,respectively.

2.2.Characterization

Powder X-ray diffraction(XRD)patterns of the samples were characterized on a D8 Advance X-Ray diffractometer(Bruker AXS,Germany)with Cu Kαradiation.X-ray photoelectron spectra(XPS)were carried out on an AXIS ULTRADLD X-ray photoelectron spectroscopy with monochromatic Al Kαradiation source.The morphologies of the powders were analyzed by a Merlin scanning electron microscope(Zeiss,German).High-resolution transmission electron microscopy(HRTEM)analysis was conducted with a Libra200FE electron microscope(Zeiss,German).The UV–vis spectra were measured by a Lambda 950 UV/vis/NIR spectrometer(Perkin Elmer,America).The Fourier IR spectroscopy was performed by using a Nicolet-670 Fourier transform infrared spectrometer(Thermo Fisher,USA).The steady Photoluminescence(PL) fluorescence spectroscopy was detected by FL3-111(Horida,Japan)at room temperature with an excitation light wavelength of 380 nm.

2.3.Photocatalytic activity and stability measurements

The photocatalytic properties of the samples were evaluated by degradation of RhB under visible light irradiation in a Xenon lamp(PLSSXE300(BF),China,Beijing)with a 400 nm cut-off filter(λ≥400 nm).In a typical experiment,0.01 g photocatalysts were added into 100 ml RhB solution(10 mg·L-1).Before illumination,the mixtures were stirred for 30 min in the dark to ensure adsorption–desorption equilibrium between the photocatalysts and RhB.After that,at a certain time interval under the visible light irradiation(0 min,5 min,10 min,20 min,30 min,60 min,90 min,120 min),the aliquot samples were taken out of the reaction suspensions and were centrifuged to remove the photocatalyst particles.The concentration variations of RhB were measured by a UV–vis spectrometer(UV2800,China).

3.Results and Discussion

3.1.XRD and XPS analysis

The XRD patterns of graphitic carbon nitride(g-C3N4),pure Bi4Ti3O12obtained by molten salt method and the Bi4Ti3O12/g-C3N4samples with different composite ratios Bi4Ti3O12/CN-5%,Bi4Ti3O12/CN-10%and Bi4Ti3O12/CN-15%were shown in Fig.1.It can be seen that the Bragg peaks of all the Bi4Ti3O12/CN composite samples are sharp indicating good crystallinity.The diffraction peaks of the asprepared Bi4Ti3O12/C3N4with different composite ratios are indexed to orthorhombic Bi4Ti3O12with the space group Aba2(JCPDS 72-1019).The peak at 30.1°is the highest intensity peak unique to the pure Bi4Ti3O12(JCPDS 72-1019)and also observed in the Bi4Ti3O12/CN composite,which corresponded to the(622)plane.In the XRD diffraction peaks of g-C3N4,the strong peak at 27.5°corresponded to the typical interplanar staking peak(002)of a conjugated aromatic system[32].However,the peak of g-C3N4was not observed in all composite samples,which could be attributed to relatively low content of g-C3N4in the composites.The peak diffraction of the g-C3N4could be covered up by the diffraction signals of Bi4Ti3O12.

Fig.1.XRD patterns of(a)g-C3N4,(b)Bi4Ti3O12,(c)Bi4Ti3O12/CN-5%,(d)Bi4Ti3O12/CN-10%and(e)B4T3O12/CN-15%.

The element composition and state of the Bi4Ti3O12/CN-10%were analyzed by XPS.As displayed in Fig.2(a),it can be found that the Bi,Ti,O,C and N elements existed in the XPS survey spectrum of the Bi4Ti3O12/CN-10%composites.The Bi4f peaks located at 158.7(Bi4f7/2)and 164.0 eV(Bi4f5/2)(Fig.2(b))can be identified as the characteristics of Bi3+.It can be seen in Fig.2(c),the high-resolution C1s spectrum was convoluted into three peaks located at 284.8,286.4 and 288.1 eV.The peak at 284.8 eV can be ascribed to adventitious carbon species,and peaks at 286.4 eV and 288.1 eV can be assigned to sp2hybridized carbon bonding covalently with N(C--N--C carbon and N--C=N carbon)[33-35].Furthermore,three peaks at 398.8 eV,399.7 and 401.1 eV were fitted in the high-resolution N1s spectrum(in Fig.2(d)),which related to three nitrogen species,as follows C=N--C,N--(C)3and C--N--H groups[36].The XPS result of C1s and N1s indicated the existence of tri-s-triazine(C6N7)units as the basic constituent block of g-C3N4and implied the successfully introduction of g-C3N4into the Bi4Ti3O12.

Fig.2.XPS survey(a),and high-resolution(b)Bi 4f,(c)C 1s,and(d)N 1s spectra of the Bi4Ti3O12/CN-10%.

3.2.Morphology

The morphological structure of Bi4Ti3O12/CN-10%was investigated by SEM,TEM and HRTEM.As shown in Fig.3(a),the Bi4Ti3O12/CN-10%sample exhibited relatively regular microsheet and nanosheet structure.It can be seen in Fig.3(b–d),the g-C3N4consisting of extremely small crystallites was stacked on the surface or closely contacted with the edge of Bi4Ti3O12sheets.This was the evidence to certify the existence of g-C3N4and Bi4Ti3O12with microsheet and nanosheet structure and also confirmed the composite with good crystallinity.

3.3.UV–vis diffuse reflectance

Fig.4 shows the UV–vis diffuse reflectance spectra of g-C3N4,Bi4Ti3O12and Bi4Ti3O12/CN-10%.It can be seen that the absorption edge of Bi4Ti3O12was about 399 nm,which corresponded to the band gap of 3.11 eV.The g-C3N4shows a distinct visible light absorption with an edge at about 465 nm,corresponding to the band gap of 2.67 eV.Comparing with pure Bi4Ti3O12,an obvious extended absorption in the visible region can be observed in the Bi4Ti3O12/CN-10%,which could be ascribed to the introduction of g-C3N4in the samples.

3.4.Photocatalytic performance

The photocatalytic properties of all samples were evaluated by the degradation of RhB under visible-light irradiation.Pure g-C3N4and Bi4Ti3O12were involved in photocatalytic degradation experiments for comparison.The change trends for RhBconcentration during the photocatalytic degradation reaction were displayed in Fig.5(a).During the dark period,a small amount of RhB was absorbed by different samples.In details,the adsorption rates were about 11.4%,18.1%and 11.6%for Bi4Ti3O12/CN-5%,Bi4Ti3O12/CN-10%and Bi4Ti3O12/CN-15%,respectively.However,the RhB degradation rate was greatly increased in the presence of the Bi4Ti3O12/CN samples,and nearly reached 100%after 120 min visible lightir radiation.Moreover,the Bi4Ti3O12/CN-10%exhibited the best efficiency compared with pure Bi4Ti3O12and g-C3N4.It could be attributed to the promotion of the heterojunction structure in the Bi4Ti3O12/CN 10%composite.And the photocatalytic degradation occurred by the Bi4Ti3O12/CN 10%sample was also better than the other composite samples,Bi4Ti3O12/CN 5%and Bi4Ti3O12/CN 15%.The result indicated that the photocatalytic performance of the Bi4Ti3O12/CN samples was highly dependent on the proper composition proportion of Bi4Ti3O12and g-C3N4.In addition,the better adsorption of Bi4Ti3O12/CN-10%sample for RhB was favorable for the improvement of dye photocatalytic degradation.The degradation process was further fitted by a first order reaction with formula=kt[37],where C0and C are the original concentration and concentration at time t,respectively,and the slope k is the kinetic rate constant.Fig.5(b)shows the corresponding reaction rate constants for pure g-C3N4,Bi4Ti3O12and Bi4Ti3O12/C3N4.The reaction rate constant k of the Bi4Ti3O12/CN-10%is approximately 2 times higher than that of pure Bi4Ti3O12and g-C3N4.Hence,the Bi4Ti3O12/CN-10%indeed showed excellent photocatalytic activity among all the samples.

Fig.3.SEM image(a),TEM images(b)–(c)and HRTEM image(d)of Bi4Ti3O12/CN-10%.

Fig.4.UV–vis diffuse reflectance spectra of g-C3N4,Bi4Ti3O12/CN-5%,Bi4Ti3O12/CN-10%,Bi4Ti3O12/CN-15%and Bi4Ti3O12.

The UV–vis spectral change trend for the RhB photocatalytic degradation by the Bi4Ti3O12/CN-10%with irradiation time increased is shown in Fig.5(c).It can be seen that the characteristic adsorption peak of RhB gradually decreased with increasing degradation time accompanied with the blue shifting of the maximum adsorption wavelength from 554 nm to 498 nm.The phenomenon is caused by the occurrence of de-ethylation in the initial photocatalytic oxidation for the RhB.It also suggested that de-ethylation played a predominant role in the whole degradation process compared to cleavage of chromophore ring structures[38].Finally,the absorption peak becomes smooth and flat,implying the nearly complete decomposition of RhB in suspensions.

Durability and stability are important properties for photocatalytic application.The reusability of the Bi4Ti3O12/CN-10%was investigated by RhB photocatalytic degradation for three cycles.As shown in Fig.5(d),there was no significant deactivation of the Bi4Ti3O12/CN-10%sample within the following three cycles,which indicated the excellent catalytic stability.

Fig.5.Photocatalytic degradation of RhB over pure g-C3N4,Bi4Ti3O12 and Bi4Ti3O12/C3N4-10%composites under visible light irradiation(a)and their corresponding reaction rate constant k(b).UV–vis spectral changes of RhB solution during the photocatalytic degradation by the Bi4Ti3O12/CN-10%at different irradiation time(c).The durability test of the Bi4Ti3O12/CN-10%for the degradation of RhB under visible light irradiation(d).

3.5.FT-IR analysis

The structures of g-C3N4,Bi4Ti3O12and Bi4Ti3O12/CN-10%and recycle Bi4Ti3O12/CN-10%were characterized by FT-IR spectrum and shown in Fig.6.For pure g-C3N4,the peak at 808 cm-1is mainly because of the existence of s-triazine ring modes[39,40].The waves at 1231 cm-1,1328 cm-1and 1407 cm-1corresponded to C--N bond stretching vibration by sp3hybridization[41-43].Moreover,peaks appeared from 3000 to 3300 cm-1may be contributed to stretching vibration by residual amino radicals,from melamine.In addition,it could also reflecthydroxy in absorbed water[44].Next,it was evidently that two distinct peaks located in 582 and 833 cm-1were discovered in both pure Bi4Ti3O12and Bi4Ti3O12/CN-10%[45,46].The former peak represents the vibration conducted by Bi--O bands[45]and the latter could be ascribed to Ti--O bands from the Bi4Ti3O12samples[46].Moreover,an interesting phenomenon was found that the peak around 830 cm-1showed some extent broaden for the Bi4Ti3O12/CN-10%,which was caused by the coexistence of Ti--O bands from Bi4Ti3O12and s-triazine ring modes fromg-C3N4.Accordingly,the mostcharacteristic peaks of Bi4Ti3O12and g-C3N4can be observed in the Bi4Ti3O12/CN-10%sample,which further suggesting the co-existence of two components.In addition,it is worth to note that after three cycled for RhB degradation,the FT-IR spectra of the recycled Bi4Ti3O12/CN-10%sample showed little changed comparing to the original sample.The result further implied that the prepared Bi4Ti3O12/CN-10%had a good stability as a photocatalysts.

Fig.6.FT-IR spectra of g-C3N4(a),Bi4Ti3O12(b),Bi4Ti3O12/CN-10%(c)and recycled Bi4Ti3O12/CN-10%(d).

3.6.Photoluminescence spectra analysis

The separation ability of holes and electrons was detected by photoluminescence spectrum(PL spectra).PL spectra of the semiconductor are available to analyze the photogenerated electron–hole pair recombination,transportation,and transition process.Under light irradiation,a semiconductor is excited to generate photo-excited electrons and holes.Fluorescence emission was formed by the energy released from recombination of electrons and holes.According to the above discussion,the higher fluorescence emission intensity always implies the more recombination of electron–hole pairs[47-49].It can be seen in Fig.7,the fluorescence emission relative intensity of the Bi4Ti3O12/CN-10%was much lower than that of pure Bi4Ti3O12and g-C3N4,which indicated the significantly better separation ability of photogenerated electron–hole pairs in Bi4Ti3O12/CN-10%and might be higher photocatalytic degradation efficiency.

Fig.7.PL spectra of g-C3N4,Bi4Ti3O12,Bi4Ti3O12/CN-10%.

3.7.Radical effect

In photocatalytic proc ess,oxydic species,such as superoxide radical(?O-2),hydroxyl radical(?OH)and hole(h+)play an important role in the photocatalytic activity[50-52].To further check into the production of active species,different scavengers were added.As blank control,none of the scavengers was added to reactants.1,4-Benzoquinone is an effective scavenger for trapping superoxide radical(?O-2)[50].EDTA-2Na has been found to have a good capture ability for holes(h+)[53],and tert-butyl alcohol was used to remove hydroxyl radical(?OH)[50].It can be seen from Fig.8(a),the photocatalytic activity for the degradation of RhB was largely decreased by the addition of 1,4-benzoquinone(1 mmol·L-1),EDTA-2Na(1 mmol·L-1)or tert-butyl alcohol(1 mmol·L-1).After 120 min irradiation,the removal rate of RhB follows the order by adding different scavengers:EDTA-2Na(65.2%)＜1,4-benzoquinone(68%)＜tert-butyl alcohol(90.9%).The reaction rate constants k for RhB degradation by adding those scavengers were also fitted by the pseudo- first-order kinesis.The result(Fig.8(b))showed that the rate constant k of RhB degradation determined in the presence of EDTA-2Na and 1,4-benzoquinone was much lower than the one obtained by adding tert-butyl alcohol,which indicated that the holes(h+)and superoxide radical(?O-2)could be the main oxidative species.

3.8.Mechanisms

The proposed electron–hole pair separation in the Bi4Ti3O12/g-C3N4composites under visible irradiation was shown in Fig.9.According to the previous reports,g-C3N4is a p type semiconductor and Bi4Ti3O12is an n type semiconductor[51].Therefore,the Bi4Ti3O12/g-C3N4could form a typical p–n heterostructure[56].Due to the formation of a p–n heterostructure,the conduction band(CB)and valence band(VB)positions of two components were shifted.In detail,the Fermi level of g-C3N4(p-type semiconductor)might be far from its VB,meanwhile,the Fermi level of Bi4Ti3O12(n-type semiconductor)might move away from its CB,separately.In this process,with the help of an internal electric field,photo-excited e-from CB of g-C3N4would move to CB of Bi4Ti3O12at the high-energy states.At the sametime,h+might transform to the VB of g-C3N4in low-energy states.Therefore,the separation efficiency of h+and photo-excited e-could be improved in the Bi4Ti3O12/g-C3N4composites.

In addition,some radicals were involved in the degradation reaction.In order to further investigate the mechanisms and process of degrade reaction,the Monk formula was used to calculate the band positions of Bi4Ti3O12/CN-10%[58].Based on the result of UV–vis diffuse reflectance absorption spectra,the band gaps of g-C3N4and Bi4Ti3O12are 2.67 eV and 3.11 eV.Moreover,the CB positions of two components were calculated to be-1.12 eV and-0.19 eV and the VB positions were calculated to be 1.55 eV and 2.92 eV for g-C3N4and Bi4Ti3O12,respectively.According to the above result,the e-concentrated on CB of Bi4Ti3O12was easily reacted by O2to produce?O2-(-0.05 eV vs.NHE)[36]in the surface of Bi4Ti3O12/g-C3N4composites because the potential of g-C3N4CB was negative enough.It suggested that Bi4Ti3O12was a good reductant in?O2-production.The result was well agreed with the above trapping experiment(as shown in Fig.7).Additionally,compared to the potential of?OH/H2O(2.27 eV vs.NHE)[59]or?OH/OH-(1.99 eV vs.NHE)[60],it is reasonable that holes were mainly involved in reaction directly,because the potential of g-C3N4VB(1.55 eV)was less positive to generate?OH.Thus,this result further confirmed h+and?O2-played the dominating roles in the RhB photocatalytic reaction over the Bi4Ti3O12/g-C3N4composites.

Fig.9.Proposed electron–hole pair separation in the Bi4Ti3O12/g-C3N4 composites under visible irradiation.

4.Conclusions

In summary,a novel Bi4Ti3O12/g-C3N4heterojunction was synthesized successfully by the ultrasonic-assisted method.According to the XRD results,g-C3N4and Bi4Ti3O12coexisted in the composite,and the XPS analysis indicated the existence of tri-s-triazine(C6N7)units as the basic constituent block of g-C3N4and the introduction of g-C3N4into the Bi4Ti3O12.From the SEM and TEM,microsheets Bi4Ti3O12stacked with nanosheets g-C3N4in the Bi4Ti3O12/g-C3N4samples,which further confirmed the contacted composite structure.According to photocatalytic experiments,the Bi4Ti3O12/CN-10%(0.06404 h-1in degradation of RhB)possessed better photocatalytic degradation for the RhB solution compared with pure Bi4Ti3O12and g-C3N4.The photocatalytic efficiency of Bi4Ti3O12/CN-10%was approximately 2 times,compared with pure g-C3N4and Bi4Ti3O12.The recycling experiments for the Bi4Ti3O12/CN-10%indicated its good stability and durability,and the result of FT-IR not only suggested the Bi--O,Ti--O and C--N bonds existed in the samples,but also indicated its good stability after 4 recycles.Moreover,the PL spectrum result showed the Bi4Ti3O12/CN-10%possessed better separation property in photo-excited electrons(e-)and holes(h+)than pure Bi4Ti3O12and g-C3N4.Based on the calculation and trapping experiment,the holes(h+)and superoxide radical(?O2-)could be the main oxidative species in photodegradation over the Bi4Ti3O12/CN-10%composites.