鋅摻雜OV-β-Bi2 O3可見光催化活性的提高：能帶結(jié)構(gòu)的調(diào)節(jié)和電荷分離的促進(jìn)

趙亞雯 張俊可 馮彩霞＊, 徐佳鵬 杜 芮虎 華 周艷梅＊, 劉山虎 劉艷紅

(1河南大學(xué)化學(xué)化工學(xué)院，河南省環(huán)境污染控制材料國際聯(lián)合實(shí)驗(yàn)室，開封 475004)

(2河南省開封生態(tài)環(huán)境監(jiān)測中心，開封 475003)

(3中國科學(xué)院理化技術(shù)研究所，北京 100190)

0 Introduction

Over the past decades,visible-light-active Bi based compounds are recommended as a category of the hottest photocatalysts,which is because Bi3+ion in the crystal lattice exhibits uniqued10configuration and 6s2lone pair electrons.Such an electronic structure contributes to promoting the transfer of photogenerated carriers and the regulation of band gap by the orbital hybridization of Bi3+with other ions[1-2].Among various Bi based photocatalysts,Bi2O3is the simplest and foremost one,thanks to its innocuity,adjustable band gap and high oxidation power of valence band(VB)hole[3-5].Generally,Bi2O3has five different polymorphs,i.e.,monoclinicα,tetragonalβ,body-centered cubicγ,face-centered cubicδand triclinicω-phases[6-9].Among them,β-Bi2O3usually exhibits the best visible-light activity for its smaller band gap and unique tunnel structure[6].However,β-Bi2O3still doesn′t have sufficientefficiency for the photocatalytic application due to its fast recombination of light-induced hole and electron.So,many researchers have tried to adopt various modification methods to improve the photocatalytic performance ofβ-Bi2O3under visible light irradiation,such as doping transition metal ions[9-11]and constructing heterojunction semiconductor[12-14].Impurity doping inβ-Bi2O3has been confirmed to be an effective strategy to improve its visible light activity.Luo et al.found that Gd-dopedβ-Bi2O3showed much higher photocatalytic activity for the photodegradation of phenol than pureβ-Bi2O3,since the doped Gd3+ion serves as an efficient scavenger to entrap photo-generated electrons thereby effectively separating photo-induced electron-hole pairs[9].Liang et al.reported that Fe-dopedβ-Bi2O3photocatalyst with 4% molar fraction of Fe displayed desired photocatalytic activity,because Fe doping provides the photocatalyst with the ability to promote the migration of electron-hole pairs,thereby adding to photocatalytic activity[10].Besides doped ions,oxygen vacancy(OV)often was introduced into semiconductor to improve the light absorption and charge separation ability of catalyst[15-17].Lu et al.pointed out that the construction of oxygen vacancies on the surfaces of photocatalyst can improve the visible light absorption,target air pollutants enrichment and electron mobility[16].However,most of the OV-modified photocatalysts are synthesized under a harsh condition such as in a hydrogen or nitrogen atmosphere or under vacuum deoxidation condition[16,19-20].The OV produced by this method is unstable and easy to be filled by oxygen in air atmosphere.Therefore,it is very essential for the development of a facial and controllable method to generate stable oxygen defect photocatalysts.

In this work,we prepared Zn-doped OV-β-Bi2O3nanoparticles(OV-Zn∶Bi2O3)with two crystal defects,OV and doped Zn2+,via a sol-gel method followed byin-situcarbon thermal reduction treatment.The OV in OV-Zn∶Bi2O3samples is stable and controllable by regulating the content of doped Zn2+.As a reference,traditionalβ-Bi2O3without OV and OV-β-Bi2O3having OV but without doped Zn2+were also synthesized.Various characterization methods were adopted to investigate the effect of OV and doped Zn2+on the improvement of photocatalytic activity of OV-Zn∶Bi2O3series.

1 Experimental

1.1 Preparation of the catalysts

OV-Zn∶Bi2O3catalyst was synthesized via a simple sol-gel method followed byin-situcarbon thermal reduction treatment.Typically,0.072 mol of citric acid monohydrate and 0.015 mol of Bi(NO3)3·5H2O were dispersed in 30 mL of distilled water and ultrasonicated for 10 min to form a transparent sol.The as-obtained sol was stirred vigorously for 30 min while the ammonia solution of disodium ethylenediaminetetra acetate(EDTA)was slowly added to afford a new sol denoted as sol A.1.5 mmol of Zn(CH3COO)2·2H2O was dissolved in 5 mL of ammonia and stirred to form sol B.The as-obtained sol B was dropped into sol A under stirring with a water-bath(80℃)for 1 h to afford sol C.After being cooled to room temperature,2 mL of sol C was carbonized in an electric furnace to obtain a viscous black product at 300℃.The viscous black product was further calcined at 400℃for 2 h to yield a yellow powder,OV-Zn∶Bi2O3-0.1 with a theoretical molar ratio(nZn/nBi)of 0.1.A series of Zn-doped OV-β-Bi2O3phtocatalysts with theoreticalnZn/nBiof 0.3,0.5 and 1.0 were prepared in the same manners while the amount of Zn(CH3COO)2·2H2O was adjusted to be 4.5,7.5 and 15.0 mmol.Corresponding products were labeled as OV-Zn∶Bi2O3-0.3,OV-Zn∶Bi2O3-0.5 and OV-Zn∶Bi2O3-1.0,respectively.Pure OV-β-Bi2O3powder with abundant oxygen vacancies but without doped Zn2+was also prepared with the same procedure in the absence of Zn(CH3COO)2·2H2O.

For comparison,traditionalβ-Bi2O3powder without OV was prepared by the precipitation method[21-22].Briefly,2 g of Bi(NO3)3·5H2O and 0.1 g of CTAB(cetyltrimethyl ammonium bromide)were both dissolved in 20 mL of nitric acid(1 mol·L-1)solution with stirring for 20 min.Then,0.4 g of oxalic acid was added in above solution and reacted under stirring for 30 min.The white precipitate was filtered,washed and calcined at 270℃to obtain the yellow powder,traditionalβ-Bi2O3.

1.2 Characterization

A JSM-7610F scanning electron microscope(SEM)was performed with an accelerating voltage of 15 kV to observe the morphology and microstructure of as-prepared traditionalβ-Bi2O3,OV-β-Bi2O3and OV-Zn∶Bi2O3series.The crystal structure of the catalysts was characterized using a Bruker D8 X-ray powder diffractometer equipped with CuKαradiation source(λ=0.154 8 nm);the tube current was 40 mA,operating voltage was 40 kV,and the scan range was 5°≤2θ≤70°.A Thermo Fisher Scientific Escalab 250Xi X-ray photoelectron spectrometer(XPS,monochromatic AlKαexcitation source)was deployed to analyze the chemical states of as-prepared photocatalysts.The ultravioletvisible light diffuse reflectance spectra(UV-Vis DRS)of as-prepared photocatalysts were recorded with a PE Lambda 950 spectrometer equipped with an integrating sphere attachment.Electron spin resonance(ESR)spectra were obtained with a Bruker E500 apparatus.The measurement was conducted at room temperature in ambient air,without vacuum-pumping.Thegtensors of the ESR signals were obtained by settinggof diphenyl picryl hydrazyl(DPPH,g=2.003 6)as the reference.The photoluminescence(PL)spectra of the products were obtained with a PuXi TU-1900 fluorescence spectrophotometer under the excitation at 380 nm.The photocurrent response and voltammogram(IV)curve were measured at CHI 650E electrochemical workstation(Chenhua Instrument Company)with a three-electrode system including a modified ITO glass as working electrode,a platinum wire as counter electrode and an Ag/AgCl electrode as reference.A 300 W xenon lamp with a 420 nm cutoff filter was used as a visible light source.

1.3 Evaluation of visible lightphotocatalytic activity

The photocatalytic activity of as-prepared photocatalysts were evaluated by the degradation of methylene blue(MB)and 2,4,6-trichlorophenol(2,4,6-TCP)in a glass cylindrical reactor under visible light irradiation(300 W xenon lamp equipped with a 420 nm cutoff filter).The reactor was surrounded by a water-cooling system so as to ensure that the photocatalytic reaction occurs at room temperature.In each experiment,100 mg of the to-be-tested photocatalysts was suspended in 150 mL MB or 2,4,6-TCP solution,which concentrations were all 10 mg·L-1.Differently,the light intensity for the degradation of MB was 23 mW·cm-2and that for 2,4,6-TCP was 45 mW·cm-2.Prior to illumination,the suspension was fiercely stirred in dark for 1 h until the absorption-desorption balance was achieved.During the photocatalytic reaction,5 mL of the suspension was taken at certain time intervals to determine the concentration of MB and 2,4,6-TCP by UV-Vis spectrophotometry (TU-1810,PuxiInstrumentCompany,China)at the maximum absorption wavelength of 665 nm and 310 nm,respectively.The degradation rate of pollutant was calculated as(c0-c)/c0×100%,wherec0refers to the initial concentration of pollutant after the adsorption-desorption equilibrium was reached,andcis the concentration of pollutant measured at a certain time interval during the photocatalytic reaction process.

2 Results and discussion

2.1 Crystal structure

Scheme 1 displays the schematic diagram for preparation process of traditionalβ-Bi2O3,OV-β-Bi2O3and OV-Zn∶Bi2O3series.For OV-β-Bi2O3and OV-Zn∶Bi2O3,the critical step marked by red circle in diagram is the carbonization of EDTA-Bi complex on an electric stove.The reductive carbon produced in the carbonization process can reduce Bi3+in the subsequent calcination process,resulting in the escaping of some oxygen atoms inβ-Bi2O3or Zn-doped Bi2O3and leading to the formation of OV-β-Bi2O3and OV-Zn∶Bi2O3.

Scheme 1 Schematic illustration of preparation for traditional β-Bi2O3(a),OV-β-Bi2O3(b)and OV-Zn∶Bi2O3 series(c)

Fig.1a depicts the XRD patterns of traditionalβ-Bi2O3,OV-β-Bi2O3and OV-Zn∶Bi2O3series with differentnZn/nBi.The diffraction peaks ofβ-Bi2O3,OV-β-Bi2O3and OV-Zn∶Bi2O3series at 2θ=27.94°,31.76°,32.69°,46.21°,46.90°,54.26°,55.48°and 57.75°are indexed to the(201),(002),(220),(222),(400),(203),(421)and(402)crystal planes of tetragonalβ-Bi2O3(PDF No.27-0050)[10-11],indicating that all as-prepared catalysts possess the same crystal structure.OV-Zn∶Bi2O3catalysts with anZn/nBiof 0.1～1.0,i.e.,OV-Zn∶Bi2O3-0.1,OV-Zn∶Bi2O3-0.3,OV-Zn∶Bi2O3-0.5 and OV-Zn∶Bi2O3-1.0,showed the same XRD patterns as traditionalβ-Bi2O3and OV-β-Bi2O3did.And all the OV-Zn∶Bi2O3photocatalysts did not show diffraction peaks of zinc oxide.Moreover,the enlarged XRD patterns shown in Fig.1b demonstrate that the(201)peak of the OV-Zn∶Bi2O3photocatalysts tended to slightly shift towards lower 2θvalue with the increase of zinc concentration.This means that as-prepared OV-Zn∶Bi2O3photocatalysts do not consist of the mixtures of Bi2O3and ZnO phases but Zn-doped tetragonal Bi2O3.It is generally recognized that there are two modes of ion doping,substitutional doping and interstitial doping.The ionic radii of Bi3+ion and Zn2+ion are 0.103 and 0.074 nm,respectively[23].On one hand,if the lattice sites of Bi3+in Bi2O3crystal are partly substituted by smaller Zn2+ion,the lattice parameters of Bi2O3would decrease in association with the slight shift of the XRD peaks towards higher 2θvalues.This supposition,however,is not supported by the above-mentioned XRD data.On the other hand,if some Zn2+ions are doped in Bi2O3crystal by way of interstitial doping,the volume of Bi2O3crystal cell would be expanded and the distance of the crystal plane(dspacing)would be increased.This supposition,indeed,is supported by relevant XRD data.Namely,according to Bragg′s law,2dsinθ=nλ,the decrease ofθrefers to the increase ofd;and the volume expansion of the OV-β-Bi2O3crystal cell after Zn doping proves that Zn2+ion is doped into OV-β-Bi2O3crystal via interstitial doping to afford the OV-Zn∶Bi2O3photocatalysts inthiswork.

Fig.1 (a)XRD patterns of as-prepared traditional β-Bi2O3,OV-β-Bi2O3 and OV-Zn∶Bi2O3 series with different nZn/nBi;(b)Enlarged(201)peak in the 2θ range of 26°～30°for OV-β-Bi2O3 and OV-Zn∶Bi2O3series

2.2 Morphology observation

Fig.2 shows the SEM images of traditionalβ-Bi2O3,OV-β-Bi2O3and OV-Zn∶Bi2O3series with different Zn content.It can be seen that both OV-β-Bi2O3and OV-Zn∶Bi2O3series exhibited similar spherical shape and had an average size of 100 nm(Fig.2a～2e).This indicates that the doping of zinc has little effect on the morphology of OV-β-Bi2O3.Compared to OV-β-Bi2O3,traditionalβ-Bi2O3prepared by the precipitation method consisted of many nano-sheets with thickness of about 10 nm(Fig.2f),which was consistent with that reported in the literature[21]and might be the reason for much larger specific surface area of 10 m2·g-1than that of OV-β-Bi2O3with a value of 2 m2·g-1.

Fig.2 SEM images of pure OV-β-Bi2O3(a),OV-Zn∶Bi2O3-0.1(b),OV-Zn∶Bi2O3-0.3(c),OV-Zn∶Bi2O3-0.5(d),OV-Zn∶Bi2O3-1.0(e)and traditional β-Bi2O3(f)

2.3 XPS analysis

The chemical states of related elements in asprepared samples were analyzed by XPS.Fig.3a shows the high-resolution XPS spectra of Bi4fin traditionalβ-Bi2O3,OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3.Compared to traditionalβ-Bi2O3,the Bi4fpeaks of OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3 shifted to the lower binding energies,indicating that the Bi atom in both samples with OV has higher electron density(or lower chemical valence)than that inβ-Bi2O3due to the charge compensation.The absence of partial oxygen atoms can reduce the coordination number of Bismuth ion leading to the decrease of binding energy of Bi4f.The Zn2p3/2and Zn2p1/2peaks of OV-Zn∶Bi2O3-0.3 were centered at 1 022.9 and 1 046.1 eV(Fig.3b),which means that the doped Zn is present in the form of Zn2+in Zn-dopedβ-Bi2O3[24].Besides,as shown in Fig.3c,the O1sspectrum of traditionalβ-Bi2O3was split into two peaks,ⅠandⅡ:peakⅠat 530.3 eV is assigned to the lattice oxygen inβ-Bi2O3crystal and peak Ⅱ at 531.4 eV is ascribed to adsorbed oxygen in the form of hydroxyl group[7,15,25-26].Different with traditionalβ-Bi2O3,a new peak Ⅲ appeared for OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3,which was situated at about 532.6 eV corresponding to the OV defect[7,27].

Fig.3 High-resolution XPS spectra of Bi4f(a),Zn2p(b)and O1s(c)for traditional β-Bi2O3,OV-β-Bi2O3 and OV-Zn∶Bi2O3-0.3 samples

2.4 ESR analysis

ESR test was adopted to confirm the existence of oxygen vacancies in OV-β-Bi2O3and OV-Zn∶Bi2O3series as a direct and sensitive way.As shown in Fig.4,no ESR signal could be observed for traditionalβ-Bi2O3.While there was a strong symmetric ESR peak for OV-β-Bi2O3and OV-Zn∶Bi2O3series both having a center atg=2.004,which isattributed to single-electron-trapped OV(VO·)with a positive charge[28-29].For OVZn∶Bi2O3series,XRD results suggested that Zn2+ion is doped into OV-β-Bi2O3crystal via interstitial doping not substitution,which leads to more positive charge in semiconductor.So,with the increase of the doped Zn2+content,the concentration of VO·decreased and the intensity of ESR signal decreased gradually.This demonstrats that the concentration of OV may be modulated by the amount of doped Zn2+in OV-Zn∶Bi2O3series.Many studies reported that,OV can not only broaden the photoabsorption range of the catalyst but also serve as trapping center for excited electrons to improve the separation efficiency of photogenerated carriers[16,30].To study the trapping role of OV,many ESR spectra under laser irradiation of different catalysts will be further compared.

Fig.4 ESR spectra of traditional β-Bi2O3,OV-β-Bi2O3 and OV-Zn∶Bi2O3 series at room temperature in air

2.5 Optical properties of as-prepared samples

It is well known that both OV and doped ions can change the optical absorption and electronic energy structure of semiconductor,which can be determined by UV-Vis DRS.Fig.5a shows the UV-Vis DRS spectra of traditionalβ-Bi2O3,OV-β-Bi2O3and OV-Zn∶Bi2O3samples with differentnZn/nBi.The absorption edge of traditionalβ-Bi2O3was approximately 460 nm.The absorption range of OV-β-Bi2O3was clearly broadened,with the absorption edge extending to approximately 500 nm.For OV-β-Bi2O3,whichhas the wider light absorption range,more electron-hole pairs can be generated to participate in photocatalytic reaction under visible light irradiation.Compared with OV-β-Bi2O3,the absorption edge of OV-Zn∶Bi2O3samples gradually blue-shifted to shorter wavelength with the increase of Zn content.However,the absorption range of nearly all OV-Zn∶Bi2O3samples,except OV-Zn∶Bi2O3-1.0,was still wider than that of traditionalβ-Bi2O3.The band gap can be estimated using the following equation(Eq.1):

Whereα,ν,AandEgare absorption coefficient,light frequency,a constant and band gap energy,respectively.The value ofnis 1 for direct transition and 4 for indirect transition of semiconductor.According to the literature[9],the value ofnis 1 forβ-Bi2O3.The band gap energies of traditionalβ-Bi2O3,OV-β-Bi2O3,OV-Zn∶Bi2O3-0.1,OV-Zn∶Bi2O3-0.3,OV-Zn∶Bi2O3-0.5 and OV-Zn∶Bi2O3-1.0 were estimated from a plot of(αhν)2vshνto be 2.72,2.40,2.46,2.50,2.53 and 2.80 eV,respectively(Fig.5b).

Fig.5 UV-Vis DRS spectra(a)and plots of(αhν)2 vs photon energy(b)of traditional β-Bi2O3,OV-β-Bi2O3 and OV-Zn∶Bi2O3 series

To further investigate the effect of Zn doping on the VB position of OV-Zn∶Bi2O3,we adopted Eq.2 to determine the VB edge position of pure OV-β-Bi2O3and OV-Zn∶Bi2O3series[31-32].

WhereEeis about 4.5 eV on the hydrogen scale;Egis the band gap of OV-β-Bi2O3and OV-Zn∶Bi2O3;EVBis the valence band edge potential;andXis the absolute electronegativity of the solid(theXvalue for Bi2O3was calculated to be 6.24,Supporting information).Simultaneously,the conduction band(CB)position of the samples can be obtained fromECB=EVB-Eg.As shown in Table 1,compared to OV-β-Bi2O3,the VB position of OV-Zn∶Bi2O3samples gradually moved down with the increase ofnZn/nBi,which suggests that the oxidizing capacity of the photoexcited holes tends to rise with the increase of Zn content.Furthermore,the VB spectra of traditionalβ-Bi2O3,OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3 as the representative of OV-Zn∶Bi2O3series were measured by XPS.As shown in Fig.6,the VB potentials of OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3 were lower than that of traditionalβ-Bi2O3for the narrow band gap due to the existence of OV,and the VB potential of OV-Zn∶Bi2O3-0.3 was higher than that of OV-β-Bi2O3,which may imply that the photo-induced holes of OV-Zn∶Bi2O3-0.3 possesses stronger oxidizing ability than that of OV-β-Bi2O3.These results are well consistent with above DRS analysis.

Table 1 Values of Eg,EVB and ECB for traditional β-Bi2O3,OV-β-Bi2O3 and OV-Zn∶Bi2O3 series

Fig.6 VB XPS spectra of traditional β-Bi2O3(a),OV-β-Bi2O3(b)and OV-Zn∶Bi2O3-0.3(c)as the representative of OV-Zn∶Bi2O3 series

2.6 Visible light photocatalytic activity

Different pollutants including MB dye and colorless phenol compound,2,4,6-TCP,were employed as the substrate to evaluate the photocatalytic activitiy of traditionalβ-Bi2O3,OV-β-Bi2O3and OV-Zn∶Bi2O3samples under visible light irradiation.Fig.7a and 7b shows the photocatalytic performance of different catalysts toward MB dye degradation with the light intensity of 23 mW·cm-2(λ=420 nm).It can be seen that,in the absence of any photocatalyst,nearly 80% of MB was retained after 3 h of visible light irradiation.In the presence of traditionalβ-Bi2O3and OV-β-Bi2O3,about 40% and 70% of MB was degraded after 4 h of visible light irradiation,respectively,which indicates that,although the specific surface area of OV-β-Bi2O3was smaller than that of traditionalβ-Bi2O3,the former promotes much higher degradation rate of MB than the latter for the wider light absorption range resulted from OV.Furthermore,OV-Zn∶Bi2O3catalysts with differentnZn/nBican more efficiently catalyze the degradation of MB under visible light irradiation as compared with OV-β-Bi2O3;and in particular,OV-Zn∶Bi2O3-0.3,with a photocatalytic efficiency similar to that of OV-Zn∶Bi2O3-0.5,can catalyze the photodegradation of up to 91% of MB within 4 h of visible light irradiation.The linear relationship between ln(c0/c)and irradiation time for MB degradation is shown in Fig.7b.The reaction rate constant(k)and linear correlation coefficient(R2)for different catalysts are summarized in Table 2.It can be seen that all as-prepared catalysts had anR2value of above 0.97,which means that the degradation of MB catalyzed byβ-Bi2O3,OV-β-Bi2O3and OV-Zn∶Bi2O3series is of the first order kinetic nature.Moreover,OVZn∶Bi2O3-0.3 had the highest rate constant among all the tested catalysts,which demonstrates that the optimalnZn/nBiof OV-Zn∶Bi2O3catalysts should be 0.3.

Table 2 Reaction rate constant(k)and correlation coefficient(R2)of traditional β-Bi2O3,OV-β-Bi2O3 and OV-Zn∶Bi2O3 series for photocatalytic degradation of MB under visible light irradiation

Fig.7 Photocatalytic degradation of MB(a)and linear fitting for first order kinetics(b)over different catalysts under visible light irradiation;Photocatalytic degradation of 2,4,6-TCP(c)and linear fitting for first order kinetics(d)over different catalysts under visible light irradiation

2,4,6-TCP,as a well-known organic pollutant in wastewater of pharmaceutical,paint and pesticide in-dustries,can cause severe nervous system and respiratory disease.Fig.7c shows the photocatalytic activity of traditionalβ-Bi2O3,OV-β-Bi2O3and OV-Zn∶Bi2O3samples toward the degradation of 2,4,6-TCP under visible light with the intensity of 45 mW·cm-2(λ=420 nm).It can be seen that,after irradiation for 100 min,about 51% and 71% of 2,4,6-TCP was degraded by OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3,respectively,while only 37% of 2,4,6-TCP was removed over traditionalβ-Bi2O3within 100 min.Mover,as illustrated in Fig.7d,the order of the reaction rate constant in the degradation of 2,4,6-TCP was,which was the same with that of the degradation of MB dye,confirming the optimalnZn/nBiof OV-Zn∶Bi2O3catalysts was 0.3.

The above systematic photocatalytic experiments revealed that,compared to traditionalβ-Bi2O3,both OV-β-Bi2O3and OV-Zn∶Bi2O3exhibited superior activity due to the introduction of OV.Moreover,the synergistic effect of OV and doped zinc ions can promote the photocatalytic activity of OV-Zn∶Bi2O3in the degradation of MB and 2,4,6-TCP under visible light.In terms of the practical applications of catalysts,the reusability and stability are two vital factors worth special emphasis.Therefore,cycling experiments of OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3 as the representative samples were carried out to evaluate their reusability for the degradation of MB under visible light irradiation.As shown in Fig.8,after 4 cycles running,the degradation rate of MB in the presence of OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3 decreased slightly.The XRD patterns of OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3 before and after four cycles of the photocatalytic degradation of MB are compared in Fig.9.It can be seen that,after four-cycle experiments,there was not any change in the crystal structure for both samples, which demonstrates that both OV-β-Bi2O3and OV-Zn∶Bi2O3possess strong structural stability during the photocatalytic degradation of MB.

Fig.8 Plots of c/c0versus irradiation time over OV-β-Bi2O3(a)and OV-Zn∶Bi2O3-0.3(c)as well as photodegradation efficiency of MB over OV-β-Bi2O3(b)and OV-Zn∶Bi2O3-0.3(d)in different recycling runs

Fig.9 XRD patterns of OV-Zn∶Bi2O3-0.3(a) and OV-β-Bi2O3(b)before and after four-cycling degradation of MB under visible light irradiation

2.7 Photocatalytic mechanism

To investigate the active species involved in the MB degradation over OV-Zn∶Bi2O3catalysts,we adopted triethanolamine(TEOA)andtert-butanol(TBA)as the radical scavengers for holes and hydroxyl radicals to conduct trapping experiments[33-35].And N2was also added to remove oxygen and prevent the formation of superoxide radicals[36].Fig.10 displays the photocatalytic activity of OV-Zn∶Bi2O3-0.3 sample for the photocatalytic degradation of MB in the presence of TEOA and TBA(their concentration in the photocatalytic system was 6 mmol·L-1).It can be seen that TEOA could strongly inhibit the photocatalytic activity of OV-Zn∶Bi2O3-0.3,which indicates that h+is the main active species for the degradation of MB.And after the introduction of TBA,the photodegradation of MB was obviously suppressed,which confirms that·OH also plays an important role in the photocatalytic degradation of MB.This is consistent with DRS results.As shown in Fig.11,although the electrons in the VB of OV-Zn∶Bi2O3-0.3 can be excited to the CB by visible light,the active·O2-radical cannot be generated,due to the lower potential of O2/·O2-(-0.33 eV(vs NHE))than that of CB potential of OV-Zn∶Bi2O3-0.3(0.49 eV(vs NHE),Table 1)[37-38].However,the holes in VB can oxidize OH-(or H2O)to generate·OH or directly oxidize MB,due to the higher VB potential of OV-Zn∶Bi2O3-0.3(2.99 eV(vs NHE),Table 1)than that of OH-/·OH(2.40 eV (vs NHE))and H2O/·OH (2.72 eV (vs NHE))[39-40].These results confirm that h+and·OH are the main active species in the photodegradation process of MB over OV-Zn∶Bi2O3-0.3 photocatalyst.

Fig.10 Effect of different scavengers on MB degradation process over OV-Zn∶Bi2O3-0.3 catalyst

Fig.11 Photocatalytic degradation mechanism of OV-β-Bi2O3 and OV-Zn∶Bi2O3 under visible light irradiation

In order to investigate the effect of OV and doped Zn2+on the improvement of photocatalytic activity,photoelectrochemical experiments were carried out to evaluate the charge carrier density and the photogenerated carrier separation efficiency of different catalysts.The density of charge carriers,which is an important parameter affecting the photocatalytic activity of catalyst,can be obtained by the following Nernst equation:

Herein,Ef1andEf2is the quasi-Fermi level of sample 1 and sample 2,respectively.Nf1andNf2is the carrier density of two samples.In addition,k,Tandeare the Boltzmann constant(1.381×10-23J·K-1),temperature and elementary charge(1.602×10-19C).With the addition of methylviologen dichloride(MVCl2)serving as the fast electron acceptor,the photocurrent onset potential in a voltammogram can be considered as the quasi-Fermi level of majority carriers[15,41-43].As shown in Fig.12a,the photocurrent onset potentials of OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3 were-0.37 and-0.41 V,both of which were higher than that of traditionalβ-Bi2O3with a value of-0.45 V.According to above Nernst equation,the carrier densities of OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3 were determined as 22.5 and 4.7 times larger than that of traditionalβ-Bi2O3,respectively.This suggests that the existence of OV can largely increase the charge carrier density of semiconductor by expanding the light absorption range of catalyst.Compared to OV-β-Bi2O3,although the carrier density of OV-Zn∶Bi2O3-0.3 was reduced for the wider band gap,the oxidation ability of photogenerated holes was enhanced and the separation efficiency of photo-generated carriers might be increased for the electron trapping role of the doped Zn2+.The transient photocurrent responses of different catalysts under visible light are shown in Fig.12b.It can be seen that,the photocurrent densities of both OV-β-Bi2O3and OV-Zn∶Bi2O3-0.3 were much larger than that of traditionalβ-Bi2O3.It demonstrates that the efficient generation and separation of photoinduced electron-hole pairs of both OV-β-Bi2O3and OVZn∶Bi2O3arise from the OV.Moreover,the photocurrent density of OV-Zn∶Bi2O3-0.3 was larger than that of OV-β-Bi2O3,suggesting that the doped Zn2+may also play an important role in promoting the separation efficiency for OV-Zn∶Bi2O3series.Fig.12c shows the Zn2pXPS spectra of OV-Zn∶Bi2O3-0.3 before and after MB degradation reaction.It can be seen that,after reaction,the binding energies of Zn2p1/2and Zn2p3/2shifted to lower energy,indicating that parts of doped Zn2+was reduced by trapped electron to a new chemical state with lower valence.This further confirms that moderate doped Zn2+can also serve as an electron trapping center to promote the separation of photogenerated carriers.The PL spectra of catalysts can also reflect the separation efficiency of photogenerated electron-hole pairs in semiconductor.It is well known that a stronger PL intensity represents lower separation efficiency and higher recombination rate of the photogenerated charge carriers.Fig.12d shows the PL spectra of different samples at the excitation wavelength of 380 nm.It can be seen thatindicating thatEOV-Zn∶Bi2O3-0.3(separation efficiency of OV-Zn∶Bi2O3-0.3)＞EOV-Zn∶Bi2O3-1.0＞EOV-β-Bi2O3＞Eβ-Bi2O3,which was consistent with transient photocurrent results of catalysts.All the above data suggest that OV and appropriate doped Zn2+can promote the separation efficiency of photogenerated carriers,however,excessive zinc may also become the recombination center,reducing the catalytic activity of OV-Zn∶Bi2O3series.

Fig.12 (a)I-V curves of traditional β-Bi2O3,OV-β-Bi2O3 and OV-Zn∶Bi2O3-0.3;(b)Photocurrent response of traditional β-Bi2O3,OV-β-Bi2O3,OV-Zn∶Bi2O3-0.3 and OV-Zn∶Bi2O3-1.0 under visible light(λ＞420 nm);(c)Comparison of Zn2p XPS spectra of OV-Zn∶Bi2O3-0.3 before and after reaction;(d)PL spectra of β-Bi2O3,OV-β-Bi2O3,OV-Zn∶Bi2O3-0.3 and OV-Zn∶Bi2O3-1.0

To further study the role of OV in promoting the separation efficiency of photogenerated carriers,the ESR spectra were recorded for OV-Zn∶Bi2O3series under laser irradiation(λ=420 nm)in air.Fig.S1(Supporting information)are the ESR spectra of OV-Zn∶Bi2O3series,where the dashed and solid lines correspond to the signals before and after laser irradiation,respectively.It can be seen that,the ESR signal intensity of VO·in all catalysts having OV could be increased under visible light irradiation,indicating that the OVs in OV-Zn∶Bi2O3series prepared in this work are very stable and there are more Vo·produced under visible light.It is well known that,besides VO·,there are two other kinds of OV,two-electron-trapped OV(effective charge=0,denoted as VO×)and no-electrontrapped OV(effective charge=+2,denoted as VO··)[27,29],both of which do not contribute to ESR signals because of the absence of unpaired electrons.Under visible light irradiation,the electrons can be photoexcited to conduction band,leaving the holes in VB.If there are trapping electron centers,i.e.,VO··or trapping hole centers VO×on the surface of catalyst,the electron of the conduction band may be trapped by surface VO··and the hole of the VB may be captured by surface VO×.Both kinds of OVs transformed into VO·and the signal intensity became stronger accordingly(Fig.11).The separation efficiency of photogenerated carriers will be also improved.In other words,Δh(Δh=hlight-hdark,hlightandhdarkrefer to the intensities of ESR signal atg=2.004 under visible light and in dark,respectively)is closely related to the separation efficiency of photogenerated carriers caused by surface OV.The dependences of Δhand the reaction constant(k)for the degradation of MB by OV-Zn∶Bi2O3series on the intensity of ESR signal for VO·(hESR,OV)are shown in Fig.13a.It can be seen that with the increase ofhESR,OV,both of Δhandkfirst increased and then decreased,suggesting that excessive OV may be detrimental to the activity of catalyst.Fig.13b shows the dependences ofhESR,OVandkfor the degradation of MB by OV-Zn∶Bi2O3series onnZn/nBi.It can be seen that,with the increase of doped Zn2+,hESR,OVdecreased and there was a peak of reaction constant situated atnZn/nBi=0.3 for OV-Zn∶Bi2O3-0.3.This suggests that the concentration of OV can be modulated by the doped zinc and excessive zinc doping may also reduce the separation efficiency of photogenerated carriers.

Fig.13 (a)Dependence of Δh and reaction rate constant(k)for the degradation of MB by OV-Zn∶Bi2O3 series on intensity of ESR signal for Vo·(hESR,OV);(b)Dependence of hESR,OVand k on nZn/nBi

Two kinds of OV on the surface of catalysts,V··oand VO×,can serve as trapping centers of electrons and holes,respectively.And moderate doped Zn2+can also act as electron capture sites to promote the separation efficiency of photogenerated carriers.Thus,the visible light activity of both OV-β-Bi2O3and OV-Zn∶Bi2O3series can be improved.Of course,compared to OV-β-Bi2O3,moderate doped Zn2+can result in the optimal energy level structure for OV-Zn∶Bi2O3,and OV-Zn∶Bi2O3-0.3 exhibited the highest activity for the degradation of MB and 2,4,6-TCP.

3 Conclusions

OV-β-Bi2O3and Zn-doped OV-β-Bi2O3series(OVZn∶Bi2O3) were prepared by a simple sol-gel method followed byin-situcarbon thermal reduction treatment.The concentration of oxygen vacancy in OV-Zn∶Bi2O3can be modulated by the content of doped zinc.The more the content of doped Zn2+is,the less the concentration of oxygen vacancy will be.The introduction of oxygen vacancy and moderate zinc can not only regu-late the energy structure of OV-Zn∶Bi2O3,but also promote the separation efficiency of photogenerated carriers.So,the visible light activity of OV-Zn∶Bi2O3series was superior to that of OV-β-Bi2O3,and the activity of OV-β-Bi2O3was better than that of traditionalβ-Bi2O3.This study offers a new method to prepare the novel visible-light-responsive catalyst with stable controllable oxygen vacancy.

Acknowledgments:The authors acknowledge the financial support provided by the Science&Technology Research Project of Henan Province(Grant No.19A150020),the Program for Science&Technology Innovation Team in Universities of Henan Province(Grant No.19IRTSTHN029),Innovation and Entrepreneurship Training Program of Henan Province(Grant No.S202010475087)and Science&Technology Development Plan Project of Kaifeng(Grant No.1808001).

Supporting information is available at http://www.wjhxxb.cn