具有不同Bi/Ti摩尔比的BiOI/TiO2(A)光催化剂的结构与性能

李慧泉 崔玉民,＊ 吴兴才 华 林 洪文珊

(1安徽阜阳师范学院化学化工学院,安徽阜阳236041;2安徽环境污染物降解与监测省级实验室,安徽阜阳236041; 3南京大学配位化学国家重点实验室,南京210093;4新加坡Linovus技术有限公司研发中心,新加坡059818)

具有不同Bi/Ti摩尔比的BiOI/TiO2(A)光催化剂的结构与性能

李慧泉1,2崔玉民1,2,＊吴兴才3,*华 林4洪文珊1

(1安徽阜阳师范学院化学化工学院,安徽阜阳236041;2安徽环境污染物降解与监测省级实验室,安徽阜阳236041;3南京大学配位化学国家重点实验室,南京210093;4新加坡Linovus技术有限公司研发中心,新加坡059818)

在室温条件下通过沉积法制备了BiOI敏化纳米锐钛矿TiO2(A)光催化剂.用X射线衍射(XRD),X射线光电子能谱(XPS),光致发光(PL)光谱和紫外-可见漫反射光谱(UV-Vis DRS)等手段对其进行了表征.通过罗丹明B(RhB)催化降解实验评价了其光催化活性.随BiOI含量增加,BiOI/TiO2(A)在370-630 nm的吸收强度增强,吸收带边红移增加,紫外和可见光催化活性先增加,当Bi/Ti摩尔比约为1.7%时,各自达到最大值,然后随BiOI含量的进一步增加而减小.1.7%BiOI/TiO2(A)的可见光活性明显高于P25,它的紫外光活性也略高于P25.在BiOI含量相近时,BiOI/TiO2(A)比BiOI/P25具有更低的光催化活性.和TiO2(A)相比,1.7%BiOI/TiO2(A)明显具有更高的紫外和可见光催化活性,这归因于它在370-630 nm的强吸收、吸收带边红移明显以及光生电子和空穴的有效转移,减少了电子-空穴对的复合.

沉积法;敏化;锐钛矿;BiOI含量;光催化

1 Introduction

Anatase TiO2(A)has been widely used as a photocatalyst for solar energy conversion and degradation of environmental pollutants because of nontoxicity,chemical stability,good photoactivity,and low cost.1-9However,the intrinsic property of TiO2(A)(wide band-gap energy,～3.2 eV)limits its photocatalytic activity in the UV light region.In order to extend TiO2(A)photocatalytic activity into the visible light region,some attempts have been made to sensitize TiO2(A).10-14Among these attempts,CdS is widely studied as an efficient sensitizer to make TiO2(A)response visible light,but it is prone to decompose and leach out hazardous Cd2+ions under photocatalytic reaction systems.15

Recently,the as-prepared BiOI micro/nanostructure materials have exhibited efficient photocatalytic activity in the degradation of organic pollutants.16-18Owing to BiOI having narrow band-gap energy(～1.8 eV),it could absorb most of visible solar light and may be a potential sensitizer to sensitize wide band-gap semiconductors.Zhang et al.19reported that BiOI/ TiO2heterostructures with different Bi/Ti molar ratios were synthesized by a soft-chemical method at 80°C and BiOI/TiO2heterostructures exhibited much higher visible light photocatalytic activity than TiO2,and the visible light photocatalytic activity enhancement of BiOI/TiO2heterostructures could be attributed to its strong absorption in the visible region and low recombination rate of the electron-hole pairs because of the heterojunction formed between BiOI and TiO2.Sensitization of oxide photocatalysts is one of the well-known methods for enhancing their photocatalytic activity.However,to the best of our knowledge,there were few reports on the BiOI as a sensitizer of TiO2(A),and the effect of Bi/Ti molar ratios on both UV and visible light photocatalytic performances was also rarely investigated.

In this work,BiOI-sensitisized TiO2(A)catalysts with varying BiOI content were synthesized by a deposition method at room temperature,and effect of BiOI contents on the structure and photocatalytic activity of BiOI/TiO2(A)catalysts were investigated in detail.The structure of the catalysts were characterized by using X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),photoluminescence(PL)spectrum,and UV-Vis diffuse reflectance spectrum(UV-Vis DRS),and the photocatalytic activities were tested by a degradation of RhB.

2 Experimental

2.1 Catalyst preparation

Tetrabutyl titanate(Ti(OC4H9)4)is chemical pure,and butyl alcohol(C4H9OH),acetic acid,ethanol,nitric acid bismuth salt(Bi(NO3)3·5H2O),potassium iodide(KI),ethylene glycol(C2H6O2)are analytical pure.

The pure TiO2(A)catalysts were prepared by a sol-gel procedure with Ti(OC4H9)4as raw materials.Ti(OC4H9)4(7.0 mL) was added dropwise into one mixture consisting of 3.0 mL of acetic acid(AR)and 30 mL of C4H9OH in a dry atmosphere under roughly stirring,then a certain of deionized water was added dropwise into above mixture solution to carry out hydrolysis until the yellowish transparent sol was obtained,which was allowed to stand for 24 h at room temperature and was dried in air at 110°C for 24 h.Thus,TiO2(A)gel precursor was obtained.Finally,TiO2(A)catalysts were gained by the thermal treatment of TiO2(A)gel precursor in air at 500°C for 3 h and grinding.

The BiOI-sensitized TiO2(A)catalysts with different BiOI contents,denoted as BiOI/TiO2(A),were prepared by a deposition method.In a typical experiment,different stoichiometric amounts of Bi(NO3)3·5H2O and 30.0 mg KI were dissolved in 20 mL ethylene glycol(AR)to obtain a clear solution I.TiO2(A)(1.0 g)was ultrasonically dispersed into deionized water to form a homogeneous mixture II.Then the solution I was added dropwise into the mixture II under strong stirring.After further agitation for 5.0 h at room temperature,the products obtained were separated centrifugally,washed with ethanol and deionized water and dried at 80°C in air.The final samples with original Bi/Ti molar ratios of 0.000,0.010,0.012,0.015,0.017, and 0.020 were denoted as 0.0%,1.0%,1.2%,1.5%,1.7%,and 2.0%BiOI/TiO2(A),respectively.

Reference BiOI-sensitized TiO2(P25)samples with original Bi/Ti molar ratios of 0.000,0.010,0.012,0.015,0.017,and 0.020 were denoted as 0.0%,1.0%,1.2%,1.5%,1.7%,and 2.0%BiOI/P25,respectively,prepared by the same method.

2.2 Catalyst characterization

X-ray diffraction were performed on a Philips Xʹpert diffractometer equipped with Ni-filtered Cu Kαradiation source(λ= 0.15418 nm).The X-ray tube was operated at 40 kV and 40 mA.XPS measurements were carried out using Multilab 2000 XPS system with a monochromatic MgKαsource and a charge neutralizer(Multilab 2000 XPS,Thermo Scientific,America). All binding energies were referenced to contaminant carbon(C 1s:284.6 eV).The PL spectra,obtained at 77 K with an excitation wavelength of 220 nm,were recorded on a CARY Eclipse (America)fluorescence spectrophotometer equipped with a Xe Lamp as an excitation source operating in the front face mode. UV-Vis DRS of the catalysts were determined with a Shimadzu UV-3600 spectrophotometer(Japan)using BaSO4as a reference.The actual Bi/Ti molar ratios of the obtained BiOI/TiO2(A)photocatalysts were detected by IRIS(INTREPID 2)inductively coupled plasma atomic emission spectrometry (ICP-AES,ICP710,Varian,America),and the results are listed in Table 1.

Table 1 Lattice parameters and cell volumes for different BiOI/TiO2(A)samples

2.3 Photocatalytic reaction

The self-degradation of RhB solution was evaluated in the absence of BiOI/TiO2photocatalyst under UV or visible light irradiation for 1.0 h.The photocatalytic activities of BiOI/TiO2(A)catalysts were evaluated by the degradation of rhodamine B(RhB)in an aqueous solution.The UV light was obtained by a high-pressure mercury lamp(300 W).The UV irradiation intensity(the wavelengths below 400 nm)of the reaction solution surface is about 15.0 mW·cm-2(UV-A radiometer,the Photoelectric Instrument Factory of Beijing Normal University). The visible light source was a Xenon-arc lamp(350 W)with the combination of a cut-off filter(＞400 nm)to eliminate UV radiation during visible light experiments.The visible irradiation intensity of the reaction solution surface is about 8.0 mW·cm-2(FZ-A radiometer,the Photoelectric Instrument Factory of Beijing Normal University).For each UV and visible light test,40 mL RhB aqueous solution(1.04×10-5mol·L-1) and 0.1 g catalyst catalysts were used.A general procedure was carried out as follows.First,RhB aqueous solution was placed into a water-jacketed reactor maintained at 25°C,and then the catalyst samples were suspended in the solution.The suspension was stirred vigorously for 1.0 h in the dark to establish the adsorption-desorption equilibrium of RhB,then irradiated under visible or UV light.About 3.0 mL solution was withdrawn from the reactor periodically and centrifuged and analyzed for the degradation of RhB using a UV-2450 UV-Vis spectrophotometer.RhB has a maximum absorbance at 554 nm,which was used as a value for monitoring RhB degradation.The absorbance was converted to the RhB concentration in accordance with a standard curve showing a linear relationship between the concentration and the absorbance at this wavelength.

3 Results and discussion

3.1 Catalyst structure

Fig.1 XRD patterns of BiOI/TiO2(A)catalysts with different BiOI contentsBi/Ti molar ratio:(a)0.0%;(b)1.0%;(c)1.2%;(d)1.5%;(e)1.7%;(f)2.0%

Fig.1 shows the XRD patterns of BiOI/TiO2(A)catalysts with different BiOI contents.It can be seen that 2.0%BiOI/ TiO2(A)exhibits a coexistence of both BiOI and anatase TiO2phases,the peaks around 2θ of 29.7°,31.7°,and 45.5°were indexed to those of tetragonal BiOI(JCPDS No.01-073-2062) and correspond to(012),(110),and(020),respectively.When the amount of BiOI is lower than 1.7%,no significant diffraction peak of BiOI could be detected,which could be ascribed to its lower content and high dispersion on the surface of TiO2(A)particles.The average crystalline sizes of TiO2(A)in the BiOI/TiO2(A)composites were calculated to be 19.6,21.2, 19.1,21.7,25.1,and 24.7 nm for 0.0%,1.0%,1.2%,1.5%, 1.7%,and 2.0%BiOI/TiO2(A),respectively,and the average crystalline sizes of BiOI in the BiOI/TiO2(A)composites were calculated to be 5.4 and 3.7 nm for 1.7%and 2.0%BiOI/TiO2(A),respectively,according to the Scherrer formula:20L=0.90λ/ βcosθ,where L is taken as crystalline size,λ is 0.154 nm,β is the full width half maximum(FWHM)measured in radians on the 2θ scale,and θis the Bragg angle for the diffraction peaks.

The lattice parameters of BiOI-sensitized samples were calculated using Braggʹs equation(2dsinθ=nλ,d:surface distance, θ:angle between incident surface and reflective surface,n:diffraction series,λ:X-ray wavelength)and a formula of(1/d2= (h2+k2)/a2+l2/c2,a,b,c:cell parameters,d:surface distance,h, k,l:indices of crystal face)from their(101),(004),and(200) diffraction peaks,and the results are listed in Table 1.It can be seen that the lattice parameters of a-axis for all the BiOI-sensitized samples are almost unchanged,while that of the c-axis decreases obviously with increase of BiOI content,indicating a lattice shrinkage along the c-axis due to the sensitization of BiOI.This lattice shrinkage may be attributed to the appearance of bismuth vacancy to reach new charge balance after the substitution of oxygen with iodine.21

In order to further determine the valence state of bismuth and iodine on the surfaces of the BiOI/TiO2(A),XPS measurements were employed to analyze the 1.7%BiOI/TiO2(A)catalyst.Fig.2(A)reveals the binding energies are 157.8 and 163.2 eV for Bi 4f7/2and Bi 4f5/2,respectively,which indicates that Bi is in the form of Bi3+assigned to BiOI.19The I 3d core level spectrum from Fig.2(B)could be observed at the binding energies of around 630.2 eV(I 3d3/2)and 618.6 eV(I 3d5/2),in agreement with that in BiOI.The above XRD and XPS results verify the existence of BiOI along with TiO2(A).

3.2 Photocatalytic activity

Fig.2 XPS spectra of 1.7%BiOI/TiO2(A)catalyst

Prior to illumination,an adsorption-desorption equilibrium between the photocatalyst and RhB was established in the dark for 1.0 h.The corresponding dark adsorption values for different samples are listed in Table 2.

The effect of BiOI sensitizing on the photocatalytic activity of BiOI/TiO2(A)has been investigated by rhodamine B(RhB) degradation in an aqueous solution under UV and visible light irradiation.Table 2 shows the UV and visible light photocatalytic activities of BiOI/TiO2(A)samples with different BiOI content.It can be seen that under UV or visible light irradiationthe degradation of rhodamine B is much lower without photocatalyst in the reaction system.In comparison,the TiO2(A)and BiOI/TiO2(A)catalysts exhibit higher photocatalytic activities than without photocatalyst for RhB degradation,and the BiOI content in the TiO2(A)exerts great influences on the photocatalytic activity of BiOI/TiO2(A)catalyst.With BiOI content increasing,the photocatalytic activities of BiOI/TiO2(A)under UV and visible light irradiation first increase,reaching a maximum around BiOI content of 1.7%,respectively,and then decrease with further increasing BiOI content.The 1.7%BiOI/ TiO2(A)catalyst exhibits much higher visible light photoactivity than P25(i.e.0.0%BiOI/P25 in Table 2),and its UV light photoactivity is slightly higher than that of P25,indicating that sensitization of BiOI in the TiO2(A)with optimum BiOI content remarkably enhances the photocatalytic activity of TiO2(A).In comparison,the UV and visible light photocatalytic activities of BiOI/TiO2(A)catalysts with similar BiOI content are lower than those of BiOI/P25 catalysts,22,23which may be attributed to the fact that the existence of an intimate contact between anatase and rutile particles in the P25 catalyst that enhances electron transfer,hindering the recombination of photogenerated electrons and holes.

Table 2 Dark adsorption values and photocatalytic activities of different catalysts under UV and visible light irradiation for 1.0 h

Fig.3 UV-Vis spectra of the RhB aqueous solution under UV light irradiation in the presence of TiO2(A)and 1.7%BiOI/TiO2(A)catalysts

The UV-Vis spectra of RhB aqueous solution as a function of UV and visible light irradiation time in the presence of TiO2(A)and 1.7%BiOI/TiO2(A)catalysts are illustrated in Fig.3 and Fig.4.It can be seen that the visible region peak intensities in the photodegradation of RhB by 1.7%BiOI/TiO2(A)de-crease more obviously than by TiO2(A)during the same irradiation time,which is in agreement with the results of Table 2. The main absorbance of the degraded solution in the presence of 1.7%BiOI/TiO2(A)catalyst gradually shifted from 554 nm to shorter wavelength as the irradiation time was increased,corresponding to the stepwise formation of a series ofN-deethylated intermediates.24Since no new peak appears,the loss of absorbance can be mainly attributed to the degradation reaction.

Fig.4 UV-Vis spectra of the RhB aqueous solution under visible light irradiation in the presence of TiO2(A)and 1.7%BiOI/TiO2(A)catalysts

To test the stability of the BiOI/TiO2(A)catalysts for the photocatalytic reaction,the 1.7%BiOI/TiO2(A)catalyst was reused for photocatalytic reaction 4 times under the same conditions and the result is shown in Fig.5.The photocatalytic efficiency of the 1.7%BiOI/TiO2(A)catalyst decreases only 7.0% and 8.5%,respectively,after four cycles under UV and visible light irradiation,which indicate that the catalyst is stable for the photocatalysis of pollutant molecules.

Fig.5 Cycling runs in photocatalytic degradation of RhB in the presence of 1.7%BiOI/TiO2(A)catalyst under UV and visible light irradiation for 2.0 h

3.3 Light absorption

The UV-Vis DRS of BiOI/TiO2(A)catalysts with different BiOI contents are shown in Fig.6.It can be seen that with increasing BiOI content,the absorption intensity increases in the 370-630 nm and the absorption band edge has a redshift.Accordingly,the photocatalytic activities of BiOI/TiO2(A)catalysts under UV and visible light irradiations increase with increasing BiOI content.When the BiOI content is higher than 1.7%,the light absorption intensity of 2.0%BiOI/TiO2(A)catalyst increases further in the 370-630 nm and the absorption band edge shifts further red but the photocatalytic activities of 2.0%BiOI/TiO2(A)catalyst under UV and visible light irradiation decrease remarkably,which may be attributed to the fact that the excessive BiOI with narrow band gap will be acted as the recombination center of electrons and holes,25impeding the photocatalytic activity on the contrary.

It can be also seen that there are two prominent absorption bands for the BiOI/TiO2(A)catalysts when the amount of BiOI is greater than 1.7%in Fig.6.The former is assigned to the absorption of anatase TiO2,and the latter is attributed to the characteristic absorption of BiOI.23The appearance of two kinds of characteristic absorption bands also testifies that the BiOI/TiO2(A)catalyst is composed of BiOI and TiO2(A).

TiO2(A)is n-type semiconductor,while BiOI is p-type semiconductor.When TiO2(A)and BiOI are brought in contact,the energy level difference between TiO2(A)and BiOI causes the electrons to flow from higher level TiO2(A)to lower level BiOI.26However,the redistribution of the electrons between BiOI and TiO2(A)is supposed to trigger an upward and downward shift of the band edges,respectively,for BiOI and TiO2(A).27In addition,BiOI particles on the surface of TiO2(A)are very small and their average crystalline sizes for 1.7%and 2.0%BiOI/TiO2(A)catalysts are less than 5.5 nm.Compared with bulk BiOI,the band gap of the BiOI particles becomes broader due to the size quantization effect,therefore the conduction band of the BiOI particle shifts to negative potentials.28,29When BiOI/TiO2(A)catalyst is exposed to UV or visible light, the electrons in the valence band of BiOI will be excited into the conduction band and then injected into the more positive conduction band of TiO2(A).So the photoelectrons are generated from BiOI and transferred across the interface between Bi-OI and TiO2(A)to the surface of TiO2(A),leaving the photogenerated holes in the valence band of BiOI.By this way,the photogenerated electron-hole pairs are effectively separated. The better separation of electrons and holes in the BiOI/TiO2(A)catalysts is confirmed by PL emission spectra of 1.7%Bi-OI/TiO2(A)and TiO2(A)catalysts in Fig.7.

Fig.6 UV-Vis DRS of BiOI/TiO2(A)catalysts with different initial BiOI contents

Fig.7 PLspectra of TiO2(A)and 1.7%BiOI/TiO2(A)catalysts recorded at-196°C with the excitation wavelength of 220 nm

Fig.7 shows the photoluminescence(PL)emission spectra of TiO2(A)and 1.7%BiOI-TiO2(A)catalysts.It is well known that the PL signals of semiconductor materials result from the recombination of photogenerated charge carriers.In general, the lower the PL intensity,the lower the recombination rate of photogenerated electron-hole pairs,and the higher the photocatalytic activity of semiconductor photocatalysts.30-32As shown in Fig.7,TiO2(A)and 1.7%BiOI/TiO2(A)catalysts show a strong and broad peak from 350 to 650 nm,and exhibit an emission peak around at 463 nm,which is an excitonic PL signal.33Compared with that of TiO2(A),the emission peak intensity of 1.7%BiOI/TiO2(A)decreases considerably,indicating that the recombination of photogenerated charge carrier is inhibited greatly in the 1.7%BiOI/TiO2(A)catalysts.Thus photogenerated electron-hole pairs are effectively separated.The efficient charge separation could increase lifetime of charge carriers and enhance the efficiency of interfacial charge transfer to adsorbed substrates,and then improve photocatalytic activity.

4 Conclusions

In summary,BiOI-sensitisized TiO2(A)photocatalysts with different BiOI contents have been successfully synthesized by a deposition method at room temperature.The BiOI content exerts great influences on the physicochemical properties and photocatalytic activities of BiOI/TiO2(A)catalysts.With increasing BiOI content,the absorption intensity of BiOI/TiO2(A)samples enhances in 370-630 nm and the redshift of absorption band edges increase,and their UV and visible light photocatalytic activities first increase,reaching the maxima around BiOI content of 1.7%,respectively,and then decrease with further increasing BiOI content.The 1.7%BiOI/TiO2(A) catalyst exhibits much higher visible light photoactivity than P25,and its UV light photoactivity is slightly higher than that of P25.The UV and visible light photocatalytic activities of BiOI/TiO2(A)catalysts with similar BiOI content are lower than those of BiOI/P25 catalysts.Compared with TiO2(A), 1.7%BiOI/TiO2(A)exhibits much higher UV and visible light photoactivity.It can be attributed to the stronger light absorption in 370-630 nm,the obvious red shift of absorption band edge,and the effective transfer of the photogenerated electrons and holes,reducing the recombination of electron-hole pairs. The 1.7%BiOI/TiO2(A)catalyst is stable for the photocatalysis of RhB.

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March 19,2012;Revised:May 15,2012;Published on Web:May 16,2012.

Structure and Properties of BiOI/TiO2(A)Photocatalysts with Different Bi/Ti Molar Ratios

LI Hui-Quan1,2CUI Yu-Min1,2,*WU Xing-Cai3,*HUA Lin4HONG Wen-Shan1
(1School of Chemistry and Chemical Engineering,Fuyang Normal College,Fuyang,236041,Anhui Province,P.R.China;2Anhui Provincical Key Laboratory for Degradation and Monitoring of Pollution of the Environment,Fuyang 236041,Anhui Province,P.R.China;3State Key Laboratory of Coordination Chemistry,Nanjing University,Nanjing 210093,P.R.China;4Research District Center,Singapore Linovus Technology Private Limited,059818,Singapore)

BiOI-sensitized nano-anatase(TiO2(A))photocatalysts were prepared by a deposition method at room temperature,and characterized by X-ray diffraction(XRD),X-ray photoelectron spectroscopy (XPS),and photoluminescence(PL),and UV-Vis diffuse reflectance spectra(UV-Vis DRS).The photocatalytic activities were evaluated by photo-degradation experiments of rhodamine B.With increasing BiOI content,the absorption intensity of BiOI/TiO2(A)increases in the 370-630 nm region and the absorption band edge redshifts.The UV and visible light photocatalytic activities increase,reaching a maximum when the Bi/Ti molar ratio is 1.7%.The 1.7%BiOI/TiO2(A)catalyst exhibits much higher visible-light photoactivity than P25,and its UV-light photoactivity is slightly higher than that of P25.The UV and visible light photocatalytic activities of BiOI/TiO2(A)with similar BiOI content are lower than those of BiOI/P25 catalysts.Compared with TiO2(A),1.7%BiOI/TiO2(A)shows higher UV and visible light photoactivities.This is attributed to the strong absorption in the 370-630 nm region,the redshift of the absorption band edge,and the effective transfer of the photogenerated electrons and holes,which reduces the recombination of electron-hole pairs.

Deposition method;Sensitized;Anatase;BiOI content;Photocatalysis

10.3866/PKU.WHXB201205161

∗Corresponding authors.CUI Yu-Min,Email:cymlh@fync.edu.cn;Tel:+86-558-2596507;Fax:+86-558-2596703.

WU Xing-Cai,Eamil:wuxingca@nju.edu.cn;Tel:+86-25-83594945;Fax:+86-25-83317761.

The project was supported by the National Natural Science Foundation of China(21171091)and Natural Science Foundation of Higher Education Institutions inAnhui Province,China(KJ2012A217,KJ2012B136).

国家自然科学基金(21171091)和安徽省高校省级自然科学基金(KJ2012A217,KJ2012B136)资助项目

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