BiOI-BiOCl 光催化

Visible-light-responsive photocatalysts x BiOBr–(1àx )BiOI

Wendeng Wang,Fuqiang Huang *,Xinping Lin,Jianhua Yang

State Key Laboratory of High Performance Ceramics and Super?ne Microstructure,Shanghai Institute of Ceramics,

Chinese Academy of Sciences,1295Ding Xi Road,Shanghai 200050,PR China

Received 2November 2006;received in revised form 11March 2007;accepted 4May 2007

Available online 17May 2007

Abstract

A new class of oxyhalide photocatalysts x BiOBr–(1àx )BiOI prepared by a soft chemical method were characterized by X-ray di?rac-tion and UV–Vis di?use re?ectance spectra.They are all visible-light-responsive materials with the bandgaps ranging from 1.92to 2.91eV.Methyl orange (MO)photocatalytic degradation experiments showed that BiOBr possessed a higher photocatalytic activity than P25(TiO 2)under UV illumination and iodine-modi?ed BiOBr exhibited high photocatlytic activities under visible-light irradiation.The high photocatalytic activity is in close relation with the deep valance band edge position and the internal electric ?elds between [Bi 2O 2]slabs and halogen anionic slabs.

ó2007Elsevier B.V.All rights reserved.

Keywords:Photocatalysis;Oxyhalide;Methyl orange

1.Introduction

As a potential solution to the recent severe problems of environment crises,destruction of organic and inorganic pollutants on semiconductor photocatalysts has been extensively studied [1–7].The main advantage of photoca-talysis is that the contaminants could be completely degraded to CO 2,H 2O and other inorganic constituents with solar energy,leaving no waste for second disposal.So far,TiO 2and its modi?ed forms are intensely studied for the oxidative decomposition of many contaminants [8].Very recently,we for the ?rst time found that the lay-ered oxyhalide photocatalyst BiOCl [9]showed a higher photocatalytic activity to degrade methyl orange (MO)under the same conditions compared with P25(TiO 2).However,due to its wide optical bandgap ($3.5eV),BiOCl can only absorb UV-light,which is merely about 3%of the solar spectrum.Because of this inherent limitation,the abundant solar energy can not be utilized e?ciently.So the development of e?cient and visible-light-driven photo-

catalysts is indispensable.But the number of such photo-catalyst materials known up to now is yet limited [10–12].BiOX (X =Cl,Br,I)compounds all crystallize in the tetragonal matlockite (PbFCl)structure,a layered struc-ture characterized by [Bi 2O 2]slabs interleaved by double slabs of halogen atoms.BiOI is a coral red material [13],which means it has a strong absorption in the visible-light region.Motivated by these facts,we are interested in the x BiOBr–(1àx )BiOI system.In the past two decades,the reported photocatalysts are oxides,sul?des,oxysu?des,nit-rides and oxynitrides [14–16].To our knowledge,x BiOBr–(1àx )BiOI are the ?rst oxyhalide photocatalysts working under visible-light (k >420nm).In the present paper,the relationship between the composition of x BiOBr–(1àx )BiOI system and its photophysical and photocata-lytic activities was investigated.2.Experimental

x BiOBr–(1àx )BiOI (x =0.0,0.25,0.50,0.75,1.0)com-pounds were prepared by a soft chemical method similar to Ref.[13].All the reagents were purchased from Sinopharm Chemical Reagent Co.(Shanghai,China)and used without further puri?cations.In a typical experiment to prepare the

1566-7367/$-see front matter ó2007Elsevier B.V.All rights reserved.doi:10.1016/j.catcom.2007.05.014

*

Corresponding author.Tel.:+862152411620;fax:+862152413903.E-mail address:huangfq@https://www.360docs.net/doc/9a8763824.html, (F.Huang).

https://www.360docs.net/doc/9a8763824.html,/locate/catcom

Available online at https://www.360docs.net/doc/9a8763824.html,

Catalysis Communications 9(2008)

8–12

BiOBr0.5I0.5sample,0.02mol Bi(NO3)3?5H2O(AR, 99.0%)powders were?rstly dissolved in15mL glacial ace-tic acid(AR,99.5%).Then the solution was added rapidly to another one containing0.01mol KBr(AR,99.0%), 0.01mol KI(AR,98.5%)and0.04mol CH3COONa (AR,99.0%)in240mL deionized water with magnetically stirring.Subsequently,the mixtures were stirred for20h. After the reaction was completed,the resulting solid prod-uct was collected by?ltration,washed several times with deionized water,and dried at120°C in air before further characterizations.

X-ray powder di?raction(XRD)patterns were obtained on a Rigaku D/max-2550V di?ractometer using Cu K a radiation(k=0.15418nm).UV–Vis di?use re?ectance spectra of the powders were measured at room temperature

on a Hitachi U-3010spectrophotometer.

The catalytic reaction was carried out to degrade10mg/ l MO aqueous solution.The UV-light photocatalytic reac-tor consists of two parts,a quartz cell with a circulating water jack to prevent any thermal catalytic e?ect and a 300W high-pressure mercury lamp with a maximum emis-sion at365nm placed inside the quartz cell.Reaction sus-pensions were prepared by adding0.2g photocatalyst powders into300mL MO aqueous solution.Visible-light photocatalytic degradations were performed in another experiment equipment.The optical system includes a 300W Xe arc lamp,cut-o??lters,and an electric fan to prevent thermal catalytic e?ect.Photocatalyst(0.1g)pow-der was suspended in200mL MO aqueous solution.In both UV and visible-light reactions,before irradiation, the suspensions were ultrasonated for20min and magnet-ically stirred in the dark for50min to establish an adsorp-tion–desorption equilibrium of the dye on the catalyst surface.At given time intervals,about5mL analytical sus-pension was taken from the reaction suspension by a syr-inge.The photocatalyst powders and the MO solution were separated by a centrifugal machine.Then the concen-tration of the?ltrate was analyzed by the Hitachi U-3010 spectrophotometer.The absorption peak of MO at 464nm was monitored as the probe of MO degradation [7,17].

3.Results and discussion

3.1.Characterizations of as-prepared powders

The XRD patterns of as-prepared samples are shown in Fig.1.The samples of BiOBr and BiOI both possess pure tetragonal phases(corresponding JCPDS-ICDD numbers 09-0393and10-0445).The di?erence between the halogen ionic radii(Br=1.82A?,I=2.06A?)is about12%[18].It is generally accepted that unlimited solubility is usually observed if the di?erence in the atomic radii of the compo-nents is not more than15%in solid solutions[19].The unlimited solid solutions formed between BiOBr and BiOI were con?rmed by the right shifts of the di?raction peaks with the increase of x(bromine content).The Br and I atoms are disordered in the halogen slabs to maintain the tetragonal crystal structure of the solid solutions.The cell constants of the as-prepared samples are summarized in Table1.Both a and c monotonically decrease with the increase of x.The patterns didn’t change after the photo-catalytic reactions,indicating that the synthesized photo-catalysts are stable.

Fig.2shows the UV–Vis di?use re?ectance spectra (DRS)of the samples.The absorption edge has a mono-tonic blue shift with increasing x.BiOBr can slightly absorb visible-light.On the contrary,the samples with x=0.25–1have intense absorptions in the visible light region.The steep shape in the visible edge and strong absorption in the visible region also indicate that

the Fig. 1.The XRD patterns of as-prepared x BiOBr–(1àx)BiOI photocatalysts.

Table1

Cell constants,bandgap(E g),pseudo-?rst order rate constant(k)of MO decomposition under visible-light irradiation and VB edge potentials (E VB)of the x BiOBr–(1àx)BiOI

x Cell constants E g(eV)k(hà1)R E VB(eV) a(A?)c(A?)

0 3.99679.1533 1.920.0840.987 2.46

0.25 3.97119.0487 2.010.5760.980 2.57

0.50 3.95268.7351 2.100.4370.977 2.66

0.75 3.93078.3475 2.220.3560.956 2.78

1.0 3.92318.0945

2.910.1460.972

3.19

Fig. 2.UV–Vis di?use re?ectance spectra of x BiOBr–(1àx)BiOI photocatalysts.

W.Wang et al./Catalysis Communications9(2008)8–129

absorption band is not ascribed to the transition from the impurity level to the conduction band,but to the intrinsic transition between the valence band and the conduction band[20–22].The bandgaps of the samples estimated from the onsets of absorption edges were listed in Table1.They range from1.92to2.91eV and increase monotonically as the contents of bromine element increase.

3.2.Photocatalytic activity

3.2.1.UV-light-induced photocatalytic activity

The dependence of photocatalytic activity on the com-position of the x BiOBr–(1àx)BiOI system under UV-light irradiation is represented in Fig.3.All the samples exhibit high photocatalytic activities and the activity increases monotonically with x increasing.When the photocatalytic reaction proceeded for28min,the removed part of MO

were49.4%,89.1%,92.2%,94.8%and100%with x ranging from0.0to1.0.The following experiments were also per-formed as comparisons:(1)a blank experiment(in the absence of the photocatalyst);(2)dark experiments(with-out irradiation);and(3)MO degradation over P25 (Degussa,average partice size21nm).In conditions(1) and(2),MO decompositions were not observable.From Fig.3,it can be concluded that BiOBr is photocatalytically more active than P25,and the performances of x BiOBr–(1àx)BiOI(x=0.25,0.50,and0.75)are comparable to P25.

3.2.2.Visible-light-driven photocatalytic activity

Fig.4displays MO degradation over the x BiOBr–(1àx)BiOI photocatalysts under visible-light irradiation (k>420nm).When the photocatalytic reactions pro-ceeded for5h,the degrees of MO decomposition were 45.5%,77.9%,84.3%,92.1%and33.4%with x increasing. BiOBr exhibits the lowest photocatalytic activity,which is just opposite to the result in the UV-light reaction where the performance of BiOBr is the highest.The activities of other photocatalysts monoclinically increase with the increase of x.The three comparison experiments men-tioned in the above UV-light-induced photocatalytic reac-tions were also performed and MO decompositions were undetectable.

In order to quantitatively understand the reaction kinet-ics of the MO degradation in our experiments,the pseudo-?rst order model as expressed by Eq.(1),which is generally used for photocatalytic degradation process if the initial concentration of pollutant is low[23],was applied

lneC e=C tT?kte1Tin which C e and C t are the concentrations of MO in solu-tion at time0(the time to obtain adsorption–desorption equilibrium)and t,respectively,and k is the pseudo-?rst order rate constant.The rate constants obtained from the data plotted in Fig.4are concluded in Table1.As it can be seen,a good correlation to the pseudo-?rst order reac-tion kinetics(R>0.95)was found.The reaction rate con-stant of BiOBr is0.084hà1,and for BiOBr0.75I0.25it is as high as0.576hà1.This indicates that the formation of the solid solution increases the reaction rate constant by about5.8times.So the composition of the photocatalysts shows its strong in?uence on the MO

degradation.

Fig.3.The dependence of photocatalytic activity on the composition of

x BiOBr–(1àx)BiOI system under UV-light

irradiation.

Fig.4.MO decomposition over the x BiOBr–(1àx)BiOI photocatalysts

under visible-light

irradiation.

Fig.5.The action spectra of x BiOBr–(1àx)BiOI(x=0.0,0.75,and1.0)

and P25(TiO2)for degrading MO.

10W.Wang et al./Catalysis Communications9(2008)8–12

The action spectra are important to evaluate a visible-light response type photocatalyst.Thus,the action spectra of BiOBr,BiOI,BiOBr0.75I0.25and P25(TiO2)for degrad-ing MO were investigated and the results are shown in Fig.5.It can be seen from the?gure that the pro?les of the action spectra of the photocatalysts are all similar to their UV–Vis re?ectance spectra as shown in Fig.2,respec-tively.The trends of the curves in Fig.5are the same,that is,with the increase of cut-o?wavelength,the photodegra-dation rates of MO decrease gradually.These facts indicate that the present catalytic reactions are driven by the inci-dent light and the light absorption of the photocatalysts governs the reaction rate.Furthermore,BiOI and BiO-Br0.75I0.25can respond to the visible-light as long as $600nm and$550nm for degrading MO,respectively. The onsets of the action spectra agree approximately with the onsets of the UV–Vis di?use re?ectance spectra corre-sponding to the bandgap of the semiconductors.These results reveal that the reactions are catalyzed by the phot-ocatalysts[24].

The above-mentioned high photocatalytic activities are believed to be in close connection with the unique layered structure and with the deep oxidative potential for the pres-ent x BiOBr–(1àx)BiOI photocatalysts.As we know,in the photocatalytic process over a semiconductor,electron–hole pairs are generated when the catalyst is illuminated with the light having an energy equal to or greater than the bandgap.The principal challenge is how to suppress the recombination of the formed electron–hole pairs[25].The atomic arrangement of the x BiOBr–(1àx)BiOI compounds consists of tetragonal[Bi2O2]slabs which are‘‘sand-wiched’’by two halogen slabs to form a[Bi2O2X2]layer along the c-axis.The formed internal electric?elds between the[Bi2O2]positive slabs and the halogen anionic slabs are believed to induce the e?cient separation of photogener-ated electron–hole pairs and then improve the photocata-lytic activity of the catalysts.

The valance band(VB)of a photocatalyst is an impor-tant factor for the e?ective photocatalytic decomposition of organic contaminants[26].The VB edge position of the x BiOBr–(1àx)BiOI photocatalysts were calculated the-oretically according to the concepts of electronegativity.So far,many chemists have calculated band potentials for semiconductors using the atomic electronegativities of the constituent atoms[27,28].Herein,the electronegativity of an atom is the arithmetic mean of the atomic electron a?n-ity and the?rst onization energy[29],other than the com-mon-de?ned term.The VB edge potential of a semiconductor at the point of zero charge can be expressed empirically by[23,26,30]

E VB?XàE et0:5E g;

where E VB is the VB edge potential,X is the electronegativ-ity of the semiconductor which is the geometric mean of the electronegativity of the constituent atoms,E e is the energy of free electrons on the hydrogen scale(%4.5eV),and E g is the bandgap energy of the semiconductor.According to this empirical expression,the VB edge potentials(E VB)of the system calculated are all higher than2.46eV(see Table 1).It is well known that H2O2and O3can oxidize many organics because they have strong oxidative potential 1.77eV(H2O2)and2.07eV(O3).Compared with them, the present photocatalysts have much stronger oxidation abilities.

It should be noted that BiOBr and BiOBr0.75I0.25, respectively,exhibit the highest photocatalytic activity in the corresponding UV and visible-light-driven (k>420nm)reactions.In the UV-light-induced photocat-alytic reactions,as mentioned above,the maximum emis-sion of the high pressure mercury lamp was365nm,the energy(3.40eV)of which is higher than the bandgaps of all samples(1.92–2.91eV).So the light energies absorbed to generate the electron–hole pairs by di?erent photocata-lysts are almost the same.Thus,photocatalyst with deeper VB position would possess a higher photocatalytic activity. As can be seen from Table1,when x increases,that is,from BiOI to BiOBr,the VB edge potential increases from 2.46eV to 3.19eV,which means the oxidation ability becomes stronger.Consequently,the photocatalytic activ-ity is made to enhance and decomposition degree of MO over x BiOBr–(1àx)BiOI photocatalysts increases mono-tonically with x increasing as shown in Fig.1.

In the visible-light-driven photocatalytic experiments by using Xe lamp,as indicated in Fig.2,more energy in the visible-light region would be utilized by the photocatalyst with x decreasing,which would result in the improvement of the photocatalytic activity.On the other hand,the VB edge potentials of the samples decrease with x decreasing, and the photocatalytic activity is expected to decline from this aspect.The mutual competition between these two e?ects leads to the result that the photocatalytic activity does not change monotonically with x decreasing but reaches a maximum at x=0.75.

4.Conclusion

The new class of oxyhalide photocatalysts x BiOBr–(1àx)BiOI(x=0,0.25,0.50,0.75)show high photocata-lytic activities for degradation of the representative organic dye(MO)in both UV and visible-light-driven reactions. The high activity is in close relation with the internal elec-tric?elds between[Bi2O2]slabs and halogen anionic slabs and the deep valance band edge position.BiOBr is photo-catalytically more active than P25under UV illumination. The mutual competition between the visible-light absorp-tion and valance band potential enables the BiOBr0.75I0.25 sample to possess the highest photocatalytic activity under visible-light(k>420nm)irradiation. Acknowledgements

This research was?nancially supported by National Sci-ence Foundation of China Grant20471068and Shanghai Fundamental Research Grant05JC14079.

W.Wang et al./Catalysis Communications9(2008)8–1211

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