Visible-light-responsive photocatalysts x BiOBr–(1Àx )BiOI
Wendeng Wang,Fuqiang Huang *,Xinping Lin,Jianhua Yang
State Key Laboratory of High Performance Ceramics and Superfine Microstructure,Shanghai Institute of Ceramics,
Chine Academy of Sciences,1295Ding Xi Road,Shanghai 200050,PR China
Received 2November 2006;received in revid form 11March 2007;accepted 4May 2007
Available online 17May 2007
翻译网Abstract
A new class of oxyhalide photocatalysts x BiOBr–(1Àx )BiOI prepared by a soft chemical method were characterized by X-ray diffrac-tion and UV–Vis diffu reflectance 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 possd a higher photocatalytic activity than P25(TiO 2)under UV illumination and iodine-modified BiOBr exhibited high photocatlytic
activities under visible-light irradiation.The high photocatalytic activity is in clo relation with the deep valance band edge position and the internal electric fields between [Bi 2O 2]slabs and halogen anionic slabs.
Ó2007Elvier B.V.All rights rerved.
Keywords:Photocatalysis;Oxyhalide;Methyl orange
1.Introduction
As a potential solution to the recent vere problems of environment cris,destruction of organic and inorganic pollutants on miconductor 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 cond disposal.So far,TiO 2and its modified forms are intenly studied for the oxidative decomposition of many contaminants [8].Very recently,we for the first 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 ($3.5eV),BiOCl can only absorb UV-light,which is merely about 3%of the solar spectrum.Becau of this inherent limitati
on,the abundant solar energy can not be utilized efficiently.So the development of efficient and visible-light-driven photo-
catalysts is indispensable.But the number of such photo-catalyst materials known up to now is yet limited [10–12].BiOX (X =Cl,Br,I)compounds all crystallize in the tetragonal matlockite (PbFCl)structure,a layered struc-ture characterized by [Bi 2O 2]slabs interleaved by double slabs of halogen atoms.BiOI is a coral red material [13],which means it has a strong absorption in the visible-light region.Motivated by the facts,we are interested in the x BiOBr–(1Àx )BiOI system.In the past two decades,the reported photocatalysts are oxides,sulfides,oxysufides,nit-rides and oxynitrides [14–16].To our knowledge,x BiOBr–(1Àx )BiOI are the first oxyhalide photocatalysts working under visible-light (k >420nm).In the prent paper,the relationship between the composition of x BiOBr–(1Àx )BiOI system and its photophysical and photocata-lytic activities was investigated.2.Experimental
x BiOBr–(1Àx )BiOI (x =0.0,0.25,0.50,0.75,1.0)com-pounds were prepared by a soft chemical method similar to Ref.[13].All the reagents were purchad from Sinopharm Chemical Reagent Co.(Shanghai,China)and ud without further purifications.In a typical experiment to prepare the
1566-7367/$-e front matter Ó2007Elvier B.V.All rights rerved.doi:10.1016/j.catcom.2007.05.014
*
Corresponding author.Tel.:+862152411620;fax:+862152413903.E-mail address:huangfq@mail. (F.Huang).
/locate/catcom
Available online at
Catalysis Communications 9(2008)
8–12
BiOBr0.5I0.5sample,0.02mol Bi(NO3)3Æ5H2O(AR, 99.0%)powders werefirstly 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.Subquently,the mixtures were stirred for20h. After the reaction was completed,the resulting solid prod-uct was collected byfiltration,washed veral times with deionized water,and dried at120°C in air before further characterizations.
X-ray powder diffraction(XRD)patterns were obtained on a Rigaku D/max-2550V diffractometer using Cu K a radiation(k=0.15418nm).UV–Vis diffu reflectance 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 effect 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 photocat
alyst 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-offfilters,and an electric fan to prevent thermal catalytic effect.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 parated by a centrifugal machine.Then the concen-tration of thefiltrate 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].
考研数学复习计划
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 posss pure tetragonal phas(corresponding JCPDS-ICDD numbers 09-0393and10-0445).The difference between the halogen ionic radii(Br=1.82A˚,I=2.06A˚)is about12%[18].It is generally accept
ed that unlimited solubility is usually obrved if the difference 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 confirmed by the right shifts of the diffraction peaks with the increa 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 decrea with the increa 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 diffu reflectance 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 inten 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),pudo-first 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 diffu reflectance 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 onts of absorption edges were listed in Table1.They range from1.92to2.91eV and increa monotonically as the contents of bromine element increa.
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 reprented in Fig.3.All the samples exhibit high photocatalytic activities and the activity increas 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 abnce 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 obrvable.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 increa with the increa 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 pudo-first order model as expresd by Eq.(1),which is generally ud for photocatalytic degradation process if the initial concentration of pollutant is low[23],was applied
lnðC e=C tÞ¼ktð1Þin 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 pudo-first order rate constant.The rate constants obtained from the data plotted in Fig.4are concluded in Table1.As it can be en,a good correlation to the pudo-first 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 increas the reaction rate constant by about5.8times.So the composition of the photocatalysts shows its strong influence 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 respon 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 en from thefigure that the profiles of the action spectra of the photocatalysts are all similar to their UV–Vis reflectance spectra as shown in Fig.2,respec-tively.The trends of the curves in Fig.5are the same,that is,with the increa of cut-offwavelength,the photodegra-dation rates of MO decrea gradually.The facts indicate that the prent 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 onts of the action spectra agree approximately with the onts of the UV–Vis diffu reflectance spectra corre-sponding to the bandgap of the miconductors.The results reveal that the reactions are catalyzed by the phot-ocfixed
microelectronicsatalysts[24].
The above-mentioned high photocatalytic activities are believed to be in clo 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 miconductor,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 electricfields between the[Bi2O2]positive slabs and the halogen anionic slabs are believed to induce the efficient paration 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 effective 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 miconductors using the atomic electronegativities of the constituent atoms[27,28].Herein,the electronegativity of an atom is the arith
metic mean of the atomic electron affin-ity and thefirst onization energy[29],other than the com-mon-defined term.The VB edge potential of a miconductor at the point of zero charge can be expresd empirically by[23,26,30]
E VB¼XÀE eþ0:5E g;
where E VB is the VB edge potential,X is the electronegativ-ity of the miconductor 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 miconductor.According to this empirical expression,the VB edge potentials(E VB)of the system calculated are all higher than2.46eV(e Table 1).It is well known that H2O2and O3can oxidize many organics becau they have strong oxidative potential 1.77eV(H2O2)and2.07eV(O3).Compared with them, the prent 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(1.92–2.91e
上海张江集团学校V).So the light energies absorbed to generate the electron–hole pairs by different photocata-lysts are almost the same.Thus,photocatalyst with deeper VB position would posss a higher photocatalytic activity. As can be en from Table1,when x increas,that is,from BiOI to BiOBr,the VB edge potential increas from 2.46eV to 3.19eV,which means the oxidation ability becomes stronger.Conquently,the photocatalytic activ-ity is made to enhance and decomposition degree of MO over x BiOBr–(1Àx)BiOI photocatalysts increas 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 decrea with x decreasing, and the photocatalytic activity is expected to decline from this aspect.The mutual competition between the two effects 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
传达英语
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 reprentative organic dye(MO)in both UV and visible-light-driven reactions. The high activity is in clo relation with the internal elec-tricfields 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 posss the highest photocatalytic activity under visible-light(k>420nm)irradiation. Acknowledgements
This rearch wasfinancially supported by National Sci-ence Foundation of China Grant20471068and Shanghai Fundamental Rearch Grant05JC14079.
W.Wang et al./Catalysis Communications9(2008)8–1211
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