Novel Heterojunction Bi2O3/SrFe12O19Magnetic Photocatalyst with Highly Enhanced Photocatalytic Activity
Taiping Xie,*,?Chenglun Liu,*,?Longjun Xu,?Jun Yang,?and Wei Zhou?
?State Key Laboratory of Coal Mine Disaster Dynamics and Control,Chongqing University,Chongqing400044,P.R.China ?College of Chemistry and Chemical Engineering,Chongqing University,Chongqing400044,P.R.China
help Bi2O3absorb more incident photons and then
expected to provide a simple preparation method for
1.INTRODUCTION
Since the photolysis water using a TiO2semiconductor electrode under UV light irradiation was found by Fujishima and Honda in1972,1the photocatalytic techniques and corresponding photocatalytic materials have attracted over-whelming attention for many years,especially in recent years, which is attributable to a fact that the compatibility of photocatalysis with modern technology is excellent.In other words,the photocatalysis can be applied in many?elds,such as environmental control or remediation,and energy-related areas. Interestingly,the application of the TiO2photocatalysis in o?set printing was found and reported by Nakata.2,3As many scientists speculated,the prospective applications of photo-catalysis still need to be continuously explored with great e?ort. In fact,the exploration of new photocatalysts is as important as the development of photocatalysis applications.4,5So?nding new photocatalysts and improving the obtained photocatalyst for better and larger contribution toward their practical applications are still essential.
Generally,two strategies can be used to enhance photo-catalytic activity of a photocatalyst.One way is to make the photocatalyst absorb more incident photons and subsequently produce more photoexcited electron?hole pairs.The other is to prevent photoproduced electron?hole pairs from recombin-ing,i.e.,enhancement of electron?hole separation e?ciency.6?8 A kind of p-type Bi2O3semiconductor was considered as a promising photocatalytic material alternative to other many binary oxides,which was attributable to a fact that Bi2O3 possessed the band gap energy in visible light range,i.e.,ca.2.8eV,which enabled itself to oxidize water and to produce some active species that could initiate oxidation reactions. However,in the practical application of wastewater treatment using photocatalyst,such as Bi2O3,TiO2,ZnO,and BiOCl, chemical scientists found a troublesome problem that the separation of photocatalyst powder from water following treatment was complicated,time-consuming,and high-cost.2,3 Suppose that the used photocatalyst could not be recycled exhaustively,the remaining photocatalyst probably caused secondary pollution,which violated the initial idea for wastewater treatment.It was encouraging to?nd that making the photocatalyst possess magnetization,namely,magnetic photocatalyst could overcome these problems.11?14
Owing to its various excellent properties,for instance, relatively large magnetization,superior coercivity,a better chemical stability,and corrosion resistivity,15as a species of hard-magnetic material,SrFe12O19has also been a focus of considerable attention in recent years.It was pleased to know that SrFe12O19could be used as magnetic substrate for magnetic catalyst,con?rmed by Pullar,16Ji,17Aziz,18and our previous investigation.19It was more interesting to?nd that as a kind of n-type semiconductor SrFe12O19also could be directly used as photodegradation reaction under visible light on account of its narrow band gap,i.e.,ca.1.86eV.11,20 Received:August28,2013
Revised:October24,2013
Published:October25,2013
In light of the aforementioned considerations,therefore,in this work,coupling SrFe 12O 19with Bi 2O 3was to prepare a novel p ?n-type heterojunction Bi 2O 3/SrFe 12O 19between p-type Bi 2O 3semiconductor and n-type SrFe 12O 19semiconduc-tor.It was well-known that a heterojunction structure with a matching band gap potential might enhance the separation e ?ciency of photoproduced electron ?hole pairs.4So the introduction of SrFe 12O 19could enhance photocatalytic ability of Bi 2O 3,as a consequence of the formation of a p ?n-type heterojunction.Meanwhile,magnetic ?eld stemming from SrFe 12O 19could make the composite photocatalyst capable of easier separation and recycling.In this work,therefore,a novel heterojunction,Bi 2O 3/SrFe 12O 19magnetic photocatalyst with the best of both worlds was fabricated and characterized.Its photocatalytic activity was preliminarily evaluated using (methylene blue)MB degradation,and a possible mechanism for the enhanced photocatalytic ability was thoroughly interpreted from three aspects.2.EXPERIMENTAL SECTION All reagents (such as Bi(NO)3,HNO 3,absolute alcohol,and sodium dodecyl benzene sulfonate obtained from Sinopharm Chemical Reagent Co.,Ltd.)were of analytical grade purity and used directly without further puri ?cation,except SrFe 12O 1921was prepared using industrial strontium residue.The water used in all experiment was deionized water. 2.1.Preparation of Pure Bi
2O 3.A certain amount of
Bi(NO)3was weighted and dissolved in 4mol/L HNO
3
solution to form a homogeneous solution.The pH of the prepared homogeneous solution was adjusted to more than 7.The chemical reaction could be obviously observed due to color change.The obtained yellow precipitate by ?ltrating was washed several times,and then was dried at 60°C.The dry
block was crushed into ?ne powder and was sintered at 550°C
for 3h to obtain pure Bi
2O 3.
2.2.Preparation of Bi 2O 3/SrFe 12O
19.Similarly,a certain
amount of Bi(NO)3was weighted and dissolved to form a
homogeneous solution (A)using 4mol/L HNO
3solution.
SrFe 12O 19with a mass ratio of 20%and a small amount of
sodium dodecyl benzene sulfonate (SDBS)were weighted and dissolved under vigorous stirring to form a suspension (B).The whole mixture was obtained by blending A solution and B suspension.The pH of the whole mixture was adjusted to 13.
Then the whole mixture was ?ltrated.The ?lter residue was
washed several times with absolute alcohol to remove the
remaining SDBS.The washed ?lter residue was dried at 60°C.
The composite Bi 2O 3/SrFe 12O 19(20wt %)was obtained by
sintering the dried ?lter residue at 550°C for 3h.The composites Bi
2O 3/SrFe
12O
19(25,30,35,and 40wt %)were
prepared by adjusting the di ?erent mass ratios of SrFe
12O 19.2.3.Material
Characterizations.Fourier transform infra-red spectroscopy (FTIR)spectra of samples were recorded on
a
Figure 1.(a ?c)Time-dependent UV ?vis absorption spectra of the MB in the presence of Bi 2O 3prepared at di ?erent pH values;(d)photocatalytic
degradation ratio of MB versus visible light (≥420nm)irradiation time by Bi 2O 3photocatalyst.
5DX FTIR(5DX,Nicolet.Co.,USA)spectrometer using KBr powder-pressed pellets.Phase identi?cation via X-ray di?rac-tion(XRD)was conducted on an X-ray di?ractometer(Bruker Advance D8)using Cu Kαirradiation at a scanning rate of4°·min?1with the2θrange of20?70°.X-ray photoelectron spectroscopy(XPS)measurements were carried out on an XPS-XSAM800(Kratos,U.K.)spectrometer with an achro-matic Al KαX-ray source and an analytical chamber with a base pressure of2×10?7Pa.The X-ray gun was operated at180W (12kV,15mA).The magnetic properties were investigated using a vibrating sample magnetometer(VSM,Lakeshore 7410)in applied?elds up to5T at room temperature.The Brunauer?Emmett?Teller(BET)special surface area was determined through N2adsorption at77K using an adsorption instrument(ASAP-2020,Micromeritics,USA).The samples’morphologies and microstructures were observed by scanning
electron microscopy(SEM,FEI,F50)and transmission electron microscopy(TEM,FEI,Tecnai G2F20).Meanwhile, Digital Micrograph software was used to analyze high-resolution transmission electron microscopy(HRTEM).The UV?vis di?use re?ectance spectra(DRS)of samples were measured using a UV?vis spectrophotometer(TU1901, China).BaSO4was used as a re?ectance standard.
2.4.Evaluation of Photocatalytic Activity.The photo-catalytic activity of Bi2O3/SrFe12O19composite was evaluated by(methylene blue)MB degradation under irradiation of a500 W Halogen lamp(λ>420nm,light intensity was110.8mW/ cm2)at the natural pH value.A200mL of10mg/L MB aqueous solution and its corresponding composite dosage of2 g/L were added into a quartz container and stirred for1h in the dark.After a given irradiation time,about3mL of the mixtures was withdrawn.Then the solution and Bi2O3/ SrFe12O19particles were separated by an extra magnet.The photocatalytic degradation process of MB was monitored by measuring its characteristic absorption at664nm with a UV?vis spectrophotometer.
3.RESULTS AND DISCUSSION
3.1.Optimum pH for Preparing Bi2O3.In previous reports,the di?erent pH values,such as8,9,and13,10,22,23 were adopted in the process of preparation of pure Bi2O3.Here, therefore,the in?uence of pH on photocatalytic activity of pure Bi2O3was evaluated to?nd a suitable pH.The results(Figure 1)remarkably showed that Bi2O3(pH=13)was superior to both Bi2O3(pH=8)and Bi2O3(pH=9)in terms of photocatalytic e?ciency under identical test conditions.The photocatalytic e?ciency of Bi2O3(pH=13)reached to97.4% after6h of photodegradation.So the optimal pH value for preparing Bi2O3was13.We speculated that the morphologies of Bi2O3preparing at di?erent pH values were di?erential.The detail had not been explored because the main aim of this work was the composite preparation and photocatalytic activity evaluation,but the investigation of thorough mechanism about this point is ongoing in our group.Therefore,pH=13was accepted in the process of the synthesis composite.
3.2.FTIR,XRD,XPS,VSM,and BET Analyses.Primary analysis of photodegradation revealed that Bi2O3/SrFe12O19(35 wt%)was the most e?cient in the MB degradation. Figure2showed FTIR spectra of pure Bi2O3and composite Bi2O3/SrFe12O19(35wt%).The intensive signal around432.8 and510.8cm?1appeared in IR spectrum of Bi2O3was ascribable to the stretching vibration of Bi?O bonds.The three characteristic peaks of SrFe12O19were at603.0,553.2,and 451.4cm?1,respectively.21Although little shift of the corresponding peaks of both Bi2O3and SrFe12O19in the FTIR spectrum of the composite was observed due to vibrational coupling between the peaks of SrFe12O19and Bi?O bond,their own characteristic peaks could be found in the FTIR pattern of the composite.In addition,the peaks at3441.7 and1631.5cm?1exhibited the stretching vibration and deformation vibration of hydroxy group(?OH)acquired from wet atmosphere.
The XRD spectra of pure Bi2O3and composite Bi2O3/ SrFe12O19(35wt%)were shown in Figure3.For comparison,
XRD pattern of pure SrFe12O19was added into Figure3.The 2θvalues of SrFe12O19were at30.4,32.3,34.2,37.1,and55.2°corresponding to(110),(107),(114),(203),and(217) di?raction phases.21The peaks observed at27.0,27.5,28.1, 33.7,and46.5°corresponded to(112),(121),(012),(112), and(041)di?raction planes of Bi2O3(No.PDF7-0398)that was a member of space group P21/C(14).The lattice parameters of the prepared Bi2O3were a=5.830?,b= 8.148?,and c=7.480?.Even if the intensity of di?raction peaks of Bi2O3was relatively stronger and the counterpart of SrFe12O19was relatively weaker,the characteristics peaks of the two materials in XRD pattern of composite could be observed. At the same time,there was no remarkable shift of di?raction peaks and no other crystalline impurities.The
above-mentioned Figure2.FTIR spectra of pure Bi2O3and Bi2O3/SrFe12O19(35wt%)
composite.
Figure3.XRD spectra of pure Bi2O3,pure SrFe12O19,and composite Bi2O3/SrFe12O19(35wt%).
analyses indicated that the as-prepared composite was desirous material,namely,composite photocatalyst Bi 2O 3/SrFe 12O 19.It was worthwhile noting that the peaks at 27.5°for both pure Bi 2O 3and Bi 2O 3/SrFe 12O 19were sharper and stronger.The relatively high intensity of the (121)peak was indicative of anisotropic growth and implied a preferred orientation of the crystallites.Indeed,Bi 2O 3should favor growth along the [121]orientation.24This meant that the introduction of SrFe 12O 19did not change the preferred orientation of the crystallites of Bi 2O 3and its crystal structure.To ?nd out the presence of elements in Bi 2O 3/SrFe 12O 19and to determine their valence states,we carried out XPS study.The binding energy peaks of Bi,Sr,Fe,and O were analyzed.The corresponding high-resolution spectra were shown in Figure 4.In Figure .4a,the peaks located at 158.6and 163.9eV were ascribed to Bi 4f 7/2and Bi 4f 5/2,respectively,con ?rming that bismuth species in Bi 2O 3/SrFe 12O 19composite were Bi 3+
cations.9In Figure 4b,the Sr 3d pro ?le was asymmetric and could be ?tted to two symmetrical peaks located at 133.2and 134.9eV corresponding to the photoelectron peaks of the Sr 3d 5/2and Sr 3d 3/2,respectively.The previous literature reported that the Sr 3d 5/2was assigned to surface Sr ?O bonds,and the Sr 3d 3/2could verify the presence of Sr 2+.11
Figure 4c showed the Fe 2p peaks were at binding energies of 711.3eV (Fe 2p 3/2)and 725.1eV (Fe 2p 1/2),which was consistent with photoelectron peaks of Fe 3+in SrFe 12O 19
system.11Similarly,O1s spectrum (Figure 4d)was also asymmetric and could be ?tted to three symmetrical peaks situating at 529.4,531.0,and 532.7eV,which were ascribable to O 2?in SrFe 12O 19system,Bi ?O bonds,and chemisorbed oxygen,7respectively,revealing that three kinds of O species in the composite.The XPS analyses indicated that the presences of Bi 2O 3and SrFe 12O 19,which was consistent of XRD and
FTIR investigations.The analysis of magnetic properties for magnetic composite
Bi 2O 3/SrFe 12O 19(35wt %)was indispensable.The magnetic hysteresis loop of the prepared composite was illustrated in Figure 5.Similarly,the magnetic hysteresis curve of SrFe
12O
19
was inserted for comparison.Their magnetic parameters were listed in Table 1.
Like the SrFe 12O 19itself,the composite also possessed a high coercivity (Hc),i.e.,337.06kA ·m ?1,revealing the composite was also a kind of hard-magnetic material that was blessed with good antidemagnetize ability,15which was favorable toward its reuse as
photocatalyst.
Figure 4.XPS analysis of composite Bi 2O 3/SrFe 12O 19(35wt
%).
Figure
5.Magnetic hysteresis loop of composite Bi 2O 3/SrFe 12O 19(35
wt %).
Although the saturation magnetization (M s )and remanent magnetization (M r )of composite decreased by 35.8%and 36.5%,respectively,compared to pure SrFe 12O 19,because of a decrease in the amount of SrFe 12O 19per gram of composite,the magnetic properties of the composite were still relatively excellent.The M s and M r of the composite were 25.42and 15.20A ·m 2·kg ?1,respectively,which was bene ?cial to its separation from liquid solution and its recycling from reaction solution using an extra magnet after use.From the aforementioned analyses,we could speculate that the synthesis process of composite did not alter crystal structure of SrFe 12O 19.Moreover,we could verify the presence of SrFe 12O 19in the composite,which further con ?rmed that the magnetic composite Bi 2O 3/SrFe 12O 19was successfully pre-pared.The pore structure of composite Bi 2O 3/SrFe 12O 19(35wt %)was investigated by N 2adsorption ?desorption isotherms.The corresponding pore size distribution and speci ?c surface area were determined using the BJH method and BET method,respectively.The N 2adsorption ?desorption isotherms and pore size distribution curve were shown in Figure 6.This isotherm could be categorized as a typical Type III isotherm,which was convex to the P /P 0axis over its entire range,11
indicating that the as-prepared composite Bi 2O 3/SrFe 12O 19
belonged to nonporous structure.This sharp increase in the adsorption isotherm was attributed to the macropore size.The most probable pore size of composite Bi 2O 3/SrFe 12O 19was 4.49nm.In addition,the single-point adsorption total pore volume of pores less than 396.320?radius at P /P 0=0.9750was 0.001256cm 3·g ?1.These results further indicated the nonporous structure.The speci ?c surface area of this composite was given by BET measurement as 1.12m 2·g ?1.So we could ignore the absorption when the composite was used to decompose MB due to nonporous structure and small speci ?c surface area. 3.3.SEM Images.Figure 7showed SEM images of pure
Bi 2O 3and composite Bi 2O 3/SrFe 12O 19(35wt %).The grain
sizes of pure Bi 2O 3and composite Bi 2O 3/SrFe 12O 19were in the range of ca.0.3?1.0μm and 0.1?0.5μm,respectively,indicating that the growth of Bi 2O 3was con ?ned due possibly to SrFe 12O 19possessing a large molecule mass.The morphology for pure Bi 2O 3was rod-like and double irregular sphere-like,but in the composite system,the morphology of Bi 2O 3was blocks of irregular shape.This demonstrated that the morphology of Bi
2O 3was di ?erent before and after
introduction of SrFe 12O 19.SrFe 12O 19changed the morphology
of Bi 2O 3but did not alter its preferential growth direction.Interestingly,a phenomenon that the lamellae-like SrFe 12O
19
inserted into Bi 2O 3was observed.In addition,the surface for
both pure Bi
2O
3and Bi 2O
3in the composite system was
smooth and no remarkable defects on the surface were found.It
was known that the surface defect of photocatalyst usually was a
recombination center for photoproduced electron and holes.5
Therefore,the smooth surface with defects was conducive to
photocatalysis.3.4.TEM and HRTEM Images.The irregular block-shaped
morphology of pure Bi 2O 3was shown in Figure 8a.The
morphology of composite Bi 2O 3/SrFe
12O 19(35wt %)was
presented in Figure 8b.It was notable to see that
some
Table 1.Magnetic Parameters of Composite Bi 2O 3/SrFe 12O 19(35wt %)and Pure SrFe 12O 191219Bi 2O 3/SrFe 12O 19
25.4215.20
337.06
Figure 6.N 2adsorption ?desorption isotherm of Bi 2O 3/SrFe 12O 19(35
wt %).Inset:the corresponding pore size
distribution.Figure
7.SEM images of pure Bi 2O 3(a)and composite Bi 2O 3/
SrFe 12O 19(35
wt %)
(b).Figure
8.TEM images of pure Bi 2O 3(a)and composite Bi 2O 3/
SrFe 12O 19(35wt %)(b);HRTEM image (c,d)at di ?erent positions
for the composite.
relatively small SrFe 12O
19laminar-shaped structures were
loaded onto the surface of Bi 2O 3,which would result in the formation of heterostructured Bi 2O 3/SrFe 12O 19.The reported hydrolysis with sintering method here was a simple process without rigorous conditions,and hence,it was a low-cost and convenient method to prepare a magnetic composite Bi 2O 3/
SrFe 12O
19with
heterojunctions.To further con ?rm this structure,HRTEM observation was carried out.From the HRTEM images (Figure 8b),we could clearly ?nd SrFe 12O 19was attached and distributed on the surface of Bi 2O 3.The interplanar spacing was ca.0.332and 0.403nm,which also corresponded to the (112)and (020)planes of Bi 2O 3,respectively.Because Bi 2O 3and SrFe 12O
19were p-type and n-type semiconductors,respectively,the heterojunction could be considered to be a well-de ?ned and well-formed p ?n-type heterojunction.3.5.Photocatalytic Activity and Stability.3.5.1.UV ?vis DRS Analysis.The optical properties of the heterojunction Bi 2O 3/SrFe 12O 19were explored by UV ?vis di ?use re ?ectance.The DRS of pure Bi 2O 3and Bi 2O 3/SrFe 12O 19composites with di ?erent mass ratios were displayed in Figure 9.A same
phenomenon for both pure Bi 2O 3and Bi
2O
3/SrFe
12O
19
composites observed was a strong absorption in UV light range of 200?400nm.It was inspiring to see that the composites showed intense absorption in a wide wavelength range from UV to visible light with absorption tail extending
into infrared region,compared to the absorption spectrum of the pure Bi
2O 3.Meanwhile,the intensity of absorption of
visible light increased along with the increase in the amount of
SrFe
12O 19,i.e.,its corresponding mass ratio.The above-
mentioned analyses revealed that SrFe
12O 19enhanced the
absorption of
Bi 2O 3in visible light region,namely,photo-
response for visible light.
The optical band gap energy (E
g )of a crystalline semi-
conductor was estimated by the formula Ahv =(h v ?E
g )n /2.5,25
The band gap energies for the prepared materials were
determined from (Ahv )2versus hv plots,as also shown in Figure 9(see insets).The E
g of pure Bi
2O
3was 2.83eV,which was in good agreement with the previous literature,7,9,26further
con ?rming the valid synthesis procedure for syntheses of Bi 2O
3
Figure 9.UV ?vis di ?use re ?ectance spectra (DRS)of composites.Inset:the corresponding band gap energy.
and composites.The E g s of composites gradually decreased with the increase in mass ratio of SrFe 12O 19,which was consistent with their corresponding di ?use re ?ectance spectra.All in all,the prepared composites not only could be considered as a UV-light-driven semiconductor photocatalyst,but also could be used as a visible-light excitation semi-conductor photocatalyst.According to intense absorption of Bi 2O 3in UV light range and some previous investigations,9,22we might know that Bi 2O 3had a better photocatalytic activity under UV light irradiation.In this work,therefore,preparing heterojunction was to make good use of sunlight by extending the absorbing light range and further to enhance the possibility of practical application.So the photocatalytic abilities of as-prepared composites were evaluated only under the visible light irradiation. 3.5.2.Photocatalytic Activity.The photocatalytic activities of composites were tested by MB degradation under visible light irradiation for 4h.The UV ?vis absorption spectra of the MB were obtained using UV ?vis spectrophotometer after a reaction interval of 1h.The results were shown in Figure 10a ?e.The intensity absorbance of MB at 664nm gradually decreased along with extension of reaction time.The peaks at 664and 288nm did not shift,and no other peak appeared,which indicated that only the pure photochemical reaction was generated.Figure 10f showed the photocatalytic degradation ratio of MB.The photocatalytic activity of pure Bi 2O 3was superior to that of Bi 2O 3/SrFe 12O 19(20and 30wt %),which revealed that in the composite system the main active central would gather on the surface of Bi 2O 3.However,the photocatalytic activity
of
Figure 10.(a ?e)Time-dependent UV ?vis absorption spectra of the MB in the presence of various composite photocatalysts;(f)photocatalytic degradation ratio of MB versus visible light (≥420nm)irradiation time using photocatalysts.
Bi2O3/SrFe12O19(35wt%)was better than the counterpart of Bi2O3/SrFe12O19(45wt%).Theoretically,E g s of Bi2O3/ SrFe12O19(35wt%)and Bi2O3/SrFe12O19(45wt%)were2.74 and2.72eV,respectively.Bi2O3/SrFe12O19(45wt%)under visible light could generate more photoinduced electrons than Bi2O3/SrFe12O19(35wt%),and the photocatalytic ability of Bi2O3/SrFe12O19(45wt%)was better than Bi2O3/SrFe12O19 (35wt%).Actually,this theoretical speculation was opposite to the practical results.On the one side,this phenomenon still could reveal Bi2O3acted as a main photocatalyst in the composite system.Moreover,only suitable mass ratio of SrFe12O19(here,it is35wt%)in the p?n-type heterojunction structure possessed the best photocatalytic ability.The above results indicated that an insu?cient SrFe12O19amount in Bi2O3/SrFe12O19heterojunction structures could not e?ectively separate photogenerated electrons and holes from hetero-junction structures,giving rise to a low photocatalytic activity. On the other side,an excess SrFe12O19amount in heterojunction structures could also decrease the photocatalytic activity because excess SrFe12O19might o?er as the recombination centers of electron?hole pairs,leading to a lower photocatalytic activity.
Provided that the photodegradation process was a pseudo-?rst-order reaction,the apparent reaction rate constant(k) could be?tted by the following equation:
?=
C C kt
ln(/)0(1) where k was the apparent reaction constant,C0was the initial concentration of MB,and C was the MB concentration at di?erent reaction time.The k values were shown in Figure11.
Among these k values,the k for Bi2O3/SrFe12O19(35wt%) was the largest.The k of Bi2O3/SrFe12O19(35wt%)(0.882 h?1)was about2.8times as high as that of pure Bi2O3(0.318 h?1).These calculated results were consistent with the above-mentioned experimental results that the degradation ratios of MB for pure Bi2O3and Bi2O3/SrFe12O19(35wt%)were97.4% after6h reaction and97.7%after4h reaction,respectively.
3.5.3.Stability and Recycle of Bi2O3/SrFe12O19.Besides the higher activity,the photostability and reusability were also indispensable for photocatalysts,especially for the magnetic photocatalyst.The prepared magnetic composite was easily recycled with an extra magnet,which was attributable to the fact that the composite(Bi2O3/SrFe12O1935wt%)possessed a higher magnetization(Table1).The photocatalysis stability of the Bi2O3/SrFe12O19(35wt%)was con?rmed by repeating the decomposition process for four times,as shown in Figure12.
The experimental results revealed the composite could be reused at least four times with no signi?cant decrease in activity, which was indicative of a higher stability.In fact,in the experimental process,we found the composite dispersibility was increasingly good after reuse.
3.6.Possible Photocatalysis Mechanism.To explain the enhanced photocatalytic activity of the Bi2O3/SrFe12O19 composite,we proposed a possible mechanism in Figures
13
Figure11.Pseudo-?rst-order rate constants of decomposition of MB
over pure Bi2O3and composites Bi2O3/SrFe12O19with di?erent mass
ratios.
Figure12.Recycle experiments of degrading MB on the composite
Bi2O3/SrFe12O19(35wt%)under visible light
irradiation.
Figure13.Schematic diagram of charge transfer between p-type Bi2O3
and n-type SrFe12O19,i.e.,p?n-type
heterojunction.
Figure14.Schematic diagram for E g matching and?ow of
photoinduced electrons for the prepared samples under visible light
irradiation.
and14.Three main reasons for the increase in the photocatalytic e?ciency of Bi2O3/SrFe12O19were(1)the formation of p?n-type heterojunction between p-type Bi2O3 and n-type SrFe12O19semiconductors,(2)magnetic?eld e?ect stemming from magnetic composite itself,and(3)as a sensitizer absorbing visible light,SrFe12O19could help Bi2O3 absorb more incident photons.The three factors might generate synergistic e?ect for the enhancement of photo-catalytic ability.The following is a detailed interpretation of the main reasons.
(1)The enhanced photocatalytic activity of Bi2O3/SrFe12O19 heterogeneous structures compared to the pure Bi2O3was partly ascribed to the formation of the p?n-type heterojunction between p-type Bi2O3and n-type SrFe12O19semiconductors (see Figure13).The conduction band(CB)and valence band (VB)potentials of the p-type Bi2O3semiconductor were0.30 and3.13eV,7,9,10respectively.The CB and VB for n-type SrFe12O19were0.20and2.06eV,20respectively.Before contact of p-type Bi2O3semiconductor and n-type SrFe12O19semi-conductor,the CB potential of n-type SrFe12O19was more negative than that of p-type Bi2O3,and the VB potential of p-type Bi2O3was more positive than that of n-type SrFe12O19. Thus,photoexcited electrons on SrFe12O19underwent vertical transfer to Bi2O3,whereas holes on Bi2O3migrated to SrFe12O19,which would produce an inner electric?eld.5In turn,the migrations of photogenerated electrons and holes would be promoted by the formed inner electric?eld,27which facilitated the charge separation and a higher photocatalytic ability.
(2)The enhanced photocatalytic activity of the composite magnetic photocatalyst Bi2O3/SrFe12O19compared to the pure Bi2O3was possibly partly attributed to magnetic?eld e?ect originating from the magnetic photocatalyst itself.The remanent magnetization(M r)of composite magnetic photo-catalyst Bi2O3/SrFe12O19(35wt%)was15.20A·m2·kg?1, which manifested the Bi2O3/SrFe12O19(35wt%)could generate a stable magnetic?eld.It was well-known that charges in stable magnetic?eld would travel in uniform motion along the direction of magnetic?eld or move in uniform circular motion around the magnetic?eld direction.28,29
A part of photoproduced electrons moved from V
B to CB along magnetic?eld direction,which kept the original motion direction of photoinduced electron,like in pure Bi2O3(Figure 14).Other part of photoinduced electrons traveling in uniform circular motion spiralled upward.The magnetic?eld was conducive to the motion of photoinduced electrons from VB to CB with two di?erent directions,which was tantamount to the increase in motion velocity of photoinduced electrons.Thus, more photoinduced electrons and holes would be produced and accumulated at the conduction band and valence band, respectively.In other words,the magnetic?eld could enhance charge collection e?ciency.Meanwhile,the recombination between photoinduced charges and holes could be inhibited through this magnetic?eld.Thus,the separation e?ciency of electrons and holes was improved,which subsequently enhanced photocatalytic activity.
(3)Owing to the color of SrFe12O19itself(black),SrFe12O19 could absorb some parts of the solar spectrum.11Moreover,the E g of SrFe12O19was1.86eV,indicating that it could absorb light in≤670nm wavelength ranges,namely,almost all the visible light ranges.In the composite photocatalyst Bi2O3/ SrFe12O19system,Bi2O3was a main photocatalyst,while SrFe12O19acted as a sensitizer absorbing visible light.SrFe12O19could help Bi2O3increase its photoresponse of visible light and absorb more photons and then produce more photoexcited electron?hole pairs.30Thus,the composite photocatalyst Bi2O3/SrFe12O19could receive more incident photons and produce more photoexcited electrons than pure Bi2O3under identical light irradiation.So the electrons and holes were free to initiate reactions with the reactants adsorbed on the photocatalyst surface,leading to the enhanced photocatalytic activity.
4.CONCLUSIONS
The magnetic composite photocatalyst Bi2O3/SrFe12O19was synthesized by hydrolysis with medium temperature sintering method.The synthesis method facilitated mass production as a result of its simple and low-cost procedure.This work was expected to provide a simple preparation method for various functional materials.In addition,this novel magnetic composite might?nd important applications in relevant?elds.
XRD study revealed that the introduction of SrFe12O19did not change the favorite growth direction of Bi2O3,[121] orientation.Micromorphology investiagtion indicated that SrFe12O19was distributed on the surface of Bi2O3to form some heterojunction structures.VSM measurements man-ifested the composite possessed better magnetic properties, which was conducive to its separation,recycling,and reuse. Three main reasons for the increase in the photocatalytic e?ciency of Bi2O3/SrFe12O19were(1)the formation of p?n-type heterojunction between p-type Bi2O3and n-type SrFe12O19semiconductors,(2)magnetic?eld e?ect stemming from magnetic composite itself generated a shunt e?ect for photoexcited electrons,and(3)as a sensitizer absorbing visible light,SrFe12O19could help Bi2O3absorb more incident photons.The common aim for the three factors was that more photoproduced electron?hole pairs were generated and less recombination for electron?hole pairs could come about prior to participation in oxidation?reduction reaction.Thus, more e?ective degradation reaction would take place to
enhance photocatalytic e?ciency.
■AUTHOR INFORMATION
Corresponding Authors
*(T.X.)Tel/Fax:+862386809361.E-mail:deartaiping@163. com.
*(C.L.)E-mail:xlclj@https://www.doczj.com/doc/846198766.html,.
Notes
The authors declare no competing?nancial interest.■ACKNOWLEDGMENTS
We would like to thank our associates,especially Ting Liao and Wenli Wu,for their valuable contributions to our research program.We want to thank the?nancial support from the National Science Foundation of China(NSFC,51374259)and Scienti?c Research Foundation of State Key Laboratory of Coal Mine Disaster Dynamics and Control(2011DA105287-KF201310).We gratefully acknowledge many important
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