Visible Light-Induced Photocatalytic Oxidation of Phenol and Aqueous Ammonia inFlowerlike Bi2Fe4O9 Suspensions

Songmei Sun, Wenzhong Wang,* Ling Zhang, and Meng ShangState Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute ofCeramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China

ReceiVed: April 1, 2009; ReVised Manuscript ReceiVed: June 9, 2009

Photocatalytic active flowerlike Bi2Fe4O9 was synthesized by a hydrothermal method. Scanning (SEM) andtransmission electron microscopy (TEM) images revealed that the photocatalysts are composed of well-crystallized nanoplates along the (100) plane. The as-prepared nano-Bi2Fe4O9 exhibited excellent photocatalyticoxidation of phenol and aqueous ammonia under visible light irradiation. Much enhanced photocatalyticperformance for phenol oxidation was observed with the assistance of a small amount of H2O2. For guidingthe further improvement of its photocatalytic activity, the relationship between the electronic structure andthe photocatalytic performance of Bi2Fe4O9 is revealed by ab initio calculations for the first time. The calculationresults indicate Bi2Fe4O9 is a multiband semiconductor, and the principle for an improved photocatalyticperformance is to increase the efficiency of electron-hole separation considering the middle energy bandthat may act as electron-hole recombination centers.

1. Introduction

It is believed that the properties of functional materialsstrongly depend on their morphology, microstructure, dimension,crystallinity, and so forth.1-3 Controllable microstructure andmorphology is important not only for fundamental research butalso for technological applications. In particular, the alignmentof nanostructure building blocks (nanoparticles, nanorods, andnanoribbons/nanoplates) into three-dimensional (3D) orderedsuperstructures has been an exciting field in recent years.4 Todate, a wide variety of functional materials, including metal,metal oxide, sulfide, hydrate, and other minerals, have beensuccessfully prepared with hierarchical structures.5-9 Synthesismethod and its corresponding process have proven to be criticalto the control of the microstructure. Hydrothermal route is oneof the most promising routes, because of its low cost andpotential advantage for large-scale production.6,9

Bi2Fe4O9 is an important material that can be used as asemiconductor gas sensor and catalyst for ammonia oxidationto NO.10-13 Besides the above-mentioned applications, a recentlystudy has reported that Bi2Fe4O9 nanoplates can act as photo-catalysts for methyl orange (MO) degradation under visible lightirradiation, while the Bi2Fe4O9 microplates only exhibit thisperformance under UV light.14 This study implied a morphol-ogy-dependent photocatalytic property of the Bi2Fe4O9 photo-catalyst. Further studies are needed to investigate the photo-catalytic performance with other morphologies such as theflowerlike structure which has not been reported up to thepresent, although the preparation of Bi2Fe4O9 has been reportedby many methods.15-18 Flowerlike morphology usually providesplenty of meso- and macropores, which can be considered astransport paths for reactants and facilitated chemical reactionswithin its hierarchical structures.19 Our previous studies havereported that flowerlike Bi2WO6, Bi2O3, and Bi5FeTi3O15

exhibited excellent photocatalytic activity.20-22 The same resultmay be obtained in the Bi2Fe4O9 system.

In addition to the morphology, the relationship between thephotocatalytic property and the electronic structure should alsobeen studied to guide the further improvement of the photo-catalytic activity. Moreover, investigations on photocatalyticperformance only by photodegradation of dyes is not convincing,as it is hard to avoid the decoloring of dyes from adsorptionand photosensitization. Photodegradation experiments on otherorganic contaminants are necessary to further evaluate thephotocatalytic performance and make preparations for itspractical applications in the future. Aqueous ammonia as oneof the major nitrogen-containing pollutants in wastewater is apotential source of oxygen depletion due to eutrophication.23,24

Both ammonium NH4+ and its conjugate base NH3 can be

present in water and wastewater. Although the photocatalyticoxidation of NH4

+/NH3 by TiO2 has been reported in severalstudies,25-28 there are few reports on the photodegradation ofNH4

+/NH3 in other systems. It is significative to start the studieson the NH4

+/NH3 degradation by other photocatalysts, such asthe prepared Bi2Fe4O9 under visible light irradiation. BesidesNH4

+/NH3, the photocatalytic degradation of phenol on Bi2Fe4O9

is also reported in the present paper. As is well known, phenolis a widely used organic chemical present in a variety ofwastewaters from various different industries,29-31 and it is quitetoxic and slowly degradable in the environment. The phenolremoval is also of high environmental concern to find a goodphotocatalyst for its oxidation.32,33

In the present paper, flowerlike Bi2Fe4O9 was hydrothermallysynthesized with the assistance of an ambient magnetic field,and the work is mainly focused on the visible light-inducedphotodegradation of NH4

+/NH3 and phenol in aqueous solution.The as-prepared Bi2Fe4O9 materials exhibited excellent photo-catalytic activity toward phenol and ammonia oxidation. Whenmixed with a small amount of H2O2, the Bi2Fe4O9 sampleshowed much enhanced photocatalytic activity. Additionally,the relationship between the photocatalytic performance and theelectronic structure was also revealed for the first time.

* Corresponding author. Tel.: +86-21-5241-5295. Fax: +86-21-5241-3122. E-mail: wzwang@mail.sic.ac.cn.

J. Phys. Chem. C 2009, 113, 12826–1283112826

10.1021/jp9029826 CCC: $40.75 2009 American Chemical SocietyPublished on Web 06/26/2009

2. Experimental Section

Sample Preparation. Typically, 1 mmol of Bi(NO3)3 ·5H2Oand 1 mmol of Fe(NO3)3 ·9H2O were dissolved into 2 mL ofHNO3 (4 M). Then a concentrated aqueous solution of NaOHwas added dropwise into the above solution until the pH valueof the suspension was 13.5. After being stirred for 2 h, thesuspension was transferred into a 50 mL Teflon-lined stainlesssteel autoclave up to 80% of the total volume. The autoclavewas heated at 180 °C for 48 h at an ambient magnetic field,and then cooled to room-temperature naturally. The resultingsamples were separated by filtration, washed with deionizedwater and absolute alcohol several times, and then dried at 60°C for 12 h. For comparison, a bulk Bi2Fe4O9 sample is alsoprepared by solid state reaction following a previous study.10

Characterization. The purity and crystallinity of the as-prepared samples were characterized by powder X-ray diffrac-tion (XRD) on a Japan Rigaku Rotaflex diffractometer equippedwith a rotating anode using Cu Ka radiation in the range of10-80°, while the voltage and electric current were held at 40kV and 100 mA, respectively. The scanning electron microscope(SEM) characterizations were performed on a Shimadzu EPMA-8705QH2 electron Probe Microanalyzer. The transmissionelectron microscope (TEM) analyses were performed by a JEOLJEM-2100Ffieldemissionelectronmicroscope.Ultraviolet-visiblediffuse reflectance spectrum (UV-vis DRS) of the sample wasmeasured by using a Hitachi U-3010 UV-vis spectrophotom-eter. Nitrogen adsorption-desorption measurements were con-ducted at 77.35K on a Micromeritics Tristar 3000 analyzer. TheBrunauer-Emmett-Teller (BET) surface area was estimatedusing the adsorption data.

Electronic Structure Calculation. First-principles calcula-tions were performed using the all-electron Blochl’s projectoraugmented wave (PAW) approach34,35 within the generalizedgradient approximation (GGA), as implemented in the highlyefficient Vienna ab initio simulation package (VASP).36,37 Thek-point meshes for Brillouin zone sampling were constructedusing the Monkhorst-Pack scheme.38 A plane wave cutoffenergy of 450 eV was used. Spin-polarized calculations wereperformed to account for the antiferromagnetic nature ofBi2Fe4O9, which had been established by neutron diffractionmeasurement.39

Photocatalytic Test. Photocatalytic activity of the flowerlikeBi2Fe4O9 was evaluated by the degradation of phenol andaqueous ammonia under visible light irradiation of a 500 WXe lamp. For the degradation of phenol, 0.1 g of the photo-catalyst was added into 100 mL of phenol solution (20 mg/L).Before illumination, the solution was stirred for 120 min in thedark in order to reach the adsorption-desorption equilibriumbetween the photocatalyst and phenol. At every 1 h, a 4-mLsuspension was sampled and centrifuged to remove the photo-catalyst particles. Then, the adsorption spectrum of the cen-trifugated solution was recorded using a Hitachi U-3010UV-vis spectrophotometer. For the degradation of aqueousammonia, 0.1 g of the as-prepared photocatalyst was added into100 mL of NH4Cl solution, in which the pH value was adjustedto 10.8 by aqueous NaOH. Before illumination, the solutionwas stirred for 120 min in dark in order to reach theadsorption-desorption equilibrium between the photocatalystand NH4

+/NH3. The concentration of NH4+/NH3 was estimated

before and after the treatment using Nesster’s reagent colori-metric method.

3. Results and Discussion

3.1. Characterizations of the Bi2Fe4O9 Sample. Figure 1shows the XRD pattern of the as-prepared Bi2Fe4O9 photocata-lyst. It is found that the photocatalyst is well crystallized in asingle phase. All of the diffraction peaks can be well indexedto orthorhombic Bi2Fe4O9 (space group Pbam(55), JCPDS 72-1832). No other possible impurities can be detected. Afterrefinement, the cell constants of Bi2Fe4O9 were calculated tobe a ) 7.94 Å, b ) 8.44 Å, and c ) 6.01 Å, which is consistentwith the data obtained from JCPDS 72-1832. The orthorhombicstructure of Bi2Fe4O9 is constructed by two formula units perunit cell,40 as shown in Figure 2. These formula units consistof evenly distributed FeO6 octahedrons and FeO4 tetrahedronswith a lower packing density than that of corresponding closedpacked structures.40 The Bi3+ ions are surrounded by eightoxygen ions with mutually orthogonal shorter BiO3 and longerBiO5 units. Crystal structures are believed to have importantinfluences on electron transfer in view of the electron scatteringcaused by the thermal disturbance of atoms. A larger atomdensity usually implied an enhanced electron scattering, whichcould change the moving directions and prolong the time neededfor the electrons to transfer to the surface. On the basis of theabove analysis, the lower packing density of Bi2Fe4O9 maycreate a higher photocatalytic activity.

The morphology and microstructure of the as-preparedBi2Fe4O9 product were studied by the microscope images. Apanoramic SEM image (Figure 3a) demonstrates that the sampleconsists of microflowers with diameters of 2-3 µm. No othermorphologies are observed, indicating a high yield of these 3Dhierarchical structures. A higher magnification SEM image(Figure 3b) shows that these flowerlike structures are in factconstructed by nanoplates with thicknesses of about 80-100nm. Such hierarchical structures were also revealed by the TEMinvestigation as shown in Figure 3c. It confirms that the Bi2Fe4O9

flowers have diameters of 2-3 µm and are built from nanoplateswith thicknesses of about 80-100 nm, in agreement with theSEM images (Figure 3a,b). A TEM image of a peeled fragmentfrom the flowers is shown in Figure 3d. It can be seen that thenanoplates are rectangle-like with different side lengths alongthe perpendicular directions. The selected-area electron diffrac-tion (SAED) pattern for the [100] zone axis recorded at onerectangle nanoplate exhibits a regular and clear diffraction spotarray (inset of Figure 3d), revealing the single-crystal nature of

Figure 1. XRD pattern of the as-prepared Bi2Fe4O9 sample. Thevertical lines at the bottom correspond to the standard XRD pattern oforthorhombic Bi2Fe4O9 (JCPDS 72-1832).

Photocatalytic Oxidation in Flowerlike Bi2Fe4O9 J. Phys. Chem. C, Vol. 113, No. 29, 2009 12827

the nanoplate and confirms that the nanoplates grow preferen-tially along the (100) plane. This anisotropic growth along the(100) plane may be ascribed to the crystal structure of Bi2Fe4O9.According to the Gibbs-Curie-Wulff theorem,41 the growthrate of a face is inversely proportional to the atom density ofthe respective plane. For orthorhombic Bi2Fe4O9, the (100) planehas a larger atom density than that of (001) and (010) planes,as shown in Figure 2. Therefore the growth speed along the[100] direction is slower than that along the [001] and [010]directions, which leads to the formation of nanoplates along(100) plane. The difference of the side lengths of the nanoplatesalong the [001] and [010] directions may also be ascribed tothe discrepancy of the atom densities between the (001) and(010) planes.

The optical absorption of the as-prepared Bi2Fe4O9 samplewas measured using an UV-vis spectrometer. As shown inFigure 4, the Bi2Fe4O9 sample has a photoabsorption from UVlight to visible light. Two absorption-edges appeared in Figure4. One is at about 610 nm, and the next is at about 850 nm.This is similar to that of previously reported data.14 Theabsorption from 610 to 850 nm was ascribed to the d-dtransitions of Fe, which will be discussed in detail later, butnot caused by impurities in the as-prepared sample.

The flowerlike Bi2Fe4O9 has a larger BET surface area (4.32m2 g-1) than that of bulk-Bi2Fe4O9 powder (0.53 m2 g-1). The

enlarged BET surface area is attributed to the decreased particlesize and the flowerlike hierarchical structures. It has beenreported that larger surface area endows higher photocatalyticactivity for the increased reactive sites and the promotedelectron-hole separation efficiency.42-44 The as-prepared nano-Bi2Fe4O9 was expected to show much higher photocatalyticperformance than its bulk material.

3.2. Photocatalytic Performance. Phenol was chosen as arepresentative model pollutant to evaluate the photocatalyticperformance of the as-prepared Bi2Fe4O9. Figure 5a shows thephotodegradation efficiencies of phenol as a function of irradia-tion time with different photocatalysts under visible-lightillumination. A decrease in the concentration of phenol wasobserved with an increase of the irradiation time, indicating thephotocatalytic degradation of the phenol in the presence offlowerlike Bi2Fe4O9. The bulk-Bi2Fe4O9 exhibited a much poorereffect compared to the flowerlike structure under the sameconditions. Meanwhile, no degradation of phenol was observedwithout the catalyst or without the light. Although only 30% ofphenol was degraded on the flowerlike Bi2Fe4O9 in 4 h, thedegradation rate was greatly increased to 74% when 50 mg/LH2O2 was introduced into the photoreaction suspension. Figure5b shows the degradation of phenol followed first-order kinetics(plots of ln(C0/C) vs time showed linear relationship). First-order rate constants were evaluated from the slopes of the ln(C0/C) vs time plots. The observed rate constant in the presence ofnano-Bi2Fe4O9/H2O2, nano-Bi2Fe4O9, and bulk-Bi2Fe4O9 were0.3707, 0.0934, and 0.0061 h-1 respectively, indicating apreferable photocatalytic performance of the nano-Bi2Fe4O9 andnano-Bi2Fe4O9/H2O2 systems.

It has been reported that the addition of H2O2 into the aqueoussuspensions of ferric oxides can result in enhanced photocatalyticactivity, ascribed to the combined effects of conduction electron

Figure 2. Schematic illustration of Bi2Fe4O9 crystal structure viewed along different directions (1 × 1 × 1 cell) to show the structural feature ofthe (001), (010), and (100) faces in the polyhedron mode.

Figure 3. (a) Low-magnification and (b) high-magnification SEMimages of the flowerlike Bi2Fe4O9. (c) TEM image of the flowerlikeBi2Fe4O9. (d) TEM image of the peeled rectangle-like Bi2Fe4O9

nanoplates (inset: SAED pattern recorded at an individual nanoplate).

Figure 4. Typical DRS of the flowerlike Bi2Fe4O9.

12828 J. Phys. Chem. C, Vol. 113, No. 29, 2009 Sun et al.

scavenging and the Fenton reaction by the dissolved ironspecies.45 Here the obvious enhancement in the rate of phenolremoval with the addition of H2O2 may be by the similar reactionprocess listed below.

H2O2 as an efficient electron scavenger and a source of •OH(eq 1) can lead to a faster degradation of phenol and preventthe electron-hole recombination. Considering the photo-Fentonreactions with H2O2 by the dissolved iron species (eqs 2-4),lattice surfaces with larger atom densities of Fe produced moreexcellent photocatalytic activity. According to the above analysison the morphology of the Bi2Fe4O9 sample, its flowerlikestructure was constructed by nanoplates with anisotropic growthalong the (100) plane, which possesses the highest atom densityof Fe among its exposed surfaces including (001), (010), and

(100) planes. From this point, the flowerlike Bi2Fe4O9 with the(100) plane as the most exposed lattice surface may havesuperior photocatalytic performance to some other morphologies.

In addition to the phenol removal, the photocatalytic degrada-tion of aqueous ammonia, a major nitrogen-containing pollutant,on the as-prepared Bi2Fe4O9 sample was performed at pH 10.8.An initial concentration of 10 mg/L NH4

+/NH3 was usedthroughout this study. As shown in Figure 6, under visible lightirradiation for 6 h, the concentration of NH4

+/NH3 decreasesfrom an initial 10 mg/L to approximately 3.2 mg/L in thepresence of the as-prepared flowerlike nano-Bi2Fe4O9 photo-catalyst. The photocatalytic performance of the bulk-Bi2Fe4O9

on the NH4+/NH3 removal was also tested. Only 32% of the

NH4+/NH3 was degraded after 6 h in the presence of bulk-

Bi2Fe4O9, indicating the excellent photocatalytic performanceof the flowerlike nano-Bi2Fe4O9. To make sure the photocatalyticdegradation of aqueous ammonia by Bi2Fe4O9 is not ascribedto a photolysis process or the volatilization of NH3, a blankNH4

+/NH3 removal experiment was conducted under visiblelight irradiation without the photocatalyst. The result indicatedthe concentration of NH4

+/NH3 was only decreased 14% afterirradiation for 6 h, indicating the real photocatalytic ability ofthe as-prepared nano-Bi2Fe4O9 sample on the NH4

+/NH3

degradation.The above experiments have shown the excellent photocata-

lytic performance of the as-prepared flowerlike Bi2Fe4O9 on thedegradation of the widely used organic contaminant phenol andthe major nitrogen-containing pollutants aqueous ammonia. Itfollows that the Bi2Fe4O9 photocatalyst may have potentialapplications in the conservation of environment. A theoreticalstudy such as the electronic structure of the Bi2Fe4O9 photo-catalyst was needed to guide the further improvement of itsphotocatalytic performance, not only limited to the experimentalstage.

3.3. Electronic Structure Analysis. Ab initio calculationswere performed to evaluate the electronic structures of Bi2Fe4O9.Figure 7a shows the band structures of Bi2Fe4O9 calculated byVASP, which indicate that the Bi2Fe4O9 is a multibandsemiconductor. There are two energy gaps divided by the middlenarrow energy bands shown in red lines with open circles inFigure 7a.

Figure 7b shows the partial and total densities of states ofBi2Fe4O9. It is clear that the conduction band bottom and themiddle narrow bands mainly consists of Fe 3d orbitals, whilethe valence band top mainly consists of O 2p orbitals. It is also

Figure 5. (a) Photodegradation efficiencies of phenol as a function ofirradiation time. (b) Kinetic linear simulation curve of phenol photo-catalytic degradation.

H2O2 + Bi2Fe4O9(e-) f •OH + OH- (1)

FeOH2+98hν

Fe2+ + •OH (2)

Fe2+ + H2O2 f Fe3+ + •OH + OH- (3)

Fe3+ + H2O2 f Fe2+ + HO2• + H+ (4)

Figure 6. Photocatalytic degradation of NH4+/NH3 in the presence of

nano-Bi2Fe4O9 and bulk-Bi2Fe4O9.

Photocatalytic Oxidation in Flowerlike Bi2Fe4O9 J. Phys. Chem. C, Vol. 113, No. 29, 2009 12829

evident that the Fe 3d band in Bi2Fe4O9 splits into mainly twoparts corresponding to the Fe t2 g and the Fe eg for therequirement of crystal stabilization. In the tetrahedral crystalfield, the Fe eg orbitals are lower than that of Fe t2 g orbitals,while in the octahedral crystal field, the Fe eg orbitals are higherthan that of Fe t2 g orbitals.

On the basis of the above analysis, three types of opticaltransitions are possible in the Bi2Fe4O9 band structure, asillustrated in Figure 7c: (i) from the valence band (Ev) to theconduction band (Ec); (ii) from the valence band to the middleband (Em); and (iii) from the middle band (Em) to the conductionband (Ec). Although the three absorptions span much of the solarspectrum, it also induced enhanced opportunities of electron-holerecombination in the middle band where the electrons and holeswill accumulate when the numbers of transitions (ii) and (iii)are not the same. That is to say, the middle band in the Bi2Fe4O9

photocatalyst may serve as a charge recombination center andlimits catalysis efficiency, as reported in the monodoped TiO2

with 3d transitional metal ions.46-48

If the photocatalytic performance is to be greatly improved,more efficient electron-hole separation is necessary. Fortunately

the middle band is not strongly localized, as shown in Figure7a,b, which means that the recombination is not inevitable ifsome actions such as the applying of electric field, the addingof electron (or hole) scavengers, the development of heteroge-neous catalysis, and so forth were taken to prevent it. Forexample, we have demonstrated the much enhanced photocata-lytic activity of Bi2Fe4O9 by adding the strong electron scavengerH2O2 into the photoreaction system.

4. Conclusion

Photocatalytic active flowerlike Bi2Fe4O9 has been success-fully synthesized by a hydrothermal method. The materials arecomposed of nanoplates that well crystallized along the (100)plane. The as-prepared nano-Bi2Fe4O9 exhibits much highervisible light-driven photocatalytic activity toward phenol andaqueous ammonia oxidation compared to its bulk material. Uponadding an appropriate amount of H2O2, a much enhancedphotocatalytic performance for phenol oxidation was observed.Ab initio calculations indicate that the Bi2Fe4O9 photocatalystis a multiband semiconductor. Although the splitting of Fe 3dorbits greatly expands the visible light absorption by bringingon a middle band between the valence band top and theconduction band bottom, the middle band may act as recom-bination centers, which indicates that efficient electron-holeseparation principally helps the improvement of photocatalyticperformance of Bi2Fe4O9.

Acknowledgment. This work is financially supported by theNational Natural Science Foundation of China (Nos. 50732004and 50672117) and the Nanotechnology Programs of Scienceand Technology Commission of Shanghai Municipality (0852nm00500).

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Photocatalytic Oxidation in Flowerlike Bi2Fe4O9 J. Phys. Chem. C, Vol. 113, No. 29, 2009 12831