Recent progress in magnetic iron oxide-semiconductor ...download.xuebalib.com/3uvbHXBEZDk.pdf · Photocatalytic degradation of toxic organic pollutants is a challenging tasks in ecological

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Cite this: Nanoscale, 2015, 7, 38

Received 26th July 2014,Accepted 22nd October 2014

DOI: 10.1039/c4nr04244a

www.rsc.org/nanoscale

Recent progress in magnetic iron oxide–semiconductor composite nanomaterials aspromising photocatalysts

Wei Wu,*a,b Changzhong Jiangc and Vellaisamy A. L. Roy*b

Photocatalytic degradation of toxic organic pollutants is a challenging tasks in ecological and environ-

mental protection. Recent research shows that the magnetic iron oxide–semiconductor composite

photocatalytic system can effectively break through the bottleneck of single-component semiconductor

oxides with low activity under visible light and the challenging recycling of the photocatalyst from the

final products. With high reactivity in visible light, magnetic iron oxide–semiconductors can be exploited

as an important magnetic recovery photocatalyst (MRP) with a bright future. On this regard, various

W. Wu

Wei Wu obtained his B.S. in2005 and M.S. in 2008 fromHunan University of Technology,and he received his PhD onMaterials Physics and Chem-istry in 2011 under the supervi-sion of Prof. Changzhong Jiangin Department of Physics,Wuhan University, China. Hethen joined the group of Prof.Daiwen Pang at the Departmentof Chemistry and MolecularScience, Wuhan University, as apostdoctoral fellow for the

design and synthesis of magnetic iron oxide –semiconductor het-erostructures. Now he is the Director and Associate Professor ofthe Laboratory of Printable Functional Nanomaterials and PrintedElectronics, School of Printing and Packaging, Wuhan University.He has published, as an author and co-author, more than 60 pub-lications in various reputed international journals. He is also anAssociate Editor of Journal of Nanoscience Letters, and editorialboard member of Advanced Science, Engineering and Medicineand Journal of Green Science and Technology, his researchinterests include the synthesis, properties, and application ofnanomaterials, printed electronics and sensors.

C. Z. Jiang

Changzhong Jiang received his B.S in 1983 from Huazhong Uni-versity of Science and Technol-ogy, M.S. in 1990 from WuhanUniversity. He obtained his PhDin 1999 from Université ClaudeBernard Lyon 1, France. Cur-rently, he has been a full pro-fessor in the Department ofPhysics, Wuhan University since2001, and he is also the Directorof Center for Ion Beam Appli-cation, Wuhan University. Hehas published, as an author and

co-author, more than 80 publications in various reputed inter-national journals, such as Physical Review Letters, Nano Letters,ACS Nano, Advanced Materials. His research interests includethe synthesis and application of low-dimension nanomaterials,magnetic materials and ion beam modification of materials.

aLaboratory of Printable Functional Nanomaterials and Printed Electronics,

School of Printing and Packaging, Wuhan University, Wuhan 430072, P. R. China.

E-mail: [email protected], [email protected] of Physics and Materials Science, City University of Hong Kong,

Hong Kong SAR, P. R. ChinacKey Laboratory of Artificial Micro- and Nano-structures of Ministry of Education,

School of Physics and Technology, Wuhan University, Wuhan 430072, P. R. China

38 | Nanoscale, 2015, 7, 38–58 This journal is © The Royal Society of Chemistry 2015

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composite structures, the charge-transfer mechanism and outstanding properties of magnetic iron

oxide–semiconductor composite nanomaterials are sketched. The latest synthesis methods and recent

progress in the photocatalytic applications of magnetic iron oxide–semiconductor composite nanoma-

terials are reviewed. The problems and challenges still need to be resolved and development strategies

are discussed.

1 Introduction

Our surrounding environment continues to become more pol-luted, and the traditional chemical methods that deal withenvironmental pollution have been unable to meet the require-ments of modern energy-saving themes and environmentalprotection. Environmental problems induced by toxic andhardly-degradable organic pollutants (such as halides, dioxins,pesticides, dyes, etc.) have posed a grave menace to humanwell-being and development in the 21st century. Photocatalysisrefers to the rate of photoreactions (oxidation/reduction)brought on by the activation of a catalyst, usually a semi-conductor oxide, through illumination under ultraviolet (UV) orvisible light. Use of semiconductor oxide nanomaterials-basedphotocatalysts to degrade organic pollutants is recognized asone of the most promising areas of research and application.1–3

On this regard, photocatalysts are regularly used in solid–liquidreaction systems, especially for the treatment of toxic waste.

However, the main restriction factor of large scale practicalapplications of semiconductor oxide photocatalysts are asfollows: (1) high recombination rate of electronic–hole pairsresulting in low quantum yield for single-component semicon-

ductor oxide photocatalysts. For instance, Sun and Bolton havereported less than 5% primary quantum yield of •OH radicalgeneration in a TiO2 suspension.

4 (2) The limitation in the har-vesting of visible light. Generally, wide bandgap semiconduc-tor oxides are employed as photocatalysts, for example, thebandgap value of anatase TiO2 is 3.2 eV, and the corres-ponding absorption wavelength is 387.5 nm, resulting inlimited light absorption of the UV region. Unfortunately thesolar spectrum consists of only 5–7% of UV light, while 46%and 47% of the spectrum consists of visible light and infraredradiation, respectively.5 (3) Poor selective adsorption and thecomplexity of intermediate products. For example, photodegra-dation reaction products such as CO2 and H2O are easilyadsorbed on the surface of TiO2 in the gas–solid photocatalystsystem due to its super-hydrophilicity and active sites.6 Thehigh oxidization potential energy of OH radicals can inducemany backward reactions, such as oxidizing the intermediatesand products converted from the as-adsorbed CO2.

7,8 (4) Thephotocatalytic treatment of a high concentration of organicpollutants from industrial waste poisons the photocatalystresulting in deactivation. In addition, it is difficult to separatea pure semiconductor oxide photocatalyst from the waste watertreating system,9 which further deactivates the photocatalysts.(5) High cost of photocatalyst industrialization. This factorlimits the industrial applications of photocatalysts, henceresearch and development of low-cost, high-performance, andrecyclable photocatalysts have became an important issue.10,11

As a hot issue, photocatalysis has witnessed a sea of changeover the past two decades with significant advancements beingmade in the preparation of novel materials and nano-structures, and the design of efficient processes for the photo-degradation of pollutants and the generation of energy. Thus,the development of a simple recyclable photocatalyst can notonly prevent excessive use of photocatalysts, but also the recov-ery of deactivated photocatalysts, thereby reducing the totalcost, and further lowering the overall usage of the photo-catalytic material. Since visible light constitutes a large frac-tion of solar energy, one of the great challenges ofphotocatalyst study is to devise new catalysts that exhibit highactivity under illumination by visible light.

Combining the magnetic iron oxide nanomaterials withsemiconductor nanomaterials to form a magnetic iron oxide–semiconductor composite photocatalyst system becomes asimple and effective method. In an iron oxide–semiconductorsystem, iron oxide has many advantages, for example, low cost,high stability and compatibility, it not only plays the role ofseparating the photocatalyst from the solution, but also it candegrade organic pollutants.12,13 The common metal oxidesemiconductors like titanium dioxide (TiO2), zinc oxide (ZnO),

V. A. L. Roy

Vellaisamy A. L. Roy obtainedhis PhD degree from Nagpur Uni-versity in 2004. Dr Vellaisamystarted his research on light-emitting materials during hisPhD, mainly on Electron SpinResonance analysis of organicmaterials and was working onthe growth of wide band gapnano-structures. Currently, he isan associate professor at theDepartment of Physics andMaterials Science, City Univer-sity of Hong Kong. His research

interests are design, synthesis, and charge transport analysis ofself-assembled nanostructures and functional materials forsensors, thin film transistors and floating gate flash memories. DrVellaisamy received the TRIL Fellowship awarded by UNESCO in2003 and an Excellent Product Award for his project prototypes onSensors and Memories at China Hitech-Fair for three consecutiveyears since 2002. He has published over 80 papers in internationalSCI journals, including Advanced Materials, AngewandteChemie International Edition, Nano Letters, ACS Nano and hispapers have been cited more than 1800 times.

Nanoscale Review

This journal is © The Royal Society of Chemistry 2015 Nanoscale, 2015, 7, 38–58 | 39

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tungsten oxide (WO3), and tin oxide (SnO2) are proven to bedynamic photocatalysts for organic dyes and pollutants, thesesemiconductors not only destroy the conjugated chromophoricsystem, but also breakdown the molecular structure of theorganic dyes and pollutants into harmless CO2 and H2O. Asmagnetic recovery photocatalytic materials, the iron oxide–semiconductor oxide photocatalyst system can effectively breakthrough the bottleneck of low activity under visible light anddemanding recycling processes from the products, and even-tually become a potential visible light responsive MRPs in thefuture.

Therefore, the design of an iron oxide–semiconductorphotocatalytic system is an essential prerequisite for bothbasic and applied research. If the system focuses on the mag-netic recovery properties, the saturation magnetization valueof used iron oxides should be no less than 1 emu g−1, in orderto separate via an external magnetic field for further reusingand regeneration. If the system focuses on the photocatalyticperformance, the used iron oxides should possess a relativelynarrow bandgap value. For instance, goethite and hematite areoften studied as photocatalysts in recent years because of theirlow band gap (2.2 eV). There are reported techniques toimprove the photocatalytic performance of an iron oxide–semi-conductor system, such as a composite heterostructure with anarrow/wide bandgap, p–n heterojunctions, noble metalloading, plasmonic structure, graphene loading, etc.14–16

Overall, an optimal iron oxide–semiconductor photocatalyticsystem design should meet the following requirements.First, the synthesis and preparation process should beboth simple, and facile with high-yield. Second, the compositesystem should exhibit an enhanced photocatalytic perform-ance remarkably superior to existing naked iron oxide andpure semiconductor materials. Third, the compositephotocata-lyst should be recycled by the external magnetic field thatfacilitates easy reuse and regeneration. Finally, the compositephotocatalyst should possess good photocorrosion resistanceability and be stable at room temperature for months.

In this review, we will first describe the structure and mech-anism of a magnetic iron oxide–semiconductor photocatalysissystem. We discuss different synthesis methods and recentadvances in magnetic iron oxide–semiconductor nanomater-ials. The potential of magnetic iron oxide–semiconductorbased materials as photocatalysts is also examined. Finally, wediscuss future prospects in realizing this technology andfurther research directions.

2 Structure and mechanism formagnetic iron oxide–semiconductorcomposite photocatalysts2.1 Magnetic iron oxide nanomaterials

As a common compound, iron oxide is widely distributed innature and can be synthesized on a large-scale. The appli-cation of small iron oxide nanoparticles has been practised in

in vitro diagnostics for more than 60 years.17 Over the past fewdecades, magnetic iron oxide nanoparticles with various mor-phologies and structures are widely fabricated because of theirimportance in basic research. On the other hand, magneticiron oxides are of great interest for researchers due to widerange of applications, including pigments, magnetic fluids,catalysis, targeted drug delivery, biosensor, magnetic reson-ance imaging, data storage, and environmental remedia-tion.13,18,19 Iron oxides are composed of Fe together with O.There are eight iron oxides known.20 Among the iron oxides,hematite (α-Fe2O3), magnetite (Fe3O4) and maghemite(γ-Fe2O3) are promising and popular candidates due to theirpolymorphism involving temperature induced phase tran-sition. These three different crystalline iron oxides haveunique biochemical, magnetic, catalytic, and other propertiesthat make them suitable for specific technical and biomedicalapplications.

Magnetic measurements of α-Fe2O3 show obvious weakferromagnetism and its saturation magnetization is often lessthan 1 emu g−1 at room temperature. However, γ-Fe2O3 andFe3O4 show saturation magnetization values up to 92 emug−1.21 More importantly, the magnetic properties of iron oxidenanoparticles are related to their size and shape. For example,Demortière and co-workers have investigated the size-depen-dence of iron oxide nanocrystals on their structural and mag-netic properties by fine tuning the size within the nanometerscale (diameters range from 2.5 to 14 nm). The evolution ofmagnetic behavior with nanoparticle size clearly emphasizesthe influence of the surface, especially on the saturation mag-netization (Ms) and the magneto-crystalline anisotropy. Dipoleinteractions and thermal dependence have also been takeninto account in the study of nanoscale size-effects on magneticproperties.22 More recently, we have reported a comparativestudy on the magnetic behavior of single and tubular clusteredFe3O4 nanoparticles. The results reveal that the coercivity ofsmall iron oxide nanoparticles could be enhanced by the com-petition between the demagnetization energy of the mor-phology and magneto-crystalline anisotropy energy.23 Choiand co-workers have prepared Fe3O4 nanoparticles withdifferent shapes, including solid nanospheres and solid/hollow nanoellipsoids. All these structures were obtained byeither adding the appropriate amount of sodium acetate(NaOAc) or using the anion exchange of β-FeOOH. All magne-tite nanoparticles exhibited ferromagnetic behaviour withdifferent values for the saturation magnetization (Ms) and coer-civity (Hc), and these values were highly dependent on theshape due to their grain size, spin disorder, shape, and surfaceanisotropy.24 In general, iron oxide nanoparticles becomesuperparamagnetic at room temperature when the size of theiron oxide nanoparticles are below ca. 15 nm, meaning thatthe thermal energy can overcome the anisotropy energy barrierof a single nanoparticle. If the semiconductor is coated on thesurface of iron oxide, the Ms value would decrease. There are anumber of magnetic properties for the characterization ofnaked iron oxides nanoparticles and iron oxide–semiconduc-tor composite nanomaterials. The most important properties

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are the type and magnetization which can be determined fromthe hysteresis loops (M–H) and zero-field cooled/field cooled(ZFC/FC, M–T) curves. As shown in Fig. 1, the saturation mag-netization (MS), remanence magnetization (Mr), and coercivity(HC) can be obtained from the hysteresis loop. When thenaked iron oxide nanoparticles and iron oxide–semiconductorcomposite nanomaterials exhibit superparamagnetic, the M–Hcurve should show no hysteresis at a certain temperature (T >TB, blocking temperature). The forward and backward magne-tization curves overlap completely.23,25

The general strategy for preparing magnetic iron oxidenanoparticles in solution is to separate the nucleation andgrowth of nanocrystals. Numerous synthetic methods havebeen developed to synthesize magnetic iron oxide NPs, includ-ing co-precipitation,26–28 high-temperature thermaldecomposition,29–31 hydrothermal and solvothermalreaction,32–34 sol–gel reactions and polyol method,25,35,36

microemulsion synthesis,37–39 sonochemical reaction,40–44

microwave-assisted synthesis45–48 and biosynthesis.49–51 Otherthan the above-mentioned methods, alternative chemical or

physical methods can also be used to synthesize magnetic ironoxide nanoparticles, such as the electrochemical methods,52–54

flow injection synthesis,55 and aerosol/vapor methods,56–58 etc.In literature, there are many reports on the fabrication of mag-netic iron oxide NPs. Here, we briefly review the recentadvances on the synthesis of magnetic iron oxide–semiconduc-tor composite nanomaterials.

2.2 Semiconductor nanomaterials

Semiconductor oxides (e.g., TiO2, ZrO2, ZnO, WO3, MoO3,SnO2, α-Fe2O3, etc.) and semiconductor sulfides (e.g., ZnS,CdS, CdSe, WS2, MoS2, etc.) can be used as catalysts for photo-induced chemical reactions due to their intrinsic electronicstructure that consists of a filled valence band (VB) and anempty conduction band (CB).59–64 When a photon with energy(hv) matches or exceeds the band gap energy (Eg) of the semi-conductor, a photogenerated electron (e−) in the valence bandis excited into the conduction band, leaving a positive hole(h+) in the valence band. The photoinduced charge carriersplay a key role in the photocatalytic degradation process. Theholes mediate the oxidation of organic compounds throughthe formation of hydroxyl radicals (•OH), and the electronsmediate redox reactions through the formation of superoxideradicals (•O2). However, the photoinduced charge carriers inthe excited states are unstable and can easily recombine, con-verting the input energy to heat and thus leading to the lowactivity of a photocatalyst.65 An ideal photocatalyst should bestable, inexpensive, non-toxic and, of course, highlyphotoactive.

On the basis of thermodynamic requirements, the VB andCB of the semiconductor photocatalyst should be located insuch a way that the oxidation potential of the hydroxyl radicals(E0(H2O/

•OH) = 2.8 V vs. NHE) and the reduction potential ofsuperoxide radicals (E0(O2/

•O2−) = −0.28 V vs. NHE), lie well

within the band gap. In other words, the redox potential of theVB holes must be sufficiently positive to produce hydroxyl rad-icals. On the other hand, the CB electrons must be sufficientlynegative to produce superoxide radicals.66 Fig. 2 shows the

Fig. 2 Band gap energy, VB and CB for a range of semiconductors on a potential scale (V) versus the normal hydrogen electrode (NHE).

Fig. 1 Schematic presentation of the typical hysteresis loops of mag-netic iron oxide nanoparticles.

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bandgap energy and band edge positions of common semicon-ductor oxides and semiconductor sulfides, along with selectedredox potentials. Obviously, the bandgap energy and band edgepositions of TiO2, ZnO, SnO2, Fe2O3, WO3 and ZrO2 are relativelygood. As already mentioned, such semiconductor materials areprone to be applied in photocatalysis due to their inherentlyfilled VB and empty CB. When these semiconducting solidsabsorb photons and hv ≥ Eg, an e− is excited from the VB to theCB. This can be expressed in the following formula: hv + semi-conductor → h+ + e−. Then the electron of the semiconductorcan be transferred to an adjacent compound.

Additionally, the choice of semiconductor materials forphotocatalytic applications rely on the consideration of photo-corrosion resistance ability. For instance, CdS and ZnO onlyhave a stable valence of +2, and can be decomposed by photo-generated holes from the VB. Furthermore, ZnO is prone to bedeactivated due to the generation of Zn(OH)2 on its surface.10

Currently, there are many methods to inhibit or delay the de-activation caused by photocorrosion, and the commonmethod is to combine with other materials and to form com-posite nanomaterials.67,68 As compared to other materials, theoxidation state of Ti in TiO2 can be reversibly changed (from+4 and +3), thereby TiO2 is more stable and suitable for photo-catalytic applications. Additionally, the anatase phase TiO2

(Eg = 3.2 eV) is more active for photocatalysis applications,even though the rutile phase TiO2 (Eg = 3.0 eV) possesses asmaller band gap, revealing the possibility of absorption oflong wavelength radiation. The CB of anatase TiO2 is morenegative compared to rutile.

In addition to the aforementioned factors, other require-ments such as low-cost, a non-toxic nature (environmentally

benign) and easy preparation should also be taken into con-sideration for photocatalytic degradation reactions.

2.3 The structure of iron oxide–semiconductor compositenanomaterials

In order to increase the range of applications of iron oxidenanoparticles, some functional materials have been intro-duced and formed as newly composite nanostructures. Com-paring with single-component nanomaterials, multiple-component nanomaterials have become the subject of exten-sive research due to the synergistic interaction effects betweeneach component, which could improve the final catalytic per-formance. Currently, wide band gap semiconductors withgood photocatalytic properties have been used to functionalizemagnetic iron oxides. In a iron oxide–semiconductor compo-site system, the magnetic iron oxide can not only separate andrecover the photocatalyst, but can also form narrow/wide bandgap semiconductor heterostructures. The narrow/wide bandgap semiconductor heterostructures can promote the separ-ation of electron and hole pairs efficiently, consequentlyincreasing the visible light utilization and finally improvingthe photocatalytic efficiency.

As shown in Fig. 3, if iron oxide nanoparticles are alwaysthe core, the structure of iron oxide–semiconductor compositenanomaterials can be simply divided into four structures:core–shell, matrix-dispersed, Janus and shell–core–shellstructures.

2.3.1 Core–shell structure. In this structure, the iron oxidecore is encapsulated by a semiconductor layer that renders thestability of the whole particle. Generally, the iron oxide nano-particles are not located at the centre of a functional semicon-

Fig. 3 Typical structure types of magnetic iron oxide–semiconductor composite nanomaterials. Blue spheres represent magnetic iron oxide nano-particles, and the non-magnetic entities and matrix materials are displayed in another other colour.

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ductor and this is known as a yolk structure. For example, Liuand co-workers have successfully synthesized variousFe3O4@TiO2 yolk–shell microspheres with different core sizes,interstitial void volumes, and shell thicknesses by controllingthe synthetic parameters.69 Li and core-workers have develo-ped a facile “hydrothermal etching assisted crystallization”method to prepare Fe3O4@TiO2 yolk–shell microspheres withultrathin nanosheets assembled as double-shell structure. Theas-obtained microspheres possess high surface area, goodstructural stability and large magnetization, the size isuniform and the shell could be tailored, which exhibits versa-tile ion-exchange capability and a remarkable catalytic per-formance.70 Indeed, the magnetic composite nanomaterialsnot only provide an improved stability of the nanoparticulatebuilding blocks, but also introduce new physical and chemicalproperties and multifunctional behaviours. In the inversecore–shell structure, the magnetic iron oxides are coated onthe surface of semiconductor materials. For instance, Luo andco-workers have fabricated highly ordered TiO2@α-Fe2O3 core–shell arrays on carbon textiles by a stepwise, seed-assisted,hydrothermal approach. The fabrication strategy is facile, cost-effective, and scalable, which opens new avenues for thedesign of optimal composite electrode materials.71 Moreover,magnetic iron oxides could be combined and coated with oneor more functional materials on the surface of another func-tional material. The above structures are all called core–shellstructures.

2.3.2 Matrix-dispersed structure. Several magnetic ironoxide nanoparticles are coated or dispersed in a semiconduc-tor matrix. Matrix-dispersed nanoparticles can be created in avariety of different photocatalytic reaction states. For example,Wang and co-workers have prepared (γ-Fe2O3@SiO2)n@TiO2

functional hybrid nanoparticles by an easy chemical route.Several γ-Fe2O3 fine particles about 15 nm in diameter as coresare distributed in the TiO2 matrix with silica as the barrierlayer between the magnetic cores and TiO2 shells has beenreported. The hybrid nanoparticles show good magneticresponse and display high photocatalytic efficiency for methyl-ene blue (MB).72

2.3.3 Janus structure. In the Janus structure, one side ismagnetic iron oxide nanoparticles, and the other side is afunctional semiconductor material. An anisotropic surfacechemical makeup is interesting for applications even withoutself-assembly. For example, Zeng and co-workers have syn-thesized multifunctional Fe3O4/TiO2 nanocomposites with aJanus structure for magnetic resonance imaging (MRI) andpotential photodynamic therapy (PDT), in which Fe3O4 is aMRI contrast agent and TiO2 is an inorganic photosensitizerfor PDT.73 Mou and co-workers have developed an asymmetricshrinkage approach for the fabrication of magnetic γ-Fe2O3/TiO2 Janus hollow bowls by constructing a precursor solutionpair with different gelation rates during the solvent evapor-ation process. The as-obtained products exhibited an efficientvisible-light photocatalytic activity and convenient magneticseparation because of the unique structure and morphology aswell as the fine magnetic properties.74

2.3.4 Shell–core–shell structure. In this structure, the mag-netic iron oxide nanoparticles are located between two func-tional semiconductor materials. Several applications requiremagnetic iron oxide nanoparticles to be embedded in non-magnetic layers to avoid aggregation and sedimentation ofmagnetic iron oxide nanoparticles as well as to endow themwith particular surface properties for specific applications. Inthis structure, the two shell layers use the same or differentsemiconductors or one layer is a non-semiconductormaterial.75

More importantly, understanding the relationship betweenthe photocatalytic performance and the microstructure is aprerequisite for widespread application. Therefore, design andcontrollable synthesis of the nanostructured photocatalysts,and further optimization of the microstructure and photo-catalytic performance are still under broad investigation.76 Aprerequisite for every possible applied structure is the propersurface properties of the magnetic composite NPs, whichdetermine their interaction with the environment. These inter-actions ultimately affect the colloidal stability and photo-catalytic efficiency of the composite particles.

2.4 Charge transfer mechanism

The charge separation mechanism in both capped semicon-ductor systems and coupled semiconductor systems involvesthe photoinduced electrons in one semiconductor beinginjected into the lower lying CB of the second semiconductor.Therefore, coupling semiconductors techniques do not alwaysimprove the photocatalytic performance by charge separation.The design of coupling semiconductor photocatalysts dependson the band structures of each component. Generally, photo-generated electrons on the CB of a higher level semiconductorare injected into the CB of a lower level semiconductor. Assuch coupled semiconductor photocatalytic systems bear greathope for next-generation solar energy harvesting and advan-cing environmental remediation techniques. Governments andresearchers have devoted considerable interests and resourcesto such fabrication, characterization, and optimization.77

In the iron oxide–semiconductor system, iron oxides can benarrow band gap semiconductors, with a band gap value forFe2O3 of 2.2 eV, they also absorb visible light. For example, thework function (ϕ) of α-Fe2O3 is 5.88 eV, which is higher thanmost common wide band gap semiconductors (TiO2 is 3.87 eV,ZnO is 4.35 eV, SnO2 is 4.3 eV, WO3 is 5.24 eV, etc.). As shownin Fig. 4a, the band configuration and photogenerated chargecarrier separation at the interface of iron oxide–semiconductor(wide band gap) under light irradiation are proposed. Underlight irradiation, the photoinduced electrons and holes areseparated at the interface of the iron oxide–semiconductor, thephotoinduced electrons in the CB of iron oxide tend to transferto that the CB of the semiconductor due to the decreasedpotential energy, and hence the coupling structure reduces theelectron–hole recombination probability and increases theelectron mobility. Thereby the electrons and holes were trans-ferred to the surface of the iron oxide and semiconductor,respectively, and finally form hydroxyl radicals (•OH). The

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superoxygen radicals (•O2) are formed by the combination ofelectrons with O2 adsorbed on the surface of the semiconductor.As a powerful oxidant, •OH can degrade many pollutes, such asorganic dyes, wastewater, and plastics. However, capped semi-conductors on the other hand have a core and shell geometry,as shown in Fig. 4b. The electrons are injected into the energylevels of the core semiconductor (on condition that it has a con-duction band potential which is lower than that of the shell).Hence, the electrons are trapped within the core particle, and isnot readily accessible for the reduction reaction.78

The introduction of an interlayer into the iron oxide–semi-conductor heterojuction for tailoring the photocatalyticefficiency is another option. As shown in Fig. 5, when the insu-lating SiO2 layer is introduced, the photogenerated electrons inthe CB of iron oxide are not able to transfer to the CB of thesemiconductor. However, from our previous reports and Chris-topher’s reports, the photogenerated electrons can still trans-fer if the thickness of the SiO2 is less than 5 nm.79,80

Therefore, the thickness of SiO2 is a key factor and responsiblefor the photocatalytic abilities of iron oxide/SiO2/semiconduc-tor systems. As an alternative, photogenerated electrons in theCB of iron oxide can transfer to the CB of the semiconductorvia a carbon interlayer, which behaves as an electron conduc-tor to enhance the electron–hole separation. For example, Hou

and co-workers have reported an interlayer of graphene thattransfers the electrons from the CB of a BiV1−xMoxO4 shell tothe CB of the Fe2O3 core in α-Fe2O3 nanorod/graphene/BiV1−x-MoxO4 core–shell heterojunction due to band alignment andpotential difference, which provides a direct path for electrontransport.81

As a classical heterostructure, the iron oxide–semiconduc-tor system has many advantages. First of all, the built-in poten-tial at the interface of iron oxide and semiconductor canpromote the separation and transport of photoinduced chargecarriers. Second, iron oxides with relatively smaller band gapssensitize the wide band gap semiconductors. Third, semi-conductor metal oxides and metal sulfides such as RuO2, NiOand IrO2, MoS2 and cobalt phosphates, can also act as effectiveco-catalysts to facilitate the surface electrochemical reaction.These co-catalysts improve charge separation, suppress therecombination of photogenerated charge carriers and lowerthe potential for electrochemical reaction.14 Although α-Fe2O3

is stable, it is prone to photocorrosion. Its photocatalyticdegradation efficiency for organic dyes needs to be improved.Therefore, as an outer layer, semiconductors such as TiO2 withexcellent electrochemical- and photochemical-stability can beused on the surface of iron oxide to improve the stability of thecatalysts.

Fig. 5 Schematic diagram showing the photogenerated charge transfer of (a) iron oxide/SiO2/semiconductors, (b) iron oxide–semiconductors and(c) iron oxide/C/semiconductors heterojunctions.

Fig. 4 Traditional charge transfer between two semiconductors with a narrow and wide band gap, depicting the isolation of reaction sites for oxi-dation and reduction in coupled semiconductor system (a); charge transfer in capped semiconductor system (b).

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3 Synthesis of magnetic iron oxide–semiconductor compositenanomaterials3.1 Seed-mediated growth strategy

As shown in Fig. 6a, the seed-mediated growth strategy is themost common method for synthesizing high-quality magneticiron oxide–semiconductor composite nanomaterials, especiallythe preparation of core–shell heterostructures. A typical growthprotocol involves the addition of magnetic iron oxide nano-particles, as seeds, to the bulk semiconductor growth. Thegrowth solution is obtained by the reduction of semiconductorprecursors. In this protocol, seeds are sequentially added tothe growth solution in order to control the rate of hetero-geneous deposition and thereby the rate of crystal growth. Manywet-chemical approaches have been used to generate ironoxide–semiconductor composite heterostructures, such as theco-precipitation, hydrothermal, and solvothermal methods,etc.82–85

For example, Chiu and co-workers have reported the syn-thesis of Fe3O4/ZnO core–shell nanoparticles by the seed-meditated growth method under nonhydrolytic conditions.Control over the thermal pyrolysis of zinc acetate give theoption to overgrow the ZnO layer on the surface of Fe3O4

seeds. These core–shell nanocrystals were magnetically separ-ated by a 0.6 T magnet, which shows high potential for usingsuch nanocrystals as recoverable catalyst materials.86 We pro-posed a facile pathway to prepare three different types of mag-netic iron oxide/TiO2 hybrid nanoparticles by the seed-mediated method. The hybrid nanoparticles are composed ofspindle, hollow, and ultrafine iron oxide nanoparticles asseeds and 3-aminopropyltriethyloxysilane as the linkerbetween the magnetic cores and TiO2 layers, respectively.About 50% to 60% of MB was decomposed in 90 min in thepresence of magnetic hybrid nanoparticles, which is higherthan pure TiO2 nanoparticles. The synthesized magnetichybrid nanoparticles display high photocatalytic efficiency andcan be used for cleaning polluted water with the help of mag-netic separation.87 Recently, Li and co-workers have reported a

versatile kinetics-controlled coating approach to fabricatehomogeneous porous TiO2 shells for multi-functional core–shell nanostructures. By simply controlling the kinetics of thehydrolysis and condensation of tetrabutyl titanate (TBOT) inethanol–ammonia mixtures, the core–shell heterostructurewith homogeneous porous TiO2 shells were fabricated withvariable diameter, geometry, and composition as seeds (e.g.,α-Fe2O3 ellipsoids, Fe3O4 spheres, SiO2 spheres, grapheneoxide sheets, and carbon spheres). This approach exhibitsmany advantages, such as facile, reproducible and the thick-ness of TiO2 shells can be tailored from 0 to 25, 45, and70 nm.88 Yuan and co-workers have prepared Fe3O4@TiO2

nanoparticles (the size is 6.7 ± 2.9 nm) by a modified sol–gelmethod. The TiO2 shell was formed by gradually adding TiCl4to the iron oxide nanoparticle gel. The as-prepared compositenanoparticles are used for targeted drug delivery.89

In addition, the semiconductor nanoparticles can also beseeds for the synthesis of iron oxide–semiconductor compositenanomaterials, and this strategy is often employed to fabricatethe iron oxide–semiconductor with a Janus structure (Fig. 6b).For instance, Buonsanti and co-workers developed a colloidalseeded-growth strategy to synthesize all-oxide semiconductor/magnetic hybrid nanocrystals in various topological arrange-ments, in which the dimensions of the constituent materialdomains were controlled independently over a wide range. TheFexOy/TiO2 composite nanorods were synthesized by using thebrookite TiO2 nanorods as seeds and Fe(CO)5 as the iron pre-cursor via a high-temperature thermal decomposition method.The preliminary magnetic and photocatalytic investigationshad highlighted that the creation of bonding heterojunctionsleads to significantly modified or even unexpected physical–chemical behaviour, relative to that offered by brookite TiO2

and FexOy alone.90,91 Zeng and co-workers first synthesizedTiO2 nanoparticles with a diameter of about 5–10 nm withferric acetylacetonate as an iron source, the multifunctionalFe3O4/TiO2 nanocomposites with a Janus structure were pre-pared by the solvent–thermal method.73 Liu and Gao first pre-pared the sheet-like TiO2 seeds by hydrothermal treatment ofTiO2 nanoparticles. Then the α-Fe2O3/TiO2 compositenanosheets were fabricated by hydrothermal treatment offerric nitrate and hydroxylamine. Results showed that thephotocatalytic activities of α-Fe2O3 make the MB degradationefficient under visible light irradiation.92 Wang and co-workerssynthesized one-dimensional (1D) heterostructures of uniformCdS nanowires separately decorated with hematite (α-Fe2O3)nanoparticles or magnetite (Fe3O4) microspheres via a two-stepsolvothermal deposition method. Each CdS nanowire had auniform diameter of 40–50 nm and a length ranging to severaltens of micrometers. Quasicubic α-Fe2O3 nanoparticles withedge lengths up to 30 nm, and Fe3O4 microspheres with dia-meters of about 200 nm produced 1D dimer-type CdS/α-Fe2O3

semiconductor heterostructures or CdS/Fe3O4 semiconductormagnetic functionally assembled heterostructures. In compari-son with the bare CdS nanowires and commercial anataseTiO2, enhanced photocatalytic activity was observed in CdS/α-Fe2O3 heterostructures.

93Fig. 6 Scheme of the preparation of iron oxide–semiconductor com-posite nanomaterials by the seed-mediated growth method.

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3.2 Step-by-step deposition strategy

Step-by-step deposition strategy is mainly used to prepare ironoxide–semiconductor composite multi-shell structures.94–96 Infact, the need for a better control over surface properties or toprotect the iron oxide itself, an interlayer was introduced inthe magnetic iron oxide–semiconductor system to form amulti-shell structure, as shown in Fig. 7.97–99 The most com-monly used interlayer materials are the SiO2 and carbon,respectively. Additionally, the interlayer can be removed bychemical corrosion or calcination.70

Silica coating can enhance dispersion in solution becausethe silica layer can screen the magnetic dipolar attractionbetween magnetic iron oxide nanoparticles, and henceincrease the stability of iron oxide nanoparticles and protectthem in acidic environments. Silica has become the most usedinterlayer material.100,101 For example, Cheng and co-workershave synthesized Fe3O4@SiO2@CeO2 microspheres with amagnetic core and mesoporous shell by a step-by-step depo-sition strategy. Such multifunctional materials were utilized tocapture phosphopeptides and catalyze the dephosphorylationsimultaneously, thereby labeling the phosphopeptides forrapid identification.102 Sarkar and co-workers synthesizedFe3O4 nanoparticles with a diameter of 20–40 nm by co-pre-cipitation method, and then a SiO2 interlayer was deposited onthe surface of Fe3O4 nanoparticles by classical Stöber method.The Fe3O4/SiO2/ZrO2 composite nanoparticles were finally fab-ricated by reducing the ZrOCl2 precursor. The thickness ofZrO2 was about 8–10 nm, and the BET surface area of the com-posite nanoparticles was up to 107 m2g−1 due to the meso-porous ZrO2 shell.103 More recently, Chi and co-workers haveprepared Fe3O4@SiO2@TiO2/Ag nanocomposites by a step-by-step deposition strategy, the as-prepared microspheres show anumber of important features as a recyclable photocatalyst: ahigh field-responsive magnetic Fe3O4 core for efficient mag-netic separation, a SiO2 interlayer for protecting the Fe3O4 corefrom chemical- and photocorrosion, and a TiO2 nano-shellwith well dispersed Ag nanoparticles for enhanced photo-catalytic activity.104

Like the SiO2 interlayer, hydrophilic carbon coating on aniron oxide nanoparticle core also endows better dispersibilityand stability. More importantly, carbon coated iron oxidenanoparticles have recently triggered enormous researchactivity due to the good chemical and thermal stability. Theintrinsic high electrical conductivity of the carbon interlayerhelps to transfer electrons.105–107 For instance, Qi and

co-workers first deposited a carbon interlayer on the surface ofFe3O4 seeds by the hydrothermal reaction of glucose, and thendeposited SnO2 on the surface of Fe3O4/C, they successfullyobtained Fe3O4/C/SnO2 composite nanoparticles.108 Shi andco-workers have prepared a core/multi-shell-structured Fe3O4/C/TiO2 magnetic photocatalyst by the vapor phase hydrolysisprocess, and the photocatalytic abilities for degradation ofmethylene blue are studied. Compared with commercialanatase TiO2, Fe3O4/C/TiO2 with low TiO2 content (37%)exhibited a relatively higher photocatalytic activity. The C inter-layer prevented the photocorrosion of Fe3O4 effectively, andthe composite nanoparticles present a good magnetic recy-cling property due to magnetic core materials.109 Liu and co-workers have fabricated one-dimensional Fe3O4/C/CdS coaxialnanochains by a magnetic field-induced assembly and micro-wave-assisted deposition method. First, one-dimensional pearlchain-like Fe3O4/C core–shell nanocables were successfullyassembled via the hydrothermal reaction of nanoscale Fe3O4

spheres with glucose in water in the presence of an externalmagnetic field. The carbonaceous layer was about 10 nm inthickness, and it acted as the stabilizer for the Fe3O4 nano-chains. Afterwards, CdS nanoparticles were deposited onFe3O4/C nanochains by a rapid microwave-irradiation route togenerate Fe3O4/C/CdS coaxial nanochains. The subsequentphotocatalytic test for organic pollutants demonstrated thatthese magnetic composites possess enhanced photocatalyticactivity as MRPs under visible light irradiation. The decoloriza-tion fraction using a sample of microwave irradiation was upto 94.7% in 20 min, and the photocatalytic performance wasstill stable after 12 cycles of degradation of RhB, the resultsrevealed that this MRP possessed excellent stability.107

3.3 Other strategies

Except the above two conventional strategies, some new syn-thesis or preparation methods have also been used to fabricateiron oxide–semiconductor composite nanomaterials, such asion implantation method, spray pyrolysis, microwave, andsonochemical method.110–112

As shown in Fig. 8, we first dropped the hematite seedsonto the surface of a clean slide and implanted Ti ions. andMagnetic monodispersed TiO2 grains filled into spindle-likehematite bi-component nanoparticles were successfully syn-thesized.113 The different implanted energy and magnetic pro-perties of the bi-component α-Fe2O3/TiO2 nanoparticles wereinvestigated. The results illustrate that the α-Fe2O3/TiO2 com-

Fig. 7 Scheme of the preparation of iron oxide–semiconductor composite nanomaterials by layer-by-layer deposition method.

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posite nanoparticles could be obtained by Ti ion implantationwith different energies, and the saturation magnetization (MS)of the samples after ion implantation were significantlyenhanced.114 Li and co-workers prepared a novel core–shellα-Fe2O3/SnO2 heterostructure by one-step flame-assisted spraypyrolysis of an iron and tin precursor. The effect of the SnO2

component was investigated for the evolution of the phasecomposition and morphology in detail. It was found that thedoping of SnO2 in Fe2O3 could effectively promote the phasetransition from γ-Fe2O3 to α-Fe2O3 during flame synthesis. Theunique morphology composed of tin doped α-Fe2O3 core andSnO2 as a shell was attributed to the solubility, segregationand second-phase surface nucleation of SnO2 in Fe2O3.

115

4 Progress on magnetic iron oxide–semiconductor compositephotocatalysts4.1 Magnetic iron oxide–metal oxide–semiconductorcomposite photocatalysts

4.1.1 Magnetic iron oxide/TiO2 photocatalysts. TiO2, themost thoroughly investigated semiconductor in the literature,seems to be the most promising photocatalytic material for the

destruction of organic pollutants. This semiconductor pro-vides the best compromise between catalytic performance andstability in aqueous media. Therefore, the magnetic ironoxide/TiO2 composite photocatalyst have become the researchfocus in recent years. Using the magnetic properties of ironoxide itself for obtaining the magnetic recoverable photo-catalyst has become an important issue in the magnetic ironoxide/TiO2 composite photocatalyst system.116–119 For instance,Wang and co-workers have reported the fabrication of core–shell Fe3O4@SiO2@TiO2 microspheres through a wet-chemicalapproach. The microspheres possess both ferromagnetic andphotocatalytic properties. The TiO2 nanoparticles on the sur-faces of the microspheres degraded organic dyes under theillumination of UV light. Furthermore, the microspheres wereeasily separated from the solution after the photocatalyticprocess due to the ferromagnetic Fe3O4 core. The photocata-lysts were recycled for further use and the degradation rate ofmethyl orange still reached 91% after 6 cycles of reuse.120 Asshown in Fig. 9, Chalasani and Vasudevan have demonstratedwater-dispersible photocatalytic Fe3O4@TiO2 core–shell mag-netic nanoparticles by anchoring β-cyclodextrin (CMCD) cav-ities to the TiO2 shell, and photocatalytically destroyedendocrine-disrupting chemicals, bisphenol A (BPA) anddibutyl phthalate, present in water. The particles, which were

Fig. 9 Scheme of the reuse of cyclodextrin-functionalized Fe3O4@TiO2 for photocatalytic degradation of endocrine-disrupting chemicals in watersupplies.121

Fig. 8 Scheme depicting α-Fe2O3/TiO2 (TiO2 grains in spindle-like α-Fe2O3) bi-component NP synthesis by ion implantation method.113

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typically 12 nm in diameter, were magnetic and removed fromthe dispersion by magnetic separation and then reused. Theconcentration of BPA solution was determined by liquidchromatography, and then irradiated under UV light for 60 min.After photodegradation of BPA, the CMCD-Fe3O4@TiO2 nano-particles that were separated from the mixtures by a magnet,and can be reused for the photodegradation of newly preparedBPA solutions. The recycle photocatalytic performance ofCMCD-Fe3O4@TiO2 for the photodegradation of BPA was excel-lent and stable, retaining 90% efficiency after 10 cycles.121 Forobtaining the magnetically recovered photocatalysts, Fe3O4 andγ-Fe2O3 were often employed due to their higher saturation mag-netization and good magnetic separation ability.

On the other hand, α-Fe2O3 has often been introduced intothe magnetic iron oxide/TiO2 composite photocatalyst in orderto use its narrow band gap properties, and to obtain magneticiron oxide/TiO2 composite heterostructures.92,122–124 Forexample, Peng and co-workers have synthesized Fe2O3/TiO2

heterostructural photocatalysts by impregnation of Fe3+ on thesurface of TiO2 and annealing at 300 °C, the compositespossess different mass ratios of Fe2O3 vs. TiO2. The photo-catalytic activities of Fe2O3/TiO2 heterocomposites, pure Fe2O3

and TiO2 were studied by the photocatalytic degrading ofOrange II dye in aqueous solution under visible light (λ >420 nm) irradiation. The Fe2O3/TiO2 heterogeneous photocata-lysts exhibited an enhanced photocatalytic ability for OrangeII, higher than either pure Fe2O3 or TiO2. The best photo-catalytic performance for Orange II could be obtained whenthe mass ratio in Fe2O3/TiO2 is 7 : 3. The results illustrate thatthe generation of heterojunctions between Fe2O3 and TiO2 iskey for improving movement and restraining the recombina-tion of photoinduced charge carriers, and finally improvingthe photocatalytic performance of Fe2O3/TiO2 composites.125

Recently, Palanisamy and co-workers have prepared Fe2O3/TiO2

(10, 30, 50, 70 and 90 wt% Fe2O3) photocatalysts by a sol–gelprocess. Mesoporous Fe2O3/TiO2 composites exhibited excel-lent photocatalytic degradation ability for 4-chlorophenol inaqueous solution under sunlight irradiation. The authorclaimed that the photogenerated electrons in the VB of TiO2

are transferred to Fe(III) ions resulting in the reduction of Fe(III)ions to Fe(II) ions. Thus the photoinduced holes in the VB ofFe2O3/TiO2 cause an oxidation reaction and decompose the

4-chlorophenol to CO2 and H2O. Meanwhile the transferredelectrons in Fe(III) ions could trigger the reduction reaction.126

4.1.2 Magnetic iron oxide/SnO2 photocatalysts. As an n-type wide-bandgap semiconductor (∼3.8 eV), tin oxide (SnO2)has proved to be a material of exceptional technological impor-tance due to its unique properties, including high stability andlithium storage capacity, and it is currently used to preparephotocatalysts. The objectives of combining magnetic ironoxide and SnO2 are the same as the iron oxide/TiO2 compositephotocatalyst system.

On the one hand is the fabrication of iron oxide/SnO2

heterostructures, for instance, Niu and co-workers have pre-pared branched SnO2/α-Fe2O3 semiconductor nanoheterostruc-tures (SNHs) of high purity by a low-cost and environmentallyfriendly hydrothermal strategy, through crystallographic-oriented epitaxial growth of SnO2 nanorods on α-Fe2O3 nano-spindles and nanocubes, respectively (Fig. 10). SnO2/α-Fe2O3

SNHs exhibited excellent visible light or UV photocatalyticability, remarkably superior to their α-Fe2O3 precursors,mainly owing to the effective electron–hole separation at theSnO2/α-Fe2O3 interface.127 Recently, Zhang and co-workershave also synthesized three-dimensional SnO2/α-Fe2O3 semi-conductor hierarchical nanoheterostructures via crystallo-graphic-oriented epitaxial growth of SnO2 onto the surface offlower-like three-dimensional iron oxide hierarchical nano-structures. For this photocatalyst, visible-light-active flower-likeFe2O3 hierarchical nanostructures were employed as a mediumto harvest the visible light and generate photoinduced chargecarriers, and SnO2 layer was employed as a charge collector totransport the photoinduced charge carriers. The SnO2/α-Fe2O3

semiconductor hierarchical heterostructures present admir-able visible-light photodegradation ability for methylene blue,which could be assigned to the wide visible-light absorptionrange, high surface area, and efficient charge carrier separ-ation of the SnO2-α-Fe2O3 heterostructures.128 Zhu and co-workers have synthesized core–shell structured α-Fe2O3@SnO2

shuttle-like composites via a facile solvothermal approach. Thephotocatalytic activities of the as-synthesized α-Fe2O3@SnO2

core–shell shuttle-like composites were studied by the photo-degradation of RhB dye under UV light irradiation (λ =365 nm), and the absorption peak of RhB diminished gradu-ally as the exposed time extended and completely disappeared

Fig. 10 Schematically illustrated formation process of hierarchically assembled SnO2/α-Fe2O3 heterostructures based on α-Fe2O3 nanospindleprecursor.127

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after 70 min. Compared with uncoated α-Fe2O3 shuttle-likenanorods, SnO2 nanoparticles, and the mixture of α-Fe2O3

nanorods and SnO2 particles, as-synthesized core–shellshuttle-like composites exhibited enhanced photodegradationabilities, suggesting that the synergistic effect of α-Fe2O3 andSnO2 was beneficial to improve the photocatalytic activity.129

On another hand, fabrication of the magnetically recover-able iron oxide/SnO2 composite photocatalyst system is alsoattractive for various reasons. As shown in Fig. 11, we have suc-cessfully synthesized the spindle-like and spherical iron oxide/SnO2 composite nanoparticles via a seed-mediated growthstrategy recently, and the as-prepared iron oxides/SnO2 core–shell heterostructures displayed enhanced visible light and UVphotodegradation activity for RhB, which is significant higherthan the uncoated a-Fe2O3 seeds and commercially availableSnO2 products. Significantly, the composite nanoparticles canbe magnetically separated from the dispersion after photo-catalytic degradation.130,131 Zhang and co-workers have fabri-cated superparamagnetic iron oxide (SPIO)@SnO2 yolk–shellheterostructures (YSHs) by a facile template approach, the as-obtained SnO2 shell is mesoporous, the thickness of the shelllayer and void spaces are both tailorable. Under UV lightirradiation for 1 h, the photodegradation activity of the as-obtained SPIO@SnO2 YSHs and commercial P25 TiO2 for RhBwere about 75% and 97%, respectively. Because SPIO cores arenot photocatalytically active, the mass of the SnO2 componentin 25 mg of SPIO@SnO2 YSHs would be less than the photo-catalyst component in the same mass of P25 TiO2.

132

4.1.3 Magnetic iron oxide/ZnO photocatalysts. Indeed, thephotocatalytic performance of pure ZnO nanomaterials is notweaker than TiO2. However, it is unstable under illuminatedaqueous solutions with Zn(OH)2 being formed on the particlesurface and resulting in catalyst deactivation. Owing to thephotocatalytic performance of ZnO, the ZnO based compositenanomaterials are still used in the photocatalytic field andhave attracted more attention in recent years.133–135 Forinstance, Liu and co-workers have synthesized the magneticnest-like γ-Fe2O3/ZnO double-shelled hollow nanostructuresvia a step-by-step process. These interesting nest-like hetero-structures consist of nanoscale ZnO flakes grown on thesurface of spherical γ-Fe2O3 particles. Significantly, these mag-netic hollow heterostructures exhibited enhanced photodegra-dation ability for different organic dyes, including MB (almost95.2% of MB could be degraded within 50 min), RhB (almost91.1% of RhB could be degraded within 50 min), and MO(almost 82.5% of MO could be degraded within 80 min), andtheir photocatalytic abilities were higher than commercial ZnOnanoparticles. The improved photodegradation ability ofγ-Fe2O3/ZnO might be attributed to the large surface area fromthe nest-like hollow structure. The photodegradation perform-ance of the as-prepared γ-Fe2O3/ZnO heterostructures was stillstable after 6 cycles without significant deterioration,suggesting that these γ-Fe2O3/ZnO heterostructures werehighly stable and could be reused many times.136 As shown inFig. 12, we have successfully fabricated mesoporous spindle-like α-Fe2O3/ZnO core–shell heterostructures by a surfactant-

Fig. 11 TEM image (a) and photograph of magnetic separation of spindle-like iron oxide/SnO2 composite nanoparticles.130

Fig. 12 Synthetic route and formation mechanism for fabricating the mesoporous hematite/ZnO core–shell composite particles.137

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free, low-cost, and environmentally friendly seed-mediatedstrategy with the help of post-annealing treatments. The thick-ness of the ZnO layer was tailored by adjusting the concen-tration of zinc precursor. Considering that both α-Fe2O3 andZnO are good photocatalytic materials, the photocatalyticactivity of the core–shell heterostructures for organic dye RhBhad been studied. It is noteworthy that the as-preparedα-Fe2O3/ZnO core–shell heterostructures displayed enhancedphotodegradation performance, clearly higher than theuncoated α-Fe2O3 seeds and commercial P25 TiO2, whichmainly attributed to the synergistic effect between the narrowand wide bandgap semiconductors and effective photogene-rated charge carrie separation at the interface of α-Fe2O3/ZnO.137

4.1.4 Others. Apart from TiO2, SnO2 and ZnO, WO3, ZrO2,Cu2O and Bi2O3 have also been investigated as potential photo-catalysts however, they are generally less photocatalyticallyactive than TiO2, and their semiconductor oxides have beenused to functionalize magnetic iron oxide nanoparticles.138,139

As shown in Fig. 13, Xi and co-workers have synthesized amagnetically recyclable Fe3O4/WO3 core–shell visible-lightphotocatalyst by a facile solvothermal epitaxial growth com-bined with a mild oxidation route. Photoelectrochemical inves-tigations verified that the core–shell structured Fe3O4/WO3 hadmore effective photoconversion capability than pure WO3 orFe3O4. At the same time, the visible-light photocatalyticability of the Fe3O4/WO3 photocatalyst exhibited significantenhancement in photodegradation of RhB. Furthermore, theFe3O4/WO3 core–shell photocatalyst was effectively recycled atleast three times without an apparent decrease in its photo-catalytic activity, which demonstrates its high stability.140

Li and co-workers have successfully synthesized mag-netic Fe3O4@C@Cu2O composites with a bean-like core–shellnanostructure by step-by-step self-assembly. The carbonaceouslayer with unavoidable hydrophilic groups inherited from thestarting materials acted as both linker and stabilizer betweenFe3O4 and Cu2O. The Fe3O4@C@Cu2O composites exhibited

ferromagnetic behaviour, and good dispersibility in aqueoussolution. Importantly, the bean-like core–shell compositesshowed universal and powerful visible-light-photocatalyticactivity for the degradation of RhB, methyl orange (MO), andalizarin red (AR) relative in comparison with commercial Cu2Oand Degussa P-25 powders.141 Wang and co-workers have syn-thesized three-dimensional flower-like hierarchicalFe3O4@Bi2O3 core–shell architecture by a facile solvothermalapproach. The diameter of the as-obtained flower-like hier-archical microsphere was ca. 420 nm and the shell was com-posed of several nanosheets with a thickness of 4–10 nm and awidth of 100–140 nm. The saturation magnetization of thesuperparamagnetic composite heterostructures was ca. 41 emug−1 at room temperature. Additionally, the Fe3O4@Bi2O3 com-posite heterostructures exhibited much higher (7–10 times)photocatalytic ability than commercial Bi2O3 particles undervisible-light irradiation. The photocatalytic activity ofFe3O4@Bi2O3 composite heterostructures did not display aclear loss in photocatalytic degradation of RhB after 6recycles.142

Recently, new semiconductor oxides have been used inphotocatalytic applications, such as V2O5,

143 Nb2O5,144,145

Ta2O5,146,147 CeO2,

148,149 Ga2O3,150 etc.151 However, reports on

the synthesis of magnetic iron oxide–semiconductor oxides arevery scarce so far, and it is worth studying various ways tointroduce semiconductor oxides into iron oxide–semiconduc-tor systems and to improve their photocatalytic ability.

4.2 Magnetic iron oxide–metal chalcogenides semiconductorcomposite photocatalysts

Currently, the proportion of using semiconductor oxides inphotocatalysts is large, however, the semiconductor sulfidessuch as ZnS, CdS, Bi2S3, SnS2, ZnSe, etc. have also been attrac-tive due to the importance of their special quantum confine-ment effect.93,152–156 As a competitive alternative, theapplication of magnetic iron oxide–semiconductor sulfides isa relatively new technology. The optical properties and photo-catalytic performance of semiconductor sulfides could bedifferent from their oxide counterparts.

For example, Liu and co-workers have reported the prepa-ration of Fe3O4/CdS nanocomposites via a sonochemical routein an aqueous solution. These Fe3O4/CdS nanocomposites dis-played fluorescence and exhibited excellent magnetic pro-perties at room temperature. Photocatalytic activity studiesconfirmed that the as-prepared nanocomposites had highphotocatalytic activity towards the photodegradation of methylorange in aqueous solution. Furthermore, the photodecompo-sition rate decreased slightly after 12 cycles of photocatalysis(89% of MO is decomposed in the last cycle).157 Their sub-sequent studies revealed that the Fe3O4/ZnS had high photo-catalytic activity towards the photodegradation of eosin Y inaqueous solution. The catalytic efficiency only decreased by5% after 15 cycles.112 As shown in Fig. 14, Shi and co-workershave synthesized α-Fe2O3/CdS corn-like nanocomposites viaCdS nanoparticles by a simple one-step wet-chemical route, inwhich preformed single-crystaline α-Fe2O3 nanorods were used

Fig. 13 Formation of the Fe3O4/WO3 core–shell structures: (a) poly-crystalline Fe3O4 microspheres; (b) Fe3O4 microspheres coated with athin layer of W18O49; (c) Fe3O4/W18O49 core–shell structures; (d) Fe3O4/WO3 core–shell structures obtained by oxidizing Fe3O4/W18O49 in air.140

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as substances for growing CdS nanoparticles. The corn-likenanocomposites exhibited superior photocatalytic perform-ance under visible light irradiation (86.7% of MB was degradedin 120 min) over pure α-Fe2O3 nanorods and CdS nanoparti-cles. The enhanced performance is attributed to the largersurface area of the corn-like structure, the crystalline nature ofthe materials and the synergy in light absorption and chargeseparation between α-Fe2O3 and CdS.158 Luo and co-workershave developed a facile and rapid synthesis of urchin-shapedFe3O4@Bi2S3 core–shell hierarchical structure through a sono-chemical method. The as-prepared Fe3O4@Bi2S3 hierarchicalcore–shell structures showed excellent photocatalytic efficiencyfor the degradation of RhB and retained the photocatalyticactivity after being recycled for five times with the help of anexternal magnetic field.159

Additionally, some ternary semiconductor sulfides have beenused in the application of photocatalysts, such as ZnxCd1−xS,ZnIn2S4, CuInS2, etc.

160–165 However, there is no literature reporton iron oxide–ternary semiconductor sulfide composite photo-catalytic systems. More importantly, the toxicity of the semicon-ductor sulfides should also be considered in practical application.

4.3 Multiple semiconductor shell photocatalysts

The synergetic effects of multiple semiconductor photocata-lysts have been extensively observed in photocatalytic degra-dations. In core–shell–shell structures, the first semiconductorshell layer can offer special active-sites for the adsorption/reac-tion of reactant/reaction intermediates. The second semicon-

ductor shell layer could also influence the overall bandconfiguration via altering the bandgap absorption and to sep-arate the photoinduced charge carriers.166–168 Moreover, semi-conductors with a narrow band gap can expand the spectralresponse range. As shown in Fig. 15, by introducing semicon-ductor heterojunction on the surface of magnetic iron oxidenanomaterials, magnetically recoverable photocatalysts areobtained.14,169–171

For example, Chen and co-workers have synthesized threetypes of ellipsoidal complex hollow structures with the shellsassembled from anatase TiO2 nanosheets with exposed (001)facets by utilizing silica-coated hematite (α-Fe2O3) nanospin-dles as the starting templates. As shown in Fig. 16, theα-Fe2O3/SiO2/SnO2/TiO2 composite can be prepared by hydro-thermal deposition of a SnO2 layer on the surface of SiO2. TheFe3O4@SnO2@TiO2 nanorattles manifested a much higherdegradation efficiency compared to Degussa P25 nanoparti-cles, as well as their analogous Fe3O4@TiO2 core–shell nano-material without the exposed (001) high-energy facets.172 Dongand co-workers have prepared the CdS modified TiO2/Fe3O4

photocatalysts by sol–gel and immersion methods. TheCdS-TiO2/Fe3O4 composites exhibited higher photocatalyticactivity than pure TiO2 and TiO2/Fe3O4 for the degradation ofReactive Brilliant Red X-3B dye (X-3B) under simulated sun-light. In addition, a gradual loss of photocatalytic activity wasobserved in the reusability test of CdS-TiO2/Fe3O4 composites,and degradation of X-3B reached 78.9% after five runs.173

Obviously, photocatalysts with multiple semiconductor shellscan effectively improve the photocatalytic abilities.

Fig. 15 Scheme of the preparation of iron oxide–multiple semiconductor layer composite photocatalysts.

Fig. 14 SEM image and photocatalytic performances of α-Fe2O3/CdS corn-like nanorods under visible light irradiation.158

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5 Summary and perspectives

Several fundamental issues must be addressed before photo-catalysts are economically viable for large scale industrialapplications. Apart from offering easy separation of the photo-catalysts from the reaction system, the magnetic iron oxide–semiconductor photocatalytic system, which interfaces chem-istry with materials science, possesses a unique position in theadvancement of heterogeneous photocatalysis. Table 1 depictsthe representative magnetic iron oxide–semiconductor photo-catalysts and their photocatalytic performances. Though a lotof effort has been made in design and fabricating magneticiron oxide–semiconductor composite photocatalytic system, itis still a field of research in modern photocatalysis and follow-ing issues are still need to be addressed.

(1) In order to improve the photocatalytic activities ofphotocatalysts, extension of the excitation wavelength, reducedcharge carrier recombination, and the promotion of activesites around the surface should be considered. Therefore, ifnoble metal nanomaterials are introduced in the magneticiron oxide–semiconductor system, the photocatalytic efficiency

could be enhanced. The photogenerated charge carriers in thenoble metal can be separated by the metal/semiconductorheterojunction. Additionally, the separated electron and holecan take part in the chemical reactions on the surface of metaland semiconductor, respectively. The absorbed photons canexcite the valence electrons of noble metals due to the surfaceplasmon resonance (SPR) effect. The energy of photoinducedelectrons is higher than the Schottky barrier resulted in cross-ing the interface and transferring to the VB of the semiconduc-tor. Numerous literature reports are dedicated to the metal–semiconductor composite photocatalytic system. However,reports on magnetic iron oxide/noble metal/semiconductorsternary photocatalysts are scarce and need to be strength-ened.174 Owing to the SPR effect, solution processed metalnanoparticles coated onto the surface of iron oxide or semi-conductors is an effective method to enhance the absorptionof visible light. However, the metal nanoparticles can also actas recombination centres resulting in inferior photocatalyticperformance due to the incorporation of chemically syn-thesized metal nanoparticles in the iron oxide–semiconductorcomposite system. Many factors can cause undesirable exciton

Fig. 16 Synthetic route (a) and SEM, and TEM images for fabricating the ellipsoidal α-Fe2O3/SiO2/SnO2/TiO2 composite photocatalytic nanomater-ials (b).172

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quenching and decrease the plasmonic effect. Indeed, coatingof metal nanoparticles with insulating materials can preventsuch recombination centres.175 More recently, we havereported a novel iron oxide/noble metal/semiconductor ternarymultilayer hybrid structure that was prepared by template syn-thesis and subsequent layer-by-layer deposition method. Threedifferent morphologies of α-Fe2O3/Ag/SiO2/SnO2 heterosturc-tures were obtained, the thickness of the insulating SiO2 inter-layer was tailored to control the coupling of noble metal silverwith tin oxide. The as-obtained α-Fe2O3/Ag/SiO2/SnO2 nano-composites exhibited enhanced catalytic abilities under UV orvisible light irradiation, higher than the commercially avail-able pure SnO2, naked α-Fe2O3 seeds and α-Fe2O3/SnO2 binarynanocomposites. Moreover, α-Fe2O3/Ag/SiO2/SnO2 exhibitedsignificant stability and recyclability because of its photodegra-dation rate maintains at 96% after 8 cycles.176

(2) The fusion of catalysis with nanotechnology continuesto generate better materials and improve their functions.Graphene and its use in photodegradation is one of the latestexamples. Its interesting electrical and mechanical properties,and high surface area make graphene a novel substrate forforming hybrid structures with a variety of nanomaterials. Theuse of graphene to enhance the efficiency of photocatalysts hasattracted much attention. Utilization of single-layer graphenesheets can not only provide a high quality two-dimensionalphotocatalyst support, but also a two-dimensional circuitboard, with an attractive potential to harness their perfect elec-trical and redox properties. There are few literature reports oncomposites of graphene with magnetic iron oxide–semi-conductors photocatalytic system.177

(3) At present, a lot of fundamental and applied research ofphotocatalysis are focused on the synthesis and modificationof new photocatalysts, nevertheless, with those endeavours,the effect of photocatalyst microstructure on their photo-catalytic performance still cannot be understood, understand-ing the relationship between these two parts is a prerequisite

for the broad application of composite nanomaterials inphotocatalysis. However, the understanding of interfaceeffects, the coupling mechanism, photocatalyst life, de-activation and the regeneration mechanism are still relativelyweak.178 As a heterogeneous catalytic reaction system, semi-conductor photocatalytic materials would be deactivated inpractical application, such as the photocatalytic efficiency ofP25 TiO2 becomes very low after 3 cycles under sunlight.Therefore, deactivation and the regeneration mechanism ofsemiconductor photocatalysts should be reinforced.

As a key issue for practical applications, the facile method toincrease the photocorrosion suppression ability, the life andstability of magnetic iron oxide–semiconductor compositephotocatalytic system must be further developed and improved.To date, the underlying photocorrosion mechanism for the ironoxide–semiconductor composite photocatalyst is not clear, andsystematic studies are necessary. Many methods have been usedto reduce the photocorrosion of pure semiconductors, such asgraphene composites,179 graphene oxide,180 quantum dot,181

etc., and these materials can be introduced to the magnetic ironoxide–semiconductor composite photocatalytic system. More-over, recycling and regeneration is also an effective method toextend the life of a deactivated photocatalyst. However, reportson the above mentioned issues are scarce so far.

(4) Although a great effort has been made in the past yearsto unravel the mechanisms of bi/ternary and multiple compo-site photocatalysts, it is still a challenge for various research-ers. To develop an efficient heterostructural photocatalysissystem for large-scale industrialization, understanding of thekinetics and mechanisms of these charge transfer processes isvery important.182 More efforts on photo-inducing chargecarrier generation, trapping, recombination, and transportingare needed to further to strengthen and improve it. Apart fromthe traditional characterization techniques, more and morephotoelectrochemical methods and techniques have beenused to study the kinetics and mechanisms of heterostructural

Table 1 The representative magnetic iron oxide–semiconductor photocatalysts and their photocatalytic performance

Structure Materials Pollutants Light resourceRate constantk (10−2 min−1) Stability performance Ref.

Binary structure Fe3O4@TiO2 Bisphenol A UV light —a 90% after 10 cycles 121α-Fe2O3@ TiO2 RhB Visible light 0.81 — 197γ-Fe2O3@SnO2 RhB UV light 0.68 — 131α-Fe2O3@ZnO RhB UV + visible light 2.4 — 137Fe3O4@WO3 MB Visible light —b No obvious decrease

after 3 cycles140

Fe3O4/ZnS Eosin Y UV light —c 95% after 15 cycles 112α-Fe2O3/CdS MB Visible light 1.68 — 158

Ternary structure Fe3O4@SiO2@TiO2 Methyl orange UV light — 91% after 6 cycles 120Graphene/TiO2/Fe3O4 RhB UV light 16 No obvious decrease

after 5 cycles177

α-Fe2O3@SnO2@Cu2O RhB UV + visible light 3.29 85% after 8 cycles 198Multiple layers structure α-Fe2O3/Ag/SiO2/SnO2 RhB UV light 0.13 — 176

Visible ligth 0.41 —UV + visible light 7.21 96% after 8 cycles

α-Fe2O3/SiO2/SnO2/TiO2 MB UV light 0.29d — 172

a Photocatalytic degradation of BPA is complete within 60 min. b Photocatalytic degradation of MB is complete within 120 min. c Photocatalyticdegradation of Eosin Y is complete within 37 min. d Photocatalytic degradation of MB is 70% within 120 min.

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photocatalysis systems. For instance, photoelectron spec-troscopy (PES) is used to measure band bending in semi-conductors, femtosecond transient reflecting grating (TRG)method is used to detect the photogenerated ultrafast relax-ation dynamic at solid/liquid interfaces, O2 photostimulateddesorption (PSD) and electronstimulated desorption (ESD) areused to study the surface photoreactions induced by the photo-excited electrons and holes in the semiconductor, etc.183–186 Atpresent, charge transfer kinetics on a short duration is wellstudied, while the charge transfer on a more extended timescaleis still unclear. Therefore, unravelling the mechanisms that playan important and key role in magnetic iron oxide–semiconduc-tor composite photocatalytic system is necessary. On thisregard, there are several mechanisms that are still not fullyunderstood and many works need to be carried out.

(5) In fact, the photodegradation of pollutants is mainlyused in a suspension of semiconductor nanomaterials in thisfield. However, from a practical point of view, there are manylimitations of using a photocatalyst suspension, such asrequirement of large photo-reactors, hard to filtrate the nano-scale photocatalyst, etc. As the catalytic mechanism of thesesynthesized photocatalysts is very complicated, the pure semicon-ductors and α-Fe2O3/semiconductor composite photocatalyticsystems are hardly recycled by external magnetic fields due totheir weak magnetic response. As shown in Fig. 17, these photo-catalysts can be printed on rigid or flexible substrates as photo-catalytic arrays or patterns, including screen printing,187–189

offset printing,190 inkjet printing,191–193 gravure printing,194

etc.195,196 These printed patterns with semiconductor photocata-lysts can also be recycled. Therefore, combining and developingmore practical methods to use these composite photocatalyticsystems should be further reinforced.

Acknowledgements

The authors thank the Hong Kong Scholars Program, NSFC(51471121, 51201115, 51171132), Young Chenguang Project ofWuhan City (2013070104010011), China Postdoctoral ScienceFoundation (2014M550406), Hubei Provincial Natural ScienceFoundation (2014CFB261) and the Fundamental ResearchFunds for the Central Universities.

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