Recent progress in magnetic iron oxide–semiconductor composite nanomaterials as promising photocatalysts

Abstract

Photocatalytic degradation of toxic organic pollutants is a challenging tasks in ecological and environmental 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 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 nanomaterials are reviewed. The problems and challenges still need to be resolved and development strategies are discussed.

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W. Wu

Wei Wu obtained his B.S. in 2005 and M.S. in 2008 from Hunan University of Technology, and he received his PhD on Materials Physics and Chemistry in 2011 under the supervision of Prof. Changzhong Jiang in Department of Physics, Wuhan University, China. He then joined the group of Prof. Daiwen Pang at the Department of Chemistry and Molecular Science, Wuhan University, as a postdoctoral fellow for the design and synthesis of magnetic iron oxide –semiconductor heterostructures. Now he is the Director and Associate Professor of the Laboratory of Printable Functional Nanomaterials and Printed Electronics, School of Printing and Packaging, Wuhan University. He has published, as an author and co-author, more than 60 publications in various reputed international journals. He is also an Associate Editor of Journal of Nanoscience Letters, and editorial board member of Advanced Science, Engineering and Medicine and Journal of Green Science and Technology, his research interests include the synthesis, properties, and application of nanomaterials, printed electronics and sensors.

C. Z. Jiang

Changzhong Jiang received his B.S in 1983 from Huazhong University of Science and Technology, M.S. in 1990 from Wuhan University. He obtained his PhD in 1999 from Université Claude Bernard Lyon 1, France. Currently, he has been a full professor in the Department of Physics, Wuhan University since 2001, and he is also the Director of Center for Ion Beam Application, Wuhan University. He has published, as an author and co-author, more than 80 publications in various reputed international journals, such as Physical Review Letters, Nano Letters, ACS Nano, Advanced Materials. His research interests include the synthesis and application of low-dimension nanomaterials, magnetic materials and ion beam modification of materials.

V. A. L. Roy

Vellaisamy A. L. Roy obtained his PhD degree from Nagpur University in 2004. Dr Vellaisamy started his research on light-emitting materials during his PhD, mainly on Electron Spin Resonance analysis of organic materials and was working on the growth of wide band gap nano-structures. Currently, he is an associate professor at the Department of Physics and Materials Science, City University of Hong Kong. His research interests are design, synthesis, and charge transport analysis of self-assembled nanostructures and functional materials for sensors, thin film transistors and floating gate flash memories. Dr Vellaisamy received the TRIL Fellowship awarded by UNESCO in 2003 and an Excellent Product Award for his project prototypes on Sensors and Memories at China Hitech-Fair for three consecutive years since 2002. He has published over 80 papers in international SCI journals, including Advanced Materials, Angewandte Chemie International Edition, Nano Letters, ACS Nano and his papers have been cited more than 1800 times.

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1 Introduction

Our surrounding environment continues to become more polluted, and the traditional chemical methods that deal with environmental pollution have been unable to meet the requirements of modern energy-saving themes and environmental protection. Environmental problems induced by toxic and hardly-degradable organic pollutants (such as halides, dioxins, pesticides, dyes, etc.) have posed a grave menace to human well-being and development in the 21^st century. Photocatalysis refers to the rate of photoreactions (oxidation/reduction) brought on by the activation of a catalyst, usually a semiconductor oxide, through illumination under ultraviolet (UV) or visible light. Use of semiconductor oxide nanomaterials-based photocatalysts to degrade organic pollutants is recognized as one of the most promising areas of research and application.^1–3 On this regard, photocatalysts are regularly used in solid–liquid reaction systems, especially for the treatment of toxic waste.

However, the main restriction factor of large scale practical applications of semiconductor oxide photocatalysts are as follows: (1) high recombination rate of electronic–hole pairs resulting in low quantum yield for single-component semiconductor oxide photocatalysts. For instance, Sun and Bolton have reported less than 5% primary quantum yield of ˙OH radical generation in a TiO₂ suspension.⁴ (2) The limitation in the harvesting of visible light. Generally, wide bandgap semiconductor oxides are employed as photocatalysts, for example, the bandgap value of anatase TiO₂ is 3.2 eV, and the corresponding absorption wavelength is 387.5 nm, resulting in limited light absorption of the UV region. Unfortunately the solar spectrum consists of only 5–7% of UV light, while 46% and 47% of the spectrum consists of visible light and infrared radiation, respectively.⁵ (3) Poor selective adsorption and the complexity of intermediate products. For example, photodegradation reaction products such as CO₂ and H₂O are easily adsorbed on the surface of TiO₂ in the gas–solid photocatalyst system due to its super-hydrophilicity and active sites.⁶ The high oxidization potential energy of OH radicals can induce many backward reactions, such as oxidizing the intermediates and products converted from the as-adsorbed CO₂.^7,8 (4) The photocatalytic treatment of a high concentration of organic pollutants from industrial waste poisons the photocatalyst resulting in deactivation. In addition, it is difficult to separate a pure semiconductor oxide photocatalyst from the waste water treating system,⁹ which further deactivates the photocatalysts. (5) High cost of photocatalyst industrialization. This factor limits the industrial applications of photocatalysts, hence research and development of low-cost, high-performance, and recyclable photocatalysts have became an important issue.^10,11

As a hot issue, photocatalysis has witnessed a sea of change over the past two decades with significant advancements being made in the preparation of novel materials and nanostructures, and the design of efficient processes for the photodegradation of pollutants and the generation of energy. Thus, the development of a simple recyclable photocatalyst can not only prevent excessive use of photocatalysts, but also the recovery of deactivated photocatalysts, thereby reducing the total cost, and further lowering the overall usage of the photocatalytic material. Since visible light constitutes a large fraction of solar energy, one of the great challenges of photocatalyst study is to devise new catalysts that exhibit high activity under illumination by visible light.

Combining the magnetic iron oxide nanomaterials with semiconductor nanomaterials to form a magnetic iron oxide–semiconductor composite photocatalyst system becomes a simple and effective method. In an iron oxide–semiconductor system, iron oxide has many advantages, for example, low cost, high stability and compatibility, it not only plays the role of separating the photocatalyst from the solution, but also it can degrade organic pollutants.^12,13 The common metal oxide semiconductors like titanium dioxide (TiO₂), zinc oxide (ZnO), tungsten oxide (WO₃), and tin oxide (SnO₂) are proven to be dynamic photocatalysts for organic dyes and pollutants, these semiconductors not only destroy the conjugated chromophoric system, but also breakdown the molecular structure of the organic dyes and pollutants into harmless CO₂ and H₂O. As magnetic recovery photocatalytic materials, the iron oxide–semiconductor oxide photocatalyst system can effectively break through the bottleneck of low activity under visible light and demanding recycling processes from the products, and eventually become a potential visible light responsive MRPs in the future.

Therefore, the design of an iron oxide–semiconductor photocatalytic system is an essential prerequisite for both basic and applied research. If the system focuses on the magnetic recovery properties, the saturation magnetization value of used iron oxides should be no less than 1 emu g⁻¹, in order to separate via an external magnetic field for further reusing and regeneration. If the system focuses on the photocatalytic performance, the used iron oxides should possess a relatively narrow bandgap value. For instance, goethite and hematite are often studied as photocatalysts in recent years because of their low band gap (2.2 eV). There are reported techniques to improve the photocatalytic performance of an iron oxide–semiconductor system, such as a composite heterostructure with a narrow/wide bandgap, p–n heterojunctions, noble metal loading, plasmonic structure, graphene loading, etc.^14–16 Overall, an optimal iron oxide–semiconductor photocatalytic system design should meet the following requirements. First, the synthesis and preparation process should be both simple, and facile with high-yield. Second, the composite system should exhibit an enhanced photocatalytic performance remarkably superior to existing naked iron oxide and pure semiconductor materials. Third, the compositephotocatalyst should be recycled by the external magnetic field that facilitates easy reuse and regeneration. Finally, the composite photocatalyst should possess good photocorrosion resistance ability and be stable at room temperature for months.

In this review, we will first describe the structure and mechanism of a magnetic iron oxide–semiconductor photocatalysis system. We discuss different synthesis methods and recent advances in magnetic iron oxide–semiconductor nanomaterials. The potential of magnetic iron oxide–semiconductor based materials as photocatalysts is also examined. Finally, we discuss future prospects in realizing this technology and further research directions.

2 Structure and mechanism for magnetic iron oxide–semiconductor composite photocatalysts

2.1 Magnetic iron oxide nanomaterials

As a common compound, iron oxide is widely distributed in nature and can be synthesized on a large-scale. The application of small iron oxide nanoparticles has been practised in in vitro diagnostics for more than 60 years.¹⁷ Over the past few decades, magnetic iron oxide nanoparticles with various morphologies and structures are widely fabricated because of their importance in basic research. On the other hand, magnetic iron oxides are of great interest for researchers due to wide range of applications, including pigments, magnetic fluids, catalysis, targeted drug delivery, biosensor, magnetic resonance imaging, data storage, and environmental remediation.^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 their polymorphism involving temperature induced phase transition. These three different crystalline iron oxides have unique biochemical, magnetic, catalytic, and other properties that make them suitable for specific technical and biomedical applications.

Magnetic measurements of α-Fe₂O₃ show obvious weak ferromagnetism and its saturation magnetization is often less than 1 emu g⁻¹ at room temperature. However, γ-Fe₂O₃ and Fe₃O₄ show saturation magnetization values up to 92 emu g⁻¹.²¹ More importantly, the magnetic properties of iron oxide nanoparticles are related to their size and shape. For example, Demortière and co-workers have investigated the size-dependence of iron oxide nanocrystals on their structural and magnetic properties by fine tuning the size within the nanometer scale (diameters range from 2.5 to 14 nm). The evolution of magnetic behavior with nanoparticle size clearly emphasizes the influence of the surface, especially on the saturation magnetization (M_s) and the magneto-crystalline anisotropy. Dipole interactions and thermal dependence have also been taken into account in the study of nanoscale size-effects on magnetic properties.²² More recently, we have reported a comparative study on the magnetic behavior of single and tubular clustered Fe₃O₄ nanoparticles. The results reveal that the coercivity of small iron oxide nanoparticles could be enhanced by the competition between the demagnetization energy of the morphology and magneto-crystalline anisotropy energy.²³ Choi and co-workers have prepared Fe₃O₄ nanoparticles with different shapes, including solid nanospheres and solid/hollow nanoellipsoids. All these structures were obtained by either adding the appropriate amount of sodium acetate (NaOAc) or using the anion exchange of β-FeOOH. All magnetite nanoparticles exhibited ferromagnetic behaviour with different values for the saturation magnetization (M_s) and coercivity (H_c), and these values were highly dependent on the shape due to their grain size, spin disorder, shape, and surface anisotropy.²⁴ In general, iron oxide nanoparticles become superparamagnetic at room temperature when the size of the iron oxide nanoparticles are below ca. 15 nm, meaning that the thermal energy can overcome the anisotropy energy barrier of a single nanoparticle. If the semiconductor is coated on the surface of iron oxide, the M_s value would decrease. There are a number of magnetic properties for the characterization of naked iron oxides nanoparticles and iron oxide–semiconductor composite nanomaterials. The most important properties are the type and magnetization which can be determined from the hysteresis loops (M–H) and zero-field cooled/field cooled (ZFC/FC, M–T) curves. As shown in Fig. 1, the saturation magnetization (M_S), remanence magnetization (M_r), and coercivity (H_C) can be obtained from the hysteresis loop. When the naked iron oxide nanoparticles and iron oxide–semiconductor composite nanomaterials exhibit superparamagnetic, the M–H curve should show no hysteresis at a certain temperature (T > T_B, blocking temperature). The forward and backward magnetization curves overlap completely.^23,25

Fig. 1 Schematic presentation of the typical hysteresis loops of magnetic iron oxide nanoparticles.

The general strategy for preparing magnetic iron oxide nanoparticles in solution is to separate the nucleation and growth of nanocrystals. Numerous synthetic methods have been developed to synthesize magnetic iron oxide NPs, including co-precipitation,^26–28 high-temperature thermal decomposition,^29–31 hydrothermal and solvothermal reaction,^32–34 sol–gel reactions and polyol method,^25,35,36 microemulsion synthesis,^37–39 sonochemical reaction,^40–44 microwave-assisted synthesis^45–48 and biosynthesis.^49–51 Other than the above-mentioned methods, alternative chemical or physical methods can also be used to synthesize magnetic iron oxide nanoparticles, such as the electrochemical methods,^52–54 flow injection synthesis,⁵⁵ and aerosol/vapor methods,^56–58etc. In literature, there are many reports on the fabrication of magnetic iron oxide NPs. Here, we briefly review the recent advances on the synthesis of magnetic iron oxide–semiconductor 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 photoinduced chemical reactions due to their intrinsic electronic structure that consists of a filled valence band (VB) and an empty conduction band (CB).^59–64 When a photon with energy (hv) matches or exceeds the band gap energy (E_g) of the semiconductor, a photogenerated electron (e⁻) in the valence band is excited into the conduction band, leaving a positive hole (h⁺) in the valence band. The photoinduced charge carriers play a key role in the photocatalytic degradation process. The holes mediate the oxidation of organic compounds through the formation of hydroxyl radicals (˙OH), and the electrons mediate redox reactions through the formation of superoxide radicals (˙O₂). However, the photoinduced charge carriers in the excited states are unstable and can easily recombine, converting the input energy to heat and thus leading to the low activity of a photocatalyst.⁶⁵ An ideal photocatalyst should be stable, inexpensive, non-toxic and, of course, highly photoactive.

On the basis of thermodynamic requirements, the VB and CB of the semiconductor photocatalyst should be located in such a way that the oxidation potential of the hydroxyl radicals (E₀(H₂O/˙OH) = 2.8 V vs. NHE) and the reduction potential of superoxide radicals (E₀(O₂/˙O₂⁻) = −0.28 V vs. NHE), lie well within the band gap. In other words, the redox potential of the VB holes must be sufficiently positive to produce hydroxyl radicals. On the other hand, the CB electrons must be sufficiently negative to produce superoxide radicals.⁶⁶Fig. 2 shows the bandgap energy and band edge positions of common semiconductor oxides and semiconductor sulfides, along with selected redox potentials. Obviously, the bandgap energy and band edge positions of TiO₂, ZnO, SnO₂, Fe₂O₃, WO₃ and ZrO₂ are relatively good. As already mentioned, such semiconductor materials are prone to be applied in photocatalysis due to their inherently filled VB and empty CB. When these semiconducting solids absorb photons and hv ≥ E_g, an e⁻ is excited from the VB to the CB. This can be expressed in the following formula: hv + semiconductor → h⁺ + e⁻. Then the electron of the semiconductor can be transferred to an adjacent compound.

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

Additionally, the choice of semiconductor materials for photocatalytic applications rely on the consideration of photocorrosion resistance ability. For instance, CdS and ZnO only have a stable valence of +2, and can be decomposed by photogenerated holes from the VB. Furthermore, ZnO is prone to be deactivated due to the generation of Zn(OH)₂ on its surface.¹⁰ Currently, there are many methods to inhibit or delay the deactivation caused by photocorrosion, and the common method is to combine with other materials and to form composite nanomaterials.^67,68 As compared to other materials, the oxidation state of Ti in TiO₂ can be reversibly changed (from +4 and +3), thereby TiO₂ is more stable and suitable for photocatalytic applications. Additionally, the anatase phase TiO₂ (E_g = 3.2 eV) is more active for photocatalysis applications, even though the rutile phase TiO₂ (E_g = 3.0 eV) possesses a smaller band gap, revealing the possibility of absorption of long wavelength radiation. The CB of anatase TiO₂ is more negative compared to rutile.

In addition to the aforementioned factors, other requirements such as low-cost, a non-toxic nature (environmentally benign) and easy preparation should also be taken into consideration for photocatalytic degradation reactions.

2.3 The structure of iron oxide–semiconductor composite nanomaterials

In order to increase the range of applications of iron oxide nanoparticles, some functional materials have been introduced and formed as newly composite nanostructures. Comparing with single-component nanomaterials, multiple-component nanomaterials have become the subject of extensive research due to the synergistic interaction effects between each component, which could improve the final catalytic performance. Currently, wide band gap semiconductors with good photocatalytic properties have been used to functionalize magnetic iron oxides. In a iron oxide–semiconductor composite system, the magnetic iron oxide can not only separate and recover the photocatalyst, but can also form narrow/wide band gap semiconductor heterostructures. The narrow/wide band gap semiconductor heterostructures can promote the separation of electron and hole pairs efficiently, consequently increasing the visible light utilization and finally improving the photocatalytic efficiency.

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

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

2.3.1 Core–shell structure. In this structure, the iron oxide core is encapsulated by a semiconductor layer that renders the stability of the whole particle. Generally, the iron oxide nanoparticles are not located at the centre of a functional semiconductor and this is known as a yolk structure. For example, Liu and co-workers have successfully synthesized various Fe₃O₄@TiO₂ yolk–shell microspheres with different core sizes, interstitial void volumes, and shell thicknesses by controlling the synthetic parameters.⁶⁹ Li and core-workers have developed a facile “hydrothermal etching assisted crystallization” method to prepare Fe₃O₄@TiO₂ yolk–shell microspheres with ultrathin nanosheets assembled as double-shell structure. The as-obtained microspheres possess high surface area, good structural stability and large magnetization, the size is uniform and the shell could be tailored, which exhibits versatile ion-exchange capability and a remarkable catalytic performance.⁷⁰ Indeed, the magnetic composite nanomaterials not only provide an improved stability of the nanoparticulate building blocks, but also introduce new physical and chemical properties and multifunctional behaviours. In the inverse core–shell structure, the magnetic iron oxides are coated on the surface of semiconductor materials. For instance, Luo and co-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 the design of optimal composite electrode materials.⁷¹ Moreover, magnetic iron oxides could be combined and coated with one or more functional materials on the surface of another functional material. The above structures are all called core–shell structures.

2.3.2 Matrix-dispersed structure. Several magnetic iron oxide nanoparticles are coated or dispersed in a semiconductor matrix. Matrix-dispersed nanoparticles can be created in a variety 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 cores are distributed in the TiO₂ matrix with silica as the barrier layer between the magnetic cores and TiO₂ shells has been reported. The hybrid nanoparticles show good magnetic response and display high photocatalytic efficiency for methylene blue (MB).⁷²

2.3.3 Janus structure. In the Janus structure, one side is magnetic iron oxide nanoparticles, and the other side is a functional semiconductor material. An anisotropic surface chemical makeup is interesting for applications even without self-assembly. For example, Zeng and co-workers have synthesized multifunctional Fe₃O₄/TiO₂ nanocomposites with a Janus structure for magnetic resonance imaging (MRI) and potential photodynamic therapy (PDT), in which Fe₃O₄ is a MRI contrast agent and TiO₂ is an inorganic photosensitizer for PDT.⁷³ Mou and co-workers have developed an asymmetric shrinkage approach for the fabrication of magnetic γ-Fe₂O₃/TiO₂ Janus hollow bowls by constructing a precursor solution pair with different gelation rates during the solvent evaporation process. The as-obtained products exhibited an efficient visible-light photocatalytic activity and convenient magnetic separation because of the unique structure and morphology as well as the fine magnetic properties.⁷⁴

2.3.4 Shell–core–shell structure. In this structure, the magnetic iron oxide nanoparticles are located between two functional semiconductor materials. Several applications require magnetic iron oxide nanoparticles to be embedded in nonmagnetic layers to avoid aggregation and sedimentation of magnetic iron oxide nanoparticles as well as to endow them with particular surface properties for specific applications. In this structure, the two shell layers use the same or different semiconductors or one layer is a non-semiconductor material.⁷⁵

More importantly, understanding the relationship between the photocatalytic performance and the microstructure is a prerequisite for widespread application. Therefore, design and controllable synthesis of the nanostructured photocatalysts, and further optimization of the microstructure and photocatalytic performance are still under broad investigation.⁷⁶ A prerequisite for every possible applied structure is the proper surface properties of the magnetic composite NPs, which determine their interaction with the environment. These interactions ultimately affect the colloidal stability and photocatalytic efficiency of the composite particles.

2.4 Charge transfer mechanism

The charge separation mechanism in both capped semiconductor systems and coupled semiconductor systems involves the photoinduced electrons in one semiconductor being injected into the lower lying CB of the second semiconductor. Therefore, coupling semiconductors techniques do not always improve the photocatalytic performance by charge separation. The design of coupling semiconductor photocatalysts depends on the band structures of each component. Generally, photogenerated electrons on the CB of a higher level semiconductor are injected into the CB of a lower level semiconductor. As such coupled semiconductor photocatalytic systems bear great hope for next-generation solar energy harvesting and advancing environmental remediation techniques. Governments and researchers have devoted considerable interests and resources to such fabrication, characterization, and optimization.⁷⁷

In the iron oxide–semiconductor system, iron oxides can be narrow band gap semiconductors, with a band gap value for Fe₂O₃ of 2.2 eV, they also absorb visible light. For example, the work function (ϕ) of α-Fe₂O₃ is 5.88 eV, which is higher than most 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 shown in Fig. 4a, the band configuration and photogenerated charge carrier separation at the interface of iron oxide–semiconductor (wide band gap) under light irradiation are proposed. Under light irradiation, the photoinduced electrons and holes are separated at the interface of the iron oxide–semiconductor, the photoinduced electrons in the CB of iron oxide tend to transfer to that the CB of the semiconductor due to the decreased potential energy, and hence the coupling structure reduces the electron–hole recombination probability and increases the electron mobility. Thereby the electrons and holes were transferred to the surface of the iron oxide and semiconductor, respectively, and finally form hydroxyl radicals (˙OH). The superoxygen radicals (˙O₂) are formed by the combination of electrons with O₂ adsorbed on the surface of the semiconductor. As a powerful oxidant, ˙OH can degrade many pollutes, such as organic dyes, wastewater, and plastics. However, capped semiconductors on the other hand have a core and shell geometry, as shown in Fig. 4b. The electrons are injected into the energy levels of the core semiconductor (on condition that it has a conduction band potential which is lower than that of the shell). Hence, the electrons are trapped within the core particle, and is not readily accessible for the reduction reaction.⁷⁸

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

The introduction of an interlayer into the iron oxide–semiconductor heterojuction for tailoring the photocatalytic efficiency is another option. As shown in Fig. 5, when the insulating SiO₂ layer is introduced, the photogenerated electrons in the CB of iron oxide are not able to transfer to the CB of the semiconductor. However, from our previous reports and Christopher's reports, the photogenerated electrons can still transfer if the thickness of the SiO₂ is less than 5 nm.^79,80 Therefore, the thickness of SiO₂ is a key factor and responsible for the photocatalytic abilities of iron oxide/SiO₂/semiconductor systems. As an alternative, photogenerated electrons in the CB of iron oxide can transfer to the CB of the semiconductor via a carbon interlayer, which behaves as an electron conductor to enhance the electron–hole separation. For example, Hou and co-workers have reported an interlayer of graphene that transfers the electrons from the CB of a BiV₁−_xMo_xO₄ shell to the CB of the Fe₂O₃ core in α-Fe₂O₃ nanorod/graphene/BiV₁−_xMo_xO₄ core–shell heterojunction due to band alignment and potential difference, which provides a direct path for electron transport.⁸¹

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.

As a classical heterostructure, the iron oxide–semiconductor system has many advantages. First of all, the built-in potential at the interface of iron oxide and semiconductor can promote the separation and transport of photoinduced charge carriers. Second, iron oxides with relatively smaller band gaps sensitize the wide band gap semiconductors. Third, semiconductor metal oxides and metal sulfides such as RuO₂, NiO and IrO₂, MoS₂ and cobalt phosphates, can also act as effective co-catalysts to facilitate the surface electrochemical reaction. These co-catalysts improve charge separation, suppress the recombination of photogenerated charge carriers and lower the potential for electrochemical reaction.¹⁴ Although α-Fe₂O₃ is stable, it is prone to photocorrosion. Its photocatalytic degradation efficiency for organic dyes needs to be improved. Therefore, as an outer layer, semiconductors such as TiO₂ with excellent electrochemical- and photochemical-stability can be used on the surface of iron oxide to improve the stability of the catalysts.

3 Synthesis of magnetic iron oxide–semiconductor composite nanomaterials

3.1 Seed-mediated growth strategy

As shown in Fig. 6a, the seed-mediated growth strategy is the most common method for synthesizing high-quality magnetic iron oxide–semiconductor composite nanomaterials, especially the preparation of core–shell heterostructures. A typical growth protocol involves the addition of magnetic iron oxide nanoparticles, as seeds, to the bulk semiconductor growth. The growth solution is obtained by the reduction of semiconductor precursors. In this protocol, seeds are sequentially added to the growth solution in order to control the rate of heterogeneous deposition and thereby the rate of crystal growth. Many wet-chemical approaches have been used to generate iron oxide–semiconductor composite heterostructures, such as the co-precipitation, hydrothermal, and solvothermal methods, etc.^82–85

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

For example, Chiu and co-workers have reported the synthesis of Fe₃O₄/ZnO core–shell nanoparticles by the seed-meditated growth method under nonhydrolytic conditions. Control over the thermal pyrolysis of zinc acetate give the option to overgrow the ZnO layer on the surface of Fe₃O₄ seeds. These core–shell nanocrystals were magnetically separated by a 0.6 T magnet, which shows high potential for using such nanocrystals as recoverable catalyst materials.⁸⁶ We proposed a facile pathway to prepare three different types of magnetic iron oxide/TiO₂ hybrid nanoparticles by the seed-mediated method. The hybrid nanoparticles are composed of spindle, hollow, and ultrafine iron oxide nanoparticles as seeds and 3-aminopropyltriethyloxysilane as the linker between the magnetic cores and TiO₂ layers, respectively. About 50% to 60% of MB was decomposed in 90 min in the presence of magnetic hybrid nanoparticles, which is higher than pure TiO₂ nanoparticles. The synthesized magnetic hybrid nanoparticles display high photocatalytic efficiency and can be used for cleaning polluted water with the help of magnetic separation.⁸⁷ Recently, Li and co-workers have reported a versatile kinetics-controlled coating approach to fabricate homogeneous porous TiO₂ shells for multi-functional core–shell nanostructures. By simply controlling the kinetics of the hydrolysis and condensation of tetrabutyl titanate (TBOT) in ethanol–ammonia mixtures, the core–shell heterostructure with homogeneous porous TiO₂ shells were fabricated with variable diameter, geometry, and composition as seeds (e.g., α-Fe₂O₃ ellipsoids, Fe₃O₄ spheres, SiO₂ spheres, graphene oxide sheets, and carbon spheres). This approach exhibits many advantages, such as facile, reproducible and the thickness of TiO₂ shells can be tailored from 0 to 25, 45, and 70 nm.⁸⁸ Yuan and co-workers have prepared Fe₃O₄@TiO₂ nanoparticles (the size is 6.7 ± 2.9 nm) by a modified sol–gel method. The TiO₂ shell was formed by gradually adding TiCl₄ to the iron oxide nanoparticle gel. The as-prepared composite nanoparticles are used for targeted drug delivery.⁸⁹

In addition, the semiconductor nanoparticles can also be seeds for the synthesis of iron oxide–semiconductor composite nanomaterials, and this strategy is often employed to fabricate the iron oxide–semiconductor with a Janus structure (Fig. 6b). For instance, Buonsanti and co-workers developed a colloidal seeded-growth strategy to synthesize all-oxide semiconductor/magnetic hybrid nanocrystals in various topological arrangements, in which the dimensions of the constituent material domains were controlled independently over a wide range. The Fe_xO_y/TiO₂ composite nanorods were synthesized by using the brookite TiO₂ nanorods as seeds and Fe(CO)₅ as the iron precursor via a high-temperature thermal decomposition method. The preliminary magnetic and photocatalytic investigations had highlighted that the creation of bonding heterojunctions leads to significantly modified or even unexpected physical–chemical behaviour, relative to that offered by brookite TiO₂ and Fe_xO_y alone.^90,91 Zeng and co-workers first synthesized TiO₂ nanoparticles with a diameter of about 5–10 nm with ferric acetylacetonate as an iron source, the multifunctional Fe₃O₄/TiO₂ nanocomposites with a Janus structure were prepared by the solvent–thermal method.⁷³ Liu and Gao first prepared the sheet-like TiO₂ seeds by hydrothermal treatment of TiO₂ nanoparticles. Then the α-Fe₂O₃/TiO₂ composite nanosheets were fabricated by hydrothermal treatment of ferric nitrate and hydroxylamine. Results showed that the photocatalytic activities of α-Fe₂O₃ make the MB degradation efficient under visible light irradiation.⁹² Wang and co-workers synthesized one-dimensional (1D) heterostructures of uniform CdS nanowires separately decorated with hematite (α-Fe₂O₃) nanoparticles or magnetite (Fe₃O₄) microspheres via a two-step solvothermal deposition method. Each CdS nanowire had a uniform diameter of 40–50 nm and a length ranging to several tens of micrometers. Quasicubic α-Fe₂O₃ nanoparticles with edge lengths up to 30 nm, and Fe₃O₄ microspheres with diameters of about 200 nm produced 1D dimer-type CdS/α-Fe₂O₃ semiconductor heterostructures or CdS/Fe₃O₄ semiconductor magnetic functionally assembled heterostructures. In comparison with the bare CdS nanowires and commercial anatase TiO₂, enhanced photocatalytic activity was observed in CdS/α-Fe₂O₃ heterostructures.⁹³

3.2 Step-by-step deposition strategy

Step-by-step deposition strategy is mainly used to prepare iron oxide–semiconductor composite multi-shell structures.^94–96 In fact, the need for a better control over surface properties or to protect the iron oxide itself, an interlayer was introduced in the magnetic iron oxide–semiconductor system to form a multi-shell structure, as shown in Fig. 7.^97–99 The most commonly used interlayer materials are the SiO₂ and carbon, respectively. Additionally, the interlayer can be removed by chemical corrosion or calcination.⁷⁰

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

Silica coating can enhance dispersion in solution because the silica layer can screen the magnetic dipolar attraction between magnetic iron oxide nanoparticles, and hence increase the stability of iron oxide nanoparticles and protect them in acidic environments. Silica has become the most used interlayer material.^100,101 For example, Cheng and co-workers have synthesized Fe₃O₄@SiO₂@CeO₂ microspheres with a magnetic core and mesoporous shell by a step-by-step deposition strategy. Such multifunctional materials were utilized to capture phosphopeptides and catalyze the dephosphorylation simultaneously, thereby labeling the phosphopeptides for rapid identification.¹⁰² Sarkar and co-workers synthesized Fe₃O₄ nanoparticles with a diameter of 20–40 nm by co-precipitation method, and then a SiO₂ interlayer was deposited on the surface of Fe₃O₄ nanoparticles by classical Stöber method. The Fe₃O₄/SiO₂/ZrO₂ composite nanoparticles were finally fabricated by reducing the ZrOCl₂ precursor. The thickness of ZrO₂ was about 8–10 nm, and the BET surface area of the composite nanoparticles was up to 107 m²g⁻¹ due to the mesoporous ZrO₂ shell.¹⁰³ More recently, Chi and co-workers have prepared Fe₃O₄@SiO₂@TiO₂/Ag nanocomposites by a step-by-step deposition strategy, the as-prepared microspheres show a number of important features as a recyclable photocatalyst: a high field-responsive magnetic Fe₃O₄ core for efficient magnetic separation, a SiO₂ interlayer for protecting the Fe₃O₄ core from chemical- and photocorrosion, and a TiO₂ nano-shell with well dispersed Ag nanoparticles for enhanced photocatalytic activity.¹⁰⁴

Like the SiO₂ interlayer, hydrophilic carbon coating on an iron oxide nanoparticle core also endows better dispersibility and stability. More importantly, carbon coated iron oxide nanoparticles have recently triggered enormous research activity due to the good chemical and thermal stability. The intrinsic high electrical conductivity of the carbon interlayer helps to transfer electrons.^105–107 For instance, Qi and co-workers first deposited a carbon interlayer on the surface of Fe₃O₄ seeds by the hydrothermal reaction of glucose, and then deposited SnO₂ on the surface of Fe₃O₄/C, they successfully obtained Fe₃O₄/C/SnO₂ composite nanoparticles.¹⁰⁸ Shi and co-workers have prepared a core/multi-shell-structured Fe₃O₄/C/TiO₂ magnetic photocatalyst by the vapor phase hydrolysis process, and the photocatalytic abilities for degradation of methylene blue are studied. Compared with commercial anatase TiO₂, Fe₃O₄/C/TiO₂ with low TiO₂ content (37%) exhibited a relatively higher photocatalytic activity. The C interlayer prevented the photocorrosion of Fe₃O₄ effectively, and the composite nanoparticles present a good magnetic recycling property due to magnetic core materials.¹⁰⁹ Liu and co-workers have fabricated one-dimensional Fe₃O₄/C/CdS coaxial nanochains by a magnetic field-induced assembly and microwave-assisted deposition method. First, one-dimensional pearl chain-like Fe₃O₄/C core–shell nanocables were successfully assembled via the hydrothermal reaction of nanoscale Fe₃O₄ spheres with glucose in water in the presence of an external magnetic field. The carbonaceous layer was about 10 nm in thickness, and it acted as the stabilizer for the Fe₃O₄ nanochains. Afterwards, CdS nanoparticles were deposited on Fe₃O₄/C nanochains by a rapid microwave-irradiation route to generate Fe₃O₄/C/CdS coaxial nanochains. The subsequent photocatalytic test for organic pollutants demonstrated that these magnetic composites possess enhanced photocatalytic activity as MRPs under visible light irradiation. The decolorization fraction using a sample of microwave irradiation was up to 94.7% in 20 min, and the photocatalytic performance was still stable after 12 cycles of degradation of RhB, the results revealed that this MRP possessed excellent stability.¹⁰⁷

3.3 Other strategies

Except the above two conventional strategies, some new synthesis or preparation methods have also been used to fabricate iron oxide–semiconductor composite nanomaterials, such as ion implantation method, spray pyrolysis, microwave, and sonochemical method.^110–112

As shown in Fig. 8, we first dropped the hematite seeds onto the surface of a clean slide and implanted Ti ions. and Magnetic monodispersed TiO₂ grains filled into spindle-like hematite bi-component nanoparticles were successfully synthesized.¹¹³ The different implanted energy and magnetic properties of the bi-component α-Fe₂O₃/TiO₂ nanoparticles were investigated. The results illustrate that the α-Fe₂O₃/TiO₂ composite nanoparticles could be obtained by Ti ion implantation with different energies, and the saturation magnetization (M_S) of the samples after ion implantation were significantly enhanced.¹¹⁴ Li and co-workers prepared a novel core–shell α-Fe₂O₃/SnO₂ heterostructure by one-step flame-assisted spray pyrolysis of an iron and tin precursor. The effect of the SnO₂ component was investigated for the evolution of the phase composition and morphology in detail. It was found that the doping of SnO₂ in Fe₂O₃ could effectively promote the phase transition from γ-Fe₂O₃ to α-Fe₂O₃ during flame synthesis. The unique morphology composed of tin doped α-Fe₂O₃ core and SnO₂ as a shell was attributed to the solubility, segregation and second-phase surface nucleation of SnO₂ in Fe₂O₃.¹¹⁵

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

4 Progress on magnetic iron oxide–semiconductor composite photocatalysts

4.1 Magnetic iron oxide–metal oxide–semiconductor composite photocatalysts

4.1.1 Magnetic iron oxide/TiO₂ photocatalysts. TiO₂, the most thoroughly investigated semiconductor in the literature, seems to be the most promising photocatalytic material for the destruction of organic pollutants. This semiconductor provides the best compromise between catalytic performance and stability in aqueous media. Therefore, the magnetic iron oxide/TiO₂ composite photocatalyst have become the research focus in recent years. Using the magnetic properties of iron oxide itself for obtaining the magnetic recoverable photocatalyst has become an important issue in the magnetic iron oxide/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-chemical approach. The microspheres possess both ferromagnetic and photocatalytic properties. The TiO₂ nanoparticles on the surfaces of the microspheres degraded organic dyes under the illumination of UV light. Furthermore, the microspheres were easily separated from the solution after the photocatalytic process due to the ferromagnetic Fe₃O₄ core. The photocatalysts were recycled for further use and the degradation rate of methyl orange still reached 91% after 6 cycles of reuse.¹²⁰ As shown in Fig. 9, Chalasani and Vasudevan have demonstrated water-dispersible photocatalytic Fe₃O₄@TiO₂ core–shell magnetic nanoparticles by anchoring β-cyclodextrin (CMCD) cavities to the TiO₂ shell, and photocatalytically destroyed endocrine-disrupting chemicals, bisphenol A (BPA) and dibutyl phthalate, present in water. The particles, which were typically 12 nm in diameter, were magnetic and removed from the dispersion by magnetic separation and then reused. The concentration of BPA solution was determined by liquid chromatography, and then irradiated under UV light for 60 min. After photodegradation of BPA, the CMCD-Fe₃O₄@TiO₂ nanoparticles that were separated from the mixtures by a magnet, and can be reused for the photodegradation of newly prepared BPA solutions. The recycle photocatalytic performance of CMCD-Fe₃O₄@TiO₂ for the photodegradation of BPA was excellent and stable, retaining 90% efficiency after 10 cycles.¹²¹ For obtaining the magnetically recovered photocatalysts, Fe₃O₄ and γ-Fe₂O₃ were often employed due to their higher saturation magnetization and good magnetic separation ability.

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

On the other hand, α-Fe₂O₃ has often been introduced into the magnetic iron oxide/TiO₂ composite photocatalyst in order to use its narrow band gap properties, and to obtain magnetic iron oxide/TiO₂ composite heterostructures.^92,122–124 For example, Peng and co-workers have synthesized Fe₂O₃/TiO₂ heterostructural photocatalysts by impregnation of Fe³⁺ on the surface of TiO₂ and annealing at 300 °C, the composites possess different mass ratios of Fe₂O₃vs. TiO₂. The photocatalytic activities of Fe₂O₃/TiO₂ heterocomposites, pure Fe₂O₃ and TiO₂ were studied by the photocatalytic degrading of Orange II dye in aqueous solution under visible light (λ > 420 nm) irradiation. The Fe₂O₃/TiO₂ heterogeneous photocatalysts exhibited an enhanced photocatalytic ability for Orange II, higher than either pure Fe₂O₃ or TiO₂. The best photocatalytic performance for Orange II could be obtained when the mass ratio in Fe₂O₃/TiO₂ is 7 : 3. The results illustrate that the generation of heterojunctions between Fe₂O₃ and TiO₂ is key for improving movement and restraining the recombination of photoinduced charge carriers, and finally improving the 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–gel process. Mesoporous Fe₂O₃/TiO₂ composites exhibited excellent photocatalytic degradation ability for 4-chlorophenol in aqueous solution under sunlight irradiation. The author claimed 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 of Fe₂O₃/TiO₂ cause an oxidation reaction and decompose the 4-chlorophenol to CO₂ and H₂O. Meanwhile the transferred electrons 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 importance due to its unique properties, including high stability and lithium storage capacity, and it is currently used to prepare photocatalysts. The objectives of combining magnetic iron oxide and SnO₂ are the same as the iron oxide/TiO₂ composite photocatalyst system.

On the one hand is the fabrication of iron oxide/SnO₂ heterostructures, for instance, Niu and co-workers have prepared branched SnO₂/α-Fe₂O₃ semiconductor nanoheterostructures (SNHs) of high purity by a low-cost and environmentally friendly hydrothermal strategy, through crystallographic-oriented epitaxial growth of SnO₂ nanorods on α-Fe₂O₃ nanospindles and nanocubes, respectively (Fig. 10). SnO₂/α-Fe₂O₃ SNHs exhibited excellent visible light or UV photocatalytic ability, remarkably superior to their α-Fe₂O₃ precursors, mainly owing to the effective electron–hole separation at the SnO₂/α-Fe₂O₃ interface.¹²⁷ Recently, Zhang and co-workers have also synthesized three-dimensional SnO₂/α-Fe₂O₃ semiconductor hierarchical nanoheterostructures via crystallographic-oriented epitaxial growth of SnO₂ onto the surface of flower-like three-dimensional iron oxide hierarchical nanostructures. For this photocatalyst, visible-light-active flower-like Fe₂O₃ hierarchical nanostructures were employed as a medium to harvest the visible light and generate photoinduced charge carriers, and SnO₂ layer was employed as a charge collector to transport the photoinduced charge carriers. The SnO₂/α-Fe₂O₃ semiconductor hierarchical heterostructures present admirable visible-light photodegradation ability for methylene blue, which could be assigned to the wide visible-light absorption range, high surface area, and efficient charge carrier separation 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. The photocatalytic activities of the as-synthesized α-Fe₂O₃@SnO₂ core–shell shuttle-like composites were studied by the photodegradation of RhB dye under UV light irradiation (λ = 365 nm), and the absorption peak of RhB diminished gradually as the exposed time extended and completely disappeared after 70 min. Compared with uncoated α-Fe₂O₃ shuttle-like nanorods, SnO₂ nanoparticles, and the mixture of α-Fe₂O₃ nanorods and SnO₂ particles, as-synthesized core–shell shuttle-like composites exhibited enhanced photodegradation abilities, suggesting that the synergistic effect of α-Fe₂O₃ and SnO₂ was beneficial to improve the photocatalytic activity.¹²⁹

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

On another hand, fabrication of the magnetically recoverable iron oxide/SnO₂ composite photocatalyst system is also attractive for various reasons. As shown in Fig. 11, we have successfully synthesized the spindle-like and spherical iron oxide/SnO₂ composite nanoparticles via a seed-mediated growth strategy recently, and the as-prepared iron oxides/SnO₂ core–shell heterostructures displayed enhanced visible light and UV photodegradation activity for RhB, which is significant higher than the uncoated a-Fe₂O₃ seeds and commercially available SnO₂ products. Significantly, the composite nanoparticles can be magnetically separated from the dispersion after photocatalytic degradation.^130,131 Zhang and co-workers have fabricated superparamagnetic iron oxide (SPIO)@SnO₂ yolk–shell heterostructures (YSHs) by a facile template approach, the as-obtained SnO₂ shell is mesoporous, the thickness of the shell layer and void spaces are both tailorable. Under UV light irradiation for 1 h, the photodegradation activity of the as-obtained SPIO@SnO₂ YSHs and commercial P25 TiO₂ for RhB were about 75% and 97%, respectively. Because SPIO cores are not photocatalytically active, the mass of the SnO₂ component in 25 mg of SPIO@SnO₂ YSHs would be less than the photocatalyst component in the same mass of P25 TiO₂.¹³²

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

4.1.3 Magnetic iron oxide/ZnO photocatalysts. Indeed, the photocatalytic performance of pure ZnO nanomaterials is not weaker than TiO₂. However, it is unstable under illuminated aqueous solutions with Zn(OH)₂ being formed on the particle surface and resulting in catalyst deactivation. Owing to the photocatalytic performance of ZnO, the ZnO based composite nanomaterials are still used in the photocatalytic field and have attracted more attention in recent years.^133–135 For instance, Liu and co-workers have synthesized the magnetic nest-like γ-Fe₂O₃/ZnO double-shelled hollow nanostructures via a step-by-step process. These interesting nest-like heterostructures consist of nanoscale ZnO flakes grown on the surface of spherical γ-Fe₂O₃ particles. Significantly, these magnetic hollow heterostructures exhibited enhanced photodegradation ability for different organic dyes, including MB (almost 95.2% of MB could be degraded within 50 min), RhB (almost 91.1% of RhB could be degraded within 50 min), and MO (almost 82.5% of MO could be degraded within 80 min), and their photocatalytic abilities were higher than commercial ZnO nanoparticles. The improved photodegradation ability of γ-Fe₂O₃/ZnO might be attributed to the large surface area from the nest-like hollow structure. The photodegradation performance of the as-prepared γ-Fe₂O₃/ZnO heterostructures was still stable after 6 cycles without significant deterioration, suggesting that these γ-Fe₂O₃/ZnO heterostructures were highly stable and could be reused many times.¹³⁶ As shown in Fig. 12, we have successfully fabricated mesoporous spindle-like α-Fe₂O₃/ZnO core–shell heterostructures by a surfactant-free, low-cost, and environmentally friendly seed-mediated strategy with the help of post-annealing treatments. The thickness of the ZnO layer was tailored by adjusting the concentration of zinc precursor. Considering that both α-Fe₂O₃ and ZnO are good photocatalytic materials, the photocatalytic activity of the core–shell heterostructures for organic dye RhB had been studied. It is noteworthy that the as-prepared α-Fe₂O₃/ZnO core–shell heterostructures displayed enhanced photodegradation performance, clearly higher than the uncoated α-Fe₂O₃ seeds and commercial P25 TiO₂, which mainly attributed to the synergistic effect between the narrow and wide bandgap semiconductors and effective photogenerated charge carrie separation at the interface of α-Fe₂O₃/ZnO.¹³⁷

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

4.1.4 Others. Apart from TiO₂, SnO₂ and ZnO, WO₃, ZrO₂, Cu₂O and Bi₂O₃ have also been investigated as potential photocatalysts however, they are generally less photocatalytically active than TiO₂, and their semiconductor oxides have been used to functionalize magnetic iron oxide nanoparticles.^138,139 As shown in Fig. 13, Xi and co-workers have synthesized a magnetically recyclable Fe₃O₄/WO₃ core–shell visible-light photocatalyst by a facile solvothermal epitaxial growth combined with a mild oxidation route. Photoelectrochemical investigations verified that the core–shell structured Fe₃O₄/WO₃ had more effective photoconversion capability than pure WO₃ or Fe₃O₄. At the same time, the visible-light photocatalytic ability of the Fe₃O₄/WO₃ photocatalyst exhibited significant enhancement in photodegradation of RhB. Furthermore, the Fe₃O₄/WO₃ core–shell photocatalyst was effectively recycled at least three times without an apparent decrease in its photocatalytic activity, which demonstrates its high stability.¹⁴⁰ Li and co-workers have successfully synthesized magnetic Fe₃O₄@C@Cu₂O composites with a bean-like core–shell nanostructure by step-by-step self-assembly. The carbonaceous layer with unavoidable hydrophilic groups inherited from the starting materials acted as both linker and stabilizer between Fe₃O₄ and Cu₂O. The Fe₃O₄@C@Cu₂O composites exhibited ferromagnetic behaviour, and good dispersibility in aqueous solution. Importantly, the bean-like core–shell composites showed universal and powerful visible-light-photocatalytic activity for the degradation of RhB, methyl orange (MO), and alizarin red (AR) relative in comparison with commercial Cu₂O and Degussa P-25 powders.¹⁴¹ Wang and co-workers have synthesized three-dimensional flower-like hierarchical Fe₃O₄@Bi₂O₃ core–shell architecture by a facile solvothermal approach. The diameter of the as-obtained flower-like hierarchical microsphere was ca. 420 nm and the shell was composed of several nanosheets with a thickness of 4–10 nm and a width of 100–140 nm. The saturation magnetization of the superparamagnetic composite heterostructures was ca. 41 emu g⁻¹ at room temperature. Additionally, the Fe₃O₄@Bi₂O₃ composite heterostructures exhibited much higher (7–10 times) photocatalytic ability than commercial Bi₂O₃ particles under visible-light irradiation. The photocatalytic activity of Fe₃O₄@Bi₂O₃ composite heterostructures did not display a clear loss in photocatalytic degradation of RhB after 6 recycles.¹⁴²

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

Recently, new semiconductor oxides have been used in photocatalytic applications, such as V₂O₅,¹⁴³ Nb₂O₅,^144,145 Ta₂O₅,^146,147 CeO₂,^148,149 Ga₂O₃,¹⁵⁰etc.¹⁵¹ However, reports on the synthesis of magnetic iron oxide–semiconductor oxides are very scarce so far, and it is worth studying various ways to introduce semiconductor oxides into iron oxide–semiconductor systems and to improve their photocatalytic ability.

4.2 Magnetic iron oxide–metal chalcogenides semiconductor composite photocatalysts

Currently, the proportion of using semiconductor oxides in photocatalysts is large, however, the semiconductor sulfides such as ZnS, CdS, Bi₂S₃, SnS₂, ZnSe, etc. have also been attractive due to the importance of their special quantum confinement effect.^93,152–156 As a competitive alternative, the application of magnetic iron oxide–semiconductor sulfides is a relatively new technology. The optical properties and photocatalytic performance of semiconductor sulfides could be different from their oxide counterparts.

For example, Liu and co-workers have reported the preparation of Fe₃O₄/CdS nanocomposites via a sonochemical route in an aqueous solution. These Fe₃O₄/CdS nanocomposites displayed fluorescence and exhibited excellent magnetic properties at room temperature. Photocatalytic activity studies confirmed that the as-prepared nanocomposites had high photocatalytic activity towards the photodegradation of methyl orange in aqueous solution. Furthermore, the photodecomposition rate decreased slightly after 12 cycles of photocatalysis (89% of MO is decomposed in the last cycle).¹⁵⁷ Their subsequent studies revealed that the Fe₃O₄/ZnS had high photocatalytic activity towards the photodegradation of eosin Y in aqueous solution. The catalytic efficiency only decreased by 5% after 15 cycles.¹¹² As shown in Fig. 14, Shi and co-workers have synthesized α-Fe₂O₃/CdS corn-like nanocomposites via CdS nanoparticles by a simple one-step wet-chemical route, in which preformed single-crystaline α-Fe₂O₃ nanorods were used as substances for growing CdS nanoparticles. The corn-like nanocomposites exhibited superior photocatalytic performance under visible light irradiation (86.7% of MB was degraded in 120 min) over pure α-Fe₂O₃ nanorods and CdS nanoparticles. The enhanced performance is attributed to the larger surface area of the corn-like structure, the crystalline nature of the materials and the synergy in light absorption and charge separation between α-Fe₂O₃ and CdS.¹⁵⁸ Luo and co-workers have developed a facile and rapid synthesis of urchin-shaped Fe₃O₄@Bi₂S₃ core–shell hierarchical structure through a sonochemical method. The as-prepared Fe₃O₄@Bi₂S₃ hierarchical core–shell structures showed excellent photocatalytic efficiency for the degradation of RhB and retained the photocatalytic activity after being recycled for five times with the help of an external magnetic field.¹⁵⁹

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

Additionally, some ternary semiconductor sulfides have been used in the application of photocatalysts, such as Zn_xCd_1−xS, ZnIn₂S₄, CuInS₂, etc.^160–165 However, there is no literature report on iron oxide–ternary semiconductor sulfide composite photocatalytic systems. More importantly, the toxicity of the semiconductor sulfides should also be considered in practical application.

4.3 Multiple semiconductor shell photocatalysts

The synergetic effects of multiple semiconductor photocatalysts have been extensively observed in photocatalytic degradations. In core–shell–shell structures, the first semiconductor shell layer can offer special active-sites for the adsorption/reaction of reactant/reaction intermediates. The second semiconductor shell layer could also influence the overall band configuration via altering the bandgap absorption and to separate the photoinduced charge carriers.^166–168 Moreover, semiconductors with a narrow band gap can expand the spectral response range. As shown in Fig. 15, by introducing semiconductor heterojunction on the surface of magnetic iron oxide nanomaterials, magnetically recoverable photocatalysts are obtained.^14,169–171

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

For example, Chen and co-workers have synthesized three types of ellipsoidal complex hollow structures with the shells assembled from anatase TiO₂ nanosheets with exposed (001) facets by utilizing silica-coated hematite (α-Fe₂O₃) nanospindles as the starting templates. As shown in Fig. 16, the α-Fe₂O₃/SiO₂/SnO₂/TiO₂ composite can be prepared by hydrothermal deposition of a SnO₂ layer on the surface of SiO₂. The Fe₃O₄@SnO₂@TiO₂ nanorattles manifested a much higher degradation efficiency compared to Degussa P25 nanoparticles, as well as their analogous Fe₃O₄@TiO₂ core–shell nanomaterial without the exposed (001) high-energy facets.¹⁷² Dong and co-workers have prepared the CdS modified TiO₂/Fe₃O₄ photocatalysts by sol–gel and immersion methods. The CdS-TiO₂/Fe₃O₄ composites exhibited higher photocatalytic activity than pure TiO₂ and TiO₂/Fe₃O₄ for the degradation of Reactive Brilliant Red X-3B dye (X-3B) under simulated sunlight. In addition, a gradual loss of photocatalytic activity was observed 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 shells can effectively improve the photocatalytic abilities.

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

5 Summary and perspectives

Several fundamental issues must be addressed before photocatalysts are economically viable for large scale industrial applications. Apart from offering easy separation of the photocatalysts from the reaction system, the magnetic iron oxide–semiconductor photocatalytic system, which interfaces chemistry with materials science, possesses a unique position in the advancement of heterogeneous photocatalysis. Table 1 depicts the representative magnetic iron oxide–semiconductor photocatalysts and their photocatalytic performances. Though a lot of effort has been made in design and fabricating magnetic iron oxide–semiconductor composite photocatalytic system, it is still a field of research in modern photocatalysis and following issues are still need to be addressed.

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

Structure	Materials	Pollutants	Light resource	Rate constant k (10^−2 min^−1)	Stability performance	Ref.
a Photocatalytic degradation of BPA is complete within 60 min. b Photocatalytic degradation of MB is complete within 120 min. c Photocatalytic degradation of Eosin Y is complete within 37 min. d Photocatalytic degradation of MB is 70% within 120 min.
Binary structure	Fe3O4@TiO2	Bisphenol A	UV light	—^a	90% after 10 cycles	121
	α-Fe2O3@ TiO2	RhB	Visible light	0.81	—	197
	γ-Fe2O3@SnO2	RhB	UV light	0.68	—	131
	α-Fe2O3@ZnO	RhB	UV + visible light	2.4	—	137
	Fe3O4@WO3	MB	Visible light	—^b	No obvious decrease after 3 cycles	140
	Fe3O4/ZnS	Eosin Y	UV light	—^c	95% after 15 cycles	112
	α-Fe2O3/CdS	MB	Visible light	1.68	—	158
Ternary structure	Fe3O4@SiO2@TiO2	Methyl orange	UV light	—	91% after 6 cycles	120
	Graphene/TiO2/Fe3O4	RhB	UV light	16	No obvious decrease after 5 cycles	177
	α-Fe2O3@SnO2@Cu2O	RhB	UV + visible light	3.29	85% after 8 cycles	198
Multiple 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	0.29^d	—	172

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(1) In order to improve the photocatalytic activities of photocatalysts, extension of the excitation wavelength, reduced charge carrier recombination, and the promotion of active sites around the surface should be considered. Therefore, if noble metal nanomaterials are introduced in the magnetic iron oxide–semiconductor system, the photocatalytic efficiency could be enhanced. The photogenerated charge carriers in the noble metal can be separated by the metal/semiconductor heterojunction. Additionally, the separated electron and hole can take part in the chemical reactions on the surface of metal and semiconductor, respectively. The absorbed photons can excite the valence electrons of noble metals due to the surface plasmon resonance (SPR) effect. The energy of photoinduced electrons is higher than the Schottky barrier resulted in crossing the interface and transferring to the VB of the semiconductor. Numerous literature reports are dedicated to the metal–semiconductor composite photocatalytic system. However, reports on magnetic iron oxide/noble metal/semiconductors ternary photocatalysts are scarce and need to be strengthened.¹⁷⁴ Owing to the SPR effect, solution processed metal nanoparticles coated onto the surface of iron oxide or semiconductors is an effective method to enhance the absorption of visible light. However, the metal nanoparticles can also act as recombination centres resulting in inferior photocatalytic performance due to the incorporation of chemically synthesized metal nanoparticles in the iron oxide–semiconductor composite system. Many factors can cause undesirable exciton quenching and decrease the plasmonic effect. Indeed, coating of metal nanoparticles with insulating materials can prevent such recombination centres.¹⁷⁵ More recently, we have reported a novel iron oxide/noble metal/semiconductor ternary multilayer hybrid structure that was prepared by template synthesis and subsequent layer-by-layer deposition method. Three different morphologies of α-Fe₂O₃/Ag/SiO₂/SnO₂ heterosturctures were obtained, the thickness of the insulating SiO₂ interlayer was tailored to control the coupling of noble metal silver with tin oxide. The as-obtained α-Fe₂O₃/Ag/SiO₂/SnO₂ nanocomposites exhibited enhanced catalytic abilities under UV or visible light irradiation, higher than the commercially available pure SnO₂, naked α-Fe₂O₃ seeds and α-Fe₂O₃/SnO₂ binary nanocomposites. Moreover, α-Fe₂O₃/Ag/SiO₂/SnO₂ exhibited significant stability and recyclability because of its photodegradation rate maintains at 96% after 8 cycles.¹⁷⁶

(2) The fusion of catalysis with nanotechnology continues to generate better materials and improve their functions. Graphene and its use in photodegradation is one of the latest examples. Its interesting electrical and mechanical properties, and high surface area make graphene a novel substrate for forming hybrid structures with a variety of nanomaterials. The use of graphene to enhance the efficiency of photocatalysts has attracted much attention. Utilization of single-layer graphene sheets can not only provide a high quality two-dimensional photocatalyst support, but also a two-dimensional circuit board, with an attractive potential to harness their perfect electrical and redox properties. There are few literature reports on composites of graphene with magnetic iron oxide–semiconductors photocatalytic system.¹⁷⁷

(3) At present, a lot of fundamental and applied research of photocatalysis are focused on the synthesis and modification of new photocatalysts, nevertheless, with those endeavours, the effect of photocatalyst microstructure on their photocatalytic performance still cannot be understood, understanding the relationship between these two parts is a prerequisite for the broad application of composite nanomaterials in photocatalysis. However, the understanding of interface effects, the coupling mechanism, photocatalyst life, deactivation and the regeneration mechanism are still relatively weak.¹⁷⁸ As a heterogeneous catalytic reaction system, semiconductor photocatalytic materials would be deactivated in practical application, such as the photocatalytic efficiency of P25 TiO₂ becomes very low after 3 cycles under sunlight. Therefore, deactivation and the regeneration mechanism of semiconductor photocatalysts should be reinforced.

As a key issue for practical applications, the facile method to increase the photocorrosion suppression ability, the life and stability of magnetic iron oxide–semiconductor composite photocatalytic system must be further developed and improved. To date, the underlying photocorrosion mechanism for the iron oxide–semiconductor composite photocatalyst is not clear, and systematic studies are necessary. Many methods have been used to reduce the photocorrosion of pure semiconductors, such as graphene composites,¹⁷⁹ graphene oxide,¹⁸⁰ quantum dot,¹⁸¹etc., and these materials can be introduced to the magnetic iron oxide–semiconductor composite photocatalytic system. Moreover, recycling and regeneration is also an effective method to extend the life of a deactivated photocatalyst. However, reports on the above mentioned issues are scarce so far.

(4) Although a great effort has been made in the past years to unravel the mechanisms of bi/ternary and multiple composite photocatalysts, it is still a challenge for various researchers. To develop an efficient heterostructural photocatalysis system for large-scale industrialization, understanding of the kinetics and mechanisms of these charge transfer processes is very important.¹⁸² More efforts on photo-inducing charge carrier generation, trapping, recombination, and transporting are needed to further to strengthen and improve it. Apart from the traditional characterization techniques, more and more photoelectrochemical methods and techniques have been used to study the kinetics and mechanisms of heterostructural photocatalysis systems. For instance, photoelectron spectroscopy (PES) is used to measure band bending in semiconductors, femtosecond transient reflecting grating (TRG) method is used to detect the photogenerated ultrafast relaxation dynamic at solid/liquid interfaces, O² photostimulated desorption (PSD) and electronstimulated desorption (ESD) are used to study the surface photoreactions induced by the photo-excited electrons and holes in the semiconductor, etc.^183–186 At present, charge transfer kinetics on a short duration is well studied, while the charge transfer on a more extended timescale is still unclear. Therefore, unravelling the mechanisms that play an important and key role in magnetic iron oxide–semiconductor composite photocatalytic system is necessary. On this regard, there are several mechanisms that are still not fully understood and many works need to be carried out.

(5) In fact, the photodegradation of pollutants is mainly used in a suspension of semiconductor nanomaterials in this field. However, from a practical point of view, there are many limitations of using a photocatalyst suspension, such as requirement of large photo-reactors, hard to filtrate the nanoscale photocatalyst, etc. As the catalytic mechanism of these synthesized photocatalysts is very complicated, the pure semiconductors and α-Fe₂O₃/semiconductor composite photocatalytic systems are hardly recycled by external magnetic fields due to their weak magnetic response. As shown in Fig. 17, these photocatalysts can be printed on rigid or flexible substrates as photocatalytic arrays or patterns, including screen printing,^187–189 offset printing,¹⁹⁰ inkjet printing,^191–193 gravure printing,¹⁹⁴etc.^195,196 These printed patterns with semiconductor photocatalysts can also be recycled. Therefore, combining and developing more practical methods to use these composite photocatalytic systems should be further reinforced.

Fig. 17 Depiction of various semiconductor solution deposition methods by printing.¹⁹⁶

Acknowledgements

The authors thank the Hong Kong Scholars Program, NSFC (51471121, 51201115, 51171132), Young Chenguang Project of Wuhan City (2013070104010011), China Postdoctoral Science Foundation (2014M550406), Hubei Provincial Natural Science Foundation (2014CFB261) and the Fundamental Research Funds for the Central Universities.

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