A Review on Iron Oxide‐Based Nanoarchitectures for Biomedical

DOI: 10.1002/smtd.201800512

Article type: Review

A Review on Iron Oxide-Based Nanoarchitectures for Biomedical, Energy Storage, and Environmental Applications

Shunsuke Tanaka, Yusuf Valentino Kaneti*, Ni Luh Wulan Septiani, Shi Xue Dou, Yoshio Bando, and Md. Shahriar A. Hossain, Jeonghun Kim, and Yusuke Yamauchi*

S. Tanaka, Dr. Y. V. Kaneti, Prof. Y. Bando International Research Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan E-mail: [email protected]

N. L. W. Septiani Department of Engineering Physics and Research Center for Nanoscience and Nanotechnology, Bandung Institute of Technology, 10 Ganesha Street, Bandung 40132, Indonesia

S. Tanaka, Prof. S. X. Dou, Prof. Y. Bando Australian Institute of Innovative Materials (AIIM), University of Wollongong, North Wollongong, New South Wales 2500, Australia

Dr. M. S. A. Hossain School of Chemical Engineering and School of Mechanical & Mining Engineering, Faculty of Engineering, Architecture and Information Technology (EAIT), The University of Queensland, Brisbane, QLD 4072, Australia

Dr. J. Kim, Prof. Y. Yamauchi

This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/smtd.201800512.

School of Chemical Engineering and School of Mechanical & Mining Engineering, Faculty of Engineering, Architecture and Information Technology (EAIT), The University of Queensland, Brisbane, QLD 4072, Australia E-mail: [email protected]

Prof. Y. Yamauchi Key Laboratory of Sensor Analysis of Tumor Marker (Ministry of Education), Shandong Key Laboratory of Biochemical Analysis, and Key Laboratory of Analytical Chemistry for Life Science in Universities of Shandong, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology (QUST), Qingdao 266042, China

Keywords: iron oxide nanoparticles, energy storage, biomedical applications, catalysis, surface modification

Abstract Iron oxide nanoarchitectures with distinct morphologies from one-dimensional (1D) to three- dimensional (3D) have been developed using various wet chemical methods. They have been employed for a wide range of applications, including energy storage, biomedical, and environmental applications. The functional properties of iron oxide nanoarchitectures depend on the size, shape, composition, magnetic properties and surface modification. To overcome the limitations of pure iron oxide nanostructures, hybridization with various inorganic materials (e.g., silica, metals, metal oxides) and carbon-based materials have been proposed. Herein, the recent advances on the preparation of various iron oxide nanoarchitectures are reviewed along with their functional applications in energy storage, biomedical, and environmental fields. Finally, the effects of various parameters on the functional performance of iron oxide nanostructures for these applications are summarized and the trends and future outlook on the development of iron oxide nanoarchitectures for these applications are also given.

1. Introduction Iron is known as the fourth most abundant element on earth. The oxide form of iron is known to exist in 16 different phases. Among them, hematite (α-Fe2O3), maghemite (γ-Fe2O3), and magnetite (Fe3O4)) are by far the most widely studied. Hematite is an n-type semiconductor with an indirect bandgap of 2.1-2.3 eV. Hematite is the most stable form of iron oxide and it can be used as a precursor to obtain Fe3O4 or γ-Fe2O3. Further, hematite is often employed in catalysis and gas sensors owing to their wide abundance, low toxicity, high sensitivity/activity, and high corrosion resistance. In the hematite crystal structure, Fe3+ ions occupy two-thirds of the octahedral sites which are confined by the almost ideal hexagonal [1] close-packed oxygen lattice (Figure 1a). In comparison, Fe3O4 possesses a cubic inverse spinel structure which consists of a cubic close packed array of oxygen ions, in which all of the Fe2+ ions occupy half of the octahedral sites and the Fe3+ are split evenly across the rest of the octahedral sites and the tetrahedral sites (Figure 1b). Fe3O4 has the lowest resistivity [2] among iron oxides as a result of its very narrow bandgap of 0.1 eV. γ-Fe2O3 is the second most stable phase of iron oxide with a narrow bandgap of 2.0 eV. Similar to Fe3O4, γ-Fe2O3 also has a cubic spinel crystal structure, however it contains only Fe3+ ions. In the crystal structure of γ-Fe2O3, the oxygen anions give rise to a cubic close-packed array in which the Fe3+ ions are distributed over tetrahedral sites and octahedral sites (Figure 1c). Typically, α-

Fe2O3 can be achieved by the oxidation of γ-Fe2O3 or Fe3O4 in air at temperatures ranging between 400-600 °C. In contrast, γ-Fe2O3 can generally be obtained from the calcination of

Fe3O4 in air at temperatures ≤250 °C. To date, iron oxide-based materials have been investigated for a wide range of applications, such as catalysis, biomedicine, environmental remediation and energy storage devices owing to their intrinsic and unique properties (including chemical, thermal, optical, electronic, and magnetic properties (Figure 1).[1, 3] In the past decade, numerous efforts have been carried out to synthesize various iron oxide nanoarchitectures from zero-dimensional (0D) to three-dimensional (3D) through various wet-chemical and gas-phase methods. Many studies have shown that synthetic parameters, such as the choice of iron precursor, type and amount of reducing agent, temperature, pH, etc., can determine the size and morphology of the resulting iron oxide products, which in turn affect their functional properties, including magnetic properties. The size, shape, and composition of the iron oxide nanostructures can affect their magnetic properties. The size determines whether the iron oxide particles display ferromagnetic or superparamagnetic behavior and affects their saturation magnetization (Ms)

(i.e., the maximum magnetization that nanoparticles (NPs) can exhibit when exposed to an external magnetic field) and blocking temperature (TB) (i.e., the temperature at which the magnetic moments of NPs become blocked during the time scale of the measurement). Further, by manipulating the morphology of iron oxide nanostructures, it is possible to tune the magnetic properties of these materials. For example, cubic Fe3O4 NPs showed a higher TB than spherical Fe3O4 NPs and pseudocubic iron oxide NPs displayed a larger Ms value than pseudospherical ones.[4, 5] Furthermore, the doping of a secondary metal to magnetic iron oxide NPs can also enhance their magnetic properties due to cationic substitution, which in turn enhances their heating efficiency for hyperthermia.[6] Therefore, proper optimization of these parameters is critical for maximizing the functional properties of the iron oxide nanostructures. Porous materials have attracted significant interests in materials research due to their high surface area, large pore volume, narrow pore size distribution, and modifiable surface properties.10, 11 In general, porous materials can be categorized into three classes based on the IUPAC (International Union of Pure and Applied Science) classification: macroporous (>50 nm), mesoporous (2-50 nm) and microporous materials (< 2 nm).12 Mesoporous and microporous materials are often referred to as nanoporous materials. In the last decade, significant research efforts have been devoted into the development of mesoporous iron oxide and ferrites (MFe2O4) through template-based methods, such as hard-templating, soft- templating, and sacrificial-templating methods. The hard-templating methods typically rely on the use of hard templates, such as mesoporous silica (e.g., KIT-6, SBA-15), mesoporous carbon, and anodic aluminium oxide (AAO).[7]

Iron oxide NPs play important roles in various biomedical applications, such as magnetic resonance imaging (MRI), drug delivery, hyperthermia, and biosensors, due to their magnetic properties, low toxicity, easy preparation, capabilities to store drugs and to react with certain biomolecules in the body, and unique enzyme-mimicking ability. The size, shape, and composition of iron oxide NPs have profound effects on their saturation magnetization and coercivity, which in turn affect their performance in these applications. For instance, the heating efficiency and contrast ability of iron oxide NPs for MRI and hyperthermia are strongly dependent on the Ms and Hc, with higher Ms leading to higher heating efficiency and contrast ability. To address the limitations of iron oxide NPs in these

bio-applications, surface modification or functionalization with polymers or biomolecules and hybridization with carbon-based materials and metals have been proposed. Beside biomedical applications, iron oxide nanoarchitectures have also been widely investigated for energy storage devices, such as lithium-ion batteries (LIBs) and supercapacitors (SCs).[3] LIBs are commonly found in portable devices and electric vehicles.[3] For LIBs, iron oxide with porous or hollow nanoarchitectures are generally preferred over solid structures. This is because they can promote better penetration of the electrolyte and improved transportation of Li+ ions in the electrode. Moreover, they can effectively buffer the structural stress caused by volume variations during the charge- discharge process. SCs, also known as electrochemical capacitors, are widely used in smart grids and power back-ups.14 Compared to LIBs, SCs exhibit short charging duration, fast energy delivery, long durability, high power density, and eco-friendliness.[11] Although, iron oxide nanostructures exhibit high specific capacity or capacitance when employed for LIBs or SCs, they often suffer from poor rate performance and poor cyclability at high rates.[3] Hence, hybridization with carbon-based materials or conductive polymers is typically necessary in order to overcome these problems. Nanoporous iron oxides have been employed in environmental remediation and detoxification of contaminants.[12] They have been used to detect and remove various pollutants (e.g., gases, contaminated chemicals, organic and biological substances) by adsorbing or catalyzing their removal. In particular, air pollution has become a widespread issue due to the burning of fossil fuels, industrial activities and vehicle emissions. For instance, carbon monoxide (CO) is a common pollutant which can cause nausea, dizziness, and serious respiratory diseases and even death at a high level of exposure due to its high toxicity and capability to bind with haemoglobin in the blood.[8] Noble metals have been used as catalysts for CO oxidation, however they are scarce, expensive and require high operating temperature.[13] As such, it is desirable to develop heterogenous catalysts using nanoporous iron oxides as supports for noble metal NPs due to their low cost, low toxicity, easy preparation, and magnetic properties (allows for the development of magnetically recyclable catalysts). This review is divided into several sections. The first section deals with the synthetic strategies for developing various iron oxide nanoarchitectures from 0D to 3D. The second section focuses on the hybridization strategies of iron oxide nanoarchitectures with carbonaceous and inorganic materials. The third section covers the applications of iron oxide

nanoarchitectures and their hybrids for biomedical, energy storage and environmental applications (especially environmental remediation of air pollution). Finally, the trend and future outlook on the fabrication of iron oxide nanoarchitectures for these applications are provided in the conclusions of this review paper.

2. Morphological Control of Iron Oxide Nanoarchitectures In this section, the design and synthesis of various types of iron oxide nanoarchitectures from zero-dimensional (0D) to two-dimensional (3D) nanoarchitectures are discussed. In principle, 0D nanoarchitectures refer to iron oxide nanoparticles with dimensions less than 100 nm on each axis. In contrast, 1D nanoarchitectures refer to iron oxide nanostructures in which the dimension of one axis is greater than 100 nm (e.g., nanowires, nanotubes, and nanorods). 2D iron oxide nanoarchitectures typically include nanosheets, nanoplates and nanodiscs, whereas 3D iron oxide nanoarchitectures typically refer to dendritic and hierarchical structures (Figure 2). The synthetic methods which have been developed for fabricating these nanostructures and the parameters influencing their formation and growth are discussed in this section. 2.1. 0D – Nanoparticles, Nanocrystals Iron oxide nanoparticles (NPs) are among some of the most widely investigated structures of iron oxide materials due to their important utilization in biomedical and catalytic applications. In the synthesis of iron oxide NPs, generally some important features, such as narrow size distribution, good dispersion, stability and high magnetic response are highly desired. The optimization of synthetic conditions, such as the choice of iron precursor and solvent, reaction temperature and time, and concentration of reducing agent are critical for achieving the above desirable properties or characteristics. Coprecipitation is by far the most popular method for synthesizing iron oxide NPs in aqueous solution. Generally, in coprecipitation method, a base (e.g., sodium hydroxide

(NaOH), ammonium hydroxide (NH4OH), or sodium carbonate (Na2CO3)) is added into a solution containing Fe(II) and Fe(III) precursors to promote the precipitation of ferrihydrites, which are thermodynamically unstable and can typically be dehydrated to generate iron oxide NPs, according to the equation[1]: 2+ 3+ - Fe + Fe +8OH ⇄ Fe(OH)2 + Fe(OH)3 → Fe3O4 + 4H2O (1)

Generally, α-FeOOH or α-Fe2O3 phase is often generated as the product when only Fe(III) precursor is used. However, different phases of iron oxide may be obtained depending on the

pH, ionic medium, and temperature. For example, Varada and co-workers demonstrated the possible tuning of the phase of the co-precipitated iron oxide nanoparticles by employing different bases.[21] Highly monodispersed acicular α-FeOOH particles with an average size of

60 nm were obtained by adding Na2CO3 into Fe(III)-containing solution. In contrast, the use of NaOH lead to the generation of acicular α-Fe2O3 particles with a higher average size of 230 nm. This phenomenon was attributed to the capabilities of different bases to maintain the solution at a constant pH. Specifically, the [OH]/[Fe] molar ratio governed the particle size and elongation by controlling the rate of hydrolysis, while in contrast, the carbonate ions promoted a constant [OH] in the solution, ensuring the pH to stay around 10 throughout the entire reaction. More recently, a bioinspired coprecipitation method has previously been used to achieve better control the resulting size of the co-precipitated Fe3O4 NPs by slowing down the rate of coprecipitation of FeIII/FeII salts through ammonia diffusion, during which ferrihydrite nanocrystals initially precipitated at low pH values and was transformed to magnetite at high pH values. Unlike direct coprecipitation which often produces small NPs with superparamagnetic properties, this bioinspired coprecipitation method enables control over the crystallization kinetics and therefore the size of the NPs by controlling the NH3 influx and the Fe concentration, thereby producing single crystals with sizes within the ferrimagnetic domain. Specifically, by adjusting the NH3 influx and total Fe concentration from 2 vol % NH3, 9 mM Fe to 5 vol% NH3, 3 mM Fe, the particle size of the Fe3O4 NPs was decreased from 15 ± 4 nm to 60 ± 21 nm. Furthermore, by introducing an optimized amount of the M6A peptide (0.3 mg/mL) during the synthesis process, spherical Fe3O4 NPs with sizes of 47 ± 12 nm were achieved . Interestingly, when a larger amount of M6A peptide was used, the formation of Fe3O4 was mostly inhibited and instead lepidocrocite was formed. This implies that the M6A peptide not only affects the nucleation process but also the formation and growth of Fe3O4. The VSM measurement of the M6A-modified Fe3O4 nanocrystals displayed ferrimagnetic behaviour with saturation magnetization (Ms) of 77 Am/kg and coercivity (Hc) of 8 mT. Although coprecipitation method provides a simple and convenient route to synthesize iron oxide NPs (especially Fe3O4 NPs) with very small diameters, it is still relatively difficult to control the size of the co-precipitated iron oxide NPs. Therefore, other methods have been developed to address the limitations of coprecipitation method. Hydrothermal and solvothermal routes provide easy methods to prepare iron oxide NPs with greater control over the phase, size, and morphology through simple tuning of

reaction parameters.[1] Furthermore, they offer some additional advantages, including low cost, high yield, moderate temperature, and do not require complex equipment. Fe3O4 NPs with tunable particle size from 15-31 nm could be synthesized by modifying the concentration of ferrous chloride and adjusting the ratio of water/ethanol mixture during the [22] hydrothermal treatment. It is found that higher concentration of FeCl2 lead to smaller nanoparticles due to the generation of a larger number of nuclei, which provided higher particle concentration and produced smaller particles. Moreover, when the proportion of ethanol in the solvent was increased, the size of the resulting Fe3O4 NPs also decreased due to the inhibition of the particle growth, thereby causing smaller particles to be produced. This size control could then be used to tune the magnetic properties. It was found that the decrease in the mean size of the Fe3O4 NPs from 22.4 to 15.4 nm reduced the specific saturation magnetization (σs) from 81.2 to 53.3 emu/g, as a result of the reduction in magnetic effective volume. Furthermore, such decrease in mean diameter resulted in a transition from ferromagnetism to superparamagnetism, as indicated by the reduced Hc values. To achieve even smaller Fe3O4 nanoparticles with tunable size from 4-6 nm and good monodispersity, solvothermal treatment of iron acetylacetonate in n-octanol with n-octylamine as the reducing agent has been employed.[23] By adjusting the volume ratios of n-octylamine to n-octanol to

4:12, 6:10, 8:8, Fe3O4 NPs with average diameters of 6, 5, and 4 nm were achieved. This indicates that the higher the ratio of n-octylamine to n-octanol, the smaller the size of the obtained Fe3O4 NPs and such observation was attributed to further restriction of the particle growth by adsorbed octylamine molecules. Interestingly, these ultrasmall Fe3O4 NPs could be well-dispersed in a variety of organic solvents, including chloroform, cyclohexane and long- carbon-chain alcohols to form a clear black solution. The magnetic measurements revealed that they were superparamagnetic and the Ms value was greatly increased from 35.7 to 52.6 and 63.1 emu g-1 with the increase in particle size from 4 to 5 and 6 nm, respectively. The decrease in Ms value with decreasing particle size was caused by the increased surface disorder and spin canting of surface Fe atoms. The sol-gel method has been used as an alternative approach to coprecipitation method for synthesizing iron oxide NPs with good control over the phase composition and particle size, owing to its simplicity and the fact that it does not require a high-pressure environment.[24] For instance, Cui et al.[25] has demonstrated the large-scale preparation (up to 60 g) of iron oxide NPs with tunable phase through a simple epoxide-assisted sol-gel route at room temperature. Here, Fe3O4 sol is initially formed by reacting FeCl2 and propylene oxide

in boiling ethanol solution and depending on the drying conditions of the sol solution, different iron oxide phases can be achieved. When the Fe3O4 sol was dried at 100 °C and further heated at 150 °C under air atmosphere, nearly monodisperse γ-Fe2O3 NPs with an average size of 4.9 nm were obtained. In contrast, when the Fe3O4 wet gel was dried in air at

150 °C, α-Fe2O3 NPs with a larger average size of 10.1 nm and wide size distribution are produced. Single phase α-Fe2O3 NPs have also been prepared through a modified Pechini sol- gel method by employing tartaric acid and ethylene glycol (EG) as chelating agents.[26] Different from traditional sol-gel method in which the NPs are part of the gel structure, in modified Pechini sol-gel method, the metal cations are trapped in the polymer gel.[27] While the sol-gel method offers some advantages as shown above, it suffers from some drawbacks, such as expensive precursor, lack of morphology control (typically spherical NPs) and difficulty in achieving small monodispersed iron oxide NPs.

Apart from the above methods, the sonochemical method has also been used for preparing Fe3O4 NPs. This method can produce interesting morphology ranging from spherical NPs to mesoporous structures. Many researchers have reported the fabrication of iron oxide NPs by sonochemical method.[29-31] For example, Gedanken group previously synthesized amorphous spherical Fe3O4 NPs with very small sizes of 3-14 nm through the sonochemical decomposition of iron pentacarbonyl (Fe(CO)5) in the presence of sodium [29] dodecyl sulfate (SDS) as a stabilizer. Interestingly, these amorphous Fe3O4 NPs showed good dispersion owing to the anionic repulsion between SDS molecules adsorbed on the surface of these Fe3O4 NPs. Such adsorption was believed to occur via ionic bonding.

Crystalline Fe3O4 NPs with an average size of 11 nm were previously prepared through the [30] sonochemical treatment of a mixture of FeCl3 and FeCl2 in a basic solution. The resulting -1 Fe3O4 NPs exhibited superparamagnetic behaviour with Ms and Hc values of 80 emu g and

10 Oe, respectively. Apart from Fe3O4, α-Fe2O3 NPs with an average size of 19 nm have also been synthesized via sonochemical treatment of FeCl3 with NaOH, followed by annealing in air at 500 °C.[31] The increase in sonication temperature from 30 to 80 °C was noted to increase the size of the resulting α-Fe2O3 NPs from 12 to 19 nm. In contrast, when the intensity of the ultrasound intensity at 80 °C was raised from 6 to 37 W cm-2, the particle size was decreased from 24 to 19 nm. Although the sonication method provides a simple and fast route for producing small iron oxide NPs (typically spherical), it is difficult to obtain iron oxide nanostructures with controllable morphology and dispersity using this method.

The fabrication of monodisperse iron oxide nanocrystals (NCs) has attracted significant interest due to their potential for a wide variety of biomedical applications, especially for those requiring precise control over their physical and magnetic properties, which are determined by the size and crystallographic phase. Monodisperse NCs generally exhibit distinct properties which are different from their bulk counterparts. Among various methods, the thermal decomposition method is particularly useful for preparing iron oxide NCs with uniform shape, high dispersity, and narrow size distribution. The thermal decomposition method typically involves the heating of organometallic or coordinated iron precursors in organic solvents at high temperatures (>300 °C), usually in the presence of stabilizers. The [32] [33] most commonly used iron precursors include iron pentacarbonyl (Fe(CO)5) , iron oleate , [34] [35] iron acetylacetonate , Fe(Cup)3 (Cup = N-nitrosophenylhydroxylamine) , ferrocene [36] [4] [4] [37] (Fe(C5H5)2) , FeCO3 , Fe2(CO3)3 , Fe3(CO)12 , and Fe–urea complex [38] ([Fe(CON2H4)6](NO3)3) . Stabilizers, such as oleylamine (OAm), oleic acid (OA), 1- octadecene, and 1-tetradecene are often added during the reaction to slow down the nucleation and growth rates of the iron oxide NCs, which result in very small size (<30 nm).[1] The Hyeon group has demonstrated the first successful large-scale preparation of monodisperse iron oxide NCs by the thermal decomposition of iron oleate complex in 1- octadecene a 320 °C, which could produce as much as 40 g.[14] The particle size of the resulting iron oxide NCs was increased when organic solvents with higher boiling points were used. This was caused by the enhanced reactivity of the iron-oleate complex in a solvent with a higher boiling point. Furthermore, the increase in the concentration of oleic acid from 3 to 4.5 μM lead to the slight increase in particle size of the resulting iron oxide NCs from 11 to 14 nm. The TB was also found to increase with increasing size of the NCs. Apart from size control, the thermal decomposition method has also been used to obtain shape-controlled iron oxide NCs with the use of stabilizers. For example, Xie et al.[28] showed the possible tuning of the shape of Mn-Zn ferrite NCs with various morphologies, such as spherical, cubic, and star-like, by adjusting the reaction temperature and controlling the molar ratio of OA/OAm. When the molar ratio of OA/OAm was set to 10:2, nearly spherical NCs (ca. 9 nm) were obtained. In contrast, reducing the OA/OAm molar ratios to 7:5 and 5:7 yield cubic (ca. 11 nm) and star-like (ca. 16 nm) NCs, respectively. It was proposed that at low amounts of OA, the stabilizing effect of OA on the {111} facets of Mn-Zn ferrite was weak and this caused a preferential growth along the [111] direction which lead to the formation of cubic NCs with

terminated {100} facets. Furthermore, as the amount of OA was continuously decreased, the continuous growth along the [111] directions promoted the eventual formation of star-like NCs with 8 corners symmetrically distributed around the cubical core. The formation of these NCs was also strongly dependent on reaction temperature and aging time. The size of the spherical NCs gradually grew with increasing time and temperature until many uniform spherical NCs were obtained when the aging time was extended from 20 to 40 min at 300 °C (Figure 3a). In the syntheses of cubic and star-like NCs, tiny quasi-spherical NCs were initially generated at lower temperatures (260-280 °C) (Figure 3b, c). However, upon increasing temperature and aging time 300 °C and 40 min, respectively, most of the NCs formed uniform cubic NCs (Figure 3b) or continued to grow along eight corners of intermediates to generate the star-like NCs. (Figure 3c). More recently, Feld et al.[4] demonstrated the possible shape and size control of superparamagnetic iron oxide NPs (SPIONs) by diluting the iron oleate with 1-octadecene (ODE) and varying the heating time, while keeping the Fe/OA molar ratio constant at 1:7. Octapod star-like NCs were achieved shortly after nucleation (kinetically-favored), in which the size quickly grew from 21 to 82 nm with a slight increase in time from 2 to 6 min. As the reaction time was increased to 30 min, these octapod star-like NCs underwent metamorphism to form cubic NCs as they are more thermodynamically-favored and an increase in size of the cubic NCs from 23 to 60 nm was observed with the increment in reaction time from 30 to 60 min. Although the thermal decomposition method can be used for the large-scale preparation of iron oxide NCs with very uniform size and shape, this method also has some disadvantages, including the use of toxic precursors and solvents, high reaction temperatures, and the fact that the resulting iron oxide NCs can only be dispersed in non-polar solvents and may exhibit some cytotoxicity. Thus, surface modifications are necessary to enable these NCs to be applied in biomedical applications.

2.2. 1D – Nanorods, Nanotubes, Nanowires 1D nanoarchitectures are known to possess enhanced electron and mass transport along with improved chemical stability compared to 0D nanoparticles.[49,68-70] They are typically formed via nucleation and subsequent anisotropic growth along one specific direction (i.e., the z- axis). Capping agents, such as surfactants and other additives are often used to provide kinetic control over the growth rates of different facets of the crystals. For instance, certain surfactants can adsorb onto x-axis and y-axis planes of iron oxide nanocrystals, thereby

enabling selective growth in the y-axis direction to generate 1D iron oxide nanostructures. To date, a wide variety of 1D iron oxide nanostructures, including nanowires, nanorods, nanospindles, and nanotubes have been reported either through template-free or template- assisted approaches. In some cases, 1D iron oxide nanoarchitectures can be achieved through very simple and convenient routes. For example, single crystalline iron oxide nanorods were previously synthesized through a facile reaction between spherical iron nanoparticles and water at 350-450 °C under argon (Ar) atmosphere. Using this approach, different phases of iron oxides could be obtained by varying the calcination temperature, with a mixture of α-

Fe2O3, Fe3O4, and FeO being the main phases observed at 350 °C. Upon further increase to

500 °C, iron and Fe3O4 were identified as the main products. The main drawback of this method is the difficulty to achieve a single pure phase of iron oxide. Alternatively, uniform α-

Fe2O3 and Fe3O4 nanorods could be simply achieved by annealing akageneite (β-FeOOH) nanorods (produced through a simple hydrolysis of FeCl3 at 80 °C) in air at temperatures ≥350 °C and by directly reducing these β-FeOOH nanorods with hydrazine at 80 °C for 6 h, respectively.[39, 40]

Single crystalline γ-Fe2O3 nanorods with tunable aspect ratios have been fabricated through the thermal decomposition of iron oleate in benzyl ether (with sodium oleate) by varying the reaction temperature.[41] It was found that the nanorods grew thicker from 2 to 5 nm with the increase in reaction temperature from 200 to 260 °C. In this method, the oleic ligand on the iron oleate complex played a significant role in directing the formation of the nanorods. In this report, the oleate group acted as a structure-directing agent by promoting the lateral growth of the nanorods to be faster than the longitudinal direction, thereby decreasing the surface energy and structure anisotropy. This in turn, caused the thickness of the nanorods to increase with increasing temperature, while the length of the nanorods remain more or less similar. These γ-Fe2O3 nanorods showed superparamagnetic behaviour at room temperature, even when their length exceeded 50 nm.

Previously, the large-scale fabrication of α-Fe2O3 nanowires was successfully achieved through a solvothermal route in the presence of nitrilotriacetic acid (NTA) as a chelating agent to form polymeric chains, followed by heat-treatment in air at 350 °C.[42] In this process, Fe3+ coordinated with NTA to form the Fe-NTA coordination compound. Under solvothermal conditions, these Fe-NTA monomers further reacted to generate longer polymeric chain products. The individual polymer chains could then self-assemble into wire- like structures owing to van der Waals force. The decomposition and removal of organic

NTA in Fe-NTA precursor nanowires at high temperatures generated α-Fe2O3 nanowires (ca.

100 nm) composed of interconnected α-Fe2O3 nanocrystals with an average particle size of 4-

5 nm. When tested for sensing applications, the prepared α-Fe2O3 nanowires displayed high sensitivities toward both ethanol and acetic acid gases at the 150 °C (30% humidity) with impressively low response/recovery time of 1-3 s. Anion-assisted hydrothermal route (with phosphate and sulfate ions) has been shown to be effective for preparing short α-Fe2O3 nanotubes (SNTs) with hollow interiors due to the 3- strong adsorption of phosphate (PO4 ) ions on surfaces parallel to the long dimension of elongated nanoparticles (c-axis) of α-Fe2O3 during nanocrystal growth and the hollow structure was generated as a result of the preferential dissolution along the c-axis of α-Fe2O3 2- [43] owing to the strong coordination effect of the sulfate (SO4 ) ions. In the absence of these two anions, only nanostars were obtained as the product. In contrast, solid nanospindles or nanoneedles were achieved when only phosphate or sulfate anions were used, respectively. The needle-like morphology originated from the preferential adsorption of sulfate anions on the {110} or {104} planes of α-Fe2O3, giving rise to elongated crystals along the c-axis.

However, it is worth noting that the adsorption of sulfate anions on α-Fe2O3 crystals is much weaker than that of phosphate anions. The short α-Fe2O3 nanotubes could be converted into short Fe3O4 nanotubes through annealing at 300 °C under a continuous H2 flow and these

Fe3O4 nanotubes could be further transformed into γ-Fe2O3 nanotubes via a simple oxidation in air at 400 °C. Based on the vibrating sample magnetometer (VSM) measurements, the as- synthesized γ-Fe2O3, Fe3O4, and α-Fe2O3 SNTs exhibited Ms values of 27.3, 64.3, and 0.5 -1 emu g at 300 K, respectively. Furthermore, the γ-Fe2O3 SNTs displayed a Hc of 1032 Oe and -1 -1 a remanent magnetization (Mr) of 0.16 emu g , compared to 193 Oe sand 15 emu g and -1 <100 Oe and 6 emu g for Fe3O4 and α-Fe2O3 SNTs, respectively. Well-ordered iron oxide nanotubes with inner diameters up to 80 nm could be synthesized through a simple potentiostatic anodization of iron foil in an EG-based [44] electrolyte containing ammonium fluoride (NH4F) and deionized water at 60 °C. The anodization conditions, including the applied potential, concentrations of H2O and NH4F, and anodization temperature together with the calcination temperature were critical in determining the morphology and structural properties of the resulting iron oxide nanotubes. The average inner diameters of the nanotubes obtained at applied potentials of 10, 30, 50, 70, and 90 V were 18, 50, 80, 100 and 125 nm, respectively, indicating an almost linear correlation between the two parameters. Furthermore, the presence of water in the electrolyte

was critical for the formation of the nanotube arrays. In the absence of water, only a dense layer of iron oxide with thickness of 340 nm was deposited on the surface of the iron foil. In contrast, at the optimum water content of 3 wt.%, a balance between the rate of formation and dissolution of the iron oxide layer was achieved, thereby leading to the generation of well- ordered nanotubes. Interestingly, the thickness of the iron oxide layer greatly increased with further rise in the concentration of water in the electrolyte, which was attributed to the higher rate of iron oxide formation compared to the dissolution rate, as a result of the presence of a large quantity of oxygen (O2) created by the splitting of water at high concentration. Their investigations also revealed that the thickness of the deposited iron oxide layer was significantly increased from 1.3 to 4.5 nm, with increasing NH4F concentration from 0.25 to

1.00 wt%, and then decreased with a further increase in NH4F concentration to 2.00 wt.%. It was proposed that higher NH4F concentration lead to higher current density, which promoted the growth of the nanotubes, while simultaneously accelerating the dissolution of the tube mouths. At 2.00 wt.% NH4F, the dissolution became very significant and the thickness of the oxide layer was reduced. Longer iron oxide nanotubes could be obtained by raising the anodization temperature from 25 to 60 °C but should be below 80 °C. Following calcination, the tube-like morphology was preserved even at high temperatures up to 600 °C, however, it started to collapse when the calcination temperature was raised above 700 °C due to further crystal growth of the iron oxide walls. Apart from template-free methods, 1D iron oxide nanoarchitectures could also be synthesized through template-based methods. For instance, Hofmeister group reported the fabrication of magnetic iron oxide nanowires constructed by Fe3O4 nanoparticles with average diameters of 9 nm by precipitating iron(II) and iron(III) salts inside nanoporous alumina membranes.[45] Interestingly, when a nanoporous membrane with a mean diameter of 100 nm was used, iron oxide fibrils were generated as the product, in contrast to iron oxide nanotubes when a nanoporous membrane with a larger mean diameter of 200 nm was used. α-

Fe2O3 nanotubes with lengths of several microns have also been fabricated by employing [15] carbon nanotubes (CNTs) as a hard template. To obtain such α-Fe2O3 nanotubes, the CNTs were initially coated with iron oxide nanoparticles through the decomposition of ferric nitrate in supercritical CO2/ethanol solution at 150 °C. This is followed by the heat treatment of the iron oxide/CNT composites in an oxygen-rich environment to remove the CNTs, thereby leaving behind α-Fe2O3 nanotubes. When tested for hydrogen sulfide (H2S) sensing, the α-

Fe2O3 nanotubes showed good sensitivity to H2S (with cathodoluminescence intensity of

around 500 absorption units), fast response and recovery properties (short response time of 22 s and recovery time of less than 100 s) and good reproducibility on exposure to 22 ppm H2S at 134 °C.

2.3. 2D – Nanoplates, Nanosheets In recent years, 2D metal oxide nanostructures have attracted significant research interests due to their high surface to volume ratio and unique physicochemical, optical and electronic properties, which render them attractive for various energy and environmental applications.[46, 47] To date, 2D iron oxide nanostructures with distinct morphologies, such as nanoplates and nanosheets have been successfully synthesized using various methods, such as thermal decomposition[48], hydrothermal[49], electrodeposition[16], and thermal oxidation,[50].

For example, crystalline γ-Fe2O3 nanoplates were previously fabricated via the thermal decomposition of iron oleate at 280 °C in the presence of oleic acid and trioctylphosphine oxide (TOPO).[48] Here, TOPO, was used to control the concentration of nuclei, which resulted in two different growth pathways. At low concentration of TOPO, the diffusional - growth pathway was observed, in which C2H5O (the residual product from the precursor reaction) acted as a third ligand and promoted the formation of nanoplates. On the contrary, high TOPO concentration caused many nuclei to be produced, thus inducing the formation of iron oxide nanoflowers. In other report, ultrathin hexagonal Fe3O4 nanoplates with thicknesses of 10-15 nm and side lengths of 250-400 nm have been successfully synthesized using a hydrothermal route by aging ferrous hydroxide under anaerobic environment (also known as Schikorr reaction) at 90 °C, according to the equation[49]:

Fe(OH)2 → Fe3O4 + H2 + 2H2O (2)

The benefit of this method is that it enables control over the thickness of the resulting Fe3O4 nanoplates by controlling the ratio of ethylene glycol (EG)/water in the solvent, with higher amount of EG leading to thinner nanosheets. Moreover, from the VSM measurements, the hexagonal Fe3O4 nanoplates were identified to be ferromagnetic with Ms and coercivity Hc values of 71.6 emu g-1 and 152.2 Oe, respectively. These values are lower than those of bulk -1 Fe3O4 (85-100 emu g and 115-150 Oe), due to the spin canting of surface Fe atoms and sharp anisotropy of the Fe3O4 nanoplates, respectively. Thin film of iron oxide nanosheets could be conveniently deposited on nickel (Ni) foils using simple electrochemical deposition by applying anodic current in nitrogen-purged

solution of ammonium ferrous sulfate, sodium sulfate, and sodium acetate, followed by annealing in air at 300 °C.[16] The formation of the as-deposited α-FeOOH nanosheets originated from the oxidation of dissolved Fe2+ to Fe3+ at the anode, which subsequently reacted with the hydroxide ions generated by the slightly alkaline electrolyte to form insoluble ferric hydroxide, which was then converted to α-FeOOH, according to the equations: Fe2+ ↔ Fe3+ (3)

3+ Fe + 3OHˉ → Fe(OH)3 ↓ (4)

Fe(OH)3 ↓ → α-FeOOH + H2O (5)

The applied current densities had a profound effect on the morphology of the deposited films, with current densities lower than 0.125 mA cm-2 resulted in the formation of nanorods whereas those above 0.125 mA cm-2 yielded nanosheets. It was proposed that larger current densities lead to larger deposition overpotentials, thereby increasing nucleation rate and promoting the formation and growth of smaller nuclei. The overlapping effect then occurred, causing these nanorods to aggregate into nanosheets. When tested in terms of their capacitive -1 performance, the annealed Fe2O3 film at 300 °C showed a specific capacitance of 146 F g at a low scan rate of 5 mV s-1 which decreased to 88 F g-1 when the scan rate was increased 40- fold to 200 mV s-1, corresponding to a moderate capacitance retention of 60.3%. Furthermore, a substantial reduction in capacitance was observed after cycling CV test for 500 cycles at a scan rate of 25 mV s-1. However, this can be partially mitigated by increasing the CV scan rate to 100 mV s-1. Apart from nanoplates and nanosheets, 2D iron oxide nanoflakes have also been fabricated by sputtering a thin film of Fe on Cu foil under inert environment, followed by a simple heating of the Fe-coated Cu foil in air at 300 °C for 5 h.[50] The electrochemical characterization of these Fe2O3 nanoflakes revealed that they exhibited a relatively high capacity of 680 mA h g–1, with good stability up to 80 cycles when cycled in the voltage range 0.005–3.0 V at a rate of 0.1 C. Additionally, they also maintained a high Coulombic efficiency of >98 % after 15 cycles.

2.4. 3D Iron Oxide Nanoarchitectures In this section, the synthetic methods for the fabrication of 3D iron oxide nanoarchitectures (aerogels, dendritic and mesoporous) are discussed. Aerogels typically refer to porous solid materials exhibiting gel-like structures with extremely low densities. Generally, aerogels

contain pores in the size range of 1-100 nm, although in most cases, the pores tend to be smaller than 20 nm. Hierarchical nanoarchitectures refer to 3D nanoarchitectures which are assembled by lower-dimensional nanostructures, such as nanorods, nanowires, nanosheets, or nanoplates. Mesoporous materials generally have a 3D structure (mostly spherical) and contain pores with diameters in the range of 2-50 nm.

2.4.1. Aerogels The distribution of active nanoscale iron oxide materials in a three-dimensional (3-D) nanoarchitecture can enable immobilization and stabilization of these nanomaterials and promote their spatial distribution for interaction with liquid- or gas-phase species.[51] 3D aerogel nanoarchitectures are most commonly synthesized via sol-gel method followed by supercritical processing, allowing for the intrinsic properties of nanopores and solid to be combined. Such nanoarchitectures are particularly appealing for applications which depend on the transport of ions or molecules to and from active surfaces. Most interestingly, iron oxide aerogels with certain nanocrystalline phases may display unique soft magnetic properties which are especially useful for magneto-optical sensors. Nanocrystalline iron oxide aerogels with mesoporous architectures were previously prepared by introducing epoxide-based proton scavengers (e.g., epichlorohydrin) to promote the sol-gel reactions of hydrolysis and polycondensation of iron(III) salts.[51] The resulting 2 -1 3 -1 FeOx aerogels exhibited high surface area (464 m g ) and large pore volume (4.0 cm g ).

Upon direct calcination in air at 260 °C, the FeOx aerogel is converted to γ-Fe2O3-Fe3O4 aerogel with poor crystallinity and relatively high surface area of 280 m2 g-1. When this mixed-phase aerogel was subjected to a secondary heating in argon atmosphere at 260 °C, the surface area was further decreased to 132 m2 g-1 with pore volume of 1.04 cm3 g-1, compared 2 -1 -1 to 143 m g and 1.04 cm g for the FeOx aerogel directly calcined in argon at 260 °C. The

FeOx aerogel calcined in air at 260 °C displayed weak superparamagnetic behaviour, unlike that calcined in argon at 260 °C which showed strong paramagnetism. As of now, various 3D hierarchical iron oxide nanostructures with distinct shapes, such as flower-like, urchin-like, chestnut-like have been fabricated using different strategies, including solvothermal[52], ultrasonic irradiation[53], and polyol-assisted self-assembly [18] process . For instance, γ-Fe2O3 and Fe3O4 chestnut-like hierarchical nanostructures (CHNs) were previously prepared by the solvothermal reaction of tin(II) chloride dihydrate

(SnCl2.2H2O) and Fe(CO)5 in N,N-dimethylformamide (DMF) at 200 °C, followed by

[52] calcination at 300 °C and 500 °C under N2 atmosphere, respectively. The Brunauer-

Emmett-Teller (BET) surface areas of the precursor Fe2O3, γ-Fe2O3, and Fe3O4 CHNs were 143.12, 96.44. and 14.90 m2 g-1, respectively, indicating the decrease in surface area with increasing calcination temperature which was attributed to the collapse of the mesopores at higher temperatures. The magnetic properties characterization of the samples revealed the ferromagnetic behaviour of the resulting γ-Fe2O3 and Fe3O4 CHNs at room temperature with -1 Ms values of 22.1 and 69.97 emu g , which were much larger than that of the precursor Fe2O3 -1 CHNs (2.1 emu g ). When tested for arsenic (As(IV)) removal, the precursor Fe2O3 CHNs showed the best performance with a high adsorption capacity of 137.5 mg g-1 followed by γ- -1 -1 Fe2O3 CHNs (101.4 mg g ) and Fe3O4 CHNs (6.07 mg g ), respectively. This trend suggested the strong relationship between adsorption capacity and surface area, with larger surface area leading to higher adsorption capacity and vice versa. Previously, 3D hierarchical urchin-like iron oxide nanostructures composed of nanotubes were fabricated via ultrasonic irradiation of ferric nitrate in 1-propanol at room temperature followed by calcination in air at 200 and 400 °C for 3 h.[53] The growth process of such nanotube-assembled urchin-like structure was proposed to involve two main processes: (i) growth of the spherical core via rapid nucleation, aggregation, and crystallization and (ii) subsequent growth of the 1D tube-like structure via oriented attachment mechanism. The calcination at 200 °C yielded γ-Fe2O3 urchin-like nanostructures 2 -1 with surface area of 282.7 m g , whereas the calcination at 400 °C yielded Fe3O4 urchin-like 2 -1 nanostructures with slightly lower surface area of 204.6 m g . Interestingly, the as- synthesized urchin-like Fe2O3 showed better adsorption performance for As(V) and Cr(IV) removal than the calcined samples with almost 100% removal at a small dosage of 2 mg for both heavy metal ions. Such observation was attributed to the increase in negative charges of the urchin-like samples after calcination and hence, they could not absorb these negatively charged metal ions. 3D flower-like iron oxide nanostructures with tunable phase have been successfully prepared by an EG-mediated self-assembly process (in the presence of FeCl3, urea and tetrabutylammonium bromide (TBAB)) followed by heat treatment under different [18] conditions. In this process, EG initially coordinated with FeCl3 to form iron alkoxide and hydrochloric acid (HCl) was generated as a by-product. The accumulation of HCl would limit the formation of iron alkoxide and hence, the addition of urea was important for neutralizing the HCl, thereby enabling the completion of the coordination reaction. The iron

alkoxide then precipitated to form nuclei which rapidly grew into primary particles with an average size of 100 nm. This was followed by the aggregation of these primary particles into microspheres which formed the core of the flower-like structure. The microspheres then combined with the rest of the primary particles to generate the flower-like structure. The calcination of the flower-like precursor in air at 450 °C for 3 h yielded 3D flower-like α-

Fe2O3, while the calcination in N2 under the same conditions yielded flower-like Fe3O4, which could be further converted to γ-Fe2O3 following annealing in air at 250 °C for 5 h. When tested for As(V) and Cr (IV) removal, the three iron oxide samples exhibited relatively similar adsorption capacities in the range of 4-6 mg g-1 at 20 °C and pH 4. Furthermore, they also showed similar performance for Orange II dye removal with adsorption capacities in the range of 40-50 mg g-1.

2.5. Mesoporous Iron Oxide Nanoarchitectures 2.5.1. Template-based methods Mesoporous materials are defined as materials containing pores with diameters between 2-50 nm. Mesoporous transition metal oxides are especially attractive because they possess d-shell electrons confined to nanosized walls, redox active internal surfaces, and interconnected pore networks.[54] Owing to these attractive characteristics, they exhibit many interesting properties in catalysis, adsorption, separation, sensing, energy storage and conversion devices, catalysis, and magnetic devices. In general, well-ordered mesoporous iron oxide can be achieved through two main pathways: hard-templating and soft-templating methods.[7] Template-based methods offer greater versatility and improved control over the pore size than non-templated methods. In hard-templating (nanocasting) method, mesoporous silicas or mesoporous carbons are typically used as the hard templates. Typically, the nanocasting route requires multiple procedures: (1) the preparation of the template; (2) the infiltration of the metal precursor onto the template, and (3) the removal of the template, resulting in replicated porous structure. The main advantage of this method is the greater controllability over the pore size and morphology. However, the template removal often requires the use of strong acids (e.g., hydrofluoric acid (HF) or hydrochloric acid (HCl)) or strong bases, such as NaOH. In contrast, the soft-templating method is simpler to perform as it does not require complex template removal using strong acid or base. The selection of the proper template is a key factor to prepare ordered mesoporous materials with well-defined frameworks and

crystalline walls. In this section, the fabrication of ordered mesoporous iron oxides through template-based methods is summarized.

2.5.1.1. Hard-templating (nanocasting) method

The hard-templating (nanocasting) method has been extensively used for the fabrication of mesoporous metals, carbons, and oxides, since it can directly replicate the structure of the original template. To date, a wide variety of hard templates has been used, including polymer microspheres, anodic aluminum oxide (AAO) and mesoporous silica.[55, 57, 59] Polymeric beads are often employed to create mesoporous materials with rigid structures. The particle size of these polymeric beads can be tuned by controlling the rate of polymerization.[60] In the report of Xu et al., polymethyl methacrylate (PMMA) was used as a hierarchically-ordered colloidal template to synthesize 3D ordered macroporous (3DOM) LaCoxFe1−xO3 (Figure 4a, b).[55] Porous alumina membranes fabricated through anodic oxidation (AAO) process have been utilized as a mold to prepare porous metals, carbons, or other nanostructures via electrodeposition, atomic layer deposition (ALD) or chemical vapor deposition (CVD). The pore sizes of the resulting porous materials typically vary between 15 and 150 nm.[54] Mesoporous silicas are frequently used as hard templates to fabricate mesoporous metal oxides because of their tunable morphology and controllable size (Figure 4c, d).[56] Furthermore, they possess good stability even under severe conditions, such as high temperature or strongly acidic environment, enabling the creation of mesoporous oxides with improved frameworks. In mesoporous silica-templated process, the silica mesopores are initially infiltrated with the metal precursor solution. Then, the metal precursor is converted into solid oxide phase through reduction or decomposition inside the pores. Finally, the mesoporous silica template is etched by using HF or NaOH solution and mesoporous oxide with replicated structure is achieved. Previously, ordered mesoporous iron oxides have been synthesized by using colloidal silica templates.[57, 58] For instance, Jiao et al. reported the synthesis of ordered mesoporous Fe3O4 and γ-Fe2O3 using mesoporous silica (KIT6) as a hard [57] template. In their report, ordered mesoporous α-Fe2O3 was initially fabricated by incorporating the iron source (ferric nitrate) into the KIT-6 template via a simple absorption at room temperature followed by calcination of the dried powder at 600 °C. Ordered mesoporous α-Fe2O3 with crystalline walls was achieved after etching the KIT-6 template with concentrated NaOH. The conversion to ordered mesoporous Fe3O4 was performed

through reduction under 5% H2-95% Ar atmosphere (Figure 4e and 4f), whereas the ordered mesoporous γ-Fe2O3 was achieved by heating the ordered mesoporous Fe3O4 in air at 150 °C for 2 h (Figure 4g). The ordered mesoporous γ-Fe2O3 exhibited a relatively large BET surface area of 86 m2 g-1, with a mesopore peak centered at 3.6 nm, indicating good replication of the pore size of the KIT-6 template. The Mössbauer measurements revealed that the ordered mesoporous γ-Fe2O3 and Fe3O4 possessed long-range magnetic ordering and exhibited magnetic freezing above 340 K despite having wall thicknesses of around 7 nm. This is unique compared to iron oxide nanoparticles with particle size below 8 nm, which would typically lose magnetic order down to much lower temperatures.

Hosseini and co-workers reported the fabrication of ordered mesoporous α-Fe2O3 using another kind of mesoporous silica template, SBA15 (Figure 4h).[58] To achieve the mesoporous α-Fe2O3, the SBA-15 template was dispersed in n-hexane and subsequently infiltrated with iron by adding ferric nitrate solution into this dispersion, followed by evaporation of the solvent, drying, and calcination of the Fe-incorporated SBA-15 template in air at 600 °C for 6 h. Finally, the SBA-15 template was etched with 2 M KOH at 70-80 °C.

The ordered mesoporous α-Fe2O3 mostly replicated the ordered mesoporous structure of the SBA-15 template, although some disordered mesopores and micropores were also observed due to the in-situ conversion of ferric nitrate to Fe2O3. The ordered mesoporous α-Fe2O3 exhibited BET surface area and pore volume of 36.8 m2 g-1 and 0.14 cm3 g-1, respectively. Furthermore, it was found that the ratio of SBA-15 to the iron precursor had a significant effect on the catalytic performance toward the decomposition of ammonium perchlorate (AP). Specifically, the specific heat generated by the AP decomposition was greater when the iron precursor to template ratio was increased. Most importantly, the mesoporous α-Fe2O3/AP composite exhibited 3 times higher specific heat (1110 J g-1) than pure AP. Apart from pure iron oxides, it is entirely possible to introduce a secondary element during the hard-templating process to generate ordered mesoporous ferrite (MFe2O4). For instance, Kleitz group have successfully created ordered mesoporous NiFe2O4 and CuFe2O4 by infiltrating a stoichiometric mixture of ferric nitrate and the secondary metal nitrate (nickel nitrate and copper nitrate) into the pores of the silica templates (KIT-6, MCM-48, and SBA-15), followed by calcination at 500 °C and 600 °C, respectively for 5 h and subsequent removal of the templates by treatment with 2 M NaOH.[61] In this process, the use of non- polar organic solvent, such as n-hexane, was essential for promoting better interaction between the silica walls and the metal nitrates by enhancing the hydrophilicity of silica and

pre-wetting its surface. All the silica-templated mesoporous ferrites displayed highly ordered structures and those obtained with MCM-48 and SBA-15 exhibited spherical and wire-like structures. Among the various-silica templated mesoporous ferrites, the MCM-48 templated 2 −1 ordered mesoporous NiFe2O4 showed the highest surface area and pore volume of 250 m g 3 −1 2 −1 and 0.32 cm g , respectively, followed by the ordered mesoporous CuFe2O4 (96 m g ). Both ordered mesoporous ferrites had an average mesopore diameter of 2.5-3.5 nm. Although silica templates are commonly employed to create rigid mesoporous materials, the elimination of silica templates typically requires the use of HF or NaOH, which are hazardous and not environmentally-friendly. Hence, over the years, soft template-method has been developed as an alternative to hard-template method.

2.5.1.2. Soft-templating method The soft-templating method typically utilizes surfactants, block copolymers or bio-polymers as templates.[7] The removal of the soft template is generally performed through dissolution in organic solvent or pyrolysis. Generally, surfactants consist of hydrophilic and hydrophobic moieties and they can form various morphologies in solutions, including spherical, cylindrical, and lamellar micelles (Figure 5a).[62] Surfactants typically become liquid crystals at a high concentration in aqueous media which then react with the inorganic precursors. In surfactant-assisted method, the synthetic conditions can directly influence the interactions between organic and inorganic interfaces. For example, the interaction between organic and inorganic interfaces involves a weak hydrogen bonding under a strong acidic environment but involves a strong electrostatic force under a strong alkaline environment.[63] In most cases, surfactant-templated porous materials are obtained by solution-phase route or evaporation induced self-assembly (EISA) route.[64] The removal of the template should be considered in accordance with the choice of template and the framework composition.

The pore size of mesoporous oxides obtained by soft-templated method is typically small because the pores are generated by low-molecular weight amphiphilic molecules, which have both short hydrophilic and hydrophobic parts.[66] The Bruce group previously prepared ordered 2D hexagonal mesoporous iron oxide (2DMIO) and 3D cubic mesoporous iron oxide (3DMIO) by employing decylamine as the soft template and Fe(III) ethoxide as the iron precursor (Figure 5b-d).[65] The structural examination of the 2DMIO indicated the presence of a large amount of disordered micropores within the walls with pore size of 10 Å,

thereby suggesting that the surfaces of the 2DMIO could actually be microporous. Furthermore, 3DMIO with the same microporous walls could also be produced using the same approach as the 2DMIO, expect with additional aging of the decylamine/Fe(III) ethoxide mixture at 150 °C. The 3DMIO exhibited an impressive surface area of 610 m2 g-1, which was about twice that of 2DMIO possibly due to the higher accessibility of pores in the 3D structure. Apart from alkyl amines, the anionic surfactant sodium dodecyl sulfate (SDS) has also been used as a template to produce mesoporous iron oxide with an average pore size of 2.5 nm and narrow pore size distribution.[67] Here, the combination of SDS (as a surfactant) and benzyl alcohol (as a co-surfactant) was essential to ensure the formation of mesoporous iron oxide with good framework rigidity. The removal of the template was achieving using a simple ethanol extraction. The mesoporous iron oxide samples obtained with SDS alone and

SDS-benzyl alcohol consisted mostly of γ-Fe2O3 phase, although other phases, such as α-

Fe2O3 and γ-FeOOH were also found. These mesoporous oxide samples exhibited disordered worm-like pores with sizes of 2.5-2.8 nm. As a result, their surface areas were also quite similar, with the SDS-templated mesoporous iron oxide exhibiting a slightly larger BET surface area of 355 m2 g-1 compared to 306 m2 g-1 for the SDS-benzyl alcohol templated one. Aside from anionic surfactants, mesoporous iron oxides have also been fabricated using cationic surfactants, as demonstrated by the work of Srivastava et al.[68], in which they successfully created mesoporous iron oxide with a very small pore size up to 7.5 nm through a supramolecular templating approach, using the sonochemical method and Fe(III) ethoxide and CTAB as the iron precursor and soft template, respectively. Similar to the case of SDS, the removal of CTAB from the pores could also be achieved by heating or ethanol extraction. It was shown that selecting the proper temperature for the ethanol extraction process was critical for ensuring complete removal of CTAB or else the surface area would be reduced as the surfactant residues still remained inside the pores. The mesoporous γ-Fe2O3 obtained by calcination of the precursor mesoporous Fe2O3 at 250 °C exhibited wormhole-like pores with high surface area of 177 m2 g-1. Additionally, they showed superparamagnetic behaviour at room temperature without any hysteresis. When tested as a catalyst for cyclohexane oxidation, surprisingly, the as-prepared (non-calcined) mesoporous Fe2O3 showed a superior conversion efficiency of 35% compared to the calcined mesoporous γ-Fe2O3. This indicated the poor activity of crystalline iron oxide for cyclohexane oxidation.

Polymeric templates are typically used to fabricate mesoporous materials with larger pore sizes. Polymers, especially block copolymers, are particularly useful as soft templates due to their enhanced stability in aqueous solution and varied molecular structures (Figure 6a).[69] The structure and morphology of the resulting mesoporous materials can be controlled by using different combinations of polymeric blocks and should be adapted to the targeted application. For example, poly(acrylic acid) (PAA) was previously used as a soft template to synthesize mesoporous Fe3O4 nano/microspheres with tunable average diameters from 50 to 200 nm (Figure 6b, c).[70] The synthesis process involves the initial fabrication of

Fe3O4/PAA microspheres by solvothermal treatment of ferric chloride in EG solution containing sodium acrylate and sodium acetate at 200 °C followed by coating with silica layer to generate the Fe3O4/PAA@SiO2 core-shell microspheres. These core-shell microspheres were then calcined at 500 °C to remove the PAA and the silica layer was etched using NaOH. The mesoporous Fe3O4 nano/microspheres possessed a large surface area of 163 m2 g-1, with an average pore diameter of 11 nm. In the synthesis process, the addition of

SiO2 coating was important to avoid the formation of large irregular mesoporous Fe3O4 particles formed by the ripening process between several mesoporous Fe3O4 microspheres at elevated temperature. The size of these mesoporous spheres could be controlled by using

Fe3O4/PAA hybrid spheres with different sizes. Magnetic properties characterization of the mesoporous Fe3O4 microspheres indicated that they exhibited soft magnetic character with -1 Ms of 48.6 emu g .

[71] Brezesinski et al. previously synthesized mesoporous α-Fe2O3 and α-FeOOH thin films by utilizing poly(isobutylene)-block-poly(ethylene oxide) (PIB–PEO) block copolymer as a soft template. In this work, the PIB-PEO block copolymer was initially dissolved in ethanol and added to a tetrahydrofuran (THF)/ethanol mixture containing Fe(III) ions. The mesoporous α-Fe2O3 thin film was then achieved by dip-coating the silicon substrates into this mixture solution followed by calcination in air at 450 °C. In comparison, the mesoporous α-FeOOH film could be obtained by decreasing the calcination temperature below 350 °C.

Interestingly, the block copolymer-templated mesoporous α-Fe2O3 thin film showed better preservation of the mesostructural order up to 400-450 °C compared to the Pluronic123- templated one, which was attributed to the higher thermal stability of the PIB–PEO template and the high hydrophilic-hydrophobic contrast of the porogen. Furthermore, the use of this novel block copolymer enabled the pore size to be enlarged up to 8 nm, higher than those

typically obtained with surfactants (Figure 6d). In a related report, Brezesinski and co- workers have also fabricated mesoporous α-Fe2O3 thin film with an average pore size of ca. 15 nm by employing poly(ethylene-co-butylene)-block-poly(ethylene oxide) diblock copolymer (PEB-PEO) (also known as KLE) as a soft template (Figure 6e).[72] The KLE- templated mesoporous α-Fe2O3 thin films obtained at 550 °C showed good in-plane periodicity with pore size averaging between 14-15 nm. In comparison, the mesoporous α-

Fe2O3 thin films obtained at 600 °C displayed lower in-plane periodicity owing to the grain growth in the pore walls which promoted the formation of elongated nanocrystals. Furthermore, the use of hydrated ferric nitrate as the iron precursor was shown to generate better and more reproducible mesoporous structure compared to ferric chloride, while also preventing the formation of α-FeOOH during calcination from 250 to 350 °C. From the above studies, it is clear that the use of block polymers as soft templates can lead to mesoporous iron oxide with enlarged pore size compared to those obtained with surfactants. Moreover, the proper selection of the metal precursor is important for obtaining mesoporous iron oxide with good mesostructural ordering. Yet, it is still a challenge to fabricate well-crystallized mesoporous iron oxides using block copolymers as the pore walls tend to collapse at very high temperatures due to possible decomposition of the polymers.

Biopolymers or biomolecules, such as DNA, protein, virus, etc. have also been used as soft templates for the fabrication of nanoporous materials owing to their abundance in nature, wide variety in molecular structures, reduced toxicity, and ease of removal (Figure 7).[73, 74] In recent years, there have been growing reports on the fabrication of nanoporous iron oxide materials templated by biomolecules or biopolymers.[75, 76] For instance, Liu et al.[75] successfully synthesized hierarchical porous iron oxide with controllable pore size from 20 nm to 50 μm using four different wood templates, including paulownia, pine, lauan and fir.

All the wood-templated porous Fe2O3 obtained at 600 °C exhibited mesopores mostly in the size range of 20-30 nm, however the pore volume of the paulownia-templated porous Fe2O3 was roughly 1.5 times higher than that of the lauan-templated one. Furthermore, when the calcination temperature was raised to 1000 °C, the average pore size increased to 60 nm, indicating the dominant presence of macropores in the wood-templated porous Fe2O3 samples. Tobacco mosaic virus (TMV) has previously been used as a bio-template to synthesize amorphous iron oxide nanotubes.[76] The growth of the iron oxide nanotubes was achieved by mixing TMV with a solution containing Fe(II) and Fe(III) under a slightly basic

condition (pH 9) followed by incubation at room temperature and subsequent purification. Calcination of these amorphous nanotubes in vacuum at 570 K and in air at 520 K lead to their conversion to amorphous mixed phase iron oxide (Fe3O4/γ-Fe2O3) nanotubes, which exhibited weakly ferro/ferrimagnetic behavior. Based on these reports, it is obvious that biopolymers or biomolecules can serve as templates to fabricate various kinds of inorganic nanomaterials, including nanoporous iron oxides. However, the extraction of these bio-templates may present some challenges for large-scale preparation. In addition, precise control over morphology and pore size has rarely been demonstrated in the case of bio-templated synthesis.

2.5.1.3. Sacrificial-templated method In recent years, porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) have gained significant attention as sacrificial templates for the synthesis of nanoporous metal oxides owing to their high surface area, large pore volume, and tunable composition. MOFs are typically prepared via coordination between metal ions and organic linkers and they can exist in various forms from 1D to 3D, which is useful for practical applications.[11, 77] MOFs can be used versatilely as sacrificial templates to prepare nanoporous carbons, oxides or their composites depending on the calcination conditions. For example, porous iron oxide nanorods have been synthesized from Fe-MIL-88 nanorods by varying the calcination conditions.[78] Specifically, the calcination of the Fe-MIL-88 nanorods in air at 380 °C led to the formation of porous α-Fe2O3 nanorods, while additional calcination of these α-Fe2O3 nanorods under N2 atmosphere resulted in their conversion to Fe3O4 nanorods (Figure 8a-c).

The resulting porous α-Fe2O3 nanorods displayed weak ferromagnetic behaviour at room temperature, whereas the Fe3O4 nanorods exhibited a stronger ferromagnetic behaviour, with -1 -1 Ms and Hc of 96 emu g (close to bulk Fe3O4 (92 emu g )) and 372 Oe, respectively. Prussian blue (PB) and Prussian blue analogue (PBA), which are a class of cyano- briged MOFs, serve as useful precursors for preparing nanoporous metal oxides owing to their ease of synthesis and tunable composition.[79] In PB or PBA-templated strategy, the metal ions serve as metal precursors, whereas the cyanide groups create pores following their removal by calcination. Hu et al.[80] discovered that the utilization of hollow PB nanocubes with large cavities could lead to the formation of porous iron oxide nanocubes consisting of a single phase rather than a mixed of phases. For instance, the calcination of large hollow PB nanocubes with average size of 190 nm at 250 °C and 400 °C lead to the formation of

crystalline porous γ-Fe2O3 and α-Fe2O3 nanocubes with relatively large BET surface areas of 130 m2 g-1 and 160 m2 g-1, respectively. In contrast, the calcination of small hollow PB nanocubes with size of 110 nm yielded amorphous Fe2O3/γ-Fe2O3 nanocubes and γ-Fe2O3/α- 2 -1 Fe2O3 nanocubes (i.e., mixed phase nanocubes) with larger BET surface areas of 420 m g and 480 m2 g-1 (Figure 8d-g). This finding indicated that with increasing size of the cavity, the diffusion of air became easier, leading to better crystallinity. The shrinkage of the PB nanocubes to smaller sizes (around 100 nm) after calcination was observed as a result of the decomposition of the cyanide groups. The magnetic measurements of these PB-derived iron oxide nanocubes indicated that the crystalline γ-Fe2O3 nanocubes exhibited higher Ms (51 -1 -1 - emu g ), Mr (3.0 emu g ), and Hc (35 Oe) than the amorphous γ-Fe2O3 nanocubes (28 emu g 1, 0.2 emu g-1, and 28 Oe, respectively).

Furthermore, our group has also demonstrated the possible size tuning of PB nanocubes from 20 to 500 nm by controlling the amount of sodium citrate, which served as a chelating agent to slow down the nucleation rate and crystal growth of PB nanocubes.[81] Specifically, when the concentration of sodium citrate was high, a lower number of nuclei was formed at the beginning of the reaction and these nuclei could then undergo crystal growth through interaction with the organic ligands [sodium hexacyanoferrate(II)] to form larger-sized nanocubes. In contrast, at low concentrations of sodium citrate, the majority of the Ni species existed as free Ni ions, so they could directly react with sodium hexacyanoferrate(II) to produce many nuclei at the beginning of the reaction, and they would grow rapidly to generate smaller‐ sized nanocubes. This citrate-assisted strategy could be exploited to prepare porous iron oxide nanocubes with controllable particle size. Due to the presence of multiple metallic species in PBA, porous ferrite nanocubes with tunable composition could be derived from PBA. For example, porous FexCo3-xO4 nanocubes have been synthesized from PBA, FeyCo1−y[Co(CN)6]0.67·nH2O nanospheres, as reported by Xuning and co-workers.[82] The amount of Fe doping was shown to significantly influence the size and morphology of the resulting porous FexCo3-xO4 nanocubes, with higher Fe doping leading to larger particle size (up to 160 nm) and more uniform nanocubes. Furthermore, when tested for bisphenol A (BPA) degradation, the porous Fe0.8Co2.2O4 nanocubes showed a high removal efficiency of 95% within 1 h after activation with peroxymonosulfate, despite the low catalyst concentration (0.1 g L-1).

Based on these studies, it is evident that by tuning the particle size and composition of the PCPs or MOFs, it is possible to control the size and composition of the derived nanoporous iron oxide products, which in turn can be used to tune their functional properties. One possible limitation is that MOF or PCP-derived porous oxides tend to inherit the original shape of their precursors, although this may be addressed by introducing surfactants or other structuring agents during the preparation of the PCPs or MOFs.

3. Hybrid materials Hybridization is a useful approach for combining the advantages of two or more materials, while also addressing their individual drawbacks. Hybrid materials can be fabricated via simple, fast and inexpensive process by electrostatic force, hydrogen bonding, or charge- transfer. The properties of hybrid materials are strongly dependent on the structure and composition of the assembling components. Currently, various materials, including carbon, silica, and noble metals have been hybridized with iron oxide nanomaterials to create highly functional hybrid materials for biomedical, energy storage, and environmental applications. The hybridization or coating of iron oxide nanostructures with carbon-based materials are common for enhancing their electrical conductivity for energy storage applications. Further, for bio-applications, carbon coating can provide protection to the magnetic core from oxidation or corrosion and enhance their dispersibility in aqueous media if the carbon coating is hydrophilic. To achieve carbon coating, the iron oxide NPs are first coated with polymer through the polymerization process, then iron oxide@carbon composite is formed following calcination at high temperatures. In the past, various carbon-containing precursors have been used to generate the carbon shell, including polydopamine[83], glucose[84], phenolic resin[85], polysaccharide[86], etc. Du et al.[85] previously showed the possible tuning of the carbon shell thickness by adjusting the concentration of resorcinol, with higher concentration leading to a thicker shell. Furthermore, the Ms value gradually decreased with the increase in the thickness of the carbon shell, due to the lower proportion of the core Fe3O4. Carbon nanotubes (CNTs) possess large specific surface area (single-walled carbon nanotubes (SWCNTs) >1600 m2 g-1 and multi-walled carbon nanotubes (MWCNTs) >430 m2 g-1) and excellent mechanical and electrical (∼5000 S cm-1) properties and thus they are commonly hybridized with iron oxide NPs for improving their electrochemical performance.[87] The deposition of iron oxide NPs on CNTs is most commonly achieved using refluxing[87], chemical vapor deposition (CVD)[88], and coprecipitation[89, 90].

Graphene is a 2D monolayer of carbon atoms with sp2 hybridization, excellent electrical and thermal conductivity, good mechanical properties, flexible porous structure, and large surface area.[94] However, it is still difficult to utilize the whole surface area due to strong π-π bonding and van der Waals interaction.[95] By hybridizing GO with metals/metal oxides, they may exhibit enhanced functional performance for various applications.[91, 96, 97]

Previously, reduced graphene oxide (rGO)/Fe2O3 was prepared through hydrolysis of FeCl3 in the GO suspension with the assistance of urea at 90 °C, followed by microwave irradiation [91] to convert the GO to rGO and to decompose Fe(OH)3 to Fe2O3. Zhao et al. reported the fabrication of iron oxide nanoflake/graphene composites through a mechanochemical synthetic route. In this process, bulk metallic iron and GO were ground in a stainless-steel grinding bowl, and the metallic iron became oxidized to form iron oxide with flake-like structure on the surface of GO. Meanwhile, GO went through reduction to form graphene.

Following calcination under Ar atmosphere, Fe3O4 nanoflake/graphene composites were [97] obtained (Figure 9a). Zhu et al. reported a more convenient method for growing Fe2O3 NPs onto rGO nanosheets in ethanol solution through a one-pot solvothermal process, in which the reduction GO to rGO was achieved in the absence of strong reducing agent and without calcination process. Here, FeCl2 and ammonia were added into the GO suspension in ethanol and the mixture solution was solvothermally-reacted in an autoclave at 170 °C which resulted the formation of Fe2O3/rGO composite. The functionalization of iron oxide nanostructures with polymers can provide enhanced biocompatibility and stability as well as improved dispersibility in aqueous media. However, polymer coating will decrease the magnetization of iron oxide NPs depending on the shell thickness due to the decrease in the magnetic core content. To date, various kinds of natural and synthetic biodegradable polymers have been used to modify the surface of iron oxide NPs, including starch[98], chitosan[99, 100], alginate[101], polyethylene glycol (PEG)[102], polymethylmethacrylate (PMMA)[103], poly(acrylic acid) (PAA)[104], polysaccharide[105], polydopamine (PDA)[106], polyethylenimine (PEI)[107], and so on. The functionalization of iron oxide nanostructures with polymers can be achieved through in situ coating during the synthesis process or post-synthesis coating. As an example, magnetic PVA gel beads were previously obtained through simultaneous formation of magnetic iron oxide NPs and cross- link of PVA chains. However, the in situ coating strategy suffers from poor control of the coating thickness and poor colloidal stability of the resulting functionalized iron oxide NPs. A more effective approach to coat polymers on iron oxide nanostructures is through post-

synthesis coating via a one-pot method, self-assembly or heterogeneous polymerization (e.g., miniemulsion polymerization and dispersion polymerization). The one-pot route typically relies on physical adsorption or chemical adsorption of functional groups on the surface of iron oxide NPs and it often leads to a core-shell structure. In contrast, the self-assembly route is achieved through covalent bonding and hydrogen bonding. The stability of iron oxide nanostructures can also be enhanced via surface modification with polymers containing multiple functional groups, such as conjugated polymers (e.g., conjugated polyelectrolyte) or amphiphilic block copolymers (e.g., polystyrene-block-poly(ethylene oxide) (PS-PEO)).[108] The emerging trend is the functionalization of iron oxide NPs with smart polymers which can impart unique properties to the iron oxide NPs upon exposure to a stimulus, such as light, temperature or pH.[109-111]

The coating of inorganic materials, such as silica (SiO2) on the surface of iron oxide

NPs (iron oxide@SiO2) can potentially enhance their functional properties for bio- applications by providing higher biocompatibility and stability in acidic environment, enhancing their dispersibility in aqueous media, and preventing aggregation.[1] Furthermore, due to the presence of abundant silanol groups on the SiO2 shell, iron oxide NP@silica could be activated with ease to provide the surface of the iron oxide NPs with various functional groups. However, a decrease in their magnetism is typically expected as a result of the SiO2 coating. There are three most common methods to generate silica shell on the surface of iron oxide NPs. By far, the most popular method is the Stöber process, in which the silica shell is formed in situ by the hydrolysis and condensation of a sol-gel precursor (e.g., tetraethoxysilane (TEOS) vinyltriethoxysilane (VTEOS), octadecyltrimethoxysilane).[112] Typically, the iron oxide NPs are first dispersed homogenously in the alcohol followed by the addition of silane (such as TEOS), and finally ammonia aqueous solution is dropped into the mixture solution and iron oxide@SiO2 is formed. The tuning of the SiO2 shell thickness can be achieved by controlling the hydrolysis and condensation of the SiO2 precursor and in some cases, surfactants may be used to provide additional control over the uniformity of the shell.[113] Another common method is the microemulsion method, in which micelles or inverse micelles are used to confine and control the silica coating on the core iron oxide. This particular method can provide good controllability of the SiO2 shell thickness but requires much effort to separate the core-shell NPs from the large quantity of surfactants employed in the microemulsion system. Ding et al.[92] have demonstrated the possible tuning of the

thickness of SiO2 shell on Fe3O4 NPs and the successful coating of SiO2 on Fe3O4 NPs with different particle size through a reverse microemulsion method. The silica coating process can be described using the La-Mer theory which can be separated into three distinct stages: increased monomer concentration stage, nucleation stage, and growth stage, as illustrated in Figure 9b. The first stage is related to the formation of TEOS monomers via hydrolysis with the addition of TEOS. When the quantity of TEOS is increased, the monomer concentration increases and when the monomer concentration exceeds the solubility concentration (Cs), a heterogeneous nucleation takes place on the surface of the Fe3O4 NPs. As the monomer concentration increases above the homogeneous nucleation threshold (Chomo), a spontaneous aggregation will occur (stage II in Figure 9b). As the size of the nucleus gradually increases, more monomers are consumed, and when the quantity of monomer decreases below the homogeneous nucleation concentration, heterogeneous nucleation will reappear (stage III in Figure 9b), in which both coating and monomers nuclei growth take place. In order to avoid the formation of core-free SiO2 particles, the monomer concentration must fulfill the condition Cs < Chomo throughout the entire reaction (Figure 9d). The thickness of the silica shell could be increased by increasing the amount of TEOS (Figure 9e-h), while the core- free SiO2 particles appeared when the TEOS content was increased above a certain amount. Further, they also found that low ammonia amount was beneficial for creating a thinner shell by restricting the hydrolysis and condensation of TEOS. The final method to obtain iron oxide@SiO2 core-shell composite is through the aerosol pyrolysis of silicon alkoxides and metal compound under a high temperature environment. The controlled fabrication of γ-

Fe2O3@SiO2 composite was previously achieved through the aerosol pyrolysis of iron pentacarbonyl and 1,1,3,3-tetramethyldisiloxane at 900-1200 °C under moist N2 atmosphere.[114] Nanosized iron oxides are used as support materials for noble metal NPs in various catalytic applications owing to their high thermal and chemical stabilities, good durability, low toxicity, and easy preparation.[13, 115] The deposition of noble metal NPs onto iron oxide supports can be conveniently achieved using the deposition-precipitation method, in which small Au NPs were formed through the rapid reduction of gold(III) chloride using a strong reducing agent (e.g., sodium borohydride).[8] The synergistic combination of iron oxide nanostructures with noble metal NPs can lead to a significant enhancement in their catalytic activities for various reactions. The catalytic performance of such iron oxide-supported noble metal NPs is strongly dependent on the size of the deposited noble metal NPs and the strength

of interaction between the catalyst and the support.[116] Hutchings et al.[117] previously prepared iron oxide-supported Au NPs by deposition-precipitation method and found that calcination at high temperatures resulted in the agglomeration of Au NPs accompanied by the increase of particle size of the Au NPs. The strength of interaction between noble metal catalyst and the metal oxide support relies on the preparation method. For example, Au NPs deposited via deposition-precipitation method have shown strong interaction with metal oxide supports, thereby leading to enhanced catalytic performance.[116] Apart from iron oxide- supported noble metal NPs, iron oxide/noble metal composites with unique core-shell, core- satellites, and dumb-bell structures have also been reported. The direct growth of metallic layers on the surface of iron oxide nanostructures has previously been attempted using microemulsion and thermal decomposition methods.[118, 119] However, due to the dissimilar nature of the surfaces and lattices of the two materials, these direct methods are rather unfavorable.[1] To overcome this, surfactants have been used to alter the surface properties and stability of the iron oxide or noble metal NPs. A more prevalent way to synthesize iron oxide/noble metal hybrids is through a multi-step method, such as seed-mediated method or microemulsion method. For example, Fe3O4@Au core-shell nanostars could be synthesized by preparing Fe3O4/Ag particles as seeds followed by the addition of the Fe3O4/Ag seeds to the Au growth solution.[120] Another feasible method is the layer-by-layer self-assembly in which the noble metal NPs spontaneously organize into ordered structures by electrostatic interaction, molecular [1] interaction, or conjugation. Previously, Fe2O3-Au and Fe3O4-Au hybrid nanorods were successfully synthesized using FeOOH-Au hybrid nanorods prepared via a layer-by-layer technique followed by a controlled annealing process. Here, the uniform deposition of Au NPs onto the surface of FeOOH nanorods was possible due to the strong electrostatic attraction between Au ions and polyelectrolyte-modified FeOOH nanorods. Unlike these methods however, dumbbell-like Fe3O4/Au (Figure 9i)and Fe3O4/Pt-Pd hybrids were previously achieved by controlling the nucleation and growth of only one Fe3O4 NP on individual Au and Pt-Pd NPs, respectively, which was possible due to the possible electron transfer between Fe and the noble metals.[93, 121] As iron oxide/noble metal hybrids tend to have low chemical stability, modifications with biocompatible co-polymers and branched copolymers can be applied to enhance their colloidal stability, corrosion/oxidation resistance, and biocompatibility. For instance, the coating of iron oxide@Au nanorods with Pluronic F127 or cationic polyelectrolyte poly(diallyldimethylamonium chloride) (PDDA) was found

to generate a much more stable dispersion (up to 20 h) than that coated with PEG.[122] Another effective strategy to improve the stability of the iron oxide/noble metal hybrids is to encapsulate them within a stable and biocompatible material, such as SiO2, to form iron oxide/noble metal@SiO2 or iron oxide@noble metal@SiO2 composites, as seen in the case of [123, 124] Fe3O4-Au@mesoporous SiO2 microspheres and Fe3O4@Au@SiO2 nanospheres.

5. Functional Applications of Iron Oxide-Based Nanoarchitectures 5.1. Biomedical applications Iron oxide nanostructures and their hybrids have played important roles in biomedical applications, not only in diagnosis but also in treatment (in vivo and in vitro). Particle size, surface functionalization, surface area and pore volume are critical parameters which can influence the physicochemical properties of these materials. To date, iron oxide nanostructures and their hybrids have been utilized in a wide variety of biomedical applications, including MRI, magnetic hyperthermia, biosensors, and drug delivery. The recent reports on the biomedical applications of iron oxide-based nanomaterials are summarized below.

5.1.1. Magnetic resonance imaging (MRI) MRI is an imaging method commonly employed in medical clinics to produce high quality images of the internal parts of human body and it plays a great importance in the diagnosis of various diseases. Ultrasmall iron oxide NPs with superparamagnetic properties can generate susceptibility effects to produce strong T2 (spin-spin relaxation process) and T2 contrast and [1] T1 effects (spin lattice relaxation process) at very low concentrations for MRI. However, it is hard to deliver these magnetic materials to the targeted site since they are transported by the reticuloendothelia system (RES) and this results in less effective detection. However, this can be solved by applying external magnetic field, surface modification or attaching a ligand that has an affinity to the receptors of the targeted cells.[125, 126] In general, the main factors which influence the contrast properties of iron oxide NPs for MRI are composition, size, morphology, surface properties, and magnetic properties. The composition of magnetic NPs can greatly influence their magnetic moments at atomic scale. The doping of metal ions into iron oxide NPs can dramatically change their magnetic moments and therefore their relaxivity values (r2 and r2*). For example, MnFe2O4 NPs showed high relaxivity value (~358 mM-1 s-1) as a result of their high magnetization value at

2+ 1.5 T, whereas the incorporation of Zn dopants in metal spinel ferrite NPs (ZnFe2O4 yielded high relaxivity values (860 mM-1 s-1) due to the high magnetization values at 4.7 T.[127] Gao et [128] al. even demonstrated the possible tuning of the T2 contrast ability of MnxFe3-xO4 NPs by tuning the Mn doping level. Here, the saturation magnetization and T2 contrast ability of

MnxFe3-xO4 NPs increased with rising Mn doping up to an optimal level of x = 0.43 (with r2 value of 904.4 mM-1 s-1 at 7.0 K) and significantly decreased at doping level beyond this value due to significant lattice distortion. Furthermore, the in vitro MRI study revealed that the SMMC-7721 cells incubated with MnxFe3-xO4 NPs displayed both enhanced T1 and T2

MR contrast at a low magnetic field of 0.5 T. Furthermore, when the MnxFe3-xO4 (x = 0.43)

NPs were used for in vivo T2-weighted MR imaging of liver at transverse plane, a maximum dark contrast was achieved only after 2 h of intravenous injection with ΔSNR (signal to noise ratio) of 81.0% at transverse plane, which was 2.9 and 9.9 times higher than Fe3O4 and

Feraheme (an approved MRI contrast agent in clinics).

The size and surface effects on the MRI relaxivity of MnFe2O4 NPs as contrast agents have been investigated by the Weller group. MnFe2O4 NPs embedded into amphiphilic polymer shell showed an increase in transverse relaxivity values with increasing particle [130] size. Furthermore, the transverse relaxivity, in particular r2* was greater for the micelle- embedded MnFe2O4 NPs compared to the polymer-coated ones. The effect of morphology on the relaxation rates of Mn-doped iron oxide NPs has been studied by the Gao group, in which 6 different morphologies, including spheres, cubes, plates, tetrahedrons, rhombohedra and octapods were compared for their contrasting properties (Figure 10a-f).[129] Their study found that T2 relaxivities increased in the order of octapods > rhombohedra > tetrahedrons > plates > cubes > spheres, which followed the trend in Ms and effective radii (r*). This finding reveals the strong dependence of T2 relaxivities on Ms and (r*). The in vitro study revealed the T2 contrast ability increased in the same order as the above trend at both 1.5 T (Figure 10g) and 7 T (Figure 10h). Furthermore, the in vivo experiments indicated that the signal changes in the liver region also followed the same trend, with the octapods capable of delivering maximum dark signal only 0.5 h after intravenous injection and remained negative until 4 h. In addition, the octapods at a quarter of standard dosage showed much higher sensitivity for tumor imaging compared with Feraheme, suggesting their excellent potential as low dosage contrast agent for highly sensitive tumor diagnosis. Further, the effects of size

and surface modification of magnetic iron oxide nanoclusters (NMIONCs) on their T2 contrast ability were investigated by using three different coated samples: PVP@NMIONC, PEI@NMIONC, and PAA@NMIONC.[131] The in vitro study found that both -1 -1 -1 -1 PAA@NMIONC (r2= 663.6 mM s ) and PEI@NMIONC (r2=542.1 mM s ) showed significant contrast enhancement in T2-weighted imaging with rising concentration, whereas PVP@NMIONC showed a weak contrasting ability. Such difference was attributed to the presence of hydrophilic groups on the surface of PAA@NMIONC (hydroxyl group) and PEI@NMIONC (carboxylic group) which could increase the contact area between NMIONCs and water molecules. In terms of size effect, smaller-sized NMIONCs yielded higher r2 values as they possessed higher Ms values, which in turn improved the T2 contrast ability. Moreover, when tested in vivo, PAA@NMIONC showed higher contrast enhancement effects than both PEI@NMIONC and PVP@NMIONC. Apart from polymeric coating (e.g., PEG, PEI, PAA), effective MRI contrast agents based on iron oxide NPs with different surface coatings have been reported in the literature. -1 -1 Previously, peptide coating has been shown to enhance the r1 (2.4 mM s ) and r2 relaxivities (217.8 mM-1 s-1) of ultrasmall SPIONs (USPIONs) with higher relaxivity ratios (>90) relative to commercially available MRI contrast agents.[132] Furthermore, peptide-coated USPIONs could show high contrast enhancement of the liver for detection of liver tumors. SiO2-coated

SPIONs (SPIONs@SiO2) demonstrated potential as T1-weighted contrast agent with good r1 -1 -1 -1 -1 [133] relaxivity of 1.2 mM s and low r2/r1 ratio of 6.5 mM s . In addition, these core-shell particles could be used to provide positive signal enhancement for in vivo T1-weighted MRI of heart in mice with maximum signal intensity achieved 2 h after intravenous administration of SPIONs@SiO2. In a related report, nanorods could be used as an effective T2-weighted -1 -1 [134] contrast agent with a high r2 value of 192 mM s . Major signal improvement was observed with increase in Fe concentration from 0.06 to 0.18 mM. Further, folic acid- conjugated Fe3O4 NPs Based on the above presented reports, it is evident that proper tuning of the size, morphology, composition, surface modification, and magnetic properties can lead to the development of highly effective contrast agents based on magnetic iron oxide NPs. In the future, more studies are needed to design magnetic iron oxide NPs which can access specific organelles for achieving excellent contrast by introducing recognition capability for special surface signatures of the targeted cells.

5.1.2. Drug Delivery System The development of drug delivery system (DDS) is crucial in order to reduce side effects and to enhance the effect of therapy in the treatment of disease. Considerable efforts have been devoted to developing DDS by many approaches, such as stimuli-responsive polymeric NPs, liposomes, metals/metal oxides, and exosomes, but there are still challenges to be overcome, including biotoxicity, targeting, difficulty of large-scale fabrication and economical availability.[135-137] In those systems, drug is entrapped, attached, absorbed or encapsulated into or onto the carrier agent. Targeted drug delivery (TDD) aims to concentrate the drug in the target tissues while decreasing the relative concentration of the drug in the rest of the tissues and overcome the biological barriers via active accumulation or an active targeting strategy.[1] Magnetic iron oxide NPs are especially useful for TDD due to their non-toxicity, low cost, biodegradability, and controllability by magnetic field (Figure 11a).[1, 138] In TDD, MNPs form the core and biocompatible components function as the shell to form core-shell structure and the drugs are encapsulated into polymeric matrix. In TDD, size, surface properties, and stability are the most crucial parameters affecting the effectiveness of the drug carrier system. In general, iron oxide NPs in the size range of 10-100 nm are preferred for intravenous injection as they have the longest blood circulation time and can penetrate well through the capillary bed. Furthermore, iron oxide NPs with hydrophobic surface are likely to adsorb at the surface of protein, resulting in low circulation time. The main parameters in TDD are the rate of adsorption and release as well as the delivery of the drug to the targeted site. Consequently, the functionalization of iron oxide NPs with biocompatible materials (e.g., organic polymers, silica, or liposomes) to resolve this issue and to enhance the drug storage and release properties.

Cheng et al. have utilized cisplatin-loaded hollow Fe3O4 NPs coupled with Herceptin (Her) to target breast cancer SK-BR-3 cells.[138] The release of cisplatin from the PEGylated

Fe3O4 NPs was highly pH dependent, with slow controlled release being observed at pH 7.4 (physiological condition) while fast release was noted at pH 5m due to etching of the porous shell. The Her-coupled PEGylated Fe3O4 NPs were found to be internalized in the acidic endosomes and lysosomes. Furthermore, they show higher cytotoxicity to the breast cancer cells after 48 h of incubation than free cisplatin, which was induced by endosomal or lysosomal pH. Human serum albumin (HSA)-coated hollow iron oxide NPs (HINPs) were previously used to load anticancer drug, doxorubicin (Dox) to target 4T1 murine breast cancer cells.[139] The free HINPs did not show inhibitive effects on cell growth, however after

Dox loading, the HINPs displayed inhibitive effects on cell viability, depending on the Dox concentration. The T41 tumor cells injected with Dox-loaded showed much higher uptake of Dox (0.75 ID/g) than free Dox (0.44 ID/g) and Doxil (0.69 ID/g) as well as greater tumor suppression effect. The in vitro assays revealed that the free HINPs were mostly stored in the cytoplasm, whereas in the case of Dox-loaded HINPs, HINPs helped the Dox to cross the cell membrane and to accumulate in the nucleus of the tumor cells. Anti-HER2/neu peptide (AHNP)-conjugated iron oxide NPs (Figure 11b, c) were previously used to load the breast cancer drug, paclitacel (PTX) and the in vivo targeting test on human HER2/neu+ SK-BR-3 breast cancer cells revealed that these NPs were accumulated in the tumour cells 6 h after injection done and were retained for at least 72 h (Figure 11d(i)).[140] The in vitro studies performed using SK-BR-3 and MDA-MB-231 breast cancer cell lines indicate that at a PTX concentration of 2.5 Nm, the PTX-loaded AHNP-conjugated iron oxide NPs showed nearly 30% enhancement in killing SK-BR-3 cells compared to MDA-MB-231 cells due to the improved cell uptake mediated by the targeting ligand AHNP (Figure 11d(ii)).

A high drug loading efficiency and a rapid drug release rate are both essential for TDD. Iron oxide NPs with hollow or porous structure and functional porous shell (e.g., carbon and SiO2) can promote the attainment of these attributes. For example, Huang and co- workers compared the drug adsorption and release capabilities of magnetic

Fe3O4@mesoporous silica (SBA-15) with NPs two distinct structures (cellular foams (MCFs), and fiber-like (FMS)).[141] Interestingly, the two samples showed different adsorption of bovine serum albumin (BSA), the model protein, in accordance with the pore size, in which Fe3O4@MCFs with pore sizes of 10-40 nm showed a higher BSA adsorption of -1 -1 191 mg g compared to at 64 mg g Fe3O4@FMSs (pore size around 8.2 nm) because BSA 3 (size of 40 × 40 × 140 Å ) could fit into the mesopores of Fe3O4@MCFs. Although the first drastic release was observed due to the adsorption on the surface of samples, the stable release is maintained over a period. Due to the larger average pore size, Fe3O4@MCFs showed a faster release of BSA within the first 48 h. In a related study, magnetic drug carrier consisting of DOX-conjugated Fe3O4 NPs as core and PEGylated porous silica as shell -1 (Fe3O4-DOX/pSiO2-PEG) exhibited a DOX loading capacity of 16.3 μg mg and they could be internalized by the cells via endocytosis.[102] Further functionalization of this porous carrier with folic acid (FA) increased its specific uptake by MCF-7 and HeLa tumor cells. The addition of silica shell created another barrier to prevent the unwanted release of the drug

during transportation. Apart from SiO2, Fe3O4 QDs/rGO composite has been shown to be capable of releasing the anesthetic lidocaine hydrochloride (LH) after 8 h, with little toxicity up to 50 μg mL-1.[142] Although major advancements have been seen in this field, more studies on the immune response during the residence time of the carrier-drug conjugate, the toxicity of carrier materials and their possible decomposition products are still needed.

5.1.3. Hyperthermia Apart from DDS, magnetic hyperthermia has been considered as a viable clinical method for cancer treatment. In magnetic hyperthermia, magnetic materials are heated under alternating magnetic field and heat is generated from the combination of internal Néel fluctuations of the particle magnetic moment, hysteresis, and the external Brownian fluctuations that all depend on the magnetic properties of iron oxide NPs.[1] Magnetic hyperthermia requires biocompatible magnetic NPs with excellent heating capabilities. The heating temperature for hyperthermia treatment mostly depends on magnetic properties of the NPs and the strength of magnetic field. Typically, cells experience apoptosis when exposed to elevating temperature from 41 to 47 °C and necrosis occurs when cells are exposed to temperatures higher than 50 °C.[143] In hyperthermia treatment, there is also the possibility that the normal cells/tissues may be damaged during the heating of tumor cells. Therefore, surface functionalization of the magnetic NPs is highly important for ensuring that these NPs target only the tumor site to minimize damage to healthy cells/tissues. The specific absorption rate (SAR) or specific loss power (SLP) is the main parameter which is used to characterize the effectiveness of NPs in generating heat during magnetic hyperthermia treatment. The heating efficiency of magnetic iron oxide NPs for magnetic hyperthermia is influenced by various factors, such as magnetic field strength, anisotropy (shape or magnetocrystalline), particle monodispersity, composition, and surface modification or functionalization. The size and shape of iron oxide NPs can have a profound effect on their heating power due to their influence on Ms and anisotropy, respectively. For example, the Baldomir group found that larger iron oxide nanocubes (40 nm) exhibited a lower SAR value than those with 20 nm size owing to minor hysteresis loss. Furthermore, nanocubes with size of 20 nm also showed a higher SAR value than nanospheres of similar size due to the favored chain formation.[144] Previously, it was shown that nanocubes possess lower surface anisotropy relative to nanospheres owing to the smaller amount of disordered spins as the surface of the nanocubes is flat and it is composed mostly of low energy <100> facets. In

contrast, the curved surface of nanospheres produces a stronger surface spin canting. In a [145] related report, Nemati et al. demonstrated that at low magnetic fields, smaller Fe3O4 nanooctapods (17 nm) displayed better heating efficiency than larger-sized ones (35 and 47 nm). However, this trend was reversed at high magnetic fields. At low magnetic fields, larger magnetic hysteresis loop area was observed for smaller nanooctapods (17 nm), whereas at higher magnetic fields. the hysteresis loop area was higher for the larger nanooctapods (47 nm) and these explained for the above observed trends. Interestingly, the magnetic field strength also had an effect on the relationship between size and heating power, as the SAR value was observed to increase with increasing size at higher magnetic fields (600 and 800

Oe). Furthermore, compared to spherical Fe3O4 NPs, the Fe3O4 nanooctapods presented a better heating efficiency, especially at high magnetic fields. Iron oxide nanorods were also reported to display higher heating efficiency for magnetic hyperthermia (862 W g-1) compared to iron oxide nanocubes (314 W g-1) and spherical iron oxide NPs (140 W g-1).[146]

This difference was promoted by larger Ms and effective anisotropy originated from the effective unidirectional shape anisotropy of these nanorods.

The incorporation of metal doping into iron oxide NPs could also effectively enhance the SAR value due to the increase in Ms from cationic substitution and the magnetic ordering in the crystal lattice. Bauer and co-workers found that the doping of Zn2+ into iron oxide nanocubes resulted in a 3-fold increase in SAR value (1019.2 W g-1) compared to the undoped nanocubes (356.2 W g-1).[147] The enhancement in heating efficiency was correlated to the increase in Ms value and therefore the overall net magnetic moment due to the selective replacement of the Fe2+ by Zn2+ in the tetrahedral sites of the spinel ferrite lattice. Similarly, -1 -1 MnFe2O4 NPs also showed a higher SAR value of 411 W g compared to 333 W g for [6] Fe3O4 NPs, indicating their superior heating efficiency. More recently, significant enhancement in heating efficiency could be achieved using exchange spring nanomagnets as they combine the best properties of magnetically hard and soft phases. The hard phase component provides a high Hc while the soft phase component promotes a higher Ms for the coupled system. In such nanomagnets, the soft phase is pinned to the hard phase at the interface, leading to a reversible demagnetization curve which is the main characteristic of these materials. Previously, exchange coupled nanomagnet composed of CoFe2O4@MnFe2O4 nanospheres (15 nm size) displayed a much higher SAR value of 2280 W g-1, compared to -1 -1 single component CoFe2O4 (443 W g ) and MnFe2O4 nanospheres (411 W g ) with average

sizes of 9 and 15 nm, respectively.[6] Furthermore, the heating efficiency of these nanomagnets could be tuned by combining different kinds of hard and soft magnetic spinel ferrites. Exchange coupled nanomagnets made of CoFe2O4@Fe3O4, Fe3O4@CoFe2O4,

CoFe2O4@MnFe2O4, MnFe2O4@CoFe2O4, and (Zn0.4Co0.6)Fe2O4@(Zn0.4Mn0.6)Fe2O4 core- shell exhibited superior high heating efficiencies between 1120 to 3886 W g-1, which were an -1 order of magnitude higher than pure Fe3O4, MnFe2O4, and CoFe2O4 NPs (339-443 W g ). Further, by tuning the shape of the core-shell nanostructures from nanospheres to nanocubes, SLP values as high as 10,600 W g-1 could be achieved as previously exhibited by

Zn0.4Fe2.6O4@CoFe2O4 core-shell nanocubes. Such high SLP was promoted by the very high

Hc of this nanomagnet resulting from exchange anisotropy. In recent years, the heat-generating properties of magnetic iron oxide NPs has been exploited for magnetically-guided cancer hyperthermia treatment.[100, 101] Often in such treatment, surface functionalization or modification is often employed to improve the biocompatibility of the magnetic iron oxide NPs and to enhance their heating efficiency. For example, the Wu group previously investigated the heating properties of Fe3O4@alginate

(Fe3O4@Alg) and galactosamine-functionalized Fe3O4@alginate (Fe3O4@Alg-GA) for [101] magnetic hyperthermia. The SAR values of Fe3O4, Fe3O4@Alg, and Fe3O4@Alg-GA were 192.8, 212.0, and 308.4 W g-1, respectively, thereby indicating the benefit of the applied surface modification and functionalization for enhancing the heating efficiency of Fe3O4 NPs.

The in vitro hyperthermia test using HepG2 cells revealed that for Fe3O4@Alg NPs, the cell viability decreased to around 60% after the magnetic field treatment. This effect was further enhanced when the cells were treated with Fe3O4@Alg-GA NPs with nearly all the cells were killed (5% viability). The enhanced hyperthermia effect was attributed to the increased internalization of the Fe3O4 NPs inside the cells. Chitosan oligosaccharide-stabilized ferrimagnetic iron oxide nanocubes (Chito-FIONs) were previously shown to be an effective heat mediator for cancer hyperthermia with a much higher SLP value (2614 W g-1) compared to single core FIONs (1792 W g-1) and commercial Feridex NPs (83 W g-1).[100] The enhanced heat generation ability of Chito-FIONs was promoted by the enhanced coercive field and stabilization of the magnetization of individual FIONs due to magnetic dipolar coupling between the neighboring FIONs in the interior of the Chito-FION particle. The localized hypothermia treatment of A459 cells treated with Chito-FIONs showed that they induced the thermal destruction of the target cancer cells more efficiently than Feridex owing to the increased facilitation of cellular apoptotic processes. Further, the hyperthermia treatment with

Chito-FIONs could induce remarkable suppression of tumor growth by 70% after 6 days, whereas the Feridex did not show any tumoricidal effect (Figure 12a, b). This phenomenon was attributed to the magnetically induced hyperthermic effects of Chito-FIONs. The histological assessment of the excised tumors revealed that unlike Ferridex, a large amount of Chito-FIONS remained within the tumor tissue after the treatment due to the strong electrostatic interaction between the cationic chitosan oligosaccharides and the negatively charged tumor cells (Figure 12c, d). The high tumor affinity of these NPs could be exploited for repeated irradiation in cancer thermotherapy. The emergence of magnetic-guided hyperthermia treatment provide the ability to analyze the local concentration of magnetic NPs trapped inside diseased tissues, thus allowing for a more defined treatment plan to avoid the side effects of overheating.[148] Moreover, the magnetic field employed in hyperthermia could be also used in magnetic imaging, thereby enabling the development of a coupled theranostic approach for cancer treatment.

5.1.4. Biosensors Biosensors rely on the immobilization of biomolecules for detecting and identifying target analytes. In biosensors, it is essential that specific biomolecules are attached to the surface of a signal transducer.[149] Following interaction with the analyte, the biological-recognition event produces an optical or electrical signal. Hence, the immobilization of biomolecules is very important for designing biosensors with high stability. In recent years, nanomaterials have aroused much interest for biosensing applications owing to their high surface to volume ratio, high surface reactivity, excellent catalytic activity, and strong adsorption capability to promote immobilization of the desired biomolecules.[99] In principle, an ideal biosensor should exhibit high sensitivity and selectivity, good stability, high resistance to aggressive/corrosive media, low fabrication cost, and possibility for automated procedure.133 For biosensing applications, the surface functionalization of iron oxide nanostructures is often necessary to promote molecular interactions with the target molecules. This section focuses on the utilization of iron oxide-based nanomaterials in colorimetric and electrochemical biosensors.

5.1.4.1. Colorimetric biosensors/Nanozymes

The enzyme-mimicking ability of iron oxide NPs was first discovered in 2007, in which ferromagnetic Fe3O4 NPs were shown to exhibit intrinsic peroxidase-like activity with catalytic behaviour identical to horseradish peroxidase (HRP).[150] Since then, many other nanomaterials, including metals, metal oxides, carbons, and metal-modified carbons have been demonstrated to show enzyme-like activities with catalytic behaviours resembling peroxidase, catalase, glucose oxidase, sulfite oxidase, haloperoxidase, superoxide dismutase and NADH peroxidase.[151-155]. Thus, the term "nanozymes" has been coined to refer to nanomaterials with enzyme-mimicking abilities and to distinguish them from externally- immobilized enzymes.[156] Iron oxide nanozymes have gained significant attention for their peroxidase-like and [156] catalase-like activities under physiological reaction conditions. Although both Fe2O3 and

Fe3O4 show peroxidase-like activity, it is generally accepted that Fe3O4 nanostructures tend to exhibit higher catalytic activity than Fe2O3 ones. To date, various substrates, such as 3,3′,5,5′- tertamethylbenzidine (TMB)[157], 2, 2'-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS)[158], and 3, 3'-diaminobenzidine (DAB)[159] have been investigated to showcase the peroxidase-like activities of iron oxide nanozymes. The enzyme-like activity of iron oxide NPs originates from the presence of ferrous ions at their which can interact with the substrate in the presence of H2O2, resulting in a color change. The mechanism follows the Fenton reaction and can be written as follows:

2AH + H2O2 → 2A + 2H2O (AH = hydrogen donor) (6)

The catalytic conversion of H2O2 and TMB follow the Michaelis-Lenten kinetics and the apparent steady-state kinetic parameters, such as KM, Vmax, and Kcat (Eq. 7 and Eq. 8) can be calculated to determine the affinity of a particular substrate to the enzyme (KM) and the [156] catalytic activity (Kcat or Kcat/KM).

υ = (Vmax[S])/(Km + [S]) (7)

Kcat =Vmax/[E] (8) where υ is the initial reaction velocity, Vmax is the maximal reaction velocity, [S] is the substrate concentration, Km is the Michaelis-Menten constant, Kcat is the catalytic efficiency, and [E] is the enzyme concentration. This color change phenomenon has been exploited for [160] [160, 161] [162] [163] [164] the detection of H2O2 , glucose , dopamine , acetylcholine , autoantibody , phenol[165], etc.

Iron oxide nanozymes have been considered as promising alternatives to natural enzyme horseradish peroxidase (HRP) due to several important advantages, including higher stability, tunable activity, and multifunctionality. Most importantly, however, various iron oxide-based nanozymes have shown much higher kinetics compared to native HRP. For instance, Fe3O4 magnetic NPs (MNPs, 300 nm in size) showed 40 times higher activity (i.e.,

KM value) than HRP for the oxidation of TMB in the presence of H2O2. The much higher activity may have arisen from the fact that the HRP molecule possessed only one Fe ion, [150] whereas Fe3O4 NPs possessed many Fe ions on their surface. Apart from higher activity, iron oxide nanozymes are also more stable under acidic and basic environments or elevated [166] temperatures. Liu et al. demonstrated that protein-modified Fe3O4 nanozyme exhibited high stability over a wide range of pH (from 1 to 12) and temperatures (4 to 90 °C). In contrast, HRP showed rapid loss of activity when the temperature was raised higher than 40 C and it also did not show any activity when the pH was lowered below 5. Furthermore,

Fe3O4 nanozyme has shown great recyclability as they could maintain high level of activity even after several cycles of usage.[165] Additionally, it is also possible to further improve the stability of Fe3O4 nanozyme via modification with RGO, as proven by the study of Qian and co-workers, in which Fe3O4 nanospheres/RGO composites stored at 4 °C could remain stable for over 3 months.[163] The other important advantage of iron oxide nanozymes is the possible tuning of their activity by modifying their size and shape, functionalization with activators or other types of nanomaterials and by doping or hybridization. The catalytic activity of iron oxide nanozymes generally decreases with increasing particle size due to the smaller surface area to interact with substrates. For instance, the catalytic activity of Fe3O4 MNPs was previously found to be in the order of 30 nm > 150 nm > 300 nm, indicating the reduction in catalytic activity with increasing particle size.[150] A similar trend was also observed in a [167] previous report by Peng et al. , in which Fe3O4 NPs with smaller average diameter of 11 nm displayed better catalytic activity than their larger-sized counterparts (20 and 150 nm).

Apart from size, the shape or morphology of the iron oxide nanozymes can also affect their catalytic activity due to the preferential affinity toward substrates. The Zhu group previously investigated three different morphologies of Fe3O4 NCs, such as cluster spheres, octahedra, and triangular plates for TMB oxidation.[170] Their study revealed the superior affinity of the Fe3O4 cluster spheres for TMB as it exhibited the highest surface area among the three samples. Interestingly, despite exhibiting very similar surface area, the Fe3O4

triangular plates showed superior activity to the Fe3O4 octahedra. This was attributed to the higher reactivity of surface with {220} planes than that with {111} planes owing to the open plane and dangling bonds. The shape effect on the peroxidase-like activity was also observed for CoFe2O4 NPs in the order of spheres > near corner-grown cubic > star-like > near cubic > polyhedral (Figure 13a, b). Furthermore, the spherical CoFe2O4 NPs (average size of 4 nm) [168] also showed higher affinity for H2O2 or TMB than both HRP and Fe3O4 NPs. Another effective method for tuning the activity of iron oxide nanozyme is through metal doping or hybridization with other nanomaterials. First, metal dopants, such as Ag, Au, and Pt can enhance the peroxidase-like activity of iron oxide nanozyme by promoting synergistic effect, polarization effect at the metal/iron oxide interface, and so on.[156] For instance, the peroxidase-like activity of Au@Fe3O4 NPs was previously found to be 1.3 times and 5.2 [171] times higher than that of pure Fe3O4 NPs and Au NPs, respectively. In a related report, dumbbell-like Au-Fe3O4 NPs displayed higher catalytic activity for H2O2 reduction than pure

Au NPs and Fe3O4 NPs, owing to polarization effect at the Au/Fe3O4 interface, which [172] enhanced the activity of the Fe3O4 NPs. Furthermore, Ag@Fe3O4 nanowires showed [173] around 6-7 times higher peroxidase-like activity than Fe3O4 NPs and Ag nanowires. Pt- modified Fe3O4 NPs (Fe3O4@Pt NPs) showed both higher affinity to TMB and enhanced catalytic activity relative to pure Fe3O4 NPs due to the improved binding between Pt NPs and [174] TMB via Pt-NH2 interaction. Apart from noble metals, transition metals (e.g., Ni, Co, Mn, Cu) which can combine with iron oxide to generate ferrite materials could also the enhance [175] the catalytic activity of iron oxide nanozyme. Phosphonate-modified Fe(1-x)MnxFe2O4 NPs, possible tuning of the peroxidase-like activity could be achieved by adjusting the proportion of Mn2+.[176] It was proposed that Mn ions (Mn2+) on the surface of the mixed ferrite NPs could form the redox couple Mn2+/Mn3+ to produce hydroxyl radicals which could catalyze the oxidation of TMB in H2O2. Similarly, cobalt ferrite (CoxFe3-xO4) MNPs also exhibited superior activity to Fe3O4 NPs for the catalytic oxidation of TMB despite the strong similarities in size distribution, shape, and crystal structure.[162] Alternatively, the enhancement in the peroxidase-like activity of iron oxide nanozyme through synergistic effects can also be achieved by hybridization with various carbon-based nanomaterials. MWCNTs possess large surface area, hollow tubular structures, and good electron transfer properties promoted by the presence of distinctive sp2 hybridized carbon bonds. Magnetic Fe3O4-multi-walled CNT (MWCNT) hybrid has been used to overcome the low pH limitations of Fenton reaction through its peroxidase-mimicking ability over a wide

[177] range of pH (1-10). Unlike pure Fe3O4 and MWCNT, the Fe3O4/MWCNT hybrid could catalyze the oxidation of TMB by H2O2 in sodium acetate buffer and generate a more intense colour change, indicating its higher peroxidase-like activity promoted by the good dispersion of the Fe3O4 NPs on the surface of MWCNTs. In other report, GO has been used to increase [161] the affinity of Fe3O4 NPs for H2O2 as well as to increase their peroxidase-like activity. The enhanced affinity was most likely due to additional contribution from the peroxidase-like activity of GO itself, as previously identified by the Qu group.[151] In addition to GO, rGO has also been utilized to improve the catalytic activity of Fe3O4 nanospheres for the oxidation of acetyl choline (Ach) due to its excellent conductivity and large surface area (Figure 13c- [163] g). In recent years, more complex hybrids, such as graphene quantum dots (GQDs)/Fe3O4 hybrids have displayed exceptional peroxidase-like activities, which were considerably higher than those of pure GQDs, pure Fe3O4 NPs, and RGO/Fe3O4 hybrid. The excellent peroxidase activity of these hybrids was promoted by the unique properties of GQDs and the synergistic interactions between the GQDs and Fe3O4 NPs. The peroxidase-like activity of iron oxide nanozyme could also be enhanced through modification with molecules possessing similar structures as the active site of natural enzyme.[156] PB has excellent electrochemical properties which can speed up electron transfer along with good catalytic performance toward the reduction of hydrogen peroxidase. The modification of Fe3O4 NPs with PB was previously shown to enhance their peroxidase-like activity by nearly three orders of magnitude and the activity was increased with increasing PB proportion.[178] Here, the PB hybridization could provide more ferrous ions to interact with the TMB substrate. Furthermore, the negatively charged surface of the PB/Fe3O4 hybrid could enhance the affinity between the catalyst and the positively charged substrate. Histidine modification has been shown to improve the affinity of Fe3O4 nanozyme to H2O2 by nearly 10-fold due to the presence of imidazole group in histidine residues which could mimic the enzyme microenvironment of natural HRP.[179] Additionally, the modification of iron oxide nanozyme with biomolecules can yield significant improvement in their peroxidase-like activity (Figure 13h). For example, DNA-capped Fe3O4 NPs showed 10 times higher activity for TMB oxidation compared to pristine Fe3O4 NPs and the enhancement in activity was more obvious with increasing length of DNA (Figure 13i, j).[169] However, replacing the TMB substrate with negatively charged ABTS substrate, the DNA inhibited the oxidation process instead. This observation revealed that the negatively charged phosphate backbone and base of DNA was responsible for the increased TMB binding as well as the enhanced

activity of DNA-capped Fe3O4 NPs. The tuning of the peroxidase-like activity of Fe3O4 nanozyme could also be achieving using natural biomolecules, such as magnetoferritin containing different iron content.[180] Similarly, the incorporation of the protein, casein onto

MNPs also enhanced their affinity toward both H2O2 and TMB in acidic solutions, while simultaneously enhancing their stability and dispersibility in aqueous medium.[166]

5.1.4.2. Electrochemical biosensors – nonenzymatic biosensors Electrochemical biosensors work by converting biological reaction into electronic signal. According to some reports, electrochemical biosensors can show better sensitivity compared to colorimetric detection along with other advantages, such as operational simplicity, real- time measurements, low cost, and high selectivity.[181] As of now, numerous efforts have been devoted to the preparation of nanomaterials for electrochemical biosensors owing to their capability to immobilize biomolecules.[149] In the past, iron oxide nanoarchitectures have been widely applied in electrochemical non-enzymatic sensing of various biomolecules, as summarized in Table 2. This is due to their good biocompatibility, easy preparation, low toxicity, and magnetic properties.[181] Although they have also been used in electrochemical enzymatic sensing of the above listed biomolecules, this topic will not be covered in this section and excellent reviews on this topic have been published elsewhere.[149, 181]

H2O2 is commonly used as oxidation agent in chemical, food, and pharmaceutical industries and several iron oxide-based electrochemical nonenzymatic sensors for H2O2 have been reported. For instance, α-Fe2O3 NPs-modified electrode showed high sensitivity (84.32 μA mM-1 cm-2), wide detection range (0.001-6.0 mM), low detection limit (1.1 μM), excellent reproducibility, good anti-interference for uric acid and ascorbic acid, and excellent long- [182] term stability for the electrochemical biosensing of H2O2. Fe3O4- poly(diallyldimethylammonium chloride) (PDDA) composite electrode prepared by layer-by- layer approach exhibited low detection limit of 1.4 μM for H2O2 along with good selectivity and good retainment of the initial response after 50 days (92.3%). Hbarc and co-workers have tested the performance of carbon paste electrodes modified with various ferric oxides (e.g., α-

Fe2O3, β-Fe2O3, γ-Fe2O3, amorphous Fe2O3, and ferrihydrite) for H2O2 reduction and [183] compared them against PB- and Fe3O4-modified carbon paste electrodes. Their study revealed the following trend: PB-modified >amorphous Fe2O3-modified > Fe3O4-modified >

α-Fe2O3-modified > β-Fe2O3 and γ-Fe2O3-modified > ferrihydrite modified. Furthermore, apart from the type of modifier, the modifier content and pH also had profound effects on the

sensitivity. For amorphous Fe2O3-modified electrode, at concentration below 2.5% or above

30%, it was not sensitive toward H2O2 and it showed maximum sensitivity at an optimum concentration of 15%. Moreover, in neutral pH environment, this particular electrode could show higher sensitivity than PB-modified electrode, while it was less effective under acidic environment.

Apart from H2O2, nonenzymatic iron oxide-based sensors have also been tested for the detection of other important analytes, such as glucose, dopamine (DA), folic acid (FA), [181] nitrite, chloroform and others. Previously, Fe3O4 NPs-modified Au electrode was tested for the voltammetric sensing of DA and it showed an impressive low detection limit of 3 x 10-8 M and good stability with only a decrease of 2.30% in sensitivity after a week.[184]

Further, β-Fe2O3-modified electrode was utilized to detect chloroform and showed a moderate sensitivity of 2.18 μA mM-1 cm-2, with a detection limit of 4.4 μM and short response time of

10.0 s. Hollow α-Fe2O3 nanopolyhedrons with exposed high index {102} facets could be used to sensitively and selectively detect nitrite in the presence of glucose and L-ascorbic acid (AA) with a fast response time (<10 s), a linear range between 0.009 and 3 mM, and a -1 [185] high sensitivity of 19.83 μA mM . The catalytic mechanism of Fe2O3 for nitride oxidation is expressed by the following equations: - - 2 Fe(III) + N(III)O2 → 2 Fe(II) + N(V)O3 or (9)

- - Fe(IV) + N(III)O2 → Fe(II) + N(V)O3 (10)

The enhanced sensitivity and fast response time were resulted from the large number of stepped atoms on the high index facets. Fe2O3 nanowire arrays were previously demonstrated to be catalytically active for the oxidation of glucose with a rapid response time of <6 s and a high sensitivity of 726.9 mA mM-1 cm-2 along with high selectivity to glucose.[186] The catalytic mechanism of Fe2O3 for the oxidation of glucose can be expressed as:

2 Fe(III) + Glucose → 2Fe(II) + Gluconolactone + H2O (11)

+ Gluconolactone + H2O → 2H + gluconate (12)

2 Fe(II) → 2 Fe(III) + 2e- (13)

The high sensitivity and rapid response of the nanowire arrays were attributed to the formation of three 3D networks which could provide large contact area between the electrode and the analyte and lower resistance to diffusion onto the electrode surface for the analyte ions due to the small size of the nanowires. Zhang et al.[187] have compared the catalytic performance of both Fe3O4 and α-Fe2O3 nanorod arrays toward glucose oxidation. They

found that the Fe3O4 nanorod arrays were more sensitive toward glucose due to the acceleration of the activity as a result of the hoping between Fe2+ and Fe3+ at the surface of

Fe3O4 and the higher conductivity of Fe3O4. The Fe3O4 nanorods arrays exhibited a -2 -1 sensitivity of 406.9 μA cm mM and low detection limit is 0.1 μM. In comparison, Fe3O4 nanotube arrays (Figure 14a) displayed a higher sensitivity of 673.3 μA cm-2 mM-1 with a similar detection limit of 0.1 μM (Figure 14b) and good selectivity for glucose sensing and such excellent catalytic performance was correlated to the tube-like structure which provided large surface to volume ratio and increased active sites.[188] To enhance the electrochemical sensing properties of iron oxide nanostructures for biomolecule detection, modifications with carbon-based materials, noble metals, and polymers have been proposed. For example, mechanochemically-synthesized Fe3O4-rGO electrode displayed a superior low detection limit of 0.03 μM and high sensitivity of 202.5 μA cm-2 mM-1 for nitrite sensing.[189] The enhanced sensitivity of this hybrid-modified electrode was due to the increased amount of adsorption sites for the negatively charged nitrite ions and increased conductivity (lower charge transfer resistance) contributed by the large surface area of rGO. Similar synergistic effect was observed in Fe3O4/RGO-modified electrode used in the detection of H2O2 which showed wide linear range between 4 μM to 1 [190] mM. In comparison to Fe3O4-rGO, hydrothermally-synthesized Fe2O3-rGO (Figure 14c) displayed a lower sensitivity of 126.9 μA cm-2 mM-1 and a higher low detection limit of 1.0 [191] μM (Figure 14d). Further, ternary PANI-Fe2O3-rGO composite (Figure 14e) exhibited superior electrocatalytic activity for the sensing of hydroquinone (HQ) [192] relative to bare

PANI and Fe2O3-rGO composite, with impressive low detection limit of 0.06 μM and linear range between 1.0 x 10-7 -5.5 x 10-4 M (Figure 14f). The improved activity of this ternary composite was attributed to the reduced diffusion length of the analyte ions due to the interconnected structure of the composite and the large surface area of the rGO sheets which offered plentiful sites for the adsorption of ions.

Apart from rGO, Fe3O4/polythiophene (PT) hybrid has also shown enhanced detection −1 for H2O2 with sensitivity of 3.1 μA mM and detection limit of 5 μM. In other report, Masud [164] et al. reported the significant enhancement in catalytic properties of porous Fe2O3 nanocubes for TMB oxidation through the loading of Au NPs. This enhancement was attributed to the provision of a larger contact area to promote improved interaction with the positively charged TMB. Metal oxides can also enhance the electroactivity of iron oxide nanostructures. As an example, Fe3O4@NiO core-shell modified electrode has shown

potential for the simultaneous detection of quercetin and tryptophan with wide linear ranges of 0.08-60 and 0.1-120 μM and detection limits of 2.18 and 14.23 nM for quercetin and tryptophan, respectively.[193] The excellent electroactivity of this core-shell hybrid was attributed to the increased active sites and anti-self-aggregation properties of NiO NPs. In a related report, microwave-synthesized Fe2O3@SiO2 exhibited high sensitivity (12.658 μA mM-1 cm-2), linear range of 0.2 nM to 2.0mM, and impressive low detection limit of 76 pM, owing to the enhanced electron transfer between the active sites and coated electrode.[194] The above studies clearly highlight the effective enhancement in sensitivity and detection limit of iron oxide-based biosensors through hybridization, which may bring them a step closer toward practical use in food-safety inspection, and environmental-pollution control. However future studies should focus on resolving problems associated with matrix interference and sensor fouling arising from the large surface area of NPs as well as improving the long-term stability of these iron oxide-based biosensors. 5.2. Energy storage applications Energy storage devices plays a key role in the development of sustainable energy systems. Although environmentally-friendly energy sources, such as wind turbines and solar cells can produce energy in a sustainable manner, their intermittent nature have prevented them from becoming the primary sources of energy. Electrochemical energy storage devices, such as lithium-ion batteries (LIBs) and supercapacitors (SCs) which store energy by means of charge/discharge of ions or electrons play significant roles in our daily lives as they are commonly found in electronic devices, electric cars, power back-ups, smart grids, and so on. In this section, the development of iron oxide-based anode materials for LIBs and SCs is reviewed.

5.2.1. Lithium-ion batteries (LIBs) Since their first development in 1991, LIBs have remained as the dominant energy storage devices in portable electronic devices and electric cars. LIBs possess several major advantages, including high energy density, long cycling life, minimal memory effect, and rapid charging.[3] However, LIBs also exhibit some disadvantages, including low power density and safety concerns. The low power density of LIBs arises from the degradation of the compounds or materials inside the batteries overtime as a result of their slow charge/discharge process. Furthermore, LIBs may explode when overcharged or overheated due to the buildup of internal gases released by the electrolyte decomposition. The

development of high-performance anode materials for LIBs can lead to higher electrochemical performance, including higher power densities, enhanced safety, and longer cycling life. Carbon-based materials, especially graphite has been widely used as the commercial anode material in LIBs, however it suffers from limited capacity (372 mAh g-1), poor cycling life, and some safety concerns.[195] Therefore, the development of new advanced anode materials with higher capacity, longer cycling life, and improved safety is strongly demanded.[196] Metal oxides have been extensively investigated as potential anode materials for LIBs since they exhibit high theoretical capacities.[197] In principle, the reversible conversion reaction between lithium ions and metal oxides lead to the formation of metal nanocrystals dispersed in a Li2O matrix, as expressed by the equation:

(M = metal) (14)

Fe2O3 can react with 6 Li per formula unit via the following reaction which gives rise to a high theoretical capacity of 1007 mAh g-1: - Fe2O3 + 6Li + 6e ↔ 3Li2O + 2Fe (15)

In comparison, Fe3O4 can store 8 Li ions per formula unit giving rise to a high theoretical -1 [198] capacity of 926 mAh g . The conversion reaction for Fe3O4 can be expressed as: + - Fe3O4 + 8 Li + 8e ↔ 4 Li2O + 3 Fe (16)

However, bulk metal oxides, including bulk iron oxides suffer from poor ion/electron transfer and fast capacity fading, resulting in much lower specific capacitance than the theoretical value.[197] Furthermore, they usually undergo large volume variations during the charge- discharge process, leading to short cycling life. Porous or hollow metal oxide materials are expected to deliver better performance for LIBs because of their large surface area and shorter diffusion paths.[199] To date, iron oxide nanostructures with different architectures from 1D to 3D have been investigated as potential anode materials for LIBs, as summarized in Table 3. 1D nanoarchitectures typically exhibit efficient charge transport along the growth direction, leading to enhanced kinetics and better rate capability. As an example, α-Fe2O3 nanorods synthesized via solvothermal treatment of ferric chloride and 1,2-diaminopropane at 220 °C displayed high initial reversible capacities of 908 mAh g-1 and 837 mAh g-1 at 0.2 C and 0.5 C rates, respectively.[200] Furthermore, these nanorods also exhibited a stable capacity of ~970 -1 -1 mAh g after 90 cycles at 0.5 C (503 mA g ). In contrast, micrometer-sized α-Fe2O3 particles

displayed a much lower specific capacity of around 200 mA g-1 after only 30 cycles at 0.5 C. + The enhanced cycling performance of the α-Fe2O3 nanorods was attributed to shorter Li diffusion paths and better strain accommodation. Iron oxide nanotubes templated by microporous organic nanotubes (produced through Sonogashira coupling between tetra(4- ethynylphenyl)methane and N,N’-di(4-iodophenyl)-4,4’-bipyridinium dichloride).[201] These iron oxide nanotubes displayed high discharge capacities of 918 mAh g-1 and 882 mAh g-1 at current densities of 0.5 A g-1 and 1 A g-1, respectively. Furthermore, a high capacity of 928 mAh g-1 was maintained after 30 cycles at 50 mA g-1. The excellent electrochemical performance of these nanotubes was contributed by their hollow tubular architecture which provided a greater amount of electrochemical reaction sites and improved structural flexibility. Furthermore, porous α-Fe2O3 nanowires prepared by the solvothermal reaction of ferric chloride with NTA at 180 °C showed a high initial discharge capacity of 1303 mAh g-1, -1 [202] which was higher than the theoretical capacity of Fe2O3 (1007 mAh g ). This extra capacity beyond the theoretical value was attributed to the decomposition of non-aqueous electrolyte during the discharge process. However, after 100 cycles, the discharge capacity was only 456 mAh g-1, corresponding to a relatively low capacity retention of 35.3%. 2D nanoarchitectures have aroused significant interest for LIBs owing to their large surface area and pore volume allowing for transport of electrons and Li-ions. Furthermore, they often show greater capability to withstand structural variation caused by lithium insertion and extraction, leading to higher reversible capacities. 2D α-Fe2O3 nanosheets prepared via coprecipitation method followed by thermal treatment in air at 400 °C showed a higher reversible capacity of 1327 mAh g-1 at a rate of 1 C, which only slightly decreased to 1215 mAh g-1 when the rate was increased to 3 C.[203] The higher reversible capacity of the α-

Fe2O3 nanosheet anode compared to theoretical value was attributed to the formation of an SEI layer via the reduction of electrolyte at low potential to form a solid electrolyte interface (SEI) layer and possible interfacial lithium storage. In comparison, the anode prepared using -1 α-Fe2O3 NPs showed a lower reversible capacity of 1006 mAh g at 1 C, which decreased -1 further to 812 mAh g . The improved rate performance of the α-Fe2O3 nanosheet anode was contributed by the smaller grain size and higher electrical conductivity of the nanosheets, leading to lower electron transport resistance. In other report, α-Fe2O3 nanosheet arrays deposited on tin foil via galvanostatic electrochemical deposition followed by calcination at 400 °C under inert atmosphere exhibited a high reversible capacity of 986.3 mAh g-1 when a current density of 100 mA g-1 was applied.[204] More impressively, a relatively high capacity

of 425.9 mAh g-1 was still maintained even when the current density was raised by 100-fold -1 to 10 A g . The excellent rate capability of the α-Fe2O3 nanosheet arrays was promoted by the rapid electron transport, large electrolyte/electrode interfaces, and good accommodation of the volume expansion by the highly interconnected structure. Hierarchical 3D nanostructures can address the drawbacks of 0D, 1D and 2D nanostructures for LIBs by providing better structural stability, improved rate performance, and enhanced cyclability.[205] Furthermore hierarchical nanostructures with hollow architectures typically also exhibit enhanced capacity retention due to better accommodation of the volume variation during cycling and significantly reduced aggregation of the assembling sub-units. In addition, hollow architectures can provide greater contact surface between the active material and the electrolyte, leading to improved electrochemical performance. For instance, hierarchical Fe3O4 hollow spheres constructed by 2D nanosheet sub-units (thickness of around 10 nm) exhibited discharge capacities of 992, 853, 716, 548, and 457 mAh g−1 at current densities of 1, 2, 4, 8, and 10 A g−1, respectively, indicating their [206] excellent rate performance. Stability-wise, these hierarchical Fe3O4 hollow spheres could maintain a high stable capacity of around 700 mAh g-1 even after 200 cycles at 3 A g-1. Mesoporous iron oxides have also been tested as anode materials for LIBs as they can provide increased contact area with the electrolyte and facilitate improved transport of electrons and Li ions. Mesoporous α-Fe2O3 synthesized using Pluronic123 (P123) as a soft template displayed a high first discharge capacity of 1730 mAh g-1 (corresponding to 10.3 -1 mol of Li per mole of Fe2O3) which was reduced to 1293 mAh g after 200 cycles at 0.2 C (200 mA g-1).[207] Furthermore, when the rate was increased to 2 C (2 A g-1), a high discharge -1 capacity of 870 mAh g was retained. In a related report, spindle-like mesoporous α-Fe2O3 derived from the calcination of the MOF, Fe-MIL-88, in air at 380 °C exhibited a high initial charge capacity of 940 mAh g-1 which was still maintained at 911 mAh g-1 after 50 cycles at [208] -1 0.2 C. In contrast, bulk Fe2O3 displayed a lower charge capacity of less than 630 mAh g after 50 cycles. Furthermore, the spindle-like mesoporous α-Fe2O3 anode exhibited higher charge capacities of 861 and 424 mAh g-1 at 1 C and 10 C, respectively, compared to the bulk -1 Fe2O3 anode (607 and 154 mAh g ). The enhanced rate performance of the mesoporous α-

Fe2O3 anode was attributed to the interconnected nanoscale subunits and smaller size of the assembling Fe2O3 NPs, leading to shorter distance for Li-ions transport.

Despite exhibiting high specific capacity, anode materials based on pure iron oxide nanostructures, typically suffer from poor high rate performance and poor cycling stability at higher rates due to large volume variation during charge/discharge process. To address these problems, iron oxide nanostructures are typically hybridized with carbon-based materials, such as reduced graphene oxide (rGO) nanosheets[96], graphene nanosheets[209], CNTs[89] or [212] carbon nanofibers (CNFs) . For example, rGO/Fe2O3 hybrid was previously synthesized through homogeneous precipitation followed by reduction of the graphene oxide with [96] hydrazine (N2H4) under microwave irradiation. The rGO/Fe2O3 anode showed high initial discharge capacity of 1693 mAh g-1 at a current density of 100 mA g-1 which decreased to -1 1027 mAh g after 50 cycles. In comparison, the discharge capacity of the pure Fe2O3 NPs rapidly decreased from to 1542 to 130 mAh g-1 only after 30 cycles. These results suggest the positive synergistic effect of rGO nanosheets and Fe2O3 NPs. Similarly, graphene nanosheets

(GNS) decorated with Fe3O4 NPs showed enhanced lithium storage performance for LIBs compared to commercial Fe3O4 and bare Fe2O3 in terms of rate capability, cycling stability, [209] and lithium storage capacity (Figure 15a-c). The GNS/Fe3O4 hybrid exhibited a high reversible capacity of 1026 mAh g-1 after 35 cycles at 700 mA g-1, whereas commercial -1 Fe3O4 and bare Fe2O3 electrodes showed lower capacities of 475 and 359 mAh g , respectively. The high reversible capacity and improved rate performance of the GNS/Fe3O4 hybrid were attributed to the creation of interleaved electron transport highways by the GNSs and the flexible confinement effect of the GNSs which limited volume expansion and prevent agglomeration of the Fe3O4 NPs during cycling. Apart from graphene-based materials, iron oxide NPs (i.e., α-Fe2O3 NPs) have also been loaded on CNFs to enhance their lithium storage performance. This composite was fabricated by electrospinning of polyacrylonitrile (PAN) in a solution containing ferric chloride followed by calcination in air at 280 °C followed by a secondary annealing in N2 atmosphere at 600 °C. The α-Fe2O3-CNF anode showed a reversible capacity of 288 mAh g-1 at a high current density of 500 mA g-1 and a constant capacity of 488 mAh g-1 after 75 cycles at 50 mA g-1.[212] Here, the 1D fibrous structure provided a continuous conducting pathway for the transport of Li ions and electrons and increased the electrode/electrolyte contact area, leading to enhanced rate performance. In addition, the CNFs helped to preserve the structural integrity of the electrode during cycling and therefore improved the cycling performance Alternatively, the lithium storage performance of iron oxide nanostructures can also be improved by coating with carbon or conductive polymer (e.g., polyaniline (PANI)).

Carbon coating can enhance the electrical performance of iron oxide nanostructures for LIBs by enhancing their electrical conductivity and preventing their aggregation during repeated charge/discharge processes, leading to improved rate performance and cycling stability, respectively.[3] The Yu group has demonstrated the possible tuning of the electrochemical performance of FeOx@C composite anode by controlling the void size of the yolk-shell [83] structure. It was found that FeOx@C composite with small void space could not retain the yolk-shell structure after cycling, indicating insufficient space to accommodate the volume expansion, thus resulting in rapid capacity fading due to the exposure of the electrode to the electrolyte which induced aggregation of the active material. On the other hand, for the

FeOx@C composite with a big void space, a large void was observed on one end of the carbon shell after cycling, suggesting that although the volume expansion was accommodated, poor contact between the FeOx core and the conductive carbon shell prevented effective transport of electrons and Li ions to from/to the core, thereby leading to lower volumetric capacity. In contrast, for the FeOx@C composite with the optimized void space, the FeOx core could freely expand or contract upon lithiation/delithiation without destroying the carbon shell. In turn, the intact carbon shell prevented the continuous rupturing and reformation of SEI by offering a stable surface for the growth of SEI layer and thus good cycling stability was achieved. This study clearly indicated the importance of optimizing the void space for achieving high cycling stability while also maintaining a relatively high volumetric energy density. In a related report, Fe3O4@C nanospindles (Figure 15d) showed an initial discharge capacity of 749 mAh g-1 at 0.5 C which decreased to 530 mAh g-1 after 80 [210] cycles (Figure 15e). In comparison, bare Fe2O3 nanospindles and commercial Fe3O4 exhibited much lower discharge capacities of 105 and 181 mAh g-1 after 80 cycles.

Furthermore, the Fe3O4@C nanospindles also showed superior rate performance than both materials, as shown in Figure 15f. At a high rate of 5 C, the specific charge capacity of -1 Fe3O4@C nanospindles remained at 190 mAh g , whereas α-Fe2O3 nanospindles and commercial Fe3O4 particles had almost zero capacities at the same rate. TEM analysis of the electrodes after several cycles revealed that the carbon coating was beneficial for protecting the inner active material and limiting the formation or decomposition of the SEI layer, thereby enhancing both rate performance and cycling stability.

Conductive polymers (e.g., PANI, poly (3,4-ethylenedioxy thiophene) (PEDOT), and polypyrrole (PPy) can provide high electrical conductivity (up to 103 S cm-1), high mechanical strength and improved structural stability during cycling.[213] For example,

hierarchical Fe2O3@PANI hollow spheres were previously investigated as potential anode material for LIBs and they were shown to deliver a stable capacity of 893 mAh g-1 after 100 [211] cycles at 0.1 C. In contrast, non-hollow Fe2O3 and urchin-like Fe2O3 electrodes exhibited lower capacities of 680 and 723 mAh g-1 after 100 cycles at 0.1 C. The improved electrochemical performance was accounted for by the lower resistance to charge transfer contributed by the increased electrode/electrolyte interfacial contact area and the overall increase in electrical conductivity. In other report, mesoporous poly(ST-AN) (PSA)-

Fe3O4@C composite microspheres derived from the carbonization of the polymer PSA via hydrothermal approach could deliver a higher capacity of 887 mAh g-1 at a rate of 1 C -1 -1 [214] relative to Fe3O4@C nanospheres (837 mAh g ) and pure Fe3O4 NPs (590 mAh g ).

Furthermore, the PSA-Fe3O4@C composite microspheres could maintain a high discharge capacity of 599 mAh g-1 after 100 cycles at 10 C. The enhanced rate performance and stability of these hybrid microspheres were accounted for by (i) the highly conductive PSA@C 3D frameworks which provided efficient electron transport and Li-ion diffusion at the electrode/electrolyte interface along with enhanced structural stability ; (ii) the carbon shell on the Fe3O4 NPs which could accommodate their large volume expansion during charge/discharge and hence prevent their aggregation during cycling, and (iii) the hierarchical mesoporous structure which provided appropriate diffusion lengths for Li-ions as well as larger contact area with the electrolyte. Apart from polymers and carbon-based materials, the coating of iron oxide nanostructures with other metal oxides can lead to higher specific capacities and better cyclability. Very recently, hollow Fe2O3@SnO2 nanorods synthesized by the inside-out

Ostwald ripening of SnO2 NPs displayed a higher reversible specific capacity of 570.7 mAh -1 -1 -1 g compared to 140 mAh g for the pure Fe2O3 nanorods at a current density of 0.2 A g [215] after 100 cycles. Furthermore, the Fe2O3@SnO2 core-shell composite exhibited good rate performance with average charge capacities of 1200, 1000, 500, 300, and 110 mAh g-1 at 0.1, 0.2, 0.5, 1, and 2 A g-1, respectively. The core-shell nature and the hierarchical porous structure of this composite was important for alleviating the volume expansion caused by the charge-discharge process and for enhancing the transport of lithium ions. To further enhance the lithium storage performance of metal oxide hybrids, additional modifications with carbon-based materials have been proposed. For instance, Fe2O3-SnO2/graphene hybrid exhibited an ultrahigh reversible capacity of 1530 mA g-1 after 200 cycles at 0.1 A g-1, along with an excellent rate capability of 615 mAh g-1 at 2 A g-1.[216] The synergistic effects of the

three composing materials combined with the highly conductive graphene matrix were responsible for the enhanced rate capability and cycling performance. In other study, complex

Fe2O3@C@MnO2@C (FCMC) multi-shell composite showed a high reversible capacity of −1 -1 1089 mAh g after 50 cycles at 0.5 A g , compared to Fe2O3@C@MnO2 (FCM), α- -1 Fe2O3@C (FC), and α-Fe2O3@MnO2 (FM) (523, 339, and 473 mAh g , respectively). The presence of the double carbon layers in this hybrid could not only prevent the core-shell FCMC from collapsing during the lithium insertion/extraction but also improve the electric conductivity and lower the transfer resistance of lithium ions.

5.2.2. Supercapacitors (SCs) SCs, also called electrochemical capacitors, have attracted significant research interests as alternatives to LIBs due to their long durability, high power density, short charging duration, fast energy delivery, and eco-friendliness. Compared with LIBs, SCs possess lower energy density, however they have almost infinite cycling lifespan.[217] Unlike LIBs, the charge storage mechanism of SCs does not involve chemical reactions and energy is simply stored electrostatically on the surface of the electrode material. Carbon-based materials are typically employed as electrodes materials for electrochemical double-layer capacitors (EDLCs) owing to their high electrical conductivity and large surface area. However, it is still difficult to utilize the whole surface area, which results in low maximum capacitance (typically ∼150- 200 F g−1). Moreover, the energy density of commercial carbon-based EDLC SCs is much lower than that of LIBs (usually around 3–5 W h kg−1).[218] On the other hand, transition metal oxides, including iron oxides with their pseudocapacitive characteristics exhibit high theoretical specific capacitance, low cost, ease of synthesis, wide abundance, and environmental friendliness. The storage mechanism of pseudocapacitors is based on the reversible redox reactions between the electrode and the electrolyte.[11] However, phase or microstructural change may occur as a result of these reactions, leading to rapid decay in capacitance. Therefore, iron oxide nanostructures have rarely been used alone as electrode materials for SCs, although some excellent electrodes based on pure iron oxide nanostructures have been reported, as seen in Table 4. For example, well-ordered α-Fe2O3 nanotube arrays (Figure 16a) produced by simple anodization of iron foils showed reasonably high specific capacitance (138 F g-1 at 1.3 A g-1), good rate performance (91 F g-1 at 12.8 A g-1) and excellent cycling life (high capacitance retention of 89% after 500 cycles at 3.9 A g-1) (Figure 16b, c).[219] The excellent electrochemical

performance of these nanotube arrays was attributed to the highly ordered tube-like structures, which provided large active surface area, fast ion transport paths and structurally- stable frameworks.

Similar to the case of LIBs, iron oxide nanostructures have been hybridized with various carbon-based materials to enhance their cycling stability and rate performance for [222] SCs. For instance, Shi et al. reported that the hybridization of Fe3O4 NPs with rGO -1 (Fe3O4:rGO= 2.8) of resulted in a higher specific capacitance of 480 F g at a current density -1 -1 -1 of 5 A g (in 1 M KOH) compared to Fe3O4 NPs (104 F g ) and pure rGO (139 F g ). Furthermore, the energy density of this hybrid was decreased from 124 to 67 W h kg-1 with -1 the increase of power density from 332 to 5506 W kg . In addition, the Fe3O4/rGO hybrid also showed good stability showing no obvious decrease in specific capacitance after 10,000 cycles at 10 A g-1. The enhanced electrochemical performance was attributed to the more efficient transport of ions across the electrolyte in the composite electrode as supported by the impedance measurements. In other report, Fe2O3@MWCNTs (Figure 16d) synthesized via atomic layer deposition (ALD) followed by calcination at 1000 °C (C-1000Fe2O3@MWNTs) showed a high specific capacitance of 787 F g-1 at 1 A g-1, which only slightly decreased to 568 F g-1 at 30 A g-1, indicating a high capacitance retention of 72% (Figure 16e).[220] In comparison, C-800Fe2O3@MWNTs and C-1200Fe2O3@MWNTs showed more rapid decline in capacitance, especially at a high current density of 20 A g-1. Furthermore, the cycling test revealed that the electrode, C-1000Fe2O3@MWNTs exhibited a high capacitance retention of 91.6% after 5000 cycles at 1 A g-1 (Figure 16f). The enhanced electrochemical performance of this hybrid was promoted by the gap between Fe2O3 nanocrystallites, which could provide additional free space for mechanical strain and volume expansion produced by the fast and long-time Faradaic reaction. Beside rGO and CNTs, graphene sheets have also been shown to be an ideal conducting support for iron oxide nanostructures due to their excellent electrical conductivity and their capability to offer more electrochemically active sites. For instance, 2D sandwich- like Fe3O4 nanosheets grown on graphene via the deposition of FeOOH nanorods followed by electrochemical transformation showed a superior specific capacitance of 326 F g-1 (in 1 M -1 -1 [223] LiOH) at 5 A g compared to 205 F g for pure Fe3O4. Furthermore, the Fe3O4 /graphene hybrid could maintain 95% of its initial capacitance after 1000 cycles at 2 A g-1 compared to only less than 50% for pure Fe3O4. The improved cycling performance was promoted by the

uniform distribution of Fe3O4 on the graphene sheets which prevented their aggregation during cycling. Beside Fe3O4, hydrothermally-synthesized Fe2O3/graphene composite aerogel (Figure 16g) has also been investigated for SC application and it showed a very impressive specific capacitance of 908 F g-1 at 2 A g-1 which could be maintained at 622 F g-1 at a higher rate of 50 A g-1 (Figure 16h).[221] In comparison, pure graphene aerogel showed a considerably lower capacitance of 150 F g-1 at the same rate, indicating the superiority of the hybrid. Furthermore, the hybrid also showed better stability with around 75% capacitance retention after 200 cycles at 20 mV s-1 compared to only 51% after 70 cycles for the pure

Fe2O3 NPs (Figure 16i). Furthermore, the introduction of nitrogen doping into graphene can further enhance their electrical conductivity, while also increasing the number of active sites. Apart from carbon-based materials, conducting polymers have also been employed to improve the rate performance and cycling stability of iron oxide electrodes due to their low cost, high conductivity, superior flexibility, variable redox states and ease of fabrication.

Previously, porous PANI/Fe3O4 hybrid created by the in-situ polymerization of aniline in the -1 presence of Fe3O4 NPs were shown to exhibit a high specific capacitance of 572 F g at 0.5 -1 [213] A g (in 1 M H2SO4 electrolyte). In contrast, pure Fe3O4 and PANI electrodes displayed much lower specific capacitances of 14 and 491 F g-1, respectively. Moreover, the

PANI/Fe3O4 hybrid could also retain 82% of its original capacitance after being cycled for -1 5000 times at 1 A g . Here, the coated PANI acted as conductive networks and the Fe3O4 NPs bridged the PANI networks, thereby providing efficient channels for ion transport, which explained for the enhanced electrochemical performance of the hybrid. Core-branch Fe2O3 nanoflakes@PPy nanoleaves with an impressive capacitance of 1167.8 F g−1 at 1 A g−1 in a [224] 0.5 M Na2SO4 electrolyte have been reported. When the current density was raised to 10 A g-1, this hybrid still maintained a specific capacitance of 113.4 F g-1, compared to only 65 F -1 g for pure Fe2O3 nanoflakes. The improved electrochemical performance was attributed to the combination of the good stability of the honeycomb-like α-Fe2O3 nanoflakes and the excellent conductivity of PPy nanoleaves. From the presented studies in this section, it is obvious that the modification of iron oxide electrodes with carbon-based materials produces better rate performance and cycling stability for both LIBs and SCs. In the future, more complex composites based on iron oxide and carbon with even better electrochemical performance may be realized. However, such composites may not be as attractive for large-scale production and hence a delicate balance

between fabrication cost and performance should be carefully considered in future design of iron oxide-based electrodes.

5.3. Environmental applications 5.3.1. Environmental remediation of air pollution – CO oxidation catalyst

The removal of air pollutants (e.g., VOCs, NOx, SOx, NH3, CO) resulting from human activities (e.g., car fumes, industrial combustion, power generation, etc.) is important for protecting the environment and the health of human beings. Among these air pollutants, CO, a colorless and odorless gas, can cause serious health effects upon exposure, including nausea, respiratory illnesses and even death. Therefore, the development of an efficient CO gas removal system is highly desired. Noble metals are widely known as the state-of-the-art catalysts for CO oxidation, however, their wider utilization has been limited by their high cost, scarcity, and possible poisoning.[225]Furthermore, noble metals are catalytically active only at high temperatures.[116] Hence, the loading of noble metal catalysts onto support materials has gained significant momentum in the past decade as a mean of reducing their consumption, while simultaneously enhancing the overall catalytic activity. Among various noble metals, Au is by far the most widely investigated noble metal catalyst for CO oxidation owing to their excellent catalytic activity, presence of Lewis acid sites, and ease of size and shape tuning. Since the first report on the high catalytic activity of oxide-supported Au NPs, there has been a significant rise in the development of low temperature CO oxidation catalysts based on iron oxide-supported Au NPs. In general, the CO oxidation occurs at interface between Au NPs and the support material (Figure 17a).[226] Previously, several factors have been reported to enhance the catalytic activity of supported Au NPs at room temperature. Cui et al. reported that the pH value during synthesis and the calcination temperature of the iron oxide support material could strongly affect their catalytic [226] performance (Figure 17b, c). The catalytic activity of FeOx/Au catalysts was observed to increase with increasing pH values from 6.7 to 11.2 and such increase was suggested to be the result of stronger interaction between Au and iron oxide support exhibited by the Au-OH- Fe or Au-Fe-O structure. This finding is in good agreement with the report of Daté and Haruta, in which the presence hydroxyl groups was discovered to be beneficial for enhancing the catalytic reaction on Au catalyst.[225] In the same study, Cui and co-workers also investigated the effect of calcination temperature of the FeOx/Au catalyst from 200 to 600 °C and found that calcination temperatures above 400 °C resulted in elimination of the surface

hydroxyl groups, leading to lower catalytic activity. Additionally, they found that the Auδ+ species was reduced to metallic Au (Au0) by CO during the catalytic test. In a related report, the Ortiz group studied the effect of temperature on the catalytic performance of Au/Fe2O3 [227] catalysts. Interestingly, the Au/Fe2O3 catalyst without any pre-treatment showed the highest catalytic activity for CO oxidation at 75 °C with a TOF value of 0.163 s-1, which was around 7 times higher than the TOF values of the calcined samples. It was suggested that pre- treatment temperature above 150 °C could eliminate lattice defects and surface adsorbed oxygen present on the Au/Fe2O3 catalysts. Furthermore, the Au NPs were completely oxidized above 200 °C and all of these factors contributed to the lowered catalytic performance of the calcined catalysts. The Gedanken group previously compared the catalytic performance of Au-loaded porous α-Fe2O3 nanorods with Au-loaded commercial α-

Fe2O3 and found that the porous Au/α-Fe2O3-nanorod catalyst containing 0.5 wt% Au (sizes -3 -1 -1 of 1-5 nm) showed a much higher specific activity of 1.11 x 10 molCO g s for CO -4 -1 -1 [228] oxidation compared to 3.36 x 10 molCO g s for the Au/commercial α-Fe2O3 catalyst. The enhanced catalytic activity was attributed to the stronger interaction between the porous

α-Fe2O3 support and Au NPs. Although there have been many reports on CO oxidation catalysis using oxide- supported Au NPs, the exact catalytic mechanism is still debated. However, several possible theories have been put forward. In oxide supported Au NPs, there are several possible factors which may influence their catalytic activity, including (i) size and shape of the Au NPs, (ii) oxidation state of the active gold species, and (iii) interaction between Au NPs and the oxide support. Some early studies have suggested the strong influence of size and shape of the Au NPs on the catalytic performance of oxide-supported Au NPs. Haruta suggested the critical size of Au NPs for CO oxidation to be 2 nm, because at this size, the fraction of atoms exposed to the surface was greater than 50%.[229] Furthermore, with decreasing particle size, the fractions of active sites, such as edge, corner or step sites also increased, thus leading to better catalytic activity. In other report, Lopez et al.[230] proposed that smaller-sized Au NPs possess a higher amount of low-coordinated Au atoms. However, this idea was opposed in a recent study by Yao and co-workers, whom did not observe the correlation between Au particle size and the number of the low-coordination Au sites or the changes in oxidation states of Au.[231] Instead, they proposed that smaller particle size is beneficial for enhancing the perimeter of the interface between Au NPs and the metal oxide support, which in turn increased the number of active sites and therefore enhanced the catalytic performance.

Chen and Goodman previously proposed that Au bilayers are more active than Au monolayers and that their presence was essential for achieving the high catalytic activity of iron oxide supported Au catalyst.[233] Furthermore, by employing aberration‐ corrected scanning transmission electron microscopy (STEM), the Hutchings group demonstrated that the presence of Au clusters with very small diameter of around 0.5 nm and containing 10 Au atoms was favorable for enhancing the catalytic performance of iron oxide-supported Au catalyst.[234] However, a more recent study by Liu et al.[235] refuted that the presence of Au bilayers with diameters of 0.5 nm and Au clusters with diameters bigger than 1 nm were not essential for achieving high catalytic activity. Regarding the oxidation state of the active Au species, there has been a consensus that cationic gold (Auδ+) is the most catalytically active Au species for CO oxidation and the catalytic activity of iron oxide-supported Au NPs generally increases with increased ratio of Auδ+/Au0.[117, 236] Apart from size and oxidation state of Au NPs, the type of support also has a strong influence on the catalytic activity as it can determine the level of interaction between the support and Au NPs. First, reducible oxides, such as Fe2O3 or TiO2 are known to exhibit considerably better catalytic performance for CO oxidation than non-reducible oxides, such as ZnO or MgO.[237] Secondly, porous oxide supports possess more defect sites (e.g., steps, edges, corners, etc.) compared to non- porous supports which are beneficial for stabilizing Au NPs and promoting the adsorption of oxygen.[116] In general, the loading of a higher amount of Au NPs tend to lower the catalytic activity due to the increasing tendency of the Au NPs to aggregate as a result of the high surface energy. Mesoporous metal oxide materials can be used as effective support materials to enable higher loading of Au NPs, while also ensuring their uniform dispersion throughout the structure. For example, our group recently demonstrated the high catalytic activity of Au- loaded mesoporous γ-Fe2O3 nanoflakes which exhibited more than 90% CO conversion at -1 -1 [8] room temperature with a high specific activity of 8.41 molCO gAu h . Due to their mesoporous structure, the γ-Fe2O3 nanoflakes could be loaded with high Au content of without significant aggregation. The excellent catalytic activity of this hybrid was attributed to: (i) the possible increased presence of defect sites on mesoporous iron oxide support which provided additional active sites for the adsorption of reactants; (ii) the mesoporous structure which ensured good dispersion of the Au NPs on the support thereby preventing their aggregation while simultaneously enhancing the contact between the Au NPs and the support;

(iii) presence of active cationic Au species, and (iv) ideal size range of the deposited Au NPs

(2-5 nm) which increased the length of Au/γ-Fe2O3 (i.e., length of active sites). In another report, we prepared Au-loaded mesoporous Fe2O3 using block copolymer-templated (poly(styrene-b-acrylic acid-b-ethylene glycol) (PS-b-PAA-b-PEG template) method, which -1 -1 [13] showed a specific activity of 0.30 molCO gAu h for room temperature CO oxidation. As shown above, iron oxide-supported Au NPs have shown excellent catalytic activity for low-temperature CO oxidation due to their strong interaction with Au NPs. However, at present, relatively few studies have investigated the effect of morphology of the iron oxide support on the catalytic performance of the supported Au NPs and such studies may be needed in the future to better understand the interaction between different structures of iron oxide nanostructures and Au NPs. Furthermore, the deposition of other forms of Au NPs aside from spherical NPs on iron oxide supports may also be of interest for future investigations.

6. Summary and Outlook

Over the past decade, iron oxide nanoarchitectures have attracted significant interest for various applications owing to their abundance, low cost, low toxicity, easy preparation, and magnetic properties. To date, the controlled fabrication of iron oxide nanostructures with various architectures from 0D to 3D have been achieved through a wide variety of wet- chemical methods, including coprecipitation, sol-gel, sonochemical, microemulsion, thermal decomposition, hydrothermal, and solvothermal methods. While low-temperature methods such as coprecipitation, sonochemical, sol-gel, and microemulsion can produce small spherical iron oxide NPs (<30 nm) in a relatively short time, achieving controlled morphology with these methods is more difficult. In contrast, high-temperature and high- pressure methods, such as hydrothermal and solvothermal methods enable better control of the size and shape of the iron oxide products. The high-pressure environment allows for unique interactions between the reactants and solvents which are not possible under ambient conditions. Highly monodispersed iron oxide NCs can be synthesized using thermal decomposition method. However, they are dispersible only in non-polar solvents and may exhibit some cytotoxicity due to the use of toxic precursors. Thus, surface modification with polymers or inorganic materials, such as SiO2 is usually necessary to enhance their biocompatibility, stability, and dispersion in aqueous media for bio-applications. In recent years, mesoporous iron oxide has gained increasing attention for catalysis and energy storage applications due to their large surface area and pore volume and interconnected pores, which are beneficial. While hard-templating method can provide good replica of the mesoporous template and good tuning of the pore size, they involve complicated procedures and use of harmful acid/base to remove the hard template. In contrast, the soft-templating method does not require as many steps and the template can be removed easily by solvent extraction. Furthermore, some soft templates, such as block copolymers can enable mesoporous iron oxide products with larger pore sizes to be obtained. However, it is still difficult to achieve good control of the pore size using soft-templated method due to the complex nature of surfactants or block copolymers. More recently, bio Iron oxide NPs, especially SPIONs are particularly useful as MRI contrast agents as they can produce strong T2 contrast and T1 effects at very low concentrations. Generally, the size, shape, composition, and magnetic properties of iron oxide NPs can affect their contrast properties for MRI. In general, the T2 contrast ability of iron oxide NPs increases with decreasing particle size due to the increase in Ms. Similarly, different morphologies of iron

oxide NPs also result in different Ms values which in turn affect their T2 contrast abilities in the order of octapods > rhombohedra > tetrahedrons > plates > cubes > spheres. Further, the doping of metal ions into iron oxide NPs can amplify their saturation magnetization values and therefore their T2 relaxivity values as they are linearly correlated. An exciting trend in this field is the combined use of magnetic iron oxide NPs for MRI and magnetic hyperthermia which provides a 'two-in-one' tool for diagnosis and treatment of cancer. Magnetic hyperthermia which relies on the generation of heat by magnetic materials upon exposure to alternating magnetic field has been proposed as a viable solution for cancer treatment. However, care should be taken that the healthy cells/tissues do not get damaged during the treatment and hence, surface functionalization is often carried out to ensure that the iron oxide NPs only target the tumor cells. The heating efficiency of magnetic iron oxide NPs for hyperthermia is highly dependent on the magnetic field strength, anisotropy (shape or magnetocrystalline), particle dispersity, composition, and surface modification or functionalization. Obviously, a higher magnetic field strength will lead to a higher heating efficiency. The size and shape of iron oxide NPs can also affect their heating efficiency as they influence the Ms and anisotropy, respectively. In general, at smaller magnetic fields, smaller iron oxide NPs show improved heating efficiency than their larger counterparts. However, this trend is reversed at high magnetic fields. Further, magnetic iron oxide nanostructures with distinct morphologies, such as nanorods and nanooctapods have shown enhanced heat-generating capabilities than nanospheres and nanocubes due to shape anisotropy effect and generally larger Ms values. The incorporation of metal doping into iron oxide NPs can also enhance the heating efficiency due to the increase in Ms and magnetic ordering in the crystal lattice. Similarly, surface modification with biomolecules can increase the internalization of the iron oxide NPs in the tumor cells, which results in enhanced hyperthermia effect upon application of the magnetic field. An emerging trend in magnetic hyperthermia is the use of exchange spring nanomagnets consisting of hard and soft magnetic spinel ferrites. In such nanomagnets, the hard phase component provides a high Hc while the soft phase component promotes a higher Ms for the coupled system. As a result, a higher heating efficiency by almost an order of magnitude can be achieved compared to single ferrite NPs. Iron oxide NPs also play important roles in drug delivery owing to their low toxicity, biodegradability, and controllability by magnetic field. Typically, in drug delivery, the magnetic iron oxide NPs form the core and biocompatible materials form the shell and the

drugs are encapsulated into the polymeric matrix. Parameters, such as size, surface properties, and stability are the most crucial parameters affecting the effectiveness of the drug carrier system. Generally, iron oxide NPs with sizes of 10-100 nm are preferred for intravenous injection as they have the longest blood circulation time and can penetrate effectively through the capillary bed. In addition, iron oxide NPs with hydrophobic surface are likely to adsorb at the surface of protein, resulting in low circulation time. The modifications of iron oxide NPs with biocompatible materials (e.g., organic polymers, silica, or liposomes) can resolve this issue, while also improving their drug storage and release properties. Based on previous studies, the use of iron oxide NPs in drug delivery generally only cause enrichment of Fe in the targeted tumor cells due to their internalization in these cells promoted by the receptors coupled on the surface of these NPs. These iron oxide NPs are typically removed from the cells within a few days after internalization. More recently, iron oxide NPs have been widely employed for the colorimetric detection of various biomolecules due to their unique peroxidase-mimicking ability under physiological conditions. The catalytic activity of iron oxide nanozymes can be tuned by modifying their size and shape, functionalization with activators or other types of nanomaterials and by doping or hybridization. The catalytic activity of iron oxide NPs for the oxidation of biomolecules generally decreases with increasing particle size due to the smaller surface area to interact with substrates. In terms of shape, iron oxide nanostructures with highly reactive surfaces containing open planes and dangling bonds are desirable for achieving higher catalytic activity. Moreover, the hybridization of iron oxide nanostructures with metals or carbons has been shown to be beneficial for enhancing their catalytic activity for the oxidation of biomolecules or enhancing their sensitivity and limit of detection for the electrochemical sensing of biomolecules as a result of synergistic effects, increased number of active sites, and/or interfacial effect. For practical applications, it is important to expand the electrochemical biosensing measurements to real-life samples, such as human blood and urine. Very recently, ternary iron oxide-based nanocomposites have shown great potential for the electrochemical sensing of uric acid in both human blood and urine samples, with very low detection limit in the picomolar range[238]. In the future, more in-depth studies into the size, shape and composition tuning of these ternary nanocomposites may improve their potential application further. Although magnetic iron oxide NPs exhibit many unique properties which endow various advantages and opportunities in biomedical applications, more studies on the

cytotoxicity of different functionalized iron oxide NPs are urgently needed to advance our understanding of the toxicity mechanism of these NPs. This is particularly important as the toxicity of iron oxide NPs may be affected by the modification of their cell/tissue distribution and clearance or metabolization. Therefore, it is desirable to implement multifunctional labels or (imaging modalities), such as fluorophores or radiotracers to the surfaces of iron oxide NPs.[1] The successful development of multifunctional iron oxide NPs is especially important for the development of theranostic nanomedicine. However, the clinical translation of functionalized iron oxide NPs has not yet been demonstrated. As such critical issues, including toxicity (or biocompatibility), in vivo and in vitro targeting efficiency, and long- term stability of these functionalized iron oxide NPs should be deeply investigated in the future. Apart from biomedical applications, iron oxide nanoarchitectures have also been extensively investigated as anode materials for LIBs or electrode materials for SCs due to their high specific capacity/capacitance, easy preparation, low cost and low toxicity. However, pure iron oxide anode materials typically suffer from poor rate performance or poor cycling stability at high rates due to aggregation or pulverization. Hence, they have been composited with carbon-based materials (e.g., rGO, CNTs, CNFs, graphene sheets) to enhance their conductivity and prevent/minimize their aggregation during cycling. Very recently, the application of iron oxide nanoarchitectures and their hybrids in energy storage has been expanded to other types of batteries, such as sodium-ion batteries and lithium- oxygen batteries. However, their utilization in these batteries is still in its infancy and further studies are needed to design high-performance iron oxide-based electrode materials for these newer batteries and to understand their storage mechanisms better.[239, 240] Furthermore, nanoporous iron oxides have also gained much attention as support materials for noble metal NPs in catalytic CO oxidation as they can provide significantly more active sites for the adsorption of oxygen and CO molecules than bulk iron oxide. Despite the various theories which have been forwarded to explain the catalytic mechanisms of iron oxide-supported Au NPs, several parameters have been conclusively identified as important in determining the catalytic activity. First, Au NPs with ideal sizes of 2-5 nm are preferred as they exhibit a higher fraction of active sites, such as edges, corners, steps. Furthermore, cationic gold, Auδ+ has been shown to be more active for CO oxidation than the metallic Au. An emerging trend in this field is the utilization of mesoporous oxides as support materials for Au NPs which

can provide good dispersion of the Au NPs while also enabling a much larger amount of Au to be accommodated.

Acknowledgements S.T. and Y.V.K. contributed equally to this work. This work was supported by Australian Research Council (ARC) Future Fellowship (FT150100479).

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Figures and tables

Figure 1. Crystal structure of (a) hematite (α-Fe2O3), (b) magnetite (Fe3O4), and (c) [1] maghemite (γ-Fe2O3). Reproduced with permission. Copyright 2015, IOP Publishing. Various applications of iron oxide-based materials, including energy storage, catalysis, biomedical, and environmental applications. Adapted with permission.[1, 8-10] Copyright 2018, The Royal Society of Chemistry; Copyright 2014, Elsevier; Copyright 2015, IOP Publishing; Copyright 2015, Elsevier.

Figure 2. Some examples of iron oxide nanoarchitectures. TEM images of (a) spherical [4] Fe3O4 and (b) cubic-like iron oxide nanocrystals. (a) Reproduced with permission. Copyright 2004, Nature Publishing Group. (b) Reproduced with permission.[14] Copyright

2018, American Chemical Society. c) TEM and (d) SEM images of 1D α-Fe2O3 nanowires [4, 15] and α-Fe2O3 nanotubes, respectively. Reproduced with permission. Copyright 2008, American Chemical Society; Copyright 2005, Wiley-VCH. Copyright 2013, American

Chemical Society. SEM images of (e) 2D α-Fe2O3 nanosheets, (f) α-Fe2O3 nanoflakes, (g) flower-like α-Fe2O3, (h) urchin-like α-Fe2O3, and (i) α-Fe2O3 polyhedrons. Reproduced with permission.[16-20] Copyright 2009, The Electrochemical Society; Copyright 2016, Elsevier; Copyright 2006, Wiley-VCH; Copyright 2012. The Royal Society of Chemistry; Copyright 2011, American Chemical Society.

Figure 3. TEM images of (a) spherical, (b) cubical, and (c) star-like Mn-Zn ferrite nanocrystals withdrawn from the reaction solution at 260, 280, and 300 °C after aging for 0, 20, and 40 min. d) Schematic illustration of the star-like nanocrystals shape evolution. Adapted with permission.[28] Copyright 2013, American Chemical Society.

Figure 4. a) SEM images of the colloidal crystal template and b) the resulting three- [55] dimensional ordered macroporous (3DOM) LaFeO3. Reproduced with permission. Copyright 2010, Elsevier. Schematic illustration of the nanocasting pathway using mesoporous silica hard templates with (c) hexagonal and (d) cubic geometries. Reproduced with permission. Copyright 2013, The Royal Society of Chemistry.[56] TEM images recorded along the [111] direction for mesoporous (e) α-Fe2O3, (f) Fe3O4, and (g) γ-Fe2O3. Reproduced with permission.[57] Copyright 2006, American Chemical Society. h) TEM image of SBA-15- templated mesoporous α-Fe2O3 viewed along and perpendicular to the direction of the hexagonal pore arrangements. Reproduced with permission.[58] Copyright 2015, Elsevier.

Figure 5. (a) Schematic phase diagram of the various morphologies formed by surfactants including spherical, reverse and cylindrical micelles, lamellar and bilayer vesicle. TEM images of 3D mesoporous iron oxide recorded along the TEM images of 3DMIO recorded along the: a) (110) direction; b) (100) direction, and c) (211) direction. Adapted with permission.[65] Copyright 2004, Wiley-VCH.

Figure 6. a) Schematic phase diagram of the various morphologies formed by block copolymers, including spherical and cylindrical micelles, vesicles, spheres with face-centered cubic (space group: Fm3m) and body-centered cubic (Im3m) packing, hexagonally packed cylinders (p6m), bicontinuous gyroid (Ia3d), F surface (Fd3m), P surfaces (Pm3n, Pn3m, or Pm3m), and lamella. Reproduced with permission.[69] Copyright 2013, The Royal Society of

Chemistry. b) Graphical representation of the fabrication of mesoporous Fe3O4 nano/microspheres with large surface area and (c) the corresponding TEM image of the [70] mesoporous Fe3O4 microspheres. Reproduced with permission. Copyright 2011, American

Chemical Society. d) A typical TEM image of mesoporous α-Fe2O3 film obtained by heat treatment at 450 °C. Reproduced with permission.[71] Copyright 2006, Wiley-VCH. e) A typical SEM image of KLE-templated mesoporous α-Fe2O3 thin films calcined at 550 °C. Reproduced with permission.[72] Copyright 2011, Wiley-VCH.

Figure 8. a) The selective preparation of hematite (α-Fe2O3) and magnetite (Fe3O4) nanorods from coordination polymer nanorods (CPP-15). SEM images of the CPP-15-derived α-Fe2O3 nanorods with an average width of 70 ± 8 nm (b) and Fe3O4 nanorods with an average width of 70 ± 12 nm (c). Reproduced with permission.[78] Copyright 2011, The Royal Society of Chemistry. d) Schematic illustrations depicting the thermal decomposition processes of solid Prussian Blue (SPB), small hollow PB (SHPB), and large hollow PB (LHPB). SEM images of several products prepared by calcination of different PB precursors at different temperatures. The applied calcination temperatures (°C) are noted in each image. e) SPB 250 °C, (f) SHPB 250 °C, (g) LHPB 250 °C. Reproduced with permission.[80] Copyright 2012, American Chemical Society.

Figure 9. a) Schematic illustration of the preparation of Fe3O4/graphene nanocomposites by a reactive solid-state milling process. Adapted with permission.[91] Copyright 2015, American Chemical Society. b) La Mer-like diagram: hydrolyzed TEOS (monomers) concentration against time on homogeneous nucleation and heterogeneous nucleation. c) The existence of

Fe3O4@SiO2 core/shell NPs and SiO2 NPs in the reaction production when C > Chomo at some moment. d) Only the existence of Fe3O4@SiO2 core/shell NPs in the reaction production when C < Chomo at any moment. TEM images of 12.2-nm Fe3O4@ SiO2 NPs obtained with different TEOS amounts: (e) 75, (f) 150, (g) 300, and (h) 600 μL (Scale bar = 20 nm). Adapted with permission.[92] Copyright 2012, American Chemical Society. i) TEM image of the dumbbell-like Fe3O4/Au hybrid synthesized via interfacial electronic transfer. Adapted with permission.[93] Copyright 2005, American Chemical Society.

Figure 10. TEM images of monodispersed manganese-doped iron oxide nanoparticles (MnIO NPs) (Mn/Fe ratio is about 1/6) with different shapes, (a) spheres (diameter of 15 nm), (b) cubes (side length of 12 nm), (c) plates (hexagonal, side length of 12 nm and thickness of 5 nm), (d) tetrahedrons (regular, side length of 25 nm), (e) rhombohedra (oblique parallelepiped, side length of 13.5 nm with a tilt angle of 60°), and (f) octapods (average edge length between two nearby arms of 30 nm and each corner angle of 40°). g) T2-weighted phantom images of MnIO with six shapes and IO nanoparticles measured at 1.5 T MRI. h)

Figure 11. a) Schematic representation of magnetic nanoparticle-based drug delivery system: these magnetic carriers concentrate at the targeted site using an external high-gradient magnetic field. After accumulation of the magnetic carrier at the target tumor site in vivo, drugs are released from the magnetic carrier and effectively taken up by the tumor cells. Reproduced with permission.[1] Copyright 2015, Springer. b) Characterization of anti- HER2/neu peptide (AHNP)-conjugated iron oxide NPs (IONP-PTX-AHNP). TEM micrographs of NPs without (b) and with (c) negative staining. Insets: enlarged images (c) pH-dependent release of cisplatin from Pt-PHNPs (19.6% Pt/ Fe). Effective in vivo targeting of HER2/neu+ SK-BR-3 breast cancer in living mice by AHNP-conjugated IONPs. IONP- AHNP-Cy5.5 or IONP-Cy5.5 was injected into mice intravenously (NP amount equivalent to 0.5 mg Fe). Fluorescence images were taken immediately before NP injection and 6, 24, 48, 72 and 96 h after injections. d) Fluorescence images of mice after NP injections. Fluorescence intensity at different time points and two groups was normalized to the same scale. Red and yellow dashed circles indicate tumors. e) Percentages of radiant efficiency in tumors to the whole body at different time points. Error bars represent standard deviation and are from three independent measurements. Reproduced with permission.[140] Copyright 2015, The Royal Society of Chemistry.

Figure 12. a) Tumor growth curves of A549 tumor xenografts following a single hyperthermia treatment with chitosan oligosaccharide- stabilized ferrimagnetic iron oxide nanocubes (Chito-FIONs) or Feridex (375 μg Fe/kg body weight). The growth curve of A549 tumors treated with Feridex alone is not shown here for a clear presentation. The arrow represents the day of particle administration. Control groups received saline. Statistically significant difference from controls, *p < 0.01, **p < 0.001. b) Relative tumor volume of different treatment groups at day 6. **p < 0.002 between two groups. c) Prussian Blue staining images of tumor sections excised at day 6. The blue region shows the intratumoral distribution of Chito-FIONs and Feridex. d) Confocal microscopic images of TUNEL-stained tumor sections. Apoptotic cells emit green fluorescence signal. Cell nuclei were also stained with DAPI (blue fluorescence). Reproduced with permission.[100] Copyright 2012, American Chemical Society.

Figure 13. a) Photographs show production of colored product upon addition of CoFe2O4 nanoparticles (NPs) to 3,3′,5,5′-tertamethylbenzidine (TMB) and o-phenylenediamine (OPD) at pH 4.0. b) The Michaelis constants Km values of the different kinds of CoFe2O4 NPs [168] toward H2O2 (the black bars) and TMB (the red bars). Adapted with permission. Copyright 2015, The Royal Society of Chemistry. c) Low- and (d) high-magnification TEM images of the Fe3O4 nanospheres/rGO composite. Photographs (e) and absorbance (f) of the reaction solution for different acetylcholine (Ach) concentrations, from left to right: 10,000, 1000, 100, 10, 1, 0.5, 0.1, 0.05, and 0.01 M. g) The linear calibration plot for ACh. Adapted with permission.[163] Copyright 2014, Elsevier. h) Oxidation of 2,2'-azino-bis(3- ethylbenzothiazoline-6-sulfonic acid) (ABTS, 1 mM) in the presence of Fe3O4 NPs (50 μg mL-1) at pH 4, and a photograph showing the oxidation of ABTS producing a green color. The reaction is inhibited by DNA modification. Kinetics of ABTS oxidation at various NaCl concentrations catalyzed by (i) bare Fe3O4 NPs and (j) DNA-capped Fe3O4 NPs, respectively. The absorbance at 420 nm was monitored. Adapted with permission.[169] Copyright 2015, The Royal Society of Chemistry.

Figure 14. a) FESEM image of Fe3O4 nanotube arrays grown on fluorine-doped tin oxide

(FTO) substrate. b) Amperometric response at 0.6 V with increasing glucose concentration [188] per 50 s for the Fe3O4 nanotube array electrode. Adapted with permission. Copyright

2017, Elsevier. c) FESEM image of the Fe3O4-reduced graphene oxide (rGO) hybrid. d) DPV responses for 10% of Fe3O4/rGO hybrid-modified GCE at different concentrations of nitrite

(10–2882 mM) in N2 saturated 0.1 M PBS (pH 4). The inset shows the corresponding calibration plot of the response peak current versus nitrite concentration. Adapted with permission.[189] Copyright 2015, The Royal Society of Chemistry. e) FESEM image of ternary PANI-Fe2O3-rGO composite. f) Differential pulse voltammetry (DPV) curves of the

PANI-Fe2O3-rGO modified GC electrode in PB solution (pH 2.5) with various concentration of hydroquinone (HQ) (0.1–550 μM); scan rate =10 mV s-1, amplitude = 0.025 V and step

Figure 15. a) FESEM image of the graphene nanosheet (GNS)/Fe3O4 composite. b) Cycling performance of the commercial Fe3O4 particles, GNS/Fe3O4 composite and bare Fe2O3 particles at a current density of 35 mA g-1. Solid symbols, discharge; hollow symbols, charge. c) Rate performance of commercial Fe3O4 particles, GNS/Fe3O4 composite, and bare Fe2O3 particles at different current densities. Adapted with permission.[209] Copyright 2010,

American Chemical Society. d) FESEM image of carbon-coated Fe3O4 nanospindles

(Fe3O4@C nanospindles). e) Cycling performance and (f) rate performance of the bare α-

(PANI) hierarchical hollow spheres. h) Cyclic performance of Fe2O3, hollow-Fe2O3 (h-

Fe2O3), and Fe2O3@PANI electrodes at a 0.1 C rate. i) Rate capability of Fe2O3, hollow-

Figure 16. a) SEM image of the α-Fe2O3 nanotube arrays (NTAs). b) Specific capacitance of

α-Fe2O3 NTAs as a function of discharge current densities. c) Cycling performance of α- -2 -1 Fe2O3 NTAs electrode at a current density of 3 mA cm (3.9 A g ). The inset shows the charge/discharge curves of the electrode in the first 10 cycles. Adapted with permission.[219]

Copyright 2011, Elsevier. d) TEM image of the hybrid of Fe2O3 and multiwalled carbon nanotubes (MWNTs) obtained after 1000 cycles of Fe2O3 deposition. e) The specific capacitances versus current densities of C-1000Fe2O3@MWNTs (calcined at 1000 °C). (f) -1 [220] Cycling performance of the C-1000Fe2O3@MWNTs at 1 A g . Adapted with permission. Copyright 2017, Elsevier. g) FESEM image of graphene/Fe2O3 composite hydrogels (the inset shows the photograph of graphene/Fe2O3 composite hydrogels). h) Capacitance versus current density plots for Fe2O3, graphene hydrogels, and graphene/Fe2O3 composite hydrogels. i) Cycling performance of -1 [221] graphene/ Fe2O3 composite hydrogels at 20 mV s . Adapted with permission. Copyright 2014, Elsevier.

Au/FeOx catalysts for low-temperature CO oxidation reaction: b) Au/Fe_O, transient and (c) -1 - Au/Fe_O stability at 30 °C. Reaction conditions: 1% CO/20% O2/79% N2, 80,000 mL h gcat 1. Reproduced with permission.[226] Copyright 2016, MDPI.

Table 1. Examples of various iron oxide nanoarchitectures reported in the literature.

Iro Template Surfa Size/ Morpholo Synthetic Applicati n ce Pore gy Approach or on R oxid surfactant area size* ef e (m2 g- (nm) . pha 1) se [241] Fe3 Nanoparti Coprecipitati SDS 70.0 D: Dye

O4 cles on 10-60 adsorptio n [242] Fe3 Nanoparti Thermolysis Oleic acid N/A D: N/A

O4 cles reaction 15-30 [243] Fe3 Nanoparti Sol-gel N/A N/A 8.5- N/A

O4 cles 15.5 [23] Fe3 Nanoparti Solvotherma CTAB, N/A D: 4- MRI

O4 cles l oleylamine 6 contrast agent FeO Nanoparti Thermal N/A N/A D: MRI [244] x cles decompositi 14-45 contrast on agent γ- Microsphe Solvotherma Tartaric 139 PS: LIBs [245]

Fe2 res l acid 6.1

O3 [245] Fe3 Microsphe Solvotherma Tartaric 122 PS: LIBs

O4 res l acid 5.9 α- Nanorods Thermal N/A 93.0 PS: Rhodami [246]

Fe2 degradation 2.2 ne B

O3 degradati on [247] α- Nanotubes Hydrotherm NH4H2PO4 N/A L: N/A

Fe2 al 250-

O3 400 [15] α- Nanotubes Thermal Carbon N/A L: A H2S

Fe2 oxidation nanotubes few sensor

O3 micro ns α- Nanowires Thermal N/A N/A D: Acetone [248]

Fe2 Oxidation 15-50 sensor

O3 α- Nanospind Self- MIL-88 Fe 75.0 PS: LIBs [208]

Fe2 les sacrificial <10

O3 template [249] Fe3 Nanoribbo Polyol N/A 115 PS: 2- LIBs

O4/ ns approach 20

Fe2

O3 [250] Fe2 Nanosheet Anion Dodecanoat 185 L: 100- Phosphate

O3 s exchange e 200 removal [251] γ- Nanosheet H2O2 N/A 98.5 L: 50 LIBs

Fe2 s oxidation

O3 [52] Fe3 Chestnut- Solvotherma N/A 14.9 D: Arsenic

O4 like l 1000- removal 1500 α- Flower- Polyol TBAB 40 D: Arsenic [18]

Fe2 like 5000- and

O3 6000 chromium removal α- Urchin- Hydrotherm N/A 150- D: LIBs [19]

Fe2 like al 180 500-

O3 600 α- Nanocube Self- PB 160 PS: N/A [80]

Fe2 s sacrificial 0.4-

O3 template 0.6 γ- Nanocube Self-sacrificial PB 130 PS: 4- N/A [80]

Fe2 s template 6

O3 [252] γ- Mesoporo Polymer‐ EO20PO70E 167 PS: Reduction

Fe2 us assist O20 4.7 of

O3 ed nitroarene self‐ s

asse mbly α- Mesoporo Sol-gel P123 137 PS: Degradati [253]

Fe2 us 3.4 on of

O3 Orange II γ- Mesoporo Sol-gel P123 100 PS: Degradati [253]

Fe2 us 5.1 on of

O3 Orange II [253] Fe3 Mesoporo Sol-gel P123 75.0 PS: Degradati

O4 us 7.1 on of Orange II

*D = diameter; L= length; PS = pore size; LIBs = lithium-ion batteries, CTAB = cetyltrimethylammonium bromide; TBAB = tetrabutylammonium bromide

Table 2. Summary of previously reported iron oxide-based materials for electrochemical biosensors.

Analyte Electrode material Sensitivity Linear range Detection Ref. (μA cm-2 mM-1) (μM) limit (μM) [254] Aprepitant Fe2O3 NPs 2.5316 0.0022-4.1 0.00038 (APPT) [255] Citric acid CTAB-stabilized Fe3O4 3.0 0.05-2.5 40 [184] Dopamine Fe3O4 NPs N/A 0.00015-0.4 0.03 [256] Dopamine Fe2O3/SWCNTs 3.44 3.2–31.8 0.36 Dopamine Nafion covered N/A 0.02-130 0.007 [257]

Fe3O4@graphene nanospheres [187] Glucose Fe3O4 nanorod arrays 406.9; 134.1 0.5-766; 766-3670 0.1 [188] Glucose Fe3O4 nanotube arrays 673.3; 71.2; 1-125; 125-1000; 0.1 9.58 1000-5000 [186] Glucose Fe2O3 nanowire arrays 726.9 15–8000 6 [258] Glucose Spruce branched Fe2O3 85.384 3–33000 1 [157] Glucose Mesoporous α-Fe2O3 N/A 1-100 1 [259] Glucose P4VP-co-PAN/Fe2O3 1382.8 2.5-580 0.58 NPs [260] H2O2 Fe2O3 nanorods arrays 181 6.6-2500 1.3

[261] H2O2 PB-modified γ-Fe2O3 94.33 100-1200 2.49 NPs [262] H2O2 Fe2O3 nanotubes/chitosan 1589 1-160 0.05

[182] H2O2 Graphene-Fe2O3-chitosan 84.32 1-6000 1.1

-5 [194] Hydrazine Fe2O3@SiO2 NPs 12.628 0.0002-2000 7.6 x 10

[192] Hydroquinone PANI-Fe2O3-rGO N/A 0.1-550 0.06

[185] Nitrite Hollow Fe2O3 19.83 9-3000 2.6 polyhedrons [263] Nitrite Fe3O4@Pt NPs N/A 0.051-7800 0.109 [189] Nitrite Fe3O4-rGO 202.5 0 5-5.8; 0.5-9500 0.03 [264] Riboflavin Flower-like Fe3O4/rGO N/A 0.3-1; 1-100 0.089 [265] Sulfite oxidase Fe3O4@GNPs N/A 0.5-1000 0.15

[266] Tryptophan β-cyclodextrin/Fe3O4 N/A 0.0008-0.30 0.5 hybrid [267] Uric acid Poly(DPA)/SiO2@Fe3O4 N/A 0.0012-0.0018 0.4 [238] Uric acid Ag-Fe2O3@PANI 128.29 0.001-0.9 0.000102 [268] Uric acid Fe3O4@C@TiO2 N/A 0.300–34.0 0.02 [269] Uric acid Fe3O4@SiO2/MWCNT 0.303 0.6-100 0.13 *PB = Prussian Blue; PDA = poly(dipicolinic acid); PV4P = poly(4-vinylpyridine)-co- poly(acrylonitrile)

Table 3. Comparison of the lithium storage performance of various iron oxide nanostructures and their hybrids for lithium-ion batteries.

Anode material Synthetic Rate Cycling Ref. approach performance performance

-1 -1 [245] γ-Fe2O3 microspheres Solvothermal 1453 mAh g at 0.2 C 697 mAh g after 110 cycles at 0.2 C

-1 -1 [245] Fe3O4 microspheres Solvothermal 1307 mAh g at 0.2 C 450 mAh g after 110 cycles at 0.2 C

-1 -1 [270] α-Fe2O3 hollow Electrospinning 948 mAh g at 2 C 1293 mAh g after nanofibers 40 cycles at 0.6 C

-1 -1 [200] α-Fe2O3 nanorods Solvothermal 837 mAh g at 0.5 C 970 mAh g after 30 cycles at 0.5 C

-1 -1 -1 [249] Fe3O4 nanoribbons Polyol 615 mAh g at 1.86 A g 1046 mAh g after 130 cycles at 0.074 A g-1

-1 -1 -1 [204] α-Fe2O3 nanosheets Electrodeposition 426 mAh g at 10 A g 901 mAh g after 100 cycles at 0.5 A

-1 g

-1 -1 [207] Mesoporous α-Fe2O3 Soft-templating 870 mAh g at 2 C 1293 mAh g after 50 cycles at 0.2 C

-1 -1 -1 [96] rGO/Fe2O3 Direct precipitation 1693 mAh g at 0.1 A g 1027 mAh g after

-1 and microwave 50 cycles at 0.1 A g irradiation Graphene Direct mixing 544 mAh g-1 at 2 A g-1 910 mAh g-1 after [271]

nanoribbons/Fe2O3 and oxidation 134 cycles at 0.2 A g-1

-1 -1 -1 [94] rGO/γ-Fe2O3 Modified sol-gel 380 mAh g at 5 A g 666 mAh g after 50 cycles at 0.5 A g-1

-1 -1 -1 [212] α-Fe2O3-CNFs Electrospinning 288 mAh g at 0.5 A g 488 mAh g after 75 -1 cycles at 0.05 A g

-1 -1 -1 [272] Fe2O3/SWCNT CVD 841 mAh g at 0.5 A g 801 mAh g after 90 -1 cycles at 0.5 A g

−1 -1 −1 [273] Fe3O4/graphene In situ reduction 617.1 mAh g at 1 A g 1049 mA h g after spheres 50 cycles at 0.05 A g-1

-1 -1 -1 [274] Fe2O3@OMCs Hydrothermal 420 mAh g at 0.5 A g 789 mAh g after 120 cycles at 0.1 A

g-1

-1 -1 [210] Fe3O4@C Hydrothermal 190 mAh g at 5 C 530 mAh g after 80 nanospindles cycles at 0.5 C

-1 -1 -1 [275] Fe3O4@C Electrospray 545 mAh g at 2 A g 525 mAh g after microspheres 300 cycles at 5 A g-1

-1 -1 -1 [276] Fe3O4@N-doped C Microwave-assisted 683 mAh g at 0.5 A g 976 mAh g after 200 cycles at 0.5 A g-1

-1 -1 [277] Fe3O4@C@PGC NaCl-assisted 858 mAh g at 5 C 792 mAh g after nanosheets 350 cycles at 5 C

-1 -1 [214] PSA-Fe3O4@C Hydrothermal 741 mAh g at 2 C 1293 mAh g after 100 cycles at 0.5 C

-1 -1 -1 [278] Fe2O3-C-Ag Reduction reaction 654 mAh g at 0.5 A g 858 mAh g after microcubes 200 cycles at 0.1 A g-1

−1 -1 −1 [279] Fe2O3@Co3O4 Hydrothermal 1050 mAh g at 0.2 A g 1005 mAh g after nanowires 50 cycles at 0.2 A g-1

−1 -1 -1 [215] Fe2O3@SnO2 Hydrothermal 500 mAh g at 0.5 A g 570.7 mAh g after nanorods 100 cycles at 0.2 A g-1 −1 -1 -1 [216] Fe2O3-SnO2/graphene Hydrothermal 1040 mAh g at 0.5 A g 1530 mAh g after 200 cycles at 0.1 A g-1 −1 -1 -1 [280] Fe2O3@C@MnO2@C Hydrothermal 975 mAh g at 0.5 A g 1089 mAh g after hybrid 50 cycles at 0.5 A g-1 *SWCNT = single-walled carbon nanotubes; CNFs = carbon nanofibers; PGC = porous graphitic carbon; rGO = reduced graphene oxide; PSA = poly (ST-AN)

Table 4. Comparison of the electrochemical performance of various iron oxide nanostructures and their hybrids for supercapacitors.

Electrode material Electrolyte Potential Rate Cycling performance Ref. range capability (V vs SCE) −1 [281] α-Fe2O3 porous nanofibers 1 M LiOH −0.1 to 348 F g at 5 80–82% capacitance 0.9 V A g-1 retention after 3000 Cycles at 100 mV s-1

-1 [219] α-Fe2O3 nanotube arrays 1 M -0.8 to 0 91 F g at 1 A 88.9 % capacitance -1 Li2SO4 V g retention after 500 cycles at 3.9 A g-1

-1 [282] α-Fe2O3@C nanococoons 1 M KOH −1.05 to 224.3 F g at 90.7 % capacitance −0.3 V 10 A g-1 retention after 2000 cycles at 10 A g-1

-1 [220] Fe2O3@MWCNTs 3 M KOH -1.3 to - 568 F g at 30 91.6% capacitance -1 0.3 V A g retention after 5000 cycles -1 at 1 A g

-1 [223] Fe3O4@rGO composite 1 M KOH -1.15 to 304 F g at 10 95% capacitance retention 0.1V A g-1 after 1000 cycles at 2 A g-1 α- 1 M -1.2 to - 98.2 F g-1 at 91.5% capacitance [283] -1 Fe2O3 mesocrystals/graphene Na2SO4 0.2 V 10 A g retention after 2000 cycles at 5 A g-1

-1 [284] Fe2O3 NDs@N-doped 2 M KOH -1 to 0 V 140 F g at 50 75.3% capacitance graphene A g-1 retention after 100000 cycles at 5 A g-1

-1 [221] Fe2O3/graphene aerogel 1 M KOH -1.05 to - 304 F g at 10 95% capacitance retention 0.3 V A g-1 after 1000 cycles at 2 A g-1

-1 [285] Fe3O4/rGO composite 1 M KOH -0.8 to 0.2 480 F g at 5 112% capacitance

V A g-1 retention after 10000

cycles at 5 A g-1

-1 [286] α-Fe2O3 nanotubes/rGO 1 M -1.0 to 0 114 F g at 5 110% capacitance

-1 Na2SO4 V A g retention after 2000 cycles at 5 A g-1

-1 [287] Fe2O3/N-doped rGO 1 M KOH -0.7 to - 350 F g 10 A 56.7% capacitance 1.1 V g-1 retention after 5000 cycles

at 4 A g-1

-1 [213] Fe3O4/PANI composite 1 M H2SO4 -0.6 to 1.0 572 F g at 82.0% capacitance V 0.5 A g-1 retention after 5000 cycles at 1 A g-1

-1 [224] Fe2O3 nanoflakes@PPy 0.5 M -1.0 to - 113 F g at 10 97.1% capacitance -1 H2SO4 0.2 V A g retention after 3000 cycles at 3 A g-1

-1 [288] Fe3O4@C@PANI 1 M KOH -0.1 to 0.6 113 F g at 10 87.8% capacitance V A g-1 retention after 3000 cycles at 2.5 A g-1

-1 [289] α-Fe2O3@MnO2 nanotubes 3 M KOH -0.4 to 0.5 290 F g at 1 85.3% capacitance V A g-1 retention after 1200 cycles at 1 A g-1 *MWCNTs = multi-walled carbon nanotubes; NDs = nanodots; PANI = polyaniline; PPy = polypyrrole

Biography Yusuf Valentino Kaneti obtained his Ph.D. degree in materials science and engineering from the University of New South Wales in 2014. He is currently working as a Research Associate at the International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Japan. His research focuses on the synthesis, characterization and application of porous nanomaterials for energy storage catalysis and gas sensors.

Yusuke Yamauchi received his Bachelor degree (2003), Master degree (2004), and Ph.D. degree (2007) from Waseda University, Japan. Then, he joined NIMS to start his own research group. In 2016, he joined the University of Wollongong as a full professor. He is currently a full professor at the School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Australia. He concurrently serves as an honorary group leader of NIMS, and a visiting professor at several universities.

Table of Contents

This review summarizes the recent progress in the fabrication of iron oxide nanoarchitectures from zero-dimensional to three-dimensional structures and their hybridization with various carbon-based materials and inorganic materials for energy storage, biomedical, and environmental applications.

Keywords: Iron Oxide Nanoparticles

A Review on Iron Oxide-Based Nanoarchitectures for Energy Storage, Biomedical and Environmental Applications Shunsuke Tanaka†, Yusuf Valentino Kaneti†*, Ni Luh Wulan Septiani, Yusuke Yamauchi*, Shi Xue Dou, Yoshio Bando, and Md. Shahriar A. Hossain