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Diversity of Iron Oxides: Mechanisms of Formation, Physical Properties and Applications

Kamil G. Gareev
Department of Micro and Nanoelectronics, Saint Petersburg Electrotechnical University “LETI”, 197022 Saint Petersburg, Russia
Engineering Centre for Microtechnology and Diagnostics, Saint Petersburg Electrotechnical University “LETI”, 197022 Saint Petersburg, Russia
Magnetochemistry 2023, 9(5), 119;
Submission received: 7 March 2023 / Revised: 14 April 2023 / Accepted: 25 April 2023 / Published: 27 April 2023


Iron oxide compounds have naturally formed during the whole of Earth’s history. Synthetic compositions with iron oxides are produced with the use of various techniques and widely used for scientific and applied purposes. This review considers an attempt to classify all the information on different iron oxide compound formation mechanisms and intended applications in biomedicine, catalysis, waste remediation, geochemistry, etc. All the literature references analyzed were divided into several groups by their number of included iron oxide compounds: compositions containing only one compound (e.g., magnetite or wüstite), including various polymorphs of iron(III) oxide (α-, β-, γ-, ε-, ζ-, δ-Fe2O3); compositions with two different distinguishable iron oxide phases (e.g., maghemite and hematite); compositions containing non-crystalline phases (amorphous iron oxide or atomic clusters); and compositions with mixed iron oxide phases (indistinguishable separate iron oxide phases). Diagrams on the distribution of the literature references between various iron oxide compounds and between various applications were built. Finally, the outlook on the perspectives of further iron oxide studies is provided.

1. Introduction

The chemistry of iron is of great interest because iron is an abundant element present in various fields [1]. The elemental abundance of oxygen, hydrogen and iron at the surface of and within the Earth’s crust has fostered widespread occurrences of iron oxides and oxyhydroxides in a diverse range of aquatic and terrestrial environments; most of the known iron oxides and oxyhydroxides are known to occur in nature [2]. Iron oxides are formed naturally through the weathering of Fe-containing rocks both on land and in the oceans and play an important role in geochemistry [3]. Iron-rich sedimentary rocks have had important implications in the evolution of Earth’s atmosphere and hydrosphere [4]. Iron oxide copper gold, apatite-magnetite and other ore deposits have very important heavy industrial applications [5,6]. There are iron oxides on the surface of Mars, in the depths of Earth, in old rusting factories, in pigeon brains and magnetotactic bacteria [7]. Despite the precise mechanism of biogenic magnetite mineralization on early Earth still being unknown, the understanding of this mechanism includes the origin of banded iron formations [8]. Iron oxides are also linked to pathological states of the human body, such as iron dysregulation in the brain and neurological disorders [9,10].
Metallurgy has been developed relating to iron and iron oxides and used for various applications including colored pigments, magnetic materials, catalysts, water oxidation, biomedical uses including therapy and diagnostics, etc. [1,11,12,13,14]. The stability of the structural incorporation of uranium into the hematite crystal structure suggests the feasibility of iron oxides for inhibiting the mobility of aqueous uranium (VI) [15]. The use of special stainless steels (i.e., Eurofer steel) for some portions of the main wall of a nuclear fusion experimental reactor may come into consideration in industrial applications of nuclear fusion; therefore, the detailed knowledge and quantification of their interactions between atoms, molecules and plasma, including electron impact ionization cross sections of iron oxide molecules, is of considerable interest [16]. Iron oxides are, furthermore, of great interest with regard to the corrosion and oxidation processes of iron metal and steel, which are mediated by the surface whose structure depends greatly on environmental variables such as temperature, oxygen or water partial pressures [17,18]. Additionally to the various Fe oxidation states, ferric oxide (Fe2O3) may be stable or metastable in the known α, β, γ, ε, ζ and δ polymorphs [19,20,21,22]. Polymorphism in this case means a possibility for a compound to exist in two or more solid phases that are isochemical but have distinct crystal structures and thus, different physical properties. Due to their different physical properties, which arise from the differences in their crystal structures, all of the polymorphs have found applications in nanotechnology [20]. Moreover, there are also not simply polymorphs of known iron oxides, but distinct compounds with the formula Fe4O5 [23]. Other iron oxides with unconventional stoichiometry, such as Fe5O6, Fe5O7, Fe7O9 and more complicated compounds have been predicted in theory, some of which have been successfully synthesized at pressures of 10 ∼ 80 GPa and annealed from high temperatures [24]. Magnetite (Fe3O4), maghemite (γ-Fe2O3) and hematite (α-Fe2O3), including in the form of the oxidized zerovalent iron core-shell structure, are widely used in heterogeneous catalysis processes and have been used as attractive alternatives for the treatment of wastewaters and soils contaminated with organic compounds [25,26,27].
Structures containing the various iron oxide crystalline or amorphous phases can either be macro- (ceramics [18]), micro- (microbial-induced precipitates [28]) or nanometer-sized (nanoparticles and nanoclusters [29]). The nucleation and growth of inorganic crystals, including iron oxides, from solutions occurs throughout geochemical, biological and synthetic systems [30]. Iron oxide biominerals are formed under “green” conditions without a loss of functionality such that they have the potential for numerous scientific as well as industrial applications [31]. Global iron cycling is driven by both abiotic and biotic reactions, and in the presence of oxygen and under circumneutral pH conditions, ferrous iron is quickly oxidized to Fe(III) and precipitates as iron oxides [32,33].
The physical properties of the various iron oxide compounds can be extremely different, e.g., magnetite and maghemite have been commonly used in biomedicine because of their saturation magnetization being the highest [34]. The ε-polymorph of Fe2O3 possesses the highest coercive force (up to 2 T at room temperature [20,35,36,37]) among the other ferrimagnetic oxides, while maghemite-magnetite nanoparticles (NPs) can have almost zero coercivity [38] with a very low difference in their average size. In some applications, e.g., in catalysts, amorphous iron oxide NPs can be more active than crystalline polymorphs of the same diameter thanks to their “dangling bonds” and higher surface–bulk ratio in their amorphous phase [39]. Based on the literature data, a generalized scheme illustrating the diversity of the known iron oxide compounds, including stable, metastable, atomic clusters and amorphous, is given in Figure 1.
The current review attempts to summarize the main information on the various iron oxide compounds to date. The next section is devoted to a comparison of some of the main physical properties of the various iron oxide compounds.

2. Physical Properties of Various Iron Oxide Compounds

An Fe–O phase diagram is given in Figure 2. According to the ratio of Fe2+ and Fe3+, the phase diagram of Fe–O can be divided into six phase zones from right to left [40].
The shown phase diagram does not take into account the polymorphisms of ferric oxides. Sakurai et al. [41] discuss the crystal structures and magnetic properties of the four phases are the following: ferrimagnetic γ-Fe2O3 with a spinel structure; ferrimagnetic ε-Fe2O3 with an orthorhombic structure; antiferromagnetic β-Fe2O3 with a bixbyite structure; and weak ferromagnetic α-Fe2O3 with a corundum structure. The observed phase transformations for Fe2O3 phases inside of the mesoporous silica matrix are due to the surface (or interface) energy GS contribution to the total free energy G = GB + (6Vm/d)GS, where GB is the free energy in the bulk, Vm is the molar volume and d is the diameter of the NP. The G vs. d curves should appear as shown in Figure 3.
Such a strong correlation between the most stable iron oxide crystal phase and the matrix makes it possible to create nanomaterials with diametrically different magnetic properties. Our team has provided the studies on the natural and synthetic silica-based systems containing inclusions of iron oxide NPs. Figure 4 illustrates the static magnetic characteristics of the silica-based systems with NPs of ε-Fe2O3 with a coercive force Hc = 1.07 T [37], γ-Fe2O3–Fe3O4 with Hc = 0.5 mT [38] and natural magnetite from the Kovdor deposit with Hc = 4 mT [6]. Thus, despite their almost identical chemical composition, the similar structures with iron oxides can possess different Hc values by more than three orders of magnitude.
Iron oxide compounds can differ not only by their coercive force, as shown above, but also by their other physical characteristics. Some of them are shown in Table 1.
The crystal structure of an iron oxide compound can be attributed to various symmetry groups: cubic for FeO [42], Fe3O4 [43] and γ-Fe2O3 [45], rhombohedral for α-Fe2O3 [44], and orthorhombic for ε-Fe2O3 [46]. Crystal structure variation influences the electron zone structure, which can be semi-metallic with an optical band gap (at 300 K) of 0.2 eV for Fe3O4 [68,69] and semiconductive for FeO with an optical band gap of 1.0 eV [46], as well as for α-, γ- and ε-Fe2O3 with an optical band gap of 1.9–2.4 eV [58,59,70,71,72,73]. There is no clear correlation between the crystal structure of an iron oxide compound and its dielectric permittivity, for which values are in the range of 12–40 for static permittivity [46,47,48,49] and 4–16 for high-frequency permittivity [46,49,50,51,52,53]. Since the various iron oxides can possess antiferromagnetic or ferrimagnetic properties, their value of saturation mass magnetization can differ by a few orders of magnitude: from 0.3 emu/g (at 300 K) for pure hematite [57,58] to 92–94 emu/g for stoichiometric magnetite [56]. Such variation in electrical and magnetic characteristics lies in the versatility of the structures containing iron oxides for their wide spectrum of possible applications. The next section is devoted to the main mechanisms of iron oxide formation disclosed in the scientific literature.

3. Mechanisms of Iron Oxide Formation

The literature data on the iron oxide formation mechanisms were divided into several groups according to the iron oxide compounds in the studied structures: the structures containing pure crystal iron oxide phases, the ones containing iron oxide atomic clusters and amorphous iron oxides, the ones containing two co-existing iron oxide crystal phases, and the ones containing three or more co-existing iron oxide phases. Their formation mechanism, either natural or synthetic, is briefly described based on the data in the cited reference. Furthermore, their existing and potential (declared) applications are also given.

3.1. The Structures Containing Pure Phases of FeO, Fe4O5, Fe3O4, and α-, β-, γ-, δ-, ε- and ζ-Polymorphs of Fe2O3

The information on the structures with a pure iron oxide crystal phase is given separately for each iron oxide compound in Table 2, Table 3, Table 4, Table 5 and Table 6.
Based on the gathered information, it was possible to determine the most frequently obtained form of the structures containing FeO (Table 2). This form is FeO NPs produced for biomedical applications via various chemical or physical routes, but not naturally originated, since this iron oxide compound is metastable under normal conditions [42,74,75,76,77,78,79,80].
As one of the stable iron oxide compounds, magnetite can be obtained from the various structures, including natural abiotic or biogenic and synthetic ones (Table 3). Fe3O4 NPs can be considered as one of the most frequently used iron oxide magnetic materials and their production techniques are mostly chemical since this approach is most suitable for biomedical applications [81,82,83,84,85,86,87,88,89]. Additionally, “green” techniques, involving the use of natural plant extracts [90,91], biomimetic formation [92] or microbial mineralization [93,94,95,96,97,98], are widely used. Other commonly described forms of magnetite are its inclusion within ore samples [6,99,100,101,102,103,104] widely used in geosciences and the external magnetite layer of metal surfaces [105,106,107,108,109], which is an important object of iron corrosion studies.
The most stable (under normal conditions) iron oxide compound, hematite, attracts the highest attention of scientists, according to the overall amount of published papers (Table 4). Similarly to magnetite, the most frequently obtained form of α-Fe2O3 is a chemically synthesized NP, predominantly used for photocatalysis, biogeochemistry and the bioremediation of toxic compounds [105,106,107,108,109,110,111,112,113,114,115,116,117,118,119]. The second important form of hematite is the natural ore with inclusions of α-Fe2O3, which is used in Earth sciences [117,118,119,120,121,122,123,124]. The use of the pure γ-Fe2O3 is quite rare. The most frequently produced form of γ-Fe2O3 is as NPs (Table 5), including chemically synthesized [125,126,127,128,129] or biogenic [93].
Structures with β-Fe2O3, ε-Fe2O3, ζ-Fe2O3, δ-Fe2O3 or Fe4O5 metastable phases (Table 6) are much less described compared to the main stable compounds of iron oxides. ε-Fe2O3 is the only one seriously applicable in biomedicine [130,131], electronics [132,133] or geosciences [134,135] and can be obtained both synthetically [37,136,137] or by extraction from various archeological objects [138,139,140]. There have also been some attempts to use β-Fe2O3 NPs in biomedicine [63], sensors and lithium-ion batteries [20,64].
Table 2. Data on the structures containing an FeO phase.
Table 2. Data on the structures containing an FeO phase.
CompositionMain Mechanisms of Iron Oxide FormationDeclared ApplicationsPhase Verification TechniquesRefs.
FeO NPsThermal decomposition of the iron(II) precursor, mechanochemical reduction of magnetite, flame synthesis, laser target interaction in liquid carrier mediaBiomedicine, electronics, spintronics, magnetic force microscopy, metastability studiesXRD 1, UV–Vis 2, MALDI-TOF MS 3, EELS 4, F-AAS 5 HAADF-STEM 6,[41,74,75,76,77,78,79,80]
Ultra-thin FeO filmOxidation of iron monocrystal surface Iron oxidation kinetics studyRMDS 7[141]
Electron-beam deposition on Au(111) surfaceIron catalysis, electronics, biomedicineSTM 8[142]
Millimeter-sized iron oxide particlesMagnetite reduction with iron as reducing agentCatalysts for ammonia synthesis TG-DSC 9[40]
FeO layer on the metal alloy surfaceInvar oxidation in a static
carbon dioxide atmosphere
Iron oxidation kinetics studyXRD, TG-DSC, TEM 10[143]
Wüstite inclusions in titanomagnetite particlesTitanomagnetite ironsand-fluidized bed reduction by hydrogenCommercial iron makingXRD[144]
FeO powderReduction of hematite in a gas-controlled electric furnaceEarth’s mantle sound velocity studiesXRD, IXS 11[145]
FeO inclusions in the mold fluxIron oxide formation in molten mold fluxStudy of the oxidation mechanism of mold flux-covered molten ironXRF 12[146]
FeO inclusions within the dense iron shellPorous hematite gas reduction under isothermal conditionsIndustrial exploitation of low-grade iron oresTG-DSC, XRD[147]
FeO clusters within the stable iron oxide matrixThe reduction of magnetite/hematite at temperatures of 400~500 °CIron catalysisQuantitative theoretical analysis[148]
1 X-ray powder diffraction. 2 UV–visible(-NIR) spectroscopy. 3 Matrix-assisted laser desorption ionization time-of-flight mass spectrometry. 4 Electron energy loss spectroscopy. 5 Flame atomic absorption spectroscopy. 6 High-angle annular dark-field scanning transmission electron microscopy. 7 Reactive molecular dynamics simulations. 8 Scanning tunneling microscopy. 9 Thermogravimetry/differential scanning calorimetry. 10 Transmission electron microscopy. 11 Inelastic X-ray scattering. 12 X-ray fluorescence.
Table 3. Data on the structures containing an Fe3O4 phase.
Table 3. Data on the structures containing an Fe3O4 phase.
CompositionMain Mechanisms of Iron Oxide FormationDeclared ApplicationsPhase Verification TechniquesRefs.
Fe3O4 NPsCo-precipitation from iron salt solution, co-precipitation from iron oxyhydroxide solution, solvothermal synthesis, electrochemical formation from a pure iron, thermal decomposition of the iron oleate complex, biomimetic process with use of a leaf extract, nucleation mediated by iron-binding protein Mms6, biogeneration with a use of amyloid peptide Aβ42Biomedicine, magnetic separation, antimicrobial and antioxidant applications, contaminant removal, black pigment production, ferrofluids TEM, XRD, SAXS 1, RS 2, FTIR 3, XPS 4, HAADF-STEM, EELS, TG-DSC, UV–Vis, PL 5, MSP 6, SAED 7[81,82,83,84,85,86,87,88,89,90,91,149,150,151,152,153,154,155,156,157,158,159,160,161,162]
Bacterial magnetosomesBacterial biomineralization, transient phosphate-rich ferric hydroxide reduction to magnetite, formation by dissimilatory iron-reducing bacteriaBiomedicine, paleomagnetism, microbial iron cycle studies, bioremediation of toxic compounds HAADF-STEM, TEM, XAS 8, SAED, XMCD 9[93,94,95,96,97,98,163]
Inclusions of Fe3O4 within ore samplesAbiotic hydrothermal mineralization, iron oxide formation derived from continental weathering, extrusive magmatic formation from iron oxide-melt liquidGeochemistry, environmental magnetism studies, early Earth iron cycle studies, origin and evolution of iron oxides studies XRD, RS, XPS, EDS 10, ICP-AES 11[99,100,101,102]
External magnetite layer on a metal surfaceOxidation of a steel surface, slow oxidation of green rusts at room temperature, high-temperature corrosion Corrosion studiesXRD, EDS, XRF, RS, XPS, AES 12[105,106,107,108,109,110]
Fe3O4 microparticlesMicrobial-induced precipitation with the use of Sporosarcina pasteuriiGreen synthesis of magnetiteEDS[28]
Aging of ferrous hydroxide gels at elevated temperaturesColloidal crud formation studiesXRD, TEM[162]
Self-assembled Fe3O4 mesocrystalline filmsHeat-up method with the use of iron(III) chloride and sodium oleateBiomedicine and industrial applicationsTEM, SAED, XAS, SAXS[163]
Fe0/Fe3O4 compositeControlled reduction of the starting Fe3O4 with H2Treatment of wastewaterMSP, XRD[164]
Magnetite nanowiresSupercritical fluid inclusion
within a mesoporous silica matrix
Soft magnetic materialsTEM, SAED, XRD, FTIR[165]
Inclusions of Fe3O4 NPsBacterial reduction of amorphous hydrous ferric oxide BiogeochemistryTEM, SAED, XRD, EDS[166]
Fe3O4 layer on the zerovalent iron surfaceSurface oxidation of iron by oxygen in an aqueous mediumOrganic pollutant removalEDS, XRD[167]
Epoxy/magnetite nanocompositesReduction of anhydrous ferric chloride by ammonium hydroxide Marine coatings of steelFTIR, XRD, TEM[168]
Iron oxide nanocomposite hydrogelCo-precipitation process by ammonium hydroxideBiomedicineXRD, TEM, TG-DSC, EDS[169]
Surface film containing Fe3O4 NPsBacterial mineralization in the air–water interface in Arctic tundra watersAnaerobic microbial carbon cycleTEM, EDS, STEM, EELS, FTIR, RS[170]
Nanocomposite hydrogel with Fe3O4 NPsReduction with ammonia from a remixed solution of FeCl3 and FeCl2BiomedicineTEM, XRF, EDS, TG-DSC, FTIR[171]
Biochar composite with Fe3O4 NPsOne-pot solvothermal method using
phoenix tree leaf-derived biochar
Treatment of wastewaterTEM, XRD, FTIR, XPS, ICP-AES[172]
Fe3O4 NP inclusions in the surface layerFormation of NPs along with cracks and pores during pre-oxidationPlasma nitriding of steelXRD[173]
Chitosan/graphene oxide composite with Fe3O4Co-precipitation of Fe3O4 and chitosan/graphene oxide Organic pollutant removalXRD, XPS, RS, FTIR[174]
Fe3O4 layer on carbon fibers of a carbon paperDeposition on the carbon paper gas diffusion layer at the cathodeCorrosion studiesXRD, EDS[175]
Mesocrystals assembled from Fe3O4 nanocubesHeat-up method with the use of iron(III) chloride and sodium oleateMesocrystal applicationsTEM[176]
Fe3O4 nanorods Formation in electron-beam-induced deposition from iron pentacarbonyl ElectronicsTEM, EELS[177]
Lipase immobilized on coated Fe3O4 NPsSolvothermal method with the use of FeCl3·6H2O and ethylene glycolBiodiesel productionTEM, XRD, FTIR[178]
Spherical mesoporous magnetite aggregatesPrecipitation from iron(III) ethoxide with ethanol in the surfactant solutionCatalysis, sustainabilityFTIR, XPS, EDS, TEM, MSP[179]
Perfluorocarbon-loaded hydrogel microcapsulesCoaxial interface shearing double emulsion methodBiomedicine[180]
Mesoporous magnetiteBall milling of Fe3O4 and SiO2 followed by partial reduction Recyclable absorbent for toxic Cr(VI) ionsTEM, XRD, XPS, ICP-AES[181]
Magnetite crystal modelLocal spin-density approximation
density-functional calculation
Magnetite electron structure studiesDensity-functional calculations[43]
Spherulite nanostructure with inclusions of Fe3O4Electron-beam irradiation of the precursor solution with iron nitrateCrystal growth dynamics studiesTEM, STEM, EDS[182]
1 Small-angle X-ray scattering. 2 Raman spectroscopy. 3 Fourier-transform infrared spectroscopy. 4 X-ray photoelectron spectroscopy. 5 Photoluminescence spectroscopy. 6 Mössbauer spectroscopy. 7 Selected area electron diffraction. 8 X-ray absorption spectroscopy. 9 X-ray magnetic circular dichroism. 10 Energy-dispersive X-ray spectroscopy. 11 Inductively coupled plasma atomic (optical) emission spectroscopy. 12 Auger electron spectroscopy.
Table 4. Data on the structures containing an α-Fe2O3 phase.
Table 4. Data on the structures containing an α-Fe2O3 phase.
CompositionMain Mechanisms of Iron Oxide FormationDeclared ApplicationsPhase Verification TechniquesRefs.
α-Fe2O3 NPsHydrothermal synthesis, precipitation from a ferric salt solution using a natural leaf extract, precipitation and aging of ferrihydrite in an oxidized system, direct transformation of α-FeOOH via high-energy ball millingBiomedicine, bioremediation of toxic compounds, photocatalysis, geochemistry, electronics, antibacterial activity studies, geochemistryXRD, FTIR, UV–Vis, EDS, TEM, RS, XPS, XAS, ICP-AES, EPR 1, HAADF-STEM, WAXS 2[105,106,107,108,109,110,111,180,181,182,183,184,185,186,187,188,189,190,191,192]
Inclusions of α-Fe2O3 in ore samplesPrecipitation from oxygenated iron-rich water or biomineralization, dissolution of Fe(III) hydroxides by Fe(III)-reducing bacteria, terrestrial subglacial oxidation of glacial iron fluvial depositionTerrestrial iron oxide concretion studies, Precambrian iron formation studies, Antarctic glacier studies, biogeochemistryEDS, RS, TEM, HAADF-STEM, XRD, SAED, FTIR, UV–Vis[112,113,114,115,116,117,118,119]
α-Fe2O3 layer on a metal surfaceAnodic potentiostatic oxidation of stainless steel sheet Anodic passivation of stainless steelAES[193]
Oxidation of steel in an O2-N2 atmosphere at high temperatureImprovement of steel coating quality TEM, EDS, GD-OES 3[194]
Corrosion of chromia-forming alloys in simplified combustion atmosphereFireside corrosion studiesEDS, XRD[195]
Porous α-Fe2O3 nanostructuresHydrothermal synthesis from FeCl3·6H2O in a microwave reactorLithium-ion batteriesXRD, TEM, SAED, XPS, TG-DTG[196]
Sol–gel transformations of precursors in self-organized nanocelluloseEnergy conversion and storageXRD, TEM, SAED, XPS, TG-DSC[197]
Martian hematite depositsPrecipitation from oxygenated iron-rich water or biomineralizationSearch for evidence of life on MarsEDS, TEM[198]
Hematite layers on sandstone grainsPrecipitation from oxidizing iron-saturated fluidGeochemistryXRD, ICP-MS 4[199]
Double-walled hematite nanotubesGrowth of Fe nanowires inside porous templates and oxidationPhotocatalysis, biomedicineXRD, EELS, HAADF-STEM, RS [200]
Coral-like and nanowire α-Fe2O3Thermal oxidation of iron foils in air- and water vapor-assisted conditionsRemoval of Cr ions from aqueous systemsXRD, RS, TEM, XPS[201]
α-Fe2O3 NPs on mineral surfacesWeathering of Fe-bearing silicate minerals or partial oxidation of Fe3O4Paleoclimate studiesXRD, TEM, SAED[202]
α-Fe2O3 nanorodsControlled aqueous growth from FeCl3·6H2O and NaNO3Photoelectrochemical water splittingXRD[203]
Inclusions of α-Fe2O3 in regolith simulantBall milling of commercial α-Fe2O3 samples in isopropyl alcoholCombustion studiesXRD, TG-DSC[204]
Inclusions of α-Fe2O3 in stone matrixBacterial mineralizationHeritage sciencesXRD, EDS, RS[205]
Inclusions of α-Fe2O3 in auriferous quartzTerrigenous abiotic mineralizationGeochemistryEDS[206]
Hematite layers on sandstone grainsTerrigenous co-precipitation with sandstone and uranium Geochemistry of radionuclidesGamma-ray spectrometry, ICP-MS[207]
Hematite inclusions encapsulated in chertDehydration of the interstitial goethite to hematite microplatesGeochemistryTEM, XRD, EDS[208]
Hollow α-Fe2O3 nanofibersElectrospinning with a use of iron chloride and poly(vinylpyrrolidone)Photoelectrochemical water splittingEDS, TEM, SAED, TG-DSC, UV–Vis[209]
Fossilized bacteria with α-Fe2O3 Biomineralization by anoxygenic photoferrotrophyBiogeochemistryRS[210]
Porous α-Fe2O3 xerogel and aerogelSol–gel synthesis from Fe(III) salts with addition of propylene oxideCatalysis, sensors, biologyTEM[211]
Iron oxide nanostructuresMicrobial Fe(II) oxidation of carbonate green rust by Fe(II)-oxidizing bacteriaPrecambrian iron formation studiesMSP[212]
Iron oxide biogenic precipitatesBacterial mineralizationBiogenic iron oxide formation studiesXAS[213]
Steel-wearing ejected debris with α-Fe2O3Steel fretting wear controlled by oxygen ingress to the contactSteel fretting wear studiesXRD[214]
α-Fe2O3 NPs on a steel surfaceOxidation of iron-bonded diamond precision-polishing wheelGrinding of hard and brittle materialsXRD, XPS, TEM[215]
Nanostructured α-Fe2O3 filmsElectrochemical anodization of steel in an alkaline solutionPhotocatalysis, anti-bioadhesionRS, UV–Vis[216]
Monodispersed micaceous α-Fe2O3Hydrothermal synthesis from iron chromium hydroxide precursorsIron chromium grinding waste recyclingICP-AES, XRD, XPS[217]
Nanoporous α-Fe2O3 layer on an iron foilAnodization of iron is an ethylene glycol and NH4F aqueous solution PhotocatalysisTEM, RS, XRD, UV–Vis, EDS, EELS[218]
Natural α-Fe2O3 from the iron depositsTerrigenous abiotic mineralizationPhotocatalytic recycling of toxic wastewaterRS, EDS, UV–Vis[219]
Nanocomposite containing α-Fe2O3Wet impregnation of Co3O4 powder with an Fe(NO3)⋅9H2O solutionCatalysisXPS, XRD, TG-DSC, EDS, TEM[220]
Stepped α-Fe2O3 (0001) surfacesFirst principles spin-polarized density-functional theory simulationChloride-induced iron depassivation studiesDensity-functional theory calculations[221]
α-Fe2O3 powderIn situ generation of iron oxide via decomposition of Fe(NO3)3·9H2O CatalysisXRD[222]
α-Fe2O3 nanorodsHydrothermal precipitation and air calcination of goethite nanorodsCatalysis, lithium-ion batteries, sensorsXRD, MSP, UV–Vis, EDS, TG-DSC[223]
α-Fe2O3 nano- and microparticlesChemically synthesized commercial α-Fe2O3 samplesMechanisms of oxide toxicity toward bacteriaFTIR, XAS[224]
α-Fe2O3 nanowiresHeating of iron wires suspended between two electric contactsVacuum electronic
TEM, EDS, XPS, RS[225]
α-Fe2O3 layer on zerovalent iron NPsIron oxide film formation under aerobic conditionsRemediation of water pollutantsTEM, FTIR, XPS, XRD[226]
Inclusions of α-Fe2O3 in rock varnishTerrigenous abiotic mineralization or biotic processesGeomicrobiologyXRD, RS, EDS[227]
Nanolayers of α-Fe2O3 in polymer compositeIron pentacarbonyl transformation with diamond anvil cells in Ar gasHigh-energy density solid studiesRS, TEM, XRD[228]
Jian ware blue-colored glaze with α-Fe2O3Calcination of a milled mix at a high temperature in oxidizing atmosphereAncient ceramics studiesXRD, UV–Vis, TEM, XPS[229]
Inclusions of α-Fe2O3 in sediment samplesMicrobial reduction of surface Fe(III) by iron-reducing bacteriaMicrobial iron reduction studiesXRD[230]
Core-shell iron/iron oxide NPsZerovalent Fe core-controlled oxidation during depositionOxide formation under e-beam radiation studiesTEM, EELS[231]
α-Fe2O3 film on a dielectric substrateLiquid-phase atomic layer deposition of crystalline hematiteCatalysis, sensors, lithium-ion batteriesXRD, UV–Vis[232]
Cube-shaped α-Fe2O3 microstructures Facile hydrothermal method using hydrated ferric nitrate and NaOHEthanol gas sensingXRD, FTIR, EDS, RS[233]
Iron oxide/Ti compositesPlasma electrolytic oxidation, impregnation and annealingPhenol photodegradationXRD, EDS, FTIR, XPS[234]
Microporous α-Fe2O3 NPsPrecipitation from iron(II) sulfate using a natural leaf extractSustainabilityXRD, UV–Vis, XPS, FTIR[235]
Inclusions of α-Fe2O3 in artificial clay Fe(OH)3 colloid mixing into chemically pure kaolinLaterite engineeringXRD[236]
Iron oxide nanotubesPotentiostatic anodization of iron foil in electrolytes containing NH4FCatalysis, sensors, supercapacitorsXRD, TEM, SAED[237]
α-Fe2O3 thin filmSpray pyrolysis from FeCl3 and methanol solutionElectrochemical supercapacitorsXRD, UV–Vis[44]
Corroded steel tube samples with α-Fe2O3Steel corrosion in an aqueous medium with oxygen and chlorinePipeline corrosion assessmentXRD, EDS, TEM, SAED[238]
Inclusions of α-Fe2O3 in stone samplesFormation by washing and leaching of a stone object by rainwaterLimestone artifact studiesRS, FTIR, EDS, XRF[239]
Iron oxide-loaded slagPrecipitation from FeCl3 solution with NaOH into melted slag Arsenic removal from waterICP-AES, XRD[240]
3D-ordered macroporous α-Fe2O3Impregnation of polymer matrices and high-temperature calcinationCatalysisXRD, TG-DSC, FTIR, SAED, UV–Vis, XPS[241]
α-Fe2O3/mesoporous silica core-shell NPsSolvothermal synthesis from ferric nitrate with sol–gel silica coatingCatalysis, biomedicineXRD, TEM, FTIR, UV–Vis[242]
α-Fe2O3 mesocrystal
Interface-driven nucleation by ferrihydrate oxidation and attachmentThermoelectronics, photonics, catalysis, photovoltaicsTEM, SAED, FTIR, EDS[243]
Hematite nanopillarsElectron-beam evaporation using anodized aluminum oxide templates with well-defined pore diametersPhotoelectrochemical water splittingXRD, XPS, UV–Vis[244]
1 Electron paramagnetic resonance spectroscopy. 2 Wide-angle X-ray scattering. 3 Glow-discharge optical emission spectrometry. 4 Inductively coupled plasma mass spectrometry.
Table 5. Data on the structures containing a γ-Fe2O3 phase.
Table 5. Data on the structures containing a γ-Fe2O3 phase.
CompositionMain Mechanisms of Iron Oxide FormationDeclared ApplicationsPhase Verification TechniquesRefs.
γ-Fe2O3 NPsSolvothermal synthesis from iron salts, bacterial mineralization, lepidocrocite calcination in an air atmosphere, hydrothermal and solvothermal synthesis from salt solutionsCatalysis, biomedicine, nucleation and formation of biogenic iron oxide studies, electronics, maghemite to hematite transition studies, sensorsTEM, EDS, SAED, XRD, XPS, EPR, FTIR, UV–Vis, ICP-AES, HAADF-STEM, MSP, in situ total scattering, XAS, SAXS, RS[93,125,126,127,128,129]
γ-Fe2O3 NPs in silica matrixGas-phase synthesis in a furnace aerosol reactor from iron pentacarbonyl BiomedicineXRD, TEM, EDS, FTIR, UV–Vis[245]
Dehydration of iron(III) hydroxide to magnetite followed by oxidationCatalysisXRD, FTIR[246]
γ-Fe2O3 powderChemically synthesized commercial γ-Fe2O3 samplesCatalytic oxidation of S(IV)ICP-MS, FTIR[247]
26-faceted maghemite polyhedronsDirect burning of ferrocene in different solvents in an alcohol lampLithium-ion batteriesXRD, TEM[248]
Magnetic polymeric NPs with γ-Fe2O3Co-precipitation of FeCl3/FeCl2·4H2O with NH4OH solutionBiomedicineTEM, TG-DSC, FTIR[249]
γ-Fe2O3 NP superlattice thin filmsChemically synthesized commercial γ-Fe2O3 samplesElectronics, optical coatingsGrazing incidence small angle X-ray scattering[250]
Maghemite-decorated graphene nanoscrollsHydrolysis of FeCl3·6H2O and W(CO)6, promoted with hydrazineEnergy storageTEM, XPS, TG-DSC, RS[251]
Hollow iron oxide NPsGas-phase vaporization synthesis of Fe NPs and oxidation to γ-Fe2O3Optics, nanoelectronicsTEM, HAADF-STEM, EDS[252]
Mesoporous iron oxideInverse micelle synthesis from Fe(NO3)3·9H2O butanol solutionArsenic removal from waterXRD, FTIR, RS, XPS[253]
Table 6. Data on the structures containing a β-Fe2O3, ε-Fe2O3, ζ-Fe2O3, δ-Fe2O3 or Fe4O5 metastable phase.
Table 6. Data on the structures containing a β-Fe2O3, ε-Fe2O3, ζ-Fe2O3, δ-Fe2O3 or Fe4O5 metastable phase.
CompositionMain Mechanisms of Iron Oxide FormationDeclared ApplicationsPhase Verification TechniquesRefs.
Pristine and co-substituted ε-Fe2O3Simulated crystal structure with a use of density-functional calculations Magnetoelectric material developmentDensity-functional theory calculations[254]
ε-Fe2O3 embedded in biomimetic graphenePrecipitation from ferric and ferrous chloride with a biocompatible polymerBiomedicineXRD, TEM, SAED, RS, XPS, TG-DSC, FTIR[130]
Epitaxially stabilized thin-film ε-Fe2O3Epitaxy on (100)-oriented yttrium-stabilized zirconia substratesElectronics, permanent magnets, biomedicineXRD, HAADF-STEM[136]
ε-Fe2O3 in ancient black glazed waresSurface iron enrichment and a firing of wares under reducing conditionsElectronics, spintronicsXRF, XAS, XRD, RS, TEM, EDS[132]
ε-Fe2O3 NPsHydrolysis of tetraethoxysilane in a solution of ferric nitrate and annealing Wireless technologies, electronicsXRD, TEM, THz-TDS 1[255]
ε-Fe2O3 inclusions in fired clay samplesStabilization of ε-Fe2O3 NPs in a matrix of silicates during firing of claysPaleomagnetismXRD, EDS[134]
Y3Fe5O12 matrix including ε-Fe2O3Formation of ε-Fe2O3 in the Y3Fe5O12 matrix using the sol–gel methodMagnetoelectric material developmentXRD, XPS, TG-DSC, FTIR[256]
δ-Fe2O3 in layered double hydroxyl Dry impregnation of layered double hydroxyl structure with ferric nitratePhotocatalysisXRD, FTIR, XRF, TG-DSC, UV–Vis[22]
ε-Fe2O3-SiO2Reverse micelle method with the use of ferric nitrateOxidative dehydrogenation of n-buteneXRD[136]
β-Fe2O3Milling of Fe2(SO4)3 and NaCl and calcination at 550 °C in air
Ga-substituted ε-Fe2O3 NPsCalcination of a mesoporous silica impregnated with metal nitratesBiomedicineXRD, XRF, TEM, ICP-MS[131]
ε-Fe2O3 in archeological brick and baked clayHigh-temperature firing of bricks and clays in airArchaeomagnetism, paleomagnetismRS[138]
ε-Fe2O3 in archeological samples, ε-Fe2O3 NPsSol–gel synthesis from ferric and barium nitrate with tetraethyl orthosilicateXRD, RS [135]
ε-Fe2O3 coatings on Si(100) substrates One-pot sol–gel recipe assisted by glycerol in an acid mediumPaleomagnetism, biomedicine, electronicsRS, XAS, EELS, HAADF-STEM[257]
ε-Fe2O3/SiO2 composite powderSol–gel synthesis from ferric and barium nitrate with tetraethyl orthosilicateElectronicsXRD, TEM[133]
ε-Fe2O3 nanorodsChemical vapor deposition from the Fe organic liquid sourcePhotocatalysis, electronicsXPS[258]
ε-Fe2O3/SiO2 compositeSol–gel synthesis from nitrate with tetraethyl orthosilicate and nitric acidElectronics, spintronics, magnetizable printingTG-DSC, XRD, TEM[259]
ε-Fe2O3 NPsImmersion of mesoporous silica with an FeSO4 or Fe(C10H9CHO) solution and high-temperature calcinationHigh-coercivity material developmentTEM, XRD, MSP, TEM, SAED[20,41]
β-Fe2O3 NPsSensors, lithium-ion batteries
Epitaxial ε-Fe2O3 films on GaN substratePulsed laser deposition on the Ga-terminated surface of the GaN (0001)ElectronicsXRD, RHEED 2, TEM, XAS, XMCD[260]
Silica-coated ε-Fe2O3 NPsSol–gel treatment of β-FeOOH nanorods with tetraethoxysilane and calcinationElectronicsXRD, TEM, EDS, MSP[261]
ε-Fe2O3 in a Hare’s Fur Jian ware High-temperature firing of local iron-rich area on the ceramic glazeMagnetoresistance materialsXRF, XAS, EDS, XRD, RS[140]
Impregnation of mesoporous silica NPs with rhodium-substituted ε-Fe2O3Electronics, magnetic force microscopy, biomedicineXRD[262]
β-Fe2O3 NPsThermally-induced solid-state reaction of NaCl with Fe2(SO4)3 in airSensors, lithium-ion batteriesXRD, MSP, TEM, SAED[20]
ζ-Fe2O3Pressure treatment of β-Fe2O3 NPs at pressures above 30 GPan/a
ε-Fe2O3 in a thin MgO(111) layerPulsed laser deposition from MgO and Fe2O3 targets ablated using a KrF laserElectronicsRHEED, XRD, neutron reflectometry[263]
Single crystal of Fe4O5Synthesis in the diamond anvil cell at
high pressure after laser heating
Solid Earth studiesDensity-functional theory calculations[23]
Nanometer-scale lamellae of Fe4O5High-pressure and high-temperature multi-anvil synthesisDeep Earth studiesXRD, TEM, SAED, EDS, STEM[264]
Powder of Fe4O5High-pressure and high-temperature direct synthesis from a mixture of Fe3O4 and FeElectronicsXRD, neutron diffraction[265]
β-Fe2O3 NPsThermally-induced solid-state reaction of NaCl with Fe2(SO4)3 in airOptoelectronics, sensors, lithium-ion batteriesXRD, MSP, TEM[64]
Hydrolysis of 2M FeCl3 in boiling water and cooling down slowly at room temperatureBiomedicineUV–Vis, TEM, XRD, FTIR, EDS, SAED[63]
1 Terahertz time-domain spectroscopy. 2 Reflection high-energy electron diffraction.

3.2. The Structures Containing Iron Oxide Atomic Clusters and an Amorphous Iron Oxide Phase

In their thermodynamic equilibrium state under normal conditions, iron oxides possess a crystal structure; therefore, there are only a few papers describing compositions with amorphous or poorly crystalline iron oxides, which are obtained via chemical [266,267,268,269] or biomineralization routes [270] and intended for various potential applications (Table 7). Iron oxide atomic clusters are a more frequently studied object and can be obtained either by synthetic chemical [271,272,273,274], physical [275] or biomimetic [276,277] techniques. Potential applications for iron oxide clusters include biomedicine [271,273], electronics [271,276], catalysis [273,274], and natural iron storage process studies [276].
The next type of iron oxide structure is ultra-thin, including two-dimensional films on metal surfaces [142,278,279,280], which are obtained using various chemical and physical techniques and can be applied to the production of molecular hydrogen [278], removal of contaminants [279], catalysis [142] and electronics [280]. Simulated iron oxide atomic clusters [281] and ultra-thin layers on a metal surface [282] are also described and can be useful for the prediction of the magnetic properties of FeOx NPs [281] and chemical water treatment technique development [282].

3.3. The Structures Containing Two Co-Existing Iron Oxide Crystal Phases

Real structures containing iron oxides in various cases are inhomogeneous, for example, due to the partial oxidation of magnetite to maghemite for synthetic [38,283,284,285,286,287,288,289,290,291,292,293], natural abiotic [6] and biogenic [294] origins. In this section, compositions with iron oxides containing two co-existing crystal phases are described (Table 8, Table 9, Table 10 and Table 11).
Crystal phases of α-Fe2O3 and γ-Fe2O3 quite rarely co-exist (Table 8), in comparison, for example, with the phases of magnetite and maghemite, according to the analyzed research articles. Nevertheless, such a combination can be found in natural and synthetic objects, including oxidized iron items [295], α/γ-Fe2O3 isoelement synthetic heterostructures with different crystal content [296], loess and paleosol [297] and saprolite soils [298] samples, graphene-iron oxide nanotube composites [299] and polyacrylonitrile/iron oxide composites [300]. The main applications of such structures are in the removal of contaminants [299,300,301], pedogenic process studies [297,298] and corrosion studies [302].
A more frequently discovered combination is the co-existence of α-Fe2O3 and Fe3O4 crystal phases (Table 9). They can be found in natural ore samples and can be explained by abiotic [303,304,305,306,307,308] or biogenic [309,310,311] processes. Besides these natural formations, such combinations of iron oxide phases can be synthetically obtained with the use of chemical [312,313,314], physical [315] or biomimetic [316] techniques. The main applications of the compositions are geosciences [306,307,310,311], biomedicine [312,314,317] and catalysis [313,316,317].
Cubic iron oxides, wüstite and magnetite can be co-existing (Table 10), despite these cases being rare, in comparison to the structures containing co-existing α-Fe2O3 and Fe3O4 phases [303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328], and can be attributed to applications of iron oxides in Earth’s mantle studies [329], porous iron growth mechanism studies [330], environmental remediation, electronics, catalysis, biomedicine and energy storage [331].
Structures containing iron oxides with a spinel structure, magnetite and maghemite, are widely used in various applications with a predominance of NPs intended for biomedicine [45,332,333,334,335,336,337] (Table 11). It is possible to propose the partial oxidation of magnetite NPs to maghemite in the vast majority of cases (except those with inert atmospheric preservation), but this effect can be distinguished only by the use of some additional instruments, including Mössbauer spectroscopy [6], zero-field and field cooling measurements to reveal a Verwey transition and high-resolution transmission electron microscopy [295] to show the crystal structure, while more widely used X-ray powder diffraction cannot resolve magnetite and maghemite [38]. The most frequently used synthetic routes used to obtain γ-Fe2O3–Fe3O4 NPs are by thermal decomposition [337,338,339,340] and chemical precipitation [332,336,341,342].
The iron oxide layer on metal surfaces is also a possible structure containing co-existing magnetite and maghemite phases. Such structures can originate from both natural [343,344] and synthetic [345,346,347,348,349] routes, generally implying chemical or electrochemical oxidation in a liquid medium. Overall, such compositions play an important role in iron corrosion studies [344,346,347,350,351]. Finally, compositions with magnetite and maghemite, presumably due to the presence of iron(II) and iron(III) cations, are actively used for the remediation of waste [348,352,353,354,355,356,357].
Table 7. Data on the structures containing iron oxide atomic clusters and an amorphous iron oxide phase.
Table 7. Data on the structures containing iron oxide atomic clusters and an amorphous iron oxide phase.
CompositionMain Mechanisms of Iron Oxide FormationDeclared ApplicationsPhase Verification TechniquesRefs.
Iron oxide atomic clustersCombustion synthesis from Fe(CO)5 mixed with hydrogen and oxygen, high irradiance laser ionization from pressed Fe2O3 and Fe3O4 tablets, biomineralization inside the ferritin shell, reaction of laser ablated iron foil with 5% O2 seeded in a helium carrier gasCatalysis, biomedicine, electronics, sensors, prediction of the magnetic properties of FeOx NPs, natural iron storage process studies, photovoltaicsMBMS 1, PMS 2, RMDS 3, TEM, LI-TOFMS 4, density-functional theory calculations, European Synchrotron Radiation Facility[271,272,273,274,275,276,277,281]
Surface iron oxide layer on metalMulticycling of an iron foil electrode between the switching potentials, formation of iron oxide species after reaction with Cr(VI) and Cu(II)Chemical water treatment, production of molecular hydrogen, removal of contaminantsRMDS, XRD, XPS, FTIR, EDS[278,279,282]
Amorphous ferric oxidesAdding Fe(II) or Fe(III) to seawaterBioavailable iron studies XAS, XRD[266]
Addition of Fe(III) to synthetic buffered solution or soluble microbial systemsChemical water treatmentUV–Vis[267]
Amorphous Fe2O3 in a silica matrixImpregnation of mesoporous silica with ferric nitrate and calcinationAntibiotic adsorptionTEM, XRD, FTIR, UV–Vis[268]
Poorly crystalline iron oxidesIron oxide biomineralization by iron-reducing bacteria GeochemistryICP-MS[270]
Amorphous iron oxide nanostructures Photothermal reaction inside a droplet
of iron(III) acetylacetonate solution
Electronics, sensorsTEM, SAED, EDS, RS[269]
Two-dimensional iron oxide on Au(111)Evaporating iron atoms, annealing and cooling down to 300 K in O2CatalysisSTM, density-functional theory calculations[142]
Iron oxide layer on zerovalent iron NPsZerovalent iron corrosion in an electrolyte solutionTreatment of contaminated aquifersUV–Vis, XAS[283]
Ferric oxide NPsProtein-promoted conversion of Fe(II) into insoluble ferric iron oxidesMitochondrial iron mishandling studiesUV–Vis[284]
Ultra-thin iron oxide
Iron oleate complex followed by selective decomposition at 150 °CBiomedicineTG-DSC, TEM, SAED, RS, XPS, FTIR[285]
High valent iron oxo complexesFluorine-substituted Fe−tetra-amidomacrocyclic ligand oxidationPhotocatalysisUV–Vis, EPR, high-resolution mass spectrometry[286]
FeO(111)-like film on Fe(110) surfaceInitial oxidation of Fe(110) in oxygen via Frank–Van der Merwe mechanismCatalysis, pigments, electronicsXPS, XAS, STM, AES, LEED 5, STS 6[280]
Colloidal Fe-FexOy composite NPsOxidation of metal NPs via a nanoscale Kirkendall processClean fuels, catalysis, electrochemical energyTEM, SAXS, WAXS, RMDS[287]
Biogenic microtubular iron oxidesBiotic formation of organic sheaths and subsequent abiotic deposition of FeCatalysis, pigmentsEDS, RS, TEM, XRD, STEM[288]
Iron oxide model thin-film electrodesThermal oxidation of pure metal iron substrates at 300 ± 5 °C in airLithium-ion batteriesRS, XPS, SIMS 7[289]
Iron(III) oxide/
hydroxide nanonetworks
Synthesis of iron(III) oxide/hydroxide xerogels from a hydrated ferric nitrateElectronics, catalysis, sensorsXPS, FTIR, XRD, TEM[290]
Fe0-iron oxide core-shell NPsPrecipitation from ferrous sulfate
with leaf extracts
Removal of nitrate in aqueous solutionEDS, XRD, FTIR[291]
Soil samples with amorphous iron oxidesAbiotic mineralization in soil pore structuresSoil weathering studiesXRD, ICP-AES[292]
Reticular pipeline cracks filled with iron oxideDecarburization and diffusive oxidation
of steel matrix
Corrosion resistance studiesEDS[293]
1 Molecular beam mass spectrometry. 2 Particle mass spectrometry. 3 Reactive molecular dynamics simulations. 4 Laser ionization orthogonal time-of-flight mass spectrometry. 5 Low-energy electron diffraction. 6 Scanning tunneling spectroscopy. 7 Secondary ion mass spectrometry.
Table 8. Data on the structures containing co-existing α-Fe2O3 and γ-Fe2O3 phases.
Table 8. Data on the structures containing co-existing α-Fe2O3 and γ-Fe2O3 phases.
CompositionMain Mechanisms of Iron Oxide FormationDeclared ApplicationsPhase Verification TechniquesRefs.
Saprolitic soil samples Aerobic weathering of Fe-bearing mineralsPedogenic process studiesXRF, UV–Vis, XRD[298]
Loess and paleosol samples with iron oxidesAerobic weathering of Fe-bearing silicate mineralsXRD, UV–Vis[297]
Oxidized iron itemsSoil iron corrosion limited by the diffusion of dissolved oxygenHeritage scienceEDS, XRD, RS[295]
Surface iron oxide layer on metalAnodic film formation on steel immersed in sour acid mediaCorrosion resistance studiesXRD, EDS[302]
Graphene-iron oxide nanotube compositeAn adept template-free hydrothermal route from ferrous sulfateRemoval of the toxic
heavy metal Cr(VI)
EDS, XRD, FTIR, UV–Vis, TEM[299]
Polyacrylonitrile/iron oxide compositeHydrothermal method for in situ growth of iron oxide; iron alkoxide hydrolysisRemoval of Congo red dye from waterFTIR, XRD, EDS, ICP-AES[300]
Carbon/FexOy magnetic compositesMechanical mixing and thermal treatment under N2 atmosphereWastewater treatmentXRD, TG-DSC, EDS, FTIR[301]
Isoelement synthetic heterostructuresHydrothermal method combined with controlled partial annealing processVisible-light
Table 9. Data on the structures containing co-existing α-Fe2O3 and Fe3O4 phases.
Table 9. Data on the structures containing co-existing α-Fe2O3 and Fe3O4 phases.
CompositionMain Mechanisms of Iron Oxide FormationDeclared ApplicationsPhase Verification TechniquesRefs.
Inclusions of iron oxides in ore samplesPrecipitation during protracted hydrothermal fluid/rock interaction, biological oxidation of Fe(II) by photoautotrophs, microbial sedimentary ferric iron flux, infiltration by hypogene and supergene fluids during or after deformationBanded iron formation studies, geochemistry, late Archean and early Paleoproterozoic studies, iron oxide copper gold system studiesICP-MS, EDS, XRF, SAED, TEM, XRD, ICP-AES, TG-DSC[303,304,305,306,307,308,309,310,311]
Surface iron oxide layer on metalTribo-oxidation wear of the cast iron discBrake system wear studiesEDS, XRD, TEM, SAED[315]
Iron oxide NPsAnodization of Fe sheet in ethylene glycol electrolyte and calcinationBiomedicine, catalysis, photovoltaics, electronicsXRD, EDS, XPS, RS, FTIR[312]
Iron oxide inclusions in concrete samplesCorrosion of a steel-reinforcing bar in air-entrained concrete with chloridesCorrosion resistance studiesEDS[318]
Iron oxide nanosheets and nanowiresThermal oxidation of iron foils in the presence of water vaporCr(VI) removalXRD, TEM, RS, UV–Vis[319]
Iron oxide hollow spheresMicrowave–hydrothermal ionic liquid method, calcination and autocatalysis PhotocatalysisXRD, TEM, UV–Vis[314]
Inclusions of iron oxides in mineralized rocksAbiotic formation of a mineral depositGeochemistryXRF[320]
Theoretically calculated iron oxide phasesRadiation-chemical oxidation of Fe depending on pH and oxygen contentPrecambrian studiesKinetics of iron oxidation calculations[321]
Iron oxide NPs supported on biogenic silicaIron oxide NP impregnation under hydrothermal conditions and calcinationRhodamine B photocatalytic degradationEDS, XRD, UV–Vis, TEM[316]
Sediment samples with inclusions of iron oxides Mineralization by variable diagenetic processesRock magnetism studiesXRD, EDS[322]
Iron oxide nanorods Sols of ferric hydroxide radiolysis in water under gamma irradiationElectronics, biomedicineXRD, TEM [314]
Spinel-bearing peridotiteOxidation of ferrous iron in olivine and pyroxene into ferric ironSerpentinization studiesFTIR, EDS[323]
Iron oxide inclusions in kaolin clay samplesAbiotic chemical precipitationClay chemistry and morphology studiesICP-AES, XRD, XRF, TG-DSC [324]
Precipitates containing iron oxide inclusionsBiomineralization by photosynthetic Fe(II)-oxidizing bacteriaBanded iron formation studiesXRD, EDS[325]
Iron-mineralized biofilmsDissolution and re-precipitation of iron oxide mineralsBioremediation of iron ore mines[326]
Iron oxide nanotubesTemplate-based electrodeposition and
calcination under oxidizing atmospheres
Biomedicine, electronics, gas sensors, catalysisTEM, XRD, SAED[317]
Iron oxide powderHydrothermal process with a use of pyrite cinder lixiviumPyrite cinder reutilizationFTIR, XRD, TEM, SAED[327]
Growth model for submarine depositsTransformation of primary (hydr)oxides via reduction by organic matterBanded iron formation studies[328]
Table 10. Data on the structures containing co-existing FeO and Fe3O4 phases.
Table 10. Data on the structures containing co-existing FeO and Fe3O4 phases.
CompositionMain Mechanisms of Iron Oxide FormationDeclared ApplicationsPhase Verification TechniquesRefs.
Fe-rich carbonates with inclusions of iron oxidesLaser heating of natural goethite in a diamond anvil cell in CO2Earth’s mantle studiesXRD, XAS, TEM, EELS, HAADF-STEM, SAED[329]
Samples with partially reduced FeO and Fe3O4Porous iron growth from wüstite in CO/CO2 and H2/H2O systemsPorous iron growth mechanism studies[330]
Fe/oxide core-shell NPsFormation of Fe3O4 during the oxidation of Fe NPs; high-temperature reduction of Fe3O4 to FeO by an electron-beamEnvironmental remediation, electronics, catalysis, biomedicine, energy storageTEM, SAED, EELS, HAADF-STEM, EDS[331]
Table 11. Data on the structures containing co-existing γ-Fe2O3 and Fe3O4 phases.
Table 11. Data on the structures containing co-existing γ-Fe2O3 and Fe3O4 phases.
CompositionMain Mechanisms of Iron Oxide FormationDeclared ApplicationsPhase Verification TechniquesRefs.
Iron oxide NPsThermal decomposition of iron oleate, continuous flow synthesis, co-precipitation of Fe3+/Fe2+ ions, aerosol spray pyrolysis with the use of ferric nitrate and ferric chloride, precipitation from iron salts with natural leaf extractBiomedicine, soil remediation, metal removal, wastewater treatment, electronics, catalysis, energy storage, groundwater remediationTEM, XRD, FTIR, SAED, TG-DSC, UV–Vis, SAXS, neutron diffraction, EDS, MSP, EELS, EPR, ICP-MS, XAS, RS[45,332,333,334,335,336,337,338,339,340,352,353,354,355,358,359,360,361,362]
Surface iron oxide layer on metalOxidation of a pure iron surface in oxygen, electrochemical reduction of lepidocrocite and ferrihydrite, in situ formation on an iron surface depending on the applied potentialIron oxidation studies, atmospheric steel corrosion studies, groundwater remediation, corrosion protection studiesXPS, XRD, XAS, RS, AES, ellipsometry[345,346,347,348,349]
Oxidation layer on archaeological steelCombined iron oxidation/iron(III) oxyhydroxide reduction without O2Corrosion studies on ancient metallic objectsEDS, RS[343]
Iron oxide-TiO2 nanorod heterostructuresPrecipitation by injection of Fe(CO)5 into stirred TiO2 containing mixtureOptoelectronics, biomedicine,
Iron oxide in nanoscrolls and nanoribbonsPrecipitation from ferric and ferrous chloride with ammonia solutionLithium-ion storage, photocatalysis, biosensorsTEM, FTIR[364]
Iron oxide hollow core/Shell NPsSolvothermal synthesis from FeCl3 and urea in ethylene glycol and calcinationBiomedicineXRD, TEM, TG-DSC, UV–Vis[365]
Thin-film nanocomposite membrane with iron oxideIn situ synthesis from aqueous solutions containing ferric chlorideBiofouling protectionEDS, TEM, XPS, UV–Vis, XRD, TG-DSC[366]
Magnetoferritin iron oxide NPsControlled mineralization from recombinant human H-chain ferritinBiomedicineTEM[367]
Iron oxide-based hollow magnetic nanoparticlesSynthesis from iron pentacarbonyl in 1-octadecene and oleylamineExchange bias studiesXRD, TEM, FTIR, MSP, F-AAS[368]
Albumin protein-based magnetic NPsCo-precipitation of FeCl2 and FeCl3 by ammonia in the presence of proteinBiomedicineTEM, TG-DSC[369]
Composite of organic matrix and iron oxide NPsThermal decomposition of iron(III) oleate complexBiomedicineTEM[370]
Iron oxide powderPhotochemical oxidation of siderite (FeCO3) by ultraviolet radiationBanded iron formation studiesXRD[371]
Interfacial iron oxide layer on iron artifactsIron corrosion in an anoxic environment after a pH increase at the interfaceAnoxic corrosion of archaeological steel studiesHAADF-STEM, RS, EDS, SAED, SIMS[344]
Iron oxide hydroxyapatite core/shell nanocompositesPrecipitation from ferric and ferrous chloride with ammonia under N2 BiomedicineTEM, FTIR, XRD, AAS, EDS[372]
Chitosan-based beads with iron oxide NPsCo-precipitation from ferric and ferrous chloride with NaOH solutionRemediation of water sourcesXRD, FTIR, TG-DSC, EDS[356]
Silica–iron oxide nanocompositeCo-precipitation from ferric and ferrous chloride with ammonia solution Toxic species removalXRD, TEM, FTIR, UV–Vis, SAED[357]
Vertical tube-shaped iron-oxide accumulationsDeep water corrosion of carbon steel Marine corrosion studiesEDS[351]
Hydrogels with embedded iron oxide NPsIn situ mineralization of iron ions in a hydrogel matrixDye removalXRD, FTIR, TG-DSC, TEM[373]
Corroded reinforced concreteIron corrosion in a laboratory corrosion chamberSteel rebar corrosion studiesXRD, EDS[350]
Porous hollow iron oxide NPs on carbon nanotubesEtching of Fe-FexOy intermediate with nitric acid aqueous solution and dryingBiomedicine, catalysis, separationTEM, XRD[374]
Iron oxide embedding of bacterial cellsBiomineralization by thermophilic iron-reducing bacteriaBiogenic iron mineral formation studiesXRD[375]
Activated carbon aerogel with iron oxide inclusionsHydrothermal synthesis from ferrous sulfate with ammonia Catalytic oxidation of pesticidesXRD, FTIR, XPS, TEM[376]
iron oxide NPs
Thermal decomposition of iron(III) acetylacetonate in triethylene glycolBiomedicineTEM, TG-DSC, FTIR, ICP-AES[377]

3.4. The Structures Containing Three or More Co-Existing Iron Oxide Phases

The co-existence of three or more iron oxide compounds in a single heterogeneous composition makes it useless to try to precisely distinguish every standalone phase. Thus, studies devoted to such a case are considered in this section and listed in Table 12. Iron oxide NPs and the surface oxide layer on metal surfaces are the largest groups of papers on the structures containing mixed iron oxide phases. Synthesis techniques for obtaining mixed iron oxide NPs include physical ones (flame synthesis [378], thermal oxidation [379] and laser ignited combustion [376]) and chemical ones (thermal decomposition [82,380] and precipitation [341], including “green” process [381]). The main application of such NPs is in biomedicine [82,378,379,380].
An iron oxide layer on the metal surface is the second commonly considered structure, which plays an important role in corrosion studies [383,384,385] and can be formed both naturally (e.g., iron carboxylate transformation in a leaf contamination on rails [386] or fireside corrosion of steel in the furnace walls in boilers [384]) and synthetically (e.g., in situ oxidation of the surface of a steel sample in a controlled atmosphere [387] or electrochemical anodization of metal in a simulated acid rain solution [385]).
Compositions with mixed iron oxides can originate via a biogenic route, including microbial direct or non-direct biomineralization [388,389,390], anoxygenic photosynthesis by a photoferrotrophic bacterium [391] and the reduction of solid ferric hydroxide by iron-reducing bacteria [392]. Biogenic iron oxide structures can be used in microbial iron reduction studies [392], banded iron formations [391] or hydrothermal vent field studies [389] and waste remediation [390].
Similarly to the above mentioned iron oxide structures, various compositions containing mixed iron oxide phases can be applied for catalytical purposes. These compositions include iron oxide NPs [379], Fe-based nanocomposites [393], silica–iron oxide nanocomposites [394] and FexOy@C spheres [395] and can be obtained with the use of chemical [394,395] or physical [379,393] synthesis.
Finally, in some cases, such structures are used for various environmental tasks, including environmental safety [396], environmental remediation [397], boreal forest studies [398], inositol phosphate selective retention in soil [399], biovermiculation studies [388] and the Earth and planetary deep interior studies [24].

3.5. The Main Characterization Techniques Used to Verify Phase Composition

From analyzing Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12, the most frequently used characterization techniques can be revealed. These techniques include X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), UV–visible(-NIR) spectroscopy (UV–Vis), selected area electron diffraction (SAED), Raman spectroscopy (RS), thermogravimetry/differential scanning calorimetry (TG-DSC), X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy (MSP) and X-ray fluorescence (XRF). The diagram showing the partial distribution between these techniques is presented in Figure 5.
In some cases, while the crystal structure of the co-existing iron oxide compounds differs significantly (e.g., for hematite and magnetite), the most common and available techniques are suitable for phase composition verification, including XRD and XRF. For instance, XRF and XRD techniques were used to determine the chemical and crystalline phase composition, accordingly, of clay samples containing iron oxides [324]. Another possible example is ε-Fe2O3, a crystal structure that is also quite distant from the other iron(III) oxide polymorphs and can be distinguished with the use of RS and XAS [258], or even with the use of a standalone XRD technique [37]. Contrary to the above, the simultaneous existence of magnetite and maghemite crystal phases cannot be adequately analyzed with XRD, SAED or some other techniques due to the similar crystal structure of these compounds and non-stoichiometry of synthetic [38] or natural [6,295] magnetite. In this case, the MSP technique can give additional information on the crystal structure, including magnetite to maghemite partial transition or the superparamagnetic state of NPs [38]. MSP can be applied to characterize samples composed of homogenously sized iron oxide NPs above the blocking temperature in a superparamagnetic regime [368]. Mössbauer measurements were performed to investigate detailed iron mineralogy compositions in magnetic fractions of fly ashes [405]. Perecin et al. showed that although Mössbauer spectra with two sextets were expected for pure magnetite, an extra sextet suggested the maghemite phase’s presence in the sample, in agreement with the FTIR results [89]. The high oxidation degree of magnetite can be confirmed by low isomer shift values [342]. MSP results can show a shift of the Morin transition in hematite upon increasing Ru3+-to-Fe3+ substitution, similar to the shift in the Morin transitions occurring in temperature-dependent magnetization measurements [224].

3.6. The Analysis of the Distribution of Iron Oxide Compounds by their Frequency of Mention

Based on the data provided in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12 (in total, more than 300 research articles were analyzed), a histogram was built (Figure 6). Compositions containing magnetite are the most frequently considered in scientific articles. This can be explained by taking into account the highest saturation magnetization of this iron oxide compound among others and also because of the wide biomedical application of magnetite NPs. The second most commonly described compound is hematite, presumably due to its use in photocatalysis and its abundance (the same for magnetite ores) in nature. The third important iron oxide compound is maghemite, since γ-Fe2O3 NPs are often used instead of magnetite NPs due to the high oxidation instability of Fe3O4 in air atmospheres and, therefore, difficulties in its preservation without an inert atmosphere or a protective shell. Other iron oxide compounds, including FeO, β-Fe2O3 and ε-Fe2O3, are metastable and/or their synthesis procedure is too complex, therefore scientific studies on them are relatively rare.

3.7. The Main Mechanisms of Iron Oxide Formation

Using the information given in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12, the generalized scheme illustrating some widely considered mechanisms of iron oxide formation was drawn (Figure 7). Terrigenous formation implies abiotic mineralization [100,102,206,219,292,324]. Extraterrestrial formation including samples from Mars [113] originated from precipitation from oxygenated iron-rich water [198] and abiotic formation in an aqueous environment of deposition [124].
Biomineralization implies bacterial magnetosome formation [94,96,97,98], formation by dissimilatory iron-reducing bacteria [95], bacterial reduction of iron hydroxide [122,166,392], biomineralization by anoxygenic photoferrotrophy [210], biomineralization inside the ferritin shell [276,277], biogeneration of magnetite with a use of the amyloid peptide Aβ42 in the case of brain diseases [160], etc.
Iron and iron alloy corrosion include chemical [106,109,238,350] and electrochemical [86,349,385] processes, either natural or intended. Ceramics firing implies the calcination of a milled mix at a high temperature in an oxidizing atmosphere [229], high-temperature firing of local iron-rich area on a ceramic glaze [140] and surface iron enrichment and firing of wares under reducing conditions [132]. Biomimetic synthesis refers to a process using natural plant extracts [91], iron oxide NP formation on biogenic silica [316], nucleation of Fe3O4 NPs mediated by the iron-binding protein Mms6 [92] and protein-promoted conversion of Fe(II) into insoluble ferric iron oxides [284].
Chemical precipitation includes co-precipitation by sodium hydroxide from an iron chloride solution [82], precipitation from Fe3+ and Fe2+ ions by urea with chitosan [149], precipitation from iron(II) sulfate heptahydrate with NaOH [158] and precipitation from iron(III) ethoxide with ethanol in the surfactant solution [179]. Physical deposition implies electron-beam deposition [142], liquid-phase atomic layer deposition [232], pulsed laser deposition on the Ga-terminated surface of a GaN (0001) [260] and chemical vapor deposition from an Fe organic liquid source [258]. Finally, mechanochemical synthesis includes mechanical mixing and thermal treatment under a N2 atmosphere [301], reactive spark plasma sintering of mechanically activated Fe powders [405] and milled zerovalent iron corrosion in anaerobic synthetic groundwater [348]. All the listed routes of iron oxide formation can be either intended (controlled) or natural (uncontrolled).

4. The Main Applications of the Structures Containing Iron Oxides

The largest amount of scientific papers being analyzed describes various biomedical applications of iron oxides, mainly magnetite/maghemite NPs. Photocatalytic oxidation and other applications in catalysis are the second major practical use of iron oxides, with a predominance of compositions with hematite. The third important field of use is electronics, including spintronics, data storage development and optoelectronics, mainly for thin ferrimagnetic or antiferromagnetic iron oxide films. Corrosion science is a quite obvious, but still very important area of application, since iron oxide passivation can provide a better reliability of steel pipelines, safety of ancient artifacts for heritage science, etc. Waste remediation is generally based on the possibility of iron oxidation from iron(II) to iron(III) and to absorb or bind inorganic and organic pollutants. Finally, serious attempts in geosciences with the use of various iron oxide compositions are still being made, despite a long history of research. Studies of natural iron ores can give much information about Earth’s evolution, including Precambrian research.
Biomedical applications of iron oxides include T2 magnetic resonance imaging (MRI). MRI contrast agents are based on superparamagnetic NPs; their nanocluster formation increases the magnetic signal and subsequently enhances imaging sensitivity or cell labeling efficiency [29]. Magnetic hyperthermia with alternating magnetic fields requires magnetic NPs having an effective heating rate to enable therapeutic applications [11]. There are diverse bioinspired approaches for the synthesis of magnetic nanochains with optimal properties for biomedical applications, including magnetically guided drug delivery [13]. The integration of magnetic NPs and organic dyes into single platforms demonstrated their use as bimodal imaging agents for both in vitro and in vivo imaging and in multifunctional platforms that perform several tasks in parallel (e.g., dual-mode imaging and photodynamic therapy or drug delivery) [14].
Iron oxide formation mechanisms are important for understanding formation processes of iron-relevant minerals in Precambrian banded iron formations, granular iron formations and associated iron-poor strata [4,33]. This understanding also includes the origin of probably the first protosensory system evolved on Earth, i.e., magnetotaxis, while the precise mechanism of biogenic magnetite mineralization on early Earth is still unknown [8]. Well-known processes of the biomineralization of iron can help in better understanding human body iron metabolism and in curing diseases linked with iron-damaged regulation [10]. Today, by far the single most important use (by volume) of iron oxides is as a source of Fe, which is subsequently processed to make steel [3]. Another important iron oxide industrial application is photoelectrochemical water splitting. It is a leading strategy for producing a promising renewable store of energy—hydrogen [12]. Iron oxides, including magnetite, maghemite and hematite, are widely used in heterogeneous catalysis processes and have been attractive alternatives for the remediation of polluted soil, groundwater and wastewater based on a heterogeneous Fenton reaction (a combination of a solid Fe-based catalyst and H2O2) [25,26]. Partially oxidized zerovalent iron NPs with a core-shell structure can be used to remediate groundwater and wastewater contaminated by chlorinated organic compounds, heavy metals, dyes and phenols [27].
Rare iron(III) oxide polymorphs can also be practically used. Thus, β-Fe2O3 has found a few applications in an electro-catalyst for the reduction of hydrogen peroxide, in optoelectronics and in red ferric pigments [19]. A very high room-temperature coercive field makes ε-Fe2O3 suitable for use in recording media; its magnetoelectric coupling and millimeter-wave ferromagnetic resonance are useful in electric/magnetic field tunable devices and for millimeter wave absorption on the walls of an interior room or on the body of a car, train or airplane [36]. In some cases, e.g., in catalysts, amorphous Fe2O3 NPs can be more active than nanocrystalline polymorphs or particles of metallic iron of the same diameter [39]. The structures of iron oxides are common to many binary systems and complex solid solutions; therefore, a rich set of isostructural compounds and solid solutions with tunable properties may be synthesized [23].
Ceramics, including composites containing inclusions of amorphous iron oxides, are suitable for various industrial applications. The process of transforming iron oxides from a glass network into a crystal nucleus was studied for the novel field of glass ceramics based on waste glass [408]. Prim et al. showed that iron oxide from a metal sheet treatment process may be used as a ceramic pigment by encapsulation in a crystalline and amorphous silica matrix [409]. Intended for hazardous waste incineration, glass ceramics containing hematite exhibited a superior compressive strength, volume density and water absorption [410]. Alumina–zirconia–titania ceramic membranes coated with a nanosized hematite layer can be applied in a combined ozonation–membrane filtration process [411]. The formation of solid solutions between mullite and transition metal cations, including iron, affects the thermal expansion of mullite ceramics through the distortion of the Al–O octahedral [412].
Less than 20 a wt% addition of iron oxide significantly lowered the softening and melting temperatures of CaO–Al2O3–MgO–SiO2-based glass ceramics [413]. The lower melting temperature leads to a significant decrease in the price of the vitrification procedure and to the suppression of heavy metal evaporation during glass melting [414]. Such a class of glass ceramics possessing excellent mechanical characteristics (bending strength of 120 MPa, hardness of 9 GPa and fracture toughness of 1.6 MPa∙m1/2) was discussed, together with the remarkable effect of their vitrification on heavy metal immobilization [415]. One of the most low-temperature techniques, the sol–gel method, which involves the hydrolysis of the precursors of constituent oxides followed by their gelation, has the potential to yield magnetic ceramics, including bioceramics, with a more flexible composition range, better homogeneity, better bioactivity and controllable porous structure [416]. Nanostructured catalyst-modified composite cathodes can be obtained by infiltrating a metal ion solution into a ceramic scaffold, followed by heating at a high temperature. A 3D heterostructured electrode decorated by amorphous iron oxide that works stably at 650 °C with an oxygen reduction reactivity comparable to that of a Pt-decorated one was obtained [417].
A wide range of metals such as gold, silver, copper, zinc, iron, platinum and palladium are fabricated in the form of NPs using algae and cyanobacteria and can be applied for infection control, diagnosis, drug delivery, biosensing and bioremediation [418]. An approach for the synthesis of highly pure, crystalline and biocompatible hematite NPs through the sole use of Psidium guajava leaf extract was proposed. The antibacterial efficacy of the obtained hematite NPs against Gram-positive as well as Gram-negative bacteria was established [419]. Both plants and microbes offer various ways to synthesize magnetite and maghemite NPs for potential dye degradation from industrial effluents from a variety of routes due to their vast genetic diversity and presence of various enzymes, respectively [420]. Co-substituted magnetite NPs were produced during the enzymatic reduction of a synthetic co-ferrihydrite using Geobacter sulfurreducens as an analogue to bioreduction processes in the natural environment to understand the natural biogeochemical cycling of cobalt in Fe-rich environments undergoing microbially mediated redox transformations [421]. Natural biogenic iron oxide extracted from banded iron formations showed high removal potential with the maximum sorption efficiency of 88.65% at a 30 g/L adsorbent dose [422].
Amorphous iron oxides are the promising material for various energetic and catalytic applications, including biomedicine. Compared to well crystalline Fe2O3, amorphous Fe2O3/graphene composite nanosheets exhibited superior sodium storage properties such as high electrochemical activity, a high initial Coulombic efficiency of 81.2% and a good rate of performance for sodium-ion batteries [423]. Efficient water oxidation catalysts have nominally amorphous mixed-metal oxide phases on their surface which are responsible for catalytic activity [424]. Amorphous iron oxide-packaged oxaliplatin prodrugs can be effective for cancer treatment, since the Fe2+/Fe3+ ions released by the amorphous iron oxide NPs produce a large amount of reactive oxygen species through Fenton’s reaction [425]. Nanosized amorphous iron oxide showed higher catalytic activity with lower oxidant consumption in comparison to Fe3O4- and Fe2O3-based clay composites [426]. Amorphous Fe2O3 nanoflakes were biosynthesized by a novel sol–gel method using Aloe vera leaf extract, and their catalytic effect on the thermal decomposition of ammonium perchlorate was investigated [427]. Amorphous Fe2O3 NPs can act as efficient and robust photocatalysts for solar H2 evolution without any cocatalysts [428]. Amorphous Fe2O3/reduced graphene oxide/carbon nanofiber films were tested as flexible and freestanding anodes for lithium-ion batteries [429].
Corrosion resistance studies involving iron oxides are also important. Thus, potassium and chlorine may interplay to accelerate the corrosion of Fe-rich oxide scales, and an understanding of this process may open up new ideas for ways to decrease corrosion in highly corrosive environments [430]. Wheat straw fiber-reinforced polyvinyl chloride composites pigmented with iron oxide pigment have better seawater corrosion resistance, including better fiber/matrix interfacial interaction, lower total discoloration and higher surface hydrophobicity, mechanical properties and thermal stability [431]. Hydroxyapatite-bioglass-Fe3O4-chitosan coatings showed an effective improvement of the surface properties, hemocompatibility and in vitro corrosion rate of a biodegradable magnesium alloy [432]. The corrosion resistance of the epoxy coating was experimentally improved using micaceous iron oxide and Al pigments [433]. In the case of stainless steel, iron oxide formation corresponds to a low pitting potential and corrosion resistance and leads to the degraded protective property of the oxide film [434].
Based on the data in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12, a diagram illustrating the main declared applications of various compositions with iron oxides was built (Figure 8).

5. Summary and Perspectives

Iron oxide compounds are widely presented in various scientific and industrial areas due to their abundance on Earth. The possibility of changing the iron oxidation state between Fe2+ and Fe3+ lies in the basement of the biogenic iron cycle, which results in band iron formation accumulations and the deposition of fossilized magnetotactic bacteria, called magnetofossils [435]. The mass production of metallurgy and large iron deposits, e.g., the Kovdor deposit [6], provides a low cost of iron oxides compositions and their applicability for different technical purposes. High biocompatibility and modern synthesis techniques, including continuous-flow, biomimetic and biogenic processes, make it possible to translate academic research to clinical practice [435,436,437].
Integrating magnetic NPs with polymers allows for the fabrication of multifunctional systems for chemotherapy and magnetic hyperthermia therapy, which can also be simultaneously monitored by utilizing the magnetic resonance imaging capabilities of magnetic nanoparticle–polymer conjugates [14]. Such systems, e.g., iron-loaded crosslinked magnetic chitosan/graphene oxide, can also be widely applied for the practical environmental remediation of wastewater effluents containing organic pollutants [174]. The novel iron oxide-based materials can be used to improve solar fuel production [110]. The use of ferritin protein as a carrier of iron oxide NPs renders it more suitable for cancer diagnosis as an effective T2 contrast agent with an expected reduced toxicity due to the prevention of NP interaction with the environment [367]. Two-phase iron oxide NPs (e.g., magnetite/maghemite core-shell structures) are promising for applications implying an intrinsic exchange bias effect [368].
As paleoenvironmental proxies, the iron abundance, speciation and isotopic composition recorded for an Archean ocean analogue in the future can assist in understanding the iron biogeochemistry in the water column and explain the information recorded in sedimentary rocks of the Precambrian ocean [104]. More empirical and experimental research is needed to quantify controlling factors of fractionation that occur with iron oxide crystallization in hydrothermal mineral systems [303]. Future work that reconstructs Archean seawater iron and Si concentrations will be crucial in evaluating the extent to which ferrous hydroxide auto-oxidation controlled the Archean iron cycle and the oxidation of the young Earth [103]. The crystal structure of iron oxides synthesized under high pressures, their bonding nature and build-up structural motifs may guide us in discovering novel iron oxide phases and will be useful in revealing the chemistry and physics of Earth and planetary deep interiors [24].
Despite the long history of iron oxide research, they continue to attract the high attention of scientists all over the world; therefore we can suppose the future fundamental and applied discoveries in this field. The main possible tendencies, which can be predicted from the current state of the research, include the further integration of various scientific analytical approaches, e.g., well-developed in geosciences and nanotechnologies [37,38,294,438], a wider implementation of “green” and biomimetic technologies [439] and a combined use of natural iron oxides and synthetic components in a single structure [6].


This work was funded by the Russian Science Foundation (Grant No. 21-19-00719).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The author declares no conflict of interest.


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Figure 1. Diversity of stable (solid line) and metastable (dashed line) iron oxides.
Figure 1. Diversity of stable (solid line) and metastable (dashed line) iron oxides.
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Figure 2. Equilibrium phase diagram for Fe–O system. Reprinted from [40], with permission from Elsevier.
Figure 2. Equilibrium phase diagram for Fe–O system. Reprinted from [40], with permission from Elsevier.
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Figure 3. Representation of free energy G vs. particle diameter d curves for the four Fe2O3 phases. Gray, blue, green and red lines represent the G values of γ-, ε-, β- and α-Fe2O3, respectively. Thick solid lines indicate the most stable Fe2O3 phases over the corresponding size ranges. Copyright 2009 by the American Chemical Society. Reprinted with permission from [41].
Figure 3. Representation of free energy G vs. particle diameter d curves for the four Fe2O3 phases. Gray, blue, green and red lines represent the G values of γ-, ε-, β- and α-Fe2O3, respectively. Thick solid lines indicate the most stable Fe2O3 phases over the corresponding size ranges. Copyright 2009 by the American Chemical Society. Reprinted with permission from [41].
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Figure 4. Static magnetic properties of various samples of silica–iron oxide systems: (a)—Hysteresis loop and backfield demagnetization curve of a sample containing ε-Fe2O3. Reprinted from [37], license CC BY 4.0; (b)—Magnetic hysteresis curve of FemOn-SiO2 synthetic colloidal nanoparticles. Reprinted from [38], with permission from Elsevier; (c)—Backfield curve and central part of the hysteresis loop of magnetite ore powder; full loop in the 1.8 T maximum field is shown in the inset. Reprinted from [6], with permission from Elsevier.
Figure 4. Static magnetic properties of various samples of silica–iron oxide systems: (a)—Hysteresis loop and backfield demagnetization curve of a sample containing ε-Fe2O3. Reprinted from [37], license CC BY 4.0; (b)—Magnetic hysteresis curve of FemOn-SiO2 synthetic colloidal nanoparticles. Reprinted from [38], with permission from Elsevier; (c)—Backfield curve and central part of the hysteresis loop of magnetite ore powder; full loop in the 1.8 T maximum field is shown in the inset. Reprinted from [6], with permission from Elsevier.
Magnetochemistry 09 00119 g004aMagnetochemistry 09 00119 g004b
Figure 5. Main characterization techniques used to verify phase composition.
Figure 5. Main characterization techniques used to verify phase composition.
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Figure 6. Distribution of iron oxide compounds by their frequency of mention (based on the analyzed research articles mentioned in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12).
Figure 6. Distribution of iron oxide compounds by their frequency of mention (based on the analyzed research articles mentioned in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11 and Table 12).
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Figure 7. Mechanisms of iron oxide formation: (a)—Terrigenous formation; (b)—Extraterrestrial formation; (c)—Biomineralization; (d)—Iron and iron alloy corrosion; (e)—Ceramics firing; (f)—Biomimetic synthesis; (g)—Chemical precipitation; (h)—Physical deposition; (j)—Mechanochemical synthesis.
Figure 7. Mechanisms of iron oxide formation: (a)—Terrigenous formation; (b)—Extraterrestrial formation; (c)—Biomineralization; (d)—Iron and iron alloy corrosion; (e)—Ceramics firing; (f)—Biomimetic synthesis; (g)—Chemical precipitation; (h)—Physical deposition; (j)—Mechanochemical synthesis.
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Figure 8. Main applications of iron oxides.
Figure 8. Main applications of iron oxides.
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Table 1. Comparison of the physical characteristics of various iron oxide compounds [42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73].
Table 1. Comparison of the physical characteristics of various iron oxide compounds [42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73].
ParameterIron Oxide Compound
Mineral nameWüstiteMagnetiteHematiteMaghemite
Crystal structureCubic
Cubic spinel
Cubic spinel
Static dielectric permittivity22.6
High-frequency dielectric permittivity10.8
Saturation mass magnetization 1 at 300 K, emu/g11–18
Curie/Neel point, K196
Optical band gap at 300 K, eV1.0
1 The values of saturation mass magnetization are given for the bulk materials.
Table 12. Data on the structures containing mixed iron oxide crystal phases [42,79,380,381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,405,406,407].
Table 12. Data on the structures containing mixed iron oxide crystal phases [42,79,380,381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,405,406,407].
CompositionMain Mechanisms of Iron Oxide FormationDeclared ApplicationsPhase Verification TechniquesRefs.
Iron oxide NPsThermal decomposition of an iron oleate in the presence of oleic acid, flame synthesis from Fe(CO)5, precipitation from ferric chloride in a natural leaf extract, thermal oxidation of polycrystalline Fe foilsBiomedicine, ferrofluids, electronics, immobilization of Cd in soils, catalysis, metal fuelPMS, TEM, EDS, TG-DSC, SAED, XRD, XPS, FTIR, UV–Vis, EELS[79,380,381,382,383,384]
Surface iron oxide layer on metalCarbon steel corrosion at room temperature, iron carboxylate transformation to iron oxides, electrochemical anodization of metal in simulated acid rain solution Corrosion resistance studies, railway industry, fireside corrosion studies EDS, XRD, XRF, RS, XPS, FTIR, TG-DSC[385,386,387,388,389]
Powder containing iron oxide microparticlesCarbothermal reduction of red mud by heating in a microwave furnaceAlumina production by-product recyclingXRD, EDS, XRF, TG-DSC[400]
Iron oxide microfibers arranged in a complex hierarchical structureThermal decomposition of Fe(CO)5 and silicone oil and microwave vaporizationEnvironmental safety, biomedicine, sensorsEDS, XRD[396]
Fe-based nanocomposite catalysts containing agglomerates of the two typesMelting of iron, aluminum and copper salts and reductionLow-temperature catalytic oxidation of COXRD, TEM, XAS, EDS[393]
Iron oxide powders containing hematite, magnetite and maghemiteChemical precipitation from ferric nitrate and ferrous sulfate and heatingInositol phosphate selective retention in soilXRD[399]
Silica–iron oxide nanocomposite with hematite, magnetite and wüstiteSilica promotion upon the reduction of amorphous iron oxide in hydrogenCatalysisXRD, TEM, EELS, STEM, SAED[394]
Iron(II) and (III) oxides inclusions in char composites Microwave pyrolysis of Moso bamboo samples with ferric chloride Syngas productionXRD[401]
Ultra-thin magnetic iron oxide films containing Fe3O4, γ-Fe2O3 and FeOThermally induced phase transformation of ultra-thin iron oxide filmsAll oxide heterostructuresXRD, XPS[402]
Surface iron(II) and (III) oxide layer on iron granulesAtomization of the molten semi-steel with a rotary cup atomizerIron powder productionTG-DSC, XRD[403]
Iron oxide (Fe3O4, γ-Fe2O3 and ɑ-Fe2O3) inclusions in fly ash samplesCoal combustion and flue gas cooling at various temperaturesSelenium adsorption by iron mineralsXRF, MSP, XPS[404]
High-pressure metastable phases mFeO⋅nFe2O3Formation of complex iron oxide crystals under high-pressure conditionsEarth and planetary deep interior studies[24]
Iron(III) oxide submicron inclusions in a biofilm on a basalt surfaceMicrobial direct or non-direct biomineralizationBiovermiculation studiesEDS[388]
Inclusions of poorly crystalline iron(III) oxides in ore samplesMicrobial biomineralizationMine remediation, waste stabilizationSIMS[390]
Iron oxide (Fe3O4, γ-Fe2O3 and ɑ-Fe2O3)/iron compositeReactive spark plasma sintering of mechanically activated Fe powdersMagnetic material developmentXRD, TG-DSC[405]
Core−shell Fe@Fe2O3 nanowires containing Fe(II) and Fe (III) oxidesFerric chloride reduction with sodium borohydride and surface oxidation Cr(VI) removalXRD, UV–Vis, TEM, XPS[406]
Iron oxide submicron particles accumulated in the cytoplasm of cellsIntracellular or extracellular microbial biomineralization Hydrothermal vent field studiesXRD, TEM, SAED, EDS[389]
Deposit samples with inclusions of iron(II) and (III) oxidesAnoxygenic photosynthesis by a photoferrotrophic bacteriumBanded iron formation studies[391]
Bovine serum albumin–iron oxide suspensionsPrecipitation from ferric nitrate with NaOH in N2 atmosphereBoreal forest studiesFTIR[398]
Iron oxide (Fe3O4, γ-Fe2O3 and ɑ-Fe2O3) magnetic short nanotubesAnion-assisted hydrothermal route by using phosphate and sulfate ionsBiomedicine, ferrofluids, electronics, spintronicsTEM, SAED, XRD[407]
Fe-biochar composites containing iron(II) and (III) oxidesPyrolysis of ferric chloride in a biochar matrix at various temperatures Arsenic removal, environmental remediationXRD, XPS, RS, FTIR[397]
FexOy@C spheres embedded with highly dispersed iron oxide NPsOne-pot hydrothermal cohydrolysis-carbonization using iron CatalysisTEM, EDS, XAS, XRD, MSP[395]
Powder with inclusions of submicron iron oxide particlesReduction of solid ferric hydroxide by iron-reducing bacteria Microbial iron reduction studiesEDS, RS[392]
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Gareev, K.G. Diversity of Iron Oxides: Mechanisms of Formation, Physical Properties and Applications. Magnetochemistry 2023, 9, 119.

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Gareev KG. Diversity of Iron Oxides: Mechanisms of Formation, Physical Properties and Applications. Magnetochemistry. 2023; 9(5):119.

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Gareev, Kamil G. 2023. "Diversity of Iron Oxides: Mechanisms of Formation, Physical Properties and Applications" Magnetochemistry 9, no. 5: 119.

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