Iron oxide or ferric oxide with the formula of Fe2O3 is an inorganic compound known to be one of the three main oxides of iron occurring naturally in a mineral called magnetite. Iron oxide has a unique supermagnetic property with an easy separation method when applied as suspension in solutions and applications in biomedical, agricultural and environmental sectors. The advantages of iron oxide powder with the ability to be modified using a number of inorganic and organic compounds namely starches, polyelectrolytes and nonionic detergents for even more applications based on their predominant surface chemistry potentials. Iron oxide is considered as the most common magnetic particle and powder to be applied in biomedical applications via applying an external magnetic field 1.
Properties of Iron Oxide
Generally, there are as many as 16 different types of iron oxide occurring naturally as a result of oxidation reaction between iron (III) and oxygen with the most common oxides as alpha iron oxide (α-Fe2O3) called hematite, gamma iron oxide (g-Fe2O3) named magnetite and Fe3O4 (hematite). Iron excites have found various applications since they are cost-effective and more importantly, prevalent. Such properties make them promising agents in numerous biological and geological process as catalysts and pigments. In addition to their industrial and commercial value, iron oxides are particularly well-known compounds in technological applications and scientific research as well. The versatile nature of iron oxides has motivated researchers to precisely concentrate on obtaining fine crystal and powders of iron oxide applications with even more uses. It has been demonstrated that particles and crystals with ferromagnetic properties show considerable magnetic, semiconducting and supermagnetic characteristics 2.
Iron oxides are among the compounds with ferromagnetic properties capable of being magnetic when placed in an external magnetic fields. This property turns out to be a desirable characteristic when they are obtained with controlled size and crystalline morphology. Fine particles of iron oxide have also been the center of attention due to their low toxicity along with their magnetic properties with an easy separation method when applied in medical diagnostics and in magnetic resonance imaging (MRI). Iron oxides applications in macroscopic scales and its reactivity is a determining factor however, they adopt pyrophoric behavior when they are obtained in smaller sizes in macron and nano scales. As a semiconductor material, iron oxide has a negative temperature efficient of resistance. This quality of high resistance α-Fe2O3 has found applications in humidity determining sensors. The attempt to synthesize favorable iron oxide powders has led to the emergence of numerous synthesis methods 3.
Ionic oxides are considered as the most studied metal oxides due to their higher surface reactivity and the so-called magnetism. Moreover, they are regarded as environmentally friendly materials and the promise of modern-age sustainable chemistry. Specifically, α-Fe2O3 shows antiferromagnetic properties at lower temperatures of -13°C and weak ferromagnetism between the temperatures -13°C up to 600°C. Preparation process is taking the advantage of precipitation and thermal decomposition approaches in the liquid phase. g-Fe2O3 with a cubic structure is basically a metastable and converted form of α-Fe2O3 at elevated temperatures. g-Fe2O3 exhibits ferromagnetic properties but the ultrafine particles smaller than 10 nm turn to be superparamagnetic. Thermal dehydration is the procedure applied for g-Fe2O3 production. Another most common method involves the careful oxidization of Fe3O4. It should be noted that gamma iron oxide particles can be obtained by thermal decomposition of iron oxalate.
Synthesis and Preparation of Iron Oxide Powder
The attempt to synthesize favorable iron oxide powder has led to development of numerous techniques known as precipitation, precursor-based technique, sol-gel, combustion hydrothermal and solvent evaporation. A study has suggested a method to obtain nanocrystalline iron oxide powder where ferric chloride is hydrolyzed at 100°C in presence of hydrochloric acid. In another method, three dimensional (3D) flower-like nanostructured iron oxide is synthesized using precursor rout and the reaction of tetra butyl ammonium bromide, ethylene glycol and ferric chloride as starting reagents at 450°C for 3 hours. The preparation of α-Fe2O3 is carried out by sol-gel method and the reagents like ethylene glycol, monomethyl ether and iron nitrate through annealing at the temperatures starting from 400 up to 700°C. In a report, the reverse micellar approach is applied to fabricate iron oxalate nanorods using cetyl-trimethyl ammonium bromide as a surfactant. In so doing, iron excite particles with spherical morphology can be obtained through decomposition of the iron oxalate nanorods at 500°C. In a method, hydrothermal technique is applied to fabricate α-Fe2O3 with micro-pine structure by autoclaving K3[(Fe(CN)6] for 2 days. The reaction between Fe(CO)5 and oleic acid takes place in dehydrated (CH3)3NO and argon atmosphere in presence of hexane as the medium to finally lead to nanocrystalline iron oxide product. In a method, a nonhydrolytic precursor is suggested based on which Fe(cupfferon)3 in dry trioctylamine at 300°C in argon atmosphere. It should be taken into consideration that most of these methods seem to have limitations like the use of expensive metal complexes as the starting reagents and difficult synthesis procedure, rare materials, strong acids and bases as well as frequent use of organic solvents 3.
Iron Oxide Powder Applications
Iron oxide is predominantly employed as the feedstock of the iron and steel industries and alloy manufacturing as a whole. Iron oxides are used in polishing applications, pigment production, magnetic recording fabrication, photocatalysis and Medicine. Fine particles of iron oxide has applications as the final polish on metallic lenses, jewelry and cosmetics. The use of iron oxide in pigment production results in the so-called pigment brown 6, pigment brown 7 and pigment red 101. Some of these pigments like pigment brown 6 and pigment red 101 are recognized and approved by the United States Food and Drug Administration with applications in cosmetics production. In addition to this, iron oxides are used in dental composite together with titanium oxide. Considering the magnetic property of iron oxides, it is most commonly employed in fabrication of different types of magnetic recordings and storages like discs for data storage and magnetic tapes. Also, iron oxides are extensively used as a photoanode agent used in solar water oxidation. However, its application in this case and its efficiency is somehow limited by a short diffusion length of 2 to 4 nm of photo-excited charge carriers as well as fast recombination that requires a large overpotential to get the reaction started. A lot of research is done in order to improve iron oxides performance based on surface functionalization and the use of substitute crystal phases such as β-Fe2O3.
The abundance of iron and its chemical and electronic state as well as its reactivity with atmospheric oxygen and many other elements lead to an enormous amount of iron oxide reserves and resources in inorganic ores and minerals. Additionally, iron oxides are widely found in most parts of the Earth with a considerably huge amount of applications as well as accessibility and cheap price. Industrial, medical, scientific and technological sectors all take the advantage of iron oxides versatility.