Preparation and applications of nano-iron oxide

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Nanomaterials and nanostructures are the most dynamic research areas in the field of new materials today, exerting a significant impact on future economic and social development; they are also the most active and application-oriented components of nanotechnology. In recent years, countries around the world have conducted extensive research on metal oxide nanoparticles, achieving significant results; among them, nano-iron oxide has attracted considerable attention due to its broad application prospects. 1 Preparation of nano-iron oxide The methods for preparing nano-iron oxide can be divided into wet and dry methods. The wet methods mainly include hydrothermal synthesis, forced hydrolysis, gel-sol method, colloid chemistry method, microemulsion method, and chemical precipitation method. Dry methods mainly include: flame thermal decomposition, vapor deposition, plasma chemical vapor deposition (PCVD) at low temperatures, solid-state methods, and laser thermal decomposition. 1.1 Wet Methods 1.1.1 Hydrothermal Method The hydrothermal method refers to the general term for chemical reactions that take place in a closed system under high temperature and pressure, within fluids such as aqueous solutions or water vapor. In 1982, the preparation of ultra-fine powders via hydrothermal reactions attracted attention both domestically and internationally. Since the reaction takes place in an aqueous solution at high temperature and pressure, it provides suitable physicochemical conditions for the formation of good microcrystalline materials through a certain form of precursor dissolution–recrystallization. Chen Xing et al. prepared a series of pure-phase, uniformly sized, and minimally agglomerated ferrite nanoparticles using the hydrothermal method. Jing Zhihong and others have also prepared iron oxide nanoparticles with various morphologies, such as diamond-shaped, spindle-shaped, and spherical. Particles prepared by the hydrothermal method have high purity, good dispersibility, a favorable crystal form, and controllable size. The reaction takes place in an autoclave, which requires significant investment in equipment and leads to higher operating costs. 1.1.2 Forced hydrolysis method: Utilizing the forced hydrolysis of metal salt solutions is an important technique for preparing uniformly dispersed nanoparticles. This method typically uses FeCl3 or Fe(NO3)3 as raw materials, and under the presence of HCl and HNO3, forces hydrolysis to produce ultra-fine nanoparticles of iron oxide in a boiling, closed, static environment or a boiling, recirculating dynamic environment. Adding some crystal growth promoters (such as NaH2PO4) during the preparation process can reduce hydrolytic precipitation and the rate of crystal growth, resulting in particles that grow in a complete and uniform manner. Li Qiaoling et al. utilized microwave heating, along with a forced hydrolysis method based on boiling reflux, to directly synthesize smooth and uniform α-Fe2O3 nanoparticles of various shapes such as spherical, ellipsoidal, spindle-shaped, and cubic. Wei Yu et al. prepared spherical α-Fe2O3 particles that were monodisperse, uniform, and had a particle size of less than 25 nm using the forced hydrolysis method. The forced hydrolysis method can produce iron oxide nanoparticles with different morphologies, but the hydrolysis concentration is usually low, generally less than 0.2 (mol.L-1). Hydrolysis is carried out under boiling conditions, thus requiring high energy consumption. 1.1.3 Gel-sol method: The gel-sol method uses alkoxides as raw materials, and hydrolysis and polycondensation reactions take place at a certain temperature. As the polycondensation reaction proceeds and the solvent evaporates, the fluid sol gradually transforms into a slightly elastic solid gel, which is then sintered at a lower temperature to yield the material to be synthesized. Liu Jianhua et al. prepared highly ordered iron oxide nanowire arrays in porous alumina templates using the gel-sol method by controlling the anodic polarization conditions. The length and diameter of the nanowires are controlled by the thickness and pore size of the template; by controlling the anodization conditions, alumina templates with a thickness of 8–10 μm and a pore size of 15–200 μm can be prepared, thereby enabling the creation of nanowires with various lengths and diameters. This method enables the large-scale preparation of iron oxide nanowires, featuring a simple process, easily controllable conditions, good reproducibility, and low cost. The gel-sol method features a low reaction temperature, resulting in particles with small sizes that can be controlled within the tens of nanometers range; this provides a solid foundation for high-density recording. The operability of its synthesis process meets the requirements for large-scale industrial production ； However, it has a longer reaction time, high costs, and is prone to cracking during drying. 1.1.4 Colloidal Chemistry: The colloidal chemistry method involves the direct formation of sols by hydrolyzing metal alkoxides or inorganic salts, or by depolymerization to form sols; thereafter, the solute is polymerized to form a gel. The gel is then dried and calcined to remove organic components, thereby yielding nanomaterials. Using high-priced iron salts as the starting material, a sol with positively charged particle surfaces is prepared by reacting them with an amount of base (such as sodium hydroxide) that is less than the theoretical amount at a certain temperature ； Anionic surfactants such as sodium dodecylbenzene sulfonate (DBS) are introduced. Due to the ionization of these surfactants in aqueous solutions, the resulting anionic groups neutralize the positively charged colloidal particles, resulting in the formation of an organic thin layer on the surface of the colloidal particles that confers oleophilic and hydrophobic properties to them. Organic solvents such as chloroform or toluene are then added to extract these particles into the organic phase; the organic solvent can be recovered and reused through vacuum distillation. Nanoparticle iron oxide is obtained by heating the residue. Yang Jun et al. used this method to prepare spherical iron oxide ultrafine particles with a particle size of 4–6 nm. The colloid chemistry method can produce ultra-fine, uniform, spherical iron oxide, but it involves a large amount of organic substances and requires strict conditions for the operating environment. 1.1.5 Microemulsion method The microemulsion method refers to the formation of an emulsion from two immiscible solvents under the action of surfactants, followed by the preparation of nanoparticles through nucleation, coalescence, agglomeration, and heat treatment within the microbubbles. Su Yunlai et al. prepared ultra-fine iron oxide using the microemulsion method, with uranium dodecylbenzenesulfonate (DBS) as a surfactant and toluene as the oil phase. The particle size distribution of the sample and the temperature-programmed reduction (TPR) tests indicate that the sample is ultra-fine Fe2O3, and its particle size is closely related to its reduction performance. The microemulsion method features a simple experimental setup, low energy consumption, and easy operation. The resulting nanoparticles have a narrow particle size distribution, which facilitates high purification, and they exhibit good dispersibility, interfacial properties, and stability. Compared to other methods, its particle size is easy to control and it has a wide range of applications, but the process operation is more difficult to control. 1.1.6 Precipitation method The precipitation method was the earliest approach used for the synthesis of metal oxide nanoparticles through liquid-phase chemical reactions. This method typically involves mixing substances with different chemical compositions in a solution, adding an appropriate precipitant to produce a precursor precipitate for the nanoparticles, and then drying or calcining this precipitate to obtain the nanoparticles. Gao Zhihua et al. prepared fibrous nanoscale α-Fe2O3 particles with an average particle size of less than 100 nm using hydrated ferric sulfate or hydrated ferric nitrate and urea as raw materials. The precipitation method has lower costs, but the precipitate is usually colloidal, making filtration difficult during washing. Precipitants tend to remain as impurities, various components may change during the precipitation process, and some of the precipitates can dissolve during washing. Furthermore, since various metals do not readily undergo precipitation reactions, the applicability of this method is limited. 1.2 Dry method: The dry method is commonly used to prepare it by gas-phase decomposition or laser decomposition, using iron carbonyl or ferrocene (FeCP2) as raw materials. Using N2 as a carrier, it is introduced into the combustion chamber at 600°C from evaporation, along with a high-velocity stream of air. It rapidly mixes with air through turbulence, resulting in intense oxidation reactions. The combustion products are quenched and separated by a cyclone to yield transparent amorphous iron oxide particles with a particle size of 5–7 nm and a specific surface area of 150 m2/g. These particles exhibit good thermal stability and dispersibility. The dry process offers advantages such as a short production workflow, a favorable operating environment, high product quality, and ultra-fine and uniform particles; however, it presents technical challenges, requires high-quality materials for the equipment, and involves significant upfront investment. 1.3 Comprehensive Methods The comprehensive method refers to the integration of the advantages of chemical and physical methods in the preparation of nanomaterials, allowing for the simultaneous synthesis and production of such materials. Examples include ultrasonic precipitation, laser precipitation, and microwave synthesis. Among these, microwave synthesis has attracted widespread attention due to its fast speed, simplicity, energy efficiency, and ability to prevent agglomeration. Jia Zhenbin et al. prepared nanocuboid and spindle-shaped α-Fe2O3 nanoparticles using microwave-induced heating; compared with conventional heating methods, the α-Fe2O3 particles obtained through microwave heating had a smaller size and more uniform distribution. 2 Applications of nano-iron oxide: As scientific research progresses, the excellent properties of nano-iron oxide have led to an increasing interest in its use in magnetic materials, transparent pigments, biomedicine, catalysts, and other fields. 2.1 Applications in magnetic materials Due to their unique superparamagnetic properties, magnetic nanomaterials hold great potential for applications in giant magnetoresistance, magnetic fluids and magnetic recording, soft magnets, permanent magnets, magnetic refrigeration, giant magnetoresistive impedance materials, as well as magneto-optical devices and magnetic detectors. Nano-iron oxide is a new type of magnetic recording material with excellent performance in terms of high magnetic recording density, approximately 10 times that of conventional iron oxide. Magnetic sensors fabricated using the giant magnetoresistive effect of iron-based nanomaterials have been developed, and magnetic fluids coated with superparamagnetic nanoparticles are also widely used in aerospace and some civilian applications as long-lasting dynamic rotary seals. Magnetic nanoparticles have a single magnetic domain structure and high coercivity; magnetic recording materials made from them can improve the signal-to-noise ratio and enhance image quality. Currently, the magnetic ultrafine particles commonly used in video tapes are needle-shaped particles of iron or iron oxide (such as needle-shaped γ--Fe2O3). 2.2 Applications in the pigment industry: Nano-iron oxide pigments not only retain the good heat resistance, weather resistance, and ultraviolet absorption properties of conventional inorganic pigments, but they can also disperse well in oily carriers. Coatings and inks formulated with these pigments exhibit satisfactory transparency. Coatings made from nano-iron oxide have high electrical conductivity, which enables them to serve as an electrostatic shield. The excellent properties of transparent iron oxide pigments also enable the production of homogeneous colored cement tiles as a substitute for colored surface coatings. The pigment is infused throughout the tiles, giving them a simple and natural texture with a matte, soft coloration. This eliminates the problem of inconsistent quality associated with cement tiles with colorful surfaces available in China. Therefore, transparent iron oxide pigments are suitable for coloring various building structures and concrete products. Transparent iron oxide pigments that can absorb light of certain wavelengths are coated onto interference-type pearlescent pigments; when mixed with glittering aluminum paste, this creates a composite pigment. Car gloss paints made with these synergistic pigments allow not only differences in color in terms of transparency, saturation, or hue to be observed when viewing the paint head-on or from the side, but also truly different colors – the so-called two-color effect. By strictly controlling the levels of arsenic and heavy metals, transparent iron oxide pigments can be used for coloring drugs, foods, cosmetics, and more. 2.3 Applications in the field of catalysis: Nano-iron oxide possesses a large specific surface area and significant surface effects, making it an excellent catalyst. Catalysts made from nanoparticles exhibit higher activity and selectivity than conventional catalysts, as well as a longer lifespan and are easier to operate. Hollow spheres coated with nano-o-Fe2O3 are placed on the surface of wastewater containing organic matter; the use of sunlight to degrade these organic substances can accelerate the wastewater treatment process. The United States, Japan, and other countries use this method to deal with the pollution caused by oil spills at sea. Nanometer α-Fe2O3 has been directly used as a catalyst for the oxidation, reduction, and synthesis of polymer materials. This catalyst can increase the cracking rate of petroleum by 1 to 5 times, and solid propellants made using it as a combustion catalyst can achieve a combustion rate 1 to 10 times higher than that of conventional propellants, which is highly beneficial for the development of high-performance rockets and missiles. 2.4 Applications in biomedicine and other fields Magnetic nanoparticles are as small as a few nanometers, with particle sizes following a normal distribution. After surface coating, ferromagnetic nanomaterials can be used as supermagnetic oxide nanomaterials in magnetic resonance imaging, where they hold significant value for disease diagnosis; they can also be utilized in the preparation of magnetic microspheres. Magnetic particles can be used for tumor thermotherapy and magnetothermal therapy. Guided by a magnetic field, these particles can target the affected areas; under the action of an alternating magnetic field, they generate a magnetic hysteresis effect that releases heat, heating the tumor areas containing these particles to between 43–48°C. This process allows for the selective destruction of cancer cells without harming normal cells. Research conducted by Dr. A. Jordan and others has shown that coating iron oxide particles with a sugar layer enables them to evade the attack of the body’s immune cells and enter tumor tissue safely. Together with an alternating magnetic field, which maintains a temperature of 45–47°C at the treatment site, these iron oxide particles can kill tumor cells while leaving nearby healthy tissues unaffected. Currently, the research on and applications of multifunctional magnetic nanobiomaterials have gradually become a focus of interest both domestically and internationally. New discoveries and technologies are continually being incorporated into the preparation of nano-iron oxide, and integrated methods that combine wet and dry techniques to leverage their complementary advantages are emerging one after another. With the development of technology and societal progress, there are increasing demands for the quality, production processes, and scale of iron oxide products. Therefore, exploring new methods for the preparation of iron oxide that meet the requirements of the times and production scales, particularly those that are pollution-free, low-energy-consuming, and highly efficient, is the trend for future development.