[Image above] Magnetite (Fe3O4) is a type of iron oxide known for its magnetism. A new open-access paper summarizes ways to synthesize this material. Credit: Bryant Olsen, Flickr (CC BY-NC 2.0)
The bright red color of bricks is one of the most obvious uses of iron oxide in our everyday lives. However, pigments are by far the only use.
As discussed earlier this month on CTT, there are many types of iron oxide that display very different properties depending on their structure. For example, the iron oxide that gives brick its red color is called hematite (α-Fe2O3), which refers to iron(III) oxide with an alpha phase crystal structure.
Magnetite (Fe3O4) and maghemite (γ-Fe2O3) are two types of iron oxide known for their magnetic properties. These iron oxides, which have similar cubic spinel crystal structures, are used for various magnetic purposes, including data storage and environmental and biomedical applications.
Synthesizing maghemite is a relatively straightforward process, as it can be obtained through oxidation of magnetite. Synthesizing magnetite, however, can take place in a variety of ways, as described in a recent open-access paper.
The paper is written by Roberto Nisticò, an independent researcher in Turin, Italy. He aimed to summarize the main synthesis protocols for magnetic iron oxide nanoparticles, and the paper is a culmination of his extensive review of the literature.
Chemical methods
Chemical approaches rely on growth of magnetite from a liquid phase through the action of further chemicals.
Among the different chemical approaches, Nisticò identifies the coprecipitation technique as the simplest and most diffuse pathway to produce magnetite. This method involves adding a base to aqueous Fe2+/Fe3+ salt solutions to form the solid magnetite. However, coprecipitation does come with a few limitations, including use of non-environmentally friendly chemicals and poor shape control of the final iron oxide nanoparticles.
“To overcome this issue, in most cases coprecipitation has been integrated with ultrasonic-assisted chemical method, which allows a better control in the nanoparticles’ size below 15 nm, or by introducing (bio)surfactants and/or stabilizers to confer a further control in sizing/shaping the final IONPs [iron oxide nanoparticles] as well as directly-forming a preservative coating (from oxidation) surrounding IONPs,” Nisticò writes.
Micro-emulsion processes are another option to chemically synthesize magnetite. These processes are based on iron-containing salts in a water/oil biphasic system, and they exploit the presence of amphiphilic (macro)molecules at the interface. While micro-emulsions have the advantages of narrow size distribution and good shape control, drawbacks include low yield and aggregation, which necessitates several post-synthesis purification steps.
Sol–gel routes are the final chemical approach highlighted in the paper. These methods are based on acid/based-catalyzed hydrolysis/(poly)condensation reactions of precursors from colloidal solutions (sol) to form a condensed network (gel) of metal oxides. Nisticò refers readers to an open-access paper by Danks et al. (2015) for a detailed analysis of the sol–gel processes.
Physical methods
Physical methods rely on exploiting physical phenomena to induce growth of magnetite from (non)-aqueous media.
The sonochemical approach is based on agitation of particles in an aqueous ferric/ferrous solution using sound energy (i.e., sonication). Iron oxide nanoparticles generated with this technique have high stability and strong magnetic properties, but the technique offers low shape control.
In contrast to the sonochemical approach, microwave irradiation uses an electromagnetic source emitting in the 1–103 mm wavelength range to excite molecules, aligning them with the external field and generating internal heat. The microwave-assisted route reduces both treatment time and energy consumption, but the synthesized iron oxide nanoparticles have low surface reactivity.
The final physical method described—the electrochemical route—involves a galvanic cell with two iron electrodes immersed in a saline solution. Only electrodes that are less than 5 centimeters apart allow for the necessary pH environment at the cathode—and, consequently, magnetite growth. This method has many advantages, including the ability to control particle size by varying the electrical parameters.
Thermal methods
Compared to other methods, thermal methods rely on higher temperature processes and result in high conversion yields.
The most diffuse thermal approach is thermal decomposition, which involves decomposing iron-containing salts from a nonaqueous mixture. The main advantage of thermal decomposition is high crystallinity of the nanoparticles, along with well-defined size and shape control. Plus, nanoparticles produced this way have hydrophobic surfaces that favor their dissolution into apolar solvents.
Spray pyrolysis is another thermal method. It involves pumping iron(III) salt-containing organic solvent drops through a nozzle to aerate in a current flow of hot gases, causing the droplets to dry in flight before falling to the bottom of the vessel. A similar process—laser pyrolysis—uses a laser source to heat a gaseous mixture of iron(III) salt-containing precursors.
Hydro- and solvothermal routes are the last two thermal methods described. These methods are based on crystallization reactions in sealed containers. These processes also generate high-crystalline iron oxide nanoparticles, plus offer high shape control by varying initial concentration of the iron precursor and time/temperature of the treatment.
Biological methods
Biological methods rely on using microbial species (mainly bacteria) to convert iron ions into iron oxide nanoparticles. In general, biological methods are sustainable processes and favor direct production of iron oxide nanoparticles with high biocompatibility. The main drawback is relative difficulty in controlling size and shape of the nanoparticles.
Table 1. Classification of the synthetic routes for production of iron oxide nanoparticles (IONPs). Credit: Nisticò, Boletín de la Sociedad Española de Cerámica y Vidrio (CC BY-NC-ND 4.0)
| Methods | Atmosphere | Temperature (°C) | Time | IONPs size distribution | IONPs shape control | IONPs yield |
| Coprecipitation | Ambient | 20–250 | min | Narrow | (Not) good | High |
| Micro-emulsion | Ambient | 20–80 | h | Narrow | Good | Low |
| Sol–gel | Ambient | 25–200 | h | Narrow | Good | Medium |
| Sonochemical | Ambient | 20–50 | min | Narrow | Bad | Medium |
| Microwave-assisted | Ambient | 100–200 | min | Medium | Good | Medium |
| Electrochemical | Ambient | 20–30 | h/days | Medium | Medium | Medium |
| T-decomposition | Inert | 100–350 | h/days | Very narrow | Very good | High |
| Spray/laser pyrolysis | Inert | >100 | min/h | Narrow | Medium | High |
| Hydro/solvothermal | High pressure | 150–220 | h/days | Very narrow | Very good | High |
| Bacteria-assisted | Ambient | 20–30 | h/days | Broad | Bad | Low |
Nisticò concludes the summary by identifying the co-precipitation chemical route and the thermal methods as the most promising processes in term of industrial feasibility and final yields. However, “For a more extensive (and generalist) discussion, this author suggests the following references as supporting literature [26], [27],” he adds.
The open-access paper, published in Boletín de la Sociedad Española de Cerámica y Vidrio, is “A synthetic guide toward the tailored production of magnetic iron oxide nanoparticles” (DOI: 10.1016/j.bsecv.2020.01.011).