Graphene-based nanoparticle composites attract considerable attention due to their unique properties and the large variety of possible applications, such as sensing, energy storage, and catalysis. The performance of these materials is strongly related to the size and composition of the nanoparticles. Recently, researchers from the University of Akron and Pennsylvania State University developed a low-cost solid-state synthesis method for the rapid and controllable formation of uniformly-sized iron oxide nanoparticles on graphene-derived substrates.
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Nanostructured materials exhibit unique chemical and physical properties which cannot be observed in bulk materials. Metal and metal oxide nanoparticles are among the most widely used nanomaterials as they offer numerous advantages.
In particular, their electrical, optical, magnetic, and catalytic properties have been exploited in a wide range of industrial applications, such as sensing, catalysis, photovoltaics, energy storage, lubricants, coatings, food processing, medical diagnostics, and drug delivery.
Iron oxide nanoparticles stand out among the metal oxides as they are inexpensive to produce, physically and chemically stable under a wide range of conditions, biocompatible, and environmentally safe. In addition, the enhanced redox activity and the high surface-area-to-volume ratio of iron oxide nanoparticles like magnetite (Fe3O4) make them excellent candidates for energy storage and chemical sensing applications.
However, the electrical conductivity of pure Fe3O4 is poor compared to other transition metal oxides. One effective strategy to circumvent the conductivity problem is to utilize an electrically conductive substrate to support the iron oxide nanoparticles.
Graphene-Based Materials Improve the Versatility of Iron Oxide Nanoparticles
Graphene-based materials, such as graphene oxide or reduced graphene oxide sheets, have shown excellent potential for developing iron oxide/graphene nanocomposites. Such hybrid nanostructures combine the redox activity of the iron nanoparticles with the unique characteristics of graphene.
Graphene oxide (GO) has a two-dimensional structure similar to graphene. Still, the single layer of carbon atoms is covalently functionalized with oxygen-containing groups, such as hydroxyls, epoxides, and carbonyls, introduced during the manufacturing process.
In contrast to pristine graphene, GO can be rapidly produced in large quantities and at a low cost by simple methods like the chemical exfoliation of graphite flakes. Using strong oxidants in the process introduces defects in the carbon lattice, thus degrading the conductive properties of GO. Although, its optical and mechanical properties suffer a lesser impact.
Fortunately, GO can regain graphene-like properties by additional reductive treatments, transforming it into reduced graphene oxide (RGO) by removing the majority of the functional groups. Being easy to prepare in large quantities from cheap GO and exhibiting graphene-like properties, including relatively good conductivity, the RGO is an excellent compromise between graphene and GO.
Finding Greener Alternatives to Traditional Hydrothermal Synthesis
Most of the existing approaches for synthesizing iron oxide-RGO nanocomposites are based on the adsorption of iron ions onto GO sheets in aqueous media to form iron hydroxide particles by hydrolysis and precipitation. Subsequent hydrothermal treatment over several hours at relatively high temperatures (typically 12 h at 80 °C) converts iron hydroxide to iron oxide, while at the same time reducing the GO to produce RGO.
In recent years, researchers have focused on developing cutting-edge environmentally friendly synthetic approaches suitable for sustainable large-scale production of novel nanocomposite materials.
Now, a research group from the University of Akron and Pennsylvania State University led by Prof. Bryan Vogt, professor of chemical engineering at Penn State, developed a rapid solid-state process for the rapid fabrication of iron oxide-RGO nanocomposites that uses microwave radiation as a cost-effective heating method.
Microwave-Assisted Synthesis for Better Control of the Nanoparticle Size
The researchers used iron nitrate adsorbed onto RGO sheets as a low-cost precursor for the iron oxide nanoparticles. The strong absorption of the microwave radiation by the RGO rapidly heats the precursor, which decomposes and produces iron oxide nanoparticles.
Microwave-assisted synthesis has many advantages. For example, the reaction time is shorter, around 5 minutes in total. Energy consumption is also less due to the efficient and uniform heating of the reaction components.
The rapid cooling, occurring after ceasing the microwave irradiation, minimized the crystal growth and resulted in the highly-uniform size distribution of the nanoparticles.
Prof. Vogt's team also discovered that even minor variations in the microwave reaction time (of less than 1 minute) led to dramatic changes in the size distribution and morphology of the iron oxide-RGO nanocomposites. By adjusting the precursor concentration and the reaction time, the researchers synthesized nanoparticles with precisely controlled sizes in the range of 4-19 nm.
Tunable Iron Oxide Nanocomposites Promise Best Performance of Future Supercapacitors
Researchers also set out to examine the implications of the particle size and morphology to understand the potential applications for their novel nanocomposite.
Here, the performance of supercapacitor electrodes, which had been prepared from the microwave-fabricated iron oxide-RGO, was explored. The electrodes' high capacitance and good cyclic performance (charge-discharge) are essential for supercapacitor applications.
By tuning the size of the iron oxide nanoparticle adsorbed onto the RGO sheets, the research team created nanocomposites delivering very high initial capacitance combined with an excellent residual capacitance after 500 charge-discharge cycles.
The microwave-assisted synthetic method enables to quickly achieve particle size tuning without optimizing many different synthetic parameters.
These results illustrate the ability of solid-state microwave processing to rapidly and controllably create small nanoparticles supported on graphene-derived substrates from low-cost precursors. The scientists envisage that the concept can be extended to a wide range of applications where metal oxide nanoparticles with tightly-controlled controlled sizes on carbon-based surfaces are needed.
Continue reading: New Research Narrows the Gap for Graphene Nanoribbon Applications.
References and Further Reading
Li, S., et al. (2021) Microwave-Enabled Size Control of Iron Oxide Nanoparticles on Reduced Graphene Oxide. Langmuir, 37 (37), 11131-11141. Available at: https://doi.org/10.1021/acs.langmuir.1c01990
Xu, S., et al. (2019) Uniform, Scalable, High-Temperature Microwave Shock for Nanoparticle Synthesis through Defect Engineering. Matter 1, 759–769. Available at: https://doi.org/10.1016/j.matt.2019.05.022
Kim, J., et al. (2019) Microwave-assisted one-pot synthesis of iron(II, III) oxide/reduced graphene oxide for an application of supercapacitor electrode. Carbon Lett., 29, 411–418. Available at: https://doi.org/10.1007/s42823-019-00045-9