Cobalt oxide nanoparticles

Though the amount of research on cobalt oxide nanoparticles is still relatively new, researchers have found that these particles have great potential for us in chemical and industrial areas.

Characteristics

Cobalt oxide nanoparticles appear white in color and are magnetic p-type semiconductors. It is interesting to note that the particles will convert into cobalt metal upon heating at 900 °C. Even though cobalt oxide nanoparticles have many beneficial uses, the substance is still toxic to humans and also aquatic life. It is harmful when swallowed and also acts as a skin irritant.

Chemical Properties^[1]

Physical Properties

Thermal Properties

Applications

Although they have been observed to readily enter cells and may possibly be exploited for hyperthermic treatment, gene therapy and drug delivery, a protective and efficient molecular shield for the nanoparticles still needs to be developed.^[2] Cobalt nanoparticles are more commonly applied to devices for energy storage.

Lithium-ion Battery

Cobalt oxide (Co₃O₄) nanoparticles anchored on a single sheet of graphene.

Cobalt oxide nanoparticles are being investigated in optimizing high performance in lithium-ion batteries. This is due to their high surface-to-volume ratio and short path length for lithium cation transport, as opposed to nickel oxide and iron oxides. Some proposals include Co₃O₄ nanoparticles anchored on graphene as a source of possible lithium battery anodes.^[3] This is synthesized by first forming Co(OH)₂ on the sheet from Co²⁺ and NH₃H₂O, which is then heated to 450 °C for two hours to yield the cobalt oxide/graphene composite. The advantageous combination of the graphene sheet’s flexibility and its strong interaction with Co₃O₄ nanoparticles prevent volume expansion/contraction and aggregation of Co₃O₄ during lithium charge and discharge processes.

Porous Co₃O₄ nanotubes are another good lithium battery anode candidate due to their higher surface-to-volume ratio than other one-dimensional nanostructures.^[4] These different types engineered nanostructures exhibited high reversible capacity and good cycle life. Diphenylalanine/cobalt oxide hybrid nanowires have a potential application as anodes for Li-ion batteries as well, also enhancing the specific capacity as an electrode.^[5]

Gas Sensor

The p-type Co₃O₄ hollow nanospheres have performed well as a gas sensor in sensing toluene, acetone, and other organic vapors with rapid response and high sensitivity to very low concentrations at low temperature. This serves to potentially monitor organic flammable and toxic gases.^[6] Co₃O₄ nanoparticles prepared on single-walled carbon nanotubes (SWCNTs) are being investigated in sensing NO_x and H₂ gases. This takes advantage of Co₃O₄’s active reactivity with the gases, as well as the electrical continuum formed within the Co₃O₄ nanoparticle/SWCNT composite. With NO_x, the resistance of the sensor decreases because of an increase in the hole density in the composite. This is because both the Co₃O₄ nanoparticles and carbon nanotubes are p-type semiconductors and NO_x is electron-accepting. Thus, the adsorbed gas molecules continue to extract electrons from them. Oppositely, the resistance of the sensor increases for H₂ because it is electron donating. “The composite shows a highly enhanced response of ~200 % upon exposure to 4 % H₂ diluted in air compared with pure SWCNTs and Co₃O₄ film” at room temperature.^[7]

Synthesis

Hydrothermal Synthesis

Hydrothermal Synthesis of Cobalt Oxide Hollow Sphere.

Hydrothermal synthesis of metal oxide hollow spheres takes place in an autoclave and proceeds through a one-pot synthesis to increase yield while reducing time and resources. To initiate the procedure, carbohydrates and metal salts are dissolved in water at 100-200 °C.^[8] Carbon spheres are produced with metal ions integrated into the hydrophobic shell. Calcination allows for removal of the carbon from the product and results in hollow metal oxide spheres. Surface area and thickness of the shell can be manipulated by varying the carbohydrate to metal salt concentration.^[9] The completion time for the procedure varies from hours to days.^[10] Hydrothermal synthesis is generally simple and being able to control size and shape is beneficial for specificity. However, the drawback of this approach is a smaller yield compared to other methods. The pH of the reaction medium, temperature, pressure, and cations used in the solution all play a role in determining the nature of the solid. This approach allows for the production of a wide range of shapes and sizes for metal oxide hollow spheres.

Solvent Free Thermal Synthesis

TEM image of cobalt oxide nanoparticle produced by heating organometallic precursor [Co-(salen)] under high temperature.

Cobalt oxide (Co₃O₄) nanoparticles are generally synthesized at low temperature; However, the tendency of Co₃O₄ to grow into complex microstructure made the addition of organic surfactant necessary. Organic surfactant is both expensive and pollutant. Using organometallic molecular as a precursor to synthesize Co₃O₄ under high temperature can ensure purity, composition, phase purity, and microstructure.^[11] Using organometallic chemistry, Co₃O₄ is synthesized by heating 0.01 mol of [bis(salicylaldehyde)ethylenediiminecobalt(II], known as [ Co-(salen) ], to 500 °C in the air. The synthesis of the organometallic molecular precursor [Co-(salen)] is prepared by first making the bis(salicylaldehyde)ethylenediimine crystal. By dissolving salicylaldehyde in methanol with drops of 1,2- ethylenediamine, 92% of bis(salicylaldehyde)ethylenediimine crystal could be purified via reflux and filtration. [Bis(salicylaldehyde)ethylenediirninccobalt(II)] is then prepared by adding cobalt(II) acetate tetrahydrate in propanol and warmed to 50 °C under nitrogen atmosphere, followed by adding bis(salicylaldehyde)ethylenediimine and purification through reflux and recrystallization.^[12]

References