An Overview of Metal Oxide Nanoparticles

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Metal oxide (MO_x) nanoparticles (MONPs) are a class of nanomaterial with interesting and diverse chemical, optical, electrical and magnetic properties. Different metals bond to oxygen forming a variety of crystal structures under specific conditions. The shape and nanostructure of these materials effects their surfaces which in turn impacts the way they interact with light, electricity, magnetic fields and other materials. Being on the nanoscale means they have increased surface area to mass ratio increase, a large proportion of the material can interact with surrounding materials, often increasing its reactivity. By controlling the morphology and size of metal oxide nanoparticles these properties can be tuned which has made them highly attractive to fields of research including biomedicine and electronics.

Examples of Metal Oxide Nanoparticles

Titanium Dioxide (TiO2) Nanoparticles

From $120

Zinc Oxide Nanoparticles

From $75

Iron Oxide (Fe₂O₃) Nanopowder

From $105

Iron Oxide (Fe₃O₄) Nanopowder

From $120

Bismuth Oxide (Bi2O3) Nanopowder

From $207

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Properties of Metal Oxide Nanoparticles

The varied and unique chemical and physical properties of metal oxide nanoparticles are a result of their low dimensionality with nanoscale morphologies. They are often high-density materials that experience changes in unit cell parameters due to size-related structural variation. As the size decreases in metal oxide nanoparticles, an increasing number of surface and interface atoms generates strain or stress and sometimes changes in crystal structure. This is observed in the cases of CuO, ZnO, SnO₂, Al₂O₃, MgO, ZrO₂, AgO, TiO₂, CeO₂, etc. The specific size of the nanoparticle can alter magnetic, conducting, chemical, and electronic properties.

Electronic Properties

Metal oxide nanoparticle's electronic structures determine their conductor, semiconductor, and insulator properties. Transition metal ions typically possess unfilled d-shells, allowing for reactive electronic transitions, wide bandgaps, superior electrical characteristics, and high dielectric constants. Metal oxides are made of electropositive metals and electronegative oxygen which creates electronic dipoles that allows the polarization of voltage in certain directions and planes.

Optical Properties

Many metal oxide nanoparticles exhibit interesting absorption, emission, and scattering of light. This can be tuned by controlling their size, shape, and composition. One of the most interesting optical properties of some metal oxides is their photocatalytic abilities. This involves the absorption of certain wavelengths of light which promotes an electron from the valence band to the conduction band. The promotion of this electron (e^-) to the CB results in the simultaneous formation of a hole (h⁺) in the VB. As a result, both reducing and oxidizing sites form which can generate reactive oxygen species (ROS).

Magnetic Properties

Some metal oxide nanoparticles, such as iron oxide and cobalt oxide, exhibit magnetic properties. Superparamagnetic iron oxide (Fe₃O₄) nanoparticles are probably the most well-known nanomagnets owing to their extensive investigations for biomedical applications. Size effects, such as the reduced coordination of the atoms at the nanoparticle surface and disorder of surface spin (spin glass), are believed to be responsible for these magnetic properties and for the appearance of new phenomena in magnetic nanoparticles compared to their bulk.

High Surface Area

The nanoscale size of metal oxide nanoparticles results in a high surface-to-volume ratio. This produces a high density of surface sites which enhances their reactivity, adsorption capacity, and catalytic activity. Doping and hybridization with covalent and noncovalent interactions also enable tuning of morphologies, crystallites, and size, generating very active surface sites, particularly attractive for sensing applications.

Property Tuning

Many metal oxide properties can be further tuned not only by altering their size but also through doping with other elements. Zinc, nickel, or manganese can modify the magnetic properties of Fe₃O₄ for use in magnetic storage, sensors, and biomedical applications such as magnetic resonance imaging (MRI) contrast agents. Titanium dioxide nanoparticles doped with transition metals like iron or cobalt show enhanced photocatalytic activity due to a shift their bandgap into the visible light spectrum. This makes them more effective in applications such as solar energy conversion and environmental remediation, where utilizing sunlight is a crucial factor.

Applications of Metal Oxide Nanoparticles

Due to the huge variety of attractive properties possessed by metal oxide nanoparticles, they have been investigated for a bread range of applications. The combination of electronic, optical and magnetic properties with such large specific surface areas means they are highly effective at many processes. This has meant they have been researched in areas including:

• Biomedicine: Many metal oxide nanoparticles are biocompatible making them useful as diagnostic agents, drug delivery systems and medical implants.
• Photocatalysis: Metal oxides such as oxides of vanadium, chromium, titanium, zinc, tin, and cerium having characteristics including, desired band gap, suitable morphology, high surface area, stability and reusability. They absorb light, which induces a charge separation process that involves the formation of positive holes that are able to oxidize organic substrates. The photocatalytic activity of metal oxide comes from two sources:
  1. The generation of ^*OH radicals by oxidation of OH^- anions
  2. The generation of O₂^- radicals by reduction of O₂. Both the radicals and anions can react with pollutants to degrade or otherwise transform them to lesser harmful by products.
  As a result, metal oxides can be used as a photocatalyst to decompose toxic organic compounds, photovoltaics, prevent fogging of glass and even split water into hydrogen and oxygen.
• Optoelectronics: Metal oxides offer different electronic structures, charge transport mechanisms, defect states, thin-film processing and therefore optoelectronic properties compared to traditional semiconductors. Transparent conductive oxides (TCOs) combine electrical conductivity with optical transparency. Due to this unique combination of properties they are established components in optoelectronic industries including, solar cells, flat panel displays, LEDs, touch screens, low-emissivity windows and more.
• Sensors: Copper(II) oxide (CuO), copper(I) oxide (Cu₂O), tin(II) oxide (SnO), tin(IV) oxide (SnO₂), zinc oxide (ZnO), nickel oxide (NiO), indium oxide (In₂O₃), and tungsten oxide (WO₃), are promising candidates with key sensing characteristics including, high sensitivity, fast response/recovery (res/rec) time, excellent reproducibility and stability, and cost-effectiveness with simple fabrication processes.
• Water treatment: Metal oxide nanoparticles have been investigated in the application waste water treatment. There are examples of enhanced pathogen disinfection and heavy metal removal, point-of-use treatment, and organic removal applications including pesticide removal.
• Batteries: Transition metal oxide nanoparticles are promising materials for energy storage devices with high performance or high conversion efficiency a because of their distinctive properties like innovative size effects, improved kinetics, greatly enhanced conductivity, activity, excellent redox properties, electrochemical stability, high specific power, and sustainability.

Synthesis of Metal Oxide Nanoparticles

Metal oxide nanoparticles are synthesised via two main strategies:

Top-down: Start with larger (macroscopic) structures and process them into nanostructures.

Bottom-up: Starting reagents are on the atomic scale and they can be reacted to form nanoparticles.

The major synthetic techniques that follow the bottom-up approach can be categorized as physical or chemical methods:

Green synthesis

Metal oxide nanoparticles can be extracted from fungi, algae, bacteria, and plants in which a variety of metabolites act as reducing agents during material synthesis. These synthetic methods are collectively referred to as bioreductions. Advantages of green synthesis include availability, safety, and versatility in the type of metabolites that could act as reducing and stabilizing agents. Bioreduction can be used alongside physical and chemical methods to improve their environmental impact.

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What are Transparent Metal Oxides?

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