Inorganic Nanoparticles: Sources, Properties, and Applications
Inorganic Nanomaterials and Nanotechnology
Inorganic nanoparticles, such as those of elemental sulfur, may be produced from natural sources. They are then released into the air, water and soil systems where they can impact ecological and human health.
On the other hand, raw materials that would otherwise be considered waste biomass (de-oiled herbs, coffee grounds and other plant by-products) can be employed in a bioreductive manner to generate natural nanoparticles.
What is a Nanomaterial?
A nanomaterial is a natural or manufactured substance that has external dimensions less than one hundred nanometers. Scientists study and create new materials on an atomic or molecular scale to obtain special properties that are not available in bulk forms. This research is called nanotechnology.
Graphene is an example of a nanomaterial with incredible physical properties. It is extremely light, has more mechanical strength than steel and excellent electrical conductivity. These qualities make it a good candidate for use in bicycle frames and batteries, for instance. Carbon nanotubes are another nanomaterial with a variety of interesting properties. These tubes are exceptionally thin and have better thermal conductivity than diamonds, along with high electric and mechanical strengths.
The unique mechanical properties of nanomaterials make them ideal for use in coatings that form self-cleaning surfaces. For example, plastic garden chairs coated with nano-titanium dioxide will be able to attract and hold water, which in turn releases dirt from the chair’s surface when it is washed.
When a material is reduced to the nanoscale, its physical properties change. Opaque substances become transparent; inert materials become catalysts; stable materials turn combustible; and insulators become conductors.
Surface and quantum effects also lead to new physical properties. For example, carbon nanotubes exhibit thermal conductivity better than diamond and mechanical strength that rivals steel. Yet they are extremely light. These qualities make them ideal for baseball bats and car bodies, and they have led to other applications like antimicrobial coatings on sports gear and military uniforms.
NPs may be constructed by top down methods that break bulk materials into their constituent parts or bottom up techniques that build nanostructures atom-by-atom or molecule-by-molecule. Colloidal synthesis is one example of the former, as are chemical synthesis of large molecules and crystal growth for the semiconductor industry. Positional assembly, a variant of the latter, takes advantage of natural chemical and physical interactions between atoms or molecules to organize them into a desired nanostructure.
Physicochemically, nanomaterials have a greater surface area than larger particles. This allows them to engage in much more chemical activity, as most chemical reactions take place at the surface of a material.
The small size of a nanoparticle also gives it more mass for its volume, making the material much more reactive than the same amount of bulk material at the same size. This is why many materials are ground down to nanoscale powders, such as coal or nacre.
Natural inorganic nanomaterials occur through crystal growth under different environmental conditions – for example, clays show complex nanostructures and volcanic eruptions produce pigments, cement and fumed silica. Brazilian crystal opals provide a beautiful play of colours caused by the diffraction and interference of light between silica spheres.
Engineered nanomaterials can be produced by top down techniques, starting with large pieces of material and using chemical and physical processes to break them down until the desired nanomaterial is formed. They can also be constructed by bottom up methods, starting with individual atoms or molecules and joining them together.
Nanotechnology is transforming many industrial fields. PNNL research on nanoscale materials is helping make products more efficient, more durable, and less expensive. It is also enabling new kinds of energy technologies. For example, cellulose nanomaterials are being engineered to convert wood chips, corn stalks, unfertilized perennial grasses and other biomass into renewable fuels for cars and trucks.
Nanomaterials are produced through a number of methods. Some, like the top down approach used to manufacture microprocessors, reduce larger structures to the nanoscale through physical processes such as cutting or milling. Other nanomaterials are assembled atom by atom or molecule using techniques such as chemical synthesis or self assembly.
Once they are produced, nanomaterials can be free or attached to a surface and may be part of an assembly that combines different materials. Examples of assemblies include oriented attachment to a surface, precise patterning on a substrate or linkages between two different nanomaterials for combined function (like the conversion of higher-energy blue light in LEDs into warmer white light).