CIEC Promoting Science at the University of York, York, UK

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Nanomaterials

The products of the chemical industry are used to produce objects that vary enormously in their size from say the iron girders for bridge building to silicon chips in microprocessors.  However, techniques are now available which make it possible to manipulate materials on the atomic or molecular scale to produce objects which are no more than a few nanometres in diameter. A nanometre is 1 x 10-9 metres (a billionth of a metre).  This is more than a 1000 times smaller than a silicon chip.

The processes used to make and manipulate such materials are known as nanotechnology and the materials or objects themselves are called nanomaterials.

 

Figure 1 Figure 1  A sunset caused by nanoparticles in the atmosphere above Palm Island in the Grenadines, the Caribbean.
By kind permission of Dr Alan Smith.
Figure 2
Figure 2  Even the finest hair on this fly has a diameter of about 5000 nanometres.
By kind permission of Bruce G Marcot.

Nanotechnology is now used in chemistry, physics, biology and engineering.  The smallness of the particles confers on them very useful properties. Some of these properties arise from the enormous increase in the surface area when, for example, a powdered material is converted into particles which are a few nanometres in diameter.  This increase in surface area will lead to an increase in the rate of any reactions which occur on the surface of the material.  The small dimensions of nanomaterials also lead to the possibility of them forming intimate mixtures with other materials with a view to enhancing the properties of the material.  In medical treatments they can be tailored to provide opportunities to target some medications more precisely.

As can be seen in Figure 3, materials or objects that need to be measured in nanometres have always existed but the techniques for manipulating materials on this scale have only been developed during the last twenty years or so.

The invention and development of the scanning tunnelling microscope (STM) in 1981 at the IBM Laboratories in Switzerland has essentially provided the basic technology for work on the nanoscale.  By scanning the surface of materials, it has not only become possible to visualise individual atoms and molecules, but we can even pick them up and move them around.  One of the defining moments was in 1989, when IBM scientists used STM to spell out the letters I-B-M in individual xenon atoms on a nickel crystal surface.


Figure 3 A scale to show the relative dimensions of various objects.
By kind permission of Dr Alan Smith.

It was the discovery of the allotrope of carbon, buckminsterfullerene, C60, (affectionately known as buckyballs) in 1985 (Figure 4), and the production of

Figure 4

Figure 4  A computer-generated model of a buckyball (buckminsterfullerene, C60) molecule.
By kind permission of Dr Alan Smith.

carbon nanotubes, and particularly the subsequent use of nanoparticles of clays, that led to the burgeoning use of nanomaterials beyond the microprocessor industry.  Carbon nanotubes resemble sheets of graphite which have been rolled into a cylinder (Figure 5), the bonding between the carbon atoms being the same as between carbon atoms in graphite.  The ends are capped with a buckyball.  The nanotubes are ca 1 nm in diameter (although some have been made which are considerably smaller, 0.4 nm) and can be as long as several nm.  They can align themselves into 'ropes' by intermolecular bonding.

Figure 5

Figure 5   A computer-generated model of a carbon nanotube.
By kind permission of Thomas Swan and Co Ltd.

As well as single walled nanotubes, double and multiwalled carbon nanotubes can be produced in a similar way (Figure 6).  These materials have been demonstrated to be the stiffest and strongest fibres known and have exceptional electrical conducting properties.

Figure 6

Figure 6  A computer-generated model of multiwalled carbon nanotubes.
By kind permission of Thomas Swan and Co Ltd.

Manufacture

Nanomaterials can either be created by cutting down macro structures to the nanoscale (top-down approach) or by assembling structures from atoms and molecules (bottom-up approach).

An example of the top-down approach is seen with the manufacture of microprocessors, in which shortwave ultra-violet and electron beams are used to cut silicon wafers that are used to produce 'circuits' with nanoscale structures (less than 50 nm).  This approach, however, tends to be very wasteful of expensive materials that are etched away during the process.

Building up materials atom by atom, or molecule by molecule, creates less waste.  The nanostructures are formed because each atom or molecule recognises its natural position, similar to ions building crystalline structures from solution.  Continuous production of nanomaterials in bulk is already established up to tonnage quantities. Among the methods used to produce them are physical and chemical vapour deposition.

In physical vapour deposition (PVD), the material is vaporised by heat in a furnace or by pulsed lasers.  The vapour is then condensed on a cool surface.  For example, single-wall carbon nanotubes can be prepared by vaporising a carbon target in a furnace at about 1500 K using a laser and allowing the vapour to condense on a cool surface.  An inert gas is bled into the reactor during the process to prevent oxidation of the carbon vapour.

In chemical vapour deposition (CVD), a reaction occurs in the vapour phase between two or more materials and/or the vapour reacts with the target material.

This bulk production technology has been used to manufacture a wide range of materials in 20-100 nm particle size ranges (nanopowders).  For example, compounds used in the electronics industry, such as silicon dioxide (from silicon hydride and oxygen) and silicon nitride (from silicon hydride and ammonia), are made in this way.  Some metals (for example, nickel and tungsten) of nano size are also prepared by CVD, by reducing their chlorides with hydrogen at high temperature.

CVD is thought to show the most promise for the production of carbon nanotubes (Figure 7).

 

Figure 7

Figure 7  A line diagram illustrating chemical vapour deposition.
By kind permission of Dr Marcello Motta, Thomas Swan and Co Ltd.

Uses

Figure 8  The glass in the Lycurgus Cup (probably made in Rome in the 4th Century AD) and now in the British Museum contains nanoparticles of silver and gold.  It is coloured green but when held up to the light, the colour changes to glowing red.
By kind permission of the Trustees of the British Museum.
Figure 8

In this section, a wide range of uses of nanomaterials is discussed, some of which are currently in production and others are in an advanced state of development.

Catalysis

Heterogeneous catalysis takes place when reactions occur on the surface of a solid catalyst. One catalytic application already developed is the use of cerium(IV) oxide nanoparticles which are added to diesel and bio-diesel fuels in very small amounts (5-10 ppm).  When diesel fuel is combusted in an engine, it is not completely oxidized and fine particles of carbon, carbon monoxide as well as unreacted fuel are emitted. The additive, cerium(IV) oxide acts as a heterogeneous catalyst and ensures that the fuel is combusted completely to carbon dioxide and water, thus leading to an improvement in fuel efficiency by as much as 4-11% and much less pollution (about 18% less particulate matter is emitted).  Because of their size, the cerium(IV) oxide nanoparticles forms a homogeneous solution in the fuel and are thus easily premixed with the fuel, which then requires no special delivery equipment and does not require any modification to the vehicle engine.

The catalysts in catalytic converters for cars are nanoparticles of alloys of palladium and rhodium.

Figure 9  Over the last few years, a bus company, Stagecoach, in the UK, has been experimenting with using cerium(IV) oxide nanoparticles added to the diesel fuel. The trials took place in a rural area (in the north west of England) and a city (London). Each trial involved 300 buses.  It found that there was a 6% and 5% fuel saving respectively. Now the company uses the additive in over 6000 buses.
By kind permission of Stagecoach Group.


Figure 9
Figure 11 Figure 10  This building in Ottawa has been built with glass that has a transparent film of titanium dioxide nanoparticles.  They catalyse the photodegradation of dirt by UV sunlight. The smaller degradation products are readily washed off by rain, thus making the windows self-cleaning and so enhancing their ability to produce a reflection.
By kind permission of Dr Alan Smith.

Plastics and glass

Carbon nanotubes are not only very strong but are also flexible. They can be twisted and bent without breaking.  Thus they are used in polymers and composites to strengthen a structure, to increase the electrical conductivity of the material and to increase heat transfer.
On the other hand, composites containing silica nanoparticles withstand the heat of a car engine and are therefore used as engine parts.  Nanocomposite materials made out of clay-reinforced polyamide, originated by Toyota, are now being used to make panels for cars, which are as much as 30% lighter than existing structures, and are twice as resistant to denting and scratching as conventional polymers.  This gives rise to lighter, therefore more fuel-efficient, cars, with corresponding environmental benefits.

Another use for cars, already in development, is in wear-resistant tyres.  Nanometre-scale clays are combined with polymers, for example poly(2-chlorobuta-1,3-diene) (neoprene), a rubber-like polymer, exploiting the extreme hardness shown by some nanomaterials.

Figure 10

Figure 11  The Airbus 380 uses about 20% composite materials containing nanomaterials, thus saving fuel.
By kind permission of Daily Telegraph/Paul Grover.

The electrical conducting properties of carbon nanotubes can also impart conductivity to polymers (including polyamides such as the nylons), whilst at the same time making them stronger than metals.  Such materials are now used in the fuel lines in cars and petrol stations, reducing risk of static build up and spark as the fuel is delivered from the pump.

Textiles and fabrics

A 'fleece' fabric has been produced that contains nanoparticles of carbon, derived from bamboo, infused into a range of fibres (rayon, polyesters and polyamides).  Through the highly absorbent, high surface areas and surface modifications of the nanoparticles, they can be used in such products to provide properties, including anti-bacterial, anti-fungal, deodorizing, thermal-regulating and static-free, yet soft and comfortable to wear.  The nanoparticles are embedded in the fibres rather than present as a coating, and are not removed from the fabric when washed.

Health and personal care

Nanotechnology will have an enormous impact in the healthcare and personal care industries, because of the extremely small dimensions of nanoparticles and their mobility.  The chemical reactivity rate, the location of effect, and the timing of a treatment are all affected by particle size.  Efficient drug delivery is being tested already.  Biological microelectromechanical devices (bioMEMS) implanted into the body to deliver doses of drugs or carry new cells to damaged tissues bring the concept of nanosurgery into being.

In the area of biomedical imaging, the use of nanoparticles as image enhancers is being developed.

Imaging probes and implant coatings can be inserted into the human body with particle sizes from 2-10 nm.  The enhanced magnetic properties of iron(III) oxide nanoparticles make them suitable for use as contrast agents in magnetic resonance imaging (MRI).  In the study and medical treatment of cancers, nanocarriers can be used for delivering imaging agents to cancer cells thus making it easier to locate the cancer cells precisely and making treatment much more effective.  One technique being tried is to inject the patient with certain nanoparticles, often gold because of its resistance to corrosion.  The gold nanoparticles that are located at a site of cancer cells can be irradiated with infrared to heat them up and destroy the nearby cancer cells.

Cosmetics are already available that contain nanoparticles, again exploiting properties arising from their incredibly high surface areas.  These include sun lotions and anti-aging creams.

Figure 12  Silver nanoparticles provide powerful antiseptic properties and are used, for example, in baby food cartons to prevent cross-contamination and in fabric dressings.
By kind permission of Dr Alan Smith.
Figure 12

Nanoscale titanium dioxide, containing very small quantities of manganese(II) oxide, is being used in sun lotions as the mixture absorbs harmful UV radiation.  The nanoparticles are so small that they are invisible to the naked eye and the lotion appears clear.  The nanoparticles are stable to light and work by reducing the harmful materials (free radicals), formed in the skin from exposure to the sun, that lead to premature skin aging and skin cancers.

Claims are being made for the effectiveness of nanoparticles in anti-aging preparations to give protection from harmful UV radiation and they can also be used to deliver vitamins that plump and soften the skin, thereby reducing wrinkles.  Zinc oxide, zinc and silver nanoparticles are being used for their anti-microbial and anti-bacterial properties in some skin preparations.

Nanoparticles of copper(I) oxide, displaying anti-fungal properties, are being incorporated into coatings, fibres, polymers, bandages, plastics and soaps.

Sports and leisure

Carbon nanotubes produce composites with epoxy resins with a tensile strength 5 -10 times higher than comparable carbon fibre reinforced materials.  Yacht masts have been produced commercially, which are up to 30 times stiffer without adding any extra weight.

Figure 13  Nano-dispersed clays and butyl rubber (poly(2-methylpropene)) provide an excellent seal for air.  The inside of tennis balls can be sprayed with a solution containing them and this makes the tennis balls much more air tight than the usual tennis ball and will maintain the air pressure, and hence their bounce, for longer.
By kind permission of Dr Alan Smith.

Other applications of carbon nanotube reinforced composites include tennis rackets, fishing rods, and car body panels used in racing cars for strength and lightness, thereby enhancing performance.  The cushioning properties of trainer shoe soles have been improved by incorporating nanoparticles thereby changing the structure of the polymeric soles, whilst at the same time extending durability because of the hardness enhanced by the presence of the nanoparticles.

Health and safety and the environment

A recent concern regarding nanoparticles is whether their small sizes and novel properties may pose significant health or environmental risks.  There is much debate over the safety and environmental impact of nanotechnology.  One particular concern is the potential cancer-causing risks from inhaled particles similar to those that were posed by asbestos fibres.

As many of the applications described above involve nanoparticles being locked into polymeric composites, film and fibres, it is not expected that such nanomaterials will pose a threat to the health and safety of users.  More important however, is the exposure of plant operators handling 'raw' nanoparticles in making these materials. Before universal application of nanotechnology can really be advanced much further, definitive studies of the effect of nanoparticles on the body and the environment must be completed.

On the other hand, some global environmental issues might well be resolved using nanomaterials.  One of the greatest potential impacts of nanotechnology on the lives of the majority of people on Earth could be desalination and purification of water, providing fresh water from the oceans and from brackish wells. Research is progressing on membranes in which carbon nanotubes are embedded to add to the strength of the membrane and make it more efficient.  Again because of the high surface areas, nanomaterials, such as some silicates, make excellent filters for trapping heavy metals and other pollutants from industrial wastewater.

The future

Nanotechnology is already having an impact in many spheres of chemical and materials science. It would seem that only our imagination will limit the widespread application of nanotechnology.  It is confidently forecast that nanotechnology-based industries will be generating sales over $1 trillion globally by 2015.

 

 

Date last amended: 18th March 2013

 

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