Self-Cleaning
Clothes
These self-cleaning
fabrics have a nanofilm coating of titanium dioxide nanoparticles, which can
break down dirt molecules, pollutants, and microorganisms when exposed to
visible and UV light. Clothes made this way could be cleaned by simply exposing
them to sunlight. Daoud and Xin have developed a method for applying the thin
film of titanium dioxide to cotton easily and inexpensively. With their method,
self-cleaning fabrics could be produced commercially and for public use. The
scientists note that self-cleaning fabrics could be especially useful for
people who don't have the time or means for washing their clothes, such as
military personnel or hikers.
Scope of use of self cleaning
fabrics:
In addition to suits, the self
cleaning fabric could be applied to hospital garments, sportswear, military
uniforms and rain coats. Other possible applications include awning material
for outdoor campers, fabrics for lawn furniture and convertible tops for cars.
How Self-Cleaning
Fabric Works
The
self-cleaning fabrics works using the photocatalytic properties of titanium
dioxide, a compound used in many new nanotechnologies solar cell applications.
The fabric is coated with a thin layer of titanium dioxide particles that
measures only 20 nanometers in diameter. When this semi-conductive layer is
exposed to light, photons with energy equal to or greater than the band gap of
the titanium dioxide excite electrons up to the conduction band. The excited
electrons within the crystal structure react with oxygen atoms in the air,
creating free-radical oxygen. These oxygen atoms are powerful oxidizing agents,
which can break down most carbon-based compounds through oxidation-reduction
reactions. In these reactions, the organic compounds (i.e. dirt, pollutants,
and micro organisms) are broken down into substances such as carbon dioxide and
water. Since the titanium dioxide only acts as a catalyst to the reactions, it
is never used up. This allows the coating to continue breaking down stains over
and over. Titanium dioxide can also destroy pathogens such as bacteria in the
presence of sunlight by breaking down the cell walls of the microorganisms
making self-cleaning fabrics especially useful in hospitals and other medical
settings.
Figure. Above is an
illustration of how self-cleaning fabric cleans itself. When photons of light
hit the layer of titanium dioxide, electrons excited up to the conduction band
react with oxygen molecules in the air. The double bond of the oxygen molecule
is broken
No More Washing
Machines?
Breakthroughs in nanotechnology
have made self-cleaning fabrics both practical and economical. With commercial
production making the technology readily available to the masses, will washing
machines and laundry detergent become obsolete?
There are several factors limiting how quickly current self-cleaning fabrics would be able to break down organic compounds. Sunlight is the best source of light for activating the self-cleaning process. A ketchup-stained shirt would have to be left outside in the sun for at least a day in order to remove the stain. However, for military persons or hikers, who are outside in the sun for long periods of time without the time or means to clean their clothes, self-cleaning fabric would be ideal. It's also important to note that the newly developed method for producing self-cleaning fabric has only been developed for cotton. Further research would be required to test ways of applying titanium dioxide nanofilms to other textiles.
Self-Cleaning Fabric
Problems
The main
reasons that self-cleaning fabrics require a lot of time to break down stains
is because titanium dioxide is very inefficient at using energy from sunlight.
The titanium dioxide serves as a catalyst for the break down of dirt molecules
by providing electrons that oxidize oxygen molecules in the surrounding air.
The electrons are freed from the titanium dioxide via the photoelectric effect.
But because of titanium dioxide's high band gap energy, only high energy blue
and UV light photons have enough energy to excite electrons to the conduction
band. High energy blue and UV light only make up 3% of the solar spectrum,
so titanium dioxide can only use a very small portion of the sun's energy to
break down stains.
Excitation of electrons to the conduction band is only the beginning of the cleaning process. These electrons must then react with oxygen atoms, which then react with the dirt particles. All of these reactions are limited by access to and the amount of freed electrons in the titanium dioxide. So for a large stain, a lot of light energy is needed before the fabric can fully break it down.
Figure. Above is
an illustration of the inefficiency of self-cleaning fabric. The titanium
dioxide's catalytic powers can only be activated by a small portion of blue
light and UV light, which is only 3% of the solar spectrum. All the rest of the
light energy is wasted
Conclusion:
Self-cleaning property will become a standard feature of future textile and
other commonly used materials to maintain hygiene and prevent the spreading of
pathogenic infection. It can also help in reducing the consumption of
chemicals, such as detergents and dry-cleaning solvents, water, and energy. But
more extensive research is required to make it economically feasible to
consumers.
