By Will Soutter
Air and Water Filtration
Cleavage for Hydrogen Production
Photocatalytic nanoparticles, as the name suggests, exhibit catalytic
activity when activated with light. The material which has been found
to have the most photocatalytic activity is titanium dioxide (TiO2
although several other semiconducting nanomaterials have been
investigatedm such as titanium disilicide (TiS2
TiO2 nanoparticles are activated by light in the UV
range, or at the very far end of blue visible light. The nanoparticles
effectively transfer the energy from this light into organic compounds,
which in most cases is enough to decompose the compound altogether.
This makes the photocatalytic particles effective as a cleaning agent.
and Easy-Cleaning Glass
One of the most common applications for this technology is not
strictly photocatalytic. No chemical reaction occurs - the surface,
which has been treated with TiO2 nanoparticles, simply
becomes highly hydrophilic (water attracting) when exposed to UV light.
This makes the water spread out over the surface, covering it in a thin
transparent film rather than beading up into droplets.
This prevents fogging of glass and shiny metal surfaces, and also
makes the surface significantly easier to clean, as organic materials
unable to adhere to the surface directly. The surface can therefore be
easily cleaned with a wipe, or a spray with fresh water - outdoor
surfaces are cleaned effectively by rainfall.
and Antibacterial Surfaces
|Figure 1. TiO2 nanoparticles can
surfaces clean for months, or even years, by catalytically decomposing
organic matter and killing off microbes with the free radicals
produced. Image credit: "Cleaning with Sunlight", Frauenhofer IGB
Some surfaces have been designed to go one step further - when UV
activated TiO2 nanoparticles catalytically decompose organic
molecules, they produce free radicals - highly reactive compounds which
are toxic to bacteria and fungi.
This makes TiO2-treated surfaces self-sterilizing, which is
extremely useful for medical
applications, and in the food industry.
Self-cleaning surfaces using photocatalytic nanoparticles have also
been developed for the sides of buildings, outdoor furniture, and glass
surfaces such as smartphone screens. Researchers at Frauenhofer IGB
have been optimizing the nanoparticle coatings for various
applications, and have produced thin-film coatings and paints which
will keep furniture free of moss and fungi for two years, and will
clean fingerprints and dirt off a smartphone with just one hour or
exposure to sunlight.
Air and Water Filtration
Photocatalytic nanoparticles have also been suggested as a treatment
or additive for water and air filters. This could help to sterilize air
in clean rooms or laboratories, and to decompose harmful volatile
organic compounds (VOCs) in exhaust gases from factories or chemical
plants before they are released to the atmosphere.
Water treatment is a less developed application of photocatalytic
technology. Water treatment systems designed for use in swimming pools
are already commercially available, but there is great interest in
using the technology, in tandem with existing treatment systems, to
make clean and safe drinking water more available in developing
countries and in remote locations. Photocatalysis could eventually form
an important part of waste water treatment and recycling.
Water Cleavage for Hydrogen Production
Although this application is currently in the research stage,
photocatalysis is considered to be an excellent candidate for
affordable production of hydrogen. Various types of nanoparticles have
been investigated as candidates for this application, including nickel,
ruthenium, and bismuth iron oxide.
At the consumption stage, hydrogen looks like a perfect fuel - it
burns efficiently, and produces nothing more harmful than pure water.
However, production of hydrogen by the methods used currently requires
huge amounts of energy, which is most likely to come from fossil fuels,
negating the green credentials of the hydrogen fuel.
Technologies like photocatalysis could open up the route to truly
carbon-neutral hydrogen, which would no doubt speed its adoption for
use in vehicles and stationary fuel cells.
There is no doubt that TiO2 nanoparticles are a
revolutionary cleaning and sterilization technology. As their use
becomes more widespread, we will see fewer and fewer dirty surfaces in
everyday life. Moreover, the ease of cleaning of TiO2-treated
surfaces will reduce the amount of harsh cleaning chemicals used, which
will have a positive environmental impact.
Research is ongoing into other photocatalytic nanomaterials,
particularly ones which could be tuned to a more specific energy range,
enabling catalysis of more specific chemical processes, rather than
just broadband decomposition. If photocatalytic chemical reactions
become common in the future, it will be a significant step towards the
sustainability of the chemical industry, as photocatalysis is a highly
efficient way to utilize the power of the sun, without having to use
the inefficient intermediary stage of photovoltaic conversion to
- "Applications of Photocatalysis" - EU ObservatoryNano
- "Photocatalytic Hydrogen Evolution under Highly Basic
Conditions" - Yamada et al., J. Am. Chem. Soc., 2011. DOI: 10.1021/ja206079e
- "Photocatalytic hydrogen evolution with Ni nanoparticles"
- Yamada et al, Energy Environ. Sci., 2011. DOI: 10.1039/C2EE03106J
- "Photocatalysis for Water Treatment" - EU ObservatoryNano