Harmful organic
molecules in the indoor air can cause adverse health effects—a problem
known as the ‘sick
building syndrome’. Current air-cleaning technologies require both energy
and upkeep, but a
promising new solution is being developed at Uppsala University
—window glass with
nanostructured coating based on titanium dioxide which uses sunlight
to remove organic
pollutants from indoor air by passing it between the inner panes of the window.
(December 9, 2015) The
preparation of such coatings, tailor-made for indoor air cleaning, was studied
by Bozhidar Stefanov and his dissertation shows how they can be made more effective.
Titanium dioxide is a white pigment often used in paints,
toothpastes and sunscreens. In the form of nanoparticles, it obtains special
properties which make it chemically very reactive. It efficiently absorbs
ultraviolet light and uses this energy to destroy organic molecules at its
surface in a process called photocatalysis. This has made titanium dioxide the
‘white knight’ of nanotechnology and the photocatalyst of choice for water and
air cleaning. Recently, pavement blocks of titanium-dioxide-impregnated cement
have been used to mitigate air pollution from outdoor pollution in cities. A
field trial has been conducted in Malmö, Sweden.
Researchers at the Division of Solid State Physics at the
Ångström Laboratory of Uppsala University are now taking the idea up a notch
and harness photocatalysis for indoors air cleaning. Using an industrial
process called ‘magnetron sputtering’, they have deposited nanostructured
surface coatings of titanium dioxide onto window glass. These coatings are
transparent, have a thickness a hundred times less than that of a human hair,
and have been shown to be very effective for removing acetaldehyde, which is a
common indoor air pollutant.
Windows comprising of such titanium-dioxide-coated glass can
be used to clean air indoors, but there is one problem. Normally pollutant
decomposition products bind strongly to the photocatalyst surface and block
active sites, thereby leading to loss of photocatalytic activity. In his thesis
Bozhidar Stefanov shows that the sputter-deposited coatings have a great
advantage connected with the exposed crystalline facets of the nanoparticles
comprising the coating. Just like the dices used in gambling, not all sides of
the titanium dioxide nanoparticles are equal in their reactivity, and some of
their crystalline facets are much more active in photocatalysis than others.
Unfortunately, these active facets correspond to only about 10 percent of the
nanoparticle surface, and in coatings made of randomly oriented nanoparticles
it is very difficult to strike a ‘lucky six’ and have the reactive facets
exposed at the film surface. During his project, Bozhidar Stefanov found a way
to tweak the deposition of sputter deposited titanium dioxide films in order to
‘cheat the dice’ and have a high probability of the very reactive titanium
dioxide facets at the surface.