By Katherine Hooper 2 February 2015
Katherine Hooper recently participated in the Swansea heats for Famelab. Read the great write up of her talk about the wonders of titanium dioxide here!
Let’s begin by considering a bottle of suncream. We may not use it very often in Swansea but on a clear bright summer’s day it is very important to help prevent sunburn. Especially if you are fair skinned and/or ginger with skin that burns more easily.
The reason we might need to protect our skin is related to the composition of sunlight. Light can be thought of as a wave and also as having energy. The shorter the wave, the less energy the light has. The light that is visible to us ranges from the longer red waves through to green and finally blue and violet as the shortest waves that we can see. This is the visible region and the majority of sunlight falls in this range. Beyond violet light is the ultraviolet region which has even shorter waves and higher energy. This is the light that can be harmful to us. Although we are shielded from most of the ultraviolet light in sunlight by the atmosphere, enough penetrates through on a clear summer’s day to be an issue.
We need something that can protect us from ultraviolet light. Luckily suncream contains a hero ingredient; tiny particles of titanium dioxide. Titanium dioxide absorbs ultraviolet light so prevents it from reaching our skin. This ability also makes titanium dioxide useful as a photocatalyst in areas such as water purification. When it absorbs ultraviolet light it produces free radicals (an atom, molecule or ion with a free bond that is highly reactive) which can break down organic dirt and kill bacteria. Note that it doesn’t do this to our skin in suncream because there are other ingredients to use these free radicals!
Titanium dioxide is also exceptionally good at scattering visible light; hence it appears very white. This leads to its most common use as a pigment in things like paint; 70% of all pigments used are titanium dioxide. It is also used to brighten up toothpaste, food colouring, medicine and cosmetics. Obviously you’d be pleased to know it is non-toxic!
So we know titanium dioxide is extensively used in many different applications. But my favourite use of titanium dioxide is as a solar cell. Now this may strike you as odd because I’ve just described how good it is at reflecting away visible light. And there isn’t enough ultraviolet light in sunlight to generate a useable amount of electricity. This is where a different tactic is needed. We need something else to absorb the visible light and to do this we use a dye. Firstly we make our tiny titanium dioxide particles into a sponge like structure – they are all joined together but there is lots of space in between. Next this sponge is immersed into a solution of dye so that the molecules can attach themselves to the titanium dioxide. When the dye absorbs the visible light it reaches an excited state, causing it to transfer an electron into the adjacent titanium dioxide. With this the titanium dioxide is very good at transporting the charge so it can take it through the sponge to be collected and put through a circuit to generate electricity.
So not only can titanium dioxide prevent sunburn, it can also brighten up almost anything, kill bacteria, and make electricity from the sun.
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