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This was another AQA GCSE Chemistry past paper exam question my neighbour's daughter (16) has. It is not dated but our inability to find a markscheme makes us guess it is 2023 as that markscheme is not yet generally published online.

The question asks why nanoparticles of titanium dioxide are used for a self-cleaning rain-activated coating on windows instead of normal titanium dioxide.

I know titanium dioxide is the bright white substance used to make white house paint. I Googled the nanoparticles and found they are transparent. I think I found the answer since obviously, we want a coating on windows to be transparent. Did I miss another obvious advantage of nanoparticles for this coating?

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According to https://welshwindows.co.uk/glassmirrors/self-cleaning-glass/

Self Cleaning Glass

How Does It Work?

Pilkington Activ™ is effectively the same as conventional glass but with a specially developed coating on the outside that has a unique dual action. Once exposed to daylight, the coating chemically reacts in two ways.

Breaking down organic dirt

Using a “photocatalytic” process, the coating reacts with ultra-violet rays present in natural daylight to break down and disintegrate organic dirt.

Washing dirt away

The second part of the process happens when rainwater hits the glass. Pilkington Activ™ is “hydrophilic” which means that instead of forming droplets, the water spreads evenly over the surface and as it runs off takes the dirt with it. Compared with conventional glass, the water also dries off very quickly without leaving unsightly “drying spots”.

The photocatalytic process referred to is the generation of free radicals at the TiO₂ surface - nanoparticles giving a high surface area.

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    $\begingroup$ Interesting. I would have thought that bandgap and phonon interactions were tuned also to increase the catalytic influence of the TiO2 particles. See pubs.rsc.org/en/content/articlelanding/2014/ta/c3ta14052k That said, I know nothing, really, about the Pilkington product. I just assumed... $\endgroup$
    – Rich
    Apr 12 at 16:47

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