https://www.thenakedscientists.com/articles/interviews/self-cleaning-glass
Self-Cleaning Glass, Interview with Professor Ivan Parkin, University College London, 25 January 2009

Meera - This week I'm at St Pancras International which reopened in November 2007 after an 800 million restoration. One of the more notable features of this restoration is the glass roof covering the famous Barlow train shed. It's the first thing that caught my attention as I walked in here. At over 110ft above street level it has an area of more than 10,000m square: using over 17,000 panels of glass. What's really interesting is that the roof isn't just made of ordinary glass. The panels have the ability to clean themselves thanks to reactions taking place on the glass at the nano scale. With me at St Pancras is Ivan Parkin, Professor of Materials Chemistry at University College, London to tell me how this self cleaning glass works.
Ivan - The self cleaning glass works under two ways. One is to something called photo-catalysis which is the action of light onto the surface of the glass to basically chomp away or eat the dirt that's on the surface. The other one's due to a process known as hydrophilicity. That is that the glass loves water and any rain water impacting on the surface will form sheets and that will wash down any dirt in a uniform fashion.
Meera - What's on the glass to cause this to happen?
Ivan - There is a coating on the outside surface of the glass and it's a chemical called titanium dioxide. Titanium dioxide's an inorganic pigment which is widely used in a whole variety of products: everything from sunscreen where it reflects away some of the sun's UV rays through to toothpaste through to the whitener responsible for the white colour in white paint o r even in paper. Titanium dioxide's present as a very thin coating on the outside surface of the glass. It's got a thickness of about 25nm which, to put it into context, the thickness of the coating relative to the thickness of the glass is the same relative ratio as the thickness of the one pence piece compared to the height of Canary Wharf tower.
Meera - How does this actually work to get rid of dirt coming onto the glass?
Ivan - There's a twofold mechanism. One: the action of sunlight on the titanium dioxide will generate a species known as electrons and holes. These electrons and holes with a specific property of titanium dioxide can migrate to the surface and they can [start] a process known as oxidation and reduction of any organic material which is present. Effectively what the titanium dioxide does - it absorbs the UV component of sunlight and this causes the degradation and break-down of any organic material, dust or debris which is on the surface of the glass. It converts it into carbon dioxide and water. One of the neat features of this is it works on the bottom of the dirt outwards and so it loosens the dirt on the materials coating by destroying the contact layer of the dirt and the glass.
Meera - What kind of dirt can actually break down? Surely not everything?
Ivan - That's right. It primarily works on organic dirt or debris. If you have inorganic material on the surface then that can be a problem. For example, if you had a self-cleaning glass which is on a seafront hotel and there was salt spray impacting on the window the self-cleaning glass would really struggle to remove the salt which would be deposited on the surface.
Meera - What then happens to get rid of the dirt?
Ivan - Any rain water impacting on the surface will form a very smooth sheet which will wash things down uniformly. The reason for that is that the action of sunlight on titanium also produces a surface which is very hydrophilic, very water-loving. Water loves wetting the surface and the action of sunlight generates effectively hydroxyl species on the surface, that it there's the ability of water to hydrogen bond the glass. That's why it wets it so effectively.
Meera - If it then creates this sheet of water on the surface how does that sheet then go away?
Ivan - It just runs down in a uniform fashion. Instead of getting dirt concentrated on the edges of the little rivulets or droplets and over time it builds up a higher and higher concentration so you kind of get a run-off pattern - the run-off pattern here is very uniform so you can't actually notice it. Everything is washed down at the same rate but it primarily works only on surfaces which have some form of slant. If you just had a perfectly horizontal surface then it would struggle because there's no gradient for the water to run off. It would then evaporate and you would still not clean very efficiently.
Meera - That makes a lot of sense because looking at the St Pancras roof, although it's arched over, every panel of glass has actually been set at an angle.
Ivan - Yes, that's right. If you look at the panels they probably have at least a 30 degree angle on any one sheet you look at. They're not only angled in an arch fashion but they're also concertinaed together in a kind of corrugated fashion so there is a ridgeline which runs down the middle of each pair of panes of glass.
Meera - Will it take a while once the glass has been fitted for the titanium dioxide to soak up enough energy and really kick in?
Ivan - Yeah, typically it will take between 12 hours on a very sunny or wet day all the way through to 24-48 hours to work effectively.
Meera - Can the properties of this coating be used for anything else as well as cleaning windows?
Ivan - Yes, that's something we're doing some research into at University College London. That is looking at the antimicrobial properties of these coatings. They actually show some very spectacular properties for destroying bacteria and viruses in the environment. For example, we've shown through some work that these coatings can destroy MRSA, E.coli and Chlostridium difficile, for example. Any of those bugs on those surfaces, under the action even sometimes of ring lighting, can be destroyed on these coated surfaces. It's a way actually, potentially of trying to help alleviate hospital acquired infections which are a massive source of death and also costs for all developed countries.