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Cleaning a window by hand with a squeegee.

Self-cleaning windows

It's a disaster! They've invented windows that can clean themselves! If, like me, you're one of the minuscule minority who actually enjoy cleaning your windows, this new technological development will fill you with horror. For the rest of humanity—the millions of people who loathe wobbling up ladders, bucket and soapy squeegee in hand—the prospect of windows that keep themselves sparkling automatically, using nothing but the sun and the rain, will seem like nothing short of a miracle. How can a simple piece of glass stay clean all by itself? Let's take a closer look!

Photo: Say goodbye to this tiresome chore! Self-cleaning glass means you can throw away your window-cleaning kit—or does it? You'll see it sold under brand names such as Pilkington Activ™ and Cardinal Neat® Glass.

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  1. What is self-cleaning glass?
  2. How does self-cleaning glass work?
  3. How does a window clean itself?
  4. Advantages and disadvantages of self-cleaning windows
  5. Not just windows
  6. Find out more

What is self-cleaning glass?

Empire State Building by Carol M. Highsmith.

Photo: The coating added to self-cleaning glass is extremely thin—like placing a dime on top of the Empire State Building. Photographs in the Carol M. Highsmith Archive, courtesy of Library of Congress, Prints and Photographs Division.

The first thing to note about self-cleaning windows is that they're not, in fact, "simple pieces of glass."

They have a very thin outer coating of titanium dioxide, a white, powdery titanium compound best known for giving that dazzling gleam to paint, toothpaste, and all kinds of other bright white things. Now if titanium dioxide is, essentially, the white in white paint, it might seem ludicrous to splash it all over a window—something we naturally want to be transparent. But the coating really is ultra-ultra thin. We're talking about putting a layer 10–25 nanometers deep on glass that might be 4mm thick, which is like sitting a dime on top of the Empire State Building! It reduces the light passing through the glass by no more than about 5 percent.

Tube of titanium white paint

Photo: Titanium dioxide, an important part of self-cleaning windows, is also the ingredient that puts the "white" into white paint. That's why this tube of artist's paint is labeled "titanium white."

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How does self-cleaning glass work?

The titanium dioxide coating cleans through a double-whammy, two-stage process: it's photocatalytic (light-activated) and hydrophilic (water-loving)—but let's not get bamboozled by the jargon. Let's find out more...


Dark red sunrise over Dorset, England.

Photo: It takes energy to shift dirt from windows, even if they clean themselves. The ultraviolet part of sunlight—the part that gives you sunburn—is the power behind self-cleaning glass.

Titanium dioxide is a photocatalyst: it's a material that makes chemical reactions happen when the right kind of light shines on it. The right kind of light for titanium dioxide is ultraviolet (UV), the super-blue, high-energy part of sunlight that our eyes can't see, but that nevertheless can give us sunburn even on a cloudy day. When ultraviolet light hits the titanium dioxide coating of a self-cleaning window, electrons are generated. These turn water molecules from the air into hydroxyl radicals that make chemical oxidation and reduction reactions take place on the coating. In effect, the hydroxyl radicals attack organic (carbon-based) dirt and chop it up into smaller pieces that are much easier for rain to wash away. Since the reactions happen on the titanium coating, on the very surface of the glass, they attack the lowest layers of the dirt, loosening encrusted muck from the glass very effectively by chipping it away from the inside out (the opposite of normal window cleaning, where you effectively scrub the dirt from the outside in). All this is illustrated in the diagram in the box below.

Electromagnetic spectrum showing position of ultraviolet light

Artwork: Ultraviolet is high-energy, high-frequency (short-wavelength) light just beyond the blue end of the visible light spectrum our eyes can see.


Glass is usually hydrophobic or "water hating": water dropped onto glass tends to "bead" (form droplets), while rain runs down windows in noticeable rivulets, leaving dirty streaks as it goes. The titanium dioxide coating changes all that: the hydroxyl radicals produced by photocatalysis make the glass hydrophilic or "water loving." Instead of staying in drops, water molecules spread out evenly across the glass in a very even sheet. So when rain hits a dirty self-cleaning window, it spreads across it like a great big cloth. Since the window is most likely vertical or mounted at an angle, the sheet of water wipes down it neatly and evenly, a bit like a rubber squeegee, and the glass dries without any streaks or smears. Magic!

How does a window clean itself?

In summary, then, here's how a titanium-dioxide-coated window gets itself clean through photocatalysis and hydrophilia:

Animation showing how a self-cleaning window works.

  1. When UV light (the yellow arrow shown on the left) shines on the titanium dioxide coating, electrons (the tiny, negatively charged particles inside atoms) are released.
  2. The electrons interact with water molecules (H2O) in the air, breaking them up into hydroxyl radicals (OH·), which are highly reactive, short-lived, uncharged forms of hydroxide ions (OH−).
  3. These agile hydroxyl radicals attack the hefty organic (carbon-based) molecules from which most dirt is made, breaking apart their chemical bonds and turning them into smaller, harmless substances such as carbon dioxide and water. This is an example of oxidation.
  4. The hydroxyl radicals also make the glass hydrophilic (water-loving). When it rains, water molecules spread evenly across it and wipe it clean like a kind of automatic squeegee!

Note that this is very similar to the process that happens in photocatalytic air purifiers.

Advantages and disadvantages of self-cleaning windows


Self-cleaning windows look great and the coating is meant to keep working for the lifetime of the window. They save time and money (window cleaning can be expensive if you hire someone to do it) and help to avoid the risk of accidents happening when people wobble up ladders with buckets of water. You can get self-cleaning windows in various different thicknesses (typically 4–10mm), with blue tints (to reduce solar glare in places such as conservatories), and with heat-reflecting inner coatings for improved energy efficiency.


That said, self-cleaning windows have quite a few drawbacks.

All told, however, the advantages seem to outweigh the drawbacks, especially if you really loathe window cleaning or your windows are inaccessible or hard to reach.

Cleaning a window by hand with a sponge.

Photo: "Ggggrrrr, you mean I still have to clean the insides?"

Not just windows

What works on a window should, theoretically, work on any large flat surface, so similar photocatalytic technology can be used in many other places. Self-cleaning solar panels and building facades have already been developed—and for fairly obvious reasons: they get dirty quickly, are often mounted high up in inaccessible places, and so can be expensive to keep clean. Although cleaning solar panels doesn't always produce that much gain in efficiency (for small domestic systems, anyway), it can be worthwhile on larger installations; but if you've ever seen a solar farm (with many hectares of solar panels), you'll know how difficult and impractical that can be. Self-cleaning technology has been developed for both solar thermal panels (ones that make hot water) and photovoltaic solar cells (ones that generate electricity).

How a titanium dioxide coated surface cleans itself.

Artwork: How a typical self-cleaning surface coating works. The surface has a substrate (base material) of glass (1) covered with polyurethane plastic (2) and a titanium dioxide coating (3) made of nanoparticles less than 100 nanometers in diameter, on which organic dirt (4) slowly builds up. When ultraviolet light hits the coating (5), photocatalyzed reactions destroy the dirt, releasing harmless carbon dioxide and water (6). Based on an artwork from Self-cleaning surface coating (photocatalysis) by Rudolf Gensler et al, Siemens AG, February 5, 2013.

Although self-cleaning technology really comes into its own for large surfaces outdoors, it can also be used more modestly indoors. Since there's obviously no sunlight inside, indoor surfaces typically need light from an ultraviolet lamp to activate the cleaning process. (Some self-cleaning surfaces have an inner layer of tiny LEDs concealed under the outer, working layer, which shine out intermittently to activate the photocatalyst.) Self-cleaning mirrors, tiles, and kitchen surfaces in the home are likely next steps; self-cleaning, antimicrobial surfaces in hospitals are another possible application. And what about self-cleaning eyeglasses and contact lenses?

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Find out more

On this website



If you want to know about the science in (much) more detail, these books are worth a look:


For technical details about how photocatalytic glass is made, it's worth browsing some of the patents in this area. These are just a few examples—and there are many more!

And why stop at self-cleaning windows? Frances Gabe invented a complete self-cleaning house, which you can find documented here. It doesn't use photocatalytic technology, however.

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Text copyright © Chris Woodford 2010, 2017. All rights reserved. Full copyright notice and terms of use.

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@misc{woodford_windows, author = "Woodford, Chris", title = "Self-cleaning windows", publisher = "Explain that Stuff", year = "2010", url = "", urldate = "2022-09-23" }

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