IMAGINE A ÒSELF-CLEANINGÓ WORLD

 

Paul Chin


You may say that I'm a dreamer, but I'm not the only one

I hope someday you'll join us, and the world will live as one

- John Lennon, Imagine

 

From the yesterday decades of yellow submarines to bell bottoms to New Kids on the Block, culture has evolved with humans, and technology is no different.  Gone will be the need to clean windows eight days a week.  Glass companies have manufactured Òself-cleaningÓ windows, billed under name brands such as PPG SuncleanTM and Pilkington ActivTM, containing a thin film of titanium dioxide (TiO2), a photocatalyst which utilizes ultraviolet light on a good day sunshine for (a) photocatalytic oxidation (PCO) of adsorbed organic materials, and (b) photo-induced superhydrophilicity (PSH) to assist with water washing of the surface by allowing water to come together instead of beading up when it rains.  Using these two TiO2 photoreactive properties may minimize the need to send washers outside of big skyscrapers to clean window exteriors, or to remove the bird ÒmishapsÓ off Hank SteinbrennerÕs luxury box window every time the Boston Red Sox score in the new Yankee Stadium.

 

Though this revolutionary technology is truly amazing, researchers canÕt just let it be.  A field analysis method is needed to characterize the TiO2 film at initial and continuing times, and various environmental conditions.  Furthermore, the creation of these Òself-cleaningÓ surfaces logically leads to consideration of testing potential organic material deposits found in outdoor air.

 

Work completed by the Ollis research group at Raleigh, NC, focused on using the organic dyes Acid Blue 9 and Reactive Black 5 for visual and quantitative PCO data on Pilkington ActivTM glass1.  The visual results showed a return of aesthetic clarity on the dye-coated ActivTM glass after 1000 min of near-UV illumination (irradiance: 1.4 mW/cm2).  Using UV-Vis spectroscopy, our lab quantitatively observed a monotonic decrease in peak absorbance as a function of time.  A two-step reaction mechanism was developed to help(!) describe the kinetics of dye degradation.  The kinetic model indicated that the colored dye forms a colored intermediate before its colorless final product.  This model can be applied to characterize the change in TiO2 kinetics at initial and continuing times.  Additionally, here comes the sun, solar experiments conducted under variable UV light intensity, relative humidity, and temperature exhibited similar dye decolorization rates compared to work completed in an indoor photoreactor.  Our model fit the solar data well even though the UV irradiance varies with time.

 

Though dyes are a good characterization material, they do not simulate organic pollutants encountered in the atmosphere.  Take a walk down Penny Lane or enter any industrialized U.S. city, and what you will find besides crime, poverty, obnoxious sport fans, and general helter skelter, is the deposition of soot on exterior surfaces (e.g., buildings, statues).

 

Work completed in our lab systematically studied various TiO2 and soot thickness to determine their effect on the PCO rate of soot2 using a quartz crystal microbalance (QCM).  Results of UV-Vis spectroscopy revealed that our lab could linearly and reproducibly deposit soot using an analytical linear rotor rotating past a hurricane lamp.  By changing the amount of soot deposited on top of the TiO2-coated QCM crystals, experiments revealed a range of behaviors from complete soot mineralization (< 0.7 mm soot thickness), to partial oxidation (1 mm thickness), to minimal soot oxidation (2 mm thickness) caused by soot screening of the incident UV light.  Alternatively, varying the TiO2 thickness did not demonstrate significant changes in the soot destruction rate because oxidation of soot in direct contact with TiO2 is the dominant reaction compared to lateral and remote soot oxidation, and the soot / TiO2 contact area is independent of TiO2 thickness.  A series / parallel reaction mechanism was successfully constructed and utilized to describe the reaction kinetics of our soot PCO for t > 1,000 min.  If we get back to previous literature3,4 for PCO of soot on thick (c.a. 1 mm) TiO2 films, we can work it out that our model5 adequately describes their data.  Finally our lab illustrated TiO2 PCO of carbon black, often studied as a model soot, though further tests were needed to elucidate the kinetic reaction mechanism.

 

While continual development of these novel Òself-cleaningÓ surfaces will transform our hard dayÕs night (and possibly put WindexTM out of business), additional research is required to perfect the TiO2 thin films.  Further experimentation of organic deposits is needed to comprehend fully the applicability of these Òself-cleaningÓ surfaces under realistic conditions.  After a long and winding road, one can imagine how this technology will take us across the universe.

 

The End.

 

 

 

1 Chin P., Ollis D.F., Catalysis Today 2007, 123 (1), 177-188.

2 Chin P., Grant C.S., Ollis D.F., ÒQuantitative Photocatalyzed Soot Oxidation on Titanium DioxideÓ, submitted to Journal of Catalysis 2008.

3 Mills A., Wang J., Crow M., Chemosphere 2006, 64 (6), 1032-1035.

4 Lee N.C., Choi W.Y., Journal of Physical Chemistry B 2002, 106 (45), 11818-11822.

5 Chin P., Roberts G.W., Ollis D.F., Industrial & Engineering Chemistry Research 2007, 46 (23), 7598-7604.