text: Heterogeneous photocatalysis is the process that underpins the activity of most architectural materials, such as glass, ceramic tiles, roof tiles, concrete, paint, and fabrics which are promoted as being self-cleaning (or air-purifying).  These photocatalytic materials facilitate the oxidative mineralisation of organic and inorganic species by ambient oxygen on their surfaces, rendering the surfaces clean and, usually, hydrophilic. In most commercial photocatalytic products the active layer is a thin, clear, colourless coating of the semiconductor anatase titania, which requires UV light to photogenerate the necessary electrons (e) and holes (h), in its conductance and valence bands, respectively, to promote the photocatalytic process. A schematic of the key processes behind the photocatalytic mineralisation of an organic pollutant on the surface of a titania photocatalyst film is illustrated in figure 1 and the overall reaction is summarised by:
  Water molecules—adsorbed to the photocatalyst—are also needed to generate the hydroxyl groups on the surface. 
  The marketing of photocatalytic products and prevention of counterfeiting is made difficult because the photocatalytic coatings are usually and necessarily invisible to the eye.  One way to achieve a visual demonstration of photocatalysis is to use a dyestuff, like methylene blue, dissolved in water, as the organic species to be mineralised, since, as the photocatalytic process proceeds, the colour of the dye disappears as it is oxidised.  This approach forms the basis of a well-established ISO test for photocatalytic activity of films ISO.  However, most photocatalyst commercial products use only a thin layer of titania (e.g. ca. 15 nm thick in self-cleaning glass) and ambient UV levels are often low (e.g. for a sunny day in the UK the UVA irradiance is only ca. 4 mW/cm).  As a consequence, the photocatalytic oxidative bleaching of methylene blue is usually very slow, taking many hours, and so inappropriate for marketing at least.