Frequently Asked Questions

Some Answers to the Most Common Questions

In addition to the published ISO tests the use of inks to rapidly screen the photocatalytic activity of self-cleaning films can be carried out, details of which are given below under further information.  We can also carry out non-ISO tests such as removal of stearic acid (also for self-cleaning surfaces), destruction of 4-chlorophenol (for powders), or activity vs. usage and activity vs. weathering plots.

It is clear that existing standards need to be used to probe the activities of photocatalytic materials subjected to extensive use under real conditions to identify the truly useful materials/products.  It is essential to probe the longevity of photocatalyst materials when exposed to realistic conditions, based on their likely area of application.  For example, an exterior photocatalyst paint, glass, tile or awning material should be tested with regard to activity stability under accelerated weather conditions.  A photocatalyst fabric (as used in clothing say) should be tested for durability with respect to repeated washing.  The air and water purification activity of photocatalyst coatings should be tested over a significant amount of time under non-laboratory conditions, i.e. using locations in cities with high levels of pollution and on real waste water streams, respectively.  Such work will expose the strengths and weaknesses of any new photocatalyst material product and help identify those with longevity, activity and, therefore, true commercial promise.   At IPS we can help with many of these studies.

In addition to the published ISO tests the use of inks to rapidly screen the photocatalytic activity of self-cleaning films can be carried out [1-3], details of which are given below under further information. We can also carry out non-ISO tests such as removal of stearic acid (also for self-cleaning surfaces) [4,5], destruction of 4-chlorophenol (for powders) [6-9], or activity vs. usage and activity vs. weathering plots.

All the inks suggested (MB, Rz, DCIP) work on the same principle, namely: as usual ultrabandgap irradiation of the semiconductor photocatalyst generates conductance band electrons (e) and valence and holes (h+).  The ink contains a sacrificial electron donor, SED, such as glycerol, which reacts irreversibly and rapidly with the photogenerated holes, leaving the photogenerated electrons to reduce the indicator ink dye molecules, D, contained within the ink film.  All these steps take place in the encapsulation medium of the polymer, HEC, which features in the ink formulation.  Unlike all the ISO tests, the photocatalyst indicator inks are extremely rapid in response, displaying a clearly visible colour change upon dye reduction, as indicated below:


[1] A. Mills, J. Hepburn and M. McFarlane, ACS Appl. Mater. Interfaces, 1 (2009) 1163.
[2] A.Mills, J. Wang, S-K. Lee and M. Simonsen, Chem. Commun., 2005, 2721.
[3] A. Mills and M.McGrady, J. Photochem. Photobiol., A,193 (2008) 228.
[4] A. Mills, A. Lepre, N. Elliott, S. Bhopal, I. P. Parkin and S.A. O’Neill, J. Photochem. Photobiol., A, 160 (2003) 213.
[5] A. Mills and J. Wang, J. Photochem. Photobiol., A, 182 (2006) 181.
[6] J. Theurich, M. Lindner and D.W. Bahnemann, Langmuir, 12 (1996) 6368.
[7] A. Mills, S. Morris and R. Davies, J. Photochem. Photobiol., A,  70 (1993) 183.
[8] A. Mills and S. Morris, J. Photochem. Photobiol., A, 71 (1993) 75.
[9] C.M. Teh and A. R. Mohamed, J. Alloys Compd., 509 (2011) 1648.

Identify what tests and how many samples you wish us to work on and we will provide you with a quote.  If this is satisfactory, upon payment and receipt of samples we will test as requested and provide a report of our findings.

We aim to have your results to you within 21 days of receipt of your sample(s).

Please contact us to discuss the appropriate tests for your sample(s).  Once this has been agreed and payment received, submit your sample(s) ready for testing.

If, under the conditions of the ISO test used, no activity is discerned, do not completely despair.  With some materials it is essential some degree of weathering/use occurs for their true potential to be revealed, as in the case of some photocatalytic paints.  For example, it has been shown for some systems from NO removing ability vs. accelerated weathering time that such films need to be used for a little while before realising their optimum performance, presumably because the pigment particles first destroy some of the organics that coat their surface and which form part of the paint formulation that bind them.   In this case, it can be argued that the presence of NOx actually helps preserve the coating, as the air-borne pollutant is oxidised in preference to the binder.  What is particularly interesting is that some paints exhibit no initial photocatalytic activity, implying that an initial assessment would indicate no activity and so possibly prompt its rejection, even though accelerated weathering or repeated use would reveal its true photocatalytic potential.

In addition, sometimes the test chosen is simply too insensitive with respect to the photocatalyst’s activity, and so inappropriate.  For example, many commercial photocatalyst self-cleaning films show little or no activity when tested using the NOx system, but are very active when assessed using the methylene blue test.  Thus, alternative test(s) may be needed.  The photocatalyst indicator inks, not an ISO standard yet, are particularly good for identifying samples with a low activity.

You will receive a written report, detailing the test(s) carried out, the conditions of the test, the raw data, and analysed/calculated results of your sample(s).  If requested, we can arrange the return of your sample, though this will incur an additional charge to cover postage and packaging.

All standards that assess the photocatalytic activity of a material, whether they be for air or water purification, self-cleaning or disinfection, provide just a snapshot of the activity of the sample.  These measured activities should NOT be assumed to be everlasting, since there are many substances that can deactivate semiconductor photocatalysts, mainly by forming inert (or at least highly recalcitrant), and/or UV-blocking, coatings. These species may be: (i) photocatalytically generated metal oxides and hydroxides, such as SiO2 (e.g. from any silicone-based cleaning solution or sealing compound), (ii) deposited metal oxides/hydroxides, from metal ions in solution (e.g. Fe2O3 from the Fe(III) ions in waste water), (iii) polymeric, coloured, UV-blocking aromatics (e.g. polyaromatics from toluene in air purification) and (iv) precipitated carbon-containing materials (such as: soot, carbonates and even dead cells).

IPS can test activity with repeated sample use if commissioned to do so to address any wear issues.  Price on application.