Photocatalyst activity indicator ink

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A photocatalyst activity indicator ink (Paii) is a substance used to identify the presence of an underlying heterogeneous photocatalyst and to measure its activity. Such inks render visible the activity of photocatalytic coatings applied to various "self-cleaning" products. The inks contain a dyestuff that reacts to ultraviolet radiation in the presence of the photocatalytic agent in the coating. They are applied to the coated product using a pen or brush, and show a color change or disappearance when exposed to ultraviolet.

Applications[edit]

A commercial Paii ink sold in a felt tip pen. It can be easily applied on a construction material (e.g. glass) to reveal the presence of a photocatalytic coating: in this case by changing its colour from blue to pink

A photocatalyst activity indicator ink[1][2][3][4][5][6][7][8][9] quickly and easily identifies the presence of an underlying heterogeneous photocatalyst and provides a measure of its activity. A heterogeneous photocatalyst is a material that uses absorbed light energy (usually UV) to drive desired reactions that would not otherwise proceed under ambient conditions. Commercial photocatalytic products, which include: architectural glass,[10][11] tiles,[12][13] concrete,[14] paint[15] and fabrics.[16] are marketed on their ability to clean their own surfaces (i.e. are self-cleaning) and the ambient air. Paiis address the industry need for a rapid, simple, inexpensive method to demonstrate and assess the activities of the usually thin, invisible to the eye, photocatalytic coatings present on self-cleaning products. A Paii, coated onto the surface of a photocatalyst material under test, works via a photoreductive mechanism, in which light absorbed by the photocatalyst drives the reduction of the dye in the paii, thereby producing a striking color change,[1][4] which can be measured through the use of a simple mobile phone camera + app, in lieu of any sophisticated analytical equipment.[17] Uses of paiis include: (i) laboratory, factory and on-site commercial photocatalyst product quality control (ii) marketing, for the rapid and striking demonstration of the efficacy of the usually invisible and otherwise slow-acting photocatalyst coating, (iii) counterfeit detection and (iv) evaluating new photocatalytic materials. The development and applications for such paiis have been reviewed in detail. [18]

Background[edit]

Figure 1. Schematic of the overall reaction, for the photocatalytic mineralisation of an organic pollutant on the surface of a titania photocatalyst film. (1) Ultra-band gap light generates electron-hole pairs. (2) Photogenerated holes migrate to the surface and can react with surface hydroxyl groups to generate hydroxyl radicals. (3) Organic pollutants are oxidised to their mineral form via these photogenerated hydroxyl radicals. (4) Photogenerated electrons can react with adsorbed oxygen to generate superoxide and subsequent other reactive species which can also oxidise organic pollutants.

Heterogeneous photocatalysis is the process that underpins the activity of most architectural materials, such as glass,[10][11] tiles,[12][13] concrete,[14] paint[15] and fabric,[16] which are promoted as being 'self-cleaning'. 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.[19] 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:

Reaction Summary (1)

The marketing of photocatalytic products and prevention of counterfeiting is made difficult because the photocatalytic coatings are usually and necessarily invisible to the eye.[19] 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.[20] This approach forms the basis of a well-established ISO test for photocatalytic activity of films ISO.[21] 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/cm2). As a consequence, the photocatalytic oxidative bleaching of methylene blue is usually very slow, taking many hours,[21] and so inappropriate for marketing at least.

Theory[edit]

Figure 2. Photocatalyst Activity Indicator Ink (Paii). Upon irradiation with UV light, photogenerated electrons (e) and holes (h+) are produced on the surface of the self-cleaning coating. The sacrificial electron donor (SED) present in the Paii ink effectively 'mops-up' the holes (h+), allowing the electrons (e) to reduce the dye (Dox) to another (usually colourless) form (Dred).

Photocatalyst activity indicator inks (Paii's) are a recent advance in the visual demonstration of photocatalysis and the assessment of the activity of photocatalyst materials.[1][2] They are an inexpensive, easy to use and provide a very quick route to demonstrating the presence of a photocatalytic film, even under low levels of UV light. Unlike the photo-oxidative bleaching of methylene blue,[20] they use the underlying semiconductor photocatalyst film to photoreduce the dye (Dox in figure 2), in the ink coating, to another (usually colourless) form, (Dred in figure 2) whilst simultaneously oxidising an easily oxidised organic species, a sacrificial electron donor (SED), such as glycerol, which is also present in the ink.[1][2][3] The kinetics of reduction of the dye in a paii have been studied in great detail.[22] Figure 2 illustrates the basic principles of operation of a Paii when applied to a product that has a thin photocatalyst film coating.

Practise[edit]

The ink is applied to the photocatalyst coating, usually using either a felt-tipped pen, air-brush, rubber stamp or paint brush, and then exposed it to sunlight or an alternative, appropriate (invariably UVA) light source. The ink identifies the presence of the photocatalyst coating by changing colour upon irradiation of the latter at a rate (usually < 10 min[1]) which provides a measure of the film's activity. For example, it has been established that the rate of change in colour of an Paii on commercial self-cleaning glass is directly related to the rate at which the glass is also able to photo-oxidatively mineralise, via reaction (1), the wax-like, natural fatty acid, stearic acid, found in finger prints.[23] By making the dyes in the ink increasing difficult to reduce chemically, for example by using: basic blue 66,[7] resazurin,[1] and acid violet 7,[8] respectively, it is possible to make Paii's which are effective on photocatalyst coatings which exhibit, respectively: low (most self-cleaning tiles), moderate (self-cleaning glass) or high (self-cleaning paints) activities. Currently Paii's are used as quality control and marketing tools in commerce and as a quick and easy way to assess and/or map the activities of new photocatalytic materials in research.[1][2][3][4] In light of the need for in situ testing of commercial photocatalyst materials, Paii labels have been developed that can be applied simply in the field on any surface to be tested, in both a non-reusable[24] and reusable[9] form.

The rate of the rapid colour change associated with photocatalyst activity indicator inks has been directly correlated with the photocatalytic oxidation of stearic acid,[1] methylene blue [25] and NOx.[8] In addition, it has also been demonstrated that such inks can be used on highly coloured and black surfaces, provided the oxidised and/or reduced form of the redox dye is luminescent,[26] and that they can be effectively used to demonstrate the activity of visible light photocatalysts.[27]

Details and videos of a recent draft ISO proposal (ISO NP21066) based on the resazurin paii have been reported recently.[28]

Applications[edit]

External video
Application of the new SunCatalyst Laboratories Paii Label
Methylene blue photocatalyst activity indicator ink (Paii) being used to verify the presence of a self-cleaning coating on a ceramic tile
  • Quality control (in laboratory, factory and on site)
  • Marketing
  • Counterfeit identification
  • Research material assessment

See also[edit]

References[edit]

  1. ^ a b c d e f g h Mills, A., Wang, J., Lee, S. & Simonsen, M. 2005, "An intelligence ink for photocatalytic films.", Chemical Communications (Cambridge, United Kingdom), no. 21, pp. 2721-2723.
  2. ^ a b c d Mills, A. & McGrady, M. 2008, "A study of new photocatalyst indicator inks.", Journal of Photochemistry and Photobiology, A: Chemistry, vol. 193, no. 2-3, pp. 228-236.
  3. ^ a b c Kafizas, A., Crick, C. & Parkin, I.P. 2010, "The combinatorial atmospheric pressure chemical vapor deposition (cAPCVD) of a gradating substitutional/interstitial N-doped anatase TiO2 thin-film; UVA and visible light photocatalytic activities.", Journal of Photochemistry and Photobiology, A: Chemistry, vol. 216, no. 2-3, pp. 156-166.
  4. ^ a b c "Home | The Intelligent Pen and Ink Company | Photocatalytic Testing Pens & EquipmentThe Intelligent Pen and Ink Company-Photocatalytic Testing Pens & Equipment". Inkintelligent.com. Retrieved 2014-06-02. 
  5. ^ A. Mills, J. Hepburn, D. Hazafy, C. O’Rourke, J. Krysa, M. Baudys, M. Zlamal, H. Bartkova, C.E. Hill, K.R. Winn, M.E. Simonsen, E.G. Søgaard, S.C. Pillai, N.S. Leyland, R. Fagan, F. Neumann, C. Lampe, T. Graumann, A simple, inexpensive method for the rapid testing of the photocatalytic activity of self-cleaning surfaces, J. Photochem. Photobiol., A, 272, 2013, 18–20.
  6. ^ A. Mills, J. Hepburn, D. Hazafy, C. O’Rourke, N. Wells, J. Krysa, M. Baudys, M. Zlamal, H. Bartkova, C. E. Hill, K. R. Winn, M. E. Simonsen, E. G. Søgaard, S. Banerjee, R. Fagan, S. C. Pillai, Photocatalytic activity indicator inks for probing a wide range of surfaces, J. Photochem. Photobiol., A, 290, 2014, 63–71.
  7. ^ a b A. Mills, C. O’Rourke, N. Wells, A smart ink for the assessment of low activity photocatalytic surfaces, Analyst, 139, 2014, 5409–5414.
  8. ^ a b c A. Mills, C. O’Rourke, K. Lawrie, S. Elouali, Assessment of the activity of photocatalytic paint using a simple smart ink designed for high activity surfaces, ACS Appl. Mater. Inter., 6, 2014, 545−552.
  9. ^ a b A. Mills, N. Wells, Smart, reusable labels for assessing self-cleaning films, Chem. Commun., 51, 2015, 4161–4163.
  10. ^ a b "First in Glass". Pilkington. Retrieved 2018-01-11. 
  11. ^ a b "Saint-Gobain". Saint-Gobain. 2014-01-07. Retrieved 2018-01-11. 
  12. ^ a b "(H.K.) Limited". TOTO. Retrieved 2018-01-11. 
  13. ^ a b Deutsche Steinzeug AG. "Deutsche Steinzeug AG - Willkommen". Deutsche-steinzeug.de. Retrieved 2014-06-02. 
  14. ^ a b "Italcementi Group". Italcementi Group. Retrieved 2018-01-11. 
  15. ^ a b Sto Ltd. "Sto Ltd. | External wall insulation, render, interiors and acoustic systems". Sto.co.uk. Retrieved 2018-01-11. 
  16. ^ a b MakMax (Taiyo Kogyo Group). "(Taiyo Kogyo Group) introducing Tensile Membrane Structures". MakMax. Retrieved 2018-01-11. 
  17. ^ Mills, Andrew; Wells, Nathan. "Indoor and outdoor monitoring of photocatalytic activity using a mobile phone app. and a photocatalytic activity indicator ink (paii)". Journal of Photochemistry and Photobiology A: Chemistry. 298: 64–67. doi:10.1016/j.jphotochem.2014.10.019. 
  18. ^ Mills, A., Wells, N., Reductive photocatalysis and smart inks, Chem. Soc. Rev., 10 (2015) 2849-64.
  19. ^ a b V. Augugliaro, V. Loddo, M. Pagliaro, G. Palmisano, L. Palmisano, "Clean by Light Irradiation: Practical Applications of Supported TiO2", RSC Publishing, Cambridge, 2010.
  20. ^ a b A. Mills, M. McFarlane “Current and possible future methods of assessing the activities of photocatalyst films”, Catalysis Today vol. 129, 2007, pp. 22–28.
  21. ^ a b ISO 10678: 2010, ‘Fine ceramics, advanced technical ceramics – determination of photocatalytic activity of surfaces in an aqueous medium by degradation of methylene blue’, ISO, Geneva, 2010.
  22. ^ Mills, Andrew; Wells, Nathan; MacKenzie, John; MacDonald, Grant. "Kinetics of reduction of a resazurin-based photocatalytic activity ink". Catalysis Today. 281: 14–20. doi:10.1016/j.cattod.2016.05.045. 
  23. ^ B. Hartzell-Baguley, R. E. Hipp, N. R. Morgan, S. L. Morgan, “Chemical Composition of Latent Fingerprints by Gas Chromatography–Mass Spectrometry. An Experiment for an Instrumental Analysis Course”, Journal of Chemical Education, vol. 84 (4), 2007, pp. 689–691.
  24. ^ Mills, A., Wells, N., Hawthorne, D., Hazafy, D., Photocatalyst activity indicating adhesive labels for use in the field, (2018) https://doi.org/10.1016/j.jphotochem.2018.01.005
  25. ^ Mills, A, Wells, N. & O’Rourke, C., Correlation between the photocatalysed oxidation of methylene blue in solution and the reduction of resazurin in a photocatalyst activity indicator ink (Rz Paii), J. Photochem. Photobiol., A, 330, 2016, 86–89.
  26. ^ Mills, A, Yusufu, D, Wells, N. & O’Rourke, C., Assessment of activity of ‘transparent and clear’ and ‘opaque and highly coloured’ photocatalytic samples using a fluorescent photocatalytic activity indicator ink, FPaii, J. Photochem. Photobiol., A, 330, 2016, 90–94.
  27. ^ Mills, Andrew; Wells, Nathan; O’Rourke, Christopher. "Probing the activities of UV and visible-light absorbing photocatalyst powders using a resazurin-based photocatalyst activity indicator ink (Rz Paii )". Journal of Photochemistry and Photobiology A: Chemistry. 338: 123–133. doi:10.1016/j.jphotochem.2017.01.030. 
  28. ^ http://www.profandrewmills.com/rz-iso-test/; accessed January 2018