Reductive photocatalysis and smart inks

Abstract

Semiconductor-sensitised photocatalysis is a well-established and growing area of research, innovation and commercialisation; the latter being mostly limited to the use of TiO2 as the semiconductor. Most of the work on semiconductor photocatalytic systems uses oxygen as the electron acceptor and explores a wide range of electron donors; such systems can be considered to be examples of oxidative photocatalysis, OP. OP underpins most current examples of commercial self-cleaning materials, such as: glass, tiles, concrete, paint and fabrics. OP, and its myriad of applications, have been reviewed extensively over the years both in this journal and elsewhere. However, the ability of TiO2, and other semiconductor sensitisers, to promote reductive photocatalysis, RP, especially of dyes, is significant and, although less well-known, is of growing importance. In such systems, the source of the electrons is some easily oxidised species, such as glycerol. One recent, significant example of a RP process is with respect to photocatalyst activity indicator inks. paiis, which provide a measure of the activity of a photocatalytic film under test via the rate of change of colour of the dye in the ink coating due to irreversible RP. In contrast, by incorporating the semiconductor sensitiser in the ink, rather than outside it, it is possible to create an effective UV dosimeter, based on RP, which can be used as a sun-burn warning indicator. In the above examples the dye is reduced irreversibly, but when the photocatalyst in an ink is used to reversibly photoreduce a dye, a novel, colourimetric oxygen-sensitive indicator ink can be created, which has commercial potential in the food packaging industry. Finally, if no dye is present in the ink, and the semiconductor photocatalyst-loaded ink film coats an easily reduced substrate, such as a metal oxide film, then it can be used to reduce the latter and so, for example, clean up tarnished steel. The above are examples of smart inks, i.e. inks that are active and provide either dynamic information (such as UV dose or O2 level) or a useful function (such as tarnish removal), and all work via a RP process and are reviewed here.

Probing the activities of UV and visible-light absorbing photocatalyst powders using a resazurin-based photocatalyst activity indicator ink (Rz Paii )

“…In recent years, this group has developed a rapid, simple method for assessing the activities of photocatalytic films, involving the use of photocatalyst activity indicator inks (paiis) [20][21][22].…”

Kinetics of reduction of a resazurin-based photocatalytic activity ink

“…To date, much of the focus of paii research has been with regard to finding the best ones for probing the different activities exhibited by the various diverse, popular commercial photocatalytic products, such as glass, tiles, paint, fabric and concrete [13,15]. Interestingly, in most cases, it has been noted that, under normal operating conditions, the kinetics of dye reduction is zero-order with respect to the concentration of the dye in the film [13].…”

“…Interestingly, in most cases, it has been noted that, under normal operating conditions, the kinetics of dye reduction is zero-order with respect to the concentration of the dye in the film [13]. However, the fundamental kinetics of reaction (2) [13].…”

“…Interestingly, in most cases, it has been noted that, under normal operating conditions, the kinetics of dye reduction is zero-order with respect to the concentration of the dye in the film [13]. However, the fundamental kinetics of reaction (2) [13]. Although it is possible to postulate a kinetic mechanism for the photocatalytic process, reaction (2), that would provide a plausible rationale for such 'zero-order' kinetics, this would be to forget that the 'normal' operating conditions, referred to above and used in most previous work, includes UV irradiation through the paii film in air.…”

“…SED, e.g. glycerol) and a polymer, hydroxyl ethyl cellulose (HEC), encapsulating agent [11][12][13]. This water-based Rz ink is used to coat the photocatalytic film under test so that, upon the subsequent ultra-bandgap irradiation of the photocatalytic sample/dried ink film combination, the photogenerated holes on the photocatalytic film react quickly and irreversibly with the glycerol SED (to form glyceraldehyde/glyceric acid), leaving the photogenerated electrons to reduce the blue-coloured Rz dye to pink-coloured resorufin, Rf.…”