Why Dyes Should Not Be Used to Test the Photocatalytic Activity of Semiconductor Oxides

M studies focusing on heterogeneous photocatalysis for water treatment report on the synthesis of novel semiconductor oxides (which can often absorb visible light) and their characterization with a broad range of techniques, as well as the test degradation of a model compound (substrate). The substrate is sometimes a dye that can be easily monitored by spectrophotometry. We discuss herein that the use of dyes in photocatalysis is very problematic. First of all, unless the full absorption spectrum is considered, it is possible to have spectral interferences by transformation intermediates, which may absorb radiation at the wavelength of the dye absorption maximum. Furthermore, dyes have an additional and more substantial problem. When degrading a model compound, one wishes to assess the ability of the photocatalyst to photogenerate reactive transient species such as surfaceor subsurface-trapped holes, hydroxyl radicals (trapped on the photocatalyst surface or free in solution), and/or trapped electrons. These transient species can then react with a wide variety of organic and inorganic substrates, accounting for the photocatalytic activity of the studied material. In contrast, any peculiar effects that the photocatalyst may have toward a particular molecule are usually not of interest (unless the aim is to obtain a selective photocatalytic degradation, which is often not the case). In fact, employing the only molecule (or one of the few) that the photocatalyst can degrade, severely limits the conclusions of such a study. Many dyes have the ability, when photoexcited, to inject an electron into the conduction band of a semiconductor. This property is widely exploited in the field of dye-sensitized solar cells (DSSC), where dyes are used together with a photocatalyst that is usually a semiconductor oxide such as TiO2. 3 In this case, radiation is absorbed by the dye, not the semiconductor oxide, differently from the typical photocatalytic setup where the photocatalyst absorbs radiation to produce reactive transients. In a DSSC, the sensitized dye degradation (which could follow the electron injection by the photoexcited dye into the conduction band of the photocatalyst) is prevented by the design of the device, but this would not be the case for an aqueous suspension. The problem with the degradation pathway of a sensitized dye is that it lacks generality, because (i) it cannot be operational with nonabsorbing molecules and (ii) for the electron transfer to be allowed, it requires compatible energy levels between the excited state(s) of the dye and the conduction band of the photocatalyst. Sensitized dye degradation is a confounding factor in the assessment of photocatalytic activity because, in the irradiated suspension, degradation could be due to either a photocatalytic process (the genuine effect one wants to highlight), a dye sensitization or both. When using dyes as model molecules, puzzling results can be obtained. Here we show experiments made using TiO2 Degussa (now Evonik) Aeroxide P25 (TiO2 P25) which is well-known to absorb radiation up to about 360 nm; thus, visible light excitation is not possible with this material. Rhodamine B was used as substrate. Figure 1 shows that the degradation of Rhodamine B in the presence of TiO2 P25 took place under different irradiation conditions. Control runs without TiO2 were also carried out, with limited or negligible Rhodamine degradation. The aqueous suspensions containing TiO2 were centrifuged before spectrophotometric measures and, by applying the same procedure to nonirradiated samples, insight was obtained into the (very limited) adsorption of the dye onto the TiO2 itself. Under simulated sunlight (Solarbox, emitting radiation above 320 nm), a semiconductor-type mechanism where TiO2 absorbs radiation and generates reactive species might be hypothesized. The same explanation is more problematic with the two lamps that emit above 380 nm, because the overlap between the lamp emission and the TiO2 absorption is, if any, extremely limited. Finally, it is very hard to imagine how a yellow lamp (emitting radiation above 480 nm, with a high emission line at 550 and a broad maximum centered at 580 nm) could be able to photoexcite TiO2 P25. One should thus conclude that a dye-sensitized process was active, which