Pierre Pichat

Titanium dioxide nanotubes (TNT) have mainly been used in dye sensitized solar cells, essentially because of a higher transport rate of electrons from the adsorbed photo-excited dye to the Ti electrode onto which TNT instead of TiO₂ nanoparticles (TNP) are attached. The dimension ranges and the two main synthesis methods of TNT are briefly indicated here. Not surprisingly, the particular and regular texture of TNT was also expected to improve the photocatalytic efficacy for pollutant removal in air and water with respect to TNP. In this short review, the validity of this expectation is checked using the regrettably small number of literature comparisons between TNT and commercialized TNP referring to films of similar thickness and layers or slurries containing an equal TiO₂ mass. Although the irradiated geometrical area differed for each study, it was identical for each comparison considered here. For the removal of toluene (methylbenzene) or acetaldehyde (ethanal) in air, the average ratio of the efficacy of TNT over that of TiO₂ P25 was about 1.5, and for the removal of dyes in water, it was around 1. This lack of major improvement with TNT compared to TNP could partially be due to TNT texture disorders as seems to be suggested by the better average performance of anodic oxidation-prepared TNT. It could also come from the fact that the properties influencing the efficacy are more numerous, their interrelations more complex and their effects more important for pollutant removal than for dye sensitized solar cells and photoelectrocatalysis where the electron transport rate is the crucial parameter.

This brief overview focuses on three of the main basic phenomena limiting the applications of photocatalysis using semiconductors for the treatment of water, so that this technology has really not gone beyond the demonstration stage aside from niche applications. These phenomena are: the recombination of photogenerated charges; the adsorption of trace organic pollutants; the spectral range of photocatalytic activity. The accomplishments and failures of the various and numerous attempts to overcome these fundamental limitations are summarized and discussed. The emphasis is on the methods susceptible to improve the use of photons, including those in the visible domain to better use solar irradiation if applicable, and to facilitate the contact between the semiconductor and the trace organic pollutants in liquid water. Despite numerous studies, these limitations have not yet been overcome for water purification, because additionally the photocatalysts must contain chemical elements that are both innocuous and inexpensive. However, combinations of TiO2 and activated carbon might provide materials of interest.

This paper outlines the basic events that occur when a semiconductor, in contact with aerated liquid water containing low concentrations of pollutants, is photoexcited. First, the factors favouring this recombination of photoproduced charges are recalled, and the difficulties in decreasing the recombination are underlined. Second, the formation and identification of the species resulting from charge transfer with O₂, H₂O and pollutants are presented. Third, the reactions involving these species are considered, and methods for assessing their respective importance are critically reviewed. Fourth, on the basis of the view that the photocatalyst surface is covered by tightly bound water layers which hinder the adsorption of many organic pollutants, the hypothesis according to which primary photocatalytic events can take place within the near-surface solution layers is discussed.

This article first indicates the various types of temperature-induced oxygen isotopic exchange (OIE) that can occur between labeled gaseous O2 and solid ox

The aim of this study was to assess in a vineyard the effect of purifying by solar photocatalysis the title rinse waters (currently most often rejected) in terms of efficacy and on-site practicality for the wine grower. The on-site, self-functioning, solar purifying unit included a corrugated-steel inclined plate of area S=1m² onto which a TiO₂-coated thin material had been slightly pressed, a tank, and an aquarium-type pump powered by a photovoltaic panel (appropriate for isolated locations). For a vineyard of area A=0.15km², the rinse water (about 90L) corresponding to each of four typical vine treatments in summer was analysed (major pesticides for each treatment, TOC, Microtox test and, in one case, BOD₅) by independent laboratories, before and after purification for 8 days. The S/A ratio tested was found insufficient even if the photocatalytic treatment markedly improved the quality of the rinse waters. From the relatively low final organic content reached in one case, it is calculated that a three-time higher S/A ratio might suffice, but new trials are necessary to determine whether it is valid for other typical cases. Inferred contribution of inorganic ions to the post-photocatalytic treatment toxicity points to the need for an additional detoxification. These field experiments have also demonstrated that the purifying prototype is robust, and easy to install and use on site by the wine grower.

The photocatalytic removal rates r of dichloroacetic acid (DCAA), 4-chlorobenzoic acid (4-CBA) and 4-chlorophenol (4-CP) in water were compared for TiO2 samples used either as a powder or as a coating on a fiber glass tissue, SiO₂ being the binder. From SEM-EDX measurements it was deduced that SiO₂ prevails over TiO₂ in the coating top layers and ¹⁸O₂-Ti₁₆O₂ isotopic exchange showed that the accessibility of O2 to TiO2 was markedly reduced when TiO₂ was thus coated. The unfavorable effect of the restricted TiO₂ accessibility on r was drastic for DCAA, much less pronounced for 4-CBA, and still smaller for 4-CP. It is inferred that DCAA can be attacked only when it directly interacts with TiO₂, whereas 4-CP can also react within the near-TiO₂ surface water layers. The 4-CBA intermediate behavior is in line with the structural similarities of 4-CBA with DCAA and 4-CP.