The photocatalytic degradation of gaseous formaldehydesa major cause of sick building syndromeswas studied using a TiO 2 thin film. TiO 2 thin films have many unique photoinduced properties, for example, self-cleaning, antifouling, and anti-bacterial functions. UV illumination of the TiO 2 thin film placed in a gaseous formaldehyde/air environment resulted in the total mineralization of formaldehyde to CO 2 and H 2 O. We invoked a Langmuir-Hinshelwood kinetic model to analyze the dependence of reaction rates on the concentration of formaldehyde. In addition, the overall decomposition rate constant for formaldehyde was comparable to that of acetaldehyde (a standard test reactant) for initial concentrations of up to 1000 ppmv. However, the apparent adsorption constant K app of formaldehyde onto TiO 2 was ca. 2.5 times larger than that of acetaldehyde. Thus in the low concentration regime, the reactivity of formaldehyde appeared to be greater than that of acetaldehyde. In like manner, a dark adsorption experiment also showed the high adsorption capacity of TiO 2 for formaldehyde. Therefore, we conclude that TiO 2 serves as both a good adsorbent and a photocatalyst for the elimination of gaseous formaldehyde.

We have probed the TiO2-mediated photomineralization of Langmuir−Blodgett (LB) films of stearic acid via atomic force microscopy (AFM) and FT-IR. In situ AFM images revealed that at the submicrometer level the photodecomposition process of stearic acid molecules on a polycrystalline anatase TiO2 film is inhomogeneous, with the various reaction initiation centers or nucleation regions being randomly distributed throughout the photocatalyst surface. Furthermore, parallel FT-IR results showed that the reaction follows pseudo-first-order kinetics. To rationalize the observed random LB film photoetching and buckling behavior, we invoked a simple reaction model that incorporates the reactivity of the TiO2 film and LB film disorganization phenomenon during the photodegradation process.

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We have studied the nature and surface morphological changes associated with the photodegradation of stearic acid LB films on TiO2(110). Interestingly, submonolayers of stearic acid consisted of circular domains of various sizesa feature very attractive for monitoring TiO2 photocatalysis by AFM. We noted that there was no bulk differential reactivity at island edges compared to the interior. This suggests that the rate of photodegradation of the stearic acid molecules is independent of their location in the island. Accordingly, the overall surface reactivity trends were similar for both partial films and complete films. Likewise, the observed inhomogeneous reactivity patterns appear to be a reflection of the transient distribution of the reaction centers.