Large-Scale MOCVD Deposition of Nanostructured TiO2 on Stainless Steel Woven: A Systematic Investigation of Photoactivity as a Function of Film Thickness

Abstract

Heterogeneous photocatalysis is considered as one of the most appealing options for the treatment of organic pollutants in water. However, its definitive translation into industrial practice is still very limited because of both the complexity of large-scale production of catalysts and the problems involved in handling the powder-based photocatalysts in the industrial plants. Here, we demonstrate that the MOCVD approach can be successfully used to prepare large-scale supported catalysts with a good photocatalytic activity towards dye degradation. The photocatalyst consisted of nanostructured TiO2 thin film deposited on a stainless steel mesh substrate. The film thickness, the morphological features, and the crystallographic properties of the different portions of the sample were correlated to the position in the reactor chamber and the reaction conditions. The photocatalytic activity was evaluated according to the international standard test ISO 10678:2010 based on methylene blue degradation. The photocatalytic activity is essentially constant (PMB over 40 µmol·m−2·h−1) throughout the film, except for the portion of sample placed at the very end of the reactor chamber, where the TiO2 film is too thin to react properly. It was assessed that a minimum film thickness of 250–300 nm is necessary to reach the maximum photocatalytic performance.

Supported MOCVD TiO2 Thin Films Grown on Modified Stainless Steel Mesh for Sensing Applications

“…High magnification SEM images (Figure 9b,d,f,h,j,l) show the distinctive morphology of titanium dioxide thin films, characterized by a compact structure of densely packed crystals with a prismatic faceted aspect, dimensionally variable in relation to the deposition time (the grain sizes tend to increase when the film thickness increases). For 20 min of deposition (Figure 9b), the morphology of the film appears different, with a petal-like structure, which is characteristic of low-thickness titania film [10]. Finally, Figure 9m,n show uncoated and TiO 2 -coated nets.…”

“…All the experiments were carried out using 50 mL of a 10M acid solution, freshly prepared before each experiment (see Supplementary Material, Table S1). The meshes were immersed into the solution under continuous stirring and maintained at different temperatures, from 25 • C (i.e., RT) up to 60 • C, and for different durations (10,15, and 25 min, see Table 1). After each etching treatment, a cleaning process was applied to remove any acid residue with deionized water and ultrasound (two successive cycles of 10 mins).…”

“…The photocatalytic activity of the SS, SS-TiO 2 , and SS-AT-TiO 2 samples was analyzed with respect to the ISO standard 10678:2010 [28] for methylene blue (MB) removal, also described in previous works [9,10]. Briefly, ISO 10678:2010 controls the determination of photocatalytic properties of surfaces in an aqueous medium through methylene blue degradation.…”

“…The generally used powder suspensions of the photocatalytic material have a high surface-to-volume ratio [5]; conversely, these systems require a processing step to remove the catalyst from aqueous suspensions and reuse it. Metal-supported photocatalyst consists of the coating of a suitable substrate with a thin layer of photoactive material to be easily recovered and reused [8,10]. Nevertheless, the immobilization of the catalyst inevitably reduces the density of surface-active sites, leading to a loss of photocatalytic activity.…”

“…[2,5,21,22]. Among them, low-pressure metal organic chemical vapor deposition (LP-MOCVD) is widely used for depositing high-quality polycrystalline TiO 2 thin films, at relatively low temperatures, with controlled deposition rates, good reproducibility, adhesion, and with high conformal properties, also on complex-shaped surfaces [10,23,24]. The characteristics of the coating, such as chemical composition, crystallinity, microstructure, and thickness, can be easily modified simply by working on the deposition parameters [25].…”