Comparison of the Photocatalytic and Photoelectrocatalytic Methyl Orange Color Removal over TiO(2-x-y)NxFy

Photocatalysis and photoelectrocatalysis are environmentally friendly methodologies for remediation pollutants present in waste waters, these methodologies employ semiconductor materials capable to generate electron hole pairs when they are illuminated [1]. Photogenerated charge carriers together with oxidizing transient species, mainly the hydroxyl radical (OH^*), can produce deep changes in the chemical structure of the contaminants [2]. One of the most commonly used semiconductor for these purposes is the TiO₂, however it is only active under UV light, therefore, many approaches have been conducted towards TiO₂ modification with different metallic and nonmetallic elements in order to reduce the gap between the bands of the oxide, allowing to use visible light for this process [3,4]. 

Photocatalytic (PC) and photoelectrocatalytic (PEC) methyl orange (MO) color removal may involve two mechanisms simultaneously [5]. The first path is the oxidative radicals’ formation such OH^* which carry the pollutant oxidation in the solution [6], and the second way refers to the direct interaction of photogenerated holes with the dye adsorbed on the semiconductor surface [7,8]. The dominant mechanism will depend on the kind of processes applied (PC or PEC), and the modification of the TiO₂ that can induce defects such oxygen vacancies in the oxide lattice, or alter the surface properties of the oxide [9].

TiO_(2-x-y)-N_x-F_y powders and thin films were prepared by combining sol gel method and hydrothermal treatment. The obtained materials were characterized by X-ray diffraction (XRD), Raman and infrared (FTIR) spectroscopies, diffuse reflectance (UV-VIS) spectrophotometry and Zeta potential measurements. Moreover, the supported films on AISI 304 stainless steel foils were characterized by scanning electron microscopy (SEM) and contact angle measurements. Photocatalytic and photoelectrocatalytic test were carried in a 5 ppm MO solution at a pH of 6.8.

The results showed that the PC Methyl orange degradation rate over fluoride modified photocatalysts (TiO_2-yF_y and TiO_(2-x-y)N_xF_y) was comparatively lower than over pristine TiO₂ and TiO_2-xN_x. However, the trend reversed when the MO degradation was carried in a photoelectrocatalytic cell (see Fig. 1). The material characterization showed that fluoride modification alter not only the crystallinity (XRD) and structure (UV-Vis and Raman) of the TiO₂, but also its surface properties (Isoelectric point and wettability), favoring the generation of radical ions in the solution. When the degradation is carried in suspension (photocatalysis) there is a high surface area available for the oxidation process favoring the direct charge transfer between the dye and the semiconductor. On the other hand, when the process is carried over a thin film (photoelectrocatalysis) there is a low contact area causing limitation in the interfacial charge transfer between the pollutant and the oxide, then the generation of radical species that travel to the bulk of the solution to carried the MO discoloration, gain relevance in the process [10].  

References

 

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