Quantification of the Photocatalytic Self-Cleaning Ability of Non-Transparent Materials.

Marco Minella,Claudio Minero

doi:10.3390/ma12030508

Abstract

The photo-induced reactivity of compounds at the surface of photocatalytic materials is used to maintain the cleanliness of the surface of glass, concretes and paints. A standard method to quantify the photocatalytic self-cleaning (SC) properties of non-transparent materials was recently published. It is based on the covering of the sample surface with a defined amount of dye and on the evaluation of the reflectance spectra of the coloured surface under irradiation. The calibration of the spectral changes allowed the quantification of the surface residual dye and the evaluation of the self-cleaning kinetics. The method was tested on seven white and coloured photocatalytic materials using methylene blue (MB), rhodamine B (RhB) and metanil yellow (MY). The main by-products of the MB photocatalytic degradation at the solid/solid interface were identified, showing that MB degradation in solution follows a path quite different from that at the solid/solid interface. Also MY showed a different order of photoreactivity. Furthermore, experiments at the solid/solid interface are more trustworthy than tests in solution for evaluating the self-cleaning ability. The differences of the photocatalytic phenomena at the solid/solid interface in comparison with the most studied photoactivated processes at the solid/liquid interface are outlined. Furthermore, photocatalytic materials showed selectivity toward some specific dyes. This encourages the use of more than one dye for the evaluation of the self-cleaning ability of a photocatalytic material.

Highlights

The effects activated at the surface of a semiconductor following the absorption of photons with energy higher than its band gap have a wide range of potential environmental applications
The photocatalytic process under irradiated semiconductors has been widely investigated for several kinds of applications, like photovoltaics [1,2,3], hydrogen production by water photosplitting or photoreforming [4], degradation of recalcitrant organic and inorganic pollutants [5,6,7,8] and the synthesis of value added chemicals [9,10,11,12,13], and for functional coatings with self-cleaning (SC)
The self-cleaning properties of a material are related to the synergistic effect of the photocatalytic degradation and, mineralization of organic compounds adsorbed at the surface, and the photoinduced superhydrophilicity of the TiO2 surface

Summary

Introduction

The effects activated at the surface of a semiconductor following the absorption of photons with energy higher than its band gap (hν ≥ Eg ) have a wide range of potential environmental applications. The insights into the photocatalytic process have undergone a successful transfer from the academic labs to the industrial fields. This process is mainly driven by the production of air purification systems and self-cleaning surfaces. The latter is perhaps the most important market for the photocatalytic materials, as photocatalytic glasses, concretes and pigments have major market value, which is growing constantly [17,18]. A soft washing (e.g., rain) of the self-cleaning materials allows the dirt residues to be washed away resulting in a clean surface [19]

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