Self-cleaning coatings based on fly ash and bismuth-photocatalysts: Bi2O3, Bi2O2CO3, BiOI, BiVO4, BiPO4

Recently, the design of photocatalytic building materials for self-cleaning, self-disinfecting, and environmental pollution remediation has resulted in increasing interest. The advantage of using solar energy and rainwater as driving force has opened a new domain for environmentally friendly building materials. This work reports the fabrication of photocatalytic self-cleaning coatings with alternative building materials such as fly ash, sodium carbonate as an alkaline activator, and bismuth-based photocatalysts deposited on concrete. The photocatalytic materials were powders of Bi2O3, Bi2O2CO3, BiOI, BiVO4, BiPO4, which were synthetized by a precipitation method at room temperature without the use of surfactants or additives. The photocatalytic self-cleaning efficiency of the coatings was evaluated according to the ISO 10678:2010 norm by using aqueous methylene blue (MB) as a model pollutant in order to measure the photoactive surface's ability to degrade dissolved organic molecules under light irradiation. The tendency of the photocatalytic self-cleaning coating activities was: Bi2O2CO3 (49%) > BiOI (30%) > BiVO4 (15%) > BiPO4 (14%) > Bi2O3 (5%) after 3 h of reaction. The best result was obtained when Bi2O2CO3 was incorporated into the coatings, and it was associated to its lowest crystallite (27 nm), flake-like morphology, and to its negatively charged surface (pH(PZC) = 11.5) at the pH of the experiments, that favored the adsorption of the cationic dye (MB) on its surface. The self-cleaning efficiency of the Bi2O2CO3 coating was investigated at different curing ages: 7, 14, and 28 days. It was observed a decrement in the photocatalytic activity with increasing the curing age. However, it was possible to restored the self-cleaning efficiency after washing the surface with water due to the removal of hydration products, which filled up the coating-pores forming diffusion barriers to both reactants and photons. (C) 2019 Elsevier Ltd. All rights reserved.