Design and fabrication of photocatalytic coatings with α/β-Bi2O3 and recycled-fly ash for environmental remediation and solar fuel generation

Abstract To promote a circular economy, alkali-activated recycled fly ash (AAFA) can be used as an alternative binder for fabricating mortar coatings. The AAFA binder can also be functionalized with photocatalysts for application in environmental remediation. Here, α/β-Bi2O3 homojunction photocatalysts were incorporated into AAFA mortar coatings to confer photocatalytic properties for two applications, namely, self-cleaning and solar fuel generation from CO2 photoconversion. The α/β-Bi2O3 homojunctions were prepared using three methods. The homojunctions had a higher proportion of the β-Bi2O3 phase compared to α-Bi2O3; additionally, they exhibited with a 2D morphology and a bandgap suitable for visible-light activation. X-ray diffraction and energy dispersive spectroscopy analysis of the mortar coatings showed that the inorganic matrix comprised two frameworks based on calcium silicate hydrate and aluminosilicate hydrate, with high porosity. The photocatalytic mortar coatings exhibited self-cleaning activity, with the highest efficiency of 31%. This result was corroborated by total organic carbon analysis, which showed ~15% mineralization. The specimens also showed high activity for CO2 photoconversion to solar fuels such as formic acid (1932 μmol/g) and methanol (6 μmol/g) under visible light. These results were related to the synergistic effect of the mesoporous framework developed after alkaline activation, more efficient charge transfer in the homojunctions, enhanced CO2 affinity, and the presence of CaO, MgO, Na2O, and SiO2 in the binder. The efficiencies for both applications are comparable to those reported previously. The present approach is potentially applicable not only for minimizing the environmental impact of waste materials by reducing the disposal of fly ash in landfills, but also for producing value-added compounds and reducing the maintenance cost of buildings.