Ecofriendly multifunctional bismuth oxyiodides pigment and paint coatings: Photocatalytic and cooling functionalities

[Display omitted] •BixOyIz pigments were synthesized by co-precipitation method.•BixOyIz exhibit multifunctional cooling and photocatalytic properties.•Microflower and microneedle structures enhance their photophysical properties.•They hold potential as air-purifying and energy-efficient building materials. This work explores the potential application of bismuth oxyiodides as “cool” and photocatalytic materials. The BiOI-orange (microflower) and Bi5O7I-white (microneedle) pigments demonstrated multifunctional properties. The enhanced photocatalytic activity is attributed to their suitable band structures for NOx pollutant degradation. The promising thermal performance compared to the uncoated metal sheets is due to their high NIR reflective and emissivity properties. In contrast, the BiOI-red (stacked-sheet structure) pigment is effective as a “cool” pigment but lacks the optimal band structure and morphology for NOx photocatalytic degradation. Incorporating these pigments into alkyd-resin paint formulations significantly reduced their photocatalytic reactivity, likely due to the degradation of the alkyd resin binder. However, the thermal performance of the paint formulations remained favorable. These results imply that BixOyIz pigments have potential as “cool” pigments for energy-efficient building facade and roof applications. To optimize their photocatalytic performance in resin-based paint formulations, further research is necessary. Exploring alternative inorganic binders is another viable option.