Top–Down Approach for the Deposition of Photoactive (Na_0.5Bi_0.5)TiO₃-Based Heterojunctions by Flame Spray: Analysis of Deposition Parameters

Visible-light photoactive (Na_0.5Bi_0.5)TiO₃ (NBT)-based heterojunctions have demonstrated their applicability in environmental remediation. The photocatalytic properties of NBT-based coatings are here reported. NBT-based materials were deposited by oxyacetylene flame spray (FS). The physicochemical properties were analyzed as a function of the spraying parameters: fuel/oxygen ratio (F/O), stand-off distance (SOD), and total gas flow (TF). A flame with reducing characteristics promotes the formation of TiO₂ (anatase and rutile) and Bi₄Ti₃O₁₂, while an oxidizing flame results in coatings rich in NBT and Bi₄Ti₃O₁₂. The SOD mainly influences the degree of crystallinity, which is higher at shorter distances. Optical properties estimated by UV–VIS diffuse reflectance confirmed an increase in light absorption after the FS process, with an E _g red shift from 3.32 eV of the NBT powder to 2.63-2.96 eV of the coatings. These values are dependent on the F/O, with a significant E _g narrowing under reducing conditions. Photoelectrochemical measurements revealed that faster electron collection is obtained due to the transformations occurring throughout the FS process while still retaining nearly 70 and 88% of the photocurrent density (j _ph) at UV and visible light, respectively. These results suggest the great potential of the FS methodology to produce multiphase photocatalytic coatings by tuning the processing parameters.