<![CDATA[In this work, ZnO/SnO2 and SnO2/ZnO composite thin films were synthesized by ultrasonic spray pyrolysis technique on silicon and glass substrates to investigate the effect of stacking order on structural, optical and catalytic properties. X-ray diffraction confirmed the existence of both hexagonal wurtzite ZnO and tetragonal rutile SnO2 phases with no detectable secondary phases confirming the high performance of the deposition technique. SEM analysis revealed a dense and homogeneous surface composed of compact nanoscale grains, while EDX confirmed high chemical purity. Optical measurements exhibited an average transmittance value of ~75% for ZnO/SnO2 and ~80% for SnO2/ZnO thin films in the visible region. Photocatalytic and sonocatalytic activities were evaluated through the degradation of the methylene blue dye. ZnO/SnO2 composite thin films exhibited enhanced photocatalytic efficiency, whilst SnO2/ZnO structures displayed higher sonocatalytic peformance, highlighting the remarkable effect of the deposition sequence on the catalytic behavior. In addition, the composites exhibited strong antibacterial activity against Pseudomonas bacteria. The ZnO/SnO2 configuration, with ZnO in contact with the agar exhibits markedly stronger antibacterial activity, as evidenced by a significant decrease in bacterial proliferation, compared to the relatively lower activity observed for the SnO2/ZnO configuration. Multiple mechanisms are responsible for this effect, including oxygen vacancies and surface electronic states, which can promote electron transfer to dissolved oxygen even under dark conditions. These findings highlight the strong influence of stacking order of ZnO/SnO2 and SnO2/ZnO systems for various applications, including sonocatalysis, photocatalysis and antibacterial activity. Copyright © 2026 by Authors, Published by BCREC Publishing Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).]]>
