Preparation of Functional, Compositionally Graded Metal Oxide Films via the Solution Precursor Plasma Spray Process

The results accumulated up to now by a number of research works tend to show that a single metal oxide semiconductor (MOS) cannot achieve the multiple objective required to make an efficient photocatalytic system, which includes not only the light response, but also the photochemical stability and the avoidance of the recombination of photogenerated electron/hole pairs. Mixing different MOS is considered a promising strategy as it induces beneficial synergistic effects. In this work, we have developed an original method to prepare compositionally graded films that are active in photodegradation applications. The selected compositionally graded films are the CuO-ZnO and CeO2-ZnO pairs that were deposited via the Solution Precursor Plasma Spray (SPPS) process. Various composition gradients were achieved using different modes of injection of the precursor solutions, generating distinct ``film configurations''. We characterized the resultant films by SEM and XRD. We determined the corresponding optical bandgap energies through UV-Vis adsorption spectrometry. Then we evaluated the photocatalytic performances of these CuO-ZnO and CeO2-ZnO films through the photodegradation of aqueous solutions of the Orange II dye. The CeO2-ZnO-M films proved capable of degrading almost entirely the Orange II dye within 2 hours, under UV light. Finally, we discuss the correlation between the various film configurations and their photodegradation efficiency. This exploratory work tackles the relatively novel field represented by the use of compositionally graded oxide films for enhanced photochemical reactivity and stresses the effectiveness and versatility of the SPPS technique for the preparation of a large variety of photocatalytic films.