ZnO/Carbon xerogel photocatalysts by low-pressure plasma treatment, the role of the carbon substrate and its plasma functionalization.

ZnO is known to be photocatalytic, but with limited performances due to the strong electron-hole recombination after irradiation. The integration of ZnO nanomaterials on a conductive and high surface area carbon substrate is thus a potential alternative to obtain a significant improvement of the photocatalytic performance. Moreover, the carbon functionalization is expected to have a significant role in the adsorption/degradation mechanisms of dye, due to the difference in wettability or surface charge. In this view, ZnO photocatalytic nanoparticles have been deposited on high surface area carbon xerogel substrate (C), using a new and original plasma process, consisting in the degradation of a solid organometallic directly on the carbon substrate (no gaseous precursor). In addition to the ZnO nanoparticle formation, the plasma treatment allows the carbon functionalization. The ZnO/C composite has been tested for the degradation of Rhodamine B (RhB) in aqueous media and compared with and O or NH plasma-treated xerogels (without nanoparticles) to identify the significant role of the substrate and its modification in the RhB adsorption and degradation mechanism. The high photocatalytic activity of ZnO/C composite is attributed to (i) the formation of small (8-10 nm) and well-crystallized ZnO nanoparticles anchored to the carbon substrate and (ii) to the modification of the xerogel surface chemistry. Indeed, O plasma treatment of the C promotes the generation of hydroxyl, carbonyl and carboxyl surface functional groups, which are polar and acidic, while the NH plasma treatment mainly leads to the formation of polar and basic amino groups. While both plasma treatments promote the formation of polar functional groups, which enhance the C wettability, the formation of acidic groups is identified as beneficial for the adsorption of the RhB dye, while basic groups are detrimental.