Dual-functional molybdenum oxide coated ceramic membrane: fabrication, characterization, separation of crude oil-in-water emulsions and photo-catalytic self-cleaning of organic foulants

The water-permeating kind of molybdenum trioxide (MoO₃) nanoparticles (NPs) coated aluminium oxide (Al₂O₃) ceramic membrane to be used as a filter for the oil–water separation system was made by depositing a thin layer of MoO₃ NPs on a porous Al₂O₃ ceramic disc by radio frequency magnetron sputtering (RFMS). The water-passing and oil-retaining features of the membrane arise from the extreme water affinity (contact angle close to 0°) and underwater oil repellence (contact angle of ∼ 154°) of the membrane surface, brought about by favourable surface liquid interfacial and optimum surface roughness. The second important functionality of the membrane is its photocatalytic self-cleaning ability to annihilate the organic contaminants accumulated during the prolonged filtration and thereby restore the original water flux. In the self-cleaning process, a broadband ultraviolet (UV) light irradiated on the membrane surface, generates electron-hole pairs to yield highly reactive hydroxides and super oxides to deactivate organic pollutants. With the membrane filter inserted in the dead-end oil–water filtration system, the water permeation flux of ∼ 800 Lm^-2h⁻¹ with oil–water separation efficiency consistently close to 100 % was achieved. The accumulation of oil-borne pollutants on the membrane surface reduced the flux by 27.77 % after 120 min, and the flux yielded by the pristine membrane was restored by the photocatalytic deactivation process by irradiating UV light for 30 min. The morphological and elemental characterizations of the membrane, along with the results of wettability studies, are also presented.