Band-Gap Physics, Photocatalytic Kinetics and Interfacial Thermodynamics of Green-Synthesised ZnO and TiO₂ Nanoparticles in Polymeric Nanocomposite Membranes for Water Purification

Abstract

Polymeric ultrafiltration membranes are central to modern water purification, but their hydrophobic surfaces foul quickly. Zinc oxide (ZnO) and titanium dioxide (TiO₂) nanoparticles can counter this, and both can be synthesised with plant extracts instead of conventional reagents. What has been missing is a physical and chemical explanation of how such particles behave, both as semiconductors and as membrane fillers. In this work, experimental data reported for plant-synthesised ZnO and TiO₂ and for ZnO- and TiO₂-modified polymeric membranes (20 peer-reviewed studies) were analysed with standard models from solid-state physics, chemical kinetics and interfacial thermodynamics. Green ZnO and TiO₂ had crystallite sizes of 15–23 nm and band gaps of 2.95–3.37 eV, placing their absorption edges at 368–420 nm. A Brus effective-mass calculation showed that quantum confinement shifts the ZnO band gap by less than 10 meV at these sizes, so band-gap narrowing of up to about 420 meV must arise from defects and surface states rather than size. Under Langmuir–Hinshelwood kinetics, the reported photocatalytic data gave apparent first-order rate constants of 0.010–0.036 min⁻¹; plant-mediated TiO₂ degraded tetracycline about 1.8 times faster than TiO₂ made without extract, in line with its twelvefold larger surface area. In membranes, optimum loadings were 0.01–3 wt%, water flux rose by a geometric mean factor of 4.8 (95% interval 2.4–9.7), and the fall in water contact angle of 13–22° corresponded to a 17–34% increase in the Young–Dupré work of adhesion of water. A Hagen–Poiseuille analysis indicated that higher surface porosity, rather than wider pores alone, must account for most of the flux gain. TiO₂ also raised the flux recovery ratio in fruit-juice ultrafiltration from 60.3% to 90.9–94.0%. The analysis identifies defect chemistry, surface charge relative to the point of zero charge, and zinc dissolution as the key variables that future experiments on green nanocomposite membranes must control.