Sequential Effect of Reaction Temperature on Physical and Chemical Properties of ZnO Nanoparticles for Water Purification in a Photocatalytic Reactor.
This study investigates the influence of reaction temperature on the coprecipitation synthesis of ZnO NPs and correlates their structural and morphological evolution with their photocatalytic performance. ZnO NPs synthesized at five temperatures, namely, 25, 35, 45, 55, and 65 °C, produced wurtzite-phase materials with varying crystallinities in the range of 77-89%. XRD, SEM, EDX, UV-vis spectroscopy, BET surface area measurement, and PL spectroscopy were used to obtain structural and compositional information about the synthesized NPs. The increase in temperature transforms the nanosheet-like structures into quasi-spherical particles with average crystallite sizes of 14-20 nm, as a function of variations in crystal defects and a high concentration of lattice distortions. Optical characterization revealed a red shift in the absorption edges, with a reduction in the bandgap (E g) to 2.75 eV. PL spectra exhibited a pronounced Stokes shift, confirming defect-mediated emission. The BET analysis revealed a maximum BET surface area of 41.14 m2/g at 45 °C, and the pore size distribution range is 1.6-48.82 nm, along with a significant change in zeta (ζ) potential. The photocatalytic activities were then determined in the degradation of Rhodamine B (Rh-B) under visible-light irradiation in a specially designed photocatalytic reactor. ZnO prepared at 45 °C exhibited the best performance, degrading up to 99% of Rh-B within 200 min, corresponding to a reaction rate constant of 0.022 min-1. Finally, an optimized ZnO sample was produced at 45 °C and directly introduced into the reactor to immobilize the catalyst, thereby avoiding the release of NPs into water streams and eliminating subsequent recovery steps. In this work, the reaction temperature has been identified as a critical determinant of ZnO defect concentration, morphology, and photocatalytic efficiency, providing a scalable route to high-performance photocatalysts for effective, environmentally friendly water purification systems.
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