Direct ink Writing of porous ZnO structures for photocatalysis: Effect of sintering temperature on rhodamine degradation and mechanical resistance

ABSTRACT The scarcity of water resources demands the development of technologies to remove contaminants from industrial wastewater in order to reclaim and reuse the water in other fields like agriculture. In this study, 3D macroporous structures are additively manufactured from commercial ZnO nanopowder to be used in the photocatalytic removal of environmental pollutants from water. Direct Ink Writing (DIW) is used for deposition of the photocatalytic 3D structures that were subsequently consolidated by Spark Plasma Sintering (SPS) using temperatures in the range 425-900 °C. The fabricated structures are microstructurally (XRD, nitrogen physisorption, SEM and EDX) and mechanically (uniaxial compression test) characterized and their photocatalytic efficiency is evaluated by analyzing the degradation of Rhodamine B (RhB). Reusability of the DIW structures is also evaluated for structures sintered at selected temperatures by determining the photocatalytic degradation of RhB during five consecutive cycles. The results prove that increasing sintering temperature increases grain size, decreases microporosity and BET specific surface area, generates oxygen vacancies, and improves compressive strength of the 3D structures, but overall, these changes do not significantly affect photocatalytic efficiency. 3D structures require higher catalyst loading and take longer to fully degrade the dye compared to the commercial nanoparticles, but the DIW 3D photocatalysts have some clear advantages in terms of easy removal from the water —avoiding costly post-treatment precipitation, filtration or centrifugation processes typically needed to remove the catalyst nanoparticles— and extraordinary reusability: despite the evident surface corrosion occurring during the photocatalysis, neither their photocatalytic efficiency nor mechanical resistance are reduced. This will translate into significant reductions in the total cost of the treatment and a reduction in the potential toxicity of the reclaimed water due to contamination from any residual catalyst nanoparticles. These findings highlight the potential of reusable DIW ZnO catalysts for sustainable industrial wastewater treatment.