Tailoring ZnO surface terminations via organic-assisted pyrolysis for highly efficient adsorption of Congo red

ZnO terminated surface was precisely tailored via an organic-assisted pyrolysis. • Zn-rich surfaces of ZnO had higher Congo red removal rate than oxygen-rich surfaces. • Coordination of Zn 2+ on Zn-rich ZnO with SO 3 − /N groups boosted dye removal. A surface-engineering strategy using organic-assisted pyrolysis was employed to precisely tailor the surface terminations of ZnO nanoparticles, dramatically enhancing their reactivity toward Congo red adsorption. ZnO nanoparticles yielded by direct pyrolysis of zinc salts had positively charged, oxygen-enriched surfaces, which facilitated efficient interfacial charge transfer yet exhibited only limited dye adsorption capacity. In contrast, organic assistance engineered ZnO nanoparticle surfaces, greatly changing their surface reactivities. Hydroxypropyl methyl cellulose promoted the production of ZnO nanoparticles with a negatively charged, zinc-deficient and oxygen-enriched surface, resulting in their poor charge transfer and high diffusion resistance, which hindered their dye adsorption. Conversely, soluble starch generated a zinc-enriched terminated surface. Despite its negative charges, this surface facilitated interfacial charge transfer. Crucially, Zn 2+ ions on the zinc-enriched surface exhibited distinct chemical states with superior coordination ability. The zinc-enriched surface exhibited a superior adsorption performance for Congo red compared to the two oxygen-enriched surfaces, with a maximum adsorption capacity for Congo red reaching 376.5 mg/g. Its adsorption process conformed to the Langmuir model and was spontaneous and exothermic. The distinctly enhanced adsorption performance stemmed from direct coordination between surface Zn 2+ ions and sulfonic and nitrogen-containing groups in the Congo red dye. In comparison, the positively charged oxygen-enriched surface relied on electrostatic adsorption, whereas the zinc-deficient and oxygen-enriched surface suffered from electrostatic repulsion, greatly reducing the adsorption efficiency. The surface engineering strategy for metal oxides, by enhancing specific metal ion coordination, provides a powerful approach to significantly boost the removal capacity of hazardous contaminants. Download: Download high-res image (350KB) Download: Download full-size image