Tuning surface termination of CeO2 nanoparticles for adsorption of anionic dyes via urea content in a simple pyrolysis technique

Abstract

CeO2 adsorbents with different terminated surfaces are fabricated by controlling urea content in simple pyrolysis methods. CeO2 adsorbent prepared with lower urea content is discovered to form a less positively charged surface with anoxic cerium-rich defect structures, inducing more difficult charge transfer in adsorbent interface and higher diffusion resistance in the interface of adsorbent and adsorbate. Thus, its adsorption isotherm is preferably fitted to Freundlich isotherms and its single adsorption site adsorbs multiple Congo red molecules, especially at higher dye concentrations. However, the adsorbent still has low adsorption ability. Oppositely, CeO2 adsorbent prepared with higher urea content induces forming a more positively charged surface with Ce-terminated structure due to higher Ce3+ concentration on its defect surface stabilized by Ce2O3 structure. CeO2 adsorbents with the Ce-terminated surface structures present easier charge transfer and lower diffusion resistance during dye adsorption. Significantly, they have much higher adsorption capacities of anionic dyes and their adsorption isotherm is better matched with Langmuir isotherms. Therefore, in the synthetic process of CeO2 adsorbents, urea content essentially regulates the arrangement structure of their terminated atoms, which greatly changes physicochemical properties of their terminal surfaces and influences the Congo red adsorption capacities.