Size-controlled synthesis of hierarchical NiO hollow microspheres and the adsorption for Congo red in water

Hierarchical NiO hollow microspheres with five different sizes have been synthesized by a simple refluxing method. The sizes of the hierarchical NiO hollow microspheres can be facilely tuned by altering the sizes of functionalized polystyrene particles (PS) templates. Firstly, Ni(OH)2 nanoflakelets assembled onto the surface of the functionalized PS to form core/shell structures. Secondly, Ni(OH)2 hollow shells were obtained after removal of the core by dissolving PS latex in toluene. Finally, through the calcinations of the Ni(OH)2 hollow shells, hierarchical NiO hollow microspheres with the diameters of 237, 308, 712, 892 and 1157 nm were obtained, respectively. Adsorption of Congo red (CR) onto the as-prepared five samples from aqueous solution was investigated. The results showed that the as-prepared NiO hollow microspheres were effective adsorbents for the removal of CR pollutant from waste water as a result of their unique hierarchical structures and high specific surface areas. Moreover, the adsorption capacities for CR are different due to the different sizes of NiO hollow microspheres. It was observed that the adsorption capacity increased with the decrease of the size of hollow microspheres. Especially, the smallest hierarchical NiO hollow microspheres have the highest adsorption capacity, which could achieve 526.3 mg/g at room temperature (pH 7.0). Most importantly, the adsorbent can be easily renewed, which will be a great potential material in real applications.

► Size-ccontrolled synthesis of hierarchical NiO hollow microspheres was reported.
► The microspheres have hierarchical structures and high specific surface areas.
► Adsorption of CR onto the samples from aqueous solution was investigated.
► The NiO microspheres were effective adsorbents for the removal of CR.
► The effects of the sizes of microspheres on adsorption capacities were discussed.