Hierarchical NiO–SiO2 composite hollow microspheres with enhanced adsorption affinity towards Congo red in water

• Hollow microspheres of hierarchical porous NiO–SiO2 particles were prepared.
• The samples exhibit strong adsorption affinity toward Congo red (CR) in water.
• The maximum adsorption capacity for CR is 204.1 mg/g for NiO–SiO2.
• CR molecules are adsorbed on the sample via physical adsorption.

Hollow microspheres and hierarchical porous nanostructured materials with desired morphologies have gained remarkable attention for their potential applications in environmental technology. In this study, NiO–SiO2 hollow microspheres were prepared by co-precipitation with SiO2 and nickel salt as precursors, followed by dipping in alkaline solution and calcination. The samples were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, nitrogen adsorption, and X-ray photoelectron spectroscopy. The synthesized hollow spheres were composed of a SiO2 shell and hierarchical porous NiO nanosheets on the surface. Adsorption experiments suggested that NiO–SiO2 composite particles were powerful adsorbents for removal of Congo red from water, with a maximum adsorption capacity of 204.1 mg/g. The high specific surface areas, hollow structures, and hierarchical porous surfaces of the hollow composite particles are suitable for various applications, including adsorption of pollutants, chemical separation, and water purification.

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