VO2(D) hollow core–shell microspheres: synthesis, methylene blue dye adsorption and their transformation into C/VOx nanoparticles

Abstract

In this work, we report the fabrication of novel VO₂(D) hollow core–shell microspheres (VO₂(D)-HCSMs) via a facile hydrothermal method. Microscopic examination revealed that the VO₂(D)-HCSMs consisted of numerous nanocrystals and had an average size of 1–10 μm and a shell thickness of 300 nm. An inside-out Ostwald ripening process was responsible for the formation of the VO₂(D)-HCSMs. Meanwhile, citric acid played dual roles, acting as a reductant for reducing vanadium pentoxide and as a morphology directing agent for producing core–shell microspheres. The resulting VO₂(D)-HCSMs exhibited excellent adsorption performance with a maximum adsorption capacity of 97.5 mg g⁻¹ for methylene blue (MB). The C/VO_x particles were generated by the calcination of the VO₂(D)-HCSMs with adsorbed MB dye at 250 °C in air for 4 h, and the microspheres showed enhanced adsorption capacity and good reusability (over 99% MB removal after four cycles) because of the existence of amorphous carbon nanowires, making them a potential adsorbent for removing organic dyes from wastewater.