Topotactic Engineering of High-Entropy (Oxy)hydroxide Nanotubes for Enhanced Photocatalysis

This study introduces a novel method for synthesizing Ce-Co-Ni-Al-Ga high-entropy (oxy)hydroxide (CeCoNiAlGa HE-OOH) nanotubes via a topotactic transformation using multiwalled carbon nanotubes (MWCNTs) as the parent crystal. CeCoNiAlGa HE-OOHs nanotubes are arranged in concentric cylinders, with high crystalline order, analogous to parent MWCNTs. CeCoNiAlGa HE-OOH nanotubes exhibit a fluorite-like crystalline structure that is supported by a distorted Ce-O framework. A neutral M-OH-M sheet stacking, resembling a partially dehydrated brucite-like layered hydroxide structure, appears to account for the multiwalled configuration of CeCoNiAlGa HE-OOH nanotubes. The fluorite-like structured CeCoNiAlGa HE-OOH (111) planes grow topotactically on the curved C (002) planes. Both the multiwalled arrangement and the stability of the fluorite-like structure are preserved from 80 to 500 ºC. CeCoNiAlGa HE-OOHs exhibit remarkably high concentration of O vacancies. Increasing the heat-treatment temperature leads to gradual dehydroxylation, indicating that HEOOHs are direct structural precursors of HEOs. Notably, the CeCoNiAlGa HE-OOHs obtained at 80 ºC have the highest OH content. They stand out for their remarkable photocatalytic activity under UV light, achieving 96% degradation of ciprofloxacin (CIP) within 45 min. The significant CIP photodegradation is attributed to the synergistic effect of abundant OH species along with O vacancies.