Nanoengineering Z‑Scheme Heterostructures in CdS Quantum Dot-Decorated Holmium-Based Metal–Organic Frameworks: Photothermal Catalytic Reduction of Hexavalent Chromium in Contaminated Waters

Designing efficient photocatalysts for the reduction of hexavalent chromium (Cr(VI)) in wastewater was crucial but challenging. Herein, a nanoscale CdS@Ho-MOF photocatalyst composite was successfully synthesized by the anchoring of CdS quantum dots within the curved channels of Ho-MOF. The nanocomposite CdS@Ho-MOF-43.66% demonstrated outstanding performance, efficiently and swiftly photocatalyzing Cr(VI) to Cr(III) in aqueous solutions, which solely utilized water as the electron donor, eliminating the need for additional photosensitizers or cocatalysts. Under visible light irradiation and acidic conditions, CdS@Ho-MOF-43.66% showed a high rate constant (k) of 1.39 min–1, a fast reduction rate of 12.41 mg Cr(VI) g–1 cata min–1, and a superior reaction efficiency of 99%. The composite material demonstrated a 5-fold and 11-fold enhancement in reaction rate compared to pure CdS quantum dots and Ho-MOF, respectively, highlighting its synergistic catalytic superiority. Impressively, the prominent performance remained remarkably consistent even after undergoing seven cycles. The formation of an indirect Z-scheme heterojunction between CdS and Ho-MOF within the nanocomposite predominantly accounted for the elevated photocatalytic performance, which enhanced the separation efficiency of photogenerated charge carriers. This study provided an avenue for the development of cost-effective and high-performance photothermal catalysts for the catalytic reduction of Cr(VI).