Synthesis of environmentally benign ultra-small copper nanoclusters-halloysite composites and their catalytic performance on contrasting azo dyes

Highlights: • Ultrafine copper nanoclusters (CuNCs) synthesised hydrothermally using only peptide. • Detail synthesis protocol reporting pH, time and temperature provided. • Multiple characterisation revealed the NCs and composites formation and properties. • CuNCs@halloysite nanotube was highly catalytic while contrasting azo dyes used. • The catalyst composite was reusable with high efficiency, stable and biocompatible. Supported metal nanoclusters (NCs) are an ideal catalytic system from their ultra-small size (< 3 nm), reactivity and confinement on support materials. Whether synthesis of such composite is feasible using copper (Cu) as catalyst on nontoxic and inexpensive support material but without using any toxic reducing agent is yet to be explored. Here, synthesis of CuNCs using only biocompatible glutathione and localised them on halloysite nanotubes (HNTs) would be a sustainable catalyst composite. Following hydrothermal reaction, composites CuNCs@HNT and CuNCs@HNT-PS were synthesised by one-step and post-synthesis methods, respectively. State-of-the-art tools, including high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy revealed NCs formation, chemical states, and confinement and stability as composite, while catalysis reaction was monitored by spectrophotometer. Both composites exhibited faster catalytic performance than did bare NCs for the degradation of contrasting model azo dyes, methylene blue (MB) and methyl orange (MO). CuNCs, CuNCs@HNT and CuNCs@HNT-PS required only 93 ± 1.0, 17.5 ± 2.5 and 27 ± 2.5 s, respectively for 100% degradation of MB whereas >90% degradation of MO occurred by 120 ± 5.21, 75 ± 3.15 and 90 ± 3.61 min, respectively. Composites showed excellent catalytic reusability and environmental nontoxicity. Therefore, as effective and safe catalysts, they can shed light on exploring further usage in the environment and industrial set-ups.