Thermal tuning of advanced Cu sol–gels for mixed oxidation state Cu/CuxOy materials

Abstract

In this report, we describe new conditions for the formation of copper gels and aerogels via epoxide addition to CuBr₂ in dimethylformamide (DMF). These CuBr₂-derived sols undergo a rapid gel transition and result in materials that have enhanced mechanical properties when compared with the gels derived from CuCl₂. Additionally, upon air-annealing, they convert to nanoscopic CuO at temperatures much lower than the CuCl₂ analogues. Moreover, annealing under nitrogen results in copper species with reduced oxidization states including Cu₂O and metallic Cu. The ratio of copper oxidation states can be controlled by simple modification of the thermal program used to anneal the materials. The thermal reduction of the copper is attributed to retained DMF ligands in the as-prepared aerogels which was confirmed by FTIR spectroscopy. These aerogel materials were characterized by powder X-ray diffraction (PXRD), physisorption, thermogravimetric analysis (TGA), and temperature programmed reduction (TPR).