In Situ In-House Powder X‑ray Diffraction Study of Zero-Valent Copper Formation in Supercritical Methanol

Nano-/microcrystalline copper is widely used in catalysts, and it has potential for being used as conductive additive to ink for inkjet printed electronics. Copper is attractive, because it has excellent electrical conductivity and low cost compared to noble metals. The nucleation and phase transitions from the precursor to the final micrometer sized Cu in supercritical methanol have been studied for the first time using in-house in situ powder X-ray diffraction (PXRD). Temperatures have a significant impact on the reduction process of Cu²⁺; at a low synthesis temperature (250 °C), it was observed how the Cu²⁺ precursor initially formed copper hydroxy nitrate (Cu₂(OH)₃NO₃) and transformed to copper­(II)­oxide (CuO), i.e., no reduction took place. At 300 °C, multiple phase transformation could be observed from initial copper hydroxy nitrate to zerovalent copper; the in situ investigations reveal the following phase transitions; Cu^II₂(OH)₃NO₃ → Cu^IIO → Cu^I₂O → Cu⁰. Increasing the synthesis temperature causes the pure Cu⁰ to form much faster; at 350 °C, it takes 8.7 min to produce phase pure Cu⁰, while at 450 °C, the formation takes ∼0.7 min. Increasing the initial concentration of Cu²⁺ in the precursor causes formation of larger Cu⁰ crystallites in the final product. Finally, the in situ observations were used as guidance for making Cu⁰ using a supercritical flow setup.