Conditions and mechanisms for the formation of nano-sized Delafossite (CuFeO2) at temperatures ≤90 °C in aqueous solution

• We solve the formation mechanism of pure CuFeO2 using sulfates as reactants.
• CuFeO2 nanoparticles crystallize on cost of green rust, Fe10O14(OH)2 and Cu2O.
• The reaction rate increases with increasing temperature and OH- concentration.
• CuFeO2 nanoparticles form at 50 °C within one week and at 70 °C within 10 h.
• 2H-polytype of CuFeO2 is favored by additional NaOH supply during pH-stat-time.

In this study, we present the mechanism of CuFeO2 formation in aqueous solution at low temperatures ≤90 °C, using sulfate salts as reactants. Furthermore, we demonstrate the influence of experimental conditions (alkalization, reaction and ageing temperature and time) on the synthesized nanoparticles. In all cases, GR–SO4, a Fe(II–III) layered double hydroxysulphate (Fe2+4Fe3+2(OH)12·SO4) and Cu2O precipitate first. During further OH− supply GR–SO4 oxidizes and forms Fe10O14(OH)2, Cu2O and CuFeO2 crystals. Due to the high pH further CuFeO2 crystals grow at the cost of the unstable intermediate products. The reaction rate increases with increasing ageing temperature, reaction pH and, in particular, NaOH concentrations in the solution. As a result, highly crystalline CuFeO2 (3R and 2H polytypes) nanoparticles showing hexagonal morphology can be synthesized at 70 °C within 10 h or at 50 °C within 1 week. The formation of 2H polytype is favored by additional OH− supply during the pH-stat time and rather low temperatures.

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