Hopper-like framework growth evolution in a cubic system: a case study of Cu₂O

Crystallization of hopper-like Cu₂O cubes is demonstrated in alkaline citrate-copper solution upon the equilibrium between complexation, precipitation and redox reactions. The complexation reactions of Na₃cit and NaOH with Cu²⁺ manipulate the current chemical reaction routes, phase transformation, and thus crystallization morphologies and compositions. During the crystallization of Cu₂O variations of mother solution such as pH and the concentration of Cu²⁺ (Cu⁺) are monitored by ex situ pH measurement and UV-Vis absorption spectroscopy. The shape evolution of Cu₂O hopper cubes is consistent with the chemical reaction routes, which demonstrates the interplay between chemical reactions and crystal growth. The appropriate reaction concentration, Na₃cit:Cu²⁺ > 2 and 1 < NaOH:Cu²⁺ < 2, has been identified for the growth of these hopper cubes. The current reaction-controllable crystal growth may further the synthesis chemistry toward rational control over crystallization.