Formation of stable Cu2O nanocrystals in a new orthorhombic crystal structure

Substantially stable Cu2O nanocrystals of 10–30 nm in size have been synthesised by an ion exchange Cu2+→Cu→Cu+ reaction in an aqueous solution (using a reducing agent NaBH4) at 80–100°C. In so small particles, a large fraction of surface atoms, i.e. 20% or even more, which suffer with a reduced coordination number of the core atoms, support the formation of the low oxidation state Cu2O oxide of copper in a stable structure. Otherwise, CuO is the most stable form of its oxides. The 10–30 nm Cu2O nanocrystals have a modified X-ray diffractogram of FCC bulk Cu2O structure. The diffractogram is very simple with a total of only seven peaks, over 0.25–0.12 nm dh k l interplanar spacing, which are successfully indexed assuming an orthorhombic crystal structure with lattice parameters a=0.421 nm, b=0.324 nm and c=0.361 nm (at 25 nm size of the sample).