BCC-Cu nanoparticles: from a transient to a stable allotrope by tuning size and reaction conditions.

Metallic copper generally adopts an FCC structure. In this work, we detect highly unusual BCC-structured Cu nanoparticles as a transient intermediate during the H reduction of a Cu precursor, [CuOBu], grafted onto the surface of partially dehydroxylated silica. The Cu BCC structure, assigned by Cu K-edge XANES and EXAFS, as well as synchrotron PXRD, converts upon heating into the most commonly found FCC allotrope. DFT calculations show that the BCC-Cu phase is in fact predicted to be more stable for small particles, and that their stability increases at lower H concentrations. Using this knowledge, we show that it is possible to synthesize BCC-structured Cu nanoparticles as a stable allotrope by reduction of the same grafted precursor either in 10% H diluted in Ar or 100% H at low temperature.