Fabrication of hollow metal oxide nanocrystals by etching cuprous oxide with metal(ii) ions: approach to the essential driving force

Abstract

Hollow metal oxide nanocrystals were prepared by etching cuprous oxide with metal ions and were applied as photoelectrodes. As a hard template, polyvinylpyrrolidone stabilized cuprous oxide (PVP–Cu₂O) and non-stabilized cuprous oxide (nPVP–Cu₂O) were synthesized by a precipitation method. Hollow iron oxide and cobalt oxide nanocrystals with a truncated octahedral morphology were fabricated by an etching reaction with transition metal(II) ions (Fe²⁺ or Co²⁺). In the etching reaction process, a cationic exchange reaction occurs between the divalent metal ion and Cu⁺ due to the higher Lewis acidity. Facet selective etching of cuprous oxide has been observed during the ionic exchange reaction of Cu⁺ and O²⁻ ions in PVP–Cu₂O complexes with transition metal(II) ions (Fe²⁺ or Co²⁺) at the surface of a (110) facet. Amorphous states of hollow metal oxide products were annealed to form α-Fe₂O₃ (hematite) and Co₃O₄ and their crystal structure was examined with X-ray diffraction and HR-TEM. The optical absorption behavior of semiconductor nanocrystals was measured with UV-vis spectroscopy to define band gap energy. The hollow hematite structure has a 2.08 eV band gap and Co₃O₄ (Co(II,III) oxide) has a 1.80 eV indirect band gap. Using these hollow nanocrystals, a metal oxide monolayer film was fabricated with a secondary growth approach and was studied for its photocatalytic properties.