Phase-controlled growth of cubic phase CuO nanoparticles by chemical vapor deposition

Cupric oxide has three polymorphs, namely, monoclinic, tetragonal, and cubic phases. The synthesis of the monoclinic phase is widely reported due to its versatile applications. However, cupric oxides with tetragonal and cubic structures were seldom investigated in the literature. In this study, the cubic phase of CuO was deposited on alumina substrate using Cu(acac)(2) as precursor. The obtained CuO with loadings of 0.2-2mmolg(-1) on alumina was determined to be pure cubic phase by XRD and XPS, which is the first report on the synthesis of pure cubic phase, to the best of our knowledge. In contrast, pure monoclinic phase was obtained when impregnation method was employed. The cubic CuO nanoparticles obtained in this work appear to be highly dispersed with a particle size of around 5nm. Moreover, the cubic CuO showed a lower chemical state and a lower reducibility as compared to monoclinic CuO supported on the same substrate. Further studies showed that the cubic phase was a metastable phase in thermodynamics and could be converted to monoclinic phase at elevated temperatures. As catalysts, the cubic CuO on alumina was found to be active and selective in the hydroxylation of phenol to catechol, hydroquinone and benzoquinone.