Metastable Cu(I)-Niobate Semiconductor with a Low-Temperature, Nanoparticle-Mediated Synthesis

A nanoparticle synthetic strategy for the preparation of a new metastable Cu(I)-niobate Is described, and that involves multipored Li3NbO4 nanoparticles as a precursor. A hydrothermal reaction of HNbO3 and LiOH center dot H2O In PEG200 and water at similar to 180 degrees C yields similar to 15-40 nm Li3NbO4 particles. These particles are subsequently used in a solvothermal copper(I)-exchange reaction with excess CuCl at 150 degrees C. Heating these products within the used CuCl flux (mp = 430 degrees C) to 450 degrees C for 30 mm yields similar to 4-12 nm Cu2Nb8O21 crystalline nanoparticles, and for a heating time of 24 h yields mu m-sized, rod-shaped crystals. The new structure was characterized by single-crystal X-ray diffraction to have a condensed network consisting of NbO7 polyhedra and chains of elongated CuO4 tetrahedra. The compound thermally decomposes starting at similar to 250 degrees C and higher temperatures, depending on the particle sizes, owing to the loss of the weakly coordinated Cu(I) cations from the structure and a concurrent disproportionation reaction at its surfaces. Thus, conventional solid-state reactions involving higher temperatures and bulk reagents have proven unsatisfactory for its synthesis. The measured bandgap size Is similar to 1.43-1.65 eV (indirect) and shows a dependence on the particle sizes. Electronic structure calculations based on density functional theory show that the bandgap transition results from the excitation of electrons at the band edges between filled Cu(I) 3d(10)-orbitals and empty Nb(V) 4d(0)-orbitals, respectively. The p-type nature of the Cu2Nb8O21 particles was confirmed in photoelectrochemical measurements on polycrystalline films that show a strong photocathodic current under visible-light irradiation in aqueous solutions. These results demonstrate the general utility of reactive nanoscale precursors in the synthetic discovery of new Cu(I)-based semiconducting oxides and which also show promise for use in solar energy conversion applications.