Copper Deficiency in the p-Type Semiconductor Cu1–xNb3O8

Abstract

The p-type semiconductor CuNb3O8 has been synthesized by solid-state and flux reactions and investigated for the effects of copper extrusion from its structure at 250–750 °C in air. High purity CuNb3O8 could be prepared by solid-state reactions at 750 °C at reaction times of 15 min and 48 h, and within a CuCl flux (10:1 molar ratio) at 750 °C at reaction times of 15 min and 12 h. The CuNb3O8 phase grows rapidly into well-faceted micrometer-sized crystals under these conditions, even with the use of Cu2O and Nb2O5 nanoparticle reactants. Heating CuNb3O8 in air to 450 °C for 3 h yields Cu-deficient Cu0.79(2)Nb3O8 that was characterized by powder X-ray Rietveld refinements (Sp. Grp. P21/a, Z = 4, a = 15.322(2) Å, b = 5.0476(6) Å, c = 7.4930(6) Å, β = 107.07(1)o, and V = 554.0(1) Å3). The parent structure of CuNb3O8 is maintained with ∼21% copper vacancies but with notably shorter Cu–O distances (by 0.16–0.27 Å) within the Cu–O–Nb1 zigzag chains down its b-axis. Copper is extruded at high temperatures in air and is oxidized to form ∼100–200 nm CuO islands on the surfaces of Cu1–xNb3O8, as characterized by electron microscopy and X-ray photoelectron spectroscopy (XPS) techniques. XPS measurements show only the Cu(II) oxidation state at the surfaces after heating in air at 450 and 550 °C. Magnetic susceptibility of the bulk powders after heating to 350 and 450 °C is consistent with the percentage of Cu(II) in the compound. Electronic structure calculations find that an increase in Cu vacancies from 0 to 25% shifts the Fermi level to lower energies, resulting in the partial oxidation of Cu(I) to Cu(II). However, higher amounts of Cu vacancies lead to a significant increase in the energy of the O 2p contributions, and which cross the Fermi level and become partially oxidized at the top of the valence band. These oxygen contributions occur over the bridging Cu–O–Nb neighbors when the Cu site is vacant. After heating to 550 °C, XPS data show the formation of a new higher energy O 1s peak that corresponds to the formation of “O–” species at this higher concentration of Cu vacancies. Light-driven bandgap transitions between the valence and conduction band edges are predicted to occur between regions of the structure having Cu vacancies to regions of the structure without Cu vacancies, respectively. This perturbation of the electronic structure of Cu-deficient Cu1–xNb3O8 could serve to drive a more effective separation of excited electron/hole pairs. Thus, these findings help shed new light on p-type Cu(I)-niobate photoelectrode films, i.e., CuNb3O8 and CuNbO3, that exhibit significant increases in their cathodic photocurrents after being heated to increasing temperatures in air.