Oxygen Self-Doping in Hollandite-Type Vanadium Oxyhydroxide Nanorods

摘要

A nonaqueous liquid-phase route involving the reaction of vanadium oxychloride with benzyl alcohol leads to the formation of single-crystalline and semiconducting VO_(1.52)(OH)0.77 nanorods with an ellipsoidal morphology, up to 500 nm in length and typically about 100 nm in diameter. Composition, structure, and morphology were thoroughly analyzed by neutron and synchrotron powder X-ray diffraction as well as by different electron microscopy techniques (SEM, (HR)TEM, EDX, and SAED). The data obtained point to a hollandite-type structure which, unlike other vanadates, contains oxide ions in the channels along the c-axis, with hydrogen atoms attached to the edge-sharing oxygen atoms, forming OH groups. According to structural probes and magnetic measurements (1.94 μ_B/V), the formal valence of vanadium is +3.81 (V~(4+)/V~(3+) atomic ratio ≈ 4). The experimentally determined density of 3.53(5) g/cm~3 is in good agreement with the proposed structure and nonstoichiometry. The temperature-dependent DC electrical conductivity exhibits Arrhenius-type behavior with a band gap of 0.64 eV. The semiconducting behavior is interpreted in terms of electron hopping between vanadium cations of different valence states (small polaron model). Ab initio density-functional calculations with a local spin density approximation including orbital potential (LSDA + U with an effective U value of 4 eV) have been employed to extract the electronic structure. These calculations propose, on the one hand, that the electronic conductivity is based on electron hopping between neighboring V~(3+) and V~(4+) sites, and, on the other hand, that the oxide ions in the channels act as electron donors, increasing the fraction of V~(3+) cations, and thus leading to self-doping. Experimental and simulated electron energy-loss spectroscopy data confirm both the presence of V~(4+) and the validity of the density-of-states calculation. Temperature-dependent magnetic susceptibility measurements indicate strongly frustrated antiferromagnetic interactions between the vanadium ions. A model involving the charge order of the V~(3+) sites is proposed to account for the observed formation of the magnetic moment below 25 K.