Valence State-Controlled Synthesis of Vanadium Oxide Nanocrystals

Abstract

We report a valence state-controlled synthesis of vanadium oxide nanocrystalline particles via a non-hydrothermal process using an alcohol and an amine ligand. A non-stoichiometric V3O5 (V4+V3+ 2O5), which is known for its difficulty of formation due to the narrow allowances in the vanadium-to-oxygen ratio, was obtained for the first time as nanocrystals in the anosovite phasea rare phase discovered only recently in bulk form. The time course of the nanocrystal formation revealed a slow seeded growth process, separated from a subsequent fast growth via Ostwald ripening. We highlight the role of vanadium precursor-to-alcohol-to-ligand ratios in precisely controlling the reduction of V5+. Polyvalence of metals, particularly the unusual stability of vanadium(II)–(V), has been considered a negative factor in achieving the targeted oxidation state in nanocrystal syntheses. In the present system, the polyvalence allowed formations of different oxide nanocrystals in a parameter-controlled manner, including anosovite V3O5 (V4+ + 2V3+) and corundum V2O3 (V3+). Such control is unprecedented in metal oxide nanocrystal syntheses.

Optical Properties of Correlated Metallic V2O3 and Anosovite V3O5 Nanocrystals and Nanocrystal Films

“…Valence State-Controlled Synthesis of Vanadium Oxide Nanocrystals. V 2 O 3 and V 3 O 5 nanocrystals were obtained selectively, using a modified hot injection method 31,40 and using benzyl alcohol (BnOH) and 1-octadecanol (ODE),…”

“…The control reaction without amine (V:BnOH:OAm = 1:7:0) confirmed the absence of this peak. In contrast, use of aliphatic primary alcohol such as ODE favored the alkyl halide elimination and aminolysis pathways and facilitated the formation of V 3 O 5 nanocrystals …”

“…The temperature was maintained at 250 °C for 90 min. For V 3 O 5 nanocrystals, 7.92 g of 1-octadecanol (ODE) was used in place of benzyl alcohol in the above, following a previously reported protocol, 31 with the molar ratio of V:ODE:OAm = 1:7:3.…”

“…26 Due to the nontriviality of preparation, anosovite V 3 O 5 was discovered only recently 30 as bulk and then as nanocrystals. 31 We characterized the properties of anosovite V 3 O 5 nanocrystals, indicating a semiconducting behavior at room temperature. For reference, another phase of V 3 O 5 , monoclinic oxyvanite (bulk), is known to be semiconducting at room temperature, with IMT at ∼430 K. 59 The expanded range of wavelength for the optical conductivity, particularly in the low energy region, will allow analysis of the spectral weight 60 for the Drude-like characteristics.…”

“…27,28 V 3 O 5 (or V 4+ V 3+ 2 O 5 ) is obtained either in monoclinic (oxyvanite) 29 or anosovite 30 crystal structure at room temperature. The existence of anosovite bulk 30 or nanocrystalline 31 V 3 O 5 has been discovered only recently because of its narrow allowances in vanadium-to-oxygen ratio 32 (1.666−1.668 ± 0.002) and higher Gibbs free energy compared to the monoclinic structure. 30 Consequently, the physical property of anosovite V 3 O 5 , in either bulk or nanocrystalline form, has remained unknown.…”