Impact of the crystal phase and 3d-valence conversion on the capacitive performance of one-dimensional MoO₂, MoO₃, and Magnéli-phase Mo₄O₁₁ nanorod-based pseudocapacitors

One-dimensional nanorods of MoO₂ MoO₃ and Magnéli-phase Mo₄O₁₁ were effortlessly prepared using the hot-filament metal oxide vapor deposition technique. Long straight and uniform nanorods were grown on indium-tin-oxide (ITO) thin film coated glass substrates. Thermal reduction and oxidation were then used to process 1D Magnéli-phase Mo₄O₁₁ nanorods synthesized at 1000, 1050, 1100, 1150, and 1200 °C into 1D MoO₂ and MoO₃ nanorods, respectively. The nanorods prepared at higher synthesis temperatures were thinner and longer. The 1D Magnéli-phase Mo₄O₁₁ nanorods consisted of various combinations of two orthorhombic (α) and monoclinic (η) crystals and varying mixtures of Mo⁴⁺, Mo⁵⁺ and Mo⁶⁺ (3d_5/2 and 3d_3/2) cations. The 1D MoO₂ nanorods were comprised of only monoclinic (η) crystals and various complex mixtures of Mo⁴⁺, Mo⁵⁺ and Mo⁶⁺ (3d_5/2 and 3d_3/2) cations. The 1D MoO₃ nanorods contained only orthorhombic (α) crystals and varying mixtures of Mo⁵⁺ and Mo⁶⁺ (3d_5/2 and 3d_3/2) cations. Comparison of the crystal phases and 3d valences showed that the MoO₂ nanorods supplied more oxidation states than the Mo₄O₁₁ or MoO₃ nanorods. According to the results of cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) measurements, and electrochemical impedance (EI) spectroscopy, the capacitive performance of the MoO₂ nanorod-based pseudocapacitor was much better than that of the MoO₃ and Magnéli-phase Mo₄O₁₁ nanorod-based pseudocapacitors. The crystal phases and 3d-valence conversions clearly had a tremendous impact on the capacitive behaviors of the MoO₂, MoO₃, and Magnéli-phase Mo₄O₁₁ nanorod-based pseudocapacitors.