Temperature-Controlled Vapor Deposition of Highly Conductive p‑Type Reduced Molybdenum Oxides by Hydrogen Reduction

Reduced molybdenum oxide, generally MoO_3–<i>x</i>, becomes increasingly metallic as the oxygen level decreases during the reduction of MoO₃ to MoO₂. Its interesting properties have recently intrigued research on MoO_3–<i>x</i> in electrical and electrochemical areas. Lacking effective tools to control the oxygen level is one of the research difficulties for MoO_3–<i>x</i>. Herein, we report facile temperature-controlled synthesis of triangular MoO₂ and square Mo₄O₁₁. The triangular and square flakes showed metallic behavior with conductivity as high as ∼940 and ∼28 S/cm in DC measurement, respectively. The decrease in oxygen level from Mo₄O₁₁ to MoO₂ affected the density of states mapped in Mo 4d orbitals, leading to higher conductivity for triangular MoO₂. Further Mott–Schottky analysis on MoO_3–<i>x</i> regrown on carbon fiber paper (CFP) revealed hole mobility of 10⁵–10⁸ cm² V^–1 s^–1. The hole carriers at high frequencies are attributed to potential oxygen acceptors, and molybdenum vacancies resulted from limited reduction power of hydrogen.