Polyelectrolyte-Assisted Oxygen Vacancies: A New Route to Defect Engineering in Molybdenum Oxide

The presence of oxygen vacancy sites fundamentally affects physical and chemical properties of materials. In this study, a dipole-containing interaction between poly(diallyldimethylammonium chloride) PDDA and α-MoO₃ is found to enable high-concentrations of surface oxygen vacancies. Thermal annealing under Ar resulted in negligible reduction of MoO₃ to MoO_{3- x} with x = 0.03 at 600 °C. In contrast, we show that the thermochemical reaction with PDDA polyelectrolyte under Ar can significantly reduce MoO₃ to MoO_{3- x} with x = 0.36 (MoO_2.64) at 600 °C. Thermal annealing under H₂ gas enhanced the substoichiometry of MoO_{3- x} from x = 0.62 to 0.98 by using PDDA at the same conditions. Density functional theory calculations, supported by experimental analysis, suggest that the vacancy sites are created through absorption of terminal site oxygen (O_t) upon decomposition of the N-C bond in the pentagonal ring of PDDA during the thermal treatment. O_t atoms are absorbed as ionic O^- and neutral O^2-, creating Mo⁵⁺-v_O^· and Mo⁴⁺-v_O^·· vacancy bipolarons and polarons, respectively. X-ray photoemission spectroscopy peak analysis indicates the ratio of charged to neutral molybdenum ions in the PDDA-processed samples increased from Mo⁴⁺/Mo⁶⁺ = 1.0 and Mo⁵⁺/Mo⁶⁺ = 3.3 when reduced at 400 °C to Mo⁴⁺/Mo⁶⁺ = 3.7 and Mo⁵⁺/Mo⁶⁺ = 2.6 when reduced at 600 °C. This is consistent with our ab initio calculation where the Mo⁴⁺-v_O^·· formation energy is 0.22 eV higher than that for Mo⁵⁺-v_O^· in the bulk of the material and 0.02 eV higher on the surface. This study reveals a new paradigm for effective enhancement of surface oxygen vacancy concentrations essential for a variety of technologies including advanced energy conversion applications such as electrochemical energy storage, catalysis, and low-temperature thermochemical water splitting.