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 alpha-MoO3 is found to enable high-concentrations of surface oxygen vacancies. Thermal annealing under Ar resulted in negligible reduction of MoO3 to MoO3-x with x = 0.03 at 600 degrees C. In contrast, we show that the thermochemical reaction with PDDA polyelectrolyte under Ar can significantly reduce MoO3 to MoO3-x with x = 0.36 (MoO2.64) at 600 degrees C. Thermal annealing under H-2 gas enhanced the substoichiometry of MoO3-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 (Or) 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 O2-, 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 degrees C to Mo4+/Mo6+ = 3.7 and Mo5+/Mo6+ = 2.6 when reduced at 600 degrees 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.