Low-Temperature Photocatalytic Hydrogen Addition to Two-Dimensional MoO₃ Nanoflakes from Isopropyl Alcohol for Enhancing Solar Energy Harvesting and Conversion

Recently, research has been focused on the plasmon resonances in semiconductors (e.g., metal oxides) with tunable resonance absorption covering visible, near-infrared, and mid-infrared regions to enhance the efficiency in the utilization of solar energy. Two-dimensional (2D) molybdenum trioxide (MoO3) nanoflakes were studied to form substoichiometric oxides with strong tunable plasmon resonance via photocatalytic hydrogen addition. Despite the classic concept that the hydrogen addition to MoO3 from pure alcohols requires elevated temperatures more than 200 °C, we report a low-temperature hydrogen addition reaction of 2D MoO3 nanoflakes with pure isopropyl alcohol under visible light irradiation. The nanoflakes were produced by a facile liquid exfoliation of the MoO3 whiskers from chemical vapor deposition. Using alcohol–water solutions at different mixing ratios, the exfoliation process can tune the dimensions of the nanoflakes, as well as their defect structures. For the photocatalytic hydrogen addition of nanoflakes with pure alcohol, the suspension temperature is below 50 °C throughout the process. Material structure characterization, optical property measurement and analysis, and photothermal study were performed thoroughly to reveal the reaction mechanism. Our study shows the reaction is initiated by the visible light absorption from their sub-bandgap defect tails and is expedited by photothermal heating without providing any additional heating. The low reaction temperature provides a new low-cost method to produce substoichiometric semiconductors with tunable plasmonic behaviors. The reaction mechanism can be extended to other photocatalytic processes of MoO3 nanostructures to improve their efficiencies in utilizing solar energy.