Production of Molybdenum Trioxide Nanosheets by Liquid Exfoliation and Their Application in High-Performance Supercapacitors

Here, we demonstrate a simple method to exfoliate layered molybdenum trioxide (MoO3) crystallites to give multilayer MoO3 nanosheets dispersed in solvents. Exfoliation is achieved by sonicating MoO3 powder in the presence of suitable solvents followed by centrifugation to remove undispersed material. This procedure works well in a range of solvents with Hildebrand solubility parameters close to 21 MPa1/2 and is consistent with the predictions of classical solubility theory. We have fully optimized this process and demonstrated methods to separate the resultant nanosheets by size. Raman spectroscopy suggests the exfoliation process does not damage the MoO3. This is supported by measurements showing that the reaggregated nanosheets display very similar photoluminescence to bulk MoO3. However, the dispersed nanosheets had distinctly different photoluminescence, indicating a decoupling of the monolayers on exfoliation. We have used liquid-exfoliated MoO3 to prepare supercapacitor electrodes that have relatively low capacitance (similar to 2 F/g at 10 mV/s) because of the low electrical conductivity of the MoO3. However, addition of carbon nanotubes beyond the percolation threshold yielded a 100-fold increase in capacitance. Some MoO3/nanotube composites displayed a capacitance as high as 540 F/g at 0.1 mV/s. This is the first example of solvent exfoliation of a layered metal oxide. We believe this work opens the way to liquid exfoliation of a wide range of layered compounds, leading to an array of new solution-processed 2D materials.