Excellent adsorption property and mechanism of oxygen vacancies-assisted hexagonal MoO3 nanosheets for methylene blue and rhodamine b dyes

The adsorption ability of oxide adsorbents is highly determined by their oxygen vacancies (OVs) and specific surface area (SSA). Herein, metastable hexagonal MoO3 (h-MoO3) nanosheets with rich OVs and high SSA were fabricated through a simple hydrothermal method to enable an excellent adsorption property. The maximum adsorption capacities of these nanosheets for rhodamine B (RhB) and methylene blue (MB) dyes are 1242 and 1433 mg/g, respectively. Furthermore, the adsorption has superior stability in a wide pH ranging from 3.0 to 10.0. Theoretical analysis shows that the equilibrium data can be well interpreted by the Langmuir isotherm model, and the adsorption kinetics can be derived by the pseudo-second-order kinetic model. The negative Gibbs energy derived from the thermodynamic calculation indicates the whole adsorption occurs spontaneously. Combining the XPS before and after adsorbing dyes, the negative zeta potentials, and the results from the adsorption energy calculation and the differential charge density simulation, this adsorption is dominated by the chemical adsorption caused by the electrostatic induction. Since the adsorption capacities and oxygen vacancies of the h-MoO3 nanosheets decrease significantly after annealing, we conclude that the rich surface oxygen vacancies in the nanosheets enable the excellent adsorption capability for MB and RhB dyes.