Size Fractionation of Two-Dimensional Sub-Nanometer Thin Manganese Dioxide Crystals towards Superior Urea Electrocatalytic Conversion.

A universal technique has been proposed to sort two-dimensional (2D) sub-nanometer thin crystals (manganese dioxide MnO2 and molybdenum disulfide MoS2 ) according to their lateral dimensions. This technique is based on tuning the zeta potential of their aqueous dispersions which induces the selective sedimentation of large-sized 2D crystals and leaves the small-sized counterparts in suspension. The electrocatalytic properties of as-obtained 2D ultrathin crystals are strongly dependent on their lateral size. As a proof-of-concept study, the small-sized MnO2 nanocrystals were tested as the electrocatalysts for the urea-oxidation reaction (UOR), which showed outstanding performance in both half reaction and full electrolytic cell. A mechanism study reveals the enhanced performance is associated with the remarkable structural properties of MnO2 including ultrathin (ca. 0.95 nm), laterally small-sized (50-200 nm), and highly exposed active centers.