Morphology Control Studies of MnTiO3 Nanostructures with Exposed {0001} Facets as a High-Performance Catalyst for Water Purification.

Novel single-crystal hexagonal MnTiO3 nanosheets with exposed {0001} facets have been synthesized via a simple one-pot hydrothermal method using NaOH as a mineralizer and tetraethylammonium hydroxide (TEAH) as a morphology controller. The intermediate morphologies of MnTiO3 nanostructures such as nanoparticles, nanowires, nanorods, and nanodiscs are trapped kinetically by adjusting the synthesis conditions. This approach enables us to elucidate the growth mechanisms of MnTiO3 nanosheets based on the tetraethylammonium cation adsorption abilities on different MnTiO3 crystal facets combined with density functional theory calculations. Dissolution and recrystallization processes are involved during the MnTiO3 crystallization. The surface-controlled MnTiO3 has been found to be effective as a catalyst for ozonation in the degradation of 4-chlorophenol (4-CP). Within typical experimental conditions (catalyst dosage = 0.3 g L-1, [4-CP]0 = 50 mg L-1, [O3] = 20 mg L-1, gas flow = 0.1 L min-1, pH 6.8, and T = 293 K), the total organic carbon (TOC) removal efficiency of 4-CP in catalytic ozonation with well-structured MnTiO3 (MnTiO3-180-10 sample) was 76.3% after 60 min, compared with only 22.1 and 38.5% TOC removal in the absence of catalyst and with uncontrolled MnTiO3 (MnTiO3-no TEAH sample), respectively. Benefiting from the high exposure percentage of {0001} facet, mixed-valences of manganese, surface hydroxyl groups, and the enrichment Lewis acid sites provided by Mn and Ti, the morphology-controlled MnTiO3 nanosheets can be applied as heterogeneous catalytic ozonation catalysts which exhibit excellent pollutant degradation. We anticipate that MnTiO3 can be a promising candidate material for the application in remediation of organic pollutants in water.