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

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.