Biomolecule-assisted route for shape-controlled synthesis of single-crystalline MnWO4nanoparticles and spontaneous assembly of polypeptide-stabilized mesocrystal microspheres

Abstract

Single-crystalline mixed metal oxide nanoparticles and 3D hierarchical mesocrystal microspheres of MnWO₄ have been synthesized on a large scale by a facile single-step hydrothermal method using Mn(NO₃)₂ and Na₂WO₄ precursors, and capping bifunctional amino acid biomolecules with different alkyl chain lengths, and water or water/ethylene glycol medium. The resulting single-crystalline MnWO₄ nanoparticles with different uniform shapes including bar, rod, square, quasi-sphere, sphere, hexagonal crystals were obtained by tuning the synthetic conditions such as the concentration and the alkyl chain length of amino acids, pH, and reaction temperature. By decreasing the Mn²⁺ and WO₄²⁻ precursor monomer concentration from 0.0150 to 0.0076 M in aqueous media, polypeptide-stabilized MnWO₄ mesocrystal hierarchical microspheres were achieved, due to the spontaneous-assembly of primary nanoparticles through the back-bone–back-bone intermolecular hydrogen bonding interactions of polypeptide chains. Nanoplatelet-based microapples with two holes on their poles were also obtained under the same synthetic conditions for the microspheres, except that the use of water/ethylene glycol (10 : 30 mL) instead of water medium. The photoluminescence (PL) results revealed that the PL emission intensity of the MnWO₄ nanobars is higher than that of the self-assembled MnWO₄ microspheres. This green chemistry method is simple and highly reproducible, using inexpensive reagents, and water as reaction solvent. The uniform MnWO₄ nanorods with the control of aspect ratio (length/width) can be produced in a large quantity as much as 16 g in a single preparation. The current approach is quite general and able to be extended to a variety of other well-defined metal oxide and mixed oxide nanomaterials with controlled shapes.