Biomolecule-assisted route for shape-controlled synthesis of single-crystalline MnWO4 nanoparticles and spontaneous assembly of polypeptide-stabilized mesocrystal microspheres

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.