Kinetic Control of MnWO4 Nanoparticles for Tailored Structural Properties

This work reports on the kinetic control of MnWO4 nanoparticles with an aim to achieve the tailored structural properties. Using citric acid as the capping agent, MnWO4 nanocrystals were prepared to show particle sizes ranging from 8 to 29 nm under hydrothermal conditions. The grain-growth kinetics for MnWO4 nanoparticles was determined to quantitatively follow the equation, D-5 = (4.59 x 10(20))te(-17.0/T), where D is the particle size at given reaction time, t, and reaction temperature, T. Systematic sample characterization using combined techniques of X-ray diffraction, transmission electron microscope, selected area electron diffraction, Barret-Emmett-Teller technique, Fourier transformed infrared spectra, UV-visible diffuse reflectance spectra, and Raman spectra indicates that particle size reduction led to an apparent lattice expansion, which is followed by lowering of lattice symmetry and band gap broadening. Different from the bulk counterparts, new vibration modes were observed in both Infrared and Raman spectra at about 913 and 930 cm(-1), respectively, which intensified monotonously with particle size reduction, leading to a picture that MnWO4 nanoparticles were terminated by surface disordered layers. All these size-dependent physical properties were closely related to the surface disorder and the relevant absorbates.