Effect of Different Solvent Ratios (Water/Ethylene Glycol) on the Growth Process of CaMoO4 Crystals and Their Optical Properties

• Published in: Crystal growth & design 2010-11, Vol.10 (11), p.4752-4768
• Main Authors: Marques, V. S, Cavalcante, L. S, Sczancoski, J. C, Alcântara, A. F. P, Orlandi, M. O, Moraes, E, Longo, E, Varela, J. A, Siu Li, M, Santos, M. R. M. C
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Cross-disciplinary physics: materials science; rheology; Electron and ion emission by liquids and solids; impact phenomena; Exact sciences and technology; Field emission, ionization, evaporation, and desorption; Materials science; Methods of crystal growth; physics of crystal growth; Nanoscale materials and structures: fabrication and characterization; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of bulk materials and thin films; Other topics in nanoscale materials and structures; Physics; Theory and models of crystal growth; physics of crystal growth, crystal morphology and orientation
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/cg100584b

In this paper, calcium molybdate (CaMoO4) crystals (meso- and nanoscale) were synthesized by the coprecipitation method using different solvent volume ratios (water/ethylene glycol). Subsequently, the obtained suspensions were processed in microwave-assisted hydrothermal/solvothermal systems at 140 °C for 1 h. These meso- and nanocrystals processed were characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman), Fourier transform infrared (FT-IR), ultraviolet−visible (UV−vis) absorption spectroscopies, field-emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. XRD patterns and FT-Raman spectra showed that these meso- and nanocrystals have a scheelite-type tetragonal structure without the presence of deleterious phases. FT-IR spectra exhibited a large absorption band situated at around 827 cm−1, which is associated with the Mo−O antisymmetric stretching vibrations into the [MoO4] clusters. FEG-SEM micrographs indicated that the ethylene glycol concentration in the aqueous solution plays an important role in the morphological evolution of CaMoO4 crystals. High-resolution TEM micrographs demonstrated that the mesocrystals consist of several aggregated nanoparticles with electron diffraction patterns of monocrystal. In addition, the differences observed in the selected area electron diffraction patterns of CaMoO4 crystals proved the coexistence of both nano- and mesostructures. First-principles quantum mechanical calculations based on the density functional theory at the B3LYP level were employed in order to understand the band structure and density of states for the CaMoO4. UV−vis absorption measurements evidenced a variation in optical band gap values (from 3.42 to 3.72 eV) for the distinct morphologies. The blue and green PL emissions observed in these crystals were ascribed to the intermediary energy levels arising from the distortions on the [MoO4] clusters due to intrinsic defects in the lattice of anisotropic/isotropic crystals.