pubs.acs.org/crystal Published on Web 10/01/2010 r 2010 American Chemical Society DOI: 10.1021/cg100584b 2010, Vol. 10 4752–4768 Effect of Different Solvent Ratios (Water/Ethylene Glycol) on the Growth Process of CaMoO 4 Crystals and Their Optical Properties V. S. Marques, † L. S. Cavalcante,* ,‡ J. C. Sczancoski, § A. F. P. Alc^ antara, † M. O. Orlandi, ‡ E. Moraes, § E. Longo, ‡ J. A. Varela, ‡ M. Siu Li, ) and M. R. M. C. Santos † † LIMAV-CCN-Quı´mica, UFPI;Universidade Federal do Piauı´, 64049-550 Teresina, PI, Brazil, ‡ UNESP;Universidade Estadual Paulista, P.O. Box 355, 14801-907 Araraquara, SP, Brazil, § LIEC;Universidade Federal de S~ ao Carlos, P.O. Box 676, 13565-905 S~ ao Carlos, SP, Brazil, and ) IFSC, Universidade de S~ ao Paulo, P.O. Box 369, 13560-970 S~ ao Carlos, SP, Brazil Received April 30, 2010; Revised Manuscript Received August 26, 2010 ABSTRACT: In this paper, calcium molybdate (CaMoO 4 ) crystals (meso- and nanoscale) were synthesized by the coprecipita- tion 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 anti- symmetric stretching vibrations into the [MoO 4 ] clusters. FEG-SEM micrographs indicated that the ethylene glycol concen- tration in the aqueous solution plays an important role in the morphological evolution of CaMoO 4 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 CaMoO 4 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 CaMoO 4 . 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 [MoO 4 ] clusters due to intrinsic defects in the lattice of anisotropic/isotropic crystals. Introduction Calcium molybdate (CaMoO 4 ) is an important material belonging to the scheelite family with tetragonal structure and space group I4 1 /a. 1,2a,b In this structure, the Mo atoms are bonded to four oxygens, forming the [MoO 4 ] clusters, while the Ca atoms are coordinated to eight oxygens, forming the [CaO 8 ] clusters. 3-7 In recent years, this molybdate has at- tracted the attention of diverse scientific and technological fields because of its wide potential for applications in acousto- optic filters, 8 solid state lasers, 9,10 white light-emitting diodes, 11 scintillators, 12 microwave dielectrics, 13,14 fluorescent lamps, 15 negative electrodes for Li þ -ion batteries, 16 and cryogenic scintillation detectors for search of 100 Mo double β decay, 17,18 catalytic properties of propane ammoxidation, 19 and so on. In terms of optical properties, this material exhibits green and/or blue luminescence emissions at room temperature when excited with wavelengths in the range from 240 to 537 nm. 19-22 However, few works in the literature have reported on the origin of the photoluminescence (PL) properties of CaMoO 4 . For example, Mikhailik et al. 23 explained that the short wave- length luminescence of this molybdate is usually caused by the intrinsic emission of the MoO 4 2- molecular complexes while its long wavelength luminescence arises from the MoO 3 oxygen- deficient defect centers. In another work, Mikhailik et al. 24 investigated the electronic transitions and luminescence decay kinetics of CaMoO 4 at low temperatures (from -265 to 27 °C) by means of ultraviolet excitation. According to these authors, the optical transitions occur in the MoO 4 2- molecular com- plexes, mainly involving the ( 1 A 1 ) ground, singlet ( 1 T 1 , 1 T 2 ), and triplet ( 3 T 1 , 3 T 2 ) levels. In this case, the electric dipole allowed 1 A 1 f 1 T 2 transitions contribute to the excitation process while the radiative transitions arise from the closely located lower-lying triplet states. Ryu et al. 25 mentioned that, besides the charge-transfer transitions within the MoO 4 2- complexes, the PL response can be influenced by morphology and particle size distribution. Marques et al. 26 attributed the origin of this optical property to the degree of structural order- disorder in the lattice. Recently, Longo et al., 27 through the first-principles quantum mechanical calculations based on the density functional theory, reported that the green and blue PL emissions of disordered CaMoO 4 powders are linked to the intrinsic slight distortion of the [MoO 4 ] tetrahedral clusters. Considering the different chemical routes, the conventional hydrothermal systems are well-known due to their versatility in the formation and crystallization of ceramic oxides at low temperatures, enabling a good control of morphologies and particle sizes. 28 On the other hand, the long processing times exhibited by this synthesis method were overcome using micro- wave radiation as energy source for this system. This innova- tion resulted in the development of equipment currently known as microwave-hydrothermal. 28 In order to obtain a good *To whom correspondence should be addressed. E-mail: laeciosc@bol. com.br.