12 Synthesis and Luminescence Properties of EuMoO 4 Octahedron-Like Microcrystals

Global claim for luminescent materials as efficient sources of energy that can supply sustained competence is growing day by day. The phosphors are facing increased challenges including concerns about global rare earth, environmental and recycling issues, and how to quickly bring to market new technological developments in the industry to meet end-user requirements. A number of applications have emerged in recent years that will change the future of the industry and new technologies like micro/nanoscale innovations and specialty phosphors are harnering increased attention. The primary drivers for growth are the expansion of key end-use applications including solid-state lighting and fluorescent lighting. Current research in nanotechnology is focused on new materials, novel phenomena, new characterization technique and fabrication of micro/nano devices.


Introduction
Global claim for luminescent materials as efficient sources of energy that can supply sustained competence is growing day by day.The phosphors are facing increased challenges including concerns about global rare earth, environmental and recycling issues, and how to quickly bring to market new technological developments in the industry to meet end-user requirements.A number of applications have emerged in recent years that will change the future of the industry and new technologies like micro/nanoscale innovations and specialty phosphors are harnering increased attention.The primary drivers for growth are the expansion of key end-use applications including solid-state lighting and fluorescent lighting.Current research in nanotechnology is focused on new materials, novel phenomena, new characterization technique and fabrication of micro/nano devices.
Rare-earth doped molybdates are excellent materials of current interest owing to their interesting optical and opto-electronic properties.Scheelite-type crystalline structures, such as metal molybdates, where molybdenum atoms are coordinated tetrahedrally, have recently attracted great attention because of their promising applications in the electrooptical field (Ryu et al., 2005,a;Gong et al., 2006).Recently, molybdates have been extensively studied due to its attractive luminescence behavior and interesting structural properties (Thongtem et al., 2010), which can be used in a number of fields, such as optic fibers (Ryu et al., 2005, b) laser host materials (Wang et al., 2007), luminescence materials (Zhang et al., 2005;Sen & Pramanik, 2001), efficient catalysts (Ishii et al., 1992), scintillation detectors (Marques et al., 2006), and microwave application (Yang et al., 2009).Among all these molybdates, EuMoO 4 is an important opto-electrical material based on its red and white LED phosphors (Rosa et al., 2009).Furthermore, the molybdates allow the doping of different rare-earth ions, which can vary its luminescence properties.Due to the rich emission spectral lines extending from ultraviolet to infrared for Eu 3+ ions, spectroscopic properties of this ion have been extensively studied in various hosts to investigate the potential applications as functional material, in the fiber laser amplifiers, quantum cutting luminescence, up-conversion laser, and tunable ultraviolet (UV) , vacuum UV solid-solidstate lasers and High-pressure x-ray diffraction study (Errandonea et al., 2011;Koch et al., 1990;Zhang et al., 2007).New and enhanced properties are expected due to size confinement in nanoscale dimensions.In this background, this research work forms a recent avenue in material science research.The field of nanophosphor development, with rare earth doped metal oxide sytems have become a separate wide field in these days.
A detailed survey on undoped and doped molybdate phosphors discusses various synthesis techniques adopted by different research groups as follows.Recently, Eu 3+ doped ZnMoO 4 with charge compensation were synthesized by solid-state reactions for phosphor-converted light-emitting diode (LED) is an important kind of solid-state illumination (Xie et al., 2010).Octahedron-like BaMoO 4 microcrystals were synthesized by the co-precipitation method at room temperature and processed in microwave-hydrothermal at 413 K for different time (Cavalcante et al., 2009).Single-crystalline BaMoO 4 microcrystals with uniform shuttle-like morphology have been successfully prepared via a facile aqueous solution mineralization process at room temperature (Wu et al., 2007).Phosphors Na 5 La(MoO 4 ) 4 :xEu 3+ and NaEu(MoO 4 ) 2 were prepared with a solid-state reaction technique and are fabricated onto near-ultraviolet/violet-emitting InGaN chips for bright red-light-emitting diodes device, respectively (Wang et al., 2006).In order to prepare fluorescent material for white Light Emitting Diodes (LEDs), a new Eu 3+ activated molybdate phosphor SrMoO 4 was fabricated with solid-state method (Xu et. al., 2007).Yttrium molybdate phosphors with fixed Er 3+ and various Yb 3+ concentrations were synthesized via a co-precipitation method (Lu et al., 2010).Tetragonal La 2 (MoO 4 ) 3 with various novel and complex 3D hierarchical architectures selfassembled from different building blocks were successfully synthesized by a hydrothermal method in EDTA mediated processes (Xu et al., 2010).Enhancement emission intensity of CaMoO 4 :Eu 3+ , Na + phosphor via Bi co-doping and Si substitution for application to white LEDs phosphors were synthesized by the conventional solid state reaction method (Marques et al., 2008).ZnMoO 4 with a rhombus sheet or flower-like structure, α-ZnMoO 4 and needle-like ZnMoO 4 .0.8H 2 O were successfully synthesized by simple hydrothermal crystallization processes with citric acid for opto-electronic applications (Zhang et al., 2010).The electronic structures of perfect crystals of barium molybdate (BaMoO4) and of crystals containing F and F+ color centers are studied within the framework of the fully relativistic self-consistent Dirac-Slater theory by using the numerically discrete variational (DV-Xα) method (Guo et al., 2009).Room temperature electrochemical growth of polycrystalline BaMoO 4 films on a metallic molybdenum substrate in a barium hydroxide aqueous solution has been studied (Xia & Fuenzalida, 2003).BaMoO 4 thin films were prepared on molybdenum substrates in Ba(OH) 2 solutions by electrochemical method at room temperature (Bi et al., 2003).Recently, on novel rare-earth activated molybdate [K x Sr 1−2x MoO 4 :Pr 3+ x (0.00 ≤ x ≤ 0.04)] light-emitting diodes (LEDs) for the replacement of current light sources (Qiuxia et al., 2011).A novel red phosphor Ca 1-2x Eu x Li x MoO 4 is a promising material for Solid-State Lighting Based on a GaN LED (Wang et al., 2005).CaMoO4 was a promising yellowish green phosphor for near-ultraviolet (NUV) InGaNbased white LED (Li et al., 2007).Highly ordered SrMoO 4 3D spherical superstructure assembled with nanosheets was synthesized via a facile and fast sonochemical route without any template (Mao et al., 2010).The indoor lighting applications are the main potential applications using either white-light-emitting diode (LEDs) made by exciting appropriate phosphors.These wide band gap materials can be implemented commercially replacing all conventional LEDs in future.A well-crystallized AMO 4 (A=Ba, Ca, Sr; M=W, Mo) films have been prepared at room temperature through a simple solution reaction in respective alkaline solution at higher pH ~ 12-14 (Dinesh et al., 2005).X-ray diffraction and Raman scattering studies on the scheelite structured barium molybdate (BaMoO 4 ) under high pressure (~5.8 GPa) shows that, it undergoes a first order phase transition to the fergusonite structure (I2/a, Z = 4)-as also observed in iso-structural barium tungstate (Panchal et al., 2006).A study on the growth kinetics and habit modification of barium molybdate single crystals in silica gel medium has been presented (Kurien & Ittyachen, 1979).Praseodymium barium molybdate (PBM) single crystals grown by gel method, reveal multiple and isolated octahedral bipyramidal crystals as well as spherulites of PBM at different depths from the surface inside the gel (Isac et al., 1996).A series of novel redemitting phosphors AgLa 0.95 Eu 0.05 (WO 4 ) 2-x (MoO 4 ) x [x=0-2] have been synthesized and their luminescence properties investigated (Sivakumar & Varadaraju, 2006).The effects of Ni 2+ doping on structural, microstructural, dielectric and electrical properties of lead molybdate PbMoO 3 have been investigated.Ferroelectric phase transition in Ni-doped lead molybdate ceramics was studied (Palai et al., 2001).
Herein, we report the controlled synthesis of EuMoO 4 microcrystals using hydrothermal method by employing PVP as surfactant along with the state of the art of this field at present.By varying the reaction time, we have reported the morphology selection and the condition to derive octahedron morphology has been discussed.The rest of the parameters like, molar ratio between initial precursor / surfactant and temperature were kept as constant.The EuMoO 4 microcrystals are found to be an excellent matrix for photoluminescence application.The Eu 3+ ion serves as a good red emitting luminescent center.Therefore, well-crystallized EuMoO 4 microcrystals have significant improvement in the phosphor properties considerably.Hence, the PL properties were also studied and reported.Also, this processing route provides the basis for a nearly low cost, low temperature method for the preparation of homogeneous nano-sized ceramics compared to any other existing methods.

Synthesis, structure, formation and morphology of the EuMoO 4 microcrystals 2.1 Synthesis
All the chemical reagents used were of analytical grade and used without further purification.In the typical synthesis, ~ 0.02 gm of PVP was first dissolved in 15 mL of deionized water under with vigorous stirring.Subsequently 0.021 mM of (NH 4 ) 6 Mo 7 O 24 •4H 2 O was added drop wise.This is followed by addition of 1 mM EuCl 2 •7H 2 O solutions drop wise to the above solution under with vigorous stirring by maintain the solution pH was adjusted up to 8. The mixture was stirred for another 10 min.Finally, the resultant solution was transferred to a Teflon-lined vessel kept in an autoclave at 135 °C for 0 -7 hrs.The resultant solid products were centrifugally separated from the suspension, washed with deionized water and absolute ethanol several times, and dried at 60 ºC in air for 6 hrs.

Structural analysis
The BaMoO 4 exhibit a scheelite-type tetragonal structure (Yang et al., 2008, Luo et al., 2009, Wu et al., 2007), which belongs to the tetragonal space group of I4 1 /a.The unit-cell presents two formula units per primitive cell which includes two formula units where the Mo atom is surrounded by four equivalents O site in tetrahedral symmetry and divalent metal shares corners with eight adjacent MoO 4 tetrahedral [Luo et al., 2009, Ryu et al., 2005a].Structure and phase purity of the EuMoO 4 microcrystals were examined using XRD analysis.From Fig. 1, it is seen that all the diffraction peaks matched well with the standard data for tetragonal scheelite-type EuMoO 4 (JCPDS, No. 22-1097; Space Group -I4 1 /a).No traces of extra peaks from other phases were observed.Therefore, all as-prepared samples were of single phase EuMoO 4 .The diffraction peaks (Fig. 1) are slightly broadened, which is characteristic of their fine nature.The average crystallite sizes of the primary particles that are calculated from peak broadening of the (112) line using the Scherrer formula was approximately 35 nm for EuMoO 4 microcrystals.

Morphological studies
The SEM and TEM images of the sample prepared with the typical procedure are shown in Fig 2 .To obtain a better understanding of the formation mechanism of the EuMoO 4 octahedral-like microstructures, products formed at different growth stages were collected for SEM measurements.Fig 2 (a-e) implies that the concentration of PVP aqueous solutions played a crucial role in the formation of EuMoO 4 crystal prepared under various growth conditions.We found that the Ostwald ripening process dominated the crystal growth process at the initial stage.Fig. 2a shows some tiny equiaxial nanoparticles with polydisperse nature, at room temperature.As is well-known, at the initial stage tiny crystalline nuclei form in a supersaturated solution that acts as the nuclei for crystallization.Then the crystal growth follows, and bigger particles grow at the expense of smaller crystals, as described by the Gibbs-Thompson equation (Peng, 2003).
where, r: is the radius of the crystal; σ: the specific surface energy; V m : molar volume of the material; S b & S r : solubility of the bulk crystals and crystals with a radius r, respectively; R, the gas constant; T, the absolute temperature.Fig. 2b shows that the processing performed at 1h favoured the formation of bipyramidal-like microparticles to some extent agglomerated nature.By increasing the reaction time to 1 hr, the resulting product formation is due to the interactions between the tails of surface-adsorbed surfactant molecules, influencing the growth process of the micro-octahedrons.At ~ 3 hrs, it could be seen that EuMoO 4 octahedral microparticles of size ~ 1 µm side length were formed (Fig 2c ), which is nearly similar to the structure of micro-octahedrons (Fig 2c).The growth rate in EuMoO 4 microcrystal is oriented preferentially along the [001] direction than in the [100] direction.In principle, we believe that the anisotropic growth in the [001] direction for the EuMoO 4 micro-octahedron is caused by differences in the surface energies on each crystal face.Probably, this preferential growth direction has stronger interatomic bonds between the [Eu 3+ ] and [MoO 4 ] 2-clusters.The microparticles synthesized with reaction time ~ 5 hrs are highly uniform, regular with smooth surface and can be dispersed well on a large scale.The PVP may form a complex the rare earth ions Pr 3+ at first.Subsequently it might bind to the surface of the growing particles after being dissociated from the rare earth ions slowly in the process of reaction.Hence, the main factor to influence the formation of the regular structures is estimated as time.Formation of the octahedral-like EuMoO 4 microstructures after hydrothermal treatment are confirmed by TEM investigation.The corresponding highresolution TEM image in Fig. 2d displays resolved lattice fringes of ( 004) planes with a spacing of 0.385 nm.Insets of Fig. 2(c, d) show the selected area electron diffraction (SAD).
The Fast Fourier Transform (FFT) pattern indicates that the microstructures are single crystalline in nature.It is inferred that at the higher reaction time (Fig 2e), growth of the microcrystals lead to preferential growth orientation All together, from the SEM images, we can clearly see that microstructural changes occurred at the very early stages and even at lower reaction time.As reaction time was prolonged, only similar and much denser microcrystals could be grown.

Formation mechanism
The relevant formation mechanism of EuMoO 4 microstructures has been interpreted in two paths: dissolution-recrystallization and as well as the effective collision rates between the small microcrystals, respectively (Cavalcante et al., 2009, Luo et al., 2009).The hydrothermal process is able to promote the effective fender-bender between the small microparticles, contributing to the growth of the microcrystals and inducing the formation of new crystallographic faces on them.Schematic (Fig. 3) shows the morphological evolution of EuMoO 4 microcrystals as a function of reaction time.It is proposed that the process for the formation of the octahedron EuMoO 4 microcrystals.The first one is the formation of the perfect small microparticles, which belongs to a typical Ostwald ripening process.Probably, the preferential growth direction has stronger interatomic bonds between the [Eu 3+ ] and [MoO 4 ] 2-ions were mixed, a highly supersaturated solution was formed, leading to the coexistence of the asymmetrical and the bipyramidal-like EuMoO 4 microparticles.It is clear that the crystalline phase of the nuclei is critical for directing the inherent shapes of the crystals owing to its distinctive symmetry and structure.As the reaction time proceeded for 1 hrs, the bipyramidal-like particles are agglomerated with a high concentration of surface defects and it is possible to note the initial formation of micro-octahedrons with welldefined structures.From 5 hrs of reaction time, the aggregation process between the small microparticles was induced by the coalescence process, resulting in the growth of microoctahedrons.This proposed crystal growth mechanism is in good agreement with the literature (Cavalcante et al., 2009, Wu et al., 2007).
Fig. 3. Schematic representation of the formation process of the octahedron-like EuMoO 4 microcrystals.

Compositional analysis
It is well known that, the XPS is known to study the composition and binding energy.Therefore, in order to further prove that the as-synthesized EuMoO 4 microcrystals are composed of relevant elements.X-ray photoelectron spectroscopy (XPS) analysis was performed to probe the surface of the particles.The structures were probed using XPS, to investigate the elementary states and binding energy values.Fig. 4a shows the XPS survey spectra of the tetragonal EuMoO 4 microcrystals.The core levels of Eu, Mo and O can be seen in Fig. 2a, in which, the XPS spectra of the tetragonal EuMoO 4 microcrystals, obtained is the range of 100 -600 eV (Eu 4d at 130 eV, Mo 3d 5/2 at 228.5 eV, Mo 3d 3/2 at 331 eV, Mo 3p 3/2 at 394 eV, Mo 3d 1/2 at 412 eV, Mo 3s at 506 eV, O 1s 531.5 eV) and 1100 -1200 eV (Eu 3d 5/2 at 1126 eV, Eu 3d 3/2 1156.5 eV), respectively.It can be seen that the binding energy data (calibrated using C (1s, 284.7 eV) as the reference) from EuMoO 4 microparticles.All these peaks are in good agreement with the literature (Moulder, et al., 1992).The composition estimated by XPS using the relative sensitivity factors of Eu, Mo and O also revealed excess oxygen in the samples.The results also reveal that the sodium ions can be eliminated from the final product after washing and calcinations.The composition estimated using XPS results are in good agreement.

Optical properties
Fluorescence spectroscopy can provide valuable information about the intermolecular interactions of molecules in molecular crystals (Lifshitz, et al., 2003).The tetragonal phased EuMoO 4 has similar structure to that of other rare-earth doped molybdate ions with scheelite-type and is expected to be a promising phosphor candidate.The presence of strong clear peaks indicates the high crystallinity of the as-prepared products, which is very beneficial for obtaining brighter luminescence bands.The luminescence properties of the EuMoO 4 phosphors were then examined.Fig. 5 presents the excitation and emission spectra of EuMoO 4 microstructures.
The excitation spectra (Fig. 5, left) were obtained by monitoring the emission of the Eu 3+ 5 D 0 → 7 F 2 transition at 616 nm.It can be observed clearly that the excitation spectra both consist of a broadband from 200 to 350 nm, which is ascribed to the O─Mo C-T transition.
The C-T band of O─Eu 3+ , which usually appears in the range of 250-300nm in the excitation spectrum (Kiselev, et al., 2008), might have overlapped with the C-T band of molybdate group and, hence, as not observed clearly.In the longer wavelength region (360-710 nm), the sharp lines are intraconfigurational 4f-4f transitions of Eu 3+ in the host lattices, and the www.intechopen.com strong excitation band at 395 and 465nm is attributed to the 7 F 0 → 5 L 6 and 7 F 0 → 5 D 2 transitions of Eu 3+ , respectively.The emission spectra of Eu 3+ (Fig. 5, right) excited under 395nm near-UV light are mainly dominated by the hypersensitive red emission, showing a strong transition 5 D 0 → 7 F 2 at 616nm and a weak 5 D 0 → 7 F 1 transition.The two peaks are assigned to the electric dipole transition and magnetic dipole transition, respectively, and the presence of electric dipole transition confirms that Eu 3+ ions are located at sites without inversion symmetry.From the PL emission spectra it is clearly attrributed that the transition from its major lines is greatly enhanced, and thus a bright red emission is observed.Since the absorption / emission process depends strongly on the nature of the lattice and its constituents, the luminescence behaviour of Eu 3+ compounds is expected to vary strongly with the preparatory conditions.This is what has been observed.The strong luminescence intensity indicated the perfection of the microstructure and good crystallization.The optical property of the EuMoO 4 are in good agreement with the literature (Kiselev, et al., 2008), that were obtained by solid-state reaction method.The trivalent europium ion (Eu 3+ ) in solids has an intricate energy level scheme with energy gaps of various magnitude and show rich emission spectral lines (Blasse & Grabmaier, 1994).Also a systematic study related to PL and the lifetime measurements are to be undertaken.Such photoluminescent materials have potential application and an efficient red phosphor candidate in the luminescence field.The result envisages an effective route to synthesize EuMoO 4 microstructures have potential application in biomedical and display systems.In summary, this novel EuMoO 4 microstructures have been successfully synthesized via a facile simple and mild hydrothermal route employing PVP as surfactant.It is interesting to observe that the microparticles are highly crystalline and with uniform octahedral-like morphology in size and shape.The use of PVP as a surfactant introduced into the reaction system is essential to obtain highly crystalline and smooth surface microparticles.A strong red emission centered at 643 nm corresponding to the 3 P 0 → 3 F 2 transition of Eu 3+ is www.intechopen.comobserved under 440 -500 nm excitation.The grown EuMoO 4 microstructures showed strong red emission upon UV illumination, finds potential application in biomedical and display devices.Altogether, this preparation methodology is found to be highly reproducible, convenient, easy, and could be extended to prepare other rare-earth doped molybdate microstructures for various applications like electroluminescent devices, optical integrated circuits, or biomarkers.

Fig. 5 .
Fig. 5.A part of room temperature PL of EuMoO 4 microstructures (A 1 -A 3 ): excitation spectra (monitored at 643 nm) and emission spectra (under 443 nm excitation) prepared by modulating the reaction time.