Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 30–36

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Effect of surface coating on optical properties of Eu3+-doped CaMoO4 nanoparticles Anees A. Ansari a,⇑, A.K. Parchur b, Manawwer Alam c, Abdallah Azzeer a a

King Abdullah Institute for Nanotechnology, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia Department of Physics, Banaras Hindu University, Varanasi 221005, India c Research Center, College of Science, King Saud University, P.O. Box 2455, Riyadh, Saudi Arabia b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 24 December 2013 Received in revised form 6 March 2014 Accepted 7 April 2014 Available online 21 April 2014 Keywords: Calcium molybdate nanoparticles Band gap energy Photoluminescence

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a b s t r a c t A simple polyol method has been used for the synthesis of CaMoO4:Eu (core), CaMoO4:Eu@CaMoO4 (core/ shell) and their silica coated CaMoO4:Eu@CaMoO4 (core/shell/shell) nanoparticles. X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), Fourier transform Raman (FT-Raman), Fourier transform infrared (FT-IR), UV/Vis absorption and photoluminescence (PL) spectroscopies techniques has been employed for their characterization. XRD patterns and FT-Raman spectra showed that these nanoparticles have a scheelite-type tetragonal structure without the presence of deleterious phases. These nanoparticles were easily dispersed in water, producing a transparent colloidal solution. The optical energy band-gap decreases after core/shell formation due to increase the crystalline size. The photoluminescence (PL) spectra of the as-synthesized core, core/shell and core/shell/shell nanoparticles measured with an excitation source wavelength of 325 nm showed that the emission intensity was increases after shell formation around the surface of core nanoparticles. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Recent interest in luminescent rare-earth ion-doped (Ln3+) calcium molybdate nanocrystals have sought out new approaches to ⇑ Corresponding author. Tel.: +966 1 4676838; fax: +966 1 4670662. E-mail address: [email protected] (A.A. Ansari). http://dx.doi.org/10.1016/j.saa.2014.04.036 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

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for synthesis of luminescent CaMoO4:Eu (core), CaMoO4:Eu@CaMoO4 (core/shell) and CaMoO4:Eu@CaMoO4@SiO2 CaMoO4:Eu@CaMoO4@SiO2 (core/ shell/shell) nanoparticles.  Investigated the effect of surface coating on core nanoparticles.  Photoluminescence properties of core and core/shell/shell nanoparticles have been examined comparatively. The observed emission intensity of electric dipole transition was dominating over magnetic dipole transition.

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synthesize such materials to take advantage of their robust optical properties most effectively [1–5]. Their high luminescence intensity, good thermal and chemical stability makes it an excellent host for luminescent materials under UV and X-ray excitation due to its luminescent –MoO4 tetrahedron cluster [1–9]. Owing to these unique luminescence properties of calcium molybdate, these nanomaterials have promising applications in various fields such

A.A. Ansari et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 30–36

as nanophosphor, solid state laser, white light emitting diodes, optical fibers, scintillator materials, humidity sensors, and catalysis [1–9]. In these applications, thermal, chemical stability and the luminescence characteristics of the nanocrystals are of prime importance to their successful implementation. However, they have important problems of rigorous low luminescent efficiency, which stringently limit their pervading application. Consequently, it will be of great interest to significantly increase the emission intensity in order to deploy their potential applications. In literature, different synthesis strategies have been used for improving the luminescent efficiency of the as-prepared luminescent Ln3+ ion-doped calcium molybdate nanocrystals. Yang et al., used polyethylene glycol (PEG), Tween-80, sodium dodecyl sulfonate (SAS) and cetyltrimethylammonium bromide (CTAB) as surfactants for preparation of CaMoO4:Eu by co-precipitation method [6]. Parchur et al., prepared the CaMoO4:Eu nanoparticles using by urea hydrolysis in ethylene glycol at low temperature and investigated the effect of concentration and temperature on luminescent properties [2,5,10]. Very recently, two groups reported the similar synthesis strategy for preparation of CaMoO4:Ln (where Ln = Pr, Tb and Eu,) nanoparticles. In this preparation method they grow CaMoO4:Ln nanoparticles by the flux growth method using Na2MoO4 as a solvent in the 1350–600 °C temperature range and investigated the luminescent properties at different temperature and pressure under different experimental conditions [3,4]. In Raju et al.’s report, spherulite morphology CaMoO4:Eu nanoparticles were synthesized by hydrothermal process and investigated the influence of crystallinity and temperature on luminescent properties of the as-prepared nanomaterials [1]. The grain size was increased as the annealing temperature increases, resulting observes the enhanced luminescence efficiency of the nanoparticles. In Jin et al.’s method, clew-like shape CaMoO4:Eu,Sm particles were prepared via a facile hydrothermal method directly in surfactant-free environment and examine the photoluminescence properties [7]. In a parallel effort, Chung et al., reported Li+/Tm3+/Yb3+ co-doped CaMoO4 upconversion phosphor prepared by complex citrate–gel method and UC luminescence properties were investigated [8]. However, due to nonradiative decay from defects on the surface of the nanocrystals, the luminescence efficiency all of the above-reported phosphor materials were usually lower compared to the corresponding bulk counterparts. To reduce these defects, the growth of a crystalline shell of a suitable inorganic material around each nanocrystal to form the core– shell structures has been regarded as an effective strategy to improve luminescent efficiency [5,11–14]. This strategy has been successfully applied in semiconductor quantum dots core/shell systems, such as CdSe/ZnS, CdSe/CdS, CdSe/ZnSe and CdS/ZnS nanocrystals [11]. This inorganic crystalline shell efficiently passivates fast nonradiative decay channels originating from surface states. In recent years, the synthesis of core/shell structural luminescent lanthanide nanocrystals as cores and inert host compounds as shells has been reported by several groups [5,12–14]. CaMoO4 as a promising host matrix, applications of these nanocrystals in biological systems have been limited due to lacking groups on the surface of the nanocrystals serving as reactive sites for coupling biomolecules. On the other hand, weak solubility of molybdate would be toxic to biological systems. Hence, much work has to be done to modify and/or functionalize the surface of nanocrystals. A simple but effective method is to grow a silica shell around the nanocrystals, forming the so-called core/shell structures. The silica coating can improve the photo-stability and bio-compatibility of the nanocrystals, due to it contain a number of –OH bonds on the surface of the nanoparticles [15,16]. It is well established method for conjugation of biomolecules to a silica surface. Up to now, very limited works have studied the composite structure of CaMoO4, the interaction between CaMoO4:Eu core

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and SiO2, as well as the interaction between CaMoO4 and semiconductor [9]. However, only a few works have been performed on the preparation and photo luminescence (PL) properties of CaMoO4:Eu@SiO2 core/shell nanocomposites [9]. In the core/shell composites, the SiO2 shell and CaMoO4:Eu core will interact with each other, which affects PL properties of the CaMoO4:Eu@SiO2 nanocomposites. However, its physical nature has not been well understood. Therefore, it is necessary to deeply study the relationship between the PL properties of Eu3+ ions and the SiO2 coating. We report here synthesis, structural, electronic and optical properties of CaMoO4:Eu (core), CaMoO4:Eu@CaMoO4 (core/shell) and CaMoO4:Eu@CaMoO4@SiO2 (core/shell/shell) nanoparticles. The effects of the crystalline size on optical, electronic and structural properties were investigated in detail on the formation of the calcium molybdate products. The as-synthesized products were characterized using X-ray powder diffraction, thermo-gravimetric analysis, optical absorption spectra, optical band gap energy, FTIR, FT-Raman and photoluminescence spectroscopy. Finally, comparative photo luminescent and electronic structural properties were studied for the core and core/shell nanostructures. Experimental section Materials Europium oxide (Eu2O3, 99.99%, Alfa Aesar, Germany), calcium carbonate (CaCO3, 99.99%, E-Merck, Germany), ammonium molybdate ((NH4)6Mo7O244H2O, 99.3%, Acros Organics), tetraethylorthosilicate (TEOS, 99 wt% analytical reagent A.R.), ethylene glycol (EG; E-Merck, Germany), urea (NH2)2CO; E-Merck, Germany), C2H5OH, HNO3 and NH4OH were used as starting materials without any further purification. Nanopure water was used for preparation of solutions. The ultrapure de-ionized water was prepared using a Milli-Q system (Millipore, Bedford, MA, USA). All other chemicals used were of reagent grade. Preparation of CaMoO4:Eu nanoparticles (core) For preparation of Eu-doped CaMoO4 nanoparticles, CaMoO4:Eu was prepared at low temperature 150 °C using urea hydrolysis in EG as a chelating agent. In a typical synthesis 0.6962 g (6.962 mmol) of CaCO3 and 0.0379 g (0.1077 mmol) of Eu2O3 were dissolved together in concentrated nitric acid (HNO3) and heated up to 80 °C to remove excess acid and neutralized by addition of distilled water. 1.2660 g (1.024 mmol) ammonium molybdate dissolved in methanol (50 mL) was mixed up in this forgoing reaction and kept for constant stirring with heating (80 °C) on hot plate for 1 h. 2.0 g urea dissolved in 50 ml EG was introduced into this reaction [5]. The reaction mixture was heated up to 150 °C for 3 h under reflux conditions until the white precipitate was appeared. The synthesized product precipitate was then collected by centrifugation, washed with distilled water and absolute ethanol four times, and dried in oven at 200 °C for 6 h for further characterization. Inductively coupled plasma atomic emission spectroscopy (ICPAES) was used to qualitatively investigate the element contents in as-prepared CaMoO4:Eu nanoparticles. The following concentrations are as (calculated are in under brackets) 4395(35.31%), 7937(63.76%) and 116(0.93%) ppb were observed for Ca, Mo and Eu, respectively, which are in good agreement with the experimental values. Preparation of CaMoO4:Eu@CaMoO4 nanoparticles (core/shell) For the preparation of CaMoO4:Eu@CaMoO4 (core/shell) nanoparticles, similar polyol process was used as discussed above.

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X-ray diffraction (XRD) of the powder samples was examined at room temperature with the use of PANalytical X’Pert X-ray diffractometer equipped with a Ni filtered using Cu Ka (k = 1.54056 Å) radiations as X-ray source. Raman spectra were recorded on a Jobin Yvon Horiba HR800 UV Raman microscope using a HeNe laser emitting at 632.8 nm. The UV/Vis absorption spectra were measured a Perkin–Elmer Lambda-40 spectrophotometer, with the sample contained in 1 cm3 stoppered quartz cell of 1 cm path length, in the range 190–600 nm. Thermogravimetric analysis (TGA) was performed with TGA/DTA, Mettler Toledo AG, Analytical CH-8603, Schwerzenbach, Switzerland. FTIR spectra were recorded on a Perkin–Elmer 580B IR spectrometer using KBr pellet technique in the range 4000–400 cm1. Photoluminescence (PL) spectra were recorded on Horiba Synapse 1024  256 pixels, size of the pixel 26 lm, detection range: 300 (efficiency 30%) to 1000 nm (efficiency: 35%). A slit width of 100 microns was employed, ensuring a spectral resolution better than 1 cm1. All measurements were performed at room temperature.

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CaMoO4:Eu@CaMoO4@SiO2 core/shell/shell nanoparticles were prepared through a versatile solution sol–gel method as follows. The synthesized CaMoO4:Eu@CaMoO4 (core/shell) nanoparticles (50 mg) were well dispersed in 50 mL of deionized water, ethanol (70 mL) and concentrated aqueous ammonia (1.0 mL) in a threeneck round-bottom (RB) flask. Afterward, 2.0 mL of tetraethyl orthosilicate (TEOS) was added drop-wise in 2 min, and the reaction was allowed to proceed for 4 h under continuous mechanical stirring. After 3 h of continuous stirring at room temperature, the silica-coated CaMoO4:Eu@CaMoO4 core/shell/shell nanoparticles were separated by centrifugation, washed four times with ethanol and dried at room temperature [15–18]. The ICP-AES analysis results of Ca, Mo, Eu and Si shows the following concentrations 5599(35.02%), 9076(56.77%), 137(0.86%) and 1176(7.36%) ppb, respectively, which is confirming the chemical composition of as-prepared CaMoO4:Eu@CaMoO4@SiO2 core/ shell/shell nanoparticles.

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Preparation of silica coated CaMoO4:Eu@CaMoO4@SiO2 nanoparticles (core/shell/shell)

the CaMoO4:Eu nanoparticles are well-crystallized and the patterns are in good agreement with a scheelite-type tetragonal structure (space group I41/a (88), in the international tables of crystallography, and point-group symmetry C64h; JCPDS Card No. 29-0351) [1,2,19,20]. In this structure, the molybdenum atoms are coordinated to four oxygen, forming –[MoO4]– clusters with tetrahedral configuration and tetrahedral polyhedra (4 vertices, 4 faces, and 6 edges). These [MoO4] clusters are slightly distorted into the matrix, as a consequence of the O–Mo–O bond angles (108.3° and 111.8°), while the calcium atoms are bonded to eight oxygens, resulting in –[CaO8]– clusters with scale nohedral configuration and snub dispenoide polyhedra (8 vertices, 12 faces, and 18 edges). Any diffraction peaks correspond to the secondary phases not verified. Thus, these results indicate that the products powders processed in thermal decomposition of urea are highly crystalline, pure and ordered at long range. As illustrated in Fig. 1c the diffraction peaks of the CaMoO4:Eu@CaMoO4@SiO2 core/shell/shell nanoparticles are sharp with decrease relative diffraction intensity with respect to the CaMoO4:Eu nanoparticles, because of the surface coating of the amorphous silica shell around the nanoparticles. Whereas, there is no defined characteristic peak for amorphous SiO2 is observed after one coating process due to the thinner property of the SiO2 layer. It suggests that the presence of the SiO2 coating significantly influence nanoparticle crystallinity, resulting the change in reflection intensity of XRD pattern. According to the Scherrer equation, the strongest peak (1 1 2) at 2h  28.7° and the peak (2 0 4) at 2h  47.25° were used to calculate the average crystallite size of the core, core/shell and core/shell/shell nanoparticles, determined to be around 20.8 ± 5, 33.5 ± 5 and 46.1 ± 5 nm, respectively. Thermal stability of the as-synthesized core, core/shell and core/shell/shell nanoparticles was determined by thermo-gravimetric analysis between ambient temperatures to 800 °C under N2 atmosphere with a heating rate of 10 °C/min. All thermograms show the similar decomposition trend of materials in two steps. In the first step the TGA indicated a minor weight loss (2%) between 25 and 325 °C, which is due to the evaporation of small amount of dangling bonds present on the surface of the nanoparticles (Fig. 2). In the second step all thermograms reveals a sluggish weight loss (4–8%) between 325 and 800 °C, it may be due to elimination or burning of surface modified amorphous silica shell [21]. Furthermore, it could be related to crystallization of scheelite type tetragonal phase of CaMoO4:Eu and further confirmed by XRD result. However, the mass of the precursor still decreases slowly till the

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1.00 g of CaMoO4:Eu were dispersed in 4 mL of distilled water containing 20 ml of EG and 2.0 g urea with constant stirring for 30 min. Typically, 0.7236 g (7.19 mmol) CaCO3 was dissolved in HNO3 acid and the excess amount of nitric acid was evaporated on hot plate by adding double distilled water. Then a solution of calcium carbonate and 1.2764 g (1.027 mmol) ammonium molybdate dissolved in methanol was injected into the foregoing mixed system, and the suspension was refluxed at 150 °C for 3 h until the precipitation is occurred. This white precipitate was centrifuged and washed four times with methanol to remove excess un-reacted reactants. The core/shell nanoparticles were collected after centrifugation and allowed to dry in ambient temperature for further characterization. The ICP-AES analysis shows the following concentrations 16,313(25.85%), 46,526(73.72%) and 276(0.44%) ppb for Ca, Mo and Eu, respectively, which confirmed the chemical composition of the as-prepared CaMoO4:Eu@CaMoO4 core/shell nanoparticles.

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Results and discussion Crystal structure and thermal properties Fig. 1 shows the XRD patterns of core (a), core/shell (b) and core/shell/shell (c) nanoparticles. The results of XRD indicate that

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temperature higher than 800 °C, revealing that the amorphous silica and shell of CaMoO4 is decomposed to crystalline CaMoO4:Eu. After 800 °C, no obvious weight loss was observed. The thermograms also confirmed the phase purity of the as-synthesized nanoparticles. Optical properties Optical absorption spectra of core, core/shell and core/shell/ shell nanoparticles dispersed in distilled water were recorded at room temperature in the range 200–800 nm as given in Fig. 3. As seen in Fig. 3a two absorption bands are recorded in the spectrum of CaMoO4:Eu (core) nanoparticles at 208 and 229 nm, which is dispersed in water. In the absorption spectra presented in Fig. 3 the concentrations of the colloidal solutions were 0.1 g/ml. It is observed that the optical absorption spectra of the prepared CaMoO4:Eu nanoparticles is closely similar to that (kmax = 210 nm) of the reported CaMoO4 nanoparticles [22]. An absorption band at kmax = 208 nm is attributed to the charge transfer transition from the oxygen (2p) ligands to the central molybdenum atom inside the MoO2 4 ion [19–20,22]. An observed shoulder at kmax = 229 nm attributed to the transition between the ground state and the charge-transfer state of the Eu–O bond (4f6 ? 4f72p1) [23,24]. The stability of the half filled 4f7 is responsible for the relatively

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low energy of these transitions. The band at 229 nm is weak because of the spin forbidden nature of the transition [24]. A clear red shift (kmax = 240 nm) in the UV absorption band of CaMoO4:Eu@CaMoO4 is observed relative to the as-prepared core (CaMoO4:Eu) nanoparticles. The shift in the absorption band toward longer wavelength indicates the formation of new chemical band structure around the surface of core nanoparticles, which enhanced the absorption band toward longer wavelength. As observed in Fig 3c the absorption spectrum of silica coated CaMoO4:Eu@CaMoO4@SiO2 core–shell nanoparticles colloidal solution show shifts longer wavelength compared to the CaMoO4:Eu and CaMoO4:Eu@CaMoO4 nanoparticles. The shift exhibited by the absorption band could be correlated to a change in coordination geometry and symmetry of the material after silica core–shell formation. This reflects that the Si–OH donor group interacts with the d-electrons of the transition metal ion, which can coordinate and enter the coordination sphere of CaMoO4:Eu@CaMoO4, bringing a change in the geometry of the metal ion [15,16]. The solubility of the nanoparticles is enhanced after silica coating and silica coated core–shell nanoparticles exhibited good chemical and photochemical stability, and a clear colloidal solution was formed. To examine the optical absorption properties and their correlation between the band gap energies and the grain sizes of the asprepared calcium molybdate nanoparticles, their UV/Vis spectra are measured in distilled water suspensions as shown in Fig. 4. The optical energy band-gap, Eg, for these nanoparticles is determined from the sharply increasing absorption region according to Tauc and Menth’s method [25]. Fig. 4 shows a plot of (ahm)2 vs. the photon energy (hm). In the high energy region of the absorption edge, (ahm)2 varied linearly with hm and the straight line behavior in the high energy region is used as prime evidence for a direct band-gap. The experimentally estimated band gaps are 4.796, 4.058, and 2.655 eV for the core, core/shell and core/shell/ shell nanoparticles, respectively. This observed band gap energy for CaMoO4:Eu nanoparticles is comparable within the range reported in the literature [19–20,22,26–28]. The increase in the band gap can be attributed to defects, local bond distortion, intrinsic surface states, and interfaces that yield localized electronic levels in the forbidden band gap. We believe that this significant difference is attributed to surface and interface intrinsic defects and quantum confinement effects linked to the nano-octahedrons. In this respect, we summarized the reported results in Table 1 and compare our obtained band gap results by using various synthetic techniques. It can be seen from Table 1 that the band gap energy is different for different authors. The band gap energy of the samples prepared with citrate base complex rout changed a little when the particle size is reduced from 53 to 20 nm [22,28]. Our core-nanoparticles, however, show a great increase of the band gap energy (Egap) when the particle size is reduced; we attribute the blue shift in absorption spectra to the quantum size effect. Fig. 5 illustrates the FT-IR spectra of core (a), core/shell (b) and core/shell/shell (c) nanoparticles synthesized by the urea based thermal decomposition method. All the spectra exhibited a weak broad band at 3445 cm1 is due to –O–H stretching vibration of physically absorbed water molecules on the surface of nanoparticles [15–18]. A strong absorption band observed at 807 cm1 (Eu mode) is related to the m3(F2) internal mode originated from the asymmetric stretching vibrations in the molybdate (MoO4) clusters [19,26]. Another sharp weak absorption band located at 428 cm1 (Eu mode) is generally associated with the m4(F2) internal modes due to the presence of asymmetric bending vibrations involved in the O–Mo–O bonds [5,10,19,26]. After silica modification on the surface of CaMoO4:Eu@CaMoO4 (core/shell) nanoparticles, these asymmetric stretching vibration modes are broader and more stronger due to mainly characteristic absorption bands of silica, such as Si–O–Si stretching, Si–OH stretching, Si–O bending, and

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Table 1 Comparative results between the optical band gap energy of CaMoO4 obtained in this work and those reported in the literature. Methods

Particle size

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Reference

Polymeric precursor method Combustion process Citrate complex route Coprecipitation method Microwave-hydrothermal method Citrate complex polymerization Polyol thermal decomposition of urea

70–251 nm (TEM) 25 nm (XRD) 20–30 nm (TEM) 1.25–4.75 lm (TEM) 50–61 nm (SEM) 25–53 (XRD) 20–46 nm (XRD)

4.0 3.7 4.7 3.6–3.4 3.4–3.9 4.8–5.1 2.6–4.7

[26] [25] [21] [19] [20] [27] Present work

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Si–O–Si bending, exhibits the absorbance in similar field [15–18]. Therefore, these bands are much stronger than the calcium molybdate nanoparticles. Two very weak bands in the spectrum of CaMoO4:Eu@CaMoO4@SiO2 core/shell/shell (c) nanoparticles at 1622 and 1408 cm1 are observed, which ascribed to the bending mode of Si–O–H groups. From these absorption bands, it is clearly known that SiO2 was successfully covered onto the surface of the core/shell nanoparticles. Raman spectroscopy was used for further structure examination by means of vibrational analysis of the as-prepared three type of calcium molybdate nanoparticles. Raman spectra of the core (a), core/shell (b) and core/shell/shell (c) nanoparticles are given in Fig. 6. The structure of the spectra is the same as the one commonly reported for scheelite type structures (crystal structure has C64h symmetry) [19,20,27]. The external or lattice phonons

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Raman Shift (cm-1) Fig. 6. FT-Raman spectra of the as-prepared (a) CaMoO4:Eu(b) CaMoO4:Eu@CaMoO4 and (c) CaMoO4:Eu@CaMoO4@SiO2 core–shell nanoparticles.

correspond to the motion of the Ca cation and the rigid molecular unit (translational modes). Vibrational modes characteristic of the scheelite phase in the tetrahedral structure are observed at room temperature. Seven internal modes, corresponding to the vibration of the MoO2 4 tetrahedron, are observed in CaMoO4. These nanoparticles presented several peaks referring to the Raman-active internal modes of MoO4 tetrahedra: t1(Ag), t3(Bg), t3(Eg), t4(Bg5), t4(Ag⁄), t2(Eg.), t2(Ag.), R(Bgd), R(Egw) and external T(Bgs Egw Egs) mode [19,20,27]. This structure is well organized in the short and long range. Moreover, the Raman spectra exhibited intense and sharp bands, indicating a strong interaction between the O–Ca–O and O–Mo–O bonds in the clusters. Table 2 shows a comparative Raman active modes results of these three samples. These results are verified from the published literature reports that all Raman-active modes are characteristic of CaMoO4 phase in

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b

5

To explore the possibilities of luminescent properties of europium ion-doped calcium molybdate nanoparticles, we carried out PL measurements at room temperature in solid phase. The room temperature emission spectra of core (a), core/shell (b) and core/ shell/shell (c) nanoparticles are shown in Fig. 7. Here, in our investigation, room temperature photoluminescence spectra are performed with an excitation wavelength (kex = 325 nm) with constant all experimental conditions such as density of emitting centers, excitation wavelength, light source powder and excitation/emission slits. The emission spectra exhibits six groups of emission lines, the emission spectrum is composed of a group of line speaking at about 536–539, 553–562, 579–596, 614, 645–655 and 694–703 nm, which are ascribed to the 5D1 ? 7F1, 5 D1 ? 7F2, 5D0 ? 7F1, 5D0 ? 7F2, 5D0 ? 7F3 and 5D0 ? 7F4 transition of the Eu3+ ions, respectively [1,2,10,29,30]. Among these emission peaks, the 5D0–7F2 (614 nm) peak is dominant in comparison with any other peaks, which is a hypersensitive force dielectric-dipole transition. It is known that the f–f transition arising from a forced electric dipole is forbidden but becomes partially allowed when the rare earth ion is situated at a low symmetry site with no inversion center. In all these nanophosphors, most of the europium ion emission transitions are found to be strongly perturbed upon doping in calcium molybdate lattice. These emission bands show multiple splitting and broadening characteristics compared with other Eu-doped nanocrystalline materials, revealing that Eu ions are occupying the Ca2+ sites without inversion symmetry. Furthermore, the broadening of spectral transitions can also be due to the energy transfer from MoO2 4 to the europium ions [1,2,10]. The emission spectrum is observed with the strongest emission peak at 614 nm, which indicates that the tetragonal phase CaMoO4 is produced. Regarding the peak position in all spectra, no shift is found when the CaMoO4 and SiO2 shells are formed around the surface of core nanoparticles. However, a difference in peaks intensity is evident, see in Fig. 7. A significant luminescent intensity enhancement is observed for the CaMoO4:Eu@CaMoO4 core/ shell nanoparticles in comparison with the CaMoO4:Eu core nanoparticles. This enhancement of emission intensity may be attributed to the fact that a significant amount of non-radiative centers existing on the surface of core nanoparticles are eliminated by the shielding effect of the undoped-CaMoO4 shell [5,12–14]. In this core/shell structure, the distance between the luminescent lanthanide ions and the surface quenchers is increased, thus reducing the nonradiative pathways and suppressing the energy quenching in energy-transfer processes. Furthermore, our group has shown strong evidence for the formation of core–shell lanthanide nanoparticles using the afore-

425

7

Photoluminescence properties

a b c

Relative Intensity (a.u.)

Relative Intensity (a.u.)

agreement with the reported in the literature [19,20,27,28]. Therefore, these results confirm the tetragonal structure for CaMoO4 powders. The small variations can be associated with the preparation method, average crystal size and structural order degree. The presence of Raman-active modes can be used to evaluate the structural order at short-range of the materials.

600

650

700

Wavelength (nm) Fig. 7. Photoluminescence spectra of the as-prepared (a) CaMoO4:Eu(b) CaMoO4:Eu@CaMoO4 and (c) CaMoO4:Eu@CaMoO4@SiO2 core–shell nanoparticles.

mentioned synthesis procedure, and we conclude the same is true for these core/shell nanoparticles [5]. It is interesting to note that the luminescent intensity for the CaMoO4:Eu@CaMoO4@SiO2 core–shell nanoparticles is higher in respect to the CaMoO4:Eu@CaMoO4 core–shell nanoparticles. The effect of crystallite size on PL intensity is clearly indicated in Fig 7c. The PL intensities at 614 nm in emission spectra gradually increase with increasing crystallite size, as shown in Fig. 7a–c, indicating the strong effect of crystallite size. Fujihara and Tokumo also reported that the luminescent properties of Eu are determined by the crystallinity and the lattice symmetry of the host crystal, rather that local distortion induced by substitution [31]. Because the surface-to volume ratio increases with decrease of the particle size, there is more chance that the atoms located near the surfaces lose their inversion symmetry. Therefore, the transition probability of 5 D0 ? 7F2 increased [5,32,33]. Here, it is believed that the amount of emitting Eu ions per SiO2 particle are increases after silica shell modification, resulting in the enhancement of the luminescent intensity. It also indicated that the silica coating decrease the surface defects of nanoparticles and increase their intensity. In generally, defects have serious implications for luminescent materials for they provide non-radiative recombination routes of electrons and holes, and decrease the emission intensity of phosphors. The coating of phosphors may reduce the defects of phosphors. Furthermore, the emission spectrum of silica coated core–shell nanoparticles is also uplifted on the base line at the low angle region, which may be caused by the diffraction between irregular molecule layers of amorphous SiO2 [34,35]. The Silica surface modified core–shell still outperform the non-silica coated CaMoO4:Eu and CaMoO4:Eu@CaMoO4 core–shell nanoparticles. This enables such biocompatible and easily dispersible nanoparticles to be employed in bio-applications without any adverse toxic effects, common with other light emitting nanoparticles that contain heavy metals (quantum dots).

Table 2 Comparative experimental Raman-active modes results of CaMoO4:Eu(b) CaMoO4: Eu@CaMoO4 and (c) CaMoO4:Eu@CaMoO4@SiO2 core–shell nanoparticles. Samples

Bg(s)

Eg(w)

Eg(w)

Bg(d)

Ag(.)

Eg(.)

Ag(⁄)

Bg(5)

Eg()

Bg()

Ag()

CaMoO4:Eu CaMoO4:Eu@ CaMoO4 CaMoO4:Eu@ CaMoO4@SiO2

– – –

– – –

141 141 141

191 191 191

320 320 321

– – –

389 389 390

– – –

791 790 791

843 843 844

876 876 876

36

A.A. Ansari et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 30–36

Conclusions In conclusion, we successfully synthesized the CaMoO4:Eu, CaMoO4:Eu@CaMoO4 and CaMoO4:Eu@CaMoO4@SiO2 core–shell nanoparticles using polyol process. It has been demonstrated that the core–shell structure significant influence the crystal structural and opto-electronic properties of the materials. The optical absorption band edges were shifted to longer wavelength from core to core–shell nanoparticles colloidal solution because of shell formation around the core nanoparticle. The experimentally estimated optical band-gap were 4.796, 4.058, and 2.655 eV for the CaMoO4:Eu, CaMoO4:Eu@CaMoO4 and CaMoO4:Eu@CaMoO4@SiO2 core–shell nanoparticles, respectively. The decrease band gap energy from core to core–shell nanoparticles, which attributed to the quantum-size effect due to the growth of the crystalline and amorphous CaMoO4 and SiO2 shell improve the crystallinity of the prepared materials, respectively. A remarkable improvement of luminescent intensity was achieved after grow a crystalline shell of CaMoO4 around the surface of CaMoO4:Eu core nanoparticles. These results confirmed that CaMoO4:Eu@CaMoO4@SiO2 core– shell nanoparticles are highly promising candidates for PL applications. References [1] G.S.R. Raju, E. Pavitra, Y.H. Ko, J.S. Yu, J. Mater. Chem. 22 (2012) 15562. [2] A.K. Parchur, R.S. Ningthoujam, S.B. Rai, G.S. Okram, R.A. Singh, M. Tyagi, S.C. Gadkari, R. Tewari, R.K. Vatsa, Dalton Trans. 40 (2011) 7595. [3] E. Cavalli, P. Boutinaud, R. Mahiou, M. Bettinelli, P. Dorenbos, Inorg. Chem. 49 (2010) 4916. [4] S. Mahlik, M. Behrendt, M. Grinberg, E. Cavalli, M. Bettinelli, J. Phys.: Condens. Matter 25 (2013) 05502. [5] A.K. Parchur, A.I. Prasad, A.A. Ansari, S.B. Rai, R.S. Ningthoujam, Dalton Trans. 41 (2012) 11032. [6] Y. Yang, X. Li, W. Feng, W. Yang, W. Li, C. Tao, J. Alloys Comp. 509 (2011) 845. [7] Y. Jin, J. Zhang, Z. Hao, X. Zhang, X.J. Wang, J. Alloys Comp. 509 (2011) L348.

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