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Feb 27, 2014 - Anees A. Ansari • Manawwer Alam •. A. K. Parchur. Received: 24 October 2013 / Accepted: 6 February 2014 / Published online: 27 February ...
Appl. Phys. A (2014) 116:1719–1728 DOI 10.1007/s00339-014-8308-4

Nd-doped calcium molybdate core and particles: synthesis, optical and photoluminescence studies Anees A. Ansari • Manawwer Alam A. K. Parchur



Received: 24 October 2013 / Accepted: 6 February 2014 / Published online: 27 February 2014 Ó Springer-Verlag Berlin Heidelberg 2014

Abstract CaMoO4:Nd (core), CaMoO4:Nd@CaMoO4 and CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles were synthesized using polyol method under urea hydrolysis. X-ray diffraction and thermogravimetric analysis were employed to examine the structural and thermal properties of the as-prepared core and core–shell nanoparticles. Optical properties of core and core–shell nanoparticles were observed to investigate the influence of surface coating on the spectra of as-prepared nanomaterials in terms of ultraviolet/visible (UVVis) absorbance, FTIR, Raman and emission spectra. The optical band gap energy calculated from the UV-Vis absorption spectrum for CaMoO4:Nd, CaMoO4:Nd@CaMoO4 and CaMoO4:Nd@CaMoO4@SiO2 nanoparticles was 3.09, 2.06 and 1.26 eV, respectively. The photoluminescence spectra of the samples showed broad charge transfer emission band of [MoO4]2- along with sharp transitions of neodymium ion in the visible and near infrared regions, respectively.

1 Introduction Recently, neodymium(Nd3?)ion-doped laser nanomaterials have been much in focus due to their unique optical

A. A. Ansari (&) King Abdullah Institute for Nanotechnology, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia e-mail: [email protected] M. Alam Research Center, College of Science, King Saud University, P.O. Box 2455, Riyadh, Saudi Arabia A. K. Parchur Department of Physics, Banaras Hindu University, Varanasi 221005, India

properties in NIR region [1–3]. Nd3? has abundant pumping levels, a four-level scheme and both relatively high absorption and emission cross-sections [1, 2]. In addition, the absorption peaks at around 850 and 1,050 nm well match with the wavelength of the laser diode pump source. It is well known that the luminescence properties of lanthanides depend considerably on the host matrix [4, 5]. Among various host materials, calcium molybdate (CaMoO4) possesses advantageous properties over fluoridebased systems, resulting from a low phonon threshold energy and more stable physical and chemical properties compared also to other oxide materials [6–8]. Moreover, the Mo6? ions in CaMoO4 matrices have strong polarization induced by large electric charge and small radius, consequently decrease symmetries, and enhance stark energy splitting in the crystal field [8–10]. Therefore, we considered CaMoO4 to be suitable host for NIR matrices. Hence, Nd3?-doped CaMoO4 nanoparticles have tremendous promise as indicators for numerous applications such as biological labeling, phosphors in lamps, solid-state lasers, white light-emitting diodes, display devices and optical amplification materials [1, 2, 6–10]. In this class materials, most previous work reported on lanthanides (Pr3?, Sm3?, Eu3?, Tb3? and Tm3?/Yb3?)doped CaMoO4 nanoparticles mainly focused on the synthesis, characterization and photoluminescence properties and their potential application in several technologies. Yang et al. [11] investigated the influence of surfactants on morphology and luminescent properties of CaMoO4:Eu3?red phosphors and observed that, surfactant did not change the crystal structure but greatly influence their morphology, resulting in the change of luminescent intensity. Jin et al. [12] synthesized Eu3? and Sm3? codoped CaMoO4 microclews via a facile hydrothermal method directly in surfactant-free environment. Recently,

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the luminescence properties of some scheelite-type tetragonal Tb3?-doped CaMoO4 at different temperatures under different experimental conditions [13] have been investigated. Parchur et al. [14, 15] studied systematically the luminescent properties of Eu3? and Tb3?-doped CaMoO4 and discussed the annealing effects on luminescent properties. In another report, Mahlik et al. [10] observed photoluminescence spectra and luminescence kinetics of Pr3?- and Tb3?-doped CaMoO4 samples at different pressures. Raju et al. [8] reported a facile and efficient strategy for the synthesis of stable Eu3?-doped CaMoO4 spherulite and proposed their applications for optical devices. Chung et al. [16] concluded that Li?/Tm3?/Yb3?)-doped CaMoO4 nanophosphors would be promising candidates for display devices. Luminescence properties of SiO2@CaMoO4:Eu3? are reported to display intense tricolor photoluminescence under ultraviolet excitation [17]. Although, several luminescent ion-doped CaMoO4 materials have been investigated in detail, there is still a lack of synthesis and optical properties of Nd3? ion-doped CaMoO4 and a shell of undoped CaMoO4 around the surface of doped nanoparticle [13–17]. A shell of undoped CaMoO4 was grown around each nanoparticle to form the core–shell structure, which has been regarded as an effective strategy to improve luminescent efficiency. This crystalline layer of CaMoO4 protects the non-radiative decay from defects on the surface of the nanoparticles. This strategy has been successfully applied in semiconductor quantum dots and lanthanide ions-doped upconversion nanomaterials [18]. In recent years, the synthesis of core–shell structural luminescent materials with luminescent lanthanide nanocrystals as cores and inert host compounds as shells has been reported by several groups [1, 15, 18–21]. These systems doped with lanthanide ions are based on two materials with similar lattice constants to avoid the formation of defects at the core–shell interface. In this structure, the distance between the luminescent lanthanide ions and the surface quenchers is increased, thus reducing the non-radiative pathways and increasing the quantum yield of nanomaterials. To have biological applications these nanoparticles must be water soluble, biocompatible and photostable [22, 23]. However, most inorganic nanoparticles are hydrophobic in solutions. It is therefore necessary to modify the surfaces of these nanoparticles with active functional groups or ligands to improve their solubility in solutions so that these nanoparticles can be bonded to biomolecules as required. The use of a silica coating over CaMoO4:Nd3?@CaMoO4 nanoparticles is an attractive alternative because the surface chemistry of silica spheres is well documented and silica is known to have benign effects in biological systems. Some reports in literature have been discussed with silica coating being a protective matrix because of its reliable chemical stability, optical

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transparency, chemical inertness, cheapness, biocompatibility, non-toxicity, photochemical stability even under laser photolysis, and easily transferred into a wide range of solvent [3, 22–24]. In this work, we report the polyol synthesis and optical properties of calcium molybdate: CaMoO4:Nd3?, CaMoO4:Nd3?@CaMoO4, and CaMoO4:Nd3?@CaMoO4@SiO2 core–shell nanoparticles. The thin silica shell was coated over the CaMoO4:Nd3?@CaMoO4 core using the modified Stober method [25]. The as-formed nanocrystals can be well dispersed in distilled water to form clear colloidal solution. In addition, the structural, optical (absorption, energy band gap and luminescence properties) and thermal stability of the synthesized nanoparticles were also studied systematically in this paper.

2 Experimental section 2.1 Materials Neodymium oxide (99.99 %, Alfa Aesar, Germany), calcium carbonate(CaCO3, 99.99 %, E-Merck, Germany), Ammonium molybdate((NH4)6Mo7O244H2O, 99.3 %, Acros Organics), Tetraethyl-orthosilicate (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 the 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. 2.2 Preparation of CaMoO4:Nd nanoparticles For typical preparation of Nd3?-doped CaMoO4 nanoparticles, CaMoO4:Nd3? was prepared at a low temperature of 150 °C using urea hydrolysis in ethylene glycol as a chelating agent. 0.7146 g of CaCO3 and 0.0121 g of Nd2O3 were dissolved together in concentrated nitric acid(HNO3) and heated up to 80 °C to remove excess acid and neutralized by addition of double distilled water. 1.27 g 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 [15]. 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 several times, and dried in oven at 200 °C for 6 h for further characterization.

Nd-doped calcium molybdate core

2.3 Preparation of CaMoO4:Nd@CaMoO4 core–shell nanoparticles For the preparation of CaMoO4:Nd@CaMoO4 core–shell nanoparticles, similar polyol process was used as discussed above. 1.00 g of CaMoO4:Nd was dispersed in 4 ml of distilled water containing 1 g of EG and 2 g urea with constant stirring for 30 min. Typically 0.723 g CaCO3 was dissolved in HNO3 acid and the excess amount of nitric acid is evaporated on hot plate by adding double distilled water. Then a solution of calcium carbonate and 1.276 gm 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 many 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. 2.4 Preparation of silica-coated CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles The CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles were prepared through a versatile solution sol–gel method as follows [3, 22, 24]. The synthesized CaMoO4:Nd@CaMoO4 nanoparticles (50 mg) were well dispersed in a mixed solution of deionized water (50 mL), ethanol (70 mL) and concentrated aqueous ammonia (1.0 mL) in a three-neck round-bottom flask. Afterward, 1.0 mL of tetraethyl orthosilicate (TEOS) was added dropwise in 2 min, and the reaction was allowed to proceed for 3–4 h under continuous mechanical stirring. After 3 h of continuous stirring at room temperature, the silicacoated CaMoO4:Nd@CaMoO4 core–shell nanoparticles were separated by centrifugation, washed several times with ethanol and dried at room temperature. 2.5 Characterization The 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 filter using ˚ ) radiations as X-ray source. Raman Cu Ka (k = 1.54056 A 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 of 190–600 nm. Thermogravimetric analysis (TGA) was performed with TGA/DTA, Mettler Toledo AG, Analytical CH-8603, Schwerzenbach,

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Switzerland. The FTIR spectra were recorded on a PerkinElmer 580B IR spectrometer using KBr pellet technique in the range of 4,000–400 cm-1. The photoluminescence (PL) spectra were recorded on Horiba Synapse 1024 9 256 pixels, size of the pixel 26 microns, detection range 300 (efficiency 30 %) to 1,000 nm (efficiency 35 %). A slit width of 100 microns was employed, ensuring a spectral resolution better than 1 cm-1. All measurements were performed at room temperature.

3 Results and discussion 3.1 Structural properties Neodymium ion-doped calcium molybdate (core) nanoparticles with their surface coating CaMoO4 and SiO2 core–shell nanoparticles were synthesized following previous reports with some slight modifications [3, 22, 24]. These core nanoparticles have lower luminescence efficiency due to quenching of the emitting surface ions by ligands and solvents. To improve the luminescence efficiency, an undoped shell of CaMoO4 was grown over the core CaMoO4:Nd nanoparticles, which shields them from ligand and solvent effects. X-ray diffraction pattern was employed to examine the crystalline nature and product purity of the prepared nanoparticles. Figure 1 shows the XRD patterns of core and core–shell nanoparticles. From the XRD patterns, we can see that all of the reflections of core and core–shell nanoparticles are identified to the scheelite-type tetragonal ˚, CaMoO4 structure with lattice parameters of a = 5.219 A ˚ and c = 11.412 A [6–8]. Meanwhile, all peak positions are well matched with those of a tetragonal phase structure known from the bulk CaMoO4 crystal (JCPDS 29-0351) ˚ , and with crystal cell parameters of a = 5.239 A ˚ [14, 15]. No extra impurity phases are c = 11.381 A detected. It indicates that Nd3?-doped CaMoO4 was formed successfully through the decomposition process of urea and chelating agent EG. As illustrated in Fig. 1c, the diffraction peaks of the silica-coated core–shell nanoparticles are highly intense with broadening and shift gradually toward larger angle as compared to the core CaMoO4:Nd nanoparticles. The broadening and shifting of XRD peaks are mainly due to three phenomena: particlesize effect, instrumental broadening, and strain broadening. Instrumental broadening has little effect on the particle size but the effect of strain broadening is noticeable. Furthermore, it may be due to little change in the crystal phase of the as-prepared core–shell nanoparticles, owing to uncontrolled growth of silica around the surface of CaMoO4:Nd@CaMoO4 core–shell nanoparticles, which induced the variation of crystallization temperature of CaMoO4:Nd. In

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100 99

c

97 96 95

a

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b

93 92

c

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323 400 316

116 215 312 224

004 200 211 114 213 204 220

b

Weight loss(%)

SiO2

101

Relative Intensity(a.u.)

98

90 0

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a

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Temperature (°C)

15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

2θ θ(Angle) Fig. 1 X-ray diffraction pattern of a CaMoO4:Nd, b CaMoO4: Nd@CaMoO4 and c CaMoO4: Nd@CaMoO4@SiO2 core–shell nanoparticles

addition, a broad band observed at around 2h = 19o in Fig. 1c belongs to the amorphous SiO2 [26]. It is suggested that the silica framework expanded the nanopores structure on the surface of CaMoO4:Nd@CaMoO4 core–shell nanoparticles and rearranged the Si–O-Si network structures without any impurities [24, 26–28].The average crystallite size of the synthesized products CaMoO4:Nd, CaMoO4:Nd@CaMoO4 and CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles was calculated from the full width at half maximum (FWHM), using the Scherrer formula for the major (112) plane and is found to be *20 and *34 and 41 nm, respectively. Comparative thermo-gravimetric analysis of Nd3?doped CaMoO4, CaMoO4:Nd@CaMoO4 and CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles under nitrogen with heating rate at 10 °C/min was carried out from ambient to 800 °C. Comparative thermal analysis of the core and core–shell nanoparticles shows remarkable thermal stability. The thermograms of three nanoparticles are closely similar, showing two stepwise decompositions. As seen in Fig. 2a–c, *3 % weight loss before the temperature gets to 340 °C is due to the evaporation of small amount of absorbed crystalline water or organic moiety in different bonding state for the present complex precursor system. In the second decomposition step, a sluggish weight loss was observed in all three thermograms from 340 to 800 °C. The slight difference in phase transition may be due to difference in duration of annealing at particular temperature. In the case of CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles, a sharp weight loss

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Fig. 2 Thermo-gravimetric analysis of a CaMoO4:Nd, b CaMoO4:Nd@CaMoO4 and c CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles

of the precursor occurs in the TG curve from 340 to 800 °C. It indicates the burning of surface-coated amorphous silica, which is also confirmed by XRD analysis. 3.2 Optical studies Ultraviolet/visible spectroscopy was employed to characterize the optical properties of the synthesized CaMoO4:Nd3?, CaMoO4:Nd@CaMoO4 and CaMoO4:Nd@ CaMoO4@SiO2 core–shell nanoparticles recorded in deionized water at room temperature (samples dispersed in distilled water with the help of an ultrasonic bath before the absorption spectra were measured), within the range of 200–600 nm as given in Fig. 3a–c. From the absorption spectra, it is clear that the samples absorb heavily in the UV region. A broad absorption band between 200 and 400 nm is attributed to charge transfer from the oxygen (2p) electrons that move into the central molybdenum atom inside the MoO42- ion [29]. It is observed that the optical absorption spectra of the prepared CaMoO4:Nd3? nanoparticles are closely similar to that (kmax = 270 nm) of the reported CaMoO4 nanoparticles (in size ranging from 10 to 30 nm) [28]. The absorption edge of CaMoO4:Nd3? nanoparticles spectrum shifts toward shorter wavelength in respect to bulk CaMoO4, which indicates that the grain size of nanoparticles decreases. This kind of blue shifting phenomenon in absorption spectrum of CaMoO4 nanomaterial has discussed in literature by many investigators [6, 7, 9, 30]. The absorption spectra of CaMoO4:Nd@CaMoO4 and their silica-coated CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles exhibit distinctively different shapes of bands in colloidal phase in the UV region. Significant changes in the

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Absorbance

Nd-doped calcium molybdate core

b c a 200

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Wavelength(nm) Fig. 3 UV-Vis absorption spectra of a CaMoO4:Nd, b CaMoO4:Nd@CaMoO4 and c CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles suspended in de-ionized water

absorption band and red shift are observed in both spectrums, which are due to modifying the surface properties of the nanoparticles. This red shift in the absorption band could be then correlated to a change in coordination geometry and symmetry of the material. This reflects that the Si–OH donor group interacts with the d-electrons of the molybdate ion, which can coordinate and enter the coordination sphere of CaMoO4:Nd@CaMoO4, bring a change in the geometry of the metal ion. Furthermore, it may be also explained by quantum confinement effects, i.e., as the size of the nanoparticles increases, the energy gap narrows, so that the absorption peak shifts to a lower energy. These observations indicate the successful coating of CaMoO4 layer and after that silica layer on the surface of CaMoO4:Nd3? core nanoparticles. It is also observed that the well-known 4f–4f absorption transitions of Nd3? ion such as 4I9/2 ? 4F3/2(*875 nm); 4 I9/2 ? 2H9/2,4F5/2(*800 nm); 4I9/2 ? 4S3/2,4F7/2 (*750 nm); 4 I9/2 ? 4G5/2,2G7/2 (*580 nm); 4I9/2 ? 2K13/2,4G7/2,4G9/2 (*520 nm) are not detected in these nanophosphor materials causing the small quantity of the Nd3? within the host lattice and high absorbance of charge transfer absorption band in the same region [3]. The optical energy band gap, Egap, for the CaMoO4:Nd3?, CaMoO4:Nd@CaMoO4 and their silica-coated CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles was determined from the sharply increasing absorption region according to Tauc and Menth’s law (Fig. 4) [31]. Figure 4 shows a plot of (ahm)2 versus 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 was used as a prime evidence for a direct band gap. The estimated optical band gap is 3.09 eV for CaMoO4:Nd3? nanoparticles. There is a very significant difference between the values of band gaps obtained in this work and those reported theoretical and experimentally for CaMoO4 in the literature reports [6, 7, 29]. For example, Marques et al. [6] have reported direct band gap values ranging from 3.68 to 3.44 eV for CaMoO4 nanoparticles synthesized using coprecipitation method. Longo et al. [7] have reported direct band gap values ranging from 3.4 to 3.97 eV for hierarchical assembly of CaMoO4 NanoOctahedrons prepared using Microwave-hydrothermal method. Most recently, Vidya et al. [29] have reported direct band gap values ranging from 3.72 to 3.990 eV for nanocrystalline CaMoO4 prepared through an autoigniting combustion technique. The decrease 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, very recently Dujradin et al. [32] reviewed luminescence and scintillation properties at the nanoscale, pointing out that this is a very open field in terms of mechanism studies (relaxation of energy, non-proportionality), as well new materials for innovative applications. Compared to the our core CaMoO4:Nd3? nanoparticles’ Eg of 3.09 eV, determined by UV-Vis spectroscopy, CaMoO4:Nd@CaMoO4 and their silica-coated CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles showed a decrease in Eg of 2.06 and 1.26 eV, respectively. These results reveal that the value of band gap energy depends on the crystallite size. It indicated that after growth of the shells of CaMoO4 and amorphous SiO2 around the surface of core (CaMoO4:Nd and CaMoO4:Nd@CaMoO4), the particle size of the nanophosphores increases and crystallinity decreases due to amorphous silica surface encapsulation. These results are in accordance with the XRD results. Surface chemistry of the synthesized CaMoO4:Nd3?, CaMoO4:Nd@CaMoO4 and CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles is examined by FTIR spectroscopy, as illustrated in the Fig. 5. It can be clearly seen that a strong stretching vibration band at around 827 and a week bending vibration band at 425 cm-1 represent the characteristic adsorption of the Mo–O (from the MoO42group) and Eu–O bond, respectively (Fig. 5a–c) [6, 7, 29]. These strong absorption bands situated at 827 cm-1 (Eu mode) are related to the m3(F2) internal mode originated from the asymmetric stretching vibrations in the [MoO4]2clusters and the other weak absorption bands located at 425 cm-1 (Au mode) are generally associated with the

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a

4.5

b

3.5

4.0

3.5

c

3.0

3.5 3.0

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3.0

2.5 2.0

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1.0

1.0

1.5 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0

0.5 2.1

Photon energy(eV)

2.4

2.7

3.0

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Photon energy(eV)

3.9

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Photon energy(eV)

Fig. 4 The plot of (aht)2 vs. photon energy(hm) of the a CaMoO4:Nd, b CaMoO4:Nd @CaMoO4 and c CaMoO4:Nd@CaMoO4@SiO2 core– shell nanoparticles

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%T

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Wavenumbers(cm ) Fig. 5 FTIR spectra of the as-prepared a CaMoO4:Nd, b CaMoO4:Nd @CaMoO4 and c CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles

m4(F2) internal modes due to the presence of asymmetric bending vibration involved in the O–Mo-O bond [6, 7, 29]. An IR broad band located at 3,419, 1,540 and 1,383 cm-1 can be ascribed to the mO–H symmetrical stretching and bending vibrations of physically absorbed water molecules on the surface of nanoparticles (Fig. 5a–c) [3, 22–24]. Because the samples were prepared in aqueous solution, the surface of particles can be covered inevitably with the absorbed water molecules. Silica modification on the surface of CaMoO4:Nd@CaMoO4 core–shell nanoparticles is confirmed by the characteristics infrared peaks of silica at 1,079, 876 and 431 cm-1, which corresponds to the bending vibration modes of Si–O–Si, Si–OH and Si–O asymmetric and symmetric stretching vibrations, respectively (Fig. 5c) [3, 22–24]. The presence of weak IR OH band at about 3,423 cm-1 suggests the existence of physically absorbed surface H2O molecules (also confirmed by TGA analysis). These surface Si–OH groups play an

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important role which includes biocompatibility in biological systems, functionality and high colloidal stability under different conditions. The X-ray diffraction pattern, optical absorption spectral analysis and FTIR spectral results confirm that silica has been successfully encapsulated on the surface of CaMoO4:Nd@CaMoO4 core–shell nanoparticles. Raman spectroscopy is an effective tool to investigate the effects of structural order and disorder. Figure 6 shows the Raman spectra of CaMoO4:Nd3?, CaMoO4:Nd@CaMoO4 and CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles recorded at room temperature. Comparative Raman spectra in this work are in good agreement with that in a previous relevant work on CaMoO4. For a perfect crystal, the first-order Raman phonon spectrum consists of broad lines that correspond to Raman-allowed zone center (C-point) modes, which obey definite polarization selection rules. The mixed polarization configuration used for these spectra allowed the simultaneous observation of all most prominent Raman modes of different symmetries. At room temperature all prominent phonon peaks are clearly resolved in the spectra. According to the literature reports, the primitive cell of CaMoO4 includes two formulaic units, the [MoO4]2- ionic group with strong covalent Mo–O bonds (Td symmetry) and the Ca2? cations, due to weak coupling between the (MoO4)2- ionic group and the Ca2? cations [6, 7, 29]. The vibrational modes observed in the Raman spectra of molybdates scheelite crystals can be classified into two groups: external and internal modes. The vibrational external modes are related to the lattice phonon, which corresponds to the motion of [CaO8] clusters and the rigid molecular cell units. The vibrational internal modes are correspondent to the vibration inside [MoO4]2-cluster units, considering the center of mass in the stationary state. In isolated [MoO4]2-, tetrahedrons have a cubic symmetry point (Td), and its vibrations are composed of four internal modes (m1(A1), m2(E1), m3(F2)) and m4(F2)), one free rotation mode mf.r.(F1), and one translation mode (F2). On the other

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Relative Intensity(a.u.)

a b c

100

150

Ag

200

250

300

350

400

-1

Ram an Shift(cm )

Eg B g

(141)

Relative Intensity(a.u.)

hand, when a (MoO4)2- tetrahedron is located in the scheelite structure, its point symmetry is reduced to S4. The spontaneous Raman spectra with the assignments of the most prominent Raman-active vibration modes of the synthesized CaMoO4:Nd3? (a), CaMoO4:Nd@CaMoO4 (b) and their silica-coated CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles are presented in Fig. 6. Some (1Bg and 1Eg) vibrational modes are not detectable, probably due to their low intensities. Moreover, the Raman spectra exhibited strong and broad bands, indicating a strong interaction between the O–Ca–O and O–Mo–O bonds in the clusters. In principle, this characteristic is normally observed in materials structurally ordered at short range. The positions of each Raman-active mode are listed in Table 1. Comparative experimental Raman active modes’ results displayed in this table indicated that the relative positions of all Raman-active modes of CaMoO4 crystals reported in this work are in good agreement with those previously reported [6, 7, 29]. As seen in Fig. 6, the observed Raman peaks in the case of CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles are highly intense with broadening in respect to other samples. No silica peak

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

is detected in the Raman spectrum due to high absorbance of CaMoO4:Nd@CaMoO4 and thin layer of silica. It suggests that the silica nanoparticles are aggregated around the surface of nanoparticles and their encapsulation greatly enhanced the stability of the Raman peaks. The shift in band positions observed on these spectra can be correlated to the structural modifications induced by the synthesis methods, mainly including the following: distortions on the [MoO4] clusters, the degree of interaction between the O– Mo–O bonds, and variations on the bond lengths, angles and/or symmetry break induced by the structural order– disorder in the lattice. The well-defined Raman-active modes of the CaMoO4 samples confirm that the samples are structurally ordered. 3.3 Photoluminescence spectral study In general, PL emission is considered a powerful tool to get informations on the electronic structure and degree of structural organization at medium range of the materials. Moreover, this optical property is sensible to the presence of energy levels within the band gap. Figure 7 shows the room temperature-recorded photoluminescence spectra of CaMoO4:Nd3?, CaMoO4:Nd@CaMoO4 and CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles using the same excitation wavelength of 325 nm. The three samples exhibited strong broad emission band in between 375 and 775 nm and some weak sharp lines in the longer wavelength region, which are due to the charge transfer band between the MoO42- and Nd3? ions and the 4fn–4fn transition of the Nd3? ions, respectively [6, 7, 15, 29]. The emission spectra were measured at room temperature because this optical property behavior can be influenced by the temperature. This broad band covers the large part of the visible electromagnetic spectrum, with a maximum emission situated at around 535 nm (green emission). This broad emission band is a typical of multiphonon and multilevel process, that is, a system in which relaxation occurs by several paths involving the participation of numerous states within the band gap of the material. Many explanations have been discussed in literature regarding the origin and mechanisms responsible for the PL emissions of molybdates. According to them the green PL emission is due to structural disorder in the CaO8–MoO4 clusters [6–8].

Table 1 Comparative experimental Raman-active modes results of CaMoO4:Nd, CaMoO4:Nd@CaMoO4 and CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles Samples

Bg(s)

Eg(w)

Eg(w)

Bg(d)

Ag(.)

Eg(.)

Ag(*)

Bg(5)

Eg(r)

Bg(r)

Ag(u)

CaMoO4:Nd



112

141

200

321



390



791

844

876

CaMoO4:Nd@ CaMoO4





141

199

320



389



790

844

876

CaMoO4:Nd@ CaMoO4@SiO2

103

112

139

195

319



387



788

841

874

123

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Relative Intensity(a.u.)

b c Relative Intensity(a.u.)

a

c a b

775

800

825

850

875

900

925

Wavelength(nm)

400

450

500

550

600

650

700

750

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

The great dependence of the PL properties on the morphology and crystallinity was investigated by Ryu et al. [33]. Liu et al. [34] made the conclusion that the chargetransfer transitions into the [MoO4]2– complex can be considered the main reason responsible for the green PL emissions and by theoretical calculations, Campos et al. [35] stated that the PL emission processes of CaMoO4 powders can be related to the existence of MoO3 and distorted MoO4 complex clusters into the lattice. Thus, it can be stated that the green PL emission at room temperature in CaMoO4 may be linked with several factors, such as: distortions on the [MoO4]tetrahedron groups caused by the different angles between O–Mo–O particle sizes, crystalline degree, morphology and surface defects. In this case, these factors promote the formation of visible-light emission centers responsible for the PL property of this material. The PL spectra of all samples displayed two characteristic multiple split emission transitions at around 799–818 and 873–915 nm corresponding to the 4F5/2 ? 4I9/2 and 4F3/2 ? 4I9/2 transitions of Nd3?, respectively [1, 2, 4, 5]. These observed transitions are attributed to the internal 4f- shell transitions of Nd3? ions. Three other transitions 4F5/2 ? 4I11/2 (1,045–1,080 nm), 4F3/2 ? 4I13/2 (1,300–1,450 nm) and 4F5/2 ? 4I15/2 could not be recorded due to the limited spectral range of the spectrofluorometer.

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As seen in Fig. 7, the highest phosphorescence intensity was obtained for CaMoO4:Nd@CaMoO4 core–shell nanoparticles in respect to the CaMoO4:Nd and CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles. A dramatic increase in overall luminescence intensity causes the formation of an undoped CaMoO4 shell which protect the dopant in the core and effectively reduces non-radiative quenching of the charge transfer transition of [MoO4]2and surface defects [15, 19–21]. This reduction in nonradiative quenching could be due to the presence of a large density of defects which act as radiation traps. It is clear that a slight line broadening and red shift CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles of the emission bands have been induced inside the structure. These changes are very likely due to Nd3?-ions site redistributions, together with lattice densification, as a result of the tightly focused femto second laser pulse processing [1, 15, 19–21]. The comparison of core and CaMoO4:Nd@CaMoO4 core–shell nanoparticles was done for the same weight percent of nanoparticles. The reduction in quenching improves the overall phosphorescence intensity of the core–shell nanomaterials. Recently some researchers have observed and discussed the improve luminescence property with core–shell nanoparticles. Haase et al. [19] were the first to report the quantum yield enhancement from 53 %for CePO4:Tb nanoparticles to 80 % for

Nd-doped calcium molybdate core

CePO4:Tb/LaPO4 core–shell nanoparticles. They attributed the significant enhancement of the quantum yield to a shell around each doped nanoparticle, which can suppress the energy-loss process on the particle surface. Veggel et al. [20] ascribed the improved quantum yield of LaF3/Ce, Te nanoparticles from 24 to 54 % to the LaF3 shells around nanoparticles. Yi et al. [21] also reported 7.4 and 29.6 times enhancement of the upconversion fluorescence intensities for NaYF4:Yb/Er(Tm)/NaYF4 core–shell nanoparticles, compared to non-coated samples. They suggested that the emission intensity enhancements primarily originated from crystalline shell with low phonon energy to reduce the quenching luminescence process. It can be seen in Fig 7c, the luminescence intensity for CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles is remarkably reduced relative to the silica-uncoated CaMoO4:Nd@CaMoO4 core–shell nanoparticles, which is similar to some literatures related to silica-coated nanophosphors [36–41]. After coating, the possible reasons of the remarkable PL intensity reductions are as follows: SiO2 coating leads to the decrease in CaMoO4:Nd@CaMoO4 amount per unit volume of the sample [42]; SiO2 is in amorphous state and the silanol (SiO–H) group which acts as the luminescence quencher may present [40]; the power of the excitation light reaching the CaMoO4:Nd@CaMoO4 core becomes weak due to the scattering and reflection of silica shell [36]. Figure 7c shows that the observed two emission transitions (4F5/2 ? 4I9/2 and 4F3/2 ? 4I9/2) of Nd3? are greatly enhanced after silica encapsulation around the surface of core nanoparticles. Whereas, the CaMoO4:Nd@CaMoO4@SiO2 core–shell nanoparticles still outperform the uncoated CaMoO4:Nd core nanoparticles. Furthermore, silica coating remained a favorable route to confer water solubility to these hydrophobic luminescent core–shell nanoparticles besides having the advantage of being able to preserve the inherent luminescence of the nanoparticles. This enables such biocompatible and easily dispersible nanoparticles to be employed in bioapplications without any adverse toxic effects, common with other light emitting nanoparticles that contain heavy metals(quantum dots).Our results strongly support the previous reports [14, 15], where the greenish-blue emission indicates that the prepared CaMoO4 samples are defect free with a complete periodic order in the long and medium wavelengths, and the emission is attributed to the [MoO4]2- tetrahedron clusters.

4 Conclusions We successfully synthesized CaMoO4:Nd and CaMoO4:Nd@CaMoO4 core–shell nanoparticles through polyol process and sol–gel chemical method was used for silica surface modification of nanoparticles. XRD studies showed

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that the as-prepared nanoparticles were single phase and possess a scheelite-type tetragonal structure. XRD, FTIR and Raman spectral analysis confirmed the phase purity and surface chemistry of the as-prepared core and core– shell nanoparticles. The optical band gap energy was determined 3.09 eV for CaMoO4:Nd nanoparticles, which decreased gradually after the growth of a crystalline undoped shell of CaMoO4 and amorphous silica around the surface of core nanoparticles. The emission spectra for CaMoO4:Nd@CaMoO4 core–shell nanoparticles demonstrate higher phosphorescence intensity relative to the core and silica-coated core–shell nanoparticles, because undoped CaMoO4 shell protects the dopants in the core and effectively reduces non-radiative quenching of the charge transfer transition of [MoO4]2- and surface defects.

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