Morphology control of uniform CaMoO4 microarchitectures and development of white light emitting phosphors by Ln doping (Ln = Dy3+, Eu3+)

1 Morphology control of uniform CaMoO 4 based self-assembled microarchitectures and development of white light emitting phosphors by Ln doping (Ln= Dy 3+ , Eu 3+ ) Mariano Laguna, Nuria O. Nuñez, * Ana I Becerro and Manuel Ocaña Instituto de Ciencia de Materiales de Sevilla, CSIC, Américo Vespucio 49, 41092, Isla de la Cartuja, Sevilla, Spain * corresponding author: [email protected] Abstract A very simple synthesis procedure based on precipitation reactions at moderated temperature (120ºC) from solutions containing calcium nitrate and sodium molybdate, using mixed solvents (polyols and water) has been developed which produces uniform tetragonal CaMoO 4 microarchitectures with different morphologies (peanuts, cocoons, spindles and spheres), which are composed by self-assembled entities. The morphology and crystal size of such assemblies could be tuned by a simple change of the nature of the components of the solvents mixture or their volumetric ratio in such mixture. All particles presented similar excitation and emission spectra arising from a charge transfer process within the MoO 42- groups. The emitted light presented a bluish-green color and its intensity was higher for the spindle-type particles. This synthesis procedure was also suitable for doping the peanut like CaMoO 4 architectures with Eu 3+ or Dy 3+ cations up to a 1% molar ratio (Ln/Ln+Ca), without altering their morphology or crystalline structure. The so prepared phosphors emitted an intense red (Eu-doped) or greenish (Dy-doped) light when excited through the MoO 42- groups excitation band indicating the presence of Page 2 of 46CrystEngComm 2 an energy transfer process from such groups to the Ln 3+ cations. Finally, a white light emitting phosphor with chromaticity coordinates x = 0.335 and y = 0.365, and a correlated color temperature of 5407 K was developed by codoping the peanut-type CaMoO 4 particles with proper amounts of Dy 3+ (0.35%) and Eu 3+ (0.15%) cations, which could find applications in white light emitting diodes. Page 3 of 46 CrystEngComm 3 1. Introduction Lanthanide(Ln) based phosphors, usually consisting of a host matrix doped with active Ln 3+ cations present several advantageous features when compared with other luminescent materials, which have favored their use for many technological applications in several fields ranging from optoelectronic (lasers, waveguides, plasma displays, LEDs, photovoltaics, photocatalysis, etc.), 1-5 to biotechnology (sensing, imaging, therapy, etc.). 6,7 These advantages include high chemical and thermal stability, photostability, high quantum yields, narrow emission bands and low toxicity, among others. 8 However, it must be also mentioned that the absorption coefficient of the Ln cations is low, which results in low intensity emissions. 9 To overcome this problem, several strategies can be adopted. One of them takes advantage of the large absorption coefficient of some inorganic anions, which are able to transfer the absorbed energy to the active Ln ions thus increasing the intensity of luminescence. 9 Among these anions are molybdates, which absorb UV light that can be transferred to some Ln 3+ ions such as Eu 3+ , Dy 3+ or Tb 3+ . 10 Because of this, a large variety of phosphors have been developed mostly based on different kinds of lanthanide 11 or alkaline earth 12-15 molybdates doped with Ln 3+ cations. It is also well known that the luminescent properties of Ln-based phosphors are affected by several factors, which include particle size and shape 16 for which, particles with uniform morphological features are usually required for most applications. Among alkaline earth molybdates, calcium molybdate (CaMoO 4 ) is perhaps the most studied host matrix for the development of Ln-based phosphors. It must be mentioned that CaMoO 4 is itself luminescent because it emits blue light when excited with UV radiation. 17 The potential applications of this kind of phosphors encompass Page 4 of 46CrystEngComm 4 biomedicine, 18 lighting, 10,13 and catalysis. 19 Up to now, several methods have been reported for the synthesis of CaMoO 4 microarchitectures with different shapes 20-27 . However, most of these methods conducted to particles with heterogeneous size and/or a certain degree of agglomeration, except in the case of the persimmon-like (2 µm) 20 particles obtained in a microwave reactor without temperature control and the spherical particles hydrothermally synthesized in the absence (5-6 µm) 22 or the presence (2 µm) 23 of a capping polymer (poly(diallyldimethyl-ammonium chloride). Herein, we report on a very simple procedure for the synthesis of highly uniform CaMoO 4 microarchitectures consisting of self-assembled nanocrystals, based on precipitation reactions at moderated temperature from solutions containing calcium nitrate and sodium molybdate, using mixed solvents (polyols and water). Interestingly, the morphology (peanuts, cocoons, spindles, spheres) and crystallinity of such assemblies can be tuned by a simple change of the nature of the solvents used for the mixture or their volumetric ratio in such mixture. This effect is discussed in terms of the variations in viscosity and dielectric constant of the solvent. This procedure is also suitable for doping the CaMoO 4 architectures with Ln cations, which is first illustrated for the Eu 3+ case, selected as a proof of concept to analyze the effect of doping on the morphological and structural features of the obtained red phosphors. Finally, white light emitting phosphors were obtained by doping the CaMoO 4 particles with proper amounts of Dy 3+ cations. It is shown that the correlated color temperature of the white light so generated can be modulated by varying the Dy 3+ doping level or by codoping with Eu 3+ cations, which provides a red component thus resulting in a warmer white light. 28 These phosphors are of particular interest for developing white LEDs, since for this technology, single phase white light emitting phosphors are being proposed to minimize Page 5 of 46 CrystEngComm 5 the inhomogeneous temporal degradation of the systems based on the simultaneous emission of three different phosphors emitting red, green and blue light, respectively. 4 2. Experimental 2.1. Reagents Calcium nitrate (Ca(NO 3 ) 2 ·4H 2 O, Aldrich 99-103%), europium nitrate (Eu(NO 3 ) 3 ·5H 2 O, Aldrich, 99.9%), dysprosium nitrate (Dy(NO 3 ) 3 ·xH 2 O, Aldrich, 99.9%) and sodium molybdate (Na 2 MoO 4 , Aldrich, ≥98%), were selected as precursors. Glycerol (GLY, Aldrich, ≥99.5%), ethylene glycol (EG, Aldrich, ≥99.5%) and mixtures GLY/H 2 O and GLY/EG were used as solvents. All chemicals were used as received. 2.2. Nanoparticle synthesis CaMoO 4 nanoparticles were prepared according to the following procedure. First, 0.02 mol dm -3 of Ca(NO 3 ) 2 and 0.1 mol dm -3 of Na 2 MoO 4 were separately dissolved in 2.5 cm 3 of the desired solvent (GLY, EG, H 2 O, GLY/H 2 O or GLY/EG mixtures). To facilitate the dissolution of reagents in GLY and EG, the solutions were mildly heated (~ 80°C) under magnetic stirring. After cooling to room temperature, both solutions were mixed and magnetically stirred. The volumetric ratio of the GLY/H 2 O and GLY/EG mixtures in the final solutions, was varied from 4/1 to 1.5/3.5, in order to investigate the effect of this parameter on the characteristics of the precipitated particles. Such solutions (total volume = 5 cm 3 ) were then aged for 20 hours in tightly closed test tubes using an oven preheated at 120°C. After aging, the resulting dispersions were cooled down to room temperature, centrifuged to remove the supernatants and washed, twice with ethanol and once with distilled water. Finally, the Page 6 of 46CrystEngComm 6 precipitates were redispersed in milliQ water or, for some analyses, dried at room temperatures. 2.3. Ln-doped CaMoO 4 nanoparticles To obtain Ln-doped CaMoO 4 phosphors we proceeded as described above for the case of pure CaMoO 4 system but incorporating the desired amount of the doping ion precursor (Eu, Dy or Eu/Dy) to the starting Ca(NO 3 ) 2 solution. The total cations concentration in these solutions was kept constant (0.02 mol dm -3 ). The Ln/(Ln+Ca) molar ratio was varied in order to investigate the effects of the doping level on the morphology and luminescent properties of the precipitated nanoparticles. 2.4. Characterization Particle shape was analized by scanning (FEGSEM, Hitachi S4800, 20 Kv) and transmission (TEM, Philips 200CM, 200 Kv) electron microscopy. For this, a droplet of an aqueous suspension of the sample was deposited on a copper grid coated with a transparent polymer and dried. Particle size was measured by counting several hundred of particles from the SEM and TEM micrographs using the free software ImageJ. The qualitative composition of the precipitated particles was assessed by energy dispersive x-ray analysis (EDX, EDX Bruker X Flash Detector 4010) coupled to the SEM microscope. The infrared spectra (FTIR) of the powdered samples diluted in KBr pellets were recorded in a Fourier transform spectrometer (Jasco FT/IR-6200). The crystalline structure of the prepared particles was identified by X-ray diffraction (XRD, Panalytical X´Pert Pro with an X-Celerator detector). Unit cell parameters were determined from the XRD data (collected at intervals of 0.02° (2θ) for an accumulation time of 1000 s) by Rietveld refinement using the X´Pert High Score Plus software. The Page 7 of 46 CrystEngComm 7 structural data for the CaMoO 4 phase was taken from Lee et al. 29 Refined parameters were: zero, background coefficients, scale factor and lattice parameters. The obtained Rwp values were in all cases in the 20-26% range. The crystallite size was estimated from the most intense XRD peak of the CaMoO 4 (112) (located at 28.7º 2θ) by using the Scherrer formula and LaB 6 as external standard. The excitation and emission spectra of the samples dispersed in water (1 mg cm −3 ) were recorded in a Horiba Jobin-Yvon Fluorolog3 spectrofluorometer operating in the front face mode. Lifetime measurements for Eu 3+ and Dy 3+ cations were obtained under pulsed excitation at λ exc 532 nm and 355 nm, respectively, using the second and third harmonics of an Nd:YAG laser (Spectra Physics model DCR 2/2A 3378) with a pulse width of 10 ns and a repetition rate of 10 Hz. Fluorescence was analyzed through an ARC monochromator model SpectraPro 500-i and then detected synchronously with an EMI-9558QB photomultiplier and recorded by a Tektronix DPO4104B-L digital oscilloscope. The CIE color coordinates were calculated from the emission spectra considering a 2° observer. The correlated color temperature value (CCT) of the white light was calculated using the empirical formula given by McCamy. 30 The photographs showing the luminescence of the phosphors deposited on Millipore filters were taken under illumination with ultraviolet radiation (λ = 254 nm) filtered from a Hg discharge lamp. 3. Results and discussion 3.1. Undoped CaMoO 4 particles The effect of the GLY/H 2 O ratio on the morphological characteristics of the particles precipitated by aging calcium nitrate (0.02 M) and sodium molybdate (0.1 M) solutions at 120°C for 20 h are summarized in Table 1 and Fig. 1. As observed, in pure glycerol Page 8 of 46CrystEngComm 8 irregular and aggregated spheres were obtained (Fig. 1A). However, important differences in particle morphology and uniformity were found when adding H 2 O to glycerol in variable GLY/H 2 O volumetric ratios (Fig. 1B-E). Thus, uniform anisometric particles with similar size (about 3 µm x 1.3 µm) (Table 1) and different morphologies including peanuts (Fig. 1B), cocoons (Fig. 1C) and spindles (Fig. 1D) were obtained for a GLY/H 2 O volumetric ratio of 4/1, 3.5/1.5 and 2.5:2.5, respectively. For a lower GLY/H 2 O ratio (1.5/3.5 and pure water), more heterogeneous and aggregated particles resulted (Fig. 1E, F). Particle shape could be also modulated by using GLY/EG mixtures as solvent. Thus, uniform spheres (Fig. 2B) with a diameter of 1.2 µm (Table 1) resulted when a mixture of GLY/EG in a volumetric ratio of 2.5:2.5 was used as solvent, whereas irregular and aggregated entities were obtained when the GLY/EG ratio was varied (Fig. 2A, C) or in pure EG (Fig. 2D). Irrespective of morphology, the uniform precipitated particles were identified by XRD as tetragonal (scheelite-type) CaMoO 4 (ICDD file: 01-077-2238) as illustrated in Fig. 3 for the spheres, spindles and peanuts (cocoons will not be hereafter considered since this morphology is intermediate between peanuts and spindles). A detailed observation of Fig. 1 and Fig. 2 also indicates that all homogeneous particles consisted of self-assembled much smaller crystals (Fig. 1B-D and 2B, inset). The size of such crystals estimated from the XRD patterns was found to be much lower for the spheres (34 nm) than for the other morphologies (96 nm for the peanuts and >100 nm, for the spindles) (Table 1). It should be noticed that the value of crystal size for the spindles is above the detection limit of the Scherrer formula (100 nm). However, the insets of Figures 1B and D clearly reveals the much higher size of the assembled crystals in the case of the spindles when compared to peanuts. This microstructure is characteristic of particles formed through an ordered aggregation process, similar to that Page 9 of 46 CrystEngComm 9 previously observed for different molybdates with scheelite structure. 22 Such mechanism was confirmed by analyzing the nature of the precipitates obtained after different aging times. Thus, in the case of the spindle-type particles, a high turbidity was detected after 1.5 min of aging, due to the precipitation of small and irregular particles (primary), which seem to aggregate to form larger spindles (Fig. 4A), which almost reached their final size after 5 min of aging (Fig. 4B). This process was accompanied by an increase of crystal size from 64 to >100 nm (Table 2), which seems to progress on prolonged aging from 5 min to 20 h, as observed by SEM (Fig. 4C). This behavior suggests that after aggregation, a recrystallization process also took place, since the reaction yield remained almost constant (~80%) during this period of aging (Table 2). A similar behavior was detected for the peanuts (Fig. 4D-F and Table 2) and the spherical particles (Fig. 4G-I and Table 2), except that the onset of precipitation was slightly slower in the first case and much slower for the latter, indicating that the reaction kinetics is faster as increasing the amount of water in the system. This result can be explained by taking into account the viscosity of the solvents (glycerol: 1.5 Pa·s, ethylene glycol: 0.01733 Pa·s, water: 0.001 Pa·s) since as increasing the amount of water, the viscosity of the solvent mixture decreases thus favoring the diffusion process required for nucleation and particle aggregation. The different size and shape of the microarchitectures obtained using different solvents are difficult to explain since the particle formation through aggregation of primary particles is a very complex process involving many stages. 31 However, it is obvious that the primary particles must be brought together, which is a diffusion controlled step and therefore influenced by the viscosity of the solvent. Once the particles are close enough, the attractive interaction between them, whose driving force is the high surface energy of the small colloidal particles, must predominate over the Page 10 of 46CrystEngComm 16 excitation at the same wavelength than Dy 3+ and the CaMoO 4 matrix. After different assays, we found that the emission spectrum of the CaMoO 4 particles codoped with a 0.35% Dy 3+ and a 0.15% Eu 3+ (Fig. 10) showed the bands due to Dy 3+ and Eu 3+ cations, as well as that of the CaMoO 4 matrix, with appropriated relative intensity so that a whiter (x = 0.335 and y = 0.365) (Fig. 9) and warmer (CCT = 5407 K) (Table 6) emission light was produced, which may be suitable for the development of LEDs for indoors illumination. 47 As expected, the addition of Eu 3+ has no important effects on particle shape (Fig. S5B), size (Table 3) or crystalline structure (Fig. S4) of the CaMoO 4 based particles, although a decrease of crystallite size with respect to the undoped sample was also detected as observed for the single doped particles (Table 4). Finally, the luminescence dynamics of the codoped sample were also evaluated to assess the possible existence of an energy transfer process between Dy 3+ and Eu 3+ in our white light emitting phosphor, which has been previously observed for other Eu 3+ -Dy 3+ codoped systems. 48 The decay curves obtained for the 4 F 9/2 - 6 H 13/2 transition of Dy 3+ (575 nm) and the 5 D 0 - 7 F 2 transition of Eu 3+ (615 nm) could be also fitted through a biexponential function (Fig. 7C, D) whose fitting parameters are shown in Table 5. As observed, the average life-time values, <τ> obtained for both Eu and Dy emissions, in the codoped sample were quite similar to those obtained for the single doped samples with the same doping level, indicating that no energy transfer between the doping cations takes place in the codoped sample and that the cooping process had no influence on the luminescence efficiency. 4. Conclusions Page 17 of 46 CrystEngComm 17 Uniform CaMoO 4 microarchitectures have been synthesized through precipitation reactions at 120 ºC from solutions containing calcium nitrate and sodium molybdate, using mixed solvents (polyols and water). The shape of such microarchitectures was found to be determined by the nature of the solvent mixture. Thus, peanuts, cocoons and spindles were obtained in glycerol-water mixtures with different polyol/water ratio, whereas spheres resulted in a ethylene glycol-water mixture. Irrespective of their shape, the obtained particles consisted of self-assembled nanocrystals which crystallized into the tetragonal CaMoO 4 phase. All these samples showed similar excitation and emission spectra arising from a charge transfer process within the MoO 42- groups. The emitted light presented a bluish-green color and its intensity was higher for the spindle-type particles, which might be associated to their higher crystallite size. This procedure was also suitable for doping the peanut like CaMoO 4 architectures with Eu 3+ or Dy 3+ cations up to a 1% molar ratio (Ln/Ln+Ca), without altering their morphology or crystalline structure. The so prepared phosphors emitted an intense red (Eu-doped) or greenish (Dy-doped) light when excited through the MoO 42- groups excitation band indicating the presence of an energy transfer process from such groups to the Ln 3+ cations. Finally, a white light emitting phosphor with chromaticity coordinates x = 0.335 and y = 0.365, and a correlated color temperature of 5407 K was developed by cooping the peanut-type CaMoO 4 particles with proper amounts of Dy 3+ (0.35%) and Eu 3+ (0.15%) cations, which could find applications in white light emitting diodes. Acknowledgements This work has been supported by the Spanish Ministry of Economy and Competitiveness (MINECO) (MAT2014-54852-R) and CSIC (Pie 201460E005). Page 18 of 46CrystEngComm 18 Page 19 of 46 CrystEngComm 19 Table 1 Shape, size and crystallite size of the CaMoO 4 particles obtained at 120°C for 20 h from solutions containing calcium nitrate and sodium molybdate in different mixed solvents (polyols and water). GLY/H 2 O (vol) GLY/EG (vol) Shape Larger dimension (µm) Shorter dimension (µm) Diameter (µm) Crystallite size (nm) 5/0 Irregular sphere - - - - 4/1 Peanut 2.84(±0.23) 1.24(±0.09) - 96 3.5/1.5 Cocoon 2.93(±0.26) 1.34(±0.1) - - 2.5/2.5 Spindle 2.78(±0.32) 1.29(±0.11) - >100 1.5/3.5 Irregular Heterogeneous Heterogeneous - - 0/5 Irregular Heterogeneous Heterogeneous - - 2.5/2.5 Sphere - - 1.2(±0.15) 34 Page 20 of 46CrystEngComm 20 Table 2 Reaction yield and crystallite size for the samples with different particle shape obtained after different reaction times. Shape Time Yield (%) Crystallite size (nm) Spindles 1.5 min 50 64 Spindles 5 min 82 >100 Spindles 20 hours 80 >100 Peanuts 3 min 82 25 Peanuts 5 min 91 38 Peanuts 20 horas 98 96 Spheres 2 hours 9.2 19 Spheres 4 hours 48 23 Spheres 20 hours 84 34 Table 3 Nominal and experimental (ICP) Ln/(Ln+Ca) mol ratio and sizes of the Lndoped CaMoO 4 particles. Ln/(Ln+Ca) Nominal (%) ICP (%) Larger dimension (µm) Shorter dimension (µm) 1% Eu 0.89% Eu 2.45 (±0.42) 1.11 (±0.16) 1% Dy 1.07% Dy 2.1 (±0.1) 1.0 (±0.05) 0.35% Dy + 0.15% Eu - 2.6 (±0.2) 1.3 (±0.1) Page 21 of 46 CrystEngComm 21 Table 4 Unit cell parameters, unit cell volumes and crystallite size measured for the Lndoped CaMoO 4 particles with peanut morphology. Ln content a=b (Å) c (Å) V(Å 3 ) Crystallite size (nm) 0% Eu 5.2268(2) 11.4424(5) 312.60 96 1% Eu 5.2273(2) 11.4467(7) 312.77 49 1% Dy 5.2273(2) 11.4439(6) 312.70 77 0.35% Dy + 0.15% Eu 5.2273(1) 11.4438(4) 312.70 69 Table 5 Lifetimes values (τ 1 , τ 2 ) and average decay time ( < τ >) for the Ln-doped CaMoO 4 particles obtained for the 5 D 0 - 7 F 2 transition of Eu 3+ (615 nm) and the 4 F 9/2 - 6 H 13/2 transition of Dy 3+ (575 nm) after pulsed excitation with λ exc 532 and 355 nm respectively, as a function of the Ln content. Emission Eu content (%) Dy content (%) I 1 τ 1 (µs) I 2 τ 2 (µs) < τ > (µs) Eu ( 615 nm) 1% - 0.647 390 ± 17 0.515 739 ± 37 600 ± 30 Eu (615 nm) 0.15% - 0.451 330 ± 20 0.712 739 ± 37 649 ± 32 Eu (615 nm) 0.15% 0.35% 0.480 340 ± 17 0.662 739 ± 37 639 ± 32 Dy (575 nm) - 1% 0.385 26 ± 1.3 0.572 200 ± 10 186 ± 9 Dy (575 nm) - 0.35% 0.245 26 ± 1.3 0.704 200 ± 10 192 ± 10 Dy (575 nm) 0.15% 0.35% 0.286 26 ± 1.3 0.640 200 ± 10 190 ± 10 Page 22 of 46CrystEngComm 22 Table 6 CIE color coordinates (x, y) and correlated color temperature (CCT) value corresponding to the emissions of the single Dy-doped and Dy-Eu bidoped CaMoO 4 samples. Sample x y CCT (K) 1% Dy 0.297 0.389 6804 0,7% Dy 0.285 0.384 7345 0,5% Dy 0.276 0.378 7821 0,35% Dy 0.269 0.372 8253 0,35%Dy + 0,15%Eu 0.335 0.365 5407 Page 23 of 46 CrystEngComm 23 References 1 J.C.G. Bünzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048. 2 X. Zhai, S. Liu, X. Liu, F. Wang, D. Zhang, G. Qin and W. Qin, J. Mater. Chem. C, 2013, 1, 1525. 3 R. P. 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Page 26 of 46CrystEngComm Fig. 4 159x111mm (150 x 150 DPI) Page 33 of 46 CrystEngComm Fig. 5 90x163mm (150 x 150 DPI) Page 34 of 46CrystEngComm Fig. 6 96x144mm (150 x 150 DPI) Page 35 of 46 CrystEngComm Fig. 7 45x31mm (300 x 300 DPI) Page 36 of 46CrystEngComm Fig. 8 91x185mm (150 x 150 DPI) Page 37 of 46 CrystEngComm Fig. 9 87x98mm (150 x 150 DPI) Page 38 of 46CrystEngComm Fig. 10 90x71mm (150 x 150 DPI) Page 39 of 46 CrystEngComm Electronic Supplementary Information Morphology control of uniform CaMoO 4 based self-assembled microarchitectures and development of white light emitting phosphors by Ln doping (Ln= Dy 3+ , Eu 3+ ) Mariano Laguna, Nuria O. Nuñez, * Ana I Becerro and Manuel Ocaña Instituto de Ciencia de Materiales de Sevilla, CSIC, Américo Vespucio 49, 41092, Isla de la Cartuja, Sevilla, Spain * corresponding author: [email protected] Page 40 of 46CrystEngComm Fig S1 FTIR spectra for the CaMoO 4 samples with different morphology. Inset: bands mark with arrows can be ascribed to the presence of polyol (ethylene glycol and/or glycerol) molecules adsorbed on the particles surface. The spectra of the pure ethylene glycol (EG) and glycerol (GLY) can be found at: EG: http://webbook.nist.gov/cgi/cbook.cgi?ID=C107211&Units=SI&Type=IR SPEC&Index=1#IR-SPEC GLY: http://webbook.nist.gov/cgi/cbook.cgi?ID=C56815&Units=SI&Type=IRSPEC&Index=1#IR-SPEC Page 41 of 46 CrystEngComm Fig S2 SEM images of the spindle-type CaMoO 4 particles with different Eu 3+ doping level (Eu/Eu+Ca mol ratio). 2 µm 1% Eu 5% Eu 0.5 µm Page 42 of 46CrystEngComm