View PDF VersionPrevious ArticleNext Article
 
DOI: 10.1039/C5RA06221G (Paper) RSC Adv., 2015, 5, 45693-45702

Facile morphology-controllable hydrothermal synthesis and color tunable luminescence properties of NaGd(MoO4)2:Eu3+,Tb3+ microcrystals†

Anming Liab, Dekang Xua, Hao Lina, Shenghong Yanga, Yuanzhi Shaoa, Yueli Zhang*a and Zhenqiang Chenb
aState Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, China. E-mail: stszyl@mail.sysu.edu.cn
bInstitute of Optoelectronic Engineering, Jinan University, Guangzhou 510632, China

Received 8th April 2015 , Accepted 12th May 2015

First published on 15th May 2015

Abstract

Well-crystallized and uniform NaGd(MoO4)2 microcrystals with morphologies of bipyramids, truncated bipyramids, quasi-cubes and tetragonal plates were selectively synthesized via a facile hydrothermal method without any additives under mild conditions. The effects of Na2MoO4/Gd(NO3)3 molar ratios and pH values of precursor solutions on the phase and morphology of the as-synthesized microcrystals were systematically investigated. The molar ratios and pH values played key roles in the selective synthesis of pure phase NaGd(MoO4)2 microcrystals with regular morphology. With increasing Na2MoO4/Gd(NO3)3 molar ratios in the appropriate pH range, the morphology of the products changed from bipyramids, truncated bipyramids, quasi-cubes to tetragonal plates, namely, the morphological truncation degree increased gradually. NaGd(MoO4)2 tetragonal microplates could be synthesized at the molar ratio of 9[thin space (1/6-em)]:[thin space (1/6-em)]1. A possible morphological formation mechanism of NaGd(MoO4)2 tetragonal microplates was proposed, i.e. nucleation-Ostwald ripening growth process. Color tunable photoluminescence properties of NaGd(MoO4)2:Eu3+,Tb3+ microcrystals were studied in detail. Among the different morphologies of NaGd(MoO4)2:5% Eu3+ microcrystals, tetragonal microplates had a broadened, red-shifted and enhanced charge transfer band in the excitation spectrum. What's more, the full-width at half-maximum for the charge transfer band of the tetragonal plates was highest (58 nm), which favored efficient excitation and absorption in the ultraviolet region. The introduction of a small amount of Tb3+ into NaGd(MoO4)2 microplates doped with Eu3+ would enhance the 5D0 → 7FJ transition of Eu3+ at 616 nm due to the energy transfer process of cross-relaxation from Tb3+ to Eu3+. Multicolor tunable luminescence from deep red, red, orange, yellow to green under 291 nm ultraviolet excitation and from red, reddish orange to pink under 380 nm near-ultraviolet excitation could be obtained in NaGd(MoO4)2 tetragonal microplates by simply adjusting the doping concentrations of Eu3+ and Tb3+, suggesting NaGd(MoO4)2:Eu3+,Tb3+ microcrystals might have practical application in optoelectronic devices, such as light emitting diodes and color display systems. This facile morphology-controlled hydrothermal synthesis strategy was simple, low-cost and environment-friendly, and might be extended to other inorganic materials.

1. Introduction

Lanthanide ion doped inorganic nano/microcrystals with a well-defined morphology have attracted much attention due to their unique physicochemical properties and therefore potential applications in numerous fields such as light emitting diodes, color displays, bio-imaging, solar cell devices and photocatalysis.1–5 As an important family of inorganic compounds, double alkaline rare-earth molybdates and tungstates have attracted much research attention because of their wide use as luminescent hosts, laser materials and catalysts. Generally, double alkaline rare-earth molybdates share scheelite type (CaWO4) crystal structure with the nominal formula ARe(MoO4)2 (where A = alkali metal cation, Re = trivalent rare-earth metal cation) and most of them have space group symmetry I41/a.6 Owing to their high chemical durability, large rare-earth ions admittance and large absorption cross-sections, double molybdate crystals have been studied in detail as laser host materials for decades.7–10 Recently, many research efforts have been devoted to synthesis and luminescence properties of double molybdate nano/microcrystals. For example, Mo et al. synthesized NaGd(MO4)2:R (M = W, Mo, R = Eu3+, Sm3+, Bi3+) red-emitting phosphors by solid-state reaction method.11 Katelnikovas et al. prepared Li3Ba2La3(MoO4)8:Eu3+ powders and ceramics, and investigated their optical properties for light emitting diodes applications.12 Wu et al. reported KEu(MoO4)2 microcrystals with different morphologies synthesized via a molten salt method using halons as flux.13 Krishnan et al. synthesized bi-pyramid (Na0.5Gd0.5)MoO4:Ln3+ microcrystals by EDTA-aided hydrothermal route, and investigated their up/down conversion luminescence, morphological formation mechanism and magnetic properties.14 Xu et al. synthesized NaEu(MoO4)2 with rugby-like microstructures and NaY(MoO4)2 microcrystals with controllable morphologies via an EDTA-mediated hydrothermal method, and investigated their photoluminescence properties.15,16 Lin et al. reported flower-like NaY(MoO4)2 microcrystals synthesized via hydrothermal method using polyvinylpyrrolidone as a template.17 Jiang et al. synthesized NaGd(MoO4)2:Eu3+/Tb3+ phosphors with octahedral morphology via a surfactant (glutamic acid) assisted hydrothermal method.18 Generally, double molybdate nano/microcrystals were prepared by conventional solid-state reaction, molten salt synthesis or additives assisted hydrothermal method. Hydrothermal method based on aqueous-phase reaction at high pressures has a lower synthesis temperature and is morphology-controllable, compared with conventional synthesis technique, such as solid state reaction and molten salt synthesis. However, additives assisted hydrothermal synthesis method, using organic surfactants, organic solvents, capping agents, or templates as additives, has several disadvantages such as toxicity, high cost and difficulty in effective removal of additives. In contrast, additives-free hydrothermal synthesis method under mild conditions is simple, low-cost, and environment-friendly. To the best of our knowledge, facile additives-free and morphology-controllable hydrothermal synthesis of NaGd(MoO4)2 microcrystals, as well as photoluminescence properties of tetragonal NaGd(MoO4)2 plates, has not been reported before.

In this paper, we report a facile morphology-controllable hydrothermal synthesis of NaGd(MoO4)2 microcrystals under mild conditions, and free of additives, such as organic surfactants and solvents, capping agents, or templates. By varying the molar ratios of Na2MoO4/Gd(NO3)3 and pH values of the precursor solutions, NaGd(MoO4)2 microcrystals with different morphologies, including bipyramids, truncated bipyramids, quasi-cubes and tetragonal plates, are synthesized successfully. A possible formation mechanism for tetragonal plate NaGd(MoO4)2 microcrystals is proposed as well. Color tunable luminescence properties of NaGd(MoO4)2:Eu3+,Tb3+ tetragonal microplates are also studied in detail.

2. Experimental section

2.1 Materials

The raw materials Gd2O3 (99.99%), Eu2O3 (99.99%), Tb4O7 (99.99%) were purchased from Sun Chemical Technology Co. Ltd. (Shanghai, China), Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and Aladdin Reagent Co. Ltd. (Shanghai, China) respectively. All other chemicals were of analytical grade. Sodium molybdate dihydrate (Na2MoO4·2H2O, 99%) was purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China). HNO3 and NaOH were purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). All chemical reagents were used as received without any further purification. Deionized water was used throughout.

2.2 Synthesis

Gd(NO3)3 was prepared by dissolving appropriate amounts of Gd2O3 in diluted nitric acid under vigorous stirring and heating in water bath. After the powders were all dissolved, the above solution was evaporated to dryness, and redissolved in appropriate amounts of deionized water to form Gd(NO3)3 solution. Eu(NO3)3 and Tb(NO3)3 were prepared in the same procedure except for using Eu2O3 and Tb4O7 instead of Gd2O3, respectively.

In a typical synthesis procedure for undoped NaGd(MoO4)2 microcrystals, a predetermined amount of Na2MoO4·2H2O was dissolved in 20 mL deionized water with strong magnetic stirring for 10 minutes to form aqueous solution. The above solution was added dropwise into Gd(NO3)3 solution (1 M) under vigorous stirring, and a white precipitate was obtained. The pH value of the obtained colloidal suspension was adjusted by adding diluted HNO3 or NaOH solution drop by drop. After additional agitation for 1 h, the as-obtained colloidal suspension was transferred to a 60 mL Teflon-lined stainless steel autoclave. Then the autoclave was sealed and maintained at 180 °C for 12 h in an oven. After cooling down to room temperature, the final precipitate products were collected by centrifugation, washed several times with deionized water and ethanol, and dried at 60 °C for 5 h in air. NaGd(MoO4)2:Eu3+,Tb3+ microcrystals were synthesized in the same procedure except that a mixed solution of Gd(NO3)3, Eu(NO3)3 and/or Tb(NO3)3 was used instead of Gd(NO3)3 solution. Typical hydrothermal synthetic parameters were as follows, Na2MoO4/Gd(NO3)3 molar ratios and pH values of the precursor solutions were 3[thin space (1/6-em)]:[thin space (1/6-em)]1 and 4.5 for bipyramids, 5[thin space (1/6-em)]:[thin space (1/6-em)]1 and 4.5 for truncated bipyramids, 6[thin space (1/6-em)]:[thin space (1/6-em)]1 and 5.0 for quasi-cubes, and 9[thin space (1/6-em)]:[thin space (1/6-em)]1 and 4.5 for tetragonal plates, respectively.

2.3 Characterization

Crystal structure and phase purity of the samples were examined by powder X-ray diffraction (XRD) performed on a Rigaku D-MAX 2200 VPC diffractometer with Cu Kα radiation (λ = 0.15405 nm) at a scanning rate of 8° min−1 in a 2 theta range of 10–80°. Scanning electron microscope (SEM, JEOL JSM-6330F) and transmission electron microscope (TEM, JEOL JEM-2010HR) were employed for the observation of the morphology. Low-resolution TEM images, high-resolution TEM (HRTEM) images and selected-area electron diffraction (SAED) patterns were performed at an accelerating voltage of 200 kV. The photoluminescence excitation and emission spectra were recorded at room temperature on an Edinburgh FLSP920 spectrometer equipped with a Czemy-Turner monochromator, using a 450 W continuous xenon lamp as excitation source and a R928 red-sensitive photomultiplier tube as detector (excitation and emission slits were both set at a width of 0.5 mm).

3. Results and discussion

3.1 Crystal structure and morphology

NaGd(MoO4)2 microcrystals with different morphologies are synthesized hydrothermally by varying the molar ratios of Na2MoO4/Gd(NO3)3 and pH values of the initial precursor solutions at 180 °C for 12 h. Crystal structures and morphologies of the as-prepared samples are characterized by powder XRD and SEM. Fig. 1 presents the SEM images and XRD patterns of the as-synthesized samples of NaGd(MoO4)2 microcrystals with different morphologies. As can be seen obviously in the SEM images of Fig. 1, the products are uniform and well-crystallized microcrystals with different morphologies including tetragonal plates, quasi-cubes, truncated bipyramids, and bipyramids. All the characteristic diffraction peaks in these XRD patterns of Fig. 1 match well with the tetragonal phase NaGd(MoO4)2 with the space group I41/a (ICDD no. 25-0828) and no additional peaks or other phases can be found, indicating that the synthesized samples are single tetragonal phase NaGd(MoO4)2 with the space group I41/a. The sharp and strong diffraction peaks indicate high crystallinity. In the XRD patterns of the tetragonal plates, the relative diffraction intensity ratio of (004)/(101) increases significantly compared with the standard diffraction patterns, implying preferentially oriented crystallization along the (001) planes, which is in good agreement with the observed morphology. The TEM images, HRTEM images and SAED patterns of bipyramids and tetragonal plates are shown in Fig. S1 (ESI†).
image file: c5ra06221g-f1.tif
Fig. 1 SEM images of the synthesized NaGd(MoO4)2 samples with different morphologies, i.e. (a) bipyramids, (b) truncated bipyramids, (c) quasi-cubes, (d) tetragonal plates. (e) XRD patterns of these samples and standard ICDD pattern (no. 25-0828).

3.2 Effects of pH values and Na2MoO4/Gd(NO3)3 molar ratios on phase and morphology

A series of NaGd(MoO4)2 microcrystals are synthesized by varying pH values and Na2MoO4/Gd(NO3)3 molar ratios of the precursor solution, while synthesizing temperature and reaction time are 180 °C and 12 h, respectively. The effects of the pH values and molar ratios on phase and morphology are systematically investigated.

Fig. 2 shows the SEM images and XRD patterns of the samples obtained at the Na2MoO4/Gd(NO3)3 molar ratio of 3[thin space (1/6-em)]:[thin space (1/6-em)]1 and different pH values varying from 3.5 to 5.5. When the pH value is 3.5, the SEM image shows that the products exhibit the morphology of submicro-bipyramids and hexagonal nanoflakes, and the XRD pattern shows impurity peaks of MoO3 and Gd2(MoO4)3. When the pH value is adjusted to 4.0 or 4.5, well-defined uniform bipyramid-like microcrystals with side length of about 2 μm are observed in the SEM images. Meanwhile, pure tetragonal phase NaGd(MoO4)2 without impurity phases are detected in the corresponding XRD patterns. Moreover, this tetragonal bipyramid-like morphology with 4-fold rotational symmetry is consistent with the symmetrical characteristic of space group I41/a. The morphology of the products become irregular at the pH value of 5.0, and when the pH value is 5.5 the products exhibit irregular sheet-like morphology. The products are pure tetragonal phase as the pH values are 5.0 and 5.5, though the peaks in the XRD pattern of samples synthesized at pH value of 5.5 are broad and weak.


image file: c5ra06221g-f2.tif
Fig. 2 SEM images of the NaGd(MoO4)2 samples synthesized at Na2MoO4/Gd(NO3)3 molar ratio of 3[thin space (1/6-em)]:[thin space (1/6-em)]1 and different pH values: (a) pH = 3.5, (b) pH = 4.0, (c) pH = 4.5, (d) pH = 5.0, (e) pH = 5.5. (f) XRD patterns of these samples and standard ICDD pattern (no. 25-0828). Traces of MoO3 (ICDD no. 35-0609), Gd2(MoO4)3 (ICDD no. 26-0655) and Gd2(MoO4)3 (ICDD no. 24-0428) are marked by ♦, ♠ and ♥ respectively.

Fig. 3 shows the SEM images and XRD patterns of the samples obtained at the Na2MoO4/Gd(NO3)3 molar ratio of 9[thin space (1/6-em)]:[thin space (1/6-em)]1 and different pH values varying from 3.5 to 5.5. When the pH value is 3.5, quasi-cubes are observed. When the pH values increase from 4.0 to 5.0, the morphologies of the products are all uniformly dispersed tetragonal plates, of which the thickness decrease with the increasing pH values. Typically, the thickness and side length of the tetragonal microplates are 0.5 and 2 μm at the pH value of 4.5. When the pH value is increased to 5.5, the samples exhibit irregular sheets. Identified by XRD analysis, all these samples synthesized at molar ratio of 9[thin space (1/6-em)]:[thin space (1/6-em)]1 are pure tetragonal phase NaGd(MoO4)2. As is shown in Fig. 3, the diffraction intensity of (004) increases remarkably in the XRD patterns of the samples synthesized at pH values of 4.0, 4.5 and 5.0. The exposed tetragonal sections of the tetragonal plates present preferentially oriented crystallization along the (001) plane in the NaGd(MoO4)2 microcrystals. A similar morphological variation along with increasing pH values is also found when the Na2MoO4/Gd(NO3)3 molar ratio is 4[thin space (1/6-em)]:[thin space (1/6-em)]1, 6[thin space (1/6-em)]:[thin space (1/6-em)]1 or 8[thin space (1/6-em)]:[thin space (1/6-em)]1 (see Fig. S2, ESI†). The XRD patterns of the products obtained at molar ratios of 4[thin space (1/6-em)]:[thin space (1/6-em)]1, 6[thin space (1/6-em)]:[thin space (1/6-em)]1 and 8[thin space (1/6-em)]:[thin space (1/6-em)]1 indicates that they are all pure phase NaGd(MoO4)2 (see Fig. S3, ESI†).


image file: c5ra06221g-f3.tif
Fig. 3 SEM images of the NaGd(MoO4)2 samples synthesized at Na2MoO4/Gd(NO3)3 molar ratio of 9[thin space (1/6-em)]:[thin space (1/6-em)]1 and different pH values: (a) pH = 3.5, (b) pH = 4.0, (c) pH = 4.5, (d) pH = 5.0, (e) pH = 5.5. (f) XRD patterns of these samples and standard ICDD pattern (no. 25-0828).

Fig. 4 presents the SEM images of the samples synthesized at pH values of 4.5 and different molar ratios varying from 3[thin space (1/6-em)]:[thin space (1/6-em)]1 to 10[thin space (1/6-em)]:[thin space (1/6-em)]1. When the molar ratio is 3[thin space (1/6-em)]:[thin space (1/6-em)]1 or 4[thin space (1/6-em)]:[thin space (1/6-em)]1, the SEM image shows uniform tetragonal bipyramids. When the molar ratio is 5[thin space (1/6-em)]:[thin space (1/6-em)]1 or 6[thin space (1/6-em)]:[thin space (1/6-em)]1, truncated bipyramids are obtained. What's more, morphological truncation degree increase with increasing Na2MoO4/Gd(NO3)3 molar ratios. When the molar ratio is 7[thin space (1/6-em)]:[thin space (1/6-em)]1 or bigger, the morphologies of the samples exhibit tetragonal plates. The thickness of the plates decreases as the molar ratio increases.


image file: c5ra06221g-f4.tif
Fig. 4 SEM images of the NaGd(MoO4)2 samples synthesized at pH values of 4.5 and different Na2MoO4/Gd(NO3)3 molar ratios: (a) 3[thin space (1/6-em)]:[thin space (1/6-em)]1, (b) 4[thin space (1/6-em)]:[thin space (1/6-em)]1, (c) 5[thin space (1/6-em)]:[thin space (1/6-em)]1, (d) 6[thin space (1/6-em)]:[thin space (1/6-em)]1, (e) 7[thin space (1/6-em)]:[thin space (1/6-em)]1, (f) 8[thin space (1/6-em)]:[thin space (1/6-em)]1, (g) 9[thin space (1/6-em)]:[thin space (1/6-em)]1, (h) 10[thin space (1/6-em)]:[thin space (1/6-em)]1. (i) XRD patterns of these samples and standard ICDD pattern (no. 25-0828).

From the above experimental results, it can be found that the pH values of the precursor solutions and the Na2MoO4/Gd(NO3)3 molar ratios have combined effects in the synthesis of pure phase NaGd(MoO4)2 microcrystals with specific regular morphology. The pH values play an important role in the formation of pure phase and regular morphology of the final products. Moreover, the Na2MoO4/Gd(NO3)3 molar ratios play crucial role in the morphology-controllable synthesis of NaGd(MoO4)2 microcrystals. For the case of tetragonal structure system, the surface energy of {001} facets is higher than {101} facets according to Donnay–Harker rules.19 A faster growth rate along the direction perpendicular to the {001} planes with higher surface energy will lead to the formation of tetragonal bipyramid morphology enclosed by exposed {101} facets. The facets with higher surface energy will disappear and not be expressed in the final equilibrium morphology as a result of the minimization of total surface energy, according to Gibbs-Wulff theorem.20,21 Thus bipyramid NaGd(MoO4)2 microcrystals can be synthesized at appropriate pH values under lower Na2MoO4/Gd(NO3)3 molar ratio conditions. Higher or lower pH value would change the growth rate of crystallographic facets with different surface energies. Consequently, the nucleation and growth behavior would be out of kinetic control and the final products tend to be irregular and impure.22 It is found that the precursor solution's pH value of about 4.5 will lead to regular and uniform morphology of products with pure phase and high crystallinity. With regard to the effects of molar ratios, higher Na2MoO4/Gd(NO3)3 molar ratio will cause higher MoO42− content. Due to electrostatic interaction, excess MoO42− ions will preferentially adsorb on {001} planes, which have a higher packing density of Na+ and Gd3+ ions than other faces.23 As a result, the reactivity and the growth rate of high-energy {001} facets will be reduced, and {001} facets will still remain in the final morphology. Therefore, under higher molar ratio conditions, there is a tendency to form tetragonal plates of which the thickness is reduced along with increasing molar ratios. On the basis of the above analyses, well-crystallized NaGd(MoO4)2 microcrystals with different morphologies can be selectively synthesized by judicious choice of Na2MoO4/Gd(NO3)3 molar ratios and pH values of the precursor solution.

3.3 Formation mechanism of NaGd(MoO4)2 micro crystals: tetragonal microplates as an example

As previously mentioned, different Na2MoO4/Gd(NO3)3 molar ratios and pH values of the precursor solution will lead to different crystal morphologies of the products, which is the key to morphology-controllable synthesis of the NaGd(MoO4)2 microcrystals. The growth of {001} facets will be inhibited when adsorbed by excess MoO42− ions. At different molar ratios in an appropriate pH range, tetragonal bipyramids, truncated bipyramids, quasi-cubes and tetragonal plates are produced respectively. The morphologies of the products transform from bipyramids, truncated bipyramids, quasi-cubes to tetragonal plates with increasing Na2MoO4/Gd(NO3)3 molar ratios at pH values of 4.5, as schematically shown in Fig. 5.
image file: c5ra06221g-f5.tif
Fig. 5 Schematic illustration of morphological transformation with increasing Na2MoO4/Gd(NO3)3 molar ratios at appropriate pH value.

Since it's hard to track the growth of nano/microcrystals in solutions directly, the formation mechanisms are mostly elucidated by investigation of morphologies and crystal structures of the samples obtained at different reaction stages. In order to further reveal the formation mechanism of NaGd(MoO4)2 microcrystals, time-dependent evolution experiments of tetragonal plates are conducted as an example. SEM images and XRD patterns of the samples synthesized at 180 °C, pH of 4.5 and molar ratio of 9[thin space (1/6-em)]:[thin space (1/6-em)]1 with different reaction time (0 h, 0.5 h, 1 h, 2 h, 6 h) are presented in Fig. 6.


image file: c5ra06221g-f6.tif
Fig. 6 SEM images of the NaGd(MoO4)2 samples synthesized at 180 °C and pH = 4.5 with different reaction times: (a) 0 h, (b) 0.5 h, (c) 1 h, (d) 2 h, (e) 6 h. (f) XRD patterns of these samples and standard ICDD pattern (no. 25-0828).

Initially, when the reaction time is 0 h or 0.5 h, the XRD patterns of the products exhibit broad and weak peaks from Gd2(MoO4)3, MoO3 and Na2MoO4, which suggests that poor-crystalline intermediates are formed in the precursor solution at the beginning of hydrothermal reaction. The corresponding products exhibit round shape sheets and irregular sheets at reaction time of 0 and 0.5 h, as is shown in SEM images of Fig. 6. When the reaction time is 1 h, the diffraction peaks in the XRD patterns become sharper and stronger, and the peaks fit well with the pure tetragonal phase NaGd(MoO4)2 (ICDD no. 25-0828), indicating that the intermediates are converted to NaGd(MoO4)2. Correspondingly, rudiments of tetragonal plates along with some small particles appear in the SEM image. These show the initial formation of pure phase NaGd(MoO4)2 microcrystals. However, small particles vanish at reaction time of 2 h, leaving tetragonal plates with slightly incomplete edges. The XRD patterns and SEM images of the products obtained at the reaction time of 2 h or longer reveal that the pure phase microplates become more intact and uniform. Eventually, well-crystallized and pure phase NaGd(MoO4)2 microplates are synthesized.

It is generally acknowledged that the kinetics and thermodynamics have comprehensive effects on the morphological formation of nano/microcrystals. For instance, the crystal size distribution is generally determined by the competition between nucleation and crystal growth rate, and the morphology evolution is mainly driven by the reduction in surface free energy. When the sodium molybdate is added into the gadolinium nitrate solution, white precipitation intermediates are formed as precursors. Then the precursors might be dissolved and converted to NaGd(MoO4)2 nuclei gradually. A small amount of rudimental NaGd(MoO4)2 tetragonal plates appears along with the proceeding hydrothermal reaction. Parts of the NaGd(MoO4)2 nuclei grow up to rudimental plates which have a lower surface free energy than the nuclei. Then the large plates grow even bigger at the expense of smaller ones due to further reduction of surface free energy through Ostwald ripening. Finally, uniform microcrystals with the morphology of tetragonal plates are formed. Based on the above analysis, the formation of NaGd(MoO4)2 tetragonal microplates is believed to follow the nucleation-Ostwald ripening growth process, as is schematically shown in Fig. 7.


image file: c5ra06221g-f7.tif
Fig. 7 Schematic illustration of the formation mechanism of NaGd(MoO4)2 microplates.

3.4 Photoluminescence properties of NaGd(MoO4)2:Eu3+,Tb3+

3.4.1 Photoluminescence properties of NaGd(MoO4)2:5% Eu3+ with different morphologies. Fig. 8 shows the photoluminescence excitation spectra of NaGd(MoO4)2:5% Eu3+ microcrystals with different morphologies, monitoring the red emission of Eu3+ at 616 nm. As is shown in Fig. 8, a broad excitation band from 250 to 350 nm is observed, which is attributed to the O2−–Mo6+ charge transfer (C-T) absorption. The other sharp lines in the range from 360 nm to 550 nm are assigned to characteristic intra-configurational f–f transitions of Eu3+ ions, i.e., 7F0 → 5D1 (535.5 nm), 7F0 → 5D2 (465.5 nm), 7F0 → 5D3 (416.5 nm), 7F0 → 5L6 (395 nm), and 7F0 → 5D4 (362.5 nm).
image file: c5ra06221g-f8.tif
Fig. 8 Excitation spectra of NaGd(MoO4)2:5% Eu3+ microcrystals with different morphologies: (a) bipyramids, (b) truncated bipyramids, (c) quasi-cubes, (d) tetragonal plates. The inset is partial excitation spectra of C-T band.

Fig. 9 shows the photoluminescence emission spectra of NaGd(MoO4)2:5% Eu3+ microcrystals with different morphologies under 465.5 nm excitation. The emission spectra consists of several 5D0 → 7FJ (J = 1, 2, 3, 4) emission lines of Eu3+ at 592, 616, 655 and 702 nm, with the strongest emission at 616 nm for J = 2. The emission peak at 616 nm is observed to split into two sub-peaks due to the Stark energy splitting. Eu3+ ions are universally known as structure probes to investigate the local environment in crystal. If the electric dipole transition 5D0 → 7F2 is dominant, the Eu3+ ions occupy sites without inversion symmetry; however, when the magnetic dipole transition 5D0 → 7F1 is dominant, the Eu3+ ions are in the sites of inversion center. In our case, the emission peak at 616 nm, which is contributed to 5D0 → 7F2 transition, is stronger than other peaks, indicating that Eu3+ ions occupy the asymmetry sites. Furthermore, the predominant emission peak at 616 nm is favorable to the high purity of red color.


image file: c5ra06221g-f9.tif
Fig. 9 Emission spectra of NaGd(MoO4)2:5% Eu3+ microcrystals with different morphologies: (a) bipyramids, (b) truncated bipyramids, (c) quasi-cubes, (d) tetragonal plates.

Among different morphologies of NaGd(MoO4)2:5% Eu3+ microcrystals (i.e. bipyramids, truncated bipyramids, quasi-cubes and tetragonal plates), their C-T bands in photoluminescence excitation spectra have a significant difference as is shown in the inset of Fig. 8. The C-T band of tetragonal microplates broadens, its peak shifts to long-wavelength direction and its intensity increases, compared to that of bipyramids (the peak of C-T band is located at 302 nm for bipyramids, and 311.5 nm for microplates). The full-width at half-maximum (FWHM) for the C-T band of tetragonal plates is about 58 nm, which is the highest among different morphologies (the FWHM is 49, 51 and 55 nm for the C-T band of bipyramids, truncated bipyramids and quasi-cubes, respectively). A broadened, red-shifted and enhanced charge transfer band in the excitation spectra favors efficient excitation and absorption in ultraviolet (UV) region for various optoelectronic device applications.

3.4.2 Color tunable photoluminescence properties of NaGd(MoO4)2:Eu3+,Tb3+ tetragonal microplates. Fig. S4 (ESI†) shows the photoluminescence excitation and emission spectra of NaGd(MoO4)2:5% Tb3+ tetragonal plates. The excitation spectrum obtained by monitoring the green emission of Tb3+ ions at 544 nm reveals a broad excitation band (with maximum at 291 nm) from 250 to 350 nm and a sharp absorption peak at 486.5 nm. The former (broad band) is ascribed to the C-T transition between Mo6+ and O2− and the latter (peak centered at 486.5 nm) is assigned to the 7F6 → 5D4 transition of Tb3+ ions. The emission spectrum under 291 nm excitation exhibits several emission peaks at 489, 544, 588 and 622 nm assigned to the characteristic 5D4 → 7FJ (J = 3–6) transitions of Tb3+ ions. The green emission at 544 nm (5D4 → 7F5 transition of Tb3+) is the strongest among the transition emissions.

The emission spectra of NaGd(MoO4)2:Eu3+, Tb3+ tetragonal plates excited by 291 nm are depicted in Fig. 10. It is observed that the emission intensity at 544 nm increases gradually with increasing Tb3+ concentrations, and the red emission intensity at 616 nm increases with increasing Eu3+ concentrations. While the relative intensity of red emission at 616 nm reaches a maximum in NaGd(MoO4)2:4% Eu3+,1% Tb3+, then decreases with further increasing Eu3+ concentrations (or decreasing Tb3+ concentrations), as can be seen in the inset of Fig. 10. This may be the result of energy transfer from Tb3+ to Eu3+ ions. As is shown in Fig. S5 (ESI†), there are overlaps between the emission spectrum of NaGd(MoO4)2:Tb3+ and the excitation spectrum of NaGd(MoO4)2:Eu3+, implying a possible energy transfer from Tb3+ to Eu3+ ions.24 As is shown in Fig. S6 (ESI†), in the excitation spectrum of NaGd(MoO4)2:1% Eu3+,4% Tb3+ monitoring the red emission of Eu3+ at 616 nm, a excitation peak of Tb3+ at 486.5 nm is observed. And the emission spectrum of NaGd(MoO4)2:1% Eu3+,4% Tb3+ under excitation of 486.5 nm (Tb3+ characteristic absorption peak) exhibits characteristic emission peak of Eu3+ ions at 616 nm (5D0 → 7F2,4). All these evidences point to the energy transfer process from Tb3+ to Eu3+, which has been observed in various hosts, such as NaGd(MoO4)2 phosphors, KLa(MoO4)2 microcrystals, CaCO3 phosphors, and Y2O3 nanorods.18,25–27 So it is speculated that there exists an energy transfer process from Tb3+ to Eu3+ ions in NaGd(MoO4)2 microplates. When a small amount of Tb3+ ions is introduced to NaGd(MoO4)2 microplates doped with Eu3+ ions, the cross-relaxation of Tb3+(5D4) + Eu3+(7F0) → Tb3+(7F4,5,6) + Eu3+(5D0,1,2) and thereafter non-radiative multiphonon relaxation from the state 5D1,2 to 5D0 of Eu3+ ions will enhance the Eu3+ 5D0 → 7FJ radiative transitions, in which the red emission of 5D0 → 7F2 is dominated (see Fig. 11 energy level scheme).27 So the red emission intensity in the sample doped with 4% Eu3+ and 1% Tb3+ is stronger than that in the sample doped with 5% Eu3+. The emission intensity of 616 nm in 5% Eu3+ doped sample is higher than that of 544 nm in 5% Tb3+ doped sample, suggesting higher luminescent efficiency of Eu3+ than that of Tb3+ in NaGd(MoO4)2 hosts under 291 nm excitation of C-T band. There is a similar phenomenon in the emission spectra of NaGd(MoO4)2:Eu3+,Tb3+ under 380 nm near-UV excitation, as is shown in Fig. S7 (ESI†).


image file: c5ra06221g-f10.tif
Fig. 10 Emission spectra of NaGd(MoO4)2:Eu3+,Tb3+ microplates with different doping concentrations excited at 291 nm: (a) 5% Eu3+, (b) 4% Eu3+, 1% Tb3+, (c) 3% Eu3+, 2% Tb3+, (d) 2% Eu3+, 3% Tb3+, (e) 1% Eu3+, 4% Tb3+, (f) 5% Tb3+. The inset is the relative intensities of red emission (616 nm) with different concentrations of Eu3+ and Tb3+ ions.

image file: c5ra06221g-f11.tif
Fig. 11 Energy level scheme of energy transfer and radiative transition process of Tb3+ and Eu3+.

Fig. 12 shows the Commission Internationale de l'éclairage (CIE, or International Commission on Illumination in English) chromaticity diagram of the emission spectra of NaGd(MoO4)2:Eu3+,Tb3+ microcrystals with different concentrations. The chromaticity coordinates shift from deep red (x = 0.656, y = 0.333) to green (x = 0.341, y = 0.565) with the increasing Tb3+ or decreasing Eu3+ under UV excitation of 291 nm. When excited at near-UV 380 nm, the chromaticity coordinates shift from red (x = 0.609, y = 0.325) to pink (x = 0.474, y = 0.298). Thus a series of visible emission colors, such as red, orange, yellow, green and pink can be tuned efficiently by adjusting the doping concentration of Eu3+ and Tb3+ ions in NaGd(MoO4)2 microcrystals under 291 nm UV excitation or 380 nm near-UV excitation.


image file: c5ra06221g-f12.tif
Fig. 12 CIE chromaticity diagram of NaGd(MoO4)2:Eu3+,Tb3+ microplates with different doping concentrations excited at 291 nm ((a) 5% Eu3+; (b) 4% Eu3+, 1% Tb3+; (c) 3% Eu3+, 2% Tb3+; (d) 2% Eu3+, 3% Tb3+; (e) 1% Eu3+, 4% Tb3+; (f) 5% Tb3+) and 380 nm ((g) 5% Eu3+; (h) 4% Eu3+, 1% Tb3+; (i) 3% Eu3+, 2% Tb3+; (j) 2% Eu3+, 3% Tb3+; (k) 1% Eu3+, 4% Tb3+).

4. Conclusions

In conclusion, well-crystallized NaGd(MoO4)2 microcrystals with the morphologies of bipyramids, truncated bipyramids, quasi-cubes and tetragonal plates are successfully synthesized via a facile additives-free hydrothermal synthesis method. The molar ratios of Na2MoO4/Gd(NO3)3 and pH values of the precursor solutions play key roles in the selective synthesis of NaGd(MoO4)2 microcrystals with different morphologies. A pH value of 4.5 is suitable for forming pure and regular morphology of the final products. The morphological truncation degree of the obtained NaGd(MoO4)2 microcrystals increases with increasing Na2MoO4/Gd(NO3)3 molar ratios. Well-crystallized and uniform NaGd(MoO4)2 tetragonal microplates are synthesized at molar ratio of 9[thin space (1/6-em)]:[thin space (1/6-em)]1 and pH value of 4.5. A possible formation mechanism of nucleation-Ostwald ripening is proposed to explain the formation and growth of NaGd(MoO4)2 tetragonal microplates. This facile morphology-controlled synthesis strategy may be extended to other inorganic materials.

C-T band in the excitation spectrum of tetragonal microplates is broadened, red-shifted and has a highest full-width at half-maximum (58 nm) among different morphologies, which favors efficient excitation and absorption in ultraviolet region for various optoelectronic device applications. The introduction of a small amount of Tb3+ ions into NaGd(MoO4)2 microplates doped with Eu3+ will enhance the 5D0 → 7FJ transitions of Eu3+ at 616 nm due to the energy transfer process of cross-relaxation from Tb3+ to Eu3+. As is confirmed by CIE chromaticity diagram, multicolor photoluminescence can be tuned efficiently in NaGd(MoO4)2 microplates from deep red, orange, yellow to green under 291 nm UV excitation and from red, reddish orange to pink under 380 nm near-UV excitation, by simply adjusting the doping concentration of Eu3+ and Tb3+. These results suggest that NaGd(MoO4)2:Eu3+,Tb3+ microcrystals may have practical applications in various optoelectronic devices, such as light emitting diodes and color display systems.

Acknowledgements

This work was supported by the National Natural Science Foundation of China under Grant no. 61176010 and no. 61172027, Guangdong Natural Science Foundation under Grant no. 2014A030311049, and the Research Foundation of IARC-SYSU under Grant no. IARC 2014-09.

References

  1. M. Haase and H. Schäfer, Angew. Chem., Int. Ed., 2011, 50, 5808–5829 CrossRef CAS PubMed.
  2. S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma and Q. Y. Zhang, Mater. Sci. Eng., R, 2010, 71, 1–34 CrossRef PubMed.
  3. S. V. Eliseeva and J. C. G. Bünzli, New J. Chem., 2011, 35, 1165–1176 RSC.
  4. H. Q. Wang, M. Batentschuk, A. Osvet, L. Pinna and C. J. Brabec, Adv. Mater., 2011, 23, 2675–2680 CrossRef CAS PubMed.
  5. A. Kudo and H. Kato, Chem. Phys. Lett., 2000, 331, 373–377 CrossRef CAS.
  6. G. Benoit, J. Véronique, A. Arnaud and G. Alain, Solid State Sci., 2011, 13, 460–467 CrossRef CAS PubMed.
  7. S. B. Stevens, C. A. Morrison, T. H. Allik, A. L. Rheingold and B. S. Haggerty, Phys. Rev. B: Condens. Matter Mater. Phys., 1991, 43, 7386–7394 CrossRef CAS.
  8. J. M. Cano-Torres, M. D. Serrano, C. Zaldo, M. Rico, X. Mateos, J. Liu, U. Griebner, V. Petrov, F. J. Valle, M. Galan and G. Viera, J. Opt. Soc. Am. B, 2006, 23, 2494–2502 CrossRef CAS.
  9. X. Han, F. Fusari, M. D. Serrano, A. A. Lagatsky, J. M. Cano-Torres, C. T. A. Brown, C. Zaldo and W. Sibbett, Opt. Express, 2010, 18, 5413–5419 CrossRef CAS PubMed.
  10. J. Tang, Y. Chen, Y. Lin, X. Gong, J. Huang, Z. Luo and Y. Huang, Opt. Mater. Express, 2012, 2, 1064–1075 CrossRef CAS.
  11. F. Mo, L. Zhou, Q. Pang, F. Gong and Z. Liang, Ceram. Int., 2012, 38, 6289–6294 CrossRef CAS PubMed.
  12. A. Katelnikovas, J. Plewa, S. Sakirzanovas, D. Dutczak, D. Enseling, F. Baur, H. Winkler, A. Kareiva and T. Jüstel, J. Mater. Chem., 2012, 22, 22126–22134 RSC.
  13. T. Wu, Y. Liu, Y. Lu, L. Wei, H. Gao and H. Chen, CrystEngComm, 2013, 15, 2761–2768 RSC.
  14. R. Krishnan and J. Thirumalai, New J. Chem., 2014, 38, 3480–3491 RSC.
  15. L. Xu, X. Yang, Z. Zhai, X. Chao, Z. Zhang and W. Hou, CrystEngComm, 2011, 13, 4921–4929 RSC.
  16. L. Xu, X. Yang, H. Lu, C. Hu and W. Hou, RSC Adv., 2014, 4, 13502–13508 RSC.
  17. H. Lin, X. Yan and X. Wang, J. Solid State Chem., 2013, 204, 266–271 CrossRef CAS PubMed.
  18. Y. Jiang, Y. Liu, G. Liu, X. Dong, J. Wang, W. Yu and Q. Dong, Opt. Mater., 2014, 36, 1865–1870 CrossRef CAS PubMed.
  19. J. D. H. Donnay and D. Harker, Am. Mineral., 1937, 22, 446–467 CAS.
  20. G. Wulff, Z. Kristallogr., 1901, 34, 449–530 CAS.
  21. T. K. Sau and A. L. Rogach, Adv. Mater., 2010, 22, 1781–1804 CrossRef CAS PubMed.
  22. X. Sun and Y. Li, Chem. Commun., 2003, 1768–1769 RSC.
  23. S. Liu, D. Yang, D. Ma, S. Wang, T. Tang and S. Huang, Chem. Commun., 2011, 47, 8013–8015 RSC.
  24. V. Naresh and S. Buddhudu, J. Lumin., 2013, 137, 15–21 CrossRef CAS PubMed.
  25. L. Hou, S. Cui, Z. Fu, Z. Wu, X. Fu and J. H. Jeong, Dalton Trans., 2014, 43, 5382–5392 RSC.
  26. Q. Cheng, Y. Dong, M. Kang and P. Zhang, J. Lumin., 2014, 156, 91–96 CrossRef CAS PubMed.
  27. Z. Liu, L. Yu, Q. Wang, Y. Tao and H. Yang, J. Lumin., 2011, 131, 12–16 CrossRef CAS PubMed.

Footnote

† Electronic supplementary information (ESI) available: TEM, HRTEM and SEM images, SAED and XRD patterns, and photoluminescence spectra of some samples (Fig. S1–S7). See DOI: 10.1039/c5ra06221g
This journal is © The Royal Society of Chemistry 2015
Click here to see how this site uses Cookies. View our privacy policy here.