Influence of microwave hydrothermal reaction factor on the morphology of NaY(MoO₄)₂ nano-/micro-structures and luminescence properties of NaY(MoO₄)₂:Tb³⁺

Abstract

The reaction conditions including exterior and interior factors in hydrothermal syntheses play very important roles. In this work, the influences of pH, Cit³⁻/Ln³⁺ and MoO₄²⁻/Ln³⁺ on the crystal structure and morphology of NaY(MoO₄)₂ microstructures derived from a microwave hydrothermal synthesis were systematically studied. It was found that certain morphologies of the NaY(MoO₄)₂:Tb³⁺ particles were only formed at critical pH values, and different amounts of Na₃Cit and Na₂(MoO₄)₂ in the reaction solution functioned in opposite ways in the crystal nucleation and growth. We also proposed that the competitive equilibrium between Na₃Cit and Na₂(MoO₄)₂ was responsible for the self-assembly behavior of the NaY(MoO₄)₂:Tb³⁺ nano-/micro-crystals. Based on the possible growth mechanism of the NaY(MoO₄)₂ microstructures, additional Cit³⁻/MoO₄²⁻/Ln³⁺ molar ratios for preparing the target products were designed and the corresponding samples were prepared. It was proven that the self-assembly behavior of the NaY(MoO₄)₂:Tb³⁺ crystals can be successfully modified under certain Cit³⁻/MoO₄²⁻/Ln³⁺ molar ratios. We also discovered that the morphology can slightly affect the luminescence intensity of the NaY(MoO₄)₂:Tb³⁺ phosphors. The temperature and concentration quenching behaviors of the NaY(MoO₄)₂:Tb³⁺ phosphors were studied. The crossover process was found to be responsible for the temperature quenching of ⁵D₄ fluorescence, and the electric dipole–dipole interaction was proven to be the physical nature of the concentration quenching and the fluorescence decay process.

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1 Introduction

The hydrothermal synthetic strategy has been a facile and effective approach for preparing many inorganic nano-/micro-architectures of fluorides, phosphates, tungstates and molybdates.^1–3 It is generally known that the processes of crystal nucleation and growth, along with self-assembly in hydrothermal reactions have significant impact on the morphologies of inorganic nano-/micro-crystals. Herein, the successful preparation of intriguing inorganic nano-/micro-architectures is greatly dependent on better control of the above processes. It is also admitted that all the controllable conditions in a hydrothermal reaction could be classified as two categories, namely interior and exterior conditions.⁴ The interior conditions mainly involve some parameters in the reaction system, such as pH values, reactant concentration, time, temperature, templates and organic additives,^5–8 while the exterior conditions generally relate to the reaction environment such as the modes of energy input.^9,10 With a sophisticated understanding and precise control of the crystal nucleation, growth and self-assembly processes, morphology modifications might be designed and conducted more easily.

Molybdates and tungstates prepared by a hydrothermal route have demonstrated many fascinating nano-/micro-structures, and a lot of effort has been devoted to controlling and modifying their microscopic morphologies. Based on adjusting both kinds of conditions, various morphologies of molybdate and tungstate nano-/microstructures have been developed. For instance, by adjusting the interior reaction system conditions, 3D bowknot-like hierarchical architectures of Y₂(WO₄)₃ were synthesized with the assistance of sodium dodecyl benzenesulfonate (SDBS);¹¹ Gd₂(WO₄)₃ micro-belts, stars and flowers formed in the presence of cetyltrimethylammonium bromide (CTAB);¹² rugby-like microstructures of NaEu(MoO₄)₂ were formed in an ethylenediaminetetraacetic acid (EDTA)-mediated hydrothermal process;¹³ and NaY(MoO₄)₂ with different morphologies (spherical, rhombic, sheet-like, and rectangular plate-like micro-structures) were also synthesized by simply adjusting the pH values and molar ratios of Y(NO₃)₃/Na₂MoO₄.¹⁴ All the while, the variation of exterior reaction environment conditions could also bring about intriguing micro-architectures. By adopting a microwave-assisted hydrothermal process, multiple morphologies of NaLa(MoO₄)₂ were synthesized without any organic additives or surfactants.¹⁵ In this case, the microwave-assisted hydrothermal method is a rapid, facile, efficient and green way to prepare inorganic nano-/micro-structures.^16–18 The reaction rate for microwave dielectric heating, which basically consists of dipolar polarization and ionic conduction, is accelerated more effectively than conventional heating. This non-classical energy source has been shown to dramatically reduce processing time, increase product yields, and enhance product purity and/or material properties compared to conventionally processed experiments. Thus microwave-assisted liquid phase routes for preparing inorganic nanomaterials have gained increasing popularity in recent years. Yet, as a branch of microwave chemistry, it hasn’t reached its full potential.

It is also worth noting that because of their outstanding physical and chemical stability, molybdates and tungstates are also ideal host matrices for luminescent materials. Strong up-conversion emissions were observed in Er³⁺/Yb³⁺, Tm³⁺/Yb³⁺ and Ho³⁺/Yb³⁺-doped NaY(WO₄)₂.⁹ NaEu(MoO₄)₂ exhibited excellent luminescence behaviors with a high color purity, which led to potential applications in LED techniques.¹³ Strong and multi-color emissions were realized in Eu³⁺/Tb³⁺ co-doped Gd₂(WO₄)₃.¹²

Though micro-/nano-molybdate structures doped with rare earth ions derived from hydrothermal routes have been widely studied, a systematic investigation of the influence of the reaction factors on the morphology of molybdates is still lacking. In this paper, we demonstrated the influence of pH, and Cit³⁺/Ln³⁺ and MoO₄²⁺/Ln³⁺ ratios on the crystal structure and morphology of the final products obtained from a microwave hydrothermal route. Various morphologies of NaY(MoO₄)₂ nano-/micro-crystals including nano-/micro-sheets and assembled micro-structures have been selectively obtained by simply controlling the reaction system conditions. The crystal nucleation, growth and self-assembly processes and possible mechanisms were investigated and discussed. Moreover, as a luminescent center, the Tb³⁺ ion was introduced into the microwave hydrothermal-reaction-derived NaY(MoO₄)₂ and its spectroscopic properties were studied.

2 Experimental

All the rare earth oxides including Y₂O₃ (99.99%) and Tb₄O₇ (99.99%) were purchased from Shanghai Second Chemical Reagent Factory (China). Other chemicals including sodium citrate (Na₃Cit) and Na₂MoO₄·2H₂O of analytical grade were purchased from Tianjin Reagent Chemicals Co. Ltd (China). All chemicals were used without any further purification.

All the lanthanide nitrates used for preparing the final samples were obtained by dissolving the rare earth oxides into dilute nitric acid, and the excess nitric acid was removed by evaporating the solution several times. In the microwave hydrothermal syntheses of the NaY(MoO₄)₂ nano-/micro-structures, the reaction factors were adjusted as listed in Table 1. It should be mentioned that the pH of the reaction solution was adjusted using NaOH (1 M) and HNO₃ (1 M). A series of samples with various Tb³⁺ concentrations were also synthesized in order to study the spectroscopic properties. As an example, here we just mention a typical process for the synthesis of NaY(MoO₄)₂:Tb³⁺ nano-sheets. First, 2 mmol of sodium citrate was dissolved into 5 mL of distilled water, and a 4 mL solution containing 2 mmol of lanthanide nitrates (1.9 mmol Y(NO₃)₃ and 0.1 mmol Tb(NO₃)₃) was subsequently added into the above solution, which formed a white colloidal precipitate. After vigorously stirring for 15 min, another 5 mL solution containing 4 mmol of Na₂MoO₄ was added, and the pH value of the mixed solution was adjusted to 8. Then the mixed solution was left under stirring for an additional 20 min. The as-obtained mixed solution was transferred to a 30 mL silica glass vessel sealed with a special cap provided by the manufacturer, and then the vessel was put into a Microwave Synthesizer (Biotage Initiator, Sweden) and irradiated for 1 h at 180 °C. After microwave heating, the vessel was naturally cooled to room temperature. The precipitate was separated by centrifugation and washed with distilled water and anhydrous ethanol several times, and then dried in air at 80 °C for 6 h. The final product was obtained through calcination of the precursor at 600 °C for 1 h.

Table 1 A brief summary of the reaction factors and the morphologies of the final products^a

No.	Doping	pH	Cit^3−/Ln^3+ (molar ratio)	MoO4^2−/Ln^3+ (molar ratio)	Morphology
a In all cases the experiment was carried out with 1 h microwave irradiation at 180 °C. Ln^3+ represents 2 mmol lanthanide ions (1.9 mmol Y(NO3)3 and 0.1 mmol Tb(NO3)3).
1	Tb^3+ doped	5	1	2	Irregular aggregation
2		6	1	2	Irregular aggregation
3		7	1	2	Nano-sheets
4		8	1	2	Nano-sheets
5		9	1	2	Irregular nano-particles
6		10	1	2	Irregular nano-particles
7		8	0.5	2	Nano-sheets
8		8	1.5	2	Micro-sheets
9		8	2	2	Micro-sheets
10		8	1.5	3	Micro-flowers
11		8	1.5	4	Micro-flowers
12		8	1.5	5	Irregular nano- and micro-sheets
13		8	1.5	6	Irregular nano-sheets
14		8	1.5	7	Irregular nano-sheets

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X-ray diffraction (XRD) patterns were collected on a PANalytical diffractometer (PANalytical Empyrean, Netherlands) in the 2θ range from 10° to 70° with graphite-monochromatized Cu-Kα1 radiation (λ = 0.15406 nm). The phase identifications were performed with the PDF-2 database provided by the International Centre for Diffraction Data (ICDD). Morphologies and microscopic structures were observed by field emission scanning electron microscopy (FE-SEM, SUPRA 55 SAPPHIRE, ZEISS, Germany) and high resolution transmission electron microscopy (HRTEM, JEM-2100F, JOEL, Japan). Fluorescence spectra and decays were obtained using a F-4600 spectrophotometer (Hitachi, Japan). The temperature controlling system, with which the samples can be accurately heated to temperatures ranging from room temperature to 723 K, was independently assembled in our lab.

3 Results and discussion

3.1 Crystal structure of products

In order to understand the influence of the reaction factors on the crystal structure, the XRD patterns of all final products derived from the microwave hydrothermal reactions were measured. Fig. 1(a) shows the XRD patterns for the samples prepared with the reaction factors of various Cit³⁻/Ln³⁺ ratios, a fixed pH of 8, and a MoO₄²⁻/Ln³⁺ ratio of 2 (see 4^th and 7–9^th rows in Table 1). Fig. 1(b) exhibits the XRD patterns for the samples prepared with different molar ratios of MoO₄²⁻/Ln³⁺ but the same pH of 8 and the identical Cit³⁻/Ln³⁺ ratio of 1.5 (see 10–14^th rows in Table 1). Fig. 1(c) depicts the XRD patterns for the samples obtained under the conditions of different pH values, the same MoO₄²⁻/Ln³⁺ ratio of 2 and the same Cit³⁻/Ln³⁺ ratio of 1 (see 1^st–6^th rows in Table 1). The XRD patterns for the samples with various Tb³⁺ concentrations synthesized under the conditions of pH = 8, MoO₄²⁻/Ln³⁺ ratio = 2 and Cit³⁻/Ln³⁺ ratio = 1.5 were also measured, and similar diffraction patterns were obtained. As examples, the XRD patterns for the samples with the highest and lowest Tb³⁺ concentrations are shown in Fig. 1(d).

Fig. 1 XRD patterns for the samples prepared under the following conditions: (a) different molar ratios of Cit³⁻/Ln³⁺ when pH = 8 and MoO₄²⁻/Ln³⁺ = 2; (b) different molar ratios of MoO₄²⁻/Ln³⁺ when pH = 8 and Cit³⁻/Ln³⁺ = 1.5; (c) different pH values when Cit³⁻/Ln³⁺ = 1 and MoO₄²⁻/Ln³⁺ = 2; (d) the samples with the lowest and highest Tb³⁺ doping concentrations prepared when pH = 8, Cit³⁻/Ln³⁺ = 1.5 and MoO₄²⁻/Ln³⁺ = 2. The last pattern at the bottom of each of (a)–(d) is the diffraction pattern plotted by using the data reported in PDF card no. 82-2369.

The XRD pattern for the pure phase NaY(MoO₄)₂ powder plotted by using the data reported in PDF card no. 82-2369 was also shown in each part of Fig. 1 at the bottom for the convenience of crystal phase identification. It can be seen that all the samples except for the one prepared at pH = 5 exist in tetragonal phase with body-centered symmetry. In Fig. 1(c) the impurity phase can be assigned to Y₂(MoO₄)₃ (PDF card no. 28-1451, the diffraction peaks were labeled by asterisks). Additionally, the HRTEM images of some samples were obtained for further characterization of the microscopic structure of these samples, as presented in Fig. S1.† It could be easily concluded that all the samples crystallized well because of these highly ordered lattice fringes to a large extent. The distances between two adjacent fringes are estimated to be about 0.31 and 0.48 nm, which matches well with the theoretical values for the (112) and (101) crystal planes, again confirming that the obtained product is tetragonal phase NaY(MoO₄)₂. From the above results, it can be concluded that the ratios of Cit³⁻/Ln³⁺ and MoO₄²⁻/Ln³⁺ in the studied extents will not affect the crystal structure, and that the products of tetragonal phase NaY(MoO₄)₂ will be obtained at a pH value higher than 6; doping with Tb³⁺ (replacement of Y³⁺ by Tb³⁺) will not cause any change in the crystal structure.

3.2 Morphology control of NaY(MoO₄)₂ nano- and micro-structures

In order to recognize how the reaction factors influence the morphologies of the final products, FE-SEM images for all the prepared samples were taken. The observed morphologies are summarized in Table 1 in the last column and the influence of pH, Cit³⁻/Ln³⁺ = 1 and MoO₄²⁻/Ln³⁺ ratios are discussed below.

3.2.1 Influence of pH. All the SEM images for the samples prepared at various pH values with Cit³⁻/Ln³⁺ = 1 and MoO₄²⁻/Ln³⁺ = 2 were measured and are shown in Fig. 2(a–h). It is found that well-defined morphologies of the final products can only be obtained when the pH value of the reaction solution reaches 7–8. When the pH value was 5, the NaY(MoO₄)₂ exhibited an irregular aggregation morphology, and the XRD pattern in Fig. 1(c) indicates that the resultant is not a pure phase. When the pH value was 6, though the product was pure phase NaY(MoO₄)₂, its morphology in Fig. 2(b) displays an amorphous form different to that obtained in the case of pH = 5, some nano-sheets with vague shapes can be observed, and it is believed that these nano-sheets may serve as the structural cell for further assembling microstructures. This fact indicates that the acid solution environment does not support the growth of pure phase NaY(MoO₄)₂ micro-/nano-structures. When the pH value is 7–8, nano-sheets with regular shapes are formed as shown in Fig. 2(c–f). The average width and length of these nano-sheets are ∼77 nm and ∼380 nm, respectively. However, the thicknesses for the nano-sheets prepared at different pH values are different: the thickness of the nano-sheets prepared at pH 7 is smaller than that of those prepared at pH 8. When the pH value of the reaction solution is higher, for example 9 and 10, irregular nano-particles were formed. Therefore, it can be concluded that an alkalescent solution environment benefits the growth and assembling of the nano-/micro-structures, and strong alkaline conditions are helpful for the growth of superfine nanoparticles.

Fig. 2 FE-SEM images of the NaY(MoO₄)₂:5 mol% Tb³⁺ samples synthesized at different pH values when the molar ratios of Cit³⁻/Ln³⁺ and MoO₄²⁻/Ln³⁺ are fixed to be 1 and 2. (a) pH = 5, (b) pH = 6 (c and d) pH = 7, (e and f) pH = 8, (g) pH = 9, (h) pH = 10.

3.2.2 Influence of the Cit³⁻/Ln³⁺ molar ratio. Organic additives in the hydrothermal synthetic environment could effectively control the product morphology by interfering with the crystal growth and assembly behavior due to their multiple roles, for example as soft templates, absorbing agents, assembly agents and reacting sources, etc. For the sake of exploring what role Na₃Cit plays in the formation of the final morphology of NaY(MoO₄)₂ nano-/micro-structures, in this work, different amounts of Na₃Cit, which was defined as the molar ratio of Cit³⁻ to Ln³⁺ (Y³⁺ + Tb³⁺), were introduced into the reaction system. SEM images of NaY(MoO₄)₂:5% Tb³⁺ nano- and micro-crystals prepared under different molar ratios of Cit³⁻/Ln³⁺ are presented in Fig. 3(a–f). When the molar ratio of Cit³⁻/Ln³⁺ is 0.5, nano-sheets with a non-uniform length ranging from ∼100 to ∼300 nm formed, as shown in Fig. 3(a) and (b). The average width of these nano-sheets is ∼73 nm. It is worth noting that the nano-sheets (see Fig. 2(e) and (f)) obtained when Cit³⁻/Ln³⁺ is 1 have the same width (∼77 nm) as those prepared at Cit³⁻/Ln³⁺ = 0.5, but their average length grows up to ∼382 nm. As displayed in Fig. 3(c) and (d), when the Cit³⁻/Ln³⁺ molar ratio is 1.5, the nano-sheets turned into micro-sheets, and the length of these micro-sheets is non-uniform. The average width and thickness of the NaY(MoO₄)₂:5% Tb³⁺ micro-sheets are ∼430 nm and 79 nm, respectively. When the molar ratio is 2, the average width and thickness of the obtained micro-sheets expanded to ∼1.7 μm and ∼250 nm, respectively, as exhibited in Fig. 3(e) and (f).

Fig. 3 FE-SEM images of NaY(MoO₄)₂:5% Tb³⁺ synthesized under different molar ratios of Cit³⁻/Ln³⁺: (a and b) 0.5, (c and d) 1.5, (e and f) 2 with pH = 8 and MoO₄²⁻/Ln³⁺ = 2.

From the above results, it can be deduced that the Na₃Cit content plays a more obvious role than the pH value in the microwave hydrothermal reaction for preparing NaY(MoO₄)₂ nano-/micro-structures. In the studied Na₃Cit content region, nano-sheets with various sizes and dimensions can be obtained, thus implying that micro-architectures with different morphologies may be assembled in this solution reaction system by controlling other experimental factors.

3.2.3 Influence of the MoO₄²⁻/Ln³⁺ molar ratio. It is well known that the reaction solution composition is also an important factor influencing the morphology of the final product. Therefore, in this section we investigate the effect of the MoO₄²⁻ content on the sample morphology. In doing so, the pH value and Cit³⁻/Ln³⁺ molar ratio are fixed to be 8 and 1.5, and the doping concentration of Tb³⁺ is 5 mol%, but the amount of Na₂MoO₄, which was presented as the molar ratio of MoO₄²⁻ to Ln³⁺ (Y³⁺ + Tb³⁺), is adjusted. FE-SEM images of all the samples derived in this sense were taken and shown in Fig. 4.

Fig. 4 FE-SEM images of NaY(MoO₄)₂:5% Tb³⁺ synthesized under the conditions of different molar ratios of MoO₄²⁻/Ln³⁺: (a and b) 3, (c and d) 4, (e) 5, (f) 6, (g) 7 when the pH value and Cit³⁻/Ln³⁺ ratio were fixed to be 8 and 1.5, respectively.

SEM images for the sample synthesized under the condition where MoO₄²⁻/Ln³⁺ = 3 are shown in Fig. 4(a) and (b). From Fig. 4(a) and (b), it is found that spherical flowers assembled from uniform nano-sheets with an average thickness of 90 nm were formed. The micro-flowers are assembled in a form where the large planes of most of the nano-sheets are randomly arrayed down in the radial direction, and the size of these micro-flowers is around 4 μm. For MoO₄²⁻/Ln³⁺ = 4, the images of the final product are displayed in Fig. 4(c) and (d), where a large number of irregularly agglomerated nano-sheets can be observed. A similar morphology is already observed when MoO₄²⁻/Ln³⁺ = 2 as shown in Fig. 3(c) and (d). Furthermore, when MoO₄²⁻/Ln³⁺ = 5, 6 and 7, the final products mainly exist as dispersed nano-sheets, the thickness and plane sizes of which are much smaller than those of the samples synthesized at other MoO₄²⁻/Ln³⁺ ratios.

From the above results it can be concluded that apart from the Cit³⁻/Ln³⁺ ratio and pH, the MoO₄²⁻/Ln³⁺ ratio is also a pivotal factor which governs the particle growth and assembly of the micro-structures. The addition of MoO₄²⁻ on the one hand can change the thickness and size of NaY(MoO₄)₂ nanosheets; on the other hand it affects the assembly of micro-structures. Therefore, to obtain the expected morphology of NaY(MoO₄)₂ nano-/micro-structures, choosing the appropriate MoO₄²⁻/Ln³⁺ ratio in the reaction system is required.

3.3 Growth and assembly mechanism analysis

In section 3.2, the influence of interior factors on the morphologies of the resultant NaY(MoO₄)₂ was discovered. From the above results, the possible crystal growth and self-assembly mechanisms may be deduced. It is a consensus that the morphology and size of a final product is closely related to the competition between crystal nucleation and crystal growth in the reaction solution system, while those two processes are determined by the inherent crystal structure and the chemical potential of the reaction solution, respectively. If the competition between nucleation and growth results in a faster net nucleation rate, the crystal size will be small and the aspect ratio of the crystals will be low. In contrast, if the growing rate is faster than the nucleation rate, then the crystal size will be large and the aspect ratio along the preferential directions will be high. It is well known that pH can affect the balance between the chemical potential and the rate of ionic motion in the precursor solution and, hence, the morphologies of the products.^6,7 In our case, when the pH value decreased or increased from the critical values (7 and 8), the nucleation and growth processes lost balance, thus the irregular aggregation formed. When the pH values ranged from 7 to 8, uniform nano-sheets as presented in Fig. 2(c–f) were formed in the hydrothermal process.

Usually, Na₃Cit plays a significant and multifunctional role in the syntheses of many inorganic micro-structures. Nano-scaled sheets formed under the Cit³⁻/Ln³⁺ molar ratios of 0.5 and 1, while they further expanded to micro-scaled sheets under the Cit³⁻/Ln³⁺ molar ratios of 1.5 and 2. Obviously, increasing the amount of Na₃Cit in reaction solutions could subsequently lead to the remarkable growth of crystal size. Micro-structures assembled from NaY(Mo/WO₄)₂ crystals have been synthesized and modified with the assistance of Na₃Cit in many works.^19,20 However, so far in our work, the self-assembly behavior of NaY(MoO₄)₂:Tb³⁺ crystals has not clearly been observed under either pH or Cit³⁻/Ln³⁺ molar ratio control. After properly increasing the amount of Na₂MoO₄ in the reaction solution, micro-spheres assembled from NaY(MoO₄)₂:Tb³⁺ micro-sheets formed, as presented in Fig. 4(a) and (b). As more Na₂MoO₄ was introduced into the reaction solution, as shown in Fig. 4(c–g), the self-assembly behavior of the NaY(MoO₄)₂:Tb³⁺ crystals was interrupted. It is also worth noting that the crystal size of these obtained samples decreased in this process when compared with the initial MoO₄²⁻/Ln³⁺ molar ratio of 2. The LaMer mechanism,^22,23 which has the conceptual separation of the nucleation and growth into two stages, could be divided into three portions: (I) a rapid increase in the concentration of the free monomers in solution, (II) the burst-nucleation significantly reduces the concentration of the free monomers in solution, (III) the following nucleation growth occurs under the control of the diffusion of monomers through the solution. An excessive amount of Na₂MoO₄ no doubt promotes the nucleation process and increases the amount of NaY(MoO₄)₂:Tb³⁺ monomers, which results in a decrease in the crystal size, as presented in Fig. 4(a–f). When an excessive amount of Na₃Cit is added to the reaction solution, the situation could be delicate. As mentioned above, Na₃Cit could function in multiple roles. It acts not only as a ligand chelating the Ln³⁺ ion that stabilizes the initial reaction system but also as a surfactant promoting the self-assembly process.^9,19 When functioning as a chelating agent, Na₃Cit restrains the crystal nucleation subsequent to the growth of the crystal, which remarkably slows down the formation rate of the precursors. However, when acting as a surfactant, it can promote crystallinity and modify the formation rate of different crystal facets and adjust the growth orientation. In the present work, increasing the amount of Na₃Cit in the reaction solution could lead to the effective growth in length and width of the crystals because of the prolonged growth time, therefore the morphology variations as shown in Fig. 2(e and f) and 3(a–f) were observed. However, there is only a slight trace of assembly behavior when micro-sheets formed as presented in Fig. 3(c–f). It could be deduced that solely increasing the amount of Na₃Cit mainly contributed to the crystal size expansion with a limited effect on the assembly behavior. In this work, Na₃Cit sufficiently reduces the concentration of the free NaY(MoO₄)₂:Tb³⁺ monomers in solution and increases the growth time, while Na₂MoO₄ significantly functions in the opposite way. Based on the above analysis, it can be concluded that Na₃Cit and Na₂MoO₄ functioned in opposite ways in crystal size control. An excessively fast or slow rate of crystal nucleation and growth is not preferable for self-assembly processes. Herein, it is proposed in Fig. 5 that only a certain competitive equilibrium could lead to the self-assembly behavior of NaY(MoO₄)₂:Tb³⁺ crystals when Na₃Cit and Na₂MoO₄ are functioning in the NaY(MoO₄)₂:Tb³⁺ crystal nucleation and growth processes.

Fig. 5 Schematic illustration of the growth and assembly processes of the NaY(MoO₄)₂:Tb³⁺ crystals.

Fig. 6 FESEM images of NaY(MoO₄)₂:5% Tb³⁺ synthesized under different molar ratios of Cit³⁻/MoO₄²⁻/Ln³⁺: (a and b) 1.75/3/1, (c and d) 2/4/1 (pH = 8).

It is suggested that the ratio of Na₃Cit/Na₂MoO₄ dominates crystal nucleation, growth and assembly processes in the reaction solution. In a typical growth and assembly process of a NaY(MoO₄)₂:Tb³⁺ crystal, the crystal nucleus first went through a nucleation process, then a growth process, which was followed by an assembly process and finally a further growth process. Any overwhelming effect of either Na₃Cit or Na₂MoO₄ in the reaction process could not only interrupt the whole process but also result in crystal morphology variation. Consequently, as displayed in Fig. 5, the self-assembly behavior of the NaY(MoO₄)₂:Tb³⁺ crystals may still depend on the balanced effect of Na₃Cit and Na₂MoO₄, which promotes a completed further growth process and results in a final assembly of microstructures.

To confirm the connection between the self-assembly behavior and the coordination function between Na₃Cit and Na₂MoO₄, additional Cit³⁻/MoO₄²⁻/Ln³⁺ molar ratios were designed and the corresponding samples were prepared. Fig. 6(a–d) display the samples prepared at Cit³⁻/MoO₄²⁻/Ln³⁺ molar ratios of 1.75/3/1 and 2/4/1, respectively. It is clearly noteworthy that the self-assembly behavior appeared in both of these samples but in different ways. Micro-spheres formed at a Cit³⁻/MoO₄²⁻/Ln³⁺ molar ratio of 1.75/3/1, while micro-flowers appeared at a Cit³⁻/MoO₄²⁻/Ln³⁺ molar ratio of 2/4/1, which results from different crystal sizes of the assembling micro-sheets.

3.4 Photoluminescence properties

3.4.1 Morphology-dependent luminescence and thermal quenching of NaY(MoO₄)₂:Tb³⁺. In order to explore the morphology and temperature dependence of the luminescent properties, the excitation and emission spectra for the NaY(MoO₄)₂:5 mol% Tb³⁺ samples synthesized with various Cit³⁻/Ln³⁺ ratios with a fixed pH of 8 and a fixed MoO₄²⁻/Ln³⁺ ratio of 2 were measured under the same experimental conditions. Fig. 7(a) displays the excitation spectra obtained by monitoring the 546 nm emission corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺. The excitation spectra consist of a broad but strong charge transfer band (O²⁺ → Mo⁶⁺) ranging from 250 to 350 nm with a maximum at around 281 nm and a sharp excitation band peaking at about 487 nm which can be attributed to ⁷F₆ → ⁵D₄ of Tb³⁺. The corresponding emission spectra measured under 281 nm excitation are shown in Fig. 7(b) where four intrinsic f–f transitions from ⁵D₄ to ⁷F₆, ⁷F₅, ⁷F₄ and ⁷F₃ peaking at 487, 546, 586 and 622 nm, respectively, are observed. All the excitation and emission spectra demonstrate similar spectral shapes and peak positions, thus implying that the Tb³⁺ ions occupy the same sites in the NaY(MoO₄)₂ crystals with different morphologies. It is also worth noting that the emission intensity increases with increasing Cit³⁻/Ln³⁺ molar ratio, which probably results partly from the crystallization grade of the samples, and partly from the packing density of the samples in the sample holder since the particle sizes are different.

Fig. 7 (a) Excitation spectra (λ_em = 546 nm) and (b) emission spectra (λ_ex = 281 nm) of NaY(MoO₄)₂:5% Tb³⁺ prepared under different Cit³⁻/Ln³⁺ molar ratios with pH = 8 and MoO₄²⁻/Ln³⁺ = 2.

The temperature dependence of the luminescence intensity is significant for evaluating luminescent materials, and also important for comprehensively understanding the thermal quenching mechanism of the phosphors to further improve the luminescence performance.^21,22 To investigate the luminescent temperature-dependent behavior of the prepared samples with different morphologies, the emission spectra of these samples were measured at temperatures ranging from 298 to 373 K. The emission spectra for the samples prepared with different Cit³⁻/Ln³⁺ molar ratios, but pH = 8 and MoO₄²⁻/Ln³⁺ = 2, were measured at different temperatures, and as an example the emission spectra for the sample derived when Cit³⁻/Ln³⁺ = 0.5 are shown in Fig. 8(a). It can be observed that the emission intensity decreases rapidly from its maximum at the initial temperature of 298 K with elevating temperatures. It is found that the sample temperature only slightly influences the fluorescent decays presented in Fig. 8(b), which suggests that the energy transfer process between the Tb³⁺ ions in NaY(MoO₄)₂ isn’t affected by the temperature, and that the multiphonon cascade nonradiative transition rate of the ⁵D₄ level is unchanged in the studied temperature region. It is necessary to mention that the emission spectra and fluorescent decays for samples with various morphologies (prepared when Cit³⁻/Ln³⁺ molar ratio is 1, 1.5 and 2) display a very similar tendency, which can be seen in Fig. S2.†

Fig. 8 (a) Emission spectra measured under 281 nm excitation and (b) fluorescent decays obtained by monitoring 546 nm emission under 281 nm excitation at different temperatures for the sample prepared at a Cit³⁻/Ln³⁺ molar ratio of 0.5. (c) Dependences of integrated luminescence intensities (λ_em = 546 nm) on the sample temperature for the samples prepared at different Cit³⁻/Ln³⁺ molar ratios. The solid dots represent the experimental data, and the solid curves are the fitting lines.

From the above results, it can be seen that the thermal quenching process of fluorescence in the NaY(MoO₄)₂:Tb³⁺ phosphors with various morphologies is governed by the crossover mechanism, thus the relationship between the luminescence intensity of the ⁵D₄ → ⁷F₅ transition and the sample temperature can be expressed as,²¹

in which I₀ represents the initial luminescence intensity, I(T) is the luminescence intensity at a given temperature T, C is a constant, k is Boltzmann’s constant, and ΔE is the activation energy for the thermal quenching process. The integrated emission intensities of the samples with different morphologies were obtained from the emission spectra and then normalized to the intensities measured at room temperature (298 K). Nonlinear fittings to the experimental data were carried out by using eqn (1), and it was found that eqn (1) fits the experimental data well for all the samples, as shown in Fig. 8(c). The ΔE values for different samples with various morphologies were obtained in the fitting processes and ranged from 0.48 to 0.60 eV. These similar ΔE values originate from the same temperature quenching mechanism.

3.4.2 Concentration quenching of NaY(MoO₄)₂:Tb³⁺. For the purpose of investigating the concentration quenching behavior of the NaY(MoO₄)₂:Tb³⁺ microstructures, samples with different Tb³⁺ concentrations were synthesized (when pH = 8, Cit³⁻/Ln³⁺ = 1.5 and MoO₄²⁻/Ln³⁺ = 2). Fig. 9(a) shows the emission spectra of these samples measured under the same conditions. It is found that all the emissions as described in Fig. 7(b) are observed, moreover the emission intensity for NaY(MoO₄)₂:Tb³⁺ increases with increasing doping concentration before reaching the maximum intensity, and then decreases with further increasing Tb³⁺ concentration. The integrated emission intensities of the ⁵D₄ → ⁷F₅ transition for all the samples were calculated and their dependence on the Tb³⁺ concentration is displayed in Fig. 9(b) as square dots. Van Uitert has developed a phenomenological model for quantitatively explaining the concentration quenching behavior in which the electric multipole interaction between luminescent centers was considered.²⁴ In this model, the relationship between fluorescence intensities and doping concentration can be mathematically expressed as follows:¹⁹in which C is the luminescent center concentration; K and β are constants for a certain system; Q represents the multipole interaction index, Q = 6, 8, or 10 for electric dipole–dipole, electric dipole–quadrupole or electric quadrupole–quadrupole interactions, respectively. In this work, eqn (2) was transformed to y = ax/(1 + bx^c) which was then used to fit the experimental data in Fig. 9(b). The fitting curve is shown as a solid curve in Fig. 9(b), and the value of c is found to be 1.81 ± 0.27, which implies that Q is nearly 6. This means that the electric dipole–dipole interaction is responsible for the fluorescence quenching process of the ⁵D₄ level, and the Tb³⁺ ions at the ⁵D₄ level would serve as donors in this energy transfer process. In order to further understand the energy transfer process, a linear fitting to the concentration quenching data in the low concentration region plotted in a double logarithmic coordinate system was carried out, and the slope for the linear function was found to be 0.30 which is much smaller than 1. This result reflects that quenching centers other than Tb³⁺ itself surely exist in the NaY(MoO₄)₂:Tb³⁺ microstructures.^25,26 Therefore, there must be some energy transfer paths involving quenching centers. In our present case the MoO₄²⁻ or some unintended doping, which was introduced together with the starting materials, may act as the quenching centers. Therefore, to advance the luminescence performance of the Tb³⁺ doped NaY(MoO₄)₂ microstructures, further efforts are required.

Fig. 9 (a) Emission spectra (λ_ex = 281 nm) of NaY(MoO₄)₂ with different Tb³⁺ doping concentrations. (b) Dependence of the integrated emission intensity of the ⁵D₄ → ⁷F₅ transition on Tb³⁺ concentration. The square dots represent the experimental data, and the continuous solid curve is the fitting line. The insert in (b) is plotted by using the same data of the concentration-dependent intensity, but in a double logarithmic coordinate system, and the straight line is a linear fitting.

3.4.3 Fluorescence decays. Fluorescence decay processes could provide further insight into the energy transfer mechanism for the ⁵D₄ level of Tb³⁺ ions, since nonradiative energy transfer has been proven by the concentration quenching behavior of NaY(MoO₄)₂:Tb³⁺. Meanwhile, nonradiative channels could lead to a shorter fluorescence lifetime because the energy transfer process guarantees more depopulating possibility for the ⁵D₄ level of Tb³⁺ ions. Fig. 10(a) presents the fluorescence decays for the ⁵D₄ level of Tb³⁺ in the samples with different Tb³⁺ doped concentrations. The solid dots in Fig. 10(a) are the experimental data which show the relatively fast decays of the NaY(MoO₄)₂:Tb³⁺ samples and obey the single exponential law as I = I₀ exp(−t/τ₀).

Fig. 10 (a) Fluorescence decays of the NaY(MoO₄)₂:x mol% Tb³⁺ (x = 2.5–30) phosphors excited at 281 nm and monitored at 546 nm. The solid dots are the experimental data, and the solid lines are the fitting lines. (b) Dependence of fluorescence lifetime of the ⁵D₄ level of Tb³⁺ on the doping concentration in NaY(MoO₄)₂:Tb³⁺ phosphors. The square dots represent the experimental data, and the continuous solid curve is the fitting line.

The average lifetime of the ⁵D₄ level of Tb³⁺ at different concentrations is evaluated based on the fluorescence decay data. Furthermore, the dependence of the average fluorescence lifetime on the different doping concentrations of Tb³⁺ is displayed in Fig. 10(b). Since a nonradiative energy transfer process could accelerate the fluorescence decay, increasing the probability of nonradiative energy transfer makes the average fluorescence lifetime decrease monotonically with increasing concentration of Tb³⁺ ions as shown in Fig. 10(b).

Dexter has established a model to reveal the relationship between the doping concentration and the fluorescence lifetime for the electric multipole interaction, which can be mathematically expressed as follows:^27,28

in which τ(c) is the average fluorescence lifetime at the present concentration c, τ₀ is the radiative transition lifetime of the luminescent level, c₀ is a constant with the same dimension as c, often called the critical concentration at which the fluorescence lifetime of the sample is half that of the radiative transition lifetime, and Q has the same physical interpretation as in eqn (2). Eqn (3) was simplified as y = τ₀/(1 + ax^s/3) and then used to fit the experimental data in Fig. 10(b). The fitting curve is presented in Fig. 10(b) as a red solid curve and the value of Q is obtained as 5.8 ± 0.05 which is very close to theoretical value of 6 indicating the electric dipole–dipole interaction between Tb³⁺ ions. This result coincides with the previous conclusion which derived from Van Uitert’s model. Thus, in this case, it is certain that the electric dipole–dipole interaction dominates in the fluorescence quenching process. Also, the fitting result of τ₀ is 0.61 ± 0.02 ms which is close to the experimental value of 0.63 ms at a 2.5% Tb³⁺ concentration.

4 Conclusions

In summary, nano- and micro-scaled architectures of NaY(MoO₄)₂:Tb³⁺ were successfully synthesized via a microwave-assisted hydrothermal process in the presence of Na₃Cit. It was found that the suitable pH value for NaY(MoO₄)₂:Tb³⁺ crystal growth was about 7–8. Morphology control was realized by adjusting the amount of Na₃Cit and Na₂(MoO₄)₂ in the reaction solution. Increasing the amount of Na₃Cit only in the reaction solution led to a remarkable growth in the crystal size, while individually increasing the amount of Na₂(MoO₄)₂ in the reaction solution resulted in the opposite phenomenon. It was also suggested that the self-assembly behavior of NaY(MoO₄)₂:Tb³⁺ crystals would happen when a certain competitive equilibrium between Na₃Cit and Na₂(MoO₄)₂ in the reaction solution was reached. A possible growth mechanism for the NaY(MoO₄)₂:Tb³⁺ crystals was also proposed. The fluorescence thermal quenching behaviors for the NaY(MoO₄)₂:Tb³⁺ phosphors with different morphologies were studied by using the crossover model. Furthermore, the electric dipole–dipole interaction was found to be responsible for both the concentration quenching behavior and the fluorescence decay process of ⁵D₄ fluorescence in the NaY(MoO₄)₂:Tb³⁺ phosphors.

Acknowledgements

This work was partially supported by NSFC (National Natural Science Foundation of China, Grant nos 21173034, 51002041, 11104023, 11104024, 11274057 and 11374044), Fundamental Research Funds for the Central Universities (Grant no. 3132014087, 3132014327 and 3132013100).

References

1. X. Wang, J. Zhuang, Q. Peng and Y. D. Li, Inorg. Chem., 2006, 45, 6661–6665.
2. Y. S. Yang, Q. Z. Wu, M. Wang, M. Zhou and X. H. Chen, Cryst. Growth Des., 2014, 14, 4864–4871.
3. A. M. Kaczmarek and R. V. Deun, Chem. Soc. Rev., 2013, 42, 8835–8848.
4. W. D. Shi, S. Y. Song and H. J. Zhang, Chem. Soc. Rev., 2013, 42, 5714–5743.
5. Y. Tian, B. J. Chen, B. N. Tian, N. S. Yu, J. S. Sun, X. P. Li, J. S. Zhang, L. H. Cheng, H. Y. Zhong, Q. Y. Meng and R. N. Hua, J. Colloid Interface Sci., 2013, 393, 44–52.
6. J. T. Han, Y. H. Huang, X. J. Wu, C. L. Wu, W. Wei, B. Peng, W. Huang and J. B. Goodenough, Adv. Mater., 2006, 18, 2145–2148.
7. X. Wang and Y. D. Li, Angew. Chem., Int. Ed., 2002, 41, 4790–4793.
8. Y. Q. Chen, S. W. Park, B. K. Moon, B. C. Choi, J. H. Jeong and C. F. Guo, CrystEngComm, 2013, 15, 8255–8261.
9. J. C. Zhang, W. Wang, B. X. Li, X. H. Zhang, X. D. Zhao, X. Y. Liu and M. Zhao, Eur. J. Inorg. Chem., 2012, 2220–2225.
10. X. X. Zhu, Q. H. Zhang, Y. G. Li and H. Z. Wang, J. Mater. Chem., 2010, 20, 1766–1771.
11. S. H. Huang, X. Zhang, L. Z. Wang, L. Bai, J. Xu, C. X. Li and P. P. Yang, Dalton Trans., 2012, 5634–5642.
12. Y. B. Zeng, Z. Q. Li, L. M. Wang and Y. J. Xiong, CrystEngComm, 2012, 14, 7043–7048.
13. L. Xu, X. Y. Yang, Z. Zhai, X. Chao, Z. H. Zhang and W. H. Hua, CrystEngComm, 2011, 13, 4921–4929.
14. J. Liu, B. Xu, C. Song, H. D. Luo, X. Zhou, L. X. Han and X. B. Yu, CrystEngComm, 2012, 14, 2936–2943.
15. J. C. Zhang, X. F. Wang, X. H. Zhang, X. D. Zhao, X. Y. Liu and L. P. Peng, Inorg. Chem. Commun., 2011, 14, 1723–1727.
16. Y. J. Zhu and F. Chen, Chem. Rev., 2014, 114, 6462–6555.
17. M. Baghbanzadeh, L. Carbone, P. D. Cozzoli and C. O. Kappe, Angew. Chem., Int. Ed., 2011, 50, 2–50.
18. I. Bilecka and M. Niederberger, Nanoscale, 2010, 2, 1358–1374.
19. Y. Tian, B. J. Chen, R. N. Hua, N. S. Yu, B. Q. Liu, J. S. Sun, L. H. Cheng, H. Y. Zhong, X. P. Li, J. S. Zhang, B. N. Tian and H. Zhong, CrystEngComm, 2012, 14, 1760–1769.
20. Z. H. Xu, C. X. Li, G. G. Li, R. T. Chai, C. Peng, D. M. Yang and J. Lin, J. Phys. Chem. C, 2010, 114, 2573–2582.
21. B. N. Tian, B. J. Chen, Y. Tian, X. P. Li, J. S. Zhang, H. Y. Zhong, L. H. Cheng, S. B. Fu, H. Zhong, Y. Z. Wang, X. Q. Zhang, H. P. Xia and R. N. Hua, J. Mater. Chem. C, 2013, 1, 2338–2344.
22. E. Cavalli, P. Boutinaud, R. Mahiou, M. Bettinelli and P. Dorenbos, Inorg. Chem., 2010, 49, 4916–4921.
23. V. K. LaMer and R. H. Dinegar, J. Am. Chem. Soc., 1950, 72, 4847–4854.
24. L. G. Van Uitert, J. Electrochem. Soc., 1967, 114, 1048–1053.
25. Y. Tian, B. J. Chen, B. N. Tian, J. S. Sun, X. P. Li, J. S. Zhang, L. H. Cheng, H. Y. Zhong, H. Zhong, Q. Y. Meng and R. N. Hua, Phys. B, 2012, 407, 2556–2559.
26. L. H. Cheng, H. Y. Zhong, J. S. Sun, X. Q. Zhang, Y. Peng, T. Yu and X. X. Zhao, J. Rare Earths, 2008, 26, 211–214.
27. D. L. Dexter and J. H. Schulman, J. Chem. Phys., 1954, 22, 1063–1070.
28. Y. Z. Wang, C. X. Lin, H. Zheng, D. P. Sun, L. Lei and B. J. Chen, J. Alloys Compd., 2013, 559, 123–128.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06915g

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