Morphology and phase transformation from NaCaSiO₃OH to Na₂Ca₂Si₂O₇ and photoluminescence evolution via Eu³⁺/Tb³⁺ doping

Abstract

A facile and controllable ethanol/water aided hydrothermal process was developed to prepare the NaCaSiO₃OH:Tb³⁺/Eu³⁺ phosphor. The morphologies were in situ constructed with the phase transformation from NaCaSiO₃OH to Na₂Ca₂Si₂O₇, and the intrinsic crystal structural transformation mechanism and the dependence of their photoluminescence tuning on the Tb³⁺/Eu³⁺ ratio have been discussed.

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The design and synthesis of rare earth (RE) ions doped luminescent materials with controllable morphologies remains a challenge owing to their size/shape-dependent properties and induced potential applications in lighting devices,¹ biological labeling,² lasers, displays,³etc. The physical–chemical properties of materials could be controlled as we desired through a good manipulation of the size and morphology.⁴ Therefore, fabrication of RE doped luminescent materials with controlled structures and morphologies is still one of the challenging issues in chemistry and materials science. Amongst them, RE doped silicate materials have been widely investigated due to the versatility in their crystal structures and tunable luminescence properties, which are suitable for application to many fields.^5–8 Usually, high-temperature solid-state reactions are employed to prepare silicate materials. So, the resultant samples appear in aggregation with an irregular shape, and such a preparation method limits the possibility of tuning the morphology and the following properties of the final products. Accordingly, various solution-based preparation techniques, such as co-precipitation,⁹ hydrothermal,¹⁰ sol–gel,¹¹ have been explored to prepare silicate materials with controllable morphologies, which is still a challenge.

Herein, we present a facile pathway toward Eu³⁺/Tb³⁺-doped Na₂Ca₂Si₂O₇ phosphors that rely on the heat treatment of the NaCaSiO₃OH:Eu³⁺/Tb³⁺ precursor prepared using a controllable hydrothermal method. By tuning the ethanol/water (EtOH/H₂O) ratio, it is possible to yield different precursor morphologies, such as rods, decahedron, distorted octahedron and spindle. Then, we synthesized Na₂Ca₂Si₂O₇ with the same particle shapes as those of NaCaSiO₃OH, as obtained from the in situ construction and phase transformation from NaCaSiO₃OH to Na₂Ca₂Si₂O₇. Additionally, the luminescence properties of Eu³⁺/Tb³⁺ doped NaCaSiO₃OH and Na₂Ca₂Si₂O₇ microstructures are comparatively investigated, and the luminescent colors of Na₂Ca₂Si₂O₇:Tb³⁺,Eu³⁺ can be adjusted from red to green depending on the Tb³⁺/Eu³⁺ ratio, which should be ascribed to the energy transfer of Tb³⁺ → Eu³⁺. The work reveals that our current hydrothermal method following a heat treatment process is a facile and efficient way to the synthesis of such phosphors with controlled morphologies.

The preparation method employed for all the presented materials is described in detail in the ESI.† Herein, the X-ray diffraction (XRD) patterns of the NaCaSiO₃OH samples prepared by the addition of different EtOH/H₂O volume ratios are shown in Fig. 1. From Fig. 1 and Fig. S1a (ESI†), it can be observed that all samples exhibit a single phase and all peaks are assigned to the monoclinic phase NaCaSiO₃OH (JCPDS card no. 25-1319). The dependence of the NaCaSiO₃OH morphology on the reaction media of the EtOH/H₂O volume ratio is illustrated well by the scanning electron microscopic (SEM) evaluation of the isolated products (see Fig. 2 and Fig. S1b, c, ESI†). Rod-like particles are obtained when the volume of H₂O is 30 ml (Fig. 2a), and some rods are agglomerated to form rod bunches. From Fig. 2b, the morphology of NaCaSiO₃OH is changing progressively to a nubby shape in the presence of 1 ml EtOH. Increasing the EtOH volume to 5 ml (EtOH/H₂O = 5/25) yields a perfect decahedron shape noted in Fig. 2c, indicating that the decahedral structure may have grown from rods. Based on the high magnification SEM image (the inset of Fig. 2c), the decahedrons with some rips are highly uniform and symmetrical, and they exhibit a bigger size compared to the sample prepared in EtOH/H₂O = 1/29. Upon addition of 10 ml EtOH, the decahedral shape transforms into octahedron one, but the two sides of the selected octahedron are not smooth (the inset of Fig. 2d). Upon further increasing the EtOH to 15 ml, the spindle shape is developed, and the centre thickness of the spindle is greater than the edge thickness. Then, under a rich amount of EtOH (20–30 ml), pure phase NaCaSiO₃OH is still formed, but the morphology of NaCaSiO₃OH particles become more irregular, even giving rise to crystal aggregates as illustrated in Fig. 2f and Fig. S1b, c (ESI†). More interestingly, it is observed that the higher the EtOH content (5–30 ml), the smaller the particle size. Therefore, we can conclude that the difference of morphologies and sizes in this surfactant/ligand-free system is mainly governed by the amount of the solvent EtOH. Generally, the growth of crystals and the formation of various morphologies are complex processes that are the cooperative results of the inherent structures and the external experimental conditions.^12–14 In our case, two kinds of solvents (H₂O and EtOH) with different polarities and saturated vapour pressures are used as reaction media, producing a distinguished morphology and a size of NaCaSiO₃OH. The polarities and saturated vapour pressures of the solvents were found to affect the products under thermal conditions by giving adjustments to the homogenization of the reactants, the amount of individual nucleus formation, the amalgamation and direction preference of growing nucleus.¹⁵ Furthermore, the ratio of two solvents would affect the solubility of the starting materials, the reaction rate and the crystallization rate of products, which result in the various morphologies, as observed in the present and other experiments.

Fig. 1 XRD patterns of NaCaSiO₃OH samples prepared in various volume ratios of EtOH to H₂O. The standard data for NaCaSiO₃OH (JCPDS card no. 25-1319) is shown as a reference.

Fig. 2 SEM images of NaCaSiO₃OH samples prepared in different volume ratios of EtOH to H₂O: 0/30 (a), 1/29 (b), 5/25 (c), 10/20 (d), 15/15 (e) and 20/10 (f). The inset images in c–f are the corresponding enlarged single particle.

The thermal stability of the NaCaSiO₃OH phase is firstly investigated by thermogravimetric and differential scanning calorimetry (TG-DSC) analysis. From the recorded TG-DSC results during the heat-treatment of NaCaSiO₃OH in air (Fig. 3a), it is apparent that the decomposition starts at 460 °C and finishes at 523 °C with the weight loss of 5.64%, which agrees with the theoretical value (5.76%) calculated from the dehydration of two NaCaSiO₃OH molecules. Hence, the phase decomposition is proposed over the temperature range of 450–530 °C. To verify the occurrence of phase transformation and find the exact phase transformation temperature, variable temperature X-ray diffraction (VT-XRD) is employed in situ. As shown in VT-XRD patterns (Fig. 3b), we find that the NaCaSiO₃OH phase is thermally stable up to 450 °C in air. At 500 °C, a mixture of the NaCaSiO₃OH phase and the newly formed Na₂Ca₂Si₂O₇ phase (ICSD card no. 95858) appears. However, all the characteristic XRD peaks coincide with the standard pattern of Na₂Ca₂Si₂O₇, when the temperature is increased to 600 °C. Even if the heating temperature rises up to 1000 °C continuously, the Na₂Ca₂Si₂O₇ phase still maintained, confirming that the temperature of phase transformation from NaCaSiO₃OH to Na₂Ca₂Si₂O₇ is about 500 °C, as observed from the TG-DSC results. Fig. 3c shows the SEM images of the resultant Na₂Ca₂Si₂O₇. More interestingly, it can be found that the morphologies of NaCaSiO₃OH can be kept in situ even if it is sintered at 700 °C to form the Na₂Ca₂Si₂O₇ phase. We believe that such a phase transformation synthesis should be a general and facile way to prepare morphology-controllable silicate materials. Moreover, Fig. 3d gives the enlarged image of the individual Na₂Ca₂Si₂O₇ particle, and the porous surface character can be observed, which implies the decomposition of NaCaSiO₃OH via evaporation of H₂O molecular and the appearance of microscopic pores on the surface of the particles. The intrinsic crystal structural transformation mechanism will also be discussed in the following section.

Fig. 3 (a) TG-DSC curves of the as-obtained NaCaSiO₃OH, (b) the VT-XRD patterns of NaCaSiO₃OH precursors (EtOH/H₂O = 5/25) by sintering them at different temperatures. (c) SEM images of resultant Na₂Ca₂Si₂O₇ when calcining the selected NaCaSiO₃OH in situ at 700 °C and (d) the enlarged image of the single Na₂Ca₂Si₂O₇ particle.

The phase transformation mechanism for the formation of the Na₂Ca₂Si₂O₇ phase with microscopic pores on the particle surface is proposed in Fig. 4. As we know, both the NaCaSiO₃OH and Na₂Ca₂Si₂O₇ belong to monoclinic phases, but their space groups are P2₁/m and C2/c respectively and their cell parameters are absolutely distinguished. Therefore, such a phase transformation doesn't belong to the topotactic transformation because two structures have unit cells without relations. Besides these, other differences of the two structures are manifested as follows: (1) Na₂Ca₂Si₂O₇ consists of Ca, Na/Ca, Na and Si sites, but NaCaSiO₃OH merely consists of Ca, Na and Si cations, no Ca/Na mixing exists in NaCaSiO₃OH; (2) networks of SiO₄ polyhedra are different, namely, there are isolated SiO₄ in NaCaSiO₃OH and two isolated groups (SiO₄ and Si₃O₁₀) in Na₂Ca₂Si₂O₇ (see Fig. 4). However, the phase relations between NaCaSiO₃OH and Na₂Ca₂Si₂O₇ should be considered. In this case we can suggest that OH groups of two SiO₃OH tetrahedra combine with two H ions from other two SiO₃OH tetrahedra forming two H₂O molecules. The remaining two SiO₃ and two SiO₄ form Si₃O₁₀ + SiO₄ as illustrated in the bottom panel of Fig. 4. The possible chemical reaction equation of the formation of Na₂Ca₂Si₂O₇ from NaCaSiO₃OH could be described as follows:

Fig. 4 The comparative description of the crystal structures of NaCaSiO₃OH and Na₂Ca₂Si₂O₇, and the transformation mechanism from NaCaSiO₃OH to Na₂Ca₂Si₂O₇.

In the phase transition process, the dehydration from NaCaSiO₃OH results in the generation of H₂O; then, a large amount of vacancies should be formed during the formation of the new Na₂Ca₂Si₂O₇ phase. The expansion of vacancies may lead to the formation of cracks and microscopic pores at the Na₂Ca₂Si₂O₇ polyhedron facets. The SEM images of calcined product Na₂Ca₂Si₂O₇ (Fig. 3d) show that the polyhedrons possess many microscopic holes, which supports the above proposed mechanism.

Our experiments have also shown that both NaCaSiO₃OH and Na₂Ca₂Si₂O₇ are good hosts for the luminescent materials formed via the Eu³⁺/Tb³⁺ doping. It should be mentioned that the doping of Eu³⁺/Tb³⁺ into NaCaSiO₃OH and Na₂Ca₂Si₂O₇ does not change the phase purity and morphology of the final products as displayed in Fig. S3 (ESI†). Herein, the luminescence properties of Eu³⁺ and Tb³⁺ singly or/and codoped NaCaSiO₃OH and Na₂Ca₂Si₂O₇ samples with a decahedron-like shape (EtOH/H₂O = 5/25) were comparatively investigated. Fig. 5a and b show the photoluminescence (PL) emission spectra of NaCaSiO₃OH:0.10Eu³⁺/0.03Tb³⁺ and Na₂Ca₂Si₂O₇:0.10Eu³⁺/0.03Tb³⁺ samples, respectively. NaCaSiO₃OH:Eu³⁺ shows a strong red luminescence originating from the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions of Eu³⁺ ions under 254 nm excitation.^16,17 For the Na₂Ca₂Si₂O₇:Eu³⁺ sample, the PL spectrum exclusively contains the characteristic emission of Eu³⁺ cantered at 611 nm. On the other hand, NaCaSiO₃OH:Tb³⁺ shows a series of strong green emission lines with a maximum at about 542 nm, which are all ascribed to ⁵D₄–⁷F_J (J = 6, 5, 4, 3) transitions of Tb³⁺ corresponding to ⁵D₄–⁷F₆ (488 nm), ⁵D₄–⁷F₅ (542 and 550 nm), ⁵D₄–⁷F₄ (583 nm) and ⁵D₄–⁷F₃ (625 nm), respectively, which are similar to other Tb³⁺-containing phosphors, such as Tb₂(MoO₄)₃.^18,19 For the Na₂Ca₂Si₂O₇:Tb³⁺, the emission spectral lines in the blue region are found to be at 416 nm (⁵D₃–⁷F₅) and 437 nm (⁵D₃–⁷F₄) with high emission intensity. However, they do not emerge in the emission spectra of NaCaSiO₃OH:Tb³⁺, which means that there is a relatively low photon energy of Na₂Ca₂Si₂O₇ compared to that of NaCaSiO₃OH. Moreover, the intensity ratio of ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions of Eu³⁺ ions in different hosts is related to the activator symmetry occupied in the lattice. The strong emission from ⁵D₀ → ⁷F₂ compared to that of ⁵D₀ → ⁷F₁ in Na₂Ca₂Si₂O₇:0.10Eu³⁺ denotes that there is relatively low symmetry for Na₂Ca₂Si₂O₇. Therefore, it is found that there is obvious difference in the emission spectra of NaCaSiO₃OH:Eu³⁺ and Na₂Ca₂Si₂O₇:Eu³⁺; as well as NaCaSiO₃OH:Tb³⁺ and Na₂Ca₂Si₂O₇:Tb³⁺. The effects can be attributed to the different coordination environments and symmetry around the Tb³⁺/Eu³⁺ ions in the NaCaSiO₃OH and Na₂Ca₂Si₂O₇ hosts.¹² Furthermore, from Fig. 5c, the spectral overlap between the PL spectrum of Na₂Ca₂Si₂O₇:Tb³⁺ and photoluminescence excitation (PLE) spectrum of Na₂Ca₂Si₂O₇:Eu³⁺ can be observed, and the PL spectra of Na₂Ca₂Si₂O₇:Tb³⁺,Eu³⁺ consist of the peaks at 611 nm and 542 nm attributed to Eu³⁺ and Tb³⁺, respectively. The results suggest that the energy transfer can occur from Tb³⁺ to Eu³⁺,²⁰ and the emitting colour can be tuned from green to yellow and red via energy transfer of Tb³⁺ → Eu³⁺ by tuning the doped Eu³⁺ concentrations as shown in Fig. 5d and the inset.

Fig. 5 PL spectra NaCaSiO₃OH:0.10Eu³⁺/0.03Tb³⁺ (a) and Na₂Ca₂Si₂O₇:0.10Eu³⁺/0.03Tb³⁺ (b) phosphors. PLE and PL spectra of Na₂Ca₂Si₂O₇:0.03Eu³⁺ (c) -I, Na₂Ca₂Si₂O₇:0.03Tb³⁺ (c) -II, and Na₂Ca₂Si₂O₇:0.03Tb³⁺,0.03Eu³⁺ (c) -III phosphors. (d) PL spectra of Na₂Ca₂Si₂O₇:0.03Tb³⁺,xEu³⁺ phosphors with different Eu³⁺ doped concentrations (x), the inset shows the corresponding CIE chromaticity diagram.

In order to further investigate the energy transfer process between Tb³⁺ and Eu³⁺, the decay curves of Tb³⁺ by monitoring the emission transition at 542 nm were measured and depicted in Fig. 6(a). The decay curve can be well fitted with a second-order exponential decay mode by using eqn (1):^21–23

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (1)

where I is the luminescence intensity, A₁ and A₂ are constants, t is the time, τ₁ and τ₂ are rapid and slow lifetime values for exponential components, respectively. Based on the eqn (1), one can obtain the A₁, A, τ₁ and τ₂ values by fitting of the decay curves. Therefore, the effective lifetime constant (τ*) can be calculated as eqn (2):

τ* = (A₁τ₁² +A₂τ₂²)/(A₁τ₁ + A₂τ₂) (2)

The effective decay time (τ*) was calculated to be 4.01, 3.88, 3.52 and 3.25 ms for Na₂Ca₂Si₂O₇:0.03Tb³⁺,xEu³⁺ with x = 0, 0.04, 0.08 and 0.10, respectively. The energy-transfer efficiency (η_T) from Tb³⁺ to Eu³⁺ ions can be expressed as the following eqn (3):^23–26

η_T = 1 − (I_S/I_S0) = 1 − (τ_S/τ_S0) (3)

where τ_S0 and τ_S stand for the lifetimes of Tb³⁺ in the absence and the presence of Eu³⁺ respectively. With the increase in the Eu³⁺ concentration, the η_T value is calculated and the results are shown in Fig. 6(b). As displayed in Fig. 6(b), the average lifetime decreased monotonically and the energy-transfer efficiency increased with the increasing Eu³⁺ content. The η_T value reached a maximum of 19% at x = 0.10.

Fig. 6 (a) The decay curves and lifetime values of Tb³⁺ in Na₂Ca₂Si₂O₇:0.03Tb³⁺,xEu³⁺ with different Eu³⁺ contents (x): x = 0, 0.04, 0.08 and 0.10. (b) Dependence of the fluorescence lifetime of the Tb³⁺ and energy transfer efficiency on doped Eu³⁺ molar concentration in Na₂Ca₂Si₂O₇:0.03Tb³⁺,xEu³⁺ samples.

In summary, we have presented for the first time a simple hydrothermal method for the synthesis of highly crystallized NaCaSiO₃OH particles with a controlled morphology/size in the presence of EtOH as both the solvent and the structure-directing agent. More importantly, the as-prepared NaCaSiO₃OH can be in situ transferred into Na₂Ca₂Si₂O₇ by the heat-treatment in air, and the morphologies of the as-prepared NaCaSiO₃OH have been kept without any agglomerations. In principle, this method offers a new alternative strategy to generate silicate materials. Moreover, when Na₂Ca₂Si₂O₇ is codoped with Eu³⁺ and Tb³⁺ with an appropriate ratio, the emission colours of the Na₂Ca₂Si₂O₇:Tb³⁺,Eu³⁺ phosphors can be tuned from green to yellow and red. It is expected that the morphology-controlled Na₂Ca₂Si₂O₇:Tb³⁺,Eu³⁺ micro-particles will provide potential applications for micro/nano functional devices.

The present work was supported by the National Natural Science Foundation of China (Grant No. 51572023 and 51272242), the Fundamental Research Funds for the Central Universities (FRF-TP-15-003A2) and the Russian Foundation for Basic Research (Grant No. 15-52-53080 GFEN_a).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cc06375f

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