Ethylenediamine-Assisted Hydrothermal Synthesis of NaCaSiO3OH

Oct 7, 2016 - ... different solvents. Sung Jun Park , Jin Young Park , Hyun Kyoung Yang , Jae-Yong Je. Inorganic Chemistry Frontiers 2017 4 (8), 1287-...
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Ethylenediamine-Assisted Hydrothermal Synthesis of NaCaSiO3OH: Controlled Morphology, Mechanism, and Luminescence Properties by Doping Eu3+/Tb3+ Mingyue Chen, Zhiguo Xia,* and Quanlin Liu The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: This paper demonstrates a facile hydrothermal method using ethylenediamine (EDA) as a “shape modifier” for the controlled synthesis of rod bunch, decanedron, spindle, flakiness, and flowerlike NaCaSiO3OH microarchitectures. The set of experimental conditions is important to obtain adjustable shape and size of NaCaSiO3OH particles, as the change in either the amount of EDA/H2O or reaction time, or the amount of NaOH. Accordingly, the crystal growth mechanism during the synthesis process is proposed, and it is found that the EDA, acting as the chelating agent and shape modifier, plays a crucial role in fine-tuning the NaCaSiO3OH morphology. Morphology evolution process of flowerlike NaCaSiO3OH as a function of NaOH is also explained in detail. Eu3+/Tb3+ doped NaCaSiO3OH samples exhibit strong red and green emission under ultraviolet excitation, corresponding to the characteristic electronic transitions of Eu3+ and Tb3+. These results imply that the morphology-tunable NaCaSiO3OH:Eu3+/Tb3+ microarchitectures with tunable luminescence properties are expected to have promising applications for micro/nano optical functional devices.

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on previous reports, we find that particles with uniform shape and narrow size distribution are highly desirable for better applications. For instance, uniform spherical particles with submicrometer size range enable fluorescent lamps to possess better resolution and brightness.20 Furthermore, nanometer size is specifically important for in vivo applications to facilitate their elimination from the body.21 Therefore, the design and control of nano- and microcrystals with defined morphologies and tunable sizes became an important yet challenging research topic. In general, a hydrothermal/solvothermal process is a better way to prepare products with perfect shape and definite size. Usually, the control of morphology, crystallinity, and size of the particles mainly depends on parameters that are the reaction temperature, source, solvent, ligand, pH, and so on.22 Saha et al. found that the shape of nanocrystals of CdS could be changed from particles to nanorods or nanotips depending on the amount of surfactant PAM.23 Recently, our group also attempted to adopt different solvent (H2O and ethanol) to prepare controlled-morphology NaCaSiO3OH; furthermore, after the calcination of NaCaSiO3OH particles, the morphology stays the same and phase transformation from NaCaSiO3OH to Na2Ca2Si2O7 occurs.24 However, until now, there is still a challenge for preparation of silicate phosphors with good shape using a ligand or surfactant through a hydrothermal/ solvothermal method.

INTRODUCTION Rare-earth (RE) doped inorganic luminescent materials have found wide applications, such as displays,1 LEDs,2,3 cell labeling,4 solar energy conversion,5 and lasers,6 due to their characteristic optical properties. In the past, many popular luminescent materials, including fluorides,7 silicates,8 nitrides,9 and phosphates,10,11 have become a growing interest in science and industry. Among them, silicates were extensively researched because of their several excellent advantages, for instance, good stability and high quantum efficiency.12 Many reports focus on the luminescent properties and phase transformation of M2SiO4:Eu2+ (M = Ca, Sr, Ba);13 especially for Ca2SiO4:Eu2+ phosphor, the emission colors can be shifted apparently from green-yellow to deep-red with increasing Eu2+ content.14 Additionally, the phase transition β → γ phase occurred as the Al3+ ions substituted for Si in the Ca2SiO4:Ce3+ phosphor.15 Akermanite (M2MgSi2O7), acting as another important host for doping RE ions, also attracts much attention. Ca2MgSi2O7:Eu2+ shows one broad emission situated at about 450 nm and another dominating around 535 nm, and Sr2MgSi2O7:Eu2+ demonstrates only a blue emission band around 468 nm.16 Generally, high-temperature solid-state reaction is primarily employed to prepare inorganic luminescent materials, because this way is simple and the targeted product can be achieved easily. Some other preparation methods including combustion,17 coprecipitation,18 and sol−gel,19 also can be used, but the morphology and size of products synthesized through the above means are irregular and segregated in most cases. Based © XXXX American Chemical Society

Received: August 5, 2016

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DOI: 10.1021/acs.inorgchem.6b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Inspired by these, we demonstrate herein a facile hydrothermal method using ethylenediamine (EDA) as a “shape modifier” for the controlled synthesis of morphology-tunable NaCaSiO3OH:Eu3+/Tb3+ microarchitectures. The morphology of the NaCaSiO3OH particles was controlled by varying the volume of EDA with respect to the H2O during the synthesis, as shown in Scheme 1. It is clearly found that rod bunch,

Table 1. Summary of the Experimental Conditions and the Corresponding Denotations for the Final Samples

Scheme 1. Different Morphologies of the NaCaSiO3OH Particles Prepared in Various Volume Ratios of EDA to H2O

EDA/H2O (mL)

NaOH (g)

sample

EDA/H2O (mL)

NaOH (g)

S-1 S-2 S-3 S-4 S-5

0/30 1/29 2/28 5/25 10/20

6 6 6 6 6

S-6 S-7 S-8 S-9 S-10

15/15 20/10 2/28 2/28 0/30

6 6 5 7 7

Ca(NO3)2 were the starting materials added to the EDA and H2O (2/ 28). Characterization. Powder X-ray diffraction (XRD) measurements were performed on a D8 Advance diffractometer (Bruker Corporation, Germany), operating at 40 kV and 35 mA with Cu Kα radiation (λ = 1.5406 Å). The scanning rate for phase identification was fixed at 8° min−1 with a 2θ range from 15° to 60°. The morphology and crystalline size of the NaCaSiO3OH samples were determined by scanning electron microscope (SEM, JEOL JSM-6510). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were carried out by a fluorescence spectrophotometer (F-4600, HITACHI, Japan) equipped with a photomultiplier tube operating at 400 V and a 150 W xenon lamp as the excitation source. The temperature dependence luminescence properties were measured on the same F-4600 spectrophotometer, and it was combined with a selfmade heating attachment and a computer-controlled electric furnace (Tianjin Orient KOJI Co., Ltd., TAP-02). The luminescence decay curves were obtained using a FLSP9200 fluorescence spectrophotometer (Edinburgh Instruments Ltd., U.K.). The internal quantum efficiency was measured using the integrated sphere on the same instrument and white BaSO4 powder as a reference to measure the absorption.

decanedron, spindle, flakiness, and flowerlike NaCaSiO3OH particles were prepared depending on various volume ratios of EDA to H2O (Scheme 1). The function of EDA was investigated, and a complexation mechanism is proposed for the fabrication of monodispersed NaCaSiO3OH microparticles. Moreover, more NaOH enables the flowerlike NaCaSiO3OH particles to be achieved when other parameters are not changed. Meanwhile, the morphology and phase purity were not related to the doping of Eu3+/Tb3+ ions. Photoluminescence property investigations imply that the morphology-tunable NaCaSiO3OH:Eu3+/Tb3+ phosphor are expected to have promising applications in fields such as light display systems and optoelectronic devices.

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sample

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RESULTS AND DISCUSSION Morphology and Crystalline Phase. Taking the one-step hydrothermal method to prepare diverse morphology of NaCaSiO3OH with the assistance of EDA as a typical example, different volume ratios of EDA to water (0/30, 1/29, 2/28, 5/ 25, 10/20, 15/15, 20/10) were selected for comparison. Figure 1 shows the XRD patterns of NaCaSiO3OH samples prepared in various EDA/H2O volume ratios. Obviously, except for sample S-2, which will be discussed later, all peaks of the samples can be well ascribed to a monoclinic phase NaCaSiO3OH (JCPDS card No. 25-1319) with lattice constants a = 5.72 Å, b = 7.06 Å, c = 5.48 Å, and β =

EXPERIMENTAL SECTION

Materials and Preparation. All the chemicals were commercially purchased and used without further purification. NaCaSiO3OH was synthesized using a facile hydrothermal method. Simply, solution A was prepared by dissolving Ca(NO3)2·4H2O (0.472 g) in deionized water and EDA (the total volume of solution, 30 mL). Solution B was obtained by dissolving Na2SiO3·9H2O (0.682 g) in 8 mL of deionized water. The mixed solutions were prepared through the dropwise addition of solution B into solution A to form a homogeneous solution with vigorously magnetic stirring for 10 min. Subsequently, 6 g of NaOH was quickly added, and after 10 min of further stirring the resultant solution was sealed in a Teflon-lined stainless autoclave (50 mL). The autoclave was heated to 200 °C and kept for 24 h, and then samples were cooled to room temperature naturally. After that, the solid material was separated by centrifuging and sequentially washed with deionized water and ethanol several times to remove the basic solution and then dried at 80 °C for 12 h. For the synthesis of various morphology NaCaSiO3OH, the volume of EDA was modulating as an x:(30 − x) volumetric ratio of EDA:H2O. The detailed experimental parameters are listed in Table 1, and the products are denoted as S-1 to S-10. Eu3+- or Tb3+-doped NaCaSiO3OH microstructures were prepared in a similar manner except that Eu(NO3)3 or Tb(NO3)3 and

Figure 1. XRD patterns of NaCaSiO3OH samples prepared in various volume ratios of EDA to H2O: 0/30 (S-1), 1/29 (S-2), 2/28 (S-3), and 20/10 (S-7). The standard data of NaCaSiO3OH (PDF card 251319) is used as reference. B

DOI: 10.1021/acs.inorgchem.6b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 122.5°, and no diffraction peaks of any other phases were detected. Moreover, the phase purity of NaCaSiO3OH: Eu3+/ Tb3+ was not changed even if the Eu3+/Tb3+ ions were doped (Figure S1). Figure 2 shows SEM images of the NaCaSiO3OH samples prepared at 200 °C with different volume ratios of EDA to

Then, in the stirring process, the chemical equilibrium in eq 1 may move to the left and the Ca−EDA complex decomposes slightly, resulting in the increase of “free” Ca2+ concentration. Meanwhile, the hydrolysis of released EDA can occur, generating the OH− ions (eq 2). A phenomenon that the solution changes from clear-transparency to turbidity can be observed; it is supposed that once the solubility product (Ksp) of Ca(OH)2 is exceeded, a Ca(OH)2 precipitation reaction will occur (eq 3). NH 2 ·(CH 2)2 ·NH 2 + 2H 2O ↔ NH3·(CH 2)2 ·(CH 2)2 ·NH32 + + 2OH−

Ca 2 + + 2OH− → Ca(OH)2 ↓

(2) (3)

Subsequently, the Na2SiO3 solution was added into the above mixture with stirring, and more white precipitate was obtained, that is CaSiO3 (eq 4). The EDA will be released completely. [Ca(NH 2 · (CH 2)· NH 2)3 ]2 + + SiO32 −

Figure 2. SEM images of NaCaSiO3OH samples prepared in different volume ratios of EDA to H2O of 1/29 (a), 2/28 (b), 5/25 (c), 10/20 (d), 15/15 (e), and 20/10 (f).

→ CaSiO3↓ + 3NH 2 · (CH 2)· NH 2

Finally, when one mixes some NaOH rapidly in this system, the ultimate product NaCaSiO3OH can be gained in the case of elevated temperature (200 °C) and enough reaction time (24 h). The reaction equations were displayed as follows (eq 5 and eq 6):

H2O. The sample S-2 synthesized with EDA/H2O = 1/29 displays rod bunches (Figure 2a). When the volume ratio of EDA to H2O is 2/28, it can be observed that the morphology of the as-prepared products greatly varies compared to sample S-2. The presence of more EDA results in the synthesis of smooth and uniform decahedron particles (Figure 2b), and the shape shows a regular crystalline orientation showing the symmetry in the structure. By increasing the EDA concentration (EDA/H2O = 5/25), both sides of decahedron are concave as shown in Figure 2c. When the ratio is 10/20, NaCaSiO3OH spindle crystals with a middle interface are the dominant morphology in the products (Figure 2d). As the ratio increases to 15/15, polyhedron-type particles become the major ones, whereas microplates are minor (Figure 2e). When the ratio is 20/10, both microplates and polyhedron are obtained, and several aggregates came forth simultaneously (Figure 2f). Furthermore, the size of NaCaSi3OH particles (S-2 to S-6) diminishes with the increasing amount of EDA. Hence, according to the experimental results, we found that the shape and size of NaCaSi3OH particles can be tuned easily through changing the EDA concentration. Formation Mechanism of NaCaSiO3OH Microarchitectures. It has been previously reported that nanostructure ZnO can be obtained at a lower temperature with the assistance of EDA; EDA was regarded as a chelating agent, and the zinc− EDA molar ratio had determinative effects on the formation of ZnO.25 Therefore, in the present EDA-assisted hydrothermal system, the complexation function between Ca2+ and EDA can be expected based on the above quasi in situ monitoring of the morphology during the NaCaSiO3OH formation process. Then, on the basis of the analysis of our experimental results together with previous reports,26,27 several formulas for explaining the formation of NaCaSiO3OH can be tentatively proposed and expressed as follows. First, Ca2+ ions are coordinated with EDA molecules to form Ca−EDA complexes in the aqueous solutions:

CaSiO3 + NaOH → NaCaSiO3(OH)↓

(5)

Ca(OH)2 + Na 2SiO3 → NaCaSiO3(OH)↓+NaOH

(6)

Another situation noteworthy to be mentioned is that sample S-2 prepared at EDA/H2O = 1/29 is not NaCaSiO3OH phase according to Figure 1. Additionally, when the volume ratio of EDA/H2O is 30/0, the NaCaSiO3OH phase still cannot be obtained. The phenomenon suggests that a suitable amount of EDA is an essential factor for the synthesis of NaCaSiO3OH. Based on the above analysis and formation mechanism, actually, the essence of regulating the volume ratio of EDA/H2O is controlling the molar ratio of EDA to Ca2+. Therefore, a small amount of EDA is not enough to form Ca2+−EDA complex completely. However, if the added amount of EDA is 30 mL, the Ca(NO3)2 cannot be dissolved without solvent. Namely, the Ca2+−EDA complex is unable to form. That is the reason why the NaCaSiO3OH phase cannot be synthesized with the addition of too little or too much EDA. Growth Mechanism Investigations. As we know, the phase formation and morphology evolution in the hydrothermal process are mainly determined by the intrinsic structure character, thermodynamic stability, and extrinsic factors such as reaction temperature, time, the amount of some organic/inorganic additive, and so on. In this paper, asprepared NaCaSiO3OH particles with various size and morphology can be successfully synthesized through controlling reaction time and the amount of EDA and NaOH. To explore the possible growth mechanisms, the function of EDA and NaOH and the influence of reaction time were investigated in detail as follows. Effect of Reaction Time. To understand the growth process of the NaCaSiO3OH particles, time-dependent experiments were carried out to study the morphological evolution with the same synthetic conditions. Figure 3 exhibits the SEM images of the intermediates (EDA/H2O = 2/28) prepared at

Ca 2+ + 3NH 2 ·(CH 2)2 ·NH 2 ↔ [Ca(NH 2 · (CH 2)2 · NH 2)3 ]2 +

(4)

(1) C

DOI: 10.1021/acs.inorgchem.6b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. XRD patterns (a) and SEM images (b−f) of NaCaSiO3OH (EDA/H2O = 2/28) after the hydrothermal treatment for 4 h (b), 10 h (c), 12 h (d), 16 h (e), and 24 h (f).

Figure 4. XRD patterns and SEM images of NaCaSiO3OH samples ((a−c) EDA/H2O = 2/28; (d−f) EDA/H2O = 0/30) with various amounts of NaOH: (b, e) 6 g of NaOH; (c, f) 7 g of NaOH.

rate in one direction with respect to other growth orientations. In a liquid-phase reaction, the type of organic ligand as well as concentration of the ligand can control the reaction outcome; organic additives are known to form a complex with the metal ion, which can change the order of free energies of different facets through their interaction with the metal surface and slow down the nucleation and growth of crystals.22,30 Furthermore, their functional groups bind on the surface of the materials and significantly affect the relative growth rates of different facets. From compositional analysis of the products using EDS (Figure S2), the obtained sample contains Ca, Na, Si, O, and C elements for NaCaSiO3OH, but the H element cannot be detected. Then we can confirm that EDA binds to the surface of the materials, because the C element that came from EDA is detected. Based on the discussion of formation mechanism, the stable Ca2+−EDA complex first forms (eq 1) so that the concentration of free Ca2+ ions in solution can be controlled. Then under hydrothermal conditions, the Ca2+−EDA complex is attacked by SiO32−, and Ca2+ ions would be released gradually leading to the growth rate of crystal nuclei, and subsequent crystal growth was relatively slow. The released EDA adsorbs selectively on the active facet of growing precursor particles. Thus, the crystal growth rate along the direction (rodlike) is retarded, and it grows preferentially along the other directions. This kinetic control results in the formation of the decahedron with the preferential growth directions. According to the observed images in Figure 2c−f, the results suggest that more EDA added to the solution may prompt the growth in other directions. Meanwhile, the observed smaller particle sizes with the addition of more EDA can be attributed to much EDA impeding the growth to bigger particle sizes. In brief, EDA complexing agent may possess double functions on the growth of the precursor. First, as a strong ligand to form a stable Ca2+−EDA complex, this way slows down the rate of nucleation and subsequent crystal growth. Second, EDA acts as a structure-directing reagent absorbing on the surface of crystals, which directly affects the growth of different crystal facets by adjusting the growth rate of different facets, resulting in the formation of different structure. Furthermore, EDA also can be applied to control the size of particles through tuning the amount of EDA. Too much EDA binding to the surface of crystals limits the further growth of the particles. Effect of NaOH. Interestingly, some of the NaCaSiO3OH decahedrons (EDA/H2O = 2/28) can stack into a threedimensional (3D) pattern, and clearly defined NaCaSiO3OH flowerlike structures were obtained as illustrated in Figure 4c.

varied reaction times and their corresponding XRD patterns. It is clearly found that the product of NaCaSiO3OH-4h presents irregular and glomerate shape (Figure 3b). The same irregular shapes still exist for NaCaSiO3OH-10h synthesized with longer reaction time, but the symmetric polyhedral structure is formed gradually as shown in Figure 3c. When the reaction time was prolonged to 12 h, the SEM image of NaCaSiO3OH-12h (Figure 3d) shows that the uniform decahedron and rod bunch shape coexist, and the length of rod bunch is approximately equal to that of decahedron. Nevertheless, the rod bunch structure disappears after heating for 24 h, and only uniform decahedron appears (Figure 3f). It implies that the final decahedron structure is derived from the growth of rod bunch in terms of the previous information. Meanwhile, another question will be proposed, that is, why the morphology of samples prepared with 4 and 10 h is disorganized as shown in Figure 3b,c. Because the reaction time is not enough to form NaCaSiO3OH phase, the products are not NaCaSiO3OH based on the XRD patterns (Figure 3a). Therefore, only if the NaCaSiO3OH is pure phase, can good morphology be obtained. As for the specific morphology or growth orientation, it may be related to the crystal structure of monoclinic phase NaCaSiO3OH and other factors. Effect of EDA. As mentioned above, the growth of NaCaSiO3OH is directly related to the function of the organic molecule EDA. The EDA is a strong chelating agent with two lone pairs of electrons associated with the two nitrogen atoms for metal ions Ca2+. The formation of stable Ca2+−EDA complexes through stronger coordination interaction reduces the generation rate of particles in solution, which is favorable for the growth of various morphologies. To examine the function of EDA in the shape evolution of NaCaSiO3OH, two controlled experiments, without EDA (S-1) and with lower EDA concentration (EDA/H2O = 2/28, S-3), were conducted for comparison. According to the analysis of XRD patterns in Figure 1, the final compounds are both NaCaSiO3OH. Without EDA, the concentration of Ca2+ in aqueous solution was high, and they could react with SiO32− and NaOH at higher temperature to form a large amount of crystal nuclei when the degree of supersaturation exceeds the critical value. So the monomer concentration of NaCaSiO3OH for crystal growth was lower, and rodlike morphology is obtained as shown in Figure 4e. Laudise et al.28,29 proposed that the growth of crystals is associated with the relative growth rate of different crystal facets, and the difference in the growth rates of various crystal facets leads to a different morphology of the crystallite. So, the rodlike structures may be due to increasing the growth D

DOI: 10.1021/acs.inorgchem.6b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry The procedures for preparing flowerlike NaCaSiO3OH were kept unchanged except for modulating the amount of NaOH. Thus, the effect of NaOH on the morphology is worthy of consideration, so that a group of contrast tests were investigated with different amounts of NaOH (5 g, 6 g, and 7 g). The phase identification of three samples indicates that, except for the sample prepared with 5 g of NaOH, the remaining samples belong to the NaCaSiO3OH phase (Figure 4a). It is because the amount of 5 g of NaOH is insufficient for completing the reaction (eq 5). According to the above investigation, the amount of 6 g is enough to form NaCaSiO3OH phase. Within a certain range, addition of more NaOH (7 g) to the solution resulted in much higher ion concentrations. The high ion concentrations generate a high chemical potential which favors the growth to flowerlike structure.31 Therefore, the excess NaOH is a necessary condition to form flowerlike NaCaSiO3OH based on Figure 4c. From Figure 4 e,f, we can see that the morphology of NaCaSiO3OH samples, both prepared without EDA, changes from rodlike to all kinds of polyhedrons when the 7 g of NaOH is added. The phenomenon is also attributed to the previous explanation that when the EDA is absent, the high chemical potential prompts the formation of polyhedron structure. Comparing Figure 4c with 4f, although the amount of NaOH is 7 g in both cases, the flowerlike NaCaSiO3OH cannot be obtained yet if the complexing agent EDA is not used. Therefore, EDA is another factor for forming flowerlike structure. As shown in Figure 5, the schematic illustration for

Figure 6. PLE and PL spectra of NaCaSiO3OH:0.03Eu3+ (a) and NaCaSiO3OH:0.03Tb3+ (b) microarchitectures. (c) Decay curves and lifetime of NaCaSiO3OH:0.03Eu3+ and NaCaSiO3OH:0.03Tb3+. (d) Temperature-dependent emission spectra of NaCaSiO3OH:0.10Tb3+.

transitions of Eu3+ ions.35 The emission spectrum is dominated by the red 5D0−7F2 (610 and 619 nm) transition of Eu3+, which is an electric-dipole-allowed transition and hypersensitive to the environment. The PL spectra of the Tb3+-doped NaCaSiO3OH sample (Figure 6b) show a strong emission with a maximum at about 542 nm, which are all ascribed to 5D4−7FJ (J = 6, 5, 4, 3) transitions of Tb3+ corresponding to 5D4−7F6 (488 nm), 5 D4−7F5 (542 and 550 nm), 5D4−7F4 (583 nm), and 5D4−7F3 (625 nm), respectively.36 The excitation spectrum of the NaCaSiO3OH:Tb3+ monitored with 542 nm consists of three strong bands and several weak bands (Figure 6b) in the range from 200 to 500 nm. The strongest excitation line located at 265 nm is due to the 4f8−4f75d1 transition of Tb3+. Additional, the photoluminescence decay curves of the NaCaSiO3OH:Eu3+/Tb3+ phosphors were measured by monitoring the emission of Tb3+ at 544 nm and Eu3+ at 619 nm. It can be seen that the lifetime is determined to be 6.48 ms (NaCaSiO3OH:Tb3+) and 2.99 ms (NaCaSiO3OH:Eu3+), respectively, which are located in the normal range of Tb3+/ Eu3+ ions (Figure 6c). The internal quantum efficiency (IQE) and thermal quenching properties of the luminescence materials are important factors in evaluating the phosphor for practical application. Therefore, the IQE values of the typical samples were measured under 365 nm excitation. The corresponding values of NaCaSiO3OH:0.10Eu3+ and NaCaSiO3OH:0.10Tb3+ are 30.2% and 50.6% respectively. Moreover, the PL spectrum of typical sample NaCaSiO3OH:0.10Tb3+ was measured as a function of temperature 25−300 °C. As given in Figure 6d, the emission intensity of NaCaSiO3OH:0.1Tb3+ even increases slightly when the ambient temperature is below 200 °C. It suggests that the NaCaSiO3OH:0.1Tb3+ phosphor has excellent thermal stability. Additionally, according to previous reports, we find that bioactive nanoparticles based on silicate (Na+0.7[(Mg5.5Li0.3)Si8O20(OH)4]−0.7) are cytocompatible and strongly interact with the cells. 3 7 This silicate (Na+0.7[(Mg5.5Li0.3)Si8O20(OH)4]−0.7) is nontoxic; by this analogy, we deduce that the NaCaSiO3OH should be also nontoxic. According to the above results, the morphologycontrolled NaCaSiO3OH:Eu3+/Tb3+ particles possess the potential applications for micro/nano functional devices.

Figure 5. Schematic illustration for the formation of flowerlike NaCaSiO3OH.

the formation of flowerlike NaCaSiO3OH is proposed, and there is a break emerging in the middle of the decahedron. Due to the higher surface energy at the middle interface of the decahedron, EDA molecules released from the decomposition of Ca2+−EDA complex will adsorb on the interface first, resulting in a decrease of the surface energy and the generation of active sites. The active sites will trigger the nucleation at the interface, promoting the formation of petal crystals extending from the interface.32 Luminescence Property investigations. Figure 6 shows the representative photoluminescence excitation (PLE) and photoluminescence (PL) emission spectra of pure NaCaSiO3OH (EDA/H2O = 2/28) doped with 0.03 mol of Eu3+ or Tb3+. The PLE spectrum of NaCaSiO3OH:Eu3+ decahedron (Figure 6a) monitored with 619 nm shows a broad band from 200 to 400 nm with a maximum at about 261 nm, which can be attributed to the charge transfer band between the Eu3+ and the surrounding oxygen anions.33,34 Under excitation of 261 nm, the NaCaSiO3OH:Eu3+ sample shows a strong red luminescence mainly located at 588, 596, 610, 619, 659, and 700 nm, which are attributed to the 5D0 → 7FJ (J = 0, 1, 2, 3, 4) E

DOI: 10.1021/acs.inorgchem.6b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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CONCLUSIONS NaCaSiO3OH microarchitectures with various morphology have been synthesized by complexing agent-assisted hydrothermal process. The shapes of NaCaSiO3OH particles (rodlike and polyhedron) mainly depend on the added content of EDA. Moreover, the two determining conditions, more NaOH and the introduction of EDA, are responsible for the formation of flowerlike structure. Based on the formation and growth mechanism of NaCaSiO3OH, EDA complexing agent possesses double functions: acting as a strong ligand to form a stable Ca2+−EDA complex so that it slows down the rate of nucleation and subsequent crystal growth; acting as a structure-directing reagent to affect the growth of different crystal facets and control the size of particles through changing its amount. The as-obtained NaCaSiO3OH:Eu3+/Tb3+ samples show strong red/green emission under ultraviolet−visible light excitation, and they are expected to have promising applications in fields such as light display, optoelectronic devices, or bioimaging.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01871. XRD patterns, SEM image, and EDS spectrum (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was supported by the National Natural Science Foundations of China (Grants 51572023, 51272242), and Fundamental Research Funds for the Central Universities (FRF-TP-15-003A2).

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DOI: 10.1021/acs.inorgchem.6b01871 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01871 Inorg. Chem. XXXX, XXX, XXX−XXX