“One-Stone–Two-Birds” Modulation for Na3ScF6-Based Novel

Jun 16, 2014 - Yangbo Wang , Bingxiao Yang , Kun Chen , Enlong Zhou , Qinghua .... Hongjin Chang , Yongsheng Zhu , Juan Xie , Hongyu Li , Botong Liu ...
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“One-Stone−Two-Birds” Modulation for Na3ScF6‑Based Novel Nanocrystals: Simultaneous Morphology Evolution and Luminescence Tuning Xianghong He†,‡ and Bing Yan*,† †

Department of Chemistry, Tongji University, Shanghai 200092, China School of Chemistry and Environmental Engineering, Jiangsu University of Technology, Changzhou, Jiangsu 213001, China

‡

S Supporting Information *

ABSTRACT: Control over the morphology, size, and crystallographic phase of nanocrystals (NCs) through impurity doping is central to the realization of their unprecedented or improved properties. Herein we present the “one-stone−twobirds” modulation including simultaneous modification of the morphology and tuning of the luminescence for Na3ScF6 based NCs via a simple doping strategy. Ce3+/Tb3+ codoped Na3ScF6 NCs with monoclinic structure and hexagonal nanoplate or nanorod morphology were obtained through a modified solvothermal method. The formation of monodisperse Na 3ScF 6-based NCs with diverse architectures closely correlates with the doping level of Tb3+. On the basis of the experimental results, the possible growth mechanism for nanoparticles is proposed. Under UV light excitation, Na3ScF6:Ce3+/Tb3+ samples exhibited characteristic emissions from both Ce3+ and Tb3+ ions. By proper variation of the amount of Tb3+ doping while maintaining Ce3+ concentration, the emission color tuned from blue to green accompanied by the shape evolution from hexagonal nanoplate to short nanorod. Furthermore, the higher quantum yield from the current nanostructures compared with those of a LaPO4-based nanophosphor indicated that this scandium-containing sample is a promising green emission phosphor candidate for lighting and display applications.

1. INTRODUCTION In the past decade, a significant body of studies has been devoted to rare-earth nanocrystals (NCs) due to their importance in both basic scientific research and widespread technological applications.1−13 Among these NCs, fluoridebased luminescent NCs are considered a promising class of nanophosphors because of their unique combination of attractive optical and chemical properties and ease of controlled synthesis.1,2,14−21 Compared with considerable work on rareearth Y3+- and Ln3+ (Ln = La−Lu)-based fluorides,14−22 scandium-containing fluoride systems have largely been overlooked.23−29 In fact, scandium is a well-known fluorophilic rareearth element with smaller ion radius and distinct atomic electron configuration, which results in optical properties different from those of Y3+/Ln3+-based fluoride NCs.23,30−32 As one important member of the scandium-containing fluoride family, Na3ScF6 can provide suitable Sc3+ sites at which trivalent rare-earth elements can be easily substituted without additional charge compensation, indicating that Na3ScF6 can be used as a good host compound for photoactive lanthanide (Ln) ions. © 2014 American Chemical Society

Consequently, the development of novel rare-earth nano- and microcrystal phosphors has led to a rebirth of interest in this compound.23−26 Most recently, the phase-controlled synthesis and upconversion emission properties of Na3ScF6:Yb3+/M3+ (M = Er, Tm, and Ho) NCs were reported.23−25 Moreover, multicolor up-conversion luminescence tuning in this system has been realized by altering Yb3+ doping concentration.24 However, to the best of our knowledge, morphology control and tunable downshifting fluorescence for Na3ScF6-based NCs still remain an open challenge. The incorporation of dopants into pure host lattices provides versatility of creating various functional nanomaterials with unprecedented properties achievable beyond what is possible in undoped counterparts.33−41 Among them, Ln ion doping of NCs has been recognized as a convenient and effective approach to induce novel properties and control the formation Received: January 3, 2014 Revised: June 10, 2014 Published: June 16, 2014 3257

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Experimental Section). As shown in Figure 1, XRD patterns of Na3ScF6 host and Ce3+ single doped and Ce3+/Tb3+ codoped

of NCs involving promoting phase transformation and tuning luminescence properties, as well as modifying the shape and size.42−49 Additionally, doping is believed to be a promising route to tailor the local environment around the lanthanide ions in the fixed host lattices, which usually results in favoring luminescence efficiency.50 In this work, uniformly distributed Na3ScF6:Ce3+/Tb3+ NCs with plate or rod morphology were obtained via a solvothermal method. A possible growth mechanism for various shapes of nanoparicles is discussed. The “one-stone−two-birds” modulation including simultaneous morphology and luminescence control has been achieved by a simple Ln doping strategy. We found that doped Ln ions not only act as emitters and sensitizers for luminescence but also can modify the shape and size of NCs without altering the crystallographic phase of the host.

2. EXPERIMENTAL SECTION Sc2O3 (4 N), Tb4O7 (5 N), Ce(NO3)3·6H2O (≥99.95%), NaOH (≥96.0%), HF (≥40.0%), concentrated nitric acid (HNO3, ≥ 68.0%), ethanol (≥99.7%), and cyclohexane (≥99.5%) were purchased from Sinopharm Chemical Reagent Co., China. Oleic acid (90 wt %) was supplied by Alfa Aesar Co., China. All of the chemicals were used as received without further purification. Rare earths oxides were separately dissolved in dilute HNO3 solution, and the residual HNO3 was removed by heating and evaporation, resulting in the formation of an aqueous solution of the corresponding RE(NO3)3. Na3ScF6 host and Ce3+ or Tb3+ single doped and Ce3+/Tb3+ codoped Na3ScF6 NCs were prepared by a modified solvothermal method. Herein we took the synthesis of Na3ScF6:Ce3+/Tb3+ (5.0/1.0 mol %) as an example. In a typical procedure, an aqueous solution containing Sc(NO3)3 (0.25 M, 3.76 mL), Ce(NO3)3 (0.05 M, 1.00 mL), and Tb(NO3)3 (0.05 M, 200 μL) was mixed with ethanol (15.0 mL), oleic acid (OA, 8.50 g), and NaOH (0.60 g) under thorough stirring. Then, 6 mmol of HF (0.30 g) was added dropwise to the mixture. After vigorous stirring at room temperature for about 30 min, the colloidal solution was transferred into a 50 mL Teflon-lined autoclave, sealed, and heated at 180 °C for 12 h. The final products were collected, washed several times with ethanol/cyclohexane, and purified by centrifugation. After drying at 50 °C under dynamic vacuum for 24 h, the Na3ScF6:Ce3+/Tb3+ sample was obtained. The synthetic procedure for Ce3+ or Tb3+ single doped Na3ScF6 NCs was the same as that used to prepare Na3ScF6:Ce3+/Tb3+, except that stoichiometric amounts of Sc(NO3)3 and Ce(NO3)3 or Tb(NO3)3 were added to sodium−oleic acid complex. LaPO4:Ce3+/Tb3+(5.0/1.0 mol %) was prepared via the above solvothermal route. 2.1. Characterization. X-ray diffraction (XRD) measurements were carried out on a Bruker D8 Advanced X-ray diffractometer with Ni filtered Cu Kα radiation (λ = 1.5406 Å) at a voltage of 40 kV and a current of 40 mA. The morphology, the selected area electron diffraction (SAED) pattern, and energy-dispersive X-ray spectroscopic (EDS) analysis were obtained with a transmission electron microscope (TEM, JEM-2010) using an accelerating voltage of 200 kV. TEM specimens were prepared by directly drying a drop of a dilute cyclohexane dispersion solution of the products on the surface of a carbon-coated copper grid. Photoluminescence (PL) excitation and emission spectra were obtained on an Edinburgh Instruments FLS920 spectrofluorimeter equipped with both continuous (450 W) and pulsed xenon lamps. Photoluminescence absolute quantum yield (QY) was determined employing an integrating sphere (150 mm diameter, BaSO4 coating) from Edinburgh FLS920 phosphorimeter. All of the measurements were obtained from powder samples and performed at room temperature.

Figure 1. XRD patterns of as-synthesized (a) Na3ScF6 host, (b) Na3ScF6:Ce3+(5.0 mol %), (c) Na3ScF6:Ce3+/Tb3+ (5.0/1.0 mol %), (d) Na3ScF6:Ce3+/Tb3+ (5.0/5.0 mol %), and (e) Na3ScF6:Ce3+/Tb3+ (5.0/10.0 mol %) NCs. The vertical bars represent the standard data of JCPDS No.47-1221.

Na3ScF6 samples exhibit well-defined peaks indicative of highly crystalline materials and can be well indexed as monoclinic Na3ScF6 (JCPDS 47-1221, space group P21/n). The calculated lattice parameters, a = 5.603, b = 5.798, and c = 8.119 Å and β = 90.6°, are quite similar to those observed for the corresponding bulk materials.51−54 For either the single doped or double doped samples, no diffraction peaks from residues or impurities have been detected, indicating the high purity of the products. The lattice structure of Na3ScF6 consists of isolated regular ScF6 octahedra linked by two crystallographically distinct Na atoms, of which one occupied a fairly regular octahedron and the other occupied an extremely distorted cubic antiprism.51−54 Compositional analysis of the products using EDS (Figure S1 and Table S1 in the Supporting Information) confirms that the chemical signatures taken within different parts of the sample are identical within experimental accuracy and that the asobtained samples contain Sc, Na, F, and Ce or (Ce + Tb) elements for Na3ScF6:Ce3+ and Na3ScF6:Ce3+/Tb3+, respectively. As for Na3ScF6:Ce3+/Tb3+ (5.0/1.0 mol %) sample, the Tb element was not detected due to its low feed content as well as being outside the determination limit. In addition, the actual atomic ratio of F, Sc, Na, and Ce or (Ce + Tb) elements was appropriately close to the designed nominal stoichiometry in each case (Table S1, in the Supporting Information). All above results confirmed that single-phase Na3ScF6-based NCs were obtained and Ce3+/Tb3+ have been successfully incorporated into host lattices. Considering the same valence states and the close ionic radii between Sc3+ [coordination number (CN) = 6, r = 0.745 Å] and Ce3+/Tb3+ (CN = 6, r = 1.03 Å for Ce3+, r = 0.92 Å for Tb3+) ions,55 Ce3+/Tb3+ ions are supposed to substitute for Sc3+ sites in Na3ScF6 host lattices. 3.2. Morphology Analysis. Figure 2 and Figure S2 (Supporting Information) show the TEM images of pure Na3ScF6, Ce3+ or Tb3+ single doped Na3ScF6, and Ce3+/Tb3+ codoped Na3ScF6 as-synthesized nanoparticles. Without addition of any dopant ions, the as-obtained NCs are irregular polyhedron shapes (panel a in Figure S2 of the Supporting

3. RESULTS AND DISCUSSION 3.1. XRD Patterns and Compositional Analysis. Pure and doped Na3ScF6 NCs were prepared via the modified solvothermal strategy (the detailed procedure is given in the 3258

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except that the Tb3+ doping content changed. Accordingly, it is reasonable to believe that the Tb3+ doping greatly influences the Na3ScF6 crystal growth rate along the c-axis. The possible formation mechanism for Na3ScF6 NCs with plate-/rod-like morphology is schematically illustrated in Figure 3. Generally, the growth process of crystals can be separated into two steps, an initial nucleating stage and a subsequent crystal growth process. Herein, in the first stage of the reaction, the oleic phase consisting of OA (RCOOH, R represents the alkyl chain), ethanol (EtOH), sodium oleate, and lanthanide oleate [RCOONa + (RCOO)3Ln] and the aqueous phase containing water, ethanol, and F− are formed in the reaction system of water−alcohol−oleic acid mixing medium.4,56 As the temperature reaches 180 °C, the oleic acid capped Na+ and Sc3+ quickly react with F− to form the Na3ScF6 based crystalline nuclei. As the reaction is prolonged, the Na3ScF6 based nuclei quickly grow, benefiting from F− diffusion from the solution phase to the surface of NCs. Previous reports have demonstrated that the size of the Ln3+ as the substituted dopant ion plays a key role in the formation of rare-earth fluoride-based nanoparticles with various morphology, such as NaYF4 and NaYbF4 NCs.40,42,49,57 Generally, the substitution ions with larger ionic radius favor the morphology tuning and size reduction because the larger the radius of Ln3+ ion, the larger the electron charge density on the surface of NCs.40,57 In our case, when Sc3+ ions in Na3ScF6 host lattice are substituted by Tb3+ ions with slightly larger radius, the increase of surface electron charge density can dramatically slow the diffusion of F− ions toward the surface of Na3ScF6:Ce3+/Tb3+ crystal nuclei because of the increase of charge repulsion.42 Different contents of Tb3+ result in various diffusion speeds of F−. The chemical potential of the crystal facet varied from the level of F− on the external surface, and the relative growth rate in different directions changed, finally leading to different crystal morphologies.58 Consequently, Na3ScF6:Ce3+/Tb3+ hexagonallike nanoplates and nanorods were formed. In a word, the formation of various morphologies is probably ascribed to the strong effect of Tb3+ as dopant on anisotropic crystal growth through surface charge modification.40,42,49,57 The presence of Ce3+ has a very limited effect on the shape and size of Na3ScF6based NCs, which may be attributed to the larger disparity in size between Sc3+ and Ce3+ relative to the case of Sc3+ and Tb3+.40,42 The selected area electron diffraction (SAED) patterns of the single hexagonal nanoplate (the right inset in Figure 2a) and nanorod (the right inset in Figure 2c) demonstrated that all of the as-obtained NCs are of single-crystalline nature. Moreover, these SAED patterns are essentially identical, which reflected that the hexagonal-like nanoplates and nanorods belonged to the same phase. The patterns can be indexed to the reflection of monoclinic Na3ScF6 structure, consistent with the XRD results presented above. 3.3. Photoluminescence (PL) of Na3ScF6:Ce3+ NCs. The excitation and emission spectra of as-obtained Na3ScF6:Ce3+ nanophosphor is presented in Figure 4a. The excitation spectra are different under different wavelength light monitoring. The excitation spectrum monitored at Ce3+ emission, 344 nm, exhibited a broad band ranging from 240 to 330 nm with maxima appearing at 260 and 290 nm; while for that monitored at 400 nm emission, a strong peak at 306 nm and weak band at 260 nm were observed. The position and profile of emission spectra varied upon different wavelength light excitation. Upon excitation at 260 nm, the emission peak was located at about

Figure 2. TEM images of Na3ScF6:Ce3+/Tb3+ with fixed concentration of Ce3+ (5.0 mol %) and different doping contents of Tb3+: (a) 1.0 mol %, (b) 5.0 mol %, (c) 10.0 mol %, (d) and 12.5 mol %. (e) Morphology evolution for Na3ScF6:Ce3+/Tb3+ NCs. Scale bars are 100, 50, 20, and 20 nm for panels a, b, c, and d, respectively. The left inset in panels a and c shows the single hexagonal nanoplate and nanorod, respectively, while the right one shows the corresponding SAED patterns.

Information). When a certain amount Ce3+ was doped into the matrix, the morphology almost remained unchanged (panels b and c in Figure S2 in the Supporting Information), whereas the size of particles decreased after introduction of Tb3+ ion (panel d in Figure S2 in the Supporting Information). For Na3ScF6:Ce3+/Tb3+(5.0/1.0 mol %) NCs, two distinct particle morphologies that include a large quantity of large hexagonallike nanoplates with a size of approximately 68.1 nm × 92.1 nm (width × length) and smaller nanorods are presented (Figure 2a). On doping with increased Tb3+ ions, the quantity of small nanorods gradually increased (Figure 2b). When the Tb3+doping level reached 10.0 mol %, as shown in Figure 2c, all the as-obtained Na3ScF6:Ce3+/Tb3+ (5.0/10.0 mol %) NCs display relatively uniform short nanorod shapes and high crystallite size uniformity, illustrating the obvious transition from nanoplate to nanorod. As more Tb3+ was introduced into the reaction system while Ce3+ concentration was fixed, the shape of nanoparticles was almost unaffected (Figure 2d). The average size of the nanorods is evaluated to be about 10.2 nm × 21.5 nm. Furthermore, the aspect ratio of NCs increased from 1.35 for nanoplates to 2.1 for nanorods. In other words, the slenderness of the NCs increased (Figure 2e). In this work, the hexagonal-like nanoplates or nanorods of Ce3+/Tb3+ codoped Na3ScF6 were obtained under identical synthetic conditions 3259

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Figure 3. Schematic illustration of the formation and evolution process for Na3ScF6-based NCs with diverse morphology.

excitation wavelength, indicating that there are at least two luminescent centers in theNa3ScF6:Ce3+ phosphor.59−70 Generally, the emission of Ce3+ has a doublet character with an average energy difference of about 2000 cm−1 due to the ground state splitting (2F5/2 and 2F7/2). The profile changes in the emission and excitation spectra can be attributed to the presence of two different Ce3+ luminescence centers.59 On basis of the asymmetric shape, the emission band of Na3ScF6:Ce3+(5.0 mol %) can be decomposed into four wellseparated Gaussian components with peak centers at 330 nm (30 303 cm−1), 354 nm (28 248 cm−1), 410 nm (24 390 cm−1), and 465 nm (21 505 cm−1) (Figure S3 of the Supporting Information). The energy difference between 330 and 354 nm and 410 and 465 nm is about 2055 and 2885 cm−1, respectively. According to precious works and the theoretical difference between the 2F5/2 and 2F7/2 levels of Ce3+ ions (about 2000 cm−1),59−69 these four peaks resulted from the ground state splitting and the different Ce3+ emitting centers. The bands centered at 330 and 354 nm are due to one Ce3+ emitting center,67 while the bands centered at 410 and 465 nm belong to another Ce3+ emitting center.71−73 In other words, the blueemitting center showed a blue emission band with its maximum around 400 nm, which can be attributed to a 5d → 4f transition of Ce3+ ion in a Sc3+ isolated octahedral site, while the ultraviolet emission center can be assumed to originate also from a Ce 3+ ion but one located at other types of sites.51−54,59,71−73 Similar emission phenomenon has also been observing in other Ce3+-doped fluorides.71−73 The relationship between the photoluminescence intensity of Ce3+ and its doping concentration in Na3ScF6:Ce3+ (x mol %) samples is depicted in Figure 4b. The emission intensity first increases with increasing doping concentration, reaching a maximum value when Ce3+ concentration is 0.05 mol of the host, and then decreases with further increasing Ce3+ content due to concentration quenching effect.60,74,75 Thus, the optimum doping concentration for Ce3+ in Na3ScF6 host is

Figure 4. (a) Excitation (left) and emission (right) spectra of Na3ScF6:Ce3+ (5.0 mol %) NCs, and (b) photoluminescent intensity of Na3ScF6:Ce3+ (x mol %) samples as a function of Ce3+ doping concentration (x mol %) under 290 nm excitation.

344 nm. However, the emission maximum was red-shifted to 400 nm when 306 nm UV light was used as the excitation source. Under excitation of 290 nm UV light, it was centered at 400 nm and exhibited a shoulder peak at 344 nm. The shapes and peak wavelengths of the emission spectra varied from the 3260

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5.0 mol % of Sc3+. And in the following work, the doping content of Ce3+ was fixed at 5.0 mol %. 3.4. PL Tuning of Na3ScF6:Ce3+/Tb3+ (5.0/y mol %, 0 ≤ y ≤ 12.5) NCs. As shown in Figure 5a, when the green

surrounding Ce3+ ions and then remarkably affected the sensitized luminescence routes of Tb3+ ions in Na3ScF6:Ce3+/ Tb3+ nanophosphor.65,66 Similar spectroscopic phenomena were also observed by another group.64−66 Furthermore, when the emission at 400 nm was monitored, the Na3ScF6:Ce3+/Tb3+ nanophosphor displayed an asymmetric and broad excitation band at around 306 nm, which is different from that monitored at 543 nm for Tb3+ ions. Thus, we speculated that there exist three luminescent centers in assynthesized Na3ScF6:Ce3+/Tb3+ NCs (i.e., blue-emitting center of Ce3+, ultraviolet-emitting center of Ce3+, and emitting center of Tb3+). Obviously, the existence of different emission centers is one of the essential prerequisites for the color-tunable fluorescence of Ce3+/Tb3+ codoped Na3ScF6 samples. Ce3+ is a well-known sensitizer for the luminescence of Tb3+ in many host compounds.59−63,65−69 Subsequently, one good strategy to enhance the luminescence of Tb3+ is to codope Ce3+ into host lattices. In this study, Ce3+ and Tb3+ ions were simultaneously doped into Na3ScF6 matrix. Herein, the presence of the Ce3+ absorption broad band (Figure 5a) as well as a significant spectral overlap between the emission band of Ce3+ and Tb3+ excitation transitions in the range 300−425 nm (Figure S4 of the Supporting Information), validated the occurrence of the energy transfer from Ce3+ to Tb3+. Ce3+ → Tb3+ energy transfer leads to highly efficient green emission of Tb3+ ions in the Na3ScF6:Ce3+/Tb3+ system, as displayed in Figure 5b. However, due to the incomplete energy transfer from Ce3+ to Tb3+,66−69 the excitation into the Ce3+ absorption band still yields the emission band from Ce3+ ions. In this case, the tuning of emission color for Na3ScF6:Ce3+/Tb3+ (5.0/y mol %, 0 ≤ y ≤ 12.5) nanophospors can be achieved by changing Tb3+ doping content (i.e., y value). We will report the detailed energy-transfer mechanism in Na3ScF6:Ce3+/Tb3+ NCs in future work. Figure 5b shows the emission spectra of Na3ScF6:Ce3+/Tb3+ (5.0/y mol %, 0 ≤ y ≤ 12.5) samples with various Tb3+-doping concentrations. Under excitation at 290 nm, each emission spectrum consists of a broad band from Ce3+ ions and a series of strong lines at 488, 543, 582, and 620 nm due to the 5D4 → 7 FJ (J = 6, 5, 4, and 3) transitions of Tb3+ ions, respectively. The green emission peak at 543 nm from 5D4 → 7F5 transition dominates the whole spectrum. With increasing Tb 3+ concentration (y value), the emission intensity of Ce3+ ions decreases monotonically, whereas the luminescent intensity of Tb3+ ions simultaneously increases, reaching a maximum value at y = 10, and then decreases with further increasing y value. Only a trace of emission from Ce3+ was detected in Na3ScF6:Ce3+/Tb3+ (5.0/12.5 mol %) NCs. The Commission Internationale de L’Eclairage (CIE) chromaticity coordinates of Na3ScF6:Ce3+/Tb3+ (5.0/y mol %) (0 ≤ y ≤ 12.5) samples are calculated and summarized in Table S2 of the Supporting Information. The CIE chromaticity diagram of the nanophosphors is depicted in Figure S5 of the Supporting Information. The color hue of Na3ScF6:Ce3+/ Tb3+(5.0/y mol %, 0 ≤ y ≤ 12.5) NCs varied from blue to green with increasing Tb3+ doping content. Furthermore, as for Na3ScF6:Ce3+/Tb3+ samples with fixed Ce3+ and Tb3+ doping contents, the photoluminescence emission color can also be changed from blue to green by adjusting the excitation wavelength from 306 nm over 290 to 260 nm (Figure S6 of the Supporting Information). Finally, considering the emission of Ce3+ and Tb3+ ions, the absolute QY of Na3ScF6:Ce3+/Tb3+ NCs was determined be

Figure 5. (a) Excitation spectra of Na3ScF6:Ce3+/Tb3+ (5.0/y mol %, 1 ≤ y ≤ 10) monitored at 543 nm. (b) Emission spectra of Na3ScF6:Ce3+/Tb3+ (5.0/y mol %) (0 ≤ y ≤ 12.5) NCs under 290 nm excitation.

emission at 543 nm corresponding to 5D4 → 7F5 emission of Tb3+ was monitored, Ce3+/Tb3+ codoped Na3ScF6 nanophosphors exhibited a broad and intense band originating from the allowed 4f → 5d transitions of Ce3+ and some weak lines coming from parity-forbidden 4f → 4f transitions of Tb3+ ions. The narrow lines beyond 330 nm are ascribed to the typical Tb3+ intra-4f8 transitions, including the peaks at 342 nm (7F6 → 5L8), 353 nm (7F6 → 5G4), 369 nm (7F6 → 5L10), and 378 nm (7F6 → 5D3). Especially, the relative intensity of two bands from Ce 3+ ion for Ce 3+/Tb 3+ codoped Na 3 ScF 6 nanophosphor shows some significant changes. For the sample doped with lower Tb3+ concentration (