Morphology-sensitive photoluminescent properties of YVO4:Ln3+ (Ln3+ = Eu3+, Sm3+, Dy3+, Tm3+) hierarchitectures

Journal of Luminescence 215 (2019) 116624

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Morphology-sensitive photoluminescent properties of YVO4:Ln3+ (Ln3+ = Eu3+, Sm3+, Dy3+, Tm3+) hierarchitectures

T

O. Tegusa, Bao Amurisanaa,b,*, Song Zhiqianga a b

Inner Mongolia Key Laboratory for Physics and Chemistry of Functional Materials, Inner Mongolia Normal University, Hohhot, 010022, China Department of Biology and Chemistry Engineering, Hohhot Vocational College, Hohhot, 010051, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydrothermal route Flower-like Hierarchitectures Morphology-sensitive photoluminescent

Novel three-dimensional YVO4:Ln3+ (Ln3+ = Eu3+, Sm3+, Dy3+, Tm3+) hierarchitectures were successfully fabricated on glass substrate by a simple, hydrothermal process by using disodium ethylenediaminetetraacetic acid (Na2H2L, where L4− = (CH2COO)2N(CH2)2N(CH2COO)24−)) as a capping agent. The shape and size of product can be tailored by controlling the reaction time and the Na2H2L/Y3+ molar ratio. As a typical morphology, YVO4:Eu3+ shows a flower-like hiearchitecture, which is composed of a primary microrod trunk and a large number of secondary nanonails. The trunk is a rectangle prism with a length of 2 μm. The nanonail branch has a length of 400 nm and a diameter of 80 nm, and its cap has a diameter of 100 nm with a thickness of 20 nm. A possible formation mechanism of this flower-like hierarchical structure was proposed, and it is investigated in detail through time evolution experiments. Under UV radiation, the thin film samples were shown the emission characteristic of Eu3+ (5D0→7F2, 618 nm), Dy3+ (4F9/2 → 6H13/2, 575 nm), Sm3+ (4G5/2 → 6H7/2, 603 nm), Tm3+ (1G4→3H6, 478 nm), and morphology-sensitive photoluminescent properties.

1. Introduction Certain recent, great contributions have been made to prepare and characterize the one-dimensional (1D) inorganic nanomaterials, such as nanorods, nanotubes, nanofibers, nanobelts, etc [1–4]. Although the 1D nanomaterials exhibit advantageous electronic structures and properties over those of bulk counterparts, the three-dimensional (3D) ordered hierarchical micro/nanomaterials with manifold morphology self-assembled by 1D nanostructures may offer much more opportunities to discover novel material properties for chemists and material scientists. The as-prepared 3D architectures can not only retain the properties of individual 1D nanostructure but can bring novel collective and cooperative properties caused by the well-ordered building blocks, whose outstanding properties allow massive potential applications in various areas [5–7]. Accordingly, the design of the 3D hierarchical micro/nanomaterials with controllable sizes and shapes, coupled by the investigation of their structure-property relationships, has become an interesting focus in materials science. Up to date, various synthetic strategies have been developed to prepare 3D superstructured inorganic nanomaterials, including top-down approach, thermal vapor deposition, template-assisted method, hydrothermal/solvothermal route, etc [8–12].

Of the inorganic functional materials, the lanthanide ion doped YVO4 is recognized as an excellent light emitting material owing to its fascinating optical characteristics, high chemical stability, and high luminescence quantum yield caused by efficient energy transfer from excited VO43− complex anions to Ln3+ activator ions [13,14]. The bulk YVO4:Eu3+ has been widely used as an efficient red emitting phosphor in color television displays and fluorescent lamps. With the fast development of modern nanoscience and nanotechnology, however, a higher request has been set to the functional materials, and the bulk and 1D nanostructured materials have not been able to meet the need to development of miniaturization, multifunction and integration of materials. In contrast to bulk and 1D nanostructured materials, the hierarchitectured nanomaterials are becoming a better candidate to meet the requests of future development. Especially, the most prominent luminescent material, YVO4:Ln3+ will play an important role in fabricating future nanodevices. Thanks to its special structure assembled from 1D nanoscale building units, YVO4:Ln3+ micro/nanosuperstructure will be the research hotspot. Recently, some research groups prepared YVO4:Ln3+ nanostructures with various morphologies and sizes using a series of methods, with appropriate Y and vanadate precursors as starting materials. Typically, You et al. fabricated YVO4:Eu3+ submicrospheres self-assembled from nanoparticles via the

* Corresponding author. Inner Mongolia Key Laboratory for Physics and Chemistry of Functional Materials, Inner Mongolia Normal University, Hohhot, 010022, China. E-mail address: [email protected] (B. Amurisana).

https://doi.org/10.1016/j.jlumin.2019.116624 Received 11 November 2018; Received in revised form 24 May 2019; Accepted 12 July 2019 Available online 13 July 2019 0022-2313/ © 2019 Published by Elsevier B.V.

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stainless-steel-lined autoclave and a piece of glass substrate vertically immersed into solution. The autoclave was heated at 250 °C for 4 h, and then waited to cool down until reached room temperature. After the reaction, the substrate was removed from the growth solution, rinsed with deionized water and ethanol, and dried at 70 °C for 5 h to obtain the final product. Additionally, different molar radio of Na2H2L/Y3+ and different reaction time at 250 °C were selected to investigate the morphological evolution of the flower-like YVO4:5% Eu3+ hierarchitectures. Following a similar procedure, self-assembled hierarchitecturs with other compositions were also prepared.

solvothermal method, using EG (ethylene glycol) and PVP (polyvinyl pyrrolidone) as solvent and surfactant, respectively [15]. Li's group has obtained a series of 3D structured YVO4:Eu3+ (e.g., hollow microspheres, potato-cakes, microhamburgers and lichee-like structures) by tuning the experimental parameters under the hydrothermal conditions [16]. Using Y(OH)CO3:Eu3+ as a sacrificial template, How's group synthesized self-assembled core-shell YVO4:Eu3+ microsphere by a twostep synthesis [17]. Subsequently, the same group reported the synthesis of YVO4:Eu3+ nanostructure with different morphologies of flowerlike, spherical, and revolving door-like multiple by the solvothermal method [18]. Tang's group prepared white light emitting YVO4:Eu3+, Tm3+, Dy3+ nanometer– and submicrometer–sized particles using an ion exchange method [19]. However, the highly symmetrical feature allows YVO4 crystal structure growth isotropicly, as a result YVO4 can hardly form anisotropic growth of 1D nanostructure and hierarchitectures built up of such a 1D nanostructure under solution condition. Thus how to develop facile, mild, easily-controlled methods to obtain hierarchically self-assembled YVO4:Ln3+ architectures with ideal and controllable morphologies, sizes, and structures are of great challenge and significance. In this work, we have prepared a series of 3D self-assembled flowerlike YVO4:Ln3+ hierarchitectures on glass substrates via a one-pot, simple hydrothermal route, using Na2H2L as chelating agent and structure-directing agent. A plausible formation mechanism has been proposed based on a systematic investigation on the assembly process. Under the UV irradiation, the as-prepared YVO4:Ln3+ superstructures showed the doped ions-typed luminescence, which can be regulated by the morphology evolution, endowing morphology-sensitive photoluminescent properties. As we know, the fabrication of such a hierarchically nanostructured YVO4:Ln3+ through hydrothermal reaction has not been reported so far.

2.3. Characterizations X-ray diffraction (XRD) was used for phase identification and crystal structural determination on a Philips PW1830 X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å; scanning rate: 4°/min in the range of 10−80°) operating at 40 kV/40 mA. The sample morphology was examined with a field emission scanning electron microscope (FESEM; Hitachi-SU8010) with an accelerating voltage of 5 kV, combined with energy dispersive X-ray spectroscopy (EDX). Transmission electron microscopy (TEM) photographs were obtained using a JEOL JEM-2010 transimission electron microscope operating at 200 kV. The PL (Photoluminescence) excitation and emission spectra of the film samples were recorded with a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The luminescence decay curves were obtained from a FLS920 fluorescence photometer Combined Steady State and Lifetime Spectrometer. All the measurements were performed at room temperature. 3. Results and discussion

2. Experimental section

3.1. Phase, composition, structure analysis

2.1. Materials

Fig. 1 shows XRD pattern of the YVO4:5% Eu3+ synthesized with Na2H2L/Y3+ mole ratio of 1.6 at 250 °C for 4 h. Compared with JCPDS (PDF #821968) for YVO4, the reflections of YVO4:5% Eu3+ thin film consist of a broad peak and some sharp peaks. In addition to the weak and broad peak at about 22° assigned to amorphous SiO2 in glass slide, the reflections are all agreed with the standard JCPDS card. No impurities peaks are observed, indicating that the Eu3+ ions have been homogenously incorporate into the host lattice. The lattice constant calculated is a = b = 7.0966 Å; c = 6.2461 Å, which are consistent well with the corresponding standard data (JCPDS, PDF #82196:

Analytical grade reagents of NH4VO3, C10H14N2Na2O8·2H2O, NH3·H2O, concentrated HNO3, KOH, and Y2O3 (99.99%), Eu2O3 (99.99%), Dy2O3 (99.99%), Sm2O3 (99.99%), Tm2O3 (99.99%) were used as the starting materials. The water used for the experiments was deionezed, and all the reagents were used without further purification. 2.2. Fabrication of YVO4:Ln3+ samples All the YVO4:Ln3+ (Ln3+ = 5% Eu3+, 1% Dy3+, 1% Sm3+, 1% Tm3+, 1% Dy3+/1% Tm3+, 1% Dy3+/2% Tm3+) samples were prepared by according to the procedures our group reported before [20–22]. The preparation process of flower-like YVO4:5% Eu3+ hierarchitecture is an example to explain the synthetic procedure of products. The typical experiment process showed as follow: 2.2.1. Pretreatment of glass substrates In this experiment, it has selected the glass slide as sample growth substrate. First, the substrate was immersed into the 5% KOH solution for 15 min, then ultrasonically cleaned for 20 min in a solution of acetone with a volume ratio of 1:1, and followed by drying it in vacuum oven at 60 °C for 4 h. 2.2.2. Preparation In a typical procedure, 0.00095 mol Y2O3 and 0.00005 mol Eu2O3 powder were added into the concentrated nitric acid, followed by magnetically stirred and heated to form a transparent aqueous solution. Subsequently, 0.002 mol NH4VO3 and right amount of Na2H2L were mixed into the solution with continuous stirring and heating until it became complete dissolution. Then, the pH of the solution was adjusted using NH3·H2O to 9. The whole mixture was then transferred into a

Fig. 1. XRD pattern of the YVO4:5% Eu3+ film sample prepared by hydrothermal reaction at 250 °C for 4 h, Na2H2L/Y3+ = 1.6. The standard JCPDS card (PDF#821968) for YVO4 is also presented for comparison. 2

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Fig. 2. SEM images of the flower-like YVO4:5% Eu3+ obtained at 250 °C for 4 h, Na2H2L/Y3+ = 1.6. (a) Low magnification SEM image. (b) Middle magnification SEM image. (c) Single flower-like hierarchitecture. (d) High magnification SEM image.

8a = b = 7.1192 Å; c = 6.2898 Å) of the tetragonal phase. The major XRD diffraction peaks appeared at 2θ = 25.1°、33.7°, 50.0°, are attributed respectively to (200), (112) and (312) planes of tetragonal phased YVO4. Interestingly, the ratios of the intensity for (200) reflection peak respectively with (112) and with (312) are 1.99 and 3.56, which are higher than the corresponding values from standard card (1.32 and 1.11), suggesting that the growth processes of YVO4:5% Eu3+ nanocrystals occur preferentially along the certain crystal axis direction. Fig. 2 depictes the SEM images of the film sample at different magnifications. A panoramic SEM image in Fig. 2a confirms that the plentiful and uniform flower-like morphologies have been produced on a large scale, showing a lot of blooming natural chrysanthemum accumulated together. No other morphologies can be observed, indicating a high yield of these flower shaped microstructures by the assistance of hydrothermal process. And all of samples possess the same morphology with an average size of 5 μm (Fig. 2b). Fig. 2c displays a representative single flower-like YVO4:5% Eu3+ hierarchitecture, which offers wellaligned nanopetal arrays with high density growth radically on the side surface of the primary stem to form flower-like superstructure. Of the nanopetal arrays, the primary stem is longest, coarsest, and rectangular shaped microrod. The whole length and diameter of the primary stem of YVO4:5% Eu3+ stem is 2 μm at longest from bottom to the top and ~400 nm, respectively, whereas the average length of the secondary nanopetals grown on the stem is ~1 μm. The magnified SEM image in Fig. 2d further reveals that the morphologies of secondarily grown nanopetals like nanonails, which have sizes with the range of 0.2–1 μm in length and 60–100 nm in diameter, respectively. Moreover, the crosssections of nanonail caps with an average thickness of ~20 nm exhibit two styles of either elliptic or rectangular. In addition to morphological information, we used TEM, HRTEM and selected-area electron diffraction (SAED) technique to examine the internal structure of the as-prepared products. Fig. 3a offers a typical TEM image of a single YVO4:5% Eu3+ flower, showing the circular shape, which is self-assembled from lots of aligned YVO4:5% Eu3+ nanonails (similar to SEM imaging in Fig. 2c) in a radial form with a diameter of 4 μm. As seen in the magnified TEM images (Fig. 3b and c),

all the secondary petals have nanonail-like structures with an average diameter of 90 nm. Fig. 3d and Fig. S1a† show enlarged TEM images of the two individual nanonails with different caps of rectangular and elliptic. The HRTEM (Fig. 3e and Fig. S1b†) and SAED (inset in Fig. 3d and Fig. S1a†) patterns further confirm the single crystalline nature of the YVO4:5% Eu3+ nanonails. Two sets of crystal lattice fringes corresponding to the (200) and (101) atomic spacings of tetragonal structured YVO4 are 0.3655 and 0.4731 nm, respectively, indicating that the growth extends to the [001] direction. The SAED pattern inserted in Fig. 3d exhibits a regular and clear diffraction spot array, which assigned to the {200} are along the zone axis [100] of YVO4. Taken the TEM images (Fig. 3d) and their corresponding SAED patterns (the insert in Fig. 3d) into consideration, it is confirmed that the nanonail petals grow along the [001] direction (Fig. 3f). Energy-dispersive X-ray spectroscopy (EDS) elemental mappings in Fig. 3g–i shows that only four elements (Y, V, O and Eu) uniformly distributed in the result samples. These results above demonstrate that the as-prepared products are YVO4:Eu3+ materials with flower-like hierarchitectures, which grow along the [001] direction. 3.2. Possible growth mechanism As reported in many previous studies, the morphology of the nanostructured materials could be controlled by tuning the different experimental conditions, such as the feeding ratio, organic additives, aging time, etc. Therefore, we studied the impacts of Na2H2L/Y3+ molar ratio and reaction time respectively on the synthesis of YVO4:5% Eu3+ microflowers. 3.2.1. Effect of the Na2H2L/Y3+ molar ratio on the morphology Owing of its chelating ability for metal ions, Na2H2L has been widely employed as a shape controller and structure-directing agent to synthesize various nanostructures, such as double tower-tip-like Cu2O [23], GdPO4·H2O:Ln3+ flower-like clusters [22], etc. Therefore, we speculate that Na2H2L might contribute a significant impact on controlling morphology and structure of the as-obtained YVO4:5% Eu3+ products. To understand the effect of Na2H2L/Y3+ molar ratio on the 3

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Fig. 3. (a–c) TME images of a single flower-like YVO4:5% Eu3+ hierarchitecture at different magnifications. (d) TEM, SADE (inset) image, and (e) HRTEM pattern of single nanonail with rectangular cap. (f) Structural representation of nanorod. (g) SEM image of a single flower-like YVO4:5% Eu3+ hierarchitecture. (h) EDS mapping images of Y, Eu, V, and O for the square region in the (g). (i) EDX spectrum of the flower-like YVO4:5% Eu3+.

250 °C for 4 h with different Na2H2L/Y3+ molar ratios of 1, 1.2, 1.4, and 2, respectively. All the diffraction peaks are consist of broad peak located at around 22°, which belongs to amorphous SiO2 in glass slide. Additionally, other reflections, including (200), (112), and (312) planes are in agreement well with the standard JCPDS cards, indicating that all the samples prepared are pure phase YVO4. The sharp and narrow peaks at around 25.1° revealed that the as-synthesized products have a high degree of crystallization. From the full-width at half-maximum (FWHM) of the strongest peaks (200), the average crystallite sizes are estimated using the Scherrer equation: D = 0.9λ/(βcosθ), where D is the crystallite size of the powers, λ is the wavelength of the X-ray (0.15406 nm) employed, β is the FWHM, θ is the diffraction angle, and the results are listed in Table 1. As shown, the crystallite size of as-prepared samples gradually increased with increasing the Na2H2L/Y3+ molar ratio. To better understand structural details of the as-synthesized samples, the calculated lattice parameters of YVO4:5% Eu3+ nanoparticles are also summarized in Table 1, and the standard data for tetragonal YVO4 are given for comparison. Clearly, the calculated lattice parameters for all samples are well consistent with the standard data of tetragonal YVO4. It is also noted that all structural parameters are strongly dependent on the crystallite size and morphology: lattice parameter a, axial ratio a/c and the cell volumes tended to decrease with increasing crystallite size from 20.67 to 27.61. In addition, the morphology of the YVO4:5% Eu3+ prepared at different Na2H2L/Y3+ molar ratios: 1 (a)-(c), 1.2 (d)-(f), 1.4 (g)-(i), and 2 (j)-(l) were monitored using SEM techniques (Fig. 5). Clearly, when the Na2H2L/Y3+ molar ratio is 1, dense nanorods arrays are tilted

Fig. 4. XRD patterns of the samples prepared with different Na2H2L/Y3+ molar ratios after reaction at 250 °C for 4 h: 1 (a), 1.2 (b), 1.4 (c), 2 (d). The standard JCPDS card (PDF#821968) for YVO4 is also presented for comparison.

growth of the flower-like superstructures, a series of contrast experiments were carried out at different amounts of Na2H2L/Y3+ molar ratio, while keeping the other synthetic conditions unchanged. The XRD pattern and SEM imagings were utilized to examine the as-synthesized products. Fig. 4 shows the XRD patterns of the samples prepared at 4

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Table 1 Cell parameters and crystallite sizes for YVO4:5% Eu3+ samples prepared with different Na2H2L/Y3+ molar ratios. Na2H2L/Y3+

JCPDS PDF#821968 1 1.4 1.6 2

Cell parameters

FWHM 3

a/Å

c/Å

a/c

V/Å

7.118 7.1154 7.1061 7.0966 7.0819

6.289 6.2599 6.2615 6.2641 6.2558

1.1318 1.1367 1.1349 1.1329 1.1321

318.64 316.93 316.19 315.47 313.75

Crystallite size

(200)

0.394 0.336 0.336 0.295

20.67 24.24 24.24 27.61

respect to their tetragonal structure, the YVO4 nanonails have the wellfaceted side surface and rectangular cross section. Compared to those prepared at the Na2H2L/Y3+ molar ratio of 1, the caps of the nanonail at a molar ratio to 1.2 change to rectangular nanosheet. With the molar ratio further increase to 1.4, the flower-like structures increased, and the corresponding sizes are 3–7 μm. Although dense petal on each entity structure (Fig. 5g–i), the as-producted flower-like structures are remain non-uniform and unperfect. Surprisingly, dense and uniform flower architectures with an average diameter of 5 μm were obtained on a large scale, as further increasing the molar ratio to 1.6 (Fig. 2). All flowers maintained their integrity after vigorous ultrasonic treatment,

randomly on surfaces (Fig. 5a and b). And the nanorods have an average length and diameter of about 1 μm and 50 nm, respectively. After a close observation (Fig. 5c), each nanorods are buttoned by small caps, showing a mushroom-like morphology. Significantly, the products show differently oriented nanonails with the further rising molar ratio to 1.2, and some flower-like hierarchical structures with an average size of 3–7 μm deposited loosely in a non-uniform manner (Fig. 5d). The magnified SEM images (Fig. 5c and f) confirm that the flowers are constructed by lots of nanonails (with the length of ~1 μm and width of ~50 nm, respectively), which grow radiating out the side surface of the primal nanorod as trunk, bringing on loose flower-like structures. With

Fig. 5. SEM images of the YVO4:5% Eu3+ samples prepared with different Na2H2L/Y3+ molar ratios: 1 (a)–(c), 1.2 (d)–(f), 1.4 (g)–(i), 2 (j)–(l). All the samples were hydrothermally obtained at 250 °C for 4 h. 5

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Fig. 6. SEM images of the samples prepared at different reaction time with Na2H2L/Y3+ molar ratios = 1.6: 0.5 h (a), (b); 2 h (c), (d); 3 h (e), (f); 5 h (g), (h).

for different reaction times, including 0.5 h, 2 h, 3 h, and 5 h, are shown in Fig. S3†. All nanostructures crystallized in a pure zircon-type tetragonal crystal form with a space group of I41/amd, and the intensities of diffraction peaks gradually increased with extending reaction time, implying that the crystalline structure of YVO4:5% Eu3+ could be improved with the reaction time. SEM images in Fig. 6a and b shows that within 0.5 h, numerous YVO4:5% Eu3+ nanoparticles formed on the surface of substrate, where the nanoparticles with the diameter of 20–40 nm congregated to form a film with the thickness of around 300 nm. The morphology of products changed from nanoparticles to flower-like structures when the reaction time was prolonged to 2 h, and a number of secondary nanorods appeared on side surface of trunk (Fig. 6c and d). When the reaction time was further increased to 3 h, all the products evolved to flower-like superstructures with a size of 4 μm, and the original nanorods changed to nanonails (Fig. 6e and f). And an

showing a high structural stability (Fig. 3). When the molar ratio was fixed at 1.8, the sample still retained perfect flower-like structures. Interestingly, their size are similar to that prepared at the molar ratio of 1.6, but the length and width of secondary petal were increased to 2 μm and 50–300 nm, respectively (Fig. S2†). Obviously, a further increasing molar ratio to 2 could also offer nanonail-built flower morphology, but the caps of nanonails disappeared (Fig. 5j–l). Based on step-by-step optimization, we find that the Na2H2L/Y3+ molar ratio plays a crucial role in regulating the growth of the flower-like YVO4:5% Eu3+ hierarchitectures.

3.2.2. Effect of the reaction time on the morphology evolution Then we investigated the effect of the reaction time on morphology by tuning growing times at a Na2H2L and Y3+ molar ratio of 1.6. The XRD patterns of as-prepared YVO4:5% Eu3+ nanostructures prepared 6

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Scheme 1. Schematic illustration of the growth mechanism: (A) formation of a YVO4:5% Eu3+ nucleus on the glass, (B) formation of 1D nanorods, (C–D) growth of secondary nanopetals, (E) formation of the bouquets of flowers.

grow into flower-like features due to the space limitation and geometric symmetry. As the reactions progress in Step E, the assembled flowers could transform into dense flower clusters.

expected product could be obtained when the reaction time was lengthened to 4 h, as evidenced in Fig. 2. Although the products still remained flower-like superstructured morphology with the continuous increase in the reaction time over 5 h (Fig. 6g and h), the secondary petals became much thicker and shorter nanorods than those prepared at 4 h, which are attributed to the crystal growth and Ostwald ripening process. In concluded, the reaction time plays an important role in forming the hierarchical YVO4:5% Eu3+ nanostructures.

3.3. Luminescence properties The photoluminescence excitation and emission spectra of YVO4:5% Eu3+ products prepared at 250 °C for 4 h with different Na2H2L/Y3+ molar ratio of 1, 1.6, and 2, respectively, are shown in Fig. 7, in which the excitation (λem) and emission (λex) wavelengths, as well as the assignments of corresponding electronic transitions are labeled. In the excitation spectra (Fig. 7a–c, left), the broad bands centered at about 289 nm are assigned to the charge-transfer bands of Eu3+−O2− resulting from electron transfer from the ligand O2− (2p6) orbitals to the empty states of 4f6 for the Eu3+ configurations [25–28]. The bands at about 323 nm are attributed to absorptions of the host lattices, which are corresponding to transitions from the 1A2 (1T1) ground state to 1A1 (1E) and 1E (1T2) excited state in terms of the molecular orbit theory [29,30]. Apart from these strong peaks, the weak lines (~395 nm) corresponding to the f-f transitions within the 4f6 configuration of Eu3+ ions can also be observed, but their intensity is much weaker than that of the charge-transfer bands (CTB), implying that the excitation of the Eu3+ ions is mainly through the VO43− groups. Interestingly, CTB of O2−−V5+(Eu3+) shifts to a shorter wavelength as the morphology of the YVO4:5% Eu3+ changes from nanorod arrays to flower-like structures. These results indicate that the excitation spectra are not only

3.2.3. Formation mechanism The formation mechanism of flower-like YVO4:5% Eu3+ hierarchical structures could be speculated on the basis of the above-mentioned experimental results. As evidenced above, both Na2H2L/Y3+ molar ratio and reaction time influence the morphology of the product, which coincides with other oxide materials [21,22]. Based on the above experimental results, we propose a five-step mechanism for the formation of the flower-like YVO4:5% Eu3+ hierarchical structures (Scheme 1). At the first stage (Step A), the H2L2− ionized into L4−, and the VO3− transformed into VO43−. It is well known that the L4− with six carboxylic groups is a strong chelating agent, and can easily coordinate with yttrium ion to form stable [YL]- complexes, showing a function of lowering the free Y3+ concentration and inhibiting the hydrolysis of the Y3+ ions in the solution, which as a result can not only regulate the kinetics of nucleation growth of the YVO4:5% Eu3+ but control its morphology. After hydrothermal processing at high temperature and pressure, the [YL]- complexes could be dissociated into Y3+ and L4−, thus, the as-dissociated Y3+ could react with VO43− to form the initial colloidal YVO4:5% Eu3+ nuclei, which occupy the active site on substrate. According to the crystal growth theory in solution, the morphology of the crystal could be determined by the relative growth rates of different crystal planes. Moreover, theoretical calculation showed that for a tetragonal rare-earth orthovanadate lattice, and the surface energy followed the order of E(100) < E(101) < E(001) [24]. Accordingly, in Step B, the relatively higher energy (001) crystal planes of newly generated nuclei will grow preferentially along [001] direction to form a 1D nanorod with the prolonged reaction time. As the reaction further proceeded, more reactant monomers could be released into the solution, as a result the L− covered the (001) plane to lower the growth rate along [001] orientation. In addition, driven by the minimization of the total energy of the system and limited space conditions, numerous tiny YVO4:5% Eu3+ crystalline nuclei appeared in the (100) plane, which might provide many high-energy sites for growing secondary nanopetals. So in Step C-D, crystal nuclei could grow along the [001] direction and radiate out from the center nanorod, and finally

Fig. 7. Excitation and emission spectra of YVO4:5% Eu3+ thin samples prepared at 250 °C for 4 h with Na2H2L/Y3+ molar ratio of 1(a), 1.6 (b) and 2 (c). 7

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strongly influenced by the microstructure surrounding V5+ (Eu3+) ions, but also related to the interaction between the O2− and V5+ (Eu3+) ions, which further modifies the excited levels of p electrons transition from the ligand O2− (2p6) orbitals to the empty states of 4f6 for the Eu3+ configurations and the 3 d orbitals of V5+ ions [31,32]. Based on SEM in Fig. 5, it is found that compared to samples prepared at other Na2H2L/Y3+ molar ratios, the crystallite size of nanorods obtained at Na2H2L/Y3+ molar ratio of 1 is smaller and their surfaces are rough, indicating that lots of defects exist on surfaces of nanorods. These facts lead to a decrease of the local symmetry of V5+ (Eu3+) and the potential field at the O2− sites in nanorods, resulting in the weakness of the covalent interaction between V and O. As a result, the energy required for transferring an electron from the O2− ion to V5+ (Eu3+) cation is low, and the corresponding charge transfer band is located at lower energy side. Conversely, the bigger the crystallite size is, the stronger potential field at the O2− sites is. Therefore, with the increase of crystallite size and the improvement of crystallinity, the CTB bands gradually shift towards shorter wavelength. The emission spectra, taken at an excitation wavelength of 323 nm, show that the all threeYVO4:5% Eu3+ samples behave the characteristic emission of Eu3+. The emission spectra consist of several groups of emission lines at about 539, 596, 618, 653, and 701 nm, which are ascribed to the 5D1→7F1 and 5D0→7FJ (J = 1, 2, 3, 4) transitions of the Eu3+ ions, respectively (Fig. 7a-c, right). Besides, it is found that the 5 D0→7F2 transition is the most intense peak compared with the other transitions. No emission from the VO34 group is observed, indicating 3+ that the energy transfer from VO3is successful. The peak po4 to Eu sitions and shapes for three samples are similar, suggesting that their luminescence mechanisms are similar. In addition, we can see that the intensity ratio (R) of 5D0→7F2 to 5D0→7F1 varies depending upon the morphology of the samples. In Fig. 7, the intensity ratios of 5D0→7F2 to 5 D0→7F1 in three samples were determined to be 4.08, 3.74 and 3.36, respectively. As reported in the previous literature [33], the whole excitation and emission process of YVO4:5% Eu3+ under UV radiation include three major steps: (i) the VO34 groups are excited by UV radiation; (ii) the excited energy is subsequently transferred to Eu3+ ions after a thermally activated energy migration through the vanadate sublattice; (iii) the excited Eu3+ ions release energy in photon form in the process of going back to the ground state. According to selective rules and JuddOfelt theory, the magnetic dipole transition 5D0→7F1 is permitted and the electric dipole transition 5D0→7F2 is forbidden, but for some case in which the local symmetry of the activator is without an inversion center, the parity forbiddance is partially permitted. In the present case, YVO4 belong to tetragonal structure and the space group is I41/amd. The point symmetry of Y atom in YO8 dodecahedra is D2d, without an inversion center, while V atom is in the center of VO4 tetrahedra (Td) [33]. When the Eu3+ ions occupy the sites of the Y3+ ions, the site symmetry of the Eu3+ ions is similar to that of the Y3+ ions, resulting in a higher intensity of the electric dipole transitions, compared to magnetic dipole transitions. Moreover, the relative intensity ratio of 5 D0→7F2 to 5D0→7F1 emission transition also depends on the local symmetry around the Eu3+ ions. This is duo to the fact that the electric dipole 5D0→7F2 transition is very sensitive to the local environment around the Eu3+ ion compared to the magnetic dipole 5D0→7F1 transition, which is hardly affected by the local environment around the ion. In Fig. 8, it is seen that the value R gradually decreases with morphology tuning from nanorod arrays to flower-like structures. This may be related to the increased ratio of surface to volume and the diminished lattice parameters, which have given rise to the change of local environment around Eu3+ ions. As listed in Table 1, with decreasing the Na2H2L/Y3+ molar ratio from 2 to 1, the axial ratio a/c increased from 1.1321 to 1.1367, and the deviation from standard data (1.1318) increased as well, which indicates a distortion of the tetragonal zircon lattices and a reduction of lattice symmetry of the structural units. In addition to the size of nanoparticles, surface defects on

Fig. 8. Excitation and emission spectra of YVO4:1% Dy3+ thin samples prepared at 250 °C for 4 h with Na2H2L/Y3+ molar ratio of 1(a), 1.6 (b) and 2 (c).

nanopaticles play an important role in determining the intensity ratio of D0→7F2 to 5D0→7F1. From Fig. 5 and Table 1, it can be seen that the average crystallite size of nanorod arrays is obviously smaller than that of other morphologies and their surface is much rough, revealing that losts of defects exist on surfaces of nanorod arrays. These defects may increase the degree of disorder, lowering the local symmetry of Eu3+ ions located at the surface of the particles. This in turn increases the transition probability of 5D0→7F2 responsible for enhancement of the red emission. We also prepared three type of YVO4:1% Dy3+ simples using a similar route with different Na2H2L/Y3+ molar ratios of 1, 1.6, and 2, respectively. Fig. 8 presents the excitation and emission spectra of three samples under excitation at 286 nm. The excitation spectra of three samples show similar spectral patterns without any band shift (Fig. 8a–c, left), but one can observe that the ratio of the 4H9/2 → 6H15/2 to 4H9/2 → 6H13/2 the transition intensity (B/Y value) in their emission spectra is decreased with changing the morphology of samples (Fig. 8a–c, left). It is well known that the 4H9/2 → 6H15/2 transition of Dy3+ is hypersensitive to the local crystal field symmetry around the activator Dy3+ ion, whereas the intensity of magnetic-dipole allowed 4 F9/2 → 6H13/2 transition is less sensitive to the symmetrical characteristic of Dy3+ ions. That is, the relative intensity of them depends strongly on the local symmetry of Dy3+ ions, and a lower symmetry of crystal field around Dy3+ ions will result in a higher B/Y value. In this case, when the morphology of samples changing from nanorod arrays to flower-like structures, the crystallite size and crystallinity of secondary nanorods increase, which could cause a lower level of disorder, and further lowering of the B/Y value. The excitation and emission spectra of Sm3+ and Tm3+ ions doped YVO4 samples are given in Fig. 9a and b, respectively. The excitation spectra of YVO4:1% Sm3+ and YVO4:1% Sm3+ samples are similar to YVO4:5% Eu3+. The emission spectrum of Sm3+ consists of the three emission lines at 567, 603, and 648 nm originated from the transitions from the 4G5/2 level to the 6H5/2, 6H7/2, and 6H9/2 levels, respectively. For Tm3+, it can be found that a typical blue emission band at 478 nm in its emission spectrum, which is corresponding to the transition of 1 G4→3H6. The energy transfer processes of YVO4:Ln3+ (Ln3+ = Dy3+, Sm3+) are as effective as YVO4:Eu3+, but the energy transfer efficiency of YVO4:Tm3+ is not complete. Fig. 10 shows the CIE chromaticity diagram of different YVO4:Ln3+ samples (Ln3+ = 5% Eu3+, 1% Dy3+, 1% Sm3+, 1% Tm3+,1% Dy3+/ 1% Tm3+, 1% Dy3+/2% Tm3+). The CIE coordinates of these samples are calculated from the luminescence spectra in Figs. 7b, 8b and 9a, 9b, S4a†, S4b†, and the correlation results are listed in Table 2. It can be found that the CIE coordinates of YVO4:Ln3+ phosphors can be regulated by tuning the activators and multiple doping. Furthermore, it has also been obtained that white light emission with the CIE chromaticity 5

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Table 2 CIE chromaticity coordinates of the YVO4:Ln3+ samples (Ln3+ = as indicated in the samples). Samples

Excitation (nm)

YVO4:5% Eu3+ YVO4:1% Dy3+ YVO4:1% Sm3+ YVO4:1% Tm3+ YVO4:1% Dy3+/1% Tm3+ YVO4:1% Dy3+/2% Tm3+

323 286 313 285 286 286

CIE coordinates (x, y) (0.602, (0.401, (0.420, (0.248, (0.347, (0.331,

0.336) 0.438) 0.243) 0.316) 0.391) 0.351)

1% Dy3+, 1% Sm3+, 1% Tm3+), have also been investigated. Fig. 11 shows the corresponding decay curves for the luminescence of the Eu3+ (λem = 618 nm, 5D0→7F2), Dy3+ (λem = 575 nm, 4F9/2 → 6H13/2), Sm3+ (λem = 603 nm, 4G5/2 → 6H7/2) and Tm3+ (λem = 476 nm, 1 G4→3H6) in YVO4 prepared by hydrothermal reaction at 250 °C for 4 h under 323, 286, 313, and 285 nm excitation, respectively. It is found that the luminescence decay curves of Eu3+, Dy3+ and Sm3+ samples can be fitted well into a double-exponential function as I= A1 exp(−t / τ1) + A2 exp(−t / τ2) , where τi (i = 1, 2) and Ai (i = 1, 2) are the decay lifetime and the fitting parameter, respectively. The double-exponential decay behavior of the activator is often observed when the excitation energy is transferred from the donor [34–36]. However, the luminescence decay curve for Tm3+ sample, deviated from a double-exponential line, indicates the presence of nonradiative 3+ processes and the energy transfer from VO3is incomplete. 4 to Tm Their average lifetime values obtained by formula [τ ] = (A1 τ12 + A1 τ22)/(A1 τ1 + A1 τ2) are 0.51, 0.093, 0.19, and 0.064 ms, respectively. These lifetimes are in accordance basically with those of the assembled-spheres YVO4:5% Ln3+ (Ln3+ = Eu3+, Dy3+, Sm3+): 0.577, 0.099, and 0.430 ms, respectively [37]. Fig. 12 shows the decay curves for the luminescence of Eu3+ (λem = 618 nm, 5D0→7F2) in YVO4:5% Eu3+ samples prepared at 250 °C for 4 h with typical Na2H2L/ Y3+ molar ratio of 1, 1.2, 1.4, 1.8 and 2. From Fig. 4, the average lifetime are determined to be 0.44, 0.48, 0.49, 0.52, and 0.54 ms, respectively. In addition, the internal quantum efficiencies of 5D0 for five samples were estimated to be 11.60% (1:1), 23.57% (1.2:1), 24.51% (1.4:1), 28.19% (1.8:1) and 31.05% (2:1), respectively, which are all much superior to those reported for its counterpart nanoparticles, e.g. 2–9% for 25–74 nm YVO4:Eu3+ nanoparticles [38] or 4.4% for 20 nm YVO4 co-doped with Eu3+/Bi3+ [39].

Fig. 9. Emission spectra for YVO4:Ln3+ (Ln3+ = 1% Sm3+ (a), 1% Tm3+ (b)).

4. Conclusion In summary, we have developed a simple one-pot hydrothermal method for the synthesis of flower-like YVO4 hierarchitectures using Na2H2L as a capping agent. The flower-like hierarchitectures are constructed by the dozens of well-aligned YVO4:Ln3+ nanonails growing radially from the surface of the stem microrod. After a series of parameter control tests, both the reaction time and the molar ratio of Na2H2L/Y3+ have important influence in tuning the morphologies, density, size, and dimension of the final products YVO4:Ln3+. The wellaligned YVO4:Ln3+ nanorod arrays change to flower-like structures when the Na2H2L/Y3+ molar ratio rises from 1 to 1.6. The morphological evolution and growth mechanism of flower-like YVO4:Ln3+ hierarchitectures have been proposed on the basis of a series of time-dependent experiments. Under ultraviolet excitation, the as-prepared YVO4:Ln3+ hierarchitectures show strong light emissions with different colors coming from different lanthanide activators. It is expected that the special YVO4:Ln3+ microstructures are very promising luminescence nanomaterials for photoelectric nanodevices in nearfuture. Such a simple and easy-control method may provide a strategy in the design and controlled synthesis of inorganic nano/micromaterials.

Fig. 10. CIE chromaticity diagram of flower-like YVO4:Ln3+ (Ln3+ = as indicated in the Fig).

coordinate of (0.331 and 0.351) is close to ideal white light (0.33 and 0.33). The decay kinetic behaviors of the YVO4:Ln3+ (Ln3+ = 5% Eu3+, 9

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Fig. 11. Luminescence decay curves of YVO4 doped with (a) Eu3+ = 5%, (b) Dy3+ = 1%, (c) Sm3+ = 1%, (d) Tm3+ = 1%.

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Fig. 12. Luminescence decay curves of YVO4:5% Eu3+ (λem = 618 nm, 5 D0→7F2) samples prepared at 250 °C for 4 h with Na2H2L/Y3+ molar ratio of 1, 1.2, 1.4, 1.8 and 2.

Appendix A. Supplementary data

[26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jlumin.2019.116624.

[37] [38] [39]

Acknowledgement This work is supported by the Inner Mongolian Natural Science Foundation (Grant No. 2018MS06030 and 2013MS0810).

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