down-converted luminescence for anti-counterfeiting and fingerprint detection

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Hexagonal sodium yttrium fluoride mesocrystals: One-pot hydrothermal synthesis, formation mechanism and multicolor up-/down-converted luminescence for anti-counterfeiting and fingerprint detection Dunpu Zhanga, Mingye Dingb,∗, Bang Dongb, Yue Zhena, Qing Changa a b

Key Laboratory of Advanced Functional Materials of Nanjing, College of Environmental Science, Nanjing Xiaozhuang University, Nanjing, 211171, China College of Science, Nanjing Forestry University, Nanjing, 210037, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Mesocrystal Luminescence Anti-counterfeiting Fingerprint identification

Currently, a precise architecture manipulation of mesocrystals with controllable morphology and tunable size remains a huge challenge and is highly desired for their practical applications. Here, we develop a facile one-pot strategy for the direct synthesis of uniform and monodispersed β-NaYF4 mesocrystals with cantaloupe-like shape and adjustable dimension. To investigate the formative mechanism of β-NaYF4 superstructures in depth, the evolution processes for the crystal structure and morphology of β-NaYF4 mesocrystals under different experimental conditions have been systematically researched and discussed. A series of experimental results demonstrate that intrinsic crystallographic structure and external factors govern the crystal growth process of hexagonal-phased NaYF4. The processes of dissolution-renucleation, anisotropic growth, subsequent oriented attachment and self-assembly are believed to be responsible for the formation of cantaloupe-shaped β-NaYF4 mesocrystals. Moreover, the luminescence properties of β-NaYF4:RE3+ (RE = Ce, Tb, Eu, Yb, Tm and Ho) mesocrystals are also studied and efficient multicolor upconverting and downshifting emissions can be easily realized. As a proof of concept experiment, the as-synthesized β-NaYF4:RE3+ phosphors are employed to demonstrate their promising applications in anti-counterfeiting and fingerprint identification.

1. Introduction Presently, preparation of highly ordered architectures with controllable morphologies and tunable sizes has gained significant interest and continues to present a challenge. As a new type of highly ordered superstructure, 3D (three-dimensional)-structured crystals termed ‘mesocrystals’ is considered as quasi-single crystals assembled from small vectorially aligned units [1–3]. Owing to their specific architectures and unique characteristics (single-crystalline-like structure, complex 3D morphology, mesoscale porosity and anisotropy in particle size and shape), novel mesocrystals hold great promise in a wide range of fields, either as structural materials, or the multifunctional nano-/ micromaterials with exciting properties [4,5]. Therefore, in the last few years, many efforts have been devoted to synthesizing many kinds of new mesocrystals, including CaCO3 [6,7], BiVO4 [8], ZnO [9], TiO2 [10], AgIn(WO4)2 [11], La9.33Si6O26 [12], LiFePO4 [13], NaxY (OH)yF3+x+y [14] and YF3 [15]. Nevertheless, owing to the complexity of nanoscale superstructure and chemical composition, it is still quite difficult to develop a simple and convenient approach to manipulate

∗

and control the crystal growth process of mesocrystals via a known formation mechanism. Especially, differing from the classical crystallization process, mesocrystals are formed through colloidal intermediates and mesoscale transformation (nanoparticle-based reaction channels) instead of the ion-by-ion attachment growth model. Thus, to further investigate the underlying fundamental theory and principle, it is highly desired to conduct an in-depth study on the observed complex phenomena of mesocrystalline growth process [16]. Hence, developing a convenient and effective method for the preparation of mesocrystals with controllable morphologies and tunable sizes is of significant importance for their practical applications. Nowadays, tremendous attentions have been given to the study of rare-earth (RE) materials for their significant potential applications ranging from solid-state lasers [17], volumetric 3D display [18], LEDs [19–21], photocatalysis [22,23], photovoltaics [24], to bioimaging [25]. Owing to their extraordinary optical, magnetic, electronic and chemical properties, rare-earth fluorides are considered as excellent host materials for upconverting (UC) and downshifting (DS) luminescence and have been extensively used in many fields such as optical

Corresponding author. E-mail addresses: [email protected], [email protected] (M. Ding).

https://doi.org/10.1016/j.ceramint.2019.07.001 Received 15 May 2019; Received in revised form 29 June 2019; Accepted 1 July 2019 0272-8842/ © 2019 Published by Elsevier Ltd.

Please cite this article as: Dunpu Zhang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.07.001

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synthesized luminescent phosphors (10 mg) were added to PVA-H2O mixed solution (4 wt% PVA). Then, the resulting mixture was sonicated for 20 min, followed by another 20 min of vigorous stirring to obtain luminescent ink. For easy of handwriting and fluorescent anti-counterfeiting, the paper with smooth surface and no background fluorescence is selected as writing substrate. Latent fingerprints collection. The latent fingerprints on different substrates such as plastic culture dish, glass slide, metal key and document envelope were collected from the same volunteer. The fingerprints were firstly printed on the surface of different substrates, and then the as-obtained powder was evenly stained sprinkled on the latent fingerprints with a feather brush. After the ridges of fingerprint were filled in the powders, the redundant phosphors could be removed via gentle brushing action.

communication [26], temperature sensors [27–29], ions detection [30–32], biolabels and bioimaging [33–35]. Among these fluorides, hexagonal-phased NaYF4 (β-NaYF4) has been currently regarded as the most efficient host for the doped lanthanide ions owing to its low vibrational energy (< 400 cm−1), high radiative emission rate and excellent chemical stability [36–38]. Since the luminescent properties of the phosphors (especially the host-sensitive lanthanide-doped materials) are greatly dependent on their morphologies and microstructures, it is reasonable that the emission intensities and peaks can be tuned by adjusting the shape, size and structure of the as-prepared β-NaYF4 crystals [39,40]. Therefore, controllable synthesis of β-phased NaYF4 host with novel architectures is a huge challenge but very promising for obtaining unique luminescent properties. Until now, various shapes and sizes of nano/micro-sized β-NaYF4 single- or poly-crystals, including spheres, rods, prisms, tubes, disks, plates and wires [41–46], have been successfully synthesized by different routes, such as molten salt [47], hydro/solvothermal [48–50] and co-precipitation [51] methods. However, up to now, the successful synthesis of monodisperse β-NaYF4 mesocrystals via one-step method has been scarcely reported. Herein, in this work, we report a simple one-step hydrothermal process for direct synthesis of hexagonal-phased NaYF4 mesocrystals with controllable architectures and tunable dimensions. Of particular interest to this research is the exploration of possible formation mechanism of 3D β-NaYF4 hierarchical mesocrystals. Thus, to understand how structural and kinetic factors govern nucleation and growth of βphased NaYF4 mesocrystals, the influences of external parameters (types and concentrations of surfactants and pH values of reaction solution) on the crystalline phase and architecture evolution have been detailedly investigated. According to a series of experimental results, the morphology evolution and growth process for the final products are preliminarily proposed. Besides, we also systematically study the DS and UC luminescent properties of lanthanide-doped β-NaYF4 mesocrystals. As a consequence, a proof of concept experiment has been performed to demonstrate the possible applications of the as-obtained phosphors in anti-counterfeiting and fingerprint identification. To the best of our knowledge, this facile and direct synthetic strategy for the preparation of β-NaYF4 mesocrystals has been reported for the first time and could extend to other complex fluorides.

2.1. Characterization X-ray diffraction (XRD) measurements were used for crystalline phase identification on a MiniFlex 600 diffractometer (Rigaku, with Cu Kα radiation, λ = 0.15406 nm). Scanning electron microscope (SEM, S4800, Hitachi) and Transmission electron microscope (TEM, JEM200CX, JEOL) were employed to observe and characterize the morphologies and sizes of the as-obtained samples. The down- and upconverted luminescence spectra were recorded with an Edinburgh FS5 spectrofluorimeter equipped with a 980 nm laser diode (MDL–III–980, China) and a 450 W Xenon light as excitation sources. The images of hand-written patterns and fingerprints were taken by a Canon EOS 70D digital camera under the excitation of 254 nm ultraviolet (UV) light or 980 nm near-infrared (NIR) light. 3. Results and discussion Crystal Structure and Morphology. To investigate the detailed structure of the as-synthesized products, SEM, TEM and XRD characterization is employed. As shown in Fig. 1a and Figure S1, the asobtained sample is composed of monodispersed cantaloupe-like granules with an average diameter of 0.62 μm and a mean length of 0.78 μm. The typical SEM image (Fig. 1b) of individual particle exhibits that each spindle-shaped granule consists of aligned nanorods with diameter of about 127 nm and length of about 263 nm, which arrange into oriented 3D architecture via a side-by-side strategy. Fig. 1c shows the TEM image and the related SAED pattern of our synthetic samples. Obviously, the SAED result presents bright and highly symmetric spots, demonstrating the “single-crystal-like” structure of the as-prepared sample. This indicates that each cantaloupe-like particle is comprised of smaller subunits with almost the same orientation. To attest to the direction of primary crystals, HRTEM and the corresponding FFT pattern are also provided, as displayed in Fig. 1d. The obvious lattice fringes with interplanar spacing of 5.10 Å can be indexed to the {1010} crystal planes, agreeing well with the FFT pattern. According to SAED pattern, TEM and HRTEM images, the length direction of primary grains and 3D-structured granule can be determined to be [0001] (c axis) and marked in Fig. 1c–d, respectively. It demonstrates that each cantaloupeshaped particle is structurally a 3D orderly arrangement of nanoscale grains along with [0001] direction. Strictly speaking, the as-obtained cantaloupe-like granules can be considered as a typical mesocrystalline material. The elemental analytical results (Fig. 1e–f) indicate that sodium (Na), yttrium (Y) and fluorine (F) elements are distributed uniformly in the whole mesocrystals. Fig. 1g presents XRD pattern of the as-prepared product. As shown, the XRD profile could be well indexed to β-NaYF4 (a = 5.96 Å, c = 3.53 Å, P6/3m) without other impurity peaks [52]. Based on the above analyses, it could be concluded that monodispersed cantaloupe-like β-NaYF4 mesocrystals with high yield and remarkable uniformity have been successfully prepared by the simple and one-pot hydrothermal approach. To investigate the formation mechanism of cantaloupe-like β-NaYF4

2. Experimental section Chemicals and Materials. All of the chemical reagents were used without further purification in this experiment. LnCl3.6H2O (99.99%, Ln = Y, Yb, Er, Eu and Tb) and CeCl3·7H2O (99.99%) were purchased from Aladdin Co., Ltd (China). NaF (98%), Na5P3O10 (STPP, 98.6%), C18H33NaO2 (NaOA, 98%), C6H5Na3O7·2H2O (Na3Cit, 99%) and ethanol (99.5%) were provided by Shanghai Macklin Biochemical Co., Ltd (China). Typical synthetic procedure for β-NaYF4:Yb/Er mesocrystals. Firstly, YCl3·6H2O (1.56 mmol, 0.4733 g), YbCl3.6H2O (0.4 mmol, 0.1550 g), ErCl3·6H2O (0.04 mmol, 0.0153 g) were dissolved in 10 mL H2O to obtain transparent rare earth chloride solution under magnetic stirring. Then 20 mL of Na5P3O10 (0.01 M, 0.0746 g) aqueous solution was added into the above solution. After continuous stirring for 20 min, 30 mL of NaF (1.2 M, 1.5425 g) aqueous solution was added into the above mixed solution. The pH values of the mixture were adjusted by adding NaOH (1 M) or HCl (1 M) aqueous solution. Subsequently, the mixture was transferred into a Teflon-lined stainless stell autoclave and heated at 180 °C for 24 h. When the autoclave reaction was completed, the precipitate was collected by centrifugation at 6000 rpm for 5 min and washed three times with deionized water and then dired in air at 80 °C for 12 h. Hexagonal-phased NaYF4:Ce/Tb/Eu and NaYF4:Yb/Tm(Ho) products were synthesized in a process similar to that for β-NaYF4:Yb/Er mesocrystals by just adding corresponding rare-earth chlorides into the precursor mixture. Preparation of luminescent ink for handwriting. Firstly, the as2

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Fig. 1. Low (a) and high (b) magnification SEM images, TEM image (c) with corresponding SAED pattern (inset in c), HRTEM image (d) with corresponding FFT pattern (inset in d), elemental mapping images (e), the corresponding EDS spectrum (f) and XRD pattern (g) of the as-prepared β-NaYF4 mesocrystals.

mesocrystals, a series of controlled experiments have been performed. Table 1 summarizes the experimental variables and the corresponding crystalline phases, morphologies and sizes of the final products. It is obvious to find that the initial reaction conditions (such as: surfactants, STPP contents, pH values) have a significant impact on the crystalline phases, morphologies and sizes of the as-prepared samples. The influences of these experimental conditions on the crystal growth process of β-NaYF4 mesocrystals will be discussed in detail and shown in the following sections. Effects of different surfactants. In the solution-phase reaction

Fig. 2. Low-magnification and high-magnification SEM images of the as-obtained samples synthesized with different surfactants: (a, b) Na3Cit, (c, d) NaOA, (e, f) STTP; and (g) the corresponding XRD patterns. All of the as-prepared samples were hydrothermally at 180 °C for 24 h.

system, organic and inorganic additives are usually regarded as “shape modifier” and play a vital role in the controlling of crystal morphologies and sizes. By appropriately selecting additives with peculiar molecular groups, the crystal growth process could be rationally modified in order

Table 1 Summary of the experimental parameters and the corresponding crystalline phases, morphologies and dimensions of the as-obtained samples. Sample

Surfactants

pH

time (h)

Crystal Phase

Crystal morphology

Diameter (μm)

Length (μm)

S1 S2 S3 S4 S5 S6 S7 S8 S9 S11 S12 S13 S14 S15 S16

Na3Cit NaOA STPP STPP STPP STPP STPP STPP STPP STPP STPP -

7 7 7 7 3 5 7 9 11 7 7 7 7 7 7

24 24 24 24 24 24 24 24 24 1 2 4 2 4 12

β-NaYF4 β-NaYF4 β-NaYF4 t-YPO4 α+β-NaYF4 β-NaYF4 β-NaYF4 β-NaYF4 β-NaYF4 α-NaYF4 α+β-NaYF4 β-NaYF4 α-NaYF4 α+β-NaYF4 β-NaYF4

Rod-like microparticles Microplates Nanoarrays Microsheets Microrods+nanoparticles Irregularity Rod-like mesocrystals Rod-like mesocrystals Rod-like mesocrystals Nanoparticles Nanorods+nanoparticles Rod-like mesocrystals Nanoparticles Nanorods+nanoparticles Mircrorods

0.6 2.9 0.2 0.01 0.4 0.6 0.7 1.9 0.2 0.6 0.2 0.5

2.5 1.6 1.0 1.56 0.8 0.8 1.1 3.3 0.8

3

2.3

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to synthesize samples with desirable morphologies and/or specific architectures [53]. Thus, three types of surfactants (Na3Cit, NaOA and STPP) are introduced into the reaction system respectively to study the influence of different surfactants on the compositions, morphologies and dimensions of the final products. Fig. 2 gives the SEM images of asobtained products synthesized with various surfactants and the corresponding XRD patterns. When Na3Cit acts as a surfactant, the as-prepared sample consists of uniform and monodisperse microplates with average diameter of 2.9 μm and thickness up to 1.3 μm (Fig. 2a), which is identified to be hexagonal-phased NaYF4 (Fig. 2g). A magnified SEM image demonstrates a symmetrical hexagonal prism-like structure of these microplates, of which top/bottom have hexagonal cross-section and some clear microcracks. When NaOA is used as an additive and other technical conditions keep unchanged, the crystalline phase and granule morphology of the as-obtained products are examined and presented in Fig. 2c, d and 2g, respectively. As shown, the integral morphology of the as-obtained sample is β-NaYF4 nanorods with average diameter of 200 nm and mean length of 1.0 μm. The higher magnification SEM image (Fig. 2d) indicates that these hexagonal nanorods possess well-defined planes at the both top/bottom ends and side surfaces. However, by using STPP as surfactant, the shape and size of the final product is quite different. The regular cantaloupe-shaped particles consist of aligned nanorods (denoted as mesocrystals) could be observed from Fig. 2e–f, and could be assigned to hexagonal NaYF4 phase (Fig. 2g). Compared with the above experimental results, it is found that the introduction of different surfactants plays an important role in the controllable preparation of β-phased NaYF4 crystals. The obvious differences in the shape and dimension of the as-obtained samples could be attributed to diverse chelating ability of surfactants with rear earth ions and absorption ability of additives on different crystal facets. On the one hand, the chelating constants of Na3Cit, NaOA and STPP are significantly different, resulting in the diverse nucleation rates of NaYF4 crystals. The chelating constant of STPP is bigger than that of Na3Cit and NaOA, which induces highest nucleation rate of crystal in the reaction system containing STPP, consequently resulting in the formation of smaller grain dimension [42,54,55]. On the other hand, the selective absorption abilities of surfactants on various surfaces of growing NaYF4 crystals lead to the different growth rates of crystallographic directions, accordingly forming β-NaYF4 crystals with different shapes and sizes [41]. More importantly, hexagonal-phased NaYF4 crystals with a new structure, named mesocrystals, have been successfully synthesized by only using STPP additive. Therefore, in the next section, we aim to investigate how STPP surfactant governs the formation of cantaloupe-like β-NaYF4 mesocrystals. Effects of STPP amounts. Here, we stress the significant effect of STPP contents on the growth process of hexagonal-phased sodium yttrium fluoride mesocrystals. Fig. 3 presents the representative SEM pictures and the typical XRD patterns of the samples synthesized with different amount of STPP additive. Without using STPP, the as-obtained microcrystals are identified as β-phased sodium yttrium fluoride and easily grow into irregular rod-like structure with mean length of 2.5 μm, as shown in Fig. 3a–c. Further observation from Fig. 3b indicates the surfaces of rod-like particles are extremely rough with obvious defects. Once STPP additive is introduced into the solution-phase reaction system, the crystal morphology of sample could change dramatically. As displayed in Fig. 3d, the product consists of large-scale, remarkable uniform and monodisperse cantaloupe-like granules with mean dimension of 0.62 μm in diameter and 0.78 μm in length. The highmagnification SEM picture (Fig. 3e) shows clearly that a great deal of nanorods self-assemble into 3D architecture. Combining with the XRD analysis result (Fig. 3f), the 3D architectures of the as-synthesized products can be considered as β-NaYF4 mesocrystals. However, the introduction of the excessive amount of STPP could result in great difference in crystalline phase, granule morphology and size of the asobtained sample. As presented in Fig. 3g, h and 3i, the sample is YPO4 with a tetragonal phase (JCPDS No. 11-0254) and displays sheet-like

Fig. 3. Low-magnification, high-magnification FE-SEM images and the corresponding XRD patterns of the as-obtained products synthesized with different amount of STTP: (a–c) 0 mmol, (d–e) 0.2 mmol, (g–i) 4 mmol. All of the asprepared samples were hydrothermally at 180 °C for 24 h and pH = 7.

shape with an average length of 1.6 μm and thickness of 11 nm. Comparing of results indicates that a moderate amount of STPP is necessary and responsible for the successful formation of cantaloupe-like β-NaYF4 mesocrystals. The function of STPP additive in the growth process may mainly have three aspects [55]: (1) providing sodium source; (2) serving as chelating agent to slow the nucleation course and the succeeding growth process; (3) acting as morphology modifier to affect its adsorption on diverse crystal faces. Effects of pH values. As a significant factor, pH values of the initial reaction solution can also exert a prominent influence on the growth process of β-phased NaYF4 mesocrystals. In a follow-up experiment, the effect of pH value on the crystalline structure, shape and size of the assynthesized products has been detailedly investigated by the integration of SEM images and the corresponding XRD patterns (Fig. 4). Unexpectedly, the sample obtained at pH = 3 is the mixture of cubicphased (α-NaYF4) and hexagonal-phased NaYF4 (β-NaYF4), which consists of spherical-like nanoparticles and a small amount of microrods (Fig. 4a). At pH = 5, pure β-phased NaYF4 crystals with irregular shape could be obtained, as displayed in Fig. 4b. When the experiment is implemented at the condition of pH = 7, the regular cantaloupe-shaped β-NaYF4 mesocrystals with average size of diameter of 0.62 μm and length up to 0.80 μm is formed (Fig. 4c). With the increasing of pH values, the morphology of the as-obtained β-NaYF4 mesocrystals keeps unchanged, but the dimensions of granules vary greatly (Fig. 4d–e). Obviously, the average diameter and length of β-phase NaYF4 mesocrystals increase gradually with the increment of the initial pH value from 7 to 11, as pictured in Figure S2. According to the above analysis, it can be inferred that pH value of the initial reaction solution greatly influences the crystal growth dynamics so as to give precise control of the dimension of β-NaYF4 mesocrystals. Effects of reaction time. To better understand the growth process of β-NaYF4 mesocrystals, the intermediate samples are collected at diverse reaction intervals for SEM and XRD characterization. Fig. 5 shows the XRD patterns and the representative SEM pictures of the intermediate products. At a short hydrothermal reaction time of 1 h, large amount of spherical-like nanoparticles with mean dimension of 40 nm can be observed from Fig. 5b. The corresponding XRD pattern (Fig. 5a) confirms the formation of phase-pure α-NaYF4. With the reaction proceeding for 2 h, a new β-phase NaYF4 generates in addition to α-phase NaYF4, as can be seen from XRD patterns of Fig. 5a. The SEM picture in Fig. 5c reveals that the intermediate product consists of short nanorods and some irregular nano-sized granules. After 4 h of reaction, the diffraction peaks of the as-obtained product are assigned to pure β-phase sodium yttrium fluoride, as illustrated in Fig. 5a. The corresponding morphology of the product is cantaloupe-like mesocrystals with remarkable uniformity and monodispersity, of which the average length and 4

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oriented attachment mode. Formation mechanism investigation. To detailedly elucidate the underlying mechanism responsible for the formation of cantaloupe-like β-NaYF4 mesocrystals, the time-dependent experiments are performed without using STPP surfactant and the other experimental conditions remain the same. Figure S3 and S4 demonstrate the phase and morphology evolution of sodium yttrium fluoride synthesized without STPP additive and the corresponding XRD and SEM evidences. Obviously, although the occurrence of dissolution-renucleation and anisotropic growth processes in the STPP-free reaction system also induces phase transformation (α → β) and shape evolution from spherical-like nanoparticle to irregular nanorods, 1D nanorods hardly self-assemble into 3D cantaloupe-like structures and finally grows into irregular structures with rod-like shape. The controlled experiments indicate that STPP additive plays a significant role in the formation of this unique selfassembled pattern. Based on the above-described researches, the possible formation mechanism of cantaloupe-like β-NaYF4 mesocrystals could be illustrated in Fig. 6. Firstly, the metastable cubic-NaYF4 nanocrystals are formed in the early stage of reaction and subsequently dissolved to provide monomers for the nucleation of hexagonal-NaYF4 seeds. Through the dissolution-reconstruction process, β-phase nucleus emerges and grows along the [0001] direction, leading to the formation of rod-like nano-sized granules. In the meanwhile, as a water-soluble anion surfactant, STPP additive interacts with the surface of granules and mostly binds on the side face of developing β-phase nanorods, resulting in the existence of surface negative electricity [57]. Owing to the gradual hydrolysis of STPP at high temperature and pressure, the surface charge of the growing granules reduce obviously, leading to the decrease of electrostatic repulsion between rod-like granules. Consequently, when the van der Waals’ force between these particles becomes dominating, the adjacent primary nanorods gradually aggregate and are directionally arranged via van der Waals attraction. Driven by minimizing the surface energy, the as-grown nanorods rotate to share the identical crystallographic orientation and subsequently these building blocks coalesce together to form the loose cantaloupe-shaped architectures via oriented attachment and self-assembly [58,59]. Eventually, the loose cantaloupe-like aggregates further assemble and become compact gradually through Ostwald ripening process, leading to the formation of 3D cantaloupe-shaped β-NaYF4 superstructures. Additionally, according to the above experimental results, it is found that the generation of sodium yttrium fluoride mesocrystals is related to these external factors, including types of surfactant, dosage of additive and pH values of initial solution. As for the categories of surfactant, sodium citrate (Na3Cit), sodium oleate (NaOA) and sodium tripolyphosphate (STPP) are regarded as anionic surfactant and all can be absorbed on the crystal facets of NaYF4 crystals through chelation reaction. However, compared to Na3Cit and NaOA, only STPP can be hydrolyzed gradually so as to decrease the surface charge of granules, thus resulting in the directional arrangement and self-assembly of the building blocks. Therefore, the hydrolysis process of STPP plays a key role and can be illustrated in Figure S5, [60,61]. Combined with the crystal growth process of NaYF4 crystals, it is inferred that moderate amount of STPP surfactant is necessary for the formation of β-phase superstructures. Once a great amount of STPP additive is introduced into the reaction system, expect for the STPP chelated with Y3+ ions, the rest of STPP could hydrolyze to release PO43− ions. Under hydrothermal conditions, the weakening of the chelating ability between STPP and Y3+ induces the gradual release of free Y3+ ions into solution, easily resulting in the nucleation of YPO4 crystals. To greatly reduce the concentration of PO43− ions in the reaction solution, the moderate amount of STPP additive is necessary for the formation of chelationinduced complexes. In additional, it is also found that the hydrolysis process is influenced by the pH value of initial solution. Obviously, the dominate phosphate species in the solution are H3PO4, H2PO4−, HPO42− and PO43−, respectively, along with the increase of initial pH value. At pH < 7, the STPP capped on the surface of growing particles

Fig. 4. SEM images of the representative samples synthesized at different pH values: (a) pH = 3, (b) pH = 5, (c) pH = 7, (d) pH = 9, (e) pH = 11; and (f) the corresponding XRD patterns. The standard data of cubic NaYF4 (α-NaYF4, JCPDS No. 39-0724) and hexagonal NaYF4 (β-NaYF4, JCPDS No. 16-0334) is shown as a reference.

diameter are about 0.68 μm and 0.52 μm, respectively (Fig. 5d). The above results demonstrate that the crystalline phase transformation (α → β) occurs along with the morphology evolution during the crystal growth of NaYF4 sample. Based on the corresponding XRD and SEM evidences, a possible evolution mechanism is proposed and described in Fig. 5e, [36,41,56]. At the beginning, the STPP surfactant in the initial solution could interact with rare-earth metal ions (Y3+) to form complexes via chelation reaction. Under high temperature and pressure, the metal chelating ability of STPP can be weakened, resulting in gradual release of Y3+ ions into the reaction solution for the purpose of controlling the nucleation and crystallization processes. Subsequently, sodium ions (Na+) and fluoride ions (F−) added in the mixture solution react with yttrium ions to generate a great amount of small crystal nucleus. In order to minimize the surface energy, these nuclei congregate quickly to form spherical-like α-NaYF4 nanoparticles through isotropic growth process because of the isotropic unit cell structure of cubic-phase NaYF4. However, because α-NaYF4 nanoparticles are thermodynamically metastable, phase transformation (α-phase → βphase) inevitably occurs once the sizes of α-phased particles increase to the critical dimension. With further reaction, more stable β-NaYF4 nuclei appears and grows through rapid dissolution of α-phase nanoparticles and subsequent release of monomers. Owing to the anisotropic unit cell structure of hexagonal-phased sodium yttrium fluoride, these seeds quickly nucleate and preferentially grow along [0001] direction. Consequently, the morphology of granule evolves from spherical-like nanoparticles to short one-dimensional (1D) nanorods through the processes of dissolution-renucleation and anisotropic growth. During the growth process, to further decrease the total surface free energy, the as-grown β-phase nanorods spontaneously self-assemble in the same direction to form 3D cantaloupe-like architectures by sharing the side surface. As a result, uniform and monodisperse β-NaYF4 mesocrystals with cantaloupe-like morphology could be obtained through the

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Fig. 5. XRD patterns (a) and the corresponding SEM images of the as-synthesized precursors at different reaction intervals: (b) 1 h, (c) 2 h, and (d) 4 h; (i) Schematic illustration of the growth progress of hexagonal-phased NaYF4 microcrystals with corresponding evidences.

hydrolyzes and the as-growing granules can aggregate through van der Waals attraction. Moreover, the higher pH value of initial solution, the more advantageous for the hydrolysis of STPP and the easier for the formation of 3D cantaloupe-like superstructure. Consequently, the dimension of the as-obtained cantaloupe-like β-NaYF4 mesocrystals increase along with the rise of pH value from 7 to 11. Multicolor Dual-mode Luminescence Properties. For the doped rare earth (RE3+) ions, the selection of suitable host lattice is crucial for achieving favorable optical performance. Among the reported host materials, hexagonal-phased NaYF4 is regarded as efficient host matrix for RE3+ activators owing to its low phonon energy and high stability. Thus, for achieving efficient UC and DS emissions, β-NaYF4:RE3+ (RE = Yb, Er, Tm, Ho, Ce, Tb and Eu) mesocrystals have been successfully prepared and characterized by XRD and SEM techniques (Figure S6 and S7). Obviously, the doping with small amount of RE3+ ions alters neither the crystalline structure nor the granules appearance of the as-formed mesocrystals. In addition, color point tuning (multicolor emission and high color purity) is a significant challenge for advancing the potential applications of luminescent materials. Herein, this study is mainly focus on the luminescent characteristics of βNaYF4:RE3+ mesocrystals in an effort to demonstrate that efficient multicolor dual-mode emissions could be easily realized through the simple doping of specified RE3+ into β-NaYF4 host lattice. Fig. 7a shows the excitation and emission spectra of Ce3+/Tb3+/Eu3+ tri-doped βNaYF4 products. Obviously, the excitation spectrum of β-NaYF4:Ce3+/ Tb3+/Eu3+ mesocrystals (λem = 541 nm, λem = 615 nm) exhibits a absorption band located in the 200–300 nm region, which is attributed to Ce3+: 4f → 5d transition. Under the excitation of 250 nm light, the as-synthesized β-NaYF4:Ce3+/Tb3+/Eu3+ phosphors exhibit typical green, yellow and red emissions. For β-NaYF4:10%Ce3+, 30%Tb3+ sample, the emission spectrum is composed of typical Tb3+ emissions

Fig. 6. Schematic illustration of the formation mechanism of hexagonal-phased NaYF4 mesocrystals.

hardly hydrolyzes, resulting that the primary building blocks can't aggregate together because of the existence of the electrostatic repulsion. On the contrary, as the pH value increased to 7, the STPP additive easily 6

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Fig. 7. (a) Photoluminescent excitation and emission spectra of β-NaYF4:10%Ce3+, (Ⅰ); β-NaYF4:10%Ce3+, 30%Tb3+ 30%Tb3+, 10%Eu3+ (Ⅱ); βNaYF4:10%Ce3+, 30%Tb3+, 1%Eu3+ (Ⅲ). (b) Absorption spectra and upconverting emission spectra of β-NaYF4:20%Yb3+, 2% Er3+ (Ⅳ); β-NaYF4:20%Yb3+, 0.5%Tm3+ (Ⅴ); β-NaYF4:20%Yb3+, 1%Ho3+ (Ⅵ). (c) CIE chromaticity diagram and the corresponding luminescent photographs of the as-obtained lanthanide-doped β-NaYF4 mesocrystals.

associated with the 5D4 → 7F6 (485 nm), 5D4 → 7F5 (541 nm), 5D4 → 7F4 (579 nm) and 5D4 → 7F3 (620 nm) transitions of Tb3+ ions. When Eu3+ ions (10 mol%) are introduced into Ce3+/Tb3+ co-doped system, three main peaks at 591 nm, 615 nm and 690 nm are assigned to Eu3+: 5D0 → 7 F1, Eu3+: 5D0 → 7F2 and Eu3+: 5D0 → 7F4 transitions, respectively. The obvious quenching of Tb3+ emissions indicates that the excitation energy can be transferred from Ce3+ sensitizer to Eu3+ activator through the terbium bridge (Ce3+ → Tb3+ → Eu3+). In the emission spectrum of β-NaYF4:10%Ce3+, 30%Tb3+, 1%Eu3+ sample, the characteristic transitions of both Tb3+ and Eu3+ ions can be observed, resulting in the yellow emission [62]. To investigate the UC luminescence behaviors of the as-prepared samples, Yb3+/Er3+, Yb3+/Tm3+ and Yb3+/Ho3+ pairs are co-doped into hexagonal-phased NaYF4 mesocrystals. Fig. 7b displays the absorption spectrum and UC emission spectra of βNaYF4:Yb3+/Ln3+ (Ln = Er, Tm and Ho) products. The absorption spectrum of β-NaYF4:Yb3+/Ln3+ mesocrystals consists of a strong broad band centered at 980 nm, which can be assigned to 2F7/2 → 2F5/2 transition of Yb3+ sensitizer. Upon 980 nm NIR laser excitation, intense yellow-green, blue and green emissions can be observed in 20%Yb3+/ 2%Er3+, 20%Yb3+/0.5%Tm3+ and 20%Yb3+/2%Ho3+ activated βNaYF4 mesocrystals. For β-NaYF4:Yb3+/Er3+ sample, the green emission bands (521 nm and 542 nm) and the comparative red emission (656 nm) can be attributed to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/ 4 3+ activator. The upconverting emission 2 → I15/2 transitions of Er 3+ spectrum of β-NaYF4:Yb /Er3+ product exhibits the characteristic transitions of Tm3+ activator, yielding intense blue emission (447 nm, Tm3+: 1D2 → 3F4; 474 nm, Tm3+: 1G4 → 3F4), weak red (646 nm, Tm3+: 1 G4 → 3H6) and NIR (698 nm, Tm3+: 3F3 → 3H6) emissions. For the case of Yb3+/Ho3+ co-doped β-NaYF4 mesocrystals, an intense green emission (536 nm) and weak red emission (656 nm) can be attributed to Ho3+: 5S2 → 5I8 and Ho3+: 5F5 → 5I8 transitions. The proposed mechanism for UC and DS luminescence is illustrated in Figure S8 and Figure S9, respectively [63,64]. As presented in Fig. 7c, under the excitation of UV and NIR light, multicolor dual-mode emissions can be easily realized by doping activators into β-NaYF4 mesocrystals. Anti-counterfeiting application. To explore the possible application of the as-obtained mesocrystals for anti-counterfeiting, downshifting and upconverting fluorescent inks have been successfully fabricated by uniformly dispersing the corresponding β-NaYF4:RE3+ (RE = Ce, Tb, Eu, Yb, Er and Tm) phosphors into the mixture of PVA and distilled water. Then the as-prepared colorless inks are used as anti-counterfeiting tools and various patterns including characters, Arabic numbers

and warning sign are handwritten on paper and banknote. As a proof of concept experiment, four types of anti-counterfeiting ink composed of β-NaYF4:RE3+ phosphors with green (RE = Ce/Tb), red (RE = Ce/Tb/ Eu) downshifting emissions and blue (RE = Yb/Tm), green (Yb/Er) upconverting emissions are employed to prepare the pre-designed patterns. As shown in Fig. 8, these handwritten patterns are invisible in daylight, but display different colors under the irradiation of 254 nm UV lamp or 980 nm NIR laser. For example, the acronym of Hangzhou Dianzi University (HDU) can be painted on paper to produce multicolor characters by using different types of fluorescent inks (Fig. 8a–b). The Chinese and English characters (‘田’, ‘日’ and ‘H’) and warning sign are patterned by handwriting to achieve dual-mode fluorescent patterns (Fig. 8c–d). To further demonstrate the practical applications of these anti-counterfeiting inks, Arabic number “100” and English character “china” are handwritten on a banknote of china by using upconverting ink and downshifting ink as anti-counterfeiting tools (Fig. 8e). Evidently, green downshifting and green upconverting emissions could be simultaneously discerned on the banknote upon irradiation with of UV light and NIR light. Undoubtedly, the as-prepared β-NaYF4:RE3+ mesocrystals with dual-mode emissions can potentially be applicable in high security anti-counterfeiting.

Fig. 8. (a–d) Various photographs of handwritten characters and signs on paper without background fluorescence under visible light, 254 nm UV lamp and 908 nm laser; (e) Performance of dual-mode anti-counterfeiting ink upon excitation with a 254 nm UV lamp and/or 980 nm NIR laser. 7

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obtained β-NaYF4:RE3+ phosphors with favorable optical performance have great prospect in anti-counterfeiting and fingerprint identification. Acknowledgement This work was supported by the National Natural Science Foundation of China (51702074) and Zhejiang Provincial Natural Science Foundation of China (LQ15E020004). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.07.001. References [1] R.-Q. Song, H. Cölfen, Mesocrystals—ordered nanoparticle superstructures, Adv. Mater. 22 (2010) 1301–1330. [2] H. Cölfen, M. Antonietti, Mesocrystals: inorganic superstructures made by highly parallel crystallization and controlled alignment, Angew. Chem. Int. Ed. 44 (2005) 5576–5591. [3] E.V. Sturm, H. Cölfen, Mesocrystals: structural and morphogenetic aspects, Chem. Soc. Rev. 45 (2016) 5821–5833. [4] L. Bergström, E.V. Sturm, G. Salazar-Alvarez, H. Cölfen, Mesocrystals in biominerals and colloidal arrays, Acc. Chem. Res. 48 (2015) 1391–1402. [5] J. Zhang, Y. Cui, Q. Qin, G. Zhang, W. Luo, W. Zheng, Nanoporous CuO mesocrystals: low-temperature synthesis and improved structure-performance relationship for energy storage system, Chem. Eng. J. 331 (2018) 326–334. [6] J. Chen, X. Wang, Y. Liu, H. Liu, F. Gao, C. Lan, B. Yang, S. Zhang, Y. Gao, pHresponsive catalytic mesocrystals for chemodynamic therapy via ultrasound-assisted Fenton reaction, Chem. Eng. J. 369 (2019) 394–402. [7] A.-W. Xu, M. Antonietti, H. Cölfen, Y.-P. Fang, Uniform hexagonal plates of vaterite CaCO3 mesocrystals formed by biomimetic mineralization, Adv. Funct. Mater. 16 (2006) 903–908. [8] L. Zhou, W. Wang, H. Xu, Controllable synthesis of three-dimensional well-defined BiVO4 mesocrystals via a facile additive-free aqueous strategy, Cryst. Growth Des. 8 (2008) 728–733. [9] M.-H. Liu, Y.-W. Chen, T.-S. Lin, C.-Y. Mou, Defective mesocrystal ZnO-supported gold catalysts: facilitating CO oxidation via vacancy defects in ZnO, ACS Catal. 8 (2018) 6862–6869. [10] L. Zhou, D. Smyth-Boyle, P. O'Brien, A facile synthesis of uniform NH4TiOF3 mesocrystals and their conversion to TiO2 mesocrystals, J. Am. Chem. Soc. 130 (2008) 1309–1320. [11] B. Hu, L.-H. Wu, S.-J. Liu, H.-B. Yao, H.-Y. Shi, G.-P. Li, S.-H. Yu, Microwave-assisted synthesis of silver indium tungsten oxide mesocrystals and their selective photocatalytic properties, Chem. Commun. 46 (2010) 2277–2279. [12] S. Yin, Y. Zeng, Z. Ye, C. Li, Z. Wang, Spindle-shaped apatite La–silicate mesocrystals grown via a multistep nucleation and growth mechanism, a new strategy for preparing ordered nanoscale superstructures, Cryst. Growth Des. 17 (2017) 1471–1475. [13] C. Lv, X. Duan, J. Deng, T. Wang, LiFePO4 mesocrystals coated with N-doped carbon from an ionic liquid for Li-ion batteries, CrystEngComm 19 (2017) 1253–1257. [14] J. Wang, B.-Q. Liu, G. Huang, Z.-J. Zhang, J.-T. Zhao, Monodisperse NaxY (OH)yF3+x–y mesocrystals with tunable morphology and chemical composition: pHmediated ion-exchange, Cryst. Growth Des. 17 (2017) 711–718. [15] Y.-J. Xu, J. Lin, Y. Lu, S.-L. Zhong, L. Wang, L. Dong, Y.-D. Wu, J. Peng, L. Zhang, X.F. Pan, W. Zhou, Y. Zhao, L.-P. Wen, S.-H. Yu, Lanthanide co-doped paramagnetic spindle-like mesocrystals for imaging and autophagy induction, Nanoscale 8 (2016) 13399–13406. [16] Y. Liu, Y. Zhang, J. Wang, Mesocrystals as a class of multifunctional materials, CrystEngComm 16 (2014) 5948–5967. [17] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, A three-color, solid-state, threedimensional display, Science 273 (1996) 1185–1189. [18] R. Deng, F. Qin, R. Chen, W. Huang, M. Hong, X. Liu, Temporal full-colour tuning through non-steady-state upconversion, Nat. Nanotechnol. 10 (2015) 237. [19] B. Li, X. Huang, H. Guo, Y. Zeng, Energy transfer and tunable photoluminescence of LaBWO6:Tb3+,Eu3+ phosphors for near-UV white LEDs, Dyes Pigments 150 (2018) 67–72. [20] P. Du, X. Huang, J.S. Yu, Facile synthesis of bifunctional Eu3+-activated NaBiF4 redemitting nanoparticles for simultaneous white light-emitting diodes and field emission displays, Chem. Eng. J. 337 (2018) 91–100. [21] P. Du, J.S. Yu, Near-ultraviolet light induced visible emissions in Er3+-activated La2MoO6 nanoparticles for solid-state lighting and non-contact thermometry, Chem. Eng. J. 327 (2017) 109–119. [22] J. Reszczyńska, T. Grzyb, J.W. Sobczak, W. Lisowski, M. Gazda, B. Ohtani, A. Zaleska, Visible light activity of rare earth metal doped (Er3+, Yb3+ or Er3+/ Yb3+) titania photocatalysts, Appl. Catal. B Environ. 163 (2015) 40–49. [23] W. Wang, M. Ding, C. Lu, Y. Ni, Z. Xu, A study on upconversion UV–vis–NIR responsive photocatalytic activity and mechanisms of hexagonal phase NaYF4:Yb3+,Tm3+@TiO2 core–shell structured photocatalyst, Appl. Catal. B Environ. 144 (2014) 379–385.

Fig. 9. (a) Schematic illustration of the process of latent fingerprints using βNaYF4:10%Ce3+, 30%Tb3+ and β-NaYF4:20%Yb3+, 2%Er3+ phosphors; (b) Brightfield, upconverting and downshifting fluorescent images of latent fingerprints impressed on plastic culture dish, glass slide, metal key and document envelope.

Latent fingerprint identification. To verify the practical applicability of the as-obtained β-NaYF4:RE3+ products, the performance of these samples for latent fingerprint detection has been tested on different surfaces, including plastic culture dish, glass slide, metal key and document envelope. Typically, two kinds of green-emitting phosphors, including β-NaYF4:Ce3+/Tb3+ downshifting powder and βNaYF4:Yb3+/Er3+ upconverting powder, are applied to identify latent fingerprint under the excitation of UV light (254 nm) or NIR light (980 nm). Fig. 9a illustrates the detailed process of latent fingerprint identification by taking advantage of bright eye-visible dual-mode emissions of β-NaYF4:RE3+ mesocrystals [65,66]. Clearly, dry fluorescent powders can be applied to the fingerprints of volunteer and quickly deposited on the ridges of fingerprints by physical adsorption. For comparison, Fig. 9b presents the images of phosphor-marked fingerprints in visible light and under the excitation of UV or NIR light. In daylight, the detailed information of latent fingerprints left on the different surfaces could hardly be recognized, which brings difficulty to criminal investigation. However, under UV or NIR irradiation, clear fluorescent images of fingerprints on various surfaces are easily observed, providing evident details of the fingerprint patterns (e. g. arches and termination points) for individual recognition. The above observations evidently suggest that the as-synthesized β-NaYF4:RE3+ phosphors can serve as a useful tool for rapid latent fingerprint detection. 4. Conclusion In summary, uniform and monodisperse β-NaYF4 mesocrystals with cantaloupe-like morphology and adjustable dimension have been successfully synthesized by a simple one-pot hydrothermal method. Through the precise control of a series of experimental conditions including choice of surfactants, STPP additive amount and the regulation of pH values, the shape evolution and formation process for hexagonalphased NaYF4 mesocrystals have been investigated in detail. The experimental results demonstrate that the intrinsic crystallographic structure of β-NaYF4 and the external factors are responsible for the formation of cantaloupe-like β-NaYF4 mesocrystals. In addition, efficient multicolor downshifting and upconverting emissions can be easily achieved via the simple doping of specified RE3+ ions into β-NaYF4 mesocrystals. The proof-of-concept experiments indicate that the as8

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