Preparation, Growth Mechanism, Upconversion, and Near-Infrared

Article Cite This: Cryst. Growth Des. 2018, 18, 1758−1767

pubs.acs.org/crystal

Preparation, Growth Mechanism, Upconversion, and Near-Infrared Photoluminescence Properties of Convex-Lens-like NaYF4 Microcrystals Doped with Various Rare Earth Ions Excited at 808 nm Yingjin Ma, Zhengwen Yang,* Hailu Zhang, Jianbei Qiu,* and Zhiguo Song College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093, China S Supporting Information *

ABSTRACT: Preparation of rare earth ions doped photoluminescence materials with controlled morphology was desired to fulfill the requirement of different applications. In the work, convex-lens-like NaYF4 microcrystals doped with various rare earth ions were prepared by adjusting preparation parameters including the reaction time, reaction temperature, NaOH concentration, ratio of oleic acid to 1-octadecene, and types of doping ions. A possible growth mechanism of convex-lenslike NaYF4 microcrystals is proposed based on reaction time and temperature-dependent morphology evolution. The formation of microconvex-lens includes the three processes of NaYF4 nanoparticles selfassemble, dissolution−nucleation, and regrowth. Doping ions dependent near-infrared and upconversion luminescence properties of convex-lenslike NaYF4 microcrystals were investigated excited at 808 nm. The visible upconversion luminescence was observed in the Er3+, Yb3+/Er3+, Nd3+/Er3+, Nd3+/Yb3+/Er3+ doped convex-lens-like NaYF4 microcrystals, and near-infrared luminescence was obtained in the Nd3+, Nd3+/Er3+, Yb3+/Er3+, Nd3+/Yb3+, Nd3+/Yb3+/Er3+ doped NaYF4 convex-lens-like NaYF4 microcrystals. The Er3+, Yb3+/Er3+, Nd3+/Er3+, Nd3+/Yb3+/Er3+ doped convex-lens-like NaYF4 microcrystals exhibit various upconversion luminescence mechanisms. The energy transfer of the Er3+ → Yb3+ and the Nd3+ → Er3+ was observed in the Yb3+/Er3+ and Nd3+/Er3+ doped convex-lens-like NaYF4 microcrystals, respectively. The upconversion emission of Nd3+/Yb3+/Er3+ doped convex-lens-like NaYF4 microcrystals is from the energy transfer mechanisms of Nd3+ → Yb3+ → Er3+.

1. INTRODUCTION Very recently, rare earth (RE) ions doped luminescent materials have attracted much attention due to their special optical properties and extensive applications in the fields of three-dimensional display, biological imaging and detection, drug delivery, solar cells, and photodynamic therapy.1−7 Among the various RE ions doped luminescent materials, the RE fluoride compounds (NaREF4) upconversion (UC) materials excited at 980 nm exhibited the higher UC luminescence efficiency due to its lower phonon energy, which was extensively reported.8−10 However, the 980 nm excitation light overlaps with the absorption of water molecules in biological tissues, which can result in the overheating of biological tissues. Thus, significant cell death and tissue damage occurred under excitation at 980 nm. In contrast to the NaREF4 UC materials upon 980 nm excitation, NaREF4 UC and nearinfrared (NIR) luminescent materials excited at 808 nm are more suitable for biological applications due to avoiding the overheating of biological tissues caused by 980 nm excitation light.11−13 Therefore, the NaREF4 phosphor with UC and NIR luminescence excited at 808 nm is very important for its applications. At present, the UC and NIR luminescence NaREF4 phosphor tridoped with Nd3+, Yb3+, and Er3+ was extensively investigated under excitation at 808 nm; however, © 2018 American Chemical Society

few investigations have been carried out on the UC luminescence Er3+ single-doped and Yb3+ and Er3+ codoped NaREF4 phosphor upon 808 nm excitation. It is well-known that the photoluminescence properties of the NaREF4 phosphors are dependent on the types of dopants and codopants, and morphology (shape and size). For example, it was reported that the sensor sensitivities of Er3+ and Yb3+ codoped NaREF4 optical thermometers were influenced by their morphologies. 14,15 The fine modification of UC luminescence color from Er3+ and Yb3+ codoped NaREF4 can be obtained by the precise control of size and morphology of UC nanocrystals.16−20 Therefore, the synthesis of RE ions doped NaREF4 phosphors with precise morphology is particularly significant for their applications. Up to now, hexagonal phase NaYF4 crystals with various sizes and morphologies such as nanoparticles, nanorods, and nanoplates have been prepared.21−26 Very recently, the Yb3+, Er3+ doped NaYF4 microplates were prepared; however, the research was focused on the UC luminescence property of NaYF4 nano- and microplates in the previous work, and the formation Received: November 29, 2017 Revised: January 21, 2018 Published: February 1, 2018 1758

DOI: 10.1021/acs.cgd.7b01667 Cryst. Growth Des. 2018, 18, 1758−1767

Crystal Growth & Design

Article

Table 1. Detailed Experimental Parameters, Average Diameter of NaYF4 Products sample

OA/ODE ratio

NaOH (mmol)

time (min)

temperature (°C)

types of doping ions

diameter (nm)

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18

4/6 4/6 4/6 4/6 4/6 4/6 4/6 4/6 3/7 5/5 4/6 4/6 4/6 4/6 4/6 4/6 4/6 4/6

5 5 5 5 5 5 5 5 5 5 3.75 6.25 5 5 5 5 5 5

10 30 50 65 80 80 80 80 80 80 80 80 80 80 80 80 80 80

305 305 305 305 305 275 290 320 305 305 305 305 305 305 305 305 305 305

2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+/2%Er3+ 2%Nd3+/10%Yb3+ 2%Nd3+/2%Er3+ 10%Yb3+/2%Er3+ 2%Nd3+ 2%Er3+ 10%Yb3+

11 21/309 419 470 719 10 17/615 660 1216 516 750 739 399 325 721 305 342 542

total volume of OA and ODE was about 20 mL, and the ratio of OA to ODE was the 4/6. The mixture was heated to 150 °C for 60 min to remove deionized water under the argon shield and continually stirred. After being cooled to the room temperature, the 2 mL NaOH and 6 mL NH4F methanol solutions were introduced into the 3-neck flask, and then the mixture was heated at 80 °C for 1 h to remove the methanol solutions. After the methanol was removed, the solution was heated at 305 °C for 80 min under the argon shield. The resulting solution was centrifuged five times with the mixture solution of 2 mL of cyclohexane and 6 mL of ethanol, and the NaYF4:Yb3+, Nd3+, Er3+ products were prepared. The crystal phases of the samples were examined by an X-ray diffraction diffractometer quipped with Cu Kα radiation. The morphology and size of products were observed by a transmission electron microscopy (TEM, JEOL 2100), a field emission scanning electron microscopy (FESEM, QUANTA 650), and a scanning probe microscopy (SPM-9600). The UC luminescence spectra and NIR luminescence spectra of the samples excited at 808 nm were collected with a HITACHI F-7000 spectrophotometer and an Edinburgh FLS980 spectrophotometer, respectively. The refractive index of products was measured by using a WAY-1S refractometer.

mechanisms of NaYF4 microplates were not investigated in detail.17,27,28 In contrast to other morphology materials, convex-lens-like materials exhibited optical focusing property at the infrared wavelengths,29,30 which may result in UC emission enhancement due to the enhancement of focusing near-infrared excitation. In this work, convex-lens-like NaYF4 microcrystals doped with various RE ions were prepared by the coprecipitation method using oleic acid as the surfactants. The influence of reaction time, reaction temperature, NaOH concentration, ratio of oleic acid to 1-octadecene and types of doping ions on the morphology, size and phase structure of convex-lens-like NaYF4 microcrystals was systematically studied. The possible growth mechanism of convex-lens-like NaYF4 microcrystals was proposed based on the reaction time and temperature-dependent morphology evolution. In addition, the NIR and UC luminescence properties and mechanisms of convex-lens-like NaYF4 microcrystals doped with various RE ions were investigated excited at 808 nm. The UC luminescence mechanisms of convex-lens-like NaYF4 microcrystals doped with Er3+, Nd3+/Er3+, Yb3+/Er3+ or Nd3+/Yb3+/ Er3+ were different under the excitation at 808 nm.

3. RESULTS AND DISCUSSION Preparation parameters will influence the structure, morphology, and size of RE ion doped NaYF4 products. The effect of the ratio of OA to ODE, NaOH concentration, reaction time, reaction temperature, and types of doping ions on the morphology, size, and structure of NaYF4:Nd3+, Yb3+, Er3+ UC materials was studied in detail. 3.1. Influence of Reaction Time and Temperature on the Morphology of NaYF4:Nd3+, Yb3+, Er3+ Products. The influence of reaction time on the crystal structure and morphology of NaYF4:Nd3+,Yb3+,Er3+ products was investigated. The X-ray diffraction (XRD) patterns of NaYF4:Nd3+,Yb3+,Er3+ products prepared at the 305 °C for 10, 30, 50, 65, and 80 min are shown in Figure 1a. The product prepared in 10 min consists of pure cubic phase NaYF4 (JCPDS No. 77-2042). Further increasing the reaction time to 30 min, the product consists of the cubic and hexagonal phase NaYF4 (JCPDS No. 16-0334). The XRD peaks of products prepared at the different reaction times from 50 to 80 min are well-indexed to the standard hexagonal NaYF4, exhibiting the formation of pure hexagonal phase NaYF4. However, the relative intensity of

2. EXPERIMENTAL SECTION High purity (99.99%) Y2O3, Yb2O3, Er2O3, Nd2O3, analytical reagent grade oleic acid (OA), 1-octadecylene (ODE), NH4F, and NaOH were used to prepare the convex-lens-like NaYF4 microcrystals as raw materials without further purification. The convex-lens-like NaYF4 microcrystals doped with different RE ions of Nd3+, Er3+, Yb3+, Nd3+/ Er3+, Nd3+/Yb3+, Yb3+/Er3+, and Nd3+/Yb3+/Er3+ were prepared by coprecipitation method. In order to obtain the influence of reaction time, reaction temperature, NaOH concentration, ratio of OA to ODE, and types of doping ions on the morphology, size, and phase structure of NaYF4 microcrystals, the various preparation parameters were used, as listed in Table 1. The S1−S18 samples were prepared followed by a similar procedure except for changing the preparation parameters listed in Table 1. As an example, the preparation of S5 sample was described as follows. First, The RECl3 (RE = Y3+, Yb3+, Er3+, and Nd3+) compounds were prepared by dissolving the corresponding RE2O3 compounds in hot HCl solution. The obtained YCl3, YbCl3, NdCl3, and ErCl3 were dissolved in deionized water to form a 0.5 mol/ L solution, respectively. The 1.72 mL YCl3, 200 μL YbCl3 and 40 μL NdCl3, 40 μL ErCl3 solutions were added into the mixture of 8 mL of oleic acid and 12 mL of ODE in a 100 mL three-necked flask. The 1759



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Figure 1. XRD patterns (a) of NaYF4:Nd3+, Yb3+, Er3+ products prepared at different reaction times; the TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared at 10 (b), 30 (c), 50 (d), 65 (e) and 80 min (f); the inset images in (c) are the enlargement TEM image and SAED pattern of the NaYF4 microplate; the insets in the (d−f) are the lateral shape of NaYF4 products; the FESEM images of NaYF4 products prepared at the 50 (g) and 80 min (h); the AFM image (i) of NaYF4 products prepared at 80 min, the inset in (i) is the curve of profile; (j) the HRTEM image and SAED pattern (inset) of convex-lens-like NaYF4 microcrystals prepared at 80 min.

diffraction peaks is not in agreement with those of the standard PDF card. In contrast to the standard diffraction peaks, the

relative intense (002) diffraction peak was observed in the all hexagonal NaYF 4 products, which suggested that the 1760



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NaYF4:Nd3+, Yb3+, Er3+ products were grown preferentially along the (002) planes. The diffraction intensity of (002) plane of hexagonal NaYF4 products was increased with the increasing of the reaction time, which suggested the larger NaYF4 products may be obtained due to their further growth. The XRD results suggested that there is a phase transformation for the NaYF4 products, and the unstable cubic phase can be transformed to the stable hexagonal phase with the increasing of the reaction time. The representative TEM images of NaYF4 products obtained under different reaction times were obtained, as shown in Figure 1b−f. It is noted that the reaction time plays a role in controlling the morphology of NaYF4:Nd3+, Yb3+, Er3+ products. At the reaction time of 10 min, the final products were composed of nonuniform nanoparticles with an average diameter of 11 nm ranging from 24 to 5 nm, as seen from Figure 1b. With increasing the reaction time to 30 min, the final products consisted of some hexagonal microplates deposited on its surface with a lot of nanoparticles, as shown in Figure 1c. With further increasing the reaction time to 50 min, all the nanoparticles completely disappeared, and the platelike NaYF4 microcrystals were formed by the further growth of NaYF4:Nd3+, Yb3+, Er3+ nanoparticles, which was demonstrated by the TEM images of corresponding lateral shapes of products given in the inset of Figure 1 and the FESEM image in Figure 1g. The FESEM image of NaYF4:Nd3+, Yb3+, Er3+ products prepared at 80 min is shown in Figure 1h, exhibiting convexlens-like NaYF4 microcrystals with the center thickness and thin edge after further growth of hexagonal microplates like NaYF4:Nd3+,Yb3+,Er3+, as shown in the TEM images of corresponding lateral shapes of products given in the inset of Figure 1. The AFM image of NaYF4:Nd3+, Yb3+, Er3+ products prepared at the 80 min was measured, as shown in the Figure 1i, which further demonstrated the formation of convex-lenslike NaYF4 microcrystals at 80 min. The reaction time has an influence on the size of NaYF4:Nd3+, Yb3+, Er3+ products, as listed in Table 1. The sizes of NaYF4:Nd3+, Yb3+, Er3+ products increased with the increasing of the reaction time. The inset image of Figure 1c exhibited the SAED pattern of NaYF4 microplates on its surface with a lot of nanoparticles. The ordered bright spots from the hexagonal phase NaYF4 and diffraction rings from the cubic phase NaYF4 were observed, which agrees well with the results measured by the XRD analysis. Figure 1j exhibited the SAED pattern (inset) and HRTEM image of thin edge of convex-lens-like NaYF 4 microcrystals prepared at 80 min. The distances between adjacent lattice fringes are both about 0.51 nm, which correspond to the d spacing of the (100) or (010) plane of the hexagonal NaYF4 product. The HRTEM image demonstrated that the convex-lens-like NaYF4 microcrystals were grown along the [100] and [010] orientation, namely, the (002) plane.31 The highly ordered bright spots suggested that the convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals are single crystalline. The refractive index of NaYF4:Nd3+, Yb3+, Er3+ products prepared at the 10, 50, and 80 min were measured, receptively, which was about 1.4270, 1.4277, and 1.4279. It can be seen that the refractive index of NaYF4:Nd3+, Yb3+, Er3+ products with the various morphologies has a slight difference. To deeply certify the morphology of convex-lenslike NaYF4 microcrystals, a diagram of electromagnetic field in two-dimensional space was simulated by a finite element solver, as shown in Figure 2. In the simulation, the structural parameters are consistent with the experiment results exhibited in Figure 1. The red spot means the source of excitation under

Figure 2. Electromagnetic field simulation of NaYF4 microcrystals based on the structural parameters exhibited in Figure 1; amplitude of magnetic field (a) and (b) the distribution of transverse magnetic (TM) polarization with the magnetic field.

the 808 nm, and the black line of convex-lens means the convex-lens-like NaYF4 microcrystals. The refractive index of the NaYF4 microcrystals is set to 1.4279, and the surrounding medium is air. Figure 2a shows the amplitude image of simulation magnetic field. It is clearly seen that when a monochromatic point source with a wavelength of 808 nm is positioned 600 nm away from the surface at the left side of the NaYF4 microcrystals, the spherical wave of source with the excitation of 808 nm is converted to plane wave. We can see the distribution of transverse magnetic (TM) polarization with the magnetic field along the z direction, as shown in Figure 2b, and the beam is almost free from divergence in propagation. The simulation demonstrates the NaYF4 microcrystals can also focus light, which means the morphology of NaYF4 microcrystals are the convex lens according to the reciprocity of optics.32 With the increasing of the reaction time, the NaYF4:Nd3+, Yb3+, Er3+ products suffered from transformation from the cubic to hexagonal phase, and the morphologies changed from nanoparticles to microplates then to micro-convex-lens. The formation mechanism of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals was inferred from the morphology changing of the products exhibited in Figure 1. A simple schematic illustration for the formation process of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals is exhibited in Figure 3. The formation of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals includes the three processes of self-assembly of NaYF4:Nd3+, Yb3+, Er3+ nanoparticles, dissolution−nucleation, and regrowth, as shown in Figure 1. For the formation of NaYF4 nanoparticles, the combination of Y3+and F− form the

Figure 3. (a−f) Schematic illustration for the formation process of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals; the blue balls and pink sheets in panels a−e represent NaYF4:Nd3+, Yb3+, Er3+ nanocrystals and microplates, respectively; the pink sheets in panel f represent convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals. 1761



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Figure 4. XRD patterns (a) of NaYF4:Nd3+, Yb3+, Er3+ crystals prepared at different reaction temperatures; the TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared at 275 °C (b), 290 °C (c), 320 °C (d); TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared at 305 °C is shown in Figure 1.

[YF4]−, and the reaction between Na+ and [YF4]− cause the fast nucleation of NaYF4,33 as exhibited in Figure 3a. The isotropic growth of cubic NaYF4:Nd3+, Yb3+, Er3+ products occurred due to its isotropic crystal structures. Thus, the small NaYF4:Nd3+, Yb3+, Er3+ nanoparticles in the cubic phase were formed at the 10 min reaction time, as exhibited in Figure 3b. At 30 min, some NaYF4:Nd3+, Yb3+, Er3+ nanoparticles were self-assembled due to their high surface energy.33 Some nanoparticles transformed from the cubic to hexagonal phase because the cubic NaYF4:Nd3+, Yb3+, Er3+ seeds are unstable, and the interfaces between transformed nanoparticles were obviously observed, as shown in Figure 1c. The further growth of hexagonal NaYF4:Nd3+, Yb3+, Er3+ results in the formation of microplates like NaYF4, as shown in Figure 1d and Figure 3. After the formation of plate-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals, some nanocrystals were attached to their surfaces. Moreover, most nanocrystals were aggregated at the centers of plate-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals, as shown in Figure 1c and Figure 3e. With the time increasing to 50 min, the cubic nanoparticles disappeared completely, as shown in the TEM images of Figure 1, which suggested that the further dissolution and regrowth of nanocrystals on the surfaces of microplates take place, resulting in the formation of convexlens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals. The reaction temperature of the coprecipitation method is an important factor for NaYF4:Nd3+, Yb3+, Er3+ preparation. NaYF4:Nd3+, Yb3+, Er3+ was prepared under various reaction temperatures. Figure 4 shows the XRD pattern of NaYF4:Nd3+, Yb3+, Er3+ products prepared at 275, 290, and 320 °C. It is noted that when the reaction temperature is 275 °C, the XRD diffraction peaks of NaYF4:Nd3+, Yb3+, Er3+ products are matched well with the standard cubic NaYF4. As the temperature increased to 290 °C, the XRD diffraction peaks of hexagonal and cubic NaYF4:Nd3+, Yb3+, Er3+ phases in the XRD pattern were observed, which indicated that the mixed

NaYF4:Nd3+, Yb3+, Er3+ phases of cubic and hexagonal coexist in the as-prepared products. Further increasing temperature to 305 (as shown in Figure 1a) and 320 °C, all the diffraction peaks are consistent with the pure hexagonal phase of NaYF4, indicating that the hexagonal NaYF4:Nd3+, Yb3+, Er3+ phase was prepared. The TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared at the various temperatures were measured, as shown in Figure 4. It can be seen from Figure 4b that the NaYF4:Nd3+, Yb3+, Er3+ products prepared under the 275 °C consists of a large number of uneven nanoparticles with an diameter of 10 nm. When the preparation temperature was 290 °C, the product morphology is composed of a lot of nanoparticles with a diameter of 17 nm and a few convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals with a diameter of 615 nm. As the reaction temperature increases to 305 °C, the nanoparticles structure completely disappears, and the convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals with uniform size were obtained, as shown in Figure 1f. The average diameter of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals was about 719 nm. With further increasing of the reaction temperature to 320 °C, 660 nm convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals with a smoother surface were observed. The formation mechanisms of convex-lens-like NaYF4 microcrystals prepared at the various temperatures are similar to that of samples prepared at different reaction times. Both dissolution−nucleation and regrowth are responsible for the convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals prepared at the various temperatures. The small NaYF4:Nd3+, Yb3+, Er3+ nanoparticles in the cubic phase were formed at the low reaction temperature (275 °C), as exhibited in the TEM image and the XRD pattern of Figure 4a. With further increasing reaction temperature to 305 °C, only hexagonal convex-lenslike NaYF4:Nd3+, Yb3+, Er3+ microcrystals were observed. However, the regrowth of crystals is not complete, and a few smaller bulges were observed on the surface of convex-lens-like 1762



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Figure 5. XRD patterns (a) of NaYF4:Nd3+, Yb3+, Er3+ products prepared by various ratios of oleic acid to 1-octadecene; the TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared by different ratios of oleic acid to 1-octadecene: (b) 3/7 and (c) 5/5; the inset is the corresponding FESEM images of NaYF4:Nd3+, Yb3+, Er3+ products.

Figure 6. XRD patterns (a) of NaYF4:Nd3+, Yb3+, Er3+ micro-convex-lens prepared by the different NaOH concentrations; the TEM images of NaYF4:Nd3+, Yb3+, Er3+ micro-convex-lens prepared by 3.75 (b) and 6.25 mmol (c) NaOH concentration; the inset in (b) and (c) is the corresponding FESEM images of NaYF4:Nd3+, Yb3+, Er3+ micro-convex-lens.

NaYF4:Nd3+, Yb3+, Er3+ microcrystals, which resulted in that the surface of micro-convex-lens was not smooth, as exhibited in the TEM image of Figure 1f. With further increasing reaction temperature to 320 °C, the hexagonal convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals with smooth surface was observed, which suggested that the regrowth process is very complete. In addition, some stripes were observed on the surface of hexagonal convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals. The distribution of nanocrystals is not uniform, and some nanocrystals were aggregated at the interface of hexagonal convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals. When they were dissolved and regrown, the formation of hexagonal convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals with some stripes resulted, as shown in Figure S1. Some nanoparticles and stripes without complete dissolution and growth were observed at the interface of hexagonal microconvex-lens, as exhibited in the TEM image of Figure 4c, supporting the mechanisms of growth of micro-convex-lens and formation of stripes. 3.2. Influence of Ratio of Oleic Acid to 1-Octadecene on Morphology of Convex-Lens-like NaYF4:Nd3+, Yb3+, Er3+ Microcrystals. Oleic acid as a surfactant has an influence on the morphology of NaYF4:Nd3+, Yb3+, Er3+ products. The convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals were prepared with different ratios of oleic acid (OA) to 1octadecene (ODE), as exhibited in Table 1. The XRD patterns of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals prepared by the 3/7 and 5/5 ratio of OA to ODE are shown in Figure 5a. On the basis of Figure 1 (XRD pattern of samples prepared by the 4/6 ratio of OA to ODE) and Figure 5, all the diffraction peaks of the three samples are correspond to the

pure hexagonal NaYF4, exhibiting high purity and good crystallinity. The quite intense (002) diffraction peak was observed in all the samples, and the (002) diffraction peak intensity was decreased while increasing the volume of OA. Figure 5b,c exhibits the TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared at different ratios of OA to ODE, and the corresponding FESEM images are given in the inset of Figure 5, panels b and c, respectively. It is noted from the TEM and FESEM images that the NaYF4:Nd3+, Yb3+, Er3+ products are made up of the hexagonal convex-lens-like microcrystals with the thickness center and thin edge. On the basis of the TEM and FESEM images, the diameters of convex-lens-like microcrystals were measured, as listed in Table 1. When the ratio of OA to ODE is 3/7, 4/6, and 5/5, the average diameter from corner to corner is about 1216, 719, and 516 nm, respectively. It is concluded that the ratio of OA to ODE influences greatly the size of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals. The diameter of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals was decreased with the increasing of the ratio of OA to ODE. For the NaYF4:Nd3+, Yb3+, Er3+ products prepared by the present hydrothermal method, the crystal growth and self-assembly coexist.33 For the crystal growth process, a large number of OA provided fewer opportunities than small number of OA for the adhesion of monomers on the surfaces of nucleation seeds. Thus, the OA as a capping ligand limits the growth of NaYF4 nanoparticles, resulting in the formation of small convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals. The previous investigations demonstrated that the OA as a capping ligand suppressed the growth of NaYF4 nanoparticles, which is consistent with the present results.34 Additionally, the self-assembly of NaYF4 nanocrystals was 1763



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Figure 7. XRD patterns of NaYF4 products doped with the different rare earth ions (a); the TEM images of NaYF4 products doped with Nd3+ (b), Er3+ (c), Yb3+ (d), Nd3+/Yb3+ (e), Nd3+/Er3+ (f), Yb3+/Er3+ (g).

influenced by the organic chelating agent such as Na3Cit.33 The OA has the cross-linking and crystal growth depressant role as Na3Cit. Increasing the OA content can lower the surface energy of the NaYF4 nanocrystals, limiting their self-assembly. Thus, the size of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals reduces with the increasing of OA content. 3.3. Influence of NaOH Concentration on Morphology of Convex-Lens-like NaYF4:Nd3+, Yb3+, Er3+ Microcrystals. To investigate the effect of NaOH concentration on the crystal structures and morphologies of NaYF4:Nd3+, Yb3+, Er3+ micro-convex-lens, the NaOH with 3.75, 5, and 6.25 mmol NaOH were used to synthesize the NaYF4:Nd3+, Yb3+, Er3+ micro-convex-lens while maintaining other experimental conditions at fixed values. Figure 6a shows the XRD patterns of NaYF4:Nd3+, Yb3+, Er3+ products prepared by 3.75 and 6.25 mmol of NaOH. On the basis of Figure 1 (NaYF4:Nd3+, Yb3+, Er3+ products prepared by 5 mmol of NaOH) and Figure 6, the XRD patterns of NaYF4:Nd3+, Yb3+, Er3+ products prepared by 3.75, 5, and 6.25 mmol NaOH are matched well with standard hexagonal NaYF4. The relatively intense (002) diffraction peak was observed in all the samples, which suggested that the NaYF4:Nd3+, Yb3+, Er3+ products were grown preferentially along the (002) planes. The TEM images and FESEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared by 3.75 and 6.25 mmol of NaOH are shown in Figure 6, panels b and c, respectively. From the TEM and FESEM images of Figure 1 (NaYF4:Nd3+, Yb3+, Er3+ products prepared by 5 mmol of NaOH) and 6, the morphology of products prepared by 3.75, 5,

and 6.25 mmol NaOH possesses the hexagon convex lens. The average diameter of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals was measured from TEM and FESEM images, as shown in Table 1. The micro-convex-lens has an average diameter of about 730 nm. XRD and the TEM demonstrated that the NaOH concentration has a slight effect on the morphology and size of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals. 3.4. Influence of Types of Doping Ions on the Morphology of Convex-Lens-like NaYF4 Microcrystals. The Nd3+, Er3+, and Yb3+ single doped NaYF4 and Nd3+/Yb3+, Nd3+/Er3+ and Yb3+/Er3+ codoped NaYF4 products were prepared with the corresponding preparation conditions as listed in Table 1. Figure 7a exhibited the XRD patterns of NaYF4 products doped with the different kinds of RE ions. All the diffraction peaks correspond to the standard hexagonal NaYF4 structure, and there are no other impure peaks observed, which implied that the different types of doping ions have no influence on the NaYF4 crystal structure. Figure 7b−g illustrates the TEM images of NaYF4 doped with the various kinds of RE ions. It is obvious that all samples possess similar morphologies of hexagonal convex-lens-like NaYF4 microcrystals regarding the types of doping ions. However, the types of doping ions have an influence on the size of the convex-lenslike NaYF4 microcrystals. The diameter of convex-lens-like NaYF4 microcrystals calculated by the TEM and FESEM images are listed in Table 1. It is clear that the different types of 1764



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the UC luminescence photos of samples shown in the inset of Figure 8 that the UC emission of the convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals is more intense than that of the plate and nanoparticle like NaYF4:Nd3+, Yb3+, Er3+ products. It is well-known that the size of NaYF4 products has an important influence on its UC emission intensity, and the UC emission intensity decreased with the decreasing of the size. As shown in the Table 1, the size of convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals is larger than that of the plate and nanoparticle like NaYF4:Nd3+, Yb3+, Er3+ products, resulting in its more intensive UC emission,35−37 as shown in the Figure 8. In addition, it was reported that the lens like materials exhibited the optical focusing property at the infrared wavelengths,29,30 which may cause the focusing of 808 nm infrared excitation around the convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals, as shown in Figure 2. Therefore, the more intense UC emission for convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ microcrystals may be attributed to the enhancement of excitation fields. 3.6. Upconversion and Near-Infrared Photoluminescence Property and Mechanisms of convex-lens-like NaYF4 Microcrystals Doped with the Various Rare Earth Ions Excited at 808 nm. The visible UC emission spectra of convex-lens-like NaYF4 microcrystals doped with the various RE ions were measured under the excitation of 808 nm. No visible UC emissions were observed for the convex-lens-like NaYF4 microcrystals doped with the Nd3+,Yb3+ or Nd3+/Yb3+ (the figure was not exhibited). But the convex-lens-like NaYF4 microcrystals doped with the Er3+, Yb3+/Er3+, Nd3+/Er3+, or Nd3+/Yb3+/Er3+ present the visible UC emission from Er3+ ions, as shown in Figure 9a. It can be seen from the UC emission photos of samples shown in the inset of Figure 9a that the convex-lens-like NaYF4 microcrystals doped with the Er3+, Yb3+/Er3+, Nd3+/Er3+, or Nd3+/Yb3+/Er3+ exhibited the various

doping ions in the NaYF4 products change the size of the micro-convex-lens. 3.5. Upconversion Photoluminescence Property of NaYF4:Nd3+, Yb3+, Er3+ with Various Morphologies Excited at 808 nm. The dependence of morphology and structure of NaYF4:Nd3+, Yb3+, Er3+ products on the UC luminescence property was investigated by excitation at 808 nm, as shown in Figure 8. It is noted that the nanoparticles,

Figure 8. UC emission spectra of the nanoparticles, microplates and convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ products upon the 808 nm excitation; the inset is the UC emission photos of samples.

micro plates and convex-lens-like NaYF4:Nd3+, Yb3+, Er3+ products were prepared at the reaction time of 10, 50, and 80 min, respectively, exhibiting the typical UC emissions from the Er3+. The 523 and 548 nm green UC emissions were attributed to the 2H11/2 → 2I15/2 and 4S3/2 → 2I15/2 transitions of Er3+, respectively, and the 660 nm red UC emission was from 4 F9/2 → 4I15/2 transition in the all samples. It can be seen from

Figure 9. UC (a) and NIR (b) emission spectra of NaYF4 samples with different doping ions upon 808 nm excitation; the inset in (a) and (b) is the UC emission photos of samples and enlargement image of NIR emission, respectively; the UC and NIR emission mechanism of NaYF4 doped with Er3+ (c), Yb3+/Er3+ (c), Nd3+/Er3+ (d) and Nd3+/Yb3+/Er3+ (e). 1765



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significantly in contrast to that of Nd3+/ Er3+ doped NaYF4 micro-convex-lens, which suggested that the Yb3+ ions play an important role in the enhancement of UC emission of Nd3+/ Yb3+/Er3+ tridoped NaYF4 micro-convex-lens. The UC emission of Nd3+/Yb3+/Er3+ doped NaYF4 micro-convex-lens is from the energy transfer mechanisms of Nd3+ → Yb3+ → Er3+, as shown in Figure 9e. The NIR spectra of convex-lens-like NaYF4 microcrystals doped with the various RE ions were measured under the excitation of 808 nm, as shown in Figure 9b. The 880, 1060, and 1336 nm NIR emission emissions of Nd3+ were observed in the Nd3+ single-doped or Nd3+/Er3+ codoped convex-lens-like NaYF4 microcrystals, which are attributed to the 4F3/2 → 4I9/2, 4 F3/2 → 4I11/2, and 4F3/2 → 4I13/2 transitions of Nd3+, respectively. For the Nd3+/Yb3+, Yb3+/Er3+, and Yb3+/Nd3+/ Er3+ codoped convex-lens-like NaYF4 microcrystals, the 978 nm NIR emission was assigned to the transition from 2F7/2 to 2F5/2 of Yb3+ ions, as shown in the inset of Figure 9b. No nearinfrared emission was observed in the Er3+ or Yb3+ single-doped NaYF4 micro-convex-lens (the figure was not exhibited). The weak 978 nm near-infrared emission of Yb3+ was observed in the Yb3+/Er3+ codoped NaYF4 micro-convex-lens. The result suggested that the energy transfer from the Er3+ ions to the Yb3+ ions occurred under the excitation of 808 nm, resulting in the weak 978 nm near-infrared emission of Yb3+. The energy transfer mechanism from the Er3+ ions to the Yb3+ ions was shown in Figure 9c. The nonradiative relaxation from the 4I9/2 populated the 4I11/2 state of the Er3+ ions. The energy transfer from the Er3+ to the Yb3+ take place by the 4I11/2 (Er3+) + 2F7/2 (Yb3+) → 4I15/2 (Er3+) + 2F5/2 (Yb3+) process, populating the 2 F5/2 state of Yb3+. Thus, the weak 978 nm near-infrared emission of Yb3+ was observed due to the transition from the 2 F5/2 to the 2F7/2.

UC emission behaviors. The convex-lens-like NaYF4 microcrystals doped with the Er3+ or Yb3+/Er3+ have the green UC emission color, exhibiting the 523 and 548 nm UC emissions of Er3+. No obvious 660 nm red UC emission peak was observed. For the convex-lens-like NaYF4 microcrystals doped with the Nd3+/Er3+, the intense 660 nm red UC emission peak and weak 523 and 548 nm green UC emission peaks of Er3+ were observed, and the UC emission color of sample is red, as shown in the inset of Figure 9a. For the convex-lens-like NaYF4 microcrystals tridoped with the Nd3+/Yb3+/Er3+, the 660 nm red UC emission and 523 and 548 nm green UC emission is very intense in contrast to NaYF4 microcrystals doped with the Er3+, Yb3+/Er3+, and Nd3+/Er3+, exhibiting the yellow UC emission color. The convex-lens-like NaYF4 microcrystals doped with the various RE ions exhibited the various UC emission behaviors, which may be caused by the various UC mechanisms. The UC emission mechanisms can be inferred by dependence of pump power (P) of excitation light on the UC intensity (I) of NaYF4 microcrystals. The relationship between the I and P could be expressed as the log I = n log P, where n is the number of absorbed photons required for emitting one UC visible photon. On the basis of the log I = n log P equation, a straight line with slope n could be obtained. The slopes of 524, 545, and 660 nm UC emissions are shown in the Figure S2 of Supporting Information, which indicated that both green and red UC emissions were from the two-photon process. The UC emission mechanism of convex-lens-like NaYF4 microcrystals doped with the Er3+, Yb3+/Er3+, Nd3+/Er3+, or Nd3+/Yb3+/Er3+ was presented in Figure 9. For the Er3+ ions single-doped NaYF4 convex-lens, the successive transition from the 4I15/2 to 4I9/2 to 2 H9/2 state of the Er3+ ions take place under the excitation at 808 nm. The Er3+ ions in the 2H9/2 state relaxed to the 2H11/2 and 4S3/2 states. The transition from the 2H11/2/4S3/2 states to the ground state leads to the green UC emission, as shown in Figure 9c. For the Yb3+/Er3+ codoped NaYF4 micro-convexlens, the Yb3+ ions cannot absorb the 808 excitation light, and the UC emission mechanism is similar to that of the Er3+ ions single-doped NaYF4 micro-convex-lens. For the Nd3+/Er3+ codoped NaYF4 micro-convex-lens, the red UC emission was observed beside the green UC emissions in contrast to the Yb3+/Er3+ codoped and Er3+ single-doped NaYF4 microconvex-lens, which suggested that the Nd3+ doping has an influence on the UC mechanism of Nd3+/Er3+ codoped NaYF4 micro-convex-lens. The UC mechanism of Nd3+/Er3+ codoped NaYF4 micro-convex-lens was proposed in Figure 9d. The Nd3+ ion has the larger absorption cross section at 808 nm than the Er3+ ion. The Nd3+ ions at the excited 4F3/2 state transfer their part energy to the adjacent Er3+ ions, causing the transition from the ground state to the 4I11/2 excited state of Er3+ ions. The energy transfer processes from the Nd3+ at the excited 4 F3/2 state to the Er3+ ions at 4I11/2 excited state populated the 4 F7/2 state of the Er3+ ions. The 2H11/2 and 4S3/2 states of the Er3+ ions were populated by the nonradiative relaxation from the 4F7/2 state of the Er3+ ions, resulting in the green UC emission. In addition, the population of 4I13/2 excited state of Er3+ ions was obtained by the nonradiative relaxation from the 4 I11/2 state of the Er3+ ions. The 4F9/2 state of the Er3+ ions was populated by the energy transfer from the Nd3+ at the excited 4 F3/2 state to the Er3+ ions at 4I13/2, resulting in the red UC emission. As shown in Figure 9a, the UC emission of Nd3+/ Yb3+/Er3+ tridoped NaYF4 micro-convex-lens was enhanced

4. CONCLUSIONS In the work, the influence of reaction time, reaction temperature, NaOH concentration, ratio of oleic acid to 1octadecene and types of doping ions on the morphology, size and phase structure of NaYF4 micrometer convex lens were systematically studied. Reaction temperature and times have an influence on the morphology of NaYF4 products. Increasing reaction temperature and time, cubic phase NaYF4 nanoparticles evolved into the hexagonal phase NaYF4 micrometer convex lens. Besides the changing of size, the morphology of NaYF4 micrometer convex lens is independent of the NaOH concentration, ratio of oleic acid to 1-octadecene, and types of doping ions. Possible growth mechanism of NaYF4 micrometer convex lens is proposed based on reaction time and temperature dependent morphology evolution. Doping-ions dependent near-infrared and upconversion luminescence properties of NaYF4 micrometer convex lens were investigated excited at the 808 nm. Excited at the 808 nm, visible upconversion luminescence was observed in the Er3+, Yb3+/ Er3+, Nd3+/Er3+, Nd3+/Yb3+/Er3+ doped NaYF4 micrometer convex, and near-infrared luminescence was obtained in the Nd3+, Nd3+/Er3+, Yb3+/Er3+, Nd3+/Yb3+, Nd3+/Yb3+/Er3+ doped NaYF4 micrometer convex lens. The upconversion luminescence mechanism of Er3+, Yb3+/Er3+, Nd3+/Er3+, Nd3+/Yb3+/ Er3+ doped NaYF4 micrometer convex lens was discussed based on the energy level scheme, exhibiting various upconversion luminescence mechanisms. 1766



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01667. Figure S1: The enlargement image of NaYF product prepared at 290°C. Figure S2: Dependence of pump power of 808 nm excitation light (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Z.Y.) E-mail: [email protected]. *(J.Q.) E-mail: [email protected]. ORCID

Zhengwen Yang: 0000-0001-6470-9244 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51762029, 11674137), and the Applied basic research key program of Yunnan Province.

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Preparation, Growth Mechanism, Upconversion, and Near-Infrared

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