Microwave-Assisted Solvothermal Synthesis of Upconverting and

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Microwave-Assisted Solvothermal Synthesis of Upconverting and Downshifting Rare-Earth-Doped LiYF4 Microparticles Nikita Panov,† Riccardo Marin,† and Eva Hemmer*,†,‡ †

University of Ottawa, Department of Chemistry and Biomolecular Science, 10 Marie Curie Street, Ottawa, Ontario K1N 6N5, Canada ‡ Centre for Advanced Materials Research (CAMaR), University of Ottawa, Ottawa, Ontario K1N 6N5, Canada

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S Supporting Information *

ABSTRACT: Growing attention toward optically active materials has prompted the development of novel synthesis methods for a more reliable and efficient access to these systems. In this regard, microwave-assisted approaches provide unique advantages over traditional solvothermal methods reliant on convectional heating: namely, significantly shorter reaction durations, more rigid reaction conditions, and thus a higher degree of reproducibility. Reported herein for the first time is a rapid synthesis of rare-earth (RE3+)-doped LiYF4 upconverting and downshifting microparticles with well-defined bipyramidal morphology and good size dispersion via a microwave-assisted solvothermal process. The suggested material growth mechanism identifies a suitable Li+ to RE3+ ion ratio, an abundance of pHsensitive acetate surface-capping ligands, and an appropriate reaction temperature/time profile as crucial for enabling a phase transformation of an intermediary yttrium ammonium fluoride phase into LiYF4 and subsequent particle ripening. The versatility of the reported method is highlighted by its extension toward the synthesis of other state of the art M(RE)F4 (M = alkali metal) optical materials: RE3+-doped LiYbF4 microparticles and β-NaGdF4 and α-NaYF4 nanoparticles. All of the obtained Yb3+/Er3+- and Yb3+/Tm3+-codoped M(RE)F4 materials exhibited characteristic upconversion emission, while downshifting capabilities were induced through Ce3+/Tb3+ codoping of LiYF4. Further attention was devoted to single-particle optical characterization via hyperspectral imaging of Yb3+/Er3+- and Yb3+/Tm3+-codoped LiYF4 microparticles to explore the spatial variability of upconversion emission within individual particles.

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LiYbF4 microparticles as well as β-NaGdF4 and α-NaYF4 nanoparticles. Thus, the proposed microwave-assisted solvothermal method displays a high degree of versatility and time efficiency, two important attributes in the synthesis of materials. Another salient characteristic of this method is the employment of a green and cost-effective ethanol/H2O mixture as solvent, which in turn enables the in situ particle surface functionalization with hydrophilic acetate ligands.10−12 This feature renders all materials synthesized via the method presented herein directly compatible with a wide range of potential applications requiring aqueous milieus. Furthermore, it circumvents postsynthesis surface modifications that often follow alternative approaches toward M(RE)F4 systems employing, for instance, high-boiling-point solvents.13 Like other state of the art M(RE)F4 materials (e.g., NaYF4, NaGdF4), LiYF4 is characterized by a low crystal lattice phonon energy.14 This is an important property in the context of optical materials, as it limits nonradiative, phonon-mediated relaxations of the RE3+ dopants and thus fosters brighter

INTRODUCTION Upconversion and downshifting of light are optical phenomena enabled by rare-earth (RE3+)-doped M(RE)F4 (M = alkali metal) materials that have significant utility in applications such as biomedical imaging, phototherapy, energy conversion, display technology, and anticounterfeiting.1−4 Systems based on the LiYF4 host matrix have great potential for facilitating these applications, given their excellent optical performance with respect to both upconversion and downshifting.3,5 However, because of its nascent status in comparison to the more rigorously investigated Na(RE)F4-based systems in the context of nano-/microscale material chemistry, rapid access to RE3+-doped LiYF4 is lacking. Addressing this lack, we here report a rapid microwave-assisted solvothermal approach yielding upconverting and downshifting RE3+-doped LiYF4 microparticles with well-defined bipyramidal morphologies. Unlike traditionally implemented high-pressure, autoclavebased solvothermal methods that are time-consuming (≥12 h) and rely on convectional heating,6−9 this microwave-assisted method yields high-quality products in 10 min. Conveniently, the developed synthesis method also grants access to other state of the art RE3+-doped M(RE)F4 host materials: namely, © XXXX American Chemical Society

Received: September 21, 2018

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DOI: 10.1021/acs.inorgchem.8b02697 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry emissions.14 LiYF4 is a particularly intriguing host matrix for upconversion (conversion of two or more lower energy photons into a single photon of higher energy) of nearinfrared (NIR) excitation light due to its capacity to generate a broad range of spectrally resolved and narrow emission lines when it is doped with the appropriate RE3+ ions.5,15,16 This attribute is accompanied by higher upconversion emission intensities in microscale systems in comparison to those of analogous nanoscale systems due to the lower surface to volume ratio of larger particles, a quality responsible for mitigating surface-related emission quenching processes.17,18 Larger upconverting LiYF4 particles are therefore advantageous with respect to emission intensity when applications are envisaged where nanoscale sizes are not a prerequisite. Today, such applications (e.g., temperature sensing,19 photocatalysis,20 and anticounterfeiting21) are almost exclusively dominated by the more thoroughly explored Na(RE)F4 microscale systems, microwave-assisted synthesis of which has been already achieved.22 Thus, extending the scope of materials that can be obtained via a rapid and reliable microwave-assisted synthesis method toward LiYF4 is opportune. Beyond its capacity to foster upconversion emission, LiYF4 is also a suitable host matrix for downshifting (conversion of a single higher energy photon into a single photon of lower energy) of ultraviolet (UV) excitation light by the Ce3+/Tb3+ dopant pair.3 However, current focus has primarily been oriented toward downshifting nanoscale systems for potential application in biomedical fields and optical devices.2−4 In fact, only a single case of Ce3+/Tb3+-mediated downshifting by a microscale system (NaYF4 host matrix) has been reported, to the best of our knowledge.23 It is thus intriguing to expand the limited scope of microscale M(RE)F4 host materials capable of fostering bright Ce3+-sensitized Tb3+ downshifting luminescence to include LiYF4. With this in mind, we herein report the rapid microwaveassisted synthesis and optical characterization of LiYF4 microparticles codoped with Yb 3+ (18%)/Er 3+ (2%) and Yb 3+ (25%)/Tm 3+ (0.5%) for upconversion and with Ce3+(2%)/Tb3+(2%) for downshifting. In addition, observation of upconversion emission from Er3+(2%)-doped LiYbF4 microparticles as well as Yb3+(18%)/Er3+(2%)-codoped βNaGdF4 and α-NaYF4 nanoparticles synthesized via the same microwave-assisted method underlines the versatility of the developed approach with respect to its suitability for a large variety of state of the art optical materials.

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Y2O3, 0.09 mmol (35.5 mg) of Yb2O3, and 0.01 mmol (3.8 mg) of Er2O3. LiYF4 codoped with 25% Yb3+ and 0.5% Tm3+ was prepared from 0.3725 mmol (84.1 mg) of Y2O3, 0.1250 mmol (49.3 mg) of Yb2O3, and 0.0025 mmol (1.0 mg) of Tm2O3. LiYF4 codoped with 2% Ce3+ and 2% Tb3+ was prepared from 0.48 mmol (108.4 mg) of Y2O3, 0.02 mmol (7.5 mg) of CeCl3, and 0.02 mmol (7.5 mg) of TbCl3. The aforementioned RE3+ dopant concentrations for the two upconverting systems reflect the optimized standards that enable bright upconversion emission from an M(RE)F4 system.5,14,24 Because no such standard yet exists for the downshifting system, a relatively low dopant concentration was used for Tb3+ and Ce3+ as a proof of concept. After complete dissolution of the RE2O3 starting materials, the HCl/H2O mixture was evaporated, and the resulting RECl3 salts were dried at 60 °C overnight. Subsequently, a mixture was prepared by combining 4.3 mmol (180.4 mg) of LiOH·H2O (Li+ to RE3+ ratio of 4.3), 4 mL of deionized H2O, 4 mL of AcOH, 8 mL of ethanol, and 4.0 mmol (148.2 mg) of NH4F. The intrinsic pH of the resulting solution was 5.3. For experiments in the pH study, the pH of this solution was adjusted to 2.0 and 7.5 with HCl and NH4OH, respectively (to reduce the risk of undesired phase formation, HCl and NH4OH were specifically chosen because ions such as Cl−, NH4+, and OH− were already present in the reaction mixture containing RECl3, LiOH, and NH4F). This solution was then added to the RECl3 salts and vigorously stirred for 30 min at room temperature. Approximately 11.5 mL of the resulting reaction mixture was transferred to a 35 mL glass microwave reaction vessel and inserted into a CEM Discover SP microwave reactor. RE3+-doped LiYF4 microparticles were grown in the microwave reactor by subjecting the reaction mixture with constant stirring (low stirring rate) to a dynamic temperature profile characterized by four distinct stages: (i) rapid increase in temperature to 210 °C (4 min), (ii) rapid cooling to 200 °C (15 s), (iii) static heating at 200 °C (10 min), and (iv) gradual cooling to 50 °C (5 min). Following reaction completion, the mixture was diluted to 25 mL with ethanol and centrifuged for 2 min at 3000 rpm (Allegra X-30R, Beckman Coulter). The obtained white product was washed twice by means of redispersion in a 35 mL 3/7 v/v H2O/ethanol mixture and subsequent centrifugation under aforementioned conditions. Finally, the washed product was redispersed in 7 mL of ethanol for storage. A complete overview of all physicochemical reaction parameters subjected to investigation and the resulting products are provided in Table S1 in the Supporting Information. Synthesis of other M(RE)F4 Systems. Er3+-doped LiYbF4 microparticles, Yb3+/Er3+-codoped β-NaGdF4 and α-NaYF4 nanoparticles were also synthesized following the aforementioned experimental protocol by replacing the existing M (Li) and/or RE (Y) constituents of M(RE)F4 by Na and/or Yb/Gd, respectively. Centrifugation conditions during the purification stage of these materials were 7 min and 10000 rpm. Characterization Techniques. The crystalline phase of the samples was determined by powder X-ray diffraction (XRD) with a Rigaku Ultima IV diffractometer (Cu Kα, λ = 1.5401 Å). The morphology and size distribution of the obtained materials were investigated by transmission electron microscopy (TEM, FEI Tecnai Spirit) and scanning electron microscopy (SEM, JEOL JSM-7500F FESEM). For TEM observations, samples were dispersed on a Formvar/carbon film supported on a 300 mesh copper grid. For characterization by SEM, RE3+-doped LiYF4 microparticles were dispersed on a glass slide and Au-sputtered (2 nm Au layer thickness) in a vacuum coater (Leica EM ACE200). For Fourier transform infrared (FTIR) spectroscopy, dried samples were mixed with potassium bromide (KBr, FTIR grade, Alfa Aesar, Ward Hill, MA, USA), and spectra on KBr pellets were recorded using a Shimadzu FTIR-8200S spectrometer. Upconversion emission spectra were obtained on dried samples dispersed on glass slides with a custombuilt hyperspectral microscope (IMA Upconversion by PhotonEtc, Montreal, Canada) equipped with a 980 nm laser diode (nonpolarized; power at the sample in the range of (4.0−4.7) × 108 mW/cm2 with a spot size of ca. 1.45 μm in diameter), an inverted optical microscope (Nikon Eclipse Ti-U), a broad-band camera for

EXPERIMENTAL PROCEDURES

Chemicals. Y 2 O 3 (99.999%), Gd 2 O 3 (99.999%), Yb 2 O 3 (99.998%), Er2O3 (99.99%), Tm2O3 (99.997%), and LiOH·H2O (98%) were purchased from Alfa Aesar (Ward Hill, MA, USA). TbCl3·6H2O (99.9%) was purchased from Strem Chemicals Inc. (Newburyport, MA, USA). CeCl3·7H2O (99.9%), NH4F (≥98%) and NH4OH (28.0−30.0% NH3 basis) were purchased from SigmaAldrich GmbH (Steinheim, Germany). HCl (36.5−38.0%), glacial acetic acid (AcOH, 99.7%), and bench-grade NaOH were purchased from Fisherbrand (USA). Ethanol (99%) was purchased from Commercial Alcohols (Brampton, Canada). All chemicals were used as received. Synthesis of Yb3+/Er3+-, Yb3+/Tm3+-, and Ce3+/Tb3+-Codoped LiYF4 Microparticles. In a typical synthesis, RECl3 precursors were prepared by dissolving a total of 0.5 mmol of the corresponding RE2O3 in 5 mL of a 3/2 v/v solution of HCl/H2O at 70 °C. As an exception from this, purchased CeCl3 and TbCl3 were used directly for the synthesis of LiYF4:Ce3+(2%)/Tb3+(2%). LiYF4 codoped with 18% Yb3+ and 2% Er3+ was prepared from 0.40 mmol (90.3 mg) of B

DOI: 10.1021/acs.inorgchem.8b02697 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry color imaging, a set of galvanometer mirrors, a Princeton Instruments SP-2360 monochromator/spectrograph, and a Princeton Instruments ProEM EMCCD camera for detection of visible emission. Optical properties of the Ce3+/Tb3+-codoped LiYF4 microparticles, dispersed in ethanol (mass concentration approximately 0.2 mg/mL) and transferred into a quartz cuvette, were investigated using a Photon Technology International fluorimeter equipped with a xenon lamp. The excitation spectrum of Ce3+ in the LiYF4 host matrix was retrieved by plotting the intensity of the Tb3+ emission signal centered at 490 nm (5D4 → 7F6 Tb3+ transition) against the respective excitation wavelength (275−315 nm in 5 nm intervals). On the basis of the results of excitation spectroscopy, 295 nm was then used as the excitation wavelength to obtain the downshifting emission spectrum of Ce3+/Tb3+-codoped LiYF4 microparticles. A photograph of the green-emitting sample under illumination with UVB irradiation (280−400 nm) produced by a Luzchem LZC 4 V photoreactor equipped with eight UVB lamps was obtained by inserting a Chroma AT560/40× band-pass filter (541−583 nm) between the sample and the camera. All data analysis and plotting were performed with the microscope’s PHySpecV2 software as well as OriginPro.

variance between microparticles of the different dopant systems.26 More specifically, the dimensions of the bipyramidal LiYF4 microparticles along the elongated and the shorter, perpendicular axes were determined to be 4.4 ± 0.4 μm × 4.0 ± 0.2 μm for the Yb3+(18%)/Er3+(2%) dopant system, 3.8 ± 0.4 μm × 3.3 ± 0.2 μm for Yb3+(25%)/Tm3+(0.5%), and 3.4 ± 0.6 μm × 3.0 ± 0.2 μm for Ce3+(2%)/Tb3+(2%). These results demonstrate good synthesis reproducibility and the capacity to yield well-defined LiYF4 microparticles with good monodispersity across the various dopant systems. Effect of pH. To gain control over the phase, size, and morphology of the target product, pH-sensitive ligands with a selective affinity for particular facets of the developing crystalline material were demonstrated to facilitate the synthesis of M(RE)F4-based materials.9,27,28 Thus, the pHdependent abundance of such growth-guiding ligands, their intrinsic chemical nature, and solvent compatibility are important physicochemical reaction parameters to consider when a new synthesis method is developed.29 For this study, the acetic acid (CH3COOH)/acetate (CH3COO−) system was selected on the basis of its excellent H2O/ethanol solvent compatibility and successful application as a particle-capping ligand in a previously reported microwave-assisted solvothermal M(RE)F4 material synthesis.10 The effect of the pHdependent abundance of the in situ generated acetate molecules on the structural and morphological characteristics of LiYF4 was demonstrated by adjusting the intrinsic acetate ion fractions in the reaction solution (pH 5.3 yielding approximately 77% acetate) to 0% and 100%. These two extremes of relative acetate abundance were induced by adjusting the pH to 2.0 and 7.5 with HCl and NH4OH, respectively (Figure S2). Employing the optimal Li+ to RE3+ ratio and reaction temperature/duration profile while acetate abundance was minimized (pH 2.0) resulted in the formation of YF3, whereas maximizing acetate abundance (pH 7.5) resulted in the formation of a phase, the identity of which could not be unambiguously discerned (Figure 2). Preferential formation of YF3 over LiYF4 in the lower pH regime has been previously observed.9,28 Ye et al. ascribed the formation of YF3 at pH 2 to the increased solubility of LiF nuclei that, at higher pH, were reported to transform into LiYF4 upon inward Y3+ and F− transport.28 The absence of the nuclei thus hindered LiYF4 formation, ultimately yielding YF3. Hence, an increased solubility of nuclei of intermediate phases at the lower pH regime of the system presented herein is also suggested to prevent the incorporation of Li+ into the matrix, ultimately resulting in the formation of YF3. With regard to morphology of the obtained products, it can be further noted that the materials products obtained at pH values of 2.0 and 7.5 display highly irregular morphologies, indicating that the functional capacity of acetate as a growth-guiding ligand is disrupted at both extremes of the pH-dependent acetic acid/acetate speciation curve. Conversely, the importance of fine-tuning the surface-capping ligand concentration to suit the needs of a particular system is evident from Figure 2A2/B2, which displays the formation of phase-pure LiYF4 microparticles with a well-defined bipyramidal morphology. In order to confirm the in situ surface functionalization (or lack thereof) of these materials with acetate ligands, FTIR analysis was performed (Figure S3). The presence of acetate or residual acetic acid on the surface of all three samples was confirmed by signature CO and C−O stretches at 1532 and 1414 cm−1, respectively

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RESULTS AND DISCUSSION Photoluminescent LiYF 4 microparticles codoped with Yb3+(18%)/Er3+(2%) and Yb3+(25%)/Tm3+(0.5%) for upconversion and with Ce3+(2%)/Tb3+(2%) for downshifting were successfully synthesized via the developed microwave-assisted solvothermal approach (Figure 1). The optimal set of reaction

Figure 1. (A, 1−3) SEM micrographs of RE3+ -doped LiYF 4 microparticles obtained under optimal microwave-assisted solvothermal conditions (pH 5.3, Li+:RE3+ = 4.3, 200 °C, 10 min). Scale bars: 4 μm. (B) XRD pattern representative of materials presented in (A) (refer to Figure S1 for XRD patterns of all materials). Reference: LiYF4 (PDF#: 01-081-1940).

parameters allowing for the synthesis of these materials was determined to be a pH of 5.3, a Li+ to RE3+ ratio of 4.3, a reaction temperature of 200 °C, and a reaction duration of 10 min. Irrespective of the dopant system, all LiYF4 microparticles obtained under the aforementioned conditions were characterized by an aspect ratio of approximately 1.1 and thus resemble octahedrons with a slight unidirectional elongation (tetragonal crystallographic phase, facets are assigned to the {101} crystallographic planes25). The difference in the cationic radii of the RE 3+ dopants and their relative doping concentrations within the LiYF4 host resulted in some size C

DOI: 10.1021/acs.inorgchem.8b02697 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (A) TEM micrographs of materials obtained at pH values of 2.0 (A1), 5.3 (A2), and 7.5 (A3) (Li+:RE3+ = 4.3, reaction temperature 200 °C, time 10 min). (B) XRD patterns of materials presented in A. References: LiYF4 (PDF#: 01-081-1940), YF3 (PDF#: 00-005-0547).

(the reader is referred to the Supporting Information for a more detailed discussion). Proposed Growth Mechanism of RE3+-Doped LiYF4. The crystallographic phase and morphology of intermediates obtained at different stages of a reaction are critical factors responsible for the formation of a specific material and its morphology. In order to propose a growth mechanism by which the RE3+-doped LiYF4 microparticles are formed under the optimal pH (5.3) and Li+ to RE3+ ratio (4.3) conditions, the duration of the particle growth stage (microwave-assisted static heating at 200 °C) was systematically increased from 1 to 5, 7.5, and 10 min. As demonstrated by the TEM and XRD results presented in Figure 3A1/B1, formation of LiYF4 was preceded by a set of Yx(NH4)yFz phases30 after 1 min. In fact, the beginning of LiYF4 formation was first observed after 5 min, as evidenced by the emerging XRD reflections characteristic of this phase and supported by the appearance of a second, microscale morphology (Figure 3A2/B2). After 7.5 min, complete disappearance of the XRD reflections attributed to the intermediary Yx(NH4)yFz phases and the consequent formation of LiYF4 as the sole reaction product was observed (Figure 3A3/B3). These observations suggest that the Yx(NH4)yFz phases served as an yttrium and fluorine reservoir for LiYF4 during a phase transformation period that took place between 1 and 7.5 min of the synthesis. Furthermore, from the TEM images presented in Figure 3A, a clear size evolution of the LiYF4 particles from approximately 2 μm (5 min) to 2.5 μm (7.5 min) and finally 4 μm (10 min) via ripening is evident. More important, however, is the obvious increase in particle size homogeneity between 5 and 10 min, a particle growth period by the end of which monodisperse LiYF4 microparticles were obtained. Thus, a 10 min heating at 200 °C is the shortest possible reaction duration during which LiYF4 microparticles with a good size monodispersity could be obtained. On this basis, it is also demonstrated that the growth mechanism of LiYF4 microparticles does not involve the initial formation of LiYF4 nanoparticles that subsequently grow in size. Rather, the initial formation of a sacrificial phase has been

Figure 3. (A) TEM micrographs of materials obtained by microwaveassisted heating at 200 °C for 1 (A1), 5 (A2), 7.5 (A3), and 10 (A4) min (pH 5.3, Li+:RE3+ = 4.3). (B) XRD patterns of materials presented in (A). References: LiYF4 (PDF#: 01-081-1940), Y3(NH4)F10 (PDF#: 01-074-2889), Y2(NH4)3F9 (PDF#: 00-043-0840), Y2(NH4)F7 (PDF#: 00-043-0847).

identified as an intermediate step toward LiYF4 microparticle formation. Effect of Li+ to RE3+ Ratio. In the context of M(RE)F4 material synthesis, the M+ to RE3+ ratio has a significant influence on the phase, size, and morphology of the targeted material and depends on the nature of the metal constituents as well as the accompanying reaction parameters.29,31 Therefore, the influence of the Li+ to RE3+ ratio on the microwaveassisted formation of RE3+-doped LiYF4 was investigated by adjusting it to 2.0, 4.3, and 7.5, while the optimal pH value of 5.3 (pH of the reaction mixture under all employed Li+ to RE3+ ratios), a reaction temperature of 200 °C, and a reaction duration of 10 min were retained (Figure 4). It is important to note that the different LiOH·H2O amounts used in this investigation fell within the buffering regime of acetic acid/ acetate (buffer produced in situ by reaction of acetic acid with LiOH) and thus did not significantly alter the intrinsic pH of the reaction solution (pH 5.3). Therefore, the results obtained by varying the amount of LiOH·H2O can be attributed to the D

DOI: 10.1021/acs.inorgchem.8b02697 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. (A) TEM micrographs of materials obtained at Li+ to RE3+ ratios of 2.0 (A1), 4.3 (A2), and 7.5 (A3) (pH 5.3, 200 °C, 10 min). (B) XRD patterns of materials presented in (A). References: Y(OH)1.57F1.43 (PDF#: 01-080-2008), LiYF4 (PDF#: 01-081-1940), Y3(NH4)F10 (PDF#: 01-074-2889).

Figure 5. (A) TEM micrographs of materials obtained by microwaveassisted heating at 150 °C for 10 min (A1) and 3 h (A2) (pH 5.3, Li+:RE3+ = 4.3). (B) XRD patterns of materials presented in (A). References: Y(OH)1.57F1.43 (PDF#: 01-080-2008), LiYF4 (PDF#: 01081-1940), Y3(NH4)F10 (PDF#: 01-074-2889).

Li+ to RE3+ ratio itself and not to the pH of the system. A Li+ to RE3+ ratio of 2.0 resulted predominantly in the formation of Y3(NH4)F10, together with nanoparticles of an unidentified phase constituting a minor fraction of the final product, as judged from the relative intensities of the XRD reflections in Figure 4B1. Note that the intermediary formation of Yx(NH4)yFz phases was also observed en route to LiYF4 at the initial stages of the proposed microparticle growth mechanism, when a higher Li+ to RE3+ ratio of 4.3 was employed (vide supra). Yet, while at such an elevated Li+ to RE3+ ratio pure LiYF4 was obtained after 10 min (Figure 4A2/ B2), employing a low Li+ to RE3+ ratio of 2.0 did not allow the phase transformation from Yx(NHy)Fz to LiYF4 to be achieved within 10 min (Figure 4A1/B1). Interestingly, further increase in the Li+ to RE3+ ratio to 7.5 resulted in the formation of yttrium hydroxy fluoride nanorods (Figure 4A3) and LiF (Figure 4B3). The XRD pattern of the obtained nanorods is in good agreement with those reported for numerous Y(OH)xFy systems with varying OH to F ratios.32−34 Their chemical composition and approximate weight fraction in the sample were determined to be Y(OH)0.75F2.25 and 73%, respectively, via Rietveld refinement. The reader is referred to Figures S4 and S5 and Table S2 in the Supporting Information for details. Effect of Reaction Temperature. To evaluate whether a lower temperature regime would be sufficient for inducing LiYF4 formation at the optimal pH (5.3) and Li+ to RE3+ ratio (4.3), the reaction temperature of the 10 min heating profile was decreased from 200 to 150 °C (refer to Figure S6 for analogous experiments conducted with Li+ to RE3+ ratios of 2.0 and 7.5) (Figure 5). As evident from Figure 5A1/B1, LiYF4 formation was not achieved at the lower temperature regime. Instead, XRD data point toward a phase mixture composed of the previously mentioned nanoparticles of an unidentified phase, Yx(NH4)yFz phases, and traces of LiF. Taking into account that Yx(NH4)yFz phases were identified as intermediate products en route to LiYF4, the reaction duration was extended from 10 min to 3 h

in order to investigate whether longer heat treatment at 150 °C can foster the phase transformation into LiYF4. As expected, formation of LiYF4 was observed after 3 h at 150 °C, yet the obtained material was not phase-pure (Figure 5A2/B2). The fact that LiYF4 microparticles were obtained together with a minor fraction of Y(OH)xFy nanorods (approximately 11 wt % as determined by Rietveld refinement, Figure S4 and Table S2) suggests that a lower reaction temperature has the effect of retarding the kinetics of the LiYF4 growth mechanism, hindering a complete phase transformation into the desired product. Ultimately, it can be concluded that temperature (over time) plays a pivotal role in the synthesis of LiYF4, as it supplies the adequate threshold energy required for rapid LiYF4 formation under the optimal pH and Li+ to RE3+ ratio conditions. Synthesis of Other M(RE)F4 Systems. Addressing the great attention M(RE)F4 upconversion host materials different from LiYF4 are receiving, we extended the developed microwave-assisted solvothermal method toward a set of other state of the art upconversion materials: namely, RE3+doped LiYbF4, β-NaGdF4, and α-NaYF4 (Figure 6). The obtained LiYbF4 is a structural analogue of LiYF4, also having a tetragonal crystallographic phase and a bipyramidal morphology. Consequently, in analogy to RE3+-doped LiYF4, microscale Er3+-doped LiYbF4 with a bipyramidal morphology was obtained (Figure 6A1/B1). However, although they have a similar aspect ratio (approximately 1.1), these microparticles were smaller (1.7 ± 0.5 μm long and 1.6 ± 0.3 μm wide) and less monodisperse. For the Yb3+/Er3+-codoped hexagonal (β)phase NaGdF4, these fall into the nanometer size range, being 107 ± 21 nm thick and 83 ± 20 nm in diameter, with an aspect ratio of 1.3 (Figure 6A2/B2). It should be noted that, to date, only a few other microwave-assisted solvothermal approaches toward β-NaGdF4 have been reported to the best of our knowledge.35,36 The crystalline phase of the obtained NaGdF4 E

DOI: 10.1021/acs.inorgchem.8b02697 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (A) SEM/TEM micrographs of RE3+-doped LiYbF4 microparticles (A1), β-NaGdF4 nanoparticles (A2), and α-NaYF4 nanoparticles (A3). (B) XRD patterns of materials presented in (A). References: LiYbF4 (PDF#: 01-071-1211), β-NaGdF4 (PDF#: 01082-4232), α-NaYF4 (PDF#: 00-006-0342).

nanoparticles constitutes an important aspect, since between the two possible crystalline phases of NaGdF4, α and β, the latter is generally considered as the superior host material in the context of upconversion.37 Consequently, addition of the solvothermal approach presented here to the limited arsenal of rapid microwave-assisted syntheses by which β- NaGdF4 can be attained is highly beneficial. Finally, we applied the developed microwave-assisted approach to the synthesis of α-phase NaYF4, gaining significantly faster access to nanoparticles (32 ± 7 nm) in comparison to a previously reported microwave-assisted solvothermal method requiring 3 h (Figure 6A3/B3).10 It is clear that the nature of the alkali- and rareearth-metal ions in the crystal lattice structure of the M(RE)F4 host material has a significant effect on the overall size and morphology of the resultant particles despite being synthesized under the same reaction conditions, which is ascribed to the intrinsic materials characteristics of each system (e.g., stable crystalline phase). Yet, most importantly, these results demonstrate that by systematic tuning of the physicochemical reaction parameters, not only does a broad range of materials become accessible but also it is possible to significantly reduce the time required for the synthesis of a particular material, which is of both economic and environmental interest. Optical Characterization of RE3+-Doped LiYF4 Microparticles. Yb3+/Er3+- and Yb3+/Tm3+-codoped LiYF4 microcrystals synthesized via the microwave-assisted solvothermal approach developed herein exhibited characteristic upconversion upon 980 nm excitation (Figure 7). Further investigation of their optical properties by means of single-particle hyperspectral imaging revealed spatial variability in upconversion emission between and within individual particles. In this context, hyperspectral imaging is a powerful characterization technique that provides simultaneously spatial (for material distribution) and spectral (for optical properties analysis) information and has only recently emerged in the field of nano-/micromaterials with scarce examples involving upconverting systems.6,38−41 Individually assessed Er3+/Yb3+- and Tm3+/Yb3+-codoped LiYF4 microparticles exhibited bright upconversion luminescence in all of their signature emission

Figure 7. Single-particle photoluminescence studies on (A) Yb3+/ Tm3+- and (B) Yb3+/Er3+-codoped LiYF4 microparticles: (1) upconversion emission spectra extracted from hyperspectral cubes (corresponding images are shown in (2)) at two selected regions of interest (ROIs) exhibiting brighter or dimmer emission from RE3+doped LiYF4 microparticles (selected ROIs are marked with bright and dark blue and green arrows, respectively, in (2) and (3)); (2) false-color hyperspectral images of the characteristic blue Tm3+ (440− 500 nm) and green Er3+ (510−570 nm) emissions (color code: dark colors indicate low emission intensity, bright colors indicate high emission intensity); (3) SEM micrographs of the same microparticles subjected to optical investigation. Scale bars: 5 μm.

bands. For the Tm3+-doped microparticles, the visible region of the optical spectrum is characterized by strong blue bands centered at 455 and 480 nm stemming from the 1D2 → 3F4 and 1 G4 → 3H6 transitions as well as a weaker red band at 650 nm ascribed to the 1G4 → 3F4 transition (Figure 7A).5 In addition, two bands centered at 790 and 801 nm, ascribed to the 1G4 → 3 H5 and 3H4 → 3H6 transitions, respectively, were observed in the NIR region.42 It is noteworthy that the integrated blue emission is approximately 4 times stronger than both the red and NIR emissions. For the Er3+-doped microparticles, the visible region of the optical spectrum was characterized by a relatively weak band centered at 523 nm and a strong band at 553 nm, both in the green range, which stem from the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions, respectively (Figure 7B).43 Further, a strong red band centered at 660 nm and ascribed to the 4F9/2 → 4I15/2 transition was detected. All emission bands for both materials were observed as wellresolved manifolds due to crystal field (Stark) splitting that is uniquely facilitated by the asymmetric crystalline environment of the LiYF4 host.5 In addition to this spectral information, F

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distribution and/or a difference in the crystal field strength experienced by dopant ions occupying different crystallographic sites within the lattice of the microparticle.7,8 Similar to the interparticle scenario, variation in electromagnetic power density experienced by RE3+ ions present at the apex versus the face of a particle may also induce intraparticle variations as a result of their different orientations in space with respect to the incident laser excitation. Moreover, uneven surface-ligand coverage has been proposed as an additional plausible explanation for the observed emission variance.7 Further investigation seeking deeper insight into single-particle photoluminescence (PL) is currently under way. Taking advantage of the versatility of the developed method with regard to the synthesis of M(RE)F4 materials hosting various combinations of RE3+ dopants, we added Ce3+/Tb3+codoped LiYF4 microparticles with strong green emission upon UV excitation to the series of luminescent microparticles (Figure 9). Depicted in Figure 9A is the excitation spectrum of

probing single particles by scanning of the excitation beam over a region of interest containing several LiYF4 microparticles yielded a map, which displays the spatial distribution of the blue Tm3+ (440−500 nm) and green Er3+ (510−570 nm) upconverted emissions (Figure 7A2/B2). In combination with SEM imaging of the same region of interest, this allowed for illustration of inter- and intraparticle differences in intensity of the dominant emission bands for both upconverting systems. It is conceivable that the interparticle variation in emission intensity is a consequence of the variation in spatial orientation of the particles with respect to the incident laser excitation and the consequent variation in electromagnetic power density experienced by the RE3+ dopants in the individual particles (a particle’s apex and face may lie in slightly different laser focus planes due to their relatively large size). To gain deeper insight into the intraparticle variation of emission intensity, the spatially resolved relative intensities of the green and red Er3+ emission bands were determined (Figure 8). Yb3+/Tm3+-codoped LiYF4

Figure 9. (A) Excitation spectrum of Ce3+ in the LiYF4 host matrix. (B) Downshifting emission spectrum of Ce3+/Tb3+-codoped LiYF4 microparticles under 295 nm excitation. (C) Photograph showing the green downshifted emission from Ce3+/Tb3+-codoped LiYF4 microparticles under broad-band UV excitation (280−400 nm). A bandpass filter (541−583 nm) was used to acquire the photograph.

Ce3+ in the LiYF4 microcrystalline host matrix, from which it is evident that the best sensitization of Tb3+ emission was achieved by exciting the material with 295 nm light. Indeed, upon UV excitation, Ce3+ undergoes a 4f → 5d electronic transition that, in contrast to the more common 4f → 4f electronic transitions of rare-earth ions, is not parity forbidden and is hence more probable (i.e., the associated absorption cross section is larger).2 An energy transfer follows from the 5d energy levels of Ce3+ to the 5Hj states of Tb3+, and a rapid nonradiative relaxation down to the Tb3+ 5D4 emitting level is then facilitated, resulting in the characteristic Tb3+ emission.2,44 Radiative relaxation from the 5D4 energy level of Tb3+ down to the 7F6 (490 nm), 7F5 (545 nm), 7F4 (580 nm), and 7 F3 (621 nm) energy levels produced four emission bands, giving rise to the observed green emission (Figure 9B and C). Evidence for the energy transfer from Ce3+ to Tb3+ is provided by photoluminescence spectroscopy on LiYF4 microparticles singly doped with Tb3+ in comparison to Ce3+/Tb3+-codoped samples (Figure S7). Optical Characterization of RE3+-Doped LiYbF4, βNaGdF4, and α-NaYF4. Finally, having successfully extended the developed microwave-assisted solvothermal method toward the synthesis of Er3+-doped LiYbF4 microparticles, Yb3+/Er3+-codoped β-NaGdF4 and α-NaYF4 nanoparticles, their capacity to foster upconversion was assessed by PL spectroscopy (Figure 10). Again, the characteristic Er3+ emission bands stemming from the 2H11/2 → 4I15/2, 4S3/2 →

Figure 8. Hyperspectral images of Yb3+ /Er 3+ -codoped LiYF 4 microparticles showing the spatial distribution of the characteristic (A) green (510−570 nm) and (B) red (627−680 nm) upconversion emission. (C) SEM micrograph of the same area of interest. (D) Intensity ratio (2H11/2 → 4I15/2, 4S3/2 → 4I15/2)GREEN:(4F9/2 → 4 I15/2)RED deduced from spectral information obtained at specific locations indicated by colored circles in (A)−(C).

microparticles were deemed less suitable for this investigation on the basis of their slightly smaller size (which would yield a less precise spatial data interpretation) along with a higher complexity of the Tm3+ electronic level configuration over that of Er3+. From the hyperspectral images displaying the spatial distribution of the green (510−570 nm) and red (627−680 nm) upconversion emissions (Figure 8A/B) and the corresponding SEM micrograph (Figure 8C), it is evident that the green to red ratio of the emission intensity at the apex of the LiYF4 particle is higher than that at the face (Figure 8D). This observed intraparticle variation of the green to red ratio is likely a consequence of an inhomogeneous RE3+ dopant G

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investigated, revealing a phase transformation from Yx(NH4)yFz to LiYF4 and the subsequent ripening of the LiYF4 microparticles. Optical studies confirmed good upconverting and downshifting capabilities of all reported M(RE)F4 systems (RE = Er, Tm, Yb) and Ce3+/Tb3+-codoped LiYF4 microparticles, respectively. Bright RE3+-doped LiYF4 microparticles presented in this work could serve as excellent candidates in applications such as photocatalysis, anticounterfeiting, and color-tunable phosphors. In addition, insight gained from single-particle photoluminescence studies conducted on Yb3+/Er3+-codoped LiYF4 microparticles provided further evidence for the existence of spatial variance in the upconversion emission intensity and green to red emission ratio among and within particles. Further extension of these studies is expected to provide knowledge valuable for the future design of optical materials featuring efficient upconverting and downshifting properties.

Figure 10. Normalized upconversion emission spectra obtained from Er3+-doped LiYbF4 as well as Yb3+/Er3+-codoped α-NaYF4, βNaGdF4, and LiYF4 under 980 nm NIR excitation.

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I15/2, and 4F9/2 → 4I15/2 transitions, respectively, were observed. In addition, α-NaYF4 enabled a weak blue emission centered at 410 nm associated with the 2H9/2 → 4I15/11 transition, traces of which could also be detected in the spectra of the other three host materials. This indicates that higher-order processes are more probable in the α-NaYF4 system. Additional insight can be gained into the optical behavior of the four different M(RE)F4 host materials by comparing the relative intensities of the green and red emission bands. From this comparison one can infer which levelsthe higher emitting levels responsible for the green emissions (2H11/2 → 4I15/2, 4S3/2 → 4I15/2) or the lower energy level feeding the red emission (4F9/2 → 4I15/2)are more likely to be populated in each system under the same experimental conditions. The green to red intensity ratios have been determined to be 0.4, 0.8, 1.1, and 1.6 for LiYbF4, NaYF4, NaGdF4, and LiYF4, respectively. Narrowing our focus on the lithium-based materials, we find it interesting to note that LiYF4 rather fosters green emission, while LiYbF4 favors the red upconversion emission. This behavior is in accordance with a previously reported observation that increasing the sensitizer concentration in a Yb3+/Er3+-codoped Li(RE)F4 system yielded a stronger red emission.45 Overall, these results are of interest in fields where color tuning is an important requisite: i.e., for potential display or lighting applications. 4

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02697.

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Overview of all investigated reaction parameters and the resulting products, XRD of RE3+-doped LiYF4 microparticles, acetic acid/acetate speciation curve, FTIR spectra, phase assignment for Y(OH)xFy nanorods and Rietveld refinement, additional discussion of the effect of the reaction temperature/time profile at Li+ to RE3+ ratios of 2.0 and 7.5, and photoluminescence spectra of Ce3+/Tb3+-codoped and Tb3+-doped LiYF4 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for E.H.: [email protected]. ORCID

Eva Hemmer: 0000-0002-9222-1219 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the University of Ottawa, the Canadian Foundation for Innovation (CFI), and the Natural Sciences and Engineering Research Council of Canada (NSERC). We would also like to thank Prof. H. Alper for his support and Prof. T. Scaiano for provision of infrastructure used for the downshifting optical study.

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CONCLUSION A microwave-assisted solvothermal synthesis was developed, providing rapid access to RE3+-doped LiYF4 microcrystals exhibiting the optical phenomena of upconversion and downshifting. Salient characteristics of this approach are the significantly reduced reaction duration, the in situ surface functionalization with acetate groups allowing for applications in aqueous media, and the use of inexpensive and more environmentally benign solvents: i.e., ethanol and water. Furthermore, the versatility of this method was highlighted by extending it toward the preparation of other state of the art M(RE)F4 (M = Li, Na; RE = Gd, Yb, Y) micro- and nanoscale materials. The method presented herein was developed by means of rigorous control of multiple physicochemical reaction parameters: namely, the reaction temperature/time profile, the Li+ to RE3+ ratio, and the initial pH of the reaction solution. The materials growth mechanism of the Li-Y-F system under optimal microwave-assisted solvothermal conditions was also

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