Accessing Mixed-Halide Upconverters Using Heterohaloacetate

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Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

Accessing Mixed-Halide Upconverters Using Heterohaloacetate Precursors K. Tauni Dissanayake,† Dinesh K. Amarasinghe,† Leopoldo Suescun,‡ and Federico A. Rabuffetti*,† †

Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States Cryssmat−Lab/DETEMA, Facultad de Química, Universidad de la República, Montevideo 11800, Uruguay

‡

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S Supporting Information *

ABSTRACT: A new type of organic−inorganic hybrid precursor to mixed-halide optical materials is described by using Ba5(CF2ClCOO)10·7H2O as an example. This heterohaloacetate belongs to the family of extended inorganic hybrids in which metal connectivity in three dimensions is achieved via metal−organooxygen−metal bridges. Thermolysis in the solid state and in solution yield crystalline BaFCl at temperatures below 300 °C. Chlorodifluoromethyl groups act as chlorine and fluorine source, making the organic moiety a de facto single-source precursor for the anionic portion of the target mixed halide. Solution thermolysis in the presence of Yb3+−Er3+ sensitizing−activator pairs yields rare-earth-doped BaFCl nanocrystals capable of NIR-to-visible photon upconversion. Analyses of the composition, morphology, structure, and luminescence of these nanocrystals demonstrate that heterohalocetates serve as dual halogen sources for mixed-halide optical materials. Findings presented in this article enlarge the synthetic toolbox to access mixed halides at temperatures compatible with solution synthesis and processing.

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of Yb3+−Er3+ pairs yields a series of mixed-halide nanocrystals capable of NIR-to-visible photon upconversion. The use of heterohaloacetic acids was inspired by the observation that trifluoromethyl groups present in mono- and bimetallic trifluoroacetates serve as a fluorine source for the synthesis of fluorides below 300 °C.16−19 Knowing that the energy of carbon−halogen bonds decreases going down the halogen group, we hypothesized that heterohalomethyl groups would serve as dual halogen sources for the solid-state and solution synthesis of mixed halides. In the following, we describe the synthesis and reactivity of Ba5(CF2ClCOO)10·7H2O. Emphasis is placed on probing the phase purity, morphology, crystal structure, and luminescence response of rare-earth-doped BaFCl nanocrystals obtained upon solution thermolysis of the hybrid precursor. Multiple compositional levers available in this family of precursors are highlighted from the perspective of extending the chemistry presented in this article to the synthesis of a broad range of mixed halides.

INTRODUCTION Mixed halides constitute an important class of optical materials used as X-ray detection and storage phosphors, photon up- and downconverters, and light absorbers in photovoltaics. Wellknown examples of this family of materials include alkali− alkaline-earth,1 alkaline-earth,2−8 rare-earth,9 and alkali−lead halides.10−14 The choice of the halogen source is determined by the form factor of the target mixed-halide (bulk, single crystal, nanocrystal, or thin film), the synthetic approach used (solidstate or solution), and also the ability to control the ratio between the two halide anions. Exercising stoichiometric control over this ratio enables fine-tuning of structural and electronic features governing the material’s optical response.1,15 With this goal in mind, a variety of single halogen sources have been proposed; these include halide salts,11,14 alkylammonium halides,10,14 benzoyl halides,12 and alkylsilyl halides.13 To the best of our knowledge, dual halogen sources containing two halogens in a single chemical entity have not been investigated. We attribute this to the fact that it seems counterintuitive that dual sources could afford enhanced control over the halide ratio than single sources. From the standpoint of exploratory synthesis, however, incorporation of dual halogen sources to the synthetic toolbox creates opportunities to exploit new reactivity patterns and seize heretofore unknown mixed-halide (and eventually oxyhalide) materials. In this work we demonstrate that dual halogen sources provide access to mixed-halide optical materials. We exemplify this concept using a commercially available heterohaloacetic acid to synthesize Ba5(CF2ClCOO)10·7H2O, an extended organic−inorganic hybrid whose thermolysis in the solid state and in solution yield BaFCl at temperatures below 300 °C. Solution thermolysis in the presence of varying concentrations © XXXX American Chemical Society

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EXPERIMENTAL SECTION

All syntheses were performed under a nitrogen atmosphere using standard Schlenk techniques. Er2O3 (99.99%), Yb2O3 (99.9%), BaCO3 (99.98%), CF3COOH (99%), CF2ClCOOH (98%), oleic acid (90%), 1-octadecene (90%), and trioctylphosphine (97%) were used as reagents. All chemicals were purchased from Sigma-Aldrich and used without further purification. CF3COOH, CF2ClCOOH, and trioctylphosphine were stored under N2. Synthesis of Ba5(CF2ClCOO)10·7H2O. Ba5(CF2ClCOO)10·7H2O was synthesized using a solvent evaporation method developed by our Received: June 18, 2019 Revised: July 29, 2019

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DOI: 10.1021/acs.chemmater.9b02384 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials group.20 First, 1 mmol of BaCO3, 3 mL of double deionized water, and 3 mL of CF2ClCOOH were added to a 50 mL two-neck round-bottom flask. A colorless, optically transparent solution was obtained. Then, the flask was connected to the Schlenk line through one of its necks; a glass adapter was placed on the other neck to allow venting of the nitrogen gas. The flask was immersed in a sand bath at 65 °C, and nitrogen gas was flown for 48 h at a constant flow rate of 200 mL min−1. After 48 h, a white polycrystalline solid was obtained at the bottom of the reaction flask. Single crystals were recovered for structural analysis. Synthesis of Er:Yb:BaFCl Nanocrystals. Er:Yb:BaFCl nanocrystals were synthesized using a hot-injection approach. Five Er:Yb:BaFCl samples were prepared, each with a total metal content of 1 mmol but with different rare-earth concentrations. These samples featured nominal total rare-earth concentrations (Er:Yb:Ba molar ratios) of 0.50 mol % (0.000500:0.00450:0.995), 1.5 mol % (0.00150:0.0135:0.985), 3.0 mol % (0.00300:0.0270:0.970), 4.5 mol % (0.00450:0.0405:0.955), and 6.0 mol % (0.00600:0.0540:0.940). In all cases, the nominal erbium concentration was fixed at 10.0% of the total rare-earth concentration, corresponding to an Er:Yb molar ratio of 1:9. Polycrystalline precursors for Er:Yb:BaFCl nanocrystals were prepared following a procedure similar to that described in the previous paragraph. BaCO3, Er2O3, and Yb2O3 were added in stoichiometric amounts to a solution containing 3 mL of double deionized water and 2 mL of CF3COOH. The resulting suspension was heated at 65 °C for 12 h until a colorless, optically transparent solution was obtained. Then, 3 mL of CF2ClCOOH were added, and the solution was stirred for a few minutes. Polycrystalline precursors were obtained after solvent evaporation for 48 h at 65 °C under a constant nitrogen flow rate of 200 mL min−1. Then, 4 mL of oleic acid and 1 mL of 1-octadecene were added to the flask containing the precursor (hereafter termed flask A). 4 mL of oleic acid and 7 mL of 1-octadecene were added to a second flask (flask B). Flasks A and B were immersed in sand baths and heated to 115 °C under vacuum (≈2 mTorr); vigorous magnetic stirring was employed throughout. After 45 min, flask B was switched to a nitrogen atmosphere, a thermocouple was placed inside the flask in direct contact with the solution, and 8 mL of trioctylphosphine was injected. Degassing oleic acid and 1-octadecene before adding trioctylphosphine was found to be critical to prevent oxidation to trioctylphosphine oxide. Then, the temperature of flask B was increased to 278 °C, and the solution in flask A was swiftly injected. The reaction proceeded for 90 min at 275 °C. Then, the flask was removed from the sand bath and quenched to room temperature by using a stream of air. Er:Yb:BaFCl nanocrystals were isolated in ≈60−70% yield following a work-up procedure described elsewhere.8 Polycrystalline samples were employed for chemical, morphological, structural, and luminescence analyses. Single-Crystal X-ray Diffraction. Single-crystal XRD analysis was performed using a Bruker X8 Apex single-crystal diffractometer. Ba5(CF2ClCOO)10·7H2O crystals required special handling: they were mixed with Paratone oil under a nitrogen atmosphere prior to mounting. A colorless crystal with approximate dimensions 0.25 × 0.38 × 0.48 mm was selected for structure determination and mounted on a microloop. Full details on the data collection, structure refinement, and structural data are given in the Supporting Information (Tables S1−S4 and Figure S1). Crystal data were deposited in the Cambridge Crystallographic Data Centre with number 1914264. Powder X-ray Diffraction (PXRD). PXRD patterns were collected using a Bruker D2 Phaser diffractometer operated at 30 kV and 10 mA. Cu Kα radiation (λ = 1.5418 Å) was employed. A nickel filter was used to remove Cu Kβ. Diffractograms were collected in the 10°−60° 2θ range using a step size of 0.025° and a step time of 0.5 s. Thermal Analysis (TGA/DTA). Simultaneous thermogravimetric and differential thermal analyses were conducted using an SDT2960 TGA-DTA analyzer (TA Instruments). Operating conditions for data collection are described elsewhere.8 Transmission Electron Imaging (TEM). TEM images were obtained using a JEOL JEM2010F (JEOL Ltd.) electron microscope operated at 200 kV. The procedure for sample preparation is described elsewhere.8 Size distribution histograms were obtained by counting 300 nanocrystals.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Elemental analyses of Er, Yb, and Ba in Er:Yb:BaFCl nanocrystals were performed using a 7700 Series ICP-MS (Agilent Technologies). 3−4 mg of powder was dissolved in 20 mL of aqua regia at 65 °C. Erbium (998 ± 4 μg mL−1, Fluka), ytterbium (1000 μg mL−1, High Purity Standards), and barium (1000 ± 2 mg L−1, Fluka) in 2% HNO3 were used as standards. Synchrotron X-ray Total Scattering. X-ray total scattering data were collected at the 11-ID-B beamline of the Advanced Photon Source at Argonne National Laboratory. An incident photon energy of 86.72 keV (λ = 0.143 Å) was employed. Scattering data were collected in transmission mode. Rietveld Analysis. Rietveld analysis was performed using the GSAS/EXPGUI software suite.21,22 Details on structural refinement are given in the Supporting Information. Pair Distribution Function Analysis (PDF). RAD23 was employed to extract pair distribution functions (G(r)) from the raw scattering data. A Qmax of 22 Å−1 was employed in the Fourier transform of normalized structure functions S(Q). Structural refinements were performed using PDFgui;24 details are given in the Supporting Information. X-ray Absorption Spectroscopy. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were collected at the 20-BM-B line of the Advanced Photon Source at Argonne National Laboratory. Spectra were collected in transmission mode at the Ba L3 edge (5247.78 eV). The collection range extended from 5097 to 5597 eV. Extraction and analysis of XANES and EXAFS spectra were performed using Athena and Artemis;25 full details are provided in the Supporting Information. Spectrofluorometry. Spectrofluorometric analyses were conducted using a Fluorolog 3-222 fluorometer (Horiba Scientific). A PSU-III-LED continuous-wave 980 nm laser (Opto Engine, LLC) was employed as the excitation source. Steady-state spectra were collected between 370 and 750 nm using a slit width of 1 nm. Time-dependent spectra were collected by operating the laser in pulsed mode. All spectra were acquired at room temperature using an excitation power density of ≈1.6 W cm−2.

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RESULTS AND DISCUSSION Ba5(CF2ClCOO)10·7H2O crystallizes in the P212121 orthorhombic space group, and its crystal structure is shown in Figures 1a and 1b. This solid belongs to the family of extended inorganic hybrids26 in which infinite barium−organooxygen− barium connectivity extends in three dimensions. Our group has reported and described several mono- and bimetallic trifluoroacetates exhibiting this type of metal connectivity.19,20,27 The observation of a similar structural pattern in a chlorodifluoroacetate underscores the ability of halogenated monocarboxylates to serve as bridging ligands to build extended hybrids. Five different barium atoms are present in the structure and feature coordination numbers ranging from 8 to 11. Coordinating atoms include oxygen and fluorine belonging to the carboxylato and chlorodifluoromethyl groups, respectively. Irregularly shaped Ba(O,F)n polyhedra are connected through corner-, edge-, and face-sharing of oxygen atoms to yield a three-dimensional framework in which channels are observed along the [100] and [101] directions. These channels are lined with the chlorodifluoromethyl groups, and because of the size of the chlorine atom, little void space is left (ca. 1.4% of the total unit cell volume, according to PLATON28). Thermal analysis and X-ray diffraction were employed to probe the thermolysis of Ba5(CF2ClCOO)10·7H2O in the solid state under a nitrogen atmosphere; the corresponding results are shown in Figures 1c and 1d. The observed total weight loss (≈53.7 wt %) is in good agreement with that expected for the quantitative decomposition to BaFCl (55.2 wt %). PXRD confirms that BaFCl is B

DOI: 10.1021/acs.chemmater.9b02384 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

with a nominal rare-earth concentration of 6.00 mol %. Diffraction patterns and microscopy images for other compositions are provided in Figures S4 and S5. All the diffraction maxima are indexed to the tetragonal P4/nmm space group of the fluorochloride phase (PDF No. 024-0096). No secondary crystalline phases are observed. Er:Yb:BaFCl nanocrystals exhibit spherical shape and average diameters in the 18.7−21.5 nm range. The nominal rare-earth concentration has no significant effect on the phase purity and morphology of the nanocrystals. Experimental rare-earth concentrations were determined using ICP-MS elemental analysis. Total ([Er] + [Yb]), ytterbium ([Yb]), and erbium concentrations ([Er]) are plotted in Figure 2c as a function of their nominal values; numerical results are provided in Table S5. The total rare-earth concentration increases linearly up to ≈5.90 mol %. Above this value, saturation is observed in samples containing a nominal rare-earth concentration of 7.50 mol %. Similar trends are observed for ytterbium and erbium. The solubility limits of ytterbium and erbium in BaFCl can therefore be estimated to be ≈5.30 and 0.60 mol %, respectively; similar values were reported by our group for ytterbium and erbium doped into SrFCl.8 Charge compensation may occur through incorporation of oxide anions in the host lattice.29−32 For samples with nominal rareearth concentrations in the 0.5−6 mol % range, experimental ytterbium-to-erbium ratios ranging from 7.1 to 9.4 are observed, in good agreement with the target value of 9.0. For these nanocrystals, calculated doping efficiencies for ytterbium and erbium are >90%. As expected, doping efficiencies drop to ≈80 (Yb) and 75% (Er) in the case of nanocrystals with a nominal concentration of 7.5 mol % (i.e., above the saturation threshold). Altogether, results from chemical and morphological analyses demonstrate the utility of chlorodifluoroacetates as precursors for the colloidal synthesis of mixed halides. These precursors offer the advantage of having the two target halogens built in, eliminating the need for external single halogen sources. In addition, cleavage of the carbon−halogen bonds occurs at temperatures compatible with those typically employed in colloidal synthetic routes. The average and local atomic structures of Er:Yb:BaFCl nanocrystals were probed using X-ray total scattering and XAFS spectroscopy. Results from structural analyses are summarized in Figure 3 for nanocrystals featuring a 3.00 mol % nominal rareearth concentration. Results for other compositions are given in the Supporting Information (Figures S6−S10 and Tables S6 and

Figure 1. Crystal structure of Ba5(CF2ClCOO)10·7H2O viewed along the (a) [100] and (b) [101] directions. The unit cell in (a) is depicted with solid lines. Only Ba(O,F)n polyhedra are shown in (b). Atom splitting in disordered positions has been omitted for clarity; only major occupancy sites are shown. (c) Thermogravimetric and differential thermal analysis of Ba5(CF2ClCOO)10·7H2O; the total weight loss is indicated. (d) Indexed PXRD pattern of the solid-state decomposition product of Ba5(CF2ClCOO)10·7H2O.

indeed the decomposition product. All diffraction maxima are indexed to the fluorochloride phase (PDF No. 024-0096); no secondary crystalline phases are observed. Thermolysis of Ba5(CF2ClCOO)10·7H2O occurs in two steps. First, coordinated water molecules are vaporized; this endothermic reaction occurs at ≈156 °C. Next, decomposition of the organic ligands and crystallization of BaFCl take place; these reactions start at ≈240 °C and are completed at ≈253 °C. Control experiments demonstrated that the thermal decomposition profile does not change in the presence of CF3COOH and of minor amounts of rare-earth ions such as Er3+ and Yb3+ (see Figures S2 and S3). Ba5(CF2ClCOO)10·7H2O was used as a precursor for the solution phase synthesis of rare-earth-doped BaFCl nanocrystals. A representative PXRD pattern and TEM image are given in Figures 2a and 2b, respectively, for Er:Yb:BaFCl nanocrystals

Figure 2. (a) Indexed PXRD pattern and (b) TEM image of Er:Yb:BaFCl nanocrystals with a nominal rare-earth concentration of 6.00 mol %. A size distribution histogram is shown in the inset of (b). (c) Experimental rare-earth concentrations in Er:Yb:BaFCl nanocrystals as a function of their nominal values. Plots for total ([Er] + [Yb]), ytterbium ([Yb]), and erbium ([Er]) concentrations are shown. Dotted lines are guides to the eye. C

DOI: 10.1021/acs.chemmater.9b02384 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 3. Structural analysis of Er:Yb:BaFCl nanocrystals with a nominal rare-earth concentration of 3.00 mol %. (a) Crystal structure of BaFCl. Backscatterer atoms included in EXAFS fits are labeled. (b) Rietveld fit of the PXRD pattern. Experimental (hollow black circles) and calculated patterns (solid red line) are shown along with the difference curve (solid blue line) and tick marks corresponding to the fluorohalide phase (vertical green bars). (c) Fits of the PDF in the 1.6−8 Å range. Experimental (hollow black circles) and calculated PDFs (solid red line) are shown along with difference curve (solid blue line). (d) Fit of the Fourier transform of the EXAFS spectrum in the 1.5−4.5 Å range. Experimental (hollow black circles) and calculated radial functions (solid black line) are shown.

S7). As shown in Figure 3a, the crystal structure of bulk BaFCl consists of a tetragonal unit cell in which metal centers have four fluorides and five chlorides as nearest neighbors. One of the chloride anions belonging to the MF4Cl5 polyhedron (Clc) sits directly above the metal center. Rietveld and pair distribution function analyses of X-ray scattering data shown in Figures 3b and 3c demonstrate that the average and local crystal structures of the nanocrystals are adequately described by a bulk-type model. Structural models derived from Rietveld refinements were used as a starting point for XANES and EXAFS analyses of the local atomic environment of barium atoms. A downshift in the Ba L3 absorption edge in the XANES spectra of Er:Yb:BaFCl nanocrystals relative to bulk BaF2 confirms the partial substitution of fluoride by chloride (see Figure S9). Experimental and calculated EXAFS spectra given in Figure 3d show that an adequate fit to the experimental radial distribution function χ(r) can be obtained in the 1.5−4.5 Å range using a bulk-type model as a starting point. Inspection of metal− halogen bond distances defining the MF4Cl5 polyhedron indicates a local expansion of the polyhedron along the c-axis (see Table S6). Structural analyses were validated using bond valence analysis. Bond valence sums computed using structural models derived from Rietveld, PDF, and EXAFS analyses yield barium valences in the 2.20−2.28 v.u. range. The ability of Er:Yb:BaFCl nanocrystals to perform NIR-tovisible light upconversion was probed using steady-state and time-resolved spectrofluorometry. A representative emission spectrum collected under 980 nm excitation is shown in Figure 4a for nanocrystals featuring a 6.00 mol % nominal rare-earth concentration. Results for other compositions are given in the Supporting Information (Figure S11). Three major emission bands arising from intraconfigurational f−f transitions of the Er3+ activator are observed upon excitation of the Yb3+ sensitizer. These transitions are 2H11/2 → 4I15/2 (525 nm, green), 4S3/2 → 4 I15/2 (545 nm, green), and 4F9/2 → 4I15/2 (660 nm, red). The integrated intensities of the corresponding emission bands increase with the total rare-earth concentration, indicating that concentration quenching does not occur (see Figure S12). Power-dependent measurements are consistent with twophoton upconversion processes for all three emission bands (see Figure S13). A very weak emission band centered at ≈410 nm is also observed. This emission corresponds to the 2H9/2 → 4 I15/2 transition of Er3+, and its appearance indicates that threephoton upconversion occurs in Er:Yb:BaFCl nanocrystals. The spectral distribution of the upconverted photons is characterized

Figure 4. (a) Emission spectrum of Er:Yb:BaFCl nanocrystals with a nominal rare-earth concentration of 6.00 mol % under 980 nm excitation (≈1.6 W cm−2). Included as insets are a magnified emission spectrum showing a violet band and a digital picture of the sample under NIR illumination. (b, c, d) Lifetimes of long- and short-lived Er3+ emitters in BaFCl and percent contributions to the emission intensities at 525, 545, and 660 nm as a function of the experimental rare-earth concentration. Dotted lines are guides to the eye.

by a red-to-green ratio in the 10−12 range and a yellow emission visible to the naked eye. Red-to-green ratios were computed as the quotient between the integrated intensity of the band centered at 660 nm and the sum of the integrated intensities of the bands centered at 525 and 545 nm. No dependence of the spectral distribution on the rare-earth concentration and the D

DOI: 10.1021/acs.chemmater.9b02384 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials excitation density is observed within the 0.5−2.3 W cm−2 range. Nanocrystals featuring a 4.50 mol % rare-earth concentration are the sole exception to this trend. For this composition, a red-togreen ratio equal to ≈25 is consistently observed for different batches; accordingly, these nanocrystals show orange red emission. Insight into the distribution and room-temperature excited-state dynamics of the Er3+ emitters in the BaFCl lattice was gained using time-resolved spectrofluorometry. Luminescence decays arising from the relaxation of the 2H11/2 (525 nm), 4 S3/2 (545 nm), and 4F9/2 (660 nm) states of Er3+ were fit with the biexponential function given by eq 1.

°C. The chlorodifluoromethyl group acted as a built-in dual source of chlorine and fluorine, eliminating the need for external chlorinating and fluorinating agents. Solution thermolysis in the presence of Yb3+−Er3+ sensitizer−activator pairs yielded Er:Yb:BaFCl upconverting nanocrystals. Nanocrystals featured spherical shape with an average diameter of ≈20 nm and rareearth concentrations up to ≈6.00 mol %. The excited-state dynamics of Er3+ emitters showed a well-defined dependence on the rare-earth concentration. Altogether, these findings show that heterohaloacetates serve as dual halogen sources to access mixed-halide optical materials at temperatures compatible with solution processing. The commercial availability of monocarboxylic acids featuring heterohalomethyl groups with varying ratios of the two halogens should make our synthetic strategy of general applicability to a broad range of mixed halides. Additionally, the ability the of halogenated monocarboxylates to bridge metals with distinct electronic and steric requirements19,27 should enable the preparation of hybrid precursors for mixed-metal mixed-halide materials. A multiplicity of synthetic levers is therefore at hand to further expand the chemistry presented in this article.

I(t ) = A + Along exp( −t /τlong) + A short exp(−t /τshort) (1)

Here, I(t) is the total luminescence intensity at time t, and A, Along, and Ashort are constants. τlong and τshort are the lifetimes of the long- and short-lived emitters, respectively. The fractional contribution of each emitter to the total luminescence intensity was computed via eqs 2 and 3. C long = Cshort

τlongAlong τlongAlong + τshortA short

τshortA short = τlongAlong + τshortA short

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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.chemmater.9b02384. (1) Experimental details on structural analysis, (2) structural determination and crystal data of Ba5(CF2ClCOO)10·7H2O, (3) additional thermal analyses of Ba5(CF2ClCOO)10·7H2, and (4) additional analyses of the chemical composition, morphology, structure, and luminescence of Er:Yb:BaFCl nanocrystals (PDF) Crystallographic data of Ba5(CF2ClCOO)10·7H2O (CIF)

(3)

Results from time-resolved luminescence analysis are summarized in Figures 4b−d, where the lifetime and fractional contribution of each emitter are plotted as a function of the experimental rare-earth concentration. Decay curves, biexponential fits, and numerical results are given in the Supporting Information (Figure S14 and Table S8). The presence of longand short-lived Er3+ emitters in BaFCl nanocrystals is in line with previous studies of rare-earth-doped alkaline-earth fluorohalide upconverters.8,33 Emission from both activators is observed across the entire doping range studied in this work. Their lifetimes and fractional contributions to the luminescence emission, however, depend on the rare-earth concentration. Lifetimes of the 2H11/2, 4S3/2, and 4F9/2 states of both emitters decrease upon increasing concentration. Lifetimes of the longlived emitter drop from ≈600 to 370 μs (2H11/2 and 4S3/2, green emission) and from ≈680 to 550 μs (4F9/2, red emission). For the short-lived emitter, lifetimes decrease from ≈170 to 80 μs (2H11/2 and 4S3/2) and from ≈180 to 140 μs (4F9/2). With regard to the fractional contributions of each type of Er3+ emitter, we note that luminescence emission is dominated by long-lived emitters at low concentrations and by short-lived emitters at high concentrations. The contribution of short-lived emitters increases from 23 to 66% (2H11/2, 525 nm), 11 to 80% (4S3/2, 545 nm), and 34 to 80% (4F9/2, 660 nm). These findings indicate that the excited-state dynamics of Er3+ emitters change in a systematic fashion upon increasing the total rare-earth concentration in the BaFCl lattice. This observation along with the nearly quantitative incorporation of rare-earth dopants demonstrate that solution thermolysis of Ba5(CF2ClCOO)10· 7H2O provides access to mixed-halide upconverting nanocrystals with well-defined composition and luminescence response.

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AUTHOR INFORMATION

Corresponding Author

*(F.A.R.) E-mail [email protected]. ORCID

Leopoldo Suescun: 0000-0002-7606-8074 Federico A. Rabuffetti: 0000-0001-8500-3427 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Science Foundation (DMR-1606917) and of Wayne State University. They also thank the Lumigen Instrument Center at Wayne State University for the use of the X-ray diffraction (National Science Foundation MRI-1427926) and electron microscopy facilities (National Science Foundation MRI0216084). This research used the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357.

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CONCLUSIONS A new organic−inorganic hybrid precursor to mixed halides was synthesized, and its crystal structure and reactivity were probed. Thermolysis of Ba5(CF2ClCOO)10·7H2O in the solid state and in solution led to phase-pure BaFCl at temperatures below 300

REFERENCES

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DOI: 10.1021/acs.chemmater.9b02384 Chem. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.chemmater.9b02384 Chem. Mater. XXXX, XXX, XXX−XXX