Synthesis and Characterization of Solvothermal Processed Calcium

Dec 10, 2013 - An evaluation of calcium tungsten oxide (CaWO4) nanoparticles' properties was conducted using the powders generated from an all-alkoxid...
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Synthesis and Characterization of Solvothermal Processed Calcium Tungstate Nanomaterials from Alkoxide Precursors Timothy J. Boyle,* Pin Yang, Khalid Hattar, Bernadette A. Hernandez-Sanchez, Michael L. Neville, and Sarah Hoppe Sandia National Laboratories, Advanced Materials Laboratory, 1001 University Boulevard Southeast, Albuquerque, New Mexico 87106, United States S Supporting Information *

ABSTRACT: An evaluation of calcium tungsten oxide (CaWO4) nanoparticles’ properties was conducted using the powders generated from an all-alkoxide solvothermal (SOLVO) route. The reaction involved a toluene/pyridine mixture of tungsten(V) ethoxide ([W(OEt)5]) with calcium bis(trimethyl silyl) amide ([Ca(N(Si(CH3)3)2]) modified in situ by a series of alcohols (H-OR) including neo-pentanol (HOCH2C(CH3)3 or H-ONep) or sterically varied aryl alcohols (H-OC6H3R2-2,6 where R = CH3 (H-DMP), CH(CH3)2 (HDIP), C(CH3)3 (DBP))]. Attempts to identify the intermediates generated from this series of reactions led to the crystallographic identification of [(OEt)4W(μ-OEt)2Ca(DBP)2] (1). Each different SOLVO generated “initial” powder was found by transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD) to be nanomaterials roughly assigned as the scheelite phase (PDF 00-041-1431); however, these initial powders displayed no luminescent behavior as determined by photoluminescence (PL) measurements. Thermal processing of these powders at 450, 650, and 750 °C yielded progressively larger and more crystalline scheelite nanoparticles. Both PL and cathodoluminescent (CL) emission (422−425 and 429 nm, respectively) were observed for the nanomaterials processed at 750 °C. Ion beam induced luminescence (IBIL, 478 nm) appeared to be in agreement with these PL and CL measurements. Further processing of the materials at 1000 °C, led to a coalescence of the particles and significant improvement in the observed PL (445 nm) and CL measurements; however, the IBIL spectrum of this material was significantly altered upon exposure. These data suggest that the smaller nanoparticles were more stable to radiation effects possibly due to the lack of energy deposits based on the short track length; whereas the larger particles appear to suffer from radiation induced structural defects. KEYWORDS: scintillators, calcium tungstates, alkoxide, luminescent, ion beam

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INTRODUCTION Self-activated, tungstate-based (WO4−2), ceramic materials have found widespread use as scintillator detectors for a variety of applications. The different cocations (i.e., Ca, Pb, Cd) employed in conjunction with this anion have been found to alter the emission and excitation properties of the final material. These phosphors are isomorphous, adopting the tetragonal dipyramidal scheelite phase. Of these fluorescent materials, we are interested in the widely used, blue emission, X-ray intensifying CaWO4 material due to its large specific gravity (6.1) and X-ray absorption coefficient that leads to a high stopping power, and potential radiation hardness. Understanding the changes wrought in the CaWO4 material’s properties upon entering the nanoregime was the focus of this investigation,1−3 especially details concerning the radiation stability of the smaller particles. The most common solid-state preparative routes to CaWO4 involve the high temperature cofiring of WO3 and CaCO3 in air at 1100 °C,4 yielding mesoscaled powders that possessed a blue emission ranging from 415 to 420 nm.4,5 Lower temperature solution routes are © 2013 American Chemical Society

valued since large-scale preparations coupled with more control over the size and shape of the final CaWO4 particles is possible. Numerous aqueous routes to a variety of CaWO4 nanomaterials have been disseminated.1−3,6−12 However, reports on organic solvent solution routes to WO42− nanomaterials have been limited, 2,13−16 including tungsten hexacarbonyl ([W(CO) 6 ]), 13,14 tungsten(IV) chloride ([WCl4 ]), 2,15 and tungsten(VI) ethoxide ([W(OEt)6])16 precursors. Our previous work on luminescent nanomaterials of WO42‑ species focused on developing solution precipitation and solvothermal (SOLVO) routes to nanomaterials of MWO4 (M = Ca, Sr, Ba, Pb, Mn, Fe, Zn) using metal alkoxide, amide, and alkyl precursors in organic solvents as a means to impart more control over the final morphologies.16 In particular, we successfully generated scheelite nanorods of CaWO4 using a solution precipitation route that involved the thermal Received: August 2, 2013 Revised: October 18, 2013 Published: December 10, 2013 965

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it has undergone the SOLVO processing conditions listed above. The yields for each of these SOLVO reactions approached 95%; however, this high value is attributed to residual C content. The individual powders were then heated sequentially at 450, 650, and 750 °C for 1 h by placing them in a ceramic boat under the circumjacent atmosphere. After thermal processing at 450 °C, the yields approximated 84% but the powder was still lightly brown. After 650 °C, the now off-white powders resulted in an 81% yield after which it remains fairly constant in weight following higher heat treatments. After each of the heat treatments, the powders were collected and the necessary analytical data were amassed prior to the next heating schedule. For the 1000 °C sample, due to the limited samples size available after firing, individual DBP, DMP, and ONep powder samples were combined for analytical analyses (i.e., IBIL and CL). [(OEt)4W(μ-OEt)2Ca(DBP)2] (1). In a glovebox under an argon atmosphere, [Ca(N(Si(CH3)3)2] (0.44 g, 1.2 mmol) was added to a vial containing a stirring mixture of [W(OEt)5] (0.50 g, 1.2 mmol) dissolved in toluene. After 10 min, H-DBP (0.51 g, 2.4 mmol) was added and the reaction turned from a pale yellow to a dark red-orange solution and allowed to stir for 24 h. After this time, the reaction mixture was allowed to set with the cap loose and any volatile material was allowed to slowly evaporate until X-ray quality crystals were isolated. The crystals consisted of a set of red-orange (major) and clear, colorless (minor) crystals. The red-orange crystals were identified as [(OEt)4W(μ-OEt)2Ca(DBP)2] (1); however, full analysis of this compound was not realized, since direct synthesis/characterization of 1 was never achieved. Attempts to obtain a useful crystal structure from the colorless crystal were not successful.

decomposition of [W(OEt)6] and [Ca(N(SiMe3)2)2]2 in a trioctylamine/oleic acid solution or in benzyl alcohol.16 Since that time, little additional information concerning solution routes to these materials have been reported. The purity of the CaWO4 nanomaterials used is critical because the inclusion of as little as 0.5 ppm impurity for any number of metals will quench the desired luminescent properties.4 Therefore, high purity and highly soluble compounds with low decomposition temperatures were of interest. This led us to explore a SOLVO alkoxide precursor route for the production of CaWO4 materials.16 The use of the commercially available oil W(V) ethoxide ([W(OEt)5]) was favored due to its high solubility. Commercially available Ca precursors were of limited utility due to their compositional purity, hydrate solvation, or solubility, which led to the use of freshly synthesized calcium bis(trimethylsilyl amide) ([Ca(N(Si(CH3)3)2]). Following the mixing of [W(OEt)5] with [Ca(N(Si(CH3)3)2] an alcoholysis reaction was undertaken using a set of alcohols (HOR = H-OCH2C(CH3)3 (H-ONep) and H-OC6H3R2-2,6 (R = CH3 (H-DMP); CH(CH3)2 (HDIP); C(CH3)3 (H-DBP)). These alcohols were selected based on our previous experience with these ligands because of their propensity to both crystallize (for identification) and cleanly transform (low percentages of carbon residue) to the oxide. The reaction (eq 1) was thought to generate the mixed metal coordination complex, and these solutions were used for production of nanomaterials under SOLVO conditions. The resulting particles were analyzed by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), cathodoluminescence (CL), photoluminescence (PL), and ion beam induced luminescence (IBIL). The IBIL studies were used to explore the initial spectral response of these nanomaterials to ion irradiation.

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INSTRUMENTATION Single Crystal X-ray Diffraction. A crystal was mounted onto a plastic loop from a pool of Fluorolube and immediately placed in a cold N2 vapor stream, on a Bruker AXS diffractometer equipped with a SMART 1000 CCD detector using graphite monochromatized MoKα radiation (λ = 0.7107 Å). Lattice determination and data collection were carried out using SMART Version 5.054 software. Data reduction was performed using SAINTPLUS Version 6.01 software and corrected for absorption using the SADABS program within the SAINT software package. The structure was solved by direct methods that yielded the heavy atoms, along with a number of the lighter atoms. Subsequent Fourier syntheses yielded the remaining light-atom positions. The hydrogen atoms were fixed in positions of ideal geometry and refined using the APEX II suite of software. The final refinement of each compound included anisotropic thermal parameters for all non-hydrogen atoms. All final CIF files were checked at http://www.iucr.org/. Additional information concerning the data collection and final structural solutions can be found by accessing CIF files through the Cambridge Crystallographic Data Base. Data collection parameters for 1 are given in Table 1. Raman Spectroscopy. Spectra were collected at room temperature on a ThermoScientific DXR SmartRaman equipped with a DXR 780 nm Laser/Filter system. Sample powders were placed in an NMR tube and analyzed between 4000 − 50 cm−1. Powder X-ray Diffraction (PXRD). Powder X-ray diffraction (XRD) was performed by a PANalytical X’Pert Pro diffractometer employing CuKα radiation (1.5406 Å) and a hgh speed RTMS X’Celerator detector in the 2θ range of 10− 100° at a scan rate of 0.15 deg/s or 0.024 deg/s on nanomaterials using a zero background holder. The XRD patterns were analyzed using JADE 9 software and indexed

[W(OEt)5 ] + [Ca(N(Si(CH3)3 )2 ] + 2H‐OR → [(OEt)3 W(μ‐OEt)2 Ca(OR)2 ] + 2H‐N(Si(CH3)3 )2 (1)

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OR = ONep, DMP, DIP, DBP

EXPERIMENTAL SECTION

All compounds described below were handled with rigorous exclusion of air and water using standard Schlenk line and glovebox techniques. All solvents were used as received (Aldrich) in Sure/Seal bottles and stored under argon, including pyridine (py), toluene, and tetrahydrofuran (THF). The following chemicals were used as received (Aldrich) and stored under argon: CaI2, [K(N(Si(CH3)3 )2], [W(OEt) 5 ], py, H-ONep, H-DMP, H-DIP, and H-DBP. [Ca(N(Si(CH3)3)2] was synthesized from the reaction of CaI2 and two equivalents of [K(N(Si(CH3)3)2] in THF.16 Nanoparticle Synthesis. In a glovebox, one equivalent of the offwhite powder [Ca(N(Si(CH3)3)2] (0.44, 1.2 mmol) was added to a toluene (∼10 mL) solution containing one equivalent of [W(OEt)5] (0.50, 1.2 mmol) in a 45 mL Parr Bomb Teflon liner. To this, two equivalents of the desired H-OR [OR = ONep (0.22 g, 2.4 mmol), HDMP (0.30 g, 2.4 mmol), H-DIP (0.43 g, 2.4 mmol), H-DBP (0.59 g, 2.4 mmol)] were added, followed by a dilution with pyridine (∼10 mL). The sample was sealed, removed from the glovebox, transferred to an oven and heated for 24 h at 185 °C. After this time, the sample was allowed to cool to room temperature. The resulting mixture was centrifuged and washed with iso-propanol three times under ambient conditions. These washed tan powders were dried in air and are identified as our initial (unprocessed) material. To help with sample identification and distinguish the precursor from the processed powder, we have added a “Δ” in front of the ligand name to indicate 966

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same sample was exposed to extended irradiation exposures with the spectra recorded constantly.

Table 1. Data Collection Parameters for 1 chemical formula formula weight temp (K) space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd (Mg/m3) μ,(Mo, Kα) (mm−1) R1 (%) (all data) wR2 (%) (all data)

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C40H69CaO8W 901.88 173(2) P2(1)/n 15.2041(7) 17.4654(8) 18.0065(8) 112.604 4414.2(3) 4 1.357 2.778 3.37 (6.77) 8.53 (10.76)

RESULTS For this project, determination of the luminescent properties of solution generated CaWO4 nanomaterials was of interest. A simple system with high purity precursors was desired, since even small levels of alternative cation impurities will quench the desired properties.4 It was decided that a SOLVO route to CaWO4 that employed an all-alkoxide precursor route would meet these demands. Four samples were generated using [W(OEt)5] and in situ generated [Ca(OR)2] where OR = ONep, DMP, DIP, and DBP. For select analytical data, representative figures are shown, with the full set of analytical data available in the Supporting Information. Solution Chemistry. Prior to the nanomaterials synthesis, we attempted to understand the solution chemistry of the various alkoxide precursor solutions. After stirring for 12 h at glovebox temperatures, the volumes of the reaction from eq 1 were drastically reduced and then set aside. For every system investigated, except for the DBP ligand, oils were isolated. For the DBP system, two different colored crystals were formed: red-orange (major) and clear (minor). The red-orange crystals were found by single crystal X-ray diffraction to be [(OEt)4W(μ-OEt)2Ca(DBP)2] (1) shown in Figure 1. The W adopts a

a R1 = Σ||Fo| − |Fc||/Σ|Fo| × 100. bwR2 = [Σw(Fo2 - Fc2)2/Σ (w | Fo|2)2]1/2 x 100

using International Center for Diffraction Data’s (ICCD) Powder Diffraction File PDF-2 2010. Transmission Electron Microscopy (TEM). An aliquot of the nanopowder dispersed in chloroform was placed directly onto a holey carbon type-A, 200 mesh, copper TEM grid purchased from Ted Pella, Inc. The aliquot was then allowed to dry. The resultant particles were examined in bright-field using a Philips CM 30 TEM operating at 300 kV accelerating voltage and equipped with a Thermo Noran System Six Energy Dispersive X-ray (EDX) System. Photoluminescence (PL) Measurements. The photoexcitation and emission spectra of nano-CaWO4 powders were measured by a standard fluorometer (PTI, QuantaMaster), using a Xe arc lamp as the light source, coupled with a double monochromator on the excitation side and a single monochromator on the emission side. PL spectra were collected at 0.25 nm per step for an accumulation time of 0.1 s. Both excitation and emission spectra were corrected with respect to the fluctuation of the light source and quantum efficiency of the photomultiplier tube. Cathodoluminescence (CL) Measurements. The prepared nano-CaWO4 powders were pressed into pellets (∼2 mm × 0.5 mm) and mounted on a SEM stub with conduction carbon tape. The CL responses were collected on an SEM equipped with optical cathodoluminescence microscope (Gatan MonoCL4). Light induced by the electron excitation (10 keV, 1.5 nA) was collected by an elliptical mirror and transferred out by fiber optics, separated by a monochromator at 0.5 nm/step, and then detected with a photomultiplier tube. Each CL spectroscopic response was collected for 30 s. Ion Beam Induced Luminescence (IBIL) and Radiation Tolerance Analyses. The IBIL and radiation tolerance analyses of the various nanoscintillators were performed using a 3 MV National Electrostatic Corporation (NEC) highbrightness 3UH-2 Pelletron machine with a new light ion microbeam chamber including in situ optical capabilities.17 The IBIL spectra were taken during the first exposure to a 3 MeV proton beam of approximately 3 mm2 on the pressed pellets. The dose rate of the irradiation was tailored for each sample in order to obtain reasonable luminescence for the Andor Shamrock SR-303i-B detector/spectrometer through the custom OM-150 triplet from Oxford Microbeam Ltd. located in the microbeam chamber. For the initial investigation of the radiation tolerance of the nanomaterials, a pristine region of the

Figure 1. Structure plot of 1. Thermal ellipsoids are drawn at the 30% level. Hydrogen atoms have been removed for clarity.

distorted octahedral geometry; whereas, the Ca was solved in a distorted tetrahedral arrangement. The metrical data indicates that the W···Ca distance is 3.56 Å. There are no alkoxide structures reported that contain both a W and Ca metal center. Of the 9 structures reported18 that contain both a Ca and a W atom in the same crystal structure, most (4 structures) are salts with no metal interactions present,19−22 two others use cyano ligands to link the metals,23,24 and the remaining three structures consist of k(2/3 −3)-(EDT-TTF) 8 [Ca(H2O)4]2(TeW6O24)·7H2O225,26 (EDT-TTF = ethylenedithiotetrathiafulvalenium) and [(CH 3 ) 4 N] 3.5 H 1.5 [PW 11 O 39 Ca(H2O2)2]·3H2O,27 and {[H(H2O)2]2[Ca(HINO)4(H2O)5(PW12O40)]}n where HINO = isonicotinic acid N-oxide. None of these represent acceptable models for 967

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are no reasonable models available, so for similar homoleptic species, the (μ-OR)−Ca−(μ-OR) (63.09°) and (μ-OR)−W− (μ-OR) (77.26) angles of 1 are smaller.18 Because of the novelty of 1, the W−(μ-OR)−Ca (av 109.5°) is unique. Attempts were made to analyze the colorless crystals; however, no meaningful data was obtained that would aid in identifying this material but they are assumed to be the Ca(OAr)2 species, since the W(OAr)x are prone to colored crystals. Because of the presence of two types of crystals, it was not possible to establish the full analytical data on either complex. However, based on the unusual complex observed at room temperature, several fundamental chemical changes have occurred. First, the W(V) metal appears to be oxidized to W(VI), which is not unusual/unexpected. It should be noted that the W(V) may persist since the W−O distances of W(V) and W(VI) cover similar ranges noted for the structure (1.8 −2.1 Å).18 For this to occur, one of the ligands must be an HOEt but this could not be distinguished metrically and no evidence of a Q-peak representing a H was located near any O atoms. The H may be disordered over the entire molecule but the high quality of the structure leads us to believe that the W(V) to W(VI) oxidation has occurred, which necessitates something being reduced. Since it is unlikely that the Ca(II) was reduced under these conditions, then it was assumed that the W(V) was both oxidized to W(VI) generating 1 and reduced to form an unidentified complex. If this occurs, then a fraction of the W(OEt)5 and the [Ca(OR)2] does not react in the same manner. Tungsten-based alkoxides tend to be colored,

the unusual complex 1 since the W−O−Ca interaction was through a nonalkoixde ligand or a W = O moiety. Tables 1 and 2, respectively list the data collection parameters and select Table 2. Metrical Data for 1 distance (Å) Ca−OR W−OR Ca−(μ-OR) W−(μ-OR) Ca···W

(av) (av) (av) (av) (av)

2.12 1.87 2.37 1.98 3.56

angle (deg) OR−Ca−OR OR−Ca−(μ-OR) (μ-OR)−Ca−(μ-OR) OR−W−OR (OR)−W−(μ-OR) (μ-OR)−W−(μ-OR) Ca−(μ-OR)−W

111.9 104.61−133.35 63.09 (av) 90.8 90.87−167.52 77.26 (av) 109.5

metrical data for 1. The Ca-OR distances are in agreement with other [Ca(OR)2] compounds, with the bridging Ca-OR distances (av 2.37 Å) being larger than those of the terminal Ca-OAr (av 2.12 Å). The W−OR distances are also in-line with the observed literature [W(OR)6] species: W−(μ-OEt) (av 1.98 Å) distance being longer than the terminal W−(OEt) (av 1.87 Å). A comparison of the angles is more difficult since there

Figure 2. Representative PXRD of CaWO4 from the Δ-DMP system (a) 450, (b) 650, (c) 750, and (d) 1000 °C. 968

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Figure 3. Raman spectra of Δ-DBP derivative after SOLVO treatment at (a) initial, (b) 450 °C, (c) 750 °C, and (d) 1000 °C.

Figure 4. TEM images for Ca−W−OR system OR = (a) Δ-ONep, (b) Δ-DMP, (c) Δ-DIP, and (d) Δ-DBP versus temperature (i) initial, (ii) 450 °C, (iii) 650 °C, and (iv) 750 °C. Scale bar 100 nm same for each figure.

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Figure 5. PL response for CaWO4 nanomaterials: (a) initial materials prepared by different H-OR, (b) the evolution of PL responses for as dried, 450°C, and 650°C Δ-DIP samples, and (c) powders processed at 650°C, (d) PL response for bulk CaWO4 materials (650/1h).33

processed powder, we have added a ‘Δ’ in front of the ligand name (e.g., Δ-ONep) to indicate it has undergone the SOLVO processing. The PXRD patterns were used to examine the long-range ordering of the samples, whereas, the local symmetry of the CaWO4 nanomaterial was probed using Raman spectroscopy. The Raman data for the various samples was collected from 50−4000 cm−1; however, typical ranges reported for Raman spectra of CaWO4 are from 50−1000 cm−1.28−31 It was hoped that these data would elucidate the luminescence behavior dependence on the ordering between Ca2+and WO42‑. The CL, PL, and IBIL data were used to determine the excitation properties of these nanomaterials. The resulting analytical data collected on these samples are discussed below based on the processing temperature: (a) initial, (b) 450, (c) 650, (c) 750, and (d) 1000 °C. Figures 2 and 3 show the PXRD patterns (ΔDMP selected as the representative powder) and Raman spectra (Δ-DBP powder as representative). The TEM images and luminescence data for all of the powders are shown in Figures 4−7, respectively. Initial Samples. After SOLVO processing of the various alcohol derivatives (Δ-ONep, Δ-DMP, Δ-DBP, and Δ-DIP), the materials isolated (referred to as initial) were found to be tan colored powders. The breadth of the PXRD peaks for these powders made conclusive assignments difficult but the peaks were consistent with the scheelite phase (PDF 00-041-1431) of CaWO4, see Supporting Information. No additional crystalline phases could be identified in these samples even after

therefore, it is assumed that the clear crystals were some form of [Ca(OEt)2]in species; however, it is of note that the true identity of this compound may be represented by several other heteroleptic amino or alkoxy arrangements. Unfortunately, all attempts to identify these crystals were not successful. At higher pressures and temperatures, a homogeneous solution is expected that consists of structures that resemble 1. For the smaller ligated species, the core interaction noted for 1 is expected but the overall nuclearity may be substantially varied. Further work to characterize these compounds is underway. However, compound 1 clearly established a close atomistic interaction between the Ca and W that would favor formation of CaWO4 materials (versus phase separation) during processing. Nanomaterials. These type of precursors favor the production of mixed metal oxide species, therefore, we undertook the production of CaWO4 nanomaterials from these solutions under SOLVO conditions. Four samples consisting of [W(OEt)5], [Ca(OR)2], and the HOR (OR = ONep, DMP, DIP, or DBP) were individually mixed in a 50/50 (v/v) solution of pyridine/toluene that was thermally treated for 24 h at 185 °C under SOLVO conditions to produce our initial powders. After this time, the insoluble fraction was isolated by centrifugation and washed with iso-propanol. These different samples were analyzed using (i) PXRD, (ii) Raman, (iii) TEM, (iv) PL, (v) CL, and (vi) IBIL. A full listing of the analytical data collected for all the samples are available in the Supporting Information. To distinguish the precursor from the 970

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be greater than 120 nm (Figure 4iv). The Raman spectra of these powders began to display the characteristic V1(Ag) ≈ 900 cm−1 and V3(Bg) near 830 cm−1, which are associated with the tetragonal phase (Figure 3c). These results are similar to reported values and observations on ceramics prepared from a polymeric precursor method.28 While literature reports for scheelite materials do not typically focus on the full Raman region,28−31 it is of note that several small peaks are present in each spectrum from 1000−2000 cm−1 (Figure 3). These peaks were consistently present throughout these spectra but their intensities begin to decrease relative to the peaks associated with the scheelite phase at the higher processing temperatures. To verify that carbonate and oxide impurities were not present in the spectrum, for powders processed at 750 °C, a comparison of neat calcium carbonate and tungsten(VI) oxide is provided in the Supporting Information. Within this region, broad peaks can also be found in the CaCO3 and WO3 spectra, however their signature peaks found with in the 1000− 50 cm−1 do not align with peaks generated by our sample. Carbon species have also been ruled out as temperatures of 750 °C are typically used to remove organics and carbonates. Finally all three samples did generate fluorescence during the experiment, as indicated by the instrument during analysis, and thus maybe producing these bands in the higher frequency region. The source of these stretches formed at 750 °C is being further explored; however, it has been reported that defect centers in CaWO4 can cause the emission of green and red bands.34 Since the PL response is generated through a direct photoexcitation, the impact that an electron excitation would have on the CaWO4 nanomaterials spectra was also explored. Using cathodoluminescent (CL) excitation, the ion excitation of each of the samples was evaluated. Figure 6a shows a representative CL spectrum for the powders processed at 750 °C. The total number of photons collected from these samples was relatively small compared to the bulk powders;33 however, the emission wavelength of our CaWO4 samples when excited with 10 kV, produce the 423−429 nm emission, which is similar to the PL emission of the powders (i.e., meso, 422 nm;33 nano, 425 nm). This indicates that the ionization process (bulk excitation) comes from the same source (i.e., the undisturbed WO42−). As noted in the literature, the regularity of the WO42− tetrahedra is critical to ensure the luminescent behavior in these scheelite materials.34−36 An alternative ion luminescent excitation can be obtained from the IBIL utilizing proton irradiation, which is often used to provide insight into neutron exposure. The Δ-DIP sample was used for all IBIL analyses due to the larger volume of powder available for this sample. Similar to the PL and CL spectra, the IBIL spectra produced by the CaWO4 scintillators were found to be highly dependent on processing, as can be seen in Figure 7a. When the 750 °C Δ-DIP sample was analyzed, multiple distinct sharp peaks emerged on the right shoulder on the previous broad singular peak that shifted to higher wavelengths. 1000 °C Samples. After processing at 1000 °C, the particles again coalesced, forming the scheelite phase as confirmed by very sharp PXRD patterns (Figure 8a) with calculated crystallite sizes being 30−45 nm in size. The TEM imaged particles that were found to form particles that were larger than 140 nm (Figure 8b). Raman data for these powders finally revealed all the expected vibrational modes associated with the scheelite phase (Figure 3d).28−31 The peak assign-

decreasing XRD scan rates. These broad peaks also made the results from the full-width half-max (fwhm) using the Scheerer equation unreliable. The particles were further characterized by Raman spectroscopy. The spectra were also broad with little indication of local symmetry ordering of the WO42− tetrahedra. Figure 3a shows the Δ-DBP Raman spectrum and was selected as a representative spectrum for these samples. Further analysis of the samples was undertaken using TEM. As can be seen in Figure 4i, the TEM images of the initial materials produced were (a) Δ-ONep 10−50 nm plates, (b) Δ-DMP 1−2 nm irregular, (c) >1 nm irregular, and (d) 1−10 nm irregular. Photoluminescence (excitation (Ex) and emission (Em)) responses for the four different CaWO4 powders revealed broad Ex spectra with a sharp transition at 382 nm with no measurable emission (see Figure 5a). The unexpected sharp transitions noted in the Ex spectra (382 nm or 3.25 eV) are not fully understood but are believed to be related to structural defects in the nanomaterials, such as an oxygen-deficient complex (i.e., WO3).5 At room temperature the width of the valence band is close to 258 nm or 4.8 eV,32 which is consistent with the sudden jump in the Ex spectra noted in the Δ-DIP and Δ-DMP spectra; however, the Δ-DBP excitation spectrum has an onset near 300 nm, which suggests that the band gap of ΔDBP nanomaterials has become smaller. Additional characterization is necessary to fully explain these unusual transitions. In an effort to determine if the lack of PL response at short wavelengths noted for these compounds was due to the particle’s crystallite size or the lack of WO42− ordering, the powders were processed at higher temperatures under atmospheric conditions. 450 °C Samples. In comparison to the initial samples, the PXRD patterns of the 450 °C processed scheelite powders (Figure 2a for Δ-DMP) increased in both the sharpness and intensity of the peaks. Using the Scherer equation, particles composed of crystallite sizes (∼5−7 nm) was calculated while the TEM images revealed that the particles that were slightly larger (10−25 nm, Figure 4ii).The Raman spectra (Figure 3b for Δ-DBP) again revealed no distinguishing vibrational modes and no change was noted for the PL response (Figure 5b). Therefore, no additional studies were performed on these powders. 650 °C Samples. Samples processed at 650 °C yielded slightly more definitive PXRD patterns that matched well with the scheelite phase (Figure 2b). The Scheerer calculated crystallite sizes ranged from 12−18 nm. The Raman spectra of these samples had several broad peaks in the 50 to 1000 cm−1 region. TEM images confirmed that larger particles (Figure 4iii; 30−80 nm) had formed. A PL response was noted only for the Δ-ONep sample, which also had larger particles (30−80 nm, Figure 4iii-a) than the other samples. Otherwise, the PL spectra of the 650 °C samples were similar to those noted previously (Figure 5b); however, there is an additional small and broad Ex peak at 250 nm that appears for these powders. As a point of reference, measurements of bulk crystalline CaWO4 materials revealed a narrow excitation peak at 261 nm and a broad emission peak at 410 nm.33 Because of the lack of a PL response for the majority of samples, other studies on these powders were not pursued. 750 °C Samples. The 750 °C treatment of the powders led to much sharper PXRD patterns (Figure 2d), which confirmed the scheelite phase of CaWO4 (PDF 00-041-1434) had been produced. The calculated crystallite size was determined to be 10−20 nm versus the TEM imaged particles that appeared to 971

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Figure 6. Luminescent properties of CaWO4 powders: (a) CL at 750 °C and (b) PL at 1000 °C.

Figure 7. Ion Beam induced luminescence (IBIL) resulting from a 3 MeV proton beam exposure on CaWO4 (Δ-DIP) pellets produced from pressing nanopowders: (a) initial IBIL spectra as a function of scintillator processing temperature and (b) resulting change in the IBIL spectra as a function of radiation damage for the 1000 °C sample over 1 h.

ments were (in cm−1): 85 (Bg), 115 (Eg), 196 (Eg) 210 (Bg) 339 v2(Ag,Bg) 400 v4(Bg), 797 v3(Eg), 838 v3(Bg), 915 28,30,31 Because of the reduction in the amount of v1(Ag). sample after processing and sample lost during analysis, the ΔONep, Δ-DMP, and Δ-DBP powders were combined in order to have enough sample to obtain meaningful PL measurements. This was deemed acceptable based on the similarity of the PXRD patterns CL, PL, and TEM images at 750 °C. The excitation (Ex 256 nm) continues to move to high angles and the Em peak resides at 445 nm. These values are in line with the bulk powders33 but the intensity of the nanomaterials’ luminosity was still considered weak (Figure 6b). The 1000 °C Δ-DIP sample was kept separate for IBIL experiments. The intensity and number of sharp peaks present in the IBIL spectra (Figure 7) increased significantly in comparison to the 750 °C sample.

Raman spectra which were found to be consistent with the scheelite patterns and possessed PL (Figure 5) and CL (Figure 6) emission maxima that were more in-line with the bulk CaWO4.33 The IBIL data (Figure 7) for the smaller particles (and lower processing temperature) were consistent with the PL and CL spectroscopic measurements; however, the larger particles (1000 °C) revealed unexpected emissions. Additional discussion concerning these results is warranted. Classically, the intrinsic luminescent behavior of the MWO4 systems is attributed to an electron transition within undisturbed WO42− complexes.1,3,5,34−37 From some studies, the more ordered the WO42− core, coupled with a disordered countercation, the better the luminescent behavior.1,3,34−37 The broad peaks of the PXRD patterns are results of the nanosized particles and the possibility of less than ideal ordering of the atoms in the nanocrystalline lattice. This is particularly true for the low temperature processed powders where the WO42− complexes might not be fully developed or ordered because of the local lattice distortion from nanomaterials with a large surface area.38 Therefore, the diminished light yield noted for the low temperature smaller CaWO4 particles may be attributed to the disordering of tetrahedral WO42−, an increase in the band

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DISCUSSION A summation of the results reveals that the smaller, lowtemperature processed nanomaterials (as identified by TEM imaging) yielded broad PXRD (Figure 2) and Raman (Figure 3) patterns. The variation of the calculated Scheerer equation size versus the observed particle size is a reflection of the crystallite size (PXRD) and not the particle size (TEM, Figure 4). The smaller crystallite and particle sizes gave no emission response for the PL or CL measurements. The higher temperature processed materials gave sharper PXRD and 972

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Figure 8. Combined powders (Δ-ONep, Δ-DMP, Δ-DBP) fired at 1000 °C (a) PXRD and (b) TEM.

characterized the donor density and flat band potential may be necessary to unequivocally verify the purity. For the CL spectrum of the CaWO4, a small background emission at 258 nm was observed between 170 and 340 nm. The source of this emission is not clear but it has been previously reported that a 258 nm excitation can cause a sharper blue photon emission at 437 nm for intrinsic CaWO4 materials.34 Additional work on determining the decay time to elucidate the emission mechanism and the different peaks observed is underway. The IBIL uses protons for the excitation source but the mechanism of luminescence must rely on the disturbance of the tetrahedral WO42−.17 Changes in the radiation tolerance of the CaWO4 nanoscintillators that were compressed into pellets can be elucidated from the degradation of the IBIL spectra as a function of dose (Figure 7b). For the smaller, less crystalline material, no effect (based on the emission spectra) from the IBIL exposure was noted. It is of note that SEM images taken of

gap,39,40 which causes a decrease in the number of electron− hole pairs created by external excitation, and the ineffectiveness of energy deposition, which would allow for most of the energy and lattice defects produced to quickly escape without producing the desired cascade excitations. In contrast, the high-temperature processed materials with well-established crystalline structure and larger sizes can effectively excite the WO42− core by the photon, electron, or ion beam. The slight red shift of the nanomaterials emission is believed to be due to the increase in particle size but requires further study. All of these phenomena described above are closely related to the reduction size of these nanomaterials. It is of note that the use of different precursors may have led to donor density variations that could impact the final PL results. While we believe the impurities in the materials would be limited due to high purity precursors ensuring only Ca and W metals are present and the low temperature loss of organic ligand, additional work to 973

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demonstrate less luminescent intensity due to the increasing of lattice disorder and band gap, as well as the reduction of interactions between materials and external excitations. Additional studies to further understand and explore these phenomena are underway.

PbWO4 nanomaterials deposited onto a surface that was exposed to a 3 MeV proton beam for select times, revealed significant nanoparticle ablation over time.41 A similar effect is expected to occur with these CaWO4 particles and experiments are underway to explore this phenomenon, since it may also contribute to the observed degradation of IBIL spectra as a function of time. However, the displacement of atoms induced by an ion beam that produce oxygen vacancies in CaWO4 has been noted previously.28 The IBIL emission peak correlated well with the size of the particles; that is, as the particle size increased, the emission was shifted toward the red: 413 (650 °C), 478 (750 °C), and 486 nm (1000 °C). For the larger CaWO4 particles [treated at 1000 °C (Δ-DIP)], the degradation of the IBIL spectral peak intensity rapidly declines during exposure, forming a new set of distinct sharp peaks in the green around 470 nm and in the red at 541 and 571 nm; however, this is not a uniform process with most of the damage occurring during the initial irradiation step. Despite the new spectra being significantly less bright, the new structure appears to be significantly stable to an ∼10−35 μA of 3 MeV proton beam exposure for 15 min. In situ optical and nearly in situ SEM characterization has suggested that ablation from the nanoparticle surface (vide infra) may in part contribute to the degradation of the spectra; however, it is of note that ablation alone does not adequately explain the observed change in luminescence properties. It is believed that lattice disorder and atom displacements induced by the highenergy proton impaction drastically reduces the IBIL emission. It is speculated that the local heating produced by the ion beam can then stabilize these defects and emission spectra. These induced defect centers produce radiative transitions and give off the green (near 550 nm) and the red (above 650 nm) emissions on the IBIL luminescence spectra, which are consistent with data reported in the literature.34 These changes would significantly alter the observed luminescence properties of the CaWO4. Further work using in situ ion irradiation TEM is underway to correlate the structural evolution to observed change in spectra as a function of dose.

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ASSOCIATED CONTENT

S Supporting Information *

Information (71 pages) pertaining to the characterization of each of the SOLVO processed CaWO4 derivatives (Δ-ONep, Δ-DMP, Δ-DIP, and Δ-DBP) is available including PXRD, Raman, PL, CL, and IBIL spectra. This information is available free of charge via the Internet at http://pubs.acs.org/. CCDC 948449 contains the supporting crystallographic data for 1, which can be obtained free of charge via http://www.ccdc.cam. ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (44) 01223−336033; e-mail: [email protected].

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Phone: (505)272-7625. Fax: (505)272-7336. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank S.B. Van Deusen (Sandia) and J. Villone (Sandia) for their technical assistance. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories. The Bruker X-ray diffractometer used for the crystal structure study was purchased via a National Science Foundation CRIF:MU award to the University of New Mexico (CHE04-43580). Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

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CONCLUSION An all alkoxide route used for the preparation of CaWO4 nanomaterials was successfully realized using a variety of alkoxide ligands. The intermediate species was identified for the larger ligated species as [(OEt)4W(μ-OEt)2Ca(DBP)2] (1) and similar species were assumed to be present for the other ligands. The final scheelite phased nanomaterials isolated for each of the SOLVO processed mixtures were fully characterized by PXRD and TEM. The PL and CL emissions were not observed for any of these materials until the powders had been processed at sufficiently high temperatures, which also led to larger sized nanoparticles. The observed changes were associated with the change in the ordering of the WO42−. The nanomaterial spectra had emissions at 425 nm (750 °C) and 445 nm (1000 °C) that are consistent with literature reports of bulk CaWO4. The IBIL spectra collected on the smaller CaWO4 nanomaterials was minimally altered upon exposure to the proton beam; whereas, it is theorized that the larger CaWO4 particles appear to have induced lattice defects, which generate additional emission in the green and red regions of the spectrum. These data imply that the fully crystalline, smaller nanomaterials may be of more interest in the pursuit of scintillator materials that can withstand extended radiation exposure. It is of note that for the smaller nanoparticles

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