Article pubs.acs.org/cm
Low-Temperature Synthesis of AMoO4 (A = Ca, Sr, Ba) Scheelite Nanocrystals Sean P. Culver,† Federico A. Rabuffetti,† Shiliang Zhou,† Matthew Mecklenburg,‡ Yan Song,† Brent C. Melot,† and Richard L. Brutchey*,† †
Department of Chemistry, University of Southern California, Los Angeles, California 90089-0744, United States Center for Electron Microscopy and Microanalysis, University of Southern California, Los Angeles, California 90089-0744, United States
‡
S Supporting Information *
ABSTRACT: An extension of the vapor diffusion sol−gel method to the synthesis of the AMoO4 (A = Ca, Sr, Ba) scheelite family of materials is reported. Sub-30 nm quasispherical nanocrystals were obtained after vapor diffusion at room temperature, followed by thermal aging at 80 °C. Rietveld analysis of X-ray diffraction data, Raman spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy demonstrated that the vapor diffusion sol−gel method affords crystalline and phase-pure AMoO4 nanocrystals with excellent compositional control. The potential lithium storage capacity of the CaMoO4 nanocrystals versus Li at a rate of C/4 was also investigated. The nanocrystals exhibited an extremely large first discharge capacity of 1300 mA h g−1, which stabilized at 250 mA h g−1 after 25 cycles. KEYWORDS: scheelite, nanocrystal, vapor diffusion, sol−gel, Li-ion battery
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temperature, ambient pressure, and near neutral pH).4 The VDSG method relies on the interfacial hydrolysis and condensation of alkoxide precursors upon diffusion of water vapor into the alkoxide solution. Previously, this method has been exclusively applied to the synthesis of perovskite oxide nanocrystals of formula A1−xA′xB1−yB′yO3 (A = Ba, Sr and B = Ti, Zr; 0 ≤ x ≤ 1, 0 ≤ y ≤ 1).4a,b Herein, we report the extension of the VDSG method to the low-temperature synthesis of sub-30 nm scheelite-structured AMoO4 nanocrystals. Chemical, structural, and morphological characterization and preliminary results on the potential utility of these nanocrystals as a Li-ion battery electrode material are provided.
INTRODUCTION Scheelite-structured alkaline earth molybdates with the formula AMoO4 (A = Ca, Sr, Ba) have been employed as functional materials in energy storage and conversion applications (e.g., solid-state phosphors, cryogenic scintillation detectors, and Liion batteries, among others).1 To harness these applications, a myriad of synthetic techniques have been exploited. Classic solid-state, sol−gel, molten salt, and hydrothermal routes have previously been used;2 these approaches require high temperature and/or pressure to achieve a crystalline and phase-pure product. Recently, more benign preparations have been developed, including microemulsion, solution precipitation, and aqueous mineralization techniques, but such methods require the use of surfactants, complexing agents, and/or mineralizers.3 Moreover, while these low-temperature techniques allow the preparation of scheelite micro- and nanocrystals spanning several micrometers to 30 nm in size, the sub-30 nm regime remains largely unexplored. Over the past few years, our group has developed a vapor diffusion sol−gel (VDSG) method that affords phase-pure metal oxide nanocrystals under ultrabenign conditions (low © 2013 American Chemical Society
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EXPERIMENTAL SECTION
Nanocrystal Synthesis. All manipulations were conducted under a nitrogen atmosphere at ambient pressure using standard Schlenk techniques. MoO2(acac)2 (95%, acac = C5H7O2) from Sigma Aldrich Received: August 26, 2013 Revised: September 19, 2013 Published: September 20, 2013 4129
dx.doi.org/10.1021/cm402867y | Chem. Mater. 2013, 25, 4129−4134
Chemistry of Materials
Article
and alkoxide solutions of Ca(OCH2CH2OCH3)2 (20 wt % in methoxyethanol), Sr(OCH2CHCH3OCH3)2 (19 wt % in methoxypropanol), and Ba(OCH2CHCH3OCH3)2 (25 wt % in methoxypropanol) from Gelest, Inc. were used as precursors. The solvents 2methoxyethanol and 2-methoxypropanol were purchased from Sigma Aldrich. All reagents were used as received. The synthetic apparatus utilized herein is described in detail elsewhere.5 Briefly, a rotameter controls the flow of the carrier gas (N2) through a bubbler housing 0.75 M aqueous HCl, which is connected via tygon tubing to a 100 mL, 3-neck round-bottom flask containing the precursor solution. Using CaMoO4 as an example target material, 0.79 mL (1.0 mmol) of Ca(OCH2CH2OCH3)2 was transferred into the reaction flask containing 326 mg (1.00 mmol) of MoO2(acac)2. The resulting mixture was diluted to 2.0 mL with 2-methoxyethanol and stirred (60 °C) for 2 h under nitrogen, after which complete dissolution of the reagents was observed, resulting in a dark reddish-brown solution. Once cool, stirring was stopped and N2/HCl/H2O vapor was allowed to flow over the reaction solution, which resulted in the formation of an opaque, off-white gel within 12 h. Vapor was allowed to flow for a total of 48 h. At this point, the flow of water vapor was stopped and the gel was thermally aged at 80 °C for 24 h under nitrogen. The resulting gel was then collected and washed three times with 10 mL of absolute ethanol; the mixture was sonicated for 5 min and centrifuged at 6500 rpm for 15 min between each wash. Ceramic yields were estimated to be 82, 88, and 90% for CaMoO4, SrMoO4, and BaMoO4, respectively. TGA results reveal that the resulting nanocrystals possess ≤5 wt % organic content (see the Supporting Information, Figure S1). Powder X-ray Diffraction (XRD). XRD patterns were collected in the 10−80° 2θ range using a Rigaku Ultima IV diffractometer operated at 44 mA and 40 kV. Cu Kα radiation (λ = 1.5406 Å) was employed. The step size and collection time were 0.0075° and 1 s per step, respectively. All patterns were recorded under ambient conditions. Rietveld Analysis. Rietveld structural refinements6a,b was carried out using the General Structure Analysis System (GSAS) software.6c The following parameters were refined: (1) scale factor, (2) background, which was modeled using a shifted Chebyshev polynomial function, (3) sample displacement, (4) peak shape, which was modeled using a modified Thomson−Cox−Hasting pseudo-Voigt,6d (5) lattice constants, (6) fractional atomic coordinates of the oxygen atom, and (7) an isotropic thermal parameter for each chemical species (i.e., UA, UMo, and UO). The usual Rwp and χ2 indicators were employed to assess the quality of the refined structural models.6e Raman Spectroscopy. Raman spectra were recorded in the 110− 1000 cm−1 range using a Horiba Xplora Raman microscope (Horiba Scientific). Laser radiation of 785 nm in wavelength was employed as the excitation source, and the power at the sample level was 50 mW. Sulfur and 4-acetamidophenol were employed as frequency standards for calibration of Raman shifts. The absolute accuracy of Raman shifts was estimated to be ±1 cm−1. All spectra were recorded under ambient conditions. Transmission Electron Microscopy (TEM). TEM images were obtained using a JEOL JEM2100F (JEOL Ltd.) electron microscope operating at 200 kV. Samples for TEM studies were prepared by dropcasting a stable suspension of nanocrystals in ethanol on a 200 mesh Cu grid coated with a lacey carbon film (Ted Pella, Inc.). Energy-Dispersive X-ray Spectroscopy (EDS). EDS spectra were obtained using a JEOL JEM2100F (JEOL Ltd.) electron microscope operating at 200 kV. Samples for EDS studies were prepared by drop-casting a stable suspension of nanocrystals in ethanol on a 400 mesh Cu grid coated with an ultrathin lacey carbon film (Ted Pella, Inc.). Grids were previously cleaned via ozone treatment for 60 min prior to sample deposition. Quantification was achieved through the use of the Cliff-Lorimer K edge k-factors for CaMoO4 and SrMoO4. Because of the inability to collect the K edge of BaMoO4, the Ba-to-Mo ratio was calculated by comparing the L edges and the corresponding k-factor. Electrochemical Testing. Galvanostatic cycling was performed using standard Swagelok-type cells. The positive electrode was prepared by mixing the active material with 30% by weight Ketjenblack
(EC60JD, AczoNobel) and ball-milling for 20 min using a Spex 8000M mill. Lithium metal foil was used as the negative electrode. The electrodes were separated by two sheets of Whatman GF/D borosilicate glass fiber saturated with 1 M LiPF6 in a 1:1 wt/wt mixture of ethylene carbonate/dimethyl carbonate. Cells were assembled in an argon-filled glovebox and typically cycled at 25 °C between 0 and 3.8 V versus Li at a rate of 1 Li+ per formula unit over 4 h (C/4). Galvanostatic cycling was carried out on a VMP3 potentiostat (BioLogic).
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RESULTS AND DISCUSSION The crystallization of metal oxides using the VDSG method progresses through kinetically controlled hydrolysis and condensation of metal alkoxides upon diffusion of water vapor.4c As previously stated, the VDSG method has been exclusively applied to the synthesis of perovskite nanocrystals (e.g., BaTiO3, SrTiO3, and BaZrO3).4a,b For this family of perovskite nanocrystals, the desired crystalline oxide products could only be obtained using bimetallic alkoxide precursors under room-temperature conditions.4e Herein, crystalline and phase-pure AMoO4 nanocrystals were synthesized using individual metal precursors (e.g., MoO2(acac)2 mixed with monometallic alkaline earth alkoxides), thereby expanding the versatility of the VDSG method to the synthesis of other families of functional materials. Upon dissolution of MoO2(acac)2 into the appropriate alcohol solution of alkaline earth alkoxide, diffusion of water vapor allows for the kinetically controlled hydrolysis and cross-polycondensation at the liquid− vapor interface within the precursor solution. Thermal aging of the resulting gel at 80 °C affords sub-30 nm AMoO 4 nanocrystals. Ceramic yields were estimated to be 82, 88, and 90% for CaMoO4, SrMoO4, and BaMoO4, respectively. The organic content was estimated by TGA to be ≤5 wt % for each material and can be attributed to unreacted surface alkoxy groups retained from the starting precursors (see the Supporting Information, Figure S1). Furthermore, while the VDSG method afforded BaTiO3, SrTiO3, and BaZrO3 nanocrystals at room temperature without the need for a thermal aging step, XRD and TEM results collected on AMoO4 samples prior to thermal aging indicated that this step was required for the scheelite materials (see the Supporting Information, Figures S2 and S3, respectively). XRD patterns of AMoO4 before and after the thermal aging step at 80 °C illustrate an increase in intensity and sharpness of the diffraction peaks upon thermal aging, which is suggestive of an increase in crystallinity. Indeed, the coexistence of amorphous and crystalline material was found in the AMoO4 samples prior to thermal aging insomuch as crystalline nuclei embedded in an amorphous matrix were observed by TEM. Additionally, the ratio of the crystalline to amorphous fraction increases upon going from CaMoO4 to SrMoO4 and finally to BaMoO4. In other words, though thermal aging was not required to initiate nucleation, it was essential to drive the amorphous-to-crystalline phase transition to completion, especially in the case of CaMoO4. XRD patterns confirming the crystallinity and phase purity of the products obtained using the VDSG method are shown in Figure 1. The diffraction maxima can all be indexed to the tetragonal scheelite structure (JCPDS, Nos. 29-0351, 85-0809, and 29-0193 for CaMoO4, SrMoO4, and BaMoO4, respectively) with no impurities present. All AMoO4 nanocrystals are isostructural, belonging to the I41/a (No. 88) space group. Upon going from CaMoO4 to SrMoO4 and finally to BaMoO4, a shift of the diffraction maxima to lower 2θ is observed, indicating an expansion of the unit cell with increasing A-site 4130
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Table 1. Rietveld Analysis of XRD Data of AMoO4 Nanocrystals CaMoO4 a (Å) c (Å) V (Å3) x, y, z O UA (Å2)a UMo (Å2)a UO (Å2)a Rwp χ2 a
5.2253(5) 11.4326(11) 312.16(9) 0.6461(2), 0.5095(2), 0.20873(8) 0.78(3) 0.98(2) 1.86(5) 7.8 1.16
SrMoO4 5.3931(7) 12.0063(17) 349.21(15) 0.6354(3), 0.5188(3), 0.20460(11) 1.31(3) 0.41(3) 1.37(6) 7.1 1.14
BaMoO4 5.5785(6) 12.8000(15) 398.33(14) 0.6184(4), 0.5242(5), 0.20106(19) 0.58(4) 1.30(4) 3.06(13) 9.1 1.16
Given as 100 × U.
with two sets of A−O distances, each set corresponding to the bond to four oxygen atoms (4 + 4). The tetrahedral oxygen coordination environment of Mo4+ can be described with a single Mo−O distance; however, two sets of O−Mo−O angles (2 + 2) were found for all three AMoO4 structures. These observations indicate that the coordination polyhedra of both A2+ and Mo4+ ions are not regular; further structural characterization is underway to clarify the dependence of the polyehdra shape on chemical composition. Regarding the variation of metal−oxygen distances with chemical composition, it should be noted that A−O and Mo−O distances increase and decrease, respectively, upon going from Ca to Sr and finally to Ba; this bond length variation is significantly more pronounced for A−O bonds, in agreement with the previously reported quasirigidity of Mo−O bonds in scheelites.9 Therefore, the observed expansion of the unit cell upon increasing the ionic radius of the alkaline-earth cation is driven by the expansion of the AO8 dodecahedra. The crystal structure of the AMoO4 scheelites was further probed using Raman spectroscopy. Corresponding spectra, along with tentative assignments of the vibrational bands, are shown in Figure 3. All vibrational bands can be assigned to the AMoO4 scheelite structure, which further confirmed the phase purity of the oxide nanocrystals.10 Vibrational bands are divided into three groups for clarity. The first group (
