Hydrothermal Synthesis of CoSb2O4: In Situ Powder X-ray

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Hydrothermal Synthesis of CoSb2O4: In Situ Powder X-ray Diffraction, Crystal Structure and Electro-chemical Properties Peter Nørby, Martin Roelsgaard, Martin Søndergaard, and Bo B. Iversen Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01421 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 7, 2016

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Crystal Growth & Design

Hydrothermal Synthesis of CoSb2O4: In Situ Powder X-ray Diffraction, Crystal Structure and Electrochemical Properties Peter Nørby, Martin Roelsgaard, Martin Søndergaard and Bo B. Iversen* Center for Materials Crystallography, Department of Chemistry and iNANO, Langelandsgade 140, DK-8000 Aarhus C, Denmark

MSb2O4 constitutes a relatively unexplored class of multinary oxides that is traditionally synthesized by high temperature solid state methods. Here, we report on a facile synthesis of CoSb2O4 under hydrothermal conditions (T=135-300 °C, 256 bar). Using in situ synchrotron powder X-ray diffraction (PXRD) the formation and growth of CoSb2O4 nanoparticles are followed in real time using different precursor stoichiometries. Phase pure CoSb2O4 can be formed at 135 °C, although the formation mechanism changes with precursor stoichiometry. The crystallite size can be fine-tuned between 14-17.5 nm under nonstoichiometric conditions, but twice as large crystallites are found in the stoichiometric case. An activation energy of 65(12) kJ/mol is obtained for the crystallization from a nonstoichiometric precursor. Modelling of Atomic Displacement Parameters obtained from Rietveld refinement of multi-temperature high resolution synchrotron PXRD data gives a Debye temperature of 331(11) K. The thermal expansion coefficients for the material was found to be αa = 6.2(1)·10-6 K-1 and αc = 3.1(4)·10-6 K-1. Electrochemical measurement shows that CoSb2O4 display a large irreversible capacity (1131 mAh/g) on the first cycle in Li-ion half cells, and that the capacity decreases significantly in the following cycles.

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INTRODUCTION Transition metal antimony(III) oxides MSb2O4 (M : Mn, Fe, Co, Ni, Zn) belonging to the schafarzikite mineral group are significantly less studied than many other binary oxides (e.g. perovskites).1–21 This is mostly due to the current lack of technological applications for these materials. It has previously been shown that MnSb2O4,5 CoSb2O4,19 FeSb2O411,17 and NiSb2O42,7,11 display antiferromagnetic ordering at low temperature (highest TN = 79 K). More recently, their applicability as anode materials in Li-batteries (LIB) has been investigated.20 It is well-known that the performance of anode materials in LIB is correlated with materials characteristics such as nanocrystallite size, crystallinity and phase purity.22 The Li-ion and electronic conductivity of the electrode materials for LIB is increased with the reduction in crystallite size, owing to faster diffusion in the smaller crystallites and access to more electrolyte.22 Hence, it is of key importance to develop synthesis methods that allow full control of both the crystallite size and phase purity. So far MSb2O4 materials have mainly been synthesized by a traditional solid state synthesis method, where the elements are mixed and heated in a sealed ampoule (700 °C for 12 hours).17,19 Westin and Nygren have investigated the decomposition of M2+-Sb3--alkoxide gels to both MSb2O6 and MSb2O4. The formation of a gel is similar to the first step in a hydrothermal synthesis, although the temperatures used in the decomposition of the gel to crystalline MSb2O4 is 620 °C,15 which is much higher than typical hydrothermal synthesis temperatures (T > 100 °C). Furthermore, the control of crystallite size and purity is difficult with this method. In contrast, the low temperature hydrothermal synthesis method has proven to be extremely versatile in the synthesis of many different oxides.23–25 Here we show for the first time that phase pure CoSb2O4 nanoparticles can be prepared via a hydrothermal synthesis route. In order to gain

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size control on the nanometer scale it is important to understand the formation and growth mechanisms taking place in the hydrothermal synthesis. In situ studies give the experimentalist an unique insight into the formation mechanism and growth of crystalline materials synthesized under hydrothermal conditions.26–32 A range of techniques can provide information about hydroand solvothermal synthesis under sub- and supercritical conditions.33–41 Here, in situ powder Xray diffraction (PXRD) is used to study the formation and growth of nanocrystalline CoSb2O4 in real time from both a stoichiometric and nonstoichiometric precursor. The results are compared with those obtained from an ex situ hydrothermal synthesis performed in an autoclave. In addition the thermal expansion, atomic displacement parameters and the Debye temperature are obtained from Rietveld refinement of high resolution multi-temperature synchrotron PXRD data. Finally, the morphology and electrochemical properties of microcrystalline CoSb2O4 was investigated.

EXPERIMENTAL Precursor synthesis SbCl3 (≥99.0%, Sigma-Aldrich), Co(CH3COO)2·4H2O (≥98.0%, Sigma-Aldrich), NaOH and HCl were used as purchased. For the in situ study solutions of SbCl3 in 4 M HCl and Co(CH3COO)2·4H2O in deionized water were utilized. The pH value of the mixture was adjusted by addition of NaOH. Upon addition of NaOH the solution changed color from light red to dark blue. In a typical synthesis, an aqueous 2.5 mL 1.00 M Co(CH3COO)2·4H2O solution was mixed with a 2.5 mL 1.33 M SbCl3 solution and subsequently 3.0 mL 8 M NaOH was added. The molar

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ratio between Sb and Co was modified to either 1.33 or 2.00 (cf. Table 1) in order to study the influence of excess Co on the formation and growth of the nanocrystallites.

Table 1 Synthesis conditions for in situ data collections at p =256 bar Compound

M(Sb)/M(Co)

135 °C

150 °C

180 °C

220 °C

1.33

x

x

x

x

2.00

x

300 °C

CoSb2O4 x

x

In situ experiments A custom-made sapphire capillary setup was used as described in detail by Becker et al.42 The experiments were carried out at beam line I711 at MAX-lab (Lund, Sweden).43 The precursor was injected into a thin single crystalline sapphire capillary (inner diameter 0.7 mm), which was subsequently pressurized to 256 bar. Heating was initiated by switching a valve directing a jet of preheated hot air onto the sapphire capillary (see Table 1 for applied temperatures). The desired temperature can be reached within approximately 20 s (see supporting information, figure S.1), and time resolved X-ray patterns with a time resolution of 5 s were recorded prior to and during the heating. Each experiment was allowed to run for ~20 minutes. The wavelengths, sample-todetector distances and instrumental contribution to the peak broadening were calibrated with a NIST LaB6 standard for each experiment (the wavelength was λ = 0.992 Å). The scattered Xrays were detected by an Oxford Diffraction Titan CCD detector. All PXRD data are presented in Q-space (Q = (4πsinθ)/λ).

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An up-scaled synthesis was made by mixing an aqueous 10 mL 0.667 M Co(CH3COO)2·4H2O solution with 10 mL 1.334 M SbCl3 solution (dissolved in 4 M HCl). Under magnetic stirring 16 mL 8.0 M NaOH was added. The autoclave was filled to half the volume with the precursor (total volume: 70 mL). The precursor was heated to 220 °C for 53 h. High resolution PXRD patterns on the powdered sample were measured at beam line BL44B2, SPring-8 (Japan) from 100 K to 300 K in steps of 50 K. A homogenous sample for the PXRD measurements was obtained by floating two times in ethanol (96%), which was subsequently packed into a 0.1 mm glass capillary. The scattered X-rays were collected on an image plate detector mounted in a Debye-Scherrer camera covering an angular 2θ-range of 2-77°, resulting in a Q-range from 0.4415.6 Å-1. The sample was further characterized by scanning electron microscopy (SEM) on a FEI Nova NanoSEM 600 and by transmission electron microscopy (TEM) on a Phillips CM20 with an acceleration voltage of 200 kV. SEM preparation was done by placing powder on conducting carbon tape. TEM samples were prepared by making a diluted suspension of the particles in ethanol and placing a droplet on a carbon-coated copper grid.

Li-battery assembly and measurements Half cells of type CR2032 were assembled with Li metal foil as both counter and reference electrode. The up-scaled product was used as the active material in the working electrode and it was mixed with Super P carbon as conductor and polyvinylidenedifluoride (PVDF) as binder in the weight ratio of 76:12:12 and coated onto Cu-foil. The typical loading of a dry electrode was ∼ 30 µm or ∼ 2 mg/cm2. The working electrodes were compressed with ∼50kN/cm2. Two pieces of 25 µm thick porous polypropylene membranes were employed as separators and the 5



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electrolyte solution was composed of 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) in the ratio of 1:1 by volume. The cells were assembled in a glove box with water and oxygen levels at ∼ 1 ppm. After assembly, the cells were discharged and charged with a constant current at 30 mA/g between 0.05 V and 1.3 V vs. Li/Li+.

Data integration and analysis Prior to integration in Fit2D44 the 2D diffraction patterns from the in situ experiments were masked in order to remove single crystalline diffraction peaks from sapphire and the beam stop. The integrated in situ PXRD patterns were analyzed in the FullProf program package45 by sequential Rietveld refinement. The volume weighted crystallite size, scale factor, and lattice parameters were extracted from each frame in the refinement. The crystallite sizes were calculated based on the Scherrer equation using the line profile parameters corrected for instrumental broadening.46 The high resolution multi-temperature PXRD patterns measured on the ex situ synthesized sample were analyzed by Rietveld refinement in the FullProf program package.45 Details on the Rietveld refinements can be found in supporting information page 1 and 9.

RESULTS AND DISCUSSION Precursor and phase identification The contour plot of the in situ PXRD data shown in Figure 1 and supporting information (figures S.2-S.9) reveal the almost instantaneously formation of phase pure CoSb2O4 under hydrothermal conditions when heat is turned towards the sample. CoSb2O4 can be synthesized at the lowest 6 ACS Paragon Plus Environment

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investigated temperature (135 °C) for both stoichiometries. The rapid formation indicates that the synthesis temperature can be reduced even further. After 1000 seconds the sample is phase pure independently on the ratio between cobalt and antimony in the precursor. Surprisingly, there is an unidentified impurity in the synthesis performed at 180 °C under stoichiometric (M(Sb)/M(Co) = 2.00) conditions. It is not present when the synthesis temperature is increased or decreased. The formation mechanism differs for the nonstoichiometric (M(Sb)/M(Co) = 1.33) and stoichiometric (M(Sb)/M(Co) = 2.00) synthesis conditions, see Figure 2. The stoichiometric precursor contains both crystalline orthorhombic Sb2O3 (Pccn) and nanocrystalline cubic Sb2O3 (Fd-3m), see supporting information figure S.8. Switching the heat towards the sample dissolves the crystalline precursor over a time period of approximately 100 seconds at 135 °C. The intensity of the reflections of CoSb2O4 increases at the expense of a decrease in intensity of the reflections of Sb2O3. Increasing synthesis temperature results in a faster dissolution of the crystalline precursor and even at 180 °C the transformation from Sb2O3 to CoSb2O4 is faster than the time resolution (5 s), see supporting information. The in situ results show that in the nonstoichiometric case the precursor is amorphous which indicates that the formation of Sb2O3 in the precursor is suppressed when the relative concentration of antimony is lowered. Since CoSb2O4 in this case is formed from an amorphous precursor it crystallizes instantaneously when heat is applied. This is seen as a sharp transition in Figure 1. This indicates a strong dependency on reactive antimony species in solution in order to form CoSb2O4. Hence, it is advantageous to have an amorphous precursor where the antimony atoms seem to be more loosely bonded. It can be rationalized that the mechanism most likely is a dissolution recrystallization mechanism where CoSb2O4 is formed as more antimony is released to the solution, which in the 7



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stoichiometric and nonstoichiometric case is from crystalline Sb2O3 and amorphous antimony material, respectively.

Figure 1. Contour plot of raw PXRD in situ data for the nanocrystals synthesized at 135 °C and with a) M(Sb)/M(Co) = 1.33 and b) M(Sb)/M(Co) = 2.00. Asterisks indicate CoSb2O4.


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Figure 2. PXRD patterns at three different times for the reaction at 135 °C with the ratio between Sb and Co of a) 1.33 and b) 2.00. c) Standard CoSb2O4 PXRD pattern. The precursor is amorphous for Sb/Co = 1.33 and crystalline for Sb/Co = 2.00.

Size evolution and growth kinetics The crystallite size is calculated from the sequential Rietveld refinement data corrected for instrumental broadening. It is extracted as a function of synthesis temperature for the nonstoichiometric precursor as shown in Figure 3. It should be noted that in a diffraction experiment the volume averaged crystallite size value is obtained, which is inversely proportional to the broadening of the Bragg peaks. The insert in Figure 3 shows that the overall growth rate of CoSb2O4 nanoparticles is almost independent of synthesis temperature, although slight differences are seen in how fast zero growth rate is achieved. At 135 °C the crystallite size stabilizes at 14 nm after approximately 300 seconds. Increasing the synthesis temperature to 220 °C results in a fast initial growth and the final crystallite size (17.5 nm) is reached within 150

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seconds after the heat has been applied. It should be noted that the estimated standard deviations in Figure 3 are smaller than the symbols.

Figure 3. The crystallite size for CoSb2O4 nanoparticles (Sb/Co = 1.33) as a function of time. The uncertainties are smaller than the marks. The insert show the growth rate as a function of time. The Johnson-Mehl-Avrami equation47,48 is utilized here to model the growth of nanocrystalline CoSb2O4 from an amorphous precursor. Although the model was initially developed for solidstate reactions it has proven to give valuable information from experiments performed under hydro- and solvothermal conditions.49–52 Both the reaction mechanism and activation energy are extractable parameters from the model. The extent of the reaction (f) is plotted as a function of time after the first appearance of the first nanocrystallites (t-t0). The data are subsequently modelled by the equation 1 exp , where the extent of the reaction is defined as V(t)/Vinf (Vinf is equal to the final stable nanocrystal volume at the specific synthesis temperature). It is assumed that the crystallites are spherical in morphology, which is supported by the peak shape modelling (see supporting information page 1). The two parameters in the equation are related to the rate constant (k) and to the mechanism (n). If the value of n is in the range from 0.54-0.62 this typically corresponds to a diffusion-controlled reaction mechanism,


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1.0-1.24 to a zero-order, first-order, or phase-boundary-controlled mechanism, and 2.0-3.0 to a nucleation and growth control mechanism.53 Figure 4a-d show t-t0 as a function of the extent of the reaction with fits for the four different synthesis temperatures. The rate constant (k) and mechanism dependent parameter (n) are obtained from the fits. It can be seen that the value of n decreases with temperature and varies from 0.9 to 0.4, however all four data points are falling outside the physical meaningful ranges as indicated with grey in Figure 4e. It can be concluded that the reaction mechanism presumable changes from something which is close to either zero or first order kinetics to a more diffusion-controlled reaction mechanism with increasing temperature, albeit the specific mechanism is uncertain. It could also be caused by different reactions occurring in parallel and further total scattering experiments may clarify the formation mechanism from amorphous to crystalline CoSb2O4. The activation energy (Ea) can be calculated by the Arrhenius equation

exp / and it is obtained as the slope when ln(k) vs. 1000/T is plotted, Figure 4f. An

activation energy of 65(12) kJ/mol is obtained for the crystallization of CoSb2O4 from an amorphous precursor. This is in good agreement with activation energies obtained in other oxide systems synthesized under hydrothermal conditions. For the formation of Fe2O3, TiO2, and αLi2TiO3 nanocrystallites activation energies of 67(15), 66(19) and 66(7) kJ/mol were estimated, respectively.49,51,54 Rossetti et al. have by a pseudo in situ method found an activation energy of the apparent crystallization of PbTiO3 of 30 kJ/mol,52 however, the initial formation was omitted in this case. The activation energy for a phase transition in nanocrystals is a factor two larger, e.g. the phase transition from β-chalcocite Cu2S to high digenite Cu2-xS has an activation energy of 171(27) kJ/mol.50 Hence, the similarity found for the activation energy for the oxides 11



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synthesized under analogous conditions is interesting, but based solely on these values a conclusion for the mechanism governing the formation cannot be drawn. A recent total X-ray scattering studies on the hydrothermal formation of nanoparticles have shown that the formation mechanism for relative simple oxides can be very complex. Going from a polymeric precursor to a highly disordered amorphous precipitate before crystallizing in the desired phase.55–57

Figure 4. (a–d) Johnson–Mehl–Avrami plots for at the syntheses at 135-220 °C in the nonstoichiometric case. (e) n-values obtained from the fits at different synthesis temperatures with grey areas indicating physical meaningful n-values. (f) Arrhenius plot.

The changes in unit cell parameters obtained from the sequential Rietveld refinement are plotted as a function of crystallite size, Figure 5. Only the changes in unit cell parameters (a-afinal, c-cfinal and V-Vfinal) are considered owing to the lack of an internal standard in the in situ experiments. It 12 ACS Paragon Plus Environment

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can be seen that both the a-axis and c-axis decrease as the crystallite grows. The change in unit cell parameters is anisotropic as the decrease is larger along the c-axis than along the a-axis. This anisotropy is also seen in the formation of SnO2 nanoparticles.58 Furthermore, there is a clear enlargement of the unit cell when the nanocrystallites are below 7 nm. At larger crystallite sizes the unit cell has almost relaxed to its final value, which indicates that the internal atomic structure changes as a function of crystallite size. The origin of this unit cell expansion with decreasing crystallite size has been ascribed to the softening of lattice vibration.59 In the present study is it not possible to distinguish between the mechanisms involved in the softening of the lattice vibration as reliable values of the correlated occupancy, atomic thermal displacements and atomic positions cannot be extracted due to insufficient data quality.

Figure 5. Changes of the unit cell parameters as a function of crystallite size. The color of the markers indicate different synthesis temperatures (red: 135 °C, blue: 150 °C, purple: 180 °C and black: 220 °C).

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The in situ PXRD data show that changing the ratio between antimony and cobalt from nonstoichiometric to stoichiometric affects the formation mechanism as shown in Figure 1 and Figure 2. Analysis of the stoichiometric data shows that the crystallite size is more than doubled when the ratio is stoichiometric compared with the nonstoichiometric case, see Figure 6. Furthermore, the growth is significantly different when the ratio is changed, as seen when comparing Figure 3 with Figure 6. In the stoichiometric case the final crystallite size at 135 °C is reached within the first 150 seconds and subsequently the crystallite size stabilizes around 34 nm. Upon an increase in synthesis temperature the initial crystallite size of approximately 35 nm increases to 44 and 52 nm at 180 °C and 300 °C, respectively. Hence, the ratio between antimony and cobalt influences the final crystallite size. Furthermore, there is both a temperature and compositional dependency for the nanoparticles to grow by Ostwald ripening, see supporting information figure S.10.58 In the stoichiometric case at 135 °C no Ostwald ripening is observed, since the scale factor can be superimposed with the crystallite volume. Hence, the nanoparticles grow because more CoSb2O4 can be formed from the precursor in solution, and thus the growth and scale factor stagnate when the precursor is depleted. When increasing the temperature to 180 °C, or changing to nonstoichiometric conditions, Ostwald ripening is observed, see supporting information figure S.10.

Figure 6. The crystallite size for CoSb2O4 nanoparticles (M(Sb)/M(Co) = 2.00). 14 ACS Paragon Plus Environment

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Ex situ autoclave study The up-scaled autoclave synthesized ex situ product has a small impurity present as seen by the Rietveld refinement in Figure 7. It is speculated that the prolonged synthesis time (53 h) might have induced decomposition of the phase pure CoSb2O4 since, as we have just shown, CoSb2O4 can be synthesized in seconds from a similar starting solution. It should be noted that there are small differences between the two methods, especially the heating rate and pressure. It has not been possible to identify the impurity phase. The impurity might also explain the light blue or greyish color of the material, which is in contrast with the previous reported pinkish brown.3,19

Figure 7. Rietveld refinement of high resolution PXRD data measured at 100 K. It is clear from the difference curve that an impurity is present in the measured sample. RBragg = 4.50, RF =2.41.

Figure 8 display a TEM picture of the sample from the autoclave synthesis, and it can be seen that at long synthesis times the particles grow into rods and cubes. The largest rods are around 400 nm in length and 100 nm in diameter, although much smaller rods are also observable. The cubes have a similar large size distribution with the smallest and largest being around 80 nm and 15



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300 nm in one dimension, respectively. No micrometer sized particles were found by SEM, see supporting information figure S.15.

Figure 8. TEM picture of autoclave sample used as anode material in Li-ion half cells.

Rietveld refinements similar to those depicted in Figure 7 were performed for the five different temperatures on the high resolution PXRD patterns, ignoring the impurity phase (see supporting information figures S.12-S.13). From these multi-temperature Rietveld refinements different physical parameters can be extracted and evaluated as a function of temperature, e.g. unit cell parameters. Figure 9 shows that there is a linear thermal expansion of the unit cell, although the c-parameter displays a deviation from linearity when compared with the a-parameter. Similarly, the unit cell thermal expansion also shows this non-linearity. The linear thermal expansion coefficients with respect to 300 K are 6.2(1)·10-6 K-1,3.1(4)·10-6 K-1 and 1.55(7)·10-5 K-1 in the a-direction and c-direction and for the unit cell, respectively. Comparing the absolute values at 300 K obtained in this study of the a-axis (8.5031(1) Å) and c-axis (5.9288(1) Å) with those reported by Koyama et al. (a = 8.500(1) Å, c=5.931(1) Å)3 and De Laune et al. (a = 8.49340(9)


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Å, c=5.92387(8) Å)19 reveal that the values are slightly larger in this study. This discrepancy might be explained by the difference in synthesis methods used in this study and the previous ones, although the absolute values differ, the relative changes investigated in this study should still be valid. None of the previously reported studies have investigated the thermal expansion of CoSb2O4. Comparing the thermal expansion coefficients with those for MnSb2O4 show that the result found in this study is slightly lower, but is in the same order of magnitude.10

Figure 9. The thermal evolution of the unit cell as extracted from the multi-temperature Rietveld refinement. The linear fits show the region where the thermal expansion coefficients are calculated.

The atomic displacement parameters (ADPs) for all atoms are shown in Figure 10. The best model is achieved when antimony is allowed to vibrate anisotropically. Independent of temperature the oxygen atoms have the smallest ADP (constrained to be identical for the two sites), whereas cobalt display the largest ADP. The isotropic equivalent ADP value for antimony is in between these two values. This is rather surprising since De Laune et al. have observed by neutron powder diffraction that antimony has a larger ADP than cobalt.19 The absolute values at 300 K are also slightly larger in the study presented here. It should be noted that antimony is 17



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surrounded by four O2- anions. The u11 for antimony is twice as large as all the other ADP elements and upon extrapolation to 0 K it approaches a finite value, which could indicate that static disorder contribute to u11. This could be a consequence of the lone electron pair present in antimony, but detailed analysis is beyond the scope of this paper.60 Furthermore, there is a decrease in electron density (void) in the center of the unit cell. However, the long axis of the ellipsoid representing the anisotropic vibration is in the ab-plane as shown in Figure 11. Hence, it is not in the direction of center.

Figure 10. Atomic displacement parameters (ADPs) for a) isotropic (Co and O) or equivalent (Sb) and b) anisotropic for antimony as a function of temperature.


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Figure 11. Illustration of the unit cell with thermal ellipsoids shown for antimony and octahedra for the cobalt atoms. The long axes for the Sb ellipsoids point either in the a- or b-direction.

The lattice dynamics of the structure can be analyzed from the ADPs, and here we approximate that the motion of the atoms can be represented by a Debye model:

3 ()/* $ ! " # %$ & + & % exp$ 1 4 ! !

Fitting the ADPs to this formula provides an estimate of the Debye temperature (see supporting information page 12 for details), which is also related to the average sound velocity in the material. If an average atomic mass of 52.3 amu is used then a Debye temperature of 331(11) K is obtained for CoSb2O4. If the ADPs of the individual atoms are fitted to the Debye formula then “Debye tempeatures” for the Co, Sb and O sublattices are 255(9), 208(2), 656(57) K, respectively.

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Electrochemical measurements Half cells of type CR2032 were measured in order to investigate the potential of CoSb2O4 as anode in LIBs. The galvanostatic cycling of the CoSb2O4 can be seen in Figure 12. The initial discharge reached 1131 mAh/g corresponding to ~15 Li-atoms reacting with CoSb2O4. This is comparable to the 1127 mAh/g reported by Jibin et al.20 and concurring with the formation of Li2O and Li3Sb leaving one Li e.g. for formation of the SEI-layer. The initial charge capacity was 417 mAh/g, which corresponds to dealloying of slightly below two formula units of Li3Sb. Furthermore, it should be noted that the unreacted cobalt in the half cells is electrochemically inactive. Future studies might reveal how cobalt is bonded in the decomposed compound and why this unusual inactivity is seen. On subsequent cycling the capacity-fade was relatively fast with only ~20 mAh/g reversible capacity remaining after the 10th cycle.

Figure 12. Potential (vs. Li/Li+) vs. capacity for CoSb2O4 under galvanostatic charge/discharge at 30 mA/g between 0.05-1.3 V for 2nd, 3rd and 10th cycle. Inset shows the initial discharge and charge curves.


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CONCLUSION We have demonstrated by in situ PXRD studies that phase pure CoSb2O4 nanoparticles can be synthesized under hydrothermal conditions at temperatures ranging from 135-300 °C. The dependency on stoichiometry of the starting solution was investigated and it was found that with the nonstoichiometric precursor the nanoparticle sizes range from 14.5-17 nm. In the case of a stoichiometric precursor the sizes are twice as large. The formation mechanism is different in the two cases as it was found that the nonstoichiometric precursor is amorphous, whereas the stoichiometric precursor is crystalline. The unit cell shows a clear enlargement for nanoparticles below 7 nm. Fitting growth curves with a Johnson-Mehl-Avrami model yields an activation of 65(12) kJ/mol for the crystallization of CoSb2O4 from a nonstoichiometric precursor. An up-scaled ex situ autoclave synthesis gave sub-micrometer CoSb2O4 particles and Rietveld refinements of multi-temperature high resolution PXRD patterns reveal that a-axis expands linear with temperature, whereas the c-axis deviates slightly from linearity. The following linear thermal expansion coefficients with respect to 300 K were obtained αa = 6.2(1)·10-6 K-1 and αc = 3.1(4)·10-6 K-1. Modelling of the ADPs gives a Debye temperature of 331(11) K for CoSb2O4. The potential as anode material in LIBs is not promising as the material exhibit a large irreversible capacity on the first cycle and the reversible capacity fades substantially on the following cycles, resulting in a capacity of ~20 mAh/g after the 10th cycle. Further material improvements have to be conducted before CoSb2O4 can be usable in battery applications.

ASSOCIATED CONTENT

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Supporting Information. Contains details on in situ data analysis, sequential Rietveld refinement and how the crystallite size is calculated from spherical harmonics. Furthermore, the contour plots for all in situ PXRD data are presented along with verification of precursor composition and Ostwald ripening. The temperature profile for the in situ setup and examples of Rietveld refinements for two frames are shown. Details on ex situ data analysis and Debye temperature calculations are presented together with representation of the whole data range for the ex situ multi-temperature high resolution data. A SEM picture of the obtained particles synthesized ex situ is displayed. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interest.

Funding Sources The work was supported by the Danish National Research Foundation (Center for Materials Crystallography, DNRF93), the Danish Research Council for Nature and Universe (Danscatt)


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and the European Community’s Seventh Framework Programme (FP7/2007-2013) CALIPSO under Grant Agreement No. 312284

ACKNOWLEDGMENT Beamline I711 at MAXLAB and the RIKEN beam line BL4402 at SPring-8 are gratefully acknowledged for beam time. Henrik L. Andersen, Dr. Espen D. Bøjesen and Dr. Mogens Christensen are thanked for assistance during the experiments at MAX-lab.

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For Table of Contents Use Only

Hydrothermal Synthesis of CoSb2O4: In Situ Powder X-ray Diffraction, Crystal Structure and Electrochemical Properties Peter Nørby, Martin Roelsgaard, Martin Søndergaard and Bo B. Iversen*

CoSb2O4 was synthesized under hydrothermal conditions and followed in real time with powder X-ray diffraction. The crystallite size evolution was studied for two different starting stoichiometries and at various synthesis temperatures. Modelling of the ADPs from multitemperature ex situ data resulted in a Debye temperature of 331(11) K.


Hydrothermal Synthesis of CoSb2O4: In Situ Powder X-ray

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