Synthesis and Structure Transformation of Ion-Exchanged Metal Cobalt Oxides Ming J. Wang,† Nian T. Suen,‡ Jin Y. Ho,‡ Hwo S. Sheu,§ Hsiang H. Chong,| Hsiao Y. Lin,‡ and Horng Y. Tang*,‡
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2738–2741
Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan, Department of Applied Chemistry, National Chi Nan UniVersity, Puli, Taiwan, National Synchrotron Radiation Research Center, Hsinchu, Taiwan, and Department of Physics, National Tsing Hua UniVersity, Hsinchu, Taiwan ReceiVed February 7, 2007; ReVised Manuscript ReceiVed September 3, 2007
ABSTRACT: A room-temperature ion exchage reaction to synthesize a series of AxCoO2•yH2O crystalline compounds has been demonstrated. The ion exchange reactions with the bilayer hydrate (BLH) Cs0.29CoO2•1.12H2O structure have been studied for a guest ion consisting of Li+, Na+, K+, Rb+, and Ag+. The experimental results reveal that only the ion-exchanged Na+-BLH displayed 4.2 K superconductivity and that the products’ structural stability was highly sensitive to the guest ion. The substitution of Rb+, K+, or Li+ stimulates the phase transformation from BLH to a monolayer hydrate (MLH) structure. Ion exchange of Ag+ ions can synthesize the silver-deficient dehydrated compound. Structures of solvated alkali cation, intermolecular, and intramolecular hydrogen bonds are correspondingly demonstrated to be the determining factors for holding the BLH structure. Introduction The discovery of bilayer hydrate (BLH) NaxCoO2•yH2O with a reported superconductivity at 4.5 K has generated broad interest in the compositional chemistry and crystal structure of hydrated materials.1 These compounds can be classified into three major groups, namely, bilayer hydrate, monolayer hydrate, and dehydrated compounds, based on the coordination differences of the cations and water molecules within the interlayer spacing. The BLH structure is particularly attractive because of its correlating physical properties and the interesting interlayer structure.1–7 To understand the advent of superconductivity for the Na+-BLH cobaltates, it is important to examine experimentally the effects of ion substitution on the BLH structure as well as the key factors holding the BLH framework. The synthesis routes of the superconducting BLHNaxCoO2•yH2O are typically via a two-step sequence of hightemperature sintering and chemical deintercalation reaction. The crystal growth of anhydrous NaxCoO2 has been reported using a floating-zone technique.8–10 The hydrated crystals are prepared through the oxidative deintercalation of an anhydrous NaxCoO2 crystal using a chemical or electrochemical method. The integrity of the NaxCoO2•yH2O hydrated crystals has run across obstacles due to the difficulties in its lattice expansion in posttreatment and the high vapor pressure of intercalated water. On the contrary, this work attempts to prepare hydrated materials by lattice contraction. Chimie Douce, or the soft chemistry method, normally offers advantages by modifying an existing host structure at a low temperature.11–13 Successful examples of the LiCoO2 and LiMnO2 systems have been extensively studied.13,14 In this work, with the assistance of the metastable BLH-Cs0.29CoO2•1.12H2O crystal structure, the use of a soft chemistry method makes a variety of hydrated metal cobalt oxides available in ambient states. The ion exchange with * To whom correspondence should be addressed. Telephone: +886-49-2910960. Fax: +886-49-2917-956. E-mail:
[email protected]. Present address: No.1 University Rd., Department of Applied Chemistry, National Chi-Nan University, Puli, Nantou, Taiwan. † Academia Sinica. ‡ National Chi Nan University. § National Synchrotron Radiation Research Center. | National Tsing Hua University.
different alkali cations shows versatility for the preparation of BLH and MLH compounds. Moreover, the ion exchange of Na+ for Cs+ ion shows the exclusive properties for maintaining the BLH structure and generating superconductivity. The ion exchange of Rb+, K+, or Li+ induces the spontaneous phase transformation from BLH to MLH structure. By introducing Ag+ ions, the capability to prepare a silver-deficient AgxCoO2 compound at room temperature is shown. The Ag+ ions attract Co-O layers to the closely packed arrangement, generating a dehydrated structure. This work demonstrates a soft chemical reaction pathway and its cation dependence for preparing BLH, monolayer hydrate (MLH), and the dehydrated structures. A fundamental problem that requires a detailed explanation is why the interlayer spacing of the alkali ions is not a function of ionic radii as well as the extended interlayer spacing’s strong dependence on the Na+ ion of the superconducting BLH structure. The correlations of the solvated ion structure, chemical stoichiometry, valence state of the metal cobalt oxides, and superconductivity are investigated. The structures of solvated alkali cation, intermolecular, and intramolecular hydrogen bonds are deduced to be the central roles for structure formation. Experimental Method The platelike BLH-Cs0.29CoO2•1.12H2O crystalline compound with ∼80 µm size is prepared in a CsOH molten flux similar to the previously reported method.11 Mechanical stirring is applied to homogenize the chemical reaction at 260 °C. To perform the ion exchange reaction, an as-prepared precursor is immersed into a saturated alkali halide solution at room temperature for 5 days. For the Ag+ cation, 50 wt% AgNO3(aq) is used as the ion exchange solution. The harvested products are rinsed with deionized water and alcohol before chemical analysis. The oxidation state of cobalt is determined by the iodometric titration method as described by Karppinen.15 Meanwhile, thermogravimetric analysis (TGA) is performed with a TA instrument in the range of 25-400 °C for water determination. Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS) are performed using a Jeol model 5200 SEM/EDS equipped with a Link data acquisition system and a semiquantitative analysis program. The chemical stoichiometry is determined by atomic absorption spectroscopy (AAS) and the SEM/ EDS in conjunction with TGA. Powder X-ray diffraction patterns are acquired using a Shimadzu XRD-7000 (Cu KR) or MacScience 18MXP system. The powder X-ray diffraction data for structure refinement are
10.1021/cg0701426 CCC: $37.00 2007 American Chemical Society Published on Web 11/15/2007
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Table 1. Chemical and Physical Properties of BLH-CsxCoO2•yH2O and Ion-Exchanged Metal Colbalt Oxidesa
a
ion
stoichiometry determined by AAS/TGA
stoichiometry determined by EDS/TGA
interlayer spacing
valence state of Co ((0.03)
magnetic property
Cs+ Na+ Li+ Ag+
Cs0.29CoO2•1.12H2O Na0.35Cs0.02CoO2•1.46H2O Li0.08Cs0.20CoO2•0.78H2O none
Cs0.31CoO2•1.14H2O Na0.38Cs0.02CoO2•1.47H2O none Ag0.75 Cs0.03CoO2
10.0(3) Å 9.8(2) Å 6.9(1) Å 6.1(1) Å
3.40 3.37 3.37 3.22
P 4.2 K Tc P P
P represents the paramagnetism.
taken at the BL01C2 beamline of the National Synchrotron Radiation Research Center (NSRRC), Taiwan. Powder XRD data are collected using a Mar345 imaging plate for a typical exposure time of 5 min and integrated by the FIT2D program. The NSRRC is running at 1.5 GeV in the top injection mode. The crystal structure is indexed by Dicvol and refined by Rietveld methods. Magnetic property is characterized by a DC superconducting quantum interference device (SQUID), Quantum Design model MPMS2. The applied magnetic field is 10 G and is measured at a temperature starting from 1.7 K.
Results and Discussion Structure of the Host Material. The Cs0.29CoO2•1.12H2O stoichiometry of the host compound determined by AAS, SEM/ EDS, and TGA is shown in Table 1. In the repetitive analysis, the determined cesium content may be slightly varied by the different levels of water rinsing that indicate an unstable solvated structure and the high mobility of the cesium ion. The chemical titration for a fresh sample has a 3.4(0) valence state for cobalt ion. However, the valence state degrades to 3.2(6) and 3.2(1) after being preserved in a saturated CsOH(aq) solution for 3 and 7 days, respectively. It reflects an intrinsic metastable property. The Co3O4 phase transformation was observed to have been completed after one month. The valence state derived from the chemical formula cannot be matched with the measured 3.4(0) valence state, indicating that the sample has a nonstoichiometry structure or H3O+ inclusion, as reported in the BLHNaxCoO2•yH2O system.16,17 The comprehensive formula Csx(H3O)zCoO2•nH2O can be derived from the cobalt valence state which is applied to have a more precise Cs0.29 (H3O)0.31CoO2•nH2O formula. It would be difficult to experimentally differentiate the values of z and n with accuracy. We thus report the values of total water content and the valence state instead of the z and n determination. The paramagnetic behavior of the BLH-Cs0.29CoO2•1.12H2O sample shown in Figure 1 is observed down to the temperature of 1.7 K. Figure 2 shows the powder X-ray diffraction pattern of the BLH-Cs0.29CoO2•1.12H2O compound. The data show the indexed peaks from the sample as well as an additional broad peak that originates from the γ-CoOOH impurity. The hexagonal j and the strong preferred orientation of the space group R3m basal plane are identified. A good agreement with the diffraction pattern is achieved, with the exception of a few much weaker peaks from the dehydrated phase. A widely expanded c-axis (c ) 30.0(9) Å) and a single isotropic peak of solid state NMR results are signs of the weak interaction between solvated Cs+ ions and Co-O layers.11 The weakly bonded Cs+ ion indicates its potential application as a host material for ion exchange reaction. Ion-Exchanged Product Holds the BLH Structure. The ion exchange of Na+ for Cs+ in the BLH-Cs0.29CoO2•1.12H2O host structure turns into a BLH-Na0.35Cs0.02CoO2•1.46H2O stoichiometry, as determined by EDS, AAS, and TGA. The superconductivity is generated by the ion exchange of Na+ for Cs+. The field-cooled and zero field-cooled magnetic susceptibility measurements of Na0.35Cs0.02CoO2•1.46H2O display a superconductivity of 4.2 K Tc as shown in Figure 1. The ideal
Figure 1. SQUID measurement of the ion-exchanged Na0.35 Cs0.02CoO2•1.46H2O sample. The applied field is 10 G, and the temperature range began from 1.7 K. The measurement of Cs0.29CoO2•1.12H2O precursor is shown in the inset.
Figure 2. Powder XRD pattern of the as-grown Cs0.29CoO2•1.12H2O compound: (∆) dehydrated phase; (*) γ-CoOOH impurity phase.
chemical composition of the BLH-NaxCoO2•yH2O superconducting phase has been proposed by Jorgensen.18 It has a refined Na composition of x ) 0.31(3) and a perfect 1:4 Na to H2O ratio. The measured chemical stoichiometry of the ionexchanged Na0.35Cs0.02CoO2•1.46H2O complies with the 1:4 ratio. Traces of cesium content may result from the entrapped Cs+ ions in its structural defects. Chemical titration obtains a 3.3(7) valence state, which is lower than the values of the precursor and reported superconducting phase.1 The valence state is reduced slightly after 5 days of ion exchange reaction, which can be attributed to the result of the γ-CoOOH phase formation as shown in Figure 3c.
2740 Crystal Growth & Design, Vol. 7, No. 12, 2007
Figure 3. XRD patterns of compounds (a) as-grown Cs0.29 CoO2•1.12H2O and (b) ion exchange of Na+ for 3 days and (c) for 5 days.
Figure 4. XRD patterns of (a) as-grown BLH-Cs0.29CoO2•1.12H2O, (b) ion exchanged Na+-BLH, and (c) Li+-MLH.
Powder X-ray analysis of the Na0.35Cs0.02CoO2•1.46H2O in Figure 3 shows the gradual reduction of CoO2 interplanar j spacing from 10.0(3) to 9.8(2) Å and the sustaining of the R3m space group in ion exchange reaction. The indexed XRD pattern is provided in the Supporting Information. Three BLHNaxCoO2•yH2O phases have been reported in past few years, namely, the R-, β-, and γ-phases, which are based on the coordination differences of CoO2 plane stacking along the c-axis direction.19–21 The hydrated γ-NaxCoO2•yH2O with a P63mmc space group shows a clear superconducting transition at 4.5 K. j phases Meanwhile, the superconducting transition of the R3m occurs at around 4.2 K for the R-phase and at 4.3 K for the β-phase. The observed Tc differs slightly from one result to another.20 Alternations of stacked layers among the CoO2 planes may cause the difference in superconductivity. The ionexchanged Na0.35Cs0.02CoO2•1.46H2O compound displays the properties of the R-phase. Ion-Exchanged Products Containing the MLH Structure. The ion exchange of Li+ ions induces the phase transition from BLH to a MLH-Li0.08Cs0.20CoO2•0.78H2O structure as shown in Figure 4c. The BLH structure cannot be sustained while the ions are exchanged with Li+, K+, or Rb+ and while the interlayer spacing is reduced to 6.9(1), 6.9(6), and 7.0(3), respectively. The chemical titration of Li+, K+, and Rb+ ionexchanged MLH products shows that the valence state is close
Wang et al.
Figure 5. SQUID measurements of ion-exchanged products harvested from the solution containing Li+/Na+ mole ratios of 1/50, 1/250, and 1/500.
to the ion-exchanged Na+-BLH compound. The hydrated structure reduced along the c-axis direction has an insignificant influence on the property of valence state. For the samples prepared by the ion exchange method, it is interesting to know why only Na+ substitution can maintain the BLH structure. On the basis of theoretical calculation, the Li+ hydrate with six coordinated water molecules has the lowest hydration energy of solvated alkali cations.22 The most stable solvated Li+ ion which apparently has the MLH structure indicates the oblique relation between the BLH structural formation and the stability of the solvated structure. The ionic radius, hydration energy, and charge density of the solvated cation seem to be unable to directly correlate with the observed results. The lattice energy which increased from the hydrogen-bonded zigzag chains in order to stabilize the solvated BLH structure was proposed by Jorgensen.18 The four-coordinated water molecule is hydrogen bonded to the CoO2 plane (intermolecular force), and an additional hydrogen is bonded to the oxygen atom of a neighboring water molecule (intramolecular force) in order to facilitate the formation of hydrogen-bonded zigzag chains. According to the model, the six-coordinated Li+ ions may possibly act as the hydrogen bond breaking additive for the BLH structure. Phase transformation from BLH to MLH could be completed while the intermolecular and intramolecular hydrogen bonds are partially broken up to certain limits. The ionexchanged MLH-Li0.08Cs0.20CoO2•0.78H2O product indicates the limit of substitution close to 1/3 of Li+ ions. The experimental results reveal that the Cs+-BLH framework can easily be collapsed by introducing alkali ions with different water coordination. Therefore, the structure of the solvated cation linking the hydrogen bonds to the BLH structure formation is proven experimentally. In order to further verify the phenomenon of counterion dependence, the sensitive SQUID measurement is adopted to probe the slightly doped system. A series of solution mixtures with a Li+/Na+ mole concentration ratio from 1/500 to 1/50 is utilized to perform the ion exchange experiments. The SQUID measurements in Figure 5 display a gradually decreased Tc from 4.2 K, while the Li+/ Na+ mole ratio is slowly increased from 1/500 to 1/50. The results of chemical titration cannot differentiate the valence difference for Li+ doped compounds. Moreover, the XRD results measured from an 18 KW instrument
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Crystal Growth & Design, Vol. 7, No. 12, 2007 2741
transformation is possible between members of the same space group by reducing the interlayer spacing. The solvated Na+ ion displays the extra stability of the BLH structure and the superconductivity in the family of hydrated alkali cobalt oxides. The results point out the structure of the solvated alkali cation and the fact that the hydrogen bonds play critical roles in coupling with the BLH fine structure and superconductivity. Further extending the applicability of this reaction sequence can generate novel structures, such as the delafossite-type silverdeficient oxide AgxCoO2. Acknowledgment. We thank the National Synchrotron Radiation Research Center in Taiwan for their support. This research was funded by the National Science Council of Taiwan, Grant Nos. 95-2745-M-001-010 and 95-2218-E-260-001. Figure 6. Rietveld refinement profiles for silver-deficient Ag0.75Cs0.03CoO2 and γ-CoOOH impurity phases. Data are refined in j Crosses are the raw powder diffraction data. The the space group R3m. solid line is the calculated diffraction pattern. A difference curve is plotted at the bottom.
cannot distinguish minor structural differences as well. This is because it is not easy to identify the compositional homogeneity of these slightly doped compounds on a microscopic scale. However, the Tc decay of the Li+-doped compounds illustrates the superconductivity to be highly dependent on the type of ions in the structure. The ordering of the Na+ ions and water molecules may be influential on the CoO2 planes and is crucial in determining the electronic structure of cobaltates. Ion-Exchanged Product with an Anhydrous Structure. The promising thermoelectric properties and potential applications of delafossite-type oxides PdCoO2 and AgCoO2 have been reported.23,24 Particularly, the ion exchange of Ag+ has demonstrated the capability to prepare a novel silver-deficient compound at room temperature. The solvated Ag+ ion with two-, three-, and four-coordinated complex structures has been simulated in an aqueous solution.25 A linearly distorted structure of the hydrated Ag+ ion has recently been found to have the lowest energy state.25 Meanwhile, the linear structure of the solvated Ag+ ion indicates that the hydrated Ag+ ion could be easily paralleled to the CoO2 plane. The ion exchange results reveal a silver-deficient Ag0.75 Cs0.03CoO2 structure with a reduced interlayer spacing. The synchrotron radiation and Rietveld analysis results shown in Figure 6 confirm the silverj space group. Furthermore, the deficient stoichiometry and R3m results of chemical titration and elemental analysis shown in Table 1 indicate that the dehydrated compound has a stoichiometric structure. The chemical formulas of the precursor compose a speculated H3O+ inclusion. The ion exchange of Ag+ with the Cs0.29 CoO2•1.12H2O precursor turns out to have a high silver content (Ag0.75 Cs0.03CoO2) and a dehydrated structure. The electrostatic forces of the Ag+ ion and Co-O layers may result in the construction of the Co-O plane in a closely packed arrangement. The energy advantage of the Ag-O lattice formation could be superior to the H3O+ binding energy, while the Ag-O bond is involved with the d-s orbital overlapping.23 In such conditions, the Cs0.29CoO2•1.12H2O precursor could be possibly transformed into a high silver formula which has a dehydrated structure. Conclusion This work demonstrates a flexible method to prepare hydrated and dehydrated cobaltates at room temperature. Structural
Supporting Information Available: Indexed powder XRD pattern of the ion-exchanged Na0.35Cs0.02CoO2•1.46H2O compound. This material is available free of charge via the Internet at http://pubs.acs.org.
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