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Anion-exchange properties of nickel–aluminum layered double hydroxide prepared by liquid phase deposition
Materials Chemistry and Physics 141 (2013) 445e453
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Anion-exchange properties of nickelealuminum layered double hydroxide prepared by liquid phase deposition Hideshi Maki, Yuki Mori, Yuzo Okumura, Minoru Mizuhata* Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, 1-1 Rokkodai-cho, Nada, Kobe 657-8501, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The optimum structure of Niefluoro complexes for NieAl LDH was determined. The anion-exchange properties of Ni eAl LDH were quantitatively evaluate. [NiF6xy(NH3)x(OH)y]nþ was more suitable than [NiF6]4 as the precursor of NieAl LDH. The improved LPD reaction achieved high purity and high crystallinity Ni eAl LDH. The anion-exchange of NieAl LDH was accelerated by the neutralization of OH anions.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 June 2012 Received in revised form 2 May 2013 Accepted 17 May 2013
The optimum coordination structure of Niefluoro complexes for the preparation of NieAl LDH by LPD process and the diverse anion-exchange properties of as-deposited NieAl on a-alumina powder were quantitatively evaluated for the industrial application of new positive material for alkali secondary batteries. The [NiF6xy(NH3)x(OH)y]nþ was more suitable than [NiF6]4 as the precursor of the deposition of NieAl LDH in the LPD reaction, and the improved LPD reaction achieved the synthesis of high purity and high crystallinity NieAl LDH. All anion-exchanged NieAl LDHs for OHe, Cle, SO2 4 e, and CH3COOeforms kept the high crystallinity and showed the enlargement of interlayer distances. The tilting angle of the intercalated CH3COO anions was about 15 . Anion-exchange capacity remained constant at a minimum of 0.8 meq g1 in pH >10, increased as pH decreased, and reached a maximum of 8 meq g1 at pH 2. Anion-exchange of OHeform of NieAl LDH was accelerated by the neutralization of hydroxide ions in interlayers, in addition, the anion-exchange capacity and the crystallinity of NieAl LDH could be controlled by the amount of doped aluminum ions. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: A. Electronic materials A. Oxides A. Multilayers D. Ageing D. Thermodynamic properties
1. Introduction Layered double hydroxides (LDHs) are widely known as hydrotalcite-like compounds, two-dimensional anionic clays, and
* Corresponding author. E-mail addresses: [email protected] (M. Mizuhata).
(H.
Maki),
[email protected]
0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.05.043
anion-exchange materials. The typical chemical composition of an 3þ xþ n x LDH can be represented by [M2þ 1exMx (OH)2] [Ax/n$yH2O] , where M2þ and M3þ represent metallic cations such as Ni2þ, Mg2þ, Zn2þ, Co2þ, Al3þ, Cr3þ, Fe3þ, and Co3þ, and An represents an anion such 2 2 as OH, Cl, NO 3 , CO3 , and SO4 [1e3]. The crystal structure of LDHs consists of positively charged octahedral hydroxide layers that are electrically neutralized by exchangeable interlayer anions and water molecules occupying the interlayer spaces [4e6].
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Recently, LDH derivatives have received considerable attention because of their anion-exchange property and capacity to intercalate anions [7], allowing them to act as catalysts [8], anion-exchangers [9e12], oxides [13,14], magnetics [15,16], and electrodes for alkaline secondary batteries [17,18]. NieAl LDH is expected to be a useful NieMH battery material owing to its high discharge capacity [19,20]. In the previous studies by other workers, the LDHs have been prepared by the coprecipitation and solegel method; however, these LDHs had low crystallinities because alkali metal cations were intercalated into their interlayers [4,21e23]. In the case of b-Ni(OH)2 a high crystallinity results in a lower chargeedischarge capacity, on the other hand, in the case of aNi(OH)2 a high crystallinity results in a better performance as an active material. The b-phase has only one channel for Hþ to diffuse, thus, Hþ exchange between OH, in contrast, the a-phase has two channels for Hþ diffusion, namely, OH and interlayer water molecules. Hence, it is not necessary to expand the diffusion channel further, instead, the improvement of low crystallinity and low purity of NieAl LDHs which have a-phase is very useful for improvement of the electrochemical performance of NieMH battery materials [18,19,24]. Therefore, Mizuhata et al. suggested the liquid phase deposition (LPD) process as a novel synthesis method for a higher crystallinity NieAl LDH [19,25,26]. In the LPD process, metal oxide and/or hydroxide thin films can be directly coated on solid substrates by simply immersing the substrates in LPD aqueous solutions. The LPD reaction spontaneously proceeds at ambient condition in a balance of two equilibrium reactions, which are principally described as follows: a hydrolysis equilibrium (ligandexchange) reaction of metal fluoride complex species (Eq. (1)) and F consumption reaction with boric acid or aluminum ion as an F scavenger (Eq. (2) or (3)) [27e32]. þ H2O 4 MOn þ xF þ 2nHþ MF(x2n) x (M; e.g. Ti, Fe, Cu, Zr, Sn, W, Mo, V, Zr, Nb, Ni)
(1)
þ H3BO3 þ 4HF / BF 4 þ H3O þ 2H2O
(2)
Al3þ þ 6HF / H3AlF6 þ 3/2H2
(3)
The LPD process also has major advantages of a successful synthesis of multilayer oxides from multicomponent solutions that consist of several metal fluoride complexes [33,34]. In addition, a high purity NieAl LDH can be synthesized because no alkali cations are present in the LPD aqueous solutions. However, in the previous study, the yield of NieAl LDH synthesis by the LPD process was too low for industrial application to NieMH battery materials because of low solubility of b-Ni(OH)2 in neutral NH4F aqueous solution, which served as a nickel source for NieAl LDH [19]. Thus in this study, the b-Ni(OH)2 was first dissolved into acidic HF aqueous solution, and NH3 aqueous solution was added subsequently in order to control the pH of the LPD aqueous solution and the coordination structure of Niefluoro complexes. A remarkable improvement in the yield of NieAl LDH synthesis by the LPD method can be expected by this improvement; therefore, the equilibrium considerations for the LPD reaction solution are indispensable in order to estimate the optimal synthesis conditions (e.g., amount of added ammonia and pH of LPD reaction solution). However, the coordination structures of Niefluoro complexes that are appropriate for the LPD process have not been clarified, nor have the anion-exchange characteristics of NieAl LDH prepared by the LPD process been clarified. This information will greatly contribute to the industrial development of NieMH batteries. In this study, the influences of the coordination structures of Niefluoro complexes on the amounts of Ni2þ ion deposited and the crystallinities of NieAl LDH on glass substrates have been
investigated, and the optimum coordination structure of Niefluoro complex for the LPD reaction has been determined. The crystallinities and chemical compositions of NieAl LDH on a-alumina powder and its various anion-exchanged NieAl LDHs have been examined. Furthermore, the pH dependences of various anionexchange capacities of NieAl LDH and the interlayer distances of anion-exchanged NieAl LDHs have also been investigated. 2. Experimental section 2.1. Chemicals All chemicals used in this work were of analytical grade. A stock solution of nickel nitrate for a potentiometric analysis was prepared by dissolving analytical grade nickel nitrate hexahydrate, Ni(NO3)26H2O purchased from Wako Pure Chemical Industries, Ltd., and was standardized by a volumetric titration with EDTA using Murexide as an indicator. The properties of a-alumina powder (Sumitomo Chemical Co., Ltd., AAe03) are following: particle diameter is 0.3 mm, mean particle size is 0.2e0.3 mm, and BET surface area is 6.5 m2 g1. Hydrogen fluoride (HF) was purchased from StellaeChemifa Inc., and all other reagents were purchased from Nacalai Tesque Inc.. 2.2. Determination of the coordination structures of Niefluoro complexes 2.2.1. Preparation of Niefluoro complexes solutions 0.045 g of Ni(NO3)2,6H2O were dissolved into ca. 20 mL of ionexchanged water and 9.75 g of 10 vol.% of aqueous HF were added. Then 15% of NH3 aqueous solution was added to the solutions, and the total volume of these solutions was adjusted to 50 mL with ionexchanged water. The ratios of total concentration, R( ¼([NH3] þ 2þ [NHþ 4 ])/CNi ), of a series of the solutions were from 0 to 33.3. All of mixture solutions were left to stand for 48 h at room temperature, in order to achieve to stable equilibrium because of slow ligandexchange of [Ni(H2O)6]2þ. 2.2.2. UVevis and HPLC measurements of Niefluoro complexes solutions UVevis absorption spectra were performed using a UVevis spectrophotometer (JASCO V7200), and all measurements were conducted within the range from 350 to 800 nm. An HPLC system (JASCO Gulliver 1500) was employed for the determination of the concentration of free F ion in Niefluoro complex solution. Chromatographic separations were conducted on an ion-exchange column (IC I-524A, Showa Denko K.K.) in an intelligent column oven (JASCO CO-2065) at 40 C. The mobile phase consisted of 2.3 103 mol L1 tris(hydroxymethyl) aminomethane aqueous solution and 2.5 103 mol L1phthalic acid aqueous solution, and the flow rate was set at 1.0 mL min1. 2.3. Syntheses of various NieAl LDH derivatives 2.3.1. Preparation of NieAl LDH on glass substrate Ni parent solution was prepared as follows (see also Scheme 1): 100 g of Ni(NO3)2,6H2O was dissolved into ca. 800 mL of ionexchanged water, and ca. 100 mL of 15% NH3 aqueous solution was added dropwisely until pH 7.5 under stirring at 90 rpm for 3 h. The precipitate of b-Ni(OH)2 was washed with ion-exchanged water, dried at room temperature, loaded into a plastic Erlenmeyer flask, and 0.5 mol L1 HF was added until the precipitate dissolved. Then, 15% NH3 aqueous solution was added dropwisely until pH 7.5, and an insoluble substance was removed by suction filtration. The total volume of the filtrate was adjusted to 1 L with
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2.4. Measurements of the time-dependent change of free concentrations of F anion in NaFeNi(NO3)2 mixture solutions Free F concentrations of Ni(NO3)2eNaF systems have been determined potentiometric analysis by use of a membrane-type F ion selective electrode I-571 purchased from Kyoto Electronics Manufacturing(Kyoto, Japan) and a single junction type reference electrode (Orion 90-01), both connected to an ionalizer (Orion 720A). All measurements were carried out at 25 1 C. E.M.F. values of 0.03 mol L1 NaF þ 0.005 or 0.03 mol L1 Ni(NO3)2 mixture solutions were collected at the interval of 15 s. The E.M.F. of an electrochemical cell can be expressed by the following equation:
h i E ¼ E00 þ glog F
(4)
0
Scheme 1. Preparation of Ni parent solution for the synthesis of NieAl LDH by the LPD process.
ion-exchanged water, and the total concentration of Ni2þ ion was determined by ICP-AES as 30 mmol L1. Degreased borosilicate glass (Matsunami Glass) as the substrate was suspended vertically into the parent solution at 50 C for 48 h. The substrate was removed from the solution, washed with distilled water, and dried at room temperature. 2.3.2. Preparation of NieAl LDHs on a-alumina powder (asdeposited NieAl LDH) NieAl LDH on a-alumina powder was prepared as follows: 80 mg of a-alumina powder was loaded into a plastic Erlenmeyer flask, and the parent solution as mentioned in Section 2.3.1 and Al(NO3)3 solution was added. The system was carried on ultrasonic treatment for 10 min before diving into bath to allow reaction at 50 C for 48 h with shaking at 100 times min1. The reaction product was collected by centrifugation, washed repeatedly ionexchanged water, and dried at room temperature for 1 day. This material is termed as as-deposited NieAl LDH. 2.3.3. Preparation of anion-exchanged NieAl LDHs for OHe, Cle, SO2 4 e, and CH3COO eforms 500 mg of as-deposited NieAl LDH was dipped into 100 mL of 6.0 mol L1 KOH aqueous solution at room temperature under N2 atmosphere for 48 h with stirring. The reaction product was collected by suction filtration, washed repeatedly with ionexchanged water and dried at room temperature for 1 day. This material is termed as OHeLDH. 500 mg of the OHeLDH was dipped into 100 mL of 1.0 mol L1 KCl, ZnSO4, or CH3COONa aqueous solution at room temperature under N2 atmosphere for 48 h with stirring. The reaction product was collected by suction filtration, washed repeatedly with ion-exchanged water and dried at room temperature for 1 day. These materials are termed as Cle LDH, SO2 4 eLDH, and CH3COO eLDH, respectively. 2.3.4. Preparation of anion-exchanged NieAl LDHs for Cleform (CleLDH) at various pH 500 mg of OHeLDHs were dipped into 100 mL of 1.0 mol L1 KCl aqueous solution at room temperature under N2 atmosphere for 48 h with stirring. The pH of a series of the solutions were adjusted from 0.11 to 13.89 with 1.0 mol L1 HCl or KOH aqueous solution. The reaction products were collected by suction filtration, washed repeatedly with ion-exchanged water and dried at room temperature for 1 day.
where E0 indicates a standard electrochemical potential of an electrochemical cell and g stands for the Nernstian slope. Before 0 and after titration procedures of the sample solutions, E0 and g were determined by the calibrating titrations. The solution of 0.01 mol L1 NaF was titrated stepwise into 20 cm3 portions of distilled water and the stable E.M.F. were measured. The g values were 56.0 1.0 mV, being quite close to the theoretical value. 2.5. Characterization of materials The crystalline structures of NieAl LDH (ICDD No. 38e0715), were investigated using an X-ray diffractometer (XRD; Rigaku RINT-TTR/S2) equipped with a scintillation detector, and a rotating Cu anode operating radiation at 50 kV and 300 mA. Using parallel beam optics formed by a multilayered mirror (Rigaku, Cross Beam Optics attachment), 2q/q scans were carried out. A scan rate of 4 per minute was applied within the range of 5e70 . The amounts of Ni and Al deposited on glass substrates were measured by an inductively coupled plasma atomic emission spectrometry (ICP-AES; HORIBA Ltd., ULTIMA 2000). The 1 1 cm2 samples of the depositions were dissolved into 20 mL of 0.17 mol L1 HNO3. Standard nickel and aluminum solutions with a concentration of 1000 ppm were used as reference samples. The components of NieAl LDH were investigated by an X-ray photoelectron spectrometer (XPS; JEOL JPS-9010MC). Measurements were conducted using Al-Ka as X-ray source (1486.6 eV) at power of 300 W (i.e. 10 kV potential and 30 mA emission current) and ca. 106 Pa. Samples were mounted on a carbon adhesive sheet, and stored into a sample chamber for ca. 12 h under vacuum for aging and removing humidity. Therein they were maintained at ambient temperature and a pressure of 100 mPa throughout the experiment. Calibration of binding energy was performed by taking C 1s electron peak (Eb ¼ 284.6 eV) as internal reference. The surface morphologies of the samples were observed by a field emission scanning electron microscope (FE-SEM; JEOL JEM6335F). An acceleration voltage of 15 kV was applied. In order to prevent the surface of the samples from electron charging up, carbon was coated on the samples using a carbon coater (CC-40FM; Meiwafosis Co. Ltd.). An EDX elemental mapping analyzer (EX23000BU, JEOL) was used to elucidate the distribution of Ni, Al, and Cl in CleLDH which was mentioned in Section 2.2.3. In order to confirm the anion-exchanges of as-deposited NieAl LDH for CleLDH, SO2 4 eLDH, and CH3COO eLDH, IR spectra was measured using an FT-IR spectrophotometer (JASCO FT-IR 615R) coupled with a diffuse reflectance attachment (JASCO DR-400) and an MCT (mercuryecadmiumetelluride) detector cooled by liquid nitrogen was used, as described in the literature [35,36]. The anion exchange amounts of CleLDH, SO2 4 eLDH, and CH3COO eLDH were determined by an HPLC system (JASCO Gulliver 1500) as mentioned in Section 2.2.2.
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3. Results and discussion 3.1. Elucidation of reaction time and appropriate coordination structure of Niefluoro complexes for the preparation of NieAl LDH by the LPD process Ni2þ ion is one of the most inert of all metal cations, and the thermodynamic stability of fluoride complex of Ni2þ ion is comparatively low as can be predicted by the HSAB (hard and soft acids and bases) concept, that is, the reaction time for the deposition of NieAl LDH is important for maximizing its yield when synthesized by the LPD process. Thus, the time dependence of free concentrations of F anions in NaFeNi(NO3)2 mixture solutions were investigated in this work, and results are presented in Fig. 1. The free concentrations of F anions contiguously decreased with the successive formation of [Ni(H2O)6nFn](2n)þ complex; however, it can be considered that it takes a fair amount of time, i.e., more than 20 h, to achieve a stable equilibrium because of the slow ligand-exchange of [Ni(H2O)6]2þ. Hence, an aging time of about 48 h for Niefluoro complex solution is indispensable to the optimized yield synthesis of NieAl LDH in the LPD process. It is a matter of course that the elucidation of the appropriate coordination structure of Niefluoro complexes is also important for the optimization of the synthesis conditions. In general, since various complexes of Ni2þ ions have a strong absorption in an ultravioletevisible region, UVevis absorption spectroscopy is typically useful for the consideration of Ni complex structures. Fig. 2 shows a comparison of the UVevis absorption spectra for Niefluoro complex solutions with the addition of aqueous NH3 after aging for 48 h. In this work, Niefluoro complex solutions typically had green or blue colors and exhibited two specific absorption bands in their UVevis spectra. The band appearing around 350e380 nm can be assigned to a transition from a ground 3A2g(F) state to an excited 3 T1g(P) state, and the band appearing around 590e640 nm can be assigned to a transition from a ground 3A2g(F) state to an excited 3 T1g(F) state. These adsorption bands shifted to shorter wavelength with an increasing amount of added aqueous NH3, and this shift is 2þ remarkable in that R( ¼([NH3] þ [NHþ 4 ])/CNi ) 23.3. The blue shift is due to shifts in the absorption bands when F ligands are replaced by NH3 and OH ligands lying toward the stronger end of the spectrochemical series. This UVevis spectroscopy provides
Fig. 1. Time dependence of free concentrations of F anion in NaFeNi(NO3)2 mixture solutions. (a) 0.03 mol L1 NaF þ 0.005 mol L1 Ni(NO3)2 mixture solution, (b) 0.03 mol L1 NaF þ 0.03 mol L1 Ni(NO3)2 mixture solution.
unambiguous evidence for the successive formation of [NiF6xy(NH3)x(OH)y]nþ from [NiF6]4 in Niefluoro complex solutions [37,38], i.e., the successive ligand substitutions of fluorine atoms bound to the central nickel atom in a [NiF6]4 anion and the formation of free fluoride anion from [NiF6]4 anion as follows: [NiF6]4 þ xNH3 þ yNH4OH # [NiF6exey(NH3)x(OH)y]nþ þ (x þ y) (5) F þ yNHþ 4 Hence, the stoichiometric coefficient of [NiF6xy(NH3)x (OH)y]nþ can be evaluated by determining the concentration of free fluoride anion with HPLC. Fig. 2 also presents HPLC chromatograms for injected Niefluoro complex solutions after the addition of 4 were aqueous NH3. The retention times of F, NO 3 , and [NiF6] found to be 2.7e2.8, 6.2e6.7, and 10.5e11.0 min in R 13.3, respectively. When the R-value increased, the integrated peak areas due to F and [NiF6]4 gradually reduced and an additional peak due to [NiF6xy(NH3)x(OH)y]nþ was observed at 11.5e13.5 min in R 16.7. Fig. 3a presents the R dependence of free F concentrations, which were calculated from the integrated HPLC peak areas due to F anions in Niefluoro complex solutions. The free F concentration non-monotonically increased with an increase in the R-value, and the free F concentration increased drastically at R ¼ 23.3. A similar trend has been observed in the R dependence of the pH of Niefluoro complex solutions. This transition point at R ¼ 23.3 is in good agreement with the results of the UVevis measurement, as mentioned above. The addition of aqueous NH3 causes the substitutions of the fluorine atoms bound to the central nickel atom in a [NiF6]4 anion by NH3 and OH groups. These results revealed that the coordination structures of Niefluoro complexes can be controlled by the addition of aqueous NH3. The amounts of Ni2þ ion deposited on glass substrates also presented a remarkable dependence on the R ratio, as shown in Fig. 3b. The deposition amounts of Ni2þ ion, determined by ICP-AES, almost vanish at R 20.0 and show a maximum at R ¼ 23.3; this finding is good agreement with the surface morphologies of the depositions on the glass substrates which were determined by FESEM. Furthermore, the crystallinities of the depositions also showed a dependence on the R ratios. Figure S1 (Supporting information) shows the XRD patterns of the deposited thin films on glass substrates. Prominent peaks appeared for the thin films deposited in 23.3 R 30.0, and all detected peaks showed the same diffraction characteristics as NieAl LDH [18,39,40]. No peaks due to a-Ni(OH)2 or b-Ni(OH)2 were detected. The relative intensity of the peak around 2q ¼ 35 can be used to estimate the crystallinity of NieAl LDH. The peak intensity of NieAl LDH samples also increased with increasing the R ratio up to 23.3, then it decreased at higher ratios. In sum, NieAl LDH samples displayed poor crystallinity at the lower and higher ratios of R, while the highest crystallinity was observed in the sample deposited at R ¼ 23.3. As discussed above, the deposition of optimized yield and the high crystallinity NieAl LDH in the LPD method can be achieved when the dissociation of F anion from the central nickel atom in a [NiF6]4 anion is the most conspicuous. Hence, [NiF6xy(NH3)x (OH)y]nþ is more suitable than [NiF6]4 as a precursor for the deposition of NieAl LDH in the LPD reaction. The following procedures are indispensable in the preparation of NieAl LDH by the LPD process: (1) formation of [NiF6xy(NH3)x(OH)y]nþ as the precursor by the addition of aqueous NH3 to Ni(NO3)2eHF mixture solution until pH 8; (2) considerable reaction time for the deposition of NieAl LDH (at least 24 h); and (3) addition of Al(NO3)3 aqueous solution as a fluoride scavenger and an aluminum dopant. It is a favor of the chemical instability of [NiF6xy(NH3)x(OH)y]nþ in the hydrolysis equilibrium reaction of metal fluoride complex species. This information will be very useful for creating the
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Fig. 2. (Left) UVevis spectra of Niefluoro complex solutions with addition of aqueous NH3. R: 0 (a), 3.3 (b), 6.7 (c), 10.0 (d), 13.3 (e), 16.7 (f), 20.0 (g), 23.3 (h), 26.7 (i), 30.0 (j), 33.3 (k). (Right) HPLC chromatograms of Niefluoro complex solutions with addition of aqueous NH3. R: 0 (a), 3.3 (b), 6.7 (c), 10.0 (d), 13.3 (e), 16.7 (f), 20.0 (g), 23.3 (h), 26.7 (i), 30.0 (j), 33.3 (k).
optimized yield and the high crystallinity syntheses of various metal oxide/hydroxides in the LPD method. 3.2. Preparation and physical properties of as-deposited NieAl LDH on a-alumina powder SEM and TEM micrographs of as-deposited NieAl LDH on aalumina powder which was prepared using the experimental conditions described in Section 3.1 are shown in Fig. 4a. In the SEM image, many irregular plate-like particles of as-deposited NieAl LDHs of more than 150 nm in diameter and 50 nm in thickness form densely on the surface of a-alumina powder. The transparent images of some alumina powders can be observed as shown in the TEM image, on the other hand, the surface of a-alumina powder cannot be confirmed in the SEM image, hence, the surface of a-alumina particles is covered completely with as-deposited NieAl LDH particles. Figure S2 (Supporting information) shows the XRD pattern of asdeposited NieAl LDH on a-alumina powder. The detected XRD
signals were due only to as-deposited NieAl LDH and a-alumina powder, so the high purity of obtained NieAl LDH is confirmed. The average crystallite size of NieAl LDH which was determined from the width of the (0 0 3) reflection using the Scherrer equation [41,42] was 7.4 nm. It is notable that this crystallite size of NieAl LDH obtained by the LPD process in this work is much greater than that obtained by solegel method (i.e., ca. 2 nm [24]). In order to identify the material quality, a compositional analysis of NieAl LDH was carried out by XPS. The survey scan information from this spectroscopic analysis is particularly useful in terms of identifying the elements present at surfaces of substrates. Figure S3 (Supporting information) shows XPS survey spectrum acquired from asdeposited NieAl LDH on a-alumina powder, which displays only peaks due to O, Ni, F, Al, and C (from internal reference) atoms. These peak positions are in good agreement with the literature [43]. The absence of any extra elements (i.e., alkali metal cations, etc.) in this qualitative survey scan analysis implies the compositional purity of as-deposited NieAl LDH on a-alumina powder. These findings
Fig. 3. (A) Variations of pH (-) and free F concentrations (C) of Niefluoro complex solutions with addition of aqueous NH3. Free F concentration was measured by HPLC. C2þ Ni : 30 mmol L1. (B) Variations of free F concentrations (C) and the deposition amounts of Ni2þ ion on glass substrates (:) with the pH of Niefluoro complex solutions, which were pH controlled by aqueous NH3.
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Fig. 4. (A) SEM (left) and TEM (right) micrographs of as-deposited NieAl LDH on a-alumina powder. (B) SEM micrograph (left) and corresponding EDX spectrum (top right) of the surface of CleLDH. In EDX spectrum, the amount of Cl anion increases in the order of gray, green, and white dots.
demonstrate the high purity and the high crystallinity obtained by this novel synthesis method for NieAl LDH, which are very valuable for industrial application of layered double hydroxides to NieMH battery materials. 3.3. Anion-exchange properties of as-deposited NieAl LDH on a-alumina powder FT-IR spectra of NieAl LDHs anion-exchanged for OHe, Cle, and CH3COOeforms are shown in Figure S4 (Supporting information) along with the wavenumbers of conspicuous bands. Since the LPD process was performed in aqueous solution of nickel nitrate, all anionic forms of anion-exchanged NieAl LDHs show the main bands around 1660 and 1380 cm1 which can be assigned to OeH vibration and NeO asymmetric stretching vibration of NO3, respectively. The bands around 1030 cm1 can be assigned to AleO stretching, AleOeH bridging, and AlOeH stretching modes. The intensity of the band around 1660 cm1 in the spectrum of OHe LDH seems to be stronger than that in the spectra of all other samples because of OeH vibration due to the intercalated OH anions. The characteristic band around 1120 cm1 in the spectrum of SO2 4 eLDH is due to SeO stretching vibration, and the bands around 1560 and SO2 4 e,
1400 cm1 can be assigned to C]O asymmetric stretching vibration (1560 nm), C]O symmetric stretching vibration (1405 nm), and CH3 symmetric bending vibration (1360 nm), respectively [44,45]. These anions can be findings suggest that OH, SO2 4 , and CH3COO intercalated into the interlayers of as-deposited NieAl LDH on aalumina powder. On the other hand, since Cl is a monatomic anion, it is difficult to obtain information about the intercalation of Cl anions from vibrational states of chemical bonds, e.g., IR and Raman spectroscopy; hence, to overcome this problem, SEMeEDX spectroscopy was employed in this work. Fig. 4b shows SEM micrograph and corresponding EDX elemental mapping analysis of the surface of CleLDH. Adequate amount and uniform distribution of the Cl anions in CleLDH has been confirmed; thus the intercalation of Cl anions can be considered successful. Since there is a possibility that the anion-exchange of intercalation compounds exerts a considerable influence on layer structure, XRD measurements of anion-exchanged NieAl LDHs were carried out, as shown in Fig. 5. All anion-exchanged samples exhibit the same diffraction characteristics as NieAl LDH [18,39,40]. The peak intensities remain after anion-exchange with any of anions in this work. This suggests the high crystallinity of all anionexchanged NieAl LDHs, and the shift toward shorter 2q values
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Fig. 5. X-ray diffraction patterns of anion-exchanged NieAl LDHs after deposition on a-alumina powder. As-deposited NieAl LDH (a), OHeLDH (b), CleLDH (c), SO2 4 eLDH (d), CH3COOeLDH (e). The marked peaks are due to a-alumina.
indicates an increase in the interlayer distance caused by the anionexchanges. The (0 0 3) interlayer distances, (d0 0 3), which calculated by Bragg’s equation are listed in Table 1 together with the ionic radii of various anions adopted in this work. The (d0 0 3) values of all anion-exchanged NieAl LDHs were greater than that of asdeposited NieAl LDH, and the values increased with the increasing ionic radii of anions. An important point for industrial application of NieAl LDHs is the stability of layered structure, namely, the insertion of water molecules to the interlayer of NieAl LDHs affects an original layer structure. To confirm this influence, XRD of NieAl LDH after the exposure to water vapor of 18 h and the vacuum drying at 200 C of 6 h were measured, and shown in Figure S5 (Supporting information). The (0 0 3) interlayer distance of dry LDH before the exposure to water vapor was 7.77 Å, and the interlayer distances of the LDH after the exposure to water vapor and the vacuum drying were 7.85 and 7.76 Å, respectively. It is obvious that the layered structure is steady after the insertion and the desorption of water molecules. The conformation of intercalated rod-like anions (e.g., CH3COO) in the interlayer of LDHs can usually be deduced from d-spacing value of resultant materials. In theory, the dimension of the long axis of an acetate anion is 6.0 Å, and the thickness of one NieAl LDH sheet can be estimated as about 4.6 Å, so it can be easily calculated that a basal spacing of 10.6 Å would be observed for a monolayer model with a perpendicular orientation of the acetate anions in the interlayer. In fact, the d-value of CH3COOeLDH was 5.98 Å; therefore, we could infer that the acetate anions may be intercalated into the interlayer of CH3COOeLDH with an interpenetrating monolayer model with a calculated tilting angle of about 15 .
Table 1 Interlayer distance (d0 0 3) of all anion-exchanged NieAl LDHs after deposition on aalumina powder and ionic radii of various anions which exist in the interlayer of respective anion-exchanged materials. (d0 As-deposited NieAl LDH OHeLDH CleLDH SO2 4 eLDH CH3COOeLDH
0 3)/Å
7.71 7.80 7.84 8.70 7.77
Ionic radii/Å e rOH ¼ 1.37 rCl ¼ 1.81 2 ¼ 2.81 rSO 4 6.0a
a The shape of an acetate anion is not spheroidal, hence this value is the dimension of the long axis of an acetate anion.
3.4. pH and amount of doped aluminum dependence of the anionexchange properties of OHeLDH A large number of hydroxide ions are contained in the interlayers of OHeLDH, which is the precursor to Cle, SO2 4 e, and CH3COOeLDHs. Accordingly, it can be expected that the neutralization of hydroxide anions in the interlayers of OHeLDH by low pH electrolyte solution would facilitate the intercalation of Cl, SO2 4 , and CH3COO anions into the interlayers. Fig. 6a indicates the dependences of the anion-exchange capacity from OHeform to Cleform and the (d0 0 3) interlayer distance of NieAl LDH on the pH of the electrolyte solutions. At the high pH range (pH >10), the anion-exchange capacity seemed to remain constant, and then the capacity increased as pH of the electrolyte solutions decreased, reaching a maximum of 8 meq g1 at pH 2. The OHeLDH dissolved completely at low pH (pH <2). The pH dependence of the (d0 0 3) interlayer distance was also similar to the ion-exchange capacity. These results suggest an acceleration of the anion-exchange of OHeLDH by the neutralization of hydroxide ions in the interlayers. The factors which influence the anion-exchange properties of OHeLDH include not only pH of the electrolyte solutions, as mentioned above, but also the amount of doped aluminum ions. It can be expected that high positive charge and small ionic radius of an Al3þ ion would bring about a decrease in the crystallinity of OHeLDH and depress its anion-exchange capacity. Fig. 7 presents the XRD patterns of OHeLDH at different values of the doped aluminum content. The lower crystallinity of OHeLDH, which was approximated from the (0 0 c) reflection peaks, is evident at higher amounts of substitution of Al3þ for Ni2þ. Fig. 6b shows the dependences of the anion-exchange ratios from OHeform to Cle form of NieAl LDHs at the different values of doped aluminum content. The anion-exchange ratio was calculated as follows:
Anion exchange ratioð%Þ ¼
Atotal Abulk 100 Cth $WLDH
(6)
where Atotal, Abulk, Cth, and WLDH indicate the total amount of added anions in OHeLDH, the amount of anions in the electrolyte solution after anion-exchange (determined by HPLC measurements), the theoretical ion-exchange capacity of OHeLDH, and the total weight of OHeLDH, respectively. The pH profiles of anion-
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Fig. 6. (A) Dependences of the anion-exchange capacity from OHeform to Cleform and the (d0 0 3) interlayer distance of NieAl LDH prepared by the LPD process on the pH of 1.0 mol L1 Cl solution. NieAl LDH dissolved completely at low pH (pH <2). Details about the Cl solution are given in the Experimental section. (B) Dependence of the anion-exchange ratio of NieAl LDH from OHeform to Cleformat different values of the doped aluminum content on the pH of 1.0 mol L1 Cl solutions. The NieAl LDH dissolved completely at low pH (pH <2). Details about the Cl solutions are given in the Experimental section. [Ni0.74Al0.26(OH)2][(OH)0.26yH2O] (C), [Ni0.62Al0.38(OH)2][(OH)0.38yH2O] (-).
Table 2 Anion-exchange ratio from OHeform to Cle, SO2 4 e, and CH3COO eforms of Nie Al LDHs for different doped aluminum contents at pH 10.5.
[Ni0.74Al0.26(OH)2][(Xn)0.26/n$yH2O] [Ni0.62Al0.38(OH)2][(Xn)0.38/n$yH2O]
X ¼ Cl
X ¼ SO2 4
X ¼ CH3COO
13.6% 7.4%
13.0% 10.8%
0.3% 1.1%
exchange ratios were similar to those of the anion-exchange capacity and the (d0 0 3) interlayer distance shown in Fig. 6a; that is, there were low exchange ratios at pH >10 and maximum exchange ratios at pH 2. Furthermore, the exchange ratio of aluminum-rich [Ni0.62Al0.38(OH)2][(OH)0.38yH2O] is smaller than that of aluminum-poor [Ni0.74Al0.26(OH)2][(OH)0.26yH2O] over the entire pH range, as expected. These results revealed that the crystallinities of NieAl LDHs strongly influence their anion-exchange properties. Anion-exchange ratios from OHeform to Cle, SO2 4 e, and CH3COOeforms of NieAl LDHs at pH 10.5 are listed in Table 2. The anion-exchange ratio of CH3COOeform is particularly low. The CH3COO anion is a monovalent anion that has a large molecular size; thus the surface charge density of the anion is comparatively low, and so the electrostatic interaction between the interlayers of OHeLDH and CH3COO anions is not particularly strong. Consequently, CH3COO anion is hard to intercalate into the interlayers of OHeLDH. This finding is in good agreement with a previous study reported by Miyata [46]. 4. Conclusions
Fig. 7. X-ray diffraction patterns of OHeLDH at different values of the doped aluminum content. [Ni0.74Al0.26(OH)2][(OH)0.26yH2O] (a), [Ni0.62Al0.38(OH)2] [(OH)0.38yH2O] (b). The peaks marked with reverse triangles (;) are due to aalumina, and the peak marked with a rhombus (A) is due to b-Ni(OH)2.
It was clarified by HPLC measurements that the addition of aqueous NH3 to a Niefluoro complex solution readily causes the substitutions of the fluorine atoms bound to the central nickel atom in a [NiF6]4 anion by NH3 and OH groups. The formation of [NiF6xy(NH3)x(OH)y]nþ as a precursor by the addition of aqueous NH3 to Ni(NO3)2eHF mixture solution until pH 8 and the 24 h aging process for Niefluoro complex solution were indispensable in the synthesis of NieAl LDH. The chemical instability of [NiF6xy(NH3)x(OH)y]nþ in the hydrolysis equilibrium reaction of metal fluoride complex species played an important role in the formation of the optimized yield and high crystallinity NieAl LDH
H. Maki et al. / Materials Chemistry and Physics 141 (2013) 445e453
in the LPD method. Furthermore, the purity of NieAl LDH prepared by our LPD process was much higher than that prepared by solegel method because there are no alkali cations in the LPD reaction solutions. These excellent chemical properties of NieAl LDH obtained by this improved LPD method are advantageous to the development of NieMH battery materials. All anion-exchanged OHeLDHs showed enlargements of the interlayer distances, while high crystallinity was maintained. These results suggest that NieAl LDH could be very useful for industrial application to ionexchange materials. Since the surface charge density of CH3COO anion is relatively low, the anion-exchange ratio to CH3COOeform was lower than with other anions. That is, the surface charge densities of anions are closely related to the anion-exchange capacity of OHeLDH. The following two factors also influenced the anion-exchange capacity of OHeLDH: (1) pH of the reaction solutions and (2) amount of doped aluminum ions. The neutralization of hydroxide anions in the interlayers of OHeLDH by low pH electrolyte solution facilitated the intercalation of various anions into the interlayers, and the decrease in the crystallinity of OHeLDH caused by excessive aluminum doping decreased the ion-exchange capacity. These results revealed that the anion-exchange capacity of OHeLDH can be optimized by the amount of doped aluminum ions and the electrostatic interaction between the interlayers of OHeLDH and various anions. The equilibrium considerations and the anion-exchange properties of NieAl LDH which were obtained in this work will provide useful guidelines for their applications not only as NieMH battery secondary materials but also as ion-exchangers and catalytic substances. Acknowledgment This study was carried out under the Energy and Environment Technologies Development Projects “Development of an Electric Energy Storage System for Grid-connection with New Energy Resources” supported by Kawasaki Heavy Industries (KHI) Ltd. and New Energy and Industrial Technology Organization (NEDO), and partially Grant-in-Aid for Scientific Research (B) (No. 22350094) by JSPS, Japan. Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. matchemphys.2013.05.043. References [1] R. Allman, Chimia 24 (1970) 99. [2] X. Duan, D.G. Evans, Layered Double Hydroxide, first ed., Springer, Berlin, 2006, p. 119. (Chapter 2).
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[3] S. Miyata, T. Kimura, Chem. Lett. 8 (1973) 843. [4] S. Aisawa, S. Takahashi, W. Ogasawara, Y. Umetsu, E. Narita, J. Solid State Chem. 162 (2001) 52. [5] S. Aisawa, H. Hirahara, K. Ishiyama, W. Ogasawara, Y. Umetsu, E. Narita, J. Solid State Chem. 174 (2003) 342. [6] S. Aisawa, H. Kudo, T. Hoshi, S. Takahashi, H. Hirahara, Y. Umetsu, E. Narita, J. Solid State Chem. 177 (2004) 3987. [7] F. Leroux, P.J. Besse, Chem. Mater. 13 (2001) 3507. [8] A.A. Bhattacharyya, G.M. Woltermann, J.S. Yoo, J.A. Karch, W.E. Cormier, Ind. Eng. Chem. Res. 27 (1988) 360. [9] A. Nakahira, T. Kubo, H. Murase, IEEE Trans. Magn. 43 (2007) 2442. [10] S.V. Prasanna, P.V. Kamath, C. Shivakumara, Mater. Res. Bull. 42 (2007) 1028. [11] T. Sato, T. Wakabayashi, M. Shimada, Ind. Eng. Chem. Prod. Res. Dev. 25 (1989) 89. [12] H. Yamada, Y. Watanabe, T. Hashimoto, K. Tamura, T. Ikoma, S. Yokoyama, J. Tanaka, Y. Moriyoshi, J. Eur. Ceram. Soc. 26 (2006) 463. [13] E. Gardener, K.M. Huntoon, T. Pinnavaia, J. Adv. Mater. 13 (2001) 1263. [14] D. Tichit, M.J.M. Ortiz, D. Francova, C. Gérardin, B. Coq, R. Durand, F. Prinetto, G. Ghiotti, Appl. Catal. A: Gen. 318 (2007) 170. [15] M. Taibi, S. Ammar, N. Jouini, F. Fievret, P. Molinie, M. Drillon, J. Mater. Chem. 12 (2002) 3238. [16] A. Nakahira, H. Murase, J. Appl. Phys. 101 (2007) 09N516. [17] P.V. Kamath, G.H.A. Therese, J. Solid State Chem. 128 (1997) 38. [18] A. Sugimoto, S. Ishida, K. Hanawa, J. Electrochem. Soc. 146 (1999) 1251. [19] A.B. Béléké, M. Mizuhata, J. Power Sources 195 (2010) 7669. [20] Z. Gao, J. Wang, Z. Li, W. Yang, B. Wang, M. Hou, Y. He, Q. Liu, T. Mann, P. Yang, M. Zhang, L. Liu, Chem. Mater. 23 (2011) 3509. [21] N. Yamaguchi, T. Nakamura, K. Tadanaga, A. Matsuda, T. Minami, M. Tatsumisago, Chem. Lett. 35 (2006) 174. [22] J. Prince, A. Montoya, G. Ferrat, J.S. Valente, Chem. Mater. 21 (2009) 5826. [23] M. Morishita, T. Kakeya, S. Ochiai, T. Ozaki, Y. Kawabe, M. Watabe, T. Sakai, J. Power Sources 193 (2009) 871. [24] Q. Song, Z. Tang, H. Guo, S.L.I. Chan, J. Power Sources 112 (2002) 428. [25] A.B. Béléké, A. Hosokawa, M. Mizuhata, J. Ceram. Soc. Japan 117 (2009) 392. [26] M. Mizuhata, A. Hosokawa, A.B. Béléké, S. Deki, Chem. Lett. 38 (2009) 972. [27] H. Nagayama, H. Honda, H. Kawahara, J. Electrochem. Soc. 135 (1988) 2012. [28] A. Hishinuma, T. Goda, M. Kitaoka, S. Hayashi, H. Kawahara, Appl. Surf. Sci. 48/ 49 (1991) 405. [29] S. Deki, Hnin Yu, Yu Ko, T. Fujita, K. Akamatsu, M. Mizuhata, A. Kajinami, Eur. Phys. J. D 16 (2001) 325. [30] S. Deki, S. Iizuka, A. Horie, M. Mizuhata, A. Kajinami, Chem. Mater. 16 (2004) 747. [31] S. Deki, S. Iizuka, K. Akamatsu, M. Mizuhata, A. Kajinami, J. Am. Ceram. Soc. 88 (2005) 731. [32] S. Deki, A. Hosokawa, A.B. Béléké, M. Mizuhata, Thin Solid Films 517 (2009) 1546. [33] S. Deki, S. Iizuka, M. Mizuhata, A. Kajinami, J. Electroanal. Chem. 584 (2005) 38. [34] K. Kuratani, M. Mizuhata, A. Kajinami, S. Deki, J. Alloys Compd. 408 (2006) 711. [35] A.B. Béléké, M. Mizuhata, A. Kajinami, S. Deki, J. Colloid Interface Sci. 268 (2003) 413. [36] A.B. Béléké, M. Mizuhata, S. Deki, Phys. Chem. Chem. Phys. 5 (2003) 2089. [37] C.W. Reimann, J. Phys. Chem. 74 (1970) 561. [38] B. Dominik, F. Heinz, J. Phys. Chem. B 107 (2003) 8547. [39] P.V. Kamath, M. Dixit, L. Indira, A.K. Shukla, V.G. Kumar, N. Munichandraiah, J. Electrochem. Soc. 141 (1994) 2956. [40] L. Lei, M. Hu, X. Gao, Y. Sun, Electrochim. Acta 54 (2008) 671. [41] A.L. Patterson, Phys. Rev. 56 (1939) 978. [42] M.T. Weller, The application and interpretation of powder X-ray diffraction data, in: Inorganic Materials Chemistry, Oxford University Press, New York, 1994, pp. 15e25. [43] N. Ikeo, Y. Iijima, N. Niimura, M. Shigematsu, M. Tazawa, S. Matsumoto, K. Kojima, Y. Nagasawa, Handbook of X-ray Photoelectron Microscopy, JEOL, 1991, p. 96. [44] V. Prevot, C. Forano, J.P. Besse, Chem. Mater. 17 (2005) 6695. [45] H. Li, J. Ma, D.G. Evans, T. Zhou, F. Li, X. Duan, Chem. Mater. 18 (2006) 4405. [46] S. Miyata, Clays Clay Miner. 28 (1980) 50.
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Anion-exchange properties of nickelealuminum layered double hydroxide prepared by liquid phase deposition Hideshi Maki, Yuki Mori, Yuzo Okumura, Minoru Mizuhata* Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, 1-1 Rokkodai-cho, Nada, Kobe 657-8501, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The optimum structure of Niefluoro complexes for NieAl LDH was determined. The anion-exchange properties of Ni eAl LDH were quantitatively evaluate. [NiF6xy(NH3)x(OH)y]nþ was more suitable than [NiF6]4 as the precursor of NieAl LDH. The improved LPD reaction achieved high purity and high crystallinity Ni eAl LDH. The anion-exchange of NieAl LDH was accelerated by the neutralization of OH anions.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 June 2012 Received in revised form 2 May 2013 Accepted 17 May 2013
The optimum coordination structure of Niefluoro complexes for the preparation of NieAl LDH by LPD process and the diverse anion-exchange properties of as-deposited NieAl on a-alumina powder were quantitatively evaluated for the industrial application of new positive material for alkali secondary batteries. The [NiF6xy(NH3)x(OH)y]nþ was more suitable than [NiF6]4 as the precursor of the deposition of NieAl LDH in the LPD reaction, and the improved LPD reaction achieved the synthesis of high purity and high crystallinity NieAl LDH. All anion-exchanged NieAl LDHs for OHe, Cle, SO2 4 e, and CH3COOeforms kept the high crystallinity and showed the enlargement of interlayer distances. The tilting angle of the intercalated CH3COO anions was about 15 . Anion-exchange capacity remained constant at a minimum of 0.8 meq g1 in pH >10, increased as pH decreased, and reached a maximum of 8 meq g1 at pH 2. Anion-exchange of OHeform of NieAl LDH was accelerated by the neutralization of hydroxide ions in interlayers, in addition, the anion-exchange capacity and the crystallinity of NieAl LDH could be controlled by the amount of doped aluminum ions. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: A. Electronic materials A. Oxides A. Multilayers D. Ageing D. Thermodynamic properties
1. Introduction Layered double hydroxides (LDHs) are widely known as hydrotalcite-like compounds, two-dimensional anionic clays, and
* Corresponding author. E-mail addresses: [email protected] (M. Mizuhata).
(H.
Maki),
[email protected]
0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.05.043
anion-exchange materials. The typical chemical composition of an 3þ xþ n x LDH can be represented by [M2þ 1exMx (OH)2] [Ax/n$yH2O] , where M2þ and M3þ represent metallic cations such as Ni2þ, Mg2þ, Zn2þ, Co2þ, Al3þ, Cr3þ, Fe3þ, and Co3þ, and An represents an anion such 2 2 as OH, Cl, NO 3 , CO3 , and SO4 [1e3]. The crystal structure of LDHs consists of positively charged octahedral hydroxide layers that are electrically neutralized by exchangeable interlayer anions and water molecules occupying the interlayer spaces [4e6].
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Recently, LDH derivatives have received considerable attention because of their anion-exchange property and capacity to intercalate anions [7], allowing them to act as catalysts [8], anion-exchangers [9e12], oxides [13,14], magnetics [15,16], and electrodes for alkaline secondary batteries [17,18]. NieAl LDH is expected to be a useful NieMH battery material owing to its high discharge capacity [19,20]. In the previous studies by other workers, the LDHs have been prepared by the coprecipitation and solegel method; however, these LDHs had low crystallinities because alkali metal cations were intercalated into their interlayers [4,21e23]. In the case of b-Ni(OH)2 a high crystallinity results in a lower chargeedischarge capacity, on the other hand, in the case of aNi(OH)2 a high crystallinity results in a better performance as an active material. The b-phase has only one channel for Hþ to diffuse, thus, Hþ exchange between OH, in contrast, the a-phase has two channels for Hþ diffusion, namely, OH and interlayer water molecules. Hence, it is not necessary to expand the diffusion channel further, instead, the improvement of low crystallinity and low purity of NieAl LDHs which have a-phase is very useful for improvement of the electrochemical performance of NieMH battery materials [18,19,24]. Therefore, Mizuhata et al. suggested the liquid phase deposition (LPD) process as a novel synthesis method for a higher crystallinity NieAl LDH [19,25,26]. In the LPD process, metal oxide and/or hydroxide thin films can be directly coated on solid substrates by simply immersing the substrates in LPD aqueous solutions. The LPD reaction spontaneously proceeds at ambient condition in a balance of two equilibrium reactions, which are principally described as follows: a hydrolysis equilibrium (ligandexchange) reaction of metal fluoride complex species (Eq. (1)) and F consumption reaction with boric acid or aluminum ion as an F scavenger (Eq. (2) or (3)) [27e32]. þ H2O 4 MOn þ xF þ 2nHþ MF(x2n) x (M; e.g. Ti, Fe, Cu, Zr, Sn, W, Mo, V, Zr, Nb, Ni)
(1)
þ H3BO3 þ 4HF / BF 4 þ H3O þ 2H2O
(2)
Al3þ þ 6HF / H3AlF6 þ 3/2H2
(3)
The LPD process also has major advantages of a successful synthesis of multilayer oxides from multicomponent solutions that consist of several metal fluoride complexes [33,34]. In addition, a high purity NieAl LDH can be synthesized because no alkali cations are present in the LPD aqueous solutions. However, in the previous study, the yield of NieAl LDH synthesis by the LPD process was too low for industrial application to NieMH battery materials because of low solubility of b-Ni(OH)2 in neutral NH4F aqueous solution, which served as a nickel source for NieAl LDH [19]. Thus in this study, the b-Ni(OH)2 was first dissolved into acidic HF aqueous solution, and NH3 aqueous solution was added subsequently in order to control the pH of the LPD aqueous solution and the coordination structure of Niefluoro complexes. A remarkable improvement in the yield of NieAl LDH synthesis by the LPD method can be expected by this improvement; therefore, the equilibrium considerations for the LPD reaction solution are indispensable in order to estimate the optimal synthesis conditions (e.g., amount of added ammonia and pH of LPD reaction solution). However, the coordination structures of Niefluoro complexes that are appropriate for the LPD process have not been clarified, nor have the anion-exchange characteristics of NieAl LDH prepared by the LPD process been clarified. This information will greatly contribute to the industrial development of NieMH batteries. In this study, the influences of the coordination structures of Niefluoro complexes on the amounts of Ni2þ ion deposited and the crystallinities of NieAl LDH on glass substrates have been
investigated, and the optimum coordination structure of Niefluoro complex for the LPD reaction has been determined. The crystallinities and chemical compositions of NieAl LDH on a-alumina powder and its various anion-exchanged NieAl LDHs have been examined. Furthermore, the pH dependences of various anionexchange capacities of NieAl LDH and the interlayer distances of anion-exchanged NieAl LDHs have also been investigated. 2. Experimental section 2.1. Chemicals All chemicals used in this work were of analytical grade. A stock solution of nickel nitrate for a potentiometric analysis was prepared by dissolving analytical grade nickel nitrate hexahydrate, Ni(NO3)26H2O purchased from Wako Pure Chemical Industries, Ltd., and was standardized by a volumetric titration with EDTA using Murexide as an indicator. The properties of a-alumina powder (Sumitomo Chemical Co., Ltd., AAe03) are following: particle diameter is 0.3 mm, mean particle size is 0.2e0.3 mm, and BET surface area is 6.5 m2 g1. Hydrogen fluoride (HF) was purchased from StellaeChemifa Inc., and all other reagents were purchased from Nacalai Tesque Inc.. 2.2. Determination of the coordination structures of Niefluoro complexes 2.2.1. Preparation of Niefluoro complexes solutions 0.045 g of Ni(NO3)2,6H2O were dissolved into ca. 20 mL of ionexchanged water and 9.75 g of 10 vol.% of aqueous HF were added. Then 15% of NH3 aqueous solution was added to the solutions, and the total volume of these solutions was adjusted to 50 mL with ionexchanged water. The ratios of total concentration, R( ¼([NH3] þ 2þ [NHþ 4 ])/CNi ), of a series of the solutions were from 0 to 33.3. All of mixture solutions were left to stand for 48 h at room temperature, in order to achieve to stable equilibrium because of slow ligandexchange of [Ni(H2O)6]2þ. 2.2.2. UVevis and HPLC measurements of Niefluoro complexes solutions UVevis absorption spectra were performed using a UVevis spectrophotometer (JASCO V7200), and all measurements were conducted within the range from 350 to 800 nm. An HPLC system (JASCO Gulliver 1500) was employed for the determination of the concentration of free F ion in Niefluoro complex solution. Chromatographic separations were conducted on an ion-exchange column (IC I-524A, Showa Denko K.K.) in an intelligent column oven (JASCO CO-2065) at 40 C. The mobile phase consisted of 2.3 103 mol L1 tris(hydroxymethyl) aminomethane aqueous solution and 2.5 103 mol L1phthalic acid aqueous solution, and the flow rate was set at 1.0 mL min1. 2.3. Syntheses of various NieAl LDH derivatives 2.3.1. Preparation of NieAl LDH on glass substrate Ni parent solution was prepared as follows (see also Scheme 1): 100 g of Ni(NO3)2,6H2O was dissolved into ca. 800 mL of ionexchanged water, and ca. 100 mL of 15% NH3 aqueous solution was added dropwisely until pH 7.5 under stirring at 90 rpm for 3 h. The precipitate of b-Ni(OH)2 was washed with ion-exchanged water, dried at room temperature, loaded into a plastic Erlenmeyer flask, and 0.5 mol L1 HF was added until the precipitate dissolved. Then, 15% NH3 aqueous solution was added dropwisely until pH 7.5, and an insoluble substance was removed by suction filtration. The total volume of the filtrate was adjusted to 1 L with
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2.4. Measurements of the time-dependent change of free concentrations of F anion in NaFeNi(NO3)2 mixture solutions Free F concentrations of Ni(NO3)2eNaF systems have been determined potentiometric analysis by use of a membrane-type F ion selective electrode I-571 purchased from Kyoto Electronics Manufacturing(Kyoto, Japan) and a single junction type reference electrode (Orion 90-01), both connected to an ionalizer (Orion 720A). All measurements were carried out at 25 1 C. E.M.F. values of 0.03 mol L1 NaF þ 0.005 or 0.03 mol L1 Ni(NO3)2 mixture solutions were collected at the interval of 15 s. The E.M.F. of an electrochemical cell can be expressed by the following equation:
h i E ¼ E00 þ glog F
(4)
0
Scheme 1. Preparation of Ni parent solution for the synthesis of NieAl LDH by the LPD process.
ion-exchanged water, and the total concentration of Ni2þ ion was determined by ICP-AES as 30 mmol L1. Degreased borosilicate glass (Matsunami Glass) as the substrate was suspended vertically into the parent solution at 50 C for 48 h. The substrate was removed from the solution, washed with distilled water, and dried at room temperature. 2.3.2. Preparation of NieAl LDHs on a-alumina powder (asdeposited NieAl LDH) NieAl LDH on a-alumina powder was prepared as follows: 80 mg of a-alumina powder was loaded into a plastic Erlenmeyer flask, and the parent solution as mentioned in Section 2.3.1 and Al(NO3)3 solution was added. The system was carried on ultrasonic treatment for 10 min before diving into bath to allow reaction at 50 C for 48 h with shaking at 100 times min1. The reaction product was collected by centrifugation, washed repeatedly ionexchanged water, and dried at room temperature for 1 day. This material is termed as as-deposited NieAl LDH. 2.3.3. Preparation of anion-exchanged NieAl LDHs for OHe, Cle, SO2 4 e, and CH3COO eforms 500 mg of as-deposited NieAl LDH was dipped into 100 mL of 6.0 mol L1 KOH aqueous solution at room temperature under N2 atmosphere for 48 h with stirring. The reaction product was collected by suction filtration, washed repeatedly with ionexchanged water and dried at room temperature for 1 day. This material is termed as OHeLDH. 500 mg of the OHeLDH was dipped into 100 mL of 1.0 mol L1 KCl, ZnSO4, or CH3COONa aqueous solution at room temperature under N2 atmosphere for 48 h with stirring. The reaction product was collected by suction filtration, washed repeatedly with ion-exchanged water and dried at room temperature for 1 day. These materials are termed as Cle LDH, SO2 4 eLDH, and CH3COO eLDH, respectively. 2.3.4. Preparation of anion-exchanged NieAl LDHs for Cleform (CleLDH) at various pH 500 mg of OHeLDHs were dipped into 100 mL of 1.0 mol L1 KCl aqueous solution at room temperature under N2 atmosphere for 48 h with stirring. The pH of a series of the solutions were adjusted from 0.11 to 13.89 with 1.0 mol L1 HCl or KOH aqueous solution. The reaction products were collected by suction filtration, washed repeatedly with ion-exchanged water and dried at room temperature for 1 day.
where E0 indicates a standard electrochemical potential of an electrochemical cell and g stands for the Nernstian slope. Before 0 and after titration procedures of the sample solutions, E0 and g were determined by the calibrating titrations. The solution of 0.01 mol L1 NaF was titrated stepwise into 20 cm3 portions of distilled water and the stable E.M.F. were measured. The g values were 56.0 1.0 mV, being quite close to the theoretical value. 2.5. Characterization of materials The crystalline structures of NieAl LDH (ICDD No. 38e0715), were investigated using an X-ray diffractometer (XRD; Rigaku RINT-TTR/S2) equipped with a scintillation detector, and a rotating Cu anode operating radiation at 50 kV and 300 mA. Using parallel beam optics formed by a multilayered mirror (Rigaku, Cross Beam Optics attachment), 2q/q scans were carried out. A scan rate of 4 per minute was applied within the range of 5e70 . The amounts of Ni and Al deposited on glass substrates were measured by an inductively coupled plasma atomic emission spectrometry (ICP-AES; HORIBA Ltd., ULTIMA 2000). The 1 1 cm2 samples of the depositions were dissolved into 20 mL of 0.17 mol L1 HNO3. Standard nickel and aluminum solutions with a concentration of 1000 ppm were used as reference samples. The components of NieAl LDH were investigated by an X-ray photoelectron spectrometer (XPS; JEOL JPS-9010MC). Measurements were conducted using Al-Ka as X-ray source (1486.6 eV) at power of 300 W (i.e. 10 kV potential and 30 mA emission current) and ca. 106 Pa. Samples were mounted on a carbon adhesive sheet, and stored into a sample chamber for ca. 12 h under vacuum for aging and removing humidity. Therein they were maintained at ambient temperature and a pressure of 100 mPa throughout the experiment. Calibration of binding energy was performed by taking C 1s electron peak (Eb ¼ 284.6 eV) as internal reference. The surface morphologies of the samples were observed by a field emission scanning electron microscope (FE-SEM; JEOL JEM6335F). An acceleration voltage of 15 kV was applied. In order to prevent the surface of the samples from electron charging up, carbon was coated on the samples using a carbon coater (CC-40FM; Meiwafosis Co. Ltd.). An EDX elemental mapping analyzer (EX23000BU, JEOL) was used to elucidate the distribution of Ni, Al, and Cl in CleLDH which was mentioned in Section 2.2.3. In order to confirm the anion-exchanges of as-deposited NieAl LDH for CleLDH, SO2 4 eLDH, and CH3COO eLDH, IR spectra was measured using an FT-IR spectrophotometer (JASCO FT-IR 615R) coupled with a diffuse reflectance attachment (JASCO DR-400) and an MCT (mercuryecadmiumetelluride) detector cooled by liquid nitrogen was used, as described in the literature [35,36]. The anion exchange amounts of CleLDH, SO2 4 eLDH, and CH3COO eLDH were determined by an HPLC system (JASCO Gulliver 1500) as mentioned in Section 2.2.2.
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3. Results and discussion 3.1. Elucidation of reaction time and appropriate coordination structure of Niefluoro complexes for the preparation of NieAl LDH by the LPD process Ni2þ ion is one of the most inert of all metal cations, and the thermodynamic stability of fluoride complex of Ni2þ ion is comparatively low as can be predicted by the HSAB (hard and soft acids and bases) concept, that is, the reaction time for the deposition of NieAl LDH is important for maximizing its yield when synthesized by the LPD process. Thus, the time dependence of free concentrations of F anions in NaFeNi(NO3)2 mixture solutions were investigated in this work, and results are presented in Fig. 1. The free concentrations of F anions contiguously decreased with the successive formation of [Ni(H2O)6nFn](2n)þ complex; however, it can be considered that it takes a fair amount of time, i.e., more than 20 h, to achieve a stable equilibrium because of the slow ligand-exchange of [Ni(H2O)6]2þ. Hence, an aging time of about 48 h for Niefluoro complex solution is indispensable to the optimized yield synthesis of NieAl LDH in the LPD process. It is a matter of course that the elucidation of the appropriate coordination structure of Niefluoro complexes is also important for the optimization of the synthesis conditions. In general, since various complexes of Ni2þ ions have a strong absorption in an ultravioletevisible region, UVevis absorption spectroscopy is typically useful for the consideration of Ni complex structures. Fig. 2 shows a comparison of the UVevis absorption spectra for Niefluoro complex solutions with the addition of aqueous NH3 after aging for 48 h. In this work, Niefluoro complex solutions typically had green or blue colors and exhibited two specific absorption bands in their UVevis spectra. The band appearing around 350e380 nm can be assigned to a transition from a ground 3A2g(F) state to an excited 3 T1g(P) state, and the band appearing around 590e640 nm can be assigned to a transition from a ground 3A2g(F) state to an excited 3 T1g(F) state. These adsorption bands shifted to shorter wavelength with an increasing amount of added aqueous NH3, and this shift is 2þ remarkable in that R( ¼([NH3] þ [NHþ 4 ])/CNi ) 23.3. The blue shift is due to shifts in the absorption bands when F ligands are replaced by NH3 and OH ligands lying toward the stronger end of the spectrochemical series. This UVevis spectroscopy provides
Fig. 1. Time dependence of free concentrations of F anion in NaFeNi(NO3)2 mixture solutions. (a) 0.03 mol L1 NaF þ 0.005 mol L1 Ni(NO3)2 mixture solution, (b) 0.03 mol L1 NaF þ 0.03 mol L1 Ni(NO3)2 mixture solution.
unambiguous evidence for the successive formation of [NiF6xy(NH3)x(OH)y]nþ from [NiF6]4 in Niefluoro complex solutions [37,38], i.e., the successive ligand substitutions of fluorine atoms bound to the central nickel atom in a [NiF6]4 anion and the formation of free fluoride anion from [NiF6]4 anion as follows: [NiF6]4 þ xNH3 þ yNH4OH # [NiF6exey(NH3)x(OH)y]nþ þ (x þ y) (5) F þ yNHþ 4 Hence, the stoichiometric coefficient of [NiF6xy(NH3)x (OH)y]nþ can be evaluated by determining the concentration of free fluoride anion with HPLC. Fig. 2 also presents HPLC chromatograms for injected Niefluoro complex solutions after the addition of 4 were aqueous NH3. The retention times of F, NO 3 , and [NiF6] found to be 2.7e2.8, 6.2e6.7, and 10.5e11.0 min in R 13.3, respectively. When the R-value increased, the integrated peak areas due to F and [NiF6]4 gradually reduced and an additional peak due to [NiF6xy(NH3)x(OH)y]nþ was observed at 11.5e13.5 min in R 16.7. Fig. 3a presents the R dependence of free F concentrations, which were calculated from the integrated HPLC peak areas due to F anions in Niefluoro complex solutions. The free F concentration non-monotonically increased with an increase in the R-value, and the free F concentration increased drastically at R ¼ 23.3. A similar trend has been observed in the R dependence of the pH of Niefluoro complex solutions. This transition point at R ¼ 23.3 is in good agreement with the results of the UVevis measurement, as mentioned above. The addition of aqueous NH3 causes the substitutions of the fluorine atoms bound to the central nickel atom in a [NiF6]4 anion by NH3 and OH groups. These results revealed that the coordination structures of Niefluoro complexes can be controlled by the addition of aqueous NH3. The amounts of Ni2þ ion deposited on glass substrates also presented a remarkable dependence on the R ratio, as shown in Fig. 3b. The deposition amounts of Ni2þ ion, determined by ICP-AES, almost vanish at R 20.0 and show a maximum at R ¼ 23.3; this finding is good agreement with the surface morphologies of the depositions on the glass substrates which were determined by FESEM. Furthermore, the crystallinities of the depositions also showed a dependence on the R ratios. Figure S1 (Supporting information) shows the XRD patterns of the deposited thin films on glass substrates. Prominent peaks appeared for the thin films deposited in 23.3 R 30.0, and all detected peaks showed the same diffraction characteristics as NieAl LDH [18,39,40]. No peaks due to a-Ni(OH)2 or b-Ni(OH)2 were detected. The relative intensity of the peak around 2q ¼ 35 can be used to estimate the crystallinity of NieAl LDH. The peak intensity of NieAl LDH samples also increased with increasing the R ratio up to 23.3, then it decreased at higher ratios. In sum, NieAl LDH samples displayed poor crystallinity at the lower and higher ratios of R, while the highest crystallinity was observed in the sample deposited at R ¼ 23.3. As discussed above, the deposition of optimized yield and the high crystallinity NieAl LDH in the LPD method can be achieved when the dissociation of F anion from the central nickel atom in a [NiF6]4 anion is the most conspicuous. Hence, [NiF6xy(NH3)x (OH)y]nþ is more suitable than [NiF6]4 as a precursor for the deposition of NieAl LDH in the LPD reaction. The following procedures are indispensable in the preparation of NieAl LDH by the LPD process: (1) formation of [NiF6xy(NH3)x(OH)y]nþ as the precursor by the addition of aqueous NH3 to Ni(NO3)2eHF mixture solution until pH 8; (2) considerable reaction time for the deposition of NieAl LDH (at least 24 h); and (3) addition of Al(NO3)3 aqueous solution as a fluoride scavenger and an aluminum dopant. It is a favor of the chemical instability of [NiF6xy(NH3)x(OH)y]nþ in the hydrolysis equilibrium reaction of metal fluoride complex species. This information will be very useful for creating the
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Fig. 2. (Left) UVevis spectra of Niefluoro complex solutions with addition of aqueous NH3. R: 0 (a), 3.3 (b), 6.7 (c), 10.0 (d), 13.3 (e), 16.7 (f), 20.0 (g), 23.3 (h), 26.7 (i), 30.0 (j), 33.3 (k). (Right) HPLC chromatograms of Niefluoro complex solutions with addition of aqueous NH3. R: 0 (a), 3.3 (b), 6.7 (c), 10.0 (d), 13.3 (e), 16.7 (f), 20.0 (g), 23.3 (h), 26.7 (i), 30.0 (j), 33.3 (k).
optimized yield and the high crystallinity syntheses of various metal oxide/hydroxides in the LPD method. 3.2. Preparation and physical properties of as-deposited NieAl LDH on a-alumina powder SEM and TEM micrographs of as-deposited NieAl LDH on aalumina powder which was prepared using the experimental conditions described in Section 3.1 are shown in Fig. 4a. In the SEM image, many irregular plate-like particles of as-deposited NieAl LDHs of more than 150 nm in diameter and 50 nm in thickness form densely on the surface of a-alumina powder. The transparent images of some alumina powders can be observed as shown in the TEM image, on the other hand, the surface of a-alumina powder cannot be confirmed in the SEM image, hence, the surface of a-alumina particles is covered completely with as-deposited NieAl LDH particles. Figure S2 (Supporting information) shows the XRD pattern of asdeposited NieAl LDH on a-alumina powder. The detected XRD
signals were due only to as-deposited NieAl LDH and a-alumina powder, so the high purity of obtained NieAl LDH is confirmed. The average crystallite size of NieAl LDH which was determined from the width of the (0 0 3) reflection using the Scherrer equation [41,42] was 7.4 nm. It is notable that this crystallite size of NieAl LDH obtained by the LPD process in this work is much greater than that obtained by solegel method (i.e., ca. 2 nm [24]). In order to identify the material quality, a compositional analysis of NieAl LDH was carried out by XPS. The survey scan information from this spectroscopic analysis is particularly useful in terms of identifying the elements present at surfaces of substrates. Figure S3 (Supporting information) shows XPS survey spectrum acquired from asdeposited NieAl LDH on a-alumina powder, which displays only peaks due to O, Ni, F, Al, and C (from internal reference) atoms. These peak positions are in good agreement with the literature [43]. The absence of any extra elements (i.e., alkali metal cations, etc.) in this qualitative survey scan analysis implies the compositional purity of as-deposited NieAl LDH on a-alumina powder. These findings
Fig. 3. (A) Variations of pH (-) and free F concentrations (C) of Niefluoro complex solutions with addition of aqueous NH3. Free F concentration was measured by HPLC. C2þ Ni : 30 mmol L1. (B) Variations of free F concentrations (C) and the deposition amounts of Ni2þ ion on glass substrates (:) with the pH of Niefluoro complex solutions, which were pH controlled by aqueous NH3.
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Fig. 4. (A) SEM (left) and TEM (right) micrographs of as-deposited NieAl LDH on a-alumina powder. (B) SEM micrograph (left) and corresponding EDX spectrum (top right) of the surface of CleLDH. In EDX spectrum, the amount of Cl anion increases in the order of gray, green, and white dots.
demonstrate the high purity and the high crystallinity obtained by this novel synthesis method for NieAl LDH, which are very valuable for industrial application of layered double hydroxides to NieMH battery materials. 3.3. Anion-exchange properties of as-deposited NieAl LDH on a-alumina powder FT-IR spectra of NieAl LDHs anion-exchanged for OHe, Cle, and CH3COOeforms are shown in Figure S4 (Supporting information) along with the wavenumbers of conspicuous bands. Since the LPD process was performed in aqueous solution of nickel nitrate, all anionic forms of anion-exchanged NieAl LDHs show the main bands around 1660 and 1380 cm1 which can be assigned to OeH vibration and NeO asymmetric stretching vibration of NO3, respectively. The bands around 1030 cm1 can be assigned to AleO stretching, AleOeH bridging, and AlOeH stretching modes. The intensity of the band around 1660 cm1 in the spectrum of OHe LDH seems to be stronger than that in the spectra of all other samples because of OeH vibration due to the intercalated OH anions. The characteristic band around 1120 cm1 in the spectrum of SO2 4 eLDH is due to SeO stretching vibration, and the bands around 1560 and SO2 4 e,
1400 cm1 can be assigned to C]O asymmetric stretching vibration (1560 nm), C]O symmetric stretching vibration (1405 nm), and CH3 symmetric bending vibration (1360 nm), respectively [44,45]. These anions can be findings suggest that OH, SO2 4 , and CH3COO intercalated into the interlayers of as-deposited NieAl LDH on aalumina powder. On the other hand, since Cl is a monatomic anion, it is difficult to obtain information about the intercalation of Cl anions from vibrational states of chemical bonds, e.g., IR and Raman spectroscopy; hence, to overcome this problem, SEMeEDX spectroscopy was employed in this work. Fig. 4b shows SEM micrograph and corresponding EDX elemental mapping analysis of the surface of CleLDH. Adequate amount and uniform distribution of the Cl anions in CleLDH has been confirmed; thus the intercalation of Cl anions can be considered successful. Since there is a possibility that the anion-exchange of intercalation compounds exerts a considerable influence on layer structure, XRD measurements of anion-exchanged NieAl LDHs were carried out, as shown in Fig. 5. All anion-exchanged samples exhibit the same diffraction characteristics as NieAl LDH [18,39,40]. The peak intensities remain after anion-exchange with any of anions in this work. This suggests the high crystallinity of all anionexchanged NieAl LDHs, and the shift toward shorter 2q values
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Fig. 5. X-ray diffraction patterns of anion-exchanged NieAl LDHs after deposition on a-alumina powder. As-deposited NieAl LDH (a), OHeLDH (b), CleLDH (c), SO2 4 eLDH (d), CH3COOeLDH (e). The marked peaks are due to a-alumina.
indicates an increase in the interlayer distance caused by the anionexchanges. The (0 0 3) interlayer distances, (d0 0 3), which calculated by Bragg’s equation are listed in Table 1 together with the ionic radii of various anions adopted in this work. The (d0 0 3) values of all anion-exchanged NieAl LDHs were greater than that of asdeposited NieAl LDH, and the values increased with the increasing ionic radii of anions. An important point for industrial application of NieAl LDHs is the stability of layered structure, namely, the insertion of water molecules to the interlayer of NieAl LDHs affects an original layer structure. To confirm this influence, XRD of NieAl LDH after the exposure to water vapor of 18 h and the vacuum drying at 200 C of 6 h were measured, and shown in Figure S5 (Supporting information). The (0 0 3) interlayer distance of dry LDH before the exposure to water vapor was 7.77 Å, and the interlayer distances of the LDH after the exposure to water vapor and the vacuum drying were 7.85 and 7.76 Å, respectively. It is obvious that the layered structure is steady after the insertion and the desorption of water molecules. The conformation of intercalated rod-like anions (e.g., CH3COO) in the interlayer of LDHs can usually be deduced from d-spacing value of resultant materials. In theory, the dimension of the long axis of an acetate anion is 6.0 Å, and the thickness of one NieAl LDH sheet can be estimated as about 4.6 Å, so it can be easily calculated that a basal spacing of 10.6 Å would be observed for a monolayer model with a perpendicular orientation of the acetate anions in the interlayer. In fact, the d-value of CH3COOeLDH was 5.98 Å; therefore, we could infer that the acetate anions may be intercalated into the interlayer of CH3COOeLDH with an interpenetrating monolayer model with a calculated tilting angle of about 15 .
Table 1 Interlayer distance (d0 0 3) of all anion-exchanged NieAl LDHs after deposition on aalumina powder and ionic radii of various anions which exist in the interlayer of respective anion-exchanged materials. (d0 As-deposited NieAl LDH OHeLDH CleLDH SO2 4 eLDH CH3COOeLDH
0 3)/Å
7.71 7.80 7.84 8.70 7.77
Ionic radii/Å e rOH ¼ 1.37 rCl ¼ 1.81 2 ¼ 2.81 rSO 4 6.0a
a The shape of an acetate anion is not spheroidal, hence this value is the dimension of the long axis of an acetate anion.
3.4. pH and amount of doped aluminum dependence of the anionexchange properties of OHeLDH A large number of hydroxide ions are contained in the interlayers of OHeLDH, which is the precursor to Cle, SO2 4 e, and CH3COOeLDHs. Accordingly, it can be expected that the neutralization of hydroxide anions in the interlayers of OHeLDH by low pH electrolyte solution would facilitate the intercalation of Cl, SO2 4 , and CH3COO anions into the interlayers. Fig. 6a indicates the dependences of the anion-exchange capacity from OHeform to Cleform and the (d0 0 3) interlayer distance of NieAl LDH on the pH of the electrolyte solutions. At the high pH range (pH >10), the anion-exchange capacity seemed to remain constant, and then the capacity increased as pH of the electrolyte solutions decreased, reaching a maximum of 8 meq g1 at pH 2. The OHeLDH dissolved completely at low pH (pH <2). The pH dependence of the (d0 0 3) interlayer distance was also similar to the ion-exchange capacity. These results suggest an acceleration of the anion-exchange of OHeLDH by the neutralization of hydroxide ions in the interlayers. The factors which influence the anion-exchange properties of OHeLDH include not only pH of the electrolyte solutions, as mentioned above, but also the amount of doped aluminum ions. It can be expected that high positive charge and small ionic radius of an Al3þ ion would bring about a decrease in the crystallinity of OHeLDH and depress its anion-exchange capacity. Fig. 7 presents the XRD patterns of OHeLDH at different values of the doped aluminum content. The lower crystallinity of OHeLDH, which was approximated from the (0 0 c) reflection peaks, is evident at higher amounts of substitution of Al3þ for Ni2þ. Fig. 6b shows the dependences of the anion-exchange ratios from OHeform to Cle form of NieAl LDHs at the different values of doped aluminum content. The anion-exchange ratio was calculated as follows:
Anion exchange ratioð%Þ ¼
Atotal Abulk 100 Cth $WLDH
(6)
where Atotal, Abulk, Cth, and WLDH indicate the total amount of added anions in OHeLDH, the amount of anions in the electrolyte solution after anion-exchange (determined by HPLC measurements), the theoretical ion-exchange capacity of OHeLDH, and the total weight of OHeLDH, respectively. The pH profiles of anion-
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Fig. 6. (A) Dependences of the anion-exchange capacity from OHeform to Cleform and the (d0 0 3) interlayer distance of NieAl LDH prepared by the LPD process on the pH of 1.0 mol L1 Cl solution. NieAl LDH dissolved completely at low pH (pH <2). Details about the Cl solution are given in the Experimental section. (B) Dependence of the anion-exchange ratio of NieAl LDH from OHeform to Cleformat different values of the doped aluminum content on the pH of 1.0 mol L1 Cl solutions. The NieAl LDH dissolved completely at low pH (pH <2). Details about the Cl solutions are given in the Experimental section. [Ni0.74Al0.26(OH)2][(OH)0.26yH2O] (C), [Ni0.62Al0.38(OH)2][(OH)0.38yH2O] (-).
Table 2 Anion-exchange ratio from OHeform to Cle, SO2 4 e, and CH3COO eforms of Nie Al LDHs for different doped aluminum contents at pH 10.5.
[Ni0.74Al0.26(OH)2][(Xn)0.26/n$yH2O] [Ni0.62Al0.38(OH)2][(Xn)0.38/n$yH2O]
X ¼ Cl
X ¼ SO2 4
X ¼ CH3COO
13.6% 7.4%
13.0% 10.8%
0.3% 1.1%
exchange ratios were similar to those of the anion-exchange capacity and the (d0 0 3) interlayer distance shown in Fig. 6a; that is, there were low exchange ratios at pH >10 and maximum exchange ratios at pH 2. Furthermore, the exchange ratio of aluminum-rich [Ni0.62Al0.38(OH)2][(OH)0.38yH2O] is smaller than that of aluminum-poor [Ni0.74Al0.26(OH)2][(OH)0.26yH2O] over the entire pH range, as expected. These results revealed that the crystallinities of NieAl LDHs strongly influence their anion-exchange properties. Anion-exchange ratios from OHeform to Cle, SO2 4 e, and CH3COOeforms of NieAl LDHs at pH 10.5 are listed in Table 2. The anion-exchange ratio of CH3COOeform is particularly low. The CH3COO anion is a monovalent anion that has a large molecular size; thus the surface charge density of the anion is comparatively low, and so the electrostatic interaction between the interlayers of OHeLDH and CH3COO anions is not particularly strong. Consequently, CH3COO anion is hard to intercalate into the interlayers of OHeLDH. This finding is in good agreement with a previous study reported by Miyata [46]. 4. Conclusions
Fig. 7. X-ray diffraction patterns of OHeLDH at different values of the doped aluminum content. [Ni0.74Al0.26(OH)2][(OH)0.26yH2O] (a), [Ni0.62Al0.38(OH)2] [(OH)0.38yH2O] (b). The peaks marked with reverse triangles (;) are due to aalumina, and the peak marked with a rhombus (A) is due to b-Ni(OH)2.
It was clarified by HPLC measurements that the addition of aqueous NH3 to a Niefluoro complex solution readily causes the substitutions of the fluorine atoms bound to the central nickel atom in a [NiF6]4 anion by NH3 and OH groups. The formation of [NiF6xy(NH3)x(OH)y]nþ as a precursor by the addition of aqueous NH3 to Ni(NO3)2eHF mixture solution until pH 8 and the 24 h aging process for Niefluoro complex solution were indispensable in the synthesis of NieAl LDH. The chemical instability of [NiF6xy(NH3)x(OH)y]nþ in the hydrolysis equilibrium reaction of metal fluoride complex species played an important role in the formation of the optimized yield and high crystallinity NieAl LDH
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in the LPD method. Furthermore, the purity of NieAl LDH prepared by our LPD process was much higher than that prepared by solegel method because there are no alkali cations in the LPD reaction solutions. These excellent chemical properties of NieAl LDH obtained by this improved LPD method are advantageous to the development of NieMH battery materials. All anion-exchanged OHeLDHs showed enlargements of the interlayer distances, while high crystallinity was maintained. These results suggest that NieAl LDH could be very useful for industrial application to ionexchange materials. Since the surface charge density of CH3COO anion is relatively low, the anion-exchange ratio to CH3COOeform was lower than with other anions. That is, the surface charge densities of anions are closely related to the anion-exchange capacity of OHeLDH. The following two factors also influenced the anion-exchange capacity of OHeLDH: (1) pH of the reaction solutions and (2) amount of doped aluminum ions. The neutralization of hydroxide anions in the interlayers of OHeLDH by low pH electrolyte solution facilitated the intercalation of various anions into the interlayers, and the decrease in the crystallinity of OHeLDH caused by excessive aluminum doping decreased the ion-exchange capacity. These results revealed that the anion-exchange capacity of OHeLDH can be optimized by the amount of doped aluminum ions and the electrostatic interaction between the interlayers of OHeLDH and various anions. The equilibrium considerations and the anion-exchange properties of NieAl LDH which were obtained in this work will provide useful guidelines for their applications not only as NieMH battery secondary materials but also as ion-exchangers and catalytic substances. Acknowledgment This study was carried out under the Energy and Environment Technologies Development Projects “Development of an Electric Energy Storage System for Grid-connection with New Energy Resources” supported by Kawasaki Heavy Industries (KHI) Ltd. and New Energy and Industrial Technology Organization (NEDO), and partially Grant-in-Aid for Scientific Research (B) (No. 22350094) by JSPS, Japan. Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. matchemphys.2013.05.043. References [1] R. Allman, Chimia 24 (1970) 99. [2] X. Duan, D.G. Evans, Layered Double Hydroxide, first ed., Springer, Berlin, 2006, p. 119. (Chapter 2).
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