Hexacyanoferrate-intercalated nickel zinc hydroxy double salts

Solid State Sciences 11 (2009) 2080–2085

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Hexacyanoferrate-intercalated nickel zinc hydroxy double salts Jacqueline T. Rajamathi a, b, N.H. Raviraj b, Mohammed F. Ahmed b, Michael Rajamathi a, * a b

Materials Research Group, Department of Chemistry, St. Joseph’s College, 36 Lalbagh Road, Bangalore 560 027, India Department of Chemistry, Bangalore University, Bangalore, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 April 2009 Received in revised form 8 August 2009 Accepted 1 September 2009 Available online 8 September 2009

When anionic clay like nickel zinc hydroxyacetate was subjected to anion exchange reaction with either hexacyanoferrate(II) or hexacyanoferrate(III) ions, the complex anion intercalation was accompanied by auto redox reactions. In both the cases a mixture of hexacyanoferrate(II) and hexacyanoferrate(III) ions was found to be intercalated in the anionic clay. The mixed anion intercalated anionic clays could be oxidized by hydrogen peroxide to get pure hexacyanoferrate(III) intercalated anionic clay. Thermal decomposition of the intercalated anionic clays yields mixed oxides of Ni, Zn and Fe. Ó 2009 Elsevier Masson SAS. All rights reserved.

Keywords: Anionic clay Hydroxysalt Intercalation Hexacyanoferrate

1. Introduction Anionic clay like layered hydroxide materials have been well studied due to their interesting interlayer chemistry and applications in varied fields [1–5]. These compounds consist of a stacking of positively charged metal hydroxide layers with anions loosely held in the interlayer region for charge neutrality. The interlayer anions can be readily exchanged with other organic/inorganic anions leading to a variety of new materials with varying properties. In addition to the well known layered double hydroxides (LDHs), a-hydroxides of divalent metals and layered hydroxy salts could also be classified as anionic clays [6]. Layered Ni(II)-Zn(II) hydroxysalts of the formula Ni3Zn2(OH)8(An)2/n$mH2O derive their structure from the zinc hydroxysalt, Zn5(OH)8(NO3)2$2H2O [7]. In these compounds, Ni2þ ions occupy 75% of the octahedral sites in brucite-like M(OH)2 layers. 25% of the octahedral sites are unoccupied and the tetrahedral sites adjacent to these vacant octahedral sites – one above and one below – are occupied by Zn2þ ions leading to a layer composition of [Ni3Zn2(OH)8]2þ. Anions are incorporated in the interlamellar region for charge compensation. These interlayer anions are exchangeable with a variety of anions such as simple inorganic anions, organic anions (alkyl and aryl carboxylates, long chain sulphate ions) and polyoxometallates [8–14]. Intercalation of complex anions have also been reported in

* Corresponding author. Tel.: þ91 80 22211429; fax: þ91 80 22245831. E-mail address: [email protected] (M. Rajamathi). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.09.001

the literature. Rojas et al have reported the intercalation of vanadates [14] and metal-EDTA complexes [15] in layered nickel zinc layered hydroxysalt and the thermal decomposition of these intercalated products. Hexacyanoferrate(II) and hexacyanoferrate(III) ions have been intercalated in LDHs. Kikkawa et al prepared Mg-Al-Fe(CN)6 LDHs with various Mg2þ/Al3þ ratios and studied their anion exchange with carbonate and perchlorate ions [16]. They also investigated molecular-sieving effect of these solids. Holgado et al reported the preparation and thermal decomposition of hexacyanoferrate(II) and hexacyanoferrate(III) exchanged Mg-Al LDHs [17]. Carpani et al have studied the intercalation of hexacyanoferrate(II) and hexacyanoferrate(III) ions in Ni-Al LDHs [18,19]. Thermal decomposition of anionic clays leads to the formation of mixed oxides whose nature depends on the cations present in the brucitic layers, calcination temperature and on the calcination time. If the interlayer anion is replaced by another metal containing anion, mixed oxides with three metal cations are obtained on calcination. del Arco et al have reported the synthesis of Mg-Al CrO4 and Zn-Al CrO4 LDHs and the study of acid and redox properties of mixed oxides obtained by calcination of these Cr containing LDHs. [20] In the case of iron containing anions, intercalation of hexacyanoferrate(II) and hexacyanoferrate(III) have been widely studied because of the easy incorporation in the interlayer due to high anion charge. Fernandez et al have studied the effect of iron on the crystallite phases formed upon thermal decomposition of Mg-Al-Fe LDHs [21]. Meng et al have reported the preparation of a magnetic material containing MgFe2O4 spinel ferrite from a hexacyanoferrate-intercalated Mg-Fe(III) LDH [22].

J.T. Rajamathi et al. / Solid State Sciences 11 (2009) 2080–2085

While hexacyanoferrate intercalation has been well studied in the case of LDHs such studies have not been carried out in the case of anionic clay like hydroxysalts. In this paper we report the synthesis, thermal decomposition and electrochemical behaviour of hexacyanoferrate-intercalated layered nickel zinc hydroxy double salt. 2. Materials and methods 2.1. Synthesis of nickel zinc hydroxyacetate The starting material nickel zinc hydroxyacetate was prepared by the method due to Morioka et al [23]. Zinc oxide powder (2.32 g) was added with vigorous stirring to 200 ml of an aqueous solution containing 10.66 g of nickel acetate tetrahydrate. The mixture was then aged at 65  C for 2 days with occasional stirring. The green solid obtained after the aging process was filtered, washed free of ions and dried in air at 65  C. 2.2. Anion exchange reactions About 0.5 mmol of nickel zinc hydroxyacetate was dispersed in 20 ml of decarbonated water by stirring overnight. 1.5 mmol of potassium hexacyanoferrate(II)/2.0 mmol of potassium hexacyanoferrate(III) was added to the dispersion and the mixture was stirred at room temperature for 24 h. The solution was allowed to settle and the supernatant solution was removed by decantation. Fresh solution of potassium hexacyanoferrate(II)/potassium hexacyanoferrate(III) was added and the mixture stirred for another 24 h. The ion-exchanged solids were separated by centrifugation, washed free of ions and dried at 65  C in air. 2.3. Oxidation of anion exchanged products using hydrogen peroxide About 0.15 g of the product obtained on anion exchange reaction with hexacyanoferrate(III) ions was stirred in 15 ml of deionised water overnight. To this mixture 15 ml of 30% of hydrogen peroxide solution was added in drops and the mixture was stirred in a closed container for 2 days. The product formed was separated by centrifugation, washed and dried at 65  C in air. 2.4. Thermal decomposition of the anion exchanged products For thermal decomposition studies w0.1 g of the hexacyanoferrate(II) and hexacyanoferrate(III) exchanged samples were kept at the desired temperature (200–800  C) for 2 h in a tubular furnace with a constant airflow. 2.5. Characterisation The solids were characterized by powder X-ray diffraction (PXRD) using a Siemens D 5005 powder diffractometer (q–2q Bragg-Brentano geometry, Cu-Ka radiation, 2 per minute), infrared (IR) spectroscopy (Nicolet model impact 400D FTIR spectrometer,

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KBr pellets, 4 cm1 resolution), and thermogravimetry (Mettler Toledo Stare SW 7.0, in N2 flow, 5  C per minute). The chemical compositions of the solids were ascertained by wet chemical methods. Ni, Zn and Fe content of the samples were determined by atomic absorption spectroscopy (AAS) using a Varian AA240 spectrometer. The OH- content was determined by dissolving a known amount of the anionic clay sample in a known excess of dilute HCl and backtitrating the unreacted acid with NaOH solution pHmetrically. Electrochemical measurements were carried out using an AUTOLAB PGSTAT 30 instrument with GPES software. The electrodes were prepared by mixing 50 mg of the solid sample with 150 mg of carbon powder. The mixture was made into a paste with epoxy resin and filled into a micropipette. Standard calomel electrode was used as the reference electrode. Platinum foil was used as auxillary electrode and copper foil was used for electrical connections. Cyclic voltammograms (CVs) were recorded in 0.1 M sodium hydroxide solution. Specific surface area measurements and N2 adsorptiondesorption studies were carried out using a Nova-1000 (Ver. 3.70) instrument. The samples were degassed at 130  C before measurement.

3. Results and discussion The composition analysis data of nickel zinc hydroxyacetate and its anion exchanged products are presented in Table 1. The mass percentages of the estimated components of the nickel zinc hydroxyacetate lead to the nominal formula, Ni3Zn2(OH)8(OAc)2$3H2O. In the complex anion exchanged samples the Fe contents suggest that the exchange is complete. The PXRD patterns of the as prepared nickel zinc hydroxyacetate and the hexacyanoferrate(II) and hexacyanoferrate(III) exchanged samples are shown in Fig. 1. The pattern of the as prepared hydroxy acetate (Fig. 1a) matches well with that reported by Morioka et al [23]. The basal spacing calculated from the 00[ reflections is 13.1 Å. The reflections at 2q ¼ 33.4 and 59.7 exhibit saw tooth shaped asymmetry indicative of turbostratic disorder which is common in nickel hydroxide materials [24]. The 00l peaks at 11.0 Å and 10.9 Å in Fig. 1b and c, respectively confirm the incorporation of hexacyanoferrate anions in the interlayer region of the hydroxysalt. The sizes of hexacyanoferrate(II) anion at different orientations were calculated to be 11.2 Å along C4 axis, 8.7 Å along C2 axis and 6.5 Å along the C3 axis [16]. The interlayer distance obtained by subtracting the layer thickness 4.6 Å from the observed basal spacings 11.0 and 10.9 Å are 6.4 and 6.3 Å, respectively for the hexacyanoferrate(II) and hexacyanoferrate(III) exchanged samples. This suggests that the hexacyanoferrate anion in the interlayer region is oriented with one of its C3 axis perpendicular to the host layer [16]. The IR spectra of nickel zinc hydroxyacetate and the anion exchanged samples are shown in Fig. 2. In all the cases the broad bands around 3450 cm1are due to the O–H stretching of M–OH groups and the hydrogen bonded interlayer water molecules. The band corresponding to the deformation mode of interlayer water

Table 1 Compositions of nickel zinc hydroxyacetate and the anion exchanged samples. Sample

Mass percentage

Nominal formula

Ni

Zn

OH

OAc

Fe

Nickel zinc hydroxyacetate Exchanged with [Fe(CN)6]4Exchanged with [Fe(CN)6]3-

27.4 28.6 28.9

21.4 21.5 21.8

23.0 20.9 21.9

19.7 – –

– 5.4 5.2

Ni3Zn2(OH)8(OAc)2$3H2O Ni3Zn2(OH)8[Fe(CN)6]0.6$3H2O Ni3Zn2(OH)8[Fe(CN)6]0.6$2.5H2O

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001

c

d 10 002

003

11

O-H bending

c

b

a

10

20

30

40

50

Transmittance (%)

Relative Intensity

O-H Stretching

b

[Fe(CN)6] [Fe(CN)6 ]

4-

3-

CN Stretching

a

Carboxylate Stretching

60

2θ (degrees) Fig. 1. PXRD patterns of nickel zinc hydroxyacetate (a); its hexacyanoferrate(II) (b) and hexacyanoferrate(III) (c) exchanged products and the product obtained on H2O2 oxidation after hexacyanoferrate(III) exchange (d).

4000

3000

2000

1000 -1

Wavenumber (cm ) Fig. 2. IR spectra of nickel zinc hydroxyacetate (a) and its hexacyanoferrate(II) (b) and hexacyanoferrate(III) (c) exchanged products.

molecules appears at 1634 cm1. In the spectrum of the hydroxyacetate (Fig. 2a) the bands at 1574 cm1 and 1408 cm1are due to the antisymmetric and symmetric stretching vibrations of the carboxylate group of the acetate ions respectively. In the IR spectra of the hexacyanoferrate(II) and hexacyanoferrate(III) exchanged hydroxy double salt (Fig. 2b and c) the acetate related absorptions are absent and new absorptions due to the hexacyanoferrate ions appear in the region 2000–2150 cm1. In Fig. 3 we show the IR spectra of these samples expanded in this region While we expect one C–N stretching band for the cyano groups of hexacya noferrate(II), the hexacyanoferrate(II) exchanged sample (Fig. 3a) shows two bands at 2096 and 2058 cm1. The hexacyanoferrate(III) exchanged sample (Fig. 3b) also shows these bands. It has been suggested that during intercalation hexacyanoferrate(II) ions are partly converted into hexacyanoferrate(III) ions and vice versa [17]. The IR spectra suggests that in both the samples we have both hexacyanoferrate(II) and hexacyanoferrate(III) ions. The band at 2058 cm1 is due to hexacyanoferrate(II) ions and that at 2096 cm1 is due to hexacyanoferrate(III) ions. From the relative intensities of these peaks we can calculate the [hexacya noferrate(II)]/[hexacyanoferrate(III)] ratios to be 1.3 and 0.9 in hexacyanoferrate(II) and hexacyanoferrate(III) exchanged samples. While the partial conversion of the hexacyanoferrate(II) ions to hexacyanoferrate(III) ions may be attributed to ligand exchange by water followed by oxidation [25,26] the mechanism of reduction of the intercalated hexacyanoferrate(III) ions to hexacyanoferrate(II) is not understood clearly. It has been speculated for a similar reduction observed in the case of hydrotalcite [26] that the reduction is caused due to partial electron transfer from the

complex to the host layers through ChN leading to a decrease in the number of 3 d electrons in the Fe atom. As nickel zinc hydroxysalt is very similar to LDHs we may assume a similar reaction causing the reduction of the intercalated hexacya noferrate(III). The TG curves of the hexacyanoferrate(II) and hexacyanoferrate(III) exchanged nickel zinc hydroxysalt are shown in Fig. 4. In both the cases the thermal decomposition takes place in two steps. The first mass loss up to 220  C corresponds to the removal of adsorbed water and water molecules in the interlayer region and corresponds to w12.5% of the total mass of the sample in both the cases. The second mass loss, centered at 400  C, is due to the dehydroxylation of the layers and loss of volatile species such as CO2 and NO2 from the interlayer anion. The fact that the TG curves of the two samples are nearly identical further confirms that the interlayer composition is almost similar in these samples. The net mass loss in the hexacyanoferrate(III) exchanged sample is w2% more than the other sample which is in line with the interlayer composition arrived at from the IR spectra (Fig. 2) and wet chemical analysis (Table 1). Though the TG curves look similar there is a slight difference in the DTG curves. The major mass loss is at 386  C in the case of hexacyanoferrate(III) intercalated sample (Fig. 4a) and 403  C in the hexacyanoferrate(II) intercalated sample (Fig. 4b). That is, the sample with higher ferrocyanide content decomposes at a higher temperature. This slight difference is due to the larger electrostatic forces of attraction between the layers and ferrocyanide anions whose charge is higher when compared to the ferricyanide anions.

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2083

100

60

b

c

15

90

50

10

40

5

25

b

mass(%)

Transmittance (%)

70

0

60 100

20

a

90 10

differential mass loss (%)

80

80

a

70

5

60

20

0

50 100

200

300

400

500

600

700

800

o

Temperature ( C)

2100

2000 Fig. 4. TG (dddd) and DTG ($$$$$$$$) curves of hexacyanoferrate(III) (a) and hexacyanoferrate(II) (b) exchanged nickel zinc hydroxysalt.

-1

Wavenumber (cm ) Fig. 3. IR spectra of hexacyanoferrate(II) (a) and hexacyanoferrate(III) (b) exchanged nickel zinc hydroxysalt and the product obtained on H2O2 oxidation after hexacyanoferrate(III) exchange (c).

In order to obtain pure hexacyanoferrate(III) intercalated hydroxysalt the hexacyanoferrate(III) exchanged sample containing a mixture of the two hexacyanoferrate ions in the interlayer was oxidized using H2O2. We have recently shown that H2O2 could be used as a mild oxidizing agent to topotactically oxidize interlayer

anions in anionic clays [27,28]. The PXRD pattern of the oxidized product (Fig. 1d) quite closely matches with that of the parent compound (Fig. 1c) indicating that the layer structure is not affected by the oxidation process. The IR spectrum of the oxidized product (Fig. 3c) is distinctly different from that of the parent compound (Fig. 3b). There is only one peak due to C–N stretching at 2096 cm1 indicating that this sample contains only hexacyanoferrate(III) ions in the interlayer.

b

a 0.00001

0.00010

0.00000

0.00005

-0.00001

Current (A)

0.00000 -0.00002 -0.00005

c

d

0.00005

0.0001

0.00000

0.0000

-0.00005

-0.0001

-0.00010 -0.0002 -1.5

-1.0

-0.5

0.0

0.5

-1.5

-1.0

-0.5

0.0

0.5

1.0

Potential (V) vs SCE Fig. 5. Cyclic voltammograms (scan rate 20 mVs1) of nickel zinc hydroxyactate (a); its hexacyanoferrate(II) (b) and hexacyanoferrate(III) (c) exchanged products and the product obtained on H2O2 oxidation after hexacyanoferrate(III) exchange (d).

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Table 2 Anodic and cathodic peaks observed in CVs of hexacyanoferrate-intercalated hydroxysalts.

Ec (V)

0.20 0.29 0.52

0.11 0.06 –

The CVs of the samples recorded at a scan rate of 20 mVs1 are shown in Fig. 5 and the observed anodic and cathodic peak potentials are listed in Table 2. The CV of nickel zinc hydroxy acetate (Fig. 5a) shows two sets of anodic and cathodic peaks. The reversible peaks observed at 0.37 V and 0.33 V are assigned to the redox reaction of nickel and the other set of peaks at 0.18 V and 0.66 V are assigned to the zinc redox couple. The CV of the hexacyanoferrate(II) exchanged sample (Fig. 5b) has an anodic peak at þ0.20 V and a cathodic peak at þ0.11 V, which are assigned to the redox system [Fe(CN)64/Fe(CN)3 6 ], in addition to the nickel redox peaks. The hexacyanoferrate(III) exchanged sample (Fig. 5c) also shows anodic and cathodic peaks corresponding to the [Fe(CN)64/ Fe(CN)3 6 ] redox couple at þ0.29 and 0.06 V. The CV of the H2O2oxidized product which has only Fe(CN)3 6 ions in the interlayer (Fig. 5d) is different from that of the parent solid (Fig. 5c). Here we observe only a broad anodic peak corresponding to the reduction of Fe(CN)3 6 . Possibly this sample has poor conductivity compared to the other samples. Hexacyanoferrate-intercalated LDHs have been found to be microporous. [17,29] In order to find if the hydroxysalt analogs exhibit similar behavior we measured the surface area porosity of the samples. The specific surface areas and the porosity data of the samples are shown in Table 3. The surface areas and porosity are poorer here compared to LDHs. The specific surface area of the hexacyanoferrate(II) intercalated hydroxysalt is higher than the hexacyanoferrate(III) intercalated hydroxysalt as observed in the case of LDHs [17]. The H2O2-oxidized sample shows higher surface area and pore volume compared to the as prepared complex ion-intercalated samples. The PXRD patterns of the thermal decomposition products of the hexacyanoferrate(III) exchanged hydroxy double salt treated at different temperatures are shown in Fig. 6. The sample retains its layered structure up to 200  C. Above 200  C removal of interlayer anions and hydroxyl groups takes place leading to the formation of mixed oxides. In the sample calcined at 400  C, the 001 peak of the anionic clay disappears completely showing the collapse of the layer structure. Peaks due to the oxide phases start appearing at this temperature. The peaks due to the oxide phases become sharper and more intense as the calcination temperature increases due to improved crystallinity and increased crystallite sizes. The PXRD pattern of the sample treated at 800  C shows reflections due to ZnO, NiO and a ferrite spinel. The intense peaks at 2.61 Å and

Table 3 Specific surface area and pore volumes of the hydroxyacetate, anion exchanged products and the product obtained on oxidation after exchange. Sample

Specific surface area m2/g

Pore volume (cm3/g)

Ni Zn hydroxyacetate Ni Zn hydroxyacetate exchanged with [Fe(CN)6]4Ni Zn hydroxyacetate exchanged with [Fe(CN)6]3After H2O2 oxidation

52.5 59.1

0.081 0.097

43.4

0.062

74.9

0.121

*

*

c

Relative Intensity

Ni Zn hydroxyacetate exchanged with [Fe(CN)6]4 Ni Zn hydroxyacetate exchanged with [Fe(CN)6]3 After H2O2 oxidation

Ea (V)

d s o

s* s* *

Fe2þ/Fe3þ couple

Sample

s o

o

b

a

10

20

30

40

50

60

2θ (degrees) Fig. 6. PXRD patterns of the products obtained on thermal decomposition of hexacyanoferrate(III) exchanged nickel zinc hydroxysalt at 200 (a); 400 (b); 600 (c); and 800  C (d). Peaks due to ZnO, NiO and spinel are marked with *, O and S, respectively.

2.48 Å confirm the presence of ZnO (JCPDS PDF 5-0664) and the intense peaks at 2.43 Å and 2.10 Å are due to NiO (JCPDS PDF 4-0835). In addition to the peaks due to ZnO and NiO there are peaks at 2.97, 2.53, 2.42, 2.1, 1.48 Å due to a spinel ferrite phase (JCPDS PDF 1-1109). This ferrite phase could be a mixed spinel of the type Ni1-xZnxFe2O4. 4. Summary The complex anions, hexacyanoferrate(II) and hexacyanoferrate(III) bearing high negative charges readily replace the interlayer acetate ions of the anionic clay, nickel zinc hydroxyacetate. In both the cases the exchange is accompanied by auto redox reactions leading to mixed anion intercalated products. The mixed anion intercalated product could be oxidized topotactically to hexacyanoferrate(III) intercalated product. Thermal decomposition of the hexacyanoferrate-intercalated anionic clay yields a mixture of oxides. Acknowledgements This work was funded by DST, New Delhi. M.R. thanks UGC, New Delhi, for having provided the IR spectrometer through CPE scheme. References [1] M. Qian, H.C. Zeng, J. Mater. Chem. 7 (3) (1997) 493. [2] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173. [3] A.J. Jacobson, in: G. Alberti, T. Bein (Eds.), Comprehensive Supramolecular Chemistry: Colloidal Dispersion of compounds with layer and chain structures, Vol. 7, Elsevier, Oxford, 1996, pp. 315–335. [4] A.I. Khan, L. Lei, A.J. Norquist, D. O’Hare, Chem. Commun. (2001) 2342. [5] S.-Y. Kwak, Y.-J. Jeong, J.-S. Park, J.-H. Choy, Solid State Ionics 151 (2002) 229. [6] M. Rajamathi, G.S. Thomas, P.V. Kamath, Proc. Ind. Acad. Sci. 113 (2001) 671. [7] W. Stahlin, H.R. Oswald, Acta Crystallogr. B 26 (1970) 860. [8] S. Yamanaka, T. Sako, K. Seki, M. Hattori, Solid State Ionics 53–56 (1992) 527. [9] R. Rojas, C. Barriga, M.A. Ulibarri, P. Malet, V. Rives, J. Mater.Chem. 12 (2002) 1071. [10] S.P. Newman, W. Jones, J. Solid State Chem. 148 (1999) 26. [11] E. Kandare, J.M. Hossenlopp, J. Phys. Chem. B 109 (2005) 8469. [12] M. Meyn, K. Beneke, G. Lagaly, Inorg. Chem. 32 (1993) 1209. [13] J.T. Rajamathi, N. Ravishankar, M. Rajamathi, Solid State Sci. 7 (2005) 195. [14] R. Rojas, C. Barriga, M.A. Ulibarri, V. Rives, J. Solid State Chem. 177 (2004) 3392.

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