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Mechanochemical synthesis of layered hydroxy salts
Materials Research Bulletin 47 (2012) 3568–3572
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Mechanochemical synthesis of layered hydroxy salts Nygil Thomas * Materials Research Group, Department of Chemistry, St. Joseph’s College, 36 Langford Road, Bangalore 560027, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 January 2012 Received in revised form 11 May 2012 Accepted 15 June 2012 Available online 23 June 2012
A simple one minute synthesis method was adapted for the preparation of layered hydroxy salts of copper, zinc, nickel and cadmium by grinding the metal salts with sodium hydroxide in a mortar. This solvent free method is environment friendly and fast. This method could be extended to the preparation of Ni/Zn hydroxy double salts. The Ni/Zn ratio could be varied from 1.2 to 1.9 by varying the metal contents of the precursor salts without the formation of any impurities in the sample. The prepared compounds had similar characteristics as that of the samples prepared by precipitation route. No sign of carbonate contamination was observed in any of the prepared samples. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds A. Layered compounds B. Chemical synthesis B. Intercalation reactions
1. Introduction LDHs (layered double hydroxides) are materials in which positively charged layers are stacked with anions intercalated in the interlayer region together with water molecules [1]. There are other layered solids like layered hydroxy salts (LHSs) also known as basic salts and alpha hydroxides which are structurally similar to LDHs [2]. LDHs and LHSs derive their structure from that of brucite, Mg(OH)2. In brucite-like hydroxides, OH ions are hexagonally closely packed and the M2+ ions occupy alternate layers of octahedral sites. Thus the structure can be described as a stacking of charge neutral M(OH)2 layers. In LDHs, the layer charge is developed by the partial substitution of divalent cations by trivalent ones and hydrated anions are intercalated between the layers to balance the charge [3]. In LHS, the cation composition of the layer consists of divalent metal ions. The existence of anion in such a structure can be explained by two common mechanisms. The excess layer charge can be created by the occurrence of octahedral sites which are unoccupied by cations, which are located tetrahedrally above and below the empty octahedron [4]. The uncompensated charge on the tetrahedral cations is balanced by intercalating anions in the interlayer. The second mechanism involves the substitution of a part of the hydroxide groups located above and below the layer by anions and water molecules [4]. The general formula of a layered hydroxy salt can be expressed as MII(OH)2 x(An )x/nnH2O where MII = Mg, Co, Ni, Cu; An = NO3 , Cl , SO42 etc. The interlayer
* Tel.: +91 80 2221 1429; fax: +91 80 2224 5831. E-mail address: [email protected]. 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.06.057
spacing of these solids is determined by the size of the anion. The commonly observed values of x are 0.5, 0.67 and 1.0 [4]. LDHs and LHSs show interesting properties such as anion mobility, anion exchange and surface basicity. Due to these properties, these compounds find many applications in varied fields such as sorption, catalysis, flame-retardation, polymer stabilization, sensing, electrochemistry, photochemistry and medicine to cite a few [5–9]. There are many methods available for the synthesis of LHSs. The common method involves the hydrolysis of divalent metal salts in the presence of a metal oxide [10,11]. This method involves a long synthesis time typically from hours to days. Another method of preparation is by the controlled precipitation of metal salts by the addition of alkaline solutions [12]. A number of LHSs have been prepared by this method. This method is time consuming and generates considerable amount of waste. Many times quantitative precipitation was not observed [13]. Urea hydrolysis method is used for the preparation of LHSs of high crystalline nature [14]. The disadvantage of this method is that CO2 produced in the decomposition of urea reacts with water and generates carbonate. This carbonate may appear as a contamination in the final sample. LHSs synthesized by polyol method were of low crystallinity and removal of polyol completely from the final product was difficult [15]. There are other methods which are not very common like heating the metal salt solution in a bomb reactor, melting reaction and hydrothermal preparation [16]. Each of the above method has its own advantages and disadvantages. One main drawback of all the above methods is the time taken to obtain the product. Since all the above methods take considerable amount of time, it will be of significance if one can develop methods with shorter preparation time.
N. Thomas / Materials Research Bulletin 47 (2012) 3568–3572
The mechanochemical method has been widely used to synthesize large variety of materials [17,18]. This solvent free method is rapid as well as environment friendly. There are different forms of mechanochemical activation used for the synthesis of novel materials. This synthesis method induces solid state reactions between the hydrated and hydroxide samples by grinding or milling. Li–Al LDH was prepared by a reaction between solid lithium and aluminium hydroxide in a mortar and then exposing the mixture with water vapour [19]. Recently, there are reports on the preparation of nitrate intercalated Mg–Al LDH by two step milling process [19]. Mg–Al nitrate LDHs were prepared by the mixing of metal salts and NaOH in a mortar [20,21]. In this paper, we describe the preparation of LHSs by simple grinding of a mixture which consists of metal salts and sodium hydroxide. The grinding time was kept as one minute for LHS and three minutes for HDS. 2. Experimental 2.1. Preparation of layered hydroxy salts All the chemicals used were of pure analytical grade. The chemical formula of the salts are as follows, Cu(NO3)23H2O, Ni(NO3)26H2O, Zn(NO3)26H2O, Cd(NO3)24H2O and NaOH. 2.18 g of copper nitrate was mixed with 0.18 g of NaOH in a mortar. The mixture was ground well for 1 min using a pestle. The resultant paste was washed with water and dried at room temperature. No precautions were taken to avoid carbonate contamination during synthesis. This procedure was repeated with zinc, cadmium and nickel nitrates using same amount of nitrate salts and NaOH.
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and back titrating the excess acid with standard NaOH using a pH meter.
3. Results and discussion Fig. 1a and b shows the PXRD patterns of the product obtained on grinding copper nitrate and NaOH before and after washing with water. Both samples exhibit all the characteristic reflections of copper hydroxy nitrate and match well with the values reported in literature [23]. The sharp and symmetrical reflections at 2u = 12.78 and its multiple at 25.98 were observed at same positions in both the samples. This proves without doubt that the formation of the layered compound happened during grinding itself. The PXRD patterns of the products match well with that of the control sample prepared by co-precipitation (Fig. 1c). In both the sample peaks after 2u = 308 are considerably broadened due to the stacking faults. Biswick et al. indexed the PXRD pattern of the copper hydroxy nitrate based on a monoclinic lattice [23]. But the phase had a few peaks matching closely to orthorhombic phase. This well ordered sample was prepared by a hydrolysis method from hydrated copper nitrate. It is difficult to distinguish these peaks in our sample due to the peak broadening. The peaks positions are directly compared with reported powder data (ICSD CC 31353) and marked in Fig. 1. The sample obtained before washing shows three additional reflections which are due to sodium nitrate (PCPDF No. 850850) formed in the reaction medium. The washed sample was completely free from any such impurities. The basal spacing of the prepared sample was 7.0 A˚ and matches well with the values reported in the literature. There was
2.2. Preparation of copper hydroxy nitrate by co-precipitation Copper hydroxy nitrate was synthesized by drop wise addition of 50 mL of sodium hydroxide (0.75 mol/L) to 20 mL boiling copper nitrate solution (3.5 mol/L) with vigorous stirring [22]. The resultant precipitate was washed and dried at room temperature. 2.3. Preparation of hydroxy double salts Stochiometric quantities of nickel acetate, zinc acetate and NaOH (Nickel(II) acetate tetrahydrate, 99.8%, 0.59 g, Zinc(II) acetate dihydrate, 99.7%, 0.44 g and NaOH, 0.46 g) were mixed in a mortar to prepare a compound of the formula Ni3Zn2(OH)8(CH3COO)2nH2O. This mixture was ground well for 3 min. The paste was washed with water and dried at room temperature. Ni/Zn ratio was varied systematically to investigate if the resultant solid had the same metal ratio as that of the precursor. 2.4. Characterization Powder X-ray diffraction (PXRD) measurements were performed on a PANalytical Xpert Pro X-ray Diffractometer using CuKa radiation (l = 0.154 nm) at 40 kV and 30 mA, at a scanning rate of 28 min 1. The infrared (IR) spectra of samples were collected using a Nicolet IR200 FT-IR spectrometer using KBr pellets, in the range 4000–400 cm 1 with 4 cm 1 resolution. The interlayer nitrate content was obtained by ion chromatography (IC) using a Metrohm 861 Advanced Compact ion chromatograph with Metrosep A Supp5 250 anion column and conductivity detector. The samples were dissolved in 1 mol/L acetic acid and diluted suitably for this purpose. The copper, zinc, cadmium and nickel contents of the samples were estimated by atomic absorption spectroscopy (Varion AA240). The OH content was obtained by dissolving a known weight of the sample in dilute HCl
Fig. 1. PXRD patterns of the cupric nitrate (a), product obtained on grinding cupric nitrate and NaOH for 1 min before washing (b), after washing (c) and the product obtained on co-precipitation (d), reflections due to sodium nitrate impurity are marked with *.
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no considerable difference in the crystalline nature of the samples synthesized by this method and by co-precipitation unlike in the case of layered double hydroxides (LDHs). There was significant loss in crystalline nature during the preparation of LDHs by mechanochemical method [21]. Wet chemical analysis showed that the copper hydroxy nitrate prepared by this method has the approximate formulae Cu2(OH)3(NO3) (Cu = 51%, NO3 = 26%). Mechanochemical synthesis method was repeated in custom made humidity chamber to study the effect of ambient humidity on the prepared samples. The ambient humidity was varied by using different salt mixtures in the humidity chamber. We could not observe any difference in the final products formed after controlled ambient atmosphere which may be due to the presence of hydrated metal salts and very short time of preparation, typically 1–3 min. Unlike zinc–aluminium and zinc–chromium layered double hydroxides (LDHs), layered hydroxy salts have no flexible hydration behaviour on exposure to humidity [4]. The IR spectra of the product obtained on grinding copper nitrate and NaOH and the control sample prepared by the precipitation method are shown in Fig. 2a and b respectively. The IR spectra of both samples match well. The broad absorption at 3545 cm 1 in both samples is due to O–H stretching vibration of the hydroxyl groups of the brucite-like sheets and water in the interlayer space. There is a sharp peak at 3400 cm 1 in both the samples which may be due to the stretching vibration of O– H groups which are not involved in the hydrogen bonding. The number and positions of the anionic IR bands depend on the coordination of ions in the layer. There are four strong absorptions in the region 1600–1000 cm 1 for copper hydroxy nitrate. The observed peaks at 1080, 1340, and 1420 cm 1 indicate the incorporation of NO3 in C2v symmetry with one of the N–O bonds parallel to the c-axis [22]. There is a sharp peak at 1380 cm 1 due to nitrate in the D3h symmetry [22]. Based on the degree of covalent character of metal–oxygen bond, it was proposed that the nitrate group in copper hydroxy nitrate is bound more strongly to the layer than its structural analogues like nickel and zinc hydroxy nitrates [23]. Fig. 3 shows the PXRD patterns of the samples obtained by the mechanochemical preparation route starting from zinc, cadmium and nickel nitrates. The product obtained from zinc nitrate (Fig. 3a) shows a pattern identical to that of Zn5(OH)8(NO3)22H2O. The
Fig. 2. R spectrum of the product obtained on grinding cupric nitrate and NaOH after washing with water (a), control sample prepared by the precipitation method (b).
Fig. 3. PXRD patterns of the samples obtained by the mechanochemical preparation starting from zinc (a), cadmium (b) and nickel nitrates (c).
Fig. 4. IR spectra of the samples obtained by the mechanochemical preparation starting from zinc (a), cadmium (b) and nickel nitrates (c).
N. Thomas / Materials Research Bulletin 47 (2012) 3568–3572
pattern is marked by an intense reflection at 9.6 A˚, due to 2 0 0 plane of the monoclinic lattice [24]. The PXRD pattern of the product obtained from cadmium nitrate precursor is shown in Fig. 3b. The pattern can be indexed based on a monoclinic system. The basal spacing of the sample is 9.4 A˚ matching with the values reported in the literature for Cd2(OH)3NO3 [25]. The product obtained from nickel nitrate (Fig. 3c) source was nearly amorphous which is quite common in the case of nickel hydroxides. The products obtained by precipitation of nickel hydroxide using ammonia or electrosynthesis show a poorly ordered pattern which is similar to our product [26]. This disordered phase is denoted as a-hydroxide. In this layered material, layers are oriented randomly with respect to one another leading to turbostatic disorder [27]. The IR spectra of the samples obtained by the mechanochemical preparation starting from zinc, cadmium and nickel nitrates are shown in Fig. 4. The product obtained from zinc (Fig. 4a) shows sharp absorption at 1384 cm 1 which is due to N–O stretching which is characteristic of the nitrate ion in the D3h symmetry [22]. This spectrum was similar to what was observed in the case of nitrate intercalated LDHs. Unlike in the case of copper hydroxy nitrates, nitrate group is not grafted to the layer here. The stretching vibrations of the OH group and water molecules in the interlayer are seen in the range of 3660–2990 cm 1. Three bands are observed in this region. The sharp peak at 3574 cm 1 may be due to stretching vibrations of O–H groups not considerably associated with the hydrogen bonding. The 3465 cm 1 band may be due to the vibrations of the OH group which are hydrogen bonded to the nitrate groups in the interlayer. The broad band at 3305 cm 1 can be assigned to stretching vibrations of O–H groups of the water. The bending vibrations of the water molecules are
Fig. 5. PXRD patterns of the products obtained on grinding different mole rations of nickel and zinc acetates with NaOH (a) Ni/Zn ratio 0.94 (a), 1.15 (b), 1.47 (c), 1.85 (d) and (e) 2.80. Reflections due to ZnO are marked with ^.
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observed at 1630 cm 1 [4,23]. The sample obtained from cadmium nitrate source (Fig. 4b) shows peaks at 1043, 1323 and 1433 cm 1 which is characteristic of grafted nitrate with C2V symmetry. There is a sharp peak at 1384 cm 1 and at 1015 cm 1 due to N–O stretching of the nitrate ion in the IR spectrum (Fig. 4c) of the sample obtained from nickel source. This pattern is characteristic of nitrate ion loosely held in the interlayer which retains its D3h structure in the interlayer. There are other bands observed which are due to Ni–O–H vibration at 658 cm 1 and Ni–O vibration at 475 cm 1 [4,26]. Our next aim was to extend this method to hydroxy double salts with two different divalent metals in the layer. Nickel–zinc hydroxy acetate (NiZn-HDS) is a well studied hydroxy salt [28–30]. In NiZn-HDS, zinc cations occupy tetrahedral sites above and below the nickel vacancies outside the hydroxide layers. Acetate ions are bound to the zinc atoms. The general formula can be given as Ni3Zn2(OH)8(CH3COO)2nH2O. NiZn-HDS are routinely synthesized by hydrothermal methods or by reacting ZnO with nickel acetate [30]. These methods take minimum of 24 h. During these preparations, the supernatant was always green coloured showing the presence of unreacted nickel (Fig. 1a of supplementary information). Fig. 5 shows the PXRD pattern of the product obtained on grinding different ratios of nickel and zinc acetates with NaOH. In Fig. 5b, a sharp and strong basal reflection close to 13 A˚ is followed by another reflection at 6.5 A˚. The non basal reflections observed at 2.66 and 1.54 A˚ are asymmetrical and weak due to the turbostatic disorder in the sample. The pattern can be indexed based on a hexagonal cell. The wet chemical analysis (Ni = 34%, Zn = 21% OH = 21% and CH3COO = 19%) shows that the product had an approximate formula Ni3Zn2(OH)8(CH3COO)22H2O. Interestingly, the washings collected was colourless indicating the complete conversion of nickel salts to product (Fig. 1b of supplementary information). Yamanaka and Rojas et al. [28,29] reported the preparation of this compound by hydrothermal method which involves heating the metal acetates in Teflon lined stainless steel bomb. They observed a limiting value of 1.5 for the Ni/Zn ratio in their samples irrespective of the starting ratio of the salts [28,29]. We wanted to investigate if we could prepare NiZn-HDS with different Ni/Zn ratio. We prepared a series of compounds with different Ni/Zn ratio in the starting acetate salts. The PXRD patterns are shown in Fig. 5. The wet chemical analysis proved that the final compounds had different Ni/Zn ratios. The Ni/
Fig. 6. IR spectrum of the product obtained on grinding nickel and zinc acetates with NaOH.
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N. Thomas / Materials Research Bulletin 47 (2012) 3568–3572
Zn ratios could be varied from 1.2 to 1.9 (Fig. 5b–d) without the formation of any impurities in the sample. As the zinc content decreased, there was a considerable loss of peak intensities due to the high scattering power of zinc compared to nickel (Fig. 5e). When the zinc content was increased, the ZnO impurity started forming (Fig. 5a) along with Ni/Zn hydroxy salt. Fig. 6 shows the IR spectrum of the product obtained on grinding nickel and zinc acetates (Ni/Zn ratio 1.47) with NaOH. The broad bands below 900 cm 1 are due to the lattice vibrations of the metal cations of the layer [29]. There are two intense bands at 1573 and 1407 cm 1 due to asymmetric and symmetric stretching vibrations of acetate anion [4]. The difference between these two bands is characteristic of the type of bonding existing between the acetate and metal. It could be monodentate, bidentate or chelating coordination. The difference of 166 cm 1 is indicative of monodentate coordination of acetate species to the metal cation [31,32]. 4. Conclusion A simple and ultrafast method was adapted to prepare layered hydroxy salts of different metals. This method is versatile as this can be used in the preparation of a variety of layered hydroxy salts like copper, zinc, cadmium and nickel. Only one minute manual grinding was sufficient to prepare the LHSs. This method can be successfully extended for the preparation of hydroxy double salts. Mechanochemical method does not require any harsh conditions like heating, refluxing or excess use of solvents. The Ni/Zn ratio could be controlled in the case of NiZn hydroxy double salts which was not possible by any conventional preparation methods. The obtained products had similar characteristics as of the samples prepared by precipitation. Acknowledgements N.T. thanks CSIR for the award of senior research fellowship and Dr. Michael Rajamathi for the permission to use the lab space.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2012 .06.057. References [1] F. Trifiro, A. Vaccari, in: J.L. Atwood, D.D. MacNicol, J.E.d. Davis, F. Vogtle (Eds.), Comprehensive Supramolecular Chemistry, vol. 7, Pergamon press, Oxford, 1996 (Chapter 10). [2] M. Rajamathi, G.S. Thomas, P.V. Kamath, Proc. Indian Acad. Sci. (Chem. Sci.) 113 (2000) 671. [3] V. Rives (Ed.), Layered Double Hydroxides: Present and Future, Nova Science Publishers Inc., New York, 2001. [4] G.G.C. Arizaga, K.G. Satyanarayana, F. Wypych, Solid State Ionics 178 (2007) 1143. [5] M.A. Aramendı´a, B. Borau, C. Jime´nez, J.M. Marinas, J.R. Ruiz, J. Urbano, Appl. Catal. A 244 (2003) 207. [6] N. Boshkov, K. Petrov, S. Vitkova, G. Raichevsky, Surf. Coat. Technol. 194 (2005) 276. [7] D. Li, Z. Tuo, D.G. Evans, X. Duan, J. Solid State Chem. 179 (2006) 3114. [8] L. Markov, R. Ionocheva, M. Marnov, K. Ivanov, Mater. Chem. Phys. 26 (1990) 493. [9] K. Petrov, A. Lyubchova, L. Markov, Polyhedron 8 (1989) 1061. [10] M. Meyn, K. Beneke, G. Lagaly, Inorg. Chem. 32 (1993) 1209. [11] S. Rouba, P. Rabu, M. Drillon, J. Solid State Chem. 118 (1995) 28. [12] M. Rajamathi, P.V. Kamath, R. Seshadri, Mater. Res. Bull. 35 (2000) 271. [13] J. Olanrewaju, B.L. Newalkar, C. Mancino, S. Komarneni, Mater. Lett. 45 (2000) 307. [14] W. Sta¨hlin, H.R. Oswald, J. Solid State Chem. 3 (1971) 256. [15] L. Poul, N. jouini, F. Fievet, Chem. Mater. 12 (2000) 3123. [16] H. Nishizawa, K. Yuasa, J. Solid State Chem. 141 (1998) 229. [17] V. Isupov, L. Chupakhina, R. Mitrofanova, J. Mater. Synth. Process. 8 (2000) 251. [18] V.P. Khusnutdinov, V.P. Isupov, Inorg. Mater. 44 (2008) 263. [19] K.R. Poeppelmeier, S.J. Hwu, Inorg. Chem. 26 (1987) 3297. [20] W. Tongamp, Q. Zhang, F. Saito, Powder Technol. 185 (2008) 43. [21] A.N. Ay, B. Zumeroglu-Karan, L. Mafra, Z. Anorg. Allg. Chem. 635 (2009) 1470. [22] S.P. Newman, W. Jones, J. Solid State Chem. 148 (1999) 26. [23] T. Biswick, W. Jones, A. Pacula, E. Serwicka, J. Solid State Chem. 179 (2006) 49. [24] W. Stahlin, H.R. Ostwald, Acta Crystallogr. B 26 (1970) 860. [25] C. Padeste, H.W. Schmalle, H.R. Oswald, Kristallografia 200 (1992) 35. [26] M. Rajamathi, P.V. Kamath, J. Power Sources 70 (1998) 118. [27] P.V. Kamath, G.H.A. Therese, J. Gopalakrishnan, J. Solid State Chem. 128 (1997) 38. [28] S. Yamanaka, K. Ando, M. Ohashi, Mater. Res. Soc. Symp. Proc. 371 (1995) 131. [29] R. Rojas, C. Barriga, M.A. Uliibarri, P. Malet, V. Rives, J. Mater. Chem. 12 (2002) 1071. [30] C. Tessier, L.G. Demourgues, C. Faure, A. Demourgues, C. Delmas, J. Mater. Chem. 10 (2000) 1185. [31] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed., John Wiley and Sons, New York, 1986. [32] J.H. Choy, Y.M. Kwon, K.S. Han, S.W. Song, S.H. Chang, Mater. Lett. 34 (1998) 356.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Mechanochemical synthesis of layered hydroxy salts Nygil Thomas * Materials Research Group, Department of Chemistry, St. Joseph’s College, 36 Langford Road, Bangalore 560027, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 January 2012 Received in revised form 11 May 2012 Accepted 15 June 2012 Available online 23 June 2012
A simple one minute synthesis method was adapted for the preparation of layered hydroxy salts of copper, zinc, nickel and cadmium by grinding the metal salts with sodium hydroxide in a mortar. This solvent free method is environment friendly and fast. This method could be extended to the preparation of Ni/Zn hydroxy double salts. The Ni/Zn ratio could be varied from 1.2 to 1.9 by varying the metal contents of the precursor salts without the formation of any impurities in the sample. The prepared compounds had similar characteristics as that of the samples prepared by precipitation route. No sign of carbonate contamination was observed in any of the prepared samples. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds A. Layered compounds B. Chemical synthesis B. Intercalation reactions
1. Introduction LDHs (layered double hydroxides) are materials in which positively charged layers are stacked with anions intercalated in the interlayer region together with water molecules [1]. There are other layered solids like layered hydroxy salts (LHSs) also known as basic salts and alpha hydroxides which are structurally similar to LDHs [2]. LDHs and LHSs derive their structure from that of brucite, Mg(OH)2. In brucite-like hydroxides, OH ions are hexagonally closely packed and the M2+ ions occupy alternate layers of octahedral sites. Thus the structure can be described as a stacking of charge neutral M(OH)2 layers. In LDHs, the layer charge is developed by the partial substitution of divalent cations by trivalent ones and hydrated anions are intercalated between the layers to balance the charge [3]. In LHS, the cation composition of the layer consists of divalent metal ions. The existence of anion in such a structure can be explained by two common mechanisms. The excess layer charge can be created by the occurrence of octahedral sites which are unoccupied by cations, which are located tetrahedrally above and below the empty octahedron [4]. The uncompensated charge on the tetrahedral cations is balanced by intercalating anions in the interlayer. The second mechanism involves the substitution of a part of the hydroxide groups located above and below the layer by anions and water molecules [4]. The general formula of a layered hydroxy salt can be expressed as MII(OH)2 x(An )x/nnH2O where MII = Mg, Co, Ni, Cu; An = NO3 , Cl , SO42 etc. The interlayer
* Tel.: +91 80 2221 1429; fax: +91 80 2224 5831. E-mail address: [email protected]. 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.06.057
spacing of these solids is determined by the size of the anion. The commonly observed values of x are 0.5, 0.67 and 1.0 [4]. LDHs and LHSs show interesting properties such as anion mobility, anion exchange and surface basicity. Due to these properties, these compounds find many applications in varied fields such as sorption, catalysis, flame-retardation, polymer stabilization, sensing, electrochemistry, photochemistry and medicine to cite a few [5–9]. There are many methods available for the synthesis of LHSs. The common method involves the hydrolysis of divalent metal salts in the presence of a metal oxide [10,11]. This method involves a long synthesis time typically from hours to days. Another method of preparation is by the controlled precipitation of metal salts by the addition of alkaline solutions [12]. A number of LHSs have been prepared by this method. This method is time consuming and generates considerable amount of waste. Many times quantitative precipitation was not observed [13]. Urea hydrolysis method is used for the preparation of LHSs of high crystalline nature [14]. The disadvantage of this method is that CO2 produced in the decomposition of urea reacts with water and generates carbonate. This carbonate may appear as a contamination in the final sample. LHSs synthesized by polyol method were of low crystallinity and removal of polyol completely from the final product was difficult [15]. There are other methods which are not very common like heating the metal salt solution in a bomb reactor, melting reaction and hydrothermal preparation [16]. Each of the above method has its own advantages and disadvantages. One main drawback of all the above methods is the time taken to obtain the product. Since all the above methods take considerable amount of time, it will be of significance if one can develop methods with shorter preparation time.
N. Thomas / Materials Research Bulletin 47 (2012) 3568–3572
The mechanochemical method has been widely used to synthesize large variety of materials [17,18]. This solvent free method is rapid as well as environment friendly. There are different forms of mechanochemical activation used for the synthesis of novel materials. This synthesis method induces solid state reactions between the hydrated and hydroxide samples by grinding or milling. Li–Al LDH was prepared by a reaction between solid lithium and aluminium hydroxide in a mortar and then exposing the mixture with water vapour [19]. Recently, there are reports on the preparation of nitrate intercalated Mg–Al LDH by two step milling process [19]. Mg–Al nitrate LDHs were prepared by the mixing of metal salts and NaOH in a mortar [20,21]. In this paper, we describe the preparation of LHSs by simple grinding of a mixture which consists of metal salts and sodium hydroxide. The grinding time was kept as one minute for LHS and three minutes for HDS. 2. Experimental 2.1. Preparation of layered hydroxy salts All the chemicals used were of pure analytical grade. The chemical formula of the salts are as follows, Cu(NO3)23H2O, Ni(NO3)26H2O, Zn(NO3)26H2O, Cd(NO3)24H2O and NaOH. 2.18 g of copper nitrate was mixed with 0.18 g of NaOH in a mortar. The mixture was ground well for 1 min using a pestle. The resultant paste was washed with water and dried at room temperature. No precautions were taken to avoid carbonate contamination during synthesis. This procedure was repeated with zinc, cadmium and nickel nitrates using same amount of nitrate salts and NaOH.
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and back titrating the excess acid with standard NaOH using a pH meter.
3. Results and discussion Fig. 1a and b shows the PXRD patterns of the product obtained on grinding copper nitrate and NaOH before and after washing with water. Both samples exhibit all the characteristic reflections of copper hydroxy nitrate and match well with the values reported in literature [23]. The sharp and symmetrical reflections at 2u = 12.78 and its multiple at 25.98 were observed at same positions in both the samples. This proves without doubt that the formation of the layered compound happened during grinding itself. The PXRD patterns of the products match well with that of the control sample prepared by co-precipitation (Fig. 1c). In both the sample peaks after 2u = 308 are considerably broadened due to the stacking faults. Biswick et al. indexed the PXRD pattern of the copper hydroxy nitrate based on a monoclinic lattice [23]. But the phase had a few peaks matching closely to orthorhombic phase. This well ordered sample was prepared by a hydrolysis method from hydrated copper nitrate. It is difficult to distinguish these peaks in our sample due to the peak broadening. The peaks positions are directly compared with reported powder data (ICSD CC 31353) and marked in Fig. 1. The sample obtained before washing shows three additional reflections which are due to sodium nitrate (PCPDF No. 850850) formed in the reaction medium. The washed sample was completely free from any such impurities. The basal spacing of the prepared sample was 7.0 A˚ and matches well with the values reported in the literature. There was
2.2. Preparation of copper hydroxy nitrate by co-precipitation Copper hydroxy nitrate was synthesized by drop wise addition of 50 mL of sodium hydroxide (0.75 mol/L) to 20 mL boiling copper nitrate solution (3.5 mol/L) with vigorous stirring [22]. The resultant precipitate was washed and dried at room temperature. 2.3. Preparation of hydroxy double salts Stochiometric quantities of nickel acetate, zinc acetate and NaOH (Nickel(II) acetate tetrahydrate, 99.8%, 0.59 g, Zinc(II) acetate dihydrate, 99.7%, 0.44 g and NaOH, 0.46 g) were mixed in a mortar to prepare a compound of the formula Ni3Zn2(OH)8(CH3COO)2nH2O. This mixture was ground well for 3 min. The paste was washed with water and dried at room temperature. Ni/Zn ratio was varied systematically to investigate if the resultant solid had the same metal ratio as that of the precursor. 2.4. Characterization Powder X-ray diffraction (PXRD) measurements were performed on a PANalytical Xpert Pro X-ray Diffractometer using CuKa radiation (l = 0.154 nm) at 40 kV and 30 mA, at a scanning rate of 28 min 1. The infrared (IR) spectra of samples were collected using a Nicolet IR200 FT-IR spectrometer using KBr pellets, in the range 4000–400 cm 1 with 4 cm 1 resolution. The interlayer nitrate content was obtained by ion chromatography (IC) using a Metrohm 861 Advanced Compact ion chromatograph with Metrosep A Supp5 250 anion column and conductivity detector. The samples were dissolved in 1 mol/L acetic acid and diluted suitably for this purpose. The copper, zinc, cadmium and nickel contents of the samples were estimated by atomic absorption spectroscopy (Varion AA240). The OH content was obtained by dissolving a known weight of the sample in dilute HCl
Fig. 1. PXRD patterns of the cupric nitrate (a), product obtained on grinding cupric nitrate and NaOH for 1 min before washing (b), after washing (c) and the product obtained on co-precipitation (d), reflections due to sodium nitrate impurity are marked with *.
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no considerable difference in the crystalline nature of the samples synthesized by this method and by co-precipitation unlike in the case of layered double hydroxides (LDHs). There was significant loss in crystalline nature during the preparation of LDHs by mechanochemical method [21]. Wet chemical analysis showed that the copper hydroxy nitrate prepared by this method has the approximate formulae Cu2(OH)3(NO3) (Cu = 51%, NO3 = 26%). Mechanochemical synthesis method was repeated in custom made humidity chamber to study the effect of ambient humidity on the prepared samples. The ambient humidity was varied by using different salt mixtures in the humidity chamber. We could not observe any difference in the final products formed after controlled ambient atmosphere which may be due to the presence of hydrated metal salts and very short time of preparation, typically 1–3 min. Unlike zinc–aluminium and zinc–chromium layered double hydroxides (LDHs), layered hydroxy salts have no flexible hydration behaviour on exposure to humidity [4]. The IR spectra of the product obtained on grinding copper nitrate and NaOH and the control sample prepared by the precipitation method are shown in Fig. 2a and b respectively. The IR spectra of both samples match well. The broad absorption at 3545 cm 1 in both samples is due to O–H stretching vibration of the hydroxyl groups of the brucite-like sheets and water in the interlayer space. There is a sharp peak at 3400 cm 1 in both the samples which may be due to the stretching vibration of O– H groups which are not involved in the hydrogen bonding. The number and positions of the anionic IR bands depend on the coordination of ions in the layer. There are four strong absorptions in the region 1600–1000 cm 1 for copper hydroxy nitrate. The observed peaks at 1080, 1340, and 1420 cm 1 indicate the incorporation of NO3 in C2v symmetry with one of the N–O bonds parallel to the c-axis [22]. There is a sharp peak at 1380 cm 1 due to nitrate in the D3h symmetry [22]. Based on the degree of covalent character of metal–oxygen bond, it was proposed that the nitrate group in copper hydroxy nitrate is bound more strongly to the layer than its structural analogues like nickel and zinc hydroxy nitrates [23]. Fig. 3 shows the PXRD patterns of the samples obtained by the mechanochemical preparation route starting from zinc, cadmium and nickel nitrates. The product obtained from zinc nitrate (Fig. 3a) shows a pattern identical to that of Zn5(OH)8(NO3)22H2O. The
Fig. 2. R spectrum of the product obtained on grinding cupric nitrate and NaOH after washing with water (a), control sample prepared by the precipitation method (b).
Fig. 3. PXRD patterns of the samples obtained by the mechanochemical preparation starting from zinc (a), cadmium (b) and nickel nitrates (c).
Fig. 4. IR spectra of the samples obtained by the mechanochemical preparation starting from zinc (a), cadmium (b) and nickel nitrates (c).
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pattern is marked by an intense reflection at 9.6 A˚, due to 2 0 0 plane of the monoclinic lattice [24]. The PXRD pattern of the product obtained from cadmium nitrate precursor is shown in Fig. 3b. The pattern can be indexed based on a monoclinic system. The basal spacing of the sample is 9.4 A˚ matching with the values reported in the literature for Cd2(OH)3NO3 [25]. The product obtained from nickel nitrate (Fig. 3c) source was nearly amorphous which is quite common in the case of nickel hydroxides. The products obtained by precipitation of nickel hydroxide using ammonia or electrosynthesis show a poorly ordered pattern which is similar to our product [26]. This disordered phase is denoted as a-hydroxide. In this layered material, layers are oriented randomly with respect to one another leading to turbostatic disorder [27]. The IR spectra of the samples obtained by the mechanochemical preparation starting from zinc, cadmium and nickel nitrates are shown in Fig. 4. The product obtained from zinc (Fig. 4a) shows sharp absorption at 1384 cm 1 which is due to N–O stretching which is characteristic of the nitrate ion in the D3h symmetry [22]. This spectrum was similar to what was observed in the case of nitrate intercalated LDHs. Unlike in the case of copper hydroxy nitrates, nitrate group is not grafted to the layer here. The stretching vibrations of the OH group and water molecules in the interlayer are seen in the range of 3660–2990 cm 1. Three bands are observed in this region. The sharp peak at 3574 cm 1 may be due to stretching vibrations of O–H groups not considerably associated with the hydrogen bonding. The 3465 cm 1 band may be due to the vibrations of the OH group which are hydrogen bonded to the nitrate groups in the interlayer. The broad band at 3305 cm 1 can be assigned to stretching vibrations of O–H groups of the water. The bending vibrations of the water molecules are
Fig. 5. PXRD patterns of the products obtained on grinding different mole rations of nickel and zinc acetates with NaOH (a) Ni/Zn ratio 0.94 (a), 1.15 (b), 1.47 (c), 1.85 (d) and (e) 2.80. Reflections due to ZnO are marked with ^.
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observed at 1630 cm 1 [4,23]. The sample obtained from cadmium nitrate source (Fig. 4b) shows peaks at 1043, 1323 and 1433 cm 1 which is characteristic of grafted nitrate with C2V symmetry. There is a sharp peak at 1384 cm 1 and at 1015 cm 1 due to N–O stretching of the nitrate ion in the IR spectrum (Fig. 4c) of the sample obtained from nickel source. This pattern is characteristic of nitrate ion loosely held in the interlayer which retains its D3h structure in the interlayer. There are other bands observed which are due to Ni–O–H vibration at 658 cm 1 and Ni–O vibration at 475 cm 1 [4,26]. Our next aim was to extend this method to hydroxy double salts with two different divalent metals in the layer. Nickel–zinc hydroxy acetate (NiZn-HDS) is a well studied hydroxy salt [28–30]. In NiZn-HDS, zinc cations occupy tetrahedral sites above and below the nickel vacancies outside the hydroxide layers. Acetate ions are bound to the zinc atoms. The general formula can be given as Ni3Zn2(OH)8(CH3COO)2nH2O. NiZn-HDS are routinely synthesized by hydrothermal methods or by reacting ZnO with nickel acetate [30]. These methods take minimum of 24 h. During these preparations, the supernatant was always green coloured showing the presence of unreacted nickel (Fig. 1a of supplementary information). Fig. 5 shows the PXRD pattern of the product obtained on grinding different ratios of nickel and zinc acetates with NaOH. In Fig. 5b, a sharp and strong basal reflection close to 13 A˚ is followed by another reflection at 6.5 A˚. The non basal reflections observed at 2.66 and 1.54 A˚ are asymmetrical and weak due to the turbostatic disorder in the sample. The pattern can be indexed based on a hexagonal cell. The wet chemical analysis (Ni = 34%, Zn = 21% OH = 21% and CH3COO = 19%) shows that the product had an approximate formula Ni3Zn2(OH)8(CH3COO)22H2O. Interestingly, the washings collected was colourless indicating the complete conversion of nickel salts to product (Fig. 1b of supplementary information). Yamanaka and Rojas et al. [28,29] reported the preparation of this compound by hydrothermal method which involves heating the metal acetates in Teflon lined stainless steel bomb. They observed a limiting value of 1.5 for the Ni/Zn ratio in their samples irrespective of the starting ratio of the salts [28,29]. We wanted to investigate if we could prepare NiZn-HDS with different Ni/Zn ratio. We prepared a series of compounds with different Ni/Zn ratio in the starting acetate salts. The PXRD patterns are shown in Fig. 5. The wet chemical analysis proved that the final compounds had different Ni/Zn ratios. The Ni/
Fig. 6. IR spectrum of the product obtained on grinding nickel and zinc acetates with NaOH.
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Zn ratios could be varied from 1.2 to 1.9 (Fig. 5b–d) without the formation of any impurities in the sample. As the zinc content decreased, there was a considerable loss of peak intensities due to the high scattering power of zinc compared to nickel (Fig. 5e). When the zinc content was increased, the ZnO impurity started forming (Fig. 5a) along with Ni/Zn hydroxy salt. Fig. 6 shows the IR spectrum of the product obtained on grinding nickel and zinc acetates (Ni/Zn ratio 1.47) with NaOH. The broad bands below 900 cm 1 are due to the lattice vibrations of the metal cations of the layer [29]. There are two intense bands at 1573 and 1407 cm 1 due to asymmetric and symmetric stretching vibrations of acetate anion [4]. The difference between these two bands is characteristic of the type of bonding existing between the acetate and metal. It could be monodentate, bidentate or chelating coordination. The difference of 166 cm 1 is indicative of monodentate coordination of acetate species to the metal cation [31,32]. 4. Conclusion A simple and ultrafast method was adapted to prepare layered hydroxy salts of different metals. This method is versatile as this can be used in the preparation of a variety of layered hydroxy salts like copper, zinc, cadmium and nickel. Only one minute manual grinding was sufficient to prepare the LHSs. This method can be successfully extended for the preparation of hydroxy double salts. Mechanochemical method does not require any harsh conditions like heating, refluxing or excess use of solvents. The Ni/Zn ratio could be controlled in the case of NiZn hydroxy double salts which was not possible by any conventional preparation methods. The obtained products had similar characteristics as of the samples prepared by precipitation. Acknowledgements N.T. thanks CSIR for the award of senior research fellowship and Dr. Michael Rajamathi for the permission to use the lab space.
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