International Journal of Inorganic Chemistry
Volume 2015 (2015), Article ID 536470, 11 pages
http://dx.doi.org/10.1155/2015/536470

Research Article

Thermal Decomposition Studies of Layered Metal Hydroxynitrates (Metal: Cu, Zn, Cu/Co, and Zn/Co)

Thimmasandra Narayan Ramesh and Theeta Lakshamaiah Madhu

Department of Studies and Research in Chemistry and Professor C N R Rao Centre for Advanced Materials, Tumkur University, Tumkur 572 103, India

Received 30 September 2014; Revised 20 December 2014; Accepted 22 December 2014

Academic Editor: Alfonso Castiñeiras

Copyright © 2015 Thimmasandra Narayan Ramesh and Theeta Lakshamaiah Madhu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Layered metal hydroxynitrates and mixed metal hydroxynitrates (copper/cobalt hydroxynitrates and zinc/cobalt hydroxynitrates at different mole ratios) were synthesized by hydrolysis of urea and metal nitrates at 140°C. Layered metal hydroxyl nitrates derive their structure from brucite mineral and generally crystallize in hexagonal and monoclinic phases. Isothermal decomposition studies of Cu₂(OH)₃(NO₃), Co₂(OH)₃(NO₃), Cu1.5Co0.5(OH)₃(NO₃), Cu1.34Co0.66(OH)₃(NO₃), Zn₅(OH)₈(NO₃)₂(H₂O)₂, Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂, and Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)₂ samples were carried out at different intervals of temperature and the structural transformations during the process were monitored using powder X-ray diffractograms. Biphasic mixture of metal hydroxynitrate/metal oxide is observed in case of cobalt/zinc based layered hydroxynitrates, while copper hydroxynitrate or copper/cobalt metal hydroxynitrate decomposes in a single step. The decomposition temperatures of layered metal hydroxynitrates and mixed layered metal hydroxides depend on the method of preparation, their composition and the nature of metal ion, and their coordination.

1. Introduction

Layered metal hydroxysalts (LHS) are used in dye adsorption, catalysts in organic reactions such as catalyst supports, fire retardants, polymer composites, electrodes, magnetism, electrochromic devices, and photocatalysts [1–5]. The structure of layered metal hydroxysalt is comprised of a hydroxyl deficient positively charged compound and the composition [M(OH)1−x]x+ where M denotes metal ion; to 1 [6]. Anions (An−) will get intercalated to compensate its charge neutrality resulting in the formula [M(OH)2−x(An−)x/n] [7, 8]. The value of “” dictates the composition of the compound; that is, when , we get layered metal hydroxide-[M(OH)₂] and, at “” = 2, metal nitrate MA₂ to obtain two end members. The structure of metal hydroxide is comprised of hexagonally close packed OH⁻ ions in which divalent metal ions occupy octahedral sites resulting in the formation of charge neutral layers. These charge neutral layers stack on top of each other having an interlamellar spacing of 4.6 Å. Intermediate values of “” will dictate the composition of the layered metal hydroxysalt. “” = 0.5 results in the formation of M(OH)(An−), and M₂(OH)₃X/M₃(OH)₄(NO₃)₂/M₅(OH)₈(NO₃)₂ will be obtained at “” = 0.67 where the above compounds crystallize in hexagonal or pseudo-hexagonal symmetry [9–13].

Several methods have been reported on the preparation of layered hydroxysalts [9–14]. Precipitation method involves addition of an alkali solution to metal nitrate solution to attain pH-6.8–7.0. Aging of metal oxide in aqueous solution of metal salt has also been adopted for the precipitation of layered hydroxysalts [3]. Hydrolysis of metal nitrates using urea/hexamine precipitates metal hydroxysalts [15, 16]. Thermodynamics and kinetic factors control the stoichiometry and composition product formation. The kinetics of the reaction can be controlled by adjusting the temperature, concentration of the reagents, pH, hydrolysis rate, and metal salt to hydrolyzing agent ratio.

Zinc, nickel, cobalt, and copper based layered hydroxysalts have been extensively studied. Cobalt and nickel based layered metal hydroxynitrates (CoHN/NiHN) crystallize in the hexagonal system and have been used as magnetic materials. While zinc hydroxynitrate is used as an anticorrosive agent and copper hydroxynitrate is used in vehicle air bags [17, 18]. Copper based hydroxynitrate (CuHN) with composition Cu₂(OH)₃(NO₃) crystallizes in orthorhombic and monoclinic crystal systems [19, 20]. The crystal structure of monoclinic copper hydroxynitrate [Cu₂(OH)₃(NO₃) or Cu(OH)1.5(NO₃)0.5] is comprised of copper hydroxide layers in which some of the hydroxyl ions () are replaced by the nitrate ions and are directly coordinated to the sheets. Copper occupies two different distorted octahedral sites within the layer and the structure of copper hydroxynitrate-Cu₂(OH)₃NO₃ is shown in Figure 1.

Figure 1: Crystal structure of copper hydroxynitrate-Cu₂(OH)₃NO₃.

Divalent metal ions or cations occupy octahedral or tetrahedral sites in the sheets of layered hydroxynitrate. One of the classic examples is zinc hydroxynitrate (ZnHN) with the composition Zn₅(OH)₈(NO₃)₂(H₂O)₂ which is comprised of hexagonal close packing of hydroxide ions in which three-fourths of the octahedral sites are occupied by divalent Zn²⁺ ions and one-fourth of the octahedral sites are vacant in the layer. Two tetrahedral sites, one above and one below the layer, will be available if one of the octahedral sites is vacant in the sheet of zinc hydroxynitrate. Zn²⁺ ion occupies tetrahedral coordination in the sheets of zinc hydroxynitrate which are coordinated by 3 OH⁻ ions on the layer and one of the OH⁻ group rom the water molecule present in the interlayer region to form tetrahedron. Nitrate ions are present in the interlamellar region and do not have any bonding or coordination with the ions present in the lattice. The formula of zinc hydroxynitrate is Zn₃(octa)Zn₂(tetra)(OH)₈(NO₃)₂(H₂O)₂ [21]. Figure 2 shows the crystal structure of zinc hydroxynitrate.

Figure 2: Crystal structure of zinc hydroxynitrate.

Extensive studies have been carried out on the decomposition of magnesium hydroxide, nickel hydroxide, and calcium hydroxide to obtain their respective metal oxides. There are also reports on the utilization of thermal energy storage density of magnesium hydroxide and calcium hydroxide during hydration-rehydration process [22, 23]. We had reported on the structural transformations during thermal decomposition of nickel hydroxide to nickel oxide, depending on the conditions of synthesis and crystallinity [24, 25]. Metal oxides have been extensively investigated in the last two decades in view of their application as magnetic materials, electrochromic devices, gas sensors, high-temperature solar selective absorbers, and catalysts [1, 26–29]. The information about the decomposition studies of layered metal hydroxysalt is important to prepare high surface area solid for adsorption, catalytic supports, and catalysts. Decomposition of layered hydroxysalt also results in formation of metal oxide. We have reported the thermal decomposition studies of cobalt and nickel based layered hydroxynitrates and their mechanism of structural transformation to metal oxides. On continuing our investigations on layered hydroxysalts, the present work is concerned with the synthesis and characterization of layered metal hydroxynitrates, layered mixed metal hydroxynitrates, and their thermal evolution studies. To the best of our knowledge, there are no reports on what is mentioned above and in this paper, we have reported on the thermal decomposition of metal hydroxynitrates (metal: Cu, Co, Zn, Cu/Co, and Zn/Co) and their phase evolution using powder X-ray diffraction analyses.

This work describes the preparation and characterization of zinc/copper/cobalt hydroxynitrate samples. The term zinc/copper/cobalt used in this paper covers not only mixed metal salts, but also a single metal ion present in the lattice of the compound. Thermal decomposition studies of CoHN, CuHN, ZnHN, Cu1−x-CoxHN (, 0.33, 0.5), and Zn1−x-CoxHN (, 0.33, 0.5) and their structural transformation have been investigated.

1.1. Experimental Section

1.1.1. Reagents

Analytical grade Co(NO₃)₂6H₂O, Cu(NO₃)₂6H₂O, and Zn(NO₃)₂6H₂O and urea were procured from Merck, India, and SD Fine Chemicals, India, and were used as received.

Stoichiometric quantities of the divalent transition metal nitrates, that is, M(NO₃)₂·6H₂O (where M = Cu/Co/Zn), were mixed with 2 g of urea and 2 mL of distilled water in 100 mL beaker. The mixture was stirred well, and beaker was covered with watch glass and placed in hot air oven maintained at 140°C for 2 h. The above mixture was periodically stirred and after 2 h, it was cooled to room temperature (RT). The solid formed was washed with distilled water and dried at room temperature till constant weight is obtained. In separate experiments, mixed metal hydroxynitrates were prepared by following the procedure reported above by taking mixed metal nitrates [Zn(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O; Cu(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O] in the mole ratios of , 0.33, 0.5. Solid solution series of layered mixed metal hydroxynitrate (, 0.33, 0.5) have been prepared and the details are given in Table 1.

Table 1: Weight of the reagents used to prepare layered metal hydroxynitrates.

Sample
Weight of zinc nitrate
Weight of cobalt nitrate
Weight of copper nitrate
Weight of urea
Volume of water (cm³)
Temperature (°C)
Duration (h)

g
moles
g
moles
g
moles
g
moles
g
moles

Cu₂(OH)₃(NO₃)
—
—
—
—
14.942
1.0
2.0
0.033
2.0
0.111
140
2

Cu1.5Co0.5(OH)₃(NO₃)
—
—
4.5
0.25
11.206
0.75
2.0
0.033
2.0
0.111
140
2

Cu1.34Co0.66(OH)₃(NO₃)
—
—
5.939
0.333
9.962
0.667
2.0
0.033
2.0
0.111
140
2

Zn₅(OH)₈(NO₃)₂(H₂O)₂)
18.399
1.0
—
—
—
—
2.0
0.033
2.0
0.111
140
2

Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂
13.8
0.75
4.49
0.25
—
—
2.0
0.033
2.0
0.111
140
2

Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)2
12.25
0.667
6
0.333
—
—
2.0
0.033
2.0
0.111
140
2

Co₂(OH)₃(NO₃)
—
—
17.999
1.0
—
—
2.0
0.033
2.0
0.111
140
2

1.1.2. Thermal Decomposition Studies

Thermal conversion of layered metal hydroxynitrates and layered mixed metal hydroxynitrate samples to their respective oxides has been carried out on the following samples: Cu₂(OH)₃(NO₃) (CuHN), Co₂(OH)₃(NO₃) (CoHN), Cu1.5Co0.5(OH)₃(NO₃) [Cu1.5Co0.5(HN)], Cu1.34Co0.66(OH)₃(NO₃) [Cu1.34Co0.66(HN)], Zn₅(OH)₈(NO₃)₂(H₂O)₂(ZnHN), Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂[Zn3.75Co1.25(HN)], and Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)₂.

In a typical procedure, approximately 100 mg of as-prepared layered metal hydroxynitrate/layered mixed metal hydroxynitrates was placed in an alumina boat and isothermally heated to different temperatures (100, 150, 200, 250, 300, 350, 400, and 500°C) for 2 h and cooled to room temperature. The details of the starting compounds are given in Table 1. The decomposed products were collected for structural characterization using powder X-ray diffraction.

2. Characterization

All the samples were characterized by powder X-ray diffraction (pXRD) using Bruker D8 advanced diffractometer (CuKα source  Å; 30 mA and 40 kV) in a 2θ range of 5° to 65°. Data was collected at a scan rate of 4°θ min⁻¹ with 2θ steps of 0.05°. X-ray diffractogram of the following compounds: Cu₂(OH)₃(NO₃), Cu₃(OH)₄(NO₃)₂(2H₂O)₂, Cu₄(OH)₆(NO₃)₂, CoO (cubic), CoO (monoclinic), Co₂(OH)₃(NO₃), CoO (cubic), Co₃O₄ (cubic), CoCu₂O₃, Zn(OH)(NO₃)(H₂O), Zn₃(OH)₄(NO₃)₂, Zn₅(OH)₈(NO₃)₂(H₂O)₂, ZnO (hexagonal), and Zn0.85Co0.15O was obtained at different experimental conditions and their decomposed products were indexed using the powder diffraction data of from ICSD database.

3. Results and Discussion

3.1. Formation Mechanism of the Precursors

Precipitation of layered metal hydroxynitrates by mixing metal nitrates with alkali at appropriate pH range from 7 to 9 does not provide control over the crystal growth [30]. Hence urea, (NH₂)₂CO, is a nontoxic, cheap, stable, and water soluble hydrolyzing agent that was used to control the precipitation of layered metal hydroxysalts and the products obtained will be homogeneous in nature. Decomposition of urea decomposes at 70°C leading to the following reaction:

In acidic or neutral conditions, HNCO will be converted to carbon dioxide and ammonia/ammonium cation:

Consumption of H⁺ ions results in an increase in the solution pH. The precipitation of layered metal hydroxynitrate involves reaction of hydroxyl ions with metal nitrate solution.

Figure 3 shows the powder X-ray diffraction patterns of copper hydroxynitrate, Cu₂(OH)₃(NO₃), Cu1.5Co0.5(OH)₃(NO₃), and Cu1.34Co0.66(OH)₃(NO₃) samples obtained by urea hydrolyses.

Figure 3: Powder X-ray diffraction patterns of copper hydroxynitrate-Cu₂(OH)₃(NO₃), cobalt substituted copper hydroxynitrate, and cobalt hydroxynitrate samples obtained by urea hydrolysis.

In the pXRD patterns of the CuHN, successive reflections, up to at least the third order, indicate the lamellar structure and could be indexed to monoclinic crystal system space group-P₂₁ using the JCPDS-ICDD 75-1779 file. The cell parameters of copper hydroxynitrate are as follows:  Å,  Å,  Å, , and  Å³ with an interlayer distance of 6.9 Å. The powder X-ray diffraction patterns of Cu₂(OH)₃(NO₃), Cu₃(OH)₄(NO₃)₂(H₂O)₂, and Cu₄(OH)₆(NO₃)₂ samples were simulated using the single crystal data obtained from ICDD database. The simulated powder X-ray diffraction pattern of Cu₂(OH)₃(NO₃) matches the observed X-ray diffraction pattern of copper hydroxynitrate (see Figure 4).

Figure 4: Simulated powder X-ray diffraction patterns of different polymorphic modifications of copper hydroxynitrate.

Thermal decomposition of layered hydroxynitrate follows dehydration, denitration, and disruption of layered framework under atmospheric conditions. The elimination of hydroxyl and nitrate groups from the precursors has great influence on the crystallinity of metal oxides, and new intermediate phase might also form during the disruption of the original structure. The copper hydroxynitrate undergoes decomposition in one step producing CuO according to the equation:

The theoretical weight loss for the reaction Cu₂(OH)₃(NO₃) → 2CuO is 33.17%.

Powder XRD was very important in the identification of the phase formation and in Figure 5 the data of the copper hydroxynitrate heated to different temperatures is shown. The XRD results show that up to 200°C the samples do not show any structural changes. Thermal decomposition of copper hydroxynitrate occurs at single step in which dehydroxylation and decomposition of the nitrate in the interlayer space take place simultaneously leading to the destruction of the layered structure (250°C) (see Figure 5). The total weight loss on isothermal heating of the copper hydroxyl nitrate sample is 31%. Copper oxide can exist in monoclinic or cubic crystal system and in Table 2 the XRD data is shown. The decomposed product could be indexed to monoclinic phase of copper oxide. Figure 6 shows the crystal structure of copper oxide (monoclinic) formed on decomposition of copper hydroxynitrate.

Table 2: XRD data of (ICSD-97597) (Zn0.85Co0.15)O (hexagonal).

2θ (°)
d-spacing (Å)

31.754
2.8179
0
1
0

34.465
2.6022
0
0
2

36.251
2.478
0
1
1

47.562
1.9117
0
1
2

56.568
1.6269
−1
2
0

62.911
1.4773
0
1
3

Figure 5: Copper hydroxynitrate-Cu₂(OH)₃(NO₃) heated to different intervals of temperature.

Figure 6: Schematic representation on the structural transformation of copper hydroxynitrate to copper oxide on isothermal heating (250°C).

3.2. Cu1.5Co0.5(OH)₃(NO₃) and Cu1.34Co0.66(OH)₃(NO₃) Series

In the structure of layered hydroxynitrate, divalent transition metal ions can be isomorphically substituted by the other divalent cations aross the whole range of composition “” which results in the formation of continuous solid solution series. Goldschmidt’s rule states that a continuous solid solution cannot be obtained if the difference in ionic radii is greater than 15% of the smallest cation. The ionic radii of Co²⁺ are 0.745 Å for high spin and 0.65 Å for low spin complexes. The ionic radius of Cu²⁺ is 0.73 Å when the coordination number is 6. Co²⁺ substitution in Cu²⁺ sites of layered metal hydroxynitrate is permitted as the difference in their ionic radii is <10% and hence the values are within the limits set by Goldschmidt, and solid solution series Cu/Co hydroxynitrates with the composition of Cu1.5Co0.5(OH)₃(NO₃) and Cu1.34Co0.66(OH)₃(NO₃) were prepared by using urea hydrolysis route and their powder X-ray diffraction patterns are shown in Figure 3. We observe structural similarities among Cu₂(OH)₃(NO₃), Cu1.5Co0.5(OH)₃(NO₃), and Cu1.34Co0.66(OH)₃(NO₃) as the peak positions in their diffraction patterns appear at similar positions in 2θ. However, the Co²⁺ and Cu²⁺ have d⁷ and d⁹ electronic configurations, respectively. Cu²⁺ ion is prone to Jahn Teller distortion and results in the irregular octahedral coordination arrangement around it. On substitution of Co²⁺ into the lattice, we observed a decrease in its irregularity in the structure of cobalt substituted copper hydroxynitrate. The pXRD patterns of Cu1.5Co0.5(OH)₃(NO₃) and Cu1.34Co0.66(OH)₃(NO₃) heated to different temperatures are shown in Figures 7 and 8, respectively. The mixed metal hydroxynitrates [Cu1.5Co0.5(OH)₃(NO₃) and Cu1.34Co0.66(OH)₃(NO₃)] undergo dehydration and denitration at lower temperatures compared to pure zinc/copper/cobalt based hydroxynitrates. It was found that on substitution of cobalt into the structure of basic copper hydroxynitrate in mixed metal hydroxynitrates, the decomposition temperature is reduced by ~50°C. The dramatic decrease in the decomposition temperature was due to decarboxylation reaction on substitution of cobalt for copper in the cobalt substitute copper hydroxynitrates.

Figure 7: Powder X-ray diffraction patterns copper hydroxynitrate-Cu1.5Co0.5(OH)₃(NO₃) heated to different intervals of temperature.

Figure 8: Powder X-ray diffraction patterns of copper hydroxynitrate-Cu1.33Co0.664(OH)₃(NO₃) heated to different intervals of temperature.

In Figure 9, the structural transformations of Cu1.5Co0.5(OH)₃(NO₃)/Cu1.34Co0.66(OH)₃(NO₃)/Cu1.0Co1.0(OH)₃(NO₃) to copper-cobalt oxide, respectively, are shown. The decomposed product, Co0.333Cu0.666O, could crystallize in cubic or monoclinic system. The powder X-ray diffraction patterns of Co0.25Cu0.75O/Co0.333Cu0.666O/Co0.5Cu0.5O shown in Figures 9 and 10 could be indexed (at temperatures >250°C) to monoclinic system.

Figure 9: Schematic representation on the structural transformation of solid solution of copper-cobalt (25%) hydroxynitrate to Cu0.75Co0.25O on isothermal heating (225°C).

Figure 10: Powder X-ray diffraction pattern of zinc hydroxynitrate-Zn₅(OH)₅(NO₃)₂(H₂O)₂ and cobalt substituted zinc hydroxynitrate samples obtained by urea hydrolysis.

Table 3 shows the decomposition temperatures of Cu₂(OH)₃(NO₃), Cu1.5Co0.5(OH)₃(NO₃), and Cu1.34Co0.66(OH)₃(NO₃), respectively.

Table 3: Decomposition temperatures for layered metal hydroxynitrate and mixed metal hydroxynitrate samples.

Sample
Decomposition temperature (°C)

Cu₂(OH)₃(NO₃)
250

Cu1.5Co0.5(OH)₃(NO₃)
225

Cu1.34Co0.66(OH)₃(NO₃)
225

Zn₅(OH)₈(NO₃)₂(H₂O)₂)
175

Zn3.75Co1 .25(OH)₈(NO₃)₂(H₂O)₂
200

Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)2
200

Co₂(OH)₃(NO₃)
225

3.3. Zinc Hydroxynitrate

Figure 10 shows the powder X-ray diffraction pattern of zinc hydroxynitrate obtained by urea hydrolysis. Zinc hydroxynitrate can crystallize in different crystal structures and depends on the composition, that is, Zn(OH)(NO₃)(H₂O), Zn₃(OH)₄(NO₃)₂, and Zn₅(OH)₈(NO₃)₂(H₂O)₂. Figure 11 shows the simulated powder X-ray diffraction patterns of Zn(OH)(NO₃)(H₂O), Zn₃(OH)₄(NO₃)₂, and Zn₅(OH)₈(NO₃)₂(H₂O)₂, respectively. The experimentally observed pattern of zinc hydroxynitrate matches the simulated powder X-ray diffraction pattern of Zn₅(OH)₈(NO₃)₂(H₂O)₂. On isothermal heating of Zn₅(OH)₈(NO₃)₂(H₂O)₂ the total weight loss observed was 50%, while theoretically the total weight loss expected was 40% indicating higher water content. Figure 12 shows the powder X-ray diffraction patterns of the Zn₅(OH)₈(NO₃)₂(H₂O)₂ samples obtained on isothermal heating at different intervals of temperature.

Figure 11: Simulated powder X-ray diffraction patterns of zinc hydroxynitrate-Zn(OH)(NO₃)(H₂O), Zn₃(OH)₄(NO₃)₂, and Zn₅(OH)₈(NO₃)₂(H₂O)₂, respectively.

Figure 12: Powder X-ray diffraction patterns of zinc hydroxy nitrate-Zn₅(OH)₅(NO₃)₂(H₂O)₂ heated to different intervals of temperature.

The powder X-ray diffraction pattern of zinc hydroxynitrate heated to 100°C shows additional peaks indicating evolution of second phase of zinc hydroxynitrate with composition Zn₃(OH)₄(NO₃)₂. The formation of Zn₃(OH)₄(NO₃)₂ could arise due to dehydration of a Zn₅(OH)₅(NO₃)₂(H₂O)₂. At 175°C, peaks due to Zn₅(OH)₅(NO₃)₂(H₂O)₂ and Zn₃(OH)₄(NO₃)₂ disappear and the zinc oxide peaks are prominently observed. Tables 4 and 5 show the XRD data of zinc oxide in hexagonal and cubic systems, respectively. The experimental data of the decomposed product obtained from Zn₅(OH)₅(NO₃)₂(H₂O)₂ can be indexed to hexagonal phase of ZnO. The decomposition of layered metal hydroxynitrate results in the absorption of heat and the process is endothermic in nature. The decomposition reaction of this type yields metal oxide and water vapour and nitrates as by-products. Figure 13 shows the crystal structure of zinc oxide formed on decomposition of zinc hydroxynitrate-Zn₅(OH)₅(NO₃)₂(H₂O)₂.

Table 4: XRD data of (ICSD-41488) ZnO (hexagonal).

2 (°)
d-spacing (Å)

31.802
2.8137
0
1
0

34.447
2.6035
0
0
2

36.29
2.4754
0
1
1

47.583
1.911
0
1
2

56.66
1.6245
−1
2
0

62.914
1.4772
0
1
3

Table 5: XRD data of (ICSD-163383) ZnO (cubic).

2 (°)
d-spacing (Å)

34.609
2.5917
1
1
1

40.176
2.2445
0
0
2

58.12
1.5871
0
2
2

Figure 13: Schematic representation on the structural transformation of solid solution of zinc hydroxynitrate to ZnO on isothermal heating (175°C).

3.4. Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂ and Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)₂

Solid solution series of Co/Zn hydroxynitrates were prepared by urea hydrolysis route. The ionic radii of Co²⁺ are 0.745 Å for high spin and 0.65 Å for low spin complexes, respectively. Zn²⁺ with ionic radius of 0.74 Å indicates its coordination number to be 6. Co²⁺ substitution in Zn²⁺ sites is permitted as the difference in the ionic radii is <10% and hence the values are within the limits set by Goldschmidt. The powder X-ray diffraction patterns of Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂ and Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)₂ are shown in Figure 10. We observe structural similarities among Zn₅(OH)₅(NO₃)₂(H₂O)₂, Cu₁ Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂, and Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)₂ as the peak positions appear at similar positions. Co²⁺ and Zn²⁺ have d⁷ and d¹⁰ electronic configurations, respectively, and both Co²⁺ and Zn²⁺ do not induce any distortion of octahedral geometry. The pXRD patterns of Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂ and Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)₂ heated to different temperatures are shown in Figures 14 and 15, respectively. The mixed metal hydroxynitrates, Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂ and Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)₂, undergo dehydration and denitration at 150 to 200°C temperatures compared to pure zinc/copper/cobalt based hydroxynitrates. The decomposition temperatures of Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂ and Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)₂ are shown in Table 3. The pXRD patterns of the decomposed products of Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂ and Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)₂ heated to 250°C could be indexed to hexagonal phase of zinc oxide. Figures 14 and 15 show the powder X-ray diffraction patterns of Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂ and Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)₂ on decomposition, respectively, to form (Zn1−xCox)O. Figure 16 shows the structural transformation of Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂ and Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)₂ on decomposition, respectively, to form (Zn1−xCox)O.

Figure 14: Powder X-ray diffraction patterns of zinc hydroxynitrate-Zn3.75Co1.25(OH)₅(NO₃)₂(H₂O)₂ heated to different intervals of temperature.

Figure 15: Powder X-ray diffraction patterns of zinc hydroxynitrate-Zn3.35Co1.667(OH)₅(NO₃)₂(H₂O)₂ heated to different intervals of temperature.

Figure 16: Schematic representation on the structural transformation of solid solution of zinc-cobalt (25%) hydroxynitrate to Zn0.75Co0.25O on isothermal heating (175°C).

Tables 6 and 7 shows the lattice parameters, expected, of different phases of layered metal hydroxynitrate and mixed metal hydroxynitrates and their respective decomposed products. Table 8 shows the percentage weight loss of the layered metal hydroxynitrate and mixed metal hydroxynitrates, respectively.

Table 6: Lattice parameters of decomposed products obtained from layered hydroxynitrate samples (expected).

Sample
(Å)
(Å)
(Å)
(°)
(°)
(°)
Space group
Crystal system
Cell volume (Å)

CuO
4.6797
3.4314
5.1362
90
99.26
90
C 12/c1
Monoclinic
81.40
4

CuO
4.245(5)
4.245(5)
4.245(5)
90
90
90
F m-3m
Cubic
76.50
4

ZnO
3.2490
3.2490
5.2070
90
90
120
P 63mc
Hexagonal
47.60
2

ZnO
4.4890
4.4890
4.4890
90
90
90
F m-3m
Cubic
90.46
4

CoO
4.260
4.260
4.260
90
90
90
F m-3m
Cubic
77.31
4

Co₃O₄
8.1893
8.1893
8.1893
90
90
90
F m-3m
Cubic
549.21
4

CoCu₂O₃
9.4085
3.9801
3.1980
90
90
90
P m m n
Orthorhombic
119.75
2

(Zn0.85Co0.15)O
3.2538
3.2538
5.2044
90
90
120
P 63mc
Hexagonal
47.72
2

Table 7: Lattice parameters of different polymorphic modifications of layered hydroxynitrate samples.

Sample
(Å)
(Å)
(Å)
(°)
(°)
(°)
Space group
Crystal system
Cell volume (Å)

Cu₂(OH)₃(NO₃)
5.605
6.087(4)
6.929(4)
90
94.48
90
P 1 211
Monoclinic
235.68
2

Cu₃(OH)₅(NO₃) (H₂O)₂
5.830(1)
6.775(1)
21.711
90
90
90
P cmn
Orthorhombic
857.55
4

Cu₄(NO₃)₂(OH)₆
5.576(2)
6.050(2)
6.896(3)
90
94.5
90
P121/m1
Monoclinic
231.92
1

Zn(OH)(NO₃) (H₂O)
17.951
3.2600
14.272
90
114.91
90
P 121/c1
Monoclinic
757.50
8

Zn₃(OH)₄(NO₃)₂
7.038
9.658(2)
11.182(2)
90
100.96
90
P 121/c1
Monoclinic
746.21
4

Zn₅(OH)₈(NO₃)₂(H₂O)₂
19.480
6.238
5.517(1)
90
93.28
90
C 12/m1
Monoclinic
669.31
2

Co₂(OH)₃(NO₃)
3.12
3.12
6.98
90
90
120
—
Hexagonal
—
—

Table 8: Total weight loss observed for layered metal hydroxynitrate samples.

Sample
% Weight loss

Cu₂(OH)₃(NO₃)
30

Cu1.5Co0.5(OH)₃(NO₃)
50

Cu1.34Co0.66(OH)₃(NO₃)
50

Zn₅(OH)₈(NO₃)₂(H₂O)₂)
50

Zn3.75Co1.25(OH)₈(NO₃)₂(H₂O)₂
50

Zn3.35Co1.65(OH)₈(NO₃)₂(H₂O)2
60

Co₂(OH)₃(NO₃)
60

4. Conclusion

Simple preparative method has been adopted for the synthesis of layered hydroxynitrates/mixed metal hydroxynitrates by urea hydrolyses. Thermal decomposition studies of mixed metal hydroxynitrates (zinc/cobalt hydroxynitrates and copper/cobalt hydroxynitrates) were investigated. The mechanism of thermal decomposition of the layered metal/mixed metal hydroxynitrates was deduced and the effect of substituting other metal cations into the structure of layered hydroxynitrate was determined. The single metal hydroxynitrate will have higher decomposition temperature than cobalt substituted copper hydroxynitrates. It was found that the mixed metal hydroxynitrate decomposed at lower temperatures than the single metal hydroxynitrate studied. The work will be used as a basis for further studies of mixed metal hydroxynitrate for the preparation of novel catalyst precursors. The mechanism of decomposition provides insight into the nature of phases formed at different temperatures which can significantly affect the catalytic activity. These metal/mixed metal hydroxynitrates with high surface area may exhibit unusual and unexpected properties for device based applications.

Conflict of Interests

The authors declare that they have no competing financial interests.

Acknowledgments

Ramesh Thimmasandra Narayan gratefully acknowledges the Council of Scientific and Industrial Research (CSIR) (project ref: 01/(2741)/13/EMR-II dated 18-04-2013), New Delhi, for the financial support. Ramesh Thimmasandra Narayan wishes to thank Professor P. Vishnu Kamath lab of Bangalore University for providing laboratory facilities. Authors gratefully thank Mr. Shivanna, Supreeth Nagendran, and Mr. Kiran of Bangalore University for their help in collection of powder-ray diffraction data. The author would like to thank Tumkur University for providing facilities.

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