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J. Phys. Chem. B 2009, 113, 13014–13017
X-ray Diffraction Studies on the Thermal Decomposition Mechanism of Nickel Hydroxide Thimmasandra Narayan Ramesh* Department of Chemistry, Central College, Bangalore UniVersity, Bangalore 560 001, India ReceiVed: July 12, 2009; ReVised Manuscript ReceiVed: August 15, 2009
Nickel hydroxide samples were prepared by using sodium hydroxide and ammonium hydroxide as precipitating agents. The powder X-ray diffraction pattern shows that the degrees of crystallinity in these samples are quite different. The thermal decomposition mechanism of these two nickel hydroxide samples has been determined using powder X-ray diffraction and thermogravimetric analysis. We observe that the transformation of nickel hydroxide to nickel oxide in the crystalline sample is via a two-phase mixture, whereas in a poorly ordered sample, it is through a single phase. This indicates that the decomposition mechanism mainly depends on the preparative conditions and the nature of the sample. Introduction Nickel hydroxide is most widely used as a positive electrode material in all Ni-based alkaline secondary cells.1 It also finds application as an active material in electrochromic devices, supercapacitors, and as a precursor for catalysts.2-4 Divalent metal hydroxides derive their structure from brucite, a mineral form of Mg(OH)2.5 The structure of nickel hydroxide comprises a stacking of hydroxyl layers having the composition [M(OH)2]. A large matrix of preparative conditions has been explored to generate a wide range of nickel hydroxide samples, of which the common procedure to prepare nickel hydroxide is a precipitation method. The precipitation conditions, such as (a) rate of addition, (b) pH at precipitation, (c) temperature of precipitation, (d) choice of aging conditions, (e) choice of appropriate salt and alkali, and (f) drying temperature, affect the outcome of the product.6 The thermal decomposition of nickel hydroxide to synthesize nickel oxide has been widely studied.7-9 The role of preparative conditions on the thermal decomposition and its influence on the changes on the powder X-ray diffraction pattern have been reported.10,11 During the thermal decomposition of nickel hydroxide, an endothermic peak in the differential thermal analysis of nickel hydroxide has been observed. This has been attributed to the release of water during the decomposition of nickel hydroxide.12 To the best of our knowledge, there are not many reports on the thermal decomposition mechanistic studies of nickel hydroxide. The aim of this article is to examine the evolution of nickel oxide from nickel hydroxide during thermal treatment of the sample at different temperatures using powder X-ray diffraction patterns. We have prepared nickel hydroxide samples with different crystallinity and investigated its role on the decomposition mechanism. Experimental Section Nickel hydroxide samples were prepared as follows.13,14 Nickel hydroxide-1 was prepared by the addition of nickel nitrate solution (1M) to a weak base, such as ammonium hydroxide (2M, 200 mL), at room temperature, and the precipitate formed was aged in the mother liquor for 2 h. The product was designated as AH-nickel hydroxide (AH- represents ammonium * To whom correspondence
[email protected].
should
be
addressed.
E-mail:
hydroxide). Nickel hydroxide-2 was obtained by the addition of nickel nitrate (1M, 50 mL) solution to a strong alkali, that is, sodium hydroxide (2M, 100 mL) solution, and the precipitate obtained was aged for 30 min in the mother liquor. The product was designated as SH-nickel hydroxide (SH- represents sodium hydroxide).The obtained slurries were filtered, washed with distilled water, and dried at 65 °C until a constant weight was attained. The samples were characterized by powder X-ray diffraction (PXRD) using a Bruker D-8 Advance X-ray powder diffractometer (Cu KR source, λ )1.5418 Å). Data were collected at a scan rate of 4° 2θ min-1 with 2θ steps of 0.05°. The thermogravimetric studies was carried out using a Mettler Toledo model 851e TG/SDTA system (heating rate ) 10° min-1). The wet chemical analysis of the samples was carried out following the procedure described elsewhere.13 The chemical composition and the weight loss of these samples are given in Table 1. Isothermal heating of nickel hydroxide samples was carried out at different intervals of temperature (RT and 100, 120, 140, 160, 175, 200, 225, 250, 300, 325, and 400 °C) for 2 h each and then cooled to room temperature. The powder X-ray diffraction patterns of these samples were measured, and the crystallite size was estimated using the Scherrer formula. In Tables 2 and 3 are given the values of crystallite size along selective directions of nickel hydroxide and nickel oxide. Results and Discussion The choice of precipitating agents, such as sodium hydroxide and nickel hydroxide, to prepare nickel hydroxide was made from the information based on the kinetics and thermodynamics that interplays and controls the product workup.6 Highly ordered nickel hydroxide can be obtained directly when ammonia is used as a precipitating agent. Ammonia is bound to form a weak complex with transition-metal ions. Therefore, addition of metal nitrate to ammonia results in the formation of a deep blue nickel ammine complex that is not stable at room temperature for a long duration and undergoes hydrolysis to generate highly ordered nickel hydroxide. A strong alkali, such as sodium hydroxide, tends to generate a highly disordered phase of nickel hydroxide at room temperature. If we need to prepare highly crystalline nickel hydroxide using a strong alkali, such as sodium hydroxide, the sample should be made at high temperatures and
10.1021/jp906578u CCC: $40.75 2009 American Chemical Society Published on Web 09/08/2009
PXRD Studies on the Thermal Decomposition of Ni(OH)2
J. Phys. Chem. B, Vol. 113, No. 39, 2009 13015
TABLE 1: Wet Chemical Analysis of Nickel Hydroxide Samples weight percentage sample AH-nickel hydroxide SH-nickel hydroxide a
Ni
OH-
H 2O
total weight loss (%)a
61.97 57.78
35.6 33.3
1.7 7
19.4 (21.45) 26.47 (27)
2+
approximate formula Ni(OH)2 · 0.1H2O Ni(OH)2 · 0.5H2O
Values in the parentheses are based on the approximate formula.
TABLE 2 (a) crystallite size (Å) in the selective directions of AH-nickel oxide temperature (°C)
(001)
(100)
(101)
(102)
(110)
(111)
155 155 158 158 232
1204 2063 2166 1083 2888
361 275 274 322 623
246 246 288 235 313
707 310 308 860 741
430 477 314 183 81
RT 100 225 240 250
(b) crystallite size (Å) in the selective directions of AH-nickel oxide temperature (°C)
(200) 70 78 196 125
300 350 400 450
(111) 65 73 205 1203
(220) 64 70 166 109
TABLE 3 (a) crystallite size (Å) in the selective directions of SH-nickel hydroxide temperature (°C)
(001)
(100)
(101)
(102)
(110)
(111)
20 13 13 13 10 10
92 79 79 79 97 97
25 25 26 26 40 38
18 16 19 18 69 18
96 126 114 84 66 97
58 48 54 36 69 49
RT 100 200 225 240 250
(b) crystallite size (Å) in the selective directions of SH-nickel oxide temperature (°C) 262 290 350 450
(200) 18 18 22 26
(111) 17 19 21 23
(220) 48 50 50 51
subject to hydrothermal conditions. Hence, we chose these two methods to prepare samples having different degrees of crystallinity. The PXRD pattern of nickel hydroxide samples obtained at room temperature using ammonia and sodium hydroxide as precipitating agents is shown in Figures 1 and 2, respectively. The reflections in the powder X-ray diffraction patterns of these samples match with the β-phase of nickel hydroxide. We observe nonuniform broadening of reflections for SHnickel hydroxide in contrast to AH-nickel hydroxide. Addition of nickel nitrate to sodium hydroxide solution results in the precipitation of nickel hydroxide as the solubility product of nickel hydroxide exceeds in strong alkaline solution. Nickel hydroxide crystallizes in two different polymorphic modifications, that is, R and β phase.1 The R phase of nickel hydroxide is metastable and hydrated with higher d-spacing (7.6 Å), initially forms at the time of precipitation of nuclei in the solution, and immediately transforms to β-nickel hydroxide. During the transformation stage, an intermediate phase designated as βbc-nickel hydroxide is formed.13 This is an intergrowth of the R and β phases. According to Ostwald’s law, the thermodynamically least stable phase is the first to be formed during crystallization from a solution. In keeping with this
expectation, SH-nickel hydroxide should be highly disordered. Among the disorders reported by us is (i) interstratification, which involves a nonintegral repeat unit along the stacking direction. In ordered β-Ni(OH)2, the composition of the first coordination shell is Ni(OH)6, whereas in the interstratified motifs, it can be represented as a combination of [Ni(OH)6-x(NO3)x] or [Ni(OH)6-x(H2O)x]. Another disorder is (ii) cation vacancies, in which case the composition changes to [Ni1-x0x(OH)6-2x(H2O)2x]. SH-nickel hydroxide is a combination of interstratification and cation vacancies and is, therefore, expected to be the least stable phase.15 The factors that contribute to the broadening of reflections in nickel hydroxide have been investigated by us in our earlier report.16 DIFFaX code enables us to simulate the PXRD patterns by incorporating structural disorders into the lattice. On the basis of DIFFaX simulation studies, we had shown that the broadening of non-(hk0) reflections can arise due to the presence of structural disorder. The powder X-ray diffraction pattern of SH-nickel hydroxide is similar to that of the NH65 nickel hydroxide sample reported in ref 15. This clearly indicates that the SH-nickel hydroxide is a structurally disordered material consisting of interstratification, cation vacancies, and stacking faults. AH-nickel hydroxide is highly ordered, indicating that the sample has very small percentages of these disorder components. We have shown in our earlier report that highly ordered nickel hydroxide requires a trace amount of stacking defects for structural stability.17 The decomposition mechanism of magnesium hydroxide based on the kinetics and the surface properties has been investigated using X-ray diffraction and electron microscopic studies.18,19 Thermal degradation studies based on electron microscopy and X-ray diffraction studies on nickel hydroxysulphate have been investigated.20,21 The general scheme of decomposition of nickel hydroxide is Ni(OH)2 f NiO + H2O. The decomposition of nickel hydroxide has been extensively studied using electron microscopy on R- and β-nickel hydroxide samples by Figlarz and co-workers.22 However, the study of crystallinity in relation to the thermal decomposition process is reported elsewhere. They show that the decomposition temperature decreases for the poorly crystalline sample.11 There is also one report on the thermodynamical size effect on the decomposition temperature of nickel hydroxide.10 A systematic investigation of nickel hydroxide samples with different degrees of crystallinity has not been investigated. We were, therefore, interested in investigating the decomposition mechanism of these two nickel hydroxide samples using X-ray diffraction. Figure 3 shows the thermograms of SH-nickel hydroxide and AH-nickel hydroxide. AH-nickel hydroxide loses weight in a single step with the weight loss of 19.4%. On heating the sample, we expect to observe changes in the crystal structure. The AH-nickel hydroxide was heated isothermally from room temperature (RT, 25 °C) to 240 °C at several steps separately. The PXRD pattern remains unaffected (see Figure 1). The AHnickel hydroxide loses only 1% weight loss in the range of 25-240 °C. At 250 °C, we observe broadening of (101) and (111) reflections with extra reflections emerging at 37.1° in 2θ. This indicates that the powder X-ray diffraction pattern on the
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J. Phys. Chem. B, Vol. 113, No. 39, 2009
Ramesh
Figure 3. Thermograms of AH-nickel hydroxide and SH-nickel hydroxide. Figure 1. PXRD patterns of AH-nickel hydroxide as a function of temperature. * is due to the sample holder.
Figure 2. PXRD patterns of SH-nickel hydroxide as a function of temperature. * is due to the sample holder.
thermal decomposition of AH-nickel hydroxide shows distinct reaction stages (see Figure 1). The crystallite size of AH-nickel hydroxide decreases continuously from room temperature to 250 °C along the (111) direction (from 430 to 81 Å) (see Table 2a). In the 250-350 °C range, the relative intensities of the nickel hydroxide phase decrease and intensities of nickel oxide peaks increase. At 350 °C, nickel hydroxide peaks completely disappear and peaks due to pure nickel oxide phase are observed. At the initial stages of the reaction, germination takes place on the edges of the platelets and propagates to the interior regions. The propagation requires sufficient thermal energy in the form of heat. The nickel oxide is in a definite and reproducible orientation relative to the nickel hydroxide, that is, (001)Ni(OH)2// (111)NiO and (110)Ni(OH)2//(110)NiO.20 The conversion requires that half the hydroxyl groups between the nickel hydroxide sheets combine with hydrogen atoms on neighboring hydroxyl groups and water molecules are eliminated. During the decomposition process, we observe a decrease in the Ni-Ni bond distance from 3.12 to 2.98 Å, exactly that required in the (111) plane of nickel oxide. The interlayer spacing changes from 4.74 to 2.42 Å. The diffraction patterns of AH-nickel hydroxide show that these changes do not occur simultaneously. Figure 1 clearly shows that the reflections related to nickel hydroxide are very sharp and the reflections corresponding to NiO are
broad. As the reaction proceeds, the dehydration interface moves from the outside to the center of each crystallite. When the sample is heated at 450 °C, the reflections of nickel oxide are narrow and the peak width has decreased, indicating that the sample is crystallized, and it is evident from the changes in the crystallite size (see Table 2b). Figure 3 shows the thermogram of SH-nickel hydroxide and displays the weight loss in two steps. The total weight loss for SH-nickel hydroxide was 28%. We have reported in an earlier paper that the first step weight loss of SH-nickel hydroxide arises due to the presence of reversible moisture content. The reversible moisture content is associated with the presence of structural disorder. The thermogram of SH-nickel hydroxide shows that the dehydration starts at 70-90 °C and proceeds until completion up to 200 °C. This results in the elimination of a water molecule associated with nickel hydroxide (see Table 1). The crystallite size decreases in all the directions up to 225 °C without significant changes in the peak positions of nickel hydroxide. The dehydration of SH-nickel hydroxide results in the broadening of reflections [selectively (001), (102), and (111)] at 100 °C (see Figure 2 and Table 3a). The crystallite size decreases in all the directions from room temperature to above 250 °C. At 262 °C, pure nickel oxide peaks are observed with complete absence of reflections for nickel hydroxide. This indicates that the cation vacancies and interstratified phases are eliminated. No information about the dimensions of the c axis was observed in the case of SH-nickel hydroxide at temperatures greater than 262 °C. Decomposition results in a gradual decrease in intensity of the reflections until they finally disappear, leaving only the reflections expected for nickel oxide. This change may be interpreted as a gradual rearrangement of the (001) or (111) planes until ordering takes place between them required for nickel oxide is formed. On heating the samples to higher temperatures, i.e, 350 to 450 °C, we observe the narrowing of the peak widths indicating the growth of crystallite size of nickel oxide particles (see Figure 2 and Table 3b). It has been mentioned earlier that SH-nickel hydroxide is a highly disordered material comprising of interstratification, cation vacancies and stacking faults. These disorders favor the decomposition to take place at lower temperatures compared to AH-nickel hydroxide sample via disruption of interlayer water molecules leading to an amorphous phase formation and then towards crystallization of nickel oxide. The transformation of amorphous to crystallite transition takes place through a topotactic reaction mechanism. Gregg et al. had proposed the thermal decomposition mechanism of magnesium hydroxide.23 In general, the decomposition
PXRD Studies on the Thermal Decomposition of Ni(OH)2 reaction of a metal hydroxide is as follows: Solid A solid B + gas, where each crystallite A decomposes to give a solid B, and the atomic positions in solid B are retained as they were in solid A during the thermal decomposition process. If this is true, we should observe an intermediate structure during the transformation process. In both the samples we do not observe such an intermediate phase. Even if such phases are formed, then it should be observed more likely in the SH-nickel hydroxide sample. Ball and Taylor proposed an alternate inhomogeneous decomposition for metal hydroxide that has donor and acceptor sites. During the thermal degradation, the metal ions migrate from the hydroxide to the oxide in the acceptor sites, while protons migrate in the opposite direction to create water molecules in the donor regions.24 It has been well-established that the NiO prepared by decomposition of brucite will bear a well-defined orientation relationship with the parent crystal; that is, the decomposition is topotactic. On the basis of our results, it indicates a trend that the inhomogeneous mechanism can be a better explanation of the formation of nickel oxide from the SH-nickel hydroxide sample. It is assumed that a crystalliteamorphous-crystal transformation takes place in the SH-nickel hydroxide sample. The results clearly show that these two nickel hydroxide samples having different degrees of crystallinity undergo decomposition using two distinct mechanisms. The decomposition mechanism depends mainly on the crystallinity and structural disorder associated with the sample. Acknowledgment. T.N.R. thanks the Council of Scientific and Industrial Research, GOI, for the award of a Research Associate Fellowship. The author gratefully acknowledges Prof. P. Vishnu Kamath for providing laboratory facilities to carry out the research work and his encouragement and generous support to publish the results. I also thank B. E. Prasad for
J. Phys. Chem. B, Vol. 113, No. 39, 2009 13017 recording X-ray diffraction patterns. The author also thanks reviewers for their useful comments. References and Notes (1) Falk, S. U.; Salkind, A. J. Alkaline Storge Batteries; Wiley: New York, 1969. (2) Fantini, M.; Gorenstein, A. Sol. Energy Mater. 1987, 16, 487. (3) Wu, M.-A.; Hiseh, H.-H. Electrochim. Acta 2008, 53, 3427. (4) Kowal, A.; Port, S. N.; Nichols, R. J. Catal. Today 1993, 32, 1209. (5) Oswald, H. R.; Asper, R. In Preparation and crystal growth of materials with layered structures; Lieth, R. M. A., Ed.; D. Reidel Publishing Company: Dordrecht, The Netherlands, 1977; Vol. 1, p 71. (6) Ramesh, T. N.; Kamath, P. V. J. Power Sources 2006, 166, 665. (7) Cronan, C. L.; Micale, F. J.; Topic, M.; Leidheiser, H.; Zettlemoyer, A. C. J. Colloid Interface Sci. 1976, 55, 546. (8) Fievet, F.; Figlarz, M. J. Catal. 1975, 39, 350. (9) Liu, K. C.; Anderson, M. A. J. Electrochem. Soc. 1996, 143, 124. (10) Boyer, J. M.; Re´petti, B.; Garrigos, R.; Meyer, M.; Be´e, A. J. Metastable Nanocryst. Mater. 2004, 22, 1. (11) Fernandez Rodriguez, J. M.; Morales, J.; Tirado, J. L. J. Mater. Sci. 1986, 21, 3668. (12) Sitakara Rao, V.; Rajendran, S.; Maiti, H. S. J. Mater. Sci. 1984, 19, 3593. (13) Jayashree, R. S.; Kamath, P. V.; Subbanna, G. N. J. Electrochem. Soc. 2000, 147, 2029. (14) Ramesh, T. N.; Kamath, P. V. Bull. Mater. Sci. 2008, 31, 169. (15) Ramesh, T. N.; Kamath, P. V.; Shivakumara, C. J. Electrochem. Soc. 2005, 152, 806. (16) Ramesh, T. N.; Jayashree, R. S.; Kamath, P. V. Clays Clay Miner. 2003, 51, 570. (17) Ramesh, T. N.; Kamath, P. V. Mater. Res. Bull. 2008, 43, 3227. (18) Goodman, J. F. Proc. R. Soc. London, Ser. A 1958, 247, 346. (19) Krylov, O. V.; Kushnerev, M. Ya.; Kiryushkin, V. V. IzV. Akad. Nauk SSSR, Ser. Khim. 1971, 10, 2155. (20) Zhang, K.; Wang, J.; Xiaoli, L.; Luying, L.; Tang, Y.; Jia, Z. J. Phys. Chem. C 2009, 113, 142. (21) Tang, Y.; Jia, A.; Jiang, Y.; Li, L.; Wang, J. Nanotechnology 2006, 17, 5686. (22) Le Bihan, S.; Figlarz, M. Thermochim. Acta 1973, 6, 319. (23) Gregg, S. J. J. Chem. Soc. 1953, 3940. (24) Ball, M. C.; Taylor, H. F. W. Mineral. Mag. 1961, 32, 754.
JP906578U
