J. Phys. Chem. C 2008, 112, 17471–17477
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Tunable Electrochemical Properties Brought About by Partial Cation Exchange in Hydrotalcite-Like Ni-Co/Co-Ni Hydroxide Nanosheets Weihua Chen, Yifu Yang,* Huixia Shao, and Jing Fan College of Chemistry and Molecular Science, Wuhan UniVersity, 430072 Wuhan, P. R. China ReceiVed: May 26, 2008; ReVised Manuscript ReceiVed: September 7, 2008
Layered bimetal (Ni-Co/Co-Ni) hydroxide nanosheets with tunable chemical composition could be successfully constructed through a one-step partial cation exchange method by immersing Ni(OH)2/Co(OH)2 in aqueous solution of the related cobalt/nickel nitrate directly under moderate conditions. XRD, SEM, EDS, HRTEM, ICP, TGA, elemental analysis, FT-IR, and UV-vis analysis were used to identify the structure and composition of such as-prepared Ni-Co/Co-Ni hydroxide nanosheets. Enormous variants from nanostructured Ni(OH)2/Co(OH)2 could be achieved through a simple progress of one-step cation exchange. The electrochemical tests show that the as-prepared nanoscale products not only perform a larger number of exchangeable electrons (1.6e) than that of the materials with approximately the same central metal composition, which were synthesized by conventional coprecipitation method, but also exhibit lower peak oxidation potential and better reversibility. These excellent electrochemical properties could be beneficial for high-energy and high-rate power application. Moreover, the results of systemic cyclic voltammogram investigations of composition-tunable layered bimetal (Ni-Co) hydroxide nanosheets display that the trend of negative shifts of anodic current peak positions is consistent with the decrease in Nis/Cos ratios. However, there exists a range of metal ratios for keeping the electrochemical properties. Importantly, the change and maintenance of electrochemical properties with the change of metal ratios of products could provide a new way of tuning and controlling property-aimed products. In addition, this simple method can be used as an effective tool to construct nanostructures with different compositions to optimize various properties. Introduction Two-dimensional (2D) layered metal hydroxides with nanostructures are attracting interest because of their applications in fields of intercalation and anion exchange,1 magnetic materials,2 and precursors of catalysts,3 as well as high-performance electrode materials of rechargeable alkaline batteries4 and supercapacitors.5 In particular, most documents touched on the application of such materials in rechargeable batteries because the necessity of overcoming the shortage of energy sources and the pollution of our environment have become more and more important.6 The reported composite materials of layered metal (Ni, Co) hydroxides have been shown effectively to have the capability to enhance battery capacity and power abilities of rechargeable alkaline batteries. This can be validated by the investigations of Ni-MH, Ni-Cd, Ni-Zn, Ni-Fe batteries.7 Among these materials, the composite Ni-Co hydroxides could be achieved via a conventional coprecipitation technique in which the additional metal ions were doped to nickel or cobalt hydroxides. However, it is very hard to overcome the agglomeration of particles during preparation by the conventional method. In addition, the coprecipitation method also requires strict control of the reaction parameters, such as pH, reaction temperature, and period, which are closely related to the features of products.8 For the purpose of obtaining composite Ni-Co hydroxides in simple and moderated conditions, alternative techniques are still in progress. Cation exchange provides a facile method to make a chemical transformation from one solid to another via insertion and exchange of central metal atoms. A range of nanocrystals of
varying composition, size, and shape have been achieved successfully by this effective and powerful method. 9-12 For example, Alivisatos et al. reported a spontaneous superlattice formed in nanorods through partial cation exchange within a CdS rod by Ag+.9b The synthesis of sodium and calcium niobate nanorods via cation exchange with K+ in K2Nb8O21 nanowires based on molten-salt reaction has been achieved by Wang et al.10 However, as far as we know, few available effective attempts on direct cation exchange reactions in the host layers of layered hydroxide materials with the creation of nanocrystals with higher structural and compositional complexity have been documented. Most importantly, partial cation exchange could bring on unexpected tunable electrochemistry properties of layered metal hydroxides. In this work, the cation exchange method was successfully adopted for the synthesis of a series of 2D Co-Ni bimetallic hydroxides with tunable electrochemical properties from pure pink powders of β-Co(OH)2. Remarkably, various structured Ni-Co biometallic hydroxide variants were achieved directly by using pure β-Ni(OH)2 as the precursor through a reversed process against the above reaction of Co(OH)2 (Scheme 1). Most importantly, prominent changes in the electrochemical properties have been brought on by the cation-exchange-induced transformation. Of course, this simple method could also be applied to the synthesis of other layered double or multi central-metalcontaining nanomaterials with different structural and compositional complexities with tunable properties.
* Corresponding author. Tel: +86-27-87218624. Fax: +86-27-68754067. E-mail:
[email protected].
NaOH was guaranteed grade; all other chemicals used in our experiments were of analytical grade. All chemicals were
Experimental Section
10.1021/jp804776j CCC: $40.75 2008 American Chemical Society Published on Web 10/15/2008
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SCHEME 1: The Relationship of the Main Materials in the Text
purchased from Shanghai Chemicals Co. Ltd. (China) and were used without further purification. The water used for solution preparation is deionized water (resistivity g18 MΩ, water purification system). Synthesis of Samples A1, A2, A3, and A4. The synthesis of layered Ni-Co hydroxides is a two-step process under hydrothermal conditions. First, the original pure β-Co(OH)2 was prepared through a hydrothermal process. Then another hydrothermal process was performed to get the Ni-Co hydroxides via replacing Co2+ with Ni2+ in the as-prepared cobalt hydroxides. Typically, an aqueous solution of 1 M NaOH (30 mL) was added to an equivalent volume of 0.5 M Co(NO3)2 solution in which the same molar amount of sodium sulfite as that of the Co(NO3)2 was dissolved for preventing the bivalent cobalt from oxidation, and the mixed solution was stirred for 30 min. The whole process was accompanied by Ar gas (99.99%) bubbling. The mixed solution was then rapidly sealed in a Teflon-lined stainless steel autoclave and placed in a 180 °C oven for 24 h. After it cooled naturally to room temperature, pink cobalt hydroxides were obtained. The second step proceeded as follows: A calculated volume of solution including the asprepared cobalt hydroxides (0.25 g) was taken out of the mixture of the primary solution and cobalt hydroxide deposition. After centrifuging, the obtained deposits were washed under strong stirring several times with deionized water. The obtained cobalt hydroxides were then soaked in an aqueous solution of nickel nitrate in which an equivalent molar amount of sodium sulfite was dissolved with the cobalt hydroxides, followed by resting at 200 °C for 24 h under hydrothermal conditions. The resulting product (A1) was prepared by adjusting the concentration of nickel nitrate (Niaq/Cos ratio ) 8/1). When the concentration of the nickel nitrate was adjusted to get the Niaq/Cos ratio to 16/1, 4/1, and 1/1, the resulting products were formed and named as A2, A3, and A4, respectively. The resulting powders of Ni-Co hydroxides were centrifuged and washed several times with water and anhydrous ethanol and then dried at 65 °C in vacuum. Synthesis of B1, B1′, and C1. When β-Ni(OH)2 was selected as the precursor for the preparation of Ni-Co hydroxides, it was synthesized first using the same procedure as the preparation of β-Co(OH)2, but without the addition of sodium sulfite. With the aforementioned second procedure, except that β-Ni(OH)2 was soaked in an aqueous solution of cobalt nitrate in which an equivalent molar amount of sodium sulfite was dissolved together with the cobalt nitrate, a pink sample, B1, was obtained (see Figure S5b of the Supporting Information). In contrast, the same experimental process but without sodium sulfite addition in the solution was carried out, and a black powder (B1′) of a mixture of NiCo2O4 and CoCo2O4 was obtained.
The reversed experimental process proceeded as follows: The as-prepared B1 sample was soaked in an aqueous solution of nickel nitrate at 200 °C for 24 h again for the other hydrothermal process. As a result, a green powder (C1) was obtained (see Figure S5b of the Supporting Information). Physical Characterization. The as-prepared samples were characterized by powder XRD (XRD-6000, Cu Ka radiation, Shimadzu, Japan), scanning electron microscopy (SEM, Sirion, Hillsboro, OR), with energy dispersive X-ray spectroscopy (EDS, Genesis 7000, EDAX, Mahwah, NJ), high-resolution transmission electron microscopy (HRTEM, JEM-2010FEF, JEOL, Tokyo, Japan), ICP-OES (Thermo, Waltham, MA), FTIR (Avatar 360, Nicolet, Madison, WI), UV-vis absorption spectroscopy (UV-3100, Shimadzu, Tokyo, Japan), thermogravimetric analysis (TGA, Q500-TGA, TA Instruments, New Castle, DE), and elemental analysis (Vario EL-III, Elementar, Hanan, Germany). Electrochemical Characterization. Cyclic voltammetry tests were carried out in a classical three-electrode cell with an electrochemistry work station (CHI660A, Chenhua instrument Ltd., Shanghai). The working electrode was the as-prepared powder microelectrode, the counter electrode was a nickel foil, and the reference electrode was a homemade Hg/HgO electrode filled with 6 M KOH solution. All potentials reported in this paper are referred to this reference electrode. The electrolyte was a degassed 6 M KOH solution. Galvanostatic charge-discharge was measured as follows: Powders of sample A1 were mixed with graphite powder and a PTFE suspension (60% suspension) in a weight ratio of 80: 15:5 with ethanol. The thoroughly mixed paste was sandwiched into two nickel forms. They were pressed at a pressure of 120 kg cm-2 at room temperature and dried at 70 °C for several hours. Galvanostatic charge-discharge studies were conducted by a battery performance-testing instrument (Land Test Equipment, CT2001A), with two pieces of negative electrodes made from commercial AB5 hydrogen storage alloys as counter electrodes on either side of the working electrode. A homemade Hg/HgO electrode was used as the reference electrode and a 6 M KOH solution as the electrolyte. Before the cycling test, the working electrode was activated by galvanostatically charging at a 0.1 C rate for 15 h, resting for 15 min, and then discharging to 0.1 V versus Hg/HgO at a 0.1 C rate for several cycles. Formal cycle performance was tested under the following scheme: charge at a 1.0 C rate for 1.2 h, rest for 15 min, and discharge at a 1.0 C rate to 0.1 V versus Hg/HgO. All the tests were conducted at 25 ( 2 °C. All capacities are normalized to the nickel content of the active materials (estimated by ICP).
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Figure 1. (a, b) Typical SEM images of the as-prepared Co(OH)2 and sample A1. Insets are their photos, respectively. (c) EDS spectrum of A1 showing the partial exchange of Co by Ni in solid platelets. (d) High-resolution TEM image of a platelet of A1. Inset is the corresponding SAED pattern.
Results and Discussion First, pure pink powders of β-Co(OH)2 were hydrothermally synthesized at 180 °C for 24 h in the presence of sodium sulfite. In this reaction, sodium sulfite was used as a reducing agent for the purpose of preventing the oxidation of the bivalent cobalt. As shown in Figure 1a, a typical SEM image reveals that the Co(OH)2 sample displays hexagonal platelike morphology with thicknesses of ∼30-40 nm and widths in the range of 300-400 nm. Since the divalent cobalt cations are easy to oxidize to the trivalent state, the hydrothermal process is very unfeasible for the synthesis of Co(II)-containing hydroxides.13 In addition, the value of the Ksp of Co(OH)3 is much smaller than that of Co(OH)2, which also goes against the cation exchange process. By using sodium sulfite as the protector, we could obtain pure Co(OH)2 directly under moderate conditions. Bands originating from Co3+ should be found in the visible region at 700 (1T1g r 1A ) in UV-vis absorption spectra.8,14 Clearly, no Co3+ could 1g be found in the resulting product, which has been verified by UV-vis absorption spectra (Figure 2). The X-ray diffraction (XRD) pattern (Figure 3) also indicates that the as-synthesized β-Co(OH)2 are in the pure brucite-like phase (JCPDS 74-1057), with lattice parameters of a ) 3.18 Å and c ) 4.65 Å. Again, the sharp diffraction peaks suggest the highly crystalline nature of the β-Co(OH)2 product. FT-IR spectra (Figure 4) display a sharp, narrow band at 3650 cm-1 that corresponds to the νO-H stretching of the geminal OH groups of the brucite-like structure. A broad band at about 3440 cm-1 is characteristic of the stretching vibration of hydroxyl groups hydrogen-bonded to H2O, and the band at about 1637 cm-1 corresponds to the bending mode of water molecules.15 The bands at low energy (less than 800 cm-1) are assigned to the bending mode M-O-H.15a A weak peak at 1385 cm-1 is assigned to the absorption peaks of the trace amounts of adsorbed nitrate
Figure 2. Visible absorption spectra of as-prepared Co(OH)2 and cation exchange induced products A1 and A4.
anions.15a,16 In addition, the weak absorption peaks of sulfite appear at 1118 and 1050 cm-1.16c,17 With the protection of sodium sulfite, the products, Ni-Co hydroxides (A1), could be obtained by cation-exchange reaction between Co(OH)2 hexagonal platelets and the aqueous solution of nickel nitrate with a molar ratio of 8:1 of nickel ions in solution (Niaq, new cation) over cobalt ions (Cos, the one to be replaced) in solid Co(OH)2 (aq and s represent the aqueous and solid environment of the given elements). As shown in Figure 3, the powder XRD pattern analysis could be indexed as the structure of hydrotalcite-like R-Ni(OH)2 (JCPDS 22-444) with lattice parameters of a ) 3.054 Å and c ) 6.973 Å. According to the results of energy dispersive X-ray spectroscopy (EDS), the central metals in sample A1 have a molar ratio, Nis/Cos, of 3.83/1. This is in good agreement qualitatively with the result of inductively coupled plasma (ICP) emission spectroscopy analysis: Nis/Cos ) 3.90/1. Here, one consequence could be
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Figure 3. XRD patterns of as-synthesized Co(OH)2 and cation exchange induced products A1.
Figure 4. FT-IR spectra of as-prepared Co(OH)2 and cation exchange induced products A1 and A4.
TABLE 1: Percentages of Elements in Samples Produced by Cation Exchange Method and Analyzed from EDS Results At % A1 A2 A3 A4 B1 B1′ C1 A1′
Ni
Co
O
S
24.94 28.82 32.22 23.06 5.01 8.52 21.21 39.66
6.52 6.28 10.54 18.92 14.87 44.27 5.79 10.12
61.95 58.00 48.81 49.39 56.65 47.21 66.60 50.22
6.59 6.90 8.43 8.63 15.26 6.40
Na
8.20
Total
Ni/Co
100 100 100 100 100 100 100 100
3.83 4.59 3.06 1.22 0.34 0.19 3.66 3.92
that the metal distribution in the sample particles is absolutely uniform. So for other samples, only EDS tests were carried out. To reveal the stoichiometry of the synthesized R-Ni(OH)2 nanostructures, elemental analysis and thermal gravimetric measurements were carried out (Table 1 and Table S1 and Figure S7 of the Supporting Information). The formula of the synthesized nickel hydroxide was determined to be Ni0.79Co0.21(OH)1.54(SO3)0.21(NO3)0.04 · 0.15H2O based on chemical and thermal gravimetric analyses. Furthermore, the FT-IR (Figure 4) also further proved this composition, which will be discussed later. The morphology of the obtained R-Ni0.79Co0.21(OH)1.54(SO3)0.21(NO3)0.04 · 0.15H2O still holds the morphology feature of the pristine Co(OH)2. A typical SEM image is shown in Figure 1b. The R-Ni0.79Co0.21(OH)1.54(SO3)0.21(NO3)0.04 · 0.15H2O platelets are not in a large-scale regular hexagon, and they are a little bit thinner (∼20-25nm), in contrast with the powders of β-Co(OH)2.
Figure 5. Normalized cyclic voltammograms of A0, A1, and A1′. They were normalized by their own anodic peak currents, respectively. Scan rate: 2 mV s-1.
The green products displayed in the inset of Figure 1b also authentically demonstrate the conversion. In Figure 1c, we could find the characteristic peaks of nickel metal ions, which also indicates that cobalt cations are, indeed, partially replaced by nickel ones. The SAED pattern (Figure 1d), which taken from the platelets shown in Figure 1b, shows many spots, all of which were identified as the diffraction from hexagonal Ni(OH)2. The appearance of the diffraction not as rings but as spots indicates the single crystalline nature of hydrotalcite-like R-Ni0.79Co0.21(OH)1.54(SO3)0.21(NO3)0.04 · 0.15H2O platelets obtained by the cation-exchange process. This is further proved by the HRTEM image of the same platelets (Figure 1d), in which the lattice fringes are clearly observed. Analogous to the reported layered metal hydroxides, we could find the insertion of SO32- (1159 and 1038 cm-1) and NO3- (1515, 1383, and 1290 cm-1) in the interlayer gallery of A1, which was determined by FT-IR (Figure 4). Although many more nitrate ions were introduced into the reaction system, the content of the intercalated SO32- is extensively higher than that of NO3- because of their basic characteristics.18 For comparison, Ni-Co hydroxide (A1′) with approximately the same ratio of nickel and cobalt as that of sample A1 was synthesized through a coprecipitation method under similar conditions. the XRD pattern of the obtained sample A1′ is shown in Figure S6a of the Supporting Information. All the diffraction peaks can be readily indexed to a pure single phase of β-Ni(OH)2 with a hexagonal structure according to the JCPDS 14-0117. The morphology of the product A1′ is characterized by SEM in Figure S6b of the Supporting Information. It is obviously observed that the as-synthesized products are dominated by nanoscale platelets with thicknesses of ∼30-40 nm and widths in the range of 100-150 nm. Although both samples A1 and A1′ have similar cation ratios, the XRD patterns and SEM images show that the structure of A1 is distinguishable from that of A1′. Cation-exchange-induced transformation produced an R phase structured Ni-Co compound A1, whereas coprecipitation-induced reaction yielded a β phase structured Ni-Co mixed compound A1′. Normalized cyclic voltammograms of these two samples are displayed in Figure 5, which indicate distinct electrochemical characteristics of these two compounds, despite both of them showing obvious negative shifts of potential positions of the oxidation and reduction peaks of the nickel ions, as compared with the pure nickel hydroxide (A0). In detail, A1 not only brought about far more negative shifts (170 mV for the oxidation peak and 110 mV for the reduction peak) of peak potentials than those of A1′ did (40 mV for the oxidation peak and 70 mV for the reduction peak), but also performed sharp
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Figure 6. Variation of the NEE of sample A1 vs the number of cycles of the battery.
Figure 7. Content relationship between central metal proportions of cation-exchange-induced products and the initial ratio of Niaq/Cos.
peak currents. That means sample A1 has lower resistance and better reversibility. The performance implies the better highrate charge-discharge ability that is beneficial for power applications. Especially, this application is urged as power sources in electric and hybrid electric vehicles. Furthermore, when sample A1 was tested as the positive active material in a Ni-MH battery, it demonstrated outstanding cycling durability for 500 cycles. The number of exchangeable electrons per nickel atom has reached about 1.6, which was determined by galvanostatic charge-discharge experiments. It means that this sample has a very high capacity, far higher than that of β-Ni(OH)2 (Figure 6).19 The number of exchangeable electrons per nickel atom (NEE) can be calculated in the following way:
NEE )
C(mA h) ⁄ mNi(g) 462(mA h g-1)
(1)
where C is the discharge capacity of nickel electrode; mNi is the mass of nickel in active materials which was obtained by the content of Ni in A1 (44.2%) analyzed by ICP emission spectroscopy, multiplied by the total weight of the active mass of the electrode; 462 mA h g-1 represents the corresponding theoretical capacity of nickel with 1 exchangeable electron per gram of nickel. It should be pointed out that the cobalt atom in sample A1 did not participate in the electrochemical cycle in the set potential window.20 To go further with this work, the molar ratios of Niaq/Cos were changed up and down to 16, 4, and 1, then green samples A2 and A3 and blue sample A4 were obtained with Nis/Cos ratios of 4.59/1, 3.06/1, and 1.22/1 (formulated as Ni0.82Co0.18(OH)1.51(SO3)0.20 (NO3)0.09 · 0.10H2O, Ni0.75Co0.25(OH)1.60(SO3)0.20 · 0.14H2O, and Ni0.55Co0.45(OH)1.60(SO3)2.20 · 0.093H2O on the basis of elemental and thermal gravimetric analyses (Table S1 and Figure S7 of the Supporting Information)), respectively (Table 1). They are in agreement with the results of FT-IR spectra (Figure 4, Figure S4a of the Supporting Information), except that of A3. Even though the FT-IR spectra of A3 display the characteristic peaks of NO3- at 1515, 1384, and 1290 cm -1, the little amount of nitrates in A3 cannot be detected though elemental analysis. Samples A2 and A3 are analogous to sample A1 in structure aspects, as shown by XRD, SEM, FT-IR, and UV-vis absorption spectra (refer to Supporting Information). However, sample A4 is much different from the others. The peaks at 500, 586, and 634 nm (20000, 17064, and 15776 cm -1) in the UV-vis absorption spectra of compound A4 (Figure 2) indicate the cobalt ions in both octahedral and tetrahedral sites. The tetrahedral cobalts impart the distinct blue color of the sample. 21 In addition, the nickels
Figure 8. The normalized cyclic voltammograms of A1, A2, A3, and A4. They were normalized by their own anodic peak currents, respectively. Scan rate: 2 mV s-1.
in sample A4 adopt an octahedral coordination. According to the XRD pattern (Figure S2a of the Supporting Information) and the formula of A4, Ni0.55Co0.45(OH)1.60(SO 3)0.20 · 0.093H2O, in which the ratio of central metals/OH is 5:8, the structure of A4 could be indexed as the hydrozincite structure type Zn5(OH)8A2 · nH2O.15b,17,22 Considering the coexisting octahedral and tetrahedral metal sites in sample A4 and the energy difference between the bands of 1204 and 1045 cm -1 in FTIR (Figure 4), A4 should belong to the structure of monoclinic Zn5(OH)8(H2O)2(NO3) 2.15b,22a,23 As documented by UV-vis absorption characterization, no Co3+ cations could be found in the resulting products A2, A3, and A4. Meanwhile, the colors identified by the naked eye also indicated the distinction between the initial pink β-Co(OH) 2 and the green or blue end products. This could also be regarded as direct evidence for cation exchange of Co2+ by Ni2+. In addition, along with the increasing molar ratios of Niaq/Cos, the molar ratios of Nis/Cos in the corresponding resulting products increase gradually (Figure 7), and the inserted nitrates increase and sulfites decrease, which is supported by FT-IR, elemental analysis, and EDS results. The results of systemic cyclic voltammogram investigations of them are shown in Figure 8, in which anodic current peaks of A2, A1, A3, and A4 appeared at 0.40, 0.37, 0.37, and 0.33V (vs Hg/HgO), respectively. Obviously, the trend of negative shifts of anodic current peak positions is consistent with the decrease in the Nis/Cos ratios. However, no clear peak position difference was found in A1 and A3, which might indicate that there exists a range of metal ratios for keeping electrochemical properties. We believe that the change and maintenance of electrochemical properties with the change in metal ratios of products could
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Figure 9. SEM images of (a) as-prepared Ni(OH)2, (b) cation-exchange-induced sample B1, (c) cation-exchange-induced B1′, (d) reversibly cationexchanged products C1 from B1.
provide a new way of tuning and controlling the properties of the products. On account of the analogous characteristics between cobalt and nickel, the reversed exchange reaction process is anticipated to take place. Accordingly, we also used in-going Co2+ ions to replace the central Ni2+ in layered nickel hydroxides. Layered Ni-Co hydroxides were obtainable through cation exchange reaction between nickel hydroxides and cobalt ions in solutions under hydrothermal conditions directly. As illustrated in Figure 9b, pink solids (B1) with irregular shape were synthesized through hydrothermal dealing of the mixture of pure β-Ni(OH)2 (JCPDS 14-117, Figure S5a of the Supporting Information) of hexagonal platelets (Figure 9a) and an aqueous solution of cobalt nitrate (Coaq/Nis ) 8/1) with the same molar amount of sodium sulfite as the protector at 200 °C for 24 h. EDS results (Table 1) indicate that nickel atoms in the solid were indubitably replaced by cobalt ions. Surprisingly, the participation of sodium ions and sulfite in the reaction system has led to a complicated product without preservation of the classic layered structure (Figure S5a of the Supporting Information). This might be caused by the participation of the abundant sodium sulfite in this reaction under such high temperature in a hydrothermal kettle. On the other hand, without the protection of sodium sulfite, the hydrothermal dealing of the mixture of pure β-Ni(OH)2 and the aqueous solution of cobalt nitrate (Coaq/Nis ) 4/1) gave rise to a mixture (B1′) of spinel-type NiCo2O4 (JCPDS 80-1542) and CoCo2O4 (JCPDS 73-1702), which was determined by XRD pattern (Figure S5a of the Supporting Information). These results agree well with the EDS analysis (Table 1). This perhaps came about because it is very easy to oxidize Co2+ to Co3+ under high-temperature hydrothermal conditions. Additionally, it is surprising to find two distinguishable morphologies in the SEM image of B1′ (Figure 9c). One morphology is the preponderant unclosed, hollow spheres with diameters of 300-500 nm. This might be NiCo2O4. According
to previous reports, the other well-defined octahedrons with edge lengths ranging from 200 to 400 nm should be considered as CoCo2O4.24 It is very interesting that a single procedure in only one reaction system gives two substances with similar structure and absolutely different shape. Through controlling the proportions of the original reaction system, these two substances should be obtained separately. When sample B1 was treated with an excess aqueous solution of nickel nitrate under hydrothermal conditions, green powders (C1) with a Nis/Cos ratio of 3.66/1 (Table 1) could be obtained. The morphology of pseudohexagonal platelets (C1) (Figure 9d) could be indexed as the structure of R phase nickel hydroxide (JCPDS 22-444, Figure S5a of the Supporting Information). No sodium was found (Table 1) in it. All this means that the reversible exchange of cobalt cations (or nickel cations) in their hydroxide solids with nickel ion (or cobalt ion) solutions can take place just accompanied by variable structures. It is notable that in the synthesis of crystalline solid state materials, an outstanding challenge is to alter the chemical composition without changing the underlying topology.25 Cation exchange was thought to be a self-sacrificing templated process,10,26 and it could be successfully used to construct hydrotalcite-like bimetallic (Ni-Co/Co-Ni) hydroxide 2D nanostructures with tunable compositions. The Ni-Co hydroxides formed as the products of a partial replacement of Co2+ in β-Co(OH)2 platelets by Ni2+ can keep the basic morphology of the original substance, which is consistent with the previous literature.9a The only difference is that the thickness of the platelets becomes a little thinner and the fringes of the particles become more irregular than the initial ones. It might be caused by the insertion of anions which neutralized the positive charge in the host layers. The cation exchange process not only constructs an enlarged distance of adjacent layers, which can allow cations passing in and out more easily, but also urges the delamination of layered Ni-Co hydroxide. In addition, the hydrothermal condition is beneficial to enhancement
Tunable Electrochemical Properties of the diffusion velocity of ions, ensuring a thorough and uniform reaction. As a result, the time period of hydrothermal treatment does not make a remarkable contribution to the structure of the products (Figure S1 of the Supporting Information). We can successfully control the composition of the resulting products by changing the molar ratios of in-going cations. More in-going cations in solutions could bring about more of these cations in the resulting solids. This type of cation-exchange reaction could take place reversibly. When β-Ni(OH)2 nanoplatelets were adopted as the initial substances, Ni2+ could be replaced by Co2+ and Na+ from solutions under hydrothermal conditions. The exchange of Ni2+ with Co2+ and Na+ could be conducted reversibly, and the resulting products had a hydrotalcite-like structure. The similarity of the ion radii of Ni2+ and Co2+ allow the exchange process to proceed reversibly; however, the structure and morphology of the precursor substances cannot be maintained anymore after cation exchange. Conclusions In this study, bimetallic or multimetallic central (Ni-Co, Co-Ni, Ni-Co-Na) hydroxides could be successfully achieved from single-metal central hydroxides (Co(OH)2, Ni(OH)2) via a cationexchange-induced single-crystal-to-single-crystal transformation process. The structure and composition of the as-prepared nanoscale products can also be tuned by controlling the reactants’ ratios. Furthermore, the exchange reaction could take place reversiblely. The result of cyclic voltammogram investigation reveals that the Ni/Co hydroxides synthesized by the cation-exchange method perform at a lower resistance and better reversibility than that of products synthesized through a coprecipitation method. The galvanostatic charge-discharge experiments of the Ni/Co hydroxides synthesized by the cation-exchange method show a very high capacity (1.6e) and outstanding cycling durability for 500 cycles. In addition, tunable electrochemical properties could be obtained by adjusting the chemical composition of the Ni-Co/Co-Ni hydroxides. The products would likely exhibit a surprising performance in other potential applications, such as supercapacitors, magnetic materials, and catalytic materials. This simple method may give a way to construct nanostructures with different compositions to optimize various properties. Acknowledgment. The financial support by the 863 National Research and Development Project Foundation of China (Grant No. 2006AA11A152) is gratefully acknowledged. Supporting Information Available: XRD patterns of samples A1 with different hydrothermal treatment times; XRD patterns and photos of samples A2, A3, A4; SEM images A4 and A3; FT-IR and UV-vis absorption spectra of samples A2 and A3; XRD patterns of the initial Ni(OH)2 powders and cation exchange induced products B1, B1′, and C1 and photos of initial Ni(OH)2, B1, and C1; XRD patterns and SEM image of Ni-Co hydroxides synthesized by the coprecipitation method. TGA and elemental analysis results of A1, A2, A3, and A4. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Han, K. S.; Guerlou-Demourgues, L.; Delmas, C. Solid State Ionics 1997, 98, 85. (b) Taibi, M.; Ammar, S.; Jouini, N.; Fie´vet, F.; Molinie´, P.; Drillon, M. J. Mater. Chem 2002, 12, 3238. (c) Z.P., Liu; Ma, R. Z.; Osada, M.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2006, 128, 4872. (d) Mavis, B.; Akinc, M. Chem. Mater. 2006, 18, 5317. (e) Ma, R. Z.; Takada, K.; Fukuda, K.; Iyi, N.; Bando, Y.; Sasaki, T. Angew. Chem. Int. Ed 2007, 46, 1.
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