Interface-Mediated Synthesis of Transition-Metal (Mn, Co, and Ni

Mar 19, 2013 - hydroxide nanoplates with tunable composition through the interface-mediated growth process. It is worth noting that, using common nitr...
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Interface-Mediated Synthesis of Transition-Metal (Mn, Co, and Ni) Hydroxide Nanoplates Peng Li,† Dingsheng Wang,‡ Qing Peng,*,‡ and Yadong Li‡ †

School of Chemistry and Chemical Engineering, Anhui University, Heifei 230601, P. R. China Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China

‡

S Supporting Information *

ABSTRACT: We report a general and efficient strategy to produce monodisperse transition-metal (Mn, Co, and Ni) hydroxide nanoplates with tunable composition through the interface-mediated growth process. It is worth noting that, using common nitrates as the precursors, the as-obtained nanoplates were prepared under hydrothermal conditions. Moreover, the possible formation mechanism of the transitionmetal hydroxide nanoplates has also been investigated. Subsequently, the resulting transition-metal hydroxides can be eventually transformed into transition-metal oxide nanoplates and lithium-ion intercalation materials through solidstate reactions, respectively. Furthermore, the electrochemical properties of the resulting nanomaterials have also been discussed in detail. This protocol may be easily extended to fabricate many other metal hydroxide and oxide nanomaterials.

1. INTRODUCTION

In earlier work, we developed the interface-controlled reaction systems to prepare monodisperse nanostructures.24−26 Herein, we are motivated to design an interface-mediated synthetic strategy to achieve transition-metal (Mn, Co, and Ni) hydroxide nanoplates and nickel-substituted cobalt hydroxides nanoplates. It is notable that the as-prepared nanoplates were obtained using common nitrates as the starting materials under hydrothermal conditions. More importantly, the as-prepared transition-metal hydroxides can be ultimately converted into corresponding transition-metal oxides and lithium-ion intercalation materials by simple solid-state reactions. While used as the anode and cathode materials, the resulting nanomaterials exhibit the characteristic electrochemical properties.

The rational design and synthesis of nanocrystals with welldefined facets and tunable compositions are of great significance in tailoring their physical and chemical properties.1−3 Transition-metal hydroxide and oxide nanocrystals have received extensive attention owing to their potential applications in lithium-ion batteries, catalysis, gas sensing, and magnetics.4−9 To date, various synthetic methods have been introduced to prepare transition-metal oxide or hydroxide nanocrystals with variable compositions and dimensions, such as thermolysis of organometallic precursors, hydrothermal method, sol−gel processes, and chemical vapor deposition process.10−13 Considerable studies have been devoted to the preparation of these nanomaterials with desired properties.14−17 Cui and co-workers have reported the synthesis of high-quality manganese dioxide nanorods and their application in lithiumion batteries.18 Chen et al. have prepared various transitionmetal oxide nanotubes and investigated their applications in gas sensors and lithium-ion batteries.19,20 Guo and Yang have described a wet chemical method to synthesize singlecrystalline nickel hydroxide polyhedrons.21 More recently, Dai and co-workers have developed a two-step strategy for growing transition-metal hydroxide and oxide nanocrystals on graphene surfaces, which show excellent electrochemical performance.22,23 However, it is still essential to explore simpler and more effective synthetic processes to manipulate the uniformity, structure, and composition of nanocrystals, which exert important effects on the properties. Among the diverse synthetic protocols, the solution-based methods have been demonstrated as the facile and low-cost routines. © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Synthesis of Transition-Metal (Mn, Co, and Ni) Hydroxide Nanoplates. 2.1.1. Ni(OH)2 Nanoplates. Typically, Ni(NO3)2·6H2O (0.5 mmol or 3 mmol) was dissolved in 1 mL of H2O. The mixed solution (2 mL of dodecanol + 4 mL of oleylamine) was added under agitation, followed by heating at 200 °C for 10 h. For the elliptical nanoplates, 3 mmol of Ni(NO3)2·6H2O was dissolved in 1 mL of H2O. Then, 8 mL of oleylamine was added under agitation, followed by heating at ∼170 °C for 10 h. 2.1.2. Co(OH)2 nanoplates. Typically, 3 mmol Co(NO3)2·6H2O was dissolved in 1 mL of H2O. The mixed solution containing 2 mL of dodecanol and 4 mL of oleylamine was added under stirring and hydrothermally treated at 200 °C for 10 h. The synthesis of cobaltsubstituted nickel hydroxides were conducted by using the mixed Received: December 17, 2012 Revised: March 11, 2013

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nitrates as precursors (Co2+/Ni2+: 1.2 mmol/1.8 mmol or 2 mmol/1 mmol) in the above reaction system. 2.1.3. Mn(OH)2 Nanoplates. A typical synthesis of Mn(OH)2 nanoplates was as follows: 1 mL of Mn(NO3)2 solution (50 wt %) was added to 9 mL of oleylamine under agitation. Then, the above solution was sealed and hydrothermally treated at 160−180 °C for 10 h. After the reaction was cooled to room temperature, the products were washed with ethanol. 2.2. Synthesis of Transition-Metal Oxide and Lithium-Ion Intercalation Materials. Transition-metal oxides were obtained by calcining the corresponding hydroxides at 350 °C in air for 3 h. For lithium-ion intercalation materials, typically, LiOH·H2O and the asprepared hydroxide nanoplates were mixed with a molar ratio, and then, 3 mL of high purity ethanol was added under agitation, followed by drying at room temperature. The mixed powder was further calcined at 700−800 °C in air for several hours. 2.3. Electrochemical Measurement. Electrochemical experiments were carried by using coin-type cells. The anode was fabricated by making a slurry of 80 wt % active materials, 10 wt % acetylene black, and 10 wt % polytetrafluoroethylene (PTFE). The cathode was prepared by compressing the mixture of the active materials, acetylene black, and polyvinylidene fluoride (PVDF) with a weight ratio of 80:10:10 onto the aluminum foil current collector. Lithium metal was used as the counter electrode, and the separator was a Celgard 2500 microporous membrane. The electrolyte was LiPF6 (1 mol/L) solution dissolved in the mixture of ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 in volume). Cell fabrication was carried out in a high-purity argon filled glovebox. 2.4. Characterization. The phase purity and crystallinity of the asobtained samples were measured on a Bruker D8 Advance X-ray powder diffractometer with Cu Kα radiation (λ = 1.5418 Å). The size and morphology of the products were examined by transmission electron microscopy (TEM) (JEOL JEM 1200EX working at 100 kV), scanning electron microscopy (SEM), and high-resolution TEM (HRTEM) (FEI Tecnai G2 F20 S-Twin operating at 200 kV).

Figure 2. Typical TEM images of the Ni(OH)2 nanoplates: (a) highresolution TEM image; (b,d) low-resolution TEM images; (c) corresponding SAED pattern; (e) XRD pattern of the products.

3. RESULTS AND DISCUSSION 3.1. Synthesis of Transition-Metal Hydroxide Nanoplates. In our system, the synthesis of transition-metal

Figure 3. SEM images: (a,c) Ni(OH)2 nanoplates; (b,d) cobaltsubstituted nickel hydroxides (the addition amounts of Co2+/Ni2+ are (b) 1.2 mmol/1.8 mmol and (d) 2 mmol/1 mmol).

Figure 1. SEM image of the Ni(OH)2 nanoplates.

Figure 4. Mn(OH)2 nanoplates: (a) SEM image, the inset shows the individual nanoplate; (b) TEM image, the inset is the corresponding SAED pattern.

hydroxide nanoplates is based on the following reaction in the presence of oleylamine:

Ni(NO3)2·6H2O (3 mmol) as a precursor in the presence of dodecanol (2 mL) and oleylamine (4 mL). Figure 1 shows the typical scanning electron microscopy (SEM) image of the asobtained Ni(OH)2 nanomaterials. It can be seen that the asprepared nanocrystals possess uniform hexagonal platelike morphology with an average thickness of ca. 25 nm.

M2 +(M = Mn, Co, Ni) + 2H 2O → M(OH)2 + 2H+

Various transition-metal (Ni, Co, and Mn) hydroxide nanoplates with uniform sizes and shapes were obtained by using common nitrates as precursors via a simple hydrothermal route. At ∼200 °C, the Ni(OH)2 nanoplates were achieved by using B

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perpendicular to the [001] zone axis direction of the hexagonal structure. The high-resolution TEM (HRTEM) image (Figure 2a) also indicates the nanoplates possess good crystallinity and that the lattice fringe is consistent with the (100) plane of hexagonal Ni(OH)2. Figure 2e shows the X-ray diffraction (XRD) pattern of the as-synthesized samples. The corresponding diffraction peaks can be clearly indexed to the hexagonal structure of Ni(OH)2 (JCPDS card No. 14-0117). The size and morphology can also be tuned by varying the synthetic conditions. As shown in Figure 3c, when the decreased amount of Ni2+ (0.5 mmol) was added in the above reaction system, hexagonal Ni(OH)2 was also obtained with widths ranging from 80 to 290 nm and thickness of 18−44 nm. Meanwhile, Ni(OH)2 nanostructures with the elliptical platelike morphology were also achieved at ∼170 °C in the presence of oleylamine (8 mL). The corresponding SEM image (Figure 3a) shows that the elliptical nanoplates are uniform with a thickness of about 20 nm. Moreover, by varying the addition amounts of Ni2+ and Co2+, cobalt-substituted nickel hydroxides with different compositions were further produced (Figure 3b,d). Figure S1 and Figure S2 (see the Supporting Information) show the energy-dispersive X-ray spectra (EDS) of the mixed metal hydroxide products, respectively. EDS analyses found that the Co/Ni atomic ratio of the samples were in agreement with the starting compositions. Similarly, this synthetic method was also applied to produce Co(OH)2 and Mn(OH)2 nanostructures with platelike shapes, respectively. When Co(NO3)2·6H2O (3 mmol) was dissolved in deionized water (1 mL), followed by heating at ∼170 °C for 10 h in the presence of oleylamine (8 mL), the as-prepared Co(OH)2 nanoplates with well-defined morphology were produced (see Figure S3a in the Supporting Information). At ∼200 °C, Co(OH)2 nanostructures with thicker platelike morphology were also prepared in the presence of dodecanol (2 mL) and oleylamine (4 mL). Figure S3b (Supporting Information) shows the SEM image of the as-synthesized Co(OH)2 nanoplates, which lay flat on the silicon wafer. The corresponding diffraction peaks in the XRD pattern (see Figure S3c in the Supporting Information) matches well with the standard JCPDS card of Co(OH)2 (JCPDS, 74-1057). Apart from the above transition-metal hydroxide, Mn(OH)2 nanoplates were also produced using the same strategy. Welldefined Mn(OH)2 hexagonal nanoplates were obtained by using common nitrate as a precursor in the presence of oleylamine. Figure 4a displays the typical SEM image of the asobtained Mn(OH)2 nanoplates. It can be observed that the nanoplates are rather thin and preferentially orient parallel to the supporting substrate. As shown in Figure 4b, the TEM image suggests that the nanoplates possess hexagonal plate-like morphology, and the corresponding SAED pattern shows hexagonally arranged diffraction spots. The purity and crystal structure of the samples were further examined by powder Xray diffraction. The diffraction peaks in the XRD pattern (Figure 5) can be clearly indexed to the hexagonal Mn(OH)2 (JCPDS Card No. 18-0787). Notably, the XRD pattern presents very strong (001) diffraction peaks, and the other diffraction peaks are rather weak. Compared to the reference card intensities, the XRD pattern of the as-obtained Mn(OH)2 nanoplates show obvious orientation. The experimental observation further indicates the highly preferential growth of Mn(OH)2 and is also consistent with the SEM results. From our experimental results, we propose the plausible formation mechanism of the as-prepared transition-metal

Figure 5. XRD pattern of the as-prepared Mn(OH)2 nanoplates.

Figure 6. Schematic illustration of the possible formation process of the as-prepared hydroxide nanoplates.

Figure 7. SEM images and XRD patterns of the as-obtained products: (a,b) NiO and (c,d) Co3O4 nanoplates.

Figure 8. As-prepared NiCo2O4 nanoplates: (a) SEM image and (b) XRD pattern.

Interestingly, the Ni(OH)2 nanoplates tend to stack in a faceto-face manner with the side edges perpendicular to the SEM silicon wafer. Transmission electron microscopy (TEM) images (Figure 2b,d) further reveal that the as-prepared Ni(OH)2 samples display well-defined hexagonal plates with an average width of ca. 100 nm. The corresponding selected area electron diffraction (SAED) pattern demonstrates that the dominant exposed hexagonal planes of Ni(OH)2 nanoplates are C

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Figure 9. Discharge curves at a rate of 0.1 C for (a) Co3O4, (b) NiO, and (c) NiCo2O4 nanoplates.

Figure 10. Discharge capacity curves versus the number of cycles at a rate of 0.1 C: (a) Li(Ni0.6Co0.4)O2 sample and (b) Li0.99NiO2 sample.

cally stable.21 Thus, the interface-mediated synthetic system and their intrinsic structure largely determine the shape and monodispersity of the as-synthesized nanocrystals. 3.2. Synthesis of Transition-Metal Oxide Nanoplates and Lithium-Ion Intercalation Materials. Additionally, the corresponding transition-metal oxides and lithium-ion intercalation materials can also be produced by simple chemical reactions. The NiO and Co3O4 nanomaterials were obtained through solid-state reactions. Figure 7a shows the as-prepared NiO nanoplates by calcining the corresponding hydroxides at 350 °C in air atmosphere for 3 h. The corresponding XRD pattern (Figure 7b) is in good agreement with the standard pattern of NiO (JCPDS Card No. 47-1049), with no peaks of impurity detected. It is notable that the plate-like morphology still remains after the high temperature reaction. As shown in Figure 7c, Co3O4 nanoplates were produced under the same calcining condition. The diffraction peaks of the as-prepared Co3O4 nanoplates (Figure 7d) have been assigned to cubic Co3O4 (JCPDS Card No. 42-1467). Moreover, NiCo2O4 nanoplates were also obtained using cobalt-substituted nickel hydroxide (the sample in Figure 3d) as a precursor. After calcination at 350 °C for 3 h, the hydroxide precursor converted into NiCo2O4 nanoplates (Figure 8a). Figure 8b shows the XRD pattern of the as-obtained NiCo2O4 nanoplates.

hydroxide nanoplates. As shown in Figure 6, a two-phase synthetic system has been applied to prepare the transitionmetal hydroxide nanoplates. Obviously, the metal salts (nitrates) are dissolved in aqueous phase. It is notable that, in our system, the upper oil phase (oleylamine) seems to act as both the coordinating solvent and the alkalescent medium. The metal ions (Mn2+, Co2+, and Ni2+) first react with oleylamine at the water/oil interfaces to generate metal−oleylamine complexes, which is similar to our reported results3b (step a). Under suitable hydrothermal conditions, the nuclei of the hydroxides are formed at the water/oil interfaces in the presence of a high concentration of oleylamine, which provides the basic environment (step b).15b In addition, based on the SEM images and XRD patterns in our experiment, we can deduce that oleylamine, as the structure-directing and activation reagents, seems to adsorb selectively onto the specific {001} crystal facets, hinder the further growth along the ⟨001⟩ direction of the hydroxide nanocrystals, and ultimately lead to the formation of the well-defined platelike morphology (step c).27,28 However, the intrinsic crystal structure of the assynthesized transition-metal (Ni, Co, and Mn) hydroxides also affects their hexagonal platelike structure. To our knowledge, nickel and cobalt hydroxides with beta phase have typical layered brucite-like structure, and the hexagonal nanoplates with exposed mostly {001} facets are usually thermodynamiD

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All diffraction peaks match well with cubic NiCo2O4 (JCPDS Card No. 20-0781). More importantly, the transition-metal hydroxides can be transformed into the corresponding lithium-ion intercalation materials. The conversion is based on the reaction between hydroxides and LiOH through the simple solid-state reactions (see Experimental Section for details). Li(Ni 0.6 Co0.4)O 2 nanoparticles were produced by sintering the mixed powder of cobalt-substituted nickel hydroxide (the sample in Figure 3b) and LiOH·3H 2 O at 700 °C. Figure S4a (Supporting Information) shows the SEM image of as-prepared Li(Ni0.6Co0.4)O2 nanomaterials. The XRD pattern (Figure S4b, Supporting Information) is in good agreement with the standard pattern of Li(Ni0.6Co0.4)O2 (JCPDS Card No. 871564). Furthermore, as shown in Figure S4c (Supporting Information), Li0.99NiO2 nanomaterials were also synthesized by heating the mixture of NiOH nanoplates and LiOH·3H2O at 800 °C in air atmosphere. The X-ray diffraction pattern (Figure S4d, Supporting Information) shows that the diffraction peaks could be indexed to Li0.99NiO2 (JCPDS Card No. 87-1553). 3.3. Electrochemical Properties. The potential electrochemical properties of the as-synthesized transition-metal oxides were investigated (Figure 9). Figure 9a shows the second discharge curve of the resulting Co3O4 nanomaterials. It can be observed that the as-prepared Co3O4 nanoplates show a long voltage plateau around 1 V, which is related to the conversion of the intermediate phase (CoO or LixCo3O4) into metallic Co.29 The specific capacity for the second discharge reaches about 970 mAh/g. Meanwhile, the first discharge curve of the as-prepared NiCo2O4 nanoplates shows the well-defined voltage plateau, and the total specific capacity reaches 1100 mAh/g (Figure 9c). Notably, the NiO nanoplates also display a distinct plateau with the initial specific capacity of ∼1170 mAh/ g for the first discharge (Figure 9b). Furthermore, we characterized the electrochemical performance of the lithiumion intercalation materials, used as the cathode material for lithium-ion batteries. Figure 10 shows the corresponding discharge capacity curves versus cycle number at a rate of 0.1 C. It is worth noting that, after 20 cycles, the capacity of the assynthesized Li(Ni0.6Co0.4)O2 nanoparticles remains as high as ∼130 mAh/g. Meanwhile, the Li0.99NiO2 nanomaterials also show relatively stable discharge capacities and an excellent cycle performance (see Figure S5 in the Supporting Information). As shown in Figure 10b, after 50 cycles, the discharge capacity still maintains above 110 mAh/g.

Article

ASSOCIATED CONTENT

S Supporting Information *

EDS spectra, SEM images, and discharge capacity curves. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(Q.P.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2011CBA00500 and 2011CB932401), the National Natural Science Foundation of China (Grant No. 20921001, 21131004, 21171105, and 21201001), Anhui Provincial Natural Science Foundation (Grant No. 1208085QB25), the Postdoctoral Fund of the Provincial Higher Education Institutions for Inorganic Chemistry of Anhui University, the 211 Project of Anhui University, and Ph.D. Start-up Fund of Anhui University.

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4. CONCLUSIONS In summary, we develop a facile and efficient interfacemediated synthetic protocol for the fabrication of transitionmetal (Mn, Co, and Ni) hydroxide nanoplates with variable composition. In addition, the as-obtained transition-metal hydroxides can be effectively converted into the corresponding oxide nanoplates and lithium-ion intercalation materials through solid-state reactions. Moreover, the as-prepared transition-metal oxide and lithium-ion intercalation materials present the characteristic electrochemical properties. Therefore, it is believed that this method may be easily extended to other metal oxide or hydroxide nanomaterials with controllable sizes and shapes. E

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