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carlosangel

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注册: 2010-01-11

[交流] 翻译一篇化学论文~~

(一些单位乱码了，不影响翻译)
High-Yield Synthesis of NiO Nanoplatelets and Their Excellent Electrochemical Performance

ABSTRACT: We demonstrate a new approach to successfully synthesize on a large scale single-crystalline nickel oxide nanoplatelets with an interesting porous structure. The as-obtained oxide nanoplatelets were characterized by a variety of techniques, such as X-ray powder diffraction, transmission electron microscopy, thermogravimetric analysis, BET surface area measurement, and electrochemical test. The electrochemical result exhibits that the electrochemical performance of the obtained nickel oxide nanoplatelets with the novel morphology has been further improved in contrast to that of the ultrafine oxide nanoparticles with average particle size of 9 nm. It suggests that the unique platelet-like nanostructure is helpful in improving the electrochemical performance of the
oxide.
                                          Introduction
   Transitional metal oxides and hydroxides, such as cobalt, iron,and nickel, have attracted increasing attention owing to their potential in a variety of field. Among them, nickel oxide, as one of the relatively few metal oxides with p-type semiconductivity, has been extensively investigated because of the stable band gap. NiO is very a prosperous material, being widely used in practice as a catalyst, as electrode material for lithium ion batteries and fuel cells, in electrochromic films, in electrochemical supercapacitors, and in dye-sensitized photocathodes.
   With advancements in all areas of industry and technology,the interest has been focused on nanoscale materials, which stems from the fact that new properties are required at this length scale and, equally important, that these properties change with their size and morphology. Thus, many methods have been
attempted to prepare nanosized nickel oxide, including nanoparticles, nanorings, nanosheets, and nanoribbons.
      In this work, we demonstrate a new approach to fabricate nickel oxide nanocrystals on a large scale attempting to find a new application or improve the existing performance. At first, the single crystalline hydroxide nanoplatelets were synthesized by hydrothermal treatment of nickel acetate and methylamine at relatively low temperature. The shape of the oxide nanoplatelets was sustained by the thermal decomposition of the hydroxide precursor, and the electrochemical performance of the unique nanostructured oxide was investigated. It is expected
to explore a more extensive potential application of transitional metal oxide.
                                       Experimental Section
         All the chemicals were of analytical grade and were used as received without further purification. At first, nickel hydroxide nanoplatelet precursor was prepared by the precipitation and hydrothermal process. In the typical procedure, 10 mmol of freshly precipitated nickel oxalate was dispersed in 20 mL of distilled water, and 20 mL of 0.4 M methylamine solution was added under magnetic stirring at room temperature. The resulting mixture was then transferred into a 60 mL Teflon-lined autoclave, which was sealed and heated at 120 °C for 12h. After the hydrothermal treatment, a green suspension was obtained. The precipitate was separated from the solution by centrifugation, washed repeatedly with distilled water, and dried at 60 °C in a vacuum. Finally, the resulting hydroxide precursor was heated at 400 °C for 2h in air to prepare the oxide product.
   The crystalline phase was identified by powder X-ray diffraction (XRD) using a Philips X’Pert Pro Super diffractometer with graphite monochromatized Cu KR radiation (λ= 1.541 78 A

. High-resolution TEM images and selected-area electron diffraction (SAED) patterns were obtained on a JEOL-2010 transmission electron microscope at an acceleration voltage of 200 kV. Thermogravimetric analysis (TG) of the sample was carried out on a Shimadzu TA-50 thermal analyzer at a heating rate of 10 °C min-1 from room temperature to 600 °C in flowing air. N2 adsorption was determined by BET measurements using a NOVA-1000e surface analyzer.
   The electrochemical performance of the oxide was examined witha Teflon cell. The cathode was a mixture of nickel oxide, acetylene black, and polyvinylidene fluoride (PVDF) with weight ratio of 85:10:5. Metallic lithium was used as the negative electrode. The electrolyte was 1 M LiPF6 in a 1:1 mixture of ethylene carbonate (EC)/diethyl carbonate (DEC). The cell was galvanostatically cycled in the range of 3.0-0.5 V. For comparison, the electrochemical performance of
ultrafine NiO nanoparticles with average particle size of 9 nm was also investigated, which was prepared by thermal decomposition of nickel oxalate nanofibers.
                                    Results and Discussion
      Figure 1 displays the X-ray diffraction (XRD) patterns of the as-prepared sample before and after calcinations. All the diffraction peaks of Figure 1a can be indexed as hexagonal β-Ni-(OH)2 with the lattice parameters a =3.128 Å and c = 4.605Å, in good agreement with the reported values (ICDD-JCPDScard No. 14-0117). No obvious peaks for other impurity were detected. Figure 2 shows the TEM images of the hydroxide precursor prepared by the hydrothermal treatment. From the images, one can see that the morphology of the hydroxide is platelet-like with average diameter of 50 nm. Many of the products are irregular hexagonal platelets.
      On the basis of the TG analysis result, the hydroxide precursor was heated at 400 °C for 2 h to ensure the complete decomposition of β-Ni(OH)2. Figure 1b shows the XRD pattern of the as-prepared NiO powder. All the reflections can be indexed to face-centered cubic (fcc) NiO phase with lattice constant a = 4.175 Å (space group Fm3hm [225]), which agrees well with the standard data (JCPDS card No. 47-1049). No peaks due to β-Ni(OH)2 were found from XRD, indicating that β-Ni(OH)2 was completely decomposed to NiO at 400 °C for 2 h, which is also confirmed by the TG measurement. The broadening of the diffraction peaks indicates the small particle size.
      Figure 3 exhibits TEM and HRTEM images of the oxide sample. From Figure 3a,b, the porous NiO nanoplatelets with average diameter of about 60 nm can be clearly observed, and the platelet-like morphology was sustained by the thermal
decomposition of the hydroxide nanoplatelet precursor at 400°C for 2 h in air. The average pore size is about 5 nm. The yield of the platelet-like structures is very high (above 90%). Figure 3c shows the representative nanoplatelets with irregular
hexagonal shape with the angels of adjacent edge of 120°, as indicated by solid lines. A high magnification of an individual nanoplatelet is shown in Figure 3d. The HRTEM image shows a well-defined crystalline structure with a lattice spacing of 1.4
Å, corresponding to the value of the (220) plane of the cubic NiO phase. The inset in Figure 3f shows the corresponding SAED pattern. The ED pattern was taken with an electron beam along the [-111] zone axis, perpendicular to the nanoplatelet
surface. After further investigation on the above results, we confirm that the surface of the nanoplatelets is the {111} plane of the cubic NiO phase, and the adjacent edges with the angles of 120° should correspond to the {110} and {101} planes. ED patterns on different individual nanoplatelets were essentially the same, indicating the single crystalline nature of the NiO nanoplatelets.
      The organic base environment provided by methylamine is vitally important for the formation of the platelet-like hydroxide. If methylamine was replaced by other alkali media, such as ammonia, the platelet-like hydroxide could also be obtained. But the average thickness of the nanoplatelets was doubled, which was not beneficial to the subsequent formation of the porous oxide nanoplatelets. BET (Brunauer-Emmett-Teller) surface areas, calculated from nitrogen adsorption isotherms, show that the surface area of the NiO nanoplatelets is 85 m2/g, which increases significantly compared with that of the nickel hydroxide nanoplatelets (21 m2/g). And the average pore size of the oxide is 4.2 nm, consistent with the TEM observation.
         The electrochemical measurements for the resulting oxide nanoplatelets were carried out at room temperature in a voltage range from 3.0 to 0.5 V at a constant current density of 0.4 mAcm-2. Figure 4 shows the voltage vs discharge capacity curves for nickel oxide nanoparticles (a) and nanoplatelets (b). For the nanoplatelets, the discharge plateau is about at 1.5 V, and the average discharge capacity can reach ca. 700 mA h g-1 for the first 20 cycles, corresponding to the intercalation of 1.95 Li per NiO, which is close to the theoretical capacity of nickel
oxide. Comparatively, dramatically fading capacity can be found for the nanoparticle sample from 620 mA h g-1 for the fifth cycle to 410 mA h g-1 the 10th cycle. The overall electrochemical process can be expressed as follows:

      In comparison with the NiO nanoparticles, the reversible capacity and cycleability of the oxide nanoplatelets have been much improved. The result indicates that the unique structure is beneficial to the improvement of electrochemical performance of nickel oxide due to the increased surface area and the porous structure, which facilitate the intercalation/deintercalation of lithium ions. A similar phenomenon was also found in the case of sheet-like V2O5 (798 mA h g-1)15 and board-like Ni(OH)2 (260 mA h g-1).4c The morphologically unique NiO nanoplatelets, as we expected, may give a new perspective for applications.
                                    Conclusions
            In summary, the work presented here reports a new method to fabricate single crystalline nickel oxide nanoplatelets with a unique porous structure on a large scale. The nickel oxide nanostructure was synthesized by thermal treatment of the hydroxide nanoplatelet precursor at 400 °C for 2 h in air. From the TEM results, the shape of the oxide nanoplatelets was sustained by the thermal decomposition of the hydroxide precursor. The surface of the oxide nanoplatelets is the {111} plane of the cubic NiO phase, and the adjacent edges with the angles of 120° should correspond to the {110} and {101} planes. ED patterns on different individual nanoplatelets were essentially the same, indicating the single crystalline nature of the NiO nanoplatelets. The result of the electrochemical test shows that the nanoplatelets exhibit an improved reversible capacity and cycleability in contrast with NiO nanoparticles. It suggests that the unique platelet-like porous structure is beneficial to the improvement of the electrochemical performance of the nickel oxide.

[ Last edited by carlosangel on 2010-2-27 at 16:38 ]

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