Preparation of Mesoporous Co3O4 Nanoparticles via Solid− Liquid

Feb 5, 2009 - (2D hexagonal structure or 3D Ia3d cubic structure) of mesoporous Co3O4 nanoparticles does not obviously affect the specific capacitance...
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J. Phys. Chem. C 2009, 113, 3887–3894

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Preparation of Mesoporous Co3O4 Nanoparticles via Solid-Liquid Route and Effects of Calcination Temperature and Textural Parameters on Their Electrochemical Capacitive Behaviors Ming-bo Zheng,† Jian Cao,† Shu-tian Liao, Jin-song Liu, Hui-qin Chen, Yu Zhao, Wei-jie Dai, Guang-bin Ji, Jie-ming Cao,* and Jie Tao Nanomaterials Research Institute, College of Materials Science and Technology, Nanjing UniVersity of Aeronautics and Astronautics, Nanjing 210016, China ReceiVed: NoVember 21, 2008; ReVised Manuscript ReceiVed: January 1, 2009

Mesoporous Co3O4 nanoparticles with different textural parameters were prepared by using mesoporous silicas, KIT-6 and SBA-15, as templates and Co(NO3)2 · 6H2O as precursor via an improved solid-liquid route. The results of N2 adsorption-desorption analysis indicated that the calcination temperature did not obviously affect the textural parameters of Co3O4 samples and the BET surface areas of Co3O4 samples could be regulated by different KIT-6 templates. The effects of calcination temperature and textural parameters on the electrochemical capacitive behaviors of Co3O4 samples were discussed. The results of electrochemical tests show the following: the capacitance value of the sample decreases slightly with the increase of the calcination temperature; the BET surface area is the crucial factor for the specific capacitance value; for mesoporous materials, large pore size and high ordering degree of mesopore facilitate ion transfer; and the meso-structure (2D hexagonal structure or 3D Ia3d cubic structure) of mesoporous Co3O4 nanoparticles does not obviously affect the specific capacitance value of the samples, but 2D hexagonal mesoporous structure is more advantageous to ion transfer than 3D Ia3d cubic mesoporous structure. 1. Introduction Electrochemical capacitors are becoming attractive energy storage systems because they have higher power density and longer cycle life than common batteries.1 On the basis of energy storage mechanisms, electrochemical capacitors are classed into two areas: (1) electrical double-layer capacitance, arising from the charge separation at the electrode/electrolyte interface, and (2) faradaic pseudocapacitance, arising from fast, reversible electrosorption or redox processes occurring at or near the solid electrode surface.1 Carbon materials with high specific surface area are widely used for electrical double-layer capacitors.2 RuO2 and hydrous ruthenium oxide possess faradaic pseudocapacitance property and exhibit much higher specific capacitance than conventional carbon materials.3 However, the high cost of these materials limits them from wide application. Hence, much effort has been aimed at searching for alternative inexpensive electrode materials with good pseudocapacitance behaviors, such as nickel oxides,4-7 cobalt oxides,8,9 and manganese oxides.10 However, these cheap transition metal oxides exhibit lower electrochemical performance compared with RuO2. Thus, improving the capacitance performance of these cheap metal oxides is a significant step. Since pseudocapacitance is an interfacial phenomena, an effective way to improve the capacitance performance is increasing the specific surface areas of the electrode materials. Mesoporous metal oxides are of wide interest for applications in many fields, such as catalysts,11 gas sensors,12 and electrochemical electrode materials,13,14 due to their large specific surface areas, ordered pore structure, and shape-selective properties. However, direct synthesis of mesoporous metal * Corresponding author. E-mail: [email protected]. † Ming-bo Zheng and Jian Cao contributed equally to this work.

oxides with surfactant micelles as a soft template is quite difficult because, compared with silica, the surfactant/oxide composite precursors are often more susceptible to lack of condensation, redox reactions, or phase transitions accompanied by thermal breakdown of the structural integrity.15,16 The hard template method for mesoporous carbons, initiated by the group of Ryoo,17 is an attractive route for the synthesis of mesoporous metal oxides. Ordered mesoporous Fe2O3,18-20 NiO,20 Co3O4,20-23 Cr2O3,20,21 In2O3,23 MnO2,24 Mn2O3, and Mn3O425 with crystalline walls have been synthesized by using mesoporous silicas as hard templates. Recently, Yue et al. developed a new solid-liquid method for the preparation of mesoporous metal oxides using KIT-6 and SBA-15 as templates.26 This method clarified the formation mechanism of metal oxides inside the pores of mesoporous silica and it was more convenient than previous methods, such as the surface modification method,27-29 the “two solvents” method,30 and the evaporation method.20,21,23 Preparation of mesoporous cheap metal oxides which were used as pseudocapacitance electrode materials has received considerable attention. Cao et al. prepared mesoporous nanocrystalline Co3O4 with a BET surface area of 212 m2/g by heattreatment of Co(OH)2 floccule, which was obtained by using polyacrylamide as additive, and the specific capacitance value of the sample was 401 F/g.31 Xing et al. prepared mesoporous NiO with a very high BET surface area of 477.7 m2/g by heattreatment of mesoporous Ni(OH)2, which was obtained by using sodium dodecyl sulfate as the soft template, and the specific capacitance value of the sample was 124 F/g.32 Wang et al. obtained mesoporous NiO with a BET surface area of 47 m2/g using SBA-15 as the hard template and the specific capacitance value of the sample was 120 F/g.33 Zhou et al. prepared mesoporous MnO2 with a BET surface area of 118 m2/g using mesoporous KIT-6 as template and the specific capacitance was

10.1021/jp810230d CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

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Figure 1. Wide-angle XRD pattern (a) and low-angle XRD pattern (b) of Co3O4-KIT-6-100-200.

220 F/g.34 Liu et al. prepared mesoporous Co3O4/RuO2 · xH2O (Co:Ru ) 19:1) using P123 as the soft template and the specific capacitance was 92 F/g.35 To the best of our knowledge, investigation of the effects of calcination temperature and textural parameters on the electrochemical capacitive behaviors of mesoporous cheap metal oxides has not been reported yet. In this work, mesoporous Co3O4 nanoparticles with different textural parameters were synthesized by using KIT-6 and SBA15 as templates via an improved solid-liquid method and their electrochemical capacitive behaviors were evaluated in 2 mol/L KOH electrolyte solution. The effects of calcination temperature and textural parameters on their electrochemical capacitive behaviors are discussed. 2. Experimental Section 2.1. Synthesis of KIT-6 and SBA-15 Silica. Threedimensional (3D) cubic Ia3d KIT-6 mesoporous silica materials were prepared according to the procedure described by Ryoo and co-workers.36 A typical preparation is as follows: 4.0 g of Pluronic P123 (EO20PO70EO20) was dissolved in 144 g of distilled water and 7.9 g of 35 wt % HCl solution with stirring at 35 °C. After complete dissolution, 4.0 g of BuOH was added. The mixture was further stirred at 35 °C for 1 h, after which 8.6 g of tetraethoxysilane (TEOS) was added to the homogeneous solution. This mixture was left under vigorous and constant stirring at 35 °C for 24 h. The synthesis was carried out in a closed polypropylene bottle. Subsequently, the mixture was aged at 100 °C for 24 h under static conditions in a Teflonlined autocalve (hydrothermal treatment process). The solid product was filtered without washing and dried at 100 °C for

Zheng et al. 24 h. Surfactant-free mesoporous materials were obtained after a brief ethanol/HCl washing and subsequent calcination at 550 °C for 5 h in air. The product was nominated as KIT-6100 (“100” denotes the hydrothermal temperature of KIT-6). In another set of experiments, the hydrothermal treatment temperature was varied from 40 to 135 °C. Two-dimensional (2D) hexagonal SBA-15 mesoporous silica material was synthesized according to the corresponding literature37 and the hydrothermal temperature was 100 °C. The product was nominated as SBA-15-100. 2.2. Synthesis of Mesoporous Co3O4. Mesoporous Co3O4 was prepared by using calcined 3D Ia3d cubic KIT-6 and 2D hexagonal SBA-15 as templates. Typically, 0.3 g of KIT-6100 was dispersed in a 10 mL solution of 0.6 g of Co(NO3)2 · 6H2O in ethanol in a glass container. The solution was stirred at room temperature for 12 h. Ethanol was removed by heating the container overnight at 40 °C. Then, the dried compound was ground for 10 min in a mortar. Afterward, the resulting powder was calcined at 200 °C for 5 h in air atmosphere. The silica template was then removed at room temperature with 5% HF aqueous solution. The black Co3O4 material was obtained by centrifugation and finally dried at 80 °C. The product was nominated as Co3O4-KIT-6-100-200. (The denomination of the sample is as follows: Co3O4-KIT-6a-b, where “KIT-6” denotes the product was prepared by using KIT-6 as template, “a” denotes the hydrothermal temperature of KIT-6, and “b” denotes the calcination temperature.) The calcination temperature was varied from 200 to 700 °C. When KIT-6-40, KIT-6-60, KIT-6-70, and KIT-6-80 were used as templates, the masses of KIT-6 templates were 0.93, 0.56, 0.48, and 0.41 g, respectively, which were adjusted according to the pore volume of the corresponding KIT-6. When KIT-6-100, KIT-6-120, and KIT-6-135 were used as template, 0.3 g of KIT-6 was used. The preparations of Co3O4-SBA-15 are similar to that of Co3O4-KIT-6: 0.3 g of SBA-15 was used as template and 0.6 g of Co(NO3)2 · 6H2O was used as the precursor. Conventional Co3O4 was prepared by calcining Co(NO3)2 · 6H2O at 500 °C directly. 2.3. Characterization. The crystal structure of the samples was characterized by X-ray diffraction (XRD) (Bruker D8 advance). The size and morphology of the samples were examined by transmission electron microscopy (TEM) (FEI, TECNAI-20)andscanningelectronmicroscopy(SEM)(LEO1530). The N2 adsorption-desorption analysis was measured on a Micromeritics ASAP 2010 instrument. 2.4. Electrochemical Tests. Electrodes for electrochemical capacitors were prepared by mixing 80 wt % of the prepared powders with 15 wt % of acetylene black and 5 wt % of polytetrafluoroethylene binder. The mixture was then pressed onto a nickel grid. Each electrode contained about 10 mg of mesoporous Co3O4. The electrodes were dried at 90 °C for 24 h. Before electrochemical tests, the electrodes were impregnated with 2 mol/L of KOH solution to guarantee that the electrode material was thoroughly wetted by electrolyte. All electrochemical measurements were done in a three-electrode experimental setup. The prepared electrode, platinum foil, and SCE electrode were used as the working, counter, and reference electrodes, respectively. All measurements were carried out in 2 mol/L of KOH electrolyte. Cyclic voltammetry (CV) and galvanostatic charge-discharge tests were conducted with a CHI660 electrochemical workstation. 3. Results and Discussions The N2 adsorption-desorption isotherms and pore size distributions for the KIT-6 samples synthesized at different

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Figure 2. SEM images of Co3O4-KIT-6-100-200 (a) and Co3O4-KIT-6-100-500 (b); TEM images of Co3O4-KIT-6-100-200 (c, e, f), Co3O4-KIT6-100-300 (g), and Co3O4-KIT-6-100-500 (h); SAED images of Co3O4-KIT-6-100-200 (d) and Co3O4-KIT-6-100-500 (i). The square area in panel e corresponds to panel f.

TABLE 1: BET Surface Area and Pore Volume of All Samples and Specific Capacitance at Different Discharge Current and Decreasing Rate of Specific Capacitance from 5 to 20 mA for Co3O4 Samples specific capacitance (F/g) at different discharge current sample name

BET surface area (m2/g)

pore vol (cm3/g)

KIT-6-40 KIT-6-60 KIT-6-70 KIT-6-80 KIT-6-100 KIT-6-120 KIT-6-135 SBA-15 Co3O4-KIT-6-100-200 Co3O4-KIT-6-100-300 Co3O4-KIT-6-100-400 Co3O4-KIT-6-100-500 Co3O4-KIT-6-100-600 Co3O4-KIT-6-100-700 conventional Co3O4-500 Co3O4-KIT-6-40-200 Co3O4-KIT-6-60-200 Co3O4-KIT-6-70-200 Co3O4-KIT-6-80-200 Co3O4-KIT-6-100-200 Co3O4-KIT-6-120-200 Co3O4-KIT-6-135-200 Co3O4-SBA-15-100-200 Co3O4-SBA-15-100-500

575.9 687.4 742.7 637.5 722.0 654.7 567.4 519.6 137.5 128.5 129.5 126.8 132.2 124.5 7.58 184.8 168.6 163.7 147.9 137.5 120.9 118.0 122.6 116.1

0.34 0.56 0.66 0.77 1.05 1.26 1.26 0.99 0.58 0.52 0.58 0.56 0.47 0.56 0.026 0.48 0.64 0.68 0.68 0.58 0.63 0.48 0.47 0.35

5 mA

10 mA

20 mA

decreasing rate of specific capacitance from 5 to 20 mA (%)

282 273 259 259

275 252 242 242

254 215 216 202

9.9 21.3 16.6 22.0

236 30.7 370 337 333 315 282 264 235 265 253

225 29.6 362 314 311 295 275 256 229 259 239

198 27.1 344 265 268 266 254 241 218 246 205

16.1 11.7 7.0 21.4 19.5 15.6 9.9 8.7 7.2 7.2 19.0

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Figure 3. N2 adsorption-desorption isotherms (a) and BJH pore size distributions from adsorption branch (b) for the mesoporous Co3O4 samples prepared at different calcination temperatures using KIT-6-100 as template. For clarification, the isotherms for 300, 400, 500, 600, and 700 °C are offset vertically by 150, 300, 500, 700, and 850 cm3 g-1, respectively.

hydrothermal temperatures (see Figure S1 in the Supporting Information) indicate that the average mesopore size of KIT-6 samples increased with the increase of the hydrothermal temperature. Textural properties for KIT-6 samples prepared at various hydrothermal temperatures are summarized in Table 1. The pore volume of KIT-6 samples increased with the increase of the hydrothermal temperature. The wide-angle XRD pattern (Figure 1a) of Co3O4-KIT-6100-200 reveals face centered cubic Co3O4 formed at 200 °C and the low-angle XRD pattern (Figure 1b) proves the bulk structural order (for the wide-angle XRD patterns of Co3O4KIT-6-100-500 and Co3O4-KIT-6-100-700 see Figure S2 in the Supporting Information). Figure 2 shows the SEM and TEM images of Co3O4-KIT6-100-200, Co3O4-KIT-6-100-300, and Co3O4-KIT-6-100-500, which indicates that the Co3O4-KIT-6 sample is composed of nanoparticles. Parts a and b of Figure 2 indicate that the calcination temperature does not obviously affect the particle size of the products. The typical diameter of the nanoparticles was estimated to be 50-200 nm, whereas the KIT-6 particle was several microns in size. Parts c, g, and h of Figure 2 indicate that all the particles have mesoporous structure. Conventional cobalt oxide particles were not observed for all samples during TEM observation, which indicates that almost all nitrates have moved into the mesopores of KIT-6 during the calcination. Figure 2e shows that the nanoparticle has Ia3d cubic mesostructure, which indicates that Co3O4 nanoparticles replicate the meso-structure of the KIT-6 template. The high-resolution TEM image (Figure 2f) reveals the crystalline wall of the Co3O4KIT-6-100-200. Selected area electron diffraction (SAED) patterns (Figure 2d,i) also reveal the crystalline walls of the mesoporous nanoparticles and correspond to the face centered cubic Co3O4. Energy-dispersion X-ray spectra analysis indicates the absence of Si (see Figure S3 in the Supporting Information). After the evaporation of ethanol, only a small amount of the nitrates migrated into the mesopores of templates. Most of the nitrates dispersed uniformly on the outside surface of silica particles. The melting point of Co(NO3)2 · 6H2O is about 55 °C and the decomposition temperature of it is about 74 °C. With the increase of temperature, the nitrates turned into a liquid phase before decomposition and moved into the mesopores of KIT-6.

Figure 4. (a) CV curves at different scan rates for Co3O4-KIT-6-100200; (b) charge-discharge curves at 10 mA for the mesoporous Co3O4 samples prepared at different calcination temperatures using KIT-6100 as template. The Arabic numerals within panel b are the calcination temperatures of the samples. The mass of the active materials is normalized to 10 mg.

The decomposition of the nitrates and crystallization of the corresponding oxides took place inside the mesopores at a higher temperature.26 After removal of the SiO2 template, mesoporous Co3O4 nanoparticles were obtained. Compared with Yue’s solid-liquid method,26 our improved solid-liquid method possessed higher yield of mesoporous Co3O4. By combing the evaporation method, the nitrates can disperse uniformly on the outside surface of the silica particle, which facilitates the nitrates move into the mesopores during the calcination. Thus, the yield of mesoporous nanoparticles can be improved.

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Figure 5. N2 adsorption-desorption isotherms (a) and BJH pore size distributions from adsorption branch (b) for the mesoporous Co3O4 samples prepared at 200 °C with KIT-6 prepared at different hydrothermal treatment temperatures as templates. For clarification, the isotherms for 60, 70, 80, 100, 120, and 135 °C are offset vertically by 200, 400, 600, 850, 1050, and 1300 cm3 g-1, respectively.

Figure 7. SEM (a) and TEM (b) images of Co3O4-SBA-15-100-500.

Figure 6. (a) CV curves for the typical samples at a scan rate of 5 mV/s; (b) charge-discharge curves at 10 mA for the mesoporous Co3O4 samples prepared at 200 °C with KIT-6 prepared at different hydrothermal treatment temperatures as templates. The Arabic numerals within panel b are the hydrothermal temperatures of the KIT-6 template. The mass of the active materials is normalized to 10 mg.

Figure 3 shows the N2 adsorption-desorption isotherms and BJH pore size distribution plots of the samples prepared by using KIT-6-100 as template and calcined at different temperatures. All the samples have a sorption isotherm with a broad capillary condensation range starting at about P/P0 ) 0.5 and extending almost to P/P0 ) 1, which indicates that the samples have a high fraction of textural porosity. The narrow pore size distribution at about 5 nm corresponds to the mesopore that was produced from the removal of pore wall of SiO2 template. The mesopore size of the products decreased slightly with the

increase of the calcination temperature. This is due to the fact that the SiO2 template shrank at high temperature and the shrinkage increased with the increase of calcination temperature. Besides the mesopore distribution, these samples also have a broad pore distribution in the range of 15-50 nm. These nanopore structures were mainly produced from the stack of the nanoparticles. The BET surface area and pore volume of the samples are shown in Table 1, which indicates that the calcination temperature does not obviously affect the textural parameters of mesoporous Co3O4. Generally, the BET surface area and pore volume of porous metal oxide decreased obviously with the increase of the calcined temperature.31,32 But, the BET surface area and pore volume of the sample prepared via the hard-template method can be retained even at 700 °C. The hardtemplate method provides a method for the preparation of the sample with both high BET surface area and high crystallization. CV and galvanostatic charge-discharge techniques were used to determine the electrochemical properties of mesoporous metal oxides. Figure 4a shows the CV curves of the Co3O4-KIT-6100-200 electrode at various scan rates (for the CV curves of other samples see Figure S4 in the Supporting Information). The curve shapes of the sample reveal that the capacitive characteristic is very distinguished from that of electric doublelayer capacitance in which case it is normally close to an ideal rectangular shape.38,39 Since solution and electrode resistance

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Figure 8. N2 adsorption-desorption isotherms (a) and BJH pore size distributions from adsorption branch (b) for Co3O4-SBA-15-100-200 and Co3O4-SBA-15-100-500.

can distort current response at the switching potential and this distortion is dependent upon the scan rate,40 the shape of the CV has changed with the scan rate increase. These results indicate that the measured capacitance is mainly based on the redox mechanism. The galvanostatic charge-discharge curves of Co3O4-KIT-6 synthesized at different temperatures and conventional Co3O4 within the potential range 0-0.45 V at the charge-discharge current of 10 mA after 50 cycles are shown in Figure 4b. The specific capacitance (Cm) can be calculated as follows: Cm ) I∆t/m∆V, where I is the current of charge-discharge, ∆t is the time of discharge, m is the mass of active materials in the work electrode, and ∆V is 0.45 V. The evaluated results (see Table 1) showed that the specific capacitance decreased slightly with the increase of the calcination temperature. It is likely that the reactive activity of the samples weakened slightly with the increase of the calcination temperature. Since long cycle life is very important for the electrochemical capacitor, the cycle charge-discharge test has been employed to examine the life of the Co3O4-KIT-6-100-200 electrode. Approximately 1% loss of specific capacitance after 300 cycles indicates that the repetitive charge-discharges do not induce noticeable degradation of the electrochemical activity. Besides, the specific capacitance of Co3O4-KIT-6-100-500 is about 8 times of that of conventional Co3O4. Although the mesoporous sample possesses better capacitive property than the conventional sample, the result also indicates that the interior surface of mesoporous sample was not fully utilized during the electrochemical reaction. This is possibly because of the weak electrical conductivity of metal oxide. Mesoporous Co3O4 samples with different textural parameters were also synthesized at 200 °C and with KIT-6 which were prepared at different hydrothermal temperatures as templates. Figure 5 shows the N2 adsorption-desorption isotherms and pore size distributions for the mesoporous Co3O4 samples. It is noticeable that Co3O4-KIT-6-40-200 has the largest mesopore and it has a broad pore size distribution. Co3O4-KIT-6-100200, Co3O4-KIT-6-120-200, and Co3O4-KIT-6-135-200 possess narrower pore size distribution than other samples, which indicates they have more ordered mesopores than other samples. Textural properties of the samples are summarized in Table 1. The BET surface areas of the mesoporous Co3O4 samples increased with the decrease of the pore volume of the corresponding KIT-6 template, which indicates that the BET surface

Figure 9. Charge-discharge curves at 10 mA for Co3O4-SBA-15100-200 and Co3O4-SBA-15-100-500. The masses of the active materials are normalized to 10 mg.

area of the product can be regulated via using different templates. Besides, all the samples possess large BET surface areas and high pore volumes, which may be important for a wide range of applications. Figure 6a shows the CV curves of typical samples at a scan rate of 5 mV/s (for the CV results of other samples see Figure S5 in the Supporting Information). The intensity of the redox peaks was enhanced with the increase of the BET surface area of the sample. The galvanostatic charge-discharge curves after 50 cycles for all samples are shown in Figure 6b. The results indicate that the specific capacitance increased with the increase of the BET surface area of the sample. Electric double-layer capacitance and pseudocapacitance are interfacial phenomenons, the increase of specific surface area can enhance the chargestorage capability. Although these samples possess different BET surface area, pore volume, and pore size distribution, electrochemical test results indicate that the BET surface area is the crucial factor for the specific capacitance value of mesoporous Co3O4. For investigation of the effect of meso-structure on the capacitive behaviors of the sample, mesoporous Co3O4 with 2D hexagonal structure was also prepared with SBA-15 as template. The SEM image (Figure 7a) reveals that Co3O4-SBA-15-100500 is also composed of nanoparticles and inherited the rodlike morphology of the SBA-15 particle. The TEM image (Figure 7b) indicates that Co3O4-SBA-15-100-500 replicates the 2D hexagonal mesoporous structure of SBA-15. Conventional cobalt oxide particles were not observed during TEM observa-

Preparation of Mesoporous Co3O4 Nanoparticles tion. The results of N2 adsorption-desorption analysis (Figure 8) indicate that Co3O4-SBA-15-100 samples possess ordered mesoporous structure and the average mesopore sizes of them are as much as those of Co3O4-KIT-6-100 samples. The CV results of the samples were similar to that of Co3O4-KIT-6-100 samples (see Figure S6 in the Supporting Information). The results of the galvanostatic charge-discharge test are shown in Figure 9 and Table 1, which indicates that the meso-structure (2D hexagonal structure or 3D Ia3d cubic structure) of mesoporous Co3O4 does not obviously affect the specific capacitance value of the samples. The specific capacitance value for each sample was tested at the discharging currents of 5, 10, and 20 mA, respectively (see Table 1). As discharge current increases, the specific capacitance decreases, which was caused by the increment of voltage drop and the insufficient use of the active material involved in the redox reaction under higher current densities.38,39 For the samples fabricated at different calcination temperatures, Co3O4-KIT-6100-200 possesses the lowest decreasing rate. For the samples obtained at 200 °C and with KIT-6 fabricated at different hydrothermal temperatures as templates, Co3O4-KIT-6-40-200 possesses the lowest decreasing rate. This may be attributed to its large mesopores, which facilitate ion transfer in the mesopores. Co3O4-KIT-6-60-200, Co3O4-KIT-6-70-200, and Co3O4KIT-6-80-200 possess a relatively large decreasing rate. From the BJH pore size distributions (Figure 5b), we can see that these samples have broad distributions in the range of 2-12.5 nm, which indicates that the ordered degree of the mesopore structure of these samples is not as good as that of Co3O4-KIT6-100-200, Co3O4-KIT-6-120-200, and Co3O4-KIT-6-135-200. Because ion transfer in the ordered mesopores is easier than that in the disordered pores,41 the samples with relatively disorderd pores have a larger decreasing rate. Although the mesopore of Co3O4-KIT-6-40-200 is relatively disordered, its large mesopore can compensate for the negative effect of its disordered mesopore structure. Besides, the decreasing rates of Co3O4-SBA-15-100 samples are lower than that of Co3O4-KIT6-100 samples. We believe that the 2D hexagonal mesoporous structure of Co3O4-SBA-15 is more advantageous to ion transfer than the 3D Ia3d cubic mesoporous structure of Co3O4-KIT-6. 4. Conclusions In summary, mesoporous Co3O4 nanoparticles with different textural parameters were prepared by using mesoporous silica KIT-6 as template via an improved solid-liquid route. The results of N2 adsorption-desorption analysis indicate that calcination temperature does not obviously affect the textural parameters and the BET surface area can be regulated by different KIT-6 templates. The results of electrochemical tests indicate the following: the capacitance value of the sample decreases slightly with the increase of calcination temperature; the BET surface area is the crucial factor for the capacitance value of the sample; and for mesoporous materials, large pore size and high ordered degree of mesopore are advantageous to the maintenance of the capacitance value at high discharging current. Mesoporous Co3O4 nanoparticles were also prepared with SBA-15 as template and their capacitive properties were also studied. The results of electrochemical tests indicate the meso-structure (2D hexagonal structure or 3D Ia3d cubic structure) of mesoporous Co3O4 does not obviously affect the specific capacitance value of the sample, but 2D hexagonal mesoporous structure is more advantageous to ion transfer than 3D Ia3d cubic mesoporous structure. Besides, the specific capacitance of Co3O4-KIT-6-40-200 with a BET surface area

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3893 of 184.8 m2/g was 370 F/g at a current density of 0.5 A/g. The specific capacitance of mesoporous Co3O4 with a BET surface area of 212 m2/g prepared by Cao et al. was 401 F/g at a current density of 0.5 A/g.31 These results show that our products possess similar electrochemical capacitive behavior with the mesoporous Co3O4 prepared by Cao et al. Acknowledgment. This work was supported by Doctor Innovation Funds of Jiangsu Province (BCXJ06-13), National Natural Science Foundation of China (50502020), Natural Science Foundation of Jiangsu Province (BK2006195), and National Natural Science Foundation of China (50701024) Supporting Information Available: The results of N2 adsorption-desorption analysis for KIT-6 templates, wide-angle XRD patterns of Co3O4-KIT-6-100-500 and Co3O4-KIT-6-100700, EDX spectrum of Co3O4-KIT-6-100-200, CV curves of Co3O4 samples obtained at different heat-treatment temperatures, CV curves of Co3O4 samples obtained with different KIT-6 as templates, and CV curves of Co3O4-SBA-15 samples. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Conway, B. Electrochemical Supercapacitors; Kluwer Academic/ Plenum Publishers: New York, 1999. (2) Qu, D. Y.; Shi, H. J. Power Sources 1998, 74, 99. (3) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699. (4) Srinivasan, V.; Weidner, J. W. J. Electrochem. Soc. 1997, 144, L210. (5) Liu, K. C.; Anderson, M. A. J. Electrochem. Soc. 1996, 143, 124. (6) Srinivasan, V.; Weidner, J. W. J. Electrochem. Soc. 2000, 147, 880. (7) Nam, K. W.; Kim, K. B. J. Electrochem. Soc. 2002, 149, A346. (8) Lin, C.; Ritter, J. A.; Popov, B. N. J. Electrochem. Soc. 1998, 145, 4097. (9) Srinivasan, V.; Weidner, J. W. J. Power Sources 2002, 108, 15. (10) Pang, S. C.; Anderson, M. A.; Chapman, T. W. J. Electrochem. Soc. 2000, 147, 444. (11) Yan, L.; Zhang, X. M.; Ren, T.; Zhang, H. P.; Wang, X. L.; Suo, J. S. Chem. Commun. 2002, 860. (12) Jiao, F.; Shaju, K. M.; Bruce, P. G. Angew. Chem., Int. Ed. 2005, 44, 6550. (13) Li, W. Y.; Xu, L. N.; Chen, J. AdV. Funct. Mater. 2005, 15, 851. (14) Li, Y. G.; Tan, B.; Wu, Y. Y. Nano Lett. 2008, 8, 265. (15) Schu¨th, F. Chem. Mater. 2001, 13, 3184. (16) Soler-illia, G. D. A; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093. (17) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743. (18) Jiao, F.; Harrison, A.; Jumas, J. C.; Chadwick, A. V.; Kockelmann, W.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 5468. (19) Jiao, F.; Jumas, J. C.; Womes, M.; Chadwick, A. V.; Harrison, A.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 12905. (20) Tian, B. Z.; Liu, X. Y.; Yang, H. F.; Xie, S. H.; Yu, C. Z.; Tu, B.; Zhao, D. Y. AdV. Mater. 2003, 15, 1370. (21) Dickinson, C.; Zhou, W. Z.; Hodgkins, R. P.; Shi, Y. F.; Zhao, D. Y.; He, H. Y. Chem. Mater. 2006, 18, 3088. (22) Rumplecker, A.; Kleitz, F.; Salabas, E. L.; Schu¨th, F. Chem. Mater. 2007, 19, 485. (23) Tian, B. Z.; Liu, X. Y.; Solovyov, L. A.; Liu, Z.; Yang, H. F.; Zhang, Z. D.; Xie, S. H.; Zhang, F. Q.; Tu, B.; Yu, C. Z.; Terasaki, O.; Zhao, D. Y. J. Am. Chem. Soc. 2004, 126, 865. (24) Jiao, F.; Bruce, P. G. AdV. Mater. 2007, 19, 657. (25) Jiao, F.; Harrison, A.; Hill, A. H.; Bruce, P. G. AdV. Mater. 2007, 19, 4063. (26) Yue, W.; Zhou, W. Z. Chem. Mater. 2007, 19, 2359. (27) Zhu, K. K.; Yue, B.; Zhou, W. Z.; He, H. Y. Chem. Commun. 2003, 98. (28) Yue, B.; Tang, H. L.; Kong, Z. P.; Zhu, K. K.; Dickinson, C.; Zhou, W. Z.; He, H. Y. Chem. Phys. Lett. 2005, 407, 83. (29) Kaleta, W.; Nowinska, K. Chem. Commun. 2001, 535. (30) Jiao, K.; Zhang, B.; Yue, B.; Ren, Y.; Liu, S. X.; Yan, S. R.; Dickinson, C.; Zhou, W. Z.; He, H. Y. Chem. Commun. 2005, 5618. (31) Cao, L.; Lu, M.; Li, H. L. J. Electrochem. Soc. 2005, 152, A871.

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