Low-Temperature Synthesis of Monodisperse 3D Manganese Oxide

Dec 10, 2008 - Ping Wang , Yu-Jing Zhao , Li-Xiong Wen , Jian-Feng Chen , and Zhi-Gang Lei ... Yu-Ting Wang , An-Hui Lu , Hui-Li Zhang , and Wen-Cui L...
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J. Phys. Chem. C 2009, 113, 54–60

Low-Temperature Synthesis of Monodisperse 3D Manganese Oxide Nanoflowers and Their Pseudocapacitance Properties Jipeng Ni, Wencong Lu,* Liangmiao Zhang, Baohua Yue, Xingfu Shang, and Yong Lv Department of Chemistry, Shanghai UniVersity, Shanghai, 200444, People’s Republic of China ReceiVed: July 22, 2008; ReVised Manuscript ReceiVed: October 26, 2008

Monodisperse manganese oxide flowerlike nanostructures have been prepared facilely at low temperature and ambient atmosphere. The effect of the reaction time on the microstructure and morphology is observed systemically by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Meanwhile, the possible formation mechanism of the flowerlike nanostructures has been proposed and discussed. It is also found that the reaction temperature has great influences on the morphology of these unique nanostructures. The results of nitrogen adsorption-desorption experiments and electrochemical measurements show that the product obtained at 40 °C for 8 h has large specific surface area, uniform pore size distribution, and excellent capacitance performance, which make it a potential supercapacitor electrode material. 1. Introduction Recently, much attention has been paid to three-dimensional (3D) nanostructures because of their structural complexities and great potential applications, such as MnO2, Al2O3, TiO2, SnO2, and Fe3O4 hollow spheres and other nanostructures.1-7 And a number of methods have been demonstrated for synthesis of 3D nanostructures with well-controlled shapes and sizes, such as hydrothermal reaction,8 chemical vapor deposition (CVD),9,10 thermal vapor transport and condensation,11,12 electrochemical deposition,13 etc. Although various techniques have been employed to synthesize 3D nanostructures, many of them demand stringent reaction conditions such as high temperature, low or high pressure, either in the vapor phase, etc.8-13 Thus, it is of great significance to develop facile, mild, and easily controlled methods to fabricate novel nanostructures. Manganese dioxides have attracted special attention due to their distinctive properties and wide applications in catalysis, ion exchange, molecular adsorption, biosensors, and particularly, energy storage.14-21 3D nanostructured manganese oxide of various morphologies have so far been prepared by several research groups. The Xie group22 and Subramanian et al.23 had synthesized urchinlike MnO2 nanostructures and porous MnO2 spherical nanostructures, respectively, via a hydrothermal method. Li et al.24 prepared MnO2 sphere networks via a homogeneous catalytic route. Even 3D dendritic nanostructures and dandelion-like microspheres have been synthesized by the Suib group25 via a hydrothermal method recently. However, few reports have been focused on the formation of monodisperse MnO2 3D flowerlike nanostructures. In addition, until now the investigation of the application of 3D nanostructured manganese dioxide as electrode materials still remains limited, although the shape and size of inorganic materials are generally believed to influence their properties and performances.26,27 Herein, we provide a simple and efficient low-temperature (40 °C) reduction route to fabricate monodisperse MnO2 3D flowerlike nanostructures. At the same time, its possible * To whom correspondence should be addressed. Phone: +86-2166133513. Fax: +86-21-66134080. E-mail: [email protected].

formation mechanism and the electrochemical performances are presented and discussed. 2. Experimental Section 2.1. Synthesis. All the reagents used in the experiment were of analytical purity and were purchased from Shanghai Chemical Reagent Company (P. R. China) and used without further purification. In a typical synthesis, 2 mmol of potassium permanganate (KMnO4) was dissolved in 160 mL of deionized water. And then 5 mL of formamide (HCONH2) was added into the above solution under stirring. The mixed solution was further stirred vigorously for 5 min before it was kept static in a water bath at 40 °C for 8 h. The black product was collected by centrifugation and washed with deionized water and alcohol to remove any possible residual reactants. Finally, the product was dried under a vacuum at 60 °C for 10 h. 2.2. Characterization. The X-ray diffraction (XRD) patterns were recorded on a Japan Rigaku D/Max-RB X-ray diffractometer with Cu KR radiation (λ ) 1.54178 Å). Transmission electron microscopy (TEM) images were taken on a JEOL JEM200CX with an accelerating voltage of 200 kV, and the highresolution transmission electron microscopy (HRTEM) image was taken on a JEOL JEM-2010F with an accelerating voltage of 200 kV. Selected-area electron diffraction (SAED) was also taken on a JEOL JEM-2010F field emission transmission electron microscope. Fourier transform infrared (FTIR) spectra were obtained on an AVATAR370 spectrometer. The nitrogen adsorption and desorption isotherms at 77 K were measured with a Micrometrics ASAP 3000 analyzer. Before measurement, the samples were degassed in vacuo at 200 °C for at least 6 h. 2.3. Electrochemical Measurements. The working electrodes of electrochemical capacitors were formed by mixing the prepared powder with 15 wt % acetylene black conductor and 5 wt % poly(tetrafluoroethylene) (PTFE) binder of the total electrode mass. A small amount of distilled water was then added to those mixtures to make them more homogeneous. The mixtures were pressed onto nickel foam current collectors (1.0 × 1.0 cm2) to fabricate electrodes. All electrochemical measurements were done in a three-electrode experimental setup. Electrochemical tests for the nano-MnO2 electrode were con-

10.1021/jp806454r CCC: $40.75  2009 American Chemical Society Published on Web 12/10/2008

Monodisperse 3D Manganese Oxide Nanoflowers

Figure 1. (a) Typical XRD pattern and (b) FTIR spectrum of the asprepared products.

ducted using a three-electrode system in which the MnO2 electrode, activated carbon electrode, and Hg/HgO (1 M LiOH) electrode were served as working, counter, and reference electrodes, respectively, and 1 M LiOH aqueous solution served as electrolyte. Cyclic voltammetry was carried out on a Solartron 1287 electrochemical interface. Galvanostatic charge/discharge tests were performed on a LAND autocycler (China). All the work was done at room temperature. 3. Results and Discussion Figure 1a shows the XRD patterns of the as-prepared products. Two main broad reflections at 2θ ) 37.4° and 65.6° indicate that the samples are poorly crystalline MnO2 (JCPDS 42-1169).28,29 More characteristics of MnO2 are also observed in its FTIR spectrum (Figure 1b). The broad band at about 3404 cm-1 could be attributed to O-H bond vibrations from the residual hydroxy groups,30 and the 1632 and 1402 cm-1 bands are normally attributed to O-H bending vibrations combined with Mn atoms,31 whereas the intense band observed at 573 cm-1 should be ascribed to the Mn-O vibrations in MnO6 octahedra.32 The IR result indicates the presence of some bound water in the MnO2 sample. The as-prepared products obtained at 40 °C for 8 h were examined by TEM and HRTEM with the SAED pattern of the nanostructures, and their typical images are displayed in Figure 2. Figure 2a reveals that the flowerlike nanostructures, which have an average size of about 35 nm, are highly yielded and monodisperse. Figure 2b shows a typical enlarged TEM image for the nanostructures, composed of a number of lamellar platelets. It should be pointed out that a large quantity of small nanoparticles was adsorbed on some of the nanoflowers, which is also supported by the HRTEM image (Figure 2c). The SAED pattern of the MnO2 particles (Figure 2d) shows broad diffused

J. Phys. Chem. C, Vol. 113, No. 1, 2009 55 polycrystalline rings, which also indicates the nanoflowers have poorly crystalline structure, coincidence well with the XRD result.33,34 To understand the formation of the novel MnO2 flowerlike nanostructures, products were collected after different reaction times. Figure 3 shows the typical TEM images of the sample prepared at different reaction times at 1/12, 1/6 h, while other conditions are kept unchanged. After 1/12 h of redox reaction, the color of the solution almost does not change, and only a small amount of solid product was collected. As shown in Figure 3a, the obtained small nanostructures were composed of smaller nanoparticles. After 1/6 h of redox reaction, the color of the solution was still purple-red, and also only a small amount of solid was obtained. At this stage, the obtained small nanostructure, average size of about 30 nm, had evolved its morphology into flowerlike nanostructures which was composed of a number of lamellar platelets (Figure 3b). After 8 h, the color of the solution changed from purple-red to yellow-brown, showing that more MnO2 was formed. At this stage, the collected monodisperse nanostructures, average size of about 35 nm, were flowerlike structures with a large quantity of small nanoparticles adsorbed on them, which had been discussed above (Figure 2). These images clearly show, with prolonged reaction times after 1/6 h, the structure of the monodisperse nanoflowers did not change much. The formation process may be explained as follows: once HCONH2 was added into KMnO4 aqueous solution, MnO2 nuclei formed immediately, which acted as the precursor. The chemical reaction can be formulated as

2MnO4- + 3HCONH2 + 4H2O f 2MnO2 V + 3CO2 v + 3NH4+ + 5OH-(1) Large amounts of the newly formed MnO2 nuclei would experience a rapid aggregation process and then fuse into each other to form flowerlike nanostructures, reducing the total energy. As the reaction going on, the freshly formed nanoparticles were adsorbed on the surface of these primary flowerlike nanostructures. To reduce system energy, some of the fresh adsorbed smaller nanoparticles fused into the flowerlike nanostructures,35 so the nanostructures obtained for 8 h were a little larger than the products obtained for 1/6 h. It is surprising that syntheses carried out on time scales ranging from 1/6 to 8 h yield products with almost similar nanostructures. This suggests a fast flowerlike structure forming process. On the basis of the above discussions, we believed that the MnO2 nanoflowers are formed through the “aggregation-fusion-adsorption-fusion” process. A schematic image of the proposed growth process is shown in Scheme 1. The reaction temperature profoundly influences the morphology and size of the samples. Interestingly, the morphology of the product changed from monodisperse nanoflowers to the coexistence of nanowires and flowerlike nanostructures when the reaction temperature increased from 40 to 99 °C and the other experimental conditions remained unchanged. The nanostructures obtained at 80 °C show that a few nanowires start to emerge from the flowerlike nanostructures (Figure 4a). This tendency is more pronounced at 99 °C with the ratio of nanowires increasing at the expense of the nanoparticles (Figure 4c). As the reaction temperature increased, the flowerlike nanostructures have the tendency of becoming nanowires. It is rationally speculated that high temperature affords more energy, which is beneficial to the oriented growth of nuclei. The diffusion of smaller MnO2 nanoparticles at low temperature is

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Figure 2. (a) Low-magnification TEM image, (b) enlarged magnification TEM image, (c) HRTEM image, and (d) SAED pattern of monodisperse MnO2 flowerlike nanostructures obtained at 40 °C for 8 h.

Figure 3. TEM images of monodisperse MnO2 3D flowerlike nanostructures obtained at 40 °C after redox times of 1/12 h (a) and 1/6 h (b), respectively.

slower than that at high temperature, which is favorable to the aggregation and adsorption of some smaller MnO2 nanostructures.36 So the monodisperse flowerlike MnO2 nanostructures can be obtained. However, when the temperature increases, thermal disturbance becomes violent. This indicates that high temperature increases the rate of mass transport (i.e., diffusion, migration, and convection) more drastically than the rate of deposition.37 It is favorable to the growth of nuclei along the low-energy direction on the surface of the flowerlike nanostructures, so the nanowire MnO2 is obtained. Figure 4, parts b and

d, shows the XRD patterns of the as-prepared products obtained at 80 and 99 °C, respectively. The XRD patterns indicate that the samples are also poorly crystalline MnO2 (JCPDS 42-1169), consistent with the samples obtained at 40 °C (Figure 1a). In the recent years, MnO2 has attracted much attention as a cathode electrode for supercapacitors because of its low cost and low toxicity.38-43 There are two mechanisms proposed for charge storage in MnO2. To crystalline samples of MnO2, the mechanism involves intercalation/extraction of protons (H3O+) or alkali cations such as Li+, Na+, K+, and so forth into the

Monodisperse 3D Manganese Oxide Nanoflowers

J. Phys. Chem. C, Vol. 113, No. 1, 2009 57

Figure 4. TEM images of the products prepared at different reaction temperatures for 8 h: (a) 80 °C and (c) 99 °C. XRD patterns of the products from different reaction temperatures: (b) 80 °C and (d) 99 °C.

Figure 5. Discharge performance of MnO2 monodisperse nanoflowers (prepared at 40 °C for 8 h) and nanowires (prepared at 99 °C for 8 h) at a constant current density of 1000 mA/g.

bulk of oxide particles with concomitant reduction/oxidation of the Mn ion.44,45

MnO2 + M+ + e- h MnOOM(M+ ) Li+, Na+, K+, or H3O+) (2) To amorphous samples of MnO2, the mechanism is mainly a surface process, which involves the adsorption/desorption of alkali cations.46

(MnO2)surface + M+ + e- h (MnOOM)surface(M+ ) Li+, Na+, K+, or H3O+) (3) To study the potential application of the monodisperse MnO2 flowerlike nanostructures (obtained at 40 °C for 8 h) as an

Figure 6. Cycle life of the monodisperse MnO2 nanoflowers and nanowires electrodes prepared at 40 and 99 °C for 8 h at 1000 mA/g in 1 M LiOH electrolyte; the inset shows charge/discharge curves of the monodisperse nanoflowers electrode in the potential range from 0 to 0.8 V at 1000 mA/g.

electrode material in manganese dioxide/activated carbon hybrid (MnO2/AC) supercapacitors, the discharge curves of the MnO2 samples prepared at different reaction temperatures with different morphologies, monodisperse nanoflowers (obtained at 40 °C for 8 h) and nanowires (obtained at 99 °C for 8 h), were measured in the voltage range of 0-0.8 V versus Hg/HgO at a constant current density of 1000 mA/g. Figure 5 shows the discharge curves of the MnO2 samples prepared at different reaction temperatures with different morphologies as cathode materials in the first discharge cycle. Notably, for the monodisperse MnO2 flowerlike nanostructures prepared at 40 °C for 8 h, a high capacity of 121.5 F/g was obtained in the first discharge process, whereas the discharge capacity of nanowires

SCHEME 1: Schematic Illustration of the Formation Process of Monodisperse MnO2 Flowerlike Nanostructures

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Figure 7. TEM images of monodisperse MnO2 nanoflowers (a) and nanowires (b) after 500 charge/discharge cycles.

obtained at 99 °C is 117.9 F/g. The results suggest that the special morphology of the monodisperse MnO2 nanoflowers and nanowires both have remarkable effects on its electrochemical performance in MnO2/AC supercapacitors. The long-term cycling stability of the composite electrodes made of the monodisperse nanoflowers (obtained at 40 °C for 8 h) and nanowires (obtained at 99 °C for 8 h) was investigated, and the variation of specific capacitance over 500 cycles is depicted in Figure 6. The specific capacitance values of the both electrodes decreased gradually. After 500 cycles of operation, the electrode could maintain ∼71% of its initial capacitance for monodisperse nanoflower MnO2 under such a very high current density of 1000 mA/g, but the electrode of the nanowires only maintained ∼58% of its first discharge capacitance. The charge/discharge process involves the alkali cations’ intercalation/extraction and adsorption/desorption. The alkali cations’ insertion results in the expansion of the lattice, which will destroy both the nanoarchitecture and the crystalline structure, thus inducing poor performance after a number of charge/discharge cycles.47 Figure 7 shows the typical TEM images for the monodisperse nanoflowers (Figure 7a) and nanowires (Figure 7b) after 500 charge/discharge cycles. After 500 cycles of operation, most of the nanoflowers maintain their morphology and are still monodisperse, but no nanowires are observed. This demonstrates that, within the voltage window of -0.2 to 0.8 V, the nanostructure of monodisperse nanoflowers have better endurance than nanowires for pseudocapacitance reactions.48 The excellent endurance and long-term stability imply that the MnO2 nanoflowers are good candidates as a material for supercapacitor electrodes. The galvanostatic charge/ discharge profile of the monodisperse MnO2 nanoflowers electrode in 1 M LiOH solution at room temperature is also presented in Figure 6. It can be seen that the charge profile is slightly curved, suggesting a pseudocapacitive characteristic. The typical rectangle-like shape of all cyclic voltammogram (CV) curves (Figure 8) measured at various scan rates in 1 M LiOH solution reveals the perfect electrochemical capacitive behavior of monodisperse MnO2 nanoflowers formed by reaction at 40 °C for 8 h. The curves at different scan rates show no peaks, indicating that the electrode is charged and discharged at a pseudoconstant rate over the complete voltammetric cycle, coinciding well with the galvanostatic charge/discharge results. It is well-known that, within the same crystalline state, the particle size and the surface area of the electrode dramatically to some extent affect the alkali cations’ adsorption/desorption rate and capacity. A high surface area can reduce the alkali cations’ adsorption/desorption rate density per unit area, which also delays the capacity loss associated with the concentration polarization to higher current density. Thus, identifying the

Figure 8. CV curves of MnO2 nanoflowers prepared at 40 °C for 8 h at scan rates of 1 and 5 mV/s (from inside to outside).

specific surface area and pore structure of the monodisperse MnO2 nanoflowers is essential to the understanding of its higher capacity. Figure 9 shows the N2 adsorption-desorption isotherms and the Barrett-Joyner-Halenda (BJH) pore size distribution curves (inset) of monodisperse MnO2 nanoflowers (Figure 9a) and nanowires (Figure 9b). The recorded adsorption and desorption isotherms for the monodisperse nanoflowers and nanowires exhibit a similar significant hysteresis loop. BJH calculations for the pore size distribution, derived from desorption data, present a bimodal distribution centered at 3 and 18 nm for monodisperse nanoflowers (Figure 9a, inset); however, the bimodal distribution centered at 3 nm is not obvious for nanowires (Figure 9b, inset). The smaller pores presumably arise from the interstitial spaces between nanoparticles within one nanoflower. The larger pores are possibly attributed to the interstitial spaces between nanostructures (i.e., nanoflowers or nanowires). The specific surface area, total pore volume, and average pore diameter for monodisperse nanoflowers and nanowires of MnO2 are listed in Table 1. Although nanoflowers and nanowires exhibit the same type of adsorption-desorption isotherms, their surface areas and pore size distributions are different. The specific surface area of 225.93 m2/g and total pore volume of 0.93620 cm3/g obtained for MnO2 nanoflowers are greater than the specific surface area of 219.17 m2/g and pore volume of 0.67372 cm3/g obtained for MnO2 nanowires. These differences indicate that MnO2 nanoflowers are more porous than MnO2 nanowires, and hence, the special high BET surface area and mesoporous structure of the monodisperse MnO2 nanoflowers provide the possibility of efficient transport of electrons in the MnO2/AC supercapacitors, which leads to the high electrochemical capacity of the monodisperse MnO2 nanoflowers.

Monodisperse 3D Manganese Oxide Nanoflowers

J. Phys. Chem. C, Vol. 113, No. 1, 2009 59 capacitance performance, which is attributed to the large specific surface area and uniform pore size distribution. Acknowledgment. Financial support to this work from the National Natural Science Foundation of China under Grant 20503015 and from the Shanghai Leading Academic Discipline Project (project no. J50101) is gratefully acknowledged. The authors thank Professors Bo Lu, Weijun Yu, Yuliang Chu, and Qiang Li from the Instrumental Analysis Center of Shanghai University for their technical assistance on XRD, TEM, SEM, and HRTEM measurements. The authors also thank Professor Liuming Yan from the Chemistry Department of Shanghai University for his valuable advice to this work. References and Notes

Figure 9. N2 adsorption-desorption isotherms and BJH pore size distribution plots (inset) of MnO2 monodisperse nanoflowers (a) and nanowires (b).

TABLE 1: Specific Surface Area and Total Pore Volume of Nanoflower and Nanowire MnO2

morphologic form

specific surface area (m2 g-1)

total pore volume (cc/g)

average pore diameter (Å)

monodisperse nanoflowers nanowires

225.93 219.16

0.93620 0.67372

159.917 132.01

4. Conclusion In summary, a facile and efficient low-temperature reaction route was provided to fabricate monodisperse MnO2 flowerlike nanostructures on a large scale. At the same time, the possible formation mechanism of aggregation-fusion-adsorption-fusion is proposed and supported by HRTEM and TEM images. It was also found that high temperature affords more energy, which is beneficial to the oriented growth of nuclei. The electrochemical study indicated that the discharge capacity of the electrode material is sensitive to the morphology; monodisperse MnO2 flowerlike nanostructures discharged with a high capacity of 121.5 F/g in the first charge-discharge process attributed to its high surface area of 225.93 m2/g and mesoporous structure. The present electrode material of monodisperse MnO2 nanoflowers also exhibited a good cycle performance under a high current density of 1000 mA/g. These results indicate that monodisperse MnO2 nanoflowers can be expected to be used in MnO2/AC supercapacitors. Monodisperse manganese oxide nanoflowers have been prepared facilely at low temperature (40 °C) and ambient atmosphere. The possible formation mechanism of the flowerlike nanostructures has been proposed and discussed. The electrode of the monodisperse manganese oxide nanoflowers has excellent

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