Hydrothermal Synthesis of Hematite Nanoparticles and Their

Jul 9, 2012 - A simple hydrothermal process for fabrication of hematite (α-Fe2O3) nanostructures with narrow size distribution was developed by using...
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Hydrothermal Synthesis of Hematite Nanoparticles and Their Electrochemical Properties Maiyong Zhu,†,‡ Ying Wang,† Dehai Meng,† Xingzhang Qin,† and Guowang Diao*,† †

College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P. R. China School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, P. R. China

‡

S Supporting Information *

ABSTRACT: A simple hydrothermal process for fabrication of hematite (α-Fe2O3) nanostructures with narrow size distribution was developed by using PVP as surfactant and NaAc as precipitation agent. The influence of experimental parameters including the concentration of the precursor, precipitation agent, stabilizing agent, and reaction time was systematically investigated to study the possible formation mechanism of α-Fe2O3. Finally, the electrochemical properties of the obtained hematite particles were studied using cyclic voltammetry and galvanostatic charge−discharge measurement by a three-electrode system. The results reveal that their specific capacitances are related to their sizes. By virtue of large surface area, the as-prepared hematite nanoparticles can present the highest capacitance (340.5 F·g−1) and an excellent long cycle life within the operated voltage window (−0.1 to 0.44 V), demonstrating that the as-prepared hematite nanoparticles can serve as one of the most excellent electrode materials for supercapacitors.

1. INTRODUCTION Owing to their unique size- and shape-dependent properties including optical, magnetic, chemical, and electrical behavior, nanostructured materials have attracted more and more interest in recent years.1−3 Especially, semiconductor nanostructures have been identified as important materials with widespread potential applications in miniaturized connectors,4 catalysts,5−7 gas sensors,8 microwave absorption,9 nanodevices, and drug delivery.10 Among all semiconductor nanomaterials, hematite (α-Fe2O3), an n-type semiconductor (Eg = 2.1 eV), is the most thermodynamically stable species of iron oxides. Hematite has been intensively studied in gas sensors,11 electromagnetic devices, solar cells,12 magnetic storage media, Li-ion batteries,1113−16 water splitting, 17−19 environmental treatment,13,20−22 and optical devices because of its nontoxicity, biodegradability, high resistance to corrosion, and low processing cost, and it can be used as a starting material to synthesize magnetite and γ-Fe2O3,which have been extensively purchased for both fundamental scientific interest and technological applications in many fields.23 Alongside the development of alternative energy sources, energy storage is still one of the greatest challenges in the 21st century. Nowadays, supercapacitors, a type of energy storage device that can supply extremely high but transient output, have been a topic of great current interest. They are applied for numerous power source applications such as auxiliary power sources for hybrid electric vehicles and short-term power sources for mobile electronic devices.24−27 Typically, supercapacitors are classified into two types depending on the © 2012 American Chemical Society

charge-storage model: electrical double-layer (EDL) supercapacitance and pseudocapacitance.25 The former comes from the charge accumulated at the electrode/electrolyte interface, the latter from the reversible redox reaction of electroactive materials.28 Compared to other energy storage devices, supercapacitors have some attractive advantages. First of all, electrode requirements are less in supercapacitors than that in batteries. Furthermore, they show higher specific power and longer cycle life than rechargeable batteries, and higher specific energy compared to conventional capacitors. The performance of supercapacitors to a great extent depends on the nature of the electrode material.29,30 Generally speaking, there are three categories of materials that have been developed as supercapacitors’ electrode materials: carbon-based materials, conducting polymers, and transition metal oxides. Carbon materials, such as activated carbons and carbon nanotubes, usually exhibit good stability but limited EDL capacitance.31−33 Conducting polymers have proven to exhibit excellent pseudocapacitance. Unfortunately, the poor cycling stability during charge/discharge processes and long response time limit their applications.34,35 Among transition metal oxides, hydrous ruthenium dioxide (RuO2) has been considered to be the most promising electrode material because of its multiple redox states and good electrical conductivity.36,37 The high cost and Received: April 26, 2012 Revised: July 7, 2012 Published: July 9, 2012 16276

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further purification. The water used in this study was deionized by milli-Q Plus system (Millipore, France), whose electrical resistivity is 18.2 MΩ. 2.2. Synthesis of Hematite Nanoparticles. The hydrothermal method was employed to synthesize hematite nanoparticles. In a typical procedure, 4 mmol FeCl3·6H2O, 40 mmol NaAc, and 1.0 g of PVP were first dissolved in 30 mL of distilled water and stirred for 2 h at 40 °C. Then, the mixture was transferred into a Teflon-lined stainless-steel autoclave and sealed for heating at 200 °C for 18 h. After the reaction, the autoclave was cooled naturally. The red solid products were collected by centrifugation and washed with distilled water and ethanol three times, respectively. Finally, the red products were dried under vacuum at 70 °C for 12 h. The quality of the product can be controlled by adjusting the amount of FeCl3·6H2O, NaAc, and PVP or the reaction time. 2.3. Characterization. The transmission electron microscopy (TEM) measurements were taken on a Tecnai-12 microscope (Philips) operated at 120 kV. The samples were dispersed in ethanol before analysis. High-resolution transmission electron microscopy (HRTEM) images of the samples were taken using a Tecnai-G2 F30 S-TWIN microscope (Philips) operating at 300 kV. The magnetic measurements were carried out on a vibrating sample magnetometer (VSM; EV7, ADE). UV−visible diffuse reflectance spectra were recorded at room temperature on a Lambda-850 UV−vis spectrophotometer (Perkin-Elmer). Powder X-ray diffraction (XRD) patterns were recorded on a D8 Advance (super speed) XRD diffractometer (Bruker). The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo Escalab 250 system using Al Kα radiation (hν = 1486.6 eV). The test chamber pressure was maintained below 2 × 10−9 torr during spectral acquisition. Fourier Transform Infrared (FTIR) Spectrometer was recorded with a Tensor 27 spectrometer (Bruker). 2.4. Electrochemical Testing. The fabrication of the working electrode was performed as follows. Typically, the electroactive materials (α-Fe2O3), graphene powder, and poly(vinylidene difluoride) (PVDF) were mixed in a mass ratio of 70:15:15 and dispersed in ethanol to form a homogeneous slurry. Then the mixture was pressed onto a nickel grid and dried at 80 °C for 12 h. For electrochemical testing, a beaker-type three-electrode cell was fabricated using the as-prepared electrode as working electrode, platinum wire electrode as counter electrode, and a saturated calomel electrode (SCE) as the reference electrode, respectively. The electrolyte used was 1 M aqueous KOH solution. All electrochemical experiments were carried out on an electrochemical workstation (CHI 660C, Chenhua, Shanghai).

toxic nature of this noble metal is a big hurdle for practical uses.38 Much effort has been devoted to purchasing more costeffective transition metal oxides to replace RuO 2 for pseudocapacitive electrode. As one of the most important transition metal oxides, hematite has been reported to be a promising electrode material for supercapacitors. For example, Wu et al. have developed hematite nanostructured films electrodeposited anodically onto nickel substrate for supercapacitors. They found that the capacitance of the film is much related to their film morphologies.39 Long and co-workers described a simple electroless deposition of conformal nanoscale iron oxide (FeOOH) on carbon nanoarchitectures for supercapacitors.40 Very recently, Muruganandham et al. have reported the electrochemical performance of various morphologies of hematites.41 Xie et al. have prepared highly ordered hematite nanotube arrays, having high specific capacitance (138 F·g−1 at 1.3 A·g−1), remarkable rate capability (91 F·g−1 at 12.8 A·g−1), and excellent cycling stability (the capacitance retention close to 89% after 500 cycles).42 It is wellknown that the effectiveness of nanostructures depends on not only the particles’ specific composition, but also morphology, crystallinity, surface property, and size. That of hematite in supercapacitance is not exceptional. Therefore, the design and synthesis of α-Fe2O3 with controlled shape and narrow size distribution will continue to be a hot subject. To date, a wealth of chemical and physical methods has been developed for the synthesis of hematite nanostructures with various shapes.43−46 These methods include sonochemical route,47 hydrothermal process,48,49 thermal decomposition of inorganic precursors,50−52 pyrolysis of Fe(acac)3,53 electrospinning,54 and microemulsion-based method.55,56 Of all above methods, the hydrothermal process was considered as the most robust one, and many groups developed this method. Lian et al. have synthesized hematite with various morphologies including mesoporous hollow microspheres, microcubes, and porous nanorods by an ionic liquid-assisted hydrothermal synthetic method.57 Chu et al. have synthesized ringlike hematite nanoparticles via a hydrothermal process using a redox reaction of Fe2+ and S2O82− in solution in the presence of poly(ethylene glycol).58 Wang et al. have prepared hematite nanocubes with a broad size distribution by the decomposition of an iron-oleate complex under hydrothermal conditions, and well-defined assembly of uniform hematite nanocubes with an average size of 15 nm can be obtained after a size selection process.59 In this article, an environmentally friendly hydrothermal route to fabricate hematite nanoparticles was reported. The morphology and the microstructure of the product were investigated in detail. The effects of some experimental parameters including reaction time, and the concentration of the precursor, stabilizing agent, and precipitation agent on the morphology and size of the product were studied systematically. The possible formation mechanism of hematite nanoparticles was presented. Furthermore, the electrochemical properties of the as-prepared hematite nanostructures were also investigated.

3. RESULTS AND DISCUSSION 3.1. Characterization of Hematite Nanoparticles. The hydrothermal method is one of the most robust procedures for fabrication of various metal oxide nanostructures for its easy conduction and high quality of the products. Many groups have developed this strategy for the preparation of hematite with controlled morphology and size. Figure 1 shows the morphology and crystal structure of hematite prepared in this work. As is shown in Figure 1A, B, polyhedronal morphology of the as-prepared hematite is in large quantity, and the average size is ∼40 nm. Figure 1 C shows a HRTEM image of a single hematite nanoparticle. The single particle has a size in the range of 30−40 nm. The lattice-resolved HRTEM image of part of a

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Poly(vinylpyrrolidone) (PVP, average molecular weight is 58 000) was purchased from Aladdin Reagent Company (Shanghai, China). All other chemicals, provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China), were analytical grade and used without 16277

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Figure 2. Typical XRD pattern of hematite nanoparticles.

where d is the mean diameter of hematite nanoparticles, K is a constant equaling 0.89 here, λ is the wavelength of the X-ray radiation (Cu Kα = 1.541 nm), β is the full width at halfmaximum, and θ is the angle at maximum peak position. The average particle diameter of hematite was calculated to be 51.4 nm, which is well consistent with the size observed by TEM (see Figure 1A and B). FT-IR spectroscopy was used to characterize the as-prepared hematite nanoparticles. Figure 3 shows the spectra of hematite

Figure 1. (A, B) Typical TEM image of Fe2O3 nanoparticles with different magnifications, (C) representative HRTEM image of α-Fe2O3 nanoparticles, (D) lattice-resolved HRTEM image of the boxed region of part C, and (E) EDS spectra of the as-prepared Fe2O3 nanoparticles.

Figure 3. FTIR spectra of the as-prepared PVP-stabilized Fe2O3 nanoparticles (a) and the stabilizing agent PVP (b).

hematite nanoparticle is shown in Figure 1D. The interplanar spacing was measured to be about 0.367 nm, which is in good agreement with the (012) crystal planes.60 Meanwhile, the EDS spectra of the as-prepared hematite nanoparticles were also investigated as shown in Figure 1E. The signal of Cu came from the substrate of the copper grid. The atom ratio of Fe/O was 30:70, which is smaller than the pure Fe2O3. The reason may be indexed to the adsorption of PVP molecules onto the surface of the as-prepared hematite nanoparticles to avoid the aggregation of hematite. Therefore, the percentage of O in the product is larger than pure hematite. X-ray diffraction technique can yield a great deal of structural information and phase identification about materials under investigation. The XRD pattern in Figure 2 confirmed formation of hematite. All XRD peaks can be unambiguously indexed to the rhombohedral phase of hematite (JCPDF no. 84-0307). No diffraction peaks of any other impurities were detected, indicating high purity and good crystallinity of the synthesized hematite. In addition, according to Debye− Scherrer equation d=

(curve a) and pure PVP (curve b). The significant difference between hematite and pure PVP lies in the low-wavenumber region. As seen in Figure 3a, the absorption peak at 545.8 cm−1 was indexed to Fe−O. The other peaks centered at 1652.9, 1461.9, and 1288.4 cm−1 were assigned to PVP adsorbed on the surface of hematite nanoparticles. However, these peaks are much weaker compared with that in pure PVP (seen in Figure 3b). The reason might be the small amount of PVP on the surface of hematite. It is noted that the peaks of PVP in our product shifted toward low wavenumber compared to pure PVP, indicating there may be certain chemical interaction between PVP and hematite. In addition, XPS (X-ray photoelectronic spectroscopy), one of the strongest techniques in characterization of materials, was employed to further reveal the structural information of the product. As shown in Figure 4a, besides Fe and O, there are also C and N in the product. The origination of N and C may be attributed to PVP, the capping agent. The high-resolution spectra of Fe and O are shown in Figure 4b and c. It is easy to observe that there are three peaks for Fe, 723.6, 718.2, and 710.5 eV from Figure 4b, which could be attributed to Fe2O3.

kλ β cos θ 16278

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Figure 4. XPS spectra of the as-prepared α-Fe2O3 nanoparticles (a) and the high-resolution XPS spectra of Fe2p (b).

It is well-known that iron oxides are famous magnetic materials, and α-Fe2O3 is not an exception. Magnetic hysteresis measurement for the as-prepared α-Fe2O3 nanoparticles is recorded on a vibrating sample magnetometer (VSM). The hysteresis curve is shown in Figure 5. It is noted that the

Figure 6. UV−visible spectra of the as prepared α-Fe2O3 nanoparticles.

3.2. Effect of Various Experimental Parameters. 3.2.1. Effect of Precursor Concentration. Different conditions were examined to understand the influence of various experimental parameters on the size and morphology of hematite. First, the effect of precursor concentration was studied. TEM images in Figure 7 clearly indicated that the concentration of the precursor influences the quality of the asprepared hematite in two aspects: size and dispersion. On one hand, as shown in Figure 7A, the size of the hematite obtained at very low concentration is very small. The diameter of hematite increases with the concentration of the precursor (see Figure 7A−E). On the other hand, the dispersion of hematite is much related to the concentration of the precursor. It is clear that the dispersion of hematite is poor when the concentration of the precursor is too low or too high. Some particles aggregate together as seen in Figure 7A and 4E. The case, however, is good when the concentration of the precursor lies between those of Figure 7A and 4E, as shown in Figure 7B and 4D. The reason for the above phenomenon can be explained by Von Weimarn theory, a general rule for the nucleation and growth of nanocrytals in solution system. It is noted that the precipitation agent (40 mmol) is excessive compared to the precursor (less than 10 mmol) in our system. A burst nucleation will occur at a very low concentration of Fe3+ in sample A, leading to aggregation of the resultant hematite particles. The same case may take place when the concentration of Fe3+ is very large, such as sample E. This result can be explained by the fact that the higher concentration of Fe3+, the larger the number of the outcome hematite particles in the

Figure 5. Magnetic hysteresis of α-Fe2O3 nanoparticles.

hysteresis loop does not reach saturation even up to the maximum applied magnetic field. From the partially enlarged inset of Figure 5, the magnetization measurement of the sample exhibits a hysteretic feature with the remanent magnetization (Mr) and coercivity (Hc), which were determined to be 0.02 emu/g and 66.8 Oe, respectively, indicating that the asprepared α-Fe2O3 nanoparticles show ferromagnetic behaviors at room temperature. However, the remanent magnetization of the as-prepared α-Fe2O3 nanoparticles is much smaller than that of commercial α-Fe2O3 (0.6 emu/g).61 The reason might be associated with the fine spherical shape of the α-Fe2O3 nanoparticles. As for the coercivity, it is relatively low, which might be caused by its small size and microstructure. Optical property of the as-prepared α-Fe2O3 nanoparticles is measured by UV−visible absorption spectra as shown in Figure 6. It is noted that the as-prepared α-Fe2O3 nanoparticles occur as absorption peaks at 308, 373, 486, and 538 nm. The first peak may be assigned to metal to ligand charge transfer spectra. The peaks centered at 373 and 486 nm correspond to 6A1 → 4E and 2(6A1) → 2( 4T1) ligand field transition of Fe 3+, respectively. As for the peak at 538 nm, it is in good agreement with the fingerprint region of the band edge of hematite. Above experimental results further reveal that the sample obtained in this work was pure α-Fe2O3.46 16279

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Figure 7. TEM images of hematite formed at different amounts of FeCl3·6H2O: (A) 2, (B) 4, (C) 6, (D) 8, (E) 10 mmol. Other conditions: 40 mmol of NaAc; 1.0 g of PVP; 30 mL of H2O; reaction temperature and time are 200 °C and 18 h.

Figure 8. TEM images of hematite prepared with different amounts of NaAc: (A) 10, (B) 15, (C) 20, (D) 30, (E) 40 mmol. Other conditions: 4 mmol of FeCl3.6H2O, 1.0 g of PVP, 30 mL of H2O, 200 °C for 18 h.

growth step may last for a comparable time. Keeping the defined concentration of the precipitation agent, the number of the nanocrystal nuclear is completely dependent on the concentration of the precursor. In other words, the higher precursor concentration leads to more nanocrystal nuclear. Also, the amount of the precursor for growth step increases

system. As for their size, it may be related to the formation process of the hematite nanoparticles. It is well-known that the process for the formation of nanoparticles in liquid phase sometimes is divided into two steps: nucleation of primary nanocrystals and growth of the nanocrystals. Typically, the nucleation step occurs within a very limited time, and the 16280

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Figure 9. TEM images of hematite obtained by using different amounts of PVP: (A) 0.00, (B) 0.25, (C) 0.50, (D) 0.75, (E) 1.00 g. Other conditions: 4 mmol of FeCl3.6H2O, 40 mmol of NaAc, 30 mL of H2O, 200 °C for 18 h.

with its concentration. So the sizes of samples from Figure 7A to Figure 7E increased in trend with the precursor concentration. 3.2.2. Effect of Precipitation Agent Concentration. Another important factor influencing the quality of hematite possibly is the concentration of NaAc, a common precipitation agent in preparation of metal oxides. The role of the precipitation agent is similar to the precursor. The typical TEM images of hematite obtained with different amounts of NaAc used are shown in Figure 8A−E. At a glance, it appears from Figure 8 that NaAc in this work mainly affects the size of the obtained hematite particles. As displayed in Figure 8, the larger size of the obtained hematite particles is preferred at a lower NaAc concentration. Especially, the size of the hematite lies in the micrometer range when the amount of NaAc used in the system is less than 20 mmol (Figure 8A and B). It seems that the size of hematite shows no vigorous change when the amount of NaAc is over 20 mmol (seen Figure 8C−E). These above regular phenomenons can also be explained by Von Weimarn theory well. At lower concentration of NaAc (less than 20 mmol), the nucleation step for the fabrication of hematite may be prevented. Under such circumstances, the growth step is preferable since there are enough precursors for it, leading to larger hematite particles. By increasing the amount of NaAc, the amount of the precursor consumed in the nucleation step increased as well. As a result, the amount of Fe3+ for the growth step is relatively less than that in samples of Figure 8A, B. 3.2.3. Effect of PVP. Besides the concentration of the precursor and the precipitation agent, the surfactant effects were also investigated in this work. Here, five samples with different amounts of PVP were prepared to figure out the role of PVP. It was found that the final quality of the product was affected by the concentration of PVP. As shown in Figure 9 A,

the dispersity of hematite nanoparticles was not satisfied when no PVP was used in the system. As the concentration of PVP increases, the hematite nanoparticles with uniform size and good dispersity can be obtained (Figure 9B−D). The size distribution became wide if the amount of PVP in the system was as much as 1.00 g, as shown in Figure 9E.The reason might be the capping effect of PVP. It is well-known that the growth of nanostructures was related to the selective adsorption organic surfactant, also called capping effect, onto particular crystallographic facets of the growing crystal.62−64 When the concentration of PVP is very low or there is no PVP in the system, the capping effect was not sufficient for the effective coverage or passivation of α-Fe2O3 crystals, which results in aggregation of the particles. If the surfactant concentration is suitable, those adsorbed on the crystals may prevent the aggregation of the particles for electrostatic repulsive or stero− barrier interaction. As for high PVP concentration, two results may occur. For one thing, surfactant in the system may selfassemble to generate some micelles or reverse micelles, which cannot induce the crystal with desired size or other properties. For another thing the capping effect was too strong to affect the growth of α-Fe2O3. 3.3. Possible Formation Mechanism. To study the growth mechanism of hematite nanoparticles, effect of reaction time on the product quality was studied by collecting samples at every interval. All hydrothermal reactions were conducted at 200 °C. As shown in Figure 10, the phase of the product is much dependent on reaction time. Yellow suspension can be fabricated if the reaction time is less than 0.5 h. The solid product in the yellow suspension could be isolated by centrifugation, which was verified to be FeOOH by XRD as shown in Figure 10a. With prolonging the reaction time, hematite could be obtained. When the reaction time was 1 h, all peaks of the XRD pattern could be attributed to hematite as 16281

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been determined to be α-Fe2O3 by XRD (Figure 10b). If the reaction time is no more than 6 h, the morphology and size distribution could not be greatly improved. The size ranged from ca. 20 nm to ca. 50 nm, and the particle became stronger than that collected only after reaction of 1 h, as shown in Figure 11C, D. As the reaction proceeded, the diameter of hematite nanoparticles continues to grow. The corresponding TEM images of the samples obtained at 12 h (Figure 11 E) showed that the size of the most particles was ca. 50 nm, although some smaller ones still existed. It is concluded from Figure 11F that the size distribution could be further improved if the reaction time was 12 h. In this study, most of experiments were set up at reaction time of 18 h since the size and morphology were more satisfied as shown in Figure 11F. As discussed above, the possible formation mechanism of hematite nanostructures may be illustrated as in the following:

Figure 10. XRD patterns of products obtained at different reaction times: (a) 0.5, (b) 1, (c) 2 h. Other conditions: 4 mmol of FeCl3·6H2O, 40 mmol of NaAc, 30 mL of H2O, 1.0 g of PVP, 200 °C.

revealed in Figure 10b. However, the intensity of the product was not very strong. This might be partly because the crystallization is not complete. With the reaction time processing, the characteristic peaks of hematite became more intense, as seen in Figure 10c. The above phenomenon reveals that the growth of the α-Fe2O3 nanocrystals is very fast under this system. Meanwhile, the morphologies of the products were also much dependent on reaction time. The typical TEM images of the time-dependent product were shown in Figure 11. It is noted that if the reaction is only a half hour, the size of the product is very small, and it is difficult to figure out its morphology, as shown in Figure 11A. This conclusion is well in agreement with the formation of FeOOH suspension, which has been confirmed before. The sample collected 1 h later (Figure 11B) showed particle shape with no unique morphology and wide size distribution. The crystalline has

CH3COO− + H 2O → CH3COOH + OH−

(1)

Fe3 + + 3OH− → FeOOH + H 2O

(2)

2FeOOH → Fe2O3 + H 2O

(3)

−

First, OH was produced by the hydrolysis of CH3COO−, which is a common step for the fabrication of metal oxides under hydrothermal system when sodium acetate acts as the precipitation agent. Second, the yellow FeOOH suspension was formed through the reaction between Fe3+ and OH−. The formation of FeOOH has been confirmed by the XRD measurement in Figure 10 during the hydrothermal process. Finally, the α-Fe2O3 nanoparticles were obtained by the dehydration of FeOOH. As for the role of PVP, it is not only the stabilizer of the α-Fe2O3 nanoparticles to protect them from aggregate but also the structure-directing agent to control the size as well as the morphology of the products.

Figure 11. TEM images of α-Fe2O3 nanoparticles collected after different reaction times: (A) 0.5, (B) 1, (C) 2, (D) 6, (E) 12, (F) 18 h. Other experimental conditions: 4 mmol of FeCl3·6H2O, 40 mmol of NaAc, 30 mL of H2O, 1.0 g of PVP, 200 °C. The scale bar is 50 nm. 16282

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3.4. Electrochemical Properties. Hematite nanostructures have been widely studied as one of the most promising electrode materials for supercapacitors due to their high conductivity at room temperature. Generally, their capacitance is mainly derived from the pseudocapacitive charging and discharging based on the following reversible redox reaction: Fe2O3 + OH− ↔ Fe2O3OH + e−

CS = iΔt /(mΔE)

(5)

where i refers to the charge (or discharge) current, Δt is the discharge time, m is the mass of the supercapacitive material, and ΔE is the potential window. The relationship between CS and the current density is shown in Figure 13b. It is easy to note from Figure 13b that the higher the current density is, the larger CS. The supercapacitance is 340.5 F·g−1 when the charge current is 1 A·g−1. Charge−discharge measurements are critical in the analysis and prediction of the active materials performance under practical operating conditions. Figure 14 shows the CD cycling

(4)

Here, the electrochemical properties of the as-prepared hematite particles were examined by a three-electrode system. The cyclic voltammetry (CV) curves of the as-prepared hematite nanoparticles at different scan rates are shown in Figure 12. It can be clearly found that there is a distinct pair of

Figure 14. Galvanostatic charge−discharge curves of α-Fe2O3 (50 nm) electrode at current density of 1 A·g−1.

Figure 12. CV curves of the electrodes made from the hematite particles for supercapacitors application in the range of −0.1 to 0.5 V at various scan rates.

behavior at the rates of 1 A·g−1 in the potential range between 0 and +0.44 V (vs saturated calomel electrode). It is observed that the curves are nonlinear and unsymmetrical, indicating the typical characteristic of an ideal capacitor behavior. Meanwhile, the voltage drop was found to be very small, suggesting the low internal resistance of the electrode. The long-term cycle stability of supercapacitors is very important for practical applications. Therefore, in the present study, the stability of the supercapacitor devices is evaluated by conducting galvanostatic charge−discharge measurements for 500 cycles in the potential range between 0 and +0.44 V. The specific capacitance as a function of cycle number is presented in Figure 15. The results indicate that the capacitors exhibit excellent cycle life.

redox peaks during the anodic and cathodic sweeps. The shape of the CV curves suggests a pseudocapacitive characteristic, distinctly different from normal electric double-layer capacitance with a rectangular CV shape. In addition, we have examined the supercapacitive performance of the as-prepared hematite nanoparticles by constant current discharge (CD) testing. Figure 13a shows the discharge curves of the as-prepared α-Fe2O3 electrode at different current density. The specific capacitance (CS), in faraday per gram, based on CD cycling technique can be calculated through the following equation

Figure 13. (a) Discharge curves of α-Fe2O3 (40 nm) electrode at different current density and (b) the relationship between the specific capacitance and the current density. 16283

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shown to exhibit high specific capacitances of 340.5 F·g−1 at the current density of 1 A·g−1. The initial capacitances can be retained well even after 500 cycles. Meanwhile, the electrochemical capacitance property is greatly dependent on their size. In a word, the results obtained in this paper suggest promising application of hematite as electrode materials for high-performance supercapacitors.

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ASSOCIATED CONTENT

S Supporting Information *

Discharge curves, specific capacitance values versus current density, initial charge−discharge cycles, and specific capacitance values versus cycle number of hematite sample sized in ∼1 μm. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 15. Specific capacitance change at a constant current of 1 A·g−1 as a function of cycle number.

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As comparison, the supercapacitance property of hematite sized in ∼1 μm (corresponding to Figure 8B) was also tested in this work. The discharge curves (Figure S1A) of the hematite microparticles sized in 1 μm displays. It is clear that the hematite sample (∼1 μm) exhibits shorter discharge time than hematite sample (∼40 nm) at the same current density, which signifies worse charge storage performance of large-sized hematite. The specific capacitance values at different current densities were calculated by eq 5 as shown in Figure S1B. It is noted that the specific capacitance values increase with the current density. This phenomenon is similar to hematite sample sized in ∼40 nm. The calculated specific capacitance value for 1 μm hematite at the current density of 1 A·g−1 was 170.8 F·g−1. Also, the cyclic performance and stability of 1 μm hematite electrode material were investigated in the supercapacitance device. The curves are nonlinear and unsymmetrical (Figure S2A), and the voltage drop was observed to be very small. The above results are very similar to hematite sample sized in ∼40 nm, suggesting the typical characteristic of an ideal capacitor property with low internal resistance of the electrode. The cycling stability of 1 μm hematite electrode was investigated at a current density of 1 A·g−1 over 500 cycles (Figure S2B). With the first-cycle capacitance of 170 F·g−1, and the capacitive retention is excellent per cycle, indicating excellent cyclability. From the above analysis, the performance of our synthesized hematite samples as electrode materials is affected greatly by their size. This phenomenon could be well explained as follows. It is widely accepted that there are two mechanisms proposed for the charge storage in nanostructured metal oxides.65 The first one mainly describes a surface process, which involves adsorption/desorption of protons H+ or alkali metal cations such as K+ from the electrolyte. The other one is based on intercalation/deintercalation of H+ or alkali cations into the bulk oxide particles with concomitant reduction/oxidation of the manganese cations. In the present study, the redox process is mainly governed by the adsorption and desorption of K+ and or H+ from the electrolyte into the hematite matrix. So the smaller the size of hematite, the larger the specific capacitance value obtained.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-514-87975436. Fax: +86-514-87975244. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant no. 20973151), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20093250110001), and the Foundation of Jiangsu Key Laboratory of Fine Petrochemical Technology.

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4. CONCLUSIONS In the present work, a facile hydrothermal route to hematite nanoparticles with narrow size distribution with FeCl3 serving as iron source and NaAc as precipitation in the presence of PVP is shown. The quality of the product depends on the experimental conditions, such as the concentration of the precursor, capping agent, and precipitation agent, as well as the reaction time. The as-prepared hematite nanoparticles are 16284

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