Synthesis of iron oxide nanoparticles at low bath temperature: Characterization and energy storage studies

Materials Science in Semiconductor Processing 16 (2013) 1837–1841

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Synthesis of iron oxide nanoparticles at low bath temperature: Characterization and energy storage studies Taher Yousefi a,n, Ahmad Nozad Golikand b, Mohammad Hossein Mashhadizadeh c a b c

NFCRS, Nuclear Science and Technology Research Institute, P.O. Box 11365/8486, Tehran, Iran Materials Research School, NSTRI, P.O. Box: 14395-836, Tehran, Iran Department of Chemistry, Tarbiat Moallem University, P.O. Box: 31979-37551, Tehran, Iran

a r t i c l e i n f o

Keywords: Iron oxide Nanoparticles Cathodic Electrodeposition

abstract Ferric oxide powders were cathodically electrodeposited at a fixed bath temperature 8 1C. The obtained powders were characterized by X-ray diffraction (XRD), scanning electron microscopy(SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and thermogramimetric analysis (TGA). Scanning electron microscopic examination of the resultant powders revealed the formation of iron oxide nanoparticles with a grain size of approximately 20 nm. The adhesive deposit is obtained by reducing bath temperature. The increased adhesion is believed to result from the reducing kinetics energy of molecules and the rate of gas bubbling the electrode surface. & 2013 Elsevier Ltd. All rights reserved.

1. Introduction Supercapacitors or electrochemical capacitors have been known for over 50 years as the potential energy storage systems the same as batteries. The commonly investigated materials for supercapacitors are transition metal oxides and conducting polymers [1]. Metal oxides show pseudocapacitance due to the Faradaic reactions between the solid material and the electrolyte depending on the voltage. In energy storage area, among various metal oxides, iron oxide is the transition metal oxide which has received increasing attention due to its low cost, abundance, and low environmental impact [2]. Until now, 16 distinct phases of iron oxides, i.e. oxides, hydroxides or oxy-hydroxides are known. These are Fe(OH)3, Fe(OH)2, Fe5HO8⋅4H2O, Fe3O4, FeO, five polymorphs of FeOOH and four of Fe2O3. Characteristics of these oxide compounds include mostly the trivalent state of the iron, low solubility and brilliant colors [3]. All iron oxides are crystalline except schwertmannite and ferrihydrite

which are poorly crystalline. Iron(III) oxides are among the most common iron compounds found in nature, which are readily synthesized. Hematite, α-Fe2O3, is the most stable of the iron oxides in ambient conditions. However, there are a number of polymorphs of the iron(III) oxide system: α-Fe2O3 (hematite); γ-Fe2O3 (maghemite); ε-Fe2O3; β-Fe2O3; and amorphous Fe2O3 [4]. In summary, iron oxides are the most important transition metal oxides of technological importance and various kinds of processes, including, as sol–gel [5], chemical precipitation [6] hydrothermal [7], and electrochemical methods (cathodic and anodic deposition) [8–10], have been used to produce nanometer-sized Fe2O3. In anodic formation of iron oxides, different phases of the iron oxides– oxyhydroxides were obtained by adjusting deposition potentials and solution composition [8] herein we present the cathodic route to synthesize Fe2O3 nanoparticles. 2. Experimental 2.1. Preparation of Fe2O3 nanoparticle

n

Corresponding author. Tel.: +98 9124865930. E-mail address: [email protected] (T. Yousefi). 1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.06.018

Iron oxide precursor was deposited directly on both sides of the stainless steel cathode (316 L, 20
20
0.5 mm3)

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under a galvanostatic mode at a cathode current density of 0.5 mA cm  2. The electrodeposition bath was Fe(NO3)3  4H2O (0.005 M, Merck) aqueous solution which was fixed at 8 1C. After electrodeposition, the deposited film was rinsed several times in deionized water and dried at room temperature for 48 h. Thereafter, the as-deposited sample was scraped from the steel electrode and subjected to further analysis. Thermal annealing was conducted in air between the room temperature and 150 1C at a heating rate of 10 1C min  1. 2.2. Characterization The obtained products (thermal annealed) were characterized by X-ray diffraction (XRD, Phillips, PW-1800), scanning electron microscopy (LEO 1455VP) and transmission electron microscopy (TEM, Phillips EM 2085). Fourier transform infrared (FTIR) spectroscopy of the sample was recorded with a KBr pellet on a VECTOR-22 (Bruker) spectrometer ranging from 400 to 4000 cm  1. 2.3. Electrochemical measurements Cyclic voltammetry (CV) was utilized to determine the electrochemical properties and specific capacitance of the fabricated iron oxide electrode in Na2SO3 (0.25 M) by means of an Autolab 302 electrochemical workstation. In a three electrode cell configuration, a platinum wire, an Ag/AgCl and a glass carbon with 0.071 cm2 area were used as counter, reference and working electrodes, respectively. Cyclic voltammetry (CV) studies were conducted within a range of  0.75– 0.15 V vs. Ag/AgCl at the scan rates of 2–5 mV s  1. The SC was calculated from integrating CV curve areas as follows: C¼

I qðdv=dtÞ

ð1Þ

where I is the average current (A), dv/dt voltage scan rate (V s  1) and q mass of the active material (g). 3. Result and discussions 3.1. TGA and DSC Fig. 1a and b. shows the TGA and DSC data for the asdeposit sample. The sample showed a total weight loss of 23 wt% in the temperature range up to 700 1C most of which achieved below 200 1C (Fig. 1a). No weight change was observed in the range of 400–700 1C. The weight loss in this region would be attributed to the liberation of the adsorbed water. Our TGA results were different from those of the literature and the total weight loss here was higher [11], in other words, at low temperature synthesis due to low kinetics of reactions, the water molecules inserted into the deposit were substantial. Also at low temperature the rate of gas elevation on the electrode surface was low and this caused an increase of water insertion to sample. The corresponding DSC data (Fig. 1b) showed a broad endothermic peak around 80 1C which was related to the endothermic process of water liberation from deposit. DSC data showed a small abroad exotherms at ∼270 1C and a relative sharp peak at ∼430 1C. The first associated with formation

Fig. 1. Thermogravimetric analyses: (a) TGA, (b) DSC of as-prepared sample.

of the γ-Fe2O3, and the second related to recrystallization of γ-Fe2O3–α-Fe2O3 [12,13]. 3.2. XRD and FTIR Fig. 2a shows the X-ray diffraction pattern of the nanoparticles calcined sample at 150 1C. Nanoparticles have two main diffraction peaks at 361 (311) and 631 (440). The X-ray diffraction intensity of (311) and (440) can be attributed to Fe2O3. Although the diffraction peaks are significantly broad because of the effect of the particle size, the XRD pattern confirmed that Fe2O3 was formed by cathodic electrodeposition. Fig. 2b shows the FTIR spectra of calcined sample within the wavelength range 400–4000 cm  1. The peak at 3500 cm  1 indicates the presence of hydroxide group [14]. The peaks observed at 1630 and 1040 cm  1 are attributed to O–H vibration modes. The peaks in the frequency region from 485 to 565 cm  1 are attributed to the characteristic Fe–O vibrations [14]. 3.3. SEM and TEM The morphologies of the nanoparticles characterized by SEM and TEM are shown in Fig. 3. The SEM images in Fig. 3 (a and b) show a smooth and uniform particle shape morphology of iron oxide. High magnification by TEM (Fig. 3c and d) revealed that the sample was composed

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of uniform and monodispersed spherical nanoparticles. The particles size was about 20 nm.μ

3.4. Electrodeposition mechanism The cathodic electrodeposition mechanism of Fe2O3 nanostructures from ferric nitrate aqueous solution is proposed as follows: (i) the reduction of water, dissolved oxygen and nitrate ions at the cathode, inducing an increase of OH  concentration; (ii) the formation of metal Fe(OH)3 on the substrate and the transformation from Fe (OH)3 to Fe2O3 [15–20]. In solution, the following electrochemical reactions: 



NO3 +H2O+2e-NO2 +2OH  

+

(2)

NO3 +7H2O+8e-NH4 +10OH 

(3)

O2+2H2O+4e-4OH 

(4)

2H2O+2e-H2+2OH 

(5)

may occur at the cathode [16–20]. These reactions cause an increase in local pH on the surface of the cathode and formation of ferric hydroxide Fig. 4. Ferric hydroxide later converts to form Fe2O3. The overall reaction can be shown as: Fig. 2. (a) XRD spectra and (b) FTIR patterns of the sample.

6H2O+6e-3H2+6OH 

Fig. 3. SEM (a–b) and TEM (c–d) of sample.

(6a)

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2Fe3++6OH  -2Fe(OH)3

(6b)

2Fe(OH)3-Fe2O3+3H2O

(6c)

2Fe3++3H2O+6e  -Fe2O3+3H2(g)

(6d)

Due to the low concentration of dissolved oxygen and nitrate ions in solution, it seems that reaction (4) is dominant in the increase of local pH and hydroxide formation. The low adhesion of deposits to the electrode surface and their spallation have been reported as common difficulties during electrogeneration of base in aqueous medium [11]. A solution to this problem is the application of lower bath temperature than room temperature which can offer important advantages such as control of the kinetic energy of solvents and deposit molecules-in other words, at lower temperature the kinetic energy of molecules is low and the adhesion of deposit is firm, thus the spallation can be prevented. Also at low temperature the rate of gas bubbling at electrode surface is reduced and the spallation of deposit into electrolyte would decrease. Another applied trick for the reduction of the deposit spallation was addition of a solvent with low dielectric constant to the electrolyte. In this way, the solvation and separation strength of electrolyte reduced and this caused lower spallation of the deposit. So we added methanol to the electrolyte to reduce the dielectric constant of solvent. 3.5. Electrochemical characterization The iron oxide nanoparticles were used in the supercapacitor and their performances were tested by means of cyclic voltammogram (CV) technique in 0.25 M Na2SO3 electrolyte. The working electrode was prepared by mixing 80 wt% active material, 15 wt% acetylene black as conductive filler and 5 wt% PTFE as a binder. A small amount of water was then added to those composites to make more homogeneous mixtures, which were pressed on the glass carbon electrode. Fig. 5 shows the iron oxide CV curves within the potential range of  7.5–0.15 V versus Ag/AgCl at different scan rates (2, 5, 10, 25 and 50 mV s  1). The maximum of specific capacitance measured by CV was 87.4 at the scan rate of

Fig. 5. CVs of the sample in different scan rates in 0.25 M Na2SO3 electrolyte. The inset is variation of specific capacitance with the scan rate.

2 mV s  1.The inset in Fig. 5. shows the variation of specific capacitance with the scan rate – in other words, the specific capacitance value decreases with increasing scan rate. The decreasing trend of the capacitance suggests that parts of the surface of the electrode are inaccessible at high charging – discharging rates, which is probably due to the diffusion effect of electrolyte within the electrode. The supercapacitor can be charged and discharged virtually at unlimited number of times. Unlike the electrochemical battery, which has a defined life-cycle, there is little wear and tear by cycling a supercapacitor. Under normal conditions, a supercapacitor fades from the original 100% capacity to 80% in 10 years. Applying higher voltages than specified ones shortens the life. The cycle charge–discharge test was carried out to examine the service life of the iron oxide electrode by cyclic voltammetry in 0.25 M Na2SO3 solution. The charge–discharge test of the electrode was repeated 1000 times. The electrode showed fairly satisfactory electrochemical stability and the capacitance decreased only 10% after 1000 charge– discharge cycles. 4. Conclusion

Fig. 4. Cathodic deposition of iron hydroxide/oxide.

The electrodeposition of Fe2O3 from an aqueous solution containing ferric nitrate produces a film of excellent adhesion through a very simple process. A serious problem during the cathodic electrodeposition of iron oxide arose from the splashing of deposit into electrolyte due to its low adhesion. To overcome this problem, the temperature of electrodeposition bath was reduced to 8 1C. The low temperature increases the adhesion of deposit by reduction of kinetics energy of molecules and the rate of gas elevation at electrode. By following this route, nanoparticles of iron oxide were successfully prepared. The sample was characterized by XRD, FTIR, TGA, SEM, and TEM. The average crystalline size distribution of iron oxide was about 20 nm. The results of this work showed that low-

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