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Facile synthesis of cobalt doped hematite nanospheres: Magnetic and their electrochemical sensing properties
Materials Chemistry and Physics 134 (2012) 590e596
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Facile synthesis of cobalt doped hematite nanospheres: Magnetic and their electrochemical sensing properties R. Suresh a, R. Prabu a, A. Vijayaraj a, K. Giribabu a, A. Stephen b, V. Narayanan a, * a b
Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Maraimalai Campus, Chennai 600025, Tamil Nadu, India Department of Nuclear Physics, University of Madras, Guindy Maraimalai Campus, Chennai 600025, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2010 Received in revised form 9 March 2012 Accepted 11 March 2012
Nanocrystalline pure a-Fe2O3 and Co-doped a-Fe2O3 powders were synthesized by the hydrolysis method. The structure and the morphology of the samples were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The magnetic property of the samples was studied by vibrating sample magnetometer (VSM) at room temperature, which showed that the Co-doped a-Fe2O3 have a weaker ferromagnetic behavior than the pure a-Fe2O3. The electrochemical sensing ability of ascorbic acid (AA) and uric acid (UA) by pure a-Fe2O3 and Co-doped a-Fe2O3 modified glassy carbon electrode (GCE) exhibited higher anodic current response with a shift in positive potential than the bare GCE. Compared with pure a-Fe2O3, Co-doped a-Fe2O3 showed enhanced electrochemical sensing performance. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Annealing Powder diffraction Magnetic properties
1. Introduction Functional metal oxide nanocrystals have been extensively investigated in the recent decade for their outstanding new properties suitable for a broad spectrum of downstream application [1e3]. Among them, nanoparticles of iron oxides are important materials due to their biocompatibility, catalytic activity and low toxicity. They have significant application in various fields such as drug delivery system [4], cancer therapy [5], magnetic resonance imaging [6], high density magnetic storage devices [7], ferro-fluids [8], rechargeable lithium batteries [9], catalysis [10], gas sensor [11] and biosensor [12], etc. Iron oxide mainly occurs in three different forms namely FeO, Fe2O3 and Fe3O4. Fe2O3 has four crystallographic phases namely, a-Fe2O3 (hematite), b-Fe2O3, g-Fe2O3 (maghemite) and 3-Fe2O3 [13]. The g-Fe2O3 and Fe3O4 have inverse spinel structure and superparamagnetic property. The a-Fe2O3 is the most stable iron oxide which has antiferromagnetism with canting ferromagnetic responses at room temperature. It has a complex defect structure in which three types of defect species namely, oxygen vacancies, Fe3þ and Fe2þ interstitials are present [14]. It has a wide range of applications such as photocatalyst for N2 fixation
* Corresponding author. Tel.: þ91 44 22202793; fax: þ91 44 22300488. E-mail address: [email protected] (V. Narayanan). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.034
[15], sorbents [16] and sensors [17], etc. As motivated by the novel properties and potential application of nanosized iron oxide, synthesis of a-Fe2O3 in nanometer scale has attracted more attention [18,19]. Many methods have been reported for the synthesis of a-Fe2O3 including forced hydrolysis [20], combustion [21], anhydrous solvent [22], sol-gel [23], wet chemical synthesis [24], microwave-hydrothermal synthesis [25e27], and spray pyrolysis [28,29], etc. Among them, forced hydrolysis of Fe3þ plays an important role in natural system. This method does not need the addition of any precipitator. Doping of different metal ions or metal oxides into Fe2O3 will find new application or improve the performance of existing applications. For instance, Niu et al. [30] found that the material mixed with rare earth oxides through a sol-gel method in citric acid system presented high gas sensitivity to gasoline. Jing et al. [31] also reported that the Co-doped g-Fe2O3 exhibits higher gas sensitivity and better selectivity to acetone and ethanol than the pure g-Fe2O3. It is noted that until now most of the researchers focused mainly on doping of metal ion into g-Fe2O3 while doping of metal ion in a-Fe2O3 and their electrochemical sensing properties have not been reported yet. In this paper, we have reported the preparation of Co-doped aFe2O3 nanopowder, by using simple hydrolysis of FeCl3 method. The prepared nanoparticles have been used to modify the GCE and remarkable enhancement in the current response with shift in anodic peak potential is observed for electrochemical oxidation of AA and UA when compared to the bare electrode.
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2. Experimental 2.1. Synthesis of a-Fe2O3 and Co-doped a-Fe2O3 nanospheres Anhydrous ferric chloride (FeCl3), hydrated cobaltous chloride (CoCl2∙6H2O) and potassium chloride (KCl) were purchased from Qualigens and used without further purification. Ascorbic acid and uric acid were purchased from Sigma and used as received. Doubly distilled water was used as the solvent. Co-doped a-Fe2O3 powders were prepared as follows [32]. The aqueous solution of anhydrous FeCl3 was heated to 100 C for 16 h. Then the yellow product obtained, FeOOH, was separated from the mother liquor with an ultra-speed centrifuge and washed with distilled water three times. The obtained yellow powder was annealed at 600 C for 6 h, and the product a-Fe2O3 was obtained. Co-doped a-Fe2O3 nanostructures were synthesized by the same method. For the preparation of Co-doped a-Fe2O3, CoCl2 salt was added to the aqueous solution of FeCl3 in four different molar percentages, 5%, 10%, 15%, and 20%, respectively. The solutions were refluxed for 16 h at the temperature of 100 C. The obtained yellow powders were annealed at 600 C for 6 h. Thus we obtained 5e20% Co-doped a-Fe2O3 nanostructures. 2.2. Characterization details The prepared samples were studied by FTIR spectroscopy using a Schimadzu FT-IR 8300 series instrument in the range of 400e4000 cm1. One milligram of each powder sample was diluted with 200 mg of vacuum-dried IR-grade KBr powder and subjected to a pressure of 10 tons. The Raman spectroscopy used in this study was the BRUKER RFS 27: Stand alone FT-Raman Spectrometer equipped with Nd: YAG 1064 nm as an excitation source. The structures of the samples were analyzed by a Rich Siefert 3000 diffractometer with Cu-Ka1 radiation (l ¼ 1.5406 Å). The UVeVis absorption spectra were obtained on a CARY 5E UVeVISNIR Spectrophotometer. The morphology of the materials was analyzed by SEM and FE-SEM using a HITACHI S600N scanning electron microscopy and a HITACHI SU6600 field emissionscanning electron microscopy, respectively. The BET specific surface area (S.S.A.) was measured using a Micromeritics (ASAP 2020) analyzer. The magnetic studies were carried at room temperature using VSM: EG & G Princeton, Applied Research Model 4500 instrument. The electrochemical experiments were performed on a CHI 600A electrochemical instrument using the asmodified electrodes and bare GCE as working electrode, a platinum wire was the counter electrode, and saturated calomel electrode (SCE) was the reference electrode.
Fig. 1. FTIR spectrum of (a) pure a-Fe2O3 and (b) a-Fe2O3(5%Co).
absorption band in the region of 3461, 1642, 1580, 1383, 574, 579, 481 and 485 cm1. The general range of 3600e3100 cm1 may be assigned for water of hydration. Hydrate also absorbs in the range of 1670e1600 cm1 [33]. This later band can be taken as another important means of identifying water of crystallization. In Fig. 1(a), the band associated with the lattice water molecule is broad, and is observed in a region of 3461 and 1642 cm1. The simultaneous presence of these two bands indicates that the water of crystallization is present in the sample. The bands at 574 and 481 cm1 in Fig. 1(a) and at 579 and 485 cm1 in Fig. 1(b) are due to the FeeO stretching vibrational modes [34]. These two bands are sharp and are of strong intensity. The intensity of the broad peaks at 3600e3000 cm1 is found to be decreasing from Fig. 1(a) to (b). The shifts observed for the above samples may be due to the particle size effect [35]. The Raman spectrum of the pure a-Fe2O3 and a-Fe2O3(5%Co) are shown in Fig. 2(a) and (b), respectively. Fig. 2(a) shows the peaks at 225, 242, 294, 411 and 499 cm1 correspond to the a-Fe2O3 phase [36], namely two A1g modes (225 and 499 cm1) and three Eg modes (242, 294 and 411 cm1). Fig. 2(b) is similar to that of Fig. 2(a). However, it shows additional peaks at 488, 519 and 680 cm1 which are characteristic of cobalt oxide [37]. It suggests that the cobalt oxide may be well dispersed in the lattices of aFe2O3. Fig. 3(a) and (b) represents the XRD pattern of pure a-Fe2O3 and a-Fe2O3(5%Co), respectively. In Fig. 3(a), the diffraction peaks are all closely matched with the standard a-Fe2O3 reflections (JCPDS No. 33-0664). It revealed that the synthesized a-Fe2O3 has an orthorhombic structure. In the case of 5%Co doped sample (Fig. 3(b)), no
2.3. Preparation of the Co-doped a-Fe2O3 modified GCE is as follows Ultrasonic agitation for 30 min was used to disperse 1 mg of Codoped a-Fe2O3 into 5 mL of acetone to make a reddish brown and homogeneous suspension. The polished GCE was coated with 5 mL of the above reddish brown suspension. The modified electrode was activated in a 0.1 M KCl solution by successive cyclic scans between 0 and þ1.2 V. Before and after each experiment, the modified electrode was washed with distilled water and reactivated by the method mentioned above. 3. Results and discussion 3.1. Structural characterization The FTIR spectrum of the pure a-Fe2O3 and a-Fe2O3(5%Co) are given in Fig. 1(a) and (b), respectively. Fig. 1(a) and (b) shows the
Fig. 2. Raman spectrum of (a) pure a-Fe2O3 and (b) a-Fe2O3(5%Co).
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ahn ¼ A(hn Eg)n . Where a is the absorption coefficient, A is a constant, h is Planck’s constant, n is the photon frequency, Eg is the band gap, and n is equal to 1/2 or 2 for the transition being direct and indirect, respectively. An extrapolation of the linear region of a plot of (ahn)2 versus hn, gives the value of the band gap of the samples. The estimated band gap was found to be 2.0 eV for aFe2O3(5%Co) and 2.29 eV for pure a-Fe2O3 which is in good agreement with a band gap value of 2.2 eV for Fe2O3 [39]. The steep absorption edge in both the samples is indicative of the uniform particle morphology and size with fairly good crystallinity [33]. The SEM micrograph of a-Fe2O3(5%Co) is shown in Fig. 5(a). It shows that the particles are in spherical shape. The nanostructure of the a-Fe2O3(5%Co) is confirmed by FE-SEM observations (Fig. 5(b)): the spherical particles have a diameter in the range of 60e90 nm. The energy dispersive spectroscopy (EDS) of the aFe2O3(5%Co) shows that the nanospheres consist of iron, cobalt and oxygen (Fig. 5(c)). The metal ion mapping (Fig. 5(d)) also confirmed the presence of Co homogenously dispersed in the a-Fe2O3 matrix. Fig. 3. XRD pattern of (a) pure a-Fe2O3 and (b) a-Fe2O3(5%Co).
additional peak is observed. However, all the peaks in the XRD pattern of doped sample are essentially of a-Fe2O3 which is in good agreement with the reported value in literature [38]. No phase containing Co additive was observed within the detection limit of the apparatus. However, all the peaks are slightly shifted to higher 2q values with increasing percentage of cobalt doping compared to pure a-Fe2O3 which is due to the smaller radius of cobalt compared to iron. The XRD peaks of pure a-Fe2O3 and a-Fe2O3(5%Co) show broadening indicating the ultrafine nature of the particles. The average crystallite size of pure and Co-doped a-Fe2O3 was calculated by Scherrer’s formula using the most intense diffraction peak at w33 (104). The estimated crystallite size of pure a-Fe2O3 is 49 nm and that of a-Fe2O3(5%Co) is 29 nm. It can be found that the crystallite size of the a-Fe2O3(5%Co) turns smaller, which means that the addition of Co2þ can effectively suppresses the a-Fe2O3 crystalline grain growth. Fig. 4(a) and (b) represents the UVevisible spectra of pure a-Fe2O3 and pure a-Fe2O3(5%Co), respectively. It is known that the fingerprint region of the band edge of hematite is 521e565 nm, whereas in Co-doped and pure a-Fe2O3 the band edge observed ranged from 560 to 570 nm. This is in good agreement with the XRD and FTIR results, i.e., all the samples are a-Fe2O3. In order to calculate the band gap, the Tauc’s relationship was used as follows:
3.2. Surface area measurement Nitrogen adsorptionedesorption measurements were conducted to determine the BET specific surface area of the nanospheres. To obtain a a-Fe2O3 powder with high specific surface area, we have to use a lower concentration of Fe3þ and a lower reaction temperature. However, the concentration we used here is 0.5 M with higher reaction and annealing temperatures than the reported values [40]. The S.S.A. of the pure a-Fe2O3 is 4.95 m2 g1 whereas the S.S.A. of the a-Fe2O3(5%Co) is 8.13 m2 g1. These results showed that the addition of 5% Co increased the specific surface area of the sample. 3.3. Magnetic property Fig. 6 illustrates the hysteresis loops of pure and 5% Co-doped aFe2O3. The hysteresis loop of pure a-Fe2O3 (Fig. 6(a)) shows weak ferromagnetic as already observed in literature [41]. It can be seen that the a-Fe2O3 nanospheres show weak ferromagnetic behavior with a magnetization of 51.2 102 emu g1 and a coercivity of 1159 Oe at room temperature. The values of magnetization and coercivity are different from those of other morphologies of Fe2O3 [42e44]. The higher magnetization and coercivity observed in the present study must be associated with the unique structure of the aFe2O3 nanospheres. However, the hysteresis loop of 5% Co-doped aFe2O3 (Fig. 6(b)) shows weak ferromagnetic behavior with a magnetization of 48 102 emu g1 and a coercivity of 863 Oe at room temperature. The variation of saturation magnetization of a-Fe2O3 and that of doped a-Fe2O3 with various concentration of cobalt is shown in Fig. 7. The Fig. 7 shows that, for 5% Co-doped a-Fe2O3, the saturation magnetization increases up to an applied field of 5100 Oe, after that it decreases slightly in comparison to pure a-Fe2O3 nanospheres. The above results indicate that for 5% Co-doped a-Fe2O3, cobalt ions are homogenously distributed in the a-Fe2O3 lattices. In case of 10e20% Co-doped samples, the saturation magnetization decreases gradually when compared with pure a-Fe2O3 nanospheres. It may be probably due to the presence of adjacent cobalt ions (at higher concentrations of cobalt) in different oxidation states (2þ/3þ) with antiferromagnetic coupling [45] in a-Fe2O3 crystallites. 3.4. Electrochemical sensing properties
Fig. 4. DRS UVevisible absorption spectrum of (a) a-Fe2O3 and (b) a-Fe2O3(5%Co).
3.4.1. Electrochemical oxidation of ascorbic acid at the modified electrode Fig. 8 shows the cyclic voltammograms (CVs) of 4 mM AA for bare and a-Fe2O3(0e20%Co) nanospheres modified GCE at a scan
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Fig. 5. SEM (a) and FE-SEM, (b) micrographs of a-Fe2O3(5%Co), (c) EDS spectrum of a-Fe2O3(5%Co), and (d) metal ion mapping of a-Fe2O3(5%Co).
rate of 50 mV/s. The electrochemical oxidation of AA proceeds through 2Hþ, 1e process and leads to a radical anion. The radical anion undergoes a fast one electron irreversible oxidation to form electro-inactive dehydroascorbic acid. It is rapidly protonated and then dehydrated to form 2,3-diketogluconic acid. At the bare GCE the CV of AA (Fig. 8(a)) shows an oxidation peak potential at 780 mV. At a-Fe2O3(0e20%Co) modified GCE (Fig. 8(bef)), well defined oxidation waves of AA were obtained. From Fig. 8, it can be seen that the current response of AA varied with the amount of added Co. It also shows that the AA anodic potential at the modified electrode was shifted to less positive direction when compared with that at the bare GCE. The AA oxidative current at the modified
electrode was largely increased relative to that at the bare GCE, indicating the AA electrocatalytic ability of the modified electrode. Especially the a-Fe2O3(5%Co) modified GCE shows higher anodic peak current response when compared with the pure a-Fe2O3 and the a-Fe2O3(10e20% Co) modified GCE. This electrocatalytic effect was attributed to the (i) larger available specific surface area of the modifying layer [46] as evidenced by the BET analysis and (ii) the decreased band gap of the sample, as demonstrated by UVevisible spectra. Fig. 9 shows the CVs of 4 mM AA with a-Fe2O3(5%Co) modified GCE at different scan rates. The shift towards higher values of the oxidation peak potential with increasing scan rates can be
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Fig. 6. Hysteresis loops of (a) pure a-Fe2O3 and (b) a-Fe2O3(5%Co).
3.4.2. Electrochemical oxidation of uric acid at the modified electrode Fig. 10 shows the CVs of 4 mM UA in 0.1 KCl for bare, pure aFe2O3 and a-Fe2O3(5e20%Co) nanospheres modified GCE at a scan rate of 50 mV/s. At the bare GCE, the CV of UA (Fig. 10(a)) shows a very broad peak at about 285 mV. It is reported that the oxidation of UA is irreversible at GCE and metal electrodes and is quasireversible at a graphite electrode [48]. The electrochemical oxidation of UA proceeds in a 2e, 2Hþ process to lead to an unstable diimine species which is then attacked by water molecules in a step-wise fashion to be converted into an imine-alcohol and then uric acid-4,5 diol. The uric acid-4,5 diol compound produced is unstable and decomposes to various products depending on the solution pH [49]. However the UA voltammogram obtained for
a-Fe2O3(0e20%Co) modified GCE showed well defined oxidation wave of UA with shift in potential and increase of the oxidation peak current. The oxidation peak potential occurs at 250e400 mV. From Fig. 10, it can be seen that the current response of UA varied with the amount of the added Co. It also shows that the UA oxidation potential at the modified electrode is shifted to more positive direction when compared with that at the bare GCE. Moreover, the UA oxidative peak current at the modified electrode was largely increased relative to that at the bare GCE, indicating the electrocatalytic ability of the modified electrode. Especially the aFe2O3(5%Co) modified GCE shows higher anodic peak current response when compared with the pure Fe2O3 and the aFe2O3(10e20%Co) modified GCE. This electrocatalytic effect was attributed to the larger available S.S.A. and the decreased band gap of the a-Fe2O3(5%Co). Fig. 11 shows the CVs of 4 mM UA on a-Fe2O3(5%Co) modified GCE at different scan rates. A shift towards higher values of the catalytic oxidation peak potential with increasing scan rates can be observed, indicating a kinetic limitation in the reaction between redox sites of a-Fe2O3(5%Co) modified GCE and UA. However, the cyclic voltammetric peak currents for UA at a-Fe2O3(5%Co)
Fig. 7. Effect of dopant concentration on saturation magnetization: (a) pure a-Fe2O3; (b) a-Fe2O3(5%Co); (c) a-Fe2O3(10%Co); (d) a-Fe2O3(15%Co); and (e) a-Fe2O3(20%Co).
Fig. 8. Cyclic voltammetric response of 4 mM AA for a-Fe2O3 sensor doped with different amounts of Co: (a) bare, (b) pure a-Fe2O3, (c) a-Fe2O3(5%Co), (d) a-Fe2O3(10% Co), (e) a-Fe2O3(15%Co), and (f) a-Fe2O3(20%Co) modified GCE.
observed, indicating a kinetic limitation in the reaction between redox sites of a-Fe2O3(5%Co) modified GCE and AA. However, the cyclic voltammetric peak currents for AA at a-Fe2O3(5%Co) modified GCE are linearly related to the square root of scan rate (inset in Fig. 9) in the range 50e400 mVs1, i.e., the process is diffusion controlled [47].
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Fig. 9. Cyclic voltammetric response of 4 mM AA for a-Fe2O3(5%Co) sensor at different scan rates: (a) 50, (b) 100, (c) 200, (d) 300, and (e) 400 mVs1. Inset figure. Square root of scan rate vs. potential.
modified GCE are linearly related to the square root of scan rate (inset in Fig. 11) in the range 50e400 mVs1, which indicates that the electron transfer reaction is controlled by a diffusion process. The above results indicate that a catalytic reaction occurs between the a-Fe2O3(5%Co) modified GCE with AA and UA. The catalytic reaction facilitates electron transfer between AA or UA and the modified electrode; as a result the electrochemical oxidation of AA or UA becomes easier. The reason for this is that the a-Fe2O3(5% Co) can act as a promoter to increase the rate of electron transfer [50,51], lower the overpotential of UA at the bare electrode, and the anodic peak shifts to a less positive potential. Since the XRD pattern of 5% Co-doped a-Fe2O3 did not show any X-ray diffraction peaks from cobalt oxide, most of the cobalt ions are well dispersed in the a-Fe2O3 lattice and suppress the crystallite growth of a-Fe2O3. Hence a-Fe2O3(5%Co) has a smaller particle size than pure a-Fe2O3, i.e., a-Fe2O3(5%Co) has a larger specific surface area. Therefore a-Fe2O3(5%Co) modified GCE shows a higher current response with a shift to a less positive potential than the
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Fig. 11. Cyclic voltammetric response of 4 mM UA for a-Fe2O3(5%Co) sensor at different scan rates: (a) 50, (b) 100, (c) 200, (d) 300, (e) 400 mVs1. Inset figure. Square root of scan rate vs. potential.
pure a-Fe2O3 modified GCE. In addition well formed nanoparticles observed by FE-SEM are also critical to improve the electrocatalytic ability of a-Fe2O3(5%Co) modified GCE. 4. Conclusion Nanocrystalline pure and 5e20% Co-doped a-Fe2O3 nanospheres were successfully prepared by the FeCl3 hydrolysis method which is a simple and cost effective method. The FTIR confirms the formation of FeeO bond in the samples. The XRD confirms the structure of the samples. The FE-SEM observation of a-Fe2O3(5%Co) shows that the particles are spherical in shape. The magnetic property of samples showed that the Co-doped a-Fe2O3 have a weak ferromagnetic behavior when compared with pure a-Fe2O3. It may be due to the presence of cobalt ions with antiferromagnetic coupling in a-Fe2O3 lattice. The electrochemical sensing properties of these nanoparticles were investigated. Both pure a-Fe2O3 and aFe2O3 (5e20% Co) modified GCE exhibited higher anodic current response for 4 mM AA and UA than the bare GCE with shifts in potential. Especially the a-Fe2O3(5%Co) modified GCE showed a larger current response with a shift in positive potential to UA and AA than the bare GCE, the pure a-Fe2O3 and other higher %Co doped a-Fe2O3 modified GCE. The enhancement of the electrochemical sensing performance of a-Fe2O3 doped with 5%Co may be attributed to the relatively larger available specific surface area with decreased band gap of the sample. Acknowledgment One of the author (RS) acknowledges the University of Madras for the financial assistance in the form of Dr Kalaignar M. Karunanidhi Endowment Scholarship. We acknowledge the FE-SEM facility provided by the National Centre for Nanoscience and Nanotechnology, University of Madras. References
Fig. 10. Cyclic voltammetric response of 4 mM UA for a-Fe2O3 sensor doped with different amounts of Co: (a) bare, (b) pure a-Fe2O3, (c) a-Fe2O3(5%Co), (d) a-Fe2O3(10% Co), (e) a-Fe2O3(15%Co), and (f) a-Fe2O3(20%Co) modified GCE.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Facile synthesis of cobalt doped hematite nanospheres: Magnetic and their electrochemical sensing properties R. Suresh a, R. Prabu a, A. Vijayaraj a, K. Giribabu a, A. Stephen b, V. Narayanan a, * a b
Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Maraimalai Campus, Chennai 600025, Tamil Nadu, India Department of Nuclear Physics, University of Madras, Guindy Maraimalai Campus, Chennai 600025, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2010 Received in revised form 9 March 2012 Accepted 11 March 2012
Nanocrystalline pure a-Fe2O3 and Co-doped a-Fe2O3 powders were synthesized by the hydrolysis method. The structure and the morphology of the samples were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The magnetic property of the samples was studied by vibrating sample magnetometer (VSM) at room temperature, which showed that the Co-doped a-Fe2O3 have a weaker ferromagnetic behavior than the pure a-Fe2O3. The electrochemical sensing ability of ascorbic acid (AA) and uric acid (UA) by pure a-Fe2O3 and Co-doped a-Fe2O3 modified glassy carbon electrode (GCE) exhibited higher anodic current response with a shift in positive potential than the bare GCE. Compared with pure a-Fe2O3, Co-doped a-Fe2O3 showed enhanced electrochemical sensing performance. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Annealing Powder diffraction Magnetic properties
1. Introduction Functional metal oxide nanocrystals have been extensively investigated in the recent decade for their outstanding new properties suitable for a broad spectrum of downstream application [1e3]. Among them, nanoparticles of iron oxides are important materials due to their biocompatibility, catalytic activity and low toxicity. They have significant application in various fields such as drug delivery system [4], cancer therapy [5], magnetic resonance imaging [6], high density magnetic storage devices [7], ferro-fluids [8], rechargeable lithium batteries [9], catalysis [10], gas sensor [11] and biosensor [12], etc. Iron oxide mainly occurs in three different forms namely FeO, Fe2O3 and Fe3O4. Fe2O3 has four crystallographic phases namely, a-Fe2O3 (hematite), b-Fe2O3, g-Fe2O3 (maghemite) and 3-Fe2O3 [13]. The g-Fe2O3 and Fe3O4 have inverse spinel structure and superparamagnetic property. The a-Fe2O3 is the most stable iron oxide which has antiferromagnetism with canting ferromagnetic responses at room temperature. It has a complex defect structure in which three types of defect species namely, oxygen vacancies, Fe3þ and Fe2þ interstitials are present [14]. It has a wide range of applications such as photocatalyst for N2 fixation
* Corresponding author. Tel.: þ91 44 22202793; fax: þ91 44 22300488. E-mail address: [email protected] (V. Narayanan). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.034
[15], sorbents [16] and sensors [17], etc. As motivated by the novel properties and potential application of nanosized iron oxide, synthesis of a-Fe2O3 in nanometer scale has attracted more attention [18,19]. Many methods have been reported for the synthesis of a-Fe2O3 including forced hydrolysis [20], combustion [21], anhydrous solvent [22], sol-gel [23], wet chemical synthesis [24], microwave-hydrothermal synthesis [25e27], and spray pyrolysis [28,29], etc. Among them, forced hydrolysis of Fe3þ plays an important role in natural system. This method does not need the addition of any precipitator. Doping of different metal ions or metal oxides into Fe2O3 will find new application or improve the performance of existing applications. For instance, Niu et al. [30] found that the material mixed with rare earth oxides through a sol-gel method in citric acid system presented high gas sensitivity to gasoline. Jing et al. [31] also reported that the Co-doped g-Fe2O3 exhibits higher gas sensitivity and better selectivity to acetone and ethanol than the pure g-Fe2O3. It is noted that until now most of the researchers focused mainly on doping of metal ion into g-Fe2O3 while doping of metal ion in a-Fe2O3 and their electrochemical sensing properties have not been reported yet. In this paper, we have reported the preparation of Co-doped aFe2O3 nanopowder, by using simple hydrolysis of FeCl3 method. The prepared nanoparticles have been used to modify the GCE and remarkable enhancement in the current response with shift in anodic peak potential is observed for electrochemical oxidation of AA and UA when compared to the bare electrode.
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2. Experimental 2.1. Synthesis of a-Fe2O3 and Co-doped a-Fe2O3 nanospheres Anhydrous ferric chloride (FeCl3), hydrated cobaltous chloride (CoCl2∙6H2O) and potassium chloride (KCl) were purchased from Qualigens and used without further purification. Ascorbic acid and uric acid were purchased from Sigma and used as received. Doubly distilled water was used as the solvent. Co-doped a-Fe2O3 powders were prepared as follows [32]. The aqueous solution of anhydrous FeCl3 was heated to 100 C for 16 h. Then the yellow product obtained, FeOOH, was separated from the mother liquor with an ultra-speed centrifuge and washed with distilled water three times. The obtained yellow powder was annealed at 600 C for 6 h, and the product a-Fe2O3 was obtained. Co-doped a-Fe2O3 nanostructures were synthesized by the same method. For the preparation of Co-doped a-Fe2O3, CoCl2 salt was added to the aqueous solution of FeCl3 in four different molar percentages, 5%, 10%, 15%, and 20%, respectively. The solutions were refluxed for 16 h at the temperature of 100 C. The obtained yellow powders were annealed at 600 C for 6 h. Thus we obtained 5e20% Co-doped a-Fe2O3 nanostructures. 2.2. Characterization details The prepared samples were studied by FTIR spectroscopy using a Schimadzu FT-IR 8300 series instrument in the range of 400e4000 cm1. One milligram of each powder sample was diluted with 200 mg of vacuum-dried IR-grade KBr powder and subjected to a pressure of 10 tons. The Raman spectroscopy used in this study was the BRUKER RFS 27: Stand alone FT-Raman Spectrometer equipped with Nd: YAG 1064 nm as an excitation source. The structures of the samples were analyzed by a Rich Siefert 3000 diffractometer with Cu-Ka1 radiation (l ¼ 1.5406 Å). The UVeVis absorption spectra were obtained on a CARY 5E UVeVISNIR Spectrophotometer. The morphology of the materials was analyzed by SEM and FE-SEM using a HITACHI S600N scanning electron microscopy and a HITACHI SU6600 field emissionscanning electron microscopy, respectively. The BET specific surface area (S.S.A.) was measured using a Micromeritics (ASAP 2020) analyzer. The magnetic studies were carried at room temperature using VSM: EG & G Princeton, Applied Research Model 4500 instrument. The electrochemical experiments were performed on a CHI 600A electrochemical instrument using the asmodified electrodes and bare GCE as working electrode, a platinum wire was the counter electrode, and saturated calomel electrode (SCE) was the reference electrode.
Fig. 1. FTIR spectrum of (a) pure a-Fe2O3 and (b) a-Fe2O3(5%Co).
absorption band in the region of 3461, 1642, 1580, 1383, 574, 579, 481 and 485 cm1. The general range of 3600e3100 cm1 may be assigned for water of hydration. Hydrate also absorbs in the range of 1670e1600 cm1 [33]. This later band can be taken as another important means of identifying water of crystallization. In Fig. 1(a), the band associated with the lattice water molecule is broad, and is observed in a region of 3461 and 1642 cm1. The simultaneous presence of these two bands indicates that the water of crystallization is present in the sample. The bands at 574 and 481 cm1 in Fig. 1(a) and at 579 and 485 cm1 in Fig. 1(b) are due to the FeeO stretching vibrational modes [34]. These two bands are sharp and are of strong intensity. The intensity of the broad peaks at 3600e3000 cm1 is found to be decreasing from Fig. 1(a) to (b). The shifts observed for the above samples may be due to the particle size effect [35]. The Raman spectrum of the pure a-Fe2O3 and a-Fe2O3(5%Co) are shown in Fig. 2(a) and (b), respectively. Fig. 2(a) shows the peaks at 225, 242, 294, 411 and 499 cm1 correspond to the a-Fe2O3 phase [36], namely two A1g modes (225 and 499 cm1) and three Eg modes (242, 294 and 411 cm1). Fig. 2(b) is similar to that of Fig. 2(a). However, it shows additional peaks at 488, 519 and 680 cm1 which are characteristic of cobalt oxide [37]. It suggests that the cobalt oxide may be well dispersed in the lattices of aFe2O3. Fig. 3(a) and (b) represents the XRD pattern of pure a-Fe2O3 and a-Fe2O3(5%Co), respectively. In Fig. 3(a), the diffraction peaks are all closely matched with the standard a-Fe2O3 reflections (JCPDS No. 33-0664). It revealed that the synthesized a-Fe2O3 has an orthorhombic structure. In the case of 5%Co doped sample (Fig. 3(b)), no
2.3. Preparation of the Co-doped a-Fe2O3 modified GCE is as follows Ultrasonic agitation for 30 min was used to disperse 1 mg of Codoped a-Fe2O3 into 5 mL of acetone to make a reddish brown and homogeneous suspension. The polished GCE was coated with 5 mL of the above reddish brown suspension. The modified electrode was activated in a 0.1 M KCl solution by successive cyclic scans between 0 and þ1.2 V. Before and after each experiment, the modified electrode was washed with distilled water and reactivated by the method mentioned above. 3. Results and discussion 3.1. Structural characterization The FTIR spectrum of the pure a-Fe2O3 and a-Fe2O3(5%Co) are given in Fig. 1(a) and (b), respectively. Fig. 1(a) and (b) shows the
Fig. 2. Raman spectrum of (a) pure a-Fe2O3 and (b) a-Fe2O3(5%Co).
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ahn ¼ A(hn Eg)n . Where a is the absorption coefficient, A is a constant, h is Planck’s constant, n is the photon frequency, Eg is the band gap, and n is equal to 1/2 or 2 for the transition being direct and indirect, respectively. An extrapolation of the linear region of a plot of (ahn)2 versus hn, gives the value of the band gap of the samples. The estimated band gap was found to be 2.0 eV for aFe2O3(5%Co) and 2.29 eV for pure a-Fe2O3 which is in good agreement with a band gap value of 2.2 eV for Fe2O3 [39]. The steep absorption edge in both the samples is indicative of the uniform particle morphology and size with fairly good crystallinity [33]. The SEM micrograph of a-Fe2O3(5%Co) is shown in Fig. 5(a). It shows that the particles are in spherical shape. The nanostructure of the a-Fe2O3(5%Co) is confirmed by FE-SEM observations (Fig. 5(b)): the spherical particles have a diameter in the range of 60e90 nm. The energy dispersive spectroscopy (EDS) of the aFe2O3(5%Co) shows that the nanospheres consist of iron, cobalt and oxygen (Fig. 5(c)). The metal ion mapping (Fig. 5(d)) also confirmed the presence of Co homogenously dispersed in the a-Fe2O3 matrix. Fig. 3. XRD pattern of (a) pure a-Fe2O3 and (b) a-Fe2O3(5%Co).
additional peak is observed. However, all the peaks in the XRD pattern of doped sample are essentially of a-Fe2O3 which is in good agreement with the reported value in literature [38]. No phase containing Co additive was observed within the detection limit of the apparatus. However, all the peaks are slightly shifted to higher 2q values with increasing percentage of cobalt doping compared to pure a-Fe2O3 which is due to the smaller radius of cobalt compared to iron. The XRD peaks of pure a-Fe2O3 and a-Fe2O3(5%Co) show broadening indicating the ultrafine nature of the particles. The average crystallite size of pure and Co-doped a-Fe2O3 was calculated by Scherrer’s formula using the most intense diffraction peak at w33 (104). The estimated crystallite size of pure a-Fe2O3 is 49 nm and that of a-Fe2O3(5%Co) is 29 nm. It can be found that the crystallite size of the a-Fe2O3(5%Co) turns smaller, which means that the addition of Co2þ can effectively suppresses the a-Fe2O3 crystalline grain growth. Fig. 4(a) and (b) represents the UVevisible spectra of pure a-Fe2O3 and pure a-Fe2O3(5%Co), respectively. It is known that the fingerprint region of the band edge of hematite is 521e565 nm, whereas in Co-doped and pure a-Fe2O3 the band edge observed ranged from 560 to 570 nm. This is in good agreement with the XRD and FTIR results, i.e., all the samples are a-Fe2O3. In order to calculate the band gap, the Tauc’s relationship was used as follows:
3.2. Surface area measurement Nitrogen adsorptionedesorption measurements were conducted to determine the BET specific surface area of the nanospheres. To obtain a a-Fe2O3 powder with high specific surface area, we have to use a lower concentration of Fe3þ and a lower reaction temperature. However, the concentration we used here is 0.5 M with higher reaction and annealing temperatures than the reported values [40]. The S.S.A. of the pure a-Fe2O3 is 4.95 m2 g1 whereas the S.S.A. of the a-Fe2O3(5%Co) is 8.13 m2 g1. These results showed that the addition of 5% Co increased the specific surface area of the sample. 3.3. Magnetic property Fig. 6 illustrates the hysteresis loops of pure and 5% Co-doped aFe2O3. The hysteresis loop of pure a-Fe2O3 (Fig. 6(a)) shows weak ferromagnetic as already observed in literature [41]. It can be seen that the a-Fe2O3 nanospheres show weak ferromagnetic behavior with a magnetization of 51.2 102 emu g1 and a coercivity of 1159 Oe at room temperature. The values of magnetization and coercivity are different from those of other morphologies of Fe2O3 [42e44]. The higher magnetization and coercivity observed in the present study must be associated with the unique structure of the aFe2O3 nanospheres. However, the hysteresis loop of 5% Co-doped aFe2O3 (Fig. 6(b)) shows weak ferromagnetic behavior with a magnetization of 48 102 emu g1 and a coercivity of 863 Oe at room temperature. The variation of saturation magnetization of a-Fe2O3 and that of doped a-Fe2O3 with various concentration of cobalt is shown in Fig. 7. The Fig. 7 shows that, for 5% Co-doped a-Fe2O3, the saturation magnetization increases up to an applied field of 5100 Oe, after that it decreases slightly in comparison to pure a-Fe2O3 nanospheres. The above results indicate that for 5% Co-doped a-Fe2O3, cobalt ions are homogenously distributed in the a-Fe2O3 lattices. In case of 10e20% Co-doped samples, the saturation magnetization decreases gradually when compared with pure a-Fe2O3 nanospheres. It may be probably due to the presence of adjacent cobalt ions (at higher concentrations of cobalt) in different oxidation states (2þ/3þ) with antiferromagnetic coupling [45] in a-Fe2O3 crystallites. 3.4. Electrochemical sensing properties
Fig. 4. DRS UVevisible absorption spectrum of (a) a-Fe2O3 and (b) a-Fe2O3(5%Co).
3.4.1. Electrochemical oxidation of ascorbic acid at the modified electrode Fig. 8 shows the cyclic voltammograms (CVs) of 4 mM AA for bare and a-Fe2O3(0e20%Co) nanospheres modified GCE at a scan
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Fig. 5. SEM (a) and FE-SEM, (b) micrographs of a-Fe2O3(5%Co), (c) EDS spectrum of a-Fe2O3(5%Co), and (d) metal ion mapping of a-Fe2O3(5%Co).
rate of 50 mV/s. The electrochemical oxidation of AA proceeds through 2Hþ, 1e process and leads to a radical anion. The radical anion undergoes a fast one electron irreversible oxidation to form electro-inactive dehydroascorbic acid. It is rapidly protonated and then dehydrated to form 2,3-diketogluconic acid. At the bare GCE the CV of AA (Fig. 8(a)) shows an oxidation peak potential at 780 mV. At a-Fe2O3(0e20%Co) modified GCE (Fig. 8(bef)), well defined oxidation waves of AA were obtained. From Fig. 8, it can be seen that the current response of AA varied with the amount of added Co. It also shows that the AA anodic potential at the modified electrode was shifted to less positive direction when compared with that at the bare GCE. The AA oxidative current at the modified
electrode was largely increased relative to that at the bare GCE, indicating the AA electrocatalytic ability of the modified electrode. Especially the a-Fe2O3(5%Co) modified GCE shows higher anodic peak current response when compared with the pure a-Fe2O3 and the a-Fe2O3(10e20% Co) modified GCE. This electrocatalytic effect was attributed to the (i) larger available specific surface area of the modifying layer [46] as evidenced by the BET analysis and (ii) the decreased band gap of the sample, as demonstrated by UVevisible spectra. Fig. 9 shows the CVs of 4 mM AA with a-Fe2O3(5%Co) modified GCE at different scan rates. The shift towards higher values of the oxidation peak potential with increasing scan rates can be
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Fig. 6. Hysteresis loops of (a) pure a-Fe2O3 and (b) a-Fe2O3(5%Co).
3.4.2. Electrochemical oxidation of uric acid at the modified electrode Fig. 10 shows the CVs of 4 mM UA in 0.1 KCl for bare, pure aFe2O3 and a-Fe2O3(5e20%Co) nanospheres modified GCE at a scan rate of 50 mV/s. At the bare GCE, the CV of UA (Fig. 10(a)) shows a very broad peak at about 285 mV. It is reported that the oxidation of UA is irreversible at GCE and metal electrodes and is quasireversible at a graphite electrode [48]. The electrochemical oxidation of UA proceeds in a 2e, 2Hþ process to lead to an unstable diimine species which is then attacked by water molecules in a step-wise fashion to be converted into an imine-alcohol and then uric acid-4,5 diol. The uric acid-4,5 diol compound produced is unstable and decomposes to various products depending on the solution pH [49]. However the UA voltammogram obtained for
a-Fe2O3(0e20%Co) modified GCE showed well defined oxidation wave of UA with shift in potential and increase of the oxidation peak current. The oxidation peak potential occurs at 250e400 mV. From Fig. 10, it can be seen that the current response of UA varied with the amount of the added Co. It also shows that the UA oxidation potential at the modified electrode is shifted to more positive direction when compared with that at the bare GCE. Moreover, the UA oxidative peak current at the modified electrode was largely increased relative to that at the bare GCE, indicating the electrocatalytic ability of the modified electrode. Especially the aFe2O3(5%Co) modified GCE shows higher anodic peak current response when compared with the pure Fe2O3 and the aFe2O3(10e20%Co) modified GCE. This electrocatalytic effect was attributed to the larger available S.S.A. and the decreased band gap of the a-Fe2O3(5%Co). Fig. 11 shows the CVs of 4 mM UA on a-Fe2O3(5%Co) modified GCE at different scan rates. A shift towards higher values of the catalytic oxidation peak potential with increasing scan rates can be observed, indicating a kinetic limitation in the reaction between redox sites of a-Fe2O3(5%Co) modified GCE and UA. However, the cyclic voltammetric peak currents for UA at a-Fe2O3(5%Co)
Fig. 7. Effect of dopant concentration on saturation magnetization: (a) pure a-Fe2O3; (b) a-Fe2O3(5%Co); (c) a-Fe2O3(10%Co); (d) a-Fe2O3(15%Co); and (e) a-Fe2O3(20%Co).
Fig. 8. Cyclic voltammetric response of 4 mM AA for a-Fe2O3 sensor doped with different amounts of Co: (a) bare, (b) pure a-Fe2O3, (c) a-Fe2O3(5%Co), (d) a-Fe2O3(10% Co), (e) a-Fe2O3(15%Co), and (f) a-Fe2O3(20%Co) modified GCE.
observed, indicating a kinetic limitation in the reaction between redox sites of a-Fe2O3(5%Co) modified GCE and AA. However, the cyclic voltammetric peak currents for AA at a-Fe2O3(5%Co) modified GCE are linearly related to the square root of scan rate (inset in Fig. 9) in the range 50e400 mVs1, i.e., the process is diffusion controlled [47].
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Fig. 9. Cyclic voltammetric response of 4 mM AA for a-Fe2O3(5%Co) sensor at different scan rates: (a) 50, (b) 100, (c) 200, (d) 300, and (e) 400 mVs1. Inset figure. Square root of scan rate vs. potential.
modified GCE are linearly related to the square root of scan rate (inset in Fig. 11) in the range 50e400 mVs1, which indicates that the electron transfer reaction is controlled by a diffusion process. The above results indicate that a catalytic reaction occurs between the a-Fe2O3(5%Co) modified GCE with AA and UA. The catalytic reaction facilitates electron transfer between AA or UA and the modified electrode; as a result the electrochemical oxidation of AA or UA becomes easier. The reason for this is that the a-Fe2O3(5% Co) can act as a promoter to increase the rate of electron transfer [50,51], lower the overpotential of UA at the bare electrode, and the anodic peak shifts to a less positive potential. Since the XRD pattern of 5% Co-doped a-Fe2O3 did not show any X-ray diffraction peaks from cobalt oxide, most of the cobalt ions are well dispersed in the a-Fe2O3 lattice and suppress the crystallite growth of a-Fe2O3. Hence a-Fe2O3(5%Co) has a smaller particle size than pure a-Fe2O3, i.e., a-Fe2O3(5%Co) has a larger specific surface area. Therefore a-Fe2O3(5%Co) modified GCE shows a higher current response with a shift to a less positive potential than the
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Fig. 11. Cyclic voltammetric response of 4 mM UA for a-Fe2O3(5%Co) sensor at different scan rates: (a) 50, (b) 100, (c) 200, (d) 300, (e) 400 mVs1. Inset figure. Square root of scan rate vs. potential.
pure a-Fe2O3 modified GCE. In addition well formed nanoparticles observed by FE-SEM are also critical to improve the electrocatalytic ability of a-Fe2O3(5%Co) modified GCE. 4. Conclusion Nanocrystalline pure and 5e20% Co-doped a-Fe2O3 nanospheres were successfully prepared by the FeCl3 hydrolysis method which is a simple and cost effective method. The FTIR confirms the formation of FeeO bond in the samples. The XRD confirms the structure of the samples. The FE-SEM observation of a-Fe2O3(5%Co) shows that the particles are spherical in shape. The magnetic property of samples showed that the Co-doped a-Fe2O3 have a weak ferromagnetic behavior when compared with pure a-Fe2O3. It may be due to the presence of cobalt ions with antiferromagnetic coupling in a-Fe2O3 lattice. The electrochemical sensing properties of these nanoparticles were investigated. Both pure a-Fe2O3 and aFe2O3 (5e20% Co) modified GCE exhibited higher anodic current response for 4 mM AA and UA than the bare GCE with shifts in potential. Especially the a-Fe2O3(5%Co) modified GCE showed a larger current response with a shift in positive potential to UA and AA than the bare GCE, the pure a-Fe2O3 and other higher %Co doped a-Fe2O3 modified GCE. The enhancement of the electrochemical sensing performance of a-Fe2O3 doped with 5%Co may be attributed to the relatively larger available specific surface area with decreased band gap of the sample. Acknowledgment One of the author (RS) acknowledges the University of Madras for the financial assistance in the form of Dr Kalaignar M. Karunanidhi Endowment Scholarship. We acknowledge the FE-SEM facility provided by the National Centre for Nanoscience and Nanotechnology, University of Madras. References
Fig. 10. Cyclic voltammetric response of 4 mM UA for a-Fe2O3 sensor doped with different amounts of Co: (a) bare, (b) pure a-Fe2O3, (c) a-Fe2O3(5%Co), (d) a-Fe2O3(10% Co), (e) a-Fe2O3(15%Co), and (f) a-Fe2O3(20%Co) modified GCE.
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