Synthesis and Optical Properties of Two Cobalt Oxides - American

J. Phys. Chem. C 2008, 112, 12225–12233

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Synthesis and Optical Properties of Two Cobalt Oxides (CoO and Co3O4) Nanofibers Produced by Electrospinning Process Nasser A. M. Barakat,†,‡,* Myung Seob Khil,‡,§ Faheem A. Sheikh,| and Hak Yong Kim‡,§,* Chemical Engineering Department, Faculty of Engineering, El-Minia UniVersity, El-Minia, Egypt; Center for Healthcare Technology DeVelopment, Department of Textile Engineering, and Department of Bionano System Engineering, Chonbuk National UniVersity, Jeonju 561-756, Republic of Korea ReceiVed: March 31, 2008; ReVised Manuscript ReceiVed: May 19, 2008

In the present study, cobalt(II) oxide (CoO), which is hard to synthesize because of the chemical activity of cobalt metal, and the popular cobalt(II, III) oxide (Co3O4) have been successfully produced in smooth and continuous nanofibrous form by using the electrospinning technique. An aqueous cobalt acetate tetrahydrate and poly(vinyl alcohol) mixture has been electrospun and vacuously dried at 80 °C. Pure, smooth and solid Co3O4 nanofibers were produced when the dried nanofiber mats were calcined in air atmosphere at 700 °C. Water gas has been prepared by a novel technique to produce pure CoO nanofibers from the original cobalt acetate/poly(vinyl alcohol) nanofiber mats. The nanofiber mats have been hydrothermally treated in the presence of carbon at 300 °C in an especially designed reactor. The invoked physiochemical analyses have affirmed formation of both oxides in nanofibrous shape. The optical properties of the obtained nanofibers have been studied. UV-visible absorbance spectra have indicated that the band gap energy differences for the Co3O4 and CoO nanofibers are 2.4 and 2.21 eV, respectively. A. Introduction As a prevalent character of the transition metals, cobalt metal has many oxidation states; accordingly, it has many oxide forms. Among the various oxide forms; cobalt(II) oxide (CoO) and cobalt(II, III) Co3O4 are especially interesting because of their exceptional physical and chemical properties. Co3O4 and its mixtures can be used as electrode materials in various applications such as oxygen evolution1–3 and reduction,4 electrochromic devices,5,6 lithium ion batteries,7–10 supercapacitors,11–13 and the protection film of cathodes in molten carbonate fuel cells.14–16 Moreover, Co3O4 is used in heterogeneous catalysis,17–19 solid state sensors,20,21 energy storage,22 and as magnetic materials.23 Structurally, the simplest cobalt oxide is CoO, the rocksalt monoxide; it is extensively used in the ceramics industry as an additive to create blue-colored glazes and enamels as well as in the chemical industry for producing cobalt(II) salts. Furthermore, cobalt(II) oxide does have distinct electrochemical properties.24–26 When CoO is added to a nickel hydroxide electrode, a conductive subnetwork is formed within the pores, electronically linking the whole active material to the current collector, and thus allowing an active material utilization rate as high as 100%.27,28 As it is well know, the physical and chemical properties are affected by size and shape; nanostructure does have prominent characteristics. Therefore, Co3O4 nanoparticles have been synthesized by various methods such as hydrothermal,10 spray pyrolysis,22 sputtering,23 thermal decomposition,1–3 sol-gel,29–32 electrochemi* Corresponding author (H.Y.K.) phone: +82 63 270 2351; fax: +82 63 270 2348; e-mail: [email protected]; (N.A.M.B.) e-mail: nasbarakat@ yahoo.com. † El-Minia University. ‡ Center for Healthcare Technology Development, Chonbuk National University. § Department of Textile Engineering, Chonbuk National University. | Department of Bionano System Engineering, College of Engineering, Chonbuk National University.

cal deposition,11,33–35 wet chemicals,36 microwave assisted,37 ionic liquid-assisted,38 and chemical vapor deposition.39,40 To the best of our knowledge, no previous work has been reported about production of cobalt monoxide nanostructures; this is because of the special synthesis requirements to force cobalt metal to be in a low oxidation state. Among the one-dimensional nanostructural materials that have taken special consideration because of their potential applications in nanodevices,41 nanofibers do have especial consideration due to their high surface-to-volume ratio. In this study, the electrospinning technique, the most popular method used for producing metal oxides nanofibers because of its simplicity and efficiency,42 was exploited to prepare CoO and Co3O4 nanofibers. A precursor has been prepared by mixing an aqueous solution of poly(vinyl alcohol) (PVA) and a cobalt acetate tetrahydrate (CoAc)/water solution. The precursor was electrospun to produce nanofiber mats. The obtained mats were calcined at different temperatures for 2 h in an air atmosphere to produce Co3O4 nanofibers. Water gas (CO + H2) can be produced by reaction of hot carbon with water vapor. This phenomenon was exploited to synthesize CoO nanofibers. The prepared nanofiber mats were heated to 300 °C for 3 h inside an especially designed reactor containing water and activated carbon. The synthesized water gas has totally eliminated the PVA polymer and has partially reduced the CoAc into cobalt(II) oxide. The prepared nanofibers were analyzed by powder X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), electron scanning microscopy (SEM), and energy dispersive X-ray analysis (EDX). The optical properties of the obtained nanofibers were studied via a UV-visible spectrophotometer. 2. Experimental Details 2.1. Materials. Cobalt(II) acetate tetrahydrate (CoAc) 98% assay (Junsei Chemical Co., Ltd., Japan) and PVA with a

10.1021/jp8027353 CCC: $40.75  2008 American Chemical Society Published on Web 07/19/2008

12226 J. Phys. Chem. C, Vol. 112, No. 32, 2008 molecular weight (MW) of 65 000 g/mol (DC Chemical Co., Ltd., South Korea) were utilized without any further modifications. Distilled water was used as a solvent. 2.2. Experimental Work. Sol-gel was prepared by mixing a 20 wt % CoAc aqueous solution and a 10 wt % PVA aqueous solution with a ratio of 1:3. The mixture was vigorously stirred at 50 °C for 5 h. The final prepared solution was placed in a plastic capillary. A copper pin connected to a high-voltage generator was inserted in the solution, and the solution was kept in the capillary by adjusting the inclination angle. A ground iron drum covered by a polyethylene sheet was serving as a counter-electrode. A voltage of 20 kV was applied to the solution. The formed nanofibers were initially dried at 80 °C for 24 h under vacuum. The calcination occurred at 500, 600, 700, and 800 °C for 2 h in an air atmosphere with a heating rate of 7 °C/min. Cobalt (II) oxide was synthesized using the reactor shown in Figure 1. The reactor was made of stainless steel with a height of 15 cm and diameter of 7 cm. The reactor was designed to perform a chemical reaction between the water vapor and hot activated carbon to produce carbon monoxide and hydrogen gases according to the following reaction

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H2O + C f CO + H2

(1)

This reaction is well-known as the water gas reaction. The experiment was conducted as follow: a sealed metallic bottle was filled with water and placed at the bottom of the reactor. An activated carbon (as a source of carbon), and nanofiber mats were placed on two meshes above the metallic bottle as shown in Figure 1. Finally, the reactor was sealed with a stainless steel cover and heated in silicon oil bath at 300 °C for 3 h. Due to heating of the reactor, the water boiled inside the bottle and the pressure increased. Therefore, water vapor streamed out with small flow rate due to the fixed cover. The hot vapor reacted with the hot carbon layer to produce water gas as indicated in reaction 1. The resulting reducing gases reacted with the nanofiber mats existing on the upper mesh. With passing time, the pressure increased inside the reactor, so some gases escaped from the reactor. No rubber seals were utilized in the cover of either of the metallic bottle or the reactor. 2.3. Characterizations. Surface morphology was studied by a scanning electron microscope (SEM, JEOL JSM-5900, Japan) equipped with energy dispersive X-ray (EDX). Information

Figure 1. Schematic diagram for the especially designed reactor to perform a reaction between water vapor and hot carbon to produce water gas: (1) the reactor, (2) the cover, (3) a bottle filled with water, (4) an activated carbon layer, and (5) the nanofiber mats.

Synthesis and Optical Properties of Two Cobalt Oxides

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12227

Figure 2. Low and high magnification SEM images for the CoAc/ PVA nanofiber mats. The nanofibers are neither agglomerated nor broken, as shown in the images.

about the phase and crystallinity was obtained by using a Rigaku X-ray diffractometer (XRD, Rigaku, Japan) with Cu KR (λ ) 1.540 A°) radiation over Bragg angles ranging from 10 to 90°. High-resolution image and selected area electron diffraction (SAED) patterns were obtained with a transmission electron microscope (TEM, JEOL JEM-2010, Japan) operated at 200 kV. Spectroscopic characterization has been investigated by Fourier-transform infrared (FT-IR), and the spectra were recorded as KBr pellets using a Varian FTS 1000 FT-IR, MidIR spectral range, cooled DTGS detector, Scimitar series, Varian Inc., Australia. Thermal properties were studied by a thermal gravimetric analyzer (TGA, Pyris1, PerkinElmer Inc., USA). The optical properties were studied using an HP 8453 UV Visible Spectroscopy System; spectra obtained were analyzed by HP ChemiStation software, 5890 series. 3. Results and Discussion 3.1. Morphology. The electrospinning technique involves the use of a high voltage to charge the surface of a polymer solution droplet and thus to induce the ejection of a liquid jet through a spinneret. Because of bending instability, the jet is subsequently stretched by many times to form continuous, ultrathin fibers. Figure 2 shows the obtained CoAc/PVA nanofiber mats after drying. Two magnifications were utilized to properly show the morphology. Figure 2A represents the low magnification image, as shown in this figure; the obtained mats were composed from separated nanofibers free of beads. The nanofibers are smooth and uniform with an average diameter of 500 nm, as can be observed from Figure 2B. Calcination of the CoAc/PVA nanofiber mats at high temperatures relatively affected the general morphology of the produced nanofibers. Panels A and B of Figure 3 reveal lowand high-magnification SEM images for nanofibers obtained after calcination of CoAc/PVA mats at 500 °C. As shown in these figures, such calcination temperature was not adequate to obtain solid and smooth nanofibers. In other words, high temperature is required to enhance the densification of the produced nanofibers since the obtained nanofibers composed form noncohesive particles, as shown in Figure 3B. Panels C and D, E and F, and G and H of Figure 3 demonstrate the low- and high-magnification SEM images for nanofibers produced at calcination temperatures of 600, 700, and 800 °C, respectively. As can be concluded from these figures, increasing the calcination temperature improved the obtained nanofibers morphology. According to these results, one can say that the optimum calcination temperature is 700 °C. As shown in panels E and F of Figure 3, the obtained nanofibers at such temperature do not have defects. Increasing the calcination temperature deformed the nanofibers by producing protrusions on the surface of the nanofibers, as shown in Figure 3, panels G and H. At relatively low calcination temperature (i.e., 600 °C), the nanofibers were solid, semismooth, and possessed some defacements (Figure 3, panels C and D).

Figure 3. Low- and high-magnification SEM images obtained after calcination the CoAc/PVA nanofiber mats at four temperatures for 2 h. At 500 (A and B), 600 (C and D), 700 (E and F), and 800 °C (G and H).

Figure 4 shows the frequency curve of the nanofibers diameters after calcination. As shown in Figure 4A, the nanofibers produced due to calcination at 500 °C laid within a wide diameters range (from 210 to 720 nm). This range has been modified with slight increasing in the calcination temperature; the diameter range of the nanofibers produced at 600 °C had lower and upper limits of 250 and 590 nm, respectively. The information gained from the frequency of diameters curves confirmed that the best calcination temperature was 700 °C. As shown in Figure 4B, the nanofibers obtained at that temperature have almost constant diameter since the diameters varied from 185 to 255 nm. Moreover, the average diameter at such temperature was small compared with the other calcination temperatures. Therefore, one can say that calcination of CoAc/ PVA nanofiber mats at 700 °C results is smooth, compacted, and relatively thin nanofibers. Increasing the calcination temperature to 800 °C broadened the diameter range as shown in Figure 4A. According to the frequency of the diameters curves, the average diameters of the produced nanofibers were 440, 338, 221, and 450 nm at calcination temperatures of 500, 600, 700, and 800 °C, respectively. Hydrothermal treatment of CoAc/PVA nanofiber mats in the designed stainless steel reactor in the presence of activated carbon resulted in smooth and compacted nanofibers as shown in Figure 5, panels A and B. According to the frequency of

12228 J. Phys. Chem. C, Vol. 112, No. 32, 2008

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Figure 4. Frequency curves for the produced nanofibers diameters after calcination of the CoAc/PVA nanofiber mats in air atmosphere at 500, 600, and 800 °C (A) and at 700 °C (B).

diameters curve of the obtained nanofibers (Figure 5C), the average diameter of these nanofibers was 421 nm. 3.2. Crystal Structures. PVA polymer does have wide utilization in the producion of metal oxide nanofibers via electrospinning methodology for its novel chemical and thermal characteristics. The hydroxyl groups deployed on the PVA chains have the capability to generate hydrogen bonds with many anions (especially those having an oxygen atom-like acetate group), which enhances the solubility of the metal salt in PVA solution. Moreover, PVA has a low decomposition temperature, so elimination of PVA from the obtained electrospun nanofiber mats is an easy task. Figure 6 shows the XRD data for the obtained nanofibers due to calcination of CoAc/ PVA nanofiber mats in air atmosphere at 500 and 700 °C. Also, XRD data of the nanofibers prepared by the hydrothermal treatment are embedded in the same figure. As can be concluded from this figure, calcination of CoAc/PVA nanofiber mats at high temperatures completely eliminated the PVA polymer and decomposed CoAc into pure cobalt(II, III) oxide. The existence of strong and sharp diffraction peaks at 2θ values of 31.35, 36.98, 44.93, 59.52, and 65.36° corresponding to (220), (311), (400), (511), and (440) main crystal planes indicates the formation of pure cubic crystalline Co3O4 (JCDPS, card no 42-1467). As can also be observed from the figure, the XRD spectra of the two samples calcined at 500 and 700 °C are similar; they exactly match the standard spectrum of Co3O4.

However, the XRD spectra for the nanofibers produced by hydrothermal treatment have manifested formation of cobalt monoxide. The strong and sharp diffraction peaks at 2θ values of 36.53, 42.52, 61.54, 73.69, and 77.52°, corresponding to (111), (200), (220), (311), and (222) crystal planes, indicate the formation of pure cubic crystalline cobalt monoxide (JCDPS, card no 09-0402). As shown in the figure, only the peaks corresponding to the cobalt monoxide crystals clearly appear without any interferrance from any peak belonging to other cobalt oxides, which affirms formation of pure cobalt monoxide. Figure 7 shows the HRTEM of the prepared nanofibers; the insets represent the selected area electron pattern diffraction images (SAED). As can be observed from Figure 7A, which reveals the HRTEM for the nanofibers obtained due to calcination of CoAc/PVA nanofiber mats at 500 °C, the nanofibers did not have good crystallinity since the atomic planes could not be identified. Moreover, SAED image reveals unsatisfied crystalline structure. Therefore, one can say bad crystallinity might be the reason for the obtained incoherent nanofibers at this temperature (Figure 3, panels A and B). Increasing the calcination temperature enhanced the crystallinity of the Co3O4 nanofibers, as shown in Figure 7, panels B-D, which represent the HRTEM of the nanofibers produced at 600, 700, and 800 °C, respectively. HRTEM results affirmed that the optimum calcination temperature was 700 °C, as shown in Figure 7C;



Figure 5. SEM images for the prepared nanofibers after hydrothermal treating of the CoAc/PVA nanofiber mats in the presence of activated carbon; (A) low magnification, (B) high magnification image. The frequency curve for the nanofiber diameters (C).

Figure 6. XRD data for the obtained nanofibers after calcination at 500 and 700 °C in air atmosphere and for the nanofibers produced by the proposed hydrothermal treatment process (CoO).

the atomic planes are parallel and are arranged in good geometry. Moreover, the SAED image demonstrates excellent crystallinity.

Figure 8 demonstrates the HRTEM and SAED micrographs for the nanofibers obtained by the proposed hydrothermal treatment technique, as shown in this figure; the prepared nanofibers have good atomic arrangement. The inset shows the SAED pattern, where the atoms are seen to be uniformly arranged. There are no dislocations or imperfections observed in the lattice planes, which indicate good crystallinity of the synthesized nanofibers. 3.3. Energy Dispersion. To understand the atomic ratio of calcined and hydrothermally treated nanofiber mats, EDX analyses were carried out. According to the molecular structure of the Co3O4, the theoretical atomic percentages of cobalt and oxygen elements are 42.86 and 57.14%, respectively; however, in case of CoO, cobalt and oxygen have the same atomic percentage. Figure 9 shows the EDX results for the nanofibers prepared by calcination of CoAc/PVA nanofiber mats at 500 and 600 °C. Also, the bottom graph in Figure 9 reveals the EDX results for the nanofibers obtained after the proposed hydrothermal treatment process. As shown in Figure 9, panels A and B, cobalt and oxygen atomic percentage for the nanofibers made by calcination at 500 and 600 °C are very close to the theoretical values of cobalt(II, III) oxide. In a case of the hydrothermally treated nanofibers, as it is observed in Figure 9C, EDX affirmed


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Figure 8. HRTEM image for the nanofibers obtained due to the proposed hydrothermal treatment process. The inset shows the SAED pattern.

Figure 7. HRTEM images for the nanofibers obtained after calcination of CoAc/PVA mats at 500 (A), 600 (B), 700 (C), and 800 °C (D). The insets show the SAED pattern.

formation of cobalt monoxide and simultaneously supported the XRD results. 3.4. Spectroscopic Characterization. Figure 10 shows the IR spectra for the obtained nanofibers after normal calcination at 500 °C, hydrothermally treated ones, and the original CoAc/ PVA nanofiber mats. In the case of the CoAc/PVA spectra, there are characteristic absorption bands of CO-2 at 1568 and 1417

Figure 9. EDX results for the nanofibers obtained after calcination of CoAc/PVA nanofiber mats in air [at 500 °C (A) and 600 °C (B)] and after hydrothermal treatment (C). The insets show cobalt and oxygen atomic percentages in each case according to the EDX quantitative analyses data.

cm-1, as well as those of CsC bonds at 1250 cm-1. Moreover, the spectrum shows a broad band centered at around 3332 cm-1, as well as weak bands at 846, 934, 1028, and 1093 cm-1. These



Figure 10. Absorbance FT-IR intensity results for CoAc/PVA nanofiber mats (A), nanofibers calcined at 500 °C for 2 h in air atmosphere (B), and nanofibers produced by the proposed hydrothermal treatment process (C).

Figure 12. TGA results for CoAc/PVA nanofiber mats in argon atmosphere (A) and the first derivative of the TGA curve (B).

Figure 11. TGA results for CoAc/PVA nanofiber mats in air atmosphere (A) and the first derivative of the TGA curve (B).

The second sharp peak denotes to the decomposition of the CoAc into cobalt(II,III) oxide since there is no weight change after this peak. Recently, Jun and Hua50 have studied the mechanism of thermal decomposition of cobalt acetate tetrahydrate in an inert gas atmosphere. They have concluded that heating of CoAc in a nitrogen atmosphere at high temperature results in the formation of cobalt monoxide. Consequently, TGA has been achieved for CoAc/PVA nanofiber mats in an argon atmosphere, the results were demonstrated in Figure 12, the first derivative curve has also been plotted. As shown in this figure, the first derivative has a broad peak at ∼420 °C. According to that previous intensive study of thermal decomposition of CoAc in an inert atmosphere, this peak represents the formation of CoO. Formation of CoO needs relatively a high temperature because it is achieved from the thermal decomposition CoCO3 according to the following reactions:

bands are attributed to stretching, wagging, and twisting vibrations of water of hydration.43 Also, the peak at 1735 cm-1 represents H2O absorbed by the nanofibers.44 The strongest and sharp peaks at 662 and 571 cm-1 in the Co3O4 spectra corresponding to the nanofibers obtained after calcination at 500 °C are assigned to Co-O bonds existing in cobalt(II,III) oxide;45,46 no other peaks appear in the spectra, which affirms the formation of pure Co3O4 oxide and simultaneously supports the XRD and EDX results. In case of the nanofibers produced by the proposed hydrothermal treatment process, the sharp and strongest peaks denoting a Co-O bond could not be observed. Moreover, the characteristics absorption bands of the acetate anion, which appear from 1590 to 600 cm-1,47 are also missing in the spectra. It is noteworthy that this FT-IR result is similar to a spectrum of pure CoO obtained in another work.48 Therefore, one can conclude that the FT-IR data supports the XRD and EDX results and confirms the formation of pure CoO nanofibers by the proposed hydrothermal process. 3.5. Thermal Properties. Figure 11 shows the results obtained from the thermal gravimetric analysis of the CoAc/ PVA nanofiber mats in air atmosphere. To obtain precise information, the first derivative of the TGA curve has been estimated and introduced in the same figure. As shown in the first derivative curve, there are two peaks at ∼201 and ∼327 °C. According to the thermal properties of PVA polymer, the first peak can be investigated as the decomposition of the PVA.49

Co(CH3COO)2 · 4H2O f Co(OH)(CH3COO) + 3H2O + CH3COOH (2) Co(OH)(CH3COO) f 0.5CoO + 0.5CoCO3 + 0.5H2O + 0.5CH3COCH3 (3) CoCO3 f CoO + CO2

(4)

The first and second reactions occur at temperatures less than 300 °C, but the last reaction needs high temperatures.51 However, in our case; XRD, EDX, and FT-IR data have affirmed formation of pure CoO at relatively low temperature by the proposed hydrothermal process. Therefore, with the consideration that the designed reactor can provide an inert atmosphere, the formation of CoO at low temperature can be explicated as a result of the following reaction,

CoCO3 + H2 f CoO + CO + H2O

(5)

where hydrogen was synthesized according to reaction 1. Otherwise, formation of pure cobalt monoxide can be ascribed to reduction of a CoAc molecule by the synthesized reducing gases. In both cases, we can say that the synthesized water gas does have a distinct role in production of CoO at relatively low temperature. 3.6. Optical Band Gap. UV-visible absorption spectra were carried out in order to characterize the optical absorbance properties of the synthesized cobalt oxides nanofibers. For semiconductor materials, the quantum confinement effect is expected if the


Barakat et al. of (Rhυ)2 versus photon energy. In a case of Co3O4, two band gaps can be detected from the plot. The first band gap can be assigned to a O2--Co2+ charge transfer process (basic optical band gap energy, or valence to conduction band excitation), whereas the second one is ascribed to O2- - Co3+ charge transfer (the Co3+ level is located below the conduction band).57,58 The optical band gap energy difference (∆E ) Eg1 - Eg2) can be calculated. Previous works have indicated that the optical properties of the Co3O4 nanoparticles mainly depend on the particle size; Xu and Zeng56 have synthesized two Co3O4 nanoparticles (5.7 and 47 nm) that have different band gaps energy differences, 1.82 and 1.38 eV, respectively. Also, Gu et al.54 have synthesized Co3O4 nanoparticles (25 nm); the corresponding band gap energy difference was 1.49 eV. Figure 13A shows the absorbance spectra for the synthesized cobalt oxides nanofibers. To estimate the band gap energies, two curves were plotted in the wavelength ranges of 200-350 (Figure 13C) and 400-900 nm (Figure 13B), these two wavelength ranges have been utilized by other researchers.54–56 From Figure 13, panels B and C, extrapolation of the linear region can give two Eg values for the Co3O4 nanofibers obtained at 500 °C (0.94 and 3.43 eV) and at 800 °C (0.98 and 3.42 eV). The optical band gap energy differences are 2.40 and 2.44 eV for Co3O4 obtained at 500 and 800 °C, respectively. As can be concluded from these results, the nanofibers shape enhances the band gap energy gap difference. The same aforementioned strategy has been invoked to check whether the prepared CoO nanofibers have two band gaps or not. As shown in Figure 13, CoO nanofibers also have two band gap energies values, 1.58 and 3.79 eV. The optical band gap energy difference is 2.21 eV. This result needs more extensive study to explain why the CoO nanofibers do have two band gaps with high difference value. Conclusion

Figure 13. UV-vis. spectra for the obtained nanofibers (A) and plot of (REPhoton)2 vs EPhoton at two ranges; 400-900 nm (B) and 200-350 nm (C) for Co3O4 nanofibers prepared at 500 and 800 °C. Also, the optical properties of the synthesized CoO nanofibers have been evaluated.

semiconductor dimension becomes smaller than the Bohr radius of the excited state, and the absorption edge is shifted to a higher energy.52–54 For a semiconductor, the absorbance in the vicinity of the onset due to the electronic transition is given by the following equation,55,56

R)

K(hV - Eg)n hV

(6)

where R is the absorption coefficient, K is a constant, Eg is the band gap, and n is a value that depends on the nature of the transition (1/2 for a direct allowed transition or 2 for an indirect allowed transition). In this case, n is equal to 1/2 for this direct allowed transition. The band gap can be estimated from a plot

In general, cobalt(II, III) oxide nanofibers can be prepared by calcination of CoAc/PVA nanofiber mats in air atmosphere. The calcination temperature does have a distinct effect on the nanofiber’s average diameter and crystallinity as well. Smooth, compact, thin, and good crystallinity Co3O4 nanofibers can be produced at a calcination temperature of 700 °C. The calcination temperature does not have noticeable effect on the band gap energy difference of the Co3O4 nanofibers; however, the nanofibers shape has good influence on the optical properties. Hydrothermal treatment of the CoAc/PVA mats at 300 °C in the presence of carbon produces pure cobalt monoxide nanofibers. The produced CoO nanofibers are continuous, smooth, and crystalline. Acknowledgment. This work is supported by the grant of postdoc program (the second-half-term of 2006), Chonbuk National University (CNU), Jeonju 561-756, Republic of Korea. Supporting Information Available: Transmittance FT-IR intensity for the CoAc/PVA, the Co3O4 nanofibers obtained due to calcination at 500 °C, and CoO nanofibers figure are available. Also, a low-density CoAc/PVA nanofiber mat SEM image is offered. These materials are available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Trasatti, S. Electrochim. Acta 1991, 36, 225. (2) Lee, Y. S.; Hu, C. C.; Wen, T. C. J. Electrochem. Soc. 1996, 143, 1218. (3) Hu, C. C.; Chen, C. A. J. Chin. Inst. Chem. Eng 1999, 30, 431.

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Synthesis and Optical Properties of Two Cobalt Oxides - American

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