poly (ethylene oxide

J Sol-Gel Sci Technol (2010) 55:235–241 DOI 10.1007/s10971-010-2239-0

ORIGINAL PAPER

Synthesis and characterization of cobalt chloride/poly(ethylene oxide) electrospun hybrid nanofibers Adurafimihan A. Abiona • John A. Ajao Samuel Chigome • Jean B. Kana Kana • Gabriel A. Osinkolu • Malik. Maaza

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Received: 4 November 2009 / Accepted: 28 April 2010 / Published online: 13 May 2010 Ó Springer Science+Business Media, LLC 2010

Abstract Cobalt chloride (CoCl2)/poly(ethylene oxide) (PEO) hybrid nanofibers with diameters ranging from 500 to 700 nm were successfully synthesized by a simple and versatile approach of electrospinning. The CoCl2/PEO hybrid nanofibers were studied by different techniques such as Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Fourier Transform Infrared (FTIR) and Thermogravimetric Analysis (TGA). One dimensional (1-D) surface roughness average (Ra1-D) along a typical nanofiber was analyzed to be 57 nm. The chemical interactions between the CoCl2 and PEO molecules showed that polar environment provided by CoCl2 for the PEO molecules aided the modification of CoCl2 molecule configuration. Keywords Electrospinning Nanofibers Cobalt chloride Surface roughness Poly (ethylene oxide)

A. A. Abiona (&) J. A. Ajao G. A. Osinkolu Materials and Electronics Division, Centre for Energy Research and Development, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria e-mail: [email protected]; [email protected] A. A. Abiona J. A. Ajao S. Chigome J. B. K. Kana Malik. Maaza Nanosciences and Nanotechnology Laboratories, Materials Research Group, iThemba LABS, P.O. Box 722, Somerset West 7129, South Africa S. Chigome Department of Chemistry, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa J. B. K. Kana Physics Department, University of Yaounde, Yaounde, Cameroon

1 Introduction Cobalt chloride (CoCl2) is one of the most important inorganic salts that has vast applications in humidity sensing due to its color changes between the anhydrous material and the hydrated complex of CoCl2vH2O (v B 6)[1]. The anhydrous materials exhibit strong absorption bands in the red wavelength range (550– 710 nm), which results in dark blue coloration, while the fully hydrated materials (CoCl26H2O) weakly absorbs in the range of 410–550 nm, which result in a light pink coloration although the material remains almost fully transparent [2]. An adequate knowledge of the concentration of water in ambient air known as relative humidity (RH) is invaluable in monitoring the environment of medical facilities, nuclear reactors, food storage and preservation facilities because it affects their output. For instance, PEO/CoCl2 hybrid has been spun coated by Tsigara et al. [3] and deposited on long-period fiber grating by Konstantaki et al. [4]. Of recent, one dimensional (1-D) nanostructural materials, such as nanofibers, nanowires or nanorods, have attracted much attention because of their desirable and enhanced characteristics when compared to their bulk counterparts. These unique characteristics of 1-D nanostructural materials, which are due to their large surface area to volume ratio [5–7], have given them great potential applications in the fields of sensors [8], catalysts [9], photonics [10] and medicine [11]. As a result of this great attention for 1-D nanostructural materials in recent times, moribund technique of electrospinning has been revived [12–14]. Electrospinning is a versatile, simple and economical technique of synthesizing 1-D nanofibers of diverse materials. It makes use of a high electric voltage (usually 5–30 kV) which is applied to polymer solution or melt. This applied voltage generates electrostatic force

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which deforms the droplet of polymer solution or melt at the end of capillary tip to a cone shape known as Taylor cone. As the electrostatic force overcomes the surface tension of the deformed drop of suspended polymer solution or melt, a jet is formed which is subsequently stretched to form a continuous nanofiber. During its passage to a collective target, the ejected, charged jet dries out, leaving non-woven mat of the nanofibers on the target. In this work, we are mainly concerned with the synthesis and characterization of 1-D nanostructured PEO/CoCl2 polymeric materials via a simple approach of electrospinning. In fact, to the best of our knowledge, there has not been any report on the synthesis of cobalt chloride/polymer hybrid nanofibers via this route. The structural changes due to this method of preparation will be reported and discussed.

2 Experimental 2.1 Materials Blue hydrated cobalt chloride (CoCl26H2O) was supplied by Sigma–Aldrich and used without further purification. Poly (ethylene oxide) (PEO) with an average molecular weight of 3 9 105 (g/mol) was equally obtained from Sigma–Aldrich Batch #: 08314JD and used without further purification while de-ionized water (H2O) was used as solvent. Fig. 1 Simple schematic drawing of single-nozzle electrospinning set-up used in this work

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2.2 Preparation of CoCl2/PEO hybrid nanofibers Blue hydrated CoCl26H2O was heated to purple dehydrated CoCl2 in an oven at 720 °C for 4 h to remove its water of hydration. Then two different solutions of 0.05 and 0.10 g of CoCl2 in 15.0 g of H2O were prepared. Added to each solution was 1.0 g of PEO to obtain two different weight ratios of CoCl2/PEO (1:20, and 1:10). The two solutions were then stirred for 48 h on a magnetic stirrer to ensure complete dissolution of PEO. 2.3 Electrospinning setup The electrospinning of CoCl2/PEO hybrid nanofibers was carried out using the set-up as shown in Fig. 1. The solutions obtained were loaded into a glass pasteur pipette. The diameter of the tip of the pipette was 1 mm. The electric field was provided by a high voltage (HV) power supply that can generate DC voltages up to 30 kV. A copper wire was inserted into the pipette to act as the electrode for charging the polymer solution. The pipette was clamped to a tripod stand located in front of a grounded target. The positive (anode) terminal of the variable high voltage transformer was attached to the copper wire inserted into the polymer solution in the pipette for electricity to be conducted through the solution. A grounded aluminium foil target was placed opposite and perpendicular to the tip of the pipette onto which the fibers were deposited. The


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pipette was tilted approximately 100 from the horizontal to maintain a droplet of solution at its tip. On the grounded aluminium foil were mounted various other substrates such as alumina and silicon wafers. The stationary grounded target was 15 cm from the charged pipette tip. The hybrid nanofibers of CoCl2/PEO solution were then collected on the target.

Microscope (SEM). Thermogravimetric analysis was carried out using Thermal Analysis Instruments Q600 simultaneous TGA/DSC equipment with horizontal balance and furnace. The samples were heated from room temperature to 1,000 °C at a rate of 10 °C/min under nitrogen atmosphere.

2.4 Characterization

3 Results and discussion

Atomic force microscopy (AFM) measurements were carried out using a Nanoscope III (Digital Instruments, Inc., Santa Barbara, CA, USA) in tapping mode in air with etched Si probe. Fourier transform infrared (FTIR) spectra were obtained using a Perkin Elmer Pragon 1000PC FTIR spectrometer. The morphology of the nanofibers was examined using a Leo-StereoScan 440 Scanning Electron

3.1 Scanning electron microscopy Figure 2 shows the morphology of non-woven 1:10 and 1:20 ratios CoCl2/PEO hybrid nanofibers deposited on alumina and silicon wafer substrates at 20 kV. Figure 2a and c show SEM images of hybrid nanofibers (1:10) with beads, junctions and coagulations while Fig. 2b and d

Fig. 2 SEM images of CoCl2/PEO nanofibers electrospun of ratio a 1:10 on alumina, b 1:20 on alumina, c 1:10 on silicon, d 1:20 on silicon

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show a well-optimized bead-free nanofibers with nearly cylindrical structure for ratio 1:20 at 20 kV. This probably shows that high concentration of CoCl2 favored the formation of beads, junctions and coagulations. This could be attributed to the effects of high polarity environment of PEO molecules. In fact, as the CoCl2 concentration increases, the polarity of the PEO environment increases and more of its oxygen atoms are exposed to attacks from its environment. Similar observations have been reported by Sui et al. [15] in their study of PEO/ ZnO composite electrospun fibers. Therefore, as the polarity of the PEO environment increases, the stretching rate of the electric force of the applied high voltage on the nanofiber increases faster than PEO molecules can withstand and as a result causes the deformation of the nanofiber instead of merely stretching it. This deformation results in the formation of beads, junctions and


coagulations observed in high concentration CoCl2/PEO (1:10 ratio) nanofibers. 3.2 Atomic force microscopy The surface topography of CoCl2/PEO hybrid nanofibers deposited on alumina substrate was also studied by tapping mode Atomic Force Microscopy (AFM). Figure 3 shows the AFM images and the height profiles of the nanofibers. It can be observed from the AFM images that the CoCl2/PEO hybrid nanofibers have uniform and smooth surface. Surface roughness average, Ra (the most commonly used surface roughness parameter), was calculated in two forms using Nanoscope III software. First, 1-D surface roughness average (Ra1-D) on a line along a nanofiber on AFM image as shown in Fig. 3a (and its height profile in Fig. 3b) is 57 nm. This analysis is similar to that reported by

Fig. 3 AFM images of CoCl2/PEO hybrid nanofibres depicting a 1-D surface roughness, Ra1-D, b height section of (a); c 2-D surface roughness, Ra2-D, d 3-D image of the nanofibres

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Dharmaraj et al. [16] except for the Ra1-D which was not determined along the nanofibers as demonstrated in this work. Secondly, 2-D surface roughness average (Ra2-D) of the AFM image as shown in Fig. 3c is 292 nm which gives general information on the surface texture and random orientation of the nanofibers within the selected area. Since surface roughness measured by the AFM depends on the size of the area selected [17], in this work the area used was 33.2 lm2. The three dimensional (3-D) image of the nanofibers was also shown in Fig. 3d. In this work, the nanofiber diameters were also measured to be in the range of 500–700 nm according to the procedure suggested by Demir et al. [17] using AFM. 3.3 FTIR spectra analysis Figure 4 shows FTIR spectra of pure PEO powder, dehydrated CoCl2 powder and CoCl2/PEO nanofibers. The bands around 1,460 and 1,348 cm-1 are ascribed to the vibrations of –CH2– group, likewise, the bands about 1,100 and 960 cm-1 are due to C–O group asymmetric stretching vibrations [10], and strong band near 2,889 cm-1 attributed to the symmetric and asymmetric C–H stretching modes as shown in Fig. 4a. Furthermore, Fig. 4 shows strong stretching vibration bands of CoCl2 particles at 3,396, 3,180, 1,596, and 600 cm-1. Being one of the polymers with hydrophilic oxygen atom and hydrophobic ethylene group aligned alternatively, PEO interaction to its environment is in two different configurations: PEO has zigzag configuration in nonpolar environment where it embeds most of its polar oxygen atoms in the PEO chains while in polar environment, the oxygen atoms are exposed to strong interaction with other species like the solvent molecule and

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other polar species in the solution as reported by Sui et al. [15]. Comparing the three spectra in Fig. 4, one could observe that the PEO vibration bands increase in intensities in CoCl2/PEO hybrid nanofiber spectrum while 1,596 cm-1 band of CoCl2 powder was broadened, weakened and red shifted to 1,633 cm-1. In addition, bands at 3,396 and 3,180 cm-1 were convoluted into broadened shoulder between 3,020 and 3,690 cm-1 depicted by an arrow. Similar observation had been reported by Tang et al. [18]. This observation suggested that there were some chemical interactions between PEO molecules and CoCl2 particles which could be as a result of polar environment, provided by CoCl2 molecules in solution, which make the oxygen atoms of PEO to stretch out which effectually modify the chemical configuration of CoCl2 molecules. 3.4 Thermogravimetric analysis (TGA) Figure 5 shows the TGA thermographs of CoCl2/PEO hybrid nanofibers, PEO powder and dehydrated CoCl2 powder. The thermograph in Fig. 5a (CoCl2/PEO hybrid nanofibers) shows three distinct steps. We ascribed the first weight loss in the temperature range from 20 to 350 °C (approximately 20%) to the gradual evaporation of solvent (water) [19]. The second weight loss in the range from 350 to 430 °C (approximately 60%) is associated with the fast thermal decomposition of PEO as also observed in Fig. 5b of PEO powder. The third mass loss in the range from 430 °C and above is associated to gradual thermal decomposition of dehydrated CoCl2 molecules. However, this observation is in sharp contrast to decomposition of CoCl2 molecules about 735 °C (melting point of CoCl2) in Fig. 5c. This contrast could be as a result of interaction between CoCl2 and PEO molecules, which has been confirmed by FTIR analysis above. The results presented here are the preliminary observations on the electrospun CoCl2/PEO hybrid nanofibers prepared via the route of electrospinning for the first time to our knowledge (after several unsuccessful attempts). Further works are in progress to study its optimization of functional performance in various application fields.

4 Conclusion

Fig. 4 FTIR spectra of PEO powder, dehydrated CoCl2 powder, CoCl2/PEO hybrid nanofibers

In this study, CoCl2/PEO hybrid nanofibers have been successfully prepared via the simple technique of electrospinning. The morphology of the nanofibers has been characterized by SEM and AFM. The AFM analysis showed that the nanofibers are cylindrical in structure and smooth with their diameter ranging from 500 to 700 nm. The chemical interactions between the CoCl2 and PEO

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Fig. 5 TG thermograph of a CoCl2/PEO hybrid nanofibres b PEO powder c dehydrated CoCl2 powder

molecules were studied by FTIR which showed that polar environment provided by CoCl2 for the PEO molecules aids the modification of CoCl2 molecule configuration. This interaction was supported by the thermal behaviour of the CoCl2/PEO hybrid nanofibers observed by TGA. Acknowledgments This work was financially supported by the following organizations; African Laser Centre (ALC), Nanoscience African Network (NANOAFNET), Abdul Salam International Centre for Theoretical Physics (ICTP) and National Research Foundation (NRF) of South Africa. The Centre for Energy Research and Development, Obafemi Awolowo University, Ile-Ife, Nigeria is also appreciated for granting two of the authors leave of absence (AAA, JAA) during the preparation of this work. Southern and Eastern Africa Network of Analytical Chemists (SEANAC) is acknowledged for funding one of the authors (SC) for his research work at iThemba LABS.

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