Optimization of the electrospinning parameters of Mn2O3 and Mn3O4 nanofibers

Optimization of the electrospinning parameters of Mn2O3 and Mn3O4 nanofibers

Available online at www.sciencedirect.com CERAMICS INTERNATIONAL Ceramics International 41 (2015) 12065–12072 Optimization of the electrospinning p...

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Available online at www.sciencedirect.com

CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 12065–12072

Optimization of the electrospinning parameters of Mn2O3 and Mn3O4 nanofibers A.M. EL-Rafei Refractories, Ceramics and Building Materials Department, National Research Centre, 33 EL Bohouth st. (former EL Tahrir st.), Dokki, Giza Egypt-P. O. 12622 Received 2 April 2015; received in revised form 4 June 2015; accepted 4 June 2015 Available online 12 June 2015

Abstract Three dimensional porous random Mn2O3 and Mn3O4 nanofibers were successfully prepared via a facile electrospinning homogenous solution of 0.006 mol of manganese nitrate 4-hydrate in 10 wt% polyvinyl alcohol followed by calcination of the as-spun nanofibers at 500, 700 and 1000 1C. The processing parameters that directly influence the morphology of the produced fibers are namely; concentration, viscosity of the solution as well as the potential of spinning as high concentration gave beaded fibers at all applied potentials. The present selected conditions were found to be; voltage 24 kV, flow rate 0.3 ml/h, the distance between the needle and collector 10 cm, viscosity 0.955 Pa S and the inner diameter of the needle 0.39 mm. X-ray diffraction results of the nanofibers showed that Mn2O3 and Mn3O4 phases were processed at two temperature; 700 1C and 1000 1C, respectively. Field emission scanning electron microscopy revealed that Mn2O3 and Mn3O4 nanofibers were fabricated with diameters in the range of 40–80 nm and 70–300 nm, respectively. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Electrospinning; B. Fibers; B. X-ray methods; D. Manganese oxides

1. Introduction Nanocrystalline metal oxides play an important role in many areas of chemical, physical and materials sciences. They can adopt a vast number of structural geometries with an electronic structure that can exhibit metallic, semiconductor or insulator characteristics [1]. During the past decade, manganese oxides have attracted considerable research interest due to their distinctive chemical and physical properties, their low cost, high natural abundance, environmental compatibility and potential applications in catalysis, ion exchange, molecular adsorption, biosensors, wastewater treatment and supercapacitors. A wide variety of manganese oxides (e.g., MnO2, Mn2O3 and Mn3O4) have been developed through various techniques [2–5]. Particularly, nanometer-scale manganese oxides with their large specific surface areas and small sizes are of great significance in some novel electrical, catalytic, and magnetic

applications as they show properties different from that of bulky materials. The success of previous applications relies on the ability to obtain cost-effective and high-quality nanosized materials with uniform grain structure [6]. Hence, many research efforts have focused on rational control of phase, shape, size, and dimensionality of nanomaterials [7]. In recent years, 1-D1 nanostructures, e.g., nanowires, nanotubes, and NFs2, have attracted great interest in physical, chemical, and biological sensor studies due to their unique properties such as small size and large surface-to-volume ratio [8]. A variety of methods including sol–gel [9], co-precipitation [10], solvolysis [11], microwave assisted synthesis [12], self-assembling hydrothermal [13], electro-deposition [14] and the electrospinning technique [15–21] have been covered for synthesis of manganese oxides.

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E-mail addresses: [email protected], [email protected] http://dx.doi.org/10.1016/j.ceramint.2015.06.022 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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Unidimensional. Nanofibers.

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Electrospinning technique is a promising technique, whose applications are rapidly growing. It is a simple, versatile, and relatively inexpensive technique for the synthesis of NFs. Electrospinning has the advantage of synthesis of continuous fibers, single-step methodology and produce uniform 1-D NFs in mass production of oxides, non-oxides and polymers, applying different types of polymers and precursors [22,23]. It is based on inducing static electrical charges on the molecules of a solution at such a density that the selfrepulsion of the charges causes the liquid to stretch into a fiber in an electric field. Provided there is no breakage in the stretched solution, a single strand of continuous fiber is formed upon solvent evaporation [24]. The transformation of polymer solutions into NFs in the electrospinning process are affected by several parameters including (a) the solution properties, such as viscosity, polymer concentration, conductivity/charge density, surface tension, polymer molecular weight and volatility of solvents; (b) processing condition, such as the field strength/applied voltage, flow rate, the distance between the tip and the collector, the needle tip design and placement, collector composition, geometry and take up velocity of the collector and (c) ambient parameters, such as temperature, humidity and atmospheric pressure [25]. PVA3 has been used widely in producing metal oxide NFs by the electrospinning method. The presence of hydroxyl groups in PVA chains, have the capability to generate hydrogen bond with many anions which enhances the solubility of the metal salt in the PVA solution. PVA is characterized by a low decomposition temperature that facilitates the removal of the organic part of the obtained electrospun mat [21]. Shao et al. [18] prepared Mn2O3 and Mn3O4 NFs with a diameter of 50–200 nm from manganese acetate and PVA. They explained that the crystalline phase and morphology of NFs were largely influenced by the temperature of calcinations. Fan and Whittingham [15] succeeded in preparing of Mn3O4 NFs using PMMA4 after complete removal of the organic matter. Mn3O4 NFs with diameters in the range of 100–600 nm have been successfully synthesized by Sahoo et al. [20] after calcination of the electrospun mats of manganese acetate and PVA at 1000 1C for 2 h. Lee et al. [17] synthesized hybrid (MnOx) NFs by using PVP5 and manganese acetate as precursors. They calcined the as-spun NFs at different temperatures to get inorganic MnOx NFs of variable compositional ratios of Mn3O4 to Mn2O3. An attempt has been made in the present work to optimize the conditions, namely, concentration of manganese salt, viscosity and potential of spinning solution to obtain uniform NFs with a relatively small diameter and in a narrow distribution range. The effect of the manganese precursor concentration of the PVA solution was first tackled and the

role of the temperature of calcination was verified to fabricate the manganese oxide NFs. 2. Materials and experimental procedure 2.1. Preparation Two compositions; 0.012 and 0.006 mol of manganese nitrate 4-hydrate 99% (BDH Laboratory Supplies, Poole, BH15 1TD, England) were dissolved in 30 ml deionized water containing 3 g of polyvinyl alcohol (Qualikems, Degree of polymerization 1700–1800) to form a viscous spinning solution of manganese oxide precursor, denoted as [M1] and [M2], respectively. The electrospinning process was performed by using Na-Bond Electrospinning device, China. The solution formed was transferred into the syringe equipped with a metallic capillary nozzle connected to a high power supply. The voltage was adjusted between 13 and 28 kV for each solution. The inner diameter of the nozzle used was 0.499 mm and its height from the collector was set at 10 cm. The flow rate selected was 0.3 ml/h. The electrospun fibers were collected on an aluminum foil. Depending on the obtained morphologies of the electrospun mats, the optimized sample was selected to be calcined at different temperatures; 500, 700 and 1000 1C for 1 h respectively, in the ambient atmosphere. A flow chart of the methodology of the prepared NFs is given in Fig. 1. 2.2. Characterization Viscosity measurements were performed to the prepared solutions at 25 1C utilizing an equipment type: RHEOPLUS/32 V3.40 21003304-33025, MCR301 SN80218500; FW3.30; Slot2; Adj1605d. The NFs prepared from both compositions spun at the different potentials were collected and examined under Field Emission Scanning Electron Microscopy, a Philips XL30 model using an accelerating voltage of 30 kV, magnification up to 400,000  and resolution for W. 3.5 nm. Samples were coated with a thin film of gold prior to examination. X-ray diffraction analysis of the as-prepared powders was performed using BRUKUR D8 ADVANCE with secondary monochromatic beam CuKα radiation at 40 kV and 40 mA. A thermo-gravimetric analysis was carried out for the as prepared manganese nitrate/PVA NFs mats utilizing SDT Q600 V20.9 Build 20 equipment, produced by TA Company, USA. The sample holder was heated in air at a rate of 10 1C/ min, in the temperature range from ambient temperature up to 800 1C. 3. Results 3.1. Microstructure of the as-spun manganese nitrate/PVA mats

3

Poly vinyl alcohol. Poly(methyl methacrylate). 5 Poly (N-vinyl pyrrolidone). 4

The morphologies of the mats spun at various potentials from 13 to 28 kV prepared from different solutions are shown

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Manganese nitrate Electrospinning Polyvinyl alcohol

Calcination Mn-nitrate/PVA NFs

Mn Oxide NFs

Deionized water

Fig. 1. The growing process of manganese oxide NFs from precursors.

Table 1 Effect of solution concentration and applied potential on the morphology and average diameter of the as-spun manganese nitrate/PVA mats. Applied voltage (kV)

Morphology and diameter of M1

Morphology and diameter of M2

13 15 17 20 22 24 26 28

Droplet/Beaded microfiber Beaded/microfiber Beaded/microfiber (300–400 nm) Beaded/microfiber (250–600 nm) Beaded/microfiber (200–400 nm) Beaded nano/microfiber (100–200 nm) Beaded nano/microfiber (90–300 nm) Beaded nano/microfiber (100–200 nm)

Beaded nano/microfiber Beaded nano/microfiber Beaded nano/microfiber Beaded nano/microfiber Beaded nano/microfiber nanofiber (40–150 nm) nanofiber (80–250 nm) Beaded nano/microfiber

in Table 1. The viscosity of the spinning solution plays an important role in determining the morphology of the obtained mats. [M1] containing the higher concentration, showed a viscosity value of 1.44 Pa S compared to [M2] with the lower viscosity of 0.955 Pa S. Accordingly, the morphology and diameter of the as-spun fibers varied. The increase in fiber diameters is the result of the increase in the viscosity of the spinning solutions. It seems that the viscosity of [M1] is relatively high that lead to non-uniform ejection of the jet. Thus, the applied electrical potential over the range (13– 28 kV) had an insignificant effect on the morphology of the produced fibers that were always beaded with larger diameters and with a broader distribution in diameter sizes, as demonstrated in Fig. 2. On the other hand, [M2] at lower values of applied electrical potential (i.e. 13–17 kV), the produced mats were beaded fibers with an average diameter of 250 nm as shown in Fig. 3 (M113, M115 and M117), while those at 20, 22 and 24 kV were smooth fibers with an average diameter of 220, 175 and 95 nm, respectively (Fig. 3, M220, M222 and M224). A further increase in the applied electrical potential resulted in an increase in diameter of the produced fibers, Fig. 3. (M226 and M228). In the light of the above results, mats of the solution [M2] spun at 24 kV was selected to be calcined at different temperatures to prepare manganese oxide. The optimal conditions were summarized in Table 2. 3.2. Thermogravimetric analysis of manganese nitrate/PVA NFs mats Fig. 4 shows TGA thermogram of manganese nitrate/PVA NFs fabricated at the selected conditions which outlined in Table 2. Loss in weight took place in the temperature range between 80 up to 500 1C. Loss of trapped water and dehydration of manganese nitrate followed by the decomposition of the manganese nitrate are accompanied by a loss in weight occurring between 80 to 250 1C. While the degradation of

(100–400 nm) (60–500 nm) (50–400 nm) (100–300 nm) (100–250 nm)

(60–300 nm)

PVA comprising dehydration on the polymer side chain results in the weight loss at 250–350 1C, the another weight loss from 350 to 500 1C is attributed to the cleavage of C–C bonds of the polymer [26]. After 500 1C, there was no alteration in weight, indicating the formation of manganese oxide. A weight gain was recorded at 700 1C. Therefore, the calcination of the asspun nanofiber was carried out at 500, 700 and 1000 1C for 1 h in air.

3.3. Phase composition of the calcined NFs XRD patterns of the samples calcined at different temperatures are shown in Fig. 5. The sample calcined at 500 1C was almost amorphous with some peaks of lower intensities belonging to Mn2O3. The main manganese oxide crystalline phases developed at 700 and 1000 1C were Mn2O3 (JCPDS 073-1826) orthorhombic structure and Mn3O4 (JCPDS 0240734) tetragonal structure, respectively.

3.4. Microstructure of NFs calcined at different temperatures The morphologies of the NFs calcined at different temperatures are shown in Fig. 6(a)–(f). FESEM of the NFs calcined at 500 1C (Fig. 6a and b) shows remnants of PVA with no obvious grains of manganese oxides. Complete removal of the PVA was achieved in those calcined at 700 1C leaving manganese oxide NFs with retained 3D6 porous structure (Fig. 6c). The measured diameter of manganese oxide NFs were about 50 nm (Fig. 6c). Higher magnification in Fig. 6d showed a single fiber composed of 40–80 nm Mn2O3 grains. Calcination at 1000 1C, results in an increase in the diameter of the fibers to reach 70–300 nm Mn3O4 (Fig. 6e and f), that were partially sintered and showing grain growth. 6

Three dimensional.

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M113

M115

M117

M120

M122

M124

M126

M128

Fig. 2. FESEM images of as-spun manganese nitrate/PVA (M1) mats obtained at different applied electrical potential; the subscriber of M1 is the applied electrical potential.

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M 2 13

M 2 17

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M 2 15

M 2 20

M 2 22

M 2 24

M 2 26

M 2 28

Fig. 3. FESEM images of as-spun manganese nitrate/PVA (M2) mats obtained at different applied electrical potential; the subscriber of M2 is the applied electrical potential.

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Table 2 Optimal parameters for the fabrication of manganese nitrate/PVA NFs. Parameter

Optimized value

Concentration of manganese nitrate Applied voltage Flow rate Needle to collector distance Inner needle diameter Viscosity

M2 24 kV 0.3 mL h 1 10 cm 0.39 mm 0.955 Pa S

120

Weight (%)

100

80

60

40

20

0

0

100

200

300

400

500

600

700

800

900

Temperature (°C)

Fig. 4. TGA of manganese nitrate/PVA (M2) mats in the air. 100

500 °C 700 °C 1000°C

Mn3O4: Mn2O3:

90 80

Intensity (A.U.)

70 60 50 40 30 20 10 0 0

10

20

30

40

50

60

70

80

90

2-Theta Fig. 5. XRD patterns of the manganese oxide NFs obtained after calcination at different temperatures for 1 h in air: (a) 500, (b) 700 1C and (c) 1000 1C.

4. Discussion NFs were successfully fabricated from manganese nitrate/ PVA solutions by electrospinning using the procedure outlined in the experimental section. Although electrospinning is an efficient method for producing continuous polymer NFs, yet there are a number of variables that can affect their morphologies namely; polymer concentration, conductivity of

the solution, rate of feed, needle inner diameter, needle tip to collector distance and electric field voltage. All these parameters significantly affect the spinability according to the character of the polymer solution [27]. The Viscosity of the spinning solution plays an important role in determining the morphology of the mats obtained. There is a proper range of viscosity of the polymer solution within which the polymer solutions are electrospinable and beyond which beads or even droplets are likely to occur [28]. Higher viscosities cause the increase in the viscoelastic force and as a result an increase in the diameter of the spun fibers. Thus, [M1] solution has a higher viscosity than [M2] solution that lead to an increase in the diameter of the spun fibers. This is explained in view of the viscoelastic force behavior that causes the thinning of the charged jet less likely to occur and as a consequence prevent the jet segment from being stretched by the constant Coulombic repulsion force [28,29]. Accordingly, this results in the onset of the bending instability to occur closer to the collector. It seems that the viscosity of M1 is too high, which in turn, leads to non-uniform ejection of the jet. Meanwhile, the effect of the applied electrical potential for the composition M1 is insignificant and the viscosity has a predominant effect on the morphology of the produced fibers i.e. beaded morphology is a characteristic all over the range of the applied electrical potential (13–28 kV). On the other hand, solution [M2], showed beaded fibers at lower values of applied electrical potential, it seems that the strength of the electric field is not sufficient to overcome the surface tension of the solution. Therefore, the beads are observed, while at 20–24 kV smooth fibers with a narrow distribution in diameter sizes are formed. Krissanasaeranee et al. [29], demonstrated that the decrease in the diameter of the fibers is attributed to the increment in the applied electrical potential that results in the increase of both the electrostatic force (which is responsible for the transport of the charged jet to the collection device) and the Coulombic repulsion force (which is responsible for the stretching of an ejected jet segment) resulting in a decrease in the diameters of the asspun fibers. Nevertheless, at higher values of applied electrical potential (26 and 28 kV), the increased electrostatic force caused both the speed and the mass flow rate of the charged jet to increase and cause the onset for the bending instability to occur closer to the collector, which in turn, cause an increase of the diameter of the fiber [28] with some beads. In other words, when the strength of the electric field is not sufficient to overcome the surface tension of the solution, the beads are observed. Only when the voltage is sufficient (to some extent) to overcome the surface tension of the solution (i.e. optimized voltage), no beads are formed. Above this value of the optimized voltage, the beads began to appear again, as the jet eventually moves around the edge of the tip with no visible Taylor cone, producing legitimate amount of beads [25]. 5. Conclusions 3D porous random Mn2O3 and Mn3O4 NFs were successfully produced by the electrospinning of manganese nitrate/PVA solutions. Electrospinning parameters were optimized in the

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Fig. 6. SEM images for calcined NFs at 500 (a, b), 700 1C (c, d) and 1000 1C (e, f).

direction that the Columbic force can overcome surface tension of the solution in order to obtain uniform NFs with lowest diameter. The optimal parameters were 24 kV of applied voltage and 0.955 Pa S. of solution viscosity. The polymer solution containing low concentration of manganese nitrate (0.006 mol) proved to be the most appropriate to fabricate porous random Mn2O3 and Mn3O4 NFs, with diameters in the range of 40– 80 nm and 70–300 nm, respectively. Acknowledgments I would like to thank Prof. Doreya Ibrahim for her time, patience, support, experience and her guidance. Also, I would

wish to thank Prof. Ninet ahmed for continuous helps and support.

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