Electrospinning of nickel oxide nanofibers: Process parameters and morphology control

Electrospinning of nickel oxide nanofibers: Process parameters and morphology control

MA TE RI A L S CH A R A CT ER IZ A TI O N 9 5 (2 0 1 4) 6 5– 7 1 Available online at www.sciencedirect.com ScienceDirect www.elsevier.com/locate/mat...

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MA TE RI A L S CH A R A CT ER IZ A TI O N 9 5 (2 0 1 4) 6 5– 7 1

Available online at www.sciencedirect.com

ScienceDirect www.elsevier.com/locate/matchar

Electrospinning of nickel oxide nanofibers: Process parameters and morphology control Abdullah Khalil, Raed Hashaikeh⁎ Institute Center for Water and Environment (iWATER), Department of Mechanical and Materials Engineering, Masdar Institute of Science and Technology, Abu Dhabi 54224, United Arab Emirates

AR TIC LE D ATA

ABSTR ACT

Article history:

In the present work, nickel oxide nanofibers with varying morphology (diameter and

Received 5 May 2014

roughness) were fabricated via electrospinning technique using a precursor composed of

Received in revised form 3 June 2014

nickel acetate and polyvinyl alcohol. It was found that the diameter and surface roughness

Accepted 5 June 2014

of individual nickel oxide nanofibers are strongly dependent upon nickel acetate concentration in the precursor. With increasing nickel acetate concentration, the diameter of nanofibers increased and the roughness decreased. An optimum concentration of nickel

Keywords:

acetate in the precursor resulted in the formation of smooth and continuous nickel oxide

Electrospinning

nanofibers whose diameter can be further controlled via electrospinning voltage. Beyond an

Nickel oxide nanofibers

optimum concentration of nickel acetate, the resulting nanofibers were found to be ‘flattened’

Morphology

and ‘wavy’ with occasional cracking across their length. Transmission electron microscopy analysis revealed that the obtained nanofibers are polycrystalline in nature. These nickel oxide nanofibers with varying morphology have potential applications in various engineering domains. © 2014 Elsevier Inc. All rights reserved.

1. Introduction Nickel oxide (NiO) nanofibers (NFs) are of great practical interest in several engineering applications such as sensing [1,2] electrochemical capacitance [3,4] photocatalysis [5,6] and other chemical catalysis [7]. Moreover, their morphology has gained significant attention due to strong dependence of properties on the morphology [8–10]. Whereas several physical techniques have been successfully employed for their synthesis, electrospinning has turned out to be a better choice because of its simplicity, economy, scaling capability [11] and control over the NF morphology [12]. Variety of metal oxide NFs have been synthesized through this technique which have shown promising potential in different engineering domains such as sensing [13], catalysis [14] and solar energy

conversion [15,16]. It has been demonstrated that electrospinning can also be employed for synthesizing pure NiO NFs using an appropriate precursor composed of NiAc salt and any suitable polymer such as PVP [1,7], PAN [17] or PVA [18]. However, no report exists regarding the morphology control of these electrospun NiO NFs. We have found that using different proportions of NiAc in the solution containing PVA as polymeric component, the diameter and roughness of NiO NFs can be easily controlled and an optimum proportion of NiAc is mandatory for obtaining smooth and continuous fibers. The diameter of these NFs can be further reduced by increasing the electrospinning voltage. These electrospun NiO NFs with varying morphology and microstructure which are producible in an economical and scalable fashion have potential applications in sensing, catalytic, magnetic and photovoltaic domains.

⁎ Corresponding author. E-mail addresses: [email protected] (A. Khalil), [email protected] (R. Hashaikeh).

http://dx.doi.org/10.1016/j.matchar.2014.06.005 1044-5803/© 2014 Elsevier Inc. All rights reserved.

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2. Materials and Methods Using nickel II acetate (NiAc) (Sigma-Aldrich) and poly(vinyl alcohol) (PVA) (Mw = 61,000, Sigma-Aldrich) as main chemicals, four types of solutions were prepared with varying salt to polymer ratios. A typical solution was prepared by mixing the specific amount of NiAc in 1 ml of water via magnetic stirring for half an hour and then 1 ml of acetic acid was added followed by magnetic stirring for another half an hour. Acetic acid was added to avoid the hydrolysis of PVA. 5 g of 15 wt.% aqueous PVA solution was then added and the solution was left for vigorous magnetic stirring until a viscous and uniform solution was formed. The four types of solutions with different salt to polymer ratios made and studied in this work, named Sol. 1, 2, 3 and 4, are shown in Fig. 1. The obtained solution was transferred to a plastic syringe having gauge 20 (internal dia = 0.603 mm) stainless steel needle at its end. A Nanon-01A electrospinning setup (MECC, Japan) was used for electrospinning. The nozzle–collector distance was kept constant at 10 cm and the voltage was varied between 20 and 29 kV while keeping the solution flow-rate constant at 0.5 ml/h. The electrospinning was carried out at room temperature and the relative humidity of nearly 60% was recorded during the process. The fibers were collected on either microscopic glass slide or aluminum foil for specific characterization. The samples were left over night in a furnace at 80 °C to remove the moisture followed by calcination in a furnace at 475 °C for 2 h under ambient conditions. The heating rate was 5 °C/min and once the calcination cycle was over, the furnace was allowed to cool down to room temperature before removing the samples. The purity of NFs was determined using energy dispersive spectroscopy (EDS) (EDAX) and X-ray diffraction (XRD) (Empyrean, PANalytical). The morphology of NFs was determined using a high resolution scanning electron microscope (SEM) (Nova NanoSEM, FEI) operating at 5 kV whereas a high resolution transmission electron microscope (HRTEM) (Tecnai F20, FEI), operating at 200 kV, was used for studying the

crystalline structure of NFs. For TEM analysis, the NFs were directly collected on Si grids having 15 nm thick Si3N4 support film. An atomic force microscope (AFM) (Cypher, Asylum Research) was used for determining the roughness of individual NFs. For AFM, amplitude modulation noncontact mode was employed for imaging with silicon cantilever (fr = 320 kHz, k = 42 nN/nm) vibrating in first mode with an amplitude of 8 nm. The tip attached at the end of cantilevers had a tip radius of 8 nm (± 2 nm).

3. Results and Discussion Fig. 2(a) shows the EDS spectrum and the corresponding quantitative results. The longest peak in the spectrum corresponds to Al which was used as a substrate for collecting the fibers. From the quantitative results, the contribution of this peak was excluded leaving only Ni and O of which the NFs are composed. The relative proportion of Ni and O was found to be nearly 80% and 20% by weight, respectively which is in good stoichiometric agreement with pure NiO. The XRD spectrum with all the indexed peaks, presented in Fig. 2(b), matches perfectly with that of pure NiO confirming the purity and crystallinity of obtained NiO NFs [19]. Fig. 3 shows the high resolution SEM images of NiO NFs obtained from different solutions. It can be seen that the NF morphology changes from ‘thin’ and ‘rough’ to ‘thick’ and ‘smooth’ as the NiAc content in the solution increases. As shown in Fig. 3(a), when the NiAc content is as low as only 0.5 wt.%, the NFs are not only very thin but also very rough and discontinuous. Due to much lower concentration of salt, the majority of space in as spun composite fibers is occupied by polymer leading to the formation of rough and discontinues NiO NFs as the polymer is selectively removed. However, when the salt content is sufficiently high, i.e. 4.5 wt.%, the as spun fibers are densely packed with salt leading to the formation of smoother and thicker NiO NFs as the polymer is removed. When the salt concentration

Fig. 1 – Four types of solutions with varying NiAc/PVA ratios.

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Fig. 2 – (a) EDS and (b) XRD spectra of obtained NiO NFs.

becomes excessive, i.e. 7.7 wt.%, the over-packing and confinement of salt molecules across the composite NFs will cause the generation of compressive stresses in the transverse direction across the NFs leading to waviness and irregularities across the NF length. This will also cause the NF cross-section to become irregular in shape, Fig. 3(d), rather than circular which is observed for lower and more optimum salt concentrations, Fig. 3(b and c). At the same time, the compressive stresses at very high salt content, sol. 4, will lead to occasional cracking of NFs across their length, as marked with dotted circles in Fig. 4, leading to smaller and more discontinuous NFs. Also observable is the ‘flattening (more belt like)’ tendency of NFs which can be attributed to strong impact of densely packed wet nanofibers on the collector

causing their flattening to some extent. Nevertheless, the smoothest and highly continuous NFs were obtained for sol. 3, i.e. NiAc = 4.5 wt.%, however with significantly large diameter as compared to the solutions based on lower NiAc concentrations. The electrospinning process, however, provides greater degrees of freedom as compared to any other process for controlling NF morphology. By increasing the electrospinning voltage or decreasing the solution flow-rate, the diameter of NFs can be further reduced [20]. Fig. 5 shows the average NF diameter for the four solutions as a function of electrospinning voltage and we found that the diameter of individual NFs can be significantly reduced by increasing the electrospinning voltage without affecting the NF morphology, as shown through SEM image insets for Sol. 3.

Fig. 3 – SEM images of NiO NFs obtained at electrospinning voltage of 20 kV from Sol. (a) 1, (b) 2, (c) 3 and (d) 4 (scale bar in all images = 100 nm).

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Fig. 4 – Low resolution SEM image of NiO NFs obtained from Sol. 4 at the electrospinning voltage of 20 kV. The cracks are marked with dotted circles.

A linear increase in the ceramic NF diameter with increasing salt content has also been reported in case of electrospun ZnO NFs which were fabricated via solution composed of zinc acetate and PVA [21]. It is important to note that the above explanation given for the increase in the NF diameter with increasing salt content is partially true. It should be realized that an increase in salt content will also lead to increased solution conductivity which will cause greater electrostatic force experienced by jet eventually leading to decreased NF diameter. It has been shown elsewhere that, while electrospinning, the NF diameter decreases with increasing zinc acetate concentration in the solution containing constant amount of dopant and this behavior was explained in terms of increasing solution conductivity with increasing salt content [22]. An increase in

Fig. 5 – Variation in the average diameter of NiO NFs as a function of electrospinning voltage. SEM image of single NF at each voltage is also shown for Sol. 3 as insets (scale bar in all insets = 100 nm).

NF diameter with increasing salt content can also be explained in the light of ‘electrohydrodynamic (EHD)’ theory [23] according to which the terminal jet diameter emerging from a solution is directly proportional to the surface tension and inversely proportional to the total current flowing through the solution (or the solution conductivity). Although the solution conductivity increases with an increasing salt content, but at the same time, the viscosity of the solution also increases (We observed drastic thickening of solution with increasing NiAc concentration.). Since the viscosity and the surface tension are directly related (assuming Newtonian fluid), an increased surface tension will tend to increase the terminal jet diameter and hence the NF diameter. It is thus evident that there exists a competition between the surface tension and the net solution conductivity in controlling the final NF diameter. If the solution is moderately conductive, the surface tension seems to be the dominant factor influencing the NF diameter. However, in the presence of any suitable dopant or if the solution is itself very conductive due to specific type of salt used, a drastic increase in the solution conductivity can overwhelm the surface tension effect leading to decrease in NF diameter with increasing salt content. The final NF diameter is thus determined by the combined effect of solution conductivity and the surface tension. The crystal structure of NiO NFs was examined via TEM. Fig. 6(a) shows the TEM image of several NFs whereas Fig. 6(b) shows HRTEM image of individual NF clearly showing that the NF is composed of tiny crystalline domains randomly attached together to give the NF shape. A further high resolution image of a portion inside a single NF is presented in Fig. 6(c). It can be seen that these individual domains, enclosed in dotted boundary, are single crystal containing well aligned atomic planes with plane spacing of 0.39 nm (ideal plane spacing = 0.41 nm for pure NiO), as shown in the inset. The selected area diffraction pattern of individual NF was found to be in the form of concentric rings, Fig. 6(d), confirming the polycrystalline nature of NFs. The TEM analysis of the NFs obtained from other solutions was found to be similar which shows that the NiAc concentration affects only the morphology of NiO NFs while the crystal structure remains unaffected. Thus the NiO NFs obtained from electrospinning are polycrystalline in nature as also reported elsewhere [6]. This is because of the thermal decomposition of precursor salt, i.e. NiAc, into small crystalline domains of NiO which merge together in a linear fashion to give the NF shape. Obtaining single-crystal NiO NFs is currently possible only through other physical techniques such as chemical vapor deposition (CVD) [24]. However, from electrospinning, it is still a challenge and requires extensive thermal treatment experimentation to analyze the possibility of converting these polycrystalline NFs into single-crystal ones. Besides the diameter, also important to observe is the variation in roughness of individual NiO NFs as a function of NiAc concentration. Fig. 7 shows the roughness variation across the length, marked with red line, of individual NiO NF obtained from different solutions. As can be seen, the roughness of NFs decreased with increasing NiAc content in the solution and the minimum roughness was found to be for the NFs obtained from Sol. 3, i.e. NiAc = 4.5 wt.%. In case of Sol. 4, the roughness was found to be nearly the same as that

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Fig. 6 – Bright-field TEM images of NiO NFs obtained from Sol. 3 at the electrospinning voltage of 29 kV. (a) Group of NFs and (b) individual NF composed of tiny crystalline domains attached together. (c) HRTEM image of a portion inside a single NF. Some individual single-crystal domains are enclosed in the dotted boundary. Inset shows a zoom-in view of a single-crystal domain showing well-aligned crystal planes with plane spacing of about 0.39 nm. (d) Selected area diffraction pattern of an individual NF, confirming its polycrystalline nature.

of Sol. 3, however, from the macroscopic perspective, the NFs are significantly wavy in nature with irregular cross-section leading to greater variance in topography, as shown by a greater length scale on the vertical axis, Fig. 7(d). It should also be noted that these roughness contours could be more qualitative in nature because the tip radius of 8 nm is comparable and likely to be even greater than the feature height at various locations across the NF leading to high uncertainty in roughness quantification. Nevertheless, since the same tip was employed under identical imaging conditions, the trend of roughness is very evident and reliable, though it could be more qualitative. It is important to develop standardized and economic methods for synthesizing NiO NFs with varying morphology. Although several physical and chemical techniques have been employed to fabricate NiO NFs [4,9] as well as employing electrospun NFs as templates for obtaining porous NiO NFs [8], the systematic control over their morphology was not achieved and reported due to low number of process variables and difficulty in adjusting those variables. Moreover, these techniques are not suitable in terms of economy and mass

production. On the other hand electrospinning, which is an economic and scalable technique, provides a number of process variables that can be easily adjusted to control the morphology of NFs. As shown in the current work, the solution composition is the key variable which has the major influence over the NiO NF morphology. Besides that, the processing variables in electrospinning which include the electrospinning voltage (also studied in the present work), solution flow-rate, electrospinning distance as well as environmental conditions such as temperature and humidity, have strong influence over NF morphology [25]. And we showed in the present work that morphology of NiO NFs can be easily varied from very thin, rough and discontinuous to very thick, smooth and continuous by adjusting the solution composition. Further, electrospinning voltage can be increased to reduce the diameter of these smooth and continuous NFs. The previous reports on electrospun NiO NFs showed very rough morphology [1,6,7,18] without any discussion about their surface roughness and such smooth and continuous NiO NFs, as obtained in current work, were not reported earlier.

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Fig. 7 – AFM topography and roughness variation across the length of individual NiO NFs obtained at electrospinning voltage of 25 kV from Sol. (a) 1, (b) 2, (c) 3 and (d) 4.

The rough and discontinuous NiO NFs are of great practical interest in applications such as fuel cell electrodes [26] and several sensing [1] and catalytic applications [5], where high specific surface area is of key importance. On the other hand, smooth and continuous NiO NFs are highly desirable in electrical [27], thermal [28] and magnetic [29] applications where minimum electron scattering from irregular nanofiber boundaries is required, especially when these NiO NFs are further reduced to obtain pure Ni NFs. Moreover, since the diameter of NFs can be easily tuned via electrospinning parameters such as voltage and flow-rate, a systematic study is possible on these NFs to determine the characteristic diameter where the quantum confinement effects become more pronounced. Thus electrospinning provides a simple and economic way of fabricating NiO NFs with varying

morphology which can be applied in plenty of engineering applications.

4. Conclusions In summary, we have demonstrated that the morphology of electrospun NiO NFs can be controlled by varying the NiAc/PVA ratio in the solution. As the NiAc content increases, the NF morphology changes from rough and discontinuous to smooth and continuous. Moreover, the diameter was found to steadily increase with increasing NiAc content. It was found that at NiAc concentration of 4.5 wt.%, the NFs possess minimum roughness with maximum continuity. Further, by increasing the electrospinning voltage, their diameter can be reduced without

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affecting their roughness and continuity. When the NiAc concentration is significantly high, i.e. 7.7 wt.%, the NiO NFs became wavy with irregular cross-section and occasional cracking was observed across their length which was attributed to the residual compressive stresses generated due to highly dense packing of salt molecules across the NF. Thus electrospinning manifests itself as an economic and versatile technique for fabricating NiO NFs with varying morphology which have potential applications in various engineering domains.

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