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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

A R T I C L E D A T A

⁎ Corresponding author.E-mail addresses: akhalil@masdar.ac.ae (A

http://dx.doi.org/10.1016/j.matchar.2014.06.01044-5803/© 2014 Elsevier Inc. All rights rese

A B S T R A C T

Article history:Received 5 May 2014Received in revised form 3 June 2014Accepted 5 June 2014

In the present work, nickel oxide nanofibers with varying morphology (diameter androughness) were fabricated via electrospinning technique using a precursor composed ofnickel acetate and polyvinyl alcohol. It was found that the diameter and surface roughnessof individual nickel oxide nanofibers are strongly dependent upon nickel acetateconcentration in the precursor. With increasing nickel acetate concentration, the diameterof nanofibers increased and the roughness decreased. An optimum concentration of nickelacetate in the precursor resulted in the formation of smooth and continuous nickel oxidenanofibers whose diameter can be further controlled via electrospinning voltage. Beyond anoptimum concentration of nickel acetate, the resulting nanofibers were found to be ‘flattened’and ‘wavy’ with occasional cracking across their length. Transmission electron microscopyanalysis revealed that the obtained nanofibers are polycrystalline in nature. These nickeloxide nanofibers with varying morphology have potential applications in various engineeringdomains.

© 2014 Elsevier Inc. All rights reserved.

Keywords:ElectrospinningNickel oxide nanofibersMorphology

1. Introduction

Nickel oxide (NiO) nanofibers (NFs) are of great practicalinterest in several engineering applications such as sensing[1,2] electrochemical capacitance [3,4] photocatalysis [5,6] andother chemical catalysis [7]. Moreover, their morphology hasgained significant attention due to strong dependence ofproperties on the morphology [8–10]. Whereas several phys-ical techniques have been successfully employed for theirsynthesis, electrospinning has turned out to be a better choicebecause of its simplicity, economy, scaling capability [11] andcontrol over the NF morphology [12]. Variety of metal oxideNFs have been synthesized through this technique whichhave shown promising potential in different engineeringdomains such as sensing [13], catalysis [14] and solar energy

. Khalil), rhashaikeh@ma

05rved.

conversion [15,16]. It has been demonstrated that electro-spinning can also be employed for synthesizing pure NiO NFsusing an appropriate precursor composed of NiAc salt and anysuitable polymer such as PVP [1,7], PAN [17] or PVA [18].However, no report exists regarding the morphology control ofthese electrospun NiO NFs. We have found that using differentproportions of NiAc in the solution containing PVA as polymericcomponent, the diameter and roughness of NiO NFs can beeasily controlled and an optimum proportion of NiAc ismandatory for obtaining smooth and continuous fibers. Thediameter of these NFs can be further reduced by increasingthe electrospinning voltage. These electrospun NiO NFs withvarying morphology and microstructure which are produciblein an economical and scalable fashion have potential applica-tions in sensing, catalytic, magnetic and photovoltaic domains.

sdar.ac.ae (R. Hashaikeh).

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2. Materials and Methods

Using nickel II acetate (NiAc) (Sigma-Aldrich) and poly(vinylalcohol) (PVA) (Mw = 61,000, Sigma-Aldrich) as main chemicals,four types of solutions were prepared with varying salt topolymer ratios. A typical solution was prepared by mixing thespecific amount of NiAc in 1 ml of water via magnetic stirringfor half an hour and then 1 ml of acetic acidwas added followedby magnetic stirring for another half an hour. Acetic acid wasadded to avoid the hydrolysis of PVA. 5 g of 15 wt.% aqueousPVA solution was then added and the solution was left forvigorous magnetic stirring until a viscous and uniform solutionwas formed. The four types of solutions with different salt topolymer ratiosmade and studied in this work, named Sol. 1, 2, 3and 4, are shown in Fig. 1.

The obtained solution was transferred to a plastic syringehaving gauge 20 (internal dia = 0.603 mm) stainless steelneedle at its end. A Nanon-01A electrospinning setup (MECC,Japan) was used for electrospinning. The nozzle–collectordistance was kept constant at 10 cm and the voltage wasvaried between 20 and 29 kV while keeping the solutionflow-rate constant at 0.5 ml/h. The electrospinning wascarried out at room temperature and the relative humidity ofnearly 60% was recorded during the process. The fibers werecollected on either microscopic glass slide or aluminum foilfor specific characterization. The samples were left over nightin a furnace at 80 °C to remove the moisture followed bycalcination in a furnace at 475 °C for 2 h under ambientconditions. The heating rate was 5 °C/min and once thecalcination cycle was over, the furnace was allowed to cooldown to room temperature before removing the samples.

The purity of NFs was determined using energy dispersivespectroscopy (EDS) (EDAX) and X-ray diffraction (XRD) (Empy-rean, PANalytical). The morphology of NFs was determinedusing a high resolution scanning electron microscope (SEM)(Nova NanoSEM, FEI) operating at 5 kV whereas a highresolution transmission electron microscope (HRTEM) (TecnaiF20, FEI), operating at 200 kV, was used for studying the

Fig. 1 – Four types of solutions w

crystalline structure of NFs. For TEM analysis, the NFswere 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 roughnessof individual NFs. For AFM, amplitude modulation non-contact mode was employed for imaging with silicon cantile-ver (fr = 320 kHz, k = 42 nN/nm) vibrating in first mode withan amplitude of 8 nm. The tip attached at the end ofcantilevers had a tip radius of 8 nm (±2 nm).

3. Results and Discussion

Fig. 2(a) shows the EDS spectrum and the correspondingquantitative results. The longest peak in the spectrum cor-responds to Al which was used as a substrate for collectingthe fibers. From the quantitative results, the contribution ofthis peakwas excluded leaving only Ni and O of which the NFsare composed. The relative proportion of Ni and O was foundto be nearly 80% and 20% by weight, respectively which isin good stoichiometric agreement with pure NiO. The XRDspectrum with all the indexed peaks, presented in Fig. 2(b),matches perfectly with that of pure NiO confirming the purityand crystallinity of obtained NiO NFs [19].

Fig. 3 shows the high resolution SEM images of NiO NFsobtained from different solutions. It can be seen that the NFmorphology changes from ‘thin’ and ‘rough’ to ‘thick’ and‘smooth’ as the NiAc content in the solution increases. Asshown in Fig. 3(a), when the NiAc content is as low as only0.5 wt.%, the NFs are not only very thin but also very roughand discontinuous. Due to much lower concentration of salt,the majority of space in as spun composite fibers is occupiedby polymer leading to the formation of rough and dis-continues 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 saltleading to the formation of smoother and thicker NiO NFsas the polymer is removed. When the salt concentration

ith varying NiAc/PVA ratios.

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

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becomes excessive, i.e. 7.7 wt.%, the over-packing and con-finement of salt molecules across the composite NFs willcause the generation of compressive stresses in the trans-verse direction across the NFs leading to waviness andirregularities across the NF length. This will also cause theNF cross-section to become irregular in shape, Fig. 3(d), ratherthan circular which is observed for lower and more optimumsalt concentrations, Fig. 3(b and c). At the same time, thecompressive stresses at very high salt content, sol. 4, will leadto occasional cracking of NFs across their length, as markedwith dotted circles in Fig. 4, leading to smaller and morediscontinuous NFs. Also observable is the ‘flattening (morebelt like)’ tendency of NFs which can be attributed to strongimpact of densely packed wet nanofibers on the collector

Fig. 3 – SEM images of NiO NFs obtained at electrospinning voltaimages = 100 nm).

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

ge of 20 kV from Sol. (a) 1, (b) 2, (c) 3 and (d) 4 (scale bar in all

Fig. 4 – Low resolution SEM image of NiO NFs obtained fromSol. 4 at the electrospinning voltage of 20 kV. The cracks aremarked with dotted circles.

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A linear increase in the ceramic NF diameter withincreasing salt content has also been reported in case ofelectrospun ZnO NFs which were fabricated via solutioncomposed of zinc acetate and PVA [21]. It is important tonote that the above explanation given for the increase in theNF diameter with increasing salt content is partially true. Itshould be realized that an increase in salt content will alsolead to increased solution conductivity which will causegreater electrostatic force experienced by jet eventuallyleading to decreased NF diameter. It has been shownelsewhere that, while electrospinning, the NF diameterdecreases with increasing zinc acetate concentration in thesolution containing constant amount of dopant and thisbehavior was explained in terms of increasing solutionconductivity with increasing salt content [22]. An increase in

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

NF diameter with increasing salt content can also beexplained in the light of ‘electrohydrodynamic (EHD)’ theory[23] according to which the terminal jet diameter emergingfrom a solution is directly proportional to the surface tensionand inversely proportional to the total current flowingthrough the solution (or the solution conductivity). Althoughthe solution conductivity increases with an increasing saltcontent, but at the same time, the viscosity of the solutionalso increases (We observed drastic thickening of solutionwith increasing NiAc concentration.). Since the viscosity andthe surface tension are directly related (assuming Newtonianfluid), an increased surface tension will tend to increase theterminal jet diameter and hence the NF diameter. It is thusevident that there exists a competition between the surfacetension and the net solution conductivity in controlling thefinal NF diameter. If the solution is moderately conductive,the surface tension seems to be the dominant factor influenc-ing the NF diameter. However, in the presence of any suitabledopant or if the solution is itself very conductive due tospecific type of salt used, a drastic increase in the solutionconductivity can overwhelm the surface tension effect lead-ing to decrease in NF diameter with increasing salt content.The final NF diameter is thus determined by the combinedeffect 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 theNF is composed of tiny crystalline domains randomlyattached together to give the NF shape. A further highresolution image of a portion inside a single NF is presentedin Fig. 6(c). It can be seen that these individual domains,enclosed in dotted boundary, are single crystal containingwell aligned atomic planes with plane spacing of 0.39 nm(ideal plane spacing = 0.41 nm for pure NiO), as shown in theinset. The selected area diffraction pattern of individual NFwas found to be in the form of concentric rings, Fig. 6(d),confirming the polycrystalline nature of NFs. The TEManalysis of the NFs obtained from other solutions was foundto be similar which shows that the NiAc concentration affectsonly the morphology of NiO NFs while the crystal structureremains unaffected. Thus the NiO NFs obtained from electro-spinning are polycrystalline in nature as also reportedelsewhere [6]. This is because of the thermal decompositionof precursor salt, i.e. NiAc, into small crystalline domains ofNiO which merge together in a linear fashion to give the NFshape. Obtaining single-crystal NiO NFs is currently possibleonly through other physical techniques such as chemicalvapor deposition (CVD) [24]. However, from electrospinning, itis still a challenge and requires extensive thermal treatmentexperimentation to analyze the possibility of converting thesepolycrystalline NFs into single-crystal ones.

Besides the diameter, also important to observe is thevariation in roughness of individual NiO NFs as a function ofNiAc concentration. Fig. 7 shows the roughness variationacross the length, marked with red line, of individual NiONF obtained from different solutions. As can be seen, theroughness of NFs decreased with increasing NiAc content inthe solution and the minimum roughness was found to be forthe NFs obtained from Sol. 3, i.e. NiAc = 4.5 wt.%. In case ofSol. 4, the roughness was found to be nearly the same as that

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-crystaldomain showing well-aligned crystal planes with plane spacing of about 0.39 nm. (d) Selected area diffraction pattern of anindividual NF, confirming its polycrystalline nature.

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of Sol. 3, however, from the macroscopic perspective, the NFsare significantly wavy in nature with irregular cross-sectionleading to greater variance in topography, as shown by agreater length scale on the vertical axis, Fig. 7(d). It should alsobe noted that these roughness contours could be morequalitative in nature because the tip radius of 8 nm iscomparable and likely to be even greater than the featureheight at various locations across the NF leading to highuncertainty in roughness quantification. Nevertheless, sincethe same tip was employed under identical imaging condi-tions, the trend of roughness is very evident and reliable,though it could be more qualitative.

It is important to develop standardized and economicmethods for synthesizing NiO NFs with varying morphology.Although several physical and chemical techniques havebeen employed to fabricate NiO NFs [4,9] as well as employingelectrospun NFs as templates for obtaining porous NiO NFs[8], the systematic control over their morphology was notachieved and reported due to low number of process variablesand difficulty in adjusting those variables. Moreover, thesetechniques are not suitable in terms of economy and mass

production. On the other hand electrospinning, which is aneconomic and scalable technique, provides a number ofprocess variables that can be easily adjusted to control themorphology of NFs. As shown in the current work, thesolution composition is the key variable which has themajor influence over the NiO NF morphology. Besides that,the processing variables in electrospinning which include theelectrospinning voltage (also studied in the present work),solution flow-rate, electrospinning distance as well as envi-ronmental conditions such as temperature and humidity,have strong influence over NF morphology [25]. And weshowed in the present work that morphology of NiO NFs canbe easily varied from very thin, rough and discontinuous tovery thick, smooth and continuous by adjusting the solutioncomposition. Further, electrospinning voltage can be in-creased to reduce the diameter of these smooth and contin-uous NFs. The previous reports on electrospun NiO NFsshowed very rough morphology [1,6,7,18] without any discus-sion about their surface roughness and such smooth andcontinuous NiO NFs, as obtained in current work, were notreported earlier.

Fig. 7 – AFM topography and roughness variation across the length of individual NiO NFs obtained at electrospinning voltage of25 kV from Sol. (a) 1, (b) 2, (c) 3 and (d) 4.

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The rough and discontinuous NiO NFs are of great practicalinterest in applications such as fuel cell electrodes [26] andseveral sensing [1] and catalytic applications [5], where highspecific surface area is of key importance. On the other hand,smooth and continuous NiO NFs are highly desirable inelectrical [27], thermal [28] and magnetic [29] applicationswhere minimum electron scattering from irregular nanofiberboundaries is required, especially when these NiO NFs arefurther reduced to obtain pure Ni NFs. Moreover, since thediameter of NFs can be easily tuned via electrospinningparameters such as voltage and flow-rate, a systematicstudy is possible on these NFs to determine the characteristicdiameter where the quantum confinement effects becomemore pronounced. Thus electrospinning provides a simpleand economic way of fabricating NiO NFs with varying

morphology which can be applied in plenty of engineeringapplications.

4. Conclusions

In summary, we have demonstrated that the morphology ofelectrospun NiO NFs can be controlled by varying the NiAc/PVAratio in the solution. As the NiAc content increases, the NFmorphology changes from rough and discontinuous to smoothand continuous. Moreover, the diameter was found to steadilyincreasewith increasing NiAc content. It was found that at NiAcconcentration of 4.5 wt.%, the NFs possess minimum rough-ness with maximum continuity. Further, by increasing theelectrospinning voltage, their diameter can be reduced without

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affecting their roughness and continuity. When the NiAcconcentration is significantly high, i.e. 7.7 wt.%, the NiO NFsbecame wavy with irregular cross-section and occasionalcracking was observed across their length which was attributedto the residual compressive stresses generated due to highlydense packing of salt molecules across the NF. Thus electro-spinning manifests itself as an economic and versatile tech-nique for fabricating NiO NFs with varying morphology whichhave potential applications in various engineering domains.

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