Fabrication and Characterizations of Silica Nanoparticles Carbon

PAN in DMF solution by continuous stirring at 60 °C for 2 h as the. 105 base polymer solution. High surface area silica nanoparticles (SiNP) were pur...
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Fabrication and Characterizations of Silica Nanoparticles Carbon Nanofiber Composite Basma I. Waisi, Sama al-jubouri, and Jeffrey R. McCutcheon Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05825 • Publication Date (Web): 27 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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Fabrication and Characterizations of Silica Nanoparticles

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Carbon Nanofiber Composite

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Basma I. Waisi a,b*, Sama M. Al-Jubouri a, Jeffrey R. McCutcheon b

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*Corresponding Author E-mail: [email protected]

7 8

a

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Baghdad, Baghdad, Iraq

Department of Chemical Engineering, College of Engineering, University of

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b

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Science and Engineering, University of Connecticut, Storrs, USA

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Abstract

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Department of Chemical and Biomolecular Engineering, Center for Environmental

Silica nanoparticles/carbon

nanofiber

composites

were

fabricated

via

14

electrospinning of the polymeric solution (polyacrylonitrile polymer in N,N-

15

dimethylformamide solvent) containing different amounts of silica nanoparticles,

16

followed by carbonization in an inert gas. The properties of surface morphology,

17

surface area, surface chemistry, and mechanical strength were characterized using

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Fourier transform scanning electron microscopy (SEM), N2 Physisorption Analyzer,

19

infrared

20

SiO2 nanoparticles with average size about 200 nm showed interesting dispersion

21

along the carbon nanofiber surface.

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nanoparticles affects the structure and properties of the carbonized nanofibers

spectroscopy

(FTIR),

and

dynamic

mechanical

analyzer

(DMA).

The results indicated that adding silica

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composites. Here, the capability to use silica nanoparticles was demonstrated to create

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high surface area carbonized nanofiber with improved mechanical strength for easier

25

handling in the applications requiring good mechanical properties materials such as

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water treatment processes. The specific surface area of the PAN/silica carbon

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nanofibers composite reached 280 m2/g which is higher than the surface area of pure

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carbon PAN nanofibers which was 19 m2/g .

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Keywords: Carbon nanofiber; Nonwoven; Silica nanoparticles; Electrospinning;

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Surface area; Flexibility.

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1 Introduction

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Generally, Engineered nanomaterials structures (including nanofibers and

34

nanoparticles) have been attracting attention due to the advantages of their small size

35

and unique properties such as the relatively high surface area that can potentially

36

improve catalytic processes, chemical reactivity, strength; and interfacial driven

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phenomena including wetting and adhesion 1. carbon nanofiber (in different forms and

38

compositions) have been widely used in various potential applications including fuel

39

cells 2, unconventional energy sources 3, hydrogen storage

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their unique structures and functionalities.

4

and batteries

5

due to

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Various precursors can be used to prepare carbon fibers such as,

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polyacrylonitrile polymer (PAN) which is a common precursor for about 90 % of

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manufactured carbon fibers due to its high melting point and high carbon yield.

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Moreover, the molecular structure of PAN can be relatively easily embedded with

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nanoscale components such as nanoparticles, nanotubes, nanowires, or catalysts to

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fabricate CNFs composite 6,7.

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The most commonly used approach to fabricate carbon nanofibers (CNFs) is

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an electrospinning method that comprises spinning of polymeric precursor fibers

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followed by thermal treatment 7. Electrospinning method to fabricate the nonwoven

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polymeric fibers has the merits of simple handling and low-cost. Moreover,

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electrospinning method has emerged as a powerful and highly versatile technique

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which uses an electrically forced fluid jet to fabricate micro- and nano-scale fibers

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from process polymeric solutions or melts using an electrically forced fluid jet 6,8.

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The thermal treatment steps include stabilization and carbonization.

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Stabilization is carried out by controlled heating of the precursor fibers in an oxidizing

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atmosphere at temperatures ranging of 180 - 300 °C. During this step, a sudden and

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rapid evolution of heat occurs along with various chemical reactions such as

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cyclization, dehydrogenation, aromatization, oxidation, and crosslinking. Stabilization

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results in slight mechanical of the weakening of the carbon backbone. While

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carbonization is carried out in an inert atmosphere at temperatures ranging of 300 -

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1000 °C 9. Carbonization is the most important step during which, fundamental

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changes occur to both chemical composition and physical properties. Therefore,

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carbonization can be considered as the heart of the CNFs fabrication process. The

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mechanical properties of CNF’s decrease significantly during this step 10

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Various nanoparticles could be used to functionalize CNFs. These

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nanoparticles are incorporated into the precursor to fabricate CNFs composite and

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create strong interactions between the nanoparticles and the fiber matrix. To impart

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functionalities to CNFs, a variety of nanoparticles could be used; these are

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incorporated into the precursor to fabricate CNFs composite strong interactions

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between the nanoparticles and the fiber matrix. Many investigations explored the use

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of composite nanofibers in different applications such as electrical devices, electrode

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materials for batteries, proton exchange membrane, supercapacitors and sensors 1,11–13.

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Combining the advantages of metal nanoparticles and polymer nanofibers can

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result in composites with unique mechanical, electrical, chemical and optical

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properties 14. Various types of inorganic nanoparticles were used to fabricate CNFs

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composites such as palladium (Pd) nanoparticles were used to produce a composite

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with high electrocatalytic activity and stability towards methanol oxidation 15. Silver

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(Ag) nanoparticles were used to fabricate high activity catalyst

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nanoparticles (SiO2) were used for porous carbon nanofibers material preparation

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required for rechargeable lithium-ion batteries by carbonizing PAN/SiO2 composite

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precursor followed by silica etching using HF acid

17

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. Also, silica

.

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However, examining the trade of mechanical properties and surface area using

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high surface area nanoparticles on CNFs composites has not been studied so far.

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Mesoporous silica materials with controlled particle size, morphology, and porosity,

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and high specific surface area reaches to (850 m2/g) along with their chemical

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stability, have made highly attractive silica matrices for a wide range of

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nanotechnological applications such as adsorption, catalysis, sensing, and separation.

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. In the present study, PAN-based CNFs containing various concentrations of

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silica

nanoparticles

(PAN/SiNF

CNFs

Composites)

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electrospinning subsequent by heat treatments. This study focused on the influence of

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the concentration of SiNP on the microstructure and properties of the CNFs. The

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morphologies and microstructures of the CNFs were studied in details using scanning

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electron microscopy (SEM), the N2 Physisorption Analyzer, and dynamic mechanical

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analyzer (DMA). SiNPs in CNFs were also characterized using infrared (FTIR)

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spectroscopy to understand the structural changes occurring during the heat

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treatments. The capability of using silica nanoparticles to create high surface area

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carbonized nanofiber compared to pure carbon nanofibers was demonstrated. In

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addition, the fabricated PAN/silica CNFs composite have excellent flexibility for

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easier handling for some applications such as water treatment.

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2 Materials and methods

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2.1 Preparation of polymer precursors

were

fabricated

via

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Polyacrylonitrile (PAN) from Scientific Polymer Products Inc. (MW avg.

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150,000 g/mol) and Dimethylformamide (DMF) from Acros Organics were used to

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prepare 14 wt. % PAN in DMF solution by continuous stirring at 60 °C for 2 h as the

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base polymer solution. High surface area silica nanoparticles (SiNP) were purchased

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from Sigma Aldrich (SiNP, particle size 200 nm).

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To prepare composite precursors, firstly a solution of SiNP with DMF solvent

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was mixed at 60 °C for 2 h, then PAN was added to the previous solution. Different

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PAN:SiNP loadings of 12:1, 5:1 and 2:1 (corresponding to 1.2, 2.8 and 7.0 wt.%

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SiNP) were prepared. The resulting solutions were mixed thoroughly at 60 °C for 2 h

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then sonicated at 60 °C for 1 h using ultrasonic bath to produce a homogeneous

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polymer/nanoparticles solution. Spinning of a homogeneous solution will help to produce

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nanofibers with well-distributed nanoparticles within all the fabricated nanofiber

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composite.

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2.2. Electrospinning precursors composite

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A high pressure syringe pump (KD Scientific) was used to dispense the

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charged solution at a constant rate of 1 ml/hr onto a grounded collector drum rotating

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at 70 rpm.

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The applied voltage was 24, 23, 22, and 20 kV (for 0, 1.2, 2.8 and 7.0 wt. % SiNP,

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respectively) and the tip to collector distance was 18 cm. The precursor mats were all

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spun at room temperature under a relative humidity of 10–20% to produce strongest

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PAN precursor mats 19.

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2.3 Fabrication of CNFs composite

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When electrospinning finished, the precursor mats were stabilized in air at 280

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°C for 1 h in a muffle furnace (Carbolite) through a ramp rate of 1 °C/min. Then, the

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sample was allowed to cool overnight before carbonization to produce stronger

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nonwovens 10. The samples were then carbonized in a tubular furnace (Lindberg Blue

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M, Thermo Scientific) in an inert nitrogen atmosphere with a 3 °C/min ramp rate at

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600 o C for 2 h as these conditions produce strongest carbonized PAN nanofibers

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During the thermal treatment steps, the white color of electrospun composite samples

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changed to brown (after stabilization), then to black (after carbonization) due to

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occurring of various reactions.

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2.4 Membrane characterization

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2.4.1 Fiber Morphology and Size

20

.

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The PAN and composite CNFs nonwoven samples were sputter coated with

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platinum and imaged using an FE-SEM (FEI Quanta). These images allow studying of

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the SiNP distribution within the carbonized nanofibers and analyzing the possible

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changes occurred in fiber structure/morphology.

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2.4.2 Surface Chemistry (FTIR)

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The surface chemistry of the PAN and composite nonwoven carbonized

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samples were analyzed using a Nicolet iS10 FT-IR (Thermo Scientific). Infrared

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spectra were recorded in the wave number range of 700–4000 cm-1 with a resolution

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of ±4 cm-1 and 16 scans per sample. The attenuated total reflection mode with a

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diamond crystal was used to scan the samples.

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2.4.3 Mechanical Properties

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The tensile strength and elasticity of the samples are the parameters which

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were used to assess the mechanical strength of the PAN and composite nanofiber

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carbonized samples. A Dynamic Mechanical Analyzer (DMA) from TA instruments

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was used for this analysis. A sample size of 3 cm, 6 mm was used for the tests which

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were performed at 25 °C and ambient humidity. All results presented were the average

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of three individual tests.

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2.4.4 Specific Surface Area

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The specific surface area was measured using N2 Physisorption Analyzer

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(Micromeritics Instrument Corporation). The samples were first degassed at 300 °C

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for 2 h and then analyzed for nitrogen sorption at 77 K. The adsorption isotherms

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were used to calculate the specific surface area by applying the BET model. All

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results presented were the average of three individual tests.

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3 Results and Discussion

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3.1. The CNF's composites morphology

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The SEM images presented in Figure 1 are for the 14 wt.% PAN:SiNP

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composite fibers prepared from polymer precursor with different SiNP weight percent

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(1.2, 2.8, and 7 wt. %). Further increasing in weight percent of SiNP is not

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recommended with these conditions to avoid the spinning difficulties due to the

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significant increasing in the viscosity of the precursor solution prepared. When the

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silica concentration loaded was relatively low (1.2 and 2.8 wt. %) (Figure 1 b and c),

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they do not show a tendency for agglomeration. The PAN:SiNP CNFs composites

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showed smooth fibrous morphologies; except for a few large SiNP agglomerates

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dispersed in the carbon fiber matrix. Further increase in the SiNP content led to

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difficult SiNP dispersion in the polymer solution as a result of increasing the

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solution's viscosity which produced lots of agglomerations during the spinning

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process as shown in (Figure 1 d).

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To assure that the silica nanoparticles have been distributed throughout the

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composite carbonized network and combined within the nanofibers, EDX analysis has

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been performed in Figure (2) for the different prepared composites carbonized

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nanofibers. The obtained results indicated the existence of SiNP peak in the different

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samples. The Silica peak is observed to be increased with increasing the silica

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nanoparticles percentage. The weight percentages of the various elements in the

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fabricated samples were concluded in Table (1).

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(a) 0.0 wt.%

(b) 1.2 wt.%

(c) 2.8 wt.%

(d) 7.0 wt.%

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Figure (1) The SEM images of 14 wt.% PAN/DMF and PAN:SiNP CNFs composites (a) 0.0 wt.% (b) 1.2 wt.% (c) 2.8 wt.% (d) 7.0 wt.% .

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Figure (2) EDX analysis of 14 wt.% PAN/DMF and PAN:SiNP CNFs composites (a) 0.0 wt.% (b) 1.2 wt.% (c) 2.8 wt.% (d) 7.0 wt.% .

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Table (1) the percentage of the concentration of each element in the various fabricated samples

Element 0.0 wt.% SiNP 1.2 wt.% SiNP 2.8 wt.% SiNP 7.0 wt.% SiNP 76.33 68.70 56.43 53.46 C K, wt.% 19.45 16.92 11.45 9.93 N K, wt.% 4.22 6.84 14.61 12.21 O K, wt.% 0 7.55 17.51 24.39 SiK, wt.% 191

3.2.

The CNF's composites surface area

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Using the high surface silica nanoparticles extensively affected the CNF's surface

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area as shown in Figure 3. Using only 1.2 wt. % of SiNP increased the surface area of

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14 wt.% PAN CNF from 19 to 60 m2/g. Increasing the SiNP loading percentage to 7.0

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wt. % resulted in tangible increasing in the CNFs surface area (reached to 280 m2/g).

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Increasing the specific surface area of the fabricated composite carbonized

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nanofibers can be explained by the effect of the embedded percentage of SiNP on the

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pore size distribution (Figure 4 a) and the adsorption isotherm curves (Figure 4 b) of

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the fabricated composite materials. The pore size of the composite carbonized

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nanofibers increased with increasing the silica nanoparticles percentage. The isotherm

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of CNFs (0 wt.% PAN:SiNP) showed very little pores in its structure. Embedding

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silica nanoparticles resulted in increase the quantity adsorbed as the relative pressure

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over the system (P/Po) was increased due to presence of the micropores in the

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fabricated composites (PAN:SiNP). Increasing the silica nanoparticles percentage

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making more micropores in the composites.

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Figure (3) The effect of SiNP on CNF's composites surface area (Average of three individual samples)

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(a)

(b)

Figure (4) The effect of SiNP on nitrogen physisorption analysis of CNF's composites. (a) Pore size distribution (b) Isothermal curves.

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3.3 The CNF's composites mechanical strength

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The results presented in Figure (5) shows an obvious effect of embedding silica

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nanoparticles within the CNFs composite on the mechanical strength. Increasing the

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SiNP percentage loading (1.2, 2.8, and 7 wt. %) decreased both of the stress (0.4, 0.2,

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and 0.1MPa) and Young's modulus (30, 10, and 4 MPa) respectively. Although

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decreasing the tensile strength of the CNFs composites with increasing the SiNP

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amount, they became less brittle because of decreasing of the Young's modulus. Low

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Young's modulus materials means high flexibility which is an acceptable property for

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some applications that need flexible materials.

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(a)

(b)

Figure (5) Mechanical properties of 14 wt.% PAN:SiNP CNF's composites (Average of three individual samples): (a) Tensile strength and (b) Young’s modulus.

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The CNF's composites FTIR

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FTIR spectra were conducted to the CNFs composite mats to analyze the

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surface chemistry, composition of PAN/SiNP CNFs composite and the possible

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interactions between PAN and SiNP. Figure (6) shows a comparison between the

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FTIR of pure PAN-based and of PAN/SiNP CNFs composite nonwoven samples

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(various SiNP loading).

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The characteristic peaks in pure PAN-based CNFs mats are generally

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associated with the aliphatic CH groups at 1270 and 1350 cm-1. The peak observed at

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1590 cm-1 was for a mixture of C꞊N, C꞊C, and N-H groups which resulted during the

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heat treatment step of the CNF fabrication

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attributed to the NH2 bending vibration and the C-O-C asymmetric stretching

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vibration of the epoxy group

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hydrolysis of nitrile groups 24.

22,23

10,21

. The peaks at 1562 and 1296 cm−1 are

. The peak appeared at 1627 m-1 is ascribed to the

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The PAN/SiNP CNFs composite mats with different SiNP contents showed

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similar peaks to those of pure PAN-based CNFs mats. Those peaks showed a gradual

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broadening and strengthening with increasing of SiNP content in the composite

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matrix. These new peaks observed confirm the good attachment of SiNP within the

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carbonized nanofibers and a favorable interaction occurring between PAN and SiNP.

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The dominant peaks at 798 and 960 cm−1 are attributed to silica nanoparticles owing

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to Si-O-Si symmetric stretching and Si-OH stretching. The peak revealed at 1638

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cm−1 was assigned to the OH bending of physisorbed water molecules 22.

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2.8 wt.% (d) 7.0 wt.%

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Conclusions

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Carbon nanofiber could be prepared with improved properties when it is embedded

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with silica nanoparticles. The pure 14 wt.% PAN CNFs prepared are brittle materials

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with a low specific surface area (19 m2/g). However, these properties were improved

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when silica nanoparticles with a high surface area have been added to the polymer

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solution. PAN/silica nanofibers composite was prepared with different loading amount

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of silica nanoparticles using the electrospinning method. The CNFs composite mats

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resulted have a high surface area (reaches to 280 m2/g) and high flexibility which

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allows easy handling for various applications.

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Acknowledgments

Figure (6) FTIR analysis of 14 wt. % PAN:SiNP CNF's composites (a) 0.0 wt.% (b) 1.2 wt.% (c)

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The authors would like to acknowledge the Higher Committee for Education Development (HCED) in Iraq for financially supporting Basma Waisi (Grant Number is 1000324) to conduct this work in the Connecticut University labs. The authors also acknowledge the University of Connecticut Center for Clean Energy Engineering for the use of the nitrogen adsorption analyser.

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