Hybrid Carbon Silica Nanofibers through Sol–Gel Electrospinning

Article pubs.acs.org/Langmuir

Hybrid Carbon Silica Nanofibers through Sol−Gel Electrospinning Tahira Pirzada,† Sara A. Arvidson,‡ Carl D. Saquing,‡ S. Sakhawat Shah,† and Saad A. Khan*,‡ †

Department of Chemistry, Quaid-i-Azam University, Islamabad 44000, Pakistan Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States

‡

S Supporting Information *

ABSTRACT: A controlled sol−gel synthesis incorporated with electrospinning is employed to produce polyacrylonitrile−silica (PAN−silica) fibers. Hybrid fibers are obtained with varying amounts of silica precursor (TEOS in DMF catalyzed by HCl) and PAN. Solution viscosity, conductivity, and surface tension are found to relate strongly to the electrospinnability of PAN−silica solutions. TGA and DSC analyses of the hybrids indicate strong intermolecular interactions, possibly between the −OH group of silica and −CN of PAN. Thermal stabilization of the hybrids at 280 °C followed by carbonization at 800 °C transforms fibers to carbon−silica hybrid nanofibers with smooth morphology and diameter ranging from 400 to 700 nm. FTIR analysis of the fibers confirms the presence of silica in the as-spun as well as the carbonized material, where the extent of carbonization is also estimated by confirming the presence of −CC and −CO peaks in the carbonized hybrids. The graphitic character of the carbon−silica fibers is confirmed through Raman studies, and the role of silica in the disorder of the carbon structure is discussed.

1. INTRODUCTION Carbon materials are recognized as high performance systems due to their superior properties, especially chemical, mechanical, and thermal stability, electrical conductivity, and biocompatibility.1 For instance, their high conductance allows the efficient electron transfer to the electrode surface for signal transduction.2 Carbons can also be prepared with high surface area to exploit their inherent catalytic properties, providing large loading capacities for reactants.2−4 Consequently, carbon materials in the form of nanotubes, nanofibers, fullerenes, aerogels, or nanoparticles are receiving attention in a variety of fields including catalyst supports,4 filters,5 hybrids for nanoelectronics and photonics,6 biosensors,1 and rechargeable batteries.7 Among all these materials, nanofibers and nanotubes deserve special attention due to their one dimensionality and large aspect ratio.8,9 However, considering the balance between limiting synthesis costs at the expense of losing precise control over carbon nanotubes morphology,9−12 the relatively simple and cost-effective technique of electrospinning provides an acceptable method of generating carbon nanofibers (via conversion from polyacrylonitrile (PAN),11,12 polyaniline, or pitch12,13) which retain the essential features of carbon nanotubes or other carbon materials.2,3 Electrospinning produces nanosized or ultrafine fibers by applying a high voltage to a capillary connected to the syringe containing the electrospinning liquid.14−19 As far as the choice of electrospinning liquid is involved, generally, a polymeric solution with sufficient viscosity and surface tension can be used for electrospinning purposes.16−19 © 2014 American Chemical Society

It has however been realized that the surface characteristics of neat carbon nanofibers are insufficient for photocatalytic reactions and photovoltaic devices.20 Moreover, electrospun fibers prepared from single polymers have limited capacity to stabilize a battery at high discharge rates due to polymer degradation and leakage of organic liquid electrolyte, which originate from the mesoporous nature of the fibrous membrane.21 To overcome these shortcomings, nanoscale inorganic additives are being used to develop organic/inorganic hybrid fibers which combine the advantages of both the polymeric material (flexibility and light weight) with that of the inorganic component (heat stability, high mechanical strength, and chemical resistance).17,18,22−24 Therefore, in recent years, the properties of carbon nanofibers have been modified either by the surface coating of electrospun nanofibers21,25 or through a one-step electrospinning process.23,26 Comparing both techniques, the one-step electrospinning process may be a better option due to its ease of processing, cost effectiveness, and better dispersion of inorganic component in the fiber structure.27 Polyacrylonitrile (PAN) is an ideal precursor for carbon because of its ease of carbonization and electrospinnability due to its high dielectric constant.22,23,28 Unlike other carbon precursors, PAN nanofibers can be used directly as electrode materials after their transformation to carbon nanofibers through stabilization and carbonization (Scheme S1 in Received: September 4, 2013 Revised: November 18, 2014 Published: December 4, 2014 15504

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Table 1. Properties of PAN−TEOS Precursor Solutions and Hybrid Fibers fiber sample

solution aging time (h)

PAN in solution (wt %)

TEOS in solution (wt %)

silica:PAN in solution (wt:wt)

viscosity (Pa s)

surface tension (dyn/cm)

conductivity (μS)

fiber diameter (nm)

PAN A B C D E F silica

1 1 1 2 3 4

10 8 6 5 5 5 5 0

0 4 8 10 10 10 10 20

0:5 3.5:5 9.3:5 14:5 14:5 14:5 14:5 5:0

1.434 0.705 0.195 0.109

40.8 38.3 37.8 36.8

54 94 107 118

627 ± 219 1590 ± 610 1285 ± 430 622 ± 185 830 ± 300 735 ± 255 845 ± 255

0.06

31

195

Supporting Information).29 As such, many research groups have functionalized PAN-based nanofibers by using silver,30,31 titania (TiO2),32 carbon nanotubes,33,34 silica,21,22,27 or metal oxides such as SnO223 or ZnO.20 Among all these hybrid materials, silica-based hybrids are considered one of the best due to high surface area, inert nature, and thermal stability.21,22,27,35 Instead of synthesizing silica through sol−gel processing, most of the research groups working on PAN−silica hybrids used either colloidal silica nanoparticles35,36 or commercially available silica to synthesize silica-filled nanofibers.21,22,27 PAN−silica fibers synthesized using silica were found to show nonuniformity in the fiber structure with silica content above 2%22 and agglomeration of silica particles with increased silica content,27 thereby severely limiting the amount of silica that could be added. Furthermore, mechanical stirring for at least 24 h is required to homogenize the fumed silica-PAN mixture22,27 which is a time-intensive process. These shortcomings in the hybrids synthesized by using fumed silica point toward the need for the development of an alternate silica source which yields better quality PAN−silica hybrids for purposes requiring uniform and homogeneous structures without any compromise on the quantity of silica content. At this point, sol−gel processing emerges as an option to generate silica which can more actively and uniformly participate in the electrospinning process. A typical sol−gel process consists of catalyzed hydrolysis and condensation of a metal alkoxide (Scheme S2 in Supporting Information).18,35,37 The choice of a catalyst plays an important role in the structure and properties of the final product since an acidic catalyst yields smaller sol particles that interconnect into a linear structure while basic catalysts produce sol particles that will aggregate into an irregular gel structure.18,35,38 One of the major benefits for using sol−gel processing is the controlled hydrolysis of silica precursor which helps to introduce coupling sites between the polymer and silica as well as among the fibers.39 This enhanced coupling may aid in producing nanofibers with more uniform structure than that of the fibers produced by using fumed silica. Moreover, transformation of silica precursor mixture into the three-dimensional silica networks, during mixing and electrospinning with the carbon precursor, has the potential to generate fibers with more homogeneous distribution of silica leading to higher functionality of the hybrid. In this work, we report the synthesis of PAN−silica hybrid nanofibers by combining sol−gel processing with electrospinning while using tetraethoxysilane, also referred to as tetraethyl orthosilicate (TEOS), as the silica precursor. Although sol−gel electrospinning has been used to generate polymer silica hybrids for various purposes,18,19,38 to the best of our knowledge, this is one of the first times TEOS has been used as silica precursor to synthesize PAN−silica fibers.

Recently, Kim et el. have fabricated silicon−carbon nanofibers by electrospinning a precursor mixture containing TEOS and PAN in N,N-dimethylformamide (DMF).40 Their approach focused on the use of TEOS and its SiOx products as catalysts to generate micropores on the surface of carbon nanofibers. The improvement of the capacitance and energy/power density values of the carbon fibers by introducing heteroatoms (silica) and by generating ultramicropores through removal of SiOx during carbonization was also studied. On the contrary, our focus is to process TEOS through a catalyzed hydrolysis and condensation reaction to transform to silica and to incorporate the silica network in the PAN fibers in such a way that it remains while PAN is transformed to carbon. The unique features of this work therefore entail (a) formation of intimately mixed carbon−silica hybrid fibers at the molecular level that contain high silica content and (b) understanding of the underlying mechanism and structure−property relationship that leads to these nanohybrids. To produce high quality carbon−silica hybrid nanofibers, we vary the concentration and ratio of PAN and silica precursor solutions while keeping the other processing parameters (temperature, TCD, voltage, and flow rate) constant. After determining the preferred conditions for fabrication of uniform fibers, the PAN fiber is converted to carbon through thermal stabilization and oxidation. The presence of silica and carbon was confirmed through FTIR analysis of the as-spun and carbonized fibers. In addition, Raman spectra revealed the graphitic character of the hybrid to be different than that of the carbonized PAN and to increase with increasing silica content. Such increase in graphitic character of the hybrid fibers with increasing content of thermally stable silica makes them possible candidates to be used in electrochemical applications such as battery and supercapacitor materials.

2. MATERIALS AND METHODS 2.1. Materials. Tetraethyl orthosilicate (TEOS, 99%) and hydrochloric acid (HCl, 37%) were supplied by Sigma-Aldrich. Polyacrylonitrile (PAN) (Mw =150 000 Da reported by the manufacturer) was provided by Scientific Polymer. N,N-Dimethylformamide (DMF, 99.9%) was supplied by Fisher Scientific. Deionized water was used throughout the experiments. All chemicals were used as-received without further purification. 2.2. Method. PAN solutions were prepared by dissolving PAN in DMF (8−11 wt %) at 60 °C. While ethanol and/or water are often chosen for the solvent phase for TEOS synthesis, PAN precipitates in both so DMF was used instead.41,42 TEOS solutions of different concentrations were prepared by adding TEOS to DMF while stirring at room temperature and then dropwise adding HCl to a concentration of 0.1 M and continuing to stir at 60 °C for 1 h. Blended solutions of PAN and TEOS were prepared by slowly adding the required volume of PAN solution to a prepared TEOS solution and stirring at 60 °C for 1 h. The compositions and nomenclature of 15505

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Figure 1. SEM micrographs of fibers resulting from electrospun solutions containing PAN and TEOS where the TEOS is held constant at 10 wt % and the PAN is varied: (P1) 4.0, (P2) 4.5, (P3) 5.0, and (P4) 5.5 wt %. the solutions are given in Table 1 and the Supporting Information (Table S1). 2.3. Electrospinning. A variable high voltage power supply (Gamma High Voltage Research, D-ES-30PN/M692) was used to provide voltage to the electrospinning solution. The solution was loaded in a 10 mL syringe with a stainless steel capillary metal hub needle (22 gauge). The positive electrode of the power supply was attached to the needle tip while the grounded electrode was connected to a metallic collector wrapped with aluminum foil. All fibers were electrospun at room temperature using a voltage of 15 kV, a tip-tocollector distance (TCD) of 15 cm, and a flow rate of 0.5 mL/h. More details on the effects of electrospinning conditions on fiber morphology are presented in the Supporting Information (Figure S1). 2.4. Carbonization. Selected PAN−silica fibers were preoxidized at 280 °C in oxygen for 2 h with a heating rate of 5 °C/min.43,44 The samples were then carbonized for 2 h under a nitrogen atmosphere at 800 °C with a heating rate of 5 °C/min.43−46 A Lindberg Blue tube furnace model 55322-3 with a 45 mm i.d. quartz tube was used. 2.5. Sample Characterization. Surface morphology and fiber diameter of the samples were analyzed by scanning electron microscopy. Selected fibers, before and after carbonization, were sputter-coated with ∼10 nm of Au with a K-550X sputter coater and subsequently analyzed by SEM with an FEI XL30 microscope with field emission gun operated under high vacuum at 5 kV. ImageJ software was used to determine average fiber diameter and standard deviation (given as ±) by measuring the diameter of 100 fibers in each case. Infrared (IR) absorption spectra of the hybrid fibers were recorded at room temperature using a Thermo Nicolet Nexus 470 FTIR. All the samples were scanned from 4000 to 400 cm−1 with a resolution of 4 cm−1. Spectra were acquired from an average of 64 scans for each sample using OMNIC software. A TA-Hi-Res 2950 thermal gravimetric analyzer was used to determine the weight loss dynamics of the fibers from 25 to 850 °C at a heating rate of 10 °C/min in air. Differential scanning calorimetry (DSC) was conducted on a TA Instruments Q2000 model calorimeter calibrated to an indium standard. Scans were carried out at heating rates of 10 °C/min under 50 mL/min N2 purge with samples of approximately 10 mg in standard aluminum pans. Steady shear rheological experiments were performed on selected samples at 25 °C with an AR-G2 rheometer using a 40 mm diameter, 2° cone and plate geometry. The rheometer was operated in soft

bearing mode for all the solutions containing TEOS due to low viscosity. Each measurement was performed at least twice, which were reproducible within 5%. A 2-probe Accumet Basic AB30 conductivity meter by Fisher Scientific provided the conductivity of the PAN/TEOS solutions while the surface tension was measured with a DuNouy Interfacial tensiometer at room temperature in triplicate. A Horiba Jobin Yvon LabRamHR VIS high resolution confocal Raman microscope system with He−Ne laser (514 nm) and Linkam heating/cooling stage was used for determining the chemical composition of fibers. A sampling time of 10 s was used for all samples.

3. RESULTS AND DISCUSSION 3.1. Fiber Morphology in Relation to Solution Properties. PAN solutions have been electrospun at concentrations ranging from about 5 to 10 wt %.12,22 To evaluate the electrospinnability of solutions containing the silica precursor in addition to PAN, an initial series of solutions were prepared containing 4.5 wt % PAN and varying TEOS concentration (corresponding SEM micrographs are available in Figure S2 of the Supporting Information). This concentration of PAN, which was lower than those previously electrospun, was chosen because of the anticipated rise in solution viscosity due to formation of the TEOS−silica network, which could impede electrospinning. We found that with 4.5 wt % PAN in solution, 5 wt % TEOS results in a solution that does not electrospin at any available applied voltage (0−40 keV) at the tip-to-collector distance of 15 cm and flow rate of 0.5 mL/h. Increasing the TEOS concentration to 10 wt % results in a solution that electrospins into fibers having some defects with an average diameter of 460 ± 165 nm. Further increases of TEOS to 15 or 20 wt % are not evaluated for their ability to electrospin because precipitation of the PAN occurred immediately upon combining the TEOS and PAN parent solutions. This precipitation of PAN can be attributed to the increased concentration of silanol groups in the silica sol− gel mixture which are capable of quickly forming hydrogen bonds with PAN, resulting in its cross-linking and producing aggregates in solution.47 15506

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better understand how the effect of the ratio of silica precursor mixture (TEOS and HCl in DMF) to PAN influences the hybrid fibers, the solutions were mixed in varying proportions, and the solution properties and electrospun fibers morphology were evaluated. Table 1 lists the amount of TEOS and PAN and the ratio of silica (supposing that TEOS hydrolyzes completely) to PAN for a series of solutions as well as the diameter of the resultant electrospun fibers. At a silica:PAN ratio of 3.5:5 (sample A), the fibers have a large fiber diameter at 1590 ± 690 nm and have smooth surface morphology relative to the resolution of the image (Figure 2A). Spinning a solution with silica:PAN ratio of 9.3:5 (sample B) again resulted in large fibers having an average diameter of 1285 ± 430 nm (Figure 2B and Figure S3). Increasing to a silica:PAN ratio of 14:5 (sample C) leads to an increase in defects and comparatively thinner fibers with average diameter of 622 ± 185 nm (Figure 2C). Therefore, it is evident that for the precursor mixtures containing more PAN the fiber diameter is larger while spinning fibers with a greater ratio of silica:PAN resulted in finer fibers, due to a lower viscosity stemming from the lower concentration of polymer in solution. However, what is more significant is that we are able to electrospin fibers even with substantial amounts of silica in the solution. This trend signifies a remarkable improvement in fiber morphology over that of the previously reported hybrid fibers electrospun from mixtures of PAN with fumed silica. Previous work reported by Ji et al.22,27 and Jung et al.21 demonstrates aggregation and irregularities in fiber morphology with rise in silica content which is not higher than 14%,21 while in our case we achieve relatively uniform fibers even when the silica content is as high as ∼37% in the fiber. The ability of these solutions to result in electrospun fibers with this low polymer content is to be attributed to the balance provided by the electrostatic repulsion, surface tension, and viscoelastic forces which favor the stabilization of the liquid jet at the given voltage and tip-tocollector distance (TCD), hence producing fibers with smooth morphology.14−16 To investigate the effect of solution viscosity on the electrospinnability of these mixtures, variation in viscosities of all the systems was studied as a function of shear rate (Figure S4). We find all solutions display Newtonian behavior over the measured range of shear rates. In addition, we find the increases

Since 10 wt % TEOS with 4.5 wt % PAN forms a solution stable for several days and electrospins fairly well, 10 wt % TEOS was held constant while the effect of PAN concentration is evaluated by comparing the morphology of fibers electrospun from varying wt % PAN. Representative micrographs are shown in Figure 1. At 4.0 wt % PAN, fibers have an average diameter of 250 ± 120 nm and exhibit defect regions including beads. Increasing the concentration of PAN to 4.5, 5.0, and 5.5 wt % significantly increases the average diameter (excluding defects) to 550 ± 150, 620 ± 185, and 960 ± 270 nm, respectively, which is consistent with increasing the amount of polymer in solution. These results are summarized in Table S1. While all of the PAN concentrations fibers exhibited some defects, the highest evaluated concentration, 5.5 wt %, seems to show more uniformity, albeit qualitatively. We did not increase the concentration further to maintain a high proportion of silica in the fibers and also to limit fiber size. Scheme 1 displays the expected interactions between silica and PAN when solutions are mixed with each other. Possible Scheme 1. Possible Intermolecular Interactions between Silica Network and PAN

interactions between silanol groups of silica and nitrogen atoms in PAN have been suggested by Ji and co-workers.22,27 Hydrogen bonding between the nascent silica network and PAN may affect the condensation mechanism of silica (Scheme 1), especially at small aging times where less of the condensation is completed before PAN is introduced. To

Figure 2. PAN−silica fibers with different silica:PAN ratios: (A) 3.5:5, (B) 9.3:5, and (C) 14:5. 15507

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in solution viscosity and polymer concentration to vary with fiber diameter (Table 1). Nevertheless, viscosity cannot be held solely responsible for the increase in fiber diameter as surface tension and conductivity of the spinning solution also play significant roles in dictating the final structure of the fiber.16,18,48 Table 1 displays the variation in surface tension and conductivity with changing silica content in the electrospinning solution. A decrease in surface tension and increase in conductivity with increasing silica content also results in enhancement of electrospinnability of the PAN silica mixture and production of thinner fibers for higher silica content.15 In each of the series above, the silica precursor (TEOS solution) is aged for 1 h before combining with the PAN solution and electrospinning. Since it is clear from Scheme S2 in the Supporting Information that the silica precursor mixture transforms from a small TEOS molecule to a polymeric network with the passage of time, the aging time (and hence the extent of the TEOS-to-silica conversion) should also play an important role in controlling fiber morphology and could provide an additional route for tailoring such morphology. To investigate how the extent of silica network formation affects the morphology of the hybrid fibers, the TEOS solution was allowed to age for varying periods of time and was then thoroughly mixed with PAN solution before electrospinning. Morphology of the electrospun fibers varied with TEOS aging time (Figure S5 in Supporting Information). Increased aging time resulted in the formation of fibers with larger diameters (Table 1, samples D−F) and more bead defects, which are the result of Raleigh instability of the electrospinning jet.15,16 Increased aging of the silica network should result in a greater number of siloxane linkages and reduced number of silanol groups since the small silica groups have combined together to form the silica network. Since PAN and silica interact via hydrogen bonding between surface silanols of silica and nitrile groups of PAN22,25 (Scheme 1), fewer available silanol groups with increased aging time appear to reduce the number of silica−PAN interactions. These fewer effective “entanglements” in the aged TEOS−PAN solutions result in more beading as seen in the fibers electrospun from aged solutions.15 Compared to precursor PAN−silica fibers, the carbonized fibers have a reduced fiber diameter (Figure 3, Figures S6 and S7 in the Supporting Information) which can be attributed to the loss of a substantial portion of PAN during densification and carbonization at elevated temperature. 41,49 During stabilized oxidation (oxidation of PAN below 300 °C in the presence of oxygen), the PAN macromolecules undergo cyclization of the nitrile group and cross-linking of the chain followed by dehydrogenation and oxidation which ultimately results in the formation of a ladder like polymeric structure (Scheme S1 in Supporting Information).29 Carbonization involves removal of non-carbon elements in the form of a variety of gases including H2O and N2 and generating a threedimensional carbon structure.22 This reaction cascade results in loss in weight of the PAN−silica fiber and reduction in fiber diameter, the extent of which depends on the percentage of PAN originally in the fiber. In case of sample A, the as-spun fibers have a higher percentage of PAN than silica due to the larger content of PAN in the electrospinning mixture. On carbonization, there is a 61% reduction in the fiber diameter for sample A (to 656 ± 205 nm), 58% for sample B (470 ± 253 nm), and 33% for sample C (415 ± 123 nm). This reduction in fiber diameter varies systematically with the percentage of PAN in the pre-carbonized fiber. Silica is thermally stable at 800 °C

Figure 3. SEM images of as-spun and carbonized fibers of PAN and silica having different silica:PAN ratios. Sample A (3.5:5) and its carbonized form (Ac); sample B (9.3:5) and its carbonized form; sample C (14:5) and its carbonized form (Cc).

(the carbonization temperature in this study) while PAN loses its non-carbon components which results in the decrease in fiber diameter.39 Having established that PAN−silica fibers can be fabricated via the sol−gel route, we now probe fundamental issues that include understanding the nature of interactions between silica and PAN, the composition of the hybrid, the extent and nature of carbonization, and the role of silica in the graphitization of the fibers. These and other issues are discussed in the subsequent sections. 3.2. Fiber Composition and PAN−Silica Interactions. Thermogravimetric analysis (TGA) of PAN, silica, and hybrid fibers was carried out in air (Figure 4) to monitor the weight loss dynamics during PAN oxidation, quantify the PAN content of fibers from the percent weight loss in the fiber, and confirm complete combustion. In the case of fibers containing PAN only, a sharp weight loss occurs at about 300 °C which has been associated with the thermal decomposition that accompanies the cyclization of the nitrile group.50 With further heating, the weight remains stable up to about 350 °C, above which a slower rate of weight loss proceeds until 655 °C and results in complete combustion of the PAN.39,41 Similarly, the hybrid with the highest percentage of PAN (A) also exhibited a sharp drop in mass around 300 °C and subsequent gradual weight loss. The hybrids with lower percentages of PAN (B and C) show a much more gradual weight loss starting a few degrees below that of the PAN-only fibers, without the plateau separating the two distinct thermal degradation zones observed for PAN. For PAN in an oxidative environment, the presence of a copolymer has been shown to broaden or inhibit thermal degradation processes such as cyclization and oxidative attack, resulting in overlap of events on thermal scans.51−53 In this 15508

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expected weight loss based on solution concentration, which is within instrument uncertainty. Differential scanning calorimetry (DSC) results for PAN, silica, and hybrid fibers carried out in nitrogen are shown in Figure 5. The Tg of PAN is seen at about 81 °C while in the

Figure 4. TGA thermograms (performed in the presence of air) of PAN, silica, and PAN−silica hybrids produced from fibers with varying silica:PAN ratios: (A) 3.5:5, (B) 9.3:5, and (C) 14:5. Figure 5. DSC thermograms (obtained in N2, exo up) of PAN, silica, and hybrid fibers containing silica:PAN ratio equal to 3.5:5 (A), 9.3:5 (B), and 14:5 (C). Curves are shifted vertically for clarity. (a) Increased scale for clarity in the vicinity of the glass transition. Arrows indicate location of Tg. (b) Expanded scale showing exotherm upon heating associated with cyclization. Arrows indicate T1, T2, and T3 for a selected sample, which are described in the text and Table 2.

manner, sample A retains the essential thermal features of PAN, while hybrids B and C exhibit increasingly broad thermal degradation zones with increasing silica content. If the hybrid silica−PAN network can be taken to approximate a co-network of silica and PAN, the broadening of the thermal degradation zones suggests a greater number of silica−PAN interactions with increasing silica content. Note that in an electrospun solution of PAN with preformed silica nanoparticles no such broadening of thermal transitions in TGA or DSC were observed.22 Table 2 lists the theoretical and measured weight loss values of the hybrid fibers, expecting that the only combustible part of the hybrid is PAN. We observe the weight loss of all hybrid fibers to be close to the theoretically expected value. In sample A, the weight loss (84.3%) is slightly less than expected (87%), which can be attributed to the incomplete combustion of PAN. Note that the PAN did not completely combust at 800 °C. Sample A contains the highest percentage of PAN; therefore, this departure from the actual value is understandable. However, samples B and C on thermal treatment display slightly higher than the expected weight loss (Table 2). It is already established that TEOS after hydrolysis can yield maximum of 28% silica; therefore, the unhydrolyzed TEOS content of the hybrid also contributes to the overall thermal degradation weight loss. Given that the weight loss at 800 °C is indicative of the amount of PAN and a part of the TEOS in the hybrids, there is good agreement with the measured and

hybrid A, the Tg is found at 97 °C (Figure 5a). This increase in Tg indicates PAN−silica intermolecular interactions reduce cooperative segmental mobility.22,25 With further increases in the percentage of silica in the hybrids, the Tg is no longer apparent, which reflects a significant increase in the number of PAN−silica interactions in addition to the decreased PAN percentage. In silica and silica-containing hybrids, there is also an endotherm centered at about 100 °C associated with the liberation of a small amount of adsorbed water. While the peak appears large at the scale plotted in Figure 5a, integration of the area indicates that this endotherm represents ≪1 wt % of the total sample in the hybrids and decreases systematically and proportionally with a decrease in the percentage of silica in the material. In PAN and hybrid fibers, there are up to three exothermic peaks between about 200 and 300 °C (Figure 5b), which are labeled T1, T2, and T3 in Table 2. The exotherm assigned to T3 occurs at about 294 °C in all PAN-containing fibers and does not vary with percentage of PAN. Hybrids B and C, with the higher percentages of silica, exhibit another peak at T2 which is

Table 2. Properties of PAN, Silica, and Hybrid Fibers fiber sample

combustible percentage (wt %)

weight lossa (%)

Tg (°C)

PAN A B C silica

100 87 72 63 0

98.8 84.3 74.1 63.5 6.7

81 97

T1,onset (°C) 228 230 229

T2, peak (°C)

T3, peak (°C)

ΔHcycb (J/g PAN)

Rvaluec

Lac (nm)

287 286

294 297 291 294

563 582 545

1.64 1.44 1.75 1.21

2.68 3.05 2.29 3.63

By TGA, calculated from weight remaining at 800 °C. bTotal integrated area associated with peaks at T1, T2, and T3, normalized to the mass of PAN determine by TGA. cAfter carbonization, calculated from Raman.

a

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the −CO peak in the hybrids reveals incomplete carbonization of oxidized PAN. Comparison of silica and PAN spectra with those of the hybrids in as-spun and carbonized states confirms the presence of silica in the hybrids, indicated by the typical silica peaks at 1080 and 800 cm−1. Silica shows a broad band at 3200−3500 cm−1 which can be assigned to the silanol groups present on the surface of silica due to the incomplete condensation reaction in an acid-catalyzed environment. This incomplete condensation in the silica is beneficial for the improvement of compatibility (mainly hydrogen bonding) between silica and other molecules (Scheme 1). The intensity of this −OH peak (which is absent in PAN) is reduced in all the hybrids but increases with silica content. Interestingly, carbonized hybrids and even carbonized PAN show this −OH band which suggests possible existence of a hydrogen-bonded −OH group on the carbonized fibers.20 Though weakened, −CN peaks in the carbonized fibers reveal incomplete carbonization of the PAN which is not unexpected since full graphitization of PAN usually requires heating PAN to around 2000 °C.31,56 The presence of −CH and −CH2 peaks in the as-spun and carbonized fibers point toward incomplete carbonization of the PAN as well as the possible presence of TEOS in the hybrids due to its incomplete hydrolysis because of low content of water in the sol−gel setup.43 The −CO peaks in all the carbonized fibers are more intense than their corresponding as-spun fibers which points to the oxidation of −CN group to −CO with the increase in PAN content in the precursor hybrid. This peak also indicates incomplete graphitization of PAN since fully carbonized PAN cannot display the −CO peak. These results are significant because studies on carbonization, typically conducted at ∼800−1000 °C, assume the polymer system gets fully converted to a carbonaceous species. 3.4. Silica Content and Graphitic Character. Figure 7 displays first order Raman spectra (≤ 1700 cm−1) of PAN and hybrid PAN−silica fibers after thermal stabilization followed by carbonization at 800 °C. All the samples demonstrate two major peaks. The band centered near 1350 cm−1 (D-band) is due to the disordered portion of carbon (because of the defects in sp3 carbon) while the band around 1600 cm−1 (G-band) indicates ordered graphitic crystallites in carbon.23,41,43 The relative intensity ratio of the D-band and G-band (R-value) characterizes the extent of disorder in the carbon structure, i.e., the lower the R-value, the higher is the number of graphitic structures in carbon. It also depends on the alignment of the graphitic planes and is therefore quite sensitive to the ratio of concentration of the graphitic edge planes and crystal boundaries relative to standard graphitic planes (i.e., the lower the R-value, the greater the amount of sp2 (graphitic) clusters that exists on the sample).44 An understanding of the graphitic nature of the carbon is relevant as it plays a major role in functional performance. Although all the carbonized fibers exhibit both D- and G-bands, their intensity and R-value vary (Table 2). Compared to reported R-value of 0.5 for fully carbonized PAN,56 all the hybrids as well as pure PAN demonstrate an R-value greater than 1, which indicates a comparatively lower content of sp2 cluster in the hybrids. Larger R-value in all the samples can be attributed to presence of disorder in the samples which is instigated by the noncarbonized content of PAN due to relatively low carbonization temperature, since PAN carbonizes completely only around 2000 °C.31,56 Except in the case of sample B, the R-value of the hybrids is found to decrease with the increase in silica content

absent in A and PAN and overlaps the T3 peak. In addition to these two sharp exotherms, all of the hybrids also exhibit a broad exotherm, the onset of which occurs at 228−230 °C, that is absent in PAN. Note there are no thermal events in silica in this region. The T3 exotherm is attributed to the instantaneous cyclization of the nitrile groups to an extended conjugated system21 and is consistent with the sharp weight loss associated with cyclization observed in PAN by TGA; in nitrogen, this event has been shown to occur at a lower temperature (294 °C, Figure 5) than when carried out in the presence of oxygen52 (300 °C, Figure 4). T1 and T2, which are only present in the hybrids, are attributed to the presence of oxygen (in the TEOS/silica); oxygen-containing comonomers have been shown to facilitate oxidation and carbonization of PAN at lower temperatures.50 3.3. Chemical Structure of Hybrid Fibers. The FTIR spectra of PAN, silica, as-spun PAN−silica, and carbonized PAN−silica fibers are shown in Figure 6. Silica shows

Figure 6. FTIR absorption spectra of silica and as-spun and carbonized (subscripted C) silica−PAN hybrids produced from precursors containing different silica:PAN ratios: (A) 3.5:5, (B) 9.3:5, and (C) 14:5.

characteristic peaks at 1096 and 800 cm−1 corresponding to Si−O−Si bond vibrations.18,54,55 Typical Si−O−Si peaks in asspun and carbonized hybrid fibers confirm the presence of silica in the hybrids even after the carbonization process. PAN displays its characteristic peaks at 2240 cm−1 for its −CN groups while peaks at 2930 and 1450 cm−1 can be ascribed to the aliphatic CH group vibration of different modes in CH and CH2;11,20 the absorption peak at 1650 cm−1 can be attributed to the stretching vibration of the −CO bond formed in the hydrolyzed PAN or the −CO bond in the residual solvent DMF.20,25 FTIR spectra of carbonized PAN display two characteristic and new peaks at 1180 and 1560 cm −1 corresponding to −CO and CC stretching vibrations.20 The peak at 810 cm−1 denotes the −CC−H bond11 which indicates that this bond is created during the carbonization of PAN (Scheme S1 in Supporting Information). Reduced intensity of −CN peak and increased intensity of −CC peaks indicate the transformation of −CN to −CC groups during the oxidized stabilization and carbonization processes (Scheme S1 in Supporting Information), while the presence of 15510

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Figure 7. Raman spectra of carbonized fibers of PAN and PAN−silica hybrids synthesized from precursor solution containing varying ratios of TEOS:silica: Ac (3.5:5), Bc (9.3:5), and Cc (14:5).

resulting in the formation of disordered carbon structures with small ring clusters. Samples A and C, on the other hand, have either very low or very high silica:PAN ratio and therefore fewer overall intermolecular interactions, which should lead to increased ordering and cluster size in the carbonized fibers. Other than the G- and D-bands, all the samples display a third band at around 1450 cm−1, which can be ascribed to the presence of impurities and/or unorganized hexagonal carbon rings or amorphous carbon.57

which is indicative of the presence of greater graphitic content in the hybrids consisting of larger amount of silica. This trend points toward possible participation of silica in facilitating the graphitization process of PAN by enhancing the formation of carbon ring clusters. An empirical formula is established by Knight and White to relate the R-value with the graphitic crystallite domain size (La).40,41,57 Using this formula (La = 4.4/R), the crystallite domain size was calculated to rise from 2.68 nm (PAN only) to 3.63 nm (sample C) with increase in the silica content, again suggesting the role of silica in generating graphitic cluster in carbonized PAN. Comparison of the crystallite size (La) measured through TEM should give a better analysis and is the focus of further studies. Sample B, on the other hand, has an R-value (1.75) slightly higher than that of PAN (1.64), which is indicative of very strong PAN silica interactions which result in disruption of the formation of carbon ring clusters in the nanofiber. This anomalous behavior of sample B compared to sample A and C can be attributed to a maximum number of interactions between PAN and silica which might have disrupted the ordering of the aromatic rings,

4. CONCLUSIONS PAN−silica hybrid fibers were synthesized through sol−gel electrospinning by using TEOS as a silica precursor. Solution viscosity, conductivity, and surface tension of the electrospinning solutions are found to significantly affect the electrospinning process. Disappearance of glass transition and broad thermal degradation zone for the hybrids with larger silica content confirms the intermolecular interaction between nitrile group of PAN and surface silanols of silica. The resultant hybrid fibers consisting of varying amount of silica and PAN 15511

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were transformed to carbon−silica fibers through stabilized oxidation followed by carbonization in the presence of nitrogen. The fiber morphology is retained despite significant mass loss during the carbonization process. FTIR studies helped to confirm the presence of silica in the hybrids as well as to estimate the extent of carbonization and oxidation of the PAN. PAN in the hybrids is found to be incompletely carbonized due to the comparatively low carbonization temperature. Upon Raman analysis, all the carbonized fibers are found to exhibit both the sp3 character (D-band) and sp2 character (G-band) of the carbon, which is in good agreement with the FTIR analysis which has already indicated presence of noncarbonized component of PAN. The R-value of all the hybrids is different than that of PAN, indicating that silica plays a role in the generation of graphitic clusters in the carbonizing PAN.

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ASSOCIATED CONTENT

S Supporting Information *

Schemes of hydrolysis and condensation reactions of the silica sol and oxidation and carbonization of PAN; SEM images of electrospun fibers under different conditions as stated in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph 919-515-4519 (S.A.K.). Present Address

C.D.S.: DuPont Central Research and Development, 200 Powder Mill Rd, Wilmington, DE 19880-0304. Author Contributions

T.P. and S.A.A. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.P. acknowledges financial support from the IRSIP program of the Higher Education Commission of Pakistan to conduct this research. The authors are grateful to Dr. Henderson and Ms. Taliman Afroz for help with Raman spectroscopy.

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