Single-Step Fabrication of Iron Single-Walled Carbon Nanotube Film from Ferrocene as a Conductive-Electrocatalyst Interlayer in Lithium-Sulfur Batteries

: Floating catalyst chemical vapor deposition (FCCVD) is a continuous and scalable method for manufacturing conductive single-walled carbon nanotube (SWCNT) thin films. Hydrocarbons or hydrocarbon derivatives have been conventionally used as carbon sources and ferrocene as a Fe nanoparticle (NP) precursor in FCCVD for the fabrication of SWCNT thin films. However, carbon, released from ferrocene decomposition, has not been well investigated for the fabrication of SWCNT thin films. Here, we have developed an FCCVD process for the fabrication of SWCNT thin films using ferrocene as a single source for the generation of catalyst NPs and carbon. Moreover, the absence of hydrocarbons and their derivatives makes the process safe, cost-effective, and environmentally friendly. We fabricated freestanding Fe-SWCNT thin films composed of small diameter nanotubes (1.2 nm) and Fe NPs, synthesized at a high yield of 0.46 mg per 30 min. Fe-SWCNT thin films exhibited good conductivity with a sheet resistance of 800 ohm/sq for 80% transmission at 550 nm. Conductive SWCNTs significantly improved sulfur utilization, with an obvious 27% increase in the capacity of lithium − sulfur batteries (LiSBs). A HNO 3 -treated Fe-SWCNT separator significantly improved the cyclic stability of LiSBs with 18% capacity loss of initial capacity compared to 32% capacity loss for polypropylene separator after 100 cycles.


■ INTRODUCTION
Single-walled carbon nanotubes (SWCNTs) have a high surface area, electrical conductivity, and mechanical strength, 1−4 which find applications in flexible displays, electrochemical energy conversion, and storage processes.−7 In addition, SWCNTs are attractive for energy storage devices.A highly conductive matrix of SWCNTs forms an efficient host in lithium−sulfur batteries (LiSBs) for better utilization of sulfur. 8Interwoven SWCNT networks provide abundant paths for the transport of lithium ions.
Floating catalyst chemical vapor deposition (FCCVD) is an appealing method for the synthesis of SWCNTs. 9FCCVD process involves precursor decomposition, catalyst nanoparticles (NPs) formation, SWCNTs nucleation, and the growth of nanotubes.The process runs continuously and produces SWCNTs in a single step, 10−12 which makes it a scalable process.Moreover, SWCNTs coming from FCCVD can be selfassembled into a microstructure to form thin films, which are used for different applications.This makes FCCVD a dry process for the SWCNT film fabrication.
To synthesize SWCNTs in FCCVD, a gaseous or liquid carbon source is required.Gaseous carbon sources such as carbon monoxide (CO), ethylene, and methane have been extensively used in FCCVD.−15 Apart from the carbon source, FCCVD requires a catalyst precursor to generate NPs for the growth of SWCNTs.Ferrocene is a commonly used catalyst precursor in FCCVD. 16It is safer to use ferrocene compared to other organometallics (cobaltocene and nickelocene). 17,18Ferrocene (C 10 H 10 Fe) is composed of iron and carbon rings.The carbon coming from ferrocene can also participate in the growth of SWCNTs. 19Recent studies investigated SWCNT film fabrication using different carbon sources. 14,15,20However, ferrocene has not been studied earlier as a carbon source alone in FCCVD for SWCNT film fabrication.
Based on the first principle of nonequilibrium molecular dynamics (MD) simulations, Page and co-workers studied ferrocene decomposition in the absence of an additional carbon source.It is proposed that ferrocene decomposition occurs via the spontaneous cleavage of Fe−C and C−H bonds. 21In the absence of an additional carbon source, Fe atoms begin clustering, which hinders the further growth of nanotubes.Their study suggested that only C 2 H x radicals as well as C atoms from an additional carbon source form carbon chains to prevent the clustering of Fe atoms, leading to the growth of SWCNTs.
Here, we report ferrocene as a carbon source as well as a catalyst precursor for the growth of a hybrid material of Fe NPs and SWCNTs (Fe-SWCNTs).This is the first-time fabrication of Fe-SWCNT films from ferrocene as a carbon source without introducing additional gaseous or liquid carbon precursors, to the best of our knowledge.We have developed a continuous, safe, and cost-effective Fe-SWCNT thin-film manufacturing process using simple chemistry.Moreover, the elimination of hydrocarbon and their derivatives (alcohols, toluene, and CO) from the SWCNT film fabrication process makes it an environmentally friendly process.The process produced conductive Fe-SWCNT films (sheet resistance of the doped sample is 800 Ohm/sq for 80% transmission at 550 nm) at a high yield.Moreover, fabricated films have iron NPs that are attached to nanotubes.−26 An additional layer of Fe-CNT over the polypropylene (PP) separator exhibits a 27% increase in the specific capacity of LiSBs.In addition, the stability of LiSBs obviously improved with an 18% capacity loss for the Fe-CNT separator compared to a 32% capacity loss for the PP separator.
■ METHODS SWCNT Synthesis.SWCNTs were grown in an FCCVD reactor.Ferrocene (Sigma-Aldrich) was introduced into the reactor using nitrogen (Woikoski) as the carrier gas.A flow of 600 cubic centimeters (ccm) of nitrogen passed through a heated ferrocene cartridge (50 °C) and carried the vapor of ferrocene into the reactor.Hydrogen (Woikoski) flow varied from 100 to 300 ccm.The temperature inside the reactor was maintained at 1100 °C.Ferrocene was used as the precursor for the formation of iron catalysts as well as a carbon source for the growth of SWCNTs.Thermal decomposition of ferrocene at high temperatures produced iron NPs.In the presence of carbon, the NPs are covered with a layer of carbon. 27A fraction of NPs grew SWCNTs in the gas phase from carbon released on the decomposition of ferrocene.The remaining NPs were attached and trapped between SWCNTs during gas phase synthesis.The Fe-SWCNT aerosol coming from the reactor outlet was filtered with the 45 mm diameter filter paper using vacuum pumping, which fabricates the Fe-SWCNT thin film.
The synthesis yield is measured by transferring the collected material on a filter to an aluminum foil.The quantity of the produced material is measured using a precise weighing balance.Aluminum foil is preferred as the SWCNT substrate over the filter to avoid moisture adsorption by the filter paper.The synthesis yield is reported here as the amount of deposit collected for 30 min of collection time.
Doping and Sheet Resistance Measurement.The SWCNT film on a filter was press-transferred onto a quartz substrate prior to doping.The doping procedure was carried out by a drop casting of 20 μL of 15 mM HAuCl 4 (HAuCl 4 3H 2 O, ACROS Organics) solution in ethanol (99.5%, ETAX) onto the 1 cm 2 SWCNT film at room temperature. 28The SWCNT film is also doped with HNO 3 by a drop casting of 20 μL of 98% HNO 3 .The sheet resistances of SWCNT films were measured with a Jandel four-point probe station.
Fabrication of Modified Separators.The filter papers were patterned according to the size of PP separators (19 mm in size Celgard 2400) that are typically used in Li-ion batteries.The aerosol coming from the reactor was collected on patterned filter paper, forming a film of a random network of Fe-SWCNTs.Fe-SWCNT thin films (1 mg) were transferred to the PP separator by the dry transfer method without involving any further posttreatment.The Fe-SWCNT film was firmly attached to the separator, forming the SWCNT-PP separator.
Fabrication of Sulfur/Carbon Composite.Sulfur/carbon (S/C) composites were fabricated using a solution-based method.First, 7.2 g of sodium thiosulfate pentahydrate (Na 2 S 2 O 3 •5H 2 O, Riedel-de Haen, M = 248.19g/mol) was dissolved in the 100 mL of deionized water (H 2 O), followed by magnetic stirring for 10 min.Further, 0.4 g of carbon black (ECP 600JD) was added to the solution, and the solution was mixed by magnetic stirring for 60 min.After mixing, 58 mL of hydrochloric acid (HCL 1 M, Sigma-Aldrich) was added to the solution dropwise.To fully complete the reaction, the final solution remained for 2 h at room temperature with magnetic stirring, after which the product was filtered and washed several times with deionized water and ethanol until the value of the pH reached 7. The obtained material was dried in an air oven at 60 °C for 12 h.Finally, the dry powder was placed in the autoclave under a nitrogen atmosphere and was heat-treated at 155 °C for 12 h, after which the S/C composite was obtained.
Material Characterization.The synthesized SWCNTs were characterized by using different techniques.The absorption spectra of the SWCNTs were measured by using a UV−vis-NIR spectrometer (Agilent Cary 5000).Raman spectra were measured using a Raman spectrometer (Thermo DXR2xi) equipped with laser 532 and 785 nm.SWCNT films were transferred to a quartz substrate before measuring the spectra.
A transmission electron microscope (TEM, JEM-2100F) was used to measure the NP size.The morphology of NPs and SWCNTs is observed by using a scanning electron microscope (SEM, SIGMA-HD).
Visualized Li 2 S 6 Adsorption.A Li 2 S 6 solution (15 mmol/ L) was prepared in a nitrogen-filled glovebox.Li 2 S (Sigma-Aldrich) and sulfur were mixed according to a molar ratio of 1:5.The mixture was added to the equal volume of 1,3-dioxolane (Sigma-Aldrich) and 1,2-dimethoxyethane (Sigma-Aldrich). 29,30The solution was stirred at 70 °C for 48 h.The obtained L 2 S 6 solution was yellow.SWCNTs were added to the solution, and the absorption of the L 2 S 6 was analyzed based on the color change of the prepared solution.
Electrochemical Measurements.The cathode was composed of active material (S/C composite), Super C65 (IMERYS), and CMC/SBR binder (TOB NEW ENERGY) (CMC: SBR ratio is 1:1).The deionized water was used as a solvent to form a slurry.The slurry was coated on the current collector (16 μm aluminum foil obtained from TOB NEW ENERGY) by using a doctor blade.The created electrode was dried under vacuum at 50 °C for 12 h.The weight ratio of the S/ The Journal of Physical Chemistry C C:C65:binder was 80:10:10.The mass loading ratio of the sulfur was about 1.2 mg/cm 2 .
The CR2016 coin cells were assembled in a glovebox under an inert argon atmosphere.The metallic lithium ribbon (Sigma-Aldrich) was cut on the disks with 16 mm diameter and 0.2 mm thickness, and these disks were used as the anode.SWCNT film attached to the microporous PP Celgard 2400 membrane or pure Celgard 2400 membrane was used as the separator.The 1  Two-coin cells were assembled for each analyzed separator.The created coin cells were stored for 12 h after assembly, and then 5 charging/discharging formation cycles with 0.05 C-Rate current (1 C = 1675 mAh/g of sulfur) were done.Further, the first cell was used for cycling stability analysis.The cycling stability of the produced cell was analyzed by galvanostatic charging/discharging of the cells in the voltage range from 1.7 to 2.8 V by applying a 0.2 C-rate current for discharging and 0.1 Crate current for charging.The second cell was used to analyze the galvanostatic charge/discharge performances and for the cycling voltammetry curves measurement.The galvanostatic charge/ discharge performances were evaluated between 1.7 and 2.8 V at various C-rates of 0.1, 0.2, 0.5, 1, and 2C (1 C = 1675 mAh/g of sulfur), and the five cycling voltammetry curves were measured in the voltage range from 1.7 to 2.8 V, at 0.1 mV/s scan rate.All tests were performed by using Arbin high-precision battery testing equipment at room temperature.

■ RESULTS AND DISCUSSION
SWCNTs were synthesized by using FCCVD.Nitrogen and hydrogen carried ferrocene from a heated cartridge into the reactor.Ferrocene was used as a carbon source, as well as a precursor for the iron catalyst.A schematic of the FCCVD reactor is shown in Figure 1a.The process produced clean and good-quality nanotubes which are visible in SEM and TEM images (Figures 1b,c, S1 and S2).The reactor produced longer and better-quality nanotubes at the optimized temperature of 1100 °C (Figure S1).Iron NPs are also obvious and are attached to nanotubes (Figure S3).We have observed Fe NPs larger in size (with a mean diameter of 11.84 nm) than previously reported for FCCVD. 31Fe-SWCNTs were collected on filter paper, which formed a random network.A continuous network of long SWCNTs produced thin films of nanotubes on filter paper as shown in Figure 1d.
Hydrogen was introduced into the reactor as a carrier gas which generated a reducing atmosphere.In addition, it facilitated the decomposition of a carbon source and prevented the formation of amorphous carbon or soot. 32Figure 2a shows the synthesis yield for different hydrogen concentrations.FCCVD process produces 0.496 mg of Fe-SWCNTs in 30 min for a hydrogen flow rate of 100 ccm.The synthesis yield decreases with the increase in hydrogen flow.The decrease in yield is faster until 200 ccm hydrogen (0.465 mg per 30 min).The further increase in hydrogen concentration leads to a small decrease in the SWCNTs yield.This shows that the presence of hydrogen has an obvious effect on yield as well as available carbon for the growth of nanotubes. 33We consider that low concentrations of hydrogen facilitate the supply of more carbon from the decomposition of ferrocene and consequently increase the growth of more nanotubes.However, a higher concentration of hydrogen is proposed to react with the carbon.This etches

The Journal of Physical Chemistry C
away some amount of carbon and reduces the carbon supply to grow nanotubes at a low yield. 34,35he effect hydrogen is further investigated using Raman spectroscopy.Raman spectroscopy is used to find the diameter and quality of nanotubes.Figure 2b shows the radial breathing mode (RBM) of SWCNTs at different hydrogen concentrations for an excitation laser of 785 nm (514 nm laser spectrum is shown in Figure S4).The RBM is the coherent vibration of carbon atoms in the radial direction.A distinct RMB suggests the synthesis of SWCNTs.RBM frequency (ω RBM ) is used to determine the diameter (d t ) of SWCNTs in an inverse relation 36,37  The RBM peaks are between 140 and 270 cm −1 as shown in Figure 2b.For 100 ccm hydrogen, there are more pronounced peaks at a higher frequency of 230 cm −1 , which shows the synthesis of small diameter SWCNTs (1 nm diameter nanotubes).However, the peaks at higher frequencies are significantly suppressed for samples collected at higher concentrations of hydrogen.This indicates that a higher concentration of hydrogen favors the synthesis of large-diameter SWCNTs.In addition, Raman spectroscopy is a useful tool to determine the quality of SWCNTs.Figure 2c shows an obvious G band, which represents the vibration of sp 2 -bonded carbon atoms in the two-dimensional hexagonal lattice of the graphite layer.The D band is related to the defect in graphitic structure or amorphous carbon in the sample.So, the I G /I D ratio is an indication of the quality of SWCNTs.Figure 2d shows that the I G /I D ratio increases with hydrogen concentration, i.e., from 100 to 250 ccm, which indicates the improvements in the quality of SWCNTs.We propose that hydrogen at higher concentrations etch away the amorphous and nongraphitic carbon which enhances the quality of nanotubes.However, for a high hydrogen concentration of 300 ccm, the D band is obviously stronger with a significantly lower I G /I D ratio.We assume that the hydrogen concentration damages the structure of SWCNTs after a threshold and creates defective SWCNTs.This deteriorates the quality of SWCNTs.
Raman spectroscopy is an efficient tool to determine the diameter of SWCNTs.However, certain excitation laser energies do not excite all of the SWCNTs in the sample.Therefore, it is difficult to measure the accurate diameter of all of the SWCNTs in the sample.Optical absorption spectroscopy is a quick and better method to measure the mean diameter and diameter distribution of all the SWCNTs in the sample. 37Figure 3a shows the absorption spectra of SWCNTs produced at different hydrogen concentrations.We clearly see the van Hove transition E 11 s of semiconducting SWCNTs.The peak of E 11 s is close to 1400 nm for a hydrogen concentration of 100 ccm.There is a steady shift in the peaks from 1400 to 1600 nm for an increase in hydrogen concentration from 100 to 250 ccm.This shift from a lower wavelength to higher wavelength with the hydrogen concentration indicates the synthesis of large-diameter SWCNTs at a higher hydrogen concentration.This is in good agreement with the Raman measurement.We do not see a clear absorption spectrum for the 300 ccm hydrogen concentration, revealing the destruction of SWCNTs at high hydrogen concentrations.Fitting the absorption spectra is a rapid and accurate method used to evaluate the diameter distribution of bulk SWCNTs. 38A MATLAB code was utilized to fit the absorption spectra in Figure 3a to obtain the diameter distributions of SWCNTs (Figure 3b−e). 38Diameter distributions of SWCNTs are given in Figure 3b−e.For 100 ccm of H 2 , SWCNTs are small in diameter with a mean diameter of 1.17 nm.The distribution moves to the higher diameter at higher hydrogen concentrations.The mean diameter of the nanotubes reaches 1.26 nm for 250 cm of H 2 .
We have clearly demonstrated the synthesis of SWCNTs from ferrocene as a catalyst precursor as well as a carbon source.We propose that the decomposition of ferrocene simultaneously proceed in two directions: (i) nucleation of iron NPs by clustering of Fe atoms and (ii) growth of carbon chains.The dissociation of cyclopentadienyl ring of ferrocene releases carbon which grows carbon chains. 21Further dissociation of ferrocene supplies enough carbon to the carbon chain for the stable growth of the carbon nanotubes.We believe that ferrocene, on decomposition, provides carbon simultaneously with the nucleation of Fe NPs.This carbon, available during the nucleation of NPs, is significantly important for the efficient growth of nanotubes.The carbon released from ferrocene immediately begins nucleation of nanotubes on the catalyst particle due to a short mean free path of carbon radicals.The presence of this carbon affects the growth and length of SWCNTs.Previous studies, using additional carbon sources, have observed obviously longer nanotubes from the ferrocenebased system than premade catalyst systems (no carbon during NPs nucleation). 5,12,20,39For instance, Hussain et al. reported the synthesis of long nanotubes (mean length 13 μm) using ethylene as a carbon source and ferrocene as an iron NP The Journal of Physical Chemistry C precursor. 6−42 We investigated the conductivity of Fe-SWCNT films.Figure S5 shows sheet resistance of the SWCNT films at different hydrogen concentrations.The conductivity of the SWCNT film improved with an increase in hydrogen concentration.The sheet resistance of the film is lowest for 250 ccm of the hydrogen, which is 3400 Ohm/sq for 80% transmission.We attribute the better conductivity of the film to improve the quality of nanotubes (G/D ratio) with hydrogen.However, the sheet resistance value is obviously high for 300 ccm hydrogen.It is assumed that a high concentration of 300 ccm damaged nanotubes and deteriorated the conductivity of the films.We further explored the conductivity of the SWCNT films of various thicknesses and transparencies of 250 ccm. Figure 4a shows the plot of sheet resistance of Fe-SWCNTs films against the transmission values.For 80% transmission at 550 nm, the sheet resistance of the pristine Fe-SWCNT film is 3400 ohm/sq.To improve the conductivity, Fe-SWCNT films were doped.The doping of a film decreases the sheet resistance due to the Fermi level shift. 43,44The successful doping of the Fe-SWCNT film is indicated by the removal of peaks in the absorption spectra in Figure 4b.The sheet resistance of Fe-SWCNT film is reduced to 800 Ohm/sq for 80% transmission at 550 nm.HNO 3 treatment further improved the conductivity of the film.This conductivity value of the film is lower than those using conventional carbon sources. 12,39,45The low conductivity of Fe-SWCNT films against transmission is attributed to a higher concentration of NPs.Iron NPs absorb the light without a significant contribution to the conductivity of Fe-SWCNT film.
−48 Fe NPs can catalyze LiPs conversion reactions to reduce the shuttling effect and improve the stability of the LiSBs.To further investigate, a Li 2 S 6 absorption test was performed.Visualized polysulfide adsorption experiment is a fast method to determine the effectiveness of the material for the conversion of polysulfides.Figure S6 shows a pristine polysulfide solution, which indicates the yellow color.However, the addition of 20 mg Fe-SWCNTs significantly changes the yellow color of the polysulfide solution after 60 min.The addition of 20 mg of HNO 3 -treated Fe-SWCNTs turned the yellow color of the polysulfide solution into colorless.This indicates the ability of Fe-SWCNTs and HNO3-treated Fe-SWCNTs for polysulfide absorption.To investigate the advantages of the prepared Fe-SWCNTs in LiSBs, the Fe-SWCNTs film was transferred to the PP separator.Prepared coated separators were used during the creation of the Li−S coin cells as it is shown in Figure 5b,c.
The comparison of the electrochemical characteristics of the created coin cells with those of commercial and SWCNT-coated separators is shown in Figure 6.CV curves of LiSBs are shown in Figure 6a.The shape of the measured CV curves did not change significantly when the PP separator was replaced with the SWCNT-coated separator.During the discharging process, the two peaks are visible at around 2.05 and 2.35 V.The peak at 2.35 V is attributed to the conversions of sulfur into high-order LiPs (Li 2 S x , 4 ≤ x ≤ 8) and the peak at 2.05 corresponds to the conversion of the higher-order LiPs into Li 2 S 2 /Li 2 S.During the charging process, the two peaks are visible that belong to the conversion of lithium sulfide to soluble LiP and finally to sulfur.However, the intensity of the peaks is higher in the case when the Fe-SWCNT-coated separators were used, especially during the lithiation process of the cathode.The highest intensity was observed for the HNO 3 -treated Fe-SWCNTs-coated separator (HNO 3 −CNT-PP) because of the increased electrical conductivity of Fe-SWCNTs after HNO 3 treatment.The charge− discharge profile of the LiSBs cell is shown in Figure 6b.The reversible specific capacity, measured at 0.05 C-rate current after the formation test, increases significantly from 748 to 948 mAh/ g when the PP separator is changed to the HNO 3 −CNT-PP separator.The increase in specific capacity is ascribed to the electrical conductivity of SWCNTs for better utilization of sulfur.The analysis of the rate performances of the created cells (Figure 6c) showed that the cell with the HNO 3 −CNT-PP separator has the best rate performance compared to the other considered cells.The decrease of the specific capacity for the cell with the HNO 3 −CNT-PP separator was 31.2% when a discharging current was changed from 0.1 to 2 C-rate.The specific capacity decreases by 46.6% and 60% for the cell with untreated Fe-SWCNTs coated separator (CNT-PP) and cell with PP separator, respectively.Moreover, the cycle stability of the CNT-PP separator and HNO 3 −CNT-PP separator obviously improved (Figure 6d).After 100 cycles, the capacity loss of the PP separator is 32% of the initial capacity.In comparison to the PP separator, the capacity loss is 1.8 times lower for the HNO 3 −CNT-PP separator (18% capacity loss) and 1.5 times lower for the CNT-PP separator (21% capacity loss).We attribute the improvement in the cycle stability of LiSBs to the electrocatalytic activity of Fe NPs for the conversion of soluble LiPs into insoluble Li 2 S 2 .We proposed that the HNO 3 treatment of Fe-SWCNTs remove the covering of Fe NPs to expose more NPs for the LiPs conversion, which has a positive effect on the cycling stability of the cell with the HNO 3 −CNT-PP separator.The obtained results showed that the HNO 3 -treated Fe-SWCNTs film can be used as an internal layer between the cathode and PP separator in LiSBs, and it is capable of improving the electrochemical performances of the energy storage system.

■ CONCLUSIONS
We have demonstrated the fabrication of Fe-SWCNTs thin films using ferrocene as an alone carbon source in a single-step and continuous process.SWCNTs were small in diameter (1.2 nm).Fe-SWCNT thin films were fabricated at a high yield of 0.46 mg per 30 min.The Fe-SWCNT films exhibited electrical

Figure 1 .
Figure 1.(a) Schematic of the FCCVD reactor for the synthesis of SWCNTs.(b) and (c) SEM and TEM images showing synthesizes of SWCNTs.(d) Freestanding Fe-SWCNT film.

Figure 2 .
Figure 2. (a) Synthesis yield of SWCNT films collected for 30 min and (b) RBM of Raman spectra of SWCNTs with hydrogen variations using the laser (785 nm).(c) G and D bands of Raman spectra of SWCNTs and (d) I G /I D ratio shows the quality of SWCNTs.

Figure 4 .
Figure 4. (a) Conductivity of SWCNT films in terms of sheet resistance versus the transmission of the films at 550 nm.(b) Absorption spectra of pristine and doped SWCNT films.