# Novel 3-Dimensional Cotton-Like Graphenated-Carbon Nanotubes Synthesized via Floating Catalyst Chemical Vapour Deposition Method for Potential Gas-Sensing Applications

Authors: Ismayadi Ismail; Md Shuhazlly Mamat; Noor Lyana Adnan; Zainab Yunusa; Intan Helina Hasan
Journal: Journal of Nanomaterials
 (2019)

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## Abstract

We reported the synthesis of graphenated-carbon nanotubes (G-CNTs) using a floating catalyst chemical vapour deposition (FCCVD) method and formed a bulk-cotton-like structure. The objectives of this work were to study the effect of the injection rate parameter of the carbon source on the formation of G-CNTs and CNTs and later to test them as an ammonia gas sensor. Ethanol, thiophene, and ferrocene were mixed and injected into FCCVD at 1150°C. The as-synthesized samples were then characterized using FESEM, HRTEM, TGA, Raman spectroscopy, XPS, and electrical conductivity measurement. We found that the injection rate of 5 ml/h was suitable for the formation of G-CNTs and a higher injection rate resulted in the formation of CNTs. Our measurement showed that the electrical conductivity response of G-CNTs was higher compared to that of CNTs. The gas-sensing performance of the gas sensor made of G-CNT materials also showed good response compared to that of CNTs. This experimental work paved the way for how we can selectively synthesize CNTs and G-CNTs via the FCCVD method, and G-CNTs have proven to be a better material for gas sensors.


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## Body

## 1. Introduction

Graphene is considered as a two-dimensional structure, whereas carbon nanotubes (CNTs) are known for one-dimensional materials. Meanwhile, a new hybrid of both having a three-dimensional structure is known as graphenated-CNTs (G-CNTs) [1–5]. The high surface area of the three-dimensional structure of the G-CNT structure provides an advantage for gas-sensing application due to the high edge density of graphene exfoliated on the outer wall of CNTs. The graphene-CNT hybrids were successfully grown by a CVD method on a copper foil coated with Si NPs under atmospheric pressure using ethanol as a precursor [1] with the temperature gradually increased to 800 or 900°C in the H2/Ar environment.A simple approach to the fabrication of G-CNT hybrid films is by direct synthesis on polycrystalline Cu substrates by a thermal CVD method. The graphene layer was synthesized on a Cu substrate in a mixture of gases Ar, H2, and CH4, and the CNTs were subsequently produced on the surface of the graphene/Cu film in a mixture of Ar, H2, and C2H2 [6].The synthesis of G-CNTs was carried out by using a catalyst-free plasma-enhanced CVD technique. Both planar CNT networks (randomly dispersed) and vertically aligned CNT arrays were used as hosts for the synthesis of G-CNT hybrid structures [7]. CNTs were first heated to 700°C in an Ar flow, and then graphene was grown off the CNTs at atmospheric pressure with CH4 (0.5 lpm) as the carbon precursor. Another work used plasma-enhanced CVD of which G-CNTs were grown using a 915 MHz microwave plasma-enhanced chemical vapour deposition (MPECVD) system [8]. The substrate was prepared by depositing 5 nm iron films onto silicon (100) wafers using a CHA electron beam evaporation system. Prior to growth, the coated substrates were heated, followed by striking and stabilizing plasma at 21 Torr and 2.1 kW of magnetron input power. Growth of the G-CNTs was accomplished by changing the gas flow to 150 sccm CH4 and 50 sccm NH3 for 360 s.The disadvantages of the research works mentioned above are the use of a conductive substrate for the formation of the graphite film such as a copper foil substrate and subsequently growing CNTs to fabricate the G-CNT hybrid. These experiments need further treatment to make G-CNTs free-standing from the substrate [1]. The innovation of our research work deals with a method in which we are able to synthesis G-CNTs without the use of the substrate. This is an advantage of our method in order to produce a large scale of G-CNTs by controlling the parameters in the experiment. The material synthesized can also be directly used as active sensing materials for gas sensors.The fundamental advantage of an integrated G-CNT structure is the high surface area three-dimensional framework of the CNTs, coupled with the high edge density of graphene [8]. Graphene edges provide significantly higher charge density and reactivity than the basal plane, but they are difficult to arrange in three-dimensional, high volume-density geometry. CNTs are readily aligned in high density geometry (i.e., a vertically aligned forest) but lack high charge density surfaces where the sidewalls of the CNTs are similar to the basal plane of graphene and exhibit low charge density except where edge defects exist. Depositing a high density of graphene, which foliates along the length of aligned CNTs, can significantly increase the total charge capacity per unit of nominal area as compared to other carbon nanostructures [9].A morphological trend can be observed as a function of temperature whereby G-CNTs emerged as an intermediate structure between CNTs at lower temperatures and vertically oriented carbon nanosheets (CNS), composed of few-layered graphene, at higher temperatures [10]. A parametric temperature series revealed that deposition temperature is the critical factor in controlling nanostructure morphology, and G-CNTs emerged as a temperature-based transitional morphology between CNTs and carbon nanostructures (CNS) [9].After scrutinizing the above-mentioned experimental works, we believed it is essential to have a single-step fabrication of cotton-like G-CNT hybrid materials using the floating catalyst chemical vapour deposition (FCCVD) technique with the intention of upscaling the process in the near future. The objectives of this work are to study the effect of the injection rate of the hydrocarbon source on the formation of the G-CNT hybrid and to test its performance for potential gas-sensing application due to its foliate structure along the CNT basal plane which was thought to be advantageous to the gas-sensing application. In this work, we proposed a direct growth of the G-CNT hybrid in the bulk form of a cotton-like structure which includes vapour-liquid-solid (VLS) mechanism growth from a metal nanoparticle: volume diffusion of carbon atoms (vapour) in metal (liquid) to crystallize CNTs (solid) effectively. Many research works on the synthesis of CNT powders via thermal chemical vapour deposition (CVD) have been carried out such as [3, 5, 11–13]; however, to the best of our knowledge, no study has been reported on the carbon feedstock effect on the formation of a G-CNT hybrid of a cotton-like structure synthesized via the FCCVD method. The G-CNTs synthesized in our research produced a bulk-cotton structure that is more flexible and easy to be manipulated. The bulk-cotton structure is then tested for ammonia gas-sensing application.


## 2. Methodology

### 2.1. Synthesis of Cotton-Like G-CNTs via FCCVD

The floating catalyst chemical vapour deposition (FCCVD) method was used in this experiment, and the schematic diagram of the setup is shown in Figure1. A total of 20 ml of ethanol was used as a hydrocarbon source, ferrocene as a catalyst precursor, and thiophene as a promoter agent. Initially, 2.4 wt% of ferrocene and 1.2 wt% of thiophene were dispersed into ethanol using ultrasonic treatment. Argon was flowed into the FCCVD at 100 sccm from the beginning of the experiment until the temperature reached 1150°C and then switched to hydrogen flow at the same temperature. The obtained reaction solution was injected into the carrier gas, and the injection rate of the carbon liquid source was varied from 5 ml/h, 10 ml/h, and 20 ml/h by using a syringe pump as shown in Table 1. The hydrogen gas flow with a rate of 150 sccm was used for all experiments. The Ar gas was then switched again to 100 sccm flow during cooling to room temperature. A continuous cotton-like structure of either G-CNTs or CNTs was formed and collected at the end of the tube reactor as in Figure 2. The samples were further characterized using FESEM, HRTEM, TGA, Raman spectroscopy, XRD, and XPS. Electrical conductivity was carried out using 2-point measurements with a Keithley model 2611 multimeter with the voltage ranging up to 10 v. The flowchart of the research work is shown in Figure 3.Figure 1
Schematic diagram of a floating catalyst CVD furnace: (1) injection pump, (2) injection tube, (3) quartz furnace tube (120 cm (length), 5 cm (outer diameter)), (4) furnace chamber, (5) furnace controller system, (6) gas regulator board, (7) inlet tube, and (8) outlet tube.Table 1
Morphological structure synthesized according to hydrocarbon feedstock flow rate and its synthesis duration.
Sample name
Carbon feedstock flow rate
Synthesis duration
Morphological structure
LFA
20 ml/h
1 hour
Entangled cotton-like CNTs
LFB
10 ml/h
2 hours
Entangled cotton-like CNTs
LFC
5 ml/h
4 hours
Entangled cotton-like G-CNTsFigure 2
Optical micrograph of cotton-like G-CNTs synthesized via the FCCVD method with a 5 ml/h injection rate of the carbon source.Figure 3
Flowchart of cotton-like G-CNT and CNT synthesis via the FCCVD method.


### 2.2. Gas-Sensing Application of Bulk-Cotton-Like CNTs and G-CNTs

The gas-sensing measurement was carried out for the as-synthesized cotton-like CNT structure of 20 and 10 ml/h denoted as LFA and LFB, respectively, and the G-CNT sample with an injection of 5 ml/h denoted as LFC. The nanomaterials were deposited onto an alumina substrate which has prepatterned electrodes made up of platinum using a drop-casting technique. Two gold wires were then connected to the platinum pads and then connected to a FlukeView 289 multimeter. The same procedure was carried out to fabricate LFA-, LFB-, and LFC-based sensors. The sensor was then put into a gas chamber which was then connected to Aalborg mass flow controllers. The controllers were then fed with air and ammonia gas. The gas flow was also controlled via a LabVIEW program designed to automate the whole process. The sensors were tested towards ammonia gas with different concentrations ranging from 600 ppm to 10,000 ppm and were carried out at room temperature (25°C). The ammonia gas was purged for 5 minutes, and synthetic air was used for recovery for 15 minutes.


## 2.1. Synthesis of Cotton-Like G-CNTs via FCCVD

The floating catalyst chemical vapour deposition (FCCVD) method was used in this experiment, and the schematic diagram of the setup is shown in Figure1. A total of 20 ml of ethanol was used as a hydrocarbon source, ferrocene as a catalyst precursor, and thiophene as a promoter agent. Initially, 2.4 wt% of ferrocene and 1.2 wt% of thiophene were dispersed into ethanol using ultrasonic treatment. Argon was flowed into the FCCVD at 100 sccm from the beginning of the experiment until the temperature reached 1150°C and then switched to hydrogen flow at the same temperature. The obtained reaction solution was injected into the carrier gas, and the injection rate of the carbon liquid source was varied from 5 ml/h, 10 ml/h, and 20 ml/h by using a syringe pump as shown in Table 1. The hydrogen gas flow with a rate of 150 sccm was used for all experiments. The Ar gas was then switched again to 100 sccm flow during cooling to room temperature. A continuous cotton-like structure of either G-CNTs or CNTs was formed and collected at the end of the tube reactor as in Figure 2. The samples were further characterized using FESEM, HRTEM, TGA, Raman spectroscopy, XRD, and XPS. Electrical conductivity was carried out using 2-point measurements with a Keithley model 2611 multimeter with the voltage ranging up to 10 v. The flowchart of the research work is shown in Figure 3.Figure 1
Schematic diagram of a floating catalyst CVD furnace: (1) injection pump, (2) injection tube, (3) quartz furnace tube (120 cm (length), 5 cm (outer diameter)), (4) furnace chamber, (5) furnace controller system, (6) gas regulator board, (7) inlet tube, and (8) outlet tube.Table 1
Morphological structure synthesized according to hydrocarbon feedstock flow rate and its synthesis duration.
Sample name
Carbon feedstock flow rate
Synthesis duration
Morphological structure
LFA
20 ml/h
1 hour
Entangled cotton-like CNTs
LFB
10 ml/h
2 hours
Entangled cotton-like CNTs
LFC
5 ml/h
4 hours
Entangled cotton-like G-CNTsFigure 2
Optical micrograph of cotton-like G-CNTs synthesized via the FCCVD method with a 5 ml/h injection rate of the carbon source.Figure 3
Flowchart of cotton-like G-CNT and CNT synthesis via the FCCVD method.


## 2.2. Gas-Sensing Application of Bulk-Cotton-Like CNTs and G-CNTs

The gas-sensing measurement was carried out for the as-synthesized cotton-like CNT structure of 20 and 10 ml/h denoted as LFA and LFB, respectively, and the G-CNT sample with an injection of 5 ml/h denoted as LFC. The nanomaterials were deposited onto an alumina substrate which has prepatterned electrodes made up of platinum using a drop-casting technique. Two gold wires were then connected to the platinum pads and then connected to a FlukeView 289 multimeter. The same procedure was carried out to fabricate LFA-, LFB-, and LFC-based sensors. The sensor was then put into a gas chamber which was then connected to Aalborg mass flow controllers. The controllers were then fed with air and ammonia gas. The gas flow was also controlled via a LabVIEW program designed to automate the whole process. The sensors were tested towards ammonia gas with different concentrations ranging from 600 ppm to 10,000 ppm and were carried out at room temperature (25°C). The ammonia gas was purged for 5 minutes, and synthetic air was used for recovery for 15 minutes.


## 3. Result and Discussion

### 3.1. Effect of Carbon Feedstock Flow Rate on Cotton-Like G-CNT Formation

Figures4(a) and 4(b) show FESEM images of identical CNT structures with hydrocarbon injection rates of 20 ml/h and 10 ml/h; however, when the injection rate was changed to 5 ml/h, we can observe different structures of samples where the existence of graphene foliates grows along the CNT sidewalls. HRTEM images in Figures 4(d) and 4(e) show the additional insight where well-aligned multiwalled CNTs were grown with the growth condition of 20 ml/h and 10 ml/h injection rates, and Figure 4(f) shows a highly packed density of graphene foliates growing along the outer wall of the CNT structure indicating that the carbon feedstock is an influential factor in determining the morphology. The summary of the synthesis condition is in Table 1.Figure 4
FESEM image of CNT and G-CNT fibers with different carbon feedstock flow rates of (a) 20 ml/h, (b) 10 ml/h, and (c) 5 ml/h and HRTEM images of CNTs and G-CNTs with injection rates of (d) 20 ml/h, (e) 10 ml/h, and (f) 5 ml/h.
(a)
(b)
(c)
(d)
(e)
(f)High-resolution transmission electron microscopy (HRTEM) images (Figure5) provide direct evidence of graphene foliates along the CNT basal plane. The hollow core, which is the main characteristic of CNTs, is clearly observed in Figure 5(a). The CNTs’ sidewalls, however, are unusually thick relative to the diameter, a consistent characteristic for the G-CNTs observed in this work. Figure 5(b) shows smaller foliates coming out from the primary “leaf” growing normal of the CNT sidewall. The specific mechanism for secondary nucleation is currently unknown and will be the topic of future studies, but defects at the active growth surface of the foliate are expected to play a role [8].Figure 5
(a) Defect from the CNT induced the growth of graphene along the CNT; (b) high magnification of G-CNT at the outer wall of CNT.Raman spectroscopy was used to characterize different bonding states present in the G-CNTs compared to usually synthesized CNTs. Figure6 shows the effects of hydrocarbon injection rate variations of 20 ml/h, 10 ml/h, and 5 ml/h, due to the formation of the CNT structure and G-CNT hybrid. Raman spectra exhibit peaks between conventional CNTs (20 and 10 ml/h) and G-CNTs (5 ml/h); for CNTs and graphene, these peaks are the D, G, and 2D peaks. The D (1332 cm-1) peak is an sp3 hybridized carbon present as impurities and dispersive defects in the graphitic structures. The G peak (1573 cm-1) is associated with the tangential stretching mode of graphite and is related to the vibration of sp2 bonded carbon atoms in a two-dimensional hexagonal lattice as explained by Dresselhaus et al. [10]. Figure 6 indicates the presence of CNTs with the appearance of the D peak; the ratio of the D band to G band (ID/IG) is utilized to estimate graphitization and the average size of graphite crystallinity. The 2D peak (2673 cm-1) is the Raman signature peak of graphitic sp2 materials. The ratio of the 2D band to G band (I2D/IG) has usually been adopted to evaluate the graphene layer number. The ID/IG of the Raman spectra of samples with 5 ml/h, 10 ml/h, and 20 ml/h carbon feedstock flow rates are 1.03, 0.97, and 1.00, respectively. The 5 ml/h carbon feedstock flow rate has the highest ID/IG ratio due to longer residence time inside the reactor tube, resulting in the catalyst being covered by carbon deposits, and thus, CNTs grew increasingly far away from the catalyst so that the role of the catalyst as an island for good CNTs to grow is reduced. G-CNTs with the 5 ml/h injection rate took 4 hours of synthesis duration causing high defect to the structure compared to samples with 20 and 10 ml/h injection rates that allowed graphene to grow between the defects in conjunction with the FESEM image (Figures 4(a) and 4(b)).Figure 6
Raman spectra of different samples with different hydrocarbon feedstock injection rates (refer to the online version for comparison).Meanwhile, theI2D/IG ratio for 5 ml/h, 10 ml/h, and 20 ml/h are 0.83, 0.81, and 0.78, respectively. It is known that the ratio of I2D/IG∼2–3 is for monolayer graphene, 2>I2D/IG>1 is for bilayer graphene, and I2D/IG<1 is for multilayer graphene [14–17]. Based on the I2D/IG ratio of Raman spectra, it can be concluded that the graphene films grown with the 5 ml/h injection rate are multilayers as observed in FESEM (Figure 4(c)).Figure7 shows the reduction in weight with the increase in temperature. An initial weight loss from ambient temperature to 300°C was attributed to the removal of absorbed water and the oxidation of volatile carbon content. A second weight loss observed at temperatures between 300°C and 800°C was attributed to the oxidation of the amorphous carbon. 20 ml/h and 10 ml/h showed thermal stability until the temperature range of 800°C-900°C when it started to have significant weight loss. This weight loss was a result of oxidation of CNTs, consistent with other studies, and we can see that 10 ml/h was more thermally stable. While the 5 ml/h sample shows more weight loss from ambient temperature to 800°C compared with other curves, this result is due to defective sites in G-CNTs which contributed to a decrease in the oxidative stability of these materials. These defects are also present at bends, Y-junctions, and kinks in nanotubes [18]. From the FESEM (Figures 4(a)–4(c)) and HRTEM (Figures 4(d)–4(f)) micrographs, we believe that air oxidation occurred preferably at these kink sites.Figure 7
TGA profile of different liquid hydrocarbon solution flow rates from ethanol as the carbon source (refer to the online version for comparison).In order to have some idea on the interlayer spacing and corresponding diffraction angle, XRD measurement was carried out and shown in Figure8. A close look at the (002) peak of all samples shows an apparent shifting of peaks indicating that there are changes in structural properties of the samples which we believed to be due to an interaction between CNTs and the graphene layer on the outer surface [19, 20]. This result can be confirmed by observing the microstructure of the samples using FESEM and HRTEM.Figure 8
XRD patterns of samples with injection rates of 5 ml/h (G-CNT), 10 ml/h (CNT 10), and 20 ml/h (CNT 20).Figure9 shows the I‐V curves with a similar trend of general, nonlinear, and symmetric shape, and this dominant characteristic can be understood as being related to graphitic properties [21]. The I‐V curve shows a low current flow at the low voltage region, but it is gradually increased at higher voltages. It was also observed that at the low voltages, the curves show nonlinearity and become linear at higher voltages. However, the sample with the 5 ml/h injection rate shows high current flow even though the low potential was applied during the testing; this was due to the existence of graphene structures being higher compared to the defect and these would help the electron mobility moving smoothly through the graphitic interfacial layer because of lower barrier resistance.Figure 9
Nonlinearity of theI‐V curves of different carbon feedstock flow rates (refer to the online version for comparison).Figure10 shows the XPS spectrum of G-CNTs obtained from the 5 ml/h carbon feedstock flow rate by determining the binding energy of photoelectrons ejected when G-CNTs are irradiated with X-rays. From the inset of Figure 10, G-CNTs obtained the C1s peak spectral indicating that unmodified carbon at 284.1 eV was attributed to a graphitic structure. The C1s spectra were composed of several characteristic peaks, such as two peaks due to the carbon-carbon interactions including C=C sp2 bonds at the binding energy of 284.21 eV and C-C sp3 bonds at 285.1 to 284.80 eV, and three relatively weak peaks including hydroxyl C-OH bonds at 286.05 eV, carbonyl C=O bonds at 288.49 eV, and a carboxyl O=C-OH bond at 290.69 eV due to π-π∗ shakeup features. C-C was likely induced by defects and graphene sheet edge formation [7]. C1s of undoped CNT is composed of three peaks: sp2 carbon (peak at about 284.3 eV), sp3 carbon (285 eV), and oxygen-related groups at about 288.5 eV, and it is also similar to those observed in nanofibers [22]. Hence, we confirmed that it was a carbon-based material. The C1s of graphite is usually observed at 284.6 eV. However, a negative shift of 0.3 eV observed in the binding energy of these multiwalled carbon nanotubes (MWNT) was explained by the weaker C-C bonds resulting from the curvature of the graphene sheets and by the larger interlayer spacing. With MWNT (30 nm in diameter) aligned along their tube axis and using spatially resolved XPS, Suzuki et al. have demonstrated that the shift of the C1s binding energy is only about 50 meV between the tips and the sidewalls [23]. The strong influence of structural defects on the electronic structure of MWNT was reported by these authors to explain this difference [24].Figure 10
C1s spectrum of G-CNT cotton fibers; the inset shows the wide range of XPS for the G-CNT sample (refer to the online version for comparison).In some ways, MWNT represents an intermediate material between graphite and single-walled carbon nanotubes (SWNT). Their multilayered structure resembles graphite crystals, and like graphite, an MWNT stably supports the type of defect known as an interstitial-vacancy bound pair. Here, we proposed a growth mechanism of G-CNTs based on the FESEM, HRTEM, XRD, and RAMAN evidence where the G-CNT sample was obtained at an injection rate of 5 ml/h that took a reaction time of 4 hours. When the reaction took place over a long duration of time, CNTs grew increasingly further away from the catalyst so that the role of the catalyst as a referrer to produce CNTs of good quality is reduced, hence, the increase of the defective structure along the sidewall (Figure11(a)). During the growth of G-CNTs, defects were formed due to the removal of amorphous carbon by H2 etching on the sidewalls (Figure 11(b)). These defects become the nucleation sites of the graphitic structure as the excessive carbon source came into the reactor to precipitate and form G-CNTs (Figure 11(c)).Figure 11
The G-CNT-proposed mechanism: (a) the CNTs grew far away from the catalyst leading to the defective structure; (b) the defect that forms due to removal of the amorphous carbon; (c) the defective site becomes a nucleation site as the excessive carbon source was fed into the reactor.
(a)
(b)
(c)


### 3.2. CNT and G-CNT Response to Ammonia Sensing

Different concentrations of ammonia were tested towards LFA-, LFB-, and LFC-based sensors (Table1). Figure 12(a) shows that there was not much change in the resistance of 296 ohms. This could be based on the structural morphology as seen from the SEM micrograph of Figure 4, which shows densely packed MWCNT which did not allow free interaction between the material and the gas molecules. The LFA sensor did not show a good response to ammonia as seen in Figure 12. The same behavior of the sensing response is seen in LFB (Figure 12(b)) which has similar structural morphology with that of LFA even though it has higher resistance in kiloohms. Similarly, the LFC-based sensor was also exposed to ammonia gas from 600 ppm to 10000 ppm. It is noticed from Figure 12(c) that there was a proportional increase in resistance when ammonia gas was injected for 5 minutes and then decreased gradually with the introduction of air into the chamber until it recovered to its baseline resistance of 145 ohms. The same sensing behavior was noticed for the different ammonia gas concentrations of 1250 ppm, 2500 ppm, 5000 ppm, 7500 ppm, and 10,000 ppm.Figure 12
Gas-sensing measurements of (a, b) CNT materials (LFA and LFB) and (c) G-CNT materials (LFC) and (d) repeatability of the gas-sensing measurement of G-CNT materials (LFC).
(a)
(b)
(c)
(d)The response to ammonia gas could be the result of good conductivity due to the presence of the graphene at the edges, which has a higher surface area. The high surface area allows for free interactions of molecules. In order to test for repeatability, the gas-sensing experiment was repeated three times using 600 ppm, as shown in Figure12(d). It could be seen that the sensor shows good repeatability towards ammonia gas as well as proper recovery to baseline. Based on equation (1), the sensitivity of the sensor was computed to be 0.014% for 10000 ppm and 0.007% in the case of 600 ppm.Young and Lin [25] have decorated Au nanoparticles onto nanotube surfaces to a thickness of approximately 5 nm as a gas sensor. Their performance is much higher than our reported work with 1 to 2% sensitivities. Meanwhile, Rigoni et al. [26] reported a threefold increase of gas sensor sensitivity by functionalizing their CNT and mixing with indium tin oxide nanoparticles. The samples in our work are observed to be low, and therefore, it is recommended that in order to enhance the sensitivity, functionalization with acids and other metal oxides and metal nanoparticles should be carried out on the pristine CNTs and G-CNTs to remove defects and impurities.Response time for the LFC-based sensor was recorded to be 100 seconds while recovery time was computed to be 480 seconds.(1)Response=∆RR%.Based on the results obtained, the response of LFA-, LFB-, and LFC-based sensors is similar to the sensing mechanism obtained from ammonia gas sensors based on CNTs as shown by Ref. [26–28]. When ammonia gas is exposed to the samples, it donates electrons to the CNTs because ammonia is an electron donor. Therefore, the increase in resistance makes the CNTs behave like a p-type semiconductor.Based on the gas-sensing results obtained, it could be seen that pristine CNT fiber-based sensors (LFA and LFB) do not show good sensing response, but G-CNTs showed good affinity to ammonia gas at room temperature which could be explained due to the hybrid structure of graphene and CNTs as confirmed from the SEM and HRTEM. In order to enhance the sensitivity of the G-CNTs for gas sensing, functionalization can be carried out to remove any impurities and defects.


## 3.1. Effect of Carbon Feedstock Flow Rate on Cotton-Like G-CNT Formation

Figures4(a) and 4(b) show FESEM images of identical CNT structures with hydrocarbon injection rates of 20 ml/h and 10 ml/h; however, when the injection rate was changed to 5 ml/h, we can observe different structures of samples where the existence of graphene foliates grows along the CNT sidewalls. HRTEM images in Figures 4(d) and 4(e) show the additional insight where well-aligned multiwalled CNTs were grown with the growth condition of 20 ml/h and 10 ml/h injection rates, and Figure 4(f) shows a highly packed density of graphene foliates growing along the outer wall of the CNT structure indicating that the carbon feedstock is an influential factor in determining the morphology. The summary of the synthesis condition is in Table 1.Figure 4
FESEM image of CNT and G-CNT fibers with different carbon feedstock flow rates of (a) 20 ml/h, (b) 10 ml/h, and (c) 5 ml/h and HRTEM images of CNTs and G-CNTs with injection rates of (d) 20 ml/h, (e) 10 ml/h, and (f) 5 ml/h.
(a)
(b)
(c)
(d)
(e)
(f)High-resolution transmission electron microscopy (HRTEM) images (Figure5) provide direct evidence of graphene foliates along the CNT basal plane. The hollow core, which is the main characteristic of CNTs, is clearly observed in Figure 5(a). The CNTs’ sidewalls, however, are unusually thick relative to the diameter, a consistent characteristic for the G-CNTs observed in this work. Figure 5(b) shows smaller foliates coming out from the primary “leaf” growing normal of the CNT sidewall. The specific mechanism for secondary nucleation is currently unknown and will be the topic of future studies, but defects at the active growth surface of the foliate are expected to play a role [8].Figure 5
(a) Defect from the CNT induced the growth of graphene along the CNT; (b) high magnification of G-CNT at the outer wall of CNT.Raman spectroscopy was used to characterize different bonding states present in the G-CNTs compared to usually synthesized CNTs. Figure6 shows the effects of hydrocarbon injection rate variations of 20 ml/h, 10 ml/h, and 5 ml/h, due to the formation of the CNT structure and G-CNT hybrid. Raman spectra exhibit peaks between conventional CNTs (20 and 10 ml/h) and G-CNTs (5 ml/h); for CNTs and graphene, these peaks are the D, G, and 2D peaks. The D (1332 cm-1) peak is an sp3 hybridized carbon present as impurities and dispersive defects in the graphitic structures. The G peak (1573 cm-1) is associated with the tangential stretching mode of graphite and is related to the vibration of sp2 bonded carbon atoms in a two-dimensional hexagonal lattice as explained by Dresselhaus et al. [10]. Figure 6 indicates the presence of CNTs with the appearance of the D peak; the ratio of the D band to G band (ID/IG) is utilized to estimate graphitization and the average size of graphite crystallinity. The 2D peak (2673 cm-1) is the Raman signature peak of graphitic sp2 materials. The ratio of the 2D band to G band (I2D/IG) has usually been adopted to evaluate the graphene layer number. The ID/IG of the Raman spectra of samples with 5 ml/h, 10 ml/h, and 20 ml/h carbon feedstock flow rates are 1.03, 0.97, and 1.00, respectively. The 5 ml/h carbon feedstock flow rate has the highest ID/IG ratio due to longer residence time inside the reactor tube, resulting in the catalyst being covered by carbon deposits, and thus, CNTs grew increasingly far away from the catalyst so that the role of the catalyst as an island for good CNTs to grow is reduced. G-CNTs with the 5 ml/h injection rate took 4 hours of synthesis duration causing high defect to the structure compared to samples with 20 and 10 ml/h injection rates that allowed graphene to grow between the defects in conjunction with the FESEM image (Figures 4(a) and 4(b)).Figure 6
Raman spectra of different samples with different hydrocarbon feedstock injection rates (refer to the online version for comparison).Meanwhile, theI2D/IG ratio for 5 ml/h, 10 ml/h, and 20 ml/h are 0.83, 0.81, and 0.78, respectively. It is known that the ratio of I2D/IG∼2–3 is for monolayer graphene, 2>I2D/IG>1 is for bilayer graphene, and I2D/IG<1 is for multilayer graphene [14–17]. Based on the I2D/IG ratio of Raman spectra, it can be concluded that the graphene films grown with the 5 ml/h injection rate are multilayers as observed in FESEM (Figure 4(c)).Figure7 shows the reduction in weight with the increase in temperature. An initial weight loss from ambient temperature to 300°C was attributed to the removal of absorbed water and the oxidation of volatile carbon content. A second weight loss observed at temperatures between 300°C and 800°C was attributed to the oxidation of the amorphous carbon. 20 ml/h and 10 ml/h showed thermal stability until the temperature range of 800°C-900°C when it started to have significant weight loss. This weight loss was a result of oxidation of CNTs, consistent with other studies, and we can see that 10 ml/h was more thermally stable. While the 5 ml/h sample shows more weight loss from ambient temperature to 800°C compared with other curves, this result is due to defective sites in G-CNTs which contributed to a decrease in the oxidative stability of these materials. These defects are also present at bends, Y-junctions, and kinks in nanotubes [18]. From the FESEM (Figures 4(a)–4(c)) and HRTEM (Figures 4(d)–4(f)) micrographs, we believe that air oxidation occurred preferably at these kink sites.Figure 7
TGA profile of different liquid hydrocarbon solution flow rates from ethanol as the carbon source (refer to the online version for comparison).In order to have some idea on the interlayer spacing and corresponding diffraction angle, XRD measurement was carried out and shown in Figure8. A close look at the (002) peak of all samples shows an apparent shifting of peaks indicating that there are changes in structural properties of the samples which we believed to be due to an interaction between CNTs and the graphene layer on the outer surface [19, 20]. This result can be confirmed by observing the microstructure of the samples using FESEM and HRTEM.Figure 8
XRD patterns of samples with injection rates of 5 ml/h (G-CNT), 10 ml/h (CNT 10), and 20 ml/h (CNT 20).Figure9 shows the I‐V curves with a similar trend of general, nonlinear, and symmetric shape, and this dominant characteristic can be understood as being related to graphitic properties [21]. The I‐V curve shows a low current flow at the low voltage region, but it is gradually increased at higher voltages. It was also observed that at the low voltages, the curves show nonlinearity and become linear at higher voltages. However, the sample with the 5 ml/h injection rate shows high current flow even though the low potential was applied during the testing; this was due to the existence of graphene structures being higher compared to the defect and these would help the electron mobility moving smoothly through the graphitic interfacial layer because of lower barrier resistance.Figure 9
Nonlinearity of theI‐V curves of different carbon feedstock flow rates (refer to the online version for comparison).Figure10 shows the XPS spectrum of G-CNTs obtained from the 5 ml/h carbon feedstock flow rate by determining the binding energy of photoelectrons ejected when G-CNTs are irradiated with X-rays. From the inset of Figure 10, G-CNTs obtained the C1s peak spectral indicating that unmodified carbon at 284.1 eV was attributed to a graphitic structure. The C1s spectra were composed of several characteristic peaks, such as two peaks due to the carbon-carbon interactions including C=C sp2 bonds at the binding energy of 284.21 eV and C-C sp3 bonds at 285.1 to 284.80 eV, and three relatively weak peaks including hydroxyl C-OH bonds at 286.05 eV, carbonyl C=O bonds at 288.49 eV, and a carboxyl O=C-OH bond at 290.69 eV due to π-π∗ shakeup features. C-C was likely induced by defects and graphene sheet edge formation [7]. C1s of undoped CNT is composed of three peaks: sp2 carbon (peak at about 284.3 eV), sp3 carbon (285 eV), and oxygen-related groups at about 288.5 eV, and it is also similar to those observed in nanofibers [22]. Hence, we confirmed that it was a carbon-based material. The C1s of graphite is usually observed at 284.6 eV. However, a negative shift of 0.3 eV observed in the binding energy of these multiwalled carbon nanotubes (MWNT) was explained by the weaker C-C bonds resulting from the curvature of the graphene sheets and by the larger interlayer spacing. With MWNT (30 nm in diameter) aligned along their tube axis and using spatially resolved XPS, Suzuki et al. have demonstrated that the shift of the C1s binding energy is only about 50 meV between the tips and the sidewalls [23]. The strong influence of structural defects on the electronic structure of MWNT was reported by these authors to explain this difference [24].Figure 10
C1s spectrum of G-CNT cotton fibers; the inset shows the wide range of XPS for the G-CNT sample (refer to the online version for comparison).In some ways, MWNT represents an intermediate material between graphite and single-walled carbon nanotubes (SWNT). Their multilayered structure resembles graphite crystals, and like graphite, an MWNT stably supports the type of defect known as an interstitial-vacancy bound pair. Here, we proposed a growth mechanism of G-CNTs based on the FESEM, HRTEM, XRD, and RAMAN evidence where the G-CNT sample was obtained at an injection rate of 5 ml/h that took a reaction time of 4 hours. When the reaction took place over a long duration of time, CNTs grew increasingly further away from the catalyst so that the role of the catalyst as a referrer to produce CNTs of good quality is reduced, hence, the increase of the defective structure along the sidewall (Figure11(a)). During the growth of G-CNTs, defects were formed due to the removal of amorphous carbon by H2 etching on the sidewalls (Figure 11(b)). These defects become the nucleation sites of the graphitic structure as the excessive carbon source came into the reactor to precipitate and form G-CNTs (Figure 11(c)).Figure 11
The G-CNT-proposed mechanism: (a) the CNTs grew far away from the catalyst leading to the defective structure; (b) the defect that forms due to removal of the amorphous carbon; (c) the defective site becomes a nucleation site as the excessive carbon source was fed into the reactor.
(a)
(b)
(c)


## 3.2. CNT and G-CNT Response to Ammonia Sensing

Different concentrations of ammonia were tested towards LFA-, LFB-, and LFC-based sensors (Table1). Figure 12(a) shows that there was not much change in the resistance of 296 ohms. This could be based on the structural morphology as seen from the SEM micrograph of Figure 4, which shows densely packed MWCNT which did not allow free interaction between the material and the gas molecules. The LFA sensor did not show a good response to ammonia as seen in Figure 12. The same behavior of the sensing response is seen in LFB (Figure 12(b)) which has similar structural morphology with that of LFA even though it has higher resistance in kiloohms. Similarly, the LFC-based sensor was also exposed to ammonia gas from 600 ppm to 10000 ppm. It is noticed from Figure 12(c) that there was a proportional increase in resistance when ammonia gas was injected for 5 minutes and then decreased gradually with the introduction of air into the chamber until it recovered to its baseline resistance of 145 ohms. The same sensing behavior was noticed for the different ammonia gas concentrations of 1250 ppm, 2500 ppm, 5000 ppm, 7500 ppm, and 10,000 ppm.Figure 12
Gas-sensing measurements of (a, b) CNT materials (LFA and LFB) and (c) G-CNT materials (LFC) and (d) repeatability of the gas-sensing measurement of G-CNT materials (LFC).
(a)
(b)
(c)
(d)The response to ammonia gas could be the result of good conductivity due to the presence of the graphene at the edges, which has a higher surface area. The high surface area allows for free interactions of molecules. In order to test for repeatability, the gas-sensing experiment was repeated three times using 600 ppm, as shown in Figure12(d). It could be seen that the sensor shows good repeatability towards ammonia gas as well as proper recovery to baseline. Based on equation (1), the sensitivity of the sensor was computed to be 0.014% for 10000 ppm and 0.007% in the case of 600 ppm.Young and Lin [25] have decorated Au nanoparticles onto nanotube surfaces to a thickness of approximately 5 nm as a gas sensor. Their performance is much higher than our reported work with 1 to 2% sensitivities. Meanwhile, Rigoni et al. [26] reported a threefold increase of gas sensor sensitivity by functionalizing their CNT and mixing with indium tin oxide nanoparticles. The samples in our work are observed to be low, and therefore, it is recommended that in order to enhance the sensitivity, functionalization with acids and other metal oxides and metal nanoparticles should be carried out on the pristine CNTs and G-CNTs to remove defects and impurities.Response time for the LFC-based sensor was recorded to be 100 seconds while recovery time was computed to be 480 seconds.(1)Response=∆RR%.Based on the results obtained, the response of LFA-, LFB-, and LFC-based sensors is similar to the sensing mechanism obtained from ammonia gas sensors based on CNTs as shown by Ref. [26–28]. When ammonia gas is exposed to the samples, it donates electrons to the CNTs because ammonia is an electron donor. Therefore, the increase in resistance makes the CNTs behave like a p-type semiconductor.Based on the gas-sensing results obtained, it could be seen that pristine CNT fiber-based sensors (LFA and LFB) do not show good sensing response, but G-CNTs showed good affinity to ammonia gas at room temperature which could be explained due to the hybrid structure of graphene and CNTs as confirmed from the SEM and HRTEM. In order to enhance the sensitivity of the G-CNTs for gas sensing, functionalization can be carried out to remove any impurities and defects.


## 4. Conclusion

We have successfully synthesized G-CNTs via the FCCVD method using ethanol as a hydrocarbon source. The injection rate of the hydrocarbon source was used as a parametric study in this work. The carbon source injection rate of 5 ml/h resulted in the formation of G-CNTs. Meanwhile, injection rates of 10 and 20 ml/h resulted in the formation of graphitic CNTs. The growth mechanism of G-CNTs synthesized was then proposed. The electrical property of G-CNTs showed better electron mobility within the structure compared to that of CNTs. This parametric study showed essential insights into the processing conditions by which high-quality CNTs and G-CNTs could be selectively synthesized. It is demonstrated that the synthesis of CNTs can be precisely controlled to customize their properties for use in various applications. We have also tested the synthesized CNTs and G-CNTs for gas-sensing applications using ammonia gas. A gas sensor made of the G-CNT structure showed good response towards ammonia gas exposure compared to gas sensors made of the CNT structure. Further study on the effect of humidity of the ammonia gas should be carried out in the future when developing the gas sensors operable at room temperature.



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*Source: 5717180-2019-10-14.xml*