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Fluorination of single-walled carbon nanotube: The effects of fluorine on structural and electrical properties
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JIEC-2871; No. of Pages 5 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Short communication
Fluorination of single-walled carbon nanotube: The effects of fluorine on structural and electrical properties Mi-Seon Park, Kyung Hoon Kim, Young-Seak Lee * Department of Applied Chemistry and Biological Engineering, Chungnam National University, Daejeon 34134, Republic of Korea
A R T I C L E I N F O
Article history: Received 22 January 2016 Received in revised form 4 March 2016 Accepted 12 March 2016 Available online xxx Keywords: Fluorination Single-walled carbon nanotube Electrical properties Raman spectroscopy Scanning tunneling microscopy
A B S T R A C T
The surfaces of single-walled carbon nanotube (SWCNT) are fluorinated at room temperature to examine their structural and electrical properties after fluorination. Fluorine functional groups are introduced on the surfaces of SWCNT via direct fluorination. The structural properties of the fluorinated SWCNT indicate that the number of defects increases due to the carbon to fluorine bond formation and that the fluorine radicals have an etching effect on the SWCNT. Thus, the structural changes of SWCNT caused by fluorination include increased diameters and changes in chirality. In addition, the conductivity of the SWCNT decreases due to the formation of carbon to fluorine bonds that prohibit the pi electron activity in SWCNT. ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Carbon nanotube (CNT) has been extensively used as promising adsorbents, electric double layer capacitors, and bio/gas sensors [1–5]. The applications of CNT depend on their properties. CNT is composed of sheets of six-membered carbon rings that are joined to form cylindrical tubes. Because CNT consists of sp2 hybridized carbon orbitals, pi electrons can move freely within their carbon layer, providing electrical conductivity. Electrical conductivity is an important property that expresses the nature of a material. When clarifying the electrical properties at the nanometer scale, scanning probe microscopy (SPM)-based techniques are interesting, because of their high lateral resolution and because they can measure the electrical properties of sample surface. Several methods can be used to change the electrical properties of materials, such as surface treatment [6–8]. Particularly, CNT is treated using chemicals, plasma, flame, corona discharge, and direct fluorination [9–12]. CNT with oxygen functional groups is favorable for introducing fluorine and nitrogen because fluorine and nitrogen each have a lone pair of electron that allows them to substitute for oxygen in functional group, a process referred to as nucleophilic substitution [13,14]. Based on the benefits of CNT and the fluorination of carbon materials, the fluorination is expected to
* Corresponding author. Tel.: +82 42 821 7007; fax: +82 42 822 6637. E-mail address: [email protected] (Y.-S. Lee).
change various CNT properties [15,16]. Other CNT treatments do not appear to show outstanding electrical property changes after CNT surface treatment because CNT is highly ordered carbon materials with a high degree of graphitization. In this study, single-walled carbon nanotube (SWCNT) was used to investigate their conductivity using the direct fluorination technique. The chemical, structural, and electrical properties of the fluorinated SWCNT (f-SWCNT) were analyzed in detail, and we observed relationships between the properties. Experimental Fluorination of SWCNT The single-walled carbon nanotube (ASP-100F, Hanwha Chemical, Republic of Korea), which was aligned, and had a purity of 60– 70 wt% and a bulk density of 0.01 g/cm3, was used after a purification in this study in order to remove amorphous carbon and metal catalyst. In the first step of purification, single-walled carbon nanotube was dry-heated at 550 8C for 80 min in dry heater under. The dry-heated single-walled carbon nanotube was mixed with aqua regia at room temperature for 1 h and then the mixtures were treated by ultrasonic homogenizer (Ultrasonic Liquid Processor VC-750, SONICS & MATERIALS Inc., USA) at 60 Hz. The physically and chemically purified single-walled carbon nanotube was named as SWCNT. The SWCNT was fluorinated using a fluorination apparatus consisting of a SUS304 reactor, a vacuum
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pump, a nickel boat, and a buffer tank connected to a gas cylinder. Fluorine gas with a purity of 99.8% was purchased from Messer Grieheim GmbH (Germany). The following fluorination conditions were used. First, the SWCNT (0.1 g) was placed in the middle of the reactor after they were dried at 120 8C in an oven to remove the physically absorbed water on the SWCNT surface. The reactor containing the SWCNT was purged with nitrogen gas (99.99%, Special Gas, Republic of Korea) twice to remove impurities that could cause side reactions, such as oxygen and moisture. After purging, the fluorine gas (0.1 bar) was injected into the reactor and held for 10 min at 25 8C. The fluorinated SWCNT was named as f-SWCNT. Characteristics of fluorinated SWCNT The various changes of the SWCNT following fluorination were investigated. X-ray photoelectron spectroscopy (XPS, MultiLab 2000, Thermo Electron cooperation, England) analysis was used to determine concentration of introduced fluorine and its type of bonding on the surfaces of the SWCNT. Al Ka (1485.6 eV) was used as the X-ray source, with an anode voltage of 14.9 keV, a filament current of 4.6 A, and an emission current of 20 mA. The structural changes of the SWCNT following the introduction of fluorine were observed using Raman spectroscopy (LabRam high resolution, Horiba Jobin-Yvon, France) with an Ar-ion laser at 514.532 nm. The surface morphology of the f-SWCNT was investigated using a transmission electron microscope (TEM, JEM-2200FS, JEOL Ltd., USA) and a scanning tunneling microscope (STM, NanoScope V, Veeco, USA), and the I–V curve was recorded. The resolution of the STM was 0.001 nm toward the X- and Y-axes and 0.01 nm toward the Z-axis. A method proposed in a previous work was adopted to characterize the conductivity of the f-SWCNT [17]. Deposition was accomplished using a wet procedure, in which samples were mechanically removed from their original layer and placed in a methanol solution. After sonication, a few drops of the sample were deposited on the HOPG and dried. The use of HOPG as a substrate, which has a high lateral conductivity, allowed us to project an image of the samples on a very flat and clean surface and use a small bias voltage. Results and discussion Surface chemical properties XPS elemental analysis was conducted to compare changes in the atoms quantity after direct fluorination. As shown in Table 1, the quantity of fluorine atoms increased to 11.11 at%. The generation of fluorine decreased the quantities of carbon and oxygen from 94.34 to 84.55 at% and from 5.66 to 4.34 at%, respectively, which is regarded as the general reaction result of carbon materials [18]. To investigate the changes in the chemical bonds of the SWCNT due to direct fluorination, untreated and fSWCNT were analyzed using C1s deconvolution, as shown in Fig. 1. The components, their peak positions and concentrations are summarized in Table 2. Four main peaks were observed at 284.5, 285.5, 286.4, and 287.4 eV for the intrinsic SWCNT, which
Table 1 XPS surface elemental analysis parameters of the fluorinated SWCNT. Sample
SWCNT f-SWCNT
Elemental contents (atomic percent) C
O
F
94.34 84.55
5.66 4.34
– 11.11
O/C (%)
F/C (%)
5.99 5.13
– 13.14
Fig. 1. Deconvolution of the core level C1s of (a) SWCNT and (b) f-SWCNT.
correspond to C–C (sp2), C5 5C (sp3), C–O, and C5 5O, respectively [5,19]. In the f-SWCNT, new components that appear at approximately 289.3 and 290.5 eV were associated with semicovalent C–F and C–F bonds [20]. We confirmed that the direct fluorination of SWCNT when subjecting the SWCNT to fluorine at 0.1 bar and room temperature results in the formation of more semi-covalent C–F. This finding corresponds with the findings of previous research of various carbon materials, such as activated carbon, activated carbon nanofibers, graphene oxide, and MWCNT [13,21,22]. Although the total quantity of fluorine functional groups was 5.58%, the number of C–C (sp2) bonds decreased. This effect is referred to as fluorine radical etching effects [2]. Structural properties Fig. 2 shows the XRD pattern of pristine SWCNT (the purified SWCNT) and f-SWCNT. The peaks appeared at 268 and 428 correspond to (0 0 2) and (1 0 0) crystal planes, respectively, of graphitic ones of the single-walled carbon nanotubes [23]. The peak intensity of f-SWCNT slightly decreased, which was revealed that fluorine was introduced on SWCNT scattering its bundles. The Raman spectra were deconvoluted to information regarding the changes in the carbon structure of the fluorinated SWCNT, as shown in Fig. 3. The Raman spectra were different after the fluorination of the SWCNT. Only two peaks were observed at approximately 1592 cm1 (G1) and 1565 cm1 (G2) for the pristine SWCNT. The G peaks (G1 and G2) correspond to the graphitic domains, which are attributed to the typical C–C stretching modes in graphite. The G2 peak was narrow, which indicated the presence of non-metallic CNT. The D1 and D2 peaks from the f-SWCNT,
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Table 2 C1s peak parameters of the fluorinated SWCNT. SWCNT
Component
C(1) C(2) C(3) C(4) C(5) C(6)
C–C (sp2) C–C (sp3) C–O C5 5O Semi-covalent C–F C–F
f-SWCNT
Peak position (eV)
Concentration (%)
Peak position (eV)
Concentration (%)
284.5 285.5 286.4 287.4 – –
78.73 14.08 5.73 1.46 – –
284.4 285.4 286.4 287.7 289.3 290.5
71.23 18.12 3.91 1.16 3.37 2.21
which are related to the defects and disorder of the carbon material, were added at approximately 1340–1355 cm1 [24]. These results were attributed to the defects generated when fluorine bonded to carbon in the SWCNT carbon layers and describes the mixing of sp2 and sp3 hybridized carbon. Table 3 shows the peak intensity ratios of ID1/IG1, ID2/IG2, and IG2/IG1 for the investigated spectral range, which provides important information regarding the structures of the carbon materials [25,26]. Specifically, the small ID1/IG1 ratio of the f-SWCNT indicates that the surface area increased. The appearance of ID2/IG2 indicated the generation of an amorphous phase. Smaller IG2/IG1 values of 0.36 to 0.25 decreased the amounts of carbon in the fluorinated materials. In summary, we suggest that fluorine attacks the SWCNT in two ways during the fluorination of SWCNT, the removal of oxygen or carbon and the formation of C–F bonds on the surfaces of the SWCNT, as discussed in section ‘‘Surface chemical properties’’. Fig. 2. XRD patterns of (a) SWCNT and (b) f-SWCNT.
Electrical properties Fig. 4 shows high resolution topographic STM images of the SWCNT and f-SWCNT. The SWCNT diameters increased from 1.3 to 1.55 nm after fluorination due to the introduction of fluorine atoms on the SWCNT surfaces [27,28]. TEM images supported an increase in diameter of f-SWCNT as shown in Fig. 5. We also calculated the chiralities of the SWCNT and f-SWCNT by using the following equation [29]: dðnmÞ ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:246 n2 þ nm þ m2
(1)
p
This is an interaction formula between diameter (d) and notation (n, m) of a carbon nanotube. pffiffiffi 3m
uðradianÞ ¼ sin1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2
(2)
ðn þ nm þ m Þ
this is an interaction formula between chiral angle (u) and notation (n, m) of a carbon nanotube. The chiral angles were 28.5 and 258 for the SWCNT and f-SWCNT. The diameter and chiral angle for each sample obtained from the STM images were substituted into Eqs. (1) and (2). The calculated results are presented in Table 4. The (10,9) CNT was the most similar to the SWCNT when considering diameter and chiral angle of 1.3 nm and 28.58, respectively. In addition, CNT with diameters of
Table 3 Structural characteristics of the fluorinated SWCNT obtained by the Raman spectra deconvolution. Sample
Fig. 3. Raman spectra of (a) SWCNT and (b) f-SWCNT.
SWCNT f-SWCNT
Intensity ratio ID1/IG1
ID2/IG2
IG2/IG1
– 0.15
– 0.77
0.36 0.25
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Fig. 4. STM and calculated chirality images of (a) SWCNT and (b) f-SWCNT.
1.3 nm was confirmed in the chirality image of the SWCNT, as shown in Fig. 4(a). For f-SWCNT, the possible CNT with a diameter of 1.55 nm was (12,11), (13,10), (16,6), (18,3), and (19,1), as shown in Fig. 4(b). Considering the angle and values shown in Table 4, carbon nanotube (13,10) is the most appropriate for the fSWCNT. The electrical properties of CNT depend on their chirality because the chirality controls the movements of the free electrons [30]. For example, for the chiral notation (n, m), the carbon nanotube is metallic when n and m are multiples of three; otherwise, the carbon nanotube is semiconducting with a band gap. The SWCNT and f-SWCNT were conformed to act as semiconductors in this study. The scansion of the tip was successively stopped on a sampling point, and the tunneling I–V curve was recorded. The I–V curve provides clear evidence of the conductivity dI/dV and the electronic density of states (r) of materials, which is expressed as follows. For a semiconductor at 0 V, there is no measurable tunneling current
occurs due to the presence of a bandgap; however, for a metal, the current–voltage spectroscopy shows linear I–V characteristics and a quantifiable tunneling current [17]. As shown in Fig. 6, we analyzed the conductivities of the samples in the region between 0.3 and 0.3 V. The dI/dV of the f-SWCNT was smaller than that of the pristine SWCNT. The conductivities of the SWCNT decreased after fluorination. Because each carbon atom has a double bond with one of its neighbors, it is possible for CNT to bond with other molecules or elements without disrupting their physical structure. However, fluorine reduces the electrical conductivity of carbon materials [31,32]. Thus, the pi electrons, which provide conductivity in the SWCNT, are fixed by forming a carbon to fluorine bond. The linear current behavior of the STM I–V curve according to the applied voltage indicates if the CNT are metal or metallic [17]. Although the studied SWCNT and f-SWCNT were semiconducting, no changes in current were observed above approximately 0.15 V.
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Conclusions In this study, the surfaces of SWCNT were modified via direct fluorination at room temperature to investigate changes in the various properties of SWCNT following fluorination. The surfaces of the fluorinated SWCNT contained fluorine functional groups in addition to carbon and oxygen. The formation of carbon to fluorine bonds indicated that sp3 and sp2 hybridized carbon were mixed, which attributed to an increase in the number of defects in the SWCNT. In addition, carbon molecules disappeared by fluorine during fluorination, which etched the carbon of the SWCNT. The SWCNT used in this study was semiconducting and resulted from the narrow peak of G2. After fluorination, the diameter increased from 1.3 to 1.55, and the chirality changed from (10,9) to (13,10). However, the semiconducting properties were not altered by fluorination. These changes resulted in lower carbon nanotube conductivity, which was attributed to the limited movement of pi electrons in the fluorinated SWCNT. Acknowledgments This research was funded by the research supporting project funded by Chungnam National University (No. 2015-1229-01). References
Fig. 5. TEM images of (a) SWCNT and (b) f-SWCNT.
Table 4 The calculated result values for the notation of the SWCNT and f-SWCNT. f-SWCNT
SWCNT (n,m)
d (nm)
u
(10,9) (11,8) (12,7) (13,6) (15,3) (16,1)
1.289 1.294 1.303 1.317 1.308 1.294
0.493 0.432 0.373 0.314 0.156 0.108
(28.38) (24.58) (21.48) (18.08) (8.958) (6.178)
(n,m)
d (nm)
u
(12,11) (13,10) (16,6) (18,3) (19,1)
1.560 1.564 1.542 1.540 1.528
0.499 0.448 0.267 0.132 0.044
(28.68) (25.78) (15.38) (7.68) (2.58)
Fig. 6. Tunneling current–voltage plot for (a) SWCNT and (b) f-SWCNT.
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JIEC-2871; No. of Pages 5 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Short communication
Fluorination of single-walled carbon nanotube: The effects of fluorine on structural and electrical properties Mi-Seon Park, Kyung Hoon Kim, Young-Seak Lee * Department of Applied Chemistry and Biological Engineering, Chungnam National University, Daejeon 34134, Republic of Korea
A R T I C L E I N F O
Article history: Received 22 January 2016 Received in revised form 4 March 2016 Accepted 12 March 2016 Available online xxx Keywords: Fluorination Single-walled carbon nanotube Electrical properties Raman spectroscopy Scanning tunneling microscopy
A B S T R A C T
The surfaces of single-walled carbon nanotube (SWCNT) are fluorinated at room temperature to examine their structural and electrical properties after fluorination. Fluorine functional groups are introduced on the surfaces of SWCNT via direct fluorination. The structural properties of the fluorinated SWCNT indicate that the number of defects increases due to the carbon to fluorine bond formation and that the fluorine radicals have an etching effect on the SWCNT. Thus, the structural changes of SWCNT caused by fluorination include increased diameters and changes in chirality. In addition, the conductivity of the SWCNT decreases due to the formation of carbon to fluorine bonds that prohibit the pi electron activity in SWCNT. ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Carbon nanotube (CNT) has been extensively used as promising adsorbents, electric double layer capacitors, and bio/gas sensors [1–5]. The applications of CNT depend on their properties. CNT is composed of sheets of six-membered carbon rings that are joined to form cylindrical tubes. Because CNT consists of sp2 hybridized carbon orbitals, pi electrons can move freely within their carbon layer, providing electrical conductivity. Electrical conductivity is an important property that expresses the nature of a material. When clarifying the electrical properties at the nanometer scale, scanning probe microscopy (SPM)-based techniques are interesting, because of their high lateral resolution and because they can measure the electrical properties of sample surface. Several methods can be used to change the electrical properties of materials, such as surface treatment [6–8]. Particularly, CNT is treated using chemicals, plasma, flame, corona discharge, and direct fluorination [9–12]. CNT with oxygen functional groups is favorable for introducing fluorine and nitrogen because fluorine and nitrogen each have a lone pair of electron that allows them to substitute for oxygen in functional group, a process referred to as nucleophilic substitution [13,14]. Based on the benefits of CNT and the fluorination of carbon materials, the fluorination is expected to
* Corresponding author. Tel.: +82 42 821 7007; fax: +82 42 822 6637. E-mail address: [email protected] (Y.-S. Lee).
change various CNT properties [15,16]. Other CNT treatments do not appear to show outstanding electrical property changes after CNT surface treatment because CNT is highly ordered carbon materials with a high degree of graphitization. In this study, single-walled carbon nanotube (SWCNT) was used to investigate their conductivity using the direct fluorination technique. The chemical, structural, and electrical properties of the fluorinated SWCNT (f-SWCNT) were analyzed in detail, and we observed relationships between the properties. Experimental Fluorination of SWCNT The single-walled carbon nanotube (ASP-100F, Hanwha Chemical, Republic of Korea), which was aligned, and had a purity of 60– 70 wt% and a bulk density of 0.01 g/cm3, was used after a purification in this study in order to remove amorphous carbon and metal catalyst. In the first step of purification, single-walled carbon nanotube was dry-heated at 550 8C for 80 min in dry heater under. The dry-heated single-walled carbon nanotube was mixed with aqua regia at room temperature for 1 h and then the mixtures were treated by ultrasonic homogenizer (Ultrasonic Liquid Processor VC-750, SONICS & MATERIALS Inc., USA) at 60 Hz. The physically and chemically purified single-walled carbon nanotube was named as SWCNT. The SWCNT was fluorinated using a fluorination apparatus consisting of a SUS304 reactor, a vacuum
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pump, a nickel boat, and a buffer tank connected to a gas cylinder. Fluorine gas with a purity of 99.8% was purchased from Messer Grieheim GmbH (Germany). The following fluorination conditions were used. First, the SWCNT (0.1 g) was placed in the middle of the reactor after they were dried at 120 8C in an oven to remove the physically absorbed water on the SWCNT surface. The reactor containing the SWCNT was purged with nitrogen gas (99.99%, Special Gas, Republic of Korea) twice to remove impurities that could cause side reactions, such as oxygen and moisture. After purging, the fluorine gas (0.1 bar) was injected into the reactor and held for 10 min at 25 8C. The fluorinated SWCNT was named as f-SWCNT. Characteristics of fluorinated SWCNT The various changes of the SWCNT following fluorination were investigated. X-ray photoelectron spectroscopy (XPS, MultiLab 2000, Thermo Electron cooperation, England) analysis was used to determine concentration of introduced fluorine and its type of bonding on the surfaces of the SWCNT. Al Ka (1485.6 eV) was used as the X-ray source, with an anode voltage of 14.9 keV, a filament current of 4.6 A, and an emission current of 20 mA. The structural changes of the SWCNT following the introduction of fluorine were observed using Raman spectroscopy (LabRam high resolution, Horiba Jobin-Yvon, France) with an Ar-ion laser at 514.532 nm. The surface morphology of the f-SWCNT was investigated using a transmission electron microscope (TEM, JEM-2200FS, JEOL Ltd., USA) and a scanning tunneling microscope (STM, NanoScope V, Veeco, USA), and the I–V curve was recorded. The resolution of the STM was 0.001 nm toward the X- and Y-axes and 0.01 nm toward the Z-axis. A method proposed in a previous work was adopted to characterize the conductivity of the f-SWCNT [17]. Deposition was accomplished using a wet procedure, in which samples were mechanically removed from their original layer and placed in a methanol solution. After sonication, a few drops of the sample were deposited on the HOPG and dried. The use of HOPG as a substrate, which has a high lateral conductivity, allowed us to project an image of the samples on a very flat and clean surface and use a small bias voltage. Results and discussion Surface chemical properties XPS elemental analysis was conducted to compare changes in the atoms quantity after direct fluorination. As shown in Table 1, the quantity of fluorine atoms increased to 11.11 at%. The generation of fluorine decreased the quantities of carbon and oxygen from 94.34 to 84.55 at% and from 5.66 to 4.34 at%, respectively, which is regarded as the general reaction result of carbon materials [18]. To investigate the changes in the chemical bonds of the SWCNT due to direct fluorination, untreated and fSWCNT were analyzed using C1s deconvolution, as shown in Fig. 1. The components, their peak positions and concentrations are summarized in Table 2. Four main peaks were observed at 284.5, 285.5, 286.4, and 287.4 eV for the intrinsic SWCNT, which
Table 1 XPS surface elemental analysis parameters of the fluorinated SWCNT. Sample
SWCNT f-SWCNT
Elemental contents (atomic percent) C
O
F
94.34 84.55
5.66 4.34
– 11.11
O/C (%)
F/C (%)
5.99 5.13
– 13.14
Fig. 1. Deconvolution of the core level C1s of (a) SWCNT and (b) f-SWCNT.
correspond to C–C (sp2), C5 5C (sp3), C–O, and C5 5O, respectively [5,19]. In the f-SWCNT, new components that appear at approximately 289.3 and 290.5 eV were associated with semicovalent C–F and C–F bonds [20]. We confirmed that the direct fluorination of SWCNT when subjecting the SWCNT to fluorine at 0.1 bar and room temperature results in the formation of more semi-covalent C–F. This finding corresponds with the findings of previous research of various carbon materials, such as activated carbon, activated carbon nanofibers, graphene oxide, and MWCNT [13,21,22]. Although the total quantity of fluorine functional groups was 5.58%, the number of C–C (sp2) bonds decreased. This effect is referred to as fluorine radical etching effects [2]. Structural properties Fig. 2 shows the XRD pattern of pristine SWCNT (the purified SWCNT) and f-SWCNT. The peaks appeared at 268 and 428 correspond to (0 0 2) and (1 0 0) crystal planes, respectively, of graphitic ones of the single-walled carbon nanotubes [23]. The peak intensity of f-SWCNT slightly decreased, which was revealed that fluorine was introduced on SWCNT scattering its bundles. The Raman spectra were deconvoluted to information regarding the changes in the carbon structure of the fluorinated SWCNT, as shown in Fig. 3. The Raman spectra were different after the fluorination of the SWCNT. Only two peaks were observed at approximately 1592 cm1 (G1) and 1565 cm1 (G2) for the pristine SWCNT. The G peaks (G1 and G2) correspond to the graphitic domains, which are attributed to the typical C–C stretching modes in graphite. The G2 peak was narrow, which indicated the presence of non-metallic CNT. The D1 and D2 peaks from the f-SWCNT,
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Table 2 C1s peak parameters of the fluorinated SWCNT. SWCNT
Component
C(1) C(2) C(3) C(4) C(5) C(6)
C–C (sp2) C–C (sp3) C–O C5 5O Semi-covalent C–F C–F
f-SWCNT
Peak position (eV)
Concentration (%)
Peak position (eV)
Concentration (%)
284.5 285.5 286.4 287.4 – –
78.73 14.08 5.73 1.46 – –
284.4 285.4 286.4 287.7 289.3 290.5
71.23 18.12 3.91 1.16 3.37 2.21
which are related to the defects and disorder of the carbon material, were added at approximately 1340–1355 cm1 [24]. These results were attributed to the defects generated when fluorine bonded to carbon in the SWCNT carbon layers and describes the mixing of sp2 and sp3 hybridized carbon. Table 3 shows the peak intensity ratios of ID1/IG1, ID2/IG2, and IG2/IG1 for the investigated spectral range, which provides important information regarding the structures of the carbon materials [25,26]. Specifically, the small ID1/IG1 ratio of the f-SWCNT indicates that the surface area increased. The appearance of ID2/IG2 indicated the generation of an amorphous phase. Smaller IG2/IG1 values of 0.36 to 0.25 decreased the amounts of carbon in the fluorinated materials. In summary, we suggest that fluorine attacks the SWCNT in two ways during the fluorination of SWCNT, the removal of oxygen or carbon and the formation of C–F bonds on the surfaces of the SWCNT, as discussed in section ‘‘Surface chemical properties’’. Fig. 2. XRD patterns of (a) SWCNT and (b) f-SWCNT.
Electrical properties Fig. 4 shows high resolution topographic STM images of the SWCNT and f-SWCNT. The SWCNT diameters increased from 1.3 to 1.55 nm after fluorination due to the introduction of fluorine atoms on the SWCNT surfaces [27,28]. TEM images supported an increase in diameter of f-SWCNT as shown in Fig. 5. We also calculated the chiralities of the SWCNT and f-SWCNT by using the following equation [29]: dðnmÞ ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:246 n2 þ nm þ m2
(1)
p
This is an interaction formula between diameter (d) and notation (n, m) of a carbon nanotube. pffiffiffi 3m
uðradianÞ ¼ sin1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2
(2)
ðn þ nm þ m Þ
this is an interaction formula between chiral angle (u) and notation (n, m) of a carbon nanotube. The chiral angles were 28.5 and 258 for the SWCNT and f-SWCNT. The diameter and chiral angle for each sample obtained from the STM images were substituted into Eqs. (1) and (2). The calculated results are presented in Table 4. The (10,9) CNT was the most similar to the SWCNT when considering diameter and chiral angle of 1.3 nm and 28.58, respectively. In addition, CNT with diameters of
Table 3 Structural characteristics of the fluorinated SWCNT obtained by the Raman spectra deconvolution. Sample
Fig. 3. Raman spectra of (a) SWCNT and (b) f-SWCNT.
SWCNT f-SWCNT
Intensity ratio ID1/IG1
ID2/IG2
IG2/IG1
– 0.15
– 0.77
0.36 0.25
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Fig. 4. STM and calculated chirality images of (a) SWCNT and (b) f-SWCNT.
1.3 nm was confirmed in the chirality image of the SWCNT, as shown in Fig. 4(a). For f-SWCNT, the possible CNT with a diameter of 1.55 nm was (12,11), (13,10), (16,6), (18,3), and (19,1), as shown in Fig. 4(b). Considering the angle and values shown in Table 4, carbon nanotube (13,10) is the most appropriate for the fSWCNT. The electrical properties of CNT depend on their chirality because the chirality controls the movements of the free electrons [30]. For example, for the chiral notation (n, m), the carbon nanotube is metallic when n and m are multiples of three; otherwise, the carbon nanotube is semiconducting with a band gap. The SWCNT and f-SWCNT were conformed to act as semiconductors in this study. The scansion of the tip was successively stopped on a sampling point, and the tunneling I–V curve was recorded. The I–V curve provides clear evidence of the conductivity dI/dV and the electronic density of states (r) of materials, which is expressed as follows. For a semiconductor at 0 V, there is no measurable tunneling current
occurs due to the presence of a bandgap; however, for a metal, the current–voltage spectroscopy shows linear I–V characteristics and a quantifiable tunneling current [17]. As shown in Fig. 6, we analyzed the conductivities of the samples in the region between 0.3 and 0.3 V. The dI/dV of the f-SWCNT was smaller than that of the pristine SWCNT. The conductivities of the SWCNT decreased after fluorination. Because each carbon atom has a double bond with one of its neighbors, it is possible for CNT to bond with other molecules or elements without disrupting their physical structure. However, fluorine reduces the electrical conductivity of carbon materials [31,32]. Thus, the pi electrons, which provide conductivity in the SWCNT, are fixed by forming a carbon to fluorine bond. The linear current behavior of the STM I–V curve according to the applied voltage indicates if the CNT are metal or metallic [17]. Although the studied SWCNT and f-SWCNT were semiconducting, no changes in current were observed above approximately 0.15 V.
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Conclusions In this study, the surfaces of SWCNT were modified via direct fluorination at room temperature to investigate changes in the various properties of SWCNT following fluorination. The surfaces of the fluorinated SWCNT contained fluorine functional groups in addition to carbon and oxygen. The formation of carbon to fluorine bonds indicated that sp3 and sp2 hybridized carbon were mixed, which attributed to an increase in the number of defects in the SWCNT. In addition, carbon molecules disappeared by fluorine during fluorination, which etched the carbon of the SWCNT. The SWCNT used in this study was semiconducting and resulted from the narrow peak of G2. After fluorination, the diameter increased from 1.3 to 1.55, and the chirality changed from (10,9) to (13,10). However, the semiconducting properties were not altered by fluorination. These changes resulted in lower carbon nanotube conductivity, which was attributed to the limited movement of pi electrons in the fluorinated SWCNT. Acknowledgments This research was funded by the research supporting project funded by Chungnam National University (No. 2015-1229-01). References
Fig. 5. TEM images of (a) SWCNT and (b) f-SWCNT.
Table 4 The calculated result values for the notation of the SWCNT and f-SWCNT. f-SWCNT
SWCNT (n,m)
d (nm)
u
(10,9) (11,8) (12,7) (13,6) (15,3) (16,1)
1.289 1.294 1.303 1.317 1.308 1.294
0.493 0.432 0.373 0.314 0.156 0.108
(28.38) (24.58) (21.48) (18.08) (8.958) (6.178)
(n,m)
d (nm)
u
(12,11) (13,10) (16,6) (18,3) (19,1)
1.560 1.564 1.542 1.540 1.528
0.499 0.448 0.267 0.132 0.044
(28.68) (25.78) (15.38) (7.68) (2.58)
Fig. 6. Tunneling current–voltage plot for (a) SWCNT and (b) f-SWCNT.
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