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Chemical vapor deposition synthesis of carbon spheres: Effects of temperature and hydrogen
Vacuum 172 (2020) 109108
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Chemical vapor deposition synthesis of carbon spheres: Effects of temperature and hydrogen Radhika Panickar a, b, *, C.B. Sobhan a, b, Sivaji Chakravorti a a b
National Institute of Technology, Calicut, 673 601, India School of Materials Science and Engineering, National Institute of Technology, Calicut, 673 601, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon spheres Chemical vapor deposition Graphitization
Carbon spheres (CS) of relatively uniform size, ranging from 200 to 350 nm, were synthesized using the noncatalytic thermal Chemical Vapor Deposition (CVD) method. The effect of temperature (650 � C–1050 � C) along with hydrogen (H2) and argon (Ar) in acetylene (C2H2) on the carbon spheres were studied using Scanning Electron Microscopy (SEM) and Raman spectroscopy. Morphological studies show that relatively uniform carbon spheres can be synthesized by pyrolysis of acetylene under H2 and Ar at temperatures above 950 � C in an at mospheric pressure CVD. The broad peaks in the XRD spectrum and ID/IG ratio of 0.85 obtained from Raman Spectroscopy reveals that the carbon spheres synthesized are amorphous in nature. The morphological studies using SEM on different concentrations of Ar and H2 during the synthesis indicate that the concentration of carrier gases profoundly affects the morphology. The presence of Ar gives smooth carbon spheres and H2 contributes to the relative uniformity of the carbon spheres synthesized. A detailed study was carried out to analyze the in fluence of the concentration of H2 using SEM, Raman spectroscopy and FTIR. The effect of low concentration of H2 along with Ar and acetylene reduces the defects and the graphitization degree obtained was 0.85. The present study also infers that high concentrations of H2 deteriorates the morphology of carbon spheres due to the excess of C–H functional group formation.
1. Introduction The applications of carbon materials are enormous irrespective of their structure, texture, and properties. Carbon has the ability to achieve any credible combination of sp, sp2, and sp3 hybridization to form a large variety of structures and morphologies, which have extensive applica tions. The production of carbon structures with exciting morphologies like fullerene [1], Fullerene-like carbon [2], carbon nanotubes [3], carbon nanofibers [4], graphene [5], carbon onions [6], carbon spheres [7], carbon nanorods [8] and mesoporous carbon structures [9] depends on the method of synthesis and chemical decomposition of the carbon precursor. Among the methods of synthesis, CVD results in interesting carbon materials with diverse morphologies and properties. The syn thesis of the carbon materials such as single or double-layer graphene [10–12] and metal catalyst assisted single wall or multiwall CNTs [13–15] can be easily achieved using a thermal CVD system. Among different carbon structures, carbon spheres get attention because of their potential applications in Li-ion batteries [16], in nanofluids for direct solar absorption [17], and as sorbent [18], to name
a few. It has been reported that the functionalized [19] and composite forms of carbon spheres [20,21] also have possible applications in many areas. Many methods have been explored in the synthesis of hollow and solid carbon spheres like cathodic arc discharge process [22], autoclave methodologies [23], catalytic assisted pyrolysis [24] and non-catalytic CVD [25]. Considering the prospective future of this material, a syn thesis route that is cost-effective, having ease of procedure, repeatability and large yield is explored using a thermal CVD setup. Synthesis of carbon spheres with a wide range of hydrocarbons, using CVD routes, both by catalytic and non-catalytic means, have been reported in the literature. Zheng et al. [26] have reported the large scale synthesis of carbon spheres from 50 nm to 1μm by pyrolysis of liquid hydrocarbons. Miao et al. [24] have reported that carbon spheres of size ranging from 400 to 2000 μm can be synthesized by kaolin supported transition metal salts CVD using metal catalysts like Fe, Co, Ni, etc. The preparation of carbon spheres under non-catalytic CVD using methane as the carbon precursor is reported by Zhang et al. [25]. Like methane, acetylene also has a major role as a carbon precursor in synthesizing carbon materials. The studies so far reported on acetylene for carbon
* Corresponding author. National Institute of Technology, Calicut, 673 601, India. E-mail address: [email protected] (R. Panickar). https://doi.org/10.1016/j.vacuum.2019.109108 Received 1 October 2019; Received in revised form 6 November 2019; Accepted 25 November 2019 Available online 28 November 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
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sphere synthesis using catalytic [27] and non-catalytic CVD [28,29] routes lack explanation on the effects of deposition parameters and reactant gases. The present work is based on non–catalytic thermal CVD synthesis of solid carbon spheres using acetylene as the reactant gas in a combined hydrogen and argon atmosphere. Mutuma et al. has reported the syn thesis of CS with acetylene as the carbon source and Ar or H2 as the carrier gas [28]. However, the CVD synthesis of CS under the combined effect of H2/Ar in C2H2 has not been explored yet. The effect of depo sition temperatures between 650 and 1050 � C and the influence of H2 and Ar on the morphology of the carbon spheres synthesized at atmo spheric pressure are studied, using Scanning Electron Microscopy. A detailed study was conducted for different concentrations of H2 in C2H2/Ar using SEM, Raman spectroscopy and FTIR to analyze the effect in CS synthesis. It has been reported that in CVD synthesis of CS, hydrogen plays a major role as the reactant gas in the synthesis of gra phene [30], MoS2 layers [31], diamond thin films [32]. In the present case also, H2 acts as a reactant gas in the synthesis of smooth CS of size range from 200 to 350 nm. Morphological studies clearly illustrate that a small amount of H2 in C2H2/Ar reduces the CS size from 400-550 nm to 200–350 nm. This shows that relative uniform CS of size range from 200 to 350 nm can be synthesized under C2H2/H2/Ar gas mixture at 100/30/200 sccm flow rates in APCVD at 950 � C. A detailed charac terization of the carbon spheres synthesized under this condition with good spheroidal structure and texture is performed using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Ultra-Violet Raman spectroscopy, Fourier Transform Infrared (FTIR) Spectroscopy, High Resolution- Transmission Electron Microscopy (HR-TEM), Selected Area Electron Diffraction Pattern (SAED), Dynamic Light Scattering (DLS), Thermogravimetric Analysis (TGA) and Brunauer–Emmett–Teller (BET).
cool to ambient temperature. Fig. 1(b) gives the schematic of CS syn thesis carried out in the thermal CVD under atmospheric pressure. In the present study, the effect of deposition temperature on the synthesis of CS was analyzed by varying the deposition temperatures from 650 � C to 1050 � C. In each experiment, the CS was deposited inside the quartz tube and was collected for further analysis. It was observed that CS was formed inside the quartz tube near to the rim of the tubular furnace which is exposed to the atmosphere as shown in Fig. 1(a). The temper ature of the quartz tube inside the tubular furnace was 950 � C and the temperature of the quartz tube at the rim area exposed to the atmo sphere was 450 � C. This suggests that pyrolysis of hydrocarbon takes place at high temperature and then condenses at low-temperature to form CS. A similar synthesis of carbon spheres has been reported using benzene as the carbon precursor in a two-zone temperature furnace at temperatures of 1000 � C and 300 � C [33]. In the CVD synthesis of CS, quartz tube/quartz boat is extensively used [26,34,35]. It has been reported that at high temperatures, the hydrocarbon can diffuse into the quartz tube by breaking the Si–O bonds, and the oxygen atoms rearrange themselves to transform from tetrahedral - quartz to HP - tridymite quartz [36]. The tridymite is formed at high temperatures of 873 � C and transforms into an amor phous transition phase. Silica is found to be more reactive in the excess of amorphous silica phase during the transition [37]. It can be under stood that at this amorphous transition phase, hydrocarbons diffuse into silica to form CS on quartz walls. 2.2. Characterization techniques The morphology and structure of the synthesized CS are analyzed using Scanning Electron Microscopy (SEM) using a Hitachi SU6600 –FESEM of resolution 1.2 nm and accelerating voltage of 5 kV, and High Resolution -Transmission Electron Microscopy (HR-TEM) using model JEM 2100, JEOL Ltd, of resolution 0.24 nm with an acceleration voltage of 200 kV. The effect of the H2 in the graphitization degree of the CS is studied using Raman Spectroscopy using WITEC Alpha 300 RA with a DPSS laser of 532 nm and a maximum power of 70 mW. X-Ray Diffraction (XRD) pattern of the CS was obtained using an X-ray diffractometer (Rigaku Miniflex 600) with Cu-kα radiation of wave length, λ ¼ 1.5406 Ao. FTIR spectra were recorded on a PerkinElmer Frontier MIR spectrometer for the analysis of the functional groups attached to the CS and the average particle size of the CS was obtained using a DLS-particle size analyzer (Nano- ZS: MalvernTm) by measuring the hydrodynamic diameter. To perform the thermal analysis of syn thesized CS, Thermogravimetric Analysis (TGA) was also conducted using STA 7200, Thermal analysis system, Hitachi high Technologies, Japan. The surface area and pore size of the CS were measured using the Brunauer–Emmett–Teller (BET) system, Belsorp Max, Microtrac BEL
2. Experimental studies 2.1. Synthesis of Carbon Spheres (CS) One of the simplest methods of synthesizing Carbon Spheres (CS) is by using a CVD setup, as shown in Fig. 1(a). A quartz tube of 100 cm length and a tubular furnace can be used for synthesizing relatively uniform carbon spheres under ambient pressure. The synthesis was carried out using acetylene (C2H2), hydrogen (H2) and argon (Ar) with 99.9% purity supplied by Bhoruka gases Ltd, India. Initially, the quartz chamber is heated to the required deposition temperature under the H2/ Ar atmosphere at a flow rate of 30/200 sccm. When the deposition temperature is reached, the hydrocarbon precursor (C2H2) is slowly introduced in the chamber at a flow rate of 100 sccm for 30 min. Ar and H2 flow are continued for another 10 min and the system is allowed to
Fig. 1. a) Thermal CVD system for carbon sphere synthesis b) Schematic of carbon spheres growth process. 2
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corporation, Japan.
CS under increased deposition temperatures. In the present study, further characterizations were done for sample CS4, CS synthesized at a deposition temperature of 950 � C under at mospheric pressure.
3. Results and discussions 3.1. Effect of temperature on Carbon Sphere synthesis
3.2. Characterization of Carbon Spheres
The pyrolysis of hydrocarbon under inert atmosphere can synthesize relatively uniform CS at high temperature under non-catalytic condi tions. The effect of temperature on the morphology of CS with acetylene as the hydrocarbon precursor is studied using SEM. The deposition temperature is varied from 650 � C to 1050 � C to find its effect on the synthesis of CS using the CVD process. Table 1 illustrates the operating conditions and precursor gas flow rate used for the CS synthesis. The samples were numbered from CS1 to CS6 in which CS4 is the optimized sample number that produced relatively uniform CS synthesis. The morphology study using SEM clarifies the effect of deposition temper ature on the synthesis of CS using CVD. It is observed that at a deposition temperature of 650 � C, acetylene forms a sticky brown coating on the quartz surface. The SEM images of carbon soot formed at different deposition tem peratures from 750 � C to 1050 � C are shown in Fig. 2. The SEM image of carbon soot formed at 750 � C shows the presence of a few sphere-like particles with irregular morphology. At 850 � C, the soot particles formed are found to be CS fused together. This is understood to be due to an insufficient temperature that prevents the growth of carbon nuclei to form uniform spheres. When the deposition temperature is increased to 950 � C, relatively uniform CS with 200–350 nm average diameter and smooth texture are obtained. CS with similar morphology is obtained at 1050 � C, but with an increased average diameter ranging from 200 to 550 nm. An increase in temperature above 950 � C results in an immense increase in the pyrolysis of hydrocarbon. This produces new nuclei to form a range of CS, with the diameter ranging from very small to big, which deteriorates the relative uniformity of the sphere synthesized. The morphological study reveals that the deposition temperature plays a major role in the synthesis of uniform CS under atmospheric pressure. However, it has been reported that in low-pressure CVD, the deposition temperature does not play a significant role in the size of the CS, whereas an increase in pressure from 3 to 15 kPa results in CS ranging from 500 nm to 5 μm [25]. From the morphological study, it can be inferred that the deposition temperature has significant role in the formation of uniform sized CS. The CS synthesized at 950 � C possesses smooth spherical structure and texture with sizes from 200 to 350 nm. Above this temperature the size increases due to the effective acceler ation of the pyrolysis of hydrocarbons at high temperatures resulting in more carbon nuclei (smaller sized CS). The size enhancement de teriorates the relative uniformity of CS synthesized. The CS synthesized usually looks like a chain of spheres formed by the accumulation of carbon spheres, because of the simultaneous growth of carbon nuclei to
A detailed analysis of the CS morphology was performed using TEM for CS synthesized at 950 � C to identify whether the CS obtained is a hollow or solid sphere. The TEM image (Fig. 3) shows that the CS conglomerated to form chains consisting of relatively uniform-sized spheres with smooth surface morphology. The chain formation of CS is due to the accumulation of spheres, because of the simultaneous growth of carbon nuclei to CS under increased deposition temperatures. It has been reported that the accretion of CS increases with the deposi tion time when the reaction time exceeds 5 min and results in chain-like conglomerated CS shown in the TEM analysis [26]. As the spheres are more than 100 nm size during the TEM analysis the electron beams were difficult to penetrate into the core of the spheres, which restricted the information on whether the CS synthesized are hollow or solid. How ever, some studies report that CS around diameters 100–500 nm possess solid structure [29,38], which implies that spheres obtained are not hollow, and are with a smooth solid structure. In the HR-TEM image of the CS (Fig. 4) small wavy flakes of thickness 1–10 nm, having graphitic structure are resolved near the edge of the spheres, and these flakes are formed at different depths following the curvature of the spheres. The HR-TEM image shows that the graphitic flakes are spherically dispersed in the entire sphere. The graphite flakes are clearly visible at the edges rather than the center, indicating an in crease in thickness or a decrease in the number of flakes parallel to the electron beam. The inset of Fig. 4 shows the SAED pattern of a single CS. The SAED pattern has two broad diffraction rings that correspond to the planes (002) and (100) which shows a good agreement to the XRD re sults. The interlayer spacing d002 of the CS is calculated as 0.36 nm, which shows a low deviation from the ideal d002 of 0.34 nm graphitic structure (JCPDS-ICDD Card no: 41–1487) reported in the literature [34]. The value of d-spacing obtained for the CS is between that of the carbon fiber (d002 -0.342), and activated carbon atoms (d002-0.363) which show a well-organized carbon structure [39]. Fig. 5 (a) shows the XRD pattern of the synthesized CS. The 2Ө values at 24.65� and 43.07� represent graphitic planes (002) and (100), respectively. In comparison with graphite, the peak 24.65 shifts slightly to a lower angle that points out the presence of a few layers of graphene stacked without correlations along the c-axis because of the heavily disordered graphitic structures. The broadened peaks at (002) plane resulted from the waving structure of the graphitic flakes confirm that the sample is amorphous in nature. The functional groups attached to the synthesized CS are obtained
Table 1 Deposition parameters and precursor gas flow rates for carbon sphere synthesis. Sample no:
Temperature (oC)
Pressure (bar)
Time of Deposition (min)
C2H2
Ar
H2
CS1 CS2 CS3 CS4
650 750 850 950
1 1 1 1
30 30 30 30
100 100 100 100
200 200 200 200
30 30 30 30
No spheres No spheres No spheres 200–350 nm Smooth spheres
CS5
1050
1
30
100
200
30
200–550 nm Smooth spheres
CS6
950
1
15
100
200
30
200–350 nm Smooth spheres
Reactant Flow Rate (sccm)
3
Size (nm)
Mean & standard deviation
267 355 443 490 233 277 317
233 � 9, 277 � 8, 317 � 12 � 10, � 22, � 24, � 15 � 9, � 8, � 12
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Fig. 2. Scanning electron microscopy images of carbon spheres synthesized at a) CS2 b) CS3 c) CS4 and d) CS5.
Fig. 3. TEM image of carbon spheres synthesized at 950 � C.
from FTIR spectroscopy as shown in Fig. 5 (b). The strong peaks at 1138 – C (bending cm 1 and 1570 cm 1 indicate the presence of C–O and C– mode) groups, respectively. The two other peaks at 2855 cm 1 and 2933 cm 1 (stretching mode) show the presence of C–H groups in the synthesized CS. Information about the type of hybridization can also be obtained from the functional groups attached as the hybridization af fects the force constant, K. The alkane and alkene in the sample reveal that both sp3 and sp2 hybridized groups could be present in the carbon sphere synthesized [40]. Apart from SEM analysis, the sphere diameter was measured also using a dynamic light absorption technique shown in Fig. 5 (c) that gave a hydrodynamic diameter of 433 nm. The particle size analyzer measures the hydrodynamic diameter of the CS suspended in water that gives the average particle size, which is greater than the particle size obtained from SEM. The thermal degradation of CS was analyzed using Thermogravimetric Analyser (TGA) in two different conditions. The TGA response obtained for CS in air and inert atmo sphere are shown in Fig. 6. The TGA data of CS shows very high thermal
Fig. 4. HR-TEM of carbon spheres showing the graphitic layers and Selected Area Electron Diffraction (SAED) pattern in the inset.
stability till 1000 � C in a nitrogen atmosphere with a weight loss of less than 4%., whereas at oxygen atmosphere the CS starts degrading at 590 � C and complete weight loss occurs towards 800 � C. The TGA results reveal that CS is a potential material for very high-temperature applications. In order to examine the surface area and the pore volume of the CS synthesized, BET analysis was carried out by measuring the adsorptiondesorption isotherms in liquid N2 atmosphere. The surface area, total pore volume and mean pore diameters measured are given in Table 2. In 4
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Fig. 5. a) X-Ray powder diffraction pattern b) FTIR spectroscopy and c) DLS obtained for carbon spheres synthesized at 950 � C under atmospheric pressure.
the case of mesoporous materials with less than 50 nm pore diameter, a very high surface area of 1400–1600 m2/g is exhibited. In the case of pristine CS, a surface area of 3.6 m2/g is exhibited [34]. Here, the CS synthesized has a mean pore diameter of 3.85 nm and a surface area of 6.48 m2/g that is slightly higher than the pristine CS. The low value of surface area relates to the smooth surface morphology and the size of the CS. A very low total pore volume of 0.0062399 cm3/g was measured for the CS that clearly determines that the CS formed are hard spheres with very little porosity. To investigate the structural fingerprint of the CVD synthesized CS, visible Raman Spectroscopy was carried out and the Raman spectra obtained depend on (i) clustering of sp2 phase (ii) presence of sp2 chains or bonds (iii) bond angle and bond length disorder and (iv) the sp3/sp2 ratio [41]. In the case of amorphous carbon materials, the Raman spectra possess two peaks: the first peak in the range of 1320 cm 1 to 1360 cm 1 is called the D-band and the second peak in the range from 1500 cm 1 and 1600 cm 1 is called the G-band. The Raman spectra were recorded (Fig. 7) for the CS synthesized at 950 � C to study the graphitic order. Two peaks were observed in the spectrum, a stronger peak at 1578 cm 1 and a weaker peak at 1336 cm 1. The stronger peak is the G-band that represents the stretching mode resulting from the in-plane vibration of the sp2 bonded carbon atoms and the weak peak is attrib uted to the out-of- plane vibration called the D-band representing the defects and disorders in the carbon material. A more relevant explana tion on the G band and D band is that the former is due to the relative motion of the sp2 carbon atoms, while the latter is linked to the breathing modes of carbon rings [42]. The Raman spectra with varying G and D bands with different positions, intensities, and widths possess
Fig. 6. TGA response of CS synthesized at 950 � C in air and inert atmosphere.
Table 2 BET characterization results for CS synthesized at 950 � C. Parameter
Value obtained
Surface area, as Total pore volume Mean pore diameter
6.48 m2/g 0.0062399 cm3/g 3.86 nm
5
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formation mechanism of the CS is depicted in Fig. 8. The pentagonal atom rings nucleate and grow to form the quasi-icosahedral shells, forming a spiral shell that acts as the core of the CS. During the depo sition process, numerous pentagons and heptagons continuously add to the surface of the spiral shell and create new nucleation sites for the graphitic flakes. These result in the stacking of randomly twisted graphitic flakes distributed on the surface without any connection among them. These graphitic flakes can be clearly observed in the HR-TEM of the CS shown in Fig. 4. The layers of the CS possess numerous non-planar P–H pairs that align along the direction parallel to the axis of graphite which significantly vary the interlayer distance, resulting in broadening of the (002) peak. 3.4. Effect of H2 and Ar in CS synthesis In this study argon (Ar) and hydrogen (H2) are the gases used for the synthesis of CS under the non-catalytic CVD method along with acety lene. The Ar introduced in the reaction chamber helps to dilute the acetylene and to create an inert atmosphere for the pyrolysis of acety lene at high temperatures. The individual and combined effects of the H2 and Ar (Table 4) on the morphology of CS were analyzed using FESEM. In Fig. 9 (a) the SEM image shows that larger spheres of with smooth surface texture were formed with Ar alone with acetylene. In Fig. 9 (b) the SEM images show distorted spheroidal shape with reduced carbon particle size when H2 alone is used with acetylene. Fig. 9 clearly depicts that Ar alone promotes the growth of smooth carbon spheres whereas H2 alone gives a negative impact on the growth and sphere morphology. These results confirm that H2 and Ar have a strong influence on the shape and texture of the CS synthesized under the non–catalytic CVD method. To further investigate the effect of Ar and H2 on the CS synthesis, the concentrations of both the gases were varied. In sample CS8, CS9 and CS4
Fig. 7. Raman spectroscopy obtained for carbon spheres synthesized at 950 � C for 30 sccm H2 under atmospheric pressure.
details on the carbon bonds and lattice vibrations of nanocrystalline to amorphous carbon materials. The D peaks or the disorder-induced modes result from the disloca tions in the lattice. As a result, the intensity of the D peak with respect to the G peak is used to determine the degree of disorder in the synthesized sample. The degree of disorder or the value of graphitization is calcu lated from the maximum intensities of the D band and G band as (ID/IG). The positions of the D and G bands in the Raman spectra can clearly reflect the structure of the amorphous carbon phases. Table 3 shows the graphitization degrees (ID/IG) reported for CS synthesized by catalytic and non-catalytic CVD methods using acetylene as the carbon source and the present work. It was observed that the degree of graphitization, ID/IG ¼ 0.85 is obtained for CS synthesized from the present work. The graphitization degree of 0.85 obtained from the ID/IG value of Raman spectroscopy implies that the value of the graphitization degree is comparatively reduced with the reported CS synthesized using CVD [26–29]. During the process of graphitization, the turbostratic carbon is converted to a more ordered three-dimensional graphite structure, and the degree of graphitization depends on the raw materials and pro cessing parameters to achieve the graphitic order. A highly disordered carbon like amorphous carbon can be structurally ordered by material degradation at high temperatures [43]. The influence of certain catalysts in catalytic- CVD also can affect the ID/IG ratio. Jose et al. [34] have reported a graphitization degree of 0.80 in CS synthesized using NiFe-LDHs as the catalytic precursor and ethylene as the carbon precursor. 3.3. Mechanism of CS formation In the carbon atom, the three possible graphitic carbon ring forma tions are pentagonal, hexagonal and heptagonal carbon rings. It has been reported that among these graphitic carbon rings the formation of curling graphitic structures are nucleated from pentagonal rings, and to maintain the smooth morphology, the pentagonal rings pair with hep tagonal rings to nucleate a non-faceted graphitic layer, and the pair of pentagon and heptagon are represented as P–H pairs [44]. The
Fig. 8. Schematic showing the formation of carbon spheres by the nucleation of pentagonal carbon rings.
Table 3 Graphitization degree reported from Raman spectroscopy of carbon spheres synthesized from acetylene using CVD. Reactant gas
Carrier gases
Deposition temperature (oC)
C2H2 C2H2 C2H2 C2H2 C2H2
H2 Ar Ar þ C N2 H2þAr
900 900 850 1000 950
D band position (cm 1333 1340 1370 1330 1336
1
)
G band position (cm 1593 1595 1600 1590 1578
*C- Catalyst. 6
1
)
Graphitization degree (ID/IG)
Reference
0.89 0.88 0.94 1.3 0.85
[28] [28] [27] [29] Present work
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Table 4 Deposition parameters for carbon sphere synthesis to study the effect of H2 and Ar in the morphology. Sample no:
Temperature (oC)
Pressure (bar)
Time of Deposition (min)
C2H2
Ar
H2
size (nm)
CS7
950
1
30
100
0
30
200–480 nm Rough spheres
CS8
950
1
30
100
60
30
300–650 nm Smooth spheres
CS9
950
1
30
100
120
30
350–550 nm Smooth spheres
CS10
950
1
30
100
200
0
250–430 nm Smooth spheres
CS4
950
1
30
100
200
30
200–350 nm Smooth spheres
CS11 CS12
950 950
1 1
30 30
100 100
200 200
60 120
No sphere morphology No sphere morphology
Reactant Flow Rate (sccm)
Morphology from SEM
Mean & Standard Deviation in size (nm)
243 � 12, 323 � 18, 462 � 10 356 � 13, 466 � 16, 620 � 5 371 � 17, 452 � 19, 526 � 5 278 � 10, 340 � 13, 398 � 12 233 � 9, 277 � 8, 317 � 12
Fig. 9. SEM images of CS synthesized at 950 � C a) with Ar alone (CS10) and (b) with H2 alone (CS7) using acetylene as carbon precursor.
the concentration of Ar were varied as 60 sccm, 120 sccm, and 200 sccm, keeping C2H2/H2 at 100/30 sccm. At 60 sccm and 120 sccm of Ar concentration are shown in Fig. 10 (a) and (b), respectively. In both cases, Carbon spheres were synthesized, but the relative uniformity of the spheres synthesized varied from 300 to 650 nm. At a low concen tration of Ar, some of the sphere morphologies obtained were rough probably due to the presence of H2. Qian et al. [45] have reported the effect of different carrier gases such as Ar, H2, and N2 in carbon sphere synthesis with toluene as the carbon precursor. To study the effect of H2 in sample CS4, CS11 and CS12 the H2 con centration was varied as 30 sccm, 60 sccm, and 120 sccm, keeping C2H2/ Ar at 100/200 sccm. Fig. 10 (c) and (d) show the SEM images of the CS synthesized at 60 sccm and 120 sccm, respectively. Higher concentra tions of H2 from 30 sccm significantly affect the surface morphology of the CS. The sphere morphology deteriorated at a high concentration of H2. In the present study, smooth carbon spheres were synthesized at different concentrations of Ar, and the addition of H2 resulted in rough spheres with enhancement in the relative uniformity of the spheres synthesized. However, in the sample CS4 (Fig. 2(c)) the combined effect of the H2/Ar at 950 � C under 100 sccm acetylene flow synthesized relatively uniform CS of 200–350 nm that are smaller sized spheres compared to the CS synthesized in C2H2/Ar gas mixture. The effect of H2 was further investigated using Raman spectra by increasing the H2 concentration from 30 sccm to 60 sccm and 120 sccm (Table 5). Fig. 11 shows the Raman spectra of CS synthesized at H2 concentrations of 60 sccm and 120 sccm, respectively. The D and G
bands obtained for H2 concentration of 60 sccm are almost similar to H2 at 30 sccm. When the amount of H2 is further increased to 120 sccm the D and G peaks slightly shift towards the right with D peak at 1344 cm 1 and G peak at 1585 cm 1. This shift in the peaks can be attributed to the presence of double bonds or unmodified graphitic areas [46]. Marto et al. [47] have reported that adding H2 into a gas mixture of Ar/N2 for amorphous carbon thin films synthesis decreased the ID/IG ratio from 1.6 to 1.1 and the G peak position changed slightly towards the right side, which shows good agreement with the present study. However, in the present case of CS synthesis, the ID/IG ratio increased from 0.85 to 0.88 as the concentration of H2 is increased from 30 sccm to 120 sccm. By observing the Raman spectra of the CS synthesized at different H2 con centrations at 60 sccm, the peak intensities of the G peak were found to be slightly larger than that of D peak, and this difference becomes prominent when the concentration is increased to 120 sccm. In general, the G and D peaks of the amorphous carbon material will have broad peaks or D peak intensity higher than G peak and it has been universally observed in carbon materials that a broader G peak and a higher in tensity of D peak observed in the Raman spectra confirms the presence of higher order defects in the material. The Raman spectra of the CS syn thesized in the present study has narrow G peak and lesser intensity of D peak with respect to G peak, which confirms the decrease of defect in CS synthesized. Apart from the intensity and peak positions of the D and G peaks, the Full Width at Half Maximum (FWHM) also has significant importance in determining the defects in the carbon materials and it can vary from 10 to 200 cm 1 from graphene to amorphous carbon materials 7
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Fig. 10. SEM images obtained for carbon spheres synthesized at 950 � C under C2H2/Ar/H2 gas mixture (a) CS8 (b) CS9 (c) CS11 and (d) CS12. Table 5 Graphitization degree obtained from Raman spectroscopy of carbon spheres synthesized under different H2 concentration. Reactant gas
Carrier gas Ar/H2 (sccm)
Deposition temperature (oC)
D band position (cm 1)
G band position (cm 1)
Graphitization degree (ID/IG)
C2H2 C2H2 C2H2
200/30 200/60 200/ 120
950 950 950
1336 1336 1344
1578 1573 1585
0.85 0.87 0.88
[48]. The FWHM values obtained for CS synthesized at different H2 concentrations were obtained from curve fitting and were found to be 75 cm 1 for 30 sccm, 51 cm 1 for 60 sccm and 83 cm 1 for 120 sccm respectively. The Raman and SEM analyses revealed that a further in crease in H2 concentration from 30 sccm causes an increase in ID/IG ratio from 0.85 to 0.88 and also deteriorates the surface morphology of the CS synthesized. It can be inferred that the effect of a very low concentration of H2 about 30 sccm can influence in reducing the defects and also the graphitization degree of CS synthesized under C2H2 and Ar. The reduction in graphitization degree is understood to be the combined effect of H2 and Ar that probably have influenced the formation of small quantities of ordered carbon due to the cross-linking between the or dered and neighboring regions to get ID/IG ¼ 0.85. Further analysis of the effect of higher concentrations of hydrogen was performed using FTIR to see whether any further changes occurred. In Fig. 12, the FTIR spectrum of CS synthesized at H2 concentrations of 30 sccm, 60 sccm and 120 sccm are given, respectively. The details of the functional groups attached to the CS synthesized at different hydrogen concentrations are listed in Table 6. In comparison with the functional groups formed in 30 sccm, at 60 sccm and 120 sccm more peaks of C–H groups are observed at 813 cm 1, 736 cm 1, and 2664 cm 1. In 120
Fig. 11. Ultra-violet laser Raman spectroscopy for carbon spheres synthesized at 950 � C under 60 sccm and 120 sccm hydrogen concentrations. (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
sccm the intensity of C–H peaks was very high compared to the C–O peak at 1122 cm 1. This implies that at higher concentrations of H2 more hydrocarbons are formed in the sample. At 60 sccm and 120 sccm, the 1 – intensity of C– – C weak stretching bond at 2100 cm is high which is understood to be due to the presence of unreduced acetylene at higher H2 concentrations. An additional peak at 2330 cm 1 was observed at – C– – O stretching mode, higher concentrations that correspond to the O– thus indicating CO2 absorption by the sample. Thus, it can be concluded from the FTIR spectra that an increase in H2 concentration results in 8
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Fig. 12. FTIR results obtained for CS synthesized at higher concentrations of Hydrogen a) 30 sccm b) 60 sccm and c) 120 sccm. Table 6 Functional groups formed at different concentrations of hydrogen from FTIR spectra. Frequency range Absorption (cm 1)
Appearance
Group
3100-3000 (2926) 2830-2695 (2664) 2349 (2330) 2140-2100 (2100) 1818 (1877) 1620-1610 (1557) 1124-1087 (1122) 1085-1050 (1085) 880 � 20 (869,813) 750 � 20 (736)
medium medium strong weak strong strong strong strong strong strong
C–H stretching C–H stretching O¼C– O stretching –– C–C stretching – C¼O stretching C¼C stretching C–O stretching C–O stretching C–H bending C–H bending
additional C–H functional groups that may be responsible for the dete rioration of the CS morphology observed in the SEM images. 4. Conclusions One of the simplest methods to synthesize relatively uniform CS ranging from 200 to 350 nm using non-catalytic thermal chemical vapor deposition under atmospheric pressure is explained in this paper. Morphological studies on SEM and TEM show smooth and spheroidal texture for the carbon spheres. The XRD results and Raman Spectroscopy reveals that the carbon spheres synthesized are amorphous carbon spheres. The surface area measurement and thermal degradation studies on carbon spheres give a mean pore diameter of 3.86 nm, and thermal stability up to 590 � C under air. The effects of the deposition tempera ture and H2 are analyzed using SEM and Raman spectroscopy. Relatively uniform CS could be synthesized at a deposition temperature of 950 � C with suitable deposition parameters. The presence of H2 and Ar has significantly influenced the morphology of the CS synthesized. Raman spectroscopy results show that the presence of H2 in C2H2/Ar has sig nificant importance in the graphitization degree, compared to the re ported CS synthesis using C2H2 as the carbon precursor. The effect of H2 was also studied using FTIR and the spectra reveal that more amount of C–H functional groups were formed at higher concentrations of H2 that resulted in the deterioration of surface morphology of the CS. The study also reveals that a further increase of H2 from 30 sccm increased the graphitization ratio. The main advantage of this CVD synthesis is the high reproducibility and large yield of about 1–2 gm of CS for 30 min of 9
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Chemical vapor deposition synthesis of carbon spheres: Effects of temperature and hydrogen Radhika Panickar a, b, *, C.B. Sobhan a, b, Sivaji Chakravorti a a b
National Institute of Technology, Calicut, 673 601, India School of Materials Science and Engineering, National Institute of Technology, Calicut, 673 601, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon spheres Chemical vapor deposition Graphitization
Carbon spheres (CS) of relatively uniform size, ranging from 200 to 350 nm, were synthesized using the noncatalytic thermal Chemical Vapor Deposition (CVD) method. The effect of temperature (650 � C–1050 � C) along with hydrogen (H2) and argon (Ar) in acetylene (C2H2) on the carbon spheres were studied using Scanning Electron Microscopy (SEM) and Raman spectroscopy. Morphological studies show that relatively uniform carbon spheres can be synthesized by pyrolysis of acetylene under H2 and Ar at temperatures above 950 � C in an at mospheric pressure CVD. The broad peaks in the XRD spectrum and ID/IG ratio of 0.85 obtained from Raman Spectroscopy reveals that the carbon spheres synthesized are amorphous in nature. The morphological studies using SEM on different concentrations of Ar and H2 during the synthesis indicate that the concentration of carrier gases profoundly affects the morphology. The presence of Ar gives smooth carbon spheres and H2 contributes to the relative uniformity of the carbon spheres synthesized. A detailed study was carried out to analyze the in fluence of the concentration of H2 using SEM, Raman spectroscopy and FTIR. The effect of low concentration of H2 along with Ar and acetylene reduces the defects and the graphitization degree obtained was 0.85. The present study also infers that high concentrations of H2 deteriorates the morphology of carbon spheres due to the excess of C–H functional group formation.
1. Introduction The applications of carbon materials are enormous irrespective of their structure, texture, and properties. Carbon has the ability to achieve any credible combination of sp, sp2, and sp3 hybridization to form a large variety of structures and morphologies, which have extensive applica tions. The production of carbon structures with exciting morphologies like fullerene [1], Fullerene-like carbon [2], carbon nanotubes [3], carbon nanofibers [4], graphene [5], carbon onions [6], carbon spheres [7], carbon nanorods [8] and mesoporous carbon structures [9] depends on the method of synthesis and chemical decomposition of the carbon precursor. Among the methods of synthesis, CVD results in interesting carbon materials with diverse morphologies and properties. The syn thesis of the carbon materials such as single or double-layer graphene [10–12] and metal catalyst assisted single wall or multiwall CNTs [13–15] can be easily achieved using a thermal CVD system. Among different carbon structures, carbon spheres get attention because of their potential applications in Li-ion batteries [16], in nanofluids for direct solar absorption [17], and as sorbent [18], to name
a few. It has been reported that the functionalized [19] and composite forms of carbon spheres [20,21] also have possible applications in many areas. Many methods have been explored in the synthesis of hollow and solid carbon spheres like cathodic arc discharge process [22], autoclave methodologies [23], catalytic assisted pyrolysis [24] and non-catalytic CVD [25]. Considering the prospective future of this material, a syn thesis route that is cost-effective, having ease of procedure, repeatability and large yield is explored using a thermal CVD setup. Synthesis of carbon spheres with a wide range of hydrocarbons, using CVD routes, both by catalytic and non-catalytic means, have been reported in the literature. Zheng et al. [26] have reported the large scale synthesis of carbon spheres from 50 nm to 1μm by pyrolysis of liquid hydrocarbons. Miao et al. [24] have reported that carbon spheres of size ranging from 400 to 2000 μm can be synthesized by kaolin supported transition metal salts CVD using metal catalysts like Fe, Co, Ni, etc. The preparation of carbon spheres under non-catalytic CVD using methane as the carbon precursor is reported by Zhang et al. [25]. Like methane, acetylene also has a major role as a carbon precursor in synthesizing carbon materials. The studies so far reported on acetylene for carbon
* Corresponding author. National Institute of Technology, Calicut, 673 601, India. E-mail address: [email protected] (R. Panickar). https://doi.org/10.1016/j.vacuum.2019.109108 Received 1 October 2019; Received in revised form 6 November 2019; Accepted 25 November 2019 Available online 28 November 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
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sphere synthesis using catalytic [27] and non-catalytic CVD [28,29] routes lack explanation on the effects of deposition parameters and reactant gases. The present work is based on non–catalytic thermal CVD synthesis of solid carbon spheres using acetylene as the reactant gas in a combined hydrogen and argon atmosphere. Mutuma et al. has reported the syn thesis of CS with acetylene as the carbon source and Ar or H2 as the carrier gas [28]. However, the CVD synthesis of CS under the combined effect of H2/Ar in C2H2 has not been explored yet. The effect of depo sition temperatures between 650 and 1050 � C and the influence of H2 and Ar on the morphology of the carbon spheres synthesized at atmo spheric pressure are studied, using Scanning Electron Microscopy. A detailed study was conducted for different concentrations of H2 in C2H2/Ar using SEM, Raman spectroscopy and FTIR to analyze the effect in CS synthesis. It has been reported that in CVD synthesis of CS, hydrogen plays a major role as the reactant gas in the synthesis of gra phene [30], MoS2 layers [31], diamond thin films [32]. In the present case also, H2 acts as a reactant gas in the synthesis of smooth CS of size range from 200 to 350 nm. Morphological studies clearly illustrate that a small amount of H2 in C2H2/Ar reduces the CS size from 400-550 nm to 200–350 nm. This shows that relative uniform CS of size range from 200 to 350 nm can be synthesized under C2H2/H2/Ar gas mixture at 100/30/200 sccm flow rates in APCVD at 950 � C. A detailed charac terization of the carbon spheres synthesized under this condition with good spheroidal structure and texture is performed using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Ultra-Violet Raman spectroscopy, Fourier Transform Infrared (FTIR) Spectroscopy, High Resolution- Transmission Electron Microscopy (HR-TEM), Selected Area Electron Diffraction Pattern (SAED), Dynamic Light Scattering (DLS), Thermogravimetric Analysis (TGA) and Brunauer–Emmett–Teller (BET).
cool to ambient temperature. Fig. 1(b) gives the schematic of CS syn thesis carried out in the thermal CVD under atmospheric pressure. In the present study, the effect of deposition temperature on the synthesis of CS was analyzed by varying the deposition temperatures from 650 � C to 1050 � C. In each experiment, the CS was deposited inside the quartz tube and was collected for further analysis. It was observed that CS was formed inside the quartz tube near to the rim of the tubular furnace which is exposed to the atmosphere as shown in Fig. 1(a). The temper ature of the quartz tube inside the tubular furnace was 950 � C and the temperature of the quartz tube at the rim area exposed to the atmo sphere was 450 � C. This suggests that pyrolysis of hydrocarbon takes place at high temperature and then condenses at low-temperature to form CS. A similar synthesis of carbon spheres has been reported using benzene as the carbon precursor in a two-zone temperature furnace at temperatures of 1000 � C and 300 � C [33]. In the CVD synthesis of CS, quartz tube/quartz boat is extensively used [26,34,35]. It has been reported that at high temperatures, the hydrocarbon can diffuse into the quartz tube by breaking the Si–O bonds, and the oxygen atoms rearrange themselves to transform from tetrahedral - quartz to HP - tridymite quartz [36]. The tridymite is formed at high temperatures of 873 � C and transforms into an amor phous transition phase. Silica is found to be more reactive in the excess of amorphous silica phase during the transition [37]. It can be under stood that at this amorphous transition phase, hydrocarbons diffuse into silica to form CS on quartz walls. 2.2. Characterization techniques The morphology and structure of the synthesized CS are analyzed using Scanning Electron Microscopy (SEM) using a Hitachi SU6600 –FESEM of resolution 1.2 nm and accelerating voltage of 5 kV, and High Resolution -Transmission Electron Microscopy (HR-TEM) using model JEM 2100, JEOL Ltd, of resolution 0.24 nm with an acceleration voltage of 200 kV. The effect of the H2 in the graphitization degree of the CS is studied using Raman Spectroscopy using WITEC Alpha 300 RA with a DPSS laser of 532 nm and a maximum power of 70 mW. X-Ray Diffraction (XRD) pattern of the CS was obtained using an X-ray diffractometer (Rigaku Miniflex 600) with Cu-kα radiation of wave length, λ ¼ 1.5406 Ao. FTIR spectra were recorded on a PerkinElmer Frontier MIR spectrometer for the analysis of the functional groups attached to the CS and the average particle size of the CS was obtained using a DLS-particle size analyzer (Nano- ZS: MalvernTm) by measuring the hydrodynamic diameter. To perform the thermal analysis of syn thesized CS, Thermogravimetric Analysis (TGA) was also conducted using STA 7200, Thermal analysis system, Hitachi high Technologies, Japan. The surface area and pore size of the CS were measured using the Brunauer–Emmett–Teller (BET) system, Belsorp Max, Microtrac BEL
2. Experimental studies 2.1. Synthesis of Carbon Spheres (CS) One of the simplest methods of synthesizing Carbon Spheres (CS) is by using a CVD setup, as shown in Fig. 1(a). A quartz tube of 100 cm length and a tubular furnace can be used for synthesizing relatively uniform carbon spheres under ambient pressure. The synthesis was carried out using acetylene (C2H2), hydrogen (H2) and argon (Ar) with 99.9% purity supplied by Bhoruka gases Ltd, India. Initially, the quartz chamber is heated to the required deposition temperature under the H2/ Ar atmosphere at a flow rate of 30/200 sccm. When the deposition temperature is reached, the hydrocarbon precursor (C2H2) is slowly introduced in the chamber at a flow rate of 100 sccm for 30 min. Ar and H2 flow are continued for another 10 min and the system is allowed to
Fig. 1. a) Thermal CVD system for carbon sphere synthesis b) Schematic of carbon spheres growth process. 2
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corporation, Japan.
CS under increased deposition temperatures. In the present study, further characterizations were done for sample CS4, CS synthesized at a deposition temperature of 950 � C under at mospheric pressure.
3. Results and discussions 3.1. Effect of temperature on Carbon Sphere synthesis
3.2. Characterization of Carbon Spheres
The pyrolysis of hydrocarbon under inert atmosphere can synthesize relatively uniform CS at high temperature under non-catalytic condi tions. The effect of temperature on the morphology of CS with acetylene as the hydrocarbon precursor is studied using SEM. The deposition temperature is varied from 650 � C to 1050 � C to find its effect on the synthesis of CS using the CVD process. Table 1 illustrates the operating conditions and precursor gas flow rate used for the CS synthesis. The samples were numbered from CS1 to CS6 in which CS4 is the optimized sample number that produced relatively uniform CS synthesis. The morphology study using SEM clarifies the effect of deposition temper ature on the synthesis of CS using CVD. It is observed that at a deposition temperature of 650 � C, acetylene forms a sticky brown coating on the quartz surface. The SEM images of carbon soot formed at different deposition tem peratures from 750 � C to 1050 � C are shown in Fig. 2. The SEM image of carbon soot formed at 750 � C shows the presence of a few sphere-like particles with irregular morphology. At 850 � C, the soot particles formed are found to be CS fused together. This is understood to be due to an insufficient temperature that prevents the growth of carbon nuclei to form uniform spheres. When the deposition temperature is increased to 950 � C, relatively uniform CS with 200–350 nm average diameter and smooth texture are obtained. CS with similar morphology is obtained at 1050 � C, but with an increased average diameter ranging from 200 to 550 nm. An increase in temperature above 950 � C results in an immense increase in the pyrolysis of hydrocarbon. This produces new nuclei to form a range of CS, with the diameter ranging from very small to big, which deteriorates the relative uniformity of the sphere synthesized. The morphological study reveals that the deposition temperature plays a major role in the synthesis of uniform CS under atmospheric pressure. However, it has been reported that in low-pressure CVD, the deposition temperature does not play a significant role in the size of the CS, whereas an increase in pressure from 3 to 15 kPa results in CS ranging from 500 nm to 5 μm [25]. From the morphological study, it can be inferred that the deposition temperature has significant role in the formation of uniform sized CS. The CS synthesized at 950 � C possesses smooth spherical structure and texture with sizes from 200 to 350 nm. Above this temperature the size increases due to the effective acceler ation of the pyrolysis of hydrocarbons at high temperatures resulting in more carbon nuclei (smaller sized CS). The size enhancement de teriorates the relative uniformity of CS synthesized. The CS synthesized usually looks like a chain of spheres formed by the accumulation of carbon spheres, because of the simultaneous growth of carbon nuclei to
A detailed analysis of the CS morphology was performed using TEM for CS synthesized at 950 � C to identify whether the CS obtained is a hollow or solid sphere. The TEM image (Fig. 3) shows that the CS conglomerated to form chains consisting of relatively uniform-sized spheres with smooth surface morphology. The chain formation of CS is due to the accumulation of spheres, because of the simultaneous growth of carbon nuclei to CS under increased deposition temperatures. It has been reported that the accretion of CS increases with the deposi tion time when the reaction time exceeds 5 min and results in chain-like conglomerated CS shown in the TEM analysis [26]. As the spheres are more than 100 nm size during the TEM analysis the electron beams were difficult to penetrate into the core of the spheres, which restricted the information on whether the CS synthesized are hollow or solid. How ever, some studies report that CS around diameters 100–500 nm possess solid structure [29,38], which implies that spheres obtained are not hollow, and are with a smooth solid structure. In the HR-TEM image of the CS (Fig. 4) small wavy flakes of thickness 1–10 nm, having graphitic structure are resolved near the edge of the spheres, and these flakes are formed at different depths following the curvature of the spheres. The HR-TEM image shows that the graphitic flakes are spherically dispersed in the entire sphere. The graphite flakes are clearly visible at the edges rather than the center, indicating an in crease in thickness or a decrease in the number of flakes parallel to the electron beam. The inset of Fig. 4 shows the SAED pattern of a single CS. The SAED pattern has two broad diffraction rings that correspond to the planes (002) and (100) which shows a good agreement to the XRD re sults. The interlayer spacing d002 of the CS is calculated as 0.36 nm, which shows a low deviation from the ideal d002 of 0.34 nm graphitic structure (JCPDS-ICDD Card no: 41–1487) reported in the literature [34]. The value of d-spacing obtained for the CS is between that of the carbon fiber (d002 -0.342), and activated carbon atoms (d002-0.363) which show a well-organized carbon structure [39]. Fig. 5 (a) shows the XRD pattern of the synthesized CS. The 2Ө values at 24.65� and 43.07� represent graphitic planes (002) and (100), respectively. In comparison with graphite, the peak 24.65 shifts slightly to a lower angle that points out the presence of a few layers of graphene stacked without correlations along the c-axis because of the heavily disordered graphitic structures. The broadened peaks at (002) plane resulted from the waving structure of the graphitic flakes confirm that the sample is amorphous in nature. The functional groups attached to the synthesized CS are obtained
Table 1 Deposition parameters and precursor gas flow rates for carbon sphere synthesis. Sample no:
Temperature (oC)
Pressure (bar)
Time of Deposition (min)
C2H2
Ar
H2
CS1 CS2 CS3 CS4
650 750 850 950
1 1 1 1
30 30 30 30
100 100 100 100
200 200 200 200
30 30 30 30
No spheres No spheres No spheres 200–350 nm Smooth spheres
CS5
1050
1
30
100
200
30
200–550 nm Smooth spheres
CS6
950
1
15
100
200
30
200–350 nm Smooth spheres
Reactant Flow Rate (sccm)
3
Size (nm)
Mean & standard deviation
267 355 443 490 233 277 317
233 � 9, 277 � 8, 317 � 12 � 10, � 22, � 24, � 15 � 9, � 8, � 12
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Fig. 2. Scanning electron microscopy images of carbon spheres synthesized at a) CS2 b) CS3 c) CS4 and d) CS5.
Fig. 3. TEM image of carbon spheres synthesized at 950 � C.
from FTIR spectroscopy as shown in Fig. 5 (b). The strong peaks at 1138 – C (bending cm 1 and 1570 cm 1 indicate the presence of C–O and C– mode) groups, respectively. The two other peaks at 2855 cm 1 and 2933 cm 1 (stretching mode) show the presence of C–H groups in the synthesized CS. Information about the type of hybridization can also be obtained from the functional groups attached as the hybridization af fects the force constant, K. The alkane and alkene in the sample reveal that both sp3 and sp2 hybridized groups could be present in the carbon sphere synthesized [40]. Apart from SEM analysis, the sphere diameter was measured also using a dynamic light absorption technique shown in Fig. 5 (c) that gave a hydrodynamic diameter of 433 nm. The particle size analyzer measures the hydrodynamic diameter of the CS suspended in water that gives the average particle size, which is greater than the particle size obtained from SEM. The thermal degradation of CS was analyzed using Thermogravimetric Analyser (TGA) in two different conditions. The TGA response obtained for CS in air and inert atmo sphere are shown in Fig. 6. The TGA data of CS shows very high thermal
Fig. 4. HR-TEM of carbon spheres showing the graphitic layers and Selected Area Electron Diffraction (SAED) pattern in the inset.
stability till 1000 � C in a nitrogen atmosphere with a weight loss of less than 4%., whereas at oxygen atmosphere the CS starts degrading at 590 � C and complete weight loss occurs towards 800 � C. The TGA results reveal that CS is a potential material for very high-temperature applications. In order to examine the surface area and the pore volume of the CS synthesized, BET analysis was carried out by measuring the adsorptiondesorption isotherms in liquid N2 atmosphere. The surface area, total pore volume and mean pore diameters measured are given in Table 2. In 4
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Fig. 5. a) X-Ray powder diffraction pattern b) FTIR spectroscopy and c) DLS obtained for carbon spheres synthesized at 950 � C under atmospheric pressure.
the case of mesoporous materials with less than 50 nm pore diameter, a very high surface area of 1400–1600 m2/g is exhibited. In the case of pristine CS, a surface area of 3.6 m2/g is exhibited [34]. Here, the CS synthesized has a mean pore diameter of 3.85 nm and a surface area of 6.48 m2/g that is slightly higher than the pristine CS. The low value of surface area relates to the smooth surface morphology and the size of the CS. A very low total pore volume of 0.0062399 cm3/g was measured for the CS that clearly determines that the CS formed are hard spheres with very little porosity. To investigate the structural fingerprint of the CVD synthesized CS, visible Raman Spectroscopy was carried out and the Raman spectra obtained depend on (i) clustering of sp2 phase (ii) presence of sp2 chains or bonds (iii) bond angle and bond length disorder and (iv) the sp3/sp2 ratio [41]. In the case of amorphous carbon materials, the Raman spectra possess two peaks: the first peak in the range of 1320 cm 1 to 1360 cm 1 is called the D-band and the second peak in the range from 1500 cm 1 and 1600 cm 1 is called the G-band. The Raman spectra were recorded (Fig. 7) for the CS synthesized at 950 � C to study the graphitic order. Two peaks were observed in the spectrum, a stronger peak at 1578 cm 1 and a weaker peak at 1336 cm 1. The stronger peak is the G-band that represents the stretching mode resulting from the in-plane vibration of the sp2 bonded carbon atoms and the weak peak is attrib uted to the out-of- plane vibration called the D-band representing the defects and disorders in the carbon material. A more relevant explana tion on the G band and D band is that the former is due to the relative motion of the sp2 carbon atoms, while the latter is linked to the breathing modes of carbon rings [42]. The Raman spectra with varying G and D bands with different positions, intensities, and widths possess
Fig. 6. TGA response of CS synthesized at 950 � C in air and inert atmosphere.
Table 2 BET characterization results for CS synthesized at 950 � C. Parameter
Value obtained
Surface area, as Total pore volume Mean pore diameter
6.48 m2/g 0.0062399 cm3/g 3.86 nm
5
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formation mechanism of the CS is depicted in Fig. 8. The pentagonal atom rings nucleate and grow to form the quasi-icosahedral shells, forming a spiral shell that acts as the core of the CS. During the depo sition process, numerous pentagons and heptagons continuously add to the surface of the spiral shell and create new nucleation sites for the graphitic flakes. These result in the stacking of randomly twisted graphitic flakes distributed on the surface without any connection among them. These graphitic flakes can be clearly observed in the HR-TEM of the CS shown in Fig. 4. The layers of the CS possess numerous non-planar P–H pairs that align along the direction parallel to the axis of graphite which significantly vary the interlayer distance, resulting in broadening of the (002) peak. 3.4. Effect of H2 and Ar in CS synthesis In this study argon (Ar) and hydrogen (H2) are the gases used for the synthesis of CS under the non-catalytic CVD method along with acety lene. The Ar introduced in the reaction chamber helps to dilute the acetylene and to create an inert atmosphere for the pyrolysis of acety lene at high temperatures. The individual and combined effects of the H2 and Ar (Table 4) on the morphology of CS were analyzed using FESEM. In Fig. 9 (a) the SEM image shows that larger spheres of with smooth surface texture were formed with Ar alone with acetylene. In Fig. 9 (b) the SEM images show distorted spheroidal shape with reduced carbon particle size when H2 alone is used with acetylene. Fig. 9 clearly depicts that Ar alone promotes the growth of smooth carbon spheres whereas H2 alone gives a negative impact on the growth and sphere morphology. These results confirm that H2 and Ar have a strong influence on the shape and texture of the CS synthesized under the non–catalytic CVD method. To further investigate the effect of Ar and H2 on the CS synthesis, the concentrations of both the gases were varied. In sample CS8, CS9 and CS4
Fig. 7. Raman spectroscopy obtained for carbon spheres synthesized at 950 � C for 30 sccm H2 under atmospheric pressure.
details on the carbon bonds and lattice vibrations of nanocrystalline to amorphous carbon materials. The D peaks or the disorder-induced modes result from the disloca tions in the lattice. As a result, the intensity of the D peak with respect to the G peak is used to determine the degree of disorder in the synthesized sample. The degree of disorder or the value of graphitization is calcu lated from the maximum intensities of the D band and G band as (ID/IG). The positions of the D and G bands in the Raman spectra can clearly reflect the structure of the amorphous carbon phases. Table 3 shows the graphitization degrees (ID/IG) reported for CS synthesized by catalytic and non-catalytic CVD methods using acetylene as the carbon source and the present work. It was observed that the degree of graphitization, ID/IG ¼ 0.85 is obtained for CS synthesized from the present work. The graphitization degree of 0.85 obtained from the ID/IG value of Raman spectroscopy implies that the value of the graphitization degree is comparatively reduced with the reported CS synthesized using CVD [26–29]. During the process of graphitization, the turbostratic carbon is converted to a more ordered three-dimensional graphite structure, and the degree of graphitization depends on the raw materials and pro cessing parameters to achieve the graphitic order. A highly disordered carbon like amorphous carbon can be structurally ordered by material degradation at high temperatures [43]. The influence of certain catalysts in catalytic- CVD also can affect the ID/IG ratio. Jose et al. [34] have reported a graphitization degree of 0.80 in CS synthesized using NiFe-LDHs as the catalytic precursor and ethylene as the carbon precursor. 3.3. Mechanism of CS formation In the carbon atom, the three possible graphitic carbon ring forma tions are pentagonal, hexagonal and heptagonal carbon rings. It has been reported that among these graphitic carbon rings the formation of curling graphitic structures are nucleated from pentagonal rings, and to maintain the smooth morphology, the pentagonal rings pair with hep tagonal rings to nucleate a non-faceted graphitic layer, and the pair of pentagon and heptagon are represented as P–H pairs [44]. The
Fig. 8. Schematic showing the formation of carbon spheres by the nucleation of pentagonal carbon rings.
Table 3 Graphitization degree reported from Raman spectroscopy of carbon spheres synthesized from acetylene using CVD. Reactant gas
Carrier gases
Deposition temperature (oC)
C2H2 C2H2 C2H2 C2H2 C2H2
H2 Ar Ar þ C N2 H2þAr
900 900 850 1000 950
D band position (cm 1333 1340 1370 1330 1336
1
)
G band position (cm 1593 1595 1600 1590 1578
*C- Catalyst. 6
1
)
Graphitization degree (ID/IG)
Reference
0.89 0.88 0.94 1.3 0.85
[28] [28] [27] [29] Present work
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Table 4 Deposition parameters for carbon sphere synthesis to study the effect of H2 and Ar in the morphology. Sample no:
Temperature (oC)
Pressure (bar)
Time of Deposition (min)
C2H2
Ar
H2
size (nm)
CS7
950
1
30
100
0
30
200–480 nm Rough spheres
CS8
950
1
30
100
60
30
300–650 nm Smooth spheres
CS9
950
1
30
100
120
30
350–550 nm Smooth spheres
CS10
950
1
30
100
200
0
250–430 nm Smooth spheres
CS4
950
1
30
100
200
30
200–350 nm Smooth spheres
CS11 CS12
950 950
1 1
30 30
100 100
200 200
60 120
No sphere morphology No sphere morphology
Reactant Flow Rate (sccm)
Morphology from SEM
Mean & Standard Deviation in size (nm)
243 � 12, 323 � 18, 462 � 10 356 � 13, 466 � 16, 620 � 5 371 � 17, 452 � 19, 526 � 5 278 � 10, 340 � 13, 398 � 12 233 � 9, 277 � 8, 317 � 12
Fig. 9. SEM images of CS synthesized at 950 � C a) with Ar alone (CS10) and (b) with H2 alone (CS7) using acetylene as carbon precursor.
the concentration of Ar were varied as 60 sccm, 120 sccm, and 200 sccm, keeping C2H2/H2 at 100/30 sccm. At 60 sccm and 120 sccm of Ar concentration are shown in Fig. 10 (a) and (b), respectively. In both cases, Carbon spheres were synthesized, but the relative uniformity of the spheres synthesized varied from 300 to 650 nm. At a low concen tration of Ar, some of the sphere morphologies obtained were rough probably due to the presence of H2. Qian et al. [45] have reported the effect of different carrier gases such as Ar, H2, and N2 in carbon sphere synthesis with toluene as the carbon precursor. To study the effect of H2 in sample CS4, CS11 and CS12 the H2 con centration was varied as 30 sccm, 60 sccm, and 120 sccm, keeping C2H2/ Ar at 100/200 sccm. Fig. 10 (c) and (d) show the SEM images of the CS synthesized at 60 sccm and 120 sccm, respectively. Higher concentra tions of H2 from 30 sccm significantly affect the surface morphology of the CS. The sphere morphology deteriorated at a high concentration of H2. In the present study, smooth carbon spheres were synthesized at different concentrations of Ar, and the addition of H2 resulted in rough spheres with enhancement in the relative uniformity of the spheres synthesized. However, in the sample CS4 (Fig. 2(c)) the combined effect of the H2/Ar at 950 � C under 100 sccm acetylene flow synthesized relatively uniform CS of 200–350 nm that are smaller sized spheres compared to the CS synthesized in C2H2/Ar gas mixture. The effect of H2 was further investigated using Raman spectra by increasing the H2 concentration from 30 sccm to 60 sccm and 120 sccm (Table 5). Fig. 11 shows the Raman spectra of CS synthesized at H2 concentrations of 60 sccm and 120 sccm, respectively. The D and G
bands obtained for H2 concentration of 60 sccm are almost similar to H2 at 30 sccm. When the amount of H2 is further increased to 120 sccm the D and G peaks slightly shift towards the right with D peak at 1344 cm 1 and G peak at 1585 cm 1. This shift in the peaks can be attributed to the presence of double bonds or unmodified graphitic areas [46]. Marto et al. [47] have reported that adding H2 into a gas mixture of Ar/N2 for amorphous carbon thin films synthesis decreased the ID/IG ratio from 1.6 to 1.1 and the G peak position changed slightly towards the right side, which shows good agreement with the present study. However, in the present case of CS synthesis, the ID/IG ratio increased from 0.85 to 0.88 as the concentration of H2 is increased from 30 sccm to 120 sccm. By observing the Raman spectra of the CS synthesized at different H2 con centrations at 60 sccm, the peak intensities of the G peak were found to be slightly larger than that of D peak, and this difference becomes prominent when the concentration is increased to 120 sccm. In general, the G and D peaks of the amorphous carbon material will have broad peaks or D peak intensity higher than G peak and it has been universally observed in carbon materials that a broader G peak and a higher in tensity of D peak observed in the Raman spectra confirms the presence of higher order defects in the material. The Raman spectra of the CS syn thesized in the present study has narrow G peak and lesser intensity of D peak with respect to G peak, which confirms the decrease of defect in CS synthesized. Apart from the intensity and peak positions of the D and G peaks, the Full Width at Half Maximum (FWHM) also has significant importance in determining the defects in the carbon materials and it can vary from 10 to 200 cm 1 from graphene to amorphous carbon materials 7
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Fig. 10. SEM images obtained for carbon spheres synthesized at 950 � C under C2H2/Ar/H2 gas mixture (a) CS8 (b) CS9 (c) CS11 and (d) CS12. Table 5 Graphitization degree obtained from Raman spectroscopy of carbon spheres synthesized under different H2 concentration. Reactant gas
Carrier gas Ar/H2 (sccm)
Deposition temperature (oC)
D band position (cm 1)
G band position (cm 1)
Graphitization degree (ID/IG)
C2H2 C2H2 C2H2
200/30 200/60 200/ 120
950 950 950
1336 1336 1344
1578 1573 1585
0.85 0.87 0.88
[48]. The FWHM values obtained for CS synthesized at different H2 concentrations were obtained from curve fitting and were found to be 75 cm 1 for 30 sccm, 51 cm 1 for 60 sccm and 83 cm 1 for 120 sccm respectively. The Raman and SEM analyses revealed that a further in crease in H2 concentration from 30 sccm causes an increase in ID/IG ratio from 0.85 to 0.88 and also deteriorates the surface morphology of the CS synthesized. It can be inferred that the effect of a very low concentration of H2 about 30 sccm can influence in reducing the defects and also the graphitization degree of CS synthesized under C2H2 and Ar. The reduction in graphitization degree is understood to be the combined effect of H2 and Ar that probably have influenced the formation of small quantities of ordered carbon due to the cross-linking between the or dered and neighboring regions to get ID/IG ¼ 0.85. Further analysis of the effect of higher concentrations of hydrogen was performed using FTIR to see whether any further changes occurred. In Fig. 12, the FTIR spectrum of CS synthesized at H2 concentrations of 30 sccm, 60 sccm and 120 sccm are given, respectively. The details of the functional groups attached to the CS synthesized at different hydrogen concentrations are listed in Table 6. In comparison with the functional groups formed in 30 sccm, at 60 sccm and 120 sccm more peaks of C–H groups are observed at 813 cm 1, 736 cm 1, and 2664 cm 1. In 120
Fig. 11. Ultra-violet laser Raman spectroscopy for carbon spheres synthesized at 950 � C under 60 sccm and 120 sccm hydrogen concentrations. (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
sccm the intensity of C–H peaks was very high compared to the C–O peak at 1122 cm 1. This implies that at higher concentrations of H2 more hydrocarbons are formed in the sample. At 60 sccm and 120 sccm, the 1 – intensity of C– – C weak stretching bond at 2100 cm is high which is understood to be due to the presence of unreduced acetylene at higher H2 concentrations. An additional peak at 2330 cm 1 was observed at – C– – O stretching mode, higher concentrations that correspond to the O– thus indicating CO2 absorption by the sample. Thus, it can be concluded from the FTIR spectra that an increase in H2 concentration results in 8
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Fig. 12. FTIR results obtained for CS synthesized at higher concentrations of Hydrogen a) 30 sccm b) 60 sccm and c) 120 sccm. Table 6 Functional groups formed at different concentrations of hydrogen from FTIR spectra. Frequency range Absorption (cm 1)
Appearance
Group
3100-3000 (2926) 2830-2695 (2664) 2349 (2330) 2140-2100 (2100) 1818 (1877) 1620-1610 (1557) 1124-1087 (1122) 1085-1050 (1085) 880 � 20 (869,813) 750 � 20 (736)
medium medium strong weak strong strong strong strong strong strong
C–H stretching C–H stretching O¼C– O stretching –– C–C stretching – C¼O stretching C¼C stretching C–O stretching C–O stretching C–H bending C–H bending
additional C–H functional groups that may be responsible for the dete rioration of the CS morphology observed in the SEM images. 4. Conclusions One of the simplest methods to synthesize relatively uniform CS ranging from 200 to 350 nm using non-catalytic thermal chemical vapor deposition under atmospheric pressure is explained in this paper. Morphological studies on SEM and TEM show smooth and spheroidal texture for the carbon spheres. The XRD results and Raman Spectroscopy reveals that the carbon spheres synthesized are amorphous carbon spheres. The surface area measurement and thermal degradation studies on carbon spheres give a mean pore diameter of 3.86 nm, and thermal stability up to 590 � C under air. The effects of the deposition tempera ture and H2 are analyzed using SEM and Raman spectroscopy. Relatively uniform CS could be synthesized at a deposition temperature of 950 � C with suitable deposition parameters. The presence of H2 and Ar has significantly influenced the morphology of the CS synthesized. Raman spectroscopy results show that the presence of H2 in C2H2/Ar has sig nificant importance in the graphitization degree, compared to the re ported CS synthesis using C2H2 as the carbon precursor. The effect of H2 was also studied using FTIR and the spectra reveal that more amount of C–H functional groups were formed at higher concentrations of H2 that resulted in the deterioration of surface morphology of the CS. The study also reveals that a further increase of H2 from 30 sccm increased the graphitization ratio. The main advantage of this CVD synthesis is the high reproducibility and large yield of about 1–2 gm of CS for 30 min of 9
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