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A new model for the synthesis of graphite encapsulated nickel nanoparticles when using organic compounds in an arc-discharge system
Diamond & Related Materials 103 (2020) 107719
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A new model for the synthesis of graphite encapsulated nickel nanoparticles when using organic compounds in an arc-discharge system
T
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Yu-Chieh Huang, Mao-Hua Teng , Tun-Hao Tsai Department of Geosciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan, R.O.C.
A R T I C LE I N FO
A B S T R A C T
Keywords: Nickel nanoparticle Organic compound Pyrolysis Arc-discharge Core-shell
Herein we propose a new “three-step” model of synthesizing graphite encapsulated nickel nanoparticles (NiGEM), including the pyrolysis reaction of organic compounds which the conventional two-step model ignores. According to the results of XRD, Raman, and TEM, we found that the Ni-GEM synthesized by using both PF resin and benzene vapor as the carbon sources has two favorable characteristics: thicker shells (~5–10 nm) and smaller particle sizes (~30 nm), which are much better than those using only PF resin or PF resin mixed with cyclohexane vapor (thinner shell less than 5 nm and larger particle sizes ~50 nm). Benzene decomposes into large aromatic molecules and tiny graphitic flakes at 1200–3500K, while cyclohexane prefers to decompose and form small hydrocarbon molecules at 1000K. As a result, the two compounds go through two different reaction paths. Benzene will decompose and directly attach onto the surface of Ni nanoparticles, forming smaller sized but thicker shell structured Ni-GEM, while cyclohexane will lead to the formation of amorphous carbon coating on the Ni-GEM. By including the above two distinct hydrocarbon pyrolysis reactions, this study modifies the conventional model and successfully explains the formation processes of Ni-GEM with very different morphologies. Furthermore, the new model may help in controlling the morphologies of other GEM nanoparticles with a number of core-metals.
1. Introduction Graphite encapsulated metal (GEM) nanoparticles are a kind of novel nanocrystalline material with a core-shell composite structure; they were first synthesized by an arc-discharge method in 1993 [1,2]. Due to their distinctive core-shell structure, GEM has many special characteristics and could have numerous applications, such as stable magnetic properties, microwave absorbent, catalyst, and hydrogen storage. Therefore, graphite encapsulated “ferromagnetic” nanoparticles, e.g., Fe-GEM and Ni-GEM, have been selected as emerging materials with great potential in biomedical technologies [3,4], energy engineering fields [5], national defense industries [6], and environmental and engineering fields [7]. However, the practical applications of GEM nanoparticles have been hindered by the insufficient production rates and the inefficient synthesis methods, e.g., using conventional carbon arc-discharge system, or high power output system [8,9]. There are two major goals in this study: one is to improve the efficiency of GEM production, and the other is to explore the possible mechanism when using liquid organic compounds as the carbon sources (hereafter called liquid carbon sources). A modified tungsten arc-discharge system, using a tungsten rod as
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the electrode, has been used to synthesize GEM nanoparticles since 1995 [10]. After almost two decades, Chiu et al. [11] and Teng et al. [12] discovered a method using liquid carbon sources and could synthesize Ni-GEM efficiently. They injected the liquid organic compounds, e.g., alcohols, benzene (C6H6), and cyclohexane (C6H12), directly into the arc-discharge area, and greatly increased the production rate and encapsulation efficiency of Ni-GEM up to 2.4 mg/s and 70–80%, respectively. However, when the organic compounds decomposed by the heat of the arc-discharge, their vapor will dilute the original dominant gas (helium) of the plasma. Because the electrical resistance increases sharply, the arc plasma becomes hard to control, and the electrical resistance of the arc fluctuates dramatically and frequently. Eventually, it will lead to the disruption of the whole experiment. In order to avoid this problem, the present experimental setup uses a new type of colloidal carbon source-phenol formaldehyde resin (PF resin) to synthesize Ni-GEM nanoparticles. The PF resin is a synthetic polymer with light yellow color, obtained from the condensation reaction of phenol and formaldehyde. When a PF resin is heated above 300°C by an arc-discharge, the resin will gradually graphitize and release both hydrogen and carbon vapors [13]. Additionally, a volatile organic compound, e.g., benzene or cyclohexane, is placed next to the
Corresponding author. E-mail address: [email protected] (M.-H. Teng).
https://doi.org/10.1016/j.diamond.2020.107719 Received 30 July 2019; Received in revised form 15 January 2020; Accepted 17 January 2020 Available online 17 January 2020 0925-9635/ © 2020 Elsevier B.V. All rights reserved.
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resin only, PF resin plus benzene vapor (hereafter called PF-bnz), and PF resin plus cyclohexane vapor (hereafter called PF-cyclo). Benzene or cyclohexane is injected into the ceramic crucible to provide additional carbon vapor, rather than directly injecting the liquid carbon source into the arc-discharge area in our previous work. The cathode, a tapered tungsten rod of 10 mm diameter (98% tungsten with 2% cerium), is fixed with a copper fixture and vertically locked into a tunable water-cooled based copper conductive tube. The vacuum chamber needs to be evacuated first and then purged with helium gas. Finally, the chamber will be filled with helium atmosphere at a pressure of 200 Torr before starting the arc-discharge. An arc-discharge will be generated by a DC current fixed at 120 A between the tungsten rod and graphite crucible. A typical variation of the arc current and voltage with time is shown in Fig. 2. (The upper line (blue) represents the voltage, and the lower line represents the DC current we used.) The arc current and voltage will usually fluctuate in a small range once the system has reached a stable condition. Due to the high temperature of the arc, the metal and the PF resin start to melt and evaporate, forming a high temperature metal pool. Through the nucleation growth of nanoparticles, metal vapor and carbon vapor will coalesce and condense into GEM nanoparticles. Note that the area where the coalescent processes occur is called the coalescence area. According to the two-step mechanism model [14], providing a homogeneous distribution of carbon and metal vapors is the key to improve the synthesis efficiency. However, the present setup still cannot guarantee a 100% ideal distribution; as a result, it is inevitable that the as-made powder contains some undesired ill-encapsulated metal particles and carbon debris. Consequently, the as-made products need to be purified. The purification procedures are not complicated. After each experiment, all the nanoparticles will be scraped off the chamber walls and submerged in a beaker with strong acid, i.e., nitric acid or hydrochloric acid, and then stirred in an ultrasonic bath to dissolve all the illencapsulated particles. Next, the well-encapsulated Ni-GEM nanoparticles can be separated by using a strong magnet. Through the purification procedures, the weight percentage of the purified GEM nanoparticles to the as-made GEM powders can be calculated and
anode, providing additional and uniform carbon vapor to the arc-discharge. The results of this method reveal that the morphology of the NiGEM nanoparticles is closely related to the concentration of the carbon vapors and the pyrolysis reactions of the organic compounds. The conventional two-step mechanism model was proposed by Elliott et al. in 1997 [14]. However, it failed to explain the new experimental results; for example, Teng et al. [12] found that the particle sizes of Ni-GEM significantly differ when using benzene or cyclohexane as the carbon source in the same setup. Because the conventional twostep model was developed based on the experimental results when using only solid carbon sources, i.e., graphite or diamond powder [15], it naturally cannot fully explain the phenomena that occurred when using liquid carbon sources, such as alcohols and other organic compounds. In this study, PF resin and two organic compounds vapor (benzene or cyclohexane) were used to synthesize Ni-GEM. Through observing the morphology, particle sizes, and graphite shell of the products, it was obvious that the synthesized Ni-GEM is affected by the pyrolysis reaction of each organic compound through different paths. According to these results, a new three-step mechanism model can be constructed. 2. Experimental 2.1. Materials and synthesis method Fig. 1 shows a schematic setup of the arc-discharge in a vacuum chamber. The chamber is specifically designed for synthesizing GEM nanoparticles, and is equipped with a modified tungsten arc-discharge system [10]. The anode is a graphite crucible loaded with 30 g nickel shots (3–25 mm diameter, 99.95+%) and 3 g phenol formaldehyde resins (purity 77%, phenol 11%, methanol 10%, and formaldehyde 2%); all the raw materials inside the graphite crucible will volatilize at the same time during the experiments. In addition, a ceramic crucible of aluminum oxide is placed next to the graphite crucible, so that the organic compounds injected into the ceramic crucible can be volatilized rapidly during the experiments. There are three basic setups for the carbon sources in this work: PF
Fig. 1. Schematics of the experimental setup of the modified tungsten arc-discharge in the vacuum chamber. 2
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Fig. 2. The variations of the DC current and voltage of the arc by time.
extent by switching the carbon sources from solid type to liquid type. Fig. 3 shows the improvement of the synthesis efficiency when using different types of carbon sources over the years. The blue words (the left coordinate axis) and bar chart correspond to the production rate, and the black words (the right coordinate axis) and line graph correspond to encapsulation efficiency. However, as mentioned earlier, using liquid carbon sources is prone to interrupt the arc and cause the experiment to fail. In order to simultaneously alleviate the arc interruption problem and enhance the production rate of Ni-GEM, we synthesized the Ni-GEM by using a colloidal type carbon source and a liquid organic compounds vapor. According to the experimental results, using only PF resin as the carbon source can avoid the arc extinction problem, and the production rate of Ni-GEM can greatly increase up to 5.6 mg/s. But the encapsulation efficiency can only reach about 30% (Fig. 3). It is worth noting that using only PF resin can achieve as high a production rate as when using a liquid carbon source, but the encapsulation efficiency is poor, similar to the results when using diamond powder as the carbon source [15]. In order to further explore this phenomenon, we first need to understand the pyrolysis reaction of PF resin. In 1995, Trick and Saliba
defined as the encapsulation efficiency of the experiments. 2.2. Characterization methods for the GEM nanoparticles High resolution transmission electron microscope (HRTEM) images and electron diffraction patterns were acquired by using a JEOL 2010F operated at the acceleration voltage at 200 kV. The phase and crystal structure of metal and graphite were analyzed by X-ray powder diffraction (XRD; X'Pert3 Powder, Panalytical). Micro-Raman spectroscopy was recorded by HORIBA-FHR1000 with laser beam size of 2 μm, and the wavelength of GaAlAs semiconductor laser was 532 nm. The average particle sizes of GEM nanoparticles were mainly derived from BET (specific surface area, NOVA2000, Quantachrome) analysis. 3. Results and discussion 3.1. Synthesis efficiency of Ni-GEM when using only PF resin as the carbon source Low production efficiency has been one of the biggest problems of the synthesis of GEM. However, the problem has been solved to a large
Fig. 3. Comparison of the encapsulation efficiencies and production rates of three selected previous studies that use different type of carbon sources. 3
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[16] investigated the gaseous product of the PF resin after pyrolysis, and found that the gas molecules contained 85% hydrogen when the temperature reached 560°C. Note that the existence of hydrogen in the arc plasma area will not only increase the working temperature of the arc, but also form unstable sp2 hybridization dangling bonds at the edge of the carbon materials, causing an etching reaction of the carbon [17]. Moreover, in 2000, Ko et al. [18] used XRD and Raman spectra to observe the microstructure changes of the solid PF resins between 300 and 2400°C; when the temperature is higher than 500°C, the resin will start to carbonize and graphitize, forming new glassy carbon nanoparticles. Based on the above studies, we know that the pyrolysis reaction of PF resin, besides generating a lot of hydrogen, is similar to that of the diamond powder in a molten metal pool; both will graphitize gradually at high temperature and provide carbon source to the arc plasma. Not surprisingly, their encapsulation efficiencies are very similar, but the pyrolysis of PF resin can also release hydrogen and form dangling bonds to trigger an etching reaction. As for the results, hydrogen plasma can help to increase the temperature and accelerate carbon formation [19,20], thus increasing the evaporation rate of raw materials and forming more nanoscale carbon. That is the reason why the production rate of using PF resin is higher than when using diamond powder, and why although the production rate can reach up to 5.6 mg/s, the encapsulation efficiency is only ~30%.
Fig. 4. The average particle sizes of Ni-GEM nanoparticles calculated by XRD, BET and TEM by using four carbon source setups. Note that the last setup shows no difference with PF-bnz.
calculated by two different methods; the first method used full width at half maximum (FWHM) of XRD diffraction peaks by Scherrer equation, with the results representing the “crystal” sizes; the second method is BET analysis that assumes a density of the nanoparticles and calculates the spherical particle size. Not surprising, both methods show the same trend: the sizes of the Ni-GEM nanoparticles made from PF-cyclo are larger than those when PF-bnz is used as carbon source. In 1976, Granqvist and Buhrman [21] found a positive correlation between particle size and gas pressure; that is, the higher the gas pressure, the larger sized particles that would be derived. The gas pressure in our chamber changes with time when using different organic compounds, as shown in Fig. 5. Once the organic compounds vaporize in the arcing vacuum chamber, they will start breaking up and re-bonding between or within molecules to form different types of organic vapors, and also simultaneously increase the chamber pressure. The highest pressure derived when using PF resins, PF-cyclo, and PFbnz as the carbon sources were 289 Torr, 622 Torr, and 490 Torr, respectively, and the results seem irrelevant to the particle sizes of GEM. Moreover, the existence of hydrogen may also affect the operating temperature of the arc [11] and increase the vaporization rate and the collision frequency. Because both the number of collisions and raw material vapor increased, the effective collision probability between these nanoparticles also increased. Consequently, it is prone to form larger sized particles. In order to determine whether the hydrogen or gas pressure will affect the particle sizes, a special PF-bnz experiment with 40 ml benzene was conducted, i.e., providing the same amount of hydrogen as in a PF-cyclo experiment where the cyclohexane is 20 ml. As the result, the average particle size was only 28.0 ± 0.9 nm, indicating that the particle size of Ni-GEM is neither dependent on gas pressure nor on hydrogen content. To further investigate the causes of the different particle sizes, we needed to observe the morphology of the products by HRTEM images, as shown in Fig. 6. The core-shell composite structure of Ni-GEMs can be clearly seen. When Ni-GEM were synthesized using PF resins only or PF-cyclo, a thinner shell with 2–4 graphite layers and thickness less than 3 nm was found in Figs. 6a and 6b. Also, many crystalline carbons with layered structure and amorphous carbon appeared besides the nanoparticles when using PF-cyclo. In contrast to the former images, the Ni-GEMs synthesized using PF-bnz had a thicker shell of about 5–10 nm (Fig. 6c and d) According to the results of the HRTEM micrographs, the products made from PF-cyclo and PF-bnz significantly differ due to the particle sizes and thickness of the graphitic shell. In 1998, Setlur et al. [17]
3.2. Second carbon sources: benzene and cyclohexene In Section 3.1, we discussed the efficiency when using a new type of colloidal carbon source PF resin, which can not only successfully avoid the arc extinction problem, but also improve the production rate of NiGEM. However, the encapsulation efficiency is disappointingly low, ~30%. In order to enhance the encapsulation efficiency, a second carbon source was added into the experimental setups to provide additional and uniform vaporized carbon source near the arc-discharge area. Table 1. shows the improvement of synthesis efficiency when adding additional organic compound vapors. Regardless of using benzene vapors or cyclohexane vapors, it can be clearly seen that both the production rates and encapsulation efficiencies increase up to ~8.1 mg/ s and ~81%, respectively. It is not difficult to conclude that adding vapor carbon sources can improve the encapsulation efficiency. Presumably, the nanoparticles have more chance to collide with carbon molecules than just in using only PF resin as the carbon source, and consequently increase the production rate and encapsulation efficiency. 3.3. The morphology changes of Ni-GEM nanoparticles Although the results of the experiments show no differences in the synthesis efficiency between using benzene or cyclohexane, we may still obtain some clues from their different particle sizes, crystal phase, and thickness of graphitic shells of the Ni-GEM. Fig. 4 shows the average particle sizes of Ni-GEM when using different carbon sources setups. The average particle sizes in using PF resin only and PF-cyclo are 39.3 ± 4.7 nm and 39.1 ± 2.6 nm, respectively. Compared to the above results, the particle sizes when using PF-bnz are much smaller, ~24.3 ± 0.7 nm. The particle sizes were Table 1 The production rates and encapsulation efficiencies of the three carbon source setups in this work: PF resin only, PF-cycle, and PF-bnz. Carbon source
Production rate (mg/s)
Encapsulation efficiency (%)
PF resin only PF-cyclo (20 ml) PF-bnz (20 ml)
5.6 8.1 8.1
29 79 81
4
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Fig. 5. The variations of the gas pressure with time while using different organic compounds as carbon sources.
Fig. 6. (a), (b) The TEM images show a thin shelled Ni-GEM nanoparticle. There is some amorphous carbon (red arrow) when using a PF-cyclo setup. (c), (d) are the Ni-GEM nanoparticle with a thick shell when using a PF-bnz setup. 5
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In order to investigate the detailed structure of the graphite shell, we focused on analyzing the (002) graphite peak for a more in-depth discussion. Fig. 9 shows the precise diffraction pattern at 2θ = 20–30° with 1°/min slow scanning speed. The main (002) graphite diffraction peak at 2θ ≈ 26.5° can be clearly seen in each pattern; another shoulder peak at 2θ ≈ 26.0° can only be seen after adding the organic compound vapors. By calculating the interlayer spacing of the carbon, the diffraction peak at 2θ = 26.0° and 2θ = 26.5° represent 0.343 nm and 0.336 nm, respectively; the standard d-spacing of the graphite should be 0.336 nm. Li et al. [22] and Matassa et al. [23] found that this phenomenon may be related to the decreased size and twisted nanostructure, causing a disordered stacking. Although the exact structure of the d-spacing = 0.343 nm remains unclear, the two different kinds of carbon can be easily found in the analysis of the XRD while using organic vapors. More detailed information on the structure of the graphitic shells of the Ni-GEM can be obtained from Raman spectra, as shown in Fig. 10. In 1970, Tuinstra and Koenig [24] first proposed an empirical formula by using the relative intensity of the D band and G band (ID/IG, R) to calculate the crystal size of the graphite (La). The G band, also called the graphite band, is located at 1580 cm−1, indicating the sp2 hybridized electron structure of carbons. The D band is associated with the defects or disordered structures around 1355 cm−1, indicating the sp3 hybridized carbons. The values of the R and crystal sizes were calculated and are listed in Table 2. When only PF resin was used as the carbon source, the spectra show a stronger G band than the D band, meaning a smaller R value and a large crystal size of graphite. In contrast, the
found the relation between hydrogen concentration and the consumption rate of carbon materials; when carbonaceous materials are under high concentration of hydrogen atmosphere, it will cause lots of unstable dangling bound at the edge of the carbon structure, causing carbon‑carbon to break more easily and form related hydrocarbon. For a more detailed discussion, an experiment using naphthalene (C10H8) as the lower hydrogen content carbon source was carried out to investigate the relation between the thickness of the graphite shell and the hydrogen concentration. If the hydrogen concentration is the main factor affecting the formation of graphite shell, the naphthalene products should have a thicker graphite shell, but this does not turn out to be the case. The product synthesized by naphthalene vapor has a thickness of graphite shell (2–4 graphitic layers with onion-like carbon particles) similar to that of cyclohexane products, which means that the shell thickness is not directly related to the hydrogen concentration; thus, the hydrogen concentration is not the main factor influencing the formation of shells. The HRTEM images also reveal the lognormal size distributions of Ni-GEM through actual measuring and counting methods. All the lognormal distributions show only one peak, meaning the resin and organic compound didn't produce very different and independent sized particles. The distribution of Ni-GEM is shown in Fig. 7. In addition to TEM images, we also used XRD and Raman spectroscopy to analyze the graphite shell in each product. Fig. 8 shows the Xray diffraction patterns of the purified Ni-GEM nanoparticles. The diffraction peaks of face-centered cubic structure nickel have much higher intensity than the graphite (002) peak at 2θ = 26.5°.
Fig. 7. The lognormal size distributions of Ni-GEM nanoparticles when using different carbon sources: (a) PF resin only, (b) PF-bnz (20 ml), (c) PF-cyclo (20 ml), and (d) PF-bnz (40 ml). 6
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Fig. 8. (a) The X-ray and (b) the electron diffraction patterns of the purified Ni-GEM nanoparticles.
Fig. 9. The precise XRD diffraction pattern at 2θ = 20–30° by using 1°/min slow scanning speed and the deconvolution peaks of Ni-GEM nanoparticles.
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Table 3 The relative reaction temperature and pyrolysis products of cyclohexane and benzene.
Table 2 The values of the R and crystal sizes by different raw materials. The relative intensity (ID/IG, R) and crystal size of the graphite (La) were calculated by following the method of Tuinstra-Koenig. ID/IG
La (nm)
PF PF PF PF
0.89 0.83 0.77 0.48
4.87 5.22 5.61 8.87
+ 40 ml Benzene + 20 ml Benzene + 40 ml Cyclohexane resin
Reactant
Products
1023–1173
Cyclohexane (C6H12)
1273–2200
Benzene (C6H6)
1-Butene (C4H8) 2-Butene (C4H8) Butadiene (C4H6) Ethylene (C2H4) Phenyl (C6H5) Biphenyl (C12H10) Triphenyl (C18H14) Acetylene (C2H2) Cyclopentadienyl (C5H5)
complicated; we can easily discover the complexity from the two compounds adopted in this work, i.e., cyclohexane and benzene. While the cyclohexane tends to form amorphous carbon on the surface of nanoparticles, benzene tends to form a graphitic shell. In addition, several other differences are also very obvious, such as particle sizes, thickness of the graphite shell, and graphite structures. To explain the differences, we must first investigate the pyrolysis behaviors of cyclohexane and benzene. The related reaction temperatures and pyrolysis products of cyclohexane and benzene are listed in Table 3, shows the different pyrolysis behaviors in a high-temperature area. The initial reaction temperature of cyclohexane is about 1000K [25]. Due to the easily broken single bond of the carbons, most reaction products are small hydrocarbon molecules [26], such as alkane and alkene substances. In contrast, the initial reaction temperature of benzene is 1273K [27,28]. Because of the stable structure of the aromatic rings, the pyrolysis reaction tends to form larger aromatics molecules, e.g., biphenyl (C12H10), triphenyl (C18H14), naphthalene (C10H8), alkyne, etc. Although the final products of both reactions are all stable graphitic structured materials, cyclohexane needs to degrade first in a relatively low temperature area, as shown in Table 4, while benzene can directly condense into crystalline graphite through phenyl radical molecules in a higher temperature area. Following the above discussions and guidance of the two-step mechanism model [14], the new model was proposed, as shown in Fig. 11. When the arc-discharge is generated, nickel and PF resins will melt and evaporate immediately because of the high temperature in the arc plasma center (about 12,000K); the ambient temperature (estimation of temperature is about 5000K as the plasma tail flame) [29–31]. Atoms will start to form “liquid-like” nanoclusters and continue to grow between temperatures at roughly 2300–1000K [32], until the temperature drops down to the lower boundary of the coalescence temperature. At this stage, the conventional phase segregation process will soon stop working. Note that the temperature distribution map in Fig. 11 simply shows the relative position between the arc plasma shape (bell shape) and the main coalescence area. If benzene is the chosen carbon source, the pyrolysis of benzene will form nano-sized graphitic molecules and attach directly onto the surface of the nanoparticles, forming a thicker shell structure. Then the thick outer shell will stop the continuing coalescence growth of the metal core, as shown in route A in Fig. 11. Moreover, benzene vapor has
Fig. 10. Raman spectra of the Ni-GEM nanoparticles synthesized by using different carbon source setups.
Raw materials
Temperature (K)
intensity of D band became stronger after adding organic vapors, which means the average crystal sizes of graphite decreased from 8.87 nm to 4.87 nm. After analyzing the above experimental data, it seems the newly formed carbon material has two special features that differ from the original graphitic shells: a larger d-space and smaller flaky crystal size.
3.4. New “three-step” mechanism model The new model is an extension of the existing conventional two-step mechanism model. The two-step model separates the formation evolution of GEM into two steps: phase segregation and catalyzation. When the carbon and metal vapor leave the arc-discharge plasma, they quickly cool and coalesce into numerous liquid-like nanoparticles. During the cooling process, the carbon will crystallize into graphite first and then segregate to the surface of the nanoparticles, thus totally or partially encapsulating the inner metal core. The collision and coalescing process between the nanoparticles continues until either the nanoparticles are too cold and rigid, or the encapsulated graphite is thick enough to prevent further coalescence. The second step “catalyzation” is specifically for certain catalyzing metals that have the ability to catalyze amorphous carbon into graphite, such as Fe, Co, and Ni. These metals can catalyze the surface amorphous carbon into graphite, making it possible for any exposed metal surface to be encapsulated by graphite at a later stage. However, Elliott et al. [14] also mention that the second step is only a support mechanism, and that the phase segregation is the main mechanism. The conventional two-step mechanism model does not consider the possibility that carbon vapor may be provided inwardly, i.e., from chamber atmosphere to the hot coalescence area. Thus, a new third step is needed to include the decomposition of liquid carbon sources. However, the pyrolysis reactions of hydrocarbon compounds are
Table 4 The relative reaction temperatures and pyrolysis products of Acetylene and Ethylene.
8
Temperature (K)
Reactant
Products
773
Acetylene (C2H2)
936
Ethylene (C2H4)
Vinylacetylene (C4H4) Benzene (C6H6) Phenyl (C6H5) Methane (CH4) Ethane (C2H6) Acetylene (C2H2)
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Fig. 11. Schematic diagram of the new three-step mechanism model of synthesize the Ni-GEM nanoparticles. The two distinct paths of the two different pyrolysis behaviors of (A) benzene and (B) cyclohexane.
amorphous carbon when using cyclohexane as carbon source, and the other is the graphitic shell when using benzene. The conventional two-step mechanism model did not consider the carbon sources provided from the outside of the arc-discharge. In this study, we successfully constructed a new “three-step” mechanism model, with the addition of the above two paths of pyrolysis reactions to the existing two-step model. The model can qualitatively explain the observed various characteristics of the Ni-GEM nanoparticles by HRTEM, XRD, BET analysis, and Raman spectra. However, the proposed three-step mechanism model is not limited to Ni-GEM; it can also be used to explore the synthesis of other encapsulated metal materials that were previously thought to be impossible to synthesize.
been shown to form graphene at the surface of the catalytic metal under 1373K by Kawasumi et al. [33] in 1981, indicating that using benzene to form nano-flake carbon materials in the arc-discharge system is possible. In contrast to benzene, cyclohexane cannot affect the coalescence growth of the nanoparticles; it can only form an amorphous carbon coating on the outside, and some crystalline carbon flake aggregation along with Ni-GEM nanoparticles, as shown in route B in Fig. 11. The organic compound pyrolysis arc-discharge method can synthesize not just Ni-GEM, as demonstrated in this study, but the method can also be used in synthesizing many other core-metaled GEM. The method, as shown in three-step model, can synthesize GEM through step three only, without the previous two steps. This means there is almost no limitation in choosing raw core metal materials to synthesize GEM nanoparticles. Though we have developed a useful model that can satisfactorily explain the experimental results qualitatively, a more quantitative description remains elusive. Much more work is needed in order to fully understand the intriguing and complicated mechanism in detail.
Yu-Chieh Huang:Conceptualization, Methodology, Resources, Writing - original draft.Mao-Hua Teng:Project administration, Funding acquisition.Tun-Hao Tsai:Validation, Formal analysis, Writing - review & editing.
4. Conclusions
Declaration of competing interest
CRediT authorship contribution statement
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The synthesis of Ni-GEM nanoparticles by using organic compound pyrolysis is a much more effective way in contrast to previous studies. Using PF resin as the carbon source can improve the production rate of Ni-GEM, from 2.4 mg/s when using injecting liquid carbon sources to 5.6 mg/s. Moreover, the use of organic vapors, e.g., cyclohexane or benzene, can dramatically increase the production rate and encapsulated efficiency up to 8.1 mg/s and 81%, respectively. The pyrolysis of organic compounds during the experiments will form two major carbon materials attaching to the surface of GEM: one is
Acknowledgment The authors would like to special thank Prof. Cheng-Yen Wen and Mr. Chuan-Yu Wei (Department of Materials Science and Engineering, National Taiwan University) for the support and operation of the 9
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HRTEM. This work was supported by the Ministry of Science and Technology, Taiwan, Grant MOST 106-2116-M-002-023 and MOST 107-2116-M-002-012.
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nanoparticles by an arc-discharge method, J. Alloys Compd. 434-435 (2007) 678–681. K.A. Trick, T.E. Saliba, Mechanisms of the pyrolysis of phenolic resin in a carbon/ phenolic composite, Carbon 33 (11) (1995) 1509–1515. A.A. Setlur, J.M. Lauerhaas, J.Y. Dai, R.P.H. Chang, Formation of nanotubes, nanowires, and nanoparticles in a hydrogen arc, Supercarbon (1998) 43–50. T.H. Ko, W.S. Kuo, Y.H. Chang, Microstructural changes of phenolic resin during pyrolysis, J. Appl. Polym. Sci. 81 (5) (2001) 1084–1089. J.Y. Geng, H.X. Wang, W.P. Sun, Modeling study on the plasma flow and heat transfer characteristics of low-power hydrogen, helium, nitrogen, and argon archeated thrusters, IEEE Trans. Plasma Sci. 42 (10) (2014) 2730–2731. Y. Guo, T. Okazaki, T. Kadoya, T. Suzuki, Y. Ando, Spectroscopic study during single-wall carbon nanotubes production by Ar, H2, and H2–Ar DC arc discharge, Diam. Relat. Mater. 14 (3–7) (2005) 887–890. C.G. Granqvist, R.A. Buhrman, Ultrafine metal particles, J. Appl. Phys. 47 (5) (1976) 2200–2216. Z.Q. Li, C.J. Lu, Z.P. Zhou, Z. Luo, X-ray diffraction patterns of graphite and turbostratic carbon, Carbon 45 (8) (2007) 1686–1695. R. Matassa, S. Orlanducci, E. Tamburri, V. Guglielmotti, D. Sordi, M.L. Terranova, M. Rossi, Characterization of carbon structures produced by graphene self-assembly, J. Appl. Crystallogr. 47 (1) (2014) 222–227. F. Tuinstra, J.L. Koenig, Raman Spectrum of Graphite, J. Chem. Phys. 53 (3) (1970) 1126–1130. A.A. Susu, A.F. Ogunye, Selective naphthene pyrolysis for ethylene with hydrogen as diluent, Thermochim. Acta 34 (2) (1979) 197–210. L.E. Gusel’nikov, V.V. Volkova, P.E. Ivanov, S.V. Inyushkin, L.V. Shevelkova, G. Zimmermann, B. Ondruschka, Direct infrared spectroscopic evidence of allyl radical and definition of the initiation step in thermal decomposition of cycloalkanes.: a comparison with very low pressure pyrolysis of alkenes, J. Analy. Appl. Pyrolysis 21 (1–2) (1991) 79–93. C. Jacobelli, G. perez, C. Polcaro, E. Possagno, R. Bassanelli, E. Lilla, Formation of isomeric terphenyls and triphenylene by pyrolysis of benzene, J. Anal. Appl. Pyrolysis 5 (3) (1983) 237–243. H.J. Singh, R.D. Kern, Pyrolysis of benzene behind reflected shock waves, Combust. Flame 54 (1–3) (1983) 49–59. A.E.F. Gick, M.B.C. Quigley, P.H. Richards, The use of electrostatic probes to measure the temperature profiles of welding arcs, J. Phys. D. Appl. Phys. 6 (16) (1973) 1941. R.M. Young, E. Pfender, Generation and behavior of fine particles in thermal plasmas—a review, Plasma Chem. Plasma Process. 5 (1) (1985) 1–37. G. Cota-Sanchez, G. Soucy, A. Huczko, H. Lange, Induction plasma synthesis of fullerenes and nanotubes using carbon black–nickel particles, Carbon 43 (15) (2005) 3153–3166. J.J. Host, V.P. Dravid, M.H. Teng, Systematic study of graphite encapsulated nickel nanocrystal synthesis with formation mechanism implications, J. Mater. Res. 13 (9) (1998) 2547–2555. S. Kawasumi, M. Egashira, H. Katsuki, Catalytic formation of graphite from benzene on iron powder, J. Catal. 68 (1) (1981) 237–241.
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A new model for the synthesis of graphite encapsulated nickel nanoparticles when using organic compounds in an arc-discharge system
T
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Yu-Chieh Huang, Mao-Hua Teng , Tun-Hao Tsai Department of Geosciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan, R.O.C.
A R T I C LE I N FO
A B S T R A C T
Keywords: Nickel nanoparticle Organic compound Pyrolysis Arc-discharge Core-shell
Herein we propose a new “three-step” model of synthesizing graphite encapsulated nickel nanoparticles (NiGEM), including the pyrolysis reaction of organic compounds which the conventional two-step model ignores. According to the results of XRD, Raman, and TEM, we found that the Ni-GEM synthesized by using both PF resin and benzene vapor as the carbon sources has two favorable characteristics: thicker shells (~5–10 nm) and smaller particle sizes (~30 nm), which are much better than those using only PF resin or PF resin mixed with cyclohexane vapor (thinner shell less than 5 nm and larger particle sizes ~50 nm). Benzene decomposes into large aromatic molecules and tiny graphitic flakes at 1200–3500K, while cyclohexane prefers to decompose and form small hydrocarbon molecules at 1000K. As a result, the two compounds go through two different reaction paths. Benzene will decompose and directly attach onto the surface of Ni nanoparticles, forming smaller sized but thicker shell structured Ni-GEM, while cyclohexane will lead to the formation of amorphous carbon coating on the Ni-GEM. By including the above two distinct hydrocarbon pyrolysis reactions, this study modifies the conventional model and successfully explains the formation processes of Ni-GEM with very different morphologies. Furthermore, the new model may help in controlling the morphologies of other GEM nanoparticles with a number of core-metals.
1. Introduction Graphite encapsulated metal (GEM) nanoparticles are a kind of novel nanocrystalline material with a core-shell composite structure; they were first synthesized by an arc-discharge method in 1993 [1,2]. Due to their distinctive core-shell structure, GEM has many special characteristics and could have numerous applications, such as stable magnetic properties, microwave absorbent, catalyst, and hydrogen storage. Therefore, graphite encapsulated “ferromagnetic” nanoparticles, e.g., Fe-GEM and Ni-GEM, have been selected as emerging materials with great potential in biomedical technologies [3,4], energy engineering fields [5], national defense industries [6], and environmental and engineering fields [7]. However, the practical applications of GEM nanoparticles have been hindered by the insufficient production rates and the inefficient synthesis methods, e.g., using conventional carbon arc-discharge system, or high power output system [8,9]. There are two major goals in this study: one is to improve the efficiency of GEM production, and the other is to explore the possible mechanism when using liquid organic compounds as the carbon sources (hereafter called liquid carbon sources). A modified tungsten arc-discharge system, using a tungsten rod as
⁎
the electrode, has been used to synthesize GEM nanoparticles since 1995 [10]. After almost two decades, Chiu et al. [11] and Teng et al. [12] discovered a method using liquid carbon sources and could synthesize Ni-GEM efficiently. They injected the liquid organic compounds, e.g., alcohols, benzene (C6H6), and cyclohexane (C6H12), directly into the arc-discharge area, and greatly increased the production rate and encapsulation efficiency of Ni-GEM up to 2.4 mg/s and 70–80%, respectively. However, when the organic compounds decomposed by the heat of the arc-discharge, their vapor will dilute the original dominant gas (helium) of the plasma. Because the electrical resistance increases sharply, the arc plasma becomes hard to control, and the electrical resistance of the arc fluctuates dramatically and frequently. Eventually, it will lead to the disruption of the whole experiment. In order to avoid this problem, the present experimental setup uses a new type of colloidal carbon source-phenol formaldehyde resin (PF resin) to synthesize Ni-GEM nanoparticles. The PF resin is a synthetic polymer with light yellow color, obtained from the condensation reaction of phenol and formaldehyde. When a PF resin is heated above 300°C by an arc-discharge, the resin will gradually graphitize and release both hydrogen and carbon vapors [13]. Additionally, a volatile organic compound, e.g., benzene or cyclohexane, is placed next to the
Corresponding author. E-mail address: [email protected] (M.-H. Teng).
https://doi.org/10.1016/j.diamond.2020.107719 Received 30 July 2019; Received in revised form 15 January 2020; Accepted 17 January 2020 Available online 17 January 2020 0925-9635/ © 2020 Elsevier B.V. All rights reserved.
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resin only, PF resin plus benzene vapor (hereafter called PF-bnz), and PF resin plus cyclohexane vapor (hereafter called PF-cyclo). Benzene or cyclohexane is injected into the ceramic crucible to provide additional carbon vapor, rather than directly injecting the liquid carbon source into the arc-discharge area in our previous work. The cathode, a tapered tungsten rod of 10 mm diameter (98% tungsten with 2% cerium), is fixed with a copper fixture and vertically locked into a tunable water-cooled based copper conductive tube. The vacuum chamber needs to be evacuated first and then purged with helium gas. Finally, the chamber will be filled with helium atmosphere at a pressure of 200 Torr before starting the arc-discharge. An arc-discharge will be generated by a DC current fixed at 120 A between the tungsten rod and graphite crucible. A typical variation of the arc current and voltage with time is shown in Fig. 2. (The upper line (blue) represents the voltage, and the lower line represents the DC current we used.) The arc current and voltage will usually fluctuate in a small range once the system has reached a stable condition. Due to the high temperature of the arc, the metal and the PF resin start to melt and evaporate, forming a high temperature metal pool. Through the nucleation growth of nanoparticles, metal vapor and carbon vapor will coalesce and condense into GEM nanoparticles. Note that the area where the coalescent processes occur is called the coalescence area. According to the two-step mechanism model [14], providing a homogeneous distribution of carbon and metal vapors is the key to improve the synthesis efficiency. However, the present setup still cannot guarantee a 100% ideal distribution; as a result, it is inevitable that the as-made powder contains some undesired ill-encapsulated metal particles and carbon debris. Consequently, the as-made products need to be purified. The purification procedures are not complicated. After each experiment, all the nanoparticles will be scraped off the chamber walls and submerged in a beaker with strong acid, i.e., nitric acid or hydrochloric acid, and then stirred in an ultrasonic bath to dissolve all the illencapsulated particles. Next, the well-encapsulated Ni-GEM nanoparticles can be separated by using a strong magnet. Through the purification procedures, the weight percentage of the purified GEM nanoparticles to the as-made GEM powders can be calculated and
anode, providing additional and uniform carbon vapor to the arc-discharge. The results of this method reveal that the morphology of the NiGEM nanoparticles is closely related to the concentration of the carbon vapors and the pyrolysis reactions of the organic compounds. The conventional two-step mechanism model was proposed by Elliott et al. in 1997 [14]. However, it failed to explain the new experimental results; for example, Teng et al. [12] found that the particle sizes of Ni-GEM significantly differ when using benzene or cyclohexane as the carbon source in the same setup. Because the conventional twostep model was developed based on the experimental results when using only solid carbon sources, i.e., graphite or diamond powder [15], it naturally cannot fully explain the phenomena that occurred when using liquid carbon sources, such as alcohols and other organic compounds. In this study, PF resin and two organic compounds vapor (benzene or cyclohexane) were used to synthesize Ni-GEM. Through observing the morphology, particle sizes, and graphite shell of the products, it was obvious that the synthesized Ni-GEM is affected by the pyrolysis reaction of each organic compound through different paths. According to these results, a new three-step mechanism model can be constructed. 2. Experimental 2.1. Materials and synthesis method Fig. 1 shows a schematic setup of the arc-discharge in a vacuum chamber. The chamber is specifically designed for synthesizing GEM nanoparticles, and is equipped with a modified tungsten arc-discharge system [10]. The anode is a graphite crucible loaded with 30 g nickel shots (3–25 mm diameter, 99.95+%) and 3 g phenol formaldehyde resins (purity 77%, phenol 11%, methanol 10%, and formaldehyde 2%); all the raw materials inside the graphite crucible will volatilize at the same time during the experiments. In addition, a ceramic crucible of aluminum oxide is placed next to the graphite crucible, so that the organic compounds injected into the ceramic crucible can be volatilized rapidly during the experiments. There are three basic setups for the carbon sources in this work: PF
Fig. 1. Schematics of the experimental setup of the modified tungsten arc-discharge in the vacuum chamber. 2
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Fig. 2. The variations of the DC current and voltage of the arc by time.
extent by switching the carbon sources from solid type to liquid type. Fig. 3 shows the improvement of the synthesis efficiency when using different types of carbon sources over the years. The blue words (the left coordinate axis) and bar chart correspond to the production rate, and the black words (the right coordinate axis) and line graph correspond to encapsulation efficiency. However, as mentioned earlier, using liquid carbon sources is prone to interrupt the arc and cause the experiment to fail. In order to simultaneously alleviate the arc interruption problem and enhance the production rate of Ni-GEM, we synthesized the Ni-GEM by using a colloidal type carbon source and a liquid organic compounds vapor. According to the experimental results, using only PF resin as the carbon source can avoid the arc extinction problem, and the production rate of Ni-GEM can greatly increase up to 5.6 mg/s. But the encapsulation efficiency can only reach about 30% (Fig. 3). It is worth noting that using only PF resin can achieve as high a production rate as when using a liquid carbon source, but the encapsulation efficiency is poor, similar to the results when using diamond powder as the carbon source [15]. In order to further explore this phenomenon, we first need to understand the pyrolysis reaction of PF resin. In 1995, Trick and Saliba
defined as the encapsulation efficiency of the experiments. 2.2. Characterization methods for the GEM nanoparticles High resolution transmission electron microscope (HRTEM) images and electron diffraction patterns were acquired by using a JEOL 2010F operated at the acceleration voltage at 200 kV. The phase and crystal structure of metal and graphite were analyzed by X-ray powder diffraction (XRD; X'Pert3 Powder, Panalytical). Micro-Raman spectroscopy was recorded by HORIBA-FHR1000 with laser beam size of 2 μm, and the wavelength of GaAlAs semiconductor laser was 532 nm. The average particle sizes of GEM nanoparticles were mainly derived from BET (specific surface area, NOVA2000, Quantachrome) analysis. 3. Results and discussion 3.1. Synthesis efficiency of Ni-GEM when using only PF resin as the carbon source Low production efficiency has been one of the biggest problems of the synthesis of GEM. However, the problem has been solved to a large
Fig. 3. Comparison of the encapsulation efficiencies and production rates of three selected previous studies that use different type of carbon sources. 3
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[16] investigated the gaseous product of the PF resin after pyrolysis, and found that the gas molecules contained 85% hydrogen when the temperature reached 560°C. Note that the existence of hydrogen in the arc plasma area will not only increase the working temperature of the arc, but also form unstable sp2 hybridization dangling bonds at the edge of the carbon materials, causing an etching reaction of the carbon [17]. Moreover, in 2000, Ko et al. [18] used XRD and Raman spectra to observe the microstructure changes of the solid PF resins between 300 and 2400°C; when the temperature is higher than 500°C, the resin will start to carbonize and graphitize, forming new glassy carbon nanoparticles. Based on the above studies, we know that the pyrolysis reaction of PF resin, besides generating a lot of hydrogen, is similar to that of the diamond powder in a molten metal pool; both will graphitize gradually at high temperature and provide carbon source to the arc plasma. Not surprisingly, their encapsulation efficiencies are very similar, but the pyrolysis of PF resin can also release hydrogen and form dangling bonds to trigger an etching reaction. As for the results, hydrogen plasma can help to increase the temperature and accelerate carbon formation [19,20], thus increasing the evaporation rate of raw materials and forming more nanoscale carbon. That is the reason why the production rate of using PF resin is higher than when using diamond powder, and why although the production rate can reach up to 5.6 mg/s, the encapsulation efficiency is only ~30%.
Fig. 4. The average particle sizes of Ni-GEM nanoparticles calculated by XRD, BET and TEM by using four carbon source setups. Note that the last setup shows no difference with PF-bnz.
calculated by two different methods; the first method used full width at half maximum (FWHM) of XRD diffraction peaks by Scherrer equation, with the results representing the “crystal” sizes; the second method is BET analysis that assumes a density of the nanoparticles and calculates the spherical particle size. Not surprising, both methods show the same trend: the sizes of the Ni-GEM nanoparticles made from PF-cyclo are larger than those when PF-bnz is used as carbon source. In 1976, Granqvist and Buhrman [21] found a positive correlation between particle size and gas pressure; that is, the higher the gas pressure, the larger sized particles that would be derived. The gas pressure in our chamber changes with time when using different organic compounds, as shown in Fig. 5. Once the organic compounds vaporize in the arcing vacuum chamber, they will start breaking up and re-bonding between or within molecules to form different types of organic vapors, and also simultaneously increase the chamber pressure. The highest pressure derived when using PF resins, PF-cyclo, and PFbnz as the carbon sources were 289 Torr, 622 Torr, and 490 Torr, respectively, and the results seem irrelevant to the particle sizes of GEM. Moreover, the existence of hydrogen may also affect the operating temperature of the arc [11] and increase the vaporization rate and the collision frequency. Because both the number of collisions and raw material vapor increased, the effective collision probability between these nanoparticles also increased. Consequently, it is prone to form larger sized particles. In order to determine whether the hydrogen or gas pressure will affect the particle sizes, a special PF-bnz experiment with 40 ml benzene was conducted, i.e., providing the same amount of hydrogen as in a PF-cyclo experiment where the cyclohexane is 20 ml. As the result, the average particle size was only 28.0 ± 0.9 nm, indicating that the particle size of Ni-GEM is neither dependent on gas pressure nor on hydrogen content. To further investigate the causes of the different particle sizes, we needed to observe the morphology of the products by HRTEM images, as shown in Fig. 6. The core-shell composite structure of Ni-GEMs can be clearly seen. When Ni-GEM were synthesized using PF resins only or PF-cyclo, a thinner shell with 2–4 graphite layers and thickness less than 3 nm was found in Figs. 6a and 6b. Also, many crystalline carbons with layered structure and amorphous carbon appeared besides the nanoparticles when using PF-cyclo. In contrast to the former images, the Ni-GEMs synthesized using PF-bnz had a thicker shell of about 5–10 nm (Fig. 6c and d) According to the results of the HRTEM micrographs, the products made from PF-cyclo and PF-bnz significantly differ due to the particle sizes and thickness of the graphitic shell. In 1998, Setlur et al. [17]
3.2. Second carbon sources: benzene and cyclohexene In Section 3.1, we discussed the efficiency when using a new type of colloidal carbon source PF resin, which can not only successfully avoid the arc extinction problem, but also improve the production rate of NiGEM. However, the encapsulation efficiency is disappointingly low, ~30%. In order to enhance the encapsulation efficiency, a second carbon source was added into the experimental setups to provide additional and uniform vaporized carbon source near the arc-discharge area. Table 1. shows the improvement of synthesis efficiency when adding additional organic compound vapors. Regardless of using benzene vapors or cyclohexane vapors, it can be clearly seen that both the production rates and encapsulation efficiencies increase up to ~8.1 mg/ s and ~81%, respectively. It is not difficult to conclude that adding vapor carbon sources can improve the encapsulation efficiency. Presumably, the nanoparticles have more chance to collide with carbon molecules than just in using only PF resin as the carbon source, and consequently increase the production rate and encapsulation efficiency. 3.3. The morphology changes of Ni-GEM nanoparticles Although the results of the experiments show no differences in the synthesis efficiency between using benzene or cyclohexane, we may still obtain some clues from their different particle sizes, crystal phase, and thickness of graphitic shells of the Ni-GEM. Fig. 4 shows the average particle sizes of Ni-GEM when using different carbon sources setups. The average particle sizes in using PF resin only and PF-cyclo are 39.3 ± 4.7 nm and 39.1 ± 2.6 nm, respectively. Compared to the above results, the particle sizes when using PF-bnz are much smaller, ~24.3 ± 0.7 nm. The particle sizes were Table 1 The production rates and encapsulation efficiencies of the three carbon source setups in this work: PF resin only, PF-cycle, and PF-bnz. Carbon source
Production rate (mg/s)
Encapsulation efficiency (%)
PF resin only PF-cyclo (20 ml) PF-bnz (20 ml)
5.6 8.1 8.1
29 79 81
4
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Fig. 5. The variations of the gas pressure with time while using different organic compounds as carbon sources.
Fig. 6. (a), (b) The TEM images show a thin shelled Ni-GEM nanoparticle. There is some amorphous carbon (red arrow) when using a PF-cyclo setup. (c), (d) are the Ni-GEM nanoparticle with a thick shell when using a PF-bnz setup. 5
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In order to investigate the detailed structure of the graphite shell, we focused on analyzing the (002) graphite peak for a more in-depth discussion. Fig. 9 shows the precise diffraction pattern at 2θ = 20–30° with 1°/min slow scanning speed. The main (002) graphite diffraction peak at 2θ ≈ 26.5° can be clearly seen in each pattern; another shoulder peak at 2θ ≈ 26.0° can only be seen after adding the organic compound vapors. By calculating the interlayer spacing of the carbon, the diffraction peak at 2θ = 26.0° and 2θ = 26.5° represent 0.343 nm and 0.336 nm, respectively; the standard d-spacing of the graphite should be 0.336 nm. Li et al. [22] and Matassa et al. [23] found that this phenomenon may be related to the decreased size and twisted nanostructure, causing a disordered stacking. Although the exact structure of the d-spacing = 0.343 nm remains unclear, the two different kinds of carbon can be easily found in the analysis of the XRD while using organic vapors. More detailed information on the structure of the graphitic shells of the Ni-GEM can be obtained from Raman spectra, as shown in Fig. 10. In 1970, Tuinstra and Koenig [24] first proposed an empirical formula by using the relative intensity of the D band and G band (ID/IG, R) to calculate the crystal size of the graphite (La). The G band, also called the graphite band, is located at 1580 cm−1, indicating the sp2 hybridized electron structure of carbons. The D band is associated with the defects or disordered structures around 1355 cm−1, indicating the sp3 hybridized carbons. The values of the R and crystal sizes were calculated and are listed in Table 2. When only PF resin was used as the carbon source, the spectra show a stronger G band than the D band, meaning a smaller R value and a large crystal size of graphite. In contrast, the
found the relation between hydrogen concentration and the consumption rate of carbon materials; when carbonaceous materials are under high concentration of hydrogen atmosphere, it will cause lots of unstable dangling bound at the edge of the carbon structure, causing carbon‑carbon to break more easily and form related hydrocarbon. For a more detailed discussion, an experiment using naphthalene (C10H8) as the lower hydrogen content carbon source was carried out to investigate the relation between the thickness of the graphite shell and the hydrogen concentration. If the hydrogen concentration is the main factor affecting the formation of graphite shell, the naphthalene products should have a thicker graphite shell, but this does not turn out to be the case. The product synthesized by naphthalene vapor has a thickness of graphite shell (2–4 graphitic layers with onion-like carbon particles) similar to that of cyclohexane products, which means that the shell thickness is not directly related to the hydrogen concentration; thus, the hydrogen concentration is not the main factor influencing the formation of shells. The HRTEM images also reveal the lognormal size distributions of Ni-GEM through actual measuring and counting methods. All the lognormal distributions show only one peak, meaning the resin and organic compound didn't produce very different and independent sized particles. The distribution of Ni-GEM is shown in Fig. 7. In addition to TEM images, we also used XRD and Raman spectroscopy to analyze the graphite shell in each product. Fig. 8 shows the Xray diffraction patterns of the purified Ni-GEM nanoparticles. The diffraction peaks of face-centered cubic structure nickel have much higher intensity than the graphite (002) peak at 2θ = 26.5°.
Fig. 7. The lognormal size distributions of Ni-GEM nanoparticles when using different carbon sources: (a) PF resin only, (b) PF-bnz (20 ml), (c) PF-cyclo (20 ml), and (d) PF-bnz (40 ml). 6
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Fig. 8. (a) The X-ray and (b) the electron diffraction patterns of the purified Ni-GEM nanoparticles.
Fig. 9. The precise XRD diffraction pattern at 2θ = 20–30° by using 1°/min slow scanning speed and the deconvolution peaks of Ni-GEM nanoparticles.
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Table 3 The relative reaction temperature and pyrolysis products of cyclohexane and benzene.
Table 2 The values of the R and crystal sizes by different raw materials. The relative intensity (ID/IG, R) and crystal size of the graphite (La) were calculated by following the method of Tuinstra-Koenig. ID/IG
La (nm)
PF PF PF PF
0.89 0.83 0.77 0.48
4.87 5.22 5.61 8.87
+ 40 ml Benzene + 20 ml Benzene + 40 ml Cyclohexane resin
Reactant
Products
1023–1173
Cyclohexane (C6H12)
1273–2200
Benzene (C6H6)
1-Butene (C4H8) 2-Butene (C4H8) Butadiene (C4H6) Ethylene (C2H4) Phenyl (C6H5) Biphenyl (C12H10) Triphenyl (C18H14) Acetylene (C2H2) Cyclopentadienyl (C5H5)
complicated; we can easily discover the complexity from the two compounds adopted in this work, i.e., cyclohexane and benzene. While the cyclohexane tends to form amorphous carbon on the surface of nanoparticles, benzene tends to form a graphitic shell. In addition, several other differences are also very obvious, such as particle sizes, thickness of the graphite shell, and graphite structures. To explain the differences, we must first investigate the pyrolysis behaviors of cyclohexane and benzene. The related reaction temperatures and pyrolysis products of cyclohexane and benzene are listed in Table 3, shows the different pyrolysis behaviors in a high-temperature area. The initial reaction temperature of cyclohexane is about 1000K [25]. Due to the easily broken single bond of the carbons, most reaction products are small hydrocarbon molecules [26], such as alkane and alkene substances. In contrast, the initial reaction temperature of benzene is 1273K [27,28]. Because of the stable structure of the aromatic rings, the pyrolysis reaction tends to form larger aromatics molecules, e.g., biphenyl (C12H10), triphenyl (C18H14), naphthalene (C10H8), alkyne, etc. Although the final products of both reactions are all stable graphitic structured materials, cyclohexane needs to degrade first in a relatively low temperature area, as shown in Table 4, while benzene can directly condense into crystalline graphite through phenyl radical molecules in a higher temperature area. Following the above discussions and guidance of the two-step mechanism model [14], the new model was proposed, as shown in Fig. 11. When the arc-discharge is generated, nickel and PF resins will melt and evaporate immediately because of the high temperature in the arc plasma center (about 12,000K); the ambient temperature (estimation of temperature is about 5000K as the plasma tail flame) [29–31]. Atoms will start to form “liquid-like” nanoclusters and continue to grow between temperatures at roughly 2300–1000K [32], until the temperature drops down to the lower boundary of the coalescence temperature. At this stage, the conventional phase segregation process will soon stop working. Note that the temperature distribution map in Fig. 11 simply shows the relative position between the arc plasma shape (bell shape) and the main coalescence area. If benzene is the chosen carbon source, the pyrolysis of benzene will form nano-sized graphitic molecules and attach directly onto the surface of the nanoparticles, forming a thicker shell structure. Then the thick outer shell will stop the continuing coalescence growth of the metal core, as shown in route A in Fig. 11. Moreover, benzene vapor has
Fig. 10. Raman spectra of the Ni-GEM nanoparticles synthesized by using different carbon source setups.
Raw materials
Temperature (K)
intensity of D band became stronger after adding organic vapors, which means the average crystal sizes of graphite decreased from 8.87 nm to 4.87 nm. After analyzing the above experimental data, it seems the newly formed carbon material has two special features that differ from the original graphitic shells: a larger d-space and smaller flaky crystal size.
3.4. New “three-step” mechanism model The new model is an extension of the existing conventional two-step mechanism model. The two-step model separates the formation evolution of GEM into two steps: phase segregation and catalyzation. When the carbon and metal vapor leave the arc-discharge plasma, they quickly cool and coalesce into numerous liquid-like nanoparticles. During the cooling process, the carbon will crystallize into graphite first and then segregate to the surface of the nanoparticles, thus totally or partially encapsulating the inner metal core. The collision and coalescing process between the nanoparticles continues until either the nanoparticles are too cold and rigid, or the encapsulated graphite is thick enough to prevent further coalescence. The second step “catalyzation” is specifically for certain catalyzing metals that have the ability to catalyze amorphous carbon into graphite, such as Fe, Co, and Ni. These metals can catalyze the surface amorphous carbon into graphite, making it possible for any exposed metal surface to be encapsulated by graphite at a later stage. However, Elliott et al. [14] also mention that the second step is only a support mechanism, and that the phase segregation is the main mechanism. The conventional two-step mechanism model does not consider the possibility that carbon vapor may be provided inwardly, i.e., from chamber atmosphere to the hot coalescence area. Thus, a new third step is needed to include the decomposition of liquid carbon sources. However, the pyrolysis reactions of hydrocarbon compounds are
Table 4 The relative reaction temperatures and pyrolysis products of Acetylene and Ethylene.
8
Temperature (K)
Reactant
Products
773
Acetylene (C2H2)
936
Ethylene (C2H4)
Vinylacetylene (C4H4) Benzene (C6H6) Phenyl (C6H5) Methane (CH4) Ethane (C2H6) Acetylene (C2H2)
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Fig. 11. Schematic diagram of the new three-step mechanism model of synthesize the Ni-GEM nanoparticles. The two distinct paths of the two different pyrolysis behaviors of (A) benzene and (B) cyclohexane.
amorphous carbon when using cyclohexane as carbon source, and the other is the graphitic shell when using benzene. The conventional two-step mechanism model did not consider the carbon sources provided from the outside of the arc-discharge. In this study, we successfully constructed a new “three-step” mechanism model, with the addition of the above two paths of pyrolysis reactions to the existing two-step model. The model can qualitatively explain the observed various characteristics of the Ni-GEM nanoparticles by HRTEM, XRD, BET analysis, and Raman spectra. However, the proposed three-step mechanism model is not limited to Ni-GEM; it can also be used to explore the synthesis of other encapsulated metal materials that were previously thought to be impossible to synthesize.
been shown to form graphene at the surface of the catalytic metal under 1373K by Kawasumi et al. [33] in 1981, indicating that using benzene to form nano-flake carbon materials in the arc-discharge system is possible. In contrast to benzene, cyclohexane cannot affect the coalescence growth of the nanoparticles; it can only form an amorphous carbon coating on the outside, and some crystalline carbon flake aggregation along with Ni-GEM nanoparticles, as shown in route B in Fig. 11. The organic compound pyrolysis arc-discharge method can synthesize not just Ni-GEM, as demonstrated in this study, but the method can also be used in synthesizing many other core-metaled GEM. The method, as shown in three-step model, can synthesize GEM through step three only, without the previous two steps. This means there is almost no limitation in choosing raw core metal materials to synthesize GEM nanoparticles. Though we have developed a useful model that can satisfactorily explain the experimental results qualitatively, a more quantitative description remains elusive. Much more work is needed in order to fully understand the intriguing and complicated mechanism in detail.
Yu-Chieh Huang:Conceptualization, Methodology, Resources, Writing - original draft.Mao-Hua Teng:Project administration, Funding acquisition.Tun-Hao Tsai:Validation, Formal analysis, Writing - review & editing.
4. Conclusions
Declaration of competing interest
CRediT authorship contribution statement
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The synthesis of Ni-GEM nanoparticles by using organic compound pyrolysis is a much more effective way in contrast to previous studies. Using PF resin as the carbon source can improve the production rate of Ni-GEM, from 2.4 mg/s when using injecting liquid carbon sources to 5.6 mg/s. Moreover, the use of organic vapors, e.g., cyclohexane or benzene, can dramatically increase the production rate and encapsulated efficiency up to 8.1 mg/s and 81%, respectively. The pyrolysis of organic compounds during the experiments will form two major carbon materials attaching to the surface of GEM: one is
Acknowledgment The authors would like to special thank Prof. Cheng-Yen Wen and Mr. Chuan-Yu Wei (Department of Materials Science and Engineering, National Taiwan University) for the support and operation of the 9
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HRTEM. This work was supported by the Ministry of Science and Technology, Taiwan, Grant MOST 106-2116-M-002-023 and MOST 107-2116-M-002-012.
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