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From carbon dots to multipods — The role of nickel particle shape and size
Diamond & Related Materials 72 (2017) 53–60
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From carbon dots to multipods — The role of nickel particle shape and size Manoko S. Maubane a,b, Shrikant S. Bhoware b, Ahmed Shaikjee b, Neil J. Coville b,⁎ a b
Microscopy and Microanalysis Unit, University of the Witwatersrand, Johannesburg 2050, South Africa DST-NRF Centre of Excellence in Strong Materials and the Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg 2050, South Africa
a r t i c l e
i n f o
Article history: Received 5 October 2016 Received in revised form 9 December 2016 Accepted 31 December 2016 Available online 3 January 2017 Keywords: Carbon nanofibers Trichloroethylene Catalyst morphology Nickel CVD
a b s t r a c t In an attempt to synthesize carbon tripod structures in high yield, we have observed that a wide range of carbon structures are formed as the Ni particle size and shape varied. In using trichloroethylene (TCE), acetylene and their mixtures as a carbon source to make carbon materials over Ni nanoparticle catalysts in a chemical vapor deposition (CVD) process, we have observed that the Ni particle size and shape impacts dramatically on the carbon structure formed. The synthesis of the Ni catalyst was achieved by reduction of Ni(acetate)2 with hydrazine (35%). Ex-situ TEM analysis of the reduced NiO particles revealed that the Ni particles underwent a morphological change in the presence of TCE (and H2) with change in (i) temperature (350–750 °C) and (ii) H2 flow rate (20–150 mL/min) and that this affected the shape of the carbon material that formed. In the absence of TCE only particle sintering occurred. At T = 350 °C the Ni particles are covered by carbon to form carbon dots. With an increase in T the carbon shapes changed to a bimodal (400 °C) and then to a tripod-like structure (450 °C) and eventually multipod-like structures (500 °C). Carbon fibers were mainly observed for T N 500 °C. Similar shape changes were observed at 450 °C as a function of reaction time and H2 flow rate. It was also found that when acetylene or an acetylene/trichloroethylene mixture was used at T = 450 °C, helical and linear fibers were produced. The TEM data reveal that the fragmentation and sintering of the Ni (by the carbon source and other factors) to give differently sized Ni particles, especially as it is affected by H2, determines the morphology of the carbon materials grown from the Ni. The TCE has the ability to restructure the Ni particles and this leads to the unusual carbon shapes observed. Thus the carbon and the catalyst influence each other during the growth of the carbon materials. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The process of producing structured carbon nanomaterials by decomposition of various hydrocarbons has attracted the attention of many researchers. Thus different shaped carbon nanomaterials such as carbon nanofibers (CNFs) [1–4], carbon nanotubes (CNTs) [5–7], as well as carbon spheres (CS) [8–10], have been produced using various sources of hydrocarbons. The CNFs produced generally have diameters between 3 and 100 nm and lengths between 0.1 and 1000 μm. The recent interest in CNFs and their potential applications emerges from their relationship to carbons with graphite like layers such as fullerenes and CNTs; e.g. for use as catalyst supports [11–15]. CNFs produced during the catalytic conversion of carbon containing gases were initially regarded as a nuisance material [11]. Studies have shown that the choice of hydrocarbon, i.e., the carbon source does influence the resulting carbon product produced [16–18]. The different hydrocarbon reactants will influence the type and amount ⁎ Corresponding author. E-mail address: [email protected] (N.J. Coville).
http://dx.doi.org/10.1016/j.diamond.2016.12.023 0925-9635/© 2017 Elsevier B.V. All rights reserved.
of carbon radicals, compounds, or species formed in the reaction zone and these impact on the CNF growth mechanism. Shaikjee et al. [19] described the effect of the use of substituted alkynes over a nickel catalyst and reported that different alkyne reactants affected the catalyst morphology and hence the morphology of the extruded carbon filaments. The use of chlorinated hydrocarbons (CHCs) as a carbon source for the synthesis of structured carbon nanomaterials has also been studied [20–23]. CHCs produced in the chemical industry (e.g. PVC) are utilized in many household and industrial products. However, these materials can break down to give toxic compounds and the materials are often found in industrial wastes [24]. The disposal of chlorinated hydrocarbon containing materials is typically achieved in hazardous waste landfills and/or by incinerator procedures [25]. Conversion of these chlorinated waste streams into a high value commodity material; e.g. carbon shaped materials, would provide yet another outlet for the waste materials. It has been reported that chlorinated hydrocarbons are effective carbon precursors that can make a range of shaped carbon materials and they are reported to produce better yields of carbon materials when compared to their equivalent non-chlorinated compounds [26–28].
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All chemicals were purchased from Sigma Aldrich and were used as received unless otherwise stated.
measured) to give the width/diameter of the CNFs. It was assumed that the Ni particle correlated with this diameter and that this allowed for the determination of the Ni particle size. This assumption was also used to determine the sizes at low carbon coverage, where the Ni particles tended to be irregular in shape. Thermal stability of the CNFs was monitored using a Perkin Elmer Thermogravimetric analyser (TGA) 4000. In a standard run, CNFs (10 mg) were placed into a high temperature alumina cup that was supported on an analytical balance located in the furnace chamber. X-ray diffraction (XRD) images were collected on a Bruker D2 phaser in Bragg Brentano geometry with a Lynxeye detector using Cu-Kα radiation at 30 kV and 10 mA. The scan range was 10° b 2θ b 90° in 0.040 steps, using a standard speed with an equivalent counting time of 1 s per step. The diffraction peaks were then compared with those of standard compounds reported in the Diffracplus evaluation package using the EVA (V11.0, rev.0, 2005) software package. Temperature programmed reduction (TPR) data were collected with a Micromeritics Auto Chem II unit. The catalyst (approximately 50 mg) was placed in a quartz tubular reactor and 5% H2/Ar was passed through the reactor (50 mL·min−1) as the temperature was raised at a rate of 10 °C·min− 1 from 50 to 850 °C. The quality of the CNFs was studied using a Jobin-Yvon T64000 Raman spectrometer. The external morphology of CNFs was studied using Field Electron and Ion (FEI) focused ion beam/scanning electron microscopy (FIB/SEM) on a Nova Nanolab 600.
2.1. Ni nanoparticle synthesis
3. Results and discussion
Ni nanoparticles (NPs) were prepared by a micro emulsion technique. In a standard reaction, 1.5 g of cetyltrimethylammonium bromide (CTAB) was dissolved in a mixture of methanol (10 mL), hexanol (10 mL) and xylene (25 mL). Ni(acetate)2 (1.0 g) was added to the reaction mixture and the mixture was stirred for 30 min at room temperature. Ni2 + was reduced by the dropwise addition of 3 mL hydrazine (35%) to the solution to form Ni nanoparticles (NPs). The precipitate was filtered and washed with 2-propanol, dried at 120 °C and calcined at 350 °C for 6 h to yield about 0.27 g of catalyst.
3.1. Characterization of the NiO catalyst
Studies using TCE as a carbon source to make shaped carbon materials has been little studied. Nieto-Marquez et al. [20] reported on the catalytic growth of a structured carbon from chloro-hydrocarbons. They produced linear fibers and carbon spheres at T = 500 and T = 650 °C respectively using TCE over a Ni/SO2 catalyst. When a non-chlorinated carbon source (acetylene) was used under identical reaction conditions, tubular structures were observed. Shaikjee and Coville [28] have reported that TCE was converted to tripod CNFs over a Ni zerogel catalyst. This study reports on the use of trichloroethylene (TCE), acetylene and their 1:1 mixtures as a carbon source to make variously shaped carbons over a Ni catalyst. What is remarkable in the study is that a wide range of carbon structures can be made carbon dots (Cdots), bimodal, trimodal, polymodal, straight CNFs, coiled CNFs) using the same Ni catalyst and that the structures can be correlated to the size and shape of the Ni particles. In this study the effect of varying reaction parameters (temperature, N2 and H2 flow rate) on the carbons produced using TCE are described. 2. Experimental
2.2. Carbon synthesis The different carbons were synthesized using a catalytic CVD method. The Ni catalyst (4 mg) was spread over a quartz boat (12 mm × 28 mm) at room temperature and placed in the centre of the quartz tube reactor (18 mm × 310 mm) that was then placed in a furnace. The furnace was then heated at 10 °C per minute while hydrogen was passed over the catalyst. Once the reaction temperature was reached (350–750 °C), the H2 flow rate was maintained at the desired temperature for 30 min prior to the introduction of TCE into the system. Different H2 flow rates were also used (20, 50, 80, 100 and 200 mL/min). Once the catalyst was reduced, nitrogen was then bubbled through TCE (held at 50 °C) at different flow rates (20, 50, 80, 100 and 120 and 150 mL/min). H2 was also used as a carrier gas. All reactions were carried out for 30 min. In separate reactions, acetylene and acetylene/TCE mixtures were also used as carbon sources. After the reaction, the carbon material formed was collected after cooling the reactor to room temperature under a constant flow of H2 (100 mL/min). 2.3. Characterization techniques CNFs and Ni were characterized by transmission electron microscopy (TEM) using a Spirit T12 instrument. About 0.5 mg of a sample to be analyzed was placed in a glass vial containing 2 mL of methanol. The mixture was then sonicated for 10 min to give a homogeneous suspension of CNFs in the solvent. A drop of the suspension was then spread on a Structure Probe, Inc. (SPI) carbon copper grid (200 mesh) and allowed to dry at room temperature. The grid was then mounted onto an exchange rod and placed into the TEM chamber and analyzed. CNFs sizes were measured from TEM images (typically about 100 particles
The morphology of the synthesized Ni catalyst was characterized using TEM. The Ni catalyst was calcined in air at 350 °C (5 °C/min heating rate) for 6 h to give NiO. Fig. 1 shows the TEM image of the synthesized NiO catalyst. The NiO catalyst particles can be seen as small agglomerates that formed clusters that took a spherical shape (Fig. 1a; see supplementary Fig. S1 for larger TEM image). The XRD pattern of the synthesized NiO confirmed that NiO had been synthesized with no other diffraction impurities (Fig. 1a inset, supplementary Fig. S2a). The peaks appearing at about 2θ = 37.2°, 43.2°, and 62.8° are indexed as (111), (200), and (220) respectively and represent the face-centered cubic (fcc) crystalline structure of NiO [29]. The NiO catalyst was further characterized using TPR to study its reduction behavior (supplementary Fig. S2b). The TPR profile shows a single reduction peak at 421 °C. The catalyst was then treated in H2 at different temperatures (400–500 °C) to study its behavior under H2. The flow rate of H2 was set at 100 mL/min and the catalyst was reduced for 30 min to give Ni particle catalysts. The TEM images of the catalyst reduced at different temperatures (400, 450 and 500, 600 and 750 °C) are shown in Fig. 1. It is observed from the TEM image that the catalyst at 400 °C is in made up of small particles that form near spherical agglomerates. This is more clearly seen in supplementary Fig. S1. When the reduction temperature was increased to 450 °C, the Ni crystallites sinter to give larger particles with smoother surfaces. At 500 and 600 °C, it is observed that the Ni particles sinter to give even bigger particles (Fig. 1d and Fig. 1e respectively). It is however not possible to quantify the size of the Ni particles as they increase in temperature. Interestingly, a further increase in temperature (750 °C) results in catalyst particle fragmentation to form smaller particles. 3.2. Synthesis of nano carbons: temperature study Reaction temperature is expected to have an influence on the type of carbons that are generated when TCE decomposes in the CVD process [30,31]. Differently shaped/sized carbons were synthesized at the different temperatures (350, 400, 450, 500, 600, and 750 °C; same N2 and H2 flow rates). In the absence of H2, no reaction took place. TEM images (Fig. 2) show that the morphology of the synthesized CNFs and Ni particles changed dramatically with temperature. At 350 °C, the Ni catalyst
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Fig. 1. TEM image of (a) NiO (inset shows corresponding XRD pattern of NiO). Images of the Ni particles after reduction are shown in (b) 400 °C, (c) 450 °C, (d) 500 °C, (e) 600 °C and (f) 750 °C under a constant flow of H2 (100 mL/min); reduction was carried out for 30 min.
is covered with a layer of carbon (shell thickness ca. 100 nm) to give Ni filled carbon dots (Fig. 2a). Carbon dots synthesized, by a CVD method using a Ni/Al composite catalyst and a CH4/H2 mixture at 600 °C, have been reported by He et al. [32]. This procedure produced a mixture of both carbon dots and CNTs. Similar results were reported more recently by Lopez et al. [33]. At 400 °C, the resulting CNFs show both monomodal and bimodal growth of carbon from the Ni particles. At this temperature, the carbon growth is still minimal and the length of the CNFs is b 200 nm. The Ni particles have different shapes and sizes and are typically N200 nm (longest dimension). This may be due to the limited decomposition of TCE at this low temperature. Nieto-Marquez et al. [16] also reported that there was a negligible carbon growth at this temperature (400 °C) using a Ni/SiO2 catalyst. At these low temperatures the NiO is reduced to give Ni catalyst particles (that have sintered relative to the
NiO starting material) but as the growth is slow, the particles are only covered with the carbon with no CNF growth seen. When the reaction temperature was increased to 450 °C (and 500 °C), major catalyst restructuring occurred which yielded tripodlike and multipod-like carbon structures, respectively (Fig. 2c, d). Fig. 2c reveals two types of carbons have been produced from two sizes of Ni particles. The larger Ni particles are seen to generate tripod [34] (Fig. 2c insert) and multipod CNFs. These Ni particles are not made up of aggregates of smaller Ni particles [28]. The Ni particles have previously been shown to have a tetrahedral shape (triangular shape in TEM 2D images [28]) with CNFs growing from 3 faces. Some multipod CNFs (N3 CNFs growing from a Ni particle) can also be seen (Fig. 2c). The smaller Ni particles show bimodal growth of CNFs from these particles. Thus two processes are operating on the Ni particles as they are formed from NiO at 450 °C, (i) fragmentation and (ii) sintering.
Fig. 2. TEM images of CNFs synthesized at (a) 350 °C, (b) 400 °C, (c) 450 °C, (d) 500 °C, (e) 600 °C and (f) 750 °C, flow rate of N2 and H2 = 100 mL/min each and reaction time = 30 min.
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Fig. 3. TEM images showing CNFs (a) at low magnification to indicate the large and small Ni particles and (b) a higher magnification image showing the small particles (450 °C, flow rate of N2 and H2 = 100 mL/min each and reaction time = 30 min).
The competition between the two processes is further illustrated in Fig. 3. It can be seen in Fig. 3a that the CNFs grow from an agglomerated Ni particle and as the agglomerated Ni particles break apart bimodal growth of CNFs occurs from these small particles (circled). Fig. 3b shows a higher magnification image of the bimodal CNF growth pattern. The larger particles undergo a shape change in the presence of the carbon source and this leads to the tripod CNF growth pattern seen [28]. The size distribution of the Ni particles (measured from the CNF diameters) are shown in Fig. 3(c, d). The smaller particles have sizes ca. 60 nm while the larger particles have sizes N150 nm. Thus, the size of the Ni particle influences the carbon growth from the particle. At a higher temperature (500 °C) the Ni particles again show two size ranges (inset to Fig. 2d) leading to bimodal growth from the smaller particles and multipods from the larger particles. The larger particles become even larger than those seen at 450 °C (due to sintering) and have a multifaceted surface that leads to multipod formation (N3 CNFs growing from a Ni particle) (Fig. 2d). Thus the number and direction of CNFs that grow from the Ni particles is determined by the size and shape of the Ni particle. At 600 °C small Ni particles dominate and no agglomerated Ni clusters are seen and as a consequence more CNFs that are bimodal are observed (Fig. 2e). The larger Ni particles are seen to be associated with linear CNFs (Fig. 2e inset). At 750 °C, due to the morphology of carbon formed, identification of the Ni catalyst particles proved difficult, thus the CNF diameters were used to determine the Ni particle sizes. The large diameter fibers (ca. N 150 nm) were different from those produced at lower temperatures (Fig. 2f). Interestingly, no carbon growth was observed at temperatures N750 °C. It is assumed that the Ni particles had become too large (sintered) at the high temperature and were now unable to grow CNFs. While it is difficult to correlate the Ni particles produced by H2 reduction (Fig. 1) with Ni particles (and CNFs) produced after carbon addition and reduction (Fig. 2) both sets of data show similar types of
changes. Thus, as the temperature increased in both cases sintering was enhanced and as the Ni particles increased in size the CNF growth patterns changed. Above a Ni particle size (ca. N 150 nm) and at high temperature (N 600 °C) classical CNF growth was observed. In summary, the changes seen in the CNF morphology between 450 and 750 °C are due to both fragmentation of reduced Ni/NiO clusters and sintering of the Ni catalyst particles as a function of temperature. It is also expected that as the temperature is altered, the composition of the gas phase intermediates and radical species change and that this can influence the sintering process [31,35]. This effect can produce different types of CNFs either by direct interaction with Ni (no shape change) or by indirect action and modification of the Ni surface/shape. Our study shows that the growth of the unusual tripod structures occurs in a very narrow temperature regime. Based on these results, and to gain further insight into the bimodal and multimodal process, 450 °C was selected to be the optimum reaction temperature for further study. At this temperature, Ni particles have grown to the appropriate size to lead to the diverse CNF structures seen, with dominance of tripods being formed. The thermal analysis of the samples synthesized at different reaction temperatures (400–750 °C) was carried out from room temperature to 900 °C under a continuous flow of air at a heating rate of 10 °C/min. The TGA and the corresponding derivative plots of the synthesized CNFs are shown in Fig. 4(a) and (b) respectively. The results show that there is no significant weight loss in air until 470 °C. This indicates that there is little or no amorphous carbon present in the synthesized material; amorphous carbon is known to decompose in the temperature range 300–500 °C [36]. The complete oxidation of CNFs is observed above 550 °C. The derivative thermogravimetric plot of the CNFs is shown in Fig. 4(b). The maximum decomposition temperature of the CNFs synthesized at 400, 450, and 500 °C, were found to be 628, 595 and 587 °C respectively. This shows that as the reaction temperature was increased from 400 to 500 °C, there is a significant decrease in thermal
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Fig. 4. TGA and the DTG plots of CNFs synthesized at (a) 400 °C, (b) 450 °C, (c) 500 °C, (d) 600 °C and (e) 750 °C, flow rate of N2 and H2 = 100 mL/min each and reaction time = 30 min.
stability of the CNFs. When reaction temperature was further increased to 600 and 750 °C, the DTG plot shows two peaks. The presence of two maxima in the DTG plot could correspond to two different forms of carbon. At 600 °C, there is a small peak at 636 °C and a much sharper peak at 672 °C. According to Porwal et al. [37], this kind of behavior is observed when both defected and non-defected CNFs are present in a sample. Defected CNFs are physically and chemically weaker and therefore are oxidized at lower temperatures when compared to the defect free CNFs that will decompose at much higher temperatures. When the temperature was further raised to 750 °C, the CNFs became more thermally stable. The added stability noted at 750 °C is usually associated with the enhanced graphitic nature of the CNFs. This increase is usually achieved by high temperature annealing [37]. The TGA of materials synthesized at 350 °C have not been reported due to the extremely small yields of product obtained.
centered at 100 nm. This is associated with the small amount of TCE that enters the system leading only to carbon coverage of Ni. When the N2 flow rate was increased to 80 mL/min, bimodal and also trimodal CNF growth is clearly seen. At this increased flow rate, the carbon content increased and the carbon deposit started to build up in two opposite directions leading to the bimodal growth (Fig. 5b). The Ni particle diameter measured from the CNF diameter was centered at 200 nm but the Ni particle can be seen to be flattened with two CNFs growing from each Ni particle (Fig. 5c). The carbon has clearly caused reconstruction of the Ni surface. A mixture of small and large CNFs (and hence Ni particles) were formed. An increase in N2 content from 80 to 100 mL/min gives trimodal CNF growth plus other shapes (Fig. 5d). At flow rates above 100 mL/min the bimodal (and monomodal) growth is again observed (Fig. 5e,f). The growth of tripod-like structures again only occurs in a very narrow parameter regime.
3.3. Synthesis of CNFs: nitrogen flow rate study 3.4. Synthesis of CNFs: hydrogen flow rate study Fig. 5 show the TEM images of the Ni particles and the carbons synthesized at 450 °C with different N2 flow rates; the N2 was used to dilute the TCE. The Ni particle size distribution (determined from the CNF diameters) is shown in supplementary Fig. S3. It is observed that at a flow rate of 20 mL/min there is a minimal amount of carbon growth and only carbon coated Ni particles with diameters centered around 200 nm are formed. At a flow rate of 50 mL/min smaller Ni particles are observed with the Ni encapsulated carbons having Ni diameters
The role of hydrogen on the formation of CNFs using various carbon sources has been described in literature [38,39]. It is reported that hydrogen has an ability to either accelerate or suppress the formation of carbon materials. Singh et al. [40] synthesized CNTs from ferrocene and toluene using CVD and reported that the length of the synthesized CNTs decreased when the hydrogen content was increased. In our experiments, a Ni catalyst particle was first reduced in H2 (100 mL/min)
Fig. 5. TEM images of CNFs produced at a N2 flow rate of (a) 20, (b) 50, (C) 80, (d) 100, (e) 120 and (f) 150 mL/min [450 °C; H2 (100 mL/min) and reaction time = 30 min].
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at 450 °C and then the H2 flow was adjusted to give different flow rates (20–150 mL/min). The TCE/N2 mixture was introduced to the system at 100 mL/min. The CNFs and Ni particles formed at different flow rates showed different morphologies and sizes (Fig. 6; supplementary Fig. S4). This change in H2 content did not have any significant influence on of the produced CNF yields (see supplementary Table S1). Monomodal and bimodal growth was observed when a H2 flow rate of 20 mL/min and 50 mL/min was used (Fig. 6a, b). The formation of the monomodal CNF growth was unexpected (Fig. 6a). This indicates that the H2 plays an important role in determining the morphology of the CNFs produced. The Ni particles have a diameter of ca. 100 nm. At both flow rates linear CNFs are seen. A possible suggestion is that the H2 and TCE interact with the Ni and that sintering and carbon growth competes to give the Ni particle sizes and shapes observed. The Ni particles in Fig. 6b show a more flattened shape again indicating that the carbon restructured the Ni in the reaction. At 80 mL/min H2 flow rate larger Ni particles are produced (ca. 200 nm diameter; supplementary Fig. S4) presumably due to Ni sintering. As the flow rate, i.e. the H2 content increased, a change from bimodal to trimodal (and larger) structures occurred. At 100 mL/min (Fig. 6d), the tripod structures dominated while at 120 mL/min multimodal growth is seen. At a flow rate of 150 mL/min much larger Ni particles form (massive sintering) and now little CNF growth is seen; clearly Ni sintering dominated the reaction Thus, beyond a certain flow rate the morphology of the catalyst (particularly size) does not favor CNF formation. This is consistent with the data noted in the temperature study where at 750 °C CNF growth decreased dramatically due to the catalyst size. It appears that a carbon layer has formed over the Ni particles and that this layer also suppresses CNF growth. This suggests that there is a minimal amount of H2 needed to initiate the reaction (presumably related to the catalyst reduction) and that over reduction led to Ni sintering and that the optimum flow rate to form tripods was found to be 100 mL/min. In summary the data are consistent with a mechanism in which H2 plays a key role in reducing the NiO and that its presence determines the size of the Ni particles formed as they sinter with temperature and time. The carbon interaction with the Ni competes with the sintering process. The TCE also plays another role — to shape the Ni particle and this affects the final carbon product morphology. 3.5. Synthesis of CNFs: effect of the carbon source Different carbon sources provide different gas species that adsorb onto the catalyst particle producing the different carbon morphologies. It has been shown that under the same reaction conditions, different CNFs such as coils and linear fibers can be produced using the same
catalyst particle but by varying the carbon source [41,42]. C2H2 produced a mixture of helical (80%; d = 80 nm) and linear CNFs (d = 150 nm) (Fig. 7a, d; supplementary Fig. S5). This difference in carbon morphology is expected from catalysts that have different sizes and morphologies i.e. small particles resulted in helical fibers and big particles favored linear growth. C2H2/TCE produced CNFs with linear morphologies (Fig. 7b, e) and various diameters. When TCE was used, a tripod morphology was observed (Fig. 7c, f). SEM images show that the morphology of the fibers is dominated by coiled, linear and tripod structures for C2H2, C2H2/TCE and TCE respectively. This correlates with the diameters (and shapes) of the Ni particles. 3.6. Raman spectroscopy of CNFs synthesized from different carbon source The effect of the carbon source on the synthesis of CNFs over Ni particle catalysts was also investigated using Raman spectroscopy. The Raman data for all the samples were recorded but only the ID/IG ratios of carbons made from TCE, C2H2/TCE and C2H2 are reported here. The ratios were found to be 0.95, 0.87 and 0.79 respectively (Fig. 8) indicating that the CNFs produced using acetylene were more graphitic as compared to CNFs produced from TCE. The TGA and the corresponding derivative plots of CNFs synthesized using TCE, TCE/C2H2 and C2H2 are shown in supplementary Fig. S6. The plots show that CNFs obtained using TCE are less thermally stable in comparison with those obtained using C2H2 and TCE/C2H2. The DTG plot shows a maximum decomposition temperature of 595, 640 and 653 °C for TCE, TCE/C2H2 and C2H2 respectively. This indicates that CNFs produced using chlorinated hydrocarbons were less thermally stable than those produced using pure C2H2 (consistent with the Raman data). Thus the presence of the chlorine plays a role in the carbon structure produced. This issue has not been explored in this study but must relate to the influence of Cl on the Ni catalyst surface. 3.7. Summary on the formation of different carbon based materials over the Ni catalyst The mechanism for CNF (and CNT) growth has been extensively discussed in the literature and has been summarized in a review by Tessonier and Su [43]. Most discussions relate to supported catalysts but the principles apply to unsupported catalysts as exemplified by the tip growth mechanism in which the metal detaches from the support as the carbon grows. A key feature relates to whether carbon atoms form CNFs (or CNTs) via (i) dissolution of carbon into the bulk of the catalyst or (ii) whether C atoms migrate on/near the catalyst surface to give the products. The enormous reconstruction that occurs during the reactions of the TCE with the Ni suggests that bulk effects are
Fig. 6. TEM images of CNFs produced at H2 flow rate of (a) 20, (b) 50, (C) 80, (d) 100, (e) 120 and (f) 150 mL/min at 450 °C. N2 (100 mL/min) and reaction time = 30 min.
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Fig. 7. TEM images of CNFs synthesized using (a) C2H2, (b) C2H2/TCE and (c) TCE and their corresponding SEM images; (d) C2H2, (e) C2H2/TCE and (f) TCE under identical reaction conditions (reaction time = 30 min and flow rate = 100 mL/min H2).
more probable and that the carbon dissolves in the Ni. In earlier studies, using acetylenes, we have shown that once a Ni particle is isolated from bulk Ni it can change shape over time in the presence of the acetylene [19]. This shape change impacts on the shape of the CNF that grows from the particle In this study a wide range of synthesis conditions were used to study the generation of carbon materials from TCE over a Ni catalyst. The results again support earlier proposals in which the carbon and catalyst (in this specific case TCE and Ni) influence each other during the growth of carbon materials. The picture that emerges from this study can be summarized as follows: 1) At low temperatures and under a limited supply of TCE, Cdots are formed. This arises since the size/shape of the Ni particles do not change significantly at the low reaction temperatures. All that occurs is fragmentation of Ni particles by carbon that also get covered by carbon. 2) This suggests that Cdots concentrations can be maximized by using low amounts of reactive carbons at low temperatures. These materials find use in many areas of study [44].
3) Increasing the reaction temperature (constant TCE concentration) results in changes in the Ni size/shape leading to a range of different structures. A competition then occurs between Ni sintering and Ni coverage by carbon. This is seen in particular at T = 450 °C where small and big Ni particle co-exist. When the Ni particle becomes larger only bimodal CNF growth occurs (see Fig. 2). 4) Increasing the TCE concentration in a TCE/N2 mix (at a set temperature; 450 °C) has a similar effect. Here the results indicate that the effect of Ni coverage dominates over the Ni sintering at high TCE concentrations. A similar effect is observed when the TCE is diluted with H2. 5) The presence of chlorine has an impact on the carbon product produced (see Fig. 7). However the role of the chlorine in the reaction is at present unknown [45]. 4. Conclusions During the synthesis of the carbons using TCE, the Ni catalyst particle changed size and shape under the controlled reaction parameters. This arose since the carbon both assisted in fragmenting the Ni particles and in covering them. A range of diverse carbon morphologies occurred that depended on and were influenced by the morphology of the Ni particle catalyst but at the same time the carbon played a role in determining the Ni size/shape. The study reveals that control of the Ni catalyst particle morphology (and the CNF morphology) is a non-trivial problem as the catalyst morphology is also influenced by temperature and H2. These variables also affect the catalyst sintering/fragmentaion and coverage processes. Our data reveal that small changes in reaction conditions can impact dramatically on the carbon structure produced (dots, bipod, tripod and multipod). No doubt the same findings observed here will also be found to affect the growth of carbon shapes on other metals. Further, the rules that correlate the carbon morphology to metal catalyst morphology still need to be determined. Acknowledgements This work was supported by the NRF, DST-NRF Centre of Excellence in Strong Materials and the University of the Witwatersrand, Johannesburg, South Africa. Appendix A. Supplementary data
Fig. 8. Raman spectra of carbons produced from (a) TCE, (b) TCE/C2H2 and (c) C2H2.
Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.diamond.2016.12.023.
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
From carbon dots to multipods — The role of nickel particle shape and size Manoko S. Maubane a,b, Shrikant S. Bhoware b, Ahmed Shaikjee b, Neil J. Coville b,⁎ a b
Microscopy and Microanalysis Unit, University of the Witwatersrand, Johannesburg 2050, South Africa DST-NRF Centre of Excellence in Strong Materials and the Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg 2050, South Africa
a r t i c l e
i n f o
Article history: Received 5 October 2016 Received in revised form 9 December 2016 Accepted 31 December 2016 Available online 3 January 2017 Keywords: Carbon nanofibers Trichloroethylene Catalyst morphology Nickel CVD
a b s t r a c t In an attempt to synthesize carbon tripod structures in high yield, we have observed that a wide range of carbon structures are formed as the Ni particle size and shape varied. In using trichloroethylene (TCE), acetylene and their mixtures as a carbon source to make carbon materials over Ni nanoparticle catalysts in a chemical vapor deposition (CVD) process, we have observed that the Ni particle size and shape impacts dramatically on the carbon structure formed. The synthesis of the Ni catalyst was achieved by reduction of Ni(acetate)2 with hydrazine (35%). Ex-situ TEM analysis of the reduced NiO particles revealed that the Ni particles underwent a morphological change in the presence of TCE (and H2) with change in (i) temperature (350–750 °C) and (ii) H2 flow rate (20–150 mL/min) and that this affected the shape of the carbon material that formed. In the absence of TCE only particle sintering occurred. At T = 350 °C the Ni particles are covered by carbon to form carbon dots. With an increase in T the carbon shapes changed to a bimodal (400 °C) and then to a tripod-like structure (450 °C) and eventually multipod-like structures (500 °C). Carbon fibers were mainly observed for T N 500 °C. Similar shape changes were observed at 450 °C as a function of reaction time and H2 flow rate. It was also found that when acetylene or an acetylene/trichloroethylene mixture was used at T = 450 °C, helical and linear fibers were produced. The TEM data reveal that the fragmentation and sintering of the Ni (by the carbon source and other factors) to give differently sized Ni particles, especially as it is affected by H2, determines the morphology of the carbon materials grown from the Ni. The TCE has the ability to restructure the Ni particles and this leads to the unusual carbon shapes observed. Thus the carbon and the catalyst influence each other during the growth of the carbon materials. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The process of producing structured carbon nanomaterials by decomposition of various hydrocarbons has attracted the attention of many researchers. Thus different shaped carbon nanomaterials such as carbon nanofibers (CNFs) [1–4], carbon nanotubes (CNTs) [5–7], as well as carbon spheres (CS) [8–10], have been produced using various sources of hydrocarbons. The CNFs produced generally have diameters between 3 and 100 nm and lengths between 0.1 and 1000 μm. The recent interest in CNFs and their potential applications emerges from their relationship to carbons with graphite like layers such as fullerenes and CNTs; e.g. for use as catalyst supports [11–15]. CNFs produced during the catalytic conversion of carbon containing gases were initially regarded as a nuisance material [11]. Studies have shown that the choice of hydrocarbon, i.e., the carbon source does influence the resulting carbon product produced [16–18]. The different hydrocarbon reactants will influence the type and amount ⁎ Corresponding author. E-mail address: [email protected] (N.J. Coville).
http://dx.doi.org/10.1016/j.diamond.2016.12.023 0925-9635/© 2017 Elsevier B.V. All rights reserved.
of carbon radicals, compounds, or species formed in the reaction zone and these impact on the CNF growth mechanism. Shaikjee et al. [19] described the effect of the use of substituted alkynes over a nickel catalyst and reported that different alkyne reactants affected the catalyst morphology and hence the morphology of the extruded carbon filaments. The use of chlorinated hydrocarbons (CHCs) as a carbon source for the synthesis of structured carbon nanomaterials has also been studied [20–23]. CHCs produced in the chemical industry (e.g. PVC) are utilized in many household and industrial products. However, these materials can break down to give toxic compounds and the materials are often found in industrial wastes [24]. The disposal of chlorinated hydrocarbon containing materials is typically achieved in hazardous waste landfills and/or by incinerator procedures [25]. Conversion of these chlorinated waste streams into a high value commodity material; e.g. carbon shaped materials, would provide yet another outlet for the waste materials. It has been reported that chlorinated hydrocarbons are effective carbon precursors that can make a range of shaped carbon materials and they are reported to produce better yields of carbon materials when compared to their equivalent non-chlorinated compounds [26–28].
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All chemicals were purchased from Sigma Aldrich and were used as received unless otherwise stated.
measured) to give the width/diameter of the CNFs. It was assumed that the Ni particle correlated with this diameter and that this allowed for the determination of the Ni particle size. This assumption was also used to determine the sizes at low carbon coverage, where the Ni particles tended to be irregular in shape. Thermal stability of the CNFs was monitored using a Perkin Elmer Thermogravimetric analyser (TGA) 4000. In a standard run, CNFs (10 mg) were placed into a high temperature alumina cup that was supported on an analytical balance located in the furnace chamber. X-ray diffraction (XRD) images were collected on a Bruker D2 phaser in Bragg Brentano geometry with a Lynxeye detector using Cu-Kα radiation at 30 kV and 10 mA. The scan range was 10° b 2θ b 90° in 0.040 steps, using a standard speed with an equivalent counting time of 1 s per step. The diffraction peaks were then compared with those of standard compounds reported in the Diffracplus evaluation package using the EVA (V11.0, rev.0, 2005) software package. Temperature programmed reduction (TPR) data were collected with a Micromeritics Auto Chem II unit. The catalyst (approximately 50 mg) was placed in a quartz tubular reactor and 5% H2/Ar was passed through the reactor (50 mL·min−1) as the temperature was raised at a rate of 10 °C·min− 1 from 50 to 850 °C. The quality of the CNFs was studied using a Jobin-Yvon T64000 Raman spectrometer. The external morphology of CNFs was studied using Field Electron and Ion (FEI) focused ion beam/scanning electron microscopy (FIB/SEM) on a Nova Nanolab 600.
2.1. Ni nanoparticle synthesis
3. Results and discussion
Ni nanoparticles (NPs) were prepared by a micro emulsion technique. In a standard reaction, 1.5 g of cetyltrimethylammonium bromide (CTAB) was dissolved in a mixture of methanol (10 mL), hexanol (10 mL) and xylene (25 mL). Ni(acetate)2 (1.0 g) was added to the reaction mixture and the mixture was stirred for 30 min at room temperature. Ni2 + was reduced by the dropwise addition of 3 mL hydrazine (35%) to the solution to form Ni nanoparticles (NPs). The precipitate was filtered and washed with 2-propanol, dried at 120 °C and calcined at 350 °C for 6 h to yield about 0.27 g of catalyst.
3.1. Characterization of the NiO catalyst
Studies using TCE as a carbon source to make shaped carbon materials has been little studied. Nieto-Marquez et al. [20] reported on the catalytic growth of a structured carbon from chloro-hydrocarbons. They produced linear fibers and carbon spheres at T = 500 and T = 650 °C respectively using TCE over a Ni/SO2 catalyst. When a non-chlorinated carbon source (acetylene) was used under identical reaction conditions, tubular structures were observed. Shaikjee and Coville [28] have reported that TCE was converted to tripod CNFs over a Ni zerogel catalyst. This study reports on the use of trichloroethylene (TCE), acetylene and their 1:1 mixtures as a carbon source to make variously shaped carbons over a Ni catalyst. What is remarkable in the study is that a wide range of carbon structures can be made carbon dots (Cdots), bimodal, trimodal, polymodal, straight CNFs, coiled CNFs) using the same Ni catalyst and that the structures can be correlated to the size and shape of the Ni particles. In this study the effect of varying reaction parameters (temperature, N2 and H2 flow rate) on the carbons produced using TCE are described. 2. Experimental
2.2. Carbon synthesis The different carbons were synthesized using a catalytic CVD method. The Ni catalyst (4 mg) was spread over a quartz boat (12 mm × 28 mm) at room temperature and placed in the centre of the quartz tube reactor (18 mm × 310 mm) that was then placed in a furnace. The furnace was then heated at 10 °C per minute while hydrogen was passed over the catalyst. Once the reaction temperature was reached (350–750 °C), the H2 flow rate was maintained at the desired temperature for 30 min prior to the introduction of TCE into the system. Different H2 flow rates were also used (20, 50, 80, 100 and 200 mL/min). Once the catalyst was reduced, nitrogen was then bubbled through TCE (held at 50 °C) at different flow rates (20, 50, 80, 100 and 120 and 150 mL/min). H2 was also used as a carrier gas. All reactions were carried out for 30 min. In separate reactions, acetylene and acetylene/TCE mixtures were also used as carbon sources. After the reaction, the carbon material formed was collected after cooling the reactor to room temperature under a constant flow of H2 (100 mL/min). 2.3. Characterization techniques CNFs and Ni were characterized by transmission electron microscopy (TEM) using a Spirit T12 instrument. About 0.5 mg of a sample to be analyzed was placed in a glass vial containing 2 mL of methanol. The mixture was then sonicated for 10 min to give a homogeneous suspension of CNFs in the solvent. A drop of the suspension was then spread on a Structure Probe, Inc. (SPI) carbon copper grid (200 mesh) and allowed to dry at room temperature. The grid was then mounted onto an exchange rod and placed into the TEM chamber and analyzed. CNFs sizes were measured from TEM images (typically about 100 particles
The morphology of the synthesized Ni catalyst was characterized using TEM. The Ni catalyst was calcined in air at 350 °C (5 °C/min heating rate) for 6 h to give NiO. Fig. 1 shows the TEM image of the synthesized NiO catalyst. The NiO catalyst particles can be seen as small agglomerates that formed clusters that took a spherical shape (Fig. 1a; see supplementary Fig. S1 for larger TEM image). The XRD pattern of the synthesized NiO confirmed that NiO had been synthesized with no other diffraction impurities (Fig. 1a inset, supplementary Fig. S2a). The peaks appearing at about 2θ = 37.2°, 43.2°, and 62.8° are indexed as (111), (200), and (220) respectively and represent the face-centered cubic (fcc) crystalline structure of NiO [29]. The NiO catalyst was further characterized using TPR to study its reduction behavior (supplementary Fig. S2b). The TPR profile shows a single reduction peak at 421 °C. The catalyst was then treated in H2 at different temperatures (400–500 °C) to study its behavior under H2. The flow rate of H2 was set at 100 mL/min and the catalyst was reduced for 30 min to give Ni particle catalysts. The TEM images of the catalyst reduced at different temperatures (400, 450 and 500, 600 and 750 °C) are shown in Fig. 1. It is observed from the TEM image that the catalyst at 400 °C is in made up of small particles that form near spherical agglomerates. This is more clearly seen in supplementary Fig. S1. When the reduction temperature was increased to 450 °C, the Ni crystallites sinter to give larger particles with smoother surfaces. At 500 and 600 °C, it is observed that the Ni particles sinter to give even bigger particles (Fig. 1d and Fig. 1e respectively). It is however not possible to quantify the size of the Ni particles as they increase in temperature. Interestingly, a further increase in temperature (750 °C) results in catalyst particle fragmentation to form smaller particles. 3.2. Synthesis of nano carbons: temperature study Reaction temperature is expected to have an influence on the type of carbons that are generated when TCE decomposes in the CVD process [30,31]. Differently shaped/sized carbons were synthesized at the different temperatures (350, 400, 450, 500, 600, and 750 °C; same N2 and H2 flow rates). In the absence of H2, no reaction took place. TEM images (Fig. 2) show that the morphology of the synthesized CNFs and Ni particles changed dramatically with temperature. At 350 °C, the Ni catalyst
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Fig. 1. TEM image of (a) NiO (inset shows corresponding XRD pattern of NiO). Images of the Ni particles after reduction are shown in (b) 400 °C, (c) 450 °C, (d) 500 °C, (e) 600 °C and (f) 750 °C under a constant flow of H2 (100 mL/min); reduction was carried out for 30 min.
is covered with a layer of carbon (shell thickness ca. 100 nm) to give Ni filled carbon dots (Fig. 2a). Carbon dots synthesized, by a CVD method using a Ni/Al composite catalyst and a CH4/H2 mixture at 600 °C, have been reported by He et al. [32]. This procedure produced a mixture of both carbon dots and CNTs. Similar results were reported more recently by Lopez et al. [33]. At 400 °C, the resulting CNFs show both monomodal and bimodal growth of carbon from the Ni particles. At this temperature, the carbon growth is still minimal and the length of the CNFs is b 200 nm. The Ni particles have different shapes and sizes and are typically N200 nm (longest dimension). This may be due to the limited decomposition of TCE at this low temperature. Nieto-Marquez et al. [16] also reported that there was a negligible carbon growth at this temperature (400 °C) using a Ni/SiO2 catalyst. At these low temperatures the NiO is reduced to give Ni catalyst particles (that have sintered relative to the
NiO starting material) but as the growth is slow, the particles are only covered with the carbon with no CNF growth seen. When the reaction temperature was increased to 450 °C (and 500 °C), major catalyst restructuring occurred which yielded tripodlike and multipod-like carbon structures, respectively (Fig. 2c, d). Fig. 2c reveals two types of carbons have been produced from two sizes of Ni particles. The larger Ni particles are seen to generate tripod [34] (Fig. 2c insert) and multipod CNFs. These Ni particles are not made up of aggregates of smaller Ni particles [28]. The Ni particles have previously been shown to have a tetrahedral shape (triangular shape in TEM 2D images [28]) with CNFs growing from 3 faces. Some multipod CNFs (N3 CNFs growing from a Ni particle) can also be seen (Fig. 2c). The smaller Ni particles show bimodal growth of CNFs from these particles. Thus two processes are operating on the Ni particles as they are formed from NiO at 450 °C, (i) fragmentation and (ii) sintering.
Fig. 2. TEM images of CNFs synthesized at (a) 350 °C, (b) 400 °C, (c) 450 °C, (d) 500 °C, (e) 600 °C and (f) 750 °C, flow rate of N2 and H2 = 100 mL/min each and reaction time = 30 min.
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Fig. 3. TEM images showing CNFs (a) at low magnification to indicate the large and small Ni particles and (b) a higher magnification image showing the small particles (450 °C, flow rate of N2 and H2 = 100 mL/min each and reaction time = 30 min).
The competition between the two processes is further illustrated in Fig. 3. It can be seen in Fig. 3a that the CNFs grow from an agglomerated Ni particle and as the agglomerated Ni particles break apart bimodal growth of CNFs occurs from these small particles (circled). Fig. 3b shows a higher magnification image of the bimodal CNF growth pattern. The larger particles undergo a shape change in the presence of the carbon source and this leads to the tripod CNF growth pattern seen [28]. The size distribution of the Ni particles (measured from the CNF diameters) are shown in Fig. 3(c, d). The smaller particles have sizes ca. 60 nm while the larger particles have sizes N150 nm. Thus, the size of the Ni particle influences the carbon growth from the particle. At a higher temperature (500 °C) the Ni particles again show two size ranges (inset to Fig. 2d) leading to bimodal growth from the smaller particles and multipods from the larger particles. The larger particles become even larger than those seen at 450 °C (due to sintering) and have a multifaceted surface that leads to multipod formation (N3 CNFs growing from a Ni particle) (Fig. 2d). Thus the number and direction of CNFs that grow from the Ni particles is determined by the size and shape of the Ni particle. At 600 °C small Ni particles dominate and no agglomerated Ni clusters are seen and as a consequence more CNFs that are bimodal are observed (Fig. 2e). The larger Ni particles are seen to be associated with linear CNFs (Fig. 2e inset). At 750 °C, due to the morphology of carbon formed, identification of the Ni catalyst particles proved difficult, thus the CNF diameters were used to determine the Ni particle sizes. The large diameter fibers (ca. N 150 nm) were different from those produced at lower temperatures (Fig. 2f). Interestingly, no carbon growth was observed at temperatures N750 °C. It is assumed that the Ni particles had become too large (sintered) at the high temperature and were now unable to grow CNFs. While it is difficult to correlate the Ni particles produced by H2 reduction (Fig. 1) with Ni particles (and CNFs) produced after carbon addition and reduction (Fig. 2) both sets of data show similar types of
changes. Thus, as the temperature increased in both cases sintering was enhanced and as the Ni particles increased in size the CNF growth patterns changed. Above a Ni particle size (ca. N 150 nm) and at high temperature (N 600 °C) classical CNF growth was observed. In summary, the changes seen in the CNF morphology between 450 and 750 °C are due to both fragmentation of reduced Ni/NiO clusters and sintering of the Ni catalyst particles as a function of temperature. It is also expected that as the temperature is altered, the composition of the gas phase intermediates and radical species change and that this can influence the sintering process [31,35]. This effect can produce different types of CNFs either by direct interaction with Ni (no shape change) or by indirect action and modification of the Ni surface/shape. Our study shows that the growth of the unusual tripod structures occurs in a very narrow temperature regime. Based on these results, and to gain further insight into the bimodal and multimodal process, 450 °C was selected to be the optimum reaction temperature for further study. At this temperature, Ni particles have grown to the appropriate size to lead to the diverse CNF structures seen, with dominance of tripods being formed. The thermal analysis of the samples synthesized at different reaction temperatures (400–750 °C) was carried out from room temperature to 900 °C under a continuous flow of air at a heating rate of 10 °C/min. The TGA and the corresponding derivative plots of the synthesized CNFs are shown in Fig. 4(a) and (b) respectively. The results show that there is no significant weight loss in air until 470 °C. This indicates that there is little or no amorphous carbon present in the synthesized material; amorphous carbon is known to decompose in the temperature range 300–500 °C [36]. The complete oxidation of CNFs is observed above 550 °C. The derivative thermogravimetric plot of the CNFs is shown in Fig. 4(b). The maximum decomposition temperature of the CNFs synthesized at 400, 450, and 500 °C, were found to be 628, 595 and 587 °C respectively. This shows that as the reaction temperature was increased from 400 to 500 °C, there is a significant decrease in thermal
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Fig. 4. TGA and the DTG plots of CNFs synthesized at (a) 400 °C, (b) 450 °C, (c) 500 °C, (d) 600 °C and (e) 750 °C, flow rate of N2 and H2 = 100 mL/min each and reaction time = 30 min.
stability of the CNFs. When reaction temperature was further increased to 600 and 750 °C, the DTG plot shows two peaks. The presence of two maxima in the DTG plot could correspond to two different forms of carbon. At 600 °C, there is a small peak at 636 °C and a much sharper peak at 672 °C. According to Porwal et al. [37], this kind of behavior is observed when both defected and non-defected CNFs are present in a sample. Defected CNFs are physically and chemically weaker and therefore are oxidized at lower temperatures when compared to the defect free CNFs that will decompose at much higher temperatures. When the temperature was further raised to 750 °C, the CNFs became more thermally stable. The added stability noted at 750 °C is usually associated with the enhanced graphitic nature of the CNFs. This increase is usually achieved by high temperature annealing [37]. The TGA of materials synthesized at 350 °C have not been reported due to the extremely small yields of product obtained.
centered at 100 nm. This is associated with the small amount of TCE that enters the system leading only to carbon coverage of Ni. When the N2 flow rate was increased to 80 mL/min, bimodal and also trimodal CNF growth is clearly seen. At this increased flow rate, the carbon content increased and the carbon deposit started to build up in two opposite directions leading to the bimodal growth (Fig. 5b). The Ni particle diameter measured from the CNF diameter was centered at 200 nm but the Ni particle can be seen to be flattened with two CNFs growing from each Ni particle (Fig. 5c). The carbon has clearly caused reconstruction of the Ni surface. A mixture of small and large CNFs (and hence Ni particles) were formed. An increase in N2 content from 80 to 100 mL/min gives trimodal CNF growth plus other shapes (Fig. 5d). At flow rates above 100 mL/min the bimodal (and monomodal) growth is again observed (Fig. 5e,f). The growth of tripod-like structures again only occurs in a very narrow parameter regime.
3.3. Synthesis of CNFs: nitrogen flow rate study 3.4. Synthesis of CNFs: hydrogen flow rate study Fig. 5 show the TEM images of the Ni particles and the carbons synthesized at 450 °C with different N2 flow rates; the N2 was used to dilute the TCE. The Ni particle size distribution (determined from the CNF diameters) is shown in supplementary Fig. S3. It is observed that at a flow rate of 20 mL/min there is a minimal amount of carbon growth and only carbon coated Ni particles with diameters centered around 200 nm are formed. At a flow rate of 50 mL/min smaller Ni particles are observed with the Ni encapsulated carbons having Ni diameters
The role of hydrogen on the formation of CNFs using various carbon sources has been described in literature [38,39]. It is reported that hydrogen has an ability to either accelerate or suppress the formation of carbon materials. Singh et al. [40] synthesized CNTs from ferrocene and toluene using CVD and reported that the length of the synthesized CNTs decreased when the hydrogen content was increased. In our experiments, a Ni catalyst particle was first reduced in H2 (100 mL/min)
Fig. 5. TEM images of CNFs produced at a N2 flow rate of (a) 20, (b) 50, (C) 80, (d) 100, (e) 120 and (f) 150 mL/min [450 °C; H2 (100 mL/min) and reaction time = 30 min].
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at 450 °C and then the H2 flow was adjusted to give different flow rates (20–150 mL/min). The TCE/N2 mixture was introduced to the system at 100 mL/min. The CNFs and Ni particles formed at different flow rates showed different morphologies and sizes (Fig. 6; supplementary Fig. S4). This change in H2 content did not have any significant influence on of the produced CNF yields (see supplementary Table S1). Monomodal and bimodal growth was observed when a H2 flow rate of 20 mL/min and 50 mL/min was used (Fig. 6a, b). The formation of the monomodal CNF growth was unexpected (Fig. 6a). This indicates that the H2 plays an important role in determining the morphology of the CNFs produced. The Ni particles have a diameter of ca. 100 nm. At both flow rates linear CNFs are seen. A possible suggestion is that the H2 and TCE interact with the Ni and that sintering and carbon growth competes to give the Ni particle sizes and shapes observed. The Ni particles in Fig. 6b show a more flattened shape again indicating that the carbon restructured the Ni in the reaction. At 80 mL/min H2 flow rate larger Ni particles are produced (ca. 200 nm diameter; supplementary Fig. S4) presumably due to Ni sintering. As the flow rate, i.e. the H2 content increased, a change from bimodal to trimodal (and larger) structures occurred. At 100 mL/min (Fig. 6d), the tripod structures dominated while at 120 mL/min multimodal growth is seen. At a flow rate of 150 mL/min much larger Ni particles form (massive sintering) and now little CNF growth is seen; clearly Ni sintering dominated the reaction Thus, beyond a certain flow rate the morphology of the catalyst (particularly size) does not favor CNF formation. This is consistent with the data noted in the temperature study where at 750 °C CNF growth decreased dramatically due to the catalyst size. It appears that a carbon layer has formed over the Ni particles and that this layer also suppresses CNF growth. This suggests that there is a minimal amount of H2 needed to initiate the reaction (presumably related to the catalyst reduction) and that over reduction led to Ni sintering and that the optimum flow rate to form tripods was found to be 100 mL/min. In summary the data are consistent with a mechanism in which H2 plays a key role in reducing the NiO and that its presence determines the size of the Ni particles formed as they sinter with temperature and time. The carbon interaction with the Ni competes with the sintering process. The TCE also plays another role — to shape the Ni particle and this affects the final carbon product morphology. 3.5. Synthesis of CNFs: effect of the carbon source Different carbon sources provide different gas species that adsorb onto the catalyst particle producing the different carbon morphologies. It has been shown that under the same reaction conditions, different CNFs such as coils and linear fibers can be produced using the same
catalyst particle but by varying the carbon source [41,42]. C2H2 produced a mixture of helical (80%; d = 80 nm) and linear CNFs (d = 150 nm) (Fig. 7a, d; supplementary Fig. S5). This difference in carbon morphology is expected from catalysts that have different sizes and morphologies i.e. small particles resulted in helical fibers and big particles favored linear growth. C2H2/TCE produced CNFs with linear morphologies (Fig. 7b, e) and various diameters. When TCE was used, a tripod morphology was observed (Fig. 7c, f). SEM images show that the morphology of the fibers is dominated by coiled, linear and tripod structures for C2H2, C2H2/TCE and TCE respectively. This correlates with the diameters (and shapes) of the Ni particles. 3.6. Raman spectroscopy of CNFs synthesized from different carbon source The effect of the carbon source on the synthesis of CNFs over Ni particle catalysts was also investigated using Raman spectroscopy. The Raman data for all the samples were recorded but only the ID/IG ratios of carbons made from TCE, C2H2/TCE and C2H2 are reported here. The ratios were found to be 0.95, 0.87 and 0.79 respectively (Fig. 8) indicating that the CNFs produced using acetylene were more graphitic as compared to CNFs produced from TCE. The TGA and the corresponding derivative plots of CNFs synthesized using TCE, TCE/C2H2 and C2H2 are shown in supplementary Fig. S6. The plots show that CNFs obtained using TCE are less thermally stable in comparison with those obtained using C2H2 and TCE/C2H2. The DTG plot shows a maximum decomposition temperature of 595, 640 and 653 °C for TCE, TCE/C2H2 and C2H2 respectively. This indicates that CNFs produced using chlorinated hydrocarbons were less thermally stable than those produced using pure C2H2 (consistent with the Raman data). Thus the presence of the chlorine plays a role in the carbon structure produced. This issue has not been explored in this study but must relate to the influence of Cl on the Ni catalyst surface. 3.7. Summary on the formation of different carbon based materials over the Ni catalyst The mechanism for CNF (and CNT) growth has been extensively discussed in the literature and has been summarized in a review by Tessonier and Su [43]. Most discussions relate to supported catalysts but the principles apply to unsupported catalysts as exemplified by the tip growth mechanism in which the metal detaches from the support as the carbon grows. A key feature relates to whether carbon atoms form CNFs (or CNTs) via (i) dissolution of carbon into the bulk of the catalyst or (ii) whether C atoms migrate on/near the catalyst surface to give the products. The enormous reconstruction that occurs during the reactions of the TCE with the Ni suggests that bulk effects are
Fig. 6. TEM images of CNFs produced at H2 flow rate of (a) 20, (b) 50, (C) 80, (d) 100, (e) 120 and (f) 150 mL/min at 450 °C. N2 (100 mL/min) and reaction time = 30 min.
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Fig. 7. TEM images of CNFs synthesized using (a) C2H2, (b) C2H2/TCE and (c) TCE and their corresponding SEM images; (d) C2H2, (e) C2H2/TCE and (f) TCE under identical reaction conditions (reaction time = 30 min and flow rate = 100 mL/min H2).
more probable and that the carbon dissolves in the Ni. In earlier studies, using acetylenes, we have shown that once a Ni particle is isolated from bulk Ni it can change shape over time in the presence of the acetylene [19]. This shape change impacts on the shape of the CNF that grows from the particle In this study a wide range of synthesis conditions were used to study the generation of carbon materials from TCE over a Ni catalyst. The results again support earlier proposals in which the carbon and catalyst (in this specific case TCE and Ni) influence each other during the growth of carbon materials. The picture that emerges from this study can be summarized as follows: 1) At low temperatures and under a limited supply of TCE, Cdots are formed. This arises since the size/shape of the Ni particles do not change significantly at the low reaction temperatures. All that occurs is fragmentation of Ni particles by carbon that also get covered by carbon. 2) This suggests that Cdots concentrations can be maximized by using low amounts of reactive carbons at low temperatures. These materials find use in many areas of study [44].
3) Increasing the reaction temperature (constant TCE concentration) results in changes in the Ni size/shape leading to a range of different structures. A competition then occurs between Ni sintering and Ni coverage by carbon. This is seen in particular at T = 450 °C where small and big Ni particle co-exist. When the Ni particle becomes larger only bimodal CNF growth occurs (see Fig. 2). 4) Increasing the TCE concentration in a TCE/N2 mix (at a set temperature; 450 °C) has a similar effect. Here the results indicate that the effect of Ni coverage dominates over the Ni sintering at high TCE concentrations. A similar effect is observed when the TCE is diluted with H2. 5) The presence of chlorine has an impact on the carbon product produced (see Fig. 7). However the role of the chlorine in the reaction is at present unknown [45]. 4. Conclusions During the synthesis of the carbons using TCE, the Ni catalyst particle changed size and shape under the controlled reaction parameters. This arose since the carbon both assisted in fragmenting the Ni particles and in covering them. A range of diverse carbon morphologies occurred that depended on and were influenced by the morphology of the Ni particle catalyst but at the same time the carbon played a role in determining the Ni size/shape. The study reveals that control of the Ni catalyst particle morphology (and the CNF morphology) is a non-trivial problem as the catalyst morphology is also influenced by temperature and H2. These variables also affect the catalyst sintering/fragmentaion and coverage processes. Our data reveal that small changes in reaction conditions can impact dramatically on the carbon structure produced (dots, bipod, tripod and multipod). No doubt the same findings observed here will also be found to affect the growth of carbon shapes on other metals. Further, the rules that correlate the carbon morphology to metal catalyst morphology still need to be determined. Acknowledgements This work was supported by the NRF, DST-NRF Centre of Excellence in Strong Materials and the University of the Witwatersrand, Johannesburg, South Africa. Appendix A. Supplementary data
Fig. 8. Raman spectra of carbons produced from (a) TCE, (b) TCE/C2H2 and (c) C2H2.
Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.diamond.2016.12.023.
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