Transmission electron microscopy investigation of substitution reactions from carbon nanotube template to silicon carbide nanowires

Substitution reactions between multiwalled carbon nanotubes and silicon monoxide vapour have been investigated using transmission electron microscopy. Different reactions occurred inside the multiwalled nanotubes and on the nanotube external surfaces, resulting in the formation of silicon carbide nanowires with a core–shell structure. The substitution reaction process and end products are strongly affected by nanotube structures and a ball milling treatment of the starting materials. Silicon carbide (SiC) is a promising semiconducting material with its unique properties including a wide band gap, high thermal conductivity and stability, and high break down electric field [1]. SiC nanorods were reported to have greater strengths than those found previously for larger SiC structures [2]. There have been various successful syntheses of one-dimensional (1D) SiC nanostructures [3]–[5]. Carbon nanotubes (CNTs) have been used as a template for the synthesis of 1D SiC and other nanostructures [6]–[8]. Most current studies have focused on the final products but the substitution process has not been fully investigated. The reaction seems not to be a simple replacement chemical reaction as the nanotube structures (cylinders or bamboo) appear to have a strong effect on the reactions. For example, SiOC nanowires are mainly produced 4 Author to whom any correspondence should be addressed. New Journal of Physics 9 (2007) 137 PII: S1367-2630(07)40664-4 1367-2630/07/010137+6$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft 2 DEUTSCHE PHYSIKALISCHE GESELLSCHAFT from the CNT template of a bamboo-type structure [9]. The segmented bamboo-like structure causes the reaction to start on the nanotube external surface and progressively transform the CNTs into a solid nanowire segment by segment. No reaction takes place inside the nanotubes. For cylindrical nanotubes, different substitution reactions could take place both inside nanotubes and on external surfaces [10]. The different end products suggest that the chemical reactions inside nanotubes might be different from surface reactions [10]. However, detailed investigations have not been performed and detailed substitution reaction mechanisms are not clear. In this paper, we demonstrate, through detailed structural characterization using high-resolution transmission electron microscopy, the transition process from cylindrical CNT template to SiC/SiO core–shell nanowires via substitution reactions. A mixture of silicon monoxide (SiO) (Aldrich, 325 mesh, 99.9%) and iron (II) phthalocyanine (FeC32N8H16 (FePc) (Aldrich)) powders in a weight ratio of 1 : 1 was mechanically milled for 100 h at room temperature in a high-energy rotating ball mill. Four hardened steel balls were used and the milling atmosphere was pure nitrogen gas at a pressure of 300 kPa. The ball milling devices and controlling parameters are described in [11], and the structural changes of the milled samples and the enhancing effect on nanotube formation are reported in [12, 13]. The milled powder was then loaded into an alumina combustion boat and placed in the middle of a horizontal tube furnace. Several Si wafers (N type doped [100] Prime, cut in an average size of 10 mm × 3 mm pre-cleaned with pure ethanol followed by drying with highpressure nitrogen gas) were used as substrates to collect deposition near the combustion boat in the downstream direction. The Si wafers were located 5 cm away from the centre of the furnace and the temperature was 200–300 ◦C lower than the centre temperature as previously calibrated using a thermocouple [14]. With Ar–H2 (5%) gas mixture at a flow rate of 50 sccm, the sample was first heated up to 1000 ◦C at a heating rate of about 20 ◦C per min and held for 30 min. After removing a piece of Si wafer coated with deposition, the temperature was further raised to 1200 ◦C. Besides the temperature change in the second experimental stage, there was no other alteration in experimental conditions. To determine the structural evolution, four Si wafers were removed at different time frames (i.e. 0, 15, 30 and 60 min, respectively) after the temperature reached 1200 ◦C. The morphology and structures of the depositions were characterized using a field emission scanning electron microscope (SEM) (Hitachi, S-4500 operated at 3 kV), a transmission electron microscope (TEM) (Philips, CM300 operated at 300 kV). Chemical compositions were examined qualitatively using x-ray energy dispersive spectroscopy (EDS) (retractable EDAX SUTW detector attached to the TEM). The SEM image in figure 1(a) shows the well-aligned CNTs in a large quantity and a typical length of 10μm, which were deposited on the Si substrate removed during the heating at 1000 ◦C. TEM analysis confirms a typical cylindrical multiwalled structure with an average outer diameter of about 15 nm as shown in figure 1(b). The formation mechanisms of the CNTs have been reported previously [15]. After further heating at 1200 ◦C for 60 min, few of the pre-formed CNTs remained on the Si substrates. Instead, a large quantity of nanowires, including nanowire bundles, was found as shown in figure 2(a). TEM analysis shown in figure 2(b) reveals that each nanowire has a uniform cross-section along the entire wire body. Counting 200 pieces of nanowires, the distribution of the nanowires outer diameter is from 10 to 65 nm with the majority population at about 40 nm, as shown in figure 2(c). A core–shell structure of the nanowires can be seen clearly from the high magnification TEM image in figure 2(d). The nanowire core is typically 15 nm in diameter. Under the same conditions of the electron beam as TEM, the EDS analysis in figure 2(e) reveals New Journal of Physics 9 (2007) 137 (http://www.njp.org/) 3 DEUTSCHE PHYSIKALISCHE GESELLSCHAFT Figure 1. CNTs produced by pyrolysis of milled FePc at 1000 ◦C: (a) SEM image shows highly oriented and packed CNTs formed on one of the Si substrates located 5 cm away from the centre of the furnace where the temperature was 200–300 ◦C lower than the centre temperature as previously calibrated using a thermocouple [14]. (b) TEM image reveals the CNTs with a cylindrical, multiwalled structure of typical diameter of 15 nm. that the outer layer of this type of nanowire has higher O and Si and lower C content, while the inner core has higher C and Si content but very low O content. This suggests a possible core–shell structure of SiC/SiO2 [16]. The presence of O in the spectrum of inner core is believed to be from shell regions lying directly above and below the core that are impossible to avoid when analyzing nanowires in this longitudinal orientation. To determine their atomic structures, TEM lattice imaging and selected area electron diffraction (SAED) were employed. Figure 2(f) with the inset reveals that: (i) a β-SiC structured core with clear twins and stacking faults has its 〈111〉-direction parallel to the nanowire axis; (ii) the outer layer is an amorphous SiO2 shell. From these comprehensive TEM analyses, it can be concluded that the SiC/SiO2 core–shell structured nanowires formed after the second heating process. These experimental results suggest that nanowires of a SiC/SiO2 composite structure were obtained from the multiwalled CNT template after the second heating. During the first heating at 1000 ◦C, most of CNT templates were formed on the Si substrates. SiO powder mainly remains solid in the combustion boat because of the high vapourization temperature (1400 ◦C). In the second heating at 1200 ◦C, some fine SiO powder was vapourized due to the pre-ball milling treatment. The ball milling effect will be discussed later. The SiO vapour was swept towards the Si wafers by the carrier gas and reacted with the pre-formed CNTs. The detailed reaction process from CNTs to SiC/SiO2 nanowires was studied by removing Si wafers at different time frames regarding to the time when the heating temperature reached 1200 ◦C in the second heating stage. The TEM images shown in figures 3(a)–(d) correspond to 0, 15, 30 and 60 min, respectively. Figure 3(a) shows part of the wall area of a single CNT, in which the hollow centre of the tube is marked by ‘H’. The parallel graphitic layers of the tube wall can be seen clearly. The external surface of the CNT is clear and the substitution reaction did not occur at this stage.After further heating for 15 min at 1200 ◦C, another wafer was removed and analysed under TEM. A thin layer of disordered SiO2 started to form on the external surface of each tube as New Journal of Physics 9 (2007) 137 (http://www.njp.org/) 4 DEUTSCHE PHYSIKALISCHE GESELLSCHAFT Figure 2. After heating at 1200 ◦C for 60 min, nanowires formed on the substrates located 5 cm away from the centre of the furnace where the temperature was 200–300 ◦C lower than the centre temperature as previously calibrated using a thermocouple [14]. (a) SEM image of a nanowires bundle. (b) TEM image shows each nanowire has a uniform cross-section along the entire wire body. (c) The distribution of the nanowires outer diameter calculated from 200 nanowires. (d) A typical nanowire with a crystalline core wrapped by an amorphous shell. (e) EDS spectra indicating that the outer shell has a higher O content and a lower C content, while the inner core has more C than O, based on relevant peak heights. (f) The lattice image with the inserted diffraction pattern, taken from the zone axis of [011], reveals that the core of the nanowire is a β-SiC structure, with twin crystals and stacking faults, with its axis in the 〈111〉-direction. Note also the amorphous SiO2 layer. shown in figure 3(b). Further heating up to 30 min leads to the formation of a thicker disordered layer of around 5−8 nm, and the outermost C-layers have become disordered as in figure 3(c), which indicates the occurrence of reactions inside the tube. It is quite possible that some SiO vapour diffuses into the tube through the open tube ends or holes on the wall. We anticipate the following reactions have taken place inside the tubes: (i) SiO + C → Si + CO/CO2 and (ii) Si + C → SiC. Due to the limited amount of SiO available inside the tube, the reaction (ii) is thus possible. Furthermore, SiC is stable with SiO under the annealing temperature and therefore no further reaction between SiC and SiO occurs [17]. Since the dimension (several nanometres) of the inner cylinder of CNTs is small, the newly formed SiC filled the hollow nanotube cylinder New Journal of Physics 9 (2007) 137 (http://www.njp.org/) 5 DEUTSCHE PHYSIKALISCHE GESELLSCHAFT Figure 3. TEM images show the transition process from CNTs to nanowires for different time frames regarding to the time when the heating temperature just reached 1200 ◦C: (a) 0 min, (b) 15 min, (c) 30 min, (d) 60 min (‘H’-the cavity of each CNT). The corresponding schematics (a′)–(d′) represent the process by which CNT could transform into SiC/SiO2 nanowires. Both images and schematics show just half of each CNT, or nanowire, along the original 1D growth axis which is labelled ‘x–x’: (a′) 0 min: Si–O vapours deposit onto the CNT surface. (b′) 15 min: Si-O vapours aggregate to form a SiO2 shell. (c′) 30 min, the SiO layer reacts with the C-layers and SiC is formed inside the tube. (d′) 60 min: SiC re-crystallizes to form a β-SiC core. The excess Si–O vapours continue to deposit on the outer surface. completely, resulting in cavity-free SiC nanowires [18]. Different reactions should take place on the external nanotube surface. The reaction (i) SiO + C → Si + CO/CO2 should occur, but the reaction (ii) is unlikely because there is sufficient SiO to oxidize the reduced pure Si. Once the C originating from the multiwalled tubes is fully used, further deposition of SiO vapour forms the amorphous SiO2 layer on the side walls of the SiC nanowires, as indicated in figure 3(d). The reactions (i) SiO + C → Si + CO/CO2 and (ii) Si + C → SiC have been observed inside CNTs [10]. Based on these experimental observations, coupled with the discussion given above, we can schematically summarize the formation mechanism of SiC/SiO2 core–shell structured nanowires as illustrated in figures 3(a ′)–(d ′). The final thickness of the shell depends on the New Journal of Physics 9 (2007) 137 (http://www.njp.org/) 6 DEUTSCHE PHYSIKALISCHE GESELLSCHAFT SiO vapour quantity. It may be possible to obtain pure SiC nanowires by removing the SiO2 layer chemically [18]. The high-energy ball milling plays a key role in the above synthesis process: (i) cylindrical CNTs were mainly produced from the FePc after a ball milling treatment while bamboo tubes were mainly obtained without the ball milling treatment [9]. The CNTs with cylindrical structures are crucial for the SiC nanowire formation; (ii) SiC/SiO2 core–shell nanowires are produced from the milled mixture of FePc + SiO, while amorphous SiOC nanowires are obtained without ball milling, indicating different substitution reactions; (iii) the lower vapourization temperature of SiO is due to the small grain and particle sizes as well as nanosized structure created by ball milling; (iv) hydrogen in the mixture carrier gas could play an important role. In summary, SiC/SiO2 core–shell structured nanowires have been produced using multiwalled CNTs as templates. Different reactions occurring inside the nanotubes and on the external surfaces lead to a core–shell structure. The substitution reaction process and end products are strongly affected by nanotube structures and a ball milling treatment of the starting materials.