J. Phys. Chem. C 2008, 112, 6723-6728
6723
Controlled CCVD Synthesis of Robust Multiwalled Carbon Nanotube Films Niina Halonen,† Krisztia´ n Korda´ s,*,† Geza To´ th,† Tero Mustonen,† Jani Ma1 klin,† Jouko Va1 ha1 kangas,† Pulickel M. Ajayan,‡,§ and Robert Vajtai⊥ Microelectronics and Materials Physics Laboratories, Department of Electrical and Information Engineering, and EMPART Research Group of Infotech, UniVersity of Oulu, P.O. Box 4500, Oulu FIN-90014, Finland, Department of Materials Science & Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, Department of Mechanical Engineering and Materials Science, Rice UniVersity, Houston, Texas 77251-1892, and Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, New York 12180 ReceiVed: NoVember 20, 2007; In Final Form: February 28, 2008
The emerging interest toward applications of robust highly aligned carbon nanotube forests requires synthesis of such films in a controlled manner. In this paper, the growth mechanism and influence of growth parameters on the structural properties of multiwalled carbon nanotube films grown by catalytic chemical vapor deposition on SiO2 surfaces are investigated. The experimental studies are complemented with computational finite element simulations of temperature and flow fields in the reactor to have a better understanding on tailored synthesis. Film thickness, mass, density, purity, alignment, and nanotube size distribution data were assessed as a function of several growth parameters by electron microscopy and electron and X-ray diffraction techniques.
Introduction
Experimental Section
Shortly after the discovery of carbon nanotubes (CNTs), several growth methods were developed and studied to synthesize different forms of CNTs in a controlled manner. Arc discharge,1 pulsed laser deposition,2 catalytic chemical vapor deposition3 (CCVD), and various special versions of this latter one such as hot wire,4 plasma-enhanced,5 and template6 CCVD are the most commonly utilized techniques today. Highquality, i.e., well-graphitized, single-walled and multiwalled CNT powders can be obtained by any of the listed methods; however, a great advantage of CVD techniques is that when applied on prepatterned substrates or catalyst particles, well-aligned CNT films self-similar to that of the predefined template can be made.7,8 This feature is essential for applications that require high and/or anisotropic thermal conductivity, outstanding mechanical or electrical materials properties with robust and/or patterned films of CNTs such as chip-cooling elements,9 large surface area supercapacitor electrodes,10 anisotropic CNT-polymer composites for strain/ pressure gauges,11 supercompressible springs,12,13 lownoise electromechanical contacts,14 and microscopic cleaning brushes.15 In this paper we discuss the effect of several growth parameters (reaction time, precursor concentration, furnace temperature, carrier gas flow) on the amount/quality of deposited films, nanotube diameter distribution, and effective density. We simulate gas flow and temperature in a horizontal tube reactor under reaction conditions and, in addition, evaluate the role of precursor gas diffusion within the nanotube forests to explain experimental findings.
Aligned multiwalled carbon nanotube (MWCNT) films are grown on patterned Si/SiO2 templates (thermal oxide, 10 × 10 mm2 size and thickness of ∼1 µm) by catalytic chemical vapor deposition (CCVD) in a single zone tube furnace (600 mm heating zone). The general process scheme is as follows. The Si/SiO2 substrates are first cleaned with acetone and then immersed into EtOH:HF (4:1) solution for ∼10 s to remove native oxide layer. After rinsing in ethanol and drying with pressurized N2, the Si/SiO2 substrates are weighted. For specimen holder a rectangular alumina boat 20 × 40 × 5 mm3 (width/ length/depth) is used in the cleaned quartz tube (1200 mm tube length, 50 mm inner diameter) at the position extending from 90 to 130 mm in the heated zone. In the boat, two Si/SiO2 wafers were placed: one to sample position 1 with a center at 100 mm deep in the furnace and another to sample position 2 having center at 120 mm deep in the furnace. The inlet and outlet ends of the quartz tube are connected to the feeding and exhaust pipelines; then the whole reactor is evacuated to a base pressure below 0.5 Torr and purged with argon. The evacuation/purging process is repeated once more to minimize contamination with oxygen and water vapor in the system. The argon flow rate is then set (10-100 mL/min), and the furnace is heated to the reaction temperature (740-830 °C). A small amount of precursor (1 mL solution of 20 g of ferrocene in 1000 mL of xylene) is injected into the evaporator column preheated to 185 °C; then the valve between the evaporator column and the reactor is opened, and the vapor is introduced into the quartz tube with the preset flow rate of argon gas. After the prefeeding step, the precursor flow rate is adjusted to 0.1 mL/min and maintained until the end of the process (10-360 min). The reaction is terminated by stopping the precursor feeding; meanwhile, the constant argon flow is maintained. The reactor heating is switched off and the tube is kept closed until cooling to 300 °C. The thickness of CNT films is determined by optical microscopy as the average of thickness measured at different
* To whom correspondence should be addressed. † University of Oulu. ‡ Department of Materials Science & Engineering, Rensselaer Polytechnic Institute. § Rice University. ⊥ Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute.
10.1021/jp7110617 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/10/2008
6724 J. Phys. Chem. C, Vol. 112, No. 17, 2008
Halonen et al.
Figure 1. (a) Field-emission scanning electron microscopy (FESEM) image of highly aligned multiwalled carbon nanotubes (60 min growth at 770 °C, reactor pressure 760 Torr, carrier Ar flow rate 40 mL/min, and precursor 20 g/L ferrocene in xylene with a feeding rate of 0.1 mL/min). Inset: MWCNT film detached from the template (side view). (b) X-ray diffraction (Cu KR) pattern of a similar film postannealed in air at 480 °C for 4 h. The low-intensity reflection for thesotherwise very intensivesC(002) plane compared to the C(100) and C(110) planes is a clear signature of the high degree of alignment in the direction perpendicular to the growth template.
Figure 2. Temporal evolution of CNT film growth. Precursor: 20 g/L ferrocene in xylene with a feeding rate of 0.1 mL/min (1 mL prefeeding at t ) 0 is applied for each experiment). Parameters: temperature 770 °C, reactor pressure 760 Torr, and carrier Ar flow rate 40 mL/min. Transmission electron microscopy images of CNTs grown for (a) 1 h, (b) 2 h, and (c) 6 h. The TEM image in upper inset shows CNTs partially coated with amorphous carbon. In the lower inset a selected area electron diffraction pattern of the coated nanotube is shown. (d) Nanotube diameter distribution in films grown for 1 and 2 h. (e) Film thickness and mass after postannealing (inset) vs growth time plots. (f) Postannealed film density F ) mannealed/(thickness × area) and graphitized content mannealed/mgrown (shown in inset) vs growth time plots.
locations of each layer. The graphitized carbon content (i.e., the amount of CNTs in the samples) was calculated as the mass ratio of postannealed and as-grown nanotube films. Postannealing of the samples was carried out in a box furnace in air at 480 °C for 2 h. The morphology of films and diameter/structure of individual nanotubes were studied by field-emission scanning electron microscopy (FESEM, Jeol JSM-6300F) and by transmission electron microscopy combined with electron diffraction (EFTEM, LEO 912 OMEGA) as well as by X-ray diffraction (XRD, Siemens D5000). Results and Discussion Effect of Reaction Time on Nanotube Growth. To analyze the temporal evolution of nanotube films, we use a 20 g/L ferrocene in xylene precursor concentration and set the reactor temperature and carrier gas flow rate to 770 °C and 40 mL/ min, respectively. The optimum reactor zone for synthesis is found by placing several growth templates along the quartz tube as described in the Experimental Section. Considerable film
growth on the substrates starts after a short incubation period of ∼10 min, which is a consequence of transient processes such as reactant drift from the evaporator toward the remote reaction zone and catalyst Fe seed formation on the SiO2 surface. As the reaction conditions are settled, nanotubes emerge from the catalyst nanoparticles, and a contiguous uniform film forms selectively on the smooth SiO2 surface. The films are well aligned as assessed by field-emission electron microscopy and X-ray diffraction (Figure 1).16 The diameter of the nanotubes is typically between 10 and 90 nm and shows a log-normal distribution (Figure 2a,d). In the case of elongated synthesis, however, the occurrence of large diameter nanotubes 50-90 nm in the samples is found to increase and the distribution curve is becoming a log-normal doublet (Figure 2b,d). The origin of the appearance of thicker nanotubes is not yet understood but might be a consequence of larger catalyst particles that form by catalyst coarsening, e.g., by deposition of iron from the gas phase on the already existing catalyst particles and/or by surface diffusion of iron atoms followed by merging with catalyst nanoparticles on the SiO2
Robust Multiwalled Carbon Nanotube Films
J. Phys. Chem. C, Vol. 112, No. 17, 2008 6725
Figure 3. CNT film growth dependence on precursor concentration. Feeding rate of 0.1 mL/min (1 mL prefeeding at t ) 0 is applied for each experiment), growth time 60 min, temperature 770 °C, reactor pressure 760 Torr, and carrier Ar flow rate 40 mL/min. (a) CNT film thickness, density, and graphitized content vs ferrocene concentration in the precursor. (b) Transmission electron microscopy images of CNTs grown using 10 and 40 g/L ferrocene in xylene. (c) Nanotube diameter distribution in films grown using 10, 20, and 40 g/L ferrocene in xylene.
template. Anyhow, the emerging thicker nanotubes in the samples explain the increasing film density (Figure 2f) we see in the first 2 h of growth. At extreme long growth durations (6 h) the nanotubes are coated with a 40-60 nm thick layer of carbonaceous deposit in which grains of iron nanoparticles are embedded (Figure 2c). The high initial growth rate of the films gradually decays with time as shown in Figure 2e. The square-root-shaped slope of temporal evolution of film thickness suggests diffusionlimited growth similar to that for oxide growth on various metals and semiconductors.17,18 The Deal-Grove diffusion-limited growth model corrected for a τ0 start delay19 h2 + Ah ) B(t τ0) fits well to the measured h thickness vs t time data in Figure 1e, where B is the parabolic rate constant and B/A is the reaction rate at the interface. To have a better insight into the kinetics of precursor transport in the nanotube forests, we estimate the mean free path λ of reactant gas molecules20 in the reactor as λ ) kBT/(x2pd2π), where kB, T, and p are Boltzmann’s constant, temperature, and gas pressure, respectively. d stands for the distance between the centers of colliding gaseous species. In a simplified picture, collisions between argon-argon, argon-xylene, and xylenexylene with the corresponding d values of 3.4, 5.2, and 6.9 Å are considered. From the experimental inlet conditions, the amount of each component and thus the weighted average cross section for the collisions can be estimated giving 〈d〉 ≈ 4.4 Å, and a corresponding mean free path is λ ≈ 170 nm (at 40 mL/ min Ar flow and 0.1 mL/min precursor feeding, T ) 1043 K and p ) 100 kPa). In fact, at such high reactor temperatures, hydrocarbons undergo thermal cracking resulting in much smaller radicals than the size of the starting xylene molecules.21-25 Accordingly, the obtained λ ≈ 170 nm is a conservative estimate of the real mean free path, and considering the intertubular spacing (∼100 nm) of nanotubes in our forests, the growth of CNT films can be limited by the diffusion. It is however worth pointing out that the growth itself is not necessarily limited by the diffusion of gaseous reactants in the forest if sufficient amount of precursor is present at the catalyst so that the rate-determining step is not the gas diffusion but some other processes. For instance, if the growth temperature is set low enough to induce strong kinetic control for carbon dissolution and diffusion in the catalyst, the amount of reactants
in the proximity of catalyst can be sufficient despite the limited gas diffusion in the forest. Accordingly, the decay of CNT film thickness growth rate should not be attributed exclusively to a limited gas diffusion in the forests. Catalyst deactivation can also cause reduced growth rate, as it was shown for both singlewalled and multiwalled CNT films grown for extended periods.26,27 The catalyst deactivation could be avoidedsand the growth of ultralong nanotubes could be achievedsby introducing trace amount of water to the reactor to oxidize amorphous carbon that forms on catalyst particles.28,29 Influence of Catalyst Precursor Concentration. The influence of catalyst precursor concentration, i.e., dissolved ferrocene in xylene, is also investigated. The growth time, reactor temperature, and carrier flow rate are set to 60 min, 770 °C, and 40 mL/min, respectively. The ferrocene in xylene concentration is 5, 10, 20, and 40 g/L with a constant precursor feeding rate of 0.1 mL/min. Using a 5 g/L precursor, the growth rate is an order of magnitude lower than that with 10 and 20 g/L solution (left panel in Figure 3 a) with very low nanotube density (middle panel in Figure 3a) and high amorphous carbon content in the deposited film (right panel in Figure 3a). In the case of a highly concentrated precursor, 40 g/L, the growth rate and film density are somewhat reduced, the nanotubes contain more defects as compared to the films obtained with 10 or 20 g/L solution, and a considerable fraction of the deposits is amorphous carbon (Figure 3a,b). The size distribution of nanotubes is fairly independent from the precursor concentration and follows the log-normal statistics as shown in Figure 3c. According to the results, there is an optimal concentration range 10-20 g/L, in which films consisting of high-quality nanotubes can be grown, whereas too diluted or too concentrated precursors do not favor for the synthesis. Once a catalyst nanoparticle is formed and starts to produce a nanotube, it is partly consumed by the nanotube itself as it takes up small amounts of metal in the inner-tubular cavity during growth. Such consumed metal particles appear as rodshaped plugs encapsulated by the nanotubes.30 If a continuous metal supplement is not provided for the catalyst at the root, the catalyst performance decays, causing reduced growth rate or eventually a termination of growth. Too slow catalyst precursor supply cannot maintain the catalyst size, while too
6726 J. Phys. Chem. C, Vol. 112, No. 17, 2008
Figure 4. Temperature dependence of CNT film growth. Precursor: 20 g/L ferrocene in xylene with a feeding rate of 0.1 mL/min. Parameters: reaction time 1 h, reactor pressure 760 Torr, and carrier Ar flow rate 40 mL/min. (a) Film thickness and graphitized content (inset) vs growth temperature. (b) CNT film mass vs growth temperature and Arrhenius plot of mass growth (inset).
rapid feeding results in inactive large metal nanoparticles that form at the root.31,32 Consequently, we may conclude the feeding rate of catalyst precursor has an optimum rangesin our system it is from 10 to 20 g/Lsin which the catalyst consumption is compensated for. Temperature Dependence of Nanotube Growth. The effect of growth temperature on the purity and quantity (mass and thickness) of synthesized CNT films is investigated in the temperature range between 740 and 830 °C using constant growth time of 60 min, carrier flow rate of 40 mL/min, and precursor concentration of 20 g/L. The active region of our reactor is close to the inletsonly 90-130 mm from the edge of the heated zone, which is a consequence of rapid decomposition of ferrocene followed by nucleation and growth of iron nanoparticles above ∼700 °C, i.e., shortly after the entrance of the furnace. As revealed by computed fluid dynamic simulations,33 both nucleation and growth steps of iron nanoparticle formation from ferrocene precursor take place within the first centimeters after entering the hot zone. The position of active growth zone is very sensitive for the reactor temperature set for the experiments. The active region moves inward or outward depending on whether the reactor temperature is in the lower or higher extreme of the experimental range. Thus, to assess growth vs temperature data, the location of growth templates had to be adjusted to the optimal position in our experiments. The growth rate shows considerable dependence on temperature (Figure 4 a), implying strong kinetic control for the process. From the Arrhenius plots in Figure 4b we find two growth regimes with corresponding activation energies of 405 ( 61 and 164 ( 12 kJ/mol for the temperature ranges of 740770 and 762-830 °C, respectively. At low synthesis temperatures (85%) at 770-800 °C, which then considerably decreases above 800 °C (lower inset in Figure 4a). This might be explained by the fast thermal decomposition xylene in the gas phase followed by the deposition of carbonaceous products on the surface of already formed nanotubes. The overall CNT film density is decreasing with temperature (upper inset in Figure 4a). The activation energy 164 ( 12 kJ/mol of this temperature regime indicates other rate-determining step than the thermal decomposition of hydrocarbons. For the catalytic growth a carbon nanotubes (and also other nanowires such as Si and Ge) the vapor-liquid-solid (VLS) theory is accepted widely. In the VLS model,35-37 it is assumed that carbonaceous species adsorb on the surface of catalyst then decompose to carbon and other products. These latter ones either leave the surface or reside there, thus gradually poisoning the catalyst. The formed carbon atoms are dissolved by the catalyst to form a supersaturated solution, from which carbon precipitates on the surface (due to any thermal instability) forming the tip of the emerging nanotube. In this process, the most energy-demanding step is the diffusion of carbon through the metal. The typical corresponding activation energies for bulk diffusion of carbon in various phases of iron are 71-83 kJ/mol in bcc Fe, 87 kJ/mol in fcc Fe, and 202 kJ/mol in fcc Fe supersaturated with C.38 Since the catalyst nanoparticles are well-saturated with carbon to produce carbonaceous precipitates on the surface, the activation energy of diffusion must be in between the above values as it was also measured to be 125 ( 13 kJ/mol by others.39 Accordingly, the value of Ea ) 164 ( 12 kJ/mol we obtained above 762 °C suggests the rate-determining step of CNT films growth in this regime is the diffusion of carbon in the catalyst iron nanoparticles. CFD Simulations of Temperature and Flow Fields in the Reactor. To have a better understanding on the growth process, both temperature and the flow conditions in the reactor have been calculated using finite element computational fluid dynamics simulations. The model boundaries included the inlet and outlet gas flow and temperatures at several points of the reactor which were measured with thermocouples. The local thermodynamic effects due to reaction enthalpies were neglected in the model, and argon is used as flow medium instead of the precursor/carrier mixture. Results showed three distinguishable flow volumes (see also Supporting Information for flow animation). (i) In the inlet region, where the precursor mixture enters into the quartz tube a relatively fast flow is seen. Then due to the lower temperature (∼100 °C), it cools and descends to the bottom part of the tube and drifts toward the heated zone. When reaching the furnace, the precursor warms up within a few centimeters of drift and
Robust Multiwalled Carbon Nanotube Films
Figure 5. Finite element modeling of temperature fields in the reactor in the case of different preset temperatures (770 and 830 °C) and carrier flow rates (30 and 100 mL/min). The heated volume is highlighted with the dotted lines.
Figure 6. Finite element computational fluid dynamic modeling of precursor flow fields in the proximity of the sample holder boat. Different preset temperatures (770 and 830 °C) and carrier flow rates (30 and 100 mL/min) are applied.
rises due to buoyancy. In this region the majority of the flow turns back and travels backward to the inlet of the quartz tube, while only a small fraction enters the inner volume of the furnace. (ii) The flow inside the furnace is laminar with a fairly uniform and slow velocity less than ∼2 mm/s. The experimentally found optimal location for nanotube growth is at the beginning of this region. (iii) The third region is the outlet volume, where again a circular flow similar to that of the inlet region is seen. Calculations of temperature fields under different carrier feeding rates showed insignificant effect on this parameter: regardless of argon feeding rate, the temperature maps are quite similar at a given reaction temperature (Figure 5). This is in good agreement with our experiments (not shown here), where the growth results were well independent of the carrier flow for the range 20-100 mL/min investigated. The simulated temperature fields show that the temperature in and around the specimen holder boat is uniform and equals that of the preset reactor temperature value. Flow field simulations (Figure 6) showed an insignificant effect of carrier feeding rate on the flow nearby the boatseven though the difference between the initial feeding rates was quite significant (40 mL/min and 100 mL). In all cases, the flow fields in the sample positions are very similar: laminar flow with very low velocity
