Structural Modification in Carbon Nanotubes by Boron Incorporation
We have synthesized boron-incorporated carbon nanotubes (CNTs) by decomposition of ferrocene and xylene in a thermal chemical vapor deposition set up using boric acid as the boron source. Scanning and transmission electron microscopy studies of the synthesized CNT samples showed that there was deterioration in crystallinity and improvement in alignment of the CNTs as the boron content in precursor solution increased from 0% to 15%. Raman analysis of these samples showed a shift of ~7 cm−1in wave number to higher side and broadening of the G band with increasing boron concentration along with an increase in intensity of the G band. Furthermore, there was an increase in the intensity of the D band along with a decrease in its wave number position with increase in boron content. We speculate that these structural modifications in the morphology and microstructure of CNTs might be due to the charge transfer from boron to the graphite matrix, resulting in shortening of the carbon–carbon bonds.
KeywordsCarbon nanotubes Boron Scanning electron microscopy Transmission electron microscopy Raman spectroscopy X-ray diffractometry
Carbon nanotubes (CNTs), an allotrope of carbon, show modifications in properties when impurities like boron, nitrogen, and lithium are deliberately introduced in their matrix [1–3]. Defects generated in CNTs due to introduction of these impurities could offer a possible route to tune their structural, physical, and electronic properties and thus have a significant bearing on a broad range of applications [1–4]. For example, it is anticipated that the width of the band gap of the CNTs changes with addition of nitrogen and boron, which could lead to tailoring of electronic properties of the material .
Carbon has four valence electrons and boron has three; thus, a boron atom substituted into graphite (also an allotrope of carbon) lattice would tend to draw electrons from neighboring carbon atoms. This results in reduction in the reactivity of carbon atoms with electronegative oxygen atoms, and thus the decrease in oxidation of graphite . Introduction of boron or nitrogen atoms could replace carbon atoms in CNTs resulting in modification in the number of charge carriers, which would eventually affect the electron field emission properties .
Boron can be incorporated into CNTs by various routes such as annealing CNTs with boron powder in a graphite crucible at high temperatures [8, 9, 10, 11]. Chemical vapor deposition (CVD) method has also been employed for incorporation of boron in CNTs by using diborane as a source material . Considering the fact that high temperature annealing causes defects into CNTs and diborane being very toxic gas, we have used boric acid, which is easy to handle, as the boron source. Furthermore, we have studied the structural modification in CNTs by varying the amount of boric acid. We show that the incorporation of small amount of boron during growth induces dramatic changes in CNT morphology.
The thermal CVD set up used for synthesis of CNTs has been described elsewhere . In brief, it consisted of two main components, namely, a furnace and a spray system consisting of a sprayer and a liquid reservoir. The one-stage tubular muffle furnace was of diameter 25.4 mm and length 220 mm. Inside this muffle was a quartz-tube of inner diameter 14 mm. The sprayer consisted of two concentric tubes with a nozzle of diameter 0.5 mm. The inner tube of the sprayer carried the solution from the reservoir to the nozzle of the sprayer. The nozzle end of the sprayer was fixed inside the quartz tube of the furnace (reactor). Argon (99.99% pure) was used as a carrier gas which flowed through the outer tube of the sprayer. It also exerted a pressure on the solution to regulate the liquid flow directed through the nozzle.
In a typical procedure, varying amounts of boric acid (5%, 10%, and 15%) was added to a fixed concentration of ferrocene and xylene solution (0.02 g/mL). After this, the solution was heated at around 100 °C to completely dissolve the boric acid and finally filtered using wattman filter paper of 0.1 pore size. The reactor was preheated to 950 °C and purged with Ar gas in order to create inert atmosphere. Subsequently, the solution from the reservoir was released at a constant rate and was atomized with the help of Ar gas till the temperature reaches to 900 °C. The soot film deposited on the reactor wall was later collected, by manually scratching the film. The samples grown with varying amounts of boric acid in the solution were named as B0NT, B5NT, B10NT, and B15NT indicating 0%, 5%, 10%, and 15% boric acid concentration, respectively.
The morphology of the grown CNT samples was studied using a scanning electron microscope (SEM: Stereo scan 360) operated at 15 kV. The microstructure of the samples was analyzed by high resolution TEM (HRTEM: Technai G2) operated at 200 kV. Raman spectroscopy (Micro-Raman T64000 Jobin Yvon triple monochromator system) was carried out at an excitation wavelength of 514.5 nm. The samples were also analyzed using X-ray diffractometry (XRD: Phillips Expert Pro-PW 3040). Elemental studies were done using energy dispersive X-ray spectroscopy (EDAX: Rontec, Quantex-Qx-1). For TEM analysis, samples were scratched from the quartz tube and were refluxed and ultrasonicated in ethanol for 3–4 h for proper dispersion. A few drops of the suspension were then transferred on to a carbon coated copper grid for TEM.
Results and Discussion
In contrast to previous reports , we have found, by using SEM and TEM techniques, that the crystallinity of CNTs deteriorates with increasing boron concentration. It is suggested from these studies that at lower concentration of boron, the boron atoms preferentially remain at the tip of the growing nanotubes with the tube body found to be of pure carbon as reported by other groups also . At higher boron concentration, the boron atom propagates along the axis of the CNT with an increase at the tip. Considering this fact, we can clearly explain the structural modifications in samples B5NT, B10NT, and B15NT as being due to the incorporation of boron. We anticipate that at low concentration of boron, i.e., at 5% (B5NT) changes are observed only at the tip of CNTs (Figs. 1b, 2a). With more boron incorporated in the samples (B10NT and B15NT), more and more defects occur along the axis of nanotubes and the crystallinity is reduced (see Figs. 2b, c, 3c, d). It is also suggested that the partial replacement of carbon by boron in the nanotubes structure reduces the local hexagonal symmetry resulting in defects in the nanotubes .
We have studied structural modifications in boron-incorporated CNTs. The EDS spectra of CNT samples were used to confirm the presence of boron. We found out that by increasing the boron concentration from 0% to 15% in the precursor solution, the alignment of the CNTs improves and the crystallinity of the samples deteriorates. At lower boron concentrations (at 5%), defects are only at the tip of the CNTs. As the boron concentration increases, it is observed that the defects are also introduced along the axis of CNTs.
One of the authors, Sangeeta Handuja is grateful to the Council of Scientific and Industrial Research (CSIR), Government of India for providing a research fellowship.