Article pubs.acs.org/JPCC
Nitrogen−Silicon Heterodoping of Carbon Nanotubes Martha Audiffred,†,‡,# Ana Laura Elías,‡ Humberto R. Gutiérrez,§ Florentino López-Urías,‡,∥ Humberto Terrones,‡,⊥ Gabriel Merino,*,# and Mauricio Terrones.*,‡,▽ †
Departmento de Química, Universidad de Guanajuato, Noria Alta s/n CP 36050, Guanajuato, Gto. México Department of Physics, The Pennsylvania State University, 104 Davey Lab, University Park, Pennsylvania 16802, United States § Department of Physics and Astronomy, University of Louisville, Louisville, Kentucky 40292, United States ∥ Advanced Materials Department, IPICYT, Camino a la Presa San José 2055, Col. Lomas 4a sección, San Luis Potosí S.L.P., 78216, México ⊥ Departamento de Física, Universidade Federal do Ceará, P.O. Box 6030, Fortaleza, CEP 60455-900, Brazil # Departamento de Física Aplicada, Centro de Investigación y de Estudios Avanzados, Unidad Mérida, Km. 6 Antigua Carretera a Progreso, Apdo. Postal 73, Cordemex, 97310, Mérida, Yuc., México ▽ Research Center for Exotic Nanocarbons (JST), Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan ‡
ABSTRACT: Si/O/N-doped single-walled carbon nanotubes (SWNTs) are synthesized using aerosol-assisted chemical vapor deposition (AACVD). The samples are characterized by Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), energy-dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). HRTEM and Raman spectroscopy studies indicate that doping plays a crucial role in the generation of stable small diameter SWNTs. In order to elucidate the role of the heterodoping (Si/N, O/N, and Si/O) on the electronic properties and stability of SWNTs, density functional theory (DFT) computations on semiconductor (10,0), semimetallic (9,0), and metallic (5,5) SWNTs are performed. It is found that in the heterodoped SWNTs substitutional nitrogen makes the inclusion of Si and O atoms more energetically favorable within the carbon lattice. Heterodoping with Si/O/N may have an important impact in the chemistry of SWNTs as they could be used for anchoring polymer chains, clusters, molecules, etc., thus leading to advanced composites and sensors.
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INTRODUCTION Carbon nanotube research has been an extremely active field since their discovery and structural identification.1,2 They possess fascinating electronic, structural, and mechanical properties, and they could be used in a wide range of electromechanical devices as well as in the fabrication of robust and/or highly conducting composites.3,4 The possibility of modifying the physical and chemical properties of pure carbon nanotubes through structural modifications,5−8 substitutional single-atom doping,9−13 and surface functionalization13−16 has been theoretically proposed and experimentally achieved. Several reports have described the synthesis of single-atom-doped single-walled carbon nanotubes (SWNTs) using different techniques such as arc discharge17−21 and chemical vapor deposition (CVD).10,22−25 These doped SWNTs could reveal very different electronic and structural changes when compared to undoped tubules. The most studied substitutional impurities in semiconductor carbon nanotubes are boron and nitrogen, which produce p- and n-type doped materials, respectively.17−19,22,26−34 In addition, it has been shown that Sidoped carbon nanotubes exhibit unique electronic and © XXXX American Chemical Society
structural properties, as well as additional intense localized states, indicating the potential of anchoring different atoms and/or molecules.11 Si-doped SWNTs have been already synthetized by our group, but it was found that when a high concentration of silicon (0.2 wt % of MTMS) is used, the formation of short nanorods composed of Si and O with metallic Fe−Si−O hemispherical tips is promoted. In these samples, it was difficult to find numerous SWNTs; nevertheless, the addition of a nitrogen precursor could help to increase the amount of nanotubes and to avoid the formation of binary Fe− Si alloys.25 Regarding co-doping, only scarce literature has been published.24,35−37 For example, heteroatomic phosphorus− nitrogen co-doped nanotubes have been studied theoretically and experimentally; here the inclusion of nitrogen atoms reduces the defect formation energy, thus stabilizing the phosphorus atom to fit in the tube lattice.35 However, further Received: December 17, 2012 Revised: March 19, 2013
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Figure 1. (a) Transmission electron microscopy image (TEM) and (b) diameter distribution of pristine carbon nanotubes. (c) TEM image, (d) diameter distribution, and (e−g) electron energy-loss spectroscopy (EELS) analysis of Si/N-doped SWNTs. Note that the EELS analysis reveals the presence of Si and N within the Si/N-doped SWNTs.
diameter of 1.6 nm. The Si/N-doped sample exhibits a narrower tube diameter distribution (between 0.7 and 1.9 nm) with an average diameter distribution shifted to smaller diameters (∼1 nm). These results are in agreement with our first-principle computations of the defect binding energy (vide infra), predicting higher stability in doped SWNTs with smaller diameters. EELS spectra (Figures 1(e−g)) on doped SWNT bundles were also recorded in order to confirm the presence of Si, Si/O, and N on the tube walls. Although the EELS measurements were performed only in SWNT bundle regions, avoiding the metal nanoparticles and amorphous carbon, nanosized particles and thin films of silicon oxide surrounding the bundles were occasionally detected. Hence, the Si EELS signal depicted in Figure 1f could be a combination of the EELS signal corresponding to doping (expected to be weak) plus SiO2 (stronger). EEL spectra of the doped nanotubes display edges at ca. 99, 284.5, and 400 eV, corresponding to the silicon L, and carbon, and nitrogen K-shells, respectively. The C−K edge exhibits a sharp π* peak (at ∼285 eV) and a wellstructured σ*-band (starting at ∼292 eV), characteristic of the sp2 hybridization, and indicating that the nanotubes have a graphitic network with high crystalline quality. The N K-edge in N-doped carbon nanotubes always exhibits a very low signal-tonoise ratio17,38,39 due to the small number of N atoms present in the tubes (typically ≤1 atom %). Figure 1g shows the nitrogen K edge after background subtraction obtained from the Si- and N-doped nanotubes, which shows two apparent features. The peak between 399 and 403 eV is attributed to
experimental and theoretical work on co-doping needs to be carried out. Thus, in this manuscript, we report the successful synthesis of Si/N O-heterodoped SWNTs. High-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy were employed to identify the presence of SWNTs, and in conjunction with chemical analysis techniques, the existence of these dopants within the tubules has been confirmed. In addition, ab initio computations based on the density functional theory (DFT) have been performed in order to understand the formation mechanism, reactivity, stability, and electronic properties of Si-, Si/N-, Si/O-, and O/N-doped SWNTs, in both metallic and semiconducting SWNTs.
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RESULTS AND DISCUSSION The produced samples consisting of pristine SWNTs and Si/Ndoped SWNTs (see Experimental Details) were characterized using a Jeol 2010F transmission electron microscope (TEM) equipped with electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX) capabilities. TEM images of the SWNT material reveal SWNT bundles of ca. 10− 30 nm in diameter. In addition, we observed metal particles, identified as Fe (using EDX), which catalyzed the nanotube growth (Figures 1a and 1c). Figures 1b and 1d depict the tube diameter distribution determined from direct measurements using TEM images as well as cross-section images of the bundles (see Figure 1c). Notice that for pristine samples the diameter distribution ranges from 0.9 to 2.6 nm with an average B
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Figure 2. X-ray photoelectron spectroscopy (XPS) analysis performed on Si/N-doped single-walled carbon nanotubes synthesized using 0.3% of benzylamine and 0.3% of methoxy-trimethyl-silane (MTMS) in the precursor solution. (a) C 1s line scan shows a sp2-hybridized C signal and a broad shoulder containing a signal coming from oxygenated carbon groups and C−N. (b) N 1s line scan exhibits a broad N signal, which could be composed by pyrrolic, pryridinic, substitutional, and gaseous N. (c) Si 2p line scans exhibiting the presence of silicon.
Figure 3. On the left are (a) low-magnification and (c) high-magnification SEM images. The right panel depicts the Auger spectra associated with the image on the left, which exhibit the presence of C, Fe, and Si signals.
electron excitations from the 1s level to π* states. The chemical environment of the nitrogen atom within the carbon network determines the shape and position of this peak. Theoretical40 and experimental17,38,39 studies have concluded that peaks corresponding to pyridine-like and pyrrole-like configurations appear at ∼399.0 and ∼400.3 eV, respectively, while the peak corresponding to highly coordinated N atoms replacing C atoms (or “graphitic” nitrogen) is found at slightly higher energies (401.0−403.0 eV). Although our samples show a small contribution to the π* peak at 399 and 400 eV, the major peak intensity is located between 401 and 403 eV, indicating that our sample contains a significant amount of substitutional nitrogen. The second feature of the N K-edge is located at energies higher than 405 eV, and it is related to the σ*-band which is characteristic of C−N materials. The Si L-edge EEL spectrum is comparable in shape to that previously reported for SiO2.41 Nevertheless, in our spectra, the Si L23 edge starts from 99 eV and shows two distinctive peaks at ca. 105 and 112 eV.
The XPS characterization (see Figure 2) was used as a complementary technique to confirm and quantify the presence of N, O, and Si and to determine their bonding configurations within the carbon nanotube walls. The C 1s line scan revealed a signal that could be attributed to several components: a main peak located at a binding energy (BE) of 284.5 eV (corresponding to sp2-hybridized C atoms)42 and low intensity signals from 286 to 289 eV, which could be due to C−N and carbon oxygenated groups, respectively.43 The N 1s line scan is broad and asymmetric and may be composed of three or more different N chemical bonding environments; a peak located at ca. 399.7 eV may originate from the presence of pyrrolic-type nitrogen, in which nitrogen atoms are bonded to a fivemembered carbon ring, and a footprint of N atoms bonded to sp2-hybridized C neighbors (CN−C, pyridinic N) with adjacent carbon vacancies at ca. 399 eV.44−46 A peak centered at around 401 eV indicates the presence of substitutional N (sp2-hybridized N bonded with three sp2-hybridized C C
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Figure 4. First- and second-order resonant Raman spectra for the pristine (undoped) and Si/N-doped SWNTs, using 514 nm (left) and 647 nm (right) excitation laser lines. (a,b) Raman spectra exhibiting G, G′ and D-bands; (c,d) Raman spectra exhibiting radial breathing modes (RBMs).
neighbors) in graphite-like structures,46 and a signal at ca. 402 eV is attributed to the existence of gaseous N adsorbed on the tube surface.46,47 From the peak intensity analysis, a nitrogen content of ca. 0.57 atom % was estimated. The Si 2p line scan exhibits a signal at ca. 104 eV, corresponding to silicon oxide.41,48 Approximately 99% of the silicon in the XPS spectrum comes from silicon oxide, and it is possible that this is masking the Si−C and Si−N signals. Auger electron spectroscopy (AES) was also carried out, and signals from carbon, iron, and silicon were detected in the spectra, thus confirming the doping of the SWNT products. The sample was fixed on carbon double-sided tape, and Auger spectra were collected over the bare tape and stub for comparison to the actual sample spectra. No obvious differences were detected in the spectra, so it was assumed that the effect of the substrate is negligible. We obtained signals from carbon, iron, and silicon. Figures 3a and 3b show a lowmagnification scanning electron microscopy (SEM) image and its associated Auger spectrum, respectively. The C, Fe, and Si signals are evident (around 285, 550−750, and 1600 eV, respectively). The iron signal shape appears to be deformed due to a charging effect. Since the sample contains SiOx particles entangled with SWNTs, the charging phenomenon was very frequently observed by various characterization techniques (FESEM, XPS, Auger). Figure 3c depicts a higher-resolution SEM image with its corresponding Auger spectrum (3d). We have focused on a region containing no observable particle contamination and consisting only of SWNTs. Here, the shape of the Si signal looks sharp, and the charging effects affect the Fe signal less (1600 and 550−750 eV, respectively).
The resonant Raman response of both pristine and doped samples using an in-Via micro-Raman spectrometer was also obtained. Two different laser excitation wavelengths were used in our experiments: 514 nm (2.41 eV) and 647 nm (1.92 eV). First- and second-order Raman spectra for both excitation wavelengths are shown in Figure 4. The radial breathing mode (RBM) frequencies (50−350 cm−1) are proportional to the inverse of the nanotube diameter (dt ≈ 1/ωRBM).23,49,50 The simultaneous Si and Si/Ox N-co-doping shifts the RBMs to higher frequencies for both excitation wavelengths (see Figures 4c and 4d), indicating the presence of smaller diameter SWNTs. The peak located at ca. 270 cm−1 corresponds to dt = 0.90 nm (Elaser = 2.41 eV), and that at ca. 290 cm−1 can be attributed to dt = 0.84 nm (Elaser = 1.92 eV). Such values are consistent with the diameter distribution observed by TEM and the computed defect binding energies (vide infra). In Figures 4a and 4b, the intensity of the D-band (or disorder-induced band ∼1350 cm−1) is higher for the Si/N, Si/ O-doped SWNT samples than for the pristine SWNTs. The Dband is activated by any structural defect (such as doping) that breaks the symmetry of the perfect hexagonal carbon lattice. The tangential phonon modes (“G-band” ∼1580 cm−1) show different Raman line shapes for metallic and semiconducting SWNTs. Scattering from semiconducting nanotubes produces Lorentzian line shapes for the components of the G-band. Metallic tubes have a broadened asymmetric G-band with an additional component due to an interference between electronic and phonon scattering channels (Breit−Wigner− Fano (BWF) resonance process). Considering the diameter distributions in Figures 1b and 1d, and according to the Kataura plot (Ejj vs 1/d),51 the 514 nm (2.41 eV) laser line will excite the electronic transitions E33 and E44 of semiconducting D
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Figure 5. Relaxed geometries for pristine (undoped) and doped (10,0) SWNTs. (a) Pristine (undoped), (b) Si-doped, (c) Si/N-doped, and (d) SiN3-doped SWNT.
in the pristine samples (G′P) and results in a splitting of the G′ feature (Figure 4a). Using ab initio computations, we find that the Si/N-doped nanotubes, having Si and N as first neighbors, show a lower binding energy than the Si/N-doped nanotubes with noncontiguous dopants. Figure 5 depicts the optimized geometries for pristine (Figure 5a) and the different Si and N heterodoped (10,0) SWNTs. As a consequence of the atomic size, in all doped systems the Si atom is displaced outwardly from the hexagonal network. Figure 5b depicts the single Si-doped case exhibiting two different Si−C bond lengths (1.73 and 1.78 Å), which are similar to those reported by Baierle et al.11 The optimized geometry of the Si/N-doped SWNT is shown in Figures 5c and 5d, sharing very similar geometrical features to the Si-doped case. The relaxed structures of doped carbon nanotubes with (9,0) and (5,5) chiralities (not shown here) are close to those computed for the (10,0) carbon nanotube. The band structures for the pristine and different Si/Ndoped (10,0) SWNTs are shown in Figure 6. Particularly, the pristine (10,0) SWNT (Figure 6a) reveals a semiconducting behavior with an electronic band gap of 0.76 eV. When the (10,0) SWNT is doped with Si, additional nondegenerate bands appear, preserving a semiconducting behavior with a band gap of 0.6 eV (see Figure 6b). When the SWNTs are codoped with Si and N atoms in a substitutional fashion, the electronic properties are drastically changed; the band structure plot exhibits an almost nondispersive band (flat band) around the Fermi level (see Figure 6c), which is due to the additional electron in N. This flat band disappears when the Si atom is passivated with a hydrogen atom leading to an electronic gap of 0.40 eV. When both dopants (Si and N) are far away, the band structure displays a metallic behavior; here the Fermi level is shifted into the conduction bands, similar to single N-doped carbon nanotubes.54 Band structures for the SiN3-doped
SWNTs in the pristine sample. The 647 nm (1.92 eV) laser line, however, will excite both the E11 and E33 interband transitions of the metallic and semiconducting nanotubes, respectively, in the same sample. This explains the slightly asymmetric G-band found in Figure 4b for the pristine sample. On the other hand, the co-doped SWNT sample exhibits a completely opposite behavior caused by the different diameter distribution (see Figure 1d). In this case, the 514 nm laser line excites the E11 transitions for metallic nanotubes, whereas the 647 nm line is in resonance with the E22 of semiconducting SWNTs. This is reflected on the asymmetric and symmetric Gbands observed in Figures 4a and 4b, respectively. It is worth mentioning that there were no significant shifts (
