Chemical State Analysis of Entrapped Nitrogen in Carbon Nanohorns

Article pubs.acs.org/JPCC

Chemical State Analysis of Entrapped Nitrogen in Carbon Nanohorns Using Soft X-ray Emission and Absorption Spectroscopy Taiji Amano and Yasuji Muramatsu* Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan

Noriaki Sano Department of Chemical Engineering, Kyoto University, Kyoto-daigaku katsura, Nishikyo-ku, Kyoto 615-8510, Japan

Jonathan D. Denlinger and Eric M. Gullikson Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States ABSTRACT: The chemical state of nitrogen in nitrogen-entrapped carbon nanohorns (N-CNHs) synthesized by submerged arc discharge in liquid nitrogen was determined by soft X-ray emission and absorption spectroscopy using synchrotron radiation. X-ray absorption spectra (XAS) and X-ray emission spectra (XES) in the CK and NK regions of the N-CNHs were measured at the Advanced Light Source. From fingerprint analysis in NK-XAS and by referring to nitrogen-containing aromatic compounds, the suggested chemical state of nitrogen in N-CNHs is imine. Theoretical analysis of NK-XES using the discrete variational Xα molecular orbital calculations elucidated that the local structure of N-CNHs is chemisorption of N2, forming pentagonal and/or hexagonal rings with carbon atoms at the graphitic layer edges, which assume an imine structure. Therefore, the entrapped nitrogen in N-CNHs is chemisorbed at the edges of the graphitic layers.

1. INTRODUCTION Carbon nanohorns (CNHs) have attracted much attention as a new carbon material for gas sensors,1 hydrogen and methane storage,2,3 electrodes,4 catalysts,5−7 and drug carriers.8 CNHs can be synthesized by various methods, such as laser ablation,9 pyrolysis of carbon−metallic compounds,10 catalytic chemical vapor deposition (CVD),11,12 and submerged arc discharge with carbon electrodes in water13 or liquid nitrogen.4,14 Although submerged arc discharge in liquid nitrogen is a novel and economical method to synthesize large quantities of CNHs, the synthesized CNHs usually contain nitrogen as an impurity.14 Entrapped nitrogen in CNHs has been detected by electron energy loss spectroscopy (EELS).14 The EELS results indicate that nitrogen can be incorporated in the graphitic layers or adsorbed as volatile CNx compounds. However, the chemical state or local structure of nitrogen in CNHs has yet to be clarified. To determine the chemical state of nitrogen in nitrogen-entrapped CNHs (denoted by N-CNHs), we measured the soft X-ray emission spectra (XES) and soft Xray absorption spectra (XAS) of N-CNHs using synchrotron radiation (SR)15 because high-resolution soft X-ray spectroscopy can elucidate the chemical/electronic structures of various nanocarbon materials.16,17 Prior to the soft X-ray spectral measurements, we calculated the electronic structure of typical CNH isomers to optimize the basal CNH cluster models for the soft X-ray spectral analysis © 2012 American Chemical Society

using the discrete variational (DV)-Xα molecular orbital (MO) method.18 The DV-Xα method is a powerful tool for X-ray spectral analysis19 and has been successfully applied to soft Xray emission and absorption spectral analyses of carbon materials.20 The DV-Xα calculations clarified that the major electronic structure of CNHs can be determined by the large hexagonal carbon layers surrounding the pentagonal rings.21 Thus, a hexagonal carbon layer model of C96H24 was selected as the basal cluster model for soft X-ray spectral analysis of CNHs. Theoretical analysis of the measured XES in the NK and CK regions of N-CNHs using chemisorption models has indicated that N2 is probably chemisorbed at the edges of hexagonal carbon layers.15 To further elucidate the chemical state of nitrogen in NCNHs, we compared NK-XAS of N-CNH with those of nitrogen-containing aromatic compounds as well as the theoretically analyzed NK-XES of N-CNH using both chemisorption and physisorption models. Herein, we present the chemical state of nitrogen in N-CNH determined by f ingerprint analysis in NK-XAS and XES supported by the DVXα calculations using chemisorption and physisorption models. Received: October 7, 2011 Revised: February 14, 2012 Published: February 27, 2012 6793

dx.doi.org/10.1021/jp209650t | J. Phys. Chem. C 2012, 116, 6793−6799

The Journal of Physical Chemistry C

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Figure 1. Standard aromatic compounds, which have various nitrogenated functional groups (A1−A12, I1−I5, AI1−AI5, AN1−AN3). A, I, AI, and AN indicate amine (−NH2, −NH−, −N< ), imine (−N), complex of amine and imine, and complex of amine and nitro (−NO2), respectively.

2. EXPERIMENTS N-CNH samples were prepared by submerged arc discharge with 99.99% purity graphite electrodes in liquid nitrogen. The arc voltage and current were tuned to 34 V and 54 A, respectively. After the 45 s discharge, water was poured into the liquid nitrogen jar. The product carbon powders were separated into two aqueous phases: floating powder and settling powder phases. According to the transmission electron microscopy (TEM) observations, the floating and settling powders were NCNHs and multiwalled carbon nanotubes (CNTs), respectively.14 The floating powder of N-CNHs and settling powder of CNTs were separated by filtration and rinsed with distilled water. Dried N-CNH and CNT powders were used as measurement samples for XAS and XES. Highly oriented pyrolytic graphite (HOPG) was used as a reference sample for graphitic carbon. For fingerprint analysis in NK-XAS, the standard samples were 25 commercially available aromatic compounds with various nitrogenated functional groups. Figure 1 shows the chemical formulas of the compounds [4-amino-p-terphenyl (A1), 1-aminopyrene (A2), 3,3′,5,5′-tetrametylbenzidine (A3), 3,3′-diaminobenzidine (A4), 2-phenylindole (A5), carbazole (A6), N,N′-diphenylbenzidine (A7), 4,4′-dimethyltriphenylamine (A8), 1-methyl-2-phenylindole (A9), 3,3′-methylenediindole (A10), N,N,N′,N′-tetramethylbenzidine (A11),

N,N,N′,N′-teraphenylbenzidine (A12), 9-phenylacridine (I1), phenazine (I2), 4,7-dimethyl-1,10-phenanthroline (I3), bathophenanthroline (I4), benzo[c]cinnoline (I5), 6-aminoquinoline (AI1), 9H-pyrido[3,4-b]indole (AI2), tetraphenylporphine (AI3), 4-(4-dimethylaminostyryl)quinoline (AI4), purine (AI5), 4-nitro-1-naphtylamine (AN1), 4-amino-4′-nitrobiphenyl (AN2), and 4-nitro-1,2-phenylenediamine (AN3)]. A, I, AI, and AN denote amine (−NH2, −NH−, −N< ), imine (−N), complex of amine and imine, and complex of amine and nitro (−NO2), respectively. These powder samples were pressed and held on indium substrates for soft X-ray spectral measurements. High-resolution soft X-ray spectral measurements using SR were performed at the Advanced Light Source (ALS). XES in the CK and NK regions were measured by a Rowland-mount grating X-ray spectrometer in beamline BL-8.0.1.22 Excitation energies of the monochromatized SR beam were tuned to 320 and 430 eV for the CK and NK regions, respectively. Theoretical energy resolutions (E/ΔE) of the XES measurements using a 600 lines/mm grating with a 10 m radius of curvature and a 40 μm slit were 600 and 460 for the CK and NK regions, respectively. Both the incident angle of the excitation beam to the sample plane and the takeoff angle of the emitted X-rays were tuned to 45°. XAS in the CK and NK regions were measured in BL-6.3.223 using the total-electronyield (TEY) method, which measures the sample photocurrent. 6794



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The E/ΔE of the XAS measurements using the 1200 lines/mm varied-line-spacing (VLS) grating and a 40 μm slit were approximately 5000 and 3000 for the CK and NK regions, respectively. The incident angles of the monochromatized SR beam to the sample plane were tuned to normal for powder samples and to 45° for HOPG. In both the XES and XAS measurements, sample degradation by SR beam irradiation was negligible.

3. RESULTS AND DISCUSSION 3.1. X-ray Absorption Spectra (XAS). Figure 2 shows the XAS in the CK to NK region of N-CNH, CNT, and HOPG,

Figure 3. Soft X-ray absorption spectra in the NK region of the NCNH relative to the standard aromatic compounds.

complex compounds (AN1−AN3) display a broad increase from 401 eV and sharp peaks at 404 eV. On the other hand, NCNH has a 4 eV wide peak at 400 eV, which is gray in Figure 3. From the f ingerprint comparison in NK-XAS between the NCNH and standard compounds, the peak structure around 400 eV of N-CNH corresponds to that of imine compounds, suggesting that the chemical state of nitrogen in N-CNH is imine (−N) and the N atoms have sp2 hybridization. 3.2. X-ray Emission Spectra (XES). Figure 4 shows the XES in the CK and NK regions of the N-CNH, CNT, and Figure 2. Soft X-ray absorption spectra in the CK−NK region of NCNH, CNT, and HOPG. Inset shows their enlarged NK spectra.

and the inset is the enlarged NK-XAS. N-CNH and CNT have CK-XAS profiles similar to that of HOPG, which exhibits a sharp π* peak at 285.5 eV and a broad σ* peak around 292 eV. These observations confirm that N-CNHs and CNTs are formed by the graphitic hexagonal carbon layers. The enlarged NK-XAS has a distinct peak at 400 eV and gently increases around 410 eV in N-CNH. However, neither CNT nor HOPG displays these characteristics. Hence, N-CNH definitely contains nitrogen, which is consistent with the EELS results.14 Comparing the absorption peak intensities between the CK and NK edges of N-CNH estimates that the atomic percentage is roughly less than 2%, demonstrating that the entrapped nitrogen in N-CNH can be regarded as an impurity. Figure 3 compares NK-XAS of N-CNH to the standard aromatic compounds with various nitrogenated functional groups. The profiles of the standard compounds depend on the nitrogenated functional groups. The amine standard compounds (A1−A12) have peaks in the region above 401 eV, whereas imine compounds (I1−I5) possess sharp peaks in the 398−400 eV region. Amine/imine-complex compounds (AI1−AI5) have spectral structures similar to the sum of the spectra from amine and imine compounds. Amine/nitro-

Figure 4. Soft X-ray emission spectra in the CK (left panel) and NK (right) regions of the N-CNH, CNT, and HOPG.

HOPG. In the CK-XES, N-CNH and CNT exhibit profiles similar to that of HOPG, which consists of a main peak at 277 eV with low-energy tailing between 265 and 273 eV and a higher shoulder around 282 eV. The main peak and higher shoulder are assigned as the σ* and π* components in the graphitic sp2-hybridized carbon atoms, respectively.20 Such a correspondence in the CK-XES among N-CNH, CNT, and HOPG means that CNHs are formed mainly by the graphitic 6795



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be regarded as the chemisorption of N2 molecules at the graphitic carbon edges. In the model of edgeI (C95NH23), one nitrogen atom substitutes for the edge carbon atom in the zigzag arrangement. In the models of innerI (C95NH24) and innerII (C94N2H24), one or two nitrogen atoms replace the inner carbon atoms at the center hexagonal ring in the C96H24 model. Cluster structures of these chemisorption models are optimized by the conventional molecular mechanics (MM2) method. In physisorption models, a N atom or N2 molecule is placed at varying distances onto the inner carbon atom or the center hexagonal ring in C96H24. The former and latter models are denoted by over and center, respectively. In the models of overI (N + C96H24) and overII (N2 + C96H24), a N atom and a N2 molecule are placed just over the carbon atoms at the center hexagonal ring, respectively. The distances between the N/N2 and the hexagonal ring vary from 1.0 to 3.0 Å. The model of centerI places a N atom on the center of the center hexagonal ring. The models of centerIIv and centerIIp place a N2 molecule vertically and parallel, respectively, to the center at the center hexagonal ring. The distances between the N/N2 and the hexagonal ring vary from 0 to 3.0 Å. The calculated DOSs are described by broadening with 0.5 eV wide Lorentzian functions to compare the measured CK- and NK-XES profiles. The MO energy is adjusted to the highest occupied molecular orbital (HOMO) at 0 eV. Figure 6 shows the occupied C-DOS of the basal C96H24 model and N-DOS of the chemisorption models and compares

hexagonal carbon layers. This result is consistent with the CKXAS in Figure 2. In NK-XES, N-CNH clearly shows double peaks at 393.5 eV (denoted by a) and 395.5 eV (b) with low-energy tailing in the 385−390 eV region. In contrast, CNT and HOPG display small peaks. These observations also support nitrogen entrapment in N-CNH. The spectral analysis using the DV-Xα method indicates that the characteristic double-peaked profile in the NK-XES of N-CNH can be a target spectrum to determine the local structure of the entrapped nitrogen. Hence, we theoretically simulated the NK-XES profile from the chemisorption and physisorption models of nitrogen in N-CNH. 3.3. Spectral Analysis of XES Using the DV-Xα Method. The measured NK-XES of N-CNH was analyzed by the DV-Xα method. A graphitic cluster model of C96H24 was used as a basal model for CNH because the electronic structures of the CNH isomers can be determined by the large hexagonal carbon layers surrounding the pentagonal rings.21 Figure 5 shows the chemisorption and physisorption models

Figure 5. Cluster models of the basal graphitic structure (C96H24), and chemisorption (edgeIIp, edgeIIh, edgeI, innerI, and innerII) and physisorption (overI, centerI, overII, centerIIv, and centerIIp) structures of a nitrogen atom and/or molecule in the C96H24.

Figure 6. Occupied C- and N-DOS of the chemisorption models. C2p-DOS of the center carbon atom (denoted by an asterisk) in the C96H24 model and the N2p-DOS of the chemisorbed nitrogen atoms in the chemisorption models are compared to the measured CK- and NK-XES of N-CNH, respectively.

for N-entrapped CNHs. The chemisorption models, which were formed by referring to other N-doped graphite models,24−26 are reported elsewhere.15 One or two nitrogen atoms bond to the edge carbon atoms or replace the inner carbon atoms. The former and latter models are denoted by edge and inner, respectively. The edgeIIp model (C96N2H22) forms a pentagonal ring by the two adsorbed adjacent nitrogen atoms with the edge carbon atoms in a zigzag arrangement, and the edgeIIh model (C92N2H22) forms a hexagonal ring with edge carbon atoms in an armchair arrangement. These models can

the measured CK- and NK-XES of N-CNH. The C2p-DOS of the C96H24 reproduces well the CK-XES profile, confirming that the local and electronic structures of the CNH can be approximated by graphitic carbon. The N2p-DOSs of the edgeIIp and edgeIIh exhibit two peaks with low-energy tailing. Such double-peaked profiles approximately reproduce the 6796



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measured NK-XES. In contrast, the N2p-DOS of edgeI shows a single peak with low-energy tailing. Both innerI and innerII models show the broad single peak in N2p-DOS. These singlepeaked profiles do not agree with the measured NK-XES profile. Consequently, the probable local structure models of N-CNH are chemisorbed edgeIIp and/or edgeIIh. Figure 7 shows the occupied N-DOS of a free N atom and N atoms from the physisorption models of overI and centerI. In

Figure 8. Occupied N-DOS of the nitrogen molecule in the physisorption models of overII, centerIIv, and centerIIp.

nitrogen atoms form sp2-hybridized orbitals. According to the thermodynamic calculations of N-doped graphene models by Xu,25 such chemisorption structures are thermodynamically stable. Figure 9 shows the unoccupied N2p*-DOS of the N2 atoms in the edgeIIp and edgeIIh models and compares them to the

Figure 7. Occupied N-DOS of the nitrogen atom in the physisorption models of overI and centerI.

both overI and centerI models, N atoms with a longer distance (d ≥ 2.0 Å) exhibit sharp N2p-DOS peaks similar to that of the free N atom. In both models, N atoms with shorter distances (d ≤ 1.0 Å) show broader N2p-DOS profiles with fine structures likely due to orbital hybridizations between N and C atoms, but these shorter distances are unrealistic. Hence, the N2p-DOSs from the overI and centerI models are inconsistent with the measured NK-XES of N-CNH. Figure 8 shows the occupied N-DOS of a free N2 molecule and of N2 molecules from three physisorption models (overII, centerIIv, and centerIIp). The free N2 molecule shows a fine structure in the N2p-DOS with a main peak at −1 eV, a higherenergy peak (HOMO) at 0 eV, and a small satellite peak at −18 eV. These peaks are attributed to hybridization between N 2p and N 2s orbitals. The N atoms with longer distances (d ≥ 2.0 Å) in the overII, centerIIv, and centerIIp models exhibit sharp N2p-DOS peaks similar to the free N2 molecules. In contrast, the N atoms with a shorter distance (d ≤ 1.0 Å) in these models show broader N2p-DOS profiles with fine structures, which may result from the orbital interaction between the N and C atoms, but these short distances are unrealistic. Consequently, the N2p-DOS profiles of these physisorption models cannot reproduce the NK-XES of N-CNH. Therefore, the physisorption models do not explain the entrapped nitrogen in N-CNH. Hence, the most probable local structure of N-CNH is the chemisorption models, such as edgeIIp and/or edgeIIh, in which N2 bonds to the edge carbon atoms in the graphitic layers. This result agrees with the fingerprint analysis (Figure 3). The chemical state of nitrogen in N-CNH is imine (−N), and the

Figure 9. Unoccupied N-DOS of the chemisorption models of edgeIIp and edgeIIh, compared to NK-XAS of N-CNH.

measured NK-XAS of N-CNH. Both models exhibit a broad π* structure in the 0−9 eV region and a σ* structure in 10−26 eV region. Although the fine structures in the π* peak differ between the edgeIIp and edgeIIh models, the broad π* region agrees well with the NK-XAS peak width at the NK edge. Therefore, the NK-XAS of N-CNH can also be qualitatively explained by the chemisorption models. 3.4. Features of N-CNH Chemical Bonding. To understand the features of chemical bonding of the chemisorbed N atoms in N-CNH, Figure 10 describes the electronic structures of the edgeIIp and edgeIIh models, where the N2 atoms are labeled as 1N and 2N, and the C atom bonded to 1N is labeled as 1C. In the edgeIIp model, the 1N 2p orbital at 0 eV, which corresponds to peak b in NK-XES, hybridizes mainly with 1N 2s, 2N 2p, 2N 2s, 1C 2p, and 1C 2s 6797



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Figure 11. Bond overlap populations (left panel) of 1N−2N and 1N− 1C bonds in edgeIIp and edgeIIh models. Electrostatic potential mappings (right) of the edgeIIp and edgeIIh models.

4. CONCLUSION To clarify the local structure and chemical state of the entrapped nitrogen in N-CNHs synthesized by submerged arc discharge with carbon electrodes in liquid nitrogen, synchrotron radiation was used to measure XAS and XES in the CK and NK regions. Fingerprint analysis in NK-XAS indicates that the chemical state of nitrogen in N-CNHs is imine (−N) with sp2-hybridized orbitals. Spectral analysis of NK-XES using the DV-Xα method suggests that N2 atoms are chemisorbed to form pentagonal and/or hexagonal rings with carbon atoms at the graphitic layers. The entrapped N atoms form covalent N− N and N−C bonds by hybridization between the N 2p, N 2s, C 2s, and C 2p orbitals.

Figure 10. Occupied N- and C-DOS of the N atoms (1N and 2N) and C atom (1C) bonding the N1 atom in the chemisorption models of edgeIIp and edgeIIh. N-DOSs are compared to the measured NK-XES of N-CNH.

orbitals, suggesting that peak b results from the N−N and N−C bonds. The 1N 2p orbital around −2 eV, which corresponds to peak a in NK-XES, hybridizes with 1N 2s, 2N 2p, and 2N 2s orbitals, indicating that peak a results from the N−N bond. In the edgeIIh model, the 1N 2p orbital at 0 eV, which corresponds to peak b, hybridizes mainly with 1N 2s, 2N 2p, and 2N 2s orbitals. On the other hand, the 1N 2p orbital around −2 eV, which corresponds to the peak a, hybridizes mainly with 1N 2s, 2N 2p, 2N 2s, and 1C 2p orbitals. Hence, peak b results from the N−N bond, whereas peak a is from the N−N and N−C bonds. The small difference in the electronic structures between the edgeIIp and edgeIIh models may be due to the resonant effect in forming the pentagonal ring (edgeIIp) and the hexagonal ring (edgeIIh). However, the double peaks in NKXES of N-CNH can be well explained from the hybridized orbitals in N−N and N−C bonds. Figure 11 shows the bond overlap population (BOP) of the N−N and N−C bonds in the edgeIIp and edgeIIh models as well as the electrostatic potential mapping displayed by VESTA.27 In the edgeIIp model, the BOP of the 1N−2N bond (0.89) is higher than that of the 1N−1C bond (0.78), indicating that the N−N bond has more covalency than the N−C bond. In the edgeIIh model, the 1N−2N bond (0.82) and the 1N−1C bond (0.86) have similar BOPs. These BOP distributions suggest that the electronic structure of the N atoms incorporated in the hexagonal ring can be more delocalized than those in a pentagonal ring. Such delocalizations in edgeIIp and edgeIIh models are consistent with the electrostatic potential mapping. The negative charge around N2 atoms in edgeIIh is spread over a wider area than that in edgeIIp. These electronic features agree with the larger resonant effect of the hexagonal ring than that of the pentagonal ring.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan under contract Nos. 20560628 and 23360291.

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REFERENCES

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Chemical State Analysis of Entrapped Nitrogen in Carbon Nanohorns

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