- Email: [email protected]
A first-principles study of nitrogen- and boron-assisted platinum adsorption on carbon nanotubes
CARBON
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available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
A first-principles study of nitrogen- and boron-assisted platinum adsorption on carbon nanotubes Yu-Hung Li, Ting-Hsiang Hung, Chun-Wei Chen* Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history:
We have performed first-principles calculations to investigate the origin of adsorption of
Received 31 July 2008
platinum on nitrogen- and boron-doped carbon nanotubes (CNTs). Our calculation results
Accepted 20 November 2008
reveal that both nitrogen- and boron-doped CNTs can assist the reactivity of platinum
Available online 7 December 2008
adsorption on the CNT surface, although the detailed mechanisms are very different. For nitrogen-doped CNTs, the enhanced adsorption results from activation of the nitrogenneighboring carbon atoms due to the large electron affinity of nitrogen. In this case, the nitrogen atoms mediate the platinum adsorption enhancement on the CNT surface. In contrast, the enhanced platinum adsorption in boron-doped CNTs can be attributed to the strong hybridization between the platinum d orbital and boron p orbital. Our results explain the experimentally observed enhanced adsorption of platinum on nitrogen-doped CNTs and also suggest that boron-doped CNTs may be a better candidate for fuel cell applications. Ó 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Since their discovery, carbon nanotubes (CNTs) have received an increasing interest due to their novel properties and potential applications in chemical sensors [1], field emission displays [2], catalysis [3–7] and nano-electronic devices [8,9]. It has been demonstrated that transition metal nanoparticles on CNTs exhibit excellent catalysis properties for various chemical reactions [10,11]. Due to the inert chemical properties of CNTs, the CNT surface is always externally functionalized before nanoparticles can be uniformly dispersed [12,13]. However, functionalization procedures sometimes require harsh treatment (such as using strong acids or long reaction times) to activate the sites on the CNT surface. These treatments may cause the deterioration of electronic properties [14,15]. Alternatively, CNTs can be internally doped with other elements to modify their electronic properties and reactivity [1,16]. Due to the small difference in atomic radii compared to carbon, nitrogen and boron atoms are the two
most widely used atomic species used to modify the electronic and chemical properties of CNTs by substituting for carbon atoms in the hexagonal graphitic structure of CNTs [1]. It has been shown that nitrogen- or boron-doped CNTs can demonstrate higher sensitivity than intrinsic CNTs to several gaseous molecules owing to drastic changes in their electronic properties as a result of the molecules binding to the doped location [17,18]. The precious metal platinum is an important and expensive element in the application of fuel cells. Reduction of platinum usage might thus facilitate the commercialization of fuel cells as a more popular and cost-effective energy source. Recently, platinum-deposited CNTs were found to exhibit better electrode performance for polymer electrolyte fuel cells (PEFC) than platinum-deposited carbon blacks (CBs) with a reduction usage of platinum [3]. Numerous methods of depositing platinum on CNTs have been reported [3–6]. In addition, uniformly dispersed platinum nanoparticles on vertically aligned nitrogen-containing CNTs show favorable electron-transfer properties with H2
* Corresponding author: Fax: +886 2 23634562. E-mail address: [email protected] (C.-W. Chen). 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.11.048
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production and methanol (CH3OH) oxidation in direct methanol fuel cells (DMFC) applications [7]. In this article, we present a detailed investigation of nitrogen- and boron-doped CNTs adsorption efficiency of platinum atoms on the CNT surface using first-principles calculations. When a CNT is doped with impurity atoms of nitrogen or boron, the local chemical reactivity changes significantly, leading to an enhancement in the binding energy of catalyst platinum atoms. The results of our calculations reveal that both nitrogen- and boron-doping of CNTs can assist the reactivity of platinum adsorption on the CNT surface. However, the detailed mechanisms for the enhancement in platinum adsorption on the nitrogen- and boron-doped CNTs were found to be very different. These results may provide useful information for future fuel cell applications, specifically in the reduction of platinum nanoparticle usage on CNT electrodes.
2.
Computational methods
First-principles calculations of total energy and electronic structure have been performed using CASTEP [19,20], which is a plane-wave, pseudopotential program based on density functional theory (DFT) and working under the local density approximation (LDA). Ion–electron interactions were modeled by a non-local real space [21], ultrasoft pseudopotential [22]. These models were constructed within a tetragonal supercell with lattice constants of a, b, and c. The lattice constants b ˚ to avoid interactions between two adjaand c were set to 20 A cent nanotubes. The lattice constant a along the tube axis was taken to be equal to the one dimensional (1D) lattice parameter of the nanotube. The tube was oriented along the x direction and the circular cross section was in the (y, z) plane. Summation over the 1D Brillouin zone (BZ) with wave vectors varying only along the tube axis was carried out with k-point sampling using a Monkhorst–Pack grid with a k-point spacing ˚ 1. A kinetic-energy cutoff of 400 eV and 12 special k of 0.02 A points were used to ensure convergence in the calculations. The structural configurations of the 10-layered isolated (8, 0) CNT were fully optimized until the force on each atom during ˚ . During the structural optirelaxation was less than 0.03 eV/A mization, the tube axis was allowed to relax. The platinum atom adsorption configurations on various nitrogen- or boron-doped CNTs were also optimized using the same computational parameters. The platinum binding energy on the nanotube surface is defined as Eb = E[Pt + tube] E[Pt] E[tube], where E[Pt + tube] represents the total energy of the platinum atom adsorbed on the nitrogen-(boron-) doped or undoped CNTs, E[tube] is the total energy of the nitrogen(boron-) doped or undoped CNT and E[Pt] is the total energy of the isolated Pt atom.
3.
Results and discussion
We have investigated the reactivity of a single platinum atom on the surface of intrinsic, nitrogen-doped and boron-doped CNTs by calculating the platinum atom binding energy on the various adsorption sites. Fig. 1a shows the relaxed structures of the platinum atom adsorbed onto an intrinsic CNT. The initial position of the platinum atom was located on the
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top of a carbon atom. After structural optimization, the platinum atom was found to move to the bridge site between two adjacent carbon atoms. The binding energy for this configuration is about 3.03 eV, which is comparable to previous results [23]. The large binding energy suggests chemisorption of the platinum atom onto the CNT surface. It is worth noting that the experimentally observed binding energy of a platinum cluster of typical size of 2–5 nm on a CNT surface [6,7] may be different from the calculated value here since no metal– metal interactions are taken into account in the adsorption of an isolated atom. The metal–metal binding interaction is expected to result in the formation of clusters or cluster coalescence into bigger clusters, which may cause surface nonwetting [24]. The calculated binding energy of an isolated platinum atom on the surface of a CNT is expected to be larger than that of a platinum cluster on a CNT surface. However, the goal of this paper is to investigate the relative reactivity of a platinum atom adsorbed on nitrogen-doped and boron-doped CNTs with respect to that on the intrinsic CNT. Our result can provide useful information when we compare the relative reactivity of platinum clusters adsorbed on the doped CNT surface compared to the intrinsic CNT counterpart. To facilitate a comparison, we have used the binding energy of a platinum atom on the sidewall of an intrinsic CNT in Fig. 1a as a reference, by averaging the binding energies of the six carbon sites in a hexagon of the CNT structure. In the following description, the sign (‘‘+’’ and ‘‘ ’’) of the binding energy of platinum atom adsorption onto a CNT surface represents a more energetically favorable (positive) or less energetically favorable (negative) configurations relative to the initial CNT structure. The reactivity of a platinum atom on various sites of a nitrogen-doped CNT was then examined by placing platinum on top of a(N), b, and c sites, as shown in Fig. 1b. With the platinum atom on the site above the nitrogen atom (configuration N1), the platinum atom moves to the bridge site between the nitrogen and carbon atoms after structural relaxation as shown in Fig. 1c. The platinum atom in the relaxed structure has a weaker binding energy of 0.33 eV, less energetically favorable with respect to that on the intrinsic CNT. With the platinum atom on top of a carbon atom (sites b and c) neighboring the nitrogen site, the binding energy of the adsorbed platinum atom is found to be larger than that on the reference intrinsic CNT, with binding energies of +0.58 eV(configuration N2) and +0.26 eV(configuration N3). Based on these results, it is clear that the platinum atom prefers to bind with the carbon atoms neighboring a nitrogen site in a nitrogen-doped CNT, instead of binding directly to the nitrogen atom. Since the nitrogen atom in the graphitelike CNT structure uses three valence electrons to form r bands, one valence electron to form a p bond, and places the remaining electron in the higher energy p* state, this leads to such ‘‘donor-like’’ behavior in a nitrogen-doped CNT [18]. The projected partial density of states (PDOS) for nitrogen and platinum atoms in the configuration N1 is shown in Fig. 2, which shows a very weak hybridization between the nitrogen p orbital and platinum d orbital. The weak bond formed between the platinum and nitrogen atoms can be seen in the contour plot of charge density distribution (inset). The large electron affinity of nitrogen leads to a reduction in electron
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Fig. 1 – (a) The relaxed structure of a Pt atom adsorbed onto an intrinsic CNT. (b) The initial positions of a Pt atom located on the top of a (N1), b (N2) and c (N3) sites of a CNT. (c), (d) and (e) represent the relaxed structures for the Pt atoms which are initially located at sites a (N1), b (N2) and c (N3).
Fig. 2 – The projected partial density of states (PDOS) of N and Pt atoms in a configuration with Pt initially located at the site a. The inset shows a contour plot of the corresponding charge density distribution for Pt adsorption on a N-doped CNT. The dashed line indicates the position of the Fermi level.
delocalization and a weakening of p–p interactions of the neighboring carbon atoms. This is a result of enhanced reactivity between neighboring carbon and platinum atoms. It is also believed that the reactivity can be further enhanced with increased CNT curvature because the degree of localization of
Fig. 3 – (a) The initial Pt atom positions for different adsorption sites on a B-doped CNT. (b), (c) and (d) represent the relaxed structures corresponding to an initial Pt atom location of the site a (B1), site b (B2) and site c (B3), respectively.
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the p electrons is enhanced [16]. The effect of nitrogen-assisted platinum adsorption on a CNT can be further enhanced when more nitrogen atoms are substituted for carbon atoms in the CNT. The binding energy is found to increase to +0.66 eV and +0.77 eV for CNT structures (not shown here) containing two and three nitrogen atoms respectively. The above result suggests that the experimental observation of enhanced adsorption of platinum or other transition metal clusters onto nitrogen-containing CNTs [7,25] does not mainly result from the direct chemical adsorption between platinum and nitrogen atoms, but from activating the neighboring carbon atoms. The nitrogen atoms therefore mediate the enhancement of platinum adsorption on the surface of CNTs. We have also considered the adsorption of a platinum atom on a boron-doped CNT surface. The adsorption sites are similar to the previous case, but now the dopant nitrogen atom is replaced by a boron atom as shown in Fig. 3a. With a platinum atom initially located above the boron atom (configuration B1), the relaxed structure exhibits an energetically favorable configuration where the platinum atom moves to
Fig. 4 – The projected partial density of states (PDOS) for B and Pt in B-doped CNT with an adsorbed Pt atom initially located at site a. The dashed line indicates the position of the Fermi level.
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the bridge site between the boron and carbon atoms as shown in Fig. 3b. Here, the binding energy is +0.36 eV relative to an intrinsic CNT. In configuration B2, where the platinum atom was initially located on the top of the neighboring carbon atom (site b), the relaxed structure shown in Fig. 3c is also an energetically favorable, with a binding energy of +0.48 eV. Therefore, the most energetically favorable sites for a platinum atom on a boron-doped CNT are found to be located at a position between the boron and carbon atoms, instead of being ended up neighboring carbon pair as seen in the similar nitrogen-doping case (configuration N2). For the adsorption site c, the relaxed structure finds that the platinum atom located in the bridge site between two carbon atoms, labeled B3 shown in Fig. 3d. This configuration has a binding energy of +0.08 eV smaller than that in the comparable adsorption site of the nitrogen-doped CNT. The calculation thus demonstrates that adsorbed platinum atoms tend to form a strong bond directly to boron in a boron-doped CNT. This can be seen further in the calculated platinum and boron PDOS for platinum adsorption on a boron-doped CNT. As shown in Fig. 4, the platinum d orbital can be strongly hybridized with the boron p orbital in a boron-doped CNT. Thus, an adsorbed platinum atom will have a larger binding energy if it is close to a boron atom in a boron-doped CNT. This effect can be further enhanced when more boron atoms are incorporated into the CNT. The binding energies for platinum atom adsorption on boron-doped CNTs with two (three) boron atoms are found to increase to +1.33 eV (+1.58 eV), again relative to a reference CNT. Our calculations reveal that although both the nitrogen and boron-doping in CNTs can assist platinum surface adsorption, the detailed mechanisms are very different. For a nitrogen-doped CNT, the enhanced adsorption results from activating the nitrogen-neighboring carbon atoms as shown above. In contrast, the enhancement in platinum adsorption on boron-doped CNTs can be attributed to the strong hybridization between the platinum d orbitals and boron p orbitals. The most energetically favorable sites for a platinum atom adsorbed onto a boron-doped CNT are around the boron atom. This enhancement in platinum adsorption is more significant for the boron-doped CNTs than it is for nitrogendoped CNTs with a similar configuration. Recently, it has been demonstrated that the loading efficiency of platinum nanoparticles on CNTs has been enhanced
Fig. 5 – The relaxed configurations for a Pt atom bonded to (a) the vacancy site of the intrinsic CNT, (b) a pyridine-like N-doped and (c) a pyridine-like B-doped CNT.
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as a result of generation of defects on CNTs [6]. This effect was investigated in detail using electron energy loss spectroscopy [26]; this work showed that the dopant nitrogen atom in a CNT can form a pyridine-like structure by bonding to only two carbon atoms. In addition, CNx nanotubes were demonstrated to be more efficient for monitoring toxic species compared to pure CNTs because of the presence of molecules strongly bound to pyridine-like sites [27]. We have therefore also examined the reactivity of platinum adsorption onto the pyridine-like nitrogen-doped and boron-doped CNT structures. The adsorption of platinum atoms onto the vacancy site of an intrinsic CNT was also calculated for comparison. Fig. 5a shows a platinum atom adsorbed on the vacancy site of an intrinsic CNT. The platinum atom is strongly bound to the vacancy site by saturating the dangling bonds of the neighboring carbon atoms, with a binding energy of +3.74 eV compared to the reference CNT. This result is close to a similar calculation of defect-induced platinum adsorption on CNTs [6], where the average platinum–carbon bond ˚ . Strong hybridization between the carbon p orbital is 1.93 A and platinum d orbital leads to a large binding energy for platinum adsorbed on a CNT vacancy site. This can be understood from the calculated PDOS overlap as shown in Fig. 6a. Fig. 5b and c shows the relaxed structures for a platinum atom initially bonded to pyridine-like nitrogen- and boron-doped CNTs. Contrary to the result for a graphite-like nitrogendoped CNT, here the platinum atom is found to be bound with the three nitrogen atoms in the pyridine-like nitrogen-doped CNT, with a binding energy of +1.18 eV. In this case, the aver˚ and the platinum– age carbon–nitrogen bond length is 1.37 A ˚ . The nitrogen atoms in nitrogen bond length is about 2.00 A the pyridine-like structure use two valence electrons to form r bonds, one to form a p bond, and the remaining two valence electrons to form a p-like lone pair state. Hybridization between the nitrogen p orbital and platinum d orbital leads to the increased binding energy for platinum adsorbed on a pyridine-like nitrogen-doped CNT compared to that in the graphite-like nitrogen-doped CNT. Due to the larger electron affinity of nitrogen, the overlap between the nitrogen p orbital and platinum d orbital is smaller compared to that between the carbon p orbital and platinum d orbital as shown in Fig. 6a. The d orbital of platinum in a nitrogen-doped CNT with the pyridine-like structure exhibits more localized behavior compared to that in the vacancy site of an intrinsic CNT. This result is consistent with the longer platinum–nitrogen bond length than the respective platinum–carbon bond. For a boron-doped CNT with a pyridine-like structure, platinum atom adsorption energy is further increased to +4.06 eV. Here, the ˚ and the platiaverage boron–carbon bond length is 1.53 A ˚ num–boron bond is 1.84 A, shorter than the platinum–carbon bond in the vacancy site of an intrinsic CNT. The boron atom in the pyridine-like structure uses two valence electrons to form r bonds, while the remaining one electron is bound strongly with the platinum atom. This can be seen from the calculated platinum and boron PDOS shown in Fig. 6c, where the very strong hybridization between the boron p orbital and platinum d orbital is evident. The platinum d orbital in a boron-doped CNT with a pyridine-like structure exhibits more delocalized behavior, leading to a large overlap interaction between neighboring boron and platinum atoms. Recently,
Fig. 6 – The corresponding PDOS for a Pt atom bonded to (a) the vacancy site of an intrinsic CNT, (b) a pyridine-like N-doped CNT, and (c) a pyridine-like B-doped CNT. The dashed line indicates the position of the Fermi level. Sankaran and Viswanathan [28] also demonstrated an enhanced adsorption activity of hydrogen storage in boron substituted CNTs. Our results strongly suggest that
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boron-doped CNTs could be a better candidate for platinum adsorption than nitrogen-doped or intrinsic CNTs.
4.
Conclusion
We have examined platinum adsorption on nitrogen- and boron-doped CNTs using first-principles calculations. Although both nitrogen- and boron-doping of CNTs can assist platinum surface adsorption, the detailed mechanisms are different. Dopant nitrogen atoms serve to mediate the enhancement in platinum adsorption by activating nitrogen-neighboring carbon atoms, due to large electron affinity of nitrogen. In contrast, the enhanced platinum adsorption in boron-doped CNTs can be mainly attributed to a strong hybridization between platinum d orbitals and boron p orbitals, leading to direct chemical bonding between the platinum and boron atoms. Our results explain the experimental observation of enhanced adsorption of platinum clusters on nitrogen-containing CNTs and also suggest that boron-doped CNTs would be a good candidate for fuel cell applications requiring maximal platinum adsorption.
Acknowledgements This work is supported by National Science Council, Taiwan (Project No. 96-2120-M-002-010 and No. 96-2120-M-001-001). The authors would like to thank Dr. M. Chhowalla in Rutgers University, U.S.A. and Dr. K.H. Chen in Academic Sinica, Taiwan for valuable discussions.
R E F E R E N C E S
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[8] Kim P, Liber CM. Nanotube nanotweezers. Science 1999;286:2148–50. [9] Tans SJ, Verschueren ARM, Dekker C. Room-temperature transistor based on a single carbon nanotube. Nature 1998;393:49–52. [10] Che G, Lakshmi BB, Fisher ER, Martin CR. Carbon nanotubule membranes for electrochemical energy storage and production. Nature 1998;393:346–9. [11] Zhang H, Qiu J, Liang C, Li Z, Wang X, Wang Y, et al. A novel approach to Co/CNTs catalyst via chemical vapor deposition of organometallic compounds. Catal Lett 2005;101(3–4):211–4. [12] Jiang K, Eitan A, Schadler LS, Ajayan PM, Siegel RW, Grobert N, et al. Selective attachment of gold nanoparticles to nitrogen-doped carbon nanotubes. Nano Lett 2003;3(3):275–7. [13] Endo M, Kim YA, Ezaka M, Osada K, Yanagisawa T, Hayashi T, et al. Selective and efficient impregnation of metal nanoparticles on cup-stacked-type carbon nanofibers. Nano Lett 2003;3(6):723–6. [14] Zhang J, Zou H, Qing Q, Yang Y, Li Q, Liu Z, et al. Effect of chemical oxidation on the structure of single-walled carbon nanotubes. J Phys Chem B 2003;107(16):3712–8. [15] Banerjee S, Hemraj-Benny T, Wong SS. Covalent surface chemistry of single-walled carbon nanotubes. Adv Mater 2005;17(1):17–29. [16] Stafstro¨m S. Reactivity of curved and planar carbon–nitride structures. Appl Phys Lett 2000;77(24):3941–3. [17] Zhang Y, Zhang D, Liu C. Novel chemical sensor for cyanides: boron-doped carbon nanotubes. J Phys Chem B 2006;110(10):4671–4. [18] Terrones M, Jorio A, Endo M, Rao AM, Kim YA, Hayashi T, et al. New direction in nanotube science. Mater Today 2004;7(10):30–45. [19] Payne MC, Teter M, Allan DC, Joannopoulos JD. Iterative minimization techniques for ab initio total-energy calculations: molecular dynamics and conjugate gradients. Rev Mod Phys 1992;64(4):1045–97. [20] Mailman V, Winkler B, White JA, Pickard CJ, Payne MC, Akhmatskaya EV, et al. Electronic structure, properties, and phase stability of inorganic crystals: a pseudopotential planewave study. Int J Quant Chem 2000;77(5):895–910. [21] King-Smith RD, Pyane MC, Lin JS. Real-space implementation of nonlocal pseudopotentials for first-principles total-energy calculations. Phys Rev B 1991;44(23):13063–6. [22] Vanderbilt D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys Rev B 1990;41(11):7892–5. [23] Durgun E, Dag S, Bagci VMK, Gu¨lseren O, Yildirim T, Ciraci S. Systematic study of adsorption single atoms on a carbon nanotube. Phys Rev B 2003;67(20):201401-1–4. [24] Maiti A, Ricca A. Metal–nanotube interactions-binding energies and wetting properties. Chem Phys Lett 2004;395:7–11. [25] Yang SH, Shin WH, Lee JW, Kim HS, Kang JK. Nitrogenmediated fabrication of transition metal–carbon nanotube hybrid materials. Appl Phys Lett 2007;90:13103-1–3. [26] Chan LH, Hong KH, Xiao DQ, Lin TC, Lai SH, Hsieh WJ, et al. Resolution of the binding configuration in nitrogen-doped carbon nanotubes. Phys Rev B 2004;70(12):125408-1–7. [27] Villalpando-Paez F, Romero AH, Munoz-Sandovale E, Martinez LM, Terrones H, Terrones M. Fabrication of vapor and gas sensors using films of aligned CNx nanotubes. Chem Phys Lett 2004;386:137–43. [28] Sankaran M, Viswanathan B. Hydrogen storage in boron substituted carbon nanotubes. Carbon 2007;45(8):1628–35.
4 7 ( 2 0 0 9 ) 8 5 0 –8 5 5
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
A first-principles study of nitrogen- and boron-assisted platinum adsorption on carbon nanotubes Yu-Hung Li, Ting-Hsiang Hung, Chun-Wei Chen* Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history:
We have performed first-principles calculations to investigate the origin of adsorption of
Received 31 July 2008
platinum on nitrogen- and boron-doped carbon nanotubes (CNTs). Our calculation results
Accepted 20 November 2008
reveal that both nitrogen- and boron-doped CNTs can assist the reactivity of platinum
Available online 7 December 2008
adsorption on the CNT surface, although the detailed mechanisms are very different. For nitrogen-doped CNTs, the enhanced adsorption results from activation of the nitrogenneighboring carbon atoms due to the large electron affinity of nitrogen. In this case, the nitrogen atoms mediate the platinum adsorption enhancement on the CNT surface. In contrast, the enhanced platinum adsorption in boron-doped CNTs can be attributed to the strong hybridization between the platinum d orbital and boron p orbital. Our results explain the experimentally observed enhanced adsorption of platinum on nitrogen-doped CNTs and also suggest that boron-doped CNTs may be a better candidate for fuel cell applications. Ó 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Since their discovery, carbon nanotubes (CNTs) have received an increasing interest due to their novel properties and potential applications in chemical sensors [1], field emission displays [2], catalysis [3–7] and nano-electronic devices [8,9]. It has been demonstrated that transition metal nanoparticles on CNTs exhibit excellent catalysis properties for various chemical reactions [10,11]. Due to the inert chemical properties of CNTs, the CNT surface is always externally functionalized before nanoparticles can be uniformly dispersed [12,13]. However, functionalization procedures sometimes require harsh treatment (such as using strong acids or long reaction times) to activate the sites on the CNT surface. These treatments may cause the deterioration of electronic properties [14,15]. Alternatively, CNTs can be internally doped with other elements to modify their electronic properties and reactivity [1,16]. Due to the small difference in atomic radii compared to carbon, nitrogen and boron atoms are the two
most widely used atomic species used to modify the electronic and chemical properties of CNTs by substituting for carbon atoms in the hexagonal graphitic structure of CNTs [1]. It has been shown that nitrogen- or boron-doped CNTs can demonstrate higher sensitivity than intrinsic CNTs to several gaseous molecules owing to drastic changes in their electronic properties as a result of the molecules binding to the doped location [17,18]. The precious metal platinum is an important and expensive element in the application of fuel cells. Reduction of platinum usage might thus facilitate the commercialization of fuel cells as a more popular and cost-effective energy source. Recently, platinum-deposited CNTs were found to exhibit better electrode performance for polymer electrolyte fuel cells (PEFC) than platinum-deposited carbon blacks (CBs) with a reduction usage of platinum [3]. Numerous methods of depositing platinum on CNTs have been reported [3–6]. In addition, uniformly dispersed platinum nanoparticles on vertically aligned nitrogen-containing CNTs show favorable electron-transfer properties with H2
* Corresponding author: Fax: +886 2 23634562. E-mail address: [email protected] (C.-W. Chen). 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.11.048
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production and methanol (CH3OH) oxidation in direct methanol fuel cells (DMFC) applications [7]. In this article, we present a detailed investigation of nitrogen- and boron-doped CNTs adsorption efficiency of platinum atoms on the CNT surface using first-principles calculations. When a CNT is doped with impurity atoms of nitrogen or boron, the local chemical reactivity changes significantly, leading to an enhancement in the binding energy of catalyst platinum atoms. The results of our calculations reveal that both nitrogen- and boron-doping of CNTs can assist the reactivity of platinum adsorption on the CNT surface. However, the detailed mechanisms for the enhancement in platinum adsorption on the nitrogen- and boron-doped CNTs were found to be very different. These results may provide useful information for future fuel cell applications, specifically in the reduction of platinum nanoparticle usage on CNT electrodes.
2.
Computational methods
First-principles calculations of total energy and electronic structure have been performed using CASTEP [19,20], which is a plane-wave, pseudopotential program based on density functional theory (DFT) and working under the local density approximation (LDA). Ion–electron interactions were modeled by a non-local real space [21], ultrasoft pseudopotential [22]. These models were constructed within a tetragonal supercell with lattice constants of a, b, and c. The lattice constants b ˚ to avoid interactions between two adjaand c were set to 20 A cent nanotubes. The lattice constant a along the tube axis was taken to be equal to the one dimensional (1D) lattice parameter of the nanotube. The tube was oriented along the x direction and the circular cross section was in the (y, z) plane. Summation over the 1D Brillouin zone (BZ) with wave vectors varying only along the tube axis was carried out with k-point sampling using a Monkhorst–Pack grid with a k-point spacing ˚ 1. A kinetic-energy cutoff of 400 eV and 12 special k of 0.02 A points were used to ensure convergence in the calculations. The structural configurations of the 10-layered isolated (8, 0) CNT were fully optimized until the force on each atom during ˚ . During the structural optirelaxation was less than 0.03 eV/A mization, the tube axis was allowed to relax. The platinum atom adsorption configurations on various nitrogen- or boron-doped CNTs were also optimized using the same computational parameters. The platinum binding energy on the nanotube surface is defined as Eb = E[Pt + tube] E[Pt] E[tube], where E[Pt + tube] represents the total energy of the platinum atom adsorbed on the nitrogen-(boron-) doped or undoped CNTs, E[tube] is the total energy of the nitrogen(boron-) doped or undoped CNT and E[Pt] is the total energy of the isolated Pt atom.
3.
Results and discussion
We have investigated the reactivity of a single platinum atom on the surface of intrinsic, nitrogen-doped and boron-doped CNTs by calculating the platinum atom binding energy on the various adsorption sites. Fig. 1a shows the relaxed structures of the platinum atom adsorbed onto an intrinsic CNT. The initial position of the platinum atom was located on the
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top of a carbon atom. After structural optimization, the platinum atom was found to move to the bridge site between two adjacent carbon atoms. The binding energy for this configuration is about 3.03 eV, which is comparable to previous results [23]. The large binding energy suggests chemisorption of the platinum atom onto the CNT surface. It is worth noting that the experimentally observed binding energy of a platinum cluster of typical size of 2–5 nm on a CNT surface [6,7] may be different from the calculated value here since no metal– metal interactions are taken into account in the adsorption of an isolated atom. The metal–metal binding interaction is expected to result in the formation of clusters or cluster coalescence into bigger clusters, which may cause surface nonwetting [24]. The calculated binding energy of an isolated platinum atom on the surface of a CNT is expected to be larger than that of a platinum cluster on a CNT surface. However, the goal of this paper is to investigate the relative reactivity of a platinum atom adsorbed on nitrogen-doped and boron-doped CNTs with respect to that on the intrinsic CNT. Our result can provide useful information when we compare the relative reactivity of platinum clusters adsorbed on the doped CNT surface compared to the intrinsic CNT counterpart. To facilitate a comparison, we have used the binding energy of a platinum atom on the sidewall of an intrinsic CNT in Fig. 1a as a reference, by averaging the binding energies of the six carbon sites in a hexagon of the CNT structure. In the following description, the sign (‘‘+’’ and ‘‘ ’’) of the binding energy of platinum atom adsorption onto a CNT surface represents a more energetically favorable (positive) or less energetically favorable (negative) configurations relative to the initial CNT structure. The reactivity of a platinum atom on various sites of a nitrogen-doped CNT was then examined by placing platinum on top of a(N), b, and c sites, as shown in Fig. 1b. With the platinum atom on the site above the nitrogen atom (configuration N1), the platinum atom moves to the bridge site between the nitrogen and carbon atoms after structural relaxation as shown in Fig. 1c. The platinum atom in the relaxed structure has a weaker binding energy of 0.33 eV, less energetically favorable with respect to that on the intrinsic CNT. With the platinum atom on top of a carbon atom (sites b and c) neighboring the nitrogen site, the binding energy of the adsorbed platinum atom is found to be larger than that on the reference intrinsic CNT, with binding energies of +0.58 eV(configuration N2) and +0.26 eV(configuration N3). Based on these results, it is clear that the platinum atom prefers to bind with the carbon atoms neighboring a nitrogen site in a nitrogen-doped CNT, instead of binding directly to the nitrogen atom. Since the nitrogen atom in the graphitelike CNT structure uses three valence electrons to form r bands, one valence electron to form a p bond, and places the remaining electron in the higher energy p* state, this leads to such ‘‘donor-like’’ behavior in a nitrogen-doped CNT [18]. The projected partial density of states (PDOS) for nitrogen and platinum atoms in the configuration N1 is shown in Fig. 2, which shows a very weak hybridization between the nitrogen p orbital and platinum d orbital. The weak bond formed between the platinum and nitrogen atoms can be seen in the contour plot of charge density distribution (inset). The large electron affinity of nitrogen leads to a reduction in electron
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Fig. 1 – (a) The relaxed structure of a Pt atom adsorbed onto an intrinsic CNT. (b) The initial positions of a Pt atom located on the top of a (N1), b (N2) and c (N3) sites of a CNT. (c), (d) and (e) represent the relaxed structures for the Pt atoms which are initially located at sites a (N1), b (N2) and c (N3).
Fig. 2 – The projected partial density of states (PDOS) of N and Pt atoms in a configuration with Pt initially located at the site a. The inset shows a contour plot of the corresponding charge density distribution for Pt adsorption on a N-doped CNT. The dashed line indicates the position of the Fermi level.
delocalization and a weakening of p–p interactions of the neighboring carbon atoms. This is a result of enhanced reactivity between neighboring carbon and platinum atoms. It is also believed that the reactivity can be further enhanced with increased CNT curvature because the degree of localization of
Fig. 3 – (a) The initial Pt atom positions for different adsorption sites on a B-doped CNT. (b), (c) and (d) represent the relaxed structures corresponding to an initial Pt atom location of the site a (B1), site b (B2) and site c (B3), respectively.
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the p electrons is enhanced [16]. The effect of nitrogen-assisted platinum adsorption on a CNT can be further enhanced when more nitrogen atoms are substituted for carbon atoms in the CNT. The binding energy is found to increase to +0.66 eV and +0.77 eV for CNT structures (not shown here) containing two and three nitrogen atoms respectively. The above result suggests that the experimental observation of enhanced adsorption of platinum or other transition metal clusters onto nitrogen-containing CNTs [7,25] does not mainly result from the direct chemical adsorption between platinum and nitrogen atoms, but from activating the neighboring carbon atoms. The nitrogen atoms therefore mediate the enhancement of platinum adsorption on the surface of CNTs. We have also considered the adsorption of a platinum atom on a boron-doped CNT surface. The adsorption sites are similar to the previous case, but now the dopant nitrogen atom is replaced by a boron atom as shown in Fig. 3a. With a platinum atom initially located above the boron atom (configuration B1), the relaxed structure exhibits an energetically favorable configuration where the platinum atom moves to
Fig. 4 – The projected partial density of states (PDOS) for B and Pt in B-doped CNT with an adsorbed Pt atom initially located at site a. The dashed line indicates the position of the Fermi level.
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the bridge site between the boron and carbon atoms as shown in Fig. 3b. Here, the binding energy is +0.36 eV relative to an intrinsic CNT. In configuration B2, where the platinum atom was initially located on the top of the neighboring carbon atom (site b), the relaxed structure shown in Fig. 3c is also an energetically favorable, with a binding energy of +0.48 eV. Therefore, the most energetically favorable sites for a platinum atom on a boron-doped CNT are found to be located at a position between the boron and carbon atoms, instead of being ended up neighboring carbon pair as seen in the similar nitrogen-doping case (configuration N2). For the adsorption site c, the relaxed structure finds that the platinum atom located in the bridge site between two carbon atoms, labeled B3 shown in Fig. 3d. This configuration has a binding energy of +0.08 eV smaller than that in the comparable adsorption site of the nitrogen-doped CNT. The calculation thus demonstrates that adsorbed platinum atoms tend to form a strong bond directly to boron in a boron-doped CNT. This can be seen further in the calculated platinum and boron PDOS for platinum adsorption on a boron-doped CNT. As shown in Fig. 4, the platinum d orbital can be strongly hybridized with the boron p orbital in a boron-doped CNT. Thus, an adsorbed platinum atom will have a larger binding energy if it is close to a boron atom in a boron-doped CNT. This effect can be further enhanced when more boron atoms are incorporated into the CNT. The binding energies for platinum atom adsorption on boron-doped CNTs with two (three) boron atoms are found to increase to +1.33 eV (+1.58 eV), again relative to a reference CNT. Our calculations reveal that although both the nitrogen and boron-doping in CNTs can assist platinum surface adsorption, the detailed mechanisms are very different. For a nitrogen-doped CNT, the enhanced adsorption results from activating the nitrogen-neighboring carbon atoms as shown above. In contrast, the enhancement in platinum adsorption on boron-doped CNTs can be attributed to the strong hybridization between the platinum d orbitals and boron p orbitals. The most energetically favorable sites for a platinum atom adsorbed onto a boron-doped CNT are around the boron atom. This enhancement in platinum adsorption is more significant for the boron-doped CNTs than it is for nitrogendoped CNTs with a similar configuration. Recently, it has been demonstrated that the loading efficiency of platinum nanoparticles on CNTs has been enhanced
Fig. 5 – The relaxed configurations for a Pt atom bonded to (a) the vacancy site of the intrinsic CNT, (b) a pyridine-like N-doped and (c) a pyridine-like B-doped CNT.
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as a result of generation of defects on CNTs [6]. This effect was investigated in detail using electron energy loss spectroscopy [26]; this work showed that the dopant nitrogen atom in a CNT can form a pyridine-like structure by bonding to only two carbon atoms. In addition, CNx nanotubes were demonstrated to be more efficient for monitoring toxic species compared to pure CNTs because of the presence of molecules strongly bound to pyridine-like sites [27]. We have therefore also examined the reactivity of platinum adsorption onto the pyridine-like nitrogen-doped and boron-doped CNT structures. The adsorption of platinum atoms onto the vacancy site of an intrinsic CNT was also calculated for comparison. Fig. 5a shows a platinum atom adsorbed on the vacancy site of an intrinsic CNT. The platinum atom is strongly bound to the vacancy site by saturating the dangling bonds of the neighboring carbon atoms, with a binding energy of +3.74 eV compared to the reference CNT. This result is close to a similar calculation of defect-induced platinum adsorption on CNTs [6], where the average platinum–carbon bond ˚ . Strong hybridization between the carbon p orbital is 1.93 A and platinum d orbital leads to a large binding energy for platinum adsorbed on a CNT vacancy site. This can be understood from the calculated PDOS overlap as shown in Fig. 6a. Fig. 5b and c shows the relaxed structures for a platinum atom initially bonded to pyridine-like nitrogen- and boron-doped CNTs. Contrary to the result for a graphite-like nitrogendoped CNT, here the platinum atom is found to be bound with the three nitrogen atoms in the pyridine-like nitrogen-doped CNT, with a binding energy of +1.18 eV. In this case, the aver˚ and the platinum– age carbon–nitrogen bond length is 1.37 A ˚ . The nitrogen atoms in nitrogen bond length is about 2.00 A the pyridine-like structure use two valence electrons to form r bonds, one to form a p bond, and the remaining two valence electrons to form a p-like lone pair state. Hybridization between the nitrogen p orbital and platinum d orbital leads to the increased binding energy for platinum adsorbed on a pyridine-like nitrogen-doped CNT compared to that in the graphite-like nitrogen-doped CNT. Due to the larger electron affinity of nitrogen, the overlap between the nitrogen p orbital and platinum d orbital is smaller compared to that between the carbon p orbital and platinum d orbital as shown in Fig. 6a. The d orbital of platinum in a nitrogen-doped CNT with the pyridine-like structure exhibits more localized behavior compared to that in the vacancy site of an intrinsic CNT. This result is consistent with the longer platinum–nitrogen bond length than the respective platinum–carbon bond. For a boron-doped CNT with a pyridine-like structure, platinum atom adsorption energy is further increased to +4.06 eV. Here, the ˚ and the platiaverage boron–carbon bond length is 1.53 A ˚ num–boron bond is 1.84 A, shorter than the platinum–carbon bond in the vacancy site of an intrinsic CNT. The boron atom in the pyridine-like structure uses two valence electrons to form r bonds, while the remaining one electron is bound strongly with the platinum atom. This can be seen from the calculated platinum and boron PDOS shown in Fig. 6c, where the very strong hybridization between the boron p orbital and platinum d orbital is evident. The platinum d orbital in a boron-doped CNT with a pyridine-like structure exhibits more delocalized behavior, leading to a large overlap interaction between neighboring boron and platinum atoms. Recently,
Fig. 6 – The corresponding PDOS for a Pt atom bonded to (a) the vacancy site of an intrinsic CNT, (b) a pyridine-like N-doped CNT, and (c) a pyridine-like B-doped CNT. The dashed line indicates the position of the Fermi level. Sankaran and Viswanathan [28] also demonstrated an enhanced adsorption activity of hydrogen storage in boron substituted CNTs. Our results strongly suggest that
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boron-doped CNTs could be a better candidate for platinum adsorption than nitrogen-doped or intrinsic CNTs.
4.
Conclusion
We have examined platinum adsorption on nitrogen- and boron-doped CNTs using first-principles calculations. Although both nitrogen- and boron-doping of CNTs can assist platinum surface adsorption, the detailed mechanisms are different. Dopant nitrogen atoms serve to mediate the enhancement in platinum adsorption by activating nitrogen-neighboring carbon atoms, due to large electron affinity of nitrogen. In contrast, the enhanced platinum adsorption in boron-doped CNTs can be mainly attributed to a strong hybridization between platinum d orbitals and boron p orbitals, leading to direct chemical bonding between the platinum and boron atoms. Our results explain the experimental observation of enhanced adsorption of platinum clusters on nitrogen-containing CNTs and also suggest that boron-doped CNTs would be a good candidate for fuel cell applications requiring maximal platinum adsorption.
Acknowledgements This work is supported by National Science Council, Taiwan (Project No. 96-2120-M-002-010 and No. 96-2120-M-001-001). The authors would like to thank Dr. M. Chhowalla in Rutgers University, U.S.A. and Dr. K.H. Chen in Academic Sinica, Taiwan for valuable discussions.
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