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First-principles study of structural and work function properties for nitrogen-doped single-walled carbon nanotubes
Accepted Manuscript Title: First-principles study of structural and work function properties for nitrogen-doped single-walled carbon nanotubes Author: Xiji Shao Detian Li Jianqiu Cai Haijun Luo Changkun Dong PII: DOI: Reference:
S0169-4332(16)30132-5 http://dx.doi.org/doi:10.1016/j.apsusc.2016.01.271 APSUSC 32503
To appear in:
APSUSC
Received date: Revised date: Accepted date:
18-12-2015 25-1-2016 29-1-2016
Please cite this article as: X. Shao, D. Li, J. Cai, H. Luo, C. Dong, Firstprinciples study of structural and work function properties for nitrogendoped single-walled carbon nanotubes, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.271 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
Substitutional nitrogen atom doping in capped (5, 5) SWNT is investigated.
Serious defects appear from breaks of C−N bonds with N contents of above
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Work function drops after N doping and may reach 4.1 eV.
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23.3 at.%.
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Graphical abstract Serious structural defects appear after breaks of C−N bonds under N doping concentration of 30 at.%. The work function drops sharply with the increase of N concentration and reaches the lowest value of 4.1 eV at 10 at.%.
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4.7 4.6
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4.5 4.4 4.3 4.2
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Work function (eV)
4.8
4.1 4.0
0.01% 0.02%
10%
20%
23.3%
30%
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Concentration of N atoms
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First-principles study of structural and work function properties for nitrogen-doped single-walled carbon nanotubes
a
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Xiji Shaoa, Detian Lia, b,*, Jianqiu Caia, Haijun Luoa, Changkun Donga, b,* Institute of Micro-nano Structures & Optoelectronics, Wenzhou University, Wenzhou,
Science and Technology on Vacuum & Cryogenics Technology and Physics
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b
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Zhejiang 325035, People’s Republic of China
Laboratory, Lanzhou Institution of Physics, Lanzhou, Gansu 730000, People’s
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Republic of China
*
Corresponding Authors: [email protected], [email protected]
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Abstract The structural and electronic properties of the capped (5, 5) single-walled carbon nanotube (SWNT), including the structural stability, the work function, and the
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charge transfer performance, are investigated for the substitutional nitrogen atom doping under different concentrations by first-principles density functional theory.
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The geometrical structure keeps almost intact with single or two N atom doping,
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while C−N bonds may break up with serious defects for N concentrations of 23.3 at.% and above. The SWNT remains metallic and the work function drops after doping due
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to the upward shift of Fermi level, leading to the increase of the electrical
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conductivity. N doping enhances the oxygen reduction activity stronger than N
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adsorption because of higher charge transfers.
Keywords
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First-principles, Single-walled carbon nanotubes, Nitrogen, Doping, Work function
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1. Introduction Impurity doping is an effective approach to modify the properties of carbon nanotubes (CNTs) on mechanical, electronic, optical, and other aspects.
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Substitutional doping with various elements, including boron [1, 2], nitrogen [1-5], and silicon [6, 7], has been investigated for different types of CNTs. The
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substitutional nitrogen atom doping (nitrogen doping for short) impacts
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significantly the device properties for many CNT applications, such as field emission [8, 9], energy storage [10], gas sensing [11, 12], composite material [13], and catalysis
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[14-17].
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Nitrogen can be viewed as an n-type carbon dopant, leading to the sensitive shift of the Fermi level (EF) [18]. The doping of nitrogen atoms in CNTs could change the
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local chemical reactivity and the work function, also generate defects in curved structures [19]. Meanwhile, nitrogen doping can change the formation of the localized
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electron-donor state near the Fermi level, resulting in the improvement of the electrical conductivity. With nitrogen doping at different sites, the local curvatures
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could be distorted, leading to the increase of the field enhancement factor for field emission [18]. Nitrogen doping can create well localized and chemically active sites [20]. In fuel cell applications, nitrogen-doped carbon nano-material electrodes showed encouraging oxygen reduction reaction (ORR) activity and long-term operation stability. Nitrogen induced charge delocalization at CNT or graphene electrode changes the O2 chemisorption mode, resulting in 3 times higher catalytic currents comparing with that at the Pt/C electrode [21, 22]. Nitrogen doping could
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enhance the electrochemical activity of carbon nanotubes in Li-ion batteries [23], and the defects from doping benefit the diffusion of lithium ions into the interwall space as storage regions [24]. Work function is a key characteristic parameter for different
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CNT applications, including field emission and FET devices. Experiments also showed that the reduction of work function benefits the ORR activity after the
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heteroatom doping [25]. The work function could drop to 4.1 to 4.5 eV level after
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nitrogen doping under different concentrations [26].
There are growing interests on the understanding of CNT properties with nitrogen
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doping, including the structural stabilities and work function behaviors under high N
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concentrations. Even nitrogen contents of up to 30 at.% were realized in the experimental investigations [27, 28], theoretical studies focused mainly on low
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concentration doping [2, 29]. In this work, we investigated the structural and electronic properties of the (5,
5) single-walled carbon nanotube (SWNT), including
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the bonding structures, the work functions, the density of states (DOS), and the charge transferring performances, for nitrogen doping with different sites and densities.
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2. Computational methods
The study is performed within the first-principles Density Functional Theory
(DFT) by using the projector-augmented plane wave (PAW) method [30, 31] of a plane wave basis implemented in the Vienna ab initio simulation package (VASP) [32, 33]. The generalized gradient approximation (GGA) with Perdew−Burke−Ernzerh (PBE) functional [34] and a plane-wave basis set with a cut-off energy of 360 eV are adopted in all computations. Following electronic states are used as valence: C, 2s22p2;
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N, 2s22p3. Two k-points mesh grids of (3 × 3 × 1) and (5 × 5 × 1) for sampling the Brillouin zone (BZ) are used in structure optimization and DOS calculation, respectively. The convergence criterion is set as 10−4 eV in energy and 10−2 eV/Å in
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force. A Methfessel−Paxton [35] electronic energy smearing of 0.15 eV has been used to improve the convergences.
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The models of the capped (5, 5) metallic SWNTs are reconstructed after nitrogen
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doping. The pristine SWNT of 90 atoms, with half of C60 as the cap at one end and ten unsaturated dangling bonds at the other end, is defined into five types of sites,
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as shown in Fig. 1. For doping of more than two atoms, N atoms substitute for C
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atoms randomly. The supercell is placed in a box with dimensions of 18 × 18 × 27 Å during all the CNT calculations under a vacuum separation of 12 Å in each
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direction to avoid the interaction between adjacent nanotubes. The positions of all atoms are fully relaxed during the geometry optimization. The work function is
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defined as WF = Φ − EF, where Φ is the vacuum level and EF is the Fermi level of the system. For the doping of n nitrogen atoms, the formation energy (FE) of the N-doped SWNT is defined as
ESWNT nC nN ESWNT nEN nEC n
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EFE
(1)
where ESWNT−nC+nN, ESWNT, EN, and EC are total energies for N-doped SWNT, pristine SWNT, N atom, and C atom, respectively. The Bader charge analysis [36-38] is carried out to determine the amounts of charge transfer between carbon atoms and nitrogen atoms.
3. Results and discussions
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With N doping under different concentrations, the CNT structures change accordingly [40-42], and the graphite sp2-hybridization could be modified to the sp3-state [4]. The C−N bond lengths (dC−N) with single N dopant at different atomic
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layers are calculated, as shown in Fig. 1b. The C−N bond lengths of 1.41 Å and 1.4 Å are slightly shorter than the graphite C−C bond length of 1.43 Å, while the neighbor
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C−C bonds are not significantly affected, agreeing well with theoretical [43] and
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experimental [44] studies. Thus, the geometrical structure of the sp2 carbon network does not change obviously with single nitrogen atom doping [45]. In contrast, for
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adsorption of nitrogen atom on the tube wall the C−N bond lengths vary widely,
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from 1.35 Å to 1.42 Å [39]. Fig. 2 shows single N atom doped SWNT structures with the Bader charge transfers illustrated. The Nitrogen atom at different sites gains 1.12
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e to 1.17 e from carbon atoms, about 0.1 e to 0.4 e more than the charge gains with N adsorption [39], suggesting better ORR enhancement effect [46, 47]. There are
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about 0.04 e to 0.17 e more charge transfers comparing with N doping in the (8, 0) SWNT [48], which may imply that CNTs of small diameters possess higher oxygen
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reduction activities.
For the doping of two N atoms, one N atom is fixed in the middle of the wall
and the other one moves from top down, corresponding to six configurations, as shown in Fig. 3. After the doping, both the tube curvatures and the C−C bond lengths around N atoms change little, similar with those after single N atom doping. When two N atoms are doped adjacently, each atom gains charges of less than 0.9 e. The charge transfers from the direct bonded C atoms accounts for about 80% of the total
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charge gains at N atoms. The formation energies with single and two N atom doping are shown in Table 1. Nitrogen doping is endothermic, and the formation energies are site related. For the
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single N atom doping, the smallest formation energy, referring to the preferable doping site, appears at the fourth layer. The formation energies with the doping of
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two N atoms are higher, and the preferable doping is for two N-atoms far apart. These
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suggest that the two N-atom doping is more stable, attributed to appearances of more sp3 states and C-N bonds which are stronger than C−C bonds [4, 49].
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The structural and electronic properties are further investigated under high N
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contents, i. e., 10 at.%, 20 at.%, 23.3 at.%, and 30 at.%, respectively, as shown in Fig. 5. Under N contents of 23.3 at.% and 30 at.%, some C−N bonds break up, creating
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serious defects. When N atoms accumulate, the interactions of C−N bonds are strengthened, which weakens the neighbor C−C bonds, leading to the extension and
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collapse of the C−C bond. Such structural deformation would change the CNT properties seriously, but may have positive impacts on some applications, including
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hydrogen storage and chemical active devices. The work functions with different N doping concentrations are investigated as
well. Generally, the work function drops after N doping, comparing with 4.89 eV for the pristine SWNT. With single N atom doping, the work function climbs for doping from the top to the tube, as illustrated in Fig. 5a. The work function reduction is related to local vacancies induced by the tube curvature increase after the N atom incorporation [50]. The structure change due to shorter C−N bond lengths will shift
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the Fermi level toward the vacuum level [51-53], resulting in the work function reduction. For two N atom doping, the work function increases roughly with two N atoms moving closer, as shown in Fig. 5b. In comparison, N atom adsorptions result
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in the increases of the work function [39]. The effective work functions under different N doping concentrations are also calculated, as shown in Fig. 5c. The work
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function drops sharply with the increase of the concentration and reaches the lowest
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value of 4.1 eV at 10 at.%, then climbs and runs up to 4.25 eV at 30 at.%. This work function increase is probably related to the change of local curvature and the increase
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of defects.
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The projected density of states (PDOS) near the Fermi levels for pristine and N-doped SWNTs are calculated, as shown in Fig. 6. After N doping, the (5, 5) SWNT
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remains metallic, and the DOS near the Fermi level shifts higher, leading to the increase of the electrical conductivity, which indicates that the hybridizations take
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place mainly between N−p and C−s & C−p orbitals, and the N−p orbit also plays a role in the electron emission at the Fermi level [26]. It also shows that the
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hybridizations are more and more pronounced with increasing N atoms. The more the N atoms doped, the higher the Fermi level shifts up, attributed to the destroy of the structural symmetry and the filling of more electrons to the C−C anti-bonding states, resulting in the reduction of work function. Meanwhile, with the increase of N contents, the DOS at Fermi level rises, benefiting the electron emission.
4. Conclusion In summary, the structural and electronic properties of the capped (5, 5) SWNT of
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90 atoms are investigated for substitutional nitrogen atom doping by the first-principles DFT calculation. The simulations are conducted for various doping sites and different N concentrations of up to 30 at.%. The geometrical structures
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change little with single or two N atom doping, but C−N bonds may extend and collapse for N concentrations of 23.3 at.% and beyond, related to the weakening of
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the C−C bonds due to stronger C−N interactions. The work function, which is doping
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site dependent, drops after N atom incorporation and reaches 4.1 eV at 10 at.% N concentration, attributed to the local vacancies induced by the tube curvature change.
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DOS analysis indicates that the hybridization takes place mainly between C−s, C−p,
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and N−p orbitals, and the N−p orbit also plays a role in the electron emission.
Acknowledgments
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This work is supported by National Natural Science Foundation of China (Grants No. 61125101 and No. 11274244), and the Foundation of Zhejiang Educational
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Committee (Grant No. Y201430416).
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Figure captions Fig. 1. (a) The structure of the pristine (5, 5) SWNT. (b) The C−N bond lengths with single N atom doping in different atomic layers, comparing with C−N bond lengths
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with N adsorption at corresponding layers [39]. Fig. 2. Bond structures of single N atom doping on different sites of (5, 5) SWNTs. (a),
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(b), (c), (d), and (e) represent N doping at the first, second, third, fourth layers, and
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the tube, respectively.
Fig. 3. Bond structures of six configurations with doping of two N atoms.
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Fig. 4. SWNT structures under high N doping concentrations. (a) 10 at.%. (b) 20 at.%.
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(c) 23.3 at.%. (d) 30 at.%.
Fig. 5. Work functions of (5, 5) SWNT with different types of N doping. (a) Single N
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atom at different sites. (b) Two N atoms under different site configurations. (c) Different doping concentrations.
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Fig. 6. DOS for pristine and N-doped (5, 5) SWNTs. (a) Pristine SWNT. (b) Single N
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doping. (c) Two N doping. (d) 10 at.% N doping. (e) 20 at.% N doping.
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1.43 1.42 1.41 1.40 1.39 1.38 1.37 1.36 1.35 1.34
N-adsorbed N-doped
Top
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Bond length of C–N
Fig. 1
Second Third
Fourth
Tube
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Site
(b)
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an
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(a)
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(b)
(c)
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cr
(a)
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Fig. 2
(e)
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ce pt
ed
M
(d)
Page 21 of 27
ip t
Fig. 3
\
(c)
cr
(b)
M
an
us
(a)
(e)
(f)
Ac
ce pt
ed
(d)
Page 22 of 27
(c)
(d)
cr
(b)
Ac
ce pt
ed
M
an
us
(a)
ip t
Fig. 4
Page 23 of 27
Fig. 5 4.8
4.7 4.6 4.5 4.4 4.3 4.2 4.1 4.0
Top
Second
Third
Fourth
4.7 4.6 4.5 4.4 4.3 4.2 4.1 4.0
Tube
1
2
3
4
6
cr
4.7 4.6 4.5 4.3 4.2 4.1 0.01% 0.02% 10%
20% 23.3% 30%
Concentration of N atoms
(a)
(b)
Ac
ce pt
ed
M
(c)
us
4.4
an
Work function (eV)
4.8
4.0
5
Site configuration
Site
ip t
Work function (eV)
Work function (eV)
4.8
Page 24 of 27
Fig. 6 40
(a)
Total
DOS
30 20
-1
0
1
2
(b)
DOS
20
cr
Total C-s C-p N-s N-p
30
10
-1
10
20
0
1
2
(d)
Total C-s C-p N-s N-p
ce pt
DOS
30
-1
ed
0 -2 40
2
M
20
1
(c)
Total C-s C-p N-s N-p
30
DOS
0
an
0 -2 40
us
0 -2
40
ip t
10
10
0 -2 40
-1
1
Total C-s C-p N-s N-p
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30
DOS
0
20
2
(e)
10
0 -2
-1
0
Energy (eV)
1
2
Page 25 of 27
Page 26 of 27
ed
ce pt
Ac
us
an
M
cr
ip t
Tables Table 1. Formation energies with single or two N atoms doping on different sites Top
Second
Third
Fourth
Tube
Formation energy (eV)
2.95
2.77
2.80
2.65
3.05
Site
1st
2nd
3rd
4th
5th
Formation energy (eV)
3.18
3.22
3.19
3.20
6th
3.86
3.81
Ac
ce pt
ed
M
an
us
cr
Two N atoms
Site
ip t
Single N atom
Page 27 of 27
S0169-4332(16)30132-5 http://dx.doi.org/doi:10.1016/j.apsusc.2016.01.271 APSUSC 32503
To appear in:
APSUSC
Received date: Revised date: Accepted date:
18-12-2015 25-1-2016 29-1-2016
Please cite this article as: X. Shao, D. Li, J. Cai, H. Luo, C. Dong, Firstprinciples study of structural and work function properties for nitrogendoped single-walled carbon nanotubes, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.01.271 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
Substitutional nitrogen atom doping in capped (5, 5) SWNT is investigated.
Serious defects appear from breaks of C−N bonds with N contents of above
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ed
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an
us
cr
Work function drops after N doping and may reach 4.1 eV.
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23.3 at.%.
Page 1 of 27
Graphical abstract Serious structural defects appear after breaks of C−N bonds under N doping concentration of 30 at.%. The work function drops sharply with the increase of N concentration and reaches the lowest value of 4.1 eV at 10 at.%.
ip t
4.7 4.6
cr
4.5 4.4 4.3 4.2
us
Work function (eV)
4.8
4.1 4.0
0.01% 0.02%
10%
20%
23.3%
30%
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ed
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Concentration of N atoms
Page 2 of 27
First-principles study of structural and work function properties for nitrogen-doped single-walled carbon nanotubes
a
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Xiji Shaoa, Detian Lia, b,*, Jianqiu Caia, Haijun Luoa, Changkun Donga, b,* Institute of Micro-nano Structures & Optoelectronics, Wenzhou University, Wenzhou,
Science and Technology on Vacuum & Cryogenics Technology and Physics
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b
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Zhejiang 325035, People’s Republic of China
Laboratory, Lanzhou Institution of Physics, Lanzhou, Gansu 730000, People’s
Ac
ce pt
ed
M
an
Republic of China
*
Corresponding Authors: [email protected], [email protected]
Page 3 of 27
Abstract The structural and electronic properties of the capped (5, 5) single-walled carbon nanotube (SWNT), including the structural stability, the work function, and the
ip t
charge transfer performance, are investigated for the substitutional nitrogen atom doping under different concentrations by first-principles density functional theory.
cr
The geometrical structure keeps almost intact with single or two N atom doping,
us
while C−N bonds may break up with serious defects for N concentrations of 23.3 at.% and above. The SWNT remains metallic and the work function drops after doping due
an
to the upward shift of Fermi level, leading to the increase of the electrical
M
conductivity. N doping enhances the oxygen reduction activity stronger than N
ed
adsorption because of higher charge transfers.
Keywords
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First-principles, Single-walled carbon nanotubes, Nitrogen, Doping, Work function
Page 4 of 27
1. Introduction Impurity doping is an effective approach to modify the properties of carbon nanotubes (CNTs) on mechanical, electronic, optical, and other aspects.
ip t
Substitutional doping with various elements, including boron [1, 2], nitrogen [1-5], and silicon [6, 7], has been investigated for different types of CNTs. The
cr
substitutional nitrogen atom doping (nitrogen doping for short) impacts
us
significantly the device properties for many CNT applications, such as field emission [8, 9], energy storage [10], gas sensing [11, 12], composite material [13], and catalysis
an
[14-17].
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Nitrogen can be viewed as an n-type carbon dopant, leading to the sensitive shift of the Fermi level (EF) [18]. The doping of nitrogen atoms in CNTs could change the
ed
local chemical reactivity and the work function, also generate defects in curved structures [19]. Meanwhile, nitrogen doping can change the formation of the localized
ce pt
electron-donor state near the Fermi level, resulting in the improvement of the electrical conductivity. With nitrogen doping at different sites, the local curvatures
Ac
could be distorted, leading to the increase of the field enhancement factor for field emission [18]. Nitrogen doping can create well localized and chemically active sites [20]. In fuel cell applications, nitrogen-doped carbon nano-material electrodes showed encouraging oxygen reduction reaction (ORR) activity and long-term operation stability. Nitrogen induced charge delocalization at CNT or graphene electrode changes the O2 chemisorption mode, resulting in 3 times higher catalytic currents comparing with that at the Pt/C electrode [21, 22]. Nitrogen doping could
Page 5 of 27
enhance the electrochemical activity of carbon nanotubes in Li-ion batteries [23], and the defects from doping benefit the diffusion of lithium ions into the interwall space as storage regions [24]. Work function is a key characteristic parameter for different
ip t
CNT applications, including field emission and FET devices. Experiments also showed that the reduction of work function benefits the ORR activity after the
cr
heteroatom doping [25]. The work function could drop to 4.1 to 4.5 eV level after
us
nitrogen doping under different concentrations [26].
There are growing interests on the understanding of CNT properties with nitrogen
an
doping, including the structural stabilities and work function behaviors under high N
M
concentrations. Even nitrogen contents of up to 30 at.% were realized in the experimental investigations [27, 28], theoretical studies focused mainly on low
ed
concentration doping [2, 29]. In this work, we investigated the structural and electronic properties of the (5,
5) single-walled carbon nanotube (SWNT), including
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the bonding structures, the work functions, the density of states (DOS), and the charge transferring performances, for nitrogen doping with different sites and densities.
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2. Computational methods
The study is performed within the first-principles Density Functional Theory
(DFT) by using the projector-augmented plane wave (PAW) method [30, 31] of a plane wave basis implemented in the Vienna ab initio simulation package (VASP) [32, 33]. The generalized gradient approximation (GGA) with Perdew−Burke−Ernzerh (PBE) functional [34] and a plane-wave basis set with a cut-off energy of 360 eV are adopted in all computations. Following electronic states are used as valence: C, 2s22p2;
Page 6 of 27
N, 2s22p3. Two k-points mesh grids of (3 × 3 × 1) and (5 × 5 × 1) for sampling the Brillouin zone (BZ) are used in structure optimization and DOS calculation, respectively. The convergence criterion is set as 10−4 eV in energy and 10−2 eV/Å in
ip t
force. A Methfessel−Paxton [35] electronic energy smearing of 0.15 eV has been used to improve the convergences.
cr
The models of the capped (5, 5) metallic SWNTs are reconstructed after nitrogen
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doping. The pristine SWNT of 90 atoms, with half of C60 as the cap at one end and ten unsaturated dangling bonds at the other end, is defined into five types of sites,
an
as shown in Fig. 1. For doping of more than two atoms, N atoms substitute for C
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atoms randomly. The supercell is placed in a box with dimensions of 18 × 18 × 27 Å during all the CNT calculations under a vacuum separation of 12 Å in each
ed
direction to avoid the interaction between adjacent nanotubes. The positions of all atoms are fully relaxed during the geometry optimization. The work function is
ce pt
defined as WF = Φ − EF, where Φ is the vacuum level and EF is the Fermi level of the system. For the doping of n nitrogen atoms, the formation energy (FE) of the N-doped SWNT is defined as
ESWNT nC nN ESWNT nEN nEC n
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EFE
(1)
where ESWNT−nC+nN, ESWNT, EN, and EC are total energies for N-doped SWNT, pristine SWNT, N atom, and C atom, respectively. The Bader charge analysis [36-38] is carried out to determine the amounts of charge transfer between carbon atoms and nitrogen atoms.
3. Results and discussions
Page 7 of 27
With N doping under different concentrations, the CNT structures change accordingly [40-42], and the graphite sp2-hybridization could be modified to the sp3-state [4]. The C−N bond lengths (dC−N) with single N dopant at different atomic
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layers are calculated, as shown in Fig. 1b. The C−N bond lengths of 1.41 Å and 1.4 Å are slightly shorter than the graphite C−C bond length of 1.43 Å, while the neighbor
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C−C bonds are not significantly affected, agreeing well with theoretical [43] and
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experimental [44] studies. Thus, the geometrical structure of the sp2 carbon network does not change obviously with single nitrogen atom doping [45]. In contrast, for
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adsorption of nitrogen atom on the tube wall the C−N bond lengths vary widely,
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from 1.35 Å to 1.42 Å [39]. Fig. 2 shows single N atom doped SWNT structures with the Bader charge transfers illustrated. The Nitrogen atom at different sites gains 1.12
ed
e to 1.17 e from carbon atoms, about 0.1 e to 0.4 e more than the charge gains with N adsorption [39], suggesting better ORR enhancement effect [46, 47]. There are
ce pt
about 0.04 e to 0.17 e more charge transfers comparing with N doping in the (8, 0) SWNT [48], which may imply that CNTs of small diameters possess higher oxygen
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reduction activities.
For the doping of two N atoms, one N atom is fixed in the middle of the wall
and the other one moves from top down, corresponding to six configurations, as shown in Fig. 3. After the doping, both the tube curvatures and the C−C bond lengths around N atoms change little, similar with those after single N atom doping. When two N atoms are doped adjacently, each atom gains charges of less than 0.9 e. The charge transfers from the direct bonded C atoms accounts for about 80% of the total
Page 8 of 27
charge gains at N atoms. The formation energies with single and two N atom doping are shown in Table 1. Nitrogen doping is endothermic, and the formation energies are site related. For the
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single N atom doping, the smallest formation energy, referring to the preferable doping site, appears at the fourth layer. The formation energies with the doping of
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two N atoms are higher, and the preferable doping is for two N-atoms far apart. These
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suggest that the two N-atom doping is more stable, attributed to appearances of more sp3 states and C-N bonds which are stronger than C−C bonds [4, 49].
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The structural and electronic properties are further investigated under high N
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contents, i. e., 10 at.%, 20 at.%, 23.3 at.%, and 30 at.%, respectively, as shown in Fig. 5. Under N contents of 23.3 at.% and 30 at.%, some C−N bonds break up, creating
ed
serious defects. When N atoms accumulate, the interactions of C−N bonds are strengthened, which weakens the neighbor C−C bonds, leading to the extension and
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collapse of the C−C bond. Such structural deformation would change the CNT properties seriously, but may have positive impacts on some applications, including
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hydrogen storage and chemical active devices. The work functions with different N doping concentrations are investigated as
well. Generally, the work function drops after N doping, comparing with 4.89 eV for the pristine SWNT. With single N atom doping, the work function climbs for doping from the top to the tube, as illustrated in Fig. 5a. The work function reduction is related to local vacancies induced by the tube curvature increase after the N atom incorporation [50]. The structure change due to shorter C−N bond lengths will shift
Page 9 of 27
the Fermi level toward the vacuum level [51-53], resulting in the work function reduction. For two N atom doping, the work function increases roughly with two N atoms moving closer, as shown in Fig. 5b. In comparison, N atom adsorptions result
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in the increases of the work function [39]. The effective work functions under different N doping concentrations are also calculated, as shown in Fig. 5c. The work
cr
function drops sharply with the increase of the concentration and reaches the lowest
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value of 4.1 eV at 10 at.%, then climbs and runs up to 4.25 eV at 30 at.%. This work function increase is probably related to the change of local curvature and the increase
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of defects.
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The projected density of states (PDOS) near the Fermi levels for pristine and N-doped SWNTs are calculated, as shown in Fig. 6. After N doping, the (5, 5) SWNT
ed
remains metallic, and the DOS near the Fermi level shifts higher, leading to the increase of the electrical conductivity, which indicates that the hybridizations take
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place mainly between N−p and C−s & C−p orbitals, and the N−p orbit also plays a role in the electron emission at the Fermi level [26]. It also shows that the
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hybridizations are more and more pronounced with increasing N atoms. The more the N atoms doped, the higher the Fermi level shifts up, attributed to the destroy of the structural symmetry and the filling of more electrons to the C−C anti-bonding states, resulting in the reduction of work function. Meanwhile, with the increase of N contents, the DOS at Fermi level rises, benefiting the electron emission.
4. Conclusion In summary, the structural and electronic properties of the capped (5, 5) SWNT of
Page 10 of 27
90 atoms are investigated for substitutional nitrogen atom doping by the first-principles DFT calculation. The simulations are conducted for various doping sites and different N concentrations of up to 30 at.%. The geometrical structures
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change little with single or two N atom doping, but C−N bonds may extend and collapse for N concentrations of 23.3 at.% and beyond, related to the weakening of
cr
the C−C bonds due to stronger C−N interactions. The work function, which is doping
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site dependent, drops after N atom incorporation and reaches 4.1 eV at 10 at.% N concentration, attributed to the local vacancies induced by the tube curvature change.
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DOS analysis indicates that the hybridization takes place mainly between C−s, C−p,
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and N−p orbitals, and the N−p orbit also plays a role in the electron emission.
Acknowledgments
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This work is supported by National Natural Science Foundation of China (Grants No. 61125101 and No. 11274244), and the Foundation of Zhejiang Educational
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Committee (Grant No. Y201430416).
Page 11 of 27
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1184–1189. [49] S.H. Lim, R.J. Li, W. Ji, J.Y. Lin, Effects of nitrogenation on single-walled
cr
carbon nanotubes within density functional theory, Phys. Rev. B 76 (2007)
us
195406−195421.
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an
carbon nanotubes and bundles, Phys. Rev. B 65 (2002) 193401−193404.
M
[51] S. Suzuki, Y. Watanabe, Y. Homma, S. Fukuba, S. Heun, A. Locatelli, Work functions of individual single-walled carbon nanotubes, Appl. Phys. Lett. 85 (2004)
ed
127–129.
[52] B. Shan, K. Cho, First-principles study of work functions of double-wall carbon
ce pt
nanotubes, Phys. Rev. B 73 (2006) 081401–081404. [53] C. Kim, B. Kim, S.M. Lee, C. Jo, Y.H. Lee, Electronic structures of capped
Ac
carbon nanotubes under electric fields, Phys. Rev. B 65 (2002) 165418–165423.
Page 18 of 27
Figure captions Fig. 1. (a) The structure of the pristine (5, 5) SWNT. (b) The C−N bond lengths with single N atom doping in different atomic layers, comparing with C−N bond lengths
ip t
with N adsorption at corresponding layers [39]. Fig. 2. Bond structures of single N atom doping on different sites of (5, 5) SWNTs. (a),
cr
(b), (c), (d), and (e) represent N doping at the first, second, third, fourth layers, and
us
the tube, respectively.
Fig. 3. Bond structures of six configurations with doping of two N atoms.
an
Fig. 4. SWNT structures under high N doping concentrations. (a) 10 at.%. (b) 20 at.%.
M
(c) 23.3 at.%. (d) 30 at.%.
Fig. 5. Work functions of (5, 5) SWNT with different types of N doping. (a) Single N
ed
atom at different sites. (b) Two N atoms under different site configurations. (c) Different doping concentrations.
ce pt
Fig. 6. DOS for pristine and N-doped (5, 5) SWNTs. (a) Pristine SWNT. (b) Single N
Ac
doping. (c) Two N doping. (d) 10 at.% N doping. (e) 20 at.% N doping.
Page 19 of 27
1.43 1.42 1.41 1.40 1.39 1.38 1.37 1.36 1.35 1.34
N-adsorbed N-doped
Top
ip t
Bond length of C–N
Fig. 1
Second Third
Fourth
Tube
cr
Site
(b)
Ac
ce pt
ed
M
an
us
(a)
Page 20 of 27
(b)
(c)
an
us
cr
(a)
ip t
Fig. 2
(e)
Ac
ce pt
ed
M
(d)
Page 21 of 27
ip t
Fig. 3
\
(c)
cr
(b)
M
an
us
(a)
(e)
(f)
Ac
ce pt
ed
(d)
Page 22 of 27
(c)
(d)
cr
(b)
Ac
ce pt
ed
M
an
us
(a)
ip t
Fig. 4
Page 23 of 27
Fig. 5 4.8
4.7 4.6 4.5 4.4 4.3 4.2 4.1 4.0
Top
Second
Third
Fourth
4.7 4.6 4.5 4.4 4.3 4.2 4.1 4.0
Tube
1
2
3
4
6
cr
4.7 4.6 4.5 4.3 4.2 4.1 0.01% 0.02% 10%
20% 23.3% 30%
Concentration of N atoms
(a)
(b)
Ac
ce pt
ed
M
(c)
us
4.4
an
Work function (eV)
4.8
4.0
5
Site configuration
Site
ip t
Work function (eV)
Work function (eV)
4.8
Page 24 of 27
Fig. 6 40
(a)
Total
DOS
30 20
-1
0
1
2
(b)
DOS
20
cr
Total C-s C-p N-s N-p
30
10
-1
10
20
0
1
2
(d)
Total C-s C-p N-s N-p
ce pt
DOS
30
-1
ed
0 -2 40
2
M
20
1
(c)
Total C-s C-p N-s N-p
30
DOS
0
an
0 -2 40
us
0 -2
40
ip t
10
10
0 -2 40
-1
1
Total C-s C-p N-s N-p
Ac
30
DOS
0
20
2
(e)
10
0 -2
-1
0
Energy (eV)
1
2
Page 25 of 27
Page 26 of 27
ed
ce pt
Ac
us
an
M
cr
ip t
Tables Table 1. Formation energies with single or two N atoms doping on different sites Top
Second
Third
Fourth
Tube
Formation energy (eV)
2.95
2.77
2.80
2.65
3.05
Site
1st
2nd
3rd
4th
5th
Formation energy (eV)
3.18
3.22
3.19
3.20
6th
3.86
3.81
Ac
ce pt
ed
M
an
us
cr
Two N atoms
Site
ip t
Single N atom
Page 27 of 27