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Non-Local and Local Electrochemical Effects of Doping Impurities on the Reactivity of Graphene Penglai Gong, Liangfeng Huang, Xiaohong Zheng, Yongsheng Zhang, and Zhi Zeng J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 28 Apr 2015 Downloaded from http://pubs.acs.org on April 29, 2015

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The Journal of Physical Chemistry

Non-Local and Local Electrochemical Effects of Doping Impurities on the Reactivity of Graphene Peng Lai Gong,† Liang Feng Huang,†,‡ Xiao Hong Zheng,† Yong Sheng Zhang,† and Zhi Zeng∗,†,¶ Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China, Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208, USA, and Department of Physics, University of Science and Technology of China, Hefei 230026, People’s Republic of China E-mail: [email protected]

∗ To

whom correspondence should be addressed Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China ‡ Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208, USA ¶ Department of Physics, University of Science and Technology of China, Hefei 230026, People’s Republic of China † Key

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Abstract

1

2

The chemical reactivity of graphene doped by B or N (B/N-G) toward H adatom has been

3

studied systematically using density functional theory. From the site dependence of the ad-

4

sorption energy of hydrogen adatom, the non-local and local charge-doping effects and local

5

strain effect of B and N impurities on the chemical reactivity of graphene are derived. The

6

non-local doping charges originate from nonlocal aromatic electron resonance, but the local

7

doping charges are bonded to the vicinity of B/N dopant as a result of its high/low inheren-

8

t chemical potential. Both of nonlocal and local charge-doping effects coexist in B-doped

9

or N-doped graphene, while nonlocal charge-doping effect will be largely suppressed in BN-

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codoped graphene. The non-locally distributed doping holes/electrons in graphene enhance

11

the stability of H adatom within the range of at least 9 Å away from the B/N dopants, while lo-

12

cally distributed holes/electrons in the vicinity of the B or N impurities only has a considerable

13

stabilizing effect on the H adatom close (∼2 Å) to the dopants. These nonlocal and local elec-

14

trochemical effects revealed here are useful for further doping-charge controlling and chemical

15

engineering in doped graphene. Our results also clarify the issue of that the dopant-induced

16

strain has a negligible effect on the enhanced stability of H adatom.

17

1. Introduction

18

Recently, graphene (G) has attracted extensive research interest due to its unusual physical and

19

chemical properties, as well as the vision in energy storage and electrode materials and so on. 1,2 It

20

has a perfect hexagonal structure with all valence electrons bonding with each other, resulting in its

21

relative inert chemical reactivity under various environments. Improving the chemical reactivity

22

of graphene, the takeup of adatoms as H, alkali metal or transition metals (TM) can be increased,

23

which is useful in the area of hydrogen storage or catalysis. 3–5

24

Charge doping is an effective method to tailor the electronic properties and improves the reac-

25

tivity of materials. It can be easily realized in graphene via charge transfer induced, for example,

26

by substrate 6–9 or adsorbate, 2,10,11 gate biasing, 4,12 and substitutional B/N dopants. 13–17 Huang 2 ACS Paragon Plus Environment

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et at. have revealed that the chemical reactivity of graphene can be improved by both electron

2

and hole doping, which enhances the thermodynamic stability of H adatom on graphene. 4 Another

3

method to realize charge doping in graphene is to introduce atomic impurities (e.g., B or N) with

4

valence number smaller (hole doping) or larger (electron doping) than that of C atom. 3,10,18–21

5

Based on the theoretical results, Muhich pointed out the fact that the binding energy between Pt

6

and graphene substrates and the capacity of Pt on substrates can be effectively increased after the

7

incorporation of impurities (B or N atoms) into substrates. 3

8

To understand the charge-doping effect on the H-graphene binding strength, we 4 have theo-

9

retically doped pristine and hydrogenated graphene with homogeneous artificial charges, and our

10

simulation results indicated that the electron occupation in the bonding π and anti-bonding π ∗

11

bands plays an important role in the chemical reactivity of graphene. Such homogeneous charge

12

doping mimics the graphene under gate bias to some extent. However, the excess charges intro-

13

duced by B or N dopants in graphene have a non-homogeneous distribution, which has been well

14

characterized by STM experiments and theoretical simulations. 22–24 On the other hand, the strain

15

field induced by dopants also has a tendency to influence the adatom stability. 25–27 It has also been

16

found that the adatoms attractively interact with vacancy-type defects in graphene within the range

17

of 20∼30 Å. 25,26 Although this nonlocal attraction has been only ascribed to the combination of

18

the strain field and strain-induced electronic effect of these defects, 25,26 the vacancies also both

19

nonlocally perturb the charge density around the defects and cause a self-doping in graphene, 28–31

20

which will probably contribute to the non-local interaction between adatom and defected graphene.

21

A clear physical picture for the strain and charge effects is required to solve this problem. B and

22

N dopants both introduce doping charges (including local doping charges in the vicinity of B or

23

N dopants and non-local doping charges in graphene matrix) and induce strain field in graphene

24

matrix, which could affect its chemical reactivity. In order to disclose these electrochemical and

25

elastochemical effects of B and N dopants in graphene, the interaction mechanism between the H

26

adatom and B/N-doped graphene needs an in-depth investigation.

27

In the present work, the interaction mechanism between the H adatom and B/N-G has been

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1

studied systematically by density functional theory (DFT) calculations. In order to reveal the

2

chemical reactivity of B/N-doped graphene, as well as to understand the effects of doping charges

3

(nonlocal and local) and strain, the adsorption energy of H adatom on various sites of B/N-G

4

substrate is calculated. It is found that the enhancement of adsorption stability of H adatom is

5

contributed by both non-local and local electrochemical effects of B and N dopants. The dopant-

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induced strain effect (elastochemical effect) on the enhanced stability of H adatom is negligible.

7

The related electronic mechanisms are thoroughly interpreted by detailed analysis.

8

2. Computational details

9

Our first-principle calculation is based on density functional theory (DFT) using Perdew-Burke-

10

Ernzerhof (PBE) 32 functional within generalized gradient approximation (GGA) as implemented

11

in Quantum Espresso package. 33 An ultrasoft pseudopotential 34 has been used to describe the

12

electron-ion interaction. The energy cutoffs for electron wave function and charge density are

13

30 and 300 Ry, respectively. The Methfessel-Paxton smearing technique 35 is adopted, with an

14

energy width of 0.01 Ry. All configurations considered in the calculations have been fully relaxed

15

until the Hellmann-Feynman forces drop below 10−3 Ry/bohr and the self-consistence-field (SCF)

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convergence threshold for electronic energy is set to be 10−8 Ry. The chemical reactivity of doped

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H (X −G) and and pristine graphene can be reflected by the adsorption energy of H adatom (Eads

18

H (G) ), which are successively calculated according to the following equations: Eads

H H X−G [email protected]−G Eads (X −G) = Etot + Etot − Etot ,

(1)

[email protected] G H H , − Etot + Etot Eads (G) = Etot

(2)

19

where X-G stands for graphene with a single B dopant (B-G), graphene with a single N dopant

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H , E X−G , E [email protected]−G , E G and E [email protected] denote (N-G) or graphene with a B-N dopant pair (BN-G). Etot tot tot tot tot

21

the total energy of an isolated H atom, X-G, H-adsorbed X-G, G, and H-adsorbed G, respectively.

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In order to reveal the chemical reactivity of B/N-doped graphene, as well as to understand the 4 ACS Paragon Plus Environment

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effects of doping charges and strain, a periodic graphene supercell of (17.06 Å×17.26 Å) with a

2

single B/N dopant [Fig. 1(a)] or a B-N dopant pair [Fig. 1(b)] is used. This relatively large su-

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percell is necessary here, because of the non-local distribution of the doping charges in B/N-doped

4

graphene. 22–24 The interlayer interaction diminishes when the separation between neighboring pe-

5

riodic layers is as large as 12 Å. In hydrogenated graphene, there is no observable direct interaction

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between different H adatoms, while the aromatic mesomeric effect 36,37 (i.e., electronic resonance)

7

in graphene will induce an indirect H–H interaction, resulting in the configuration dependence

8

of adsorption stability and magnetism of hydrogenated graphene. 38–41 However, the response of

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the strengths of the C-H and C-C bonds to charge doping does not qualitatively vary in different

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adatom configurations. 4,42 It has also been indicated that if the H–H interaction is neglected, the

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B/N dopant has the same enhancing effect on the stabilities of single and multiple H adatoms on

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graphene. 20 Therefore, the single H adatom is chosen here to study the electrochemical and elas-

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tochemical effects of B and N dopants. 4,42 The adsorption sites considered here are categorized

14

into two groups, i.e., the armchair path (AP) and zigzag path (ZP), which are respectively the A1-

15

B2-A3-B4-A5-B6-A7-B8 and A1′ -B2′ -A3′ -B4′ -A5′ -B6′ -A7′ -B8′ paths labeled in Fig. 1(a). For

16

BN-G, only the C1-C2-C3-D3-D2-D1 path has been considered [Fig. 1(b)]. These paths also play

17

important roles in the dynamic behavior of H adatom 41,43 and the stability of graphene nanos-

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tructures. 44,45 These adsorption-site groups are named as [email protected], [email protected], [email protected],

19

[email protected] and [email protected], respectively. To further clarify such nomenclature, the [email protected]

20

G configuration, for instance, represents that the H atom is adsorbed on the carbon site within the

21

AP of B-G.

22

H (X − G)) depends on the charge doping (i.e., elecThe stability of H adatom on B/N-G (Eads

23

trochemical effect) and dopant-induced strain effect (i.e., elastochemical effect), which can be

24

expressed as H H Eads (X −G) = Eε (X −G)+EQ (X −G)+Eads (G),

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H (G) is the adsorption energy of H adatom on neutral equilibrium G, and E (X-G) and where Eads ε H (X-G), respecEQ (X-G) are the contributions of dopant-induced strain and charge doping to Eads

tively. In order to evaluate Eε (X-G), the stability of H adatom on the pristine graphene Eε (G) with a homogeneous mechanical strain is calculated firstly (Fig. 2). The dopant-induced strain of one C atom at S site (Strain(CS ), here S stands for the labeled C atom site in Figs. 1(a) and 1(b)) within AP or ZP of B/N-G, is defined as,

Strain(CS ) =

l1 +l2 +l3 3

− l0

l0

(4)

1

where l1 , l2 and l3 are the bond lengths of three nearest neighbors of CS . l0 is the C–C bond length

2

in equilibrium pristine graphene (1.422 Å). Using the derived Strain(CS ) (Equ. 4) as an input data,

3

we can obtain the Eε (X-G) from Eε (G) curve with the strain (Fig. 2). Then, the corresponding

4

EQ (X − G) can be derived by Equ. 3. Only part of Eε (B-G) and Eε (N-G) is displayed in Fig. 2.

5

Although B and N tend to bond with each other in graphene due to their Coulombic attraction, 24

6

in order to reveal the underlying electrochemical mechanism, B and N are separated in BN-G

7

[Fig. 1(b)], which enables us simultaneously demonstrate the nonlocal and local natures of doping

8

charges introduced by B and N. If setting B and N close to each other, the nonlocal nature can not be

9

derived from the short-range annihilation between holes and electrons. On the other hand, in BN-

10

G, the B and N dopants can occupy the same or different sublattice with similar electrochemical

11

effects. The former is chosen here due to its lower energy. These could be well verified by the

12

carrier distributions in BN-G with (1) B and N bonding with each other and (2) B and N occupying

13

the same sublattice (Fig. S1 in the online Supporting Information), as well as their comparison

14

with Fig. 4(a-c).

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3. Results and discussion

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B and N not only introduce doping charges (holes or electrons) but also induce the strain in

17

graphene matrix. Both charge doping and strain can affect the chemical reactivity of graphene, 6 ACS Paragon Plus Environment

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H ) on various carbon sites within which can be reflected by the adsorption energy of H adatom (Eads

2

H (B/N-G) oscillatorily decreases with the AP or ZP of B/N-G. As shown in Figs. 3(a)-3(d), Eads

3

increasing the distance between the adsorption site and the B or N dopant (R), and asymptotically

4

H (G)=0.82 eV of pristine graphene. This energetic oscillation is related with approaches the Eads

5

the effect of Friedel oscillation 46–48 in the charge distribution, which will be discussed in detail

6

H (B/N-G) along at the last paragraph of this section. In addition, the oscillation amplitude in Eads

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the AP [Figs. 3(a) and 3(c)] is larger than that along the ZP [Figs. 3(b) and 3(d)]. These re-

8

H (B/N-G) as well as the nonlocal effect of B sults indicate the adsorption-site dependence of Eads

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H (B-G) is ∼0.1 eV larger than (N) dopant in B-G (N-G). Mechanisms will be analyzed later. Eads

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H (N-G), implicating that the effect of holes on the stability of H adatom exceeds that of elecEads

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trons, which agrees well with Denis’s results 49 and Huang’s results 42 of doped and hydrogenated

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graphene with artificial charges. The same results demonstrate that the electrochemical effect of

13

B/N dopants prevails over the inter-atom orbital hybridization (B-C, N-C). When H is adsorbed

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H (BN-G) has a fast decay down to the on a site in the AP of BN-codoped graphene (BN-G), Eads

15

H (G) (0.82 eV) within a range about 2.45 Å [Figs. 3(e)-3(f)]. This indicates the vicinity of Eads

16

local effect of a BN dopant pair in BN-G. The strain of B/N dopants in graphene can influence the

17

stability of H adatom. The contribution from the strain of B/N dopants (Eε (B/N-G)) is displayed in

18

H (G)), Figs. 3(a)-3(f), for a better view, Eε (B/N-G) has been up-shifted by 0.82 eV (the value of Eads

19

labeled by Eε (B/N-G). In H-adsorbed B/N-G systems, the variation of of Eε (B/N-G) is only . 40

20

H is negligible. Our results are expected meV, which demonstrates that strain contribution to Eads

21

to deeply understand the effect of the strain of defects on the adatom. For example, the nonlocal

22

enhancement of the adatom stability has been ascribed to the combination of the strain field and

23

strain-induced electronic effect of these defects. 25,26 However, the vacancies also both nonlocally

24

perturb the charge density around the defects and cause a self-doping in graphene. 28–31 Similar as

25

to charge doping effect of B/N dopants, the self-doping effect of defects is also an electrochemical

26

effect that will mostly contribute to the interaction between adatom and defective graphene since

27

H by ∼20 meV. Our results clarify the issue of that the a biaxial strain of 2% only enhances Eads



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dopant-induced strain has a negligible effect on the enhanced stability of H adatom. In the rest,

2

the strain effect of B and N dopants on the chemical reactivity of B/N-doped graphene will not be

3

involved, and the discussion will concentrate on their significant electrochemical effect.

4

To reveal the electrochemical effect of B and N dopants on the graphene reactivity (or stability

5

of H adatom), we need further investigations on the related electronic mechanisms. Homogeneous

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charge doping by gate biasing to graphene can weaken the strength of an aromatic Π bond ( π

7

and π ∗ ) via electron occupation in π or π ∗ bands, leading to the improved adsorption energy of

8

H adatom. 4 However, B or N dopants can introduce non-homogeneous doping charges (holes or

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electrons) and the doping charges density differs from one C site to another. 22–24 These doping

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charges of B or N dopants include the non-locally distributed holes/electrons in graphene matrix

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and the locally distributed holes/electrons close to dopants. The former originates from aromatic

12

electron resonance 36,37 (or called mesomeric effect) and thus can easily spread in graphene matrix

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(i.e., non-local electrochemical effect), while the later is bonded to the vicinity of B/N dopants as a

14

result of their inherent chemical potential (i.e., local electrochemical effect). When H is adsorbed

15

on a C atom, it needs to break the Π bond, the strength of which is in relationship with the dis-

16

tribution of doping charges (carriers). Holes in B-G [Fig. 4(a)] and electrons in N-G [Fig. 4(b)]

17

clearly show the non-local distribution, along with the behavior of oscillatory decrease with the

18

increased distance from B or N dopants. Moreover, the oscillation of carriers density n in AP is

19

much stronger than that in ZP. For BN-G, both electrons and holes coexist in graphene matrix, and

20

the distribution is displayed in Fig. 4(c). It exhibits that the non-local doping charges are largely

21

canceled out and only part of local doping charges persist, in contrast to doping charges [Figs. 4(a)

22

and 4(b)]. We have also examined the electrochemical effect of BN-doped graphene with B and N

23

occupying the same graphene sublattice (Fig. S1 in the Supporting Information). In this case, the

24

carriers distribution shows that the nonlocal charge-doping effect is also largely suppressed (simi-

25

lar as Fig. 4(c)). Accordingly, the nonlocal nature of the electrochemical effect does not depend on

26

the sublattice occupation. The above results indicate that both non-local electrochemical effect and

27

local electrochemical effect do coexist in B-G or N-G, while the non-local electrochemical effect

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will be largely suppressed in BN-G. We can also derive the non-local electrochemical effect and

2

H (B/N-G) with R increasing. It can local electrochemical effect in B/N-G from the behavior of Eads

3

H (BN-G) has a fast decay down to nearly the E H (G) for pristine G within a range be seen that Eads ads

4

H (B/N-G) in about 2.45 Å [Figs. 3(e)-3(f)], inferring the local electrochemical effect. However, Eads

5

B-G or N-G has no obvious decaying variation within such short range [Figs. 3(a) and 3(d)], and

6

instead has a long-ranged oscillatorily decaying trend, with that the decaying range is at least 9 Å.

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H (B/N-G) is caused by the Friedel oscillation 46–48 in charge densiIn addition, the oscillation in Eads

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ty. Thus, the energy oscillation itself is also another proof for the nonlocal electrochemical effect of

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B and N dopants. These results have testified the local and non-local electrochemical effect in B-G

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and N-G. The special distribution of carries indicates the different strength of Π bond at a certain

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H (B/N-G) carbon site. If we carefully compare n at each carbon site within AP or ZP (Fig. 4) to Eads

12

[Figs. 3(a)-3(f)] at the corresponding site, they unexpectedly show the same behavior. We found

13

the correlation between carriers density n and the adsorption energy of H adatom on B/N-doped

14

graphene, which reveals that the nature of chemical reactivity of B/N-G is dependent on n induced

15

by B or N dopants. The carriers distribution of BN-G [Fig. 4(c)] absolutely reflects the electro-

16

chemical effect of B and N dopants. Thus, it is unnecessary to calculate the adsorption energy of

17

H (B/N-G) correlation stresses that the local H adatom in other paths (e.g., zigzag path). The n-Eads

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electrochemical effect and non-local electrochemical effect of B/N-G have a local stabilizing effect

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on H adsorption at a certain C site. In addition, our calculations show that B-doped and N-doped

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graphene are non-magnetic, which is consistent with other reports. 50,51 This is because the nonlo-

21

cal carriers induced by B and N dopants are very itinerant and their electronic exchange interaction

22

(driving force for spin polarization) is relatively small. The adsorption of H tends to induce spin

23

polarization in graphene, however, the magnetic state also has a nonlocal characteristic, leading to

24

a weak magnetism. 4 This magnetic state can be easily annihilated by the nonlocal carriers from B

25

and N dopants. Even in the pristine graphene with H adatom, the nonmagnetic-magnetic energy

26

H by the doping charge here difference is only 0.04 eV, which is far less than the increase in Eads

27

(up to 1.0 eV). Thus, the perturbation from magnetism on the electrochemical effect of B and N

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dopants can be safely neglected. Recently, Denis pointed out that the 3p elements have a potential

2

to modulate the chemical reactivity of graphene. 49 Further investigation on electrochemical and

3

elastochemical effect of 3p elements on the chemical reactivity of graphene will be done soon.

4

It has been illustrated above that the nonlocal and local charges introduced by B/N dopan-

5

t enhance the adatom stability, and we will further show in the following that these two kinds of

6

charges also can influence (be well probed by) the H-graphene electron transfer by Löwdin popula-

7

tion 52 analysis. The net charge of H atom (QH ) adsorbed on various sites of B-G or N-G surfaces is

8

plotted in Fig. 5, where the dashed line denotes the net charge of H adsorbed on pristine graphene

9

H (B/N-G) [Figs. 3(a)-3(d)], QH (B/N-G) also presents an oscillation QH (G) (0.2 e). Similar as to Eads

10

behavior. This is related with the nonlocal Friedel oscillation 46–48 of electronic states in defect-

11

ed graphene, resulting in the inhomogeneous distribution of holes/electrons in the two sublattices

12

of graphene (Fig. 4). The affinity of C to electron is dependent on its possessing charge density,

13

which then influences the electron transfer between H adatom and graphene matrix. There are both

14

nonlocal and local doping charges on the A1 site [Fig. 1(a)], while only nonlocal ones on other far-

15

ther sites, therefore, the oscillation amplitude of QH (B/N-G) at the A1 site is much larger than that

16

at other sites. In addition, QH (B-G) increases as R increases when H is adsorbed on B-G system,

17

while the opposite result happens for H-adsorbed N-G. The value of QH (B/N-G) approaches 0.2

18

e when H is far away from B or N dopants. On the other hand, our results indicate that holes (in-

19

duced by B) at a C site enhance electron transfer from H to the substrate, while electrons (induced

20

by N) weaken it. These phenomena arose from charge transfer are in accordance to previous results

21

relative to charge doping by gate biasing to graphene with H adsorption, 4 but not well explained

22

yet. Here we could can understand the charge transfer from the effect of holes/electrons density on

23

the affinity of C to electron. For B-G system, B introduces holes to C atoms of graphene matrix,

24

leading to the increase of affinity of C atoms that makes electron transfer from H to the matrix

25

when H is adsorbed on a certain C atom. However, N introduces electrons to C atoms of graphene

26

matrix, leading to the decrease of affinity that makes electrons transfer from matrix to graphene. It

27

should be pointed out that the charge transfer between H adatom and graphene is only a reflection

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of the doping-charge density of B/N-G systems, as well as its characteristics, while the adsorption

2

stability is governed by the electrochemical effect of B and N dopants, but not by the H-graphene

3

electron transfer.

4

4. Conclusions

5

By density functional theory (DFT) calculations, the interaction mechanism between the H adatom

6

and B/N-doped graphene (B/N-G) has been investigated systematically. From the site dependence

7

of the adsorption energy of hydrogen adatom, both non-local and local electrochemical effects of

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B and N impurities contribute to the enhanced chemical reactivity of graphene. The non-local

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doping charges originate from nonlocal aromatic electron resonance, and the local doping charges

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are bonded to the vicinity of B/N dopant as a result of its high/low inherent chemical potential.

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Both nonlocal and local charge-doping effect do coexist in B-doped or N-doped graphene, whereas

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nonlocal charge-doping effect has been largely suppressed in BN-codoped graphene. The nonlocal

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electrochemical effect enhances the stability of H adatom within the range of at least 9 Å away

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from the B/N dopants, while the local electrochemical effect only has a considerable stabilizing

15

effect on the H adatom close (∼2 Å) to the dopants. Our results also clarify the issue of that the

16

dopant-induced strain has a negligible effect on the enhanced stability of H adatom. Finally, We

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have demonstrated that the adsorption stability of H adatom is independent on the H-graphene

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charge transfer, whereas the charge transfer reflects the doping-charge density of B/N-G systems

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and further confirms the existence of nonlocal and local electrochemical effects of B and N dopants.

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Acknowledgement

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The authors wish to thank Dayong Liu for his supercomputer technical support. This work is

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supported by the National Science Foundation of China under Grant Nos. 11204305, 11174289

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and U1230202 (NSAF), the special Funds for Major State Basic Research Project of China (973)

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under Grant No. 2012CB933702, Hefei Center for Physical Science and Technology under Grant 11 ACS Paragon Plus Environment

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no. 2012FXZY004 and Director Grants of CASHIPS. The calculations were performed in Center

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for Computational Science of CASHIPS, the ScGrid of Supercomputing Center and Computer Net

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work Information Center of Chinese Academy of Science.

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Figure 1: (Color online) The periodic graphene supercell (17.06 × 17.26 ) with (a) a single B/N dopant and (b) a B-N dopant pair for the H adatom adsorption on the carbon site within armchair path (AP) or zigzag path (ZP). The dashed box shows the supercell and the labels denote the carbon site (shaded circle) for H adsorption. The green and red solid lines highlight the AP and ZP, respectively.

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0.04

Pristine-G B-G

0.02 E (eV)

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0.00 -0.02

A1

N-G

A3 A1

A3

B2 B4 B4 B2

-0.04 -1.0

-0.5

0.0

0.5

1.0

Strain (%)

Figure 2: (Color online) The strain contribution (Eε ) to the adsorption energy of H adatom.

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Figure 3: (Color online) The adsorption energy of H adatom on the carbon site within the armchair path (AP) or the zigzag path (ZP) as a function of distance from the B (or N) dopant site for (a-b) B-G, (c-d) N-G and (e-f) BN-G. The dotted line denotes the adsorption energy (0.82 eV) of H ′ adatom on pristine graphene. Eε = Eε − 0.82 eV .

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Figure 4: (Color online) (a) The holes distribution in B-G, (b) electrons distribution in N-G and (c) holes and electrons distributions in BN-G. The isovalue for the charge density is 0.0005e/bohr3 . The cyan and the red charge density denote holes and electrons, respectively.

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0.28

[email protected]

0.26

[email protected]

H

0.24

Q

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0.22 0.20 0.18 0.16 0.14 A1

B2

A3

B4

A5

B6

A7

B8

Adsorption site

Figure 5: (Color online) The net charge of H adsorbed at C atom site within AP or ZP of B-G and N-G surface. The dotted line denotes the net charge (0.2 e) of H adsorbed on pristine graphene.

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TOC page

The adsorption energy of H adatom on the carbon site within the armchair path (AP) or the zigzag path (ZP) as a function of distance from the B (or N) dopant site for B-G (Left) and N-G (Right). Inset the carriers (holes or electrons) distributions of B-doped and N-doped graphene are plotted.

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