Adsorption and Catalytic Activation of O2 Molecule on the Surface of

Article pubs.acs.org/JPCC

Adsorption and Catalytic Activation of O2 Molecule on the Surface of Au-Doped Graphene under an External Electric Field Teng Zhang,†,‡,§ Qingzhong Xue,*,†,‡,§ Meixia Shan,‡ Zhiyong Jiao,‡ Xiaoyan Zhou,‡ Cuicui Ling,‡ and Zifeng Yan*,† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, 266580, Shandong, P. R. China College of Science, China University of Petroleum, Qingdao, 266580, Shandong, P. R. China § Key Laboratory of New Energy Physics & Materials Science in Universities of Shandong, Qingdao, 266580, Shandong, P. R. China ‡

S Supporting Information *

ABSTRACT: The interaction between oxygen molecule (O2) and metaldoped graphene has always been a heated discussed issue because O2 plays an important role in the graphene-based gas-storage materials, sensing platforms, and catalysts. In this article, the effect of an external electric field on the interaction between O2 and Au-doped graphene is studied using densityfunctional theory (DFT) calculations. The simulations show that O2 vertically moves away from Au-doped graphene substrate under a positive electric field, whereas under a negative electric field, accompanied by the vertical pushing out movement, O2 also moves toward the specific Au atom horizontally. Besides, the adsorption energy (Ead) of O2 is dramatically changed with electric field. A negative electric field strengthens the interaction between O2 and Au-doped graphene substrate, resulting in an enhanced Ead; the corresponding O−O distance (dO−O) is also elongated, while Ead is decreased and dO−O is shortened under a positive electric field. Because dO−O of the adsorbed O2 correlates with its catalytic activation, the findings can provide a new avenue to tune the O2 adsorption process onto Au-doped graphene substrate and may be useful in the future applications of graphene-based nanocatalyst. adsorption of CO onto Al-doped graphene surface.31 By using DFT calculations, the CO oxidation processes on different kinds of metal-doped graphene are reported,18−20 which open up a new avenue to fabricate graphene-based catalysts. Moreover, Zhou et al. presents an effective method to control the stabilization and the catalytic activities of metal nanoclusters supported on graphene via tuning the mechanical strain in graphene.17 According to their research, a relatively modest tensile strain (10%) applied in graphene increases the adsorption energies by at least 100%, and a small strain of 5% in graphene reduces the reaction barrier of the catalyzed CO oxidation process from ∼3.0 eV (without strain) to less than 0.2 eV. However, to use these metal−graphene complexes in reality, a full understanding of the interaction between O2 molecule and graphene-based materials is needed. First, although metaladsorbed/doped graphene is widely accepted as a potential H2 storage material, if we take O2 into consideration, then things can be totally different. A series of investigations that specially focus on the effect of O2 during the H2 storage process are reported.30,32,33 This research indicates that O2 interferes with the H2 adsorption process by either blocking the adsorption site

1. INTRODUCTION Recently, graphene has attracted tremendous interest due to its unique properties1−4 and promising applications.5−9 Metal− graphene complex, which combines the large surface areas of graphene and the high chemical activities of metal atoms together, has to some extent contributed to the wide applications of graphene in gas storage materials,10−12 sensing platforms,6,13−16 catalysts,7,17−26 and so forth.8,27 Experimentally, Pd−graphene13 and Au-reduced graphene oxide (rGO)14 composites are synthesized and serve as excellent gas sensors. The high catalytic activity of Pt−graphene composite for the methanol oxidation reaction is revealed by Yoo et al.23 Theoretically, the interactions between various gas molecules and metal-doped graphene have been discussed extensively using density functional theory (DFT) calculations. In general, it is demonstrated that Li,10 Al,28 Ti,29 and other metal atoms29,30 adsorbed/doped graphene with each metal atom adsorbed as many as four hydrogen molecules (H2), may be potential outstanding H2 storage materials. Besides, using DFT calculations and Monte Carlo simulations, Wang et al. propose that Ti−GO can be an ideal sorbent for carbon monoxide (CO) capture and separation.12 Compared with pure graphene, a huge enhancement of CO sensitivity of Al-doped graphene is reported by Ao et al.; they attribute the enhancement to the large amount of shallow acceptor states introduced by the © 2012 American Chemical Society

Received: July 25, 2012 Published: August 30, 2012 19918

dx.doi.org/10.1021/jp3073359 | J. Phys. Chem. C 2012, 116, 19918−19924

The Journal of Physical Chemistry C

Article

or by irreversible oxidation of the metal decoration.33 Second, the adsorption of O2 on the metal-doped graphene surface can affect the magnetic, electronic, and atomic properties of graphene, which would have a huge impact on the sensing properties of graphene.34 Lastly, considering the key role that O2 plays in a variety of reaction processes such as CO oxidation35,36 and oxygen reduction reaction (ORR),37 the O2 adsorption process onto various catalysts surface is regarded as the first step toward a systemic investigation on the catalytic performance of the catalyst. The structural configurations and the electronic properties for O2 adsorbed onto pristine and various transition-metal-embedded graphene are studied in detail, and it is suggested that Ti and Au may be the best choices among all transition-metal elements as graphene-based catalyst.38 The structural and electronic properties for O2 adsorbed onto Pt13-decorated defective graphene is discussed by Lim et al.39,40 Gao et al. especially focus on the adsorption of O2 on Au- and Au2-doped graphene like hexagonal boron nitride (h-BN); they suggest that the adsorption and catalytic activation of O2 on the h-BN-supported Au and Au2 can be effectively tuned by the interaction with the support via electron pushing and donor/acceptor mechanisms.41 In short, the interaction between O2 and various metal-doped graphene deserves comprehensive investigations to realize the applications of graphene in nanocatalysts, sensing platforms, and so forth. Recently, the external electric field has been identified as a regular and effective method to modulate the corresponding physical properties of graphene.42−44 Compared with other approaches, the electric field has several advantages for practical applications, such as being clean, easily acquirable, and adjustable in both direction and intensity.45 For instance, Zhou et al. report that applying an electric field to a fully hydrogenated graphene sheet can unload hydrogen atoms on one side, while keeping the hydrogen atoms on the other side, thus forming a half-hydrogenated graphene sheet.42 Besides, the effect of an electric field on different gas molecules interacting with metal-doped graphene has been widely discussed, ranging from gas adsorption and desorption44,46,47 to decomposition.48−50 It is claimed that an electric field can be used to control the H2 adsorption/desorption process on the Li-doped graphene.44 The correlation between an electric field and the adsorption/desorption behavior of a CO molecule on the Aldoped graphene is identified.46 Ao et al. propose an alternative approach to hydrogenate graphene by using an external electric field.48 According to their research, the hydrogenation process would occur smoothly without any potential barrier when the applied electric field increased above −0.02 au. Another interesting investigation is reported by Lv et al., who suggest that the electric field may be an effective strategy for the capture and decomposition of nitrogen monoxide (N2O) on the surface of Al- and Ga-doped graphene.50 Although the importance of electric field is widely recognized, the effect of an electric field on the interaction between O2 molecule and metal-doped graphene has seldomly been reported. In this study, we specifically focus on the influence of an electric field on the interaction between O2 molecule and Au-doped graphene because the Au−graphene complex is widely used as gas sensors14 and catalysts.7,22,25,26

In general, calculations based on the local density approximation (LDA) tend to overestimate the binding energy and underestimate the atomic equilibrium distances,52,53 so the generalized gradient approximation (GGA) with the Perdew− Burke−Ernzerhof (PBE) function,54 which is widely adopted to describe the interaction between gas molecules and graphene,20,39,46 is used for describing the exchange−correlation interaction in this study. The double numerical plus polarization (DNP), which has a computational precision being comparable to the Gaussian split-valence basis set 6-31 g**, has been applied in the expanded electronic wave function.50 DFT semicore pseudopotentials (DSPPs) core treatment55 is employed for the relativistic effects. In addition, the selfconsistent field (SCF) calculations are performed with a convergence criterion of 10−6 au on the total energy. To ensure high-quality results, we chose a real-space global orbital cutoff radius as high as 6.5 Å and a smearing point of 0.004 Ha in all calculations. A 4 × 4 hexagonal graphene supercell containing 32 atoms is built, and one carbon atom is substituted by an Au atom, which approaches the isolated impurity limit. A vacuum thickness of 20 Å is employed along the z direction of the graphene sheets to avoid the interactions from neighboring molecules. Monkhorst−Pack special k-points of 6 × 6 × 2 meshes are used to represent the Brillouin zone for all slab models. Moreover, the electric field is applied with the direction perpendicular to the graphene plane, in the upward (defined as “+”) or downward (defined as ‘‘−’’) direction. To investigate further the intensity-dependent behavior, we vary the electric field imposed on the O2-adsorbed Au-doped graphene system from −1.5 to 1.5 V/Å with a step of 0.25 V/Å.

3. RESULTS AND DISCUSSION As a preliminary test, we have optimized the atomic structures of the isolated O2 molecule and the Au-doped graphene. Using the aforementioned method, we calculate the bond length (dO−O) 1.23 Å for O2, and the average bond length between the Au atom and each neighboring carbon atom (dAu−C) is 2.109 Å. These results correspond well with previously published results.20,38,56 The most stable configuration for O2 adsorbed onto Au-doped graphene is recorded in Figure 1. Here the deformation density Δρ is defined as Δρ = ρO + Au ‐ doped graphene − (ρAu ‐ doped graphene + ρO ) 2

2

(1)

where ρO2+ Au‑doped graphene, ρAu‑doped graphene, and ρO2 represent the total charge densities of the O2-adsorbed Au-doped graphene, the Au-doped graphene sheet before O2 adsorption, and the isolated O2 molecule, respectively. According to Figure 1, after geometrical optimization, the O2 molecule is inclined to lie parallel to the graphene surface, and dO−O is elongated to ∼1.36 Å; besides, the adsorption energy (Ead) is calculated to be −1.47 eV, and the calculated dO−O and Ead match previous reported data.20 In this article, Ead is defined as follows E ad=EO2 + Au ‐ doped graphene − (EAu ‐ doped graphene + EO2)

(2)

where EO2+Au‑doped graphene, EAu‑doped graphene, and EO2 stand for the total energies of the O2-adsorbed Au-doped graphene, the isolated Au-doped graphene sheet, and the O2 molecule separately. The Mulliken charge population analysis shows that ∼0.4 electrons are transferred from the Au-doped

2. MODELS AND METHODS Our spin-unrestricted DFT computations are carried out using the DMol3 code51 embedded in the Materials Studio software. 19919



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also increases under a positive electric field, and it is inclined to decrease under a negative electric field smaller than −1.0 V/Å; further increase in the electric field would again push the adsorbed O2 away from the Au atom. Coupled to Figure 2, the possible explanations to the trend presented in Figure 4 can be given as follows: the adsorbed O2 molecule vertically moves away from Au-doped graphene under a positive electric field, which increases dAu−O and dC3−O; however, under a negative electric field, besides vertically moving away from the substrate, the O2 molecule also moves toward the Au atom horizontally, which to some extent decreases dAu−O. To identify further the effect of “horizontal movement” on the structural transformations for O2 adsorbed onto Au-doped graphene under electric field, we also calculate the angle Φ (O−Au−C3). The definition of Φ is shown as follows: Figure 1. Top and side views of the geometric structure (a) and the electronic deformation density (b) for O2 adsorbed onto Au-doped graphene. Red, gray, and yellow balls in panel a represent oxygen, carbon, and gold atoms, respectively; red and blue regions in panel b indicate the electron accumulation and loss.

Φ(O − Au − C3) = 1/2(Φ(O1 − Au − C3) + Φ(O2 − Au − C3))

(3)

Graphical description of Φ(O1−Au−C3) and Φ(O2−Au−C3) without electric field can be found in Figure 3b. Figure 5 presents the changes of Φ under different electric fields, more detailed information of horizontally fixed and relaxed Φ are provided in the Supporting Information. By comparing relaxed Φ with horizontally fixed Φ, we are able to separate “horizontal movement” from the structural transformation process induced by the electric field. Compared with horizontally fixed Φ, relaxed Φ undergoes more dramatical changes under a negative electric field; this suggests that horizontal movement of the adsorbed O2 accounts for a large part of the structural transformation process under a negative electric field, in accordance with our findings in Figures 2 and 4. While under a positive electric field, vertical movement dominates the interacting process because relaxed Φ varies slightly compared with horizontally fixed Φ. 3.2. Effects of the Electric Field on the Ead and dO−O. Figure 6 records Ead and dO−O variations under different electric fields. In general, the Ead variation under electric field roughly

graphene substrate to the corresponding O2, which indicates that O2 is an acceptor. 3.1. Effects of the Electric Field on the Geometrical Transformations of O2-Adsorbed Au-Doped Graphene. Figure 2 records the structural configurations for O2 adsorbed onto Au-doped graphene under different electric fields. To facilitate our analysis, several atoms C1−3, O1−2 are defined and recorded in Figure 3a. Considering that the O2 molecule lies parallel to the graphene surface, which means the distance between Au−O1 (C3−O1) and Au−O2 (C3−O2) is approximately the same, the distance Au−O (dAu−O) and C3−O (dC3− O) is calculated by averaging the distance between Au, C3 atom, and two corresponding O atoms. Figure 4 describes the changes of dAu−O and dC3−O under different electric fields. Clearly, dC3−O increases regardless of the direction of electric field, which indicates that O2 moves away from the C3. dAu−O

Figure 2. Optimized structures for O2 adsorbed onto Au-doped graphene under different electric fields. The plots a−e record the relaxed models with the electric field increases from −1.5 to 1.5 V/Å with a variation of 0.75 V/Å. 19920



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Figure 3. (a) Definition of several useful atoms: C1−3, O1−2. (b) Graphical description of Φ(O1−Au−C3) and Φ(O2−Au−C3) without electric field.

Figure 4. Changes in dAu−O and dC3−O under different electric fields.

Figure 6. Ead and dO−O shifts under different electric fields.

the longer the dO−O will be, so dO−O is elongated to about 1.47 Å under −1.5 V/Å, and it is shortened by 0.06 Å under 1.5 V/ Å. According to Zhou et al., the enlargement of dO−O indicates significant weakening of the O−O bond and contributes to a better catalytic activity;38 besides, Gao et al. also propose that a slight increase in dO−O of the adsorbed O2 molecule implies its catalytic activation.41 Therefore, the increase in dO−O under a negative electric field may enhance the catalytic activity of Audoped graphene. Besides, because dO−O is correlated with the direction and intensity of the electric field, we suggest that the catalytic activity of Au-doped graphene can be effectively tuned by the additional electric field. In the following part, we aim to give detailed explanations to the Ead variations and dO−O shifts under electric field using charge-transfer analysis and electronic deformation density images. According to previous researches,40,41 the activation and reactivity of the adsorbed O2 are strongly affected by the charge transferred from the Au-hybridized 5d6s orbitals to the antibonding 2π* orbital of O2. Compared with the system without electric field, additional 0.55 electrons are transferred from Au-doped graphene substrate to adsorbed O2 molecule under a −1.5 V/Å, and a positive electric field as high as 1.5 V/ Å continues to push electrons from the adsorbed O2 molecule back to the corresponding graphene substrate; these are shown

Figure 5. Variation of Φ under different electric fields.

exhibits a second-order trend; according to Hyman et al.,57 this is attributed to the second-order stark effect. Ead increases to more than −2.9 eV under −1.5 V/Å, a more than 90% enhancement compared with that without electric field. Under a positive electric field, Ead gradually decreases to less than −1.0 eV, which indicates that the interaction between O2 and Audoped graphene is deteriorated by the positive electric field. For dO−O, it exhibits a close relationship with Ead: the larger the Ead, 19921



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in Figure 7a. As is described in Figure 7, the defective graphene substrate acts as the main electron donator that continually

Figure 8. Graphical descriptions of the “electron transfer” mechanism under different electric fields.

discussed, under a positive electric field, the adsorbed O2 molecule combined with the Au atom acts as the charge donator. So even though the defective graphene substrate receives more than 0.5 electrons from the charge donator under 1.5 V/Å, only 0.4 electrons are transferred from the adsorbed O2 molecule. The charge transferred from the adsorbed O2 molecule and the Au atom back to the defective graphene substrate under a positive electric field is recorded in Table 1. Table 1. Charge Transfered from the Adsorbed O2 and the Au Atom Back to the Defective Graphene Substrate under a Positive Electric Field F/(V/Å)

0.25

0.5

0.75

1

1.25

1.5

from adsorbed O2 from Au atom

0.08 0.009

0.154 0.023

0.219 0.041

0.282 0.06

0.341 0.083

0.401 0.107

Moreover, coupled to Figure 7, it seems that the larger the positive electric field, the more charge would be transferred back to the graphene substrate from the Au atom, and relatively less charge would be transferred downward from the adsorbed O2 molecule; this can explain the diminishing trend of Ead and dO−O under a positive electric field. The electronic deformation density images for O2 adsorbed onto Au-doped graphene under different electric fields are presented in Figure 9, and it can be an effective tool to straighten out the interaction between adsorbed O2 molecule and Au-doped graphene substrate. To make it more clear, slice images of the electronic deformation density are recorded in Figure 10. In Figure 10, the preferential electron accumulation sites are mainly localized on particular atoms; this implies the chemisorption of O2 molecule on Au-doped graphene substrate. Because the red regions around the adsorbed O2 become larger and thicker under a negative electric field compared with that without electric field, the interaction between adsorbed O2 and graphene substrate would be strengthened, resulting in an enhanced Ead. Moreover, the enlarged red regions also indicate that more electrons are transferred upward from Au-doped graphene to the adsorbed O2 molecule, in correspondence with our aforementioned conclusion, whereas the reduced Ead under a positive electric field can be explained by a weaker interaction between the adsorbed O2 molecule and Au-doped graphene substrate, and this is quite obvious because the red regions become smaller and thinner under a positive electric field, as presented in Figures 9 and 10.

Figure 7. (a) Charge localized on the O2 molecule and the corresponding defective graphene substrate under different electric fields. (b) Charge localized on Au, C1, C2, and C3 under different electric fields.

donates electrons to the adsorbed O2 under a negative electric field; however, fewer than 0.01 electrons are transferred from Au atom to the O2 molecule during this process. This suggests that under a negative electric field the Au atom mainly acts as the bridge that transfers electrons from the defective graphene to the adsorbed O2. Under a positive electric field, instead of being an electron acceptor, the Au atom also donates ∼0.1 electrons back to the defective graphene. The “electron transfer” mechanism can be explained as follows: under a negative electric field, the electrons are transferred from Audoped graphene to the adsorbed O2, whereas if we apply a positive electric field to the system, the adsorbed O2 molecule and the Au atom together push electrons back to the defective graphene. The graphical descriptions of the “electron transfer” mechanism under different electric fields are provided in Figure 8. According to Lim et al., dO−O correlates with the charge localized on the adsorbed O2 molecule.40 In this study, under a negative electric field, the electrons continue to flow from Audoped graphene up to adsorbed O 2 molecule, which contributes to the elongation of dO−O and the enhancement of Ead; however, it is a bit complicated to explain the Ead variation under a positive electric field. As we have previously 19922



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Figure 9. Electronic deformation density images for O2 molecule adsorbed onto Au-doped graphene under different electric fields. (a−c) System under −1.5, 0, and 1.5 V/Å separately. The red and blue regions show the electron accumulation and loss.

Figure 10. Slice images of the electronic deformation density for O2 molecule adsorbed onto Au-doped graphene under different electric fields. (a− c) System under −1.5, 0, and 1.5 V/Å separately. The red and blue regions show the electron accumulation and loss.

4. CONCLUSIONS In summary, the structural configurations and the binding mechanism for O2 adsorbed onto Au-doped graphene under different electric fields are discussed in detail using DFT calculations. Structurally, under a positive electric field, the vertical pushing out mechanism dominates the transformation process, whereas if we apply a negative electric field to the system, accompanied by the vertical pushing out movement, the adsorbed O2 molecule also moves toward the corresponding Au atom horizontally. Ead is enhanced and dO−O is extended under a negative electric field, whereas the interaction between O2 molecule and Au-doped graphene is deteriorated to some extent by a positive electric field, with decreased Ead and shortened dO−O. Because the elongation of adsorbed O2 indicates its catalytic activation, we suggest that an additional negative electric field may enhance the catalytic performance of Au-doped graphene. Finally, it is recommended that the O2 adsorption process onto Au-doped graphene substrate can be greatly influenced by the external electric field, and the catalytic performance of Au-doped graphene may be effective tuned by an additional electric field.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

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ACKNOWLEDGMENTS

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REFERENCES

This work is supported by the Fundamental Research Funds for the Central Universities (11CX05002A, 11CX0460A), the Natural Science Foundation of Shandong Province (ZR2010AL009, ZR2011AL023), and Qingdao Science & Technology Program (12-1-4-7-(1)-jch).

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S Supporting Information *

Definition of horizontally fixed and relaxed Φ. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected] (Q. Z. X.); zfyancat@ upc.edu.cn (Z. F. Y.). Tel: 86-0532-86983366. Fax: 86-053286983366. Notes

The authors declare no competing financial interest. 19923



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Adsorption and Catalytic Activation of O2 Molecule on the Surface of

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