Reversible Transition of Graphene from Hydrophobic to Hydrophilic in

Article pubs.acs.org/JPCC

Reversible Transition of Graphene from Hydrophobic to Hydrophilic in the Presence of an Electric Field Q. G. Jiang,† Z. M. Ao,*,‡ D. W. Chu,‡ and Q. Jiang*,† †

Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, and School of Materials Science and Engineering, Jilin University, Changchun 130022, China ‡ School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia ABSTRACT: The catalytic effect of a perpendicular electric field on the reversible transition of graphene with water from hydrophobic to hydrophilic has been investigated by using first principles calculations. It is found that a negative electric field F can reduce the energy barrier for H2O dissociative adsorption on graphene, while a positive electric field has an opposite effect. Under F = −0.39 V/Å, the energy barrier becomes negative and the dissociative adsorption occurs smoothly without any potential barrier, which results in hydrophilic graphene. For the desorption of H and OH from graphene after the dissociative adsorption of the H2O molecule, the positive electric field of F = 0.36 V/Å leads to a negative desorption energy barrier and the desorption progress is present automatically, making the graphene hydrophobic again. Therefore, the electric field can act as a switch to reversibly change the graphene from hydrophobic to hydrophilic in the presence of water vapor.

1. INTRODUCTION Graphene has attracted enormous interest due to its unique physical properties ever since its experimental observation.1 The potential applications of graphene significantly depend on the surface wettability, which is influenced by both the morphology2 and the chemical composition3 of the solid surface. Density functional theory (DFT) calculations4 and experimental observations5 indicate that graphene is strongly hydrophobic. The hydrophobic graphene surface can reduce possible liquid deposition and prevent contamination of advanced nanoelectromechanical systems. Furthermore, controllable manipulation of the wetting properties of materials has various applications, such as in biomaterials and microfluidic devices.6,7 The reversible transition of graphene from hydrophobic to hydrophilic has not been studied extensively. Very recently, Zhang et al. achieved a reversible superhydrophobic to superhydrophilic transition on a graphene film via alternation of ultraviolet (UV) irradiation and dark storage of the film.8 UV light is considered to produce hydrophilic OH groups, which are kinetically favorable for the physical adsorption of H2O at these sites. When the graphene film is placed in the dark, the surface evolves back to its original state, which restores the hydrophobicity of the film. However, the UV irradiation method is time-consumingit needs about 12 hand may hinder some special applications. Therefore, an alternative method is demanded for efficiently controlling the reversible wettability of graphene. It is well-known that manipulation of the electric field is a powerful method to modify the chemical potentials of materials. For example, the energy barrier of molecular hydrogen dissociative adsorption on graphene disappears in the presence of a strong electric field.9 The electric field is © 2012 American Chemical Society

considered to be a simple, versatile, and effective external stimulus to switch smart surface wettability, due to its ability to control the surface chemistry or morphology in a few seconds or less.10 Therefore, an alternative approach that efficiently changes the graphene from hydrophobic to hydrophilic may be realized through the exposure of graphene to a humid circumstance in the presence of an electric field. In addition, when a reversing electric field is applied, the dissociated H and OH group may combine into H2O again. Therefore, the dissociative adsorption and recombination of H2O on graphene with a perpendicular electric field will be investigated in this work through DFT calculations.

2. COMPUTATIONAL METHODS All calculations are implemented by Dmol3 code.11 The generalized gradient approximation (GGA) with Perdew− Burke−Ernzerhof (PBE) functional is employed to describe exchange and correlation effects.12 Double numerical plus polarization (DNP) is employed as the basis set. The convergence tolerance of energy of 10−5 hartree is taken (1 hartree = 27.21 eV), and the maximal allowed force and displacement are 0.002 hartree/Å and 0.005 Å, respectively. A smearing of 0.005 hartree is applied to achieve accurate electronic convergence. The effect of the exchange−correlation functional on the calculated adsorption energies is quite large and systematic, whereas the effect on the calculated energy barriers is much smaller.13 To investigate the minimum energy pathway for water dissociative adsorption on graphene, linear synchronous transit/quadratic synchronous transit (LST/ Received: May 24, 2012 Revised: August 22, 2012 Published: August 22, 2012 19321

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Figure 1. Initial (a) and final (b) structures for H2O molecule dissociative adsorption on graphene. The initial structure is a molecular H2O physically adsorbed at the hollow site of a carbon hexagon. The final structure is an H atom and an OH group chemically adsorbed on two face-byface carbon atoms of a carbon hexagon. The gray, white, and red balls are carbon, hydrogen, and oxygen atoms, respectively. The direction of the electric field is indicated by the arrow. The direction of charge transfer is opposite that of the electric field. The numbers in (b) represent the possible positions of the OH group.

QST)14 and nudged elastic band (NEB)15 tools in Dmol3 code are used, which have been well validated to find the structure of the transition state and the minimum energy pathway. In the simulation, three-dimensional periodic boundary conditions are taken. The computer simulation cell consists of a 3 × 3 graphene supercell with a vacuum width of 20 Å above the layer to minimize the interlayer interaction. The k-point is set to 12 × 12 × 1, and all atoms are allowed to relax. The Grimme scheme16 is used in all calculations to correct the van der Waals forces. For a H2O molecule adsorbed on graphene, the adsorption energy Ead is determined by Ead = E H 2O/graphene − (Egraphene + E H2O)

Table 1. Adsorption Energies (Ead) and Distances of the O Atom above the Graphene Surface (d) of One H2O Molecule on Graphene with Different Orientations and Positions orientation up

parallel

down

(1) V

where EH2O/graphene, Egraphene, and EH2O are total energies of the H2O/graphene system, the isolated graphene, and a H2O molecule in the same slab, respectively.

position

Ead (meV)

d (Å)

bridge top hollow bridge top hollow bridge top hollow bridge top hollow

−149 −145 −156 −170 −170 −176 −207 −207 −213 −212 −212 −212

3.428 3.476 3.407 3.470 3.474 3.435 3.556 3.540 3.530 3.337 3.300 3.229

graphene with an OH bond pointing to graphene and another OH bond parallel to graphene. This may be caused by the fact that the small energetic difference of 1 meV between the two configurations shows the same stability. In addition, this disagreement has little effect on our transition state search calculations due to the little difference between them. In order to study the effect of the simulation cell size on the results, we also performed calculations with H2O adsorbed on the hollow site using a 4 × 4 supercell and found adsorption energy Ead = −218 meV, which is 2.5% larger than the result obtained with the 3 × 3 supercell. The difference is in the acceptable range. For the product, graphene with chemically adsorbed H and OH on graphene, there are three positions of OH relative to the position of H in the six-member ring, which are denoted as positions 1, 2, and 3 in Figure 1b. The three configurations are named p1, p2, and p3. According to the DFT+D calculations, the relative total energies compared to p1 are 1.054 and −0.019 eV for p2 and p3, respectively. p1 and p3 have the same energetic level. Thus, both reaction paths for the dissociative

3. RESULTS AND DISCUSSION For H2O molecule adsorption on graphene, there are three different adsorption sites as shown in Figure 1a: the hollow, bridge, and top sites. At each site, the OH bonds may point up, down, or parallel to the graphene surface. The orientations with an OH bond pointing to graphene and another OH bond parallel to graphene are also considered (named “V” orientation). Based on the DFT+D calculations with consideration of the van der Waals force through the Grimme scheme16 and eq 1, the obtained Ead values are listed in Table 1. In general, Ead values more likely depend on the orientation rather than the position of the H2O, which is consistent with the result of a previous report.17 Based on our calculations, the favorable adsorption site of the H2O is at the hollow site with two OH bonds pointing down to the graphene surface, as shown in Figure 1a, which is adopted as the initial structure in the subsequent transition state search calculations. Our result differs from that in ref 17, where H2O prefers to adsorb on 19322

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Figure 2. Reaction pathway of H2O molecule dissociative adsorption on graphene from the initial structure to p3 under different electric fields. IS, TS, FS, and 1 and 2 represent initial structure, transition structure, final structure, and energy minimum states 1 and 2, respectively. Their atomic structures are given by the insets. The energy of IS is taken to be 0. The unit of F is V/Å, while that of Ebar, Er, E′bar, and E′r is eV, where Ebar is the energy barrier and Er is the reaction energy; E′bar and E′r are the energy barrier and reaction energy for the reaction from state 1 to state 2.

In addition, both configurations of the initial and final states reconstruct spontaneously in the presence of the electric field, as shown in Figure 2. For example, under F = 0.36 V/Å (Figure 2b), the geometry of the reactant is optimized from the IS to the energy minimum state, state 1, where the positive F pushes the H2O upward away from graphene and 0.242e is transferred from the H2O molecule to graphene (Table 2). Therefore, the

adsorption of H2O on graphene from the initial structure to p1 and p3 may occur and both are considered. We first study the pathway from the favorite configuration of the H2O physisorption on graphene shown in Figure 1a to the configuration of p3 shown in Figure 1b for the reaction of the H2O molecule dissociative adsorption on graphene. After LST/ QST and NEB calculations, it is found that the reaction barrier Ebar is 3.455 eV for this pathway as shown in Figure 2a. This Ebar value corresponds to the literature datum of 3.599 eV.18 From Figure 2a, it is also known that the reaction requires two steps: H2O is first dissociated into H and OH (from initial structure IS to transition structure TS) with Ebar = 3.455 eV; then H and OH move to the exact top sites of C from TS to the final structure FS with an energy release of 0.921 eV. As a result, a total of 2.534 eV is needed for the H2O dissociative adsorption on graphene, where the H2O dissociation with larger Ebar value is the rate-limiting step. It is known that a perpendicular electric field (F) leads to polarization of the charge density and affects the dissociative adsorption of H2O on graphene. To investigate the effect of F, we apply an electric field perpendicularly on the H2O/graphene system; the direction of F is shown in Figure 1. The pathways for this reaction in the presence of an electric field with different intensities are shown in Figure 2, where IS, TS, FS, Ebar = ETS − EIS, and reaction energy Er = EFS − EIS for the H2O dissociative adsorption on graphene under different electric field intensities are given. In general, Ebar and Er increase with increasing F as shown in Figure 2 when F > 0, while they decrease with decreasing F if F < 0.

Table 2. C−H Bond Lengths (lC−H), H−O Bond Lengths (lH−O), and Mulliken Atomic Charges (QH2O) of the Reactant in the Presence of Different Electric Fields (V/Å)a lC−H (Å) lH−O (Å) QH2O (e) a

F = 0.36

F=0

F = −0.20

F = −0.39

3.681 0.979 0.242

2.995 0.971 −0.002

2.151 0.981 −0.075

1.780 1.026 −0.299

The carbon atom is the nearest one to the hydrogen atom.

negatively charged oxygen atom is upward due to the Coulomb repulsion between the oxygen atom and graphene at state 1. For the product under F = 0.36 V/Å, its geometry is optimized from the FS to the energy minimum state, state 2. Therefore, in the presence of the electric field, the H2O dissociative adsorption reaction is from state 1 to state 2, the energy barrier, E′bar = ETS − E1, is 3.779 eV, and the reaction energy, E′r = E2 −E1, is 3.615 eV. E′bar and E′r for the other reaction paths are also indicated in Figure 2. 19323

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Figure 3. Reaction pathway of H2O molecule dissociative adsorption on graphene from the initial structure to p1 under different electric fields. The meanings of the symbols and numbers are the same as in Figure 2.

−0.006 eV, while a small energy barrier of 0.164 eV needs to be overcome from state 2 to TS. In other words, the H2O recombination reaction takes place easily at room temperature. Therefore, the desorption of OH and H will make graphene become hydrophobic again. We also did the calculation without the consideration of van der Waals forces. E′bar values are 3.973, 3.594, 1.923, and 0.226 eV when F = 0.036, 0, −0.20, and −0.39 V/Å, respectively. We define ΔE′bar = E′bar(DFT) − E′bar(DFT+D), where E′bar(DFT) and E′bar(DFT+D) are the energy barriers within the DFT and DFT+D methods, respectively. Based on this definition, ΔE′bar is 0.194, 0.139, 0.018, and −0.009 eV under F = 0.036, 0, −0.20, and −0.39 V/ Å, respectively. This shows that as long as F > −0.20 V/Å, the van der Waals forces facilitate the reaction. This result is understandable since the distance between H2O and graphene increases and the attractive force plays a main role under a positive electric field, which facilitates the dissociative adsorption of H2O on graphene. Under a negative electric field, the distance between H2O and graphene gradually decreases and the repulsive force dominates, which may hinder the dissociative adsorption of H2O on graphene. The pathway for the dissociative adsorption of H2O molecule on graphene from the initial structure to p1 was then studied. The reaction barrier Ebar is 3.223 eV for this pathway as shown in Figure 3a, corresponding to the literature datum of 3.183 eV.18 From Figure 3b to 3d, both configurations of the initial and final states reconstruct spontaneously under F. The change of reaction barrier is similar to that in Figure 2. When F = −0.39 V/Å, E′bar from state 1 to TS is 0.266 eV, which is similar to the E′bar = 0.235 eV in Figure 2d. However, the reaction energy E′r from state 1 to state 2 is 0.244 eV, which means that state 2 is less stable than state 1. In contrast, E′r = 0.020 eV in

At F = −0.20 V/Å (Figure 2c), the geometry of the reactant is optimized from IS to state 1, where the negative F pulls the H2O down toward graphene. Due to the charge transfer from graphene to the H2O under F (Table 2), as an electron acceptor, the OH group points to graphene at state 1 under a negative F. Therefore, the number of electrons in H2O decreases with increasing F from −0.39 to 0.36 V/Å (Table 2), while the bond length of the C−H bond lC−H increases due to the positively charged H atom, and the bond length of the H−O bond lH−O decreases because of the negatively charged OH. It is interesting to note that Ebar = −1.164 eV < 0 (the energy difference between IS and TS, as shown in Figure 2d) for the dissociation of adsorbed H2O on graphene under F = −0.39 V/ Å. Along the reaction coordinate, the reactant is now reconstructed to state 1 and releases 1.399 eV of energy that can be used to support the subsequent step from state 1 to TS. Usually, surface reactions at ambient temperature may occur when Ebar < 0.75 eV19 while the barrier E′bar is 0.235 eV from state 1 to TS. Thus, the negative F can catalyze the dissociative adsorption of H2O on graphene and change the reaction process from endothermic to exothermic. When F < −0.39 V/ Å, H2O automatically dissociates into H and OH by the geometry optimization method. Therefore, the presence of the chemically adsorbed hydrophilic OH group makes graphene hydrophilic. Now we consider the reaction of H + OH → H2O when F is removed. As the reverse reaction of the dissociative adsorption, FS and IS shown in Figure 2a are now respectively IS and FS with Er = −2.534 eV and Ebar = 0.921 eV. As Ebar > 0.75 eV, this reaction hardly occurs at ambient temperature. However, if a positive F of 0.36 V/Å is added, Er = −3.717 eV and Ebar = 19324

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Figure 4. PDOS of hydrogen, oxygen, and carbon atoms of IS in the presence of different electric fields, where the carbon atom is the nearest one to the hydrogen atom. The black, blue, and red curves are the PDOS of the p orbitals of the C atoms and the O atoms, and the s orbitals of the H atoms of the reactant, respectively. The vertical lines indicate the Fermi level. Inset: (α) HOMO and (β) LUMO of H2O (blue and yellow indicate different signs of the orbital wave function). The molecular orbitals of H2O are also labeled.

and a C−H bond is formed. Therefore, negative F can enhance the reactivity of the reactant and lower Ebar for the dissociative adsorption of H2O on graphene.

Figure 2d. As a result, the reaction from the initial structure to p3 is preferred. The effect of F on Ebar can be understood by considering the interaction between the bands of H2O and graphene through analyzing the partial density of states (PDOS). In order to understand how H2O interacts with the graphene substrate, the highest occupied orbital (HOMO) and lowest unoccupied molecule orbital (LUMO) of the adsorbed H2O are also considered. The HOMO (1b1) is completely located on O, while the LUMO (4a1) is fully located on H (see inset of Figure 4a). The HOMO and LUMO of H2O and their relative positions with respect to the Dirac point determine the charge transfer between graphene and H2O.17 As H points to the carbon layer, the LUMO of H2O interacts with graphene surface and accepts a small charge of 0.02e from graphene in the absence of the electric field, based on the Mulliken charge analysis. Thus, the interaction between the H s and C p bands determines the interaction between H2O and graphene. Since the interaction between C and H is very weak as shown in Figure 4a, a large Ebar is present for H2O dissociative adsorption on graphene as shown in Figure 2a. When F is added, such as F = 0.36 V/Å, a charge of 0.242e is transferred from H2O to C, inducing the increase of the C−H distance. The peaks of the O−H band shift to the left as shown in Figure 4b, indicating the enhancement of O−H interaction and the difficulty of H2O dissociation. However, at F = −0.20 V/Å and F = −0.39 V/Å, 0.075e and 0.299e are transferred from C to H2O, respectively. The peaks of the O−H band shift to the right with decreasing F as shown in Figure 4c,d, which implies that the O−H interaction decreases and the H2O dissociation becomes easier. At F = −0.39 V/Å (Figure 4d), the O−H bond is even broken

4. CONCLUSION In summary, the dissociative adsorption of H2O on graphene with F has been investigated by using first principles calculations. It is found that a negative F can act as a catalyst to facilitate H2O dissociative adsorption on graphene, which induces graphene to be hydrophilic. If F > 0, the opposite effect is found that desorption of H and OH from graphene is facilitated, inducing graphene to be hydrophobic again. The origin of the effect of F on Ebar is discussed through analyzing the interaction between the s/p orbital band of H/O and the p orbital band of C. As a result, F acts as a switch to reversibly change the graphene from hydrophobic to hydrophilic.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.M.A.); [email protected] (Q.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the National Key Basic Research, Development Program (Grant 2010CB631001), and the High Performance Computing Center (HPCC) of Jilin University for supercomputer time. Z.M.A. acknowledges financial support from the Vice-Chancellor’s Postdoctoral Research Fellowship Program of the University of New South 19325

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Wales (SIR50/PS19184), and the Faculty Grant of the University of New South Wales (IR001/PS27218).

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REFERENCES

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