Adsorption and Dissociation of Ammonia on Graphene Oxides: A First

Article pubs.acs.org/JPCC

Adsorption and Dissociation of Ammonia on Graphene Oxides: A First-Principles Study Shaobin Tang† and Zexing Cao*,‡ †

Key Laboratory of Organo-Pharmaceutical Chemistry of Jiangxi Province, Gannan Normal University, Ganzhou 341000, China State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

‡

S Supporting Information *

ABSTRACT: The interactions of ammonia with graphene oxides (GO) were studied by density functional theory calculations. Our results indicate that the adsorption of NH3 on GO is generally stronger than that on graphene because of the presence of diverse active defect sites, such as the hydroxyl and epoxy functional groups and their neighboring carbon atoms. These surface oxygen sites can form OH···N and O···HN hydrogen bonds with NH3 and enhance charge transfers from NH3 to the graphene oxide. The dissociation of the adsorbed NH3 into the chemisorbed NH2 or NH species through the H atom abstractions leads to hydroxyl group hydrogenation and ring-opening of epoxy group. The reactions of NH3 with the hydroxyl and epoxy groups are predicted to be exothermic with different energy barriers, depending on the oxidation species and the atomic arrangement of these groups. The hydroxyl group exhibits relatively higher reactivity toward hydrogen abstraction from the adsorbed NH3 than the epoxy group in GO with a single oxygen group. The presence of a neighboring OH group may activate the oxygen groups to facilitate the surface reaction of NH3. Followed by the ring-opening of the epoxy group, the newly formed hydroxyl group can be removed by the second H atom abstraction from NH2. The calculated density of states of the adsorbed systems also reveals strong interactions between GO and NH3. The calculated results show good agreement with available experimental observations.

1. INTRODUCTION Graphene, a flat monolayer of carbon atoms arranged in a hexagonal structure, has stimulated extensive research activities because of its novel electronic, optical, and mechanical properties.1−6 It is important to realize the reliable control of the type and the density of charge carriers by doping for the graphene-based electronics. Chemical modification, such as the adsorption of external atoms and small molecules, is an effective route to tune the electronic and magnetic properties of lowdimensional nanomaterials.7−14 Many experimental and theoretical works13−21 show that the interactions of NO2 and NH3 with sp2 carbon-based nanomaterials may change the local carrier concentration and result in a remarkable fluctuation of conductivity, and thus, they have promising applications in the design of chemical detectors. However, only weak adsorption was found in most cases when these molecules interact with pristine graphene nanomaterials. The introduction of defect and active sites on graphene may effectively improve the adsorption of molecules onto materials.22−24 Graphene oxides (GO), in which the sp2 carbon surface is modified by oxygen functional groups, have emerged as a new class of carbon-based nanoscale materials.25−29 Graphene oxides are an important precursor for the preparation of large-scale graphene by thermal and chemical reduction of GO.30−34 Using the density functional method, Gao et al.35 and Kim et al.36 investigated the reaction mechanisms for deoxygenation of GO with hydrazine. The chemically reduced GO (rGO) leads to significant restoration of the sp2 carbon © 2012 American Chemical Society

network but is still unable to completely remove all oxygencontaining groups.37 The residual oxygen functional groups on rGO may provide the active defective sites enhancing the interaction of molecules with graphene. Recently, experimental studies38−40 have shown that the reduced graphene oxide or chemically converted graphene can be utilized as highperformance molecular sensors, such as for NO2 and NH3. Our recent calculations41 indicate that the presence of oxygencontaining groups on GO may be responsible for increasing the binding energies and enhancing charge transfers from nitrogen oxides to GO, which is consistent with experimental observations.38−40 Modification of graphene with ammonia and its molecular radicals has been of emerging importance in manipulating physical and chemical properties of materials.42−45 The experimental works by Chiu et al.43 show that nitrogencontaining radicals bond readily to a carbon lattice to form covalent bonds upon graphene exposure to NH3 plasma. On the basis of first principles calculations, it was reported that nitrene radicals attached to graphene may introduce band gaps, depending on the levels of functionalization.44 The adsorption of NH3 on graphene and single-walled carbon nanotubes (SWNT) results in an electron charge transfer from the molecule to these materials, acting as a donor characReceived: December 19, 2011 Revised: February 24, 2012 Published: March 27, 2012 8778

dx.doi.org/10.1021/jp212218w | J. Phys. Chem. C 2012, 116, 8778−8791

The Journal of Physical Chemistry C

Article

Figure 1. Schematic representation of (a) 4 × 4 supercell of graphene and (b) graphene oxides containing one epoxy functional group and indicating the binding sites A−D of other hydroxyl or epoxy groups. (c) Schematic representation of the charge transfer dependence on the applied electric field in +E along graphene → adsorbate and −E along its reverse directions. (d) The net electron-charge transfers from NH3 to graphene as a function of the external electric field in +E and −E directions shown in part c.

ter.13,14,19,20 Recently, experimental work46 has shown that graphite oxides can be used as excellent adsorbents of NH3, and its surface and structure play an important role in adsorption of molecules. More importantly, X-ray photoelectron spectroscopy and electrical transport measurements by Dai et al.47 indicated that chemical N-doping occurs and is accompanied by the reducation of GO through annealing of GO in NH3. It was proposed that the oxygen-conatining groups in GO may be responsible for reactions with NH3. Despite these important contributions, the detailed adsorption mechanisms for the ammonia molecule on graphene oxides are less known, and an atomic-scale understanding of the effect of the active defect sites on the interaction of NH3 with GO is highly required. In this paper, using density-functional theory calculations, we investigate the adsorption and dissociation of NH3 on graphene oxides. The mechanisms for the interaction of NH3 with GO and the removal of oxygencontaining groups with NH3, as well as the adsorption doping effect on their electronic properties, were explored.

considered to separate the layer and its images in the direction perpendicular to graphene plane. The 2D Brillouin zone was sampled by 7 × 7 × 1 k-points within the Monkhorst−Pack scheme.50 During geometry optimization, the whole configuration was allowed to relax until all of the force components on any atom were less than 10−3 au, and 15 × 15 × 1 k-points chosen according to the Monkhorst―Pack method along the periodic direction were used to acquire the electronic properties. We used the linear/quadratic synchronous transit51 methods to estimate the transition-state barriers. The charge transfer from NH3 to GO was calculated on the basis of Mulliken population analyses. The structural features of GO have been extensively investigated, both experimentally and theoretically.25−29,52−63 It is widely accepted that the oxygen-containing functional groups on the graphene basal plane exist in the form of hydroxyl (OH) and epoxy (−O−) groups,52,59−61 although new oxidation species on the surface of graphene, such as the carbonyl group (CO) and epoxy pair, have also been reported.53,54,57,58 The exploration of the relative ordering of epoxy and hydroxyl groups has been subject of several theoretical studies for the thermal reduction and oxidative unzipping of graphene.64 Recently, using spectroscopic tools and ab initio calculations, Larciprete et al.64 suggested that the formation of surface lactones as precursors from epoxides plays an important role in thermal reduction of GO at higher O coverage. Experimental results by carbon-13 (13C) solid-state nuclear magnetic resonance52,63 showed that the hydroxyl and epoxide functional groups of GO were located on graphene basal planes close to each other. On the basis of theoretical calculations, various atomic configurations of GO were proposed. It was found that the epoxide and hydroxyl groups

2. COMPUTATIONAL DETAILS The interaction of NH3 with graphene oxides was studied by density functional theory calculations using the DMol3 package.48 In all-electron calculations by DMol3, the density functional of the local-spin density approximation with the exchange−correlation potential parametrized by Perdew and Wang49 were adopted. All the calculations were performed within the spin-polarized frame. The double numerical plus polarization function basis set and a real-space cutoff of 4.5 Å were used. Periodic boundary conditions with a supercell of 4 × 4 graphene unit cells composed of 32 carbon atoms were employed in the calculations. A vacuum region of 12 Å was 8779



Article

Figure 2. Optimized structures for the adsorption and dissociation of NH3 on graphene oxides containing only an epoxide functional group. (a) O− NH3 and (e) 2O−NH3 with the adsorption of NH3 on GO. (b−d) The N−H bond dissociation of NH3 in O−NH3 leading to the NH2 group chemisorbed at sites A, B, and C indicated by part a, respectively. (f) 2O−NH2−1 and (g) 2O−NH2−2 with similar epoxide group opening as parts b−d. (h) 2O-NH with the dissociation of NH2 from part g, leading to NH species. The side view and top view of all structures are given, and the distances are shown in angstroms.

energetically prefer to aggregate on the graphene plane;55,62,65 thus, the graphene oxides in our computational models contain hydroxyl and epoxide functional groups. When the surface of GO includes several oxygen functional groups, the initial structures are constructed with these groups close to each other. Figure 1a and b shows the schematic representation of graphene (C32) and graphene oxides containing binding sites for oxygen groups, respectively. In our calculations, the effect of a single oxygen group and both OH and −O− groups on adsorption of NH3 are discussed. In the presence of one epoxide functional group (Figure 1b), the other epoxy group can bridge over two nearest-neighbor carbon atoms A and B (denoted as 2O) on the same side relative to the existing group, although the structure of 2O is only 2.3 kcal/mol in energy higher than that of the opposite side.62 For GO with two hydroxyl functional groups, the atomic configurations with two OH groups attached to the carbon atoms A and B at the opposite side each other (called as the 1,2-hydroxyl group pair, defined as 2OH-1) or bound to carbon atoms A and C at the same side (2OH-2) have been considered.55,62 The structure

with the 1,2-hydroxyl group pair is energetically the most favorable one for all adsorptions of two OH groups on the graphene plane. Similarly, when graphene oxides contain both OH and O groups, the hydroxyl group may be bound to carbon atom A or C at the opposite side relative to the epoxy group, named as OH−O, and this is consistent with the 1,2-ether oxygens proposed by previous density-functional calculations.65 Although the OH−O with 1,2-ether oxygens is energetically more favorable than other atomic arrangements of these groups, the structure with OH at the same side (O−OH) is only 5.7 kcal/mol higher than OH-O. Accordingly, the interaction of NH3 with O−OH was also investigated here. To evaluate the interaction of NH3 with graphene oxides, the binding energies (Eb) are calculated by E b = [E(M) + E(GO)] − E(GO−M)

(1)

where E(GO) and E(M) are the total energies of isolated GO and free ammonia molecule, respectively, and E(GO−M) is the total energy of graphene oxide with the adsorbed or dissociative-adsorbed NH3. Note that the positive binding 8780



Article

energies correspond to exothermicity for adsorption and dissociation of NH3 on GO.

Table 1. Summary of Calculated Results for Adsorption and Dissociation of NH3 on Graphene Oxides Containing only Epoxide Groups: the Adsorbed Species from NH3 (AS), the Binding Energy (Eb in kcal/mol), the Distances of the Hydrogen Bond OH···N (d1) and the C−N Bond (d2), and the Charge Transfers from the Adsorbed Molecule to GO (ΔQ)

3. RESULTS AND DISCUSSION 3.1. Interaction of Ammonia with Pristine Graphene. For comparison, we first investigate the physisorption of the ammonia molecule on pristine graphene. Figure 1c shows the optimized structure of NH3 adsorbed on graphene with the N atom pointing to the surface. The adsorption of NH3 on the graphene is slightly exothermic by 2.5 kcal/mol, compared with 0.35−0.71 kcal/mol by PBE20 and 4.14 kcal/mol for NH3 on (10,0) SWNT.19 In contrast to p-type doping by NO2,41 the adsorbed NH3 behaves as a donor character with 0.02e charge transfer from the molecule to graphene, in agreement with 0.026e by PBE.20 The electron carrier doping induced by adsorption of NH3 on graphene is consistent with the experimental observations for the sensor response mechanism.13,14 Charge transfers between the adsorbed species and graphene producing the local carrier concentration changes in materials may be sensitive to the presence of an external electric field. Here, we investigate the variation of charge-transfer features for the graphene with physisorption of NH3 as subjected to electric fields. As shown in Figure 1c, two directions perpendicular to the graphene surface for the applied external electric field (i.e. along graphene → adsorbate (+E) and adsorbate → graphene (−E)) are considered. The calculated results show that the electronic properties of charge transfer can be well modulated by the applied external electric field (Figure 1d). In the presence of +E, the charge transfers from NH3 to graphene decrease as the electric field increases. In contrast, the reverse electric field increases the electron charge transfer, indicating the deep electron carrier doping. This variation of chargetransfer features contrasts with the results for adsorptions of nitrogen oxides on graphene, where the external electric field along +E increases the charge transfer from adsorbate to graphene because of the acceptor character of NO2.41 3.2. Interaction of Ammonia with GO Containing Single Epoxide Group. Previous experiments38−40,66show that the chemically converted graphene or reduced graphene oxides are promising for design of high-performance molecular sensors because of the presence of active defect sites provided by the oxygen-containing groups. Furthermore, the oxygen groups in GO may be responsible for reactions with NH3 and covalent C−N bond formation.46,47 Herein, we investigate interactions of the ammonia molecule with GOs or reduced GOs. Figure 2 and Table 1 show the optimized structures and summary of calculated results for the interaction of NH3 with graphene oxides with a single epoxide functional group. The adsorption of NH3 on one epoxide group (Figure 1a) is predicted to be exothermic by 7.1 kcal/mol as a result of the electrostatic attraction between H and O atoms,36 larger than 2.5 kcal/mol for pristine graphene, implying that the epoxide group improves the interaction of the molecule with materials. As shown in Figure 2a, the distance between the H and O atoms is 2.11 Å. The H atom pointing to the O is transferred from NH3 to the oxygen group after adsorption, leading to the N−H bond dissociation of NH3 and epoxide ring opening. The dissociated NH2 species may be bound to different carbon sitesA, B, and C indicated by Figure 2ato form the covalent C−N bond, defined as O−NH2−i (i = A, B, and C) as shown in Figure 2b−d, respectively. Total energy calculations

structures O−NH3 O−NH2−A O−NH2−B O−NH2−C 2O−NH3 2O−NH2−1 2O−NH2−2 2O−NH

AS NH3 NH2 NH2 NH2 NH3 NH2 NH2 NH

Eb 7.1 14.7 −27.1 10.1 7.4 15.2 −5.3 15.6

d1(Å)

d2(Å)

a

2.11 1.79 1.75 1.58 2.22a 1.59 1.61 1.76

1.5 1.52 1.5 1.48 1.48 1.48

ΔQ (e) 0.02 0.37 0.43 0.37 0.02 0.39 0.4 0.5

a The distance between the H of NH3 and the O atoms due to the electrostatic attraction.

show that the binding of NH2 to carbon site A is energetically the most favorable one, with a binding energy of 14.7 kcal/mol, compared with sites B and C. It is surprising that the epoxide ring opening for binding site B of NH2 (Figure 2c) is energetically unfavorable because such N−H bond cleavage is endothermic by 27.1 kcal/mol. As shown in Figure 2c, the energetically unfavorable structure may be due to dissociation of NH3 into NH2 at the carbon site B, leading to 1,3-addition of graphene. The two carbon atoms connecting the formed OH and NH2 belong to the same sublattice of graphene, resulting in the structure with ferromagnetic ground state.67,68 Such an addition destroying the sublattice balance of graphene is energetically less favorable than the reaction with two formed groups located at different sublattices. The structures show that the OH···N hydrogen bonds between newly generated OH and NH2 groups are formed with distances of 1.79, 1.75, and 1.58 Å for sites A, B, and C, respectively. The notable charge transfers of 0.37−0.43e from NH3 to GO, larger than the physisorption of NH3 on GO and pristine graphene, are found due to the surface reactions. The reaction pathways for the adsorption and dissociation of NH3 on GO are investigated using the transition state search method. Figure 3a and b shows the relative energy profiles and the atomic configurations of initial, transition, and final states for the formation of O−NH2−i (i = A and C). The energy barrier for the N−H bond dissociation of NH3 leading to NH2 species located at the C site is predicted to be 23.4 kcal/mol; however, the barrier is reduced to 16.5 kcal/mol when the dissociated NH2 group is adsorbed at the carbon A site, suggesting that such ring opening of the epoxy group with NH3 is kinetically more favorable (see Figure 2b). The dissociation energy of NH3 for binding site A is −6.9 kcal/mol, larger than the −2.9 kcal/mol of site C. Therefore, the epoxide ring opening is expected occur via the H atom abstraction from NH3. The predicted epoxide ring-opening mechanisms with NH3 are consistent with previous experimental works46 for GO used as adsorbents of NH3 and theoretical results36 for epoxide reduction with hydrazine, in which the abstraction of an H atom in NH2NH2 leads to the formation of a hydroxyl group. Similarly, the adsorption of NH3 on GO containing two epoxide groups with the atomic arrangement of 2O, defined as 2O−NH3 (Figure 2e), is exothermic by 7.4 kcal/mol due to the electrostatic attraction. The H atom in NH3 attacks one oxygen 8781



Article

calculations show that the structure of GO with a 1,2-hydroxyl functional group pair (2OH−1) is 8.5 kcal/mol in energy more stable than the configuration with the two hydroxyl groups at the same side (2OH−2), which is consistent with the previous results.62 As shown in Table 2 and Figure 4, the adsorptions of NH3 on GO with one OH group (Figure 4a), and on 2OH−1 (Figure 4c) and 2OH−2 (Figure 4e), named OH−NH3, 2OH−1−NH3, and 2OH−2−NH3−1, result in larger binding energies of 18.6, 18.4, and 20.9 kcal/mol, respectively, larger than the physisorption of NH3 on pristine graphene. The interaction of hydrogen bonds OH···N between the NH3 and OH groups with separation of 1.61−1.67 Å may be responsible for the larger binding energies. The corresponding charge transfers of 0.11e from the molecule to GO is found, which is attributed to the hydrogen bonds OH···N. The important role of the hydrogen bond in improving the molecule adsorption and charge transfer has also been reported by our recent calculations for adsorption of nitrogen oxides on GO41 and other theoretical studies.24 In contrast, the adsorption of NH3 with an H atom pointing to the O of 2OH−2 (Figure 4f) is weak, with a binding energy of 1.6 kcal/mol. Similar to the case of the epoxide group, the H atom from NH3 attacks the O of the OH of GO after adsorption, resulting in the OH hydrogenation and N−H bond dissociation of NH3, as shown in Figure 4b, d, g, and h. The predicted binding energies for OH hydrogenation of GO with one hydroxyl group (defined as OH−NH2) and a 1,2-hydroxyl group pair (2OH− 1−NH2) are 14.7 and 16.6 kcal/mol, in comparison with the values for the epoxide ring opening, respectively. When two hydroxyl groups are attached to the surface of graphene at the same side (2OH−2), however, the N−H bond dissociation of NH3 due to H atom abstraction, defined as 2OH−2−NH2 (Figure 4g), is predicted to be endothermic by 6.9 kcal. Interestingly, the second H atom abstraction from adsorbed NH2 leading to another OH hydrogenation of 2OH−2−NH2 (see Figure 4h) is energetically favorable, with a larger exothermicity of 21.6 kcal/mol due to reducing the surface strain introduced by the OH group. In Figure 4b, d, and g, those OH hydrogenations result in the formation of H2O, which interacts with the adsorbed NH2 through the hydrogen bonds OH···N with distances of 1.67−1.83 Å. Figure 5a and b presents relative energy profiles for the OH hydrogenation of GO with one and two hydroxyl functional groups. The transition state search (Figure 5a) shows that the dehydroxylation of GO by the H atom abstraction from NH3 is achieved by overcoming an energy barrier of 14.2 kcal/mol and with exothermicity of 6.6 kcal/mol relative to the physisorbed state OH−NH3−2. The predicted mechanism for OH hydrogenation is compared with the recent report for the reduction of HO−graphene with N2H4 with a barrier of 3.5 kcal/mol.36 For GO including a 1,2-hydroxyl group pair (Figure 5b), the energy barrier for an H atom abstraction from NH3 is only 2.5 kcal/mol, obviously lower than the value for the case of one hydroxyl group. Such a small energy barrier may be attributed to the initial state (defined as 2OH−1−NH3−2) with adsorption of NH3 at the carbon atom neighboring the hydroxyl group pair, leading to a 4-fold coordinated N atom. Therefore, the presence of another OH group at the opposite side relative to the adsorbed NH3 activates the oxygen group to facilitate OH hydrogenation. The formation of product (2OH− 1−NH2) is 8.7 kcal/mol lower in energy than the initial state. The results show that the reduction of an OH group of GO with NH3 can be realized even at low temperature, in

Figure 3. Relative energy profiles for the adsorption and dissociation of NH3 on GO containing a single epoxide group. All energies (in kcal/mol) in parts a−c are relative to the initial structures, and the top and side views of optimized configurations (distance in Å) of initial, transition, intermediate, and final states are shown. The epoxide group opening for the N−H bond dissociation of NH3 leading to NH2 located at sites (a) C and (b) A indicated by Figure 2a. (c) Two epoxide groups opening for dissociation of NH3, leading to chemisorbed NH species. Note that other carbon atoms are omitted.

group after NH3 adsorption, resulting in the formation of an OH group and N−H bond dissociation of NH3. As shown in Figure 3f and g and Table 1, the binding of dissociated NH2 to the carbon atom 1 connecting the epoxide group attacked by the H (named as 2O−NH2−1) is found to be exothermic by 15.2 kcal/mol, whereas the dissociation reaction of NH3 leading to the NH2 group located at the neighboring carbon atom 2 (2O−NH2−2) is energetically unfavorable because it is endothermic by 5.3 kcal/mol. Owing to the presence of another neighboring epoxide group, the second H atom abstraction from NH2 leads to the dissociation of NH2 into NH species connecting two neighboring carbon atoms with a binding energy of 15.6 kcal/mol, as shown in Figure 2h (defined as 2O−NH). Similar to GO with one epoxide group, the newly formed OH for two epoxide groups interacts with the chemisorbed NH2 or NH species through the hydrogen bonds OH···N with distances of 1.59−1.76 Å. The larger charge transfers from NH3 to GO are estimated to be 0.39−0.5e because of the dissociation reaction of NH3. Figure 3c shows the reaction pathways for two epoxide rings opening with NH3. Conversion of the first epoxide group (2O− NH3) into the hydroxyl group (2O−NH2−1) is predicted to have exothermicity of 7.9 kcal/mol with an energy barrier of 18.6 kcal/mol in comparison with 16.5 kcal/mol for GO with one epoxide group (Figure 3b). However, the second H atom abstraction from NH2 (2O−NH) is kinetically unfavorable because a relatively high barrier of 30 kcal/mol with respect to 2O−NH2−1 must be overcome. The predicted exothermicity for the second epoxide ring opening is 8.4 kcal/mol, slightly larger than the first H abstraction. 3.3. Interaction of Ammonia with GO Containing Single Hydroxyl Group. The interactions of NH3 with GO containing one and two hydroxyl groups are discussed. Figure 4 and Table 2 present the structures and calculated results for the adsorption and dissociation of NH3 on GO. Total energy 8782



Article

Figure 4. Optimized structures for the adsorption and dissociation of NH3 on graphene oxides containing only a hydroxyl functional group. (a) OH−NH3, (c) 2OH−1−NH3, (e) 2OH−2−NH3−1, and (f) 2OH−2−NH3−2 with the adsorption of NH3 on GO through the hydrogen bonds OH···N or HO···H. (b) OH−NH2, (d) 2OH−1−NH2, (g) 2OH−2−NH2, and (h) 2OH−2−NH with the N−H bond dissociation of adsorbed NH3 or NH2 leading to a nOH group hydrogenation. The side view and top view of all structures are given, and the distances are shown in angstroms.

O through the H atom abstraction leads to the epoxy ring opening. Figure 6b−e and Table 3 show the optimized structures and calculated results. There are several sites A, B, C, and D for the binding of dissociated NH2, indicated by Figure 6a, defined as OH−O−NH2−i (i = A, B, C, and D), respectively. The dissociated NH2 is energetically preferable to be adsorbed at carbon atom site A (Figure 6b) with a binding energy of 30.6 kcal/mol, compared with other sites. However, the N−H bond dissociation reactions for the binding sites B and C of NH2 (Figure 6c and d) are predicted to be endothermic by 32 and 7.6 kcal/mol, respectively. Thus, the stabilities of GO after epoxy ring opening depend not only on the binding site of NH2 relative to the existing OH but also on the site of newly formed OH. The calculated structures show that the hydrogen bonds OH···N between NH2 and new OH are formed with distances of 1.59−1.69 Å, similar to GO with a single epoxide functional group.

agreement with the experimental observation for thermal annealing of GO in ammonia.47 3.4. Interaction of Ammonia with GO Containing Both Epoxy and Hydroxyl Group. Usually, the surface of GO contanis both epoxy and hydroxyl groups in the intermediate region between O-rich and H-rich conditions.52−62 In the following discussion, our investigation will focus on the interaction of NH3 with GO with both epoxy and hydroxyl groups. In the presence of one epoxy group, the structure of GO with one hydroxyl group attached to the neighboring carbon atom at the opposite side with respect to the existing epoxide (OH−O) is 5.7 kcal/mol in energy lower than that of OH at the same side (O−OH). Figure 6a shows the atomic structure of OH−O. The adsorption of NH3 at the epoxy group of OH−O is exothermic by 7.2 kcal/mol due to the electrostatic attraction. Similar to GO with a single oxygen group, the N−H bond dissociation of adsorbed NH3 on OH− 8783



Article

kcal/mol, compared with 16.5−23.4 kcal/mol for GO with a single epoxy group. The lower barrier results from the existing OH group at the opposite side activating the epoxy group. The formation of the final state (OH−O−NH2−A) is predicted to have a larger exothermicity of 23 kcal/mol relative to the initial state (OH−O−NH3−A). In addition, the interaction of NH3 with an existing OH group of OH−O (Figure 6f) is also calculated. The larger binding energy of 20 kcal/mol and notable charge transfer of 0.11e from the molecule to GO are found as a result of the formation of a hydrogen bond, consistent with the results for GO with single hydroxyl group. In the case of two oxygen groups at the same side of graphene (O−OH), the adsorption and dissociation of NH3 induced by both the epoxide and hydroxyl groups are also considered. The structures and calculated results are shown in Figure 7 and Table 3. As discussed above, the formation of a hydrogen bond between the adsorbate and hydroxyl group improves the adsorption of NH3 on GO, giving rise to the binding energy of 16.3 kcal/mol and charge transfer of 0.11e (Figure 7a and Table 3). When NH3 interacts with the epoxide group from two different directions close to carbon sites A and B (indicated by Figure 7a) relying on the electrostatic attraction, the H atom abstractions from NH3 result in the

Table 2. Summary of Calculated Results for Adsorption and Dissociation of NH3 on Graphene Oxides Containing only Hydroxyl Group: the Adsorbed Species from NH3 (AS), the Binding Energy (Eb in kcal/mol), the Distances of the Hydrogen Bond OH···N (d1) and the C−N Bond (d2), and the Charge Transfers from the Adsorbed Molecule to GO (ΔQ) structures

AS

Eb

d1(Å)

OH−NH3 OH−NH2 2OH−1−NH3 2OH−1−NH2 2OH−2−NH3−1 2OH−2−NH3−2 2OH−2−NH2 2OH−2−NH

NH3 NH2 NH3 NH2 NH3 NH3 NH2 NH

18.6 14.7 18.4 16.6 20.9 1.6 −6.9 21.6

1.67 1.81 1.66 1.83 1.61 2.16a 1.67 1.72

d2(Å) 1.5 1.49

1.52 1.47

ΔQ (e) 0.11 0.34 0.11 0.34 0.11 0.01 0.37 0.47

a The distance between the H of NH3 and the O atoms due to the electrostatic attraction.

The predicted reaction mechanisms (Figure 5c) show that the epoxy group opening of OH−O for binding site A is likely to be quite facile because of the lower energy barrier of 4.2

Figure 5. Relative energy profiles for the adsorption and dissociation of NH3 on GO. All energies (in kcal/mol) in parts a−d are relative to the initial structures, and the top and side views of optimized configurations (distance in Å) of initial, transition, and final states are shown. Hydroxyl group hydrogenation for GO with (a) a single OH and (b) two OH groups on both sides and (d) both O and OH groups at the same side. (c) Epoxide group opening for GO with both O and OH groups at two sides. Note that other carbon atoms are omitted. 8784



Article

Figure 6. Optimized structures for the adsorption and dissociation of NH3 on graphene oxides containing both epoxide and hydroxyl functional groups at both sides. (a) Atomic structure of OH−O. (b−e) OH−O−NH2−i (i = A, B, C, and D) with epoxide group opening due to the N−H bond dissociation of NH3, leading to NH2 species chemisorbed at sites A−D indicated in part a, respectively. (f) OH−O−NH3 with adsorption of NH3 through hydrogen bond interaction.

attacks the O of the existing OH (O−OH−NH), leading to the formation of H2O and NH species (Figure 7e). The N−H bond dissociation of NH2 is estimated to be exothermic by 19.3 kcal/mol, with a charge transfer of 0.51e from NH3 to GO. In Figure 7e, two hydrogen bonds between H2O and the other two groups with corresponding distances of 1.62 and 1.72 Å are formed. In comparison with the epoxide opening (Figure 7d), the H atom of adsorbed NH3 at the domain close to the carbon site B may first attack the O of the existing OH instead of the epoxide group (Figure 7c), leading to dehydroxylation of GO with a binding energy of 14.5 kcal/mol (Table 3), slightly larger than the 13.8 kcal/mol for the epoxide group first attacked by H atom. Figures 5d and 8a show relative energy profiles for adsorption and dissociation of NH3 on O−OH, respectively. The calculated results indicate that when NH3 is adsorbed on the epoxy group of O−OH close to the carbon atom site B due to the electrostatic attraction, the H atom from NH3 prefers to attack the epoxide group rather than the hydroxyl group kinetically because the barrier for ring opening of the epoxy group is 11.2 kcal/mol, whereas the latter is 25.4 kcal/mol. The predicted exothermicity for formation of the OH group (Figure 5d) and H2O (Figure 8a) are 5.5 and 6.9 kcal/mol with respect to the initial states, respectively. Comparing with the single oxygen group (Figure 2), the energy barrier for the epoxide group opening of GO with both epoxide and neighboring hydroxyl groups at the same side is reduced from 16.5 to 23.4 to 11.2 kcal/mol, suggesting that this reaction with the assistance of the hydroxyl group should be more facile. Such results are also confirmed by the case of two oxygen groups at the opposite side (Figure 5c). Herein, we investigate the adsorption behavior of NH3 on GO with the combination of O + 2OH. The atomic configuration of GO with the epoxy group and its nearestneighbor 1,2-hydroxyl pair, defined as O−2OH as shown in Figure 9a, is energetically the most favorable one among all atomic arrangements of these groups.55,62 In Figure 9a and b,

Table 3. Summary of Calculated Results for Adsorption and Dissociation of NH3 on Graphene Oxides Containing Both Epoxide and Hydroxyl Functional Groups: the Adsorbed Species from NH3 (AS), the Binding Energy (Eb in kcal/ mol), the Distances of the Hydrogen Bond OH···N (d1) and the C−N Bond (d2), and the Charge Transfers from the Adsorbed Molecule to GO (ΔQ) structures

AS

Eb

d1(Å)

d2(Å)

ΔQ (e)

OH−O−NH2−A OH−O−NH2−B OH−O−NH2−C OH−O−NH2−D OH−O−NH3 O−OH−NH3 O−OH−NH2−A−O O−OH−NH2−B−OH O−OH−NH2−B−O O−OH−NH O−2OH−NH3−1 O−2OH−NH3−2 O−2OH−NH3−A O−2OH−NH2 O−2OH−NH

NH2 NH2 NH2 NH2 NH3 NH3 NH2 NH2 NH2 NH2 NH3 NH3 NH3 NH2 NH

30.6 −32 −7.6 20.9 20 16.3 7.1 14.5 13.8 19.3 13.8 20.2 1.4 17.8 15.4

1.6 1.69 1.59 1.62 1.65 1.62 1.58 1.83 1.51 1.72 1.62 1.64

1.49 1.51 1.49 1.49

0.37 0.37 0.37 0.37 0.11 0.11 0.39 0.37 0.4 0.51 0.11 0.11 0.57 0.35 0.42

1.64 1.99

1.49 1.48 1.49 1.48

1.56 1.49 1.5

epoxide opening and the NH2 chemisorbed at carbon sites A (Figure 7b) and B (Figure 7d), defined as O−OH−NH2−A−O and O−OH−NH2−B−O, respectively. Total energy calculations show that the ring opening of the epoxy group for the B site with an exothermicity of 13.8 kcal/mol is more favorable in energy than that of the A site (7.1 kcal/mol). From Figure 7b and d, it is found that the formation of many hydrogen bonds between newly formed OH and NH2 and existing OH groups is useful to stabilize those structures, consistent with previous reports for the chainlike structures formed by hydroxyl groups on GO.55,62 Followed by the epoxide opening (Figure 7d), the second H atom from the NH2 8785



Article

Figure 7. Optimized structures for the adsorption and dissociation of NH3 on graphene oxides containing both epoxide and hydroxyl functional groups at the same side. (a) O−OH−NH3 with adsorption of NH3. (b) O−OH−NH2−A−O, (c) O−OH−NH2−B−OH, (d) O−OH−NH2−B−O, and (e) O−OH−NH with the N−H bond dissociation of NH3 or chemisorbed NH2. The H atom abstractions from NH3 or NH2 lead to epoxide group opening in parts b and d and hydroxyl group hydrogenation in parts c and e. The side and top views of all structures are given, and the distances are shown in angstroms.

Figure 8. Relative energy profiles for the adsorption and dissociation of NH3 on GO. All energies (in kcal/mol) in parts a−c are relative to the initial structures, and the top and side views of the optimized configurations (distance in Å) of initial, transition, intermediate, and final states are shown. (a) Epoxide group opening for GO containing both O and OH groups at the same side. (b) Epoxide group opening and subsequent OH hydrogenation for GO with both epoxide and 1,2-hydroxyl pair. Note that other carbon atoms are omitted.

kcal/mol lower in energy than the initial state without N−H bond dissociation (Figure 8b and Table 3). Comparing with the reaction pathways for the epoxide group opening of GO (Figures 3, 5c, and 8a, b), the presence of a neighboring OH group may activate the epoxide group to be easily attacked by the H atom of NH3, suggesting an improving reaction rate consistent with the recent report for the formation mechanism of carbonyl pairs.58 Followed by the ring opening of epoxy group, the second H atom may transfer from NH2 to an existing OH group at the same side, giving rise to the formation of H2O as shown in Figure 9f (defined as O−2OH−NH). However, such a OH hydrogenation reaction is energetically unfavorable because the formation of product is predicted to be endothermic by 2.4

the adsorptions of NH3 on O−2OH with the N atom pointing to the OH group at the same side and opposite side relative to the epoxy group lead to the formation of hydrogen bonds with binding energies of 13.8 and 20.2 kcal/mol (Table 3), respectively. Similar to GO with only a 1,2-hydroxyl pair (Figure 5b), as shown in Figure 9d, a 4-fold coordinated N atom is formed as NH3 is adsorbed at carbon site A, indicated by Figure 9a. This adsorption is predicted to be slightly exothermic by 1.4 kcal/ mol. For same reason, with Figure 5b, the H atom abstraction from NH3 leading to epoxide group opening (Figure 9e) is expected to be facile because of the lower energy barrier of 2.5 kcal/mol by the predicted reaction pathway (Figure 8b). The formation of the OH group in the state O−2OH−NH2 is 16.4 8786



Article

Figure 9. Optimized structures for the adsorption and dissociation of NH3 on graphene oxides containing both epoxide and a 1,2-hydroxyl pair. (a) Atomic structure of O−2OH. (b, c) Adsorptions of NH3 on GO through a hydrogen bond. (d) O−2OH−NH3−A with adsorption of NH3 at carbon site A shown in part a. (e) O−2OH−NH2 and (f) O−2OH−NH with N−H bond dissociation of NH3 or NH2, leading to epoxide group opening and OH hydrogenation, respectively. The side and top views of all structures are given, and the distances are shown in angstroms.

Table 4. Calculated Binding Energies, Eb, and Reaction Energies, Er, (kcal/mol) for Adsorption and Dissociation of NH3 on Graphene Oxides Containing Several Epoxide and Hydroxyl Functional Groups by 4 × 4, 5 × 5, and 6 × 6 Graphene Supercells Structures 4×4 Eb 2O−NH2−1 OH−O−NH3−A OH−O−NH2−A O−2OH−NH3−A O−2OH−NH2

15.2 7.6 30.6 1.4 17.8

5×5 Er

Eb 15.1 7.6 29.3 1 19.3

−23a −16.4b

6×6 Er

Eb

Er

−21.7a

15.1 7.6 29.2 0.8 19.4

−21.6a

−18.3b

−18.6b

The reaction energy for conversion from OH−O−NH−A to OH−O−NH2−A. The reaction energy for conversion from O−2OH−NH3−A to O−2OH−NH2. a

b

supercells, as shown in Figure S3 in the Supporting Information. 3.5. Epoxide Reduction with NH3. Recently, the experimental works by Dai et al.47 showed that thermal annealing of GO in NH3 was effective for GO reduction, in which oxygen groups were removed through its reactivity toward NH3. As discussed above, the reduction of OH in GO is predicted to be facile through the H atom abstraction of NH3. Herein, we investigate the reaction mechanism for epoxide reduction after epoxide group opening with NH3. Figure 10a and b shows the relative energy profiles for the reduction of a newly formed hydroxyl group in GO with only one O group. When the epoxide group was opened by H atom abstraction from NH3 (Figure 10a and b), the H atom of the OH group pointed to the N atom of NH2 because of the formation of hydrogen bond. To realize the second H atom transfer from NH2 to the O of OH, the O atom should be close to one of the H's of the NH2. For this purpose, the introduction of intermediate states O−NH2−C1 (Figure 10a) and OH− NH2−A1 (Figure 10b) by rotating the OH goup along the C− O bond is easily achieved by overcoming the small energy barriers of 12 and 11.7 kcal/mol, respectively, although the

kcal/mol by overcoming a high energy barrier of 25 kcal/mol relative to the first H atom transfer (see Figure 9(b). Owing to the presence of multiple O and OH groups on the surface of GO, the 4 × 4 supercell used in our models might be at the limit for a suitable description of the interaction or surface reaction of NH3 with GO structurally. For comparison, the coverage effect of oxygen groups on the relative energies and barriers for the adsorption and dissociation of NH3 on GOs was estimated by using the larger 5 × 5 and 6 × 6 graphene supercells. The calculated results for selected structures with several oxygen groups are shown in Table 4 and Figures S1−S3 in the Supporting Information. As Table 4 shows, these predicted binding energies and relative energies from different supercells considered here are comparable. For example, the exothermicity difference between 4 × 4 and larger supercells for the conversion from OH−O−NH−A to OH−O−NH2−A (O−2OH−NH3−A to O−2OH−NH2) is less than 1.4 kcal/ mol (2.2 kcal/mol), and these thermodynamic values are gradually converged at the 5 × 5 supercells. Similarly, the predicted barrier of 2.5 kcal/mol for the surface process from O−2OH−NH3−A to O−2OH−NH2 (Figure 8b) is only 0.3 kcal/mol higher than 2.2 kcal/mol by larger graphene 8787



Article

Figure 10. Relative energy profiles for the reduction of a newly formed hydroxyl group starting from the epoxide group opening. The OH hydrogenation for the N−H bond dissociation of NH2 located at carbon sites (a) C and (b) A indicated by Figure 2a. All energies (in kcal/mol) in parts a and b are relative to O−NH3−C (Figure 2d) and O−NH3−A (Figure 2b), corresponding to the products in Figure 3a and b, respectively. The top view of the optimized configurations (distances in Å) of initial, transition, intermediate, and final states are shown. Note that other carbon atoms are omitted.

epoxide reduction with NH3 is compared with the recent results for the hydrazine reduction of GO.35,36 Our calculated results suggest that the adsorption and dissociation of NH3 on GO may be responsible for the removal of epoxide and hydroxyl functional groups, which is consistent with the experimental observations.47 3.6. Electronic Properties of GO with Adsorption and Dissociation of NH3. To have insight into the interaction of NH3 with GO, we calculated the total density of states (TDOS) and the projected density of states (PDOS) of GO containing one 1,2-hydroxyl pair (C32−2OH) and one epoxide functional group (C32−O) without and with adsorption and dissociation of NH3, as well as the free molecule state (Figure 11). Band structure calculations and TDOS reveal that the GO systems of C32−2OH and C32−O are semiconductors with different band gaps (Figure 11a and b), consistent with the energy gap of graphene tuning by varying the oxidation level.62 According to the TDOS of 2OH−1−NH2 (Figure 11b(2)), 2OH−1−NH3 (Figure 11c(1)), and O−NH2−A (Figure 11c(2)), the semiconducting properties of GO are less affected by the interaction of NH3. When the adsorbed NH3 molecule on GO is decomposed into chemisorbed NH2 species as a result of the H atom abstraction (Figures 4d and 2b), the electrons from the NH2 group (Figure 11b(2) and c(3)) are in a wide energy range between ∼−9 and −2 eV, compared with those states localized at −0.5 and −5.5 eV below the Fermi level for free NH2 (Figure 11b(2)), suggesting the formation of a covalent bond between GO and NH2. In contrast, in Figure 11b(2), the electron states from OH are strongly localized at energy levels of −2, −4, and −8 eV due to the OH hydrogenation in 2OH−1−NH2, compared with the wide peaks for the PDOS of the OH of GO (Figure 11b(3)) before NH3 adsorption. The changes in the DOS before and after NH3 decomposition lead to the larger charge transfer.

corresponding reaction steps are endothermic by 7.1 and 11.6 kcal/mol. Because the OH and NH2 groups of O−NH2−C are bound to the same C−C bond (Figure 10a), the O atom can approach the H of NH2 with a distance of 1.94 Å after OH rotation. As a result, the second H transfer occurs with an energy barrier of 25.9 kcal/mol relative to the intermediate state O−NH2−C1, leading to OH hydrogenation (Figure 10a). The formation of product (O−NH−H 2 O) is slightly endothermic by 0.2 kcal/mol relative to the initial state (O− NH2−C). For the intermediate state O−NH2−A1 (Figure 10b), although the O atom was pointed to the NH2 group after OH rotation, the large distance between the O and H of NH2 is still unsuitable for the second H transfer from NH2. Similarly, the reaction pathway (Figure 10b) shows that the structure with the H atom close to the O of OH can be realized through NH2 rotation along the C−N bond. This reaction step leading to the second intermediate state (O−NH2−A2) is not a barrier process, with an exothermicity of 6 kcal/mol relative to O− NH2−A1. Finally, the OH group is removed as a result of the second H abstraction from NH2 by overcoming the barrier of 25.4 kcal/mol. The formation of product is exothermic by 5.2 kcal/mol with respect to the state O−NH2−A2. Comparing with the two pathways for ring opening of the epoxy group leading to NH2 adsorbed at sites A and C (Figure 3a and 3b), the route with site A (Figure 3b) is more favorable than that of site C, both kinetically and thermodynamically. Followed by the epoxide group opening, however, the mechanisms of newly formed OH reduction for the sites A and C are almost the same except for the step with NH2 rotation for site A (Figure 10a and b). On the basis of calculated barriers, the second H transfer is the ratedetermining step in the whole de-epoxidation process, which may be realized through thermal annealing of GO in NH3 at temperatures of 300−500 °C.47 The predicted mechanism for 8788



Article

Figure 11. (a) The density of states (DOS) of graphene oxides containing (a(1)) a 1,2-hydroxyl pair and (a(2)) one epoxide functional group, and the total DOS and PDOS (a(3)) of free NH3. (b) PDOS of NH2 from free NH3 (b(1)), TDOS and PDOS for 2OH−1−NH2 (b(2)), and PDOS of OH group from GO (b(3)). (c) TDOS and PDOS for 2OH−1−NH3 (c(1)), TDOS of O−NH2−A (c(2)), and PDOS of NH2 from O−NH2−A (c(3)). The corresponding structures of O−NH2−A, 2OH−1−NH3, and 2OH−1−NH2 are shown in Figures 2b and 4c, and d, respectively. The Fermi level is set to 0.

The reactions of NH3 with the hydroxyl and epoxy groups are generally predicted to be exothermic, and the predicted barriers show remarkable dependence on the oxidation species and atomic arrangement of these groups. The presence of a neighboring OH group at the opposite side may activate the oxygen groups to facilitate the reaction of NH3 with GO. The de-epoxidation reaction is mainly controlled by two steps, including the ring opening of the epoxy group and the hydrogenation of the newly formed OH, and the consecutive hydrogen abstractions from NH3 are involved in the surface reaction. The adsorption and dissociation of NH3 on GOs may be responsible for removal of the oxygen groups and modify the structural and electronic properties of GOs. The calculated DOS and PDOS also suggest the remarkable charge transfers from NH3 to GO due to the formation of surface hydrogen bonds. Furthermore, the electronic properties of charge transfer from the adsorbed NH3 to the graphene can be well tuned by the applied external electric field, and thus, the graphene oxides, including the active defect sites, provided by the oxygen functional groups have potential applications for chemical functionalization of graphene-based nanomaterials and design of novel chemical sensor devices.

From the TDOS and PDOS of free NH3 (Figure 11a(3)), the electrons of the HOMO are mostly contributed by the N atom. When NH3 is adsorbed on C32−2OH through the hydrogen bond OH···N (2OH−1−NH3), the HOMO almost becomes an empty state because of the hybridization of this orbital with conduction band of GO (see the PDOS in Figure 11c(1)), giving rise to a 0.11e charge transfer from the N atom of the molecule to GO. The adsorption has less influence on the electronic states located at a deep energy level, which have the comparable contribution by N and H atoms of NH3. Such a charge transfer mechanism has also been reported in our recent studies.41

4. CONCLUSIONS The first-principles calculations have been used to investigate the interaction of NH3 with the graphene oxides containing the hydroxyl and epoxy functional groups. Our calculations reveal that the presence of diverse active defect sites on GOs can strengthen the adsorption of NH3 on the surface and enhance charge transfers from NH3 to GO. The oxygen groups of GOs can induce dissociation of the adsorbed NH3 into the chemisorbed NH2 or NH species by the H atom abstractions, leading to the removal of surface oxygen species through the hydroxyl group hydrogenation and the ring opening of epoxy group. 8789



■

Article

(21) Wehling, T. O.; Novoselov, K. S.; Morozov, S. V.; Vdovin, E. E.; Katsnelson, M. I.; Geim, A. K.; Lichtenstein, A. I. Nano Lett. 2008, 8, 173. (22) Tang, S.; Cao, Z. J. Chem. Phys. 2009, 131, 114706. (23) Andzelm, J.; Govind, N.; Maiti, A. Chem. Phys. Lett. 2006, 421, 58. (24) Robinson, J. A.; Snow, E. S.; Bǎdescu, S. C.; Reinecke, T. L.; Perkins, F. K. Nano Lett. 2006, 6, 1747. (25) Watcharotone, S.; Dikin, D. A.; Stankovich, S.; Piner, R.; Jung, I.; Dommett, G. H. B.; Evmenenko, G.; Wu, S.-E.; Chen, S.-F.; Liu, C.P.; Nguyen, S. T.; Ruoff, R. S. Nano Lett. 2007, 7, 1888. (26) Eda, G.; Fanchini, G.; Chhowalla, M. Nat. Nanotechnol. 2008, 3, 270. (27) Eda, G.; Lin, Y.-Y.; Miller, S.; Chen, C.-W.; Su, W.-F.; Chhowalla, M. Appl. Phys. Lett. 2008, 92, 233305. (28) Becerril, H. A.; Mao, J. L., Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. ACS Nano 2008, 2, 463. (29) Wu, J.; Becerril, H. A.; Bao, Z.; Liu, Z.; Chen, Y.; Peumans, P. Appl. Phys. Lett. 2008, 92, 263302. (30) Wang, X.; Zhi, L.; Mullen, K. Nano Lett. 2008, 8, 323. (31) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Nano Lett 2007, 7, 3499. (32) Chen, C.; Yang, Q.-H.; Yang, Y.; Lv, W.; Wen, Y.; Hou, P.-X.; Wang, M.; Cheng, H.-M. Adv. Mater. 2009, 21, 1. (33) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282. (34) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Nature 2007, 448, 457. (35) Gao, X.; Jang, J.; Nagase, S. J. Phys. Chem. C 2010, 114, 832. (36) Kim, M. C.; Hwang, G. S.; Ruoff, R. S. J. Chem. Phys. 2009, 131, 064704. (37) Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B. Nano Lett. 2007, 7, 3394. (38) Fowler, J. D.; Allen, M. J.; Tung, V. C.; Yang, Y.; Kaner, R. B.; Weiller, B. H. ACS Nano 2009, 3, 301. (39) Dan, Y.; Lu, Y.; Kybert, N. J.; Luo, Z.; Johnson, A. T. C. Nano Lett. 2009, 9, 1472. (40) Robinson, J. T.; Perkins, F. K.; Snow, E. S.; Wei, Z.; Sheehan, P. E. Nano Lett. 2008, 8, 3137. (41) Tang, S.; Cao, Z. J. Chem. Phys. 2011, 134, 044710. (42) Wang, X. R.; Li, X. L.; Zhang, L.; Yoon, Y. K.; Weber, P. K.; Wang, H. L.; Guo, J.; Dai, H. J. Science 2009, 324, 768. (43) Lin, Y.-C.; Lin, C.-Y.; Chiu, P.-W. Appl. Phys. Lett. 2010, 96, 133110. (44) Denis, P. A.; Iribarne, F. J. Phys. Chem. C 2011, 115, 195. (45) Kan, E.-j.; Li, Z.; Yang, J.; Hou, J. G. J. Am. Chem. Soc. 2008, 130, 4224. (46) Seredych, M.; Bandosz, T. J. J. Phys. Chem. C 2007, 111, 15596. (47) Li, X.; Wang, H.; Robinson, J. T.; Sanchez, H.; Diankov, G.; Dai, H. J. Am. Chem. Soc. 2009, 131, 15939. (48) Delley, B. J. Chem. Phys. 1990, 92, 508. (49) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244. (50) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. (51) Govind, N.; Petersen, M.; Fitzgerald, G.; King-Smith, D.; Andzelm. J. Comput. Mater. Sci. 2003, 28, 250. (52) Cai, W.; Piner, R. D.; Stadermann, F. J.; Park, S.; Shaibat, M. A.; Ishii, Y.; Yang, D.; Velamakanni, A.; An, S. J.; Stoller, M.; An, J.; Chen, D.; Ruoff, R. S. Science 2008, 321, 1815. (53) Bagri, A.; Grantab, R.; Medhekar, N. V.; Shenoy, V. B. J. Phys. Chem. C 2010, 114, 12053. (54) Bagri, A.; Mattevi, C.; Acik, M.; Chabal, Y. J.; Chhowalla, M.; Shenoy, V. B. Nat. Chem. 2010, 2, 581. (55) Yan, J.-A.; Xian, L.; Chou, M. Y. Phys. Rev. Lett. 2009, 103, 086802. (56) Wang, L.; Lee, K.; Sun, Y.-Y.; Lucking, M.; Chen, Z.; Zhao, J. J.; Zhang, S. B. ACS Nano 2009, 3, 2995.

ASSOCIATED CONTENT

S Supporting Information *

Optimized structures and relative energy profiles for the adsorption and dissociation of NH3 on GO with larger graphene supercells. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: +86-592-2183047. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21103026 and 21133007) and the Ministry of Science and Technology (2011CB808504 and 2012CB214900).

■

REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. Science 2004, 306, 666. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197. (3) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451. (4) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (5) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. Rev. Mod. Phys. 2009, 81, 109. (6) Zhang, Y.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201. (7) Elias, D. C.; Nair, R. R.; Mohiuddin, T. M. G.; Morozov, S. V.; Blake, P.; Halsall, M. P.; Ferrari, A. C.; Boukhvalov, D. W.; Katsnelson, M. I.; Geim, A. K.; Novoselov, K. S. Science 2009, 323, 610. (8) Sofo, J. O.; Chaudhari, A. S.; Barber, G. D. Phys. Rev. B 2007, 75, 153401. (9) Lu, N.; Li, Z.; Yang, J. J. Phys. Chem. C 2009, 113, 16741. (10) Tang, S.; Zhang, S. J. Phys. Chem. C 2011, 115, 16644. (11) Nair, R. R.; Ren, W.; Jalil, R.; Riaz, I.; Kravets, V. G.; Britnell, L.; Blake, P.; Schedin, F.; Mayorov, A. S.; Yuan, S.; Katsnelson, M. I.; Cheng, H.-M.; Strupinski, W.; Bulusheva, L. G.; Okotrub, A. V.; Grigorieva, I. V.; Grigorenko, A. N.; Novoselov, K. S.; Geim, A. K. Small 2010, 6, 2877. (12) Jeon, K.-J.; Lee, Z.; Pollak, E.; Moreschini, L.; Bostwick, A.; Park, C.-M.; Mendelsberg, R.; Radmilovic, V.; Kostecki, R.; Richardson, T. J.; Rotenberg, E. ACS Nano 2011, 5, 1042. (13) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (14) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nature 2007, 6, 652. (15) Ellison, M. D.; Crotty, M. J.; Koh, D.; Spray, R. L.; Tate, K. E. J. Phys. Chem. B 2004, 108, 7938. (16) Qi, P.; Vermesh, O.; Grecu, M.; Javey, A.; Wang, Q.; Dai, H.; Peng, S.; Cho, K. J. Nano Lett. 2003, 3, 347. (17) Valentini, L.; Armentano, I.; Kenny, J. M.; Cantalini, C.; Lozzi, L.; Santucci, S. Appl. Phys. Lett. 2003, 82, 961. (18) Peng, S.; Cho, K.; Qi, P.; Dai, H. Chem. Phys. Lett. 2004, 387, 271. (19) Chang, H.; Lee, J. D.; Lee, S. M.; Lee, Y. H. Appl. Phys. Lett. 2001, 79, 3863. (20) Leenaerts, O.; Partoens, B.; Peeters, F. M. Phys. Rev. B 2008, 77, 125416. 8790



Article

(57) Zhang, W.; Carravetta, V.; Li, Z.; Luo, Y.; Yang, J. J. Chem. Phys. 2009, 131, 244505. (58) Li, Z.; Zhang, W.; Luo, Y.; Yang, J.; Hou, J. G. J. Am. Chem. Soc. 2009, 131, 6320. (59) Lerf, A.; He, H.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998, 102, 4477. (60) Li, X.; Zhang, G.; Bai, X.; Sun, X.; Wang, X.; Wang, E.; Dai, H. Nat. Nanotechnol. 2008, 3, 538. (61) Gao, W.; Alemany, L. B.; Ci, L.; Ajayan, P. M. Nat. Chem. 2009, 1, 403. (62) Yan, J.-A.; Chou, M. Y. Phys. Rev. B 2010, 82, 125403. (63) Jeong, H.-K.; Lee, Y. P.; Lahaye, R. J. W. E.; Park, M.-H.; An, K. H.; Kim, I. J.; Yang, C.-W.; Park, C. Y.; Ruoff, R. S.; Lee, Y. H. J. Am. Chem. Soc. 2008, 130, 1362. (64) Larciprete, R.; Fabris, S.; Sun, T.; Lacovig, P.; Baraldi, A.; Lizzit, S. J. Am. Chem. Soc. 2011, 133, 17315. (65) Lahaye, R. J. W. E.; Jeong, H. K.; Park, C. Y.; Lee, Y. H. Phys. Rev. B 2009, 79, 125435. (66) Dua, V.; Surwade, S. P.; Ammu, S.; Agnihotra, S. R.; Jain, S.; Roberts, K. E.; Park, S.; Ruoff, R. S.; Manohar, S. K. Angew. Chem., Int. Ed. 2010, 122, 2200. (67) Gao, X.; Zhao, Y.; Liu, B.; Xiang, H.; Zhang, S. B. Nanoscale 2012, 4, 1171. (68) Gao, X.; Wang, Y.; Liu, X.; Chan, T.-L.; Irle, S.; Zhaoa, Y.; Zhang, S. B. Phys. Chem. Chem. Phys. 2011, 13, 19449.

8791


Adsorption and Dissociation of Ammonia on Graphene Oxides: A First

Recommend Documents