ARTICLE pubs.acs.org/JPCC
Density Functional Theory Study of O2 and NO Adsorption on Heteroatom-Doped Graphenes Including the van der Waals Interaction Anup Pramanik† and Hong Seok Kang*,‡ † ‡
Institute of Engineering Research, Jeonju University, Hyoja-dong, Wansan-ku, Chonju, Chonbuk 560-759, Republic of Korea Department of Nano and Advanced Materials, College of Engineering, Jeonju University, Hyoja-dong, Wansan-ku, Chonju, Chonbuk 560-759, Republic of Korea ABSTRACT: On the basis of the PBE-D2 calculation that empirically includes van der Waals interaction to the standard GGA approximation of Perdew, Berke, and Ernzerhof, we have investigated the adsorption of paramagnetic O2 and NO on pristine, N-doped, and P-doped graphene. We found that the van der Waals interaction makes an important contribution to the physisorption energy and to the adsorption geometry of these gases in pristine and N-doped graphene. A detailed band-structure calculation shows that the electrostatic interaction due to charge transfer is also important, causing their adsorption on 2N-doped graphene to be appreciably stronger than that on pristine graphene or 1N-doped graphene. In the case of the adsorption of two molecules on 2N-doped graphene, spins of two adsorbed molecules couple differently depending upon the kind of gas molecules. Meanwhile, chemisorption of two O2 atoms leaves the 2P-doped graphene a nonmagnetic semiconductor, while adsorption of two NO molecules turns the system into a magnetic semiconductor.
1. INTRODUCTION Graphene is a novel two-dimensional material with applications in nanoelectronics due to its extremely high electron mobility, which is a consequence of the linear dispersion relation in the band structure of the π and π* states around the Dirac point.1 It exhibits ballistic transport on a submicrometer scale and can be heavily doped without significant loss of mobility.1,2 In addition, graphene is also potentially useful in spintronics, such as the realization of spin qubits, because of its long spin-coherence time and long spin relaxation time.3 Therefore, graphene is a promising candidate for future electronics, considering that silicon-based electronics is expected to encounter a fundamental limit at the spatial scale below 10 nm. However, the controllable generation of the semiconducting gap is still under discussion, because it is a zero-gap semiconductor. As an example, a quantum confinement of 1 nm induces a band gap of approximately 1 eV in graphene nanoribbons (GNRs).4 The desirable sensor properties of carbon nanotubes (CNTs) are already known. For example, the electrical resistance of a semiconducting single-walled CNT changes dramatically upon exposure to gaseous molecules such as NO2 and NH3.5 Graphene is also useful as a gas sensor for NO2, NH3, H2O, and CO.6 Hall measurements have revealed n- or p-type effects depending upon the type of gas molecules. These gases have been detected at remarkably low concentrations, and the environmentally hazardous NO2 gas was detected at the single-molecule level. r 2011 American Chemical Society
The extreme sensitivity of graphene can be attributed to the facts that (1) graphene is a strictly two-dimensional material with few crystal defects, whose whole volume can be exposed to the surface adsorbate, and (2) graphene is highly conductive, even in the limit of no charge carriers,7 where a few extra electrons can cause notable changes in the carrier concentration. Using calculations based on density-functional theory (DFT), the adsorption of NH3 molecules was found to significantly influence the electronic and transport properties of GNRs.8 Wehling et al. found that open-shell NO2 molecule is a strong acceptor from graphene, whereas the closed-shell dimer N2O4 causes only weak doping.9 In the work, this effect was shown to be attributed to graphene’s unique density of states (DOS). Another hazardous gas belonging to the same NOx series is NO. Recently, Leenaerts et al. theoretically studied the adsorption of the gas molecule on pristine graphene using the generalized gradient approximation (GGA) of Perdew, Burke, and Enzerhof (PBE),10 showing that the gas molecule can be weakly physisorbed on pristine graphene.11 However, their calculation requires a careful reconsideration, because the GGA cannot describe the van der Waals interaction, which may be crucial in understanding the physisorption. For example, the GGA calculation suggests that two graphene layers do not interact. In Received: January 25, 2011 Revised: March 16, 2011 Published: May 16, 2011 10971
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Table 1. Magnetic Moment (μ), Binding Energy (Eb), and Geometrical Parameters of Graphene with Adsorbed O2 or NO Moleculesa Ebb (eV) adsorbent
adsorbate (AB)
μ (μB)
PBE
PBE-D2
lABc (Å)
lCAd (Å)
lXBe (Å)
GrS
O2
1.90
0.04
0.11
1.24 (1.24)
3.09 (3.39)
3.09 (3.39)
N-doped GrS
O2
1.79
0.15
0.24
1.26 (1.26)
3.01 (3.15)
2.93 (3.22)
2N-doped GrS P-doped GrS
2O2 O2
0.00 0.00
0.20 1.14
0.30 1.26
1.26 (1.26) 1.56 (1.56)
2.91 (3.08)
2.94 (3.17) 1.62,f 1.73g (1.62, 1.73)
2P-doped GrS
2O2
0.00
0.97
1.12
1.57 (1.57)
GrS
NO
1.00
0.03
0.12
1.18 (1.17)
2.91 (3.17)
3.19 (3.49)
N-doped GrS
NO
1.10
0.10
0.21
1.19 (1.19)
2.79 (3.01)
2.96 (3.26)
2N-doped GrS
2NO
2.39
0.15
0.27
1.19 (1.19)
2.64 (2.92)
P-doped GrS
NO
2.00
0.50
0.61
1.25 (1.25)
1.79h (1.79)
2P-doped GrS
2NO
2.00
0.21
0.33
1.26, 1.24I (1.25, 1.21)
1.79,h 1.86j (1.80, 2.08)
1.61,f 1.72g (1.61, 1.72)
2.91 (3.15)
a
The numbers without parentheses denote data from the PBE-D2 calculation, while those in parentheses denote corresponding values from the PBE calculation. b Binding energy of the gas (AB) per molecule to the graphene surface. c Interatomic AB distance of the adsorbed gas molecule (AB). d Interatomic distance between Cgraphene and atom A of adsorbed molecule. A represents an O or N atom when the adsorbed molecule is O2 or NO, respectively. e Interatomic distance between the doped atom (X = C, N, or P) and atom B of the adsorbed molecule. B represents an O atom when the adsorbed molecule is O2 or NO. f Equatorial PO distance. g Axial PO distance. h PN distance. I NO distance of the second adsorbate. j PN distance of the second adsorbate.
addition, the chemical doping of graphene with N and P atoms may affect the binding of the gas as well as the sensing properties of the graphene toward the gas. We recall that nitrogen doping in graphene was recently achieved by NH3 annealing after Nþ-ion irradiation and chemical-vapor deposition.12 Phosphorus doping was achieved in singled-walled carbon nanotubes, leading us to expect that a similar doping in graphene can be realized in the near future.13 As in the case of CNTs, the doping can open a practical method to tune the electronic structure of graphene. In particular, the breaking of the symmetry can even introduce a band gap, which can affect the gas-adsorption abilities of the graphene. In this respect, Zhang et al. recently investigated the adsorption of CO, NO, NO2, and NH3 on graphene using a calculation based on the local-density approximation (LDA).14 However, it has been known that the LDA substantially overestimates the binding energy and underestimates the bonding distance. In this work, we will theoretically investigate the adsorption of NO and O2 gas on pristine, N-doped, and P-doped graphene using the PBE-D2, which implements Grimme’s approach to empirically include the van der Waals interaction to the PBE.15 A detailed band-structure calculation will be performed to understand the adsorption. In this respect, we recall that adsorption of O2 on pristine graphene was recently investigated using a calculation based on the LDA.16 The adsorption of O2 doped with various types of heteroatoms was also investigated using a calculation based on the PBE.17 Clearly, a detailed understanding of physics and chemistry behind O2 physisorption or chemisorption on doped graphene, including the role of the van der Waals interaction and the magnetic coupling between adsorbed paramagnetic molecules, is crucial for its application in electronics.
2. THEORETICAL METHODS Geometry optimizations were carried out using the Vienna ab initio simulation package (VASP).18 Electronion interactions were described by the projector-augmented wave (PAW) method,
which is basically a frozen-core all-electron calculation.19 To elucidate the role of the van der Waals interaction in the adsorption, we performed parallel calculations using the PBE-D2 and the PBE. For structure optimization, atoms were relaxed to the direction of the HellmannFeynman force using the conjugate gradient method until a stringent convergence criterion (=0.03 eV/Å) was satisfied. To simulate a two-dimensional graphene sheet (GrS), we used a 4 4 supercell model that consists of 32 atoms in the XY plane, where the optimized lattice constant along the two directions was 9.864 Å. k-point sampling was done using Γ-centered 6 6 1 k-points.
3. RESULTS AND DISCUSSION First, we focused on the binding of triplet molecular oxygen (3O2) on graphene. To this end, we investigated the binding of an oxygen molecule on the GrS. We considered two different configurations of O2. In the parallel configuration (P), the two oxygen atoms of the molecule are parallel to the graphene plane on top of a pair of CdC atoms, while they are located on the graphene plane in the vertical configuration (V) on top of a carbon atom. Our calculation indicates that the former configuration in GrS is more stable than the latter by 0.03 eV. Therefore, our description will concentrate on the P configuration. In the configuration, the oxygen molecule is physisorbed on graphene, as indicated by the small binding energies (Eb = 0.11 eV from the PBE-D2 calculation) shown in Table 1. Note that our PBE-D2 data are in excellent agreement with the experimental binding energy of 0.1 eV,20 while the PBE binding is substantially weaker. This observation also indicates that the van der Waals interaction plays an important role in the physisorption. Our separate analysis of the band structure of the GrSO2 shows that the binding involves a small charge transfer from the valence band of the graphene to the π* (O2) states, as indicated by the shift of the Fermi level by 0.04 eV. In fact, the spin-down π* (O2) states act as acceptor levels just above the Fermi level. Besides this small charge transfer, little change is introduced to the band structure upon O2 adsorption. In short, the oxygen 10972
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Table 2. Binding Energy (Eb) of O2 to the N-Doped GrS in Various Configurationsa
Table 3. First (Ed1) and Second (Ed2) Doping Energies for 2N- and 2P-Doped GrS
configuration
position of O2
Eb (eV)
dopant atom
Ed1 (eV)
Ed2 (eV)
P1
NC5
0.24
N
0.83
0.90
P2 P3
C1C2 C2C3
0.22 0.22
P
2.70
1.79
V1
N
0.20
V2
C1
0.19
V3
C2
0.17
V4
C3
0.17
a
P and V denote parallel and vertical orientations, respectively. The atom numbering is defined in Figure 1. For P configurations, two oxygen atoms of the oxygen molecule are located on top of the two atoms of the graphene specified in the second column. For V configurations, one of the two oxygen atoms is located on top of the atom of the graphene specified in the second column.
Figure 1. Optimized geometry of N-doped GrSO2.
binding introduces a weak p-type effect to the graphene. Because the charge transfer is small, the GrSO2 remains quasi-semimetallic. In fact, the OO distance (=1.24 Å) increases by 0.01 Å upon adsorption. Meanwhile, the measured CgrapheneO distance (=3.09 Å) does not indicate the formation of a chemical bond between the graphene and the oxygen molecule. [Unless specified, all geometrical parameters will refer to the PBE-D2optimized structure.] Therefore, the O2 adsorption can be ascribed to the van der Waals interaction supplemented by an electrostatic interaction resulting from the charge transfer. [In fact, Table 1 shows that the oxygen molecule is separated from the graphene plane by 3.39 Å in the PBE-optimized structure, which is appreciably longer than that (=3.09 Å) in the PBE-D2-optimized structure.] As a result, the GrSO2 becomes a magnetic system with a magnetic moment (=1.90 μB) close to that (=2.0 μB) of an isolated oxygen molecule, where the spin density is almost entirely concentrated on the oxygen molecule. The situation is somewhat different in N- or P-doped graphene, in which one of the carbon atoms is replaced by N or P. Table 2 compares the Eb values of oxygen adsorption on N-doped GrS for various configurations. Figure 1 indicates the labeling of the different carbon atoms. As in the case of O2 binding on the pristine GrS, the parallel configurations are more stable than the vertical configurations. Among the three parallel configurations considered, that (P1) with two oxygen atoms on top of the NdC bond is the most stable. Table 1 shows that the
oxygen binding in P1 is stronger than that on the pristine graphene, which is also found when excluding the van der Waals interaction. In fact, the OO distance (=1.26 Å) is longer than that in the pristine GrSO2 system, indicating a larger charge transfer to the O2 molecule from an electron-rich nitrogen atom. In addition, the oxygen molecule lies closer to the graphene plane than in the pristine GrSO2, as indicated by the NgrapheneO distance (=2.93 Å) being shorter than the CgrapheneO distance in the pristine GrSO2. We also note that the distance is appreciably shorter than that (=3.22 Å) in the PBE-optimized structure, also indicating the important role of the van der Waals interaction in the adsorption. The spin polarization is also concentrated on the O2 molecule, and the magnetic moment (=1.79 μB) of the system is similar to that of an isolated O2 molecule. We have also considered the adsorption of two oxygen molecules on 2N-doped GrS, that is, the graphene supercell in which two nitrogen atoms are doped far from each other. To estimate the relative ease of the second N-doping in 2N-doped GrS with respect to the first N-doping, we have calculated the energy of the first (Ed1) and the second (Ed2) N-doping from the processes: 4 4 graphene þ (1/2) N2 f N-doped 4 4 graphene þ C and N-doped 4 4 graphene þ (1/2) N2 f 2N-doped 4 4 graphene þ C, where the chemical potential of the C was taken from that of a carbon atom in the graphene. Table 3 shows that the second N-doping can be realized once the first doping is done, although it is slightly more difficult to achieve. In 2N-doped GrS2O2, we have assumed that both of the two oxygen molecules adopt P1 configuration centered at each nitrogen site. However, the two oxygen molecules can still adopt syn and anti configurations with respect to each other, in which they are located on the same side and opposite sides of the graphene plane, respectively. Our calculation indicates that the anti configuration is more stable than the syn configuration by 0.03 eV, which can be understood if we note that the electrostatic repulsion between the negatively charged oxygen molecules is minimized in the anti configuration. The binding energy per oxygen molecule (0.20 and 0.30 eV without and with the van der Waals interaction, respectively) shown in Table 1 is appreciably larger than that for 1N-doped GrS, indicating that oxygen molecules are adsorbed more strongly on N-doped GrS with a higher N content. In accordance with this observation, Table 1 shows that the CgrapheneO distance (=2.91 Å) is shorter than that (=3.01 Å) in N-doped GrSO2, which is also shorter than that (=3.08 Å) in the PBE-optimized structure. In the adsorption, the van der Waals interaction contributes to roughly 1/3 of the total binding energy. Figure 2ad gives a comparison of the band structures of pristine GrS, N-doped GrS, and N-doped GrSO2 systems. In N-doped GrS, the N-doping opens a band gap of 0.20 eV at the Dirac point, that is, point K due to symmetry breaking. The Fermi level is shifted upward by 0.81 eV, which is reminiscent of an n-type effect caused by N doping. The band structure of the N-doped GrSO2 system is quite similar to that of N-doped GrS 10973
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Figure 2. Band structures of pristine GrS (a), N-doped GrS (b), and N-doped GrSO2 for spin-up (c) and spin-down (d) states. The direct and reciprocal lattice vectors and the high-symmetry points are described in Figure 2e. Fermi level is set to energy zero.
except that two spin-polarized π*(O2) states are introduced. Therefore, the magnetic moment (=1.79 μB) of the system is close to that of an isolated O2 molecule. The system also exhibits an overall n-type effect when compared to the case of pristine GrS. As compared to the case of N-doped GrS, the Fermi level is shifted upward by 0.11 eV due to a weak p-type effect introduced by the adsorbed O2 molecule. The fact that the shift is larger than that (=0.04 eV) for the pristine GrSO2 system clearly indicates a larger charge transfer in N-doped GrSO2, which is consistent with a larger binding energy of O2 on the N-doped GrS. 2Ndoped GrS2O2, that is, 2N-doped GrS with two oxygen molecules, exhibits an antiferromagnetic (AFM) coupling of localized spins in the two oxygen molecules, giving rise to a net magnetic moment of zero. In fact, we found that the AFM state is more stable than the ferromagnetic (FM) state by 5 meV. Although not explicitly shown here, the band structure of the AFM state exhibits two flat π*(O2) states just above the Fermi level and other two flat π*(O2) states within 1.93 eV below the Fermi level. Here, we delve into the case of P-doped graphene. Different from the case of the N atom in N-doped GrS, the P atom in P-doped GrS adopts a partial sp3 configuration and protrudes from the graphene plane by 1.32 Å along the Z axis. The partial sp3 configuration of the P atom is borne out in the bond lengths (=1.76 Å) of the three CP bonds, which are between that (=1.84 Å) in a typical CP single bond and that (=1.62 Å) in the
planar sp2 configuration. In P-doped GrSO2, O2 binds to the P atom covalently, forming two PO single bonds, that is, PO1 and PO2 bonds in Figure 3, in such a way that the P atom adopts a sp3d hybridization. As a result, the pentavalent P atom forms trigonal bipyramidal bonds. Two equivalent C atoms (=C2 and C3) of the graphene and one O atom (=O1) occupy three equatorial sites with bond lengths of 1.74 and 1.62 Å, respectively, while one C atom (=C1) of the graphene and the other O atom (=O2) occupy axial sites with longer bond lengths (=1.78 and 1.74 Å, respectively). A similar bonding geometry was also observed for O2 adsorption on M-doped graphene (M = Al, Si, P, Cr, and Mn) from a previous PBE calculation, indicating that the chemisorption geometry is correctly predicted by the PBE.17 Table 1 shows that the binding energy of O2 on the P-doped GrS, which is 1.14 and 1.26 eV from the PBE and the PBE-D2 calculations, respectively, is much larger than that on N-doped GrS. Our PBE binding energy is in a reasonable agreement with that (=1.04 eV) obtained from the previous PBE calculation on the 6 6 P-doped GrS.17 Next, we have also investigated the binding of two oxygen molecules on 2P-doped GrS, where two P atoms are located far away. First, we estimate whether 2P-doping is possible. Table 3 shows that the energy of the second P-doping, Ed2 (=1.79 eV), is appreciably smaller than that for the first P-doping, Ed1 (=2.70 eV), suggesting that the 2P-doping on the GrS can be easily 10974
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Figure 3. Optimized geometry of P-doped GrSO2 projected onto the XY (a) and XZ (b) planes.
realized after 1P-doping occurs. [Here, the energy of doping is defined similarly to the case of N-doping except that the chemical potential of P was taken from one-quarter of the energy of a P4 molecule.] In 2P-doped GrS, one of the two P atoms protrudes from the graphene plane, while the other sinks below the plane. Therefore, the graphene plane becomes highly ragged. This deformation is certainly responsible for the observed ease of the second P-doping. In short, we can expect that the P-doping should mostly occur in pairs. When two oxygen molecules are adsorbed, therefore, they also adopt an anti configuration with respect to each other. The binding geometry is similar to that in the 1P-doped GrSO2 system described above, and the binding energy per oxygen molecules (=1.12 eV) is also comparable to that of 1P-doped GrSO2. Figure 4a and b shows the band structures of P-doped GrS. We find that it is a magnetic semiconductor, which exhibits a flat spinpolarized band (n = 65) very close to the Fermi level. In fact, our separate calculation on the density of states (DOS) confirms that P-doped GrS is a magnetic semiconductor. Our separate analysis shows that the band represents a delocalized π-band with a specific symmetry, not a localized state around the P atom, indicating that the spin polarization is also delocalized over the whole graphene plane. It is very interesting to note that the system exhibits p-magnetism with a magnetic moment of 1.0 μB. It is also worth noting that a gap opening (1.19 eV), which is much larger than that in N-doped GrS, is introduced between the valence band (n = 64) and the conduction band (n = 66) of pristine GrS at the symmetry point K. A comparison of Figure 4a and b with Figure 4c shows that O2 adsorption turns the system from a magnetic semiconductor into a nonmagnetic metal. We recall that a vanishing of the magnetic moment upon O2 adsorption was also noticed by Dai and Yuan.17 This observation also shows that the O2 adsorption on the P-doped GrS will be accompanied by a drastic increase in the electrical conductivity. Unlike the case of P-doped GrS, the band structure of 2Pdoped GrS shown in Figure 4d indicates that it is a nonmagnetic semiconductor. Two new bands (n = 65 and 66) are introduced around the Fermi level, and one of them becomes completely filled. Therefore, they form the valence band and the conduction band of the system. They correspond to bonding and antibonding combinations of two bands (n = 65 in the P-doped GrS)
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originated from the doping of two P atoms. Because the energy splitting (>0.54 eV, which can be estimated from the band gap of 2P-doped GrS2O2) due to the bonding and antibonding interactions of the two bands is larger than the small exchange splitting (=0.25 eV) shown in Figure 4a and b, the system becomes nonmagnetic. Therefore, P-doped GrS is either magnetic or nonmagnetic depending upon the number of doped atoms. As Figure 4e shows, the adsorption of two O2 molecules does not change the semiconducting character of the system. Our next goal was to investigate the possibility of binding NO gas on graphene. The Eb data in Table 1 show that it can be physisorbed on the pristine GrS and N-doped GrS with binding energies (=0.12 and 0.21 eV) similar to that of the O2 molecule and that the van der Waals interaction also plays an important role. We recall that the binding energy of the NO to the pristine GrS obtained from our calculation is much larger than that (e0.03 eV) obtained from Leenaerts et al.’s PBE calculation11 but significantly smaller than that (=0.40 eV) from Zhang et al.’s LDA calculation.14 In fact, the CgrapheneNNO (=2.91 Å) and CgrapheneONO (=3.19 Å) distances in our calculation are much shorter than those (g3.76 Å) from the PBE calculation. Table 1 shows that those distances are also shorter than those (=3.17 and 3.49 Å) in our PBE-optimized structures, also indicating the importance of the van der Waals interaction in the adsorption. In the optimized geometry of the pristine GrSNO, the NO bond is almost parallel to the graphene plane. The two atoms of the NO molecule are located above a CdC bond of the pristine graphene. Table 4 compares the binding energy among the various possible configurations of NO adsorption to the N-doped GrS, showing that P1 is the most stable. In the optimized geometry of P1, the NO molecule lies above the NdC bond of the graphene in such a way that the O and N atoms of the NO molecule are located above the N and C atoms of the N-doped GrS. The NO distance (=1.19 Å) of the NO molecule is slightly longer than that (=1.18 Å) of an isolated NO molecule, also indicating a small charge transfer from the GrS to the molecule. In addition, the CgrapheneNNO (=2.79 Å) and NgrapheneONO (=2.96 Å) distances indicate that the NO molecule in the N-doped GrSNO lies closer to the graphene plane than does the NO molecule in the pristine GrSNO. [Table 1 also shows that the NO molecule is more distant from the graphene plane in the PBE-optimized structure. In fact, the van der Waals interaction contributes almost 50% to the total adsorption energy.] However, the separation is still too large to accommodate any covalent interaction between the GrS and the NO molecule. It is worth mentioning that the binding energy (=0.02 eV) of the configuration in which the N and O atoms of the NO molecule are located above the N and C atoms of the N-doped GrS is much smaller. We have also calculated the binding energy of two NO molecules on 2N-doped GrS. When the two NO molecules are located in the anti configuration, the binding energy per NO molecule (=0.27 eV) is slightly larger than the binding energy of an NO molecule on the N-doped GrS. Figure 5 shows the band structure of N-doped GrSNO. When compared to the case of N-doped GrS shown in Figure 2b, the Fermi energy is shifted downward by 0.06 eV, clearly indicating a weak p-type effect of the NO molecule. For the spin-up states, n = 70 and 73, which correspond to the valence band and the conduction band of the pristine GrS, respectively, are interspersed by two localized π*(NO) states (n = 71 and 72) at 0.08 and 0.11 eV. The spin-down states are located at higher 10975
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Figure 4. Band structure of P-doped GrS for spin-up (a) and spin-down (b) states, P-doped GrSO2 (c), 2P-doped GrS (d), and 2P-doped GrS2O2 (e). Fermi level is set to energy zero.
Table 4. Binding Energy (Eb) of NO to the N-Doped GrS in Various Configurationsa position of NO
Eb (eV)
P1
NC5
0.21
P2
C5N
0.18
P3
C1C2
0.18
P4
C2C3
0.17
P5
C3C2
0.19
V1 (N) V2 (O)
N N
0.18 0.15
orientation
a
P and V denote parallel and vertical orientations, respectively. The atom numbering is defined in Figure 1. For P configurations, the oxygen and nitrogen atoms of the NO molecule are located sequentially on top of the two atoms of the graphene specified in the second column, respectively. For vertical configurations, the atom in the parentheses is located on top of the atom of the graphene specified in the second column.
(=0.73 and 1.78 eV) energies due to the exchange splitting. Therefore, the magnetic moment of the system is 1.10 μB, and
the spin-polarization due to the filling of a π*(NO)v state is localized on the NO molecule. For 2N-doped GrS2NO, our separate analysis shows that the Fermi level is shifted downward by 0.22 eV with respect to that of the 2N-doped GrS, clearly indicating a charge transfer to each NO molecule that is larger than that on the N-doped GrSNO. As was described in the previous paragraph, this observation is consistent with the larger binding energy of an O2 molecule on the 2N-doped GrS than on the N-doped GrS. Contrary to the case of the 2N-doped GrS2O2 system, the magnetic moment (=2.39 μB) is approximately twice of that for the N-doped GrSNO. The system exhibits a ferromagnetic coupling of the spins in two NO molecules. In fact, we find that the FM state is more stable than the AFM state by 16 meV. Here, we investigate NO adsorption on P-doped GrS. In the most stable configuration (V1), the binding energy (=0.61 eV) of the molecule is larger than that on N-doped GrS. Still, the binding is weaker than that of O2. In accordance with this result, the geometry of the complex shown in Figure 6 indicates that it can be thought of as weak chemisorption, considering that a weak PN bond (=1.79 Å) is formed between the graphene and the 10976
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Figure 5. Band structure of N-doped GrSNO for spin-up (a) and spin-down (b) states. Fermi level is set to energy zero.
Figure 6. Optimized geometry of P-doped GrSNO.
molecule. The NO bond length (=1.25 Å) is significantly elongated upon adsorption. The configuration is more stable than the configuration in which a PO bond is formed instead of a PN bond by 0.77 eV. A calculation of the adsorption of two NO molecules in anti configuration on the 2P-doped GrS indicates that the binding energy per NO molecule (=0.33 eV) is smaller than that on P-doped GrS. However, all of the local geometrical parameters are similar to those in P-doped GrS NO, indicating that it can be also considered weak chemisorption. Figure 7 shows the band structure of P-doped GrSNO. It shows that the NO adsorption leaves the magnetic semiconducting character of the P-doped GrS unchanged. This is clearly different from the case of O2 adsorption in which the magnetic moment disappears. Therefore, the P-doped GrS can be used to distinguish those two kinds of paramagnetic molecules using the change of magnetic moment. For the spin-up states, two localized π*(NO) states are introduced just below the Fermi level and are responsible for spin-polarization of the system with a magnetic moment of 2.00 μB. This observation clearly indicates the p-type effect on the graphene, that is, a transfer of one electron from the graphene to the NO molecule, because an isolated NO molecule
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Figure 7. Band structure of P-doped GrSNO for spin-up (a) and spin-down (b) states. Fermi level is set to energy zero.
has only one spin-polarized electron in one of the two π*(NO)v states. We recall that the NO bond (=1.25 Å) is enlogated to a double bond upon the adsorption mentioned above, which is due to the filling of another π*(NO)v state. A comparison of the figure with the band structure of P-doped GrS indicates that the transfer comes from the π-band, that is, n = 65v in Figure 4a, which moves upward upon NO adsoption. The weak PN bond is formed by the secondary interaction of those two π*(NO)v states with the atomic orbitals of the P atom with appropriate symmetry. Our separate calculation shows that 2P-doped GrS 2NO exhibits a magnetic moment of 2.00 μB, which is the same as that of P-doped GrSNO. Because there are two NO molecules, four localized π*(NO) states are introduced between the valence and the conduction bands of the 2P-doped graphene. The spins of the two NO molecules interact not ferromagnetically but ferrimagnetically, resulting in two spin-polarized electrons in spin-up states.
4. CONCLUSION On the basis of the PBE-D2 calculation, we have investigated O2 and NO adsorption on pristine, N-doped, and P-doped graphene. The excellent agreement of the O2 binding energy on pristine graphene with the experimental data clearly indicates the validity of our PBE-D2 calculation in describing the van der Waals interaction.21 We found that the van der Waals interaction plays an important contribution to the physisorption energy as well as to the adsorption geometry of these gases in pristine and N-doped graphene. For example, the van der Waals interaction contributes ∼50% of total binding energy of an NO molecule on N-doped graphene. In addition, the interaction forces these gases closer to the graphene plane. The electrostatic interaction due to charge transfer is also important, causing their adsorption on 10977
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The Journal of Physical Chemistry C N-doped graphene, particularly the 2N-doped graphene, to be stronger than that on pristine graphene. Therefore, the adsorption energy will depend on the doping concentration of N-doped graphene. In 2N-doped graphene with two O2 or NO molecules, the spins of two adsorbed molecules couple differently depending upon the kind of adsorbed gas molecule. When the former molecules are adsorbed, an antiferromagnetic coupling is observed, while spins couple ferromagnetically when the latter molecules are adsorbed. This observation suggests that magnetic properties of N-doped graphene might be used for quantitative sensing of NO gas. To clarify this possibility, a more extensive investigation may be necessary for the adsorption of the gas on the multiply N-doped graphene system. The adsorption of these gases on P-doped graphene is stronger than that on pristine or N-doped graphene. Clearly, O2-adsorption is a kind of normal chemisorption, while NOadsorption corresponds to weak chemisorption. We were able to determine the adsorption geometry of P-doped GrS-NO, which had not been indentified in the previous work. P-doping mostly occurs in pairs. Adsorption of two O2 molecules leaves the 2Pdoped graphene a nonmagnetic semiconductor, while adsorption of two NO atoms turns the system into a magnetic semiconductor. Therefore, multiply P-doped graphene can be used to distinguish O2 and NO gases using magnetic properties, which is certainly not possible with multiply N-doped graphene.
ARTICLE
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (Grant 2010-0007815). Some of the computations were performed using a supercomputer at the Korea Institute of Science and Technology Information under contract no. KSC2008-S03-0008. ’ REFERENCES (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (3) Tombros, N.; Jozsa, C.; Popinciuc, M.; Jonkman, H. T.; van Wees, B. Nature 2007, 448, 571. (4) (a) Han, M. Y.; Ozyilmaz, B.; Zhang, Y.; Kim, P. Phys. Rev. Lett. 2007, 98, 206805. (b) Li, X.; Wang, X.; Zhang, L.; Lee, S.; Dai, H. Science 2008, 319, 1229. (c) Taloli, S.; Umari, P.; Souza, M. M. D. Phys. Status Solidi B 2009, 246, 2572. (5) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (6) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nat. Mater. 2007, 6, 652. (7) (a) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (b) Zhang, Y.; Tan, J. W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201. (8) Huang, B.; Li, Z.; Liu, Z.; Zhou, G.; Hao, S.; Wu, J.; Gu, B.-L.; Duan, W. J. Phys. Chem. C 2008, 112, 13442. (9) Wehling, T. O.; Novoselov, K. S.; Morozov, S. V.; Vdovin, E. E.; Katnelson, M. I.; Geim, A. K.; Lichtenstein, A. I. Nano Lett. 2008, 8, 173. 10978
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