Graphene Supported on Hematite Surfaces: A Density Functional

Article pubs.acs.org/JPCC

Graphene Supported on Hematite Surfaces: A Density Functional Study Manh-Thuong Nguyen* and Ralph Gebauer The Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, 34151 Trieste, Italy ABSTRACT: Structural, electronic, and chemical properties of graphene supported on hematite (α-Fe2O3) (0001) surfaces are investigated, employing density functional theory calculations. Apart from pristine surface−graphene systems, also the interaction with various technologically relevant adsorbates is addressed. Both ferryl-terminated (SO) and single ironterminated (SFe) surfaces of α-Fe2O3 are considered. While the graphene sheet is nearly unaltered when in contact with SFe, a sizable amount of graphene-to-surface charge transfer is found in the case of SO. This charge transfer leads to rather strong binding and is accompanied by a remarkable shift of the Fermi energy away from the Dirac point in graphene. In a second step, hydrogen atoms and hydroxyl groups are introduced as adsorbants. A strong site-selectivity behavior for the adsorption is found in SO-supported graphene (G/SO), where the presence of adsorbates can lead to the formation of covalent surface−graphene bonds. This is not the case for SFe-supported graphene (G/SFe) where no strong site selectivity is observed. The hydrogen dissociation reaction on G/SO, which is found to be exothermic, is simulated using the nudged elastic band method. The strong electric dipole forming at the G/SO interface is found to influence strongly the adsorption of water molecules on graphene. Finally, it is shown that upon adsorption of alkali metal atoms on G/SO, the Fermi level of the system can be shifted back toward the Dirac point of graphene, counteracting the charge transfer effects due to the interaction with the hematite surface.

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INTRODUCTION The physics and chemistry of monolayer graphene on metallic surfaces is a very active and fruitful research area.1 Much less is known about graphene on insulating surfaces, especially on metal oxides, because of inherent difficulties in the fabrication of such systems. However, for many prospective applications in nanoscale devices, heterostructures with nonmetallic substrates are essential. For example, graphene/metal oxide composites have recently been shown to possess a huge potential for energy applications.2 A better understanding of such hybrid systems is thus very desirable. In this paper we report on density functional theory (DFT)based calculations, which provide atomic-scale insights into the structure and energetics of graphene on a metal oxide surface. Our model system is graphene supported on α-Fe2O3(0001) surfaces. α-Fe2O3, also known as hematite, is an abundant and stable oxide of iron that has many technological applications, especially in photocatalysis.3 Its (0001) surface is one of two natural growth faces4 and well studied.5−7 Graphene/hematite complexes have numerous energy conversion and storage applications. For instance, graphene has recently been introduced to hematite photoanodes to improve photocurrent density and incident photo-to-current efficiency in water splitting.8 In addition, composites of hematite nanoparticles on graphene or graphene oxide have also been considered as advanced electrode materials for rechargeable Li-ion batteries.9,10 Because of the layered structures of hematite along the [0001] direction, different terminations are possible. We considered two stable surface terminations that have been © 2014 American Chemical Society

experimentally observed, ferryl-terminated and single ironterminated surfaces (SO and SFe).5,7 These two surface terminations can coexist at, for example, a temperature of 973 K and an oxygen pressure of 40 mbar.5 Equilibrium positions of graphene on these surfaces are first determined, and then electronic properties of graphene−surface systems are investigated. The reactivity of these systems to H atoms, OH groups, and H2O molecules are also studied. Finally, we examine the modification of graphene density of states upon adsorption of Na and Li atoms on the graphene−hematite interfaces.

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METHODS AND MODELS Spin-polarized plane-wave density functional theory calculations were carried out using the Quantum ESPRESSO package.11 We employed the PBE+U (density functional proposed by Perdew, Burke, and Ernzerhof, coupled with the U interaction in the Hubbard model, U=4.2 eV for the Fe 3d states) formalism12,13 and ultrasoft pseudopotentials.14,15 A kinetic energy cutoff of 40 Ry for the wave function and 320 Ry for the charge density was used. A force convergence threshold of 10−4 eV/Å was used for structural optimizations. The nudged elastic band (NEB) approach, 16 with a force convergence threshold of 5 × 10−2 eV/Å, was used to estimate reaction barriers. Received: February 2, 2014 Revised: March 17, 2014 Published: March 21, 2014 8455

dx.doi.org/10.1021/jp501171t | J. Phys. Chem. C 2014, 118, 8455−8461

The Journal of Physical Chemistry C

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used. A vacuum layer of more than 20 Å was added between slabs to ensure minimal interactions between them. Additionally, all atoms were allowed to relax in geometry optimizations. To sample the Brillouin zone in the self-consistent calculations we used a 6 × 6 × 1 k-point mesh for geometry optimization processes and a 12 × 12 × 1 k-point grid for the calculation of the projected density of states (PDOS). Positions of graphene on each surface were defined by considering the high symmetry sites, i.e., on-top (a C atom), bridge (the middle of a CC bond), and hollow (the center of a C hexagon) located above the topmost Fe (O) atom of SFe (SO), Figure 1. Consequently, the on-top position of graphene on SO means one of the eight graphene atoms in the unit cell is placed above the ferryl group. Calculations in this work were conducted using the same lateral unit cell size for graphene in vacuum and graphene on surfaces. Strain-induced properties of graphene are thus treated on an equal footing in all cases.

The adsorption energy of a molecule M on a surface S is defined as ΔEM − S = EM − S − EM − ES

(1)

where EX is the energy of system X in its fully relaxed geometry. The electron density difference is defined as ΔρM − S = ρM − S − ρM − ρS

(2)

where ρX is the electron density of system X in the bonded geometry. Hematite surfaces were modeled with slabs of 1 × 1 periodicity, the thickness of SFe is four oxygen−iron double layers (correspondingly, 20 atoms), and SO is generated by symmetrically adding one oxygen on top of the outermost Fe of SFe on both slab sides. Antiferromagnetic ordering in the bulk was considered to describe the spin configuration in the slabs. Graphene was modeled with 2 × 2 periodicity. Figure 1 shows such a hematite−graphene hybrid system. Experimentally, the hematite lattice constant is about 2% larger than that of the graphene. Our calculations predict that this mismatch is approximately 3%, as the lattice constant of hematite is more overestimated than that of graphene by the density functional

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GRAPHENE ON HEMATITE SURFACES Energetic and Geometric Properties. According to the calculated potential energy surfaces, shown in Figure 1 d, the equilibrium graphene−surface distance is about 3.1 Å (G/SO) and 3.5 Å (G/SFe). These values are rather large, implying no evidence of covalent chemical bonds between graphene and the surface. Note that the precise values of these distances, in particular for the case of G/SFe, are not reliable given the inability of semilocal density functionals to account for van der Waals dispersion interactions. However, as shown below, the more interesting G/SO system exhibits strong dipolar character, and the presence of adsorbates leads to covalent bonds between the surface and graphene sheet. Such interactions are well represented at this level of theory. Data shown in Table 1 reveal that the site-selectivity effect is weak in both SO and SFe. In other words, the adsorption energy Table 1. Graphene on SO and SFea SO

SFe

position

ΔE (meV)/C

h0 (Å)

position

ΔE (meV)/C

h0 (Å)

on-top bridge hollow

−26 −25 −25

3.08 3.08 3.08

on-top bridge hollow

−2 −2 −2

3.47 3.47 3.47

a Position is defined in the text, ΔE(eV)/C is the adsorption energy per C atom of graphene on the surfaces, and h0 is the equilibrium graphene−surface distance.

of graphene on these surfaces is almost the same in the on-top, bridge, and hollow adsorption positions. The adsorption of graphene on SO and SFe is −26 and −2 meV per carbon atom, respectively. Graphene−surface interactions do not lead to any significant geometry modification. In the following, for the on-top position, the C atom of graphene located right above the topmost O/Fe atom is named Cr (“C reference”). We shall only consider this adsorption configuration because, as shown below, Cr can form a strong chemical bond with O upon the adsorption of H and OH. The on-top position facilitates this bond. Electronic Properties. Modification of graphene’s electronic properties induced by an underlying solid substrate is an intriguing issue. One of the most interesting problems is the response of the graphene Dirac cone to the presence of the surface. In this subsection we shall investigate the shift of the

Figure 1. Graphene on SO: (a) Arrows indicate the surface unit cell. C, O, and Fe are in gray, red, and pink, respectively. (b) h is the graphene−surface distance. (c) Adsorption positions: the red circle implies a topmost oxygen atom of the surface. (d) Potential energy surface of graphene−surface systems against h. 8456



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graphene-covered substrate.22 Our calculations yield work functions for graphene, SFe, SO, G/SFe, and G/SO of 4.54 eV (which is in excellent agreement with the experimental value of 4.56 eV23), 4.40 eV, 7.39 eV, 4.44 eV, and 5.12 eV, respectively. The similarity in work function between SFe and graphene indicates that their Fermi energies are at the same level relative to vacuum. Additionally, the work function of G/SFe is almost the same. This implies that there is no electron transfer between graphene and SFe. The G/SO work function of 5.12 eV, compared to 4.54 eV of graphene, suggests a shift down to lower energy of the Fermi level with respect to the Dirac point, i.e., there is an amount of electrons transferred from graphene to the surface. We note that on both SO and SFe, graphene is not spin-polarized. The magnetic moment of 3.92 μB on the topmost Fe atom of SFe is not changed upon graphene adsorption while that of the topmost O atom of SO is changed from −0.18 to −0.15 μB as a result of charge redistribution.

Dirac point and charge transfer in graphene−hematite interfaces. Figure 2a shows the PDOS on graphene in the G/SO and G/ SFe complexes. While the Dirac point is almost unchanged in

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ATOMS AND MOLECULES ON HEMATITE-SUPPORTED GRAPHENE The monovalent species H and OH are products of the dissociation reaction of H2 and H2O. Their presence can be detected and identified on graphene during experimental processes.19 H and OH are important intermediates in the process of water splitting. In this section, we study their adsorption properties on G/SO and G/SFe. As addressed above, the on-top structure of graphene on the surfaces is considered as the starting point for the adsorption of X (X = H, OH). Positions just above C atoms are energetically favorable adsorption sites of monovalent X on graphene.19 Figure 3a

Figure 2. (a) Spin-up PDOS on graphene in the G/SO and G/SFe complexes (respective spin-down PDOS is almost identical). (b) Electron density difference: isosurface value is at 0.0015 au, accumulation is in orange, and depletion is in green. (c) xy-integrated electron density difference along the z direction: the topmost surface atom is set at z = 0, and arrows indicate the position of z0.

G/SFe, it is shifted by about 0.7 eV to higher energy with respect to the Fermi level in G/SO. It can thus be concluded that graphene remains almost neutral upon adsorption on SFe while it becomes positively doped on SO. This conclusion is accompanied by the electron density difference calculated using eq 2 and visualized in Figure 2b. In the G/SO system, electrons are accumulated in the space around the surface topmost atom while they are depleted in the space right below the graphene sheet. The isosurface in this case also suggests that graphene loses π electrons to the surface. The electron density difference in the case of G/SFe is negligible; accordingly, there is a zero electron transfer between graphene and SFe. Quantitatively, to estimate the amount of charge transfer, we determined Δρ graphene =

z1

Lx

∫z ∫0 ∫0 0

Ly

Δρ(x , y , z) dx dydz

Figure 3. X (X = H, OH) on hematite-supported graphene: (a) Two adsorption positions of X on graphene in the unit cell are denoted by the blue filled circles, and the red filled circles imply the topmost O (Fe) atoms of the underlying surface. (b, c) Induced charge density, Δρ = ρX−G−S − ρX − ρG − ρS, at an isosurface of 0.01 au. Accumulation charge is in orange, and depletion charge is in green for the H−G/SO and OH−G/SO structures.

(3)

where Δρ(x,y,z) is determined by eq 2, Lx and Ly are the lateral unit cell sizes, z0 is a grid point in between graphene and the surface that satisfies ∫ ∫ Δρ(x,y,z0) dxdy = 0, and z1 is the midpoint of the vacuum layer. Equation 3 leads to an amount of 0.15 and 0.0 electrons transferred from graphene to SO and SFe, respectively. This fully agrees with the Bader analysis,21 which predicts a charge transfer of 0.16 and 0.0 electrons for the two cases, respectively. The graphene−SO interaction has thus an ionic bonding character. The charge transfer can also be interpreted by considering the difference in work function between graphene and the

and Table 2 show two adsorption structures of X and properties of the most stable structure. The one with X located above Cr is not considered because it hinders Cr−O(Fe) bonds. In this section, we provide an account of how the underlying surface changes the chemical reactivity of the supported graphene sheet. H and OH. Our calculations reveal that the hematite surfaces improve the adsorption strength of H on graphene. Quantitatively, the adsorption energy of H on graphene is 8457



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Table 2. Properties of X (X = H, OH) on Graphene (G) and Surface-Supported Graphene (G/SO and G/SFe)a G/SO X

ΔEAI

ΔEAII

d0Fe−O

H OH

−2.96 −2.56

−1.14 −0.75

1.61 1.61

dFe−O

dC−O

dX−Cr

μ0O

μO

1.87 1.88

1.39 1.40

1.12 1.46

−0.15 −0.15

0.16 0.17

μFe

ΔE

dX−Cr

3.90 3.87

−0.98 −0.64

1.13 1.50

G/SFe

G

X

ΔEAI

ΔEAII

dCr−Fe

dX−Cr

μ0Fe

H OH

−1.10 −0.82

−1.08 −0.73

3.28 2.55

1.13 1.49

3.92 3.92

a Binding energy ΔE of X and the rest in eV, distance d in Å, magnetic moment μ in μB. Superscript 0 implies before X adsorption. Properties are determined in the most stable geometry.

−0.98 eV while that of H on G/SFe is −1.10 eV and, dramatically, on G/SO it is −2.96 eV. This remarkable change can be seen as a consequence of geometric constraints; an analysis of H on G/SO will shed light on this issue. In structure AI, H binds to the nearest C neighbor of Cr, leading to a strong chemical bond of Cr with the topmost O of SO, evidenced by the O−Cr distance of 3.08 Å reduced to 1.39 Å upon H adsorption (see Figure 3b, Table 1, and Table 2), implying that O is strongly shifted outward from SO. Such a vertical displacement strongly enhances the stability of the adsorbates.17,18 The strong geometric modification of the graphene sheet, Figure 1b, is associated with the formation of two chemical bonds C−H and Cr−O where the two atoms C and Cr show sp3 hybridization. The C−H bond length is also slightly reduced by SO. Structure AII with an adsorption energy of −1.14 eV is considerably less stable than AI. In this structure, the Cr−O distance of 2.96 Å implies a weak interaction between the two atoms. Consistently, the adsorption energy is slightly larger than in the case of graphene without SO. If a strong Cr−O chemical bond was formed, it would lead (together with the CH bond) to two unpaired electrons in benzol rings, reducing the stability of the system. More importantly, the relatively long distance between C−H and Cr− O bonds make the graphene lattice less deformed, impeding their strength. Similar to the coadsorption of atoms/functional groups at close ranges on both sides of free-standing graphene,19,20 H and O interact with atoms of the same C− C bond in a cooperative fashion, strongly modifying the C lattice, mutually enhancing their bonding with the graphene sheet, and thereby stabilizing the configuration. Note that if the Cr−O distance is kept fixed, the H−G/SO complex would be 1.7 eV less stable. The flexibility of graphene and the Fe−O bond of SO is thus a key factor contributing to the highly stable adsorption of H in structure AI. This strong site-selectivity for H adsorption on G/SO is not present in the G/SFe system. Here, the adsorption energies for the AI and AII configurations are almost the same. This is the consequence of graphene being almost unbound on SFe, as discussed above. Because the graphene−surface separation is large, the H adsorption is very little influenced by the presence of SFe. Only slight changes in the H adsorption energy and Cr− Fe bond length also prove that the effect of SFe on the chemical reactivity of graphene is minor. Similar to H, OH prefers direct bonding to a carbon atom of graphene. Data in Table 2 show the same binding behavior as H, namely, the adsorption energy of OH is remarkably increased, and site-selectivity is pronounced, when the substrate comes into play. Also in this case, SO appears to have a stronger influence. Consistent with the modification of the adsorption

energy in the more stable structure, AI, the C−Oa (index a indicates of adsorbate OH) bond becomes shorter. The interaction between graphene and the topmost O atom of SO is also strengthened, as indicated by the reduction of the O−Cr bond length from 3.08 Å to 1.40 Å. The Fe−O bond of SO is elongated from 1.61 to 1.88 Å. Structural flexibility is thus again a key factor. Figure 3b,c provides further insight from an electronic structure point of view on the bonding in X−G/SO complexes. The depletion of the electron density in the O−Fe bond indicates that it is weakened. In the space between O and Cr, the electron accumulation shows the covalent characteristics of the bonding. The X−G/SO bonding is thus changed from ionic to covalent character upon adsorption on the graphene sheet. H2 and H2O. H2 and H2O molecules are known to weakly bind to graphene,24,25 and their dissociation reaction on freestanding graphene is endothermic,24,26 i.e., the dissociated state is less stable than the predissociation state. It is demonstrated that applying strain on graphene can change the reaction from endothermic to exothermic.24,27 Here, we show that the underlying SO can also strongly modify the potential energy surface of hydrogen and water dissociation reactions. We examined different adsorption structures of molecular hydrogen on graphene, either parallel or perpendicular to the carbon sheet, at the on-top, bridge, and hollow sites. These structures are almost equally stable, with a molecule−graphene spacing of 3.7 Å and a tiny adsorption energy of about −0.01 eV. The dissociated state of hydrogen is inspected where two atoms are located as described in Figure 4a. In the case of graphene, the most stable one, SIII, is 0.88 eV more stable than SII, the second most stable structure. In the case of G/SO, structure SI appears to be 0.44 eV more stable than SII. The potential energy surface of the two atoms on graphene can thus be flatter in the presence of SO, similar to the case of two oxygen atoms on a Ag(111)-supported nanographene.28 In the dissociated state, the adsorption energy per H is −1.85 eV (on graphene) and −2.50 eV (on G/SO). Moreover, the calculated dissociation energy of molecular hydrogen (in vacuum) is 2.26 eV per atom, larger than the desorption energy on graphene and smaller than that on G/SO. In other words, molecular hydrogen dissociates on G/SO but does not on pristine graphene. This dramatic change can be attributed to the graphene−SO interactions through the Cr and O atoms. Figure 4b shows the dissociation pathway of molecular hydrogen on graphene and on G/SO. On graphene, in the initial state (IS) the hydrogen atom is located on a hollow site. When the reaction proceeds, two hydrogen atoms approach two opposite on-top sites of a carbon hexagon, overcoming a reaction barrier of 3.14 eV. In the G/SO case, on the other hand, in the IS a 8458



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while on G/SO, water-up is 0.07 eV more stable than waterdown. The water−G/SO dipole−dipole interaction is roughly estimated as ΔEd − d =

μH OΔμz 2

4πε0R3

(5)

(4)

where ε0 is the vacuum permittivity, and R is the distance between the two dipole moments. Δμz was assumed to be at z0 (see Figure 2) and μH2O at water’s center of mass. It appeared that ΔEd−d(water-up) − ΔEd−d(water-down) = −0.06 eV, in agreement with the DFT result. As mentioned above, a vacuum layer of 20 Å can ensure minimal interactions between slabs. To verify this point, we tested this system (H2O/G/SO) with symmetric and asymmetric slabs, and it appeared that there was no detectable difference between these cases. Regarding the dissociation state of water, the most stable pair of H and OH on G/SO is also SI. However, unlike the hydrogen case, this state of water is less stable than its predissociation state. The total adsorption energy of H and OH on G/SO is −4.67 eV while the dissociation energy of H2O → H + OH is 5.17 eV, which means that the reaction on G/SO is endothermic. However, we have previously seen that the total adsorption energy of single H and OH is −5.52 eV, which is larger than the binding energy of coadsorbed H and OH in the same unit cell (−4.67 eV). Accordingly, H2O will exothermically dissociate on G/SO at a low coverage. A likely scenario is that once the dissociation reaction takes place, one of the products diffuses away, searching for an available Cr−ferryl contact, thereby lowering the energy of the whole system. Alkali Metal Atoms. On the one hand, the Dirac point of graphene is shifted up to higher energy upon adsorption on SO. On the other hand, upon adsorption of alkali metal atoms, the Dirac point is shifted down to lower energy.29 It is thus worth to investigate the response of the Dirac point when metal atoms are placed on G/SO. Here we considered Li and Na. From our calculations, the hollow sites of graphene are energetically favorable. The adsorption energy is almost the same if Li (Na) is located on different carbon hexagons. Figure 5a shows the PDOS on graphene in different systems, Li (Na) on graphene and Li (Na) on G/SO. Consistent with a previous work,29 graphene becomes negatively doped upon adsorption of these atoms, and the Dirac point is 1.4 eV (Li adsorption) and 1.1 eV (Na adsorption), shifted down to lower energy. In the Li(Na)/Gr/SO structure, graphene is sandwiched between an electron donor and an electron acceptor. The shift magnitude is expected to be not larger than in Li(Na)/Gr and G/SO. Our calculations indeed predict the Fermi level to be 1.2 eV (0.8 eV) higher than the Dirac point in the sandwich system. If the Na coverage is reduced by half, this value amounts to 0.6 eV. Additionally, from the Bader analysis, Li (Na) loses 0.82 (0.54) electrons, and graphene gains 0.47 (0.24) electrons in this system. An illustration of the charge redistribution in this case is shown in Figure 5b. It can thus be suggested that one can recover the Dirac point at the Fermi level by adsorbing metal atoms with suitable electron affinity and/or coverage.

resulting in a value of 3.1 D. Given a relatively large dipole moment of water, 1.86 D, the dipole−dipole interaction can be a key factor. Next, we calculated the energy of water-up and water-down, placed 4 Å above graphene, G/SFe, and G/SO. On graphene and G/SFe, the energies of water-up and water-down are the same (within 0.1 and 2 meV difference, respectively)

SUMMARY We have investigated properties of hematite-supported graphene using DFT calculations. Two selected hematite (0001) surfaces, namely, single metal-terminated and ferrylterminated surfaces, influence the supported graphene sheet

Figure 4. (a) H and H (H and OH) on graphene and G/SO at different positions as denoted by the blue filled circles. The red filled circles imply the topmost O atoms of SO, and EH−H is the energy of the two H atoms in each position. (b) Dissociation pathway of H2 on graphene and G/SO.

hydrogen molecule is located above a C atom, and then two H atoms move toward two C atoms adjacent to Cr. The reaction barrier in this case is 1.62 eV. In the two-H adsorption structure SI, the second H reduces the Cr−O bond length from 1.39 to 1.35 Å, implying that it makes the bond stronger. Note, however, that the adsorption energy of a single H atom at G/SO is −2.98 eV while that per H atom in SI is −2.50 eV. Accordingly, if the hydrogen coverage is twice reduced, i.e., the ratio H:Cr−O is 1:1, upon molecular hydrogen dissociation, one H atom will diffuse away from its initial position in SI so that each H atom will be fully stabilized by one Cr−O contact, just like the single H atom case presented above. For water on graphene, we considered the up (with the two H atoms pointing away from graphene, called accordingly water-up) and down (with two H atoms pointing to graphene, water-down) geometries. In addition, the rotation of water around its C2 axis and high symmetry adsorption sites above the graphene sheet were also taken into account. The difference in energy of these structures is less than 0.01 eV, and the adsorption energy in the most favorable case, a water-down molecule at a hollow site, is just about −0.02 eV. On G/SO, this scenario is completely changed. Water-down is far less favorable than water-up. This can be interpreted by the following. As previously pointed out, the graphene-to-surface charge transfer in G/SO leads to an induced dipole moment of pointing outward from the surface, which can be evaluated by Δμz = −e

∫ ∫ ∫ zΔρ(x , y , z)dxdydz

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the CINECA Award H2OSPLITHP10B6BMAA, 2013, for the availability of high performance computing resources and support.

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Figure 5. (a) Spin-up PDOS on graphene in the Li−graphene (Li−G), Na−graphene (Na−G), G/SO, and G/SFe complexes (respective spindown PDOS is almost identical). Na−2G/SO implies a system with half of a Na atom in a unit cell. (b) Electron density difference: isosurface value is at 0.001 au. Accumulation is in orange, and depletion is in green.

differently. While the former leaves the graphene almost unchanged, the latter shifts the Dirac point to higher energy by 0.7 eV. Correspondingly, there is an almost zero electron transfer from graphene to SFe while about 0.15 electrons per unit cell are donated to SO, developing an induced dipole moment of 3.1 D at the interface. SO strongly enhances the graphene chemical reactivity, by the interaction between the ferryl group and the closest carbon atom. Particularly, the adsorption of a hydrogen atom or a hydroxyl group on G/SO is about 2 eV more stable than on free-standing graphene. As a result, the dissociation of molecular hydrogen or water on G/ SO is energetically possible. Introducing Li or Na adatoms on G/SO shifts the Dirac point to lower energy. One thus can manipulate the Dirac point around the Fermi level by using adatoms of varying electron affinity and/or at suitable coverage. We have shown that graphene on surfaces of a metal oxide semiconductor can be electronically nearly unaltered or strongly disturbed depending on details of the surface terminations. The understanding of such behavior is essential for technological applications of graphene/semiconductor hybrid systems. To conclude, graphene interacts with SO more strongly than with SFe because of a sizable amount of charge transfer to the surface which is associated with a shift of the graphene Dirac point. SO-supported graphene appears to be much more chemically reactive considering the adsorption of H, OH, H2, and H2O, which can lead to exothermic dissociation of such molecules. Charge transfer in the interface of graphene and SO results in a dipole moment that strongly effects the orientation of molecular water on G/SO. The position of the Dirac point in the case of G/SO can be tuned by altering alkali metal adatom types and/or coverage.

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