Xin
Zhang
*ab and
Shaoqing
Wang
a
aShenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 110016 Shenyang, Liaoning, China. E-mail: xzhang17b@imr.ac.cn
bSchool of Materials Science and Engineering, University of Science and Technology of China, 110016 Shenyang, Liaoning, China
First published on 14th October 2019
The binding energy, bond length, projected density of states and differential charge density of graphene–metal interfaces are investigated using a first-principles method in which a single layer graphene is adsorbed on the low-index metal surfaces such as the (111), (110) and (100) surfaces. The bond length results show the graphene sheet has a different degree of buckling after graphene is adsorbed on the (110) and (100) surfaces of metals. The projected density of states and the differential charge density results confirm the adsorption of graphene on the Ni(111), Co(111), Ni(110), Co(110) and Cu(110) surfaces is chemisorption due to the strong orbital coupling effect and the obvious charge accumulation between the carbon and metal atoms, while the adsorption of graphene on the Cu(111) surface is physical adsorption owing to the absence of the orbital coupling effect and the charge accumulation between the carbon and Cu atoms. Interestingly, the adsorption of graphene on the (100) surface of Ni, Co and Cu is all physical and chemical mixed adsorption because there are the strong orbital coupling effect and the apparent charge accumulation between the carbon and metal atoms in some parts of these surfaces while there are almost no orbital coupling effects and charge accumulation between the carbon and metal atoms in other parts.
The (111), (110) and (100) surfaces are the most basic and important low-index metal surfaces. In particular, the close-packed structure of the (111) surface of Ni, Co and Cu has been commonly used to make graphene–metal contacts owing to their structural resemblance. The difference is the adsorption of graphene on the (111) surface of Ni and Co is chemisorption while the adsorption of graphene on the Cu(111) surface is physical adsorption. However, the lack of hexagonal symmetry of (110) and (100) surfaces results in less theoretical studies on graphene–metal contacts. Therefore, in this work, in order to explore the adsorption mechanism of graphene on metal surfaces, taking Ni, Co and Cu as examples, the interfaces between graphene and low-index metal surfaces such as (111), (110) and (100) surfaces are investigated by using first-principles calculations at the level of density functional theory (DFT). The results obtained by the present study are not only expected to explain the adsorption mechanism of graphene on the metal surfaces, but also to provide help for the research of graphene-based composites and carbon nanomaterials.
Fig. 1 The top and side views of initial configurations ((a) graphene–Ni(111), (b) graphene–Ni(110), (c) graphene–Ni(100) and (d) graphene–Cu(100)). |
g–Ni(111) | g–Co(111) | g–Cu(111) | g–Ni(110) | g–Co(110) | g–Cu(110) | g–Ni(100) | g–Co(100) | g–Cu(100) | |
---|---|---|---|---|---|---|---|---|---|
a Ref. 28. b Ref. 43. | |||||||||
a g (Å) | 9.784 | 9.784 | 9.784 | 4.892 | 4.892 | 4.892 | 4.892 | 4.892 | 4.892 |
b g (Å) | 9.784 | 9.784 | 9.784 | 16.947 | 16.947 | 16.947 | 16.947 | 16.947 | 12.710 |
a m (Å) | 9.666 | 9.527 | 9.968 | 4.833 | 4.763 | 4.984 | 4.833 | 4.763 | 4.984 |
b m (Å) | 9.666 | 9.527 | 9.968 | 17.086 | 16.841 | 17.621 | 16.915 | 16.672 | 12.460 |
a% | 1.221 | 2.698 | −1.846 | 1.221 | 2.698 | −1.846 | 1.221 | 2.698 | −1.846 |
b% | 1.221 | 2.698 | −1.846 | −0.814 | 0.629 | −3.825 | 0.189 | 1.649 | 2.006 |
d eq (Å) | 2.016 | 1.976 | 3.004 | 2.004 | 2.028 | 2.257 | 2.165 | 2.209 | 2.734 |
d Refeq (Å) | 2.05a | 2.05a | 3.26a | 2.03b | — | — | 2.13b | — | — |
E b (eV per C) | 0.135 | 0.259 | 0.022 | 0.178 | 0.221 | 0.017 | 0.175 | 0.162 | 0.028 |
E Refb (eV per C) | 0.125a | 0.160a | 0.033a | 0.209b | — | — | 0.180b | — | — |
Buckling (Å) | 0.0010 | 0.0102 | 0.0006 | 0.2016 | 0.3694 | 0.1192 | 1.0178 | 1.2743 | 0.8924 |
BucklingRef (Å) | 0.00a | — | — | 0.29b | — | — | 0.69b | — | — |
Eb = (Egraphene + Emetal surface) − Egraphene–metal interface |
In our calculations, all graphene-metal interface models can be confirmed to be stable according to the definition of the binding energy. All the initial configurations are shown in Fig. 1. When graphene is adsorbed on the (111) surface of metals, the average equilibrium interfacial distance (deq) and the binding energy (Eb) show the adsorption types of graphene on the (111) surface of metals can be divided into chemisorption and physical adsorption according to the literature.27 To be more specific, when deq is less than 2.3 Å and Eb is greater than 0.1 eV per C, the adsorption of graphene on the (111) surface of metals is chemisorption, while deq is greater than 3 Å and Eb is less than 0.04 eV per C, the adsorption of graphene on the (111) surface of metals is physical adsorption. Combining the above criterion of adsorption type and our calculation results, it can be found the adsorption of graphene on the (111) surface of Ni and Co is chemisorption, while the adsorption of graphene on Cu(111) surface is physical adsorption. However, for graphene adsorbed on the (110) surface of metals, the average equilibrium interfacial distances are less than 2.3 Å, the binding energies of the graphene–Ni(110) interface and the graphene–Co(110) interface are greater than 0.1 eV per C while the binding energy of the graphene–Cu(110) interface is less than 0.04 eV per C. Unlike graphene adsorbed on the (111) surface of metals, the graphene sheet shows a certain degree of buckling. Therefore, the adsorption type of graphene on the (110) surface of metals needs to be further investigated. In terms of graphene adsorbed on the (100) surface of Ni and Co, the average equilibrium interfacial distances are less than 2.3 Å and the binding energies are 0.175 and 0.162 eV per C, respectively. Notably, when graphene is adsorbed on Cu(100) surface, the average equilibrium interfacial distance is more than 2.3 Å but less than 3 Å, and the binding energy is still less than 0.04 eV per C. However, the adsorption type of graphene on the (100) surface of metals also needs to be further investigated because the graphene sheet has a greater degree of buckling after graphene is adsorbed on the (100) surface. In conclusion, it is unreasonable to analyse the adsorption types of graphene on the (110) and (100) surfaces of metals by combining the binding energy and the average equilibrium interfacial distance due to no consideration for the buckling on the graphene sheet. In order to determine the adsorption types of graphene on different surfaces of Ni, Co and Cu more accurately, the bond lengths (r) between carbon and metal atoms at the interface can be used as a new criterion since the adsorption types of graphene on the (111) surface of Ni, Co and Cu can be determined. And the bond length (r) analysis will be discussed in the following paragraphs.
Fig. 2 The top and side views of local configurations before ((a), (b) and (c)) and after ((a′), (b′) and (c′)) the adsorption of graphene on the (111) surface of Ni, Co and Cu. |
r (Å) | C–Ni(111) | C–Co(111) | C–Cu(111) |
---|---|---|---|
r 1–1 | 2.116 | 2.068 | 3.043 |
r 2–1 | 2.149 | 2.117 | 3.152 |
r 3–2 | 2.116 | 2.068 | 3.043 |
r 4–2 | 2.149 | 2.117 | 3.152 |
r 5–3 | 2.116 | 2.068 | 3.043 |
r 6–3 | 2.149 | 2.117 | 3.152 |
r 7–4 | 2.116 | 2.068 | 3.043 |
r 8–4 | 2.149 | 2.117 | 3.152 |
Unlike graphene adsorbed on the (111) surface of Ni, Co and Cu, the graphene sheet shows a certain of buckling after graphene is adsorbed on the (110) surface of metals from the side views as shown in Fig. 3. Table 3 gives the bond lengths (r) of carbon and metal atoms induced by the adsorption of graphene on the (110) surface of metals. The maximum and minimum bond lengths (r) between carbon and Ni atoms are 2.357 Å and 2.009 Å, respectively. And the maximum and minimum bond lengths (r) between carbon and Co atoms are 2.398 Å and 2.039 Å, respectively. According to the new criterion mentioned above, it is known that the adsorption of graphene on the (110) surface of Ni and Co is chemisorption. However, for the graphene–Cu(110) interface, the minimum bond length (r) between carbon and Cu atoms is 2.218 Å, which is very close to 2.2 Å, while the maximum bond length (r) between carbon and Cu atoms is 2.770 Å, which is slightly larger than 2.2 Å, but less than 3 Å. In general, the adsorption of graphene on Cu(110) surface is also chemisorption, but the interaction between carbon and Cu atoms is slightly weaker than that between carbon and Ni (or Co) atoms.
Fig. 3 The top and side views of local configurations before ((a), (b) and (c)) and after ((a′), (b′) and (c′)) the adsorption of graphene on the (110) surface of Ni, Co and Cu. |
r (Å) | C–Ni(110) | C–Co(110) | C–Cu(110) |
---|---|---|---|
r 1–1 | 2.078 | 2.039 | 2.218 |
r 2–2 | 2.009 | 2.040 | 2.270 |
r 3–3 | 2.357 | 2.398 | 2.770 |
r 4–3 | 2.357 | 2.398 | 2.770 |
r 5–4 | 2.009 | 2.040 | 2.270 |
r 6–5 | 2.078 | 2.039 | 2.218 |
An interesting phenomenon is that the graphene sheet shows a large of buckling after graphene is adsorbed on the (100) surface of Ni, Co and Cu from the side views as shown in Fig. 4. In addition, there are obvious changes in the relative positions of carbon and Ni atoms observed from the top views as shown in Fig. 4. Table 4 shows significant changes in the bond lengths (r) of carbon and metal atoms after optimization. The bond lengths (r) of majority carbon and Ni atoms are less than 2.2 Å, and the bond lengths (r) of minority carbon and Ni atoms are larger than 3 Å. Similarly, the bond lengths (r) of majority carbon and Co atoms are about 2.2 Å, and the bond lengths (r) of minority carbon and Co atoms are larger than 3 Å. According to the new criterion mentioned above, it can be concluded that the adsorption of graphene on the (100) surface of Ni and Co is physical and chemical mixed adsorption. Unlike the graphene–Ni(100) interface and the graphene–Co(100) interface, the bond lengths (r) of minority carbon and Cu atoms are about 2.2 Å, and the bond lengths of some carbon and Cu atoms are close to 3 Å while the bond lengths (r) of other carbon and Cu atoms are larger than 3 Å, which indicates the interactions between carbon and Cu atoms are slightly weaker than those between carbon and Ni (or Co) atoms. That's to say, the adsorption of graphene on the Cu(100) surface is the coexistence of physical adsorption and weak chemisorption. In other words, the adsorption of graphene on the (100) surface of Ni, Co and Cu is all physical and chemical mixed adsorption.
Fig. 4 The top and side views of local configurations before ((a), (b) and (c)) and after ((a′), (b′) and (c′)) the adsorption of graphene on the (100) surface of Ni, Co and Cu. |
r (Å) | C–Ni(100) | C–Co(100) | C–Cu(100) |
---|---|---|---|
r 1–1 | 3.001 | 3.164 | 3.278 |
r 2–2 | 2.130 | 2.129 | 2.843 |
r 3–3 | 1.970 | 1.966 | 2.321 |
r 4–4 | 1.998 | 1.997 | 2.321 |
r 5–5 | 1.998 | 1.997 | 2.843 |
r 6–6 | 1.970 | 1.966 | 3.278 |
r 7–7 | 2.130 | 2.129 | — |
r 8–8 | 3.001 | 3.164 | — |
According to the geometric analysis above, the adsorption types of graphene on the metal surfaces are initially determined. Chemisorption process will not only have structural changes, such as the bond lengths (r) and the relative positions between the carbon and metal atoms, but also have chemical changes, such as the coupling effect between atomic orbitals and charge accumulation between atoms. Therefore, in order to further verify adsorption types of graphene on the metal surfaces, we calculated the projected density of states (PDOS) and the differential charge density in the following paragraphs, respectively.
There exists the strong orbital coupling effect between the carbon and metal atoms after graphene is adsorbed on the Ni(111), Co(111), Ni(110), Co(110) and Cu(110) surfaces, that is to say, the adsorption of graphene on these metal surfaces is chemisorption. In more detail, for the graphene–Ni(111) interface, many overlapping peaks between the C-pz orbital and the Ni-dz2 orbital are found in the energy range from −6 to −4.5 eV, −4 to −2 eV and −1 to 0 eV after graphene is adsorbed on the Ni(111) surface as shown in Fig. 5(a′), which shows the formation of covalent bonds between the C-pz orbital and the Ni-dz2 orbital. Still, the electron occupied states of the Ni-dz2 orbital increase significantly above the Fermi energy. Similarly, in the case of the graphene–Co(111) interface, the same conclusions can be drawn according to the projected density of states results in Fig. 5(b′). After graphene is adsorbed on the Ni(110) surface, for carbon and Ni atoms near the configuration edge, many overlapping peaks in the energy range from −5 to −3.5 eV, −3 to −1 eV, and −0.5 to 0 eV are found between the C-pz orbital and the Ni-dz2 orbital as shown in Fig. 5(c′), indicating the C-pz orbital and the Ni-dz2 orbital form covalent bonds. Above the Fermi energy, the electron occupied states of the Ni-dz2 orbital increase obviously. In addition, for carbon and Ni atoms near the configuration centre, a few overlapping peaks between the C-pz orbital and the Ni-dx2−y2 orbital in the energy range from −3 to 0 eV, which indicates the formation of covalent bonds between the C-pz orbital and the Ni-dx2−y2 orbital. Above the Fermi energy, the electron occupied states of the Ni-dx2−y2 orbital also increase apparently. Similar to the graphene–Ni(110) interface, the same conclusions can also be drawn for the graphene–Co(110) interface according to the projected density of states results in Fig. 5(d′). However, when it comes to the graphene–Cu(110) interface, for carbon and Cu atoms near the configuration edge, a few overlapping peaks are found between the C-pz orbital and the Cu-s orbital in the energy range from −5 to −4.5 eV, −3 to −2.5 eV, −2 to −1.5 eV and −0.5 to 0 eV as shown in Fig. 5(e′), showing the formation of covalent bonds between the C-pz orbital and the Cu-s orbital. Above the Fermi energy, the electron occupied states of the Cu-s orbital increase. Moreover, some overlapping peaks between the C-pz orbital and the Cu-dz2 orbital in the energy range from −5.5 to −3.5 eV and −3 to −1.5 eV as shown in Fig. 5(f′) indicate the C-pz orbital and the Cu-dz2 orbital form covalent bonds but the amount of overlapping peaks is less than that between the C-pz orbital and the Ni-dz2 (or Co-dz2) orbital, illustrating the interaction between the C-pz orbital and the Cu-dz2 orbital is weaker. Above the Fermi energy, the electron occupied states of the Cu-dz2 orbital also increase remarkably. For carbon and Cu atoms near the configuration centre, a few overlapping peaks are found between the C-pz orbital and the Cu-dx2−y2 orbital as shown in Fig. 5(f′) in the energy range from −4.5 to −1.5 eV, demonstrating the C-pz orbital and the Cu-dx2−y2 orbital form covalent bonds. What's more, the electron occupied states of the Cu-dx2−y2 orbital also increase obviously above the Fermi energy.
However, there is no orbital coupling effect between the carbon and Cu atoms after graphene is adsorbed on the Cu(111) surface. Thus, the adsorption of graphene on the Cu(111) surface is only physical adsorption. More specifically, there are slight changes in the PDOS of the C-pz orbital but the projected densities of states of the Cu-s and Cu-dz2 orbitals remain almost unchanged and there are no overlapping peaks found between the C-pz orbital and the Cu-s and Cu-dz2 orbitals as shown in Fig. 6(a′) and (b′), which indicates the C-pz orbital does not form covalent bonds with the Cu-s and Cu-dz2 orbitals. Still, the electron occupied states of the Cu-s and Cu-dz2 orbitals remain almost unchanged.
Interestingly, there exists the strong orbital coupling effect between the carbon and metal atoms in some parts of the model while there is no orbital coupling effect between the carbon and metal atoms in other parts of the model after graphene is adsorbed on the (100) surface of Ni, Co and Cu. Therefore, the adsorption of graphene on the (100) surface of Ni, Co and Cu is all physical and chemical mixed adsorption. When it comes to the graphene–Ni(100) interface, for carbon and Ni atoms near the configuration centre, there are lots of overlapping peaks found between the C-pz orbital and the Ni-dz2 orbital in the energy range from −5.5 to −1.5 eV, and −0.5 to 0 eV as shown in Fig. 7(a′), demonstrating the C-pz orbital and the Ni-dz2 orbital form covalent bonds. Still, the electron occupied states of the Ni-dz2 orbital above the Fermi energy increase obviously. However, for carbon and Ni atoms near the configuration edge, there are no overlapping peaks found between the C-pz orbital and the Ni-dz2 orbital below the Fermi energy as shown in Fig. 7(a′′), which shows no formation of covalent bonds between the C-pz orbital and the Ni-dz2 orbital. Besides, the electron occupied states of the Ni-dz2 orbital above the Fermi energy also remain almost unchanged. Similarly, as far as the graphene–Co(100) interface is concerned, the same conclusions can be drawn according to Fig. 7(b′) and (b′′). However, as far as the graphene–Cu(100) interface is concerned, for carbon and Cu atoms near the configuration centre, several overlapping peaks are found between the C-pz orbital and the Cu-s orbital in the energy range from −5.5 to −5 eV and −2 to −1.5 eV as shown in Fig. 7(c′), which indicates the C-pz orbital and the Cu-s orbital form covalent bonds. Above the Fermi energy, the electron occupied states of the Cu-s orbital increase. What's more, the overlapping peaks between the C-pz orbital and the Cu-dz2 orbital mainly concentrate in the energy range from −4 to −1 eV as shown in Fig. 7(d′), which also shows the formation of covalent bonds between the C-pz orbital and the Cu-dz2 orbital. Still, the electron occupied states of the Cu-dz2 orbital increase apparently above the Fermi energy. It's worth noting that the amount of overlapping peaks between the C-pz orbital and the Cu-dz2 orbital is less than that of the central carbon atoms adsorbed on the Ni(100) and Co(100) surfaces, which indicates the interaction between the C-pz orbital and the Cu-dz2 orbital is weaker. However, for carbon and Cu atoms near the configuration edge, there are no overlapping peaks found between the C-pz orbital and the Cu-s and Cu-dz2 orbitals as shown in Fig. 7(c′′) and (d′′), showing the C-pz orbital does not form covalent bonds with the Cu-s and Cu-dz2 orbitals. Besides, the electron occupied states of the Cu-s and Cu-dz2 orbitals remain almost unchanged.
Fig. 8 The differential charge density plots induced by the adsorption of graphene on (a) Ni(111), (b) Co(111), (c) Cu(111). |
Fig. 9 The differential charge density plots induced by the adsorption of graphene on (a) Ni(110), (b) Co(110), (c) Cu(110). |
Fig. 10 The differential charge density plots induced by the adsorption of graphene on (a) Ni(100), (b) Co(100), (c) Cu(100). |
Notably, there are few reports about the differential charge densities after graphene is adsorbed on the (110) and (100) surfaces of metals. For the graphene–Ni(110) interface as shown in Fig. 9(a), there exists a lot of charge accumulation between the carbon and Ni atoms at the interface, which confirms the carbon and Ni atoms form covalent bonds. That's to say, the adsorption of graphene on the Ni(110) surface is chemisorption. When it comes to the graphene–Co(110) interface as shown in Fig. 9(b), the same conclusion can be drawn that the adsorption of graphene on the Co(110) interface is also chemisorption. In terms of the graphene–Cu(110) interface as shown in Fig. 9(c), there also exists many charge accumulation between the carbon and Cu atoms at the interface. The result shows the carbon and Cu atoms also form covalent bonds, which confirms the adsorption of graphene on the Cu(110) surface is chemisorption but the interaction at the graphene–Cu(110) interface is weaker than that at the graphene–Ni(110) interface and at the graphene–Co(110) interface. Therefore, to be more accurate, the adsorption of graphene on the Cu(110) surface is weak chemisorption.
An interesting phenomenon is that the graphene sheet shows a large of buckling after graphene is adsorbed on the (100) surface of Ni, Co and Cu, which makes the interactions between the carbon and metal atoms have some differences. In terms of the graphene–Ni(100) interface as shown in Fig. 10(a), for carbon and Ni atoms near the configuration edge, there is almost no charge transfer found between them, illustrating the interactions between them are only van der Waals force. Therefore, the adsorption of the carbon atoms near the configuration edge on the Ni(100) surface is physical adsorption. However, for carbon and Ni atoms near the configuration centre, there is a lot of charge accumulation between them at the interface, which explains why the carbon and Ni atoms form covalent bonds. In other words, the adsorption of the carbon atoms near the configuration centre on the Ni(100) surface is chemisorption. Therefore, the adsorption of graphene on the Ni(100) surface is the coexistence of physical adsorption and chemisorption. Similar differential charge density result can be found for the graphene–Co(100) interface as shown in Fig. 10(b), that's to say, the adsorption of graphene on the Co(100) surface is also physical and chemical mixed adsorption. Compared with the graphene–Ni(100) interface and graphene–Co(100) interface, for the carbon and Cu atoms near the configuration edge as shown in Fig. 10(c), almost no charge accumulation is found between them at the interface, which shows the carbon and Cu atoms don't form covalent bonds, that is, the adsorption of the carbon atoms near the configuration edge on the Cu(100) surface is physical adsorption. On the contrary, for carbon and Cu atoms near the configuration centre, charge accumulation is found at the interface, illustrating that the carbon and Cu atoms form covalent bonds. In other words, the adsorption of the carbon atoms near the configuration centre on the Cu(100) surface is chemisorption. Therefore, the adsorption of graphene on the (100) surface of Ni, Co and Cu is all physical and chemical mixed adsorption.
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