Oxygen assisted ultrafast growth of graphene

Chapter 4. New Mechanisms on Graphene Chemical Vapor Deposition Growth

4.2 Elements modulation on graphene growth

4.2.1 Oxygen assisted ultrafast growth of graphene

In the experimental results of our collaborator Xu et al., it was found that with the introduction of oxygen, the growth behavior of graphene domains could be quite different¹¹⁵. As shown in Fig. 4.1a and b, the equipment design of the experiments ensured the oxygen supply to the back side of the Cu foil while on the front side there was no oxygen. With the optical images of the graphene domains grown within a very short time period on both the back (Fig. 4.1d) and front (Fig. 4.1e) side of the Cu foil, it could be easily seen that with the presence of oxygen, the graphene domain is large and round, in a totally different growth mode from that without oxygen where the graphene domain is small and in star-like shape. In addition, oxygen was also greatly increasing the growth rate of graphene to 60 μm/s as shown in Fig. 4.1f-j.

Figure 4.1 (a) Schematic diagram of the experimental equipment for the growth of graphene on Cu foil with continuous oxygen supply. (b) Side view of the equipment in (a). (c) AES of the SiO2/Si substrate before and after annealing for the growth of graphene with oxygen supply. Optical image of graphene domains grown on the (d) back and (e) front of the Cu foils in the equipment of (a). (f-h) Optical images of the graphene domain synthesized at t = 0 s, 2 s and 5 s, respectively. (i) Domain size of graphene vs growth time. (j) Coverage of graphene vs growth time. Reproduced from Ref.¹¹⁵. Copyright@2016, Springer Nature.

According to the PFT simulations (Fig. 4.2), it was demonstrated that the growth of the large round-shape domains on the back side is because that the graphene was grown under the attachment-limited growth mode where the limited step is the attachment of the carbon sources to the growing edge of graphene, while on the front side the growth of small star-like shape domains is because of the diffusion-limited growth of graphene where the limited step is the diffusion of the carbon sources. Considering the only difference of the growth conditions between these two is the existence of oxygen on the back side of the Cu foil, it is believed that

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oxygen here greatly enhanced the carbon flux one the back which changed the graphene growth from the diffusion-limited mode to the attachment-limited mode.

Figure 4.2 (a-c) Graphene domains and (d-f) concentration maps of the carbon sources obtained by PFT simulations, with the carbon sources landing rates of 0.001 (a,d), 0.1 (b,e), and 4 (c,f), respectively.

Reproduced from Ref.¹¹⁵. Copyright@2016, Springer Nature.

But how could oxygen increase the carbon flux? It was calculated that with the existence of surface oxygen on the Cu(100) surface, the methane decomposition rate is greatly enhanced as the reaction barriers for the four steps’ dehydrogenation are all reduced. Normal CH4

decomposition on Cu(100) surface includes four steps of reactions:

𝐶𝐻₄^𝐶𝑢→ 𝐶𝐻₃+ 𝐻, (4.1) 𝐶𝐻₃^𝐶𝑢→ 𝐶𝐻₂+ 𝐻, (4.2) 𝐶𝐻₂^𝐶𝑢→ 𝐶𝐻 + 𝐻, (4.3)

𝐶𝐻^𝐶𝑢→ 𝐶 + 𝐻. (4.4) The energy barriers for each reaction are 1.57, 1.37, 0.69, and 1.47 eV, respectively, and the reaction energies are 0.84, 0.53, -0.02, and 0.70 eV, respectively (red line in Fig. 4.3 a, c, e, and g). While with the participation of surface oxygen on Cu(100), the active atom could help stabilize the leaving H atom at the transition state just, as shown in the bottom of Fig. 4.3b, d, f, and h, where in all the transition states, when the bonding between the leaving H atom and

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the CHx species is breaking, a bond between the leaving H atom and the O atom is also forming.

So the reactions are actually written as

𝐶𝐻₄+ 𝑂^𝐶𝑢→ 𝐶𝐻₃+ 𝑂𝐻, (4.5) 𝐶𝐻₃+ 𝑂^𝐶𝑢→ 𝐶𝐻₂+ 𝑂𝐻, (4.6) 𝐶𝐻₂+ 𝑂^𝐶𝑢→ 𝐶𝐻 + 𝑂𝐻, (4.7) 𝐶𝐻 + 𝑂^𝐶𝑢→ 𝐶 + 𝑂𝐻. (4.8)

Figure 4.3 Energy barriers of the (a) first, (c) second, (e) third, and (g) fourth step of the CH4

decomposition reactions on the Cu(100) surface with (black line) and without (red) oxygen. MEPs of the (b) first, (d) second, (f) third, and (h) fourth step of the CH4 decomposition reactions with (at bottom

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with black arrowheads) and without oxygen (at top with red arrowheads). White, black, orange, and red spheres represent H, C, Cu and O atoms, respectively.

Therefore, the energy barriers for the methane decomposition are reduced to 0.62, 1.29, 0.53, and 1.11 eV, respectively. Besides, because the formation of the product OH is more stable than H on the Cu(100) surface, the reaction energies of the decomposition are decreased to 0.24, 0.53, -0.43, and -0.11 eV, respectively. The reduction of the decomposition barriers could greatly increase the dissociation rate of CH4 by orders of magnitude, and therefore promote the concentration of the carbon precursors. To show a whole picture of oxygen’s effect on methane dissociation, Fig. 4.4 plots the energy profile during the full decomposition process of CH4 with and without oxygen’s assistance. It indicates that, no matter which step of the decomposition is the rate-limiting step, the methane dehydrogenation is accelerated as the entire energy like with oxygen (black line) is under that without oxygen (red line).

Figure 4.4 Energy profile for the full decomposition of CH4 with and without oxygen supply on Cu(100) surface.

So with DFT calculations, it was proved that surface oxygen on Cu foil could decrease the methane decomposition barrier and therefore increase the carbon flux, which leading to the promotion of the graphene growth rate and the change of the growth mode from diffusion limited to attachment limited.