First-principles analysis of C₂H₂ molecule diffusion and its dissociation process on the ferromagnetic bcc-Fe(110) surface

Using the projector-augmented plane wave method, we study diffusion and dissociation processes of C₂H₂ molecules on the ferromagnetic bcc-Fe(110) surface and investigate the formation process of graphene created by C₂H₂ molecules. The most stable site for C₂H₂ on the Fe surface is a hollow site and its adsorption energy is -3.5 eV. In order to study the diffusion process of the C₂H₂ molecule, the barrier height energies for the C atom, C₂-dimer and CH as well as the C₂H₂ molecule are estimated using the nudged elastic band method. The barrier height energy for C₂H₂ is 0.71 eV and this indicates that the C ₂H₂ diffuses easily on this FM bcc-Fe(110) surface. We further investigate the two step dissociation process of C₂H ₂ on Fe. The first step is the dissociation of C₂H ₂ into C₂H and H, and the second step is that of C ₂H into C₂ and H. Their dissociation energies are 0.9 and 1.2 eV, respectively. These energies are relatively small compared to the dissociation energy 7.5 eV of C₂H₂ into C₂Hand Hin the vacuum. Thus, the Fe surface shows catalytic effects. We further investigate the initial formation process of graphene by increasing the coverage of C₂H₂. The formation process of the benzene molecule on the FM bcc(110) surface is also discussed. We find that there exists a critical coverage of C₂H₂ which characterizes the beginning of the formation of the graphene.