Direct in situ observations of single Fe atom catalytic processes and anomalous diffusion at graphene edges

Single-atom catalysts are of great interest because of their high efficiency. In the case of chemically deposited sp² carbon, the implementation of a single transition metal atom for growth can provide crucial insight into the formation mechanisms of graphene and carbon nanotubes. This knowledge is particularly important if we are to overcome fabrication difficulties in these materials and fully take advantage of their distinct band structures and physical properties. In this work, we present atomically resolved transmission EM in situ investigations of single Fe atoms at graphene edges. Our in situ observations show individual iron atoms diffusing along an edge either removing or adding carbon atoms (viz., catalytic action). The experimental observations of the catalytic behavior of a single Fe atom are in excellent agreement with supporting theoretical studies. In addition, the kinetics of Fe atoms at graphene edges are shown to exhibit anomalous diffusion, which again, is in agreement with our theoretical investigations.Defects in graphene, including vacancies (1), dislocations (2), grain boundaries (3), and edges (4), are currently of interest, because they open up a variety of ways with which to tune the properties of pristine graphene. Dopant atoms in graphene are also of tremendous interest (e.g., transition metal atoms, which substitute carbon in graphene sheets, have theoretically been shown to have unusual magnetic or catalytic properties) (5). Single atoms or small clusters in graphene [e.g., N (6) and Fe (7–9)] have been directly observed with high-resolution transmission EM (TEM). Si dopant species have been directly observed with scanning TEM (10). In these cases, the atoms were embedded within the graphene. In addition, single Au (11, 12) and Al (12) atoms have also been observed (using TEM or scanning TEM) to be absorbed at the edges of graphene. The interactions between single metal atoms and graphene edges are complicated because of the different types of trapping states at the edges (11). In addition, some theoretical works suggest that the atomic configurations at graphene edges are greatly affected by nearby transition metal atoms (13–15).Here, we examine individual Fe atoms residing at graphene edges. Single metal atom catalysts have recently been proposed as a means to maximize catalytic efficiency (16, 17). Thus, atoms at graphene edges are of great interest for their catalytic potential, particularly to gain insight to the catalytic growth of sp² carbon (e.g., graphene) by transition metals. By means of low-voltage aberration-corrected TEM (LVACTEM) (18), the atomic configurations of an Fe atom at a graphene edge can be determined, and in addition, the dynamics of a single Fe atom can be recorded. Pentagon–hexagon transitions are observed and reveal the catalytic addition (growth) or removal processes. These processes are in excellent agreement with our ab initio and molecular dynamics (MD) simulations, which are also in agreement with a previously proposed catalytic growth model for carbon nanotubes (19). In addition, the kinetics of Fe atoms at graphene edges are shown to exhibit anomalous diffusion, which again, is in agreement with our theoretical investigations.Previous studies have shown that the 2D behavior of individual atoms over the basal plane in graphene exhibits Brownian motion. From this information, the 2D diffusion coefficient as well as the average Arrhenius activation energy could be extracted (20, 21). The spatial probability of a particle in Brownian motion follows a Gaussian distribution. In this case, the mean square displacement [MSD; <R²(t)>] is dependent linearly with respect to time, and the gradient corresponds to the diffusion coefficient (D) and is written as <R²(t)> = 2nDt (where n is dimensionality) (22, 23). Similar to previous observations of an Au atom at graphene edges (11), our in-depth experimental observations reveal that the diffusion of a single (Fe) atom along the edge of single-layer graphene is directly related to the atomic configuration of the graphene edge. Moreover, the random 1D diffusion of an Fe atom along a graphene edge does not follow Brownian motion but exhibits anomalous diffusion (namely, sub- or superdiffusion). Subdiffusion is attributed to the presence of trapping configurations. Without such trapping configurations, superdiffusion (levy flight) is obtained.Details on the preparation and transfer method of the chemical vapor deposition-grown graphene specimens used in this work can be found elsewhere in the literature (24–26) and SI Appendix. The transferred synthetic graphene is found to occasionally have holes, which can be augmented on electron beam irradiation in a low-voltage C_s aberration-corrected transmission electron microscope (80 kV acceleration voltage). These freshly in situ-derived edges from the augmented holes provide a clean [previous electron energy loss spectrum and TEM study confirmed no oxygen or nitrogen species on these edges (27) as well as no hydrogen (28)] and convenient platform to study the 1D motion of adsorbed Fe atoms. Because of the larger binding energy of an Fe atom at graphene open edges as opposed to binding at the basal plane, Fe atoms tend to reside at the edges. The Fe atoms are present as remnants from the transfer process, in which FeCl₃ was used as an etchant to remove the underlying Ni–Mo over which the graphene was synthesized. Our electron energy loss spectrum studies previously confirmed that these atoms are Fe (26). In our in situ LVACTEM, the atoms are imaged with dark-contrast conditions to optimize imaging (29). The specific experimental condition can be found in Materials and Methods.