Surface Chemistry of Ruthenium and Graphene on Ruthenium Studied by Scanning Tunneling Microscopy
- Author(s): Feng, Xiaofeng
- Advisor(s): Salmeron, Miquel B
- et al.
Fundamental studies of surface chemistry processes on metals may lead to breakthroughs in many fields including heterogeneous catalysis, corrosion, and electrochemistry. We are motivated to study the surface chemistry of Ruthenium (Ru), which is a versatile catalyst. In this dissertation, I present our studies of surface chemistry phenomena occurring on Ru and graphene/Ru at the molecular level using scanning tunneling microscopy (STM). The results are divided into two parts. The first part is the adsorption and interactions of small molecules on Ru(0001) surface, including the adsorption of CO2 and the coadsorption of CO and H2, towards a better understanding of the fundamental steps in catalytic reactions such as Fischer-Tropsch synthesis (FTS). The second part is about water adsorption and reactions on graphene/Ru(0001) surface and resulting graphene flakes.
First, the adsorption and interaction of CO2 molecules on Ru(0001) surface was studied. The CO2 molecules adsorbed on Ru(0001) at 77 K form primarily an unordered structure with a few small 2×2 domains. The desorption of CO2 molecules occurs after annealing to 250 K, while the CO2 molecules remaining on the Ru surface form trimers, probably with a cyclic structure. The trimers present alternating orientations on neighboring Ru terraces, due to the three-fold hollow adsorption site of CO2 molecules, which determines the sites for three interacting molecules. The intermolecular interaction between CO2 molecules is facilitated and mediated by the Ru substrate.
Second, the coadsorption and interaction of CO and H on Ru(0001) surface were studied, towards an understanding of the precursor state in FTS. The coadsorbed CO and H form an unordered structure on Ru upon adsorption at 77 K, which segregate to triangular CO islands that are separated by H domains after annealing to 150~200 K. In the islands, CO molecules are compressed to a nearly (1×1) pattern due to the strong repulsion of H. They finally evolve to a homogeneously mixed structure after annealing to 300~350 K. This mixed structure can be derived from other samples with different initial CO/H ratios or coverages after annealing, indicating that the CO-H mixing is energetically favorable and might be the precursor state for CO-H reaction.
In addition, we are very interested in surface chemistry of epitaxial graphene, specifically the adsorption and reaction of small molecules on graphene films, as the environmentally abundant molecules, such as water and oxygen, can adsorb and influence their properties. We investigated the adsorption and reactions of water, oxygen, hydrogen and ammonia on epitaxial graphene grown on Ru and Cu substrates, and found that on Ru(0001) graphene line defects are extremely fragile towards chemical attack by water, which could split the graphene film into numerous fragments at temperatures as low as 90 K, followed by the intercalation of water under the graphene. On Cu(111), water can also split graphene but far less effectively, indicating that the chemical nature of the substrate strongly affects the reactivity of C-C bonds in epitaxial graphene. Interestingly, no such effects were observed with other molecules, including oxygen, hydrogen and ammonia also studied here, which indicates the unique reactivity of water with graphene line defects.
The water-induced splitting of graphene on Ru resulted in numerous graphene flakes that were displaced onto other graphene areas. We found that the flakes exhibit a facile sliding behavior on the graphene substrate even at a low temperature of 5 K, a phenomenon that is at the heart of superlubricity inherent in incommensurate interfaces. The flakes are only stable when sitting commensurately on the underlying graphene layer. Once switched to an incommensurate state, the flakes can diffuse over distances of tens of nanometers until another commensurate state is reached finally. Our observations of the superlubric sliding behavior of graphene flakes can benefit the understanding of nanotribological phenomena and their applications in nanomechanical systems.
Finally, we also studied the electronic screening effect in stacked graphene flakes on Ru and Cu substrates using STM. The apparent height of a graphene layer reflects its density of states near the Fermi level and then the doping level, which provides a simple method to study the electronic screening effect in multilayer graphene films. It was thus revealed that the strong doping effect in the first graphene layer on Ru is weakened by 15 times in the second one, and almost eliminated in the third and fourth layer. In contrast, the charge transfer from Ru can be blocked effectively by a water layer intercalated underneath the graphene, indicating that water or other molecules might be a potential dielectric material for graphene devices.