Kinetics and Thermodynamics at the Solution/Solid Interface: A Scanning Tunneling Microscopy Study
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A detailed investigation on the nature of stability of surface structures of octaethylporphyrins (OEP) on graphite and Au(111) has been performed using scanning tunneling microscopy (STM). In-situ temperature dependent studies on the monolayer of these porphyrins show that the surface structure is stable up to 70 °C and that the molecules do not exchange between the surface and solution. Hence, at temperatures lower than 70 °C the monolayer is controlled by kinetics and the rates of desorption are extremely slow. In order to extract kinetic parameters ex-situ annealing was performed at higher temperatures. Substrate effects are studied by measuring and comparing the rates of desorption of CoOEP from 1-phenyloctane/graphite and 1-phenyloctane/Au(111) interfaces. Significant desorption from Au(111) starts at 135 °C whereas that from graphite starts at 90 °C and the rate of desorption from graphite is 2 orders of magnitude greater than from Au(111). In a different study to understand the effects of the central metal of the porphyrin on the adsorption strength, it was shown that the metal-free and metallated OEP adsorb on graphite with similar energy. This shows that the adsorption strength strongly depends on the interaction between the porphyrin core and the graphite surface and that the interaction between central metal and graphite is minimal. In any case, the adsorption energy for OEP on to graphite and Au(111) ranges from 100 – 130 kJ/mol. Substrate effects on oxygen binding to CoOEP adsorbed on graphite and on Au(111) were also studied using STM. At temperatures close to room temperature oxygen binds to CoOEP on graphite where as it does not bind on Au(111) even at -25 °C. It was shown that the binding of oxygen to CoOEP-graphite is a dynamic equilibrium process. Temperature and pressure dependent studies were performed to extract thermodynamic quantities such as ΔG, ΔH, and ΔS values for the oxygenation process.