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Desorption Kinetics and Activation Energy for Cobalt Octaethylporphyrin from Graphite at the Phenyloctane Solution–Graphite Interface: An STM Study

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journal contribution
posted on 2015-04-30, 00:00 authored by Ashish Bhattarai, Ursula Mazur, K. W. Hipps
Temperature-dependent desorption rates and desorption energies are determined from a monolayer assembly at the solution–solid (SS) interface. Scanning tunneling microscopy (STM) was used to measure molecular-scale temperature-dependent desorption of cobalt­(II) octaethylporphyrin (CoOEP) at the phenyloctane solution–highly ordered pyrolytic graphite (HOPG) interface. At lower temperatures, monolayer formation of metal­(II) octaethylporphyrin (MOEP) on HOPG from solution was found to be completely controlled by kinetics, and the adlayer formed was stable up to 70 °C. Significant desorption of CoOEP from the HOPG surface was observed above 80 °C on a time scale of hours. CoOEP desorbs from HOPG into phenyloctane at a rate of 0.0055 ± 0.0007 min–1 at 90 °C, 0.013 ± 0.001 min–1 at 100 °C, and 0.033 ± 0.003 min–1 at 110 °C. From these temperature- and time-dependent measurements, assuming an Arrhenius rate law, the activation energy of molecular desorption at the SS interface was determined using studies solely based on STM. The desorption energy of CoOEP from HOPG into phenyloctane is determined to be 1.05 × 102 ± 0.03 × 102 kJ/mol. NiOEP desorption occurs at a slower rate and is homogeneous across HOPG terraces, unlike the inhomogeneous desorption observed on Au(111). A previous study performed on Au(111) reported that the rate of desorption of CoOEP is 0.004 min–1 at 135 °C. The calculated desorption rate on HOPG in this work is 0.22 min–1, making the rate of desorption of CoOEP from HOPG 2 orders of magnitude greater than from Au(111). On the other hand, for solution concentrations of the order of 100 μM, a dense monolayer is formed within seconds. For this fast adsorption process, where a full monolayer coverage occurs, the surface coverage of MOEP on both surfaces was determined by the relative concentration of each species in the phenyloctane solution. The rates of adsorption (for concentrations near 100 μM) are found to be within 20% of each other. The surface structures of both the NiOEP and CoOEP on HOPG and Au(111) are very similar and can be described by A = 1.30 ± 0.04 nm, B = 1.40 ± 0.04 nm, and α = 57° ± 2° with an area of 1.50 ± 0.08 nm2/molecule.