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

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  Bhattarai, Ashish, Mazur, Ursula, Hipps, K. W. The Journal of   Physical Chemistry C 2015 119 (17)   9386-9394

  Abstract: 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.

  Address (URL): http://dx.doi.org/10.1021/acs.jpcc.5b01444