The adsorption and self-assembly behaviors of a carboxyl-group-terminated alkanethiol, 11-mercaptoundecanoic acid (MUA), on Au(111) electrodes in an electrochemical system are studied using in situ scanning tunneling microscopy. The effect of applied potential on the phase evolution of the MUA adlayer is investigated and compared with those reported for alkanethiols with various terminal groups. The results show that the MUA molecules initially adsorb in a lie-down orientation, organizing into ordered domains with a stripe structure. With further adsorption of MUA molecules, the alkyl chains lift off from the substrate, forming a more condensed phase with an arrangement of (root 3 x root 3). This phase evolution is similar to those reported for other alkanethiols. However, the adsorption process of MUA is much slower and a disordered transition phase (gamma phase) exists between the stripe and saturation phases. The gamma phase converts back to the stripe phase when the electrode potential is shifted from 0.2 to 0.4 V, following which the phase evolution cannot proceed further to the saturation phase. These results are contrary to those observed for other alkanethiols and are attributed to the interaction of the terminal COOH group with the substrate at positive potentials. Under the electrode potential, the molecules bind to the substrate via both head and end groups, triggering a lie-down orientation and decreasing the mobility of adsorbed molecules. As a result, the adlayer remains in the stripe phase and further phase evolution is inhibited.
關聯:
Journal of Physical Chemistry C, v.118 n.2, pp.989-993
