%Nanoscale Imaging of Molecular Adsorption of Metal Complexes on the Surface of a Hydrotalcite Crystal

Abstract

Atomic force microscopy (AFM) and scanning tunnel microscopy (STM) observations have been performed on the surfaces of a hydrotalcite (HT) crystal ([Mg6Al2(OH)16](CO3)1/2·Cl·2H2O) and HT crystals ion-exchanged with [Fe(CN)6]3- and [Fe(CN)6]4- (denoted by HT−[Fe(CN)6]3- and HT−[Fe(CN)6]4-, respectively). The AFM image of a HT crystal surface in contact with an aqueous solution of 0.1 M Na2SO4 shows two-dimensional periodicity with the unit lattice of a = 0.31 ± 0.02 nm, b = 0.31 ± 0.02 nm, and α = 58 ± 3°. This corresponds to the lattice made by the external hydroxyl groups on a basal plane. No ion-exchanged anion was observed by the AFM method, indicating that an external anion is so weakly bound to a surface that it is removed under the scanning operation of an AFM tip. The STM image of the same crystal under air shows a two-dimensional lattice with a = 0.75 ± 0.04 nm, b = 1.10 ± 0.03 nm, and α = 70 ± 3°. One bright spot in the image exists at every 1.1 [Mg6Al2(OH)16]2+ unit, leading to the conclusion that the spot represents a chloride ion adsorbed on a surface. The STM image of the same crystal at less negative bias voltage exhibits a different two-dimensional lattice with a = 0.62 ± 0.03 nm, b = 0.62 ± 0.03 nm, and α = 65 ± 3°. This is ascribed to the lattice made by the aluminum atoms at the octahedral sites inside a layer. The STM images of HT−[Fe(CN)6]3- and HT−[Fe(CN)6]4- crystals in contact with an aqueous solution of 0.1 M Na2SO4 show the two-dimensional lattices of a = 1.43 ± 0.06 nm, b = 1.86 ± 0.06 nm, and α = 90 ± 4° and a = 2.20 ± 0.06 nm, b = 2.20 ± 0.06 nm, and α = 75 ± 3°, respectively. These are ascribed to the molecular layers of [Fe(CN)6]3- and [Fe(CN)6]4- on a surface, respectively. The causes for the ordered molecular adsorption are discussed.