Self-assembly of zinc oxide thin films modified with tetrasulfonated metallophthalocyanines by one-step electrodeposition

Cathodic electrodeposition in an aqueous mixed solution of zinc nitrate and water-soluble tetrasulfonated metallophthalocyanines (TSPcMs), in which M = Zn(II) (TSPcZn), AI(III)[OH] (TSPcAl), or Si(IV)[OH](2) (TSPcSi), resulted in a self-assembled growth of zinc oxide (ZnO) thin films whose surface is modified by TSPcMs. It has been found that the adsorption of TSPcM onto the growing surface of ZnO strongly affects the crystal growth and the orientation of the ZnO crystallites. The effect was most prominently seen with TSPcSi, creating a film looking like stacking disks aligned perpendicular to the substrate. Crystallographic studies by X-ray diffraction and TEM observation coupled with the selected area electron beam diffraction have revealed that; thin platelike crystals, whose planes and edges correspond to the (002) and (100) crystal faces, respectively, are aligned in the same orientation around the c-axis within the stacks. The evolution of this unique structure is interpreted as arising from the preferential adsorption of TSPcMs at the (002) planes, leading to film growth preferentially in the (100) direction. Optical analysis of the films also revealed high order in the interactions among TSPcMs. Because of the strong intermolecular attraction, TSPcZn forms multilayers of pi-stacking aggregates on ZnO as confirmed by the characteristic blueshift of the Q-band absorption. Thus, TSPcZn could be condensed at a very high concentration exceeding 1 M in the deposited film. These surface aggregates were completely removed by dipping the film in a solution of cationic detergent, cetyltrimethylammonium chloride, while only leaving surface-bound monomeric TSPcZn. By contrast, the aggregation was hindered for TSPcA1 and TSPcSi because of the presence of axially coordinated OH-, and monomeric adsorption was found in the electrodeposited films.