Formation of catalytic Pd on ZnO thin films for electroless metal deposition

The formation process of highly dispersed Pd on ZnO thin films as a catalyst for electroless metal deposition was studied in detail by means of adsorption experiments combined with X-ray photoelectron spectroscopy measurements. The chemical state of the yellow-brown species adsorbed on ZnO thin film during the catalyzation process in acidic PdCl2 solution (pH 2.5) was determined to be Pd(OH)(2). It was found that the adsorption of Pd on the ZnO thin film is closely related to the partial dissolution of ZnO in acidic solution, which results in an increase of the pH in the vicinity of ZnO surface, while the pH of the overall PdCl2 solution remains essentially unchanged (similar to 2.5) within the period of the catalyzation treatment (<10 min). This pH increase results in partial hydrolysis of Pd-containing ions; and thus Pd(OH)(2) is formed near the surface of the ZnO thin film. The Pd(OH)(2) thus produced is then efficiently adsorbed on the ZnO surface, which has been made highly porous by its partial dissolution in the acidic solution. In the subsequent electroless metal deposition step, Pd(OH)(2) is first reduced to Pd-(o) by the reductant, e.g., HCHO, contained in the deposition bath, and then the Pd particles act as catalytic nuclei for the deposition reaction. The Pd catalyst prepared in this fashion was found to be highly dispersed and can initiate several types of electroless metal deposition reactions.