Adsorption of Zirconium−Phosphonate Multilayers onto Phosphate-Derivatized Glassy Carbon Substrates

Two methods were developed to modify glassy carbon surfaces with phosphoric acid groups. In the first method, phosphoryl chloride was reacted directly with oxides on the glassy carbon surface. Oxidative and reductive pretreatment of the polished surface improved the phosphate site density, as determined by X-ray photoelectron spectroscopy. In the second method, the carbon surface was modified with an ω-aminoalcohol by electrochemical oxidation, prior to phosphorylation, yielding similiar phosphate site densities. Zirconium 1,10-decanediylbis(phosphonic acid) (Zr-DBP) multilayers were deposited upon these modified glassy carbon substrates. Variable takeoff angle XPS studies and the steady increase of zirconium and of phosphorus with each metal−phosphonate deposition indicate regular adsorption of each multilayer. Higher than ideal zirconium-to-phosphorus ratios are attributed to deposition kinetics, signal attenuation, and the random character of the glassy carbon surface. Zr−DBP layers exhibited ellipsometric thickness of 17 Å or higher, plausibly due to adsorption of excess ionic species into disordered regions or pockets of the film. Contact angle measurements concur with this picture of an imperfect film. The ability of the films to insulate the carbon interface from solution species was investigated by cyclic voltammetry of potassium ferricyanide; heterogeneous electron transfer was significantly hindered after one to two Zr−DBP adsorption steps.