The adsorption of dyes on zinc oxide: The effect of non-stoicheiometry

The adsorption of uranine and rhodamine B, from both aqueous and alcoholic solutions, on to high-purity zinc oxide has been determined, and the results compared with the adsorption on zinc oxide powders having a wide range of non-stoicheiometric composition induced by doping with monovalent and trivalent cations. It is shown that, from aqueous solution, total adsorption of the acid dye uranine on undoped zinc oxide comprises approximately 10% chemisorption, and 90% adsorption of lower energy. Both chemisorption and lower energy adsorption were markedly increased when the zinc oxide was made more strongly n-type by doping with aluminium. Doping with lithium to produce an electron deficiency resulted in a reduction of both types of adsorption to approximately one-half of their original value. From alcoholic solution, only chemisorption is affected by the non-stoicheiometry of the zinc oxide. It is suggested that the lower energy adsorption from alcoholic solution is true physisorption, and that from aqueous solution is a weak type of chemisorption by electrostatic forces. Adsorption of the basic dye rhodamine B, from both aqueous and alcoholic solutions, on to surfaces of doped and undoped zinc oxide is exclusively chemisorption. The chemisorption is increased by doping with lithium, and decreased by aluminium doping.