Quantitative study of the charge transfer in chemisorption; oxygen chemisorption on ZnO

A new experimental approach is presented for studying the charge-transfer process involved in the chemisorption on polar semiconductors. This approach utilizes the surface piezoelectric effect, contact potential difference measurements, and surface photovoltage spectroscopy. From the study of oxygen adsorption on ZnO it was found that the rate of electron transfer varies exponentially with the surface barrier height and is proportional to the oxygen pressure (from 10−3 to 20 Torr). Furthermore, it was found that the charge transfer is characterized by a thermal activation energy of about 0.72 eV. At room temperature this activation energy constitutes the most significant rate-limiting factor and is largely responsible for the extremely slow rate of chemisorption. A model for chemisorption was developed in which the thermal activation is treated as an intermediate nonelectronic step involving metastable activated surface states. Upon capturing electrons from the bulk these states become stable surface states. A rate equation was derived through which the capture cross section of the activated surface states was calculated to be 10−16 cm2, in contrast to the unrealistically small value of 10−29 to 10−22 cm2 obtained with exclusively electronic models.