Adsorption of 1,2-diaminoethane on ZnO thin films from p-xylene

The interaction of 1,2-diaminoethane (DAE) with ZnO thin films prepared by electrodeposition and magnetron sputtering was investigated by X-ray photoelectron spectroscopy (XPS). The samples were exposed to organic solution of 0.5 M DAE-p-xylene in an Ar atmosphere glove box (O2 and H2O <5 ppm), directly connected to the XPS analysis chamber by an anaerobic and anhydrous transfer system. A clear interaction of DAE with the ZnO surface is evidenced by the presence of a high intensity N1s peak at BE = 399.5 ± 0.2 eV and C1s at BE = 286.3 ± 0.2 eV which are attributed to C-N bonding. The atomic ratio C:N was very close to 1:1 consistent with the molecular, non-dissociative adsorption of DAE on the ZnO layer. No significant difference in adsorption of DAE was observed for three different ZnO surfaces despite slight differences in their acid/base properties as evidenced by the O/OH ratio. The results are interpreted in terms of adsorption on Brönsted acid sites. A uniform layer model was used to approximate the DAE film thickness, which was found to be around 10 Å on three studied samples. The N1s and C1sB signals were observed to decrease on sample exposure to vacuum and/or X-ray irradiation and additional N1sB peak appeared at lower binding energy at around 398.5 ± 0.2 eV. This is interpreted by the desorption and modification of DAE, indicating low stability of the adsorbed state on ZnO. The exposure to water of the sample with adsorbed DAE causes a significant decrease of the N1sA and C1sB peak intensities attributed to the adsorbed DAE molecule, demonstrating the instability of the DAE-ZnO interface in water.