The nature of self-assembled octadecylphosphonic acid (ODPA) layers on copper substrates

Hypothesis: The self-assembly of amphiphilic molecules onto solid substrates can result both in the formation of monolayers and multilayers. However, on oxidized and non-oxidized copper (Cu), only monolayer formation was reported for phosphonic acids possessing one phosphate head group. Here, the adsorption of octadecylphosphonic acid (ODPA) on Cu substrates through a self-assembly process was investigated with the initial hypothesis of monolayer formation. Experiments: The relative amount of ODPA adsorbed on a Cu substrate was determined by infrared reflection/absorption spectroscopy (IRRAS) and by atomic force microscopy (AFM) investigations before and after ODPA deposition. X-ray photoelectron spectroscopy (XPS) with sputtering was used to characterize the nature of the layers. Findings: The results show that the thickness of the ODPA layer increased with deposition time, and after 1 h a multilayer film with a thickness of some tens of nm was formed. The film was robust and required long-time sonication for removal. The origin of the film robustness was attributed to the release of Cu ions, resulting in the formation of Cu-ODPA complexes with Cu ions in the form of Cu(I). Preadsorbing a monolayer of octadecylthiol (ODT) onto the Cu resulted in no ODPA adsorption, since the release of Cu(I) ions was abolished. (C) 2020 Elsevier Inc. All rights reserved.

Automated summary

The adsorption of octadecylphosphonic acid (ODPA) on copper substrates was studied, showing that a multilayer film was formed with increasing deposition time. The robustness of the film was attributed to the formation of Cu-ODPA complexes. Preadsorbing a monolayer of octadecylthiol (ODT) prevented the adsorption of ODPA by abolishing the release of Cu(I) ions.