Formation of thick aminophenyl films from aminobenzenediazonium ion in the absence of a reduction source

Aminophenyl films, electrografted to conducting substrates from a solution of the corresponding diazonium ion, are a useful platform for building up functional surfaces. In our hands, reproducible preparation of aminophenyl films via electrografting is difficult, suggesting competing grafting pathways. To investigate the grafting process without the possibility of reduction of the diazonium ion by the substrate, we have used a spin-coated and cured SU-8 substrate that is nonconducting and very smooth (rms surface roughness 0.43 nm). After in situ formation of the aminobenzenediazonium ion (50 mM) in acidic solution, the substrate was added to the solution in the presence and absence of reducing agents (hypophosphorous acid and iron powder). At short reaction times, the films prepared with and without reducing agent have the same thickness and composition (as revealed by X-ray photoelectron spectroscopy). However, in the presence of a reducing agent, films reach a limiting thickness of 7-8 nm after 10 min, whereas, in the absence of a reducing agent, strong film growth continues, giving a film thickness of 14 nm after 120 min. This behavior contrasts with that of other diazonium ions which, in the absence of an applied potential, a reducing agent, or a reducing substrate, give only very thin films after long reaction times.