Electrochemical deposition of polyborate monolayers at Ag(111) electrodes

Non-Faradaic electrodeposition of polyborate monolayers at Agr(111) in aqueous solutions containing sodium borate (Na2B4O7) is reported. Electrodeposition is characterized by a very sharp voltammetic wave with half-wave potential E-1/2 approximate to -1.2 V vs Hg/Hg2SO4 and surface charge density sigma 20 +/- 5 mu C/cm(2). X-ray photoelectron spectroscopy and electrochemical quartz crystal microbalance measurements demonstrate that monolayer formation occurs at potentials positive E-1/2. The voltammetric behavior suggests that electrodeposition is associated with the polymeric condensation of B(OH)(4)(-) and B(OH)(3), both species produced by the dissociation of B4O72- in the bulk solution (B4O72- + 7H(2)O reversible arrow 2B(OH)(4)(-) + 2B(OH)(3)). The dependence of the voltammetric wave shape on scan rate is consistent with the nucleation and growth of an ordered polyborate monolayer. Deposition of the monolayer is also observed in boric acid/NaOH solutions, strongly supporting the proposed mechanism.