Surface Modification of n-MoS₂ Electrodes with a Viologen Based Redox Polymer: Persistent Attachment of a Polysiloxane via a Thin SnO₂ Adhesion Layer

A procedure for surface modification of n-MoS₂ electrodes with a polymer formed from hydrolysis of N,N′-bis[p-(trimethoxysilyl)benzyl]-4,4′-bipyridinium, BPQ²⁺, is described. (BPQ²⁺)_n is persistently confined onto the surface of n-MoS₂ crystals in a sandwich structure, n-MoS₂/SnO₂/(BPQ²⁺)_n. The n-MoS₂ crystal is first modified by electrochemical deposition of small islands of tin, followed by oxidation to give islands of SnO₂. The (BPQ²⁺)_n is confined onto the resulting surface via standard procedures for this polysiloxane system: electrochemical reduction of the viologen in aqueous electrolyte. The SnO₂ at the interface between n-MoS₂ crystal and (BPQ²⁺)_n functions as an adhesive layer to bind the polymer to the MoS₂. Without SnC₂, the attachment of the coating of (BPQ²⁺)_n on MoS₂ is not durable. Although a coating of SnC₂ shifts the flat band potential of the electrode ∼0.1 V negatively (depending on pH and SnO₂ coverage) the essential behavior of the semiconductor/electrolyte interface is not significantly altered. (BPQ²⁺)_n electrostatically incorporates a high concentration of I⁻ from dilute (0.005 M) aqueous concentration, even when competing with 0.05 M KCl. The photooxidation of the highly concentrated I^∲ at the (BPQ²⁺)_n modified n-MoS₂ electrode surface suppresses anodic photocorrosion in aqueous solution.