Covalent Attachment of Polyoxometalates to Passivated Si(111) Substrates: A Stable and Electronic Defect-Free Si|POM Platform

Polyoxometalates (POMs) have been investigated as multiredox components in functional molecular materials, and as a result the significance of irreversible POM attachment on electrode surfaces has increased. Achieving covalent immobilization on high-quality silicon surfaces remains a challenge, however, as prevailing methods couple POMs to unpassivated hydride-terminated Si substrates, which are prone to deleterious surface oxidation. Herein, we demonstrate an improved approach for covalent POM immobilization via secondary functionalization of Si(111)-mixed monolayers. Specifically, a carboxylate-functionalized Keggin-type polyoxometalate, [PW 11 O 39 (Ge(CH 2 ) 2 COOH)] 4–, was bound to phenylethylamine surface linkers on methyl-passivated Si(111). Current–voltage ( J – V ) analysis of POM-modified n + -type Si electrodes revealed multiple discrete redox transitions ( E 1/2,W1 = −880 mV; E 1/2,W2 = −1260 mV vs Fc/Fc + ). Both J – V (Laviron, k ET,W1 = 4 s –1 ) and electrochemical impedance spectroscopy (EIS) analyses ( k ET,W1 = 5 s –1 and k ET,W2 = 6 s –1 ) revealed consistent interfacial electron-transfer kinetics that are commensurate with other Si|POM systems. Importantly, these electrodes were of such high electronic quality that photoelectrochemical function of POM-modified p-type Si photoelectrodes was displayed. An experimental photovoltage was observed for p-Si(111)|POM, and Mott–Schottky analysis (dark conditions) revealed a systematic increase in the barrier heights (Φ B ) of POM-modified p-Si photoelectrodes relative to control samples (ΔΦ B = 120 meV). However, non-ideal trends observed between ΔΦ B and Δ V on for the photoelectrochemical reduction of methyl viologen at these illuminated photoelectrodes revealed that the functional outcome of this Si|POM system is defined by a thermodynamic interplay between charge equilibration in the Si substrate and interfacial electric field effects of the POM molecular overlayer. These results thus provide a platform for the further development and study of POM-modified (photo)electrode systems.