Covalent Attachment Of Polyoxometalates To Passivated Si(111) Substrates: A Stable And Electronic Defect-Free Si Vertical Bar 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, [PW11O39(Ge(CH2)(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,E-W1 = -880 mV; E-1/2,E-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 vertical bar 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)vertical bar POM, and Mott-Schottky analysis (dark conditions) revealed a systematic increase in the barrier heights (FB) of POM-modified p-Si photoelectrodes relative to control samples (Delta Phi(B) = 120 meV). However, non-ideal trends observed between Delta Phi(B) and Delta 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.