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, [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.

Automated summary

Polyoxometalates (POMs) have been investigated as multiredox components in functional molecular materials, and an improved approach for covalent POM immobilization on high-quality silicon surfaces has been demonstrated. The POM-modified Si electrodes showed multiple redox transitions and high-quality electronic properties, with potential for photoelectrochemical applications. The study provides insights into the thermodynamic interplay between the Si substrate and POM molecular overlayer in defining the functional outcome of the Si|POM system.