Covalent Attachment of Polyoxometalates to Passivated Si(111) Substrates: A Stable and Electronic Defect-Free Si|POM Platform
Joseph M. Gurrentz, Michael J. Rose
Abstract
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)2COOH)]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 (E1/2,W1 = −880 mV; E1/2,W2 = −1260 mV vs Fc/Fc+). Both J–V (Laviron, kET,W1 = 4 s–1) and electrochemical impedance spectroscopy (EIS) analyses (kET,W1 = 5 s–1 and kET,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 (ΦB) of POM-modified p-Si photoelectrodes relative to control samples (ΔΦB = 120 meV). However, non-ideal trends observed between ΔΦB and ΔVon 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.