Polyoxometalates (POMs) have been increasingly investigated for use as multiredox components in functional molecular materials, and the significance of irreversible POM attachment on electrode surfaces has grown in tandem. Achieving covalent immobilization on high-quality silicon surfaces remains a challenge, however, as prevailing methods couple POMs to unpassivated hydride-terminated Si substrates that are prone to deleterious surface oxidation. This work focuses on an alternative 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- has been bound to phenylethylamine surface linkers on methyl-passivated Si(111). Current-voltage (J-V) analysis of POM-modified n+-type Si electrodes reveals multiple discrete redox transitions (E1/2,W1 = -880 ± 85 mV; E1/2,W2 -1260 ± 90 mV vs. Fc/Fc+ at 50 mV/s) and interfacial electron transfer kinetics that, while slow (~4 s-1), are commensurate with other covalently bound Si|POM systems. Significantly, these electrodes are of sufficient quality that the photoelectrochemical behavior of POM-modified p-type Si photoelectrodes has also been evaluated. A 230 ± 110 mV shift is observed in the E1/2 values of the POM species immobilized on illuminated POM-modified p-type Si photoelectrodes. In addition, Mott-Schottky analysis reveals a large positive interfacial dipole is generated upon POM immobilization that is reflected by a 120 ± 10 mV increase in the barrier height of POM-modified p-Si photoelectrodes relative to control samples. This system thus provides a platform for the further development and study of POM-modified (photo)electrode systems.

Interfacial electron transfer and dipole modulation of polyoxometalates covalently attached to passivated Si(111) substrates

ACS Spring Meeting 2021

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