Covalent Grafting of Polyoxometalate Hybrids onto Flat Silicon/Silicon Oxide: Insights from POMs Layers on Oxides

Abstract

Immobilization of polyoxometalates (POMs) onto oxides is relevant to many applications in the fields of catalysis, energy conversion/storage or molecular electronics. Optimization and understanding the molecule/oxide interface is crucial to rationally improve the performance of the final molecular materials. We herein describe the synthesis and covalent grafting of POM hybrids with remote carboxylic acid functions onto flat Si/SiO2 substrates. Special attention has been paid to the characterization of the molecular layer and to the description of the POM anchoring mode at the oxide interface through the use of various characterization techniques, including ellipsometry, AFM, XPS and FTIR. Finally, electron transport properties were probed in a vertical junction configuration and energy level diagrams have been drawn and discussed in relation with the POM molecular electronic features inferred from cyclic-voltammetry, UVvisible absorption spectra and theoretical calculations. The electronic properties of these POMbased molecular junctions are driven by the POM LUMO (d-orbitals) whatever the nature of the tether or the anchoring group.

Redox-controlled conductance of polyoxometalate molecular junctions

“…At the nanoscale, monolayers (or few monolayers) of POMs assembled on conducting surfaces by various approaches (covalent grafting, electrostatic immobilization, layer-by-layer deposition, etc.) and contacted by scanning probe microscopy have been used to measure the conductance of various POMs, [7][8][9][10][11][12] but the redox state was not systematically tuned and controlled. It is only recently that at the single molecule level, using scanning tunneling microscopy, Linnenberg et al demonstrated a step-by-step increase of the POM (Lindqvisttype V 6 ) conductance up to the 4-electron reduced state.…”

Polyoxometalates-Functionalized Electrodes for (Photo)Electrocatalytic Applications: Recent Advances and Prospects

“…Lacunary Keggin [PW 11 O 39 ] 7– -type POM hybrids have been covalently grafted on Si/SiO 2 substrates through carboxylic acid binding units . However, the presence of electrically insulating SiO 2 layer precluded the use of these modified surfaces for photoelectrocatalytic purposes.…”

“…Lacunary Keggin [PW 11 O 39 ] 7− -type POM hybrids have been covalently grafted on Si/SiO 2 substrates through carboxylic acid binding units. 279 However, the presence of electrically insulating SiO 2 layer precluded the use of these modified surfaces for photoelectrocatalytic purposes. Improved interfacial electron transfer properties have been obtained when these hybrid assemblies were covalently attached to hydrogenterminated, oxide-free Si (Si−H) surfaces through the aryldiazonium chemistry.…”

Dye-Sensitized Photocathodes: Boosting Photoelectrochemical Performances with Polyoxometalate Electron Transfer Mediators

“…These compounds can display remote active sites for extra transition metal coordination, assembly in advanced architectures, or covalent anchorage onto substrates . Some of the authors of this manuscript have been deeply engaged in the synthesis and implementation of POM hybrids, and we recently described the immobilization of [PW 11 O 39 {SnC 6 H 4 C 6 H 4 COOH}] 4– (POM-COOH) onto Si/SiO 2 wafers and the grafting of [PM 11 O 39 {M′C 6 H 4 C 6 H 4 –N 2 + }] 3– (M = W, Mo, M′ = Ge, Sn) on various electrodes, including glassy carbon, graphene, and silicon. , We herein report the amplification effect of a cografted POM on the photoelectrochemical activity of a dye-sensitized photocathode.…”