Charge retention of soft-landed phosphotungstate Keggin anions on self-assembled monolayers

Abstract

Soft landing of mass-selected ions onto surfaces often results in partial loss of charge that may affect the structure and reactivity of deposited species. In this study, Keggin phosphotungstate anions in two selected charge states, PW₁₂O₄₀³⁻ (WPOM³⁻) and PW₁₂O₄₀²⁻ (WPOM²⁻), were soft-landed onto different self-assembled monolayer (SAM) surfaces and examined using in situ infrared reflection absorption spectroscopy (IRRAS) and density functional theory (DFT) calculations. Partial retention of the 3− charge was observed when WPOM³⁻ was soft-landed onto the fluorinated SAM (FSAM), while the charge state distribution was dominated by the 2− charge after both WPOM³⁻ and WPOM²⁻ were deposited onto a hydrophilic alkylthiol SAM terminated with cationic NH₃⁺ functional groups (NH₃⁺SAM). We found that during the course of the soft landing of WPOM³⁻, the relative abundance of WPOM³⁻ on FSAM decreased while that of WPOM²⁻ increased. We propose that the higher stability of immobilized WPOM²⁻ in comparison with WPOM³⁻ makes it the preferred charge state of WPOM on both the FSAM and NH₃⁺SAM. We also observe weaker binding of WPOM anions to SAMs in comparison with phosphomolybdate ions (MoPOM) reported previously (J. Phys. Chem. C, 2014, 118, 27611–27622). The weaker binding of WPOM to SAMs is attributed to the lower reactivity of WPOM reported in the literature. This study demonstrates that both the charge retention and the reactivity of deposited anionic POM clusters on surfaces are determined by the type of addenda metal atoms in the cluster.