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Self-Assembled Bilayers on Nanocrystalline Metal Oxides: Exploring the Non-Innocent Nature of the Linking Ions Jamie C Wang, Kyle Violette, Omotola O. Ogunsolu, Seda Cekli, Eric Lambers, Hadi M. Fares, and Kenneth Hanson Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01964 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017
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Self-Assembled Bilayers on Nanocrystalline Metal Oxides: Exploring the Non-Innocent Nature of the Linking Ions Jamie C. Wang†‡, Kyle Violette†‡, Omotola O. Ogunsolu∥, Seda Cekli§, Eric Lambers∫, Hadi M. Fares†, Kenneth Hanson†∥*
† Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32304, USA ∥ Materials Science and Engineering, Florida State University, Tallahassee, Florida 32306, USA § Department of Chemistry, University of Florida, Gainesville, Florida 32611, USA ∫ Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA ‡These authors contributed equally.
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ABSTRACT
Self-assembled bilayer on nanocrystalline metal oxide films is an increasingly popular strategy for modulating electron and energy transfer at the dye-semiconductor interfaces. A majority of the work to date has relied on ZrII and ZnIV linking ions to assemble the films. In this report, we demonstrate that several different cations (CdII, CuII, FeII, LaIII, MnII, and SnIV) are not only effective in generating the bilayer assemblies but also have a profound influence on the stability and photophysical properties of the films. Bilayer films with ZrIV ions exhibited the highest photostability on both TiO2 and ZrO2. Despite the metal ions having minimal influence on the absorption/emission energies and oxidation potentials of the dye, bilayers composed of CuII, FeII, and MnII exhibit significant excited state quenching. The excited state quenching reduces the injection yield but also, for CuII and MnII bilayers, significantly slows back electron transfer kinetics.
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INTRODUCTION Self-assembly of molecular multi-layers via metal ion linkages is an increasingly popular strategy for manipulating energy and electron transfer events at molecule-semiconductor interfaces.1-3 Self-assembled films have shown promise in applications including dye-sensitized solar cells (DSSCs),4 dye-sensitized photoelectrosynthesis cells (DSPECs),5-6 photon upconversion solar cells,7-8 and molecular p-n junctions.9 The multilayer films are formed using a simple, step-wise soaking procedure.10-16 Briefly for the bilayer film, first molecule is bound to high surface area metal oxide (MO2) via surface binding groups. Then metal ions are coordinated to the terminal functional groups (CO2H or PO3H2) of the first molecular layer. Finally, a second molecular layer is bound to the linking metal ions (Figure 1a). Historically, the most common method for adhering multiple molecular units to a metal oxide surface was using co-deposition and/or depositing pre-formed molecular dyads. Unfortunately, due to surface area limitations, co-deposition decreases the total loading for each molecule. The decreased loading can be supplemented by increasing the film thickness but it also increases the likelihood of losses due to recombination.17-18 Pre-formed molecular assemblies typically require a multistep synthesis for each new dyad, with cumulatively low yields and low surface loadings (
