Abstract
Metal oxide-organic hybrid semiconductors exhibit specific properties depending not only on their composition but also on the synthesis procedure, and particularly on the functionalization method, determining the interaction between the two components. Surface adsorption is the most common way to prepare organic-modified metal oxides. Here a simple sol–gel route is described as an alternative, finely controlled strategy to synthesize titanium oxide-based materials containing organic molecules coordinated to the metal. The effect of the molecular structure of the ligands on the surface properties of the hybrids is studied using three enediols able to form charge transfer complexes: catechol, dopamine, and ascorbic acid. For each system, the process conditions driving the transition from the sol to chemical, physical, or particulate gels are explored. The structural, optical, and photoelectrochemical characterization of the amorphous hybrid materials shows analogies and differences related to the organic component. In particular, electron paramagnetic resonance (EPR) spectroscopy at room temperature reveals the presence of organic radical species with different evolution and stability, and photocurrent measurements prove the effective photosensitization of TiO2 in the visible range induced by interfacial ligand-to-metal charge transfer.
Highlights
The conjugation of inorganic materials with organic compounds provides emerging functional properties to the resulting hybrid materials
The rate, extent, and mechanism of these reactions depend on different factors, including the structure of the complexing ligand, its concentration relative to the metal, the amount of water, the properties of the solvent, and the solution pH, which in turn determine the structural properties of the products.[56,63,64]
We explored the evolution of hybrid sols, i.e., the colloids produced by hydrolysis and partial condensation of titanium alkoxide species modified with the enediol ligands, with the aim to understand the conditions promoting homogeneous gelation or precipitation
Summary
The conjugation of inorganic materials with organic compounds provides emerging functional properties to the resulting hybrid materials. Adding the ligand to the titanium precursor solution before hydrolysis and condensation reactions yields coordination complexes with modified reactivity, which, in suitable conditions, stabilize the sol or promote the growth of homogeneous chemical gels.[54,55] Depending on the molecular structure of the ligand and on the concentration of the reagents, a variety of metal oxo-clusters can be formed, working as building blocks for metal−organic frameworks, polyoxometalates, nanostructured composites, and other hybrid materials with a specific architecture.[54,56] the structural and morphological features of the products and the content of organic phase can be finely regulated by the process variables We have applied such sol−gel strategy for the synthesis of TiO2-diketonate amorphous materials, showing unusual surface stabilization of superoxide radicals and oxidative activity in the dark.[21,57−59] each complexant requires specific processing conditions to yield the desired product. The effect of the composition and synthesis conditions on the characteristics of the interfacial charge transfer processes involved in the Tiligand coordinative complexes was evaluated
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