Spontaneous Production of Ultrastable Reactive Oxygen Species on Titanium Oxide Surfaces Modified with Organic Ligands

Abstract

The spontaneous formation and long-term surface stabilization of superoxide radicals are observed on specific TiO 2 hybrid materials in which titanium is coordinated to an organic ligand. Here the rationale for this uncommon phenomenon is investigated by a synergistic theoretical and experimental approach involving density functional theory (DFT) calculations and spectroscopic techniques. Stoichiometric and reduced anatase (101) surfaces modified with acetylacetone, dibenzoylmethane, and catechol are comparatively examined. These results reveal that the interaction between organic ligands and adsorbed O 2 molecules improves when O vacancies are present on the external layer of the surface, promoting O 2 reduction. The electronic features of the ligand play a pivotal role for both an effective electronic interaction with the surface and the stabilization of the generated reactive oxygen species. These results agree with experimental data showing that sol-gel-derived Ti-diketonate hybrid oxides spontaneously produce very persistent superoxide radicals under ambient conditions, thus holding a high intrinsic oxidative activity.

TiO2-Acetylacetone as an Efficient Source of Superoxide Radicals under Reduced Power Visible Light: Photocatalytic Degradation of Chlorophenol and Tetracycline

“…One of the main goals of our study was understanding the roles of ROS ( • O 2 − and •OH) in photocatalysis performed by TiO 2 -A300 CTC, using BQ and IPA scavengers. The photodegradation of 4-CP and TC under visible light with the addition of • O 2 − scavenger confirms the efficient generation of • O 2 − radicals by TiO 2 -ACAC CTC (Figures 3 and 5), in accordance with the EPR characterization and DFT studies carried out on LMCT complexes [16,25,26]. On the other hand, the •OH radicals were not efficiently generated by TiO 2 -A300 CTC under visible light, resulting in the lower photodegradation of 4-CP than the photodegradation of TC, since •OH radicals are the dominant ROS in the 4-CP photodegradation [4,7,32], while • O 2 − radicals play a key role in TC degradation [5].…”

“…Experimental analyses performed by electron paramagnetic resonance (EPR) [16,25] and theoretical studies based on density functional theory (DFT) reported for the TiO 2 -Acetylacetone [25] and TiO 2 -Thiosalicylic acid [26] complexes demonstrate the capacity of the LMCT mechanism to produce superoxide ( • O 2 − ). On the other hand, Fourier transform infrared (FTIR) analysis reveals the presence of adsorbed hydroxyls on the surface of these LMCT complexes [16,[20][21][22]25] that may participate in the formation of •OH if electronic holes exist in the valence band (VB) of TiO 2 . However, to the best of our knowledge, the individual potential of ROS in the LMCT systems has not been evaluated for photocatalytic applications.…”

Recent Advances in Endocrine Disrupting Compounds Degradation through Metal Oxide-Based Nanomaterials

“…These can give the ability to activate molecular O 2 by spontaneous reduction to superoxide, as recently observed for titanium and zirconium oxides with coordinated diketone ligands [48][49][50][51]. On the surface of such hybrid materials the generated •O 2 − radicals can be stabilized for exceptionally long times, leading to oxidative degradation of organic water contaminants even in dark conditions [50,52].…”

Interfacial Charge Transfer Complexes in TiO2-Enediol Hybrids Synthesized by Sol–Gel

“…It can be induced by relatively small organic molecules that do not absorb visible light on their own. ,, Among them, catechol is the most studied. Its interaction with TiO 2 sols, nanoparticles, and surfaces has been investigated in detail by experiments, − computational methods, or both. − The electron injection upon the formation of Ti-catecholate complexes results in an absorption extended up to about 600 nm and a considerable lifetime of charge separation, which has been linked to a partial delocalization in the TiO 2 lattice, slowing down the recombination (back electron transfer) . Consequently, TiO 2 -catechol materials have shown enhanced photocatalytic activity under visible light in water splitting, − selective oxidation of amines, Cr­(VI) reduction, and inactivation of bacteria .…”

“…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. , 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. , Thus, 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 TiO 2 -diketonate amorphous materials, showing unusual surface stabilization of superoxide radicals and oxidative activity in the dark. ,− However, each complexant requires specific processing conditions to yield the desired product. A similar basic idea inspired the synthesis of crystalline organo-titania containing 4,6-dihydroxypyrimidine or p -phenylenediamine, with visible light photocatalytic activity .…”