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

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

Oxidative Degradation of Different Chlorinated Phenoxyalkanoic Acid Herbicides by a Hybrid ZrO₂ Gel-Derived Catalyst without Light Irradiation

The oxidative degradation of 2-methyl-4-chlorophenoxyacetic acid (MCPA), 4-(4-chloro-2-methylphenoxy)­butanoic acid (MCPB), 4-chlorophenoxyacetic acid (4-CPA) and 2,4-dichlorophenoxyacetic acid (2,4 D) by ZrO₂-acetylacetonate hybrid catalyst (HSGZ) without light irradiation was assessed. The thermal stability of the catalyst was investigated by thermogravimetry, differential thermal analysis, and Fourier transform infrared spectroscopy. For each herbicide, a virtually complete removal in about 3 days without light irradiation at room temperature was achieved. The removal kinetics of the herbicides has been satisfactorily characterized by a double-stage physico-mathematical model, in the hypothesis that a first-order adsorption on HSGZ surface is followed by the herbicide degradation, catalytically driven by HSGZ surface groups. The long-term use of the HSGZ catalyst was assessed by repeated-batch tests. The specific cost for unit-volume removal of herbicide was evaluated by a detailed cost analysis showing that it is comparable with those pertaining to alternative methods

Remediation of Waters Contaminated with MCPA by the Yeasts <i>Lipomyces starkeyi</i> Entrapped in a Sol−Gel Zirconia Matrix

A single-stage sol−gel route was set to entrap yeast cells of Lipomyces starkeyi in a zirconia (ZrO2) matrix, and the remediation ability of the resulting catalyst toward a phenoxy acid herbicide, 4-chloro-2-methylphenoxyacetic acid (MCPA), was studied. It was found that the experimental procedure allowed a high dispersion of the microorganisms into the zirconia gel matrix; the ZrO2 matrix exhibited a significant sorption capacity of the herbicide, and the entrapped cells showed a degradative activity toward MCPA. The combination of these effects leads to a nearly total removal efficiency (>97%) of the herbicide at 30 °C within 1 h incubation time from a solution containing a very high concentration of MCPA (200 mg L−1). On the basis of the experimental evidence, a removal mechanism was proposed involving in the first step the sorption of the herbicide molecules on the ZrO2 matrix, followed by the microbial degradation operated by the entrapped yeasts, the metabolic activity of which appear enhanced under the microenvironmental conditions established within the zirconia matrix. Repeated batch tests of sorption/degradation of entrapped Lipomyces showed that the removal efficiency retained almost the same value of 97.3% after 3 batch tests, with only a subsequent slight decrease, probably due to the progressive saturation of the zirconia matrix

Hybrid Hemp Particles as Functional Fillers for the Manufacturing of Hydrophobic and Anti-icing Epoxy Composite Coatings

The development of hydrophobic composite coatings is of great interest for several applications in the aerospace industry. Functionalized microparticles can be obtained from waste fabrics and employed as fillers to prepare sustainable hydrophobic epoxy-based coatings. Following a waste-to-wealth approach, a novel hydrophobic epoxy-based composite including hemp microparticles (HMPs) functionalized with waterglass solution, 3-aminopropyl triethoxysilane, polypropylene-graft-maleic anhydride, and either hexadecyltrimethoxysilane or 1H,1H,2H,2H-perfluorooctyltriethoxysilane is presented. The resulting epoxy coatings based on hydrophobic HMPs were cast on aeronautical carbon fiber-reinforced panels to improve their anti-icing performance. Wettability and anti-icing behavior of the prepared composites were investigated at 25 °C and −30 °C (complete icing time), respectively. Samples cast with the composite coating can achieve up to 30 °C higher water contact angle and doubled icing time than aeronautical panels treated with unfilled epoxy resin. A low content (2 wt %) of tailored HMPs causes an increase of ∼26% in the glass transition temperature of the coatings compared to pristine resin, confirming the good interaction between the hemp filler and epoxy matrix at the interphase. Finally, atomic force microscopy reveals that the HMPs can induce the formation of a hierarchical structure on the surface of casted panels. This rough morphology, combined with the silane activity, allows the preparation of aeronautical substrates with enhanced hydrophobicity, anti-icing capability, and thermal stability

Use of a New Hybrid Sol–Gel Zirconia Matrix in the Removal of the Herbicide MCPA: A Sorption/Degradation Process

A class II hybrid sol–gel material was prepared starting from zirconium­(IV) propoxide and 2,4-pentanedione and its catalytic activity in the removal of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) was revealed. The thermal and structural characterization, performed by thermogravimetry, differential thermal analysis, and diffuse reflectance Fourier transform infrared spectroscopy, demonstrated the hybrid nature of the material. The structure of the material can be described as a polymeric network of zirconium oxo clusters, on the surface of which large part of Zr⁴⁺ ions are involved in strong complexation equilibria with acetylacetonate (<i>acac)</i> ligands. The incubation of MCPA in the presence of this material yielded an herbicide removal fraction up to 98%. A two-step mechanism was proposed for the MCPA removal, in which a reversible first-order adsorption of the herbicide is followed by its catalytic degradation. The nature of the products of the MCPA catalytic degradation as well as the reaction conditions adopted do not support typical oxidation pathways involving radicals, suggesting the existence of a different mechanism in which the Zr⁴⁺:<i>acac</i> enol-type complex can act as Lewis acid catalyst

In Situ P‑Modified Hybrid Silica–Epoxy Nanocomposites via a Green Hydrolytic Sol–Gel Route for Flame-Retardant Applications

Flame retardance of epoxy resins is usually imparted using suitable additives and/or properly modified curing agents. Herein, via a two-step green synthetic procedure, the chemical modification of the epoxy matrix with reactive silicon and phosphorus precursors is explored to obtain nanocomposites with intrinsic flame-retardant features. Nanoscale phase separation occurs in the first step, forming an inverse micelle system in which polar nanodomains act as nanoreactors for the hydrolysis of silanes (Si precursors), giving rise to silica lamellar nanocrystals (SLNCs). In the second step, inside the silica nanodomains, the formation of stable Si–O–P bonds occurs because the reactivity of phosphoric acid (P precursor) with the oxirane rings of the polymer chain is balanced by its tendency to diffuse into polar nanodomains. Intriguingly, the use of phosphoric acid alone in epoxy composite manufacturing leads to a wormlike morphology of the network, whereas its addition in the presence of silanes results in the formation of SLNCs with a thinner interlayer distance. The morphology of the hybrid Si/P–epoxy nanocomposites, comprising organic and inorganic co-continuous phases, can confer, through a prevalent mechanism in the condensed phase, interesting flame-retardant performances, namely, the absence of dripping during vertical burning tests, the formation of a large amount of coherent char after combustion, and a remarkable reduction (up to 27.7%) in the peak of heat release rate. The above characteristics make these nanostructured hybrid materials very promising for the manufacturing of epoxy systems with enhanced fire behavior (e.g., coatings, sealants, matrices for reinforced composites), even containing a low amount of specific flame retardants and thus keeping good viscoelastic properties