Espectroscopia de espalhamento Raman intensificado por superfície na detecção de poluentes orgânicos persistentes e no monitoramento da degradação fotocatalítica em nanotubos de titanato modificados com AgNP e CuxO

Abstract

This PhD thesis aimed to address the urgency for new technologies for treating effluents containing persistent organic pollutants (POP), such as dyes, pharmaceuticals, and pesticides, which resist conventional purification methods due to their high chemical stability besides its intrinsic toxicity. The relevance of this work lies in the development of plasmonic hybrid catalysts for photocatalysis and the central application of surface-enhanced Raman scattering (SERS) spectroscopy as a high-performance analytical tool. The methodology involved the hydrothermal synthesis of titanate nanotubes (TiNT) and their subsequent modification with silver nanoparticles (AgNP) and copper oxides (CuxONP) nanoparticles, resulting in hybrid catalysts involving semiconductor-semiconductor and semiconductor-metal heterojunctions. The synthesis stage involved the initial fabrication of anatase TiO2 with approximately 98% purity by titanium isopropoxide hydrolysis and calcination at 640 °C for 1 h, this precursor of TiNT being obtained via the hydrothermal method in a strongly alkaline medium. Spectroscopic characterization confirmed both the successful formation of TiNT and its modifications, either isolated or simultaneously with AgNP and CuxONP, thus constituting binary and ternary heterojunctions. Diffuse reflectance and photoluminescence spectroscopies demonstrated a reduction in the energy gap from 3.01 eV (λ ≈ 412 nm) for TiNT to 2.57 eV (λ ≈ 482 nm) upon simultaneous modification with both nanomaterials, a crucial outcome for its application in visible-light-driven photocatalysis. The use of SERS spectroscopy demonstrated its dual power, superior to conventional techniques, through (i) the sensitive detection of pure molecules at submilimolar levels, surpassing the capability of UV-VIS spectroscopy, and (ii) the detailed monitoring of degradation reactions, allowing for the structural characterization of by-products and the elucidation of reaction mechanisms. For the crystal violet dye, SERRS monitoring revealed N-demethylation processes and the cleavage of the chromophore system. In the case of the pharmaceutical penicillin G, the technique enabled the identification of the β-lactam ring opening and the hydrolysis of amides, even when UV-VIS spectroscopy proved inconclusive due to the overlapping of metabolite bands. For the pesticides tebuthiuron and imidacloprid, the technique allowed for the inference of adsorption geometries on the metallic surface and the identification of initial oxidation stages in the substituent groups. In summary, this work establishes SERS spectroscopy as an indispensable methodology for modern environmental chemistry, uniting sensitive detection, the characterization of complex mixtures, and refined mechanistic investigation of Advanced Oxidative Processes.