https://repositorio.ufms.br/handle/123456789/3608 2026-05-30T01:34:19Z 2026-05-30T01:34:19Z https://repositorio.ufms.br/handle/123456789/14408 2026-05-28T17:27:45Z 2026-01-01T00:00:00Z

Título: Influência dos Parâmetros de Anodização no Crescimento de Nanotubos de TiO2 Visando Sua Aplicação na Fotodegradação de Contaminantes Orgânicos Abstract: Titanium dioxide (TiO2) is a semiconductor with a prominent position for various applications, mainly due to its chemical stability, non-toxicity, biocompatibility, and environmental friendliness. Among these applications, photocatalytic processes for the degradation of organic compounds and energy storage through the production of green hydrogen stand out. However, despite its promise, many research groups are still seeking to optimize its properties in order to enhance the photocatalytic properties of TiO2 and make its production method simple, inexpensive, and environmentally viable. In this sense, scaling up materials from bulk to nanostructures represents a significant increase in the photocatalytic efficiency of TiO2, as it provides more adsorption/reactive sites and transport pathways. Within this context, this work presents a systematic study of the growth mechanisms of titanium nanotubes (TNTs) using the anodic oxidation technique. For this purpose, metallic titanium (Ti) plates were used as anodes and a metallic copper (Cu) plate as cathodes. Ammonium fluoride (NH4F) solution was used as the electrolyte. Voltages of 10V, 20V, and 30V were used, and anodizing times of 30 min, 60 min, and 120 min were applied. During electrocorrosion, the electrolyte was kept under magnetic stirring and at room temperature by a water cooling system. The distance between the electrodes was kept fixed at 0.5 cm in all electrocorrosions. Once the process was finished, the samples were subjected to heat treatment in a tubular furnace for 2 hours at 450ºC, with a ramp of 5ºC/min, at ambient pressure. The properties of the non-woven fabrics (NTF) were studied by X-ray diffraction, Raman scattering, and Scanning Electron Microscopy. Finally, the photocatalytic properties of the TNTs were evaluated by photodegradation tests of methylene blue dye using a UV light source, which showed greater effectiveness in samples anodized for 60 min at 30 V. Tipo: Dissertação

2026-01-01T00:00:00Z https://repositorio.ufms.br/handle/123456789/14378 2026-05-13T14:55:38Z 2026-02-26T00:00:00Z

Título: Magnetic Properties of the frustrated J1-J2 square lattice with Kondo interactions Abstract: The magnetic and electronic properties of materials are governed by the complex interplay between localized exchange interactions and coupling with itinerant electrons. This work investigates the competition between magnetic frustration and the Kondo effect in a J1 − J2 square lattice. Magnetic frustration is introduced through competing nearest-neighbor (J1) and next-nearest-neighbor (J2) interactions. The Kondo effect, governed by the coupling JK, introduces a nonmagnetic screening mechanism via the formation of Kondo inglets. To solve the resulting Hamiltonian, we employ a rigorous Cluster Mean-Field (CMF) approximation coupled with exact diagonalization on a 4-site cluster. The methodology is validated by reproducing the known J1 − J2 phase diagram, which shows the suppression of the N´eel temperature (TN ) at the point of maximum frustration (J2/J1 ≈ 0.5). The results demonstrate that the introduction of JK further suppresses long-range magnetic order, promoting the formation of a Kondo singlet state, particularly in the highly frustrated regime. These findings characterize the quantum boundaries between magnetic and non-magnetic phases and provide a crucial foundation for mapping the full phase diagram across varying coupling strengths and electron densities. Tipo: Dissertação

2026-02-26T00:00:00Z https://repositorio.ufms.br/handle/123456789/14326 2026-03-03T21:24:20Z 2025-01-01T00:00:00Z

Título: SÍNTESE DE NANOPARTÍCULAS À BASE DE COBRE PARA APLICAÇÃO COMO ÂNODOS DE CÉLULAS A COMBUSTÍVEL MICROFLUÍDICAS IMPRESSAS EM 3D ALIMENTADAS POR METANOL Abstract: Technological advances have increased the demand for low-power energy converters to supply portable devices such as laptops, smartphones, and smartwatches. This demand arises from growing social automation – communication mediated by electronic devices – and the widespread use of portable systems in healthcare, defense, and entertainment. In this context, miniaturized fuel cells (µFCs) have emerged as promising alternatives. However, their commercial implementation remains limited by: (i) unstable catalysts with low electrocatalytic activity and (ii) architectures that restrict power output. This work aimed to synthesize new materials for application as anodes in microfluidic µFCs. A simple, rapid, small-scale, and surfactant-free route was proposed for the synthesis of Cu nanoparticles (NPs) at room temperature under three distinct conditions: stationary (ST), magnetic stirring (MS), and ultrasonic bath (US). The catalytic activity toward methanol electrooxidation in alkaline medium was evaluated using electrochemical and spectroelectrochemical techniques, revealing higher current densities for the US-synthesized NPs. In situ FTIR analyses confirmed that all materials promote methanol oxidation to carbonate via a formate intermediate, without detectable CO formation. The US sample exhibited superior performance, associated with a higher surface proportion of Cu(OH)2, lower carbon contamination, and greater availability of active sites. At higher potentials, this sample further displayed distinct behavior, involving an additional step of formate formation and its subsequent conversion to carbonate (or CO2 in the thin-layer configuration), suggesting a more efficient reaction mechanism. Finally, a mixed-media µFC was constructed and fed with methanol in alkaline medium and sodium persulfate in acidic medium. The device, comprising a Carbon Paper™ (CP) anode modified with US-synthesized Cu NPs and a metal-free CP cathode, achieved a maximum current density of 0.56 mA cm-2 and a peak power density of 26 µW cm-2 at 100 µL min-1. These results confirm that the membrane-free configuration optimizes the performance of each half-cell and demonstrate the feasibility of µFCs with noble-metal-free anodes and fully metal-free cathodes, representing a promising alternative for sustainable energy conversion technologies. Keywords: fuel cell; microfluidic; methanol electrooxidation; copper anode; metal-free cathode; in situ FTIR; energy conversion. Tipo: Tese

2025-01-01T00:00:00Z https://repositorio.ufms.br/handle/123456789/14197 2026-01-13T16:24:16Z 2025-01-01T00:00:00Z

Título: Correlação entre spins localizados e itinerantes com interações de troca Kondo e magnéticas em um cluster hexagonal (tipo Grafeno) Abstract: The study of magnetic properties in many-body systems remains a fundamental challenge in Condensed Matter Physics, mainly due to the competition between strong electronic correlations, collective quantum effects, and constraints imposed by spatial geometry. Simplified models, particularly finite spin arrangements, are widely used to investigate essential phenomena such as antiferromagnetic interactions, magnetic frustration, singlet formation, and the Kondo coupling between localized and itinerant moments. In this dissertation, we analyze a hexagonal cluster similar to that found in graphene composed of six sites, a structure that preserves relevant symmetries and enables a controlled examination of the influence of itinerant electrons on the behavior of localized spins. Each site hosts a fixed magnetic moment, while itinerant electrons move through the lattice, allowing a direct investigation of the competition between electronic obility and exchange interactions. The theoretical description is based on the Ising–Kondo and Heisenberg–Kondo Hamiltonians, which incorporate kinetic-energy terms, magnetic interactions between localized spins, and local Kondo coupling. The quantum basis of the model is constructed using a binary representation of states, and the numerical solution is obtained through exact diagonalization and the Lanczos method. These matrices are large and grow rapidly with the number of itinerant electrons, which motivates the use of high-performance computational resources such as CENAPAD (the National High-Performance Computing Center) at UNICAMP, allowing the calculation of the relevant magnetic correlations. The analysis focuses on the quantities D, F and S², which characterize, respectively, the correlation between localized spins, the correlation between localized and itinerant spins, and the total spin of the system. The results show that the hexagonal geometry plays a crucial role in shaping the distribution of correlations. Strong Kondo interactions promote the screening of localized spins, while increasing temperature tends to suppress magnetic correlations. Thus, the hexagonal cluster proves to be an effective platform for understanding, in a controlled manner, the competition between different magnetic mechanisms in correlated systems, providing a solid foundation for future studies involving other geometries and interaction regimes. Tipo: Dissertação

2025-01-01T00:00:00Z