Explorando nanomateriais : estudo experimental e teórico das propriedades do NiO e propriedades óticas de defeitos em h-BN

Abstract

The study of nanomaterials has become increasingly fascinating, as their size can generate unusual behavior, altering their properties. For this study, nickel oxide (NiO) and hexagonal boron nitride (h-BN) were chosen and it could be seen that both have adjustable electronic and optical properties, making them eligible for various applications, such as for example nanoelectronic devices. Regarding NiO, a systematic theoretical and experimental investigation of its properties was developed. NiO nanoparticles were synthesized by the coprecipitation method (Cp) and calcined at different temperatures (350◦C - 650◦C), allowing the formation of samples with different crystallite sizes (8.73 - 32.40 nm). To characterize the samples, Thermogravimetric analysis (TG/DTG), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FITR), Raman Spectroscopy, Visible-Ultraviolet and Near Infrared Spectroscopy (UV-Vis-NIR) and electrocatalytic performance for the Oxygen Evolution Reaction (OER). Furthermore, the optical results obtained by UV-Vis-NIR measurements were also simulated by Density Functional Theory (DFT) for a better understanding of the experimentally observed behavior. Regarding h-BN, several types of punctual defects were inserted in its crystalline structure, such as vacancies and/or antisites and substitutional impurities of a carbon atom and the electronic and optical properties were theoretically investigated using DFT. Both materials have their properties strongly related to their structure. NiO stood out for presenting changes in bandgap energy values as a function of crystallite size, excellent optical conductivity and electrocatalytic activity. In the case of h-BN, it started to behave like a semiconductor or a conductor depending on the presence of defects. All defects inserted in h-BN were associated with a source of single photon emission, exhibiting extra absorption of light in the infrared and visible regions. Photon emission by a two-dimensional material opens up new applications in quantum engineering. Finally, the theoretical study proved to be a powerful auxiliary tool to predict and/or better understand some behaviors of nanomaterials. Despite the computational limitations and the difficulty of describing exchange and correlation effects, such as that presented by nickel’s 3d orbitals, it was possible to obtain satisfactory results, within the same order of magnitude.