Síntese rápida de nanobastões de pentóxido de nióbio – com e sem doping com nitrogênio - pelos métodos hidrotérmico simples e hidrotérmico assistido por micro-ondas, e sua caracterização para aplicações fotoeletroquímicas

Devido ao aumento de demanda energética, limitação dos recursos fósseis e aumento da consciência ambiental, tornou-se essencial buscar processos mais eficientes, diante deste cenário a proposta deste trabalho é a sintetização de nanoestruturas homogêneas de pentóxido de nióbio (Nb2O5) utilizando um método hidrotérmico simples e um método rápido hidrotérmico assistido por micro-ondas. Placas de nióbio foram usadas como matéria-prima sem a adição de HF corrosivo nem qualquer agente diretor. Realizou-se diferentes tratamentos térmicos para realização de dopagem com N2 e cristalização após o processo hidrotermal simples. A composição , morfologia das estruturas e resposta fotoeletroquímica foram determinadas por microscopia eletrônica de varredura (MEV), difração de raios-X (DRX) e espectroscopias UV-vis, Raman e voltametria linear. Os resultados mostram que a irradiação de micro-ondas reduziu o tempo de síntese de 48h para 1h e 2h. Imagens de MEV revelam a formação de matrizes de nanobastões estruturadas pelo método hidrotérmico assistido por micro-ondas a 200 ºC. O band gap dos nanobastões sugere uma mudança no comportamento do semicondutor. Esses resultados indicam que o método hidrotérmico assistido por micro-ondas é uma técnica rápida e barata para a produção de matrizes de nanobastões de Nb2O5 homogêneas para diferentes aplicações fotoeletrocatalíticas.Due to the increase in energy demand, limited fossil resources and increased environmental awareness, it has become essential to seek more efficient processes. simple and fast microwaveassisted hydrothermal method. Niobium plates were used as raw material without the addition of corrosive HF or any directing agent. Different heat treatments were carried out to perform N2 doping and crystallization after the simple hydrothermal process. The composition, morphology of structures and photoelectrochemical response were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and UV-vis, Raman and linear voltammetry spectroscopy. The results show that microwave irradiation reduced the synthesis time from 48h to 1h and 2h. SEM images reveal the formation of nanorod arrays structured by the microwaveassisted hydrothermal method at 200 ºC. The band gap of the nanorods suggests a change in the behavior of the semiconductor. These results indicate that the microwave-assisted hydrothermal method is a fast and inexpensive technique for producing homogeneous Nb2O5 nanorod arrays for different photoelectrocatalytic applications

Revealing the true impact of interstitial and substitutional nitrogen doping in TiO2 on photoelectrochemical applications

Application of photocatalysts that strongly absorb within the visible range is a common strategy to improve the efficiency of photoelectrochemical (PEC) systems; this may translate to high photocurrents, but it is not always the case. Here, we show that nitrogen doping enhances visible light absorption of TiO2; however, it does not necessarily result in improved PEC performance. Depending on the applied external potential, N-doping can improve, or degrade, PEC performance either under water oxidation conditions or via hole scavenging (Na2S/Na2SO3). In this work, we developed a holistic approach to evaluate the true impact of N doping in TiO2 on PEC performance. Interstitial and substitutional N doping are experimentally explored for the first time through a simple and novel PEC approach which complemented X-ray photoelectron analyses. Using this approach, we show that interstitial N doping of anatase TiO2 dominates up to 400 °C and substitutional doping up to ca. 600 °C, without rutile formation. This reveals that the bottleneck for doping higher N-concentrations in TiO2 is the direct transformation to thermodynamically favorable N-rich phases, such as TiN/Ti2N at 700 °C, inhibiting the formation of rutile phase. Transmission electron microscopy revealed that N doping proceeds mainly from the inner to the outer tube walls via nitridation and follows a preferential pathway from interstitial to substitutional doping. Direct PEC experimental evidence on visible light activation of N doped TiO2, and the location of interband states, showed acceptor levels of 1.0 eV for substitutional and 0.7 eV for interstitial doping above the TiO2 valence band maximum. In addition, due to O vacancies and Ti3+ species, donor levels below the conduction band minimum were also created. These levels act as trapping/recombination centers for charge carriers and, therefore, the gain in the visible range due to N doping does not translate to an improved PEC performance by these structural defects. Ultimately, we show that whilst there is a benefit of visible light absorption through N doping in TiO2, the PEC performance of the samples only surpasses pristine TiO2 at relatively high biasing (>0.3 V vs. Ag/AgCl)