Navegando por Autor "Martins, Guilherme Mendes"
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Item Hydrothermal synthesis and polarized micro-Raman spectroscopy of copper molybdates.(2018) Martins, Guilherme Mendes; Coelho, Pâmela de Oliveira; Moreira, Roberto Luiz; Dias, AndersonConventional and microwave-assisted hydrothermal methods were used to synthesize Cu3(MoO4)2(OH)2 ceramics. Different experimental conditions of temperature, time and precipitation rate were employed to explore the growth mechanisms, structural and morphological properties of the obtained copper molybdates. The use of microwave-assisted reactors at 150 °C/10–30 min produced a mixture of copper molybdate hydrates, while only Cu3(MoO4)2(OH)2 was obtained by using microwave processing at longer times (above 120 min) or conventional hydrothermal reactors (110–250 °C/24 h). Experiments conducted after fast and slow coprecipitation rates resulted in different crystallographic phases and morphologies. Flower-like and rod-like micrometer-sized ceramics were produced with high anisotropy and single-crystalline nature. A better understanding on the growth mechanisms, as well as on the structural and morphological characteristics of copper molybdates was attained. Based upon the relative enhancement of Raman bands in parallel or in perpendicular configuration, a symmetry assignment of 34 (17Ag + 17Bg) external modes for Cu3(MoO4)2(OH)2 ceramics (42 bands are expected) was possible. Besides, five (of six possible) internal OH-stretching modes were identified in the spectral region above 3200 cm−1.Item Investigation of polymorphism and vibrational properties of MnMoO4 microcrystals prepared by a hydrothermal process.(2018) Martins, Guilherme Mendes; Coelho, Pâmela de Oliveira; Siqueira, Kisla Prislen Félix; Moreira, Roberto Luiz; Dias, AndersonMnMoO4 microcrystals were synthesized by hydrothermal methods, and their structural, morphological, and vibrational properties were investigated. Conventional reactors and microwave-heated hydrothermal vessels were used to synthesize micrometer-sized MnMoO4 polymorphs at different temperature and time conditions. MnMoO4·H2O crystals were obtained in both reactors at temperatures below 200 °C, for times up to 24 h. These microcrystals belong to the triclinic P1̅ space group, and their 26 characteristic Raman bands were identified. A monoclinic MnMoO4 polymorph belonging to the P2/c structure was synthesized at temperatures higher than 200 °C only into conventional hydrothermal reactors. Polarized micro-Raman analyses have evidenced and allowed the assignment of 18 phonon modes predicted by group-theory calculations for this polymorph. Another MnMoO4 polymorph, within the monoclinic C2/m space group, was obtained by two processing routes: (i) heating the P2/c microcrystals at 600 °C; or (ii) heating the MnMoO4·H2O phase at 250 °C. For this monoclinic phase, 33 Raman-active bands were identified and assigned, in very good agreement with group-theoretical calculations, which predict 36 modes for the C2/m polymorph. Well-faceted, highly crystalline microcrystals were clearly observed by scanning and transmission electron microscopies, in perfect agreement with X-ray diffraction and Raman spectroscopic analysis. Finally, the appearance of characteristic phonon modes related to molybdenum in octahedral coordination suggests an incipient crystallization of a new, unreported α-MnMoO4 polymorph, at least in a short-range degree.Item Síntese e caracterização de molibdatos de metais de transição.(2017) Martins, Guilherme Mendes; Dias, Anderson; Dias, Anderson; Moreira, Roberto Luiz; Siqueira, Kisla Prislen FélixNeste trabalho, foram investigados molibdatos de metais de transição (MnMoO4, CuMoO4, ZnMoO4, NiMoO4 e CoMoO4), hidroximolibdatos e compostos tipo Natrocalcita. Os processos de síntese utilizados foram a hidrotérmica convencional e a hidrotérmica assistida por micro-ondas. No reator micro-ondas, foi fixada a temperatura em 150ºC e os tempos variaram de 10 a 120 minutos. As condições dos experimentos realizados no reator Parr foram temperaturas entre 110ºC e 250ºC e tempo fixo de 24 horas. De acordo com os resultados obtidos, realizaram-se procedimentos adicionais para alguns conjuntos de materiais, tais como tratamentos térmicos, aumento do tempo de preparo no reator Parr e sínteses com controle da velocidade da reação. Foram obtidas fases puras e cristalinas, além de materiais contendo mistura de fases, comprovadas pelas técnicas de difração de raios X e espectroscopia Raman. Os compostos TMoO4 (T = Mn, Co, Ni), em sua forma anidra, exibiram estruturas monoclínicas (C2/m e P2/c); já em sua forma hidratada, a estrutura observada foi a triclínica (P-1). Os hidroximolibdatos produzidos exibiram em um caso a estrutura ortorrômbica (Pnnm) e em sua maioria eram fases monoclínicas (P21/n). No caso das fases tipo Natrocalcita sintetizadas, estruturas monoclínicas (C2/m) foram observadas. Realizaram-se, também, medidas de espectroscopia Raman polarizado para algumas amostras monoclínicas, podendo-se assim identificar e atribuir seus modos de vibração ativos em Raman com o auxílio da teoria de grupos. A diferença das bandas Raman observadas em função da coordenação (octaédrica ou tetraédrica) do átomo de molibdênio na estrutura TMoO4 foi evidenciada; dessa forma, foi possível relatar a existência de uma fase de MnMoO4 ainda não reportada pela literatura. Através das medidas de microscopia eletrônica (varredura e de transmissão), pode-se obter informações sobre a morfologia e estrutura dos compostos estudados. Foram observados materiais com características monocristalinas e partículas nanoestruturadas com diferentes morfologias.