Abstract: Protonated titanate nanotubes (HTiNts) were synthesized using the hydrothermal method, followed by proton exchange with acid. Gold nanoparticles decorated titanate nanotubes (AuHTiNts) were obtained by the reduction of AuCl4- under vigorous stirring with an aqueous solution of HTiNts containing 1 wt% of ascorbic acid. To investigate the surface structural chemistry of the titanate and AuHTiNts, the following characterization methods were used: scanning and transmission electron microscopy, X-ray diffraction, UV-Vis diffuse reflectance spectroscopy, Raman spectroscopy and X-ray photo-electron spectroscopy. The mean internal and external diameters for titanate nanotubes were found to be 5.46 ± 0.08 nm and 8.42 ± 0.03 nm, respectively, whereas the mean diameter of gold nanoparticles was measured to be 9.68 nm ± 0.03 nm. The as-synthesized AuHTiNts was used as photocatalyst in hydrogen production from glycerol as a sacrificial agent. The enhancement in the production of hydrogen, using the heterogeneous AuHTiNts catalysts, can be attributed to the intrinsic catalytic potential of gold as well as its interactions with titanate nanostructure.

Neste trabalho, nanopartículas de dióxido de titânio (TiO2) cristalino, com um tamanho de cristalito médio (e.g., 8 nm) e área de superfície (e.g., 192 m² g-1) variáveis, foram sintetizadas em fase anatase pura, utilizando H2O2 como agente de redução da taxa de hidrólise dos íons de titânio, em solução de isopropanol (IP) como meio de reação. As nanopartículas de TiO2 foram caracterizadas por meio das técnicas de difratometria de raios X de pó (XRD), espectroscopia Raman e microscopia eletrônica de transmissão (TEM). A alteração dos parâmetros de síntese permitiu o controle do tamanho das nanopartículas e evitou o processo de coalescência. Foi determinada uma dependência entre o número de onda Raman e o tamanho dos nanocristais, o que é bastante útil para uma rápida verificação do tamanho de nanocristais de TiO2.

In this work, crystalline titanium dioxide (TiO2) nanoparticles with variable average crystallite sizes (e.g., 8 nm) and surface areas (e.g., 192 m² g-1) were synthesized in pure anatase phase using H2O2 to reduce the hydrolysis rate of the titanium ions. An isopropanol (IP) solution was employed as the reaction medium. The TiO2 nanoparticles were characterized by powder X-ray diffraction analysis (XRD), Raman spectroscopy and transmission electron microscopy (TEM). By changing the synthesis parameters it was possible to control nanoparticle size and avoid the coalescence process. A dependence of the Raman wavenumber on the nanocrystal sizes was determined, which is quite useful for a quick check of the size of TiO2 nanocrystals.

Este trabalho relata o estudo das propriedades estruturais, morfológicas e vibracionais dos nanotubos e das nanofitas de titanato obtidos a partir de estruturas bidimensionais por meio de tratamento hidrotérmico do TiO2 em solução aquosa de NaOH. As propriedades físicas destas nanoestruturas, como preparadas e tratadas termicamente, são discutidas e comparadas com seus similares "bulks" (Na2Ti3O7 e Na2Ti6O13). Os resultados obtidos por várias técnicas de caracterização nos permitem concluir que as paredes dos nanotubos e das nanofitas, como preparados, são isoestruturais ao composto Na2Ti3O7. Contudo, nos nanotubos as ligações químicas são deformadas por causa da curvatura das paredes, enquanto nas nanofitas as camadas apresentam apenas uma desordem estrutural provocada pelo efeito de tamanho. As características térmicas das nanofitas são similares àquelas observadas para os nanotubos, nos quais as mudanças estruturais e morfológicas levam para a formação de grandes bastões (bulks) com uma mistura de fase Na2Ti3O7 e Na2Ti6O13. Concluímos que os nanotubos e as nanofitas de titanato têm a mesma composição química, igual a Na2-xHxTi3O7·nH2O (0 < x < 2). Também podemos sugerir que a espectroscopia Raman pode ser usada para uma fácil e rápida identificação das mudanças morfológicas e estruturais das nanoestruturas de titanato.

This work reports the structural, morphological and vibrational properties of titanate nanotubes and nanoribbons obtained from bidimensional structures by hydrothermal treatment of TiO2 in aqueous NaOH solutions. The physical properties of these as-synthesized and heat-treated nanostructures are discussed in comparison with their bulk (Na2Ti3O7 and Na2Ti6O13) counterparts. The results obtained from several characterization techniques allowed to conclude that the layers of both as-synthesized nanotubes and nanoribbons are isostructural to the Na2Ti3O7 compound. However, in the nanotubes, the chemical bonds are deformed because of the curvature of walls while in the nanoribbons the layers present structural disorder due only the size effects. The thermal behavior of titanate nanoribbons is similar to those observed for titanate nanotubes. When thermally treated titanate nanoribbons change to bulk with a phase mixing of Na2Ti3O7 and Na2Ti6O13. We conclude in this work that the chemical composition of both the titanate nanotubes and the nanoribbons is the same, Na2-xHxTi3O7·nH2O (0 < x < 2) and Raman spectroscopy can be used for an easy and quick identification of both morphology and structure changes of the nanosized titanates.