Development of nanostructures in hydrothermally grown
TiO2
Authors: Lábár János , Ezzeldien Mohammed , Ebnalwaled Khaled , Németh Regina , Gubicza Jenő 
Published: December 2016

Development of nanostructure as a function of production temperature in hydrothermal processing of TiO
2
is reported here. Nanostructured TiO
2
was grown in autoclave at 3 different temperatures of 130, 170 and 200 °C. The samples were referred to as T‐130, T‐170 and T‐200 here. Powder of TiO
2
with the particle size of 100‐200 nm was mixed with NaOH and closed in an autoclave for 24 h at the respective temperatures. The resulting material was cooled to room temperature, washed in distilled water several times, followed by washing in diluted hydrochloric acid (pH=1.6) and again in distilled water until pH=7 was restored. The final product was dried a 400°C.

The structure of the as‐received powder samples was first examined by X‐ray diffraction (XRD). XRD suggested that all specimens are mixture of anatase and a monoclinic titanate phase, and the ratio of the two phases varied with changing the processing temperature. The analysis of the dependence of the width of the diffraction peaks as a function of the length of the scattering vector gave the average values of the crystallite size in the two phases, however provided no clue either about the shape of these crystallites or the separation of the phases between different morphological entities. Therefore, a detailed transmission electron microscopy (TEM) analysis was performed on the three samples.

Morphology and phase distribution were studied by two transmission electron microscopes after dispersing the “powder” sample in distilled water or alcohol and dropped on “Quantifoil” holey carbon support films. Bright field (BF) and dark field (DF) images together with selected area electron diffraction (SAED) patterns were recorded on imaging plates (IP) in a Philips CM‐20 TEM, operated at 200 kV and equipped with a Bruker X‐ray detector (EDS). Higher resolution images and SAED patterns were also recorded on a GATAN Orius CCD in a JEOL 3010 TEM operated at 300 kV. The EDS showed that the samples contained a few atomic percent Na. It was proved that the samples contain nanofibers and equiaxed nanoparticles. The SAED patterns were processed with the “ProcessDiffraction” program [1]. The measured 2D patterns were converted into an XRD‐like 1D intensity distribution by averaging over ellipses (that correct for the minor elliptical distortion caused by the lenses of the TEM). Additionally, maximal intensity values over the ellipses were also rendered to detect faint features, caused by the presence of a small number of diffraction spots, not forming a complete ring. The SAED patterns recorded from a collection of large number of nano‐components gave diffraction peaks at the same positions as XRD.

Individual fibers (Fig. 1) were examined by SAED (Fig.2) whenever they protruded from the bunch of fibers. For sample T‐130 these nanofibers look like nanotubes (Fig. 1). However the number of layers at the “wall” is occasionally different at the two sides of the same “tube”, which indicate that it might also be a rolled‐up sheet. All diffraction patterns contained well‐defined reciprocal lattice planes with less characteristic spots within the planes. None of them contained the diffraction spots characteristic of anatase (although the patterns from the bunch did contain them). DF images recorded by the anatase lines showed that this phase is always present in the small equiaxed nanoparticles (Fig. 3). High resolution (HRTEM) images recorded from sample T‐200 showed filled nanofibers in contrast to nanotubes (Fig. 4). They looked as covered with small particles. Sample T‐170 was between the previous two extremes, as expected.