REPOSITORY > RESULTS

Abstract

Polycrystalline, BaTiO3 nanorods and SrTiO3 nanotubes were synthesized by the electrophoretic
deposition (EPD) of a barium titanate sol and a strontium titanate sol into 2D ordered anodic
aluminium oxide templates (AAO) with a subsequent thermal treatment and template removal.
The BaTiO3 nanorods grown within the AAO template membranes had diameters ranging
from 150 to 200 nm, with an average length of 10-25 μm. The following processing parameters
were systematically varied in order to obtain BaTiO3 nanorods with the most uniform
morphology: applied potential, time of deposition, annealing temperature and annealing time.
The obtained BaTiO3 nanorods were always polycrystalline and were composed of wellcrystallized
xenomorphic pseudo-cubic BaTiO3 grains, ranging from 10 to 30 nm. Grain
boundaries between the BaTiO3 grains contained an approximately 1-nm-thick amorphous phase
with the composition matching BaTiO3. It was shown that the processed nanorods were
homogeneous with respect to their chemical composition and grain size along their longer axis.
In addition to cubic polymorph hexagonal BaTiO3 polymorph was observed as intergrowth of
more or less dense sequences of (111) twins. Its formation was triggered by the reduction of Ti4+
to Ti3+ as a consequence of highly reducing atmosphere during the annealing process. When
AAO templates containing the BT sol were annealed in an oxygen atmosphere the presence of
the hexagonal polymorph was significantly reduced.
Electrical measurements performed on single polycrystalline BaTiO3 nanorods showed
resistivity values between 10 and 100 ohm·cm, which is in a good agreement with typical
reported values for oxygen-deficient barium titanate. Single BaTiO3 nanorods were tested as
proof-of-concept humidity sensors. The measurements of the electrical resistivity of the single
BaTiO3 nanorods in a varying humidity environment showed a reproducible response, thus
demonstrating that single BaTiO3 nanorods can be integrated into complex circuit architectures
with the functional capacities of a humidity nanosensor.
Polycrystalline SrTiO3 nanotubes that were obtained by the electrophoretic deposition of
a ST sol into AAO membranes were dense and polycrystalline with outer diameters of
approximately 200 nm, inner diameters of approximately 180 nm and lengths ranging from a few
micrometres up to a few tens of micrometres.
A unique phenomenon of the self-ordering of SrTiO3 nano-cubes inside the AAO
template was observed that was not yet reported for any other material prepared by the EPD of
sols into templates and subsequent annealing. This unusual crystallization mechanism resulted in
the assembly of highly-organized SrTiO3 nanocrystals in a perfect cube-to-cube arrangement.
Namely, homogeneous nucleation inside the AAO pores produces a critical number of SrTiO3
crystallites, which leads to their self-organization as soon as the nanocrystals reach the sizes that
equal to the mean free distance between the nuclei. Due to steric constraints the crystals start to
rearrange in order to most efficiently fill the available surface on the pore walls. This process
leads to the formation of domains containing a large number of idiomorphic SrTiO3 nano-cubes
that are self-aligned into an almost perfect cube-on-cube and cube-to-wall registry, which cover
the walls. The described mechanism shows the ability of nanocrystals with well defined
morphologies to adopt to spatial constraints and self-organize into desired architectures. By
optimizing the processing parameters one should be able to control the rates of nucleation and
growth, the morphology dictated by the crystallography of the nanocrystal units, exploit the selforganization
mechanism to design layers of any desired material in a chosen crystallographic
orientation in virtually any geometrical setting.
The electric properties of single SrTiO3 nanotubes doped with Fe to modify the materials
conductivity were investigated by measuring their current-voltage (I-V) characteristics at room
temperature. The results showed ideal linear I-V characteristics with the electrical resistivity of a
single Fe-doped SrTiO3 nanotube of 35 ohm·cm. Additionally, a single Fe-doped SrTiO3
nanotube-based device was tested as a UV photodetector. A repeatable and reversible response
under the UV radiation was found with short response and recovery times, which indicates that
such prototype devices can be used as UV photodetectors.