Photoelectrochemical response of TiO2/ZnO2 nanotubes annealed under diferent atmospheres [Dataset]

The influence of three annealing atmospheres (air, nitrogen and argon) and the use of controlled hydrodynamic conditions (from 0 to 5000 rpm) on morphological, structural, chemical and photoelectrochemical properties of TiO2 nanotubes have been evaluated. For this purpose, different characterization techniques have been used: Raman Confocal Laser Spectroscopy, X-Ray Photoelectron Spectroscopy, Mott-Schottky analysis and photoelectrochemical water splitting tests. According to the results, it can be concluded that both hydrodynamic conditions and annealing in non-oxidizing atmospheres improve the photoelectrochemical response of the TiO2 nanotubes. This fact has been attributed to the oxygen vacancies formed after annealing in argon and nitrogen atmospheres and also to the presence of nitrogen into the TiO2 lattice due to the thermal treatment in the nitrogen atmosphere.

Description of methods used for collection/generation of data: 

- Synthesis of TiO2 nanotubes by anodization

Anodization of titanium was carried out at room temperature. For
anodization, the titanium sample was the anode and a platinum foil
was used as the cathode of the process. The anodization electrolyte
was an ethylene glycol based solution with 0.05 M of NH4F and 1 M
of H2O. The titanium sample was connected to a Rotating Disk Electrode
(RDE) to control the hydrodynamic conditions during anodization.
Different rotation speeds were applied: 0 (stagnant), 2500 and
5000 rpm. A multimeter in series was also connected to register the
anodization current density during the process. A potential of 55 V
was applied for 30 min. After anodization, the titanium sample was
rinsed with distilled water and ethanol and then dried with air. Finally,
samples were cut in a slice of approximately 0.5 cm.
In order to transform the amorphous TiO2 anodized nanostructures,
a heat treatment was carried out. In this way, samples were annealed
at 450 °C for 1 h to obtain anatase crystalline TiO2 nanostructures. Different
atmospheres were used (air, argon and nitrogen) to evaluate the
influence of the atmosphere during the heat treatment. For annealing
in argon and nitrogen atmospheres, samples were introduced in a
tubular oven and the gas was bubbled for 30 min prior the heat treatment,
in order to remove the air contained in the tube.

- Structural and compositional characterisation of the nanostructures:

Raman Spectroscopy measurements were carried out with a Confocal
Laser microscope with Raman spectroscopy (WITec). In order to
determine the crystalline structure of the samples they were illuminated
with a 633 nm (red laser) using 420 μW.

X-Ray Photoelectron Spectroscopy spectra were collected using Al-
K monochromatized radiation (1486.6 eV) at 3 mA×12 kV. The scanning
step energies were 200 eV to measure the whole energy band and
50 eV to selectively measure elements.

- Electrochemical characterisation of the nanostructures:

Mott-Schottky plots under dark conditions (in the absence of illumination)
for the samples anodized under hydrodynamic conditions
(at 5000 rpm) and annealed in air, argon and nitrogen atmospheres
were obtained applying an initial potential of 0.5 VAg/AgCl and sweeping
the potential from that value to −0.4 VAg/AgCl at a frequency of
5 kHz. The amplitude of the signal was 10 mV.

Water splitting measurements were performed to evaluate which of
the TiO2 nanostructures (obtained under different hydrodynamic conditions
and annealing atmospheres) presented the best photocurrent
response. For this purpose, TiO2 nanostructures were used as photoanodes
in the photoelectrochemical water splitting with sunlight. Photoelectrochemical
water splitting tests were carried out in a three
electrode electrochemical cell connected to a potentiostat, with a
1 M KOH solution as electrolyte. The working electrode was the
TiO2 nanostructure with an exposed area of 0.26 cm2, an Ag/AgCl
(3 M KCl) was the reference electrode and a platinum foil was the
counter electrode. A potential scan from − 0.8 VAg/AgCl to 0.5 VAg/AgCl
with a scan rate of 2 mV s−1 was performed and the photocurrent density
values generated were recorded by chopped light irradiation (60 s
in the dark and 20 s in the light).

Stability measurements were used to evaluate the resistance of
TiO2 nanostructures to photocorrosion. In these tests, a potential of
0.5 VAg/AgCl was applied under light irradiation for one hour.