The preparation and study of supported TiO 2 for photocatalytic application in solar cell devices is a relevant research field. Thin films of TiO 2 prepared on Ti by thermal oxidation in a wide range of temperatures (450 °C - 900 °C) were characterized by electrochemical impedance spectroscopy, potentiometry and amperometry. This material presents photoelectrochemical activity, which depends dramatically of the oxidation temperature and the exposition time at the studied temperatures. The flatband potential as well as the donor density and the resistance to the charge transfer were measured. All these parameters are temperature dependent, and the optimal values are observed on the photoelectrodes prepared at 750 °C. This result is consistent with the photochemical response reported in the literature for thin films of Ti/TiO 2 prepared under similar conditions.
Ti/TiO2 photoelectrode becomes one of the relevant materials in studies related to the application towards clean energy using the solar radiation [
When clean metallic Ti is exposed to air, a very thin film of TiO2 grows up on the surface. If this system is heated in air atmosphere, the oxide films increases but crystallographic morphological changes take place on the film [
It has been found that current measured in chronoamperometric experiments increases when the material is exposed to visible light. We have found that the thermal treatment of the material leads to a larger photocurrent output when films are prepared at the temperature of 750˚C [
Electrochemical impedance spectroscopy (EIS) allows us to get information on the number of charge carriers, the flatband potential and the resistance to the charge transfer [
The flatband potential is related to the space charge layer which is generated when the electrode is dipped into the solution. An electric field arises between the semiconductor surface and the solution. If the electric field increases, the recombination hole-electron in the semiconductor film decreases [
We are interested in material with higher yields in connection with the conversion of solar light in chemical energy. In order to accomplish with this target, we expect to prepare photoelectrodes presenting a small resistance to charge transfer, a larger number of charge carriers and a big flatband potential should be expected.
The main objective of this study was the characterization of TiO2 films grown on Ti surface after thermal treatment at different temperatures with the help of the electrochemical impedance spectroscopy and the comparison of the results with other studies performed in our laboratory with the help of amperometry and potentiometry methods as reported previously [
A set of 11 plates of Ti (Johnson Mattey, 99.7%) of approximately 100 mm2 were employed in all experimental conditions. The plates were polished till the surface becomes mirror-like using a 1 µm and 0.3 µm particle size alumina suspension (Büehler). These plates were heated in an oven in atmosphere of air at a rate of 20˚C∙min−1. As soon the desired temperature was reached, the plates were taken out of the oven. Four of these plates were heated till 450˚C, 600˚C, 750˚C and 900˚C and taken out of the oven when the temperature reached the desired value. These temperatures will be referred as quenching temperatures. Another set of three plates were allowed to be heated 30 minutes at 450˚C, 600˚C and 750˚C respectively. Finally, a third set was 3 plates were heated 60 minutes at 450˚C, 600˚C and 750˚C. A reference plate remains without thermal treatment.
The reference and the 10 plates, once cooled, were covered by an epoxy resin leaving a section of 64 mm2 for photo-electrochemical experiments.
The cell consisting of three electrodes, the photoelectrode as a working electrode (WE) with Pt electrodes symmetrically arranged as counter electrodes and a reference of Hg/HgO were employed in all measurements.
All experiments were carried out at 25˚C. A 6 M KOH (p.a.) aqueous solution was the electrochemical media. Measurements under light irradiation were carried out employing a dichroic lamp of 250 W (Zurich MR16) settled at 16 cm of the photoelectrode surface.
The equivalent circuit shown in
The Rct values were obtained from each photoelectrode taking at the repose potential an impedance spectrum in the range 100 kHz to 1 MHz. These results were fitting using a standard soft for this sort of calculations.
The analysis of the Mott-Schottky plot [
In Equation (1), e0 is the elemental charge, Nd is the donor density, ε is the dielectric constant of TiO2, ε0 is the permittivity in the vacuum, V is the applied potential, Vfb is the flatband potential, k is the Boltzmann constant and T is the temperature of the solution.
A linear dependence between
The Csc is measured as the imaginary component (Im) in impedance measurements at the frequency f (in Hertz) using an electrode of area A, as given by Equation (2),
Spectrochemical impedance measurements were carried out employing an impedance analyzer Zahner IM6. An external overvoltage of 5 mV was chosen and applied in all experiments performed with this equipment.
The measurements were carried out in short circuited conditions irradiating the photoelectrode against an electrode of Pt. Both electrodes were dipped in the electrolyte solution. The photocurrent was measured with a microamperimeter Kipp and Zoner (AL4 model).
A similar set of experiments were carried out in open circuit conditions with the photoelectrodes in darkness and under irradiation. These experiments were used for characterizing the photoelectrodes and for measuring the photoactivity of the materials under studies. These potential difference against the Hg/HgO electrode was recorded with a potentiostat PAR model 263 A.
The potential of the photoelectrodes in darkness and under irradiation as well as the output photocurrent is listed in
As seen in this Table, excluding the reference, the response of the photoelectrodes depend on the quenching temperature and the time of heating of the material at a given temperature. Even though the behavior is strongly dependent how the material was handled (temperature quenching and time of heating at a given temperature during 30 and 60 minutes), the best photochemical answer is observed in the material prepared at 750˚C. This behavior is consistent with previous results obtained in our laboratory [
An interesting behavior can be observed in
Photoelectrode | Experimental conditions | |||
---|---|---|---|---|
Vca, dark (V) | Vca, light (V) | DV | Ilight (mA∙cm−2) | |
Reference | −1.056 | −1.056 | 0 | -- |
450˚C | −0.456 | −0.605 | 0.149 | 5.94 |
450.30 | −0.444 | −0.668 | 0.224 | 6.72 |
450.60 | −0.566 | −0.661 | 0.095 | 7.50 |
600˚C | −0.312 | −0.547 | 0.235 | 7.97 |
600.30 | −0.116 | −0.626 | 0.510 | 12.19 |
600.60 | −0.527 | −0.636 | 0.109 | 19.06 |
750˚C | +0.137 | −0.663 | 0.800 | 41.56 |
750.30 | +0.097 | −0.667 | 0.764 | 13.13 |
750.60 | +0.070 | −0.662 | 0.732 | 10.63 |
900˚C | −0.068 | −0.622 | 0.554 | -- |
Moreover, the largest output photocurrent is obtained from the material prepared at 750˚C. Except for the reference and the photoelectrode prepared at 900˚C; these results are reproducible, even though the measurements were carried out several days after their preparation.
The behavior of these magnitudes show that the photoelectrode quenched at 750˚C presents the optimal photocurrent. These results can be correlated with the optimal thickness and the ratio of the anatase/rutile crystalline forms on the films as seen by X-ray diffraction and SEM studies reported in earlier studies [
A small current intensity (1 mA) is recorded in the blank and the photoelectrode prepared at 900˚C. However, these values decays to negligible values (<1 µA) in a few minutes. Some instability are observed which can be associated to morphological changes on the electrodes, i.e., if the mirror-like Ti electrode is dipped alone into the solution, there are visual changes in the reflectance of the surface, but when is connected to the Pt counter electrode, the visual appearance of the surface changes, and a dark-grayish tonality is developed. This behavior is not observed on the other photoelectrodes, which show stability and reproducibility as reported in the literature [
The thickness or amount of the TiO2 on the surfaces should increase regularly with the time of heating leading to a better performance of the photoelectrochemical behavior. At 750˚C, the photocurrent is the highest observed in our experiments, but it decreases by a factor 3 to 4 with the heating time of this photoelectrode.
Nyquist diagrams obtained on the photoelectrodes prepared at 750˚C are shown in
Nyquist diagrams shown dramatic differences when the measurements are carried out in darkness and under illumination. A significant decreasing the charge transfer resistance in the illuminated photoelectrode is recorded, as consequence of the catalytic effect of the TiO2 under irradiation.
In
Photoelectrode | Experimental results in darkness and under irradiation | |||||
---|---|---|---|---|---|---|
Rct,dark. (KW∙cm2) | n,dark | Rct,light (KW∙cm2) | n,light | |||
Reference | 2.88 | 0.951 | 0.734 | 0.939 | ||
450 | 450.16 | 0.964 | 83.731 | 0.961 | ||
450.30 | 915.20 | 0.981 | 57.315 | 0.971 | ||
450.60 | 324.04 | 0.962 | 26.378 | 0.952 | ||
600 | 875.65 | 0.890 | 41.286 | 0.913 | ||
600.30 | 502.40 | 0.962 | 28.206 | 0.934 | ||
600.60 | 406.33 | 0.957 | 23.739 | 0.943 | ||
750 | 2.14 | 0.976 | 0.463 | 0.936 | ||
750.30 | 0.986 | 0.953 | 0.703 | 0.959 | ||
750.60 | 1.04 | 0.970 | 0.762 | 0.961 | ||
As seen in
Plots of
The slopes and the intercepts of MS plots are also temperature dependent as seen in these Figures. New sets of photoelectrodes were prepared to perform these experiments as described in section 2.1.
As usual, experiments were performed in a wide range of frequencies, but those showing a reasonable electrochemical behavior were selected as relevant in the construction of MS diagrams.
Experimental results are resumed in
The flatband potentials do not show a significant difference below 750˚C, but at this temperature a much higher value is measured.
The information presented in
Vint is the potential in the erfase between the support and the TiO2. A rough estimation can be obtained is this value is assumed smaller than Vint, as observed on other systems [
The simplified expression leads to an approximate linear dependence between w and
This proportionality between these variables allows us to estimate the dielectric constant of the film, assuming that the natural thickness of the film is of the order 10 - 20 nm. Therefore, taking a value of 15 nm, an applying the Equation (3) assuming that
Sample | Experimental results and estimated thickness of the SCL | |||||
---|---|---|---|---|---|---|
Nd (cm−3) | Increase of TiO2 mass (%) from TG (ref. 5) | w/nm | ||||
Reference (5011.9 Hz) | 3.02 × 1020 | 1.07 | 0 | 15 | ||
450˚C (31,623 Hz) | 1.04 × 1020 | 1.10 | 0.0675 | 45 | ||
600˚C (63,096 Hz) | 1.04 × 1018 | 1.09 | 0.0862 | 160 | ||
750˚C (100,000 Hz) | 8.67 × 1017 | 1.80 | 0.2339 | 226 | ||
be seen that the donor density decreases with the thickness as observed in by R. van de Krol et al., [
Studies performed on films of anatase on indium-tin oxide showed that the donor density decreases with the thickness. Values of 2.9 × 1019, 7.9 × 1018 and 4.1 × 1016 cm−3 have been evaluated for thickness of 40, 80 and 120 nm respectively (See reference [
Photoelectrodes of Ti/TiO2 prepared by thermal treatment at different temperatures and at different time of heating (450˚C, 600˚C, 750˚C and 900˚C) show different behavior in darkness and under light illumination. The photoactivity of the semiconductor measured by the charge transfer resistance, the flatband potential and the number of charge carriers are also temperature dependent as well as the temperature and the heating time at a given temperature.
Under the present experimental conditions, the electrodes prepared at 750˚C showed the best photoresponse under irradiation, which could be applied in the design photoelectrochemical devices in PEC.
The authors thank the support by grants from the National Agency of Science and Technology (ANPCyT-PICT 01482/06), CONICET (PIP 157/08) and UNLP (11-X486) as well as to Ing. L. Gassa for employing of the equipments in her laboratory, Dr. Ricardo Tucceri for the critical comments on the manuscript and Lic Dario Scolari for his assistance on the computational software. MP is member of Comisión de Investigaciones de la Provincia de Buenos Aires and ALC is member of CONICET.
Marcos M.Pedemonte,Alberto L.Capparelli, (2015) Physicochemical Characterization of Photoelectrodes of Ti/TiO2 Prepared by Thermal Oxidation of Titanium. Energy and Power Engineering,07,403-411. doi: 10.4236/epe.2015.79038