Role of hydrogen in the electronic transport through passivating TiO2 films

0360-3199/92 $5.00 + 0.00 Pergamon Press Ltd. © 1992 International Associationfor Hydrogen Energy.

htt. J. Hydrogen Energy, Vol. 17, No. 3, pp. 205-209, 1992.

Printed in Great Britain.

ROLE OF HYDROGEN IN THE ELECTRONIC TRANSPORT THROUGH PASSIVATING TiO2 FILMS SU-IL PYUN and CHANG-HA KIM Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, #373-1 Kusong Dong, Daejon, Korea (Received for publication 10 December 1991)

Abstract--The role of hydrogen in determining electronic transport in passivating TiO2 films is investigated by using a.c. impedance spectroscopy in a 0.1 M NaOH solution. The passive film was prepared on titanium galvanostatically with l0 mAcm 2 at a formation potential of 50 V (SCE) in a 0.5 M H2SO4 solution. Hydrogen was injected into the fresh passivating TiO2 film by scanning the applied potential range from -1.7 to -1.5 V(SCE) and back at a rate of 2 mV s - i in a 0. l M NaOH solution. Hydrogen injection into the TiO2 film increases a.c. conductivity as well as donor concentration. The experimental results suggest that hydrogen donates an electron to the conduction band inside the passive film. The frequency dependence ofa.c, conductivity is discussed in terms of electronic hopping between deep donor levels.

1. INTRODUCTION Passivating TiO2 films have been one of the more commonly used semiconductive electrodes to attract interest in recent years. Particularly, the TiO2 films have been studied extensively as a photoanode for the photoelectrolysis of water [ 1, 2]. Hydrogen doping was electrolytically conducted into the TiO2 film to improve the overall efficiency of the photoanode. It has been observed that the donor concentration as well as the space charge capacitance of the TiO2 films largely increase as hydrogen is injected into the films [3, 4]. Also, it has been reported that hydrogen injected into the TiO2 films markedly increases the photocurrent of the films [5]. However, the same authors [ 3 - 5 ] did not consider the influence of hydrogen on the electronic transport in the passivating TiO2 film. The aim of this paper is to obtain an understanding of the role of hydrogen in determining the electronic transport through the passivating TiO2 film. For this purpose, the donor concentration was determined from the M o t t Schottky relationship and the a.c. conductivity was measured as a function of frequency. 2. EXPERIMENTAL Specimen preparation and impedance measurement were carried out at 300 K in a fiat cell (EG&G Model KO 235) which had an exposed surface area of 1 cm 2. A platinum foil and a saturated calomel electrode were used as the counter electrode and reference electrode, respectively. A passivated titanium electrode used as a working electrode was prepared from titanium foil of 99.99% purity (Alfa products). The titanium specimen was etched in a 1 : 4 : 5 mixture of HF (48%), HNO3 (65%) and distilled

water. Anodically passivating TiO2 films were prepared galvanostatically with a current density of 10 mA cm -2 until the formation potential, 50 V(SCE), was reached. An aqueous 0.5 M H2504 solution used as eleectrolyte was previously deaerated by bubbling with purified nitrogen for 24 h. Film thickness was measured to be 106 nm with an ellipsometer (Gaertner Scientific Corp.) at a beam wavelength of 632 nm. The crystal structure of the TiO2 film was examined by using X-ray diffraction (Rigaku DMax III). Hydrogen injection was performed into the fresh passivating TiO2 films by scanning the potential range from - 1 . 7 to - 1 . 5 V(SCE) and back at a rate of 2 mV s -~. The electrolyte used was a 0.1 M NaOH solution deaerated by bubbling with purified nitrogen for 24 h. Impedance measurement of the TiO2 film was conducted in an aqueous 0.1 M NaOH solution with a two-phase lockin amplifier (EG&G Model 5208) and a potentiostat (EG&G Model 273) by superimposing an a.c. voltage of 5 mV amplitude ranging from 10 -2 to 104 HZ on a d.c. potential. The d.c. potential ranged between - 1 and 3 V(SCE). The NaOH solution used as electrolyte was previously deaerated by bubbling with purified nitrogen for 24 h. A microcomputer was used to control the lock-in amplifier and the potentiostat, and to analyse the measured data. The a.c.-conductivity was determined from the measured a.c. impedance. At the flatband potential, the space charge region has been compressed to zero thickness. The capacitance is then associated only with the surface charge distribution and the resistance is a measure of the resistivity of the passive film [6]. The a.c. conductivity, o, of the film is given by R = d/(aA)

205

(1)

206

SU-IL PYUN and CHANG-HA KIM

where R is the resistance of the passive film at the flatband potential, d is the film thickness and A is the area of specimen. 3. RESULTS Figure 1 shows the X-ray diffraction pattern obtained from the fresh TiO2 film on the titanium substrate. Beside the (002) and (011) peaks of the titanium substrate, the (004) and (112) peaks of anatase structure and the (200) and (111) peaks of rutile structure are simultaneously observed from the passivating TiO2 film. The TiO2 film is actually composed of crystallized modifications of anatase and

60

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0 38.0

38.5

39.0

39.5

Diffroction Angle

40.0

/

40.5

41.0

degree

rutile. This coincides with the experimental results reported previously [7]. Figure 2 shows the Nyquist plot obtained from the hydrogen-injected Tie2 film at frequencies ranging from l0 2 to l04 Hz. The Nyquist plot involves simply a semicircle, and not the Warburg impedance, which represents a kind of resistance to proton diffusion, indicating that electrochemical reaction inside the film is mainly due to electron motion. Figure 3 shows the space charge capacitance ,(Csc) of the fresh Tie2 film and of the hydrogen-injected Tie2 film as a function of the applied potential. The space charge capacitance of the passive films decreases with increasing applied potential until a constant value is reached in the potential range higher than 2 V(SCE), and the space charge capacitance is increased by hydrogen-injection into the film in the total potential range. The result indicates that the passive film formed on titanium behaves as an n-type semiconducting film. The nonlinear Mott-Schottky plots of the fresh and hydrogen-injected films are shown in Fig. 4. A linear relationship between Cs~2 and V is found to hold only in the potential range from Cs~2 = 0 to the inflection point and the instantaneous slopes of the Mott-Schottky plot decrease gradually with increasing applied potential in the potential range more positive than that at the inflection point irrespective of hydrogen injection. Since defect-doped Tie2 has a relative dielectric constant, K, ranging from 100 to 1000 [8], the value of K should be first determined before donor concentration, Nd,

Fig. 1. X-ray diffraction patterns obtained from the passivating TiO2 film anodically formed at a potential of 50 V(SCE) in the 0.5 M H2SO4 solution.

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Fig. 2. Nyquist plot obtained from the hydrogen-injected passivating Tie2 film at frequencies ranging from 10- z to l0 4 Hz. The fresh passive film was anodically formed at a potential of 50 V(SCE) in the 0.5 M H2SO4 solution. Hydrogen injection into the fresh film was electrolytically performed in the 0.1 M NaOH solution.

0

|

I

|

I

0

1

2

3

Applied Potential / VscE

Fig. 3. Applied potential dependence of the space charge capacitance for the fresh and hydrogen-injected passivating Tie2 films at 103 Hz. The fresh passive film was anodically formed at a potential of 50 V(SCE) in the 0.5 M H2SO4 solution. Hydrogen injection into the fresh film was elcctrolytically performed in the 0.1 M NaOH solution.

TRANSPORT THROUGH PASSIVATING TiO2 FILMS 600

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Fig. 4. Mott-Schottky plots obtained from the flesh and hydrogen-injected passivating TiO2 films at 103 Hz. The fresh passive film was anodically formed at a potential of 50 V(SCE) in the 0.5 M H2SO4 solution. Hydrogen injection into the fresh film was electrolytically performed in the 0.1 M NaOH solution.

Fig. 5. Frequency dependence of the relative dielectric constant for the fresh and hydrogen-injected passivating TiO2 films. The fresh passive film was anodically formed at a potential of 50 V(SCE) in the 0.5 M H2SO4 solution. Hydrogen injection into the fresh film was electrolytically performed in the 0.1 M NaOH solution.

is estimated. When the thickness of the space charge layer is equal to the film thickness for thin films, Csc becomes constant with respect to applied potential, V. In Fig. 3, Csc corresponds approximately to that measured at the applied potential of 3 V(SCE). Thus, regarding the film as a parallel plate capacitor and knowing the film thickness, d, K can be calculated by:

Schottky plot. As hydrogen is injected into the TiO2 film, the donor concentration increases over the total frequency range as shown in Fig. 6.

Csc = (KeoA)/d

(2)

where Csc is the space charge capacitance of the passivating TiO2 film, K is the relative dielectric constant of the passivating TiO2 film, and t0 is the permittivity of free space. Figure 5 shows the frequency dependence of the relative dielectric constant for the fresh and hydrogeninjected TiO2 films. The value of K is higher for the hydrogen-injected film than that for the fresh film. From the Mott-Schottky plot, the value of the flatband potential, Vfb, and the donor concentration, Nd, can be calculated by using the Mott-Schottky relationship [ 1 , 2 , 9 , 10]. Csc2 = [2/(qNdKeoA2)] ( V - Vro - k T / q )

? E o I

E) " .~,

o

•

Fresh

•

Hydrogen-Injected Film

E ¢O- j

(3)

where q is the electronic charge, V is the applied potential, Vro is the flatband potential, k is the Boltzmann constant and T is the absolute temperature. The flatband potential is estimated to be - 0 . 8 3 V(SCE) for the fresh film and - 0 . 8 0 V ( S C E ) for the hydrogen-injected film from extrapolation of Cs~2 to Cs~z = 0. We obtained the donor concentration in the TiO2 film from equation (3) by taking value of the slope of the linear portion from the M o t t -

Film

0 10

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I

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i

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,

i

,

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Fig. 6. Frequency dependence of the donor concentration for the fresh and hydrogen-injected passivating TiO2 films. The fresh passive film was anodically formed at a potential of 50 V(SCE) in the 0.5 M H2SO4 solution. Hydrogen injection into the fresh film was electrolytically performed in the 0. ! M NaOH solution.

208

SU-IL PYUN and CHANG-HA KIM ,

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Fig. 7. Frequency dependence of a.c. conductivityfor the fresh and hydrogen-injectedpassivating TiO2 films. The fresh passive film was anodicallyformed at a potentialof 50 V(SCE) in the 0.5 M H2SO4 solution. Hydrogen injection into the fresh film was electrolytically performed in the 0.1 M NaOH solution.

Information about the nature of the electronic conduction mechanism in semiconductors can be obtained from a.c. conductivity measurement. One presents the frequency dependence of a.c. conductivity in the form of [11-13] o = constant x cos

(4)

where ~0 is the angular frequency and the exponent, s, is defined as d(ln o)/d(ln co). In Fig. 7, a.c. conductivity is plotted against frequency for the fresh and hydrogen-injected passivating TiO2 films. The value of a.c. conductivity is enhanced by hydrogeninjection into the film. The increases in a.c.-conductivity of the hydrogen-injected film may be caused by the increment of the electron concentration due to the ionization of hydrogen. It is calculated from Fig. 7 that the values of s obtained from the fresh and hydrogen-injected films amount to 0.5 and 0.48, respectively. 4. DISCUSSION Song et al. [ 14] suggested from the analysis of permeation curves of hydrogen that protons move by hopping in the iron oxide film in the relatively lower frequency range of 10 -3 to 10 -I Hz. In the lower frequency range, the impedance of the hydrogen-injected iron oxide film is determined by the Warburg impedance. In the present work, we infer from Fig. 2 that a.c. impedance measured on the hydrogen-injected TiO2 film is caused by electron motion. Thus, a.c. conductivity represents electronic conductivity.

When electrons are excited from shallow donor levels into the conduction band and the flow in the conduction band, a.c. conductivity does not depend on frequency, at least in the frequency range up to 108 Hz. On the other hand, when electron conduction occurs by electronic hopping between trap sites of deep donor levels originating from inhomogeneous lattice defects, grain boundaries and dislocations, etc., one expects the a.c. conductivity to increase with frequency [ 15 ]. It is reported from the analysis of the electronic structure of bulk TiO2 surfaces by using a tight-binding extended Hiickel calculation [16] and from the photoelectrochemical measurements on iron-implanted TiO2 film [ 17] that the deep donor levels are located at about 1.0 eV below the conduction band. In the present work, the frequency dependence of a.c. conductivity of the fresh TiO2 film indicates electronic hopping between deep donor levels. This suggestion is in good agreement with the work of Kennedy and Frese [18]. They suggested deep donors exist from the analysis of the two slopes of the Mott-Schottky plot, and that electrons move by hopping between deep donors in polycrystalline Fe203. Intrinsic defects of the passivating TiO2 film such as oxygen vacancies and titanium ions are not themselves sufficient to explain the increases in the space charge capacitance and donor concentration in the hydrogeninjected film. These increases in the hydrogen-injected film indicate that, inside the film, hydrogen is ionized and simultaneously donates electron to the conduction band, H = H÷ + e .

(5)

From the increased electronic conductivity by hydrogen injection (Fig. 7), it is clear that inside the passive film hydrogen acts as not a deep donor but a shallow donor. The value of s for the fresh TiO2 film appears to be s~milar to that of the hydrogen-injected film as shown in Fig. 7, indicating that the electronic transport occurs mainly by hopping in both the fresh and hydrogen-injected TiO2 films. Hydrogen injected into the TiO2 film does not cause proton hopping and contributes favourably to electronic hopping. 5. CONCLUSIONS On the basis of the experimental results, we draw some conclusions on the role of hydrogen in the electronic transport through the passivating TiO2 film in a 0.1 M NaOH solution. (1) The electronic conductivity as well as the donor concentration are increased by hydrogen-injection into the TiO2 film, indicating that inside the passive film hydrogen acts as a shallow donor and exists as a proton, H ÷. (2) From the results that the values of s are approximately the same for both the fresh and hydrogen-injected TiO2 films, it is concluded that, inside passivating TiO2 films, electrons move by electronic hopping between the deep donor levels. Hydrogen injected into the film provides a shallow donor site to electronic transport and hence helps the electronic hopping.

TRANSPORT THROUGH PASSIVATING TiO2 FILMS Acknowledgements--The authors acknowledge the financial support of the Ministry of Science and Technology and Korea Advanced Institute of Science and Technology, Daejon, Korea. One of the authors (S.-I. P) thanks KOSEF and DFG for supporting his stay at Max-Planck Institut fiir Eisenforschung, Diisseldorf, Germany, for this work. REFERENCES 1. K. Leitner, J. W. Schultze, and U. Stimming, J. Electrochem. Soc. 133, 1561 (1986). 2. M. Nakao, R. Schumacher, and R. N. Schindler, J. Electrochem. Soc. 133, 2308 (1986). 3. C. K. Dyer and J. S. L. Leach, J. Electrochem. Soc. 125, 23 (1978). 4. C. K. Dyer and J. S. L. Leach, Electrochim. Acta 23, 1387 (1978). 5. M. F. Weber, L. C. Schumacher and M. J. Dignam, J. Electrochem. Soc. 129, 2022 (1982). 6. T. O. Rouse and J. L. Weininger, J. Electrochem. Soc. 113, 184 (1966).

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