Department of Chemistry, University of Geneva
1211 Geneva 4, Switzerland.
Photoelectrochemical properties of mesoporous colloidal anatase
films continue to attract large attention in connection with their potential application in
liquid-junction photovoltaic cells and/or in the field of water and air decontamination.
The current understanding of the charge transport phenomena occurring in
nano-structured semiconductor films filled with an electrolyte is largely influenced by
the photoelectrochemical characteristics of particulate CdS and CdSe films reported
by Hodes et al. [ 1]. However, while the so-called " electron
diffusion model " [ 2] apparently accounts correctly for the observed
behavior of the CdS and CdSe networks, it is clearly inadequate to describe the
essential features of nanosized anatase TiO2 films.
Careful measurements, performed using the anatase films of various
thicknesses subjected to the bandgap illumination in the presence of a number of
redox couples in the solution, showed that none of the predictions of the electron
diffusion model is, in fact, fulfilled by this system [ 3].
While, according to the latter model, the quantum efficiency of the photocurrent is
expected to decrease with increasing film thickness - exceeding the penetration depth
of the incident light, concomitant with a red shift of the maximum photoresponse, the
actual spectral photoresponse of the nanostructured anatase films is observed to
exhibit a maximum at ca. 300 nm, irrespective the film thickness, with the
photocurrents increasing in parallel with the film thickness till ca 10 m m. In other terms, the quantum yield reaches a
maximum under conditions where the film thickness exceeds by two-three orders of
magnitude the optical penetration depth.
In addition, even very thick (> 40
m m) anatase films, in contact with
solutions containing efficient hole scavengers (such as organic acids, alcohols etc.)
still exhibit excellent photocurrent-voltage characteristics indicative of small resistance
losses.
To explain the peculiar behavior of nanostructured anatase
TiO2 films, both under the bandgap UV illumination and in the
dye-sensitized configuration, under white light illumination, the self-doping, occurring
at the initial stages of the photocurrent flow across the film, is proposed. The transient
charging of the film continues till the critical electron concentration in the donor level
of anatase (ca 1019 cm-3) - allowing the
insulator-metal (Mott) transition [4] - is reached. This transition is accompanied by an
important increase in the electrical conductivity of anatase. The proposed mechanism
of self-doping constitutes an unique feature of nano-porous semiconductor networks,
penetrated by the electrolyte, which offer the possibility of excess charge
compensation by an adjustment of the cation concentration in the Helmhotz layer.
Due to their high surface to volume ratio, the increase of the surface concentration of
cations required to compensate a 1019 cm-3 excess
of electrons within the nanoparticles remains quite small.
Such an explanation is consistent with the observation that the
photocurrent-voltage characteristics of the nanostructured anatase electrodes are
practically unaffected by the thickness of the films being employed. High quantum
yields of the photocurrent observed for the films illuminated with short wavelengths of
the incident light (characterized by large absorption coefficients) support equally the
view that the charge recombination takes place principally within and close to the
illuminated portion of the film
References
| (1) | G. Hodes, I.D.J. Howell, L.M. Peter. J. Electrochem. Soc. 139: 3136-3140 (1992). |
| (2) | S. Södergren, A. Hagfeldt, J. Olsson, S.E. Lindquist. J. Phys. Chem. 98: 5552-5556 (1994). |
| (3) | A. Wahl and J. Augustynski, J.Phys.Chem.B, 102: 7820 (1998). |
| (4) | H. Tang, K. Prasad, R. Sanjinés, P.E. Schmid, F. Lévy. J.Appl. Phys. 75: 2042-2047, (1994). |
Acknowledgment.
This work was supported by the Swiss National Science Foundation.