VOLUME 100, NUMBER 20, MAY 16, 1996
© Copyright 1996 by the American Chemical Society
LETTERS Photocapacitance of Nanocrystalline Oxide Semiconductor Films: Band-Edge Movement in Mesoporous TiO2 Electrodes during UV Illumination Anders Hagfeldt,† Ulrika Bjo1 rkste´ n, and Michael Gra1 tzel* Institut de Chimie Physique, Ecole Polytechnique Fe´ de´ rale de Lausanne, CH-1015 Lausanne, Switzerland ReceiVed: July 6, 1995; In Final Form: January 9, 1996X
Impedance measurements of mesoporous TiO2 electrodes in contact with aqueous and nonaqueous electrolytes have been performed in the dark and under UV illumination. In the dark the differential capacitance was found to be dominated by the back contact. Bandgap excitation of the TiO2 film turned on a capacitance associated with the mesoporous semiconductor layer. This is interpreted in terms of trapping of photogenerated holes in surface states leading to unpinning of the energy bands. The unpinning of the energy bands during UV illumination was confirmed by a dye-sensitization experiment.
Introduction Mesoporous oxide semiconductor films1,2 have become the focus of many recent investigations. Research and development of these systems is expanding rapidly, and one expects to find a variety of important applications. Thus, dye-sensitized mesoporous TiO2 electrodes have shown strikingly high photovoltaic conversion efficiencies.3-7 Solar cell efficiencies up to 10% at simulated solar intensity (AM 1.5, 96.4 mW/cm2) have been obtained showing good stability.4 Efficient lithium intercalation by these electrodes has been exploited for use in batteries8,9 and electrochromic devices.10-12 The application of mesoporous TiO2 electrodes for photocatalysis has also been studied.13 The mesoporous oxide semiconductor films are constituted by nanometer sized TiO2 (anatase) particles which are electrically interconnected by sintering. The films are distinguished by a very high internal surface area. They are normally prepared by sol-gel procedures. A porous network is constituted by the
voids present between the semiconductor particles. These are interconnected and are filled with an electrolyte. The behavior of nanocrystalline films1,2,14,15 is often strikingly different from that of compact semiconductor electrodes. Fundamental questions concern the mechanism of charge percolation through the mesoporous semiconductor film and the electric potential distribution in these systems.2 Photoeffects observed with these electrodes may be rationalized in terms of an array of weakly coupled colloidal particles. In this model light induced charge carrier separation is mainly controlled by kinetics at the semiconductor/electrolyte interface rather than by an electric field associated with a space charge layer. The present study concerns impedance measurements in the dark and under illumination of mesoporous TiO2 electrodes. These experiments have given information on the electronic properties of the films in the dark and during illumination. The interpretation of these results was confirmed by a dye-sensitization experiment. Experimental Section
†
Present address: Department of Physical Chemistry, Box 532, S-751 21 Uppsala, Sweden. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)01856-9 CCC: $12.00
Nanocrystalline TiO2 film electrodes were prepared as previously described.3,4 Briefly, a paste of 15 nm sized (diameter) colloidal TiO2 particles autoclaved at 200 °C was © 1996 American Chemical Society
8046 J. Phys. Chem., Vol. 100, No. 20, 1996 spread on a conducting back-contact consisting of a titanium sheet in contrast to previous work where transparent conducting glass was employed. After air drying, the electrode was fired for 30 min at 450 °C in air. The resulting film thickness was 3.5-4 µm. X-ray diffraction measurements showed that the colloidal TiO2 film had 100% anatase crystal structure. SEM studies showed the film to be porous from the outer layers to the back contact. The roughness factor, defined as the ratio between the real and projected surface of the film, was determined to be 800 by adsorption of cis-bis(thiocyanato)bis(2,2-bipyridyl-4,4′-dicarboxylate)ruthenium(II) dye.4 An autolab (Eco Chimie) electrochemical analyzer was used to perform the electrochemical measurements. The TiO2 electrode formed the working electrode (area 1-2 cm2) of a three-electrode cell with a Pt-grid counter electrode and a standard calomel electrode (SCE) as reference. All potentials refer to SCE unless otherwise indicated. An Ag/AgCl (aqueous saturated KCl in water) electrode was used as a reference for experiments with acetonitrile as a solvent. This reference electrode was separated from the working electrode compartment by a bridge containing the same electrolyte as the test solution, i.e., 0.1 M NaClO4 in acetonitrile. Before the measurements, the electrolytes were purged with Ar for at least 10 min to remove dissolved oxygen. All measurements were performed at room temperature. The experimental procedure for a given electrode and electrolyte started by performing two cyclic voltammograms. The cyclic voltammetry was performed between a potential close to the potential of the conduction band edge, Vcb, and a potential significantly positive of Vcb, removing, e.g., O2 on the TiO2 surface. The subsequent impedance measurements were carried out by applying the starting potential for 10 min and then performing a potential scan with a potential step of 0.1 V and an equilibration time of 30 s at each potential. Using a longer equilibration time (60 s) did not significantly change the capacitance values. There were very small variations in the measured capacitance values dependent on the potential scan direction. The amplitude of the ac sinusoidal wave was 10 mV, the integration time 10 or 15 s and the frequency 1 or 100 Hz. The concentration of the supporting electrolyte was 0.1 M. The dependence of differential capacitance values on salt concentration and the size of the cations has been discussed by Kay et al.16 Photocurrent action spectra with and without background UV irradiation were performed on a dye-sensitized nanocrystalline TiO2 electrode. An Oriel xenon lamp (450 W) equipped with a water filter and a UG1 bandpass filter was used for the steadystate background UV illumination. The light intensity was 35 W/m2. The photocurrent action spectra were measured with a halogen lamp as a light source in conjunction with a 420 nm cutoff filter. The halogen lamp was chopped and the chopper signal was fed to the reference channel of an EG&G 5206 twophase lockin analyzer. The output from the potentiostat (Pine Instruments RDE3) was connected to the signal channel so that the in-phase photocurrent could be detected. An Oriel Model 77250 was used as a monochromator. The light power was measured with a YSI-Kettering 65A radiometer. The photoelectrochemical experiments employed the dye sensitized TiO2 film incorporated into a thin-layer sandwich-type cell. A platinized TCO glass was used as a counter electrode. The counter electrode was placed directly on top of the dye coated TiO2 film, and both electrodes were clamped tightly together. A thin layer of electrolyte (0.1 M NaClO4, 0.1 M LiI, and 0.1 M NaOH in ethanol) was attracted into the inter electrode space by capillary suction. The dye coated TiO2 film was illuminated
Letters
Figure 1. C-2-V diagram of a nanocrystalline TiO2 electrode (squares), heat-treated Ti plate (circles), and a Ti plate (crosses). The heat treatment was performed together with the sintering of the nanocrystalline film. The frequency was 1 Hz. The electrolyte was 0.1 M CsNO3 and CsOH in water, pH ) 11.7. Electrode area 2.1 cm2.
through the conducting glass support. The conversion efficiencies (the incident monochromatic photon-to-current conversion efficiency (IPCE)) reported are overall yields which are uncorrected for losses due to light absorption and reflection by the conducting glass support. The sensitizer used was the ruthenium complex [Ru(II)LL′(NCS)], where L ) 4,4′-COOH-2,2′biquinoline, and L′ ) bis(2,6-bis(1′-methylbenzimidazol-2′-yl)pyridine.17 The complex has a reduction potential of about -0.8 V vs SCE and an absorption maximum at 574 nm. Coating of the TiO2 surface with the dye was carried out by soaking the film overnight in a saturated solution of the Ru complex in dry ethanol and 10% DMSO. Th dye coating was done immediately after annealing of the electrode at 450 °C for 30 min. Results and Discussion The mesoporous TiO2 anatase films examined in the present study were deposited on titanium sheets acting as conducting support. Since the films are annealed in air at 450 °C, a thin oxide layer is formed between the Ti sheet and the TiO2 nanoparticles, avoiding direct exposure of the metal to the electrolyte. With conducting glass as a support, such a contact is unavoidable due to the porous nature of the nanocrystalline TiO2 films.18 In the present system, the contact electrode and the mesoporous film consist of the same material, i.e., TiO2 anatase. The compact film of TiO2 formed on the titanium metal is likely to be n-doped by diffusion of low-valency titanium ions into the oxide,19 while the particle film is nearly intrinsic. Since both are in series connected, the capacitance of the whole film under reverse bias is expected to be determined by that of the compact part alone where a depletion layer is formed.20 No such space charge layer can be produced in the mesoporous films due to the small size of the TiO2 particles and their insulating character.1,21,22 It is therefore expected that the capacitance response of the film does not reflect the impedance of the interconnected particles but rather that of the blocking underlayer of TiO2 which exhibits the lowest capacitance under depletion conditions. The predictions are borne out by the experiments reported in Figure 1 showing the dark capacitance of the nanocrystalline TiO2 film supported on the titanium sheet together with blank measurements on the sheet itself. At the surface of a titanium a passive oxide layer will always be present, and this grows further under heat treatment. Since the colloidal TiO2 film deposited on the Ti plate is sintered at 400-500 °C, the relevant
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J. Phys. Chem., Vol. 100, No. 20, 1996 8047
Figure 3. (a) Photocurrent action spectra for nanocrystalline TiO2 films coated with a RuLL′(NCS) dye, where L ) 4,4′-carboxy-2,2′biquinoline and L′ ) 2,6-bis(1-methylbenzimidazol-2′-yl pyridine.17 The measurements were performed with (filled circles) and without (squares) background UV irradiation from a Xe lamp. The incident photon-tocurrent conversion efficiency is plotted as a function of excitation wavelength. The electrolyte was 0.1 M NaClO4, 0.1 M LiI, and 0.1 M NaOH in ethanol. Electrode area 1 cm2. The cell was operated in the short-circuit mode. (b) Absorption spectrum of the RuLL′(NCS) dye-coated TiO2 film.
Figure 2. C -V diagram of the nanocrystalline TiO2 electrode in dark (circles) and under illumination (squares). The electrolytes were (a) 0.1 M CsNO3 and CsOH in water, pH ) 11.7, (b) 0.1 M NaClO4 in ethanol and (c) 0.1 M NaClO4 in acetonitrile with Ag/AgCl (saturated KCl in water) as the reference electrode. The frequency was 1 Hz. Electrode area 2.1 cm2. -2
blank experiment is the one where the Ti plate is heat treated for the same time and under the same conditions as the sintered colloidal film electrode. The results are presented in the conventional Mott-Schottky diagram where 1/C2 is plotted versus the applied potential. The observed differential capacitance increases only slightly by depositing the colloidal TiO2 film on the Ti plate. Hence the measured capacitance is dominated by the back contact, in agreement with the expectations. The effect of bandgap excitation of the mesoporous TiO2 electrode on its capacitance is displayed in Figure 2. The effect of UV illumination is to shift the capacitance response in the
Mott-Schottky representation by 0.5 V anodically. The increase in capacitance detected under illumination was not detected in the blank experiment with the back contact alone and hence is entirely due to the contribution of the nanocrystalline layer. A similar finding was first reported in 1980 for WS2 and MoS2 single-crystal electrodes23 and was attributed to the unpinning of bands during illumination. The origin of this effect can be multifold. In most cases it has been explained by trapping of minority carriers in surface states. Provided that a sufficient number of surface states is available, a considerable charge can be stored, leading to a change of the band position. An extended midbandgap state in nanocrystalline TiO2 films has been suggested from conductivity measurements by Ko¨nenkamp et al.22 In the mesoporous TiO2 electrode photogenerated holes could be trapped in these surface states, giving rise to a positive shift of Vcb under illumination. Apart from aqueous electrolytes the capacitance behavior was also examined in ethanol and acetonitrile and the results are shown in Figure 2. Both systems show an increase in
8048 J. Phys. Chem., Vol. 100, No. 20, 1996 capacitance under illumination. For ethanol- and acetonitrilebased electrolytes this increase in capacitance under illumination could also clearly be observed at a frequency of 100 Hz, whereas in water no effect was detected at the higher frequency indicating a lower rate of surface trap filling for the aqueous ambient. The sensitivity of the photo effect to the nature of the ambient confirms that the states involved in the capacitance charging of the film are at the surface and not in the interior of the particles.24 Blank experiments performed with annealed titanium plates in the three electrolytes did not show any significant effect of illumination. To further confirm the positive shift of Vcb during UV irradiation, electron injection from a surface-adsorbed chargetransfer sensitizer into the TiO2 conduction band was investigated. The idea of this experiment was to employ such a type of dye whose standard redox potential in the excited state is located below the conduction band edge preventing electron injection to occur. If the bandgap excitation of the nanocrystalline film induces a positive shift of the band edge, the injection should be facilitated. Photocurrent action spectra of the RuLL′(NCS) dye adsorbed on the TiO2 surface were thus taken with and without background UV irradiation and the results can be seen in Figure 3a. The open squares indicate the photocurrents obtained in the absence of UV light bias, whereas the filled circles denote the resulting photocurrents under bandgap excitation of the TiO2. For comparison, the absorption spectrum of the dye-coated electrode is also presented in Figure 3b. The expected trend is clearly observed. Without the UV light bias a small cathodic photocurrent is obtained. A striking switch to the anodic direction is induced by UV-background illumination. In both cases the photocurrent action spectra resemble the absorption spectrum of the RuLL′(NCS) dye adsorbed to the TiO2 surface. A blank experiment with and without UV background illumination conducted on a nonsensitized TiO2 electrode gave no photocurrent response due to visible illumination. These results support the interpretation that the increase in capacitance under illumination is due to the trapping of positive minority carriers at the semiconductor surface leading to a shift in the band edges. This displacement assists greatly the charge injection process which manifests itself in the appearance of the anodic photocurrent.25 Conclusion The differential capacitance of mesoporous TiO2 films was found to be determined in the dark by that of the support. During illumination of the film a capacitance in the nanocrystalline semiconducting layer was switched on. This is attributed to trapping of photogenerated holes in surface states leading to unpinning of the energy bands. The unpinning of the energy bands during UV illumination was confirmed by a
Letters dye-sensitization experiment. A Ru complex adsorbed on the TiO2 surface which normally, after excitation by visible light, does not inject electrons into the TiO2 conduction band due to a too positive reduction potential, was shown to give rise to a photoanodic current under UV light bias. Acknowledgment. This work was supported by a grant from the Swiss National Science Foundation. References and Notes (1) O’Regan, B.; Moser, J.; Anderson, M.; Gra¨tzel, M. J. Phys. Chem. 1990, 94, 8720. (2) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (3) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (4) Nazeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (5) Knoedler, R.; Sopka, J.; Harbach, F.; Gru¨nling, H. W. Sol. Energy Mater. Sol. Cells 1993, 30, 277. (6) Hagfeldt, A.; Didriksson, B.; Palmqvist, T.; Lindstro¨m, H.; So¨dergren, S.; Rensmo, H.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cells 1994, 31, 481. (7) Smestad, G. The Spectrum 1994, 7, 16. (8) Huang, S.; Kavan, L.; Kay, A.; Gra¨tzel, M. J. ActiVe PassiVe Electron. Comput. Comput. 1995, 18, 23. (9) Huang, S.; Kavan, L.; Exnar, I.; Gra¨tzel, M. J. Electrochem. Soc. 1995, 142, L142. (10) Hagfeldt, A.; Vlachopoulos, N.; Gra¨tzel, M. J. Electrochem. Soc. 1994, 141, L82. (11) Hagfeldt, A.; Vlachopoulos, N.; Gilbert, S.; Gra¨tzel, M. Proc. SPIEInt. Soc. Opt. Eng. (Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XIII) 1994, 2255, 297. (12) Marguerettaz, X.; O’Neill, R.; Fitzmaurice, D. J. Am. Chem. Soc. 1994, 116, 2629. (13) Vinodgopal, K.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97, 9040. (14) Hagfeldt, A.; Bjo¨rkste´n, U.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cells 1992, 27, 293. (15) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. Soc. 1992, 139, 3136. (16) Kay, A.; Humphry-Baker, R.; Gra¨tzel, M. J. Phys. Chem. 1994, 98, 952. (17) Synthesized by Dr. M. Nazeeruddin at our institute. (18) Kavan, L.; O’Regan, B.; Kay, A.; Gra¨tzel, M. J. Electroanal. Chem. 1993, 346, 291. (19) Augustynski, J. Struct. Bonding 1988, 69, 3. (20) Blum, O.; Ko¨nig, U. Appl. Surf. Sci. 1995, 86, 417. (21) Ko¨nenkamp, R.; Henninger, R.; Hoyer, P. J. Phys. Chem. 1993, 97, 7328. (22) Ko¨nenkamp, R.; Henninger, R. Appl. Phys. A 1994, 58, 87. (23) (a) Memming, R., Kelly, J. J., Connoly, Ed. Photochemical ConVersion and Storage of Solar Energy; Academic Press: New York, 1980, p 243. (b) Kelly, J. J.; Memming, R. J. Electrochem. Soc. 1982, 129, 730. (24) Gerischer, H. J. Electrochem. Soc. 1966, 113, 1174. (25) One of the referees pointed out that the anodic photocurrent induced by visible light in the presence of sensitizer and under background UV illumination may be due to the reaction of the valence band holes with the excited dye. However this is unlikely to happen due to the presence of iodide in the electrolyte acting as efficient hole scavenger.
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