Charge Carrier Transport in Nanostructured Anatase TiO2 Films

Publication Date (Web): September 10, 1998. Copyright ..... Design, synthesis and photoelectrochemical properties of hexagonal metallomacrocycles base...
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J. Phys. Chem. B 1998, 102, 7820-7828

Charge Carrier Transport in Nanostructured Anatase TiO2 Films Assisted by the Self-Doping of Nanoparticles Axel Wahl and Jan Augustynski* UniVersite´ de Gene` Ve, De´ partement de Chimie Mine´ rale, Analytique et Applique´ e, 1211 Gene` Ve 4, Switzerland ReceiVed: March 6, 1998; In Final Form: July 26, 1998

The photoelectrochemical behavior of nanostructured TiO2 (consisting mainly of anatase) films, with thicknesses ranging from ca. 0.6 to 45 µm, was examined under the band gap UV illumination. The shape of the spectral photoresponses exhibiting a maximum at 300 nm, irrespective the film thickness, and the excellent photocurrent-voltage characteristics of the thickest films are inconsistent with the previously proposed models aimed at describing electron transport in such nanoporous semiconductor networks. It was found that even electrophoretically deposited nanoparticulate films, simply dried at room temperature, behaved essentially as the high-temperature-annealed films. An explanation of the particular behavior of the nanostructured anatase films, in terms of the self-doping occurring at the initial stages of the photocurrent flow, is proposed. This initial film charging causes an insulator-metal (Mott) transition in a donor band of anatase, accompanied by a sharp rise in conductivity of the nanoparticles. Such a self-doping appears as a special feature of nanostructured semiconductor films filled with the electrolyte and characterized by large surface-area-tovolume ratios. This offers a convenient way of compensating the excess charge within the semiconductor by the adjustment of the ion concentration in the Helmholtz layer.

Introduction Several important advances in the field of (photo)electrochemical energy conversion and storage rely on the use of mesoporous nanocrystalline semiconductor films.1-6 The initial interest in the films of this kind, consisting of a network of colloidal semiconductor particles with diameters in the range of a few nanometers penetrated by the electrolyte, was associated with the investigation of the quantum size effect.7,8 These early studies demonstrated a striking difference between the behavior of solid-state and liquid electrolyte junctions involving such films,7 pointing at the essential role played by the species present in the solution. Recognizing that the small size of the particles forming the films, which are unable to sustain significant electric field in their bulk, impeded formation of an effective space charge layer, Hodes et al.7,8 suggested that the separation of charges occurred directly at the particle surface through the transfer to the solution species. Using similar arguments, So¨dergren et al.9 described the transport of charge carriers across the nanocrystalline films in terms of diffusion due to the gradient of electrochemical potential. In an attempt to increase the surface area of dye-sensitized electrodes over which the preadsorbed dye molecules may capture incident photons, O’Regan and Gra¨tzel1,10 employed colloidal anatase TiO2 films composed of a large number of interconnected nanoparticles (ca. 15 nm in diameter) stacked on a conducting glass support. Surprisingly enough, despite the low doping level and poor conductivity of the anatase particles, the solar cells based on relatively thick (10-15 µm) TiO2 films of this kind exhibit excellent photocurrent-voltage characteristics with fill factors exceeding 0.7.1 Various studies conducted with other dye-sensitized nanocrystalline semiconductor materials (SnO2, ZnO)11 indicated that the characteristics of those materials did not, for the moment, equal the excellent characteristics of the anatase-based cells. In this paper, we report on photoelectrochemical characteristics of nanostructured anatase and rutile TiO2 films under band

gap UV illumination. Photocurrent measurements performed as a function of the film thickness, the nature of the redox species present in the solution, the light wavelength, and the intensity contribute to illustrate the anomalous, complex behavior of such films. In particular, “the electron diffusion model” appears inadequate to describe the principal features of the nanostructured anatase films, and an alternative explanation is proposed. Experimental Section Preparation and Structural Characterization of TiO2 Films. Nanostructured TiO2 films were formed by attaching P25 ca. 75% anatase/25% rutile (Degussa), A-HRS anatase, and R-SM2 rutile (both from Tioxide) powders to the conducting glass support. The support consisted of an F-doped SnO2 film (0.5 µm thick) deposited on glass (Libbey Owens Ford, 10 Ω/square). A suspension of the TiO2 powder in DMF containing dissolved poly(vinylidenefluoride), PVDF (typical composition: 1 g P25 and 0.4 g PVDF per 20 cm3 DMF), was applied to the substrate, dried in air for 30 min at 25 °C and then for 40 min at 100 °C to evaporate the solvent, and finally annealed for 1 h at 450 °C.12 To avoid cracking of the films during firing, the thickness of the individual deposited layers did not exceed 1 µm. Thicker films were formed by multiple application (up to 50 times) and heat treatment of the standard ca. 1 µm thick layer. In Figure 1 are displayed typical SEM images of the nanostructured TiO2 films, obtained at 30 kV accelerating voltage using a Hitachi S-900 scanning electron microscope. Parts a and b of Figure 1 represent a top view, and Figure 1c shows a cross-sectional view of a ca. 0.6 µm thick film composed of P25 particles. Aggregates of partly fused particles are separated by nanopores of different sizes. The thickness of the TiO2 films was determined with a Tencor Alpha Step 200

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Figure 1. Scanning electron micrographs of a particulate TiO2 (P25) film showing surface view (a, b) and cross-sectional views (c, d) of a onelayer (c) and a three-layer (d) deposit. Magnification is the following: (a) 140000×; (b) 70000×; (c) 93333×; (d) 14000×.

profilometer. X-ray diffraction patterns of the P25 powder and the corresponding partly sintered films showed similar ratios of the peaks associated with anatase and rutile. Although previous X-ray photoelectron spectroscopic analyses of the sintered films, prepared according to the procedure described above, revealed only the usual amount of carbon contamination,12 we decided to repeat various series of measurements using films formed by electrophoretic deposition of P25 particles. The deposits were obtained from a suspension of 0.5 g of P25 powder in 10 cm3 of demineralized water. For example, a voltage of 4 V applied for 20 s between a SnO2/ glass cathode and a Pt counter electrode yielded a ca. 15 µm thick deposit. Freshly formed films were first dried and then heated for 1 h at 450 °C in air. Photoelectrochemical Measurements. Photoelectrochemical experiments involving variable light intensities were carried out using a Spectra Physics model 2025-04 argon-ion laser (334.0, 351.1, and 363.8 nm emission lines) equipped with a series of neutral density filters. The absolute intensity of the incident light was measured with a laser power/energy meter (model DG, Newport Instruments). The wavelength photoresponse (i.e., quantum efficiency of the photocurrent vs excitation wavelength) of the TiO2 electrodes was determined using a 500 W xenon lamp (Ushio UXL-

502HSO) set in an Oriel model 66021 housing and a Multispec 257 monochromator (Oriel) with a bandwidth of 4 nm. The absolute intensity of the incident light from the monochromator was measured with a model 730 A radiometer/photometer from Optronic Lab. The light power was approximately 660 µW cm-2 at λ ) 300 nm and 1680 µW cm-2 at λ ) 380 nm. The photoelectrochemical experiments were carried out in a two-compartment Teflon cell equipped with a quartz window by illuminating the TiO2 electrode from the side of the film/ solution interface (except for the measurements reported in Figure 10). The cell contained about 30 cm3 of carefully deaerated solution. The counter electrode consisted of a platinum sheet, and Hg2Cl2/Hg/Cl- was the reference electrode. All potentials are quoted with respect to the reversible hydrogen electrode (RHE) in the same solution. The solutions were prepared from reagent grade chemicals and twice-distilled water. The electrochemical equipment consisted of an Elpan potentiostat and an Elpan waveform generator controlled by dedicated microcomputer employing Keithley DAS 20 hardware. Results and Discussion A remarkable feature of the nanostructured anatase TiO2 photoelectrodes consists of an unprecedented sensitivity of the

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Figure 2. Photocurrent-potential curves for a TiO2 electrode consisting of three successive layers of P25 nanoparticles recorded in 0.1 M HClO4 (a), 0.1 M HClO4/0.1 M MeOH (b), and 0.1 M HClO4/0.1 M HCOOH (c) under the full output illumination of a 150 W xenon lamp.

delivered photocurrent to the nature of the hole scavenger present in the solution. Such a behavior is illustrated in Figure 2, showing photocurrent-voltage (iph-E) characteristics of a TiO2 film, composed of P25 nanoparticles, recorded in the background electrolyte (0.1 M HClO4) alone and with additions of methanol and formic acid. Although the saturation photocurrent attained in the presence of HCOOH in the solution is about 20 times larger than that corresponding to the photooxidation of water (curve a), it is still twice as large as the photocurrent observed in the solution containing CH3OH (curve b). Usually, an increase in the photocurrent was accompanied by a negative shift of its onset potential Eph. A similar shift of Eph has previously been observed for the bulk polycrystalline anatase photoelectrodes following addition to the solution of the species undergoing preferential photooxidation with respect to water.13,14 However, in the case of the bulk samples, the associated increase of the photocurrent (attributable essentially to the partial occurrence of the photocurrent doubling) does not exceed 20-30%. The unusually strong influence of the nature of the electron donor upon the amount of the photocurrent (observed till now only in the case of particulate anatase films)12,15 is in agreement with the suggestion of Hodes et al.8 following their study of colloidal CdS and CdSe films, indicating that “in such systems, charge separation occurs not by a space-charge layer but rather by differing rates of electron and hole transfer into solution”. This is, in fact, the case for formic acid and methanol undergoing rapid charge transfer at the anatase surface, leading, in both cases, to CO2 as a final product. Importantly, the intermediates HCOO• and CH3O• are oxidized even more easily than the starting reactants (both are able to inject electrons into the conduction band of TiO2) and the reduction of CO2 at the surface of TiO2 is a slow process. The situation is totally different for the photooxidation of water at anatase TiO2, producing in the first step the Tis-O• radical species acting as effective recombination centers for the conduction band electrons.14,16 It is to be noted that curve a in Figure 2 reflects the general behavior, at the particulate anatase films, of reversible redox couples where the initial hole transfer leads to the formation of an easily reducible species (which can be either the final product or an intermediate of the photooxidation reaction). In fact, we obtained photocurrent-voltage characteristics similar to curve a in Figure 2 in the case of photooxidation of both hydroquinone and iodide ions.

Wahl and Augustynski Although the highly selective behavior of the nanostructured anatase films toward the photooxidation of various species in the solution is essentially consistent with the model proposed by Hodes et al.,8 this is not the case for other features exhibited by these anatase samples. The measurements performed using colloidal CdS and CdSe films showed that their spectral responses were strongly affected by the thickness of the semiconductor layer. The general trend was toward the decrease of the observed quantum yield with increasing film thickness accompanied by a red shift of the maximum photoresponse.8 These observations were explained in terms of recombination losses at crystal boundaries occurring in the semiconductor films that are significantly thicker than the penetration depth of the incident light. The authors predicted also, in such a case, resistance losses leading to a deteriorated iph-E behavior. Assuming that the transport of the majority charge carriers in a nanostructured semiconductor film takes place via diffusion, So¨dergren et al.9 derived a relationship for the spectral response of the photocurrent as a function of the film thickness. According to this model, for the films with thicknesses d exceeding the diffusion length of electrons L, the quantum efficiency of the photocurrent should reach a maximum when

R ) 1/d

(1)

R being the optical absorption coefficient. Again, the maximum is expected to be shifted toward the band edge with the thickness of the anatase film increasing from ca. 1 to more than 40 µm. However, the results of our measurements carried out with nanostructured P25 films of various thicknesses (ranging from ca. 1 to 45 µm) apparently do not follow predictions of the above models. Figure 3A displays photon-to-current efficiency versus potential plots for a ca. 2.5 µm thick film illuminated with 300 and 380 nm wavelengths and obtained in 0.1 M aqueous HClO4 with and without addition of formic acid. Despite the fact that the 300 nm light is absorbed very close to the outer film/electrolyte interface, much farther from the back contact than the 380 nm light, the short-wavelength photocurrent yield is almost 3 times larger than that for the 380 nm illumination. The optical penetration depths (R-1) for 300 and 380 nm wavelengths in rutile, based on the absorption spectrum given by Eagles,17 are 10 nm and 1 µm, respectively. Taking into account that anatase films formed by sputtering exhibit somewhat lower absorbance than similar rutile films18 and assuming that the porosity of our 2.5 µm thick film (Figure 1d) is close to 60%, we can estimate the actual penetration depths for 300 and 380 nm light to be on the order of 30 nm and 3 µm, respectively. Consequently, the highest photocurrent efficiencies represented in Figure 3A correspond to the situation where the hole-electron pairs are generated in an extremely thin outer portion of the TiO2 films, ca. 2.5 µm from the back contact. On the other hand, the corresponding quantum yields decrease significantly in the case where the light (λ ) 380 nm) penetrates more or less the whole film. These results, which are apparently inconsistent with the existing models for photocurrent generation in nanocrystalline semiconductor films, suggest that the electrons flowing to the back contact suffer, in fact, recombination principally inside the illuminated portion of the film. The larger the illuminated region of the film, where the holes are photogenerated, traversed by the electrons, the smaller the actual maximum photocurrent yield. This view is consistent with the characteristic shape of the spectral photoresponses shown in Figure 4, which will be examined in more detail later. Photocurrent efficiency versus potential (IPCE-

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Figure 3. Photocurrent efficiency (IPCE) vs potential curves for the nanostructured anatase (P25) films of various thicknesses, 2.5 µm (A), 10 µm (B), and 45 µm thick (C), illuminated with 300 nm (a, b) and 380 nm (a′, b′) monochromatic light in 0.1 M HClO4 (a, a′) and 0.1 M HClO4/0.1 M HCOOH (b, b′).

Figure 4. Spectral photoresponses for the nanostructured anatase (P25) films, 2.5 µm (a), 10 µm (b), and 45 µm thick (c), recorded in 0.1 M HClO4/0.1 M MeOH (A), 0.1 M HClO4/0.1 M HCOOH (B), and 0.1 M HClO4 (C).

E) plots recorded for a much thicker (ca 10 µm) anatase film, represented in Figure 3B, follow still the same trend, with the maximum efficiencies for both the 300 and 380 nm illumination increasing markedly with respect to those for the ca. 2.5 µm thick film. Such an increase is easier to understand in the case of the longer-wavelength illumination where the 380 nm light eventually could not be completely absorbed within the 2.5 µm thick film. Although the latter argument obviously does not hold for the shorter (i.e., 300 nm) wavelength illumination where the observed increase of the photocurrent may possibly be related to a changed film morphology, the corresponding data in Figure 3B support a virtual absence of recombination in the unilluminated region of the film. Further increase of the TiO2 film thickness (up to ca. 45 µm) did not produce significant changes in the amount of the photocurrent observed in the

solution containing formic acid (Figure 3C). It is to be noted that despite the light absorption taking place at a very large distance from the back contact, the photocurrent rises steeply to reach a maximum corresponding, for λ ) 300 nm, to the quantum yield of about 140% (due, in part, to the occurrence of the photocurrent-doubling mechanism).19 This indicates, in addition to a small extent of recombination, quite low resistive losses across the film. Although significantly different, the saturation photocurrents for both the thinner and the thickest TiO2 film examined in this study are, in fact, attained under practically identical bias. The above-described behavior of the nanostructured anatase films was observed in all cases in which the solution contained an efficient scavenger of the photogenerated holes, such as formic or oxalic acid, methanol, or glucose. However, for the

7824 J. Phys. Chem. B, Vol. 102, No. 40, 1998 photooxidation reactions involving a large extent of recombination, an increase in the film thickness caused an opposite effect, i.e, a decrease of the photocurrent, although the quantum yields corresponding to short-wavelength illumination remained still higher than those observed at longer wavelengths. This kind of behavior is represented by IPCE-E and IPCE-λ plots for the photooxidation of water shown in Figures 3 and 4, respectively. In parts A-C of Figure 4 are displayed spectral photocurrent responses for the nanostructured anatase films of different thicknesses corresponding to the photooxidation of formic acid, methanol, and water. In the first two cases, the plots exhibit a pronounced maximum at ca. 300 nm irrespective the actual thickness of the film. These results are at variance with the predictions of the “electron diffusion model” of charge carrier transport in semiconducting nanostructured films,9 according to which the maximum of the spectral photoresponse is expected to be red-shifted with increasing film thickness (cf. eq 1). The actual dependence of the photoresponse on the film thickness is clearly affected by the nature of the hole scavenger present in the solution. Thus, for rapid (and irreversible) photooxidation reactions, the quantum efficiency of the photocurrent increases with increasing the film thickness (up to ca. 10 µm) over the entire range of wavelengths. In the case of the photooxidation of water, leading to the formation of readily reducible Tis-O• species, the IPCE values at shorter wavelengths follow an opposite tendency, decreasing moderately for thicker films. An abrupt decrease of the quantum yield for the 2.5 µm thick film (more rapid than for the thicker films), observed for the wavelengths larger than 360 nm (Figure 4C), is explained by the optical penetration depth exceeding the thickness of the TiO2 film. Various measurements performed with the nanostructured TiO2 films deposited from the dispersions containing the PVDF binder were duplicated using the films formed by electrophoresis of a suspension of P25 particles in pure water. The results (including the amount of the photocurrent) were closely similar to those shown in Figures 2-4. The data displayed in Figures 3 and 4 reflect the photocurrent response of nanostructured anatase films to low-intensity illumination (cf. Experimental Section). Additional measurements, performed using argon-ion laser light (334, 351.1, and 363.8 nm emission lines), revealed that the actual relationship between the film thickness and the amount of the photocurrent was, in fact, strongly affected by the light intensity. As shown in plot b of Figure 5, under 300 mW illumination (focused on 0.28 cm2 of the film surface) the maximum photocurrents measured in a solution of formic acid were almost independent of the film thickness for the films consisting of 3-12 consecutive layers with thicknesses ranging from 2.5 to 10 µm. However, thinner films, such as the 1 µm thick one, delivered much lower photocurrents. These results are closely similar to those previously reported for the nanostructured anatase films exposed to a 0.1 M NaOH/0.1 M CH3OH solution and illuminated with the full output of a 150 W xenon lamp.15 Importantly, when the full output of the argon-ion laser was replaced by a strongly attenuated illumination (using a 0.006 transmission filter), the observed photocurrents showed again a substantial increase with increasing film thickness (cf. plot a in Figure 5). Thus, the pronounced dependence of the photocurrent on the film thickness, extending to thicknesses of ca. 10 µm, appears as a typical feature of the nanostructured anatase films subjected to a low-intensity illumination.

Wahl and Augustynski

Figure 5. Variation of the saturation photocurrent of the nanostructured anatase (P25) films as a function of the film thickness recorded in 0.1 M HClO4/0.1 M HCOOH using the full output of an argon-ion laser (334, 351.1, and 363.8 nm lines) corresponding to 300 mW/0.28 cm2 (b) and illuminated through a 0.6% neutral filter (a).

Figure 6. Saturation photocurrents for a 2.5 µm thick nanostructured anatase (P25) film plotted as a function of light intensity in 0.1 M HClO4 (A) and 0.1 M HClO4/0.1 M HCOOH (B). The full output illumination corresponded to 300 mW/0.28 cm2 of argon-ion laser light (334, 351.1, and 363.8 nm).

In Figure 6 are shown the maximum photocurrents (corresponding to the plateau of the iph-E curve) plotted against the light (334-363.8 nm) intensity. Although in the case of photooxidation of water the photocurrent is linear with light intensity (Figure 6A), the photocurrents recorded in the solution containing formic acid strongly deviate from linearity (Figure 6B) apparently because of the rate-limiting diffusion and, especially, adsorption of HCOOH within the TiO2 film. Accordingly, it is the relationship between the amount of the photocurrent and the film thickness observed under low-intensity illumination, i.e., in the absence of the effects related to the

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Figure 7. Scanning electron micrographs showing surface views of particulate TiO2 films consisting of rutile Tioxide (R-SM2) (a) and anatase Tioxide (A-HRS) (b). Magnification is 76666×.

transport and adsorption of reactants, that is to be considered as typical of the behavior of nanostructured anatase photoelectrodes. Almost all studies devoted till now to the photoelectrochemical characterization of nanostructured TiO2 films are in fact concerned with the anatase form of this semiconductor. It is useful to recall, in this connection, that aqueous suspensions of the rutile photocatalyst exhibit, in general, lower or even much lower activity than similar anatase samples.20-23 Our preliminary results12 showed that in an analogous way the nanostructured rutile TiO2 films delivered almost negligible photocurrents in comparison with those observed for the anatase photoelectrodes. Such behavior, which is in contrast with that of the corresponding bulk films exhibiting comparable photoelectrochemical activities,19 indicates the essential role played in the case of the nanostructured films by the interfacial kinetics and the feasibility of the charge carrier transport. Because the results of those previous measurements could be affected by significantly differing sizes of the anatase and rutile powders employed, we decided to compare both kinds of nanostructured films again using closely similar A-HRS (anatase) and R-SM2 (rutile) Tioxide photocatalysts, with mean diameters of 150 and 210 nm, respectively. As shown by scanning electron micrographs in Figure 7, both films exhibit almost identical morphologies. Photocurrent versus potential curves, recorded in the supporting electrolyte and after addition of methanol and formic acid to the solution, fully confirmed the striking difference in photoactivity between the two forms of TiO2. As shown in Figure 8, the photocurrents observed for the rutile film are, in fact, 30-50 times lower than those for the anatase counterpart. On the other hand, the comparison of the iph-E curves in Figure 8B with those displayed in Figure 2, recorded for a film of similar thickness composed of much smaller P25 particles, shows significant difference both in the amount of the photocurrent and in the shape of the curves. Poorer photocurrentvoltage characteristics of the film formed from the larger anatase particles may be due to larger recombination losses and higher contact resistance. When considering largely differing photoelectrochemical behavior of nanostructured anatase and rutile films, it is important to recall that also physical properties of these two forms of TiO2 are far from being identical. Recent studies of polycrystalline films of anatase and rutile18 (and of the anatase

Figure 8. Photocurrent-potential curves for TiO2 rutile (R-SM2) (A) and TiO2 anatase (A-HRS) (B) nanostructured films consisting of three consecutive layers recorded in 0.1 M HClO4 (a), 0.1 M HClO4/0.1 M MeOH (b), and 0.1 M HClO4/0.1 M HCOOH (c) under the full output illumination of a 150 W xenon lamp.

single crystal24) revealed in particular important differences in their electrical properties. In the case of anatase films submitted to thermal treatment in a vacuum, with donor concentrations approaching 1019 cm-3, the authors found evidence for a Mott (insulator-metal) transition occurring in a shallow donor band

7826 J. Phys. Chem. B, Vol. 102, No. 40, 1998 of the semiconductor. This finding is consistent with the electrical resistivity of the anatase films strongly decreasing after the vacuum treatment and becoming practically independent of the temperature. The Hall effect measurements performed with these films showed a maximum in the Hall constant with decreasing temperature, indicative of a transition in the conduction mechanism.18 None of these effects was observed for the rutile films with high carrier concentration (>1019 cm-3), which exhibit a less pronounced decrease in resistivity following the thermal treatment in a vacuum. An important conclusion drawn by Tang et al.18 from the results of their resistivity and Hall effect measurements is that the insulator-metal transition occurs in a donor band of the reduced anatase films. This is supported by the fact that both the corresponding resistivity against temperature and Hall constant versus temperature plots remain flat to low temperatures at which the population of electrons in the conduction band of anatase falls to a low level. As postulated by the authors,18 the conduction via the donor band of doped anatase still remains the dominant process at room temperature. The absence of the Mott transition in the donor band of heavily doped rutile is to be assigned to a small Bohr radius (aH) of the donor state, comparable with the interionic distance in the rutile lattice.18 In contrast, the donor radius aH in anatase, deduced by Tang et al.18 on the basis of the critical donor concentration for the Mott transition, is almost 6 times larger. The large donor radius in anatase is the consequence of a small electron effective mass m* ≈ 1m0 , 20 times smaller than that in rutile. It is useful to recall, in this connection, a large improvement in the photocurrent-voltage characteristics of the anatase films, deposited on titanium by a sol-gel method, induced by high-temperature (>500 °C) treatment in argon.19,25 Diffusion of Ti atoms from the support into the film creates a profile of donor concentration sufficient to allow the insulator-metal transition in a more or less large portion of the film adjacent to the back contact. This is, however, not the case for the nanostructured films consisting normally of poorly doped anatase particles. The estimated donor concentration in such films, of about ND ) 1017 cm-3,10,26 is in fact almost 2 orders of magnitude lower than the critical value of ND for which the Mott transition is expected to take place. It is interesting to note, in this connection, that the additional thermal treatment in argon performed with the first version of nanostructured anatase films used in the dye-sensitized cells10 was rapidly abandoned.1 Clearly, the central question to be answered is how an initially poorly conducting network of anatase nanoparticles allows excellent photocurrent-voltage characteristics either in a dyesensitized liquid-junction solar cell or under direct band gap (and, more especially, the short wavelength) illumination in the presence of a suitable hole scavenger. A possible explanation is that an adjustment in the electron population within the donor band of anatase occurs at the initial stages of the film illumination before the steady-state photocurrent is attained. Once the critical carrier concentration close to 1019 cm-3 is reached, the unilluminated (and probably also the weakly illuminated) region of the anatase film exposed to a UV light undergoes the Mott transition accompanied by a more than 100fold increase in conductivity.18 Although in the case of a bulk anatase electrode the compensation of an excess of negative charge would require a simultaneous injection into the solid phase of protons or alkali cations, the situation is totally different for the nanostructured anatase film, with pores filled with the electrolyte, exhibiting an exceptionally high surface-to-volume ratio. Assuming that

Wahl and Augustynski roughly 2/3 of the total surface area (∼55 m2 g-1) of the P25 particles is actually in contact with the electrolyte, we can estimate that the electrochemically active surface area of the nanostructured film is on the order of 1.5 × 106 cm2 per cm3 of the solid phase. This implies that a negligible adjustment, ca. 6 × 1012 ions cm-2 (or 1 ion per 15 nm2), in the concentration of cations in the Helmholtz layer suffices to compensate for the excess charge carrier concentration of 1019 cm-3 within the P25 nanoparticles. The above value is to be compared with a typical variation in the surface-charge density of an oxide with pH of the solution, which is about 5 µC cm-2 per ∆pH ) 1,27 i.e., 5 times larger. Salafsky et al.28 have recently described time-resolved microwave conductivity measurements performed with dyesensitized, 10 µm thick nanostructured anatase films. The illumination of the sample with continuous white light of 100 mW cm-2 was accompanied by a significant increase in the conductivity extending over a period of several seconds (in fact, the conductivity was still rising when, after a 6 s illumination period, the light was turned off). Importantly, when the film was illuminated with less intense light, the observed microwave conductivity was much (more than 10 times) lower. These results establish a direct connection between charging of the nanostructured film through the electrons injected to the conduction band of anatase by the photoexcited dye on one hand and the rise in the film conductivity on the other. Cao et al.29 reached, in fact, a similar conclusion on the basis of their measurements of transient photocurrent response of a dye-sensitized nanocrystalline anatase cell. The authors noted that their results could be interpreted in terms of the “electron diffusion model”9 only by assuming that the diffusion coefficient for electrons within the film depends on the intensity of the incident light. To explain the fact that the observed rise time of the photocurrent became shorter with increasing light intensity, Cao et al. assumed the diffusion coefficient for electrons was proportional to the electron concentration within the nanostructured film. Again, these results are consistent with the initial buildup of the electron concentration in the particulate anatase film after switching on the light, leading to an increased conductivity.30 The preparation of the nanostructured semiconducting oxide films involves, in general, a high-temperature (g400 °C) annealing intended to produce partial sintering of the nanoparticles and to establish among them an electrical contact.10 In an attempt to obtain deposits of P25 nanoparticles at microelectrodes arrays,31 which can hardly support a heat treatment above 150 °C, we examined in detail the influence of the annealing temperature on the photoelectrochemical characteristics of the electrophoretically deposited films. Surprisingly enough, the films that were simply dried at room temperature behaved essentially in a similar way as the annealed samples. In particular, the shapes of the spectral photoresponses of a film dried at 25 °C and of a film annealed at 400 °C were practically identical (cf. Figure 9). Considering that for the 300 nm illumination the optical penetration depth R-1 is 2-3 orders of magnitude lower than the film thickness, one cannot invoke the presence of “in situ” photoexcited electrons29 to explain the apparent low resistance of the contact regions among nonannealed nanoparticles. It is important to realize that in the absence of charging (self-doping), such a network of nanoparticles maintained together by the van der Waals-London interactions can be expected to exhibit much higher resistance than a similar film consisting of partly sintered particles. This is related to the presence of more numerous trap sites in the contact regions

Charge Carrier Transport in TiO2

Figure 9. Spectral photoresponses for the nanostructured anatase (P25) films obtained by electrophoretic deposition without final heat treatment (a) and after annealing at 400 °C (b), recorded in 0.1 M HClO4/0.1 M HCOOH.

that are able to deplete the nanoparticles of free carriers below the original 1017 cm-3 level. This problem has been discussed in detail in the context of Hall effect measurements performed with bulk polycrystalline semiconductor samples consisting of small crystallites, i.e., including the large area of grain boundaries.18,32 The situation will be completely changed as a consequence of the proposed film charging owing (i) to trap filling and (ii) to the strong increase of the carrier concentration. It is to be noted, in this connection, that once the critical carrier concentration corresponding to the Mott transition, ND = 1019 cm-3, is reached, the depletion layer will become relatively thin in comparison with the size of nanoparticles, ca. 25 nm. Using  ) 31 as the static dielectric constant of anatase,18 we obtain a Debye length LD ) [0kT/(NDe2)]1/2 close to 2 nm. Conclusions Several aspects of the photoelectrochemical behavior of the nanostructured anatase films appear anomalous in light of previously proposed models describing the charge carrier transport in such networks of semiconductor particles. This concerns in particular the fact that the films with thicknesses largely (by at least 2 orders of magnitude) exceeding the penetration depth of the incident UV light exhibit excellent photocurrent-voltage characteristics consistent with quite small resistance losses. This is the case even for electrophoretically deposited films that did not undergo any annealing. We propose an explanation of the behavior of nanostructured anatase TiO2 films both under band gap UV illumination and, in the dye-sensitized configuration, under white light illumination33 in terms of the self-doping occurring at the initial stages of the photocurrent flow across the film. The transient charging of the film continues till the critical electron concentration in the donor level of anatase (ca. 1019 cm-3), allowing for the insulator-metal (Mott) transition, is reached. This transition is accompanied by an important increase in the electrical conductivity of anatase. The proposed mechanism of self-doping constitutes a unique feature of nanoporous 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. In the case of 25 nm diameter P25 particles, the increase of the surface concentration of monovalent cations required to compensate for a 1019 cm-3 excess of electrons within the nanoparticles is on the order of only 1 ion per 15 nm2.

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Figure 10. Spectral photoresponses for a ca. 5 µm thick P25 film recorded from the front side (a, b) and from the back side (a′, b′) in 0.1 M HClO4 (a, a′) and 0.1 M HClO4/0.1 M HCOOH (b, b′).

For relatively thick anatase films illuminated with the band gap UV light, the proposed occurrence of the insulator-metal transition, as a consequence of the self-doping of nanoparticles, actually provides a means for extending the back contact close to the illuminated region of the film. 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 IPCE values observed for the films illuminated with short wavelengths of the incident light (characterized by large absorption coefficients, R) support the view that the charge recombination takes place principally within and close to the illuminated portion of the film. This is also consistent with the fact that the spectral photoresponses of the nanostructured anatase films recorded from the front side and from the back side (through a quartz/SnO2 substrate) were almost identical (cf. Figure 10). The deviation observed in the range of short wavelengths, for which the optical penetration depth R-1 is on the order of or significantly smaller than the particle size, is to be attributed to a large extent of sintering of the first few layers of P25 particles close to the substrate. Acknowledgment. We thank Dr. H. Cachet of LP15 du CNRS-PARIS, Physique des Liquides et Electrochimie, for the supply of the SnO2-coated quartz samples. The authors also thank Dr. V. Shklover, Institute of Crystallography and Petrography, ETH, Zu¨rich, for carrying out SEM analyses. This research was supported by the Swiss National Science Foundation. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Nazeerudin, M. K.; Kay, A.; Rodicio, I.; Humphrey-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (3) Hagfeldt, A.; Vlachopoulos, N.; Gra¨tzel, M. J. Electrochem. Soc. 1994, 141, 82. (4) Vinodgopal, K.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97, 9040. (5) Huang, S. Y.; Kavan, L.; Exnar, I.; Gra¨tzel, M. J. Electrochem. Soc. 1995, 142, L142. (6) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (7) Hodes, G.; Albu-Yaron, A.; Decker, F.; Motisuke, P. Phys. ReV. B 1987, 8, 4215. (8) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. Soc. 1992, 139, 3136.

7828 J. Phys. Chem. B, Vol. 102, No. 40, 1998 (9) So¨dergren, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S. E. J. Phys. Chem. 1994, 98, 5552. (10) O’Regan, B.; Moser, J.; Anderson, M.; Gra¨tzel, M. J. Phys. Chem. 1990, 94, 8720. (11) (a) Liu, D.; Fessenden, R. W.; Hug, G. L.; Kamat, P. V. J. Phys. Chem. B 1997, 101, 2583. (b) Rensmo, H.; Keis, K.; Lindstro¨m, H.; So¨dergren, S.; Solbrand, A.; Hagfeldt, A.; Lindquist, S. E. J. Phys. Chem. B 1997, 101, 2598. (12) Wahl, A.; Ulmann, M.; Carroy, A.; Augustynski, J. J. Chem. Soc., Chem. Commun. 1994, 2277. (13) Ulmann, M.; Augustynski, J. Chem. Phys. Lett. 1987, 141, 154. (14) Augustynski, J. Struct. Bonding 1988, 69, 1. (15) Augustynski, J.; Carroy, A.; Wahl, A. In Nanostructured materials in electrochemistry Meyer. G. J., Searson, P. C., Eds. The Electrochemical Society Proceedings Series 95-8; Pennington, NJ, 1995; p 88. (16) Ulmann, M.; De Tacconi, N. R.; Augustynski, J. J. Phys. Chem. 1986, 90, 6523. (17) Eagles, D. M., Jr. Phys. Chem. Solids 1964, 25, 1243. (18) Tang, H.; Prasad, K.; Sanjine´s, R.; Schmid, P. E.; Le´vy, F. J. Appl. Phys. 1994, 75, 2042. (19) Wahl, A.; Ulmann, M.; Carroy, A.; Jermann, B.; Dolata, M.; Kedzierzawski, P.; Chatelain, C.; Monnier, A.; Augustynski, J. J. Electroanal. Chem. 1995, 396, 41. (20) Okamoto, K.; Yamamoto, A.; Tanaka, H.; Itaya, A. Bull. Chem. Soc. Jpn. 1985, 58, 2015. (21) Augugliaro, V.; Palmisano, L.; Sclafani, A.; Minero, C.; Pelizzetti, E. Toxicol. EnViron. Chem. 1988, 16, 89. (22) Sclafani, A.; Palmisano, L.; Schiavello, M. J. Phys. Chem. 1990, 94, 829. (23) Sclafani, A.; Herrmann, J. M. J. Phys. Chem. 1996, 100, 13655. (24) Forro, L.; Chauvet, O.; Emin, D.; Zuppiroli, L.; Berger, H.; Le´vy, F. J. Appl. Phys. 1994, 75, 633.

Wahl and Augustynski (25) Stalder, C.; Augustynski, J. J. Electrochem. Soc. 1979, 126, 2007. (26) Cao, F.; Oskam, G.; Searson, P. C.; Stipkala, J. M.; Heimer, T. A.; Farzad, F.; Meyer, G. J. J. Phys. Chem. 1995, 99, 11974. (27) Furlong, D. N.; Yates, D. E.; Healy, T. W. In Electrodes of conductiVe metallic oxides; Trasatti, S., Ed.; Elsevier: Amsterdam, 1981; Part B, Chapter 8, p 367. (28) Salafsky, J. S.; Lubberhuizen, W. H.; Van Fassen, E.; Schropp, R. E. I. J. Phys. Chem. B 1998, 102, 766. (29) Cao, F.; Oskam, G.; Meyer, P. C.; Searson, P. C. J. Phys. Chem. 1996, 100, 17021. (30) It is to be noted, in this connection, that the photocharging of small semiconductor particles deposited on an insulating support or in the form of a suspension in an aqueous electrolyte has been reported previously to produce a shift of the absorption edge toward shorter wavelengths (the so-called Burnstein shift) (cf.: Liu, C. Y.; Bard, A. J. J. Phys. Chem. 1989, 93, 3232 and references therein). This effect was assigned to a shift of the Fermi level into the conduction band of the semiconductor due to the increased electron concentration within the nanoparticles. Interestingly, in the case when the irradiated dispersion of a CdS colloid contained an efficient hole scavenger and no electron acceptor other than water molecules (i.e., a situation similar to that of the nanostructured anatase film illuminated in a deaerated solution of formic acid), the relaxation of the colloid in the dark to a nondegenerate state was quite slow, taking several seconds. (31) Lopez, C.; Augustynski, J.; Fiaccabrino, J. C.; Koudelka-Hep, M. Presented at the Joint Meeting of the Electrochemical Society/ISE, Paris, 1997; Abstract 1145. (32) Orton, J. W.; Powell, M. J. Rep. Prog. Phys. 1980, 43, 1263. (33) The situation prevailing in the unilluminated region of the film, located between the outer part absorbing the UV light and the back contact, and that in the dye-sensitized film irradiated with the white light are both characterized by the virtual absence of minority charge carriers.