J. Phys. Chem. B 2001, 105, 3023-3026
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Enhancement of the Photoinduced Hydrophilic Conversion Rate of TiO2 Film Electrode Surfaces by Anodic Polarization Nobuyuki Sakai,† Akira Fujishima,‡ Toshiya Watanabe,† and Kazuhito Hashimoto*,† Research Center for AdVanced Science and Technology, The UniVersity of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan, and Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: September 12, 2000; In Final Form: January 27, 2001
Changes in hydrophilicity of TiO2 film electrode surfaces by UV light irradiation in an aqueous solution were investigated under potential-controlled conditions. Although no change was observed when the electrode potential of TiO2 was set around its flat-band potential Efb, the photoinduced hydrophilic conversion proceeded at potentials positive of Efb. The hydrophilic conversion rate increased with larger positive potentials, but the addition of hole scavengers decreased the rate, suggesting that the diffusion of photogenerated holes to the surface is the important process for the hydrophilic conversion. Based on these results, we propose a model in which the photoinduced hydrophilicity is initiated by two-hole trapping by a surface lattice oxygen, producing an oxygen defect, followed by the dissociative adsorption of a water molecule at the defect site.
Introduction Although the photoinduced hydrophilicity of the TiO2 surface was first reported only a few years ago,1 this property has opened various new applications of TiO2 coatings such as antifogging and self-cleaning.2 For example, TiO2-coated mirrors give a clear view even on a rainy day, and stains adsorbed on TiO2-coated substrates can be easily washed off by rainwater. One of the well-known characteristics of TiO2 is its strong oxidation power by the photogenerated holes.3-13 One may then think that the hydrophilic conversion is caused by the decomposition of adsorbed organic contaminants. Actually, the surface could indeed become hydrophilic as a result of the decomposition of organic contaminants adsorbed on the surface. However, the following experimental results indicate that, besides the decomposition of adsorbed organic contaminants, an alternate mechanism must exist for the hydrophilic conversion of a TiO2 surface: (1) sonication converted the photoproduced highly hydrophilic surface back to the original less hydrophilic one immediately (within 5-10 min);14 (2) strontium titanate, which has almost the same photocatalytic oxidation power as TiO2, does not become hydrophilic by means of UV irradiation;15 and (3) the hydrophilic surface of TiO2 after UV irradiation consists of large numbers of closely spaced hydrophilic domains with sizes of several tens of nanometers, which was revealed by atomic force microscopic (AFM) observation.1,16 Besides the experiments shown above, we have studied the photoinduced hydrophilic conversion process by various physical methods. The XPS experiments suggested that Ti3+ and oxygen defects were produced on the rutile single-crystal surface after UV irradiation in UHV,17,18 and when this surface was exposed to water, a peak assigned to dissociative water (hydroxyl groups) appeared in the O 1s spectrum.19,20 In addition, our previous FTIR data showed that UV irradiation * To whom correspondence should be addressed. † Research Center for Advanced Science and Technology, The University of Tokyo. ‡ Department of Applied Chemistry, School of Engineering, The University of Tokyo.
increases the intensity of the IR band at 3695 cm-1 which is assigned to the stretching of a hydroxyl group adsorbed on a surface defect site.16,21 Based on these results, we have proposed a mechanism for the hydrophilic conversion in which electrons and holes produced by UV irradiation are trapped by surface Ti4+ and O2- ions, producing Ti3+ and oxygen defects, respectively, followed by the dissociative adsorption of water molecules at the defect sites. This model means that the photoinduced hydrophilic surface consists of partially reduced TiO2. However, careful XPS measurements did not detect a signal for Ti3+ at all, even after UV irradiation in ambient air,17 although a shoulder due to the OH formation clearly appeared on the O 1s peak.14,17 The mechanism described above does not account for this XPS results. In the present paper, we have studied the photoinduced hydrophilic conversion of TiO2 film in an aqueous electrolyte under potential-controlled conditions to elucidate the mechanism in which the new hydroxyl groups are formed. Now we can obtain information on the roles of electrons and holes in the photoinduced process separately. Experimental Section SnO2-coated glass plates were used as substrates for the preparation of polycrystalline TiO2 films. The substrates were partially dipped into the titanium tetraisopropoxide solution, leaving one-third of the area uncoated. The samples were sintered at 500 °C for 30 min in air, which resulted in polycrystalline TiO2 (anatase) films. The thickness and roughness Ra of the TiO2 films were estimated to be 240 and 1.7 nm by focused laser microscopy and AFM observations, respectively. To fabricate the TiO2 film electrode, a copper wire was attached to the conductive substrate remaining uncovered, with silver paste. After the silver contact dried, the copper lead wire was covered with an epoxy, so that only the TiO2 surface was exposed. The photoelectrochemical measurements were carried out in a standard three-electrode, single-compartment glass cell, fitted
10.1021/jp003212r CCC: $20.00 © 2001 American Chemical Society Published on Web 03/28/2001
3024 J. Phys. Chem. B, Vol. 105, No. 15, 2001
Figure 1. Changes in absorbance at 700 nm between those measured at each applied potential and those measured at -0.1 V for a polycrystalline TiO2 film electrode.
with a quartz window, with a potentiostat and an X-Y recorder coupled with a function generator. The polycrystalline TiO2 thin film electrode served as the working electrode. The counter and reference electrodes were platinum and Ag/AgCl/saturated KCl(aq), respectively. All of the electrode potential values are given with respect to Ag/AgCl/KCl(aq) in the present paper. A standard buffer solution (pH 6.86) containing 0.2 mol dm-3 sodium sulfate was employed as the supporting electrolyte after being deaerated by bubbling with N2. A cylindrical black-light lamp (BLB) was employed as the excitation light source, and the UV light intensity was controlled to be approximately 0.1 mW/cm2. The hydrophilicity of the sample surface was evaluated by contact angle measurements by use of a commercial contact angle meter (CA-X, Kyowa Interface Science, Japan). The sample was taken from the electrolyte solution and washed thoroughly with pure deionized water, followed by drying the surface with dry N2 gas, and then the contact angle was measured. The contact angle was measured at least 6 times with the same sample, and the average value was used. Results and Discussion First, we determined the flat-band potential of the present anatase polycrystalline film electrode. Because of the optical transparency of the electrode, we were able to adopt the spectroelectrochemical method22 to estimate the flat-band potential, which is described as follows. Optical absorption spectra of the film electrode were measured following polarization for 60 s at various applied potentials. The electrode potential was changed from the rest potential (+0.02 V) in the negative direction by 0.1-V steps. The absorbance difference at 700 nm measured for each potential vs. -0.1 V as a reference was plotted against the potential. The results, shown in Figure 1, indicate that an increase in absorption starts at potentials slightly more positive than -0.8 V. This result indicates that the flatband potential is around -0.8 V, which is coincident with that reported by Boschloo and Fitzmaurice (-0.77 V vs Ag/AgCl, pH 6.86).22 Figure 2 shows current-voltage curves for the film electrode in the dark and under UV irradiation. In the dark, no anodic current was observed. Cathodic current flowed at potentials more negative than -0.8 V. Under UV irradiation, however, anodic photocurrent was also observed, and its saturation value was proportional to the light intensity. These characteristics are typical for n-type semiconductor electrodes.23 The quantum
Sakai et al.
Figure 2. Current-voltage curves for a polycrystalline TiO2 film electrode in buffer solution (pH 6.86) containing 0.2 mol dm-3 Na2SO4: (a) in the dark, (b) under the UV irradiation (10 mW/cm2), and (c) under reduced UV irradiation (6 mW/cm2).
Figure 3. Changes in the contact angle of the TiO2 electrode surface at open circuit: (a) in the dark and (b) under 0.1 mW/cm2 UV irradiation. The rest potentials for (a) and (b) were +0.02 and -0.24 V, respectively. For (c), the UV irradiation was conducted in air.
efficiency (φ) of the photocurrent was estimated to be approximately 30% at +1.0 V. The contact angle of the anatase polycrystalline film was 48.5° ( 3.9° (average ( standard deviation) after being kept for a long time (at least 2 months) in the dark under ambient conditions. A simple dipping of the TiO2 film into the aqueous solution decreased the contact angle to a certain extent (to ca. 20°), even in the dark. However, the films soon exhibited constant values, and we used these values as the initial contact angles of the surfaces in Figure 3. Curves a and b in Figure 3 show the changes of the water contact angle of the film electrode surface in the aqueous electrolyte solution under open circuit conditions in the dark and under UV irradiation, respectively. The potentials were +0.02 V and -0.24 V in the dark and under UV irradiation, respectively. No changes of contact angle were observed in the dark. After being subjected to UV light with an intensity of 0.1 mW/cm2 for 1 h, however, the contact angle of the electrode surface decreased from 19.1° ( 0.4° to 11.3° ( 0.7°, making the surface slightly more hydrophilic. Curve c in this figure represents the case in which the film was irradiated in air at the same UV light intensity. The contact angle decreased from 27.2° ( 0.8° to 8.6° ( 1.3° after 1 h, showing that the hydrophilic conversion rate is faster in air than in the aqueous electrolyte solution at open circuit. Figure 4 shows the changes of the water contact angle in the
Conversion Rate of TiO2 Electrode Surfaces
Figure 4. Dependence of the photoinduced hydrophilic conversion of the TiO2 electrode surface on the electrode potential: (a) -0.8 V; (b) -0.5 V; (c) -0.2 V; (d) +0.2 V; (e) +0.5 V; (f) +0.8 V. The inset shows a plot of the reciprocal of the contact angle vs UV irradiation time. The symbols used in the inset are the same as those used in the figure.
Figure 5. Dependence of the rate constant of the hydrophilic conversion on the electrode potential. The arrow in the figure indicates the value of the rate constant for the UV irradiation in air.
aqueous electrolyte solution at various electrode potentials. Curve a represents the case for the flat-band condition (-0.8 V), and no change in contact angle was observed, even under UV irradiation. Under anodic polarization, however, photoinduced hydrophilic conversion proceeded. It can clearly be seen that the hydrophilic conversion rate increases with larger positive polarization potentials. Therefore, the faster conversion rate in air than in water at open circuit described above is explained by the larger band bending in air compared to that in water.24 When the reciprocal of the contact angle is plotted against the irradiation time, linear relationships between these two parameters are obtained, as shown in the inset of Figure 4, and thus we tentatively define the slope of the straight line as the apparent rate constant for the hydrophilic conversion.25 In Figure 5 are plotted the apparent rate constants defined in this way against the TiO2 electrode potential. The arrow in this figure represents the value of the apparent rate constant observed for irradiation in air. It should be noted that the conversion rates at potentials more positive than +0.0 V were greater than that in air. The potential dependence resembles that of the currentvoltage curves of the electrode under UV irradiation (Figure 2, curves b and c). These data suggest that the hydrophilic conversion is caused by the photogenerated carriers. Specifically, under UV irradiation at the flat-band potential (-0.8 V), most of the excited electrons and holes recombine. In contrast, when
J. Phys. Chem. B, Vol. 105, No. 15, 2001 3025
Figure 6. Changes in the water contact angle of the TiO2 electrode at +0.5 V under UV irradiation (a) in the absence and in the presence of (b) 0.001, (c) 0.01, (d) 0.1, and (e) 0.33 mol dm-3 of sodium sulfite.
the TiO2 electrode is under positive polarization, the charge separation proceeds efficiently, and holes are transferred at the interface with water. By applying a negative bias, the contact angle first decreased slightly even in the dark. After reaching a stationary state with a contact angle of 13.2° ( 0.2°, however, the hydrophilicity did not change even with UV irradiation (data not shown). Thus we know that the photoinduced hydrophilic conversion proceeds only under anodic polarization. Figure 6 shows the changes of the contact angle at +0.5 V under UV irradiation in aqueous electrolyte in the absence and presence of sodium sulfite, which is known as a hole scavenger.26 The hydrophilic conversion rate was clearly lowered by increasing the concentration of sodium sulfite. All of the above results show that the hydrophilic conversion is induced by the photogenerated holes diffusing to the TiO2 surface. In other words, the photogenerated electrons do not directly play an important role in the surface hydrophilic conversion. We can now reasonably explain the photoinduced hydrophilic conversion process of the TiO2 electrode surface as follows. The photoproduced electron and hole diffuse to the counter electrode and the TiO2 electrode surface, respectively, reducing a proton and being trapped at a lattice oxygen (O2-(l)).27 These processes are the same as those occurring in photoelectrochemical water cleavage with TiO2 electrodes, in which the trapped hole is further transferred to an adsorbed water molecule, eventually producing a molecule of O2. For the present hydrophilic conversion process, however, the lattice oxygen, after trapping a hole (O-(l)), may be further oxidized by a second hole to form a neutral O• radical. By coupling two neutral radicals thus formed, an O2 molecule is produced, and two oxygen defects are formed on the surface. The coordination number of the Ti ions next to the oxygen defects changes from 6 to 5, and one water molecule spontaneously adsorbs on the each defect dissociatively.19,28-30 These processes are summarized in Figure 7. The photoinduced hydrophilicity of TiO2 film photocatalysts may be explained by almost the same processes as those of the electrode, the only exception being that the electron trapped at the surface Ti site as Ti3+ is consumed by adsorbed molecular oxygen, producing O2- at the TiO2 surface. This is the reason we could not detect a Ti3+ signal with XPS after the irradiation of TiO2 film photocatalysts in ambient air. It is expected that the formation of oxygen defects occurs more readily at doubly coordinated lattice oxygen sites (bridging
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Sakai et al. on the surface properties because the reaction is on the twodimensional surface. Acknowledgment. We express gratitude to Prof. D. A. Tryk for careful reading of the manuscript. N.S. acknowledges the financial support of the Japan Society for the Promotion of Science (JSPS). References and Notes
Figure 7. Proposed mechanism for the photoinduced hydrophilic conversion of the TiO2 surface: (A) originally less hydrophilic and (B) photoinduced highly hydrophilic.
oxygens) than at triply coordinated sites.31 Therefore, based on the above mechanism, the photoinduced hydrophilic conversion rate should depend on the crystal face. In fact, we previously reported that rutile (110) and (100) faces, both containing bridging oxygens, exhibit much faster hydrophilic conversion rates than does the (001) face, which contains no bridging oxygens.17 The present mechanism suggests that two oxygen defects are created simultaneously to form one oxygen molecule. It is possible that the presence of the two new hydroxyl groups in close proximity to each other may destabilize neighboring surface lattice oxygens, which could cause them to be more easily photoejected. This could be one of the reasons why the hydrophilic sites form domain structures, as was observed on the rutile (110) surface by friction force microscopy.1,16 Conclusion In the present photoelectrochemical experiments, we showed that the diffusion of photogenerated holes to the surface is the important process for the hydrophilic conversion, whereas the photogenerated electrons do not directly play an important role in the process. Based on these results, we have proposed a mechanism for the photoinduced hydrophilic conversion of the TiO2 surface. In the anodically polarized TiO2 electrode, the photoproduced electron diffuses to the counter electrode and the hole first diffuses to the surface, being trapped at surface lattice oxygen. The lattice oxygen trapping hole is further oxidized by the second hole, creating a neutral oxygen followed by the formation of oxygen defect. Then a water molecule adsorbs on the defect sites dissociatively. As a result, a new hydroxyl group is produced, which is responsible for the highly hydrophilic surface. For a film form TiO2 (photocatalyst), almost the same mechanism should exist in the photoinduced hydrophilic conversion, except that the photogenerated electron is trapped at the adsorbed oxygen molecule, forming O2-. Although TiO2 has been believed to be stable under UV irradiation, the proposed mechanism is based on the assumption that topmost surface atoms are removed both in neutral aqueous solution and even in air. These processes are similar to those of the well-known photocorrosion process of metal oxides in aqueous solution. Even though the quantum efficiency of this photoreaction is very low on TiO2, it induces a drastic change
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