Characterization of TiO2 Photocatalysis in the Gas Phase as a

Thus, the photocatalytic system in the gas phase should consist of anodes, ... If the photocatalytic activity per unit area of the micro-cell is highe...
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J. Phys. Chem. B 2001, 105, 10016-10021

Characterization of TiO2 Photocatalysis in the Gas Phase as a Photoelectrochemical System: Behavior of Salt-Modified Systems Yoshihisa Ohko, Tetsu Tatsuma,† and Akira Fujishima* Department of Applied Chemistry, School of Engineering, UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: April 18, 2001; In Final Form: August 9, 2001

Photocatalytic degradation rates of 2-propanol to acetone were examined by using TiO2 films modified with Pd stripes whose interval was varied from 25 µm to 5 mm, to characterize their photoelectrochemical behavior in the gas phase. Even though the ratio of Pd area to bare TiO2 area was kept constant (1:1), the photocatalytic activity increased as the stripe interval was decreased, in the case where a certain amount of NaCl had been deposited on the Pd-modified TiO2. This is indicative of the formation of photoelectrochemical micro-cells in the vicinity of the Pd deposited on the TiO2 thin film in the gas phase. The width of the micro-cell was, for example, estimated to be e 250 µm when 0.2 µg cm-2 of NaCl had been deposited on the sample film. The micro-cell formation required adsorbed water on the TiO2 surface, which should be essential for the dissociation of NaCl; the adsorbed water layer in which NaCl is dissolved should act as the electrolyte of the micro-cell. The width of the micro-cell varied from the order of µm to mm, depending strongly on the amount of NaCl (0.02-20 µg cm-2). On the other hand, the width of the micro-cell at the salt-free TiO2 surface was found to be less than 25 µm. Comparison between the experimentally observed dependence and the theoretically simulated one suggests that ionic conductivity in the adsorbed water layer is suppressed, probably due to electrostatic and/or chemical interaction between the ions and the TiO2 surface.

Introduction Photocatalytic reactions1 of semiconductors have been studied extensively over the past 30 years in a wide variety of research fields, including water splitting, removal of heavy metals, and environmental purification of air and water.2-10 Among many kinds of semiconductors, TiO2 in particular has beneficial characteristics, such as chemical and physical stability, as well as the strong oxidizing power of the photogenerated holes, with which most organic compounds can be oxidized to carbon dioxide at ambient temperature and pressure. It is well-known that TiO2 photocatalysis in an electrolyte solution proceeds as follows: photons with energies greater than the band gap are absorbed by TiO2, then electron-hole pairs are generated in the TiO2. The photogenerated holes are transferred to the surface, where they can oxidize surfaceadsorbed molecules (e.g., H2O). On the other hand, the photoexcited electrons are transferred to the bulk, and if the TiO2 is electrically connected to a metal that is placed in the same solution, the electrons are further transferred to the metal, where they can reduce molecules adsorbed on the surface (e.g., O2). In this case, the photocatalysis consists of reactions in a photoelectrochemical system with the TiO2 anode, the metal cathode, and the electrolyte. Unless the TiO2 is combined with a metal, the photoexcited electrons are assumed to be transferred to a metallic impurity adsorbed on the TiO2 or a nonirradiated region of the TiO2, which also acts as a cathode.11,12 In general, the photocatalytic activity of TiO2 is increased by depositing * Author to whom all correspondence should be addressed. Phone: +813-3812-9276. Fax: +81-3-3812-6227. E-mail: [email protected]. † Current address: Institute of Industrial Science, University of Tokyo, 4-61 Komaba, Meguro-ku, Tokyo 153-8505, Japan.

metals such as palladium (Pd) and platinum (Pt) on TiO2, because the deposited metals expedite the transport of photogenerated electrons to reducible molecules and thus improve the charge separation efficiency.13-17 As well as in the liquid phase, the photocatalytic reactions proceed in the gas phase.9,10,18-23 The gas-phase reactions are also assumed to proceed as photoelectrochemical reactions at oxidation sites (anodes) and reduction sites (cathodes) separately. Such photoelectrochemical reactions should require electrolytes as do ordinary electrochemical reactions. In the TiO2 photocatalysis in the gas phase, acidic surface hydroxyl groups and/or electrolytic impurities that are dissociated in water adsorbed on the TiO2 surface may act as electrolytes. Thus, the photocatalytic system in the gas phase should consist of anodes, cathodes, and electrolytes at the TiO2 surface. However, to the best of our knowledge, photocatalysis in the gas phase has not yet been characterized systematically as a photoelectrochemical system. To systematize the present investigation, we modified a TiO2 thin film with Pd stripes (schematic illustrations are shown in Figure 1) and characterized it in the gas phase. In a boundary between a region of the bare TiO2 and a Pd stripe, a photoelectrochemical micro-cell (hereafter referred to simply as a micro-cell) should be formed by the TiO2 as the anode, the Pd as the cathode, and the adsorbed water (containing ionic species) as the electrolyte (Figure 1a). As the width and interval between the Pd stripes decrease, while the ratio of the widths of the Pd and bare TiO2 is kept constant (1:1 in our experiments), the number of the micro-cells increases (Figure 1b). If the photocatalytic activity per unit area of the micro-cell is higher than that in the bare TiO2 region that is apart from the Pd stripes, the total activity should increase with the number of the microcells. However, since the width of the micro-cell should have a lower limit, the total activity must reach a maximum when

10.1021/jp011459y CCC: $20.00 © 2001 American Chemical Society Published on Web 09/22/2001

TiO2 Photocatalysis in the Gas Phase

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Figure 2. Dependence of the rates of acetone production per unit TiO2 area on the relative humidity in the photocatalytic degradation of 2-propanol with bare TiO2 (O) and TiO2 modified with Pd stripes (B, inter-stripe gap was 25 µm) in the gas phase (light intensity, 2 mW cm-2; initial concentration of 2-propanol, 3000 ppm). The samples were thoroughly rinsed with distilled water and then dried prior to UV illumination in order to remove impurities on the surface.

Experimental Section

Figure 1. Procedure to characterize the width of the photoelectrochemical micro-cell formed in the gas phase on TiO2 modified with Pd stripes whose width is the same as the inter-stripe gap. A broken circle indicates the micro-cell consisting of a TiO2 anode (hatched region), Pd cathode, and adsorbed water layer as the electrolyte. Each figure expresses the situation as follows: (a) a few micro-cells are formed (their contribution to the overall activity might be negligible), (b) many micro-cells are formed (their contribution may be significant), (c) micro-cells cover the whole exposed TiO2 surface (further improvement in the activity cannot be expected). See text for further details.

the interval between the Pd stripes (stripe interval) is about twice as wide as the micro-cell width, and then, the total activity will not increase any more even if the interval is further decreased (Figure 1c). In the present work, the stripe interval was changed over 3 orders of magnitude (25 µm to 5 mm) to estimate the width of the micro-cell in the gas phase. We used an indium tin oxide (ITO)-coated glass plate as the substrate for the TiO2 thin film so as to prevent the electrical resistance of the TiO2 from limiting the width. The width of the micro-cell should therefore be limited by the ionic conductivity of the electrolyte on the TiO2 surface; thus, the width can be an index of the ionic conductivity. Sodium chloride (NaCl) was deposited on the sample film in the range of 0.2 to 200 µg cm-2 in order to control the ionic conductivity of the adsorbed water. As a result, we were able to estimate experimentally the width of the microcell and its dependence on the ionic conductivity of the adsorbed water layer. In addition, the results were compared with those simulated on the basis of an equivalent circuit for the microcell. Such characterization of the TiO2 photocatalysis in the gas phase as a photoelectrochemical system helps us to recognize the importance of ionic conductivity of the adsorbed water layer on TiO2 surfaces, which has not received much attention so far. Additionally, the information obtained in this work would be important in designing and constructing more sophisticated photocatalytic systems in the gas phase, especially, systems in which anodic and cathodic sites are separated for improvement of charge separation efficiency and/or photoproduct separation efficiency. The latter can be improved by separating the oxidation product (A + h+ f Aox) and the reduction product (B + e- f Bred) to avoid indirect recombination (Aox + Bred f A + B).

The TiO2 film was prepared on an ITO glass substrate by a spray-pyrolysis technique from a 0.05 M ethanol solution of bis(2,4-pentanedionato) titanium oxide (Tokyo Kasei) at 500 °C.24 Consequently, the thickness of the TiO2 films was ca. 1.2 µm. Pd stripes of the same width as the interval were electrochemically deposited onto this TiO2 film using a photolithographically prepared photoresist mask in a 1 mM PdCl2 aqueous solution at -200 mV vs SCE at pH 2. Finally, the photoresist mask was removed in a furnace at 500 °C. The stripe interval was varied in the range of 25 µm to 5 mm. Before photocatalytic experiments, the TiO2 film was rinsed with double-distilled water, and dried in a vacuum. An O2 (20%)-N2 gas mixture that was partly passed through a water humidifier in order to adjust the relative humidity to a given value was used to fill a 17-mL Pyrex glass vessel. The relative humidity was confirmed by using a thermo-hydrometer (Shinei, TRH-CA). 2-Propanol-saturated gas was injected into the vessel using a syringe to give an initial 2-propanol concentration of 3000 ppmv. For the photocatalyic decomposition of gaseous 2-propanol, the Pd-modified (or bare) TiO2 thin film was illuminated with a Hg-Xe lamp (Hayashi Tokei, Luminar Ace 210) together with a 365-nm band-pass filter. The UV intensity was 2 mW cm-2 measured using a UV power meter (TOPCON UVR-1). The 2-propanol and acetone concentrations were measured using gas chromatography (Shimadzu Model GC-8A) equipped with a 5-m PEG1000 column and a flame ionization detector, using N2 as the carrier gas. To change the ionic conductivity of the adsorbed water layer on the surface, a given amount of an aqueous NaCl solution was dropped on the sample surface and dried in a vacuum before the photocatalytic experiments. Results Effect of Humidity. The Pd-modified TiO2 (or unmodified TiO2) was illuminated at room temperature (27 °C) after equilibrium between gaseous and adsorbed 2-propanol was achieved, as evidenced by a constant 2-propanol concentration in the gas phase. In the case of a bare TiO2 sample, the rate of acetone generation was 1.2 × 10-8 mol min-1 cm-2 of TiO2 in dry air, and then it increased to 1.8 × 10-8 mol min-1 cm-2 of TiO2 at a relative humidity (RH) of 50%, but decreased to 1.2 × 10-8 mol min-1 cm-2 of TiO2 at 95% RH (Figure 2). As we discuss later, the initial increase in the acetone generation rate

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Figure 3. Dependence of the rates of photocatalytic acetone production per unit TiO2 area on the amount of NaCl deposited on bare TiO2 (n) and TiO2 modified with Pd stripes (B, inter-stripe gap was 25 µm). The relative humidity was adjusted to 50%; other experimental conditions were the same as those in Figure 2. A NaCl aqueous solution was deposited on the clean samples and then dried before UV illumination.

Figure 4. Dependence of the ratio of the acetone generation rates for the Pd-modified TiO2 films to those for bare TiO2 films (AGRTiO2-Pd/ AGRTiO2) in the photocatalytic degradation of 2-propanol in the gas phase on the gap distance between Pd stripes (relative humidity is adjusted to 50%; the other experimental conditions were the same as those in Figure 2). The amounts of NaCl deposited on the samples were 20 (B), 2 (O), 0.2 (2), and 0.02( 4) µg cm-2.

with relative humidity suggests that the 2-propanol degradation reaction involves •OH radicals generated from water oxidation, while the subsequent decrease is explained in terms of interference of excess water adsorption with mass transfer of 2-propanol and/or oxygen to the TiO2 surface. The Pd-modified TiO2 samples exhibited almost the same activities per unit TiO2 area, even though the stripe interval was 25 µm, which was the minimum value used in this experiment. This indicates that the dimension of the micro-cell is much smaller than 25 µm over the whole humidity range, even if the micro-cells form. Incidentally, as described above, the mass transfer may control the overall reaction rate at around 90% RH. On the other hand, at 50% RH, the reaction rate was higher and the apparent quantum yield was ca. 5%, which is close to the maximum value for oxidation of 2-propanol to acetone by TiO2.21 In addition, the reaction rate was not influenced by forced-convection.22 Thus, the present system is not controlled solely by mass transfer at the humidity of 50%. In the following experiments, the relative humidity was adjusted to 50%, because the improvement of the intrinsic activity by the formation of the micro-cells cannot be observed under mass transfer-limited conditions. Effect of NaCl. Next, we examined the effect of NaCl on the Pd-modified TiO2 films (stripe interval was 25 µm). An NaCl solution was cast and water was evaporated before initiation of the photocatalytic reactions. The result was that the rate of acetone generation increased with the amount of deposited NaCl (Figure 3) up to 0.2 µg cm-2. On the other hand, the rate of acetone generation for the bare TiO2 sample decreased gradually as the amount of NaCl increased. That may be because of the interference of an adsorbed water layer, the amount of which increases with that of NaCl,25 with the mass transfer of 2-propanol and/or oxygen molecules to the TiO2 surface. Also, Cl- may act as a catalyst poison. In either case, the Pd-modified TiO2 exhibited higher rates of acetone generation (per unit TiO2 area) than the bare TiO2 when the NaCl amount was 0.02-20 µg cm-2. These results indicate that the contribution of the micro-cells is no longer negligible as a result of improvement of the ionic conductivity of the adsorbed water layer. Effect of Stripe Interval. We also examined the photocatalytic degradation of 2-propanol by using various Pd-modified TiO2 samples with different stripe intervals. We plotted the ratios of the acetone generation rates for the Pd-modified TiO2 samples (AGRTiO2-Pd) to those for the bare TiO2 (AGRTiO2) as a function of the stripe interval in Figure 4. When the amount of NaCl deposited was 0.02 µg cm-2, the values of the ratio

(AGRTiO2-Pd/AGRTiO2) were 1, regardless of the stripe interval. On the other hand, when the amount of NaCl was 0.2 µg cm-2, the values of the ratio were larger than 1 when the interval was 50 µm or less, while the ratio decreased to 1 when the interval was g 250 µm. When the amount of NaCl deposited was 2 µg cm-2, the value of the ratio was greater than 1 even at a stripe interval of 5 mm, and furthermore, when the NaCl amount was 20 µg cm-2, the ratio was almost 2 over the whole range examined. Discussion Photocatalytic Oxidation of 2-Propanol. In this study, we chose 2-propanol as a reactant in order to evaluate activities of the TiO2 films modified with Pd stripes. This is because the photocatalytic process of 2-propanol oxidation to acetone is well characterized and simple, as we have reported previously.21,22 The oxidation of one 2-propanol molecule to acetone involves only a single photogenerated •OH radical or hole, with negligible contribution from chain reactions:

CH3CH(OH)CH3 + •OH (or h+) f CH3C•(OH)CH3 + H2O (or H+) (1) CH3C•(OH)CH3 f CH3COCH3 + H+ + e-

(2)

where the electron is trapped finally by O2 (to give O2-• or H2O2). The higher rate of acetone generation at 50% RH than that in dry air in Figure 2 indicates that •OH radical is involved in the reaction (eq 1) at least in part because H2O is necessary for •OH generation (H2O + h+ f •OH + H+). On the other hand, the lower rate of acetone generation at 100% RH than that at 50% may be due to the interference of an excess amount of adsorbed water with mass transfer of 2-propanol and/or oxygen molecules to the TiO2 surface, as described above. Formation of the Photoelectrochemical Micro-Cell. In the present system, the counter reaction is the reduction of adsorbed oxygen molecules. Pd is known to have catalytic activity for the electrochemical reduction of molecular oxygen. Therefore, if a Pd cathode is combined with a TiO2 anode effectively, the overall activity may increase as a result of improved charge separation efficiency and/or facilitated oxygen reduction. The photocatalytic activity of a TiO2 sample modified with Pd stripes was better than that of a bare TiO2 sample under certain conditions, for example when 0.2 µg cm-2 of NaCl was

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TABLE 1: Relationship between the Amount of Deposited NaCl and the Estimated Width of the Photoelectrochemical Micro-cells Formed on the Pd-Modified TiO2 in the Gas Phase. amount of deposited NaCl (µg cm-2)

width of the micro-cell (µm)

0.02 0.2 2 20

,13 < 13 e 250 g 2500

deposited on the Pd-modified TiO2 with a stripe interval of 50 µm or less (Figure 4). Thus, we conclude that the Pd stripes function as cathodes; Pd cathodes and TiO2 anodes are combined not only electronically, but also ionically via adsorbed water layer containing NaCl, to form the micro-cells described in the Introduction. In addition, as the stripe interval decreases, the activity ratio AGRTiO2-Pd/AGRTiO2 increases, and finally a constant value is reached (Figure 4). This behavior is also clearly expected on the basis of our model as described in the Introduction. Since the NaCl deposition inhibits the photocatalytic activity of bare TiO2 (Figure 3), if NaCl is deposited preferentially on the Pd regions of the Pd-modified TiO2 samples (i.e., smaller amounts of NaCl are deposited on the bare TiO2 regions), this could be responsible for the greater activity of the Pd-modified samples. If this is true, however, the activity should not depend on the stripe interval, since the Pd-TiO2 area ratio was kept constant (i.e., unity). Furthermore, since the TiO2 surfaces are more hydrophilic than the Pd surfaces, NaCl may be deposited preferentially on the bare TiO2 regions. On the other hand, in dry air, the photocatalytic activity of a TiO2 sample modified with Pd stripes was not improved in comparison with a bare TiO2 sample even though 20 µg cm-2 of NaCl was deposited. This is probably because NaCl can contribute to ionic conduction at the TiO2 surface only when a certain minimum amount of water is adsorbed on the surface. This is quite reasonable, because an appropriate solvent such as water is necessary for dissociation of a salt such as NaCl to ions. Incidentally, Tada et al.28 have recently reported that a patterned TiO2/SnO2 bilayer type photocatalyst (stripe interval ) 1 mm) exhibits higher photocatalytic activity in the gas phase than a nonpatterned sample without addition of a salt. Although their results are very interesting, no information about the humidity is given, so that it is not clear whether the temperature was higher or lower than the dew-point. In addition, since they have not varied the stripe interval, it is not possible to obtain any information about the formation of the micro-cells and the function of an electrolyte in the gas phase. Thus, we cannot compare our results with theirs. Width of the Micro-Cell. There are two possible reasons why the activity ratio AGRTiO2-Pd/AGRTiO2 reaches a plateau in Figure 4: one reason is that the width of the Pd stripes limits the width of the cell, and a second is that another factor (e.g., mass transfer) limits the overall reaction. If the former holds, the width of the micro-cell is close to the half of the stripe width at which the plateau value is reached, as explained in the Introduction and in the description of Figure 1. Thus, the size of the micro-cell can be estimated. If the latter holds, however, the width of the micro-cell may be smaller than the estimated one. Even so, a larger micro-cell should give rise to an improvement of the overall photocatalytic activity (and the activity ratio) even at a larger stripe interval, which gives a smaller number of micro-cells. Table 1 summarizes the widths

Figure 5. Electrical equivalent circuits for the photoelectrochemical micro-cell formed on the Pd-modified TiO2 film. Each component is explained in the text. Circuit (b) is simplified from circuit (a) assuming steady state.

of the micro-cells thus estimated from the data in Figure 4. It is clear that a greater amount of NaCl results in the formation of a larger micro-cell size. This should be because the ionic conductivity of the adsorbed water layer improves as the NaCl concentration and/or the thickness of the layer increase, and the micro-cell dimension increases with the ionic conductivity, as expected from our model. Simulation Based on an Equivalent Circuit. The relationship between the width of the micro-photoelectrochemical cell in the gas phase and the amount of deposited NaCl was simulated on the basis of the equivalent circuit shown in Figure 5. The circuit for the micro-cell (Figure 5a) is characterized by a photovoltage , a bulk resistance rt for TiO2, a bulk resistance R of the electrolyte (adsorbed water layer) in a unit compartment, a reaction resistance ra and a double-layer capacitance Ca at the TiO2 anode/electrolyte interface in the unit compartment, and a reaction resistance rc and double-layer capacitance Cc at the Pd cathode/electrolyte interface. To analyze the photocatalysis in the steady state, a direct current model was assumed so that Ca and Cc were neglected, and therefore the circuit was simplified as shown in Figure 5b. In this circuit, resistance r consists of ra, rc, and rt. We simulated the voltage Vn (n ) 1, 2, 3, ...) applied to r when the relative humidity is 50% and the amount of deposited NaCl is 2 µg cm-2. Besides, the stripe length was assumed to be 1 cm, although the length does not affect the Vn values and the micro-cell width. The width of the unit compartment was set to 25 µm. Since almost the same results were obtained when the width was 50 µm, the width of 25 µm was sufficiently small. The photovoltage E was assumed to be 3.2 V according to the band gap of general anatase-type TiO2. Reaction resistance r was estimated as follows: under the present experimental conditions, the acetone generation rate was 2 × 10-8 mol min-1 cm-2 for the Pd-modified TiO2 sample in Figure 3. Therefore, the reaction current per unit compartment is ca. 8 × 10-8 A considering that one electron participates in generating one molecule of acetone in eqs 1 and 2. The potential for •OH generation (H2O + h+f •OH + H+) is +2.8 V (SHE, pH 0.7),26 and that for oxygen reduction (O2 + e- f O2-•) is +0.9 V (SHE, pH 0.7).27 Therefore, the voltage applied to r should be greater than 1.9 (= 2.8-0.9) V and smaller than 3.2 V. Thus, r is ca. 4 × 107 [= 3.2/(8 × 10-8)] Ω. Next, resistance R in the unit compartment at 50% RH was estimated as follows: the

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Figure 6. Simulated curves of the voltage applied to resistance component r in Figure 5 (b). The solid and broken curves were obtained for R/r ) 5 × 10-5 and 5 × 10-4, respectively. The voltage required for the photocatalytic oxidation of 2-propanol (1.9 V) is also indicated by a dotted line.

amounts of adsorbed water were 4 and 7 µg cm-2 when the amounts of deposited NaCl were 1 and 10 µg cm-2, respectively.25 Thus, in the present case, the amount of adsorbed water should be > 4 µg cm-2. The cross-sectional area of the adsorbed water layer is therefore 4 × 10-6 cm2. Considering the ionic conductivity of saturated NaCl solution (0.3 Ω-1 cm-1) and the width of the unit compartment (25 µm), R was estimated to be < 2 × 103 Ω . On the basis of the r (4 × 107 Ω) and R (2 × 103 Ω) values, R/r was assumed to be 5 × 10-5 (eq 3). The voltage Vn applied to resistance r (Figure 5b) decreases by a factor of R (eq 3) with departing from the Pd.

R ) Vn/Vn+1 ) 1 + 0.5R/r + x[(1 + 0.5R/r)2 - 1] (n ) 1, 2, ...) (3) The Vn values calculated from eq 3 were plotted in Figure 6 (solid curve). Since a potential difference of 1.9 V is necessary for the photocatalytic reactions to occur as described above, the simulated width of the micro-cell should be about 1.8 mm. This value is larger than the one estimated experimentally (e 250 µm). The simulated value contains certain errors arising from the estimation of the r and R values. Also, since resistance r includes not only ra and rc but also rt, the Vn value required for the micro-cell formation is larger than 1.9 V; the simulated value may be overestimated. On the basis of the simulation, a 10-fold increase in the R/r value causes a 4-fold decrease in the width of the micro-cell (Figure 6). However, the experimentally estimated width decreased by almost 2 orders of magnitude as the amount of deposited NaCl was decreased from 2 µg cm-2 to 0.2 µg cm-2 (Figure 4). This large difference between the theoretical expectation and the experimental result suggests that the ionic conductivity of the adsorbed water layer decreases with the salt concentration much more dramatically than does that of a bulk solution. The decrease in the conductivity may be explained in terms of the degree of NaCl dissociation and/or the mobility of the dissociated ions. The degree of dissociation might be suppressed as a result of interactions between water molecules and the TiO2 surface, which may limit the freedom of reorientation of water molecules and lower the dielectric properties. The mobility of the dissociated ions may also be affected by electrostatic and chemical interactions of the ions with the TiO2 surface. Among these, the latter effect may be more sensitive to an ionic concentration change. These effects may also be responsible for the difference between the experimentally estimated micro-cell width (e 250 µm) and the estimated one (1.8 mm).

We have demonstrated that photoelectrochemical micro-cells are formed in the vicinity of Pd deposited on a TiO2 thin film, on which NaCl has been deposited but on which there is no bulk liquid solution (i.e., in the gas phase). However, the microcell formation requires adsorbed water on the TiO2 surface, which should be essential for the dissociation of NaCl; the adsorbed water layer in which NaCl is dissolved should act as the electrolyte of the micro-cell. The width of the micro-cell varies from the order of micrometers to millimeters, depending strongly on the coverage of NaCl. Comparison between the experimentally observed dependence and the theoretically simulated one suggests that the ionic conductivity of the adsorbed water layer containing NaCl is suppressed in comparison with the corresponding bulk solution, probably due to electrostatic and/or chemical interactions between the ions and the TiO2 surface. The dimension of the micro-cell at the salt-free TiO2 surface was revealed to be smaller than 25 µm over the whole humidity range, even if the micro-cells form. We are currently trying to find the formation of the micro-cell and to evaluate its size at the salt-free TiO2 surfaces by using another system. In the case of NaCl as an electrolyte, it may be removed easily under wet conditions. However, if the TiO2 surface is chemically modified with an electrolyte that has sufficient stability against TiO2 photocatalysis and high ionic conductivity, it might be possible to construct novel photocatalytic systems in the gas phase with high charge separation efficiency and/or high photoproduct separation efficiency. In addition, photoelectrochemical micro-cells fabricated for the use in the liquid phase29 would also become applicable to gas-phase photocatalysis as a consequence of such a modification. Acknowledgment. We express gratitude to Professor D. A. Tryk for a careful reading of the manuscript and Dr. K. Honda for assistance with the theoretical simulations. We are grateful to the Ministry of Education, Culture, Sports, Science and Technology of Japan for financial support. This work was supported also by a grant from New Energy and Industrial Technology Development Organization in 2001, Japan (No. 00X25020x for Y. Ohko), the Kawakami Memorial Foundation (for T. Tatsuma), and Iketani Science and Technology Foundation (No. 0131014-A for T. Tatsuma). References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Pleskov, Y. V.; Gurevich, Y. Y. Semiconductor Photoelectrochemistry, Plenum: New York, 1986. (3) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1979, 100, 4317. (4) Photocatalytic Purification and Treatment of Water and Air; Ollis, D. E., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993. (5) Photocatalysis: Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; Wiley-Interscience: Amsterdam, 1989. (6) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (7) Hagfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49. (8) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (9) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis: Fundamentals and Applications; BKC, Inc.: Tokyo, 1999. (10) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C 2000, 1, 1. (11) Reiche, H.; Dunn, W. W.; Bard, A. J. J. Phys. Chem. 1979, 83, 2248. (12) Kobayashi, T.; Taniguchi, Y.; Yoneyama, H.; Tamura, H. J. Phys. Chem. 1983, 87, 768. (13) Kawai, T.; Sakata, T. Nature 1979, 282, 283. (14) Dunn, W. W.; Aikawa, Y.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 3445.

TiO2 Photocatalysis in the Gas Phase (15) Pichat, P.; Mozzanega, M. N.; Disdier, J.; Herrmann, J. M. NouV. J. Chim. 1982, 6, 559. (16) Wold, A. Chem. Mater. 1993, 5, 280. (17) Wang, C.-M.; Heller, A.; Gerischer, H. J. Am. Chem. Soc. 1992, 114, 5230. (18) Ibusuki, T.; Takeuchi, K. J. Mol. Catal. 1994, 88, 93. (19) Fu, X.; Clark, L. A.; Yang, Q.; Anderson, M. A. EnViron. Sci. Technol. 1996, 30, 647. (20) Sitkiewitz, S.; Heller, A. New J. Chem. 1996, 20, 233. (21) Ohko, Y.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. A 1997, 101, 8057. (22) Ohko, Y.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. B 1998, 101, 2699. (23) Romeas, V.; Pichat, P.; Guillard, C.; Chopin, T.; Lehaut, C. Ind. Eng. Chem. Res. 1999, 38, 3878.

J. Phys. Chem. B, Vol. 105, No. 41, 2001 10021 (24) Ikeda, K.; Sakai, H.; Baba, R.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. B 1997, 101, 2617. (25) Huang, J.; Shinohara, T.; Tsujikawa, S. Zairyo-to-Kankyo 1999, 48, 575. (26) Electrochemical Data - A Handbook for Electrochemists in Industry and UniVersities; Dobos, D., Ed.; Elsevier Science: Amsterdam, 1975; p 253. (27) Vracar, L. M.; Sepa, D. B.; Damjanovic, A. J. Electrochem. Soc. 1986, 133, 1835. (28) Tada, H.; Hattori, A.; Tokihisa, Y.; Imai, K.; Tohge, N.; Ito S. J. Phys. Chem. B 2000, 104, 4585. (29) Tatsuma, T.; Ikezawa, A.; Ohko, Y.; Miwa, T.; Fujishima, A. Electrochem. Solid-State Lett. 2000, 3, 467.