Influence of Sub-Band-Gap States on Light Induced Long-Lasting

Feb 6, 2009 - Influence of Sub-Band-Gap States on Light Induced Long-Lasting Super-Hydrophilic Behavior of TiO2. V. Spagnol, H. Cachet, B. Baroux and ...
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J. Phys. Chem. C 2009, 113, 3793–3799

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Influence of Sub-Band-Gap States on Light Induced Long-Lasting Super-Hydrophilic Behavior of TiO2 V. Spagnol,† H. Cachet,† B. Baroux,‡ and E. Sutter*,† UniVersite´ Pierre et Marie Curie-Paris 6, Laboratoire Interfaces et Syste`mes Electrochimiques, CNRS, UPR15-LISE, 4 place Jussieu, 75005 Paris, France, and Laboratoire Science et Inge´nierie des Mate´riaux et Proce´de´s, INPG, Domaine UniVersitaire, 1130 rue de la Piscine, B.P. 75, 38402 Saint Martin d’He`res Cedex, France ReceiVed: September 25, 2008; ReVised Manuscript ReceiVed: January 8, 2009

The photo-induced superhydrophilic behavior of nanocolumnar TiO2 layers was investigated by means of contact angle measurements and electrochemical impedance spectroscopy (EIS). Two types of layers were compared: the first type of layers showed long-lasting superhydrophilic behavior after UV exposure (even during storage in the dark), whereas in the second type of layers, no surface wettability conversion was observed. EIS measurements showed that the two layers were characterized by a high dielectric constant that still increased upon light exposure. A main difference between the two types of sample concerned the distribution of the trap states in the gap before UV exposure. Moreover, after UV exposure, new traps were generated in the gap but only for the superhydrophilic layer. These results are discussed, taking into account the well-admitted photoinduced hydrophilic mechanism, involving hole trapping at the surface of the layer and generation of oxygen vacancies. It clearly appeared that hole-trapping at the surface is determined by long-lasting electron-trapping in the bulk of the oxide layer, enabling efficient and long-lasting charge separation. A model is suggested for estimating the contact angle variation using the electrowetting theory. 1. Introduction R. Wang et al.1 first reported the generation of highly hydrophilic TiO2 surfaces by UV exposure in 1997, providing self-cleaning and antifogging properties to this material. It is now well admitted that the mechanism of photoinduced wettability is quite different from that of conventional photocatalysis.2 The accepted mechanism for photoinduced aqueous wettability on TiO2 is the following: first, absorption of UV photons results in the generation of conduction band electrons and valence band holes. While electrons reduce Ti(IV) cations to Ti(III), holes migrate to the TiO2 surface where they oxidize bridging O2- anions. The later reaction leads to the expulsion of an O atom followed by adsorption of water molecules at the resulting vacancy site, thereby producing new OH groups and increasing the hydrophilicity of the surface.3,4 Results obtained on single-crystals showed that the effect depends on the crystal plane and on the presence of oxygen bridging sites at the surface, where dissociative water adsorption occurs.5 Sakai et al.4 stated that only the photogenerated holes play a key role in the surface hydrophilic conversion, whereas photogenerated electrons do not directly influence the process. In most cases photoinduced superhydrophilicity is only maintained under UV exposure and is quickly suppressed when the surface is stored in the dark, due to the replacement of hydroxyl groups by oxygen from air.5 To our best knowledge, long-lasting superhydrophilic behavior of TiO2 layers has not been discussed before, but some authors reported that addition of SiO2 into a TiO2 film allows the superhydrophilic behavior to be maintained for a certain time * To whom correspondence should be addressed. Phone: 331 44 27 41 68. Fax: 331 44 27 40 74. E-mail: [email protected]. † Universite´ Pierre et Marie Curie-Paris. ‡ INPG-St Martin d’He`res.

in the dark.6,7 This effect has been explained in terms of enhanced acidity at the SiO2-TiO2 interfaces. In general, the photon-to-charge carrier conversion efficiency is determined by the competition between recombination and charge transfer reactions. In the case of TiO2, it has been reported that the kinetics of charge recombination are largely controlled by the electronic occupancy of conduction band/trap states within TiO2.8 Due to their different behavior as compared with bulk systems, many works have been performed during the past decade on the charge transfer mechanisms in nanosized TiO2. In nanocrystalline titanium dioxide, in particular, the existence of high density sub-bandgap or trap states has been highlighted.9-15 Berger et al.13 also reported strong dependence of the trap states concentration on the morphological structure of the films. Whereas in photocatalytic systems or in dye-sensitized solar cells, the influence of charge recombination or charge trapping on the overall quantum efficiency is well admitted, it is generally not taken into consideration to explain hydrophilic behavior. Nevertheless, if long-lasting superhydrophilic behavior is observed, efficient charge separation is likely to occur. The aim of the present work is thus first to determine the parameters responsible for long-lasting superhydrophilic properties of titanium dioxide films with nanocolumnar structure and then to correlate this behavior with the electronic properties of the film. Surface wettability conversion on the nanocolumnar titanium dioxide films was followed by means of contact angle measurements and the electronic properties by electrochemical impedance spectroscopy (EIS). For a better understanding of the overall mechanism of long-lasting wettability conversion, two types of nanocolumnar TiO2 layers were compared: on one hand, samples presenting a photoinduced superhydrophilic behavior

10.1021/jp8085182 CCC: $40.75  2009 American Chemical Society Published on Web 02/06/2009

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and on the other hand, samples that do not become superhydrophilic upon UV exposure. Impedance measurements were performed in the dark for the two types of samples, before and after exposure, in order to measure the long-lasting UV-induced modifications. For interpretation of EIS diagrams, the electrical circuit described in ref 14 was used. In this work, the UV-induced modifications on the electrochemical characteristics of TiO2 layers will be used to highlight the mechanism of long-lasting photoinduced superhydrophilic behavior. The results will be discussed taking into account the well-admitted photo- induced hydrophilic mechanism, involving creation of oxygen vacancies at the surface with dissociative water adsorption. 2. Experimental Section Electrodes and Solutions. TiO2 films were deposited on stainless steel (AISI 304 thickness 0.2 mm) by Physical Vapor Deposition (provided by Arcelor-Mittal), using two different reactors in which DC-reactive magnetron sputtering was performed in a mixture of argon and oxygen with a titanium target as sputter source. Samples (1 cm × 1 cm) were cleaned in an ultrasonic bath first with ethanol and then with deionized water (Millipore, 18 MΩ.cm) and dried with blowing N2 gas. Side and back surfaces of the sample were covered with an insulating resin (Araldite). For electrical contact, a conductive wire was soldered onto the rear of the steel sample after removing the passive layer. Samples were finally cleaned as mentioned previously. All the measurements were performed in 1 mol/L deaerated Na2SO4 solution (deionized water (Millipore) and Na2SO4 · 3H2O (PROLABO)). Before each experiment, oxygen was removed from the solution through N2 bubbling during 1 h (purity 99.995%). Electrochemical Cell and Apparatus. Electrochemical measurements were performed at room temperature in a standard three-electrode cell. All potentials were measured against and referred to a saturated calomel reference electrode (SCE), whereas a platinum grid was used as a counter electrode. An electrochemical interface (Solartron SI 1287) and a frequency response analyzer (Solartron 1250) were used for impedance measurements which were performed in potentiostatic conditions (between 0.2 V/SCE and -0.8 V/SCE by 25 mV or 50 mV steps), over frequencies ranging from 0.1 Hz to 65 kHz, using a 10 mV sinusoidal potential modulation. Impedance data were fitted with ZSimpWim software (Princeton Applied Research). A 125 W mercury vapor lamp (HPR 125 W (Philips)) was used as the UV light source (wavelength range 300-400 nm). Protocol for Impedance Measurements in Potentiostatic Conditions. Samples were immersed in the electrolyte during 1 h before impedance measurements. The first measurement was performed at the open-circuit potential (OCP). Then the applied potential value was changed and held during one minute, before another impedance measurement was performed. After this first set of experiments, the sample was left in the dark in the electrolyte during 12 h before UV exposure (3 h). Impedance measurements were then performed in the dark again, first at the OCP and then at various applied potentials as stated previously. Contact Angle Measurements. The photoinduced hydrophilicity of the sample was evaluated by following the in-time variation of the water contact angle under and after UV exposure in a water-saturated atmosphere. UV light was provided by a 100 W black light bulb (B-100AP, Ultraviolet Product Co. Ltd.)

Figure 1. SEM picture of a cross section of a TiO2 film deposited on stainless steel.

associated with a 365 nm filter. Real-time experiments were performed under constant temperature (20 °C) by using a ¨ SS G10 goniometer connected with a video camera. KRU Deionized water droplets (volume 0.5 µL) were placed on the sample and water contact angle variations were measured using DSA-software. 3. Results and Discussion In the present work, two types of samples were considered: the samples with photoinduced hydrophilic behavior are designed as superhydrophilic (SH) and the samples with no superhydrophilic behavior upon UV exposure are designed as nonsuper-hydrophilic (NSH). In the case of SH samples, films of various thickness (from 30 to 145 nm) were studied. 3.1. Characterization of TiO2 Films. Figure 1 shows a cross section SEM photograph of a SH type TiO2 film deposited on stainless steel. The TiO2 film is compact, and made of 20 nmdiameter nanocolumns. The thickness of the TiO2 films is about 100 nm. The 100 nm thick NSH TiO2 layer has exactly the same morphology and is therefore not represented here. Whatever the thickness of the TiO2 deposit is, all the studied layers showed a morphology similar to that of Figure 1, that is, a nanocolumnar structure with 20 nm diameter nanocolumns. X-ray diffraction analysis revealed, in all the samples, a polycrystalline anatase structure of the film. The only difference between SH and NSH layers was detected by secondary-ion mass spectroscopy analyses (SIMS): the comparison of 1H and (1H + 16O) signals between the two types of samples showed 100-fold higher signals in the SH layer as compared with the NSH one, indicating high water content in the SH TiO2 layers. 3.2. Contact Angle Measurements. Figure 2 shows the effect of UV exposure on the contact angle of a water droplet on SH and NSH 100 nm thick TiO2 films. The contact angle on the SH TiO2 layer decreases from 70° to a value close to 0°, after 40 min of exposure, whereas for the same light exposure conditions, the contact angle of the NSH sample does not vary, even for longer UV duration. Influence of the Thickness of the SH Layers on the Decrease Rate of the Contact Angle upon UV Exposure. The plot of the contact angle decrease as a function of UV exposure time is reported in the insert of Figure 3 for several thickness of SH TiO2 layers. It appears that the rate of contact angle variation increases when the thickness of the layer increases. From these results, the exposure time necessary to reach a contact angle of

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Figure 2. Variation of contact angle on SH and NSH TiO2 layers as a function of UV exposure time. Figure 4. Contact angle increase as a function of storage time in the dark, after 40 min (squares), 1 h (circles), and 2 h (triangles) of UV exposure (40 min is the minimum exposure time necessary to have superhydrophilic behavior).

Figure 3. Variation of UV exposure time needed to reach a contact angle of 10°, as a function of the thickness of a SH TiO2 layer. Inset: contact angle decrease as a function of exposure time for different thickness of the layer.

10° is plotted in Figure 3 as a function of the film thickness (between 30 and 145 nm) and shows an exponential decrease. ReWersibility of the Hydrophilic BehaWior after Storage in the Dark. Once the films exhibited a superhydrophilic behavior, additional measurements were performed to measure the rate of contact angle increase after storage in the dark for various initial UV exposure times. As an example, Figure 4 shows the results obtained for three SH 100 nm thick layers. All samples need a 40 min UV exposure to reach a contact angle of 0°, but for two of them, UV exposure was extended, respectively, to 1 and 2 h. It clearly appears from Figure 4 that the recovery of the initial contact in the dark is significantly lower when the initial exposure duration is longer. All these results (Figures 2-4) show that the bulk of the layer acts like a “reservoir” able to store light induced modifications, which is determining for the long-lasting photoinduced superhydrophilicity of the surface of the layer. 3.3. Influence of UV Exposure on the Open-Circuit Potential. For open-circuit measurements, samples were irradiated and kept in the dark in the electrochemical cell in the 3-electrode configuration. Upon UV exposure, the open-circuit potential decreases from -0.1 to -0.7 V/SCE for the SH layer, whereas it only reaches -0.4 V/SCE for the NHS sample (Figure 5a). During storage in the dark after UV exposure (Figure 5b), the open-circuit potential of a SH sample keeps a very negative value, even after 80 min, whereas the open-circuit potential of the NSH sample reaches a value close to its initial value within the same time. We can thus conclude that in the

Figure 5. Evolution with time of the open-circuit potential in 1 mol L-1 Na2SO4 solution (a) during UV exposure of SH and NSH TiO2 layers (b) during storage in the dark, for SH layers after 45 min (open circles) and 90 min (triangles) of UV exposure and for a NHS layer after 45 min (squares) of UV exposure.

case of a SH TiO2 film, UV exposure induces long-lasting modifications due to the storage of photoinjected negative charges in trapping states. 3.4. Impedance Measurements. There is large amount of literature concerning sub-band gap states in TiO2.8-12,15,16 They may be extended below the conduction band (Urbach tail) and stay in the surface or in the bulk. Also, they may have localized energy. Those are the generally assigned as surface states. Accordingly, the storage effect mentioned above is expected to be correlated with some charge trapping in the layer during UV

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Figure 6. Equivalent circuit used to characterize the electrochemical behavior of the SH and NSH layers in the dark, before and after UV exposure.

exposure. Thus impedance measurements were performed with special attention to the behavior of surface states before and after UV exposure. All the measurements were performed in the dark in order to take only into account the long-lasting lightinduced modifications. The equivalent circuit used in order to characterize the electrochemical behavior of a substrate/TiO2/ electrolyte system has been described elsewhere14 and is reported in Figure 6. A (RSS, CSS, WSS) series circuit is added in parallel to a basic RT-CPE circuit and the result was in series with an electrolyte resistance RS. From the values of Rs, Rt, and CPE, an equivalent capacity value could be calculated as exposed in reference,14 and this capacity value is characteristic of the space charge layer capacitance. The parallel disposition of the RT-CPE circuit with (RSS, CSS, WSS) means that band bending in the charge space layer of the semiconductor and surface states filling occur simultaneously when the potential changes. This model enables data fitting with a relative error for real and imaginary parts of the impedance of less than 1% over the whole frequency range. RSS and CSS are associated, respectively, with surface state resistance and capacitance. The Warburg element, Wss, accounts for some delay in the surface states response. Thus, we suggest that the filling of states is associated with some mass transfer process, such as hydrogen diffusion. According to refs 14 and 17, the dielectric constant εr of the layer can be determined from Mott-Schottky diagrams (C-2 ) f(E)) in the potential range in which the space charge layer of the semiconductor is fully depleted, that is, at high enough anodic potentials (above 0 V in Figure 7). Figure 7 shows two examples: the first one (Figure 7a) is the Mott-Schottky diagram of a 145 nm thick SH TiO2 layer, before and after UV exposure. The second example (Figure 7b) corresponds to a 100 nm thick NSH layer before and after exposure. From the Mott-Schottky diagrams, a flat band potential (EFB) value around -0.5 V can be estimated for the different layers. Results Concerning the Variation of the Dielectric Constant. Before UV Exposure. The values of the dielectric constant extracted from the capacitance values reported in Mott-Schottky diagrams, for two 100 nm thick layers, before and after light exposure are presented in Table 1. The comparison between 100 nm thick SH and NSH layers clearly shows that a SH layer has a higher εr value (εr ) 310) than a NSH layer (εr ) 140). Moreover, both these values are surprisingly high as compared with values usually reported in the literature (εr ) 55 for polycrystalline anatase in17). The high values obtained for the capacitance and the corresponding εr in SH layers have been discussed in ref 14. As shown in section 3.1, the main difference in physicochemical properties between the two types of layer is their water content. Because the determined value is still much higher for SH than for NSH samples, we suggest that the high water content of the SH layers is associated with the high value of εr. Nevertheless, it is not clear if such a high value is due to bulk dielectric value or to the increase of the polarizability of the layer.

Figure 7. Mott-Schottky plots of TiO2 film in 1 mol L-1 Na2SO4 solution in the dark, before and after UV exposure (a) for a 145 nm thick SH layer and (b) for a 100 nm thick NSH layer.

TABLE 1: Dielectric Constants of Nano-Columnar TiO2 Layers Before and After UV Exposure hydrophilic behavior

thickness (nm)

εr before UV exposure

εr after UV exposure

SH NSH

105 100

310 140

620 280

After UV Exposure. After UV exposure, the dielectric constant value of both layers undergoes a 2-fold increase (εr ) 620 for SH and εr ) 280 for NSH for the 100 nm thick layers, in Table 1). The increase of εr after UV exposure, for the two types of layers, has been attributed in ref 14 to UV-driven hydrogen insertion into the layer. Nevertheless, since the increase occurs in both SH and NSH films, this εr increase is not likely to be correlated with the superhydrophilic behavior and will therefore not be further discussed in this paper. Results Concerning Sub-Band Gap States. The Css, Rss, and Wss values extracted from the equivalent circuit of Figure 6 are reported in Table 2 for three different potential values before and after UV exposure of the two types of layers (SH and NSH). Before UV Exposure. The surface state capacitance values Css before UV exposure are plotted in Figures 8a and 9a as a function the applied potential for the two 100 nm thick TiO2 layers. The SH layer (Figure 8a) and the NSH layer (Figure 9a) show very different behaviors. Whereas the NSH layer presents randomly distributed surface state capacitance values, the SH layer displays an exponential dependence on the applied potential. This behavior is characteristic of an exponential energetic distribution of states in the band gap (sometimes called Urbach tail).8 In addition, a capacitance peak appears at about -0.25 V, corresponding to a nearly monoenergetic state in the

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TABLE 2: Fitted Values from the Model in Figure 6, at some Selected Potentials, Before and After UV Exposure for Two Types of Layer (100 nm Thickness) SH layer

NSH layer

E (V/SCE) Rss (Ω cm2) CSS (µF cm-2) Wss (Ω cm2) before UV exposure after UV exposure

-0.1 -0.4 -0.6 -0.1 -0.4 -0.6

72880 2.4 1.1 1637 0.6 1.3

2.9 23 52 20 36 40

3 6 6 2 6 8

× × × × × ×

-6

10 10-4 10-3 10-5 10-4 10-3

Figure 8. Surface state capacitance values Css calculated using the model in Figure 6, before (a) and after (b) UV exposure for a 100 nm thick SH layer.

Figure 9. Surface state capacitance values Css calculated using the model in Figure 6, before (a) and after (b) UV exposure for a 100 nm thick NSH layer.

gap with a corresponding charge value close to 3.6 µC cm-2. Such a combination has often been reported for nanostructured TiO2 layers.9,16 The Rss values reported in Table 2 shows that Rss drastically decreases in SH layer changing from 73000 to 2 Ω cm-2 when the applied potential changes from -0.1 to -0.4 V, corresponding to a significant decrease of the surface state resistance. On the contrary, in the NSH layer, in the same potential range, Rss values only decrease from >100000 to about 2000 Ω cm-2. The calculation of the corresponding time constant CssRss in Table 2 shows a very smaller value for SH layers than for NSH layers in the potential range negative to -0.4 V, indicating that charge trapping in surface states is much faster in SH layer than in NSH layer. After UV Exposure. After UV exposure, another significant difference appears between the two types of sample. For SH layers (Figure 8b), capacitance values relative to new surface states appear in the [0.1; -0.2 V] potential range, that is, in a potential range far from the conduction band edge. On the contrary, for NSH layers (Figure 9b), new states appear after

Rss (Ω cm2) CSS (µF cm-2) Wss (Ω cm2)

CssRss 0.21 5.5 × 5.7 × 3.3 × 2.2 × 5.2 ×

10-5 10-5 10-2 10-5 10-5

689000 2011 31 15000 6600 130

2 57 106 10 330 1120

1 2 9 9 5 8

× × × × × ×

-5

10 10-4 10-4 10-6 10-5 10-4

CssRss 1.38 0.11 3.3 × 10-3 0.15 2.2 0.15

UV exposure but mainly in a potential range negative to -0.4 V, that is, very close to the conduction band edge (Efb is about -0.5 V/SCE). 3.5. Mechanism of Charge Transfer in SH and NSH Layers. In the present work, impedance measurements clearly evidenced different charging mechanisms in SH and in NSH TiO2 layers. Before UV exposure, a different distribution of energy states in the band gap is observed in the two cases (Figures 8a and 9a). After UV exposure, new states appear in the two types of layer, but in a different potential range, whereas for the SH layer, new states appear in the [0.1; -0.2 V] potential range, the main modifications in the NSH layer are localized close to the conduction band at potentials negative to -0.4 V (Figures 8b and 9b). In the following, those new states are assigned as “photo-generated states”, though it is yet not clear if they are really photogenerated or if they are just charged or redistributed. Figure 5 shows that UV exposure induces a shift of the opencircuit potential, which is more negative for the SH than for the NSH sample. Moreover, for the SH sample, after UV disruption, the open-circuit potential is pinned at a value more negative than the potential characteristic of the photogenerated states, indicating that all the states generated in the gap are electron-filled in a long-lasting way. On the contrary, for NHS layers, the open-circuit potential after UV disruption reaches rapidly a value close to its initial value in the dark, confirming the absence of stable photogenerated state in the gap. The UV induced modifications described above have to be set back in the context of the well-admitted mechanism of photoinduced superhydrophilic behavior of TiO2,3,4 in which formation of oxygen vacancies at the surface results from the reaction between a photohole and an oxygen ion of the surface, leading to dissociative water adsorption at the oxygen vacancies. Berger et al.18 performed EPR and IR analyses at low temperature (90 K) to reduce the rate of hole-electron recombination in TiO2 particles after UV exposure. They clearly evidenced that photogenerated holes are O- species, produced from lattice O2- ions, and that photogenerated electrons are detected as Ti3+ species. On the basis of their observation, the following mechanism has been suggested

2-

O

hν f e-CB + h+VB

(1)

e-CB + Ti4+ f Ti3+ trapped electron

(2)

+ h+VB f OL-

(trapped hole in the lattice)

(3)

3,4

In the overall theory of hydrophilic conversion, oxygen vacancies are formed after trapping of a second hole, according to eq 4

OL- + h+VB f 1⁄2O2 + 0(oxygen vacancy)

(4)

While in ref 18 charge separation was reversed via charge recombination when the temperature was raised from 90 to 298

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K, in the present work, charge separation at 298 K in the SH layer exhibited lifetimes of hours, enabling the present electrochemical study. Our measurements, performed in a potential range far from the valence band edge, could only evidence electron trapping after UV exposure. But since long-lasting superhydrophilic behavior seems to be correlated with stable photogenerated electron traps, we can conclude that hole trapping at the surface is determined by long-lasting electron trapping in the bulk of the layer, enabling efficient and persistent charge separation. The strong negative shift of the open-circuit potential after UV exposure of SH layers shows that there is no compensation between the number of trapped electrons and the holes. This is due to the fact that a large ratio of photogenerated holes is transferred to the electrolyte for oxidation reactions, whereas the photogenerated electrons accumulate in the trap states. Another important result of this study is that efficient longlasting electron storage in the layer needs a specific energetic distribution of states in the band gap, before any light exposure. Indeed, the super hydrophilic behavior is only generated in TiO2 layers with an exponential energetic distribution of states in the upper part of the gap and a monoenergetic state. Quantification of the Contact Angle Decrease after UV Exposure. In SH layers in which the requested distribution is observed, UV exposure leads to the pinning of the open-circuit potential at negative values (-0.7 V in Figure 5), similar to a permanent cathodic polarization. Because the open-circuit potential after UV exposure is more negative than the potential characteristic of the sub-band gap states, electron trapping in these states is likely to occur. It is well-known that accumulation of surface charge affects the redistribution of potential drop over the depletion layer of the semiconductor and the Helmholtz layer. This can lead to Fermi level pinning, thereby changing the potential drop ∆ΦH in the Helmholtz layer at the oxide/ electrolyte interface. It can be considered that the potential applied across the double layer is no more negligible in comparison with the potential applied across the depletion layer when the surface state capacitance Css reaches a value similar or higher than the double layer capacitance (i.e., Css g about 30 µF cm-2). From Figure 8, values close to or higher than 30 µF cm-2 are observed for the SH layer in the whole potential range under study. We can then at first approximate consider that the observed shift of the open circuit potential from -0.1 to -0.7 V results in a potential drop of -0.6 V across the Helmholtz layer. The response of the water contact angle upon applying a voltage ∆U across a double layer at a metal/electrolyte interface can be quantified using the electrowetting theory.19 According to this approach (and neglecting the possible influence of surface roughness),

cos θ ) cos θY +

CH (∆U)2 2σlv

(5)

with θ and θY, the contact angles, respectively, after and before applying a potential drop across the double layer, CH, the capacitance of the double layer, and σlv, the interfacial tension between water and air (σlv ) 0.072 J m-2). In the electrowetting theory, ∆U of eq 5 is the difference between the applied potential and the potential of zero charge. In the present work, eq 5 was used for the estimation of the contact angle variation resulting from a potential drop of 0.6 V across the double layer. Taking CH ) 30 µF cm-2 for the double layer capacitance and θY ) 70° for the initial value of the contact

angle, we found for cos θ a value very close to 1, which corresponds to a superhydrophilic behavior. In the case of the NSH layer, because electron trapping only occurs in states close to the conduction band edge, no Fermi level pinning on the surface states is expected. This means that the potential drop induced through UV exposure occurs mainly within the space charge layer of the semiconductor. In the electrowetting theory, eq 5 is then replaced by

cos θ ) cos θY +

ε0εd (∆U)2 2dσlv

(6)

with εd and d, respectively, the dielectric constant and the thickness of the layer, and ε0, the permittivity of free space. Taking θY ) 70°, εd ) 390 (from Table 1), d ) 100 nm, and ∆U ) 0.4 V (from Figure 5b) for the potential drop across the TiO2, a value close to 0.38 for cos θ, that is, a contact angle of 68° are calculated. This result is in good agreement with the observed constant values of the contact angle upon UV exposure for NSH layers. Chemical Nature of Electron Carrier Traps. Whereas the mechanisms of photoinduced hydrophilicity clearly assigns trapped photoholes to O-L or oxygen vacancies at the surface of TiO2,3,4 the chemical nature of electron traps is still a matter of discussion. Electron trap states characterized by an exponential energetic distribution in the gap are generally attributed to Ti(IV) sites in the TiO2 lattice9,13,16,18 (see eq 2) or to Ti-OH sites,20,21 in which photoelectrons are trapped according to

Ti(IV) - OH + e- f Ti(III) - OH

(7)

But the location of surface states in other defects in the lattice or at grain boundaries have also been suggested.13 Because, in the present work, the main difference in composition between SH and NSH samples is the higher water content of SH films, we suggest that water molecules in the layer could generate states enabling long transit time of the photoinjected electrons. This assumption is based on some recent works,22,23 in which water molecules adsorbed on TiO2 particles have been shown to increase the efficiency of electron trapping. Nevertheless, further investigations are necessary for a better identification of the electron trap states 4. Conclusion The comparison between long-lasting superhydrophilic behavior and nonsuper- hydrophilic behavior of TiO2 layers showed that the rate of contact angle decrease and increase, respectively, during and after UV exposure, can be correlated with the filling of electron traps in the band gap of the semiconducting oxide. This leads to the conclusion that the photoinduced superhydrophilic behavior is a complexe mechanism involving several contributions: (1) an oxidized surface with vacancy sites; and (2) the electron-trapping in the bulk of the layer alloying inhibition of charge carrier recombination and creating a potential drop across the double layer suitable for electrowetting. Efficient electron trapping seems to be correlated with an exponential distribution of the electron trap states and/or with the presence of a monoenergetic state before any UV exposure. Such a situation could be generated by the presence of water molecules in the nanocolumnar structure of the layer, since only TiO2 layers with high water content showed long-lasting hydrophilic behavior. Memory effect of the superhydrophilicity is thus largely dependent on the columnar structure and the deposition process of the oxide layer.

Hydrophilic Behavior of TiO2 The value and stability of the photoinduced shift of the opencircuit potential provide a first estimation of the value and the persistence of the contact angle, by means of the electro wetting theory. Acknowledgment. The authors wish to thank ArcelorMittal company for financial support and supplying the TiO2 deposited samples, Stephan Borensztajn (CNRS, UPR15-LISE) for SEM pictures, Patrick Choquet (CRP-Gabriel Lippmann) for SIMS analyses, and J.C. Joud and M. Langlet (INP Grenoble) for contact angle measurements and fruitful discussions Supporting Information Available: Experimental details. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (2) Miyauchi, M.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Chem. Mater. 2002, 14, 2812. (3) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (4) Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2001, 105, 3023. (5) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188.

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3799 (6) Machida, M.; Norimoto, K.; Watanabe, T.; Hashimoto, K.; Fujishima, A. J. Mater. Sci. 1999, 34, 2569. (7) Permpoon, S.; Berthome´, G.; Baroux, B.; Joud, J.; Langlet, M. J. Mater. Sci. 2006, 41, 7650. (8) Haque, S. A.; Tachibana, Y.; Willis, R. L.; Moser, J. E.; Gratzel, M.; Klug, D. R.; Durrant, J. R. J. Phys. Chem. B 2000, 104, 538. (9) Bisquert, J. Phys. Chem. Chem. Phys. 2008, 10, 49. (10) Boschloo, G.; Fitzmaurice, D. J. Electrochem. Soc. 2000, 147, 1117. (11) Wang, H.; He, J.; Boschloo, G.; Lindstrom, H.; Hagfeldt, A.; Lindquist, S. E. J. Phys. Chem. B 2001, 105, 2529. (12) Salvador, P.; Hidalgo, G.; Zaban, A.; Bisquert, J. J. Phys. Chem. B 2005, 109, 15915. (13) Berger, T.; Lana-Villarreal, T.; Monllor-Satoca, D.; Gomez, R. J. Phys. Chem. C 2007, 111, 9936. (14) Spagnol, V.; Sutter, E.; Debiemme-Chouvy, C.; Cachet, H.; Baroux, B. Electrochim. Acta 2008, doi:10.1016/j.electacta.2008.08.070. (15) Fabregat-Santiago, F.; Mora-Sero, I.; Garcia-Belmonte, G.; Bisquert, J. J. Phys. Chem. B 2003, 107, 758. (16) Bisquert, J.; Fabregat-Santiago, F.; Mora-Sero´, I.; Garcia-Belmonte, G.; Barea, E. M.; Palomares, E. Inorg. Chim. Acta 2008, 361, 684. (17) van de Krol, R.; Goossens, A.; Schoonman, J. J. Electrochem. Soc. 1997, 144, 1723. (18) Berger, T.; Sterrer, M.; Diwald, O.; Knozinger, E.; Panayotov, D.; Thompson, T. L.; Yates, J. T. J. Phys. Chem. B 2005, 109, 6061. (19) Mugele, F.; Baret, J.-C. J. Phys.: Condens. Matter 2005, 17, R705. (20) Boschloo, G.; Fitzmaurice, D. J. Phys. Chem. B 1999, 103, 2228. (21) Szczepankiewicz, S. H.; Colussi, A. J.; Hoffmann, M. R. J. Phys. Chem. B 2000, 104, 9842. (22) Warren, D. S.; Shapira, Y.; Kisch, H.; McQuillan, A. J. J. Phys. Chem. C 2007, 111, 14286. (23) Panayotov, D. A.; Yates, J. J. T. Chem. Phys. Lett. 2005, 410, 11.

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