Efficient Charge Transport through a Metal Oxide Semiconductor in

Jul 10, 2007 - Koji Sone,†,‡ Masahiro Teraguchi,‡,§ Takashi Kaneko,‡ Toshiki Aoki,‡,§ and Masayuki Yagi*,†,‡. Faculty of Education and...
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J. Phys. Chem. C 2007, 111, 11636-11641

Efficient Charge Transport through a Metal Oxide Semiconductor in the Nanocomposite Film with Tris(2,2′-bipyridine)ruthenium(II) Koji Sone,†,‡ Masahiro Teraguchi,‡,§ Takashi Kaneko,‡ Toshiki Aoki,‡,§ and Masayuki Yagi*,†,‡ Faculty of Education and Human Sciences, Faculty of Engineering, and Center for Transdisciplinary Research, Niigata UniVersity, 8050 Ikarashi-2, Niigata 950-2181, Japan ReceiVed: April 18, 2007; In Final Form: May 18, 2007

A nanocomposite film of tungsten trioxide (WO3) and [Ru(bpy)3]2+ (bpy ) 2,2′-bipyridine) (denoted as RuWO3) was prepared from an aqueous colloidal triad solution containing peroxotungstic acid, [Ru(bpy)3]2+, and poly(sodium 4-styrenesulfonate) by an electrodeposition technique. The electrochemical features of the Ru-WO3 film were investigated using cyclic voltammetry (CV) and potential-step chronoamperospectrometry (PSCAS) techniques, compared with those in a [Ru(bpy)3]2+/Nafion (Ru-Nf) film. PSCAS data spectrophotometrically showed that RuII is completely oxidized for 0.3 s in the Ru-WO3 film, in contrast to the corresponding reaction completed for 30 s in the Ru-Nf film. The apparent diffusion coefficient (Dapp ) (0.1-1.1) × 10-7 cm2 s-1) for charge transport (CT) by a RuII/RuIII redox in the Ru-WO3 film was higher than that (2.4 × 10-10 cm2 s-1) in the Ru-Nf film by 2 or 3 orders of magnitude. Activation energy (Ea) for CT (14.1 kJ mol-1) for the Ru-WO3 film was 3.5 times lower than that (Ea ) 49.8 kJ mol-1) for the Ru-Nf film. The lower Ea could be responsible for the faster CT in the Ru-WO3 film than the Ru-Nf film. Dapp in the Ru-WO3 film increased linearly with an increase of the final applied potential (Ef) for PSCAS from 1.2 to 1.5 V vs SCE and saturated above Ef ) 1.5 V. The mechanism for CT in the Ru-WO3 film is proposed, in which electrons are injected from [Ru(bpy)3]2+ into the conduction band (CB) of WO3 and go through there to a collector electrode.

Introduction Recently, much attention has been paid to nanocomposite films containing functional molecules that are applicable to electronic or photoelectronic devices such as sensors,1 electrocatalysis cells,2,3 batteries,4 and solar cells.5,6 For fabrication of molecular devices using functional molecules-containing nanocomposite films, it is important not only to develop a new molecule with a superior function but also to improve charge transport (CT) from a collector electrode to a center molecule in the film. In a solid film containing a redox molecule, CT takes place by physical displacement of the molecule and/or by charge hopping between them, involving counterion movement and redox equilibrium. CT should be treated as a diffusion process regardless of these mechanisms, and the apparent diffusion coefficient (Dapp) is generally given to be 10-8 to 10-12 cm2 s-1 in the solid films.7-17 Nanoporous semiconductor films, in particular of TiO2, adsorbing functional redox molecules as a monolayer are potential nanocomposite films.18,19 In such semiconductor/ functional redox molecule films, electrochemical oxidation of the adsorbed molecules does not occur in the case with their redox potential lying above the flat band potential (EFP) of n-type semiconductors due to the energy barrier of the space-charge layer formed at an interfacial heterojunction (a so-called Schottky junction).20 However, Gra¨tzel’s group has found that phosphonated triarylamine adsorbed on nanocrystalline films * Corresponding author. Telephone and fax: +81-25-262-7151. Email: [email protected]. † Faculty of Education and Human Sciences. ‡ Center for Transdisciplinary Research. § Faculty of Engineering.

of metal oxide semiconductors (TiO2, ZrO2, and Al2O3) as a monolayer displays a reversible electrochemical response of triarylamine, though its redox potential lies above the EFP of semiconductors.21 Dapp for CT by oxidation of triarylamine increased with their coverage on the metal oxides, and Dapp at the full coverage was a range of 3.5 × 10-10 to 1.1 × 10-7 cm2 s-1. On the basis of the concentration-dependent Dapp, the mechanism of CT in the film was proposed in which charge injected from the collector electrode is transported by lateral charge hopping between center molecules within the monolayer on the metal oxides.21,22 [Ru(bpy)3]2+ and its derivatives are versatile functional molecules that have been applied to a wide variety of molecular devices due to its stable redox properties and potential ability for visible-light-photochemical reactions.23,24 A nanocomposite film of n-type WO3 semiconductor and [Ru(bpy)3]2+ was reported to be prepared by an electrodeposition technique.25,26 It significantly showed the redox of RuII/RuIII on cyclic voltammetry (CV), although its redox potential (1.03 V vs SCE) lies above the EFP ) 0.09 V of WO3.25 This is special because it cannot be observed in a WO3/[Ru(bpy)3]2+ film prepared by solvent evaporation from a WO3 suspension containing [Ru(bpy)3]2+. To the best of our knowledge, this is the first case in which the redox response of RuII/RuIII has been given in any composite film of n-type semiconductor films and Ru(II) complex derivatives. This result encouraged us to reveal the detailed electrochemical features and the CT mechanism of the Ru-WO3 film. Herein we report that CT by oxidation of RuII to RuIII in the Ru-WO3 film depends on the applied potential, and Dapp ranges from 1.0 × 10-8 to 1.1 × 10-7 cm2 s-1. The mechanism of CT in the Ru-WO3 film is proposed in which electrons are injected from [Ru(bpy)3]2+ into the conduction

10.1021/jp073020x CCC: $37.00 © 2007 American Chemical Society Published on Web 07/10/2007

Charge Transport in WO3/[Ru(bpy)3]2+ Nanocomposite Film

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band (CB) of WO3 and go through there to a collector electrode. To give a better understanding of the featured electrochemical reaction and CT in the Ru-WO3 film, the spectro-electrochemical data will be compared with those in a [Ru(bpy)3]2+/Nafion (Ru-Nf) film whose CT characteristics are well-defined; CT is known to take place by a charge hopping mechanism between the Ru centers, and Dapp is on the order of 10-9 to 10-10 cm2 s-1.14-17 Experimental Details Materials. [Ru(bpy)3]Cl2‚6H2O, poly(sodium 4-styrenesulfonate) (PSS, Mw ) 70 000), and Nafion (Nf) perfluorinated resin solution (5 wt % alcoholic solution) were purchased from Aldrich Chemical Co., Inc. Tungsten powder and hydrogen peroxide (30%) were purchased from Kanto Chemical Co., Inc. All reagents were used as received. Preparations. Ru-WO3 Film. Tungsten powder (0.92 g, 5.0 mmol) was dissolved in 10 mL of a 30% hydrogen peroxide solution to prepare a peroxotungstic acid (PTA) solution. Excess hydrogen peroxide was decomposed by Pt black, and ethanol was then added to the solution for stabilizing PTA to prepare a 30 vol % ethanol solution containing 100 mM PTA (based on the W concentration). [Ru(bpy)3]2+ and PSS were added to the PTA solution to prepare a 30 vol % ethanol solution containing 1 mM [Ru(bpy)3]2+, 50 mM PTA, and 30 mM PSS. The solution turns into a colloidal triad solution of [Ru(bpy)3]2+, PTA, and PSS at room temperature for 5 h, and the colloidal solution is favorable for electrodeposition. A nanocomposite film of WO3 and [Ru(bpy)3]2+ was cathodically electrodeposited from the colloidal triad solution on an indium tin oxide (ITO) electrode with stirring under the potentiostatic conditions (-0.45 V vs Ag/AgCl) to 1.0 C cm-2, and then treated by cathodic polarization at -0.5 V vs SCE in a 0.1 M HNO3 aqueous solution to complete the electrodeposition. Amorphous WO3 in the RuWO3 film was indicated by X-ray diffraction measurement. The thickness of the film was measured by a scanning electron microscopic technique to be 560 ( 24 nm (on average) under the typical conditions. The UV-visible absorption spectrum of the film exhibited a metal-to-ligand charge-transfer band of [Ru(bpy)3]2+ at λmax ) 459 nm. The coverage (ΓRu) of [Ru(bpy)3]2+ was calculated to be (2.0-3.4) × 10-8 mol cm-2 from the absorbance (A459) at 459 nm and its molar absorption coefficient (14 600 M-1 cm-1) in an aqueous solution. A neat WO3 film was also cathodically electrodeposited from a 30 vol % ethanol solution containing 50 mM PTA and 30 mM PSS on an ITO electrode under the same conditions as the preparation for the Ru-WO3 film. Ru-Nf Film. Nafion perfluorinated resin solution (5 wt % alcoholic solution) was diluted to 2.5 wt % with CH3OH before use. A Nf film was prepared by casting 10 µL of the 2.5 wt % Nf solution on an ITO electrode (area, 1.0 cm2). A Nf-coated ITO electrode was immersed in an aqueous solution of [Ru(bpy)3]2+ to incorporate it into the Nf film. The film thickness was measured to be 0.76 µm from interference spectroscopic data and refractive index (1.35) of Nf using a film thickness analysis software (Lambda Vision, TF-Lab). ΓRu was calculated to be 2.1 × 10-8 mol cm-2 from the absorbance (A450 ) 0.31) at λmax ) 450 nm, similarly to the Ru-WO3 film. Measurements. The electrochemical measurements were conducted by an electrochemical analyzer (Hokuto Denko, HZ3000). A conventional single-compartment electrochemical cell was equipped with a modified working electrode, an SCE reference electrode, and a platinum wire counter electrode. The spectroelectrochemical measurements were conducted in a

Figure 1. Cyclic voltammograms of a Ru-WO3 film in a 0.1 M KNO3 aqueous solution (pH ) 1.2) as measured at various scan rates. The scan rate is 5, 10, 20, 50, 100, and 200 mV s-1. Inset shows the plot of anodic peaks around 1.1 V vs scan rate. ΓRu ) 2.3 × 10-8 mol cm-2.

spectrophotometrical cell by combining a photodiode array spectrophotometer (Shimadzu, Multispec-1500) with the electrochemical analyzer. Results and Discussion CV of a neat WO3 film in an aqueous electrolyte solution exhibited a redox response of HxWO3/WO3 below 0.26 V, not giving any redox response in the range from 0.26 to 1.5 V (not shown). When 1 mM [Ru(bpy)3]2+ is contained in the electrolyte solution, the redox response of RuII/RuIII was not given on CV of the neat WO3 film because an n-type Schottky heterojunction is formed at an interface between the WO3 film and the electrolyte solution. This demonstrates that the WO3 film is so superficially densified that [Ru(bpy)3]2+ in the solution cannot diffuse through the WO3 film to the ITO surface. However, CV of the Ru-WO3 film in an aqueous electrolyte solution gave a stable redox wave of RuII/RuIII at 1.03 V,27 in addition to a redox wave of HxWO3/WO3 below 0.09 V, as shown in Figure 1. When CV was measured with the scan rate (V/(V s-1)) changed, the anodic peak current around 1.1 V increased linearly with V in the range of 5-100 mV s-1 though it deviated downward over 100 mV s-1 (inset of Figure 1). The linear relationship suggests that [Ru(bpy)3]2+ performs as it is adsorbed on WO3 in the electrochemical reaction. This electrochemical reaction is mechanistically different from that for the Ru-Nf film under the comparative conditions; the anodic peak current of RuII/RuIII increased linearly with V1/2 on CV of the Ru-Nf film, consistent with diffusion-controlled CT in the film (not shown). To reveal kinetics and mechanism of CT by a RuII/RuIII redox in the Ru-WO3 film, a PSCAS technique was employed; it is useful to obtain not only amperometric data but also spectroscopic data for the evident observation of a real change of the Ru center. The in situ visible absorption spectral change in a potential step from 0.4 to 1.7 V is shown in Figure 2A. A459 decreased very quickly by oxidation of RuII, which completed for 0.3 s (inset of Figure 2A). This is very fast when compared with the corresponding spectral change for the Ru-Nf film under comparable conditions, in which it takes more than 30 s for RuII to be completely oxidized, as shown in Figure 2B. Three possible mechanisms can be considered to explain CT in the Ru-WO3 film, as illustrated in Scheme 1A-C: (1) by physical diffusion of the Ru center, (2) by charge hopping

11638 J. Phys. Chem. C, Vol. 111, No. 31, 2007

Sone et al. SCHEME 1: Possible Mechanisms for Charge Transport in a Ru-WO3 Film

Figure 2. In situ absorption spectral changes in PSCAS measurement of (A) a Ru-WO3 film (ΓRu ) 2.0 × 10-8 mol cm-2) and (B) a RuNf film (ΓRu ) 2.1 × 10-8 mol cm-2) in a potential step from 0.4 to 1.7 V vs SCE in a 0.1 M KNO3 aqueous solution (pH ) 1.2). Inset shows the A459 and A450 changes with time for the Ru-WO3 film and the Nf-Ru film, respectively.

between Ru centers, and (3) through CB of WO3. When CT occurs by the first mechanism, Dapp should be independent of the coverage of the Ru center (Scheme 1A). Whatever the case may be, the first mechanism is unlikely because [Ru(bpy)3]2+ is not allowed to diffuse in the film (vide supra). As for the second mechanism, Dapp should increase with an increase of the coverage of the Ru center when CT occurs by charge hopping as it is related to the concentration according to the Dahms-Ruff equation28-30 and so forth12,31 (Scheme 1B). In the present preparation method of the Ru-WO3 film, the formation of triadic colloid in the solution for electrodeposition is important (see Experimental Details), and it strongly depends on the Ru concentration in the solution. Since the Ru concentration conditions for the triadic colloid to be formed stably in the solution are restricted, it was difficult to obtain the Ru-WO3 film with a different Ru coverage in a wide range by changing the Ru concentration in the solution in electrodeposition. However, the Ru concentration conditions under which the triadic colloid is allowed to be formed stably have been enlarged by replacing the solvent from ethanol/water (3:7) to ethanol/ acetonitrile/water (3:5:2), and WO3/[Ru(bpy)3]2+ films with different Ru coverage in the range of 0.29-0.67 M (in concentration) have been prepared by electrodeposition from the colloidal solution. (The detailed preparation and characterization of the WO3/[Ru(bpy)3]2+ film will be reported sepa-

rately.) The preliminary PSCAS measurement of the prepared film showed that Dapp ((1.4 ( 0.9) × 10-8 cm2 s-1) is independent of the Ru coverage in the film. This result could exclude the possibility of the charge hopping mechanism in the present Ru-WO3 film. The possibility of the third mechanism (through CB of the WO3 matrix) could be evaluated by examining an influence of applied potential on Dapp because electron transport in semiconductor solid films should be controlled by conductivity correlated to a potential gradient, in addition to diffusion based on its concentration gradient (Scheme 1C). When CT occurs by a diffusion process in a solid film, the plot of j vs t-1/2 provides a straight line according to the Cottrell equation,

j ) nFc0(Dapp/πt)1/2

(1)

where j, n, F, c0, and t are the current density (A cm-2), the number of electrons, the Faraday constant, the concentration (mol cm-3) of a redox center in the film, and time (s), respectively. Dapp given from the slope of the plots should be independent of final applied potential (Ef) for PSCAS when Ef is sufficiently higher than the redox potential of a redox center (for oxidative CT). PSCAS measurements were conducted in a potential step from 0.4 V to Ef changed in the range of Ef ) 1.2-1.8 V to characterize CT in the Ru-WO3 film. The amperometric data in PSCAS measurements were analyzed assuming the diffusion-controlled CT according to the Cottrell equation. Cottrell plots of j vs t-1/2 for the Ru-WO3 film exhibited sigmoid curves, and they significantly depended on

Charge Transport in WO3/[Ru(bpy)3]2+ Nanocomposite Film

Figure 3. Cottrell plots in PSCAS measurement of a Ru-WO3 film (black symbols, ΓRu ) 2.0 × 10-8 mol cm-2) and a Ru-Nf film (gray symbols, ΓRu ) 2.1 × 10-8 mol cm-2) in a 0.1 M KNO3 aqueous solution (pH ) 1.2) as measured with different final applied potential (Ef) for PSCAS from Ef ) 1.2 to 1.8 V vs SCE.

Figure 4. Dependence of Dapp on Ef in PSCAS measurement for a Ru-WO3 film (closed circles, ΓRu ) 2.0 × 10-8 mol cm-2) and for a Ru-Nf film (open circles, ΓRu ) 2.1 × 10-8 mol cm-2) at 25 °C.

Ef, as shown by black plots in Figure 3. On the contrary, Cottrell plots for the Ru-Nf film exhibited a straight line at each Ef, and they did not change by Ef under the same conditions as the Ru-WO3 film (gray plots in Figure 3). This result shows that CT in the Ru-WO3 film is not simply controlled by a diffusion process. The plots of the nearly plateaued j value at an initial time for the sigmoid curve can be explained by an IR drop resulting from the high current density generated (14.3 mA cm-2 for Ef ) 1.5 V) because a linear relationship was given between the initial current at t ) 10 ms and Ef (Figure S1 of the Supporting Information). As the current decreases with time, the contribution of the IR drop to j becomes negligible to give the significant decrease of j, which could represent the neat CT process (excluding the IR drop influence). Dapp was provided from the slopes of the j decrease at each Ef. The plots of Dapp vs Ef for the Ru-WO3 film are shown in Figure 4 including data in the Ru-Nf film. Dapp ((0.1-1.1) × 10-7 cm2 s-1) for the RuWO3 film was 55-480 times higher than those Dapp ((2.4 ( 0.05) × 10-10 cm2 s-1) for the Ru-Nf film under the same conditions. It increased linearly with Ef and thereafter saturated above Ef ) 1.5 V. The linear relationship between Dapp and Ef below Ef ) 1.5 V is consistent with the fact that an electron migration rate in a solid semiconductor is proportional to a

J. Phys. Chem. C, Vol. 111, No. 31, 2007 11639

Figure 5. Dependence of Dapp on Ef in PSCAS measurement for a neat WO3 film at 25 °C.

potential gradient as is well-defined by electron carrier mobility with a dimension of cm2 V-1 s-1. This result suggests that electrons are transferred through CB of WO3 from RuII to the ITO electrode in the Ru-WO3 film. As Ef increases further, electron transport through CB of WO3 could become faster and must be controlled at some Ef conditions by diffusion of counterions at the interfacial liquid phase that is inevitably involved in CT. This could explain that Dapp is saturated above Ef ) 1.5 V in Figure 4. The saturated Dapp value (1.1 × 10-7 cm2 s-1) for the Ru-WO3 film was lower than well-known diffusion coefficients (10-6 to 10-5 cm-2 s-1) of ions in an aqueous solution. This could suggest that diffusion of counterions in the Ru-WO3 film is significantly slower than ionic diffusion in an aqueous solution, possibly due to some interaction with the Ru-WO3 matrix. To characterize electron transport through CB of WO3, CT by oxidation of HxWO3 to WO3 in the neat WO3 film (without [Ru(bpy)3]2+) was investigated. The PSCAS measurements were conducted for the neat WO3 film in a potential step from -0.5 V to Ef changed in the range of Ef ) 0.4-0.9 V, and Cottrell plots (Figure S2 of the Supporting Information) gave sigmoid curves and significantly depended on Ef, similar to the case of the Ru-WO3 film. The nearly plateaued j values at the initial time for the sigmoid curve are also indicated to be caused by an IR drop. Dapp values were calculated from the slopes of the j decrease at each Ef using c0 ) 2.2 × 10-3 mol cm-3 that is obtained from the coverage of reduced WO3 and the film thickness. Dapp increased linearly with Ef and deviated downward above 0.7 V (Figure 5), demonstrating that electron transport through CB of WO3 definitely depends on Ef. The result similar to dependency of Dapp on Ef for the Ru-WO3 film significantly supports that CT occurs through CB of WO3 in the Ru-WO3 film. The downward deviation of Dapp above Ef ) 0.7 V could be also explained by control by counterion diffusion in an interfacial liquid phase, as is the case for the Ru-WO3 film. However, the highest Dapp (3.8 × 10-6 cm2 s-1 at Ef ) 0.9 V) for the neat WO3 film was 35 times higher than the saturated Dapp value (1.1 × 10-7 cm2 s-1) for the Ru-WO3 film, the former of which is consistent with well-known diffusion coefficients (10-6 to 10-5 cm-2 s-1) of ions in an aqueous solution. This might suggest that diffusion of counterions at the interfacial liquid phase in the Ru-WO3 film is slower than that in the neat WO3 film. For the electron transport through CB in the Ru-WO3 film, an ohmic contact must be formed at the interface between WO3 and [Ru(bpy)3]2+, allowing for either an electron injection from RuII to WO3 or an electron removal from WO3 to RuIII.

11640 J. Phys. Chem. C, Vol. 111, No. 31, 2007 However, a Schottky heterojunction is still kept at the interface between WO3 and the electrolyte solution because the electrochemical response of [Fe(CN)6]4- (redox potential ) 0.16 V vs SCE) was not observed in a CV from 0 to 0.5 V of the RuWO3 film in an electrolyte solution containing 10 mM [Fe(CN)6]4-. The electron-transfer channel for the ohmic contact could be formed by the electrostatic interaction between [Ru(bpy)3]2+ and partially reduced WO3-. This is supported by the local element analysis data on the film surface by an electron probe microanalysis technique indicating that the content of cationic [Ru(bpy)3]2+ is higher than the sum of anionic PSS unit and Cl- contents (as counter anions for [Ru(bpy)3]Cl2).25 This means that the positive charge of [Ru(bpy)3]2+ could be neutralized by partially reduced WO3-. Since the Cottrell plots approximately gave a straight line if the current decrease by the IR drop was excluded, the analysis assuming the diffusion-controlled CT could be appropriate for the CT in the Ru-WO3 film. On the other hand, the scan rate dependence of CV suggested that [Ru(bpy)3]2+ performs as it is adsorbed on WO3 in the electrochemical reaction (Figure 1). It means that the CV data do not suggest the diffusion-controlled CT. Nevertheless, the analysis of PSCAS data does not conflict with the CV data if the different measurement methodology between PSCAS and CV is taken into account. For a potentialstep technique applying sufficiently high Ef, the electron injection from RuII to CB of WO3 could be enough fast compared with the electron transport through CB, and consequently the amperometric data could be controlled by the electron transport process. Meanwhile, since for a potential sweep technique the electron injection could not be sufficiently faster than the electron transport through CB under the scan rate conditions below 100 mV s-1, the CV data in Figure 1 are considered to have suggested that the Ru center performs as it is adsorbed on WO3. The temperature dependence of CV shows a clear contrast between the Ru-WO3 film and the Ru-Nf film. The anodic current peak of the RuII/RuIII redox in CV increased by a factor of 4.3 with the temperature increase of 5-35 °C for the RuNf film (Figure 6B), whereas it hardly changed with the same temperature change for the Ru-WO3 film (Figure 6A). To evaluate the temperature dependence of CT in the Ru-WO3 film and the Ru-Nf film, PSCAS measurements in a potential step from 0.4 to 1.5 V were carried out with the temperature changed in the range of 5-35 °C. The slope of the linear Cottrell plots for the Ru-Nf film increased with the temperature increase (Figure S3 of the Supporting Information). For the Cottrell plots of the Ru-WO3 film, the slope after t ) 250 ms, corresponding to the neat CT process, hardly seemed to change with the temperature increase, though the plateaued j value at an initial time by an IR drop increased considerably (Figure S3 of the Supporting Information). Dapp increased by 8.0 times with the temperature increase from 5 to 35 °C for the Ru-Nf film, with Dapp increasing by 1.8 times with the same temperature change for the Ru-WO3 film (inset of Figure 7). Arrhenius plots of Dapp provided a straight line in the range of 5-35 °C for respective films, as shown in Figure 7. Activation energy (Ea ) 14.1 kJ mol-1) given from the slope of the straight line for the Ru-WO3 film was slightly higher but close to Ea (10.4 kJ mol-1) for CT by oxidation of HxWO3 to WO3 in the neat WO3 film as obtained by a PSCAS technique in a potential step from -0.5 to +0.4 V in the same temperature range. The close Ea could support the CT mechanism through CB of WO3 in the Ru-WO3 film. Ea (14.1 kJ mol-1) for the Ru-WO3 film was 3.5 times lower than that (Ea ) 49.8 kJ mol-1) for the Ru-Nf

Sone et al.

Figure 6. Cyclic voltammograms of (A) a Ru-WO3 film (ΓRu ) 3.4 × 10-8 mol cm-2) and (B) a Ru-Nf film (ΓRu ) 2.1 × 10-8 mol cm-2) in a 0.1 M KNO3 aqueous solution (pH ) 1.2) as measured at 100 mV s-1 and different temperature.

Figure 7. Arrhenius plots of Dapp in PSCAS measurement for (a) a Ru-WO3 film (ΓRu, 2.4 × 10-8 mol cm-2) and (b) a Ru-Nf film (ΓRu, 2.3 × 10-8 mol cm-2). The inset shows temperature dependence of Dapp for (a) the Ru-WO3 film and (b) the Ru-Nf film.

film. For the charge hopping mechanism in the Ru-Nf film, the successive self-exchange electron-transfer reaction occurs between the Ru centers attached strongly on a Nf matrix,32 involving a local bounded movement of the center to assist the electron-transfer reaction.9,12,33 It could require significant Ea due to intermediate formation for the electron-transfer reaction involving rearrangements of the coordination sphere and solvation. The electron transport through CB may not need Ea as significant as that for the intermediate formation in the charge

Charge Transport in WO3/[Ru(bpy)3]2+ Nanocomposite Film hopping mechanism. The lower Ea could be responsible for the faster CT in the Ru-WO3 film than the Ru-Nf film. Conclusion CT by a RuII/RuIII redox in the Ru-WO3 film was found to occur through CB of WO3. This led to electrochemical oxidation of [Ru(bpy)3]2+ in n-type WO3 semiconductor film with EFP below its redox potential. The possibility of CT through CB in nanocomposite films of semiconductor/functional molecule could provide a wide variety of its application and new concept for design of nanocomposite films. Acknowledgment. This research was partially supported by Grant-in-Aid for Young Scientists (B) from the Ministry of Education, Culture, Sports, Science and Technology (Grant No. 16750113) and Grant for Promotion of Niigata University Research Projects. Fellowship grant was provided by The Niigata Engineering Promotion, Inc. (K.S.). Supporting Information Available: Cottrell plots of RuWO3 film and neat WO3 film. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wang, H.; Xu, G.; Dong, S. Anal. Chim. Acta 2003, 480, 285. (2) Liu, C.; Hu, J.; Hu, J.; Tanga, H. Electroanalysis 2006, 18, 478. (3) Tsiafoulis, C. G.; Trikalitis, P. N.; Prodromidis, M. I. Electrochem. Commun. 2005, 7, 1398. (4) Torres-Gomez, G.; Tejada-Rosales, E. M.; Gomez-Romero, P. Chem. Mater. 2001, 13, 3693. (5) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci, S.; DeAngelis, F.; DiCenso, D.; Nazeeruddin, M. K.; Graetzel, M. J. Am. Chem. Soc. 2006, 128, 16701. (6) Xue, J.; Rand, B. P.; Uchida, S.; Forrest, S. R. AdV. Mater. 2005, 17, 66. (7) Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker, Inc.: New York, 1996; Vol. 18.

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