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ARTICLES Influence of Aspect Ratio of TiO2 Nanorods on the Photocatalytic Decomposition of Formic Acid Hyeong Jin Yun,† Hyunjoo Lee,‡ Ji Bong Joo,† Wooyoung Kim,† and Jongheop Yi*,† School of Chemical and Bioengineering, Seoul National UniVersity, Seoul 443-749, South Korea, and Department of Chemical and Biomolecular Engineering, Yonsei UniVersity, Seoul 120-749, South Korea ReceiVed: September 29, 2008; ReVised Manuscript ReceiVed: NoVember 24, 2008
A variety of shape-controlled TiO2 nanoparticles, such as spheres, short ellipsoidal rods with a low aspect ratio (low AR), and long ellipsoidal rods with a high aspect ratio (high AR), were synthesized by a gel-sol method. X-ray diffractometer and ultraviolet diffuse reflectance spectroscopy analyses revealed that the synthesized TiO2 nanoparticles have the same anatase structure with a band-gap energy (Eg) of 3.2 eV regardless of nanoparticle shape. Electrochemical impedance spectroscopy (EIS) results showed that the increasing aspect ratio of TiO2 nanoparticles was accompanied by increases in the charge-transfer rate between nanoparticle surfaces and electrolyte, the space charge capacitance on the surface of TiO2 nanoparticles, and electrochemical double-layer capacitance at the interfacial region of the electrolyte. The amount of electron donors in the nanoparticles increased with increasing aspect ratio of nanoparticles, which is in good agreement with EIS results. Use of high-AR ellipsoidal TiO2 nanoparticles as photocatalysts resulted in enhanced current density and, consequently, an increase in the photocatalytic decomposition rate of formic acid. 1. Introduction Titanium dioxide (TiO2) is an n-type semiconductor material with a wide band-gap energy (Eg) of 3.0-3.2 eV.1-4 Photogenerated electrons (e-) and holes (h+) serve for red-ox reaction in dye-sensitized solar cells (DSSCs),3,5-9 hydrogen evolution via water splitting,10-12 air and water purification,1,4,13-15 lithiumion batteries,16 photocatalytic devices,17 etc. Photocatalytic water purification using TiO2 has received much attention as an alternative method for treating water contaminated by toxic organic chemicals. It is considered to be more economical than other methods. Purification is achieved via the charge transfer between photocatalysts and water solution containing organic contaminants under light illumination, leading to oxidative decomposition of hydrocarbon contaminants that yield carbon dioxide. Thus, the amount of e-/h+ on the photocatalyst surface is a key factor in determining the photocatalytic reaction rate. One-dimensional (1-D) TiO2 nanostructures such as ellipsoidal nanoparticles,7,18,19 nanowires,20 and nanotubes15,21 have been investigated extensively due to their superior electrochemical properties, which are attributed to dimensional anisotropy. For 1-D nanostructured crystals the space charge region is well constructed along the longitudinal direction of TiO2 nanocrystal, meaning that photogenerated electrons can flow in the direction of the crystal length. Increased delocalization of electrons at 1-D nanostructured crystals can lead to a remarkable decrease in e-/h+ recombination probability. Consequently, larger numbers of e- and h+ exist on the active sites of the nanocrystal * To whom correspondence should be addressed. Phone: +82-2-8807438. Fax: +82-2-877-2012. E-mail:
[email protected]. † Seoul National University. ‡ Yonsei University.
surface, resulting in higher activity compared with spherical nanoparticles. Joo et al. reported that TiO2 nanorods have superior photocatalytic inactivation of E. coli compared with Degussa P-25:19 99% of the E. coli was inactivated after 92 and 140 min by TiO2 nanorods and P-25, respectively. Kang et al. fabricated a nanorod-based DSSC that showed improved performance compared with nanoparticle-based DSSC.7 Various methods of synthesizing shape-controlled TiO2 nanoparticles have recently been introduced, including hydrothermal,22,23 solverthermal,24,25 template synthesis,2637 and gel-sol methods.28-30 The gel-sol method enables shape-controlled nanoparticles to be obtained in large quantities via controlled condensation from highly viscous Ti(OH)4 gel.28 TiO2 precipitate is obtained from the gel via a dissolution-recrystallization process in an inhomogeneous system. This method is favorable not only for synthesizing monodispersed nanoparticles in a simple process but also for controlling their size and shape.30 Although many groups have synthesized shape-controlled TiO2 nanoparticles, few reports have verified the shape effect of TiO2 nanoparticles on photocatalytic activity.7,19,31 To address this shortcoming, we synthesized TiO2 nanospheres and ellipsoidal nanorods with different aspect ratios using various surfactants through the gel-sol method. We demonstrated the influence of aspect ratio of TiO2 nanorods on the photocatalytic decomposition of formic acid by characterizing the electrochemical behavior using electrochemical impedance spectroscopy and chronoamperometry. The influence of the aspect ratio of TiO2 nanorods was demonstrated by comparing the electrochemical behavior of each shape with photocatalytic activity in the oxidation of formic acid.
10.1021/jp808604t CCC: $40.75 2009 American Chemical Society Published on Web 02/03/2009
Influence of Aspect Ratio of TiO2 Nanorods 2. Experimental Section 2.1. Preparation of Shape-Controlled TiO2 Nanoparticles. Shape-controlled TiO2 nanoparticles were synthesized by the gel-sol method. A total of 0.1 mol of titanium tetraisopropoxide (TTIP, Advanced Materials Institute Co. Ltd., 98%) was added to 0.2 mol of triethanolamine (TEOA, Fluka, 99%); deionized water (DI water) was then added to achieve 200 mL of aqueous solution. The resulting turbid white solution gradually turned transparent light yellow. Twenty milliliters of this solution was mixed with 20 mL of 0.02 M of oleic acid (OA, Samchun, 99.9%), 0.2 M diethylamine (DEA, Aldrich, 99.5%), and 0.1 M diethylenetriamine (DETA, Aldrich, 99%) aqueous solution to synthesize spherical, low-AR, and high-AR ellipsoidal TiO2 nanoparticles, respectively. These surfactants serve as surfacecapping agents. The mixtures were placed into Pyrex bottles and heated at 100 °C for 24 h. The resulting pale yellow-colored gel was transferred to a Teflon-lined autoclave and heated at 250 °C for 72 h, yielding white TiO2 precipitate. The precipitate was centrifuged at 3000 rpm for 10 min and washed several times with DI water to remove residual surfactants. Finally, TiO2 powder was obtained by drying at 70 °C for 12 h. Each organic surfactant on the surface of TiO2 nanoparticles was completely removed by burning out at 450 °C for 3 h under air. 2.2. Characterization of Nanoparticles. The shapes of synthesized TiO2 nanoparticles were characterized using an energy-filtering transmission electron microscope (EF-TEM, Libra 120-Carl Zeiss, 80 kV). The microstructure was examined using a high-resolution transmission electron microscope (HRTEM, JEM 3010-JEOL, 300 kV) and X-ray diffractometer (XRD, D/max-2500/PC-Rigaku) with Cu KR radiation (wavelength ) 0.154 nm) as an incident beam working at 50 kV and 100 mA. The optical absorption ability of TiO2 powder was measured by ultraviolet diffuse reflectance spectroscopy (UVDRS, V670-Jasco) to estimate Eg. 2.3. Electrochemical Analyses. Electrochemical analyses were performed using a standard three-electrode cell with a platinum-coated titanium mesh of 2.5 cm2 used as a counter electrode. The substrate was ITO glass (1.5 × 2.0 cm) connected to the platinum electrode using conductive carbon tape. A total of 0.4 mg of synthesized TiO2 nanoparticles was dropped on the ITO glass, which is used as a working electrode for electrochemical impedance spectroscopy (EIS) and chronoamperometry experiments. A saturated Ag/AgCl electrode was used as a reference electrode. Test electrolyte was prepared by dissolving formic acid (Samchun, 99%) with a concentration of 10 mM in 10 mM of NaClO4 (Samchun, 98%) aqueous supporting electrolyte. NaClO4 has very little effect on electrochemical response. Prior to each experiment the electrolyte was deaerated by passing nitrogen gas through it for 5 min. EIS and chronoamperometry experiments were conducted using a computer-controlled potentiostat (Iviumstat, Ivium). EIS was performed in the frequency range between 1 kHz and 10 mHz with an ac voltage amplitude of 10 mV at a dc bias of 1.0 V. The impedance spectra were interpreted by a nonlinear leastsquares fitting procedure using commercial software (ZsimpWin). A Xe arc lamp (300 W, Oriel) was used as a light source for UV light irradiation in photoelectrochemical analyses. 2.4. Photocatalytic Activity. A total of 0.1 g of each shape of TiO2 nanoparticle was suspended in 200 mL of 1 mM aqueous formic acid solution under vigorous stirring. The concentrations of formic acid were measured by highperformance liquid chromatography (HPLC). Target compounds were quantified using HPLC on a 150 × 4.6 mm Optimapak C18 column connected to a UV detector (2487 dual λ absor-
J. Phys. Chem. C, Vol. 113, No. 8, 2009 3051
Figure 1. TEM images of TiO2 nanoparticles controlled in aspect ratio by (a) 0.02 M of oleic acid (OA), (b) 0.2 M of diethylamine (DEA), and (c) 0.1 M of diethylenetriamine (DETA) aqueous solution. The average diameter of spherical TiO2 nanoparticles is 24.7 ( 4.3 nm. The aspect ratios of low-AR and high-AR ellipsoidal TiO2 nanoparticles are 3.5:1 and 5.8:1, respectively. Low-AR ellipsoidal TiO2 nanoparticles have average dimensions of 132.4 ( 17.8 nm in length and 38.5 ( 5.1 nm in width; high-AR ellipsoidal TiO2 nanoparticles have average dimensions of 156.0 ( 22.6 nm in length and 27.4 ( 5.2 nm in width. The insets show HR-TEM images of individual nanoparticles. Interplanar distances in the crystal lattice indicate a TiO2 anatase structure. Ellipsoidal nanoparticles are elongated in the [001] direction of the anatase phase.
bance detector; Waters). The detection wavelength was set to 210 nm. The mobile phase was an aqueous solution of 0.1 vol. % phosphoric acid pumped at a flow rate of 1 mL/min. A total of 100 µL of specimen was injected into the column to measure the concentration of formic acid. 3. Results and Discussion 3.1. Characterization of Shape-Controlled TiO2 Nanoparticles. Figure 1 shows the diverse shapes of TiO2 nanoparticles synthesized by the gel-sol method. Three types of shape-
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Figure 2. XRD spectra of each shape of as-synthesized TiO2 nanoparticles: (a) spherical, (b) low-AR, and (c) high-AR ellipsoidal TiO2 nanoparticles. The spectra show anatase structure, regardless of particle shapes.
controlled nanoparticles were synthesized: spherical nanoparticles (24.7 ( 4.3 nm), ellipsoidal nanorods with a low aspect ratio (low AR) of 3.5:1, and ellipsoidal nanorods with a high aspect ratio (high AR) of 5.8:1, as shown in Figure 1a, 1b, and 1c, respectively. The spherical and ellipsoidal TiO2 nanoparticles show an approximately monodispersed distribution in their sizes and shapes. For all of the synthesized nanoparticles, lattice arrays mainly show interplanar distances corresponding to [101], indicating an anatase structure. The ellipsoidal nanoparticles are elongated in the [001] direction of the anatase phase. The lowAR ellipsoidal TiO2 nanoparticles have average dimensions of 132.4 ( 17.8 nm in length and 38.5 ( 5.1 nm in width; the high-AR ellipsoidal TiO2 nanoparticles are 156.0 ( 22.6 nm in length and 27.4 ( 5.2 nm in width. Figure 2 shows X-ray diffraction (XRD) patterns of the spherical, low-AR, and high-AR ellipsoidal TiO2 nanoparticles. The characteristic peaks correspond to anatase-structured TiO2 with crystallographic preferred orientations,32 regardless of the aspect ratio of TiO2 nanorods. It is well known that anatasestructured TiO2 shows a band-gap energy (Eg) of 3.2 eV. In this work, the Eg of shape-controlled anatase nanoparticles was examined by a UV-DRS spectroscope, revealing a typical value of Eg of anatase-structured TiO2, regardless of particle shape (see Supporting Information, Figure S1). Therefore, we can selectively compare the influence of aspect ratio on electrochemical reactions by utilizing TiO2 nanoparticles of different shapes while maintaining the same microstructure and Eg. As noted in the Experimental Section, various surfactants were used to control the aspect ratio of TiO2 nanorods. The presence of surfactants on the nanoparticle surface can disturb the adsorption of formic acid molecules (which are reactants) and hydrogen ions (used as sacrificial species). For example, oleic acids used to synthesize spherical TiO2 nanoparticles show strongly hydrophobic behaviors that hinder the access of reductant molecules and sacrificial ions in aqueous electrolyte. These surfactant layers for each nanoparticle shape were removed by calcination at 450 °C for 3 h in air atmosphere. The calcination temperature was determined by examining weight changes of as-synthesized TiO2 nanoparticles using thermogravimetric analysis (TGA) performed in the temperature range of 25-600 °C (see Supporting Information, Figure S2). Once the temperature approached 210 °C, the weight of the TiO2 nanoparticles began to decrease drastically until around 450 °C, from which the weight was largely invariant with increasing temperature. Compared with as-synthesized TiO2
Figure 3. (a) Schematic diagram of the electron flow path from the bulk electrolyte to the nanoparticulated TiO2 electrode. (b) Equivalent electric circuit of the electrochemical system for the photocatalytic decomposition of formic acid by TiO2 nanoparticles. RΩ: bulk electrolyte resistance. Qdl: electrochemical double-layer capacitance. Csc: space charge capacitance. Rct: charge-transfer resistance. Zw: Warburg impedance.
nanoparticles, we found no significant change in the particle shapes and microstructures (see Supporting Information, Figure S3). These calcined TiO2 nanoparticles were used to investigate the electrochemical behavior and photocatalytic decomposition of formic acid in aqueous systems. 3.2. Photoelectrochemical Analyses. Electrochemical impedance spectroscopy was used to characterize electrochemical interfacial reactions. The photocatalytic decomposition of formic acid can be explained as an electrochemical oxidation reaction in which reactants supply electrons to an anode. It is assumed that the electrons supplied via oxidation of formic acid pass through the interfacial region by both faradaic and nonfaradaic processes, as shown in Figure 3a. In a faradaic process, charges are transferred across the electrode-electrolyte interface governed by Faraday’s law, which states that the mass of a substance altered at the electrode during electrolysis is directly proportional to the quantity of electricity transferred at the electrode. From an electrochemical perspective, the faradaic process consists of two main mechanisms: charge transfer and mass transfer. Nonfaradaic processes also induce an electrical double layer in solution and a space charge region in the electrode surface for electrochemical reactions. The charges on the side of the electrolyte are made up of excess ions around the electrode surface. An electrochemical double layer is formed by an array of charged species, and oriented dipoles exist at the electrodeelectrolyte interface, as shown in Figure 3a. Moreover, when a semiconductor electrode is immersed in the electrolyte, the excess charges are distributed in the space charge region of the electrode that arises from the band-bending effect.33 These two
Influence of Aspect Ratio of TiO2 Nanorods
J. Phys. Chem. C, Vol. 113, No. 8, 2009 3053 TABLE 1: Parameters Extracted from Fitted Results of EIS Spectra for Each Shape of TiO2 Nanoparticle in an Aqueous Solution of Formic Acida Q, Ssecn
RΩ, kΩ Csc, µF sphere 3.759 rod (low AR) 3.159 rod (high AR) 3.991 a
Figure 4. Nyquist plots for each shape of TiO2 nanoparticle in aqueous solution of formic acid under UV light illumination: (a) spherical, (b) low-AR, and (c) high-AR ellipsoidal TiO2 nanoparticles. Symbols and solid lines indicate the experimental data and fitted curves, respectively.
nonfaradaic processes can affect charge flow through the interfacial layer. The equivalent circuit for the photocatalytic decomposition of formic acid by TiO2 nanoparticles can be designed with a serial combination of bulk electrolyte resistance (RΩ) and electrochemical interfacial reaction impedance represented as a parallel combination of the electrochemical double-layer capacitance (Qdl), space charge capacitance (Csc), and faradaic process consisting of charge-transfer resistance (Rct) and masstransfer impedance (Zw), as shown in Figure 3b. In this circuit the constant phase element (CPE) Q can replace the electrochemical double-layer capacitance at the electrode-electrolyte interface and Zw indicates Warburg impedance, which represents mass-transfer resistance. The impedance of CPE is given as follows34
1 Q) YO(jω)n
(1)
where YO is the admittance magnitude of CPE and n is the exponent related to the phase angle φ by φ ) n(π/2). In the present work the value of n is fixed at 0.75 to elucidate the relation between particle shape and the capacitance of the electrochemical double layer. Figure 4 shows that the aspect ratio of TiO2 nanorods affects the electrochemical impedance in the photocatalytic oxidation of formic acid. Each marker in Figure 4 represents experimental impedance data for photocatalytic oxidation performed by shapecontrolled TiO2 nanoparticles, while the solid lines correspond to theoretical impedance spectra calculated from the electrochemical equivalent circuit shown in Figure 3b. The theoretical curves are in good agreement with the experimental impedance data, thereby validating the proposed equivalent circuit in this system. The semicircles apparent in Figure 4 indicate that the charge-transfer-limited process is strongly related to electron movement in the electrode-electrolyte interface region. The diameters tend to decrease significantly with increasing aspect ratio of TiO2 nanoparticles, indicating that both charge-transfer resistance and capacitive reactance decrease with increasing aspect ratio. The extracted parameters for the circuit elements are summarized in Table 1. The charge-transfer resistances of TiO2 nanoparticles in 10 mM formic acid tend to decrease remarkably from 405.4 to 174.3 kΩ with increasing aspect ratio of nanoparticles. This finding indicates that the electron-transfer
Rct, kΩ
Zw, Ssec0.5
-6
12.68 1.550 × 10 405.4 6.110 × 10-5 13.72 1.778 × 10-6 294.6 5.711 × 10-5 13.92 2.649 × 10-6 173.1 9.827 × 10-5
Calculated using the software ZsimpWin.
rate from electrolyte to the electrode surface is enhanced in highAR ellipsoidal nanoparticles, for which photogenerated electrons can easily flow to the bulk electrode because the space charge region is well developed along the longitudinal direction of ellipsoidal nanoparticles. This leads to a reduction in the probability of e-/h+ recombination; consequently, the large number of holes on the surface can induce electron movement toward the electrode, resulting in an increase in the photocatalytic oxidation rate of formic acid and affecting the capacitive reactance. Table 1 shows that both the space charge and electrochemical double-layer capacitance increase slightly with increasing aspect ratio. The higher excess charges in the capacitive region probably result from the longer lifetime of charges on the active sites at high-AR ellipsoidal TiO2 nanoparticles; consequently, the capacitive reactance shows a marked reduction. To support the EIS results, the concentrations of electron donors with photocatalytic oxidation by TiO2 nanoparticles were calculated by the Mott-Schottky equation using the potentiodynamic EIS technique. In the case of a semiconductor-electrolyte interface Csc-2 is given as follows35,36
(
)(
1 2 kT -(E - Efb) ) 2 eεε N e Csc 0 D
)
(2)
where E is the applied potential, Efb is the flat band potential, e is electronic charge, ε is the dielectric constant of the electrode material, ε0 is the permittivity of free space, ND is the concentration of electron donors, k is the Boltzmann constant, and T is operating temperature. From this equation a plot of Csc-2 vs E is expected to be linear, and the slope can be used to obtain ND. The relation between Csc-2 and E can be obtained using the potentiodynamic EIS technique. Applied potential is scanned from 0.2 to 1.0 V with an interval of 0.2 V, and Csc is calculated by fitting the experimental data to a theoretical curve corresponding to the circuit shown in Figure 3b. Figure 5 shows the relations between Csc-2 and E when each type of shapecontrolled TiO2 nanoparticle is used in the photocatalytic decomposition of formic acid. Linear relations are obtained for all nanoparticle shapes, which are shown for typical semiconductor electrodes. The concentration of electron donors, as calculated from the slope of the relation, is 1.85 × 1016 cm-3 for spherical TiO2 nanoparticles, 2.42 × 1016 cm-3 for low-AR ellipsoidal nanoparticles, and 3.72 × 1016 cm-3 for high-AR ellipsoidal nanoparticles. The lifetime of e- and h+ on the active site under light illumination increases with the increasing aspect ratio of TiO2 nanoparticles due to a reduction in e-/h+ recombination probability. The greater number of electron donors at high-AR ellipsoidal TiO2 nanoparticles leads to an increase in the current flow rate (Figure 6). To measure photogenerated current density, chronoamperometry experiments were performed in electrolyte containing 10 mM formic acid and 10 mM NaClO4 on variously shaped TiO2 nanoparticles at a potential of 1.0 V with turning on/off
3054 J. Phys. Chem. C, Vol. 113, No. 8, 2009
Figure 5. Mott-Schottky plot for each shape of TiO2 nanoparticle in aqueous solution of formic acid (10 mM) as an estimate of the concentration of electron donors: (a) spherical, (b) low-AR, and (c) high-AR ellipsoidal TiO2 nanoparticles.
Figure 6. Current density transient with light on/off every 20 s for 5 min for each shape of TiO2 nanoparticle in the aqueous solution of formic acid (10 mM): (a) spherical, (b) low-AR, and (c) high-AR ellipsoidal TiO2 nanoparticles. The length of the arrow denotes 1 µA cm-2.
light. The photogenerated current density can be regarded as equivalent to photocatalytic activity. The current densities measured at spherical, low-AR ellipsoidal, and high-AR ellipsoidal TiO2 nanoparticles in formic acid were 0.56, 0.78, and 1.49 µA cm-2, respectively. The higher concentration of electron donors or reduced resistance in both the space charge region and electrochemical double layer leads to the enhancement of photogenerated current density with increasing aspect ratio of nanoparticles. 3.3. Photocatalytic Activity. Figure 7 shows the photocatalytic activities obtained when different shapes of TiO2 nanoparticles were used as photocatalysts. The concentration of formic acid decreased sharply when high-AR ellipsoidal TiO2 nanoparticles were used, while the spherical TiO2 nanoparticles showed poor activities. Large numbers of photogenerated e-/ h+ pairs on the surface of high-AR ellipsoidal nanoparticles led to the rapid oxidation of formic acid compared with the rate of oxidation for spherical nanoparticles. These findings are in agreement with electrochemical results. High current density at high-AR ellipsoidal TiO2 nanoparticles under light illumination gives rise to superior activity in the photodecomposition of formic acid. 4. Conclusions Anatase TiO2 nanoparticles with various aspect ratios (i.e., spheres, low-AR ellipsoidal rods, and high-AR ellipsoidal rods)
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Figure 7. Photocatalytic decomposition of formic acid (a) without TiO2 nanoparticles using each shape of TiO2 nanoparticle as a photocatalyst (1 mM): (b) spherical, (c) low-AR, and (d) high-AR ellipsoidal TiO2 nanoparticles.
were synthesized using a gel-sol method. We undertook electrochemical studies of the photocatalytic decomposition of formic acid for each aspect ratio of TiO2 nanorods using EIS and chronoamperometry methods. The charge-transfer resistance between TiO2 nanoparticles and aqueous solution of formic acid tends to decrease with increasing aspect ratio of TiO2 nanoparticles. Moreover, the space charge capacitance in the surface of nanoparticles and electrochemical double-layer capacitance were enhanced at high-AR ellipsoidal TiO2 nanoparticles. A decrease in the probability of e-/h+ recombination led to an increase in electron donors at high-AR TiO2 nanoparticles, leading in turn to increases in the charge-transfer rate, space charge, and double-layer capacitance. Consequently, photocatalytic activity increased when high-AR ellipsoidal TiO2 nanoparticles were used for the photocatalytic decomposition of formic acid. Acknowledgment. This work was supported by grant No. R01-2006-000-10239-0 awarded by the Basic Research Program of the Korea Science & Engineering Foundation. Supporting Information Available: DRS absorption spectra of each shaped TiO2 nanoparticles; TGA plot of (a) spherical, (b) low-AR, (c) high-AR ellipsoidal TiO2 nanoparticles; shapes and microstructures of shape-controlled TiO2 nanoparticles after calcination. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) McMurray, T. A.; Byrne, J. A.; Dunlop, P. S. M.; McAdams, E. T. J. Appl. Electrochem. 2005, 35, 723. (2) Randeniya, L. K.; Bendavid, A.; Martin, P. J.; Preston, E. W. J. Phys. Chem. C 2007, 111, 18334. (3) Koo, B.; Park, J.; Kim, Y.; Choi, S.-H.; Sung, Y.-E.; Hyeon, T. J. Phys. Chem. B 2006, 110, 24318. (4) Sun, B.; Smirniotis, P. G.; Boolchand, P. Langmuir 2005, 21, 11397. (5) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (6) Nelson, J.; Haque, S. A.; Klug, D. R.; Durrant, J. R. Phys. ReV. Lett. B 2001, 63, 205321. (7) Kang, S. H.; Choi, S.-H.; Kang, M.-S.; Kim, J.-Y.; Kim, H.-S.; Hyeon, T.; Sung, Y.-E. AdV. Mater. 2008, 20, 54. (8) Nakamura, R.; Tanaka, T.; Nakato, Y. J. Phys. Chem. B 2004, 108, 10617. (9) Waita, S. M.; Aduda, B. O.; Mwabora, J. M.; Granqvist, C. G.; Lindquist, S.-E.; Niklasson, G. A.; Hagfeldt, A.; Boschloo, G. J. Electroanal. Chem. 2007, 605, 151. (10) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (11) Sato, S.; White, M. Chem. Phys. Lett. 1980, 72, 83. (12) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146.
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