J. Phys. Chem. C 2010, 114, 11515–11521
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Photoelectrochemical Behavior of Alizarin Modified TiO2 Films Yesica Di Iorio,† Herna´n B. Rodrı´guez,‡ Enrique San Roma´n,‡ and Marı´a A. Grela*,† Departamento de Quı´mica, UniVersidad Nacional de Mar del Plata, Funes 3350, B7602AYL Mar del Plata, Argentina, and INQUIMAE, Facultad de Ciencias Exactas y Naturales, UniVersidad de Buenos Aires, Ciudad UniVersitaria, Pabello´n 2, 1428 Buenos Aires, Argentina ReceiVed: March 15, 2010; ReVised Manuscript ReceiVed: May 19, 2010
Photocurrent voltage curves obtained under visible light excitation of alizarin molecules chemisorbed to nanoporous TiO2 films show both anodic and cathodic currents. The potential at which the sign reversal occurs depends on the electrolyte pH, the presence of acceptors, and the dye coverage, but as a general rule, it occurs at potentials ca. 600-700 mV more positive than the flat band potential. Negative photocurrents are accounted by efficient electron discharge to the electrolyte mediated by the ligand. Cathodic photocurrents are only observed at pH values higher than ca. 4.0 and go through a maximum at intermediate alizarin loadings. This phenomenon is ascribed to the progressive reparation of surface states by alizarin which hampers carrier transport through the TiO2 matrix and decreases electron discharge to the electrolyte solution. 1. Introduction The photoelectrochemical behavior of nanocrystalline TiO2 films modified by chemisorption of organic compounds and transition metal complexes is of particular relevance for the development of dye sensitized solar cells and electronic or photonic devices for information processing, sensing, and computation.1-3 Furtado and co-workers have recently designed an optoelectronic XOR and INH logic gate based on a TiO2 dye-sensitized Gra¨tzel-type cell.4 The proposed device uses a ruthenium trinuclear complex [Ru3O(ac)6(py)2(pzCO2H)]PF6 (ac ) acetyl, py ) pyridine, and pz ) pyrazine) as the sensitizer. In this setup, semiconductor excitation leads to the development of anodic photocurrents; however, since the reduction potential of the chemisorbed sensitizer lies between the CB and the I3-/Icouple, a photocurrent sign-reversal is observed upon excitation of the sensitizer at suitable wavelengths. The development of spontaneous cathodic currents, between 350 and 500 nm, originates from the reductive quenching of the excited state of the chromophore by the TiO2 layer, followed by the reduction of I3- to I- by the reduced complex. Photocurrent sign-reversal may also be modulated by the potential. Szacilowski, et al. reported on various light-driven OR and XOR programmable logical devices based on cyanoferrate complexes5 bound to titanium dioxide through the cyano bridge. In these devices, the photocurrent sign depends on the irradiation wavelength and on the oxidation state of the surface complex, which is also controlled by the applied potential. Since the excited state of the iron(III) complexes cannot inject electrons in the TiO2 conduction band, only anodic photocurrents, originated by TiO2 photon absorption, are observed under the prevalence of the oxidized state. However, the excitation of iron(II) surface complexes mainly results in negative photocurrents. Briefly, the development of cathodic photocurrents is attributed to the electron transfer from the conduction band of the semiconductor to an acceptor in solution after the formation of a depletion layer. * To whom correspondence should be addressed. E-mail:
[email protected]. † Universidad Nacional de Mar del Plata. ‡ Universidad de Buenos Aires.
The phenomenom described as photoelectrochemical photocurrent switching, PEPS, has also been observed for carminic acid (7-D-glucopyranosyl-3,5,6,8-tetrahydroxy-1-methyl-9,10dioxo-9,10-dihydroanthracene-2-carboxylic acid, CA) bound to titanium dioxide through its carboxylic functionality.6 However, in this case, the photocurrent switching has been ascribed to the dual electron donor/acceptor character of the adsorbed organic molecule. Electrochemically amphoteric species bound to semiconductor surfaces are attractive systems from both basic and applied perspectives. As reported elsewhere, 1,2-dihydroxy9,10-anthraquinone (alizarin, A) forms strong chelates with titanium dioxide nanoparticles.7-9 Alizarin is an interesting molecule bearing two redox-active quinonoid fragments, the 9,10-dioxo and the 1,2-catechol-like fragment, which may act as electron acceptor or electron donor groups, respectively.10,11 We have recently shown that, although the visible excitation of the red alizarin molecule leads to a very efficient sensitization of Cr(VI) reduction, the direction of the electron transfer process may be reverted from A f TiO2 to TiO2 f A by UV excitation in the presence of a sacrificial electron donor.10,11 In this paper, we examine the photoelectrochemical behavior of A@TiO2 films and discuss the possible mechanisms for the development of cathodic currents under visible light excitation. 2. Experimental Section 2.1. Materials. 1,2-Dihydroxy-9,10-anthracenedione (alizarin, A) and acetonitrile (Cicarelli) were of the highest purity available and used as received. KCr(NH3)2(SCN)4 was prepared from the Reinecke salt12 (Aldrich) and recrystallized from warm water as described in ref 12. 2-Phenylacetic acid (Sigma) was recrystallized from carbon tetrachloride (Merck) prior to use. Aqueous solutions were prepared with deionized ultrapure water of resistivity >18.0 MΩ cm. TiO2 films were prepared from a commercially available TiO2 paste based on nanosized particles (Solaronix, Ti-nanoxide T; average diameter, d ) 13 nm). 2.2. Methods. Nanoporous TiO2 films of 1 cm2 geometric area, unless specified, were prepared on transparent conducting glass slides (indium tin oxide (Aldrich), surface resistivity: 15-25 Ω/square). Briefly, a drop of the Ti-nanoxide T sol was
10.1021/jp102354m 2010 American Chemical Society Published on Web 06/14/2010
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Di Iorio et al. Spectroelectrochemical experiments were performed by incorporating a 3 cm3 glass electrochemical cell in the sample compartment of an Ocean Optics diode array spectrophotometer. The electrolyte was an aqueous solution of NaClO4 0.2 M, pH 3.0 thoroughly deaerated by bubbling with Ar prior to experiments. A high pressure Hg-Xe lamp coupled to a Kratos-Schoeffel monochromator or blue LEDs (Phillips, LUXEON III, 3W, LXHL-LB3C, and LXHL-LR3C) were used as illumination sources in the UV and visible regions, respectively. According to manufacturer specifications, the LED output has a Gaussian profile with a spectral half-with of 25 (20) nm and a maximum at 470 (455) nm for LB3C (LR3C) models. The incident photon flux impinging on the cell was estimated by chemical actinometry using the Reinecke salt or 2-phenylacetic acid, in the visible and UV regions, respectively.12,13
Figure 1. Comparison between the spectrum of a 2 × 10-4 M solution of the free dye in acetonitile and the normalized absorption spectra of nanocrystalline titanium dioxide films obtained after dipping the semiconductor electrode in acetonitrile solution of various alizarin concentrations: 5 × 10-5 M (cyan), 1 × 10-4 M (blue), 5 × 10-4 M (dark cyan), and 1× 10-3 M (purple). Inset: UV-vis absorption spectra of TiO2 films supported on a conducting glass obtained before (dotted line) and after (solid line) alizarin chemisorption.
placed near the end of the electrode. By capillarity action, when the drop is touched with a microscope slide, it slips all along its border. Using an adhesive tape as frame and spacer, a thin uniform film was formed on the surface of the conductive substrate by raking off the excess of the colloidal paste. The films were then dried at room temperature, heated at 400 °C (at 5 °C/min), and then left at this temperature for 2 h. Semiconductor electrodes were dipped in acetonitrile dye solution of variable concentrations for 30 min in the dark and afterward thoroughly washed with distilled water and left in the electrolyte solution (see below) for an additional 30 min before use. The UV-vis spectra of the films were recorded with an integrating sphere accessory (Hitachi U-3210 model, 60 mm sphere diameter, opening ratio ) 7.8%). By this procedure, reproducible spectra with maximum absorbances around 0.7 units at λmax ) 475 nm (see Figure 1) were obtained at maximum coverage (i.e., after dipping the films in 1 mM acetonitrile alizarin solution). Complementary measurements were performed on a Shimadzu UV-3101 double beam spectrophotometer equipped with an integrating sphere (Shimadzu ISR-260) in the diffuse transmittance mode. The entrance windows were adjusted to the film size. Absorbances were calculated as A ) -log(Td), where Td is the diffuse transmittance. This procedure neglects film reflectance. Spectra were obtained by subtracting the spectra of naked films. The photoelectrochemical studies were carried out in a 20 mL square single-compartment, three-electrode cell, using a commercial potentiostat/galvanostat, TEQ-04, Argentina, interfaced to a personal computer. The alizarin coated nanocrystalline TiO2 film was used as the working electrode. In addition, Ag/ AgCl and a Pt foil serve as reference and counter electrodes, respectively. The working electrode was illuminated from the back-side (light first hits the supporting conductive glass). The photocurrent potential curves were determined at room temperature, using 1.0 M KNO3, as electrolyte unless specified. Solution pH was adjusted with HClO4 or NaOH. At least 20 min before use, the solution was equilibrated with N2 (99.99%) or air. Absorption spectra of the films were routinely checked before and after the photoelectrochemical experiments, and the electrodes were discarded when the difference was significant.
3. Results 3.1. Optical and Electrochemical Characterization of Alizarin Modified Films. The inspection of electrode surfaces was carried out with a high-resolution scanning electron microscope (SEM, Hitachi S-5000). The analysis reveals that the films have a nanoporous structure with an approximate thickness of 2 µm. The normalized absorption spectra of the nanocrystalline titanium dioxide films obtained after dipping the semiconductor electrode in the alizarin solution at different concentrations are shown in Figure 1. Spectra were derived from diffuse transmittance measurements and obtained as the difference in the absorbances of the loaded and naked TiO2 films. As previously discussed in the literature, the band centered at ca. 475 nm is the result of alizarin chemisorption and evidences a relative large red shift in the visible band of the free ligand which is observed at 425 nm in acetonitrile solutions.9-11 The analysis of transient absorption spectroscopy studies has revealed that, despite the high strong coupling between TiO2 and alizarin, the band is better described as a localized S0 f S1 excitation instead of a direct charge-transfer absorption. This conclusion is further supported by a more recent electroabsorption spectroscopy study based on the Stark effect14 and is fully consistent with theoretical studies.15 The adsorption isotherm was examined by derivatizing the electrodes from acetonitrile solutions of different alizarin concentrations, C, and measuring the absorbance, A, of the films at 475 nm. We used the linearized form of the Langmuir isotherm
1 1 1 + ) Γ Γmax ΓmaxKC
(1)
to test the results. In the above expression K is the equilibrium binding constant, Γ, the amount of adsorbed alizarin on the surface, and C, the (molar) concentration of the species in the medium in contact with the solid surface. Γ is related to the absorbance of the films, A475, by the expression
Γ)
A475 Υε475
(2)
where ε is the molar absorption coefficient of alizarin adsorbed on the films and Υ the rugosity of the film. By combining eq 1 and 2, we obtained
1 1 1 + ) A Amax AmaxKC
(3)
Behavior of Alizarin Modified TiO2 Films
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SCHEME 1: Energy Diagram Depicting the Transfer of Charge under Different Bias, to Account for Anodic (Left) and Cathodic (Right) Photocurrents
thus, if the adsorption data follows the Langmuir isotherm, a plot of 1/A vs 1/C should give a straight line, allowing the determination of the binding constant and the absorbance of a semiconductor film bearing a monolayer of the ligand Amax. In spite of the fact that the surface of the nanoporous electrode is not uniform and the basic assumption of equivalent adsorption sites is unlikely, a linear regression of the above-mentioned plot shows a good correlation with K ) 4.2 × 104 M-1 and Amax ) 0.7 as the y intercept.16 Thus, we were able to evaluate the surface coverage, θ, of the films at different loadings, as
θ)
A Amax
(4)
It is also worth noticing that K is in the range of values obtained for the binding constant of other enediol ligands;9 thus, the derived value is consistent with experimental evidence as well as current assumptions regarding the nature of alizarin interaction with the TiO2 surface.7,9 In fact, although other ways of binding are possible for alizarin, preliminary theoretical calculations using the MSINDO formalism under the cyclic cluster model indicate that the binding to the titanium surface through the 1,2 hydroxyl groups yields the most stable complex.17 To determine the molar absorption coefficient of alizarin adsorbed on the films, known amounts of dilute alizarin solution in acetonitrile were carefully loaded at the surface of the films, the solvent was evaporated and their absorbance were determined as detailed in the experimental section. The derived value at 475 nm, the wavelength of maximum absorption, ε475 ) 5.4 × 103 M-1 cm-1, is in fair agreement with previous reports obtained in ethanolic sols of titanium dioxide ε500) 8.7 × 103 M-1cm-1.9 This information was combined with the molecular surface of alizarin S, to obtain an estimation of Υ, the roughness factor of the electrodes, defined as the ratio of the effective surface area of the nanocrystalline layer to its projected area18
Υ ) ΑmNAS/ε
The redox properties of the adsorbed ligand have been derived by some of us from dark cyclic voltammetric experiments of alizarin derivatized TiO2 films under N2 atmosphere.11 Using the reported values for the oxidation, E1/2 (A•+/A) ) 0.96 V, and reduction, E1/2 (A/A•-) ) -0.5 V vs Ag/AgCl, of the ground state molecules, we estimated the corresponding values of excited alizarin molecules as20
E1/2(A · + /A*) ) E1/2(A · + /A) - E00
(6)
E1/2(A*/A•-) ) E1/2(A/A•-) + E00
(7)
In the above equations, E00 ) 2.48 eV is the energy of the 0-0 transition.7a The results, E1/2 (A •+/A*) ) -1.52 V and E1/2 (A*/A•-) ) 1.98 V, are depicted in the energy diagram (Scheme 1). Figure 2 shows the successive spectra obtained by biasing the films in the dark at -700 mV vs Ag/AgCl, in a N2 saturated atmosphere. Similar spectral changes are apparent by photo-
(5)
From a geometry optimization under the DFT formalism using the three-parameter exchange functional of Becke B3LYP,19 we obtained, S ) 230 Å2. Using this figure we estimate an approximate roughness factor, Υ ) 1.8 × 103 for the nanoporous electrodes.
Figure 2. Upper: Successive spectra obtained by electrochemical reduction of the films at -700 mV using NaClO4 0.2 M as electrolyte at pH 3.0 in the absence of air. Lower: Spectra of a nanoparticulate sol of (a) fully oxidized A@TiO2 and (b) reduced AH2@TiO2 complexes (see ref 11). The inset in each graph shows the differences between the initial and final spectra.
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Figure 3. Photocurrent-voltage (s) and dark current-voltage (- - -) curves obtained during +300 to -300 mV scans at 1 mV/s. The dotted curve represents the result obtained under interrupted illumination. The curves were obtained in air saturated 1.0 M KNO3 solution at pH 6.0. Irradiation conditions, λ) 455 ( 20 nm; incident photons, 1.64 × 1016 cm-2 s-1; film absorbance, 0.29. The potential is given vs Ag/AgCl.
chemical UV reduction of A@TiO2 observed in a nanoparticulate sol in the presence of ethanol and account for the electron acceptor properties of the dye. 3.2. Photocurrent Generation as a Function of the Externally Applied Bias. UV irradiation (at 330 ( 10 nm) of a bare TiO2 electrode results in anodic currents when the electrode is polarized above the flatband potential.21 The same behavior is found when alizarin-modified electrodes are illuminated at this wavelength, where the dye barely absorbs. However, at wavelengths longer than that corresponding to the bandgap where the dye is selectively excited, a current reversal is observed, as below a certain potential cathodic photocurrents begin to flow. The potential at which the photocurrent sign is reversed depends on the atmosphere composition (air/N2), the electrolyte and pH, and the degree of alizarin coverage, as we examine separately below, but as a general rule the onset of negative photocurrents occurs at potentials ca. 600-700 mV less negative than Vfb for pure TiO2. Figure 3 shows typical j-V curves obtained under interrupted visible illumination at λ ) 455 ( 20 nm, scanning the applied potential from +300 to -300 mV at 1 mV/s. Sign reversal is observed nearby 0 V at pH 6.0 in air saturated KNO3. Following standard conventions, anodic and cathodic photocurrents are taken as positive and negative, respectively. As reported elsewhere, the adsorption of an organic molecule to the surface of the semiconductor can modify its electronic properties.6,22-24 A precise determination of the flatband potential is often complicated by the existence of surface states and other imperfections at the oxide surface.25 As a crude approximation, we determined the potential at which photocurrent begins to flow, Vph, to obtain a rough estimate of the flatband potential of the semiconductor. By irradiating aqueous air saturated solutions at λ ) 330 nm, we obtained Vph/mV ) -490 and -725 vs Ag/AgCl, at pH 2.0 and 6.0, respectively, in good agreement with previous results, Vfb/mV ) -475 and -715 derived from spectroelectrochemical studies of bare TiO2 electrodes. For alizarin-modified TiO2 electrodes, the corresponding estimations for the onset of the photocurrent occurs at -340 and -600 mV vs Ag/AgCl, suggesting an anodic shift of ca. 135 ( 15 mV in the flatband potential, very close to the difference obtained in reference 6 for carminic acid using a different approach.21,26
Di Iorio et al.
Figure 4. Photocurrent as a function of applied external bias at various pHs. The curves were obtained in air saturated 1.0 M KNO3 electrolyte solution. Irradiation conditions, λ) 455 ( 20 nm; incident photons, 1.64 × 1016 cm-2 s-1; film absorbance, 0.2.
Figure 5. Photocurrent as a function of applied external bias voltage, determined at pH 6.0 using an aqueous saturated 1.0 M KNO3 electrolyte under (a) N2 and (b) air-saturated conditions. Curve (c) shows the results of adding the redox couple [I3-]/[I-] to the 1.0 M KNO3 electrolyte solution in the air-saturated atmosphere. ([I3-] ) 0.15 mM, [I-] ) 1.5 mM). Irradiation conditions and film absorbance as in Figure 4.
Figures 4 and 5 show the dependence of induced photocurrents on electrolyte pH and composition. In these and the following figures, the plotted current density, j, represents the difference between steady-state values, jl, determined under illumination ca. 5 min after steps to selected potentials and that measured at the same potentials in the dark, jd. Photocurrent determinations were examined at electrolyte pHs between 1.5 and 6.0. Alkaline pHs were not explored to avoid ligand desorption and degradation. As shown in Figure 4, cathodic photocurrents occur at pH values in excess of 4.0; that is, at more acidic conditions, anodic photocurrents prevail at all the explored potentials. It is interesting to note that exhaustively purged N2 solutions still show the presence of negative photoinduced currents; however, the onset of cathodic photocurrents occurs at more negative potentials, and the maximum absolute values are at least a factor of 3 lower. On the other hand, the presence of the redox couple [I3-]/[I-] increases the light induced flow in both directions, as expected.6 (See Figure 5.) The upper graph in Figure 6 shows the changes in the current density, at selected potential biases, obtained in air saturated
Behavior of Alizarin Modified TiO2 Films
J. Phys. Chem. C, Vol. 114, No. 26, 2010 11519 It is worthwhile mentioning that the photocurrent action spectra determined at positive and negative applied potentials closely follows the absorbance of the film; thus, both cathodic and anodic currents originate from the absorption of light by the alizarin complex. (See the Supporting Information.) 4. Discussion
Figure 6. Upper: Changes in photocurrent (right) and IPCE values (left) at selected potential bias as a function of the film absorbance. Lower: Apparent photocurrent quantum yields, ηPC, as a function of the dye coverage. All measurements were performed in air saturated 1.0 M KNO3 electrolyte solution at pH 6.0. Irradiation conditions, λ) 470 ( 25 nm; incident photons, 8.4 × 1015 cm-2 s-1. The potential is given vs Ag/AgCl electrode.
electrolyte solutions at pH 6.0 after illumination of films of various absorbances, i.e., with different alizarin loadings. The same electrode was used to avoid differences in the thickness of the film and possible defects on the surface which can affect the j-V curves.27 The estimated incident photon to current conversion yields (IPCE) defined by the equation
IPCE )
j[µA cm-2]/F
∫λ φλ[µEinstein cm-2 s-1] dλ
(8)
are indicated at the right ordinate axis of this figure. In the above equation, F is the Faraday constant, and ∫λ φλ[µ Einstein cm-2 s-1] dλ represents the integrated incident photon flux, which peaks at 470 nm. Negative IPCE values correspond to cathodic photocurrents and indicate that charge flows from the ITO surface to the electrolyte. Apparent photocurrent quantum yields, ηPC, were estimated as the ratio between IPCE values and f, the fraction of the incident light that is absorbed by the films as determined by the diffuse transmittance spectra of the films, i.e., neglecting light reflection at the cell window. The results plotted as a function of dye coverage showed that cathodic photocurrents go through a maximum at 25% of dye coverage.
Photocurrent generation and collection involving bare and modified nanoporous crystalline TiO2 electrodes have been analyzed in detail in the literature.28-33 It has been recognized that because the crystallite size is too small to support an effective depletion layer the photocurrent generation is governed by the dynamics of the electron transfer process at the interface rather than been controlled by the potential gradient over the space charge region.34 Nevertheless, an electrochemical positive bias with respect to the flatband usually improves charge separation.25,35 As indicated in Figure 3, the modulation of the Fermi level of the nanocrystalline TiO2 film, Vf, through the application of an external potential to the supporting ITO, results in the development of anodic photocurrents at positive bias showing characteristic spikes upon shutter opening due to fast charge recombination. Moving the Fermi level of the film toward the conduction band is expected to cause an increase in the rate of charge recombination, or inhibit charge injection.31 However, we observed that, as Vf becomes more negative (in direction of the flatband), the sign of j is reversed. Cathodic photocurrents have previously been detected using naked TiO2 electrodes in alkaline electrolytes.36 It has been recognized that, under TiO2 excitation, the transfer of both electrons and holes to the ITO surface may occur simultaneously. Thus, it has been rationalized that the net photocurrent collected may be anodic or cathodic depending on the relative rates of the charge transfer process at the interface and the efficiency of charge percolation through the porous network. Since in acidic solutions the hole capture by water is faster than the charge transfer to dissolved oxygen (which is usually the electron acceptor), the majority of charge carriers are often collected in the underlying conductive glass substrate and anodic photocurrents are typically observed.36,37 However, since the position of the band edges is shifted by -60 mV per pH unit, a pH increase weakens the oxidizing power of valence band holes and favors the rates of interfacial electron capture.38,39 Notice that the redox potential for O2 reduction also shifts with pH depending on speciation. Reported redox potentials derived from homogeneous studies indicate E(O2 + H+/HO2•) ) -78 mV at pH 0 and E(O2/O2•-) ) -367 mV at pH 7 or higher, both values quoted vs Ag/AgCl. Given that the pK value of HO2 ) 4.8,40,41 the driving force for O2 reduction rises with pH above ca. 6. It is apparent that this fact, together with the preferential adsorption of O2 in alkaline solutions, accounts for the development of cathodic photocurrents in bare n-TiO2 films.36,42 Following visible excitation of dye modified TiO2 electrodes, a positive hole is created on the sensitizer at the semiconductor electrolyte interface and an electron is injected directly into the conduction band or a surface state. As in the case of naked TiO2 films, the photocurrent sign is also expected to be controlled by the kinetics of electron transfer at the semiconductor/ electrolyte and semiconductor/substrate interface.43 Since A•+ cannot oxidize water molecules, in the absence of added donors, D, satisfying the condition E1/2 (A•+/A) > E1/2 (D•+/D), depletion of A•+ at the interface could eventually occur by hydrolysis or
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oxidation with O2. On the other hand, as in the case of naked electrodes, removal of electrons at the semiconductor electrolyte interface may be accomplished by oxygen, however, this process is known to be rather inefficient in air saturated slightly acidic or neutral electrolytes.36 We recall that the prevalence of a cathodic photocurrent requires an efficient charge transport mechanism through the semiconductor in the interbandgap region. Since, at potentials more positive than the flatband, the Fermi level of the ITO electrode lies below the bottom of the conduction band of TiO2 particles, we would not expect electrons in the conductor support to enter the TiO2 film. Thus, electron transport should involve electron hopping through alizarin molecules and/or between surface states through the TiO2 matrix, from the ITO to the electrolyte solution. As widely documented, nanoporous electrodes offer a high surface area, leaving at the interface a great number of dangling bonds originated in interrupted lattice periodicity.24,29,35 It is also worth mentioning that the energy and abundance of surface states rises with the electrolyte pH.44 We favor the idea that surface states are involved in the mechanism of charge percolation and allow carrier transport through the otherwise insulating TiO2 matrix. Moreover, considering the dual character of alizarin molecules, we propose that the easily reductive excited state of the ligand may be involved in the mechanism of generation of cathodic photocurrents, acting as an intermediate in oxygen reduction. Thus, at negative applied bias, electrons from the ITO may transported through the TiO2 layer by surface states before been captured in the excited state of the alizarin radical anion and discharged to the electrolyte solution. Similar ideas have previously been considered in reference 6 to rationalize the photocurrent switching mechanism in carminic acid modified TiO2 electrodes. The dependence of photoinduced currents on pH and oxygen concentration shown in Figures 4 and 5 provides support to the suggested mechanism. As previously stated, the abundance of surface states rises with the electrolyte pH, and also a pH increase enhances deprotonation of surface Ti-OH groups and facilitates chemical adsorption of oxygen molecules through a charge transfer interaction with surface Ti-O- groups. The above argument implies that chemically adsorbed O2 molecules will probably remain attached to the surface even under extensive N2-purging, accounting for the existence of cathodic photocurrents under anaerobic conditions, as suggested in ref 36. The coordination of Ti(IV) centers by alizarin ligand is expected to modify the energy and distribution of intraband states.8a,45 The reparation of surface states by chemisorption of organic molecules is widely documented in the literature.8,45,46 The most direct experimental evidence of this fact is the rate enhancement of O2 reduction as result of surface complexation with catechol, salicylic, and phtalic acids and the surface reconstruction upon ascorbic adsoption on anatase, evidenced by the change of the pentacoordinate Ti surface atoms to octahedral geometry observed by X-ray absorption fine structure (EXAFS) and X-ray absorption near edge (XANES) spectroscopies.8b,46 We suggest that the progressive reparation of surface states with the amount of dye loading may help to explain the existence of the maximum in Figure 6, assuming that the decrease in the density of surface states at high dye coverage limits the electron transport through the TiO2 film. It is worthwhile to notice that, if the electron transport is dominated by electron hopping between alizarin molecules, instead of surface states, we would
Di Iorio et al. not expect to observe cathodic photocurrents at low alizarin coverages; rather, we would have observed a threshold, i.e., a minimum surface coverage for electronic conductivity. On the other hand, we verify that excitation of alizarin molecules on Al2O3 films supported on the conductive glass does not produce any photocurrent. This observation, though not conclusive because nanocrystalline TiO2 offers a higher surface area, is consistent with the idea that the semiconductor does not behave as an inert support. To summarize, Scheme 1 shows the energy levels and the electron transfer pathways under different bias. At positive applied potentials, a fraction of the injected electrons that escape recombination manage to percolate through the nanoporous film and are collected as a positive photocurrent. At negative bias, electrons from the ITO are transported through the TiO2 layer by surface states before been captured in the excited state of the alizarin radical anion and discharged to the electrolyte solution. Finally, the fact that no cathodic photocurrents in the whole range of potentials examined (-300 e V/mV e +300) could be observed under TiO2 excitation (λ ) 330 ( 10 nm, N2 or air-saturated atmosphere, 1.5 e pH e 7.0) may provide a piece of valuable information and illustrate the possible use of photoelectrochemical studies to contribute to the understanding of the dynamics and mechanisms involved in charge transfer process. We suggest that the development of cathodic photocurrents in alizarin modified TiO2 electrodes is probably linked to the low reactivity of adsorbed A•+, which results in a very poor collection efficiency of anodic photocurrents due to recombination. However, under TiO2 excitation, the simultaneous formation of more energetic holes results in effective water oxidation and is decisive to turn the electron conduction to the ITO surface faster than hole collection, accounting for the net anodic photocurrents. 5. Conclusions Photoelectrochemical photocurrent switching, PEPS, has been observed for alizarin molecules chemisorbed in nanoporous TiO2 electrodes. Sign reversal of the induced photocurrent is ascribed to the dual electron donor/acceptor character of the ligand. Negative IPCE values, corresponding to cathodic photocurrents, display a maximum at intermediate alizarin loadings, suggesting that surface states may be involved in the mechanism of carrier transport through the TiO2 matrix. This work illustrates that electrochemically amphoteric species strongly bound to semiconductor surfaces, as alizarin, may be attractive for the design of logical devices. Acknowledgment. This work was financially supported by the University of Mar del Plata and the National Research Council of Argentina (CONICET), Project PIP 319. M.A.G. and E.S.R. are members of the research staff of CONICET. Y.D.I. and H.B.R. thank CONICET for a doctoral and a postdoctoral fellowship, respectively. Supporting Information Available: Photocurrent action spectrum of alizarin modified TiO2 films (25% coverage) obtained under air saturated conditions using 1.0 M KNO3 electrolyte solution at pH 6.0. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Biancardo, M.; Bignozzi, C. A.; Doyle, H.; Redmond, G. Chem. Commun. 2005, 3918–3920.
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