J. Phys. Chem. C 2008, 112, 10233–10241
10233
Electronic Excited State of Alizarin Dye Adsorbed on TiO2 Nanoparticles: A Study by Electroabsorption (Stark Effect) Spectroscopy Agnieszka Nawrocka and Stanisław Krawczyk* Institute of Physics, Maria Curie-Skłodowska UniVersity, 20-031 Lublin, Poland ReceiVed: October 23, 2007; ReVised Manuscript ReceiVed: March 8, 2008
Alizarin is one of the dyes extensively investigated as an example of a molecule capable of serving as a light absorber and an electron donor in model systems designed for the new type solar cells. Using the Stark effect measurements for alizarin, both free in solution and adsorbed to TiO2 nanoparticles, the question has been addressed whether the excited-state orbital of adsorbed alizarin extends into the solid and involves the orbitals of the Ti atom or remains localized within the alizarin molecule. Because an important role can be played by the electric field at the charged surface of the nanoparticles, the field was modulated by changing the pH of the medium. The results reveal a substantial dipole moment change on the electronic excitation of the alizarin-TiO2 system, |∆µ| ≈ 10 Debye units or slightly more. The observed dependence of the absorption maximum and the measured |∆µ| on pH were used to distinguish between ∆µ directed toward the nanoparticle surface and that corresponding to the intrinsic rearrangement of electrons within alizarin or, in reverse direction, corresponding to more- or less-complete electron transfer from alizarin onto the orbitals of Ti and adjacent atoms comprising a localized surface (or a delocalized conductive) state. The results qualitatively contradict a significant dye-to-solid charge-transfer character of the electronic transition. It is shown that they can be interpreted in a self-consistent way by considering, in a first approximation, the light absorption by alizarin monoanion subject to the electric field generated by the charged nanoparticle surface. SCHEME 1
Introduction Different approaches are currently being pursued for the synthesis and studies of metal oxide nanoparticles, motivated by their potential technological applications. Some of the anticipated applications are the decontamination of water, photocatalysis,1 color photography,2 and, perhaps the most important, their use to build large-area solid-liquid interfaces in dye-sensitized semiconductor solar cells.2,3 These functions are linked with the photosensitization of wide band gap oxide semiconductors by organic molecules; the process is especially effective on nanostructured surfaces containing metal atoms within a distorted crystal structure.4 Alizarin (Scheme 1) is extensively used in photosensitization studies of metal oxides. Its visible absorption spectrum undergoes a large red shift when adsorbed to titanium dioxide, usually interpreted as due to the charge transfer character acquired by the electronic transition. Alizarin adsorbed on TiO2 nanoparticles forms complexes in different aqueous media from acidic, with pH ∼1.7 or 3,5–7 up to pH 12.5 The infrared data8 and structure calculations9–12 indicate that the dye is covalently linked to the surface Ti atom in a chelating bidentate mode involving the oxygen atoms of its two hydroxy groups. In general, two cases of photosensitization by adsorbed dye have been considered. One involves the direct injection of an electron from the dye to the nanoparticle, which is ascribed to dyes such as catechol on TiO213–15 and to the transition-metal complexes,16,17 and is generally referred to as interfacial chargetransfer absorption.18 In this direct mechanism, partial charge transfer is instantaneous and takes place during the absorption process. The initially formed Franck-Condon state has a hole on the dye and an extra electron on the nanoparticle. The second * Corresponding author e-mail:
[email protected].
type of sensitization occurs through indirect injection; an excited state localized on the adsorbed dye molecule is initially formed, which then transfers an electron to the solid.19–21 The direct injection mechanism is considered to be related to the new charge-transfer band in the absorption spectrum that is observed when the dye is attached to the particle, whereas no new band appears in the absorption spectrum in the case of the indirect mechanism. The 6 fs time constant for electron injection from electronically excited alizarin into TiO2, considered to proceed through the charge-transfer optical transition,22 makes this process one of the fastest charge-separation reactions reported and studied in real time. A question pertaining to both mechanisms, which is very relevant to the practical applications, and concerning the charge-transfer character of the light absorption by the adsorbed dye is whether the electron transfer involves a localized surface state or a delocalized conductive state enabling the movement of the electron toward the cell electrode. The back electron transfer from the reduced nanoparticle to the oxidized dye following either type of injection proceeds much slower and is generally nonexponential, pointing to a diversity of surface states involved.23 The studies of dye-metal oxide surface interactions have been concentrated on the kinetics of the forward and back electron transfer and their dependence on factors such as the
10.1021/jp710252h CCC: $40.75 2008 American Chemical Society Published on Web 06/14/2008
10234 J. Phys. Chem. C, Vol. 112, No. 27, 2008 chemical nature (TiO2, ZrO2)7 and crystal structure24 of the solid, pH-dependent modulation of the conduction band energy,23,25,26 and the oxidation potential of the dye.24,27 Other experimental studies focused on the properties of the (quasi)stationary electronic states attained by the excited electrons and donated by both the dye and adsorption center and participating in the electron transfer. The interpretations of experimental5–7,23 and computational10–12 studies of the alizarin-TiO2 system point to the charge-transfer character of the electronic transition, but an essentially localized excitation of the dye, followed by subsequent electron transfer to a localized surface state and then by spreading the electron into the conduction band states, was also concluded from both experimental28 and computational11 studies. In fact, the relations between the locally excited states and charge-transfer states in alizarin, catechol, and other enediol dyes adsorbed on an oxide surface have not yet been directly addressed in experiment. The aim of this study is to characterize the redistribution of the electronic charge in the electronic transition in alizarin adsorbed on the surface of TiO2 nanoparticles. This is done using the electroabsorption (Stark) spectroscopy, which is an adequate spectroscopic technique providing information on the changes in the dipole moment and polarizability between the groundstate and the Franck-Condon excited-state of the free alizarin and of the dye-semiconductor complex. The change in dipole moment, which measures the amount of charge transfer associated with an electronic transition, is compared for the neutral form of the dye and its mono- and dianion with that for alizarin-TiO2 system undergoing electronic excitation, and the results are analyzed in terms of alizarin-TiO2 charge-transfer transition or a transition localized on the adsorbed alizarin molecule.
Nawrocka and Krawczyk in a stream of cold nitrogen vapor, which resulted in the formation of transparent glass, the samples were transferred into an optical flow cryostat (Optistat, Oxford Instruments) and kept at temperatures of 100-120 K, higher than the liquid nitrogen temperature but minimizing the risk of sample cracking in longlasting experiments. The spectra were recorded using light from a 450 W xenon lamp dispersed by a computer-controlled grating monochromator. The electric field-induced change in absorbance was induced by sinusoidal voltage of 1000-1500 V rms applied to the sample and detected as the AC component (rms) of the transmitted light intensity, ∆I, with a digital lock-in amplifier (SR 830, Stanford Research Systems) at the second harmonic frequency in order to measure the quadratic Stark effect. This signal was recorded at time intervals of 3-10 s per point, depending on detector’s time constant, and recalculated into the rms value of absorbance modulation using the expression ∆A ) ∆I/(2.303I). The DC component of the transmitted light intensity (I) was recorded simultaneously with ∆I and was also used to calculate the absorption spectrum. Multiple scanning and averaging of spectra was employed to improve the signalto-noise ratio. The values of electro-optical parameters are given below as the mean value and the standard deviation in a series of measurements with at least four samples. According to theory, the electric field-induced absorbance change ∆A for an isotropic sample containing immobilized dye molecules is proportional to the square of the electric field and can be expressed, in general, as a linear combination of the absorption spectrum and its first and second derivatives with respect to the wavenumber ν.29,30 With the neglect of the field dependence of the transition intensity, which can be relevant only for weak or forbidden transitions, the change in absorbance can be reduced to the derivative terms (eq 1).30,31
∆A ) a1ν
Materials and Methods Titanium tetraisopropoxide (Ti[OCH(CH3)2]4) and alizarin (1,2-dihydroxyanthraquinone, Scheme 1) were purchased from Aldrich and were used without further purification. Water was prepared by distillation from potassium permanganate. Other solvents used were of analytical purity. TiO2 nanoparticles were prepared by the hydrolysis of Ti(IV) tetraisopropoxide according to the published procedure.27 A solution of 5 mL of Ti[OCH(CH3)2]4 dissolved in 95 mL of 2-propanol was added dropwise (1 mL/min) to 900 mL of water at 2 °C and pH 1.5 (adjusted with HNO3). The solution was continuously stirred for 10-12 h until a transparent colloid was formed. The colloid solution was concentrated at 35-40 °C with a rotary evaporator and then dried with a stream of nitrogen to a white powder. For the measurement of the Stark spectra the TiO2 nanoparticles were dissolved in 60% ethylene glycol-water (3:2, v/v) to the final concentration of 1.18 M. The pH was adjusted by adding HNO3 or NaOH. Sample solutions of alizarin adsorbed on TiO2 were prepared by adding alizarin powder to the TiO2 colloid and stirring for 1.5 h. Room-temperature absorption spectra were recorded with a Shimadzu UV-160A spectrophotometer. Raman spectra were recorded with the Renishaw InVia spectrometer with an excitation wavelength of 514.5 nm. The spectra of alizarin in the forms of monoanion, dianion, and adsorbed on TiO2 were measured using 60% ethylene glycol in water as solvent; alizarin in the neutral form was dissolved in anhydrous ethanol. For measurement of electroabsorption spectra, the solutions were applied into a thin (0.08 mm) cuvette constructed from 1 mm thick glass plates with conductive layers of indium-tin oxide serving as electrodes. After quick freezing
d(A ⁄ ν) d2(A ⁄ ν) + a2ν dν dν2
(1)
The coefficients a1 and a2 depend on ∆r and ∆µ, the excitedminus-ground-state differences of the molecular polarizability tensor and of the permanent dipole moment,
a1 )
(fF)2 5 Tr(∆R) + 15√2hc 2
[
3 1 (3cos2 χ - 1) p(∆R) · p - Tr(∆R) 2 2
)]
(2)
(f|∆µ|)2F2 [( 3cos2 δ - 1)cos2 χ + 2 - cos2 δ] 10√2h2c2
(3)
(
a2 )
where F is the applied electric field intensity, Tr(∆r) means the sum of diagonal components of ∆r, p is a unit vector along the transition dipole moment, and the angle δ is between the transition moment and the vector ∆µ. The 2 factors in the denominator account for detection of the signal at the second harmonic frequency of the applied sinusoidal voltage. For a molecule with polarizability in one direction much larger than the perpendicular polarizability components the tensor, ∆r reduces to a scalar (uniaxial) polarizability ∆R and (eq 2) simplifies to eq 4,
a1 )
∆R(fF)2 [( 3cos2 γ - 1)cos2 χ + 2 - cos2 γ] 10√2hc
(4)
where γ is the angle between the polarizability axis and the transition moment. The values of a1 and a2 were obtained by fitting the ∆Α spectrum with a least-squares procedure using the linear
Alizarin Dye Adsorbed on TiO2 Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 27, 2008 10235
Figure 1. Room-temperature absorption spectra of the neutral, monoanion, and dianion forms of alizarin (84 µM/L) in ethylene glycol/water ) 3:2 (v/v). The values of solution pH are quoted at each curve.
Figure 2. Absorption spectra of TiO2-alizarin complex in 60% ethylene glycol in water at different pH values at 295 K. Concentrations: TiO2, 36 mM; alizarin, 84 µM. The spectra represent the final form of the complex reached at sufficiently high TiO2 concentration.
combination of absorption derivatives according to eq 1. The angles γ and δ were estimated from the slope of the linear dependence of a1 and a2 on cos2 χ, where χ is the angle between the applied electric field F and the electric vector of light. The latter was varied in the experiment by setting the sample at different angles of incidence in p-polarized measuring light. The angles γ and δ were then used to calculate ∆R and |∆µ| from eqs 3 and 4. It should be noted that the scalar ∆R representing the uniaxial polarizability is equal to Tr(∆r), which can be obtained from measurements at χ ) 54.7° using eq 2. The values of electro-optical parameters quoted here include the local field factor f, that is, they are reported as f2∆R and f|∆µ|. The value of f is 1.33 for glycerol-water glass.31,32 The ab initio and semiempirical calculations of Hartree-Fock type were performed with HyperChem v.533 using more than 100 singly excited configurations. Ab initio excitation energies were scaled with the factor 0.6.
range 12.5-13, respectively. Lowering the temperature enhanced the degree of dissociation, and the formation of alizarin mono- and dianions was essentially complete in frozen solutions with pH slightly above neutral and at pH 11, respectively. Overall, this dependence on pH is in agreement with the pK values of 6.6 and 12.4 for alizarin in dioxane/water (1:2) at room temperature.34 Absorption spectra of alizarin adsorbed on TiO2 at different pH at room temperature are shown in Figure 2. Upon binding to the TiO2 nanoparticles, the absorption maximum of alizarin is positioned in a narrow spectral range between 490 and 520 nm depending on pH. The absorption maximum of the complex at pH 3 was located at 495 nm, that is, 65 nm (or 3000 cm-1) to the red from the position for the free dye. At pH above 7, a blue shift from 530 to 505 nm was observed, reflecting the adsorption of the alizarin monoanion to the TiO2 nanoparticle. Using the ethylene glycol-water mixed solvent we observed the binding of alizarin to TiO2, which causes a shift of the absorption maximum from 571 nm (for dianion) to 512 nm, that is, by ∼2100 cm-1 toward the violet. The absorption spectra of alizarin bound to TiO2 in the pH range from 3 to 13 are always represented by a wide unstructured band (at room temperature) with the maximum located within a relatively narrow range from 495 to 512 nm. This behavior of bound alizarin suggests the same form of alizarin is associated with TiO2 surface in the whole pH range, with the absorption band near 500 nm showing a noticeable red shift with the increase of pH. The resonance Raman (RR) spectra of alizarin in the presence of TiO2 nanoparticles in 60% ethylene glycol obtained at pH 3, 7, and 11 show the frequencies and relative intensities characteristic for the dye bound to TiO2 observed in other works5,6 (see Supporting Information). They confirm the binding of alizarin to TiO2 without essential differences in its electronic structure in these diverse conditions. Electroabsorption Spectroscopy of Free Alizarin. Representative low-temperature absorption and electroabsorption spectra and electroabsorption spectral components for alizarin in the neutral, monoanion, and dianion forms are shown in Figures 3–5, respectively. The electro-optical parameters of free alizarin and that bound to TiO2 at different pH are presented in Table 1. The study of the free dye in the neutral form at low pH was not possible in water/ethylene glycol because of poor solubility of alizarin in acidic media. However, the similarity of the position and shape of the absorption spectrum in ethanol
Results Adsorption of Alizarin on TiO2 at Room Temperature. The binding of alizarin to TiO2 in 60% ethylene glycol was monitored by absorption and resonance Raman spectroscopy at room temperature. Selected absorption spectra of free alizarin are presented in Figure 1. Consistent with other studies of alizarin in buffered water-dioxane34 and of an alizarin derivative in binary solvent mixtures,35 the formation of alizarin dissociation products with red-shifted absorption spectra was observed in ethylene glycol-water, accompanying the change of pH from 3 to 13. The spectra indicate the subsequent formation of mono- and dianions of the dye. In acidic solutions (pH < 5) the absorption spectrum of alizarin has the maximum at 435 nm, corresponding to the neutral molecule. With increasing pH of the solvent, the absorption switches to new bands at longer wavelengths. At pH 7 the band with maximum at 435 nm is still observed with a shoulder at about 535 nm, indicating the presence of both the neutral and the monoanion forms. A further increase of pH to 10 causes an increase of sample absorbance, and the transitory absorption maximum at 530 nm corresponds to the alizarin monoanion. The spectra at higher pH are clearly structured with vibronic transitions at 612 and 571 nm and a shoulder at 535 nm, and they can be assigned to the dianion form of the dye. The values of pK1 and pK2 for alizarin in 60% ethylene glycol were found at 6.7 and in the
10236 J. Phys. Chem. C, Vol. 112, No. 27, 2008
Nawrocka and Krawczyk
Figure 3. Electroabsorption data for the neutral form of alizarin in ethanol at 100 K, in an rms electric field of 162 500 V/cm. (A) absorption spectrum, (B) Stark spectrum (points) and the fitting curve, and (C) the fit components. The fit in panel B (continuous line) was calculated for the spectral range 15 000-25 000 cm-1 but is shown further to the violet (dashed line), indicating the presence of the next electronic transition (see text).
Figure 4. Spectra of the alizarin monoanion (2.78 mM) in ethylene glycol/H2O ) 3:2 (v/v), pH 10.5. (A) absorption spectrum, (B) electroabsorption spectrum (points) with the fit (continuous line), and (C) the fit components. The dashed line in panel B is the fit with derivatives shifted by 150 cm-1 toward the violet (see text). Electric field intensity (rms), 125 000 V/cm; temperature, 120 K.
observed in the whole range of temperatures to those at room temperature in water/dioxane at pH < 5,34 in methanol,7 and of the acidic solution in ethylene glycol (Figure 1) confirms that the neutral form of the dye is also in ethanol at 100 K in our experiments. The Stark spectrum of the neutral alizarin shown in Figure 4B can be adequately described by the linear combination of the absorption derivatives in quite a wide spectral range up to 25 000 cm-1. The Stark spectrum and the second derivative of the absorption spectrum exhibit vibrational structure with a spacing of about 1200 cm-1. This points to the ∼1200 cm-1 vibrational modes composed of ring and C-O stretches as the normal coordinates coupled with the electronic excitation of the neutral alizarin molecule. However, there are no strong lines close to this frequency in the resonance Raman spectrum of neutral alizarin obtained with excitation at 375 nm.6 This inconsistency can be partly due to the excitation far from the resonance conditions, but another and probably more important reason for this inconsistency follows from the Stark spectrum in Figure 3B. The discrepancy between the fitting curve and experimental data observed there for frequencies above 25 000 cm-1 indicates that there is another electronic transition at ∼27 000 cm-1 (370 nm) that corresponds to the weak band discernible in this region of the absorption spectrum in Figure 3A, which is most probably associated with a different set of electro-optical parameters. A weak band at about 390 nm is also discernible on the blue side of the absorption band of an alizarin derivative in water and in mixed solvents with methanol and dimethylsulfoxide at low pH.35 Thus, the resonance of exciting light (375 nm, 26 666 cm-1)6 with this transition can make the Raman spectrum of neutral alizarin incompatible with
the vibrational structure observed in the lower energy absorption band investigated here. The difference in the dipole moments between the ground and excited states, 4.4 D (see Table 1), reflects the significant intramolecular transfer of electronic charge on the electronic transition in the neutral alizarin molecule. Indeed, the quantum chemical calculations at different levels of theory10,36 describe the HOMO orbital of alizarin as localized on the two hydroxy oxygens and adjacent carbon atoms, and the LUMO as uniformly delocalized over the entire molecule, similarly to the 1,4-DHAQ (quinizarin).37 The change in electron distribution thus corresponds to the transfer of electron charge from the oxygen atoms toward the center of the molecule. The structureless absorption spectrum of the alizarin monoanion (Figure 4) is similar to that at room temperature with the maximum at 530 nm, but the low temperature causes the shift of the absorption maximum to 521 nm (19200 cm-1) and the appearance of weak signs of the vibrational structure. The vibrational progression with the spacing of about (1400 ( 100) cm-1 is seen more clearly in the electroabsorption spectrum. The monoanion exhibits significantly larger values of the dipole moment change (10 D)and of the polarizability difference compared to the neutral alizarin and to the dianion (cf. Table 1). These characteristics make the monoanion molecule the one most susceptible to the influence of electric fields arising in solution or at the TiO2 surface. The large value of |∆µ| seems to be the reason for a 9 nm (320 cm-1) hypsochromic shift of the monoanion absorption band upon going to the low temperature, which can be ascribed to the rise of polarity of the solvent on cooling and freezing, an effect observed for polar solutes in several glassy solvents.38 The
Alizarin Dye Adsorbed on TiO2 Nanoparticles
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Figure 5. Spectra of the alizarin dianion (2.78 mM) in ethylene glycol/ H2O ) 3:2 (v/v), pH 13. (A) absorption spectrum, (B) electroabsorption spectrum (points) with the fit (continuous line), and (C) the fit components. The dashed line in panel B is the fit with derivatives shifted by 100 cm-1 toward the violet (see text). Electric field intensity (rms), 125 000 V/cm; temperature, 120 K.
TABLE 1: Electro-optical Parameters for Alizarin Free in Solution and for the Alizarin-TiO2 complex at Different pH Valuesa alizarin (alizarin)-1 (alizarin)-2 alizarin/TiO2
pH
νmax/λmax (cm-1/nm)
∆R (Å3)
|∆µ| (D)
(EtOH) 10 13 3
22 750/439 19 200/521 17 550/570 20 200/495b 20 520/487.5 19 800/505b 19 962/501 19 530/512b 19 585/510.5
50 ( 5 190 ( 40 300 ( 60
4.4 ( 0.2 9.7 ( 1 3.2 ( 0.3
350 ( 20
9.4 ( 0.6
7 11
b
300 ( 40 10.5 ( 0.5 310 ( 40 11.7 ( 0.7
a Low temperature data are quoted, except for those annotated. Data at room temperature.
increase of transition energy on lowering temperature indicates that the vector ∆µ has an opposite direction to the reaction field produced by the polarized solvent environment. The approximation of the Stark spectrum for the monoanion with the absorption derivatives exhibits a weaker vibrational structure of the fit than that in the experimental Stark spectrum (Figure 4B). This deficiency can not be removed by adding a third term proportional to the absorption spectrum to the fitting curve defined by eq 1, which could account for the transition polarizability.30,31 This term was always returned by the leastsquares fitting procedure with a negligibly small amplitude. A significant improvement can be obtained only by fitting ∆A with the derivatives of the absorption spectrum shifted toward the violet by 100-150 cm-1; the result for a 150 cm-1 shift is shown in Figure 4B. This observation suggests that the inhomogeneous broadening of the absorption and Stark spectra is correlated with a slightly stronger electrochromism of the spectral components absorbing at ≈100-200 cm-1 higher
Figure 6. Spectra of alizarin adsorbed on TiO2 at pH 11. (A) Absorption spectrum, (B) electroabsorption spectrum (points) with the fit (continuous line), and (C) the fit components for alizarin (2.78 mM) in the presence of 1.18 M TiO2, which corresponds to a ratio of alizarinto-surface Ti atoms of 1:88. The dashed line in panel B is the fit with shifted derivatives (see text). Solvent: ethylene glycol/H2O ) 3:2 (v/ v); temperature, 120 K; electric field strength, 125 kV/cm.
energies than the other ones at slightly lower energies. Also, for the dianion form of alizarin a significantly better fit can be obtained with a +100 cm-1 shift (cf. Figure 5B), resulting in a e20% increase in |∆µ| and a e30% decrease in ∆R for the higher-energy components. These estimates for the dianion are also the largest ones found for all forms of alizarin investigated in this work and thus set the upper limits of data modification by accounting for the spectral inhomogeneity in such a manner. Electroabsorption Spectroscopy of Alizarin Adsorbed on TiO2. The samples with different ratios of alizarin to TiO2 were tested to check for the possible effect of the surface density of the dye. The nanoparticle diameter d was assumed here to be 6 nm, as in the original estimates27 for nanoparticles prepared with the method adapted here. The molar concentration of surface titanium atoms was calculated using the formula that has been derived by counting the Ti atoms on different crystallographic planes and edges of an approximately spherical nanoparticle:4,8 [Tisurf] ) [TiO2] × 12.5/d, where d is the nanoparticle diameter in Ångstroms. We used this formula to estimate the ratio of alizarin to surface titanium atoms by directly comparing the molar concentration of alizarin with [Tisurf]. The molar ratio of alizarin to the surface Ti atoms was between 1:22 and 1:175 in our experiments. No noticeable effect on the spectra and electrooptical parameters was observed at different proportions of alizarin to the adsorption sites. Figures 6 and 7 show the absorption and electroabsorption spectra of alizarin adsorbed on TiO2 nanoparticles at pH 11 and 3, respectively. The shape of the spectra remained roughly similar in the whole range of pH 3-11, but the increase of pH was always associated with a red shift of the absorption maximum at normal and low temperatures and with a noticeably larger value of the excited-minus-ground-state dipole moment
10238 J. Phys. Chem. C, Vol. 112, No. 27, 2008
Figure 7. Spectra of alizarin adsorbed on TiO2 at pH 3. (A) Absorption spectrum, (B) electroabsorption spectrum (points) with the fit (continuous line), and (C) the fit components for alizarin (2.78 mM) in the presence of 590 mM TiO2, which corresponds to 44 surface Ti atoms per one molecule of alizarin. The dashed line in panel B is the fit with shifted derivatives (see text). Solvent: ethylene glycol/H2O ) 3:2 (v/ v); temperature, 120 K; electric field strength, 150 kV/cm.
difference |∆µ|. The mean data from three series of experiments at pH 3, 7, and 11, each with at least four samples, are given in Table 1. They indicate (i) the shift of the absorption maximum from 20 520 cm-1 (487.5 nm) to 19 585 cm-1 (510.5 nm) and (ii) the change of |∆µ| by 2.3 D between pH 3 and 11. In the whole range of pH examined, the fits to electroabsorption spectra of alizarin on TiO2 display deviations from the experimental points toward lower wavenumbers (cf. Figures 6 and 7). The use of the derivatives shifted toward higher wavenumbers resulted in a decrease of ∆R from 300 Å3 to about 200 Å3 and a relatively smaller increase of |∆µ| not exceeding 10%, that is, by less than 1 D. The best fits corresponding to the shift by +200 cm-1 are shown for comparison in Figures 6B and 7B. We can not give a plausible explanation for the small but systematic shift between the absorption and Stark spectra indicated by the fitting procedure for the free and adsorbed forms of alizarin. Although for the free dye the diversity of electrooptical parameters may result, for example, from hydrogen bonding and solwatochromic effect, in adsorbed alizarin it may result from the inequivalence of adsorption sites indicated by the studies of catalytic properties of TiO2 nanoparticles,4 multicomponent electron transfer rates in the alizarin-TiO2 system,7,23 and resonance Raman spectra.6 Nevertheless, the data estimated from either modified or unmodified fits exhibit the same relations and thus support the same conclusions elucidated below. Because there are no good arguments to justify a particular modification of the fitting method, we present and discuss in the following the mean data obtained with the unambiguous procedure of directly fitting the Stark spectra with unshifted derivatives.
Nawrocka and Krawczyk The orientation of the dipole moment change and polarizability axis (angles δ and γ in eqs 3 and 4) with respect to the transition moment were found to be similar for alizarin forms both free and bound to TiO2, and the values of both angles were always between 15 and 30°. Electronic Structure Calculations. The results of calculations with the ab initio method using the 6-31G basis functions set are presented in Table 2. Quantitatively similar values of |∆µ| were also obtained with the 3-21G basis set, and the results of the ab initio calculations can also be reproduced in many respects with the semiempirical methods ZINDO or AM1. The ab initio results correctly reproduce both the red shift of absorption spectra (in the order: neutral, monoanion, and dianion, cf. Figure 1) and the rise of the oscillator strength in the same sequence, in agreement with the experimental data presented above and found in other studies for absorption and fluorescence spectra of neutral and dissociated alizarin.34,35 For all three forms of alizarin the calculated ∆µ vectors were found to be directed roughly along the long molecular axis, in the direction meaning the electron density shift from the HOMO localized largely on the hydroxy oxygens and neighboring carbon atoms toward the highly symmetrical LUMO more uniformly spread over the three conjugated rings. This picture of electron redistribution is in agreement with the results of calculations with both the Hartree-Fock and TDDFT methods for the neutral form of alizarin36 as well as with the semiempirical PM3 calculations for the closely related 1,4-dihydroxyanthraquinone (quinizarin).37 It also agrees with the experimental determination of |∆µ| for quinizarin39 from the Stark shift of quasilinear spectra, which exhibits a larger dipole in the ground state (2.46 D) diminishing upon electronic excitation by |∆µ| ) 1.64 D to the value of 0.82 D. The two middle rows in Table 2 contain data for two possible tautomers of the alizarin monoanion, in which the hydrogen at O1 is bonded to the keto oxygen at C10 (structure I) or with the oxygen at C2 (II). Our experimental data can not be used to distinguish one of these structures since the calculations suggest similar values of the vector ∆µ (10 D) and its orientation with respect to the transition moment. However, the calculated absorption wavelength (516 nm) for tautomer II is more close to the experimental value of 521 nm. Also, the total energy for tautomer II in the ground-state calculated with AM1 was found to be lower by 1.1 kcal/mol. On this basis, we suggest the C1-O-H · · · · O-C2 structure as more likely to occur in the alizarin monoanion. Discussion General Remarks. The absorption spectra obtained in this work remain in agreement with the spectral data for the free form of the dye in different dissociation states and with the spectral characteristics of alizarin adsorbed on TiO1 nanoparticles described in the literature. The results show that the adsorption of alizarin on TiO2 nanoparticles in 60% ethylene glycol/water can occur at high pH, like in the water solution.5 Under these conditions the dianion form of the dye is formed with maximum absorption at 571 nm, which shifts to 512 nm upon binding with TiO2. It has been reported that dyes that bind to the TiO2 nanoparticle surface in aqueous solution might be desorbed at pH higher than 8.5.40 However, another study23 did not corroborate this caution for water-methanol (4:1, v/v) solutions at pH 9 and confirmed binding of alizarin to TiO2. We observed good stability of adsorbed alizarin at pH 11 for over one month, as indicated by the absorption spectra. There were also no signs of the free alizarin dianion, which would
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TABLE 2: Results of Ab Initio 6-31G Calculations Involving Singly Excited Configurations for the Lowest Singlet Excited States of Alizarin, Its Monoanion in the Two Tautomeric Forms, and Its Dianion alizarin (alizarin)-1 I. C10dO · · · · H-O-C1 II. C1-O-H · · · O-C2 (alizarin)-2
|µG| (D)
|µE| (D)
|∆µ| (D)
2.58
4.26
6.54
clearly show up at 16 200 cm-1 (617 nm), especially in the low temperature electroabsorption spectra. Also, a large increase in dye solubility in the presence of TiO2 was observed, which allowed us to prepare the concentrated samples for Stark effect measurements. We thus conclude that, in the whole range of pH investigated, the formation of a dye-TiO2 complex is accompanied by the characteristic structureless absorption band near 500 nm, suggesting the same form of adsorbed dye independent of the adsorption conditions, as also indicated by the resonance Raman spectra. Does the Electronic Transition Have the Alizarin-to-TiO2 Charge-transfer Character? The absorption band of adsorbed alizarin near 500 nm is usually compared with the 430 nm band of the neutral form of alizarin observed in acidic media, and the large shift upon adsorption on TiO2 is considered to reflect the strong electronic coupling of the dye with the surface Ti atoms and to indicate the charge-transfer character of this electronic transition.5–8,23 This transition is tentatively considered to result from the electronic coupling of alizarin with the delocalized conduction band states in TiO2 or, as more specifically indicated by quantum chemical calculations,11,41 with localized surface states extending a few atoms inside TiO2. However, the dissociative mode of binding of alizarin with its two hydroxy oxygens forming a five-membered ring in the bidentate fashion with one surface Ti atom suggests the dissociated forms of alizarin as more appropriate reference states for assessing the modification in the adsorbate’s electronic structure caused by the binding with TiO2. The main results of this work are the electro-optical parameters associated with the lowest electronic transition in the free alizarin in its different dissociation states and in the alizarin adsorbed on the TiO2 nanoparticles. The results provide a reference data for the analysis of the electronic excited state in the latter system. The charge-transfer character of the electronic transition in alizarin-TiO2 involving a partial transfer of an electron from alizarin onto the TiIV atom would be associated with a dipole moment change vector ∆µCT directed outward from the nanoparticle surface. Such a charge redistribution would be sensitive to the electric field at the nanoparticle surface, the intensity and direction of which can be modified by changing the pH. The positive charge on the nanoparticle surface at low pH (i.e., below the isoelectric point at pH 542,43) would enhance the shift of the electronic charge in the excited-state toward the titanium atom, thus leading to an increase of |∆µCT| at low pH compared to that at high pH. The same interaction would also cause a downshift of the transition energy at low pH. However, these effects, which can be expected for an electronic transition with a significant alizarin-to-TiO2 electron transfer, are opposite to what was observed for the measured value of |∆µ| and for the position of the absorption band maximum of adsorbed alizarin. The resonance Raman spectra (see Supporting Information) do not indicate a significant change of the electronic structure of adsorbed alizarin in the wide range of pH investigated. Also, alizarin has no additional side groups whose dissociation could influence the position of its absorption maximum like in
10.6 10.0 6.2
ν (cm-1)
λ (nm)
f
24
24 210
413
0.265
45 38 2 (∼0)
17 540 19 380 17 000
570 516 589
0.392 0.487 0.982
δ (deg)
adsorbed eosin43 or phenylfluorone.44 The insensitivity of the absorption band positions to pH was also confirmed for each of the neutral, monoanion, and dianion forms of alizarin by the experimentally observed switching of one spectrum into another, without a gradual pH-dependent shift. In this context it seems reasonable to assume that the main source of the pH-dependent spectral changes observed for alizarin-TiO2 is the interaction of the adsorbed dye with the static electric field at the nanoparticle surface. We thus conclude that the electronic excited Franck-Condon state corresponding to the 500 nm absorption band of adsorbed alizarin should be considered, at least in the first approximation, as essentially localized on the dye molecule subject to the surface electric field. Alizarin Monoanion As the Adsorbed Form of the Dye. The data presented above indicate that the position and structure of the absorption band and the values of |∆µ| and ∆R of adsorbed alizarin are similar to the analogous quantities for the alizarin monoanion. There is also a regular increase of |∆µ| of alizarin-TiO2 complex from 9.4 to 11.7 D and a shift of the absorption maximum from 487.5 to 510 nm, which accompany the change of pH from 3 to 11. The latter observation is similar to the 15 nm red shift between pH 1.7 and pH 10 for alizarin on TiO2 in aqueous solution (cf. Figure 1 in ref 5) and to a 26 nm shift between pH 2 and 9 in water-ethanol (4:1).23 Guided by the similarity of electro-optical data for adsorbed alizarin to those for its monoanion and neglecting the effects related to charge transfer interactions with TiO2, we consider the monoanion form of the dye as the one most closely corresponding to the adsorbed alizarin at a semiquantitative level of description presented below. This attempt also gains support in the analogy between the configuration of the hydrogen atom in the C1-O-H group of the monoanion that, after the dissociation of the more labile hydroxy group at C2, can be shared by the two hydroxy oxygens and, on the other hand, the bidentate C1-O-Ti-OC2 chain with the Ti atom carrying a slightly positive charge in place of the hydrogen atom. Below we assess the plausibility of this picture. The long axis of an alizarin molecule with two hydroxy oxygens linked to a surface Ti atom would be tilted at 60° to the surface normal and thus to the vector of the static electric field generated by the nanoparticle’s surface charge. However, the precise direction of the electric field near the surface of a nanoparticle can not be a priori assumed since the nanoparticle surface presents a set of crystalline planes and the adsorption may preferably occur near their edges. Also, the layer of adsorbed ions is present, causing local differences in the surface field intensity, the effect of which on the absorption spectrum is illustrated by temperature-dependent solvatochromism of the monoanion (see Results section). These factors can significantly contribute to the observed inhomogeneity in the electroabsorption spectrum. An example configuration of adsorbed alizarin is shown in Scheme 2 together with the difference dipole ∆µ calculated for the monoanion. It is remarkable that the direction of ∆µ in the three dissociation states examined is always toward the ring containing the hydroxy oxygens and at less than 20°
10240 J. Phys. Chem. C, Vol. 112, No. 27, 2008
Nawrocka and Krawczyk
SCHEME 2
charge in the perpendicular direction, which could be related to interactions of the keto group. Such interactions of alizarin with Zr cations in the organic modified silicate polymer were considered45 as one of two modes of alizarin binding through the C9 keto and C1 hydroxy oxygens or through the oxygens of two hydroxy groups. The conserved direction of ∆µ at low angle to the transition moment seems to indicate the binding of alizarin with TiO2 nanoparticles through the hydroxy oxygens as more preferable, possibly because of a steric hindrance that could occur if the alizarin molecule had to bind with C9-O and C1-O to the locally flat TiO2 surface.
from the long molecular axis. At low pH the electric field F generated by the charged surface induces an increase in the excitation energy compared to the reverse situation at high pH. The spectral shift ∆ν ) νpH3 - νpH11 should then be given by eq 5
∆ν ) -
∆µ(FpH3 - FpH11) hc
(5)
The absorption shift by 670 cm-1 (at normal temperature) between pH 3 and pH 11, combined with |∆µ| ) 10 D means the change of the electric field intensity projected onto the vector ∆µ by |∆F| ) 4 MV/cm. This estimate neglects the shift that could arise from the molecular polarizability (eq 6).
∆ν )
-∆R(F2pH3 - F2pH11) 2hc
(6)
Since there is a reversal of F at the isoelectric point at pH 5,42,43 the absolute values of the effective field vectors should each be somewhat less than the estimated vectorial difference |∆F|. Inserting the value |∆F|2 for the difference in (eq 6) one can estimate the maximum spectral shift due to the polarizability (taken to be 300 Å3 for the adsorbed alizarin, cf. Table 1) as equal to 132 cm-1. This result is an overestimate because it assumes the direction of the axis of largest polarizability parallel to ∆µ, which is not necessarily the case. Moreover, the shift related to the polarizability would be toward the red for both field directions, so that the effects of F will partly cancel, as reflected by the difference in eq 6. Therefore, the contribution of polarizability to the spectral shift is rather small compared with the shift due to the interaction of permanent dipole moments with the electric field of the nanoparticle. The surface electric field can, in principle, induce the pHdependent contributions to the intrinsic permanent dipoles; the difference of which in the ground and excited electronic states contributes to the measured value of |∆µ|. An estimate of effect using the formula |∆µind| ) ∆R|∆F|, with ∆R ) 300 Å3 for the adsorbed alizarin, leads to |∆µind| ) 4 D. However, the mean values of electro-optical parameters obtained from the plain fits overestimate ∆R, and a value closer to 200 Å3 would be more appropriate for molecules most intensely contributing to the electroabsorption spectra. Therefore, the value of |∆µind| can be about 3 D, and thus more close to the difference of the value of |∆µ| at pH 3 and 11 (2.3 D). Another factor that can play a role in these estimates is the direction of the vector ∆µind, which can only partly add to the value of ∆µ. For ∆µ directed as in Scheme 2, ∆µind should decrease the intrinsic ∆µ at low pH and increase it at high pH, in agreement with the experimental observations. Because the calculated transition moments for the neutral, monoanion, dianion, and adsorbed forms of alizarin are roughly parallel to the long molecular axis, the small values of the angle δ between ∆µ and the transition moment found in the experiment point to the lack of significant displacement of
Conclusions A complete set of electro-optical characteristics for the three forms of free alizarin in solution were obtained and compared with the spectra of the dye adsorbed on TiO2. The electro-optical characteristics and other spectral features are compatible with the monoanion form of the dye as the molecular species most closely corresponding to the adsorbed alizarin. The pH-induced shift of the absorption band and the change in ∆µ of adsorbed alizarin can be explained by the interaction of the adsorbed dye with the electric field generated by charged nanoparticles. In this respect, the results of this study are an electronic counterpart of the static Stark effect in adsorbed OH groups induced by the electric field of photoexcited electrons in TiO2, as observed by vibrational spectroscopy.46 To our knowledge, no computational results concerning the electronic charge redistribution, for example, dipole moments of the ground and excited electronic states, have been published for alizarin adsorbed on TiO2. The HOMO and LUMO orbitals of alizarin bound to a single Ti atom in a model system suggest a substantial electron density on Ti in the HOMO and its shift toward the conjugated rings of alizarin in the LUMO,36 but the same authors stress the weak optical activity of the dye-solid charge-transfer states in the case of adsorbed alizarin.10,36 The HOMO and LUMO orbitals36 seem to indicate a similar direction of electron movement as in the isolated molecule, that is, from the peripheral oxygen and titanium atoms toward the molecule center. Such charge redistribution would be qualitatively consistent with the pH-dependent effects observed in this study, provided the electro-optical characteristics of the transition involving the titanium orbitals (particularly the value of |∆µ|) are similar to those for the alizarin monoanion. A good confirmation of the involvement of Ti orbitals in the binding of alizarin would be the resonance enhancement of Ti-O stretching vibrations in Raman spectra. However, no vibrational modes attributable to TiO2 were observed under resonance excitation at ∼500 nm in Raman spectra of adsorbed alizarin,6 most of which should appear near and below 600 cm-1 as in TiO2 crystal.47 In the absence of evidence for the alizarin-TiO2 charge transfer transition, the self-consistent interpretation of the experimental results given above seems to be sufficiently supported by the experimental data. Acknowledgment. The authors acknowledge financial support from the Polish Ministry of Science and Higher Education. Supporting Information Available: Resonance Raman spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69.
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