- Email: [email protected]
Investigation of the interaction between a sulfonated azo dye (AO7) and a TiO2 surface
9 July 1999
Chemical Physics Letters 307 Ž1999. 397–406 www.elsevier.nlrlocatercplett
Investigation of the interaction between a sulfonated azo dye žAO7 / and a TiO 2 surface C. Bauer
a,)
, P. Jacques b, A. Kalt
a
a
b
Laboratoire de Chimie Textile, Ecole Nationale Superieure de Chimie de Mulhouse, 3 rue A. Werner F 68093 Mulhouse, France ´ Departement de Photochimie Generale, UMR CNRS N87525 Ecole Nationale Superieure de Chimie de Mulhouse, 3 rue A. Werner F ´ ´ ´ ´ 68093 Mulhouse, France Received 19 March 1999; in final form 23 April 1999
Abstract The adsorption of an azo dye, Acid Orange 7 ŽAO7., on a TiO 2 surface was studied using FT–IR spectroscopy and adsorption isotherm parameters. The major FT–IR bands of the dye have been assigned in the range 1000–1900 cmy1. Within the AO7–TiO 2 complex, the dye appears to exist in its hydrazone form. The interaction of AO7 with the TiO 2 surface is strong, leading to a surface inner-sphere-type complex. The dye is linked to three Ti IV surface metallic cations through two oxygen atoms from the sulfonate group and the oxygen atom of the carbonyl group of the hydrazone tautomer. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The intimate interactions between organic compounds and inorganic surfaces are the governing factors for reactions occurring at interfaces. Detailed structural information on surface complexes would be essential for qualitative and quantitative descriptions of many interfacial processes. Applications can be found in various fields, like molecular geochemistry, catalysis, environmental engineering, for instance, the removal of contaminants from ground water or industrial waste waters. In the field of biology, it was recently discovered that the sulfonated azo dyes Evans Blue and Congo Red bind to the HIV protease and reverse transcriptase, inhibiting viral replication and are therefore potential pharmaceutical agents w1x. At present, we are probing the photodegradation of dyes on TiO 2 with the intention of discolouring the waste water from dye houses. In the present study, we selected Acid Orange 7 ŽAO7. which is commonly used as a model compound for the photodegradation of dyes w2–9x. We studied its interaction with titanium dioxide, a semiconductor which appears to be one of the most attractive photocatalysts because it is largely available, cheap, non-toxic and not prone to photocorrosion. It has received considerable attention and a vast body of literature documents the applications of this semiconductor to solar energy conversion w10x and water purification w11x.
)
Corresponding author. Fax: q33 3 89 33 68 05; e-mail: [email protected]
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 5 1 8 - 7
398
C. Bauer et al.r Chemical Physics Letters 307 (1999) 397–406
In all processes initiated by interfacial charge carrier transfer, a key factor that governs the efficiency is the electronic coupling between the molecular adsorbed state of the dye and the solid state of TiO 2 . Note, for example, that for the ruthenium complexes which exhibit the most important solar energy conversion efficiency, the type of coordination of the dyes is not yet clear. Some authors propose an ester-type linkage Žmonodentate mononuclear. through the carboxylate groups w12–15x, others have concluded that the dye is linked through a bidentate mononuclear or binuclear coordination type w16,17x. For a detailed understanding of the photoreactions which can occur later, it is important to investigate the nature of the interaction of the adsorbates with the oxide surface at the molecular level. It is of special interest whether an adsorbate is adsorbed by a specific chemical interaction, e.g. is coordinated with surface sites Žchemisorbed as a inner sphere complex. or whether it is less specifically adsorbed by electrostatic forces Žphysisorption as an outer-sphere complex.. The type of bonding structure is likely to be a deciding factor in the charge distribution mechanisms that may occur under irradiation at the semiconductor–dye interface. Thus, we have studied the attachment of the sulfonated azo dye AO7 to a TiO 2 surface at the molecular level by using transmission and diffuse reflectance FT–IR spectroscopy and isotherm adsorption parameters. Some experiments were carried out with ZnO, a semiconductor with a similar band gap, for comparison.
2. Materials and methods Acid Orange 7 was obtained from Aldrich; it is salt free and HPLC pure. TiO 2 P25 Ž80% anatase, 20% rutile. came from Degussa AG with an elementary particle size of 30 nm and a BET specific surface area of 50 m2 P gy1 . ZnO was obtained from Aldrich. All experiments were carried out with doubly distilled water. Transmission FT–IR spectra were recorded on a Nicolet 710 spectrophotometer with 4 cmy1 resolution and 30 scans. Diffuse reflectance FT–IR spectra were performed on a Brucker IFS 66 spectrophotometer at 4 cmy1 resolution fitted with a MCI detector and equipped with a Spectra Tech 8251 sphere. Measurements of the aggregates diameters particle analysis of TiO 2 were performed on a Coulter N4 light-scattering system. Spectrophotometric measurements were performed on a Perkin–Elmer UV–VIS spectrophotometer. 2.1. Preparation of AO7–TiO2 and AO7–ZnO complexes The samples were prepared by a 100 ml suspension of oxide Ž10 grl. which contained 4 = 10y5 mol P ly1 AO7 at natural pH ŽTiO 2 : 4.5 and ZnO: 7.0. for 4 h. The colored powders were separated by filtration through a Millipore membrane Ž D s 450 nm.. The AO7 concentration in the aqueous phase was determined by UVrVIS spectrophotometry at lmax s 485 nm. The dye coverage, calculated by solving the mass balance of the system was 1.15 = 10y6 mol P gy1 for TiO 2 and 2.8 = 10y7 mol P gy1 for ZnO. The samples were dried at 608C under air ventilation in the dark. 2.2. FT–IR spectroscopy analysis Spectra of isolated AO7 were obtained by dilution in KBr Ž1:100.. Transmission spectra of AO7–TiO 2 were performed on pellets of 20 mg of pure complex, spectra of AO7–ZnO were performed on pellets consisting of a mixture of 20 mg of AO7–ZnO powder and 30 mg of KBr for better cohesion of the sample. Contributions from the FT–IR bands of TiO 2 and ZnO were substracted. All diffuse reflectance spectra were performed on pure complexes deposited on a sample holder. 2.3. Adsorption isotherms For the adsorption measurements, 200 mg of TiO 2 were added to 100 ml of freshly prepared AO7 solutions of known concentration in the range 10y4 –10y3 M at natural pH ŽTiO 2 pH s 6.0 and ZnO pH s 7.2.. The
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suspension was sonicated for 15 min and stirred for 14 h at 302 K in the dark to reach equilibrium. After filtration, a sample was taken to determine the AO7 concentration in the aqueous phase Ceq by UVrVIS spectrophotometry at l max s 485 nm. The amount of AO7 adsorbed Ž Nads . on the oxide surface was determined from the expression: Nads .s
VDC W
,
where V is the volume of aqueous suspension, W is the weight of oxide and DC s C0 y Ceq is the decrease in AO7 concentration after reaching equilibrium, and Ceq represents the AO7 concentration in the aqueous phase at equilibrium, C0 being the initial concentration.
3. Results 3.1. FT–IR spectroscopy: bands attribution of Acid Orange 7 Phenylazonaphtol dyes are known to undergo a tautomeric equilibrium w18x. FT–IR spectroscopic data about this azo-hydrazone tautomerism often presents conflicting results due to the fact that the analytically useful group vibrations are intermixed with those of the aromatic rings. A critical analysis of this data permits the complete attribution of all bands between 1000 and 1900 cmy1 in the fingerprint area of the dye molecule. The results are summarized in Table 1.
The most intense bands are situated at 1005, 1036, 1124, 1180, 1209 and 1506 cmy1 . Following the results of a detailed investigation of the interaction of sodium dodecyl benzenesulfonate with an alumina surface w19x, we assign the band at 1005 to the phenyl ring mode 1 using Varsanyi ` notation w20x and the two bands at 1036 y1 Ž and 1124 cm to the coupling between benzene mode 1 and ns SO 3 .. We also link the bands at 1180 and 1209 cmy1 to the nas ŽSO 3 . stretching mode. For the band at 1506 cmy1 , Hadzi ˇ w21x and Morgan w22x have shown that Ž . it is related to the bending vibration mode d N–H of the H form of the azo dye. Indeed, this band disappears in the spectrum the 2-phenylazo-1-methoxy-naphtalene and is shifted about 70 cmy1 towards lower frequencies after deuterium substitution. Characteristic bands of phenyl ring vibrations are localized at 1597, 1480 and 1451 cmy1 .
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Table 1 Acid Orange 7 FT–IR bands assignment between 1000 and 1900 cmy1 Wavenumber Žcmy1 .
Assignment
Ref.
1005 1036 1124 1180 1209 1232 1253 1451 1480 1506 1553 1570 1597 1622
axial 18a . axial 1 q nsŽSOy 3 . axial 1 q nsŽSOy 3 . nas ŽSOy 3 . nas ŽSOy 3 n ŽN–N. Žhydrazone. n ŽC–N. Žhydrazone. n ŽC5C. aromatic n ŽC5C. aromatic d ŽN–H. n ŽC5N. q d ŽN–H. n ŽC5O. n ŽC5C. aromatic Ždisbustituted 1,4. n ŽC5C. aromaticq n ŽC5N.
w19x w19x w19x w19x w19x w25,27x w25,27x w25,27x w27x w21,23x w24x w23x w25,27x w27x
As a general rule, carbonyl stretching vibration modes are situated between 1850 and 1600 cmy1 , but for the H form of the dye, it was shown using 18 O isotope labelling that n ŽC s O. occurs at 1570 cmy1 w23x. Because of the highly delocalized structure of the H form and the intramolecular hydrogen bond, the double bond character of the carbonyl group is greatly reduced. The band at 1253 cmy1 is assigned to the vibration stretching mode n ŽC–N. of the H tautomer. At last, bands at 1553 and 1622 cmy1 are assigned to a combination of vibrations involving N–H bending and –N5C stretching of the ŽNH–N5C. group w24x and a combination of phenyl ring vibrations with stretching of ŽC5N. group respectively associated to the H tautomer. This can explain why some authors attributed the band at 1622 cmy1 to the carbonyl group stretching mode of the H form. Using Raman spectroscopy, Saito et al. w25x have linked the stretching vibration mode n ŽN5N. of the azo tautomer to the strong band at 1372 cmy1 observed in a basic medium. This strong band was not observed in the present work, suggesting that AO7 exits mainly in the H form. Finally, FT–IR characteristic bands of the H form of this azo dye are found at 1622, 1570, 1553, 1506 and 1253 cmy1 . 3.2. FT–IR spectra modifications of AO7 after adsorption on TiO2 Following the adsorption of AO7 on semiconductor surfaces, three types of modifications are expected to occur in the FT–IR spectra of the complexes: Ži. vibrational frequencies of the adsorbate may be shifted, Žii. intensities of the bands can be affected considerably, Žiii. new bands associated to the interaction between the AO7 ligand and TiO 2 substrate can appear. Transmission FT–IR spectra of isolated AO7, AO7 adsorbed on ZnO, and AO7 adsorbed on TiO 2 are shown in Fig. 1. Surprisingly, no broadening of the bands is observed after adsorption, a broadening which was for instance observed in the phthalate–TiO 2 complex w26x. This may suggest a homogeneity in the adsorption sites. However, the spectrum of AO7 adsorbed on TiO 2 is strongly modified with respect to the isolated AO7 molecule. The most striking feature is the collapse of the bands linked to the sulfonate stretching vibration mode. Another difference is the disappearance of the band at 1570 cmy1 corresponding to the carbonyl stretching vibration mode of the H tautomer.
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Fig. 1. Transmission FT–IR spectra of AO7 isolated ŽA., adsorbed on ZnO ŽB. and TiO 2 ŽC..
The band at 1180 cmy1 due to nas ŽSO 3 . is shifted to 1168 cmy1 with a considerable decrease in intensity after adsorption, while the band associated with nsŽSO 3 . at 1124 cmy1 is only slightly affected. The ratio of the intensities nas ŽSO 3 .rnsŽSO 3 . s 0.2 for adsorbed AO7 is very different from the one of isolated AO7 nas ŽSO 3 .rnsŽSO 3 . s 0.87. The band at 1253 cmy1 associated to n ŽC–N. of the H tautomer is shifted to higher frequencies Ž1260 cmy1 ., reflecting an enhancement of the electron density. The other bands arising from the combination of n ŽSO 3 . with ring vibrations undergo a very small shift. At 1232 cmy1 a new band appears which is attributed to n ŽN–N. of the H form w27x. In the spectra of isolated AO7, this band is eclipsed by the strong bands associated with n ŽSO 3 .. Spectra of isolated AO7, AO7–TiO 2 and AO7–ZnO obtained by diffuse reflectance are presented in Fig. 2. They are quasi-similar to those obtained by transmission. The only notable difference is the presence of a band at 1280 cmy1 which is not present in the transmission spectrum of AO7–TiO 2 . We suggest that this band corresponds to the following interaction:
The electronic density along C5O is redistributed in favour of the Ti–O dative bond. This interaction leads to a charge transfer from the carbonyl group to the Ti IV electron acceptor center on the TiO 2 surface.
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Fig. 2. Diffuse reflectance FT–IR spectra of AO7 isolated ŽA., adsorbed on ZnO ŽB. and TiO 2 ŽC..
3.3. FT–IR spectra modifications of AO7 after adsorption on ZnO The spectrum of AO7 adsorbed on ZnO undergoes fewer modifications. The intensities of sulfonate stretching vibration modes are only slightly affected. Only the band at 1570 cmy1 due to n ŽC5O. of the H tautomer is affected by a diminishing of the intensity and a slight shift to 1565 cmy1 . The ratio of the intensities of nas ŽSO 3 .rnsŽSO 3 . s 0.84 for the sulfonate group of AO7 adsorbed on ZnO is very close to the ratio nas ŽSO 3 .rnsŽSO 3 . s 0.87 for isolated AO7. Thus, the sulfonate group is not affected by the adsorption on the ZnO surface. This suggests that the interaction between the sulfonate group of AO7 and the ZnO surface is weak. 3.4. Adsorption isotherms The interfacial adsorption–desorption equilibrium is treated in terms of a competition between solute and solvent molecules for adsorption sites w28x: solvent Ž ads. . q solute Ž bulk . | solute Ž ads. . q solvent Ž bulk . . It follows a Langmuir adsorption isotherm that can be written: Nads s
Ns KCeq 1 q KCeq
,
Ž 1.
where Ceq is the concentration in AO7 in the bulk after reaching the adsorption equilibrium, Nads is the amount of AO7 adsorbed on the oxide, K is the association constant and Ns is the total number of accessible adsorption sites. The number of moles of AO7 adsorbed per gram of oxide versus Ceq is showed in Fig. 3a. In Fig. 3b, the
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Fig. 3. Ža. AO7 adsorption isotherms on TiO 2 Ž`. and ZnO Žv .. Žb. Data points linearised according to Eq. Ž2..
adsorption isotherms are plotted in accordance with the linearised form Ž2. where Nmax is the maximum amount of AO7 at monolayer coverage. Ceq 1 Ceq s q . Ž 2. Nads KNmax Nmax From Eq. Ž2., we obtain the binding constants K TiO2 s 18000 My1 and K ZnO s 1100 My1 for the AO7–TiO 2 and AO7–ZnO complexes, respectively, and the amounts of AO7 at monolayer coverage Nmax ŽTiO 2 . s 2.56 = 10y6 mol P gy1 and Nmax ŽZnO. s 1.96 = 10y6 mol P gy1. From the K values, we calculate the adsorption standard free enthalpies of 24.6 kJ P moly1 and 17.6 kJ P moly1 for TiO 2 and ZnO, respectively. The high value of the binding constant K for TiO 2 implies a strong interaction of AO7 with the TiO 2 surface. The difference in the K values between the both semiconductors is in accordance with the importance of the FT–IR spectrum modification of AO7 within the complex and illustrates clearly the difference of the binding states between the dye and both oxides. 4. Discussion Using FT–IR spectroscopy it is possible to gain information about the type of coordination of the ligand with the metallic ion. For the sulfonate group, three types of coordination can be considered: unidentate, bidentate mononuclear, bidentate binuclear.
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We exclude the possibility that all three oxygen atoms of the SO 3 group interact with the Ti IV cation because this would require the 1,4-phenyl ring axis and C 3v axis of the SO 3 group to be normal to the surface. In this situation, in view of the H tautomer conformation, it would be impossible for the carbonyl group to interact with a surface Ti IV cation. Deacon and Phillips w29x postulated an empirical rule for the carboxylate ŽCOOy. ; that has been confirmed by, an ab initio molecular orbital study w30x. It relates D nas – s , the frequency separation between the COOy antisymmetric and symmetric stretches, and the type of coordination of the COOy group to the metallic ion. This rule can be expressed as follows D nas – s unidentate) D nas – s Žisolated. ) D nas – s bidentate with D nas – s s nasŽCOOy. y nsŽCOOy. . But this law hardly allows us to distinguish between bidentate mononuclear and bidentate binuclear from the stretching vibrations of the group. Table 2 gives the values of D nas – s for the sulfonate group for isolated AO7, AO7–ZnO and AO7–TiO 2 obtained by transmission FT–IR spectroscopy. Assuming that the rule of Deacon and Phillips can also be applied to asymmetric and symmetric stretching vibrations modes of the sulfonate group would imply a bidentate coordination type for the AO7–TiO 2 complex and that only one oxygen atom interacts with the surface for the AO7–ZnO complex. This reflects the specificity of the surface properties of both semiconductors. It should be noted that data taken from diffuse reflectance spectra lead to the same conclusion. AO7 molecules are bound to surface Ti IV cations in a process that involves the substitution of surface coordinated OHy. Therefore, complexation of the TiO 2 surface at the TiO 2raqueous solution interface will be limited by the availability of Ti–OH sites that participe in a Lewis acid–base ligand exchange reaction. Rodriguez et al. w31x have found the number Ns of available OH surface groups at maximum monolayer coverage to be Ns s 1.79 sites P nmy2 or 2.97 = 10y6 mol P my2 . When the TiO 2 powder is dispersed in water, the 30 nm non-porous crystallites aggregate in a regular dispersion. We have found the size of these agglomerates to be D s 0.6 mm, in close agreement with Ref. w32x. Therefore, we assume that the AO7 molecules only have access to a specific area of 2.5 m2 P gy1 which leads to a value of Ns s 7.425 = 10y6 mol P gy1 . We can determine the number of OHy group exchanges during the chemisorption per molecule of AO7 by using the value Ns and the parameter Nmax calculated from adsorption isotherm parameters. From the specific area available for AO7 molecules during the experiment adsorption Nmax s 2.56 = 10y6 P mol P gy1 . We calculate the ratio NsrNmax s 2.9 which gives the adsorption exchange stoechiometry. From this value, we deduce that three oxygen atoms of an AO7 molecule are involved in its adsorption on TiO 2 to form an AO7–TiO 2 complex with a bridged configuration. The molecular structure of the dye–semiconductor complex AO7–TiO 2 is illustrated in Fig. 4. On one side, two oxygen atoms from the sulfonate group are linked to two
Table 2 Bands position of n ŽSO 3 . Compound
nas SO 3 Žcmy1 .
n SO 3 Žcmy1 .
D nas – s Žcmy1 .
AO7 AO7–TiO 2 AO7–ZnO
1180 1168 1185
1124 1120 1121
56 48 64
Data from spectra obtained by transmission.
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Fig. 4. Adsorption geometry of AO7 on TiO 2 .
Ti IV cations in a bidentate binuclear coordination type complex; on the other, the oxygen atom from the carbonyl group of the H tautomer is linked to a Ti IV cation in a unidentate coordination type complex. This is in agreement with the fact that in anatase the most stable faces Ž001. and Ž011. contain fivefold-coordinated Ti IV cations which are Lewis acid sites. Initially at the TiO 2rH 2 O interface these cations fix OH groups to reach their stable octahedral coordination. During adsorption of an AO7 molecule, three OH groups situated on three different surface Ti IV cations are replaced by three oxygen atoms from AO7. There is a strong overlap between the 3d orbitals of the titanium atoms and the 2p orbitals of the oxygen atoms and the Ti–O bonds between the AO7 molecule and the surface Ti IV cations have a strong covalent or dative character. This affirmation is supported by recent works which proved using several techniques that each formiate anion are bound to two titanium atoms on a TiO 2 surface w33x and that a significant amount of the Ti–O interaction is covalent in nature w34x. In conclusion, it should be underlined that the adsorption of AO7 occurs through a Lewis acid–base reaction which implies the formation of an inner-sphere complex. In view of photoinduced interfacial charge transfer processes, this means that the electronic coupling between AO7 and TiO 2 falls in the strong and adiabatic limit coupling regime with the following important implications: ultrafast electron transfer, hot carrier injection, surface states suppression, etc.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x w22x
W.H. Ojala, E.A. Sudbeck, L.K. Lu, T.I. Richardson, R.E. Lovrien, W.B. Gleason, J. Am. Chem. Soc. 118 Ž1996. 2131. K. Hustert, R.G. Zepp, Chemosphere 24 Ž1992. 335. K. Vinodgopal, I. Bedja, S. Hotchandani, P.V. Kamat, Langmuir 10 Ž1994. 1767. K. Vinodgopal, P.V. Kamat, Environ. Sci. Technol. 29 Ž1995. 841. J. Bandara, C. Morrison, J. Kiwi, C. Pulgarin, P. Peringer, J. Photochem. Photobiol. A Chem. 99 Ž1996. 57. C. Morrison, J. Bandara, J. Kiwi, Adv. Oxid. Technol. 1 Ž1996. 160. J. Bandara, V. Nadtochenko, J. Kiwi, C. Pulgarin, Wat. Sci. Tech. 35 Ž1997. 87. J. Donlagic, J. Levec, Ind. Eng. Chem. Res. 36 Ž1997. 3480. M. Nango, T. Iwasaki, Y. Takeuchi, Y. Kurono, J. Tokuda, R. Oura, Langmuir 14 Ž1998. 3272. B. O’Regan, M. Gratzel, Nature 353 Ž1991. 737. ¨ M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 Ž1995. 69. S. Anderson, E.C. Constable, M.P. Dare-Edwards, J.B. Goodenough, A. Hamnett, K.R. Seddon, R.D. Wright, Nature 280 Ž1979. 571. K. Murakoshi, G. Kano, Y. Wada, S. Yanagida, H. Miyazaki, M. Matsumoto, S. Murasawa, J. Electroanal. Chem. 396 Ž1995. 27. R. Argazzi, C.A. Bignozzi, T.A. Heimer, F.N. Castellano, G.J. Meyer, Inorg. Chem. 33 Ž1994. 5741. G.J. Meyer, J. Chem. Educ. 74 Ž1997. 652. N.W. Duffy, K.D. Dobson, K.C. Gordon, B.H. Robinson, A.J. McQuillan, Chem. Phys. Lett. 266 Ž1997. 451. K.S. Finnie, J.R. Bartlett, J.L. Woolfrey, Langmuir 14 Ž1998. 2744. P.F. Gordon, P. Gregory, Organic Chemistry in Colour, Springer, Berlin, 1987. R. Sperline, Y. Song, H. Freiser, Langmuir 10 Ž1994. 37. G. Varsanyi, Vibrational Spectra of Benzene Derivatives, Academic Press, New York, 1969. ` D. Hadzi, ˇ J. Chem. Soc. Ž1956. 2143. J. Morgan, J. Chem. Soc. Ž1961. 2151.
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E. Lippert, D. Samuel, E. Fischer, Ber. Bunsenges. Physik. Chem. 69 Ž1965. 155. J. Connor, D. Fine, J. Chem. Soc. Dalton Ž1981. 559. Y. Saito, B. Kim, K. Machida, T. Uno, Bull. Chem. Soc. Jpn. 47 Ž1974. 2111. S. Tunesi, M. Anderson, Langmuir 8 Ž1992. 487. J. Kelemen, S. Moss, H. Sauter, T. Winkler, Dyes Pigments 3 Ž1981. 27. A.W. Adamson, Physical Chemistry of Surfaces, Wiley, New York, 1982. G. Deacon, R. Phillips, Coord. Chem. Rev. 33 Ž1980. 227. M. Nara, H. Torii, M.M. Tasumi, J. Phys. Chem. 100 Ž1996. 19812. R. Rodriguez, M. Blesa, A. Regazzoni, J. Colloid Interface Sci. 100 Ž1996. 177, 122. M. Cabrera, O. Alfano, A. Cassano, J. Phys. Chem. 100 Ž1996. 20043. S.A. Chambers, S. Thevuthasan, Y.J. Kim, G.S. Herman, Z. Wang, E. Tober, R. Ynzunza, J. Morais, C.H.F. Peden, K. Ferris, C.S. Fadley, Chem. Phys. Lett. 267 Ž1997. 51. w34x A. Labouriau, W.L. Earl, Chem. Phys. Lett. 270 Ž1997. 278.
Chemical Physics Letters 307 Ž1999. 397–406 www.elsevier.nlrlocatercplett
Investigation of the interaction between a sulfonated azo dye žAO7 / and a TiO 2 surface C. Bauer
a,)
, P. Jacques b, A. Kalt
a
a
b
Laboratoire de Chimie Textile, Ecole Nationale Superieure de Chimie de Mulhouse, 3 rue A. Werner F 68093 Mulhouse, France ´ Departement de Photochimie Generale, UMR CNRS N87525 Ecole Nationale Superieure de Chimie de Mulhouse, 3 rue A. Werner F ´ ´ ´ ´ 68093 Mulhouse, France Received 19 March 1999; in final form 23 April 1999
Abstract The adsorption of an azo dye, Acid Orange 7 ŽAO7., on a TiO 2 surface was studied using FT–IR spectroscopy and adsorption isotherm parameters. The major FT–IR bands of the dye have been assigned in the range 1000–1900 cmy1. Within the AO7–TiO 2 complex, the dye appears to exist in its hydrazone form. The interaction of AO7 with the TiO 2 surface is strong, leading to a surface inner-sphere-type complex. The dye is linked to three Ti IV surface metallic cations through two oxygen atoms from the sulfonate group and the oxygen atom of the carbonyl group of the hydrazone tautomer. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The intimate interactions between organic compounds and inorganic surfaces are the governing factors for reactions occurring at interfaces. Detailed structural information on surface complexes would be essential for qualitative and quantitative descriptions of many interfacial processes. Applications can be found in various fields, like molecular geochemistry, catalysis, environmental engineering, for instance, the removal of contaminants from ground water or industrial waste waters. In the field of biology, it was recently discovered that the sulfonated azo dyes Evans Blue and Congo Red bind to the HIV protease and reverse transcriptase, inhibiting viral replication and are therefore potential pharmaceutical agents w1x. At present, we are probing the photodegradation of dyes on TiO 2 with the intention of discolouring the waste water from dye houses. In the present study, we selected Acid Orange 7 ŽAO7. which is commonly used as a model compound for the photodegradation of dyes w2–9x. We studied its interaction with titanium dioxide, a semiconductor which appears to be one of the most attractive photocatalysts because it is largely available, cheap, non-toxic and not prone to photocorrosion. It has received considerable attention and a vast body of literature documents the applications of this semiconductor to solar energy conversion w10x and water purification w11x.
)
Corresponding author. Fax: q33 3 89 33 68 05; e-mail: [email protected]
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 5 1 8 - 7
398
C. Bauer et al.r Chemical Physics Letters 307 (1999) 397–406
In all processes initiated by interfacial charge carrier transfer, a key factor that governs the efficiency is the electronic coupling between the molecular adsorbed state of the dye and the solid state of TiO 2 . Note, for example, that for the ruthenium complexes which exhibit the most important solar energy conversion efficiency, the type of coordination of the dyes is not yet clear. Some authors propose an ester-type linkage Žmonodentate mononuclear. through the carboxylate groups w12–15x, others have concluded that the dye is linked through a bidentate mononuclear or binuclear coordination type w16,17x. For a detailed understanding of the photoreactions which can occur later, it is important to investigate the nature of the interaction of the adsorbates with the oxide surface at the molecular level. It is of special interest whether an adsorbate is adsorbed by a specific chemical interaction, e.g. is coordinated with surface sites Žchemisorbed as a inner sphere complex. or whether it is less specifically adsorbed by electrostatic forces Žphysisorption as an outer-sphere complex.. The type of bonding structure is likely to be a deciding factor in the charge distribution mechanisms that may occur under irradiation at the semiconductor–dye interface. Thus, we have studied the attachment of the sulfonated azo dye AO7 to a TiO 2 surface at the molecular level by using transmission and diffuse reflectance FT–IR spectroscopy and isotherm adsorption parameters. Some experiments were carried out with ZnO, a semiconductor with a similar band gap, for comparison.
2. Materials and methods Acid Orange 7 was obtained from Aldrich; it is salt free and HPLC pure. TiO 2 P25 Ž80% anatase, 20% rutile. came from Degussa AG with an elementary particle size of 30 nm and a BET specific surface area of 50 m2 P gy1 . ZnO was obtained from Aldrich. All experiments were carried out with doubly distilled water. Transmission FT–IR spectra were recorded on a Nicolet 710 spectrophotometer with 4 cmy1 resolution and 30 scans. Diffuse reflectance FT–IR spectra were performed on a Brucker IFS 66 spectrophotometer at 4 cmy1 resolution fitted with a MCI detector and equipped with a Spectra Tech 8251 sphere. Measurements of the aggregates diameters particle analysis of TiO 2 were performed on a Coulter N4 light-scattering system. Spectrophotometric measurements were performed on a Perkin–Elmer UV–VIS spectrophotometer. 2.1. Preparation of AO7–TiO2 and AO7–ZnO complexes The samples were prepared by a 100 ml suspension of oxide Ž10 grl. which contained 4 = 10y5 mol P ly1 AO7 at natural pH ŽTiO 2 : 4.5 and ZnO: 7.0. for 4 h. The colored powders were separated by filtration through a Millipore membrane Ž D s 450 nm.. The AO7 concentration in the aqueous phase was determined by UVrVIS spectrophotometry at lmax s 485 nm. The dye coverage, calculated by solving the mass balance of the system was 1.15 = 10y6 mol P gy1 for TiO 2 and 2.8 = 10y7 mol P gy1 for ZnO. The samples were dried at 608C under air ventilation in the dark. 2.2. FT–IR spectroscopy analysis Spectra of isolated AO7 were obtained by dilution in KBr Ž1:100.. Transmission spectra of AO7–TiO 2 were performed on pellets of 20 mg of pure complex, spectra of AO7–ZnO were performed on pellets consisting of a mixture of 20 mg of AO7–ZnO powder and 30 mg of KBr for better cohesion of the sample. Contributions from the FT–IR bands of TiO 2 and ZnO were substracted. All diffuse reflectance spectra were performed on pure complexes deposited on a sample holder. 2.3. Adsorption isotherms For the adsorption measurements, 200 mg of TiO 2 were added to 100 ml of freshly prepared AO7 solutions of known concentration in the range 10y4 –10y3 M at natural pH ŽTiO 2 pH s 6.0 and ZnO pH s 7.2.. The
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suspension was sonicated for 15 min and stirred for 14 h at 302 K in the dark to reach equilibrium. After filtration, a sample was taken to determine the AO7 concentration in the aqueous phase Ceq by UVrVIS spectrophotometry at l max s 485 nm. The amount of AO7 adsorbed Ž Nads . on the oxide surface was determined from the expression: Nads .s
VDC W
,
where V is the volume of aqueous suspension, W is the weight of oxide and DC s C0 y Ceq is the decrease in AO7 concentration after reaching equilibrium, and Ceq represents the AO7 concentration in the aqueous phase at equilibrium, C0 being the initial concentration.
3. Results 3.1. FT–IR spectroscopy: bands attribution of Acid Orange 7 Phenylazonaphtol dyes are known to undergo a tautomeric equilibrium w18x. FT–IR spectroscopic data about this azo-hydrazone tautomerism often presents conflicting results due to the fact that the analytically useful group vibrations are intermixed with those of the aromatic rings. A critical analysis of this data permits the complete attribution of all bands between 1000 and 1900 cmy1 in the fingerprint area of the dye molecule. The results are summarized in Table 1.
The most intense bands are situated at 1005, 1036, 1124, 1180, 1209 and 1506 cmy1 . Following the results of a detailed investigation of the interaction of sodium dodecyl benzenesulfonate with an alumina surface w19x, we assign the band at 1005 to the phenyl ring mode 1 using Varsanyi ` notation w20x and the two bands at 1036 y1 Ž and 1124 cm to the coupling between benzene mode 1 and ns SO 3 .. We also link the bands at 1180 and 1209 cmy1 to the nas ŽSO 3 . stretching mode. For the band at 1506 cmy1 , Hadzi ˇ w21x and Morgan w22x have shown that Ž . it is related to the bending vibration mode d N–H of the H form of the azo dye. Indeed, this band disappears in the spectrum the 2-phenylazo-1-methoxy-naphtalene and is shifted about 70 cmy1 towards lower frequencies after deuterium substitution. Characteristic bands of phenyl ring vibrations are localized at 1597, 1480 and 1451 cmy1 .
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Table 1 Acid Orange 7 FT–IR bands assignment between 1000 and 1900 cmy1 Wavenumber Žcmy1 .
Assignment
Ref.
1005 1036 1124 1180 1209 1232 1253 1451 1480 1506 1553 1570 1597 1622
axial 18a . axial 1 q nsŽSOy 3 . axial 1 q nsŽSOy 3 . nas ŽSOy 3 . nas ŽSOy 3 n ŽN–N. Žhydrazone. n ŽC–N. Žhydrazone. n ŽC5C. aromatic n ŽC5C. aromatic d ŽN–H. n ŽC5N. q d ŽN–H. n ŽC5O. n ŽC5C. aromatic Ždisbustituted 1,4. n ŽC5C. aromaticq n ŽC5N.
w19x w19x w19x w19x w19x w25,27x w25,27x w25,27x w27x w21,23x w24x w23x w25,27x w27x
As a general rule, carbonyl stretching vibration modes are situated between 1850 and 1600 cmy1 , but for the H form of the dye, it was shown using 18 O isotope labelling that n ŽC s O. occurs at 1570 cmy1 w23x. Because of the highly delocalized structure of the H form and the intramolecular hydrogen bond, the double bond character of the carbonyl group is greatly reduced. The band at 1253 cmy1 is assigned to the vibration stretching mode n ŽC–N. of the H tautomer. At last, bands at 1553 and 1622 cmy1 are assigned to a combination of vibrations involving N–H bending and –N5C stretching of the ŽNH–N5C. group w24x and a combination of phenyl ring vibrations with stretching of ŽC5N. group respectively associated to the H tautomer. This can explain why some authors attributed the band at 1622 cmy1 to the carbonyl group stretching mode of the H form. Using Raman spectroscopy, Saito et al. w25x have linked the stretching vibration mode n ŽN5N. of the azo tautomer to the strong band at 1372 cmy1 observed in a basic medium. This strong band was not observed in the present work, suggesting that AO7 exits mainly in the H form. Finally, FT–IR characteristic bands of the H form of this azo dye are found at 1622, 1570, 1553, 1506 and 1253 cmy1 . 3.2. FT–IR spectra modifications of AO7 after adsorption on TiO2 Following the adsorption of AO7 on semiconductor surfaces, three types of modifications are expected to occur in the FT–IR spectra of the complexes: Ži. vibrational frequencies of the adsorbate may be shifted, Žii. intensities of the bands can be affected considerably, Žiii. new bands associated to the interaction between the AO7 ligand and TiO 2 substrate can appear. Transmission FT–IR spectra of isolated AO7, AO7 adsorbed on ZnO, and AO7 adsorbed on TiO 2 are shown in Fig. 1. Surprisingly, no broadening of the bands is observed after adsorption, a broadening which was for instance observed in the phthalate–TiO 2 complex w26x. This may suggest a homogeneity in the adsorption sites. However, the spectrum of AO7 adsorbed on TiO 2 is strongly modified with respect to the isolated AO7 molecule. The most striking feature is the collapse of the bands linked to the sulfonate stretching vibration mode. Another difference is the disappearance of the band at 1570 cmy1 corresponding to the carbonyl stretching vibration mode of the H tautomer.
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Fig. 1. Transmission FT–IR spectra of AO7 isolated ŽA., adsorbed on ZnO ŽB. and TiO 2 ŽC..
The band at 1180 cmy1 due to nas ŽSO 3 . is shifted to 1168 cmy1 with a considerable decrease in intensity after adsorption, while the band associated with nsŽSO 3 . at 1124 cmy1 is only slightly affected. The ratio of the intensities nas ŽSO 3 .rnsŽSO 3 . s 0.2 for adsorbed AO7 is very different from the one of isolated AO7 nas ŽSO 3 .rnsŽSO 3 . s 0.87. The band at 1253 cmy1 associated to n ŽC–N. of the H tautomer is shifted to higher frequencies Ž1260 cmy1 ., reflecting an enhancement of the electron density. The other bands arising from the combination of n ŽSO 3 . with ring vibrations undergo a very small shift. At 1232 cmy1 a new band appears which is attributed to n ŽN–N. of the H form w27x. In the spectra of isolated AO7, this band is eclipsed by the strong bands associated with n ŽSO 3 .. Spectra of isolated AO7, AO7–TiO 2 and AO7–ZnO obtained by diffuse reflectance are presented in Fig. 2. They are quasi-similar to those obtained by transmission. The only notable difference is the presence of a band at 1280 cmy1 which is not present in the transmission spectrum of AO7–TiO 2 . We suggest that this band corresponds to the following interaction:
The electronic density along C5O is redistributed in favour of the Ti–O dative bond. This interaction leads to a charge transfer from the carbonyl group to the Ti IV electron acceptor center on the TiO 2 surface.
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Fig. 2. Diffuse reflectance FT–IR spectra of AO7 isolated ŽA., adsorbed on ZnO ŽB. and TiO 2 ŽC..
3.3. FT–IR spectra modifications of AO7 after adsorption on ZnO The spectrum of AO7 adsorbed on ZnO undergoes fewer modifications. The intensities of sulfonate stretching vibration modes are only slightly affected. Only the band at 1570 cmy1 due to n ŽC5O. of the H tautomer is affected by a diminishing of the intensity and a slight shift to 1565 cmy1 . The ratio of the intensities of nas ŽSO 3 .rnsŽSO 3 . s 0.84 for the sulfonate group of AO7 adsorbed on ZnO is very close to the ratio nas ŽSO 3 .rnsŽSO 3 . s 0.87 for isolated AO7. Thus, the sulfonate group is not affected by the adsorption on the ZnO surface. This suggests that the interaction between the sulfonate group of AO7 and the ZnO surface is weak. 3.4. Adsorption isotherms The interfacial adsorption–desorption equilibrium is treated in terms of a competition between solute and solvent molecules for adsorption sites w28x: solvent Ž ads. . q solute Ž bulk . | solute Ž ads. . q solvent Ž bulk . . It follows a Langmuir adsorption isotherm that can be written: Nads s
Ns KCeq 1 q KCeq
,
Ž 1.
where Ceq is the concentration in AO7 in the bulk after reaching the adsorption equilibrium, Nads is the amount of AO7 adsorbed on the oxide, K is the association constant and Ns is the total number of accessible adsorption sites. The number of moles of AO7 adsorbed per gram of oxide versus Ceq is showed in Fig. 3a. In Fig. 3b, the
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Fig. 3. Ža. AO7 adsorption isotherms on TiO 2 Ž`. and ZnO Žv .. Žb. Data points linearised according to Eq. Ž2..
adsorption isotherms are plotted in accordance with the linearised form Ž2. where Nmax is the maximum amount of AO7 at monolayer coverage. Ceq 1 Ceq s q . Ž 2. Nads KNmax Nmax From Eq. Ž2., we obtain the binding constants K TiO2 s 18000 My1 and K ZnO s 1100 My1 for the AO7–TiO 2 and AO7–ZnO complexes, respectively, and the amounts of AO7 at monolayer coverage Nmax ŽTiO 2 . s 2.56 = 10y6 mol P gy1 and Nmax ŽZnO. s 1.96 = 10y6 mol P gy1. From the K values, we calculate the adsorption standard free enthalpies of 24.6 kJ P moly1 and 17.6 kJ P moly1 for TiO 2 and ZnO, respectively. The high value of the binding constant K for TiO 2 implies a strong interaction of AO7 with the TiO 2 surface. The difference in the K values between the both semiconductors is in accordance with the importance of the FT–IR spectrum modification of AO7 within the complex and illustrates clearly the difference of the binding states between the dye and both oxides. 4. Discussion Using FT–IR spectroscopy it is possible to gain information about the type of coordination of the ligand with the metallic ion. For the sulfonate group, three types of coordination can be considered: unidentate, bidentate mononuclear, bidentate binuclear.
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We exclude the possibility that all three oxygen atoms of the SO 3 group interact with the Ti IV cation because this would require the 1,4-phenyl ring axis and C 3v axis of the SO 3 group to be normal to the surface. In this situation, in view of the H tautomer conformation, it would be impossible for the carbonyl group to interact with a surface Ti IV cation. Deacon and Phillips w29x postulated an empirical rule for the carboxylate ŽCOOy. ; that has been confirmed by, an ab initio molecular orbital study w30x. It relates D nas – s , the frequency separation between the COOy antisymmetric and symmetric stretches, and the type of coordination of the COOy group to the metallic ion. This rule can be expressed as follows D nas – s unidentate) D nas – s Žisolated. ) D nas – s bidentate with D nas – s s nasŽCOOy. y nsŽCOOy. . But this law hardly allows us to distinguish between bidentate mononuclear and bidentate binuclear from the stretching vibrations of the group. Table 2 gives the values of D nas – s for the sulfonate group for isolated AO7, AO7–ZnO and AO7–TiO 2 obtained by transmission FT–IR spectroscopy. Assuming that the rule of Deacon and Phillips can also be applied to asymmetric and symmetric stretching vibrations modes of the sulfonate group would imply a bidentate coordination type for the AO7–TiO 2 complex and that only one oxygen atom interacts with the surface for the AO7–ZnO complex. This reflects the specificity of the surface properties of both semiconductors. It should be noted that data taken from diffuse reflectance spectra lead to the same conclusion. AO7 molecules are bound to surface Ti IV cations in a process that involves the substitution of surface coordinated OHy. Therefore, complexation of the TiO 2 surface at the TiO 2raqueous solution interface will be limited by the availability of Ti–OH sites that participe in a Lewis acid–base ligand exchange reaction. Rodriguez et al. w31x have found the number Ns of available OH surface groups at maximum monolayer coverage to be Ns s 1.79 sites P nmy2 or 2.97 = 10y6 mol P my2 . When the TiO 2 powder is dispersed in water, the 30 nm non-porous crystallites aggregate in a regular dispersion. We have found the size of these agglomerates to be D s 0.6 mm, in close agreement with Ref. w32x. Therefore, we assume that the AO7 molecules only have access to a specific area of 2.5 m2 P gy1 which leads to a value of Ns s 7.425 = 10y6 mol P gy1 . We can determine the number of OHy group exchanges during the chemisorption per molecule of AO7 by using the value Ns and the parameter Nmax calculated from adsorption isotherm parameters. From the specific area available for AO7 molecules during the experiment adsorption Nmax s 2.56 = 10y6 P mol P gy1 . We calculate the ratio NsrNmax s 2.9 which gives the adsorption exchange stoechiometry. From this value, we deduce that three oxygen atoms of an AO7 molecule are involved in its adsorption on TiO 2 to form an AO7–TiO 2 complex with a bridged configuration. The molecular structure of the dye–semiconductor complex AO7–TiO 2 is illustrated in Fig. 4. On one side, two oxygen atoms from the sulfonate group are linked to two
Table 2 Bands position of n ŽSO 3 . Compound
nas SO 3 Žcmy1 .
n SO 3 Žcmy1 .
D nas – s Žcmy1 .
AO7 AO7–TiO 2 AO7–ZnO
1180 1168 1185
1124 1120 1121
56 48 64
Data from spectra obtained by transmission.
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Fig. 4. Adsorption geometry of AO7 on TiO 2 .
Ti IV cations in a bidentate binuclear coordination type complex; on the other, the oxygen atom from the carbonyl group of the H tautomer is linked to a Ti IV cation in a unidentate coordination type complex. This is in agreement with the fact that in anatase the most stable faces Ž001. and Ž011. contain fivefold-coordinated Ti IV cations which are Lewis acid sites. Initially at the TiO 2rH 2 O interface these cations fix OH groups to reach their stable octahedral coordination. During adsorption of an AO7 molecule, three OH groups situated on three different surface Ti IV cations are replaced by three oxygen atoms from AO7. There is a strong overlap between the 3d orbitals of the titanium atoms and the 2p orbitals of the oxygen atoms and the Ti–O bonds between the AO7 molecule and the surface Ti IV cations have a strong covalent or dative character. This affirmation is supported by recent works which proved using several techniques that each formiate anion are bound to two titanium atoms on a TiO 2 surface w33x and that a significant amount of the Ti–O interaction is covalent in nature w34x. In conclusion, it should be underlined that the adsorption of AO7 occurs through a Lewis acid–base reaction which implies the formation of an inner-sphere complex. In view of photoinduced interfacial charge transfer processes, this means that the electronic coupling between AO7 and TiO 2 falls in the strong and adiabatic limit coupling regime with the following important implications: ultrafast electron transfer, hot carrier injection, surface states suppression, etc.
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