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Monitoring Hydrous Metal Oxide Surface Charge and Adsorption by STIRS Kevin D. Dobson, Paul A. Connor, and A. James McQuillan* Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand Received October 30, 1996. In Final Form: March 27, 1997X The first in situ infrared spectroscopic measurements of surface excess solvated ion concentrations in the electrical double layer resulting from metal oxide surface charge are reported. Enhanced concentrations of perchlorate and tetramethylammonium ions have been detected at the surface of a TiO2 gel film. A new technique, surface titration by internal reflection spectroscopy (STIRS), has been used in which spectra are recorded after flow-induced, stepwise pH changes. A measured isoelectric point for TiO2 of 5 concurs with those from other methods. The perturbing effect of specific adsorption on the TiO2 surface charge is clearly indicated by the STIRS spectra of the system containing oxalate ion.
Introduction Solid-aqueous solution interfaces are of great practical importance, but we have only recently begun to probe their molecular (surface enhanced Raman spectroscopy (SERS) and Fourier transform infrared (FTIR) spectroscopy)1-5 and microscopic (scanning tunneling microscopy (STM) and atomic force microscopy (AFM))6-8 structures, details of which are fundamental to their physical and chemical behavior. The surface charge and adsorption properties of metal oxide surfaces in aqueous environments have long been studied by macroscopic measurements.9 However the molecular models which have been proposed to account for this macroscopic behavior have been largely hypothetical, supported by little in situ spectroscopic evidence. The application of structure sensitive infrared absorption spectroscopy to aqueous systems has in the past been impeded by the strong infrared absorptions of water. This difficulty has recently been overcome by the use of internal reflection techniques10 and the power of FTIR difference spectroscopy, resulting in a resurgence in the application of infrared spectroscopy to aqueous systems.11 This Letter reports the first use of a new in situ infrared spectroscopic technique, surface titration by internal reflection spectroscopy (STIRS), to study nonspecific and specific adsorption9 at TiO2aqueous solution interfaces and to determine the TiO2 isoelectric point. Materials and Methods TiO2 films were formed by evaporation of aqueous TiO2 sols12 onto a 45° single internal reflection ZnSe prism (Harrick). The films so formed are about 1 µm thick. Further details of their * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 1, 1997. (1) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783-826. (2) Garrell, R. L. Anal. Chem. 1989, 61, 401A-411A. (3) Iwasita, T.; Nart, F. C. Adv. Electrochem. Sci. Eng. 1995, 4, 123216. (4) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1986, 2, 203210. (5) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1992, 8, 21832191. (6) Liu, H. Y.; Fan, F. R. F.; Lin, C. W.; Bard, A. J. J. Am. Chem. Soc. 1986, 108, 3838-3839. (7) Hansma, P. K.; Elings, V. B.; Marti. O.; Bracker, C. E. Science 1988, 242, 209-216. (8) Manne, S.; Hansma, P. K.; Massie. J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183-186. (9) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: Oxford, 1989; Vol. 1. (10) Mirabella, F. M. Appl. Spectrosc. Rev. 1985, 21, 45-178. (11) Hester, R. E., Girling R. B. eds. Spectroscopy of Biological Molecules; Royal Society of Chemistry: Cambridge, 1991. (12) Connor, P. A.; Dobson, K. D.; McQuillan, A. J. Langmuir 1995, 11, 4193-4195.
S0743-7463(96)01053-0 CCC: $14.00
Figure 1. The hemispherical glass chamber and 45° ZnSe single internal reflection element used for STIRS experiments. properties will be reported elsewhere.13 A hemispherical glass chamber was sealed to the ZnSe prism surface by an O-ring as shown in Figure 1, and solutions were passed through the chamber at 4.5 mL min-1 using a peristaltic pump. A series of aqueous solutions of different pH but of constant ionic strength of 5 × 10-3 M were made up with 18 MΩ cm-1 MilliQ water, tetramethylammonium hydroxide [TMAOH] (BDH), tetramethylammonium perchlorate [TMAP] (ICN Pharmaceuticals), and perchloric acid (Reidel de Haen). The solutions of highest and lowest pH contained only TMAOH and perchloric acid, respectively, while that containing only TMAP gave the medial data point. The more alkaline and the more acidic solutions contained TMAOH/TMAP and TMAP/perchloric acid mixtures, respectively. All solutions were freshly prepared prior to the measurements. In the STIRS technique14 an acid-base titration of the surface of the oxide is carried out by flowing solutions of stepwise decreasing pH over the oxide surface. This results in an initially decreasing negative and subsequently increasing positive surface charge as the pH is reduced. A solution of 5 × 10-3 M NaOH was firstly pumped through the system and used to obtain a reference spectrum at the highest pH. This first solution also served to remove any stray surface contamination arising from the method of film formation. Subsequently, data were collected for solutions beginning with 5 × 10-3 M TMAOH and continuing with solutions of decreasing pH. Thus the resultant differential spectra indicate absorbance changes relative to the TiO2 film in contact with the NaOH reference solution. Infrared spectra (64 scans at 4 cm-1 resolution) were collected with a Digilab FTS60 spectrometer using Win-IR software and were sampled 20 min after each solution change. The infrared signals reached a plateau about 15 min after each solution composition change. The above experiment was also carried out with solutions containing an additional 5 × 10-5 M sodium oxalate (BDH). The measured absorbances, Ai, of the STIRS signals of the TMA+ and of the ClO4- ions may be related to surface charge via the surface excess concentrations, Γi, of these ions. Beer’s law states that the amount of light absorbed is proportional to the number of absorbing species. Thus, in a transmission experiment, the absorbance of a given number of ions is the same irrespective of whether those ions are distributed uniformly through solution or concentrated near an interface. This assumes the molar absorption coefficients of solvated ions near the (13) Dobson, K. D.; McQuillan, A. J. Langmuir, in press. (14) New Zealand Patent Application no. 286876, filed 24/6/96.
© 1997 American Chemical Society
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interface, i.e., ions within the electrical double layer, are unchanged from those in bulk solution. The Beer-Lambert law for homogeneous solutions is Ai ) icil where ci is the concentration of species i, i is the molar absorption coefficient of species i at a given wavelength, and l is the optical pathlength through solution. If species i is present as a surface excess and the contribution to the absorbance from the bulk homogeneous solution species is negligible, the relationship for a planar surface becomes simply
Ai ) iΓi
(1)
For a porous film the ratio of the true surface area of the film to the cross-sectional area of the film sampled by the radiation, ra, is greater than 1. Thus the apparent surface excess concentration Γi′ from absorption spectroscopy of a cross section of the film is larger than the true surface excess concentration Γi where
Γi′ ) Γira
(2)
and Γi is directly proportional to the surface charge which is of opposite polarity to that of ion i. However, in internal reflection spectroscopy the Beer-Lambert Law becomes Ai ) i ci de where de is an effective pathlength10 which depends on the refractive indices of the internal reflection element and the less dense medium, on the sample thickness and on the wavelength of the light. In the case where the internal reflection element has a coating of a porous film of thickness t which is in contact with a solution, Equation (1) becomes
Ai ) iΓi′ (de/t)
Figure 2. (a) STIRS spectra showing excess TMA+ and ClO4at the surface of a TiO2 sol-gel film on a 45° ZnSe internal reflection element and in equilibrium with aqueous solutions of different pH. (b) The infrared spectrum of an aqueous TMA perchlorate solution, at 10 times the STIRS experiment concentration, obtained with the same ZnSe internal reflection element. Spectrum offset below zero absorbance.
(3)
where Γi′ is an apparent surface excess concentration and de/t is a correction for the effective path length being different from the film thickness. Γi′ may be calculated for a given film thickness, t, and film refractive index, n2, using the following relationship for de, derived from the equations of Sperline et al.15 and Mirabella.10
3n21 cos θ dp
de ) (1 - e-2t/dp)
dp )
(1 - n212)
λ1 2π(sin θ - n212)1/2 2
where n21 ) n2/n1, λ1 ) λ/n1, n1 ) 2.43 (for ZnSe), and θ ) 45°. In the absence of ra data for the TiO2 sol-gel films, Γi′ values were calculated from the absorbances in the STIRS spectra using eq 3. The molar absorption coefficients were obtained from the infrared absorption spectrum of an aqueous 0.05 M TMAP solution recorded by internal reflection with the same ZnSe prism. A film thickness of 0.7 µm from SEM data and a film refractive index of 1.5 from mass considerations were used for the calculations.
Results The STIRS infrared spectra shown in Figure 2a were obtained over a pH range from 11.7 to 2.3 using a TiO2 gel-coated ZnSe prism. Figure 2b shows the internal reflection infrared spectrum of a 5 × 10-2 M aqueous TMAP solution recorded in the absence of a TiO2 film on the ZnSe prism. The prominent absorption at 1486 cm-1 is due to the ν15 antisymmetric CH3 deformation16 of TMA+ and that at 1104 cm-1 is due to the ν3 antisymmetric Cl-O stretch17 of the ClO4- ion. A small absorption due to bulk solution ClO4- is present in all spectra except that of the pH ) 11.7 solution. The small band at 1385 cm-1 at more (15) Sperline, R. P.; Muralidharan, S.; Freiser, H. Langmuir 1987, 3, 198-202. (16) Berg, R. W. Spectrochim. Acta 1978, 34A, 655-659. (17) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd ed.; Wiley: New York, 1978.
Figure 3. Apparent surface excess concentrations of ClO4and TMA+ ions on a TiO2 gel film from solutions of various pH containing these ions at 5 × 10-3 M. The data were derived from the spectra given in Figure 2a and have been fitted to a fourth-order polynomial.
acidic pH arises from a trace amount of carbonate specifically adsorbed from solution.13 Figure 3 shows the variation with pH of the apparent surface excess concentrations of the TMA+ and ClO4- ions. These data were derived from the absorbances and molar absorption coefficients of the 1486 and 1104 cm-1 absorptions, respectively, with corrections for the bulk solution contributions. A fourth-order polynomial was fitted to the Figure 3 data. Comparison of the present data for Γi′ with literature surface charge data for titanium dioxide9 indicates that ra is of the order of 102. Figure 4a shows STIRS spectra for the system that gave the data in Figure 2a but containing an additional 5 × 10-5 M sodium oxalate. A small absorption due to bulk solution ClO4- is again present but is of constant absorbance in spectra with pH below 11.7. Figure 4b shows the spectrum of an aqueous 0.1 M sodium oxalate solution. Discussion The striking feature of the STIRS spectra in Figure 2a is the considerable enhancement of the TMA+ and ClO4absorptions, under alkaline and acidic conditions, respectively, in comparison with the infrared spectrum expected from the bulk solution. It is significant that the spectral bands arising from these ions are unchanged in wavenumber and bandwidth from those of the ions in bulk
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Figure 4. (a) STIRS spectra showing surface excess TMA+ and oxalate at a TiO2 sol-gel film on a 45° ZnSe internal reflection element and in equilibrium with aqueous solutions of different pH, each containing 5 × 10-5 M oxalate ion. (b) The infrared spectrum of an aqueous 0.1 M sodium oxalate solution obtained with the same ZnSe internal reflection element. Spectrum offset below zero absorbance.
Letters
comparable to that in the system previously discussed, but there is now no evidence for perchlorate adsorption in acidic solutions. However, below the isoelectric point there are now large infrared absorptions at 1409 and 1262 cm-1 which are markedly different from those of oxalate ion in solution (Figure 4b) and are due to specificallyadsorbed oxalate ion.19 This adsorption reduces the net surface charge to near zero as there is now no detectable surface excess of ClO4- over the small absorption from the bulk solution. Coordinative adsorption of bidentate ligands such as oxalate ion is characteristic of metal oxide surface chemistry and this has already been shown in a previous internal reflectance infrared study of TiO2, ZrO2, and Al2O3 sol-gel films12 and in a recent study of photosensitizer adsorption to TiO2.20 Electrostatic forces still appear to play a role in the oxalate adsorption which only occurs in the pH range where the surface charge would have been positive in the absence of oxalate. The oxalate absorptions also increase with decreasing pH reflecting the expected trend in surface charge. We have found, when the oxalate concentration is reduced to below 10-5 M, that some surface excess perchlorate can be detected in addition to the adsorbed oxalate. Thus, even very low concentrations of coordinating adsorbates have a profound influence on metal oxide surface chemistry. Conclusions A new internal reflection infrared spectroscopic technique (STIRS) has been developed to monitor surface charge and adsorption of thin metal oxide films in aqueous solutions. A strength of STIRS is its ability to directly distinguish between specific and nonspecific adsorption and to indicate the chemical nature of the specific adsorption. The method also reveals the interdependence of surface charge and specific adsorption. The STIRS technique appears applicable to surface chemical studies in many natural systems.
solution. This is expected in nonspecific adsorption of fully solvated ions, but these spectral data appear to be its first confirmation by vibrational spectroscopy. The enhanced infrared absorptions must arise from localized concentration increases in the double layer due to the TiO2 surface charge and are readily detectable due to the large surface area sampled. The apparent surface excess concentrations of TMA+ and ClO4- ions, as shown in Figure 3, are proportional to the net negative and net positive surface charges, respectively. The plot shows a similar pH dependence to previous surface charge determinations.9 The present data points to an isoelectric point at pH ≈ 5, and this is in good agreement with determinations by other methods18 on TiO2 derived from TiCl4. However, with a trace of oxalate ion added to this system the spectra are markedly altered. Figure 4 shows that the TMA+ adsorption above the isoelectric point is
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(18) Ardizzone, S.; Trasatti, S. Adv. Colloid Interface Sci. 1996, 64, 173-251.
(19) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587-3597. (20) Duffy, N. W.; Dobson, K. D.; Gordon, K. C.; Robinson, B. H.; McQuillan, A. J. Chem. Phys. Lett. 1997, 266, 451-455.
Acknowledgment. Kevin Dobson and Paul Connor thank the University of Otago for postgraduate scholarships. We thank Charles Clark for valuable discussions. This work was supported by the Research Committee and Division of Sciences of the University of Otago.
