Langmuir 1995,11, 4193-4195
4193
New Sol-Gel Attenuated Total Reflection Infrared Spectroscopic Method for Analysis of Adsorption at Metal Oxide Surfaces in Aqueous Solutions. Chelation of TiO2, Zr02, and A203 Surfaces by Catechol, 8-Quinolinol, and Acetylacetone Paul A. Connor, Kevin D. Dobson, and A. James McQuillan" Department of Chemistry, University of Otago, P. 0. Box 56, Dunedin, New Zealand Received June 13, 1995. In Final Form: August 28, 1995@ A new sol-gel method has been used t o study, by in situ internal reflection infrared spectroscopy, the adsorption of coordinating ligands onto metal oxide surfaces from dilute aqueous solutions. Good sensitivity has been achieved in spectra obtained from a single internal reflection for catechol, 8-quinolinol, and acetylacetone adsorbed t o TiOz, ZrO2, and AI203 gel layers. The spectra indicate that the adsorbates bind to surface metal ions as bidentate ligands and that more than one ligand may be bound to some surface metal ions.
Introduction Infrared spectroscopic studies have played a significant role in establishing the nature of surface groups and adsorbed species present at metallgas and metal oxide/ gas interfaces of importance in catalytic processes. However, there are many natural and technologically significant systems involving wet metal oxide surfaces where the presence of water has deterred infrared spectroscopic analysis. Inferences about the molecular details of interfacial processes in such systems have largely been obtained by indirect means, often by removing the aqueous phase. A particular example is found in the photoelectrochemical schemes for solar energy conversion where the photosensitization of titanium dioxide is achieved by adsorption of dyes which absorb visible light.2 In these systems the nature of the adsorbate-oxide binding is poorly understood but is generally believed to arise from chelation of surface titanium ion^.^-^ In situ spectroscopic studies are needed to clarify the nature and mechanism of adsorption in these photoelectrochemical systems and in many other systems containing hydrous oxide surfaces. Recently, however, two types of i n situ infrared spectroscopic studies of wet metal oxide surfaces have been reported. Both types of study have utilized the advantages of internal reflection or attenuated total reflectance (ATR) spectroscopy for aqueous systems and the power of FTIR difference spectroscopy. In the first type of work6-8 colloidal solutions or aqueous suspensions of dispersed metal oxides are usually placed in contact with a cylindrical internal reflection element and sensitivity limitations due to the inherently low colloid concentration are reduced by multiple reflections. The second approach is to coat a n internal reflection element with a thin layer of metal
* To whom correspondence should be addressed.
Abstract published inAduanceACSAbstracts, October 15,1995. (1)Kiselev,V. F.; Krylov, 0.V. Adsorption and Catalysis on Transition Metals and their Oxides. In Springer Series in Surface Sciences; Ertl, G., Gomer, R., Eds.; Springer-Verlag: Berlin, 1989; Vol. 9. (2) Gratzel, M. Comments Inorg. Chem. 1991, 12, 93. (3)Frei, H.; Fitzmaurice, D. J.;Gratzel, M. Langmuir 1990,6, 198. (4) Moser, J.;Punchihewa, S.; Infelta, P. F.; Gratzel, M. Langmuir 1991, 7, 3012. (5) Redmond, G.; Fitzmaurice, D.; Gratzel, M. J. Phys. Chem. 1993, @
97. - . , 6961 - - - -. (6) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1986,2,203. (7) Tunesi, S.; Anderson, M. A. Langmuir 1992, 8, 487. ( 8 )Tejedor-Tejedor, M. I.; Yost, E. C.; Anderson, M. A. Langmuir 1990, 6, 979.
oxideg-12by sputtering or by drying off small volumes of aqueous suspensions. This method shows greater promise, giving stronger adsorbate signals from more stable substrates. The present paper reports a new, more general approach to the study of metal oxidelaqueous solution interfaces. A thin transparent metal oxide gel layer is deposited on a horizontal surface of a n internal reflection element by room temperature evaporation of a n aqueous metal oxide sol. When this porous gel layer is placed in contact with a dilute aqueous solution of a n adsorbing species, infrared spectra of adsorbates are readily detected with good sensitivity, even with a single internal reflection. These substrates enable concentration and pH effects to be studied, and accounts of such work will follow this preliminary report. I n the presently reported work i n situ infrared spectra of catechol, 8-quinolinol, and acetylacetone adsorbed from aqueous solutions onto titanium dioxide, zirconium dioxide, and alumina sol-gel layers have been recorded. The infrared spectral data is used to reveal the nature of adsorbate binding to these oxide surfaces.
Materials and Methods Titanium dioxide sols containing 0.1 mol dm-3 of Ti02 were prepared by hydrolysis of titanium(1V)chloride (Aldrich 99.9%) at 0 "C and dialyzed to a pH of approximately 2.6. Zirconium dioxide sols containing 0.5 mol dm-3 ZrOz were prepared by hydrolysis of zirconium(1V)chloride (Aldrich99.9%)at 0 "C and dialyzed to pH = 3.7. The ZrOz sol was diluted to 0.05 mol dm-3 ZrOz to optimize gel stability. Alumina sols containing 0.025 mol dm-3&03 were prepared by hydrolysis of aluminum chloride hexahydrate (BDH,reagent grade)at 20 "C with aqueous sodium hydroxide solution to pH = 7. Deionized (Milli-Q)water was used for all aqueous solutions and for washing apparatus. Gel layers were prepared by overnight room temperature evaporation of 100 pL of each sol on the horizontal surface of a 45" single reflection ZnSe ATR prism (Harrick Scientific Corporation) giving an approximate gel area of 2 cm2. The Ti02 gels are derived from sols which contain some crystallineanatase but are substantially amorph~us.'~The ZrOn gels are also expected to be a mixture of crystalline (baddeleyite) and amorphous material. Gel layers were removed from the ZnSe surface by washing with acetone, followed by polishing with an (9) Sperline, R. P.; Song, Y . ;Freiser, H. Langmuir 1992, 8 , 2183. (10) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1994, 10, 37. (11) Couzis, A,; Erdogan, G. Langmuir 1993,9, 3414. (12) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587. (13)Mozer, J.; Gratzel, M. J. Am. Chem. SOC.1983, 105, 6547.
0743-746319512411-4193$09.00/0 0 1995 American Chemical Society
Letters
4194 Langmuir, Vol. 11, No. 11, 1995 E
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Figure 1. Scanning electron micrograph of a ZrO2 sol-gel layer deposited on an aluminum disc. Angle of view was 50" from normal. The scale bar corresponds to 5 pm. aqueous alumina powder (0.075 pm) slurry on a polishing microcloth (Buehler),brief sonication in water, and rinsing with water. Dilute solutions of adsorbate species were placed in contact with the gel layers on the ZnSe prism using a hemispherical glass chamber sealed to the surface of the prism with an O-ring. Infrared spectra were recorded with a Digilab FTS6O spectrometer fitted with a DTGS detector and a Harrick prism liquid cell accessory. Each spectrum was calculated from 64 scans at 4 cm-l resolution. FTIR spectra of the geyadsorbate systems were recorded with respect to H20 on each gel layer as reference. Solution spectra were recorded on the uncoated ZnSe prism with respect to a water reference. Scanningelectron microscopy (SEM) was carried out with a Cambridge S360 instrument. Catechol (BDH),8-quinolinol(Riedel de Haen), acetylacetone (Fisons), and sodium hydroxide (May & Baker) were all reagent grade.
Results Evaporation at 20 "C of an aqueous metal oxide sol spread on a planar surf-aceresults in a transparent circular gel deposit which is of relatively uniform depth in the center. This central region is surrounded by a gel annulus, which is of greater depth but amounts to less than 20% of the total surface area covered. The evanescent wave of the internally reflected infrared radiation sampled the central part of the gel layer. Figure 1 shows a SEM micrograph of part of the central region of a ZrO2 gel layer deposited on an aluminum disc. The thickness of the gel coating, as revealed by the vacuum-formed cracks seen in the SEM, is less than 1 pm. This micrograph closely resembles those from the Ti02 and A 1 2 0 3 gels deposited in the same manner. Parts a and b of Figure 2 show the infrared spectra from ZrO2 gel layers in contact with M aqueous catechol and M aqueous 8-quinolinol solutions, respectively. Very similar infrared spectra were obtained M from Ti02 and A 1 2 0 3 gel layers in contact with aqueous catechol and with M aqueous 8-quinolinol solutions. Figure 2c shows the infrared spectrum from M aqueous acetylacetone solution in contact with a Ti02 gel layer. The infrared spectrum from a ZrO2 gel layer in contact with M aqueous acetylacetone solution is very similar to that shown in Figure 2c; however, no acetylacetone adsorption to the A 1 2 0 3 gel surface was detected. In the absence of gel layers on the ZnSe prism, infrared absorptions of all solution species were undetectableat the given concentrations. In contrast to the behavior of bidentate ligands, typical monodentate ligands gave no evidence of coordinative adsorption onto the oxide gel surfaces.
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W avenumber/cm-' Figure 2. Infrared spectra of (a)catechol adsorbed to ZrO:! gel M aqueous catechol solution, (b)8-quinolinoladsorbed from to ZrOg from M aqueous 8-quinolinol solution, and (c) acetylacetone adsorbed to Ti02 gel from M aqueous acetylacetone solution.
The wavenumbers of the most prominent absorption bands in all the recorded spectra are listed in Tables 1 and 2, together with the wavenumbers of the most prominent infrared absorption bands of catecholate, 8-quinolinolate, and acetylacetonate ions in aqueous solution.
Discussion Thin aqueous metal oxide gel layers provide stable substrates for the study of adsorbate interactions with metal oxide surfaces. The gel layers are sufficientlythin that the absorptionsfrom metal-oxygen stretchingmodes below 1000 cm-l, which usually restrict catalyst infrared studies, are no longer a problem. However, the layers are of sufficient thickness and surface area to allow quality spectra of adsorbates to be readily obtained from a single internal reflection. This approach promises to be applicable to a wide range of substrates. Catechol Adsorption. Catechol has a particular affinity for metal ions in high oxidation states14and has been shown to bind strongly to the surface of colloidal TiO2, significantlyenhancing interfacialelectron transfer rates.4 It has been speculated that the binding involves chelation of surface titanium(1V)ions: but no supporting spectroscopicevidence has been presented. The infrared spectra of catechol adsorbed to TiO2, to ZrO2, and to A 1 2 0 3 gels are very similar and correspond closely to that of the catecholate dianion (Table 1). The spectra are dominated by two strong bands near 1480 and 1250cm-l. It is clear that catechol binds to the oxide surfaces in the bidentate dianion form.
Langmuir, Vol. 11, No. 11, 1995 4195
Letters Table 1. Wavenumbers (f4cm-l) of Principal Infrared Absorption Bands of Catechol and 8-QuinolinolAdsorbed to TiOz, ZrOz, and A 1 2 0 3 Sol-Gel Layers from M Aqueous Solutions and of Catecholate and 8-Quinolinolate Ions in Aqueous Solution
is present.16 Thus it appears that the 8-quinolinoladsorbs to the metal ions in the oxide surfaces in the 8-quinolinolate ion form and that the binding is bidentate. The absorption close to 1100 cm-l has been attributed to the aryl-oxygen stretch vibration17 and this is significantly o n Ti02 o n ZrOz o n A1203 i n HzO" shifted by binding of the 8-quinolinolate ion to each of the oxide surfaces. catechol 1581 1481 1485 1489 1473 Acetylacetone Adsorption. Acetylacetone (acac) is 1446 1451 1428 a well-known bidentate ligand which has been employed 1333 1322 1335 to modify the behavior of titanium alkoxide precursors of 1277 1284 Ti02 sol-gels.ls The infrared spectra of acetylacetone 1258 1256 1257 1256 adsorbed to Ti02 (Figure 2c) and to ZrOz gels are very 1210 1215 1227 1207 similar in appearance, and the wavenumbers of the 1192 1202 principal absorptions for the Ti02 and ZrOzsurface species 1101 1100 1100 1099 1021 1022 1024 as shown in Table 2 concur closely. The infrared spectra 863 869 863 correspond more closely to those of the acetylacetonate 798 794 than to those of acetylacetone in aqueous solution. 747 748 746 The spectrum of the Ti02 surface species closely corre8-quinolinol 1578 1577 1579 1555 sponds to that previously reportedlafor Ti02 derived from 1497 1500 1502 1495 hydrolysis of titanium isopropoxide complexed with acac. 1467 1470 1471 1456 Comparisons of the Ti02 adsorbate infrared band wave1379 1380 1385 1384 numbers with those of known titanium(1V) acetylacetone 1321 1321 1328 1316 complexes20-24indicate closest similarity with those of 1270 1277 1281 1280 1107 1107 1110 1100 the [TiO(acac)z]zdimerz4(see Table 2). This suggests that 822 825 819 the oxo-bridged dimer is a reasonable model for the TiOd 740 740 744 acac surface complex and that each surface titanium atom is coordinated to two acac ligands. Aqueous solutions of 0.1 M catechol i n 1M N a O H a n d 0.05 M 8-quinolinol in 0.1 M NaOH. Comparisons of the ZrOz adsorbate wavenumbers with those of known zirconium(1V)acac c o m p l e x e ~involving ~~,~~ Table 2. Wavenumbers (G/cm-l) of Principal Infrared from one to four acac ligands suggest that either two or Absorption Bands of Acetylacetone Adsorbed to Ti02 and three acac ligands are coordinated to each surface ZrOz Sol-Gel Layers from M Aqueous Solutions, of zirconium. However the close similarity of the spectra of Acetylacetonate Ions in Aqueous Solution, and of acetylacetone adsorbed to Ti02 and to ZrOz gels points [TiO(acac)zIzc,) toward identical surface stoichiometries, probably involvon Ti02 on ZrOz in Hz0" [ T i O ( a c a c ) ~ l ~ ~ ~ , b ing two acac ligands for each surface titanium or zirconium. acetylacetone
1579 1533 1433 1365
1576 1529 1430 1375
1287
1282
1492 1422 1362 1311 1186
1027 935
1028 930
998 946
1579 1517 1430 1356 1272 1184 1121 1020 927
"Aqueous solution of 0.1 M acetylacetone i n 0.1 M NaOH. Preparation from Smith, G. D., e t al.24
Comparison of the infrared spectrum from the TiOJ catechol system with those of some catecholato complexes of titanium(IV)14shows closest correspondence with that of the bis(pz-oxo)dimer &[TiO(cat)2]2.2HzO which exhibits two strong infrared absorptions a t 1475 and 1247 cm-'. If this compound is a good model for the TiOz-catecholate surface complex then one or, a t most, two catecholate ligands are bound to each surface Ti(1V). 8-Quinolinol Adsorption. 8-Quinolinol is a metal chelating agent which has been used to photosensitize platinum-modified Ti02 in photochemical Hz generating experiments.I5 The similarity in appearance of the spectra of 8-quinolinol adsorbed to TiOz, ZrOz, and A 1 2 0 3 gels and the close correspondence of the wavenumbers of their absorption bands (Table 1)indicate that a common type of surface binding occurs in these systems. The absorption band wavenumbers (Table 1)are very similar to those for metal 8-quinolinolate salts where bidentate coordination (14)Borgias, B. A.; Cooper, S. R.; Koh, Y. B.; Raymond, K. N. Inorg. Chem. 1984,23, 1009. (15) Houlding, H.; Gratzel, M. J . A m . Chem. Soc. 1983, 105, 5695. (16) Fanning, J. C.; Jonassen, H. B. J. Inorg. Nucl. Chem. 1963,25, 2910.
Conclusions The application of in situ internal reflection FTIR difference spectroscopy has shown, with good sensitivity, that catechol, 8-quinolinol, and acetylacetone adsorb from aqueous solution onto TiOz, ZrOz, and A 1 2 0 3 sol-gel surfaces as anions. By comparison with known coordination complexes, the binding mode and possible surface stoichiometries of the adsorbed species have been established. The present results point toward bidentate binding in all cases. The pz-oxobridged coordination compounds appear to be good models for the adsorptive chelation to metal oxide surfaces.
Acknowledgment. P.A.C. and K.D.D. are grateful to the University of Otago for Postgraduate Scholarships. The support of the University of Otago Research Committee and the Division of Sciences is acknowledged. We thank Sue Johnstone of the University of Otago Dental School for the SEM work. LA950463W (17) Ohkaku, N.; Nakamoto, K. Inorg. Chem. 1971, 10,798. (18)Livage, J. In Better Ceramics Through Chemistry II; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; Materials Research Society Symposium Proceedings; Mater. Res. SOC.: Pittsburgh, 1986;Vol. 73, p 717. (19) Ernstbrunner, E. E. J . Chem. SOC.A 1970, 1558. (20) Bradley, D. C.; Holloway, C. E. J. Chem. SOC.A 1969, 282. (21) Collis, R. E. J . Chem. SOC.A 1969, 1895. (22) Holloway, C. E.; Sentek, A. E. Can. J . Chem. 1971,49, 519. (23) Serpone, N.; Bird, P. H.; Somogyvari, A,; Bickley, D. G. Inorg. Chem. 1977, 16, 2381. (24) Smith, D. G.; Caughlan, C. N.; Campbell, J. A. Inorg. Chem. 1972, 11, 2989. (25) Fay, R. C.; Pinnavia, T. J . Inorg. Chem. 1968, 7, 508. (26) Saxena, U. B.; Rai, A. K.; Mathur, V. K.; Mehotra, R. C.; Radford, D. J . Chem. SOC.A 1970. 904
