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Langmuir 2001, 17, 816-820
IR Spectroscopy of Surface Water and Hydroxyl Species on Nanocrystalline TiO2 Films Kim S. Finnie,* David J. Cassidy, John R. Bartlett, and James L. Woolfrey Materials Division, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, NSW 2234, Australia Received June 30, 2000. In Final Form: October 23, 2000 The speciation of water and hydroxyl groups bound to the surface of a nanocrystalline titania film has been investigated by in-situ infrared spectroscopy as a function of temperature. Calibration of the absorbance of the δ(H2O) mode at 1625 cm-1 by thermogravimetry has enabled an estimation of the concentration of surface H2O present during thermal dehydration of the films, which varied from 5 to 0.65 molecules per nm2 over the temperature range 27-150 °C. Two types of coordinated H2O and both terminally bound and bridging hydroxyls have been identified by the temperature-dependent behavior of their corresponding O-H stretching modes. Hydrogen bonding was observed between coordinated H2O and terminally bound hydroxyls (ν(OH) ) 3730 cm-1), whereas bridging hydroxyls (ν(OH) ) 3670 cm-1) do not appear to be affected by similar H-bonding.
Introduction In a recent study, we investigated the binding of a ruthenium(II) dye to the surface of nanocrystalline TiO2 (anatase) films, using IR and Raman spectroscopy.1Such dye-sensitized films form the anode of the photoelectrochemical cell developed for solar energy conversion by Gra¨tzel et al.2 In the course of optimizing the film dyeing procedure, we noted the effect of various surface treatments on the extent of dye uptake by the films, which suggested that attachment of the dye molecules involves interaction with hydroxyl groups on the surface of the TiO2. Although initial IR measurements revealed a number of bands in the ν(OH) stretching region (38003400 cm-1), it was unclear to what extent the presence of coordinated surface water would influence the spectra. The present paper reports a systematic IR study of this spectral region, to enable the functional groups present on the film’s surface to be identified. It was noted that under controlled conditions there appeared to be a reproducible and temperature-dependent quantity of water present at surface temperatures in excess of 100 °C. We have employed this apparent equilibrium to determine an absorption coefficient for δ(H2O) at 1625 cm-1, enabling in-situ determinations of sorbed H2O concentration at temperatures from ambient to 150 °C. There are a large number of reports of vibrational spectroscopic studies probing the surface of anatase, presumably due to its widespread use as a catalyst by industry.3 These have involved both direct observation of hydroxyl vibrations and the use of appropriate probe molecules to investigate acid/base characteristics. However, the surface properties of TiO2 depend strongly on how it is prepared.4 In particular, particle size and the presence of surface impurities (e.g., chloride, sulfate, etc.) from the various synthetic procedures used are important (1) Finnie, K. S.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 1998, 14, 2744. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (3) (a) Yates, D. J. C. J. Phys. Chem. 1965, 65, 746. (b) Tanaka, K.; White, J. M. J. Phys. Chem. 1982, 86, 4708. (c) Spinicci, R.; Tofanari, A. Appl. Catal. 1985, 14, 261. (d) Morterra, C. J. Chem. Soc., Faraday Trans. 1 1988, 84, 1617. (4) Hadjiivanov, K. I.; Klissurski, D. G. Chem. Soc. Rev. 1996, 25, 61.
factors giving rise to such variability. A notable disadvantage in studying the transmission IR spectrum of polycrystalline TiO2 is the necessity to press a selfsupporting wafer, resulting in transmission difficulties and the possibility of modifying the oxide’s surface under pressure. In the present study, we have made use of the conductive glass substrate of the film, which acts as one terminal of the photoelectrochemical cell; while transparent to visible light, it is sufficiently conducting to completely reflect IR wavelengths. IR spectra were recorded by measuring light that is passed through the film and reflected back off the conductive glass interface. Apart from a sharp band due to the TiO2 at 880 cm-1 and broad absorption at lower wavenumbers, the spectral features due to surface species are very weak, with typical band heights of
