Langmuir 2007, 23, 9551-9554
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Interactions of 4-Chlorophenol with TiO2 Polycrystalline Surfaces: A Study of Environmental Interfaces by NEXAFS, XPS, and UPS Alexander Orlov,* David J. Watson, Federico J. Williams, Mintcho Tikhov, and Richard M. Lambert Chemistry Department, Cambridge UniVersity, Lensfield Road CB2 1EW, U.K. ReceiVed May 3, 2007. In Final Form: July 20, 2007 Despite a significant body of literature on the photocatalytic decomposition of 4-chlorophenol by TiO2 at liquid/solid and gas/solid interfaces, a fundamental understanding of the interaction of 4-chlorophenol with TiO2 is lacking. We present the first study of this interaction under well-defined UHV conditions by means of NEXAFS, time-dependent XPS, and UPS. XPS data show that the molecule adsorbs with the carbon framework intact and no scission of the C-Cl bond. The NEXAFS results indicate a coverage-dependent tilted geometry for the adsorbed molecule that is attached to the surface via a phenolate link. In contrast, because of the absence of an OH functionality, 1,2,4trichlorobenzene lies flat. The adsorption of 4-chlorophenol is accompanied by a decrease in the TiO2 work function pointing to adsorbate f substrate charge transfer. Interestingly, the UPS data suggest that 4-chlorophenol adsorption leads to the photosensitization of TiO2, thus providing a basis for understanding recent results on the visible light photocatalytic activity of TiO2 for 4-chlorophenol decomposition. It is also found that the molecule is stable against thermal decomposition at temperatures of up to ∼473 K, which is well above the temperature range used for photocatalytic decomposition studies.
1. Introduction The interactions of 4-chlorophenol with TiO2 surfaces, especially in regard to photocatalytic decomposition, are significant in the environmental area. This molecule, which is used in the production of dyes, drugs, and fungicides, is one of the most studied in the field, primarily because of issues related to its toxicity and persistence in the environment. Although its removal from wastewater is usually performed by conventional methods such as biological treatment, 4-chlorophenol is not easily biodegradable, requiring a long residence time to achieve complete degradation under ambient conditions. Photocatalytic oxidation is therefore a promising alternative to existing methods. TiO2 dioxide is the most widely used semiconductor for photocatalytic applications. It exhibits significant activity for the decomposition of a wide range of organic molecules under UV irradiation.1 Pure TiO2 requires UV radiation, as determined by its band gap of 3.1-3.3 eV. Efforts to improve the intrinsic efficiency of TiO2 have included doping by transition metals, sensitization, application of composite semiconductors, and the addition of noble metals. Surface sensitization by various organic molecules has been employed for both environmental2 and solar cell3 applications. Even though TiO2 single-crystal surfaces have been extensively studied under well-defined UHV conditions by many groups,4 corresponding studies on polycrystalline TiO2 surfaces are much rarer,5 despite their importance in a range of catalytic applications, including photocatalysis.2 Thus, although over 100 papers have been published on the photocatalytic degradation of 4-chlorophenol by TiO2 or TiO2-based materials,6,7 there have been no corresponding fundamental studies of this system under UHV conditions. Studies at atmospheric pressure have been aimed at
elucidating the interaction of 4-chlorophenol with TiO2 surfaces by means of DRIFTS6 and diffuse reflectance UV-vis spectroscopy (DRUVS),8 but available information is very limited. Here, by means of measurements carried out under well-defined conditions, we address the structure, bonding, and molecular mechanism of 4-chlorophenol interaction with polycrystalline TiO2, the critical reaction-initiating step in photocatalysis. Some experiments were also carried out using 1,2,4-trichlorobenzene, another species of environmental concern:9 these data provide a useful comparison with the 4-chlorphenol results. UPS, XPS, and NEXAFS were used to obtain insight into the orientation of adsorbed 4-chlorophenol and its mode of bonding to the surface. An examination of the valence-level photoemission suggests that the system may be subject to photosensitization that would account for recently reported visible light photoactivity.8 2. Experimental Section 4-Chlorophenol interaction with the TiO2 surface was examined by near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy and time-dependent X-ray photoelectron spectroscopy (XPS) performed at the ELETTRA synchrotron radiation facility in Trieste (Italy). Photon energies used for the Ti 2p, C 1s, O 1s, and Cl 2p XP spectra were chosen to be 550, 400, 650, and 300 eV, respectively, thus providing high surface sensitivity according to the inelastic mean free path dependence on kinetic energy. The beamline provided high photon flux in the 260-320 eV range used for NEXAFS. The NEXAFS data were acquired at two photon angles of incidence of θ ) 90 and 10° using partial electron yield detection. The absolute energy scale calibration was carried out by referring to the absorption features due to weak carbon contamination of the optics (285 eV). Experiments on 1,2,4-trichlorobenzene/TiO2 were conducted using the same setup. XPS (Mg KR) and corresponding UPS (He I) experiments were performed at Cambridge using a VG ADES system equipped with a rotatable hemispherical analyzer, dual Mg/Al X-ray
* Corresponding author. E-mail:
[email protected]. (1) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69-96. (2) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735-758. (3) Oregan, B.; Gratzel, M. Nature 1991, 353, 737-740. (4) Diebold, U. Surf. Sci. Rep. 2003, 48, 53-229. (5) Sykes, E. C. H.; Tikhov, M. S.; Lambert, R. M. J. Phys. Chem. B 2002, 106, 7290-7294.
(6) Stafford, U.; Gray, K. A.; Kamat, P. V.; Varma, A. Chem. Phys. Lett. 1993, 205, 55-61. (7) Orlov, A.; Jefferson, D. A.; Macleod, N.; Lambert, R. M. Catal. Lett. 2004, 92, 41-47. (8) Kim, S.; Choi, W. J. Phys. Chem. B 2005, 109, 5143-5149. (9) Adrian, L.; Szewzyk, U.; Wecke, J.; Gorisch, H. Nature 2000, 408, 580583.
10.1021/la7012792 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/14/2007
9552 Langmuir, Vol. 23, No. 19, 2007
Letters
anode, and helium discharge lamp. The 10 × 10 × 1 mm3 polycrystalline titanium sample (Advent, 99.6%) was mechanically polished and cleaned in vacuum by Ar+ sputtering/annealing cycles until no impurities were detectable by XPS. A polycrystalline TiO2 thin film was then produced in situ by controlled oxidation in 6 × 10-6 mbar of oxygen at 500 K for 8 min followed by cooling to 300 K in the same oxygen atmosphere. With this preparation procedure, a fully oxidized TiO2 thin film was produced showing no detectable impurities within the XPS sampling depth.5 The Ti 2p binding energy, the observed band gap of 3.36 eV, and the measured work function of 5.35 eV are in excellent agreement with literature values reported for the anatase form of TiO2.5 The films were tested for stability under the photon beam, and no changes were observed over the experimental time scale. 4-Chlorophenol (Aldrich) at room temperature is a clear crystalline solid with a vapor pressure of approximately 1 mbar at room temperature. For successful dosing, it was necessary to heat both the glass sample reservoir and the leak valve to about 340 K to avoid condensation within the dosing assembly: the resulting 4-chlorophenol vapor pressure in this assembly was ∼6 mbar. Before each dose, the whole assembly was pumped out by a diffusion pump, and the purity of the 4-chloropenol admitted to the UHV chamber was monitored by mass spectrometry. This procedure resulted in clean, reproducible dosing. 1,2,4-Trichlorobenzene (liquid at room temperature, vapor pressure
