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Molecular Species on Nanoparticulate Anatase TiO2 Film Detected by Sum Frequency Generation: Trace Hydrocarbons and Hydroxyl Groups Chuan-yi Wang, Henning Groenzin, and Mary Jane Shultz* Pearson Laboratory, Department of Chemistry, Tufts University, Medford, Massachusetts 02155 Received March 31, 2003. In Final Form: June 4, 2003 Visible-infrared sum frequency generation (SFG) has been applied to probe molecular species on the surface of nanoparticulate anatase TiO2 films. A trace hydrocarbon film on the surface, not detected by Fourier transform infrared spectroscopy (FTIR), is easily sensed with SFG. The first direct observation of hydroxyl groups (-OH) on the TiO2 film surface by SFG is reported. A broad vibrational band with multiple peaks in the region of 3500-3800 cm-1 ascribed to OH groups reflects the surface heterogeneity of the material.
I. Introduction In the past two decades, TiO2 has become one of most extensively studied metal oxides due to its applications in many fields including heterogeneous photocatalysisbased environmental cleanup,1 phototherapy,2 solar cells,3 antifogging and self-cleaning windows,4 antibacterial construction materials,5 cosmetics,6 and white pigments.7 Since many applications of TiO2 are heavily dependent on its surface properties, several experimental investigations have aimed to determine reaction sites and mechanisms on the surface of TiO2, e.g., X-ray photoelectron spectroscopy (XPS),8,9 atomic force microscopy (AFM),10 Fourier transform infrared spectroscopy (FTIR),8,11-13 Raman spectroscopy,14,15 and high-resolution electron-energy-loss spectroscopy (HREELS).16,17 Important results have been obtained with these techniques but they are limited because the techniques are either not surface-sensitive at the molecular level (FTIR, Raman), require an ultrahigh vacuum (UHV) environment (XPS, HREELS), or cannot * To whom correspondence should be addressed: Tel 617 627 4810; fax 617 627 3443; e-mail
[email protected]. (1) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69-96. (2) Wamer, W. G.; Yin, J. J.; Wei, R. R. Free Radic. Biol. Med. 1997, 23, 851-858. (3) Oregan, B.; Gratzel, M. Nature 1991, 353, 737-740. (4) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431432. (5) Wolfrum, E. J.; Huang, J.; Blake, D. M.; Maness, P. C.; Huang, Z.; Fiest, J.; Jacoby, W. A. Environ. Sci. Technol. 2002, 36, 3412-3419. (6) Ohshima, K.; Tsuto, K.; Okuyama, K.; Tohge, N. Kag. Kog. Ronbunshu 1997, 23, 237-242. (7) Johnson, R. W.; Thiele, E. S.; French, R. H. Tappi J. 1997, 80, 233-239. (8) Erdem, B.; Hunsicker, R. A.; Simmons, G. W.; Sudol, E. D.; Dimonie, V. L.; El-Aasser, M. S. Langmuir 2001, 17, 2664-2669. (9) McCafferty, E.; Wightman, J. P. Surf. Interface Anal. 1998, 26, 549-564. (10) Fukui, K.-I.; Iwasawa, Y. Surf. Sci. 2000, 464, L719-L726. (11) Nakamura, R.; Ueda, K.; Sato, S. Langmuir 2001, 17, 22982300. (12) Liao, L.-F.; Wu, W.-C.; Chuang, C.-C.; Lin, J.-L. J. Phys. Chem. B 2001, 105, 5928-5934. (13) Tanaka, K.; White, J. M. J. Phys. Chem. 1982, 86, 4708-4714. (14) Brazdil, J. F.; Yeager, E. B. J. Phys. Chem. 1981, 85, 10051014. (15) Gutierrez-Alejandre, A.; Ramirez, J.; Busca, G. Langmuir 1998, 14, 630-639. (16) Henderson, M. A. Surf. Sci. 1996, 355, 151-166. (17) Henderson, M. A. J. Phys. Chem. B 1997, 101, 221-229.
distinguish among molecular species on nanoparticulate surfaces (AFM). In contrast, sum frequency generation (SFG) spectroscopy has been shown to be an effective surface/interface probe.18-27 As an optical spectroscopy, SFG can be used in any environment that is accessible to both visible and infrared beams. As a vibrational spectroscopy, SFG is specific for both the species and the environment. Among the most intriguing properties of TiO2 is its switch between hydrophobic and hydrophilic character upon irradiation with ultraviolet light (UV).4 In ambient conditions, a surface coated with a transparent nanoparticulate TiO2 film is hydrophobic and water beads on the surface, e.g., resulting in fogging of a glass surface. However, after irradiation with UV light, the surface becomes hydrophilic, water wets the surface, and glass with an irradiated coating remains clear in a high humidity environment. This paper presents evidence that this hydrophobic/hydrophilic switch is due to photooxidation of a previously undetected hydrocarbon film on the coating. Many important applications of TiO2, like its photocatalytic properties, involve surface hydroxyl groups. For example, by trapping photogenerated holes at the TiO2 surface, hydroxyl groups turn into hydroxyl radicals, •OH, which play a central role in the photodegradation of organic pollutants.28 In addition, hydroxyl groups often work as joints for assembling TiO2 with other materials such as conductive glass to form electrodes or with dyes to form TiO2-based solar cells. There are many reports of vibrational spectroscopic studies probing the surface hydroxyl groups on TiO2 by FTIR. Background interference from (18) Shen, Y. R. Nature 1989, 337, 519-525. (19) Shen, Y. R. Solid State Commun. 1998, 108, 399-406. (20) Shultz, M. J.; Baldelli, S.; Schnitzer, C.; Simonelli, D. J. Phys. Chem. B 2002, 106, 5313-5324. (21) Shultz, M. J.; Schnitzer, C.; Simonelli, D.; Baldelli, S. Int. Rev. Phys. Chem. 2000, 19, 123-153. (22) Richmond, G. L. Annu. Rev. Phys. Chem. 2001, 52, 357-389. (23) Richmond, G. L. Chem. Rev. 2002, 102, 2693-2724. (24) Ma, G.; Allen, H. C. J. Am. Chem. Soc. 2002, 124, 9374-9375. (25) Chen, Z.; Ward, R.; Tian, Y.; Baldelli, S.; Opdahl, A.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 2000, 122, 10615-10620. (26) Chen, Q.; Zhang, D.; Somorjai, G.; Bertozzi, C. R. J. Am. Chem. Soc. 1999, 121, 446-447. (27) Kim, J.; Cremer, P. S. J. Am. Chem. Soc. 2000, 122, 1237112372. (28) Wang, C. Y.; Rabani, J.; Bahnemann, D. W.; Dohrmann, J. K. J. Photochem. Photobiol. A: Chem. 2002, 148, 169-176.
10.1021/la0345542 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/01/2003
Molecular Species on Nanoparticulate Anatase TiO2 Film
bulk-trapped water masks the IR signal of surface OH as the surface groups have a lower number density and a relatively weak signal. Diffuse reflection or total internal reflection IR offers improved sampling of the surface; nonetheless, the OH signal is dominated by bulk water. These linear-IR studies form the motivation for direct detection of surface groups. This paper reports observation of OH groups sans bulk interference by use of SFG. Evidence is also presented showing that the surface hydroxyl groups are involved in the chemisorption of methanol to the surface of TiO2. SFG is surface-specific due to the second-order nature of the process. Under the electric-dipole approximation, the second-order susceptibility is zero in any medium with inversion symmetry, e.g., the gas phase or liquid solution and many solids. At a surface or interface where the inversion symmetry is broken, the second-order susceptibility is nonzero and SFG is symmetry-allowed. Thus for any medium in which the constituent particles possess a sufficiently random orientation that the medium can be considered to be isotropic, e.g., the nanoparticulate TiO2 film, the SFG signal can only originate from the surface of the medium. The signal intensity is proportional to the square of the second-order nonlinear susceptibility of the medium, χ(2)(ωSF ) ωvis + ωIR). The second-order nonlinear susceptibility can be decomposed into a nonresonant term and a resonant term:
χ(2) ) χNR(2) + χR(2)
(1)
(2)
The resonant term χR depends on the density of surface molecules (N) and the molecular hyperpolarizability (β) averaged over all molecular orientations on the surface:
χR(2) ) N〈β〉/0
(2)
where 0 is the permittivity of free space. Within the electric dipole approximation, β can be expressed as29,30
βq,lmn )
Rq,lmµq,n 2p(ωq - ωIR - iΓq)
(3)
where Rlm, µn, ωq, ωIR, and Γq are the Raman tensor element, IR transition dipole moment, resonant frequency, frequency of IR beam, and damping constant of the qth molecular vibrational mode, respectively. As given by eq 3, vibrational modes are only SF-active if they are both IR- and Raman-active. The SFG intensity is enhanced when the frequency of the infrared beam is in resonance with an SF-active molecular vibrational mode. In addition, the SFG signal is polarization-dependent, enabling detection of the average of molecular orientation at the surface or interface. The inherent surface sensitivity in conjunction with the vibrational information and orientation dependence makes SFG a powerful method of studying molecules at surfaces. In the present work, molecular information on the surface of nanoparticulate TiO2 films has been probed by SFG. For the first time, a layer of trace hydrocarbon contaminants and the spectral signature of surface hydroxyl groups (-OH) on a thin film sample prepared under ambient conditions are revealed.
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Figure 1. Schematic of experimental SFG setup. (As given by momentum matching, in the present work θSF is within the range 51.1°-51.5°) of the colloidal TiO2 solution and the area of the film. The films used in this work are ca. 500 nm thick, as monitored by a profilometer (Sloan Dek Tak). Seed TiO2 nanoparticles were prepared by controlled hydrolysis of TiCl4 as reported by Kormann et al.31 The average size of the TiO2 particles in the colloidal solution is ca. 2.4 nm, and the crystal structure is anatase as determined by regular and high-resolution transmission electron microscopy (TEM and HRTEM). UV irradiation to clean up the surface of the TiO2 film, when necessary, is achieved by means of a 1000 W ozone-free xenon lamp (Oriel Instruments) equipped with a 10-cm water filter to remove IR. The TiO2 film on a CaF2 substrate was mounted on a vacuum cell for sum frequency generation experiments. SFG is accomplished by temporally and spatially overlapping a 532 nm visible and a tunable IR beam on the TiO2 film surface at incident angles of 50° and 60°, respectively, relative to the surface normal. The beam diameters are 1.0 and 0.5 mm, respectively. The experimental setup is shown schematically in Figure 1. The visible beam is produced by frequency doubling of a 1064 nm laser pulse generated from a passive-mode locking Nd:YAG laser (EKSPLA PL2143A, 20 ps, 20 Hz) in KTP. The tunable IR beam is obtained by means of a LaserVision KTP/KTA/AgGaSe2 OPG/OPA (optical parametric generation/optical parametric amplification) system. Using the 1064 nm laser pulse as a pump beam, the system produces an output tunable from 710 to 860 nm and from 1.4 to 13 µm. The SFG signal from the sample surface is collected in reflection and detected with a photomultiplier tube after filtering by a polarizer and monochromator. The IR input energy is monitored during the experiment by an energy meter (Molectron J8LP + Energy Max500). The surface vibrational spectrum, normalized to the input beam intensities and referenced to a signal from silver, is obtained by measuring the SFG signal as a function of input IR frequency. Each data point, taken at 4 cm-1 intervals, is averaged over 3000 pulses with a gated integrator (Stanford Research Systems, SR250) and stored in a personal computer. All SFG spectra in the present work are obtained with an ssp polarization combination (s-polarized SF, s-polarized visible, p-polarized IR) that is sensitive to the projection of the IR dipole onto the surface normal.
III. Results
Optically transparent films composed of nanoparticulate TiO2 were fabricated by deposition and drying of an aqueous, colloidal TiO2 solution. The thickness is determined by the concentration
Figure 2 shows the FTIR absorption spectrum of a freshly prepared TiO2 film in the 2500-4000 cm-1 region. Only one broad peak centered at ca. 3400 cm-1 is observed. This is consistent with a previously reported internal reflection infrared spectrum of a TiO2 sol-gel film in air.32 The broad peak has been attributed to surface-adsorbed hydroxyl groups on TiO2.32 Combined with the reported SFG studies, this feature is more accurately assigned to bulk water trapped in the TiO2 matrix during preparation. Figure 2 shows no resonant features not attributable to
(29) Bell, G. R.; Bain, C. D.; Ward, R. N. J. Chem. Soc., Faraday Trans. 1996, 92, 515-523. (30) Guyot-Sionnest, P.; Superfine, R.; Hunt, J. H.; Shen, Y. R. Chem. Phys. Lett. 1988, 144, 1-5.
(31) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1988, 92, 5196-5201. (32) Connor, P. A.; Dobson, K. D.; McQuillan, A. J. Langmuir 1999, 15, 2402-2408.
II. Experimental Section
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Figure 2. FTIR absorption spectrum of a freshly prepared TiO2 film.
Figure 3. SFG vibrational spectra of a TiO2 film in the CHstretching region before and after UV irradiation [Evis ∼ 70 µJ, EIR ∼ 200 µJ, ssp polarization (SF output, visible input, and IR input are s-, s-, and p-polarized, respectively)].
water, indicating that the TiO2 surface is clean within the sensitivity of transmission FTIR. The SFG spectra from the surface of the same TiO2 film as for the FTIR are shown in Figures 3 and 4. Figure 3 shows the SFG spectrum in the CH stretching region, 2800-3025 cm-1, of the TiO2 film before and after UV irradiation. Prior to UV treatment (b), the freshly prepared TiO2 film shows two resonant SFG features between 2800 and 3000 cm-1. These features are not detected by transmission FTIR (cf. Figure 2), mainly because the FTIR spectrum in that region is dominated by the broad band of hydrogen-bonded OH. Vibrations in this region are typically due to CH3 and/or -CH2- groups. The presence of these features prior to UV treatment indicates that the freshly prepared TiO2 film surface under ambient conditions has a contaminating hydrocarbon layer. The persistence of these features under evacuation indicates that the hydrocarbon layer is strongly adsorbed to the surface. Upon UV irradiation (Figure 3, O), the TiO2 surface is cleaned as evidenced by the disappearance of the corresponding SFG resonances. This surface cleanup is consistent with known TiO2-assisted photocatalysis,33 the mechanism of which is shown in Figure 5. The CH stretch signature returns with the same SFG features when the UV-cleaned surface is exposed to ambient conditions for about a day. The spectral region from 3500 to 3800 cm-1 is often associated with OH groups that lack hydrogen bonding. (33) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. Rev. 1995, 95, 735-758.
Wang et al.
This is called the “free OH” region. SFG spectra on the TiO2 film surface in the free OH region with and without UV irradiation treatment are shown in Figure 4a. The OH SFG signal is greatly enhanced by UV treatment of the TiO2 surface. On the UV-irradiated neat TiO2 film surface, multiple broad bands appear in this region (Figure 4, O). Assignment of these multiple broad bands as surface hydroxyl groups is verified by quenching of the SFG resonances with the addition of methanol (Figure 4b, b). This is consistent with previous experiments on TiO2 powder samples.34 The region from 3000 to 3500 cm-1 contains features due to hydrogen-bonded OH groups. The SFG response in the hydrogen-bonding region is also shown in Figure 4b. No remarkable SFG resonance is observed. Figure 6 shows the SFG spectra of methanol adsorbed on the same TiO2 film with and without UV pretreatment. There are four resonant peaks located at 2828, 2855, 2935, and 2968 cm-1, respectively. The peaks at 2855 and 2968 cm-1 arise from molecular methanol, and the other two peaks are attributed to methoxy chemisorbed to the TiO2 film surface. The assignment is supported by the change of the SFG signal with both pressure and temperature. When the system is evacuated or heated, the SFG resonances arising from molecular methanol are greatly decreased while the other two peaks are little affected. The two peaks that are insensitive to pressure and temperature arise from a species that is strongly adsorbed on TiO2, i.e., methoxy. Although the chemisorbed species are strongly bonded on the surface, for sample recycling when necessary, they can also be removed by exposing the film to UV light. As seen in Figure 6, the UV pretreatment of the TiO2 film has a significant effect on the SFG signal arising from the methoxy. IV. Discussion Trace Hydrocarbon Contaminants and Surface Wettability. Reversible wetting by means of UV irradiation is one of the most intriguing phenomena recently observed for TiO2.4 A fundamental explanation of this photoinduced hydrophobic-hydrophilic conversion under ambient condition has been lacking. On the basis of FTIR and XPS studies,35,36 the conversion was explained by an increase of surface defects due to the UV irradiation. It is not surprising that such defects are detected under UHV conditions. However, under ambient conditions where oxygen and water vapor are present, the conversion persists. Under such conditions the lifetime of the surface defects is too short to account for persistence of the hydrophilic surface. In a more recent work,37 the surface wettability conversion has been explained in terms of surface defects generated by UV irradiation resulting in an increased surface OH density. The experimental data presented here give a supplement to the above explanation. A clean TiO2 surface is, in principle, occupied by either oxygen or hydroxyl groups. Both are hydrophilic. Hydrophobicity is commonly associated with a hydrocarbon layer on the surface. The FTIR spectrum of Figure 2 suggests that such a hydrocarbon layer is not present and that the TiO2 surface is clean. Neither has such a hydrocarbon layer been reported (34) Suda, Y.; Morimoto, T.; Nagao, M. Langmuir 1987, 3, 99-104. (35) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10, 135-138. (36) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188-2194. (37) Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 1028-1035.
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Figure 4. SFG vibrational spectra with ssp polarization in the free OH and hydrogen-bonding region from a TiO2 film: (a) preand post-UV irradiation; (b) post-UV treatment and with 1 Torr of methanol. Inset is a Lorentzian fit of the spectrum in the free OH region from UV-pretreated TiO2 film in the absence of methanol.
Figure 5. Mechanism for cleanup of a TiO2 film by UV irradiation (Eg is the band gap energy; CB is the conduction band energy; VB is the valence band energy; D is an electron donor; A is an electron acceptor).
Figure 6. SFG spectra of methanol adsorbed on the TiO2 film in the CH region (ssp polarization)
in the literature for TiO2 films under ambient conditions. In contrast, the SFG spectrum of the same film (Figure 3) shows a clear signature in the CH-stretch region, indicating hydrocarbon species that were undetected by FTIR. This hydrocarbon layer seen by SFG explains the hydrophobicity of the TiO2 film. Furthermore, the SFG experiments show that this hydrocarbon layer is removed by UV irradiation, consistent with the known photocatalytic capabilities of TiO2. As mentioned above, this hydrocarbon layer is regenerated upon exposure of the film to ambient conditions for about a day. UV irradiation is also expected to increase the number of surface OH groups as follows. Irradiation increases the number of surface defects. Defect sites are related to unsaturated Ti, which is often compensated by water, readily present under ambient conditions. Part of the additional water dissociates to form surface hydroxyl
groups, further increasing the hydrophilicity of the surface. The increased surface hydroxyl group concentration upon UV irradiation is observed by SFG (cf. Figure 4a): Before and after UV irradiation the SFG features in the OH region are the same, but the SFG intensity is roughly doubled upon irradiation. The surface OH groups will be discussed in greater detail in the following section. Surface OH Groups. The TiO2 surface OH groups have previously been studied by FTIR spectroscopy, and successful detection of the surface groups has been reported by some groups.38-40 However, this technique is not specifically sensitive to the surface, so surface features must be deduced by subtraction, which can over- or undercompensate, particularly as the sample changes. Reflection-absorption IR has also been used with some success. However, as addressed by Finnie et al.,39 there are inevitably intense interference bands in the FTIR measurement, which must be fitted and removed to distinguish the weak features (
