Verification of the Photoadsorption of H2O Molecules on TiO2

The interactions between H2O molecules and photoreduced TiO2 surfaces were investigated by spectroscopic methods such as Fourier transform infrared ...
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J. Phys. Chem. C 2007, 111, 9811-9817

9811

Verification of the Photoadsorption of H2O Molecules on TiO2 Semiconductor Surfaces by Vibrational Absorption Spectroscopy Masato Takeuchi,† Gianmario Martra,‡ Salvatore Coluccia,‡ and Masakazu Anpo*,† Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture UniVersity, 1-1 Naka-ku, Gakuen-cho, Sakai, Osaka, 599-8531, Japan, and Dipartimento di Chimica IFM and NIS Center of Excellence, UniVersita di Torino, Via P. Giuria 7, 10125, Torino, Italy ReceiVed: December 24, 2006; In Final Form: March 22, 2007

The interactions between H2O molecules and photoreduced TiO2 surfaces were investigated by spectroscopic methods such as Fourier transform infrared (FT-IR) and UV-visible-near-infrared (NIR) spectroscopic absorption measurements. When TiO2 powders were irradiated with UV light in the absence of O2, the white color of the TiO2 powders changed to blue-gray and the H2O molecules simultaneously desorbed from the TiO2 surfaces due to the heating effect from the light source. The H2O molecules could hardly readsorb on such photoreduced TiO2 surfaces, which could remain stable in the absence of O2 for a long time. The photoformed holes trapped on the TiO2 surfaces are immediately consumed to oxidize the lattice oxygen and/or surface hydroxyl groups, resulting in the formation of oxygen vacancies, while the photoformed electrons are trapped on the Ti4+ sites to produce Ti3+ sites in the absence of O2 as electron scavengers. Hence, such photoreduced TiO2 surfaces, on which the photoformed electrons are trapped, can be represented as “negatively charged surfaces” or “electron-rich surfaces”. H2O molecules, which are strongly polarized due to the high electronegativity of the O atoms, are hardly able to interact with such electron-rich surfaces due to repulsion. Moreover, when the TiO2 surfaces are irradiated with UV light in the presence of O2, the oxygen vacancies are quickly oxidized and the electrons trapped on the Ti3+ sites are immediately scavenged by O2 molecules. Such a consuming process of the negative charges for the oxygen vacancies and trapped electrons may be the driving force for the photoadsorption of O2 molecules on TiO2 semiconducting photocatalysts. The desorption of H2O and the simultaneous adsorption of O2 during UV light irradiation on the TiO2 surfaces were also confirmed by Q-mass analysis.

Introduction Since the discovery of the photoinduced superhydrophilicity of TiO2 surfaces in 1997,1 TiO2 thin films prepared on various substrates have been widely applied for self-cleaning, antifogging, antibacterial and stainproofing materials.2-4 The mechanism proposed behind this photodriven phenomenon is that the H2O molecules dissociatively adsorb on the oxygen vacancies of the TiO2 surfaces, resulting in an increase of the surface hydroxyl groups of the TiO2 surfaces during UV light irradiation.2-7 When UV light is turned off, the high wettability of the TiO2 surfaces gradually disappears. This disappearance in the dark suggested that the newly formed hydroxyl groups on the TiO2 surfaces by UV light were oxidized by O2, followed by recovery of the bridging oxygen sites.5-7 However, we have already reported that the superhydrophilic state of TiO2 surfaces obtained by UV light irradiation easily disappears by evacuation at room temperature for only 5 min.8 Since the hydroxyl groups of the oxide surfaces cannot be removed by degassing at room temperature (rt), if the newly formed surface hydroxyl groups are the origin of this photodriven phenomenon, the superhydrophilicity of the TiO2 surfaces should be maintained even after evacuation at rt. It is generally known that when solid surfaces are irradiated with light, the temperature of the surfaces always * To whom correspondence should be addressed: e-mail anpo@ chem.osakafu-u.ac.jp. † Osaka Prefecture University. ‡ Universita di Torino.

increases by the heating effect from the light source. Especially, in the case of semiconductor oxides, most of the incident light energy is lost in the recombination process of the photoformed electron-hole pairs as an exothermic (nonradiative) process. This corresponds to the increase in the entropy of solid surfaces by light irradiation. From thermodynamic considerations, the adsorption of H2O on solid surfaces can never be promoted during light irradiation since such adsorption is quite sensitive to temperature changes.8 White et al. have recently revealed that the oxygen vacancies are not related to the photodriven hydrophilic conversion of TiO2 surfaces, as analyzed by TPD measurements of TiO2 (110) single crystals.9-10 However, analyses of the TiO2 surfaces under high vacuum conditions cannot clarify the origin of the high wettability of TiO2 surfaces since the superhydrophilicity of TiO2 surfaces easily disappears under vacuum conditions. For these reasons, the adsorption states of H2O molecules on TiO2 surfaces during UV light irradiation should be discussed by experimental results measured under ambient conditions. In our previous paper,8 the photodriven hydrophilic conversion of TiO2 surfaces was proposed to originate from the desorption of H2O molecules from the TiO2 surfaces by a heating effect from light source, and the partial elimination of hydrocarbons, by photocatalytic decomposition under UV light irradiation. In this work, the photoadsorption of H2O molecules on TiO2 semiconductor surfaces during UV light irradiation was verified by vibrational spectroscopies such as FT-IR and UV-visible-

10.1021/jp0689159 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/12/2007

9812 J. Phys. Chem. C, Vol. 111, No. 27, 2007 near-infrared (NIR) spectroscopy, microcalorimetry of the adsorption heat of the H2O molecules. Moreover, the interactions between the H2O molecules and oxygen vacancies formed on the photoreduced TiO2 surfaces by UV light irradiation in the absence of O2 have been investigated in detail. Experiments The diffuse reflectance UV-vis-NIR absorption spectra of TiO2 powders (Degussa, P-25) were recorded in air on a UVvis-NIR spectrophotometer equipped with an integrating sphere (Varian, Cary-5). The baseline was calibrated by BaSO4 in the diffuse reflectance mode. UV light irradiation was carried out by a medium-pressure Hg lamp (Polymer401, Helios Italquartz srl, Italy) through a H2O filter in a Pyrex glass vessel to avoid the heating effect of the Hg lamp. The Pyrex glass vessel works as a cutoff filter for UV light shorter than ca. 300 nm. The irradiation distance between the light source and powder samples was adjusted to ca. 50 cm. The temperature of the TiO2 powder during UV light irradiation was measured by a thermocouple. FT-IR spectra of the TiO2 samples were recorded in transmittance mode by a FT-IR spectrophotometer equipped with a DTGS detector (JASCO, FT/IR 660Plus, resolution 4 cm-1). TiO2 powder samples were pressed into self-supporting pellets. The baseline was calibrated by air in transmittance mode. UV light irradiation was carried out by a conventional 500 W superhigh-pressure Hg lamp (Ushio, USH-500BY). UV light from the Hg lamp was guided by a fiber scope and focused on the pellet placed in an in situ FT-IR cell (CaF2 windows) connected with a vacuum line (10-7 kPa range). The adsorption isotherm and adsorption heat of the H2O molecules on the TiO2 surfaces were measured by microcalorimetry. Prior to measurements, the TiO2 samples placed in a Pyrex cell were irradiated with UV light in air by a mediumpressure Hg lamp through a H2O filter for 3 h and then outgassed at 303 K (thermostat temperature of the microcalorimeter) for 1 h. For comparison, the TiO2 samples before UV light irradiation were also measured in the same way. The desorption profiles of H2O (m/z ) 18) and O2 (m/z ) 32) from the TiO2 surfaces during UV light irradiation were measured in a high-vacuum chamber of 10-9 kPa range by a gas desorption analyzer (ANELVA, M-QA100TS) equipped with a quadrupole mass analyzer. The TiO2 powder (30 mg) in a quartz cell was outgassed at 723 K for 2 h, treated in O2 (ca. 6.7 kPa) at the same temperature for 2 h, and then outgassed at 373 K for 2 h. H2O of the vapor pressure and O2 of ca. 2.8 kPa were admitted into the quartz cell for 30 min until saturated absorption was reached. Prior to desorption measurements, the H2O and O2 gases in the cell were outgassed at rt for 30 min. UV light was irradiated on the TiO2 samples by a 500 W superhigh-pressure Hg lamp (Ushio, USH-500BY) through a H2O filter in Pyrex glass (cutoff wavelength ca. 300 nm). Results and Discussion The UV-vis-NIR absorption spectra of the TiO2 powders for (A) the TiO2 sample irradiated with UV light in the presence of only H2O vapor (ca. 2.0 kPa); (B) the sample stored in dark for 53 h after step A; (C) the sample exposed to O2 of ca. 20 kPa after step B; and (D) the sample irradiated with UV light under the coexistence of H2O and O2 gases after step C are shown in Figure 1A-D. The typical absorption band edge of the TiO2 semiconductor was observed at around 400 nm. Figure 1A′-D′ show the enlarged spectra in NIR regions. The absorption bands at 1940 and 1450 nm can be assigned to the combination bands, such as (δ + ν3) and (ν1 + ν3), of the H2O

Takeuchi et al. molecules adsorbed on the TiO2 surfaces (ν1, symmetric stretching; δ, bending; ν3, asymmetric stretching).8,11-18 Small absorptions at around 2250 and 1380 nm can be assigned to the combination (ν + δ) and overtone (2ν) of the silanol groups for the quartz cell, respectively.8,14,16-18 Since these peaks did not show any changes even after evacuation of the blank quartz cell at 1073 K, the silanol groups were found to exist in the bulk of the quartz cell. When the TiO2 samples adsorbed with H2O molecules (spectrum a) were irradiated with UV light in the absence of O2 (only H2O vapor of ca. 2.0 kPa existed in the cell), the broad absorption in vis-NIR regions increased to show the blue-gray color of the TiO2 powder (Figure 1A, spectra b and c), and the combination (ν3 + δ) bands at 1940 nm due to the adsorbed H2O molecules simultaneously decreased (Figure 1A′, spectra b and c). The broad absorption in the vis-NIR regions can be attributed to the conductive electrons in the conduction band of the TiO2 semiconductor.19-23 Although the TiO2 samples were irradiated through a H2O filter in a Pyrex glass vessel (H2O filter thickness ca. 10 mm), the temperature of the samples increased up to ca. 35 °C from an initial temperature of ca. 25 °C. These results clearly indicate that the TiO2 surfaces were reduced by UV light irradiation in the absence of O2 and the H2O molecules were desorbed from the TiO2 surfaces by the heating effect. Even after the photoreduced TiO2 samples were placed in the dark for 53 h, the broad absorption due to the conductive electrons on the TiO2 semiconductor could hardly return to their initial state before UV light irradiation. Furthermore, although sufficient amounts of H2O vapor existed in the quartz cell, the H2O molecules did not readsorb on the photoreduced TiO2 surfaces (Figure 1B′, spectra d-f). These results suggest that the photoreduced TiO2 surfaces are very stable in the absence of O2 for long periods and hardly interact with the H2O molecules. Since a fraction of the photoformed electrons are trapped on the Ti4+ to produce the Ti3+ sites while the majority remains in the conduction band,23 the photoreduced TiO2 surfaces can be represented as “electron-rich surfaces” or “negatively charged surfaces”. It seems that the strongly polarized H2O molecules due to the high electronegativity of the O atoms hardly interact with such negatively charged surfaces. Moreover, when the TiO2 surfaces were irradiated with UV light in the presence of methanol vapor, the color of the TiO2 powders was found to quickly change to blue-gray (data not shown here). These results clearly indicate that the photoformed holes were consumed to mainly oxidize the methanol and the lattice oxygen of the TiO2 surfaces, although the photoformed electrons were not consumed by O2 as electron scavengers. This phenomenon can be explained by the photocatalytic production of H2 from a methanol solution as the sacrificial reagent.24,25 As shown in Figure 1C,C′, when sufficient amounts of O2 (ca. 20 kPa) were admitted into the cell, the broad absorption in vis-NIR regions immediately decreased and the combination bands of the adsorbed H2O molecules at around 1940 nm were recovered. This means that sufficient amounts of O2 molecules not only oxidize the oxygen vacancies but also consume the electrons trapped on the Ti3+ sites to form O2- superanion radicals. This interaction between the O2 molecules and electrons trapped on the photoreduced TiO2 surfaces may represent the driving force for the photoadsorption of O2 on the TiO2 photocatalysts to form the active oxygen species.23,26-36 When the electrons trapped on such photoreduced TiO2 surfaces are consumed by O2 molecules, the adsorption of H2O molecules could be immediately recovered. And when the TiO2 powders

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Figure 1. Diffuse reflectance UV-vis-NIR absorption spectra of the TiO2 powder: (A, A′) Under UV light irradiation in the presence of H2O vapor (ca. 2.0 kPa) (a) after adsorption of H2O for 14 h before UV light irradiation; (b) UV light irradiation for 1 h; (c) UV light irradiation for 3 h. (B, B′) Sample c stored in the dark for (d) 1 h, (e) 22 h, and (f) 53 h. (C, C′) O2 of ca. 20 kPa was admitted to the sample (f), which was then stored in dark for (g) 5 min and (h) 14 h. (D, D′) Sample h was irradiated with UV light in the presence of H2O vapor and O2 for (h) 1 h and (j) 3 h.

were irradiated with UV light in the coexistence of H2O (vapor pressure) and O2 (ca. 20 kPa), the TiO2 surfaces were not photoreduced and small amounts of H2O adsorbed on the surfaces were desorbed, as shown in Figure 1D,D′. As reported

in previous work, when the TiO2 powders were irradiated with UV light in air, the desorption of H2O molecules by the heating effect from a light source could also be observed.8 Although the data are not shown here, the same results could be obtained

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Figure 2. FT-IR spectra of TiO2 sample: (A) outgassing at rt in dark for 30 min; pressure in the FT-IR cell, in air, was 0.81, 0.09, 0.008, 0.0005, and 0.0002 kPa (from top to bottom); (B) the TiO2 sample after step A was exposed to air for 5, 10, 15, 30, 60, and 90 min (from bottom to top); (C) the TiO2 pellet after step B was irradiated with UV light in air for 2, 10, 15, and 30 min (temperature of the sample increased up to 26.0, 26.3, 26.9, and 27.1 °C, respectively) (from top to bottom); (D) the TiO2 pellet after step C was outgassed at rt for 30 min under UV light irradiation, pressure in the FT-IR cell, in air, was 0.80, 0.04, 0.008, 0.0007, and 0.0002 kPa (from top to bottom).

for H2O molecules adsorbed on ZnO semiconductor powders but not for SiO2 insulator powders. Detailed investigations on the H2O molecules and hydroxyl groups of the TiO2 surfaces were also carried out by FT-IR measurements. The FT-IR spectra of the TiO2 sample under evacuation at rt in dark are shown in Figure 2A. The broad absorption at around 3700-2800 cm-1 and a sharp peak at 1640 cm-1 can be assigned to the stretching and bending modes of the physisorbed H2O molecules, respectively.37-40 The small absorption peaks at 3631 and 1620 cm-1, which remained after outgassing of the TiO2 sample at rt, were assigned to the chemisorbed H2O molecules directly interacting with the Ti4+ cation sites.37,39-42 These results clearly indicate that the physisorbed H2O molecules on the TiO2 surfaces can be easily removed, while in contrast, the chemisorbed H2O molecules cannot be removed by outgassing at rt As shown in Figure 2B, when this TiO2 pellet was exposed to air, the adsorption of H2O on the TiO2 surfaces immediately recovered to the initial states within 1 h. This TiO2 pellet was then irradiated with UV light in air. As shown in Figure 2C, the physisorbed H2O molecules observed at 3700-2800 cm-1 slightly decreased in proportion to the UV irradiation time. UV light guided from a 500 W highpressure Hg lamp by a glass fiber was focused on the TiO2 surfaces through a H2O filter; however, the temperature of the TiO2 sample slightly increased up to 27.1 °C from the initial temperature of 25.4 °C before UV irradiation. The desorption of the physisorbed H2O molecules from the TiO2 surfaces during UV light irradiation could be confirmed not only by NIR but also by FT-IR (mid-IR) spectroscopies. Moreover, in order to verify whether the chemisorbed H2O molecules and/or the hydroxyl groups on the TiO2 surfaces increased by UV light irradiation, the TiO2 sample was outgassed under UV light

Figure 3. Comparison of FT-IR spectra of the TiO2 pellet before and after UV light irradiation (measured after outgassing at rt for 30 min).

irradiation (Figure 2D). A comparison of the FT-IR spectra of the TiO2 surfaces in vacuum before and after UV light irradiation is shown in Figure 3. The chemisorbed H2O molecules and surface hydroxyl groups on the TiO2 surfaces did not show any changes by UV light irradiation. The increase in the surface hydroxyl groups and chemisorbed H2O by UV light irradiation of the TiO2 surfaces could not be experimentally observed by vibrational spectroscopy. Adsorption isotherms of the H2O molecules on the TiO2 surfaces before and after UV light irradiation as observed by microcalorimetry measurements are shown in Figure 4. The equilibrium adsorption amounts of H2O molecules on the TiO2 surfaces showed good reproducibility for each measurement. Moreover, the TiO2 surfaces did not show any significant differences in the equilibrium adsorption amounts of H2O

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Figure 4. Adsorption isotherms of H2O molecules on the TiO2 samples (A) before and (B) after UV light irradiation for 3 h, as determined by microcalorimetry measurements.

Figure 5. Effect of UV light irradiation on the desorption profiles of (A) H2O and (B) O2 from the TiO2 surfaces.

molecules before and after UV irradiation. These results clearly indicate that the TiO2 surfaces after UV light irradiation did not dramatically change their surface properties and absorb larger amounts of H2O than before irradiation. The adsorption heat of H2O molecules on the TiO2 surfaces before and after irradiation were estimated to be 68.6 and 73.9 kJ/mol, respectively. Since the condensation heat of H2O is known to be 44 kJ/mol, the H2O molecules adsorbed on the TiO2 surfaces are much more stable than H2O on a liquid/gas interface due to the surface relaxation energies. These differences could also be confirmed by the differences in the NIR absorption spectra of H2O adsorbed on the TiO2 surfaces and liquid H2O.18 The UV light irradiation effect on the desorption profiles of H2O and O2 molecules from the TiO2 surfaces measured by a Q-mass analyzer is shown in Figure 5. Although the apparatus setup was completely the same as temperature-programmed desorption (TPD) measurements, the desorption profiles were obtained under UV light irradiation instead of heating of the samples. Desorption of H2O was found to increase during irradiation, but desorption of O2 decreased. Desorption of H2O molecules during UV light irradiation showed good correspondence with the results obtained from NIR and FT-IR measurements. In contrast, the decrease in O2 desorption during irradiation can be associated with photoadsorption of O2 molecules on the TiO2 semiconductor. Moreover, photoadsorption of O2 molecules has been reported to be promoted on TiO2 surfaces saturated with H2O molecules.26,27,32,34 This may be explained by the consumption of the photoformed holes to

oxidize the H2O molecules to form OH radicals, causing the relative concentration of the photoformed electrons trapped on the TiO2 surfaces to increase and resulting in an increase in photoadsorption of the O2 molecules. With regard to the photoinduced superhydrophilic properties of the TiO2 surfaces, it is often mentioned that this phenomenon is achieved by an increase in the amount of H2O molecules adsorbed on the TiO2 surfaces during UV light irradiation. However, as already mentioned, it is difficult to irradiate UV light on solid surfaces without any temperature changes. Also, recombination of the photoformed electron-holes on semiconductor photocatalysts is an exothermic process. The adsorption of H2O molecules, thus, cannot be increased on such temperature-heightened solid surfaces during light irradiation. The blank FT-IR spectra measured by a Bruker IFS28 apparatus in Torino (Italy) and a JASCO FT/IR 660Plus in Osaka (Japan) are shown in Figure 6, panels A and B, respectively. These blank spectra were measured by use of only FT-IR apparatuses (mercury-cadmium-telluride, MCT, detector) without any cells. After fresh liquid N2 was placed in the MCT detector, the background was immediately corrected and the blank FTIR spectra were then measured at several time intervals. A main absorption band at 3250 cm-1 and a shoulder band at 3400 cm-1 were found to increase in proportion with the progression of time. Since the same absorption bands could not be observed with a DTGS detector, these absorption spectra were associated not with the samples themselves but with the MCT detector of the FT-IR apparatus. The absorption bands at around 3250 cm-1

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Figure 6. Blank FT-IR spectra measured by (A) Bruker IFS28 in Torino, Italy and (B) Jasco FT/IR 660Plus in Osaka, Japan. Baseline was corrected in air immediately after liquid N2 was placed inside the MCT detector. The blank spectra were then measured at several time intervals.

SCHEME 1: Interaction Models of the H2O Molecules on UV Light-Irradiated TiO2 Surfaces in the Absence and Presence of O2 Molecules

can be assigned to the ice formed in liquid N2 of the MCT detector since the ice shows an OH stretching vibration at around 3400-3200 cm-1.41,43 The intensities of the absorption bands at 3250 cm-1 measured in Osaka were much higher than those measured in Torino. Sharp noises at around 3600-4000 cm-1 due to H2O vapor in the spectra measured in Osaka were also larger as compared to the spectra measured in Torino. This can be explained by the higher humidity in Japan. The increase in the absorption bands at 3250 cm-1 was found to level off after 2 h, caused by the saturation of ice formation in the liquid N2. Conclusions H2O molecules adsorbed on TiO2 surfaces were confirmed to desorb during UV light irradiation caused by the heating effect from the light source. When the TiO2 surfaces are irradiated with UV light in the absence of O2, the electrons trapped on the Ti3+ sites are not scavenged by O2 and the holes trapped on the O2- sites oxidize the lattice oxygen to form oxygen vacancies, resulting in photoreduced TiO2 surfaces that show a blue-gray coloring, as shown in Scheme 1. Such photoreduced TiO2 surfaces could hardly adsorb the H2O molecules. When the TiO2 surfaces are irradiated with UV light in the presence of O2, the surface-trapped electrons are scavenged by O2 and

the photoreduced surfaces are immediately oxidized. In this way, the TiO2 surfaces retain its white color under UV light irradiation. However, the increase in the adsorption of H2O molecules on the TiO2 semiconductor surfaces during UV light irradiation has yet to be confirmed experimentally. Moreover, the process behind the scavenging of the electrons trapped on the photoreduced TiO2 surfaces by O2 can be associated with the driving force for photoadsorption of the O2 molecules. References and Notes (1) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (2) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis Fundamentals and Applications; BKC, Inc.: 1999; references therein. (3) Tryk, D. A.; Fujishima, A.; Honda, K. Electrochim. Acta. 2000, 45, 2363. (4) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C: Photochem. ReV. 2000, 1, 1 and references therein. (5) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. AdV. Mater. 1998, 10, 2, 135. (6) Sakai, N.; Wang, R.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Langmuir 1998, 14, 5918. (7) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 1999, 103, 2188. (8) Takeuchi, M.; Sakamoto, K.; Martra, G.; Coluccia, S.; Anpo, M. J. Phys. Chem. B 2005, 109, 15422.

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