J. Phys. Chem. B 2001, 105, 6987-6992
6987
Remote Oxidation of Organic Compounds by UV-Irradiated TiO2 via the Gas Phase Tetsu Tatsuma,† Shin-ichiro Tachibana, and Akira Fujishima* Department of Applied Chemistry, School of Engineering, UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: March 23, 2001
The remote oxidation of organic materials via the gas phase was studied in detail. A TiO2-coated glass plate was faced to an organic film separated by a small gap (50 µm to 2.2 mm), and the TiO2 was irradiated with UV light. As a result, aromatic and aliphatic substances were oxygenated and decomposed to generate CO2 by active oxygen species that were generated at the TiO2 surface and transported in the gas phase. The amount of CO2 generated during the irradiation with 1 mol of photons was estimated to be 2 × 10-4 mol for polystyrene oxidation and 3 × 10-5 mol for polyethylene oxidation. The active oxygen species that oxidizes the organic substrates is most likely to be HO•. Strong evidence for this is the result that the organic materials were oxidized by chemically generated HO• in a similar way.
Introduction TiO2 photocatalysts1 have been applied to glass, tile, filters, and many other materials that possess self-cleaning, deodorizing, self-sterilizing, antifogging, and air-cleaning functions.2-5 Thus, TiO2 is now a very important material for the improvement of our quality of life. Some of these characteristics are based on gas-phase photocatalytic reactions, which have been believed to be surface reactions, namely, the reaction of a substrate on the TiO2 surface with holes (h+) and/or active oxygen species adsorbed on the surface. It has been reported that active oxygen species, which may include hydroxyl radical (HO•), superoxide anion (O2-•), hydrogen peroxide (H2O2), and singlet oxygen (1O2), are generated on TiO2 surfaces in the gas phase6-10 and that some of these active oxygen species may diffuse twodimensionally on the TiO2 surface.8,10 However, we have recently reported the remote bleaching of a dye, methylene blue (MB), by UV-irradiated TiO2 via the gas phase for the first time.9 A TiO2-coated glass plate was faced to a glass plate coated with MB, separated by a small gap (12.5500 µm), and the TiO2 coating was irradiated with UV light (Figure 1a). As a result, bleaching of the MB was observed by means of visible spectroscopy only in the presence of oxygen and was inhibited by ethanol. Therefore, the observed bleaching is most probably caused by active oxygen species that are generated at the TiO2 surface and transported in the gas phase. In the present work, we employed some other organic materials as probes for active oxygen species and found that both aromatic and aliphatic hydrocarbons are oxygenated and decomposed to generate CO2. On the basis of these results and comparison with the corresponding reactions with some active oxygen species (HO•, O2-•, and H2O2), we conclude that the species that oxidizes the organic substrates is most likely to be HO•. The information obtained in this work should be important from the viewpoint of the safety of TiO2-based materials because most researchers working on photocatalysts have not considered the desorption of HO• from the TiO2 surface into the gas phase. † Present address: Institute of Industrial Science, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8505, Japan.
Figure 1. Experimental setups for the remote oxidation experiment (a) and the control experiment (b).
Additionally, the remote oxidation effect would be of significance with respect to the catalytic efficiency of TiO2 in air because not only substrates adsorbed on the TiO2 surfaces but also those in the gas phase in the vicinity of the TiO2 may react during UV-light irradiation. Development of novel uses of TiO2 and related photocatalysts may also be facilitated. For instance, the remote oxidation process can be exploited as a simple dry process for the modification of the physical and chemical characteristics and morphology of organic and inorganic surfaces. Such a technique might become an alternative to conventional plasma treatment and wet chemical oxidation processes. In particular, with this remote oxidation method, microscopic patterning and etching are potentially possible by using a TiO2 film irradiated with patterned UV light (like a photolithographic technique) or a small TiO2 tip with an appropriate UV source. Experimental Section Preparation of TiO2 Coatings. A TiO2 aqueous solid STS21 (Ishihara Sangyo, Japan) was diluted with water (80 vol %), then sonicated for 1 h, and coated on a Pyrex glass plate (5 cm × 5 cm) by spin-coating at ∼2000 rpm for 10 s. The TiO2 was calcined at 450 °C for 30 min to obtain an anatase TiO2-coated glass plate. The resulting TiO2 coating was irradiated with a
10.1021/jp011108j CCC: $20.00 © 2001 American Chemical Society Published on Web 06/28/2001
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black-light-type lamp (approximately 300-400 nm, ∼1.8 mW/ cm2) overnight before each experiment to clean the surface. Preparation of Organic Probes. MB, electrochemically polymerized MB (polyMB),11-13 rhodamine 6G (Rh6G), poly(2-methoxyaniline-5-sulfonic acid)14 (PMAS) (a gift from Mitsubishi Rayon), polystyrene (PS), a glass treated with octadecyltriethoxysilane (ODS-glass), and polyethylene (PE) (from Aldrich) were used as organic probes for active oxygen species. A glass plate and an ITO electrode were used as substrates for these organic probes. In the case of MB and Rh6G, a frosted glass (roughness scale observed by SEM, ∼3 µm) was dipped in a 10-mM MB or Rh6G aqueous solution for 1 h and then rinsed with water. PolyMB was deposited on an ITO electrode by potential cycling between -0.4 and +1.2 V vs Ag/AgCl (6 cycles) at 50 mV/s in a 0.1 M KCl aqueous solution containing 0.25 mM MB (adjusted to pH 9 with sodium borate). For PMAS, a 0.032 wt % aqueous solution was cast on a substrate (75-80 µL/cm2), and water was evaporated under ambient conditions. A PS coating was prepared by applying a 0.17 wt % toluene solution of PS onto a substrate (typically 43 µL/cm2 × 12 times, unless otherwise noted) followed by evaporation of toluene under ambient conditions. An ODSglass was prepared by soaking a glass plate in a 10 vol % toluene solution of octadecyltriethoxysilane for 2 h. A PE film was also prepared by casting a heptane solution (80 °C). Remote Oxidation Experiments. A substrate coated with an organic probe was faced to the TiO2 coating with a small intervening gap (12.5 µm to 2.2 mm). The gap was controlled by use of polyimide film gaskets, as shown in Figure 1a. The TiO2 coating was irradiated with a black-light-type lamp (UV intensity, ∼1.8 mW/cm2) or Hg-Xe lamp (Luminar Ace, Hayashi Tokei; UV intensity, ∼10 mW/cm2) from the back. A control experiment was carried out by mounting the TiO2-coated glass plate upside down, as shown in Figure 1b. Control experiments were carried out basically after remote oxidation experiments, by using the same TiO2 coating and the same polyimide film. Incidentally, even when the remote oxidation experiments were conducted after the control experiments, almost the same results were obtained. In experiments to reveal effects of possible scavengers for HO• (mannitol and histidine), a TiO2 film coated with a scavenger (by evaporation of a 40µL aliquot of a 2-mM aqueous solution) was used in place of bare TiO2. Analysis of Organic Probes. Visible spectra of an organic probe-coated substrate (glass or ITO) were conducted using a spectrophotometer (Shimadzu UV2400PC). X-ray photoelectron spectroscopy (XPS) of PS, ODS-glass, and PE surfaces was performed with a PHI 5600 XPS spectrometer (Perkin-Elmer). Water contact angles of organic film surfaces were measured by use of an optical microscope equipped for contact angle measurements. The amount of CO2 generated during the remote oxidation was determined by gas chromatography (GC) using a GC-8A (Shimadzu) equipped with a Porapak-Q column, a methanizer, and a flame ionization detector with N2 as the carrier gas. Results and Discussion Dye Probes. We have recently reported the remote bleaching of MB (methylene blue) by UV-irradiated TiO2 in the gas phase.9 Although the mechanism of the bleaching reaction has not yet been elucidated, we have ruled out the possibility that it is a simple reduction to leucoMB (eq 1).
MB+ + 2e- + H+ f leucoMB
(1)
Although leucoMB is known to be reoxidized to MB by oxygen dissolved in water,15,16 the visible spectrum for MB subjected to remote bleaching did not revert to the original spectrum even after a treatment with air-saturated water.9 To study this in further detail, we prepared electrochemically deposited polyMB film on an ITO electrode surface. After remote bleaching of polyMB on ITO in air (gap between TiO2 and polyMB, 50 µm; irradiated with the black-light-type lamp for ca. 1000 min), the electrode was polarized at +0.4 V vs Ag/AgCl, at which potential reduced polyMB is reoxidized,11-13 for 10 min in 0.1 M KCl aqueous solution (adjusted to pH 9 with sodium borate). However, the visible spectrum did not revert to the original one; thus, the remote bleaching of polyMB is also not a simple reduction to the leuco form. Incidentally, Rh6G also exhibited remote bleaching behavior similar to that of MB. Effects of Scavengers Coated on TiO2. To examine the effects of scavengers for HO• (mannitol and histidine), the scavenger-coated TiO2 was used for the remote bleaching experiments (gap, 12.5 µm; irradiated with the black-light-type lamp for 120 min). MB was employed as an organic probe. The scavenger coatings suppressed the remote bleaching of MB almost completely. One of the explanations is that HO•, which is scavenged by the coatings, is involved in the remote bleaching reaction. Another explanation is that the scavenger coating physically blocks the approach of oxygen, which is required for the generation of active oxygen species, to the TiO2 surface. The desorption of active oxygen species from the TiO2 surface could also be blocked. Additionally, scavengers may trap the holes generated at the TiO2 surface. PMAS Probe. Next, we used a polyaniline derivative, PMAS [poly(2-methoxyaniline-5-sulfonic acid)]. This water-soluble conducting polymer is commercially available, and reduction and oxidation of as-obtained PMAS (partially oxidized) give rise to different changes in the visible spectrum. Thus, we can know whether PMAS is reduced or oxidized by means of visible spectroscopy. A PMAS film coated on a glass plate was faced to TiO2, separated by a small gap (50 µm), and the TiO2 was irradiated with UV light in air with the Hg-Xe lamp (Figure 1a). As a result, the absorbance peak at 472 nm decreased, while absorption at >510 nm increased in the course of the experiment (240 min) (Figure 2a). These changes are characteristic of the oxidation of polyaniline derivatives; actually, almost the same changes were observed when the as-obtained PMAS coated on an ITO electrode was electrochemically oxidized in an acetonitrile-based electrolyte solution. In contrast, no significant changes were observed in a control experiment (the TiO2-coated glass plate was mounted upside down, Figure 1b). Therefore, we can conclude that the PMAS is oxidized by the UV-irradiated TiO2 via the gas phase. When the UV irradiation was continued for a longer period (>240 min), the absorbance decreased gradually in the whole wavelength range examined. Figure 2b shows time courses for the absorbance at 472 nm and that at 800 nm. These decreases should be indicative of the irreversible oxidation (oxygenation or decomposition) of PMAS. PS Probe. This irreversible oxidation was examined in further detail by use of PS (polystyrene) as an organic probe because of its simpler structure and lack of reversible redox activity. A PS film coated on a glass plate was faced to a TiO2-coated glass plate (gap, 50 µm), and the TiO2 was irradiated with UV light (Hg-Xe lamp). Changes in the water contact angle of the PS (coated on a glass plate) surface were measured (Figure 3). The decrease in the contact angle observed (from ∼90° to ∼30°) is indicative of increased hydrophilicity. This should have resulted
Remote Oxidation of Organic Compounds
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Figure 4. Dependence of the O/C atomic ratio of PS film surfaces measured by XPS on the UV irradiation time in the remote oxidation experiments (gap, 50 µm; irradiated with the Hg-Xe lamp). Each data point was obtained with an independently prepared PS film.
Figure 2. Changes in the spectra (a) and time courses of the absorbance at 472 nm and that at 800 nm (b) of a PMAS coating during the remote oxidation experiment (gap, 50 µm; irradiated with the Hg-Xe lamp).
Figure 3. Time course of the water contact angle of the surface of a PS coating on a glass plate during the remote oxidation experiment (gap, 50 µm; irradiated with the Hg-Xe lamp).
from introduction of oxygen-containing functional groups on PS. In contrast, changes in the contact angle observed in the control experiments were negligible (less than 3° for at least 1500 min in a control experiment with UV and no TiO2; less than 1° for at least 2900 h with TiO2 and no UV). To verify the presence of oxygen atoms on or in the PS film, XPS analysis was carried out. We found that oxygen atoms were introduced onto the PS surface. The O/C atomic ratio for the surface region increased with increasing irradiation time, and nearly constant values were obtained after about 2000 min (Figure 4). No oxygen was detected for control experiments (TiO2 is mounted upside down and irradiated with UV light for >1000 min). The oxygen incorporated into PS by the remote oxidation (irradiated for 4100 min) was completely removed by Ar sputtering at 1 kV for 88 min. This etching condition is so mild that the etched depth is expected to be less than 100 nm. Also, no significant change could be observed in the transmission IR spectrum of the PS film. We can therefore conclude that only
the surface region of the PS film is oxygenated in the remote oxidation. High-resolution XPS spectra for C 1s peaks were also obtained and are shown in Figure 5. It is clear that the shoulder at around 286-292 eV increased in intensity with increasing irradiation time. This shoulder basically corresponds to carbon atoms bound to oxygen atoms.17 Alcohol and ether functionalities (C-O) are reflected by a peak at around 286.5 eV. A peak at 288 eV is known to correspond to aldehyde and ketone functionalities (CdO), and a peak at 289 eV is ascribed to carboxyl and ester groups (COO). It is therefore evident that the PS is oxygenated in the present experiment. This oxygenation is probably initiated by an active oxygen species generated at the UV-irradiated TiO2. Incidentally, it is known that these functional groups are introduced on PS by an argon or nitrogen plasma treatment followed by exposure to oxygen17,18 and by a photooxidation process.19 The shoulders of the C 1s peaks were deconvoluted on the basis of the respective functional groups mentioned above so as to evaluate the ratio of the number of COO groups to the total number of C-O, CdO, and COO groups. As the irradiation time increased from 120 to 2305 min, this ratio increased from 0.04 to 0.43. This obviously shows that the oxidation proceeds more extensively as the treatment time increases. The generation of COO may reflect an increase in the number of COOH terminal groups because the generation of esters is not very likely in the present system. For PS, COOH end groups can be generated as a consequence of oxidative scission of the polymer backbone or oxidative ring opening of benzene rings, and the former reaction is known to be more preferable.17,18 In either event, the remote oxidation can give rise to severe degradation of PS. To examine the degradation of PS, a PS film that had been subjected to remote oxidation (ca. 3600 min) was soaked in water (
