J. Phys. Chem. B 2004, 108, 19373-19379
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Influence of Adsorbed Water on Phonon and UV-Induced IR Absorptions of TiO2 Photocatalytic Particle Films David S. Warren and A. James McQuillan* Department of Chemistry, UniVersity of Otago, P.O. Box 56, Dunedin, New Zealand ReceiVed: June 29, 2004; In Final Form: October 1, 2004
The role of adsorbed water in photocatalytic processes on TiO2 surfaces has been addressed using ATR-IR spectroscopy. Changes in the IR spectra of thin films of Degussa P25 TiO2 photocatalyst have been observed in the presence of adsorbed water and under near-UV light. Removal of adsorbed water results in the appearance of absorption peaks at 828, 745, and 685 cm-1 attributed to TiO2 surface phonon modes. UV irradiation of the dry TiO2 results in a broad IR absorption which toward lower wavenumbers increases in absorbance, reaches a peak at 880 cm-1, and then drops sharply to a minimum absorption at about 750 cm-1. From observations of the behavior of P25 thin films under a drying regime and comparisons with transient IR studies, the broad absorption generated by the UV light has been assigned to excitation of shallow trap electrons to the conduction band with an excitation energy of ∼0.1 eV.
Introduction There have been many studies which have revealed the products of TiO2 photocatalytic reactions but the detailed mechanisms of interfacial photocatalytic processes have still to be established.1,2 A better understanding of the influence of UV irradiation on the surface structure of TiO2 and the nature of adsorbed species should provide strategies for improving the efficiency of photocatalysis. Adsorbed species such as water, oxygen, and peroxide are expected to affect the kinetics of photocatalysis but their influences have not yet been clearly determined. During the process of photocatalysis, photogenerated electrons and holes are believed to be trapped at surface Ti4+OH centers forming Ti3+ sites and surface OH radicals, respectively.
e- + Ti4+OH f Ti3+OH h+ + Ti4+OH f Ti4+OH• The role of water associated with these trap sites is the source of considerable interest. Recently, Szczepankiewicz et al.2 have used infrared spectroscopy to identify traps associated with particular O-H stretch absorptions on photocatalytic TiO2 and found the decay over minutes of a signal they assigned to mobile carriers to be influenced by adsorbed water. While surface spectroscopic methods are potentially capable of providing new structural information about surface species and trapping sites, there are only some surface spectroscopies which are applicable under working photocatalytic conditions. Infrared (IR) spectra of surface species are particularly sensitive to environment, and IR spectroscopy has been widely used to provide detailed information about catalyst surfaces. Although it has not been widely recognized, IR spectroscopy can be used for in situ studies during UV irradiation of thin TiO2 photocatalytic films under gaseous atmospheres and under aqueous solutions. Consequently, there have been few in situ * Corresponding author. E-mail:
[email protected]; phone: +64 3 4797928; fax: +64 3 4797906.
IR studies of TiO2-related surface species during photocatalytic processes. Ekstro¨m and McQuillan3 used attenuated total reflection infrared (ATR-IR) spectroscopy to monitor the photoconversion of adsorbed glyoxalate to adsorbed oxalate on P25 TiO2 under aqueous solution. They also observed, with 365nm irradiation, the emergence of a peak at 878 cm-1 which was attributed to changes in the TiO2 surface structure or to surface peroxo species. Nakamura et al.4 used multiple internal reflection IR spectroscopy to identify TiOOH as a primary intermediate species from the oxygen reduction reaction on nanocrystalline TiO2 samples in aqueous solution. They observed a UV-induced asymmetric peak at 943 cm-1 from a 91% rutile/9% anatase sample which was attributed to surface peroxo species. Further multiple internal reflection IR work by Nakamura et al.5 on 100% rutile particles in aqueous solution has shown a UV-induced asymmetric peak at 838 cm-1 which was attributed to surface peroxo intermediate species of the oxygen photoevolution reaction. Szczepankiewicz et al. have used diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to study the behavior of P25 TiO2 photocatalysts under vacuum and controlled atmospheres.2,6,7 Band-gap irradiation produced a broad baseline feature with increasing absorption to longer wavelength (λ) which showed a λ1.7 dependence. The evidence that this spectral feature was not seen in the presence of oxygen was used to assign the feature to electrons coupled to acoustic phonons altering the refractive index of the surface TiO2. The relaxation time of this feature was affected by adsorbed water. They assigned TiO-H stretching absorptions at 3716 and 3647 cm-1 to deep traps for both electrons and holes, respectively. They proposed trapping of electrons in an excited state whose relaxation is assisted by surface hydration. The behavior of the TiO-H stretch peaks under UV irradiation was used as evidence that the electrons in shallow traps couple with surface phonons creating a two-dimensional delocalized surface state. Yoshihara et al.8 used time-resolved spectroscopy to study an anatase nanoparticle film and observed a transient broad absorption with a λ1.7 intensity dependence in the 400-2500 nm range. The broad absorption was assigned to free electrons
10.1021/jp0471812 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/17/2004
19374 J. Phys. Chem. B, Vol. 108, No. 50, 2004 and they also observed different spectral features which were assigned to trapped holes and to trapped electrons. Yamakata et al.9-11 used microsecond time-resolved IR spectroscopy to study the behavior of TiO2 P25 particle films under UV irradiation. They reported a similar broad absorption between 3000 and 1000 cm-1 showing a λ1.5 dependence. Exposure of the film to oxygen quenched the absorption and on this basis it was assigned to excitations to the conduction band (CB) of electrons in shallow Ti4+ traps at the photocatalyst surface or to intra-CB transitions of electrons thermally excited after being initially trapped. The presence of water vapor prolonged the transient signal and this behavior was ascribed to a holeconsuming reaction involving water lowering the probability of electron-hole recombination. Spectroscopic studies on other wide band gap semiconductor materials (ZnO, ZnS)12,13 have also reported the presence of a broad IR band that was attributed variously to the absorptions of shallowly trapped electrons, free carrier absorption, and intra-conduction-band transitions.14 In general, there is a lack of consensus about the dominant contributions to the broad IR absorptions induced by band gap irradiation of semiconductor particle systems. Some time ago, the behavior of free carriers in response to changes in surface hydration of germanium films was studied (Brattain-Bardeen ambient cycle).15-18 The presence of water on the germanium surface resulted in a reduction in surface conductivity which was explained in terms of an increase in shallow trapping sites. Water adsorbs at the five coordinated Ti4+ centers present on TiO2 surfaces either dissociatively, resulting in the formation of surface hydroxyl groups, or more commonly without dissociation. Although a variety of crystal faces occur in polycrystalline materials, the (110) face predominates in rutile while in anatase the (101) and (100) faces are in the majority. Temperature-programmed desorption (TPD) studies and density functional theory calculations indicate that the molecular adsorption of water predominates on perfect single-crystal surfaces in these situations.19,20,21 On the rutile (100) and anatase (001) minority surfaces, there is an initial dissociative adsorption followed by overlayers of molecularly adsorbed water. TPD studies are carried out in ultrahigh vacuum (UHV) and the conditions in such UHV studies do not correspond to those in real photocatalytic systems where there is likely to be a relatively large amount of adsorbed molecular water under ambient conditions. The most commonly used TiO2 photocatalyst is Degussa P25 which is a polycrystalline mixture of anatase and rutile. In the absence of adsorbed impurities, the only vibrational modes of TiO2 observed above ∼1000 cm-1 are the sharp OH stretch vibrations at 3400-3700 cm-1 when the TiO2 is fully dehydrated.2,22,23 Most vibrational spectroscopic data on TiO2 is on the low-frequency modes of well-defined single-crystal anatase or rutile surfaces. Of particular relevance to this paper are the infrared active longitudinal optical (LO) and Fuchs-Kliewer or surface optical (SO) phonon modes of TiO2 which have been reported in the 900-600 cm-1 spectral region. Surface phonons arise from the incomplete coordination of the surface layer of ions and have generally higher frequencies than those of bulk phonons.24 Table 1 shows the pertinent observed wavenumbers and assignments of the high-resolution electron energy loss spectroscopy (HREELS) and IR spectroscopy data on singlecrystal and polycrystalline TiO2 samples. Rutile has been more thoroughly studied, especially the most stable (110) face. In general, the rutile data shows predominant assignments to LO modes above 800 cm-1 and to SO modes about 760 cm-1. Ocana31,32 and Busca33 have shown that rutile powder phonon
Warren and McQuillan TABLE 1: Longitudinal Optical (LO) and Surface Optical (SO) Phonon Modes in 900-600 cm-1 Region from HREELS and IR Spectroscopy of TiO2 Materials sample
technique
single crystal IR oxide film (100)
IR HREELS
(110) (110) (110) (110) powder powder powder
EELS HREELS HREELS IR IR IR IR
(001) HREELS (100) (001) HREELS (100) single crystal IR pol. reflect. powder
IR
wavenumber/cm-1 assignments reference rutile 806 811 828, 809 812,842, 766 765 762 755 757 630, 655 720 630, 655, 675, 720, 620 anatase 736 784 736 784 755 876 800, 650
LO (Eu) LO (A2u) LO LO SO SO LO (Eu & A2u) SO SO
SO (A2u) SO (A2u) SO (A2u) SO (A2u) LO (A2u) LO (Eu)
25 26 27 28 29 19 30 31 32 33 34 35 36 33
frequencies are modified by surface morphology. The few anatase studies show both LO and SO modes in the 900-650 cm-1 region. In the P25 anatase/rutile mixture, it is likely that LO and SO modes will be observed from both phases. Recently, it has also been shown in HREELS studies that metal and metal oxide overlayers on rutile (110) strongly attenuate surface phonon signals because of electromagnetic field screening effects.37 The structure of metal oxide surfaces are influenced by the presence of adsorbed species37 and this can impact upon observed phonon frequencies.38 Reflectivity studies of rutile single crystals39 show that LO phonon modes around 800 cm-1 couple with the electron plasmon mode. The 900-600 cm-1 region therefore appears to offer a potentially rich source of information regarding the behavior of TiO2 surfaces during photocatalysis. Using ATR-IR spectroscopy, we have investigated the influence of adsorbed water and UV light on the infrared spectrum of Degussa P25 TiO2 under conditions approaching those of real photocatalytic systems. In some previous IR experiments on TiO2 photocatalytic materials, the use of windows such as CaF2 or BaF29 has limited the low-frequency cutoff to the 1100-900 cm-1 region. However, in attenuated total reflection infrared (ATR-IR) spectroscopy, the use of ZnSe and diamond/ZnSe prisms allows IR investigations of thin films down to 600 cm-1, which includes the region where bulk and surface phonons have been reported.19,25-36 In the present work, changes in the IR spectrum of P25 thin films were observed upon exposure to liquid water (H2O and D2O). Further investigation of these spectral changes via dehydration of thin films under nitrogen of different humidities led to the observation of spectroscopic features that have been assigned to phonons. When the effect of UV irradiation on these features was investigated, a broad IR absorption due to transitions of electrons in shallow traps was detected. This initial work is currently being extended to further test the preliminary assignment of the spectral features presented in this paper. Materials and Methods Degussa P25 TiO2, containing about 80% anatase and 20% rutile with surface area23 of 51 m2 g-1 and consisting of 2530 nm particles,40 was used in the photocatalysis experiments. Anatase (Aldrich, 99.9%) and rutile (Aldrich, 99.9%) powders
Influence of Adsorbed Water
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Figure 1. Schematic diagram of ATR-IR diamond-faced ZnSe prism and glass flow cell for UV irradiation of photocatalyst thin films.
were used to record IR spectra of these phases. Suspensions of TiO2 were prepared with deionized water (Millipore, Milli-Q, resistivity 18 MΩ cm). Thin films were prepared by depositing 7 µL of a 0.05 mol L-1 P25 TiO2 aqueous suspension on the ATR prism and then drying under a water pump vacuum for 20 min. A film thickness of ∼0.5 µm was determined by atomic force microscopy. Infrared spectra of the films, from evidence in the ∼3400 and ∼1640 cm-1 regions, show a low water content which is similar to that of the original P25 powder under ambient conditions (22% humidity). Nitrogen (oxygen free, BOC) was used to control the humidity of P25 thin films. The humidity of nitrogen flowed over the thin films was varied by mixing dry nitrogen and water-saturated nitrogen flows. The resultant humidity was measured to (1% using a Vaisala HMP45A humidity probe. Infrared spectra were recorded using Bio-Rad Digilab FTS60 and Digilab FTS4000 spectrometers. The resolution was 4 cm-1 and spectra were averaged over 64 scans. Irradiation of samples with UV light as shown in Figure 1 was carried out using a 365-nm “black light” source (UVP Inc, 4 W) giving about 700 µW cm-2 at the sample. A DuraSamplIR 3-mm diameter diamond-faced 3 reflection ZnSe prism (ASI SensIR Technologies) was used in the ATR-IR measurements. This accessory has a chemically inert diamond layer surface which isolates from the TiO2 (Eg ) 3.2 eV) any potential influence of the ZnSe (Eg ) 2.7 eV) band-gap excitation. A glass flow cell made in this department was attached to the ATR prism surface via an O-ring and used to apply either a vacuum (10-2 Torr) or a flow of gas (ambient pressure) to the prism surface. Prior to preparation of the TiO2 thin films, the ATR prism surfaces were cleaned by polishing with 0.015 µm γ-alumina (BDH, polishing grade) and rinsed with Milli-Q water. Results and Discussion Influence of Bulk Water on P25 TiO2 Infrared Spectrum. Degussa P25 has been used for many studies of photocatalysis but its infrared spectrum has received little attention. The ATRIR spectrum in the 1060-670 cm-1 region of a thin film, prepared as previously described in this paper, is shown as spectrum c in Figure 2. The spectrum shows sharply increasing absorption below ∼1000 cm-1 with the only details being two broad shoulders around 820 and 750 cm-1. These shoulders in the generally increasing absorption to lower wavenumbers are found in the TiO2 spectral region that is known to contain phonon modes.35 Figure 3 shows the corresponding ATR-IR spectra for anatase, rutile, and P25 powders. Comparison of the spectra in Figure 3 indicates that the absorptions around 820 and 750 cm-1 arise from the anatase present in the P25. The ATR-IR spectrum of water in the same region can be seen in Figure 2, spectrum a. This spectrum shows an increasing and featureless absorption below ∼1000 cm-1 because of the
Figure 2. ATR-IR spectra of (a) H2O, (b) P25 TiO2 thin film under H2O, (c) P25 TiO2 thin film, and (d) P25 TiO2 thin film under D2O. Background spectra for a and c are from bare ZnSe. Background spectra for b and d are from the P25 TiO2 thin film on ZnSe. For clarity, the spectra are offset on the absorbance scale.
Figure 3. ATR-IR spectra of anatase, rutile, and P25 TiO2 powders on ZnSe prism surface.
water libration modes. Spectrum b of Figure 2 shows the changes from spectrum c when water is flowed over the thin film. In this spectrum, there is also a generally similar rise in absorption below 1000 cm-1. However, spectrum b also shows two noticeable absorption losses which occur at similar wavenumbers to those of the shoulders in spectrum c of the thin film not immersed in water. The general increase in absorbance to lower wavenumbers in spectrum b can be largely attributed to the water librational mode absorptions. The absorption losses at about 820 and 750 cm-1 must be due to changes in the infrared spectrum of the TiO2 or of the surface when the TiO2 is immersed in water. The surface of TiO2 is terminated with hydroxyl groups which have clearly resolved stretching mode absorptions when the surface is fully dehydrated.2,22,41 The corresponding TiO2 hydroxyl bending modes are as yet unreported but may occur in the spectral region of present interest. Such hydroxyl bending mode absorptions are shifted to much lower frequencies by exchange with D2O. Spectrum d is that corresponding to b using D2O in place of H2O. This spectrum shows two clear absorption losses at about 830 and 750 cm-1 which occur at similar wavenumbers to those shown less distinctly in spectrum b. The pronounced librational absorptions of the water spectrum a are not found in spectrum d because the D2O librational mode absorptions occur at lower wavenumbers. Thus, the presence of a bulk water phase on the TiO2
19376 J. Phys. Chem. B, Vol. 108, No. 50, 2004
Figure 4. Changes in the ATR-IR spectrum of a P25 TiO2 thin film resulting from (a) exposure of the film to a water-saturated nitrogen flow and (b) reduction of the gas humidity to 0.1%. Background spectrum for a is from the P25 TiO2 thin film before exposure and for b from film after exposure to water-saturated nitrogen gas flow.
results in losses in absorption of what appear to be phonon modes. The exact assignment of these phonon modes at this stage is not possible but it is clear that the absorbance losses are comparable in intensity to those of the absorptions shown by the TiO2 P25 sample in the same spectral range (Figure 2). Influence of Adsorbed Water on P25 Phonon Absorptions. Spectroscopic investigations of TiO2 surfaces are mainly carried out on single crystals and in the absence of adsorbed water. Variation in the amount of molecularly adsorbed water may be achieved by changing the humidity of an inert gas at atmospheric pressure. These are conditions which resemble those under which photocatalysis may be observed in practical situations. Figure 4 spectrum a shows the difference spectrum arising from increasing the humidity to 100% in a nitrogen flow from that of the film under ambient conditions (22% humidity). An increase in adsorbed water is shown by the peak at 1646 cm-1 which is characteristic of the bending mode of liquid water. An additional component of the liquid water spectrum is the increasing absorption below 1000 cm-1 because of water librations (see Figure 2 spectrum a). There are pronounced absorption losses in Figure 4 spectrum a below about 900 cm-1 with peaks at 828, 745, and 685 cm-1. Figure 4 spectrum b shows the corresponding spectral change caused by changing the nitrogen humidity from 100% to that of dry nitrogen (0.1%). The reduction in the amount of adsorbed water is seen in the absorption losses at 1646 cm-1 and below ∼1000 cm-1. There are also three distinct absorption gain peaks at 828, 745, and 685 cm-1 superimposed on the broad librational mode absorption loss. The behavior and wavenumbers of these peaks strongly suggest that they have the same origin as the spectral loss features seen previously (spectra b and d of Figure 2) when water was flowed over a P25 thin film. There are also minor spectral features at 1201, ∼1130, and 1065 cm-1 which may be due to phonon combination and overtone bands.27 We have carried out the corresponding water adsorption/ desorption experiments using nitrogen humidified with D2O instead of H2O and the spectral results are shown in Figure 5. The spectral features associated with water are shifted to lower wavenumber with the bending mode now at 1210 cm-1 and the influence of the broad librational absorption is shifted almost outside the spectral range of interest. As with H2O, there are three characteristic peaks below 900 cm-1 but they are now seen at 836, 770, and 700 cm-1. The absence of a pronounced
Warren and McQuillan
Figure 5. Changes in the ATR-IR spectrum of a P25 TiO2 thin film resulting from (a) exposure of the film to a D2O-saturated nitrogen flow and (b) reduction of the gas humidity to 0.1%. Background spectrum for a is from the P25 TiO2 thin film before exposure and for b from film after exposure to the D2O-saturated gas flow.
Figure 6. Changes in the ATR-IR spectrum induced by 3 min UV irradiation of a P25 TiO2 thin film under a pressure of 10-2 Torr. Background spectrum is from the P25 TiO2 thin film dried under 10-2 Torr before UV irradiation.
shift in the peaks to lower wavenumbers indicates that they do not arise from bending modes of TiO2 surface hydroxyl groups. There are small peak shifts to higher wavenumbers compared with the H2O data. Furthermore, the shifts are not uniform which may be due to the nature of polycrystalline materials and the manner in which surface water influences surface phonons associated with different crystal faces. The presence of adsorbed water must therefore play an important role in the activity of the modes giving rise to the observed three peaks. The evidence considered so far supports the assignment of these spectral features to surface phonons. Such features have been observed in HREELS studies of water adsorption on rutile (110)19 although their behavior when the amount of adsorbed water was varied was not specifically discussed. Several HREELS reports30,42,43 attribute attenuation in TiO2 (110) surface phonon peaks from multilayer coverage of metals and metal oxides to screening of the incident electron beam field. A similar attenuation mechanism may therefore apply to the present IR absorption measurements when adsorbed water is present on the TiO2 surface. UV-Induced Infrared Absorption of a P25 TiO2 Film. When a P25 thin film was dried at 10-2 Torr pressure and then irradiated by UV light, the resultant absorption difference spectrum given in Figure 6 was obtained. The baseline absorbance rises from 1900 cm-1 toward lower wavenumbers and
Influence of Adsorbed Water peaks at ∼880 cm-1. There is then a sharp absorbance drop with a minimum at ∼750 cm-1, a shoulder at ∼810 cm-1, and an absorbance increase to the low wavenumber cutoff. The absorbance of the spectrum reached a maximum within 3 min of the start of UV irradiation. The spectrum shows some noise at about ∼1850 cm-1 because of the absorption edge of the diamond internal reflection element. A broad UV-induced IR absorption that shows an intensity increase with wavelength has been observed by several groups studying TiO2 particle films.2,8,9 Presently, it is considered that there are two general sources of the phenomenon: intraband transitions of CB electrons8 and excitation of shallowly trapped electrons to the CB.9,10,11 We will consider each of these possible mechanisms in turn. It is well known that free carriers in the conduction band behave as an electron plasma. This phenomenon manifests itself in reflectance spectra of nonpolar semiconductors as a broad IR absorption having the following intensity/wavelength relationship44,45
R ) Aλp where R is the absorption coefficient, λ is the wavelength, A is a proportionality constant, and p is the scattering constant. The value of p depends on the interaction of the plasmon with acoustic phonons, optical phonons, or ionized impurities. The absorption reaches a broad maximum at what is described as a plasma frequency. The frequency at the reflectivity minimum (absorption maximum) is dependent upon the number of charge carriers present in the sample. Dipole oscillations in polar semiconductors (e.g., TiO2) result in both longitudinal and transverse phonons. Because of the longitudinal nature of the plasmon, it can couple strongly with LO phonons and produce a new plasmon-phonon coupled mode. Baumard and Gervais39 observed this in rutile (110) single crystals and used a dielectric function model to decouple the observed LO mode into pure phonon and pure plasmon features. By varying the number of defects in the crystal, they controlled the number of charge carriers and observed the effect of this variation on the coupled mode. They found that relatively small changes in the number of carriers produced significant changes in the rutile spectrum, both in the frequency of the observed LO mode and its intensity. The spectrum in Figure 6 shows a change in absorbance over the first three minutes of UV irradiation, yet the wavenumber of the peak at 880 cm-1 shows no variation. We have also observed that there is also no change in the peak wavenumber when the UV irradiation is stopped and the absorption decays with time. Recently, Turner et al.46 used time-resolved terahertz spectroscopy (TRTS) to study the transient photoconductivity of dye-sensitized P25 TiO2 particle films and extracted a value of ∼105 cm-1 for the plasma frequency at 77 K. This would seem to rule out the 880 cm-1 feature as being due to the absorptions of charge carriers in the conduction band. Yamakata et al.10 used time-resolved IR spectroscopy to study the kinetics of photogenerated electrons in a P25 TiO2 film. They observed an increasing absorption from 3000 to 900 cm-1. The transient absorption was attributed to photogenerated electrons trapped in shallow mid-gap states within picoseconds of irradiation. They discussed two possible origins for the absorption arising from these electrons trapped on coordinatively unsaturated Ti atoms (Figure 7): first (Figure 7a), as a result of intraband transitions of electrons within the conduction band (CB); second (Figure 7c), because of direct optical transition from the trap state to the CB. While both mechanisms would give very similar broad IR absorptions, the latter mechanism
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Figure 7. Processes involving electrons after optical promotion into CB (after Yamakata et al.).10 (a) Intraband excitation. (b) Electrons trapped in shallow traps. (c) Excitation of shallowly trapped electrons into CB.
would be characterized by an absorption cutoff associated with the energy gap between the trap state and the CB. They suggested that this cutoff may occur below their 900 cm-1 experimental limit. The probability of such transitions decreases with increasing energy giving rise to the absorbance decrease to higher wavenumbers. This mechanism and proposed spectral identification appears to fit our observations. If this assignment is confirmed, the 880 cm-1 peak would correspond to a shallow trap at ∼0.1 eV below the conduction band which is comparable with previous literature values9-11,47 for TiO2 shallow traps. As pointed out by Yamakata et al.,10 free hole transitions in the valence band and hole transitions from mid-gap states are unlikely because of the lack of structure in the observed spectra and the much larger energy gap reported between the valence band and the hole trap state. The transient absorptions observed by Yoshihara et al.8 and attributed to holes consisted of absorptions peaking at 1200 and 520 nm showing no significant mid-IR absorption. Influence of UV Irradiation and Loss of Adsorbed Water on P25 TiO2 Spectrum. Having examined the separate influences of water desorption and UV irradiation on the P25 TiO2 infrared spectrum, it was of interest to investigate a system upon which both influences impinge. A P25 thin film was irradiated under air at ambient humidity for 15 min before being exposed to a dry nitrogen flow. Figure 8 shows the spectral changes over the subsequent 15 min until the spectrum became time independent. This is a much longer temporal effect than seen in the Figure 6 spectrum which reached maximum absorbance within 3 min of simultaneous initiation of spectral data collection and of UV irradiation. The Figure 8 spectrum shows a progressive baseline change with time which rises to a peak at ∼880 cm-1 with an adjacent absorption drop to ∼800 cm-1 and an absorption increase at 750 cm-1. There is also a progressive loss of adsorbed water observed at ∼1640 cm-1 and the appearance of some minor peaks at ∼1450 cm-1. The growth of these minor peaks appears to correlate with the growth of the 750 cm-1 peak and are likely to be due to phonon overtone or combination bands.27 The emergence of the peak at 750 cm-1 in Figure 8 correlates with the water removal, as previously seen in Figure 4. The broad asymmetric peak at 880 cm-1 arises from UV irradiation as seen in Figure 6. Thus, the final Figure 8 spectrum is in effect a combination of Figure 4, spectrum b, and Figure 6. The absorbance of the 880 cm-1 peak in Figure 6 is much greater than that reached in the Figure 8 spectra. This is likely to be due to the vacuum conditions leaving less adsorbed water
19378 J. Phys. Chem. B, Vol. 108, No. 50, 2004
Figure 8. Changes in the ATR-IR spectrum of a P25 TiO2 thin film over 1, 3, 7, 10, and 15 min resulting from flowed dry nitrogen dehydration while under constant UV irradiation. The spectra show the changes from the start of the dry gas flow until no further spectral change was observed. Background spectrum for all spectra shown was from the P25 TiO2 thin film which had been under constant UV irradiation for 15 min before start of gas flow. For clarity, the spectra are offset on the absorbance scale.
on the TiO2 surface than under the dry nitrogen flow used to obtain the spectra in Figure 8. The increase in the intensity of the 880 cm-1 peak indicates that removal of water increases the number of electrons being excited from the shallow traps but the possible role of adsorbed water in this phenomenon needs further discussion. Weng et al.,12 in a study of back-electron-transfer kinetics in dyesensitized nanoparticulate TiO2, suggest a model of electron injection proposing that (1) injected electrons randomly populate a small region near the adsorbate molecule, (2) spatial diffusion between trap sites is negligible on the nanosecond time scale, and (3) injected electrons relax from shallow traps to deep trap states. It is well known that deep traps act as recombination centers in semiconductor materials.15-18,44,45,48 Hoffmann and co-workers7 have proposed that initially trapped electrons occupy an excited state trap and then relax into what is effectively a recombination center where electron-hole pairs recombine. They suggested that the process of relaxation is mediated by the presence of water. Thus, water removal would inhibit the relaxation of shallow trapped electrons, allowing more excitations from the shallow traps into the CB. This model appears to contradict the findings of Yamakata et al.9 who in their time-resolved IR studies found that for times up to 1 s the presence of water enhances the transient signal attributed to excitation of shallow trapped electrons. However, given the difference in the time scale of the work reported here and that of the Hoffmann group2,6,7 compared to that of Yamakata et al.,9 there may be two routes for electron-hole recombination. Electrons in shallow traps ∼0.1 eV below the CB may recombine either directly with holes in a fast process or via a much slower process that involves the presence of adsorbed water. The fast process is hindered by the presence of adsorbed water while the slower process is facilitated by adsorbed water. Given the hydration-dependent lifetime (seconds to hours) of the broad IR signal reported by Hoffmann, it would seem that the slower water-mediated process is dominant for electronhole recombination. Without considering a comprehensive mechanistic scheme, an alternative scenario resulting in the accumulation of electrons would be an increased trapping of holes as water is removed in
Warren and McQuillan such a way as to prevent electron-hole recombination. The surface of the photocatalyst sample consists of TiOH groups and adsorbed water linked by hydrogen bonds. Removal of water is likely to change the energetics of the trapping process and influence the trapping efficiency. Observation relating to any such influence of adsorbed water is more likely to be seen using UV-vis spectroscopic techniques rather than in the infrared. In several previous ATR-IR studies during UV irradiation of TiO2 photocatalytic films,3-5 a broad peak in the 1000-800 cm-1 region has been observed with an asymmetric tail extending to higher wavenumbers. There is a striking similarity of the present results with those of Nakamura et al.4 The wavenumber of the broad peak in these studies was different with different photocatalysts and thus appears to be related to the phase composition of the TiO2 photocatalyst material. Our observations in the present study attribute this absorption to the optical excitation of electrons present in shallow traps. The variation in observed peak wavenumber in different studies appears to be a consequence of variations in the depth of these shallow traps on different materials.8 We have confirmed this view in preliminary ATR-IR work on different TiO2 photocatalyst materials and this will be reported elsewhere. Also, the IR absorption, which we attribute to shallowly trapped electrons, is observed from UV-irradiated TiO2 films having adsorbed intermediate species and fully immersed in aqueous media.3-5 This appears to be additional evidence that species such as adsorbed water and other ligands play a key role in determining the fate of photogenerated charge carriers at TiO2 surfaces. Ligand species adsorbing from aqueous solution displace adsorbed water from TiO2 surfaces and this process may deactivate some trapping states associated with adsorbed water. Alternatively, adsorbates may act as more efficient hole scavengers than water, preventing electron-hole recombination. Evidence of the nature of the shallow traps may lie within the results regarding the TiO2 adsorbate-sensitive phonon modes reported in the earlier part of this work. An IR absorption attributed to a surface optical phonon mode was observed at 828 cm-1 which is energetically close to the shallow trap. The difference between the wavenumber of the SO phonon and that of the 880 cm-1 peak lies within the band tail of the CB. The proximity of the SO phonon and trap excitation energies suggests the possibility of coupling between these modes which would impact upon observed spectral features. Thus, the SO phonon modes which are sensitive to adsorbed water are likely to be involved in the trapping mechanism. This situation matches proposals by others that the shallowly trapped electrons are present as a 2D delocalized surface state on TiO2 particles.6,31 We are investigating further the nature of the relationship between trapped electrons and surface phonons and the roles which adsorbed water and ligands play in photocatalytic processes. Conclusions 1. ATR-IR spectroscopy of P25 TiO2 photocatalyst particle films in contact with liquid H2O show IR absorption decreases at ∼820 and 750 cm-1 when the bulk water is removed. These absorptions do not shift appreciably in the corresponding experiments with liquid D2O. Comparison of anatase, rutile, and P25 powder spectra show that these absorptions appear to arise from the anatase phase which predominates in P25 TiO2. 2. Reduction in the amount of H2O adsorbed on a P25 TiO2 particle film under nitrogen gas results in the growth of IR peaks at 828, 745, and 685 cm-1, some of which correspond to the peaks observed in the related experiments with liquid water.
Influence of Adsorbed Water These absorptions show large absorbance decreases when the amount of adsorbed water is increased by an increase in gas humidity. In the corresponding experiments with D2O, the peaks do not show the wavenumber shifts that would indicate bending modes of surface OH groups. On the basis of these observations and literature data, the IR absorptions have been assigned to TiO2 surface phonon modes having absorptions strongly influenced by water adsorption/desorption. 3. Irradiation at 365 nm of a P25 TiO2 particle film at 10-2 Torr pressure produces a broad IR absorption with absorbance increasing to lower wavenumber and peaking at 880 cm-1. This feature is ascribed to the IR-induced transition to the CB of electrons present in shallow traps ∼0.1 eV below the CB. 4. Concurrent use of a dry nitrogen flow to remove adsorbed water from a P25 TiO2 thin film and UV irradiation results in a broad IR absorption the growth of which appears to correlate with the removal of water from the surface. The absorption growth is much slower than that of a film dehydrated under 10-2 Torr vacuum prior to irradiation. These observations support previous reports that adsorbed water is involved in the mechanism of electron trapping. 5.ATR-IR spectroscopy has been used to show that TiO2 phonon modes are sensitive to the nature of adsorbed molecules. The approach has considerable potential to reveal new information about surface processes in photocatalysis. Acknowledgment. Scholarship and equipment support for this work came from Johnson Matthey (U.K.) Ltd and the University of Otago. We acknowledge valuable suggestions from referees, Michael Pepper of Cambridge University and David Cahen of the Wiezmann Institute. References and Notes (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (2) Szczepankiewicz, S. H.; Moss, J. A.; Hoffmann, M. R. J. Phys. Chem. B 2000, 104, 9842. (3) Ekstro¨m, N. G.; McQuillan, A. J. J. Phys. Chem. B 1999, 103, 10562. (4) Nakamura, R.; Imanishi, A.; Murakoshi, K.; Nakato, Y. J. Am. Chem. Soc. 2003, 125, 7443. (5) Nakamura, R.; Nakato, Y. J. Am. Chem. Soc. 2004, 126, 1290. (6) Szczepankiewicz, S. H.; Moss, J. A.; Hoffmann, M. R. J. Phys. Chem. B 2002, 106, 7654. (7) Szczepankiewicz, S. H.; Moss, J. A.; Hoffmann, M. R. J. Phys. Chem. B 2002, 106, 2922. (8) Yoshihara, T.; Katoh, R.; Furube, A.; Tamaki, Y.; Murai, M.; Hara, K.; Murata H.; Arakawa, H.; Tachiya, M. J. Phys. Chem. B 2004, 108, 3817. (9) Yamakata, A.; Ishibashi, T.; Onishi, H. J. Phys. Chem. B 2001, 105, 7258. (10) Yamakata, A.; Ishibashi, T.; Onishi, H. Chem. Phys. Lett. 2001, 333, 271.
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