Surface Phases of TiO2 Nanoparticles Studied by UV Raman

Apr 30, 2008 - Surface phases of TiO2 nanoparticles (30 ∼ 200 nm) were studied by UV Raman spectroscopy and FT-IR spectroscopy with CO and CO2 as ...
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J. Phys. Chem. C 2008, 112, 7710–7716

Surface Phases of TiO2 Nanoparticles Studied by UV Raman Spectroscopy and FT-IR Spectroscopy Weiguang Su, Jing Zhang, Zhaochi Feng, Tao Chen, Pinliang Ying, and Can Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China ReceiVed: December 17, 2007; ReVised Manuscript ReceiVed: February 26, 2008

Surface phases of TiO2 nanoparticles (30 ∼ 200 nm) were studied by UV Raman spectroscopy and FT-IR spectroscopy with CO and CO2 as probe molecules. UV Raman spectroscopy can differentiate the surface phase structure of TiO2 calcined at different temperatures. IR spectra of adsorbed CO and CO2 on TiO2 calcined at different temperatures are in good agreement with the results from UV Raman spectra. IR results evidently confirm that UV Raman spectroscopy is a surface-sensitive technique for TiO2. Both UV Raman and IR spectra indicate that the crystalline phase of TiO2 in the surface region is usually different from that in the bulk which is characterized by XRD. CO is weakly adsorbed on Ti4+ ions of anatase phase but is hardly adsorbed on those of rutile phase at room temperature. Adsorbed CO2 on anatase phase produces mainly bidentate carbonate, while on rutile phase produces mainly bicarbonate species. These results suggest that the surface Lewis acidity of anatase phase is stronger than that of rutile phase, and the concentration of cus Ti4+-O2- pairs on the surface of anatase phase is much higher than that on rutile phase; however, the basicity of surface OH groups of rutile phase is stronger than that of anatase phase. 1. Introduction Titania (TiO2) has been widely used as a catalytic material (either a catalyst or a catalyst support) for many catalytic reactions including nitrogen oxides reduction by hydrocarbons,1,2 CO hydrogenation and oxidation,3–5 and hydrocarbon selective oxidation.6,7 TiO2 is also an effective photocatalyst8–10 particularly for the photo-oxidation of organic pollutants.11–13 The most used crystalline phases of titania are the anatase and rutile phases.14 The two phases usually exhibit different behavior in catalytic reactions15 particularly in photocatalysis.16 It has been reported that the surface properties of anatase and rutile exhibit considerable differences. Rutile is characterized by a surface where the dissociation of adsorbed molecules is much easier than on anatase.17,18 For example, adsorption of methanol is dissociative on rutile, whereas it is coordinated on anatase.17 These essential differences in the surface chemistry of the two TiO2 phases result in their different catalytic properties. The crystalline phase in the surface region is usually different from that in the bulk for many metal oxides, such as ZrO219 and TiO2.20 It is well-known that catalytic performance of a catalyst largely depends on the surface properties because catalytic reactions take place on the surface.21 Therefore, it is very necessary to characterize the surface phases and then to correlate the surface phases with their catalytic properties. However, the surface phases are not easy to differentiate from the bulk phase because of the lack of suitable techniques that can detect the surface phases of a catalyst, particularly for the catalyst in nanoparticles. In our previous work, we studied the phase transition of zirconia (ZrO2) from tetragonal phase to monoclinic phase,19 titania (TiO2) from anatase phase to rutile phase20 by UV Raman spectroscopy, visible Raman spectroscopy, and XRD. The results from UV Raman spectroscopy are * To whom correspondence should be addressed. Phone: +86 411 84379070. Fax: +86 411 84694447. E-mail: [email protected]. Homepage: http://www.canli.dicp.ac.cn.

quite different from those of visible Raman spectroscopy and XRD. We proposed that this inconsistency may be because UV Raman spectroscopy provides the information mainly from the surface region, while visible Raman spectroscopy and XRD provide information mainly from the bulk. The surface properties of TiO2 were investigated by FT-IR spectroscopy with the adsorption of probe molecules,22–27 like CO and CO2.18,28–37 CO, a weak Lewis base, was usually chosen to test the Lewis acid sites, whereas CO2 was used to investigate the Lewis basic sites on metal oxides.31 CO interacts with the surface cationic centers and this interaction can change the C-O stretching frequency.38 Different surface species can be formed from the CO2 adsorption on the surface with different acid–base characters.30,35,39,40 The reaction with surface OH groups gives rise to the formation of bicarbonate species. The adsorption of CO2 on basic sites (coordinatively unsaturated oxygen anions) usually forms monodentate carbonate, whereas the adsorption of CO2 on an acid metal ion with its neighboring basic oxygen often produces surface bidentate carbonate species. The differences in the surroundings of Ti4+ and O2- on the various surface phases of TiO2 may lead to different adspecies for CO and CO2 adsorption. There have been many IR studies of CO and CO2 adsorption on TiO2.18,28–37 However, the “anatase” and “rutile” phases which were routinely examined by XRD actually refer to the bulk phases; it is difficult for XRD to detect the surface phases of TiO2 nanoparticles, particularly when TiO2 is in the transition state from anatase to rutile. Therefore, it is hard to correlate the adsorption of CO and CO2 with the surface phases of TiO2 because the surface phases are not really determined for most cases. In the present work, the surface phases of TiO2 calcined at different temperatures were first characterized by UV Raman spectroscopy, and then IR spectroscopy was used to study the adsorption of CO and CO2 on TiO2 whose surface phases were well-characterized. We found that the results from the IR spectra

10.1021/jp7118422 CCC: $40.75  2008 American Chemical Society Published on Web 04/30/2008

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of adsorbed CO and CO2 on TiO2 are in good accordance with the results from UV Raman spectroscopy but are different from those of XRD. The IR spectra of adsorbed CO and CO2 directly reflect the surface information of TiO2; therefore, the IR results further confirm that the information detected by UV Raman spectroscopy is mainly from the surface region of TiO2. Specifically, UV Raman spectroscopy can sensitively detect the surface phases of TiO2, and IR spectroscopy of adsorbed CO and CO2 can differentiate the adspecies on the different surface phases of TiO2. 2. Experimental Section 2.1. IR Spectroscopy. Two types of TiO2 powders were investigated: (i) TiO2 prepared by the precipitation method as described previously,20 and (ii) commercial TiO2 Degussa P25 (∼56 m2/g, anatase 80%, rutile 20%) denoted as TiO2 (P25). Both samples were pressed into self-supporting wafers (ca. 15 mg/cm2) and mounted inside an IR cell for FT-IR spectroscopy. IR experiments were then performed as follows: (1) Adsorption of CO and CO2 on TiO2 prepared by precipitation method. Prior to the adsorption measurements, the TiO2 samples were activated by a calcination at 723 K for 60 min and an evacuation at the same temperature, subsequently cooled to room temperature (ca. 300 K). (2) Low-temperature adsorption. The TiO2 pellets were mounted inside a variable temperature Oxford cryostat model OptistatDN-V (77 to 500 K) with two ZnSe windows. The TiO2 sample in the cell was heated to 373 K under vacuum for 4 h. The temperature of TiO2 sample was measured by a platinum resistance thermometer. After the heating, the sample was cooled to 80 K for 30 min with liquid nitrogen, and the gas dosing experiments were carried out. (3) Room temperature (RT) adsorption. The TiO2 (P25) sample was heated to 723 K in air for 60 min, then evacuated at 723 K for 120 min. After the heating, 15 Torr O2 was introduced into the cell as the sample was cooled to 423 K. After the sample temperature reached room temperature (ca. 300 K), the cell was evacuated for gas dosing. All infrared spectra were collected with a resolution of 4 cm-1 and 64 scans by a Fourier transform infrared spectrometer (Nicolet NEXUS 470) with an MCT detector. All of the spectra shown here are in the absorbance mode, and their backgrounds were recorded before admitting the adsorbed gas under corresponding experimental conditions of the spectra. 2.2. UV Raman Spectroscopy. UV Raman spectra were measured at room temperature with a Jobin-Yvon T64000 triplestage spectrograph with spectral resolution of 2 cm-1. The laser line at 325 nm of a He-Cd laser was used as an excitation source with an output of 25 mW. The power of laser at the sample was about 3.0 mW. 2.3. X-ray Powder Diffraction (XRD). Crystalline phases of TiO2 calcined at different temperatures were characterized by X-ray diffraction (XRD) using the packed powder method. XRD patterns were obtained on a Rigaku MiniFlex diffractometer with a Cu KR radiation source. Diffraction patterns were collected from 20° to 80° at a speed of 5°/min. 3. Results and Discussion 3.1. Phase Transformation of TiO2 Calcined at Different Temperatures. 3.1.1. XRD Patterns of TiO2 Calcined at Different Temperatures. Figure 1 shows the XRD patterns of TiO2 calcined at different temperatures. The “A” and “R” in the figure denote the anatase phase and rutile phase, respectively. When the sample was calcined at 500 °C, sharp peaks at 2θ )

Figure 1. XRD patterns of TiO2 calcined at different temperatures. The “A” and “R” denote the anatase phase and rutile phase, respectively. The symbol “90 %R” represents that the weight fraction of rutile phase in the sample is 90% based on XRD pattern.

Figure 2. UV Raman spectra of TiO2 calcined at different temperatures and the mechanical mixture with 1:1, 1:3, and 1:10 ratios of pure anatase phase to pure rutile phase with the excitation line at 325 nm. The “A” and “R” denote the anatase and rutile phases, respectively. The symbol “84 %R” represents that the weight fraction of rutile phase in the sample is 84% based on UV Raman spectrum.

25.6°, 38.1°, 48.3°, 54.2°, and 55.3° are observed. These peaks represent the indices of (101), (004), (200), (105), and (211) planes of anatase phase, respectively.41 This result indicates that the phase of the sample is in anatase after calcination at 500 °C. When the sample was calcined at 680 °C, strong peaks are observed at 2θ ) 27.7°, 36.3°, 41.5°, and 54.6°, which correspond to the indices of (110), (101), (111), and (211) planes of rutile phase,41 and the diffraction peaks of anatase phase greatly diminish in intensity. The symbol “90 %R” represents that the weight fraction of rutile phase in the sample, WR, is about 90% estimated according to the following formula:42

WR )

1 1 + 0.884 [ (Aana ⁄ Arut)]

where Aana and Arut represent the X-ray integrated intensities of anatase (101) and rutile (110) diffraction peaks, respectively. This result shows that most anatase phase has changed into rutile phase after calcination at 680 °C. The diffraction peaks assigned to anatase phase disappear at 750 °C, indicating that the anatase phase completely changes into rutile phase, namely, WR is 100%. The diffraction peaks of rutile phase become quite strong and sharp after the sample was calcined at 800 °C, owing to the high crystallinity of rutile phase, WR is also 100%. 3.1.2. UV Raman Spectra of TiO2 Calcined at Different Temperatures. The laser line at 325 nm was selected as the excitation source for the UV Raman spectra. Figure 2 shows the UV Raman spectra of TiO2 calcined at different temperatures. When the sample was calcined at 500 °C, strong bands at 143, 196, 395, 516, and 639 cm-1 are observed. All of these

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Figure 3. IR spectra of CO2 adsorbed at RT on TiO2 calcined at different temperatures. The “A:R” represents the mechanical mixture with different ratios of pure anatase phase to pure rutile phase.

bands are assigned to the anatase phase, and they can be attributed to the five Raman-active modes of anatse phase with the symmetries of Eg, Eg, B1g, A1g, and Eg, respectively.43 This result is well accordant with that of XRD (Figure 1). After calcination at 680 °C, the UV Raman spectrum is essentially the same as that of the sample calcined at 500 °C, which suggests that the sample is still in anatase; however, according to the XRD pattern, most of the anatase phase has changed into rutile as shown in Figure 1. When the sample was calcined at 750 °C, a new band at 613 cm-1 and two weak bands at 236 and 446 cm-1 appear, which are due to the A1g, two-phonon scattering and Eg modes of rutile phase44 respectively. At the same time, the intensities of the bands due to anatase phase greatly decrease, but they are still strong in the UV Raman spectrum. UV Raman spectrum of the sample calcined at 750 °C indicates that the sample is in the mixed phases of anatase and rutile, and the content of rutile is about 84%,20 while the XRD pattern shows that the same sample is totally in rutile. The characteristic bands due to anatase phase disappear, and the sample is totally in rutile phase after calcination at 800 °C according to the UV Raman spectrum (Figure 2). Obviously, there are distinct differences between the results from the UV Raman spectra and the XRD patterns, which suggest that the two techniques detect the different regions of TiO2. 3.2. IR Spectra of CO2 and CO Adsorbed on TiO2 Calcined at Different Temperatures. 3.2.1. IR Spectra of CO2 Adsorbed at RT on TiO2 Calcined at Different Temperatures. In order to investigate the surface phases of TiO2, we choose CO2 and CO as probe molecules for IR spectroscopy. Figure 3 displays the IR spectra of adsorbed CO2 at RT on TiO2 calcined at different temperatures. A number of IR bands in the range of 1800 -1000 cm-1 are observed when CO2 was adsorbed on TiO2. These bands are mainly due to various types of carbonatelike (CO3) and bicarbonate (HCO3) species.29–32,34–37,39 The intensities of all bands are reduced somewhat under vacuum (spectra not shown), indicating that some of these species desorb in vacuum. By comparing TiO2 calcined at 500 °C and TiO2 calcined at 680 °C, the XRD patterns indicate that the phase compositions of the two TiO2 samples are very different, while the IR spectra of adsorbed CO2 on the two TiO2 samples are almost the same. This suggests that the surface properties or surface phases of the two TiO2 samples are almost the same. In the cases of TiO2 calcined at 750 °C and TiO2 calcined at 800 °C, the XRD patterns show that the two TiO2 samples are both in rutile; however, the results from the IR spectra of adsorbed CO2 on them are quite different. It is shown that the surface chemistry of the two rutile phases is different in fact. Anyway, the results

Su et al. from the IR spectra of adsorbed CO2 are quite different from those of XRD patterns; however, the results between IR spectroscopy and UV Raman spectroscopy agree well with each other as shown in Figure 2. As we all know, IR spectra of adsorbed probe molecules supply information mainly from the surface while XRD gives information mainly from the bulk. Accordingly, above results confirm that UV Raman spectroscopy is also a surface-sensitive technique and can provide information mainly from the surface region of TiO2, and the surface phase is quite different from the bulk phase during the phase transformation of TiO2. From above discussion, the bulk and surface phase compositions of TiO2 can be well-characterized by XRD and UV Raman spectroscopy, respectively. For brevity, according to the results from Figure 1 and Figure 2, the TiO2 samples calcined at 500, 680, 750, and 800 °C are identified as TiO2 (anatase), TiO2 (B-90%R, S-A), TiO2 (B-R, S-84%R), and TiO2 (rutile), respectively. The “anatase” indicates that the surface and bulk phases of TiO2 are both in anatase, namely, pure anatase phase, so does the “rutile”. In the symbol of TiO2 (B-mR, S-nR), the “B” and “S” respectively mean the bulk and surface phases, and the m and n denote the rutile content in the samples based on XRD patterns and UV Raman spectra. In order to compare, pure anatase phase and pure rutile phase of TiO2, namely, TiO2 (anatase) and TiO2 (rutile) samples, were mechanically mixed at a given weight ratio and ground carefully to mix sufficiently. In Figure 3, in the case of anatase (TiO2 calcined at 500 °C), the IR bands at 1589 and 1315 cm-1 are attributed to the Vas (CO3) and Vs (CO3) modes of surface bidentate carbonate species, respectively.29,39 The band at 1055 cm-1 is also assigned to bidentate carbonate.30 The two bands, located at 1434 and 1222 cm-1, can be attributed to the bicarbonate (HCO3) species, whose IR modes are Vs (CO3) and δ (HO), respectively.30,39 A band at 3609 cm-1 is also observed (spectra not shown), which is assigned to the V (OH) mode of HCO3.35,36,39 The band at 1377 cm-1 is ascribed to the Vs (CO3) mode of monodentate carbonate species29,36 on the surface. This band vanishes quickly under evacuation at RT. Four bands at 1706, 1606, 1416, and 1220 cm-1 are observed after CO2 adsorption on rutile (TiO2 calcined at 800 °C). The band at 1706 cm-1 is attributed to the Vas (CO3) mode of bridged carbonate.29,39 The band at 1606 cm-1 is also assigned to bidentate carbonate. The other two bands at 1416 and 1220 cm-1 are ascribed to the bicarbonate (HCO3) species. Table 1 summarizes the assignments of infrared bands of surface species adsorbed on the surface of anatase and rutile phases. By comparing the IR spectrum of adsorbed CO2 on anatase with that on rutile, several differences can be found: (i) The bands at 1377, 1315, and 1055 cm-1 which are observed on anatase are not detected in the case of rutile; (ii) The intensities of the bands assigned to bidentate carbonate on TiO2 greatly diminish from anatase to rutile, where, on the other hand, the amount of surface bicarbonate (HCO3) species largely increases; and (iii) The frequencies of some bands of the adspecies show slight differences between anatase and rutile. For example, a band at 1434 cm-1 is observed for anatase, but it shifts to 1416 cm-1 for rutile, (iv) Additionally, the band at 1589 cm-1 is the strongest one when CO2 is adsorbed on anatase, while in the case of rutile, the band at 1416 cm-1 is the strongest one. From these results, anatase and rutile can be clearly distinguished from each other on the basis of the IR spectra of adsorbed CO2, suggesting that the surface acid–base properties or more specifically the surface structures of anatase and rutile are very different.

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TABLE 1: Assignments of Infrared Bands of Surface Species Formed from the Adsorption of CO and CO2 on the Surface of Anatase and Rutile Phases TiO2 anatase

rutile

probe IR bands molecules temperature (cm-1) CO

RT

CO

80 K

CO2

RT

CO

80 K

CO2

RT

2206 2188 2177 2151 1589 1315 1055 1434 1222 1377 1670 1245 2170 2157 1606 1416 1220 1706

species Ti4+-CO Ti4+-CO Ti4+-CO OH···CO bidentate carbonate bidentate carbonate bidentate carbonate bicarbonate bicarbonate monodentate carbonate CO2- carboxylate CO2- carboxylate Ti4+-CO OH···CO bidentate carbonate bicarbonate bicarbonate bridged carbonate

IR spectrum of adsorbed CO2 on TiO2 (B-R, S-84%R) shows four bands: 1596, 1427, 1317, and 1221 cm-1. The band at 1427 cm-1 is the strongest one while the band at 1596 cm-1 is rather weak; this phenomenon is very similar to CO2 adsorption on the rutile phase. However, a broad band at 1317 cm-1 ascribed to CO2 adsorbed on the anatase suggests that some anatase phase also exists on the surface of the sample. The IR spectrum of adsorbed CO2 suggests that the surface of TiO2 (B-R, S-84%R) is in the mixed phases of anatase and rutile, and rutile phase is predominant in the surface phases; this result is strongly supported by the UV Raman spectrum of the same sample as shown in Figure 2. Figure 3 also displays the IR spectra of adsorbed CO2 on the mechanical mixture with 1:1, 1:3, and 1:10 ratios of anatase phase to rutile phase. The relative intensity ratios of the bands at 1596 and 1427 cm-1 gradually decrease with increasing rutile content. At the same time, it is evident that IR spectra of CO2 adsorbed on TiO2 (B-R, S-84%R) and the mechanical mixture sample (A:R ) 1:10) are almost the same. Because CO2 is considered as an acidic probe, it adsorbs selectively on basic sites of the metal oxide surfaces. On anatase, adsorbed CO2 gives abundant bidentate carbonate, while on rutile, CO2 adsorption produces mostly bicarbonate species. Bachiller-Baeza et al.45 and Bell et al.46 have found that the crystallographic structure of ZrO2 polymorphs is the main factor determining the nature and density of the surface sites involved in the adsorption of CO2. According to the literature,40 in general, the surface bidentate carbonate complexes are formed involving the interaction with Lewis acid–base pairs sites (cus Ti4+-O2- centers), and the bicarbonate species are produced by a reaction of CO2 with basic OH groups. Hence, the different type and number of adspecies may be due to different kinds of Lewis acid–base sites and OH groups exposed on the surface of anatase and rutile. The slight variations in the frequencies of IR bands of the same adspecies on anatase and rutile can be attributed to some differences in the surface structures of cations and anions exposed on the surface of the two phases. Thermal activation in vacuum of metal oxides at high temperatures (723 K) may cause the creation of surface oxygen vacancies. The concentration of surface oxygen vacancies on rutile is much less than that on anatase because of higher crystallinity and smaller Brunauer–Emmett–Teller surface area

Figure 4. IR spectra of CO adsorbed at RT on TiO2 with different phase compositions. The “anatase” and “rutile” denote pure anatase phase and pure rutile phase, respectively. The “A:R” represents the mechanical mixture with different ratios of pure anatase phase to pure rutile phase. In the symbol of TiO2 (B-mR, S-nR), the “B” and “S” respectively mean the bulk and surface phases, and the m and n denote the rutile content in the samples based on XRD patterns and UV Raman spectra.

(SBET) of rutile. Thus, the higher concentration of surface bidentate carbonate on anatase can be ascribed to the much higher concentration of cus Ti4+-O2- pairs on the surface of anatase than that on rutile. The stronger the basicity of the surface OH groups, the greater the reactivity with CO2 to produce bicarbonate species. The larger amount of bicarbonate species on rutile indicates that the basicity of the surface OH groups of rutile is stronger than that of anatase. This result is supported by the work of Primet et al.,47 who also pointed out that OH groups of anatase are more protonic than those of rutile. 3.2.2. IR Spectra of CO Adsorbed at RT on TiO2 with Different Phase Compositions. Figure 4 shows the IR spectra of adsorbed CO at RT on TiO2 with different phase compositions. On the surface of anatase, CO adsorption gives a strong band at 2188 cm-1 attributed to adsorbed CO on Ti4+ cations at terrace sites exposed on well-dehydrated samples and a shoulder at 2206 cm-1 assigned to Ti4+ cations at step sites. 28,33 However, adsorbed CO on the surface of rutile is hardly observed in IR spectrum at room temperature. Obviously, the adsorption behavior of CO on anatase and rutile is largely different, suggesting that the surfaces of the two phases are chemically different. Thus, the band at 2188 cm-1 of adsorbed CO can be tentatively used to determine the surface anatase phase of TiO2. Figure 4 also displays the IR spectra of adsorbed CO on the mechanical mixture with different ratios of anatase phase to rutile phase. With increasing rutile content, the intensity of the band at 2188 cm-1 due to CO adsorbed on anatase nearly linearly decreases. An evident band at 2188 cm-1 is observed when CO is adsorbed on TiO2 (B-90%R, S-A), which is very similar to the IR spectrum of CO adsorbed on the pure anatase phase. It clearly suggests that the surface phase of TiO2 (B-90%R, S-A) is mainly the anatase phase. This result is in good agreement with that of the UV Raman spectrum (Figure 2). Furthermore, for the TiO2 (B-R, S-84%R) sample, again, a weak band at 2188 cm-1 assigned to CO adsorbed on anatase is observed, suggesting that the TiO2 sample still contains a small portion of anatase phase on the surface. It is obvious that CO adsorption can also probe the surface phases of TiO2, and the results from IR spectra and UV Raman spectra are in good agreement with each other. Once more, on the basis of the results from the IR spectra of adsorbed CO, it is shown that UV Raman spectroscopy is a surface-sensitive technique for TiO2. The bulk phase and surface phase are not always the same in the phase transformation of TiO2. Zhang et al.48 proposed that the activation energy for

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Figure 5. IR spectra of CO adsorbed at 80 K on TiO2 with different phase compositions. The “anatase” and “rutile” denote pure anatase phase and pure rutile phase, respectively. The “A:R” represents the mechanical mixture with different ratios of pure anatase phase to pure rutile phase. In the symbol of TiO2 (B-mR, S-nR), the “B” and “S” respectively mean the bulk and surface phases, and the m and n denote the rutile content in the samples based on XRD patterns and UV Raman spectra.

surface nucleation is expected to be higher than that for interface nucleation during the phase transformation of TiO2. Therefore, we observed the phenomenon that the phase transformation of agglomerated TiO2 particles takes place from its bulk region and then extends to its surface region because the rutile phase nucleates at the interfaces of contacting anatase particles.20 By comparing the results from IR spectra of adsorbed CO on anatase and rutile, it is clearly shown that CO is much more easily adsorbed on surface Ti4+ sites of anatase than those of rutile. This may be due to distinct surroundings of surface Ti4+ ions on the anatase and rutile phases. In the case of rutile, the surface fivefold coordinated Ti4+ sites are more efficiently shielded by the surrounding oxygen atoms and possibly inwardly displaced. Another possibility is that the BET surface area (SBET) of rutile after a high temperature calcination procedure is rather smaller than that of anatase. As a result, the amount of surface coordinatively unsaturated Ti4+ cations which can adsorb CO on rutile is much less. 3.2.3. IR Spectra of CO Adsorbed at 80 K on TiO2 with Different Phase Compositions. Figure 5 shows the IR spectra of CO adsorbed on different TiO2 samples at 80 K. On exposure of anatase to CO at 80 K, two bands at 2177 cm-1 and 2151 cm-1 are found. The band at 2177 cm-1 is assigned to CO adsorbed on Ti4+ cations on regular faces.28 The band at 2151 cm-1, highly reversible by decreasing the CO coverage (not shown), is attributed to CO interaction with OH groups49 on the surface of anatase. However, a broad peak centered at 2157 cm-1 appears when CO is adsorbed on rutile at low temperature. This band is due to the interaction of CO with OH groups on the surface of rutile.30 A weak shoulder at approximately 2170 cm-1 is also detected, which can be assigned to CO adsorbed on surface Ti4+ cations of rutile.28,30 Figure 5 also displays the IR spectra of adsorbed CO at 80 K on the mechanical mixture with 1:1, 1:3, and 1:10 ratios of anatase phase to rutile phase. Again, the two bands at 2177 cm-1 and 2151 cm-1 appear for all three samples. With the increase of rutile content, the intensity ratios of the band at 2176 cm-1 to the band at 2151 cm-1 gradually decrease. For the TiO2 (B-90%R, S-A) sample, CO adsorption gives a main peak at 2176 cm-1and a very weak peak at 2151 cm-1, which shows that most of the surface phase is in anatase. For CO adsorption on TiO2 (B-R, S-84%R), two bands at 2176 cm-1 and 2152 cm-1 (weak) are detected. The band at 2176 cm-1 is a characteristic band of CO adsorbed on anatase. This result suggests that some anatase phase still remains on the surface

Su et al.

Figure 6. XRD patterns of TiO2 (P25) calcined at different temperatures. The “A” and “R” denote the anatase and rutile phases, respectively. The number above each pattern represents the content of rutile phase in the sample based on XRD pattern.

of the sample, which agrees well with the result of the UV Raman spectrum (Figure 2). In a word, it is evident that the results of IR spectra of CO2 adsorption and CO adsorption on the TiO2 samples are in accordance with each other. The results from IR spectra are quite consistent with those from UV Raman spectra, too. It is concluded that UV Raman spectroscopy is more surface-sensitive than XRD for TiO2, and the anatase phase in the surface region can remain at relatively higher temperatures than it can be in the bulk region of TiO2. IR spectroscopy with CO as probe molecule can sensitively probe the surface phases of TiO2. The blue shift of the stretching mode of CO adsorbed on surface cations with respect to CO in gas phase (2143 cm-1) increases as the Lewis acid strength of the adsorbing centers increases,50 and then the differences in the frequencies of the stretching bands of CO adsorbed at 80 K on anatase and rutile obviously indicate that the surface fivefold coordinated Ti4+ ions exposed on anatase exhibit a stronger Lewis acidity than those exposed on rutile. 3.3. Phase Transformation of TiO2 (P25) Calcined at Different Temperatures. 3.3.1. XRD Patterns of TiO2(P25) Calcined at Different Temperatures. TiO2 (P25), a commercial sample, is widely used in catalysis and has received much attention.29,30,34,37 From the above discussion, the results from UV Raman spectroscopy and IR spectroscopy for TiO2 are wellconsistent with each other, and IR spectroscopy of adsorbed CO and CO2 can probe the surface phases of TiO2. Similar results are obtained in the cases of TiO2 (P25) samples as follows. Figure 6 shows the XRD patterns of TiO2(P25) calcined at different temperatures. For TiO2(P25) before calcination, sharp peaks assigned to both the anatase and the rutile phases appear, and the rutile content in TiO2(P25) is about 20% according to the XRD pattern.42 The diffraction peaks of anatase phase gradually diminish in intensity, and the diffraction patterns of rutile phase become predominant with the calcination temperatures from 650 to 725 °C. The contents of rutile phase are increased from 44% to 94% respectively for the samples calcined at 650 and 725 °C based on XRD patterns.42 When TiO2(P25) is calcined at 820 °C, the diffraction peaks assigned to anatase phase disappear, and only the peaks due to the rutile phase are observed, indicating that the anatase phase completely changes into the rutile phase. The rutile content in the samples calcined at different temperatures estimated from XRD patterns is presented in Figure 6, too. 3.3.2. UV Raman Spectra of TiO2 (P25) Calcined at Different Temperatures. Figure 7 shows the UV Raman spectra of TiO2 (P25) samples calcined at different temperatures. For

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Figure 7. UV Raman spectra of TiO2 (P25) calcined at different temperatures with the excitation line at 325 nm. The “A” and “R” denote the surface anatase phase and surface rutile phase based on UV Raman spectra, respectively. The numbers beside some spectra represent the content of surface rutile phase in the samples based on UV Raman spectra.

Figure 8. IR spectra of CO2 adsorbed at RT on TiO2 (P25) samples with different surface phase compositions. The “A” and “R” denote the surface anatase phase and surface rutile phase based on UV Raman spectra, respectively. The numbers beside some spectra represent the content of surface rutile phase in the samples based on UV Raman spectra.

TiO2 (P25) before calcination, five strong bands at about 142, 195, 395, 515, and 637 cm-1 are observed, and all of these bands are assigned to the anatase phase.43 This result shows that the surface of TiO2 (P25) is in anatase according to the UV Raman spectrum, which is inconsistent with the result of XRD (Figure 6). It is proposed that the rutile phase in TiO2 (P25) is more concentrated in the bulk. When the sample was calcined at 650 °C, the UV Raman spectrum is almost the same as that of TiO2 (P25) before calcination, meaning the surface is still in anatase. After calcination at 680 °C, a new band at 614 cm-1 due to the rutile phase appears in the UV Raman spectrum.44 The intensities of the bands due to the anatase phase gradually diminish, and the bands of rutile phase become predominant with the calcination temperatures from 680 to 725 °C. The contents of surface rutile phase estimated from UV Raman spectra of the above two samples are 65% and 92%, respectively.20 The characteristic bands due to anatase phase disappear, and the surface of the sample is in rutile after calcination at 820 °C. Figure 7 also shows the content of the surface rutile phase in the samples calcined at different temperatures estimated from UV Raman spectra. Unlike TiO2 prepared by the precipitation method, the results between XRD patterns and UV Raman spectra of TiO2 (P25) calcined at different temperatures are generally accordant with each other. It is shown that the scheme for the phase transformation of TiO2 (P25) with increasing calcination temperatures is different from that of TiO2 prepared by the precipitation method, which may be due to their different preparation methods. It seems that the phase transformations of TiO2 (P25) in the bulk and surface are nearly simultaneous. 3.3.3. IR Spectra of CO2 Adsorbed at RT on TiO2 (P25) Calcined at Different Temperatures. Figure 8 displays the IR spectra of adsorbed CO2 at RT on TiO2 (P25) calcined at different temperatures. These spectra are qualitatively similar to those on TiO2 prepared by the precipitation method. The surface phase of TiO2 (P25) is in anatase, as shown in Figure 7. IR bands at 1671, 1629, 1580, 1404, 1352, 1245, 1220, and 1056 cm-1 are observed when CO2 is adsorbed on TiO2 (P25). According to previous literature and the stabilities of different surface adspecies under vacuum, the bands at 1580, 1352, and 1056 cm-1 are attributed to bidentate carbonate species.29,30,35 The bands at 1670 and 1245 cm-1 may be due to CO2carboxylate species adsorbed on Ti3+ sites as a result of charge transfer.37,39,51,52 Furthermore, components at 1629, 1404, and 1220 cm-1 are assigned to the bicarbonate (HCO3) species.30 From the UV Raman spectrum, the surface of TiO2 (P25) is in anatase after calcination at 650 °C (Figure 7). The IR spectrum

of CO2 adsorbed on TiO2 (P25) calcined at 650 °C is almost the same as that on TiO2 (P25). Once again, the results of the IR spectra agree well with those of the UV Raman spectra. UV Raman spectra indicate that the surface anatase phase gradually changes into rutile and the surface is in a mixture of the two phases with the calcination temperatures from 680 to 725 °C. The contents of surface rutile phase are increased from 65% to 92% respectively for TiO2 (P25) calcined at 680 and 725 °C based on the UV Raman spectra (Figure 7). Accordingly, the IR spectra of the adspecies formed from CO2 adsorption also change greatly. The intensities of the IR bands at 1671 and 1245 cm-1 due to the CO2- carboxylate species decrease with increasing the calcination temperatures. The bands at 1629 and 1056 cm-1 disappear when TiO2 (P25) was calcined at 725 °C. However, the bands at 1414 and 1220 cm-1 ascribed to the bicarbonate (HCO3) species grow in intensity progressively for TiO2 (P25) calcined at elevated temperatures. Furthermore, the band at 1580 cm-1 attributed to bidentate carbonate quickly disappears, and a new band at 1598 cm-1 occurs. According to the UV Raman spectrum, the surface of the TiO2 (P25) sample is totally in rutile after calcination at 820 °C (Figure 7). CO2 adsorption on the surface rutile phase gives three IR bands at 1598, 1414, and 1220 cm-1. The band at 1598 cm-1 is assigned to bidentate carbonate on the surface of the rutile phase,29,39 and the other two bands are attributed to bicarbonate on the rutile phase.30,35 It is again suggested that the surface acid–base characters of anatase and rutile are very different. Furthermore, the surface phase compositions determine the surface acid–base properties of TiO2 and, consequently, the type and number of adsorbed species formed from the adsorption of CO2. By comparing Figure 7 with Figure 8, it is obvious that the IR spectrum of CO2 adsorbed on anatase is quite different from that on rutile. It is also shown that the results from UV Raman spectra and IR spectra of adsorbed CO2 on TiO2 (P25) calcined at different temperatures are in good agreement with each other. In conclusion, irrespective of the different sources of TiO2 samples (TiO2 prepared by precipitation method or TiO2 (P25)), the adsorbed species produced by CO2 adsorption mainly depend on the surface phases of TiO2. Both UV Raman spectroscopy and IR spectroscopy of adsorbed CO2 can characterize the different surface phases of TiO2. 4. Conclusions The results from UV Raman spectra and IR spectra of adsorbed CO and CO2 are in good accordance with each other but are different from those of XRD for TiO2 calcined at

7716 J. Phys. Chem. C, Vol. 112, No. 20, 2008 different temperatures. UV Raman spectroscopy is a surfacesensitive technique for TiO2, both UV Raman spectroscopy and IR spectroscopy of adsorbed CO and CO2 can sensitively probe the surface phases of TiO2. The crystalline phase in the surface region is usually different from that in the bulk during the phase transformation of TiO2. CO is weakly adsorbed on Ti4+ ions of the anatase phase, while it is hardly adsorbed on rutile phase at room temperature, and the IR bands of adsorbed CO on rutile phase can be detected at 80 K. This suggests that the surface Lewis acidity of the anatase phase is stronger than that of the rutile phase. CO2 adsorption on the anatase phase produces mainly bidentate carbonate while bicarbonate species are mainly formed on the rutile phase, which indicates that the concentration of cus Ti4+-O2- acid–base pairs on the anatase phase is much higher than that on the rutile phase; however, the basicity of the surface OH groups of the rutile phase is stronger than that of the anatase phase. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (NSFC, Grants 20673112 and 20590363) and the National Basic Research Program of China (Grants 2003CB214504 and 2005CB221407). References and Notes (1) Védrine, J. C. (Ed.); Eurocat oxide, Catal. Today 1994, 20, 1. (2) Védrine, J. C. (Ed.); Eurocat oxide, Catal. Today 2000, 56, 329. (3) Vannice, M. A. J. Catal. 1982, 74, 199. (4) Boccuzzi, F.; Guglielminotti, E.; Martra, G.; Cerrato, G. J. Catal. 1994, 146, 449. (5) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301. (6) Wainwright, M. S.; Forster, N. R. Catal. ReV. 1979, 19, 211. (7) Hayashi, T.; Tsubota, S.; Haruta, M. Ind. Eng. Chem. Res. 1995, 34, 2298. (8) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (9) Kudo, A.; Domen, K.; Maruya, K.; Onishi, T. J. Catal. 1992, 135, 300. (10) Anpo, M. Bull. Chem. Soc. Jpn. 2004, 77, 1427. (11) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (12) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388 (6641), 431. (13) Park, D. R.; Zhang, J. L.; Ikeue, K.; Yamashita, H.; Anpo, M. J. Catal. 1999, 185, 114. (14) Kostov, I. Mineralogy, Nauka i IzkustVo, Sofia 1973. (15) Hadjiivanov, K.; Klissurski, D. Chem. Soc. ReV. 1996, 29, 61. (16) Li, G. H.; Gray, K. A. Chem. Phys. 2007, 339, 173.

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