二氧化鈦的紫外拉曼光譜研究外文翻譯

1、<p><b>　　附 錄</b></p><p>　　UV Raman Spectroscopic Study on TiO2. I. Phase Transformation at the Surface and in the Bulk</p><p>　　Jing Zhang, Meijun Li, Zhaochi Feng, Jun Chen, a

2、nd Can Li*</p><p>　　State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,</p><p>　　P. O. Box 110, Dalian 116023, China</p><p>　　Recei

3、Ved: September 16, 2005; In Final Form: NoVember 4, 2005</p><p>　　Phase transformation of TiO2 from anatase to rutile is studied by UV Raman spectroscopy excited by 325and 244 nm lasers, visible Raman spectr

4、oscopy excited by 532 nm laser, X-ray diffraction (XRD), andtransmission electron microscopy (TEM). UV Raman spectroscopy is found to be more sensitive to the surfaceregion of TiO2 than visible Raman spectroscopy and XRD

5、 because TiO2 strongly absorbs UV light. Theanatase phase is detected by UV Raman spectroscopy for the sample calcined at higher temperatur</p><p>　　1. Introduction</p><p>　　Titania (TiO2) has b

6、een widely studied because of its uniqueoptical and chemical properties in catalysis,[1]photocatalysis,[2]sensitivity to humidity and gas,[3,4] nonlinear optics,[5] photoluminescence,[6]and so on. The two main kinds of c

7、rystalline TiO2,anatase and rutile, exhibit different physical and chemicalproperties. It is well-known that the anatase phase is suitablefor catalysts and supports,[7] while the rutile phase is used for optical and elec

8、tronic purposes because of its high diel</p><p>　　Although at ambient pressure and temperature the rutile phase is more thermodynamically stable than the anatase phase,[12]anatase is the common phase rather

9、than rutile because anatase is kinetically stable in nanocrystalline TiO2 at relatively low temperatures.[13] It is believed that the anatase phase transforms to the rutile phase over a wide range of temperatures.[14]The

10、refore, understanding and controlling of the crystalline phase and the process of phase transformation of TiO2 are importan</p><p>　　Many studies[13-31] have been done to understand the process of the phase

11、transformation of TiO2. Zhang et al.[15]proposed that the mechanism of the anatase-rutile phase transformation was temperature-dependent according to the kinetic data from X-ray diffraction (XRD). On the basis of transmi

12、ssion and scanning electron microscopies, Gouma et al.[16] suggested that rutile nuclei formed on the surface of coarser anatase particles and the newly transformed rutile particles grew at the expense of n</p>&l

13、t;p>　　Catalytic performance of TiO2 largely depends on the surface properties, especially the surface phase, because catalytic reaction takes place on the surface. The surface phase of TiO2 should be responsible for i

14、ts photocatalytic activity because not</p><p>　　only the photoinduced reactions take place on the surface[32] but also the photoexcited electrons and holes might migrate through the surface region. Therefore

15、, the surface phase of TiO2, which is exposed to the light source, should play a crucial role in photocatalysis. However, the surface phase of TiO2, particularly during the phase transformation, has not been investigated

16、. The challenging questions still remain: is the phase in the surface region the same as that in the bulk region, or how </p><p>　　UV Raman spectroscopy is found to be more sensitive to the surface phase of

17、a solid sample when the sample absorbs UV light.[33]We studied the phase transition of zirconia (ZrO2) from tetragonal phase to monoclinic phase by UV Raman spectroscopy, visible Raman spectroscopy, and XRD.[33] These re

18、sults clearly indicated that the surface phase of ZrO2 is usually different from the bulk phase of ZrO2 and the phase transforma-tion of ZrO2 starts from its surface region and then gradually develops int</p><

19、p>　　These findings lead us to further investigate the phase transformation in the surface region of TiO2 by UV Raman spectroscopy as TiO2 also strongly absorbs UV light. In this study, we compared the Raman spectra of

20、 TiO2 calcined at different temperatures with excitation lines in the UV and visible regions. XRD and transmission electron microscopy (TEM) were also recorded to understand the process of phase transformation of TiO2. I

21、t was found that the results of UV Raman</p><p>　　spectra are different from those of visible Raman spectra and XRD patterns. The anatase phase of TiO2 at the surface region can remain at relatively higher t

22、emperatures than that in the bulk at elevated calcination temperatures; namely, the anatase phase in the inner region of the agglomerated TiO2 particles turns out to change into the rutile phase more easily than that in

23、the outer surface region of the agglomerated TiO2 particles.</p><p>　　The literature[15,17,19] proposed the mechanism that phase transformation of TiO2 might start at the interfaces of contacting anatase par

24、ticles. If the anatase particles of TiO2 are separated, the phase transformation of TiO2 from anatase to rutile couldbe retarded or prohibited. Jing et al.[34]showed that La3+ did not enter the crystal lattices of TiO2 a

25、nd was uniformly dispersed onto TiO2 in the form of lanthana (La2O3) particles with small size. To verify the above assumption, this study also</p><p>　　2. Experimental Section</p><p>　　2.1. Cat

26、alyst Preparation. </p><p>　　2.1.1. Preparation of TiO2. TiO2 was prepared by precipitation method. To 100 mL of anhydrous ethanol was added 20 mL of titanium(IV) n-butoxide (Ti(OBu)4). This solution was add

27、ed to a mixture solution of deionized water and 100 mL of anhydrous ethanol. The molar ratio of the water/Ti(OBu)4 was 75. After the formed white precipitate was stirred continuously for 24 h, it was filtered and washed

28、twice with deionized water and anhydrous ethanol. Finally, the sample was dried at 100 °C and calcined</p><p>　　2.1.2. Preparation of La2O3-CoVered TiO2 (La2O3/TiO2). The above TiO2 powder calcined at 5

29、00 °C was used as a support. The critical La2O3 loading corresponding to monolayer coverage of La2O3 on the grain surface of TiO2 is 0.27 g/100 m2. [35,36] On the basis of the BET surface area of the TiO2 support (5

30、4.3 m2/g), the monolayer dispersion capacity can also be expressed as 15 wt % La2O3 of the weight of TiO2. La2O3/TiO2 samples, containing different amounts of La2O3(0.5-6 wt %) were prepared by a</p><p>　　ke

31、pt at 110 °C overnight, it was calcined at 900 °C in air for 4 h. A TiO2 sample was prepared by calcining the TiO2 support at 900 °C for 4 h (denoted as TiO2-900) for comparison with the La2O3/TiO2 sample.

32、 Pure La2O3 was obtained by calcining La(NO3)3·6H2O at 550 °C for 4 h.</p><p>　　2.2. Characterization. </p><p>　　2.2.1. UV Raman Spectroscopy. UV Raman spectra were measured at room te

33、mperature with a Jobin-Yvon T64000 triple-stage spectrograph with spectral resolution of 2 cm-1. The laser line at 325 nm of a He-Cd laser was used as an exciting source with an output of 25 mW. The power of laser at the

34、 sample was about 3.0 mW. The 244 nm line from a Coherent Innova 300 Fred laser was used as another</p><p>　　excitation source. The power of the 244 nm line at sample was below 1.0 mW.</p><p>　　

35、2.2.2. Visible Raman Spectroscopy. Visible Raman spectra were recorded at room temperature on a Jobin-Yvon U1000 scanning double monochromator with the spectral resolution of 4 cm-1. The line at 532 nm from a DPSS 532 Mo

36、del 200 532 nm single-frequency laser was used as the excitation source.</p><p>　　2.2.3. X-ray Powder Diffraction (XRD), TEM, and UltraViolet-Visible Diffuse Reflectance Spectroscopy. XRD patterns were obtai

37、ned 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. TEM was taken on a JEM-2011 TEM for estimating particle size

38、 and morphology. UV-vis diffuse reflectance spectra were recorded on a JASCO V-550 UV-vis spectrophotometer.</p><p>　　2.2.4. Brunauer-Emmett-Teller (BET) Specific Surface Area. The BET surface area of the Ti

39、O2 support was measured by nitrogen adsorption at 77 K using a Micromeritics ASAP 2000 adsorption analyzer.</p><p>　　3. Results</p><p>　　3.1. Spectral Characteristics of Anatase and Rutile TiO2.

40、The anatase and rutile phases of TiO2 can be sensitively identified by Raman spectroscopy based on their Raman spectra. The anatase phase shows major Raman bands at 144, 197, 399, 515, 519 (superimposed with the 515 cm-1

41、 band), and 639 cm-1.[37] These bands can be attributed to the six Raman-active modes of anatase phase with the symmetries of Eg, Eg, B1g, A1g, B1g, and Eg, respectively.[37] The typical Raman bands due to rutile phase a

42、ppe</p><p>　　Figure 2 shows UV-vis diffuse reflectance spectra of the TiO2 sample calcined at 500 and 800 °C (the TiO2 sample is in the anatase phase and rutile phase, respectively). For the anatase pha

43、se, the maximum absorption and the absorption band edge can be estimated to be around 324 and 400 nm, respectively. The maximum absorption and the absorption band edge shift to a little longer wavelength for the rutile p

44、hase.[39]</p><p>　　By comparing the Raman spectra of the anatase (Figure 1A) or rutile phase (Figure 1B) excited by 532, 325, and 244 nm lines, it is found that the relative intensities of characteristic ban

45、ds due to anatase or rutile phase in the high-frequency region are different. For the anatase phase (Figure 1A), the band at 638 cm-1 is the strongest one in the Raman spectrum with the excitation line at 325 or 532 nm,

46、while the band at 395 cm-1 is the strongest one in the Raman spectrum with the excitation</p><p>　　line at 244 nm.</p><p>　　For the rutile phase (Figure 1B), the intensities of the bands at 445

47、and 612 cm-1 are comparable in the visible Raman spectrum. The intensity of the band at 612 cm-1 is stronger than that of the band at 445 cm-1 in the Raman spectrum with the excitation line at 325 nm, and the reverse is

48、true for the Raman spectrum with the excitation line at 244 nm. In addition, for the rutile phase, a band at approximately 826 cm-1 appears in the UV Raman spectra. Some investigations show that the rutile phas</p>

49、<p>　　The fact that the relative intensities of the Raman bands of anatase phase or rutile phase are different for UV Raman spectroscopy and visible Raman spectroscopy are mainly due to the UV resonance Raman effe

50、ct because the laser lines at 325 and 244 nm are in the electronic absorption region of TiO2 (Figure 2). There is no resonance Raman effect observed for the TiO2 sample excited by visible laser line, because the line at

51、532 nm is outside the absorption region of TiO2 (Figure 2). Therefore, fo</p><p>　　3.2. Semiquantitative Analysis of the Phase Composition of TiO2 by XRD and Raman Spectroscopy. The weight fraction of the ru

52、tile phase in the TiO2 sample, WR, can be estimated from the XRD peak intensities using the following formula:[41]</p><p>　　where Aana and Arut represent the X-ray integrated intensities of anatase (101) and

53、 rutile (110) diffraction peaks, respectively.</p><p>　　To estimate the weight fraction of the rutile phase in the TiO2 sample by Raman spectroscopy, pure anatase phase and pure rutile phase of the TiO2 samp

54、le, which have been prepared by calcination of TiO2 powder at 500 and 800 °C for 4 h, were</p><p>　　mechanically mixed at given weight ratio and ground carefully to mix sufficiently.</p><p>

55、;　　Figure 3A displays the visible Raman spectra of the mechanical mixture with 1:1, 1:5, 1:10, 1:15, 5:1, and 10:1 ratios of anatase phase to rutile phase. The relationship between the area ratios of the visible Raman ba

56、nd at 395 cm-1 for anatase phase to the band at 445 cm-1 for rutile phase (A395 cm-1/A445 cm-1) and the weight ratios of anatase phase to rutile phase (WA/WR) is plotted in Figure 3B. It can be seen that a linear relatio

57、nship between the band area ratios and the weight ratios of ana</p><p>　　Figure 4A presents the UV Raman spectra of the mechanical mixture with 1:1, 1:2, 1:4, 1:6, 1:10, and 1:15 ratios of anatase phase to r

58、utile phase with the excitation line at 325 nm. Figure 4B shows the plot of the area ratios of the UV Raman band at</p><p>　　612 cm-1 for rutile phase to the band at 638 cm-1 for anatase phase (A612 cm-1/A63

59、8 cm-1) versus the weight ratios of rutile phase to anatase phase (WR/WA). There is also a linear relationship between the band area ratios and the weight ratios of rutile phase to anatase phase.</p><p>　　3.

60、3. Phase Transformation of TiO2 at Elevated Calcination Temperatures. </p><p>　　3.3.1. XRD Patterns and Visible Raman Spectra of TiO2 Calcined at Different Temperatures. Figure 5 shows the XRD patterns of Ti

61、O2 calcined at different temperatures. The “A” and “R” in the figure denote the anatase and rutile phases, respectively. For the sample before calcination, diffraction peaks due to the crystalline phase are not observed,

62、 suggesting that the sample is still in the amorphous phase. When the sample was calcined at 200 °C, weak and broad peaks at 2=25.5°, 37.9°, 48.2°, 53.8°</p><p>　　suggest that some p

63、ortions of the amorphous phase transform into the anatase phase. The diffraction peaks due to anatase phase develop with increasing the temperature of calcination. When the calcination temperature was increased to 500 &#

64、176;C, the diffraction peaks due to anatase phase became narrow and intense in intensity. This indicates that the crystallinity of the anatase phase is further improved.[44]</p><p>　　When the sample was calc

65、ined at 550 °C, weak peaks were observed at 2=27.6°, 36.1°, 41.2°, and 54.3°, which correspond to the indices of (110), (101), (111), and (211) planes of rutile phase. [43]This indicates that the

66、 anatase phase starts to transform into the rutile phase at 550 °C. The diffraction peaks of anatase phase gradually diminish in intensity and the diffraction patterns of rutile phase become predominant with the cal

67、cination temperatures from 580 to 700 °C. These results clearly sh</p><p>　　Figure 6 displays the visible Raman spectra of TiO2 calcined at different temperatures. For the sample before calcination, two

68、 broad bands at about 430 and 605 cm-1 are observed, indicating that the sample is in the amorphous phase.[19] For the sample calcined at 200 °C, a Raman band at 143 cm-1 is observed and the high-frequency region sh

69、ows interference from the fluorescence background, which might come from organic species. After calcination at 300 °C, other characteristic bands of anatase ph</p><p>　　It was found that, when the sampl

70、e was calcined at 400 °C, the fluorescence disappeared, possibly because the organic residues were removed by the oxidation. The bands of anatase phase increased in intensity and decreased in line width when the sam

71、ple was calcined at 500 °C. This result suggests that the crystallinity of the anatase phase is greatly improved,[18] which is confirmed by XRD (Figure 5). The enlarged section of Figure 6 shows the Raman spectrum o

72、f the high-frequency region of the sa</p><p>　　When the sample was calcined at 580 °C, other two characteristic bands were observed at 235 and 612 cm-1 due to Raman-active modes of rutile phase. Figure

73、7 shows the rutile content is 13.6% and 10.9% based on the visible Raman spectrum and XRD pattern of the sample calcined at 580 °C. The intensities of the bands of rutile phase (235, 445, and 612 cm-1) increased ste

74、adily while those of the bands of anatase phase (195, 395, 515, and 638 cm-1) decreased when the calcination temperatures were ele</p><p>　　The Raman spectrum of the sample calcined at 700 °C shows main

75、ly the characteristic bands of rutile phase, but the very weak bands of anatase phase are still observed (Figure 6). When the sample was calcined at 750 °C, the bands of anatase phase disappeared and only the bands

76、due to rutile phase (143, 235, 445, and 612 cm-1) were observed. These results indicate that the anatase phase completely transforms into the rutile phase and are consistent with the results from XRD (Figure 5). When the

77、 te</p><p>　　Both the results of XRD and visible Raman spectra (Figures 5 and 6) show that the anatase phase appears at around 200 °C and perfect anatase phase is formed after calcination at temperature

78、s of 400-500 °C. The rutile phase starts to form at 550 °C, and the anatase phase completely transforms into the rutile phase at 750 °C. The signals of visible Raman spectra come mainly from the bulk regio

79、n of TiO2 because the TiO2 sample is transparent in the visible region (Figure 2).[33] XRD is known as a b</p><p>　　3.3.2. UV Raman Spectra of TiO2 Calcined at Different Temperatures. UV-vis diffuse reflecta

80、nce spectra (Figure 2) clearly show that TiO2 has strong electronic absorption in the UV region. Thus, the UV Raman spectra excited by a UV laser line contain more signal from the surface skin region than the bulk of the

81、 TiO2 sample because the signal from the bulk is attenuated sharply due to the strong absorption.[33] Therefore, if</p><p>　　a UV laser line in the absorption region of TiO2 is used as the excitation source

82、of Raman spectroscopy, the information from UV Raman spectra is often different from that of visible Raman spectra.</p><p>　　The laser line at 325 nm was selected as the excitation source of the UV Raman spe

83、ctra. The UV Raman spectra and the content of the rutile phase of the TiO2 sample calcined at different temperatures are shown in parts A and B, respectively, of Figure 8. When the sample was calcined at 200 or 300 °

84、;C, the Raman band at 143 cm-1 with a shoulder band at 195 cm-1 and three broad bands at 395, 515, and 638 cm-1 were observed, indicating that the anatase phase is formed in the sample. However, the low </p><p

85、>　　All bands assigned to anatase phase become sharp and strong after calcination at 500 °C (Figure 8A). These results are in agreement with those of XRD and visible Raman spectra (Figures 5 and 6). The UV Raman s

86、pectra of the sample with the calcination temperatures from 550 to 680 °C are essentially the same as those of the sample calcined at 500 °C (Figure 8A). However, according to the XRD patterns and visible Raman

87、 spectra (Figures 5 and 6), the anatase phase starts to transform into the rutile</p><p>　　After calcination at 700 °C, a new band at 612 cm-1 and two weak bands at 235 and 445 cm-1 due to rutile phase

88、appear while the intensities of the bands of anatase phase begin to decrease (Figure 8A). On the basis of the UV Raman spectrum and XRD pattern of the sample calcined at 700 °C, the rutile content in the sample is 5

89、6.1% and 97.0%, respectively (Figure 8B). It is found that the rutile content estimated by UV Raman spectroscopy is far less than that estimated by XRD.</p><p>　　When the sample was calcined at 750 °C,

90、the intensities of the bands due to rutile phase increased, but the intensities of the bands due to anatase phase were still strong in the UV Raman spectra (Figure 8A). The UV Raman spectrum of the sample calcined at 750

91、 °C indicates that the rutile content is 84.3% (Figure 8B). However, the results of XRD and visible Raman spectrum (Figures 5 and 6) suggest that the anatase phase totally transformed into the rutile phase after the

92、 sample was calcined at 7</p><p>　　between the results from the UV Raman spectra, visible Raman spectra, and XRD patterns. It seems that the anatase phase remains at relatively higher temperatures when detec

93、ted by UV Raman spectroscopy than by XRD and visible Raman spectroscopy.</p><p>　　Another UV laser line at 244 nm was also selected as the excitation source of UV Raman spectroscopy in order to get further i

94、nsights into the phase transformation of TiO2. The results of the UV Raman spectra of TiO2 calcined at different temperatures with the excitation line at 244 nm are presented in Figure 9. When the sample was calcined at

95、200 °C, four broad bands were observed at 143, 395, 515, and 638 cm-1, which clearly indicate that the anatase phase exists in the sample. The intensities </p><p>　　bands due to anatase phase disappeare

96、d, while the bands of rutile phase developed. This result indicates that the sample calcined at 800 °C is in the rutile phase. It is interesting to note that the results of the UV Raman spectra with the excitation l

97、ines at</p><p>　　325 and 244 nm are in agreement with each other but are different from those of XRD patterns and visible Raman spectra.</p><p>　　3.3.3. TEM of the TiO2 Sample Calcined at Differ

98、ent Temperatures. TEM was used to characterize the microstructure of the TiO2 sample calcined at 500, 600, and 800 °C (shown in Figure 10). Most particles in the sample calcined at 500 °C exhibit diameters in a

99、 range between 10 and 30 nm (Figure 10a). On the other hand, remarkable agglomeration is observed for the TiO2 sample calcined at 500 °C. The particle size increases after calcination at 600 °C (Figure 10b). Ac