2022年二氧化钛的紫外拉曼光谱研究外文翻译

1、附 录UV Raman Spectroscopic Study on TiO2. I. Phase Transformation at the Surface and in the BulkJing Zhang, Meijun Li, Zhaochi Feng, Jun Chen, 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

2、: September 16, 2022; In Final Form: NoVember 4, 2022 Phase transformation of TiO2 from anatase to rutile is studied by UV Raman spectroscopy excited by 325and 244 nm lasers, visible Raman spectroscopy excited by 532 nm laser, X-ray diffraction XRD, andtransmission electron microscopy TEM. UV Raman

3、spectroscopy is found to be more sensitive to the surfaceregion of TiO2 than visible Raman spectroscopy and XRD because TiO2 strongly absorbs UV light. Theanatase phase is detected by UV Raman spectroscopy for the sample calcined at higher temperatures thanwhen it is detected by visible Raman spectr

4、oscopy and XRD. The inconsistency in the results from the abovethree techniques suggests that the anatase phase of TiO2 at the surface region can remain at relatively highercalcination temperatures than that in the bulk during the phase transformation. The TEM results show thatsmall particles agglom

5、erate into big particles when the TiO 2 sample is calcined at elevated temperatures andthe agglomeration of the TiO2 particles is along with the phase transformation from anatase to rutile. It is suggested that the rutile phase starts to form at the interfaces between the anatase particles in the ag

6、glomerated TiO 2 particles; namely, the anatase phase in the inner region of the agglomerated TiO 2 particles turns out to change into the rutile phase more easily than that in the outer surface region of the agglomerated TiO2 particles.When the anatase particles of TiO2 are covered with highly disp

7、ersed La2O3, the phase transformation in both the bulk and surface regions is significantly retarded, owing to avoiding direct contact of the anatase particles and occupying the surface defect sites of the anatase particles by La 2O3.1. Introduction Titania TiO 2 has been widely studied because of i

8、ts uniqueoptical and chemical properties in catalysis,1photocatalysis,2sensitivity to humidity and gas,3,4 nonlinear optics,5 photoluminescence,6and so on. The two main kinds of crystalline TiO2,anatase and rutile, exhibit different physical and chemicalproperties. It is well-known that the anatase

9、phase is suitablefor catalysts and supports,7while the rutile phase is used for optical and electronic purposes because of its high dielectric constant and high refractive index.8It has been well demonstrated that the crystalline phase of TiO2 plays a significant role in catalytic reactions, especia

10、lly photocatalysis.9-11Some studies have claimed that the anatase phase was more active than the rutile phase in photocatalysis. 9,10Although at ambient pressure and temperature the rutile phase is more thermodynamically stable than the anatase phase, 12anatase is the common phase rather than rutile

11、 because anatase is kinetically stable in nanocrystalline TiO 2 at relatively low temperatures.13 It is believed that the anatase phase transforms to the rutile phase over a wide range of temperatures.14Therefore, understanding and controlling of the crystalline phase and the process of phase transf

12、ormation of TiO 2 are important, though they are difficult.Many studies13-31 have been done to understand the process of the phase transformation of TiO2. Zhang et al.15proposed that the mechanism of the anatase-rutile phase transformation was temperature-dependent according to the kinetic data from

13、 X -ray diffraction XRD. On the basis of transmission and scanning electron microscopies, Gouma et al.16suggested that rutile nuclei formed on the surface of coarser anatase particles and the newly transformed rutile particles grew at the expense of neighboring anatase particles. Penn et al. 17sugge

14、sted that the formation of rutile nuclei at twin interfaces of anatase particles heated hydrothermally.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 res

15、ponsible for its photocatalytic activity because not only the photoinduced reactions take place on the surface 32 but also the photoexcited electrons and holes might migrate through the surface region. Therefore, the surface phase of TiO 2, which is exposed to the light source, should play a crucial

16、 role in photocatalysis. However, the surface phase of TiO2, particularly during the phase transformation, has not been investigated. The challenging questions still remain: is the phase in the surface region the same as that in the bulk region, or how does the phase in the surface region of TiO2 pa

17、rticle change during the phase transformation of its bulk. The difficulty in answering the above questions was mainly due to lacking suitable techniques that can sensitively detect the surface phase of TiO UV Raman spectroscopy is found to be more sensitive to the surface phase of a solid sample whe

18、n the sample absorbs UV light.33We studied the phase transition of zirconia ZrO2 from tetragonal phase to monoclinic phase by UV Raman spectroscopy, visible Raman spectroscopy, and XRD.33These results clearly indicated that the surface phase of ZrO2 is usually different from the bulk phase of ZrO 2

19、and the phase transforma-tion of ZrO 2 starts from its surface region and then gradually develops into its bulk when the ZrO2 with tetragonal phase is calcined at elevated temperatures.These findings lead us to further investigate the phase transformation in the surface region of TiO2 by UV Raman sp

20、ectroscopy as TiO2 also strongly absorbs UV light. In this study, we compared the Raman spectra of TiO 2 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 transfor

21、mation of TiO2. It was found that the results of UV Raman spectra are different from those of visible Raman spectra and XRD patterns. The anatase phase of TiO 2 at the surface region can remain at relatively higher temperatures than that in the bulk at elevated calcination temperatures; namely, the

22、anatase phase in the inner region of the agglomerated TiO2 particles turns out to change into the rutile phase more easily than that in the outer surface region of the agglomerated TiO 2 particles.The literature 15,17,19 proposed the mechanism that phase transformation of TiO 2 might start at the in

23、terfaces of contacting anatase particles. If the anatase particles of TiO2 are separated, the phase transformation of TiO 2 from anatase to rutile couldbe retarded or prohibited. Jing et al. 34showed that La 3+ did not enter the crystal lattices of TiO2 and was uniformly dispersed onto TiO2 in the f

24、orm of lanthana La2O3 particles with small size. To verify the above assumption, this study also prepared the anatase phase of TiO 2 sample covered with La2O3 and characterized the above sample by visible Raman spectroscopy and UV Raman spectroscopy. The results of the two types of Raman spectra are

25、 in agreement with each other and show that the TiO2 particle covered with La2O3 can retain its anatase phase both in the bulk and in the surface region even after calcination at 900 2. Experimental Section C.2.1. Catalyst Preparation. 2.1.1. Preparation of TiO2. TiO2 was prepared by precipitation m

26、ethod. To 100 mL of anhydrous ethanol was added 20 mL of titaniumIV n-butoxide TiOBu 4. This solution was added to a mixture solution of deionized water and 100 mL of anhydrous ethanol. The molar ratio of the water/TiOBu 4 was 75. After the formed white precipitate was stirred continuously for 24 h,

27、 it was filtered and washed twice with deionized water and anhydrous ethanol. Finally, the sample was dried at 100 C and calcined in air at temperatures from 200 to 800 C for 4 h, and then cooled to temperature.2.1.2. Preparation of La2O3-CoVered TiO2 La 2O3/TiO2. The above TiO2 powder calcined at 5

28、00 C was used as a support. The critical La 2O3 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 TiO 2 support 54.3 m2/g, the monolayer dispersion capacity can also be expressed as 15 wt % La 2O3 of

29、the weight of TiO2. La2O3/TiO 2 samples, containing different amounts of La 2O30.5-6 wt % were prepared by a wet impregnation method. The support was impregnated with aqueous solution of various concentrations of lanthanum nitrate LaNO336H2O and subsequently stirred in a hot water bath until it was

30、dried. After the sample waskept at 110 C overnight, it was calcined at 900 h. A TiO 2 sample was prepared by calcining the TiO 2 support at 900 C for 4 h denoted as TiO 2-900 for comparison with the La2O3/TiO2 sample. Pure La2O3 was obtained by calcining LaNO336H2O at 550 2.2. Characterization. C fo

31、r 4 h.2.2.1. UV Raman Spectroscopy. UV Raman spectra were measured at room temperature 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 sa

32、mple was about 3.0 mW. The 244 nm line from a Coherent Innova 300 Fred laser was used as anotherexcitation source. The power of the 244 nm line at sample was below 1.0 mW.2.2.2. Visible Raman Spectroscopy. Visible Raman spectra were recorded at room temperature on a Jobin-Yvon U1000 scanning double

33、monochromator with the spectral resolution of 4 cm-1. The line at 532 nm from a DPSS 532 Model 200 532 nm single-frequency laser was used as the excitation source.2.2.3. X-ray Powder Diffraction XRD, TEM, and Ultra Violet-Visible Diffuse Reflectance Spectroscopy. XRD patterns were obtained on a Riga

34、ku 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-2022 TEM for estimating particle size and morphology. UV -vis diffuse reflectance spectra were recorded on a JASCO V-550 UV-vis spectrophotometer.2

35、.2.4. Brunauer-Emmett-Teller BET Specific Surface Area. The BET surface area of the TiO2 support was measured by nitrogen adsorption at 77 K using a Micromeritics ASAP 2022 adsorption analyzer.3. Results 3.1. Spectral Characteristics of Anatase and Rutile TiO2.The anatase and rutile phases of TiO 2

36、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-1band, and 639 cm-1.37These bands can be attributed to the six Raman-active modes of anatase phase with the symmetries o

37、f Eg, Eg, B1g, A1g, B1g, and Eg, respectively.37 The typical Raman bands due to rutile phase appear at 143 superimposed with the 144 cm-1 band due to anatase phase, 235, 447, and 612 cm-1, which can be ascribed to the B1g, two-phonon scattering, Eg, and A1g modes of rutile phase, respectively.38 Add

38、itionally, the band at 144 cm-1 is the strongest one for the anatase phase and the band at 143 cm-1 is the weakest one for the rutile phase. Parts A and B, respectively, of Figure 1display the Raman spectra of TiOcalcined at 500 and 800 C with excitation lines at 532, 325, and 244 nm. Obviously, bot

39、h visible Raman spectra and UV Raman spectra show that the TiO 2 sample is in the anatase phase Figure 1A and rutile phase Figure 1B.Figure 2 shows UV-vis diffuse reflectance spectra of the TiO2 sample calcined at 500 and 800 C the TiO 2 sample is in the anatase phase and rutile phase, respectively.

40、 For the anatase phase, 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 phase. 39By comparing the Raman spectra of the anatase Figure

41、 1A or rutile phase Figure 1B excited by 532, 325, and 244 nm lines, it is found that the relative intensities of characteristic bands 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 sp

42、ectrum with the excitation line at 325 or 532 nm, while the band at 395 cm-1 is the strongest one in the Raman spectrum with the excitation line at 244 nm.-1 For the rutile phase Figure 1B, the intensities of the bands at 445 and 612 cm-1 is are comparable in the visible Raman spectrum. The intensit

43、y of the band at 612 cm 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 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

44、spectra. Some investigations show that the rutile phase of TiO2 exhibits a weak band at 826 cm-1 assigned to the B2g mode.38,40The 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

45、due to the UV resonance Raman effect because the laser lines at 325 and 244 nm are in the electronic absorption region of TiO 2 Figure 2. There is no resonance Raman effect observed for the TiO2 sample excited by visible laser line, because the line at 532 nm is outside the absorption region of TiO2

46、 Figure 2. Therefore, for the anatase or rutile phase, the Raman spectroscopic characteristics in the visible Raman spectrum are different from those in the UV Raman spectrum. When the UV laser line with different wavelengths is used as the excitation source, the resonance enhancement effect on the

47、Raman bands of anatase or rutile phase is different. For example, for the rutile phase Figure 1B, the band at 612 cm-1 is easily resonance enhanced when the excitation wavelength is 325 nm. Among all the characteristic bands of the rutile phase, the extent of resonance enhancement of 445 cm-1 is the

48、 strongest when the 244 nm laser is used as the excitation source Figure 1B.3.2. Semiquantitative Analysis of the Phase Composition of TiO 2 by XRD and Raman Spectroscopy. The weight fraction of the rutile phase in the TiO2 sample, WR, can be estimated from the XRD peak intensities using the followi

49、ng formula:41WR110.884（A anaA rut）where Aana and Arut represent the X-ray integrated intensities of anatase 101 and rutile 110 diffraction peaks, respectively.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

50、 TiO 2 sample, which have been prepared by calcination of TiO2 powder at 500 and 800 C for 4 h, were mechanically mixed at given weight ratio and ground carefully to mix sufficiently.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 ratio

51、s of anatase phase to rutile phase. The relationship between the area ratios of the visible Raman band at 395 cm-1 for anatase phase to the band at 445 cm-1for 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

52、 linear relationship between the band area ratios and the weight ratios of anatase phase to rutile phase in the mixture is obtained. The rutile content in the Degussa P25, which usually consists of roughly about 80% anatase and 20% rutile phase, 42 was estimated by this plot. Our Raman result indica

53、tes that the rutile content in the Degussa P25 is about 18.7%, which is close to the known result. Thus, the above linear relationship based on visible Raman spectroscopy can be used to estimate the rutile content in TiO Figure 4A presents the UV Raman spectra of the mechanical mixture with 1:1, 1:2

54、, 1:4, 1:6, 1:10, and 1:15 ratios of anatase phase to rutile phase with the excitation line at 325 nm. Figure 4B shows the plot of the area ratios of the UV Raman band at 612 cm-1 for rutile phase to the band at 638 cm-1 for anatase phase A612 cm-1/A638 cm-1 versus the weight ratios of rutile phase

55、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.3.3. Phase Transformation of TiO 2 at Elevated Calcination Temperatures. 3.3.1. XRD Patterns and Visible Raman Spectra of TiO2 Calcined at Different Tempera

56、tures. Figure 5 shows the XRD patterns of TiO2 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, suggesting that the sample is sti

57、ll 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 , and 55.0 were observed. These peaks represent the indices of 101, 004, 200, 105, and 211 planes of anatase phase, respectively. 43 These resultssuggest that some portions of the a

58、morphous 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 C, the diffraction peaks due to anatase phase became narrow and intense in intensity. This indicat

59、es that the crystallinity of the anatase phase is further improved. 44When the sample was calcined at 550 C, weak peaks were observed at 236.1 , 41.2 , and 54.3 , which correspond to the indices of 110, 101, 111, and 211 planes of rutile phase. 43This indicates that the anatase phase starts to trans

60、form 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 calcination temperatures from 580 to 700 C. These results clearly show that the phase transformation from anatase to ru