附录UV Raman Spectroscopic Study on TiO2. I. Phase Transformation at theSurface and in theBulkJing 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, ChinaRecei V ed: September 16, 2005; In Final Form: No V ember 4, 2005Phase 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 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 spectroscopy 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 agglomerate into big particles when the TiO2sample is calcined at elevated temperatures andthe agglomeration of the TiO2 particles is along with the phase transformation from anatase to rutile. It issuggested that the rutile phase starts to form at the interfaces between the anatase particles in the agglomeratedTiO2 particles; namely, the anatase phase in the inner region of the agglomerated TiO2 particles turns out tochange 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 dispersed La2O3, the phase transformation inboth the bulk and surface regions is significantly retarded, owing to avoiding direct contact of the anataseparticles and occupying the surface defect sites of the anatase particles by La2O3.1. IntroductionTitania (TiO2) has been 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 crystalline 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 foroptical and electronic purposes because of its high dielectricconstant and high refractive index.[8]It has been well demonstratedthat the crystalline phase of TiO2 plays a significant rolein catalytic reactions, especially photocatalysis.[9-11]Some studieshave claimed that the anatasephase was more active than therutile phase in photocatalysis.[9,10]Although at ambient pressure and temperature the rutile phaseis morethermodynamically stable than the anatase phase,[12]anatase is the common phase rather than rutile because anataseis kinetically stable in nanocrystalline TiO2at relatively lowtemperatures.[13] It is believed that the anatase phase transformsto the rutile phase over a wide range of temperatures.[14]Therefore, understanding and controlling of the crystalline phaseand the process of phase transformation of TiO2 are important,though they are difficult.Many studies[13-31]have been done to understand the processof the phase transformation of TiO2. Zhang et al.[15]proposedthat the mechanism of the anatase-rutile phase transformationwas temperature-dependent according to the kinetic data fromX-ray diffraction (XRD). On the basis of transmission andscanning electron microscopies, Gouma et al.[16] suggested thatrutile nuclei formed on the surface of coarser anatase particlesand the newly transformed rutile particles grew at the expenseof neighboring anatase particles. Penn et al.[17]suggested thatthe formation of rutile nuclei at twin interfaces of anataseparticles heated hydrothermally.Catalytic performance of TiO2 largely depends on the surfaceproperties, especially the surface phase, because catalyticreaction takes place on the surface. The surface phase of TiO2should be responsible for its photocatalytic activity because notonly the photoinduced reactions take place on the surface[32] butalso the photoexcited electrons and holes might migrate throughthe surface region. Therefore, the surface phase of TiO2, whichis exposed to the light source, should play a crucial role inphotocatalysis. However, the surface phase of TiO2, particularlyduring the phase transformation, has not been investigated. Thechallenging questions still remain: is the phase in the surfaceregion the same as that in the bulk region, or how does thephase in the surface region of TiO2 particle change during thephase transformation of its bulk? The difficulty in answeringthe above questions was mainly due to lacking suitabletechniques that can sensitively detect the surface phase of TiO2.UV Raman spectroscopy is found to be more sensitive tothe surface phase of a solid sample when the sample absorbsUV light.[33]We studied the phase transition of zirconia (ZrO2)from tetragonal phase to monoclinic phase by UV Ramanspectroscopy, visible Raman spectroscopy, and XRD.[33] Theseresults clearly indicated that the surface phase of ZrO2 is usuallydifferent from the bulk phase of ZrO2 and the phase transforma-tion of ZrO2 starts from its surface region and then graduallydevelops into its bulk when the ZrO2 with tetragonal phase iscalcined at elevated temperatures.These findings lead us to further investigate the phasetransformation in the surface region of TiO2 by UV Ramanspectroscopy as TiO2 also strongly absorbs UV light. In thisstudy, we compared the Raman spectra of TiO2 calcined atdifferent temperatures with excitation lines in the UV and visibleregions. XRD and transmission electron microscopy (TEM)were also recorded to understand the process of phase transformationof TiO2. It was found that the results of UV Ramanspectra are different from those of visible Raman spectra andXRD patterns. The anatase phase of TiO2 at the surface regioncan remain at relatively higher temperatures than that in thebulk at elevated calcination temperatures; namely, the anatasephase in the inner region of the agglomerated TiO2 particlesturns out to change into the rutile phase more easily than thatin the outer surface region of the agglomerated TiO2 particles.The literature[15,17,19] proposed the mechanism that phasetransformation of TiO2 might start at the interfaces of contactinganatase particles. 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 notenter the crystal lattices of TiO2 and was uniformly dispersedonto TiO2in the form of lanthana (La2O3) particles with smallsize. To verify the above assumption, this study also preparedthe anatase phase of TiO2 sample covered with La2O3 andcharacterized the above sample by visible Raman spectroscopyand UV Raman spectroscopy. The results of the two types ofRaman spectra are in agreement with each other and show thatthe TiO2 particle covered with La2O3 can retain its anatase phaseboth in the bulk and in the surface region even after calcinationat 900 °C.2. Experimental Section2.1. Catalyst Preparation.2.1.1. Preparation of TiO2. TiO2was prepared by precipitation method. To 100 mL of anhydrousethanol was added 20 mL of titanium(IV) n-butoxide(Ti(OBu)4). This solution was added to a mixture solution ofdeionized water and 100 mL of anhydrous ethanol. The molarratio of the water/Ti(OBu)4was 75. After the formed whiteprecipitate was stirred continuously for 24 h, it was filtered andwashed twice with deionized water and anhydrous ethanol.Finally, the sample was dried at 100 °C and calcined in air attemperatures from 200 to 800 °C for 4 h, and then cooled toroom temperature.2.1.2. Preparation of La2O3-Co V ered TiO2(La2O3/TiO2). Theabove TiO2powder calcined at 500 °C was used as a support.The critical La2O3 loading corresponding to monolayer coverageof La2O3 on the grain surface of TiO2 is 0.27 g/100 m2. [35,36] Onthe basis of the BET surface area of the TiO2 support (54.3m2/g), the monolayer dispersion capacity can also be expressedas 15 wt % La2O3 of the weight of TiO2. La2O3/TiO2 samples,containing different amounts of La2O3(0.5-6 wt %) wereprepared by a wet impregnation method. The support wasimpregnated with aqueous solution of various concentrationsof lanthanum nitrate (La(NO3)3·6H2O) andsubsequently stirredin a hot water bath until it was dried. After the sample waskept at 110 °C overnight, it was calcined at 900 °C in air for 4h. A TiO2 sample was prepared by calcining the TiO2supportat 900 °C for 4 h (denoted as TiO2-900) for comparison withthe La2O3/TiO2 sample. Pure La2O3was obtained by calciningLa(NO3)3·6H2O at 550 °C for 4 h.2.2. Characterization.2.2.1. UV Raman Spectroscopy. UVRaman spectra were measured at room temperature with a Jobin-Yvon T64000 triple-stage spectrograph with spectral resolutionof 2 cm-1. The laser line at 325 nm of a He-Cd laser wasused as an exciting source with an output of 25 mW. The powerof laser at the sample was about3.0 mW. The 244 nm linefrom a Coherent Innova 300 Fred laser was used as anotherexcitation source. The power of the 244 nm line at sample wasbelow 1.0 mW.2.2.2. Visible Raman Spectroscopy. Visible Raman spectrawere recorded at room temperature on a Jobin-Yvon U1000scanning double monochromator with the spectral resolutionof 4 cm-1. The line at 532 nm from a DPSS 532 Model 200532 nmsingle-frequency laser was used as the excitation source.2.2.3. X-ray Powder Diffraction (XRD), TEM, and Ultra V iolet-Visible Diffuse Reflectance Spectroscopy. XRD patterns wereobtained on a Rigaku MiniFlex diffractometer with a Cu KRradiation source. Diffraction patterns were collected from 20°to 80°at a speed of 5°/min. TEM was taken on a JEM-2011TEM for estimating particle size and morphology. UV-visdiffuse reflectance spectra were recorded on a JASCO V-550UV-vis spectrophotometer.2.2.4. Brunauer-Emmett-Teller (BET) Specific SurfaceArea. The BET surface area of the TiO2 support was measuredby nitrogen adsorption at 77 K using a Micromeritics ASAP2000 adsorption analyzer.3. Results3.1. Spectral Characteristics of Anatase and Rutile TiO2.The anatase and rutile phases of TiO2 can be sensitivelyidentified 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 band), and 639cm-1.[37] These bands can be attributed to the six Raman-activemodes of anatase phase with the symmetries of Eg, Eg, B1g,A1g, B1g, and Eg, respectively.[37] The typical Raman bands dueto rutile phase appear at 143 (superimposed with the 144 cm-1band due to anatase phase), 235, 447, and 612 cm-1, whichcan be ascribed to the B1g, two-phonon scattering, Eg, and A1gmodes of rutile phase, respectively.38Additionally, the band at144 cm-1is the strongest one for the anatase phase and theband at 143 cm-1 is the weakest one for the rutile phase. PartsA and B, respectively, of Figure 1display the Raman spectra ofTiO2 calcined at 500 and 800 °C with excitation lines at 532,325, and 244 nm. Obviously, both visible Raman spectra andUV Raman spectra show that the TiO2 sample is in the anatasephase (Figure 1A) and rutile phase (Figure 1B).Figure 2 shows UV-vis diffuse reflectance spectra of theTiO2 sample calcined at 500 and 800 °C (the TiO2 sample is inthe anatase phase and rutile phase, respectively). For the anatasephase, the maximum absorption and the absorption band edgecan be estimated to be around 324 and 400 nm, respectively.The maximum absorption and the absorption band edge shiftto a little longer wavelength for the rutile phase.[39]By comparing the Raman spectra of the anatase (Figure 1A)or rutile phase (Figure 1B) excited by 532, 325, and 244 nmlines, it is found that the relative intensities of characteristicbands due to anatase or rutile phase in the high-frequency regionare different. For the anatase phase (Figure 1A), the band at638 cm-1 is the strongest one in the Raman spectrum with theexcitation line at 325 or 532 nm, while the band at 395 cm-1 isthe strongest one in the Raman spectrum with the excitationline at 244 nm.For the rutile phase (Figure 1B), the intensities of the bandsat 445 and 612 cm-1 are comparable in the visible Ramanspectrum. The intensity of the band at 612 cm-1 is strongerthan that of the band at 445 cm-1 in the Raman spectrum withthe excitation line at 325 nm, and the reverse is true for theRaman spectrum with the excitation line at 244 nm. In addition,for the rutile phase, a band at approximately 826 cm-1 appearsin the UV Raman spectra. Some investigations show that therutile phase of TiO2 exhibits a weak band at 826 cm-1 assignedto the B2g mode.[38,40]The fact that the relative intensities of the Raman bands ofanatase phase or rutile phase are different for UV Ramanspectroscopy and visible Raman spectroscopy are mainly dueto the UV resonance Raman effect because the laser lines at325 and 244 nm are in the electronic absorption region of TiO 2(Figure 2). There is no resonance Raman effect observed forthe TiO 2 sample excited by visible laser line, because the lineat 532 nm is outside the absorption region of TiO 2 (Figure 2).Therefore, for the anatase or rutile phase, the Raman spectroscopiccharacteristics in the visible Raman spectrum are differentfrom those in the UV Raman spectrum. When the UV laserline with different wavelengths is used as the excitation source,the resonance enhancement effect on the Raman bands ofanatase or rutile phase is different. For example, for the rutilephase (Figure 1B), the band at 612 cm -1 is easily resonanceenhanced when the excitation wavelength is 325 nm. Amongall the characteristic bands of the rutile phase, the extent ofresonance enhancement of 445 cm -1 is the strongest when the244 nm laser is used as the excitation source (Figure 1B).3.2. Semiquantitative Analysis of the Phase Compositionof TiO 2 by XRD and Raman Spectroscopy. The weightfraction of the rutile phase in the TiO 2 sample, W R, can beestimated from the XRD peak intensities using the followingformula:[41]]84.801[1）（rut ana A A W R +=where A ana and A rut represent the X-ray integrated intensities ofanatase (101) and rutile (110) diffraction peaks, respectively.To estimate the weight fraction of the rutile phase in the TiO 2sample by Raman spectroscopy, pure anatase phase and purerutile phase of the TiO 2 sample, which have been prepared bycalcination of TiO 2 powder at 500 and 800 °C for 4 h, weremechanically mixed at given weight ratio and ground carefullyto mix sufficiently. Figure 3A displays the visible Raman spectra of the mechanicalmixture with 1:1, 1:5, 1:10, 1:15, 5:1, and 10:1 ratios ofanatase phase to rutile phase. The relationship between the arearatios of the visible Raman band at 395 cm-1 for anatase phaseto the band at 445 cm -1 for rutile phase (A 395 cm -1/A 445 cm -1) andthe weight ratios of anatase phase to rutile phase (W A /W R ) isplotted in Figure 3B. It can be seen that a linear relationshipbetween the band area ratios and the weight ratios of anatasephase to rutile phase in the mixture is obtained. The rutilecontent in the Degussa P25, which usually consists of roughlyabout 80% anatase and 20% rutile phase,42 was estimated bythis plot. Our Raman result indicates that the rutile content inthe Degussa P25 is about 18.7%, which is close to the knownresult. Thus, the above linear relationship based on visibleRaman spectroscopy can be used to estimate the rutile contentin TiO 2.Figure 4A presents the UV Raman spectra of the mechanicalmixture with 1:1, 1:2, 1:4, 1:6, 1:10, and 1:15 ratios of anatasephase to rutile phase with the excitation line at 325 nm. Figure4B shows the plot of the area ratios of the UV Raman band at612 cm-1 for rutile phase to the band at 638 cm-1 for anatasephase (A 612 cm -1/A 638 cm -1) versus the weight ratios of rutile phaseto anatase phase (W R/W A). There is also a linear relationshipbetween the band area ratios and the weight ratios of rutile phaseto anatase phase.3.3. Phase Transformation of TiO 2 at Elevated CalcinationTemperatures.3.3.1. XRD Patterns and Visible Raman Spectraof TiO2Calcined at Different Temperatures. Figure 5 showsthe XRD patterns of TiO2calcined at different temperatures.The “A” and “R” in the figure denote the anatase and rutilephases, respectively. For the sample before calcination, diffractionpeaks due to the crystalline phase are not observed,suggesting that the sample is still in the amorphous phase. Whenthe sample was calcined at 200 °C, weak and broad peaks at2θ=25.5°, 37.9°, 48.2°, 53.8°, and 55.0° were observed. Thesepeaks represent the indices of (101), (004), (200), (105), and(211) planes of anatase phase, respectively.[43] These results suggest that some portions of the amorphous phase transforminto the anatase phase. The diffraction peaks due to anatasephase develop with increasing the temperature of calcination.When the calcination temperature was increased to 500 °C, thediffraction peaks due to anatase phase became narrow andintense in intensity. This indicates that the crystallinity of theanatase phase is further improved.[44]When the sample was calcined at 550 °C, weak peaks wereobserved at 2θ=27.6°, 36.1°, 41.2°, and 54.3°, whichcorrespond to the indices of (110), (101), (111), and (211) planesof rutile phase. [43]This indicates that the anatase phase starts totransform into the rutile phase at 550 °C. The diffraction peaksof anatase phase gradually diminish in intensity and thediffraction patterns of rutile phase become predominant withthe calcination temperatures from 580 to 700 °C. These resultsclearly show that the phase transformation from anatase to rutileprogressively proceeds at the elevated temperatures. The diffractionpeaks assigned to anatase phase disappear at 750 °C,indicating that the anatase phase completely changes into therutile phase. The diffraction peaks of rutile phase became quitestrong and sharp after the sample was calcined at 800 °C, owingto the high crystallinity of the rutile phase.Figure 6 displays the visible Raman spectra of TiO2 calcinedat different temperatures. For the sample before calcination, twobroad bands at about 430 and 605 cm-1are observed, indicatingthat the sample is in the amorphous phase.[19]For the samplecalcined at 200 °C, a Raman band at 143 cm-1 is observed andthe high-frequency region shows interference from the fluorescencebackground, which might come from organic species.After calcination at 300 °C, other characteristic bands of anatasephase appear at 195, 395, 515, and 638 cm-1, but some portionsof the sample may exist in the amorphous phase because thereis still a broad background in Figure 6.It was found that, when the sample was calcined at 400 °C,the fluorescence disappeared, possibly because the organicresidues were removed by the oxidation. The bands of anatasephase increased in intensity and decreased in line width whenthe sample was calcined at 500 °C. This result suggests thatthe crystallinity of the anatase phase is greatly improved,[18] whichis confirmed by XRD (Figure 5). The enlarged section of Figure6 shows the Raman spectrum of the high-frequency region ofthe sample calcined at 500 °C. Besides the bands at 395, 515,and 638 cm-1, two very weak bands at 320 and 796 cm-1areobserved. These two bands can be assigned to a two-phononscattering band and a first overtone of B1g at 396 cm-1,respectively. [37]It is noteworthy that a very weak band appearsat 445 cm-1 due to rutile phase for the sample calcined at 550°C. This indicates that the anatase phase starts to change intothe rutile phase at 550 °C. This result is in good agreementwith that of XRD patterns (Figure 5). The weightpercentageof the rutile phase in the samples calcined at different temperatureswas estimated by visible Raman spectroscopy and XRD(shown in Figure 7). As seen from Figure 7, the rutile contentestimated from visible Raman spectrum and XRD pattern ofthe sample calcined at 550 °C is 4.2% and 5.7%, respectively.It can be seen that the rutile contents estimated by visible Ramanspectroscopy and XRD are also in accordance with each other.When the sample was calcined at 580 °C, other twocharacteristic bands were observed at 235 and 612 cm-1 due toRaman-active modes of rutile phase. Figure 7 shows the rutilecontent is 13.6% and 10.9% based on the visible Ramanspectrum and XRD pattern of the sample calcined at 580 °C.The intensities of the bands of rutile phase (235, 445, and 612cm-1) increased steadily while those of the bands of anatasephase (195, 395, 515, and 638 cm-1) decreased when thecalcination temperatures were elevated from 600 to 680 °C(Figure 6). These results suggest that the TiO2 sample undergoesthe phase transformation from anatase to rutile gradually. Therutile content was estimated for the samples calcined from 600to 680 °C based on the visible Raman spectra. The results showthat the content of the rutile phase is increased from 33.1% to91.2% respectively for the samples calcined at 600 and 680 °C(Figure 7). The XRD results corresponding to the above twosamples indicate that the rutile content is changed from 32.9%to 90.7% (Figure 7). These results clearly show that the rutilecontent in the sample estimated by visible Raman spectroscopyis agreement with that estimated by XRD.The Raman spectrum of the sample calcined at 700 °C showsmainly the characteristic bands of rutile phase, but the very weakbands of anatase phase are still observed (Figure 6). When thesample was calcined at 750 °C, the bands of anatase phasedisappeared and only the bands due to rutile phase (143, 235,445, and 612 cm-1) were observed. These results indicate thatthe anatase phase completely transforms into the rutile phaseand are consistent with the results from XRD (Figure 5). Whenthe temperature was increased to 800 °C, the characteristic bandsdue to rutile phase increased in intensity further.Both the results of XRD and visible Raman spectra (Figures5 and 6) show that the anatase phase appears at around 200 °Cand perfect anatase phase is formed after calcination at temperaturesof 400-500 °C. The rutile phase starts to form at 550°C, and the anatase phase completely transforms into the rutilephase at 750 °C. The signals of visible Raman spectra comemainly from the bulk region of TiO2because the TiO2 sampleis transparent in the visible region (Figure 2).[33]XRD is knownas a bulk-sensitive method. Therefore, it is essentially inagreement between the results of visible Raman spectra andXRD patterns.3.3.2. UV Raman Spectra of TiO2 Calcined at DifferentTemperatures. UV-vis diffuse reflectance spectra (Figure 2)clearly show that TiO2 has strong electronic absorption in theUV region. Thus, the UV Raman spectra excited by a UV laserline contain more signal from the surface skin region than thebulk of the TiO2 sample because the signal from the bulk isattenuated sharply due to the strong absorption.[33] Therefore, ifa UV laser line in the absorption region of TiO2is used as theexcitation source of Raman spectroscopy, the information fromUV Raman spectra is often different fromthat of visible Ramanspectra.The laser line at 325 nm was selected as the excitation sourceof the UV Raman spectra. The UV Raman spectra and thecontent of the rutile phase of the TiO2 sample calcined atdifferent temperatures are shown in parts A and B, respectively,of Figure 8. When the sample was calcined at 200 or 300 °C,the Raman band at 143 cm-1 with a shoulder band at 195 cm-1and 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 intensity and the broad band indicate thatthe amorphous phase still remains in the sample. It can be seenthat the fluorescence in the high-frequency region can be avoidedwhen the UV laser line is used as the excitation line. However,the corresponding visible Raman spectra (Figure 6) showinterference from the fluorescence.All bands assigned to anatase phase become sharp and strongafter calcination at 500 °C (Figure 8A). These results are inagreement with those of XRD and visible Raman spectra(Figures 5 and 6). The UV Raman spectra of the sample withthe calcination temperatures from 550 to 680 °C are essentiallythe same as those of the sample calcined at 500 °C (Figure 8A).However, according to the XRD patterns and visible Ramanspectra (Figures 5 and 6), the anatase phase starts to transforminto the rutile phase at only 550 °C and the anatase phasegradually changes into the rutile phase in the temperature rangeof 550-680 °C.After calcination at 700 °C, a new band at 612 cm-1 andtwo weak bands at 235 and 445 cm-1 due to rutile phase appearwhile the intensities of the bands of anatase phase begin todecrease (Figure 8A). On the basis of the UV Raman spectrumand XRD pattern of the sample calcined at 700 °C, the rutilecontent in the sample is 56.1% and 97.0%, respectively (Figure8B). It is found that the rutile content estimated by UV Ramanspectroscopy is far less than that estimated by XRD.When the sample was calcined at 750 °C, the intensities ofthe bands due to rutile phase increased, but the intensities ofthe bands due to anatase phase were still strong in the UVRaman spectra (Figure 8A). The UV Raman spectrum of thesample calcined at 750 °C indicates that the rutile content is84.3% (Figure 8B). However, the results of XRD and visibleRaman spectrum (Figures 5 and 6) suggest that the anatase phasetotally transformed into the rutile phase after the sample wascalcined at 750 °C. The characteristic bands due to anatase phasedisappear and the sample is in the rutile phase after calcinationat 800 °C (Figure 8A). Obviously, there are distinct differences between the results from the UV Raman spectra, visible Ramanspectra, and XRD patterns. It seems that the anatase phaseremains at relatively higher temperatures when detected by UVRaman spectroscopy than by XRD and visible Raman spectroscopy. Another UV laser line at 244 nm was also selected as theexcitation source of UV Raman spectroscopy in order to getfurther insights into the phase transformation of TiO2. Theresults of the UV Raman spectra of TiO2 calcined at differenttemperatures with the excitation line at 244 nm are presentedin Figure 9. When the sample was calcined at 200 °C, four broadbands were observed at 143, 395, 515, and 638 cm-1, whichclearly indicate that the anatase phase exists in the sample. Theintensities of the Raman bands due to anatase phase (143, 395,515, and 638 cm-1) become strong after calcination at 500 °C.The UV Raman spectra hardly change for the sample calcinedatdifferent temperatures even up to 680 °C. The characteristicbands (445 and 612 cm-1) of rutile phase appear only whenthe calcination temperature exceeds 700 °C. This result is ingood agreement with that from the UV Raman spectrum of thesample calcined at 700 °C with 325 nm excitation (Figure 8A).The intensities of the bands due to anatase phase (395, 515,and 638 cm-1) decrease while those of bands assigned to rutilephase (445 and 612 cm-1) increase after calcination at 750 °C(Figure 9). When the sample was calcined at 800 °C, the Ramanbands due to anatase phase disappeared, while the bands of rutilephase developed. This result indicates that the sample calcinedat 800 °C is in the rutile phase. It is interesting to note that theresults of the UV Raman spectra with the excitation lines at325 and 244 nm are in agreement with each other but aredifferent from those of XRD patterns and visible Raman spectra.3.3.3. TEM of the TiO2 Sample Calcined at DifferentTemperatures. TEM was used to characterize the microstructureof the TiO2 sample calcined at 500, 600, and 800 °C (shown inFigure 10). Most particles in the sample calcined at 500 °Cexhibit diameters in a range between 10 and 30 nm (Figure10a). On the other hand, remarkable agglomeration is observedfor the TiO2 sample calcined at 500 °C. The particle sizeincreases after calcination at 600 °C (Figure 10b). Accordingto the results of XRD and visible Raman spectra (Figures 5and 6), the sample undergoes the phase transformation fromanatase to rutile gradually in the temperature range of 550-680 °C. These results imply that the phase transformation andgrowth of the particle size are interrelated. Many researchers[45-49]reported similar phenomena. Kumar et al.45 attributed thisparticle size growth to the higher atomic mobility because ofbond breakage during the phase transformation. When thecalcination temperature was increased to 800 °C, TiO2 particlesfurther grew and the particle size could be as large as about200 nm (Figure 10c).3.4. Visible Raman Spectra and UV Raman Spectra ofLa2O3/TiO2 with Increasing La2O3 Loading. The literature[15,17,29]proposed the mechanism that the phase transformationof TiO2might start at the interfaces of contacting anataseparticles. If direct contact between anatase particles of TiO2 isavoided, the phase transformation of TiO2 from anatase to rutilecould be retarded or prohibited. This assumption may be verifiedby covering the surface of anatase TiO2 with an additive. Inthis work, we used La2O3dispersed on anatase TiO2to preventthe anatase particles from contacting directly because it wasreported that La2O3could be highly dispersed on anataseTiO2.[34-36]Figure 11A displays the visible Raman spectra of La2O3/TiO2with increasing La2O3 loading. TiO2 support is in the anatasephase because only characteristic bands (143, 195, 395, 515,and 638 cm-1) due to anatase phase are observed. When theTiO2 support was calcined at 900 °C (TiO2-900), the Ramanspectrum gave the characteristic bands of rutile phase, indicatingthat the TiO2-900 sample was in the rutile phase. When thesample with 0.5 wt % La2O3 loading was calcined at 900 °C,the Raman spectrum drastically changed, as compared to theTiO2-900 sample. Only characteristic bands of anatase phasewere observed, suggesting that the TiO2 sample retains itsanatase phase when La2O3 loading is 0.5 wt % while the TiO2-900 sample is in the rutile phase. The。
