1、附录 UV Raman Spectroscopic Study on TiO2. I. Phase Transformation at the Surface and in theBulk Jing 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, 2005； In Final Form: NoVember 4, 2005 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 Ra
3、man 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 sp
4、ectroscopy 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 ag
5、glomerate into big particles when the TiO2 sample 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
6、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 disper
7、sed 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. Introduction Titania (TiO2) has been widely studied because of its
8、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 pha
9、se 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.8It has been well demonstratedthat the crystalline phase of TiO2 plays a significant rolein catalytic reactions, especially ph
10、otocatalysis.9-11Some studieshave claimed that the anatasephase was more active than therutile phase in photocatalysis.9,10 Although at ambient pressure and temperature the rutile phaseis more thermodynamically stable than the anatase phase,12anatase is the common phase rather than rutile because an
11、ataseis kinetically stable in nanocrystalline TiO2 at relatively lowtemperatures.13 It is believed that the anatase phase transformsto the rutile phase over a wide range of temperatures.14Therefore, understanding and controlling of the crystalline phaseand the process of phase transformation of TiO2
12、 are important,though they are difficult. Many studies13-31have been done to understand the processof the phase transformation of TiO2. Zhang et al.15proposedthat the mechanism of the anatase-rutile phase transformationwas temperature-dependent according to the kinetic data fromX-ray diffraction (XR
13、D). 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.17suggested thatthe formation of
14、 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 photocatalyt
15、ic activity because not only the photoinduced reactions take place on the surface32 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, t
16、he 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 transform
17、ation 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.33We studied
18、 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
19、 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 ligh
20、t. 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
21、Raman spectra 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 TiO
22、2 particlesturns out to change into the rutile phase more easily than thatin the outer surface region of the agglomerated TiO2 particles. The literature15,17,19 proposed the mechanism that phasetransformation of TiO2 might start at the interfaces of contactinganatase particles. If the anatase partic
23、les of TiO2 are separated,the phase transformation of TiO2 from anatase to rutile couldbe retarded or prohibited. Jing et al.34showed that La3+ did notenter the crystal lattices of TiO2 and was uniformly dispersedonto TiO2 in the form of lanthana (La2O3) particles with smallsize. To verify the above
24、 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
25、 La2O3 can retain its anatase phaseboth in the bulk and in the surface region even after calcinationat 900 C. 2. Experimental Section 2.1. 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-butox
26、ide(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)4 was 75. After the formed whiteprecipitate was stirred continuously for 24 h, it was filtered andwashed twice with deionized water and anhydrous ethanol
27、.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-CoVered TiO2 (La2O3/TiO2). Theabove TiO2 powder calcined at 500 C was used as a support.The critical La2O3 loading corresponding to mo
28、nolayer 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.
29、5-6 wt %) wereprepared by a wet impregnation method. The support wasimpregnated with aqueous solution of various concentrationsof lanthanum nitrate (La(NO3)36H2O) andsubsequently stirredin a hot water bath until it was dried. After the sample was kept at 110 C overnight, it was calcined at 900 C in
30、air for 4h. A TiO2 sample was prepared by calcining the TiO2 supportat 900 C for 4 h (denoted as TiO2-900) for comparison withthe La2O3/TiO2 sample. Pure La2O3 was obtained by calciningLa(NO3)36H2O at 550 C for 4 h. 2.2. Characterization. 2.2.1. UV Raman Spectroscopy. UVRaman spectra were measured a
31、t 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 about 3.0 mW. The 244 nm linefrom a Coherent Innova 300 Fred l
32、aser was used as another excitation 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 nm
