Raman spectroscopic study on pressure-induced amorphization in nanocrystalline anatase (TiO2)

Raman spectroscopic study on pressure-induced amorphization innanocrystalline anatase (TiO 2)Zhongwu Wang *,S.K.SaxenaCenter for Study of Matter at extreme conditions (CeSMEC),Florida International University,VH-150,University Park,Miami,FL 33199,USA Received 16November 2000;received in revised form 29January 2001;accepted 30January 2001by F.J.DiSalvoAbstractA raman spectroscopic investigation was carried out to study the pressure-induced phase transformation in nanocrystalline anatase (TiO 2)up to 37GPa.As compared to a macrocrystalline solid which transforms to the a -PbO 2structure at 2.6±4.5GPa,we found that the nano-anatase phase remains stable to pressures as high as ,24GPa,and then transforms to an amorphous phase.The new amorphous phase is quenchable upon release of pressure to ambient condition.An increased surface energy may stabilize the nano-anatase,suppressing the appearance of anatase-to-a -PbO 2phase transformation (2.6±4.5GPa in bulk anatase),and then leads to the amorphization of nano-anatase under strong compression.q 2001Elsevier Science Ltd.All rights reserved.Keywords:A.Nanostructures;D.Phase transition;D.Order±disorder effects;D.Raman spectroscopy PACS :62.50.1p;61.82.Rx;64.70.Kb1.IntroductionNanocrystalline solids are de®ned as materials with crystal size between 10±100nm.They have attracted signi®cant interest due to their special properties [1].Numerous nanocrystalline metals,ceramics and semi-conductors have been synthesized and studied [2,3].Nano-crystalline materials have large surface to volume ratios and fewer dislocations as compared to bulk samples.There is lattice relaxation at grain boundaries.Each of these characters can have signi®cant impact on the physical and electronic properties of the material.Studies have shown that various properties such as stability,hardness,melting temperature,sintering ability,compressibility and electronic structure can be dependent upon particle size [4±9].Nanocrystalline TiO 2has been the subject of considerable experimental and theoretical work in recent years [10±13].The mechanical,optical,electrical behavior,and vibrational modes of nano-TiO 2have been widely studied [10,12,13].In order to better understand the physical and vibrationalproperties of nano-TiO 2,the internal strain and grain boundary structure of nano-TiO 2have been extensively investigated by transmission electron microscopy,X-ray diffraction,and Raman and FTIR spectroscopy [10,11,12,13].But there is little study on the pressure dependence of phase transformations in nano-TiO 2.At ambient pressure,bulk TiO 2crystallizes in three modi-®cations:rutile,brookite and anatase.Because of the simpli-city of synthesis with application in materials science (e.g.nano-techniques)and the relevance to geophysics,high-pressure behavior of bulk anatase has been investigated experimentally by using Raman spectroscopy and X-ray diffraction [14±18].To explore the different dynamics caused by external pressure in nanoparticle materials,we investigated the high-pressure behavior of nano-anatase by using DAC technique together with in-situ Raman spectro-scopy.Here,we report on the pressure-induced amorphiza-tion in nanosized (the average size of 9nm)anatase (TiO 2),and its pressure dependence of Raman vibrational modes.2.ExperimentalThe sample used here was commercial anataseSolid State Communications 118(2001)75±780038-1098/01/$-see front matter q 2001Elsevier ScienceLtd.Allrights reserved.PII:S0038-1098(01)00046-1*Corresponding author.Tel.:11-305-348-3445;fax:11-305-348-3070.E-mail address:zwang04@® (Z.Wang).nanomaterial,with a particle size within 7±11nm.It was identi®ed with X-ray diffraction.At atmospheric pressure,the powder anatase exhibited spectral features similar to these of single crystal [10,19],but the Raman peak in low wave number (141cm 21in bulk-and 158cm 21in nano-anatase)is a little different [12±14],and this is likely caused by heating and plasma effect from laser.High pressure Raman measurements were carried out at room temperature by using a gaseketed high pressure Diamond Anvil Cell (DAC)and Raman spectrometer in the back scattering con®guration.An argon ion laser in combination with an additional Ti 31:sapphire laser was operated at 785nm.This design can effectively suppress the strong ¯uorescence effect of diamond [20].The laser power was operated at 30±50mw (after ®lter)to excite the sample.The Raman spectra were collected by using high throughput holographic imaging spectrograph with volume transmission grating,holographic notch ®lter and thermoelectrically cooled CCD detector with the resolution of 4Rcm 21[20].A 4:1mixture of methanol:ethanol was used as pressure medium.This mixture is known to produce a hydrostatic environment to about 10GPa.Pressures were determined using the pressure dependent spectral shift of the sharp ruby ¯uorescence R 1line [21].The R 1line ¯uorescence was excited by the argon green laser with the wave number of 514.5nm.The sample was placed in a 301steel gasket hole 115m m in initial thickness and 150m m in diameter with a ruby chip.The gasket hole was then completely ®lled with the alcohol mixture to ensure a hydrostatic environ-ment.Moreover,another run without any pressure medium was carried out to check the effect of deviatoric stress on Raman scattering.3.Results and discussionThe Raman spectra of nano-crystalline anatase with a 4:1mixture of methanol/ethanol at high pressures are shown in Fig.1.A signi®cant spectral change occurs at pressure of 24GPa.As is well known,anatase belongs to the spacegroup D 194h (I41/amd)and the primitive unit cell contains two formula units [14].According to the factor group analy-sis,the six modes (A 1g 12B 1g 13E g )are Raman active and the three modes (A 1u 12E u )infrared active [14].At low pressure,all Raman active modes are in reasonable agree-ment with these reported previously (Fig.1)[10,13].Because of the plasma effect from Ti 31:sapphire laser and heating in sample,the lowest wave number mode (E g )in 162cm 21is a little higher than that (158cm 21)observed previously at ambient condition by using an argon ion laser of 488nm [10,13],and also shows a relatively weak peak.Our studies on other powders including garnets and oxides show a similar phenomenon,which can be effectively suppressed under high pressures [20].This can also be found in Fig.1,showing that the above Raman peak inlow wave-number mode (E g )gets stronger with increasing pressure.As shown in Fig.2,the four strong Raman peaks,assigned to two E g modes and one B 1g mode and one A 1g 1B 1g duplicated mode,shift to higher frequencies under compression.The three high wave-number modes have similar Raman shift slope of 2.7cm 21/GPa,similar to these observed in bulk materials [17].One low wave number mode (E g )shows a lower slope of 1.5cm 21/GPa,much lower than that in bulk materials [17].With increasing pressure to about 24GPa,all Raman modes disappear.In stead,a halo starts to arise and remains up to our investi-gated pressure of 37GPa (Fig.1).It means that a pressure-induced amorphization occurs at pressure of about 24GPa.Upon release of pressure,the amorphous phase was quench-able to ambient pressure.In the run of free pressure medium,the spectral depen-dence of pressure is the almost same as that with pressure medium,and the amorphization occurs at same pressure of 24GPa.Also,a few ruby chips in the loading sample were used to examine the pressure gradient,indicating that no remarkable pressure gradient cross the sample exists at pressure lower than 10GPa.This implies that the nano-crystalline anatase may have a low bulk modulus as compared with the bulk anatase (179GPa)[15].Z.Wang,S.K.Saxena /Solid State Communications 118(2001)75±7876Fig.1.High pressure Raman spectra of nano-anatase at room temperature.The spectra were collected from sample with a pres-sure medium of a 4:1mixture of methanol/ethanol.The run without pressure medium has the same spectral dependence on pressure.For the bulk crystalline anatase,both Raman and X-ray diffraction studies indicate that anatase transforms to the orthorhombic columbite phase(a-PbO2structure with space group Pbcn,60)at2.6±4.5GPa,and then to the monoclinic baddeleyite phase(ZrO2structure with space group P21/c,14)at13GPa[14±19].Upon decompression, the baddeleyite phase transforms to the a-PbO2phase at about7GPa[16].As compared to the bulk materials, nano-anatase has a higher ratio of surface energy to volume and a ratio of lower defect number to volume[2,3].Also, nano-crystalline anatase may have different physical proper-ties,likely having a lower bulk modulus.Thus,a high energy kinetic hindrance may exists in the phase trans-formation of anatase-to-a-PbO2due to the size-induced effect,so it is likely to result in anatase stable up to very high pressure(24GPa),and then in turn to transform to an amorphous phase,rather than a new a-PbO2phase or ZrO2 phase at high pressure.4.ConclusionAn in-situ Raman spectroscopic study of nanocrystalline anatase with an average size of9nm has been conducted up to37GPa.It is shown that the pressure-induced amorphiza-tion in nano-anatase takes place at pressures around24GPa. The decrease of the particle size results in an anatase phase stable up to very high pressure,and suppresses the appear-ance of the anatase-a-PbO2phase transformation observed in the corresponding bulk material.It is suggested that a higher ratio of surface energy to volume and a ratio of defect number to volume as well as the size-induced effect in nano-anatase compared with the bulk material may stabilize TiO2 in the original low pressure anatase phase,and then lead to the amorphization of nano-anatase under strong compres-sion.AcknowledgementsWe thank Florida International University for the research was made possible through®nancial support from the Division of sponsored Research at FIU.We also thank Peter Lazor for invaluable discussion,Debby Arnold for editorial and Peter Liermann for technical assistance.References[1]H.Gleiter,Prog.Mater.Sci.33(1989)223±315.[2]R.W.Siegel,Mater.Sci.Engng.A168(1993)189±197.[3]A.P.Alivisatos,P.F.Barbara,A.W.Castleman,J.Chang,D.A.Dixon,M.L.Klein,G.L.McLendon,ler,M.A.Ratner, P.J.Rossky,S.I.Stupp,M.E.Thompson,Adv.Mater.10 (1998)1297±1336.[4]C.-C.Chen,A.B.Herhold,C.S.Johnson,A.P.Alivisatos,Science276(1997)398±401.[5]S.H.Tolbert,A.B.Herhold,L.E.Brus,A.P.Alivisatos,Phys.Rev.Lett.76(1996)4384±4387.[6]S.H.Tolbert,A.P.Alivisatos,J.Chem.Phys.102(1995)4642±4671.[7]S.B.Qadri,J.Yang,B.R.Ratna,E.F.Skelton,J.Z.Hu,Appl.Phys.Lett.69(1996)2205±2207.[8]J.Z.Jiang,L.Gerward,R.Secco,R.Frost,J.Appl.Phys.87(2000)2658±2660.Z.Wang,S.K.Saxena/Solid State Communications118(2001)75±7877Fig.2.Pressure dependence of the peak frequencies of the Raman active modes of anatase up to24GPa at room temperature.[9]A.N.Goldstein,C.M.Echer,A.P.Alivisatos,Science256(1992)l425±1428.[10]W.F.Zhang,Y.L.He,M.S.Zhang,Z.Yin,Q.Chen,J.Phys.D.Appl.Phys.33(2000)912±916.[11]S.D.Mo,W.Y.Ching,Phys.Rev.B.51(1995)13023±13032.[12]D.Bersani,P.P.Lottici,X.Z.Ding,Appl.Phys.Lett.72(1998)73±75.[13]J.C.Parker,R.W.Siegel,Appl.Phys.Lett.57(1990)943±945.[14]M.Nicol,M.Y.Fong,J.Chem.Phys.54(1971)3167.[15]T.Arlt,M.Bermejo,M.A.Blanco,L.Gerward,J.Z.Jiang,J.S.Olsen,J.M.Recio,Phys.Rev.B61(2000)14414±14419.[16]J.K.Dewhurst,J.E.Lowther,Phys.Rev.B54(1996)R3673±R3676.[17]T.Ohsaka,S.Yamaoka,O.Shimomura,Solid State Commun.30(1979)345±347.[18]J.F.Mammone,S.K.Sharma,M.Nicol,Solid State Commun.34(1980)799±802.[19]T.Ohsaka,F.Izumi,Y.Fujiki,J.Raman Spectrosc.7(1978)321.[20]O.N.Shebanova,zor,Z.Wang,submitted for publica-tion.[21]H.K.Mao,P.W.Bell,J.W.Shaner,D.J.Steinberg,J.Appl.Phys.49(1978)3276±3284.Z.Wang,S.K.Saxena/Solid State Communications118(2001)75±78 78。