TEM Investigation and FBB Model Explanation to the Phase Relationships between(1)

TEM Investigation and FBB Model Explanation to the Phase Relationships between Titanates and Titanium DioxidesHongwei Liu,Eric R.Waclawik,Zhanfeng Zheng,Dongjiang Yang,Xuebin Ke,Huaiyong Zhu,and Ray L.Frost*Discipline of Chemistry,Faculty of Science and Technology,Queensland Uni V ersity of Technology,Brisbane QLD 4001,AustraliaRecei V ed:April 23,2010;Re V ised Manuscript Recei V ed:May 31,2010Sodium and hydrogen titanates fibers and three TiO 2polymorphs (TiO 2(B),anatase,and rutile)were prepared from inorganic titanium compounds by hydrothermal reaction and calcination.The nature and morphologies of the nanofibers with layered structure were investigated by means of X-ray diffraction (XRD)and transmission electron microscopy (TEM).It was revealed that the phase transitions between titanates and titania (via proton titanate)could be interpreted systemically by the fundamental building blocks (FBB)model.The reaction also requires interconversion between some corner-linked and edge-linked pairs of TiO 6octahedrons.A generalized relationship of phase transitions between sodium/hydrogen titanates and titania by the wet-chemical process was proposed.A duplex pentagonal prism scheme was implied to summarize all of the phase relationships between titanates and titania.I.IntroductionMuch attention has been put into the design and fabrication of nanostructures based on metal oxides in the past decade because of their peculiar electronic and optic properties and their potential applications in technology.1-5There is great interest in the development of titanates and TiO 2-based solids with nanoscale dimensions and high morphological specificity 6,7such as nanofibers,8nanosheets,9and nanotubes 10because of their de-monstratedpotentialinsolarenergyconversion,11photocatalysis,12,13photovoltaic devices,14,15and as carrier for metallic nanopar-ticles.16Hydrothermal treatment of different TiO 2precursors in a highly alkaline medium is a powerful way to prepare nanotubes and nanowires,but the information about the structure of the synthesized nanostructures is still unclear despite intensive investigations.In contradiction with previous investigations,Sun et al.17report that as-synthesized nanotubes are titanates and can be described as Na x H 2-x Ti 3O 7.Thermal treatment of these materials leads to the formation of different titanates with the general formula Na 2Ti n O 2n +1.Recently,it was also mentioned that as-synthesized nanotubes or nanorods have an even more complex structure,Na x H 2-x Ti n O 2n +1·y H 2O.18-20Treatment of these nanoparticles with a HCl solution produced nanotubes and nanorods of H 2Ti 3O 7.Although the importance of continuing efforts to develop alternate approaches to the synthesis of the nanostructures has been realized,4the potential for controlled reactions of these nanostructures has not drawn significant attention.In fact,phase transitions at moderate temperature are strongly preferred for constructing inorganic structures on nanometer scale (nano-scale)21because the delicate nanostructures can easily be lost at high temperatures due to sintering.The present authors’recent work has described a reversible scheme of transitions between nanoscale titanium dioxides to and from nanoscale titanates.22We found for the first time that phase transitions from the titanate nanostructures to TiO 2polymorphs take place readilyin wet-chemical processes at temperatures close to ambient.Furthermore,the resultant TiO 2nanocrystals can react with concentrated NaOH solution,yielding hollow titanate nanotubes.In the present work,we report phase transitions from titanate fibers to hexatitanate,TiO 2(B),and anatase.The phase transitions are systematically explained by a fundamental building blocks (FBB)model,and a stereoschematic is implied on the basis of the above explanation.II.Experimental Section1.Sample Preparation.NaOH pellets and HNO 3(both are AR grade from Aldrich)and TiOSO 4·x H 2O (98%,from Fluka)were used in the synthesis.Titanate nanofibers in this study were prepared via a hydrothermal reaction between a concen-trated NaOH solution and an inorganic titanium salt.15Specif-ically,10.7g of TiOSO 4·H 2O was dissolved into 80mL of water and stirred until becoming clear.The resultant TiOSO 4solution was mixed with 100mL of 15M NaOH solution while stirring.The mixture (white suspension)was then transferred into a 200mL Teflon-lined stainless steel autoclave and kept at a temperature between ambient and 200°C for 48h to yield titanate precipitates via a hydrothermal reaction.The white precipitate in the autoclaved mixture was recovered by cen-trifugation and washed with deionized water four times by dispersing the wet cake into 100mL of water and recovering the solid by centrifugation.The sodium titanate product was labeled as T3.T3was calcinated at 500°C for 16h under air flow to obtain a new titanate T6.The sodium titanates T3and T6were then neutralized using 0.1M HCl solution and washed with water to remove most of the sodium ions.We dried the resultant hydrogen titanate (H-titanate;denoted as T3-H and T6-H,respectively)at 100°C for 16h and then dispersed this into a dilute (0.05M)HNO 3solution,at temperatures selected to form anatase (TA).On the other hand,the resultant hydrogen titanate (H-titanate)was carried out at 500°C in an atmosphere of air flow (20cm 3/min)to obtain TiO 2(B)(denoted as “TB”).Standard laboratory safety precautions,including the use of appropriate hoods,*Corresponding author.E-mail:r.frost@.au.J.Phys.Chem.C 2010,114,11430–114341143010.1021/jp103644x 2010American Chemical SocietyPublished on Web 06/14/2010vessels,and safety gloves,were required when handling the strong acids and bases.2.Sample Characterization.The microstructures and mor-phologies of products were investigated using transmission electron microscopy (TEM)and X-ray diffraction (XRD)techniques.TEM images were recorded on a Philips CM20TEM,employing an accelerating voltage of 200kV and double-tilt holder.High-resolution transmission electron microscopy (HRTEM)was carried out on a Philips Tecnai F20under an accelerating voltage of 200kV.XRD patterns of the sample powders were recorded using a Shimadzu XRD-6000diffrac-tometer,equipped with a graphite monochromator.Cu K R radiation (λ)0.15418nm)and a fixed power source (40kV and 40mA)were used.The samples were scanned at a rate of 1°(2θ)/min over a range of 2-80°,which covers the main characteristic diffraction peaks of the titanates,anatase,and TiO 2(B).X-ray photoelectron spectroscopy (XPS)spectra were recorded in an ESCALAB 250spectrometer,and Al K R radiation was used as the X-ray source.The C1s peak at 284.5eV was used as a reference for the calibration of the binding energy (BE)scale.III.Results and Discussion3.1.Phase Transformations from Titanate to Titania.The preparation procedure of the samples used here is shown in Scheme 1summarized from the viewpoint of phase transforma-tion.The phase transformations were investigated by X-ray diffraction (Figure 1).The directly obtained product (T3,Figure 1A)from hydro-thermal reaction is consistent mainly with tritanate phase (Na 2Ti 3O 7,monoclinic,S.G.P 21/m ,a )0.8566nm,b )0.3804nm,c )0.9133nm, )101.57°,ICSD #250000),althoughthere are some impurities,which is similar to the result given by Kolen’ko et al.23X-ray photoelectron spectroscopy (XPS)of the surface indicated that this washed product possessed a sodium content of ∼10wt %Na.The T3was converted to T6shown in Figure 1B (Na 2Ti 6O 13,monoclinic,S.G.C 2/m ,a )1.513nm,b )0.3745nm,c )0.9159nm, )99.30°,ICSD #23877)after calcining at 500°C.Proton titanate T6-H (H 2Ti 6O 13,Figure 1C)was obtained by ion-exchange of the Na with H from the respective sodium titanate T6.It is noted that the basal structure of proton titanate was not changed,although the ion-exchange process results in the decrease of the d -spacing in the layered structure.Calcining the proton titanate T6-H yielded TB shown in Figure 1D (TiO 2(B),monoclinic,S.G.C 2/m ,a )0.6524nm,b )0.3740nm,c )01.218nm, )107.05°,ICSD #41056),TA shown in Figure 1E (anatase,tetragonal,S.G.I 41/amd ,a )0.3782nm,c )0.9502nm,ICSD #63711),depending on the calcination temperature.It is noteworthy that the fibril morphology was heritage from T3for all of the above samples.All of the obtained titania phases can react with concentrated NaOH solution and yield trititanate.In addition,the phases involved here exhibit a complicated transition relationship,which has been summarized in Figure 2.All titanates on the green plane in Figure 2can be converted into titania directly by an acid-assisted wet-chemical process as reported earlier.163.2.TEM Observation.The phase transition process was also confirmed by transmission electron microscopy (TEM)and corresponding electron diffraction (ED)patterns,as shown in Figure 3.The lattice parameters for the samples were calculated from ED patterns and listed in Table 1.Figure 3A shows a typical low-magnification TEM image of as-prepared nanofibers of T3sample.The corresponding ED patterns (Figure 3B)indicate that it has a high crystallinity of the nanostructures,which are also confirmed by XRD.The ED patterns can be indexed as the monoclinic C 2/m space group,using the Na 2Ti 3O 7parameters.Figure 3C shows a typical low-magnification TEM image of as-prepared nanofibers of T6sample.The correspond-ing ED patterns (Figure 3D)indicate that this sample has also a high crystallinity of the nanostructures,which are confirmed by XRD.The ED patterns can be indexed as the monoclinic P 21space group,using the Na 2Ti 6O 13parameters.The zone axis is [001].Figure 3E shows a low-magnification image of proton titanate T6-H (H 2Ti 6O 13),a product of ion exchange of T6.The powder X-ray diffraction pattern of the sample reveals that the crystal structure is almost the same as that of T6because the positions of peaks of the sample are almost coincident with those of T6.It could be deduced that the structure of the sample is the sameSCHEME 1:Schematic Showing Phase Transformation Relationships between the Samples Used in This StudyaaT3(6)represents trititanate (hexatitanate).T3(6)-H represents proton trititanate(hexatitanate).Figure 1.Powder X-ray diffraction patterns of the titania-basednanofibers.Figure 2.A generalized phase transition between sodium-and hydrogen-titanates,anatase,TiO 2(B),and rutile by the wet-chemical process.Phase Relationships between Titanates and Titanium Dioxides J.Phys.Chem.C,Vol.114,No.26,201011431as T6based on the relationship of the crystal structure between T3and T3-H.The ED taken down [001]is shown in Figure 2F.The lattice parameters determined by ED are about 1.46nm for a ,0.382nm for b ,and 1.30for c ,which slightly depart from those of T6.Figure 3G and I show the TEM micrograph of TB (TiO 2(B))and TA (anatase)samples.The corresponding ED for TB sample (Figure 3H)down [001]can be indexed as the monoclinic C 2/m space group,using the TiO 2(B)parameters.The correspondingED down [67j 1]for TA is shown in Figure 3J,and can be indexed as the tetragonal I 41/amd space group,using the anatase parameters.3.3.FBB Model Explanation to Phase Transitions be-tween Titanates and Titania.There are many ways developed to describe the structures and the relationships between inorganic crystals.Veblen has discussed the utility of the poly somatic-series approach.24It cannot be simply applied to describe the TiO 2polymorphs,primarily because of the absence of slabs common to all polymorphs.In this work,the titanium oxide structures will be discussed on the approach based on the use of fundamental building blocks (FBB),25emphasized by the similarities between the structures.Banfield 26has used this model to represent the relationships among the TiO 2minerals.All of the structures except rutile related in this study can be constructed from a unit composed of four edge-sharing octa-hedrons,which are shown in Figure4.The difference between the corner-shared octahedrons and edge-shared octahedrons is marked with arrows.Thus,polyhedral representation of the structures of titanate and titania in this work can be described as Figure 5.It is shown in Figure 5the polyhedral representation of the structures of titanate and titania.All of the projections are viewed down [010]except rutile (down [001]).In Figure 5a,every three TiO 6octahedrons form a slab by edge-sharing,and the slabs connect with each other to form a layer of TiO 6octahedrons in corner-linked mode.Thus,upper-layer and lower-layer TiO 6octahedrons form layered-structural trititanate.If atomic sites between TiO 6slab layers are occupied by sodium ions,it is sodium titanate (Figure 5a).If they are taken by hydrogen ions,it is hydrogen trititanate (Figure 5b).If TiO 6layers in trititanate are connected in corner-linked mode,it will change into microporous structure.Thus,sodium hexati-tanate and hydrogen hexatitanate can be obtained as shown in Figure 5c and d.Given the absence of ions,if all TiO 6octahedrons are corner-linked,we can get rutile (Figure 5g).Similarly,if all octahedrons are edge-linked,we can get TiO 2(B)(Figure 5e)or anatase (Figure 5f).By applying the FBB model,the present authors have successfully explained the structural evolution in several transi-tion systems,such as the absorption of ammonium ions,27bivalent Ba and Sr cations 28,29by trititanate,and conversion of trititanate to hexatitanate.30A PBB schematic representing the transition of Na 2Ti 3O 7to TiO 2(B)is implied in Figure 6.There are four steps that are needed for converting this structural transition.Figure 6A shows an undistorted representation of the Na 2Ti 3O 7structure viewed down [010].The conversion from Na 2Ti 3O 7to TiO 2(B)requires replacement of Na cations with hydrogen ions,whichcorre-Figure 3.TEM microstructure of the samples used in this study.PanelsA,C,E,G,and I are T3,T6,T6-H,TB,and TA fibers,respectively.Panels B,D,F,H,and J are the electron diffraction patterns corresponding to the above samples in the same sequence.TABLE 1:Phase Compositions and Crystallographic Parameters for Nanofiberscrystal structureparameters (nm)reference phasecomposition space group a bc(deg)Ti3Na 2Ti n O 2n +1,n )3,4,9C 2/m 1.5130.3750.91699.3T6Na 2Ti 6O 13P21 1.560.378 1.22105Ti6-H a H 2Ti 6O 13P21 1.460.38 1.30105TB TiO 2(B)C 2/m 1.2160.3740.651107.29TAanataseI 41/amd0.37890.9537aCrystalline parameters for Ti6-H are measured in this study.11432J.Phys.Chem.C,Vol.114,No.26,2010Liu et al.sponds to step I.The reaction also requires interconversion between some corner-linked and edge-linked pairs of Ti -O 6octahedra,which have been illustrated in Figure 6b and c.The slab of TiO 6octahedron in trititanate is -3-3-3-;that is,every three edge-shared octahedrons connect mutually by corner-sharing,which will become -2-2-2-after conversion in step II (Figure 6c)and step III (Figure 6d).The structure of TiO 2(B)(Figure 6e)thus becomes reality by relative slipping of upper layer and lower layer (step III)followed by dehydration (step IV),as shown in the enlarged inset of Figure 6.IV.ConclusionThe phase transformations from sodium trititanate to hexati-tanate and from proton titanate to TiO 2(B)and anatase were investigated by XRD and TEM.It was revealed that the phase transitions and morphological derivation between titanates,anatase,and TiO 2(B)(via proton titanate)could be interpreted systemically by the fundamental building blocks model (FBB).A generalized relationship of phase transitions between sodiumand hydrogen titanates,anatase,TiO 2(B),and rutile by the wet-chemical process was then proposed.A duplex pentagonal prism scheme was then implied to summarize all of the relationships between titanates and titania including this work and previous work.It is the first time in which seven compounds of titanium have been made into one stereoscheme,showing the complex connections among them.Acknowledgment.The financial and infra-structure support of the Queensland University of Technology Inorganic Materials Research Program of the School of Physical and Chemical Sciences is gratefully acknowledged.The Australian Research Council (ARC)is thanked for funding the instrumentation.References and Notes(1)Li,D.;Xia,Y.Ad V .Mater.2004,16,1151–1170.(2)Limmer,S.J.;Cao,G.Ad V .Mater.2003,15,427–431.(3)Xia,Y.;Yang,P.;Sun,Y.;Wu,Y.;Mayers,B.;Gates,B.;Yin,Y.;Kim,F.;Yan,H.Ad V .Mater.2003,15,353–389.(4)Rao,C.N.R.;Deepak,F.L.;Gundiah,G.;Govindaraj,A.Prog.Solid State Chem.2003,31,5–147.(5)Patzke,G.R.;Krumeich,F.;Nesper,R.Angew.Chem.,Int.Ed.2002,41,2446–2461.(6)Hennings,D.;Klee,M.;Waser,R.Ad V .Mater.1991,3,334–340.(7)Newnham,R.E.MRS Bull.1997,22,20–33.(8)Gao,X.P.;Zhu,H.Y.;Pan,G.L.;Ye,S.H.;Lan,Y.;Wu,F.;Song,D.Y.J.Phys.Chem.B 2004,108,2886–2872.(9)Sukpirom,N.;Lerner,M.M.Chem.Mater.2001,13,2179–2185.(10)Kasuga,T.;Hiramatsu,M.;Hoson,A.;Sekino,T.;Niihara,ngmuir 1998,14,3160–3163.(11)Gra ¨tzel,M.Nature 2001,414,338–344.(12)Fujishima,A.;Hashimoto,K.;Watanabe,T.TiO 2Photocatalysis Fundamentals and Applications ;BKC,Inc.:Tokyo,1999.Figure 4.A scheme of functional buildingblocks.Figure 5.Polyhedral representation of the structures of titanate and titania.All of the projections are down [010]except rutile (down[001]).Figure 6.Diagram illustrating the stepwise conversion of Na 2Ti 3O 7(a)to TiO 2(B)(d).Parts (b)and (c)are the proposed intermediate.Phase Relationships between Titanates and Titanium Dioxides J.Phys.Chem.C,Vol.114,No.26,201011433(13)Linsebigler,A.L.;Lu,G.;Yates,J.T.Chem.Re V.1995,95,735–758.(14)Sugiura,T.;Yoshida,T.;Minoura,H.Electrochem.Solid-State Lett. 1998,1,175–177.(15)Tokudome,H.;Miyauchi,mun.2004,958–959.(16)Huang,J.G.;Kunitake,T.;Onoue,mun.2004, 1008–1009.(17)Sun,X.;Li,Y.Chem.-Eur.J.2003,9,2229–2238.(18)Armstrong,G.;Armstrong,A.R.;Canales,J.;Bruce,P.G.Chem. Commun.2005,2454–2456.(19)Armstrong,A.R.;Armstrong,G.;Canales,J.;Garsia,R.;Bruce, P.G.Ad V.Mater.2005,17,862–865.(20)Armstrong,A.R.;Armstrong,G.;Canales,J.;Bruce,P.G.Angew. Chem.,Int.Ed.2004,43,2286–2288.(21)Zhu,H.Y.;Gao,X.P.;Lan,Y.;Song,D.Y.;Xi,Y.X.;Zhao, J.C.J.Am.Chem.Soc.2004,126,8380–8381.(22)Zhu,H.Y.;Lan,Y.;Gao,X.P.;Ringer,S.P.;Zheng,Z.F.;Song,D.Y.;Zhao,J.C.J.Am.Chem.Soc.2005,127,6730–6736.(23)Kolen’ko,Y.V.;Kovnir,K.A.;Gavrilov,A.I.;Garshev,A.V.; Frantti,J.;Lebedev,O.I.;Churagulov,B.R.;Gustaaf Van Tendeloo,G.V.; Yoshimura,M.J.Phys.Chem.B2006,110,4030–4038.(24)Veblen,D.R.Am.Mineral.1991,76,801–826.(25)Moor,P.B.Am.Mineral.1986,71,540–546.(26)Banfield,J.F.;Veblen,D.R.Am.Mineral.1992,77,545–557.(27)Yang,D.J.;Zheng,Z.F.;Zhu,H.Y.;Liu,H.W.;Gao,X.P.Ad V. Mater.2008,20,2777–2781.(28)Liu,H.W.;Yang,D.J.;Waclawik,E.R.;Ke,X.B.;Zheng,Z.F.; Zhu,H.Y.;Frost,R.L.J.Raman Spectrosc.,in press.(29)Liu,H.W.;Zheng,Z.F.;Yang,D.J.;Waclawik,E.R.;Ke,X.B.; Zhu,H.Y.;Frost,R.L.J.Raman Spectrosc.,in press.(30)Yang,D.J.;Zheng,Z.F.;Yuan,Y.;Liu,H.W.;Waclawik,E.R.; Ke,X.B.;Xie,M.X.;Zhu,H.Y.Phys.Chem.Chem.Phys.2010,12, 1271–1277.JP103644X11434J.Phys.Chem.C,Vol.114,No.26,2010Liu et al.。