titanium oxide nanotubes, nanofibers and nanowires

Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 Titanium oxide nanotubes, nanofibers and nanowires Zhong-Yong Yuan, Bao-Lian Su ∗ Laboratory of Inorganic Materials Chemistry, I.S.I.S., The University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium Available online 25 May 2004 Abstract A simple one-step hydrothermal reaction among TiO 2 powders and alkaline solution has been developed to synthesize low-dimensional titanate nanostructures. The morphologies of the obtained nanomaterials depend on the process parameters: the structure of raw material, the nature and concentration of alkaline solution, reaction temperature and time, which suggests that the nanostructure synthesis could be controllable. Trititanate nanotubes with the diameters of about 10 nm were synthesized via the hydrothermal reaction of TiO 2 crystals of either anatase or rutile phase and NaOH solution in the temperature range of 100–160 ◦ C. Nanofibers with an interlinked structure were formed when amorphous TiO 2 or commercial TiOSO 4 was treated with NaOH at 100–160 ◦ C. Pentatitanate nanoribbons with high aspect ratio were obtained by autoclaving of either crystalline or amorphous TiO 2 in NaOH solution at the temperature above 180 ◦ C. Octatitanate nanowires with the diameters of 5–10nm were prepared from TiO 2 particles treated with KOH solution. These nanostructures were analyzed by a range of methods including powder X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), energy-dispersive X-ray spectroscopy (EDX) and infrared spectroscopy (IR). © 2004 Elsevier B.V. All rights reserved. Keywords: Titania; Titanate; Nanotube; Nanofiber; Nanowire 1. Introduction Low-dimensional nanostructured materials have attracted considerable attention recently due to their unique physical properties and their potential applications in nanoelectronics and optoelectronic nanodevices. Dimensionality is a crucial factor in determining the properties of nanomaterials [1] and, thus, the control of size and shape is of great interest. In con- trast to size control, shape control of particulates is a more difficult and challenging topic. The tubes, flakes or fibers with the size range in the nanometer region are expected to possess novel properties [2–4]. Many efforts are being devoted to develop new methods (for example, so called “bottom-up” and “top-down” methods) [5] for synthesizing one-dimensional nanomaterials such as nanowires and nan- otubes. However, soft-chemical routes are specially focused with much interest on the preparation of such nanostructures of metastable oxide materials, though general methods are not yet available. ∗ Corresponding author. Tel.: +32-81-724531; fax: +32-81-725414. E-mail address: bao-lian.su@fundp.ac.be (B L. Su). Titanium oxide is a n-type semiconductor and a typical photocatalyst, attracting much attention from both funda- mental and practical viewpoints. It has been used in many industrial areas including environmental purification, so- lar cell, gas sensors, pigments and cosmetics [6,7].To explore novel approaches for the nanostructured titanias of various nature with the control of the particle size in nanometer-scale and the morphology is quite interesting, since the performance of titania in its various applica- tions depends on its crystalline phase state, dimensions and morphology [8].TiO 2 tubes [9–11] and fibers [12,13] have been prepared using a sol–gel template processing making use of porous alumina, polymer fibers, or super- molecular compound as a template, and their diameters were normally larger than 50 nm. The walls of the titania nanotubules, prepared by deposition in porous alumina membrane, consisted of anatase nanoparticles (10–20 nm size) and contained mesopores arising from the spaces be- tween the anatase particles [14]. Smaller TiO 2 tubes with a diameter of about 8nm were also reported [15], however, their structure and formation mechanism are in question [16]. It is found that the titanium oxide nanotubes pro- duced via the reaction of TiO 2 crystals and NaOH aqueous 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.04.030 174 Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 solution had the structure of neither anatase nor rutile phase [16]. Sasaki et al. [4,17,18] reported a colloidal suspension of TiO 2 sheets by the swelling and subsequent exfolia- tion of a layered titanate in aqueous tetrabutylammonium (TBA) hydroxide. After heating above 400 ◦ C, anatase thin flakes were formed [4]. However, a considerable drawback to this synthetic approach lies in the process required for exfoliation of the titania sheets, which requires a relatively long time, a low solids content, and a high TBA concen- tration. These factors make the approach seem unfavorable for large-scale reactions and the production of low-cost materials [19]. In this paper, we report the preparation of titanium oxide nanotubes, nanofibers and nanosheets by a hydrothermal soft-chemical process. It is found that these nanoparticles can be shape-controlled by the hydrother- mal temperature and the crystallization of TiO 2 precur- sors. 2. Experimental The hydrolysis of an ethanol solution of titanium triiso- propoxide [Ti(O i C 3 H 7 ) 4 ] gave, after aging, the precipita- tion of amorphous TiO 2 gel. After calcination at 600 ◦ C for 2 h, the obtained amorphous TiO 2 gel transformed to crystalline powders of pure anatase phase. The hydrolyzed amorphous gel, crystalline anatase, and commercial TiO 2 powders (P-25; a mixture of crystalline rutile and anatase phases with surface area of 52 m 2 /g) were used as the TiO 2 precursors for the wet chemical nanoparticle prepa- ration. 0.1–0.3g of the precursor was mixed with 20ml of NaOH or KOH aqueous solution with the concentra- tion of 4–20 mol/L, followed by hydrothermal treatment at 100–250 ◦ C in a Teflon-lined autoclave for one or two days. The treated powders were washed throughly with water and 0.1 mol/L HCl aqueous solution until the pH value of the washing solution lower than 7, and subse- quently filtered and dried at 60 ◦ C. The structural and chemical natures of the obtained materials were studied us- ing powder X-ray diffraction (XRD; Philips PW1820 with CuK␣ radiation), infrared spectroscopy (IR; Perkin-Elmer Spectrum-2000), N 2 adsorption analysis (Micromeritics Tristar 3000 system), scanning electron microscopy (SEM; Philips XL-20), transmission electron microscopy (TEM; Jeol JEM-2010 at 200kV, Philips CM-200 at 200kV and Philips TECNAI-10 at 100 kV) and energy-dispersive X-ray spectroscopy (EDX). 3. Results and discussion 3.1. Nanotubes Titanium oxide nanotubes can be formed in the range of reaction temperature of 100–180 ◦ C when either crys- talline anatase or rutile or commercial P-25 was used as the raw materials. The yield of nanotubes increased with the hydrothermal temperature when the temperature was in the range of 100–150 ◦ C, resulting in the higher yield of nanotubes of 80–90%, based on the TEM observations. Little nanotubes formed when the temperature was lower than 100 ◦ C or higher than 180 ◦ C. This is in agreement with the BET surface areas of the products (Fig. 1). The surface areas of the products are also influenced by the particle size of the raw materials. The commercial P-25 powder is known to be very fine with a particle size of 25–30 nm, while the laboratory-made anatase particles are relatively large (more than 50 nm). Correspondingly, the surface areas of the produced nanotubes from P-25 is higher than that from the lab-made anatase (Fig. 1), though the effect of the particle size of the raw materi- als to the yield of the produced nanotubes is quite small in comparison to the hydrothermal temperature. Addition- ally, the concentration of NaOH solution is also a critical parameter during the formation of nanotubes. Little nan- otubes were observed when the NaOH concertration is lower than 5 mol/L or as strong as 20 mol/L. High yield of nanotubes can be obtained when the NaOH concentration was 10–15 mol/L, and their surface areas can be as large as 350 m 2 /g. Fig. 2a shows an overall view of titania nanotubes, re- vealing large quantity of tubular materials with narrow size distribution. The diameters of the tubular materials are al- most uniform around 8–10 nm, and the length range from several tens to several hundreds of nanometers. High reso- lution transmission electron microscopy (HRTEM) images (Fig. 2b) clearly show that the tubular structures are well crystalline tubes with multiple shells of 1–5 layers, and a shell spacing is about 0.78 nm. The tubes are hollow and Fig. 1. BET surface areas of the products obtained from anatase and P-25 with 10 mol/L NaOH at different temperatures for 24h, and the products from P-25 with different content of NaOH at 110 ◦ C for 20 h. Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 175 Fig. 2. HRTEM images of titania nanotubes: (a) overall view; (b) detailed view on the tube structure. open ended with an inner diameter of about 5–6 nm. There are many defects presented in the shell layers of the tubes. The tubes have been shown to be of circular cross-section, by rotating the sample holder along the tube axis during the observation. Besides nanotubes, other phases includ- ing rolled-up sheets and flat sheets can be found in the samples. Normally, the known nanotubes of carbon [20] and in- organic dichalcogenide MX 2 (M = W, Mo; X = S, Se) [21–23] have uniform structure with the layers evenly dis- tributed on either side. Such uniform nanotube structure can also be seen in titania nanotubes (Fig. 3a). However, in some titania nanotubes, the shell layers are unequally dis- tributed on either side of the tubes, for example two lay- ers on one side and three or four layers on another side (Fig. 3b). Considering the existence of rolled-up sheets, the tubes can be described as scroll-type rather than con- centric structure. This is similar to the tubular structure of vanadium oxide nanotubes synthesized hydrothermally by Krumeich et al. [24]. Fig. 4 shows a rolling-up nan- otube together with a rolled-up sheet with two layers on one side, which provide strong evidence for the dispar- ity of the layers of the tube walls. The rolling-up nan- otube is bended in the middle, having an unequal diameter. Since the numbers of the layers on both sides of the nan- otubes could differ by one or two layers (Fig. 3b) and the thin sheets with two or more layers have often been ob- served, the nanotubes are reasonably believed to be formed by rolling of a one- or two-layered sheet of titanium ox- ide. 176 Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 Fig. 3. HRTEM images of single nanotubes with the different shell numbers. The EDX analysis of an area containing a large amount of tubular particles reveal the existence of Na, Ti and O elements. However, when only the freestanding individual nanotubes were selected for EDX study, sodium was not detected. Infrared spectroscopy of the dried sample indicated that OH bonds constantly exist in the nanotubes (Fig. 5), implying that the nanotubes might be the H-form of titanates rather than titania. Fig. 6 shows the XRD patterns of the products obtained from anatase with 10 mol/L NaOH solution at different temperatures. The diffraction peaks of unreacted anatase Fig. 4. A typical HRTEM image showing one rolling-up nanotube. Fig. 5. Infrared spectra of the products obtained from with: (a) anatase and NaOH (10mol/L) at 150 ◦ C; (b) amorphous TiO 2 and NaOH (10 mol/L) at 180 ◦ C; (c) anatase and NaOH (10 mol/L) at 220 ◦ C; and (d) anatase and KOH (10mol/L) at 200 ◦ C. The samples were degassed at 150 ◦ C overnight before IR measurements. can be seen in the product obtained at 100 ◦ C, and only a small yield of nanotubes (about 30%) was observed by TEM. The products obtained in the temperature range of 130–180 ◦ C show some broadened diffraction peaks with the intensities of progressively temperature-depended in- crease. The broadening of diffraction peaks is due to the nanometer size of the tubes and the bending of some atom planes of the tubes. The structure of the nanotubes can be indexed as trititanate H 2 Ti 3 O 7 [25]. These nanotubes can be stable at the calcination temperature lower than 400 ◦ C. However, after calcination at 540 ◦ C, the nanotubes have sintered into nanorods with a perfect circular cross-section perpendicular to its axis (Fig. 7). No changes of the di- ameters of the nanorods were observed by tilting the angles. 3.2. Nanofibers When amorphous TiO 2 powders were used as the raw materials, no nanotubes were found after the similar Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 177 Fig. 6. XRD patterns of the products obtained from anatase with 10mol/L NaOH at different temperatures for 24 h. hydrothermal chemical treatment in the NaOH solution. The products obtained with NaOH concentration of 5–15 mol/L at the temperature range of 100–160 ◦ C are in the mor- phology of non-tubular needle-shaped fibers. Fig. 8a shows a low-magnification TEM image of the obtained titanium oxide nanofibers. A large number of nanofibers with the thickness of 5–30nm and the length of a few tens to sev- eral hundreds of micrometres are interlaced to form an intertexture-like hierarchical structure. Most of nanofibers have a curl-shaped morphology, and the surface of the nanofibers is uneven. Fig. 8b shows a HRTEM image of a nanofiber, indicating that nanofibers are composed of layered structure. A fringe spacing of about 0.7 nm can be seen in the lattice resolved image, which is similar to the shell spacing of the nanotubes. Many defects and dis- locations were observed in the stucture of the nanofibers (Fig. 8b). The electron diffraction patterns of these nanofibers pre- sented diffuse rings of amorphous phases. However, the phase of nanofibers is not truly amorphous. XRD patterns shown in Fig. 9 support the HRTEM results of the layered structure. The patterns are similar to that from the nanotubes shown in Fig. 6, though we did not found hollow nanotubes in our specimens synthesized from amorphous TiO 2 in- stead of crystalline titania. EDX of the samples revealed that they consist of titanium and oxygen. The structure of these nanofiber materials should be similar to that expected for a layered titanate of nominal H 2 Ti n O 2n+1 , probably an analogue of titanate H 2 Ti 3 O 7 . Nanoscale fibrous structures can also be obtained with good yields when commercial TiOSO 4 powders were used as the starting materials. XRD and HRTEM results suggest that nanofibers started from TiOSO 4 have the similar inter- linked structure and composition as those from amorphous TiO 2 (Figs. 8 and 9). N 2 adsorption–desorption isotherms of the synthesized nanofibers revealed the presence of meso- pores (Fig. 10), probably arising from the interparticle space, meshy holes of intertextural nanofibers. BET surface areas of these fibrous materials are quiet large, values of 267, 354 and 375m 2 /g being obtained when amorphous TiO 2 pow- ders reacted with NaOH (10 mol/L) solution at 100, 130 and 150 ◦ C, respectively. Values of 325 and 371 m 2 /g obtained when TiOSO 4 powder was used as raw material treated with NaOH solution at 100 and 150 ◦ C, respectively. It is expected that such interlinked titanium oxide nanofiberous materials will attract interest on catalysis. The morphol- ogy of nanofibers can remain unchanged after calcination at 540 ◦ C, though surface layers became amorphous after high-temperature calcination. 3.3. Nanoribbons When the hydrothermal temperature was in the range of 180–250 ◦ C with the NaOH concentration of 5–15mol/L, the products are mainly nanoribbons with very high yields (almost 100%) whatever the raw materials are crystalline or amorphous TiO 2 powders. SEM observations reveal that the products consist of a large quantity of wirelike nanostruc- tures with typical lengths in the range of several hundreds to several tens micrometers. Fig. 11a is a low-magnification TEM image of TiO 2 powders treated with 10 mol/L NaOH solution at 200 ◦ C, showing the ribbon-like structure. The width of the ribbons varies from 30 to 500 nm, and the ge- ometry of the ribbons is uniform. The nanoribbon structure has a high aspect ratio with a quite small thickness. Fig. 11b shows an individual nanoribbon with a rolled end. From the rolled region one can clearly see that the ribbon is very thin (≤5 nm). Fig. 12 shows the XRD patterns of the synthesized and calcined nanoribbons. The patterns of the as-synthesized nanoribbons can be readily indexed to a monoclinic phase of H 2 Ti 5 O 11 ·H 2 O. The infrared spectra of the nanorib- bons shown in Fig. 5 clearly reflect two kinds of protonic species. Three absorption bands centered at 3400, 1630 and 950 cm −1 can be assigned to O–H stretching mode for in- terlayer water, oxonium ions and hydroxyl groups, H–O–H bending for water and oxonium ions, and O–H bending for hydroxyls, respectively [26]. After calcination at the temperature range of 400–600 ◦ C, the materials are com- pletely dehydrated and re-crystallized into the metastable form of titanium dioxide, TiO 2 (B) [26,27] (Fig. 12). In the temperature range 700–900 ◦ C, the metastable TiO 2 was transformed into anatase, which was then changed into rutile. Fig. 11c is a TEM image of a nanoribbon after calcination at 540 ◦ C for 2 h, showing many holes on the surface of the nanoribbons, which may be due to dehydration. Fig. 13a is a typical HRTEM image of a nanoribbon with well-defined structure, growing along the [0 0 2] di- rection. Two sets of lattice fringes can be observed in the 178 Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 Fig. 7. HRTEM images of nanotubes after calcined at 540 ◦ C, recorded on the specimen holder tilted at different angles: (a) y-axis, −15 ◦ ; (b) y-axis, +12 ◦ ; (c) x-axis, −15 ◦ ; (d) x-axis, +12 ◦ . lattice-resolved image (Fig. 13b). The fringes parallel to the ribbon axis (or [002] plane) correspond to an interplan- nar distance of about 0.65nm. Another set of fringes with smaller spacing of 0.36 nm skewed in the direction of the ribbon axis at an angle can correspond to [1 1 0] plane of the H 2 Ti 5 O 11 ·H 2 O crystal structure. The nanocrystals viewed along the [1 0 0] axis, oriented parallel to the electron beam. The corresponding selected area electron diffrac- tion (SAED) pattern (Fig. 13c) can be indexed to be the [2 0 0] crystal zone of pentatitanate, which could match the main diffraction peaks at 0.82 nm of d-spacing in the XRD patterns. HRTEM images also displayed that some of the nanoribbons consist of two side layers of amorphous phases, especially at the end parts of ribbons (Fig. 13d). The thick- ness ratio of the side and core layers is less than 1/15. The nanoribbons can also bend along the ribbon axis during the growth. 3.4. Nanowires When KOH solution was used to react with TiO 2 pow- ders instead of NaOH solution, the products were nei- ther tubular nor ribbonlike, but wirelike. Fig. 14a shows a typical low-magnification TEM image of the obtained nanowires. The nanowires are almost uniform with a di- ameter of 5–10nm and a length in the range of several micrometers to several tens micrometers. Such nanowires can be synthesized with good yields in the reaction tem- perature range of 130–240 ◦ C. The concentration of KOH solution in the range of 4–25 mol/L did not affect the Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 179 Fig. 8. (a) Low-magnification TEM image of titanium oxide nanofibers, showing a well-interlinked structure; (b) HRTEM image of a nanofiber, revealing the layered structure. structure of the products, as revealed by XRD and TEM results. The powder XRD patterns of the nanowires are shown in Fig. 15, which can be indexed as a single phase of K 2 Ti 8 O 17 with monoclinic structure. No other polymorphs of titania are observed. The broadening of the diffraction peaks indicates small size of nanocrystals. The crystallite size calculated by the Sherrer formula is about 10 nm. EDX and chemical analysis reveal the existence of Ti, K and O in the sample with a molar ratio of K:Ti close to 1:4.Fig. 14b is the corresponding SAED pattern from Fig. 14a, which is consistent with the result of XRD. Fig. 14c shows HRTEM image of single nanowires. One set of distinctive lattice fringes parallel to the nanowire axis is observed. The fringe spacing is about 0.78nm, which corresponds well Fig. 9. XRD patterns: (a) amorphous TiO 2 precursor; (b–d) products from amorphous TiO 2 gel treated with 10 mol/L NaOH solution at 150, 130 and 100 ◦ C, respectively; (e) calcined sample of (d) at 540 ◦ C; (f and g) products from TiOSO 4 treated with 10 mol/L NaOH at 100 and 150 ◦ C, respectively. with the reported interplanar distance for [200] plane of K 2 Ti 8 O 17 . The growth direction of these titanate nanowires is along the [0 1 0] crystal direction. Some nanowires aligned well to present a form of coaxial nanocables or nanoropes (Fig. 14d). Several individual nanowires coiled together. Conventionally, K 2 Ti 8 O 17 can be prepared by ion- exchange reaction of K 2 Ti 4 O 9 followed by a thermal treatment of KHTi 4 O 9 ·nH 2 O [27–29]. In the present work, K 2 Ti 8 O 17 nanowires were synthesized by a sim- ple hydrothermal reaction of TiO 2 particles and KOH solution, and the growths of fine nanowires are oriented along [010] direction. N 2 adsorption analysis revealed that these nanowires have a high specific surface area of 250–320 m 2 /g, which should be expected to attract much interest in the application of catalysis. Moreover, since K 2 Ti 8 O 17 has high ion conductivity because of easy hole generation compared to K 2 Ti 6 O 13 [28], it is expected to be Fig. 10. N 2 adsorption–desorption isotherms of nanofibers obtained: (a) from amorphous TiO 2 gel at 130 ◦ C and (b) from TiOSO 4 at 100 ◦ C. 180 Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 Fig. 11. (a) Low-magnification TEM images of titania nanoribbons; (b) one nanoribbon with a rolled end revealing the small thickness; (c) TEM image of nanoribbons after calcination at 540 ◦ C. useful as a novel functional material. Calcination at a con- ventional Muffle above 600 ◦ C, K 2 Ti 8 O 17 nanowires tend to decompose to K 2 Ti 6 O 13 and TiO 2 (anatase and rutile) (Fig. 15). 3.5. Formation mechanism It has been found that the morphology of the obtained products depends on the process parameters: the structure of raw material, the nature and concentration of alkaline solution, reaction temperature and time. The hydrother- mal reaction of crystalline TiO 2 and concentrated NaOH solution at the temperature of 100–160 ◦ C results in the production of nanotubes, whilst amorphous TiO 2 treated at the same conditions results in the production of nanofibers. Either crystalline or amorphous TiO 2 can be used as raw material to produce nanoribbons when the synthesis was performed with NaOH solution at the temperature above Fig. 12. XRD patterns of nanoribbons: (a) as-synthesized from amorphous TiO 2 gel with 10 mol/L NaOH at 180 ◦ C; (b) after calcination of (a) at 540 ◦ C; (c) as-synthesized from anatase powders with 10mol/L NaOH at 220 ◦ C; (d–g) after calcination of (c) at 400, 500, 600, 700 ◦ C, respectively. A: anatase; B: TiO 2 (B). 180 ◦ C. Whereas nanowires were obtained in the KOH, and nanoparticles were observed in the LiOH treated samples. The crystalline structure of the crystalline TiO 2 poly- morphs (such as anatase and rutile) is described with rep- resentative Ti–O 6 octahedra which share vertices edges to build up the three-dimensional framework of the oxide. It can be proposed that some of Ti–O–Ti bonds of the raw materials are broken when reacted with alkaline solution, and layered titanates composed of octahedral TiO 6 units with the complication of alkali metal ions are formed in the form of thin small sheets. Under autoclaving, the ti- tanate sheets were exfoliated into nanosheets with one or two layers, and then the nanosheets rolled into nanotubes with a slow growth rate possibly due to the high concen- tration of NaOH. Simultaneously, nanosheets with several trititanate layers were formed three-dimensionally, which may be difficult to roll up completely, so the edges of these nanosheets were often bent (Figs. 2 and 4). The Na + ions attached in nanotubes and nanosheets could be exchanged and removed after washing with water and diluted acid so- lution. Titanium in the amorphous TiO 2 gel is octahedrally coordinated by both oxygen atoms, which linked with ti- tanium atoms of adjacent octahedra, and –O–H groups, possessing a different structure from crystalline titania Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 181 Fig. 13. (a) HRTEM image of a single nanoribbon with a well-defined structure; (b) a local enlarged image of (a); (c) SAED pattern; (d) HRTEM image of a nanoribbon with amorphous sides. polymorphs. When reacted with NaOH, some of Ti–O–H (and Ti–O–Ti) bonds are broken by the cauterization of caustic soda solution to generate new coordina- tions. Then the octahedra interact between each other to give long-rang order, partly transform into layered structure of titanate, and self-assemble into thin titanate nanosheets. With the process of hydrothermal reac- tion, more and more nanosheets could be formed with anisotropic growth. Some of individual sheets may merge to form nanofibers and further interlink into a hierar- chical intertextural structure. Most of nanofibers can interlink chemically rather than physically. Similar phenom- ena were taken place when TiOSO 4 was used as the raw material, since two of oxygen in the octahedral TiO 6 units were coordinated with SO 4 2− in place of another titanium atom of adjacent octahedra in crystalline titania. It is be- lieved that local order of the structure of the raw materials may be important to orientedly formnanostructure of shaped oxides. When the hydrothermal temperature was higher than 180 ◦ C, the forms of the products should be controlled mainly by thermodynamics. With the increase of the temperature (>160 ◦ C), the nanoribbons grew accom- panied with the decrease of titanate nanotubes. And the crystalline structure transformed from trititanate to a fairly stable phase of pentatitanate H 2 Ti 5 O 11 ·H 2 O. Since the nanoribbons curved along the ribbon axis were observed, it is reasonable to believe that the mor- phology is temperature-depended. The form of flat nanoribbons is stable during elevated-temperature autoclav- ing. K 2 Ti 8 O 17 nanowires were formed in the autoclaving of TiO 2 powders and KOH solution. The TEM observation of the growth process of the nanowires reveals that nanowires 182 Z Y. Yuan, B L. Su /Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 Fig. 14. (a) Low-magnification TEM images of titanate nanowires; (b) corresponding SAED pattern from the region of (a); (c) HRTEM image of nanowires showing lattice fringes of 0.78nm parallel to the nanowire axis; (d) HRTEM images of a bundle of nanowires and its corresponding SAED pattern (insert). grew out directly from the titania particles. When raw material of anatase reacted with KOH solution, some of Ti–O–Ti bonds of titania crystals are broken, and layered octatitanates are formed on the titania surface along the (0 1 0) lattice plans of TiO 2 . Their (2 0 0) plans may parallel to the (101) lattice plans of TiO 2 . Further, hydrothermal reaction cause the nanowires grow out along the [0 1 0] direction. [...]...Z.-Y Yuan, B.-L Su / Colloids and Surfaces A: Physicochem Eng Aspects 241 (2004) 173–183 183 Acknowledgements This work was supported by the Belgian Federal Government PAI-IUAP-5/01 project and the European Program of InterReg III (Programme France-Wallonie-Flandre, FW-2.1.5) References Fig 15 Powder XRD pattern of the as-synthesized titanate nanowires and after calcination at different temperatures... at the temperature above 180 ◦ C Octatitanate nanowires with the diameters of 5–10 nm were prepared from TiO2 particles treated with KOH solution These synthesized nanostructured materials may be significant on the understanding dimensionally confined transport phenomena and developing new catalyst materials Ion-exchange and intercalation of other metal ions and organic molecules may lead to functionalization... K2 Ti8 O17 ; H: K2 Ti6 O13 ; A: anatase; R: rutile 4 Conclusions Low-dimensional nanostructured titanium oxide materials have been prepared controllably Trititanate nanotubes with the diameters of about 10 nm were synthesized via the hydrothermal reaction of TiO2 crystals of either anatase or rutile phase and NaOH solution in the temperature range of 100–160 ◦ C Nanofibers with an interlinked structure... functionalization of titanate nanostructures for future applications [1] J Hu, T.W Odom, C.M Lieber, Acc Chem Res 32 (1999) 435 [2] R Saito, G Dresselhaus, M.S Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998 [3] Characterization of Nanophase Materials, Z.L Wang (Ed.), Wiley-VCH, Weinheim, 2000 [4] T Sasaki, S Nakano, S Yamauchi, M Watanabe, Chem Mater 9 (1997)... F Bieri, B Schnyder, R Nesper, J Am Chem Soc 121 (1999) 8324 [25] Q Chen, G.H Du, S Zhang, L.M Peng, Acta Cryst B 58 (2002) 587 [26] T Sasaki, Y Komatsu, Y Fujiki, Chem Mater 4 (1992) 894 [27] R Marchand, L Brohan, M Tournoux, Mater Res Bull 15 (1980) 1129 [28] C.T Lee, M.H Um, H Kumazawa, J Am Ceram Soc 83 (2000) 1098 [29] K Sasaki, Y Fujiki, J Solid State Chem 83 (1989) 45 . Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 173–183 Titanium oxide nanotubes, nanofibers and nanowires Zhong-Yong Yuan,. CM-200 at 200kV and Philips TECNAI-10 at 100 kV) and energy-dispersive X-ray spectroscopy (EDX). 3. Results and discussion 3.1. Nanotubes Titanium oxide nanotubes