facile synthesis and characterization of novel nanocomposites

Materials Chemistry and Physics 100 (2006) 507–512 Facile synthesis and characterization of novel nanocomposites of titanate nanotubes and rutile nanocrystals Jiaguo Yu ∗ , Huogen Yu State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Received 23 July 2005; received in revised form 20 January 2006; accepted 4 February 2006 Abstract The nanocomposites of one-dimensional (1D) titanate nanotubes and 0D rutile nanocrystals were fabricated by hydrothermal treatment of bulky rutile TiO 2 powders in a 10 M NaOH solution without using any templates and catalysts. The as-prepared samples were characterized with transmis- sion electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area, Fourier transform infrared spectroscopy (FTIR), UV–visible spectrophotometry (UV–vis), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found that many small rutile nanocrystal particles of about 5 nm could uniformly attach to the outer surface and in the inner of the titanate nanotubes, forming an interesting and novel nanocomposite structure. Adjusting reaction time could control the amount of rutile nanoparticles in the nanocomposites. With increasing reaction time, the specific surface areas, porosity, pore volume, UV absorption and band gap energies of the nanocomposites gradually increased due to the fact that rutile particles were steadily turned into the tubular nanocomposites, finally completely formed titanate nanotubes. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocomposites; Nanotubes; Nanocrystals; Fabrication; Hydrothermal reaction 1. Introduction The controlled synthesis of inorganic materials with specific size and morphology is an important aspect in the development of new materials in many fields such as advanced materials, catalysis, medicine, electronics, ceramics, pigments, cosmet- ics, etc. [1,2]. Since the discovery of carbon nanotubes in 1991 [3], one-dimensional (1D) nanostructured materials (nanotubes, nanobelts, nanowires and nanorods) have attracted consider- able attention due to their distinctive geometries, novel physical and chemical properties, and potential applications in numer- ous areas such as nanoscale electronics and photonics [4–7]. One-dimensional nanostructured TiO 2 are of great interest for possible applications to high effect solar cell [8], photocatalysts [9,10], gas sensor [11], molecular straws [12] and semiconduc- tor devices and so on where the tuning of the pore size and overall morphology are crucial. The titanium oxide nanotubes are of particular interest, since they have larger surface area and higher photocatalytic activity [13].TiO 2 tubes with diameters ∗ Corresponding author. Tel.: +86 2787883610; fax: +86 2787880261. E-mail address: jiaguoyu@yahoo.com (J. Yu). of 70–100 nm were produced using a sol–gel processing [14] or porous alumina as template [15,16]. The synthesis of smaller TiO 2 tubes with a diameter of about 8 nm were also reported by Kasuga et al. by treating TiO 2 in the alkaline solution [17,18]. This chemical process opened a novel route to form TiO 2 nan- otubes with the crystalline wall. Although many methods have been used to fabricate 1D nanostructured TiO 2 (such as nanotubes [19–21], nanobelts [22,23] and nanowires [24,25]), including sol–gel, templates and hydrothermal synthesis, these methods have mainly been concentrated on monomorphic 1D nanostructures, such as nan- otubes, nanobelts and nanowires, etc. However, the synthesis of the nanocomposites of 1D nanostructures and 0D nanocrys- talline remains a challenge to materials scientists [26,27]. Herein, we report that the nanocomposites of titanate nan- otubes and rutile nanocrystals can be easily obtained by a simple hydrothermal treatment of bulky rutile TiO 2 particles in a 10 M NaOH solution without using any templates and catalysts. 2. Experimental In a typical synthesis, titania powders (0.5 g, about 50–300 nm in size, pre- pared by calcining P25 at 900 ◦ C for 2 h) and an aqueous solution of NaOH (10 M, 150 ml) were placed into a 200 ml Teflon-lined autoclave. The mixture 0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2006.02.002 508 J. Yu, H. Yu / Materials Chemistry and Physics 100 (2006) 507–512 was stirred for 10 minto form a milk-like suspension, sealed and hydrothermally treated at 140 ◦ C for 24, 48, 96 and 144 h, respectively. The white precipitate was collected and washed with distilled water until a pH value near 6 was reached. The precipitate was then ground in alcohol followed by ultrasonic-assisted dis- persion. After a second filtration and alcohol washing step, the sample was oven-dried at 80 ◦ C for more than 4 h. An X-ray diffractometer (XRD, Bruker D8 Advance XRD with Cu K␣ radiation) was used to characterize the crystalline phase of the products. The Fig. 1. TEM (a), HRTEM images (b–e) and SAED pattern (f) of the starting material (rutile) (a) and the products (b–f) prepared by a hydrothermal reactionat140 ◦ C for 48 (b), 96 (c) and 144 h (d–f). J. Yu, H. Yu / Materials Chemistry and Physics 100 (2006) 507–512 509 accelerating voltage and the applied current were 40 and 40 mA, respectively. X-ray photoelectron spectroscopy measurements were performed on a PHI Quantum 2000 XPS system with a monochromatic Al K␣ source and a charge neutralizer. The binding energies were referenced to the C1s peak of the sur- face adventitious carbon at 284.8 eV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images, which were obtained using a JEOL TEM-2010F at acceleration voltage of 200 kV, were used to observe or determine the morphology, size and identity of nanoneedles. Raman spectra of the powder samples on a glass slide were measured using a Renishaw 1000 micro-Raman system. Fifty times magnification objectives were selected. The excitation source used was an Argon ion laser operating at 514.5 nm with an output power of 20 mW. UV–vis absorption spectra of the samples were obtained for the dry-pressed disk samples using a UV–vis spectrophotometer (Cary 100 Scan Spectrophotometers, Varian, U.S.A.). Infrared (IR) spectra on pellets of the samples mixed with KBr were recorded on a Nicolet Magna 560 FTIR spectrometer at a resolution of 4 cm −1 . The Brunauer–Emmett–Teller (BET) surface area (S BET ) and pore size distribution were determined using a Micromeritics ASAP 2010 nitrogen adsorption apparatus. All the samples mea- sured were degassed at 100 ◦ C before the actual measurements. 3. Results and discussion Fig. 1 shows the TEM, HRTEM images and SAED pattern of the starting material (rutile) and the products. The particle size of the starting material is from several tens to several hundreds of nanometers (Fig. 1a). Fig. 1b and c show the TEM images of the products obtained by a hydrothermal reaction at 140 ◦ C for 48 and 96 h, respectively. The parent long titanate nanotubes were formed by a hydrothermal reaction of rutile TiO 2 ina10M NaOH solution. Many rutile nanocrystals of 5–10 nm attached to the outer surface of the titanate nanotubes and some rutile nanocrystals of about 5 nm also existed in the inner of the nan- otubes, forming an interesting composite structurethat possesses both the surface properties of rutile nanocrystals and most mor- phology and mechanical properties of titanate nanotubes. The phase structures of the nanocomposites were confirmed by the XRD patterns of the samples. The diffraction peaks from both rutile and the parent titanate could be observed in the reaction products (Fig. 2b and c). To the best of our knowledge, this is the first time to observe this novel morphological structure of the nanocomposites of rutile nanocrystals and titanate nanotubes. Fig. 2. XRD patterns of the starting material (a) and the products prepared by hydrothermal reactions at 140 ◦ C for 48 (b), 96 (c) and 144 h (d). We expect that it would be complicated and difficult to delib- erately create such a peculiar structure of binary composition on nanoscale. Usually, the catalytic reactions take place on the surface of the catalyst. Catalysts with a particle size at a scale of several nanometers should exhibit a superior activity because of its large specific surface. However, nanoparticles have a strong tendency to agglomerate into larger particles, resulting in an adverse effect on catalyst performance. Furthermore, it is very hard to recover them after the reaction, leading to a potential difficulty in downstream separation. Continuing efforts have been made to develop alternate approaches to synthesize the structures that can be separated readily and have the superior per- formance of nanocrystals. Importantly, this delicate composite structure was achieved through a simple hydrothermal reac- tion of common rutile TO 2 powders and a concentrated NaOH solution. Fig. 1d is a low magnification TEM image of the products prepared by a hydrothermal reaction at 140 ◦ C for 144 h, which shows large quantity of tubular materials with narrow size dis- tribution. The diameters of the tubular materials are around 8–10 nm and the lengths range from several tens to several hun- dreds of nanometers. A typical selected area electron diffraction pattern (SAED) taken from an area containing a large amount of nanotubes is shown in Fig. 2f. Unlike that reported by Kasuga et al. [17,18], we found that the electron diffraction patterns cannot be assigned to rutile or anatse TiO 2 , but be assigned to titanate (H 2 Ti 3 O 7 ). Fig. 1e shows HRTEM images of the sample (d). The HRTEM images clearly show that (1) the tubular structures are well crystalline tubes with multiple shells, with an inner shell diameter of about 8.0 nm, a shell spacing of about 0.75 nm and an average tube diameter of about 12.0 nm; (2) the structures of different shells are well correlated; (3) the tubes are open ended. Further investigations showed that adjusting hydrothermal reaction time could control the amount of rutile nanocrystals in the nanocomposites. Fig. 2 shows the XRD patterns of the start- ing material and the products. It can be seen that when reaction time reaches 48 h, the diffraction peaks of titanate nanotubes appear [20,26,27]. With increasing reaction time, the peaks of rutile disappeared steadily and the intensity of the diffraction peak of titanate enhanced. At 144 h, all rutile phases were com- pletely turned into titanate nanotubes. Fig. 2d is an XRD profile taken from the products obtained at 140 ◦ C for 144 h, showing that the phase structure of the nanotubes agreed with neither anatase nor rutile phase of TiO 2 . XRD diffraction and SEAD image indicated that the structure of the present tubular materi- als was similar to that expected for a layered titanate H 2 Ti 3 O 7 [26,27]. In order to determine the chemical composition of the prod- ucts, XPS experiments were performed on the products obtained at different reaction time. Fig. 3 shows a typical X-ray photoelec- tron spectroscopy (XPS) survey spectrum of the nanocomposites obtained at 140 ◦ C for 96 h. Only Ti and O peaks were observed in Fig. 3 together with a C peak that was attributed to the adventi- tious hydrocarbon from the XPS instrument itself. Further XPS analysis showed that the product consisted of Ti and O in an atomic ratio of about 1:2.3. Ignoring the possible existence of undetectable light elements such as H in the sample, the titanium 510 J. Yu, H. Yu / Materials Chemistry and Physics 100 (2006) 507–512 Fig. 3. XPS survey spectrum of the nanocomposites prepared by hydrothermal reaction at 140 ◦ C for 96 h. nanotube should in general be called TiO x nanotubes instead of TiO 2 , and we expected that the oxygen composition deviation from TiO 2 might play an important role in the formation of our nanotubes. The similar results were also obtained for the nan- otubes obtained at 140 ◦ C for 48 and 144 h. Energy-dispersive X-ray (EDX) analysis also showed that there was no sodium in the resulting product (as shown in Fig. 4). The yield and purity of the nanocomposites or nanotubes are estimated to be higher than 95 and 90%, respectively, based on the TEM, XRD, XPS and EDX results. The tube-like structures of the products were further exam- ined using BET analysis, FTIR, UV–vis and Laser Raman spec- tra. Table 1 shows effect of reaction time on BET specific surface areas and pore parameters of the products. It could be seen that with increasing reaction time, the specific surface areas, porosity and pore volume of the tubular materials gradually increased, which was ascribed to the fact that rutile particles in size of 50–300 nm were steadily turned into the tubular nanocompos- ites, finally completely formed titanate nanotubes. The pore size of the samples was about 8–10 nm and almost kept the same due to the diameter of the tubular materials having no obvious change. Fig. 5 shows the FTIR spectra of the starting material and the product prepared at different reaction time. There is a large amount of water and hydroxyl groups existed in the tubular products because of the existence of a bending vibration of H O H at 1630 cm −1 , and a strong stretching vibration of O H Fig. 4. EDX spectrum of the nanocomposites prepared by hydrothermal reaction at 140 ◦ C for 96 h. Table 1 Effect of reaction time on BET specific surface areas and pore parameters of the tubular products Reaction time (h) S BET a (m 2 g −1 ) Porosity b Pore volume c (ml g −1 ) Pore size d (nm) 0 2.9 1.8 0.005 6.6 24 29.9 22.1 0.061 7.9 48 81.4 42.1 0.196 9.6 96 154.0 59.7 0.400 10.4 144 244.5 66.7 0.542 8.9 a BET surface area calculated from the linear part of the BET plot (P/P 0 = 0.05 − 0.3). b The porosity is estimated from the pore volume determined using the adsorp- tion branch of the N 2 isotherm curve at the P/P 0 = 0.995 single point. c Total pore volume, taken from the volume of N 2 adsorbed at P/P 0 = 0.995. d Average pore diameter,estimated using the adsorption branch of the isotherm and the Barrett–Joyner–Halenda (BJH) formula. at 3400 cm −1 . Notably, the amount of adsorbed water in thetubu- lar materials increased with increasing reaction time, owing to the increase of the specific surface areas and pore volumes. J. Yu, H. Yu / Materials Chemistry and Physics 100 (2006) 507–512 511 Fig. 6. UV–vis absorption spectra of the starting material (a) and the product obtained at 140 ◦ C for 48 (b), 96 (c) and 144 h (d). spectrum are due to Ti O Ti [28]. For the product obtained at 140 ◦ C for 96 (c) and 144 h (d), the peaks at 250 and 620 cm −1 had almost disappeared and new peaks were observed near 290 and 660–760 cm −1 . The peaks at 290 and 660–760 cm −1 might be suggested to be due to H O Ti, as reported for Na 2 O–TiO 2 glass [29]. Although the exact formation mechanism for these 1D and 0D nanocomposite structures is still unclear, it is obvious that the growth of the nanocomposite structures is not template- directed. Since the raw materials used in our synthesis are oxides and NaOH, it is possible that the nanocomposite structures are formed by a simple reaction-crystallization process (RC) [5], in which bulky TiO 2 particles gradually react with NaOH to form water soluble Na 2 Ti 3 O 7 in solution under hydrother- mal treatment. Na 2 Ti 3 O 7 is then turned into H 2 Ti 3 O 7 via acid treatment. The unreacted TiO 2 particles serve as the sites for the heterogeneous nucleation of the titanate nanotubes, which subsequently grow into tube-shaped nanocomposite structures through a reaction-crystallization mechanism. Fig. 7. Laser Raman spectra of the starting material (a) and the product obtained at 140 ◦ C for 48 (b), 96 (c) and 144 h (d). 4. Conclusions In conclusion, the nanocomposites of titanate nanotubes and rutile nanocrystals could be easily synthesized by a simple hydrothermal reaction of bulky rutile powders and 10 M NaOH solution without using any templates and catalysts. 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Kozuka, S. Sakka, J. Mater. Sci. 26 (1991) 248. . Materials Chemistry and Physics 100 (2006) 507–512 Facile synthesis and characterization of novel nanocomposites of titanate nanotubes and rutile nanocrystals Jiaguo. this novel morphological structure of the nanocomposites of rutile nanocrystals and titanate nanotubes. Fig. 2. XRD patterns of the starting material (a) and