CRYSTAL GROWTH & DESIGN
Synthesis of Titania Nanostructure Films via TiCl4 Evaporation-Deposition Route
2007 VOL. 7, NO. 4 815-819
Bo Chi* and Tetsuro Jin National Institute of AdVanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan ReceiVed August 1, 2006; ReVised Manuscript ReceiVed January 5, 2007
ABSTRACT: In this paper, nanostructured TiO2 films on a glass slide were synthesized through a simple evaporation-deposition route using volatile TiCl4 as the titania precursors. TiO2 nanostructures such as nanorod, nanofiber, ribbon, and network films on etched or bare glass slides were prepared by simply adjusting the heating condition of TiCl4 in an autoclave. The research found that the heating temperature, heating time, and humidity in an autoclave are the major factors that affect the formation of various TiO2 nanostructures. Introduction Nanostructured TiO2 has attracted great attention for its wide applications in many fields, such as photocatalysis,1 high efficient solar cells,2 biological coatings,3 sensors,4 and photoluminescence.5 Many techniques have been applied to prepare TiO2 nanostructures as nanowires,6 nanofibers,7 and nanotubes.8 However, thin films and coatings of these nanostructures TiO2 are often more desirable for applications involving catalysis, sensors, solar cells, and electrodes. According to this consideration, efforts have been made to prepare TiO2 nanostructured films.9,10 The techniques developed to prepare TiO2 nanostructured films include DC magnetron sputtering,11 sol-gel,12 liquidphase deposition,13 hydrothermal treatment,14 etc. Recently, Wu et al.15 synthesized TiO2 nanorods on fused silica and Si(100) substrates using a template- and catalyst-free metal-organic chemical vapor deposition method. Liu et al.16 reported that they directly prepared large arrays of oriented TiO2based nanotubes and continuous films by hydrothermal treatment of nanoparticle TiO2-coated Ti foils. Holmes et al.17 have prepared well-ordered and thermally stable mesoporous titania films on silicon wafers or glass slides by a spin-coating method up to 850 °C. Balkus et al.18-19 have fabricated TiO2 nanofibers and core-shell structures using mesoporous molecular sieves as hard templates and found that the nanofiber diameters deeply depend on the pore size of the mesoporous template. TiCl4 is a kind of Ti-containing chloride that is highly volatile and can be hydrolyzed even in ambient atmosphere conditions. It has been used as a Ti source to prepare TiO2 nanopowders and other nanostructures for application.20,21 In this paper, we present a facile method to prepare nanostructured TiO2 transparent films on glass slides through a TiCl4 evaporation-deposition route at a relatively low temperature. Various TiO2 nanostructures on slides can be prepared by simply adjusting the heating conditions, such as temperature, time, and humidity of the autoclave, of TiCl4 in an autoclave. Experimental Section Before the deposition of TiO2, the glass slides were washed with acetone and distilled water in ultrasonic cleaner. Except for the bare glass slide (named the GS slide), some GS slides were also sputtered with a layer of Au on one side (AS slide) and some were etched in 1 M NaOH solution at 130 °C for 5 h in a Teflon-lined autoclave (NS slide). The evaporation-deposition process was conducted in a 100 * To whom correspondence should be addressed. Tel: +81-72-751-9642. Fax: +81-72-751-9627. E-mail:
[email protected].
Scheme 1.
a
Setup of the Glass Slide and TiCl4 in the Autoclavea
(a) Without distilled water and (b) with distilled water.
Table 1. Preparation Conditions of the Films and Their Products substrate
temp (°C)
time (h)
NS-1 NS-2 NS-3
NaOH-etched slide NaOH-etched slide NaOH-etched slide
90 90 110
12 24 24
NS-4a NS-5b GS-1 GS-2 AS-1 silica template
NaOH-etched slide NaOH-etched slide bare glass slide bare glass slide Au-coated slide mesoporous silica
90 90 90 90 90 90
12 12 6 24 24 24
sample
products nanorod nanofiber sheet-covered nanofiber indefinite particle nanorod network nanoribbon short fiber nanofiber
a Before being moved into the drying oven, the film was exposed to ambient temperature for 30 min for further hydrolysis. b The film was prepared by adding 0.1 g of distilled water in the autoclave during the heating duration.
mL Teflon-lined autoclave using volatile TiCl4 as the Ti source. Five milliliters of TiCl4 was added into a 20 mL beaker. A size-suitable glass slide was placed at the top of the beaker without completely covering the beaker for the evaporation of TiCl4. The schematic setup is shown in Scheme 1a. The beaker was transferred into a Teflonlined autoclave and heated to the temperature of 90 or 110 °C for different hours under autogenerated pressure. Then, the films were moved out from the autoclaves and immediately transferred into a 100 °C drying oven. For comparison, some of the films were exposed at ambient temperature for 30 min before being transferred into a drying oven. After they were dried for 5 h, the films were then calcined at 400 °C for 2 h. To investigate the influence of humidity on the film, films were also prepared by adding 0.1 g of distilled water in the autoclave, which was separated from the beaker containing TiCl4 during the heating duration (Scheme 1b). A mesoporous silica film-covered slide was also prepared as a hard template for the growth of titania nanostructures.
10.1021/cg060515n CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007
816 Crystal Growth & Design, Vol. 7, No. 4, 2007
Chi and Jin study. The crystal structures were measured by a powder X-ray diffractometer (XRD, Shimadzu, XRD-6000) with Cu KR radiation (λ ) 0.15406 nm) at 40 kV and 30 mA. The morphologies and nanostructures of TiO2 films were observed using a field emission scanning electron microscopy (FE-SEM, Hitachi S-5000) and transmission electron microscopy (TEM, Hitachi H-9000).
Results and Discussions
Figure 1. XRD patterns of TiO2 films synthesized on NaOH-etched glass slides (NS) and bare glass slides (GS). The meaning of sample names can found in Table 1. The detailed synthesis process of the mesoporous silica film was similar with that reported by Fukuoka et al.22 The summary of the samples is listed in Table 1. For characterization, the film on the downside of the slide (Scheme 1a) that directly faced the TiCl4 vapor was used for comparison and
XRD patterns of the typical products of the films are shown in Figure 1. Only TiO2 anatase was formed in all samples. The hydrolysis of TiCl4 can yield either anatase or rutile TiO2 depending on the synthetic conditions used.20,21 On the NS slides, the diffraction peaks of TiO2 anatase are more obvious than that on the GS slides, meaning that more TiO2 anatase was deposited on the NS slides. It was found that the diffraction intensities of anatase increase with the increase of heating time or temperature, meaning that more TiO2 was formed under the conditions. Figure 2 shows the SEM result of the products on NS slides (NS-1, NS-2, and NS-3) and the mesoporous silica template. It was found that a uniform TiO2 nanorod film was obtained after
Figure 2. SEM images of TiO2 films on the NaOH-etched glass slides NS-1 (a and b), NS-2 (c), and NS-3 (d and e) and the mesoporous silica template (f).
Synthesis of Titania Nanostructure Films
Figure 3. TEM image and SAED result of a TiO2 nanorod.
Figure 4. SEM images of TiO2 films on the NaOH-etched glass slides NS-4 (a) and NS-5 (b).
heating at 90 °C for 12 h on the NS-1 sample (Figure 2a,b). These short nanorods have the length of about 1 µm and a diameter of about 80 nm. Most of the nanorods are connecting with others at one top, while some individual nanorods could also be observed. The HRTEM image of a single nanorod and
Crystal Growth & Design, Vol. 7, No. 4, 2007 817
the selected-area electron diffraction (SAED) pattern are shown in Figure 3, revealing that the nanorods had high crystallinity and grew along the [001] direction. The fast growth along the [001] direction is typical of the anatase phase and results from the 1.4 times higher surface energy of the {001} surfaces in comparison to that of the {101} surfaces, as predicted by the Donnay-Harker rules.23,24 Banfield proposed another mechanism of rapid growth on {001} faces, in which only the {001}surfaces can generate continual reactive sites for the crystal growth.25,26 After the heating time was extended to 24 h (NS-2), the product was nanofibers (Figure 2c). The size and length of these nanofibers are more irregular than those of the nanorod film of NS-1. When the heating temperature was increased to 110 °C for 24 h (NS-3), the surface was covered with a layer of thin sheets (Figure 2d). However, under the sheet layer of the surface, nanofibers still existed in the inner of the film (Figure 2e). Uniform long nanofibers with a smaller diameter were obtained as compared with the sample of NS-1 and NS-2 when mesoporous silica film was used as the hard template (Figure 2f). Balkus et al. studied the formation of TiO2 nanofibers using mesoporous silica as a hard template and found that the mesopores could store TiCl4 vapor for hydrolysis after being exposed to the atmosphere.18 It is easy to understand the formation of nanostructures using mesoporous silica as the hard template for the preparation of nanostructured materials.27,28 The mesopores could act as a hard template, and then, nanostructure products can form inside the pores or extend to the outside. At present study, there are no mesopores for TiCl4 vapor to store or embed, and then, the vapor is hydrolyzed when it is exposed to the atmosphere to form nanostructures. The possible route of the formation of nanorods and nanofibers on NS slide is different to the route that mesoporous silica used as a hard template. Although some researchers have confirmed that the major formation period of TiO2 nanostructures happened after TiCl4-adsorbed films were taken out of the autoclaves,18,19 the hydrolysis of TiCl4 during the heating period should also be taken into account. Because the volatile TiCl4 is highly sensitive to moisture, it is possible for TiCl4 to hydrolyze and form TiO2 nuclei during the heating period because of the existence of the small amount of water vapor in the autoclave. The rough surface of the NS slide could act as the growing sites, and the autogenerated pressure in the autoclave could promote the growth of TiO2 nuclei and then nanostructures. After the heating period, the short contact of the NS slide with the atmosphere during transfer to a drying oven leads to the further hydrolysis of TiCl4 and the growth of the nanostructures. On the other hand, if the hydrolysis of TiCl4 only took place after the slide was moved out of the autoclaves, the influence of the heating time or heating temperature on the final product could not bring such a large difference of the products. So, it is possible to assume that the formation of nanostructures first took place during the heating period in the autoclave and then continued to grow through the hydrolysis of TiCl4 in the atmosphere after the slides were moved out of the autoclaves. On the basis of the above discussion, the formation of the sheet layer on the nanofibers surface was related with the large amount of TiCl4 vapor in the space piled up by the nanofibers. At the beginning stage, the water vapor in the autoclave would lead to the hydrolysis of TiCl4 and the formation of nanofibers. After the exhaustion of water vapor, TiCl4 vapor was embedded and stored into the space formed by the irregular accumulation of nanofibers. After the stored TiCl4 vapor was suddenly exposed to the atmosphere and met a large amount of water
818 Crystal Growth & Design, Vol. 7, No. 4, 2007
Chi and Jin
Figure 5. SEM images of TiO2 films on the bare glass slides of GS-1 (a and b) and GS-2 (c and d).
vapor, the hydrolysis reaction started again, which led to the formation of a sheet layer of TiO2. Figure 4 shows SEM micrographs of NS-4 and NS-5 products. As compared with the film (NS-1) that was immediately transferred into the high-temperature drying oven, the film (NS-4) that was first exposed to ambient temperature for 30 min after it was taken out from the autoclave shows an indefinite structure. Although nanorods could also be observed, they were of less amount and not as definite as those in the NS-1 film. The whole surface was covered with a layer of smallsized TiO2 particles. The sudden contact of the TiCl4 vapor with moisture in the atmosphere leads to the further formation of TiO2 particles in the film. These fresh-formed TiO2 particles cover the surface of the nanorods and form the as-observed surface appearance. When a small amount of distilled water was added in the autoclave during the heating process, the high humidity promoted the hydrolysis of TiCl4 easily, which resulted in the formation of a uniform flat TiO2 film (NS-5), and no special structure of TiO2 was observed. The result confirmed that high humidity or long exposure in ambient atmosphere would lead to the irregular growth of titania structure and thus irregular appearance of films. To further investigate the effect of the rough surface of NS slide on the formation of nanofibers and nanorods, the bare glass slides (GS) were used as substrates to prepare TiO2 films. Figure 5 shows the obtained TiO2 films on the GS slides. Unlike the obtained films on the NS slides, a nanostructured TiO2 network film (Figure 5a) was formed after the GS slide was treated in an autoclave at 90 °C for 6 h (GS-1). The network structure extends deeply into the inner layer of the film. The frame of the network was combined with short nanorods or nanofibers (Figure 5b), with a rough outer surface. After they were heated at 90 °C for 24 h (GS-2), TiO2 ribbon films (Figure 5c) were synthesized. The ribbons show a thickness of 50 nm and a
Figure 6. SEM images of TiO2 films on Au-sputtered slide AS-1.
triangle edge. Some ribbon clusters could also be found in the low magnification view (Figure 5d). Without the role of the rough surface, TiO2 formed through the hydrolysis of TiCl4 was difficult to form oriented long fibers or rods. It has been
Synthesis of Titania Nanostructure Films Scheme 2.
Crystal Growth & Design, Vol. 7, No. 4, 2007 819
Simple Scheme of the Formation of TiO2 Nanostructures on an NS Slide (Route I) and an GS Slide (Route II)
confirmed that TiO2 fibers on the bare glass slide were difficult to prepare using the similar process.18 The formation mechanism here is to some extent similar with the vapor-solid process.29 After the start of the nucleation, a very small amount of TiO2 nuclei could have the opportunity to deposit on the smooth surface of GS slide (GS) as compared with that on the rough surface of the NS slide. Then, the growth of these nuclei could lead to the formation of the short rodlike TiO2.30 Finally, the rodlike TiO2 connected with each other to form a network structure. When the AS slide was used, TiO2 of short nanofibers was prepared (Figure 6). The short fiber product shows a similar appearance to that in the TiO2 network (Figure 5b). The fibers had rough surfaces and inhomogeneous lengths. The sputtered Au particle layers on the slide surface may serve as nucleation centers for the growth of TiO2. On the basis of the above discussion, the simple scheme of the formation of TiO2 nanostructure films on NS and GS slide is shown in Scheme 2. The roughness of the slide surface determined the amount of nuclei formed at the starting stage. The large amount of the nuclei on NS slide leads to the formation of TiO2 nanorods and fibers. The amount of the nuclei on the smooth glass slide is relatively small, and the growth of these nuclei leads to a loose network structure. The relatively low humidity in the autoclave helps to decrease the hydrolysis rate of TiCl4, which results in the formation of a regular nanostructure of titania. When the humidity increases, the fast hydrolysis of TiCl4 produces irregular titania films on slide surface regardless of the roughness of the slide. Conclusions Different nanostructured TiO2 films were synthesized through a simple evaporation-deposition route using volatile TiCl4 as the precursor. TiO2 nanorods, nanofibers, networks, and ribbons were prepared on rough or bare glass slides. The surface roughness of the slides, the humidity for TiCl4 hydrolysis, and the heating conditions are the main factors that affect the formation of TiO2 nanostructured films. Acknowledgment. We thank M. Makino and C. Uetani for the help of TEM and FE-SEM observation and Dr. Weiyou Yang for the suggestive discussions. B.C. thanks the Japanese Society for Promotion of Science for a JSPS Postdoctoral Fellowship.
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CG060515N
