ARTICLE pubs.acs.org/crystal
Growth and Optical Properties of Rectangular Hollow Tube TiO2 Crystals with Rutile-Type Structure Ken Niwa,* Takashi Taguchi, Tomoharu Tokunaga, and Masashi Hasegawa Department of Materials Science and Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan ABSTRACT:
Rectangular hollow tube crystals of the rutile-type TiO2 have been successfully grown in supercritical water at about 2 GPa for the first time using a diamond-anvil cell combined with the infrared laser heating system. The scanning electron microscope (SEM) and transmission electron microscope (TEM) observations indicate that they are grown along the [001] direction in a few tens of micrometer lengths. It is also found that they are surrounded by the {110} face with its wall thickness of ∼200 nm. Details of the morphology, optical property, and growth mechanism of the obtained crystals are discussed.
’ INTRODUCTION Materials with tubular structure are very attractive in terms of their novel physical and chemical properties owing to the unique morphology. Therefore, many studies have been carried out for various classes of chemicals,118 such as carbon,1 BN,3,4 and Al2O3,8 etc., in order to fabricate tubular materials. Although many tubular materials have been reported so far, tubular single crystals with a rectangular hollow-shaped cross section have been limited.1015,17,18 Rectangular hollow tube crystals for various other component materials are interesting and important issues not only for fundamental understanding of the crystal growth mechanism but also in view of novel functional materials. Liu et al. succeeded to grow the square-shaped SnO2 tube arrays on the quartz substrates using a combustion chemical vapor deposition (CVD) method.10,11 They concluded that the direct vaporsolid (VS) process was a dominant mechanism for growth of the square-shaped SnO2 tube arrays. On the other hand, Song et al. reported rectangular-shaped AgIn(WO4)2 nanotubes prepared by the hydrothermal method.14 According to their study, a two-dimensional layer intermediate was formed owing to the difference of bond lengths in the crystalline AgIn(WO4)2; then it undergoes rolling to form the tubular structure under the hydrothermal condition. This mechanism seems to be suitable for anisotropic intrinsic crystalline structural materials. Recently, Cho et al. succeeded in growing single-crystal SrNb2O6 nanotubes with a rhombic cross section by the hydrothermal method.18 They clarified the formation process of the singlecrystal SrNb2O6 nanotubes on the basis of condition-controlled r 2011 American Chemical Society
experiments, such as duration time, temperature, and pH, and concluded that the diffusion-limited aggregation process1921 played an important role for growth with the rhombic crosssection shape. In the present study, we focused on rutile-type titanium dioxide (TiO2) which has been widely used as a photocatalyst, solar cell, and pigment, etc.22 The rutile-type structure exists in various functional materials with the composition of MX2.23 Three polymorphs (anatase, rutile, and brookite type) exist in TiO2 at ambient conditions; however, most of the tubular TiO2 have been reported to crystallize into the anatase-type structure or its related ones.2428 There are only a few studies which have reported the fabrication of tubular TiO2 with the rutile-type structure, although rutile-type TiO2 is stable at ambient conditions. Recently, Eder et al. reported the fabrication of “pure rutile nanotubes” by a solgel method using the carbon nanotube template at ambient conditions.29 However, their nanotubes consist of an aggregate of polycrystalline rutile-type TiO2 nanoparticles. Therefore, single crystals of rutile-type TiO2 with tubular structure have not been reported so far. Recently, we first succeeded in growing rectangular hollow tube crystals of rutile-type GeO2 in supercritical oxygen above 5 GPa using a laser-heated diamond-anvil cell (LHDAC).30 This indicates that high-pressure techniques and supercritical fluids Received: May 11, 2011 Revised: August 7, 2011 Published: August 30, 2011 4427
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Figure 1. Schematic illustration of the diamond-anvil cell. Dot-lined area is enlarged, and the sample chamber is shown.
could be powerful tools for growth of tubular morphological materials. Rectangular hollow tube single crystals of the rutiletype structure have been reported only for GeO217 and SnO21012 so far. In this study, we performed single-crystal growth experiments of rutile-type TiO2 with a tubular structure in supercritical water at about 2 GPa using LHDAC. Here, we describe the details of their morphology, optical property, and growth mechanism.
’ EXPERIMENTAL SECTION The diamond-anvil cell (DAC), which is a high-pressure generation apparatus, combined with the infrared laser heating system (fiber laser, wavelength λ = 1090 nm) was used for crystal growth experiments under high pressure and temperature.17,30 Details of the DAC and laser heating system were described elsewhere.30 A stainless steel plate with a sample hole with a diameter of 300 μm was used as a gasket. A tiny pure rectangle-shaped titanium plate (150 μm 100 μm 30 μm, purity of 99.5%) was loaded into the sample hole. Then it was filled with distilled water along with a ruby pressure maker,24 as shown in the schematic illustration of Figure 1. Several independent crystal growth experiments were carried out at about 2 GPa. After increasing pressure at room temperature, the infrared laser was focused with ∼30 μm in diameter and the titanium plate was irradiated through the diamond anvils from both sides. The laser-irradiated position was fixed during heating. After laser irradiation for 1 s, the sample was cooled quickly to room temperature by stopping laser irradiation. The sample temperature was not measured. However, since the intense thermal radiation was observed during the laser heating, the temperature of the heated spot might reach above ∼2500 K. The ambient recovered samples were characterized by X-ray diffraction (XRD) measurements, Raman scattering measurements, and field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) observations. XRD measurements were carried out using Cu Kα radiation with DebyeScherrer geometry. The ambient recovered sample was placed on the microgrid. TEM observation was carried out at 200 kV. An Ar+ laser (λ = 514.5 nm) with a focusing area of ∼20 μm was used as the excitation source for Raman scattering measurements and irradiated to the sample at ambient condition. The Raman scattering lights were dispersive using a grating spectrometer and detected using a liquid-N2-cooled CCD detector.
’ RESULTS AND DISCUSSION Figure 2ac shows photos of the sample chamber. Before laser heating, a rectangle-shaped titanium plate was surrounded by ice-VII,32 which is one of the high-pressure crystalline polymorphs of H2O and stable above about 2 GPa at room temperature (Figure 2a). On laser heating, intense thermal radiation was observed from the heated spot and the surrounded ice-VII was melted and became supercritical water (Figure 2b). After being kept for about 1 s, the sample was quenched to room
Figure 2. Photos of sample chamber (a) before heating at 2.3 GPa and room temperature, (b) during heating, and (c) after heating. (d) SEM image of the titanium plate which was worn in the hole of approximately 30 μm in diameter by laser irradiation. The scale bars in ac correspond to 100 μm.
Figure 3. XRD pattern of the ambient recovered sample. Diffraction peaks of the rutile-type TiO2 are indexed. Asterisk marks indicate unknown peaks.
temperature by stopping laser irradiation (Figure 2c). Figure 2d shows the SEM image of the sample recovered into the ambient pressure. A circle hole was observed at the laser-heated spot of the reacted sample. It should be noted that needle-like products were observed around the laser-heated spot. The XRD pattern of the ambient recovered sample in Figure 2d is shown in Figure 3. Most of the diffraction peaks can be identified with the crystalline phases of the titanium (P63/mmc), rutile-type TiO2 (P42/mnm), and Ti2O3 (R3c). The strong diffraction peaks assigned to titanium correspond to the part of the titanium plate which was not irradiated by the laser beam because the laser-heated area (∼30 μmj) was much smaller than the size of initial loaded titanium plate (150 μm 100 μm 30 μm). In addition, the whole titanium plate was exposed to an X-ray of 300 μm in diameter. Therefore, the unreacted titanium was detected intensively in the diffraction profile. The reason for the existence of Ti2O3 is described later. Since the amount of Ti2O3 is very small, we concluded that the main synthesized product is rutile-type 4428
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Figure 4. SEM and TEM images of the synthesized products. (a) Aggregate located around the heated spot. (b) High-magnification image of a. (c) TEM image of the aggregation of the synthesized products.
Figure 6. Raman spectra of the aggregate of the rectangular hollow tube crystals (A) and reagent TiO2 (B) which is polycrystalline with an averaged grain size of 12 μm.
Figure 5. (a) High-magnification TEM image of Figure 4c. Upper and lower inset correspond to the lattice and electron diffraction images, respectively. (b) Relationship between the crystal morphology and structure. (c and d) Schematic illustration of the atomic arrangement and polyhedral connection viewed from the [001] direction. The wall thickness of about 200 nm corresponds to about 620 (110) layers.
TiO2 in the present study. The lattice parameters of the rutiletype TiO2 are a = 4.593(2) Å and c = 2.952(4) Å, which are consistent with the reported ones.33 Figure 4a shows the high-magnification SEM image of the needle-like products in Figure 2d. This indicates that a lot of needle-like products are accumulated around the laser-heated hole. Figure 4b corresponds to the further high-magnification SEM image of the synthesized products. As clearly seen, most of these elongated products have openings at both ends with irregular shape. This indicates that they are tubular. Energydispersive spectroscopy (EDS) analysis of the single tube demonstrated that it was composed of titanium and oxygen with nearly a Ti:O = 1/2 ratio. This analysis and XRD measurement conclude that the tube products are rutile-type TiO2. Figure 4c shows the transmission electron image of the aggregation of tubular products. From this image the obtained products are found to be definitely a hollow tube, and the length of these hollow tube materials is between a few and a few tens of micrometers. Figure 5a shows the high-magnification TEM image of Figure 4c, lattice one (upper inset of Figure 5a), and electron
diffraction one (lower inset of Figure 5a) of the hollow tube product. It is found that no steps are observed on the surface. The wall thickness of this hollow tube is about 200 nm, which might correspond to the maximum wall thickness in the present study. The fringe spacings perpendicular and parallel to the longitudinal direction in the electron diffraction image were estimated to be 0.32 and 0.29 nm, respectively. These correspond to the lattice spacing values of the (110) and (001) faces of rutile-type TiO2. Therefore, on the basis of TEM analysis and XRD measurements it is concluded that the rectangular hollow tube products are single crystals of rutile-type TiO2 and that they are grown along the [001] direction and surrounded by the {110} face. The relationship between the crystal morphology and the structure is shown in Figure 5b. The schematic illustration of the atomic arrangement viewed from [001] direction is also shown (Figure 5c and 5d). Since the wall thickness is about 200 nm, this corresponds to about 620 layers of the {110} plane. The Raman spectrum of the grown rectangular hollow tube TiO2 crystals which accumulated on the titanium plate is shown in Figure 6. Here, the Ar+ laser was able to be focused with a ∼20 μm diameter. The spectrum of the pure reagent rutile-type TiO2 (99.9%) is also shown for comparison. According to previous studies for the rutile-type TiO2, four active Raman modes (B1g, Eg, A1g, and B2g) are given.3436 In this measurement, the B2g mode was out of the observation region. It is found that the peak profile of the rectangular hollow tube crystals is consistent with that of the reagent rutile-type TiO2 and previous studies3436 with respect to the peak position and relative 4429
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Crystal Growth & Design intensity. The peaks positions corresponding to Eg (∼450 cm1) and A1g (∼610 cm1) show no significant red shifts, which have been observed in the reported nanosized crystals of TiO2.37 On the other hand, no clear peak corresponding to the B1g mode is observed in our rectangular hollow tube crystals, although the peak intensity of the B1g mode is originally weak and the signalto-noise ratio is lower than the reagent one. The Raman spectrum of the ∼200 nm thin film rutile-type TiO2, of which the {110} plane has been preferentially grown, was also reported by Omari et al.38 According to their study, the intensity of the B1g mode was relatively weak compared to that of the Eg and A1g modes. Here, the B1g mode is associated with the vibration between Ti and O on the (001) plane. The wall thickness of the rectangular tube crystals in this study is 200 nm. This might result in the lower intensity of the B1g mode in the present rectangular hollow tube crystals as well as the reported thin film. There is only one study which reported on the synthesis of the rutile-type TiO2 nanotubes. Eder et al. recently succeeded in the synthesis of “pure rutile nanotubes” by a solgel method at ambient pressure.29 However, their synthesized nanotubes consist of polycrystalline nanosized rutile-type TiO2, and they are not hollow tube single crystals. Therefore, our result is the first success of the rectangular hollow tube single crystals of the rutiletype TiO2 with a length of a few tens of micrometers. On the other hand, Jafarkhani et al. recently reported the synthesis of TiO2 nanoparticles by irradiating the pulsed Nd:YAG laser to the titanium plate located in pure water at ambient pressure.39 However, no tubular materials were obtained in their study. This means that supercritical water under high pressure is necessary to grow rutile-type TiO2 crystals having the novel “rectangular hollow” morphology. The growth process of the present rectangular hollow tube rutile-type TiO2 crystals is definitely important. However, chemical synthesis of TiO2 should be considered as the first step before the growth process of rectangular hollow tube crystals. The following chemical reaction is proposed at the heated surface of the titanium plate, when the infrared laser was irradiated to the titanium plate, which was surrounded by ice-VII under high pressure Ti þ 2H2 O f TiO2 þ 2H2 The resultant hydrogen might dissolve into the surrounded H2O, and it was difficult to be detected. The ice-VII surrounding the titanium plate was melted immediately after laser irradiating the titanium plate and became supercritical water (Figure 2a and 2b). We detected a very small amount of Ti2O3 in the XRD profile. In the present crystal growth experiments a large temperature gradient existed in the heated spot, and this might result in the various oxidation states (Ti, Ti3+, and Ti4+). Therefore, Ti2O3 was synthesized in addition to TiO2 (so-called magneli phase in the TiO system40,41). However, the amount of Ti2O3 is very small, and no other TiO2 polymorphs (anatase or brookite type) were observed in the diffraction profile. Furthermore, although the present crystal growth experiments were carried out at about 2 GPa under water saturation environment, our result is consistent with the phase diagram of TiO2, which was determined by high-pressure and -temperature experiments.42,43 This indicates that there is no significant effect of water on the change in the phase diagram of TiO2. In order to understand the formation mechanism of tubular materials with rectangular hollow shape, many researchers have proposed some growth mechanisms10,11,14,18 such as the
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vaporsolid process using substitute materials, rolling of the two-dimensional layer, and the diffusion-limited aggregation process, etc., on the basis of their condition-controlled experiments so far. While in the present study the crystals were grown in an extremely short period (
