CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 3 281-283
Communications Novel Crystal Growth from a Two-Dimensionally Bound Nanoscopic System. Formation of Oriented Anatase Nanocrystals from Titania Nanosheets Katsutoshi Fukuda,†,‡ Takayoshi Sasaki,*,†,# Mamoru Watanabe,† Izumi Nakai,‡ Katsuhiko Inaba,§ and Kazuhiko Omote§ Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan, Department of Applied Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo, 162-8601, Japan, CREST, Japan Science and Technology Corporation (JST), and X-ray Research Laboratory, Rigaku Corporation, 3-9-12 Matsubaracho, Akishima, Tokyo, 196-8666, Japan Received December 18, 2002;
Revised Manuscript Received January 30, 2003
ABSTRACT: A monolayer film of unilamellar titania crystallites consisting of two planes of Ti/O atoms and two additional planes of O atoms underwent an interesting structural change by heat treatment at 800 °C and above to produce anatase nanocrystals oriented along the c-axis. This phenomenon is distinct from phase transformation of the bulk phase in terms of the significantly higher crystallization temperature of anatase and the preferential orientation of nanocrystals. This may be peculiar to two-dimensional nanosystems, providing a new route for the fabrication of oriented nanocrystals. Nanomaterials including nanoparticles,1-3 nanotubes,4,5 and nanosheets6-11 have recently attracted intense interest from fundamental and practical points of view. Such materials offer intriguing physical and chemical properties associated with size and shape on a nanometer scale. Examples of these properties include size-quantization effects for semiconductor nanocrystallites, and the semiconducting to metallic properties of carbon nanotubes depending on the helicity. Many novel phenomena peculiar to the nanoscopic regime are expected to remain as yet undiscovered. Recently several groups have reported pioneering work to control crystal growth in the nanometer-scale range. Johnson and co-workers have studied annealing effects on superlattice materials fabricated by molecular beam epitaxy (MBE).12,13 Whitesides’ group has examined the crystal growth of CaCO3 on a micropatterned surface of selfassembled monolayers with various functional groups.14 Both groups have demonstrated novel crystal growth behavior by which the crystal size and orientation can be controlled. Here, we report a thermally induced structural change of a monolayer film of titania nanosheets: Twodimensional crystallites composed of only four atomic planes transformed into anatase nanocrystals preferentially oriented along the c-axis at a temperature of 800 °C and above, a change that is not seen in the bulk material. To the best of our knowledge, the crystallization from such an ultimately thin system has not been explored before. * Corresponding author: e-mail:
[email protected]; fax: +8129-854-9061. † National Institute for Materials Science. ‡ Tokyo University of Science. # CREST, Japan Science and Technology Corporation (JST). § Rigaku Corporation.
Unilamellar titania nanosheets of Ti1-δO24δ- (δ ≈ 0.09) were obtained by delamination of a layered titanate, H0.7Ti1.82500.175O4‚H2O (0 ) vacancy), into single layers.7,8,15,16 The resulting two-dimensional crystallites have a molecular thickness (∼0.7 nm), consisting of two edgeshared TiO6 octahedra. In contrast, the lateral dimensions of these crystallites range from several hundred nanometers to several micrometers.17 A monolayer of these titania nanosheets can be deposited by electrostatic self-assembly onto a positively charged surface.18,19 A Si wafer chip (1 × 5 cm2) to be used as a substrate was cleaned by a procedure described previously,18,19 and then was treated with an aqueous solution of polyethylenimine (PEI, pH ) 9, 2.5 g dm-3) for 20 min. The substrate primed with PEI was immersed in a colloidal suspension of nanosheets (pH ) 9, 80 mg dm-3) for 20 min. Deposition under this condition resulted in monolayer coverage with the nanosheets except for minor areas of uncovered surface and nanosheet overlaps.19 Transformation of the monolayer film upon heat treatment was studied by various characterization techniques. Figure 1 shows normalized Ti-K-edge X-ray absorption near-edge structure (XANES) spectra for the monolayer film of nanosheets after heating at 600, 700, 800, and 900 °C for 1 h. Measurements were performed by total reflection fluorescence XANES technique with synchrotronradiated X-ray at the Photon Factory BL-12C and a 19 element Ge-solid state detector, which allows high-quality data to be acquired for monolayer films containing a limited quantity of Ti (0.7 × 10-7 g cm-2). The spectral profiles in the preedge (4965-4973 eV) and absorption edge region (4980-4990 eV) were found to be very sensitive to structural modification. There was little change in the XANES
10.1021/cg025619m CCC: $25.00 © 2003 American Chemical Society Published on Web 03/05/2003
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Figure 1. Normalized Ti-K-edge XANES spectra for (a) as-grown film and for samples heated at (b) 600 °C, (c) 700 °C, (d) 800 °C, and (e) 900 °C. The data for (f) anatase is shown as a reference.
spectra up to 700 °C. On the other hand, peaks characteristic of anatase appeared overlapping the profile of nanosheets at 800 °C. A sharp peak at 4984 eV and fine structures at the preedge were clearly observed at this temperature. Upon further heating at 900 °C, the spectrum was identifiable as a single phase of anatase. This result suggests that nanosheets started to change into anatase at 800 °C, and the change was complete by 900 °C. The structural change was also followed by in-plane diffraction data (Figure 2), obtained using a four-axis X-ray diffraction (XRD) instrument (ATX-G, Rigaku) with CuKR (λ ) 0.1542 nm) radiation from a rotating anode source.20 The as-grown film exhibited sharp two-dimensional reflections at 2θ ) 38.6°, 48.4°, and 62.5°, indexable to 11, 20, and 02, respectively, for a face-centered rectangular unit cell (0.38 nm × 0.30 nm) of the nanosheet.21 The pattern appeared to remain unchanged through the heating process except for the almost loss of 11 and 02 peaks. However, the high-resolution peak profile around 48° indicated a clear change. The peak became broad by heating at 600 and 700 °C. This suggests some structural disorder, although the nanosheet structure basically remained unchanged. On further heating, the peak tended to shift to a lower angular region. The final peak position was 48.0°, being consistent with that of 200 reflection of anatase in the literature data.22 The poorly resolved nature of the profiles makes conclusive analysis difficult, but the films at 800 °C may be identified as an intermediate, composed of anatase and nanosheets. Then why did the anatase film formed at 900 °C exhibit only one reflection? A polycrystalline sample of anatase exhibits several diffraction lines in this angular range, with the 101 peak at 2θ ) 25.4° being the strongest. This diffraction feature may be accounted for by the preferred orientation of anatase along the c-axis with respect to the substrate surface. This is further supported by reflected high-energy electron diffraction (RHEED) data (not shown) showing rows of diffraction spots perpendicular to the substrate. The topographical change in the heating process was examined by tapping-mode atomic force microscopy (Seiko Instrument SPA400 AFM system) using a Si tip cantilever (force constant 20 N m-1) (Figure 3). The nanosheets were clearly visualized as lamellar objects with a thickness of
