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J. Phys. Chem. B 2001, 105, 6116-6121
Two-Dimensional Diffraction of Molecular Nanosheet Crystallites of Titanium Oxide Takayoshi Sasaki,* Yasuo Ebina, Yoshizo Kitami, and Mamoru Watanabe AdVanced Materials Laboratory,† National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Tetsuo Oikawa JEOL, Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan ReceiVed: February 2, 2001; In Final Form: April 18, 2001
We have examined two-dimensional diffraction features of novel nanosheet crystallites of quasi-TiO2. A single crystallite gave a face-centered orthogonal array of hl diffraction spots representing a 0.38 nm × 0.30 nm periodicity, which reflects the two-dimensional atomic arrangement in the nanosheet. Diffraction collected from plural crystallites was composed of concentric circles as a result of their random orientation in the TEM grid plane, which were transformed into ovals on tilting the sample stage. Analysis of the “oval-shaped” electron diffraction pattern allowed the direct probing of the reciprocal lattice geometry and its intensity distribution. The reconstructed reciprocal space is of nested cylinders for multiple nanosheets, and consequently, a set of parallel diffuse rods for one crystallite, being diagnostic of the two-dimensional crystal. The intensity modulation along the reciprocal rods demonstrates the molecular feature of the nanosheet and possible adsorption of water molecules on it.
Introduction In the past decade, there has been considerable progress in the field of physics and chemistry of nanoscale materials including ultrafine particles and clusters.1-7 The most prominent examples, fullerenes6 and nanotubes,7 have demonstrated that their dimensions in a nanometer range and novel shapes evolve a wide range of intriguing properties. Recently two-dimensional semiconductor crystallites of titanium and niobium oxides have been synthesized by delaminating precursor layered crystals into their elementary layers.8,9 These nanosheet materials can be regarded as new classes of nanosized semiconductors, which are characterized by high crystallinity and well-defined chemical composition as well as extremely high anisotropy with an ultrathin thickness. These unusual features result in distinctive physicochemical properties in comparison with conventional nanocrystallites mostly in spherical shape which have been intensively investigated.4,5 For example, the nanosheet crystallite of Ti1-δO24δ- (δ ∼ 0.09) shows very sharp optical absorption peak which is appreciably blue-shifted relative to the band gap energy of bulk titanium oxides.10 Structural information on these nanosheet semiconductors is essential for deep understanding of their physical and chemical properties. Furthermore, diffraction phenomenon from such a unique system with exceeding two-dimensionality is a very interesting topic from a viewpoint of diffraction theory. Following the pioneering work on two-dimensional systems such as carbon black11 and clay minerals,12 Frindt and co-workers13 have carried out elaborate structural study on delaminated twodimensional crystallites of transition metal dichalcogenides. Their reports focused on powder diffraction profile from * Corresponding author. Fax: +81-298-54-9061. E-mail:
[email protected]. † Reorganized from National Institute for Research in Inorganic Materials.
randomly oriented crystallites, which expresses the diffraction data onto one parameter axis or the magnitude of scattering vector. To our knowledge, there has been no report describing straightforward exploration of the reciprocal space of nanosheet crystallites, which should provide valuable and detailed information on their structure. Therefore, in the present study, we examined structural aspects of the quasi-TiO2 nanosheet on the basis of electron diffraction data, directly probing the geometry of the two-dimensional reciprocal lattice and structure factors associated with the atomic arrangement along the sheet thickness. Experimental Section Reagents and Materials. Reagents such as Cs2CO3 and TiO2 were of 99.99% purity or higher (Rare Metallic, Co.). All the other chemicals were of analytical grade. All water used was purified to a resistivity of >17 MΩ cm by being filtered through a Milli-Q reagent water system. Preparation of the quasi-TiO2 nanosheets involves delamination of a layered Cs titanate, Cs0.7Ti1.82500.175O4 (0 : vacancy). The polycrystalline sample used in this study was composed of platy microcrystals of average dimensions in sub-micrometer range, which was prepared by calcination of a stoichiometric mixture of Cs2CO3 and TiO2 at 1073 K.14,15 In a typical exfoliation procedure, its acid-exchanged product of H0.7Ti1.82500.175O4‚H2O was shaken vigorously with an aqueous solution of tetrabutylammonium hydroxide, (C4H9)4NOH, at ambient temperature.8 The equivalent dose of the agent with respect to the exchangeable protons in H0.7Ti1.82500.175O4‚H2O is important to proceed exfoliation to completion, which produces colloidal single layers, namely, the nanosheet crystallite of Ti1-δO24δ- (δ ∼ 0.09).8b Instrumentation. Transmission electron micrographs (TEM) and electron diffraction data were taken by a JEM 1010 electron
10.1021/jp010421i CCC: $20.00 © 2001 American Chemical Society Published on Web 06/07/2001
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Figure 1. Atomic architecture of the nanosheet crystallites of Ti1-δO24δ-: (a) top view, (b) side view. Red and blue circles represent Ti and O atoms, respectively. The axis notation is taken by following to that for the orthorhombic crystal structure of the parent layered titanate.
microscope operated at an accelerating voltage of 100 kV. A drop of the diluted colloidal suspension of the nanosheets (0.04 g dm-3) was placed onto a holey carbon grid and was passed through it by giving gentle vibration. After being air-dried, the specimen was allowed to TEM observations. The electron diffraction data were recorded with an imaging plate and their intensity was read out as a function of scattering vector to give a conventional pattern. The data were obtained from a thin sector region with a central angle of 2° to correct the geometrical effect or Lorentz factor. To obtain accurate peak positions and their intensities, the camera length and the baseline contribution from amorphous carbon on the grid were corrected by Au diffraction rings and background data measured under the identical conditions, respectively. The nanosheet thickness was examined by atomic force microscopy (AFM) and spectroscopic ellipsometry for a monolayer or multilayer film of the titania nanosheets deposited on a Si wafer substrate precoated with polyethylenimine (PEI). The samples were prepared by self-assembling layer-by-layer the negatively charged titania nanosheets and polydiallyldimethylammonium ions (PDDA) onto the substrate. Experimental details were similar to those described before.16 AFM images were obtained by a Digital Instrument Nanoscope III in the air. The measurements were carried out in tapping mode with a silicon tip (force constant ) 41 N cm-1). The ellipsometric measurements were performed by a JovinYbon UVISEL/DH10 spectroscopic ellipsometer equipped with a photoelastic modulator and a 75 W Xe lamp as a light source. The ellipsometric parameters, ψ and ∆,17 were measured over a wavelength range of 240-830 nm at a fixed incident angle of 75° for a film of Ti1-δO2/(PDDA/Ti1-δO2)9. The obtained curves were fitted with a multilayer model of Si/SiO2/PEI/film to obtain dispersion parameters for this system. The composite film of Ti1-δO2 and PDDA was treated as a uniform medium in the fitting. Assuming that the resultant parameters are valid for different layer-pair numbers, the film thickness in the multilayer growth process was monitored by analyzing a singlewavelength data at 633 nm. Results and Discussion Two-Dimensional Diffraction Features. The titania nanosheet of Ti1-δO24δ- is to correspond to the individual host layer of the parent titanate Cs0.7Ti1.82500.175O4,14b its atomic architecture being illustrated in Figure 1. Titanium atom is coordinated with
Figure 2. Transmission electron micrograph of Ti1-δO24δ- nanosheet crystallites (a), and diffraction patterns from single nanosheet (b) and multiple nanosheets (c). The diffraction rings 1-4 in (c) are 11, 20, 02, and 22 reflections.
six oxygen atoms and resulting octahedra are joined via edgesharing to produce the two-dimensional crystallite. Its thickness is well below 1 nm, being consisted of two edge-shared TiO6 octahedra. The TEM imaging allowed the identification of individual nanosheet crystallites. The lateral size ranging from 0.1 to 1 µm coincides with dimensions of the parent layered microcrystals, indicating that fracture of the two-dimensional sheets into smaller pieces is negligible. The observed contrasts were very faint and, more importantly, uniform within a crystallite and similar from one crystallite to another. There are some spots where two crystallites were overlapped and their contrast was approximately double of that of other portions. These features imply that the crystallites are very thin and likely to be unilamellar.
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Figure 3. Diffraction patterns as a function of tilting angle. (a) 15°, (b) 30°, (c) 40°. The ring pattern in Figure 2c corresponds to one without tilt or perpendicular alignment of electron beam with respect to the nanosheet face.
Figure 4. Reconstructed reciprocal space for single crystallite (a) and multiple crystallites (b).
The selected area electron diffraction from one nanosheet crystallite (Figure 2b) displayed an orthogonal array of sharp spots, which indicates single-crystal quality as well as the high crystallinity of the nanosheets. The data can be indexed as hl reflections for a two-dimensional lattice of 0.38 nm × 0.30 nm, being compatible with the “unit cell” enclosed by the yellow lines in Figure 1. The extinction of hl spots with h + l ) 2n + 1 is attributable to its face-centered symmetry when projected along the b-axis. Data acquisition from multiple crystallites using a larger aperture gave a concentric ring pattern (Figure 2c). This is due to the parallel settlement of the nanosheet crystallites onto the TEM grid surface and their random rotation in that plane with respect to each other. Geometry of the Reciprocal Space. The reciprocal space perpendicular to the hl lattice plane was explored by tilting the sample stage, which brought about deformation of the concentric diffraction rings into “ovals” (see Figure 3). Their shorter radii were kept unchanged from radii of corresponding circles before tilting. The degree of the deformation, i.e., a ratio of longer and shorter radii, was 1.02, 1.04, 1.06, 1.11, 1.16, 1.21, 1.32, and 1.41 at the tilting angle of 10°, 15°, 20°, 25°, 30°, 35°, 40°, and 45°, respectively. These values are approximately equal to 1/cos φ where φ is the tilting angle. This “oval” diffraction feature can be accounted for by the single-sheet nature of the crystallites. The periodic atomic arrangement in the nanosheet and absence of a repeating unit along the sheet normal should give the two-dimensional lattice spots in the sheet plane which extend as diffuse rods in its perpendicular direction, as depicted in Figure 4a. The random azimuthal orientation of the nanosheets in the TEM grid plane should result in the composite reciprocal space being constituted of nested cylinders (Figure 4b). A cross
section of these cylinders by a plane tilted with respect to the horizontal can evidently explain the oval diffraction patterns and the tilting angle dependence of the eccentric ratio of the ovals. The oval diffraction pattern could result from a twodimensional system where nanosheets are turbostratically stacked. In this case, there is no sheet-to-sheet registry involving random lateral displacement and/or rotation with respect to each other. Due to the loss of three-dimensional coherence in a particle, the resulting diffraction pattern is very similar to the unilamellar system. The only difference is the presence of 0k0 basal diffraction series besides two-dimensional hl rods. If particles composed of turbostratically stacked nanosheets are preferred oriented along the stacking direction, an oval pattern should be observed when the electron beam is set tilted from the stacking axis. However, we can distinguish the single-sheet system from this turbostratic stacking on the basis of the selected area diffraction from one crystallite. If a particle is composed of plural nanosheets turbostratically stacked, its diffraction data should be a simple sum of two-dimensional lattice spots from each nanosheet. A concentric ring pattern should be observed for a particle composed of ample number of the nanosheets. This was not the case when we collected the diffraction data from one crystallite. The single-crystal pattern of sharp spots exemplified by Figure 2b was always detected. Another misleading case is a polycrystalline sample with strong preferred orientation. In contrast to the turbostratic stacking, each particle has three-dimensional order and is aligned along a particular crystal axis. The composite reciprocal space for this so-called fibrous texture comprises successive layers of concentric rings as a consequence of random orientation about the fiber axis. Its section by certain angle with respect to the oriented axis does not give a continuous oval shape but a series of broken arcs associated with different lattice planes.18 This is definitely different from the observations in the present study. Intensity Distribution in the Reciprocal Space. The intensity was apparently inhomogeneous along the ovals. This modulation should correspond to square of the structure factor or molecular transform arisen from the atomic arrangement along the nanosheet normal, which is important for understanding structural features of the nanosheet crystallite. We employed an imaging plate to collect the oval diffraction pattern which was obtained by tilting the ring pattern by 45°, and read out a series of diffraction data as a function of an angle, φ, taken from the oval short axis (see Figure 5). As can be understood by Figure 4, the intensity distribution along the hl rods can be followed quantitatively by this procedure. It is to be pointed
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Figure 5. Diffraction patterns as a function of the read-out angle, φ. The geometrical relationship of φ on the ovals is shown beside the graph. The oval diffraction pattern was obtained by tilting the ring pattern by 45°.
out that the kinematical effect on the intensities, which is usually significant in electron diffraction, need not to be taken into consideration because the crystallites are ultrathin in this study. The trace at φ ) 0 corresponds to the hl pattern which revealed sharp diffraction profiles attributable to the face-centered orthogonal lattice. With increasing φ, the peak locations shifted to a larger scattering vector region and several hl diffuse rods with h + l * 2n became perceptible. These do not have an intensity at k ) 0 because of the face-centered symmetry. The intensity variations for each hl reciprocal rod are plotted as a function of scattering vector along k, as depicted in Figure 6a.19 The complex modulation obtained is attributable to the presence of two or more atomic planes, more precisely, O, Ti/ O, Ti/O, and O atom planes along the b-axis (see Figure 1). Note that structure factors for two-dimensional materials with a single atom plane such as graphite decrease monotonically. The two-dimensional structure factor, F(hl), for the quasiTiO2 nanosheet can be calculated by the following equation:
F(hl) )
∑j fj exp 2πi(hxj + kyj + lzj)
(1)
where fj, xj, yj, zj are scattering amplitudes and fractional coordinates of constituent Ti and O atoms in the twodimensional crystallite, respectively.20 Positional parameters used are based on the single-crystal data of the parent titanate, Cs0.7Ti1.82500.175O4.14b For the two-dimensional symmetry of this system, the index, k, is taken as continuous while those, h and l, are integer. The calculated traces (Figure 6b) agreed qualitatively with the experimental data, which confirms again that the atomic arrangement was not perturbed significantly even after disintegration from the bulk crystalline state into the ultrathin system in the molecular-level thickness. Nevertheless, there is a recognizable difference in terms of relative intensities. In the observed data, the reflections with h + l ) 2n appeared to be stronger than the others, when compared with the calculated trace. One of the most ordinary explanations for such a discrepancy is some structural modification, e. g., small distortion in atomic arrangement. However, our preliminary simulations did not reproduce the observed changes to a satisfactory extent. Moreover, Raman vibrational spectra did not change appreciably upon exfoliation into the nanosheet crystallites from the bulk titanate crystal, denying a significant structural change. On the other hand, some adsorbed species on the nanosheet surface may rather explain the difference. One of the most probable adsorbates may be water molecules because the
Figure 6. (a) Intensity variation along the hl reciprocal rods. (b) Square of structure factors calculated on the basis of the atomic architecture of the nanosheet. (c) Square of structure factors calculated for the hydrated nanosheet (refer to a model in Figure 7).
nanosheets were deposited from the aqueous colloidal suspension. Addition of water molecules at appropriate locations can considerably improve matching between the observed and calculated intensity data. Figure 6c illustrates one of such examples where the intensities of the reflections such as 20, 02, 22 were enhanced by approximately twice while the others were kept unchanged. This simulation is based on a hydration model (see Figure 7) in which O atoms of water molecules are placed just above and below Ti and O atoms in the nanosheet keeping reasonable interatomic distances.21 Water molecules are located at a distance of 0.25 nm from the surface O atoms, the dimension of which is based on the hydration behavior of layered titanates including the parent titanate, H0.7Ti1.82500.175O4‚
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Figure 7. Structural model for a hydrated nanosheet crystallite. Closed, open, and shaded circles represent Ti atom, O atom, and H2O molecules, respectively. All the water sites are assumed to be half occupied.
Figure 9. Ellipsometric thickness in the layer-by-layer assembling process. Titania nanosheets and polycations were alternately adsorbed as shown by closed and open circles, respectively. Each data point represents an averaged value from 10 different spots and error bars designate three times of standard deviations. The broken line designates the initial priming process.
1.31 nm, is again compatible with the presence of adsorbed water molecules on the nanosheet. In summary, the reciprocal space of the exfoliated nanosheet crystallite was reconstructed in terms of geometry and intensity modulation, which clearly demonstrates the genuine twodimensional nature for Ti1-δO24δ- having a thickness of molecular level. This structural analysis may make an important contribution to the elucidation of the semiconducting properties and chemical reactivities (adsorption, photocatalytic activities, etc.) of novel nanosheet materials such as Ti1-δO24δ-. References and Notes
Figure 8. AFM image of the titania nanosheets deposited on a Si wafer.
H2O. Zero-, mono-, and bilayer hydrates were formed depending on interlayer cations and their population and a difference in their gallery heights was 0.20-0.30 nm.8a,15,22 The nanosheet with a monolayer of water molecules on both sides is around 1.2 nm () 0.70 + 0.25 × 2) thick. Actually the thickness of 1.2-1.3 nm was experimentally detected by AFM as exemplified by the typical image for nanosheet crystallites deposited on a Si wafer substrate (Figure 8). The observed features having a lateral dimension of submicrometer are consistent with the morphology of the titania nanosheets. A height difference of ∼1.2 nm was frequently encountered between the nanosheet terrace and the bare substrate surface, as depicted in the roughness profile. The ellipsometric data also supported the above discussion. Figure 9 shows the evolution of the multilayer film thickness in the self-assembly process of titania nanosheets and PDDA where a monolayer of each component was adsorbed at a time. The reproducible thickness increment was observed in each adsorption for each component. The average value for Ti1-δO2,
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Molecular Nanosheet Crystallites of Titanium Oxide J. Phys. C: Solid State Phys. 1987, 20, 4043. (c) Yang, D.; Sandoval, S. J.; Divigalpitiya, W. M. R.; Irwin, J. C.; Frindt, R. F. Phys. ReV. B 1991, 43, 12053. (d) Yang, D.; Frindt, R. F. J. Appl. Phys. 1996, 79, 2376. (14) (a) Hervieu, M.; Raveau, B. ReV. Chim. Miner. 1981, 18, 642. (b) Grey, I. E.; Li, C.; Madsen, I. C.; Watts, J. A. J. Solid State Chem. 1987, 66, 7. (15) Sasaki, T.; Watanabe, M.; Michiue, Y.; Komatsu, Y.; Izumi, F.; Takenouchi, S. Chem. Mater. 1995, 7, 1001. (16) Sasaki, T.; Ebina, Y.; Watanabe, M.; Decher, G. Chem. Commun. 2000, 2163. (17) These parameters are defined by an equation of r˜p/r˜s ) (tan ψ)‚ei∆ where r˜p and r˜s are the complex amplitude reflection coefficients for pand s-polarized light, respectively. (18) Hirsch, P. B.; Howie, A.; Nicholson, R. B.; Pashley, D. W.; Whelan, M. J. Electron Microscopy of Thin Crystals; Butterworth: London, 1965; pp 108-119. (19) Because the oval pattern obtained on 45° tilting was analyzed, the
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