J. Phys. Chem. B 1997, 101, 10159-10161
10159
Semiconductor Nanosheet Crystallites of Quasi-TiO2 and Their Optical Properties Takayoshi Sasaki* and Mamoru Watanabe National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan ReceiVed: August 25, 1997; In Final Form: October 13, 1997X
We have studied the optical properties of novel semiconductor nanosheets of Ti1-δ0δO24δ- (δ ∼ 0.09; 0, vacancy) which were obtained by delaminating a layered titanate into elementary host layers. A uniform thickness in subnanometer scale as well as high crystallinity resulted in a sharp absorption onset with a well-developed peak assignable to excitonic transition. The absorption was pronouncedly blue shifted (>1.4 eV) relative to the band edge for bulk TiO2, which is attributed to size quantization effects. Room-temperature photoluminescence spectra showed resonant emission and well-structured fluorescence extending into a lower energy region.
Extremely small-sized or nanostructured semiconductors have been one of the most exciting and intriguing topics in the past decade in the fields of both physics and chemistry, since they exhibit novel physicochemical properties as a reflection of the transition from molecules to bulk solid materials.1-3 A change in electronic structure is typically noticed by a blue shift of the band edge in optical absorption spectra with decreasing particle size, which is induced by exciton confinement in a restricted space. This so-called size quantization effect has clearly been observed for several materials, e.g., CdS nanocrystallites 1.4 eV is very large in comparison with values for the small particles of TiO2 which are mostly less than 0.5 eV.9-12 These small shifts reported have stimulated controversy for their origin, i.e., quantum confinement or other reasons. Serpone et al.13 have concluded that no size effects were observed for TiO2 particles of dimensions >2 nm. Apparent blue shifts observed particularly for diluted specimens were attributed to Franck-Condon type transitions involving a larger energy difference instead of a relatively weak indirect transition (3.19 eV) corresponding to the bandgap of bulk TiO2. They demonstrated that the bandgap transition was detectable for a high-loading sample. On the other hand, this was not the case for the colloidal nanosheets here. By taking into accounts this fact as well as the large magnitude of the energy shift, the spectral changes upon delamination are likely to be associated with size quantization. The fact that the thickness of the nanosheets is comparable to or below the theoretically predicted size of exciton in TiO215 also supports this inference. The bandgap energy shift, ∆Eg, by exciton confinement in anisotropic two-dimensional crystallites is formulated as follows:21-23
∆Eg )
(
)
h2 1 1 h2 + + 8µxz L 2 L 2 8µyLy2 x z
(1)
where h is Plank’s constant, µxz, µy are reduced effective mass of exciton, and Lx, Ly, Lz are crystallite dimensions. Here suffixes x, z, and y express parallel and perpendicular directions with respect to the sheet, respectively. Since Lx, Lz . Ly for the nanosheet,24 the first term can be ignored. In other words, the blue shift is predominantly governed by the sheet thickness. The thickness is constituted by two TiO6 octahedra joined via edge-sharing which may be comparable to a hypothetical binuclear complex compound. Thus the absorption features observed should be very similar to those for molecular species, an end member in the course of decreasing particle size. The sharp absorption threshold is consistent with this situation. The colloidal nanosheets exhibited strong photoluminescence as exemplified by Figure 5. The spectra were measured by a Hitachi F-4500 spectrofluorometer with an excitation wavelength of 200 nm.25 The emission onset moved steadily to a shorter wavelength region until the colloid concentration of 0.007 g dm-3 was reached. Below this concentration, the luminescence did not change further. This apparent shift may be ascribed to reabsorption of the emission. Thus we consider the data for the diluted samples (Figure 5d,e) as intrinsic fluorescence.
Letters
J. Phys. Chem. B, Vol. 101, No. 49, 1997 10161 References and Notes
Figure 5. Room-temperature fluorescence spectra of the colloidal suspension: (a) 0.35, (b) 0.07, (c) 0.0175, (d) 0.007, (e) 0.0035 g dm-3.
The onset of ∼270 nm is close to the absorption maximum and consequently is assignable to resonant luminescence. Besides it, a series of peaks extended into a longer wavelength region. Energy differences between neighboring peaks exclude the possibility for coupling with vibrational modes. The wellstructured feature may rather suggest some interband levels that are generated by the Ti site vacancies intrinsically incorporated as given by δ in the chemical formula of Ti1-δ0δO24δ-. It is known that such defects give rise to impurity levels, but resulting fluorescence data for bulk materials are usually very broad. The contrasting sharp peaks observed even at room temperature may also be attributable to the subnanometer-thickness and its uniformity. In summary, the colloidal semiconductor nanosheets of quasiTiO2 exhibited the sharp optical absorption features and interesting photoluminescence, which have been discussed in terms of their novel flaky morphology with ultrathin dimensions comparable to molecules. Although further study including theoretical calculations is required to ascertain the interpretations above, the nanosheet crystallites could be used as a high-quality model system. Investigations on them will shed light upon the photophysical properties of semiconductor nanoparticles. Acknowledgment. The authors are grateful to Professor Y. Nosaka of Nagaoka University of Technology for his fruitful discussion. Thanks are due to Mr. H. Yamada of Hitachi Corporation for his assistance in obtaining fluorescence spectra.
(1) Henglein, A. Chem. ReV. (Washington, D.C.) 1989, 89, 1861. (2) Brus, L. E. Appl. Phys. A 1991, 53, 465. (3) Kanemitsu, Y. Phys. Rep. 1995, 263, 1. (4) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (5) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (6) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (7) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (8) Bahnemann, D. W. Photochemical ConVersion and Storage of Solar Energy; Pelizzetti, E., Schiavello, M., Eds.; Kluwer Academic Publications: Dordrecht, 1991. (9) Anpo, M.; Shima, T.; Kodama, S.; Kubokawa, Y. J. Phys. Chem. 1987, 91, 4305. (10) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1988, 92, 5196. (11) Kavan, L.; Stoto, T.; Gra¨tzel, M.; Fitzmaurice, D.; Shklover, V. J. Phys. Chem. 1993, 97, 9493. (12) Joselevich, E.; Willner, I. J. Phys. Chem. 1994, 98, 7628. (13) Serpone, N.; Lawless, D.; Khairutdinov, R. J. Phys. Chem. 1995, 99, 16646. (14) Gra¨tzel, M. Heterogeneous Photochemical Electron Transfer; CRC Press: Boca Raton, FL, 1986. (15) An exciton radius (Rexc) in TiO2 is theoretically predicted to be 0.75-1.90 nm10 or as small as 0.30 nm.14 (16) Sasaki, T.; Watanabe, M.; Hashizume, H.; Yamada, H.; Nakazawa, H. J. Am. Chem. Soc. 1996, 118, 8329. (17) Grey, I. E.; Li, C.; Madsen, I. C.; Watts, J. A. J. Solid State Chem. 1987, 66, 7. (18) Sasaki, T.; Watanabe, M.; Michiue, Y.; Komatsu, Y.; Izumi, F.; Takenouchi, S. Chem. Mater. 1995, 7, 1001. (19) Evidence for exfoliation into single sheets is based on the following XRD data on colloidal aggregates centrifuged from the suspensions.16 (i) The aggregate immediately after the centrifugation was substantially amorphous, in contrast to sharp diffraction lines for the starting titanate. (ii) Upon drying, the broad amorphous-like pattern totally changed into sharp basal diffraction series, indicating evolution of a lamellar structure with an intersheet distance of 1.75 nm. (iii) The restacking process was initiated from a trace amount of a paired assembly of the nanosheets the spacing between which is >10 nm. This was suggested from line profile analyses on a series of basal reflections appearing in a small-angle scattering region that were observed at the very beginning of drying. (20) Kim, Y. I.; Atherton, S. J.; Brigham, E. S.; Mallouk, T. E. J. Phys. Chem. 1993, 97, 11802. (21) Shinada, M.; Sugano, S. J. Phys. Soc. Jpn. 1966, 21, 1936. (22) Sandroff, C. J.; Hwang, D. M.; Chung, W. M. Phys. ReV. B 1986, 33, 5953. (23) Sandroff, C. J.; Kelty, S. P.; Hwang, D. M. J. Chem. Phys. 1986, 85, 2237. (24) Lateral dimensions are in a range of 0.1-1 µm on the basis of TEM observations while the sheet thickness is 0.75 nm. (25) No care was taken to eliminate oxygen from the colloidal suspensions.
