14597
2007, 111, 14597-14601 Published on Web 09/18/2007
Effect of Size and Shape of Nanocrystalline TiO2 on Photogenerated Charges. An EPR Study Nada M. Dimitrijevic,*,†,‡ Zoran V. Saponjic,† Bryan M. Rabatic,‡ Oleg G. Poluektov,† and Tijana Rajh‡ Chemistry DiVision and Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: July 18, 2007; In Final Form: August 16, 2007
The continuous wave and pulsed electron paramagnetic resonance study of the photogenerated charges in differently shaped anatase nano-objects dispersed in aqueous solution is presented. Excitation of TiO2 nanoobjects (spherical, faceted, rods, and bricks) at temperatures T2 (spherical), observed for lattice-trapped electrons in faceted particles would suggest suppressed charge recombination in these nanocrystallites compared to spherical particles of the same size in analogy to the natural photosynthesis where electron spin-correlation slows down the spontaneous decay of the charge-separated state. Additionally, eventual formation of a long-lived triplet state in photoexcited TiO2 would further increase the effective lifetime of the electron-hole separated state and explain its high photocatalytic efficiency. However, unlike quantum dots, there is not enough experimental evidence for the formation of a triplet in bare TiO2 and further studies are needed. Acknowledgment. The work was performed under the auspices of the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Contract DE-AC02-06CH11357. Supporting Information Available: Field-swept echo spectra of anatase nano-objects and values for g-tensors and spinspin relaxation times of photogenerated charges. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Saponjic, Z. V.; Dimitrijevic, N. M.; Tiede, D. M.; Goshe, J. A.; Zuo, X.; Chen, L. X.; Barnard, A. S.; Zapol, P.; Curtiss, L.; Rajh, T. AdV. Mater. 2005, 17, 965.
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