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Preparation and characterization of quantum size zinc oxide: a detailed spectroscopic study

Bahnemann, Detlef W. and Kormann, Claudius and Hoffmann, Michael R. (1987) Preparation and characterization of quantum size zinc oxide: a detailed spectroscopic study. Journal of Physical Chemistry, 91 (14). pp. 3789-3798. ISSN 0022-3654. https://resolver.caltech.edu/CaltechAUTHORS:20150817-150959345

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Abstract

We report the synthesis of transparent colloidal suspensions of small zinc oxide particles in water, 2-propano1, and acetonitrile. Quantum (Q)-size effects are observed during particle growth and qualitatively interpreted by using a simple molecular orbital (MO) picture. The particles at the final stage of growth are approximately spherical in shape and consist of 2000-3000 ZnO molecules. They exhibit many of the photophysical properties of bulk zinc oxide. However, pronounced shifts in the absorption spectrum during the illumination of anoxic suspensions of ZnO reveal a distinctively different behavior of these small particles. Fluorescence spectra of the ZnO sols suggest that adsorbed electron relays are necessary to shuttle electrons from the conduction band into lower lying traps. Two fluorescence maxima are observed at the final growth stage of the ZnO particles. The bandgap fluorescence at 365 nm has an extremely short lifetime (τ < 100 ps), while the visible luminescence at 520 nm exhibits a slower biexponential decay (i.e., τ = 14 and 190 ns). The latter fluorescence is attributed to photogenerated electrons tunneling to preexisting, trapped holes. The low overall fluorescence quantum yield of Ф = 0.03 measured in these zinc oxide suspensions is indicative of radiationless transitions accompanying the emissions. A pronounced pH dependence of the Stern-Volmer constants obtained with various ionic substances, that effectively quench the 520-nm emission, is explained by specific adsorption to the charged particle surface. The zero point of charge (pH_(zpc)) of the aqueous colloidal suspension was determined to be 9.3 ± 0.2 by several independent methods.


Item Type:Article
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http://dx.doi.org/10.1021/j100298a015DOIArticle
http://pubs.acs.org/doi/abs/10.1021/j100298a015PublisherArticle
ORCID:
AuthorORCID
Hoffmann, Michael R.0000-0001-6495-1946
Additional Information:© 1987 American Chemical Society. Received: November 5, 1986: In Final Form: March 18, 1987. Publication Date: July 1987. We gratefully acknowledge the financial support by the US. EPA (CR812356-01-0) and in particular we want to thank Dr. Marcia Dodge for her support. We appreciate extensive scientific discussions with Prof. Arnim Henglein (Hahn-Meitner Institute Berlin) who also supplied a preprint of his recent work^(23) thereby stimulating our ideas concerning the nonlinear optical effects. We enjoyed a very pleasant collaboration with Prof. Douglas Magde (University of California, San Diego) during the fluorescence decay studies and like to thank him and Ms. Gale Rojas for their help. D.W.B. thanks the Hahn-Meitner Institut for granting him a leave of absence.
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Funding AgencyGrant Number
Environmental Protection Agency (EPA)CR812356-01-0
Issue or Number:14
Record Number:CaltechAUTHORS:20150817-150959345
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20150817-150959345
Official Citation:Preparation and characterization of quantum size zinc oxide: a detailed spectroscopic study Detlef W. Bahnemann, Claudius. Kormann, and Michael R. Hoffmann The Journal of Physical Chemistry 1987 91 (14), 3789-3798 DOI: 10.1021/j100298a015
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:59656
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:18 Aug 2015 15:51
Last Modified:03 Mar 2020 13:01

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