A new mild synthesis and optical properties of colloidal ZnO nanocrystals in dimethylformamide/ethanol solutions
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A new mild synthesis and optical properties of colloidal ZnO nanocrystals in dimethylformamide/ethanol solutions Yaroslav V. Panasyuk, Oleskandra E. Rayevska, Oleksandr L. Stroyuk, and Stepan Ya. Kuchmiy L.V. Pysarzhevsky Institute of Physical Chemistry of National Academy of Sciences of Ukraine, 31 Nauky av., 03028, Kyiv, Ukraine ABSTRACT A green and mild synthesis of colloidal zinc oxide nanocrystals in ethanol/dimethylformamide mixtures was introduced which allows to produce stable crystalline ZnO particles and tailor their average size in the range of 2.8−4.5 nm by varying temperature and duration of postsynthesis ageing. An increase in dimethylformamide fraction in the mixture results in acceleration of ZnO nanocrystals ripening. Colloidal ZnO nanocrystals emit broadband photoluminescence in the range of 2−3 eV with the quantum yields of up to 12 %. INTRODUCTION Interest in colloidal zinc oxide nanocrystals (NCs) is stimulated by a broad variety of their application in advanced technologies such as light-emitting devices, luminescent biomarkers, solar cells, etc. [1]. Introduction of ZnO NCs into these technologies requires simple and green synthetic ways to ZnO NCs with controlled average size and photoluminescence (PL). Today among the mild syntheses of ZnO NCs probably the most popular remains an approach introduced in 1980s based on reaction of zinc salts with inorganic bases in absolute aliphatic alcohols [2−7]. Despite the popularity the approach has a number of limitations impeding considerably progress in application of ZnO NCs. In particular, strict requirements to the alcohol dryness are dictated by a low aggregation stability of colloidal ZnO NCs in the presence of even small amounts of water and by the fact that in wet alcohols only Zn(OH)2 forms which then only slowly dehydrates and converts into ZnO. Another limitation is the necessity of prolonged post-synthesis thermal treatment of zinc oxide colloids at around 60 °C which imparts ZnO NCs with high aggregation stability and crystallinity but makes impossible controlled and smooth variation of the ZnO NC size. Finally, the quantum yields of the broadband PL in the visible part of the spectrum, which is typical for colloidal ZnO NCs produced by this method, are typically not higher than 4–5%. The present paper shows that using mixtures of ethanol with a second moderately polar solvent – dimethylformamide (DMFA) as dispersive media for preparation of colloidal ZnO NCs allows to simplify considerably the synthetic procedure allowing at the same time to tailor the average size and the absorption band threshold of ZnO NCs, as well as to increase the PL quantum yields of up to 10−12%. EXPERIMENTAL DETAILS Colloidal ZnO NCs were synthesized via reaction between Zn(CH3COO)2 and NaOH in DMFA/ethanol mixtures containing a varied amount of DMFA. Typically, two types of colloids were prepared containing either 3 v% or 40 v% of DMFA. In the first case, 0.3 mL of 1.0 M zinc acetate solution in DMFA were added to 5.7 mL absolute ethanol and then this solution w
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