A Simple Colloidal Synthesis for Gram-Quantity Production of Water-Soluble ZnS Nanocrystal Powders
✍ Scribed by Richard Kho; Claudia L. Torres-Martı́nez; Rajesh K. Mehra
- Publisher
- Elsevier Science
- Year
- 2000
- Tongue
- English
- Weight
- 180 KB
- Volume
- 227
- Category
- Article
- ISSN
- 0021-9797
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✦ Synopsis
A simple, inexpensive, and reproducible procedure is described for large-scale synthesis of highly stable nanocrystalline ZnS powders. Cysteine-capped ZnS nanocrystals (NCs) were produced by a colloidal aqueous synthesis, employing a ligand-competition mechanism in which sulfide was introduced into a preformed zinc-cysteine solution. The synthesis procedure resulted in highly concentrated ZnS NC solutions (∼100 mM) which could be ethanolprecipitated, redissolved, and dried to produce fine powders stable for more than 30 months at 4 • C. The NC powders were readily dissolved in aqueous solvents to concentrations as high as 300 mM. ZnS NCs could be prepared without cysteine capping, but only at extremely dilute concentrations (∼0.2 mM ZnSO 4 ) as per Sooklal et al. J. Phys. Chem. 100, 4551 (1996). The 30-month-old ZnS NC powders retained their original optical and photocatalytic properties and could be handled much like routine shelf chemicals, unaffected by ambient air or moderate moisture and temperature. UV/vis absorption spectroscopy showed band gap energies (E g ) ranging from 4.82 eV (257 nm λ max ) to 4.47 eV (277 nm λ max ) for ZnS samples prepared with 0.25-2.0 initial sulfide ratios (as compared to zinc). Samples stored at 4 • C for 30 months showed equivalent band gap energies and spectral profiles. The average NC particle size was estimated to be 6.08 ± 0.76 nm by high-resolution transmission electron microscopy. Selected-area electron diffraction and X-ray diffraction analyses concurred in suggesting a hexagonal crystal structure, with diffractions near d = 3.1, 1.9, and 1.6 Å. The average NC composition of size-fractionated samples was estimated to be Cys 1 Zn 7 S 6 . p-Nitrophenol, a model organic, was photocatalytically degraded using 30-month-old ZnS NC powders dissolved in an aqueous buffer. Rates of degradation (first-order rate constant k = 0.261 min -1 ; t 1/2 = 2.66 min) were comparable to those of experiments using freshly prepared ZnS NCs (first-order rate constant k = 0.247 min -1 ; t 1/2 = 2.80 min), further demonstrating the longterm stability of thus-produced NC powders.