Growth kinetics and long-term stability of CdS nanoparticles in aqueous solution under ambient conditions
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The ubiquity of naturally occurring nanoparticles in the aquatic environment is now widely accepted, but a better understanding of the conditions that promote their formation and persistence is needed. Using cadmium sulfide (CdS) as a model metal sulfide species, thiolate-capped CdS nanoparticles were prepared in the laboratory to evaluate how aquatic conditions influence metal sulfide nanoparticle growth and stability. This work examines CdS nanoparticle growth directly in aqueous solution at room temperature by utilizing the size-dependent spectroscopic properties of semiconductors detectable by UV/vis. CdS nanoparticle growth was governed by oriented attachment, a non-classical mechanism of crystallization in which small precursor nanoparticles coalesce to form larger nanoparticle products. Nanoparticle growth was slowed with increasing capping agent and decreasing ionic strength. In addition to examining the short-term (hours) growth of the nanoparticles, a long-term study was conducted in which cysteine-capped CdS nanoparticles were monitored over 3 weeks in solutions of various ionic strengths. The long-term study revealed an apparent shift from small nanoparticles to nanoparticles twice their original size, suggesting nanoparticle growth may continue through oriented attachment over longer time scales. High-ionic strength solutions resulted in salt-induced aggregation and eventual settling of nanoparticles within days, whereas low-ionic strength solutions were stable against settling over the course of the experiment. Sulfide recovery from cysteine-capped CdS nanoparticles as acid volatile sulfide was nearly quantitative after 2 weeks in fully oxygenated water, demonstrating significantly slowed oxidation of sulfide when complexed to Cd(II) within CdS nanoparticles. The nanoparticles were also shown to be resistant to oxidation by Fe(III) (hydr)oxide. This study illustrates that aggregation, rather than chemical oxidation, is likely more important to the lifetime of many metal sulfide nanoparticles in the aquatic environment.
KeywordsOriented attachment Metal sulfide nanoparticles Nanoparticle growth kinetics Cadmium sulfide Colloids
The authors acknowledge Debbie Powell and Kirk Czymmek (Delware Biotechnology Institute) for help with TEM sample preparation and image analysis and Rovshan Mahmudov (UD Civil and Environmental Engineering) for help with DLS measurements. The authors would like to thank Dominic DiToro (UD Civil and Environmental Engineering) for thoughtful discussions that lead us to oriented attachment as a growth mechanism. This work was supported by the National Science Foundation under Grant No. EPS-0447610 (Delaware EPSCoR Seed Grant). Additional support was provided by a subcontract to the University of Delaware from the NASA Astrobiology grant to UC, Berkeley.
- Gilbert B, Banfield JF (2005) Molecular-scale processes involving nanoparticulate minerals in biogeochemical systems. Mol Geomicrobiol 59:109–155Google Scholar
- Hsu-Kim H, Mullaugh KM, Tsang JJ, Yucel M, Luther GW (2008) Formation of Zn- and Fe-sulfides near hydrothermal vents at the Eastern Lau Spreading Center: implications for sulfide bioavailability to chemoautotrophs. Geochem Trans 9Google Scholar
- Moreau JW, Webb RI, Banfield JF (2004) Ultrastructure, aggregation-state, and crystal growth of biogenic nanocrystalline sphalerite and wurtzite. Am Miner 89:950–960Google Scholar
- Morrison ID, Ross S (2002) Colloidal dispersions: suspensions, emulsions and foams. John Wiley and Sons, Inc, New YorkGoogle Scholar
- Stumm W (1992) Chemistry of the solid–water interface: processes at the mineral–water and particle–water interface in natural systems. John Wiley & Sons, New YorkGoogle Scholar