"Passivation is required mostly in colloidal solutions, which is the cheap way to produce nanoparticles. Imagine the surface of the nanostructure: there are ligands, also called surfactants, binding to the surface," explains Zherebetskyy. "The surfactants define a lot of the chemical and physical properties of the nanoparticles."
"We can synthesize a very beautiful nanostructure, and know how to control even the shape. But how to control the shape is related to how you passivate a surface during the growth process, and exactly how ligands passivate the surface [and how electronic structures happen] has never been well understood," adds
The first step in making a PbS nanocrystal is to dissolve lead oxide in hot oleic acid. This forms one of the precursor molecules, which is lead plus long oleate ligands, and a byproduct of water. "You heat the precursors [to dry them], so people thought that all the water had been evaporated," explains Wang.
"People were really puzzled about how the surface can be passivated," he continues. The nanocrystals have an excess of lead ions relative to sulfate, which means that a surfactant of 2- charge is needed to passivate each extra 2+ lead ion. Each oleic acid molecule (oleate) has charge 1-, but experiments show that the number of extra lead atoms is roughly equal to the number of oleates. Thus it does not make sense that the precursor behaves as though it is passivated.
But doing calculations and following the synthesis processes suggested to Zherebetskyy and Wang that there might still be water in the precursor molecules: indeed, a series of spectroscopic experiments showed that the water binds strongly to the precursors and serves as a source of hydroxyl groups, charge 1-, that can also allow passivation.
"Oleates are big. Imagine them like a tube," explains Zherebetskyy. "The radius of this tube is too large to form such a dense packing that completely passivates the lead atoms." That is, they are too big to cram around the lead without interfering with each other. His research was an effort to find what 'something else' was needed to fully passivate the nanocrystal.
Keywords for this news article include: Chemicals, Chemistry, Nanotechnology, Emerging Technologies,
Our reports deliver fact-based news of research and discoveries from around the world. Copyright 2014, NewsRx LLC
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