A seed-mediated approach was applied to synthesize gold (Au) nanoparticles (NP) by using twin tail alkylammonium cationic surfactants such as 12-6-12 and 12-0-12 as capping agents in aqueous phase at ambient conditions. The growth of Au NP was monitored by changing the amount of seed. Spherical NP (10–50nm) and nanorods (aspect ratio = 2–3) were obtained in the presence of 12-6-12 as capping agent; their shape and size systematically deformed because of anisotropic growth with a decrease in the amount of seed. In contrast, when 12-0-12 was used as a capping agent, no anisotropic growth was observed. An effective liquid/solid interfacial adsorption of 12-0-12 prevented anisotropic growth which led to precise morphologies. This was not observed in the case of 12-6-12 because of the presence of a spacer which restricted an effective interfacial adsorption because of the steric factors. XPS and FTIR studies clearly indicated the presence of a surfactant film on the surface of Au NP, while XRD analysis demonstrated a difference in the preferential adsorption of 12-6-12 and 12-0-12 at different crystal planes of fcc geometry which resulted in a difference in their capping behaviors.

Aqueous phase synthesis at 80 °C was carried out to synthesize lead sulfide (PbS) nanocrystals (NC) and microcrystals (MC) by using cationic twin-tail surfactants (TTS) such as 12-0-12, 10-2-10, 12-2-12, and 14-2-14 as capping agents in the concentration range from 0.1 -to 2 mM. The effect of hydrophobicity on the shape and size of PbS NC was evaluated by choosing DTAB as a reference surfactant for all TTS. TEM micrographs of PbS MC synthesized in the presence of DTAB indicated the formation of star-shaped MC with sizes between 3 -and 5 µm. An increase in the hydrophobicity, by introducing another tail in the basic structure of DTAB to make 12-0-12, significantly controlled the shape and size and lead to the formation of well-defined nanocubes and spheres 50-100 nm in size. Similarly, the effect of the hydrocarbon tail length on the shape controlled synthesis of PbS NC was systematically evaluated. Pyrene fluorescence measurements were used to determine the variation in the degree of hydrophobicity with respect to both chemical structure as well as concentration of TTS. It was concluded that a stronger hydrophobic character and higher concentration produced well-defined geometries of PbS NC. No significant concentration effect within a range of 0.1-2 mM DTAB and 10-2-10 was observed on the morphology of PbS NC probably due to a much weaker hydrophobicity of these surfactants. An attempt was made to present all TEM results in a schematic phase diagram. This phase diagram provided the best correlation between the shape and the size of PbS NC and the surfactant parameters (i.e., hydrophobicity and concentration effects). Apart from this, shape dependence UV-vis absorbance was also noted and discussed in context with an overall preview of all shapes of PbS NC/MC obtained. The shape controlled synthesis of PbS NC was obtained due to the preferential adsorption of TTS on the {111} crystal planes that directed the overall growth predominantly at the {100} planes. FTIR measurements were used to evaluate the adsorption of TTS on the PbS surface. A large shift in the stretching vibrations of TTS head functional groups suggested their orientation toward the PbS surface. This was further supported by the high-resolution XPS spectra of C 1s and N 1s of adsorbed TTS on the PbS surface. An effective interfacial adsorption of TTS on the surface of PbS NC driven by a stronger hydrophobic character is the key to achieve controlled PbS NC growth at the nanoscale.

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