We report a dramatically improved synthesis of colloidal gold nanorods (NRs) using a binary surfactant mixture composed of hexadecyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL). Both thin (diameter <25 nm) and thicker (diameter >30 nm) gold NRs with exceptional monodispersity and broadly tunable longitudinal surface plasmon resonance can be synthesized using seeded growth at reduced CTAB concentrations (as low as 0.037 M). The CTAB-NaOL binary surfactant mixture overcomes the difficulty of growing uniform thick gold NRs often associated with the single-component CTAB system and greatly expands the dimensions of gold NRs that are accessible through a one-pot seeded growth process. Gold NRs with large overall dimensions and thus high scattering/absorption ratios are ideal for scattering-based applications such as biolabeling as well as the enhancement of optical processes.
We demonstrate for the first time that monodisperse gold nanorods (NRs) with broadly tunable dimensions and longitudinal surface plasmon resonances can be synthesized using a bromide-free surfactant mixture composed of alkyltrimethylammonium chloride and sodium oleate. It is found that uniform gold NRs can be obtained even with an iodide concentration approaching 100 μM in the growth solution. In contrast to conventional wisdom, our results provide conclusive evidence that neither bromide as the surfactant counterion nor a high concentration of bromide ions in the growth solution is essential for gold NR formation. Correlated electron microscopy study of three-dimensional structures of gold NRs reveals a previously unprecedented octagonal prismatic structure enclosed predominantly by high index {310} crystal planes. These findings should have profound implications for a comprehensive mechanistic understanding of seeded growth of anisotropic metal nanocrystals.
Supporting InformationABSTRACT: The importance of co-adsorption for applications of porous materials in gas separation has motivated fundamental studies, which have initially focused on the comparison of the binding energies of different gas molecules in the pores (i.e. energetics) and their overall transport. By examining the competitive co-adsorption of several small molecules in M-MOF-74 (M= Mg, Co, Ni) with in-situ infrared spectroscopy and ab initio simulations, we find that the binding energy at the most favorable (metal) site is not a sufficient indicator for prediction of molecular adsorption and stability in MOFs. Instead, the occupation of the open metal sites is governed by kinetics, whereby the interaction of the guest molecules with the MOF organic linkers controls the reaction barrier for molecular exchange. Specifically, the displacement of CO 2 adsorbed at the metal center by other molecules such as H 2 O, NH 3 , SO 2 , NO, NO 2 , N 2 , O 2 , and CH 4 is mainly observed for H 2 O and NH 3 , even though SO 2 , NO, and NO 2 , have higher binding energies (~70-90 kJ/mol) to metal sites than that of CO 2 (38 to 48 kJ/mol) and slightly higher than water (~60-80 kJ/mol). DFT simulations evaluate the barriers for H 2 O CO 2 and SO 2 CO 2 exchange to be ∼ 13 and 20 kJ/mol, respectively, explaining the slow exchange of CO 2 by SO 2 , compared to water. Furthermore, the calculations reveal that the kinetic barrier for this exchange is determined by the specifics of the interaction of the second guest molecule (e.g., H 2 O or SO 2 ) with the MOF ligands. Hydrogen bonding of H 2 O molecules with the nearby oxygen of the organic linker is found to facilitate the positioning of the H 2 O oxygen atom towards the metal center, thus reducing the exchange barrier. In contrast, SO 2 molecules interact with the distant benzene site, away from the metal center, hindering the exchange process. Similar considerations apply to the other molecules, accounting for much easier CO 2 exchange for NH 3 than for NO, NO 2 , CH 4 , O 2 , and N 2 molecules. In this work, critical parameters such as kinetic barrier and exchange pathway are first unveiled and provide insight into the mechanism of competitive co-adsorption, underscoring the need of combined studies, using spectroscopic methods and ab initio simulations to uncover the atomistic interactions of small molecules in MOFs that directly influence co-adsorption.
Insight into the structural variation of metal organic framework materials upon hydration.
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