Surface heterogeneity of nanoparticles has been mostly studied in terms of different facets and boundary defects. While it is typically assumed that single types of ligands are homogeneously distributed on nanoparticle facets, which is valid only for monodentate ligands such as thiols, we observed that citrate molecules are heterogeneously distributed on facets of gold nanoparticles (AuNPs). The organic layers were inversely probed by ligand-templated metal depositions. Platinum islands were epitaxially grown on AuNP facets via reduction of Pt ions by ascorbic acids in solution, where citrate layers act as pre-existing templates. Local surface regions without Pt islands are interpreted as regions of densely-packed ligand layers formed, which were characterized using high-resolution TEM. We found a pH-dependent distribution of 5 - 10 nm domains of the densely-packed citrate layer on AuNP facets. At pH 4.5 citrate molecules form both loose and dense layers, whereas at pH 5 - 7 citrate molecules form only homogeneously-distributed loose layers. Fully-protonated citric acids (H3Citrate) are likely responsible for the formation of dense layers, due to the versatile intermolecular hydrogen bonds on gold surfaces, suggested by a simple structural modeling on Au(111). Detailed metal-molecule interactions are proposed with considering aurophilicity of Au(I) adatoms, depending on metal oxidation states and citrate speciation. We conclude that deprotonated citrate species including H2Citrate- and HCitrate2- interact with Au(I) adatoms only through carboxylate groups whereas citric acids directly adsorb on metallic Au(0) surfaces through both the hydroxyl group and carboxylic moieties.

Heterogeneity of surface coverage of organic ligands on single facets of gold nanoparticles due to inhomogeneous metal-molecule interactions

254th National Meeting (2017)

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