Structural Features of PbS Nanocube Monolayers upon Treatment with Mono- and Dicarboxylic Acids and Thiols at a Liquid-Air Interface.

This paper describes the ordering of PbS nanocubes (NCs) within free-standing monolayers (suspended on acetonitrile), upon exchanging the native oleate ligands for a series of thiolate and carboxylate ligands at the liquid-air interface. Treatment with either carboxylic acids or thiols effectively decreases the inter-NC separation of nearest-neighbor particles without etching the NC surface. Dicarboxylic acids and dithiols bridge neighboring NCs with an interparticle separation that is consistent with fully extended, bridging ligands. Monocarboxylic acids and monothiols separate NCs by an amount governed by their length, with long-chain ligands showing significant intercalation. (1)H NMR spectroscopy shows carboxylic acids are more effective at replacing the native oleate than are thiols, which we ascribe to the lower pKa values of carboxylic acids. The fast exchange that occurs upon treatment with monocarboxylic acids kinetically traps the clusters of particles in nonclosed packed geometries, so monolayers treated with monocarboxylic acids are, on average, less ordered than those treated with monothiols. Ex situ electron microscopy and grazing incidence small-angle X-ray scattering (GISAXS) analyses of deposited films on Si/SiO2 substrates show that NCs exchanged with nonbridging ligands pack more efficiently at long length scales than do NCs exchanged with bridging ligands, due primarily to the creation of defects within the NC lattice in response to the rigidity of the bridging ligand.

Fluoride adsorption onto granular ferric hydroxide: effects of ionic strength, pH, surface loading, and major co-existing anions.

Fluoride adsorption onto granular ferric hydroxide (GFH) was investigated using batch methods, under various ionic strength, pH, surface loading, and major co-existing anion conditions. Adsorption of fluoride on GFH included an initial fast adsorption phase followed by a slow adsorption phase. Within the pH range of 2-11, fluoride adsorption equilibrium was not affected by ionic strength, but was significantly affected by pH. Maximum adsorption was achieved in the pH range of 3-6.5. Under the same pH condition, fluoride adsorption followed the Freundlich isotherm, indicating that the GFH surface was heterogeneous. X-ray photoelectron spectroscopy (XPS) and attenuated total reflection-infrared (ATR-IR) spectroscopy data showed evidence for fluoride sorption on the GFH surface via inner-sphere complexation accompanying increased hydrogen bonding and surface hydroxylation. Major anions, including phosphate, bicarbonate, sulfate, and chloride, reduced fluoride adsorption in the following order: H(2)PO(4)(-)>HCO(3)(-)>SO(4)(2-)>Cl(-).