Spontaneous Assembly of a Monolayer of Charged Gold Nanocrystals at the Water/Oil Interface

Abstract

Novel materials based on 2D and 3D arrays of nanocrystalline building blocks can be prepared from colloidal suspensions by controlled processing. There is a large effort in this field due to the exciting electrical, [1][2][3] magnetic [4,5] and optical [6,7] properties of such systems, which can be tuned by the size of the building blocks and the interactions between them. Ordered arrays of uncharged, sterically stabilized nanocrystals have been successfully obtained by a subtle destabilization of the suspension [8] or by assembly at a liquid/air [9][10][11] or liquid/liquid interface. [12] There is, however, an important class of suspensions that are stable owing to the nanocrystal surface charge. Assembly of uncapped, charged nanocrystals into arrays is considerably more difficult as destabilization of a suspension usually leads to uncontrolled coagulation. [13] Herein, we report surprising results that may lead to novel routes for the controlled fabrication of materials from chargestabilized nanocrystal colloids. We have observed that gold nanocrystals spontaneously form a monolayer at the water/oil interface if the surface charge of the nanocrystals is gradually reduced. The separation between the nanocrystals in the layer is smaller than the width of the diffuse electrical double layer. Nevertheless, coagulation of the particles into clumps does not occur. The monolayers are remarkably robust and can be easily transferred to substrates, opening the way to technological applications. The spontaneous 2D assembly of charged nanocrystals is qualitatively described in terms of a reduction of the water/oil interfacial energy upon particle adsorption counteracted by electrostatic repulsion in the film.Suspensions of sterically stabilized and supposedly uncharged gold nanocrystals have been used as model systems for the study of self-assembly. A wealth of gold nanocrystal structures have been reported [9,11,14] and their optoelectrical properties are still under investigation. The synthesis of charge-stabilized gold sols is well established. The processing of such sols has been focused on the capping of charged nanocrystals with organic molecules to allow transfer to a

Dip‐Coated Gold Nanoparticle Electrodes for Aqueous‐Solution‐Processed Large‐Area Solar Cells

“…To obtain the gold NPs electrode, a close-packed of ethanol molecules and weak interaction between the citrate and gold NPs. [ 27 ] This is confi rmed by the increase of transmittance in the range of 3200-3600 cm −1 after dip-coating process ( Figure S2, Supporting Information).…”

Gold Nanofilm Redox Catalysis for Oxygen Reduction at Soft Interfaces

“…Following the calculations of ref. [54], but utilising a viscosity of 0.527 mPa s instead of 0.038 mPa s for TFT [55], and avoiding the sign error in the original paper, the resulting values are presented in the Table A1. The transfer energy is calculated as follows:…”

“…In addition, the active area available for electrochemical reactions can be considered as the projected area available for diffusion. This area is roughly equal on both sides of the interface (most likely, particles are split equally by the interface due to stability reasons [1,54,55]), so A w =A o was chosen equal to unity. As the bulk concentrations of both DMFc and O 2 are of similar magnitude, and considering that O 2 is present in both phases and at least two DMFc molecules are needed for each reacted O 2 molecule, the surface concentration ratio c O /c DMFc % 1.…”

Interfacial Redox Catalysis on Gold Nanofilms at Soft Interfaces

“…Several methods exist to induce the self-assembly of AuNPs at soft interfaces. Since Reincke and co-workers 24 reported the spontaneous self-assembly of AuNPs at a water | heptane interface following the addition of alcohol, numerous alternative approaches have been described to trigger the assembly of metallic NPs at soft interfaces. 15,25,26 The alcohol injection method is still one of the most convenient but requires introducing a substantial volume of alcohol (at least 10 vol %) 27À29 close to the interface and lacks a precise control of the interfacial surface coverage of AuNPs in the film.…”