Toward dynamic control over TiO2 nanocrystal monolayer-by-monolayer film formation by electrophoretic deposition in nonpolar solvents

Electrophoretic Deposition of Single Nanoparticles

The controlled assembly of colloid particles on a solid substrate has always been a major challenge in colloid and surface science. Here we provide an overview of electrophoretic deposition (EPD) of single charge-stabilized nanoparticles. We demonstrate that surface templated EPD (STEPD) assembly, which combines EPD with top-down nanofabrication, allows a wide range of nanoparticles to be built up into arbitrary structures with high speed, scalability, and excellent fidelity. We will also discuss some of the current colloid chemical limitations and challenges in STEPD assembly for sub-10 nm nanoparticles and for the fabrication of densely packed single particle arrays.

Quantitatively controlled electrophoretic deposition of nanocrystal films from non-aqueous suspensions

This study presents a novel method to correlate the mass and charge transfer kinetics during the electrophoretic deposition of nanocrystal films by using a purpose-built double quartz crystal microbalance combined with simultaneous current-measurement. Our data support a multistep process for film formation: generation of charged nanocrystal flux, charge transfer at the electrode, and polarization of neutral nanocrystals near the electrode surface. The polarized particles are then subject to dielectrophoretic forces that reduce diffusion away from the interface, generating a sufficiently high neutral particle concentration at the interface to form a film. The correlation of mass and charge transfer enables quantification of the nanocrystal charge, the fraction of charged nanocrystals, and the initial sticking coefficient of the particles. These quantities permit calculation of the film thickness, providing a theoretical basis for using concentration and voltage as process parameters to grow films of targeted thicknesses.

Electrophoretic mobility and stability of SiO2 nanoparticles in the solutions of AOT in n-hexadecane-chloroform mixtures

Electrophoretic mobility of SiO₂ nanoparticles in a n-hexadecane-chloroform mixture depending on AOT concentration and chloroform content was determined. It was shown that an increase in chloroform content and a decrease in AOT concentration cause a growth in electrophoretic mobility. The use of the values of Debye lengths (characteristic thickness) of the diffuse part of the electric double layer (EDL) that were determined previously allowed us to calculate the electrokinetic potential and to evaluate the stability of organosols. The obtained data were in good correlation with the dynamics of temporal changes of hydrodynamic radius and the intensity of light scattering. Organosols may be used for heteroaggregation (sorption) of Au and Ag nanoparticles on SiO₂ .

Mesoscale Particle-Based Model of Electrophoretic Deposition

We present and evaluate a semiempirical particle-based model of electrophoretic deposition using extensive mesoscale simulations. We analyze particle configurations in order to observe how colloids accumulate at the electrode and arrange into deposits. In agreement with existing continuum models, the thickness of the deposit increases linearly in time during deposition. Resulting colloidal deposits exhibit a transition between highly ordered and bulk disordered regions that can give rise to an appreciable density gradient under certain simulated conditions. The overall volume fraction increases and falls within a narrow range as the driving force due to the electric field increases and repulsive intercolloidal interactions decrease. We postulate ordering and stacking within the initial layer(s) dramatically impacts the microstructure of the deposits. We find a combination of parameters, i.e., electric field and suspension properties, whose interplay enhances colloidal ordering beyond the commonly known approach of only reducing the driving force.

Low-temperature synthesis and electrophoretic deposition of shape-controlled titanium dioxide nanocrystals

A comprehensive, low-temperature strategy for obtaining optimized, dense and nanostructured TiO₂ thin films is proposed.

X-ray scattering as a liquid and solid phase probe of ordering within sub-monolayers of iron oxide nanoparticles fabricated by electrophoretic deposition

Order within sub-monolayers of nanoparticles, fabricated by electrophoretic deposition, was assessed during nanoparticle deposition in a liquid suspension and after the films had dried by grazing-incidence small-angle X-ray scattering. Experiments were performed in a custom-made, liquid-phase cell. The results indicated that ordering occurred during the drying event.

Statistical assessment of order within systems of nanoparticles: determining the efficacy of patterned substrates to facilitate ordering within nanoparticle monolayers fabricated through electrophoretic deposition

The degree of order within nanoparticle monolayers deposited through electrophoretic deposition on lithographically patterned and unpatterned substrates was analyzed using four complementary measures of order: Voronoi-cell edge-fraction entropy, local bond-orientation order parameter, translational order parameter, and anisotropy order parameter. From these measures of order, we determined that the pattern had an influence on some aspects of the ordering within the nanoparticle monolayer but had no effect on others. The Voronoi-cell edge-fraction entropy did not measurably change due to the pattern, indicating that the pattern has no effect on the number of defects present. The translational order parameter also had no change due to the pattern. The local bond-orientation order parameter had a measurable change, indicating the pattern increased the bond ordering slightly. Also, the anisotropy order parameter developed herein indicated an increase in order. The direction of the increased order corresponded with the direction of the anisotropy designed on the patterned substrate, strongly suggesting that the pattern drives the particles to become more ordered.

Binder-free and carbon-free nanoparticle batteries: a method for nanoparticle electrodes without polymeric binders or carbon black

In this work, we have developed a new fabrication method for nanoparticle (NP) assemblies for Li-ion battery electrodes that require no additional support or conductive materials such as polymeric binders or carbon black. By eliminating these additives, we are able to improve the battery capacity/weight ratio. The NP film is formed by using electrophoretic deposition (EPD) of colloidally synthesized, monodisperse cobalt NPs that are transformed through the nanoscale Kirkendall effect into hollow Co(3)O(4). EPD forms a network of NPs that are mechanically very robust and electrically connected, enabling them to act as the Li-ion battery anode. The morphology change through cycles indicates stable 5-10 nm NPs form after the first lithiation remained throughout the cycling process. This NP-film battery made without binders and conductive additives shows high gravimetric (>830 mAh/g) and volumetric capacities (>2100 mAh/cm(3)) even after 50 cycles. Because similar films made from drop-casting do not perform well under equal conditions, EPD is seen as the critical step to create good contacts between the particles and electrodes resulting in this significant improvement in battery electrode assembly. This is a promising system for colloidal nanoparticles and a template for investigating the mechanism of lithiation and delithiation of NPs.