Article pubs.acs.org/Langmuir
Toward Dynamic Control over TiO2 Nanocrystal Monolayer-byMonolayer Film Formation by Electrophoretic Deposition in Nonpolar Solvents I. Gonzalo-Juan,†,‡ Alex J. Krejci,†,‡ and J. H. Dickerson†,‡ †
Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37234-0106, United States Vanderbilt Institute of Nanoscale Science and Engineering, Vanderbilt University, Nashville, Tennessee 37234-0106, United States
‡
ABSTRACT: The controlled electrophoretic deposition of monolayers and ultrathin films of 4.0 nm TiO2 nanocrystals from stable, nonpolar solvent-based suspensions is reported. Stable suspensions were prepared in hexane, and the electrophoretic mobility of the nanocrystals was enhanced by a combination of a liquid−liquid extraction followed by mechanical surfactant removal by high-speed centrifugation. The controlled evolution of the density of TiO2 nanocrystal monolayers was studied by transmission electron microscopy and optical transmittance spectroscopy. Ultrathin films were assembled while maintaining monolayer-by-monolayer growth and uniform density of the film. A time-dependent, equivalent circuit model has been proposed to characterize the electrophoretic current that was recorded during our experiments. Further, we demonstrate that the proposed model, coupled with the mobility, provides a means to estimate the deposition rate and, hence, the time necessary to fabricate a submonolayer, a monolayer, and multilayers of nanocrystals.
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INTRODUCTION Nanocrystals (NCs) of anatase titanium dioxide (TiO2) have increasingly been employed in thin films due to their attractive size-dependent optical properties in dye-sensitized solar cells, photocatalysis, photochromic and electochromic devices, batteries, and other applications.1−6 Precise control of the morphology and the size of these nanoscale materials is central to the fine-tuning of their physical properties, such as electrical conductivity, magnetic coercivity, optical absorbance, and mechanical strength. TiO2 thin films, both nanostructured and single crystalline, have been prepared using a variety of process including plasma enhanced chemical vapor deposition,7 sputtering,8,9 metal organic chemical vapor deposition,10 spray pyrolysis,11 chemical bath deposition,12 electrochemical deposition,13,14 electrophoretic deposition (EPD),15−17 sol−gel,18 and photodeposition.19 Of the aforementioned techniques, EPD is one of the more promising for the assembly of nanocrystals. Increasing interest in EPD has been stimulated by four important advantages that this technique possesses: 1) versatility, because EPD allows for self-supported, shaped materials, and coatings across a large range of length scales (nanometer scale to meter scale); 2) low overall cost of the process, including equipment, materials, and chemicals; 3) reliability, which can be increased by accurate control of the process kinetics; and 4) deposition site specificity, as cast materials are deposited only at the desired locations.20−22 Hence, EPD is a facile approach that offers a simple design setup and substantial thickness control at rapid deposition rates to assemble particles of any size and shape. Recently, EPD conducted on nanocrystals that were suspended in nonpolar © 2012 American Chemical Society
solvents has increased in popularity, in contrast to the more traditional approach of using polar solvents (methanol, acetone, water, etc.) to produce films, coatings, and casts.23−29 Two of the driving factors for using nonpolar solvents are: 1) to suppress the van der Waals interactions between nanocrystals while in suspension, which frequently results in particle agglomeration or aggregation before the nanocrystals reach the electrode; and 2) to minimize electrolysis of the suspension solvent, which leads to difficult-to-control hydrodynamics and electrohydrodyamics of the suspension. In nonpolar solvent-based suspensions, the origin of charge on the NCs differs from the origin of charge on NCs in polar solvent-based suspensions. This gives rise to some marked differences in nonpolar solvent EPD and conventional EPD. Although materials dispersed in nonpolar solvents generally do not acquire charge, recent studies have shown that the addition of appropriate amphiphilic surfactant, typically hydrophilic head-hydrophobic tail surfactants, during the synthesis can impart charge on particles in nonpolar solvents.30−33 The adsorption of these organic molecules onto the surface of hydrophilic NCs, like oxides, facilitate the formation of inverse micelles in the suspension with the head facing the NC surface and the tail in contact with the nonpolar solvent.30,34 The main characteristics of these systems are: 1) that thermal energy is large enough to facilitate the surface reaction between the NC and the surfactant; and 2) that the charge of the surface is Received: December 28, 2011 Revised: February 20, 2012 Published: February 21, 2012 5295
dx.doi.org/10.1021/la205124s | Langmuir 2012, 28, 5295−5301
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spectroscopy measurements of the nanocrystal films were recorded using a Cary 5000 spectrophotometer. The absorption measurements not only confirmed the composition of the nanocrystals in suspension but also allowed us to monitor the nanocrystal concentration in the suspension before and after each EPD experiment. From this data, we constructed an absorption calibration curve, according to the Beer− Lambert Law, to determine the mass of nanocrystals deposited during EPD. Both the hydrodynamic diameter of the nanocrystals and the electrophoretic mobility were measured in suspension (hexane) on a Malvern Zetasizer Nano ZS system. The surface morphologies of the monolayers were analyzed on an Agilent 5400 series atomic force microscope, working in tapping mode (PPP-NCHR AFM tips, probe radius
