Elsevier

Applied Surface Science

Volume 332, 30 March 2015, Pages 318-327
Applied Surface Science

Structure analysis of layer-by-layer multilayer films of colloidal particles

https://doi.org/10.1016/j.apsusc.2015.01.171Get rights and content

Highlights

  • We used the RSA model to mimic layer-by-layer deposition of hard particles.

  • We formed multilayers of micron-sized latex particles at various conditions.

  • The coverage of 0.3 is optimal for producing a uniform, constant-porosity film.

  • We used the fluorescence microscopy to determine the 2D pair-correlation functions.

  • We found a good agreement between our theoretical and experimental results.

Abstract

We have mimicked the layer-by-layer self-assembling process of monodisperse colloidal particles at a solid–liquid interface using the extended random sequential adsorption model of hard spheres. We have studied five multilayer structures of similar thickness, each created at a different single-layer surface coverage. For each multilayer, we have determined its particle volume fraction as a function of distance from the interface. Additionally, we have characterized the film structure in terms of 2D and 3D pair-correlation functions. We have found that the coverage of about 0.3 is optimal for producing a uniform, constant-porosity multilayer in a minimum number of adsorption cycles. The single-layer coverage has also a significant effect on the primary maximum of 2D radial distribution function. In the case of multilayer with the coverage lower than 0.30 the 2D pair-correlation functions of even layers exhibit maxima decreasing with the increase in the layer number. We have verified our theoretical predictions experimentally. We have used fluorescence microscopy to determine the 2D pair-correlation functions for the second, third, and fourth layers of multilayer formed of micron-sized spherical latex particles. We have found a good agreement between our theoretical and experimental results, which confirms the validity of the extended RSA model.

Introduction

Recent advances in the nanotechnology have made possible the production of highly refined colloidal particles with controlled surface and bulk properties. There have been published novel methods of production of, e.g., surface-activated, magnetic, conductive, biocompatible, fluorescent, hollow, or nanoporous particles [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Exploiting their ability to self-organize we can synthesize sophisticated functional nanomaterials, which opens a world of novel applications for nanoparticle multilayer structures. Colloidal multilayers are highly porous and exhibit a textured surface, which make them potentially attractive as separation membranes, heterogeneous catalysts, antireflective coatings, and optical elements, e.g., interferometers. Because of major practical importance, the mono- and bi-layer deposition of particles has been extensively studied both theoretically [12], [13], [14], [15], [16], [17], [18], [19], [20] and experimentally [18], [19], [20], [21], [22]. However, despite their increasing significance for the preparation of nano-structured porous materials, functional multilayers of more than two particle layers are much less discussed in the literature [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], and thus less known. In particular, little is known about the effect of formation conditions on the obtained structure.

A common experimental method for fabricating a multilayer with certain characteristics is layer-by-layer (LbL) adsorption of composing particles [33], [34], [35], [36], [37]. It is based on the adsorption of consecutive layers by immersing the substrate alternately in suspensions of colloidal particles (or solutions of macromolecules) with positive and negative surface charges. The structure of multilayer obtained in this way depends strongly on the number of adsorption cycles, amount of particles adsorbed in each cycle, and interactions in the system. Therefore, the LbL method allows controlling the various parameters of multilayer, such as the thickness, roughness, porosity, or specific surface area [29], [30]. Also, the procedures for the LbL assembly are relatively simple and can be automated. It is worth mentioning that the LbL method is widely used for the preparation of polyelectrolyte multilayer films of desired composition and functionality on solid substrates [38].

A reliable interpretation of experimental measurements requires adequate theoretical models and results. One of our main theoretical tools for a better understanding of the particle deposition mechanism is the computer modeling of the process. Using a numerical approach we can study the kinetics of multilayer formation and then analyze it to also gain insight into its structure. The advantage of such modeling is that in addition to the global properties of the multilayer, we can also study its local properties, which play a major role in the mass transport, especially in heterogeneous systems. Thus, computer modeling can be a powerful tool in the study of structural and kinetic aspects of deposition of macromolecules, proteins, and colloidal particles [24], [25], [29], [30], [39], [40], [41], [42]. There are several advanced algorithms suitable for modeling of colloidal systems [43], especially helpful for understanding the behavior of particles in highly concentrated suspensions. In the dilute regime, however, where the particle–particle interactions can be neglected, the more sophisticated method, such as Brownian dynamics or Molecular Dynamics are in good agreement with statistical–geometric models, such as random sequential adsorption (RSA) and ballistic deposition [44].

The goal of this work is to conduct in-depth analysis of spherical particle multilayers produced using the LbL technique. In what follows we have assumed that particle deposition occurs from dilute suspensions. Therefore, for the sake of simplicity and efficiency, we have simulated our multilayers with the extended RSA model [29]. Then, we have characterized the structure of generated multilayers in terms of the distribution of particle volume fraction in the direction perpendicular to the adsorption surface. We have also calculated the 2D and 3D pair-correlation functions. Additionally, to verify our model experimentally, we have compared the theoretical predictions with experimental results. We have found a good agreement between the theory and experiment.

Section snippets

Theoretical model

In this paper we have investigated the five multilayers of hard monodisperse particles, generated as described in detail in Ref. [29]. We have used the extended RSA model mimicking the process of LbL deposition of alternating particles of type 1 (odd layers) and type 2 (even layers) on a homogeneous substrate. Obviously, whether the surface charge of odd layer particles is negative or positive, the resulting multilayer structure is the same, as long as these particles’ surface charge is

Experimental details

To test our theoretical predictions we have determined experimentally the 2D pair-correlation function for multilayers of micron-sized polystyrene latex particles. Specifically, we have studied the outer layers of multilayers composed of two, three, and four single layers, each formed at the coverage about 0.1, which we have denoted as L2, L3, and L4, respectively. In addition, we have also determined the 2D pair-correlation function for a bilayer at the coverage about 0.3, denoted as L2x.

We

Results and discussion

As we demonstrated in Refs. [29], [30], multilayers of the same thickness, built from the same particles at different single-layer coverages can vary considerably in their surface and transport properties. Here we have reported an in-depth structure analysis of the obtained multilayers, i.e., the distribution of particle volume fraction, as well as 2D and 3D pair-correlation functions. In what follows we have presented results of our numerical simulations obtained for the five multilayers with

Conclusions

We have applied the extended RSA model for simulating multilayer adsorption of hard spheres to mimic the LbL process of spherical monodisperse particles. We have generated five multilayers of similar thickness at various single-layer surface coverage. We have analyzed in-depth their structure in terms of the particle volume distribution and pair-correlation functions. We have also verified our theoretical predictions experimentally.

The particle volume fraction exhibits two types of change with

Acknowledgments

This work was supported by the EU Human Capital Operation Program, Polish Project No. POKL.04.0101-00-434/08-00, the grant FUNANO No. POIG.01.01.02-12-028/09-05, and the Ministry of Science and Higher Education grant No. N N204 347737.

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