WO2012075309A1

WO2012075309A1 - Spin spray layer-by-layer assembly systems and methods

Info

Publication number: WO2012075309A1
Application number: PCT/US2011/062919
Authority: WO
Inventors: Andre D. Taylor; David Kohn; Forrest Gittleson; Xiaokai Li
Original assignee: Yale University
Priority date: 2010-12-01
Filing date: 2011-12-01
Publication date: 2012-06-07

Abstract

An apparatus/system and associated method for formation of layer-by-layer materials are provided that includes a spinning disc or substrate, a plurality of atomizing nozzles directed to the spinning disc or substrate, each of the plurality of nozzles in communication with a source of fluid, and a heat source directed to the spinning disc or substrate for delivering heat energy to the spinning disc or substrate. The disclosed apparatus/system and associated method have widespread industrial and research application, and greatly increase the speed with which LBL materials may be formed.

Description

SPIN SPRAY LAYER-BY-LAYER

ASSEMBLY SYSTEMS AND METHODS

BACKGROUND

1. Technical Field

The present disclosure is directed to systems and methods for efficiently and effectively undertaking layer-by-layer assembly of nanostructures. The present disclosure further relates to advantageous apparatus/systems for layer-by-layer assembly and processing modalities that achieve high quality nanostructures in a more time effective manner.

2. Background Art

Layer-by-layer (LBL) assembly has emerged out of a range of disciplines including biology, chemistry, chemical engineering, materials science, mechanical engineering, electrical engineering, and applied physics. This interdisciplinary area is promising for a wide variety of industries, especially for energy conversion and storage (e.g., fuel cells, solar cells and batteries). However, layer-by-layer assembly techniques are not limited in potential applicability to energy/storage; rather, there are diverse applications in other industries/technologies, e.g., the pharmaceutical industry (e.g., to achieve desired time release parameters), defense industry applications, membrane separation technologies, sensor technology, etc.

Some of the technical challenges that have delayed commercialization of direct alcohol fuel cells (DAFCs) pertain to materials performance due to poor assembly of the catalyst interface. In the past few years, it has been demonstrated that layer-by- layer (LBL) assembly can controllably blend disparate materials together with nanometer level control; however, a key issue that remains is the slow throughput of this process coupled with the large amount of wasted materials. FIG. 1 shows a schematic illustration of cycle times for a conventional dipping LBL assembly process for a PEO PAA film. In the traditional layer-by-layer assembly process ~ as shown in the left side of FIG. 1 ~ the film stays in contact with the polyelectrolyte solutions and rinsing fluids for sufficiently long periods of time to achieve a thermodynamic equilibrium at each stage. For example, the cycle time for a conventional dipping LBL process is 26 minutes or 45 hours for a 100 bilayer film. The amount of the material deposited onto the substrate is thus determined by the adsorption/desorption equilibrium conditions. Exploration of integrative membrane-electrode assembly methods as well as a comprehensive assessment of materials for DAFCs is critical for rapid nanomanufacturing and transformative progress in this field.

SUMMARY

The present disclosure provides a spin spray layer-by-layer apparatus/system and associated processing methods/techniques that advantageously reduce traditional layer-by-layer processing times by 2-3 orders of magnitude. The disclosed spin spray layer-by-layer (SSLBL) assembly

apparatus/system and associated method offer an integrated approach to the formation of self- assembled polyelectrolyte composites. Exemplary embodiments of the disclosed apparatus/system utilize a plurality of atomizing nozzles to deliver desired constituents, e.g., polyanionic and polycationic polymer systems, to a spinning substrate. In exemplary implementations, the disclosed apparatus/system also advantageously delivers rinse fluid, e.g., from one or more separate atomizing nozzles, between constituent delivery. A source of heat is also generally provided so as to control/promote drying of individual layers.

Thus, in illustrative applications of the present disclosure, fixed volume water based processing may be performed to generate alkaline direct alcohol fuel cells (e.g., methanol and ethanol). The disclosed apparatus/systems and associated methods may also be employed to generate a range of fuel cell systems (e.g., hydrogen, hydrazine, or other hydrocarbons) and composite applications (e.g., drug delivery, solar cells and the like). The disclosed spray spin layer-by- layer assembly apparatus/systems and associated methods may be used to generate ultra thin (submicron) films, e.g., through the alteration of polyanionic and polycationic polymer systems. These molecular level blends would be difficult to achieve using conventional means of fabricating materials composites. Assembly and processing parameters may be selected according to the present disclosure to achieve desired results, e.g., parameters such as polyelectrolyte selection, ionic strength, pH, and solution concentration (nanocolloids and poiyions) may be varied to nanomanufacture a series of functional thin film membranes with tunable porosity, strength, and conductivity.

In an exemplary embodiment, a system for layer-by-layer assembly of a thin film, catalyst, and/or membrane is disclosed. The system includes a spinning substrate, atomizing nozzles, and a heat source. The atomizing nozzles are directed to the spinning substrate. Each of the nozzles are in communication with a source of fluid. The heat source directed to the spinning substrate for delivering heat energy to the spinning substrate.

In another exemplary embodiment, a method for layer-by-layer assembly of a thin film, catalyst, membrane is disclosed. The method includes providing a plurality of atomizing nozzles and a spinning disc or substrate, delivering constituents to the spinning disc or substrate from the plurality of atomizing nozzles in an alternating sequence so as to form a product in a layer-by-layer manner, and delivering heat energy to the spinning disc or substrate so as to influence the drying cycle of the product on a layer-by- layer basis.

In yet another exemplary embodiments, a system for layer-by-layer assembly of a thin film, catalyst, or membrane is disclosed. The system includes a spinning disc; a first nozzle, a second nozzle, and a control system. The first nozzle is directed to the spinning disc and is in communication with a first polyelectrolyte. A first solenoid controls ejection of the first polyelectrolyte from the first nozzle. The second nozzle is directed to the spinning disc and is in communication with a second polyelectrolyte. A second solenoid controls ejection of the second polyelectrolyte from the second nozzle. The control system operates the first and second solenoids to control a quantity of the first and second polyelectrolytes being disposed on the disc and further controls a speed at which the spinning disc rotates.

The disclosed spray spin layer-by-layer assembly apparatus/system and associated method may be employed to achieve material properties that are comparable ~ if not superior ~ to traditional layer- by-layer films with the added advantages of high throughput and increased material utilization. Thin film materials assembled according to the present disclosure may be used to generate thicker (e.g., 10 μηι) freestanding catalyst layers having substantial industrial utility.

Previous attempts have been made to improve this processing time such as, spray layer-by- layer processing , spin-assisted layer-by-layer processing, and roll to roll layer-by-layer processing. However, these approaches have drawbacks. For example, the above approaches typically do not adequately address material waste. Recently, a spin spray layer-by-layer (SSLBL) technique has been described where an amount of the material deposited is determined by its net quantity in the fluid layer that remains on the spinning disk at the end of the drying step. By adjusting process parameters, such as the polyelectrolyte solution concentration, the amount of fluid sprayed on the disk, spinning speed (which affects the drainage rate of the fluid layer), and the drying time, one can achieve a greater control of the layer-by-layer deposition process than is available in the usual dipping layer-by- layer method shown in FIG. IB. For example, the cycle time for the SSLBL process can be about 2 seconds (e.g., about 3 minutes for a 100 bilayer or about 30 minutes for a free standing 1000 bilayer film).

This greater degree of control provides new opportunities for acceleration of the film buildup and for manipulating the microstructure of the resulting nano-composite film. However, the additional flexibility also brings new challenges in designing apparatus/systems and processing

techniques/methods for attaining a rapid film growth rate, while simultaneously achieving desirable film properties (e.g., electric conductivity, permeability, and strength). The foregoing challenges are met by the apparatus/systems and methods disclosed herein, as will be apparent from the detailed description which follows, particularly when read in conjunction with the accompanying figures.

Additional features, functions and advantages of the disclosed spin spray layer-by-layer apparatus/system and associated method will be apparent from the detailed description which follows.

BRIEF DESCRIPTION OF THE FIGURES

To assist those of skill in the art in making and using the disclosed apparatus/systems and associated methods, reference is made to the accompanying figures wherein:

FIG. 1A shows a schematic illustration of cycle times for a conventional dipping LBL assembly process for a PEO/PAA film.

FIG. IB shows a schematic illustration of cycle times for a spray spin LBL assembly process for a PEO/PAA film.

FIG. 2 is a schematic illustration of an exemplary SSLBL apparatus/system according to the present disclosure in which atomized droplets form a lamella phase are spread across the top of a rotating substrate. Various polyelectrol te conformations are dependent on solution conditions.

FIGS. 3 and 4 are additional schematic illustrations of exemplary SSLBL apparatus/systems according to the present disclosure.

FIG. 5 provides plots of the angular evolution of a rod-like particle tumbling in shear flow in a parallel-wall channel as calculated using applicable algorithms. In particular, evolution of linear chains of spears undergoing tumbling motion in a parallel wall channel are shown.

FIG. 6 provides SEM images comparing a traditional polyethylene oxide/polyacrlylic acid (PEO/PAA) LBL film to an SSLBL film with similar striation patterns. Insets -100-bilayer LBL film, 1000 bilayer SSLBL. Fabrication times as set forth in FIGS. I A and IB.

DESCRIPTION OF EXEMPLARY BMBODIMENTfSI As noted above, layer-by-layer assembly systems and techniques that may be employed to generate desired materials in a rapid and reliable manner are highly desirable in many fields and industries. For example, explorations of integrative membrane-electrode-assembly methods as well as a comprehensive assessment of high performance materials tailored for direct alcohol fuel cell (DAFC) operation are critical for brisk progress in this field. As disclosed herein, the advantageous spin spray layer-by- layer (SSLBL) assembly apparatus/systems and associated methods of the present disclosure can be used to rapidly improve upon conventional layer-by-layer processing to create desired materials, e.g., membrane electrode assemblies for alkaline DAFCs. Exemplary embodiments of the SSLBL apparatus/system and related methods can use an aqueous based processing technique. In exemplary embodiments, the aqueous based processing technique can utilize a fixed volume electrostatic assembly. As used herein, "layer-by-layer" (LBL) assembly involves repeated, sequential exposure of a substrate to one or more constituents.

Of note, catalyst development for direct alcohol fuel cells has been generally elusive. This can be attributed to the high cost and poor utilization of the materials used in developing these catalysts. The majority of the cost typically arises from the precious metal based catalysts (i.e., Platinum,

Palladium). Supported metal particles on carbon nanotubes offer advantages over traditional carbon black materials due to improved access to triple phase boundary regions, higher electrical conductivity of the support, and electrochemical stability. It has been demonstrated that these nanocolloids can be dispersed with functional polyelectroiytes to create freestanding films via LBL assembly. While these films exhibited an order of magnitude higher Pt utilization compared to conventional catalysts, diffusion limited LBL construction of these films required 5 days of continuous processing according to conventional LBL techniques.

The present disclosure dramatically changes the time-consuming limitations associated with conventional LBL techniques. Exemplary embodiments of the present disclosure can be implemented in various fields of technology. For example, applications of exemplary embodiments of the SSLBL apparatus/systems can range from sensors to electrochemical devices to biomedical applications or to organic electronics. Using the disclosed apparatus/sy stems and associated methods to rapidly generate LBL assemblies, it is possible to effectively study a range of issues/parameters, e.g., the interactions of new nanocolloids with new polyelectrolyte LBL systems. In this way, a fundamental understanding can be developed on how these materials can be strategically arranged to control film conductivity, porosity, and morphology. The interfacial fuel cell catalyst materials for this study are particularly interesting because similar interfacial challenges are being investigated for solar cells, batteries, water purification, or ion selective membranes.

With reference to FIG. 2, an exemplary SSLBL apparatus/system 100 according to the present disclosure is depicted that offers several elements of control that could be advantageous for film growth, including solution volume, flow rate, concentration, and rinse. The disclosed

apparatus/system 100 includes a plurality of atomizing nozzles:

• Nozzle 110 which is in communication with a supply of polyelectrolyte 1 12 and is adapted to deliver atomized droplets 1 14 of polyelectrolyte to a centrally located spinning disc/substrate 102;

• Nozzle 120 which is in communication with a supply of deionized water 122 (i.e., rinse solution) and is adapted to deliver atomized droplets of DI water to the centrally located spinning disc/substrate 102; and

• Nozzle 130 which is in communication with a supply of polyelectrolyte 132 and is adapted to deliver atomized droplets of polyelectrolyte to the centrally located spinning disc/substrate 102.

For traditional LBL film systems, typically less than 1% of the polymers or colloids in the initial solution are incorporated into the film. Material conservation becomes essential when considering, for example, a fuel cell that uses single walled carbon nanotubes (~$1000/g) that are decorated with Pt. If the solution sprayed on the substrate only evaporates (no spin off), than all the particles or polymers sprayed will be deposited independent of charge distributions.

The exemplary SSLBL apparatus/system 100 of FIG. 2 shows three nozzles 1 10, 120, 130, but the present disclosure is not limited to the disclosed arrangement. Rather, a greater number of nozzles may be provided, e.g., if it is desired to introduce more than two constituents to the LBL assembly. In addition, elimination of the "rinse" nozzle 120 may be feasible, e.g., if each constituent layer is permitted and/or induced to form a layer of desired thickness and to sufficiently dry prior to delivery of the next constituent layer. In this regard, it is contemplated that a heat source 140, e.g., a blower for delivery of heated air/gas to the substrate surface may be advantageously included in the exemplary apparatus/system. The source of heat could be adapted to deliver heat to the substrate surface at predetermined times and for predetermined periods of time, or may be adapted to deliver heat to the substrate surface on a continuous basis.

The disc/substrate 102 is adapted to spin to establish desired shear forces and to facilitate formation of distinct layers 104 of desired thickness on the disc/substrate 102. The sprayed droplets are dispersed substantially uniformly across the disc/substrate surface (or atop the previously deposited layer) and any excess material is removed from the disc/substrate 102 through applicable centrifugal forces. In exemplary embodiments of the present disclosure, the spin rate, cycle time for atomization nozzle operations, solution concentrations, constituent selection, and temperature/flow rate of the heat source are among the variables that may be controlled to achieve desired LBL assembly using the disclosed SSLBL apparatus/system 100 and associated methods. For example, the timing of the rinse application during hydration of the underlying film could affect the bond between the recently deposited materials with the underlying layer. If the film is too dry, this could allow too much entanglement, preventing the removal of the excess material, thus again affecting the quality of the film.

Operation of the nozzles 110, 120, 130 is generally controlled by solenoid valves 116, 126, 128, respectively, that are, in turn, controlled by a processor 162 of a control system 160 that is programmed to control the cycle time and cycle sequence of droplet delivery to the spinning disc/substrate 102. Additionally, the heat source 140 and rotation of the substrate/disc 102 can be controlled by the processor 162. Thus, the apparatus/system is generally designed for automated control, whereby variables such as cycle time, rinse time, spinning speed, hot air temperature and/or flow rate, are controlled by the programmed processor. The processing variables may be modified with ease, thereby allowing rapid modification of operating conditions to achieve desired results.

Turning to FIGS. 3 and 4, further schematic illustrations of an exemplary apparatus/system 200 according to the present disclosure are provided. The apparatus/system 200 includes three spaced atomizing nozzles 210, 220, 230 positioned above and pointed at a centrally located disc/substrate region 202. The nozzles 210, 220, 230 are in communication with discrete sources of materials 212, 222, 232, respectively, and may be repositioned, as desired, to ensure accurate delivery of atomized droplets to the spinning disc/substrate 202. A motor (not shown) can drive rotation of the

disc/substrate 202. One skilled in the art will recognize that a conventional motor drive can be positioned below the base of the apparatus housing 250. A heat source (not shown) is generally positioned above the disc/substrate to deliver heated air/gas to the disc/substrate 202 so as to enhance drying of individual layers, e.g., a hot air blower.

Similar to the system 100 of FIG. 2, the operation of the atomizing nozzles 210, 220, 230 is generally controlled by solenoid valves that are, in turn, controlled by a processor that is programmed to control the cycle time and cycle sequence of droplet delivery to the spinning disc/substrate 202. Thus, the apparatus/system 200 is generally designed for automated control, whereby variables such as cycle time, rinse time, spinning speed, hot air temperature and/or flow rate, are controlled by the programmed processor. The processing variables may be modified with ease, thereby allowing rapid modification of operating conditions to achieve desired results.

The concentration and volume of constituent delivery from each atomizing nozzle (other than the rinse nozzle) may be advantageously selected so as to deliver only so much material as is necessary to form the desired layer thickness on the disc/substrate. In selecting a

concentration/volume for each such constituent, it is appropriate to take into consideration the potential that a small fraction of the material will not adhere to the disc/substrate (or the underlying layer), e.g., based on the fact that some droplets can miss the disc/substrate 102 and/or 202 and some droplets will be spun free of the disc/substrate 102 and/or 202 prior to adhering with respect thereto. Accordingly an appropriate "overage" is generally calculated into the material concentration/volume. Nonetheless, the disclosed SSLBL apparatus/system 100 and/or 200 and associated methods dramatically increase the efficiency of the LBL process by minimizing material waste. In addition and as noted previously, the disclosed SSLBL significantly increases the speed with which LBL assembly is achieved relative to previous LBL systems/techniques.

Exemplary embodiments of the systems 100, 200 provide an integrated approach to the formation of self-assembled polyelectrolyte composites by utilizing, for example, an atomized fixed volume water based processing method such that ultrathin (e.g., submicron) films having molecular level blends are generated through the alteration of polyanionic and polycationic polymer systems Three particular applications of the disclosed SSLBL technology of the present disclosure are

DAFCs, alkaline fuel cells and catalysts. Each of these noted exemplary applications is briefly discussed below:

Direct Alcohol Fuel Cells. DAFCs based on liquid fuels have attracted enormous attention as power sources for portable electronic devices and fuel -cell vehicles due to the much higher energy density of liquid fuels than gaseous fuels such as hydrogen. Among various liquid fuels, ethanol is particularly attractive because it is less toxic than methanol, is readily available, and can be produced from renewable sources (i.e., biofuel). In comparison to acidic fuel cells, alkaline fuel cells can have faster kinetics associated with the oxygen reduction reaction and can be implemented using less expensive non-precious metals, such as silver catalysts and/or perovskite type oxides. A thorough discussion of the advantages, disadvantages, and the rationale leading towards the pursuit of alkaline fuels cells is covered in "Prospects for alkaline anion-exchange memberanes in temperature fuel cells", by J.R. Varcoe and R.C.T. Slade in Fuel Cells, 2005.5(2), pages 187-200.

Alkaline Fuel Cells. One of the challenges with alkaline fuel cells is the need to prevent the formation of carbonates created at the anode by the reaction of OH^" with the CO and/or C0₂ resulting from the electro-oxidation reaction. These salt precipitates (i.e., Na₂C0₃ or K₂C0₃, depending on the electrolyte chemistry) would greatly decrease the power and long term performance of the alkaline cells by fouling the electrode catalysts and blocking the pores of the electrolyte. Previous methods used to circumvent this issue required the use of very expensive ultra pure hydrogen and oxygen. Recently, it has been shown that the use of an Anion Exchange Membrane (AEM) to facilitate hydroxide ion transport in place of basic aqueous electrolyte solution can be used to eliminate this issue, yielding an order of magnitude increase in power generation. These AEMs can be used to make systems very similar to the proton exchange membranes (PEMs) used in more traditional fuel cell systems. These early studies have been followed up in the past few years by reports of various aromatic polymers functionalized with basic groups such as N-pyridinium, N-alkyl benzylammonium, and other aminated polymers that offer promising hydroxide conductivities. These studies have shown that these systems are effective in transporting hydroxide ions and preventing the formation of carbonate species. In fact, the initial results are encouraging with observed ionic conductivities ranging from 8 to 10 mS/cm, which is an order of magnitude lower than values observed for the protonic conductivity in Nafion® PEM fuel cells. Similar to Nafion® fuel cells, however, there is room for improvement for not only these membranes, but also the polymeric components necessary for the catalyst layer.

Catalyst Layer. One of the most desirable properties of an ionomer for use in the catalyst layer is high water solubility and a low boiling point (such as ethanol and (n- or 2-) propanol), due to ease of handling and removal during electrode preparation. Previously it has been found that when one attempts to increase the ion exchange capacity in AEM or PEM systems, the mechanical properties (hence durability) of the films are penalized. In addition, when the pore sizes of the films are too large due to increased swelling, fuel crossover of polar molecules becomes an issue. Although it has been shown that swelling of Nafion® fuel cells in an aqueous solution of ethanol is greater than in methanol, ethanol has been shown to have a lower permeation rate due to its molecular size. From a fuel balance perspective, it has also been shown that permeated ethanol to the cathode exhibits a less serious effect on the cell performance when compared to methanol, due to both ethanol' s smaller permeability and slower electrochemical oxidation kinetics over the Pt C cathode. Covalently crossl inked analogs of these polymers can be mechanically stable, but the decreased water uptake and ion mobility, along with insolubility, restricts this approach as a viable catalyst layer advancement.

The disclosed SSLBL apparatus/systems and associated methods offer advantageous opportunities to address the limitations described above with respect to the highlighted potential applications, e.g., by utilizing polyelectrolyte complexation to generate highly stable ionically conductive media for hydroxide ion transport through the membrane and catalyst layer.

Polyelectrolyte multilayer assembly typically involves the formation of thin films through the alternating adsorption of positively and negatively charged polymer species under ambient conditions. The disclosed SSLBL method allows the fine-tuning of polyanions and polycations to create ionically crosslinked continuous films. The disclosed SSLBL methods also have utility for microelectrochemical devices, including organic solar cells and battery applications. Recent work in this area has been focused on hydrogen fuel cells, particularly in the formation of the catalyst layers that incorporate high performance nanostructured materials. According to the present disclosure, this nanoscale blend approach to the formation of ultrathin functional nano-assemblies using SSLBL permits development of high performance nanomaterial catalysts and permits such nanomaterials to be incorporated into an alkaline direct ethanol fuel cell architecture.

Thus, the apparatus/systems and associated methods of the present disclosure may be used to develop (i) high performance nanomaterial catalysts that are active for ethanol oxidation and oxygen reduction in alkali mediums, e.g., by decorating carbon nanotubes and nanofibers with selected transition metals using supercritical fluids, (ii) rapid nanomanufacturing of high performance nanomaterials and polyelectrolyte polymers into functional thin film catalyst layers, such rapid manufacturing benefiting from swift film growth, precise use of materials, materials in one cycle are not dependent on the specific composition of previous cycles, and increased flexibility of layered films built with different materials in different layers, and (iii) variation of polycationic basic polymers with polyanions coupled with nanocolloids to create a new generation of high performance functional thin films. Film growth modulation can be achieved by changing a variety of deposition parameters, such as solution pH, salt content, rinse times, and drying times.

As discussed above, the disclosed SSLBL process involves many parameters (e.g., the delivered amount of material, flow strength, and drying time), which control not only the time required to produce a nanocomposite film, but also affect the film microstructure and macroscopic properties. To assess the role of different parameters and to support the design of optimal fabrication protocols, a fundamental understanding of physical phenomena that govern the growth dynamics and the microstructure of the film are desired. Modeling efforts may be employed to gain a better understanding of these issues, e.g., (i) hydrodynamic modeling of the drying liquid layer on the spinning disk; (ii) an analysis of the effect of the fluid flow on the deposition process; and (iii) an investigation of the effect of layer adsorption conditions on the properties of the LBL films. More particularly, hydrodynamic modeling of the spin/dry process may aid development of the optimal SSLBL protocol by directly evaluating the amount of material retained in the film after the completion of a given spray/spin/dry step. Determination of the time-dependent concentration profile in the fluid film covering the spinning substrate is also valuable. Information obtained from such calculations may be used to establish which key factors control the properties of the nanocomposite film. For example, the role of the amount of material deposited in each step, deposition time, concentration of the solution, and the flow rate during the deposition process may be ascertained. The dynamics of a layer of the polyelectrolyte solution on the spinning disk may be determined using lubrication approximation.

Assuming axial symmetry of the system, the evolution of the local thickness of the fluid layer H is governed by the continuity equation.

where / is time, r is the radial coordinate, J_e is evaporation flux, and

is the fluid velocity driven by the capillary forces (first term) and by the centrifugal force (second term). Here η is the fluid velocity, σ is the interfacial tension, p is the fluid density, and ω is the angular velocity of the spinning disk. Assuming fast diffusion across the fluid layer, the concentration c of the dissolved polyelectrolyte is determined from the continuity equation: ₍₁.₃₎

dt r dr

Approximate solutions of Eqs. (1.1)-(1.3) have been derived in previous analyses of spin-coating processes, but these approximations are often insufficient at low polymer concentrations used in the SSLBL method. The effect of flow on the deposition process is also a potentially important parameter. At large spinning velocities and for a sufficiently thick fluid layer (of the order of 10 um), the shear rate γ can achieve values exceeding 10⁴ s^"1. At such high shear rates, fluid flow can cause undersired film erosion. The flow also can significantly affect conformation of polyelectrolyte molecules and orientation of nanotubes in the solution, thus it can influence the microstructure of the deposited layer. Fluid flow can be used to align nanotubes on the adsorbing substrate in a microfluidic channel and a similar ordering can be expected in the disclosed SSLBL system. Such an ordering can facilitate rapid flow-guided fabrication of nonisotropic microstructured films. FIG. 5 provides plots of the angular evolution of a rod-like particle tumbling in shear flow in a parallel-wall channel as calculated using applicable algorithms. In particular, evolution of linear chains of spears undergoing tumbling motion in a parallel wall channel are shown. The upper graph shows normalized period of motion versus distance from a channel wall for different chain lengths N. The lower graph shows evolution of the orientation of the chain.

To elucidate the effect of applied flow on the deposition process, a set of numerical simulations of the dynamics of macromolecules and nanotubes in shear flow in the presence of a planar substrate may be performed. In the simulations, the nanotubes may be modeled as linear conglomerates of spheres. It has been shown that such models faithfully capture hydrodynamic interactions of rod-like particles. In simulations of the disclosed SSLBL system, hydrodynamic interactions between the particles and the substrate may be described applying Cartesian-representation algorithms. Using such algorithms, the angular evolution of a rod-like particle tumbling in shear flow in a parallel-wall channel has been established, as illustrated in FIG. 3.

The nanotubes may be solubilized using PSS or Nafion polyelectrolyte. In simulations, nanotube-polyelectrolyte complexes may be described using a coarse-grained bead-spring model for the polyelectrolyte and a rigid chain of spheres for the nanotube. The orientational evolution of nanotube-polyelectrolyte complexes in shear flow may be investigated in the presence of a planar substrate. In addition, adsorption of such complexes on the substrate may be studied. Numerical simulations allow an assessment of feasibility of fabrication of nanostructured films with nanotubes aligned by the flow, e.g., for potential photovoltaic and nanoelectronic applications. The effect of layer adsorption conditions on the properties of the LBL film may also be determined. Preliminary investigations show that a nanocomposite film structure that is very similar to the one obtained in traditional dip LBL deposition techniques can be achieved in a rapid SSLBL process, as shown in FIGS. 6 in which SEM images compare a traditional Polyethylene

Oxide/Polyacrylic Acid (PEO/PAA) LBL film (upper image) to an SSLBL film (lower image) with similar striation patterns. However, it has also been demonstrated that films of different roughness can be fabricated by varying parameters such as the solution concentration and drying time.

Understanding how and why different parameters of the SSLBL process affect film properties is important for developing a robust and flexible platform for rapid fabrication of nanocomposite films for fuel-cell applications. Although the present disclosure has been described with reference to exemplary embodiments and implementations of the disclosed SSLBL apparatus/system and associated method, the present disclosure is not limited by or to such exemplary embodiments and/or implementations. Rather, the present disclosure may be modified, varied and/or enhanced without departing from the spirit and/or scope of the present disclosure.

Claims

1. A system for layer-by-layer assembly, comprising:

a. a spinning substrate;

b. a plurality of atomizing nozzles directed to the spinning substrate, each of the plurality of nozzles in communication with a source of fluid; and

c. a heat source directed to the spinning substrate for delivering heat energy to the spinning substrate.

2. The system according to claim 1, wherein at least one of the plurality of atomizing nozzles is in communication with a rinse fluid.

3. The system according to claim 1, wherein the heat source is a hot air blower.

4. The system according to claim 1, further comprising a control system for controlling

parameters associated with operation of the apparatus/system.

5. The system according to claim 4, wherein the control system controls the cycle time of the plurality of atomizing nozzles.

6. The system according to claim 4, wherein the control system controls operation of solenoid valves associated with the plurality of atomizing nozzles.

7. The system according to claim 4, wherein the control system controls a quantity of material to be delivered to the disc, a flow strength of the material, and drying time.

8. The system according to claim 1, wherein the plurality of atomizing nozzles are adapted to deliver constituent materials to the spinning disc or substrate in alternating fashion so as to assemble a product in a layer-by-layer manner.

9. The system according to claim 1, wherein the atomizing nozzles eject the fluid source onto the spinning disc to form a nanomaterial catalyst active for ethanol oxidation and oxygen reduction in alkali mediums.

10. A method for layer-by-layer assembly, comprising: a. providing a plurality of atomizing nozzles and a spinning disc or substrate; b. delivering constituents to the spinning disc or substrate from the plurality of atomizing nozzles in an alternating sequence so as to form a product in a layer-by-layer manner; c. delivering heat energy to the spinning disc or substrate so as to influence the drying cycle of the product on a layer-by-layer basis.

11. The method of claim 10, further comprising providing a processor to control operation of the atomizing nozzles.

12. The method according to claim 1 1, further comprising controlling a quantity of material to be delivered to the disc, a flow strength of the material, and drying time via the processor.

13. The method of claim 10, wherein at least one of the atomizing nozzles delivers a rinse fluid to the spinning disc or substrate as part of the alternating sequence.

14. The method of claim 10, wherein delivering constituents comprises delivering a first constituent to the disc to form a first layer and delivering a second constituent to the disc to form a second layer.

15. The method of claim 14, wherein the first and second layers are variations of polycationic basic polymers with polyanions coupled with nanocolloids.

16. A system for layer-by-layer assembly, comprising:

a. a spinning disc; b. a first nozzle directed to the spinning disc, the first nozzle being in communication with a first polyelectrolyte and a first solenoid to control ejection of the first polyelectrolyte from the first nozzle;

c. a second nozzle directed to the spinning disc, the second nozzle being in

communication with a second polyelectrolyte and a second solenoid to control ejection of the second polyelectrolyte from the second nozzle; and

d. a control system to operate the first and second solenoids to control a quantity of the first and second polyelectrolytes being disposed on the disc, the control system further controlling a speed at which the spinning disc rotates.

17. The system according to claim 16, further comprising a heat source directed to the spinning disc for delivering heat energy to the spinning substrate.

18. The system according to claim 17, wherein the control system controls the operation of the heat source.

19. The system according to claim 16, further comprising: a. A third nozzle directed to the spinning disc, the third nozzle being in communication with a rinse material and a third solenoid to control ejection of the rinse material from the third nozzle.

20. The system according to claim 19, wherein the control system operates the third solenoid to control a quantity of rinse material being disposed on the spinning disc, the rinse material being disposed on the spinning disc between formations of thin film layers.