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Nanobrick wall multilayer thin films grown faster and stronger using electrophoretic deposition

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 185703 (http://iopscience.iop.org/0957-4484/26/18/185703) View the table of contents for this issue, or go to the journal homepage for more

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Nanotechnology Nanotechnology 26 (2015) 185703 (7pp)

doi:10.1088/0957-4484/26/18/185703

Nanobrick wall multilayer thin films grown faster and stronger using electrophoretic deposition Chungyeon Cho1, Kevin L Wallace1, David A Hagen1, Bart Stevens1, Oren Regev2 and Jaime C Grunlan1 1 2

Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel

E-mail: [email protected] Received 15 December 2014, revised 22 February 2015 Accepted for publication 16 March 2015 Published 15 April 2015 Abstract

In an effort to speed up the layer-by-layer (LbL) deposition technique, electrophoretic deposition (EPD) is employed with weak polyelectrolytes and clay nanoplatelets. The introduction of an electric field results in nearly an order of magnitude increase in thickness relative to conventional LbL deposition for a given number of deposited layers. A higher clay concentration also results with the EPD–LbL process, which produces higher modulus and strength with fewer deposited layers. A 20 quadlayer (QL) assembly of linear polyethyleneimine (LPEI)/poly(acrylic acid)/ LPEI/clay has an elastic modulus of 45 GPa, tensile strength of 70 MPa, and thickness of 4.4 μm. Traditional LbL requires 40 QL to achieve the same thickness, with lower modulus and strength. This study reveals how these films grow and maintain a highly ordered nanobrick wall structure that is commonly associated with LbL deposition. Fewer layers required to achieve improved properties will open up many new opportunities for this multifunctional thin film deposition technique. S Online supplementary data available from stacks.iop.org/NANO/26/185703/mmedia Keywords: layer-by-layer assembly, electrophoretic deposition, elastic modulus, polymer nanocomposites, clay (Some figures may appear in colour only in the online journal) barrier (better than SiOx), and flame retardant properties [6– 10]. Despite its promise, the greatest hurdle for widespread usage of the LbL approach is long deposition times and the large number of layers often required to achieve a given property. Spin-coating or spraying are faster techniques, but they can result in loss of structural control [11–15]. Combining traditional LbL assembly with electrophoretic deposition (EPD) provides an opportunity to circumvent both of these problems. EPD employs an electric field to induce migration of suspended colloids toward an electrode (i.e., electrophoresis) [16–18]. The colloids then experience a London–Van der Waals attractive force that accelerates their deposition and formation of a highly organized nanocoating on the electrode

1. Introduction Electrostatically-driven, layer-by-layer (LbL) assembly of thin films became quite popular over the past twenty years due to its simplicity and versatility, although the technique traces its roots back nearly 50 years [1, 2]. The LbL method produces polyelectrolyte (PE) multilayers with tunable properties, controlled at the nanoscale [3–5]. Classic LbL deposition can make use of a wide spectrum of charged building blocks, providing an array of functionalities that can be incorporated into these thin films. For example, LbL-prepared PE-clay multilayer nanocomposites exhibit near complete intercalation by polymeric mortar at a clay concentration above 70 wt%, which results in superb mechanical, gas

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Figure 1. (A) Molecular structures of polyelectrolytes and clay (from [28]). The clay structure is reprinted with permission from AAAS. (B) Schematic diagram of the EPD process to assemble LPEI/PAA/LPEI/MMT quadlayer nanocomposite films.

2. Experimental

[19]. EPD has been successfully used to obtain uniform carbon nanomaterials (such as graphene and graphene oxide films) and polymer nanocomposites [20–23]. For example, Tang et al achieved ultra-sensitive electrochemical detection of nitroaromatic explosives, which results from the effective adsorption ability and outstanding electrocatalytic activity of reduced graphene oxide films [20]. A few reports of EPD– LbL assembly dealing with oppositely charged PE pairs or inorganic nanoparticles have also been reported [24, 25]. Ko et al used an electric field to control the assembly of weakly charged PEs, exploring the ability to control thickness over timescales that are much shorter than the conventional LbL dipping method [24]. Very little work has been done regarding LbL complexation between PEs and clay nanoplatelets in electric fields, which are expected to exhibit unique growth, morphology, and mechanical properties. In the present work, EPD was combined with LbL deposition to improve both deposition speed and mechanical properties with fewer layers, while retaining the structure of a classic immersion-based LbL process. In this case, cationic linear polyethyleneimine (LPEI), anionic polyacrylic acid (PAA), LPEI and negatively-charged montmorillonite (MMT) clay nanoplatelets were deposited as quadlayers (QLs), with and without an externally applied field (figure 1). The LPEI/PAA LbL system was chosen because its physical and mechanical properties can be controlled in response to either an externally applied electric field or pH variations [24, 26]. These weak PEs can adopt various conformations in the presence of an electric field due to attraction between charged groups and the field and hydrodynamic forces as they move through solution [27]. This unique combination of EPD and LbL self-assembly produces better mechanical behavior and accelerated film growth. An electric field of 0.8 V produces a 30 QL film with a thickness of 7.4 μm. This same thickness requires 50 QL without the field. This faster and thicker growth makes LbL deposition more amenable to commercial production of functional thin films.

2.1. Materials

Poly(acrylic acid) (PAA, MW = 100 000 g mol−1, 35 wt% aqueous solution) was purchased from Sigma-Aldrich (Milwaukee, WI). Linear poly(ethyleneimine) (LPEI, MW = 40 000 g mol−1) was purchased from Polysciences. Southern Clay Products, Inc. (Gonzales, TX) supplied natural, untreated MMT (trade name Cloisite NA+). This clay has a cationic exchange capacity of 0.926 meq g−1 and is negatively-charged in deionized (DI) water. MMT platelets have a density of 2.86 g cm−3, diameter of 10–1000 nm, and thickness of 1 nm. All chemicals were used as received without further purification. DI water with a specific resistance greater than 18 MΩ cm−1 was used in all aqueous solutions and rinsing procedures. All of the aqueous solutions were adjusted to the appropriate pH using 0.1 and 1 M HCl or NaOH, prior to assembly.

2.2. Substrates

Indium tin oxide (ITO)-coated glass slides (Delta Technologies. Loveland, Colorado), with a surface resistivity of 5–15 Ω, were used as substrates for the build-up of multilayers. Prior to use, ITO substrates were rinsed repeatedly with DI water and subjected to standard RCA treatment (H2O: H2O2:NH4OH = 5:1:1 in volume at 90 °C for 5 min) to remove organic residue and particles from the surface, followed by rinsing with water. ITO-coated poly(ethylene terephthalate) (PET) film with a thickness of 127 μm and a surface resistivity of 100 Ω was purchased from SigmaAldrich. While corona or plasma treatment are usually used for the build-up of multilayers on silicon or PET films to induce surface charge and improve adhesion, no surface treatment was used on ITO-coated substrates because they take on a negative charge in water [24]. Polished Ti/Au crystals with a resonance frequency of 5 MHz were purchased from Maxtek, Inc. (Cypress, CA) and used to characterize 2

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cutting ∼90 nm thick sections, using an Ultra 45° diamond knife (Diatome, Hatfield, PA), onto 300 mesh copper grids. TEM images of the multilayers containing the MMT particles were imaged with a Tecnai G2 F20 FE-TEM (FEI, Hillsboro, OR, energy) at an accelerating voltage of 200 kV and analyzed using Digital Micrograph Software 3.0. The thermogravimetric analysis of the MMT particles was measured with a Q50 (TA Instruments, New Castle, DE). Each sample was run under air at a scan rate of 10 °C min−1. The mass of deposited layers was measured with a research quartz crystal microbalance (RQCM, Infinicon, East Syracuse, NY). The quartz crystal was blown with compressed nitrogen gas prior to being left on the microbalance to analyze the mass change of both LbL and EPD–LbL thin films. For QCM analysis in the presence of the electric field, the quartz crystal and Pt coil were used as the working electrode and counter electrode, respectively. Electrodes were vertically oriented and separated by 3 cm in the solution and samples were rinsed in between the deposition steps. A Bruker AXS D8 Advanced Bragg-Brentano x-ray diffractometer (CuKα, λ = 1.541 Å; Bruker AXS Inc., Madison, WI) was used for glancing angle XRD. Uniaxial, in-plane tensile tests were conducted with a Q800 dynamic mechanical analyzer (DMA) from TA Instruments at room temperature under ambient (30% RH) conditions. 20–30 QL EPD–LbL and 50–100 QL LbL assemblies on ITO-coated PET were easily delaminated as the free-standing films without any further treatments. Assembled free-standing films then were cut into strips (10 mm in length and 4 mm in width) and measured in controlled strain rate mode with a preload of 0.01 N and a strain ramp rate of 0.05% min−1.

deposited mass per layer with a quartz crystal microbalance (QCM). 2.3. LbL assembly

Aqueous solutions of 0.1 wt% LPEI and 0.2 wt% PAA, and anionic suspensions of 1 wt% MMT, were prepared by simply rolling for 24 h to ensure homogeneity. Multilayers were deposited using a home-built robotic system in which each treated substrate was first dipped into the LPEI solution for 5 min and rinsed with DI water. The positively charged sample was then immersed in the PAA solution for 5 min, followed again by a rinsing step. The substrate was then submerged in LPEI and MMT solutions for 1 min with DI water rinsing steps in between, which results in one deposition sequence of a LPEI/PAA/LPEI/MMT QL. After this initial QL was deposited, the remaining number of QLs were created using 1 min deposition until the desired number of QLs was achieved. Unless otherwise stated, the outermost layer of the multilayers was MMT. 2.4. Electrophoretic deposition

An Epsilon 851 electrochemical workstation (BASi Instrumentation, West Lafayette, IN) was used to supply the potential to the substrates. An ITO glass slide or ITO-coated PET and platinum wire were used as the working and counter electrode, respectively. No reference electrode was used. The electrodes were vertically oriented, with a distance of 3 cm between them. The schematic diagram of the cell for EPD is shown in figure 1, along with structures of the ingredients used in this study. For the adsorption of the cationic LPEI, a negative bias was applied to the ITO substrate and a positive bias was applied for anionic PAA and MMT adsorption. A direct-current voltage was varied from 0.2 to 0.8 V, between the substrate and counter electrode, during the deposition step. The deposition time for the initial bilayer of LPEI/PAA was 5 min, followed by 1 min for the remaining layers, with water rinsing in between each deposition step. The films were air-dried overnight before being measured with a profilometer to assess film thickness.

3. Results and discussion 3.1. Film growth

Figure 2(A) shows the pH-dependent growth of LPEI/PAA/ LPEI/MMT QLs. In the absence of an electric field, these films grow exponentially at pH 4 and 5, and linearly at pH 3 and 6. The thickest film (150 nm at 10 QL) is obtained at pH 4. The different thickness profile is dictated by both the pH-controlled charge density of these PEs and their ‘in-andout’ diffusion during the assembly [29]. The charge density of weak PEs, such as PAA (pKa ∼ 5.5) and LPEI (pKa ∼ 4–5) used in this study, is determined by their degree of ionization when they are close to the pKa. Both polymers are only weakly charged and adsorb in a more coiled conformation, creating a relatively thick film [30]. At pH levels far from the pKa, at least one of the polymers is fully charged (at pH 3 for LPEI or pH 6 for PAA) and assumes a flattened conformation due to self-repulsion. In this case, a very thin film is formed [31, 32]. The partially charged PEs at pH 4 or 5 possess high chain mobility that results in interlayer diffusion and mixing [31, 33–35]. EPD–LbL involves application of an electrical potential to a conductive substrate. A negative bias is applied when the electrode is in the cationic LPEI solution, while negatively-

2.5. Characterization

AFM images were taken with a Digital Instruments Nanoscope in tapping mode (scan rate 0.5 Hz) using NanoProbe TESP tips under ambient conditions. Field-emission scanning electron microscopy (FE-SEM) was done with a JEOL JSM7500F FE-SEM. In order to capture cross-sectional images, the multilayers were immersed in liquid nitrogen and fractured with a diamond cutter. Energy-dispersive x-ray (EDX) analysis was conducted with an Oxford system microanalyzer attached to the FE-SEM. The thickness of the films were determined by a profilometer (KLA-Tencor Instruments P-6), with 2 μm radius stylus and 1 mg stylus force. Each sample was measured 10 times at different locations, with three samples of each type being measured, to get the reported average. Samples for TEM were prepared by embedding the film in Epofix (EMS, Hatfield, PA) resin overnight and 3

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Figure 2. Film thickness as a function of LPEI/PAA/LPEI/MMT quadlayers deposited at different pH levels using (A) LbL and (B) EPD– LbL with electric field of 0.8 V. Growth of LPEI/PAA bilayers at pH 4 is also shown in (A).

charged PAA or MMT are electrophoretically drawn to the electrode upon the application of a positive potential. The electric field greatly enhances the migration of PEs towards the electrode, yielding exponential growth at all pH values, as shown in figure 2(B) [24, 36]. The film thickness of pH 4 LPEI/PAA/LPEI/MMT, prepared using EPD–LbL with a potential of 0.8 V, is nearly an order of magnitude higher at 10 QL (920 nm) than traditional LbL deposition (150 nm). This dramatic difference stems from the energetic state of mutlilayers during the assembly. Each adsorption step follows a charge reversal due to overcompensation at the film/solution interface for traditional LbL. This process builds an energy barrier at the interface and restricts polymer deposition due to coulombic repulsion, so the deposition in the absence of an applied voltage is self-limiting [37, 38]. In the EPD process, the applied field enhances the electrophoretic mobility and overcomes the repulsion, allowing faster deposition and greater ionic complexation between polymer and clay [39– 41]. It is important to note that the exponential growth of the LPEI/PAA pair is disrupted by the presence of the intercalated MMT platelets, acting as a blocking layer, when clay is deposited in sequence with these weak PEs. The exponential growth of the PEs is still retained, but its extent is reduced (figures 2, S1 and S3) [9, 24]. This can be explained by the fact that inclusion of the MMT nanoplatelets has little effect on the ‘in-and-out’ diffusion mechanism of exponential LbL film formation, as previously reported by Podsiadlo et al [42]. These clay-containing multilayers allow polymers to permeate and reptate between the clay particles, creating PE complexes on the surface of the multilayers. A range of potentials (0.2–0.8 V) were applied to ITO glass at pH 4. As expected, greater potential produced greater thickness (figure S1). Above 0.8 V, films became dark-brown, indicating damage (figure S2), presumably due to ITO reduction at higher electric field strength (i.e. large current density). Reduction rendered the entire ITO substrate unstable for further analysis [43]. Thermogravimetric analysis was used to generate the clay as a function of QLs deposited, as shown in figure 3. While the clay concentration in both

Figure 3. Clay concentration in LbL and EPD–LbL films as a function of LPEI/PAA/LPEI/MMT QL deposited at pH 4. Concentration at 10 QL (or less) was determined with QCM, while TGA was used beyond 10 QL.

systems decreased with the number of QLs deposited, both film thickness and mass increase, indicating that only the polymers are responsible for the exponential growth [9]. Thickness (figure 2) and mass growth (figure S3) both show that the EPD–LbL process results in thicker layer formation at shorter fabrication times. The rest of this study focuses on EPD–LbL performed at pH 4 and 0.8 V, which is the thickest growing system. 3.2. Film morphology

The AFM-generated surface morphology of LPEI/PAA/LPEI/ MMT assemblies reveals what appear to be individual clay nanoplatelets deposited at 2 QL, as shown in figure 4. In both LbL and EPD–LbL films with more QL, MMT platelets are increasingly overlapping and aggregated, making it difficult to find distinct boundaries of individual clay. The surface of EPD–LbL films appear more aggregated at 10 QL, exhibiting a cobblestone structure filled with larger clay particles 4

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Figure 4. AFM height images of LbL and EPD–LbL films at 2 and 10 QL.

(0.6 ∼ 0.8 μm in size). The adsorbed cationic LPEI acts as the ‘glue’ for the negatively charged MMT platelets, allowing denser packing with fewer QLs relative to traditional LbL. EDX spectra of 10 QL LbL and EPD–LbL display aluminum and silica (representing MMT) on the surface in addition to expected peaks of carbon and oxygen, as shown in figure S5. EDX mapping quantitatively shows much higher concentration of oxygen than carbon, confirming the clay aggregation is more significant within the EPD–LbL coatings. Traditional LbL films show relatively low surface roughness (30 nm for 10 QL in figure S4), while high roughness is observed in EPD–LbL assemblies (145 nm for 10 QL). Greater EPD–LbL roughness is likely due to a few platelets overlapping one another, incomplete exfoliation of the clay, loopier conformation of PEs deposited in layers, and/or polymer-stimulated stacking [44, 45]. In addition to increasing thickness, this higher surface roughness could be one of the reasons that EPD–LbL films become more opaque with fewer deposited layers, while traditional LbL films remain visually transparent up to 20 QL (figures S2(A) and (B)). Cross-sectional micrographs of the EPD–LbL films show a layered organic-inorganic hybrid resembling a brick-and-mortar structure (figure 5) [46]. A uniformly layered structure originates from the tightly packed MMT layers parallel to the surface to maximize the attractive energy as well as the preferable diffusion of polymers beneath and atop the adsorbed MMT sheets [47–49]. Furthermore, the MMT orientation in the film is not affected by the ‘in-andout’ diffusion of the PEs, in agreement with thickness measurements at pH 4 (figure 2). Similar trends were obtained for

Figure 5. TEM and SEM (insert) cross-sectional micrographs of a 30

QL EPD–LbL film assembled at pH 4. The black and white lines indicate clay platelets and polyelectrolytes, respectively.

the LbL films (figure S6). The thickness profile obtained from free standing films also highlights the rapid growth of the EPD–LbL films without sacrificing near-perfectly oriented clay structures (figure S7). Clay intercalation in between PE layers, as evidenced by TEM, is supported by XRD for the LPEI/PAA/LPEI/MMT assemblies in figure 6. The basal spacing (d001) of natural Na-MMT clay was 1.14 nm from a 2Ɵ peak at 7.73° due to the gallery spacing between the stacked platelets, which is similar to literature values [46]. Both LbL and EPD–LbL films exhibited 15% greater clay spacing (d001 = 1.33 nm) with respect to the neat Na-MMT. 5

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Figure 6. XRD patterns for neat MMT (black line), 30 QL EPD–LbL

(red line) and 50 QL LbL (blue line) assemblies.

This implies that the PEs are intercalated into the clay during deposition. 3.3. Nanobrick wall mechanical properties

Static uniaxial tensile testing was performed with a DMA to determine the elastic modulus, ultimate strain, and ultimate tensile strength of free-standing EPD–LbL films (figure S8 shows representative stress–strain curves). Figure 7 compares the mechanical properties of the EPD–LbL, traditional LbL and LPEI/PAA assemblies prepared without clay (see also supporting information, table S1). Films below 6 μm thick could not be peeled away from the ITO substrate (20 and 50 QL for the EPD–LbL and LbL, respectively). As expected, the presence of clay significantly increases the elastic modulus of these films (2–4 X), along with the ultimate tensile strength. These increased mechanical properties are due to the highly ordered nanobrick wall structure (figure 5) and stiff MMT platelets [50–52]. The largest elastic modulus (45 GPa from EPD–LbL of 20 QL) corresponds to the highest MMT content, and is 2 to 3 times greater than previously reported polymer-clay nanocomposites in LbL or bulk from [23, 47]. Only with hundreds of layers and covalent cross-linking has an LbL film achieved greater modulus properties [8]. The positive bias to the substrate facilitates the adsorption of more negatively-charged MMT nanoplatelets, which are highly oriented with their largest dimension parallel to the substrate (figure 5). Differing growth rates and MMT deposition makes the direct comparison between LbL and EPD–LbL assemblies difficult, but multilayers with similar thickness (∼7.5 μm) show that EPD–LbL has a slightly higher elastic modulus and ultimate tensile strength. This high mechanical performance with fewer number of cycles over traditional LbL also highlights the superiority of the combination process. Mechanical properties of these nanocomposites deteriorate with additional QLs, for both traditional and EPD–LbL, due to a gradual decrease in clay concentration (figure 7). Although they could not be measured, it is likely that EPD–LbL films with 10 (or fewer) QLs have improved

Figure 7. Elastic modulus and ultimate tensile strength of LbL

(empty squares) and EPD–LbL (filled squares) films as a function of QL deposited and clay concentration. LPEI/PAA/LPEI/MMT quadlayers were deposited at pH 4, as well as the 400-bilayer LPEI/ PAA film (indicated with a star).

mechanical properties. Since the post-heat treatment causes covalent cross-linking between LPEI and PAA, the influence of thermal cross-linking on the mechanical properties of the PE nanocomposites is now being studied [53].

4. Conclusions Applying an electric field during the LbL assembly of PEs and clay enhances clay concentration, film growth, and as a consequence, thin film mechanical properties. At an optimal electric field of 0.8 V, a 1 μm thick 10 QL EPD–LbL polymer nanocomposite is produced with 74 wt% clay. This is nearly almost 6 times thicker than traditional LbL (150 nm in thickness and only 64.5 wt% clay at 10 QL). A well-aligned nanobrick wall structure forms with both LbL and EPD–LbL, but the greater clay concentration with EPD–LbL improves the elastic modulus and ultimate tensile strength of these films. This straightforward combination of electric field and LbL assembly is a simple and fast film growth approach, yielding thick and strong films with fewer deposition cycles than LbL alone. It is likely that this approach could be used with other charged nanoparticles (e.g., graphene oxide) to 6

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yield additional property improvements. Additionally, this advance is very promising for large-scale industrial use due to minimized deposition steps and rapid processing that produces high performance films (i.e., the advantages of traditional LbL assembly are not sacrificed).

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Nanobrick wall multilayer thin films grown faster and stronger using electrophoretic deposition.

In an effort to speed up the layer-by-layer (LbL) deposition technique, electrophoretic deposition (EPD) is employed with weak polyelectrolytes and cl...
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