Signatures of Van Der Waals and Electrostatic Forces in the

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Signatures of Van Der Waals and Electrostatic Forces in the Deposition of Nanoparticle Assemblies Ekhlas Homede, Anna Zigelman, Ludmila Abezgauz, and Ofer Manor J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02052 • Publication Date (Web): 25 Aug 2018 Downloaded from http://pubs.acs.org on August 26, 2018

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The Journal of Physical Chemistry Letters

Signatures of Van Der Waals and Electrostatic Forces in the Deposition of Nanoparticle Assemblies Ekhlas Homede, Anna Zigelman, Ludmila Abezgauz, and Ofer Manor∗ Wolfson Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa, Israel 32000 E-mail: [email protected]

∗

To whom correspondence should be addressed

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ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

We evaporate aqueous suspensions in a micro-chamber to explore the connection between the morphology of the nanoparticle deposits at nanometer resolutions and at micrometer and hundreds of micrometer resolutions. Repulsive or weakly attractive electrical double layer and van der Waals surface forces render the deposition of detached particles and small aggregates at nanometer resolutions. However, strongly attractive surface forces render the dense deposition of large aggregates. At greater length resolutions, the deposit morphology is further governed by evaporation-mediated transport of particles in the volatile suspension. We use experiment and theory to show that the contributions of the different mechanisms to the deposit morphology are mediated by particle coagulation and by particle adsorption to the substrate. The nanometer deposit morphology and particle transport render the morphology of the deposits at greater length resolutions, where it may take the shape of crude or smooth particulate micro-patterns or continuous particulate coating layers.

Graphical TOC Entry

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Investigating the evaporation of a colloidal suspension is not only aimed at realizing the array of physical mechanisms that govern this complex system, but also at capitalizing on the ability to introduce the assembly of diverse types of colloids. In particular, one endeavors to control the structure of complex particulate morphologies in the sub-micron resolution through the process of pattern deposition. 1,2 The dark ring deposits which appear following the evaporation of coffee and wine drops and the intrinsically uneven marks of paint to appear following the evaporation of the volatile carrier liquid are examples for encountering the process of pattern deposition in our daily life. Despite being a common phenomenon, the drying of a volatile suspension is a complex event, underlying an array of physical mechanisms. 3 So far, there is a striking number of studies on mechanisms of pattern deposition of nanoparticles, 4–10 and many potential applications are found, e.g. printing, coating, device fabrication, and painting. 11 Moreover, there are several outstanding applications of the self assembly of nanoparticles. Particulate structures range from two-dimensional arrays of nanoparticles to three dimensional colloidal structures that reach macroscopic dimensions support ultra sensitive biosensors and highly conductive nanowires. 12 The self assembly of colloidal particles requires inter-particle interactions. The most ubiquitous subset of inter-particle interactions are the van de Waals (VdW) and electrical double layer (EDL) forces. 13 The VdW force between particles is usually attractive; it is often considered as the untoward mechanism, which supports coagulation in colloidal dispersions. The EDL force may be either attractive, repulsive, or both (at different particle separations). The interplay between these two mechanisms is responsible for many of the morphologies encountered in self assembled colloidal structures. Examples are periodic arrays of colloids (colloidal crystals) that clarified the early stage of crystallization in dilute solutions 14 and photonic crystals which are made from arrays of colloidal particles. Moreover, monodisperse nanometer and micrometer sized spheres in a suspension can spontaneously form closepacked colloidal crystal arrays. Spherical colloids typically pack into face-centered cubic (fcc), hexagonal close-packed (hcp), or random hexagonally close-packed (rhcp) arrays, fcc

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being entropically favored over hcp. 15,16 Thorough investigations on the connection between the VdW and EDL forces and the macro-scale shape of the deposit of colloidal particles from volatile drops of suspensions were performed by Bhardwaj et al. and Duglaya et al., 17,18 who altered the pH levels of the suspensions, thus mainly altering the charge on the particles and the EDL force. The deposits that were obtained from drops of the volatile suspensions took the shape of rings, scattered aggregates, or homogeneous coating. In similar studies researchers added charged surfactants or electrolytes to volatile suspensions. Charged surfactants were used to alter the surface charge of the suspended particles and hence alter the EDL force. 19–21 Electrolytes were used to reduce the Debye length of the EDL and hence screen the EDL force. 5,22,23 Moreover, recently, Zigelman and Manor used theory in order to analyze the direct contribution of the VdW and EDL forces to the morphology of the deposit of colloidal particles from a drop of a volatile suspension 24,25 and from a volatile suspension in an open rectangular chamber 26 . Here we study the ‘signatures’ of VdW and EDL forces in the morphology of colloidal deposits at nanometer and micrometer resolutions. Volatile suspensions of nano particles support the pattern deposition of colloidal structures under conditions which facilitate a well defined interplay between the contributions of surface forces and transport mechanisms to the deposition process. We designed a rectangular micro-chamber of 125 µm inner gap and of width and length of two and one centimeters, respectively, for the deposition experiment depicted in Figure 1. The thin geometry of the micro-chamber allows for a strict control over the ambient conditions in the chamber, as well as over the rate of the diffusion-mediated evaporation of the volatile suspension. In addition, the geometry of the chamber renders a linear three phase contact line between the suspension, vapor, and the substrate, which simplifies the analysis and eliminates contributions to the shape of the deposit from a curved contact line. Moreover, the micro-chamber was kept under conditions of humidity (30-40% relative humidity), temperature (60 ◦ C), and airborne impurities (less than 100 particles per cubic fit).

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In the experiment, we injected an approximate volume of V ≈ 70 µl of a suspension of 50 nm latex particles into the micro-chamber. The studied suspensions consisted of four different particle concentrations (0.01, 0.05, 0.1, 0.5 %wt particle solids). In addition, we added different concentrations of sodium chloride (NaCl) salt to alter the ion strength in the suspensions and hence the surface forces between the particles and between the particles and the substrate of the chamber. Following the evaporation of the suspensions we observed deposits on the substrate of the micro-chamber. Both the concentrations of the particles and of the salt appeared to inflict an immense influence on the morphology of the particulate deposits at different length scales. Moreover, at large particle and salt concentrations, these contributions shadowed the classic contributions to pattern deposition from the transport of particles toward the edge of the liquid meniscus, exchanging the formation of stripes with the formation of a continuous coating layer. In order to investigate the origin of the interplay between surface forces and transport contributions to the process of pattern deposition, one must assess the nanometer scale morphology of the deposits at the level of the individual particles. For example, see the atomic force microscopy (AFM) scan of particulate structures, which is given in Figure 2 for particle concentration of 0.01% and salt concentrations of 2 × 10−2 to 4 × 10−2 M. At the lower salt concentration one observes that the deposit is composed of detached colloidal particles and small aggregates that are attached to the solid substrate. At the intermediate salt concentration one observes that the deposit is composed of much greater aggregates that cover the solid substrate more densely. At the maximum salt concentration one observes that the surface of the substrate is fully covered by a thick layer of large aggregates. The results shown in Figure 2 correlate well with the interaction energy VT , calculated for the different cases considered in this paper and with the energy barriers to the attachment of particles (the maxima of the interaction energy) in the suspension and to the substrate of the chamber, shown in 3. The interaction potentials in the figures are obtained as the sum of the VdW and EDL forces between either two similar particles or between a particle and

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the substrate for various salt concentrations. We performed numerical calculations of the EDL force by solving the full Poison-Boltzmann equation between two spherical particles or a spherical particle and a flat substrate (see Supporting Information), subject to the concentration of salt and the ζ-potential on the particles and on the substrate. The measured ζ-potentials are summarized in Supporting Information. In addition, the Hamaker constant, AH , for calculating the VdW force was found to be approximately AH = 2.8 × 10−20 J for all cases according to the closed form approximation for the nonretraded Hamaker constant for two macroscopic phases, interacting across a different medium, on the basis of the Lifshitz theory 27 . Note that the energy barriers are greater in the case of the particle-substrate interactions than in the case of the particle-particle interactions. Moreover, an increase in salt concentration reduces the energy barriers depicted in 3. At a salt concentration of 10−1 M the energy barriers appear to vanish in both cases, so that the interaction energy between particles and between the particles and the substrates translates to pure attraction. In fact, the contribution of the VdW and EDL forces to the deposition process is mediated through the rates of particle coagulation in the suspension and particle adsorption to the substrate. The corresponding rates in our experiments are dependent on the energy barriers for particle attachment. Moreover, the rates of particle coagulation and adsorption are further dependent on the local concentration and size of the particles and aggregates in the suspension and their rate of diffusion, according to the Smoluchowski and the interaction-force boundary layer theorems, respectively 28 . The ratio between the characteristic rates by which particles adsorb to the substrate and the initial concentration of the particles in the pres −1 R s→∞ ence of an interaction potential, VT , may be given by kF ≡ D s=0 g 1 eVT /kB T − 1 ds , where D, s, kB and T are the coefficient of particle diffusion, minimum separation between an interacting particle and the substrate, the Boltzmann constant, and temperature. Moreover, the ratio between the characteristic rates by which particles coagulate and the initial R s→∞ concentration of the particles is r ≡ Dφ0 /[2 s=0 g 2 eVT /kB T ds/(s + 2a)2 ], where φ0 and a are the initial concentration and the radius of the colloidal particles. The functions of sepa-

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ration and particle radius, g 1 and g 2 , introduce the hydrodynamic resistance to the process of attachment; exact expressions for g 1 and g 2 are given elsewhere, 29 although here we used the corresponding Padé approximations. 30,31 In a previous work Zigelman and Manor 24–26 showed that in a system of pattern deposition the contributions from particle coagulations and adsorption to the morphology of the deposit are scaled by the transport rate of particles in the volatile liquid. The effective contributions from particle adsorption and particle coagulation to the deposition process may be quantified by the dimensionless numbers, −1 g 1 eVT /kB T − 1 ds , J/ρ

(1)

Dρφ0 H c ≡ Hr = R s→∞ , Co 2 2J/ρ 4J s=0 g eVT /kB T ds/(s + 2a)2

(2)

D kF Da ≡ = J/ρ

R s→∞ s=0

and

respectively, where J, ρ, and H are the characteristic rate for evaporation, the density of the volatile liquid, and the thickness of the chamber, respectively. The separation variation of the interaction energy VT for the different cases considered in this paper is given in Figure 3. c in eqs (1) and (2) are given in the insets for different particle The magnitudes of Da and Co and salt concentrations. We now return to the nanometer morphology of the deposits, given in Figure 2. The c in Figure 3 at a particle concentration of 0.01% and corresponding values of Da and Co salt concentrations of 2 × 10−2 to 4 × 10−2 show qualitative agreement between theory and experiment. An increase in the size of deposited aggregates and their surface coverage, c when increasing salt concentration, corresponds to the increase in the magnitude of Co. c demonstrates the increase in the contribution of surface forces to the The increase in Co morphology of the deposit in respect to the contribution of particle transport. In order to generalize our observation at a greater length scale we conducted further AFM scans over substrate areas of 50 × 50 µm. The scans are depicted in Figure 4. The

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thickness of the deposits in the form of stripes increases from 140.8 nm to 574.6 nm as the particles concentration increases from 0.01% w/w to 0.5 % w/w. However, when increasing salt concentration, the width of each stripe increases while its height decreases (at least at the greater particles concentrations). For example, increasing the salt concentration from 10−2 to 2 × 10−2 M at 0.5% particle concentration renders the thickness of the stripe decrease from 574.6 nm to 227.7 nm and the corresponding width of the deposit increases from ∼15 µm to ∼45 µm. Moreover, the corresponding dimensionless number that gives the c is 0.03 and scaled contribution of particle coagulation to the morphology of the deposit, Co, 0.54 for salt concentrations of 10−2 and 2 × 10−2 M, respectively. The contribution to the morphology of the deposit from the adsorption of particles to the substrate vanishes at these salt concentrations. Thus, it appears that the difference between the two cases is associated with the faster coagulation of particles, which render a larger aggregates of particles in the case of the greater salt concentration. We further show in the figure a similar qualitative response of the deposit morphology to increasing the salt concentration for different initial particle concentrations. Further generalizing our insights over a far greater area of the substrate, we show in Figure 5 images of deposits on the silicon oxide substrate, which were taken by optical microscopy. Both the particle and salt concentrations contribute to the morphology of the deposits. At the lowest salt concentration (10−2 M), the deposits appear as periodically parallel stripes that become thicker and denser as the particle concentration increases (Figure 5a1,b1,c1,d1). Moreover, in the salt concentration range between 10−2 M and 3 × 10−2 M we observe the formation of deposits in the form of parallel stripes, excluding the case in which the salt concentration is 3 × 10−2 M and the particle concentration is 0.01% in which we observe the deposit of a continuous coating layer. In a salt concentration of 4 × 10−2 we observe deposits in the shapes of stripes (particle concentration of 0.1%) and in the shapes of continuous coating films (other particle concentrations). A salt concentration at the range between 5 × 10−2 M and 10−1 M renders the formation of continuous coating films. In general, the

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deposits become thicker and denser when increasing the initial concentration of the suspended particles and cover greater area of the substrate when increasing the concentration of the salt. c and Da. The magnitude In Figure 5 we further give the corresponding magnitudes of Co c increases with increasing both the salt and the particles concentration. The magnitude of Co of Da increases from zero at low salt concentrations to numbers at the order of hundreds at a salt concentration of 5 × 10−2 M. At a salt concentration of 10−1 M, the interaction force boundary layer theorem becomes obsolete in the absence of an energy barrier to the attachment of particles to the substrate, indicating that the rate of adsorption of particles to the substrate is fast with respect to the rate by which the particles are transported in the volatile suspension. The consequence is the deposition of continuous films whose thickness and roughness appear in the figure to increase when increasing the initial concentration of particles. c indicates that larger aggregates of particles are convected A larger magnitude of Co from the bulk toward the receding contact line during the evaporation of the suspension, which results in a cruder deposit morphology. The aggregates which constitute the “building blocks” of the deposit increase in size. In addition, a larger magnitude of Da indicates that the adhesion of these aggregates to the surface becomes increasingly preferable relatively to the transport of the particles in the suspension. When the adhesion becomes more dominant, the aggregates are adsorbed to the surface faster than they are transported to the contact line and these cases support the formation of a continuous deposition layer. In conclusion, we commence the analysis by examining the deposits at the nanometer scale of the particles on a substrate area of 1 × 1 µm2 . The inner nanometer-scale structure of the deposit traverses from a partial coverage of the substrate by detached particles and small aggregates to a thick coverage of the substrate by large aggregates of particles. In the former case, the rate of particle transport exceeds the rates of particle coagulation and adsorption. In the later case the rates of particle coagulation and adsorption are comparable

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or exceed the rate of particle transport. In the next step we consider the deposits on a substrate area of 50 × 50 µm2 . The contributions of both particle and salt concentrations to the morphology of the deposits are appreciable. In particular, larger aggregates render coarser deposit morphology, which is spread over a greater substrate area, connecting our findings to the previous observation of the deposit morphology on a substrate area of 1 × 1 µm2 . Finally, we consider the morphology of the deposits on a substrate area of approximately 1000 × 1000 µm2 . The findings indicate that the transition between deposits in the shape of stripes and continuous coating films is connected to the effective contribution of particle adsorption to the substrate. The deposit roughness is connected to the effective contribution of particle coagulation, similarly to our observations on a substrate area of 50 × 50 µm2 . The findings in this study connect the macroscopic shape of the deposits to the nanometer level of the single nanoparticle or aggregate on the substrate. The particulate deposits appear coarser and on a greater substrate area when increasing ion strength and the concentration of the suspensions of nanoparticles. We connect the contribution of particle concentration and ion strength to the rate of particle coagulation and particle adsorption to the substrate. These mechanisms in turn are connected to the morphology of the deposits that range from the level of scattered detached 50 nm nanoparticles to the level of large aggregates that are of hundreds of nanometers thick and are densely placed on substrate areas of 1 × 1 µm2 , 50 × 50 µm2 , and 1000 × 1000 µm2 . Moreover, the deposit morphologies at the different length scales which were examined are in agreement with the effective theoretical contributions of particle coagulation and adsorption to the process of deposition. These effective contributions are calculated from the surface forces and rates of particle transport in the volatile suspensions.

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Experimental Section The suspension samples that were studied are commercially available polystyrene beads in water (Polyscience) with averaged diameter of 50 nm. The particles suspensions were diluted by adding HPLC water to four different concentrations (0.01, 0.05, 0.1, 0.5 %wt particle solids). Varied concentrations (10−4 , 10−3 , 10−2 , 10−1 M) of NaCl (Spectrum 99% purity, Sigma Aldrich), were used in order to modify the Debye length of the EDL force in the investigated system. Silicon oxide was used as a substrate (100, 550 µm thickness, with 1 µm wet thermal oxide layer, Silicon Materials) and glass (microscope slide, Marienfeld) was used as a cover for the rectangular micro-chamber in our experiments. The silicon substrate and the glass cover were cleaned with Acetone (99.8% purity, GADOT), Ethanol (96% purity, GADOT), Iso propyl alcohol (99.8% purity, GADOT), then rinsed by Milli-Q water and blow-dried with air. The inner thickness of the chamber was 125 µm, according to the choice of the polyamide film (Kapton, DuPont) spacer, which separates between the substrate and the glass cover. The length and width of the chamber were 2 and 1 cm, respectively. The thin geometry of the chamber further supports good control over the distribution of temperature in the liquid, and simple analysis of the dynamic position of the contact line. The rate of evaporation, J/ρ, was measured experimentally by tracking the dynamic change in the position of the contact line during the evaporation of the suspension under a microscope. The measurement gives the variation in the dynamic rate of evaporation as function of the separation between the contact line and the open end of the chamber. The zeta potential of the particles and the substrates was measured for different salt concentrations using a SurPASS electrokenetic analyzer (Anton Paar GmbH) and Zetasizer ZSP (Malvern), respectively. The patterns formed after the evaporation were examined by optical microscopy (Eclipse, Ni-E, Nikon) and atomic force microscopy (AFM). A commercial AFM instrument (Dimension 3100 with Nanoscope IIIa controller, Veeco Instruments Inc.) equipped with a scanner (100 µm X 100 µm) was used to image the samples in a tapping mode in air. Silicon cantilevers with a normal resonance frequency of 160 kHz and spring 11



constants of 5 N/m (NSC14/AlBs, MikroMasch, Estonia) were used.

Acknowledgements We acknowledge support of this research by the German Israel Foundation for Scientific Research and Development (GIF) under grant number I-1361-401.10/2016.

Figure 1: (a) 3D and (b) side view illustrations of the experimental system, comprising a silicon oxide substrate, a Kapton spacer, and a glass cover.

Figure 2: Locally magnified 3D AFM scans of deposits on a substrate area of 1 × 1 µm2 , where in all cases the particle weight concentration is 0.01% and the concentration of the salt is (a) 2 × 10−2 M NaCl, (b) 3 × 10−2 M NaCl, and (c) 4 × 10−2 M NaCl salt. 12


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Figure 3: Separation variation of the interaction potential between (a) two particles and (b) a particle and the substrate, where the curves represents different salt concentrations in which the ζ−potential of the particles and the substrate are given in Supporting Information, where κ is the Debye length of the EDL of ions, and the insets give the salt concentration variation c (the different curves represent of the dimensionless coagulation and adsorption numbers, Co different particle concentrations) and Da (for all particle concentrations), respectively, on log-linear scales. Further detail about the calculations is given in Supporting Information.

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Figure 4: AFM characterizations of the deposits over a scan area of 50 × 50 µm2 (a thickness profile is given under each image), which is shown in the first two rows in Figure 5, where each column represents different particle concentration and each row represents different salt concentration (10−2 , 2 × 10−2 M NaCl).

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Figure 5: Deposits (black) of 50 nm polystyrene particles on the silicon oxide substrate (orange), where each column of images was obtained at different initial weight concentration of the suspensions (from left to right: 0.01%, 0.05%, 0.1%, 0.5%) and each row was obtained at different salt concentration (from up, down: 10−2 , 2 × 10−2 , 3 × 10−2 , 4 × 10−2 , 5 × 10−2 , 10−1 M NaCl). The adsorption number, Da, given to the left is the same for each row; it is absent from the bottom row of images due to the absence of an energy barrier for the c is specific for each attachment of particles to the substrate. The coagulation number Co image. Scale bar=100 µm.

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Supporting Information Available In Supporting Information we further summarize the measured ζ-potential values of the particles and the substrate; explain the numerical methods, which we used in order to calculate the exact electrical double layer force between spherical particles and a spherical particle and the substrate; and explain the numerical methods used for calculating the corresponding rate of particle coagulation and adsorption as function of the van der Waals and the electrical double layer forces in the suspension, obtaining the characteristic ratios between the rates of particle coagulation (or particle adsorption) and the rate of particle transport in the volatile liquid, which we use for the analysis of the experimental findings. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Signatures of Van Der Waals and Electrostatic Forces in the

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