Role of Atomic Layer Functionalization in Building Scalable Bottom-Up

Dec 15, 2016 - Building three-dimensional (3D) structures from their constituent zero-, one-, and two-dimensional nanoscale building blocks in a botto...
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Role of Atomic Layer Functionalization in Building Scalable Bottom-Up Assembly of Ultra-Low Density Multifunctional ThreeDimensional Nanostructures Peter Samora Owuor,† Thierry Tsafack,† Hye Yoon Hwang,† Ok-Kyung Park,† Sehmus Ozden,§ Sanjit Bhowmick,‡ Syed Asif Syed Amanulla,‡ Robert Vajtai,† Jun Lou,*,† Chandra Sekhar Tiwary,*,† and Pulickel M. Ajayan*,† †

Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, United States Hysitron, Inc., Minneapolis, Minnesota 55344, United States § Materials Physics and Application Divison, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ‡

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ABSTRACT: Building three-dimensional (3D) structures from their constituent zero-, one-, and two-dimensional nanoscale building blocks in a bottom-up assembly is considered the holey grail of nanotechnology. However, fabricating such 3D nanostructures at ambient conditions still remains a challenge. Here, we demonstrate an easily scalable facile method to fabricate 3D nanostructures made up of entirely zero-dimensional silicon dioxide (SiO2) nanoparticles. By combining functional groups and vacuum filtration, we fabricate lightweight and highly structural stable 3D SiO2 materials. Further synergistic effect of material is shown by addition of a 2D material, graphene oxide (GO) as reinforcement which results in 15-fold increase in stiffness. Molecular dynamics (MD) simulations are used to understand the interaction between silane functional groups (3-aminopropyl triethoxysilane) and SiO2 nanoparticles thus confirming the reinforcement capability of GO. In addition, the material is stable under high temperature and offers a costeffective alternative to both fire-retardant and oil absorption materials. KEYWORDS: silicon dioxide (SiO2), graphene oxide (GO), stiffness, functionalization, molecular dynamics, silanes nterconnecting nanosize materials (CNTs,1,2 graphene,3,4 hBN,5 metallic nanoparticles,6 and ceramics7) is a fertile ground to build 3D architectures for wide applications in automotive, aerospace, biomedicine, electronics packaging, energy storage, conversion, etc. In order to connect CNTs and other carbon-based materials, electrical, mechanical, and chemical methods can be used. Among all, the chemical methods of connecting nanomaterials are simple and easily scalable. The chemical functionalization of CNTs can produce 3D porous interconnected structures and improve mechanical properties.1 Extending this method to interconnecting inorganic and hybrid nanomaterials can be an interesting and promising research area. Inorganic nanoparticles are wellknown for their chemical stability,8 high-temperature resistance,8 and lightweight.9,10 In this class of materials, silicon dioxide (SiO2) nanoparticles are preferred due to their low toxicity,11−13 stability,8 and ability to be functionalized by a wide range of chemical functional groups.8 The chemical nature

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of SiO2 permits easy attachments of organic moieties which has made SiO2 nanoparticles widely applicable as reinforcing fillers in polymers,14 strengthening fillers in concrete, additives in rubber/plastics,15 and nontoxic platforms for drug delivery.16,17 Furthermore, the hydrolyzable nature of the bonds provides an effective stress relaxation mechanism between the interfaces of organic and inorganic materials, which leads to improved durability and adhesion.17 However, directly fabricating lightweight nanostructured materials using inorganic nanoparticles as building blocks has proven elusive. Two-dimensional (2D) materials such as graphene oxide (GO),18 exfoliated clays, and one-dimensional (1D) materials such as carbon nanotubes were explored to build three-dimensional (3D) lightweight nanostructures to be Received: October 27, 2016 Accepted: December 15, 2016 Published: December 15, 2016 806

DOI: 10.1021/acsnano.6b07249 ACS Nano 2017, 11, 806−813

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ACS Nano used as effective flame retardants.19 However, the difficulties in handling and fabricating such materials render them unsuitable for large-scale flame-retardant applications (buildings, automobiles, airplanes, etc.). In this work, we demonstrate that functionalized SiO2 nanoparticles can form well-defined 3D nanostructured materials. By employing an easily scalable simple vacuum filtration method, we assemble a 3D structure of SiO2 from its constituent zero-dimensional (0D) nanoparticles. We show that vacuum filtration with its versatility can be used to fabricate new advanced materials with tailored properties. We believe this is an easy method to build a 3D structure based entirely on its constituents 0D building blocks using simple chemistry. We further correlate mechanical properties with the degree of functionalization and show a close relationship between stiffness and functionalization. Functionalized SiO2 exhibits high stiffness and structural stability compared to nonfunctionalized SiO2. Furthermore, reinforcing the SiO2 with small amount of GO as a filler, we observe a multifold increase in strength and stiffness. Abundant functional groups on GO nanosheets provide more anchoring points for the functionalized SiO2. Our material could be used in various applications including lightweight, chemically resistant, and fire-retardant materials.

RESULTS AND DISCUSSION Vacuum-assisted filtration was used to fabricate the 3D SiO2 structure. Vacuum filtration has been used widely to prepare carbon nanotubes films,20 GO films,21,22 and other 2D materials.23 As depicted by the schematic representation of the method in Figure 1a, different types of silane at required concentration were first added to water and left for 8 h before the addition of a specified amount of SiO2 nanoparticles. The extended time of silane in water is crucial for complete hydrolysis.24 Addition of SiO2 was accompanied by magnetic stirring for 3 h to ensure homogeneous distribution of the silane coupling agents on the SiO2 nanoparticles. Later filtration was done, and the samples were left for a day to ensure all the solvent has been removed. Figure 1b shows the proposed mechanism of silane modification on SiO2 nanoparticles. Typically, SiO2 nanoparticles contain hydroxyl groups on their surfaces.25−27 Silanol derivatives from hydrolysis of silane in deionized water (DI) are reacted with SiO2 where hydroxyl groups in silanol are covalently bonded to the hydroxyl on the surface of SiO2 through a condensation reaction by modifying the SiO2 with long alkyl chains. The functionalized SiO2 (fSiO2) then forms a network of −Si−O−Si− bonds resulting in a random 3D structure when excess silanol and DI water are filtered out and dried in an oven at 50 °C for 3 h. X-ray photo spectroscopy (XPS) characterization was used to quantify the chemical groups on samples (Figure S1). As expected, nonfunctionalized samples exhibit only Si and O chemical elements. Functionalized SiO2 and GO-reinforced samples on the other hand show addition of C and N elements possibly from the silane functional groups. Further analysis reveals reduction of O element from nonfunctionalized SiO2 mainly due to their reaction with silanols to form networks. Optical and digital images of the silane-deficient SiO2, silane functionalized (fSiO2) and GO (fSiO2/GO)-reinforced functionalized 3D structures are shown in Figure S2. Nonfunctionalized SiO2 can still form a 3D structure, but weak bonding between SiO2 particles does not allow them to form stable structures (inset images in Figure S2). High-magnification optical image shows

Figure 1. Fabrication methodology for functionalized (fSiO2) material. (a) Hydrolyzation of silane couplings in water to form reactive silanol groups; addition of SiO2 nanoparticles followed by vacuum filtration and later heating at 50 °C for 3 h. (b) Proposed mechanism of reaction between silanol groups with SiO2 nanoparticles; reactive silanol groups attach themselves to the oxygen molecules on the SiO2. Depending on silane coupling, bi-, tri-, etc., connection can be achieved. This connection is responsible for covalent bonds between nanoparticles resulting in a random 3D structure. (c) SEM of nonfunctionalized SiO2 showing large cracks within the material due to lack of functional groups. (d) Functionalized SiO2 with negligible cracks within the sample. TEM images show tendency of nonfunctionalized SiO2 nanoparticles to segregate and separate from each other (e). Functionalized nanoparticles exhibit a characteristic of clustering (f).

isolated crumps of the SiO2 nanoparticles. The weak forces between SiO2 nanoparticles are not enough to maintain the structural stability of the materials in large scale. Unlike silanedeficient SiO2 samples, silane-treated SiO2 tend to congregate together in large interconnected chunks of SiO2 forming large 3D structures. In addition, the digital image shows the structure composed of a 3D structure with minimal observable manufacturing defects. Silane coupling of SiO2 introduces chemical bonding responsible for maintaining high attraction forces among SiO2 nanoparticles. The addition of GO to the fSiO2 further improves the structural stability of the materials as shown by optical and digital images of the sample. To further understand the morphology of the SiO2 samples, highresolution scanning electron microscope images (SEM) were taken. The SEM images reveal major cracks on SiO2 samples (Figure 1c). The absence of chemical interactions among SiO2 nanoparticles could be the reason for these large-scale cracks on the sample. On the other hand, SEM images of fSiO2 samples 807

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ACS Nano show large area SiO2 crumpled together without any observable cracks (Figure 1d) most likely due to the presence of silane functional groups. Microstructural analysis of fSiO2/GO samples shows large areas anchoring of SiO2 nanoparticles on GO sheets (Figure S3). GO has been shown to possess a lot of functional groups on the basal plane.28 It is theorized that functional groups such as carboxyl, hydroxyl, etc. provide more anchoring sites for SiO2 nanoparticles to adhere on the surface of GO sheets. Further high-resolution transmission electron microscope (TEM) reveals better connection of fSiO2 and GOreinforced samples compared to SiO2 samples. As observed in the optical images (Figure S2), SiO2 shows scattered small areas of SiO2 nanoparticles possibly due to the lack of strong interactions between individual SiO2 nanoparticles (Figure 1e). Contrary to SiO2 samples, fSiO2 samples exhibit large groups of nanoparticles in close proximity (Figure 1f). The effect of functionalization on the mechanical properties of different 3D SiO2 structures was investigated using a dynamical mechanical analysis (DMA) instrument. Samples were loaded to a compression strain of 0.25% and held in an isothermal mode for a number of specified cycles. We first correlated the degree of functionalizing to the mechanical response. Silane-fSiO2 shows a significantly higher stiffness compared to that of nonfunctionalized silicon dioxide samples (SiO2) (Figure 2a). Unlike nonfunctionalized SiO2 samples, we observe a self-stiffening behavior of fSiO2 in all concentrations of silane (Figure S4). The highest stiffening behavior is exhibited by 1 vol % silane concentration. The general trend is that a higher concentration of silane leads to a higher stiffness. A higher silane concentration leads to more silanol formation in water which in turn leads to a higher number of networks within the structure, hence the direct improvement in stiffness with increase in concentration. However, 10 vol % silane concentration shows low stiffening in comparison to 5 vol % silane concentration due to functionalization saturation. Saturation of silane within the nanoparticles could result in a thin, shell-like cover around the nanoparticles. Owing to this thin layer, when the structure is loaded, there is a tendency of the nanoparticles to slide over each other, unlike the unsaturated silane functionalized-nanoparticles which tend to lock over each other when loaded hence the decrease stiffness observed. Self-stiffening has been observed in model random packed granular chains.29 The self-stiffening in such systems was attributed to the length of connected granular chains. The long chains are essential to form high-density entanglements which make them tighten and lock in the presence of strain to prevent any failure due to shear. The presence of functional groups may allow the SiO2 nanoparticles to form long chains which entangle and self-stiffen. For nonfunctionalized SiO2, this connection between SiO2 does not exist, hence easily broken apart in the presence of a force. An observation of the samples after the test showed a good structural integrity of fSiO2. Unlike SiO2 samples which broke into small pieces after the test, fSiO2 and fSiO2/GO samples maintained their structures with no noticeable failure (inset of Figure 2a). The addition of GO to the fSiO2 (fSiO2/GO) makes it 15 times stiffer than the nonfunctionalized SiO2. The extraordinary stiffness exhibited by the addition of GO could be explained by an increased interaction due to functional groups on the surfaces of GO such as epoxide, carboxyl, hydroxyl, etc.28,30 These moieties may create covalent bonding as well as noncovalent interactions between GO and fSiO2 surfaces. It can be theorized that there

Figure 2. Mechanical behavior of fSiO2. (a) Discernable stiffening of fSiO2 in a cyclic compressive test, fSiO2 has a stiffness more than 4-fold compared to nonfunctionalized SiO2 sample. After loading, nonfunctionalized sample completely collapsed into small pieces, while functionalized and GO-reinforced samples show no observable damage (insets). The addition of GO, greatly enhances the stiffness to more than an order of magnitude in comparison to nonfunctionalized SiO2. (b) Load−displacement curves showing better elastic deformation behavior of fSiO2/GO and fSiO2. Nonfunctionalized sample fails at very low loads (0.01N). (c) Frequency controlled test to understand the stability of functional groups on the SiO2 at high loading cycles. As expected at high frequency, nonfunctionalized SiO2 loses its stability unlike functionalized and GO-reinforced samples which exhibit constant stiffness without noticeable loss in stiffness. (d) Nanoindentation test on two types of silane coupling agents and nonfunctionalized SiO2 nanoparticles. APTS (i) fSiO2 takes an extremely large force compared to N1-(3-trimethoxysilylpropyl) diethylenediamine (ii) and nonfunctionalized SiO2 nanoparticles (iii and inset). (e, f) SEM images in situ during nanoindentation test. Nonfunctionalized SiO2 structure breaks at very low loads (e), and once the crack develops, it propagates in all directions. On the other hand, the fSiO2 structure does not show any crack development (f).

is a tendency to form covalent bonds between the GO and SiO2. In fact, as shown by scanning electron microscope earlier, we observe a high density of SiO2 nanoparticles attached to GO nanosheets. Load−displacement curves are shown in Figure 2b. Such curves should normally exhibit two regions for materials built from particulate building blocks: deformation stage and densification stage. For nonfunctionalized SiO2, the deformation stage is completely absent due to failure of the sample at very low loads of 0.01N (preload force). The small force is enough to break the SiO2 samples. For this sample, densification commences at a displacement of 120 μm. The 808

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images during indentation show that SiO2 deformation originates from the indentor tip, a stress concentrated area. When loaded, weak forces between SiO2 nanoparticles make them easily breakable, and cracks start to propagate in all directions as shown in Figure 2e. This type of failure is repeated no matter where the indentation location is chosen on the material. As opposed to nonfunctionalized samples, silanetreated 3D SiO2 does not show any noticeable failure for the same load (Figure 2f). The presence of covalent bonding on SiO2 nanoparticles does not allow for the loading force to easily fracture the material. We have so far clearly demonstrated that functionalization SiO2 with inorganic functionalized graphene and 2D graphene sheet results in multifold improvement in stiffness, elastic modulus, and fracture resistance. The atomic scale of functional groups and 2D GO sheets is hypothesized to play a crucial role in interconnecting and building SiO2 3D porous architecture, which cannot be observed directly in experiments. Therefore, a detailed MD simulation was performed. The generation of the computer model for pristine amorphous SiO2 nanoparticles (pSiO2) used in the hydrogen passivated (hSiO2 or SiO2), APTS functionalized (fSiO2), and APTS functionalized with GO 3D models as well as the calculation of mechanical properties of the aforementioned structures were done by employing a combination of the scientific visualization and analysis software for atomistic simulation, OVITO,34 and classical molecular dynamics (MD) with its numerical implementation in the large-scale atomic/molecular massively parallel simulator35 (LAMMPS). The details are described in the simulation details section. Using OVITO,34 a 35 Å diameter spherical nanoparticle of pristine amorphous SiO2 (pSiO2) is carved out of aSiO2. Additionally, hydrogen atoms are attached to roughly 5% of the surface of pSiO2 nanoparticle to create the hydrogen-passivated nanoparticle (hSiO2 or SiO2 for simplicity), and APTS functional groups are attached to roughly 4% of the surface of pSiO2 nanoparticle to create the APTS-fSiO2 nanoparticle (fSiO2). The APTS/pSiO2 interface, in light green highlight of Figure 3, encompasses a silicon atom and three highly reactive oxygen atoms to be attached to the surface of pSiO2. The 3D structure made up of fSiO2 nanoparticles and both epoxide and hydroxyl groups attached to 35 Å × 25 Å graphene nanoribbons was created, connecting APTS nitrogen endings to carbon atoms belonging to the previously described GO nanoribbons. Such a GO structure, i.e., GO-h2e2, has been chosen for its stability36 and its likelihood to match the experimental randomness of graphene functionalization. As the box compression/tension or loading/unloading occurred in the z-direction, the respective stresses and strains at each step were computed. The above description of the creation of 3D models of SiO2, fSiO2, and fSiO2/GO is illustrated in Figure 3. The uniaxial stress−strain relationship depicted in Figure 4 is qualitative in agreement with the experimental mechanical testing in Figure 2d. In fact, under the same 25% uniaxial load, the addition of GO to the fSiO2 3D structure provides ∼200% more maximum stress than the fSiO2 and ∼600% more maximum stress than SiO2. Such a superior mechanical performance on the part of fSiO2/GO and fSiO2 over SiO2 is attributed to the interfacial bonding between nanoparticles and GO nanosheet ribbons and among nanoparticles. Assuming the percentage of hydrogen passivation at the interface is enough to reduce the amount of O−O, O−Si, or Si−Si bonds at the nanoparticle interface, the presence of hydrogen atoms

deformation stage for functionalized samples is characterized by a nonlinear curve with “pop-in” features showing possible local shear failure of the sample, and they take as much as 1N before reaching densification stage. The addition of GO in the 3D SiO2 structure results in a much stiffer sample taking as much load as the fSiO2 without apparent transition between deformation and densification stages. To compare effects of different functional groups, we performed additional similar load−unload experiments of these SiO2 samples functionalized by different chemical groups. The aforementioned comparison reveals that silane functional groups are better compared to epoxide functional groups (Figure S5). The epoxy functional group has a comparable stiffness to nonfunctionalized SiO2 but four times less than silane-treated SiO2. Further comparisons between different types of silane coupling agents show that 3-aminopropyl triethoxysilane (APTS) exhibits a higher stiffness than all the silanes tested such as N1-(3-trimethoxysilylpropyl) diethylenetri-amine and (3 glycidyloxypropyl) trimethoxysilane. The high stiffness induced by APTS could be attributed to its shorter alkyl spacer. This shortness of alkyl spacer has been shown to be as a result of stronger electron interaction between the functionality and the silicon atom.31 In all the silanes used to functionalize the SiO2 nanoparticles, APTS had the shortest alkyl spacer. What’s more, frequency controlled test also shows high stiffness with an increase of frequency for fSiO2 and fSiO2/GO samples. At higher frequencies, SiO2 starts to lose its stiffness due to the lack of connection between individual SiO 2 nanoparticles (Figure 2c). We further monitored the concentration of GO concentration in the SiO2−silane structure and observed that stiffness increases with GO concentration. However, concentrations of GO above 1% do not show any further improvement in the stiffness (Figure S6). This observation could be due to agglomeration of GO nanosheets at higher concentration where the majority of moieties could be shielded from the fSiO2 nanoparticles, thereby limiting covalent linkages. Though the stiffness of 1% GO is