Aqueous Assembly of Oxide and Fluoride Nanoparticles into 3D

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Interface-Rich Materials and Assemblies

Aqueous assembly of oxide and fluoride nanoparticles into 3D microassemblies Shanying Cui, Xin N. Guan, Eliana Ghantous, John J. Vajo, Matthew Lucas, Ming-Siao Hsiao, Lawrence F. Drummy, Josh Collins, Abigail Juhl, Christopher Stephen Roper, and Adam F Gross Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01662 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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Aqueous assembly of oxide and fluoride nanoparticles into 3D microassemblies Shanying Cui1, Xin N. Guan1, Eliana Ghantous1, John J. Vajo1, Matthew Lucas2, Ming-Siao Hsiao2, Lawrence F. Drummy2, Josh Collins3, Abigail Juhl2, Christopher S. Roper1, Adam F. Gross1* 1

HRL Laboratories, LLC, 3011 Malibu Canyon Road, Malibu CA, 90265

2

Air Force Research Labs, 2941 Hobson Way, Wright Patterson Air Force Base OH, 45433

3

Intelligent Material Solutions, Inc., 201 Washington Road, Princeton NJ, 08540

Abstract

We demonstrate rapid (~mm3/(hr*L)) organic ligand-free self-assembly of three dimensional, >50 µm single-domain microassemblies containing up to 107 individual aligned nanoparticles through a scalable aqueous process. Organization and alignment of aqueous solution-dispersed nanoparticles is induced through decreasing their pH-dependent surface charge to induce dense packing without organic ligands that could be temperature-sensitive or absorb infrared light in some applications. This process is exhibited by transforming both dispersed iron oxide hydroxide nanorods (FeOOH) and lithium yttrium fluoride (LiYF4) nanoparticles into high packing density

ACS Paragon Plus Environment

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microassemblies. The approach is generalizable to nanomaterials with pH dependent surface charge (e.g. oxides, fluorides, and sulfides) for applications requiring long range alignment of nanostructures as well as high packing density.

Introduction Individual nanoparticles possess size-tunable magnetic,1 optical,2 mechanical,3 electrical,4 or catalytic5 properties. However, incorporating nanoparticles in macroscopic length scale structures typically involves either encasing them in a matrix that dilutes their magnetic, electrical, or optical properties, or sintering them together and thus losing their size dependent properties.6 In contrast, nanoparticle assembly forms dense, three dimensional (3D) solids that retain the size-dependent properties of individual particles while also permitting emergent collective properties.7 Most nanoparticle assembly techniques create larger structures of nanoparticles through controlling attractive and/or repulsive interparticle forces.8 Examples of existing assembly methods include capillary force assisted drying,9,10 organic ligand assisted assembly,11,12 electrostatic assembly of oppositely charged nanoparticles,13 chemical bonding triggered assembly,14 and external force induced densification.15 Capillary force assisted drying produces micrometer to millimeter dimension agglomerates on substrates by overcoming Brownian motion and solvation based dispersion forces with a compressive force at a liquid-vapor interface.16 Organic ligand assisted assembly creates dense and aligned superparticles by altering the solubility and dispersion of ligand covered nanoparticles in a solvent.11,17 Instead of altering solubility, electrostatic attractions can crystallize two populations of metal nanoparticles coated with oppositely charged ligands into larger structures.18 Alternatively, ligands with reactive


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chemical groups can bring nanoparticles together through covalent14 or hydrogen bonding19 to fabricate complex assemblies of nanoparticles. For example, DNA ligands enable controlled hydrogen bonding and the combination of multiple nanoparticle types and shapes into one assembly.19 Lastly, exposing a suspension of dispersed nanoparticles in solution to an external field can induce deposition and ordering on a substrate to form up to approximately one micrometer thick layers of aligned semiconducting nanorods on a substrate.15 A crucial aspect of all the methods described above is that they require organic ligands for the initial nanoparticle dispersions. Although enabling controlled assembly, organic ligands on nanoparticles can inhibit electrical conductivity,20 create optical absorptions in the infrared, and photodegrade yielding additional infrared optical absorptions.21,22 While techniques exist to exchange organic ligands for inorganic anions after deposition on surfaces,20,23 it is not known if the assembled nanoparticles will remain intact after these processes. Instead, if nanoparticles are assembled without organic ligands present, the resulting solid can possess electrical connectivity and be free of infrared absorbing ligands. Thus a method to assemble nanoparticles without the use of organic ligands is a useful addition to existing nanoassembly approaches. Examples of organic ligand free assembly processes show the feasibility of this approach. Ligand-free assembly of spherical core-shell nanoparticles were recently demonstrated by suspending the particles in water-in-oil micelles and removing the discrete phase,24 but the process has only been demonstrated on spherical colloids. Fang et al demonstrated the first organic ligand-free, combined nanoparticle synthesis and assembly process to sheets of tightly packed goethite nanorods stacked approximately 2 to 3 layers high.25 The assembly process was driven by a decrease in surface charge during rod formation and assembly, which was induced by the decomposition of urea. Urea is a thermal base generator and the shift in pH towards the


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isoelectric point of FeOOH reduced electrostatic repulsions, leading to agglomeration of nanorods into tightly packed, aligned nanorod sheets a few layers high. In Fang et al., the synthesis and assembly process were combined in one step. However, this process could be applicable generally to a much wider set of materials, if the synthesis and assembly steps were fully decoupled and the electrostatic repulsion temporally carefully controlled in solution. In this work, we present an organic ligand-free assembly method generalizable to oxides, fluorides, and sulfides, based on titrating the pH dependent surface charge26 of separately synthesized nanoparticles to enable new large-scale applications of assembled nanomaterials. Two materials, FeOOH and LiYF4, were chosen for assembly to show that this process has applicability to magnetic and optical materials. Assembled FeOOH nanorods can potentially be reduced into Fe3O4 or Fe nanorods to form an organized, magnetic nanostructured material. LiYF4 can contain upconverting optical dopants, and is transparent to light from 0.15 to 7.5 µm if no infrared absorbing ligands are present.27 Using FeOOH nanorods as our first demonstration material, we present large 3D microassemblies (>50 um in diameter) of aligned nanorods with order persisting through the whole microassembly. The assembly is shown to be driven by a uniform and slow decrease of electrostatic repulsion between nanoparticles, and the conditions required to achieve such control are explored. This approach builds upon the work of Fang et al25. Specifically, we decouple their combined synthesis and assembly process and show the assembly step as an independent process. Furthermore, we demonstrate that the decoupled process is applicable more generally than just for FeOOH by assembling independently synthesized

lithium yttrium fluoride tetragonal bipyramidal nanoparticles into

>100 µm

microassemblies This second demonstration shows that surface charge titration can function on


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materials with pH dependent surface charges and is not limited to oxides or one nanoparticle shape. Experimental Section β-FeOOH nanorods, both undoped and doped with aluminum (FeOOH and Al/FeOOH) are synthesized through a hydrothermal route similar to previous work.25 1 mmol FeCl3, 0.1 mmol Al(NO3)3, and 2 mmol urea are dissolved in water, and placed in a Teflon-lined stainless steel autoclave. The sealed vessel is heated to 120 ºC at a rate of 1 ºC/min and held for 2 hours. The vessel is plunged into cold water while still warm to prevent any assembly during the synthesis of the rods. Undoped FeOOH nanorods are synthesized following the same procedure, omitting the aluminum salt. The resulting nanorods are 180-250 nm in length and 40-50 nm in diameter (Figure S1a). Tetragonal bipyramidial lithium yttrium fluoride (LiYF4) nanoparticles (Intelligent Materials Solutions, Inc) are stabilized in dimethyl sulfoxide (DMSO) solution with tetrafluoroborate (BF4) ligands (Figure S1b). Tetrafluoroborate ligands are known to hydrolyze into fluoroboric acid upon contact with water28 and thus are not present during the subsequent assembly process. The isoelectric points of undoped FeOOH, Al-doped FeOOH, and LiYF4 nanoparticles are determined with zeta potential measurement using a Malvern Zetasizer Nano ZS with a MPT-2 pH autotitrator. X-ray diffraction was performed using a PANalytical X’Pert Pro diffractometer. To assemble the nanoparticles, 50-60 mg of nanoparticles are re-suspended in 8 mL of water. 4 mg of urea are added to the FeOOH assembly solutions whereas 8 mg of urea are added to the LiYF4 assembly solutions. Dilute hydrochloric acid is added to lower the pH to 3, and the solution is then placed in a Teflon-lined autoclave. The sealed vessel is heated to 90 to 120 ºC at


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a rate of 1 ºC/min, held for 2 - 4 hours, and left to cool in the oven overnight. The ending pH is typically within 1 pH unit of the target isoelectric point for the nanoparticles in question. For Al/FeOOH nanorods, the ending pH is between 7-8 and for LiYF4 nanoparticles, the ending pH is between 8-9. The solution is centrifuged to concentrate the microassemblies of nanoparticles and remove extraneous salt. The microassembly concentrate is reconstituted with DI water or isopropanol. The resulting microassemblies are deposited on a TEM grid and analyzed with both SEM and x-ray tomography. Distribution of microassembly size is determined through optical profilometry (See supplemental information for more experimental details of the analyses). Results and Discussion Upon titrating the surface charge with controlled urea decomposition, the Al doped β-FeOOH nanorods assemble to form ordered three dimensional microassemblies, as exemplified in Figure 1a. Each microassembly extends 10-80 µm in each of three orthogonal directions, and consists of packed sheets of nanorod layers (Figure 1b and c). On average, each microassembly contains > 80 layers of nanorod sheets and >107 individual nanorods. We calculate that there are 1500-2000 microassemblies present in every mL of assembly solution by counting microassemblies greater than 10 µm in diameter found in every 10 µL of solution deposited on a TEM grid. The alignment of nanorods indicates controlled self-assembly and not stochastic agglomeration. To ensure the observed microassemblies are not a product of capillary drying forces during solvent evaporation, some samples were freeze dried after assembly and before microscopy thus avoiding liquid-vapor interfaces, and still showed 3D microassemblies consisting of packed sheets of layered nanorods (Figure S2). The most frequent orientation of the nanorods is in layers with the axis of each nanorod aligned normal to the layer, similar to smectic stacking in liquid crystals (Figure 1d). Occasionally, the microassemblies show nematic ordering (Figure 1e) with


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general alignment of the nanorods, but no positional order. Smectic packing is known to occur from assembly at higher nanorod concentrations than nematic packing.6

Thus the smectic

packed microassemblies may occur when microassemblies form earlier in the pH titration and the nematic microassemblies may result after smectic microassemblies have formed and reduced the nanorod concentration in solution.

Figure 1. Approximately 100 µm wide ordered 3D microassembly formed in a single assembly step from Al/FeOOH nanorods imaged using scanning electron microscopy. (a-c) A representative microassembly progressively magnified, (d) a smectic phase microassembly, and (e) a nematic phase microassembly. Insets show a low magnification image of each microassembly. X-ray tomography determines that the nanorod order persists inside the microassemblies, and not only on the surface as imaged through electron microscopy. An image in the center of a smectic microassembly (Figure 2) shows that order is present through the center of the microassembly.


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Each line in the image is measured to be 0.18 - 0.22 µm thick and represents one sheet of aligned nanorods. A video (supporting information) shows 2D cross-sections through the entire 20 µm x 45 µm x 50 µm microassembly, demonstrating that each microassembly is a single domain and not an agglomeration of smaller, unaligned domains.

Figure 2. Slice from 3D reconstruction of phase contrast X-ray computed tomography of one Al/FeOOH microassembly in epoxy, showing order through the depth and not just at the surface. Each line is one layer of an aligned sheet of nanorods in a smectic phase. We hypothesize that the assembly is driven by a decreasing interparticle electrostatic repulsion, and that the microassembly yield and quality depends on the uniformity and slowness of pH change (Figure 3). Zeta-potential measurements of the synthesized undoped and aluminumdoped FeOOH nanorods show shifting of surface charges with pH and demonstrate the electrostatic interactions present during the assembly process (Figure 3a). Agglomeration occurs when the pH induces the nanoparticle surface to progress from a positive charge, where the electrostatic forces repel each other, to near zero charge, where van der Waal attraction dominates (Figure S3). Executing the pH titration slowly and uniformly throughout the solution is thought to be critical to the assembly process. Standard methods of pH titration, e.g. burette titration, result in temporarily localized excess concentrations of acid or base, which yield non-


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uniform, rapid, and uncontrolled agglomeration of nanoparticles29. However, this titration method uses thermal decomposition of uniformly dissolved urea to control pH (Figure 3b) which should be much more uniform than methods like burette titration. At elevated temperatures, urea hydrolyses into ammonia and hydroxide ions to produce alkaline conditions.25,30 CO(NH2)2 + H2O → 2NH3 + CO2

(1)

NH3 + H2O → NH4+ + OH-

(2)

Figure 3: (a) Zeta-potential titration curve of undoped and Al-doped β-FeOOH (IEPFeOOH = 6; IEPAl/FeOOH = 7.6) shows a composition dependent shift in IEP. (b) Urea decomposition with time and temperature in a sealed round bottom flask demonstrates the pH change that induces assembly of nanorods. The temperature and pH of the system are shown in red and blue curves, respectively. The importance of a slow rate of pH change near the nanoparticles’ isoelectric point on assembly quality is demonstrated with FeOOH and Al/FeOOH nanorods. The isoelectric point (IEP), defined as the pH where the net surface charge is zero, was determined to be at pH 6 and 7.6 for FeOOH and Al/FeOOH, respectively (Figure 3a). The shift of the IEP to higher pH is attributed to the presence of Fe2O3 (IEP = pH 8-9)31 and of Al2O3 (IEP = pH 7-9)26 in Al/FeOOH. X-ray


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diffraction of dried nanorods synthesized with aluminum salt shows approximately 15% composition of Fe2O3, whereas nanorods synthesized without the aluminum salt results in pure FeOOH (Figure S4). While only a small amount of elemental aluminum remains in the Al-doped FeOOH nanorods (2 x 104 µm3 in volume, which consists of >107 nanorods each). On average, a slow cooled system results in a 4x greater microassembly total volume than a fast cooled system. This quantitative measurement of microassembly size affirms the importance of a gradual approach to the nanoparticle isoelectric point in such electrostatic self-assembly methods. The estimated assembly rate of a fast and slow cooled reaction is calculated to be, respectively, 6.5 x 10-4 and 2.0 x 10-3 mm3 of assembled Al/FeOOH per hour per milliliter of


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solution. In future studies, in-situ pH measurements and assembly visualization can further elucidate the ideal rate of zeta-potential change per hour and the pH at which aggregation begins to occur. A feedback system between the nanoparticle suspension pH and the temperature could lead to a dynamic and finer controlled assembly system.

Figure 4. Particle size distribution of microassembly volume for Al/FeOOH assembled under a slow and fast cooling method. Number of microassemblies: Nfast=18, Nslow=40. The effect of cooling rate on microassembly volume is shown by 60% having volume

Aqueous Assembly of Oxide and Fluoride Nanoparticles into 3D

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