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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Synthesis of Inorganic/Organic Hybrid Materials via Vapor Deposition onto Liquid Surfaces Mark M. De Luna, Prathamesh Karandikar, and Malancha Gupta* Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089, United States
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S Supporting Information *
ABSTRACT: Hybrid materials have a multitude of uses in electronics, catalysis, and drug delivery. Here, we combine sputter deposition and initiated chemical vapor deposition onto low-vapor-pressure liquids to fabricate inorganic/organic hybrid materials. We demonstrate that polymer nanoparticles can be decorated with metal nanoparticles and nanoparticle dispersions can be encapsulated within polymer shells or within gel beads. The generality of our synthetic route allows for a variety of hybrid materials to be fabricated by tuning the viscosity of the liquid, solubility of the monomer, and surface tension of the polymer and liquid. KEYWORDS: synthesis, hybrid, polymer, nanoparticles, hydrogels, encapsulation
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Our previous studies have shown that the deposition of polymers onto low-vapor-pressure liquids via the iCVD process yielded different morphologies such as nanoparticles, films, and gels based on the surface tension and viscosity of the liquid and the monomer solubility in the liquid.9−11 The inorganic materials are deposited onto the low-vapor-pressure liquids via direct-current (dc) magnetron sputtering, which has been shown to create inorganic thin films,12 nanoparticles at the vapor−liquid interface,13 or nanoparticles within the bulk of the liquid.14 Recently, our group elucidated the effects of the liquid viscosity and surface tension on the resulting metal morphology in dc magnetron sputtering.15 Despite the advantages of vapor-phase deposition methods, little work has been done to combine inorganic and organic deposition methods to create hybrid structures. Here, we demonstrate for the first time that iCVD and sputtering can be combined to fabricate a variety of unique hybrid materials composed of inorganic nanoparticles and polymers by tuning the viscosity of the liquid, solubility of the monomer, and surface tension of the polymer and liquid. In order to synthesize polymer nanoparticles decorated with metal nanoparticles, we deposited the metal and polymer onto low-surface-energy liquids, which allowed for the formation and submergence of inorganic and organic components into the bulk liquid. As shown in Figure 1a, metal was first sputtered onto the liquid to create dispersed nanoparticles. Polymer nanoparticles were deposited onto the resulting dispersion, which was then subsequently centrifuged to fabricate decorated nanoparticles. We have previously shown that a negative spreading coefficient (S) is required for particle
he development of new synthetic routes for the production of hybrid materials composed of inorganic and organic components is important for the fabrication of materials with enhanced properties. Hybrid materials have been shown to have a variety of applications in optics,1 sensors,2 electronics,3 and catalysis.4 For instance, Winter and co-workers incorporated iron oxide nanoparticles and quantum dots into amphiphilic polymeric micelles via solution-phase self-assembly, which enabled magnetic control and fluorescent tracking of these hybrid nanostructures for applications in theraputics.5 Additionally, Á lvarez-Paino et al. decorated poly(dopamine methacrylamide-co-ethylene glycol dimethacrylate) particles with gold and iron oxide nanoparticles via solution-phase chemical reactions for the catalytic reduction of 4-nitrophenol and 4-aminophenol.6 In this paper, we demonstrate a versatile method to create hybrid materials with control over the size, functionality, and morphologies of both the organic and inorganic phases. We combine two vapor-phase deposition processes, which eliminates the need for surfactants and organic solvents. The polymers and metals are deposited onto reuseable low-vaporpressure liquids [silicone oils and ionic liquids (ILs)], which allows for the formation of a variety of hybrid materials such as polymer nanoparticles decorated with metal nanoparticles, encapsulated nanoparticle dispersions, and polymer gel beads with embedded metal nanoparticles. The polymer is deposited onto liquid surfaces via initiated chemical vapor deposition (iCVD). Initiator and monomer precursors are flown into a vacuum chamber where the initiator is thermally cleaved by a heated filament array to initiate free-radical polymerization.7 Polymer deposition occurs at moderate substrate and filament temperatures, which allows for retention of the chemical functionality, as confirmed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS).8 © XXXX American Chemical Society
Received: October 21, 2018 Accepted: December 3, 2018
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DOI: 10.1021/acsanm.8b01888 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Nano Materials
Figure 1. (a) Schematic representation of the fabrication process to decorate polymer nanoparticles with metal nanoparticles. (b) TEM micrograph of a PHEMA nanoparticle decorated with gold nanoparticles. (c) TEM micrograph of a P4VP nanoparticle decorated with gold nanoparticles.
with the gold because of the weaker interaction of the lone pairs of the oxygen groups with gold compared to the lone pair of the pyridine groups with gold.20 The iCVD process parameters can be optimized to achieve a narrower size distribution. For example, previous studies have shown that lowering the deposition rate of the polymer leads to a lower standard deviation in particle sizes.16 The silicone oil serves as a model liquid system because of the neutral nature of the oil, which allows the polymer and metal to interact freely compared to ILs, where electrostatic repulsion can inhibit interactions between the polymer and metal.22 Since the silicone oil does not react with the polymer or metal, the decorated nanoparticles can be separated out and the oil reused, allowing for large-scale synthesis of these particles. Polymer nanoparticles decorated with gold nanoparticles are useful for catalytic reactions such as the reduction of nitrophenols,6 which is important for applications in the pharmaceutical and organic synthesis industries. We estimate that we can fabricate ∼1011 decorated P4VP nanoparticles on a single 3 × 3 cm wafer containing 100 μL of silicone oil over 10 min of deposition time. We can fabricate larger hybrid particles by modifying the iCVD parameters. Typically, the iCVD process uses reactor conditions in which the ratio of the monomer partial pressure (Pm) to the monomer saturation pressure (Psat) is less than 1, which leads to undersaturation of the precursors.7 We previously demonstrated that we can form larger polymer particles by introducing the monomer into the deposition chamber at Pm/Psat > 1, leading to the condensation of monomer droplets onto the silicone oil.18 During condensation, the monomer droplets nucleate at the vapor−liquid interface and are readily wet by the silicone oil, which leads to the engulfment of these droplets into the liquid. The monomer droplets grow via coalescence in the bulk liquid. The monomer molecules within the droplets are subsequently polymerized via a free-radical mechanism to form polymer chains. The molecular weight of a polymer chain is typically ∼100 kDa in the iCVD process,16 and therefore there are approximately ∼10000 polymer chains within each resulting polymer particle. In order to fabricate larger hybrid particles, first we deposited gold nanoparticles onto 100 cSt silicone oil via sputtering. Then we condensed 4-vinylpyridine (4VP) droplets onto this
formation via iCVD of polymers onto liquid substrates in which S is defined as S = γLV(1 + cos θ) − 2γPV, where γLV is the liquid−vapor surface tension, θ is the advancing contact angle of the liquid on the polymer, and γPV is the polymer− vapor surface tension.9 The polymer particles will either remain at the vapor−liquid interface16,17 or submerge into the bulk17,18 depending on the energy required for submergence (ΔG), which is defined as ΔG = πr2γLV(1 − |cos θe|)2, where r is the particle radius and θe is the equilibrium contact angle. We chose to deposit poly(4-vinylpyridine) (P4VP) and poly(2-hydroxyethyl methacrylate) (PHEMA) onto 100 cSt silicone oil because the spreading coefficient of these systems is negative, ensuring the formation of polymer particles, and the energy required for submerging is zero because of the 0° contact angle of silicone oil on these polymers, which causes the particles to submerge into the liquid. Similarly, 100 cSt silicone oil readily wets gold surfaces (4° contact angle) because of the high surface energy of the gold (1500 mN/ m),19 which results in the submersion of gold nanoparticles sputtered onto the silicone oil surface.15 A low-viscosity silicone oil (100 cSt) was chosen because gold sputtered onto higher viscosities (≥350 cSt) leads to the formation of dense films rather than submerging nanoparticles.15 After the deposition of gold and polymer onto the silicone oil, the silicone oil was placed in a centrifuge tube and centrifuged for 30 min to complex the gold nanoparticles with the polymer nanoparticles. Computational models have shown that neutral sputtered metals, such as gold, complex through weak van der Waals interactions with the lone pairs in nitrogen and oxygen atoms,20,21 and therefore we expect that gold interacts with the pyridine moieties of P4VP and the hydroxyl groups of PHEMA. Sputtering gold onto the silicone oil yielded nanoparticles with an average radius of 5 nm. Dynamic light scattering (DLS) data showed that PHEMA nanoparticles decorated with gold nanoparticles have an average radius of 110 ± 38 nm. Figure 1b shows a representative decorated PHEMA nanoparticle with a radius of 150 nm. DLS data showed that P4VP nanoparticles decorated with gold nanoparticles have an average radius of 82 ± 20 nm. Figure 1c shows a representative decorated P4VP nanoparticle with a radius of 95 nm. As exhibited by the lower number of gold nanoparticles in Figure 1b, PHEMA complexes less readily B
DOI: 10.1021/acsanm.8b01888 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Nano Materials dispersion and subsequently polymerized the monomer to fabricate decorated nanoparticles (Figure 2a). Figure 2b shows
Figure 2. (a) Schematic representation of the fabrication process to decorate polymer nanoparticles with metal nanoparticles via condensing. (b) TEM micrograph of P4VP nanoparticles decorated with gold nanoparticles. (c) Magnified view of a representative nanoparticle.
Figure 3. (a) Schematic diagram of the process to encapsulate dispersed gold nanoparticles within a polymer shell and (b) the resulting encapsulated marble. (c) Schematic diagram of the process with the steps reversed and (d) the resulting encapsulated marble. (e) TEM micrograph of the extracted liquid from the coated marble in part b and the liquid blotted on chromatography paper (inset). (f) TEM micrograph of the extracted liquid from the coated marble in part d and the liquid blotted on chromatography paper (inset).
that we form particles with a heterogeneous size distribution because of coalescence of the monomer droplets prior to polymerization. Figure 2c shows a representative decorated nanoparticle with a diameter of 500 nm, which demonstrates that we can achieve a larger particle size by changing process parameters independent of the substrate viscosity. A narrower particle size distribution can be achieved by increasing the substrate viscosity in order to inhibit the coalescence of monomer droplets prior to polymerization.18 We can combine the iCVD and sputtering processes to encapsulate nanoparticle dispersions within polymer shells by coating liquid marbles. The ability to encapsulate nanoparticles can be useful for applications such as drug delivery and imaging, which require the controlled release of nanoparticles at specific locations. 23 In our process, the chemical composition and thickness of the polymer shell can be tuned to include functionality such as thermoresponsiveness or pHresponsiveness to control the release. One approach to fabricating spherical dispersions that can be encapsulated is to use the concept of liquid marbles.24,25 Liquid marbles are fabricated by rolling hydrophilic droplets onto hydrophobic grains24 or particles25 to stabilize the liquids in a spherical shape. In order to encapsulate gold nanoparticles, we first sputtered gold onto an IL. We chose 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) because of its ability to form nanoparticle dispersions.26 A 10 μL droplet of this solution was rolled in 35-μm-diameter PTFE particles to fabricate spherical marbles (Figure 3a). Poly(1H,1H,2H,2Hperfluorodecyl acrylate-co-ethylene glycol diacrylate) was deposited onto the marbles via the iCVD process in order to create a polymer shell.27 The marbles were coated on a bed of PTFE particles to prevent the marbles from attaching to the Petri dish during deposition (Figure 3b). 1H,1H,2H,2Hperfluorodecyl acrylate (PFDA) was chosen as the monomer because poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) has a positive spreading coefficient and therefore a film is formed at the vapor−liquid interface.9,27 The cross-linking molecule ethylene glycol diacrylate (EGDA) was copolymerized with PFDA to increase the robustness of the resulting
polymer shell. The PTFE particles on the surface of the marble get incorporated into the polymer shell, which enhances the mechanical strength of the shell. XPS analysis showed that the atomic composition of the top (polymer−vapor) and bottom (polymer−liquid) sides of the shell consisted of only carbon, oxygen, and fluorine, and no gold was present in the shell. Figure 3c shows the order of the steps reversed by first fabricating the liquid marbles from pure IL and then subsequently depositing gold before performing polymer deposition. The resulting encapsulated marble is darker, indicating that the gold remains mostly on the surface of the marble likely because the PTFE particles act as a solid barrier, preventing most of the sputtered gold from getting incorporated into the IL (Figure 3d). We performed TEM analysis on the liquid extracted from the marbles, which confirmed a much higher concentration of gold nanoparticles when the marble is fabricated by first sputtering gold onto the IL versus when gold is sputtered after marble formation (Figure 3e,f). Additionally, the extracted liquid was also spotted onto chromatography paper. There are more nanoparticles when sputtering occurs before marble formation, as evidenced by the darker area on the paper (insets of Figure 3e,f). We can easily encapsulate ∼500 nanoparticle dispersions of 10 μL each in a single deposition because our reactor chamber has a deposition area of ∼400 cm2. An advantage of using liquid substrates instead of solid substrates during vapor deposition is the ability of precursors to absorb into the liquid. Our previous work showed that [emim][BF4] readily absorbs the monomer 2-hydroxyethyl methacrylate (HEMA), which can be subsequently polymerized to form a gel bead.28 We can use this absorbing system to embed dispersed nanoparticles within gels, as shown in Figure 4a. Gold was first sputtered onto [emim][BF4] to create C
DOI: 10.1021/acsanm.8b01888 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials
metal nanoparticles were fabricated by using a low-viscosity and low-surface-energy silicone oil, which caused both the metal nanoparticles and polymer nanoparticles to submerge. We showed that the size of the decorated particles could be increased by condensing the monomer during the iCVD process. Dispersions of gold nanoparticles were encapsulated by fabricating liquid marbles and coating the marble with a cross-linked fluoropolymer. We also embedded gold nanoparticles within the gel beds by sputtering metal onto an IL, absorbing monomer into the resulting dispersion, and polymerizing the monomer. The versatility of the iCVD and sputter deposition methods allows for the fabrication of other functional hybrid materials. For example, the functionality of the monomer can be varied to make polymers with properties such as thermo- or photoresponsiveness, and the inorganic phase can be independently tuned by switching the sputtering target to another metal such as silver or platinum.
Figure 4. (a) Schematic of the process to embed gold nanoparticles within gel beads. (b) Stereoscopic (left) and contact-angle (right) images of the resulting gel bead. (c) Stereoscopic (left) and contactangle (right) images of the gel bead that is formed when the gold is sputtered after formation of the gel bead.
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ASSOCIATED CONTENT
S Supporting Information *
a nanoparticle dispersion. In order to keep this dispersion spherical during iCVD coating, we fabricated a hydrophobic surface by coating chromatography paper with PPFDA. The hydrophobicity of the coated paper was achieved through a combination of the roughness of the paper and the low surface energy of the PPFDA polymer.29 The contact angle of [emim][BF4] with and without dispersed gold on the PPFDA-coated paper was 130°. After the gold dispersion was placed on the coated paper, HEMA monomer was absorbed into the spherical droplet and subsequently polymerized by introducing free radicals (Figure 4a). We chose to first absorb the monomer before polymerization because the simultaneous introduction of the monomer and free radicals results in PHEMA deposition onto the PPFDAcoated paper which changes the paper back to hydrophilic and thereby causes the IL droplet to spread. By sequential absorption of the monomer and then subsequent polymerization, the gel beads maintained the same contact angle (130°) after fabrication (Figure 4b). The presence of gold within the gel was confirmed via EDS analysis. Reversing the steps by sputtering onto a gel bead after polymerization instead of adding the gold before absorption causes the gel bead to wet through the chromatography paper because the surface properties of the PPFDA-coated paper change back to hydrophilic during sputter deposition because of the high surface energy of the metal (Figure 4c). However, spreading of the gel bead is inhibited because of the viscoelastic properties of the gel,10 as evidenced by the hemispherical shape. We expect that the gold remains at the surface of the gel bead in this case because the gelation process increases the viscosity of the IL droplet, preventing submersion of the sputtered gold.15 Therefore, in order to embed gold nanoparticles within a gel bead, the gold must first be dispersed within the IL droplet before monomer absorption and subsequent polymerization. The ability to embed metal nanoparticles within gel networks is useful for applications in catalysis, electronics, optics, and sensing.30−32 However, common synthesis methods lack versatility in changing the shape of the gel, limiting their applications.32 In contrast, we can easily extend our process to other geometries, such as thin films or microspheres. In conclusion, we showed that we can combine two vaporphase deposition processes, iCVD and dc magnetron sputtering, to create novel hybrid materials with embedded inorganic nanoparticles. Polymer nanoparticles decorated with
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b01888. Experimental details, including the materials and characterization methods used (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Mark M. De Luna: 0000-0003-4192-5432 Malancha Gupta: 0000-0002-6828-7445 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.M.D.L. is supported by a National Science Foundation Graduate Research Fellowship under Grant DGE-1418060. REFERENCES
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DOI: 10.1021/acsanm.8b01888 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
