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Controllable Synthesis and Surface Wettability of Flower-Shaped Silver Nanocube-Organosilica Hybrid Colloidal Nanoparticles Yangyi Sun,† Min Chen,† Shuxue Zhou,† Jing Hu,†,‡ and Limin Wu*,† †
Department of Materials Science and State Key Laboratory of Molecular Engineering of Polymers, the Advanced Coatings Research Center of MEC, Fudan University, Shanghai 200433, China and ‡Shanghai Research Institute of Fragrance & Flavor Industry, Shanghai, 200232, China
ABSTRACT Synthesis of hybrid colloidal particles with complex and hierarchical structures
is attracting much interest theoretically and technically in recent years, but still remains a tremendous challenge. Here, we present a mild and controllable wet-chemical method for the synthesis of silver nanocube (Ag NC)-organosilica hybrid particles with finely tuned numbers (with one, two, three, four, five, or six) and sizes of organosilica petals, by simply controlling the affinity with Ag NC/nature, amount, and prehydrolysis process of alkoxysilanes. The morphologies of hybrid colloidal particles have an obvious influence on the surface wettability of the hybrid particle-based films. More and larger organosilica petals can increase the surface hydrophobicity of the hybrid particle-based films. KEYWORDS: flower-shaped particles . Ag NC-organosilica hybrids . controllable . surface wettability
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ybrid colloidal nanoparticles, which are composed of multiple functional components, have attracted considerable interest in recent years because integration of functional materials with specific optical, electronic, or magnetic properties into one unit can not only inherit the properties of both components but also exhibit some new exciting physical-chemical properties by the coupling, which can make people deeply understand some fundamental scientific issues for exploring novel properties of this kind of materials16 and find their potential applications in energy utilization,7 optics,8,9 magnetic,10 and biomedical sensing,11 etc. To date, significant research efforts have been mainly devoted to design and fabricate hybrid nanoparticles with controllable complex nanostructures, including concentric coreshell,12,13 eccentric core shell,1416 Janus,17,18 dimers,19,20 and flowershaped,2124 especially the noble metalbased (e.g., Au and Ag) hybrid colloidal particles due to their prominent properties. Nonetheless, these structure-tailored noble metal-based hybrid nanoparticles are primarily focused on noble metalmetal2528 SUN ET AL.
and noble metalsemiconductor nanocrystals10,11,2931 because the heterogeneous epitaxial overgrowth mechanism between the crystal-phase materials is based on the minimization of the interfacial energy theory, while the noble metal-amorphous hybrid nanoparticles are usually very hard to obtain due to the atomic networks of amorphous materials which are usually mechanically flexible and not suitable for the heterogeneous epitaxial overgrowth mechanism.32,33 For example, eccentric/ Janus Au-SiO232 and Au-TiO234 hybrid metalamorphous nanoparticles were successfully synthesized by tuning the interfacial reaction, but more complex and hierarchical structures, e.g., flower-like types metal-amorphous nanoparticles, have not been reported to the best of our knowledge.18,21,22 In this study, we have successfully synthesized the first flower-shaped Ag nanocube (Ag NC)-organosilica (oSiO2) hybrid particles with well-defined nanostructures through a feasible and controllable wet-chemical method. In this approach, when the prehydrolyzed alkoxysilanes were added into the system consisting of Ag NCs, these alkoxysilanes continued solgel reaction VOL. XXX
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* Address correspondence to [email protected]. Received for review September 25, 2015 and accepted November 13, 2015. Published online 10.1021/acsnano.5b06051 C XXXX American Chemical Society
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RESULT AND DISCUSSION Figure 1 demonstrates the single-crystal Ag seeds are almost cubic with the edge length of about 65 ( 5 nm, and their UVvis extinction spectrum shows a main LSPR band with a maximum at 468 nm and two additional but relatively weak peaks at 350 and 390 nm, indicating the formation of Ag NCs with good monodispersity and sharp corners. When the solution of SHMPS and ODS was directly mixed with the dispersion of Ag NCs to produce hybrid nanoparticles, some free oSiO2 particles were observed because these small molecules alkoxysilanes easily went for solgel reaction to produce the homogeneous nucleation of oSiO2 besides heterogeneous nucleation on Ag NCs (Figure S1). Fortunately, when the SHMPS/ODS mixture was prehydrolyzed in the mixture solution of water and ethanol at 40 °C for 2 h and then reacted with the dispersion of Ag NCs, monodisperse and high-yield hybrid colloidal particles were obtained. This is probably because the prehydrolysis process promotes the chemical reaction between SHMPS and ODS molecules to produce oligomers, and the silanol groups of these oligomers can form stable emulsion droplets in aqueous phase (Figure 2).35 Figure 3 presents the obtained Ag NC-oSiO2 hybrid colloidal particles from different amounts of prehydrolyzed alkoxysilanes. In all hybrid reaction systems, nearly no isolated oSiO2 particles were observed, indicating that the oSiO2 particles mainly nucleate
heterogeneously on the Ag NC seeds. At a low amount of alkoxysilanes (550 μL, Figure 3a, 3a1, and 3a2), only one oSiO2 petal grows on the one surface of Ag NC seed, forming obvious Janus structure with the spherical oSiO2 particles of about 100 nm in mean diameter. The yield of this Janus structure is as high as 95%, as determined by statistical analysis of a transmission electron microscopy (TEM) image with over about 200 particles. When 800 μL of alkoxysilanes is used, dumbbell-like hybrid particles with two oSiO2 petals parallel connected by one Ag NC are obtained (Figure 3b, 3b1, and 3b2), and these two oSiO2 petals have nearly the same diameter of about 90 nm. As the amount of alkoxysilanes is further increased to 950 μL, three oSiO2 petals with around 80 nm in diameter grow on the three sides of one Ag NC (Figure 3c, 3c1, and 3c2). When 1050 μL of alkoxysilanes is used (Figure 3d, 3d1, and 3d2), four oSiO2 petals with around 70 nm in diameter are nearly symmetrically distributed on the four facets of one Ag NC. When alkoxysilanes are further increased to 1200 μL, five oSiO2 particles grow on one Ag NC, and the mean size is around 65 nm (Figure 3e, 3e1, and 3e2). At more alkoxysilanes, e.g., 1350 μL, six oSiO2 petals with about 60 nm in diameter are surrounding one Ag NC (Figure 3f, 3f1, and 3f2). These results clearly indicate that the morphologies of the Ag NC-oSiO2 hybrid particles can be controllably tuned by the amount of alkoxysilanes, and all oSiO2 petals grow around the Ag NCs as possible by geometrical symmetry, except for the Janus structure.
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to directly form flower-shaped Ag NC-oSiO2 hybrid colloidal particles with tunable numbers (with one, two, three, four, five, or six-) and sizes of petals. Compared to the previously reported strategies, the method we report here is rather mild and easily massproduced, and the obtained hybrid particles with tunable morphologies and sizes exhibit various surface wettability on their film surfaces. Accordingly, we believe this study could open a new pathway and possibility for the rational design and synthesis of other complex and hierarchical noble-amorphous heterostructures to create new materials.
Figure 2. (a) The solution obtained upon 1 min of mixing. (b) The emulsion obtained after the hydrolysis-emulsification process. (c) Optical image of the prehydrolyzed emulsion.
Figure 1. (a) TEM image of Ag NCs. (b) Extinction spectrum of Ag NCs. SUN ET AL.
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Figure S2 shows the relative proportions of the different nanostructures obtained from different amounts of alkoxysilanes, calculated by a statistical analysis of the TEM images based on around 200 particles for each experiment. The yields of the obtained hybrid colloidal particles are 97%, 67%, 91%, 84%, 84%, and 95% for Janus, two-, three-, four-, five-, or six-petal particles, respectively. And almost no free oSiO2 particles are observed in the TEM images. The corresponding DLS data of the obtained hybrid colloidal particles are shown in Figure S3. The polydispersity index (PDI) values of these hybrid particles are generally smaller than 0.10, indicating that the particles are relatively monodisperse without aggregation and coupling. This synthetic process is very mild and simple and only needs to control the alkoxysilane types and their SUN ET AL.
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Figure 3. TEM images of the flower-shaped Ag NC-oSiO2 particles with different numbers of petal: (a1) one, (b1) two, (c1) three, (d1) four, (e1) five, (f1) six; (a2f2) corresponding high-magnification TEM images, and (a3f3) corresponding cartoons.
prehydrolysis processes for fabrication of various hybrid morphologies. Generally, in a multiple-component system synthesized by depositing growth material on the seeds, the growth model for either homogeneous nucleation or heterogeneous nucleation is mainly dependent upon the interfacial reaction between the seeds and the other growing materials.4,13 This can be referred as the wettability/interaction between the seed and the growing material. If the growing material can wet extremely well on or has good interaction with the seed nanoparticle (the contact angle is 150°), then there is a high-energy landscape between the precursor atoms produced by the growing material and the seed. In this case, once the concentration of precursor atoms is supersaturated, homogeneous nucleation emerges to produce free pure particles. When the contact angle locates in the range of 30°150°, the growth mode depends on the reaction conditions, to form eccentric coreshell, Janus, flower-like, and other structures.33 Thus, the ability to tune the interfacial reaction is essential for the rational design and synthesis of different hybrid particles with complex nanostructures, especially for amorphous materials. When the sole SHMPS precursor was used in our reaction system as the growth material, it could heterogeneously nucleate on the Ag NCs surface even at a very low amount (Figure S4). This is because SHMPS can attach on the surfaces of Ag NCs by a strong AgS bonds,36 which can significantly improve the Ag NCoSiO2 interaction thus the “wetting” ability of oSiO2 on the Ag NCs seeds, guaranteeing the heterogeneous nucleation of oSiO2 on Ag NCs. When some relatively hydrophobic alkoxysilanes with different lengths of carbon chain, such as MPS, HDS, and ODS combined with SHMPS, were used, the oligomers derived from the hydrolysis and condensation reactions of these alkoxysilanes have different wettability on Ag NCs. When SHMPS/MPS mixture was prehydrolyzed and emulsified, eccentric coreshell structure with each Ag NC encapsulated inside one oSiO2 particle accompanied by some free oSiO2 particles were obtained. Increasing the content of alkoxysilanes only increased the size of oSiO2 rather than changing the eccentric coreshell structure (Figure S5). When different amounts of SHMPS/HDS mixture were added, Janus and dimer Ag NC-oSiO2 hybrid particles rather than flower-shaped Ag NC-oSiO2 hybrid particles were obtained (Figure S6). When SHMPS/ODS mixture was used, different morphologies including flower-shaped Ag NC-oSiO2 hybrid nanoparticles can be obtained as shown above (Figure 3). These results clearly demonstrate the feasibility of choosing different alkoxysilanes to regulate complex Ag NC-oSiO2 nanostructures.
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Figure 4. General schematic representation of nucleation and growth of oSiO2 on the surface of Ag NC.
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These structural transitions of Ag NC-oSiO2 hybrid particles can probably be explained thermodynamically by the interfacial energy analysis. In our reaction system, the total interfacial energy (Einterface) should be related to the free energies of Ag NCs (δAg), oSiO2 (δoSiO2), and the solvent (δsol) and can be expressed as follows: Einterface= σAg‑oSiO2 AAg‑oSiO2 þ σAg‑sol AAg‑sol þ σoSiO2‑sol AoSiO2‑sol (where σ is the surface energy of a given facet, A is its area), and σAg‑sol = σAg‑oSiO2 þ σoSiO2‑sol cos θ (where θ is the contact angle) from Young's equation.34 Owing to the increasing hydrophobicity of alkoxysilanes from MPS to HDS and ODS, the wettability of these oligomers derived from the solgel process of these alkoxysilanes to the waterborne PVP-stabilized Ag surfaces is becoming worse and worse, thus the contact angle θ increases. And because the σAg‑sol and σoSiO2‑sol are constants with the same H2O/ethanol solvent, thus the increase of contact angle θ will increase σAg‑oSiO2, that is σAg‑MPS < σAg‑HDS < σAg‑ODS.37 According to the Einterface equation, the contact area, AAg‑oSiO2, should be the minimum because the σAg‑oSiO2 of ODS is the highest. Consequently, the growing oligomers from the solgel reaction of SHMPS and ODS tend to form spherical domains on the surfaces of Ag NCs to minimize the Ag NC-oSiO2 interface energy to form a Janus structure rather than to expand the Ag NC-oSiO2 interface area to produce an eccentric coreshell structure. On the other hand, the prehydrolysis process of alkoxysilanes is also vital to the formation of flowershaped Ag NC-oSiO2 nanostructures. As the prehydrolysis temperature increased from 25 and 40 to 50 °C, the size of oSiO2 petals decreased. Too short prehydrolysis time, e.g., 1 h, did not avail the formation of monodisperse oSiO2 petals, while too long time, e.g., of 5 h, formed some free oSiO2 particles due to homogeneous nucleation (Figure S7). This prehydrolysis process actually produced oligomers with SiOH groups, which can, on one hand, stabilize the emulsion droplets as emulsifiers, and on the other hand, increase the combination of SHMPS and ODS. The higher the temperature, the more the hydrophilic species are, and the smaller the mean size of emulsion drops is.38 The longer the hydrolysis time, the more the oligomers are.39 Based on these experimental results and discussion, we propose a possible mechanism for the formation of the flower-shaped Ag NC-oSiO2 hybrid particles as shown in Figure 4. When the prehydrolysized alkoxysilanes were added into the system with Ag NCs, the reaction kinetics of hydrolysis and condensation of oSiO2 networks can be controlled by the amount of prehydrolysized alkoxysilanes. Without Ag cubes, very small pure oSiO2 particles were formed at a small amount of alkoxysilanes, e.g., 550 μL. With the increasing amount of alkoxysilanes to 800, 1200, and 1500 μL, more and larger pure oSiO2 particles were obtained
(Figure S8). In the presence of Ag NC seeds, when a small amount of alkoxysilanes, e.g., 300 μL, was used, although the hydrolysiscondensation rate of alkoxysilanes is slow to result in a low concentration of SH-contained oligomers, a very small oSiO2 domain was nucleated on the surface of Ag NC (Figure S9a). This indicates that the initial heterogeneous nucleation barrier of SH-contained oligomers is lower than homogeneous nucleation. These forming oSiO2 continued to grow on the existing oSiO2 domains of Ag NCs (Figure S9b-e) rather than forming new nucleation sites on the other faces of Ag NCs, because the interfacial energy of oSiO2-oSiO2 is very low due to the similar SiOSi components.13,36,40 On the other hand, the initial nucleation of oSiO2 may consume plenty of SH-contained oligomers to attach on the Ag NCs surface, the subsequent production of SHcontained oligomers is not sufficient to give new nucleation sites on the Ag surfaces. This suggests that the nucleation of oSiO2 is mainly dependent on the concentration of initial SH-contained oligomers, while the growth of oSiO2 is mainly dependent on the strength of SiOSi bonding interactions. When much higher amounts, e.g., between 550 and 1350 μL, were used, different numbers of oSiO2 petals can be constructed on the surfaces of Ag NCs. This can be explained as follows: As the amount of the prehydrolyzed alkoxysilanes increases, the reaction kinetics of formation of oSiO2 is greatly enhanced,41 and a fast reaction may build a larger number of SH-contained oligomers around a Ag NC seed, thus facilitating the initial nucleation of SH-contained oligomers with Ag NC faces to form active nucleation sites.25 Once the nucleation sites are formed, the subsequent growth of oSiO2 producing tunable flower-shaped Ag NC-oSiO2 hybrid particles with one to six oSiO2 petals occurs. Moreover, the oSiO2 petals prefer to nucleate symmetrically along the facets of Ag NCs, owing to the balance between the repulsive force of negative oSiO2 petals and an attractive force toward the Ag NC surface by the strong AgS interaction.42 Moreover, no obvious free oSiO2 nanoparticles were found in these systems (Figure 3), indicating that heterogeneous nucleation and growth of oSiO2 dominate the whole reaction system. When the amount of alkoxysilanes was further increased to 1500 μL, the oSiO2 particles would coat more than one Ag NC seed to make aggregation of Ag VOL. XXX
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CONCLUSIONS In summary, we have demonstrated a mild and feasible wet-chemical approach to controllably synthesize the flower-shaped Ag NC-oSiO2 hybrid colloidal
EXPERIMENTAL METHOD Materials. All chemicals were analytical grade and used as received. Ethylene glycol (EG, 99.5%), acetone (99.5%), ethanol (99.7%), and ammonia (28 wt %) were purchased from Sinopharm Chemical Reagent Co. (China). Polyvinylpyrrolidone (PVP, Mw = 55,000) and sodium hydrosulfide (NaHS) were
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NC-oSiO2 hybrid particles with some free irregular oSiO2 particles (Figure S9f). This is not difficult to understand: As the amount of alkoxysilanes is further increased, the hydrolysis and condensation reactions of alkoxysilanes are becoming faster, to produce supersaturated oligomers for the formation of nuclei in the medium and a continual cascading of nuclei, resulting in homogeneous nucleation to form free oSiO2 particles and the continual cascading of nuclei to make the aggregation of the particles.12 The size of oSiO2 petals on the hybrid particles can be easily tuned by the relative volume ratios of SHMPS to ODS. For example, for Janus Ag NC-oSiO2 hybrid particles, at equal amounts of prehydrolyzed alkoxysilane dispersion and SHMPS, being 450 and 210 μL, respectively, as the ODS amount increases from 50 and 70 to 95 μL, the mean diameter of oSiO2 petals increases from 60 and 80 to 150 nm, respectively (Figure 5). One more example, for six-petal Ag NC-oSiO2 hybrid particles, when the amounts of prehydrolyzed alkoxysilane dispersion and SHMPS are kept at 1350 and 210 μL, respectively, but the amount of ODS increases from 50 and 70 to 95 μL, the mean size of oSiO2 petals also gradually increases from 60 and 75 to 85 nm, as shown in Figure 6. EDS analysis also indicates that the C/S atomic ratio increases with the increase of ODS amount (Table S1). The morphologies and the sizes of oSiO2 petals have a tremendous influence on the surface wettability of the hybrid particle-based films. As the oSiO2 petal numbers of Ag NC-oSiO2 hybrid particles increases from one, two, three, four, and five to six, the WCAs of the corresponding particulate films increase from 87° to 130°, as shown in Figure 7. Moreover, as the size of oSiO2 petal increases due to more ODS amounts used, the WCAs of the corresponding particulate films increase from 130° to 156°, displaying a superhydrophobic property. This is because the more and larger oSiO2 petals benefit the formation of micronano hierarchical structures of particulate films, while the hydrophobic long CH2 groups of ODS can provide low surface energy to further improve the superhydrophobic property of particulate films.43,44
Figure 5. TEM and corresponding high-magnification TEM images of the Janus Ag NC-oSiO2 particles with different SHMPS/ODS volume ratios: (a, a1) 210:50, (b, b1) 210:70, (c, c1) 210:95.
Figure 6. TEM and corresponding high-magnification TEM images of the six-petal Ag NCs-oSiO2 particles with different SHMPS/ODS volume ratios: (a, a1) 210:50, (b, b1) 210:70, (c, c1) 210:95.
nanoparticles. The morphologies (with one, two, three, four, five, or six oSiO2 petals) and the sizes of these hybrid nanoparticles can be easily tuned by the amounts, affinity with Ag, and prehydrolysis process of alkoxysilanes. As different morphologies and petal sizes obviously influence the surface wettability of these hybrid particle-based films, a superhydrophobic film can even be directly fabricated from six-petal Ag NC-oSiO2 particulate film without any post-hydrophobization treatment. This study provides another feasibile way of fabricating hybrid particles with tunable morphologies, and the method we present here is versatile and can be readily extended for syntheses of other complex and hierarchical hybrid colloidal particles with various metals to exhibit specific properties and applications, such as the surface plasmon resonance of Ag NCs and the tunable optical films.
obtained from Sigma-Aldrich Chemical. Silver nitrate (AgNO3, 99.9%), mercaptopropyltrimethoxysilane (SHMPS, HS(CH2)3Si(OCH3)3, 95%), 3-(trimethoxysilyl)propyl methacrylate (MPS, CH3CH2CCOO(CH2)3Si(OCH3)3, 97%), hexadecyltrimethoxysilane (HDS, CH3(CH2)15Si(OCH3)3, 97%), and octadecyltrimethoxysilane (ODS, CH3(CH2)17Si(OCH3)3, 90%) were purchased from
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ARTICLE Figure 7. (a) Change of the WCAs for the Ag NC-oSiO2 hybrid particulate films as a function of different numbers of petals. (bd) SEM images and WCAs for the six-petal AgNCs-oSiO2 particulate films as a function of different ODS contents: 50, 70, and 95 μL. Aladdin Chemical Reagent Co. Deionized water was used in all experiments. Synthesis of Ag NCs. Ag NCs were synthesized using the sulfide-mediated polyol process.45 Typically, 180 mL of EG was added into a 250 mL three-neck round-bottom flask and heated in an oil bath preset at 150 °C under magnetic stirring. After stirring under uncapped conditions for 1 h, the flask was capped, degassed by a flow of argon for 15 min, and then quickly injected by 2.1 mL of NaHS (3 mM in EG), followed by injection of 45 mL of PVP (20 mg/mL in EG). When the temperature was maintained at 150 °C, 15 mL of AgNO3 (48 mg/mL in EG) was quickly added into the above solution which was allowed to proceed for 25 min. The as-synthesized Ag NCs in EG were cooled to room temperature in an ice bath and then stored in the refrigerator for subsequent uses. Synthesis of Flower-Shaped Ag NCs-oSiO2 Hybrid Particles. The flower-shaped Ag NCs-oSiO2 hybrid particles were synthesized from the prehydrolyzed alkoxysilanes as follows: In a typical synthesis, the Ag NCs were isolated by precipitating 2.0 g of the assynthesized Ag NCs-EG solution with an addition of 6.0 g acetone, followed by centrifugation at 15000 rpm for 5 min, and then redispersed into 20 mL of H2O and 10 mL of ethanol to form the mixture solution A. After ultrasonication for 15 min, the well-dispersed solution A was stirred in a water bath at room temperature for 30 min and then heated to 60 °C within 30 min, followed by addition of 0.50 mL ammonia (28 wt %). After 30 s, 550 μL of dispersion of the prehydrolyzed alkoxysilanes B was rapidly injected into the above mixture and reacted for another 3 h. The as-obtained Ag NC-oSiO2 hybrid particles were centrifuged and washed four times with ethanol and then redispersed in 2 mL of ethanol. The dispersion B was prepared in advance by adding 210 μL SHMPS and 50 μL ODS into the mixture of 6 mL water and 3 mL ethanol to prehydrolyze under vigorous stirring at 40 °C for 2 h. The numbers and sizes of organosilica petals could be increased by increasing the concentration of prehydrolyzed alkoxysilanes, as shown Table S2. By using the same procedure as above with various volume ratios of SHMPS to ODS or SHMPS/MPS and SHMPS/
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HDS, the various Ag NC-oSiO2 hybrid colloidal particles were obtained. Fabrication of Ag NC-oSiO2 Hybrid Particle-Based Films. The flowershaped Ag NC-oSiO2 hybrid colloidal particles were ultrasonically dispersed in ethanol for a few minutes to ensure uniform dispersion and then cast on the precleaned silicon wafer by spin-coating. This procedure could be repeated until a thin particulate film was obtained. After drying in an oven at 60 °C for 2 h, the particulate film was achieved. Characterization. TEM images were taken on a transmission electron microscope operating at 200 kV. The samples were dispersed in ethanol and then dried on a holey carbon film Cu grid. UVvis absorption spectra of Ag NC (background solution: 99.7% ethanol) were recorded at room temperature on the U-4100 spectrophotometer (Hitachi, Japan) using quartz cuvettes with an optical path of 1 cm. Dynamic light scattering (DLS) measurements were carried out on the diluted ethanol solutions to produce particle sizes and size distribution using a Nano-ZS90 (Malvern). Scanning electron microscopy (SEM) measurements were performed using a Philips XL 30 emission microscope at an accelerating voltage of 30 kV. Elemental analysis was performed by energy-dispersive X-ray spectroscopies (EDX) conducted on a FEI JEOL JSM-6700 field-emission scanning electron microscope. The surface wettability of particulate film was characterized by the water contact angle (WCA) by OCA15 contact angle analyzer (Dataphysics, Germany) averaged over 3 fresh spots using 3 μL deionized water. Conflict of Interest: The authors declare no competing financial interest. Acknowledgment. Financial supports of this research from the National Natural Science Foundation of China (51322307, 51273218, 51133001 and 21374018), National “863” Foundation (2013AA031801), Science and Technology Foundation of Ministry of Education of China (20110071130002), Science and Technology Foundation of Shanghai (13JC1407800), and “Shu Guang” project supported and Shanghai Rising-Star Program (14QA1403300) are appreciated.
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Additional Tables, TEM images and EDX analysis (PDF)
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Colloid Spheres. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 3272–3279. Tang, C.; Zhang, C. L.; Sun, Y. J.; Liang, F. X.; Wang, Q.; Li, J. L.; Qu, X. Z.; Yang, Z. Z. Janus Anisotropic Hybrid Particles with Tunable Size from Patchy Composite Spheres. Macromolecules 2013, 46, 188–193. Sacanna, S.; Korpics, M.; Rodriguez, K.; Colón-Meléndez, L.; Kim, S. H.; Pine, D. J.; Yi, G. R. Shaping Colloids for SelfAssembly. Nat. Commun. 2013, 4, 1688. Wang, F.; Cheng, S.; Bao, Z. H.; Wang, J. F. Anisotropic Overgrowth of Metal Heterostructures Induced by a SiteSelective Silica Coating. Angew. Chem. 2013, 39, 10534– 10538. Perro, A.; Duguet, E.; Lambert, O.; Taveau, J. C.; BourgeatLami, E.; Ravaine, S. A Chemical Synthetic Route towards “Colloidal Molecules. Angew. Chem. 2009, 121, 367–371. Désert, A.; Hubert, C.; Fu, Z.; Moulet, L.; Majimel, J.; Barboteau, P.; Thill, A.; Lansalot, M.; Bourgeat-Lami, E.; Duguet, E.; et al. Synthesis and Site-Specific Functionalization of Tetravalent, Hexavalent, and Dodecavalent Silica Particles. Angew. Chem. 2013, 125, 11274–11278. Chen, Y. Z.; Zeng, D. Q.; Cortie, M. B.; Dowd, A.; Guo, H. Z.; Wang, J. B.; Peng, D. L. Seed-Induced Growth of FlowerLike AuNiZnO MetalSemiconductor Hybrid Nanocrystals for Photocatalytic Applications. Small 2015, 11, 1460–1469. Datskos, P.; Cullen, D. A.; Sharma, J. Step-by-Step Growth of Complex Oxide Microstructures. Angew. Chem., Int. Ed. 2015, 54, 9011–9015. Zhu, C.; Zeng, J.; Tao, J.; Johnson, M. C.; Schmidt-Krey, I.; Blubaugh, L.; Zhu, Y. M.; Gu, Z. Z.; Xia, Y. N. Kinetically Controlled Overgrowth of Ag or Au on Pd Nanocrystal Seeds: From Hybrid Dimers to Nonconcentric and Concentric Bimetallic Nanocrystals. J. Am. Chem. Soc. 2012, 134, 15822–15831. Lee, J. H.; Kim, G. H.; Nam, J. M. Directional Synthesis and Assembly of Bimetallic Nanosnowmen with DNA. J. Am. Chem. Soc. 2012, 134, 5456–5459. Feng, Y. H.; He, J. T.; Wang, H.; Tay, Y. Y.; Sun, H.; Zhu, L. F.; Chen, H. Y. An Unconventional Role of Ligand in Continuously Tuning of MetalMetal Interfacial Strain. J. Am. Chem. Soc. 2012, 134, 2004–2007. Zhang, W. Q.; Yang, J. Z.; Lu, X. M. Tailoring Galvanic Replacement Reaction for the Preparation of Pt/Ag Bimetallic Hollow Nanostructures with Controlled Number of Voids. ACS Nano 2012, 6, 7397–7405. Shi, W. L.; Zeng, H.; Sahoo, Y.; Ohulchanskyy, T. Y.; Ding, Y.; Wang, Z. L.; Swihart, M.; Prasad, P. N. A General Approach to Binary and Ternary Hybrid Nanocrystals. Nano Lett. 2006, 6, 875–881. Fan, F. R.; Ding, Y.; Liu, D. Y.; Tian, Z. Q.; Wang, Z. L. FacetSelective Epitaxial Growth of Heterogeneous Nanostructures of Semiconductor and Metal: ZnO Nanorods on Ag Nanocrystals. J. Am. Chem. Soc. 2009, 131, 12036– 12037. Schick, I.; Lorenz, S.; Gehrig, D.; Schilmann, A. M.; Bauer, H.; Panthöfer, M.; Fischer, K.; Strand, D.; Laquai, F.; Tremel, W. Multifunctional Two-Photon Active Silica-Coated Au@MnO Janus Particles for Selective Dual Functionalization and Imaging. J. Am. Chem. Soc. 2014, 136, 2473–2483. Chen, T.; Chen, G.; Xing, S. X.; Wu, T.; Chen, H. Y. Scalable Routes to Janus Au-SiO2 and Ternary Ag-Au-SiO2 Nanoparticles. Chem. Mater. 2010, 22, 3826–3828. Sun, Y. G. Interfaced Heterogeneous Nanodimers. Natl. Sci. Rev. 2015, 3, 329–348. Seh, Z. W.; Liu, S. H.; Zhang, S. Y.; Bharathi, M. S.; Ramanarayan, H.; Low, M.; Shah, K. W.; Zhang, Y. W.; Han, M. Y. Anisotropic Growth of Titania onto Various Gold Nanostructures: Synthesis, Theoretical Understanding, and Optimization for Catalysis. Angew. Chem. 2011, 125, 10322–10325. Lu, Z. D.; Sun, L.; Nguyen, K.; Gao, C. B.; Yin, Y. D. Formation Mechanism and Size Control in One-Pot Synthesis of Mercapto-Silica Colloidal Spheres. Langmuir 2011, 27, 3372–3380.
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36. Sun, Y. Y.; Chen, M.; Wang, Z. B.; Wu, L. M. Facile Synthesis of Asymmetric AgOrganosilica Hybrid Nanoparticles with Tunable Morphologies and Optical Properties. Chem. Commun. 2014, 50, 5767–5770. 37. Li, L.; Zhang, L. Y.; Xing, S. X.; Wang, T. T.; Luo, S. R.; Zhang, X. Q.; Liu, C.; Su, Z. M.; Wang, C. G. Generalized Approach to the Synthesis of Reversible Concentric and Eccentric Polymer-Coated Nanostructures. Small 2013, 9, 825–830. 38. Wang, Q. B.; Liu, Y.; Yan, H. Mechanism of a Self-Templating Synthesis of Monodispersed Hollow Silica Nanospheres with Tunable Size and Shell thickness. Chem. Commun. 2007, 2339–2341. 39. Sun, Y. Y.; Yin, Y. Y.; Chen, M.; Zhou, S. X.; Wu, L. M. One-Step Facile Synthesis of Monodisperse Raspberry-Like P(S-MPSAA) Colloidal Particles. Polym. Chem. 2013, 4, 3020–3027. 40. Zhang, L.; Zhang, F.; Dong, W. F.; Song, J. F.; Huo, Q. S.; Sun, H. B. Magnetic-Mesoporous Janus Nanoparticles. Chem. Commun. 2011, 47, 1225–1227. 41. LaMer, V. K.; Dinegar, R. H. Theory, Production and Mechanism of Formation of Monodispersed Hydrosols. J. Am. Chem. Soc. 1950, 72, 4847–4854. 42. Noya, E. G.; Vega, C.; Doye, J. P.; Louis, A. A. The Stability of A Crystal with Diamond Structure for Patchy Particles with Tetrahedral Aymmetry. J. Chem. Phys. 2010, 132, 234511. 43. He, J.; Yu, B. Y.; Hourwitz, M. J.; Liu, Y. J.; Perez, M. T.; Yang, J.; Nie, Z. H. Wet-Chemical Synthesis of Amphiphilic Rodlike Silica Particles and Their Molecular Mimetic Assembly in Selective Solvents. Angew. Chem. 2012, 124, 3688–3693. 44. Xu, D. Z.; Wang, M. Z.; Ge, X. W.; Lam, M. H-W.; Ge, X. P. Fabrication of Raspberry SiO2 /Polystyrene Particles and Superhydrophobic Particulate Film with High Adhesive Force. J. Mater. Chem. 2012, 22, 5784–5791. 45. Zhang, Q.; Cobley, C.; Au, L.; Mckiernan, M.; Schwartz, A.; Wen, L. P.; Chen, J. Y.; Xia, Y. N. Production of Ag Nanocubes on A Scale of 0.1g per Batch by Protecting the NaHSMediated Polyol Synthesis with Argon. ACS Appl. Mater. Interfaces 2009, 1, 2044–2048.
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Controllable Synthesis and Surface Wettability of Flower-Shaped Silver Nanocube-Organosilica Hybrid Colloidal Nanoparticles Yangyi Sun,† Min Chen,† Shuxue Zhou,† Jing Hu,†,‡ and Limin Wu*,† †
Department of Materials Science and State Key Laboratory of Molecular Engineering of Polymers, the Advanced Coatings Research Center of MEC, Fudan University, Shanghai 200433, China and ‡Shanghai Research Institute of Fragrance & Flavor Industry, Shanghai, 200232, China
ABSTRACT Synthesis of hybrid colloidal particles with complex and hierarchical structures
is attracting much interest theoretically and technically in recent years, but still remains a tremendous challenge. Here, we present a mild and controllable wet-chemical method for the synthesis of silver nanocube (Ag NC)-organosilica hybrid particles with finely tuned numbers (with one, two, three, four, five, or six) and sizes of organosilica petals, by simply controlling the affinity with Ag NC/nature, amount, and prehydrolysis process of alkoxysilanes. The morphologies of hybrid colloidal particles have an obvious influence on the surface wettability of the hybrid particle-based films. More and larger organosilica petals can increase the surface hydrophobicity of the hybrid particle-based films. KEYWORDS: flower-shaped particles . Ag NC-organosilica hybrids . controllable . surface wettability
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ybrid colloidal nanoparticles, which are composed of multiple functional components, have attracted considerable interest in recent years because integration of functional materials with specific optical, electronic, or magnetic properties into one unit can not only inherit the properties of both components but also exhibit some new exciting physical-chemical properties by the coupling, which can make people deeply understand some fundamental scientific issues for exploring novel properties of this kind of materials16 and find their potential applications in energy utilization,7 optics,8,9 magnetic,10 and biomedical sensing,11 etc. To date, significant research efforts have been mainly devoted to design and fabricate hybrid nanoparticles with controllable complex nanostructures, including concentric coreshell,12,13 eccentric core shell,1416 Janus,17,18 dimers,19,20 and flowershaped,2124 especially the noble metalbased (e.g., Au and Ag) hybrid colloidal particles due to their prominent properties. Nonetheless, these structure-tailored noble metal-based hybrid nanoparticles are primarily focused on noble metalmetal2528 SUN ET AL.
and noble metalsemiconductor nanocrystals10,11,2931 because the heterogeneous epitaxial overgrowth mechanism between the crystal-phase materials is based on the minimization of the interfacial energy theory, while the noble metal-amorphous hybrid nanoparticles are usually very hard to obtain due to the atomic networks of amorphous materials which are usually mechanically flexible and not suitable for the heterogeneous epitaxial overgrowth mechanism.32,33 For example, eccentric/ Janus Au-SiO232 and Au-TiO234 hybrid metalamorphous nanoparticles were successfully synthesized by tuning the interfacial reaction, but more complex and hierarchical structures, e.g., flower-like types metal-amorphous nanoparticles, have not been reported to the best of our knowledge.18,21,22 In this study, we have successfully synthesized the first flower-shaped Ag nanocube (Ag NC)-organosilica (oSiO2) hybrid particles with well-defined nanostructures through a feasible and controllable wet-chemical method. In this approach, when the prehydrolyzed alkoxysilanes were added into the system consisting of Ag NCs, these alkoxysilanes continued solgel reaction VOL. XXX
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* Address correspondence to [email protected]. Received for review September 25, 2015 and accepted November 13, 2015. Published online 10.1021/acsnano.5b06051 C XXXX American Chemical Society
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RESULT AND DISCUSSION Figure 1 demonstrates the single-crystal Ag seeds are almost cubic with the edge length of about 65 ( 5 nm, and their UVvis extinction spectrum shows a main LSPR band with a maximum at 468 nm and two additional but relatively weak peaks at 350 and 390 nm, indicating the formation of Ag NCs with good monodispersity and sharp corners. When the solution of SHMPS and ODS was directly mixed with the dispersion of Ag NCs to produce hybrid nanoparticles, some free oSiO2 particles were observed because these small molecules alkoxysilanes easily went for solgel reaction to produce the homogeneous nucleation of oSiO2 besides heterogeneous nucleation on Ag NCs (Figure S1). Fortunately, when the SHMPS/ODS mixture was prehydrolyzed in the mixture solution of water and ethanol at 40 °C for 2 h and then reacted with the dispersion of Ag NCs, monodisperse and high-yield hybrid colloidal particles were obtained. This is probably because the prehydrolysis process promotes the chemical reaction between SHMPS and ODS molecules to produce oligomers, and the silanol groups of these oligomers can form stable emulsion droplets in aqueous phase (Figure 2).35 Figure 3 presents the obtained Ag NC-oSiO2 hybrid colloidal particles from different amounts of prehydrolyzed alkoxysilanes. In all hybrid reaction systems, nearly no isolated oSiO2 particles were observed, indicating that the oSiO2 particles mainly nucleate
heterogeneously on the Ag NC seeds. At a low amount of alkoxysilanes (550 μL, Figure 3a, 3a1, and 3a2), only one oSiO2 petal grows on the one surface of Ag NC seed, forming obvious Janus structure with the spherical oSiO2 particles of about 100 nm in mean diameter. The yield of this Janus structure is as high as 95%, as determined by statistical analysis of a transmission electron microscopy (TEM) image with over about 200 particles. When 800 μL of alkoxysilanes is used, dumbbell-like hybrid particles with two oSiO2 petals parallel connected by one Ag NC are obtained (Figure 3b, 3b1, and 3b2), and these two oSiO2 petals have nearly the same diameter of about 90 nm. As the amount of alkoxysilanes is further increased to 950 μL, three oSiO2 petals with around 80 nm in diameter grow on the three sides of one Ag NC (Figure 3c, 3c1, and 3c2). When 1050 μL of alkoxysilanes is used (Figure 3d, 3d1, and 3d2), four oSiO2 petals with around 70 nm in diameter are nearly symmetrically distributed on the four facets of one Ag NC. When alkoxysilanes are further increased to 1200 μL, five oSiO2 particles grow on one Ag NC, and the mean size is around 65 nm (Figure 3e, 3e1, and 3e2). At more alkoxysilanes, e.g., 1350 μL, six oSiO2 petals with about 60 nm in diameter are surrounding one Ag NC (Figure 3f, 3f1, and 3f2). These results clearly indicate that the morphologies of the Ag NC-oSiO2 hybrid particles can be controllably tuned by the amount of alkoxysilanes, and all oSiO2 petals grow around the Ag NCs as possible by geometrical symmetry, except for the Janus structure.
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to directly form flower-shaped Ag NC-oSiO2 hybrid colloidal particles with tunable numbers (with one, two, three, four, five, or six-) and sizes of petals. Compared to the previously reported strategies, the method we report here is rather mild and easily massproduced, and the obtained hybrid particles with tunable morphologies and sizes exhibit various surface wettability on their film surfaces. Accordingly, we believe this study could open a new pathway and possibility for the rational design and synthesis of other complex and hierarchical noble-amorphous heterostructures to create new materials.
Figure 2. (a) The solution obtained upon 1 min of mixing. (b) The emulsion obtained after the hydrolysis-emulsification process. (c) Optical image of the prehydrolyzed emulsion.
Figure 1. (a) TEM image of Ag NCs. (b) Extinction spectrum of Ag NCs. SUN ET AL.
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Figure S2 shows the relative proportions of the different nanostructures obtained from different amounts of alkoxysilanes, calculated by a statistical analysis of the TEM images based on around 200 particles for each experiment. The yields of the obtained hybrid colloidal particles are 97%, 67%, 91%, 84%, 84%, and 95% for Janus, two-, three-, four-, five-, or six-petal particles, respectively. And almost no free oSiO2 particles are observed in the TEM images. The corresponding DLS data of the obtained hybrid colloidal particles are shown in Figure S3. The polydispersity index (PDI) values of these hybrid particles are generally smaller than 0.10, indicating that the particles are relatively monodisperse without aggregation and coupling. This synthetic process is very mild and simple and only needs to control the alkoxysilane types and their SUN ET AL.
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Figure 3. TEM images of the flower-shaped Ag NC-oSiO2 particles with different numbers of petal: (a1) one, (b1) two, (c1) three, (d1) four, (e1) five, (f1) six; (a2f2) corresponding high-magnification TEM images, and (a3f3) corresponding cartoons.
prehydrolysis processes for fabrication of various hybrid morphologies. Generally, in a multiple-component system synthesized by depositing growth material on the seeds, the growth model for either homogeneous nucleation or heterogeneous nucleation is mainly dependent upon the interfacial reaction between the seeds and the other growing materials.4,13 This can be referred as the wettability/interaction between the seed and the growing material. If the growing material can wet extremely well on or has good interaction with the seed nanoparticle (the contact angle is 150°), then there is a high-energy landscape between the precursor atoms produced by the growing material and the seed. In this case, once the concentration of precursor atoms is supersaturated, homogeneous nucleation emerges to produce free pure particles. When the contact angle locates in the range of 30°150°, the growth mode depends on the reaction conditions, to form eccentric coreshell, Janus, flower-like, and other structures.33 Thus, the ability to tune the interfacial reaction is essential for the rational design and synthesis of different hybrid particles with complex nanostructures, especially for amorphous materials. When the sole SHMPS precursor was used in our reaction system as the growth material, it could heterogeneously nucleate on the Ag NCs surface even at a very low amount (Figure S4). This is because SHMPS can attach on the surfaces of Ag NCs by a strong AgS bonds,36 which can significantly improve the Ag NCoSiO2 interaction thus the “wetting” ability of oSiO2 on the Ag NCs seeds, guaranteeing the heterogeneous nucleation of oSiO2 on Ag NCs. When some relatively hydrophobic alkoxysilanes with different lengths of carbon chain, such as MPS, HDS, and ODS combined with SHMPS, were used, the oligomers derived from the hydrolysis and condensation reactions of these alkoxysilanes have different wettability on Ag NCs. When SHMPS/MPS mixture was prehydrolyzed and emulsified, eccentric coreshell structure with each Ag NC encapsulated inside one oSiO2 particle accompanied by some free oSiO2 particles were obtained. Increasing the content of alkoxysilanes only increased the size of oSiO2 rather than changing the eccentric coreshell structure (Figure S5). When different amounts of SHMPS/HDS mixture were added, Janus and dimer Ag NC-oSiO2 hybrid particles rather than flower-shaped Ag NC-oSiO2 hybrid particles were obtained (Figure S6). When SHMPS/ODS mixture was used, different morphologies including flower-shaped Ag NC-oSiO2 hybrid nanoparticles can be obtained as shown above (Figure 3). These results clearly demonstrate the feasibility of choosing different alkoxysilanes to regulate complex Ag NC-oSiO2 nanostructures.
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Figure 4. General schematic representation of nucleation and growth of oSiO2 on the surface of Ag NC.
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These structural transitions of Ag NC-oSiO2 hybrid particles can probably be explained thermodynamically by the interfacial energy analysis. In our reaction system, the total interfacial energy (Einterface) should be related to the free energies of Ag NCs (δAg), oSiO2 (δoSiO2), and the solvent (δsol) and can be expressed as follows: Einterface= σAg‑oSiO2 AAg‑oSiO2 þ σAg‑sol AAg‑sol þ σoSiO2‑sol AoSiO2‑sol (where σ is the surface energy of a given facet, A is its area), and σAg‑sol = σAg‑oSiO2 þ σoSiO2‑sol cos θ (where θ is the contact angle) from Young's equation.34 Owing to the increasing hydrophobicity of alkoxysilanes from MPS to HDS and ODS, the wettability of these oligomers derived from the solgel process of these alkoxysilanes to the waterborne PVP-stabilized Ag surfaces is becoming worse and worse, thus the contact angle θ increases. And because the σAg‑sol and σoSiO2‑sol are constants with the same H2O/ethanol solvent, thus the increase of contact angle θ will increase σAg‑oSiO2, that is σAg‑MPS < σAg‑HDS < σAg‑ODS.37 According to the Einterface equation, the contact area, AAg‑oSiO2, should be the minimum because the σAg‑oSiO2 of ODS is the highest. Consequently, the growing oligomers from the solgel reaction of SHMPS and ODS tend to form spherical domains on the surfaces of Ag NCs to minimize the Ag NC-oSiO2 interface energy to form a Janus structure rather than to expand the Ag NC-oSiO2 interface area to produce an eccentric coreshell structure. On the other hand, the prehydrolysis process of alkoxysilanes is also vital to the formation of flowershaped Ag NC-oSiO2 nanostructures. As the prehydrolysis temperature increased from 25 and 40 to 50 °C, the size of oSiO2 petals decreased. Too short prehydrolysis time, e.g., 1 h, did not avail the formation of monodisperse oSiO2 petals, while too long time, e.g., of 5 h, formed some free oSiO2 particles due to homogeneous nucleation (Figure S7). This prehydrolysis process actually produced oligomers with SiOH groups, which can, on one hand, stabilize the emulsion droplets as emulsifiers, and on the other hand, increase the combination of SHMPS and ODS. The higher the temperature, the more the hydrophilic species are, and the smaller the mean size of emulsion drops is.38 The longer the hydrolysis time, the more the oligomers are.39 Based on these experimental results and discussion, we propose a possible mechanism for the formation of the flower-shaped Ag NC-oSiO2 hybrid particles as shown in Figure 4. When the prehydrolysized alkoxysilanes were added into the system with Ag NCs, the reaction kinetics of hydrolysis and condensation of oSiO2 networks can be controlled by the amount of prehydrolysized alkoxysilanes. Without Ag cubes, very small pure oSiO2 particles were formed at a small amount of alkoxysilanes, e.g., 550 μL. With the increasing amount of alkoxysilanes to 800, 1200, and 1500 μL, more and larger pure oSiO2 particles were obtained
(Figure S8). In the presence of Ag NC seeds, when a small amount of alkoxysilanes, e.g., 300 μL, was used, although the hydrolysiscondensation rate of alkoxysilanes is slow to result in a low concentration of SH-contained oligomers, a very small oSiO2 domain was nucleated on the surface of Ag NC (Figure S9a). This indicates that the initial heterogeneous nucleation barrier of SH-contained oligomers is lower than homogeneous nucleation. These forming oSiO2 continued to grow on the existing oSiO2 domains of Ag NCs (Figure S9b-e) rather than forming new nucleation sites on the other faces of Ag NCs, because the interfacial energy of oSiO2-oSiO2 is very low due to the similar SiOSi components.13,36,40 On the other hand, the initial nucleation of oSiO2 may consume plenty of SH-contained oligomers to attach on the Ag NCs surface, the subsequent production of SHcontained oligomers is not sufficient to give new nucleation sites on the Ag surfaces. This suggests that the nucleation of oSiO2 is mainly dependent on the concentration of initial SH-contained oligomers, while the growth of oSiO2 is mainly dependent on the strength of SiOSi bonding interactions. When much higher amounts, e.g., between 550 and 1350 μL, were used, different numbers of oSiO2 petals can be constructed on the surfaces of Ag NCs. This can be explained as follows: As the amount of the prehydrolyzed alkoxysilanes increases, the reaction kinetics of formation of oSiO2 is greatly enhanced,41 and a fast reaction may build a larger number of SH-contained oligomers around a Ag NC seed, thus facilitating the initial nucleation of SH-contained oligomers with Ag NC faces to form active nucleation sites.25 Once the nucleation sites are formed, the subsequent growth of oSiO2 producing tunable flower-shaped Ag NC-oSiO2 hybrid particles with one to six oSiO2 petals occurs. Moreover, the oSiO2 petals prefer to nucleate symmetrically along the facets of Ag NCs, owing to the balance between the repulsive force of negative oSiO2 petals and an attractive force toward the Ag NC surface by the strong AgS interaction.42 Moreover, no obvious free oSiO2 nanoparticles were found in these systems (Figure 3), indicating that heterogeneous nucleation and growth of oSiO2 dominate the whole reaction system. When the amount of alkoxysilanes was further increased to 1500 μL, the oSiO2 particles would coat more than one Ag NC seed to make aggregation of Ag VOL. XXX
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CONCLUSIONS In summary, we have demonstrated a mild and feasible wet-chemical approach to controllably synthesize the flower-shaped Ag NC-oSiO2 hybrid colloidal
EXPERIMENTAL METHOD Materials. All chemicals were analytical grade and used as received. Ethylene glycol (EG, 99.5%), acetone (99.5%), ethanol (99.7%), and ammonia (28 wt %) were purchased from Sinopharm Chemical Reagent Co. (China). Polyvinylpyrrolidone (PVP, Mw = 55,000) and sodium hydrosulfide (NaHS) were
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NC-oSiO2 hybrid particles with some free irregular oSiO2 particles (Figure S9f). This is not difficult to understand: As the amount of alkoxysilanes is further increased, the hydrolysis and condensation reactions of alkoxysilanes are becoming faster, to produce supersaturated oligomers for the formation of nuclei in the medium and a continual cascading of nuclei, resulting in homogeneous nucleation to form free oSiO2 particles and the continual cascading of nuclei to make the aggregation of the particles.12 The size of oSiO2 petals on the hybrid particles can be easily tuned by the relative volume ratios of SHMPS to ODS. For example, for Janus Ag NC-oSiO2 hybrid particles, at equal amounts of prehydrolyzed alkoxysilane dispersion and SHMPS, being 450 and 210 μL, respectively, as the ODS amount increases from 50 and 70 to 95 μL, the mean diameter of oSiO2 petals increases from 60 and 80 to 150 nm, respectively (Figure 5). One more example, for six-petal Ag NC-oSiO2 hybrid particles, when the amounts of prehydrolyzed alkoxysilane dispersion and SHMPS are kept at 1350 and 210 μL, respectively, but the amount of ODS increases from 50 and 70 to 95 μL, the mean size of oSiO2 petals also gradually increases from 60 and 75 to 85 nm, as shown in Figure 6. EDS analysis also indicates that the C/S atomic ratio increases with the increase of ODS amount (Table S1). The morphologies and the sizes of oSiO2 petals have a tremendous influence on the surface wettability of the hybrid particle-based films. As the oSiO2 petal numbers of Ag NC-oSiO2 hybrid particles increases from one, two, three, four, and five to six, the WCAs of the corresponding particulate films increase from 87° to 130°, as shown in Figure 7. Moreover, as the size of oSiO2 petal increases due to more ODS amounts used, the WCAs of the corresponding particulate films increase from 130° to 156°, displaying a superhydrophobic property. This is because the more and larger oSiO2 petals benefit the formation of micronano hierarchical structures of particulate films, while the hydrophobic long CH2 groups of ODS can provide low surface energy to further improve the superhydrophobic property of particulate films.43,44
Figure 5. TEM and corresponding high-magnification TEM images of the Janus Ag NC-oSiO2 particles with different SHMPS/ODS volume ratios: (a, a1) 210:50, (b, b1) 210:70, (c, c1) 210:95.
Figure 6. TEM and corresponding high-magnification TEM images of the six-petal Ag NCs-oSiO2 particles with different SHMPS/ODS volume ratios: (a, a1) 210:50, (b, b1) 210:70, (c, c1) 210:95.
nanoparticles. The morphologies (with one, two, three, four, five, or six oSiO2 petals) and the sizes of these hybrid nanoparticles can be easily tuned by the amounts, affinity with Ag, and prehydrolysis process of alkoxysilanes. As different morphologies and petal sizes obviously influence the surface wettability of these hybrid particle-based films, a superhydrophobic film can even be directly fabricated from six-petal Ag NC-oSiO2 particulate film without any post-hydrophobization treatment. This study provides another feasibile way of fabricating hybrid particles with tunable morphologies, and the method we present here is versatile and can be readily extended for syntheses of other complex and hierarchical hybrid colloidal particles with various metals to exhibit specific properties and applications, such as the surface plasmon resonance of Ag NCs and the tunable optical films.
obtained from Sigma-Aldrich Chemical. Silver nitrate (AgNO3, 99.9%), mercaptopropyltrimethoxysilane (SHMPS, HS(CH2)3Si(OCH3)3, 95%), 3-(trimethoxysilyl)propyl methacrylate (MPS, CH3CH2CCOO(CH2)3Si(OCH3)3, 97%), hexadecyltrimethoxysilane (HDS, CH3(CH2)15Si(OCH3)3, 97%), and octadecyltrimethoxysilane (ODS, CH3(CH2)17Si(OCH3)3, 90%) were purchased from
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ARTICLE Figure 7. (a) Change of the WCAs for the Ag NC-oSiO2 hybrid particulate films as a function of different numbers of petals. (bd) SEM images and WCAs for the six-petal AgNCs-oSiO2 particulate films as a function of different ODS contents: 50, 70, and 95 μL. Aladdin Chemical Reagent Co. Deionized water was used in all experiments. Synthesis of Ag NCs. Ag NCs were synthesized using the sulfide-mediated polyol process.45 Typically, 180 mL of EG was added into a 250 mL three-neck round-bottom flask and heated in an oil bath preset at 150 °C under magnetic stirring. After stirring under uncapped conditions for 1 h, the flask was capped, degassed by a flow of argon for 15 min, and then quickly injected by 2.1 mL of NaHS (3 mM in EG), followed by injection of 45 mL of PVP (20 mg/mL in EG). When the temperature was maintained at 150 °C, 15 mL of AgNO3 (48 mg/mL in EG) was quickly added into the above solution which was allowed to proceed for 25 min. The as-synthesized Ag NCs in EG were cooled to room temperature in an ice bath and then stored in the refrigerator for subsequent uses. Synthesis of Flower-Shaped Ag NCs-oSiO2 Hybrid Particles. The flower-shaped Ag NCs-oSiO2 hybrid particles were synthesized from the prehydrolyzed alkoxysilanes as follows: In a typical synthesis, the Ag NCs were isolated by precipitating 2.0 g of the assynthesized Ag NCs-EG solution with an addition of 6.0 g acetone, followed by centrifugation at 15000 rpm for 5 min, and then redispersed into 20 mL of H2O and 10 mL of ethanol to form the mixture solution A. After ultrasonication for 15 min, the well-dispersed solution A was stirred in a water bath at room temperature for 30 min and then heated to 60 °C within 30 min, followed by addition of 0.50 mL ammonia (28 wt %). After 30 s, 550 μL of dispersion of the prehydrolyzed alkoxysilanes B was rapidly injected into the above mixture and reacted for another 3 h. The as-obtained Ag NC-oSiO2 hybrid particles were centrifuged and washed four times with ethanol and then redispersed in 2 mL of ethanol. The dispersion B was prepared in advance by adding 210 μL SHMPS and 50 μL ODS into the mixture of 6 mL water and 3 mL ethanol to prehydrolyze under vigorous stirring at 40 °C for 2 h. The numbers and sizes of organosilica petals could be increased by increasing the concentration of prehydrolyzed alkoxysilanes, as shown Table S2. By using the same procedure as above with various volume ratios of SHMPS to ODS or SHMPS/MPS and SHMPS/
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HDS, the various Ag NC-oSiO2 hybrid colloidal particles were obtained. Fabrication of Ag NC-oSiO2 Hybrid Particle-Based Films. The flowershaped Ag NC-oSiO2 hybrid colloidal particles were ultrasonically dispersed in ethanol for a few minutes to ensure uniform dispersion and then cast on the precleaned silicon wafer by spin-coating. This procedure could be repeated until a thin particulate film was obtained. After drying in an oven at 60 °C for 2 h, the particulate film was achieved. Characterization. TEM images were taken on a transmission electron microscope operating at 200 kV. The samples were dispersed in ethanol and then dried on a holey carbon film Cu grid. UVvis absorption spectra of Ag NC (background solution: 99.7% ethanol) were recorded at room temperature on the U-4100 spectrophotometer (Hitachi, Japan) using quartz cuvettes with an optical path of 1 cm. Dynamic light scattering (DLS) measurements were carried out on the diluted ethanol solutions to produce particle sizes and size distribution using a Nano-ZS90 (Malvern). Scanning electron microscopy (SEM) measurements were performed using a Philips XL 30 emission microscope at an accelerating voltage of 30 kV. Elemental analysis was performed by energy-dispersive X-ray spectroscopies (EDX) conducted on a FEI JEOL JSM-6700 field-emission scanning electron microscope. The surface wettability of particulate film was characterized by the water contact angle (WCA) by OCA15 contact angle analyzer (Dataphysics, Germany) averaged over 3 fresh spots using 3 μL deionized water. Conflict of Interest: The authors declare no competing financial interest. Acknowledgment. Financial supports of this research from the National Natural Science Foundation of China (51322307, 51273218, 51133001 and 21374018), National “863” Foundation (2013AA031801), Science and Technology Foundation of Ministry of Education of China (20110071130002), Science and Technology Foundation of Shanghai (13JC1407800), and “Shu Guang” project supported and Shanghai Rising-Star Program (14QA1403300) are appreciated.
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Additional Tables, TEM images and EDX analysis (PDF)
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