Abstract
We have used a thiol-functionalized polyhedral oligomeric silsesquioxane (SH-POSS) as a protective group for the preparation of POSS-protected gold nanoparticles (POSS–Au NPs).
The organic/inorganic hybrid SH-POSS NPs exhibited an interesting platelike morphology arising from steric hindrance between the isobutyl groups of SH-POSS. An XRD study of the SH-POSS crystal revealed the relatively large interstice (1.64-nm-lattice constant a >
1.3-nm-diameter SH-POSS) on the basal plane of the unit cell, which resulted in a platelike morphology having lateral dimensions on the order of a few micrometers and thicknesses of a few hundred nanometers. In addition to behaving as a stabilizer for the preparation of Au NPs, an excess of SH-POSS colloids led to the formation of a crystalline template that incorporated the POSS–Au hybrid NPs on its surface, providing a unique fernlike microstructure. After removal of the POSS template (through sublimation and decomposition to a silica char) at 350 °C in air for 1 h, nanosized Au islands having diameters of 50–100 nm and thicknesses of 2–20 nm were sintered onto the substrate. As a result, SH-POSS is an excellent protective group for the preparation of Au NPs of high stability in the powder state.
In addition, we suspect that SH-POSS crystals could be used to disperse Au NPs onto substrates for a wide range of applications (e.g., Au catalysts).
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4-1 Introduction
Self-assembly of nanoparticles (NPs)—i.e., the controlled organization of NPs into ordered or hierarchical structures—allows coupling of their size- and shape-dependent properties to obtain potentially useful materials for optoelectronics, sensing and imaging, and biomedical applications.1–4 A broad range of targeted self-assembled structures can be produced by organizing NPs exhibiting compositional heterogeneity;5–8 they can be realized either by synthesizing NPs from several different materials or by selectively attaching organic molecules to different sites of the NPs.9–11 Compositional heterogeneity makes NPs conceptually similar to amphiphilic molecules (e.g., surfactants or block copolymers) and allows the thermodynamic approach of self-assembly to be used to form “colloidal molecules” in energetically favorable structures possessing unique properties.12–22 Ordered monolayers of NPs over large surface areas are sensitive to the presence of defects on the substrate’s surface, which can destroy the assembly. In contrast, large ordered arrays of NPs can be obtained through judicious choice of the types of chemical interactions between the particles and the substrate.23,24 In this paper, we report the preparation of ordered fernlike 3D microstructures of assembled POSS–Au hybrid NPs on to a large-scale crystalline POSS template. Alkyl-functionalized polyhedral oligomeric silsesquioxane (alkyl POSS) derivatives are well-known organic/inorganic hybrid NPs comprising 0.5-nm-diameter siloxane cages and eight alkyl chains.25 Thus, alkyl POSS molecules can be regarded as colloids (a dispersed phase and a dispersion medium) in solution. Interestingly, platelike morphologies of POSS crystals, with lateral dimensions on the order of a few micrometers and thicknesses of a few hundred nanometers, have been described several times previously.26–29 Therefore, in this study we employed a thiol-functionalized POSS as (i) a protective group to stabilize Au NPs in solution and (ii) a novel template for the incorporation of Au NPs into thin films.
Thiol compounds are often used to form monolayers on the surfaces of Au NPs via the formation of dynamic S–Au covalent bonds.30 The concentration of the Au NPs is usually
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maintained at less than 5 wt% to prevent irreversible aggregation. Thus, the self-assembly of Au NPs in condensed phases is difficult because of the strong tendency to form a concentrated droplet, rather than a large-scale thin film, during evaporation of the solvent.
Schmid et al. used 3-mercaptopropylcyclopentyl-POSS to quantitatively exchange the PPh3
ligands in (PPh3)12Au55Cl6 in an attempt to obtain POSS–Au NPs stabilized through Au–S bonds; instead, they obtained amorphous structures.31 Naka et al.32,33 and Rotello et al.34,35 have prepared POSS–Au hybrid NPs that were stabilized through electrostatic interactions with the HCl salts of octa(3-aminopropyl)octasilsesquioxane or through hydrogen bonding recognition processes with diaminopyridine-monofunctionalized octasilsesquioxane. Such strong intermolecular interactions (i.e., electrostatic and hydrogen bonding interactions) would suppress the thin-film character of POSS during crystallization. In an attempt to exploit the “crystalline template” of alkyl POSS colloids to self-assemble Au NPs, we turned our attention toward the use of a thiol-monofunctionalized 3-mercaptopropylisobutyl-POSS (herein denoted “SH-POSS”; Figure 4-1a). We synthesized SH-POSS-protected Au NPs (POSS–Au) using the method developed by Brust et al., in which HAuCl4 was transferred into the toluene phase with tetraoctylammonium bromide (TOAB) and then reduced with sodium borohydride (NaBH4) in the presence of SH-POSS.36,37 Thus, interesting large-scale POSS–Au hybrid microstructures36,38—featuring 2~4-nm-diameter Au NPs surrounded by 1.3-nm-diameter POSS colloids—could be constructed from a crystalline POSS template (an excess of SH-POSS).33–35 In addition, at temperatures above 250 °C, most alkyl POSS derivatives can be sublimed, with the residual alkyl POSS (ca. 7.5 wt%) decomposing and oxidizing into a silica char (a network of POSS cages).39,40 Therefore, we obtained Au-island films through subsequent sintering of well-dispersed Au NPs on the large fernlike microstructures in air at 350 °C.
In this study, we first investigated the characteristic of SH-POSS crystals using power X-ray diffraction analysis; we found that steric hindrance of the alkyl groups on the POSS
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cages dominated the formation of fernlike or dendritic crystals and provided relatively large interstices on the crystal surface, similar to those observed for NH4Cl, NH4Br, and CsCl crystals because of large interstices resulting from ionic repulsion between halide anions.41 We suspected that it would be reasonable to self-assemble POSS-surrounded Au NPs onto the relatively large interstices on the surface of excess SH-POSS crystals because of the strong tendency for POSS units to aggregate. This novel template—the SH-POSS crystal—can form on several substrates, including water, silicon wafers, and glass slides. Because of its easy and reproducible sample preparation process, the fernlike microstructures of the POSS–Au hybrids can be analyzed using many microscopic techniques, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical microscopy (OM).
4-2 Experimental Section 4-2.1 Materials.
3-Mercaptopropyl isobutyl-POSS (SH-POSS) was purchased from Hybrid Plastics, Inc.
1-Dodecanethiol (SH-C12, >98%, Sigma–Aldrich), tetraoctylammonium bromide (TOAB, 98%, ACROS), hydrogen tetrachloroauate(III) trihydrate (HAuCl4, ACS grade, ACROS), sodium borohydride (NaBH4, >96%, Fluka), and HPLC-grade solvents were used as received.
4-2.2 Preparation of Au Ps.
The molar feeding ratio (n/m) of HAuCl4 to thiol compounds was the only controlling factor under the reaction conditions. SH-POSS and SH-C12 were selected to compare the effects of the protecting groups on the self-assembly of the Au NPs. A solution of HAuCl4
(0.12 g) in water (100 mL, 6.03 mmol/L) was mixed with a solution of TOAB in toluene (6.03 mmol/L, 100 mL). The two-phase mixture was stirred vigorously until all the AuCl4–
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ions were transferred into the organic layer; SH-POSS or SH-C12 was then added to the organic phase. Freshly prepared aqueous NaBH4 (301.5 mmol/L, 100 mL) was slowly added with vigorous stirring. After further stirring for 3 h, the organic phase was separated, the toluene was evaporated using a rotary evaporator, and the residue was mixed with ethanol (400 mL) to remove excess TOAB. The mixture was maintained at –18 °C for 4 h and then the dark brown precipitate was filtered off and washed twice with ethanol. The Au NPs of C12–Au (n/m = 1) and POSS–Au1 (n/m = 1) were obtained in yields of 53.75 and 36.31 wt%.
4-2.3 Analytical Procedures.
For the TEM images, three drops of a toluene solution (40.9 µL; 10 mg/mL of POSS–Au1; 50 mg/mL of SH-POSS) were placed onto a water surface having a diameter of 5 cm. After air-drying at 25 °C for 30 min, the aggregates were transferred to a carbon-coated Cu TEM grid. For the AFM, SEM, and OM analyses, one drop of dilute toluene solutions (13.7µL, 10 mg/mL of POSS–Au1 and 50 mg/mL of SH-POSS) was placed onto a wafer and then they were air-dried at 25 °C.
4-2.4 Measurements.
A TA Instruments thermogravimetric analyzer (TGA), operated at a scan rate of 20 °C over temperatures ranging from 30 to 800 °C under a nitrogen purge of 40 mL/min, was used to record TGA thermograms of samples on a platinum holder. A Hitachi H-7500 transmission electron microscope (100 kV) was used to record TEM images of dilute solutions and aggregated POSS-Au1, SH-POSS, and C12-Au. The XRD patterns were collected using a D8 Advance powder X-ray diffractometer (Cu K, 40 kV/40 mA; Bruker, Germany).
Transmission electron microscopy (TEM) with electron diffraction analysis and energy dispersive X-ray spectra are performed by Philips Tecnai G2 F20 at 200 kV. The atomic force
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microscopy (AFM) employed in this study is a Digital Instruments Veeco Dimension 5000 Scanning Probe Microscope (Veeco Metrology Group). The AFM tapping mode with a 5-10 nm radius silicon tip was used to scan the cranial suture and cranial bone surface. The displacement resolution of AFM is about 0.1 nm.
4-3 Results and Discussion 4-3.1 POSS Crystals on Au Ps
Although it is difficult to evaluate the exact size of an alkyl POSS derivative because of the flexibility of the alkyl chain, the total diagonal length of alkyl POSS can be considered as a measure of its size. SH-POSS can be regarded as a 1.3-nm-diameter organic/inorganic hybrid colloid (dSH-POSS) because it possesses 0.4-nm-long isobutyl chains on its 0.53-nm-diameter inorganic siloxane cage.21,32,42 Once the critical particle concentration is reached, a sharp transition from dispersed colloids to a condensed crystalline-like behavior is observed. The transition from the colloid liquid (dispersed state) to the colloidal crystalline phase (condensed state) is first order in nature, similar to the phase transitions from liquid to solid observed in molecular systems.43 Thus, the crystallography of the condensed POSS state can be also studied through X-ray diffraction analyses (XRD). Figure 4-1b reveals that the powder XRD (wavelength: 1.5418 Å) patterns of the SH-POSS crystals displayed four major diffraction angles (2θ) and d-spacing distances (dhkl)—8.03° (11.00 Å), 10.78° (8.20 Å), 11.92° (7.42 Å), and 18.73° (4.69 Å)—that corresponded to the (hkl) diffraction planes (101), (110), (102), and (113) for a hexagonal POSS crystal with the following lattice parameters: γ
= 120°; α = β = 90°. According to the hexagonal unit cell, the lattice parameters (a = b = 16.4 Å; c = 17.4 Å) were calculated using Eq. (4-1) and Table 4-1. When the crystalline model of the 1.3-nm-diameter SH-POSS spheres was made (Figure 4-1b), we observed relatively large interstices (lattice parameter a of 16.4 Å > SH-POSS diameter of 1.3 nm) on the basal plane of a SH-POSS unit cell. For hexagonal close packing (HCP) of hard spheres having a
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diameter ds, Eq. (4-2) suggests that the z-pitch distance (dz) of the ABA repeating layers in the axial direction is 0.816×ds. Herein, the lattice parameters a and c for HCP are equal to the diameter of the hard spheres (ds) and twice the z-pitch distance (2dz) for the two repeating layers, respectively. Thus, the dimensional ratio c/a for HCP is calculated to be 1.63 in Figure 4-2a.
Figure 4-1. (a) Chemical structure and 3D model of SH-POSS; (b) WAXS spectrum and cartoon representation of the molecular packing in a SH-POSS crystal.
Table 4-1. Crystal parameters of SH-POSS powders
line 2θ dhkl plane (hkl) a C
(o) (Å) hk l 4/3(h2+hk+k2) l2 (Å) (Å)
1 8.03 11.00 101 1.33 1 17.387
2 10.78 8.20 110 4.00 0 16.394 ---
3 11.92 7.42 102 1.33 4 17.392
4 18.73 4.73 113 4.00 9 17.385
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Figure 4-2. Calculated layer-to-layer thicknesses dz for (a) ABA two-repeating HCP system and (b) SH-POSS system for only z-directional close packing.
Assuming non-overlapping packing in the axial direction (z direction), Eq. (4-3) suggests that the theoretical z-pitch distance (dz*) of an SH-POSS crystal is 8.91 Å, a value larger than the experimental (Figure 4-1b and 4-2b) z-pitch distance (dz,SHPOSS) of 8.7 Å (i.e., lattice parameter c/2). Unlike the HCP of hard spheres, we speculate that the substrate-supported SH-POSS colloids can approach one another to pack into the basal plane with a relatively large interstice of a crystal. Because of repulsion between the sterically demanding isobutyl groups on the POSS cages, the intermolecular distance is slightly larger than the diameter of the SH-POSS colloid. The free SH-POSS colloid in solution can then
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pack onto the surface interstices of the first layer. For non-supported free SH-POSS colloids, the soft organic shell of SH-POSS colloids can overlap slightly with the low-level SH-POSS colloids, forming a three-dimensional crystal. Thus, the dimension c/a of SH-POSS crystal is calculated to be 1.06 for platelike unit cells (i.e., less than the value of 1.63 for the HCP of hard spheres). In addition, the difference in the intermolecular distances between the basal plane and the axial direction results in major crystal growth in two approximately orthogonal directions parallel to the substrate, leading to the ordered aggregation of platelike unit cells into fernlike or dendritic crystals having lateral dimensions on the order of a few micrometers and thicknesses of a few hundred nanometers.26–29
Steric interactions of the alkyl chains on the POSS cages appear to dominate the low dimensional ratio c/a (1.06 < 1.63) of the unit cell of a SH-POSS crystal. For bulky molecules without alkyl chains, C6044,45
molecules and octahydro-POSS [(HSiO1.5)8, 424.74 g/mol],46 possess lattice dimensional ratios (c/a) of 1.62 and 1.68, respectively, in their hexagonal unit cells—values quite close to the idea dimensional ratio (1.63) expected for the HCP of hard spheres. Fernlike and dendritic crystals of inorganic salts, such as NH4Cl, NH4Br, and CsCl, are commonly observed.41 In these cases, repulsion of hydrated halide anions induces loose packing on the basal plane of a unit cell, allowing cations to insert into the kink sites (interstices) after dehydration. Thus, we infer that the formation of fernlike or dendritic SH-POSS crystals was dominated by steric hindrance between the solvated isobutyl chains on the POSS cages.
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4-3.2 Character of POSS–Au Hybrids
Because bulky SH-POSS colloids can pack loosely into crystals on various substrates, we expected them to be excellent protective agents for the dispersion of Au NPs after becoming anchored onto their surfaces through Au–S bonds. We synthesized the SH-POSS-protected Au NPs (POSS–Au) through the reduction of HAuCl4 in the presence of SH-POSS. Because HAuCl4 is water-soluble, a phase-transfer agent (TOAB) was required to transfer it into the toluene phase. TOAB also transfers the reducing agent (NaBH4) into toluene. Using this approach, we obtained POSS–Au1 and POSS–Au2 at HAuCl4-to-SH-POSS feeding molar ratios (n:m) of 1:1 and 1:0.5, respectively.
1-Dodecanethiol (C12-SH)-protected Au NPs (C12-Au; n:m = 1:1) were also prepared as a control (Table 4-2). We used many techniques to analyze our POSS–Au and C12-Au hybrid NPs—OM, SEM, TEM, AFM, Fourier transform infrared (FTIR) spectroscopy, ESCA, small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), dynamic light scattering (DLS), and thermogravimetric analysis (TGA). Because Au atoms scatter electrons efficiently as a result of their high mass number, the sizes of the Au cores are easily measured using TEM (i.e., high the contrast between the Au NPs and thiol compounds). To disperse the POSS–Au or C12–Au NPs, we prepared dilute samples for TEM imaging by (i) placing a drop of dilute solution (1 mg/mL) onto a carbon-coated copper grid, (ii) blotting away the excess solution using a strip of filter paper, and (ii) air-drying at 25 °C for 3 min. Thus, through statistical analysis using the Gatan image process, we obtained Au core sizes for C12–Au, POSS–Au1, and POSS–Au2 of 2.0 ± 0.7, 1.8 ± 0.9, and 2.8 ± 0.9 nm (Figure 4-3), respectively. These values suggest that SH-POSS plays the same role as that of the well-known C12-SH for the dispersion of Au NPs in toluene.31
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Table 4-2. Compositions and element analysis results of thiol-protected gold nanoparticles.
Entry
State Yielda
Thiol
n/mc
Input
Output
(25 °C) (%) TGAdICP-MSeSEM-EDX XPS
C12-Au Liquid 53.8 1-Dodecanthiol 1 2.61 --- --- --- POSS-Au1 Solid 36.3 SH-POSS 1 2.76 2.42 3.25 2.47 POSS-Au2 Liquid N.A.b SH-POSS 2 --- 6.02 6.75 ---
[a] determined by weights [b] a mixture of gold nanoparticles with tetraoctylammonium bromide [c] feeding molar ratio (n/m) of gold salts to thiol compounds [d] determined by the char yield at 700 °C [e] determined by atomic fraction of gold and silicon after digestion.
Figure 4-3. (a) TEM images (×200k, top), (b) schematic particle distributions, and (c) probability size distributions of dilute C12-Au, POSS-Au1, and POSS-Au2.
In addition to the dispersed state, we also studied the condensed states of POSS–Au and C12–Au.43 After evaporation of the toluene, we extracted TOAB and excess NaBH4 into
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methanol to obtain a pure powder for POSS–Au1 and a sticky material for C12–Au. Based on their char yields at 700 °C, TGA revealed n/m ratios for POSS–Au1 and C12-Au of 2.76 and 2.61 (Figure 4-4), respectively. Thus, in the preparation of Au NPs using the water/toluene two-phase method,36,37 the loss of SH-POSS and C12-SH resulted in higher experimental n/m ratios for both POSS–Au1 and C12–Au. With its low content of SH-POSS, we could not purify POSS–Au2 because the amorphous SH-POSS units became redispersed in methanol.
Theoretically, if most SH-POSS colloids are bound onto the spherical surfaces of Au NPs, crystallization of SH-POSS colloids would be suppressed, as was the case for POSS–Au2 (see the FTIR in Figure 4-5). In contrast, POSS–Au1 was a powderlike crystal (see the inserted graphs in Figure 4-6b). Thus, we needed to determine the packing mode of the SH-POSS colloids on the spherical surface of Au NPs (Figure 4-6a). The number of SH-POSS colloids absorbed on the surface of each Au NP can be calculated in terms of the n/m ratio, based on the bulk Au density of 19.3 g/cm3 and the packing mode of a SH-POSS crystal. Because of the large interstices on the basal plane of a SH-POSS unit cell, we supposed that the SH-POSS colloids would pack into a bilayer shell on the surfaces of the Au cores, thereby reducing thermodynamically disfavored contact between the Au cores and toluene (Figure 4-6a). The SH-POSS colloids in the first layer are chemically absorbed onto the Au cores through Au–S bonds; in contrast, those in the second layer can be classified as physically absorbed through POSS–POSS recognition.35 As a result, the theoretical n/m ratios of POSS–Au1 were 14.48 and 4.24 for the monolayer and bilayer, respectively, of 1.3-nm-diameter SH-POSS spheres surrounding 1.84-nm-diameter Au cores. To determine the mode of assembly of the SH-POSS bilayer on the Au core, we performed an SAXS study on the POSS–Au1 powder (Figure 4-6b). The broad band at a scattering vector (q) of 1.41 nm–1 corresponds to a spacing distance of 4.45 nm, indicating the center-to-center distance between two Au cores (Figure 4-6a). Thus, the surface-to-surface distance of 2.60 nm can be assigned to the aggregation of the protective POSS bilayer encapsulating 1.84-nm-diameter
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Au NPs.19 In addition, DLS analysis of POSS–Au1 revealed an average size (Dh) of 7.9 nm, slightly larger than the predicted size (6.23 nm) because of reversible absorption of toluene-solvated SH-POSS colloids onto the surfaces of the Au NPs (Figure 4-7). We also performed WAXS (Figure 4-6b) analyses to ensure that the feeding of SH-POSS was more than that required for the POSS-bilayer-protected Au NPs to form a “pure” SH-POSS crystal.
In addition to the four sharp diffraction bands for the hexagonal POSS crystal, the WAXS spectrum of POSS–Au1 displayed another four broad diffraction angles (2θ) and d-spacing distances (dhkl)—38.60° (2.33 Å), 44.43° (2.04 Å), 65.10° (1.43 Å), and 78.01° (1.22 Å)—corresponding to the (hkl) diffraction planes (111), (200), (220), and (311) for a face-centered cubic (FCC) lattice Au phase.47 Thus, the excess SH-POSS colloids play an important role in the aggregation of the POSS–Au hybrid NPs. Moreover, the powder POSS–Au1 containing bulky POSS exhibited higher stability for long-term storage than did C12–Au because the dynamic nature of the Au–S bonds led to local aggregation through exchange of thiol-functional ligands (see the aggregation in Figures 4-7 and 4-8). The repulsion interaction between POSS colloids can prevent the exchange of Au-S bonds to stabilize Au NPs.
Figure 4-4. TGA thermograms of (a) SH-C12, (b) C12-Au, (c) SH-POSS, and (d) POSS-Au1 in N2 and (e) SH-POSS air and (f) POSS-Au1 air in air.
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Figure 4-5. FTIR spectra of (a) TOAB, (b) SH-C12, (c) C12-Au, (d) SH-POSS, (e) POSS-Au1, and (f) POSS-Au2.
Figure 4-6. (a) cartoon representation of the molecular packing in the SH-POSS bilayer-protected POSS–Au1, with the center-to-center distance between two Au cores highlighted; (b) SAXS and WAXS spectra of POSS–Au1 powders. The insert photograph is a crystal powder of POSS–Au1.
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Figure 4-7. DLS analyses of (a) C12-Au, (b) POSS-Au1, and (c) SH-POSS.
Figure 4-8. TEM images of C12-Au aggregations.
4-3.3 TEM and AFM Analyses of POSS–Au Micro- and anostructures
A spherical Au core with the diameter of 1.84 nm is composed of an average of 192.4 Au atoms based on the Au density of 19.3 g/cm3 (POSS-Au1 in Figure 4-9). For the first layer, the centers of the 1.3-nm-diameter SH-POSS units are positioned on the spherical shell of 3.14 nm diameter (i.e., 1.84 + 1.3) occupying an area of 30.97 nm2 to incorporate a
A spherical Au core with the diameter of 1.84 nm is composed of an average of 192.4 Au atoms based on the Au density of 19.3 g/cm3 (POSS-Au1 in Figure 4-9). For the first layer, the centers of the 1.3-nm-diameter SH-POSS units are positioned on the spherical shell of 3.14 nm diameter (i.e., 1.84 + 1.3) occupying an area of 30.97 nm2 to incorporate a