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Tubular Hybrids: A Nanoparticle – Molecular Network Priyadarshi Ranjan, Sreejith Shankar, Ronit Popovitz-Biro, Sidney R Cohen, Iddo Pinkas, Reshef Tenne, Michal Lahav, and Milko E. van der Boom Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03125 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018
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Langmuir
Tubular Hybrids: A Nanoparticle – Molecular Network Priyadarshi Ranjan†,‡, Sreejith Shankar†,¶, Ronit Popovitz-Biro§, Sidney R. Cohen§, Iddo Pinkas§, Reshef Tenne‡.*, Michal Lahav†, Milko E. van der Boom†,* †
Department of Organic Chemistry, The Weizmann Institute of Science, 7610001 Rehovot, Israel Department of Materials and Interfaces, The Weizmann Institute of Science, 7610001 Rehovot, Israel § Department of Chemical Research Support, The Weizmann Institute of Science, Rehovot 7610001, Israel ‡
ABSTRACT: We report here a new methodology for the formation of free-standing nanotubes composed of individual gold nanoparticles crosslinked by coordination complexes or porphyrin molecules using WS2 nanotubes (INT-WS2) as a template. Our method consists of three steps: (i) the coverage of these robust inorganic materials with monodispersed and dense monolayers of gold nanoparticles, (ii) formation of a molecular-AuNP network by exposing these decorated tubes to solutions containing a ruthenium polypyridyl complex or meso-tetra(4-pyridyl)porphyrin, (iii) removal of the INT-WS2 template with a hydrogen peroxide (H2O2) solution. Nanoindentation with atomic force microscopy (AFM) of the template-free AuNP-tubes indicate a radial elastic modulus of 4 GPa. The template-free AuNP-molecular tubes are characterized using scanning and transmission electron microscopy (SEM and TEM, respectively), energy-dispersive X-ray spectroscopy, and micro-Raman spectroscopy. The methodology provides a convenient and scalable strategy for the realization of molecular-AuNP tubes with defined length and diameter depending on the dimensions of the template.
INTRODUCTION The design and formation of organized and well-defined 1D and 2D structures based on individual plasmonic metallic particles and functional organic molecules or metal complexes is a great challenge. Materials based on metallic particles have been designed using bottom-up and top-down approaches. However, relatively few hollow structures composed of NPs (i.e., spheres, tubes) have been reported.1,2 Rubinstein and coworkers reported an example of the templated synthesis of metallic nanoparticle nanotubes.3,4 Their multiwalled nanotubes consist of fused nanoparticles generated using an outer template of aluminum oxide having nanopores functionalized with an aminosilane to allow binding of citrate-capped AuNPs.
Removal of the template under basic conditions resulted in elongated nanostructures. In another example, microporous organic nanotubes were used as templates for the formation of particulate Fe2O3–based nanotubes.5,6 The literature reports on diverse nanotubular structures, including tubes made out of biomaterials,7-9 and coordination polymers.10-12 However, to the best of our knowledge, tubular architectures consisting of metallic nanoparticles stitched together via molecular bridging moieties are unknown. In this paper, we report on a method for the surface functionalization of intrinsically inert inorganic nanotubes (INTs)13-18 composed transition-metal chalcogenides (WS2). We show here how the chemically inert surfaces of these
Scheme 1. Schematic rendering of the structures of the molecular cross-linkers (1,2), the decoration of the INT-WS2 with AuNPs (AuNP/INT-WS2) crosslinking of the surface-bound AuNPs, and removal of the inorganic template (INT-WS2) to generate free-standing RuAuNP/NT or PorAuNP/NT. The average interparticle spacing is similar to the dimensions of the crosslinkers (1,2).
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inorganic materials can be homogeneously coated with uniform monolayers of AuNPs by interfacial lattice matching involving the individual components. The synthetic procedures available for the functionalization of such inorganic materials with NPs are often surface defect guided.19 Furthermore, we demonstrate here the formation of molecular-AuNP nanotubes using INTs as an inner template. These single-walled hybrids consist of continuous networks of AuNPs and ruthenium polypyridyl complexes (1) or porphyrin molecules (2). The advantages of chosen cross-linkers include (i) high extinction coefficients (which facilitates the use of optical spectroscopy to follow the process), (ii) stability under the experimental conditions, (iii) strong and stable bonding between the NPs and between NP and INT-WS2, (iv) ease of synthesis/availability, (v) well-studied coordinative interactions with AuNPs, and (vi) Either easy to synthesize (1) or commercially available (2). These INT-WS2 are first decorated with a single layer of AuNPs and subsequently cross-linked with the bridging molecular component. Although these transition metal dichalcogenide materials are robust, they react with hydrogen peroxide (H2O2). Reaction of our assemblies with H2O2 results in oxidative removal of the inorganic template, generating freestanding molecular-metallic NP nanotubes (Scheme 1). The use of sulfur-terminated molecules was avoided since such compounds may undergo unwanted oxidative side reactions during the removal of the inorganic templates. The strategy described here not only provides a unique and unprecedented methodology to surface functionalize the otherwise inert INT-WS2, but also allows the generation of hollow tubes of molecularly crosslinked, individual AuNPs via a straightforward and efficient 3step protocol.
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AuNP/INT-WS2 preparation was carried out in a straightforward procedure, by sonication of suspension of INTWS2 in toluene with the preformed AuNPs (∅AuNP = 5 nm). The use of ice-cold solutions resulted in optimal coating of the nanotubes with AuNPs. The process is near quantitative in yield with respect to the amount of INT-WS2 and AuNPs used for the reaction. All solutions and suspensions were cooled in ice before adding or mixing. These AuNPINT-WS2 hybrid structures were characterized by scanning and transmission electron microscopy (SEM and TEM) revealing monodispersed and uniform coatings of a single layer of NPs (Figures 2A left and middle, 2B, 3A). The average interparticle spacing is 1.6–2 nm.
RESULTS AND DISCUSSION To demonstrate the feasibility of the oxidative removal of the INT-WS2 template, we initially carried out a blank reaction between pristine INT-WS2 and a 15% hydrogen peroxide (H2O2) solution at room temperature. TEM images showing the progress of the reaction after 1 h are shown in Figure 1A-C. It appears that the ends of the nanotubes are more susceptible to oxidation. Photographs confirm the complete disappearance of the black tubes after 15 h (Figure 1D). Likewise, timedependent UV/Vis spectroscopy revealed the gradual disappearance of the characteristic bands of the pristine INTWS2 over 15 h (Figure 1E).
Figure 2. TEM and SEM images. (A) left and middle: INT-WS2 decorated with AuNPs, and right: RuAuNP/INT-WS2. (B) Left: HRTEM image of AuNP/INT-WS2; middle: HRTEM image of the area marked in yellow on the left image at the interface of the AuNPs and INT-WS2. The white lines indicate the lattice continuation between AuNPs (111) and INT-WS2 (013). Right: image displaying the corresponding Fast Fourier Transform (FFT) showing lattice matching (circle). ∅AuNP = 5 nm. (C) RuAuNP/INTWS2 with the INT-WS2 template being partially etched with H2O2, (D) Free standing RuAuNP/NT and porAuNP/NT. For clarity only the two images of porAuNP/NT have labels.
Figure 1: (A-C) TEM images showing pristine INT-WS2 reacting with aqueous H2O2 (15%) after 1 h. (D) Photographs of the reaction solution showing the pristine INT-WS2 at t = 0 (left) and the complete disappearance of the black tubes after 15 h (right). (E) UV/Vis spectra of the reaction mixture recorded at t = 0 (black), t = 5 h (red), t = 10 h (blue), t = 15 h (green).
The first step of our method to prepare the tubular hybrids is the decoration of the surface of INT-WS2 with tetraoctylammonium bromide (TOAB)-capped AuNPs (Scheme 1). The
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Langmuir Freshly prepared AuNP/INT-WS2 were used for the second step. The reaction of a dispersion of AuNP/INT-WS2 with [Ru(mbpy-py)3][PF6]2 (1) in DMF/toluene (1:1; v/v) under icecold conditions resulted in formation of RuAuNP/INT-WS2. The isostructural osmium complex [Os(mbpy-py)3][PF6]2 is known to induce aggregation of AuNPs because of electrostatic interactions and 1-coordination of the vinylpyridine moieties to the gold surfaces.20
The removal of the INT-WS2 template of the AuNPdecorated tubes to obtain the free-standing tubular hybrids was carried out using H2O2. RuAuNP/INT-WS2 and porAuNP/INT-WS2 were dispersed in an aqueous H2O2 solution (15%) and kept at 4C for 24 h. The low temperature ensured the stability of the coordinative binding of AuNPs and the molecular components 1 or 2, thereby only the inorganic template reacted with H2O2. This treatment resulted in the formation of freestanding nanotubes comprised of AuNPs held together by the molecular components (RuAuNP/NT and porAuNP/NT). The INT-WS2 template is oxidized and removed by H2O2 from these tubular single-layered structures as confirmed by the disappearance of the features characteristic of the WS2 vdW layer spacing (Figures 2D, 3C). Energydispersive X-ray spectroscopy (EDS) of the as-synthesized RuAuNP/NTs shows a high gold atomic fraction of 87 at%, while only minor amounts of tungsten (6 at%) and sulfur (7 at%) remained (Figure S2). The interparticle distances (~2.0 nm) are similar to the distance between the N-pyridine moieties of [Ru(mbpy-py)3][PF6]2 (1; 2.3 nm) and of meso-tetra(4pyridyl)porphine (2; 1.5 nm). The shape, size and crystallinity of the AuNPs is not affected by our etching procedure. However, impact of the etching process on the packing of the NPs cannot be rigorously excluded. The molecular component is essential. The removal of the INT-WS2 template from AuNP/INT-WS2 (where AuNPs are not cross-linked by the molecules) did not result in tubular hybrids, confirming that the molecular component is essential. The reaction process between RuAuNP/INT-WS2 or porAuNP/INT-WS2 and H2O2 was monitored by ex-situ Raman spectroscopy showing the gradual disappearance of the characteristic bands at = 353 cm-1 (E2g) and = 421 cm-1 (A1g) of the INT-WS2 (Figures 4; S3).
Figure 3. TEM images of (A) AuNPINT-WS2, (B) RuAuNP/INT-WS2 with the INT-WS2 template being partially etched with H2O2, (C) RuAuNP/NT. The arrows in the magnified images on the right highlight the etching of the inorganic template (INT).
SEM and TEM images of RuAuNP/INT-WS2 ascertained that the dense and homogeneous AuNP-coating remained intact (Figure 2A, right). Figures 2C and 3B show the nanostructures with partially removed INT-WS2 template. The UV/Vis spectrum of RuAuNP/INT-WS2 drop-casted on quartz slides shows the absorption minima of the AuNP-decorated INTs at = 622 nm (A exciton), = 518 nm (B exciton), and the characteristic metal-to-ligand-charge transfer (MLCT) band at = 492 nm of the ruthenium polypyridyl complex (Figure S1A). The reaction of a dispersion of AuNP/INT-WS2 with meso-tetra(4-pyridyl)porphyrin (por; 2) also resulted in the incorporation of the molecular component with retention of the dense and uniform AuNP-decoration, as shown by SEM and UV/Vis spectroscopy (Figures 2,3, S1B). The UV/Vis spectrum of porAuNP/INT-WS2 shows the distinct and intense Soret band of the porphyrin at = 437 nm in addition to the bands of the AuNP-decorated INTs. For both RuAuNP/INT-WS2 and porAuNP/INT-WS2 the absorption bands are slightly shifted, relative to the individual molecular and inorganic components (Figure S1A,B). Our observations indicate that the binding affinity of the pyridine-based molecules to the AuNP surface is higher than the TOAB-capping layer, but these compounds do not strip the AuNPs off the surface of the INT-WS2 template under the applied reaction conditions. Multiple binding of the sulfur groups of the INT-WS2 ensures a reasonably stable coating of AuNPs which can further be chemically functionalized.
Figure 4. Raman spectra of (A) RuAuNP/INT-WS2, (B) RuAuNP/NT, (C) Enlargement of the area marked in B; RuAuNP/NT (black), and [Ru(mbpy-py)3][PF6]2 (green). These measurements were performed on single nanostructures dropcasted on silicon substrates. The Raman signature of the silicon substrate (*) is evident in the spectra.
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the NPs not strongly interconnected. The elastic behavior of the template-free RuAuNP/NT was studied by nanoindentation with atomic force microscopy (AFM) and compared with the properties of RuAuNP/INT-WS2. The radial modulus of the structures under small deformations can be estimated by AFM nanoindentation (see experimental section).24,25 RuAuNP/INT-WS2 has a ~50% larger modulus than that of the etched tube. The reduced stiffness of the RuAuNP/NT reflects the diminished mechanical stability after removing the INT-WS2 template even though the mechanical properties of the RuAuNP shell itself should not change much when the INT-WS2 is stripped away. In any case, the estimated radial modulus for the unetched tube is 6 Gpa, about an order of magnitude lower than the elastic modulus of Au metal. The value for the etched tube is 4 GPa, similar to modulus value recently measured by AFM for metalorganic frameworks (MOFs).26
Interestingly, the resonance Raman bands of [Ru(mbpypy)3][PF6]2 (1) and meso-tetra(4-pyridyl)porphyrin (2) are not clearly observable in the starting material probably due to depletion of ground state electrons (Figures 4A, S3A). However, these characteristic bands (= 1100-1800 cm-1) are apparent in the freestanding RuAuNP/NT and porAuNP/NT structures (Figures 4B-C, S3B), unambiguously demonstrating the presence of [Ru(mbpy-py)3] (1) and meso-tetra(4pyridyl)porphyrin (2) in these tubular hybrids. The ruthenium complex is unlikely to be affected by the etching process with H2O2. The characteristic metal-to-ligand charge transfer (MLCT) band is clearly visible after dissolving the Cl-derivative of the ruthenium complex in an aqueous solution containing H2O2 (30%) for 15 h in the UV/Vis spectrum (Figure S4). The Cl-derivative was used instead of the PF6 salt to enhance the solubility in water. Mass spectrometry showed the molecular mass of the ruthenium complex [m/z = 956.29 (M+-Cl-), 460.79 (M2+-2Cl-)]. Similar results were obtained with the porphyrin derivative. Although this molecular compound is considered to be very stable,21 it is probably oxidized by the H2O2 as judged by both UV/Vis and mass spectrometry (m/z = 633.36). Nevertheless, it did not affect the formation of the unique freestanding hybrid nanotubes of individual AuNPs. The decoration of INT-WS2 with metallic NPs is a selflimiting reaction; only monolayers of NPs are formed on the surface of the tubes (Figures 2-3). Interestingly, the ruthenium polypyridyl complex of RuAuNP/INT-WS2 and of RuAuNP/NT allows binding of additional amounts of AuNPs to the surfaces of these tubes. Reaction of RuAuNP/INT-WS2 and RuAuNP/NT with a toluene dispersion of TOAB-capped AuNPs resulted in walls consisting of densely packed NPs as shown by SEM imaging (Figures S5, S6). Polypyridyl complexes can induce aggregation of metallic nanoparticles via electrostatic interactions.20 In our case, 1-coordination of free vinylpyridine moieties can also contribute to binding of additional AuNPs to the surface. The radial modulus of INT-WS2 nanotubes was previously measured.22,23 These measurements, and their analyses are nontrivial since shear between the internal, multi-walled layers plays a significant role in the force balance. The measurements reported here address a different question – namely, the nature of the cohesion within the outer NP shell, and changes occurring when the supporting INT-WS2 is removed. The larger size of these nanotubes allows, when using small indentation depths, analytical contact mechanics analysis. Although this semiquantitative analysis ignores the non-idealities of this system, it is useful for direct comparison of the INT-WS2 supported and the unsupported tubes, and additionally provides a rough approximation of the modulus value. By keeping the indentation depths to 10–15% of the tube thickness, and using large-radius probes, a semi-quantitative comparison between the supported and unsupported nanotubes is obtained. The topographic image and profiles in Figure 5 show that removal of the INT results in flattening of the structure, leaving a broader and flatter tube, with somewhat irregular shape. The height profile of a representative RuAuNP/NT is ~30 nm, which is 3 the estimated combined thickness of two AuNPs walls (5 nm each; Figure 5B). The empty tube is robust mechanically and multiple AFM scans with loads at least an order of magnitude greater than typical AFM scan forces do not destroy or alter the structures. This would not be possible were
Figure 5. Peak-force images of RuAuNP/INT-WS2 (A) and RuAuNP/NT (B). The upper frames show the topographic image with nanotube dimensions, indicated by the profile shown below.
CONCLUDING REMARKS A new bottom-up approach for the formation of singlelayered tubular structures consisting of a continuous network of both metallic nanoparticles and structurally well-defined metal complexes or porphyrins has been introduced. The chemical steps involved in the overall process are efficient and straightforward: (i) surface functionalization of onedimensional (1D) nanomaterials of transition-metal chalcogenides by metallic nanoparticles, (ii) cross-linking of the nanoparticles with a molecular component, (iii) oxidative removal of the 1D-template. The use of two very different molecules to generate very similar hybrid materials suggest that such tubular structures can be generated with a wide variety of molecular components and physicochemical properties. The single-walls of the tubular structures can be chemically and structurally modified by exposing the tubes to dispersions of gold nanoparticles. The large number of nano- and microscale structures reported based on transition-metal chalcogenides is diverse and growing,27,28 suggesting that many different freestanding hybrids could be generated using procedures shown in this study. For example, various MS2-based INTs and IFs have been reported since the discovery of WS2-based nanotubes about 25 years ago.13 INT-WS2 can find industrial applications in electrical and optical devices and in catalysis. A substantial number of lubrication-based products containing fullerene-like IF-WS2 nanoparticles are already in the marketplace.16-18 These facts imply that the present 3-step method is scalable to large quantities with readily accessible templates. Diverse functions of different tubular structures have been extensively demonstrated.29-32 Our study provides an efficient method to decorate inorganic nanostructures with a high density
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Langmuir the Si tip becomes dulled under scanning and leads to changes in its dimensions. This results in uncertainty of approximately 15% in the modulus values obtained. Reaction of INT-WS2 with Hydrogen Peroxide. INTWS2 (0.4 mg) was dispersed in 15% H2O2 (500 L). The reaction progress was monitored using an integrating sphere spectrophotometer. After 15 h, the black colored dispersion dissappeared and a colorless solution remained. The remnants were analyzed by SEM and TEM. Decoration of INT-WS2 with Gold Nanoparticles. INT-WS2 (0.4 mg) was dispersed in toluene (1.0 mL) via sonication for 3 min. Subsequently, the dispersion was ice-cooled for 1 h and an ice-cold toluene solution (200 µL) of TOAB-capped AuNPs (2.67 mg/mL; ∅AuNP = 5 nm). was added. The mixture was sonicated for 1 min and allowed to react for 30 min at 0°C. This step was repeated 3 times (600 µL of the AuNP-tolune solution was added in total). Perfoming this coating step without ice, causes the nanoparticles to coalesce. Reaction of AuNP/INT-WS2 with [Ru(mbpy-py)3][PF6]2 (1). a) A solution of [Ru(mbpy-py)3][PF6]2 (1; 5.0 mg, 4.0 µmol in 100 µL DMF) was added drop-wise to a dispersion of AuNP/INT-WS2 (0.4 mg) in toluene (3.0 mL) with manualy shaking. Subsequently, the reaction mixture was kept at icecold conditions for 20 min. and occasionally shaken manually. The dispersion was washed with toluene/DMF (3 5 mL; 10:1 v/v), the product was allowed to settle and decanted. The extinction spectra of the remaining dispersion (RuAuNP/INTWS2) was measured in 3.0 mL toluene/DMF (10:1 v/v). b) A dispersion of AuNP/INT-WS2 (0.4 mg, 200 µL water/toluene 3:1, v/v) was drop-casted on a silicon substrate and dried under a gentle flow of air. Subsequently an ice-cold solution of [Ru(mbpy-py)3][PF6]2 (1; 5.0 mg, 4.0 µmol; 200 µL DMF/toluene 1:1, v/v) was drop-casted on the silicon wafer and allowed to react for 20 min. Then, the substrate was washed by immersing in acetonitrile (10 mL; 3) and dried under a flow of air. Reaction of AuNP/INT-WS2 with meso-tetra(4pyridyl)porphine (2). A dispersion of AuNP/INT-WS2 (0.4 mg, 200 µL water/toluene 3:1, v/v) was drop-casted on a silicon substrate and dried under a gentle flow of air. Subsequently an ice-cold solution of meso-tetra(4-pyridyl)porphine (2; 5.0 mg, 8.0 µmol; 200 µL DMF/CHCl3 1:1, v/v) was drop-casted on the Si wafer and allowed to react for 20 mins. Then, the substrate was washed by immersion in CHCl3 (10 mL; 3) and dried under a flow of air.
coating of nanoparticles, irrespective of the defect density on the nanotube surface.33,34 Combining NPs with other functional materials using our method can lead to composites with new properties. Superimposing the morphology of the NPs with the geometry of the nanotube results in surface-to-volume ratios that can advance the development of new catalytic processes.35 Moreover, the nanoparticle – molecular networks introduced here might have interesting properties related to artificial photosynthesis. EXPERIMENTAL SECTION Materials and Methods INT-WS2 was supplied by NanoMaterials Ltd, Israel. The length and diameter of INT-WS2 are in the range 1-20 µm to 30150 nm, respectively. The TOAB-capped AuNPs (∅AuNP = 5 nm)36 and [Ru(mbpy-py)3][PF6]2 (1)37 were prepared according to published procedures. NaAuCl42H2O was purchased from Alfa Aesar. Meso-tetra(4-pyridyl)porphine (2) was purchased from Strem Chemicals. Tetraoctylammonium bromide (TOAB) was purchased from Sigma Aldrich (98%) and Chem-Impex International (99.4%), and was used as received. HRTEM images were obtained using a Tecnai F30 electron microscope (UT, FEI) operating at an accelerating voltage of 300 kV with a Gatan charge-coupled device (CCD) camera. TEM images were obtained with CM120 (ST, Philips) operating at an accelerating voltage of 120 kV with a Gatan Ultrascan 1000 CCD camera. Energy dispersive X-ray spectroscopy (EDS) was also performed on a CM120 (ST, Philips) equipped with an EDAX Genesis EDS system. TEM images were also obtained with a JEM 2100 (HT) TEM (JEOL) with a Gatan Ultrascan 1000 CCD camera. Scanning electron microscope (SEM) images were collected with an Ultra 55 FEG (Zeiss). UV-Vis spectra were obtained using Varian Cary 100 spectrophotometers (in double beam transmission mode). Absorption data were obtained by scatter-free absorbance spectra collected on a CLARiTY 1000 spectrophotometer (Bogart, GA, USA). Raman measurements from 50 cm-1 to 1800 cm-1 were made on a LabRAM HR Evolution (Horiba, France) 532 nm, 2 mW maximum power on the sample. The instrument is equipped with an 800 mm spectrograph allowing for very high spectral resolution and low stray light. The pixel resolution is ~1.8 cm-1 when working with a 600 gr/mm grating and a 532 nm laser. The sample was illuminated using several microscope objectives, like the 100 objective (MPlanFL N NA 0.9, Olympus, Japan). The LabRAM measured using a 1024 256 pixel Open Electrode front illuminated CCD camera cooled to -60°C (Syncerity, Horiba, USA). The system utilizes an open confocal microscope (Olympus BXFM) with spatial resolution better than m. The measurements were obtained with 532 nm laser focused on individual nanotubes. Atomic Force Microscopy (AFM) was performed using a Multimode AFM with Nanoscope V controller (Bruker, Santa Barbara, CA, USA). Measurements were made in the peakforce QNMTM mode whereby simultaneously with the topographic image, a force-distance curve is measured at each pixel. From these curves, radial modulus at a given indentation depth can be estimated analytically by presuming a cylindrical nanotube geometry.24 The probe used was TAP525 (Bruker) with nominal force constant of 200 N/m. Sample deformation under the tip was kept to about 3 nm, and the tip radius was 60 20 nm. The large uncertainty in this parameter is due to the fact that under the high loads used, on the order of a µNewton,
Formation of porAuNP/NT.
freestanding
RuAuNP/NT
and
A silicon wafer with RuAuNP/INT-WS2 was immersed in an aqueous solution of H2O2 (15%; 500 L) and kept at 4ºC for 24 h. Alternatively RuAuNP/INT-WS2 can be added to the aqueous solution of H2O2. The resulting RuAuNP/NT was characterized by SEM and TEM. The sample was drop-casted on a quartz slide for spectrophotometer measurement. The sample was drop-casted upon a silicon wafer for Raman spectroscopic measurements. A sample was also prepared after 18 h and analyzed by SEM, TEM and Raman spectroscopy. All the samples were dried in high vacuum for 2 days before analysis.
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PorAuNP/INT-WS2 on a silicon wafer was immersed in a H2O2 (15%; 500 L) solution at 4C for 24 h. The resulting dispersion was analyzed by SEM and TEM showing the formation of porAuNP/NT. All the samples were dried in high vacuum for 2 days before analysis. The sample was transferred by drop-casting on a quartz slide or Si surface for Raman measurement, respectively.
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Supporting Information. Figures S1-S6. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION Corresponding Author
[email protected],
[email protected] (11) (12)
Present Addresses (13)
¶National
Institute for Interdisciplinary Sciences and Technology (CSIR - NIIST), Industrial Estate Po, Thiruvananthapuram, Kerala 695019, India
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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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ACKNOWLEDGMENT We thank the Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging grant No. 7208214, and the Perlman Family Foundation; the Israel Science Foundation grant No. 265/12, the Kimmel Center for Nanoscale Science grant No. 43535000350000, the German-Israel Foundation (GIF) grant No. 712053, the Irving and Azelle Waltcher Foundations in honor of Prof. M. Levy grant No. 720821, the EU project ITN-“MoWSeS” grant No. 317451, the G. M. J. Schmidt Minerva Center for Supramolecular Chemistry grant No. 434000340000. MEvdB is the incumbent of Bruce A. Pearlman Chair in Synthetic Organic Chemistry.
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