Metallic Nanocrystal Ripening on Inorganic Surfaces

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Article Cite This: ACS Omega 2018, 3, 6533−6539

Metallic Nanocrystal Ripening on Inorganic Surfaces Priyadarshi Ranjan,†,‡ Ifat Kaplan-Ashiri,§ Ronit Popovitz-Biro,§ Sidney R. Cohen,§ Lothar Houben,§ Reshef Tenne,*,‡ Michal Lahav,† and Milko E. van der Boom*,† †

Department of Organic Chemistry, ‡Department of Materials and Interfaces, and §Department of Chemical Research Support, The Weizmann Institute of Science, Rehovot 7610001, Israel

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S Supporting Information *

ABSTRACT: In this paper, we demonstrate the formation of hybrid nanostructures consisting of two distinctive components mainly in a one-to-one ratio. Thermolysis of inorganic nanotubes (INT) and closed-cage, inorganic fullerene-like (IF) nanoparticles decorated with a dense coating of metallic nanoparticles (M = Au, Ag, Pd) results in migration of relatively small NPs or surface-enhanced diffusion of atoms or clusters, generating larger particles (ripening). AuNP growth on the surface of INTs has been captured in real time using in situ electron microscopy measurements. Reaction of the AuNP-decorated INTs with an alkylthiol results in a chemically induced NP fusion process at room temperature. The NPs do not dissociate from the surfaces of the INTs and IFs, but for proximate IFs we observed fusion between AuNPs originating from different IFs.

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INTRODUCTION The generation of hybrid nano- and micromaterials consisting of structurally different components to acquire new functionalities is a tremendous challenge. The diversity of possible hybrid nano- and micromaterials and their potential applications are countless.1,2 Nanomaterials have been developed into real-world applications and are commercialized (i.e., carbonand metal-based materials).3−5 As an example, MS2-based inorganic nanotubes (INT) and closed-cage, inorganic fullerene-like (IF) nanoparticles were discovered about 25 years ago and are nowadays produced on a large-scale, enabling their use in diverse applications.6−8 Large efforts are being devoted to engineer structurally well-defined hybrid nano- and micromaterials that fulfill the many requirements needed to find competitive applications.9−11 The predesign of such hybrid materials and their properties using bottom-up synthetic approaches requires a large knowledge base, which is rapidly expanding because of the increasing number of excellent fundamental studies. We have introduced recently an efficient one-step method for coating INT and IF nanoparticles of WS 2 and MoS 2 respectively, by uniform monolayers of monodispersed metallic nanoparticles (MNPs; M = Au, Ag, Pd).12 Formation of multiple sulfur−metal coordination bonds and lattice matching between the surfaces of these two-dimensional nanomaterials composed of transition-metal chalcogenides (MS2; M = W or Mo) and the MNPs are the driving forces for the formation of the homogenous coatings (Scheme 1). In contrast to previous studies,13 where decoration of the INT by MNP was defectdriven and hence nonuniform, the decoration of the nanotubes is chemically driven and hence highly uniform and controllable. The interaction between the NPs of group 11 elements and the © 2018 American Chemical Society

INT/IF results in narrowing of the optical band gap of INT/IF, increased work function, and improved surface-enhanced Raman scattering (SERS). These hybrid materials can be further functionalized by the introduction of fluorescent and structurally well-defined metal complexes, and their solubility can be significantly enhanced by tetraoctylammonium bromide (TOAB)-capping layer exchange with alkyl thiols. Furthermore, we have shown that the surface-bound AuNPs can be crosslinked with chromophores (metal complexes or porphyrins) having pyridine binding sites. Subsequent oxidative removal of the inner INT with hydrogen peroxide resulted in the formation of freestanding tubular assemblies consisting of a nanoparticle−molecular network. The overall structure of these single-walled tubular assemblies resemble that of the inorganic template.14 In this paper, we demonstrate that the decorated MNP/INTWS2 (M = Au, Ag, Pd) and AuNP/IF-MoS2 can undergo thermally and chemically induced processes leading to onsurface NP growth. This NP-growth process occurs on the outer surfaces of the inorganic structures. The thermolysis of MNP/INT-WS2 results in the formation of unique hybrid nanomaterials consisting predominantly of a single relatively large NP per INT-WS2. Both in situ and ex situ monitoring of this process for AuNP/INT-WS2 by scanning electron microscopy (SEM) captures the growth of the AuNPs on the surface of INT-WS2. These experiments indicate the formation of a crystalline structure by on-surface particle diffusion. The lability of the surface-bound monolayer of AuNPs is further Received: April 21, 2018 Accepted: May 22, 2018 Published: June 18, 2018 6533

DOI: 10.1021/acsomega.8b00779 ACS Omega 2018, 3, 6533−6539

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Scheme 1. Schematic Rendering Showing the Homogeneous Decoration of the Surfaces of INT-WS2 and IF-MoS2 with MNPs (M = Au, Ag, Pd)12 Followed by Thermolysis or Chemically Induced Ripening of the MNPs on the Surfaces of the Inorganic Structures

demonstrated by atomic force microscopy (AFM) by “shaving” the AuNP-monolayer from the surface of the INT-WS2.

AuNP/INT-WS2 confirm the monolayer structure of the nanoparticle coating (Figure 2). Freshly prepared materials were used to demonstrate the on-surface fusion of the NPs.

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RESULTS AND DISCUSSION Physical attachment of TOAB-capped MNPs (M = Au, Ag, Pd) of various diameters (⌀ AuNP = 5 nm; ⌀ AgNP = 17 nm; ⌀ PdNP = 2 nm) onto the outer surface of INT-WS2 and IFMoS2 was achieved using our previously reported synthetic route (Scheme 1; inorganic decoration).12 For example, adding a dispersion of TOAB-coated AuNPs in toluene (600 μL, 2.67 mg/mL) to a dispersion of INT-WS2 in toluene (1.0 mL, 0.4 mg) resulted in the quantitative formation of the desired AuNP/INT-WS2. SEM and transmission electron microscopy (TEM) confirmed the formation of uniform coatings of a monolayer of NPs. Representative SEM images of the MNPdecorated INT-WS2 and IF-MoS2 are shown in Figure 1. The average NP spacing of the dense coatings is ∼2 nm. Tilt series of high-resolution scanning transmission electron microscopy (STEM) high-angle annular dark field (HAADF) images of

Figure 2. HAADF STEM tomography of a decorated AuNP/INTWS2. (a) Single image taken from a full tilt series of images. (b−d) Slices from the tomogram showing orthogonal cut planes through the nanotube. The scale bars are 50 nm.

The AuNP/INT-WS2 was drop-casted on a preheated (150 °C) silicon substrate using a hot plate. SEM measurements showed the formation of new hybrid materials after 3 h of continuous heating in air, consisting of a relatively large AuNP near an end of the INT-WS2 (Figure 3a). The one-to-one relationship and the absence of free AuNPs indicate a mechanism in which the AuNPs remain bound to the surface of the INT-WS2 during thermolysis. High-resolution SEM images of an individual AuNP/INT-WS2 after heating unambiguously show the formation of crystalline AuNPs. Examples of AuNPs having a decahedron structure are shown in Figure 3b,c.15 The radius of these AuNPs ranges from 20 to 40 nm. Ex situ monitoring of the thermolysis of AuNP/INTWS2 by SEM at different reaction times (0−180 min) captures the growth of smaller AuNPs into increasingly larger ones randomly distributed on the INT-WS2 (Figure 3d−g). This process culminated in formation of one single crystalline structure (Figure 3h). We also monitored the thermolysis of AuNP/INT-WS2 by ex situ Raman spectroscopy showing the gradual disappearance of the SERS peaks (Figure 3i). The signal intensity of the characteristic bands at ν = 353 cm−1 (E2g) and ν = 421 cm−1 (A1g) of INT-WS2 decreases concurrently with the decreasing number of AuNPs. The position of these bands and the ratios

Figure 1. SEM images showing examples of decorated MNP/INTWS2 (a) M = Au; (c) M = Ag; (d) M = Pd) and AuNP/IF-MoS2 (b) with MNPs. 6534


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Figure 3. Thermolysis of AuNP/INT-WS2 on a Si[100] wafer at 150 °C in air. (a) Schematic showing the thermolysis process. (b,c) SEM images of AuNP/INT-WS2 after heating for 3 h. (d−h) SEM images taken at different stages of the thermolysis process. (i) Raman spectra taken at different stages of the thermolysis process. The inset shows an exponential increase of the diameter of the AuNPs during heating (R2 = 0.98).

Figure 4. Thermolysis of AuNP/IF-MoS2 on a Si[100] wafer at 150 °C in air. (a) Schematic showing the thermolysis process. (b) SEM images of AuNP/IF-MoS2 after heating for 3 h. The red circles emphasize the formed AuNP-nanodecahedrons. (c) SEM images of AuNP/IF-MoS2 after heating for 24 h. (d) TEM image showing a crystalline AuNP-nanodecahedron attached to the surface of IF-MoS2.

of their intensities remain constant indicating that the structure of the INT-WS2 is not affected.16 Thermolysis of AuNP/IF-MoS2 (Figure 1b) was carried out using similar reaction conditions as for AuNP/INT-WS2. SEM images of the thermodynamic products show again the formation of AuNP-nanodecahedrons (Figure 4). The close proximity of the IF-MoS2 resulted in inter-IF fusing of these AuNPs after 3 h (Figure 4b). The AuNPs continue to grow as judged from the SEM images recorded after 24 h reaction time, suggesting some mobility of the new crystalline Au structures between the IFs (Figure 4c), although they remain bound to the MoS2 surface (Figure 4d). SEM images recorded after the thermolysis of AgNP/INTWS2 and PdNP/INT-WS2 also show the disappearance of the dense NP coatings of monodispersed MNPs and the generation of new materials consisting of a relatively large single NP attached to the surface of the INT-WS2 (Figure 5). Apparently, there is no transfer of the new MNPs between the INT-WS2. No free NPs were observed, indicating that the NP-growth occurs exclusively on the surface of the INT-WS2. To gain further insight into the AuNP growth, we monitored in situ the thermolysis of AuNP/INT-WS2 by SEM under high vacuum (10−5 mbar). A movie and real-time images showing this thermolysis process at 150 °C are provided in the Supporting Information (Movie S1 and Figure S1). Although we applied the same temperature as for the thermolysis of AuNP/INT-WS2 in air, the reaction under vacuum is ∼7.5×

Figure 5. Thermolysis of MNP/INT-WS2 (M = Ag, Pd) on a Si[100] wafer at 150 °C in air. (a) SEM images of AgNP/INT-WS2 after heating for 5 h. (b,c) SEM images of PdNP/INT-WS2 after heating for 4 h.

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microscope is capable of measuring temperature differences on the order of 1 K, but no difference in the heat dissipation was observed between the AuNP-decorated and native nanotubes. This null result was likely due to the fact that the AuNPs were mechanically removed from the surface of INT-WS2 by the thermal scan process. Indeed, SEM micrographs of the samples scanned by AFM revealed that the AuNPs had been selectively removed from the scanned regions (Figure 7). This measure-

faster and was completed in 24 min. Energy transfer from the electron beam may lead to additional activation. It is likely that the TOAB-capping layer of the AuNPs is further destabilized under these conditions, thus facilitating surface-enhanced diffusion. The movie unambiguously shows the growth of the NPs on the surface of the INT-WS2. We have previously reported that reaction of AuNP/INTWS2 with an alkylthiol results in the TOAB-alkylthiol-capping layer exchange.12 The exchange did not affect the structure of the AuNP monolayer. The hydrophobic coating enhances the solubility of the hybrid INT in organic solvents. We found here conditions that induce the fusion of the surface-bound AuNPs in toluene at room temperature by using 11-mercaptoundecanoic acid and AuNP/INT-WS2. TEM and SEM images obtained after ∼6 h reaction time show that a monolayer of the relatively small AuNPs has been transformed into many larger AuNPs attached to the INT-WS2, that is, ripening (Figure 6). This chemically induced AuNP fusion at room

Figure 7. Mechanical removal of AuNPs from AuNP/INT-WS2 by AFM. Top: Schematic showing the mechanical removal of AuNPs by the AFM tip from the surface of INT-WS2 (not to scale). Bottom: SEM image and inset showing the AuNP-decorated and mechanically removed INT-WS2 surface.

ment provides supporting evidence for the mobility of the AuNPs on the INT. A previous study of the interaction between AuNPs and INT-WS2 found that a normal force of ∼60 nN was required to move the AuNPs along the defect-free INT-WS2 surface.13 In those studies, the AuNPs subsequently became stuck on edge defects at the INT cap, where the binding force was much stronger, on the order of a chemical bond. However, those measurements were made with the AFM probe in contact, leading to substantial shear force. Here, only intermittent contact was made in the semicontact mode. Analytical expressions can be used to estimate both the average and peak forces applied per cycle.17 Presuming a radial modulus of 4 GPa for the nanotube,18,19 a tip radius of 50 nm, and the measured modulation amplitude of 150 nm, we estimate average forces of ∼1 nN and peak forces of ∼400 nN. Further studies, using different conditions, could reveal the minimum force required for the AuNP displacement. Overall, the results observed here are in good agreement with mobility of AuNPs on the surface of the INT-WS2.

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Figure 6. Chemical-induced fusion of AuNPs. SEM (a,b) and TEM (c) images after reacting AuNP/INT-WS2 with 11-mercaptoundecanoic acid in toluene at room temperature.

CONCLUDING REMARKS Unique nanostructures have been demonstrated consisting of two distinctive structures mainly in a one-to-one ratio. In situ electron microscopy provided valuable fundamental information, as it has done in the past for phase transformation and recrystallization processes for other inorganic materials.20,21 Growth of metallic particles on various surfaces from smaller particles can occur through different mechanisms, including vapor-phase diffusion (Ostwald ripening) or/and substratesurface diffusion of smaller particles or atoms to generate larger structures.22−27 A vapor-phase diffusion process is not likely to occur under the experimental conditions in air at 150 °C. We

temperature can be a result of competing Au−S interactions between the thiol groups of the 11-mercaptoundecanoic acid and those of the INT-WS2 surface. Weakening the binding of the AuNPs to the surface of the INT-WS2 by reducing the number of Au−S interactions will enhance their mobility and fusion. Under the applied reaction conditions, no free AuNPs were observed, rendering a dissociation−association route unlikely in this reaction as well. We attempted to measure the heat dissipation of the AuNP/ INT-WS2 and INT-WS2 using AFM. The scanning thermal 6536


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time was 10 s. The samples were drop-casted on a silicon wafer. Individual nanotubes were placed at 45° with respect to the XY plane. Electron Tomography. Tilt series of high-resolution STEM HAADF images were acquired on an FEI F20 TWIN electron microscope. The tilt increment was 2° in a range of 132°. Tomograms were reconstructed using the iterative simultaneous reconstruction technique, after cross-correlation alignment and iterative feedback displacement refinement.41,42 MeVisLab was used for the visualization of tomogram data. AFM measurements were performed using an NTEGRA AFM with Smena head (NT-MDT, Zelenograd). A module for performing scanning thermal microscopyVertiSense (App Nano, Mt. View, CA) was applied to investigate the thermal properties of decorated and uncoated nanotubes. VTP-200 probes, with nominal spring constant of 10 N/m, were used in the semicontact mode. The module was operated in the heat dissipation mode, where the laser alignment on the probe was adjusted to heat the tip to approximately 20 K above room temperature, so that mappings of heat dissipation could be obtained simultaneously with the topography. SEM micrographs were made of the sample before and after imaging the nanotubes by AFM. Thermolysis of MNP/INT-WS2 (M = Au, Ag, Pd) and AuNP/ IF-MoS2 on a Si[100] Wafer in Air. A dispersion of MNP/ INT-WS2 (M = Au, Ag, Pd) or AuNP/IF-MoS2 (20 μL, toluene, 0.13 mg/mL) is drop-casted on a Si[100] substrate (0.2 cm × 0.2 cm), and excess of solvent is removed using Kim wipe. Then, the substrate is transferred to a preheated hot plate (150 °C) and heated for 3 h (AuNP/INT-WS2), 5 h (AgNP/ INT-WS2), 4 h (PdNP/INT-WS2), and 3 h (AuNP/IF-MoS2). In Situ Monitoring the Thermolysis of AuNP/INT-WS2 by SEM on a Cu Substrate under Vacuum. A dispersion of AuNP/INT-WS2 (20 μL, toluene, 0.13 mg/mL) is drop-casted on a copper substrate, and excess of solvent is removed using Kim wipe. The Cu substrate is placed in an XL30 ESEM-FEG (FEI) and is heated to 150 °C using a heating stage (Kammrath and Weiss 800 °C heating module) under high vacuum mode (10−5 mbar). The fusion of the AuNPs to form Aunanostructures occurs within 24 min. Images were captured using 1 frame/s, and a movie was generated by stacking these images. Reaction of AuNP/INT-WS2 with 11-Mercaptoundecanoic Acid. A dispersion of AuNPs (600 μL, toluene, 2.67 mg/mL) was added to a dispersion of INT-WS2 (1.0 mL, toluene, 0.4 mg). AuNP/INT-WS2 slowly precipitated as judged by absolute spectroscopy. Subsequently, 11-mercaptoundecanoic acid (200 μL, toluene, 7.14 mg/mL) was added after 2 h, and the reaction mixture was analyzed by absolute spectroscopy. The materials were analyzed by SEM and TEM after 6 h to confirm that the treatment with the thiol did not result in the release of the AuNPs. Mechanical Removal of AuNPs from AuNP/INT-WS2 by AFM. 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. The sample was analyzed by low magnification SEM, followed by AFM measurements, after which the SEM was applied again to observe changes in the scanned hybrid nanostructures.

have shown that destabilizing the capping layer by adding alkyl thiol results in nanoparticle fusion at room temperature (Figure 6). In our study, migration of metallic particles (atoms and clusters thereof) and subsequent coalescence is likely the dominant mechanistic pathway. This mechanism could be considered as a modified Volmer−Weber 3D island growth, facilitated by several factors: (i) a low interfacial energy between AuNPs and INT-WS2, (ii) small contact area of AuNPs on the nanotube due to faceting, and (iii) favorable energy of formation of the Au nanocrystal. The fact that the final crystalline shape is similar in all cases supports the hypothesis of weak interaction with the surface. The movement of the gold atoms and clusters on the surface of INT-WS2 has to proceed with both breaking and forming of Au−S bonds, making the overall process likely to be thermodynamically neutral. Large NPs/clusters have more Au−S interactions and are expected to be less mobile. The Au−S bond is 40−50 kcal/ mol and expected to break upon heating as is well-known for alkylthiol-based monolayers on Au substrates.28 This proposed particle-migration mechanism is in agreement with, for example, the thermal-induced fusion of AuNPs on GaN nanowires.22 The mobility is remarkable as the lengths of some of the used INT-WS2 are orders of magnitudes larger than those of the initial MNPs. No dissociation−association pathway of the NPs is apparent. However, we did observe that the use of closely packed IFs resulted in fusion of the AuNPs from different IFs, indicating that cross-over pathways are accessible. Because the feedstock of the final MNPs during the thermolysis processes is the smaller MNPs bound to the same INT-WS2, the size of the final MNP should reflect the dimensions of the supporting inorganic nanostructures. Shape control and uniformity of NPs and related nanostructures are highly sought after.29−35 The use of homogeneous inorganic nano- and microstructures as the template can result in a new approach to generate metallic structures with a high uniformity and useful mechanical properties.14,36

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EXPERIMENTAL SECTION Materials and Methods. INT-WS2 was supplied by NanoMaterials Ltd, Israel. The length and diameter of the INT-WS2 are in the range 1−20 μm to 30−100 nm, respectively. TOAB-coated AuNPs (⌀ AuNP = 5 nm),37,38 AgNPs (⌀ AgNP = 17 nm),39 and PdNPs (⌀ AuNP = 2 nm)40 were prepared according to published procedures. NaAuCl4· 2H2O, AgCl, and PdCl2 were purchased from Alfa Aesar. Sodium citrate monobasic and sodium borohydride were purchased from Sigma-Aldrich. 11-Mercaptoundecanoic acid was purchased from Sigma-Aldrich. TOAB was purchased from Sigma-Aldrich (98%) and from Chem-Impex International (99.4%) and was used as received. Decoration of the NPs onto the outer surface of INT-WS2 and IF-MoS2 was achieved using our previously reported method.12 TEM images were obtained with a JEM 2100 (HT) TEM (JEOL) system operating at an accelerating voltage of 200 kV and equipped with a Gatan UltraScan 1000 CCD camera. SEM images were collected with an Ultra 55 FEG (Zeiss). UV−vis spectra were obtained by 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 spectra were obtained with a Renishaw micro Raman imaging microscope with a 632.8 nm He−Ne laser. The laser power was 25 mW with a 10× objective used for focusing. The acquisition 6537


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00779. Examples of real-time observation by in situ SEM of the ripening of AuNPs on INT-WS2 (PDF) Ripening of AuNPs on the surface of INT-WS2 (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.T.). *E-mail: [email protected] (M.E.v.d.B.). ORCID

Reshef Tenne: 0000-0003-4071-0325 Milko E. van der Boom: 0000-0003-4102-4220 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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-Israeli 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, and the G. M. J. Schmidt Minerva Center for Supramolecular Chemistry grant no. 434000340000.

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