Structures and Mechanisms in the Growth of Hybrid Ru–Cu2S

May 2, 2012 - structures at 210 °C and Ru-nanonets at 220 °C. The goal of this study .... This leads to the final product of free Ru(0) dots separat...
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Structures and Mechanisms in the Growth of Hybrid Ru−Cu2S Nanoparticles: From Cages to Nanonets Kathy Vinokurov, Janet E. Macdonald,† and Uri Banin* Institute of Chemistry and The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel S Supporting Information *

ABSTRACT: Combining metal and semiconductor segments with well-defined morphologies on a single hybrid nanoparticle provides functionality benefiting from the joint and possibly also synergetic properties of the disparate components. We have recently reported the synthesis of a novel family of Ru nano-inorganic caged (NICed) copper(I) sulfide hybrid nanoparticles, which were grown through a mechanism of selective edge growth of the Ru on the copper(I) sulfide seeds. In this work we investigate the effect of reaction conditions on the Ru−Cu2S products. There is an extraordinary sensitivity to reaction temperature in which four product structures were discovered upon varying the reaction temperature from 190 to 220 °C. The products changed from homogeneous nuclei of Ru along with the free Cu2S seed at lower temperature, to Ru nano-inorganic caged copper(I) sulfide, to long thin Ru structures protruding from the seed surface at the higher temperature range. The resulting materials were imaged and characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), high-angle annular dark field-scanning TEM (HAADF-STEM), and powder Xray diffraction. Differential scanning calorimetric (DSC) analysis of the Cu2S template nanoparticles revealed an endothermic peak at the specific temperature for selective edge growth of Ru, and was assigned to a surface change on the seed particle. Competition between homogeneous nucleation of the secondary material Ru and heterogeneous nucleation on the seed Cu2S nanoparticle leading to a rich reaction landscape is discussed. KEYWORDS: hybrid nanoparticles, cages, copper sulfide, ruthenium, selective growth

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INTRODUCTION Nanoparticles (NPs) attract significant interest because of the new chemical and physical properties that depend on their size, shape and composition. Hybrid NPs which contain a combination of two or more different materials add a level of complexity in behavior and function that also opens opportunities to various applications.1,2 For example, in the field of photocatalysis,3−5 hybrid NPs of semiconductor−metal combinations have shown to perform redox reactions with high efficiency. In photovoltaic devices, hybrid NPs can be engineered to spatially separate the electron−hole pairs and to increase charge carrier density.6,7 In the field of biology and medicine, hybrid NPs can combine on a single marker fluorescence tagging along with magnetic contrast for magnetic resonance imaging.8−10 Although synthesis of different hybrid nanoparticles has already been developed,11 the control of the reactions and understanding of the growth mechanisms remain challenging due to the unique combinations of two disparate materials on one nanosystem. To control the topology of the desired hybrid nanostructure, several parameters should be brought into consideration: (1) lattice constant mismatch and structural differences between the two materials; (2) interfacial energy among the materials; (3) surface accessibility/reactivity; (4) miscibility of the materials; (5) presence of polar facets; (6) surface defects. © 2012 American Chemical Society

In the case when the lattice constants of the two dissimilar materials do not differ significantly, and the interfacial energy is low enough, the formation of core−shell type NPs is favored, as demonstrated for semiconductor NPs.12 In other cases, combining metal and semiconductor components, more elaborate structures may form,13 for example, selective growth of metal tips on the apexes of semiconductor nanorods.14 Such nanorods have polar tips which are more reactive than their side facets, due to the higher surface energy that can lead to the preferential nucleation of the second material at these locations. In other cases, defects on nanorod facets can appear and provide additional sites for nucleation of the second material.15−19 Additional innovative configurations are cage-like structures, which hold interesting potential in NPs development, because of their high surface area and the accessibility to the inner region of the cage. Metal nanometric cage structures have been synthesized by galvanic displacement starting from Ag-cubes and yielding open Au frames.20 These were studied as catalysts,21 drug-delivery carriers, biomedical diagnosis agents,22 and as contrast agents for spectroscopic optical coherence tomography.23 Another approach for hollow NPs employed the Kirkendall effect, providing for example Pt yolk/CoO shell NPs Received: February 1, 2012 Revised: April 24, 2012 Published: May 2, 2012 1822

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°C caused by the injection of the cooler Ru(acac)3/octylether solution resulted in different products (Figures 1 and 2). Such extraordinary sensitivity to reaction temperature had not been reported for hybrid NP synthesis previously.

showing catalytic activity in ethylene hydrogenation reaction.24 We have recently reported on a new type of selective edge growth mechanism leading to a cage structured hybrid metal semiconductor NP.1 Ruthenium metal was grown selectively on the edges of 14 nm faceted copper(I) sulfide seeds, yielding a first example of hybrid nano-inorganic caged (NICed) nanoparticles. Such structures with metal frames are of interest in sensing, catalysis and additional areas. These applications require to tune the hybrid nanocage morphology. In this work, we report an extraordinary sensitivity of metal edge growth to reaction temperature. Cage growth was achieved at 205 °C with a very low tolerance, which requires optimal control of the reaction temperature. Additional product phases were discovered while varying the reaction temperature from 190 to 220 °C. At low temperatures, separate ruthenium dots were observed surrounding the Cu2S seed particles, whereas at higher temperatures, string-like Ru structures protruding from the seed surface were created. Ru Cage structures are formed in a narrow window of temperature around 205 °C, while further heating leads to string-=like Ru structures at 210 °C and Ru-nanonets at 220 °C. The goal of this study was to explore the reaction mechanism and to shed light on the extraordinary thermal sensitivity of the selective edge growth phenomenon.

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Figure 1. Formation of separate Ru(0) dots surrounding the Cu2S seeds at (a−c) 190 °C and Ru−Cu2S nanoinorganic cages formation at (d−f) 205 °C. (a,d) TEM images of the products at 190 and 205 °C, respectively. (b, e) HAADF-STEM images of the corresponding phases. (c, f) HRTEM images of the structures.

EXPERIMENTAL SECTION

General Information. The following chemicals (from Aldrich Chemical Co.) were used without further purification: Copper(II) acetylacetonate (Cu(acac)2, 99.99%), Ruthenium(III) acetylacetonate (Ru(acac)3, 97%), dodecanethiol (≥98%), octadecylamine (ODA, 98%), octylether (99%), neocuproine, phenanthroline (≥99%), chloroform (Anhydrous, 99%), isopropanol (Anhydrous, 99.5%). Standard Schlenk and glovebox techniques for inert chemical treatments were employed throughout. Synthesis. Ru-NICed copper(I) sulfide nanoparticles were prepared by a procedure reported previously.1 During the course of the development of the procedure, several variables were optimized to achieve the desired selective edge growth: the purity of the Cu2S seed solution, the seed to Ru(acac)3 ratio, and especially the reaction temperature. Cu2S seeds are formed by the decomposition of Cu(acac)2 in excess dodecanethiol at 200 °C.25 The dodecanethiol acts as the solvent, surfactant and, notably, the sulfur source. Others have shown that this synthesis yields a byproduct of alkenes, resulting presumably from the reductive elimination of the carbon chains of the thiol as the sulfur of the thiols become incorporated into the growing Cu2S seed. The main impurity in the Cu2S seed solution was a polymerlike byproduct and is likely a result of the polymerization of these alkenes combined with some free thiols. Inconveniently, it was not readily soluble in common laboratory solvents. Instead, purification was achieved by allowing the seeds to slowly settle (∼1 day) from suspensions in chloroform, which allowed the seeds to sink and the organic polymer to rise. Improper cleaning of the seeds led to inconsistencies in the amount of Ru(acac)3 necessary to be added in the cage growth step. When the seeds were clean from excess ligand, the molar ratio of Ru(acac)3 to Cu in the Cu2S seeds was set to be 1:12(± 2) (see the Supporting Information for comment on molar ratio). Cage growth is performed by heating a solution of the Cu2S seeds (Figure S1 in the Supporting Information) in octadecylamine and injecting a solution of Ru(acac)3 in octylether at 205 °C. An extraordinary sensitivity to reaction temperature was discovered. This requires control of the reaction temperature to within 2−3 °C in order to achieve the desired products. Standard techniques in NP syntheses, where typically room temperature solutions of reagents are injected into precursor solutions at several hundred degrees, did not suffice. This method causes a drop in the reaction solution temperature that is difficult to reproduce. Indeed, even a temperature reduction of only 5

Figure 2. Formation of Cu2S seeds with (a−c) surrounding Ru-strings structures at 210 °C and (d−f) surrounding Ru-nets structures at 220 °C. (a, d) TEM images of the products at 210 and 220 °C, respectively. (b, e) HAADF-STEM images of the corresponding phases. (c, f) HRTEM images of the structures.

To fully control the reaction temperature and avoid dips during injection a heated syringe was employed. In this procedure, the Ru(acac)3 solution was first brought up to the reaction temperature in the heated syringe before the injection, thereby minimizing the temperature fluctuations of the reaction solution after the injection to less than 2 °C. It was only with this level control that improved understanding of nucleation and growth kinetics of the system could be attained. Microscopic Measurements. Transmission electron microscopy (TEM) was performed on a Tecnai T12 G2 Spirit at 120 keV. Samples for TEM were prepared by deposition of a drop of particle chloroform solution onto a carbon-coated copper grid. High-resolution TEM (HRTEM) and high-angle annular dark-field scanning TEM (HAADF1823

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STEM) were performed on a scanning transmission electron microscope Tecnai F20 G2. Composition Characterizations. Powder X-ray Diffraction (XRD) patterns were obtained using Cu Kα photons from a Philips PW1830/40 diffractometer operated at 30 mA and 40 kV. Samples were prepared by deposition of a particle chloroform solution on a low background scattering quartz substrate. The solvent was evaporated by gentle heating until a thin sample layer was formed. Thermal Measurements. Differential Scanning Calorimetry (DSC) was performed by METLLER TOLEDO - DSC823e model and STARe software. The system was calibrated with the melting points of indium and zinc in the standard 40 μL aluminum crucible. Samples were prepared by deposition of a particle chloroform solution in a 40 μL Al crucible and the solvent evaporated and dried leaving behind the sample powder. Five different DSC runs were carried out, with a sample mass ranging from 2.3 mg to 9.3 mg, with good reproducibility. The displayed result is for a sample with a mass of 8.0 ± 0.1 mg. The heating/cooling rate of the sample was 1 °C/min under N2 gas flow of 50 mL/min.

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RESULTS AND DISCUSSION The reaction mechanism investigation revealed four distinct product phases as a result of changing the reaction temperature from 190 to 220 °C. The products were characterized by TEM, HRTEM and HAADF-STEM (Figures 1 and 2). The HAADFSTEM method gives a much stronger signal for the heavier ruthenium, as the signal is approximately proportional to the square of the atomic number (z-contrast imaging). At reaction temperature of 190 °C, free Ru(0) dots were observed separate from the Cu2S seeds (Figure 1a−c) indicating homogeneous nucleation of the Ru. In contrast, at higher temperature of 205 °C the Ru nucleated and grew preferentially on the crystal edges of the seeds forming the hybrid nanoinorganic cage structures (Figure 1d−f). The formation of the Ru cages can be inferred from the high contrast band patterns identified by TEM as analyzed in detail in previous work.1 Increasing the reaction temperature to even higher values of 210 °C, resulted in string-like Ru structures that originate on the Cu2S seeds but extend into solution, the length of which increased with increasing the reaction temperature to 220 °C (Figure 2). The string structures were confirmed to be crystalline Ru, first by leaching out the Cu2S with a solution of neocuproine/phenanthroline in toluene, which is known to bind preferentially to copper(I) species.25 The remaining black powder was analyzed by XRD and gave broad peaks at angles corresponding to hcp ruthenium (Figure 3). At 210 °C, it is clear that the short string structures originate from the Cu2S seeds; however at 220 °C, the stringlike Ru structures were so dominant that it was often difficult to identify the seed origins. STEM images of the net structures (Figure 2e) show four- and six-sided rings of Ru, indicating the former presence of a seed that may have been lost through oxidative processes in the synthesis or workup. To comprehend the reaction temperature sensitivity, an understanding of the mechanism and the competing reactions is necessary. The formation of NPs of a single material may be divided into two steps: nucleation and growth.26−28 In the present case, dealing with hybrid NPs, there are several competing processes occurring that will influence the products; homogeneous nucleation of Ru and heterogeneous nucleation on the Cu2S seeds; growth of Ru on existing Ru areas and growth of Ru along crystallographic edges of the Cu2S seeds; There is also the possibility of coalescence of Ru particles in solution. The rate of all of these processes is dependent on reaction temperature. This is affected also by the existence of

Figure 3. Ru(0) network structure obtained after leaching out the copper sulfide. (a) HRTEM image. (b) Powder XRD spectra of the Ru nets. Red lines at the bottom indicate the position of bulk hcp Ru diffraction peaks. The additional sharp peak at 50.3° is due to the ODA ligands. Inset: TEM image of Ru network analyzed by XRD.

competing reduction routes, either reduction of Ru by Cu1+ in the Cu2S seeds or reduction of Ru by the amine solvent. Since ruthenium did not nucleate on the seed edge at 190 °C, we infer that the activation energy requirements for heterogeneous nucleation of Ru upon the Cu2S seed were not met. This leads to the final product of free Ru(0) dots separate from the hexagonal Cu2S seed crystals (Figure 1a−c). In a control reaction without Cu2S seeds, similar Ru dots resulted (Figure 4). This indicates that the amine solvent acts as the preferential reductant for the Ru3+ at this lower temperature to yield the homogeneous nucleation of Ru NPs. At higher temperatures, free Ru(0) in the solution nucleate, but nucleation also occurs on the seeds (Figure 2). The vermicular growth phenomenon (Figure 2d) was previously observed for Ru nanoparticle growth using hydrogen reduction and in the presence of dodecyl- and hexadecylamine ligands at low reaction temperatures.29 Its formation may be attributed to the coalescence of Ru particles into net structures 1824

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nuclei. Both routes would lead to the resulting Ru nets, similar to those discussed above.29 In light of the observation of edge nucleation of Ru on the Cu2S seeds at 205 °C and to help identify the nature of this abrupt change in reactivity, a differential scanning calorimetry (DSC) analysis was performed on the Cu2S seeds (Figure 5).

Figure 5. Differential scanning calorimetry (DSC) analysis on Cu2S seeds. The measurements were carried out in N2 atmosphere. The heating (red curve, left axis) and cooling (blue curve, right axis) rates were 1 °C/min. Note the irreversible endothermic peak at 203 °C of the heating curve highlighted by the circle. See text for details.

An endothermic peak was observed at 203 °C upon heating, at a temperature consistent with the abrupt change in reactivity. The lowest temperature features observed in the DSC were an endothermic peak at 87 °C upon heating and an exothermic peak at 65 °C upon cooling. This pattern indicates a reversible phase transition, which is assigned to a crystal phase transition. The complicated and low symmetry crystal structures of the copper(I) sulfides (low chaclocite, djurelite, etc.) at room temperature are known to simplify to a disordered hexagonal crystal structure (high chalcocite) at elevated temperature due to temperature dependent mobility of the copper atoms.33,34 This is reported to occur at 93−103.5 °C for the bulk depending on the precise stoichiometry of the copper sulfide. The lowering of this transition temperature in the Cu2S seed nanoparticles is consistent with a recent study of the phase transition in Cu2S rods.35,36 Such behavior is also observed in the depression of the melting point for small NPs due to the contribution of surface tension.37 Of particular note is that no further thermal changes were observed in the crystal structure from above 100 to 240 °C excluding the endothermic peak at 203 °C. Considering first a possible phase change in the organic ligands on the crystal faces, previous work for alkane thiols on Au particles showed that this took place at significantly lower temperatures.38,39 We propose that this peak results from a change in the particle surface related to the dissociation of the ligands at the highly reactive edges of the seed particles, due to reduced cohesive interactions. This proposed dissociation process transforms the particles from being fully passivated by dodecanethiol to being reactive to Ru3+ at the edges of the NPs. In addition to its large effect on the nucleation step, the reaction temperature also has a considerable influence on the growth stage of ruthenium and on the resulting product (Figure 6). At 205 °C the Ru growth occurred exclusively along the

Figure 4. Ru(0) dots formed upon heating Ru(acac)3 dissolved in octylether (see text). (a) TEM image of the Ru(0) dots. (b) Powder XRD spectra of the Ru(0) particles. Red lines at the bottom are the position of bulk hcp Ru diffraction peaks. The additional sharp peaks at 36.6, 39.7, and 50.3° are due to the ODA ligands.

and this may be related to the mechanism of directed attachment that was invoked to describe the formation of nanowires and necklace-like formations in semiconductor30,31 and metal32 nanoparticles. In a control reaction with Ru alone at 220 °C, both isolated Ru dots and net structures were observed (Figure S2 in Supporting Information), supporting the mechanism of Ru particle coalescence also in our case. Therefore, the vermicular Ru growth onto the Cu2S seeds likely involves the following reaction pathways. One process that takes place is the reduction of Ru3+ by the Cu1+ on the seed surface forming nuclei. In addition, Ru3+ can be reduced by the amine solvent, either onto existing Ru particles nucleated on the Cu2S seeds, or independently in solution forming separated Ru dots. Vermicular growth can therefore take place either from direct growth of the Ru onto the Ru nuclei on the seeds, or via coalescence of the separated Ru particles with these 1825

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ruthenium will not nucleate on the seed edge until the temperature will reach 205 °C. The next observation was that selective metal edge growth will not proceed unless the thermal conditions will be preserved. The narrow temperature region for selective edge growth of the metal may be attributed to the dependence of the seed surface reactivity on temperature. This was supported by DSC analysis which showed a change at 203 °C. Varying the reaction temperature revealed formation of further phases combining the Cu2S seeds with Ru nanostructures. The study illustrates the complexity in reaction paths present in the growth of hybrid nanoparticles.

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ASSOCIATED CONTENT

S Supporting Information *

Comment on reactants ratio. TEM of superlattices of Cu2S seeds. TEM data for Ru dots and nets. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 6. Reaction mechanism divided into two steps: (a) Nucleation of Ru onto Cu2S seeds (dark spots assigned to small Ru nucleation centers are marked by the arrows), and (b−d) growth and coalescence steps which lead to different products: (b) Ru−Cu2S NICs, (c) stringlike structures, (d) netlike structures.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +972-26584253. Fax: +972-26584148.

Cu2S seed edges (Figure 6a), whereas at higher temperature of 210 °C, the ruthenium additionally grew from existing Ru areas on the seeds and protruded out into solution as a stringlike structure. At an even higher temperature of 220 °C, a netlike structure of Ru formed. Ru selective growth on the seed edges may be promoted by reduction by Cu1+ in the seed surface. At higher temperatures, where reduction by the amine solvent is more facile, growth preferentially occurs on existing Ru dots on the seed surface in addition to the coalescence of separated Ru particles discussed above, leading to the formation of the net and string like structures. This system presents behavior that at first seems contrary to prior knowledge that heterogeneous nucleation on solid surfaces has lower Gibbs energy (ΔG) and hence should be faster than homogeneous nucleation.26,27 Here instead, homogeneous nucleation is preferable at the lower temperature of 190 °C, and heterogeneous nucleation occurs at a specific temperature of 205 °C with little variation. We explain this as a change in reductant from the amine solvent to the Cu1+ in the seeds. To this end, we also used dodecylamine (DDA) instead of octadecylamine as the coordinating solvent with similar results albeit less convenient since the DDA requires more stringent degassing. On the other hand, attempts to use nonamine coordinating solvent such as myristic acid did not yield the desired products. The activation energy for ruthenium reduction by the Cu1+ must change near 205 °C, which can indicate a change on the seed surface increasing its surface reactivity toward heterogeneous Ru nucleation. The observation of the irreversible endothermic peak in the DSC analysis of the seed Cu2S NPs is consistent with such a surface change.

Present Address †

Department of Chemistry, Vanderbilt University, 7330 Stevenson Centre, PMB 351822, Nashville, TN, USA 37235.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC Grant [246841]. U.B. thanks the Alfred and Erica Larisch Memorial Chair. We thank Dr. Inna Popov of the Unit for Nanocharacterization of the Hebrew University for assistance in the EM characterization.

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DEDICATION We dedicate this manuscript to our dear colleague Professor Dr. Dieter Fenske from the Karlsruhe Institute of Technology celebrating his 70th birthday.

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CONCLUSIONS We have studied the selective growth mechanism leading to cage structured hybrid ruthenium semiconductor NPs. The most influencing parameters on the reaction pathways were identified, mainly focusing on the reaction extraordinary thermal sensitivity. Diverse growth paths were discovered, leading to four different products, influenced mainly by fairly minor changes in the temperature conditions. The first observation for understanding the mechanism was that 1826

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