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Synthesis of Au-CdS@CdSe Hybrid Nanoparticles with Highly Reactive Gold Domain Yuriy Khalavka, Sebastian Harms, Andreas Henkel, Malte S. Strozyk, Rubén Ahijado-Guzmán, and Carsten Sönnichsen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02756 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017
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Synthesis of Au-CdS@CdSe Hybrid Nanoparticles with Highly Reactive Gold Domain Yuriy Khalavka1,2,3, Sebastian Harms1, Andreas Henkel4, Malte Strozyk1, Rubén Ahijado-Guzmán1 and Carsten Sönnichsen1* 1.
Institute of Physical Chemistry, University of Mainz, Duesbergweg 10-14, 55128 Mainz, Germany. Yuriy Fedkovych Chernivtsi National University, Kotsiubynsky St. 2, 58012, Chernivtsi, Ukraine. 3. Graduate School of Materials Science in Mainz, Staudingerweg 9, D-55128 Mainz, Germany. 4. Data Center, University Mainz, Anselm-Franz-von-Bentzel-Weg 12, 55128 Mainz, Germany. 2.
KEYWORDS Gold, semiconductor, hybrid nanoparticles, synthesis of hybrid nanoparticles in water, photochemistry, nanoparticle growth. ABSTRACT: We propose a novel route to synthesize semiconductor-gold hybrid nanoparticles directly in water resulting in much larger gold domains than previous protocols (up to 50 nm) with very reactive surfaces which allow further functionalization. This method advances the possibility of self-assembly into complex structures with catalytic activity towards the reduction of nitrocompounds by hydrides. The large size of these gold domains in hybrid particles supports efficient light scattering at the plasmon resonance frequency, making such structures attractive for single particle studies.
Combining two or more materials in one nanostructure not only allows to produce materials with multiple functionalities, but also to create new properties which are based on interactions between the used components.1 A striking example of such new material properties of hybrid materials are nanoparticles consisting of a semiconductor and a noble metal (mostly gold) domain (Au-SChybrids). The direct electronic interactions between excitons in the semiconductor domain and plasmons in the noble metal domain lead to novel optical and photochemical effects which cannot be observed in mixtures of the two nanoparticle types.2 The collective oscillation of the conduction electrons in noble metal nanoparticles at the plasmon frequency enhances the interaction with light by many orders of magnitude, known as ‘antenna effect’. This antenna effect is the strongest in metal domains in the size regime around 50 nm.3 So far, most Au-SC-hybrids were synthesized in organic solvents (usually chloroform) following the method of Uri Banin and coworkers 4-10 which usually result in gold domains smaller than 10 nm attached to one side of the semiconductor rod. Using UVlight induced growth, we reach gold domain diameters of up to 20 nm.11 In our experience, hybrid particles with gold domains larger than 20 nm in diameter are not stable in organic solvents, form aggregates and finally weld together. The same applies to the alternative procedure which uses thermally induced reduction of Au-oleate complexes.12,13 Since most known protocols for the synthesis of gold nanoparticles in the size regime of 50 nm use water as a solvent, 14 a possible strategy to create Au-SC-hybrids with large gold domains
consists of switching to water based procedures. The high dielectric constant of water supposedly decreases the gold domain’s van-der-Waals attraction preventing their aggregation and subsequent welding even after long reaction times. However, previous attempts of growing Au and Pt on CdS-rods in water have only led to random growth of small metal domains with no control over their size and location.15 Here, we report a novel strategy using a water/ethylene glycol mixture which leads to matchstick-type hybrid particles with one large, size-tunable gold domain (CdS rod with a Au particle on one tip). Our synthesis not only results in hybrid particles with the desired large gold domains, but solves another problem of previous synthetic routes: The difficulties with the selective functionalization of the Au and CdS surfaces after the phase transfer to aqueous media. Selective functionalization is important for self-assembling particles in functional arrangements and is most often performed in water due to the large number of available ligand–receptor systems. Most ligands used for the transfer of hybrid nanoparticles from an organic solvent to water (thio-PEG or mercaptoundecanoic acid) cover both metallic and CdS surfaces equally well and strongly which makes any further functionalization (especially selective functionalization) difficult and considerably decrease catalytic activities.16 Furthermore, the efficiency of the charge-transfer through monolayer of thiols is poor.17 In our new approach, the gold domain is only loosely covered by surfactants which allows to employ it in most of the established gold surface functionalization strategies. We start with CdS rods (70x6 nm) as substrates which are grown from CdSe-seeds according to established methods.18 These CdS
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rods are one of the most popular one-dimensional semiconductor nanostrustructures with well-established control of the size and aspect-ratio. The CdS rods are transferred to aqueous media using mercaptoundecanoic acid (MUA).14 After the addition of polyvinylpyrrolidone (PVP), ethylene glycol (EG) and chloroauric acid (HAuCl4), the solution is irradiated by UV-light which leads to the selective growth of a gold domain on one end of the CdS-rods (Figure 1). The transmission electron microscopy (TEM) characterization confirms a relatively narrow size distribution of the gold domains (below 20 %.)
Figure 1. Synthesis and characterization of Au-SC hybrids. (a) Scheme of the hybrids synthesis including the transfer from chloroform to water. (b) TEM image of CdS nanorods after the transfer to water. (c) TEM image of hybrids produced under typical synthetic conditions. The diameter of the gold domains can be adjusted by varying either the Au3+/CdS-ratio or the irradiation time (Figure 2a-c). Monitoring the reaction by optical extinction spectroscopy (‘UVVIS’) enables a precise adjustment of the gold domain size. During irradiation, the pure CdS-extinction spectrum evolves into the hybrid spectrum with a well-defined plasmon-resonance-peak at 540-560 nm. The height and spectral position of the plasmon peak strongly depend on the size of the gold domain and can be used to terminate the reaction once the desired size is reached (Figure 2d). In organic solvents, the growth of gold domains on CdS rods is caused by the light induced excitation of valence band electrons in CdS which is followed by a migration of electrons into the gold domain.11 In our case, the ethylene glycol can act as a reducing agent by itself. In previous studies, El-Sayed and co-workers observed gold reduction under UV irradiation in the presence of ethylene glycol.19 However, in a control experiment with gold seeds instead of CdS rods, we observed considerable differences in particle growth dynamics –a long lag time followed by a very rapid domain growth (Figure 2d). We have not observed selfnucleation even at the highest Au3+ concentrations. These observations strongly suggest that our protocol leads to the same gold domain growth mechanism as observed in the organic phase.20
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In our case, the particles are coated by PVP, a surfactant known to stabilize gold nanoparticles without completely blocking the gold surface to other compounds.21 To test if the gold domain of our hybrid nanoparticles is indeed available for further chemical modifications, we performed two experiments: Forming a silver layer and catalytic reduction of 4-nitrophenol. For the silver layer formation, we purified hybrids by centrifugation and used them (instead of pure gold particles) in an established silver layer growth procedure.22 This resulted in a silver shell around the gold domain (Figure 3a).
Figure 2. Kinetics of gold domain formation on CdS rods. (a-c), TEM images of hybrid particles at different stages during their growth (t=15, 30, 45 min (d), Evolution of the optical extinction maximum of hybrid particles (red line) and pure gold particles (black line). The reduction of 4-nitrophenol by sodium borohydride is catalyzed by the surface gold particles.23 Therefore, we choose it as a model reaction to test catalytic properties of the hybrids to demonstrate the surface accessibility. We spectroscopically monitored the reaction at the specific 4-nitrophenol absorption band at 420 nm (Figure 3b). In contrast to hybrid particles grown in organic media and transferred to water with MUA, the water grown hybrids show a rapid reduction of 4-nitrophenol over time comparable to the activity of the same size gold spheres. (Figure 3b inset and Supporting Information). We attribute the absorbance increase at 500-530 nm during the reaction to the “plasmon charging effect” proving the role of the gold domain as the reaction center.
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Figure 3. Reactivity of the gold domain surface. (a), EDXlinescan of Au-SC hybrid gold domain which was successfully coated with a layer of silver. Image size is 100 nm. (b), Reduction of 4-Nitrophenol with NaBH4 catalyzed by Au-SC hybrids after 1s, 600s and 900s (dark blue to light blue) with positive and negative control (red and black dashed lines, respectively, corresponding to no Au-SCs and only Au-SCs). Inset: Extinction of 4Nitrophenol over time in the presence of Au-SCs synthesized by the water based method (blue line) and in the presence of Au-SCs synthesized in organic media (red line).
We observed that an increase of the HAuCl4 concentration leads to a prismatic shape of up to 20% gold domains (Fig 5). This observation is in accordance with the model of the kineticallycontrolled growth of metal nanocrystals proposed by N. Ravishankar.25 Weak reduction agents like EG provide reduction potentials at which symmetry breaking is likely to occur, which favors the formation of 2D structures. The presence of semiconductor seeds further increases the probability of symmetry braking. The resulting particles show a shovel-like structure with very characteristic defects in the place of the gold-semiconductor interface. Raising the pH to 11 increases the reduction potential of EG which leads to a faster gold reduction and more irregular flower-like shapes (Figure 5c) even without UV-light. Figures 5d and e shows that prismatic domains are bound with (111) surfaces. Both shovel- and flower-shaped hybrids are new in the pool of CdS-Au nanostructures.
d)
e)
Figure 5. Hybrid structures produced at pH=4 (a) , 6 (b) and 11 (c) showing decreasing number of the particles with 2D -gold domains with pH increase. Scale bar – 120 nm Top (d) and side (e) HRTEM view of shovel-like nanohybrids.
Figure 4. (a, b), Visible appearance of light scattering properties of hybrid nanoparticles. Au-SC hybrid samples taken after 10, 20 and 30 minutes of irradiation time displaying increasing scattering efficiencies. (c), Real color image of individual Au-SC hybrids in a dark-field microscope. (d), Linewidth and resonance energy of plasmon scattering spectra of many individual Au-SC hybrids recorded by dark-field spectroscopy. Plasmonic light scattering greatly increases along with particle diameter and becomes comparable to the above mentioned absorption of about 40 nm. Indeed, suspensions of hybrid particles clearly display strong light scattering (Figure 4a/b). Furthermore, the gold domains are large enough to enable single particle observations in a dark-field microscope (Figure 4c/d).24 Summarized, statistics of more than 100 measured spectra show resonance positions in the range of lower energies (2-2,1 eV) for hybrids than for pure gold nanospheres of the same diameter (range of 2,3 eV) because of the presence of a semiconductor which increases the average refractive index around the particle.
Our novel synthesis route for Au-CdS@CdSe hybrid nanoparticles, which enabled us to grow gold domains larger than 20 nm in diameter while retaining an active and accessible gold domain, provides an adjustable system to observe plasmon-exciton interactions and photochemical charging effects.26-28 The accessible gold domain provides exciting opportunities for self-assembling match-stick like inorganic particles (CdS rod with a Au sphere on one tip) into complex artificial structures and offers new possibilities to explore catalytic applications.29
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors approved the final version of the manuscript.
Funding Sources
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We acknowledge Nano SciEra EU support. This work was financially supported by the ERC grant 259640 (“SingleSense”). Yuriy Khalavka was partially supported by Graduate school of excellence MAINZ, BMBF and SFFR (F70/156).
ASSOCIATED CONTENT Supporting Information. Materials. Synthesis. Optical characterization. Control Experiments.
ACKNOWLEDGMENT We thank Dr. Luigi Carbone, Dr. Tatiana Gorelik for insightful discussions. REFERENCES (1) Banin, U. Ben Shachar Y. and Vinokurov K. Hybrid Semiconductor-Metal Nanoparticles: From Architecture to Function. Chem. Mater., 2014, 26, 97-110. (2) Costi, R.; Saunders, A. E.; Banin, U. Colloidal Hybrid Nanostructures: A New Type of Functional Materials. Angew. Chem. Int. Edit., 2010, 49, 4878–4897. (3) Sönnichsen, C. Plasmons in metal nanostructures. PhD thesis, University of Munich, 2001, 130 p. (4) Sheldon, M. T.; Trudeau, P. E.; Mokari, T.; Wang, L. W.; Alivisatos, A. P. Enhanced semiconductor nanocrystal conductance via solution grown contacts. Nano Lett., 2009, 9, 3676-3682. (5) Mokari, T.; Costi, R.; Sztrum, C. G.; Rabani, E.; Banin, U. Formation Of Symmetric And Asymmetric Metal–Semiconductor Hybrid Nanoparticles. Phys. Stat. Sol. (b), 2006, 243, 3952–3958. (6) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Selective Growth of Metal Tips onto Semiconductor Quantum Rods and Tetrapods. Science, 2004, 304, 1787-1790. (7) Menagen, G.; Macdonald, J. E.; Shemesh, Y.; Popov, I.; Banin, U. Au Growth On Semiconductor Nanorods: Photoinduced Versus Thermal Growth Mechanisms. J. Am. Chem. Soc., 2009, 131, 1740617411. (8) Menagen, G.; Mocatta, D.; Salant, A.; Popov, I.; Dorfs, D.; Banin, U. Selective Gold Growth on CdSe Seeded CdS Nanorods Chem. Mater., 2008, 20, 6900-6902. (9) Khalavka, Y.; Sönnichsen, C. Growth of Gold Tips onto Hyperbranched CdTe Nanostructures. Adv. Mater., 2008, 20, 588- 591. (10) Figuerola, A.; Huis, M. v.; Zanella, M.; Genovese, A.; Marras, S.; Falqui, A.; Zandbergen, H. W.; Cingolani, R.; Manna, L. Epitaxial CdSe-Au Nanocrystal Heterostructures by Thermal Annealing. Nano Lett., 2010, 10, 3028-3036. (11) Carbone, L.; Jakab, A.; Khalavka, Y.; Sönnichsen, C. LightControlled One-Sided Growth of Large Plasmonic Gold Domains on Quantum Rods Observed on the Single Particle Level. Nano Lett., 2009, 9, 3710–3714. (12) Khon, E.; Hewa-Kasakarage, N. N.; Nemitz, I.; Acharya, K.; Zamkov, M. Tuning the Morphology Of Au/Cds Nanocomposites Through Temperature-Controlled Reduction Of Gold-Oleate Complexes. Chem. Mater., 2010, 22, 5929–5936. (13) Khon, E.; Mereshchenko, A.; Tarnovsky, A. N.; Acharya, K.; Klinkova, A.; Hewa-Kasakarage, N. N.; Nemitz, I.; Zamkov, M. Suppression Of The Plasmon Resonance In Au/Cds Colloidal Nanocomposites. Nano. Lett., 2011, 11, 1792–1799. (14) Yang P., Zheng J., Xu Y., Zhang Q., and Jiang L. Colloidal Synthesis and Applications of Plasmonic Metal Nanoparticles. Adv. Mater., 2016, 28, 10508–10517
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