Functional Nanometer-Sized Clusters of Transition Metals: Synthesis, Properties and Applications: Volume 7 (Smart Materials Series) - Hardcover

Hardcover

ISBN 10: 1849738246 ISBN 13: 9781849738248

Publisher: Royal Society of Chemistry, 2014

Synopsis

Edited by active researchers in the area, the book provides a valuable reference for researchers in the area of functional nanomaterials. It also provides a guide for graduate students, academic and industrial researchers interested in the fundamentals of the materials or their applications.

About this title

About the Author

Wei Chen received his Ph.D. in electrochemistry from Xiamen University under the direction of Professor Shi-Gang Sun in 2003. Following his graduate studies, he began working as a postdoctoral associate in the area of synthesis and the property studies of metal nanoclusters at University of California-Santa Cruz. He is currently a full professor at State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. His research interests include the controlled synthesis, characterization, and applications of nanomaterials, especially the metal nanoclusters, in fuel cells, solar cells, optical devices; interfacial engineering and electron transfer properties of functional metal nanoparticles; surface electrochemistry, electroanalytical chemistry and spectroelectrochemistry.

Shaowei Chen completed his undergraduate education at the University of Science and Technology of China with a B.Sc. degree in Chemical Physics in 1991. He then attended Cornell University, receiving his M.Sc. and Ph.D. degrees in Chemistry in 1993 and 1996, respectively. Following a postdoctoral appointment at the University of North Carolina at Chapel Hill, he started his independent career in 1998 in Southern Illinois University – Carbondale. He moved to the University of California – Santa Cruz in 2004 and is currently a Professor of Chemistry. His research is mainly focused on nanoscale functional materials and their electron transfer chemistry. So far he has published more than 100 research articles in peer-reviewed journals, co-edited two monographs and contributed five book chapters.

From the Back Cover

Metal nanoclusters, which bridge metal atoms and nanocrystals, are gaining attention due to their unique chemical and physical properties which differ greatly from their corresponding large nanoparticles and molecular compounds. Their electronic and optical properties are of particular interest for their use in sensing, optoelectronics, photovoltaics and catalysis.

The book highlights recent progress and challenges in size-controlled synthesis, size-dependent properties, characterization and applications of metal nanoclusters. Specific topics include organochalcogenolate-stabilized metal nanoparticles, water-soluble fluorescent silver nanoclusters, thiolate-protected Au and Ag nanoclusters, DNA-templated metal nanoclusters, fluorescent platinum nanoclusters and janus nanoparticles by interfacial engineering.

Edited by active researchers in the area, the book provides a valuable reference for researchers in the area of functional nanomaterials. It also provides a guide for graduate students, academic and industrial researchers interested in the fundamentals of the materials or their applications.

Excerpt. © Reprinted by permission. All rights reserved.

Functional Nanometer-Sized Clusters of Transition Metals

Synthesis, Properties and Applications

By Wei Chen, Shaowei Chen

The Royal Society of Chemistry

Copyright © 2014 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-84973-824-8

Contents

Chapter 1 Mechanistic Insights into the Brust–Schiffrin Synthesis of Organochalcogenolate-Stabilized Metal Nanoparticles Yuan Gao, Yangwei Liu, Ying Li, Oksana Zaluzhna and YuYe J. Tong, 1, 
Chapter 2 New Strategies and Synthetic Routes to Synthesize Fluorescent Atomic Quantum Clusters Beatriz Santiago González and M. Arturo López-Quintela, 25, 
Chapter 3 Silver Magic-Number Clusters and Their Properties Brian A. Ashenfelter, Anil Desireddy, Jingshu Guo, Brian E. Conn, Wendell P. Griffith and Terry P. Bigioni, 51, 
Chapter 4 Synthesis and Applications of Water-Soluble Fluorescent Silver Nanoclusters Hangxun Xu and Kenneth S. Suslick, 80, 
Chapter 5 Synthesis and Applications of Silver Nanoclusters Protected by Polymers, Protein, Peptide and Short Molecules X. Yang and E. K. Wang, 100, 
Chapter 6 Novel Synthetic Strategies for Thiolate-Protected Au and Ag Nanoclusters: Towards Atomic Precision and Strong Luminescence Xun Yuan, Qiaofeng Yao, Yong Yu, Zhentao Luo and Jianping Xie, 131, 
Chapter 7 Noble Metal Clusters in Protein Templates Thalappil Pradeep, Ananya Baksi and Paulrajpillai Lourdu Xavier, 169, 
Chapter 8 Metal(0) Clusters in Catalysis Noelia Vilar-Vidal, José Rivas and M. Arturo López-Quintela, 226, 
Chapter 9 Metal Nanoclusters: Size-Controlled Synthesis and Size-Dependent Catalytic Activity Yizhong Lu and Wei Chen, 261, 
Chapter 10 Metal Clusters in Catalysis Seiji Yamazoe and Tatsuya Tsukuda, 291, 
Chapter 11 In Silico Studies of Functional Transition Metal Nanoclusters Lichang Wang and Pamela C. Ubaldo, 323, 
Chapter 12 DNA-Templated Metal Nanoclusters and Their Applications Zhixue Zhou and Shaojun Dong, 352, 
Chapter 13 Synthesis of Fluorescent Platinum Nanoclusters for Biomedical Imaging Shin-ichi Tanaka and Yasushi Inouye, 391, 
Chapter 14 Janus Nanoparticles by Interfacial Engineering Yang Song, Xiaojun Liu and Shaowei Chen, 407, 
Subject Index, 434, 

CHAPTER 1

Mechanistic Insights into the Brust–Schiffrin Synthesis of Organochalcogenolate-Stabilized Metal Nanoparticles

YUAN GAO, YANGWEI LIU, YING LI, OKSANA ZALUZHNA AND YUYE J. TONG

Department of Chemistry, Georgetown University, 37th & O Streets, NW, Washington, DC 20057, USA

1.1 Introduction

Metal nanoparticles (NPs) made of tens, hundreds, or thousands of atoms can have tunable chemical and physical properties as a function of NP size (number of atoms), elemental composition, and/or chemical environment (ligand-stabilized, matrix-embedded, or structurally-encaged). These NPs are artificial atoms and novel building blocks for new materials that hold novel physicochemical properties as compared to the existing (atomic/molecular) materials. It is expected that these novel materials will enable widespread technological breakthroughs in the not too distant future, for instance in molecular and/or nano-electronics and clean energy generation. Within this broad context, organoligands, particularly organothio-late-stabilized metal (mainly Au) NPs, have been subjected to intensive research over the last two decades due to their potential applications in nano-optics, nano-electronics, (bio)sensing and medicinal science (theranostics).

The first step towards any practical applications of metal NPs is the synthesis of these metal NPs, preferably air-stable and of homogeneous size distribution and known chemical composition. Among many synthetic methods, the Brust–Schiffrin two-phase method (BSM) synthesis worked out by Brust, Schiffrin, and company in 1994, including its late variants, is definitively the most widely employed synthetic approach to make <5 nm organo-ligand-stabilized metal NPs. Briefly, a typical BSM consists of three steps: Step 1, metal ions are phase transferred (PT-ed) from an aqueous to an organic phase (usually toluene or benzene) with a PT reagent (usually tetraoctylammonium bromide (TOAB), i.e. R4NBr, R = C8H17). Step 2, organochalcogen-containing ligand (usually RSH) is added to the separated organic phase during which AuIII cations can be reduced to AuI cations. Step 3, metal ions residing in the separated organic phase are reduced into M0 by a reducing reagent like NaBH4 during which organochalcogenolate-protected metal NPs are formed.

Despite the prevailing use of the BSM in the synthesis of sub-5 nm metal (mainly Au) NPs (according to Thomson Reuters' Web of Knowledge, the original paper has accumulated a current number of citations as high as 3755, and counting), mechanistic details of the BSM synthesis have been sketchy until very recently. A long-held belief concerning the metal precursor in the synthesis of metal NPs, probably due to earlier papers by Whetten et al., has been that the metal-thiolate polymer, [AuISR]n, is the metal ion precursor of metal NPs. However, a recent paper by Goulet and Lennox has shown that the metal–TOA+ complex, [TOA][AuIBr2], can also be the major metal ion precursor. Our ensuing studies have not only confirmed the results of Goulet and Lennox, but also proposed that the BSM synthesis is an inverse micelle based approach based on their proton NMR results and showed via Raman spectroscopic study that the Au–S bond does not form until the formation of Au NPs. In this chapter, we will review and discuss in various degrees of detail the relevant chemistry involved, particularly the role of encapsulated water, in the BSM synthesis of alkyl-chalcogenolate-stabilized metal NPs unravelled after the paper of Goulet and Lennox and highlight the similarity and difference when ligands containing different chalcogen elements (S, Se, or Te) are used as the starting source of the NP-stabilizing agents.

1.2 Phase Transfer of Metal Ions: Formation of Inverse Micelle Encapsulated Water

1.2.1 Proton NMR Evidence of Encapsulated Water

The experimental evidence of possible encapsulated water by TOAB in an organic phase came first from the observation of a large down-field shift (~2 ppm, due largely to the appearance of hydrogen bonding among the water molecules that strongly suggests the formation of water aggregates) of the water proton peak in C6D6 containing dissolved TOAB (0.03 mmol of TOAB in 0.8 mL C6D6) as compared to that of pure C6D6 after both being mixed with 0.105 mL Milli-Q water (18.2 MΩ) and the extra water layer being then removed, as shown in Figure 1.1. The water peak at 2.43 ppm was also reported in the Goulet and Lennox paper.

More convincing and detailed evidence of the inverse micelle formation is shown in Figure 1.2, proton NMR spectra of a series of samples prepared by mixing various amounts of TOAB with 0.8 mL C6D6 and 0.210 mL Milli-Q water and then separating the undissolved water. The two clearly distinguishable regimes enable the critical micelle concentration (CMC) to be determined: the intersection of the two dashed lines gives the CMC of TOAB in C6D6 = 7.5 mM, which is about 4 to 5 times smaller than the TOAB concentrations generally used in a typical BSM synthesis of metal NPs. That is, under the normal condition of the BSM synthesis, inverse micelles enclosed by TOAB are formed.

Unlike other well-known inverse micelle systems, such as sodium 2-ethylhexylsulfosuccinate (AOT), whose size can be readily varied by changing water/surfactant ratio, our proton NMR data show that the inverse micelles of TOAB can be saturated even with a very small amount of water, as shown by the proton NMR spectra in Figure 1.3A and the corresponding normalized peak integrals in Figure 1.3B. The samples for the spectra in Figure 1.3A were prepared with the same amount of TOAB (0.03 mmol) in 0.8 mL C6D6 but mixed with different amounts of water before the organic phase being separated for the NMR measurements. Interestingly, the amount of encapsulated water remained constant even if the water amount varied from 0.0425 mL to 0.21 mL, so did the peak positions of the encapsulated water and –CH2N+.

With the information in Figure 1.3, we can estimate the average size of the inverse micelles. According to Figure 1.3B, the average number of protons from the H2O in Samples (a–d) is 5.11 per TOAB. After subtracting the number of protons from the H2O saturated in C6D6 (i.e. 1.63 per TOAB (Sample (f)), the number of protons from the H2O encapsulated in the inverse micelles formed by TOAB is 3.48, which means that the ratio of the encapsulated H2O to TOAB is 1.74. Assuming that one molecule of TOAB is surrounded by one molecule of H2O, 57% of the encapsulated H2O might be located in the outer layer of the inverse micelle core. Together with the fact that the H2O volume per molecule is 0.03 nm3 at 20 °C (density of H2O: 0.998 g mL-1), the diameter of the inverse micelle core, i.e. the part of the H2O encapsulated in the inverse micelle of TOAB, is ~2.5 nm, which is at the small end of known inverse micelles.

1.2.2 Phase Transfer of Metal Ions: Formation of Metal Complex

The first step of the BSM synthesis is the phase transfer (PT) of metal ions from the initial aqueous solution to the organic phase using TOAB as PT agent. The proton spectra in Figure 1.4A were obtained after the PT of different amounts of Au ions into the organic phase. The corresponding normalized proton peak integrals and sample preparation parameters are collected in Figure 1.4B. In addition to constant C6D6 volume (0.8 mL) and amount of TOAB (0.03 mmol), the volumes of the aqueous solutions were kept the same as used for Figure 1.3. Thus, the corresponding TOAB : Au ratios were 1 : 1, 2 : 1, 3 : 1, and 5 : 1 for spectra (g) to (j) respectively. For comparison, the 1H NMR spectra of the synthesized [TOA][AuBr4] complex (0.0125 mmol) in 0.8 mL of C6D6 with (0.09 mL) and without H2O mixing are shown in spectra (k) and (l) respectively.

As can be seen in Figure 1.4A, the introduction of metal species not only caused clear variation of peak positions of the encapsulated water and α-proton in –CH2N+, but also the amount of encapsulated water. The pair (H2O/–CH2N+) peak values are now down-field shifted as the Au content decreases: 0.70/2.97, 2.34/3.18, 2.63/3.22, and 2.69/3.26 ppm for TOAB : Au ratios of 1 : 1, 2 : 1, 3 : 1, and 5 : 1, respectively. Notice that the α-1H peak position of –CH2N+ in the TOAB : Au = 1 : 1 case [Figure 1.4(g)] is the same as that of the synthesized [TOA][AuBr4] complex [Figure 1.4(k), (l)]. This indicates strongly that one [TOA]+ cation was associated with one [AuIIIX4]-anion with X = Br- or/and Cl-. As the TOAB : Au ratio increases, which corresponds to a decrease of Au content because a constant TOAB amount was used, more and more [TOA]+ cations can be associated with halogen anions (e.g., Br- or Cl-) other than the AuIII complex. However, the fact that only one α-1H peak of –CH2N+ was observed in all cases is a sign of fast exchange between the AuIII complex units and the halogen anions in the inverse micelles.

Now taking 2.97 ppm [Figure 1.4(g), (k), (l)] and 3.34 ppm [Figure 1.3(a) to (d)] as the α-1H peak positions of the –CH2N+ in two extremities, i.e., all AuIII units vs. all halogen anions, we calculated the expected peak position using the fast-exchange model δ(α-1 H) = 2.97y + 3.34(1 - y) (in ppm) where y is the fraction of the AuIII units among the total anions (i.e., [AuX4]- and X1). We found 3.16, 3.22, and 3.27 ppm for TOAB : Au ratios of 2 : 1, 3 : 1, and 5 : 1, respectively. These should be compared with the experimentally observed values: 3.18, 3.22, and 3.26 ppm [Figure 1.4(h) to (j)]. That these two sets of values are in excellent agreement indicates strongly that the partitioning of the AuIII units and the halogen anions in a given inverse micelle follows the nominal stoichiometry.

Figure 1.5 presents the 1H NMR spectra of samples containing Ag ions in the organic phase after the PT. The samples were prepared with 0.8 mL C6D6, 0.03 mmol of TOAB, 0.4 mL aqueous solution of AgNO3 of different concentrations. After the PT of Ag ions, the organic phase was collected and used for NMR measurements. As can be seen from the figure, the peak positions of water are very similar to those in Figure 1.4A, indicating the formation of inverse micelles. Moreover, the same trend in down-field shift and increase of water amount were observed as the Ag content decreases in the order TOAB : Ag = 3 : 1, 3.5 : 1, 4 : 1, and 5 : 1. The peak integrals for the encapsulated water (normalized by setting that of α-1H of –CH2N+ to 8) are 2.7, 2.9, 3.4, and 3.7 respectively, which are also close to those found in the Au case (see Figure 1.4B).

We show in Figure 1.6 the 1H NMR spectra of the organic phase after PT of PdII (from PdCl2) or PtVI (from H2PtCl6). The samples were again prepared with constant C6D6 volume (0.8 mL) and constant TOAB amount (0.03 mmol) but various metal content: TOAB : Pd or Pt = 1 : 1, 2 : 1, 3 : 1, and 5 : 1 for spectra (a) to (d) respectively. Interestingly, as the metal content decreases, we observe again the down-field shift and amplitude increase of the water peak for both PdII and PtVI samples, as those observed for the Au and Ag samples in Figures 1.4A and 1.5, respectively, also suggesting the formation of inverse micelles. However, the trend of variation for the α-1H peak of – CH2N+ in the PdII case (left panel in Figure 1.6) is opposite to those of the three other metals, for which the chemical reason is still unclear but probably has to do with the difference in the type of complex structure formed with TOA+. Notwithstanding such difference, the water aggregates behave in a remarkably similar fashion that is related directly to the formation of inverse micelles, as clearly alluded to by the results shown in Figure 1.6.

1.3 Addition of Ligand: Reduction of Metal Complex or Formation of Polymeric Species

1.3.1 Alkylthiols RSH

The second step in a typical BSM synthesis is to add ligand to the organic phase that contains PT-ed metal ions. This is the step where divergence exists in the literature as to what precursor species become the PT-ed AuIII ions. Earlier work by Whetten asserted that the AuIII ions were reduced to AuI and formed [AuIRS]n polymeric species. This assertion has been widely accepted, cited, or/and assumed as the Au precursor species in the BSM synthesis until the Goulet and Lennox paper in which [TOA][AuIX4] and [TOA][AuIX2] complexes were shown to be the relevant metal ion precursors, which also applies to Ag and Cu. Our work confirms this important discovery by showing that no metal–sulfur bond is formed during this step, as presented in Figure 1.7.

It is expected that a Au–S bond will form when RSH self-assembles on a Au surface or polymeric [AuSR]n species are formed. This is indeed what we have observed, as shown in Figure 1.7 by the presence of a Au–SR vibrational band at 327 cm-1 in the Raman spectra (c) and (d) for C12SH self-assembling on a rough Au surface and for the synthesized [AuSR]n-like polymer respectively. However, no such vibrational band was observed when C12SH was added to an organic phase of TOAB that contained PT-ed Au ions obtained in the first step of a BSM synthesis (see spectrum (e) in Figure 1.7), indicating no formation of the Au–S bond that is expected for the presence of polymeric [AuSR]n species. The appearance of a Au–Br band at 209 cm-1 in spectrum (e) is direct experimental evidence confirming the exchange of Br- from TOAB with Cl- in the original (AuCl4)-.

Figure 1.8 shows the Raman spectra of species formed in the 2nd step of the BSM synthesis of Ag (A) and Cu (B) NPs and of some reference materials. Again, no Ag–S or Cu–S vibrational band was observed among the species formed in the 2nd step of the BSM synthesis: spectrum (c) in Figure 1.8A and B. No RS–SR but RS–H observed in the Ag case indicates that no reduction of AgI took place. On the other hand, a strong RS–SR band was observed in the Cu case, illustrating that the added C12SH reduced CuII to CuI without forming a Cu–S bond.

Figure 1.9A presents the proton NMR spectra of the organic phase obtained in the 2nd step of the BSM synthesis for samples (g) to (k) shown in Figure 1.4A, together with those of reference disulfide (spectrum (r)) and thiol (spectrum (s)). The formation of disulfide (peak at 2.58 ppm) in all samples indicates that the added thiol reduced AuIII to AuI ions, which was also observed by Goulet and Lennox previously. The appearance of even more down-field shifted water peaks evidences the acidification of the encapsulated water by the reaction

[TOA][AuIIIX4] + 2RSH [right arrow] or [vector] [TOA][AuIX2] + RSSR + 2HX (1.1)

in which acidic protons were generated. That is, the existing inverse micelle encapsulated water or/and organic-solvent-dissolved water provided a hydrophilic receiving medium for the reaction-generated protons, enabling Reaction (1.1) to proceed forward readily.

(Continues...)

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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2014
• Language: English
• ISBN 10: 1849738246
• ISBN 13: 9781849738248
• Binding: Hardcover
• Number of pages: 463
• Editor: Wei Chen