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Theses and Dissertations2014
M4Ag44 (p-MBA) 30-4 molecular nanoparticles
Brian E. Conn University of Toledo
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A Master’s Thesis Titled
M4Ag44(p-MBA)30 Molecular Nanoparticles by
Brian E. Conn
Submitted to the Graduate Faculty as partial fulfillment of the requirements for The Master of Science Degree in Chemistry
_______________________________________
Dr. Terry Bigioni, Committee Chair
_______________________________________ Dr. Wendell P. Griffith, Committee Member
_______________________________________ Dr. Dragon Isailovic, Committee Member
_______________________________________ Dr. Patricia R. Komuniecki
Dean of College of Graduate Studies
The University of Toledo August 2014
Copyright 2014, Brian Conn.
This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.
An abstract of
Ag44(p-MBA)30-4 Molecular Nanoparticles by
Brian E. Conn
Submitted to the Graduate Faculty as partial fulfillment of the requirements for The Master of Science Degree in Chemistry
The University of Toledo August 2014
In recent years, molecular nanoparticles have attracted much attention due to their unique physical, optical, and electronic properties. The properties of molecular
nanoparticles are shown to deviate from their larger bulk counterparts, due to quantum confinement effects and large surface-to-volume ratios. As the size of the nanoparticle shrinks to a cluster of metal atoms (<3 nm in diameter), there is an emergence of a HOMO-LUMO band gap, which is not present in transitional d-block metals. The
HOMO-LUMO band gap gives rise to discrete electronic states, leading to new chemical and physical properties. Molecular nanoparticles have had a substantial impact across a diverse range of fields, including catalysis, sensing, photochemistry, optoelectronic, energy conversion, and medicine.
Currently many of the synthetic procedures for molecular nanoparticles require low temperatures, long incubation times, multistep purification and hazardous reagents that produce low yields and polydisperse molecular nanoparticles with poor stability.
Although silver has very desirable physical properties, good relative abundance and low cost, gold molecular nanoparticles have been widely favored owing to their proved
stability and ease of use. Unlike gold, silver is notorious for its susceptibility to oxidation,
i.e., tarnishing, which has limited the development of silver-based nanotechnologies.
Despite two decades of synthetic efforts, silver molecular nanoparticles that are inert or have long-term stability have remained unrealized. Herein we report a simple synthetic protocol for producing ultrastable M4Ag44(p-MBA)30 nanoparticles as a single-sized molecular product and in exceptionally large quantities. The stability, purity, and yield are substantially better than other metal nanoparticles, including gold, due to several stabilization mechanisms.
Also, reported are the structural and mechanical properties of extended crystalline solids of Na4Ag44(p-MBA)30 from large-scale quantum-mechanical simulations based on the atomically-precise X-ray measured structure. Calculations show that cohesion is derived from hydrogen bonds between bundled p-MBA ligands and that the superlattice’s mechanical response to hydrostatic compression is characterized by a molecular-solid-like bulk modulus B0 = 16.7 GPa, exhibiting anomalous pressure softening and a compression-induced transition to a soft-solid phase. Such a transition involves ligand flexure, which causes gear-like correlated chiral rotation of the nanoparticles.
Acknowledgements
During the time I spent completing the requirements for my master of chemistry, many professors, staff workers, and students have helped me along the way. I would like to acknowledge the people that have helped me obtain my master of chemistry. First and foremost, I would like to thank my research advisor Dr. Terry P. Bigioni whose guidance allowed me to become a successful graduate student. During my studies, Dr. Bigioni showed me what is necessary to be a competent and productive research scientist through mentorship and scientific discussion.
I would like to also extend my deepest gratitude to my committee members Drs. Wendell Griffith, and Dragan Isailovic. I want to give thanks to Dr. K. Kirschbaum, Dr. P. Burckel, Y. Kim, and L. Hanson for all their help using the I-Center and interpreting the collected data. I would like to acknowledge Anthony Kaminski for all his help in the stockroom. I would like to thank the University of Toledo Chemistry Department for allowing me the opportunity to purse a higher education.
Lastly, I would like to give thanks to all my friends, present and past lab members for all their support. I would like to give an extra special thanks to my lab mentor, Anil Desireddy for all his advice and words of encouragement. Also, would like to thank Brian Ashenfelter, Charak Joshi, and Brad Monahan for all the insightful laboratory
discussions. Most importantly, I would like to thank my mother Trish, father Fred, older brother David and the rest of my family members for their years of patience and love.
Table of Contents Abstract……….. iii Acknowledgements………..v Table of Contents………....vi List of Tables……… ix List of Figures………. x List of Abbreviations ...xv 1 Introduction………. 1 1.1 “Small is Different”………...1
1.2 Electronics Configurations of Nanoclusters………..3
1.3 Molecular Nanoclusters……….5
1.4 References………10
2 Ultrastable silver nanoparticles………..15
2.1 Ultrastable silver nanoparticles………15
2.2 Methods Summary………...27
2.2.1 Synthetic Methods………27
2.2.2 X-ray Crystallography………..27
2.2.3 Computational Methods………28
2.3 References………29
3.1 Introductions………33
3.2 Experimental………35
3.2.1 Chemicals………..35
3.2.2 Synthesis………...36
3.3.3 Electrospray Ionization Mass Spectrometry (ESI-MS)……37
3.2.4 Optical Measurements………..37
3.2.5 NMR……….38
3.2.5 Stability Measurments………..38
3.3 Results and Discussion………39
3.3.1 Synthesis………...39
3.3.2 Coordinating Solvent………43
3.3.3 Stability and Seeding………47
3.3.4 Decay………49
3.4 References………52
4 Silver nanoparticle superlattice 4.1 Hydrogen bonded structure and mechanical chiral response of a silver nanoparticle superlattice………..55
4.2 Methods………67
4.2.1 Computational Methods………67
4.3 References………68
Appendix A………..76
Appendix B………...………99
References………115
List of Tables
A1 Radial distances from the single crystal of Na4Ag44(p-MBA)30………94
A2 Interatomic distances from the single crystal Na4Ag44(p-MBA)30………95
B1 Average and standard deviations( in parenthesis) of the radial distances of the metals core atoms organized in shells in the Na4Ag44 (p-MBA)30 nanoparticle..100
List of Figures
1-1 (a) The mass spectrum of the sodium clusters in the gas phase with N = 4-75. (b) The energy levels of the delocalize orbitals of a gas phase sodium cluster with
N =40.………..3
1-2 A series of gold thiolated molecular nanoparticles separated by polyacrylamide gel electrophoresis (PAGE), and gold content was determined by electrospray
ionization mass spectroscopy………..7
1-3 Ag:SG and Au:SG bands from the same PAGE gel, using concentrations of 15 and 30 mg/mL, respectively. The number of atoms in each Au:SG band is
indicated………..8
1-4 UV/Vis absorption spectra of Ag44(SR)30-4 molecular nanoparticle synthesis with different aryl-thiol ligands……….10
2-1 Absorption spectrum of the M4Ag44(p-MBA)30 raw product solution (red line) synthesized in the presence of “seed” M4Ag44(p-MBA)30 clusters (open circles). Inset: 140 grams of M4Ag44(p-MBA)30 clusters pictured with two 1 oz. silver coins for scale. The dish is 18 cm in diameter………..17
2-2 Electrospray-ionization mass spectra (ESI-MS) of the final product without size
separation shows that only one species is present………18
2-4 Projected densities of states (PDOS) and orbital images………...26
3-1 M4Ag44(p-MBA)30 nanoparticles shown in solid and in solution forms. The dice represent the four Platonic solids that can be found in the structure……….34
3-2 M4Ag44(p-MBA)30 absorbance. Molar absorptivity of M4Ag44(p-MBA)30 nanoparticles in DMF solution. Inset shows the absorbance as a function of
energy……….40
3-3 ESI-MS of M4Ag44(p-MBA)30. (A) Raw product, containing excess alkali metal counterions. (B) Fully protonated final product………42
3-4 M4Ag44(p-MBA)30 crystal structure. Space-filling view of the structure down a 3-fold axis. Ligand bundling creates gaps that leave the Ag atoms in the mounts
3-5 H1 NMR and NOSY of M4Ag44(p-MBA)30. (left-a) Aromatic protons are shown
for nanoparticles in DMF. (right-b) NOSY shows coupling between bridging ligands and ligands at the base of the mounts, as well as coupling between ligands at the base of the mounts.(blue) exposed to chemical attack. Color scheme: grey – C; orange – O; yellow – S; blue – Ag in mounts; green – Ag in decahedral outer
core shell………45
3-6 Spectral evolution during M4Ag44(p-MBA)30 synthesis. Silver thiolates at 0 min are reduced to immediately form M4Ag44(p-MBA)30 nanoparticles, with
characteristic spectra observed beginning after 1 min and persisting for the entire
3-7 Temporal stability of M4Ag44(p-MBA)30 in DMSO. Aged DMSO solutions of M4Ag44(p-MBA)30 retain their characteristic spectra, indicating that no other nanoparticle species were produced. Absorbances were not rescaled…………..50
3-8 Polymerization of M4Ag44(p-MBA)30. (left) Dissolving the product in neat water leads to the formation of plasmonic Ag nanoparticles. (right) Electron micrograph of the resultant plasmonic nanoparticles………51
4-1 Ag44(p-MBA)30.Na4 superlattice structure……….59 4-2 Superlattice compression, and rotational structural transition………...61 4-3 Intra- and inter- nanoparticle distances and angles induced by the applied
compression (V/V0) of the superlattice……….64
A1. Gel electrophoresis of Ag clusters run on the same gel. (A) Glutathione-capped Ag clusters, with Ag32(SG)19 indicated. (B) p-MBA-capped Ag clusters, with three major bands observed. (C) Absorption spectra of the three prominent gel bands, offset for clarity………..80
A2. (a) ESI-MS of the final product without size separation shows that only one species is present, with other peaks accounted to different charge states, fragments and non-specific dimerization. (b) Isolated Ag44L30−4 ions spontaneously fragment into Ag43L28−3 and AgL2− when desolvated, where L is p-MBA. Inset: the
experimental data (black) were fit (blue) using a simulation of the Ag44L30−4 ion isotopic distribution (red bars) and that of its Na salt (green bars)………82
A4. (A) Gel electrophoresis of Au:SG clusters synthesized with and without Au25SG18 cluster seeds. Synthesis with seeds (middle) shows that the Au25SG18 clusters reacted to form larger clusters. Adding seeds after the synthesis shows the result expected had the Au25SG18 clusters been inert. (B) Comparison of
M4Ag44(p-MBA)30 and Au25SG18 solution ambient stability, with linear fits shown in red. Open symbols for Au indicate that the spectra contain other species…....87
A5. Optical micrographs of typical crystals of M4Ag44(pMBA)30 clusters using (left) episcopic and (right) diascopic illumination………89
A6. Radial atomic distance distribution, with respect to the center of the x-ray determined structure of the Ag44(pMBA)30-4 cluster. The radial distances of the 6 outer sulfur atoms is given in black (centered about 8.2 Å). A Gaussian convolution with σ = 0.03 A was used………..92
B1(a) Optimal structure of the Na4Ag44(p-MBA)30 nanoparticle showing silver core and p-MBA ligands………..99
B1(b) Two views (a and b) of the p-MBA ligand-ligand binding between two
neighboring silver nanoparticles, both located in the same layer of the superlattice………...101
B1(c) Two views (a and b) of the p-MBA ligand-ligand binding between two
neighboring silver nanoparticles, located in neighboring layers of the superlattice………..102
B2(a) Intralayer and interlayer NP rotations. Configuration of two neighboring
nanoparticles, both located in the same layer of the superlattice………108
B2(b) Intralayer and interlayer NP rotations. Configuration of two neighboring
nanoparticles, located in neighboring layers of the superlattice, denoted as α and β………109
B2(c,d) Configurations of the superlattice viewed normal to the (a,b) [or (x,y) plane]
plane (as in Figure B2(a). The configuration in (C) corresponds to V/V0 = 1.0 and the one in (d) corresponds to the end of the compression process at V/V0 =0.71………110
B3 Torsion angles plotted versus the compression parameter V/V0……….112
B4 Compression-induced changes in the structure of the Na4Ag44(p-MBA)30 nanoparticle in the superlattice………113
List of Abbreviations
APS ...Ammonium Persulfate
CTAB...Cetyl Trimethyl Ammonium Bromide
DCTB ...Trans-2-[3-(4-tetra-butylphenyl)-2-methyl-2-propenylidene]
malononitrile
DHB ...2,4 Dihydroxy Benzoicacid
DLVO ...Derjaguin and Landau, Verwey and Overbeek DMF ...Dimethyl Formammide
DMSO...Dimethyl Sulfoxide
EDS ...Energy Dispersive Spectroscopy
ESI-MS ...Electrospray Ionization Mass Spectrometry
FFT...Fast Fourier Transformation
GGA ...Generalized Gradient Approximation GSH...Glutathione
HDMS ...High Definition Mass Spectrometer HOMO ...Highest Occupied Molecular Orbital
IBANs ...Intensely and Broadly Absorbing Nanoparticles
ICP-OES ...Inductively Coupled Plasma Optical Emission Spectrometry
LDI...Laser Desorption/Ionization
LUMO...Lowest Unoccupied Molecular Orbital MALDI ...Matrix Assisted Laser Desorption/Ionization MEF ...Metal Enhanced Fluorescence
MS...Mass Spectrometry
PAGE ...Polyacrylamide Gel Electrophoresis PDOS ...Projected Density of States
p-MBA ...para-Meracapto Benzoic Acid
SC-XRD...Single Crystal X-ray Diffraction SDS ...Sodium Dodecyl Sulfate
SEM ...Scanning Electron Microscope SERS...Surface-Enhanced Raman Scattering
STEM...Scanning Transmission Electron Microscope
TDDFT...Time-Dependent Density Functional Theory TEM ...Transmission Electron Microscope
TEMED...Tetramethylethylenediamine THAM...Tris(hydroxymethyl) Aminomethane THF ...Tetrahydro Furan TMAD...Tetramethylammonium decanoate TOAB...Tetraoctylammoniumbromide TOF ...Time-of-Flight
Chapter 1
Introduction
1.1 “Small is Different”
Molecular nanoparticles are a collection of atoms in a finite aggregation that range between 2-1000 atoms.1-3 In the nanometer’s size regime, the properties of bulk materials transition into atoms and molecules, and it is during this transition that nanoclusters may yield divergently different properties from their bulk counterpart.1, 4 The phenomenon of quantum confinement plays a dominant role in the emergence of unique size-dependent physical and chemical properties of a molecular nanoparticle when it is in the size range of <3 nm.1,2,3,5 The unique size-dependent properties of molecular nanoparticles allow them to have a substantial impact across a diverse range of fields, including catalysis,6 sensing,7 photochemistry,8 optoelectronics,9,10 energy conversion,11 and medicine.12 The characteristics of the nanoclusters’ unique properties arise from their large surface-to-volume ratios, 1,2,4 quantum confinement, 1-5,13 and structural and
energetic size effects.1,2,3,13
As the volume of the molecular nanoparticles decreases, the ratio of the surface area-to-volume increases.1,2,4 This increased surface area-to-volume means the
constituent surface atoms have fewer nearest neighbors, and therefore, fewer satisfied bonds. 4 These unsatisfied bonds require additional energy to maintain the bond distance
between the surface atoms and the atoms of the underlying layer. 4 This added energy proves to have a significant role in determining the overall stability, the structure, and energetics of the nanoparticle, and rationalizes why the addition or removal of one atom may have a remarkable altering effect to the nanoparticle’s integrity.1
The small sizes of molecular nanoparticles can generate multiple size effects that will dictate their chemical, physical, and electrical properties, and subsequently allow for these properties to deviate from those of the bulk materials.1-6, 13-15 The unique properties of molecular nanoparticles may be attributed to the quantum confinement of their
electrons.13-15 The quantum confinement of the electrons yield quantized energy levels that generate a highest occupied molecular orbital and a lowest unoccupied molecular orbital (HOMO-LUMO) band gap13-16. Possession of a HOMO-LUMO gap is common for molecules, but for transition d-block metals it is considered an anomaly. When metals are reduced to just a few atoms in diameter, their continuous conduction band transforms into discrete energy levels12-14. The physical and chemical properties of molecular nanoparticles are strongly dependent on the electronic transition between the HOMO and LUMO gap14, 15. Therefore, as the molecular nanoparticles becomes smaller (or larger) the electronic transitions between its HOMO-LUMO gap are altered, and so are the chemical and physical properties of the nanoparticle. Once the molecular nanoparticle exceeds a certain size limit, further enlargement of the cluster proves to have minimal effect on its properties. However, because shrinking a material into the nanometer size range produces new and unique properties this validates the statement that “small is different”.3,17,18
3
1.2 Electronics Configurations of Molecular Nanoparticles
Elements with noble gas configurations are chemically inert due to their electronic shells being closed.19 The stability of molecular nanoparticles are strongly dependent on their electronic configurations. Knight et al. discovered when sodium cluster are formed in the gas phase there is mass distribution pattern. 20 As shown in Figure 1-1 (a), the sodium clusters mass abundances were predominately higher for clusters with N = 8, 20, 40, 58 and 92, where N is the number of sodium atoms in the cluster.20,21 The relative abundances of the sodium clusters can be associated with their relative stability. 19-21 Sodium cluster with N = 8, 20, 40, 58 and 92 are shown to have the higher stability, because they have a complete electronic shell configuration, as shown in Figure 1-1 (b) of N = 40. 19-21 When electronic shells are filled, they produce a potential energy minimum that lowers the free energy, which supplies additional stability to the molecular nanoparticles.19,22
Figure 1-1: (a) The mass spectrum of the sodium clusters in the gas phase with N =
4-75.20 (b) The energy levels of the delocalize orbitals of a gas phase sodium cluster with N = 40.21 Reprinted with permission from reference 20. Copyright 1993 American Physics Society.
Figure 1-1: Sodium has one free 3s electron and is known to form abundant
gas-phase clusters of specific numbers of atoms (a) Gas gas-phase abundance of sodium clusters of different size (b) Effective potential of Na40 cluster as function of radius. Reprinted with permission from reference 19 (copyright 1993, APS)
spectrometry (ESI-MS).7-10 If crystallized, their total structure can even be determined by single-crystal X-ray diffraction (sc-XRD).4,11-14
Magic-number theory was developed to explain the stability of certain cluster sizes in a gas phase cluster beam.5,15-18 Cluster stability was found to depend on two things: (1) electronic shell closings and (2) geometric shell closings.5,6,18-20 This magic-number theory has now been successfully applied to explain the anomalous stability of thiolate-passivated gold clusters in the condensed phase.5
Sodium has one free 3s electron and is known to form abundant gas-phase clusters of specific numbers of atoms, as shown in Figure 1-1a.19 This is due to the stability created by an electronically closed shell, as shown in Figure 1-1b.19
1.2 Electronic structure theory (electronic shell closing model)
In the gas phase there are limited cluster-to-cluster interactions, which are assumed in the ideal gas law. The limited cluster-to-cluster interactions limit the
aggregation process of molecular nanoparticles, but the cluster-to-cluster interactions are not completely impeded in the gas phase. This is why even though N = 8, 20, 40, 58 and 92 sodium cluster have the highest stability; they are not the only sodium clusters present in the gas phase. In contrast to the gas phase, the condensed phase of clusters have significant cluster-to-cluster interactions. Therefore, when clusters are in the condensed phase, they must be a protected by a capping group that stabilize and prevent
nanoparticles aggregation by hindering the interactions between neighboring clusters. The protecting or capping groups on clusters are called ligands, and are directly coordinated to the metal core of the cluster. 19 The ligand is usually an organic compound that attaches to the metal core of the nanoparticle by covalent bonds or as a weak Lewis base.19 Once the ligands are coordinated to the metal cluster, they contribute to the overall electronic configuration of the cluster. 19 A theory was developed that determines the electron configuration of a cluster or molecular nanoparticles, and is analogous to atomic theory and the Aufbau rule.19
The electrons of molecular nanoparticles behave as “superatom electronic complexes” rather than individual atoms.19, 22 “Superatom electronic complexes” have metal atoms coordinated to localized ligands, which contributes to the overall electronic shell configuration.19,22 Therefore, both the metal core and ligands attributed to the closing of the electron shell configuration for “superatom complexes”.19 Walters et al. developed the superatom electronic theory, which predicts the chemical identity and stability of simple metal molecular nanoparticles. 19 The superatom electronic theory
accounts for the delocalized orbitals of the molecular nanoparticle, and the electric shell configuration is 1S2|1P6|1D10|2S2 1F14|2P6 1G18|...19 The electronic shell filling rules are predicted by the equation:
N* = Nν
A– M – z
Where N* is the shell- closing electron count, N is the number of metal atoms, νA is the number of valence electrons of the metal atom, M is the number of electron withdrawing (or localizing) ligands, and z is the overall charge on the cluster. 19
1.3 Molecular Nanoparticles
Nanoparticles have been used as early as in the 4th century23, but it wasn’t until the mid 1800’s that Michael Faraday first published his discovery of colloidal gold nanoparticles.23 Over the next century there was very little research pertaining to nanoparticles, and this was due to the limiting characterization techniques. The
emergence of the high resolution microscopes, such as SEM, TEM, STM and AFM, as well as, popularized nanotechnology writings of Richard Feynman and Eric Drexler in their titled works “There’s plenty of room at the bottom,” and “Engine of creations,” respectfully, led to cultivation of interest in nanoscience. The unique optical, structural, electronic, and physical properties of nanoparticles are what sustained the simulation of research in nanomaterials. 1-3,5,19,22,24,25 One of the major synthetic milestones for nanoparticles was by Brust et al., where they reported the synthesis of highly stablized thiol capped gold nanoparticles.26 The synthesis is now referred to as the Brust-Schiffrin synthesis, and it allowed for the wide availability of gold nanoparticles for research.26
Whetten et al. published a landmark paper in the field of molecular nanoparticles in 1996, in which they predicted, isolated, and characterized a set of gold molecular nanoparticles.27 This paper was monumental because it showed that nanoparticles could be defined as molecular systems. The idea that nanoparticles could be viewed as
molecules was a contemporary notion, because at the current time only polydisperse (i.e., plus and minus hundred of atoms) nanoparticles were observed. Having an isolated single-sized molecular nanoparticle allows for the use of molecular type characterizations techniques, such, as mass spectrometry and single crystal x-ray diffraction. The single-size molecular nanoparticle along with molecular characterization techniques provides a pathway to study the fundamentals of nanoparticles size-dependent properties. While in the case of polydisperse nanoparticles, the assortment of different sizes would provide inconsistency when trying to accurately determine the size-dependent properties of nanoparticles.27
To study the size dependent chemical and physical properties of molecular nanoparticles, the nanoparticles must be isolated and chemically inert. Negishi et al., were the first to report the synthesis and isolation of an all-thiol family of gold
nanoparticles.28,29 The gold molecular nanoparticles were separated by polyacrylamide gel electrophoresis (PAGE), and in Figure 1-2 the extracted gold molecular nanoparticles can be viewed in solution.28,29 The molecular formula of each extracted gold nanoparticle was determined by electrospray ionization mass spectroscopy (ESI-MS). The work by Negishi et al. provided a great system to probe the chemical properties of a series of molecular gold nanoparticles, but Negishi et al. study didn’t reveal any information about the atomic structure of a molecular nanoparticle.28,29
Figure 1-2: A series of gold thiolated molecular nanoparticles separated by polyacrylamide gel electrophoresis (PAGE), and gold content was determined by
electrospray ionization mass spectroscopy. Reprinted with permission from reference 29. Copyright 2005 American Chemical Society.
It was two years after Negishi et al. published their work on all-thiol molecular gold nanoparticles that the Kornberg group successfully produced a single crystal of an all-thiol molecular gold nanoparticle with the molecular formula Au102(p-MBA)44.30 The
crystal structure of Au102(p-MBA)44 revealed its spherical atomic structure, with an all
metal core and capped with 2D Au-SR-Au-SR-Au and Au-SR-Au staples that passivate the gold core.30 SR represent a thiolated ligand. The electronic structure satisfies
supramolecular theory by having a 58 electron closed shell. 19,30 The crystal structure of Au25(SR)18, Au38(SR), have since been reported after the crystallization of Au102
(p-MBA)44. 30-33 Each of the three molecular nanoparticle’ crystal structures have a 13-atom
8
determine if the trends displayed in the family of gold molecular nanoparticles are
specific to only gold nanoparticles or to all metal nanoparticles, it is vital that other metal molecular nanoparticles are explored.
In recent years, there has been an effort to examine non-gold molecular
nanoparticles in regards of finding generalizable rules for molecular nanoparticles. Silver molecular nanoparticles have been explored to test the generality of molecular
nanoparticle stability and formation. Silver is a less noble than gold, but silver posses superior optical and antibacterial properties, and two orders of magnitude cheaper than gold. Kumar et al. was the first to report the synthesis and separation of a family of silver molecular nanoparticles.34 It was shown that the family of
silver-Figure 1-3: Ag:SG and Au:SG bands from the same PAGE gel, using concentrations of
15 and 30 mg/mL, respectively. The number of atoms in each Au:SG band is indicated.34 Reprinted with permission from reference 34. Copyright 2010 American Chemical Society.
glutathione were related to the family of gold-glutathione nanoparticles synthesized by Negishi et al, but the relationship was not a direct comparison.34 In Figure 1-3, the PAGE separations of the Ag and Au glutathione can be interpreted as similar by their individual
spectra were consistent with signatures previously attributed to
metal-bound ligands.
15Different synthesis conditions were able to change the overall
composition of the raw product but did not change the component
clusters. For example, changing the solvent composition, Ag:GSH
ratio, and reduction rate shifted the mass distribution toward either
larger or smaller cluster sizes (see Supporting Information), while
the relative positions and colors of the PAGE bands were
independent of reaction conditions. The pattern and colors of the
bands were always reproduced with only variations in their
abundance, strongly suggesting the molecular precision of
magic-number clusters.
The similarities between Au and Ag allow comparisons to be
made between the patterns of PAGE bands for each family of
clusters. Both Au and Ag contribute one free electron per atom to
the clusters, so the same pattern of electronic shell closings as a
function of the number of core atoms is expected for each metal.
Further, the atomic sizes and alkyl thiol packing densities are almost
identical for bulk Au and Ag.
16The same pattern of atomic shell
closings is therefore expected, and the charge and electrophoretic
mobility for the same size clusters should also be similar. A similar
pattern of bands is therefore expected.
The patterns of bands for Au and Ag clusters run on the same
gel are shown in Figure 2. Although there is some correspondence
between individual bands, it is immediately apparent that the mass
distribution is quite different for the two families of clusters. This
suggests that the most stable structures may be different since the
most abundant Au:SG and Ag:SG clusters are not necessarily the
same size. This could be due to a difference in Au-S and Ag-S
chemistry, or it could indicate a need to consider new models.
There are also striking differences in the optical properties of
Au and Ag clusters. The optical density of Ag is significantly higher
than that of Au, as can be seen in Figure 2. The integrated spectrum
(280-1100 nm) of the raw Ag:SG mixture was almost double that
of Au:SG (see Supporting Information). The absorption spectra of
the Ag bands are also quite different compared with Au:SG
clusters.
4The Ag spectra are less complex, containing a few
well-defined peaks as well as more subtle features, as shown in Figure
3 and Table 1.
Closed electronic shells lead to large HOMO-LUMO gaps,
which impart stability to clusters and can be observed as
high-energy absorption onsets in optical spectra. The measured onset
energy for light absorption decreases monotonically with increasing
Ag:SG cluster size (Table 1), as might be expected based on
quantum mechanical or statistical arguments.
17The position of the
first identifiable peak changes nonmonotonically, however, as might
be expected based on electronic shell closings and particle
symmetry.
The stability of Ag:SG clusters was excellent in dry powder form.
prepared clusters and powder stored in air for 8 months. Stability
in aqueous solution was size dependent, however. While the
smallest particles appeared to be stable, bands 14 and higher were
lost after ∼1 day in aqueous solution. This is consistent with the
idea that smaller clusters would have enhanced stability due to their
larger HOMO-LUMO gaps. All cluster sizes were stable within
the gel.
In conclusion, we have demonstrated the existence of a family
of small glutathione-ligated Ag clusters. The appearance and
properties of each family member was independent of reaction
conditions, consistent with the molecular precision of magic-number
clusters. Although these Ag:SG clusters are related to Au:SG
clusters, they are not a simple extension of the Au model. This
suggests that the rules for determining the most stable structure
could be different for Ag:SG, due to differences in Au and Ag
chemistry. Alternatively, condensed-phase magic-number cluster
theories may need to be more complex than currently believed.
Acknowledgment. The authors would like to thank Drs. Don
Ronning and Robert Whetten for useful discussions and Ms. Vidhi
Mishra and Mr. Anil Desireddy for their kind assistance.
Supporting Information Available: Synthetic methods, PAGE,
UV-vis, XRD, STEM, EDS, and NMR results. This material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. AdV. Mater. 1996, 8, 428–433.
(2) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 2630–2641. (3) Negishi, Y.; Takasugi, Y.; Sato, S.; Yao, H.; Kimura, K.; Tsukuda, T. J. Am.
Chem. Soc. 2004, 126, 6518–6519.
(4) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 5261– 5270.
(5) Negishi, Y.; Chaki, N. K.; Shichibu, Y.; Whetten, R. L.; Tsukuda, T. J. Am. Chem. Soc. 2007, 129, 11322–11323.
(6) Akola, J.; Walter, M.; Whetten, R. L.; Ha¨kkinen, H.; Gro¨nbeck, H. J. Am.
Figure 2. Ag:SG and Au:SG bands from the same PAGE gel, using
concentrations of 15 and 30 mg/mL, respectively. The number of atoms in each Au:SG band is indicated.4
Figure 3. Optical absorption spectra of four selected Ag:SG bands, as labeled, chosen based on abundance and the quality of separation. Spectra were taken of clusters inside the gel.
Table 1. Salient Features in the Optical Spectraa
Abs. onset
(eV) first peak(eV) second peak(eV) third peak(eV)
band 2 2.1 2.51 2.82 3.12
band 6 1.6 2.01 2.55 3.75
band 9 1.3 2.18 2.65 3.70
band 13 (1.0) 1.90 2.32 3.42
aParentheses indicate estimate by extrapolation.
band overlap, but the mass distribution between the individual bands are not the same.34 The varying mass distribution in the silver and gold glutathione nanoparticles can be rationalized by the different chemistry of the metal atoms with the organic thiol ligand, which suggest the generalized rules for gold molecular nanoparticles may not be directly applicable to silver containing molecular nanoparticles.34
While there have been many solution-phase methods for synthesizing gold and silver molecular nanoparticles there is one synthesis that has been significantly
remarkable. The synthesis by Bakr et al. was reported to yield a single-sized silver molecular nanoparticle.23, 35,36 At the time many of the solution-phase methods produced a series of different size molecular nanoparticles (as mentioned above), but none of the syntheses yields only a single-size product.23, 30,3-436 The silver molecular nanoparticle was classified as an intensely and broadly absorbing nanoparticle (IBAN), and its molecular formula was determined by ESI-MS to be Ag44(SR)30-4.23,35,36 The IBAN’s spectrum can be viewed in Figure 1-4, and is categorized by its 8 distant optical peaks that span the entire visible spectrum. 23,35,36 Ag44(SR)30-4 was reported to be stable for 18 months in the freezer, and accredits its stability to the 18 electron count closed shell configuration. 23,36 The Ag
44(SR)30-4 molecular nanoparticle is a special system that needs further exploration, so a deeper understanding of why only a single size molecular
Figure 1-4: UV/Vis absorption spectra of Ag44(SR)30-4 molecular nanoparticle synthesis with different aryl-thiol ligands. Reprinted with permission from reference 35. Copyright 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
1.4 References
1. Jortner, J. Cluster size effects. Atoms, Molecules and Clusters 1992, 24, 247-275. 2. Heer, W., A. The physics of simple metal clusters: experimental aspects and simple models. Reviews of Modern Physics, 1993, 65.
3. Barnett, R., N., Yannouleas, C., and Landman, U. Small can be different. Z. Phys. D,
1993, 26, 119-125
4. Cao, Guozhong. Nanostructures & nanomaterials synthesis, properties & applications. Imperial College Press, London, 2004.
5. Zhu, M., Aikens, C. M., Hollander, F.J., Schatz, G. C., Jin, R. J. Correlating the Crystal Structure of A Thiol-Protected Au25 Cluster and Optical Properties. J. Am.
Chem Soc. 2008, 130, 5883
7. Anker, J.N. et al., Biosensing with plasmonic nanosensors. Nat. Mater. 2008, 7, 442-453.
8. Jin, R. et al., Controlling anisotropic nanoparticle growth through plasmon excitation.
Nature. 2003, 425, 487-490.
9. Maier, S.A. et al., Plasmonics - A route to nanoscale optical devices. Adv. Mat. 2001, 13, 1501-1505.
10. Noginov, M.A. et al., Demonstration of a spaser-based nanolaser. Nature. 2009, 460, 1110-1113.
11. Atwater, H.A., Polman, A. Plasmonics for improved photovoltaic devices. Nat. Mater.
2010, 9, 205-213.
12. Arvizo, R.R., et al., Intrinsic therapeutic applications of noble metal nanoparticles: past, present and future. Chem. Soc. Rev. 2012, 41, 2943-2970.
13. Häkkinen, H. in Fronttier of Nanoscience. Ligand-protected Gold Nanoclusters as Superatoms - Insights from Theory and Computations, Johnston R.L., Wilcoxon J. Ed., Elsevier: Great Britian, Vol 3, p 129-157.
14. Jin, R. Quantum sized, thiolate-protected gold nanoclusters. Nanoscal. 2010, 2, 343- 362
15. Sattler, K. The Energy Gap of Clusters Nanoparticles, and Quantum Dots. In Handbook of thin film materials, Nalwa, H. S., Ed., Academic Press: San Diego, 2002. Vol. 5, p 61-97
16. Atkins, P. W., Julio P. Physical chemistry, 9th ed., W. H. Freeman and Co: New York, 2010.
17. El-Sayed, M., A. Small Is Different: Shape-, Size-, and Composition-Dependent Properties of Some Colloidal Semiconductor Nanocrystals. Acc. Chem. Res., 2004, 37 (5), pp 326–333.
18. Landman, U., Luedtke, W., D. Small is different: energetic, structural, thermal and mechanical properties of passivated nanocluster aseemblies. Faraday Discuss. 2004, 125, 1-22
19. Walter ,M., Akola, J., Lopez-Acevedo, O., Jadzinsky, P. D., Calero. G., Ackerson, C. J., Whetten, R. L., Grönbeck, H., Häkkinen, H. A unified view of ligand-protected gold clusters as superatom complexes, PROC. NATL. ACAD. SCI. 2008, 105, 9157 20. Knight, W.D., Clemenger, K., deHeer, W. A., Saunders, W. A., Chou, M. Y., Cohen, M. L. Electronic shell structure and abundances of sodium clusters Phys. Rev. Lett. 1984, 2141
21. Eberhardt, W. Clusters as new materials. Surface science 2002, 500, 242-270
22. Harkness, K. M., et al. Ag44(SR)30-4: a silver-thiolate superatom complex. Nanoscale
2012, 4(14), 4269-4274
23. Faraday, M. Experimental relations of gold (and Other Metals) to Light.
Philosophical Transactions of the Royal Society of London, 1857, 147, 145-181.
24. Whetten, R. L. et al. Crystal structures of molecular gold nanocrystal arrays. Acc.
Chem. Res. 1999, 32, 397–406.
25. Templeton, A. C., Wuelfing, P. W., Murray, R. W., Monolayer-protected cluster molecules. Acc. Chem. Res. 2000, 33, 27-36
26. Brust, M., Walker, M., Bethell, D., Schriffin, D. J., Whyman, R. Synthesis of Thiol-derivatised gold nanoparticles in a two-phase liquid-liquid system. J. Chem. So., Chem.
Commu., 1994, 801.
27. Whetten, R., L. et al. Nanocrystal gold molecules. Adv. Mater. 1996, 8.
28. Negishi, Y., Takasugi, Y., Sato, S., Yao, H., Kimura, K., Tsukuda, T. Magic-numbered Aun clusters protected by glutathione monolayers (n = 18, 21, 25, 28, 32, 39): isolation and spectroscopic characterization. J. Am. Chem. Soc., 2004, 126 (21), 6518–6519. 29. Negishi, Y.; Nobusada, K.; Tsukuda, T. Glutathione-protected gold clusters revisited, J.
Am. Chem. Soc. 2005, 127, 5261-5270.
30. Jadzinksy, P. D., Calero, G., Ackerson, C. J., Bushnell, D. A., Kornberg, R. D., Strucuture of a thiol monolayer-protected gold nanoparticle at 1.1 A resolution.
Science. 2007, 318, 430-433.
31. Zhu, M., Aikens, C. M., Hollander, F. J., Schatz, G. C., Jin, R. Correlating the crystal strucuture of a thiol-protected Au25 cluster and optical properties. J. Am. Chem. Soc.,
2008, 130 (18), 5883-5885.
32. Heaven, M. W., Dass, A., White, P. S., Holt, K. M., Murrary, R. W. Crystal strucuture of gold nanoparticle [N(C8H17)4][Au25(SCH2CH2Ph)18]. J. Am. Chem. Soc., 2008, 13 (12), 3754-3755.
33. Qian, H., Eckenhoff, W. T., Zhu, Y., Pintauer, T., Jin, R. Total structure determination of thiolated-protected Au38 nanoparticles. J. Am. Chem. Soc., 2010, 132 (24), 8280-8281.
34. Kumar, S., Bolan, M. D., Bigioni, T. P., Glutathione-stabilizd magic-numbered silver cluster compounds. J. Am. Chem. Soc. 2010, 132 13141-13143.
35. Bakr, O. M., Amendola, V., Aikens, C. M., Wenseleers, W., Li. R., Negro, L. D., Schatz, G. C., Stellacci, F. Silver nanoparticles with broad multiband linear optical absorption. Angew. Chem. Int. Ed. 2009, 48, 5921-5926.
36. Pelton, M., Tang, Y., Bakr, O. M., Stellacci, F. Long-lived charged-separated states in ligand-stabilized silver clusters. J. Am. Chem. Soc. 2012, 134, 11856-11859.
Chapter 2
2.1 ULTRASTABLE SILVER NANOPARTICLES
Noble metal nanoparticles have had a deep impact across a diverse range of fields,
including catalysis,1 sensing,2 photochemistry,3 optoelectronics,4,5 energy conversion,6 and medicine.7 Although silver has very desirable physical properties, good relative abundance, and low cost, gold nanoparticles have been widely favored due to their proven stability and ease of use. Unlike gold, silver is notorious for its susceptibility to oxidation, i.e. tarnishing, which has limited the development of important silver-based nanomaterials. Despite two decades of synthetic efforts, inert or long-term stable Ag nanoparticles remain unrealized. Herein we report a simple synthetic protocol for producing ultrastable M4Ag44 (p-MBA)30 nanoparticles as a self-selecting single-sized molecular product in exceptionally large quantities, with quantitative yield, and without the need for size sorting. The stability, purity, and yield are substantially better than other metal nanoparticles, including gold, owing to an effective stabilization mechanism. The particular size and stoichiometry of the product was found to be immune to variations in synthesis parameters. The unique chemical stability and structural, electronic and optical properties are understood using
---*Reprinted from the Desireddy, A., Conn, B. E., Guo, J., Yoon, B., Barnett, R. N., Monahan, B. M., Kirschbaum, K., Griffith, W. P., Whetten, R. L., Landman, U., Bigioni, T. P. Ultrastable Silver Nanoparticles. Nature 501, 399–402 (2013). Copyright © Nature
first-principles electronic structure theory based on an experimental single-crystal X-ray structure. While several structures have been determined for protected gold nanoclusters, 8-12 none has been reported to-date for silver nanoparticles.
The total structure of a thiolate-protected silver nanocluster reported here uncovers the unique structure of the silver-thiolate protecting layer, consisting of Ag2S5 mounts. The exceptional stability of the nanoparticle is attributed to a closed-shell 18-electron configuration with a large HOMO-LUMO gap, an ultrastable 32-silver atom excavated-dodecahedral13 core consisting of a hollow 12-Ag atom icosahedron encapsulated by a 20-Ag atom dodecahedron, and the choice of protective coordinating ligands. The facile synthesis of large quantities of pure molecular product promises to make this class of materials widely available for further research and technology development.14-18
All-aromatic silver-thiolate clusters with a ~1.2 nm core diameter have been discovered only recently.19 Electrospray-ionization mass spectrometry (ESI-MS) identified these as discrete molecular complexes, Ag44(SR)30−4.20 Complexity of preparation and handling have proven limiting, however, as these clusters shared the typical vulnerabilities of Ag nanoparticles.
We have developed an entirely new approach for the preparation of ultrastable silver nanoparticles in semi-aqueous solution with an all-aromatic p-mercaptobenzoic acid (p-MBA) protecting ligand shell.8,19 With judicious choice of solvent conditions and stabilizing agents we have transformed these fragile and unstable Ag complexes into chemically inert materials with unprecedented stability.
The synthesis involves the reduction of a soluble precursor in semiaqueous solution and in the presence of alkali metal cations and a coordinating solvent. The straightforward protocol produces a pure molecular material without size separations and achieves near quantitative yield in exceedingly large quantities (see Figure 2-1 inset). The product can be dried and fully redispersed in protic, aprotic, and nonpolar solvents with no loss of material or change in chemical identity.
Figure 2-1. Optical absorption and material sample. Absorption spectrum of the
M4Ag44(p-MBA)30 raw product solution (red line) synthesized in the presence of “seed” M4Ag44(p-MBA)30 clusters (open circles). Inset: 140 grams of M4Ag44(p-MBA)30 clusters pictured with two 1 oz. silver coins for scale. The dish is 18 cm in diameter.
The absorption spectrum of the raw product is highly structured (see Figure 2-1) and identical to that of the purified material, with an onset at about 1100 nm (~ 1.1 eV). ESI-MS of the raw product (see Figure 2-2) identified several ion species that were all attributed to a single cluster size, the pure Ag44(p-MBA)30−4 complex (m/z 2336). The Ag43(p-MBA)28−3 complex (m/z 2975) was attributed to electrostatic destabilization and spontaneous fragmentation of Ag44(p-MBA)30−4 upon desolvation (see Appendix A). The experimental data only matched the simulated isotopic distribution for the fully protonated species (see Appendix A), therefore the entire -4 charge was carried by the silver core rather than by the carboxylates. Four alkali counterions (M) were identified by elemental analysis, giving M4Ag44(p-MBA)30 as the molecular formula.
Figure 2-2. Electrospray-ionization mass spectra. ESI-MS of the final product without
size separation shows that only one species is present. Peaks from 3000-3200 m/z are fragments with -3 charge state and the broad intensity at 3500 m/z is attributed to nonspecific dimerization of fragments with -5 total charge state. (see Appendix A for further details)
The synthesis of M4Ag44(p-MBA)30 has characteristics unlike any nanoparticle preparation. Normally single-sized products are isolated by attrition, wherein the less stable sizes are either destroyed or converted into the most stable size.21-23 Direct synthesis of a truly single-sized molecular product with yields >95% indicates that these clusters are more stable than any other known cluster species. Furthermore, the particular size, composition, and stoichiometry of the nanocluster product was found to be immune to changes in experimental parameters (e.g. solvent composition, reactant concentrations) for the size-sorting-free synthesis method developed and employed by us here.
The profound difference between canonical nanoparticle syntheses and the present work was demonstrated by synthesizing M4Ag44(p-MBA)30 clusters in the presence of existing M4Ag44(p-MBA)30 clusters. Normally the existing nanoparticles would act as seeds and grow at the expense of new particle nucleation.24 Instead, M
4Ag44(p-MBA)30 clusters were formed with identical yield and chemical identity, with or without these seeds (Figure 2-1). Once formed, M4Ag44(p-MBA)30 clusters were extraordinarily stable and unreactive, behaving as completely inert molecules rather than as typical nanoparticles. When an analogous reaction was performed with Au25(SG)18 (SG = glutathionate), the clusters were not inert but rather acted as seeds (see Appendix A).
The long-term stability of solutions of M4Ag44(p-MBA)30 clusters were also superior to those of Au25(SG)18 clusters. The ambient decay rates of M4Ag44(p-MBA)30 cluster solutions were ~7 times slower than those of Au25(SG)18 cluster solutions (see Appendix, Figure A4B). Therefore, under both ambient (mildly oxidizing) and reducing conditions the M4Ag44(p-MBA)30 clusters proved to be more noble than even the highly stable Au25(SG)18
cluster.21 When discussing relative stabilities of protected nanoclusters caution should be exercised with regard to the ligands used and the environmental conditions (see Appendix A). Indeed, experiments in our laboratory have shown that Au25(p-MBA)18 is too unstable for meaningful temporal stability measurements to be made.
This remarkable inertness under reducing conditions implies that the synthesis of new clusters can carry on without regard for existing clusters in the reaction vessel, giving impetus to scaling up the reaction. Indeed, it has been possible to produce 140 grams of the final M4Ag44(p-MBA)30 product from a single reaction (see Figure 2-1 inset), although kilogram-scale syntheses should be easily achievable. We note that 140 g is three orders of magnitude larger than typical nanoparticle preparations.
Clues as to the origins of the extraordinary stability of M4Ag44(p-MBA)30 as well as the structure of the thiol surface layers that protect silver nanoparticles have been revealed by single-crystal x-ray diffraction. Na4Ag44(p-MBA)30 clusters were crystallized from DMF solution, with rhombus-shaped crystals forming after 1-3 days. The entire structure of the cluster was determined by single-crystal x-ray crystallography (see Appendix A), and is shown in Figs. 2-3(a,b).
The crystal structure has exceptionally high symmetry, containing elements that exhibit four of the five Platonic solids. The all-silver core consists of a hollow icosahedron (Ag12 inner core) within a dodecahedron (Ag20 outer core), forming an Ag32 excavated-dodecahedral core with icosahedral symmetry (Figure 2-3 c,d); interestingly, a hollow core has been recently put forward theoretically for another thiol-protected Ag cluster.25 The 20 atoms of the outer core occupy two distinct environments. Eight Ag atoms within the
dodecahedral outer core define the vertices of a cube (light green in Figure 2-3), the faces of which contain the remaining 12 Ag atoms in pairs (dark green in Figure 2-3) and are capped in such a way as to create an overall octahedral shape for the particle (Figure 2-3f). Namely, four sulfur atoms from the p-MBA ligands are located on each face of the cube, such that the 24 sulfurs define a slightly distorted rhombicuboctahedron, an Archimedean solid (see Figure 2-3e). Each face then receives an additional Ag2S group to complete the inorganic part of the structure and the octahedral shape.
The capping units are complex three-dimensional structures and unlike anything seen in gold clusters (1D)8-12 or in silver-thiolate materials (2D).26 They can be viewed most simply as an Ag2S5 mount, with four S atoms acting as legs that connect it to the Ag32 core. These four S atoms are bridged by a pair of Ag atoms, which are in turn bridged by a terminal S atom. Each sawhorse-shaped mount straddles a pair of Ag atoms (dark green in Figure 2-3) of the intact Ag32 core. Altogether, six such Ag2S5 mounts comprise the entire layer protecting the compact, quasi-spherical Ag32 core.
Figure 2-3. X-ray crystal structure obtained from a Na4Ag44(p-MBA)30 crystal. (a) Complete cluster structure showing silver core and p-MBA ligands (see colour scheme below). (b) Space-filling view down a 3-fold axis. Note face-to-face and edge-to-face pi stacking in the groupings of two and three ligands, resulting in considerable void space. (c, d) The Ag32 excavated-dodecahedral core consists of an inner 12-atom (hollow) icosahedron (red) whose atoms do not contact sulfur, encapsulated by a 20-atom dodecahedron (green). The 8 atoms of the dodecahedron colored in light green define a cube, with pairs of dark green Ag atoms located above the faces. (e) Sulfur atoms are arranged in a slightly distorted rhombicuboctahedron with S atoms in the triangular faces coordinating to the light green Ag atoms of the 20-atom dodecahedron. (f) Six faces of the rhombicuboctahedron are capped with an Ag2S unit with the bridging S atom tilted off axis, completing the inorganic structure. (g) Two Ag atoms (dark green) on each face could be excised from the cluster to create Ag4S5 capping mount structures, leaving a cubic Ag202+ core. The distance between the two Ag atoms at the bottom of the mount and the nearest Ag atoms of the Ag20 core is 2.83 Å, resulting in strong mount-to-core coupling. (h) An Alternative Ag2S5 capping structure can be visualized as a sawhorse-shaped mount that straddles the dark green Ag atoms of the intact dodecahedral Ag32 core. The Ag2S5 mount is better defined than the Ag4S5 mount (see g) since its Ag atoms are separated by a larger distance (> 3.1 Å) from the nearest atoms of the Ag32 core, resulting in a weaker mount-to-core interaction. Colour scheme: grey – carbon; orange – oxygen; blue – exterior silver atoms in the mounts; gold – bridging sulfur atoms in the mounts. For interatomic distances see the Appendix A.
The two relatively exposed Ag atoms27 on each side of the six mounts can acquire effective protection from the coordinating solvent, consistent with experimental observations. If this protection is lost, the clusters can polymerize to form larger plasmonic Ag nanoparticles. The uniqueness of this structure and the structural perfection of this cluster are reflected in its remarkable stability and its immunity to compositional changes.
Further insight into the bonding and electronic structure of the Ag44(p-MBA)30 cluster was gained through extensive first-principles calculations.27 Figure 4 shows the projected densities of states (PDOS, see Appendix A for details) calculated via density functional theory (DFT) based on the experimental configuration of the entire cluster (Figure 2-3a); the PDOS reflects the angular momenta (l) symmetries of the cluster’s
orbitals,28 denoted as S, P, D, F…(corresponding to l = 0, 1, 2, 3,…). The first outstanding feature observed is the relatively large energy gap between the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals, ΔHL = 0.78 eV, conferring stability to the cluster and endowing it with resistance to chemical attack. The first calculated optically allowed transition occurs at 0.98 eV, however, from the 1D HOMO to the 1F LUMO+1 orbital, in fair agreement with the measured onset of optical absorption ( ≤ 1.1 eV, see Figure 2-1).
The wavefunctions of the cluster exhibit both localized and delocalized character. The localized states are derived from the atomic Ag 4d electrons and are located at the middle of the energy spectrum whereas the delocalized states are derived from the atomic Ag 5s electrons and can be found near the top and bottom of the electronic spectrum.28 These delocalized cluster states can be assigned angular momentum symmetries following the electronic cluster shell model (CSM),28-30 with a (superatom) Aufbau rule: 1S2 |1P6 |1D10 | 2S2 |1F14 |…. Here, the vertical lines denote shell-closures, which are associated with a magic numbers, i.e. closed-shell electronic structures are accompanied by the opening of a stabilizing energy gap. The number of electrons not engaged in bonding to sulfur (thiolates) is given by the electron count n* = νNAg – NL – Z, where ν = 1 is the valence for Ag (5s1), NAg is the number of Ag atoms, NL is the number of anionic thiolate ligands, and Z is the overall cluster charge. For the case of Ag44 (p-MBA)30-4, the electron count is n* = 18, which corresponds to the stable superatom with the Aufbau shell filling 1S2 |1P6 |1D10 |.
The 18-electron superatom shell-closure and accompanying energy gap stabilization enables a deeper insight into the structure of the protective silver-thiolate layer. Two alternative motifs for the aforementioned capping-ligand mounts can be constructed, as shown in Figure 2-3. The cluster can be decomposed into six Ag4(SR)51- mount units and a cubic Ag202+ core (Figure 2-3g), where the formal charges result from the valences assigned to the silver atoms (+1) and SR thiolate ligands (-1). Alternatively, the cluster can be decomposed into six Ag2(SR)53- mounts and a quasi-spherical Ag3214+ core (Figure 2-3h). In both cases the cluster cores contain eighteen Ag 5s electrons.
Electronic structure calculations on the two proposed cores, as extracted from the x-ray determined structure, provide a way to differentiate these two competing motifs. The PDOS of the Ag3214+ cluster core (Figure 2-4b) bears a great similarity to that of the entire cluster, including a large HOMO-LUMO gap (Figure 2-4a) and a marked degeneracy of the 1D10 superatom orbitals, reflecting an approximate spherical symmetry of the effective potential governing the motion of the delocalized electrons of the cluster core (the corresponding superatom orbital shapes are shown at the top of Figure2- 4). In contrast, similar analysis for the Ag202+ cluster core results in a spectrum that differs considerably from that of the complete particle, and in particular does not exhibit a gap (Figure 2-4c), reflecting strong coupling of the this core to the Ag4(SR)5 mounts. The Ag2(SR)5mounts can therefore be thought of as the operative capping unit and the Ag32 as the natural choice for the core.
Figure 2-4. Projected densities of states (PDOS) and orbital images. (a) PDOS
calculated for the Ag44(SC6H5)30-4 cluster with all atoms at the x-ray determined positions (see Methods section, and SI). Different colors correspond to the various angular momentum contributions S, P, D, F, G, H, and I, as shown on the right. The Fermi energy EF is the energy in the middle of the HOMO-LUMO gap, ΔHL = 0.78 eV. The inset shows an image of the HOMO 1D superatom orbital superimposed on the structure of the cluster; different colors of the orbital (blue and pink) correspond to different signs of the wavefunction. The 18-electron gap is marked. (b) PDOS of the Ag3214+ core, as extracted from the measured structure, corresponding to the Ag2S53- mount motif. Selected superatom orbitals are shown at the top of the Figure, with the energies and angular momenta marked. The 18-electron gap ΔHL = 1.29 eV is noted. (c) PDOS of the Ag202+ core, as extracted from the measured structure, corresponding to the Ag4S51- mount motif. Note the absence of an 18-electron gap.
2.2 METHODS SUMMARY
2.2.1 Synthetic Methods.
M4Ag44(p-MBA)30 clusters are prepared in essentially quantitative yield by a simple 3-step procedure, namely (i) generation of a Ag(I)-p-MBA precursor; (ii) reduction of the precursor to product; and (iii) removal of byproducts. Afterward, the pure substance is protonated.
(i) Aqueous AgNO3 is combined with ethanolic p-MBA in excess to form the insoluble Ag(I)-p-MBA precursor. The pH is then adjusted to 9 with CsOH to solubilize the precursor and further adjusted to 12 to stabilize the final cluster product. (ii) Aqueous NaBH4 is added dropwise with stirring and is allowed to incubate for an hour. The final dark red solution yields an alkali metal salt of the final M4Ag44(p-MBA)30 product. (iii) The clusters are separated from the reaction mixture by precipitation with DMF. The carboxylates are then protonated with acetic acid until the cluster completely dissolve in DMF, yielding the stoichiometric M4Ag44(p-MBA)30 final product. This final product is isolated by precipitation.
2.2.2 X-ray Crystallography.
Data were collected for an 80 x 70 x 50 µm3 crystal at 150 K with a Bruker Apex Duo diffractometer (CuKα = 1.54178 Å) equipped with an APEX II CCD detector. The structure was solved and refined using the Bruker SHELXTL software package, with space group R-3c. All eight crystallographically independent Ag atoms were obtained by direct
methods and all remaining non-hydrogen atoms were located with subsequent difference Fourier techniques. The refinement converged to R1 = 5.2% with a maximum resolution of 0.83 Å. The highest residual electron density was 1.036 e/Å3.
2.2.3 Computational Methods.
The VASP-DFT package was used, with a plane-wave basis, kinetic energy cutoff of 400 eV, PAW pseudopotentials,31 and the PW91 generalized gradient approximation (GGA) for exchange-correlation .32,33 In structural optimizations, convergence was achieved for forces <0.001 eV/Å. Calculations were performed for the X-ray determined Ag44(SC6H5)30-4 structure ( hydrogens were added and their positions were relaxed, average d(C-H) = 1.09 Å).
2.3 References and Notes
1. Heiz, U., Landman, U. Nanocatalysis. Springer-Verlag, Berlin, 2007.
2. Anker, J.N. et al., Biosensing with plasmonic nanosensors. Nat. Mater. 2008, 7 442-453.
3. Jin, R. et al., Controlling anisotropic nanoparticle growth through plasmon excitation.
Nature. 2003, 425, 487-490.
4. Maier, S.A. et al., Plasmonics - A route to nanoscale optical devices. Adv. Mat. 2001, 13 1501-1505.
5. Noginov, M.A. et al., Demonstration of a spaser-based nanolaser. Nature. 2009, 460 1110-1113.
6. Atwater, H.A. & Polman, A., Plasmonics for improved photovoltaic devices. Nat.
Mater. 2010, 9, 205-213.
7. Arvizo, R.R. et al., Intrinsic therapeutic applications of noble metal nanoparticles: past, present and future. Chem. Soc. Rev. 2012, 41 2943-2970.
8. Jadzinsky, P.D., Calero, G., Ackerson, C.J., Bushnell, D.A., & Kornberg, R.D., Structure of a thiol monolayer-protected gold nanoparticle at 1.1 Å resolution.
Science. 2007, 318, 430-433.
9. Heaven, M.W., Dass, A., White, P.S., Holt, K.M., & Murray, R.W., Crystal structure of the gold nanoparticle [N(C8H17)4][Au25(SCH2CH2Ph)18]. J. Am. Chem. Soc. 2008, 130 (12), 3754-3755.
10. Zhu, M., Aikens, C.M., Hollander, F.J., Schatz, G.C., & Jin, R., Correlating the crystal structure of a thiol-protected Au25 cluster and optical properties. J. Am. Chem.
11. Qian, H., Eckenhoff, W.T., Zhu, Y., Pintauer, T., & Jin, R., Total structure
determination of thiolate-protected Au38 nanoparticles. J. Am. Chem. Soc. 2010, 132, 8280–8281.
12. Zeng, C. et al., Total structure of the golden nanocrystal Au36(SR)24. Angew. Chem.
Int. Ed. 2012, 51 (52), 13114-13118.
13. Williams, R., The Geometrical Foundation of Natural Structure. Dover, New York, 1979.
14. Kratchmer, W., Lamb, L.D., Fostiropoulos, K., & Huffman, D.R., Solid C60: a new form of carbon. Nature. 1990, 318, 354-358.
15. Ebbesen, T.W. & Ajayan, P.M., Large-scale synthesis of carbon nanotubes. Nature.
1992, 358, 220-222.
16. Bethune, D.S. et al., Cobalt-catalysed growth of carbon nano-tubes with single-atomic-layer walls. Nature. 1993, 363 605-607.
17. Murray, C.B., Norris, D.J., & Bawendi, M.G., Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites. J. Am.
Chem. Soc. 1993, 113, 8706-8715.
18. Brust, M., Walker, M., Bethell, D., Schiffrin, D.J., & Whyman, R., Synthesis of thiol-derivatised gold nanoparticles in a two-phase liquid-siquid System. J. Chem. Soc.,
Chem. Commun. 1994, 801-802.
19. Bakr, O.M. et al., Silver nanoparticles with broad multiband linear optical absorption.
Angew. Chem. Int. Ed. 2009, 48, 5921-5926.
20. Harkness, K.M. et al., Ag44(SR)304−: a silver–thiolate superatom complex. Nanoscale.
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