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SYNERGISTIC EFFECT OF TWO FOREIGN METAL IONS
ON SHAPE-SELECTIVE SYNTHESIS OF GOLD
NANOCRYSTALS
TRAN TRONG TOAN
NATIONAL UNIVERSITY OF SINGAPORE
2011
SYNERGISTIC EFFECT OF TWO FOREIGN METAL IONS
ON SHAPE-SELECTIVE SYNTHESIS OF
GOLD NANOCRYSTALS
TRAN TRONG TOAN
(B.Sc. (Hons.), University of Science Ho Chi Minh City)
A THESIS SUBMITTED
FOR THE DEGREE OF MASTER OF ENGINEERING
DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2011
ACKNOWLEDGEMENTS
First of all, I would like to express my deep gratitude towards the following people
who have helped me complete the thesis.
A special thank to my research supervisor, Assistant Professor Lu Xianmao, for
offering an opportunity to me to be a part of his research group. I want to thank him
for his invaluable support and all the guidance throughout the course of study. I would
also like to thank my thesis examiners, Professor Zeng Hua Chun and Assistant
Professor Saif A. Khan for their advice, guidance and encouragement throughout my
MEng study.
All the professional officers and lab technologists, Mr. Chia Phai Ann, Dr. Yuan Ze
Liang, Ms. Lee Chai Keng, Ms. Li Xiang, Dr. Yang Liming, Ms. Li Fengmei, and
other staffs who have unconditionally helped me in many administrative works as
well as experiments and have willingly shared their knowledge and expertise to
further enhance my studying process.
My colleagues, Dr. Sun Zhipeng, Ms. Zhang Weiqing, Mr. Shaik Firdoz, and all the
final year students for all their kind supports they provided.
Finally, I want to specially thank my parents who have given me all what they have
for their unconditional support and their love. I also want to thank my girlfriend for
her non-stop support during my study.
i
Table of Contents
Acknowledgement .................................................................................................. i
Table of Contents ................................................................................................. ii
Summary.............................................................................................................. iv
Nomenclature ....................................................................................................... vi
List of Figures..................................................................................................... vii
List of Tables ....................................................................................................... ix
Chapter 1. Introduction........................................................................................ 1
1.1. Background ................................................................................................. 1
1.2. Research objectives ..................................................................................... 2
1.3. References ....................................................................................................3
Chapter 2. Literature Review .............................................................................. 4
2.1. Shape-controlled synthesis of noble metal nanocrystals............................. 4
2.1.1. Nucleation and growth of metal nanocrystals ..................................... 4
2.1.2. Chemical methods for synthesis of metal nanocrystals
with controlled shapes ................................................................................... 6
2.1.2.1. Seeded-growth route.................................................................. 6
2.1.2.2. Hydrothermal route.................................................................... 8
2.1.2.3. Electrochemical route ................................................................ 8
2.1.2.4. Photochemical route .................................................................. 9
2.1.2.5. Polyol route.............................................................................. 10
ii
2.2. Synthesis and catalytic properties of metal nanocrystals
with high-index facets ..................................................................................... 12
2.3. References ..................................................................................................14
Chapter 3. Shape-controlled Synthesis of Au Nanocrystals
with High-index Facets .......................................................................................17
3.1. Shape-selective growth of polyhedral gold nanocrystals
with high-index facets .......................................................................................17
3.1.1. Introduction ..................................................................................... 17
3.1.2. Experimental Section ...................................................................... 19
3.1.3. Results & discussion ....................................................................... 22
3.1.4. Conclusion....................................................................................... 39
3.2. References ..................................................................................................41
Chapter 4. Conclusions and Recommendations for Future Work................. 43
4.1. Conclusions ............................................................................................... 43
4.2. Recommendations for Future work........................................................... 44
4.3. References ................................................................................................. 48
iii
Summary
Shape-controlled synthesis of metal nanocrystals has been widely investigated
for the last several decades because of its ability to tailor the morphology of metal
nanocrystals, and therefore, their physical and chemical properties. These properties,
which greatly differ from their bulk counterparts, are highly dependent on the size and
the shape of the nanocrystals. Metal nanocrystals with many shapes such as cubes,
octahedra, cubotahedra, icosahedra, plates, rods, and wires in various sizes have been
synthesized. However, these nanocrystals are mainly enclosed by low Miller-index
facets (i.e. {111}, {100}, and {110}). Recently, much focus has been given to metal
nanocrystals with high-index facets due to their superior catalytic properties to those
bounded by low-index facets. The metal nanocrystals with high-index facets are,
however, difficult to be prepared due to the fact that high-index facets are not as
stable as those low-index ones during the synthetic period.
In this work, we present the facile PDDA-mediated polyol route for synthesis
of a series of novel Au nanocrystals, namely, truncated octahedra bounded by both
{111} and {310} facets, truncated ditetragonal prisms exclusively enclosed by {310}
facets, and bipyramids with exposed {117} facets by simply varying the ratio of Ag
and Pd ions. The synergistic effect of Ag and Pd ions on the formation of the novel
Au nanocrystals was studied. In our experimental conditions, the underpotential
deposition (UPD) of Ag on Au surface was believed to inhibit the growth along
directions, therefore lead to the formation of {110} facets on Au nanocrystals.
Palladium ions could, on the other hand, also take part in the deposition on Au surface
and stabilize {100} facets. Together, Ag and Pd ions enabled the growth of {310}
facets on the Au nanocrystals as {310} facets are composed of {110} and {100}
iv
subfacets. Since the Au nanocrystals obtained in this report possess high-index facets,
they are expected to be promising candidates for many catalytic applications.
v
Nomenclature
CTAB
Cetyltrimethylammonium bromide
CTAC
Cetyltrimethylammonium chloride
EDX
Energy dispersive X-ray spectroscopy
EG
Ethylene glycol
FESEM
Field-emission scanning electron microscopy
ICP-MS
Inductive coupling plasma mass spectrometry
PDDA
Poly(diallyldimethylamonium chloride)
PEG
Polyethyleneglycol
PVP
Polyvinylpyrrolidone
SAED
Selected area electron diffraction
SEM
Scanning electron microscopy
TEM
Transmission electron microscopy
vi
List of Figures
Figure 3.1. (A) Low and (B) high magnification SEM images of Au truncated
ditetragonal prisms showing well-defined structures with sharp edges and apexes. (C)
HRSEM of a group of Au truncated ditetragonal prisms. (D) TEM images of Au
truncated ditetragonal prisms showing its cross-section. (E) High magnification of a
truncated ditetragonal prism (inset) exhibiting (200) d-spacing of fcc Au. (F) The
schematic drawings at different views of an Au nanoprism ....................................... 23
Figure 3.2. Determination of facets of Au truncated ditetragonal prisms from
different views (A) top view (cross-section) and (B) side view. The result indicates
that Au truncated ditetragonal prisms are bound by 12 {310} facets. Note that image
(A) and (B) were taken from different truncated ditetragonal prisms. (C), (D)
Schematic drawing of truncated ditetragonal prisms with their theoretical angles. (E)
Atomic model of Au (310) facet including (110) and (100) subfacets....................... 24
Figure 3.3. (A), (B), (C) and (D) Schematic models for Au truncated ditetragonal
prisms at different views illustrating for (E), (F), (G) and (H) the corresponding TEM
images. (I), (J), (K) and (L) the ED patterns that consistently show all [310] zone
axes. Note that the SAED patterns were taken from different Au nanoprisms .......... 25
Figure 3.4. Schematic models for other configurations of Au truncated ditetragonal
prisms which differ from the Au nanoprisms in Figure 3.1. (A) one sloping face pair
(at one end) rotated 90° around the principle axis, (B) one vertical half rotated 90° so
that two side faces become two new sloping faces at two ends, and (C) one sloping
face pair (at one end) rotated 90° around the principle axis and one vertical half
rotated 90° so that two side faces become two new sloping faces at two ends (i.e.
combining (A) and (B)) .............................................................................................. 25
Figure 3.5. (A) Low and (B) high magnification SEM images of Au bipyramids. Inset
of Figure 3B clearly shows a pentagonal cross-section of an exceptionally big
bipyramid. Inset scale bar is 100 nm. (C) TEM image of Au bipyramids. (D) HRTEM
image of a bipyramid (inset) describes the (111) d-spacing. Inset scale bar is 20 nm.
(E) The corresponding ED pattern showing the superposition of [110] and [111] zones
of fcc structure. (F) Schematic drawing of a bipyramid ............................................. 27
Figure 3.6. (A) TEM image of an Au nanobipyramid with defined width base (W,
yellow line) and height of half (Hhf, red line). (B) Model of half of pentagonal
bipyramid and formula that exhibits the relationship between morphological
measurements (i.e. W and Hhf) and Miller index of the bipyramidal facets. By
measuring few tens of Au bipyramids in TEM images, we could determine the
average Hhf/W ratio of 2.18 which means that the Au bipyramids obtained in this
work enclosed by the high-index {117} facets........................................................... 27
Figure 3.7. (A) Low magnification SEM image of Au truncated octahedra. Inset
shows schematic model of a truncated octahedron that exposes both {111} and {310}
facets. (B) High magnification SEM image of truncated octahedra with a superposed
drawing frame on single truncated octahedra shows the consistency with the
schematic model. (C) TEM image of Au truncated octahedra with the inset showing
vii
(200) d-spacing of Au fcc. (D), (E) ED patterns of Au truncated octahedra clearly
show [310] and [111] zone axes. (F) Schematic drawing showing the morphological
relationship between an octahedron and a truncated octahedron................................ 28
Figure 3.8. EDX analyses of Au nanostructures: (A) truncated ditetragonal prisms,
(B) bipyramids and (C) truncated octahedra............................................................... 30
Figure 3.9. XPS analyses of Au nanostructures: (A) truncated ditetragonal prisms,
(B) bipyramids and (C) truncated octahedra............................................................... 31
Figure 3.10. XRD patterns of Au nanostructures: (A) truncated ditetragonal prisms,
(B) bipyramids and (C) truncated octahedra............................................................... 33
Figure 3.11. UV-vis spectra of Au truncated ditetragonal prisms, bipyramids and
truncated octahedra ..................................................................................................... 34
Figure 3.12. Au nanostructures synthesized at different temperature: (A, C and E) at
140 °C and (B, D and F) at 170 °C. The procedures were similar to those used for the
syntheses of Au truncated ditetragonal prisms, bipyramids and truncated octahedra
except that no NaCl was used for the truncated ditetragonal prism synthesis. (A, B)
Truncated ditetragonal prisms with the longest lengths of 52 and 30 nm, (C, D)
bipyramids with lengths of 53 and 40 nm and (E, F) truncated octahedra with
diameters of 75 and 32 nm.......................................................................................... 35
Figure 3.13. Au nanostructures obtained without the addition of Pd2+. The
concentration of AuCl4- in these experiments was kept the same as previously. The
concentrations of Ag+ are as follows: (A) 0.024 mM; (B) 0.096 mM; (C) 0.476 mM
..................................................................................................................................... 37
Figure 3.14. Au nanoparticles synthesized without the addition of Ag+. (A) [Pd2+] = 0
mM, 195 °C, 30 min; (B) [Pd2+] = 0.06 mM, 120 °C, 12 h; (C) [Pd2+] = 0.12 mM, 120
°C, 12 h ....................................................................................................................... 39
viii
vii
List of Tables
Table 3.1. Atomic composition based on EDX, ICP-MS and XPS of Au truncated
ditetragonal prisms, bipyramids and {310} truncated octahedra.................................32
ix
Chapter 1. Introduction
1.1. Background
Noble metal nanoparticles are excellent catalysts for many chemical
transformations due to their much higher surface-to-volume ratio than the bulk
materials.1-3 Since the catalytic properties of metal nanoparticles are highly dependent
on the morphology,3-8 control of their shape and size holds great promise for the
preparation of catalysts with improved performance.3,9,10
Noble metal nanocrystals with high-index facets are known to provide high
catalytic activities because of their high density of low-coordinated surface atoms that
can serve as active sites for breaking chemical bonds.11,12 Therefore, synthesis of
metal nanocrystals with high-index facets has been of much interest to numerous
investigators during the past decade. However, it still remains challenging to fabricate
such nanocrystals because of their high surface energy and thus low stability.
Recently, metal nanocrystals bounded by high-index facets such as Pt and Pd
tetrahexahedral (THH) nanocrystals have been synthesized using electrochemical
method.13,14 The Pt and Pd THH particles have exhibited 2-6 times higher catalytic
activity per unit surface area than the commercial catalysts toward ethanol
electrooxidation. These works, therefore, shed new light to the synthesis of metal
nanocrystals enclosed by high-index facets for catalysis, although the electrochemical
approach is limited to small-scale production. Wet chemical synthesis is promising
for large-scale preparation of nanocrystals.15,16 However, the current wet chemical
routes still lack the ability to simultaneously control over the shape and size of metal
nanocrystals bounded by high-index facets.
1
1.2. Research objectives
The synthesis of metal nanocrystals with high-index facets using wet chemical
methods is currently of intensive focus. Moreover, the study of catalytic properties of
metal nanocrystals bounded by high-index facets as well as the use of these
nanocrystals as the building blocks for more complex heterometallic nanostructures
are promising topics in nanoscience and nanotechnology. So far, by using a modified
polyol process in combined with the use of Ag(I) and Pd(II) as foreign ions, we have
successfully synthesized Au nanocrystals with exposed high-index facets including
truncated octahedra enclosed by both {111} and {310} facets, truncated ditetragonal
prisms bounded by twelve {310} facets, and bipyramids with {117} facets.
2
1.3. References
(1)
Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett. 2000, 2, 2385.
(2)
Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc.
2002, 124, 7642.
(3)
Wang, D. S.; Xie, T.; Li, Y. D. Nano Res. 2009, 2, 30.
(4)
Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343.
(5)
Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663.
(6)
Tao, A. R.; Habas, S.; Yang, P. Small 2008, 4, 310.
(7)
Xu, R.; Wang, D. S.; Zhang, J. T.; Li, Y. D. Chemistry Asian J. 2006, 1, 888.
(8)
Schmidt, E.; Vargas, A.; Mallat, T.; Baiker, A. J. Am. Chem. Soc. 2009, 131,
12358.
(9)
Xia, Y. N.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem. Int. Ed. 2009,
48, 60.
(10)
Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005,
105, 1025.
(11)
Somorjai, G. A.; Blakely, D. W. Nature 1975, 258, 580.
(12)
Somorjai, G. A. Science 1985, 227, 902.
(13)
Tian, N.; Zhou, Z.-Y.; Sun, S.-G.; Ding, Y.; Wang, Z. L. Science 2007, 316,
732.
(14)
Tian, N.; Zhou, Z.-Y.; Yu, N.-F.; Wang, L.-Y.; Sun, S.-G. J. Am. Chem. Soc.
2010, 132, 7580.
(15)
Ma, Y.; Kuang, Q.; Jiang, Z.; Xie, Z.; Huang, R.; Zheng, L. Angew. Chem. Int.
Ed. 2008, 47, 8901.
(16)
Ming, T.; Feng, W.; Tang, Q.; Wang, F.; Sun, L.; Wang, J.; Yan, C. J. Am.
Chem. Soc. 2009, 131, 16350.
3
Chapter 2. Literature Review
2.1. Shape-control synthesis of noble metal nanocrystals
In order to control the shape and size of metal nanocrystals, one should know
how they are created and grown. From these understandings, one can basically choose
the appropriate synthetic method to selectively fabricate the desired shapes and sizes
of the metal nanocrystals. Thus, in this part, a brief discussion of the growth
mechanism of metal nanocrystals is introduced, followed by a review of various
chemical methods in shape-controlled synthesis of noble metal nanocrystals.
2.1.1. Nucleation and growth of metal nanocrystals
Chemical synthesis of nanoparticles involves either decomposition or reduction
of metal precursors.
For the decomposition route, nucleation stage is considered to follow the LaMer
diagram.1 Briefly, under suitable conditions the number of metal atoms increases with
time. As this concentration reaches supersaturation stage, the nucleation events start
to happen and precursor concentration drops accordingly. In case the atomic
concentration sinking too fast, no more homogeneous nucleation can occur, leading to
uniform size of the nuclei. With the non-stop addition of new metal atoms from the
bulk solution, the nuclei develop into nanocrystals and then cease to grow when the
equilibrium state is achieved between surface atoms and the atoms remaining in the
bulk solution.2
For the reduction route, the chemical precursors are to be reduced into atoms
before these atoms agglomerate with each other to form nuclei. Afterwards, these
nuclei keep growing in size through an autocatalytic process in which the newly born
4
atoms are continuously added onto the nuclei surfaces. Finally, these nuclei grow into
nanoparticles with much bigger sizes.2
During the growth from nuclei to nanocrystals, firstly, the nuclei grow and form
seed with the presence of facets due to the fact that thermal fluctuation is no longer
energetically sufficient to randomly change the morphology of the nuclei.2 The seeds,
at this stage, must take their own configurations either single-crystals, singly twinned
or multiply twinned structures. This stage can be considered as the most important
stage to define the final shape and structure of the resultant nanocrystals because the
configuration (i.e., single-crystalline, singly twinned or multiply twinned) taken by
the seed will also be the resultant configuration for the nanocrystals later on.
With single-crystalline seeds, the final nanocrystals would accept either
polyhedral or anisotropic structures. For polyhedral shapes, the seeds will take the
octahedral forms if R (ratio of growth rate along to directions.) is equal
to 1.73, cuboctahedral forms if R = 0.87 and cubic shapes if R = 0.58. Therefore, for
fcc nanocrystals, R value is a very important parameter to control if one expects to
exclusively produce one of the three polyhedrons. For anisotropic structures which
are the consequences of symmetry breaking effect, octagonal rod and bar can be
formed from cuboctahedron and cube, respectively, through the so-called surface
passivation.
With singly twinned seeds, the resultant nanocrystals could be either right
bipyramids or beams which are favorable shapes for nanocrystals with one twinned
plane located in the middle.
With multiply twinned (usually penta-twinned) seeds, three possible shapes have
been obtained, namely, decahedron, icosahedron and pentagonal rod. While
5
decahedron and icosahedron are composed of certain numbers of identical tetrahedra
subunits, pentagonal rod is formed by five elongated tetrahedra which share one
common edge.
Finally, with plate-like seeds having stacking faults, the resultant nanocrystals
will take the hexagonal or triangular plate shape.
2.1.2. Chemical methods for synthesis of metal nanocrystals with controlled
shapes
Current wet chemical methods for shape-controlled synthesis of metal
nanocrystals
mainly
include
seeded-growth,
hydrothermal,
electrochemical,
photochemical and polyol routes. Each method has its advantages and disadvantages
and can find applications in different areas.
2.1.2.1. Seeded-growth route
Seeded-growth method is a two- or multi-stage chemical process. At the first
stage, metal precursor is quickly reduced in aqueous solution with high surfactant
concentration by using a strong reducing agent (usually NaBH4). Under such a
concentrated-surfactant condition, metal seeds formed are very small, about 3-5 nm in
diameter.3-5 These preformed-seeds are subsequently added into the so-called “growth
solution” that contains suitable concentrations of the metal precursor, surfactant and a
mild reducing agent. The ability to control the shape and size of the resultant
nanocrystals relies on the rational input ratio between seeds, precursor, and surfactant.
This method has been widely used to control the shape and size of metal nanocrystals
as it can separate the nucleation stage from the growth stage.
6
Seeded-growth method has been reported by the Murphy’s group in the synthesis
of spherical and rod-like Au nanoparticles.3 This method has been employed to
synthesize various shapes of Au nanoparticles including cubes, octahedra, rods, and
multipods.4
Seeded-growth has also been adopted and modified by other groups to further
improve its ability to produce various shapes of gold and other noble metal
nanoparticles with uniform sizes. For example, El-Sayed and co-workers modified the
synthesis of Au nanorods with the use of CTAB-capped seeds and the addition of
trace AgNO3 that could boost the yield of single-crystalline Au nanorods up to 99%.6
Guyot-Sionnest et al. reported the growth of either Au nanorods or bipyramids by
using single-crystalline or multiply twinned seeds.5 Recently, Huang et al. presented a
facile seed-mediated growth with the use of Cu UPD on Pd nanocrystals to synthesize
monodisperse, long Pd nanorods.7 Very recently, Xu and co-workers performed the
growth of uniform Pd polyhedral nanoparticles, namely, cubes, octahedra, rhombic
cuboctahedra and their intermediate forms by controlling KI concentration and
reaction temperature.8
Although seeded-growth has been considered as one of the most powerful
methods for synthesizing metal nanoparticles, it strictly requires the very accurate
conditions for making seeds such as pH value and concentration of the strong
reducing agent. Additionally, metal nanoparticles synthesized by this method are
usually very difficult to be stored for a long time.
67
8
2.1.2.2. Hydrothermal route
Hydrothermal method involves a process in which metal precursor, surfactant and
solvent (normally water) are first mixed together at room temperature. The whole
reaction solution is then transferred into a Teflon vessel that is closely sealed by the
external metal shell. The system is subsequently heated up to a high temperature
which is usually higher than the boiling point of the solvent in the system. The
nucleation and growth stage are to be one after another to finally produce the metal
nanocrystals. The hydrothermal pathway has attracted much attention due to its
simple one-step reaction but can provide a wide range of shapes of metal
nanocrystals. For example, Quian et al. reported the procedure for synthesis of Ag
nanowires by using a simple hydrothermal method.9 Dong et al. used PDDA-mediated
hydrothermal route to obtain Ag nanocubes, Au nanoplates, Pd and Pt
nanopolyhedra.10 Recently, monodisperse Au octahedra with different sizes have been
synthesized by using sodium citrate as mild reducing agent.11 Zheng and co-workers,
for the first time, have presented a new hydrothermal route to synthesize uniform Pd
nanowires.12
Although this method is facile, it is usually time- and energy-consuming, and it
needs to be done under highly safe conditions.
2.1.2.3. Electrochemical route
Electrochemical method relies on the trigger of redox chemical reactions by using
an external applied voltage. This method can be applied with or without nanoporous
template (i.e., hard template) such as anodized-aluminum oxides13. For the
electrochemical method with hard template, deposition on one face of the membrane
8
with a metal layer is first prepared so that this layer can serve as a cathode for electrocoating. Subsequently, desired metal precursors are to be reduced and delivered inside
the pore channels of the membrane. The shape and size of metal nanoparticles can be
rationally controlled by varying the potential, deposition time, and surfactant during
the electrochemical process13. This method has a main advantage that it can be
applied to fabricate nanostructures of most of metals. Electrochemical method has
been developed by many research groups.13-16
Despite of its wide range of synthesis of metal nanoparticles, electrochemical
route cannot be considered for large-scale applications due to its high cost and low
yield of product.
2.1.2.4. Photochemical route
Photochemical route is the chemical process in which irradiation of light is used
to reduce metal ions into metal atoms with or without pre-formed nanopaticles in the
presence of suitable surfactant in the solution. This method has been known to be very
effective in the synthesis of Ag and Au nanocrystals. For instance, Mirkin et al.
reported that Ag nanoprisms could be obtained via the transformation of Ag
nanospheres under irradiation of fluorescent light.17 Using this method, they obtained
Ag nanoprism with sizes ranging from 40 to 120 nm.18 Au nanorods was also
fabricated using UV irradiation by Yang and co-workers.19 Recently, transformation
of Ag nanoplates into rounded Ag nanoplates with increased thickness has been
observed by using UV light irradition.20 Very recently, the Mirkin’s group has
synthesized Ag right bipyramids with a very high yield (>95%) by using halogen
lamp irradiation.21
9
Photochemical synthesis can be considered as an effective and green method (i.e.,
without the use of strong reducing agent, low reaction temperature). However, this
method usually gives rise to low yield of product, and it can be only performed in the
syntheses of Ag and Au nanocrystals.
2.1.2.5. Polyol route
Polyol route has been known as a powerful method to control the shape and size
of metal nanocrystals. In this method, either ethylene glycol (EG) or other polyols
such as 1,5-pentadiol and polyethyleneglycol (PEG) are used to serve as both the
solvent and reducing agent in the reaction. Polyvinylpyrrolidone (PVP) or its
copolymer with different molecular weight is used as both the capping agent and
stabilizer where PVP and metal precursor are able to form complex compounds. The
reduction power in the polyol method can be easily tuned by adjusting the reaction
temperature since EG becomes easier to be oxidized at higher temperature. The
method was first introduced by Fievet et al. in the late 90’s.22 Great enhancement has
been made by the Xia’s group who discovered the so-called “oxidative etching
process” and “surface passivation”
on Pd and Ag nanocrystals. By using these
strategies, Xia et al. have successfully controlled the shape and size of Ag, Pd and Pt
nanoparticles.
Silver
nanostructures,
namely,
nanocubes,23-27
nanowires,28-31
nanobipyramids,27,32 nanobeams,33 nanorices and nanobars27 have been obtained by
rational control of foreign ions such as Cl¯, Br¯ and Fe3+.
Additionally, a series of Pd nanocrystals have been synthesized with the similar
strategies, namely, nanocubes,34,35 nanoboxes and nanocages,36 nanoplates,37
nanowires, nanobipyramids,38 and nanobars and nanorods.39
10
Though more difficult to control the shape, Pt nanocrystals have also been
prepared in different morphologies such as Pt nanowires, nanooctahedra, nanoplates,
nanomultipods.40
Moreover, Au nanocrystals with various shapes have been also fabricated based
on the modified polyol syntheses in which a trace amount of Ag+ is used. For
example, Yang et al. reported the synthesis of Au nanocrystals by using PVP as
surface-regulating agent, EG as a solvent heated up at 280 °C.41 While Au
nanotetrahedra and nanoicosahedra were obtained without the absence of Ag ions, Au
nanocubes were synthesized by adding a trace amount of Ag ions. Song and coworkers prepared Au nanooctahedra, nanocuboctahedra and nanocubes by simply
adjusting the ratio of Ag to Au ions in the 1,5-pentadiol.42
The polyol method has a drawback that the PVP bounded on the surface of assynthesized nanoparticles is difficult to completely remove. This limitation inhibits
some applications of metal nanocrystals synthesized by the polyol synthesis,
especially in biomedical applications. Therefore, the prominent post-treatment of
those metal nanocrystals is of great necessity for this method to be promising for
biomedical applications.
11
2.2. Synthesis and catalytic properties of metal nanocrystals with high-index
facets
High-index facets are facets composed of periodic combination of two or more
microfacets of low Miller-indices (i.e., {111}, {100} and {110}). The high-index
facets of noble metal crystals can serve as active sites for breaking chemical bonds
due to their high density of ledges, steps and kinks.43,44 Synthesis of metal
nanostructures with high-index facets has become an increasingly important research
topic due to the fact that high-index surfaces usually exhibit superior catalytic
properties in many chemical reactions.
Noble metal nanostructures bounded by high-index facets have been mainly
synthesized using two methods: electrochemical approach45,46 and seeded-growth
synthesis.47,48 In electrochemical method, Pt nanoparticles with high-index facets (i.e.,
tetrahexahedra or THH) were synthesized through the adsorption and desorption of
oxygen onto Pt surface inspired by the square wave potential45. THH Pt nanocrystal is
composed of twenty-four (730) high-index faces which comprise (310) and (210)
subfacets. The THH nanocrystals are surprisingly stable even under strict condition
such as 800 °C. The reason for this high stability can be explained that the adsorption
and desorption of oxygen on the Pt surface can stabilize the high-index facets.
Although such nanocrystals bounded by high-index facets can be formed by using this
method, the feasibility of up-scaling and ease of processing should be further
improved to make those nanocrystals useful in catalytic applications.
Seeded-growth synthesis of Au nanocrystals with high-index facets has been
reported by Xie et al.47 By reducing HAuCl4 in the presence cetyltrimethylammonium
chloride (CTAC) and ascorbic acid (AA), THH Au nanocrystals bounded by 24
12
11
{211} facets were obtained in high yield. On the same trend, Wang et al. successfully
synthesized elongated THH Au nanocrystals in high yield (~95%).48 In their
synthesis, the amount of seed and pH adjustment were claimed to be the crucial
factors responsible for the formation of these Au nanocrystals. Inspired by these two
works on the synthesis of nanocrytals with high-index facets, several reports have
recently been introduced to further improve the yield and size range.49-51 Very
recently, Mirkin et al. have prepared gold nanocrystals with a unique shape called
“concave cube”52. This structure can be described as a cube with six concave square
pyramids on its faces (in contrast to tetrahexohedron that possesses six convex square
pyramids on six faces).
Metal nanocrystals bounded by high-index facets have been renowned for their
superior catalytic activities to those of the nanocrystals with low-index facets. For
example, THH Pt nanocrystals exhibit the 200% and 400% higher catalytic activities
of electro-oxidation compared with that of 3.2 nm Pt/C commercialized catalyst for
ethanol and formic acid, respectively.45 In addition, the trisoctahedra Au (TOH)
nanocrystals displayed different electrochemical behavior from that of polycrystalline
Au and Au nanocrystals with low-index facets.47
13
12
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16
Chapter 3. Shape-controlled synthesis of Au Nanocrystals with
High-index Facets
3.1. Shape-selective growth of polyhedral gold nanocrystals with high-index
facets
3.1.1. Introduction
As discussed previously, shape-controlled synthesis of noble metal nanocrystals
in solution-phase have relied on the flexibility of choosing reaction parameters such
as precursor, solvent, surfactant and foreign ions. Among these strategies,
introduction of foreign metal ions, especially silver ions, in the synthesis of gold
nanocrystals has shown drastic morphology-selection effect. Au nanoparticles with
tailored shapes including cube,1-4 octahedron,2-4 nanorod,2,5-7 bipyramid,8-11 and
plate12 have been successfully synthesized in aqueous or polyol solvents in the
presence of trace amount of Ag ions. The influence of silver ions has been recognized
for nearly a decade in the growth of Au nanorods and polyhedral nanocrystals.
Murphy and co-workers firstly discovered that adding Ag ions to the growth solution
of Au in a seed-mediated approach can significantly improve the yield of single
crystalline Au nanorods; while for the same method but without Ag+, only
polycrystalline nanorods can be obtained.8 Later, Yang et al. extended the use of Ag+
to preparation of Au nanocubes of high yield in ethylene glycol.1 This method was
further developed by Song and co-workers to generate Au octahedra and
cuboctahedra.3,4 Recently, the use of Ag+ in N-alkylmidazole was reported in which
Au octahedra, cubes, rhombic dodecahedra and high-index tetrahexahedra (THH)
17
were successfully synthesized.13 The Ag underpotential deposition (UPD) on Au
surface refers to a process in which the Ag layer can be deposited on the Au surface at
the potential much positive than the Nernst potential for the reduction of Ag. This
deposition of Ag usually appears to be one or two atomic layers on Au surface that
are able to adjust the growing rate of different facets of Au (i.e., {110}, {100} and
{111}). In addition, the presence of Cl- are consider to further boost the Ag UPD shift
by some hundreds mV which is critically necessary for the morphological control of
Au nanocrystals.14
Compared to silver, palladium has received much less attention in shape-selective
growth of Au nanocrystals. Although Pd and Au have been both used in some
reactions, focus has been mainly on the formation of Au-Pd bi-metallic structures
especially core-shell nanoparticles.15-19 For example, Yacaman and co-workers
conducted successive reduction of PdCl2 and HAuCl4 in ethylene glycol using PVP as
the protective agent. The 5-nm particles formed this way show a three-layer core-shell
structure with a Au-Pd alloy inner core, an intermediate layer rich of Au, and a third
layer of Pd-rich alloy.20 A one-step aqueous synthesis was reported by Han et al. who
found that Au@Pd core-shell particles with an octahedral shape were formed because
Au(III) was preferentially reduced over Pd(II) in the presence of CTAC, thus Au
octahedral were formed first followed by deposition of Pd on the surface to give the
core-shell structure.17 Recently, Krichevski and Markovich found that Pd doping may
induce growth of Au nanowires, where small Pd nuclei formed in situ can reduce the
intermediate Au+ species and the incorporation of Pd into the growing Au
nanostructures induced nanowire formation in high yield.21
Due to the different behavior of Ag and Pd ions when they are involved in the Au
nanocrystal synthesis, one would expect that combining these two foreign metal ions
18
in the synthesis of Au nanocrystals would show synergistic effect on controlling the
shapes of the resultant particles. Indeed, although scarcely reported, controlled growth
of nanocrystals in tri-metallic nanocrystal systems has been noticed recently. LizMarzan et al. examined the influence of Ag ions on the growth of Pt on Au nanorods
and found that in the presence of Ag+, the deposition of Pt takes place on the rod tips;
while without Ag+, homogeneous coating of Pt on rod surface are obtained. This was
attributed to the UPD of Ag on Au(110) which causes slower growth of Pt on {110}
faces compared to those on {100} and {111} faces.22 Here, we report the shapeselective synthesis of Au nanocrytals in the presence of two foreign metal ions – Ag
and
Pd.
A
facile
one-pot
polyol
synthesis
was
employed
with
poly(diallyldimethylammonium chloride) (PDDA) as the capping agent. For the first
time, a series of Au nanostructures, namely, Au truncated ditetragonal nanoprisms
bounded by twelve {310} facets, bipyramids enclosed with {117} high-index facets,
and truncated octahedra with exposed {111} and {310} facets, were synthesized by
simply varying the ratio of Ag and Pd ions.
3.1.2. Experimental Section
Ethylene glycol (EG, Sigma-Aldrich), chloroauric acid trihydrate (HAuCl4⋅3H2O,
Alfa Aesa), silver nitrate (AgNO3, Sigma-Aldrich), palladium(II) chloride (PdCl2,
Alfa Aesar), poly(diallyldimethylammonium chloride) solution (PDDA, 20%, MW =
200 000-350 000, Aldrich), poly(vinyl pyrrolidone) (PVP, MW = 55 000, Aldrich),
hydrochloric acid (HCl, 37%, Merck) and sodium chloride (NaCl, Sigma-Aldrich)
were used as adopted without any further purification. 10 mM H2PdCl4 solution was
prepared by dissolving 35.6 mg of PdCl2 in 20 ml of 20 mM HCl at 100 °C till a
19
transparent solution was obtained. The water used throughout this work was 18.2 MΩ
ultrapure deionized water.
In a typical synthesis of Au truncated ditetragonal prisms, 0.2 ml of PDDA (20%
in H2O) solution and 10 ml of EG were mixed in a glass vial using magnetic stirrer at
room temperature. To this solution, 18.8 μl of 0.5 M HAuCl4, 2 μl of 500 mM
AgNO3, 62.5 μl of 10 mM H2PdCl4, and 430 μl of de-ionized H2O were added under
stirring. The volumes of AgNO3 and H2PdCl4 aqueous solutions were controlled so
that the final concentrations of Au, Ag and Pd were 0.895 mM, 0.024 mM, and 0.06
mM, respectively. It was found that for the synthesis of nanoprisms, adding 10 μl of 1
M NaCl aqueous solutions to the reactions could further improve the yield. This
solution (in EG) was then kept mixing for 5 min before closely sealed with the cap.
The vial was then heated up in an oil bath at 120 °C for 12 h with stirring. The
solution color changed from yellow to colorless gradually after heated up at 120 °C
for 2 h. After 3.5 h, the solution became pale pink and finally stopped at reddish
brown at 12 h. The product was harvested by centrifugation and washed with acetone
once and with water five times to remove excess PDDA on the surface of the
particles. The as-synthesized Au nanocrystals were finally store in water for further
uses.
The synthetic procedure for gold bipyramids was similar to the Au nanoprisms,
except that the Ag+ concentration was increased to 0.476 mM (10 μl of 500 mM
AgNO3). For the reactions of truncated octahedra, Ag+ concentration was reduced to
0.024 mM (25 μl of 10 mM AgNO3). Note that the purification was carried out in
case of Au truncated octahedra to remove minor amount of big Au octahedra and the
washing with saturated NaCl was applied in case of Au bipyramids to remove the
20
AgCl cluster in the resultant solution for more accurate elemental analysis and better
imaging.
Scanning Electron Microscopy (SEM) images were taken using a JEOL JSM6700F operating at 10 kV. Transmission electron microscopy (TEM) images, electron
diffraction (ED) patterns and energy dispersive X-ray spectroscopy (EDX) spectra
were acquired on a JEOL JEM-2010F operating at 200 kV. High-resolution
transmission electron microscopy (HRTEM) images were obtained using a JEOL
JEM-2100F operating at 200 kV. Samples for TEM and SEM were prepared by drop
casting the sample solutions onto carbon-coated copper grids and silicon substrates,
respectively. The TEM and SEM samples were then rinsed for a few hours with
deionized water to remove excess polymers, followed by drying at 60 °C in air. The
X-ray diffraction (XRD) spectra were acquired using a SHIMADZU XRD-6000
diffractometer equipped with a Cu Kα radiation source (λ = 1.5418 Å) from samples
deposited onto a plastic substrate. The scan rate and step size used were 1 (deg/min)
and 0.02 (deg). UV-visible spectra of the Au nanocrystals were recorded using a
SHIMADZU UV-1601 spectrometer with plastic cuvettes of 1-cm path length at room
temperature. X-ray photoelectron spectroscopy (XPS) analyses were performed using
a Kratos AXIS Ultra DLD spectrometer equipped with an Al Kα monochromatized
X-ray source with a penetration depth of ~3 nm. Inductively coupled plasma mass
spectrometry (ICP-MS) measurements were carried out on an Agilent 7500 ICP-MS
instrument. Samples were prepared by dissolving nanocrystals using freshly made
aqua regia followed by diluting with DI-H2O.
21
3.1.3. Result & discussion
Figure 3.1A-D are representative SEM and TEM images of the Au nanocrystals
obtained from reactions with molar ratio of Au:Pd:Ag = 15:1:1.6. Most of particles
(>95%) exhibit well-defined facets with average longest edge length of ~195 nm,
respectively. SEM images at high magnifications (Figure 3.1B, C) revealed that the
majority of the particles exhibit a ditetragonal prism shape with truncated ends. The
particles with this shape are enclosed with twelve faces – eight side-faces parallel to
the principle axis and four terminating faces located at both ends of the prism. A
schematic drawing of this truncated ditetragonal prism is presented in Figure 3.1F,
which illustrates the projections of such a particle at different viewing angles. The
TEM projection along the principle axis parallel to the prism side-faces show a
ditetragonal cross-section (Figure 3.1D), with measured inner angles alternating
between 143° and 127°. The measured angles match well with those calculated from a
ditetragonal prism with {310} facets, which alternate between 143.1 and 126.9 degree
(Figure 3.2).23,24 This indicates that the facets of the Au nanocrystals are {310}. To
further confirm the facets of the truncated prisms, we recorded electron diffraction
(ED) patterns of a number of particles. Figure 3.3 shows four representative ED
patterns and the corresponding TEM images at different project angels. We found that
all the patterns can be indexed with a zone axis of [310].
22
24
Figure 3.1. (A) Low- and (B) high-magnification SEM images of Au truncated
ditetragonal prisms showing well-defined structures with sharp edges and apexes. (C)
HRSEM of a group of Au truncated ditetragonal prisms. (D) TEM images of Au
truncated ditetragonal prisms showing its cross-section. (E) High magnification of a
truncated ditetragonal prism (inset) exhibiting (200) d-spacing of fcc Au. (F) The
schematic drawings at different views of an Au nanoprism.
23
Figure 3.2. Determination of facets of Au truncated ditetragonal prisms from
different views (A) top view (cross-section) and (B) side view. The result indicates
that Au truncated ditetragonal prisms are bounded by 12 {310} facets. Note that
image (A) and (B) were taken from different truncated ditetragonal prisms. (C), (D)
Schematic drawing of truncated ditetragonal prisms with their theoretical angles. (E)
Atomic model of Au (310) facet including (110) and (100) subfacets.
In addition to the typical shape illustrated in Figure 3.1F, the truncated
ditetragonal prisms also exhibited a few other forms, but all with 12 exposed {310}
faces. Based on SEM and TEM images, we carefully established the models of the
particles with different arrangement of the faces (Figure 3.4). The three additional
configurations that can be described based upon the deviations from the one described
in Figure 3.1.
24
Figure 3.3. (A), (B), (C) and (D) Schematic models for Au truncated ditetragonal
prisms at different views illustrating for (E), (F), (G) and (H) the corresponding TEM
images. (I), (J), (K) and (L) the ED patterns that consistently show all [310] zone
axes. Note that the SAED patterns were taken from different Au nanoprisms.
Figure 3.4. Schematic models for other configurations of Au truncated ditetragonal
prisms which differ from the Au nanoprisms in Figure 3.1.
In addition to truncated nanoprism particles, we also synthesized Au bipyramids
in high yield following the same reaction procedure but with higher concentration of
silver ions. Figure 3.5 shows the morphology of the Au bipyramids obtained at a ratio
of Au:Pd:Ag = 15:1:8 with a reaction temperature of 120 °C (same as for the case of
25
Au prisms). FESEM images (Figure 3.5A, B) indicate that the Au bipyramids have
smooth side faces with truncated tips. The truncated tips of the bipyramid showed a
pentagonal shape (Figure 3.5B, inset). The average base length and height of the
bipyramids are 50 and 127 nm, respectively. TEM image of the Au bipyramids
(Figure 3.5C) displayed twinned planes, indicating that the bipyramid-shaped Au
particles have a five-fold twinned structure. This 5-fold twinned structure has been
previously observed from nanorods and bipyramids by various research groups.6,9-11,25
High-resolution TEM (HRTEM) of an Au bipyramid (Figure 3.5D) revealed lattice
fringes with planar distances of 0.23 nm which matches well with the d-spacing of
(111) lattice plane of gold. Selected-area electron diffraction pattern (Figure 3.5E) of
the same particle showed the superposition of two sets of patterns characteristic of
[111] and [110] zone diffraction of fcc gold. This result not only confirmed the 5-fold
twinned structures of Au bipyramids but also revealed the growth direction of
as described by previous works on Au bipyramids.9-11 A schematic model for a Au
bipyramid is showed in Figure 3.5F. Additionally, based on the formula for indexing
pentagonal bipyramid facets established by Sun et al.,26 we proposed that the Au
bipyramids are bounded by {117} facets (Figure 3.6). The same result was also
reported previously by Guyot-Sionnest and co-workers.9
26
Figure 3.5. (A) Low and (B) high magnification SEM images of Au bipyramids. Inset
of Figure 3B clearly shows a pentagonal cross-section of an exceptionally big
bipyramid. Inset scale bar is 100 nm. (C) TEM image of Au bipyramids. (D) HRTEM
image of a bipyramid (inset) describes the (111) d-spacing. Inset scale bar is 20 nm.
(E) The corresponding ED pattern showing the superposition of [110] and [111] zones
of fcc structure. (F) Schematic drawing of a bipyramid.
Figure 3.6. (A) TEM image of an Au nanobipyramid with defined width base (W,
yellow line) and height of half (Hhf, red line). (B) Model of half of pentagonal
bipyramid and formula that exhibits the relationship between morphological
measurements (i.e., W and Hhf) and Miller index of the bipyramidal facets. By
measuring few tens of Au bipyramids in TEM images, we determined the average
Hhf/W ratio of 2.18 corresponding to high-index {117} facets.
In recognition of the effect of silver ions on controlling the shape of the Au
nanocrystals, we also lowered the concentration of the Ag+ in the reactions, which led
27
to the formation of truncated octahedra enclosed by 8 {111} and 24 {310} facets.
Figure 3.7A shows the truncated octahedral nanocrystals prepared at a ratio of
Au:Pd:Ag = 15:1:0.4. SEM at higher magnification (Figure 3.7B) revealed that the
exposed faces of the nanocrystals are pentagons and hexagons, with each hexagonal
face surrounded by 6 pentagonal ones. The homogeneous contrast under TEM (Figure
3.7C) is attributed to the single-crystallinity of the particles. Electron diffraction
patterns taken from a number of Au nanocrystals showed either [310] or [111] zone
axes (Figure 3.7D-E), indicating that the exposed faces include both {310} and {111}
facets. Based on the careful analysis of SEM images and diffraction patterns, we
proposed that each truncated Au octahedral nanocrystal contains 8 {111} facets in
hexagonal shape and 24 {310} facets in pentagonal shape. While the {111} facets are
created from faces of an octahedron, the {310} facets are formed at the 6 vertexes of
the octahedron (Figure 3.7F).
Figure 3.7. (A) Low magnification SEM image of Au truncated octahedra. Inset
shows schematic model of a truncated octahedron that exposes both {111} and {310}
facets. (B) High magnification SEM image of truncated octahedra with a superposed
drawing frame on single truncated octahedra shows the consistency with the
28
schematic model. (C) TEM image of Au truncated octahedra with the inset showing
(200) d-spacing of Au fcc. (D), (E) ED patterns of Au truncated octahedra clearly
show [310] and [111] zone axes. (F) Schematic drawing showing the morphological
relationship between an octahedron and a truncated octahedron.
The compositions of the nanocrystals were analyzed using EDX on SEM. It was
found that the nanoprisms are mainly composed of Au (>98 at%), with a very small
amount of Ag (~0.4%) and Pd (~1.2%) (Figure 3.8). This is consistent with the ICPMS measurement, which gives 98.3%, 1.12%, and 0.58% for Au, Ag and Pd,
respectively. However, XPS analysis showed much higher atomic percentages of Ag
and Pd, which account for ~11% and ~19%, respectively (Figure 3.9). Since the
penetration depth of XPS is ~3 nm, the much higher concentrations of Ag and Pd
from XPS measurements indicate that Ag and Pd are mainly located at the surface of
the nanocrystals. This result agrees well with previous works where UPD of Ag was
used to control the shape of Au nanoparticles.3,27 The EDX, ICP-MS and XPS
analyses of the bipyramids and truncated octahedra showed similar composition
profiles - the nanocrystals are mainly composed of Au with trace amount of Pd and
Ag on the surface. Table 3.1 summarizes the elemental analyses of the Au
nanocrystals with three different shapes.
29
Figure 3.8. EDX analyses of Au nanostructures: (A) truncated ditetragonal prisms,
(B) bipyramids and (C) truncated octahedra.
30
Figure 3.9. XPS analyses of Au nanostructures: (A) truncated ditetragonal prisms,
(B) bipyramids and (C) truncated octahedra.
31
Table 3.1. Atomic composition based on EDX, ICP-MS and XPS of Au truncated
ditetragonal prisms, bipyramids and {310} truncated octahedra.
EDX
ICP-MS
XPS
Au
98.48
98.30
70.18
Ag
0.40
1.12
10.79
Pd
1.12
0.58
19.03
EDX
ICP-MS
XPS
Au
96.61
92.93
58.93
Ag
1.50
5.97
18.47
Pd
1.89
1.10
22.60
EDX
ICP-MS
XPS
Au
99.19
97.56
74.71
Ag
0.24
1.94
10.35
Pd
0.57
0.50
14.94
Atomic %
Atomic %
Atomic %
Figure 3.10 shows the XRD patterns of the Au ditetragonal prisms, bipyramids,
and truncated octahedra, respectively. The peaks positions of all three XRD patterns
match well with those of gold with fcc crystal structure. The ditetragonal prisms
exhibited a much stronger (200) reflection peak than the bipyramids and truncated
32
octahedral particles. The intensity ratios of (111) to (200) and (111) to (220) for
ditetragonal prisms are 0.37 and 1.33, respectively, which are much lower than those
of standard powder sample of fcc gold (1.92 and 3.12, respectively). The relatively
stronger (200) and (220) reflections than (111) may be attributed to the abundance of
(310) facets of the nanocrystals since the high-index (310) facet is composed of (100)
terrace and (110) step denoted by 3(100)x(110) (Figure 3.2). The XRD pattern of Au
bipyramids (Figure 3.10B) clearly matches the standard pattern of fcc Au. The
polycrystallinity of these nanoparticles were revealed by the peaks splitting.28 The
strong (111) peak could be attributed to the abundance of {111} planes derived from
this structures. Same result was reported by Wu et al. for Au nanobipyramids.29 The
very strong (111) peak for the truncated octahedra could be explained that the
particles were preferentially seated on their {111} facets parallel to the substrate.
Figure 3.10. XRD patterns of Au nanostructures: (A) truncated ditetragonal prisms,
(B) bipyramids and (C) truncated octahedra.
33
35
Figure 3.11. UV-vis spectra of Au truncated ditetragonal prisms, bipyramids and
truncated octahedra.
UV-vis spectra of the Au nanocrystals with three different shapes were presented
in Figure 3.11. The ditetragonal prisms exhibit a strong surface plasmon resonance
(SPR) peak at 600 nm and another broader peak at 836 nm corresponding to the
transversal and longitudinal vibrations of the particles, respectively. The Au
bipyramids show a sharp, strong peak at 775 nm and another broad peak at 533 nm.
Unlike the ditetragonal prisms and bipyramids, Au truncated octahedra showed only
one peak at 578 nm due to the more symmetric configuration.
Au nanostructures including Au ditetragonal prisms, bipyramids and truncated
octahedra can be synthesized with different sizes by simply adjusting reaction
temperature. By increasing reaction temperature from 120 °C to 140 °C and 170 °C,
we have obtained Au ditetragonal prisms with longest edge lengths of 52 (Figure
3.12A) and 30 nm (Figure 3.12B), bipyramids with lengths of 53 (Figure 3.12C) and
40 nm (Figure 3.12D), and truncated octahedra with diameters of 75 (Figure 3.12E)
and 32 nm (Figure 3.12F).
34
Figure 3.12. Au nanostructures synthesized at different temperature: (A, C and E) at
1400C and (B, D and F) at 1700C. The procedures were similar to those used for the
syntheses of Au truncated ditetragonal prisms, bipyramids and truncated octahedra
except that no NaCl was used for the truncated ditetragonal prism synthesis. (A, B)
Nanobars with the longest lengths of 52 and 30 nm, (C, D) bipyramids with lengths of
53 and 40 nm and (E, F) truncated octahedra with diameters of 75 and 32 nm.
It is clear that at higher temperatures, smaller Au nanostructures were formed
since higher temperatures favor the formation of larger number of nuclei that causes
smaller extent of nanoparticle growth.
The effect of Ag ions on controlling the shape of Au nanocrystals has been
studied extensively.14 The role of silver ions in the growth of Au nanocrystals with
different shapes can be multi-faceted. One possibility is that the adsorption of silver
halide monolayer on Au {110} faces could inhibit or slow down the growth of Au in
the direction perpendicular to these facets.14 Another explanation is because of the
35
different UPD shifts of silver on gold surfaces which decrease following the order
{110}>{100}>{111}.9 The different UPD shifts indicate that adatom deposition of
Ag on Au is easier for {110} and {100} that {111} facets, which causes symmetry
breaking effect and thus the appearance of {110} and {100} facets.
In order to interpret the role of Ag+ and Pd2+ in the process, we firstly conducted
experiments with the same conditions as described previously but in the absence of
Pd2+. Figure 3.13 shows the Au nanocrystal formed at [Pd2+] = 0 for three different
Ag concentrations. Without Pd2+ addition, truncated octahedra, mixtures of multiplefacets crystals, mixtures of faceted particles and bipyramids in a wide range of sizes
appeared (Figure 3.13A-C, respectively). It is worth noting that with low and medium
concentrations of Ag ions added in the reactions, edge-truncated octahedra (Figure
3.13A) and edge-truncated faceted-nanocrystals (Figure 3.13B) were obtained. These
structures appeared to be exposed with {110} facets.
The growth mechanism of the Au nanocrystals in the absence of Pd2+ can be
described as follows. At first, Au3+, Ag+ and PDDA can form complex at room
temperature upon mixing. As the reactant solution is heated up, Au3+ and Ag+ are
reduced into Au0 and Ag0 atoms with the protection of PDDA. Since the reduction
potential of Au3+/Au is much higher than that of Ag+/Ag, the resulting Ag atoms can
be re-oxidized by Au3+. Consequently, only Au atoms exist in the solution during this
period. Due to the extremely high surface energy of the as-formed Au atoms, they
tend to aggregate to form the so-called nuclei with a size of a few nanometers under
the protection of PDDA macromolecules. At this period, the PDDA-protected Au
nuclei keep growing in the presence of underpotential deposition (UPD) of Ag, and
their shape profile may vary randomly. As the Au nuclei grow to certain sizes, they
begin to take their own shape since the coverage of PDDA macromolecules and the
36
UPD of Ag may become more energetically favorable than the thermal effect.
Because the PDDA macromolecules contain Cl- anions which could form AgCl
precipitate in the presence of Ag+ cations, it is believed that PDDA could influence
the UPD shift of Ag, leading to the preferential deposition on {110} facets of the Au
nanoparticles. For this reason, the growth of Au nanoparticles perpendicular to the
{110} facets is strongly inhibited. As a result, the Au nanocrystals with {110} facets
remain in the final products. Therefore, we believe that the UPD of Ag on the surface
of Au nanocrystals inhibits the growth along directions, leading to the
formation of the {110} facets.
At the very high Ag+ concentration (0.476 mM), there was the formation of Au
nanobipyramids with fivefold twinned structure (Figure 3.13C). This multiply
twinned structure could be due to the interference of AgCl species which is mostly
insoluble in ethylene glycol30 leading to the formation of Au mutiply twinned nuclei.
These multiply twinned nuclei were further developed into Au nanobipyramids via a
continuous deposition of Au adatoms on the surface of the pre-formed nuclei.
Figure 3.13. Au nanostructures obtained without the addition of Pd2+. The
concentration of AuCl4- in these experiments was kept the same as previously. The
concentrations of Ag+ are as follows: (A) 0.024 mM; (B) 0.096 mM; (C) 0.476 mM.
Pd deposition (either under- or over-potential) may also take place on the surface
of Au crystals to form epitaxial layer. It has been found that the tendency of alloy
formation of Pd on Au decrease following the order of {110}>{100}>{111}, which is
37
the same as that of Ag. The role of Pd2+ can be concluded based on another set of
experiments without adding Ag ions. It is clear that in the absence of Pd2+ only Au
octahedra with uniform size can be obtained (at 195 °C) (Figure 3.14A). At [Pd2+] =
0.06 mM, the mixture of vertex-truncated nanocrystals were harvested, namely,
octahedra, tetrahedra and twinned plates (Figure 3.14B). Higher concentration of Pd2+
only led to the formation of similar mixture of Au nanocrystals obtained previously
(Figure 3.14C). The truncation at vertices of those Au nanocrystals is, in fact, the
indication for the formation of {100} facets of the nanocrystals which might be
attributed to the deposition of Pd on Au nanocrystal surface.
The growth mechanism of the Au nanocrystals in the absence of Ag+ can be
interpreted similar to the above process (i.e. in the absence of Pd2+). In this process,
the as-formed Au nanoparticles were affected by the UPD of Pd in combination with
the coverage of PDDA macromolecules leading to the exclusive inhibition of the
growth perpendicular to the {100} facets. Therefore, the resultant Au nanocrystals
appeared to have certain exposure of {100} facets. These results clearly indicate that
the deposition of Pd on Au nanocrystal surface induces the growth of {100} facets.
Overall, Ag and Pd ions, which stabilize the {110} and {100} facets,
respectively, together are able to inhibit the growth along directions that
results in the growth of {310} facets existing in the Au nanocrystals obtained in this
work.
38
Figure 3.14. Au nanoparticles synthesized without the addition of Ag+. (A) [Pd2+] = 0
mM, 195 °C, 30 min; (B) [Pd2+] = 0.06 mM, 120 °C, 12 h; (C) [Pd2+] = 0.12 mM, 120
°C, 12 h.
3.1.4. Conclusion
We have presented a new route to Au nanoparticles with high-index facets
including Au truncated ditetragonal prisms enclosed by 12 {310} high-index facets,
bipyramids bounded by {117} stepped facets and truncated octahedra with exposed of
8 {111} and 24 {310} facets. By using a facile one-pot PDDA-mediated polyol
process combined with the synergistic effect of Ag and Pd ions, we have obtained
those Au nanocrytals in high yield, monodispersity and wide range of sizes. In this
process, UPD of Ag serve to inhibit the growth along direction that leads to the
formation of {110} facets of Au nanocrystals. The presence of Pd2+, on the other
hand, stabilizes the {100} facets of Au nanocrystals. Therefore, together, Ag and Pd
ions enable the growth of {310} facets that lead to the formation of Au truncated
octahedra partially enclosed by {310} facets and truncated ditetragonal prisms totally
bounded by {310} facets. Very high Ag ion concentration which could induce the
existence of AgCl species was appeared to interfere with the construction of Au
adatoms on the Au nuclei leading to the formation of multiply twinned nuclei. These
nuclei could finally grow further into the Au nanobipyramids with penta-twinned
structures. The Au truncated ditetragonal prisms and truncated octahedra with the
{310} high-index facets could be excellent candidates for catalytic applications due to
39
their abundance in unsaturated atomic steps, ledges and kinks. The Au bipyramids
with {117} stepped faces were synthesized in the highest yield ever using our method.
Because of their high yield and monodispersity, these bipyramids could not only serve
as catalysts for chemical reactions but also act as good substrate for surface-enhanced
Raman scattering (SERS) and contrast agent for biomedical applications.
40
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Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957.
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Wu, H.-Y.; Huang, W.-L.; Huang, M. H. Cryst. Growth Des. 2007, 7, 831.
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Huang, X.; Neretina, S.; El-Sayed, M. A. Adv. Mater. 2009, 21, 4880.
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Jana, N. R.; Gearheart, L.; Murphy, C. J. Adv. Mater. 2001, 13, 1389.
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Liu, M.; Guyot-Sionnest, P. J. Phys. Chem. B 2005, 109, 22192.
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Kou, X.; Zhang, S.; Tsung, C.-K.; Yeung, M. H.; Shi, Q.; Stucky, G. D.; Sun,
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Kou, X.; Ni, W.; Tsung, C.-K.; Chan, K.; Lin, H.-Q.; Stucky, G. D.; Wang, J.
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Grzelczak, M.; Pérez-Juste, J.; Mulvaney, P.; Liz-Marzán, L. M. Chem. Soc.
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Xiang, Y.; Wu, X.; Liu, D.; Jiang, X.; Chu, W.; Li, Z.; Ma, Y.; Zhou, W.; Xie,
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Fan, F.-R.; Liu, D.-Y.; Wu, Y.-F.; Duan, S.; Xie, Z.-X.; Jiang, Z.-Y.; Tian, Z.Q. J. Am. Chem. Soc. 2008, 130, 6949.
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Ding, Y.; Fan, F.; Tian, Z.; Wang, Z. L. J. Am. Chem. Soc. 2010.
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Lim, B.; Kobayashi, H.; Yu, T.; Wang, J.; Kim, M. J.; Li, Z.-Y.; Rycenga, M.;
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42
Chapter 4. Conclusions and Recommendation for Future Work
4.1 Conclusions
The main focus of this thesis is to devise a new approach for synthesis of Au
nanocrystals with novel shapes and with high-index facets, and novel heterometallic
nanostructures based on the Au nanocrystals obtained. First, by using the modified
PDDA-mediated polyol process, we have successfully synthesized Au nanocrystals
with high-index facets, including truncated octahedra bounded by both {111} and
{310} facets, truncated ditetragonal prisms enclosed by {310} facets, and bipyramids
with {117} facets. In this process, UPD of Ag serve to inhibit the growth along
direction that leads to the formation of {110} facets of Au nanocrystals. The presence
of Pd2+, on the other hand, stabilizes the {100} facets of Au nanocrystals. Therefore,
together, Ag and Pd ions enable the growth of {310} facets that lead to the formation
of Au truncated octahedra partially enclosed by {310} facets and truncated
ditetragonal prisms totally bounded by {310} facets. Very high Ag ion concentration
which could induce the existence of AgCl species was appeared to interfere with the
construction of Au adatoms on the Au nuclei leading to the formation of multiply
twinned nuclei. These nuclei could finally grow further into the Au nanobipyramids
with penta-twinned structures. The Au truncated ditetragonal prisms and truncated
octahedra with the {310} high-index facets as well as the Au bipyramids could be
excellent candidates for catalytic applications due to their abundance in unsaturated
atomic steps, ledges and kinks.
43
4.2 Recommendations for Future work
Gold has recently been recognized as the very good catalyst for many reactions
such as hydrogenation of alkenes, dienes and alkynes, selective oxidations of alcohols
and alkenes and activation of carbonyl groups.1-4 Gold nanocrystals with high-index
facets have electrochemically proved to be more active than Au nanocrystals enclosed
by low-index facet (i.e., {111}, {100} and {110}).5,6 However, to date, the
applications of these Au nanocrystals with high-index facets in chemical synthesis
have not been reported yet. Besides, bimetallic nanostructures, made of two different
metal elements, have long been known as excellent catalysts owing to their
synergistic effects between the two different metal components.7 Among bimetallic
structures, gold-palladium core-shell (Au@Pd) has proved to be one of the most
attractive catalysts due to synergistic effect from the Au core and highly-strained Pd
shell leading to the high catalytic activity for many chemical syntheses.8
The application of novel nanocrystals with high-index facets or controlled
bimetallic structures in catalysis are therefore of technical and scientific interest. In
this work, we have successfully synthesized a series of high-index Au nanocrystals,
namely, truncated ditetragonal prisms bounded by 12 {310} facets, bipyramids
enclosed by {117} facets, and {310} and {111} truncated octahedra that can
theoretically provide superior catalytic properties. In addition, we have also prepared
Au@Pd nanorods which contain the high density of grooves and defects. These
nanocrystals can be applied as catalysts for reactions such as direct formation of
hydrogen peroxide from the reaction of H2 and O2.
Hydrogen peroxide has been considered as the significantly important commodity
chemicals for fine chemical industry and household uses. Approximately a few
44
million tons of H2O2 is produced every year. The production of H2O2 is of economic
importance for pharmaceutical industry and daily uses.
Previously, H2O2 was synthesized by using the indirect anthraquinone which was
first invented by Riedl and co-workers in 1939. The process includes the
hydrogenation of a substituted anthraquinone using metal catalyst such as Ni or Pd to
form the diol9. The successive oxidation of anthraquinol using oxygen-enriched air
recovers the initial anthraquinone and produces H2O2.
This process is still used for commercial H2O2 supply even though it mainly
depends on the effective recycle of anthraquinone which is a highly expensive
chemical. In addition, the production of H2O2 must be carried out on large scale for
lowering the manufacturing expenses and the transportation of concentrated H2O2 can
be hazardously dangerous that could end up with unexpected explosion. Therefore,
there has been a strong need for a small-scale and safe process for producing H2O2 in
which H2O2 is directly produced from H2 and O2.
The direct synthesis of H2O2 has been pursued by many research groups.9-20 The
synthesis of hydrogen peroxide was first catalyzed by Pd colloids in the presence of
H2 and O2.10 Afterwards, Hutchings et al. discovered that Al2O3 supported Au catalyst
could produce much higher selectivity as compared to Al2O3 supported Pd
catalyst.13,14 Interestingly, in the same work, the authors also showed that the
supported Au:Pd (1:1 by wt) catalysts gave rise to the significantly higher rate of
H2O2 formation than the pure Au catalyst due to the synergistic effect of Pd serving as
a promoter for Au catalyst. Since then, many reports of direct synthesis of H2O2 using
supported Au:Pd catalyst have been published.9,16-19,21 Among those, supported Au-
45
50
49
core Pd-shell catalyst appears to be very efficient for catalyzing the formation of H2O2
with high production rate and selectivity.18,19
While most reports on direct synthesis of H2O2 focus on in situ preparation of
supported Au, Pd, and Au:Pd catalysts (i.e., impregnation or co-precipitation), few is
on ex situ preparation of these catalysts. Very recently, Lundsford and co-workers has
devised a novel route to synthesize the supported Pd nanoparticles for catalyzing the
direct formation of H2O2.22 In their work, the as-prepared Pd nanoparticles were
immobilized on the XC-72 carbon black support to use as catalysts for the synthesis
of H2O2. By making a comparison between the supported Pd nanoparticles prepared
ex situ and in situ (i.e., impregnation), they have proved that the specific activity and
selectivity of the supported Pd nanoparticles prepared ex situ are significantly higher
than that of the conventionally prepared Pd catalyst.
In view of the above findings, we propose the direct synthesis of H2O2 using our
Au nanocrystals and Au@Pd nanorods immobilized ex situ on support as the catalyst.
In this proposed work, the Au nanocrystals including truncated ditetragonal prisms
and bipyramids, and Au@Pd nanorods with the smallest sizes (30 nm, 40 nm and 40
nm for Au prisms, bipyramids, and Au@Pd nanorods, respectively) will be used as
the active components of the supported catalysts. The immobilization of the Au and
Au-Pd catalysts will follow the procedure developed by Lundsford et al.22 Briefly, the
as-synthesized Au and Au-Pd nanoparticles and carbon black will be mixed in toluene
followed by drying in oven to obtain supported catalyst. The synthesis of H2O2 can be
performed in a stainless steel autoclave with a H2/O2 ratio of 1:2. Gas analysis for H2
and O2 will be recorded by gas chromatography. The conversion of H2 will be
calculated based on the gas analyses before and after the reaction. The yield of H2O2
can be estimated by titration of the final solution with acidified Ce(SO4)2.
46
Subsequently, Ce(SO4)2 solution is standardized against (NH4)2Fe(SO4)2.6H2O by
using ferroin as indicator.14
The performance of the supported Au catalyst can be evaluated based upon their
productivity and selectivity for H2O2 formation. The productivity of the catalysts is
defined as the mole of H2O2 generated per kg of catalysts per hour. The selectivity of
the catalyst is defined as the mole of H2O2 generated per hour divided by the mole of
H2 consumed per hour. Although the Au and Au-Pd nanoparticles used as the active
components of the supported catalyst have larger sizes than the conventional Au or Pd
nanoparicles used in previous reports in this field, it is expected that the supported
catalyst will provide high productivity and selectivity because of the following two
reasons:
i.
The Au nanocrystals are bounded by high-index facets, namely, {310} or
{117} which are composed of low-coordinated atoms at the steps, ledges and
kinks that serve as highly active sites for breaking and forming of chemical
bonds. The surface of the Au nanocrystals including ditetragonal prisms and
bipyramids are constituted of Au and Pd that can synergistically give rise to
superior catalytic activities for H2O2 synthesis.
ii.
The Au@Pd nanorods which contain Au-core and Pd-highly strained shell
may exhibit synergistic effect enhancing the catalytic activities. In addition,
the Au@Pd nanorods possess the high density of grooves and defects which
may act as highly active sites for chemical reactions.
47
51
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2008, 47, 6221.
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[...]... followed by a review of various chemical methods in shape- controlled synthesis of noble metal nanocrystals 2.1.1 Nucleation and growth of metal nanocrystals Chemical synthesis of nanoparticles involves either decomposition or reduction of metal precursors For the decomposition route, nucleation stage is considered to follow the LaMer diagram.1 Briefly, under suitable conditions the number of metal atoms increases... 2.1 Shape- control synthesis of noble metal nanocrystals In order to control the shape and size of metal nanocrystals, one should know how they are created and grown From these understandings, one can basically choose the appropriate synthetic method to selectively fabricate the desired shapes and sizes of the metal nanocrystals Thus, in this part, a brief discussion of the growth mechanism of metal nanocrystals. .. R.; Personick, M L.; Zhang, K.; Li, S.; Mirkin, C A J Am Chem Soc 2010, 132, 14012 16 Chapter 3 Shape- controlled synthesis of Au Nanocrystals with High-index Facets 3.1 Shape- selective growth of polyhedral gold nanocrystals with high-index facets 3.1.1 Introduction As discussed previously, shape- controlled synthesis of noble metal nanocrystals in solution-phase have relied on the flexibility of choosing... have relied on the flexibility of choosing reaction parameters such as precursor, solvent, surfactant and foreign ions Among these strategies, introduction of foreign metal ions, especially silver ions, in the synthesis of gold nanocrystals has shown drastic morphology-selection effect Au nanoparticles with tailored shapes including cube,1-4 octahedron,2-4 nanorod,2,5-7 bipyramid,8-11 and plate12 have... applications of metal nanocrystals synthesized by the polyol synthesis, especially in biomedical applications Therefore, the prominent post-treatment of those metal nanocrystals is of great necessity for this method to be promising for biomedical applications 11 2.2 Synthesis and catalytic properties of metal nanocrystals with high-index facets High-index facets are facets composed of periodic combination of two. .. nanocrystals would show synergistic effect on controlling the shapes of the resultant particles Indeed, although scarcely reported, controlled growth of nanocrystals in tri-metallic nanocrystal systems has been noticed recently LizMarzan et al examined the influence of Ag ions on the growth of Pt on Au nanorods and found that in the presence of Ag+, the deposition of Pt takes place on the rod tips; while... homogeneous coating of Pt on rod surface are obtained This was attributed to the UPD of Ag on Au(110) which causes slower growth of Pt on {110} faces compared to those on {100} and {111} faces.22 Here, we report the shapeselective synthesis of Au nanocrytals in the presence of two foreign metal ions – Ag and Pd A facile one-pot polyol synthesis was employed with poly(diallyldimethylammonium chloride) (PDDA)... surfactant concentration by using a strong reducing agent (usually NaBH4) Under such a concentrated-surfactant condition, metal seeds formed are very small, about 3-5 nm in diameter.3-5 These preformed-seeds are subsequently added into the so-called “growth solution” that contains suitable concentrations of the metal precursor, surfactant and a mild reducing agent The ability to control the shape and size of. .. cuboctahedra and their intermediate forms by controlling KI concentration and reaction temperature.8 Although seeded-growth has been considered as one of the most powerful methods for synthesizing metal nanoparticles, it strictly requires the very accurate conditions for making seeds such as pH value and concentration of the strong reducing agent Additionally, metal nanoparticles synthesized by this method... morphological control of Au nanocrystals. 14 Compared to silver, palladium has received much less attention in shape- selective growth of Au nanocrystals Although Pd and Au have been both used in some reactions, focus has been mainly on the formation of Au-Pd bi-metallic structures especially core-shell nanoparticles.15-19 For example, Yacaman and co-workers conducted successive reduction of PdCl2 and .. .SYNERGISTIC EFFECT OF TWO FOREIGN METAL IONS ON SHAPE- SELECTIVE SYNTHESIS OF GOLD NANOCRYSTALS TRAN TRONG TOAN (B.Sc (Hons.), University of Science Ho Chi Minh City)... ratio of Ag and Pd ions The synergistic effect of Ag and Pd ions on the formation of the novel Au nanocrystals was studied In our experimental conditions, the underpotential deposition (UPD) of. .. reaction parameters such as precursor, solvent, surfactant and foreign ions Among these strategies, introduction of foreign metal ions, especially silver ions, in the synthesis of gold nanocrystals