Ionic liquid-assisted synthesis of thorned gold plates comprising three-branched nanotip arrays

Article abstract of DOI:10.1039/c0cc05116k

Hierarchically structured, thorned gold plates comprising regular, three-branched nanotip arrays were synthesized by a facile, one-pot reduction process in mixed solvents of the ionic liquid [BMIM][PF₆] and formamide without additives. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0cc05116k

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COMMUNICATION
Ionic liquid-assisted synthesis of thorned gold plates comprising
three-branched nanotip arraysw
Yao Qin,ab Yin Song, Teng Huang and Limin Qi*
a
a
a
Received 23rd November 2010, Accepted 22nd December 2010
DOI: 10.1039/c0cc05116k
1
3
nanoimprint lithography, as well as nanosphere lithography,
14
Hierarchically structured, thorned gold plates comprising regular,
three-branched nanotip arrays were synthesized by a facile, one-
pot reduction process in mixed solvents of the ionic liquid
were developed for preparing gold nanotip arrays. It would be
desirable to explore efficient wet chemical routes towards the
controlled growth of planar gold nanotip arrays. Recently, ionic
liquids have shown great potential in the solution-phase
[
6
BMIM][PF ] and formamide without additives.
15
Gold nanostructures have attracted significant research interest
because of their unique physical, chemical, and biocompatible
1
properties, as well as promising applications. Since the intrinsic
preparation of tailored inorganic nanostructures. We fabri-
cated hierarchical, three-fold symmetrical, gold dendrites by the
reaction between a Zn plate and a solution of HAuCl
4
in the
properties and relevant applications of gold nanostructures are
largely determined by their size and shape, there has been
remarkable progress over the past decade in the shape-controlled
ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate
16
([BMIM][PF
of novel hierarchically ordered, thorned gold plates comprising
three-branched nanotip arrays by the reduction of HAuCl in
mixed solvents of [BMIM][PF ] and formamide without
additives.
The synthesis of the thorned gold plates with three-branched
nanotips was achieved simply by reducing HAuCl in mixed
[BMIM][PF ]/formamide solvents where formamide acted as
both a reducing agent and a cosolvent. In a typical synthesis,
500 mL of [BMIM][PF ] containing 10 mM HAuCl was mixed
with 500 mL formamide in a plastic tube at room temperature to
give a yellowish HAuCl solution (5 mM), which was then kept
]). Herein, we report a facile, one-pot synthesis
6
2
synthesis of gold nanocrystals. In particular, gold nanoparticles
4
3
4
5
with sharp tips (e.g., rods/wires, triangles, and stars/dendrites )
are attracting increasing interest since they can sustain large
electromagnetic fields at their ends, vertices, or tip apexes upon
excitation with light of appropriate energy, which makes them
promising candidates for applications in ultrasensitive detection
6
4
6
6–8
based on surface enhanced spectroscopies. It is notable that
the seeded growth method was recently employed for the
fabrication of dendritic gold nanostructures with a fractal
6
4
9
7
architecture and thorned nanowires comprising sharp tips.
However, it remains a challenge to realize the facile synthesis
of hierarchical gold structures comprising regular nanotip
arrays.
4
in an oven at 95 1C and held for B24 h. The resulting precipitate
was washed with acetone thoroughly and characterized by
scanning electron microscopy (SEM, Hitachi S4800, 15 kV),
transmission electron microscopy (TEM, FEI Tecnai F30, 300
kV), and powder X-ray diffraction (XRD, Rigaku Dmax-2000,
Cu-Ka).
Surface-enhanced Raman scattering (SERS) is a sensitive
analytic technique and powerful tool for fundamental studies
of surface species, and hence considerable efforts have been
devoted to the controlled fabrication of novel plasmonic nano-
structures as highly sensitive, robust, and reproducible SERS
Fig. 1 shows representative SEM images of the gold product
6
obtained after 24 h of aging at 95 1C in mixed [BMIM][PF ]/
1
0
substrates. Particularly, planar nanotip arrays with high tip
density are appealing SERS substrates as the sharp tips and the
small gaps between adjacent tips can significantly enhance the
local electromagnetic fields, leading to high SERS enhancement.
In this regard, various energy- or time-consuming procedures,
formamide solvents with a volume ratio of 1 : 1. As shown in
Fig. 1a, the product consists of hexagonal plates typically
about 2–4 mm in width and 500–600 nm in thickness together
with some relatively smaller particles. The related XRD and
energy-dispersive X-ray spectroscopy (EDS) characterizations
show that the product is pure gold crystals of the cubic
structure (Fig. S1, ESIw). Fig. 1b shows an enlarged SEM
image of the gold plates, which suggests that the well-defined
hexagonal plates show thorned surfaces comprising densely
aligned nanotips on both the top and bottom planes. A high-
magnification top-view clearly shows dense arrays of three-
branched nanotips standing vertically on the top surface of the
gold plate (Fig. 1c), while a high-magnification side-view
suggests that the three-branched nanotips actually adopt a
cone-like shape exhibiting sharp tip apexes (Fig. 1d). Each
+
11
12
such as Ar -ion irradiation, electron beam lithography,
a
Beijing National Laboratory for Molecular Sciences, State Key
Laboratory for Structural Chemistry of Unstable and Stable Species,
College of Chemistry, Peking University, Beijing 100871, People’s
Republic of China. E-mail: liminqi@pku.edu.cn
The Institute for Advanced Materials and Nano Biomedicine, Tongji
University, Shanghai 200092, People’s Republic of China
b
w Electronic supplementary information (ESI) available: XRD pattern,
EDS spectrum and additional SEM images (Fig. S1,2), and Raman
measurements. See DOI: 10.1039/c0cc05116k
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2985–2987 2985

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that the hexagonal plate is a single crystal with the top face of the
(111) crystal plane and with the six edges perpendicular to the
h110i directions, which is consistent with the crystal orientation
of the final hexagonal thorned plates. When the reaction time
was prolonged to 12 h, the hexagonal plates became considerably
thicker with pristine nanotips appearing on the surface (Fig. 2d).
This result suggests a two-step growth process for the thorned
gold plates, namely, relatively flat, hexagonal gold plates with the
top (111) plane formed first, which was followed by the vertical
growth of the [111]-oriented gold nanotips with three side
branches parallel to the {110} planes on the top surfaces of the
gold plates. The formation of the [111]-oriented nanotips with a
threefold symmetry could result from the kinetically controlled
growth process under nonequilibrium conditions with a moder-
1
7
ate driving force, similar to the formation of hierarchical [111]-
16
oriented gold dendrites in [BMIM][PF6].
It was found that an appropriate mixing ratio of [BMIM][PF6]
and formamide in the solution was essential to the formation of
the well-defined, thorned gold plates. As shown in Fig. 3a, when
Fig. 1 SEM images of thorned gold plates with three-branched
nanotips obtained after 24 of aging at 95 1C in mixed
BMIM][PF ]/formamide solvents with a volume ratio of 1 : 1.
h
[
6
the [BMIM][PF
: 1 to 3 : 7, only thorned gold microparticles with some irregular
tips were obtained. If the [BMIM][PF ]/formamide ratio was
6
]/formamide volume ratio was decreased from
nanotip has three side branches perpendicular to the plate
surface with a threefold symmetry, and each side branch
approximately corresponds to a right-angled triangle less than
1
6
increased from 1 : 1 to 7 : 3, gold plates with mildly rugged
surfaces were obtained under otherwise similar conditions
20 nm in thickness, B50 nm in base side width, and 70–80 nm
in height. The three-branched gold nanotips with a threefold
(Fig. 3b). Here formamide played the roles of both reducing
symmetry are reminiscent of the threefold symmetrical gold
1
agent and cosolvent whereas [BMIM][PF ] acted mainly as an
6
6
6
dendrites obtained in [BMIM][PF ], indicating that each
inert viscous medium. The relative concentration of the ionic
liquid can considerably influence the ion diffusivity and reaction
plate is a single crystal with the flat (111) crystal face and
each nanotip perpendicular to the plate face is a single crystal
oriented along the [111] direction with the three side branches
rate in the solution. At a low [BMIM][PF
:7, the reaction occurred rather fast and the driving force
would be pretty high, resulting in thorned gold microparticles,
whereas at a high [BMIM][PF ]/formamide ratio of 7 :3, the
reaction took place quite slowly, resulting in gold plates with
mildly rugged surfaces. When the [BMIM][PF ]/formamide ratio
6
]/formamide ratio of
3
ꢀ
ꢀ
ꢀ
parallel to the (110), (011), and (101) planes, respectively.
The gold products obtained at earlier stages of the formation
of the thorned gold plates with three-branched nanotips were
examined by SEM and TEM, and the results are shown in Fig. 2.
After 5 h of reaction, hexagonal gold plates typically about
6
6
was kept at 1 : 1, a delicate balance between the diffusion rate
and the reaction rate or a moderate driving force was achieved,
leading to appropriate nonequilibrium conditions for the growth
2–4 mm in width and less than 100 nm in thickness were formed
together with some small particles (Fig. 2a). The top surface of
16
the plate was somewhat rugged showing some visible wrinkles
of hierarchical sharp gold nanotips with a three-fold symmetry.
(
Fig. 2b). Fig. 2c presents a typical TEM image and the related
The significantly lowered ion diffusivity and reaction rate owing
to the presence of ionic liquid could prevent uncontrolled over-
growth of gold crystals.
electron diffraction (ED) pattern of a single plate, which shows
A seed-mediated growth experiment was designed to grow
nanotip arrays on smooth gold sheets with larger surface
areas. Firstly, smooth gold sheets with the top (111) faces,
which were typically tens of micrometres across and tens of
nanometres thick (Fig. S2a,b, ESIw), were fabricated through
1
8
a polyol process following the literature. Secondly, a growth
Fig. 2 SEM (a,b,d) and TEM (c) images of gold plates obtained after
different aging times: (a–c) 5 h, (d) 12 h. Inset shows the related ED
pattern.
Fig.
[BMIM][PF
the enlarged images.
3
SEM images of gold products obtained at different
6
]/formamide volume ratios: (a) 3 : 7, (b) 7 : 3. Insets show
2
986 Chem. Commun., 2011, 47, 2985–2987
This journal is c The Royal Society of Chemistry 2011

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enhancement in Raman scattering on the thorned gold plate
may be ascribed to the giant local electromagnetic fields at the
apexes of densely arrayed nanotips and the small gaps between
adjacent nanotips as well as adjacent side branches.
In conclusion, the facile, one-pot synthesis of unique
thorned gold plates comprising three-branched nanotip arrays,
which exhibited enhanced SERS response, was achieved by the
4 6
reduction of HAuCl in mixed solvents of [BMIM][PF ] and
formamide without additives. This work may open new avenues
towards large-area, hierarchical plasmonic structures with
unusual patterns and useful properties.
Fig. 4 Raman spectrum of solid PATP (a) and SERS spectra of
PATP adsorbed on a smooth gold sheet (b) and a thorned gold
plate (c).
This work was supported by NSFC (20873002, 21073005,
20633010, and 50821061), MOST (2007CB936201), and the
Program for Young Excellent Talents in Tongji University.
solution was prepared by aging the [BMIM][PF
6
]/formamide
Notes and references
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4
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6
the seeds for gold deposition at 95 1C. After 2.5 h of growth,
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ESIw). When the growth time was increased to 24 h, almost all
the gold sheets grew considerably thicker and comprised
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defined three-branched nanotips was evident (Fig. S2d, ESIw).
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6
´
´
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´
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´
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using the smooth Au sheets as a reference sample (See ESIw for
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excited with the 633 nm laser line are presented in Fig. 4. The
SERS spectrum of the PATP molecules adsorbed on the thorned
´
8
9
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ꢀ1
Au plate exhibits the bands at 1006, 1074, and 1180 cm
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1
ꢀ
436 cm assigned to b modes, and the band at 1574 cm
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ꢀ1
1
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19
formed by both a
1
and b
2
modes. Generally, the a
1
modes of
PATP are predominantly enhanced through the electromagnetic
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2
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1
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ꢀ
1
1
574 cm for the thorned Au plate are about 35 and 74 times
1
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stronger than those for a smooth Au sheet, respectively, while the
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2
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This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2985–2987 2987