punitive supervision

1. In summary, a novel and simple constituent-template dis-
placement process has been introduced to incorporate nano-
sized particles into MSTFs. Combined FTIR, SAXRD,
HREM/EDS, and UV-vis spectroscopy proves that nanosized
TiO2 have been formed into pore channels of the MSTFs and
the hexagonal mesostructure of parent materials is still re-
tained. XPS results further confirmed that there are at least
two coordination states present to the titanium species in
TiO2/MSTFs. The first is an anatase-like nanosized TiO2
phase and the second is an isolated [TiO4] unit in the frame-
work or on the pore surface. More importantly, with a similar
process after carrying out pore-surface organic modification,
we have successfully incorporated metallic silver into MSTFs
as well as the already obtained results in powder mesoporous
materials;[8]
the related paper is in preparation.
Experimental
Characterization: SAXRD patterns were collected with a Rigaku D/MAX-
diffractometer, equipped with a rotating anode, using Cu Ka radiation (k =
1.5418 Š, 40 kV, 60 mA) over the range of 1.8 £ 2h £ 10 with a step size of
0.002. HREM images and ED patterns were recorded on a JEOL 200CX elec-
tron microscope operated at 160 kV. EDS spectra were obtained from an at-
tached Oxford Link ISIS energy-dispersive spectrometer fixed on a JEM-2010
electron microscope operated at 200 kV. FT-IR spectra were obtained on Nico-
let 7000-C or Bio-Rad FTS-185 spectrometer with 4 cm±1
resolution. Powder
samples scratched from substrates were dispersed in KBr for IR analysis. UV-
vis absorption spectra were collected on the Shimadzu UV-3101PC spectrome-
ter. XPS analysis was carried out at VG MicroLab MKII analyser using Mg Ka
radiation as X-ray source. All the spectra was corrected with the C 1s
(285.0 eV) band.
Received: October 29, 2001
Final version: March 15, 2002
±
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Large-Scale Synthesis of Uniform Silver
Nanowires Through a Soft, Self-Seeding, Polyol
Process**
By Yugang Sun and Younan Xia*
Metal nanowires have been the focus of many recent stud-
ies because of their potential use as active components or in-
terconnects in fabricating electronic, photonic, and sensing
devices.[1]
They also provide an ideal system to experimentally
probe transport properties under various physical confine-
ments (e.g., quantized conductance and localization effects).[2]
In these regards, silver nanowires seem to be particularly in-
teresting to synthesize and study because bulk silver exhibits
the highest electrical and thermal conductivities among all
metals. Silver has also been extensively exploited in a variety
of applications that range from catalysis, through electronics,
to photonics and photography.[3]
The performance of silver in
most of these applications could be significantly enhanced by
processing silver into wire-like nanostructures with well-con-
trolled dimensions. For instance, the loading of silver in poly-
mer composites (used as conductive pastes in electronic pack-
aging) could be greatly reduced by replacing nanoparticles
with nanowires or nanorods with relatively higher aspect
ratios.[4]
Silver nanowires have been successfully synthesized by tem-
plating against various types of one-dimensional structures,
such as channels in alumina or polymer membranes,[5]
meso-
porous materials,[6]
carbon nanotubes,[7]
block co-polymers,[8]
DNA chains,[9]
rod-shaped micelles,[10]
calix[4]hydroquinone
Adv. Mater. 2002, 14, No. 11, June 5 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 0935-9648/02/1106-0833 $ 17.50+.50/0 833
COMMUNICATIONS
±
[*] Prof. Y. Xia, Dr. Y. Sun
Department of Chemistry, University of Washington
Seattle, WA 98195 (USA)
E-mail: xia@chem.washington.edu
[**] This work has been supported in part by an AFOSR-DURINT subcon-
tract from SUNY-Buffalo, a Career Award from the NSF (DMR-
9983893), and a Fellowship from the David and Lucile Packard Founda-
tion. Y.X. is an Alfred P. Sloan Research Fellow (2000±2002). We thank
Dr. B. Gates, Y. Yin, B. Mayers, and Y. Lu for their help with electron
and X-ray diffraction studies.

2. nanotubes,[11]
and steps or edges on solid substrates.[12]
As
shown recently by Murphy et al., the use of physical templates
can ensure a good control over the morphology of the final
products and thus to obtain nanowires of metals with high as-
pect ratios.[13]
On the other hand, the use of templates may
complicate the synthetic procedure and limit the scale at
which a material can be processed in each synthesis. One can
overcome these difficulties by developing solution-phase
methods that involve no physical templates. To this end, a
number of direct, chemical methods have recently been dem-
onstrated for processing silver into wire-like nanostructures.
For example, silver nanowires have been synthesized by
reducing AgNO3 with a developer in the presence of AgBr
nanocrystallites,[14]
by arc discharging between two silver elec-
trodes immersed in an aqueous NaNO3 solution,[15]
or by ex-
truding from zeolite pores.[16]
Silver nanorods have been pro-
duced by irradiating AgNO3 solution with ultraviolet light in
the presence of poly(vinyl alcohol).[17]
The final products of
all these chemical syntheses were, however, characterized by
problems such as relatively low yields, low aspect ratios, irreg-
ular morphologies, non-uniformity in size, or polycrystalline
domain structures. As a result, it is still desired to develop a
high-yield method capable of generating silver nanowires in
moderately copious quantities and with well-controlled di-
mensions, crystallinities, and morphologies.
We have recently demonstrated a solution-phase method
that generates silver nanowires by reducing silver nitrate with
ethylene glycol in the presence of poly(vinyl pyrrolidone)
(PVP).[18]
In this so-called polyol process, which has been ex-
tensively exploited by many research groups to synthesize
metal particles,[19]
the ethylene glycol acts as both solvent and
reducing agent. The key to the formation of wire-like nano-
structures is the introduction of exotic seeds (e.g., platinum
nanoparticles) to the reaction mixture.[20]
When silver nitrate
is reduced in the presence of these seeds, silver nanoparticles
with a bimodal size distribution will be formed through het-
erogeneous and homogeneous nucleation, respectively. In the
course of refluxing, the nanoparticles having larger sizes will
grow at the expense of smaller ones (Ostwald ripening).[21]
With the help of PVP, which can probably control the growth
rates of various faces of silver by coordinating to the sur-
faces,[22]
these silver nanoparticles can be directed to grow
into nanowires having uniform diameters. We have demon-
strated the potential of this polyol process by generating bi-
crystalline nanowires of silver with diameters in the range of
30±50 nm, and lengths of up to ~50 lm.[18]
Here we demon-
strate that silver nanowires with similar quality and quantity
could also be routinely produced using a self-seeding process,
in which silver nitrate and PVP solutions (in ethylene glycol)
were simultaneously injected into refluxed ethylene glycol
through a two-channel syringe pump. By controlling the injec-
tion rate, the silver nanoparticles formed at the initial stage of
the reduction reaction could serve as seeds for the subsequent
growth of silver nanowires. The exclusion of exotic seeds is fa-
vorable for the synthesis of silver nanowires with lower levels
of impurity. The use of a syringe pump also provides a better
way to control the nucleation step, and has allowed us to sys-
tematically investigate the influence of ªcoordination re-
agentº (including its chemical structure, molecular weight,
and its molar ratio relative to silver nitrate) on the yield and
quality of silver nanowires synthesized using this soft, solu-
tion-phase approach.
Depending on the experimental conditions, the as-synthe-
sized products of silver nanowires might contain some nano-
particles (20 wt.-%).[18]
When diluted with acetone (~ five
times by volume) and centrifuged at 2000 rpm, the nanowires
would settle down to the bottom of the container and were
efficiently separated from the nanoparticles. Figure 1A shows
the SEM image of some purified silver nanowires that had a
mean diameter of ~60±8 nm. This image indicated the
straightness along the longitudinal axis, the level of purifica-
tion, and the copiousness in quantity that we could routinely
achieve through this synthetic approach. Figure 1B shows an
XRD pattern of these nanowires, and all diffraction peaks
could be indexed to the face centered cubic (fcc) phase of sil-
ver. The lattice constant calculated from this XRD pattern
was 4.092 Š, which was very close to the reported data
(a=4.0862 Š, JCPDS File No. 04-0783). Figure 1C gives the
TEM image of three such nanowires, indicating the uniformity
in diameter along each wire. Previous studies have suggested
a low threshold for twinning parallel to the {111} faces of an
fcc metal such as silver or gold.[23]
These materials tend to
grow as bicrystals twinned at {111} planes. Figure 1D shows a
TEM image of the end of an individual silver nanowire. The
contrast in this image clearly shows that there is a twin plane
situated parallel to its longitudinal axis that divides this wire
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Fig. 1. A) SEM and C) TEM images of uniform silver nanowires that were
synthesized via the self-seeding, polyol process. These nanowires were separat-
ed from silver nanoparticles through centrifugation. B) XRD pattern of these
nanowires, indicating the fcc structure of silver. D) The TEM image of an indi-
vidual silver nanowire, showing the bicrystalline structure that is characteristic
of silver nanowires synthesized using the present method. The [111] twin plane
in the middle of this nanowire is indicated by an arrow. The inset gives the
microdiffraction pattern recorded by focusing the electron beam on this wire.
The molar ratio between the repeating unit of PVP (Mw»55 000, n»500) and
AgNO3 was 1.5:1.

3. into two halves. The electron microdiffraction pattern (inset)
consists of two sets of spots corresponding to either side of
this wire. Each set could be independently indexed to reveal
the corresponding growth direction. Measurements of trans-
port properties at room temperature suggest that these silver
nanowires were electrically continuous with a conductivity of
~105
S cm±1
(the value for bulk silver is 6.2”105
S cm±1
).
The morphologies and aspect ratios of these silver nano-
wires strongly depend on the molar ratio between the repeat-
ing unit of PVP and AgNO3. Figure 2 shows TEM and SEM
images of three as-synthesized products obtained at different
molar ratios. PVP with an average molecular weight of 55 000
was used for these studies. There were only silver nanoparti-
cles present in the product when the molar ratio between
PVP and AgNO3 was higher than 15:1. Figure 2A shows the
TEM image of a product synthesized at the molar ratio of
15:1. This sample was essentially composed of silver nanopar-
ticles with an average diameter of ~20 nm. Figure 2B shows
an SEM image of the product when the molar ratio was re-
duced to 6:1. This sample contained a mixture of relatively
short nanowires (~45 nm in diameter and ~2 lm in length)
and nanoparticles of silver. Compared with the product shown
in Figure 1, which was synthesized with a molar ratio of 1.5:1,
we could conclude that decreasing the molar ratio of PVP to
AgNO3 was favorable for the formation of silver nanowires
with higher aspect ratios (as high as 1000). When this molar
ratio was further reduced to less than 1:1, the yield and mono-
dispersity of silver nanowires became unacceptable for practi-
cal use. Figure 2C shows the SEM image of a product gener-
ated with a molar ratio of 0.6:1. Most of these wires were
significantly thicker as compared to the sample in Figure 1,
and some of them were non-uniform along the longitudinal
axis.
The degree of polymerization of PVP (n, the average num-
ber of repeating units in one PVP macromolecule) was also
found to play an important role in determining the yield and
quality of the silver nanowires. Figure 3 shows the TEM and
SEM images of several samples synthesized with PVP of dif-
ferent molecular weights. The molar ratio between the repeat-
ing unit of PVP and AgNO3 was 6:1 for all these studies be-
cause Figure 2B shows that a narrow distribution in length
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COMMUNICATIONS
Fig. 2. A) TEM and B,C) SEM images of silver nanostructures (as-synthesized)
that were obtained when different molar ratios between the repeating unit of
PVP (Mw»55 000, n»500) and AgNO3 were used: A) 15:1, B) 6:1, and C) 0.6:1.
Fig. 3. A,B) TEM and C) SEM images of silver nanostructures that were ob-
tained when PVP with different degrees of polymerization were used: A) n=1
(1-ethyl-2-pyrrolidinone), B) n»90, and C) n»11 700. The molar ratio between
the repeating unit of PVP and AgNO3 was 6:1 for all these syntheses. The image
shown in (C) was taken from the as-synthesized sample, and no centrifugation
was involved.

4. could be obtained at this molar ratio. Figure 3A shows the
TEM image of silver nanostructures obtained with 1-ethyl-2-
pyrrolidinone (n=1) as the coordination reagent. In this case,
silver nanoparticles with highly irregular shapes and morphol-
ogies were formed. Figure 3B gives the TEM image of a prod-
uct synthesized with PVP of n»90 as the coordination reagent.
At this molecular weight, rod-shaped silver nanoparticles
were formed. Nanowires with higher aspect ratios and more
uniform diameters were obtained by using PVP with higher n
values. Figure 3C shows an SEM image of the product ob-
tained with PVP of n»11 700. These silver nanowires are more
uniform than those shown in Figure 2B, with an average diam-
eter and length of ~55 nm and ~4.8 lm, respectively. Note
that this sample was essentially free of nanoparticles, and no
centrifugation was required to recover the nanowires. The ex-
act function of PVP is still not clear at the moment. We sus-
pect that the PVP macromolecules might coil around the
nanowires as they grow along their longitudinal directions.
The use of a coordination reagent (polymeric or mono-
meric) to control the morphological evolution of metal nano-
structures in the solution phase has been extensively explored
in previous studies.[21]
It is generally accepted that the coordi-
nation reagent kinetically controls the growth rates of various
faces of a metal through selective adsorption and desorption
on these surfaces. There seems to be a selectivity between the
metal surfaces and the functional groups of the coordination
reagent. We have tried several other commonly used coordi-
nation polymers for the synthesis of silver nanowires and only
PVP worked. Figure 4 shows the SEM and TEM images of
two as-synthesized silver nanostructures when PVP was re-
placed with an equal amount of poly(ethylene oxide) (PEO,
Mw»100 000) and poly(vinyl alcohol) (PVA, Mw»50 000±
85 000). These images indicate that only PEO had the same
function as PVP to promote the formation of silver nanostruc-
tures with wire-like morphologies (Fig. 4A), albeit the yield
and uniformity of silver nanowires obtained with PEO as the
coordination reagent were much worse as compared to the
system based on PVP. These results also imply the importance
of the O±Ag coordination bond in forming silver nanostruc-
tures with wire-like morphologies.[24]
The oxygen atoms in
PVA can easily form intramolecular hydrogen bonds due to
the strong interactions between hydroxyl groups. These hy-
drogen bonds may cause the PVA molecules to fold into coils
and make it difficult for the O atoms to interact with silver
surfaces. A systematic study on the interactions between var-
ious coordination reagents and silver surfaces is under way in
our group. We believe that the synthetic approach described
in this paper can also be extended to many other metals by se-
lecting appropriate coordination reagents.
In summary, silver nanowires with uniform diameters, high
aspect ratios, and bicrystallinity have been synthesized at high
yields via a self-seeding process that used PVP as the coordi-
nation reagent. It was found that both the molecular weight of
PVP and the molar ratio of its repeating unit relative to
AgNO3 played important roles in determining the morphol-
ogy and dimensions of these silver nanostructures. Uniform
nanowires with aspect ratios as high as ~1000 could be rou-
tinely synthesized through this approach by using PVP with
relatively high molecular weights (~55 000) and a molar ratio
of ~1.5:1. We believe that it should be possible to synthesize
silver nanowires with smaller diameters and higher aspect
ratios than those described in this paper by controlling other
experimental parameters (such as the refluxing temperature
and the injection rates for the solutions).[18]
These uniform
silver nanowires should find uses in a variety of areas such as
catalysis, electronics, photonics, and electrochemistry.
Experimental
In a typical synthesis, 3 mL ethylene glycol solution (0.1 M) of silver nitrate
(Aldrich) and 3 mL ethylene glycol solution (0.6 M) of poly(vinyl pyrrolidone)
(PVP, Aldrich) or other coordination reagents were injected into 5 mL ethylene
glycol (anhydrous, 99.8%, Aldrich) refluxed at 160 C using a two-channel sy-
ringe pump (KDS-200, Stoelting Co., Wood Dale, IL) at a rate of ~0.3 mL min±1
.
After injection, the reaction mixture was further refluxed at 160 C for 60 min.
Magnetic stirring was continuously applied throughout the entire process of re-
duction and wire growth. The silver nanowires could be separated from particles
through centrifugation. In this case, the reaction mixture was diluted with ace-
tone (5±10 times by volume) and centrifuged at 2000 rpm for ~20 min. The
supernatant containing silver particles could be easily removed using a pipette.
This centrifugation procedure was repeated several times until the supernatant
became colorless. The sample (~30 mg in each synthesis) shown in Figure 1 rep-
resents a typical example of silver nanowires purified through this centrifugation
process. All other samples (Figs. 2±4) did not go through centrifugation.
Transmission electron microscopy (TEM) and electron diffraction studies
were performed with a JEOL-1200XII microscope operated at 80 kV. Samples
for TEM were diluted 50 times with 18 MX cm±1
water and then dropped on
copper grids and allowed to dry in a fume hood. SEM measurements were car-
ried out with a field-emission microscope (JEOL, 6300F) operated at an accel-
eration voltage of 15 kV. SEM samples were diluted 20 times with 18 MX cm±1
water and then deposited on silicon substrates. The X-ray diffraction (XRD)
pattern was recorded on a powder sample using a Philips PW1710 diffractome-
ter with Cu Ka radiation (k=1.54056 Š) and a graphite monochromator at a
scanning rate of 0.02 degrees per second in 2h ranging from 30 to 80.
Received: January 14, 2002
Final version: March 8, 2002
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COMMUNICATIONS
Fig. 4. A) SEM and B) TEM images of the as-synthesized, silver products when
PVP was replaced with other ªcoordinationº reagents: A) poly(ethylene oxide)
(PEO, Mw»100 000) and B) poly(vinyl alcohol) (PVA, Mw»50 000±85 000).

5. ±
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Preparation of Smooth Single-Crystal Mn3O4
Nanowires**
By Wenzhong Wang,* Congkang Xu, Guanghou Wang,*
Yingkai Liu, and Changlin Zheng
Considerable research has recently focused on manganese
oxides due to their ion-exchange, molecular adsorption, cata-
lytic, electrochemical, and magnetic properties.[1±3]
These ma-
terials have also attracted interest recently as an electrochro-
mic material for anodic coloration,[4]
since they change color
reversibly from brown (colored) to yellow (bleached). Mn3O4
is known to be an active catalyst for several processes, such as
the oxidation of methane and carbon monoxide[5]
or the selec-
tive reduction of nitrobenzene.[6]
Moreover, in recent studies,
different polymorphs of Mn3O4 (hausmannite)[7]
have been
found to be active and stable catalysts for the combustion of
organic compounds at temperatures of 373±773 K. These
combustion-related catalytic technologies are of interest in
relation to several air-pollution problems, to limit the emis-
sion of NOx and volatile organic compounds from waste gases
of different origins.[8]
One-dimensional (1D) nanoscale materials have stimulated
great interest recently because of their unique electronic,
optical, and mechanical properties and their potential applica-
tions in nanodevices.[9±11]
Many methods have been devised to
synthesize 1D nanosized materials, such as arc discharge,[12]
laser ablation,[13,14]
templating,[15,16]
from solution,[17±19]
and
others.[20±22]
We have developed an interesting new approach
that compliments other approaches for making nanowires.
Herein, we report on a novel and simple approach to the
synthesis of single-crystal Mn3O4 nanowires that exploits a
one-step, solid-state reaction to prepare nanometer-diameter
precursors at ambient temperature that subsequently deposit
Mn3O4 nanowires in NaCl flux. Our approach requires
neither complex apparatus and sophisticated techniques nor
metal catalysts and/or templates.
The X-ray diffraction (XRD) pattern of the as-synthesized
Mn3O4 nanowires is shown in Figure 1. All diffraction peaks
can be perfectly indexed to the tetragonal Mn3O4 structure.
The Mn3O4 lattice constants obtained by refinement of the
XRD data of the nanowires are a = 5.735 and c = 9.416 Š,
which are consistent with those of bulk Mn3O4 (International
Centre for Diffraction Data (ICDD), PDF File No.24-734).
The morphology of the nanowires was observed by trans-
mission electron microscopy (TEM). TEM images of the
as-prepared nanowires are shown in Figure 2. It can be seen
Adv. Mater. 2002, 14, No. 11, June 5 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 0935-9648/02/1106-0837 $ 17.50+.50/0 837
COMMUNICATIONS
______________________
±
[*] Dr. W. Z. Wang,[+]
Prof. G. H. Wang,[+]
C. K. Xu, Y. K. Liu, C. L. Zheng
National Laboratory of Solid State Microstructures
and Department of Physics, Nanjing University
Nanjing 210093 (China)
E-mail: wangqun@nju.edu.cn
[+] Second address: Structure Research Laboratory, University of Science
and Technology of China, Hefei, Anhui 230026, China.
[**] This work was supported by the National Natural Science Foundation of
the People's Republic of China (No. 29890210, 10023001, and 10074024).