Novel silver nanostructures from silver mirror reaction on reactive substrates

Article abstract of DOI:10.1021/jp0515838

Novel silver clusters have been prepared by simply carrying out the silver mirror reaction on certain reactive substrates. Leaflike fractal silver microstructures and perpendicularly aligned silver nanosheets were produced on a commercially available copper foil and sandpaper-rubbed copper foil, respectively. The surface features of copper foils and the chemical state of Cu atoms play important roles in regulating the morphological structures of the resulting silver clusters. Silver nanoclusters with various morphologies ranging from the leaflike to flowerlike hierarchical structures can be produced from the silver mirror reaction on commercially available copper foils after being treated with a dilute aqueous HCl solution under different conditions. The aqueous solution of silver nanosheets shows an optical absorption spectrum with a broad light-scattering peak at about 350 nm, compared to a corresponding surface plasmon absorption band around 430 nm for silver nanoparticles from the conventional silver mirror reaction on glass. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0515838

J. Phys. Chem. B 2005, 109, 13985-13990
13985
Novel Silver Nanostructures from Silver Mirror Reaction on Reactive Substrates
Liangti Qu and Liming Dai*
Department of Chemical and Materials Engineering, School of Engineering, The UniVersity of Dayton,
300 College Park, Dayton, Ohio 45469-0240
ReceiVed: March 29, 2005; In Final Form: June 2, 2005
Novel silver clusters have been prepared by simply carrying out the silver mirror reaction on certain reactive
substrates. Leaflike fractal silver microstructures and perpendicularly aligned silver nanosheets were produced
on a commercially available copper foil and sandpaper-rubbed copper foil, respectively. The surface features
of copper foils and the chemical state of Cu atoms play important roles in regulating the morphological
structures of the resulting silver clusters. Silver nanoclusters with various morphologies ranging from the
leaflike to flowerlike hierarchical structures can be produced from the silver mirror reaction on commercially
available copper foils after being treated with a dilute aqueous HCl solution under different conditions. The
aqueous solution of silver nanosheets shows an optical absorption spectrum with a broad light-scattering
peak at about 350 nm, compared to a corresponding surface plasmon absorption band around 430 nm for
silver nanoparticles from the conventional silver mirror reaction on glass.
Introduction
Experimental Section
Materials. AgNO3, Ag(NH3)2+, glucose, and tartaric acid
were purchased from Sigma-Aldrich and used as received.
Copper foils were purchased from EMS US, while sandpapers
(both 180-C and 600-B) were obtained from GatorGrit, Finland.
The CatorGrit 180-C sandpaper is rougher than its 600-B
counterpart. HCl used was analytical grade from Fluka. The
silver mirror solution3 contains a freshly prepared silver complex
The recent development of nanotechnology has opened up
novel fundamental and applied frontiers in materials science
and engineering. At the nanometer scale, the wavelike properties
of electrons inside matter and atomic interactions are influenced
by the size and shape (surface) of the material.1 As a
consequence, changes in physicochemical properties (for ex-
ample, melting points, magnetic, optic, and electronic properties)
may be observed through a reduction in size, even without any
compositional change.1 New phenomena, such as the confine-
ment-induced quantization effect, could also occur when the
size of materials becomes comparable to the deBroglie wave-
length of charge carriers inside.1 By creating nanostructures,
therefore, it is possible to control the fundamental properties
of materials through the size/shape effects. This should, in
principle, allow us to develop new materials and advanced
devices of desirable properties and functions for numerous
applications.
Historically, the silver mirror reaction has been and is still
being used as an efficient method for preparing thin film
coatings of silver nanoparticles.2 Since glass plates have been
being used extensively as the substrate for the silver mirror
reaction, almost no attention has been paid to possible influence
of the substrate surface on the silver nanostructures thus formed.
To our surprise, we have recently observed novel silver
nanostructures produced from the classical silver mirror reaction
by simply changing the glass plate substrate to certain reactive
metal foils. We found that the substrate surface plays an
important role in regulating the silver nanoclusters thus formed
and their hierarchical structures. In the present paper, we
demonstrated this simple, but very effective, approach to various
novel hierarchical structures of silver nanoclusters by carrying
out the silver mirror reaction on copper foils of controllable
surface features. A possible formation mechanism for the newly
observed silver nanoclusters and their hierarchical structures are
also discussed.
of Ag(NH3)2 (6.375 × 10-3 M), glucose (3.405 × 10-2 M),
+
and tartaric acid (4.005 × 10-3 M). In a typical experiment,
silver microstructures were produced by immersing a Cu foil
into the silver mirror solution for a certain time at room
temperature, followed by rinsing with a large amount of distilled
water.
Characterization. Scanning electron microscopic (SEM) and
energy-dispersive X-ray analysis (EDX, or EDS: energy
dispersive spectroscopy) were carried out by using a Philips
XL30 FEG scanning electron microscope equipped with an
energy-dispersive X-ray detector. UV-vis spectra were recorded
on a Lambda 900 UV/VIS/NIR Spectrometer (PerkinElmer).
X-ray diffraction (XRD) measurements were performed on a
Rigaku X-ray generator at 50 kV and 150 mA using a
monochromatic Cu KR X-ray beam.
Results and Discussion
To start with, we carried out the control experiment by
immersing a smooth glass plate (Figure 1a) into a freshly
+
prepared aqueous silver mirror solution3 containing Ag(NH3)2
(6.375 × 10-3 M), glucose (3.405 × 10-2 M), and tartaric acid
(4.005 × 10-3 M) for several hours. As expected, the classical
silver mirror reaction led to the coating of silver nanoparticles
on the surface of the glass plate with regular size and shape
(Figure 1b,c).
When the above silver mirror reaction was performed on a
commercially available copper foil (Figure 2a) under the same
conditions, however, large scale leaf-like microstructures of a
fractal geometry were formed (Figure 2b,c). SEM images taken
* Corresponding author. E-mail: ldai@udayton.edu.
10.1021/jp0515838 CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/30/2005

13986 J. Phys. Chem. B, Vol. 109, No. 29, 2005
Qu and Dai
Figure 1. SEM images of (a) smooth glass plate, (b) silver nanostructures deposited on the glass surface, and (c) as for (b) under a higher magnification.
Figure 2. SEM images of (a) surface of the Cu foil, (b,c) leaflike silver microstructures formed on the Cu surface, (d,e) as for (c) taken from
different areas under a higher magnification, and (f) EDX spectrum of the silver nanoclusters.
under a higher magnification (e.g., Figure 2d,e) clearly show
that these “leaves” are composed of a large number of polygonal
nanosheets with a thickness of about 200 nm self-assembled
into a 3D hierarchical structure. The EDX analyses on the cluster
“building blocks” (Figure 2f) clearly show a major peak of silver
with a much weaker peak for Cu, arising from the copper
substrate.
Figure 3a shows a typical XRD pattern for the as-prepared
Ag nanoclusters. Consistent with previously reported XRD
patterns for highly crystalline silver nanoclustes,3,4 three dif-
fraction peaks characteristic of a face-centered cubic (fcc)
crystalline structure are clearly evident in Figure 3a. These
diffraction peaks can be attributed to the (111), (200), and (220)
planes (JCPDS, File No. 04-0783). The corresponding electron

Novel Ag Nanostructures from Ag Mirror Reaction
J. Phys. Chem. B, Vol. 109, No. 29, 2005 13987
Figure 3. Typical (a) XRD and (b) ED patterns for the as-prepared Ag nanoclusters.
Figure 4. EDX spectra of the (a) commercially available and (b) sandpaper-rubbed copper foils.
diffraction (ED) pattern of the silver nanoparticles (Figure 3b)
also shows fringes related to the (111), (200), and (220) planes
of pure fcc silver.4
randomly interconnected silver nanosheets within the spherical
microcluster stand out from the surface like a silver “flower”.
These nanosheets are very thin with a thickness less than 100
nm. After rubbing the copper foil more extensively with a finer
sandpaper (GatorGrit 600-B, Figure 5b), the same silver mirror
reaction produced a more tightly packed nanostructured silver
coating (Figure 5d,f,h) with an increased number of the silver
“flowers” (Figure 5d) made from perpendicularly aligned silver
sheets of about 200-nm thick (Figure 5f).
In view of the above observation and a recent report on the
formation of silver nano-inukshuks on germanium via a galvanic
displacement reaction,5a we anticipate that the following reaction
occurs with copper atoms actively participating in the silver
mirror reaction as the reductive reagent:
The commercially available copper foils are often partially
covered by copper oxides arising from air oxidation, as indicated
by those scattered dark dots in Figure 2a and verified by the
EDX analysis (Figure 4a). To perform the silver mirror reaction
on pure copper surfaces, we used sandpapers to remove the
oxide coating. The effectiveness of removal of the copper oxides
by rubbing with sandpapers is confirmed by the absence of the
oxygen peak in Figure 4b.
Having effectively removed the copper oxide layer, we used
the freshly prepared pure copper surface as the substrate for
the silver mirror reaction. In contrast to Figure 2, Figure 5 shows
completely different silver nanostructures formed on the sand-
paper-rubbed copper foil from the same silver mirror solution
under the same reaction conditions. On the copper foil rubbed
by a rough sandpaper (GatorGrit 180-C, Figure 5a), densely
packed silver nanostructures with somewhat scattered spherical
microclusters formed (Figure 5c). Close inspections of the
nanostructured surface and spherical microcluster under higher
magnifications show similar “building blocks” of perpendicu-
larly aligned silver nanosheets in both areas (Figure 5e,g). The
Cu(s) + 2Ag+(aq) f Cu2+(aq) + 2Ag(s)
(1)
The above process is not inconsistent with our SEM images
shown in Figures 2 and 5. For the commercially available copper
foil partially coated with spotlike oxide defects (Figure 2a), the
above reaction could take place at the sites where pure Cu atoms
are in direct contact with the silver mirror solution. Continuous
reduction of Ag+ ions by Cu atoms requires electron-transfers

13988 J. Phys. Chem. B, Vol. 109, No. 29, 2005
Qu and Dai
Figure 5. SEM images of Cu foils rubbed with the (a) 180-C and (b) 600-B sandpapers, and silver nanostructures from the silver mirror reaction
on the corresponding Cu foils (c,e,g and d,f,h for the 180-C and 600-B sandpaper-rubbed Cu substrates, respectively) under different magnifications.
from the underlying Cu atoms, apart from the concomitant
reduction of Ag+ ions by the classical silver mirror reducing
reagent (i.e., glucose and tartaric acid) in the solution. As the
presence of the electron-rich copper oxide defects could set up
a fractal potential gradient over the surface,5,6 the fractal
“leaflike” silver nanostructures parallel to the surface were
formed in this particular case (Figure 2b-e). Once the insulating
copper oxide layer was removed by rubbing with sandpapers,
the underlying Cu atoms over the whole substrate surface
actively participated in Reaction 1 to produce the densely packed
silver nanostructures (Figure 5). As seen in Figure 5, the rubbing
process created line-shaped structures of Cu atoms with a high
packing density on the sandpaper-rubbed copper foil (Figure
5a,b), which allowed for a preferential growth of the silver
nanosheets along the direction perpendicular to the substrate
surface (Figure 5e-h). More silver species have been deposited
along the protruding surface lines over the sandpaper-rubbed
copper foils (Figure 5a,b), most probably due to the surface

Novel Ag Nanostructures from Ag Mirror Reaction
J. Phys. Chem. B, Vol. 109, No. 29, 2005 13989
Figure 6. SEM images of Cu foils treated with an aqueous solution of HCl (0.5 M) for (a) 5 min and (b) 60 min, and silver nanostructures from
the silver mirror reaction on the corresponding Cu foils (c) for (a) and (d) for (b).
Figure 7. SEM images of silver nanostructures formed by reacting (a) a commercially available Cu foil with an aqueous solution of AgNO3 (6.375
+
× 10-3 M), and (b) a sandpaper (180-c)-rubbed Cu foil with an aqueous solution of Ag(NH3)2 (6.375 × 10-3 M).
potential gradient. Similarly, the point-type protruding surface
sites in Figure 5a,b have led to a faster growth of the silver
nanoclusters in a form of the standing-out silver “flowers”
(Figure 5c-f). Comparing Figure 5b with Figure 5a, the copper
foil rubbed with a finer sandpaper produced more point-type
protruding surface sites (Figure 5b) for the formation of more
flowerlike silver microclusters in Figure 5d than in Figure 5c.
An important check for Reaction 1 is provided by the silver
mirror reaction to be carried out by immersing copper foils in
an aqueous solution of AgNO3 (6.375 × 10-3 M) or Ag(NH3)2
+
(6.375 × 10-3 M) in the absence of any other reductive reagent.
Silver nanostructures similar to those observed earlier on the
commercially available (Figure 2b,c) and sandpaper-rubbed
(Figure 5c,e) copper foils are seen in Figure 7 (parts a and b,
respectively). Clearly, therefore, Cu atoms indeed participated
in Reaction 1 as an efficient reductive reagent. Furthermore,
we have also found that other metals of a low work function
(e.g., Al foil) also underwent a reduction process similar to that
shown in Reaction 1, indicating that the methodology developed
in this study for producing novel silver nanostructures could
be regarded as a versatile approach.
As can be seen from Figures 2a and 5a,b, the rubbing with
sandpapers caused severe surface alterations of the copper foils.
A more controllable and less harsh chemical treatment has been
investigated for removal of the surface copper oxide layers by
immersing the commercially available copper foils in a dilute
aqueous HCl solution (0.5 M) for a predetermined period of
time. Depending on the acid concentration and acid-treatment
time, surfaces of different roughness with (Figure 6a) and
without (Figure 6b) the oxide defect sites can be prepared to
support the growth of silver nanoclusters with a large variety
of morphologies, ranging from the leaflike (Figure 6c) to
flowerlike (Figure 6d) hierarchical structures.
It is appropriate here to mention that the silver nanosheets
produced in the present study possess optoelectronic properties
different from those of silver nanoparticles prepared by the more
conventional silver mirror reaction. While an aqueous solution
of nanoparticles from the conventional silver mirror reaction

13990 J. Phys. Chem. B, Vol. 109, No. 29, 2005
Qu and Dai
reagent. The surface morphology and chemical state of Cu foils
play important roles in regulating the structure features, and
hence properties, of the resulting silver clusters. While an
aqueous solution of the silver nanoparticles formed on a glass
plate from the conventional silver mirror reaction shows a broad
surface plasmon absorption peak around 430 nm, the corre-
sponding solution of the silver nanosheets prepared from the
same silver mirror reaction on copper foils gives a light-
scattering band over 350 nm. This work represents a general
approach to the preparation of a large variety of new silver
nanomaterials of potential applications in many areas, as other
metals of a low work function (e.g., Al foil) have also been
demonstrated to undergo similar galvanic displacement reac-
tions.
Note Added in Proof. After the submission of our manu-
script, we note that He et al. recently reported the preparation
of silver nanosheets on an aluminum foil (He, Y.; Wu, X. F.;
Lu, G. W.; Shi, G. Q. Nanotechnology 2005, 16, 791).
Figure 8. UV-vis spectra of (a) silver nanoparticles and (b) nanosheets
dispersed in water by sonication.
Acknowledgment. We are grateful for financial support from
the National Science Foundation (NSF-CCF-0403130), Ameri-
can Chemical Society (ACS-PRF 39060-AC5M), NEDO In-
ternational Joint Research Grant (04IT4), the Materials and
Manufacturing Directorate of the Air Force Research Labora-
tory, The Dayton Development Coaltition, and Wright Brothers
Institute. We also thank Kyung Min Lee and Amanda Schrand
for their help with the X-ray and electron diffraction measure-
ments, respectively. The use of facilities in the NEST Center
at UD is greatly appreciated.
(Figure 1b,c) shows a typical UV-vis spectrum with a strong
surface plasmon absorption band around 430 nm (curve a, Figure
8) characteristic of silver nanoparticles,4,7,8 the corresponding
spectrum for the silver nanosheets prepared in the present study
(curve b, Figure 8) reveals a light-scattering peak over 350 nm
due to the relatively large particle size.9 These observations
clearly indicate that properties (e.g., optical absorption) of silver
clusters are strongly dependent on their size and symmetry.
Therefore, there is considerable room for tailoring the structure-
property relationship of the silver clusters resulting from the
silver mirror reaction by controlling the substrate nature and
its surface features.
References and Notes
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Conclusions
In summary, we have demonstrated that novel silver clusters
could be reproducibly prepared by simply carrying out the silver
mirror reaction on certain reactive substrates with specific
surface features. While leaflike fractal silver microstructures
were produced from the conventional silver mirror reaction on
a commercially available copper foil with spotlike oxide defects,
perpendicularly aligned silver nanosheets were prepared under
the same reaction conditions on the copper foils after having
been rubbed with sandpapers. Silver nanoclusters with various
morphologies ranging from the leaflike to flowerlike hierarchical
structures have been produced from the silver mirror reaction
by removing the copper oxide layer from commercially available
copper foils through relatively benign acid (e.g., dilute HCl)
treatments under different conditions. Control experiments on
(3) Zhang, S. H.; Xie, Z. X.; Jiang, Z. Y.; Xu, X.; Xiang, J.; Huang,
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+
reactions of AgNO3 or Ag(NH3)2 with copper foils in the
absence of any other reductive reagent indicated that Cu atoms
actively participated in the silver mirror reaction as a reductive