Fabrication of a Flowerlike Ag Microsphere Film with Applications in Catalysis and as a SERS Substrate

Article abstract of DOI:10.1002/ejic.201800119

Three-dimensional flowerlike Ag microstructures were assembled in situ on a substrate to fabricate Ag microflower (Ag MF) films. The morphology and size of the Ag MFs can be easily controlled by varying the reaction time. The mechanism of growth of Ag MFs was investigated by SEM. The shape evolution revealed that the Ag MFs are assembled from thin nanosheets composed of Ag nanoparticles (NPs), and the elemental composition of the NPs was evidenced by energy-dispersive X-ray spectroscopic analysis. The potential of the film as a catalyst in the reduction of nitrophenols was investigated. The reusability of the catalyst was estimated by recycling catalysis of 4-nitrophenol. Moreover, the Ag MF film could be used as a versatile substrate for surface-enhanced Raman scattering (SERS) for detection of Rhodamine 6G with a detection limit as low as 1 × 10^–7 m. The results demonstrated that the Ag MF film exhibits excellent SERS enhancement ability and catalytic activity.

Full text of DOI:10.1002/ejic.201800119

A Journal of
Accepted Article
Title: Fabrication of flower like Ag microspheres film with its application
in catalysis and as SERS substrates
Authors: Qijun Dai, Lu Li, Chungang Wang, Changli Lv, Zhongmin Su,
and Fang Chai
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800119
Link to VoR: http://dx.doi.org/10.1002/ejic.201800119

European Journal of Inorganic Chemistry
10.1002/ejic.201800119
FULL PAPER
Fabrication of flower like Ag microspheres film with its
application in catalysis and as SERS substrates
Qijun Dai,[a,b], Lu Li, Chungang Wang, [a] Changli Lv, * [a] Zhongmin Su* [a] and Fang Chai* [a,b]
[a]
Abstract: Three-dimensional flower like Ag microstructures were
assembled in situ on the substrate to fabricate Ag micro-flower (Ag
MFs) film. The morphology and size of the Ag structures of these Ag
MFs can be easily controlled by varying reaction time. And the
mechanism of growth of Ag MFs was also investigated by SEM. The
shape evolution revealed the Ag MFs was assembled with thin
nanosheets which composed of Ag nanoparticles, and the elemental
composition of the nanoparticles was evidenced by X-ray
spectroscopy analysis. Its potential which used as a catalyst in the
reaction of reduction of nitrophenols was investigated. The
reusability of the catalyst of the Ag MFs film was also estimated by
recycling catalysis of 4-nitrophenol. Moreover, the Ag MFs film could
be used as a versatile Surface Enhanced Raman Scattering (SERS)
substrate for detection of the R6G molecules with a detection limit as
Chang and co-workers fabricated a thin Ag film into nanocrystals
(NCs) with various shapes including nanoflowers by cyclic
scanning electrodeposition method.11 And You’s group prepared
a rose-like Ag mesoparticles on screen-printed carbon electrode
using cyclic voltammetric methods, which was use as surface-
enhanced Raman scattering substrate.12 Pillai and co-workers
presented a facile synthetic approach that involves the use of
sustainable nanocellulose for the synthesis of flower-like Ag
nanostructure with optical properties.13 Among them, flower like
structure of Ag particle which was usually assembled by nano-
or submicro-plates was one of most remarkable materials in all
reported Ag structures due to its highly branched structures,
large specific surface area and rough surface. There are a large
number of reports on the synthesis of flower like Ag nan- micro-
structure with their excellent applications in catalysis, biological
labeling, chemical sensing, electrocatalysis, SERS detection and
so on.3, 14-17
−
7
low as 1× 10 M. The results demonstrate that the Ag MFs film
exhibit excellent SERS enhancement ability and catalytic activity.
However, mostly used various surfactants during the controllable
preparation inevitably blight the activity of Ag NPs used as
catalysts, substrates and assembling units. Therefore, the
shape-, size-controllable synthesis on the large scale is still a
significant challenge, especially under the mild condition of
without any additive surfactants. Furthermore, the sustainable
reuse of the noble metal materials is strongly advocated due to
the cost of noble metals is one of the key limitations for their
practical application.18 The recovery of expensive catalysts after
catalytic reactions and reusing them without the loss of activity
are essential features for sustainable process development.19 To
Introduction
Large-scale production of metallic nanocrystals have attracted
great attention due to their fascinating size- and shape-
dependent properties for various applications, such as catalysis,
sensing, plasmonics and photothermal therapy.1 In particular,
the significant efforts have promoted the development of the
shape controlled synthesis of silver nanoparticles in past
decades.2-7 The synthesis of the Ag particles with different
shapes and sizes are currently widely investigated due to shape
control shows a greater versatility for tuning the properties of the
Ag nanostructures. So, tremendous efforts have been made to
improve the efficiency and save resources, the possible solution
is of urgent need to exploit. Nevertheless, there are only two
more effective way including the use of magnetically retrievable
Ag nanoparticles20 and fabricating the large scale Ag surface on
search for
a versatile method to precisely control the
morphology of Ag NPs over the past decade, due to the
application such as catalyst can be strongly affected by particle
size and shape.2,5,7,8 Recently, a class of sophisticated attractive
structures of Ag has been synthesized by electrochemical
method, chemical deposition method, galvanic replacement and
substrate.
The fabrication of nanostructured metal surfaces (such as Au
21
and Ag)
can be treated as a efficacious process in efficient
green chemical synthesis, which could provide outstanding
applications in recyclable heterogeneous catalysis, biological
labeling, SERS and electrocatalysis.11,22,23 Though, fabricating
the silver microcrystals in large-scale is of scientific significance,
greatly underdeveloped as the lack of rational design strategies,
which meaning that the applications of these desirable
superstructures remain limited because of the complexity of their
synthesis or low yield during fabrication.1
8
so on. For example, Nie and co-workers reported flower-like
silver hierarchical microcrystals through reducing silver nitrate by
L-ascorbic acid in nitric acid-mediated systems with high
9
catalytic activity. Xu’ s group designed a kind of porous Ag
platelet structures and exhibited excellent catalytic activity.10
,11,12
Stimulated by this, we have synthesized flower like Ag
microstructures and assembled in situ on the substrate to
fabricate Ag micro-flower (Ag MFs) film by a low cost and facile
synthetic route, a two-step process seed-assisted auto-catalytic
reaction. To investigate the catalytic activity of the Ag MFs, the
hydride reduction from 4-nitrophenol (4-NP) to 4-aminophenol
[
[
a]
b]
Qijun Dai, Lu Li, Chungang Wang, Changli Lv, Zhongmin Su and
Fang Chai
Institute of Functional Material Chemistry, Faculty of Chemistry
Northeast Normal University
Changchun, 130024, P.R. China
E-mail: lucl055@nenu.edu.cn, zmsu@nenu.edu.cn
Qijun Dai, Fang Chai
(
4
4-AP) by NaBH is used as a model reaction, due to the
Faculty of Chemistry
process of reaction can be monitored by ultraviolet–visible
absorption spectroscopy. And more importantly, it is of great
significant to produce the 4-AP from reduction of the 4-NP,
owing to 4-NP is the most ordinary organic pollutant and
Harbin Normal University
Harbin, 150025, P.R. China
E-mail: fangchai@gmail.com
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201800119
FULL PAPER
carcinogenic to humankind and wildlife, while its amino
derivative, 4-AP is an important intermediate for the manufacture
of analgesic and antipyretic drugs.18,19, 24 In this work, the Ag
MFs film exhibit excellent catalytic activity and reusability for the
reduction of nitrophenols in aqueous solution. Compared with
reported method, this method can be regarded as a feasible and
sustainable method for reuse the noble metal.
Otherwise, as we know, the Surface-enhanced Raman
scattering (SERS) by using the Ag clusters or aggregated has
been demonstrated as a powerful tool for ultrasensitive chemical
detection.12,13,25-27 As mentioned previously, flower-like Ag
self-assembling was still in progress. When the reaction was
finished at 8 min, the Ag flower structure of film was pronounced
in large domains, and longer reaction time seems in favor of the
growth of Ag MFs structure. The Fig. 1d presents that the large
scale film was assembled by independent Ag flowers with the
diameter of 300-450 nm. The highly branched Ag microflower
formed which enlarged twice than the reaction at 3 min. The
sparse petals can be observed folded by several layer of
intercrossed nanosheets (Fig. 1e). The thinness of Ag
nanosheet was still remained at about 20 nm, but the growth
proceeded along the dendritic direction (Fig. 1f, Fig. S2). The Ag
NPs which arranged in a dendritic Ag thin nanosheet, were
clearly visible with the diameter of 30-50 nm. Accompany the
increase of reaction time, the Ag nanosheets grew about twice
bigger gradually at the cost of little particles.27 The EDS
measurements further confirmed the Ag MFs film respectively
(Fig. S3a and S3b), and proved the existence of only Ag atoms
without Au atoms. This result indicated that the Au nanoseeds
were used as precursor to fabricate a nanosheets-assembled Ag
flower-like structure by seed-assisted auto-catalytic reactions.29
particles have
a unique morphology and rough surface,
rendering them suitable active substrates for SERS. And a class
of sophisticated flower-like Ag structures with different sizes are
currently widely investigated for developing a highly efficient
1
5-17
SERS substrate.
So, in this work, the Ag MFs film is also
attempt to using as SERS substrate for detection of R6G.26
Results and Discussion
The fabrication of the Ag MFs film was a two steps process, the
+
seeds fastened on the substrate and grew in the Ag solution. By
immersing in the APTMS solution, the glass and silicon slices
were pretreated by the -NH
soaked in 15 nm Au NPs (Fig. S1) to stick the Au NPs as seeds
by -NH decorated on the substrates. These seeds are
2
. In succession, the substrate was
2
necessary to obtain well-ordered flower-like Ag sturctures and
well-dispersed on the substrate. The gold nanoseeds modified
substrate was then immersed in the Ag+ solution with the
hydroquinone at low pH. The Ag nanoflowers were emerged
sited on the seeds quickly by the seed-assisted auto-catalytic
reaction, which can be tuned to create a huge variety of
nanoparticle morphologies. 28 The large field Ag NFs assembled
to well-distributed Ag film.
Figure 1. SEM images of Ag MFs film with different reaction time (a, b and c:
3
min, d, e and f: 8 min).
The FE-SEM images revealed that the morphology of the Ag
MFs film were assembled uniformly by a large number of
spherical flower like structure with a rough surface by this simple
method. The Ag MFs film formed by 3 min (Fig. 1a) and 8 min
The TEM was used to evaluate the morphology of the Ag micro
flower which was peeled off from the substrate. The single Ag
MF was a spherical shape (Figure 2a) with diameter of around
(
Fig. 1d) were presented in a larger field of view. As can be seen
1
.2 μm, and the flimsy nanosheet at the edge (Figure 2b) can be
from the Fig. 1a-c, the large scale equidistribution of silver flower
was produced within 3 min. The spherical flower-like Ag
nanostructures have been formed with the diameter of 150-200
nm. And most of them were assembled by large scale fan-shape
thin Ag nanosheet with thickness of 20 nm. As can be observed
observed clearly, indicating that the flower consisted of a lot of
nanosheets, which were consistent with those observed under
SEM investigation. And through extensive investigation by
selected area electron diffraction (SAED), we found that the
subunits of Ag MFs single layer Ag nanosheets exhibited single
crystalline nature. The high-resolution TEM (HRTEM) image of
the nanosheet at the edge of flower was shown in Fig. 2c, which
depicted the continuous lattice fringes in the visible range,
indicating its single crystal-line nature. And clear lattice structure
of the Ag nanosheet can be seen, the lattice spacing was
measured to be about 0.23 nm (inset of Fig. 2c), which matched
the interplanar distance of the (111) planes of Ag face-centered
cubic (fcc) structure. The chemical compositions of Ag MF were
determined by energy dispersive X-ray (EDS) (Fig. 2d), which
proved the presence of the Ag element, and other peaks
originated from the copper and carbon from the TEM grid.
(
Fig. 1b), the semicircle petals were assembled by single
flabellate lamellar nanosheets by across each other with the
diameter of 100-150 nm. It seems that the Ag nanosheets were
at the initial stage of assembling, but the flowerlike structures
have already been recognizable. More accurately, as can be
detected from the sparse petals in Fig. 1c, the assembled
lamellar nanosheets were composed of the dendritic silver
nanoparticles. These results indicated that the Ag flower
structures were formed within 3 min preliminarily. The Ag
nanosheets composed of flower structure were expanded by Ag
nanoparticles extending dendritic growth. Though most of Ag
nanosheets accumulated together, the process of growth and
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European Journal of Inorganic Chemistry
10.1002/ejic.201800119
FULL PAPER
be pointed out that the amount of Au NPs loading on the
substrate can affected the growth of the Ag MFs film. The Ag
MFs films with the substrate soaking different time in Au NPs
were compared in Fig. S4. The less Au NPs loading on the
substrated resulted in the inadequacy growth of the Ag MFs,
while the redundant soakin led to the big block on the film. Thus
the optimized soaking time was determined at 10-15 min.
Figure 2. TEM of the Ag microflower a. the edge of the flower, b. the nano
sheet of the edge of Ag flower (inset: SEAD pattern of Ag nanosheet), c.
HRTEM images of single layer of Ag nano sheet with lattice parameters of
0.23 nm (inset), d. EDS of the Ag MFs.
In order to understand the morphological evolution of the Ag
MFs film, the growth was monitored by different reaction time,
and the structures grown at different stages were examined by
SEM. When the reaction started, the spherical Ag nanoparticles
were produced through seed-mediated growth immediately and
spread on the substrate uniformly in a large scale (Fig. 3a). The
Ag NPs were formed by reducing on the embedded Au nano
beads with smooth surface and a diameter of 100-120 nm (inset
of Fig. 3a). The precipitation speed of Ag NPs was appropriate,
at which nanosheets can be obtained and then assembled into
Figure 3. SEM image of shape evolution of the Ag MFs film with increasing
duration of reaction (a.1 min, b.3 min, c.5 min, d.8 min)
The catalytic activity of Ag MFs film was tested as
heterogeneous catalysis catalyst by a model reaction reduction
of nitrophenol to aminophenol, due to the catalytic
hydrogenation of nitrophenol by noble metal particles was
considered as a green process.9, 24, 30-33 The time resolutions of
the kinetic measurements were limited by the integration times
for UV-vis spectra collection. Firstly, the reduction of 4-
nitrothiophenol (4-NTP) was first tested by using Ag MFs film as
a catalyst. It is clear to be seen that the yellow fading and
eventual bleaching, during the catalytic reactions, the complete
hydrogenation of 4-NTP into 4-aminothiophenol (4-ATP) has
been occurred. Fig. 4 reveals the successive UV-visible (UV-vis)
absorption spectra for the 4-NTP can be transferred to 4-ATP
catalyzed by Ag MFs film within 10 min exhibiting the high
activities. As the catalytic reaction proceeds, the absorption
peak at 410 nm originating from 4-NTP gradually decreases,
indicating the continuous reduction of 4-ATP. To estimate the
efficiency of the catalyst, the predetermined calibration curve
has been confirmed in Fig. S5. And to give a quantitative
description about the catalytic performance, the apparent rate
3D architectures (Fig. 3b). Obviously, with the increase of
reaction time, the surface roughness of Ag NPs increased, since
the growth and aggregation of Ag nanosheets became faster,
which was much faster than Ostwald ripening Au seeds
sacrificed smaller crystallites, finally led to the rough surface of
silver particles by aggregation grown (Fig. 3b-d). 9, 29
With the addition of the reaction time, the single layer of Ag nano
sheets were assembled to aggregated sheets (Fig. 3c). Fig. 3d
shows the typical FESEM image of Ag MFs film, in which a
nanosheet-assembled flower-like structure with the size of 400-
600 nm can be seen. The resulted highly branched Ag MFs
structure evoluted accompanied reaction time, the process of
formation was highly reproducible. Based on these findings, it
can be deduced that the size and shape of Ag nanoparticles are
highly dependent on the reaction time at milder reducing
conditions. The growth of flower structure was clearly
observable as the seed-assisted auto-catalytic reaction was still
proceeded. The distinct flower-like framework possessing the
largest surface area produced after 3 min, and the absolute Ag
MFs film can be fabricated within 8 min, potentially making
subsequent seed-assisted auto-catalytic reaction more efficient
for construction of subsequent nanostructures. 30 And it needs to
constant of the reducing reaction was calculated. Since NaBH
is far excessive for the concentration of 4-NTP, reaction kinetics
is independent of NaBH concentration, it is reasonable to
regard the concentration of NaBH as a constant throughout the
reaction. Thus, the reduction kinetics can be conformed to be
pseudo-first-order kinetics.34 As shown in the inset of ln(C
/C )
0
4
4
4
t
exhibits a linear relationship with the reaction time. The apparent
rate constants was calculate to 0.2166 min-1 (R =0.9879). As is
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201800119
FULL PAPER
well known, the 4-NTP is more difficult to reduction than 4-NP
due to the existence of the structure of –SH, which indicated the
good catalytic activity of Ag MFs film. Owing to the high surface
areas of Ag MFs, which favouring to the interactions between
the reactants and the catalyst surface. And the stability of Ag
MFs film was also evaluated by SEM and EDS after the use of
catalyst, due to the property of metal nanoparticles exhibited
strongly dependent on their size, shape, morphology and
composition.11 As revealed by the SEM (Fig. S6), the Ag MFs
film still remained the flower structure after the splashing by the
solution in the reaction, which was ensured the high surface
areas and favor to improve the catalytic activity. And the EDS
manifested that the main element was still Ag element, indicated
the good stability of the Ag MFs film, which permitted the utility
as a catalyst in a next turn.
To evaluate SERS performance, the R6G was selected as the
model SERS probe, due to its large cross section and well-
characterized Raman bands. As revealed in Fig. 6a, all the
feature peaks of R6G at 1646, 1570, 1533, 1504, 1356, 1311,
1186, 769 and 606 cm−1 were observed and consistent with
other research studies, and peaks of R6G at 1186, 1311, 1356,
1504, and 1646 cm−1 were observed and assigned to C–H in-
plane bending, C–O–C stretching, and C–C stretching of the
aromatic ring.14 However, the Ag NPs (Fig. 3a, prepared by the
process of the fabrication of the Ag MFs) showed very weak
SERS ability (black curve), the SERS activity of petals of Ag
MFs was enhanced a lot (red curves), which indicated the Ag
petals improved the SERS signal of R6G. This clearly shows the
ability of the Ag MFs film to enhancing the detection sensitivity.
And Fig. 6b shows the SERS spectra of different concentrations
of R6G adsorbed onto Ag MFs film, the SERS spectra of the
R6G decreased with the reducing concentration. Many main
peaks still can be distinctly detected in the spectra as the R6G
concentration was down to 10-7 M, demonstrating the high
sensitivity of the Ag MFs film. Thus, the Ag MFs film is both an
excellent candidate for SERS substrate and catalysis.
The Ag MFs were fabricated on the substrate which provided a
feasibility of recycle. The Fig. 5 illustrates the recyclable usage
of the Ag MFs film in the reaction of reduction of 4-NP. The
reusability of Ag MFs film was investigated in detail by using it
as catalyst in recycle reactions of reduction of 4-NP. The original
4
3
-NP solution was light yellow with a maximum absorption at
17 nm, which could not be reduced by NaBH without catalyst.
, the formation of 4-
4
After adding the aqueous solution of NaBH
4
nitrophenolate anions caused the maximum absorption to shift to
about 400 nm (Fig. S7, S8). Meanwhile, the absorbance at 300
nm related to aminophenol slightly increased and led to a small
peak there. As demonstrated in the Fig.5, the 4 successive
reduction cycles were performed using a slice of Ag MFs film
fabricated on glass as the catalyst. As revealed in Fig. 5a and b,
1
00% reduction of 4-NP was achieved within 12 min in the first
two cycles with the kinetic rate constants of 6.54×10 s-1 and
-
3
-3
-1
4.47×10 s respectively. However, the latter two cycles, more
than 97% 4-NP were reduced within 14 and 18 min with the
kinetic rate constants of 3.38×10-3 s-1 and 2.61×10-3 s-1
respectively (Fig. 5c and d). These results suggest that the Ag
MFs film can be successfully recycled for at least 4 successive
cycles with a stable conversion efficiency of around 97 % (Fig.
Figure 5. UV-vis absorption spectra of the reduction of 4-NP by NaBH
4
th
5e and inset).
st
nd
rd
catalyzed by the Ag MFs film for recorded for the (a)1 , (b) 2 , (d) 3 , (d) 4 ,
Insets: The corresponding plots of ln(C(t)/C(0)) versus reaction time for the
(
reduction of 4-NP.)
Figure 4. The absorption spectra of catalytic reduction of 4-NTP to 4-ATP by
fresh solution of NaBH in the presence of Ag MFs film as catalyst. (Inset: The
plot of ln(C(t)/C(0)) versus reaction time for the reduction of 4-NTP.)
Figure 6. SERS spectra of R6G obtained by dropping 10 μL R6G ethanol
4
-
4
solution (a) on Ag NPs and Ag MFs with 10 M R6G, (b) with different
-4
-11
concentrations of R6G (10 -10 M) on Ag MFs substrate.
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European Journal of Inorganic Chemistry
10.1002/ejic.201800119
FULL PAPER
Conclusions
Raman measurements were recorded by
a Raman microscope
(Princeton Instruments (HORIBA HR800)) with an excitation laser
wavelength of 488 nm.
In summary, three-dimensional flower like Ag MFs films were
fabricated, the growth mechanism of flower like Ag
microstructures was also investigated by SEM, indicated the
morphology and size of the Ag micro-flowers can be easily
controlled by varying the reaction time. This technique provides
great flexibility in tuning the shapes of Ag micro-flower on the
film. Significantly catalytic activity and reusability demonstrates
that the Ag MFs film is an excellent candidate for high active
catalyst. The Ag MFs film exhibited obvious enhancement
signals with all concentrations of R6G solutions, and the main
Acknowledgements
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (Project No.
21205024), the National Science Foundation for Post-doctoral
Scientists of China (Grant No. 2012M520659, 2013T60307).
Keywords: sliver • flower like structure• film • catalysis
• Surface-enhanced Raman scattering
Raman bands were still visible even at a concentration as low
as10−7 M. Therefore, the Ag MFs film exhibited great potentials
in catalysis and SERS applications.
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European Journal of Inorganic Chemistry
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Key Topic*
[
a,b]
[a]
Qijun Dai, , Lu Li, Chungang Wang,
[
a]
Changli Lv, * [a] Zhongmin Su* [a] and
Fang Chai *[a,b]
Page No. – Page No.
Fabrication of flower like Ag
microspheres film with its application
in catalysis and as SERS substrates
*Flower like Ag microstructures film with its application in catalysis and SERS.
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