Preparation and characteristics of molecularly homogeneous Ag/AgCl nano-heterostructures via a two-step synthesis

Article abstract of DOI:10.1039/c4ra16325g

Ag/AgCl nano heterostructures with different degrees of molecular homogeneity and high photocatalytic activity have been created through a two-step synthesis in ethanol via an in situ oxidation route at room temperature. The result offers an alternative method for synthesizing molecular homogeneous metal/semiconductor nanocomposites. The heterostructures with AgCl ratios of 50% and 80% revealed excellent performance for the photocatalytic degradation of methyl orange molecules. This technique had the advantages of convenient operation, low cost, and mass production and built up a great molecularly homogeneous composite structure of Ag/AgCl which exhibited high photocatalytic activity and stability towards the decomposition of organic methyl orange.

Full text of DOI:10.1039/c4ra16325g

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Preparation and characteristics of molecularly
homogeneous Ag/AgCl nano-heterostructures via
Cite this: RSC Adv., 2015, 5, 17210
a two-step synthesis
a
Ping Yang, Changchao Jia,a Haiyan He,a Ling Chena and Katarzyna Matras-
*
Postolekb
Ag/AgCl nano heterostructures with different degrees of molecular homogeneity and high photocatalytic
activity have been created through a two-step synthesis in ethanol via an in situ oxidation route at room
temperature. The result offers an alternative method for synthesizing molecular homogeneous metal/
semiconductor nanocomposites. The heterostructures with AgCl ratios of 50% and 80% revealed
excellent performance for the photocatalytic degradation of methyl orange molecules. This technique
had the advantages of convenient operation, low cost, and mass production and built up a great
molecularly homogeneous composite structure of Ag/AgCl which exhibited high photocatalytic activity
and stability towards the decomposition of organic methyl orange.
Received 13th December 2014
Accepted 3rd February 2015
DOI: 10.1039/c4ra16325g
www.rsc.org/advances
acetate for highly efficient sunlight-driven photocatalysts.13
Cube-like Ag/AgCl was created using CH2Cl2 to slow release
chloride source.14 To improve the photocatalytic activity, Liu
Introduction
Photocatalysis is the acceleration of a photoreaction in the
presence of a catalyst and is becoming promising in many
applications that involve the utilization of solar energy.1 In
recent years, attention has been paid to design Ag/AgX (X ¼ Cl,
Br, I) composite materials due to the surface Plasmon reso-
nance (SPR) of metallic Ag and their excellent photocatalytic
activity and high stability.2,3 Efficient and stable visible-light
driven Ag@AgCl (or Ag@AgBr) photocatalysts have been fabri-
cated through a direct reaction between AgNO3 and HCl fol-
lowed by converting some Ag+ ions to Ag0 species via UV
irradiation or through an ion–exchange reaction between the
aqueous solution of Ag2MoO4 and HCl (or HBr).4–8 Hierarchical
porous AgCl@Ag hollow architectures revealed highly enhanced
visible light photocatalytic activity.9–12 Furthermore, both high
photocatalysis behavior and yields are important for research
and applications.
Ag/AgCl nano heterostructures exhibit an ideal performance
in photocatalysis. Regular nano heterostructures such as
nanowires and cubes revealed high photolysis activity. However,
these nanomaterials have to be fabricated using a complex
procedure with low yields,1,2,9 which signicantly limits their
practical applications. In addition, Ag/AgCl heterostructures
with different morphologies have been synthesized in recent
years.13–15 For example, Chen and co-workers reported cube-like
Ag/AgCl structures fabricated from sodium chloride and silver
et al. fabricated Ag/AgCl heterostructures using graphene oxide
as a capping agent to adjust the size and shape and increase
surface area for enhancing visible light photocatalytic perfor-
mance. Although these heterostructures revealed high photo-
catalysis activity, a limitation still retained for applications
because of the difficulty of the controlling of preparation
process. There is an urgent need to nd a simple and facile
method to synthesize Ag/AgCl nano heterostructures for
improving their performance and applications.
It is well known, an ideal photocatalysis material has three
basic features including fast migration rate of charge carrier,
efficient separation of photo-generated electron–hole pairs, and
a large specic surface area. Recent advances in plasmonic
photocatalysis have revealed that integrated plasmonic metal
nanostructures generating local electro-magnetic-elds allow
efficient separation of photo-induced electron–hole.16 To
increase the surface areas of AgX-based photocatalysts, a variety
of spherical, cubic, one-dimensional, and porous nano-
structures have been fabricated by using various synthetic
routes.17 Ag/AgCl nano heterostructures revealed a high photo-
catalytic performance in UV and visible-light regions for the
photo-degradation of methylorange (MO). Previous studies have
been reported that heterostructures was created from Ag
nanowires with a single crystalline structure for the fast
migration rate of the charge carrier.18 Such heterostructure
exhibited a high photocatalytic activity due to their uniform
component distribution of Ag and AgCl. In addition, these
materials with regular morphologies request special prepara-
tion procedure which limited their applications. There were
aSchool of Material Science and Engineering, University of Jinan, Jinan, 250022, P. R.
China. E-mail: mse_yangp@ujn.edu.cn
bFaculty of Chemical Engineering and Technology, Cracow University of Technology,
Krakow, 31-155, Poland
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very few reports for Ag/AgCl nano heterostructures regarding its function adsorption instrument (MFA-140 of Beijing Builder
molecular homogeneity and microstructure. company). Specic surface area was calculated by the Brunauer–
Herein, we have reported a two-step synthesis to fabricate Emmett–Teller (BET) method using the adsorption data at the
molecularly homogeneous Ag/AgCl nano heterostructures as range from P/P0 ¼ 0.05 to 0.35 just below the capillary
highly efficient photocatalysis which can drive degradation of condensation, and the pore diameter distribution curve was
organics. Moreover, it has been found that the morphology of derived from the desorption branch by the BJH method.
the Ag/AgCl heterostructure is different in that of Ag particles.
The components of the obtained heterostructures can be easily
controlled by adjusting the feeding amount of FeCl3 in solu-
Photocatalytic tests
The aqueous solution of methyl orange (MO) was used as target
pollutants to evaluate the Ag/AgCl nano heterostructures with
different AgCl under ultraviolet (UV) light irradiation. 0.01 g of
photocatalyst was used for the degradation of 20 mL MO solu-
tion (10 mM) under a (12 W) UV light. The distance between the
target solution and the lamp was 30 cm. Before irradiation,
samples were stirred for 30 min in dark to make sure the
adsorption equilibrium. 2 mL of solution was taken out from
the test sample every a certain time. The absorption spectra of
samples were recorded by a conventional UV-vis spectrometer
(Hitachi U-4100).
tions. The morphology, size, and photocatalytic activity of
samples depended strongly on their compositions. The Ag/AgCl
heterostructures prepared using AgCl/Ag molar ratios of 50%
and 80% revealed great photocatalytic behavior. Because of
facial synthesis, enhanced photolysis performance, and high
yield, this result is utilizable for practical applications.
Experimental
Synthesis of samples
All of the chemicals used in this work were of analytical reagent
grade. For the synthesis of Ag/AgCl nano heterostructures,
typically, AgNO3 of 0.08 g was dissolved in 20 mL of ethanol.
Meanwhile, NaOH of 0.03 g was dissolved in 20 mL of ethanol.
The ethanol solution of NaOH was added into the ethanol
solution of AgNO3 drop by drop with stirring. Sometime later,
loose Ag particles (sample 1) were prepared. The FeCl3 ethanol
solution of 10 mL was injected into the above-mentioned
solution of the Ag particles with certain feeding rates. The
mixture was stirred for 3 h to ensure the reaction carried out
completely. The resulting samples were rinsed with de-ionized
water (resistivity ꢀ18 MU cm) and centrifuged at 4000 rpm to
remove unreacted chemicals. The preparation conditions of
Ag/AgCl loose heterostructures (samples 2–9) were illustrated in
Table 1.
Results and discussion
A two-step synthesis was used for creating Ag/AgCl nano
heterostructures as demonstrated in Scheme 1. Ag particles
were rstly prepared through the reaction of AgNO3 and NaOH
in ethanol. The oxidization of Ag into Ag+ ions by Fe3+ ions then
occurred and combined with Clꢁ ions to form AgCl immedi-
ately. The reaction is below.
Ag + Fe3+ + Clꢁ / Fe2+ + AgClY
The half reactions are:
Ag + Clꢁ / AgClY + e (EAgCl/Ag ¼ +0.223 V)
q
Characterization
À
Á
The phase composition of samples was identied by using an
X-ray diffractometer (Bruker D8-Advance, Germany). The
morphologies and sizes of samples were taken on a eld-
emission scanning electron microscope (FESEM, QUANTA
250 FEG, FEI, USA). N2 adsorption–desorption isotherms were
obtained at liquid nitrogen temperature (77 K) using a multi-
Fe3þ þ e/Fe2þ EFe3þ
¼ þ 0:771 V
q
=Fe2þ
Table 1 shows the composition of samples. Fig. 1 shows the
scanning electron microscopy (SEM) images of as-prepared Ag
Table 1 Preparation conditions of Ag/AgCl heterostructures and rate
constant k of MO decomposition reaction
Sample
AgCl molar ratio (%)
Reaction rate constant k
1
2
3
4
5
6
7
8
9
0
N/A
10
30
50
60
70
80
90
100
0.0009
0.0016
0.0824
0.0159
0.0250
0.0810
0.0107
0.0207
Scheme 1 Preparation procedure of Ag/AgCl heterostructures.
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particles (sample 1 as illustrated in Table 1). The Ag particles
revealed two morphologies including irregular particles and
long rods as shown in Fig. 1a and b. To conrm the formation of
Ag phase, Fig. 2 shows the X-ray diffraction (XRD) pattern of
sample 1. All of XRD peaks in Fig. 2 matched with those of Ag
materials. This indicates single Ag phase was obtained although
two morphologies were observed. When the solution of NaOH
was dropped into the ethanol solution of Ag+, the nucleation
and growth of Ag occurred because of a quick reaction rate.
Because silver seeds (single and multiple crystalline) with a
variety of crystal structure might be produced in the initial
stage, nally two morphologies of irregular particles and long
rods were created. In practically, inhomogeneous nucleation
and growth resulted in large Ag rod formed as shown in Fig. 1b.
Fig. 3 shows the SEM images of Ag/AgCl heterostructures.
Compared with sample 1 (Ag), no long rod was observed in
Ag/AgCl nano heterostructures even though in the case of a low
AgCl ratio (e.g., 10%). This indicates the reaction of Ag0 trans-
ferred into Ag+ occurred quickly. Such reaction resulted in the
destroying of rod structures and the formation of Ag/AgCl nano
heterostructures. In the case of AgCl ratio of 10%, the sizes of
the heterostructure are from several tens to 150 nm. With
increasing the amount of AgCl, the size distribution became
narrow. As a result, the Ag/AgCl heterostructures consisted of
small particles for the formation of loose structures. The
composition of the Ag/AgCl heterostructures was identied by
energy dispersive spectroscopy (EDS) analysis as shown in
Fig. 3 SEM images of Ag/AgCl heterostructures. (a) Sample 2 with 10%
AgCl. (b) Sample 3 with 30% AgCl. (c) Sample 4 with 50% AgCl. (d)
Sample 7 with 80% AgCl. (e) Sample 9 with 100% AgCl. Their images
with high magnification are shown in a1, b1, c1, and d1.
Fig. 4. The result indicates that the AgCl component is homogeneously distributed. This is ascribed to Ag particles with
a loose structure.
The formation mechanism of Ag/AgCl nano heterostructures
was summarized to the transformation of Ag particles in situ. In
the case of a low FeCl3 ratio, inhomogeneous Ag/AgCl hetero-
structures formed as shown in Scheme 1 while molecularly
homogeneous Ag/AgCl heterostructures were fabricated in the
case of a large amount of FeCl3. The size and morphologies of
the heterostructures differs from those of Ag particles. Because
˚
of the difference of the lattice constant of AgCl (5.54 A) and Ag
+
˚
(4.09 A), the oxidation of Ag into Ag resulted in crystalline
stress which led to the reconstruction of monomers. The
diffusion of reaction agents in loose Ag particles resulted in
molecularly homogeneous nano heterostructures formed.
To conrm the formation of Ag/AgCl heterostructures, Fig. 5
shows the XRD patterns of the heterostructures. The XRD peaks
Fig. 1 SEM images of as-prepared Ag particles (sample 1). (a and b)
Same specimen under different magnification.
of samples consisted of Ag (JCPDS no. 04-0783) and AgCl crys-
tals (JCPDS no. 31-1238). No peaks from other crystal types are
observed. With increasing the amount of FeCl3, the intensity of
the XRD peaks of the Ag crystals decreased gradually, whereas
the intensity of the peaks of AgCl crystals increased signi-
cantly. This is conrmed the phase transfer between Ag and
AgCl.
Fig. 6 shows the N2-sorption isotherms and pore size distri-
bution of samples 4 and 7. The corresponding nitrogen
isotherm is of type III isotherms (BDDT classication). The
analysis is the standard method for determining surface areas
from nitrogen adsorption isotherms. The result suggests that
the specic surface areas of samples 4 and 7 are 20.5 and
Fig. 2 XRD pattern of Ag particles (sample 1).
17212 | RSC Adv., 2015, 5, 17210–17215
19.7 m2 gꢁ1, respectively. The pore-size distribution is further
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Fig. 4 Element distribution of line scanning via EDS line scanning
analysis.
conrmed by the corresponding pore-size distribution curve in
the insets in Fig. 6. The pore size of samples 4 and 7 are 5.6 and
7.6 nm, respectively.
The photocatalytic activity of Ag/AgCl heterostructures was
evaluated by the degradation of MO under UV irradiation. Fig. 7
shows the degradation proles of samples with different
Ag/AgCl ratios. The area on the le of dashed line (negative
value in X axis) represents the time used for adsorption–
desorption equilibrium. The heterostructures revealed excellent
photocatalytic activity when the molar ratios of AgCl are 50% Fig. 6 N2-sorption isotherms and pore size distribution of samples. (a)
Sample 4. (b) Sample 7. Insets in (a) and (b) show the pore size
distribution.
and 80%. The reproducibility of experiments further conrmed
this result. Although the mechanism for samples with AgCl of
50% and 80% is not clearly, this is mostly ascribed that the ratio
of Ag and AgCl affect the separation of the photogenerated
migration to the metal surfaces also protects AgCl from photo-
carriers. The enhanced photocatalytic behavior of the hetero-
reduction, and makes it stable.1
structure is assigned to the efficient separation of the photo-
As plotted in Fig. 7, C/C0 and the reaction time (t) exhibits a
generated carriers between the Ag and AgCl in the
linear relationship, indicating that the decomposition reaction
heterostructure.17,19 The Fermi level of metallic Ag is lower than
of MO molecules follows the rst-order kinetics.
that of conduction band of the AgCl semiconductor,5 facili-
tating the migration of photo-excited electrons to the metal
surfaces. Such charge transfer divides electrons and holes into
ꢁdC/dt ¼ kC
two distinct regions, and effectively suppresses the recombi-
nation of electronic–hole pairs. In addition, the electron
where, C is the concentration of the MO molecules, C0 is the
initial concentration of the MO molecules, t is reaction time,
and k is the rate constant. From results shown in Fig. 7, the rate
constant is illustrated in Table 1. In the heterostructure, Ag
works as electron trap and strongly absorbs light, whereas AgCl
offers Cl for the degradation of MO molecules. The ratio of Ag
and AgCl plays an important role for the photocatalytic activity
of the nano heterostructures. In the case of low AgCl molar
ratio, the heterostructure contained inhomogeneous Ag
domains which affect the photolytic activity. With increasing
the amount of AgCl, the heterostructure becomes molecularly
homogeneous. In the cases of AgCl of 50% and 80%, the pho-
togenerated electron–hole pairs were separated efficiently.
Furthermore, the result from the investigation of N2 adsorp-
tion–desorption isotherms indicates that the heterostructure
did not reveal a porous structure even though it is loose
according to SEM observation. This result also supported the
heterostructure with high photocatalytic activity. For porous
AgCl/Ag nanostructures, the enhanced photocatalytic activity
cannot be expected because the interconnected ligaments in the
Fig. 5 XRD patterns of Ag/AgCl heterostructures. (a) Sample 2 with
10% of AgCl. (b) Sample 3 with 30% of AgCl. (c), Sample 4 with 50% of
AgCl. (d) Sample 9 with 100% of AgCl.
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Scheme 2 Illustration of photocatalytic mechanism of molecularly
homogeneous Ag/AgCl nano heterostructures.
Fig. 7 Photocatalytic degradation profile of MO under UV light irra-
diation using Ag/AgCl nano heterostructures with different AgCl molar
ratios.
increased the photocatalytic redox reaction.19 These photo-
generated electrons and holes were separated. Because holes
were transferred to the surfaces of AgCl particles, Clꢁ ions on
the AgCl surfaces were oxidized to Cl0 atoms, which enables to
oxidize MO and hence reduced to Clꢁ ions again. On the
other, the electron transference to Ag surfaces prevents
photo-reduction of AgCl.19 Therefore, the Ag/AgCl nano-
heterostructures exhibited good activity and stability.
porous heterostructures may act as straight routes for the
transportation of photogenerated electrons and holes.
To investigate the photostability of the Ag/AgCl sample for
degradation of MO under UV and visible light irradiation, the
sample was repeatedly used for 4 times aer separation by
centrifugation. Fig. 8 shows the photostability performances of
Ag/AgCl sample with an AgCl molar ratio of 80% for degradation
of MO upon UV light irradiation. At beginning, the adsorption
occurred in dark. This is ascribed to the pore structure of
samples. The photocatalytic performance of the sample
continually decreased, and the degradation efficiency of MO
was about 50% aer 4 cycles. The decrease of the activity may be
attributed to the photocorrosion of the Ag/AgCl heterostructure
under UV irradiation resulting from partial AgCl transformation
into metallic silver.20
An illustration of Ag/AgCl nano heterostructure formation
and photocatalytic mechanism is depicted in Scheme 2. The
Ag/AgCl nano heterostructures are highly photosensitive mate-
rials because of their composition. Molecularly homogeneous
heterostructures make the separation of electrons and holes
easily. Photons were rstly adsorbed by Ag and then photo-
generated carriers were separated by AgCl to carry out the
photodegradation of MO. Furthermore, the photo-generated
electrons and holes arise from the dipolar character of the
surface plasmon state of Ag molecules, which substantially
Conclusions
Molecularly homogeneous Ag/AgCl nano heterostructures have
been fabricated by a facile and novel two-step synthesis. Ag
particles were prepared in ethanol using AgNO3 and NaOH. The
molar ratio of Ag and AgCl can be easily adjusted by changing
the amount of FeCl3. The heterostructures with AgCl ratios of
50% and 80% revealed excellent performance for the photo-
catalytic degradation of MO molecules. Because of high yield,
facile route, and ideal properties, this rational synthetic route at
room-temperature may be easily extended to fabricate other
photocatalytic nano heterostructures and applied in some elds
such as electronics, sensing, photonics and catalysis.
Acknowledgements
This work was supported in part by the project from National
Research Program of China (973 Program, 2013CB632401), the
program for Taishan Scholars, the projects from National
Natural Science Foundation of China (51202090) and
Outstanding Young Scientists Foundation Grant of Shandong
Province (BS2012CL004 and BS2012CL006), and Graduate
Innovation Foundation of University of Jinan, GIFUJN
YCXZ13001.
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Fig. 8 Photostability performances of Ag/AgCl sample with AgCl
molar ratio of 80% for degradation of MO under UV light irradiation.
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