Silver chromate and silver dichromate nanostructures: Sonochemical synthesis, characterization, and photocatalytic properties

Article abstract of DOI:10.1016/j.materresbull.2013.02.025

In this work, Ag₂CrO₄ and Ag₂Cr ₂O₇ nanostructures have been produced via a sonochemical method using silver salicylate as precursor. Besides silver salicylate, Na ₂CrO₄ and (NH₄)₂Cr₂O ₇ as starting reagents were applied. To investigate the effect of preparation parameters on the morphology and particle size of Ag ₂CrO₄ and Ag₂Cr₂O₇, sonication time, type of surfactant and its concentration were changed. The as-produced nanostructures were characterized by techniques like powder X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. The scanning electron micrographs showed that particle-like and rod-like nanostructures of Ag₂CrO₄ and Ag₂Cr ₂O₇ were produced using different surfactants. To investigate the catalytic properties of Ag₂CrO₄ nanoparticles, photooxidation of methyl orange (MO) was performed. According to the obtained results, it was found that the methyl orange degradation was about 87.3% after 280 min irradiation of visible light.

Full text of DOI:10.1016/j.materresbull.2013.02.025

Materials Research Bulletin 48 (2013) 2084–2094
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Silver chromate and silver dichromate nanostructures: Sonochemical synthesis,
characterization, and photocatalytic properties
Faezeh Soofivand a, Fatemeh Mohandes b, Masoud Salavati-Niasari a,b,
*
a Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran
b Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
In this work, Ag2CrO4 and Ag2Cr2O7 nanostructures have been produced via a sonochemical method
using silver salicylate as precursor. Besides silver salicylate, Na2CrO4 and (NH4)2Cr2O7 as starting
reagents were applied. To investigate the effect of preparation parameters on the morphology and
particle size of Ag2CrO4 and Ag2Cr2O7, sonication time, type of surfactant and its concentration were
changed. The as-produced nanostructures were characterized by techniques like powder X-ray
diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. The scanning electron
micrographs showed that particle-like and rod-like nanostructures of Ag2CrO4 and Ag2Cr2O7 were
produced using different surfactants. To investigate the catalytic properties of Ag2CrO4 nanoparticles,
photooxidation of methyl orange (MO) was performed. According to the obtained results, it was found
that the methyl orange degradation was about 87.3% after 280 min irradiation of visible light.
ß 2013 Elsevier Ltd. All rights reserved.
Received 4 August 2012
Received in revised form 16 January 2013
Accepted 12 February 2013
Available online 26 February 2013
Keywords:
A. Nanostructures
A. Surfaces
C. Thermogravimetric analysis (TGA)
D. Catalytic properties
1. Introduction
and nanowires by emulsion liquid membranes was reported by Liu
et al. [14].
In recent years, the preparation of low-dimensional nanos-
tructures has been intensively pursued because of their useful
applications in various areas [1,2]. Among these nanostructured
materials, it was found that silver chromate is a good visible-light
sensitive photocatalyst [3]. Silver chromate can be used as cathode
for lithium cells [4–6]. Suthanthiraraj and Premchand developed a
new solid electrolyte system involving CuI and Ag2CrO4, and
studied its ion transport, electrical and electrochemical properties
[7]. On the other hand, the visible-light sensitive materials
including Cr element for photocatalytic water spiliting [8,9], and
multi-metal oxides involving Ag(I) for organic photooxidation
under visible-light irradiation [10,11] have been attractive many
attentions. So far, several methods have been reported for the
synthesis of Ag2CrO4 nanostructures. Liu et al. prepared necklace
structures of Ag2CrO4 composed of single crystalline nanorods by
using template [12]. Alamdari et al. have synthesized Ag2CrO4
nanoparticles by precipitation method, and investigated the effect
of silver and chromate concentrations, flow rate of reagent
addition and temperature on the particle size of silver chromate
[13]. In addition, biomimetic synthesis of Ag2CrO4 quasi-nanorods
Recently, the development of a simple and fast synthetic method
that can control the shape and particle size of nanostructures under
ambientconditionsisremainedanimportant topicofinvestigations.
The ultrasonic processes as a fast, convenient, and economical
method have been widely used to generate novel nanostructured
materials under ambient conditions [15–19]. The chemical and
mechanical effects of ultrasound arise from acoustic cavitation,
which is the formation, growth, and implosive collapse of bubbles in
a
liquid medium. When solution is exposed to ultrasound
irradiation, bubbles in the solution are implosively by acoustic
fields; as a result, the temperature can be as high as 5000 K, the
pressure can reach over 1800 atm, and the cooling rate can be in
excess of 1010 K/s. The collapse of bubbles can also induce a shock
wave in the solution and drive impact of liquid to the surface of
particles. Therefore, various nanostructures can be obtained under
these conditions [20]. Besides using sonication, metal coordination
compounds as precursor have been widely used to fabricate nano-
sized materials as they represent an important interface between
synthetic chemistry and materials science [21–26]. Herein, we have
beensynthesizedAg2CrO4 and Ag2Cr2O7 nanostructures usinga new
silver-organic precursor in the presence of ultrasonic irradiation. In
addition, the effect of preparation parameters such as sonication
time, type of precursor and surfactant on the particle size and
morphology of the final products was studied by using scanning
electron microscopy (SEM). In this work, methyl orange (MO)
degradation was carried out to study the photocatalytic activity of
Ag2CrO4 nanoparticles.
* Corresponding author at: Institute of Nano Science and Nano Technology,
University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran.
Tel.: +98 361 5912383; fax: +98 361 5552935.
E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).
0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2013.02.025

F. Soofivand et al. / Materials Research Bulletin 48 (2013) 2084–2094
2085
Table 1
Preparation conditions for Ag2CrO4 and Ag2Cr2O7.
Sample
Sonication time (min)
Surfactant (g)
Morphology
A1
A2
A3
A4
A5
A6
A7a
A8
A9
A10
A11
B1
5
10
20
50
120
–
–
Rod-like structures with irregular dimensions
–
Capsule-like; diameters 100–110 nm and lengths 220–250 nm
Capsule-like; diameters 120–130 nm and lengths 250–260 nm
Rod-like; diameters 80–85 nm and lengths 90–100 nm
Plate-like and rod-like nanostructures
–
–
–
–
Micrometer irregular shapes plus a small amount of nano-rods
10
10
10
10
10
5
–
Agglomerated particles with particle size of between 220 and 400 nm
Nanoparticles and a small amount of nano-rods
Nanoparticles; 45–50 nm
SDS (0.1)
SDS (0.5)
SDS (1.0)
Fused rhombic-like sheets composed of nanoparticles
Regular nano-rods; diameters 90–95 nm and lengths 180–190 nm
Plate-like micro/nanostructures
PEO (0.5)
–
B2
10
30
50
–
–
Irregular shapes
B3
–
Plate-like micro/nanostructures
B4
–
Plate-like micro/nanostructures
B5
–
Agglomerated microparticles
B6
10
10
10
SDS (0.1)
PEO (0.1)
–
Nanoparticles; 15–20 nm
B7
B8a
Plate-like; diameters 95–100 nm and lengths 700 nm–1 mm
Agglomerated particles with particle size of between 300 and 600 nm
a
AgNO3 was used instead of [Ag(HSal)].
2. Experimental
2.3. Synthesis of Ag2CrO4 and Ag2Cr2O7 nanostructures
2.1. Materials and characterization
In a general procedure, Ag2CrO4 nanostructures were
prepared by reaction between [Ag(HSal)] and Na2CrO4 pre-
cursors with molar ratio of 2:1. At first, 2 mmol of silver
salicylate was dissolved in 50 mL of hot distilled water under
magnetic stirring, and then sodium chromate solution (1 mmol
of Na2CrO4 dissolved in 50 mL H2O) was sequentially introduced
to the silver salicylate solution under magnetic stirring. During
the addiction process, the color of the solution became red-
brown. The rate of sodium chromate addition to silver salicylate
solution in all experiments was 3 mL/min. The as-obtained
solution was exposed to ultrasound irradiation under ambient
air for 5, 10, 20, and 50 min, respectively. The red-brown colored
precipitates obtained under various conditions (Table 1) were
All the chemicals used in this work were commercially available
and employed without further purification. Silver nitrate, sodium
salicylate, sodium chromate, diammonium dichromate, sodium
dodecyl sulfate (SDS), and polyethylene oxide (PEO) were
purchased from Merck Chemical Company. A multiwave ultrasonic
generator (Sonicator 3000; Bandeline, MS 72, Germany), equipped
with
a converter/transducer and titanium oscillator (horn),
12.5 mm in diameter, operating at 20 kHz with a power of
50 W/cm2 was used for the ultrasonic irradiation. FT-IR spectra
were recorded on Magna-IR, spectrometer 550 Nicolet in KBr
pellets in the range 400–4000 cmꢀ1. Powder X-ray diffraction
(XRD) patterns were collected from a diffractometer of Philips
Company with X’PertPro monochromatized Cu K
˚
(l = 1.54 A). Microscopic morphology of products was visualized
a radiation
by SEM (LEO 1455VP), and TEM images were obtained on a Philips
EM208S transmission electron microscope with an accelerating
voltage of 100 kV. The energy dispersive spectrometry (EDS)
analyses were studied by XL30, Philips microscope. Thermogravi-
metric and differential thermal analyses (TGA/DTA) were carried
out with Pyris Diamond Perkin-Elmer under air atmosphere at a
heating rate of 10 8C/min from room temperature to 500 8C. The
UV–vis diffuse reflectance spectra of the samples were studied by
UV–vis spectrophotometer (Shimadzu, UV-2550, Japan). X-ray
photoelectron spectra (XPS) were recorded on a twin anode,
XR3E2, X-ray photoelectron spectrometer, using Al Ka radiation as
the exciting source. Nuclear magnetic resonance (1H NMR)
spectrum of silver(I) salicylate precursor was recorded by BRUKER
(400 MHz) in DMSO-d6.
2.2. Synthesis of [Ag(Hsal)] precursor
In a typical synthesis to produce [Ag(HSal)] precursor, 3 mol of
AgNO3 was dissolved in 40 mL of distilled water, and then sodium
salicylate solution (3 mol of sodium salicylate dissolved in 40 mL of
distilled water) was added gradually into the above solutions with
vigorous stirring. After stirring for 15 min, the as-synthesized
white precipitate was isolated and washed with distilled water and
ethanol several times to remove impurities, and dried at 50 8C in
vacuum. The [Ag(HSal)] precursor was characterized by FT-IR, 1H
NMR, and TGA/DTA.
Fig. 1. (a) TGA/DTA curves and (b) 1H NMR spectrum of [Ag(HSal)] precursor.

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F. Soofivand et al. / Materials Research Bulletin 48 (2013) 2084–2094
Table 2
FT-IR absorption peaks of salicylate [29].
Peak (cmꢀ1
)
Assignments
CX (17a), CX (14a), t(C–O)h
467 m
d
d
537 s
g
CCC (18a)
Breathing (2)
CC (6) Umbrella (4)
CH (19A), COO
CH (7), CH (19B)
CH (14)
nCC (5)
670 s
dCC (8)
700 vs, 743 vs
810 vs
d
g
g
d
d
861 vs
g
984 sbr
1030 m
1084 m
d
CC (10)
1142 m
dCH (3)
1202 wbr
1299 vs
n
(C–O)h
d
n
n
(O–H)h
1377 vvs
CC (13a) (13b)
CC (16a,16b)
1468 vs, 1485 vs
1582 vs, 1588 s
1652 m
vC5O
Benzene fingers and n(OH)
2680 mbr, 2830 m, 2910 m
nCH
collected and washed repeatedly with distilled water and
ethanol several times and finally dried at 50 8C in vacuum.
The synthesis processes of Ag2Cr2O7 and Ag2CrO4 nanostruc-
tures were the same, but (NH4)2Cr2O7 was applied instead of
Na2CrO4 for the preparation of Ag2Cr2O7. The as-synthesized
products were characterized by TEM, SEM, XRD, XPS, EDS, and
FT-IR.
Fig. 2. FT-IR spectra of (a) sodium salicylate and (b) [Ag(HSal)] precursor.
Fig. 3. SEM images of Ag2CrO4 obtained from samples (a) A1, (b) A2, (c) A3, (d) A4, (e) A5 and (f) A6.

F. Soofivand et al. / Materials Research Bulletin 48 (2013) 2084–2094
2087
weight loss observed at 276.9 8C, and the second weight loss
shown at 313.8 8C were corresponding to silver salicylate
decomposition to AgO and Ag2O, respectively. These weight loss
steps with a total mass loss of 56.3% (calcd 55.80%) are shown as
one exothermic stage in the DTA curve presented in Fig. 1a. Mass
loss calculations showed that the final decomposition products
were AgO and Ag2O. At high temperature (up to 320 8C), a
backward movement was seen in the TGA curve of silver
salicylate. This phenomenon may be related to the generation of
an inert atmosphere of product gas in the crucible, which drove
out O2 gas for a while, causing the formation of some metal or
lower valence oxides. When oxygen diffused again in the
crucible, the metal or lower oxide oxidized to the normal oxide
[28]. According to TGA analysis, it was found that silver salicylate
synthesized as precursor has no coordinated or crystallization
water molecules in its structure. To further study about the
structure of silver salicylate, 1H NMR (400 MHz) spectrum was
recorded. Fig. 1b shows the 1H NMR spectrum of silver salicylate.
The multiple peaks appeared at the chemical shifts of
6.6–7.8 ppm, and the sharp peak appeared at 14.35 ppm could
be assigned to aromatic protons, and the proton of phenolic
hydroxyl group, respectively. The chemical shift of hydroxyl
group showed that this group could interact with silver ions as
shown in Fig. 1b.
FT-IR spectroscopy was applied to confirm the formation of
silver salicylate from sodium salicylate and AgNO3. FT-IR spectra
of sodium salicylate and the as-prepared silver salicylate are
shown in Fig. 2a and b, respectively. By comparing the IR spectrum
of sodium salicylate (Fig. 2a) and the IR spectrum of silver
salicylate (Fig. 2b), it was found that salicylate ligand has been
coordinated to Ag+ ion, and the [Ag(HSal)] complex formed. All
distinct absorption peaks of salicylate were illustrated in Table 2,
and every absorption peak was assigned to corresponding
vibration [29].
Fig. 4. SEM images of Ag2CrO4 synthesized with AgNO3 as precursor.
2.4. Photocatalytic measurements
The photocatalytic activity of Ag2CrO4 nanoparticles with
particle size of 40–45 nm obtained from sample A9 was tested
by using MO solution. The degradation reaction was carried out in
a quartz photocatalytic reactor. The photocatalytic degradation
was carried out with 0.05 g of MO solution containing 0.12 g of
Ag2CrO4. This mixture was aerated for 30 min to reach adsorption
equilibrium. Then, the mixture was placed inside the photoreactor
in which the vessel was 40 cm away from the UV and 25 cm away
from visible sources of 400 W Osram lamps. The quartz vessel and
light sources were placed inside a black box equipped with a fan to
prevent UV leakage. The experiments were performed at room
temperature and pH of the MO solution was adjusted 2–3 [27].
Aliquots of the mixture were taken at periodic intervals during the
irradiation, and after centrifugation they were analyzed with the
UV–vis spectrometer. The MO degradation percentage was
calculated by Eq. (1) as follows:
3.2. Characterization of Ag2CrO4 nanostructures
To investigate the effect of sonication time on the morphology
of Ag2CrO4, SEM images of the as-synthesized samples were taken
after sonication for 5 (sample A1), 10 (sample A2), 20 (sample A3),
50 (sample A4) and 120 min (sample A5). SEM images of samples
A1–A5 are shown in Fig. 3a–e. As shown in Fig. 3a, most of the
formed particles after sonication for 5 min have rod-like shapes.
After sonication for 10 and 20 min, the length of the nanorods
formed in the initial stage (5 min) decreased, and then capsule-
like nanostructures of Ag2CrO4 were obtained (Fig. 3b and c). By
increasing the sonication time from 20 to 50 min, both diameter
and length of the capsule-like nanostructures formed in the
middle stage (20 min) decreased (Fig. 3d). In Fig. 3e, SEM image of
sample A5 obtained after sonication for 120 min is shown. As
shown in Fig. 3e, morphology of sample A5 is composed of rod-like
and plate-like nanostructures. The increase in sonication time
shows that energy is continuously added to the reaction, and this
hinders the growth of Ag2CrO4 nanorods. To make a comparison,
we have carried out the experiment in the absence of ultrasound
irradiation with magnetic stirring at room temperature. SEM
image of Ag2CrO4 prepared by using [Ag(HSal)] and Na2CrO4 with
molar ratio of 2:1 after stirring for 2 h (sample A6) is shown in
Fig. 3f. The morphology of Ag2CrO4 powders obtained with
magnetic stirring was composed of irregular shapes in an
aggregated state plus a small amount of nanorods. It can be
found that sonication is a key factor to form 1-D Ag2CrO4
nanostructures, such as nanocapsules and nanorods. The forma-
tion mechanism of rod-like nanostructures in the presence of
ultrasonic irradiation has been proposed. So, with the aid of
sonication, the formed bubbles collapse asymmetrically, resulting
A0 ꢀ At
D:P: ðtÞ ¼
ꢁ 100
(1)
A0
where A0 and At are the absorbance value of solution at 0 and t min,
respectively.
3. Results and discussion
3.1. Characterization of [Ag(Hsal)] precursor
To determine the number of coordinated or crystallization
water molecules of silver salicylate complex, thermogravimetric
and differential thermal analyses (TGA/DTA) were carried out
between 30 and 500 8C in air. Fig. 1a shows the TGA and DTA
curves of [Ag(HSal)]. As shown in Fig. 1a, two weight loss steps
are shown in the TGA curve. According to the TGA curve, the first

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Fig. 5. SEM images of Ag2CrO4 obtained from samples (a) A8, (b) A9, (c) A10, and (d) A11.
in the generation of high-speed microjets. Because of the high
velocity of these microjets, the chance of collision between the
nanoparticles formed in the initial stage increased. At last, more
nanoparticles fused in an oriented manner, and then Ag2CrO4
nanorods prepared [30].
To study the effect of silver precursor on the morphology of
Ag2CrO4, the experiment was done using AgNO3 (sample A7). The
morphology of sample A7 obtained after sonication for 10 min was
studied by scanning electron microscope. SEM images of sample
A7 are shown in Fig. 4a and b. When AgNO3 was used as silver
precursor, particle-like microstructures of Ag2CrO4 with particle
size of 220–400 nm were obtained. By comparing SEM image of
sample A2 (synthesized by silver salicylate after sonication for
10 min) and SEM image of sample A7 (synthesized by silver nitrate
after sonication for 10 min), it was found that capsule-like
nanostructures and particle-like microstructures of Ag2CrO4 have
been formed with the aid of [Ag(HSal)] and AgNO3, respectively. In
fact, the morphology and particle size of sample A2 and sample A7
were different. This phenomenon is explained as follows. By using
[Ag(HSal)], the available salicylate ligand in the solution was
adsorbed onto the surface of Ag2CrO4 nuclei through carboxylate
and hydroxyl groups, which leads to the self-assembly of Ag2CrO4
Fig. 6. (a) EDS spectrum, (b) FT-IR spectrum, (c) XRD pattern of sample A9 synthesized by SDS, and (d) XRD pattern of sample A11 synthesized by PEO.

F. Soofivand et al. / Materials Research Bulletin 48 (2013) 2084–2094
2089
Fig. 7. UV–vis diffuse reflectance spectrum of Ag2CrO4 obtained from A9.
nuclei. As a result, more nanoparticles fused in an oriented manner,
and then Ag2CrO4 capsule-like nanostructures formed. So that, the
as-mentioned proposed mechanism could not be occurred by
AgNO3. Therefore, Ag2CrO4 microparticles were formed in the
presence of AgNO3 as precursor.
It is well-known that the presence of surfactant during the
production of nano-sized materials has a great effect on the shape
and particle size of products. Therefore, much attention has been
paid to the study of such supramolecular structures, which can
play an important role as both template and microreactore for
producing nanomaterials. For example, sodium dodecyl sulfate
(SDS) molecules as an anionic surfactant can self-aggregate into
cubic, hexagonal and lamellar structures [31,32]. In this work,
sodium dodecyl sulfate (SDS) molecules were used, where the
amount of SDS was 0.1, 0.5 and 1.0 g. When the amount of SDS was
0.1 g (sample A8), the morphology of Ag2CrO4 was composed of
nanoparticles and a small amount of nano-rods (Fig. 5a). When the
amount of SDS was increased from 0.1 to 0.5 g (sample A9),
Ag2CrO4 nanoparticles with particle size of 45–50 nm were
produced, and the presence of nanorods disappeared (Fig. 5b).
On the other hand, by increasing the amount of SDS from 0.1 to
0.5 g, the particle size of Ag2CrO4 decreased. By using 1.0 g of SDS
(sample A10), fused rhombic-like sheets composed of nanoparti-
cles were obtained (Fig. 5c). The obtained results showed that by
using SDS, morphology of Ag2CrO4 changed from rod-like to
particle-like nanostructures. Besides SDS, polyethylene oxide
(PEO) as a soft template was studied to fabricate Ag2CrO4
nanostructures. In this case, the amount of PEO with molecular
weight of 600,000 was 0.5 g and sonication was done for 10 min
(sample A11). As shown in Fig. 5d, rod-like nanostructures of
Ag2CrO4 with diameters of 90–100 nm and lengths of 300–400 nm
were prepared by using PEO as surfactant. By comparing SEM
image of Ag2CrO4 nanorods synthesized without using PEO
(Fig. 3b) and SEM image of Ag2CrO4 nanorods synthesized by
PEO (Fig. 5d) after sonication for 10 min, it was found that the
dimensions of nanorods decreased in the presence of PEO due to its
steric hindrance effect.
Fig. 8. Photocatalytic MO degradation of Ag2CrO4 nanoparticles obtained from
sample A9 (a) under visible light, and (b) under UV light.
To further confirm the chemical composition of the as-prepared
Ag2CrO4, EDS spectrum was recorded for sample A9. In the EDS
spectrum of sample A9 (Fig. 6a), Ag and Cr elements are detected.
Fig. 6b shows FT-IR spectrum of sample A9 in the range 400–
4000 cmꢀ1. The FT-IR spectrum shows a strong absorption band at
888 cmꢀ1 and a shoulder at about 860 cmꢀ1, which may be
assigned to the n3 and n1 vibration modes of Cr–O, respectively
[33,34]. No impurities of surfactant and precursor are detected
from this spectrum. The crystal structure of sample A9 was
determined by powder X-ray diffraction method. Fig. 6c and d
shows XRD patterns of Ag2CrO4 synthesized by SDS (sample A9)
and PEO (sample A11), respectively. All the reflection peaks in
Fig. 6c and d can be readily indexed to orthorhombic phase
Fig. 9. (a) SEM image and (b) XRD pattern of Ag2CrO4 nanoparticles after
photocatalytic reaction.

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F. Soofivand et al. / Materials Research Bulletin 48 (2013) 2084–2094
Fig. 10. SEM images of Ag2Cr2O7 obtained from samples (a) B1, (b) B2, (c) B3, (d) B4, and (e) B5.
Ag2CrO4 with Pmnb space group (JCPDS No. 26-0952). No
impurities are detected from these patterns. The particle size of
sample A9 and sample A11 was calculated from the major
diffraction peaks of the corresponding Ag2CrO4 using the Scherrer
formula [35]. The particle sizes calculated of sample A9 and sample
A11 were 43 and 92 nm, respectively. Fig. 7 shows UV–vis diffuse
reflectance spectrum of Ag2CrO4 obtained from sample A9. In the
UV–vis spectrum of the product, three absorption peaks are seen at
268, 359, and 576 nm. This spectrum has a good agreement with
the UV–vis spectrum of Ag2CrO4 nanoparticles prepared by
Ouyang et al. [3]. As we are aware, there is a suitable correlation
between crystal structure and absorption spectrum of materials. It
was found that the Ag2CrO4 crystal consists of two kinds of
tetrahedra, AgO4 and CrO4 tetrahedron, and one kind of octahe-
dron, AgO6 octahedron. Since 3d-electron numbers of the Cr(VI)
and Ag(I) ions are 0 and 10, respectively, there is no crystal field
effect. Therefore, the absorption spectrum of Ag2CrO4 shows only
one absorption band in the visible region (576 nm) [36]. Two
absorption peaks centered at 268 and 359 nm can be attributed to
the charge transfer from oxygen as ligand to Cr(VI) species without
any 3d-electron [34].
Fig. 8a and b. As shown in Fig. 8, the MO degradation percentage of
Ag2CrO4 nanoparticles increased rapidly after the Osram lamp
turned on. According to photocatalytic calculations by Eq. (1), the
MO degradation was about 87.3% after 280 min irradiation of
visible light. These results indicated that Ag2CrO4 nanoparticles are
novel photocatalyst under visible light. To investigate the
morphology and chemical composition of the photocatalyst after
photocatalytic treatment, high magnification SEM image and XRD
pattern of the Ag2CrO4 nanoparticles used were studied. Fig. 9a and
b presents the SEM micrograph and XRD pattern of the Ag2CrO4
powders collected after photooxidation of MO, respectively.
Particle-like nanostructures with grain size of about 42 nm could
be clearly observed in Fig. 9a. All diffraction peaks in Fig. 9b can be
indexed as orthorhombic Ag2CrO4 structure (JCPDS No. 26-0952).
SEM and XRD analyses indicated that the morphology and crystal
structure of Ag2CrO4 nanoparticles before and after photocatalytic
treatment were the same. Therefore, Ag2CrO4 nanoparticles
synthesized by the present method are introduced as a high-
stable photocatalyst.
3.3. Characterization of Ag2Cr2O7 nanostructures
In this work, the photocatalytic activity of Ag2CrO4 nanopar-
ticles (sample A9) was studied by photooxidation of MO under
visible and UV light irradiation. The obtained results are shown in
To study the effect of sonication time on the morphology of
Ag2Cr2O7, SEM images of samples synthesized after sonication

F. Soofivand et al. / Materials Research Bulletin 48 (2013) 2084–2094
2091
Fig. 11. SEM images of Ag2Cr2O7 synthesized with AgNO3 as precursor.
Fig. 12. SEM images of Ag2Cr2O7 obtained from samples (a) B6, and (b) B7.
for 5 (sample B1), 10 (sample B2), 30 (sample B3), and 50 min
(sample B4) were taken. The SEM images of samples B1–B4 are
shown in Fig. 10a–d. As shown in Fig. 10a and b, irregular plate-
like microstructures of Ag2Cr2O7 were obtained after sonication
for 5 and 10 min. When sonication time increased to 30 and
50 min, the major morphology of products was plate-like
nanostructures (Fig. 10c and d). Besides, by increasing the
sonication time, the diameters of the as-formed plates decreased.
To investigate the effect of ultrasound irradiation on the
morphology of Ag2Cr2O7, sample B5 was prepared without
the solution could control the shape and crystal growth of
Ag2Cr2O7.
One of the other strategies for decreasing the particle size of
nanostructures is the application of active surface reagents.
Sodium dodecyl sulfate (SDS) and polyethylene oxide (PEO) as
anionic and polymeric surfactants can decrease the growth of
nanoparticles due to their steric hindrance effects. Ag2Cr2O7
nanostructures were synthesized by using [Ag(HSal)] in the
presence of 0.1 g of SDS and PEO after sonication for 10 min
(samples B6 and B7). SEM images of samples B6 and B7 are shown
in Fig. 12a and b, respectively. The agglomerated nanoparticles
with particle size of 50–60 nm were obtained in the presence of
SDS. The particle size of sample B6 synthesized by SDS was lower
than that of sample B2 prepared without SDS. Fig. 12b shows SEM
image of sample B7 synthesized using PEO. As shown in Fig. 12b,
plate-like nanostructures of Ag2Cr2O7 with diameters of about 95–
sonication. When
a solution of diammonium dichromate
(1 mmol of (NH4)2Cr2O7 dissolved in 50 mL of distilled water)
was added dropwise into a solution of [Ag(HSal)] (2 mmol of
silver salicylate dissolved in 50 mL of distilled water), Ag2Cr2O7
precipitate was obtained after magnetic stirring for 50 min at
room temperature. SEM image of sample B5 is shown in Fig. 10e.
It is interesting that the plate-like micro/nanostructures
disappeared and Ag2Cr2O7 microstructures were formed in the
absence of ultrasound irradiation. These results indicated that
100 nm and lengths of 700 nm–1
PEO.
mm were obtained with the aid of
sonication is favorable to produce micro/nano-plats as 2-D
To investigate the chemical composition of silver dichromate,
EDS spectrum was recorded for plate-like Ag2Cr2O7 nanostruc-
tures (sample B7). In the EDS spectrum of sample B7 (Fig. 13a), Ag
and Cr elements are detected. The crystal structure of sample B7
was determined by powder X-ray diffraction method. Fig. 13b
shows XRD pattern of sample B7. All reflection peaks in Fig. 13b
can be readily indexed to Ag2Cr2O7 (JCPDS No. 01-0963). No
impurities are detected from this pattern. This indicated that pure
Ag2Cr2O7 was obtained under the present synthetic conditions.
The broad XRD reflection peaks in Fig. 13b indicate the
nanocrystalline nature of Ag2Cr2O7 synthesized in the presence
of ultrasound irradiation. The average crystal diameter calculated
by Scherrer formula was about 95 nm. Fig. 13c shows FT-IR
spectrum of sample B7 in the range 400–4000 cmꢀ1. The FT-IR
spectrum of Ag2Cr2O7 obtained from sample B7 shows a strong
absorption band at 888 cmꢀ1 and a shoulder at 860 cmꢀ1, which
2ꢀ
structures. We can assume that Cr2O7
ions in the solution
combine with silver ions released of the [Ag(HSal)] precursor to
yield Ag2Cr2O7 nuclei. Under ultrasound irradiation, self-
assembly of the primary particles occurred to form plate-like
nanostructures of Ag2Cr2O7.
The effect of silver precursor was investigated in the case of
Ag2Cr2O7 nanostructures by using AgNO3 instead of [Ag(HSal)].
Fig. 11 shows SEM images of Ag2Cr2O7 powders generated by
AgNO3 after sonication for 10 min. According to Fig. 11, agglom-
erated particles with particle size of between 300 and 600 nm
have been formed. By comparing the SEM images in Fig. 10a–d and
the SEM images in Fig. 11, it can be seen that plate-like
nanostructures and agglomerated microparticles have been
generated with [Ag(HSal)]and AgNO3, respectively. The as-
obtained results proved that the presence of salicylate ligand in

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F. Soofivand et al. / Materials Research Bulletin 48 (2013) 2084–2094
Fig. 14. TEM images of plate-like Ag2Cr2O7 nanostructures synthesized by PEO.
reference peak. The presence of peaks attributed to O, Cr, and Ag
elements in the XPS spectrum presented in Fig. 15a indicates the
high purity of Ag2Cr2O7 synthesized by this method. In Fig. 15b,
the peak centered at 531.4 eV was attributed to the O 1s. The two
peaks located at 574.5 and 580.2 eV were attributable to the
Cr 2p(3/2) and Cr 2p(1/2), respectively (Fig. 15c). In Fig. 15d,
the two strong peaks at the Ag region of 369.2 and 375.4 eV
are assigned to Ag 3d(5/2) and Ag 3d(3/2), respectively. The
quantitative analysis of Ag2Cr2O7 nanoparticles by XPS data
gave a rough atomic ratio Ag:Cr:O as nearly 1:1:3.5 (Ag: 18.12%;
Cr: 18.36%; O: 63.52%), which confirms the purity of the
nanoparticles.
Fig. 13. (a) EDS spectrum, (b) XRD pattern, and (c) FT-IR spectrum of Ag2Cr2O7
nanostructures synthesized by PEO.
may be assigned to the n3 and n1 vibration modes of Cr–O,
respectively [34,35]. The broad absorption band centered at
3432 cmꢀ1 and a weak peak at 1627 cmꢀ1 are attributable to the
n
(OH) stretching and bending vibrations, respectively, which
The discussion for the mechanism formation of the products
can be assumed as follows. When silver salicylate molecules were
dissolved in water, silver cations (Ag+) and salicylate anions
(HOC6H4COOꢀ) released. The as-released Ag+ cations could react
indicates the presence of physisorbed water molecules linked to
Ag2Cr2O7 [37]. No impurities of surfactant and precursor are
detected from this spectrum.
2ꢀ
2ꢀ
To further investigate the morphology of the products, TEM
images of Ag2Cr2O7 obtained by using PEO (sample B7) were
taken, and shown in Fig. 14. In Fig. 14a and b, it can be observed
that 1-D nanostructures involving plate-like and rod-like shapes
have been produced in the presence of PEO. Besides, the presence
of agglomerated particles can be seen in these figures. The
lengths and diameters of these 1-D nanostructures estimated by
TEM images are between 600–1200 nm and 90–200 nm,
respectively. The average crystal diameter calculated by Scherrer
formula for sample B7 was in good agreement with the diameter
of 1-D nanostructures estimated by TEM images. X-ray photo-
electron spectra (XPS) were applied for the evaluation of surface
composition and purity of Ag2Cr2O7 nanoparticles. The XPS
survey spectrum in Fig. 15a shows that the peak centered at
285.5 eV can be assigned to the binding energy of C 1s as
with the Cr2O4
and Cr2O7
to form Ag2CrO4 and Ag2Cr2O7
nuclei, respectively. Then the obtained mixture was exposed to
ultrasound irradiation for 5, 10, 20, and 50 min, respectively. Under
ultrasound irradiation, self-assembly of the primary particles
occurred to form different morphologies of the products. In
addition, we can propose that the role of salicylate anions in this
strategy is as capping agent or surfactant. By comparing the SEM
images of the products synthesized by AgNO3 (Figs. 4 and 11) with
the SEM images of the products synthesized by [Ag(HSal)] (Figs. 3
and 10), it was found that nano-sized Ag2CrO4 and Ag2Cr2O7 were
obtained by using [Ag(HSal)]. Therefore, the presence of the
salicylate anions in the solution has a key role to control crystal
growth and morphology due to its steric hindrance effect. The main
chemical reaction for the preparation of Ag2CrO4 and Ag2Cr2O7 are
as follows:

F. Soofivand et al. / Materials Research Bulletin 48 (2013) 2084–2094
2093
Fig. 15. XPS survey spectrum of Ag2Cr2O7 nanoparticles prepared by SDS (a), XPS spectra of the O 1s (b), Cr 2p (c), and Ag 3d (d) regions.
50 ꢃ
C
½AgðHOC6H4COOÞꢂ þ H2Oꢀ!Agþ þ HOC6H4COOꢀ
salicylate and AgNO3. Ag2CrO4 nanostructures with different
morphologies such as nano-rods, nano-capsules, and nanoparticles
have been synthesized under different conditions in the presence
of ultrasonic irradiation. When Ag2CrO4 nanoparticles were used
as photocatalyst, the MO degradation percentage was about 87.3%
after 280 min irradiation of visible light. SEM and XRD results of
Ag2CrO4 nanoparticles after photocatalytic treatment indicated
that the Ag2CrO4 nanoparticles synthesized by this method are a
high-stable photocatalyst. In this work, for the first time, Ag2Cr2O7
nanostructures with different shapes including nano-plates,
nanoparticles, and nano-rods have been synthesized by [Ag(HSal)]
and (NH4)2Cr2O7 as starting reagents by sonication. FT-IR, EDS,
XRD, and XPS analyses proved high purity of the as-prepared
Ag2Cr2O7 nanostructures.
2Agþ þ CrO42ꢀ ! Ag2CrO4
2Agþ þ Cr2O72ꢀ ! Ag2Cr2O7
To further consider the ultrasonic irradiation effect on the
formation of Ag2CrO4 and Ag2Cr2O7 nanostructures, more attention
has been paid to the mechanical and chemical effects of sonication.
The chemical effects are related to the free radicals generated by the
sonolysis of solvent molecules inside collapsing cavitation bubbles.
The chemical effects of ultrasound can drive chemical reactions such
as oxidation, reduction, and decomposition [15]. On the other hand,
the mechanical effects are mainly caused by hydrodynamic shear
forces appearing in the surrounding of oscillating or collapsing
cavitation bubbles [38]. Protein release from yeast cells, and DNA or
RNA formation and membrane permeability of yeast cells are
examples of the mechanical effects [38,39]. In the case of
sonochemical synthesis of Ag2CrO4 and Ag2Cr2O7 nanostructures,
the free radicals formed by the sonolysis of H2O as solvent could not
drive the chemical reactions like oxidation, reduction, and etc., since
the oxidation numbers of Cr(VI) and Ag(I) were constant during the
sonochemical reaction. Therefore, the chemical effects of sonication
were not important for the formation of Ag2CrO4 and Ag2Cr2O7
nanostructures. When different morphologies of Ag2CrO4 and
Ag2Cr2O7 were obtained with and without sonication, we concluded
that mechanical effects of ultrasound play a key role on the self-
assembly and orientation of Ag2CrO4 and Ag2Cr2O7 nuclei. When
sonication was applied, 1-D nanostructures of Ag2CrO4 (nanorods)
were obtained, while the morphology of Ag2CrO4 synthesized
withoutsonicationwasirregular (microparticles). Ontheother hand,
Ag2Cr2O7 plate-like nanostructures and agglomerated microparti-
cles have been obtained with and without sonication, respectively.
Acknowledgements
Authors are grateful to Council of University of Kashan, Doctor
Masood Hamadanian from Mechanical Chemistry Department of
University of Kashan and Maryam Shamshiri, degree of M.Sc. in
Mechanical Chemistry Department of University of Kashan for
providing financial support to undertake this work.
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