Controlled synthesis of peony-shaped photocatalyst grains of Ag3PO4/Zn3(PO4)2 by coprecipitation and recrystallization technology

Article abstract of DOI:10.1039/c5ra05063d

Peony-shaped grains of a Ag₃PO₄/Zn₃(PO₄)₂ composite have been synthesized at room temperature through coprecipitation and recrystallization technology with the coexistence of Zn²⁺ and Ag⁺ in a aqueous solution. The simultaneous presence of high-concentration Ac^- ions plays a key role in the initial formation of flower-shaped grains of Zn₃(PO₄)₂ being the large-area host of this composite. Compared with pure Ag₃PO₄ crystals, the obtained composite exhibits enhanced visible-light-driven photocatalytic activity and stability for decomposition of Rhodamine B in aqueous solution. This is attributed to effective hole-electron pair transport and optimal Ag₃PO₄ dispersion achieved by confining Ag₃PO₄ to the as-prepared Zn₃(PO₄)₂ framework. This journal is

Full text of DOI:10.1039/c5ra05063d

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Controlled Synthesis of Peony-Shaped Photocatalyst Grains
of Ag PO /Zn (PO ) by Coprecipitation and
3
4
3
4 2
Recrystallization Technology
a
Ma Peiyan , Yu Yong ,Yu Hongjian , Che Liyuan ,Wang Ling and Xiong Yan
a
c
a
b
b,*
a
School of Chemistry , Chemical engineering and Life science, Wuhan University of Technology,
Wuhan, Hubei 430070, PR China
b
School of Materials Science & Engineering, Hubei Province Key Laboratory of Green Materials
for Light Industry, Hubei University of Technology, Wuhan, Hubei 430023, PR China
c
State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan
University of Technology, Wuhan, Hubei 430070, PR China
3 4 3 4 2
Peony-shaped grains of a Ag PO /Zn (PO ) composite has been synthesized at room temperature
2
+
+
through coprecipitation and recrystallization technology with the coexistence of Zn and Ag in a
-
aqueous solution.The simultaneous presence of high-concentration Ac ions plays a key role in the
initial formation of flower-shaped grains of Zn
Compared with pure Ag PO crystals, the obtained composite exhibits enhanced
visible-light-driven photocatalytic activity and stability for decomposition of Rhodamine B in
aqueous solution. This is attributed to effective hole-electron pair transport and optimal Ag PO
dispersion achieved by confining Ag PO to the as-prepared Zn (PO framework.
Keywords Ag PO /Zn (PO composite; ion exchange reaction; visible-light-driven
3 4 2
(PO ) being the large- area host of this composite.
3
4
3
4
3
4
3
4 2
)
3
4
3
4 2
)
photocatalytic activity
1
Introduction
In order to resolve environmental and energy issues, great efforts have been devoted for the
1
removal of pollutants from water by developing more active visible-light-driven photocatalysts .
One such photocatalyst, Ag
photocatalytic degradation of dyes . It has been reported that Ag
photocatalytic activity than N-doped TiO on the degradation of methylene blue aqueous solution
3 4
PO , has recently drawn attention due to its high efficiency in the
2, 3
3 4
PO crystals exhibit higher
2
3 4
under visible light irradiation. In addition, Ag PO crystals with high- energy crystal faces, porous
4
4
Ag
3
PO
, and Ag PO
3
4
-based heterostructured photocatalysts possess excellent performance for
5
-9
3 4
dyes degradation . However, the stability of Ag PO suffers from the dissolution of active
1

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+
Ag -ions in the water and consequently results in the performance deterioration in repeated
1
cycles . Hence, effective methods should be explored to improve the stability of Ag
0
3 4
PO in
photocatalysis process.
Immobilization of inorganic photocatalysts on inert supports has been attractive because it can
help to stabilize the photocatalysts. Some suitable supports not only possess the ability of
decreasing the recombination rate of electron-hole pairs but supplying a great amount of active
sites. In addition, supports with excellent absorption performance can concentrate the pollutants
from the solution to the photoactive sites of the photocatalysts. Different materials have been used
1
3
1
12
13
14
as suitable supports for the photocatalysts, such as Al
2
O
, TiO
2
, graphene , SBA-15 and
15
3 4
C N . Those supports can reduce the contact area of water with the catalysts and contribute to
improving the stability of the target materials. The structural and morphological characteristics of
the supports play important roles in the catalytic behavior. Therefore, choosing suitable supports is
necessary for the enhancement of photocatalytic activity of Ag PO .
3
4
Flower-shaped inorganic semiconductor materials have been widely applied in the area of solar
1
6
17
cell and photocatalysis due to their high specific surface area and effective hole-electron pair
transport through the interconnected network. Template molecules containing strong coordinating
groups have been used to mediate the formation of flower-like materials with large surface area. It
1
8
19
is worth noting that flower-shaped Au , CdS , Cu
20, 21
2
22
and ZnO crystal aggregates have
3
(PO
4
)
been successfully synthesized with the participation of DNA, l-cysteine, BSA or ascorbate,
respectively. However, the hybridization of inorganic materials with organic molecules would
decrease the degree of crystallinity and material properties. Subsequent treatment of hybridized
samples at temperature as high as 600-800°C is inevitable. Until now, there have been no reports
of flower-shaped inorganic crystals made from pure inorganic components by means of facile
synthetic routes in water.
For the first time, we report a novel method for creating peony-shaped grains of
Ag PO /Zn (PO ) composite at room temperature in water without the addition of any organic
3
4
3
4 2
molecules. The compound Zn (PO ) is a common host lattice for fluorescent materials like
3 4 2
2
3
24
25
Zn (PO ) :Hf , Zn (PO ) :Eu and Zn (PO ) :Dy . The environmentally friendly material will
3
4 2
3
4 2
3
4 2
3 4
be applied in this study as the framework for Ag PO crystals. In the preparation process,
2

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-
large-area Zn (PO ) framework is firstly precipitated ruled by Ac -ions and, secondly, Ag PO is
3
4 2
3
4
precipitated onto the surface of the Zn
to form hierarchically flower-like structure, leading to the formation of well-distributed
Ag PO /Zn (PO composite catalysts.
3 4 2 3 4
(PO ) support. Lastly, Ag PO nanoparticles self-aggregate
3
4
3
4 2
)
2
2
. Results and discussion
.1 Morphology and structure of the composites
The crystallinity and purity of the as-synthesized samples were investigated by XRD recordings. A
XRD pattern of a typical Ag PO /Zn (PO composite is shown in Fig. 1a. The diffraction peaks
denoted by “#” and “*” symbols are assigned to cubic Ag PO (JCPDS 06-0505) and
orthorhombic Zn (PO •4H
O (JCPDS 74−1778), respectively. SEM images of the corresponding
3
4
3
4 2
)
3
4
3
4
)
2
2
sample are displayed at different magnifications in Fig. 1b-d. It illustrates that the typical
morphology of the sample grains is a well-defined peony-shaped three-dimension structure with a
diameter in the range of 4−8 ꢀm. The peony-shaped grains are assembled by many densely
arranged nano-thin sheet-shaped “petals”. A close-up SEM view of this flower-like microstructure
reveals that the sheets are about 100 nm in thickness, as seen in Fig.1d. The microscopic
characteristic of flower-shaped composite was further analyzed by TEM (Fig.1S). TEM image
verifies the microstructure of flower-like Ag PO /Zn (PO ) assembled with nano -thick plates
3
4
3
4 2
(
Fig.1Sa). Obvious contrast can be observed from the edge (Fig.1Sb). The microstructures indicate
Ag PO is covered by Zn (PO ) and the optimal Ag PO dispersion is achieved by confining
3
4
3
4 2
3
4
Ag PO to the as-prepared Zn (PO ) framework.
3
4
3
4 2
.
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3 4 3 4 2
Fig.1 The XRD pattern and SEM images of a typical Ag PO /Zn (PO ) composite sample.
(
(
a) XRD pattern. The diffraction peaks denoted by “#” are assigned to cubic Ag PO
3 4
JCPDS 06-0505). The other peaks denoted by signals of sea star are assigned to
orthorhombic Zn
magnification.
3
(PO
4
)
2
•4H
2
O (JCPDS 74−1778); (b)-(d) SEM images at different
4

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Fig. 2 Elemental maps of a flat surface of a flower from as-synthesized sample.
EDX analysis of the thin sheets forming the typical composite grains register the major elements
Zn, Ag, O and P, as seen from Fig. 2S. The element signal of Al results from the sample support.
The estimated atom ratio of Ag to Zn is 2.4. Elemental maps of the same sample clearly
demonstrate uniform distribution of Zn, Ag, O and P elements over the flat surface, see Fig. 2.
Hence, the typical sample is mainly composed of well-mixed Ag
formation of a Ag PO /Zn (PO composite is further confirmed by measuring XPS spectra. The
binding energy of Ag 3d5/2 and Ag 3d3/2 states corresponding to 368 and 374 eV peaks,
3 4 3 4 2
PO and Zn (PO ) phases. The
3
4
3
4 2
)
2
6
respectively, are seen in Fig. 3a and are ascribed to the Ag-ion in Ag
3 4
PO . The binding energy of
Zn 2p1/2 and Zn 2p2/3 located at 1045eV and 1022 eV, respectively, and seen in Fig. 3b are
2
+
27
3 4 2
associated with Zn in Zn (PO ) .
2
3 4 3 4 2
.2 Formation process of peony- shaped grains of the Ag PO /Zn (PO ) composite
XRD patterns of the Ag
3
PO
4
/Zn (PO
3
4
)
2
3
composites prepared with different AgNO concentration
(
at a fixed mole ratio of Ag PO
3
4
/Zn (PO )
3 4 2
/KH PO ) are summarized in Fig. 4a. With the increase
2
4
of AgNO
3
3 4
concentration, the diffraction peaks of Ag PO become stronger，indicating the
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crystalline degree of Ag PO is improved gradually. The SEM morphology of Ag PO /Zn (PO )
4 2
3
4
3
4
3
3
composite grains prepared with 0.05M AgNO solution is seen in Fig. 4b. The obtained sample
consists of a large number of hierarchical crystals with an average size of 10-20 ꢀm. The crystal
agglomerates are assembled by octahedron-shaped crystals with a size of 4ꢀm. For the sample
prepared with 0.1M AgNO
with a size of 5 ꢀm and the building crystallites change from micro-sized octahedrons to thin
plates, see Fig. 4c. Further increase of the AgNO solution concentration (0.2 M) results in the
formation of peony-shaped grain aggregates. These are assembled by nano-thick plates or “petals”,
as demonstrated earlier by SEM in Fig. 1b. The morphology of pure Zn (PO crystals as a
reference consist of open and cross-linked interiors, see Fig. 4d. With the increase of reactant
concentration, more Zn (PO and Ag PO crystals are obtained resulting in that flower-shaped
3
solution, hierarchical crystal aggregates evolve into flower-like grains
3
3
4 2
)
3
)
4 2
3
4
composite grains with more “petals”. Increasing petals bring out more exposed crystal faces,
which exhibit the superiority in the process of photocatalysis.
Ag 3d
368 eV
Zn 2p
1022eV
a
b
3
74 eV
1045eV
3
78 376 374 372 370 368 366 364
1060 1050 1040 1030 1020 1010
Binding energy(eV)
Binding energy(eV)
Fig. 3 XPS analysis of the as-synthesized sample, see text.
4 2
To further elucidate the formation process of peony-shaped Ag PO /Zn (PO ) crystal aggregates,
3
4
3
the time dependent morphological evolution was examined by SEM. Corresponding XRD pattern
and SEM images of the samples at different reaction periods are summarized in Fig.3S and Fig
5
a-d. SEM images of the formed precipitates show that band-shaped AgAc (saturated solution)
seen initially (2 min) changes to porous Zn (PO ) at a reaction time of 5 min, compare Fig. 4S
3 4 2
and 5a. After 20 min, tiny Ag PO nanoparticles appear on the surface of these porous Zn (PO )
4 2
3
4
3
3 4
grains (Fig.5b). When the reaction time is prolonged to 1.5h, more and larger Ag PO crystals
6

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grow and the flower-like structure is further pronounced (Fig. 5c). After 4-h reaction time the
image of a peony-like composite grains around 4-8 ꢀm are seen assembled by separated
nano-sheets, see Fig. 5d. With a further extension of the reaction time, the morphology of the
composite remains nearly unchanged, as shown earlier for 5 h in Fig.1d.
Fig.4 XRD patterns and SEM images of the samples prepared with different AgNO
concentration.(a) XRD patterns of Ag PO /Zn (PO composites prepared with different AgNO
concentration and a fixed mole ratio of Ag PO /Zn (PO /KH PO . The diffraction peaks
denoted by “•” symbols are attributed to orthorhombic Zn (PO •4H
O (JCPDS 74−1778 ) and
the other diffraction peaks are attributed to cubic Ag PO (JCPDS 06-0505). (b) and (c)
Corresponding SEM images of Ag PO /Zn (PO ) composites prepared with 0.05M and 0.1M
3
3
4
3
4
)
2
3
3
4
3
4
)
2
2
4
3
4
)
2
2
3
4
3
4
3
4 2
AgNO solution. (d) SEM image of pure Zn (PO ) extracted from the solution before the
3
3
4 2
3 4
formation of Ag PO .
-
To validate the function of Ac for the formation of peony-shaped Ag
3 4 3 4 2
PO /Zn (PO ) composite
3 4
grains, control experiments of two kinds of pure Ag PO crystals were performed. Firstly, when
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Ag PO crystals are prepared by the direct reaction of 20 ml of 0.2M AgNO and 20 ml of 0.1M
3
4
3
KH
2
PO
4
solution the Ag PO
3
4
crystals formed are composed of small particles with a size of
2
00-500 nm and some polyhedron particles with a size larger than 2 ꢀm, as shown in Fig. 6a.
Secondly, after replacing the AgNO solution with an AgAc solution, micron-sized Ag PO
particles with several morphologies form, e.g. spherical, triangular pyramid or cubic as seen in Fig.
3
3
4
6
b. These experiments indicate that flower-like Ag
3
PO
4
could not create without Zn
3 4 2
(PO )
framework. It is also shown that the morphology of Zn
3 4 2
(PO )
grains extracted from the solution
before the formation of Ag
irregular sheet-shaped Zn (PO
reaction of Zn(NO
Fig. 6c. It is clear that the open and porous Zn
3
PO
4
are open and porous frame construction, see Fig. 4d. However,
3
4 2
) crystals with micro-scaled thickness are obtained by the direct
3
)
2
solution (0.15M, 20 ml) and KH
2
PO
4
solution (0.1M, 20 ml), as shown in
-
crystals form in the presence of Ac . For
3
(PO
)
4 2
2
further analysis, an Ag PO /Zn (PO ) composite was prepared with Zn(NO ) as Zn source and
+
3
4
3
4 2
3 2
with other experimental conditions unchanged. The obtained Ag PO /Zn (PO ) sample consists of
4 2
3
4
3
a majority of tetrahedral particles with a size of 15 ꢀm and a small quantity of cubic particles with
a size of 5 ꢀm, as seen in Fig. 6d. The observed morphology is obviously different from that of the
-
composite obtained in the presence of Ac . Therefore, the interaction between
Ag
3
PO
4
/Zn (PO )
3 4 2
-
2+
Ac and Zn on the surface of Zn
3
(PO
4
)
2
is the main driving force for forming porous Zn
(PO crystals originating at these
Zn -binding sites result in the formation of open crystal aggregates, which is essential for the
3 4 2
(PO )
framework. Secondary nucleation and growth of Zn
3
4 2
)
2
+
formation of peony-shaped Ag
3
PO
4
/ Zn (PO
3
)
4 2
composite grains with a large surface area.
8

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Fig. 5 SEM images of samples extracted from the solution for preparing peony-shaped
Ag PO /Zn (PO ) composite at different reaction times. In (a) 5min; (b) 20min; (c)1.5 h; and (d)
3
4
3
4 2
4
h.
In the reaction process, the following reactions (1) and (2) should be involved in the synthesis
process of Ag
3
PO
4
/Zn
3
(PO
4
)
2
composite:
2
+
Zn (aq) + 2H
-
+
3
2
PO
4
(aq) = Zn
3
(PO
PO
(s) ⇋ 2Ag
4
)
2
(s) + 4H
(1)
(2)
(3)
+
Ag (aq) + H
-
+
3
2
PO
4
(aq) = Ag
3
4
(s) + 2H
+
Ag (aq) + Zn
2+
(s) + 3Zn (aq)
6
3
(PO
4
)
2
3
PO
4
Both Ag
transformation reaction occur from Zn
The total equilibrium constant for the transformation reaction is calculated to be 0.459
3
PO
4
and Zn
3
(PO
4
)
2
contain phosphate anions, therefore we should consider whether the
3
(PO (A precipitate) to Ag PO (A precipitate).
4
)
2
3
B
2
3
4
3 4
B
-
16
according to the reaction (3) and the solubility product constants of Ag
3
PO
4
(1.4×10 ) and
-33
3 4 2
Zn (PO ) (9.0×10 ), which means that the spontaneous trend at room temperature is small. A
supplementary experiment was conducted to verify it. At room temperature, 20 ml of 0.15M
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Zn(NO ) solution was mixed with 20 ml of 0.1M KH PO solution to obtain pure Zn (PO )
4 2
3
2
2
4
3
precipitate, then 20 ml of 0.2M AgNO
precipitate did not change its color, which indicates that after Zn
difficult to transform to Ag PO
Fig.5S). It is believed that the formation of the peony-shaped Ag
3
solution was added . After reacting for 2h, the white
3
4 2
(PO ) crystallizes, it is
+
3
4
by ion-exchange process even in the Ag saturated solution (see
3
4 3 4 2
PO / Zn (PO ) composite
involves a coprecipitation process. In the reaction process, the color change of the precipitates
+
can be observed from the photos taken at different time (Fig.6S). Slow release of Ag from
oversaturated AgAc decreases the formation speed of Ag
appear after Zn (PO has precipitated completely
On the basis of the above observations, the detailed formation process of peony-shaped
3 4 3 4
PO , therefore Ag PO precipitates
3
4
)
2
.
Ag
3
PO
4
/Zn
3
(PO
4
)
2
composite undergoes three stages: (1) Oversaturated AgAc precipitates from
-
the solution. (2) Ac -ions induces the formation of an open Zn (PO ) skeleton. (3) The slow
3
4 2
+
release of Ag from supersaturated AgAc offers enough time for the initial formation of a
Zn (PO ) skeleton .After that, Ag PO nucleates on the surface of Zn (PO ) skeleton. Based on
4 2
3
4 2
3
4
3
the above comparative experiments, we believe that the Ag
dissolution, recrystallization, and oriented attachment. With the presence of supersaturated
AgAc, the tiny Ag PO particles formed on the surface of Zn (PO skeleton undergo a crystal
relocation process, which is a thermodynamic process. The larger Ag PO particles continue to
grow and become thinner depending on the dissolution of the smaller Ag PO particles. The
dissolved ions will initiate anisotropic regrowth of nano Ag PO particles on the surface of the
preexisted sheets to form peony-shaped structures. As the process continues and the solution
keep supersaturated, nanosheets continue to grow until small Ag PO particles disappear.
3 4
PO flowers involve a sequence of
3
4
3
4 2
)
3
4
3
4
3
4
3
4
Thereby the uniform flower-like composite were fabricated via a dissolution−recrystallization
process, which is the driving force for the assembling of the hierarchical structure, such as
2
8
29
30
flower-like α-Ni(OH) microspheres and anatase TiO2 , 3D- network polyaniline , porous
2
3
ZnO-ZnSe nanocomposites , and porous CoFe O microspheres .
1
32
2
4
10

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Fig. 6 SEM images of (a) pure Ag
3 4 3
PO crystals prepared with 0.2M AgNO solution (20ml)
and 0.1M KH PO (20ml); (b) pure Ag PO crystals prepared with 0.2M AgAc solution (20ml)
2
4
3
4
and 0.1M KH PO4 (20ml); (c) Zn (PO ) crystals prepared with same volume of 0.15M
4 2
2
3
Zn(NO
3
)
2
solution and 0.1M KH
2
PO
4
solution; (d) Ag
3
PO
4
/Zn
3
(PO
4
)
2
composite prepared with
PO (20ml).
0
.2M AgNO
3
solution (20ml), 0.15M Zn(NO
3
)
2
solution (20ml) and 0.1M KH
2
4
2
.3 Photocatalytic activity
Zn (PO )
4 2
3
Z
n (PO ) /Ag PO conposite
3
4 2
3
4
300
400
500 600
Wavelength(nm)
700
800
Fig. 7
UV-Vis diffusive absorption spectra of porous Zn (PO ) support and as-prepared
3 4 2
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peony-shaped grains of the Ag PO /Zn (PO ) composite.
3
4
3
4 2
The UV-vis diffusive absorption spectra of as-prepared peony-shaped Ag PO /Zn (PO )
4 2
3
4
3
composite grains and the Zn
the region ranging from 250 to 400 nm, corresponding to the absorption of Zn
the absorption in the UV region, the Ag PO /Zn (PO composite can also absorb visible light
with a wavelength range from 420 to 530 nm, which corresponds to the absorption range of
pure Ag PO (Fig. 7Sa). Thus, valence electrons of Ag PO confined in the framework of
Zn (PO can still be excited to the conduction band state by absorbing visible light, which
suggests that the Ag
Sb indicates the bandgap of Zn
3 4 2
(PO ) support are shown in Fig. 7. Both display an absorption in
3
3
3
4 2
(PO ) . Besides
3
4
3
4 2
)
3
4
3
4
3
4 2
)
3
PO
4
/Zn
3
(PO
4
)
2
composite could be used as visible light photocatalysts. Fig.
2
7
3
(PO
4
)
2
is 4.5eV based on the (αhv) -hv curve.
Photoluminescence (PL) spectra of pure Zn
Ag PO /Zn (PO composite were compared to inquire into the migration and separation
efficiency of the photo-generated electrons and holes , as well as the surface vacancies. The
emission peaks of Zn (PO and flower-like Ag PO /Zn (PO composite show similar
characteristic (Fig.8Sa). For Ag PO /Zn (PO ) composite , the intensity of the peaks is weaker
3 4 2 3 4
(PO ) , pure Ag PO and peony-shaped
3
4
3
4 2
)
3
4
)
2
3
4
3
4 2
)
3
4
3
4 2
probably due to the covering of defect sites by Ag PO . For pure Ag PO , the broad PL peak
4
3
4
3
centered at 557 nm is originated from the recombination of photoexcited electrons with holes
3
4
(
Fig.8Sb). The PL peak of Ag
that the combination of photogenerated electrons and holes occurred over Ag
The photodegradation of RhB aqueous solution in the presence of peony-shaped grains of
Ag PO /Zn (PO composite under visible light illumination is shown in Fig. 8a. The
3
PO
4
disappears after the presence of Zn
3
(PO
4
)
2
, which indicates
3
PO is inhibited.
4
3
4
3
4 2
)
absorption peak of RhB at 554 nm decreases gradually and disappears in less than 60 min,
suggesting a complete cleavage of RhB chromophores. Furthermore, the photodegradation
curves of RhB in the form of C/C
photocatalytic performance of Ag
The photocatalytic reaction rate of Ag
complete degradation of RhB dye over pure Ag
4 2
of the dyes is removed over the Ag PO /Zn (PO )
0
as a function of irradiation time are utilized to compare the
PO /Zn (PO with that of pure Ag PO , as shown in Fig. 8b.
PO /Zn (PO is faster than that of pure Ag PO . The
PO crystals takes about 170 min. Close to 100%
composite within 60 min, whereas only 50%
3
4
3
4
)
2
3
4
3
4
3
4
)
2
3
4
3
4
3
4
3
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of the dyes are degraded for pure Ag PO catalyst during the same time duration.
3
4
3 4 3 4 2
To study the stability of the photocatalyst, Ag PO /Zn (PO ) composite grains used in above
degradation process was collected and reused for additional RhB degradation cycles. After
another 4 cycles of photodegradation of RhB solution under same visible light exposure time,
the photocatalytic activity of Ag
3 4 3 4 2
PO /Zn (PO ) composite shows a slight loss, see Fig. 8c.
Therefore, the composite catalyst could be used more than 200 min, which exceeds the stability
3
5
of pure Ag
In order to explore the essential reasons for the enhanced performance produced by Zn
framework for Ag PO , active species trapping experiments were carried out. Three substances,
including Isopropanol, benzoquinone (BZQ), and EDTA-2Na, were added at the beginning of the
3 4
PO .
3
4 2
(PO )
3
4
–
+
catalytic process as the scavengers of •OH, •O
2
and h , respectively. Fig.9S shows the
photocatalytic curves of RhB solution over peony- shaped Ag PO /Zn (PO ) with the presence of
3
4
3
4 2
different quenchers. The results show that the degradation trend of RhB has been completely
inhibited by the addition of BZQ. While about 20 percent of degradation efficiency was lost after
–
+
2
Isopropanol and EDTA-2Na were added, respectively. Consequently, •O , •OH and h involve in
the process of degrading dyes. Based on the previous report, the oxidization of RhB dyes over
+
Ag
electrons. O
Ag) also consumes photogenerated electrons. For common Ag
3
PO
4
mainly depends on the h . Two reactions will occur after the formation of photogenerated
–
2
+
-
2
molecules trap the electrons to produce •O
and photoreduction reaction (Ag + e →
PO -based materials, the second
radicals. For the peony-shaped
3
4
–
reaction dominates, resulting in the failure of producing •O
2
Ag
3
PO
4
/Zn
3
(PO
4
)
2, the effective combination of Ag
3
PO
4
with Zn
3
(PO
4
)
2
by the coprecipitation
+
technology separates the electrons from the Ag , leading to the occurrence of •O
–
2
-related reaction.
PO react
On the other hand, with the assistance of Zn
3
(PO
4
)
2
,partial holes on the surface of Ag
3
4
-
with OH to produce •OH species, which has a strong oxidizing ability to decompose organic
chemicals.
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1.0
0.8
0.6
0.4
0.2
0.0
1.0
.8
.6
.4
0.2
.0
Zn (PO ) /Ag PO
4
0
min
min
3
4
2
3
a
b
2
pure Ag PO
4
4min
0
3
10min
14min
20min
24min
32min
36min
40min
44min
52min
60min
0
0
0
0
50
100
150
200
300
400
500 600
Wavelength(nm)
700
800
t(min)
1.0
0.8
0.6
0.4
0.2
0.0
1run
2run
3run
4run
c
0
50
100
150
200
250
300
Irradiation time(min)
Fig. 8 (a) The photodegradation of RhB aqueous solution in the presence of peony-shaped grains
of Ag PO / Zn (PO ) composite under visible light illumination. (b) The photodegradation curves
3
4
3
4 2
0
of RhB in the form of C/C as a function of irradiation time. (c) Cycling runs in the
3 4 3 4 2
photodegradation of RhB in the presence of used Ag PO / Zn (PO ) composite under visible-light
irradiation, cf. text.
The analysis of band structure of Zn
(PO are calculated to be 0.59 and 5.09eV, respectively (The calculation was
described in Support Information), therefore Zn (PO can accept the photogenerated electrons
from Ag PO
According to the detailed investigation, three factors are accounted for the excellent
3 4 2 3 4
(PO ) and Ag PO are illustrated in Fig.9. The ECB and
E
VB of Zn
3
4 2
)
3
4 2
)
3
4
.
performance of peony- shaped Ag
3
PO
4
/Zn
3
(PO
4
)
2
. Firstly, the specific surface area of peony-
2
shaped Ag PO /Zn (PO ) is 28.20 m g , which is higher than that of pure Ag PO (13.71 m
−1
2
3
4
3
4 2
3
4
−1
g ) and helps to increase the photocatalytic reaction sites. Secondly, three active species with
–
+
high oxidizing ability, concluding •O , •OH and h , involve in the process of degrading dyes.
2
Lastly, photogenerated electrons can transferred from the conduction band of Ag
3 4
PO to that of
3
4
)
2
3
4 2
Zn (PO . The special microstructure regulated by Zn (PO ) promotes the quick migration of
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DOI: 10.1039/C5RA05063D
carriers, leading to the improvement of the separation efficiency of photo-generated electrons
and holes.
Fig.9. Schematic diagram of the photodegradation mechanism of peony- shaped Zn (PO ) /
3 4 2
3 4
Ag PO under visible light.
3
Experimental
3
.1 Materials
All the reagents utilized in the present work were of analytical grade and were used without
further purification.
3
.2 Synthesis
The preparation procedure takes place in water solution. 20 ml of 0.2M AgNO
drop-wise to 20 ml of 0.15M Zn(Ac) at room temperature. The obtained mixture (containing
supersaturated AgAc) was stirred for 1h. Thereafter 20 ml of 0.1M KH PO solution was added
and a great amount of white precipitate formed immediately. The color of the precipitate changed
3
was added
2
2
4
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slowly from white to yellow during prolonged reaction time for 3h. Afterwards, the solution was
transferred to a closed steel autoclave and heated at 110℃ for 2h. Subsequently, the precipitate
was collected by centrifugation, thoroughly rinsed with distilled water and absolute ethanol before
drying at 60℃ for 6 h. As a reference pure Ag
mixing 20 ml of 0.1M KH PO with 20 ml of 0.2M AgAc.
For exploring the formation of the obtained peony-shaped Ag
different AgNO concentrations were used (0, 0.05, 0.1 and 0.2M) while keeping the same mole
ratio of AgNO , Zn(Ac) and KH PO
3 4
PO crystals were prepared in a similar way by
2
4
3 4 3 4 2
PO /Zn (PO ) composite grains
3
3
2
2
4
.
3
.3 Characterization
Powder X-ray diffraction (XRD) was carried out with a Rigaku D/max-2400 powder X-ray
diffractometer. The morphologies of the as-synthesized samples were observed using
JSM-5610LV scanning electron microscope (SEM) and Hitachi H-800 transmission electron
microscope (TEM). X-ray photoelectron spectroscopy (XPS) was performed using an
ESCALABMK II X-ray photoelectron spectrometer. UV-Vis diffusive absorption spectra were
recorded on a Shimadzu UV2550 spectrophotometer. PL study was carried out on an Opticon 2
Luminescence Spectrometer with the excitation wavelength at 325 nm.
3
.4 Photocatalytic experiments
The photocatalytic activity was investigated through the decomposition of a Rhodamine B (RhB)
aqueous solution at room temperature. The reaction system, containing 100 ml of RhB solution
-5
with an initial concentration of 10 M and 50 mg of the tested photocatalysts, were placed in a
cylindrical quartz vessel to record the absorption spectrum. Prior to irradiation, the reaction
system was magnetically stirred in darkness for 0.5h to reach equilibrium. A visible light
irradiation (λ＞400nm) through the quartz vessel was provided by a 150W Xe-lamp with
UV-light filter. A sequence of UV-Vis spectra were collected at fixed time intervals to measure the
RhB degradation by time.
4
Conclusions
In this paper, we present the first report on coprecipitation and recrystallization technology for
16

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-
the fabrication of peony-shaped Ag PO /Zn (PO ) composite grains. The Ac ion is the key
3
4
3
4 2
factor for ruling the initial precipitation of peony-shaped grains as it induce the formation of an
+
open Zn
AgAc precipitate that slows the formation rate of Ag
well-distributed Ag PO upon the framework surfaces of the host. Compared with pure Ag
crystals, the obtained Ag PO /Zn (PO composite exhibits excellent visible-light-driven
photocatalytic activity for the decomposition of RhB aqueous solution. This work illustrates a
new way for fabrication of Ag PO containing composites from pure inorganic components and
3
(PO
4
)
2
support. The support also reacts with excess of free Ag -ions in water from
3
PO
4
, leading to the formation of
PO
3
4
3
4
3
4
3
4 2
)
3
4
promotes the practical applications of this catalyst in addressing environmental and energy
issues.
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (No.
5
1402097) , Natural Science Foundation of Hubei Province of China (112-45111811) and
Fundamental Research Funds for the Central Universities (WUT: 2015IB002).
The authors are grateful to Prof. Thommy Eirkssion for his careful work on revising and
polishing this paper.
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