Photocatalytic degradation of 4-chlorophenol over Ag/MFe2O4 (M = Co, Zn, Cu, and Ni) prepared by a modified chemical co-precipitation method: A comparative study

Article abstract of DOI:10.1039/c5ra04608d

In this study, MFe₂O₄ (M = Zn, Cu, Co and Ni) nanoparticles were prepared by a modified chemical co-precipitation method followed by annealing. Then, Ag/MFe₂O₄ nanoparticles were synthesised via a wetness impregnation method. The structural properties of the samples were systematically investigated by XRD, SEM, TEM, EDX, DRS, BET, SPV and FT-IR techniques. The photocatalytic performances of the prepared MFe₂O₄ and Ag/MFe₂O₄ samples were comparatively studied by the photodegradation of 4-chlorophenol under xenon lamp illumination in a FTIR reaction cell. The photodegradation rate of 4-chlorophenol was 90% for the Ag/CoFe₂O₄ sample. The cheap cost and easy synthesis of the photocatalysts indicate the potential for a better thermal disposal of chlorophenol pollutants at lower temperatures.

Full text of DOI:10.1039/c5ra04608d

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2 4
Photocatalytic Degradation of 4-Chlorophenol over Ag/MFe O (M= Co, Zn, Cu,
and Ni) Prepared by a Modified Chemical Co-precipitation Method:
A Comparative Study
ab*
b
a
b
b
Zhengru Zhu , Feiya Liu , Hongbo Zhang , Jinfeng Zhang and Lu Han
a
Key Laboratory of Subsurface Hydrology and Ecological Effect in Arid Region of
Ministry of Education
b
Research Center of Hydrology and Water Source, School of Urban and
Environmental Science, Liaoning Normal University, Dalian, 116029, China.
*
Corresponding author.
Tel.: +86ꢀ411ꢀ8215ꢀ8364.
Fax: +86ꢀ411ꢀ8215ꢀ8542
Eꢀmail:zhengruzhu@gmail.com
1

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Abstract
In this study, MFe
2
4
O (M= Zn, Cu, Co and Ni) nanoparticles were prepared to use a
modified chemistry coꢀprecipitation methods followed by annealing, respectively.
Then, Ag/MFe O nanoparticles were synthesised via a wetness impregnation method.
2
4
The structural properties of the samples were systematically investigated by XRD,
SEM, TEM, EDX, DRS, BET, SPV and FTꢀIR techniques. The photocatalytic
2 4 2 4
performances of the prepared MFe O and Ag/MFe O samples were comparatively
studied by the photodegradation of 4ꢀChlorophenol under xenon lamp illumination in
FTIR reaction cell. The photodegradation rate of 4ꢀChlorophenol in photocatalytic
2 4
process was 90% on Ag/CoFe O sample. The cheap cost and easy synthesized
process of the photocatalysts indicate a better potential for the thermal disposal of
chlorophenols pollutants at lower temperatures.
Keywords: 4ꢀChlorophenol; Photocatalytic oxidation; Coꢀprecipitation methods;
Characterization methods; Surface photovoltage.
2

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1
. Introduction
Over the past decades, chlorophenols represent an important organic pollutant of
water pollutants [1]. They are listed one of 129 priority pollutants by U.S. EPA [2]. In
950s, 4ꢀchlorophenol (4ꢀCP) was a typical pollutant among these chlorophenols,
1
which has used as disinfectants, antiseptics and herbicides, and has also applied in the
largeꢀscale disinfection of drinking water [2ꢀ6]. It can result in negative effects such
as carcinogenic, teratogenic, mutagenic and acute toxicity. So it is difficult to dispose
these pollutants by usual treatment methods. Compared to the conventional several
treatment processes, photocatalysis is a effective method which transformed
chlorophenol organic compounds into small inorganic moleculars.
2
TiO is known as an efficient photocatalyst because of its nontoxicity, lowꢀcost,
and photostability, which has led to many promising applications in photovoltaics [7],
photocatalysis [8], photoꢀelectrochromics and sensors [9]. J. Theurich et al. reported
that TiO ꢀbased materials showed preeminent photocatalytic activities in the
2
photodegradation of 4ꢀCP under UVꢀirradiation [10]. P. Rangsunvigit et al. reported
the addition of Pt in TiO
was obtained with 1%Pt/TiO
0%SiO ꢀTiO which has highest adsorption capacity [11]. However, TiO
2
2
can improve the catalytic activity and the highest activity
2
. For TiO ꢀSiO , the highest activity was achieved with
2
2
1
2
2
nanomaterials normally are transparent in the visible light region. By developing new
visible light inducted catalysts, it is possible to satisfy a sustainable energy demands.
2 4
Recently, some composite oxides such as spinel AB O had better selectivity and
sensitivity to certain gases than singleꢀmetal oxides . Nanoparticles of spinel ferrites
3

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MFe
have great interest for addressing relationship between physical chemistry properties
and their crystal structure [12]. Some synthetic methods to prepare MFe ferrites
2 4
O (M = Zn, Co, Ni, Cu etc.) are becoming a new type catalyst which people
2
O
4
such as solꢀgel [13], microꢀemulsion [14, 15], citrate gel [16], precipitation [17],
precursor techniques [18ꢀ20] host template [21], and mechanical alloying [22] have
been developped. The chemical coꢀprecipitation method is more suitable to prepare
nanostructures oxides because its simplicity and good control of grain size. Veronica
B. et al. have reported superparamagnetism and interparticle interactions in ZnFe
2 4
O
nanocrystals [23]. The method could be also extended to synthesize CoFe O ,
2
4
CuFe
NiFe
temperature, while present finite coercivity of 353 Oe at 5 K. S. Saadi et al. [25] and
H. Yang et al. [26] studied photoassisted hydrogen evolution over CuFe , and they
found CuFe O exhibited a promising photocatalytic activity. S. Li et al. [27] have
2
O
4
and NiFe
2 4
O nanocrystals. J. Jiang et al. [24]reported nanostructured
2
4
O via a refluxing route exhibited a typical superparamagnetic behavior at room
2
O
4
2
4
studied the fabrication and characterization of TiO
2
/BaAl
2
O
4
. J. Okal et al. [28] have
nanoparticles.
which
reported the catalytic combustion of methane over Ru/ZnAl
2
O
4
C.Ragupathi et al. [29] have studied the photocatalytic activities of ZnAl
prepared by the microwave method.
2 4
O
The supported noble metal catalysts have better activities and handling properties.
Moreover, the synthesized conditions are mild . The noble metal/semiconductor oxide
composites are greatly attractive because of their enhanced efficiency of
photocatalytic activity. It was reported that the addition of noble metals Pt, Au, Rh in
4

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2
TiO can enhance photocatalytic efficiency [30]. An efficient support, including
activated carbon [31], silica [32], alumina [33] and zeolite [34], could improve the
activity, recycling, and selectivity of Ag catalyst systems. Recently, Yang Liu et al.
[
35] reported that Ag nanoparticles in the TiO mesoporous were not only an
2
antimicrobial but also an intensifier for photocatalysis. Jia Ren et al. [36] reported
Agꢀloaded Bi WO photocatalysts exhibited significantly increase in photocatalytic
2
6
activity in inactivating E. coli, a Gramꢀnegative bacterium, and S. epidermidis, a
Grampositive bacterium under visible light irradiation (λ >420 nm). Recently, spinel
type oxides catalysts have been used as supports for Ru and Pd and a distinct
metalꢀsupport interaction have been found [37, 38]. However, the photocatalytic
properties and the activities of these spinelꢀtype oxides for degradation of 4ꢀCP have
not been reported.
2 4
In this work, MFe O (M = Zn, Co, Ni and Cu) nanoparticles with brickꢀlike spinel
superstructure were prepared by a modified chemical coꢀprecipitation method along
with post calcinations at 673 K, 773 K and 873 K, and then 1 % wt. noble metal Ag
loaded on the MFe
2
O
4
(M = Zn, Co, Ni and Cu) (673K) by a wetness impregnation
nanomaterials. These MFe and Ag/MFe
method, which produced Ag/MFe
2
O
4
2
O
4
2 4
O
catalysts were studied the photocatalytic degradation of 4ꢀCP under xenon lamp
irradiation . The photocatalytic mechanism is discussed, thereby pointing out the way
of environmental purification need new active catalysts.
2
. Experimental section
5

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2
.1 Materials and methods
The MFe (M = Zn, Co, Ni and Cu) catalysts with brickꢀlike superstructure were
prepared by a coꢀprecipitation method. The starting materials were FeSO ·7H O and
MSO ·nH O (M= Zn, Co, Ni and Cu). 0.1 M (100 mL) solution of FeSO ·7H O and
2
O
4
4
2
4
2
4
2
0
4 2
.2 M (100 mL) MSO ·nH O were mixed in deionized water. 0.3 M (25 mL) solution
of Na was prepared and added to the salt solution. The solution was kept
2 2 4
C O
continuous stirring under 353 K and allowed to cool slowly until room temperature. A
ferrioxalate precipitate appeared. Then the precipitate was washed twice by distilled
water and ethanol, respectively. The separated precipitate was dried overnight at 373K.
The precursor was divided into three portions and calcined at 673K, 773K and 873K
for 2 h, respectively. Consequently, MFe
produced. The samples were named in Table 1, respectively.
The catalyst of MFe (M= Zn, Co, Ni and Cu) (calcined at 673K) supported Ag
2 4
O (M = Zn, Co, Ni and Cu) samples were
2
O
4
was prepared by an impregnation strategy [39]. AgNO solution was used as the
3
precursor. The silver loading was 1 wt%. After evaporation, the catalyst was then
◦
dried at 100 C for 12 h and calcined at 673K in air for 6 h, finally, the catalyst was
slowly cooled until room temperature. Then Ag/MFe
samples were named in Table 1, respectively.
2 4
O (M = Zn, Co, Ni and Cu)
All chemicals in this work were analytical grade reagents and used as starting
materials without further purification.
2
.2 Characterizations of catalysts
The crystal structures of the MFe
2
4 2 4
O and Ag/MFe O (M = Zn, Co, Ni and Cu)
6

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samples were examined by Xꢀray diffraction (XRD, Rigaku D/max) with Cu Ka
◦
radiation (λ = 0.15418 nm) over the 2θ range of 20ꢀ80 . The surface area was obtained
2
through the Brunauer–Emmet–Teller (BET), and N gas adsorptionꢀdesorption
isotherms were determined by a Micromeritics ASAPꢀ2000 equipment. Pore volume
was calculated by the BJH model. The light absorption properties were measured
using a UVꢀVis diffuse reflectance spectrophotometer (JASCO, UVꢀ550) with a
wavelength range of 200–800 nm. The morphology of these prepared samples was
characterized by scanning electronic microscopy (SEM) with a JSMꢀ6700 LV and
transmission electron microscope (TEM, Hitachi 800 system at 200 kV). Energy
dispersive Xꢀray analysis (EDX, Horiba 7593 H) was performed to determine the
elemental concentration distribution on the sample. Fourier transform infrared (FTIR,
−
1
BRUKER VERTEX 70) spectrum was recorded in the range 4000–450 cm with 4
−1
cm resolution. XPS data were obtained by using a PerkinꢀElmer PHI 5600 electron
+
spectrometer which using achromatic Al Kα radiation (1486.6 eV) with Ar sputtering
to remove the surface layer of the sample.
The characteristics of photogenerated charge carriers on spectrum was examined by
a lockꢀinꢀbased surface photovoltage (SPV) measurement system. The system consists
of a monochromatic light by a monochromator (Omniꢀλ3005) and a xenon lamp (500
W), a photovoltaic sample cell and a lockꢀin amplifier (SR830ꢀDSP) with an optical
chopper (model SR540). The AC photovoltage signal of the sample was obtained
using a capacitor structure. The highest intensity monochromic light on spectrum was
−2
less than 80 ꢁW cm . The effective overlapping area of the two electrodes is about 1
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2
cm for test. The corresponding phase spectra were taken with the SPV spectra by a
computer. The SPV measurement was carried out in ambient environment and the raw
data were used directly.
2
.3 Measurements of photocatalytic activities
The photodegradation of 4ꢀchlorophenol was performed under an xenon lamp
2
（
XQꢀ500W）with an intensity of 40 mW/cm . The reaction system was shown in
6 4
selfꢀmade photochemical reactor [40]. The initial concentration of the 4ꢀCP (C H ClO)
−
1
solution was 20 mg L in the test. Reaction suspensions are prepared by adding 0.01
g catalyst powder into 4ꢀCP solution (100 mL). The reaction suspension was stirred in
a magnetic stirrer in the dark for 30 minutes to reach an adsorption/desorption
equilibrium. Then the test was performed under the xenon lamp irradiation. After a
given irradiation time, the 4.0 mL samples are withdrawn, and the collected samples
are centrifuged at 6000 rpm for 5 minutes in order to separate the photocatalyst from
the 4ꢀCP solution. The absorption spectra of 4ꢀCP are measured by a UVꢀVis
spectrophotometer.
3
. Results and discussion
3
.1 XRD analysis
From Fig. 1a, the undoped NiFe O samples Ni1, Ni2, Ni3 and Agꢀdoped NiFe O
4
2
4
2
sample Ag/Ni can be assigned to the inverse spinel structure, according to the
standard XRD patterns (JCPDS Patterns No. 21ꢀ1272). The peaks with 2θ values of
3
0.6°, 36.0°, 44.0°, 57.8° and 63.2° were correspond to the crystal planes (220), (311),
8

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2 4
(400), (511), and (440) of crystalline NiFe O , respectively.
The crystallite size was calculated by the Scherrer equation:
D = 0.89λ/βbcosθ
Where D is the average crystal size, λ is the wavelength of Cu Ka (0.15406 nm), β
is the full width at half maximum, θ is the diffraction angle [41].
The average crystallite size calculates from the (311) peak of the crystal plane using
Scherrer’s equation. In this case the average crystallite sizes were 8 nm for Ni1, 10
nm for Ni2, 14 nm for Ni3 and 8 nm for Ag/Ni (See Table 1).
Figure 1b shows the XRD patterns of undoped CoFe O samples Co1, Co2, Co3
2
4
and Agꢀdoped CoFe
product is CoFe
values of 30.7° (with index number of 220), 36.3° (311), 43.6° (400), 57.9° (511), and
3.2° (440) reveal the formation of CoFe . The average crystallite sizes of these
2
O
4
sample Ag/Co samples. The results indicate that the final
2
O with the expected inverse spinel structure. The peaks with 2θ
4
6
2 4
O
samples are about 12 nm (Co1), 14 nm (Co2), 16 nm (Co3), and 11 nm (Ag/Co),
respectively.
The XRD patterns of undoped CuFe
2 4
O samples Cu1, Cu2, Cu3 and Agꢀdoped
CuFe sample Ag/Cu samples are shown in Fig. 1c. In the XRD pattern of Cu1 and
2
O
4
Ag/Cu, the phase of copper ferrite appears, and the diffraction angle and intensity of
the characteristic peaks are well consistent with that of the standard JCPDS card NO.
3
4ꢀ0425. And the copper ferrite with spinel crystal structure phase can be directly
formed after calcination (673K), and it does not require other synthesis techniques
[
2 4
42]. There exists a small amount of CuO impurity phase in the asꢀobtained CuFe O
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with increasing of the calcination temperature (773K and 873K). From XRD data of
Fig. 1c, the average sizes of these samples are about 26 nm (Cu1), 22 nm (Cu2), 21
nm (Cu3), and 21 nm (Ag/Cu), respectively.
Figure 1d shows the XRD patterns of asꢀsynthesized precursor for ZnFe O and
2
4
Zn1, Zn2, Zn3 and Ag/Zn samples. It was noted that the diffraction peaks of
asꢀsynthesized precursor are not the phase of zinc ferrite. The observed diffraction
peaks from (220), (311), (400), (422), (511) and (440) crystal planes are well matched
with the cubic spinel structure of ZnFe
XRD data of Fig. 1d, the average sizes of these samples are about 14 nm (Zn1), 15 nm
Zn2), 18 nm (Zn3), and 13 nm (Ag/Zn), respectively. The average crystallite size of
Ag/MFe sample seems smaller than the undoped MFe The phenomenon was
. Some of the MFe
2 4
O phase (JCPDS card no. 22ꢀ1012). From
(
2
O
4
2 4
O .
probably by reason of the reaction between MFe
particles were destroyed into smaller ones [43].
2
O
4
and HNO
3
2 4
O
The crystallinity will increase with the rise of the calcining temperature. Though, a
little CuO and Fe phase existed in the powders which calcined below 773 K [44].
The augmentation in the calcination temperature of CuFe sample led to a decrease
in the degree of ordering and crystallite size of both CuO and Fe phases. This
solids
2
O
3
2
O
4
2
O
3
decrease may be due to the solidꢀstate reaction between CuO and Fe
yielding nanocrystalline CuFe O catalyst [45].
2 3
O
2
4
From Fig. 1, the peaks with Ag species can’t be observed due to the low doping
content of Ag in the samples. And also with the increasing of calcination temperature,
2 4
the peak intensity of MFe O samples increases. It indicates that calcining
1
0

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temperature plays a role in the formation of spinel crystal structure and morphology.
The average crystallite size of Ag/MFe sample seems smaller than undoped
MFe sample. The results are consistent with the description by X. Li et al. reported
46].
The specific surface area of the MFe
Table 2. The surface area of Ag/Co, Ag/Zn, Ag/Ni, Ag/Cu is 187, 166, 153.5 and
2
O
4
2
O
4
[
2
4 2 4
O and Ag/MFe O samples are showing in
2
−
1
1
42.3 m g , respectively. The Ag/CoFe O mesoporous has the highest specific
2 4
surface area. And the pore volume of Ag/Co, Ag/Zn, Ag/Ni, Ag/Cu is 0.12, 0.162,
3
ꢀ1
0
.181 and 0.20 cm g . The results shows that the specific surface area of the samples
which loading silver are greater than others. With the increase of the specific surface
area, the pore volume will be reduced.
3
.2 SEM and TEM analysis
For Fig. 2, the SEM images show the morphology of Co1 (Fig. 2a), Co2 (Fig. 2c),
Co3 (Fig. 2b), and Ag/Co (Fig. 2d). For Fig. 3, the SEM images show the morphology
of Ni1 (Fig. 3a), Ni2 (Fig. 3c), Ni3 (Fig. 3b) and Ag/Ni (Fig. 3d). The CoFe and
NiFe powders calcined at different temperatures are completely composed of
timber”. Fig. 2e shows the TEM micrograph of Ag/Co. Fig. 3e and 3f shows the
2
O
4
2
O
4
“
TEM micrograph of Ag/Ni. Both the porous blocks (Ag/Ni and Ag/Co) were formed
through the agglomeration of numerous Ag/Ni and Ag/Co nanoparticles, respectively.
Figure 4 presents the TEM images of the Cu1 and Ag/Cu samples. It is found
that the blocks (Fig. 4a) are formed through the agglomeration of numerous
2 4
nanocrystalline CuFe O particles. Fig. 4b shows the TEM micrograph of the Ag/Cu
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sample. The nanoparticles distinctly exhibit narrow size distribution with size ranging
from 20 to 30 nm. Figure 5 presents the TEM images of the Zn1 and Ag/Zn samples.
It is found that the blocks (Fig. 5a) are formed through the agglomeration of numerous
nanocrystalline ZnFe O particles. Figure 5b shows the TEM micrograph of the
2
4
Ag/Zn sample. The nanoparticles distinctly exhibit narrow size distribution with size
ranging of 20 nm. These results are in good agreement with the XRD analysis.
From the above images, we cannot distinguish the Ag species in the samples. So
the EDS patterns of the prepared Cu1 and Ag/Zn were measured. For Fig. 4c, the EDS
spectrum exhibits Cu, Fe and O peaks (Fig. 4c) and also reflects the atomic ratio of
Cu, Fe and O in the prepared Cu1 sample (Table 3). For Fig. 5c, the peaks with
metallic silver are identified by EDX patterns of the Ag/Zn sample. Table 4 exhibits
the atomic ratio of the elements in the prepared Ag/Zn sample.
3
.3 DRS analysis
The UVꢀVis spectra of these MFe O and Ag/MFe O (M= Zn, Co, Ni and Cu)
2
4
2
4
2 4 2 4
samples were shown in Fig. 6. These MFe O and Ag/MFe O samples have strong
absorption in the range of whole spectrum (200ꢀ800 nm). As shown in Fig. 6, we
clearly observed the characteristic bands of absorption spectra at 500–700 nm. More
widen and strong absorption spectra are found for Ag/MFe
Due to the mesoporous structure, the photoꢀgenerated electronꢀhole pairs in the
Ag/MFe nanocrystal are separated via diffusion, mainly. With the improved
separation efficiency of photoꢀinduced electron–hole pairs into charge carriers, the
SPV response and photocatalytic activity of Ag/MFe are higher than MFe
2 4
O samples.
2
O
4
2
O
4
2 4
O
1
2

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samples. Moreover, due to that the Fermi level of Ag (ca. ꢀ 4.3 eV) [47] is much lower
than the conduction band edge of the MFe catalysts in electronic energy [48], the
Ag species probably adsorbed onto the surface of MFe play a key role in trapping
and transporting the photoꢀgenerated electrons. Therefore, the Ag/MFe O sample
2
O
4
2
O
4
2
4
exhibits the higher photocatalytic activity than MFe
For the Ag/CoFe sample, the absorption intensity is visible stronger than other
sample may have a better absorption capability of visible
2 4
O .
2
O
4
samples. So the Ag/CoFe
light.
2 4
O
3
.4 FTIR analysis
The FTIR spectra of these MFe
samples were seen in Fig. 7. The main absorption bands γ1 and γ2 around 600 and
2
4 2 4
O and Ag/MFe O (M= Zn, Co, Ni and Cu)
−1
4
00 cm are attributed to the stretching vibration of the tetrahedral and octahedral
sites, respectively, which are responsible for the formation of MFe [49]. And the
band at 580 cm is attributed to the FeꢀO bond vibration of the samples [50, 51].
2
O
4
−
1
−
1
−1
−1
The peaks with 3455 cm , 1340 cm and 1610 cm can be assigned to the
stretching vibrations of hydrogenꢀbonded surface water molecules and hydroxyl
−
1
groups [52]. 1185 cm is attributed to CꢀO stretching vibration mode [53]. For Fig.
−1
−1
7
c and 7d, the peak around 1000 cm (930 and 1030 cm ) corresponds to C–H
stretch vibration in C=C bonds [54, 55].
3
.5 XPS analysis
The XPS analysis of the MFe
2
O
4
(M= Zn, Cu, Co and Ni) and Ag/MFe
2
O
4
samples
and
are shown in Fig. 8ꢀ11. Fig. 8 shows the high resolution spectra of ZnFe
2
O
4
1
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Ag/ZnFe
2 4
O samples. Fig. 8a shows the Zn peaks with Zn 2p3/2 and Zn 2p1/2 at
1
023.4 eV and 1047.9 eV, respectively. Fe peaks with Fe 2p1/2 and Fe 2p3/2 at 728.9
eV and 715 eV, and O peak with O 1s at 534.1 eV are showing in Fig. 8bꢀc. Which
indicate that the existing of ZnFe O sample. Fig. 8d shows the spectra of the Ag peak
2
4
with Ag 3d 3/2 at 372.1 eV. No contaminant species were observed within the
sensitivity of the technique.
Fig. 9 shows the high resolution XPS spectra of Co 2p, Fe 2p, O 1s and Ag 3d
respectively. In fig. 9a, two peaks appeared in 780.31 eV and 788.2 eV, which are
attributable to Co 2p3/2. Form fig. 9b, the peaks of Fe 2p3/2 and Fe 2p1/2 are located
in 710.98 eV and 724.25 eV. Fig. 9c shows the XPS spectrum of C1s at 530.5eV.
These peaks indicate the existing form of the CoFe
spectra of the Ag peak with Ag 3d 5/2 at 368.8 eV.
2 4
O sample. Fig. 9d shows the XPS
2 4 2 4
Fig.10 shows the XPS spectra of NiFe O and Ag/NiFe O samples. The peak
located at 860.2eV is attributed to the Ni 2p 3/2 as shown in fig. 10a. Form fig. 10b,
the peaks of Fe 2p3/2 and Fe 2p1/2 are located in 710.9 eV and 725.2 eV. Fig. 10c
represents the XPS spectra of O 1s with a single peak at 530 eV. The spectrum show
that the Ag 3d 3/2 is approximately at 372.6 eV in fig. 10d.
From fig. 11, it can be clearly noticed that the XPS spectra of CuFe
2 4
O and
Ag/CuFe O samples. Fig. 11a shows the peaks of Cu with Cu 2p3/2 at 936.7 eV. The
2
4
Fe2p shows two peaks at 728.9 and 715.7eV which are attributable to Fe 2p1/2 and Fe
p3/2 binding energy. Fig. 11c shows O peak with O 1s at 534.5 eV. Fig.11d shows
the XPS spectra of the Ag peak with Ag 3d 3/2 at 372.8 eV.
2
1
4

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3
.6 SPV analysis
Figure 12 shows the SPV amplitude and phase spectra for these MFe
Ag/MFe (M= Zn, Co, Ni and Cu) samples. The Ag/MFe samples exhibit more
distinguished SPV response than those MFe O samples. The higher SPV signal may
2
4
O and
2
O
4
2 4
O
2
4
implies the higher separation rate of photoꢀgenerated charge carriers [56]. From Fig.
2b, 12d, 12f and 12h, the SPV phase values for these Ag/MFe samples below 600
nm are about 90°. It indicates that the photoꢀgenerated electrons are accumulated to
the surface of sample [57]. It could be observed by comparison that MFe samples
have smaller phase retardation. The electronic band gaps of the dispersed nanocrystals
in the “timberꢀlike” aggregation of Ag/MFe would improve the redox activity of
their photoꢀgenerated electrons and holes. For Fig. 12g, the SPV spectrum of
Ag/ZnFe sample has a 50 nm blue shift with respected to ZnFe . Moreover, the
SPV signal of Ag/CoFe samples is significantly larger than CoFe and other
1
2 4
O
2
O
4
2
O
4
2
O
4
2 4
O
2
O
4
2 4
O
samples (Fig. 12a). For Ag/CoFe O , the efficiency of charge separation is higher than
2
4
2 4
other samples, so that the Ag/CoFe O higher photocatalytic activity.
3
.7 Comparison on the photocatalytic activities of the MFe
samples
Figure 13 shows that the comparison on the photocatalytic activities of the MFe
2
O
4
and Ag/MFe
2
O
4
2
O
4
and Ag/MFe O (M=Co, Zn, Ni and Cu) samples is studied by the photodegradation
2
4
of 4ꢀCP solution under xenon lamp illumination. After 120 minutes, the 4ꢀCP solution
cannot be photodegraded under xenon lamp irradiation without any catalyst, while the
4
2 4 2 4
ꢀCP solution can be photodegraded by the MFe O and Ag/MFe O samples,
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5

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2 4
respectively. The degradation ratios of the 4ꢀCP solution with the MFe O and
Ag/MFe
photodegradation ratio of the 4ꢀCP solution with the Ag/CoFe
And the photodegradation ratio with the Ag/ZnFe O , Ag/NiFe O , Ag/CuFe O
4
2
O
4
samples are shown in Table 1 and Fig. 13a. The results show that the
2
O
4
sample is up to 90%.
2
4
2
4
2
samples are 88%, 86%, 81%, respectively. So it implies that the Ag/CoFe
has the higher photocatalytic activities than other samples.
2 4
O sample
There are many influence factors which have an impact on decomposition rate,
such as the following: (1) The MFe nanoparticle absorbs photon energy greater
2
O
4
than the band gap energy and generates photoꢀgenerated electron−hole pairs in the
bulk. (2) The photoexcited carriers separate and move to the surface without
+
recombination. (3) Water molecules (or H ) are reduced and oxidized by the
2 2
photoꢀgenerated electrons and holes to produce H and O , respectively [58]. (4)
Redox reactions with adsorbed reactants [59]. Metal properties have some influence,
but these factors are more important than the nature of the metal on the degradation
efficiency, obviously.
According to XRD study, the crystallite size of Ag/MFe
2 4
O sample is smaller than
that of the MFe O sample. The Ag/MFe O sample has more hydrogenꢀbonded，
2
4
2
4
surface water molecules and hydroxyl groups adsorbed at its surface due to its smaller
crystallite size. The radical groups have been shown to be capable of contributing to
the oxidation process of organic substances. With the decreasing of the particle size,
the catalytic activity of the Ag/CoFe
2
O
4
sample is enhanced because of the widened
sample for
band gap. Moreover, Ag dopants makes the induction of Ag/MFe
2 4
O
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6

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2 4 2 4
visible light gets more intense from the DRS analysis of the MFe O and Ag/MFe O
(M=Co, Zn, Ni and Cu) samples. And also small particles of the semiconductor
became almost totally depleted upon immersion in the electrolyte (allowing for large
photovoltages) [60].
We learned that there are many loading quantity through reading literatures, such as
0
.05wt% [61], 0.98wt%, 1.3wt% [62], 1.5wt% [63] 3.2wt% [64] and 12wt% [65]. It is
concluded that the sample which Ag loading 1wt% has the strongest adsorption
capacity through experiments. The loading of Ag are not only an antimicrobial but
also an intensifier for photocatalysis. Three factors are ascribed to the excellent
2 4
antibacterial ability of the mesoporous Ag/MFe O sample. First, it has high specific
surface area and small particle size, which would result in better activity. Second, Ag
nanoparticles are an excellent antimicrobial material even though a tiny amount of Ag
doped in the pores of Ag/MFe
nanoparticles could act as trapping center for photoꢀgenerated electrons in the
conduction band of MFe , leaving the holes in the valence band of MFe and
2 4
O (Ag/M<0.01, mole ratio). Furthermore, the Ag
2
O
4
2 4
O
inhibiting the recombination of electrons and holes. These factors will lead to the
improved quantum efficiency for photochemical reaction [66, 67]. So it implies the
Ag/CoFe
other samples.
.8 The stability and reusability of the MFe
Ni) catalysts.
We also completed the stability experiments of the MFe
2 4
O sample calcined at 673K shows the enhanced photocatalytic activity than
3
2 4 2 4
O and Ag/MFe O (M= Co, Zn, Cu, and
2
4 2 4
O and Ag/MFe O
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samples by five reaction cycles under the same xenon lamp illumination test condition.
The photodegradation ratio of the 4ꢀCP over Ag/CoFe was shown in fig. 14a and it
2
O
4
is up to 75.3% after the fifth repetition, which is slightly lower than that (90%) with
the fresh Ag/CoFe O catalyst. And the photodegradation ratio of the Ag/ZnFe O ,
2
4
2
4
Ag/NiFe
5.2%, respectively. The results show that there is good stability and reusability for
the MFe and Ag/MFe catalyst.
2 4 2 4
O , Ag/CuFe O samples which after the fifth repetition are 71%, 68.1%,
6
2
O
4
2 4
O
3
.9 Intermediate Analysis for photocatalytic degradation of 4ꢀCP
The main intermediates of photocatalytic oxidation 4ꢀCP were hydroquinone (HQ),
benzoquinone (BQ), and hydroxyhydroquinone (HHQ) by HPLC measurement.
During the dark surrounding, adsorption/desorption processes were equilibrated.
The quick disappearance of the 4ꢀCP during the first 30 mins due to their adsorption
on the samples surface [68].Intermediates which produced by the photoꢀoxidation of
4
ꢀCP interact with the catalyst on the surface so that they can be released to the gas
phase. Hydroxyl groups on the surface of catalysts are able to react with intermediates,
which is then retained on the catalyst surface. Subsequently it can be converted into
2 2
other products, like CO and H O [69]
The 4ꢀCP was almost full degradation, but 44% of the total organic carbon was still
present in the solution in 120 minutes. We can learn from the phenomenon that the
degradation efficiency was greater than the mineralization efficiency. Therefore, there
are transient organic intermediates present in the photocatalytic system. The
intermediate products were subsequently oxidized by heterojunction anode rapidly
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8

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[
70]. We can come to a conclusion that we should prolong the illumination time for
complete mineralization [71].
4
. Conclusions
2 4
In summary, MFe O (M= Zn, Cu, Co and Ni) nanocrystals were synthesized by a
modified chemical coꢀprecipitation method under different calcining temperature
673K, 773K and 873K). Then, based on the MFe (673K), through the traditional
wetness impregnation strategy, we prepared the Ag/MFe (M= Zn, Cu, Co and Ni)
catalysts. Within the visible region of the spectra, these Ag/MFe O nanocrystals
(
2 4
O
2
O
4
2
4
possess attractive photovoltage response, enhanced photocatalytic activities and good
stability and reusability in degradation of 4ꢀCP. The Ag/CoFe catalyst with spinel
crystal structure annealing in 673K ensures the adsorbates into the active sites and
higher photoꢀinduced reactivity. So Ag/CoFe nanoparticles investigated here have
2 4
O
2
O
4
an expected and remarkable photocatalytic activity and could be potentially applied in
environmental purification in the future.
Acknowledgement
The project was supported by Open Research Fund of Key Laboratory of
Subsurface Hydrology and Ecological Effect in Arid Region of Ministry of Education
(No. 2014G1502033) (The funds did not sure last time. Please give your
understanding of the inconvenience caused), Engineering Research Center of
Groundwater and EcoꢀEnvironment of Shaanxi Province, National Nature Science
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9

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Foundation of China (No. 20877013 and NSFCꢀRGC 21061160495), the National
High Technology Research and Development Program of China (863 Program) (No.
2
010AA064902), the Major State Basic Research Development Program of China
(973 Program) (No. 2011CB936002), the Excellent Talents Program of Liaoning
Provincial University (LR2010090) and the Key Laboratory of Industrial Ecology and
Environmental Engineering, China Ministry of Education.
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