Accepted Manuscript Title: Synthesis and characterization of Ag3 PO4 immobilized with graphene oxide (GO) for enhanced photocatalytic activity and stability over 2,4-dichlorophenol under visible light irradiation Author: Xiao-juan Chen You-zhi Dai Xing-yan Wang Jing Guo Tan-hua Liu Fen-fang Li PII: DOI: Reference:
S0304-3894(15)00034-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.01.032 HAZMAT 16533
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
13-12-2014 8-1-2015 10-1-2015
Please cite this article as: Xiao-juan Chen, You-zhi Dai, Xing-yan Wang, Jing Guo, Tan-hua Liu, Fen-fang Li, Synthesis and characterization of Ag3PO4 immobilized with graphene oxide (GO) for enhanced photocatalytic activity and stability over 2,4-dichlorophenol under visible light irradiation, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.01.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and characterization of Ag3PO4 immobilized with graphene oxide (GO) for enhanced photocatalytic activity and stabilityover 2,4-dichlorophenol under visiblelight irradiation
Xiao-juan Chena, You-zhiDaia,*, Xing-yanWanga, Jing Guoa, Tan-huaLiua, Fen-fang
Department of Environmental Science and Engineering, Xiangtan University,
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a
PT
Lia,b
b
of
Environmental
Science,Changsha
Environmental
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Department
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Xiangtan 411105, PR China
Corresponding author
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*
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Vocational College, Changsha410004,PR China
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Tel.: +86 73158292231;fax:+86 73158292231
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E-mail address:
[email protected] 1
Protection
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Abstract A
2
series
ofvisible-light
responsive
photocatalysts
prepared
usingAg3PO4 immobilized with graphene oxide (GO) with varying GO content
4
wereobtained by an electrostatically driven method, and 2,4-dichlorophenol (2,4-DCP)
5
was used to evaluate theperformance of thephotocatalysts. The composites exhibited
6
superior photocatalytic activity and stability compared with pure Ag3PO4. When the
7
content ofGO was 5%, the degradation efficiency of 2,4-DCP could reach 98.95%,
8
and 55.91% of the total organic (TOC) content was removed within 60 min irradiation.
9
Meanwhile, the efficiency of 91.77% was achieved for 2,4-DCP degradation even
10
after four times of recycling in the photocatalysis/Ag3PO4-GO(5%) system. Reactive
11
species of O2•−,OH• and h+ were considered as the main participants foroxidizing
12
2,4-DCP, as confirmed by the free radical capture experiments.And some organic
13
intermediates
14
benzoquinone(BZQ), 2-chlorohydroquinone and hydroxyhydroquinone (HHQ) were
15
detected by comparison with the standard retentiontimes from the high performance
16
liquid chromatography (HPLC). In short, the enhanced photocatalyticproperty
17
ofAg3PO4-GO was closely related to the strong absorption ability of GO relative to
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(4-CP),
hydroquinone
(HQ),
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4-chlorophenol
2,4-DCP, the effective separation of photogeneratedelectron-hole pairs, and the excellent electron capture capabilityof GO.
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including
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Keywords: Ag3PO4-GO; visible light photocatalysis; 2,4-dichlorophenol; degradation
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intermediates; photocatalytic mechanism 2
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1. Introduction Semiconductor photocatalysis, an environmental friendly technology, has been
2
widelyused
for
removal
anddegradation
of
pollutants,
water
purification
4
anddisinfection[1,2].To increase the utilization efficiency of sunlight and improve the
5
photocatalyticactivity, novel visible-light-driven photocatalysts with high activity and
6
stabilitymust be developed [2,3]. To date, semiconductor photocatalysts of Ag-based
7
[4-6], BiVO4[7], WO3[8], and N- or S- doped TiO2[9, 10]act.have been recognized
8
and all of them exhibit excellent photocatalytic performance under visible light
9
irradiation.
organic
dye
Ag3PO4,
decomposition
shows
and
good
10
photooxidativecapabilities
O2evolution
11
fromwater[4]. The O2evolution efficiency inAg3PO4photocatalytic system can reach
12
approximately 90% with AgNO3 as a scavenger, which is significantly higher than the
13
other reported semiconductor (20%) [4].
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especially
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Ag-basedphotocatalysts,
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Unfortunately, the unwanted and uncontrolled photocorrosionwould inevitably
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become a main obstacle for the Ag3PO4application[11]. Photocorrosion results mainly
16
from theslight solubility of Ag3PO4 in solution and the characteristics of the
17
energy-band structure. The conduction band energy of Ag3PO4 is 0.45 eV, which is
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higher than the reduced potential of H2O/H2, making the capture of photoinduced electrons by H2O impossible if there are no other scavengers in the solution. Thus, the
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electrons could only be adopted by Ag+ released from the crystal lattice of Ag3PO4,
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leading to the deposition of Ag0 on the surface [4]. This phenomenon not only
22
destroys the structure of Ag3PO4 but also reduces thelight absorption efficiency of 3
Ag3PO4, which sequentially influences thephotocatalytic activity and stability of
2
Ag3PO4. To address this issue, some support materials such as CNTs [12-15],
3
bentonite[16], polyacrylonitrile nanofiber [17,18], attapulgite[19], hydroxyapatite[20],
4
layered double hydroxides[21], and flaky layered double hydroxides [22] act., have
5
been used to immobilize with the Ag3PO4. These studies confirm that the support
6
materials in the composite can act as anelectron acceptor to suppress the charge
7
recombination andprevent the photocorrosion of Ag3PO4, ultimately enhance the
8
photocatalytic activity and stability of Ag3PO4.
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Among the novel support materials, graphene(GR)with large specific surface
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(2630 m2g-1) and carrier mobility (200000 cm2 V-1S-1), has been proven to be a
11
promising support material in the field of water treatment, sterilization, and
12
capacitance[23-27]. Fortunately, the graphene oxide (GO)formed from the oxidation
13
of GR, containsvarious functional groups(such ashydroxyl, carboxyl and epoxy
14
groups)[24], making the GOexhibit gooddispersibilityin aqueous solution and
15
excellent affinity for many pollutants (including heavy metals, dyes, and
16
phenols)[23,24].Because the photocatalytic reaction is a surface-controlled process [2],
17
the effectivecontact of the containments and hybrid catalysts favors the improvement
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of the photocatalytic activity as well.
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Though several groups have reported the photocatalytic performance of
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Ag3PO4 immobilization with GR or GO [28-33], thephotocatalytic mechanism of the
21
composites and the degradation pathways of the pollutants are still to be further
22
explored. In this study, a series of Ag3PO4-GO composites with different GO content 4
were prepared by the electrostatically driven method and the characteristics of the
2
as-prepared materials were detected systematically. Moreover, 2,4-DCP was used to
3
evaluate the photocatalytic performance of the composites, and reactive species,
4
degradation intermediates as well as thephotocatalytic mechanism were also
5
investigated in detail.
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2. Experimental
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2.1. Materials
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Graphite was purchased from Tianjin Instituteof Chemical Reagents.Other
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chemicals are of analytical grade and were used without further purification. Ultrapure water was used throughout thisstudy.
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2.2. Preparation of graphite oxide (GO)
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Graphite oxide (GO) was preparedbased on the Hummers method using the
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strong oxidant, KMnO4[34]. In atypical synthesis, three processes including low-,
14
middle- and high-temperature reactions were necessary. First, 0.3 g NaNO2 was
15
dissolved completely in 50 mL of H2SO4solution. Then, the temperature of the
16
solution was cooled to 0 °C with an ice-water bath, 2 g graphitepowder and 7 g
17
KMnO4were slowly added, and the temperature was maintained below 20 °C for 1 h
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of reaction.The solution was thenheated to 35 °C and stirred for another 2 h. When 45 mL ultrapure water was added, the solution temperature was suddenly increased to
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90 °C, and the solutionwas reacted for 15 min. H2O2 (10 mL) and H2O (90 mL) were
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finally added to end the reaction, and the yellow suspension was then filtered and
22
washed with 1 M HCl. The precipitate was dispersed into 600 mL H2O and 5
1
centrifuged (10000 rpm, 15 min) repeatedly with ultrapure water until the pH of the
2
solution was near neutral. The brown solid obtainedwas collected and vacuum-dried
3
and finally crushed and sieved to 80mesh to achieve a uniform sample.
4
2.3. Synthesis of Ag3PO4-GO The electrostatically driven method was used to synthesize a series of
6
Ag3PO4-GO composite photocatalystswith different GO content. First, a definite
7
amountof GO was added to 100 mL ultrapure water and then treated with ultrasound
8
for 5 hoursat 150 W. Then, 40 mL AgNO3(0.03 mol) solution was added tothe above
9
GO dispersion and vigorously stirredfor 12 h to make Ag+ combine adequately with
10
the surface of GO. Afterwards, another 40 mLNa2HPO4(0.01mol) solution was
11
addeddropwise into the above mixture and maintained with stirring for 1 h. Finally,
12
the precipitates were collected and washed repeatedly with ultrapure water and
13
vacuum-dried overnight at 60 °C,then crushed and sieved to 80mesh. According to
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the amount of additional GO, six sampleswith initial GO content of 0.5%, 1%, 2%,
15
5%, 10% and 15% were prepared. Theschematic illustration of the synthesis pathway
16
for Ag3PO4-GO is shown in Scheme 1. For comparison, pure Ag3PO4 was
17
alsoprepared by the same method without addition of GO.
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2.4. Characterization
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Phasesof the as-prepared materials were collected on a Rigaku D/max 2500 PC
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X-ray diffractometer(XRD) equipped with Cu Kα radiation(40 kV, 100 mA)at a rate
21
of 4.0°/min over a 2θrange of 10-90°.Fourier transform infrared (FT-IR) spectra of
22
the products were recorded over the 400-4000 nm wavelength range usinga 6
1
Nicolet380
spectrometer.Brunauer-Emmett-Teller
2
materialsobtained was performed on a NOVA-2200e analyzer (Quantachrome).
3
Morphologiesof the samples were characterized by scanning electron microscopy
4
(SEM, JSM-6360LV, JEOL) andhigh-resolutiontransmission electron microscopy
5
(HRTEM, FEITecnai G2 F20 S-TWIN) with the energy-dispersive X-ray
6
spectroscopy
7
materialswasperformedonthe laser particle size analyzer(WJL-602). The X-ray
8
photoelectron spectra (XPS, Thermo Scientific, ESCALAB 250) were recorded using
9
Al Ka radiation.Ultraviolet-visible spectroscopy (UV-Vis) diffusive reflectance
10
spectra of the materials were performed on aUV-2550spectrophotometer (Shimadzu)
11
over theanalysis range from 200 to 800 nm.
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2.5. Photocatalyticactivity experiments
analysis.The
size
analysis
distributionof
the
of
the
as-prepared
adopted
to
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was
evaluate
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2,4-DCP
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photocatalyticactivity
apparatus(BL-GHX-V,
15
BilangBiological Science and Technology Co., Ltd., Xi'an)using a 300 W Xe lamp
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with an ultraviolet cutoff filter (providing visible light≥ 400 nm)as the light source. In
17
each experiment, 25 mg of photocatalyst was added to a 50 mL 2,4-DCP solution
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photoreaction
of the
as-preparedphotocatalysts
ataninitial concentration of 20 mg/L. Prior to illumination,the solution was magnetically stirred in the dark for 30 min to reachthe adsorption-desorption
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the
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in
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(EDS)
(BET)
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equilibrium of2,4-DCPon the surface of thephotocatalyst. Then, the solution was
21
exposed to Xe lamp irradiation. As the reaction proceeded, the pH of the solution was
22
measured by a pH analyzer (PHS-3C, Shanghai Jinghong Scientific Instrument Co., 7
1
Ltd.).The sample was also taken out and filteredimmediately with a0.45 µm
2
membrane. The concentration of the residual 2,4-DCP and the TOC of the reacted
3
solution would be measured by the high performance liquid chromatography
4
(HPLC,Hitachi L-2000) and TOC (Shimadzu, TOC-VCPH) analyzer.
5
2.6. Photocatalyticstability experiments Photocatalysts of Ag3PO4 and Ag3PO4-GO (5%) were used repeatedly to analyze
6
the degradation efficiency of 2,4-DCP for evaluating the stability of the photocatalysts.
8
The experiments were similar to photocatalytic experiments. After reaction in each
9
run, the solution was extracted to detect the concentration of 2,4-DCP, while the
10
photocatalysts were collected and vacuum-dried overnight at 60 °C, then crushed and
11
sieved to 80 mesh for the next use.
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2.7. Analysis of reactive species
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Free radical capture experiments were used to ascertain the reactive species in
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the photodegradation process of 2,4-DCP, and tert-butanol (t-BuOH) was chosenas
15
the
16
ethylenediaminetetraacetate(EDTA-Na2) was chosenas thehole (h+) scavenger,
17
benzoquinone (BZQ) was chosenas thesuperoxide radical(O2 •
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(OH•)
radical
scavenger,
disodium
−
) scavenger.
Thedetailed free radical capture experimental processeswere similar to photocatalytic experiments.
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3. Results and discussion
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3.1. Characterization of the photocatalysts
22
Morphologies of the as-prepared materials were characterized by SEM, and the 8
results are shown in Fig. 1. Graphite powders display a lamellar structure (see Fig.
2
1(a)), while the edge is activated and stripped (see Fig. 1(b)) after the oxidation of
3
KMnO4, which favors the formation of sheet structures under ultrasound [31]. The
4
as-prepared pure Ag3PO4possesses an irregular sphere-like morphology withan
5
average diameter of approximately 400 nm (see Fig. 1(c)). When GO dispersion was
6
introduced into the syntheticsystem, the diameter of Ag3PO4 gradually decreased with
7
GO content increasing (see Fig. 1(d-i)and Table 1), indicating an obvious tailoring
8
roleof GO relative to the size of theAg3PO4particles in the composites [28], mainly
9
because the self-assembly of positively charged Ag+ on negatively charged GO sheets
10
driven by the electrostatic interaction, hinders the generationof the Ag3PO4 seed
11
particles and controls the growth of the Ag3PO4particles[28,30].Further analysis of the
12
HRTEM images for Ag3PO4-GO (5%) composite, as shown in Fig. 2(a), suggests that
13
the Ag3PO4particles are immobilized with GO sheets. The lattice fringe spacing is
14
determined to be0.246 nm, corresponding to the (211) planes of Ag3PO4 (see Fig. 2(b))
15
[5,7]. The clear diffraction rings in Fig. 2(c) reveal the polycrystallinenature of
16
Ag3PO4.Energy
17
(5%)compositephotocatalyst was also recorded and shown in Fig. 2(d), wherethe
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(EDS)
pattern
of
the
Ag3PO4-GO
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X-ray
signals fromAg, P, and O arewell attributed to Ag3PO4. The signalfor C is from GO, while the signalfor Cu is mainly fromthe impurities on the substrate fixed sample.
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dispersive
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Meanwhile, BET data shown in Table 1 indicate an increased specific surface area of
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photocatalysts as the GO content increases. Thatis to say, the combination of GO with
22
Ag3PO4 actuallyimproves the surface area of Ag3 PO4-GO composites, which would 9
1
be beneficial for the enhancement of photocatalytic activity[31].
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Fig. 3 depicts XRD patterns of the as-prepared materials. All of the diffraction
3
peaks of pure Ag3PO4 and Ag3PO4-GO composites could readily be indexed to the
4
sphere-likestructure
5
characteristicdiffraction peaks for GO are observed in the patternbecause of the low
6
diffractionintensity of GO[28], indicating that the introduction of GO influencesonly
7
the diffractionintensity of Ag3PO4, not the peak structure.
Ag3PO4
(JCPDS
No.
06-0505).No
obvious
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Fig. 4(a). Compared with the spectrumof Ag3PO4, an obvious enhancedabsorbance
10
both in UV and visible light regions was found when GO was incorporated, and the
11
absorbance intensity increased as the GO content increased. The band gap energy (Eg)
12
of a semiconductor is an important optical property parameter for evaluation of the
13
formation and transformation of photoinduced electrons and holes, and the Egcan be
14
calculated bythe following formula[35]:
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ℎ =
ℎ −
/
(1)
whereα, h, v, Eg and A are the absorption coefficient, the Planck constant, thelight
16
frequency, the band gap energy, and a constant, respectively.Among these parameters, n is determined from the type of optical transitionof a semiconductor and for Ag3PO4, n = 1 for a direct transition whilen = 4 for an indirecttransition. Therefore, theband
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gap energy of the prepared materials could be elicited from aplot of light energy
20
(αhv)2 versus energy (hv) (n = 1, shown in Fig. 4(b)), and the results are listed in
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Table 1. The band gap energy decreases withincreasing GO content, also indicating 10
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the enhancement of visible light absorbance. In addition, the valence band (VB) edge
2
position ofAg3PO4could be estimated according to the concept ofelectronegativity,
3
and the electronegativity of an atom is thearithmetic mean of the atomic electron
4
affinity and the first ionizationenergy [4,35]. Thus, the VB potential of Ag3PO4at
5
thepoint of zero charge could be calculated by the following empiricalequation: =
−
+ 0.5
(2)
where EVB is the valence band edge potential; X is the electronegativity of Ag3PO4,
7
which is the geometric mean of the electronegativityof the constituent atoms (5.96 eV)
8
[4]; Ec is the energy of free electronson the hydrogen scale (approximately 4.5 eV);
9
and Eg is band gap energy of Ag3PO4 (2.42 eV calculated from Fig. 4(b)). The
10
conduction band (CB) edge potential(ECB) can be determined by ECB=EVB − Eg.
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Consequently, EVB and ECB positions of Ag3PO4are estimated to be 2.67 and 0.25
12
eV/NHE,respectively.
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3.2. Photocatalytic activityof Ag3PO4-GO
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To investigate thephotocatalytic activities of the as-prepared photocatalysts,
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several photocatalyticexperiments using the samples with different GO content as
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well as pure Ag3PO4 were carried out for 2,4-DCP degradation under visible light
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irradiation. The results are shown in Fig. 5(a), and the data for the 2,4-DCP degradationby Ag3PO4-GO (10%) and Ag3PO4-GO (15%) are not shown in the figure
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because 2,4-DCP ismostly adsorbed by the samples (see Table 1). As observed in Fig.
20
5(a), the self-photolysis of 2,4-DCP in theabsence of any catalysts can be neglected.
21
When
Ag3PO4
was
immobilized
withGO, 11
all
the
composites
exhibited
higherphotocatalytic activity than pure Ag3PO4. Moreover, the degradation efficiency
2
of 2,4-DCPincreased with theincreasing content of GO under the same reaction time,
3
and as the GO contentincreased to 5%, 98.43% of the2,4-DCPwas decomposed under
4
60 min irradiation. Meanwhile, it is not difficult to find that theadsorption efficiency
5
of composites for 2,4-DCP increases from 7.23% to 28.87% along with the GO
6
content increasing from 0.5% to 5%. This trend is consistent with the total
7
photodegradation efficiency of 2,4-DCP, indicating the significant role of adsorption
8
in promoting the photocatalysis. As the photocatalytic reaction is a surface-controlled
9
process, efficient contact between containments and hybrid catalysts favors the
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photodegradation of containments [2].
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Apparent degradation rate constants for 2,4-DCP were also calculated by plots of
12
-ln(C/C0) versus irradiation time, as described in Fig. 5(b), and the values are shown
13
in Table 1. The results indicate that all the experiment data fit a first-order kinetic
14
model well, and introduction of GO significantlyfacilitates the degradation rate of
15
2,4-DCP. When theGO content is 5%, the apparentrate constant is 0.0600 min-1,
16
which isapproximately 5.21 timesof pure Ag3PO4.
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3.3. Analysis of reactive species
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Reactive species are the main participants for pollutant degradation in
photocatalytic processes. When Ag3PO4 is irradiated by light, electron-hole (e--h+)
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pairs were generated, which maypartially recombine internallyinAg3PO4 andmay
21
partially recombine as transforming to surface of Ag3PO4.The residual effective holes
22
(h+) directly participate in the oxidation of pollutants while the isolated electrons (e-) 12
1
result in a series of chain reactions to form other reactive species, which could also
2
oxidize pollutants [28,31]. Therefore, analysis of reactive species is significant for
3
exploration of thephotocatalytic mechanism. In this study, t-BuOHas a hydroxyl radical (OH•) scavenger, EDTA-Na2 as ahole
5
(h+) scavenger and BZQ as asuperoxide radical (O2•−) scavenger were introducedinto
6
the photocatalytic system of composite Ag3PO4-GO (5%)toascertain the dominant
7
species in the degradation process of 2,4-DCP.Degradation efficiency of 2,4-DCP
8
under these scavengers is shown in Fig. 6. The adsorption efficiency of 2,4-DCPon
9
the composites decreased in all the reaction systems as the reactive species scavenger
10
was added, which maybe because of the competitive adsorption between 2,4-DCP and
11
the additives. When 1 mM OH• scavenger (t-BuOH) or h+scavenger (EDTA-Na2) was
12
addedto the reaction system, the photocatalytic activity of Ag3PO4-GO (5%) over
13
2,4-DCPevidently decreased from 98.95% to 31.56% and 10.95%, respectively. When
14
1 mMO2•−scavenger (BZQ)was added, inactivation of Ag3PO4-GO (5%)photocatalyst
15
was observed as rarely degradation of2,4-DCP. The above results illustrate that allthe
16
OH•, h+and O2• − contribute to the high photocatalyticactivity of Ag3PO4-GOfor
17
2,4-DCP degradation. The most active speciesis O2•−, which may be produced by the
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reductionof O2 molecules adsorbed on the catalyst surface by photoexcited electrons. 3.4. Intermediatesof2,4-DCP photodegradation
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To further understand the degradation products and pathway of 2,4-DCP in
21
thephotocatalysis/Ag3PO4-GO system, photocatalytic experiments with prolonged
22
times of irradiation (120 min)were conducted. Fig. 7(a) shows the removal 13
1
efficiencies (%) of 2,4-DCP and TOC at different irradiation times. In 120
2
min,approximately 43.86% of the TOCstill remained, while 98.95% of 2,4-DCP was
3
removed.Themineralization
4
degradationefficiency,implying that there are transient organic intermediates formed
5
in the photocatalytic processes. Moreover, the mineralization efficiency changes
6
slightly after 60 min of irradiation, indicating that some of theintermediates are
7
difficult to be degraded further.
is
remarkably
less
than
the
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Meanwhile, the concentration of TOC and chlorine (Cl-) in the solution at
9
different reaction timeswere also measured, and the resultsare presented inFig. 7(b).
10
As seen, 6.20 mg/L Cl- was detected after 120 min irradiation, which accounted for
11
72.19% of the theoreticalquantity (8.589 mg/L). This resultalso leads to the
12
conclusion that there are transient organic intermediates with Cl-present in the
13
photocatalytic system.
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Furthermore, the pH of the solution at different reaction times was also detected
15
by the pH analyzer, and the results are described in Fig. 7(c). The pH of the solution
16
decreased overall, but there was a fluctuation at a certain time interval in each
17
photocatalytic experiment. This phenomenon may be explained by the formation of
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some acidic intermediates that gradually decompose as the irradiation time is prolonged.
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Based on the above analysis, degradation intermediates of 2,4-DCP were
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detected bycomparison of retention times with the standards from the HPLC
22
chromatograms under the same operating conditions. Fig. 8 illustrates the HPLC plots 14
1
of the 2,4-DCP photodegradation solution at different irradiation times.Structural
2
formulas of the intermediate products detected are labeled in thechromatograms at
3
their relative peak positions. The peak indicating 2,4-DCPgradually decreases as the
4
reaction time increases. By comparison of the retention time with the standard under
5
the same operating conditions, 4-chlorophenol (4-CP), hydroquinone (HQ),
6
benzoquinone(BZQ), 2-chlorohydroquinone and hydroxyhydroquinone (HHQ) were
7
found
8
photocatalysis/Ag3PO4-GO system.A dechlorination reaction could be presumed to
9
bethe most important degradation step of 2,4-DCP and some more refractory organic
10
substances formed in the photocatalytic process. Thus, the degradation pathway could
11
be illustrated as Scheme 2.
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3.5. Photocatalytic stability of Ag3PO4-GO
main
degradation
intermediates
of
2,4-DCP
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The photocatalyticstability of Ag3PO4 and Ag3PO4-GO (5%) were investigated
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relative to 2,4-DCP degradationby repeating the reaction for four times. The
15
degradation efficiency of 2,4-DCPat different recycle times is shown in Fig. 9(a).The
16
resultsshow that the degradation efficiency of Ag3PO4-GO (5%) relative to
17
2,4-DCPafter four times recyclingwas rather stable andremained about91.77%, which
D
TE
EP
is much higher than that of the pure Ag3PO4 (approximately 34.78%) in this study. Meanwhile,this value is also much higher thanthe degradation efficiency of orange
A
19
CC
18
M
13
20
methyl (MO)(about 55%) and bisphenol A(about 90%) over the Ag3PO4-GO
21
composite
22
[29,30].Furthermore,XRD spectraofAg3PO4 and Ag3PO4-GO (5%) (shown in Fig.
after
four
times
recyclingreported
15
in
the
reference
1
9(b)) after fourrecycle times were also recorded in an effort to understand the
2
structure stability. Ag3PO4was clearly shown to be partiallydecomposed to Ag0, while
3
no obvious Ag0was observed with the Ag3PO4-GO (5%) sample, confirming a higher
4
stability of the Ag3PO4-GO (5%) composite.
5
3.6. Discussion of the photocatalyticmechanism On the basis of the results described above, the photocatalytic mechanism of the
7
composite photocatalyst Ag3PO4-GOwas proposed, as shown in Fig. 10(c). In the
8
photocatalytic process, electron-hole pairsare formed when Ag3PO4 (conduction band:
9
0.25 eV, valence band: 2.67 eV)is irradiated by visible light,and electrons at the
10
valence band (VB) are excited to the conduction band (CB), inducing the separation
11
with holes in the VB. Then, a series of chain reactionswill provide more reactive
12
species for 2,4-DCP degradation.
A
N
U
SC
RI
PT
6
The existence of GO in the composite promoted the chain reactions and played a
14
significant role in the enhanced photocatalytic activity and stability. First, the high
15
surface
16
photocatalyticreaction sites, which has the benefit ofimproving the photocatalytic
17
activity.Second, GO acts as an effective acceptor of the photoexcited electrons,
D
GO
provides
manymore
active
adsorption
sites
and
EP
TE
of
making O2•− radicals producedby the reductionof O2 molecules adsorbed on catalyst surface. Third, from the FT-IR spectra(shown in Fig. 10(a))of the GO, Ag3PO4-GO
A
19
area
CC
18
M
13
20
(5%) before and after photocatalysis,almost all of the characteristic peaks of GO
21
disappear or weaken after photocatalysis, including C=Ostretching vibrations of
22
COOH groups (1721.04 cm-1), O-Hdeformation vibrations of COOH groups (1625.31 16
cm-1), O-Hdeformation vibrations of tertiary C-OH (1398.49 cm-1), andC-O stretching
2
vibrations of epoxy groups (1056.81 cm-1)[29].Moreover, the C 1s XPS
3
spectrum(shown in Fig. 10(b)) ofAg3PO4-GO (5%) after photocatalysissuggeststhat
4
the XPS peak area ratios of the C=O (286.08 eV), C-O-C (287.50 eV), and C-OH
5
(289.33 eV) bonds to the C=C (284.98 eV) bondareobviously decreased compared
6
with that of GO under the same binding energy.These results indicate the
7
partialreduction of functional groups in GO during the course of thephotocatalysis,
8
leading to the formation of reduced GO which can serve as the transformation
9
medium for photoexcited electrons. On the one hand, the phenomenonseriously
10
promotes the separation of electron-hole pairs, resulting in an increased quantity of
11
holes for 2,4-DCP degradation. On the other hand, the phenomenondecreases the
12
probability of combination of electrons and Ag3PO4, offering better stability and
13
recyclability of the composite. The transportation and mobility of electrons on
14
reduced GO sheets are very rapid for the specific π-conjugated structure of reduced
15
GO, so more photogenerated electron-hole pairs are continuously produced working
16
in this way[24, 28]. Therefore, 2,4-DCP is degraded byradicals of h+ andO2 •− as well
17
as OH• generated by the reaction of H2O andholes.
RI
SC
U
N
A
M
D
TE
EP
CC
4. Conclusions
A
18
PT
1
19
A series of Ag3PO4-GO composite photocatalysts with different GO content were
20
prepared and used for 2,4-DCP degradation. The composites exhibited superior
21
visible-light photocatalytic activity and stability compared with pure Ag3PO4. In the
22
photocatalytic process, reactive speciesof O2• − , OH• and h+ participated in the 17
1
oxidation of 2,4-DCP, and some more refractoryorganic intermediates were formed.
2
Approximately 55.91% of the2,4-DCP or intermediates were mineralized under 60
3
min irradiation, indicating the great potential application of theAg3PO4-GO composite
4
in the photocatalytic degradation of refractory organic pollutants.
5 6
Acknowledgements This work was supported by thePostgraduate Innovative Research Project of
8
Hunan Province (No.CX2014B269),the OpenFoundation ofthe Innovation Platform in
9
Higher Education of Hunan Province (No.11k070), and the Science and Technology of
Hunan
11
No.2013FJ3071).
Provincial
Science
Department
(Grant
A
12
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13
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[35] M. Ge, N. Zhu, Y.P. Zhao, J. Li, L. Liu, Sunlight-assisted degradation of dye
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8
Chem. C 112 (2008) 19841-19845.
N
11
A
CC
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TE
D
M
A
12
23
Figure captions
2
Fig. 1.SEM images of (a) graphite, (b) graphite oxide, (c) Ag3PO4, (d) Ag3PO4-GO
3
(0.5%), (e) Ag3PO4-GO (1%), (f) Ag3PO4-GO (2%), (g) Ag3PO4-GO (5%), (h)
4
Ag3PO4-GO (10%), (i) Ag3PO4-GO (15%).
5
Fig. 2.(a)HRTEM image, (b)lattice fringe image, (c) selected area electron diffraction
6
(SAED) pattern, and(d) EDS patternof the Ag3PO4-GO (5%) composite.
7
Fig. 3.XRD patterns of the as-prepared materials.
8
Fig. 4. (a) UV-Vis diffuse reflectancespectra of the as-prepared composites, (b) plots
9
of (αhv)2 versus energy(hv) for band gap energy analysis.
SC
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1
Fig. 5. (a) Photocatalytic activity of different samples toward 2,4-DCP degradation
11
under visible light irradiation, (b) plots of -ln(C/C0 ) versus irradiation time.
12
Fig. 6.Photodegradation efficiency of 2,4-DCP under different scavengers.
13
Fig. 7. Plots of (a) removal efficiencies (%) of 2,4-DCP and TOC versus reaction time,
14
(b) concentrations of TOC and Cl- versus reaction time, and (c) changes of pH values
15
during the course of 2,4-DCP photodegradation.
16
Fig. 8.Evolution of HPLC chromatograms for 2,4-DCPphotocatalytic solutions using
17
Ag3PO4-GO (5%) catalyst.The iconograph shows the magnified graph of HPLC, and
N
A
M
D
TE
EP
the degradation intermediates are labeled. Fig. 9. (a)Photodegradationefficiencies of 2,4-DCP with Ag3PO4 and Ag3PO4-GO
A
19
CC
18
U
10
20
(5%) in different recycle runs, (b) XRD spectra of Ag3PO4 and Ag3PO4-GO (5%)
21
recycled four times.
22
Fig. 10. (a) FT-IR spectra of GO, Ag3PO4-GO (5%) before and after photocatalysis, 24
(b)C1s XPS spectra of the Ag3PO4-GO (5%) after photocatalysis, the inset Table
2
showsthe peak area ratios ofC=O, C-O-C, C-OHbonds to the C=C bond for GO [36]
3
and reacted Ag3PO4-GO (5%) respectively,(c)the proposed photocatalytic mechanism
4
for 2,4-DCP degradation using Ag3PO4-GO.
A
CC
EP
TE
D
M
A
N
U
SC
RI
PT
1
25
TE
EP
CC
A D
PT
RI
SC
U
N
A
M
Fig. 1.
26
27
TE
EP
CC
A D
PT
RI
SC
U
N
A
M
TE
EP
CC
A D
PT
RI
SC
U
N
A
M
Fig. 2.
28
PT RI SC U N
C
A
D
M
3.11 0.61 3.03
TE
O P Ag
EP CC A
O
Ag
Cu P
2
0.08 0.03 0.26 Cu
Ag
0
(d)
Elements Atomic (%) Uncert. (%)
Cu
4
6 8 Energy (keV)
29
Ag
20
Ag
25
1
Fig.3
2 3
RI
SC U N
45 60 75 2θ (degree)
A
30
M
15
A
CC
EP
TE
D
4
30
90
0
Intensity (cps)
14000 12000 10000 8000 6000 4000 2000
PT
Ag PO 3 4 Ag PO -GO (0.5%) 3 4 Ag PO -GO (1%) 3 4 Ag PO -GO (2%) 3 4 Ag PO -GO (5%) 3 4 Ag PO -GO (10%) 3 4 Ag PO -GO (15%) 3 4
Fig. 4.
1.4
(a)
1.0 0.8
PT
(3) (2) Ag3PO4 (1)
RI SC
400 500 600 Wavelength (nm)
700
800
N
(b)
(7)
M
10
(6)
D
2
(α hv) (e V )
(5) (4)
U
300
12
8
TE
2
(6)
Ag3PO4-GO (0.5%)
14
6 4
EP
2
Ag3PO4-GO (10%)
Ag3PO4-GO (2%) Ag3PO4-GO (1%)
0.4
0.0 200
CC
(7)
0.6
0.2
A
Ag3PO4-GO (15%)
Ag3PO4-GO (5%)
A
Absorbance (a.u.)
1.2
(5) (4) (3) (2)
(1)
2 0
1.6
2.0
2.4
2.8 3.2 hv (eV)
31
3.6
4.0
Fig.5.
20 16 12 8
RI
Ag3PO4-GO (0.5%) Ag3PO4-GO (1%)
4
Ag3PO4-GO (2%) Ag3PO4-GO (5%)
-30
-15
0 15 30 Time (min)
45
60
A
N
U
0
PT
Blank Ag3PO4
SC
Concentration (mg/L)
Visible-light irradiation (a)
Dark
(b)
M
Ag3PO4 Ag3PO4-GO (0.5%)
4
TE
3
Ag3PO4-GO (5%)
A
CC
EP
-ln(C/C0)
D
Ag3PO4-GO (1%) Ag3PO4-GO (2%)
2 1 0
0
10
20 30 40 Time (min)
32
50
60
Fig.6.
Dark
1.0
Visible-light irradiation
0.8 C/C0
0.6
PT
Ag3PO4-GO (5%) with 1mM t-BuOH with 1mM EDTA-Na2
0.4
RI
with 1mM BZQ
0
15 30 Time (min)
U
-15
A
CC
EP
TE
D
M
A
N
0.0 -30
SC
0.2
33
45
60
Fig. 7.
Removal efficiency (%)
100
(a) 98.95%
80 60
RI
PT
40 20 40
60 80 Time (min)
U
20
100
120
9.0
A
N
0
SC
2,4-DCP TOC
(b)
M
0
D
7.5
TE
6.0 4.5
EP
Concentration (mg/L)
56.14%
adsorption efficiency: 28.87%
A
CC
3.0
TOC Cl
1.5 0.0
0
20
40
60 80 Time (min)
34
100
120
8.0
(c)
Ag3PO4-GO (2%) Ag3PO4-GO (1%) Ag3PO4-GO (0.5%)
7.6
pH
increasing increasing
pH
7.2
increasing
6.8 increasing
-20
0
20 40 60 Time (min)
PT
6.4 -20
0
20 40 60 Time (min)
80 100 120
A
CC
EP
TE
D
M
A
N
U
SC
6.0
RI
Ag3PO4-GO (5%)
35
Fig. 8.
O
OH
0 min 10 min 20 min 40 min 60 min 120 min
Cl OH
OH
OH
O
OH
Cl
OH Cl OH
OH
Cl
4.9 5.6 Time (min)
6.3
7.0
4 6 Time (min)
U
2
A
CC
EP
TE
D
M
A
N
0
4.2
SC
3.5
RI
PT
OH
36
8
10
Fig.9.
(a)
Ag3PO4-GO(5%)
80 60
PT
Ag3PO4
RI
40
0
SC
20
1st recycle 2nd recycle 3rd recycle 4th recycle
N
U
Photocatalytic efficiency (%)
100
A
(b)
M D TE
EP
Intensity (cps)
Ag3PO4-GO(5%) after photocatalysis
Ag3PO4 after photocatalysis
A
CC
Ag0
15
30
45 60 2θ (degree)
37
75
90
Fig. 10.
C-O 1056.81
U
SC
before photocatalysis after photocatalysis
RI
GO
PT
1721.04 1398.49 C=O 1625.31 O-H O-H
(a)
600
M
A
N
3600 3000 2400 1800 1200 Wavelength (nm)
(b)
D
Samples GO Composite 100 100 C=C 28 C-OH 27 45 C-O-C 59 11 C=O 9
C-O-C
284.98 eV 286.08 eV 287.50 eV 289.33 eV
C-OH C=O
A
CC
EP
TE
C=C
294 292 290 288 286 284 282 280 Binding Energy (eV) 38
TE
EP
CC
A D
PT
RI
SC
U
N
A
M
(c)
39
1 2
Table 1
3
Size distribution, BET, band gap energyof the as-prepared materials and
4
adsorption/degradation performances of different samples to 2,4-DCP.
5
Samples
Size
BET
Adsorption
Degradation
distribution
surface
efficiency
efficiency
Kinetic analysis
Band gap energy
area (m g )
(%)
(%)
Ag3PO4
392.63
0.137
0.89
50.41
0.0115
0.9799
2.42
Ag3PO4-GO (0.5%)
382.28
1.666
7.23
62.21
0.0131
0.9694
2.39
Ag3PO4-GO (1%)
345.15
2.560
15.49
78.62
0.0216
0.9819
2.31
Ag3PO4-GO (2%)
314.64
4.160
23.53
85.59
0.0260
0.9563
2.27
Ag3PO4-GO (5%)
243.92
7.553
28.87
98.43
0.0600
0.9503
2.10
Ag3PO4-GO (10%)
206.49
8.705
77.39
--
--
--
1.96
Ag3PO4-GO (15%)
169.17
9.147
98.73
--
--
--
1.73
RI
SC U
R2
PT
D90 (nm)
Kapp (min-1)
2 -1
Note: D90, the particle size corresponding to 90% of the cumulative particle size
7
distribution.
A
CC
EP
TE
D
M
A
N
6
40
(eV)
PT RI SC
A
CC
EP
TE
D
M
A
N
U
Scheme 1.Schematic illustration of the synthesis pathway of Ag3PO4-GO composites.
41
PT RI of
the
SC
pathway
intermediate
U
2.Producing
N
Scheme
A
CC
EP
TE
D
M
A
photodegradation.
42
productsfor
2,4-DCP