A new fabrication of AgX (X = Br, I)-TiO2 nanoparticles immobilized on polyacrylonitrile (PAN) nanofibers with high photocatalytic activity and renewable property

Article abstract of DOI:10.1039/c5ra19235h

A highly efficient visible light-driven AgX-TiO₂/PAN (X = Br, I) photocatalyst was synthesized by means of a combination of the electrospinning technique, solvothermal synthesis, physical adsorption and gas/solid reaction. The components, morphological and optical properties of the photocatalysts were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR). The as-prepared composites exhibited excellent photocatalytic efficiency for the degradation of methyl orange (MO), methylene blue (MB), acid red 18, sodium fluorescein, xylenol orange and phenol under visible light irradiation. Compared with pure PAN, AgX/PAN and TiO₂/PAN, AgX-TiO₂/PAN showed much higher photocatalytic activity in degrading MO. In addition, AgX-TiO₂/PAN had a certain photochemical stability and could be regenerated easily. The application of PAN nanofibers made it easy to separate the catalysts from an aqueous solution without any loss. The degradation of MO in the presence of different scavengers suggested that holes and O₂^- were the main reactive species and holes played the predominant role. Thus, a possible two-stage photocatalytic mechanism associated with AgX-TiO₂/PAN was proposed.

Full text of DOI:10.1039/c5ra19235h

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ARTICLE
A new fabrication of AgX (X= Br, I)-TiO2 nanoparticles immobilized
on polyacrylonitrile (PAN) nanofibers with high photocatalytic
activity and renewable property
Received 00th January 20xx,
Accepted 00th January 20xx
Dandan Yu,a Jie Bai, *a Haiou Liang,a Junzhong Wang,a and Chunping Lia
DOI: 10.1039/x0xx00000x
A highly efficient visible light-driven AgX-TiO2/PAN (X= Br, I) photocatalyst was synthesized by means of a combination of
the electrospinning technique, solvothermal synthesis, physical adsorption and gas/solid reaction. The component,
morphological and optical properties of the photocatalysts were characterized by X-ray diffraction (XRD), field-emission
scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy
(DRS) and Fourier transform infrared spectroscopy (FTIR). The as-prepared composites exhibited the excellent
photocatalytic efficiency for the degradation of methyl orange (MO), methylene blue (MB), acid red 18, sodium
fluorescence, xylenol orange and phenol under visible light irradiation. Compared with pure PAN, AgX/PAN and TiO2/PAN,
AgX-TiO2/PAN showed the much higher photocatalytic activity in degrading MO. In addition, AgX-TiO2/PAN had a certain
photochemical stability and could be regenerated easily. The application of PAN nanofibers made it easy separate the
catalysts from an aqueous solution without any loss. The degradation of MO in the presence of different scavengers
suggested that holes and •O2ˉ were the main reactive species and holes played the predominant role. Thus, a possible two-
stage photocatalytic mechanism associated with AgX-TiO2/PAN was proposed.
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In general, silver halides are unstable under light irradiation, which
greatly affects their applications in photocatalytic reactions. Many
researchers have reported that AgX could maintain its stability by
coupling with other support materials such as TiO2,16-22 ZnO23-25 and
BiOX.5,26,27 Hu et al. have prepared AgI/TiO2 photocatalysts by
deposition-precipitation method and the catalyst’s activity was
maintained effectively for degradation of K-2G after six cycling runs
under visible light irradiation without the destruction from AgI.16
According to previous researches, the photosensitivity of AgX makes
Introduction
Currently, semiconductor photocatalysis has attracted remarkable
attention in solving environment and energy problems.1-4 From the
semiconductor photocatalysts investigated, TiO2 is considered to be
a desirable material for the elimination of organic pollutants due to
its high activity, stability, low cost and nontoxicity.5-7 However, the
practical application of TiO2 has been greatly hampered by the
inefficient utilization of solar energy because of its wide band gap
(anatase, 3.2 eV; rutile, 3.0 eV) and the rapid recombination of
photogenerated electron-hole pairs.5-10 There are some different
approaches to be explored to extend the absorption edge of TiO2
into the visible region, such as nonmetal and metal doping,7,8
anionic or cationic doping,10 noble metal deposition11 and coupling
with other narrow band gap semiconductors.12
it feasible synthesize
nanoparticles with TiO2.
a novel photocatalyst by coupling AgX
In addition, TiO2 particles are easy to agglomerate and difficult
to recover after being used for photocatalytic reactions.28,29 To
overcome these hurdles, many efforts have been made to
immobilize TiO2 nanoparticles on various supporters such as
zeolite,31 carbon nanofiber,32 carbon nanotubes33 and graphene.30,34
Owing to the advantages of fine stability and easily availability,36-39
polyacrylonitrile (PAN) nanofibers obtained from an effective and
economical electrospinning technique may be promising materials
for the immobilization of catalysts as follows: (1) the large specific
surface area is beneficial to the high exposure level of
photocatalysts nanoparticles and further enhance the
photocatalytic activity,28,35,39 (2) the randomly arrayed nanofibers
favour the separation, recovery and reuse of photocatalysts,39 (3)
PAN nanofibers are hydrophobic with a low density and easily fixed
at the proper position of the reactors, which could maximize the
efficiency of light utilization by avoiding the hindrance of light
penetration.29 Su et al. have successfully fabricated TiO2/PAN hybrid
Silver halides (AgX, X= Br, I), the important narrow band gap
semiconductors, have been recognized as photosensitive materials
and extensively used in photographic films.13-15 Under light
irradiation, silver halides absorb photons and electron-hole pairs
can be liberated.15,16 The photogenerated electrons are easily
captured by the Ag+, leading to formation of silver atoms (Ag0).14-16
a.Chemical Engineering College, Inner Mongolia University of Technology, Huhhot
010051, People’s Republic of China. E-mail: baijie@imut.edu.cn (J. Bai); Fax: +86
471 6575722; Tel.: +86 471 6575722.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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nanofibers by electrospinning and hydrothermal processes.35 The
as-prepared catalysts showed the high removal efficiencies of SO2
and NO in the UV light photocatalysis oxidation of flue gas.
Therefore, it is a great ideal to prepare a recyclable photocatalyst by
immobilizing TiO2 on the surface of PAN nanofibers.
Preparation of AgX-TiO2/PAN (X= Br, I) nanocomposites
AgX-TiO2/PAN (X= Br, I) was synthesized as follows: 0.0250 g
AgNO3 was dissolved in 100 ml deionized water under dark
condition. Then 0.4 g of TiO2/PAN hybrid nanofibers was added
subsequently by physical adsorption for 6 h with stirring constantly.
The composites contained Ag+ were filtered, washed three times
using deionized water and transferred to a vacuum oven to dry at
90 °C for 3 h. To prepare AgX-TiO2/PAN, the obtained Ag (I)-
TiO2/PAN composites were exposed in I2 atmosphere for 48 h (or Br2
atmosphere for 24 h). Finally, they were placed in a vacuum oven to
evaporate residual I2 (or Br2) at 90 °C for 4 h. And the as-prepared
sample was named AgX (10%)-TiO2/PAN. By changing the dosage of
AgNO3 to 0.0050 g, 0.0100 g, 0.0150 g and 0.0200 g, respectively, a
series of AgX-TiO2/PAN photocatalysts was prepared and labeled as
AgX (y%)-TiO2/PAN, where y% represents the molar ratio of
AgX/TiO2. For comparison, AgX/PAN (X= Br, I) was synthesized
through similar processes by using 0.0250 g AgNO3 and 0.3 g PAN
without solvothermal treatment.
Based on the above statements, a novel photocatalyst AgX (X =
Br, I)-TiO2 nanoparticles immobilized on PAN nanofibers were
successful prepared through the electrospinning, solvothermal
synthesis,29 physical adsorption process and gas/solid reaction.38 In
the present work, a series of photocatalysts with different molar
ratio between AgX and TiO2 had been fabricated and the
photocatalytic activities of the as-prepared catalysts were evaluated
by decomposing different organics (methyl orange, acid red 18,
methylene blue, xylenol orange, sodium fluorescence and phenol)
under visible light irradiation. Moreover, a recycling test was
conducted to investigate the photochemical stability and reusability
of catalysts and a possible photocatalytic mechanism of the highly
enhanced performance was also proposed.
Experimental Section
Characterization
To study the crystal phase and crystalline of the samples, X-ray
diffraction (XRD, Rigaku Ultima IV, Japan) patterns were performed
in a range of 2θ from 10° to 90° with a scanning rate of 2°/min. The
morphologies of the products were observed by field-emission
scanning electron microscopy (FE-SEM, FEGQUANTAN 650) and
transmission electron microscopy (TEM, F20 S-TWIN, Tecnai). The
UV-vis diffuse reflectance spectra (DRS) were measured with a
Shimadzu UV (3600)-vis spectrophotometer and BaSO4 was used as
a reference material. Fourier transform infrared spectra (FTIR) were
recorded from KBr pellet in a Nicolet Nexus 670 spectrophotometer
with a range of 400-4000 cm-1.
Materials
Polyacrylonitrile (PAN, Mw = 80 000) was purchased from Kunshan
Hongyu Plastics Co., Ltd. N,N-dimethylformamide (C3H7NO, AR,
99.5%) was purchased from Tianjin Fuyu Fine Chemical Co., Ltd.
Tetra-n-butyl titanate (Ti(OC4H9)4, AR, 98.5%) was purchased from
Xiya Reagent. Acetic acid (CH3COOH, AR, 99.5%) was provided by
Beijing Chemicals Co. Silver nitrate (AgNO3, AR, 99.8%), absolute
ethyl alcohol (C2H6O, AR, 99.7%) and bromine (Br2, AR, 99.5%) were
purchased from Sinopharm Chemical Reagent Co., Ltd. Iodine (I2, AR,
99.8%) was supplied by Tianjin Chemical Reagent Factory. The
above chemical reagents were used as received without future
treatment.
Photocatalytic performance
Synthesis of TiO2/PAN hybrid nanofibers
The photocatalytic activities of the as-prepared samples were
measured by degrading methyl orange under visible light (λ ≥ 400
nm) irradiation at ambient condition. The light source was a 300 W
Xe arc lamp (Beijing Perfectlight Technology Co., Ltd) with a 400 nm
cutoff filter. In a typical procedure, 0.2 g photocatalysts were
dispersed in 100 ml of a 5 mg/l aqueous solution of MO placed in a
beaker. Prior to irradiation, the suspension was magnetically stirred
in dark for 30 min to establish adsorption-desorption equilibrium.
The experiments were carried out under room air-equilibrated
conditions. The light was focused onto the breaker. After visible light
irradiation at a given time intervals, 3 ml suspensions were sampled
and filtered by membrane filters (0.22 μm pore size). The
concentrations of dye aqueous solution were analyzed by UV-vis
spectrophotometer (UV-1800, Mapada) at a maximum adsorption
wavelength of MO (λ = 464 nm). The photocatalytic performance of
TiO2/PAN, AgBr/PAN and AgI/PAN was also measured under the
same conditions.
Polyacrylonitrile (PAN) was dissolved in N,N-dimethylformamide
(DMF) with vigorous stirring to form a homogeneous 8 wt%
PAN/DMF solution for the subsequent electrospinning process. The
solution was placed in a glass dropper. A self-made copper ring was
wrapped around the top of glass dropper and connected to the
anode of a high voltage generator. The applied direct current
voltage was 17 kV. A piece of aluminium foil connected to the
cathode was placed in 15 cm distance from the tip of the dropper to
collect PAN nanofibers. Secondly,
1 ml Tetra-n-butyl titanate
(Ti(OC4H9)4) and 0.15 ml acetic acid (CH3COOH) were respectively
added into 20 ml continuous stirred absolute ethyl alcohol. Then
0.15 g of PAN nanofibers was dispersed into the above mixture
solution followed by 24 h physical adsorption. Subsequently, the
mixtures were transferred into a 100 ml Teflon-lined stainless
autoclave and kept at 180 °C for 9 h. After cooling to room
temperature naturally, the products were collected, washed several
times with water and dried at 80 °C for 2 h in a vacuum oven. Thus,
TiO2/PAN hybrid nanofibers were fabricated.
In order to demonstrate the as-prepared samples could degrade
different organic pollutants, the photocatalytic activity of AgX-
TiO2/PAN was measured by monitoring the decomposition of five
2
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organics (acid red 18, methylene blue, sodium fluorescence, xylenol Catalysts characterization
orange and phenol) aqueous solutions at the same conditions. For
The crystalline structure of the as-prepared samples was
determined by X-ray diffraction (XRD) measurements. XRD patterns
of TiO2/PAN, AgBr (10%)-TiO2/PAN and AgI (10%)-TiO2/PAN are
shown in Fig. 1. All samples exhibited the diffraction peaks of
anatase TiO2 (JCPDS file No. 21-1272), which proved that the
solvothermal treatment was beneficial to the formation of anatase
TiO2.29,31,35 The wide peaks appeared in the range of 15° and 20° had
been detected, which should be attributed to PAN polymer phase.35
In addition to the diffraction peaks of TiO2 and PAN, the peaks at 2θ
= 31.28° and 44.63° were indexed to AgBr (200) and (220) (JCPDS
file No. 06-0438),40 while the peak around 2θ = 22.42° should be
assigned to β-AgI (100) (JCPDS file No. 09-0374).6,16,20,21 No other
diffraction peaks were displayed in the as-prepared AgX (10%)-
TiO2/PAN. These results revealed that AgX (X= Br, I) nanoparticles
could be synthesized by gas/solid reaction.
detecting the reactive species during photocatalytic degradation,
holes (h+), hydroxyl radicals (•OH), superoxide radical (•O2ˉ) and 1O2
were investigated by adding 0.2 mM ethylenediamine tetraacetic
acid disodium salt (Na2EDTA), 2 mM isopropanol (IPA), 0.2 mM p-
benzoquinone (BQ) and 0.2 mM sodium azide (NaN3), respectively.
The photocatalytic conditions were similar to the above
experiments.
To investigate the photochemical stability of the catalysts, the
recycling tests of AgX-TiO2/PAN with the optimum component,
which could completely degrade MO, were conducted for five times.
At the end of each cycle, the catalysts were collected by filtration,
washed with deionized water and dried at 80 °C for 1 h. Then fresh
MO solution was mixed with the used catalysts for the next cycle.
Results and discussion
Synthetic procedures
The overall fabrication procedures of AgX-TiO2/PAN (X= Br, I)
nanocomposites are illustrated in Scheme 1. It started with the
preparation of PAN nanofibers by electrospinning 8 wt% PAN/DMF
homogeneous solution, proceeding with dispersing electrospun PAN
nanofibers in a Ti(OC4H9)4/ethanol solution and then following to be
physical adsorption for 24 h. Then, TiO2 nanoparticles embedded on
PAN nanofibers were fabricated by the alcoholysis of Ti(OC4H9)4
during solvothermal process, which promoted the formation of TiO2
with highly crystallization and uniform size. The as-prepared
TiO2/PAN hybrid nanofibers were added in an AgNO3 aqueous
solution and Ag (I) could be absorbed on their surface. Finally, AgX-
TiO2/PAN nanocomposites were fabricated by transforming Ag (I)
into AgX through evaporated iodine or bromine.
Fig. 1 XRD patterns of (a) TiO2/PAN, (b) AgBr (10%)-TiO2/PAN and (c)
AgI (10%)-TiO2/PAN.
(I)
(II)
solvothermal
electrospinning
PAN
+
DMF
Ti(OC4H9)4
TiO2/PAN hybrid nanofibers
(III)
PAN nanofibers
physical
adsorption
AgNO3
(V)
(IV)
gas/solid
reaction
visible light
photocatalysis
recycled
I2 or Br2
organics
AgX-TiO2/PAN nanocomposites
Ag (I)-TiO2/PAN
AgX
TiO2
Ag (I)
Fig. 2 FE-SEM images of (a) PAN nanofibers, (b) TiO2/PAN, (c) AgBr
Scheme 1. Schematic illustration for the preparation of AgX-TiO2/
(10%) -TiO2/PAN and (d) AgI (10%)-TiO2/PAN.
PAN (X= Br, I) nanocomposites.
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The microstructures of the obtained samples were examined by
field-emission scanning electron microscopy (FE-SEM). Fig. 2a shows
The morphologies of the obtained samples were determined by
the FE-SEM image of PAN nanofibers, which served as supporters in TEM images. The low resolution TEM image of TiO2/PAN was
the as-prepared photocatalysts. It could be clearly seen that PAN presented in Fig. 3a. Small nanoparticles were uniformly distributed
nanofibers possessed a smooth surface and a uniform diameter. As on the surface of PAN nanofibers, while the interplanar spacing of
observed in Fig. 2b, TiO2/PAN hybrid nanofibers still maintained the 0.352 nm corresponded to TiO2 (101) (Fig. 3b),40,42 indicating the
morphology and structure of the original PAN nanofibers after formation of anatase TiO2 after solvothermal treatment. As shown
solvothermal treatment. Compared with pure PAN nanofibers, in Fig. 3c and d, after modified by AgX (X= Br, I) nanoparticles, no
uniform TiO2 nanoparticles were compactly grown on the surface of obvious aggregation was found in the AgX-TiO2/PAN samples.The
PAN nanofibers. In consequence, Ag+ ions were coupled with TiO2 TEM images (Fig. 3e and f) exhibited the existence of AgX and the
nanoparticles rather than PAN nanofibers via physical adsorption HRTEM images (Fig. 3g and h) further confirmed the formation of
process. AgX (X= Br, I) nanoparticles were generated without the heterojuctions between AgX and TiO2. STEM-HADDP pictures
aggregation via gas/solid reaction (shown in Fig. 2c and d). It could were also used to clarify the distribution of different elements in
be presumed that AgX nanoparticles had intimate contact with TiO2. AgX-TiO2/PAN nanocomposites (Fig. 4a and b). PAN nanofibers were
mainly composed of C and N, which distribution was similar to the
structures of composite catalyst. Through the location of Ti and O, it
could be presumed TiO2 nanoparticles were uniformly and densely
coated on the surface of PAN nanofibers. There were sporadic signs
of Ag and X elements due to their low content.
(a)
(b)
Fig. 3 TEM (a), HRTEM (b) images of TiO2/PAN (a, b); TEM (c,d,e,f)
and HRTEM (g,h) images of AgBr (10%)-TiO2/PAN (c,e,g) and AgI
(10%)-TiO2/PAN (d, f, h)
Fig.
4 STEM-HAADF images of the as-prepared AgBr (10%)-
TiO2/PAN(a) and AgI (10%)-TiO2/PAN (b).
4
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absorption peaks at 2244 cm-1 were attributed to the stretching
Fig. 5 gives the diffuse reflectance UV-vis spectra (DRS) of vibration of -C≡N- in PAN.39,41 The peak at 1740 cm−1 might originate
different photocatalysts. As shown in Fig. 5a and b, two absorption from the vibration of C=O bonds existed in the hydrolyzed PAN
bands presented in the range of 200-300 nm attributing to the nanofibers or residual DMF.39 In comparison to pure PAN, the FTIR
contribution of PAN polymer, while a visible-light absorption band spectra of TiO2/PAN (Fig. 6b) exhibited a broad absorption band
could be observed after loading AgX on the surface of PAN below 1000 cm-1 corresponding to Ti-O-Ti vibration, which proved
nanofibers, expecially for AgI, due to the light adsorption of AgX. Fig. the formation of TiO2.42 In addition to characteristic absorption
4c exhibits a strong adsorption peak below 390 nm, which should peaks of PAN and TiO2, no different peaks were observed in AgX
be attributed to the band gap energy of anatase TiO2 (3.2 eV).18,44 (10%)-TiO2/PAN. Furthermore, a shift of the -C≡N- vibration was not
Except for the adsorption peak of TiO2, the as-prepared AgBr- detected in AgX (10%)-TiO2/PAN, which indicated that there was no
TiO2/PAN had a little absorption in the visible light region.18,42,43,44 any bond formation between -C≡N- group in AgX and PAN.41 So, it
Moreover, AgI-TiO2/PAN exhibited a strong absorption band around could be concluded that AgX nanoparticles did not connect to the
400-436 nm, which should be assigned to the presence of AgI.16,19,20 PAN nanofibers.
The diffuse reflectance UV-vis spectra future demonstrated the
formation of AgX (X= Br, I).
a
6
4
2
0
a
b
c
d
e
f
PAN
AgBr/PAN
TiO2/PAN
AgBr (2%)-TiO2/PAN
AgBr (4%)-TiO2/PAN
AgBr (6%)-TiO2/PAN
AgBr (8%)-TiO2/PAN
AgBr (10%)-TiO2/PAN
A
d
g
h
b
c
e
d
a
b
c
PAN
TiO2/PAN
AgBr (10%)-TiO2/PAN
AgI (10%)-TiO2/PAN
b
g
h
c
f
d
a
3500 3000 2500 2000 1500 1000 500
Wavelength (nm)
200
300
400
500
600
Wavelength (nm)
Fig. 6 FTIR spectra of (a) PAN nanofibers, (b) TiO2/PAN, (c) AgBr
(10%)-TiO2/PAN and (d) AgI (10%)-TiO2/PAN.
a
b
c
d
e
f
PAN
AgI/PAN
TiO2/PAN
AgI (2%)-TiO2/PAN
AgI (4%)-TiO2/PAN
AgI (6%)-TiO2/PAN
AgI (8%)-TiO2/PAN
AgI (10%)-TiO2/PAN
B
4
3
2
1
Visible light photocatalytic activity and two-stage photocatalytic
mechanism of AgX-TiO2/PAN
The photocatalytic activities of the as-prepared AgX-TiO2/PAN were
evaluated by visible-light degradation of MO aqueous solution. For
comparison, the performances of pure PAN, AgX/PAN and TiO2/PAN
were also investigated. The photodegradation curves of MO over
the different samples as a function of visible light irradiation time
were plotted in Fig. 7. The degradation rate was represented as the
variation of (1-C/C0) with irradiation time, in which C0 and C were
the concentration of MO at initial time and at certain time interval,
respectively. As shown in Fig. 7a, nearly 10% of MO was absorbed
by pure PAN nanofibers in dark condition, while the decolorization
rate of MO kept unchanged after the light was turned on. It
indicated that PAN nanofibers, a supporter in the as-prepared
catalysts, only had adsorption ability toward MO, which was
beneficial for the degradation of MO. Compared with TiO2/PAN and
AgX/PAN (shown in Fig. 7b and c), AgX-TiO2/PAN catalysts exhibited
the much higher photocatalytic activity. As shown in Fig. 7(d-h), the
g
h
g
h
d
b
a
e
c
f
200
300
400
Wavelength (nm)
500
600
Fig. 5 UV-vis diffuse reflectance spectra of (a) PAN nanofibers, (b)
AgX/PAN, (c) TiO2/PAN and (d)-(h) AgX (y%)-TiO2/PAN-n ( y= 2, 4, 6,
8, 10)
The FTIR spectra of pure PAN, TiO2/PAN, AgBr (10%)-TiO2/PAN
and AgI (10%)-TiO2/PAN are shown in Fig. 6. The characteristic
molar ratios of AgX/TiO2 had
a significant impact on the
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photodegradation activity. In the as-prepared AgX-TiO2/PAN system, different organics and AgI (10%)-TiO2/PAN showed much higher
when the AgX content increased, the AgX nanoparticles were grown photocatalytic activity than AgBr (10%)-TiO2/PAN for the same
dispersedly on the surface of TiO2, which was beneficial for the light organic. Except for phenol solution, the color of organic pollutants
adsorption. Moreover, the effective contacted interface area gradually diminished as the irradiation time increased, suggesting
between AgX and TiO2 increased as the AgX ratio rose, promoting that the chromophoric groups had been destroyed. It could be
the separation of electron-hole pairs.47 Thus, the photocatalytic presumed that continuing to extend the illumination time, the
degradation efficiency of MO increased with the AgX loading. As organics, such as phenol and xylenol orange, could be degraded
shown in Fig. 7h, most of MO had been removed in the presence of completely. In summary, the as-prepared AgX-TiO2/PAN catalysts
AgX (10%)-TiO2/PAN after illumination for 4.5 h.
exhibited the excellent photocatalytic activities for the degradation
of methylene blue, sodium fluorescein, phenol, xylenol orange and
acid red 18, which could extend their application in the field of
degrading different organic pollutants in waste water.
0.0
a
b
c
A
0.2
0.4
0.6
0.8
1.0
0.0
a
b
c
d
e
20mg/l Sodium fluorescein
20mg/l Methylene blue
30mg/l Xylenol orange
20mg/l Phenol
0.2
0.4
0.6
0.8
1.0
e
d
5mg/l Acid red 18
a
PAN
TiO2/PAN
AgBr/PAN
AgBr (2%)-TiO2/PAN
AgBr (4%)-TiO2/PAN
AgBr (6%)-TiO2/PAN
AgBr (8%)-TiO2/PAN
AgBr (10%)-TiO2/PAN
b
c
d
e
f
d
e
f
g
c
g
h
h
b
0
1
2
3
4
5
a
Time (h)
A
0
1
2
3
4
5
Time (h)
0.0
0.2
0.4
0.6
0.8
1.0
a
b
c
B
0.0
0.2
0.4
0.6
0.8
1.0
a
b
c
d
e
20mg/l Sodium fluorescein
5mg/l Acid red 18
20mg/l Methylene blue
20mg/l Phenol
30mg/l Xylenol orange
a
b
c
d
e
f
PAN
TiO2/PAN
AgI/PAN
AgI (2%)-TiO2/PAN
AgI (4%)-TiO2/PAN
AgI (6%)-TiO2/PAN
AgI (8%)-TiO2/PAN
AgI (10%)-TiO2/PAN
d
e
d
e
f
g
h
g
h
c
0
1
2
3
4
5
b
a
Time (h)
B
0
1
2
3
4
5
Fig. 7 The degradation curves of MO over (a) PAN, (b) TiO2/PAN, (c)
Time (h)
AgX/PAN and (d)-(h) AgX (y%)-TiO2/PAN ( y= 2, 4, 6, 8, 10)
Fig. 8 The adsorption and photodegradation of different oragnics
To confirm the universal degradation ability of the as-prepared (100 ml) in aqueous solutions containing 0.2 g AgBr (10%)-TiO2/PAN
catalysts, the photocatalytic activities toward different organics (A) or AgI (10%)-TiO2/PAN (B).
were evaluated in the presence of AgBr (10%)-TiO2/PAN and AgI
(10%)-TiO2/PAN. The degradation curves against irradiation time
were plotted in Fig. 8A and B. The initial concentrations of
Recycling experiments were carried out to evaluate the stability
methylene blue, sodium fluorescein, phenol, xylenol orange and and durability of AgX (10%)-TiO2/PAN for MO degradation.
acid red 18 were 20mg/l, 20mg/l, 20mg/l, 30mg/l and 5mg/l, Notably, the catalysts could be easily separated from an aqueous
respectively. It was clear that the adsorption ability and degradation solution without any loss due to the application of membranous
efficiency of the as-prepared AgX (10%)-TiO2/PAN varied with PAN. In each cycle, the adsorption process was conducted in dark
6
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condition for 30 min to establish adsorption-desorption equilibrium
and then the irradiation process was lasted for 4.5 h After 4.5 h of
.
visible light irradiation, the catalysts were filtered, washed with
distilled water and dried at 80 °C for 1 h. The recycled tests were
performed for five times and described in Fig. 9. A slightly reduction
of photocatalytic activity could be observed after the second run,
while the degradation efficiency of MO was reduced obviously in
the fourth and fifth runs, which was attributed to the
photocorrosion of AgX. The decomposition of AgX could be proved
by the diffuse reflectance UV-vis spectra (shown in Fig. 10b), in
which a strong adsorption band appeared in visible region was
attributed to the surface plasma resonance (SPR) of Ag
nanoparticles.15 The catalysts recycled for five times were put in I2
(or Br2) atmosphere for regeneration, which could be convinced by
the UV-vis DRS of the regenerated AgX-TiO2/PAN (Fig. 10c).
Additional experiments showed that the regenerated catalysts
regain high photocatalytic performance. These results demonstrated
6
4
2
0
a
b
c
AgBr (10%)-TiO2/PAN
AgBr (10%)-TiO2/PAN (5 runs)
AgBr (10%)-TiO2/PAN (regenerated)
b
c
a
A
200
300
400
500
600
Wavelength (nm)
that the as-prepared AgX-TiO2/PAN catalysts had
photochemical stability and were easily regenerated.
a
certain
6
a
b
c
AgI (10%)-TiO2/PAN
AgI (10%)-TiO2/PAN (5 runs)
AgI (10%)-TiO2/PAN (regenerated)
0.0
4
2
0
1st
2nd
5th
3rd
4th
0.2
0.4
0.6
0.8
1.0
b
c
a
B
200
300
400
500
600
Wavelength (nm)
A
0
5
10
15
Time (h)
20
25
30
Fig. 10 The UV-vis diffuse reflectance spectra of (a) fresh AgX (10%)-
TiO2/PAN, (b) AgX (10%)-TiO2/PAN (recycled for five runs) and (c)
AgX (10%)-TiO2/PAN (regenerated).
0.0
0.2
0.4
0.6
0.8
1.0
To illustrate the photocatalytic mechanism of AgX-TiO2/PAN (X=
Br, I), a series of control experiments with different scavengers were
carried out to investigate the generation and contribution of
reactive species such as h+, •OH, •O2− and 1O2, during visible light
photocatalysis. In this study, 2 mM IPA was added to quench •OH in
3rd
1st
2nd
4th
5th
−
the solution,16,39 0.2 mM Na2EDTA for h+,47 0.2 mM BQ for •O2
42,43,46,47
1
and 0.2 mM NaN3 for O2.42,43 Fig. 11A and B show the
photocatalytic degradation curves of MO over AgBr (10%)-TiO2/PAN
and AgI (10%)-TiO2/PAN in the presence of different scavengers. The
addition of NaN3 and IPA scavengers had a slightly reduction in MO
degradation efficiency, which implied that 1O2 and •OH were not
the main reactive species for MO degradation. The most depressed
degradation rate occurred in the presence of Na2EDTA, indicating
B
0
5
10
15
20
25
30
Time (h)
Fig. 9 The cycling degradation efficiency for MO of 0.2 g AgBr (10%)- that h+ was the most significant reactive species. The presence of
TiO2/PAN (A) or AgI (10%)-TiO2/PAN (B) under visible-light BQ also dramatically inhibited the removal efficiency of MO, which
−
irradiation.
suggested that •O2 played an important role in the degradation of
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ARTICLE
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MO.
organics. The relevant reactions could be shown in Table 1.
As the illumination time extended, AgX could be decomposed
into Ag0. Therefore, fresh AgX-TiO2/PAN was turned into Ag-AgX-
TiO2/PAN. In the newly constructed system, the electron-hole pairs
could also be formed in Ag0 owing to their surface plasmon resona-
nce (SPR) under visible light irradiation.42,43,45,46 Considering the
SPR-induced local electromagnetic field and the polarization effect
of negatively charged AgX surface, the photoinduced electrons and
holes from Ag0 would migrate to the completely different directions.
The photoinduced electrons in Ag0 might migrate to the conduction
band of TiO2, while the holes would transfer from Ag0 to valance
band of AgX particles, leading to the efficient separation of electron-
hole pairs in the Ag0.42,43,50 Besides, the electrons generated from
AgX could be injected into Ag0 and immediately migrate to the CB of
TiO2.42,43 The electrons transferred from AgX could significantly
inhibit the reaction where Ag+ ions of AgX might capture the
electrons generated in the CB of AgX, resulting in the decomposition
of AgX.43 The generation process of reactive species in this system
was similar to that of fresh AgX-TiO2/PAN. The major reaction steps
were summarized in Table 2. As for the regenerated catalysts,
charge transfer in photocatalytic process would go through the
same procedure occurred in fresh AgX- TiO2/PAN.
0.0
0.2
0.4
0.6
0.8
1.0
A
Na2EDTA
BQ
IPA
no scavenger
0.2 mM Na2EDTA
0.2 mM BQ
0.2 mM NaN3
2 mM IPA
NaN3
no scavenger
0
1
2
3
4
5
Time (h)
0.0
0.2
0.4
0.6
0.8
1.0
B
Na2EDTA
BQ
O2
CB
e- e- e-
CB
organic pollutants
e-
e-
e-
no scavenger
0.2 mM Na2EDTA
0.2 mM BQ
0.2 mM NaN3
2 mM IPA
IPA
-
.O2
NaN3
products
no scavenger
products
CO2+H2O
0
1
2
3
4
5
X-
CO2+H2O
Time (h)
h+ h+ h+
VB
organic pollutants
X0
VB
TiO2
AgX
Fig. 11 Photodegradation curves of MO over 0.2 g AgBr (10%)-
TiO2/PAN (A) or AgI (10%)-TiO2/PAN (B) under visible light
irradiation without and with the presence of different scavengers.
+
h
(I)
On the basis of above experimental results and related
researches, the photodegradation mechanism of AgX-TiO2/PAN (X=
Br, I) under visible light irradiation was proposed and illustrated in
Scheme 2. The photocatalytic process could be divided into two
stages for the decomposition of AgX. At the first stage, under visible
light irradiation, AgX nanoparticles could be excited to produce
photoinduced electrons (eˉ) and holes (h+), due to their narrow
band gap (AgBr, 2.6 eV; AgI, 2.8eV).43,48 The generated eˉ in AgX
could migrate to the conduction band (CB) of TiO2 for the less
negative CB of TiO2 as compared to that of AgX.48,49 Then the
electrons would be trapped by surface absorbed O2 to form•O2ˉ and
reactive oxygen species such as •OH and 1O2 could be further
generated. The left holes (h+) might transfer to the interface
between AgX and TiO2 and oxidize Xˉ to X0. X0 could oxidize organic
pollutants while being reduced to Xˉ. Meanwhile, •O2ˉ, •OH and 1O2
also had a strong oxidizing ability to degrade organic pollutants.
Therefore, the efficient separation of electron-hole pairs was
achieved, which facilitated the photocatalytic degradation of
O2
CB
e-
e-
CB
e-
organic
pollutants
e-
e -
e-
.O2
-
products
products
CO2+H2O
h+
X-
CO2+H2O
h+ h+
VB
Ag
h+
organic pollutants
X0
VB
AgX
TiO2
(II)
Scheme 2. Schematic illustration of two-stage photocatalytic
mechanism within AgX-TiO2/PAN under visible light irradiation.
8
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nanoparticles were densely and uniformly distributed on the surface
Table 1. Relevant reactions happened in the first stage of of PAN nanofibers and AgX nanoparticles were closely combined
photocatalytic process with TiO2 rather than PAN nanofibers. The fabricated catalysts
exhibited highly visible light photocatalytic performance in
a
Reactions
No.
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
degrading different organics. But most of all, they could be easily
separated and regenerated. Based on the experimental results, a
possible photocatalytic process was also proposed. It could be
concluded that the high photocatalytic activity of AgX-TiO2/PAN was
attributed to the well contact between AgX and TiO2 and the
efficient separation of photogenerated electron-hole pairs. The
excellent photocatalytic performance and reusable property of AgX-
TiO2/PAN enabled it to be a promising material in the field of
environmental remediation.
AgX + hv → AgX (eˉ + h＋)
AgX ( eˉ + h＋) + TiO2 → AgX ( h＋) + TiO2 (eˉ)
TiO2 (eˉ) + O2 → •O2ˉ + TiO2
•O2ˉ+ H＋ → HO2•
HO2• + TiO2 (eˉ) + H＋→ H2O2 + TiO2
H2O2 +TiO2 (eˉ) → •OH + OHˉ + TiO2
•O2ˉ + •OH → 1O2 + OHˉ
Acknowledgements
The authors gratefully acknowledge the support from the National
Natural Science Foundation of China (no. 21266016) and Specialized
Research Fund for the Doctoral Program of Higher Education (no.
20121514120004).
AgX ( h＋) + Xˉ→ AgX + X0
X0 + organics → products + Xˉ
•O2ˉ, •OH, 1O2 + organics → products
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