Research Article
pubs.acs.org/journal/ascecg
Facile In Situ Self-Sacrifice Approach to Ternary Hierarchical
Architecture Ag/AgX (X = Cl, Br, I)/AgIO3 Distinctively Promoting
Visible-Light Photocatalysis with Composition-Dependent
Mechanism
Chao Zeng,† Yingmo Hu,*,† Yuxi Guo,† Tierui Zhang,‡ Fan Dong,§ Yihe Zhang,† and Hongwei Huang*,†
†
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials,
School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China
‡
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
§
Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering,
Chongqing Technology and Business University, Chongqing 400067, China
S Supporting Information
*
ABSTRACT: Three series of ternary hierarchical architecture photocatalysts Ag/AgX
(X = Cl, Br, I)/AgIO3 were fabricated for the first time by a facile in situ ion-exchange
route. The novel ternary architectures are confirmed by XRD, XPS, SEM, TEM, EDX, and
EDX mapping. In contrast to pristine AgIO3, the Ag/AgX (X = Cl, Br, I)/AgIO3 composites
show extended absorption edges and highly boosted photoabsorption in the visible region,
which are separately ascribed to the intrinsic absorption of AgX and the surface plasmon resonance
(SPR) effect of Ag species. The photocatalysis activity of Ag/AgX (X = Cl, Br, I)/AgIO3
composites is studied and compared via photodegradation of methyl orange (MO) under
visible-light (λ > 420 nm) irradiation. It is interesting to find that the activity enhancement
levels are different for Ag/AgX (X = Cl, Br, I)/AgIO3 with four types of photocatalytic
mechanism, which are closely related to the type of AgX or the component content in
Ag/AgX (X = Cl, Br, I)/AgIO3. The separation behaviors of charge carrier were also
systematically investigated by the PL and EIS. The study may furnish new perspective into
controllable fabrication of hierarchical architecture photocatalysts with multiform photocatalytic mechanism.
KEYWORDS: Photocatalyst, Ag, AgX (X = Cl, Br, I), AgIO3, Visible-light
■
INTRODUCTION
Silver-containing materials show great promise, including
Ag3PO4,6 Ag2CO3,7 Ag3VO4,8 AgX (X = Cl, Br, I),9 and so on.
They exhibit superior photocatalytic activity for water splitting
and pollutant photodegradation. However, the above silvercontaining photocatalysts still suffer from the downsides of high
recombination rate of hole−electrons in the phtocatalysis
process, which seriously restrict their practical applications.
Thus, heterojunction photocatalytsts are fabricated to improve
the catalytic performance. For instance, Ag3PO4/TiO2,10
AgPO4/WS2,11 etc. exhibit higher photocatalytic activity than
pristine AgPO4 for degrading rhodamine B or methylene blue.
Recently, AgIO3 was reported as a new photocatalyst, which can
not only efficiently photodegrade dye12 but also convert CO2 to
CH4 and CO.13 However, it has very weak absorption in the visible
light region. Therefore, efforts should be made to enhance the
visible light absorption and inhibit hole−electron recombination
of AgIO3. As previously described, developing heterostructure is
Semiconductor photocatalysts have attracted great attention
owing to their unique abilities for environmental purification
and energy generation.1,2 Among the various semiconductor
materials, titanium dioxide (TiO2) has been a widely researched
photocatalyst because of its low cost, long-term stability, and
nontoxicity. However, pristine TiO2 exhibits low quantum yields
and inferior visible-light utilization efficiency due to its wide
band gap (3.0−3.2 eV), which extremely restricts its practical
application. Therefore, it is urgent to develop novel visiblelight-driven photocatalysts. Currently, there are two main strategies to obtain visible-light-driven photocatalysts. The first one is
to broaden the photoresponse of TiO2 into the visible region
by doping metal or nonmetal elements, coupling TiO2 with
conducting polymer, and building a heterostructure construction.3
Another alternative strategy is to exploit new materials with a small
band gap, such as sulfides, oxides, and oxygen acid salts.4,5
However, to date, these photocatalysts are still unappeasable for
practical application due to the weak photoreactivity and low
stability.
© 2016 American Chemical Society
Received: February 18, 2016
Revised: April 5, 2016
Published: April 18, 2016
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an effective avenue to realize the above purpose. On the other
hand, surface plasmon resonance (SPR) of Ag species can endow
the Ag nanoparticles with stronger visible light absorption.
For example, Yang et al. prepared a Ag/TiO2 plasmonic photocatalyst with high photocatalytic activity for degradation of
gaseous acetone under visible light irradiation.14 Jiang et al.
reported a Ag/BiOCl plasmonic photocatalyst that possesses
efficient photoactivity under excitation of visible light.15 Besides,
Ag/AgCl composite material shows noteworthy photocatalysis
activity under visible light irradiation.16 Consequently, it is
available to employ the SPR technique to broaden the response
region of AgIO3. In a word, combining developing a heterostructure and utilizing an SPR technique is highly desirable and
anticipated.
In this work, we developed novel Ag/AgX (X = Cl, Br, I)/AgIO3
ternary heterojunction photocatalysts by a facile in situ ionexchange route for the first time, which are heterostructural and
contain surface plasmon resonance (SPR). The photocatalysis
properties of the Ag/AgX (X = Cl, Br, I)/AgIO3 composite are
measured by decomposition of MO removal under visible light
illumination (λ > 420 nm). The composite photocatalysts all
depict greater photocatalytic activity under irradiation of visible
light. The formation of the heterostructure photocatalysts and the
diverse photocatalytic mechanism of three series composites are
studied in detail.
■
EXPERIMENTAL SECTION
Preparation of the Photocatalyst. All starting materials were of
analytical grade and used as received. The hydrothermal method was
used to prepare AgIO3. Representatively, 2 mmol of AgNO3 and
stoichiometric I2O5 were dissolved in 40 mL of deionized water
with stirring. Then, the resulting white suspension was transferred into
a 50 mL Teflon-lined stainless steel autoclave and heated at 180 °C for
24 h. After natural cooling, the product was collected by filtration,
washed repeatedly with ethanol and distilled water, and then maintained
at 80 °C for 10 h.
The Ag/AgX (X = Cl, Br, I)/AgIO3 composites were synthesized by a
facile in situ ion-exchange method between KX (X = Cl, Br, I) and a
AgIO3 precursor. A total of 1 mmol of pristine precursor AgIO3 was
dissolved in 20 mL of distilled water, and then the aqueous solution KX
was added into the AgIO3 solution stepwise under magnetic agitation.
After stirring for 5 h at room temperature, the suspension was filtrated,
washed, and dried in a desiccator overnight. The as-prepared samples
with molar ratios of KX/AgIO3 of 20%, 40%, 60%, 80%, and 100% are
marked as 20%, 40%, 60%, 80%, and 100% Ag/AgX (X = Cl, Br, I)/AgIO3,
respectively. Ag/AgX (X = Cl, Br, I) was fabricated by a precipitation
route with KX (X = Cl, Br, I) and AgNO3.
Characterization. X-ray powder diffraction (XRD) was recorded
on a Bruker D8 focus with graphite monochromatized Cu Kα radiation
(40 kV/40 mA). Scanning electron microscopy (SEM) images were
carried out on a Hitachi S-4800 field emission scanning electron
microscope operated at 10.0 kV. Transmission electron microscopy
(TEM) and high-resolution TEM (HRTEM) were evaluated by
JEM-2100 electron microscopy (JEOL, Japan). UV−vis diffuse
reflectance spectra (DRS) were performed with a Varian Cary 5000
UV−vis spectrophotometer. The X-ray photoelectron spectroscopy
(XPS) was identified on an ESCALAB 250xi (ThermoFsher, England)
electron spectrometer. The photoluminescence emission (PL) spectra
were conducted using a Hitachi F-4600 fluorescence spectrophotometer
PL system with a xenon lamp (400 V, 150 W) as an excitation source.
All the above-mentioned measurements were taken at room temperature.
Photocatalytic Activity. The photocatalytic activities of Ag/AgX
(X = Cl, Br, I)/AgIO3 were tested by photocatalytic decomposition
of methyl orange (MO) under visible light with a 500 W Xe lamp
(λ > 420 nm). In a typical procedure, 20 mg of photocatalyst was
ultraphonically suspended into 40 mL of MO (2× 10−5 mol/L) aqueous
solution. Before photoreaction, the suspension was magnetically stirred
Figure 1. XRD patterns of AgIO3, AgX (X = Cl, Br, I), and Ag/AgX
(X = Cl, Br, I)/AgIO3 samples.
in darkness for 1 h to obtain an adsorption−desorption equilibrium.
Later, about 3 mL of the liquid was taken at a certain period time
and separated through centrifugation to obtain the supernatant.
The concentration of MO liquid was analyzed by measuring the
absorbance at the characteristic band of 464 nm on a Cary 5000 UV−vis
spectrophotometer. The temporal absorption spectra of MO liquid were
obtained by the Cary 5000 UV−vis spectrophotometer.
Active Species Trapping Experiment. To detect the active
species generated in the photocatalytic process, disodium ethylenediaminetetraacetate (EDTA-2Na), ethylene glycol (IPA), and
1,4-benzoquinone (BQ) were chosen as the hole (h+), hydroxyl radical
(•OH), and superoxide radical (•O2−) scavengers, respectively.
Typically, 20 mg of photocatalyst with different scavengers (1 mmol)
were ultraphonically dispersed in MO aqueous solution (40 mL,
2× 10−5 mol/L), and the following processes were similar to the above
MO photodegradation experiment.
Photoelectrochemical Measurement. The electrochemical
impedance spectra (EIS) are recorded in a standard three-electrode
system of the electrochemical station (CHI-660B, China) with Na2SO4
(0.1 M) as the electrolyte solution. The measurement is performed with
irradiation of a 300 W xenon lamp equipped with a 420 nm filter at 0.0 V.
The saturated calomel electrode (SCE) was chosen to be the reference
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Figure 2. Typical XPS survey spectra (a) and Ag 3d spectra (b) of Ag/AgX (X = Cl, Br, I)/AgIO3 and a high-resolution XPS spectrum for Ag 3d of 100%
Ag/AgCl/AgIO3 (c), 80% Ag/AgBr/AgIO3 (d), 40% Ag/AgI/AgIO3, (e) and 100% Ag/AgI/AgIO3 (f).
electrode, and the platinum wires were employed as the counter
electrode. The working electrode was the AgIO3, Ag/AgBr, and 80%
Ag/AgBr/AgIO3 films coated on ITO.
To investigate the element composition and the chemical
states of the as-obtained Ag/AgX (X = Cl, Br, I)/AgIO3 sample,
X-ray photoelectron spectroscopy (XPS) analysis was performed
and is shown in Figure 2. The main peaks corresponding to Ag
3d, I 3d, O 1s, Cl 2p, Br 3d, and C 1s all can be found in their
perspective samples, and the C peak is due to the adventitious
hydrocarbon of the XPS instrument (Figure 2a). Due to the
distinct binding energy between Ag and X (X = Cl, Br, I),
AgCl, AgBr, and AgI exhibit different positions of the Ag peak
(Figure 2b). Figure 2c exhibits that the Ag 3d spectra of
Ag/100%AgCl/AgIO3 could be divided into two sets of bands.
The two peaks at 373.4 and 367.4 eV are attributed to Ag 3d3/2
and Ag 3d5/2 of Ag+; meanwhile the peaks at 374.1 and 368.0 eV
can be assigned to Ag 3d3/2 and Ag 3d5/2 of Ag0 species, respectively.17,18 For the 80% Ag/AgBr/AgIO3 sample (Figure 2a), the
obvious peaks of Ag, Br, I, O, and C elements can be observed.
From Figure 2d, typical peaks of Ag 3d can be deconvoluted into
two different peaks. The bands at 373.7 and 367.7 eV are ascribed
■
RESULTS AND DISCUSSION
The XRD patterns of the pristine AgIO3, Ag/AgX (X = Cl, Br, I),
and Ag/AgX (X = Cl, Br, I)/AgIO3 composites are depicted
in Figure 1. Take the Ag/AgCl/AgIO3 series as an instance
(Figure 1a). It can be found that all the diffraction peaks can be
well-indexed to the standard data of AgIO3 (ICSD # 14100) and
AgCl (JCPDS # 1−1013), and the characteristic peaks of AgCl
gradually strengthen with raising the KCl content. In addition,
the existence of metallic Ag has been confirmed through the step
scanning XRD (2θ ranging from 37.5° to 38.5°) in samples,
which is attributed to the decomposition of AgCl. Similar results
are also observed in Ag/AgBr/AgIO3 (Figure 1b) and Ag/AgI/AgIO3
(Figure 1c) series. These results evidenced that the ternary
Ag/AgX (X = Cl, Br, I)/AgIO3 composites are obtained.
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to Ag+, and the other two bands centered at 374.7 and 368.6 eV
are ascribed to Ag0 species. With regard to 40% Ag/AgI/AgIO3
and 100% Ag/AgI/AgIO3 (Figure 2a), the XPS signals from Ag,
Cl, I, O, and C elements can be seen. The XPS spectra of Ag 3d
for 40% Ag/AgI/AgIO3 and 100% Ag/AgI/AgIO3 consist of two
peaks at ∼374 and ∼368 eV. In the 40% Ag/AgI/AgIO3 sample
(Figure 2e), the peaks at 374.5 and 368.5 eV match Ag0 species,
and the other two peaks centered at 373.8 and 367.8 eV are
resulted from Ag+. For 100% Ag/AgI/AgIO3 (Figure 2f), the
peaks at 374.4 and 368.4 eV are ascribed to Ag0 species, and the
other two peaks centered at 373.8 and 367.8 eV are related to
Ag+. The XPS analyses confirmed the coexistence of Ag+ and Ag0
species in the Ag/AgX (X = Cl, Br, I)/AgIO3 sample.
To study the morphology and structure of the as-prepared
ternary composites, scanning electron microscopy (SEM)
images of AgIO3, Ag/AgX (X = Cl, Br, I), and Ag/AgX
(X = Cl, Br, I)/AgIO3 were obtained (Figure 3). The AgIO3
samples and shown in Figure 4. The Be element is taken as a
contrast, and the signal of Al is due to the utilizing of aluminum
foil. The result strongly demonstrated the formation of ternary
hierarchical architecture of Ag/AgCl/AgIO3. SEM images of
Ag/AgX (X= Br, I) and Ag/AgX (X= Br, I)/AgIO3 with different
X− (X = Br, I)/IO3− ratios are shown in Figure 5. It also can be
seen that the Ag nanoparticles and AgX (X = Br, I) uniformly
inlaid on the surface of AgIO3 particles, and their amounts also
orderly rise with increasing the molar ratio of X− (X = Br, I)/IO3−.
On the basis of the above observation, the formation process of
the Ag/AgX (X = Cl, Br, I)/AgIO3 ternary hierarchical composites
is illustrated in Scheme 1. When the KX (X = Cl, Br, I) solution
was added into a AgIO3 suspension, AgX (X = Cl, Br, I) nanosheets would generate via an ion-exchange reaction. Meanwhile, part of the as-generated AgX sheets would decompose into
Ag particles in this process. As the appearance of Ag particles is
mainly induced by outer light, they formed on the surface
of AgX. Thus, the ternary hierarchical architectures of Ag/AgX
(X = Cl, Br, I)/AgIO3 are constructed. To further inspect the
elemental composition and distribution of Ag/AgBr/AgIO3, EDX
mapping is recorded by taking 80% Ag/AgBr/AgIO3 as an
instance. As shown in Figure 6, the O, Br, Ag, and I elements are all
homogeneously distributed in the 80% Ag/AgBr/AgIO3 sample.
To corroborate the interfacial interaction between the different
phases, TEM and HRTEM images of 100% Ag/AgCl/AgIO3 are
conducted and displayed in Figure 7. The two sets of adjacent
fringes with intervals of 0.30 and 2.37 nm correspond well to the
(211) lattice of AgIO3 and (111) lattice of Ag, respectively.
However, AgCl cannot be observed because of its instability under
electron beam irradiation. This similar phenomenon has been
reported in BiVO4/Ag/AgCl.19
The light absorption property is an important factor for
photocatalysts, which can be studied by diffuse reflection
spectroscopy (DRS). Figure 8 displays the diffuse reflection spectra
of AgIO3, Ag/AgX (X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/
AgIO3 composites. It can be seen that the absorption edge of AgIO3
is located at about 380 nm, according to the reported value.13 The
as-prepared Ag/AgX photocatalysts show strong absorption in the
visible range with an obvious absorption peak at ∼550 nm in
contrast to the pure AgX (X = Cl, Br, I) photocatalysts as
reported,16,20,21 which should be ascribed to the surface plasmon
resonance (SPR) of the Ag particle. With increasing the ratio of
X− (X = Cl, Br, I) to IO3−, the absorption edges of the Ag/AgX
(X = Cl, Br, I)/AgIO3 composites continuously red-shift to the
visible light region, which are due to the effect of AgX (X = Cl, Br, I).
Meanwhile, the monotonic strengthening of the visible-light
response for Ag/AgX (X = Cl, Br, I)/AgIO3 composites is attributed to the SPR effect of incremental Ag particles on the surface of
the composite. Light-absorption enhancement of the photocatalyst
in the whole visible-light region is believed to be beneficial to
photodegradation. In addition, the DRS result also supports the
existence of Ag species in the Ag/AgX (X = Cl, Br, I)/AgIO3
photocatalysts.
To calculate the band gap of AgIO3, an equation22 on the basis
of the classical Tauc method, αhv = (αhv − Eg)n, is employed.
AgIO3 is reported as an indirect-transition-allowed semiconductor. Therefore, the value of n for AgIO3 is 2, and band
gap Eg of AgIO3 is calculated to be 3.18 eV. Besides, the
conduction band (CB) position (ECB) and valence band (VB)
position (EVB) of AgIO3 can be calculated through the following
empirical equation:23 ECB = X − Ec − 0.5Eg, EVB = ECB + Eg. For
AgIO3, electronegativity (X) is estimated to be 6.71 eV.13
Figure 3. SEM images of AgIO3 (a), Ag/AgCl (f), and the full-range
Ag/AgCl/AgIO3 composites (b−e). The particles marked by a white
arrow are Ag, and sheets marked by a black arrow are AgCl.
single crystal (Figure 3a) possesses a spindly shaped morphology
and smooth surfaces with a diameter of ∼5 μm. Figure 3f displays
the SEM image of the Ag/AgCl product. It is obvious that the
smooth surface of AgCl (∼1um in size) is uniformly covered
by Ag nanoparticles with a size of ∼50 nm. SEM images of
Ag/AgCl/AgIO3 composites with different Cl−/ IO3− ratios were
illustrated in Figure 3b−e. It can be observed that all of the
Ag/AgCl/AgIO3 heterostructural composites contain the three
phases of Ag, AgCl, and AgIO3. Particularly, the amount of Ag
nanoparticles and AgCl assembled on the surface of AgIO3
gradually increases with enhancing the molar ratio of Cl−/IO3−.
To provide direct evidence for confirming the coexistence of the
three Ag, AgCl, and AgIO3 phases in the Ag/AgCl/AgIO3
composite, EDX was performed on 100% Ag/AgCl/AgIO3
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Figure 4. SEM and EDX images of the 100% Ag/AgCl/AgIO3 composite.
Accordingly, the ECB and EVB for AgIO3 are calculated to be 0.68
and 3.86 eV, respectively.
Disintegrating MO was selected to evaluate the photocatalytic
activities of AgIO3, Ag/AgX (X = Cl, Br, I), and Ag/AgX (X = Cl,
Br, I)/AgIO3 composites under visible light irradiation (λ > 420 nm).
The absorbance−time curves of those samples are depicted
in Figures S1−S3. Without catalyst, MO could only be slightly
degraded, which can be neglected. The pseudo-first-order model
based on the Langmuir−Hinshelwood (LH) kinetics model
was employed to calculate the reaction kinetics of the photodegradation process of MO quantitatively, as shown in the
following equation:24
and Figure 9c, the 60%, 80%, and 100% Ag/AgBr/AgIO3
composites show stronger photocatalytic activity than the pristine
Ag/AgBr and AgIO3, evidencing that the charge separation and
transfer was improved in 60%/80%/100% Ag/AgBr/AgIO3
composites. The 80% Ag/AgBr/AgIO3 composite shows the
highest photocatalytic activity, which could degrade 98% of MO
molecules after visible light irradiation (λ > 420 nm) for 50 min.
The apparent rate constant obtained for the 80% Ag/AgBr/AgIO3
composite is 0.074 min−1, which is 82.2 and 1.62 times higher than
that of AgIO3 and Ag/AgBr. For X = I, all the Ag/AgI/AgIO3
composites with different I−/IO3− ratios show higher photocatalytic
activity than the AgIO3 and Ag/AgI (Figure 9e and Figure S3).
As the ratio of I−/IO3− was increased from 20% to 60%, the rate
constant of Ag/AgI/AgIO3 ascends first and reaches the maximum of 0.019 min−1 at 40% Ag/AgI/AgIO3, which is 22.0 times
that of AgIO3 and 6.62 times that of Ag/AgI (0.0030 min−1), and
then decreases. In particular, the photocatalytic efficiency
continuously increases with elevating the I−/IO3− ratio from
60% to 100%. The decomposition rate constant of MO over
100% Ag/AgI/AgIO3 is 0.019 min−1, reaching 21.3 times that of
AgIO3 and 6.40 times that of Ag/AgI (0.0030 min−1). All the
above experimental results indicate that the photochemical
property of AgIO3 can be significantly improved by constructing
Ag/AgX (X = Cl, Br, I)/AgIO3 architectures, and they may
possess different photocatalytic mechanisms.
To test the stability of the as-obtained photocatalysts in the
photocatalytic reaction, an 80% Ag/AgBr/AgIO3 sample was
chosen for five-cycle recycling experiments for degradation of MO.
ln(C0/C) = kappt
Here, kapp represents the apparent pseudo-first-order rate
constant (min−1), C0 is the initial MO concentration (mg/L),
and C is the instantaneous concentration (mol/L) of MO
solution at time t. As exhibited in Figure 9a and Figure S1, AgIO3
photocatalyst exhibited worse photocatalytic activity than the
other samples in the X = Cl series. With increasing the ratio of
Cl−/IO3− from 20% to 100%, the photocatalytic activity of
Ag/AgCl/AgIO3 gradually goes up, and Ag/AgCl shows the
highest decomposition rate (0.074 min−1), which reaches
83.8 times that of AgIO3 (0.00089 min−1) and 1.48 times that
of 100% Ag/AgCl/AgIO3 (0.051 min−1). The Ag/AgCl sample
can decompose 97% of MO only under 40 min of visible light
irradiation (λ > 420 nm). For X = Br, as displayed in Figure S2
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Figure 5. SEM images of 20% Ag/AgBr/AgIO3 (a), 40% Ag/AgBr/AgIO3 (b), 60% Ag/AgBr/AgIO3 (c), 80% Ag/AgBr/AgIO3 (d), 100% Ag/AgBr/AgIO3
(e), Ag/AgBr (f), 20% Ag/AgI/AgIO3 (g), 40% Ag/AgI/AgIO3 (h), 60% Ag/AgI/AgIO3 (i), 80% Ag/AgI/AgIO3 (j), 100% Ag/AgI/AgIO3 (k), and Ag/AgI (j).
As shown in Figure S4, there is no large activity loss in the degradetion of MO under the irradiation of visible light (λ > 420 nm).
Moreover, compared with the 80% Ag/AgBr/AgIO3 sample
before photoreaction, the XRD pattern after the photocatalysis
reaction has no changes. These results demonstrate the high
stability of 100% Ag/AgBr/AgIO3.
Photoluminescence spectroscopy (PL) and electrochemical
impedance spectra (EIS) are widely used to monitor the separation
Scheme 1. Schematic Illustration of Fabrication Process for
the Ag/AgX (X = Cl, Br, I)/AgIO3 Composites
Figure 6. EDX mapping of 80% Ag/AgBr/AgIO3.
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Figure 7. TEM and HRTEM images of the as-obtained 100% Ag/AgCl/AgIO3 sample.
and transfer efficiencies of a photogenerated charge carrier. For PL
spectra, generally, high emission intensity indicates a high
recombination rate of the photogenerated carriers and a low
photocatalytic activity.25 The PL spectra of the AgIO3, Ag/AgX
(X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/AgIO 3
heterojunctional samples at room temperature are exhibited in
Figure 10a−c. For the X = Cl series (Figure 12a), they all show
similar emission peaks centering around 467 nm. One can see that
the Ag/AgCl sample shows the lowest intensity, suggesting the
lowest recombination rate of photogenerated charge carrier
compared to other samples. The inhibited recombination of the
holes and electrons should be assigned to the strong interaction
between Ag and AgCl. For X = Br (Figure 10b), with increasing
the ratio of Br−/IO3−, the emission intensity of Ag/AgBr/AgIO3
samples decreases monotonically and reaches a minimum when
the ratio of Br−/IO3− is 80%. Then, the emission intensity
enhances with further increasing the Br−/IO3− ratio. Similarly,
40% and 100% Ag/AgI/AgIO3 show the lowest peak intensities,
which also confirm their higher photocatalytic activity (Figure 10c).
This is also in good agreement with the MO photodegradation
results. Figure 10d displays the EIS Nyquist plots of AgIO3,
Ag/AgBr, and 80% Ag/AgBr/AgIO3 composite. It can be found
that the arc radius of the Ag/AgBr/AgIO3 composite is apparently
smaller than that of AgIO3 and Ag/AgBr samples, revealing a high
efficiency of charge transfer on the surface of 80% Ag/AgBr/AgIO3.
The results of PL and EIS confirm that the Ag/AgBr/AgIO3
photocatalyst has very efficient charge transfer and a low
recombination rate of hole and electron, which may be due to
the cooperative action of Ag, AgBr, and AgIO3.
As we know, there are various reactive species (•O2−, h+,
and •OH) that directly participate in the photocatalytic degradation
process. To disclose the effect of reactive species generated over
Ag/AgX (X = Cl, Br, I)/AgIO3 composites in the photocatalytic
process, active species trapping experiments were carried out,
and benzoquinone (BQ), disodium ethylenediaminetetraacetate
(EDTA-2Na), and isopropanol (IPA) were utilized to quench •O2−,
h+, and •OH in the degradation process, respectively.26 For
100% Ag/AgCl/AgIO3 and 80% Ag/AgBr/AgIO3, it can be
found from Figure 11a and b that the addition of 1 mM IPA only
slightly affects the photocatalytic degradation of MO, demonstrating that hydroxyl radicals •OH have little influence in
the photocatalytic process. However, when benzoquinone (BQ,
1 mM) and disodium ethylenediaminetetraacetate (EDTA-2Na,
1 mM) were added to the reaction system, the degradation of MO
was largely suppressed. This result suggests that •O2− and h+
should be the main reactive species for 100% Ag/AgCl/AgIO3
Figure 8. UV−vis diffuse reflectance spectra for the AgIO 3 ,
Ag/AgX (X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/AgIO3 composites.
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Figure 9. Apparent rate constants for the photodegradation of MO over AgIO3, Ag/AgCl, and Ag/AgCl/AgIO3 composites (a); AgIO3, Ag/AgBr,
and Ag/AgBr/AgIO3 composites (c); AgIO3, Ag/AgI, and Ag/AgI/AgIO3 composites (e) under the irradiation of visible light (λ > 420 nm).
Temporal absorption spectra of MO under visible light irradiation (λ > 420 nm) for 100% Ag/AgCl/AgIO3 (b), 80% Ag/AgBr/AgIO3 (d), and 40%
Ag/AgI/AgIO3 (f).
and 80% Ag/AgBr/AgIO3. And h+ plays a greater role in MO
photodegradation than •O2−. With regard to 40% Ag/AgI/AgIO3,
the addition of EDTA-2Na and BQ all greatly affected the
degradation of MO. However, •O2− plays a more dominant role
than h+. In contrast, for 100% Ag/AgI/AgIO3, more h+ participates
in the MO photodegradation reaction than •O2−. The active
species experiments demonstrated that the Ag/AgX (X = Cl,
Br, I)/AgIO3 heterostructural photocatalysts possess a compositiondependent photocatalytic mechanism.
Different photocatalytic mechanisms on the basis of band
structure over 100% Ag/AgCl/AgIO3, 80% Ag/AgBr/AgIO3,
40% Ag/AgI/AgIO3, and 100% Ag/AgI/AgIO3 are proposed
and schematically illustrated in Figure 12. From the previous
report,27 one can deduce the band energy level of AgCl
(ECB= 0.22 eV, EVB= 2.98 eV), AgBr (ECB= 0.07 eV, EVB= 2.67
eV), and AgI (ECB= −0.15 eV, EVB= 2.65 eV). In the case of 100%
Ag/AgCl/AgIO3 heterocatalyst (Figure 12a), AgCl cannot
absorb visible light (λ > 420 nm) because of its large band gap.
Nevertheless, Ag particles can absorb visible light and thus
induce the appearance of photogenerate electrons and holes
owing to the dipolar character and SPR of metallic Ag.28,29 Then,
the electrons would transfer to the CB of AgCl and further
transfer to the more positive CB position of AgIO3, which would
weaken the reducing capacity of electrons. Though the redox
potential of CB in AgIO3 is too positive compared to that of
O2/•O2− to produce •O2−, the Fermi level of AgIO3 would shift
to align the energy level of AgIO3 and its surrounding medium.13
Thus, the reducing reaction of O2 to •O2− can take place.
Because of the weakened reducing capacity of electrons, the
holes play a more important role than electrons (or •O2−) for
photodegradation of MO. As AgCl has a more negative CB level
which can endow electrons with stronger reducing capacity, the
Ag/AgCl photocatalyst exhibits better photocatalytic activity
than the Ag/AgCl/AgIO3 sample. For Ag/AgBr/AgIO3 samples,
AgBr can absorb visible light (λ > 420 nm). Thus, both the Ag
nanoparticles and AgBr can produce electrons and holes under
visible light illumination. The electrons generated from Ag
particles and AgBr all transfer to the CB of AgIO3. The relatively
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Figure 10. PL spectra of pure AgIO3, Ag/AgX (X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/AgIO3 composites under an excitation of 350 nm (a−c) and an
EIS Nynquist plot (d) of AgIO3, 80% Ag/AgBr/AgIO3, and Ag/AgBr.
Figure 11. Photocatalytic degradation of MO over the 100% Ag/AgCl/AgIO3 (a), 80% Ag/AgBr/AgIO3 (b), 40% Ag/AgI/AgIO3 (c), and 100%
Ag/AgI/AgIO3 (d) photocatalysts alone and with the addition of EDTA-2Na, BQ, or IPA.
Different from Ag/AgCl/AgIO3, the electrons originated from
AgBr itself in the Ag/AgBr/AgIO3 also transfer to a more positive
CB of AgIO3, improving the charge separation. So the existence
positive CB position of AgIO3 decreases the oxidating ability
of •O2−. Consequently, the holes take a more important role than
electrons in photodegradation of MO, as shown in Figure 12b.
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composites also originates from the Ag SPR and band charge
transfer, two types of photocatalytic mechanisms were observed
in Ag/AgI/AgIO3 which are closely associated with their relative
content of components. These results are confirmed by the PL,
EIS, and active species trapping experiments. Our work may pave
a new way to fabrication of hierarchical architecture with tunable
photochemical properties.
■
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acssuschemeng.6b00348.
Figures S1−S5 (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected].
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
Figure 12. Schematic diagrams with different charge-transfer mechanisms of 100% Ag/AgCl/AgIO3 (a), 80% Ag/AgBr/AgIO3 (b), 40%
Ag/AgI/AgIO3 (c), and 100% Ag/AgI/AgIO3 (d).
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundations of China (Grant No. 51302251 and 51372233) and
the Fundamental Research Funds for the Central Universities
(No. 2652013052, 2652015296, 2652015439).
of AgIO3 is crucial to enhancing the photocatalytic performance
of the Ag/AgBr/AgIO3 ternary heterostructure, and 80%
Ag/AgBr/AgIO3 thus shows better photoactivity than the
Ag/AgBr sample. According to the active species trapping
experiments, the 40% Ag/AgI/AgIO3 and 100% Ag/AgI/AgIO3
possess different photocatalytic mechanisms. Both Ag and AgI
can be excited by visible light (λ > 420 nm) to generate electrons
and holes. The as-generated •O2− from electrons would transfer
to the CB of AgIO3, and h+ would be accumulated on the VB of
AgI. The 40% Ag/AgI/AgIO3 (Figure 12c) has more AgIO3 in
components, which permits more •O2− to participate in the
photocatalytic process than holes. Consequently, •O2− plays a
different role than h+ in photodegradation of MO over 40%
Ag/AgI/AgIO3 photocatalyst. As for the 100% Ag/AgI/AgIO3
(Figure 12d), the contents of Ag and AgI covered on AgIO3
increase. This would allow h+ to have more of a platform to
directly oxidate the pollutant. Thus, h+ has a more important
effect on degradation of MO over 100% Ag/AgI/AgIO3.30 All in
all, by coordinating the SPR of Ag and heterostructure as well as
components in Ag/AgX (X = Cl, Br, I)/AgIO3, the photocatalytic activity can be optimized, and a different photocatalytic
mechanism is understood.
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