Enhancing the visible-light-induced photocatalytic activity of AgNbO3 by loading Ag@AgCl nanoparticles

Article abstract of DOI:10.1039/c5ra06803g

A new visible-light-driven plasmonic photocatalyst Ag@AgCl/AgNbO₃ is prepared via loading with Ag@AgCl nanoparticles by an impregnating precipitation photoreduction method. The physical and chemical properties of catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and UV-Visible diffusion reflectance spectra. In comparison with pristine AgNbO₃, Ag@AgCl/AgNbO₃ exhibits a high visible-light-induced photocatalytic activity for degradation of methylene blue. Moreover, the photocatalytic mechanism is also discussed. The photoexcited electrons on the surface of the silver nanoparticles are injected due to surface plasmon resonance, and formed radical groups (O₂^-,·HOO, ·OH and Cl⁰), which enhanced the photocatalytic activity of Ag@AgCl/AgNbO₃ in visible-light.

Full text of DOI:10.1039/c5ra06803g

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Enhancing the visible-light-induced photocatalytic
activity of AgNbO3 by loading Ag@AgCl
nanoparticles
Cite this: RSC Adv., 2015, 5, 59970
*
*
Leifei Yang, Junbo Liu, Haibo Chang and Shanshan Tang
A new visible-light-driven plasmonic photocatalyst Ag@AgCl/AgNbO3 is prepared via loading with Ag@AgCl
nanoparticles by an impregnating precipitation photoreduction method. The physical and chemical
properties of catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy,
scanning electron microscopy and UV-Visible diffusion reflectance spectra. In comparison with pristine
AgNbO3, Ag@AgCl/AgNbO3 exhibits a high visible-light-induced photocatalytic activity for degradation of
methylene blue. Moreover, the photocatalytic mechanism is also discussed. The photoexcited electrons
on the surface of the silver nanoparticles are injected due to surface plasmon resonance, and formed
radical groups (O2ꢀ,$HOO, $OH and Cl0), which enhanced the photocatalytic activity of Ag@AgCl/
AgNbO3 in visible-light.
Received 15th April 2015
Accepted 6th July 2015
DOI: 10.1039/c5ra06803g
www.rsc.org/advances
photocatalysts Ag/AgX have been developed and have aroused
broad interesting.16–20 Wang et al. reported Ag@AgCl, Ag@AgBr
Introduction
and Ag@AgCl–AgI plasmonic photocatalysts, which have been
prepared by an ion-exchange method and light-induced reac-
tion.21–23 Wang et al. indicated that AgBr/AgNbO3 photocatalytic
activity was greatly enhanced in comparison with pure
AgNbO3.24
Normally, AgNbO3 particles were prepared by traditional
high-temperature solid-state reaction, which require calcina-
tion of oxide and nitrate precursors at temperatures in excess of
900 ꢁC with frequent grindings.25 So chemical methods, such
as the use of sol–gel precursors or molten salts as reaction
media, have been adopted for the synthesis of oxide, but these
methods involve oen complex operating procedures.7 The
mild hydrothermal method is an attractive route to prepare the
inorganic solids due to the relatively mild conditions, one step
synthetic procedure and controllable particle size
distribution.26,27
Nowadays, the development of photocatalysts with visible-light
response has been studied extensively from the viewpoint of the
utilization of solar light energy. Semiconductor photocatalysis
has attracted a great deal of attention as a useful technique of
water splitting and decontamination treatment in polluted
water.1,2 Silver niobate, AgNbO3 with a perovskite structure is a
multifunctional material with extensive application potential in
photocatalysis, microwave communications and microelec-
tronic technology.3–5 With a band gap of 2.8 eV, AgNbO3 absorbs
into the visible spectrum and has shown signicant visible-light
activity for O2 evolution from an aqueous silver nitrate solution,
which acts as the oxidizing agent.6 However, pristine AgNbO3
photocatalytic activity for decomposition of organic pollutants
is low. Therefore, much progress has been made to improve the
photocatalytic activity by doping metal ions and metal oxides on
the surface of AgNbO3.7–9
In this work, we have successfully synthesized AgNbO3
particles by hydrothermal method. Ag@AgCl/AgNbO3 photo-
catalyst was synthesized by depositing AgCl nanoparticles on
the AgNbO3 powders and then reducing partial Ag+ ions in the
AgCl particles to Ag0 species under xenon lamp irradiation. The
visible-light photocatalytic activity of prepared samples was
measured by photocatalytic degradation of methylene blue
(MB), and the mechanism has been discussed.
In particular, photocatalysts modied with noble metals like
Au and Ag have received more and more attention.10–13 Noble
metals exhibit unique optical properties due to the surface
plasmon resonance (SPR). Zhou et al. showed14 that TiO2 lm
modied by Ag can signicantly enhance the visible-light pho-
tocatalytic activity. Because the SPR can dramatically amplify
the absorption of visible-light. And Zhou et al. also reported
microwave hydrothermal preparation and visible-light photo-
activity of plasmonic photocatalyst Ag–TiO2 nanocomposite
hollow spheres.15 Moreover, high efficient plasmonic
Results and discussion
Catalyst characterization
College of Resource and Environmental Science, Jilin Agricultural University,
Changchun 130118, Jilin Province, China. E-mail: liujb@mail.ccut.edu.cn; jlchhb@
163.com; Fax: +86-0431-84532955
XRD was used to determine the phase structure of the AgNbO3
and Ag@AgCl/AgNbO3. Fig. 1(a) is the XRD pattern of AgNbO3,
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which indicates perovskite-type diffraction patterns. All peaks
in the pattern can be indexed using AgNbO3 perovskite struc-
ture (JCPDS Card no. 52-0405) and their corresponding crys-
talline planes were marked. No characteristic peaks belonging
to other impurities were detected, which indicated that pure
precursors had been synthesized. The XRD patterns of
Ag@AgCl/AgNbO3 (AgNO3 concentrations is 0.5–2.0 M) showed
that, in comparison with Fig. 1a, new strong diffraction peaks
appear at the positions about 27.8ꢁ, 55.1ꢁ and 74.6ꢁ, corre-
sponding to (111), (311) and (331) diffraction peaks of AgCl
(JCPDS Card no. 85-1355), which are marked with 7 in the
Fig. 1(b–g). The diffraction peaks of AgCl appear in Fig. 1(b–g)
due to the following chemical reaction (1). Ag atoms produced
via photochemical or photocatalytic reduction reaction of AgCl
under xenon lamp light in the presence of AgNbO3 by
formula (2). Ag atoms aggregated to form small silver nano-
crystals, and deposited on the surface of AgCl particles.28
However, there are no the diffraction peaks of metallic Ag in
Fig. 1(b–g), because a small amount of Ag nanoparticles have
deposited on the surface of AgNbO3 below the detection limit of
XRD analysis.
Fig. 2 XPS survey spectrum of Ag@AgCl/AgNbO3 obtained in the
presence of AgNO3: 1.5 M.
corresponding to their angular momentum of electrons. Ag
3d3/2 and Ag 3d5/2 peaks were identied at 374.4 eV and 368.3 eV
in Ag@AgCl/AgNbO3, respectively. The difference of two peaks
is 6.1 eV from binding energy, which reveals that the silver is of
partial metallic nature on the surface of AgNbO3.28,29 Zhang
et al.30 also have reported that the peaks at 374.3 eV and 368.6 eV
can be attributed to Ag0, whereas the peaks at 367.7 eV and
373.7 eV can be attributed to Ag+. The Fig. 3(b) shows the XPS
spectrum of the Cl peak regions in the Ag@AgCl/AgNbO3. Two
peaks at 199.6 eV and 198.0 eV appear in the Cl 2p spectrum,
AgNO3 + HCl /AgCl + HNO3
AgCl / Ag + Cl
(1)
(2)
(3)
corresponding to the binding energies of Cl 2p1/2 and Cl 2p3/2
respectively, with a doublet separation of 1.6 eV.24
,
nAg0 / (Ag0)n
Fig. 4 shows typical SEM images of the as-prepared AgNbO3
and Ag@AgCl/AgNbO3 (AgNO3 concentration is 1.5 M) particles.
From Fig. 4(a), it can be found that the cube AgNbO3 aggre-
gated, and particle size is larger than 1 mm. Fig. 4(b) displays the
SEM image of Ag@AgCl/AgNbO3 aer the precipitation and
reduction process. The aggregates particles were broken and
dispersed by the ultrasound. The particle size obviously
The surface element composition and chemical state of
AgNbO3 and Ag@AgCl were further analyzed by X-ray photo-
electron spectroscopy. Fig. 2 shows the XPS spectrum of the Ag,
Nb, O, C and Cl peak regions in the Ag@AgCl/AgNbO3 (AgNO3
concentration is 1.5 M) in a wide energy range. The C contam-
ination was probably connected with long time exposure to
atmosphere or the adventitious hydrocarbon from the XPS
instrument itself. The Cl obviously appeared in the spectra of
Cl 2p peak, showing the AgCl was successfully modicated on
the surface of AgNbO3, which is in accordance with the XRD
analysis.
Fig. 3(a) shows the XPS spectra of the Ag peak regions in the
AgNbO3 and Ag@AgCl/AgNbO3 (AgNO3 concentration is 1.5 M).
As it is clearly seen the Ag 3d spectra consist of two peaks
Fig. 1 XRD patterns of AgNbO3 (a) and Ag@AgCl/AgNbO3 (b–g)
obtained in the presence of AgNO3: (b) 0.5 M, (c) 1.0 M, (d) 1.25 M, (e)
1.5 M, (f) 1.75 M, (g) 2.0 M.
Fig. 3 XPS spectra of AgNbO3 and Ag@AgCl/AgNbO3: (a) Ag, (b) Cl.
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decreases. Fig. 4(c) is TEM image of pure AgNbO3 particles.
Fig. 4(d) is TEM image of Ag@AgCl/AgNbO3 particles (AgNO3
concentration is 1.5 M), whose surfaces have been covered
obviously with a large number of Ag@AgCl particles.
To investigate the optical properties of the AgNbO3 and
Ag@AgCl/AgNbO3, the samples were analyzed by diffuse
reectance spectra. As illustrated in Fig. 5(a), each of the
samples absorbs well in the visible spectrum. Compared with
pure AgNbO3 whose wave absorption edge is about 510 nm, the
absorption threshold edge of Ag@AgCl/AgNbO3 (AgNO3
concentration is 1.5 M) is about 590 nm. Because the AgCl was
irradiated under xenon lamp to get partial Ag0 nanoparticles on
the surface of AgCl particles. This photocatalyst exhibited a high
photocatalytic activity and good stability under visible light
irradiation owing to SPR absorption by Ag nanoparticles and the
efficient charge separation at the Ag nanoparticles.31–33
The optical band gap Eg of a semiconductor could be
deduced according to the following equation (Ahn)2 ¼ hv ꢀ Eg,
where A means the absorption coefficient, h is planck's
constant, n is the incident photon frequency, and Eg is the band
gap. Fig. 5(b) showed the Eg of AgNbO3 was elicited to be
2.75 eV, and Eg of Ag@AgCl/AgNbO3 (AgNO3 concentration is
1.5 M) was found to be 2.55 eV. This result indicated that doped
Ag@AgCl nanoparticles on the surface of AgNbO3 could narrow
the band gap of catalysts, which might be conducive to improve
the photocatalytic activity of the composite.
The photocatalytic activity of the samples were evaluated by
photocatalytic degradation decolorization of methylene blue
(MB) aqueous solution under visible-light irradiation. Pure
AgNbO3 and Ag@AgCl/AgNbO3 were chosen as the reference
photocatalysts for comparison. The photocatalytic results are
shown in Fig. 6, before irradiation, the MB solution containing
the catalyst was kept in the dark for 30 min to obtain the
Fig. 5 (a) UV-vis DRS spectra for AgNbO3 and Ag@AgCl/AgNbO3
obtained in the presence of AgNO3: 1.5 M; (b) band gap of
photocatalysts.
adsorption–decolorization equilibrium state. Pure AgNbO3
exhibited stronger adsorptive capacities for MB in the dark aer
30 min, while as the loading amount of Ag@AgCl increased, the
adsorption becomes smaller gradually. Aer 2 h visible-light
irradiation, the degradation rate for MB of AgNbO3 (AgNO3
concentrations was 0) catalyst was only 20.2%. While for
Ag@AgCl/AgNbO3 (AgNO3 concentrations were 0.5 M, 1.0 M,
1.25 M, 1.5 M, 1.75 M and 2.0 M, respectively), the corre-
sponding degradation rate constant values E were estimated to
be 34.9%, 42.2%, 49.9%, 56.9%, 44.3% and 29.9%, respectively.
We can see that when concentration of AgNO3 was 1.5 M, the
photocatalytic effect was optimum, and the degradation rate of
2 h reached to 56.9%. Compared with the AgNbO3 on the
degradation rate of MB, it increased by 36.7%. Aer the loading
Ag@AgCl nanoparticles on the surface of AgNbO3, the optical
response of the photocatalyst was extended, which was due to
the SPR effect. The photocatalytic activity of the Ag@AgCl/
AgNbO3 composite was increased observably with the
increasing AgNO3 content. The photocatalytic activity decreased
Fig. 6 Photocatalytic decolorization rate of MB of Ag@AgCl/AgNbO3
Fig. 4 SEM and TEM images of AgNbO3 (a and c) and Ag@AgCl/ obtained in the presence of AgNO3 : 0 M, 0.5 M, 1.0 M, 1.25 M, 1.5 M,
AgNbO3 (b and d).
1.75 M, 2.0 M.
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Photocatalytic mechanism
Under visible light, AgNbO3 has a certain degree of light
absorption. It is well-known that in the photocatalytic process, the
electron of the valence band of the AgNbO3 can be excited when
illuminated by light of appropriate wavelength (equal to or greater
than the band gap energy). And then the electrons are elevated to
the unoccupied conduction band, creating electron–hole pairs
which are utilized to initiate redox chemistries with surface
absorbed substrates. The electron–hole recombination is the
principle reason for the decrease of the photocatalytic efficiency.
Under visible-light irradiation, photo-generated electron–
hole pairs are formed in Ag nanoparticles (NPs) due to surface
plasmon resonance. The photoexcited electrons at the silver NPs
are injected into the AgNbO3 conduction band (Fig. 9), silver
nanoparticles and the injected electrons can be transferred to
the ubiquitously present molecular oxygen to form active species
O2ꢀ,$HOO, H2O2 and $OH. Meanwhile, the holes transfer to the
surface of the AgCl particles due to the surface of AgCl particles
with negatively surface charged. The transferred holes will cause
the oxidation of Clꢀ ions to Cl0 atoms.16–18 These active species
will result in the degradation and mineralization of MB. Thus
the Ag NPs can be rapidly regenerated and the Ag@AgCl/AgNbO3
system remained stable. The major reaction steps in this plas-
monic photocatalytic mechanism under visible light irradiation
are summarized by eqn (4)–(12) as follows.17,19,20
Fig. 7 Absorption spectra changes of MB under visible light irradiation
for AgNbO3 (a) and Ag@AgCl/AgNbO3 (b) obtained in the presence of
AgNO3: 1.5 M.
when AgNO3 concentration was over 1.5 M. Mainly because
large amounts of Ag@AgCl were loaded on the part of active
center of AgNbO3, which reduces the reactive group in the
solution and decreases photocatalytic performance.
Ag-NPs + hn(visible) / Ag-NPs*
Ag-NPs* + AgNbO3 / Ag-NPs+c(h+) + AgNbO3(eꢀ)
AgNbO3(eꢀ) + O2 / AgNbO3 + O2ꢀc
O2ꢀc + H+ / cHOO
(4)
(5)
Furthermore, the temporal absorption spectra variation of
MB aqueous solution under the visible-light irradiation in the
present of AgNbO3 and Ag@AgCl/AgNbO3 were showed in Fig. 7.
It was obviously found that degradation rate for MB of
Ag@AgCl/AgNbO3 was much higher than that of pure AgNbO3.
Fig. 8 showed that decomposition of MB increases signi-
cantly with the increasing of AgNO3 concentration from 0 to
2.0 M. It reached a maximum kinetic rate constant at 1.5 M
AgNO3, and then decreases with the further increasing of AgNO3
concentration. On the basis of the above results and discussion,
we concluded that the optimum AgNO3 concentration is 1.5 M,
which is consistent with the result of photocurrent. The pho-
tocatalytic activity of Ag@AgCl/AgNbO3 is about 2 times higher
than that of pure AgNbO3.
(6)
(7)
eꢀ + cHOO + H+ / H2O2
(8)
H2O2 + eꢀ / cOH + OHꢀ
(9)
MB + cOH / CO2 + H2O + Clꢀ
Ag-NPs+c(h+) + Clꢀ / Ag-NPs + Cl0
(10)
(11)
MB + Cl0 (or cOH or O2ꢀc or H2O2) / CO2 + H2O + Clꢀ (12)
Fig. 8 Kinetic rate constants for different concentration of AgNO3.
Fig. 9 Photocatalytic mechanism of Ag@AgCl/AgNbO3 composites.
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equilibrium concentration of MB (at reaction time 0). A and A0
are the corresponding values of absorbance, respectively.
Experimental
Sample preparation
All chemicals used in this study were received from Shanghai
Chemical Regent Factory of China. The AgNbO3 samples were
prepared by hydrothermal method, as we reported previously.34
Ag@AgCl nanoparticles were deposited on the surface of as-
synthesized AgNbO3 powders via an impregnating-
precipitation-photoreduction method.35 The AgNbO3 powders
(0.2 g) were added to 50 mL of deionized water, and the
suspension was sonicated for 10 min at room temperature.
10 mL of 0.5–2.0 mol Lꢀ1 AgNO3 solution and 10 mL 0.5–2.0 mol
Lꢀ1 HCl aqueous solution were added and stirred for 20 min,
respectively. The mixture were collected by washed with deion-
ized water and dried at 80 ꢁC for 10 h. Finally, the powders were
irradiated with the xenon lamp (350 W) for 30 min. They then
obtained a dark color, revealing the presence of silver particles.
Conclusion
In summary, Ag@AgCl/AgNbO3 particles are successfully
prepared by precipitation and photoreduction reaction. Partial
Ag+ ions of the AgCl particles were reduced to Ag0 species under
xenon light irradiation. Ag@AgCl/AgNbO3 exhibits strong
absorption in the whole visible-light region, and reveals much
higher photocatalytic activity for the degradation of MB under
visible-light irradiation than pure AgNbO3 owing to surface
plasmon resonance. The photoexcited electrons on the surface
of the silver nanoparticles are injected and formed active
species O2ꢀ,$HOO, H2O2 and $OH. The holes transfer to the
surface of the AgCl particles to form Cl0 atoms. Therefore, they
can be used as efficient visible-light-induced material in
wastewater treatment and air purication.
Characterizations of the prepared composites
Acknowledgements
The powder XRD data were collected on a Rigaku D/Max 2500
The National Natural Science Foundation of China (No.
21302062) is gratefully acknowledged. The Science and tech-
nology developmental plan (No. 20130206099SF), and Science
and Technology Develop-ment Plan of Jilin Province (No.
20150101018JC) also supported this work.
V/PC X-ray diffractometer (Tokyo, Japan) with CuKa radiation
˚
(l ¼ 1.5418 A) at 50 kV and 200 mA at room temperature by step
scanning mode in the range 20ꢁ # 2q # 80ꢁ with increments of
0.02ꢁ. X-ray photoelectron spectroscopy (XPS) was performed
with a PHI 1600 spectroscope using MgKa X-ray source for
excitation. The nanoparticle morphology was measured using a
scanning electronic microscope (SEM, JEOL JSM-7001F). UV-vis
diffuse reectance spectra (DRS) were recorded on a UV-vis
spectrophotometer (Hitachi U-4100) with BaSO4 as the reec-
tance standard material.
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