A novel visible-light-driven photocatalyst Ag2O/AgI with highly enhanced photocatalytic performances

Article abstract of DOI:10.1016/j.jallcom.2017.01.106

A series of heterojunction Ag₂O/AgI photocatalysts with different Ag₂O contents were successfully prepared though a facile two-step precipitation process at room temperature. The as-synthesized samples were measured in detail by XRD, EDS, XPS, SEM and DRS. The results showed that pherical Ag₂O nanoparticles with diameter about 100–300?nm were uniformly distributed on the surface of AgI leading to the formation of Ag₂O/AgI heterojunction. The photocatalytic activities of the obtained photocatalysts were evaluated by photocatalytic degradation of Rhodamine B (RhB) under visible light irradiation. The Ag₂O/AgI with molar ratio of 1:1 showed the highest photocatalytic activities compared to the pure Ag₂O and AgI with almost all RhB decomposed in 25?min. During photocatalytic process the visible light induced photogenerated electrons can be captured by Ag⁺ions on the surface lattice of heterojunction to form plasmonic Ag nanoparticles, which may promote the interfacial charge transfer. The enhanced photocatalytic activity was then largely attributed to the synergetic effects of heterojunction Ag₂O/AgI/Ag and the efficient separation of photogenerated electron–hole pairs. This research may provide a novel Ag₂O/AgI heterojunction with assistant of plasmonic Ag NPs to generate efficient, stable, and recyclable visible-light-driven plasmonic photocatalysts. The photocatalytic reaction kinetics and possible photocatalytic mechanism for this highly efficient photocatalysts were also proposed.

Full text of DOI:10.1016/j.jallcom.2017.01.106

Journal of Alloys and Compounds 701 (2017) 163e169
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Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
A novel visible-light-driven photocatalyst Ag2O/AgI with highly
enhanced photocatalytic performances
,
*
Duo Heng Cui a, Yi Fan Zheng b, Xu Chun Song a
a Department of Chemistry, Fujian Normal University, Fuzhou 350007, PR China
b Research Center of Analysis and Measurement, Zhejiang University of Technology, Hangzhou 310014, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of heterojunction Ag2O/AgI photocatalysts with different Ag2O contents were successfully pre-
pared though a facile two-step precipitation process at room temperature. The as-synthesized samples
were measured in detail by XRD, EDS, XPS, SEM and DRS. The results showed that pherical Ag2O
nanoparticles with diameter about 100e300 nm were uniformly distributed on the surface of AgI leading
to the formation of Ag2O/AgI heterojunction. The photocatalytic activities of the obtained photocatalysts
were evaluated by photocatalytic degradation of Rhodamine B (RhB) under visible light irradiation. The
Ag2O/AgI with molar ratio of 1:1 showed the highest photocatalytic activities compared to the pure Ag2O
and AgI with almost all RhB decomposed in 25 min. During photocatalytic process the visible light
induced photogenerated electrons can be captured by Agþ ions on the surface lattice of heterojunction to
form plasmonic Ag nanoparticles, which may promote the interfacial charge transfer. The enhanced
photocatalytic activity was then largely attributed to the synergetic effects of heterojunction Ag2O/AgI/Ag
and the efficient separation of photogenerated electronehole pairs. This research may provide a novel
Ag2O/AgI heterojunction with assistant of plasmonic Ag NPs to generate efficient, stable, and recyclable
visible-light-driven plasmonic photocatalysts. The photocatalytic reaction kinetics and possible photo-
catalytic mechanism for this highly efficient photocatalysts were also proposed.
Received 17 October 2016
Received in revised form
12 December 2016
Accepted 12 January 2017
Available online 16 January 2017
Keywords:
Ag2O/AgI
Ag NPs
Photocatalytic
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
initial photocatalytic activity [7,8], have aroused more and more
attention.
In recent decades, semiconductor-based photocatalysts have
attracted considerable attention for their availability of solar energy
conversion and potential application on solving the aquatic envi-
ronmental issues. Among them, titanium dioxide (TiO2) with high
physical and chemical stability, low price, nontoxicity, effectivity
has been regarded as one of the most promising semiconductor
catalysts and extensively used in the degradation of organic pol-
lutants [1,2]. However, relatively wide band gap of TiO2 limits its
photo-response only to ultraviolet range, which occupies only a
small portion of solar light (less than 5%) [3]. Moreover, there is a
deficiency in rapid recombination of photogenerated electron-hole
pairs before they transfer to the photocatalyst surface, which leads
to low quantum efficiency [4]. Therefore, it is indispensable to
exploit novel visible light induced photocatalysts with high activ-
ities [5,6]. Recently, Ag-based semiconductors, which exhibit high
Among the Ag-based semiconductors, Ag2O with favorable
narrow band gap (1.2 eV) is suitable for applications in photo-
catalysis in the visible light region. It has been found to have high
photocatalytic activity under visible light irradiation with also
much high stability [9e13]. Ag2O nanoparticles as visible light
active components have also been used to combine with other
semiconductors or metal and nonmetal to form composited pho-
tocatalysts, such as ZnO/Ag2O [14], Ag2O/TiO2 [9,15], Ag2O/g-C3N4
[16], Ag2O/AgBr [17], Ag2O/Bi2WO6 [18] and Ag/AgI/BiOI [6], which
all displayed excellent photocatalytic performance under visible
light irradiation.
Recently, AgI with a small band gap (2.80 eV) have been coupled
with TiO2 as a highly efficient visible light sensitive material, which
also showed enhanced visible light induce performance [19].
However, silver halides (AgX, X ¼ Cl, Br, I), as photosensitive
semiconductor materials, is instable in pure crystal form, which
have been commonly applied in photographic films [20]. If the AgX
materials are coated on certain semiconductor materials, it could
* Corresponding author.
E-mail address: songxuchunfj@163.com (X.C. Song).
http://dx.doi.org/10.1016/j.jallcom.2017.01.106
0925-8388/© 2017 Elsevier B.V. All rights reserved.

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D.H. Cui et al. / Journal of Alloys and Compounds 701 (2017) 163e169
improve the stability and the photocatalytic activity of the hybrid
materials, such as AgX/Ag3PO4 [21,22], AgI/TiO2 [1,2,23,24], AgX/
BiPO4 [25e27] AgX/Ag2CO3(X ¼ Cl, I) [28], AgI/Bi2O2CO3 [29] and so
on. Thus, we have attempted to combine Ag2O with AgI to construct
Ag2O/AgI composites, and to the best of our knowledge there is no
report on this kind novel Ag2O/AgI heterojunction.
photoelectrons spectroscopy (XPS, ThermoESCALAB250, USA) was
employed to examine the elemental compositions and chemical
status of the samples. The UVevis diffuse reflectance spectra (DRS)
of the products were recorded on an UVevis spectrophotometer
(Lambda 850) equipped with an integrated sphere and using BaSO4
as reference.
In this work, Ag2O/AgI photocatalysts were successfully syn-
thesized through a facile two-step precipitation process at room
temperature. The as-prepared products were characterized by XRD,
SEM, XPS, DRS, and EDS technologies. Unexpectedly, light-reducted
Ag0 were generated owning to the reduction of AgI on the series of
Ag2O/AgI composites with different content of Ag2O, during the
photocatalytic degradation of Rhodamine B (RhB) solution under
visible light irradiation at room temperature, which can be
confirmed by the results of XPS analysis. Thereby, there are the
loaded noble metal nanoparticles (Ag NPs) in these systems which
may cause localized surface plasmon resonance (SPR) effect
[30e32]. The Ag2O/AgI with the assistant of plasmonic Ag NPs
exhibits strong visible light absorption and superior photocatalytic
activity compared with pure Ag2O and AgI. The photocatalytic
enhancement is supposed to stem from the strong SPR of Ag NPs on
the surface of composites, which increase effective charge transfer
and impede the recombination of the photogenerated electron-
hole pairs during the photocatalytic reaction [33]. Finally, the
probable mechanism of the composites was also proposed in detail.
2.3. Photocatalytic experiments
The photocatalytic activity of the synthesized Ag2O/AgI, pure
AgI and Ag2O samples was evaluated with degradation of Rhoda-
mine B (RhB) solution under visible light irradiation. In each
experiment, 0.1 g of the as-prepared photocatalyst was dispersed
within 200 mL of RhB dye solution with a concentration of 10 mg/L
in a 500 mL quartz beaker. Prior to irradiation, the suspension was
placed in dark and stirred continuously without irradiation for
30 min to establish the adsorptionedesorption equilibrium of the
RhB dye on the surface of the photocatalyst. After that, the sus-
pension was subjected to visible light irradiation and the source of
visible light was a 300 W Xe lamp with 420 nm cut off filter. During
irradiation, at intervals of every 5 min, about 3 mL of suspension
was continually collected and centrifuged at 10,000 rpm to remove
the photocatalyst particles. The concentration of solution was
analyzed by measuring the maximum absorbance at its character-
istic adsorption peak of 553 nm for RhB using a UV759S UVeVis
spectrophotometer.
2. Experimental section
3. Results and discussion
2.1. Sample preparation
Crystalline phase structure of the as-prepared samples was
investigated by X-ray power diffraction. Fig. 1 shows the XRD pat-
terns of the AgI, the Ag2O and the Ag2O/AgI heterojunction com-
positions with different molar ratios of Ag2O. It can be seen that all
the observed diffraction peaks [(100), (002), (101), (102), (110),
All chemicals were of analytical purity and applied without
further purification. The Ag2O/AgI composites were prepared by a
facile two-step precipitation method at room temperature. In a
typical synthesis, 0.166 g of KI and 0.1699 g of AgNO3 were dis-
solved in 20 mL deionized water under constant stirring at room
temperature, respectively. Subsequently, the AgNO3 solution was
dropped into the KI solution with continuous stirring, respectively.
After 30 min of stirring, the resulting yellow suspension was then
filtered, washed several times with deionized water, and finally
dried at 50 ꢀC for 12 h.
The obtained products of AgI (0.2348 g) were dispersed into
20 mL distilled water under magnetic stirring for 30 min. Then,
0.1699 g of AgNO3 dispersed into 10 mL of distilled water and the
solution was added dropwise into the above suspension for 30 min
of stirring. The 10 mL of NaOH (0.04 g) solution was subsequently
added to the above mixture and stirred vigorously for 30 min.
Finally, the final product was centrifuged, washed with deionized
water, and dried at 50 ꢀC for 12 h. For comparison, other Ag2O/AgI
composites with theoretical molar ratios of Ag2O to AgI (1:2, 1:1,
2:1) were also prepared under the same conditions. Meanwhile,
pure AgI and Ag2O particles were also prepared according to the
same preparation procedure of Ag2O/AgI composites without
addition of Ag2O or AgI.
(103), (112)] of the pure AgI can be indexed to a hexagonal b-AgI
phase (JCPDS card no. 09-0374), while all the diffraction peaks of
pure Ag2O are indexed to (110), (111), (200), (220), (311) planes of
cubic Ag2O (JCPDS Card No. 76-1393). The XRD patterns of Ag2O/AgI
heterojunction are comprised of two phases from AgI and Ag2O,
which indicates the coexistence of both AgI and Ag2O phases.
Moreover, no diffraction peaks assignable to Ag0 (JCPDS no.65-
2871) phase can be observed, indicating that the Ag2O in the
2.2. Characterization
The X-ray diffraction (XRD) studies for phase identification were
performed using an X-ray diffractometer (Thermo ARL SCINTAG
X'TRA) with CuK
voltage and current maintained at 40 kV and 40 mA and with the 2
a
radiation (
l
¼ 0.154,056 nm), the operation
q
ranging from 10ꢀ to 80ꢀ. Scanning electron microscopy (SEM) im-
ages of the obtained products were taken with a Hitachi S-4700
field emission scanning electron microscope (SEM, scanning volt-
ages 15 kV). EDS analysis were performed on an Energy-dispersive
X-ray detector (EDS, Thermo Noran VANTAG-ESI). The X-ray
Fig. 1. XRD patterns of pure Ag2O, AgI, and Ag2O/AgI with different molar ratios.

D.H. Cui et al. / Journal of Alloys and Compounds 701 (2017) 163e169
165
Ag2O/AgI composites has not decomposed during the synthesis
process of as-prepared samples.
Fig. 2 presents the surface morphologies of the AgI, the Ag2O
and the Ag2O/AgI heterojunction with the molar ratio of 1:1. The
SEM image of the pure AgI shown in Fig. 2a revealed that the ob-
tained sample has a smooth surface and features an irregularly
ratio of I:O existed in the Ag2O/AgI (mole ratio ¼ 1:1) is calculated
according to the EDS results to be 0.93: 1. STEM-EDS elemental
mapping was also applied to obtain the elemental distributions in
the Ag2O/AgI heterojunctions (Fig. 2f). It can be observed that Ag
and I elements are homogeneously distributed in the AgI area,
while O element from Ag2O is distributed in the areas without AgI.
The surface elemental compositions and chemical status of the
Ag2O/AgI (mole ratio ¼ 1:1) composites were analyzed by XPS as
shown in Fig. 3. Fig. 3a presents that the two strong peaks with a
binding energy of 373.7 eV, 367.7 eV are attributed to Ag 3d3/2 and
Ag 3d5/2 spectra of Agþ related to the AgeO or AgeI chemical
bonding in Ag2O/AgI, respectively. Fig. 3b shows that both of Ag
3d5/2 and 3d3/2 peaks can be deconvoluted into two separate peaks.
The peaks at 373.6 eV and 367.7 eV can be assigned to Agþ ions, and
those at 374.9 eV and 368.8 eV are ascribed to Ag0. This means the
formation of Ag0 during the RhB photodegradation process. Fig. 3c
spherical-like microparticle structure with a diameter of 2e10 mm.
In contrast, the images of Ag2O in Fig. 2b has a spherical shape with
smaller sizes of around 100e300 nm than those of AgI. For the
Ag2O/AgI samples, as shown in Fig. 2c and d, some Ag2O nano-
particles can be investigated nicely adhere to the AgI microparticles
surface, indicating the formation of an intimate interfacial contact
between the AgI and Ag2O in the Ag2O/AgI composite. In addition,
the element compositions of Ag2O/AgI were determined by EDS
analysis (Fig. 2e). The peaks of Ag, I and O elements were detected
in Ag2O/AgI, which confirms the existence of Ag2O/AgI. The molar
Fig. 2. SEM images of (a) AgI, (b) Ag2O, (c) Ag2O/AgI (1:1), respectively. (d) TEM image, (e) EDS pattern, (f) STEM image of the Ag2O/AgI (1:1).

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D.H. Cui et al. / Journal of Alloys and Compounds 701 (2017) 163e169
dominant peak at 531.2 eV and another higher energy peaks at
approximately 530.7 eV. The two peaks can be indexed to AgeO
bonding and OeH in the hydroxyl groups.
To investigate the optical properties of the obtained samples,
the UVevis diffuse reflectance spectra are measured for AgI, Ag2O
and the Ag2O/AgI heterojunctions with various mole ratios, as
shown in Fig. 4. The results present that pure AgI has absorption
edge around 450 nm in the visible light region. For Ag2O/AgI het-
erojunctions, its absorption intensity is enhanced gradually and
higher than that of AgI with increasing of the Ag2O content, and the
absorption edge of Ag2O/AgI shows an obvious red shift compared
with the absorption of pure AgI due to the spectral superposition of
Ag2O and AgI, which indicates the formation of the Ag2O/AgI het-
erojunction composites is in favor of improving the utilization of
visible light and enhancing their photocatalytic activity.
Photocatalytic performances of pure AgI, Ag2O and Ag2O/AgI
heterojunctions with different molar ratios of Ag2O and AgI are
evaluated by measuring the degradation of RhB under the visible
light irradiation, as shown in Fig. 5a. The results exhibit that the
RhB decomposition rate for pure Ag2O and AgI after 25min visible
light irradiation can reach only 40% and 42%. However, there is a
significant increase of photodegradation efficiency of RhB for Ag2O/
AgI heterojunctions composites, which is higher than those of the
pure AgI and the Ag2O. In addition, with increasing the molar ratio
of Ag2O to AgI, the photocatalytic activity of Ag2O/AgI hetero-
junction has been promoted gradually with the degradation rate
reaching 88% for Ag2O/AgI (1:2) and a maximum rate of 99% for
Ag2O/AgI (1:1). However, the photocatalytic degradation rate de-
creases to 85% with the molar ratio is further increased owning to
the excessive trapping sites resulted from excessive Ag2O content in
the Ag2O/AgI composite, which may promote the recombination of
photogenerated electronehole pairs. With the mole ratio of 1:1,
Ag2O/AgI exhibits the highest photocatalytic activity, suggesting
that the optimal mole ratio of Ag2O and AgI is 1:1. The visible light
absorption spectral changes of the degredation of RhB dye by the
Ag2O/AgI (1:1) catalyst was shown in Fig. 5b. It is observed that the
absorbance peak at 552 nm becomes weak significantly in the
presence of the Ag2O/AgI heterostructure (1:1) as the irradiation
time increases (Fig. 5b), which suggests that RhB was effectively
photodegraded over the Ag2O/AgI heterojunctions after 25 min.
The total organic carbon was also measured which showed
approximately 47.6% of removing after 25 min of reaction time. In
Fig. 3. High-resolution XPS spectra for the Ag 3d3/2 and Ag 3d5/2 of the fresh and used
Ag2O/AgI (1:1) (a) before photocatalysis and (b) after photocatalysis; (c) The High-
resolution XPS spectra of O1s.
displays the XPS spectrum of the O 1s region of Ag2O/AgI (mole
ratio ¼ 1:1), which can be seen the O 1s peak consisted of a
Fig. 4. UVevis diffuses reflectance spectra of pure AgI, Ag2O and Ag2O/AgI composites
(1:1, 1:2, 2:1).

D.H. Cui et al. / Journal of Alloys and Compounds 701 (2017) 163e169
167
Fig. 6. The plot of ln(C/C0) versus irradiation time curves of RhB degradation over AgI,
Ag2O and Ag2O/AgI composites (1:1, 1:2, 2:1).
0.074 minꢁ1, respectively. Thus, Ag2O/AgI (1:1) heterojunctions
show the highest photocatalytic activity under visible light irradi-
ation. To study the stability of the photocatalysts, Ag2O/AgI (1:1)
heterojunctions was tested in recycling reaction and the result is
displayed in Fig. 7. It can be discovered that although there is
deactivation of catalyst in every cycling runs, the photocatalytic
activity of Ag2O/AgI is still retained nearly 90% of its original pho-
tocatalytic activity in the third experimental run, suggesting that
the heterojunctions catalyst possesses excellent stability.
To further investigate the photocatalysis mechanism of RhB
degradation in detail, it's necessary to ascertain the main active
species including hydroxyl radicals (ꢂOH), superoxide radical (ꢂO2-
)
and holes (hþ), which participate in the photocatalytic process. The
radical scavengers of benzoquinone (BQ), isopropyl alcohol and
ammonium oxalate are added to the RhB dye solution as the
quenchers of ꢂO2-, ꢂOH and hþ, respectively. It can be observed that
the presence of BQ and isopropyl alcohol significantly inhibited the
degradation, which indicates the ꢂO2- and ꢂOH generated in the
reaction process (see Fig. 8). However, when the ammonium oxa-
late is added to the system, the degradation rate of RhB has a slight
Fig. 5. (a) The photocatalytic activity of pure AgI, Ag2O and Ag2O/AgI composites with
different Ag2O contents, (b) visible light absorption spectral changes of the degreda-
tion of RhB dye by the Ag2O/AgI (1:1) catalyst.
addition, the specific surface areas of those heterostructures were
measured to investigate the influencing factors during the photo-
catalytic reaction. The result shows that the SBET values of Ag2O,
Ag2O/AgI (2:1), Ag2O/AgI (1:1), Ag2O/AgI (1:2) and AgI were 18.2,
15.5, 14.7, 12.3 and 8.1 m2/g, respectively. As can be seen the BET
specific surface area increases with the increase of Ag2O content.
However, Ag2O/AgI (1:1) other than Ag2O showed the highest
photocatalytic activity, indicating that the BET surface areas may
not be the major factor in affecting the photocatalytic activity.
The RhB degradation process corresponding to pseudo-first-
order reaction kinetics can be discussed with
a
Lang-
muireHinshel wood model (at low dye concentration):
lnðC=C0Þ ¼ kt
Where k is the pseudo-first-order rate constant (minꢁ1) and t is the
irradiation time. Moreover, C/C0 is the ratio of the dye concentration
at various intervals of irradiation time (min) and equilibrium con-
centration of the dyes after 30 min dark adsorption. As shown in
Fig. 6b, the plots of ln (C/C0) against the irradiation time (t) have a
nearly linear relationship. The rate constants (k) of the AgI, Ag2O
and Ag2O/AgI heterojunctions (mole ratio ¼ 1:2, 1:1, 2:1) for the
degradation kinetics of RhB are 0.024, 0.027, 0.079, 0.130,
Fig. 7. The recycling efficiency of the Ag2O/AgI composites (1:1).

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D.H. Cui et al. / Journal of Alloys and Compounds 701 (2017) 163e169
band (VB) of AgI, Ag and Ag2O react directly with H2O to produce
oxidative ꢂOH radicals, which induces the subsequent degradation
of the RhB dye. Thus, the heterostructured Ag2O/AgI composites
have the excellent photocatalytic activity deriving from the assis-
tant of plasmonic Ag NPs and the generating ꢂOH and ꢂO2ꢁ radicals.
4. Conclusion
In summary, Ag2O/AgI heterojunction composites doped with
different Ag2O are successfully synthesized via a facile two-step
precipitation process at room temperature. The results display
that the Ag2O nanoparticles are dispersed uniformly on the surface
of AgI, which results in the formation of heterojunction. Moreover,
Ag0 NPs can be generated on Ag2O/AgI composites after irradiation
by visible light, which was confirmed by XPS analysis. Thus, the
Ag2O/AgI with the assistant of plasmonic Ag NPs is beneficial for
interfacial charge transfer and has greatly enhanced photocatalytic
activity in comparison with that of Ag2O and AgI for photo-
degradation of RhB under visible light irradiation. The mole ratio of
the Ag2O/AgI composites plays a crucial role in the photocatalytic
performance and the optimized molar ratio was 1:1. The present
work provides a highly efficient Ag2O/AgI with plasmon-induced
Ag NPs heterostructured photocatalyst, which is a promising pho-
tocatalyst for organic pollutants abatement.
Fig. 8. Trapping experiment of active species during the photocatalytic degradation of
RhB over Ag2O/AgI composites (1:1) under visible light irradiation.
decrease, implying that holes can be produced but not the main
reactive species. Thus, the ꢂO2- and ꢂOH as the reactive species play a
crucial role on this photocatalytic process.
The possible mechanisms for degradation of RhB is proposed
and shown in Scheme 1. According to the above XPS analysis, the
presence of Ag0 was confirmed which is owning to the partial
reduction Agþ ions on the surface of Ag2O/AgI during the RhB
photodegradation process. Therefore, under visible light irradia-
tion, the photogenerated electronehole pairs can be formed on the
surface of the excited Ag2O/AgI and the Ag NPs. The electrons (eꢁ)
on the conduction band (CB) of AgI will transfer to the Ag NPs.
Sequentially, owning to the fact that the conduction band of AgI has
a more negative potential than that of Ag2O, the energetic electrons
on the Ag NPs as the best conductive metals can migrate quickly to
the CB of Ag2O and ultimately can be trapped by the surface
Compliance with ethical standard
Conflict of interest the authors declare that they have no con-
flict of interest.
Acknowledgment
This work is financially supported by the National Nature Sci-
ence Foundation of China (No. 21273034).
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Scheme 1. Photocatalytic mechanism scheme of Ag2O/AgI under visible light illumi-
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