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DOI: 10.1002/chem.201203408
Ag/AgBr/Co–Ni–NO3Layered Double Hydroxide Nanocomposites with
Highly Adsorptive and Photocatalytic Properties
Hai Fan, Jianying Zhu, Jianchao Sun, Shenxiang Zhang, and Shiyun Ai*[a]
Introduction
Layered double hydroxides (LDHs), due to their special lay-
ered structure, have received growing interest in fields such
as catalysis, photocatalysis, adsorption, electrochemistry, and
biotechnology.[1] As a class of inorganic layered materials,
LDHs consist of positively charged brucite-like hydroxide
sheets and charge-balancing interlayer anions. The interlayer
bonding is relatively weak; therefore, LDHs exhibit excel-
lent ability to exchange with organic and inorganic anions.
Application of LDHs as adsorbents to selectively remove
anionic pollutants from aqueous solutions has recently at-
tracted considerable attention.[2] The adsorption process is
effective in removing pollutants,[3] but it only transfers them
from one medium to another and leaves them in the envi-
ronment.
Photocatalytic degradation is an attractive solution due to
the ability to completely degrade organic contaminants to
carbon dioxide, water, and mineral acids, rather than trans-
fer them.[4] A variety of photocatalysts have shown high
photocatalytic activity,[5] such as TiO2,[6] ZnO,[7] Ag3PO4,[8]
bismuth vanadate,[9] and BaBiO3.[10] However, they are un-
suitable for treating large amounts of wastewater for eco-
nomic reasons. Incorporation of photocatalysts in adsorbents
is a promising strategy for wastewater treatment. The pollu-
tants would not only be adsorbed on the nanocomposites,
but also degraded, and this could greatly increase the waste-
water treatment capacity. Till now, only a few such nano-
composites have been reported,[11] and most of them are
based on TiO2and ZnO. Besides conventional semiconduc-
tors such as TiO2and ZnO, which need to be stimulated by
an artificial source of UV light in the photocatalysis process,
it is still highly desirable to develop efficient visible-light-
driven photocatalysts that predominantly take advantage of
solar energy and indoor illumination in the visible-to-infra-
red range. There are two general strategies to prepare visi-
ble-light-driven photocatalysts. One involves modifying TiO2
or ZnO by doping with elements such as N, C, and S, combi-
nation with metals (Pt or Pd) or other semiconductors, and
addition of quantum dots or dyes on the TiO2surface for
better light sensitization.[5a] The other is exploiting novel
photocatalytic materials, such as silver halides, WO3, nio-
bates, tantalates, and vanadates.[12]
Recently, silver/silver halide-based (Ag/AgX, X=Br, Cl)
nanomaterials have been developed as photocatalysts which
display excellent plasmonic photocatalytic performance in
the degradation of pollutants under visible-light irradia-
tion.[13] Although bare AgX is unstable on light irradiation,
because photogenerated electrons will combine with mobile
interstitial Ag ions to form Ag atoms and ultimately Ag
clusters, the metallic Ag0species existing on the surface are
able to inhibit the decomposition of AgX, and thus provide
Ag/AgX with plasmon-induced photostability and high visi-
ble-light photoactivity toward degradation of pollutants in
the aqueous phase.[14] Furthermore, Ag/AgX supported on
substrates such as TiO2,[15] Al2O3,[16] BiOBr,[17] and graphene
oxide[18] has shown considerable visible-light activity in de-
composition of organic pollutants. Such supported Ag/AgX
catalysts can display high photocatalytic activity and also
maintain optical stability to some extent. However, to the
Abstract: A facile anion-exchange pre-
cipitation method was used to synthe-
size bifunctional Ag/AgBr/Co–Ni–NO3
layered double hydroxide (LDH) nano-
composites by adding AgNO3solution
to a suspension of Co–Ni–Br LDH.
The Ag/AgBr nanoparticles were
highly dispersed on the sheets of Co–
Ni–NO3LDH. The prepared nanocom-
posites were used to adsorb and photo-
catalytically degrade organic pollutants
from water. Without light illumination,
the nanocomposites quickly adsorbed
methyl orange, and the adsorptive ca-
pacity, which can reach 230 mg g1,is
much higher than those of Co–Ni–Br
LDH, Ag/AgBr, and activated carbon.
The photocatalytic activities of the
nanocomposites for the removal of
dyes and phenol are higher than those
of Co–Ni–Br LDH and Ag/AgBr. The
proposed method can be applied to
prepare other LDH/silver salt compo-
sites. The high absorptive capacity and
good photocatalytic activity of such
nanostructures could have wide appli-
cations in wastewater treatment.
Keywords: adsorption ·layered
compounds ·nanostructures ·pho-
tocatalysts ·silver
[a] H. Fan, J. Y. Zhu, J. C. Sun, S. X. Zhang, Prof. S. Y. Ai
College of Chemistry and Material Science
Shandong Agriculture University
Taian, 271018, Shandong (P. R. China)
Fax : (
+
86)538-8242251
E-mail: chemashy@yahoo.com.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203408.
Chem. Eur. J. 2013,19, 2523 – 2530 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2523
FULL PAPER
best of our knowledge, few reports have been related to the
synthesis of Ag/AgBr and LDH nanocomposites with both
high absorptive capacity and high photocatalytic activity.
Layered double hydroxides containing Co or Ni ions have
aroused great interest due to their potential applications in
adsorption, catalysis, and electrochemistry.[19] Recently,
Liang et al. prepared Co–Ni–Br LDHs directly by a topo-
chemical method.[20] The Br anions can be intercalated into
the interlayers of LDHs by partial oxidation of Co2
+
to
Co3
+
in brucite-like Co–Ni hydroxides with an excess of
bromine in acetonitrile. It is well-known that Br anions can
react with Ag ions to form AgBr. This inspired us to design
bifunctional composites by combining Co–Ni LDHs with ad-
sorptive ability and Ag/AgBr with photocatalytic activity.
Herein, we report a facile anion-exchange precipitation
method to synthesize bifunctional Ag/AgBr/Co–Ni–NO3
LDH nanocomposites by adding AgNO3solution to suspen-
sions of Co–Ni–Br LDH. The Ag/AgBr nanoparticles were
found to be highly dispersed on the sheets of Co–Ni–NO3
LDH. The adsorptive and photocatalytic properties of the
prepared nanocomposites were investigated in detail. Ag/
AgBr/Co–Ni–NO3shows superior adsorptive capacity and
good visible-light-driven photocatalytic activities, which can
greatly increase the ability to remove pollutants.
Results and Discussion
Fabrication of Ag/AgBr/Co–Ni–NO3LDH nanocomposites
is shown schematically in Scheme 1. First, Co–Ni–Br LDH
sheets were prepared by a topochemical synthesis
method.[20] Then, by adding AgNO3solution to the Co–Ni–
Br LDH suspension while stirring, Brions in the LDH in-
terlayers were released and reacted with Ag
+
to form AgBr
nanoparticles on the surface of the LDH sheets, while NO3
ions entered the LDH interlayers to offset the charge imbal-
ance caused by removal of Br. Finally, under visible-light ir-
radiation, AgBr nanoparticles were partially reduced to Ag0
species,[17,21] and Ag/AgBr nanoparticles were anchored on
the Co–Ni–NO3LDH sheets. The as-prepared nanocompo-
sites with both layer structure and photocatalyst-loaded
sheets not only have high adsorption capacity, but also show
high visible-light-driven photocatalytic activity and hence
promising performance in water treatment.
Formation of the Ag/AgBr/Co–Ni–NO3LDH nanocom-
posites was clearly demonstrated by XRD, SEM, UV/Vis
diffuse-reflectance spectroscopy, and FTIR spectroscopy.
The XRD pattern in Figure 1A, which shows characteristic
peaks at 11.290 and 22.724
8
corresponding to the (003) and
(006) crystalline planes and sharp basal reflections attributa-
ble to an interlayer distance of 0.78 nm, is consistent with
literature reports[20] and clearly suggests successful formation
of Co–Ni–Br LDH.
After AgNO3solution was added to the suspension of
Co–Ni–Br LDH, the XRD pattern (Figure 1 B) revealed the
coexistence of LDH, AgBr, and Ag phases. The peaks as-
signed as “&” can be indexed as AgBr phase (JCPDS 06-
0438) with hexagonal lattice parameters of a=0.3153 nm
and c=0.4632 nm. The peaks assigned as “#” can be indexed
as (111) crystalline plane of Ag phase (JCPDS 65-2871).
The peaks assigned as “*” can be indexed as Co-Ni LDH.
The interlayer distance varied from 0.78 nm in Co–Ni–Br to
0.88 nm in Ag/AgBr/Co–Ni–NO3LDH nanocomposites.
This change clearly indicated that Co–Ni–Br LDH was com-
pletely transformed into Co–Ni–NO3LDH, which is consis-
tent with literature reports.[22] The above results indicated
that the Ag/AgBr/Co–Ni–NO3LDH nanocomposites have
been successfully synthesized.
The morphologies of the products were characterized by
SEM (Figure 2), which showed
that irregular Co–Ni–Br LDH
sheets with thicknesses of sev-
eral tens of nanometers were
obtained by the topochemical
synthesis method. The low-
magnification image of Ag/
AgBr/Co–Ni–NO3LDH nano-
composites (Figure 2 B) shows
clearly that Ag/AgBr nanopar-
ticles with diameters of 50–
150 nm are highly dispersed on
Scheme 1. Illustration of the concept for formation of Ag/AgBr/Co–Ni–NO3LDH nanocomposites by the
anion-exchange precipitation method.
Figure 1. XRD patterns of the products. A) Co–Ni–Br LDH. B) Ag/
AgBr/Co–Ni–NO3LDH nanocomposites. “*” denotes LDH phase, “&”
denotes AgBr phase, and “#” denotes Ag phase.
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the Co–Ni–NO3LDH sheets. Figure 2C and D show that
the Ag/AgBr nanoparticles are almost uniformly anchored
on both sides of the sheets. Furthermore, the morphology
and size of the Co–Ni–NO3LDH sheets in the nanocompo-
sites did not change obviously compared to those of Co–Ni–
Br LDH sheets, that is, the LDH sheets are stable during
formation of the nanocomposites. The HRTEM images in
Figure 2E and F clearly indicate that uniform Ag/AgBr
nanoparticles with diameters of 50–150 nm are highly dis-
persed on the surface of LDHs sheets, consistent with obser-
vations from the SEM images. The composition of the prod-
uct was further examined by energy dispersive X-ray spec-
troscopy (EDS). Figure 2 G shows a typical ED spectrum of
the product. Co, Ni, Ag, Br, and O could all be detected, be-
sides the Cu and C signals from the grid used in the TEM
measurement. Weak Cl signals could also be detected, due
to residual Clanions from the raw materials. The Co:Ni
and Ag:Br molar ratios are about 6:1 and 3:4, respectively.
Energy dispersive spectra on various regions of the product
containing different nanoparticles were similar. Ag is clearly
in excess relative to the composition of AgBr, that is, AgBr
nanoparticles are covered with Ag.
Figure 3 shows the UV/Vis diffuse-reflectance spectra of
the samples. In contrast to Co–Ni–Br LDH, which has two
absorption peaks at 620 and 660 nm, Ag/AgBr-Co–Ni–NO3
displays obvious absorption in the visible region from 450 to
800 nm. The remarkable photoabsorption enhancement of
Ag/AgBr-Co–Ni–NO3LDH stems from the surface plasmon
resonance (SPR) of Ag nanoparticles,[23] and further con-
firms the existence of metallic Ag species in the product.
The FTIR spectra of the products are shown in Figure 4.
In the spectrum of Co–Ni–Br, broad absorption peaks in the
region of 3477 cm1could be assigned to the OH stretching
mode of the LDH layer and interlayer water molecules. The
corresponding HOH vibration appears at about 1631 cm1.
Apart from the identical absorption peaks to Co–Ni–Br, the
spectrum of Ag/AgBr/Co–Ni–NO3also shows a very strong
absorption peak at 1384 cm1, which can be attributed to the
n3stretching vibration of NO3.[24] It clearly indicates that
NO3anions have been exchanged into the interlayers of
Co–Ni LDH, due to the release of Br anions to form AgBr.
Ag/AgBr/Co–Ni–NO3shows high adsorption and good
photocatalytic abilities for removal of organic waste from
water. Therefore, we investigated in detail the adsorptive
and photocatalytic properties by using methyl orange (MO)
as a typical organic water pollutant. The adsorption spectra
in Figure 5A show fast initial adsorption within 5 min, while
Figure 2. A) SEM image of Co–Ni–Br LDH sheets. B) Low-magnification
SEM image of Ag/AgBr/Co–Ni–NO3LDH nanocomposites. C, D) High-
magnification SEM images of Ag/AgBr/Co–Ni–NO3LDH nanocompo-
sites. E, F) HRTEM images of the Ag/AgBr/Co–Ni–NO3LDH nanocom-
posites. G) Typical ED spectrum of Ag/AgBr/Co–Ni–NO3LDH nano-
composites.
Figure 3. UV/Vis diffuse-reflectance spectra of Co–Ni–Br LDH and Ag/
AgBr/Co–Ni–NO3nanocomposites.
Chem. Eur. J. 2013,19, 2523 – 2530 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 2525
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Ag/AgBr/Co–Ni–NO3LDH Nanocomposites
after 20 min negligible adsorption of MO indicated that the
adsorption equilibrium state was reached. The high adsorp-
tion velocity is probably due to the better affinity between
MO molecules and LDH.
Figure 5B shows the adsorption spectra of MO solutions
with fixed initial concentration of 25 mg L1after treatment
with different amounts of Ag/AgBr/Co–Ni–NO3nanocom-
posite. With 0.05 gL1Ag/AgBr/Co–Ni–NO3, about 55 %
MO could be removed. With increasing amount of Ag/
AgBr/Co–Ni–NO3nanocomposite, the amount of MO re-
maining in the water gradually decreased, and MO could be
almost completely removed from the water by a Ag/AgBr/
Co–Ni–NO3loading of 0.15 gL1. The adsorption properties
of Ag/AgBr/Co–Ni–NO3LDH, Co–Ni–Br LDH, and Ag/
AgBr were compared under the same experimental condi-
tions (0.2 gL1adsorbent and 25 mgL1MO). As shown in
Figure 5C, about 99.9 % MO could be removed by Ag/
AgBr/Co–Ni–NO3, while Co–Ni–Br and Ag/AgBr removed
85 and 2.8% of MO respectively. The adsorption isotherms
of the products (Figure 5D) showed that the highest MO
adsorption capacity of the as-synthesized Co–Ni–Br and the
commercial activated carbon (20–80 mesh) are about 195
and 140 mgg1, respectively, while as-synthesized Ag/AgBr/
Co–Ni–NO3has a significantly higher MO adsorption ca-
pacity of 230 mg g1, and Ag/AgBr shows negligible adsorp-
tion in comparison with Ag/AgBr/Co–Ni–NO3. Thus, Ag/
AgBr/Co–Ni–NO3nanocomposites have superior adsorption
properties compared to commercial activated carbon, Co–
Ni–NO3, and Ag/AgBr. Reportedly, anion exchange is the
main reason for the good adsorption property of LDHs, in
addition to surface adsorption.[25] The basal spacing of
0.78 nm in Co–Ni–Br LDH is enlarged to 0.88 nm in Ag/
AgBr/Co–Ni–NO3LDH, and thus more MO molecules can
enter the LDH interlayers by anion exchange, and the ad-
sorption capacity is greatly increased. Furthermore, surface
adsorption may also influence the adsorption capacity. The
BET surface areas of as-prepared samples were 25.68 m2g1
Figure 4. FTIR spectra of Co–Ni–Br LDH and Ag/AgBr/Co–Ni–NO3
nanocomposites.
Figure 5. A) UV/Vis spectra of MO solutions adsorbed by Ag/AgBr/Co–Ni–NO3nanocomposites with different adsorption times. B) UV/Vis absorption
spectra of MO solutions after being treated with different amounts of Ag/AgBr/Co–Ni–NO3nanocomposites ; the initial concentration of MO was
25 mgL1. C) UV/Vis absorption spectra of MO solutions after being treated by different samples: a) without adsorbent and with b) Ag/AgBr, c) Co–Ni–
Br, and d) Ag/AgBr/Co–Ni–NO3nanocomposites under the same conditions (adsorbent concentration 0.2 g L1, MO concentration 25 mgL1). Inset pho-
tographs 1–4 correspond to curves a–d. D) Adsorption isotherms of MO with Ag/AgBr/Co–Ni–NO3nanocomposites, Co–Ni–Br, and active carbon.
www.chemeurj.org 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2013,19, 2523 – 2530
2526
S. Y. Ai et al.
for Co–Ni–NO3LDH and 19.90 m2g1for Ag/AgBr/Co–Ni–
NO3LDH, that is, the BET surface area of the latter is only
slightly smaller than that of the former. Therefore, although
surface adsorption may make a certain contribution to ad-
sorption capacity, the enlarged LDH interlayer space is the
main reason for the enhanced adsorption capacity of/AgBr/
Co–Ni–NO3LDH relative to Co–Ni–NO3LDH.
We also investigated the selective adsorption properties of
Ag/AgBr/Co–Ni–NO3LDH. A mixed solution containing
anionic dye MO and cationic dye rhodamine B (RhB) in
equal concentrations (25 mg L1) was prepared, and adsorp-
tion spectra measured after treatment with 0.15 gL1of Ag/
AgBr/Co–Ni–NO3nanocomposite. About 96.5% of MO but
only 5.5% of RhB could be removed (see Supporting Infor-
mation, Figure S1). The adsorption selectivity of the Ag/
AgBr/Co–Ni–NO3nanocomposites can reach 17.5:1, and
further verified that anion-exchange plays a major role in
the adsorption of MO.
Besides superior adsorption property, Ag/AgBr/Co–Ni–
NO3also shows good photocatalytic ability. We first studied
the photocatalytic efficiency of nanocomposites towards
MO after saturated adsorption simply by measuring the
UV/Vis spectra of MO solutions. However, the photocata-
lytic efficiency is not obvious even after visible-light irradia-
tion for more than 1 h, because a dynamic adsorption bal-
ance that exists in the solution at high MO concentration
disturbed measurement of the UV/Vis spectrum. When we
decreased the concentration of the MO solution to 5 mgL1
and added Ag/AgBr/Co–Ni–NO3nanocomposites, MO was
rapidly adsorbed by the nanocomposites and the solution
bleached, and this prevented measurement of the photocata-
lytic property of the composites by means of the color
change of the solution. The above phenomena indicate that
adsorption has a great influence on determination of the
photocatalytic activity. To solve this problem and determine
the photocatalytic activity of the composites, an anion-ex-
change method was used to exchange the MO molecules
after the solution had been irradiated for a period of time.
Photodegradation of MO was carried out with a photoca-
talyst concentration of 0.10 gL1and an MO concentration
of 25 mg L1. After adsorption and subsequent irradiation
for different times, 1.5 molL1NaNO3solution in ethanol/
water (1/1) was added. The solution was centrifuged and
then analyzed by UV/Vis spectroscopy. When no NaNO3
was added to the solution, MO was almost completely ad-
sorbed. After NaNO3was added to the solution, MO mole-
cules were released from the LDH. Therefore, by this
method, it can be determined whether the adsorbed MO
molecules in the LDH could be photodegraded during irra-
diation. The bottom curve in Figure 6 shows the UV/Vis
data of MO solution after adsorption by Ag/AgBr/Co–Ni–
NO3, irradiation, and then addition of NaNO3solution.
After adsorption for 20 min, almost all MO molecules were
adsorbed. After the resulting solution was treated with
NaNO3solution, MO molecules could be released from the
catalyst. When the MO solution was irradiated and NaNO3
solution was subsequently added, the peak intensity was
much lower than without irradiation. This clearly indicated
that the MO molecules could be photodegraded after ad-
sorption in the LDH.
To further evaluate the photocatalytic performance of our
photocatalysts, Ag/AgBr, Co–Ni–Br LDH, and Ag/AgBr/
Co–Ni–NO3were compared under the same experimental
conditions. Ag/AgBr/Co–Ni–NO3showed the highest ad-
sorption capacity for MO, and Ag/AgBr the lowest. After
the three post-adsorption solutions were treated with
NaNO3solution, the peak intensity of MO in the three solu-
tions was almost the same, that is, the adsorbed MO mole-
cules were released. After the MO solutions were irradiated,
NaNO3solution was added, and the irradiated solutions
showed obvious decreases in peak intensity compared to
those without irradiation.
As shown in Figure 6, both Co–Ni–Br and Ag/AgBr show
photocatalytic activity toward MO, but Ag/AgBr/Co–Ni–
NO3displays the highest photocatalytic activity and de-
grades almost all MO dye molecules in 120 min, while Ag/
AgBr and Co–Ni–Br repectively resulted in about 63 and
15% degradation under visible-light irradiation for 120 min
at an MO concentration of 25 mg L1. Thus, immobilizing
Ag/AgBr nanoparticles on Co–Ni–NO3sheets greatly en-
hanced the photocatalytic activity.
To further verify the photocatalytic activity of the Ag/
AgBr/Co–Ni–NO3nanocomposites, we chose cationic dye
RhB and phenol as pollutants. Because LDHs show little
adsorption of cationic dyes and phenol, adsorption has no
obvious affect on determination of the photocatalytic activi-
ty. As shown in Figure 7 A and B, after reaching adsorption
equilibrium, Ag/AgBr/Co–Ni–NO3shows higher photocata-
lytic activity than Co–Ni–NO3and Ag/AgBr for degradation
of RhB and phenol, and this further verifying the high pho-
tocatalytic activity of the prepared Ag/AgBr/Co–Ni–NO3
nanocomposites.
Possible applications of the products were also investigat-
ed. In recycling experiments of MO adsorption and photoca-
Figure 6. Photodegradation of MO by different photocatalysts under visi-
ble-light irradiation (l400 nm). A) UV/Vis spectra of MO solutions
after adsorption. B) UV/Vis spectra of MO solutions after release of MO
molecules by adding a 1.5 mol L1solution of NaNO3in ethanol/water
ACHTUNGTRENNUNG(1/1 v/v). C) UV/Vis spectra of MO solutions after irradiation for
different times and then release of MO molecules.
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Ag/AgBr/Co–Ni–NO3LDH Nanocomposites
talysis (see Supporting Information, Figure S2), the adsorp-
tion efficiency of Ag/AgBr/Co–Ni–NO3nanocomposites de-
creased slightly after five cycles of adsorption and regenera-
tion with a 1.5 molL1solution of NaNO3in ethanol/water
(1/1 v/v). The decrease in adsorption capacity may be attrib-
uted to the decreasing crystallinity of the adsorbents, loss of
the adsorbents, pollution by carbonate in solution, and the
presence of residual MO during the regeneration proc-
ess.[22,26] The photocatalytic efficiency showed an inconspicu-
ous decrease after five cycles. Furthermore, the XRD pat-
terns (see Supporting Information, Figure S3) of Ag/AgBr/
Co–Ni–NO3after recycling experiments of adsorption and
photocatalysis clearly show the layer structure of LDH and
Ag/AgBr phase. The basal spacing d003 of the LDH decreas-
es compared to the freshly prepared product, due to pollu-
tion of the LDH by carbonate.[27] However, this did not ob-
viously decrease the adsorption and photocatalytic activity
of the product according to the results of the recycling ex-
periments. The above results suggest that the Ag/AgBr/Co–
Ni–NO3nanocomposites can be used as efficient and rela-
tively stable pollutant-removal agents in water treatment
with highly adsorptive and visible-light-driven photocatalytic
properties.
On the basis of the above-mentioned experimental facts
and analysis, we could propose an explanation for the en-
hanced photocatalytic performance observed for our Ag/
AgBr/Co–Ni–NO3nanocomposites (Figure 8). Under visi-
ble-light irradiation, photogenerated electron–hole pairs are
formed on the surface of Ag NPs owing to SPR. The elec-
trons are transferred from the photoexcited Ag NPs to the
conduction band (CB) of AgBr.[28] The electrons would be
trapped by molecular oxygen in solution to form O2and
other oxidative species.[29] After releasing the photoexcited
electrons, Ag NPs would shift to more positive potential to
generate positively charged Ag NPsn
+
, which are considered
to be one of the primary active species. By oxidation of the
organic pollutants or accepting electrons from the valence
band (VB) of AgBr, Ag NPsn
+
revert to primal Ag
NPs.[30,13c] Meanwhile, the photoinduced holes in the VB of
AgBr could directly oxide the organic pollutants. It is well
known that OH groups are located on the surface of LDH
materials. Surface OH groups of LDHs were reported by
Zhao et al. to play a key role in the photocatalytic reaction
of a dye, owing to their accepting photoholes to form hy-
droxyl radicals (COH).[31] Because AgBr is anchored on the
surface of Co–Ni LDH in our experiment, the photoinduced
holes in the VB of AgBr can more easily induce formation
of COH from OH groups of the LDH. These oxidative spe-
cies will result in degradation of dyes and other pollutants.
On the other hand, the high adsorptive capacity of Ag/
AgBr/Co–Ni–NO3for MO molecules may lead to contact of
more MO molecules with the photocatalyst, and this may
contribute partially to the enhanced photocatalytic activity
of the Ag/AgBr/Co–Ni–NO3nanocomposites.[11c,32] The spe-
cific surface area also plays an important role in determining
the photocatalytic activity of a photocatalyst. The BET sur-
face areas of the Co–Ni–NO3LDH and Ag/AgBr/Co–Ni–
NO3LDH were measured to be 25.68 and 19.90 m2g1, re-
spectively. Since the BET surface area of Ag/AgBr/Co–Ni–
NO3LDH is only slightly smaller than that of Co–Ni–NO3
LDH, the BET surface area has a limited influence on the
difference in photocatalytic activities between Co–Ni–NO3
and Ag/AgBr/Co–Ni–NO3LDH. Furthermore, the highly
dispersed Ag/AgBr nanoparticles on the surface of Co–Ni–
NO3sheets are smaller than bare Ag/AgBr nanoparticles,
and thus have a larger surface area accessible to MO mole-
cules, while suppressed recombination of electron–hole pairs
in Ag/AgBr/Co–Ni–NO3may also contribute to the en-
hanced photocatalytic performance.[18,33]
The above method can be used to prepare other LDH/Ag
salt composites, such as AgCl/Co(OH)2, AgCl/Zn–Cr LDH,
and Ag2WO4/Ni–Cr LDH. The XRD patterns (see Support-
ing Information, Figures S4–S6) of the composites were
Figure 7. Photodegradation of RhB (A) and phenol (B) by different
photocatalysts under visible-light irradiation (l400 nm) after reaching
adsorption equilibrium.
Figure 8. Schematic photocatalytic mechanism of Ag/AgBr/Co–Ni–NO3
LDH nanocomposites under visible-light illumination.
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S. Y. Ai et al.
characterized, and preliminary results indicate that these
composites were successfully prepared by this method, that
is the method described herein is a general strategy to pre-
pare LDH/silver salt composites. Further work is progress.
Conclusion
We have reported a facile anion-exchange precipitation
method to prepare novel Ag/AgBr/Co–Ni–NO3LDH nano-
composites by adding AgNO3solution to a suspension of
Co–Ni–Br LDH. The Ag/AgBr nanoparticles are highly dis-
persed on the Co–Ni–NO3LDH sheets. The nanocomposites
show high adsorptive capacity and highly selective adsorp-
tion for anionic dyes. Furthermore, the Ag/AgBr/Co–Ni–
NO3LDH nanocomposites show enhanced visible-light pho-
tocatalytic activity for photodegradation of MO, RhB, and
phenol as model pollutants. This method can be applied to
prepare other LDH/silver salt nanocomposites due to the in-
terlayer anion-exchange property of LDHs. Such special
nanostructures could have wide applications in wastewater
treatment.
Experimental Section
Synthesis of brucite-like Co–Ni hydroxide: Preparations of Co–Ni hy-
droxide and Co–Ni–Br LDH were similar to literature methods with a
little modification.[1] Typically, 0.5 mmol of cobalt chloride (CoCl2·6 H2O),
0.25 mmol of nickel chloride (NiCl2·6 H2O), and 4.5 mmol of hexamethy-
lenetetramine (HMT) were dissolved in 100 mL of distilled water. The
CoCl2–NiCl2–HMT solution was transferred to a 120 mL Teflon-lined
stainless steel autoclave. The autoclave was sealed and maintained at
95
8
C for 5 h, and then allowed to cool to room temperature. The final
product was collected by filtration, washed with absolute ethanol and dis-
tilled water several times each, and finally air-dried at room temperature.
Topochemical oxidation to synthesize of Co–Ni–Br LDH: To prepare
Co–Ni–Br LDH, as-synthesized brucite-like Co–Ni hydroxide (2 mmol,
0.186 g) was dispersed in 100 mL of acetonitrile containing 6.67 mmol of
bromine. The mixture was sealed in an airtight capped flask and magneti-
cally stirred for 24 h to promote complete transformation. The yellow-
green product, identified as bromide-intercalated Co–Ni LDH, was col-
lected by centrifugation and washed with copious anhydrous ethanol to
remove the excess bromine adsorbed on the powder.
Preparation of Ag/AgBr/Co–Ni LDH nanocomposite : To prepare AgBr/
Co–Ni LDH nanocomposite, 0.257 g of Co–Ni–Br LDH was dispersed in
100 mL of absolute alcohol. AgNO3solution (15 mL 0.5 moL L1) was
dropped into the above solution during sonication. After 10 min of fur-
ther reaction, the dark brown product was collected by centrifugation,
washed with distilled water and absolute ethanol several times each, and
finally air-dried at room temperature.
Preparation of Ag/AgBr: For comparison, Ag/AgBr was prepared by
direct precipitation reaction of AgNO3and KBr in aqueous solution fol-
lowed by light reduction.
Adsorption of anionic dyes: The adsorption experiments evaluated the
effect of different products on MO removal. Adsorption was carried out
by placing products in 100 mL of MO solution in the dark, under magnet-
ic stirring at room temperature. The adsorption of MO was monitored on
a UV/Vis spectrophotometer.
Photocatalytic degradation of dyes and phenol: The photocatalytic per-
formance of the as-prepared products was evaluated by decomposition of
MO, RhB, and phenol under visible-light irradiation at room tempera-
ture. A 300 W Xe arc lamp was used as light source with a 400 nm cutoff
filter. The distance between the liquid surface of the suspension and the
light source was set to about 10 cm. The photodegradation experiments
were performed with the sample powder (10 mg) suspended in solutions
of the pollutants (100 mL; MO 25, RhB 20, and phenol 20 mg L1) with
constant stirring. Prior to irradiation, the suspensions were stirred in the
dark for 20 min to ensure adsorption/desorption equilibrium. During irra-
diation, the solution was kept at room temperature by cooling with flow-
ing water. After irradiation, to avoid the influence of adsorption on pho-
tocatalysis, the adsorbed MO was released to the solution by adding a
solution of 1.5 molL1NaNO3in ethanol/water (1/1 v/v). MO photode-
gradation was then analyzed on a UV/Vis spectrophotometer (Shimadzu
UV 2550).
Characterization: The structure and morphology of products were exam-
ined by field-emission SEM (FESEM, JEOL JSM-6700F). The crystal
phase was studied by powder XRD analysis (Bruker D8-Advance X-ray
diffractometer; CuKa,l=1.5406 ). HRTEM imaging in association with
energy-dispersive X-ray spectroscopic (EDS) analysis was performed by
using a H-7650 TEM with an acceleration voltage of 200 kV. The specific
surface areas of the products were measured by nitrogen adsorption on
the basis of the Brunauer–Emmett–Teller equation with a surface area
analyzer (Micromeritics, ASAP 2020M
+
C). FTIR spectra in transmis-
sion mode were recorded on a Nicolet Impact 400 D FTIR spectrometer
(4000–400 cm, 4 cm resolution, KBr pellet). UV/Vis diffuse-reflectance
spectra were collected on a Shimadzu UV 2550 spectrophotometer.
Acknowledgements
This work was supported by the National Natural Science Foundation of
China (No. 21075078), the Natural Science Foundation of Shandong
Province, China (No. ZR2010M005) and the Promotive Research Fund
for Excellent Young and Middle-aged Scientisits of Shandong Province
(No. BS2011L033).
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Received: September 23, 2012
Published online: December 28, 2012
www.chemeurj.org 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2013,19, 2523 – 2530
2530
S. Y. Ai et al.
