Ternary Layered Double Hydroxides (LDHs) Based on Co-, Cu-Substituted ZnAl for the Design of Efficient Photocatalysts

Article abstract of DOI:10.1002/ejic.201601213

Co-, Cu-substituted ZnAl ternary layered double hydroxides (LDHs) were synthesized and explored as efficient photocatalysts for dye degradation. LDH materials were fully characterized with various methods including elemental analysis, X-ray diffraction, i

Full text of DOI:10.1002/ejic.201601213

A Journal of
Accepted Article
Title: Ternary layered double hydroxides (LDH) based on ZnAl
substituted with Co, Cu for efficient photocatalysts design
Authors: Sanghoon Kim, Jean Fahel, Pierrick Durand, Erwan Andre,
and Cédric CARTERET
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European Journal of Inorganic Chemistry
10.1002/ejic.201601213
FULL PAPER
Ternary layered double hydroxides (LDH) based on ZnAl
substituted with Co, Cu for efficient photocatalysts design
[
a]
[a]
[b]
[a]
[a]
Sanghoon Kim, Jean Fahel, Pierrick Durand, Erwan André, and Cédric Carteret*
Dedication ((optional))
[
4,9-11]
Abstract: Co, Cu substituted ZnAl ternary layered double hydroxides
LDH) were synthesized and explored as efficient photocatalysts for
to modify their inherent properties.
Indeed, various doping
[
12]
(
techniques like impregnation,
grafting of nanoparticles,
[
13]
dye degradation. LDH materials were fully characterized using various
methods including elemental analysis, X-ray diffraction, Infrared
spectroscopy, Raman spectroscopy and transmission electron
microscopy, which revealed that LDH materials were well crystallized
without any amorphous phase as impurity. In addition, the plates of
LDH were well dispersed as being easily distinguished from one plate
to other plate, and their size could be also controlled by varying Co,
Cu substitution ratio. As application, the adsorption characteristics
and the photocatalytic degradation of orange II, a model pollutant,
were carried out. In particular, Co substituted LDH exhibited an
excellent photocatalytic activity with the optimal substitution ratio of
Co/Zn of 1/3. Interestingly, LDH materials having a higher ratio didn’t
show any improvement of the photocatalytic activity, suggesting that
the amount of Zn is the crucial factor. Furthermore, the photocatalytic
quantum dot
or composite with other materials (i.e.
[
14]
graphene) have been proposed, nevertheless these methods
require multiple steps synthesis or high energy input like thermal
treatment.
In the other hand, layered double hydroxides (LDH) and
derivatives (like layered mixed oxides, LMO) have been widely
[
15,16]
studied as a new class of photocatalytic materials.
Indeed, as
known as hydrotalcite(HT)-like compounds, LDH have received
great attention for the past decades, due to their facile fabrication
and tuning abilities with various applications such as anion
[
17]
[18]
[19]
exchange system, drug delivery vector and catalyst. The
great potential of LDH could be explained by the fact that the
[
20]
metal cations are uniformly distributed at the atomic-scale. In
addition, the properties of LDH mainly depend on the nature of
2
+
3+
performance of Zn1.5Co0.5Al
1
(Co/Zn = 1/3) was compared with other
metal ions, the ratio of M /M as well as the identity of anion
[
21]
materials like metal oxide prepared by heat or microwave treatment,
and the result could prove the efficiency of LDH materials of this work.
Therefore, Co, Cu substitution into ZnAl ternary LDH could be a
straightforward approach for designing efficient photocatalysts with
various potential applications, such as remediation of wastewater.
ions, which could be easily tailored by simple ratio adjustment
during the preparation. As a matter of the fact, LDH could be a
good candidate for designing the photoactive materials through
physicochemical properties tuning such as ternary metal
[
22,23]
[24]
substitution
reported the influence of Co, Cu substitution in the framework of
MgAl-CO LDH for designing efficient catalysts for H assisted
Dark Fenton-like reaction for organic pollutants degradation.
synergic effect of mixing two transition metals was observed as
slow reaction rate steps could be avoided. However, adding H
or exchange of anion ions. Recently, our group
3
2 2
O
[
25]
A
Introduction
2
O
2
The remediation of wastewater has been of great interest along
as oxidizing agent might be an obstacle in some practical
applications, thus it would be worth to improve this system by
using photoactive metal (i.e. Zn) as main component of LDH
materials.
[
1]
with the rapid growth of cosmetics or leather industries that have
been considered as main responsible for the water pollution with
harmful organic molecules. Among various solutions proposed for
this demand, one of the mostly used methods consists of the
photocatalytic degradation using semiconductor materials based
For that reason, we demonstrate, herein, the synthesis of efficient
3
photocatalysts based ZnAl-CO LDH that are partially substituted
[
2,3]
[4,5]
on Ti
or Zn
which show high photosensitivity, low toxicity,
with Co and Cu. Indeed, although some papers have already
reported the effect of Co or Cu substitution, their main products
are layered mixed oxides (LMO), generally obtained by
[
6,7]
stability as well as low-cost production.
However, the wide
band gap and fast recombination of electrons and holes after light
irradiation are one of the main obstacles of these semiconductor
[12]
[14]
calcination or microwave treatment that could be considered
as high energy consumption method. In addition, the
photocatalytic properties of LDH has been often ignored and has
not been widely investigated. However, the work that we present
in this paper deals with only Co or Cu substituted LDH, which can
be obtained by simple and fast co-precipitation method without
any post-synthesis step. The synthesized LDH were
characterized by various techniques, including XRD, FT-IR,
Raman, DR UV-Vis spectroscopies, TEM and ICP-AES. The
adsorption experiments and the photocatalytic degradation of a
model pollutant, orange II, were carried out, which confirmed the
excellent catalytic ability of these LDH. Indeed, contrary to what
is considered for azo dyes as non-toxic compound, some of these
8
materials , especially when these materials are used without any
activation or tuning. Therefore, doping with other metals like
cobalt, copper, iron or manganese have been proposed in order
[
a]
b]
Dr. S. Kim, J. Fahel, Dr. E. André, Prof. C. Carteret, Institut Jean
Barriol, LCPME UMR 7564, CNRS, Université de Lorraine, F-54600
Villers-lès-Nancy, France
E-mail: cedric.carteret@univ-lorraine.fr, www.lcpme.cnrs-nancy.fr
Dr. P. Durand, Institut Jean Barriol CRM2 UMR 7036, CNRS,
Université de Lorraine, F-54506 Vandœuvre-lès-Nancy, France
[
Supporting information for this article is given via a link at the end of
the document.
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European Journal of Inorganic Chemistry
10.1002/ejic.201601213
FULL PAPER
[
1]
dyes like orange II have been found to be mutagenic, therefore
the degradation study of orange II could offer solid background for
catalysts design as investigated in this study. Especially,
photoluminescence (PL) experiments confirmed an effective
generation of OH radical from LDH materials. Furthermore, the
degradation profile of adsorbed dye onto LDH surface was
investigated, suggesting more detailed information on the
degradation reaction. LDH materials were also compared with
other reported materials like metal oxides, generally prepared by
heat treatment, and the result showed
a good catalytic
performance of LDH materials of this work, having higher, or in
the same range of order in terms of degradation rate of orange II.
Figure 1. XRD diffraction data: A) copper substitution into Zn
substitution into Zn
2 1
Al and B) cobalt
2
Al1.
Results and Discussion
The effect of Cu, Co substitution on FT-IR spectra was also
investigated and given in Figure 2. First, all infrared spectra of
LDHs showed similar features; broad absorption between 2800-
Characterization of LDH materials
-
1
-1
3
700 cm and 1550-1670 cm , assigned to O-H stretching mode
The formula and the atomic ratio of synthesized LDH materials
were determined using ICP-AES elementary analysis and the
result are summarized in Table S1. It should be noted that very
small deviations between initial atomic ratios in solution and final
atomic ratios were observed.
of the basal layer and the interlayer water, and bending mode of
the interlayer water, respectively. The O-H stretching was found
-
1
at lower wavenumber compared to free OH (3600 cm ). An
-
1
intense vibrational band at 1355 cm , due to asymmetric
2
-
3
stretching vibration of carbonate (CO ) anions, was also shifts to
The XRD patterns of pristine ZnAl LDH and substituted with Co or
Cu are depicted in Figure 1. All reflections of these materials are
indexed on a hexagonal lattice with Ṝ3m rhombohedra symmetry,
commonly used for the description of LDH structures and no
change on the nature of the polytype (3R stacking) is observed
upon substitution. The value of the refined lattice parameters a
-
1
lower wavenumbers compared to free carbonate (1380 cm ).
These two features are mainly due to the hydrogen bonding
network between the intercalated anion, water molecules and the
basal hydroxide group. Moving to the low wavenumber region
-
1
(
under 1000 cm ), which gives the information on the vibrational
mode of the lattice, all LDH materials showed that the vibrational
bending mode of hydroxyl group of the basal layer (HO-M-OH) at
and c are reported in Table S1. The lattice parameter a = 2d(110)
,
which depends on the atomic radius of the metals and the
composition of the (110) reflection, evolves a bit with the
substitution of Co or Cu (Figure S1) due to the small difference
-
1
-1
7
69 cm , 606 cm shift towards higher wavenumbers with higher
Co or Cu substitution (expanded region spectra given in Figure
-
1
2
+
2+
S3). The translational modes of the lattice found at 548 cm also
move to higher wavenumber as Co or Co content increases.
Indeed, these shifts were much pronounced along with
broadening of peaks for Cu substitution, compared to Co
substitution, probably due to a possible Jahn–Teller distortion that
between the atomic radius of the metals, Zn (74 pm), Cu (72
2
+
pm) and Co (72 pm). On the other hand, the lattice parameter c
didn’t evolve with the substitution of Co or Cu, since the interlayer
distance depends mainly on the nature of the interlayer anion and
its orientation, and the extent of hydration. The small deviation on
the value of c may be due to the difference of hydration state. The
small FWHM and the resolution of the (110) and (113) reflections
are a clear evidence to long-range order of the metals within the
sheets even with Co or Cu substitution. Interestingly, the XRD
peaks became broader along with Co or Cu substitution, probably
due to the decrease in the size of the particles upon to Cu or Co
substitution. Indeed, the insertion of ternary atoms, Co or Cu, in
the layers could lead to the distortion, resulting in broadening the
2
+
cause c-axis elongation, as often observed for Cu octahedral
[
26]
compounds.
(
110) plane. In addition, regarding Cu insertion, Jahn–Teller effect
could also induce the distortion, which was confirmed by FT-IR
experiments. (see FT-IR part for discussion) TEM images validate
2
the results, where Zn Al particle size is 1.5 - 2 times larger than
Zn1.5Cu0.5Al and Zn1.5Co0.5Al (Figure 5, discussed later). In
addition, the program GSAS/EXPGUI (Figure S2) was used for
Le Bail extraction in the space group Ṝ3m in order to verify the
purity and refine the lattice parameters of some LDH materials
substituted with high contents of Cu or Cu. Owing to the
complexity of the structure, only the cell dimensions, parameters
of the pseudo-Voigt profile shape function and the background
were refined.
2 1
Figure 2. FT-IR spectra: A) copper substitution into Zn Al and B) cobalt
substitution into Zn Al .
2
1
The purity of these materials was also checked out by Raman
spectroscopy. All Raman spectra (Figure 3) present the same
features and no additional vibration bands assigned to
amorphous phase has been detected. The lattice vibrational
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European Journal of Inorganic Chemistry
10.1002/ejic.201601213
FULL PAPER
-
1
-1
bands ca. 554 cm and 493 cm characteristics of the brucite
due to the agglomeration of plate-like particles that was confirmed
by transmission electron microscopy experiments as discussed
below. The morphology of LDH was investigated using TEM. TEM
images (Figure 5) of selected LDH materials showed the
agglomeration of hydrotalcites plates deposited on one to another.
This morphology was also observed in other LDH materials
[
21]
sheets remains upon substitution by comparing to pristine LDH.
A progressive inversion in intensity between these two bands is
observed for LDH substituted with Cu. The substitution of Co into
the ZnAl framework induces only a weak continuous shift (almost
linear with the amount of Co) of the most intense band from 554
-
1
[30]
to 551 cm . In addition, the translation and symmetric stretching
vibration modes of intercalated carbonate are observed ca. 152
obtained using co-precipitation as reported previously.
Interestingly, the size of LDH slightly decreased with increasing
Co or Cu substitution contents, as already observed in XRD
experiments that showed the broadness of peak for Co or Cu
-
1
and 1060 cm , respectively.
substituted LDH. For instance, Zn
2 1
Al layered double hydroxide
(
Figure 5A) showed a typical hexagonal formed plates with the
mean size of 255 nm with standard deviation of 49 nm, by
counting several dozen particles. Then, the size of LDH plates
particles decreased with the substitution, giving 140 ± 31 nm and
1
5
61 ± 36 nm for Zn1.5Cu0.5Al
1 1
(Figure 5B) and Zn1.5Co0.5Al (Figure
C), respectively (TEM images for other LDH, see Figure S6).
This might be due to the presence of ternary ions (Co or Cu),
which might limit the crystal growth.
2 1
Figure 3. Raman spectra: A) copper substitution into Zn Al and B) cobalt
2 1
substitution into Zn Al .
DR UV-Vis experiments for LDH were performed and the spectra
are given in Figure 4. For all LDH, an intense absorption was
observed below 300 nm region, which can be assigned to charge
[
27]
2 1
transfer in ZnAl framework. Afterward, when the pristine Zn Al
was substituted with Co or Cu, this band became broader and its
cut off shifted toward higher wavelength, which is quite enough to
cover UV range of 254 nm that we used for photocatalysis
experiments (more detailed discussion in the part 3.2. Catalytic
performance of LDH materials). In addition, a broad peak around
2 1 1 1
Figure 5. TEM image of A) Zn Al , B) Zn1.5Cu0.5Al and C) Zn1.5Co0.5Al .
Catalytic performance of LDH materials
Dye adsorption
7
50 nm and 550 nm for Cu and Co substitution, respectively, was
The adsorption isotherm experiments were carried out to
investigate the adsorption phenomena of orange II before UV light
irradiation. During the adsorption study, the pH value of the
solutions was adjusted to 8.0, which is the pH of the solution
containing 30 ppm of orange II that was further used for the
photocatalytic degradation. First, Figure 6 depicts the adsorption
isotherms of orange II on LDH materials. The maximum quantity
of orange II adsorbed at equilibrium (qmax) varies as a function of
LDH materials. For example, 55 mg / g as qmax was recorded for
2
2
2+
observed due to the transition of
E
g
(D) ->
T
4
2g (D) of Cu in
[
28]
4
octahedral configuration and the transition A
2
(F) -> T1(P) of
2
+ [29]
octahedral species of Co . Besides, the band-gap of LDH was
also calculated and given in Figure S4.
Zn
1 1 1 2 1
Co Al , while Zn Al showed only 21 mg / g for qmax value
(
Table 1). Indeed, the adsorption of organic molecules onto LDH
materials is mainly due to the surface charge and the surface area,
especially, the first one varies much as cobalt or copper was
substituted in ZnAl LDH framework, while the surface area of LDH
2
-1
material is generally around 15 m g and this value doesn’t vary
much as a function of LDH composition (Table S2). Therefore, it
could be assumed that the difference in the adsorption can be
mainly explained by the zeta potential as observed in our previous
Figure 4. Diffuse reflectance UV-Vis spectra: A) copper substitution into Zn
and B) cobalt substitution into Zn Al
2 1
Al
[
25]
2
1
.
work. Besides, a possibility of exchange of intercalated layered
species was neglected since the intercalated molecule of this
2
-
N
2
adsorption-desorption experiments were carried out for some
work, CO
3
is known for its high stability and could not be
selected LDH materials and their isotherms, pore volume as well
as specific area are given in Figure S5 and Table S2. The type IV
adsorption isotherms with H3-type hysteresis loops are observed,
exchanged with other anionic ions like orange II, which has low
2
- [31]
3
charge density compared to CO .
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European Journal of Inorganic Chemistry
10.1002/ejic.201601213
FULL PAPER
2
−
be absorbed by oxygen that form superoxide radicals (O ∙). Both
of these radicals are the principal responsible species for the
[
28]
oxidative degradation of organic pollutants. In addition, when
LDH materials are constituted by two or three different metal ions,
the charge transfer between metals ions would occur via the oxo-
bridge link (e.g. Zn-O-Cu or Zn-O-Co). This charge transfer
phenomena could reduce electrons-holes recombination, which is
[
38,39]
one of main obstacle for Zn based photoactive materials.
As
a matter of fact, an efficient LDH based catalyst could be achieved
by substitution of ternary metals ions. Apart from the characteristic
of metal hydroxides sheets, the intercalated anions play an
Figure 6. Adsorption isotherm of orange II at 298 K: A) copper substitution into
important role. Several papers have claimed that
a high
2 1 2 1
Zn Al and B) cobalt substitution into Zn Al and fitting graphs using Langmuir
photocatalytic activity was observed for carbonate intercalated
LDH, probable due to a synergetic effect of carbonate that
isotherm model. (qmax (mg/g) is the maximum adsorption capacity, C
the concentration of dye in the solution)
e
(mg/L) is
[
40]
reduces electrons and roles recombination. Therefore, it must
be worth to investigate the photocatalytic activity of Co or Cu
substituted Zn based LDH with carbonate as intercalated anion.
The adsorption isotherms data were then fitted using the
Langmuir (fitting graph included in Figure 6) isotherms model. As
shown in Figure 6 and Table 1, the isotherms data were well fitted
2
with the Langmuir model, giving R > 0.99 for all LDH materials.
In addition, by comparing the Langmuir constants, it could be also
concluded that orange II adsorption rate into LDH materials
decreases along with increasing of the maximum adsorption
capacity (qmax).
Table 1. Fitting parameters for Langmuir isotherm model
Zn
2
Al
1
Zn1.75Cu0.25Al
33.4
1
Zn1.5Cu0.5Al
42.3
1
1 1 1
Zn Cu Al
[
a]
q
max (mg/g)
25.7
25.8
44.4
4.4
Figure 7. Orange II degradation using LDH under UV irradiation: A) copper
[
b]
K
L
2
(L/mg)
10.7
4.3
2 1 2 1
substitution into Zn Al and B) cobalt substitution into Zn Al .
[c]
R
0.986
0.992
0.995
Zn1.5Co0.5Al
50.6
0.990
First, in the absence of the either LDH material (catalyst) or light
source, no dye degradation was observed (data not shown in
Figure 7), showing only the adsorption of orange II on the catalyst
from 20 to 70 %, which depends on the composition of LDH
-
-
-
-
Zn1.75Co0.25Al
33.8
1
1
1 1 1
Zn Co Al
q
max (mg/g)
55.0
3.8
(
Figure 7). Then, when the adsorption equilibrium was reached,
K
L
2
(L/mg)
5.4
3.4
the degradation reaction was initiated by UV light irradiation.
Since the dye adsorption highly varies according to the
composition of the catalysts, all degradation data were
normalized with respect to the concentration at the adsorption
equilibrium for clarity reasons (Figure 8).
R
0.995
0.993
0.990
[
a] maximum adsorption capacity, [b] Langmuir constant and [c] coefficient of
determination
Photocatalytic degradation of orange II
To evaluate the catalytic ability of the designed LDH materials,
the photocatalytic degradation of a model dye, orange II was
investigated under UV irradiation. Indeed, catalytic degradation
reactions have been proposed as highly advanced oxidation
[
32,33]
processes (AOPs)
and widely used for the remediation of the
[
34]
wastewater in lieu of traditional methods like chlorination
or
[
35]
filtration. Generally, the hydroxyl and superoxide radicals that
are catalytically generated by photoactive materials are
[
36,37]
responsible for organic pollutants degradation.
photocatalysis process for LDH materials, 3d electrons of metal
ions (Co, Cu or Zn) in LDH (which would be considered as MO
Indeed, in the
Figure 8. Orange II degradation profile normalized with respect to the
concentration at the adsorption equilibrium of each catalyst: A) copper
6
2 1 2 1
substitution into Zn Al and B) cobalt substitution into Zn Al .
octahedron structure) are excited and then promoted from the
valence band (VB) to the conduction band (CB), giving holes and
electrons. Afterwards, the holes could react with the water,
generating hydroxyl radicals (OH∙), whereas the electrons could
As shown in Figure 8, one can notice that Co or Cu substituted
ZnAl LDH materials are more efficient for the photo-degradation
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European Journal of Inorganic Chemistry
10.1002/ejic.201601213
FULL PAPER
of orange II than the pristine ZnAl LDH, due to both the charge
transfer via oxo-bridge and the narrower band gap (Figure S4).
Indeed, as observed in Figure 4, UV absorbance of Co or Cu
substituted ZnAl was extended from 220 nm to 310 nm, 370 nm,
respectively, thus these LDHs could effectively absorb UV light of
higher than that of Zn1.5Cu0.5Al
1
, which also agrees with the result
of photocatalytic activity. Therefore, the fast electron transfer
ensured by cobalt would make the difference of degradation rate,
which is also in good agreement with some results reported
[
22]
previously.
2
54 nm that we used in this study. Afterward, when comparing
Another interesting finding in this work is that when Co or Cu
orange II degradation rate, Co-substituted LDH materials showed
higher photocatalytic activity compared to Cu-substitution. (Figure
substitution exceeds the ratio of 1/3 (Co/Zn or Cu/Zn, e.g.
1 1 1 1 1 1
Zn Co Al or Zn Cu Al ), the photocatalytic degradation rate
8
A vs 8B and Table S3 for pseudo-first-order kinetic model fitting
showed only a linear relation with the amount of Zn. Indeed, by
normalizing the reaction rate with the amount of Zn, it was found
that the pseudo-first-order kinetic model of orange II degradation
parameters) Indeed, Co has been largely used as doping agent
[
36,41]
not only for the design of visible light adsorption catalysts
but
also for the charge separation and the charge transport into Zn
atoms in LDH materials, which restrains the electrons and holes
1 1 1 1
for Zn1.5Co0.5Al and Zn Co Al could be superposed (Figure S8).
Their similar kinetic rate constants suggest that Zn is the limiting
species for the photocatalytic reaction. The same result was
4
recombination . In case of Cu substitution, the charge separation
could be the main reason for the photocatalytic activity, but its
effect might not be significant in this study. Moreover, its narrow
bandgap compared to Co substitution didn’t give a significant
impact on the catalytic performance since our light source is UV
light of 254 nm that can be effectively absorbed by both Co and
Cu substituted materials. However, when the catalytic ability of
these ZnAl based materials was tested under UV-Vis light
observed for the dye degradation using Zn0.5Co1.5Al
shown). However, when lowering this ratio to 1/7 (Zn1.75Co0.25Al
or 1/19 (Zn1.9Co0.1Al ), the degradation rate did not remarkably
change, giving only 6 - 12 % of the improvement, compared to the
pristine Zn Al (Figure 8). This controversial result might be
1
(data not
1
)
1
2
1
explained by the configuration of the divalent octahedra in these
II
materials. Indeed, each M octahedron is in contact with exactly
II
(
sunlight of a sunny day in May in France), only small difference
three M octahedra. In the case of Zn1.5Co0.5Al
1
, supposing an
was observed between Cu and Co substituted, which was 60 %
and 70 % of orange II degradation, respectively. This gain of the
catalytic performance for Cu substitution could be due to the
extension of UV-Vis absorption spectra up to 350 nm, whereas for
Co substitution, UV-Vis absorption spectra doesn’t exceed to 300
nm, as seen in Figure 4. In addition, the broad peak around 550
nm and 750 nm for Co and Cu substitution imply that the materials
could absorb visible light, then catalyze the degradation reaction.
More precise scientific data using standard visible light source will
be reported in a further study.
ordered distribution of Co, each Zn is in contact with exactly one
Co and each Co is in contact with three Zn. As Zn is the limiting
species for the photocatalytic reaction, this would correspond to
the maximum efficiency. More Co would not improve the
efficiency of the reaction and would diminish the amount of Zn
catalytic centers, whereas less Co would create Zn octahedra not
in contact with Co and thus less reactive. This behavior was also
confirmed by comparing the generation ability of hydroxyl radicals
for Zn1.5Co0.5Al
1
and Zn1.75Co0.25Al
1
as observed by
photoluminescence spectra (Figure 9). Afterwards, a possible
synergistic effect of Co and Cu co-substitution was also
investigated (Characterization data given in Figure S9). Contrary
[
22]
to what was expected and reported. no synergistic effect was
(Figure
S10). The optimal composition of LDH for photo-degradation of
the dye is Zn1.5Co0.5Al . Finally, the dye solution after the
observed for Zn1.5Co0.25Cu0.25Al
1 1
and Zn1.0Co0.5Cu0.5Al
1
degradation reaction was analyzed using ICP-AES to investigate
metals leaching phenomena. This is quite important for the
practical use of catalysts not only for their stability but also their
environment compatibility since leached metals from catalysts
would be more harmful than target pollutants. After 6 h of
photocatalytic reaction, very small quantity of metal ions was
Figure 9. PL spectral changes of a basic solution of terephthalic acid during UV
light irradiation for A) Zn1.5Cu0.5Al
Zn1.75Cu0.5Al
1
/ Zn1.75Cu0.5Al
1
and B) Zn1.5Co0.5Al
1
/
leached (less than 0.4 wt.%) for Zn1.5Co0.5Al
of Zn1.5Cu0.5Al , this phenomenon was well suppressed, showing
less than 0.1 wt.% of metals leaching (Table S4). Besides, the
cycling performance of Zn1.5Co0.5Al was also carried out, and it
1
. In particular, in case
1
.
1
This difference of the catalytic ability was also confirmed by the
photoluminescence (PL) experiments using terephthalic acid,
which reacts with hydroxyl radical (OH∙) to give a fluorescent
molecule, 2-hydroxyterephthalic acid (excitation at 315 nm, PL
peak at 425 nm, see Figure S7 for the detailed mechanism). Since
the PL intensity is proportional to the amount of hydroxyl radical,
this experiment could give a direct evidence of photocatalytic
1
gave almost the same performance for up to 3 times use with only
small variation that could be considered as experiment deviation
(
data not shown), which confirmed a good stability of LDH
materials along with ICP-AES experiment. As a result, one can
suggest that this kind of LDH materials could be offer a promising
platform for designing photocatalysts with detailed information
about the effect of substituted transition metals, especially
through various LDH activation techniques like calcination.
Indeed, preliminary results (Figure S11, S12) on the
[
42]
ability of LDH materials, as widely reported in the literature.
Figure 9 shows the PL spectra change of a basic solution of
terephthalic acid during UV light irradiation. It is clearly observed
1
that the amount of hydroxyl radical generated by Zn1.5Co0.5Al is
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European Journal of Inorganic Chemistry
10.1002/ejic.201601213
FULL PAPER
photocatalytic degradation of orange II using LDH activated by
calcination at different temperatures showed an inverse
relationship between transition metals content and degradation
rate, probably due the destruction of LDH structure. Work is
underway to investigate completely this phenomenon and the
results will be reported elsewhere, however, a brief discussion
about this result was given in SI of this paper.
Total
Remov
-
48
61
94
90
[
b]
al
[
a] Total degradation: photocatalytic degradation both in solution and in catalyst,
not taking into account dye adsorption, [b] Total removal: dye adsorption and
degradation in solution. (Initial dye concentration
concentration = 0.5 g / L)
= 30 ppm, catalyst
Degradation profile of adsorbed dye onto the catalyst surface
Evaluation of photocatalytic ability of the catalyst
The photocatalytic ability of Zn1.5Co0.5Al was evaluated by
Despite of the large quantity of adsorption, the degradation of
adsorbed dye during the photo-catalysis has been often
1
[
9,31,43]
neglected in most of the studies,
because of several
comparing the degradation rate with some materials in the
literature. When taking into consideration low light intensity of this
study (7 W), the normalized degradation rate (with respect to the
reasons like the difficulty of dye separation from the catalyst.
However, it is worth to investigate the degradation profile of
adsorbed dye during the catalysis because the real catalytic
performance of the materials could be obtained only by taking into
account both the degradation rate of adsorbed dye and the dye in
the solution. Figure 10 showed the variation of orange II adsorbed
amount of the catalyst for a given amount of dye) for Zn1.5Co0.5Al
1
could be in the same range of order, compared to that of other
materials (Table 3). Moreover, this result could be also
comparable with the result for TiO
considered as the standard material for photocatalytic reactions
Figure S11 for orange II degradation using P25). Therefore, it can
2
Degussa P25 that is
1 1
on two LDH materials, Zn1.5Co0.5Al and Zn1.5Cu0.5Al as function
of time. It appears that the degradation of orange II adsorbed on
LDH is a little slower than that in the solution, but still remained in
the same range of order, when comparing the kinetic constant
(
be assumed that the competitive photocatalytic activity of the
present LDH material could be explained by the homogenously
dispersed Co, which would suppress holes-electrons
recombination and improve charge transfer in ZnAl LDH
framework.
(
0.2679 for solution vs 0.2164 for adsorbed dye degradation using
Zn1.5Co0.5Al1,). The total dye degradation rate, which is the sum of
dye degradation in solution and in the LDH material, was given in
Table 2.
From the overall results of this study, one can conclude that Co
or Cu substitution into ZnAl LDH is simply very effective way to
design photocatalysts for the degradation of harmful azo-dyes like
orange II. However, it would be worth to mention that for the
industrial use of the catalysts of this study, more detailed
degradation pathway should be investigated, as toxic by-products
could be generated, because of partial degradation. Thus, the
limit of this study could be overcome using GS-MS during catalytic
study or total organic carbon (TOC) analysis after total
degradation.
Table 3. Comparison of the catalytic performance for orange II degradation
using different catalysts.
1 1
Figure 10. Degradation of orange II adsorbed on Zn1.5Co0.5Al and Zn1.5Cu0.5Al .
-
1
Catalyst
Condition
r (mg∙h )
Ref.
Table 2. Degradation and removal amount of orange II after 6 h of irradiation
This
Zn1.5Co0.5Al
1
-LDH
λ = 254 nm, 7 W, pH = 8
3.9
work
Zn
2
Al
1
Zn1.9Cu0.1
Zn1.5Cu0.5
Zn
47
77
1
Cu
1
Al
1
Zn
1 1 1
Co Al
Al
1
Al
1
Same
Degussa P25 (TiO
Degussa P25 (TiO
ZnCr-LDH
2
2
)
)
λ = 254 nm, 7 W, pH = 8
4.8
condition
Total
Degrad
24
30
33
31
[a]
400 W , pH = 4
25.0
0.9
[44]
[31]
[
a]
ation
λ = 365 nm, 125 W, pH
Total
Remov
=
8
38
-
49
55
72
Zn
50
[
b]
al
λ > 320 nm, 75 W, pH =
.5
γ-FeOOH
19.1
3.2
[45]
[46]
[47]
6
Zn1.9Co0.1
Zn1.75Co0.2
Zn1.5Co0.5
1
1 1
Co Al
Al
1
5
Al
1
Al
1
γ-Fe
2
O
3
and
2
Fe Si
4
O
10(OH)
2
λ = 254 nm, 8 W, pH = 3
Total
composite
Degrad
-
31
36
77
[
a]
ation
λ = 365 nm, 13 W, pH =
7
Mo-doped titania
5.2
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European Journal of Inorganic Chemistry
10.1002/ejic.201601213
FULL PAPER
[
a] Degradation rate normalized with respect to 0.5
g
/
L
as catalyst
concentration of dye, the adsorption process was performed until its
adsorption equilibrium (generally 1.5 h). All dye adsorption tests were
performed at room temperature.
concentration and the wavelength not mentioned in the paper.
Photocatalytic performances of LDH materials for dye degradation
Conclusions
The photocatalytic performance of LDH materials (catalyst) was
investigated using orange II as a model pollutant. 50 mg of the catalyst
was added to 120 mL of a cylindrical quartz cell containing 100 mL of 30
ppm of orange II aqueous solution. Then, the suspension was kept in dark
under stirring at least for 2 h to achieve adsorption-desorption equilibrium.
Then the irradiation was carried out, at room temperature, using a home-
made reactor equipped with a UV lamp (low pressure, light power 7 W,
In summary, we demonstrate the design of efficient
photocatalysts based on ZnAl LDH substituted with Co, Cu. The
materials showed well-crystallized structure without any
amorphous phase as confirmed by XRD and Raman
spectroscopy. FT-IR, UV-Vis spectroscopies also evidenced of
the successful substitution of transition metals. Afterwards, these
LDH materials showed an excellent photocatalytic activity toward
the degradation of orange II, a model pollutant, compared to
pristine ZnAl LDH. Moreover, the optimal substitution ratio of
Co(or Cu)/Zn was found, as of 1/3, beyond which the degradation
rate decreased. The performance of the synthesized LDH
materials was compared with other materials reported previously,
suggesting that LDH materials of this study could be a promising
candidate as catalyst. As a result, the substitution of Co, Cu in the
framework of ZnAl LDH could be a straightforward and facile way
to obtain efficient photocatalysts that could be applied in various
reactions, including abatement of organic pollutants. Furthermore,
these LDH materials could be combined with other materials such
−
5
-1 -1
2
54 nm, Iincident = 10 Einstein L s ) which specification was reported in
[
50,51]
previous papers from our institute.
Under this condition, heating effect
on the reaction medium by light source is negligible. At given intervals, 2
mL of solution was withdrawn and filtered using 0.2 µm syringe filter to
separate the catalyst and then measured using
a
UV–Vis
spectrophotometer. The photocatalytic degradation of orange II was
determined using pre-established standard calibration curve at 486 nm. All
catalytic performance tests were performed at room temperature.
Analysis of Orange II adsorbed on the catalyst
Orange II adsorbed on the catalyst was quantified after 2, 4 and 6 h of
reaction time. The catalyst was first filtered, then dried at 25 °C for 3 days.
Then, adsorbed orange II was separated from LDH materials by dissolving
3
in aqueous HNO solution (0.05 M). The concentration was determined
spectrophotometrically using standard calibration curve established for
[
48]
[49]
as mesoporous silica
or metallic nanoparticles
for new
orange II dissolved in HNO solution.
3
applications in catalytic processes.
Characterization
The XRD measurements were performed using a Panalytical X’Pert Pro
diffractometer equipped with
monochromator (Kα = 1.5406 Å), 0.02 rad Soller slits, programmable
divergence and anti-scatter slits (the irradiated area was fixed to
0mm x 10mm) and an X’Celerator detector. The X’Celerator detector was
a Cu tube, a Ge(111) incident-beam
Experimental Section
1
Chemical
1
Zn(NO
3
)
2
6H
2
O, CuCl
2
2H
2
O, Al(NO
3
)
3
9H
2
O, Co(NO
3
)
2
6H
2
O, Na
2
CO
3
,
used as “scanning line detector (1D)” with 2.122° active length. Data
collection was carried out in the scattering angle range 5–70° with a
0.0167° step over 90 min. DR UV-Vis spectra were recorded using a UV-
visible-NIR spectrometer (Cary 6000, Agilent). FT-IR spectra were
recorded on a Nicolet 8700 transform infrared spectrometer equipped with
NaOH, orange II, HNO
3
and ammonia solution (25 %) were purchased
from Sigma-Aldrich. Water was purified using a Milli-Q pack system. All
reagents were used as received without further purification.
Synthesis of layered double hydroxides (LDH)
-1
a DTGS detector by accumulating during 1 min at 4 cm resolution
-
1
between 400 and 4000 cm . Data were collected from finely ground
samples placed over ATR crystal. The Raman analysis was performed
using a Jobin Yvon T64000 apparatus equipped with a charged coupled
device detector (CCD) and motorized XYZ stage. The samples were
placed on a glass and mounted in the focal plane of an Olympus X50
LDH materials were synthesized by co-precipitation at constant pH (CPC)
[
29]
[25]
method as reported in the literature, and also optimized in our group
0 mL of aqueous solution containing a given amount of metal salts (total
5
-
1
cation concentration = 0.4 M) was slowly added (0.3 mL∙min ) to 20 mL of
ammonia-nitrate buffer solution (pH = 9, 0.1 M, mixture of ammonia
2
objective. The spot is around 2 μm . Exciting radiation at 532.14 nm was
solution with HNO
3
) using a peristaltic pump while keeping the pH fixed at
CO and 0.1 M of NaOH.
used with a laser power at 50 mW. Optical density filter of 1.3 and 1.6 for
LDH substituted with Cu or Co are used respectively, to prevent the
pH = 9 using a mixture solution of 0.25 M of Na
2
3
The as-synthesized materials were transferred into sealed Teflon
autoclaves and maintained at 100°C for 20 h. Afterwards, the resulting
materials were centrifuged and washed with deionized water to remove
the residual ions. Then, the materials were dried at 25°C for 3 days. Indeed,
the concentration of metal cations and its addition rate into buffer solution
are crucial factor to obtain LDH materials with well dispersed plates. The
-
1
degradation of these materials. The spectral resolution is about 4 cm and
-
1
the precision of the wavenumber is better than 1 cm . PL spectra were
measured with a Fluorolog 3 (Jobin Yvon). 100 mL of an aqueous solution
-
4
-3
of terephthalic acid (5 * 10 M) with NaOH (2 * 10 M) was used for
detection of OH radical generated by 50 mg of LDH, which is exactly the
same experimental condition as dye degradation test. Zeta potential value
x y 1
samples are denoted Zn2-x-yCo Cu Al , with 0 ≤ x, y ≤ 1 and 0 ≤ x+y ≤ 1.
were recorded using a Malvern 3000HSA. N
2
adsorption-desorption
Dye adsorption test
isotherms were determined on a Micromeritics Tristar 3100 at −196°C. The
pore volume and the specific surface area were calculated by the BET
5
0 mg of LDH materials (adsorbent) was added to 100 mL of orange II
(
Brunauer, Emmett, Teller) method. TEM analysis was performed using
(
Scheme S1) aqueous solution (pH 8.0) with the initial concentration within
Philips CM200 microscope, operated at an accelerating voltage of 200 kV.
the range of 15-300 mg/L. The mixture was then kept in dark under
magnetic stirring at room temperature. At given intervals, 2 mL of solution
was withdrawn and the liquid was separated using 0.2 µm syringe filter.
The adsorption was monitored by UV-Vis measurement. For each
Acknowledgements
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European Journal of Inorganic Chemistry
10.1002/ejic.201601213
FULL PAPER
The authors would like to thank to Claire Genois for performing
ICP-AES analysis, Sylvie Migot for TEM measurement and
Stéphane Parent for photoluminescence experiment. The authors
acknowledge SRSMC for Zetasizer and photoluminescence
facilities. Financial support was received from the French Ministry
of Higher Education (MRES), the French National Scientific
Centre (CNRS). JF acknowledges the French Ministry of Higher
Education for PhD grant. SK and JK contributed equally to this
work.
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European Journal of Inorganic Chemistry
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FULL PAPER
Co, Cu substituted ZnAl ternary
layered double hydroxides (LDH)
were synthesized and explored as
efficient photocatalysts for dye
degradation. The suppression of
electrons and holes recombination
the narrow band gap induced by Co,
Cu enhanced dye degradation even
without activation of LDH.
Co, Cu substituted ZnAl ternary
layered double hydroxides (LDH)
Sanghoon, Kim, Jean Fahel, Pierrick
Durand, Erwan André, Cédric Carteret*
Page No. – Page No.
Ternary layered double hydroxides
(LDH) based on ZnAl substituted
with Co, Cu for efficient
photocatalysts design
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