Enhanced Photocatalytic Activity of a Molybdate-Intercalated Iron-Based Layered Double Hydroxide

Article abstract of DOI:10.1002/ejic.201601191

Herein, we report the design and successful fabrication of new Mg/Fe layered double hydroxides (LDHs) with different Mg/Fe molar ratios (2:1, 3:1, and 4:1) by a coprecipitation method. The prepared Mg/Fe (4:1) LDH sample with high crystallinity and phase

Full text of DOI:10.1002/ejic.201601191

A Journal of
Accepted Article
Title: Enhanced Photocatalytic Activity of Molybdate Intercalated Fe-based Layered
Double Hydroxide
Authors: Lagnamayee Mohapatra; Dhananjaya Patra; Kulamani Parida; Syed Javaid
Zaidi
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201601191
Link to VoR: https://doi.org/10.1002/ejic.201601191

European Journal of Inorganic Chemistry
10.1002/ejic.201601191
Enhanced Photocatalytic Activity of Molybdate Intercalated Fe-based
Layered Double Hydroxide
Lagnamayee Mohapatra,*a Dhananjaya Patra,b Kulamani Parida, *c Syed Javaid Zaidi, *a
a Center for Advanced Materials (CAM), Qatar University (QU), Doha, Qatar
E-mail: mlagnamayee@gmail.com
Email: szaidi@qu.edu.qa
b Department of Chemistry, Texas A&M University at Qatar
E-mail: dhananjaya.patra@qata.tamu.edu
c Centre for Nano Science and Nano-Technology, ITER, Siksha ‘O’ Anusandhan University, India
E-mail: kulamaniparida@ soauniversity.ac.in
Keywords: Photocatalysis, Methyl orange, Degradation, wastewater treatment, Charge
separation, Oxo-bridged linkage
Abstract
Herein, we design and successful fabrication of novel Fe-based Mg/Fe Layered double
hydroxides (LDHs) with different molar ratios (2:1, 3:1, and 4:1) by co-precipitation method.
Among all, the prepared Mg/Fe (4:1) LDH sample with high crystallinity and phase purity was
further modified by intercalating molybdate anion. The prepared catalysts were subsequently
examined their structural, morphological, optical, electrochemical and photocatalytic properties.
In comparison with Mg/Fe (4:1) LDH, the photocatalytic activity for degradation of methyl
orange (MO) dye was enhanced surprisingly by intercalating molybdate into the interlayer of
Mg/Fe (4:1) LDHs following apparent first order kinetics. The radical trapping experiments
-•
demonstrate that h+, OH• and O2 radicals are the dominant reactive species for pollutant
degradation. The highest photocatalytic activity of molybdate intercalated LDH is attributed
mainly to the combined effect of layered structure with Oxo-bridged Mg (II)-O-Fe (III) linkage
and intercalated molybdate anion which act as the visible light absorption center towards higher
wavelength, and can promote charge carrier trapping, as well as hinder the photogenerated
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European Journal of Inorganic Chemistry
10.1002/ejic.201601191
electron–hole recombination. The time-resolved photoluminescence spectra (TRPL) serves to
prolong the charge separation under visible light.
Introduction
Degradation of residual organic dyes from different sources (pharmaceutical industries, textile
industries, paper industries, dye and dye intermediates industries, etc.) are becoming one of the
most notorious issues in the 21st century for both human beings and ecosystems. Among all
chemically synthesized dye-stuffs, azo dyes containing azo groups (R-N=N-R') are unable to
noticeably degrade during the biological treatment or any chemical and physical processes. In
recent years one of the conventional method is “photocatalysis” based on the advanced oxidation
process that can oxidizes the organic pollutants from waste water. Moreover, photocatalysis is
one of the green technologies and has been recognized as an effective way to utilize solar energy
for destruction of aromatic rings of organic dyes.1-3Currently, various semiconductor
photocatalysts such as SnO2, CdS, WO3, In2O3, TiO2, ZnO, ZrO2, and different layered materials
as well as composites have been investigated for photocatalytic demineralization of different
pollutants.4-16
Layered double hydroxides (LDHs) are found to exhibit good alternatives which have
been studied extensively for a wide range of photocatalytic applications possess a layered
structure of general formula [M(II)1−x M(III)x(OH)2]x+ [Ax/n H20]x-, where M(II)and M(III) are
metal cations and ‘A’ is an intercalate anion. When divalent cations is substituted by the trivalent
cations, the positive charge is generated in the brucite layer and is balanced by the presence of
anions in the interlayer space.17 The brucite layer of LDHs are constituted by sheets of edge
shared MO6 octahedrons and act as a doped semiconductor. Recently, both modified and
unmodified LDHs have been reported in the field of selective photocatalysis.18-25
Most polyoxo metalates also showed excellent photocatalytic activity due to their unique
electronic structures, crystal structure, energy band structure and charge carriers transfer
efficiency.26-27 Likewise, incorporation of polyoxometalates in the interlayer region of a layered
compound is an encouraging photocatalyst for enhancing the photocatalytic activity.28 The
interlayer space of a lamellar semiconductor played a vital role for high catalytic activity.29-
30Therefore, introducing polyoxometallates into the interlayer space towards photoactivity is
challenging.
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European Journal of Inorganic Chemistry
10.1002/ejic.201601191
In this study, we report fabrication different molar ratios (4:1, 3:1, 2:1) of Mg/Fe LDH
and modified it by intercalating molybdate for photo activities. As explained earlier, layered
materials use their interlayer space with intercalated anions, edge shared octahedron and Oxo-
bridging linkage could retard the electron hole recombination. In an attempt to enhances the light
harvesting capacity, and reduce the electron hole recombination with an encouraging results in
terms of degradation rate and degradation time, we synthesize molybdate intercalated LDHs. The
enhancement of photocatalytic activity of as prepared LDHs has been also explained.
Results and discussion
Structural properties
The XRD patterns of Mg/Fe-2, Mg/Fe-3 Mg/Fe-4 and Mg/Fe-4/Mo LDH catalysts illustrate in
Figure 1. This figure reveals the typical reflections of layered double hydroxides with the peaks
for d(003) and d(006) at low 2Ɵ angle and the planes for d(101), d(015), d(018), d(110) and
d(113) planes at high 2Ɵ angle in case of Mg/Fe-4.31,32 In this case, all the reflection peaks (00l)
could be indexed brucite like structure with the hexagonal unit cells. The basal spacing (sum of
one brucite-like layer and interlayer of LDH) corresponding to d (003) was 7.7 A˚ (2θ ≈
11.4°).The plane d(003) reflection responds hydrotalcite-type materials and its intensity is
associated to the crystallinity degree of the material. In the case of Mg/Fe-3 and Mg/Fe-2, some
additional peaks were observed; however, a single crystalline peak was found in Mg/Fe-4 LDH
case. The presence of a large amount of Fe3+ in the in the salt solution resulted lower crystallinity
and less phase purity due to the difference of the ionic radii of the Mg2+ and Fe3+ Also the
.
”
solubility product of “Mg (OH) 2 is 1.8 x 10-11 far below than that of Fe (OH) 3 = 6 x 10-38. So, by
taking large amount of Fe3+ in the salt solution, the product of LDH does not yield single-phase.
The degree of crystallinity of Mg/Fe-4 is higher than others. The degree of crystallinity and
phase purity are also the significant factors for the photocatalytic activity.33 For Mg/Fe-4/Mo
LDH, the basal spacing value is expanded to 9.7 A˚ after intercalation of molybdate (scheme 1).
The absence of any peak at 7.7 A˚, which would be the absence of carbonate or nitrate in the
interlayer of Mg/Fe-4/Mo. The average crystallite sizes of the materials were calculated using
Debye−Scherrer equation,
D = 0.9λ / (β cos θ)
(1)
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European Journal of Inorganic Chemistry
10.1002/ejic.201601191
where D is the average crystallite size, λ is the wavelength of incident X-ray = 1.5418 Å, β is the
corrected full width at half-maximum of d(003) reflections, and θ is the diffraction angle for the
(hkl) plane.34 The thickness of crystallite sizes towards d(003) plane for Mg/Fe-4 and Mg/Fe-
4/Mo are 25.4 and 16.0 nm respectively.
Transmission electron microscopy (TEM) further confirms the architecture of the as-
prepared sample Mg/Fe-4 and Mg/Fe-4/Mo (shown as figure 2 a, b). For Mg/Fe-4, the TEM
figure revealed that the material consists of typical plate like particle with hexagonal shape with
a clean surface; however, Mg/Fe-4/Mo exhibits irregular morphology with uniform dispersion.20
The selected area diffraction (SAED) pattern of materials shows clear diffraction rings verified
crystalline nature of Mg/Fe-4, the particle shape is regular hexagonal and the diffraction pattern
was very strong, indicating a highly crystalline phase which confirmed from our PXRD section.
-
No any significant change in the morphology of Mg/Fe-4/Mo, even after intercalation of MoO4
in the interlayer of LDH. The corresponding SAED pattern is shown in Figure 2(c, d) and this
results are in good agreement with the XRD analysis. Figure 2 (e, f) reveals the field-emission
scanning electron microscopy (FESEM) images of the Mg/Fe-4 and Mg/Fe-4/Mo LDHs. The
image of Mg/Fe-4 shows that the nature of LDH particles, which approximately looks plate-like
morphology and stacked on top of each other also some of them are m irregular aggregates. The
morphology is not altered after intercalation of molybdate.
Further evidence for the structural confirmation of the synthesized materials were
provided by FTIR spectroscopy in the frequency range 4000−500 cm−1 (Figure 3). The broad and
strong absorption peak between 3600 and 3300 cm−1 was due to the stretching vibration of
structural −OH groups from LDH layers as well as from the interlayer H2O molecules. The HOH
bending band (ν2) of water was showed at 1641 cm−1 which is also close to that of bulk water.
Intercalated carbonate anions gave a strong absorption peak at around 1384 cm−1 associated with
the carbonate ions which indicates D3h symmetry and only one mode of vibration, which is
infrared active.The bands observed below 1000 cm-1 such as the M-O-H vibration at 667 cm-1
and the O-M-O vibration around 550 cm-1 are identified as the lattice vibration modes.35
Furthermore, the stretching vibrations at 665 cm−1 is interpreted to Mo–O–Mo. The band at 856
cm-1 is related to MoO42- and the band at 914 cm−1for Mo =O stretching.36-37
The nitrogen sorption isotherm curve confirmed the pore size distribution and surface
area of the materials. As can be observed in Figure 1, both the materials exhibit the type IV
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European Journal of Inorganic Chemistry
10.1002/ejic.201601191
isotherm with a distinct H3 type of hysteresis loop based on the standard definition of
International Union of Pure and Applied Chemistry (IUPAC). Such a combination of type IV
isotherm and type H3 hysteresis loop indicates a characteristic of the mesoporous structure of
the material.38 According to Brunauer−Emmett− Teller (BET) analysis, the BET surface area of
Mg/Fe-4 and Mg/Fe-4/Mo LDHs was found to be 62 and 84 m2/g, respectively,. The increased
surface area of Mg/Fe-4/Mo LDH is due to the higher degree of expansion by intercalating
-
MoO4 in the interlayer of Mg/Fe LDH. The plots of the pore size distribution for analyzing pore
size and pore volume of the materials, are investigated by desorption branch of the Barrett-
Joyner-Halenda (BJH) method as shown in figure 4 (Inset). The pore size ranging from 3.0 to 50
nm indicates the result of the accumulation of the nanocrystals with mesoporosity. The
mesoporosity of the materials allowed rapid diffusion and supplies the efficient transport
pathways for the photogenerated charge carriers, which is favorable to the photocatalytic
activity.39 The large surface area and porous structure make Mg/Fe-4/Mo LDHs an excellent
material toward photocatalysis.
Optical properties
The optical property of catalysts were examined using the UV-Vis DRS. It was carried out to
investigate the state of transition metals incorporated into the layered framework and also
changes the absorption properties by intercalating the anions in the interlayer region of the
LDHs. As presented in figure 5, the broad absorption band in case of Mg/Fe LDH is due to the
metal ligand charge transfer band of O2p →Fe3+ and the metal-metal-charge-transfer spectra
(MMCT) of Mg2+-O-Fe3+. The MMCT for an oxo-bridged bimetallic system was defined to be an
excitation transition of an electron from one metal to the other with different oxidation states,
which is known to absorb visible light and even near-IR light. Previously, some groups
IV
designated the oxo-bridged systems like TiIV -O-CuI, and Ti
-O-CeIII , respectively for
photocatalysis.40,41 Furthermore, parida and coauthors reported the visible-light induced MMCT
of different oxo-bridged system towards photocatalysis.24,30,42 All results demonstrated that, the
hetero-bimetallic oxo-bridged systems induced MMCT and acts as a photo induced redox
center.43 In the case of Mg/Fe-4/Mo LDH, the absorption edge shifted towards near IR region is
due to the highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-
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European Journal of Inorganic Chemistry
10.1002/ejic.201601191
LUMO) oxygen-metal charge transfer spectra (OMCT) of Interlayer molybdate where the
HOMO is mainly derived from the O 2p orbitals and the LUMO is from the Mo 4d orbitals.
For a crystalline semiconductor, the optical band gap is calculated by Touc plot:
αh=A(h-Eg)n/2
(2)
Where α,, A and Eg are the absorption co-efficient, light frequency proportionality constant
and band gap energy, respectively.The values of n and Eg were determined using the published
methods.42,44 Here, n describes the type of transition in a semiconductor. If n = 1/2 responds for
direct transition and n =2 for indirect transition. In our case, the value of n was determined to be
1/2 from their absorption spectra. So, optical transitions for both LDH materials are directly
allowed. The band gap energy (Eg) can be estimated from the intercept of the tangents to the
plots of (ah)2 against h photon energy as shown in Figure 5(insert figures). The band gap of
Mg/Fe-4/Mo (2.06 eV) is indicates a narrow band gap compared to that of Mg/Fe-4 LDH (2.3
eV).
Photoluminscence study
The efficient charge separation and transfer of photo generated exactions are essential for
the enhanced photocatalytic activity for a photocatalyst. Photoluminscence (PL) spectroscopy
provides the overview of the migration and separation efficiency of charge-carrier trapping, the
existence of surface defects and oxygen vacancies of the material. At certain excitation
wavelength, the electrons on the valence band (VB) excited to the conductance band (CB) and
again back to valence giving rise to PL signal. High PL intensity is considered to reflect a high
recombination rate of photogenerated electron hole pairs.45 Figure 6 displays the PL spectra of
Mg/Fe-4 and Mg/Fe-4/Mo LDH samples in the range of 340–500 nm with the excitation
wavelength of 320 nm. All LDH materials showed two strong PL emission peaks at around at
415 nm and 450 nm due to the trapping of charge carriers on its surface and the defect sites of
LDHs respectively.42,46,47The pure LDH has a high-intensity emission peak and after
intercalating molybdate, the PL signal becomes significantly quenched. A more active separation
of electron-hole pairs was observed after incorporation of Molybdate in LDH. This result
showed, an efficient charge separation in Mg/Fe-4/Mo LDH, suppressing the recombination of
charge carriers and significantly enhanced photocatalytic activities.
Time-resolved photoluminescence study
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European Journal of Inorganic Chemistry
10.1002/ejic.201601191
The recombination of photogenerated excitons (e- and h+) can be qualitatively investigated. To
determine the lifetime of charge carriers, we have studied the time resolved photoluminescence
42
decay (TRPL) spectra (Figure 7).
The curves have been fitted with bi-exponential decay
response functions defined by the following equation48, 49
:
( )
(3)
which contains two lifetimes ( 1 and 2), and the corresponding amplitudes (A1 and A2). The
intensity average lifetime of both the materials were calculated to make an overall comparison of
the emission decay behavior. The calculated average lifetimes of Mg/Fe-4 is 310 ps and Mg/Fe-
4/Mo is 980 ps (Table 1). This result demonstrates that, HOMO-LUMO transitions of interlayer
molybdate prohibit the recombination of electron and holes. Both the results of PL showed that
with Mg/Fe-4/Mo LDH, the PL intensity as well as the time period for charge separation are
improved, which are very promising result for the photocatalytic reactions.
Photo electrochemical properties
To better understand the mechanism,we further investigated the photoelectrochemical
properties of the material by photocurrent measurement . The result of photocurrent
measurements for both the materials confirmed that both are p-type semiconductor and serving
as photocathodes [figure 8(a)]. The photocurrent onset potentials (valence band edge) of both
Mg/Fe-4 LDH and Mg/Fe-4/Mo LDH were observed at +2.04V and +2.14 V vs. Ag/AgCl at pH
6.5. In our case Mg/Fe-4/Mo LDH (+2.14V) showed a higher oxidation ability (more positive
•
valence band potential) than Mg/Fe-4 LDH (+2.08V) which are more positive than OH /OH-
(1.19V) and suggesting that both could ability to produce OH radical.43,50 Furthermore, we have
done the EIS analysis to recognize the charge transfer properties. Figure 8(b) shows EIS Nyquist
plots of Mg/Fe-4 and Mg/Fe-4/Mo LDH electrodes. It could be seen that, in the presence of
visible light irradiation, the diameter of the Nyquist circle of Mg/Fe-4/Mo was significantly
lower than that of the pristine Mg/Fe-4 LDH. The smaller diameter of the arc implied lower
charge transfer resistance and enhance the separation and transfer efficiency of photogenerated
electron and holes, which corresponds superior photocatalytic activity.51This result demonstrates
that the intercalation of molybdate in Mg/Fe-4 LDH can dramatically enhance the photocatalytic
efficiency.
Catalytic activities
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European Journal of Inorganic Chemistry
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The photocatalytic activities of Mg/Fe-4 and Mg/Fe-4/Mo LDH samples were
investigated by degrading organic pollutant like methyl orange (MO) under solar light (λ>400
nm) irradiation. Figure 9 (a) showed the rate of decolorized of MO over Mg/Fe-4 and Mg/Fe-
4/Mo LDH materials. As the highest chemical stability of MO, the photolysis of the dye (without
catalyst) under visible irradiation was negligible. Prior to photocatalytic experiment, the
adsorption properties of MO over two as prepared catalysts were examined in the dark condition
up to 30 minutes. The adsorption/desorption equilibriums of the dye over two samples are totally
established within that time period. With respect to the same initial MO concentration, only 15%
and 20% MO are adsorbed over Mg/Fe-4 LDH and Mg/Fe-4/Mo LDH, respectively, indicating
that Mg/Fe-4/Mo showed high adsorption ability to adsorb MO in aqueous solution due to high
basal spacing corresponding high surface area. Then the solution was stirred under visible light
irradiation for photocatalytic reaction. The result directed that, Mg/Fe-4/Mo presented the most
impressive photocatalytic degradation ability, and completely decomposed MO dye molecules
within 60 min. By contrast, at the same reacting time, the removal rate of MO dye over Mg/Fe-4
was only ~60% and about 90% decomposed after 180 minutes. The excellent photocatalytic
activity was showed in the case of Mg/Fe-4/Mo LDH under solar-light irradiation.
Correspondingly, different experiments were investigated using the initial MO
concentrations varying from 50 mg/L to 100 mg/L with materials doses 1g/L to know the effect
of concentration of dye in the degradation process over Mg/Fe-4/Mo LDH. The photo
degradation rate of MO was about 90% degradation rate in case of both cases (50 mg/L, 80mg/L
initial dye concentration) and 48.39% degradation rate in case of 100mg/L initial dye
44
concentration. At higher dye initial concentration, the approach of the radiation of photons to
the catalyst surface is hindered, thereby, reducing the photocatalytic activity .So we carried out
all the reactions in 80ppm dye concentration with 1g/L materials doses. As methyl orange dye
regarded as an anionic dye and the point of zero charge (PZC) of both the LDHs are 7.5 and 8.0.
When PH > PZC, the surface of the catalyst exhibit a negative charged surface and the below
PZC, the MO dye can adsorbed in to the surface of the catalysts easily via coulombic interaction.
Hence, the weak acid or neutral is the ideal condition for MO degradation over LDHs. So we
proposed that the best range of pH for MO degradation was under weak acid condition.
Furthermore, the structure of layered materials with hydroxide sheets is vanished below pH 3.
52,53
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European Journal of Inorganic Chemistry
10.1002/ejic.201601191
In addition, the kinetic curve of MO photocatalytic degradation was also investigated by
fitting its kinetic curve according to an approximated pseudo-first-order process. Figure 9 (b)
showed the plots of ln (C0/C) versus irradiation time and the fitting results of these plots.
ln (C0/C ) = kappt
(4)
where C is the instantaneous concentration of MO solution at time t (mol/L), C0 is the initial MO
concentration (mol/L), and kapp is the apparent pseudo-first-order rate constant (min−1).
The t parameters which indicates the time required to degrade half of the initial concentration
½
of dye can be calculated by using the following equation:
t½ = 0.693/kapp
(5)
The apparent reaction rate constant k app and t for the sample Mg/Fe-4 and Mg/Fe-4/Mo LDH
½
was 0.021 min-1 (t =32 min ), and 0.057 min-1 (t =11 min) respectively, suggesting a
½
½
preferable photocatalytic performance of the sample Mg/Fe-4/Mo LDH.
The stability of the materials were evaluated by recycling the photocatalysts for the
degradation of MO under visible irradiation for 3 hours. After each run, the fresh MO dye
solution was used for each photocatalytic experiment .Also the catalysts were collected, washed
and then reused. As shown in Figure 9©, there was no significant decrease of the efficiency after
four successive cycles, the photocatalytic efficiency decreased only slightly after four successive
cycles, indicating that the photocatalysts were photo- and chemically stable.
The chemical oxygen demand (COD) was widely used to measure the organic strength of
the organic pollutant which indicate the mineralization of methyl orange dye. In our case, COD
method was performed by acidic dichromate method. Figure 9(d) shows the disappearance of
COD with respect to time. It can be seen that there is a gradual reduction in the COD values and
the efficiency of COD was about maximum of 70% and 80% efficiency for Mg/Fe-4 LDH and
Mg/Fe-4/Mo LDH respectively.32,43
Mechanistic Aspects
Mg/Fe LDH constitute the sheets of edge shared Mg (OH)6 and Fe(OH)6 octahedral units.
In FeO6, the top of the VB is mainly composed of O 2p orbital and the bottom of the CB Fe3+ (3d
orbital). When a semiconductor photocatalyst (Mg/Fe-4 LDH) is exposed by photon of energy
equal to or greater than the band-gap energy, the valence band electron of O 2p orbital of FeO6
was excited to the conduction band of Fe3+ (3d orbital) resulting hole and electron in the valence
band and conductance band respectively. The photogenerated holes are trapped by surface
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European Journal of Inorganic Chemistry
10.1002/ejic.201601191
absorbed water to form hydroxyl radicals (OH•), and the photogenerated electrons are absorbed
•-
by oxygen to form super oxide radicals (O2 ).25 The hydroxyl radical and superoxide radicals are
active species helps for photodegradation of organic pollutants.
As LDHs has cation doping capacity at the MO6 octahedrons of brucite layers,
consequently it act as a doped semiconductor toward photocatalysis and the higher valent cation
act as a dopant. Recently, the photocatalytic performance of the Fe doped semiconductor have
been investigated many groups.54 Recent study explained the photo functional LDHs with ternary
cations were demonstrated as an effective dopant to show excellent results towards
photocatalysis. Also, the good crystallinity of LDHs can be the essential factor for photocatalysis
which depends upon various metal cations and interlayer anions as well as different metal
ratios.55
According to the Kabakcioglu density functional theory, the molybdate bearing Td
symmetry with highest occupied molecular orbital (HOMO) which is nonbonding fully filled
oxygen 2p orbital and lowest unoccupied molecular orbital (LUMO) contributing Mo (4d
orbital).56This study also confirmed that, when Mg/Fe-4/Mo photocatalyst absorbed photon
energy under visible light irradiation, the excited electron induces oxygen-metal charge transfer
spectra (OMCT) which has a high oxidizing power to oxidize dye compounds can be represented
as follows and schematically represented in scheme 2:
−
+
Mg/Fe-4/Mo
+
h → Mg/Fe-4/Mo
(
eCB
+
hVB
)
(6)
(7)
Mg/Fe-4/Mo (hVB ) + H2 O→ Mg/Fe-4/Mo + H+
Dye + OH• → Degradation product (DP)
+
OH
•
+
(8)
(9)
−
•
−
(
eCB ) + O →
+ O
Mg/Fe-4/Mo
2
Mg/Fe-4/Mo
−•
2
O2 + Dye → Degradation product (DP)
(10)
Investigation of active species:
The significantly enhanced photocatalytic activity of the Mg/Fe-4/Mo LDHs catalysts
motivated us to investigate the active species in degradation process. Generally, photo induced
-.
holes (h+), hydroxyl radicals (OH.) and superoxide radicals (O2 ) are the main reactive species
involved in the photocatalytic degradation of dyes. Hence the trapping experiments were
conducted (as shown in figure 10) by taking various scavengers, including disodium salt of
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European Journal of Inorganic Chemistry
10.1002/ejic.201601191
ethylene-diamine-tetra-acetic-acid (Na2-EDTA) for holes (h+), benzoquinone (BQ) for
superoxide radicals, and isopropanol (IPA) for hydroxyl radicals (OH.) for detecting the active
species in the photodegradation of MO dye. When holes (h+) reacts with oxalate ion,it produced
active carbondioxide anion radial and hence can reduce degradation process. Similarly,
benzoquinone and isopropanol (IPA) inhibit the degradation process.
-•
BQ+O2 → BQ-• + O2
(11)
(12)
C2O42- + h+ → CO2 + CO2
-•
The % of MO degradation was significantly decreased by adding IPA, indicating that
OH radicals were foremost active species in this photo degradation reaction. Likewise, the
addition of Na2-EDTA and BQ decreased on the degradation of MO, confirming that the holes
and superoxide radicals were also dominant active species. The above results confirmed that the
-.
degradation process was mainly owing to direct holes, OH. and O2 radicals.43
Factors Affecting the Photocatalytic Degradation
1. Oxo bridged system
A feature of the structure of LDH is the connecting of different metal ions to each other
by oxygen atoms having different metal- oxygen- metal sites. The metal- oxygen- metal sites of
LDHs reduce charge recombination via a metal-to metal charge transfer (MMCT) mechanism. In
the pioneering work by Fri groups, a variety of oxo-bridged systems have been developed for
photocatalysis.41The visible light photo excitation of MMCT transition was followed by electro
transfer from one metal to another metal (acceptor ),which result in the formation of transition
species. The MMCT of the oxo bridged system are suitable for the photocatalysis. MMCT
groups act as visible light active electron pumps with adjustable redox potential that depends
upon the selected donor acceptor metals and its oxidation states. Fe-based Oxo-bridging LDH
photocatalysts help to prevent the recombination of charge carriers and enhanced the photo
activity. 57
2. Generation of the Hydroxyl Radical
The hydroxyl radical plays an important role for the degradation of the organic dyes.
Henceforth, it is crucial to investigate the generation of hydroxyl radicals formed by each
photocatalyst. Terpthalic acid (TA) by using PL technique is used as a probe molecule for
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European Journal of Inorganic Chemistry
10.1002/ejic.201601191
detection of OH• radicals. The PL emission spectrum excited at 315ꢀnm of the solution containing
TA and when it reacts with OH• radicals, produced 2-hydroxyterephthalic acid (TAOH) and
gives a fluorescence signal at 425 nm (equation 13).42 Figure 11 illustrates the changes of PL
spectra of all the as prepared materials after reaction. It can be revealed that, the PL intensity is
larger in case of Mg/Fe-4/Mo LDHs than the Mg/Fe-4 LDHs at wavelength range of 350–
550ꢀnm. According to recent reports, the signal of PL is only caused by the reaction of TA with
•OH radicals formed on the photocatalyst during visible-light irradiation and the PL intensity is
directly associated to the number of hydroxyl radicals produced by the photocatalysts.58 Thus,
the highest PL intensity of Mg/Fe-4/Mo LDH confirms the generation of a more number of
hydroxyl radicals gives fruitful result compared to Mg/Fe-4 LDH photocatalyst.
3. Lowering of Electron−Hole Recombination and prolonged charge carriers
The photocatalytic activity of the photocatalyst and the separation efficiency of the photo
generated charge carriers are closely related. The PL emission spectra mostly result for the
recombination of e- and h+ charge carriers of the semiconductor. In the case of the Mg/Fe-4/Mo,
the PL signal becomes significantly quenched after intercalating molybdate. This is due to the
intercalated molybdate ion. The highest quenching indicating the lower degree of recombination
of the electron–hole pairs with the highest photocatalytic activity. Moreover, the increase of the
photoluminescence lifetime can be inversely related to the radiative recombination of charge
carriers.59From TRPL analysis, we concluded that, Mg/Fe-4/Mo LDH has about 3 times longer
decay time than Mg/Fe-4 LDH. The longevity of long lived charge carriers are beneficial for the
photocatalytic reactions.
4. Generation of High Photocurrent and lower charge transfer resistance
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European Journal of Inorganic Chemistry
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The higher the applied potential, the higher the photocurrent generated by the samples.
The Mg/Fe-4/Mo LDH has higher photocurrent density as compared to Mg/Fe-4 LDH implies
that, the availability of photoelectrons is responsible for the degradation of dyes. Hence, the
interlayer molybdate play an important role for increasing the photocurrent density as well as
increasing the effective charge carriers’ separation and improve photocatalytic activity. Also the
higher oxidation ability of Mg/Fe-4/Mo LDH attributed to the high MO dye degradation.
Conclusions
In this work, we have successfully synthesized the visible light-driven Mg/Fe-4LDH and
molybdate intercalated Mg/Fe-4 LDH and their photocatalytic activity towards photocatalytic
MO dye (100ppm) degradation investigated. The photocatalytic result indicate that the Mg/Fe-4
LDH sample possess 90(%) degradation after 3hrs whereas Mg/Fe-4/Mo LDH can absolutely
degrade within 45 minutes under simulated visible light irradiation. The materials have good
stability and have demonstrated easy reusability with easy reusability and good stability. The
superior result was attributed due to the synergistic effect of high band gap energy, ability to
utilize broad bands in the solar spectrum and high surface area. On the basis of the active species
trapping experiments, photoluminescence study and the photo electrochemical measurements,
the factors of enhanced photo activity of the Mg/Fe-4/Mo LDH was discussed. The prolonged
lifetime of photogenerated charge carriers and faster interfacial charge-transfer rate of Mg/Fe-
4/Mo LDH could improve photo activity. These modified photo catalysts can be reused easily
several times without substantial loss of the activity, which is alternative green material towards
degradation of organic dyes. This work can be a basis for future work for designing other novel
intercalating materials with remarkable properties, for applications in photocatalytic degradation
of different organic pollutants, photocatalytic water splitting to produce hydrogen, photocatalytic
CO2 conversion to future renewable fuels.
Experimental Section
The Mg/Fe LDH materials used in this study were prepared by co-precipitation process i.e. a
mixed Mg(NO3)3.6H2O and Fe(NO3)2.6H2O solution with Mg/Fe desired M (II)/M (III) molar
ratios (4:1,3:1,1:1) were titrated drop wise with sodium hydroxide (1.0 M) mixed alkali with
vigorous stirring at room temperature (35±20 C) up to pH 9.0. The resulting suspension
transferred to the Teflon coated stainless steel autoclave for hydrothermal treatment at 1200C for
10 h, then filtered and washed with deionised water until the pH of the filtrate was around 7.0
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European Journal of Inorganic Chemistry
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and then dried at 60oC for 24 h. LDHs prepared with variation of molar ratios 4:1, 3:1 and 2:1
are named as Mg/Fe-4, Mg/Fe-3 and Mg/Fe-2. The details of the preparation procedure of
-
intercalated MoO4 anions over Mg/Fe-4 LDHs has been reported elsewhere.31
The samples were characterized by powder X-ray diffraction (PXRD) measurements,
high resolution transmission electron microscopy (HRTEM), field emission scanning electron
microscopy (FESEM),Fourier transform infrared (FTIR) spectra, diffused reflectance UV-vis
(DRUV-vis) spectra, photoluminescence (PL) spectra, time resolved photoluminescence (TRPL)
spectra. Rigaku Miniflex was used to record the powder X-ray diffraction (PXRD) ranging from
3º to 70ᵒ with a scan rate of 5°/min using Cu K radiation (λ = 1.54Å).Transmission electron
microscope images were carried out by TEM FEI, TECNAI G 20, and TWIN at 200 kV
and FESEM images were obtained using a HITACHI 3400N microscope. The Fourier transform
infrared (FTIR) spectra of the samples were recorded on a Varian FTIR spectrophotometer. BET
surface areas of the samples were analyzed by the multipoint N2 adsorption–desorption method
at liquid nitrogen temperature in a constant-volume adsorption apparatus (BELSORP-mini, BEL
JAPAN). UV-vis (DRUV-vis) spectra of the catalyst samples were taken with Varian Cary IE
UV-vis spectrophotometer. Photoluminescence spectral measurements were performed on a
Perkin-Elmer (LS 55) fluorescence spectrophotometer. Time-resolved photoluminescence (PL)
spectra were measured at room temperature using a Fluoromax-4 spectrofluorometer (HORIBA
Scientific). The photocurrent and electrochemical impedance spectroscopy (EIS) were measured
with an electrochemical analyzer (CHI 660B Chenhua Instrument Company).
Photo electro chemical experiments, the Mg/Fe-4 and Mg/Fe-4/Mo LDH film electrodes
were prepared by electrophoretic deposition by the previous method by using a potentiostat
(ITECH IT6834).The current potential measurements were carried in a three-electrode
electrochemical cell with Ag/AgCl electrodes, a platinum wire and the prepared electrode were
used as reference, counter, and working electrodes, respectively. The potential of the working
electrode was controlled by a potentiostat and the cell was filled with an aqueous solution of
Na2SO4 (0.1 M) at pH around 6.5. A scanning potentiostat (Gamry 3000) was used to measure
photo currents at a scan rate of 10 mV/s. A 100 W ozone-free xenon lamp (Abet Technologies,
USA) was used as the light source, with simulate sunlight at 100mW/cm2. The Nyquist plots
were recorded within 100 MHz to 100 kHz frequency range.
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European Journal of Inorganic Chemistry
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For a typical photocatalytic experiment, 20 mg of catalyst was added to an aqueous solution
of methyl orange (usually 20 ml of 80 ppm) in different 100 ml quartz reactors. A 500 W Xe
lamp was used as the light source with a 420 nm cut-off filter and the measured light intensity of
120 mW cm2. Prior to irradiation, the suspension was magnetically stirred under dark for 30
minute to establish the adsorption and desorption equilibrium, then suspension were exposed to
lamp for 180 minutes. After irradiation, the suspension was centrifuged and the dye content was
concentration of the dye was monitored using a UV-visible spectrometer (Cary 100 model EL
96043181) at different time intervals and the blank reaction was also carried out by the same
procedure without adding any catalyst named as photolysis reaction. The absorbance at 450 nm
was used for estimating the concentration of the methyl orange dye. And the degradation of dye
is expressed by C/C0, where C0 is the residual concentration and C is the initial concentration of
the dye.
Acknowledgements
This work is financially supported by Center for advanced materials, Qatar University.
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Schemes
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European Journal of Inorganic Chemistry
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Scheme 1. Schematic representation of nitrate/carbonate exchange with Molybdate ion in layered
double hydroxide; Basal spacing calculated from XRD analysis.
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European Journal of Inorganic Chemistry
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Scheme 2. Schematic representation of detailed the interaction of dye with LDH and the
degradation mechanism over Mg/Fe-4/Mo LDH.
Figures
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European Journal of Inorganic Chemistry
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Figure 1. X-ray diffraction patterns of (a) Mg/Fe-4 LDH (b) Mg/Fe-4/Mo LDH(c) Mg/Fe-3
LDH (d) Mg/Fe-2 LDH.
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European Journal of Inorganic Chemistry
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Figure 2. TEM images of (a) Mg/Fe-4 LDH (b) Mg/Fe-4/Mo LDH , SAED patterns of (c)
Mg/Fe-4 LDH (d) Mg/Fe-4/Mo LDH, FESEM image of (e) Mg/Fe-4 LDH (f) Mg/Fe-4/Mo
LDH.
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European Journal of Inorganic Chemistry
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Figure 3. FTIR spectra of (a) Mg/Fe-4 LDH (b) Mg/Fe-4/Mo LDH.
Figure 4. Nitrogen sorption isotherm of (a) Mg/Fe-4 LDH (b) Mg/Fe-4/Mo LDH.
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Figure 5. The UV-vis DRS spectra of LDHs (Inset: The specific absorption band edges
calculated from DRS).
Figure 6. Fluorescence spectra of all the as prepared LDHs.
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European Journal of Inorganic Chemistry
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Figure 7. Time-resolved photoluminescence spectra for (a) Mg/Fe-4 LDH (b) Mg/Fe-4/Mo LDH
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European Journal of Inorganic Chemistry
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Figure 8. The photocurrent density (I-V) curves and The EIS Nyquist plots of (a) Mg/Fe-4 LDH
(b) Mg/Fe-4/Mo LDH.
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European Journal of Inorganic Chemistry
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European Journal of Inorganic Chemistry
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Figure 9. (a) Photocatalytic degradation of MO in aqueous solution over LDHs. (b) Pseudo-first-
order kinetics of MO for LDHs. (c) Regeneration study of MO in recycle run over Mg/Fe-4/Mo
LDH.(d)Photocatalytic degradation of MO by COD Mg/L.
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Figure 10. Photocatalysis test by taking different quenchers over Mg/Fe-4 LDH and Mg/Fe-
4/Mo LDH.
Figure 11. PL spectral changes with visible-light irradiation time over different prepared
materials in 5 × 10-5 M basic solution of Terpthalic acid.
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