Synthesis and characterization of Mn/Co/Ti LDH and its utilization as a photocatalyst in visible light assisted degradation of aqueous Rhodamine B

Article abstract of DOI:10.1039/c6ra24288j

Luminescent Mn/Co/Ti LDH, synthesized by a single step hydrothermal route, has been found to be optically responsive for utilization as a highly efficient photocatalyst in destruction of the cationic dye Rhodamine B, in aqueous solution under visible light irradiation. The material has been found to be better than the commercial catalysts like MnO, CoO, TiO₂ and Degussa P25. Multiple techniques like XRD, XPS, FT-IR, EIS, TG, UV-visible DRS, PL, TRES, N₂-sorption-desorption, dynamic light scattering, TEM-EDS and AFM analyses were used to characterize the LDH. The results indicated Mn/Co/Ti LDH to have a multilayered crystalline structure with hexagonal morphology that carries metal ions in mixed valences, oxygen vacancies, defect states, thermal stability, narrow band gap, high surface area, and electrostatic surface charge variation with pH. The photocatalytic activity of the LDH could be co-related with the structural aspects such as oxidation states, narrow band gap, high surface area and existence of defects. The active species responsible for photocatalysis have been evaluated with EPR, terephthalic acid fluorescence probe and indirect radical-hole trapping experiments. The photodegradation mechanism involves electron and hole hopping across the material and also photosensitization of the dye. Ex situ¹H-NMR and GC-MS analyses of the colorless end products of Rhodamine B destruction provide further insight into the reaction mechanism. The complete mineralization of the decolorized end product of degradation was evaluated with TOC analysis. The results indicate the potential for using multi metal incorporated LDH in destroying dyes and their degradation products in industrial wastewater.

Full text of DOI:10.1039/c6ra24288j

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DOI: 10.1039/C6RA24288J
Synthesis and characterization of Mn/Co/Ti LDH and its utilization as a photocatalyst in
visible light assisted degradation of aqueous Rhodamine B†
Priyadarshi Roy Chowdhury and Krishna G. Bhattacharyya*
Department of Chemistry, Gauhati University, Guwahati 781014, Assam, India
* Tel: +91 (0) 9864031987; Fax: +91 (0) 361 2570599
Eꢀmail: kgbhattacharyya@gmail.com
Abstract
Luminescent Mn/Co/Ti LDH, synthesized by a single step hydrothermal route, has been found to
be optically responsive for utilization as a highly efficient photocatalyst in destruction of the
cationic dye Rhodamine B, in aqueous solution under visible light irradiation. The material has
been found to be better than the commercial catalysts like MnO, CoO, TiO2 and Degussa P25.
Multiple techniques like XRD, XPS, FTꢀIR, EIS, TG, UVꢀvisible DRS, PL, TRES, N2ꢀsorptionꢀ
desorption, dynamic light scattering, TEMꢀEDS and AFM analyses were used to characterize the
LDH. The results indicated Mn/Co/Ti LDH to be having a multilayered crystalline structure with
hexagonal morphology that carries metal ions in mixed valences, oxygen vacancies, defect
states, thermal stability, narrow band gap, high surface area, and electrostatic surface charge
variation with pH. The photocatalytic activity of the LDH could be coꢀrelated with the structural
aspects such as oxidation states, narrow band gap, high surface area and existence of defects. The
active species responsible for photocatalysis have been evaluated with EPR, terephthalic acid
fluorescence probe and indirect radicalꢀhole trapping experiments. The photodegradation
mechanism involves electron and hole hopping across the material and also by photosensitization
of the dye. Exꢀsitu 1HꢀNMR and GCꢀMS analyses of the colorless end products of Rhodamine B
destruction provides further insight into the reaction mechanism. The complete mineralization of
the decolorized end product of degradation was evaluated with TOC analysis. The results
indicate the potential for using multi metal incorporated LDH in destroying dyes and their
degradation products in industrial wastewater.
Keywords: Photoluminescence, defect states, Fermiꢀlevel pinning, hydrothermal LDH synthesis
with three metals; Rhodamine B and visible light photocatalysis.
1. Introduction
Photocatalysis occurs through the process of generation of electrons and holes in the appropriate
materials.1 The widespread importance of heterogeneous photocatalysis has been thought to be
due to the consequences of FujishimaꢀHonda effect (1972).2 Photocatalysis has been explored as
a solution to deal with problems of large scale environmental pollution.3 It has been shown that
visible light induced photocatalysis could be cost effective in treating with effluents containing a
†
Electronic Supplementary Information (ESI) available: Xꢀray diffraction data,
characterizations and photocatalysis results associated with this work.
1

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wide variety of organics and dyes.4, 5 Dyes are the copious sources of coloured organics
extensively used in textile industries, and discharged in comparatively high concentration in the
effluents, making their removal ineffective in the conventional train of unit processes like
filtration, coagulation, biological oxidation and adsorption on activated carbon, etc. Hence, it has
become imperative to develop ecoꢀfriendly, simple and economical techniques for the removal of
the hazardous dyes from water.6, 7 One class of materials that have received wide attention in this
regard is the layered double hydroxides (LDHs) having the general formula,
[M2+ M3+ (OH)2](An−)x/n·yH2O, where M2+ and M3+ are divalent and trivalent cations, and An−
1−x
x
is the charge balancing interlayer anion. As a result of relatively weak interlayer bonding, LDH
exhibits expanding properties with variability in the composition and finds application in highly
efficient photochemical reactions. Due to high surface area, narrow band gap, presence of defect
sites along with nꢀtype semiconductor characteristics and high recyclability, the LDH is expected
to play a major role in costꢀeffective photocatalytic degradation of carcinogenic and neurotoxic
dyes from textile industries.5, 7, 8
In this work, the synthesis and characterization of a novel 2:1:1 Mn/Co/Ti LDH are reported
with application in visible light assisted photodegradation of aqueous Rhodamine B. The
mechanism of LDH synthesis is discussed from the viewpoint of nucleation and self assembly,
induced by Ostwald ripening.8 Characterizations of the LDH shows an effective separation of eꢀꢀ
h+ pair, narrow band gap, high surface area with defect states, and all these properties have been
suggested to contribute to its remarkable performance as a photocatalyst. On the basis of the
results, valuable insights into the mechanism of photocatalysis of the dye have been suggested
with electronꢀhole hopping conduction model as the basis. The results from ex situ 1HꢀNMR and
GCꢀMS analyses of the colorless degradation products of the dye have been used to elucidate the
reaction mechanism of Rhodamine B degradation. The work thus exhibits that Mn/Co/Ti LDH
could be an important addition to costꢀeffective photocatalysts for large scale environmental
waste water treatment.
2. Experimental section
2.1. Materials
All the chemicals used were analytical grade reagents. TiO2 (particle diameter 30 nm, surface
area 60 m2gꢀ1, band gap 3.17 eV; product name Degussa P25) having high levels of
photocatalytic activity and available from Degussa Corp. was used as a reference photocatalyst.
MnO2 (surface area 90 m2gꢀ1, band gap 2.5 eV) and CoO (surface area 85 m2gꢀ1; band gap ~ 2.68
eV), associated with many redox systems, were purchased from Sigma Aldrich, Germany and
were also used as reference photocatalysts. TiO2 (anatase phase; surface area 100 m2gꢀ1, band
gap ~ 3.26 eV) being primarily known for its high charge separation and improvement of the
photocatalytic performance was purchased from Merck Chemicals Co. USA and was used
without further purification as reference catalyst. Decarbonated water was used throughout the
experiments.
2

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2.2. Synthesis of Mn/Co/Ti LDH
2:1:1 Mn/Co/Ti LDH was synthesized by a single step hydrothermal route taking 10.98 g
Mn(NO3)2.4H2O, 5.89 g Co(NO3)2.6H2O, 1.1 ml TiCl4 and 3.0 g urea in 100 ml decarbonated
water, followed by vigorous stirring and subsequent hydrothermal ageing5 of the mixture in a
Teflon lined autoclave at 140 °C for 36 h. The crystalline product is extracted, washed with
decarbonated water and dried in air at room temperature. The LDH synthesis is schematically
illustrated in Fig. 1.
Fig. 1. Scheme of hydrothermal synthesis of 2:1:1 Mn/Co/Ti LDH
2.3. Characterizations
The powder Xꢀray diffraction (XRD) pattern of the Mn/Co/Ti LDH was obtained using a
PANalytical X'Pert PRO diffractometer using monochromated CuKα Xꢀray radiation (λ = 1.5418
Å) at 25 mA at 35 kV with a scanning rate of 4° minꢀ1 and 2θ angle of 5 to 80°. Xꢀray
photoelectron spectroscopy (XPS) was done with a PHI Quantum 5900 ESCA instrument in
order to identify the chemical states and understand the nature of bonding taking place between
the different groups present on the LDH surface. The XPS sample was irradiated with Al Kα Xꢀ
rays under ultrahigh vacuum conditions. FTꢀIR analysis was performed with a Shimadzu FTꢀIR
3000 spectrometer, the palette having LDH:KBr in the ratio of 1:100. The electrochemical
impedance spectroscopy (EIS) analysis was carried out with a CHI 760E electrochemical
workstation with a threeꢀelectrode configuration, (a) Pt wire (counter electrode) (b) saturated
calomel electrode (SCE; reference electrode) and (c) working electrode (LDH deposited on ITO
glass). A 0.5 M Na2SO4 solution was used as the electrolyte during the impedance
measurements. Thermogravimetric analysis (TG) from 32 to 750 °C was done in a Mettler
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Toledo Thermal Analyser in an inert atmosphere with a heating rate of 10 °C/min. The UV–
visible diffuse reflectance spectroscopy of the LDH was investigated on a Hitachi U4100
spectrometer, equipped with an integrating sphere attachment and using BaSO4 background.
Photoluminescence (PL) analysis at different excitation wavelengths was investigated using a
Hitachi Fꢀ2500 FL spectrophotometer with a Xe lamp as the excitation source. Timeꢀresolved
photoluminescence (TRPL) measurements were done at room temperature using a PicoQuant
Fluo Time 300 lifetime spectrophotometer. The specific surface area and pore size distribution of
the LDH was investigated by low temperature multipoint N2ꢀsorptionꢀdesorption measurements
following Brunauer–Emmett–Teller (BET) method using a Micromeritics Tristar 3000 V6.08
analyzer. Prior to the analysis, the LDH was degassed at 100 °C under vacuum for 3 h to remove
the moisture content. The variation of electrostatic surface charge of the LDH with pH was
measured using a Malvern Zetasizer Nano ZS instrument. The morphology, multilayered
structure, lattice fringes of the product were characterized by transmission electron microscopy
(TEM) using a JEOL JEMꢀ2010 TEM instrument at an accelerating voltage of 200 kV,
associated with energy dispersive Xꢀray spectroscopy (EDS) arrangement for analyzing
elemental composition. The tapping mode atomic force microscopy (AFM; Cypher S Oxford
instruments) was employed to obtain information about the layered structure. The high resolution
AFM imaging was achieved using a Si tip of 10 nm radius. The threeꢀdimensional images were
recorded simultaneously in 512 x 512 pixels with 1–2 Hz scan rate. The presence of hydroxyl
radical (•OH), generated in Mn/Co/Ti LDH during its use as a photocatalyst was measured with
ꢁ
a terephthalic acid (TA) fluorescence probe. The superoxide radicals (O•ꢀ ) generated in the
LDH during photocatalysis were investigated by electron paramagnetic resonance (EPR)
spectroscopy recorded with a Bruker EMX Xꢀband spectrometer with a field modulation of 100
kHz. The photocatalytic reactions were monitored with respect to the dye using a Shimadzu 1800
UVꢀvisible spectrometer. The 1HꢀNMR analysis of the colorless end products of
photodegradation, was done at 800 MHz with a Bruker Advance spectrometer. A 20 K signal
1
averaging was followed in HꢀNMR experiments with 90° and other pulses being calibrated
using standard methods. The excitation sculpting pulse sequence was effectively used for H2O
suppression which was obtained directly from Bruker pulse programme library.11 The carrier
frequency was located on H2O resonance for maximizing the suppression of H2O. A relaxation
delay of 2 s was followed for complete relaxation of H2O signal. D2O was added to the sample
1
prior to HꢀNMR analysis, for signal locking and shimming. The colourless products of
degradation were further identified using a Varian 3900 GCꢀSaturn 2100T GC/MS under column
temperature of 45 °C (1 min) and 45–250 °C (5 °C/min, hold time: 5 min). On the basis of
1
product identification by ex situ HꢀNMR and GCꢀMS analyses, the reaction mechanisms have
been proposed for the degradation of the dye. The complete mineralization of the degraded dye
metabolite was analyzed by measuring the Total Organic Carbon (TOC) using a TOC analyzer
(ANATOC II, SGE International Pty. Ltd., Australia). In order to determine the carbon mass
balance associated with degradation, arrangements were made to connect the photocatalytic
reactor with the gas chromatograph, ion chromatograph and TOC analyser.
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2.4. Photocatalysis
The visible light assisted photocatalysis of aqueous Rhodamine B was carried out in a cost
effective specially designed photocatalytic device until the dye solution turned colourless.
2.4.1. Photocatalytic device
The photocatalytic reactions were done in a photocatalytic device comprising of a stainless steel
chamber fitted with a tungsten lamp (Philips 38941ꢀ1; PS 25, Frostꢀ6100) as a visible light
source, equipped with lens (λ > 400 nm) to allow the collimated beam of visible light to be
incident onto the reaction mixture, taken in a double walled beaker. The lens is adjusted with
respect to the light source, in such a way so as to produce a collimated beam having an intensity
of 150 W/m2 reaching the reactants present within the beaker. A continuous circulation of water
through the outer jacket ensures a constant temperature in the reaction system. The double
walled beaker is placed over a magnetic stirrer for vigorous stirring of the reactants. The device
is attached to a voltage stabilizer in order to avoid overꢀvoltage and under voltage protection (of
specification: high voltage cutꢀoff: 290 V; 240 V output, capacity 1.3 Amps, input 90 ꢀ 290 V
range; > 95 % efficiency), connected to an ACꢀsource. The core portion photocatalytic device is
schematically illustrated in Fig.2.
Fig. 2. Schematic design of the core portion visible light photocatalytic device
2.4.2. Photocatalytic reaction procedure
The photocatalytic experiments were performed at 30 °C (room temperature) in a double walled
beaker with continuous circulation of running water. In the degradation experiments, the
reactants were vigorously stirred for 30 min in the dark to establish the adsorptionꢀdesorption
equilibrium between the catalyst and the dye prior to visible light exposure. During this period,
the reaction mixture of the test dye was analysed by taking out 5 ml aliquots at every 5 min
intervals and analyzing its centrifugate by UVꢀvis spectrophotometer. Each photocatalytic
reaction was carried out with 200 ml of 10ꢀ5 M solution of Rhodamine B (Fig. S1; ESI†). During
photocatalysis, at every 10 min intervals, 5 ml aliquots were taken out, centrifuged to remove
solid LDH particles and the centrifugate was analyzed for the unconverted dye at λmax of 553 nm.
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The decolorization efficiency of the dye was evaluated using the relationship C/Co against time,
where C and Co represent the absorbance at a particular interval and at t = 0 respectively. The
decrease in the absorbance of the dye to almost zero after 60 min of visible light exposure
indicated the completion of the decolorization process. Control reactions were performed for the
dye in the dark for 60 min under the same set of conditions. The effects of pH on the
photocatalytic activities of the test dyes were investigated by variation of the pH of the reaction
mixtures from 4.0 to 11.0 by adding either 0.1 M HCl or NaOH. Catalyst loadings of 5.0, 10.0,
15.0 and 20.0 mg in 200 ml dye solution was used to investigate the effects of the same on RhB
photoꢀdegradation at pH 11.0 with dye concentration of 10ꢀ5 M. The photocatalytic conversion
was carried out with three concentrations of the dye of 1.0 x 10ꢀ3, 1.0 x 10ꢀ4 and 1.0 x 10ꢀ5 M
with LDH loading of 15.0 mg in 200 ml of dye solution separately. The photocatalytic efficiency
of Mn/Co/Ti LDH was compared with that of commercially available pure MnO2, CoO, TiO2
and Degussa P25 (15.0 mg in 200 ml of dye solutions) for degradation of 1.0 x 10ꢀ5 M dye
solution. The kinetics of photodegradation was also evaluated under optimum conditions
(concentration 1.0 x 10ꢀ5 M; catalyst 15.0 mg and pH 11, 30 °C). The photocatalytic efficiency of
the LDH was followed separately in the presence of the quenchers like 2NaꢀEDTA (h+
scavenger), nꢀbutanol (•OH scavenger), benzoquinone (O2•− scavenger) added at the beginning
of the dye degradation reaction in order to establish the roles played by •OH, h+ and O2•− species,
for establishing the semiconduction mechanism of photocatalysis. The FTꢀIR data of the
colourless end products of dye degradation were compared with those of the pure dyes to
evaluate whether adsorption or degradation dominated the decolorization reactions. The
colourless end products of photocatalysis were identified by 1HꢀNMR and GCꢀMS techniques for
establishing the mechanism of RhB degradation.
3. Results and discussion
3.1. Xꢀray diffraction study (XRD)
The powder XRD pattern of the LDH (Fig. 3) under investigation shows the characteristic Bragg
2ꢀ
reflections of the hexagonal LDH phase with interlayer CO3 ions, similar to those previously
reported for the Xꢀray diffraction data of LDHs.5, 7, 9, 12, 13, 14 A wellꢀdefined series of (00l) Bragg
reflections indicates the presence of parallel layered houseꢀof cards stacking of the LDH
nanoparticles. The presence of (003), (006) and (009) reflections at 2θ values of 13.34°, 26.68°
and 40.02° respectively indicates the incorporation of CO32ꢀ ions and H2O molecules within the
LDH lattice. The dꢀspacing corresponding to the (003) peak was 0.664 nm (2θ ≈ 13.340) and that
of (110) was 0.313 nm (2θ ≈ 28.56°). Since the basal spacing of the synthesized LDH is similar
2ꢀ
to that of Ti containing LDHs, it is likely that the interlayer CO3 and H2O molecules have
retained the usual LDH pattern. The (110) and (101) peaks at 2θ values of 28.56° and 37.43°
indicate the existence of the anatase phase of TiO2 in the synthesized LDH.3 The presence of
these narrow and sharp diffraction peaks indicate good crystallinity. The other diffraction peaks
(018), (100), (0111), (113) and (1013) could also be indexed to typical LDHs. The XRD
parameters are illustrated in S2 (ESI†).
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Fig. 3. Xꢀray diffractogram of 2:1:1 Mn/Co/Ti LDH
3.2. Xꢀray photoelectron study (XPS)
The XPS survey spectrum carried out using monochromatic AlKα Xꢀray source reveals the
existence of elements in different chemical states on the LDH surface (Fig. 4). Thus, Mn is
present in +(III) and +(II) states in the LDH. Ti is present in +(III) and +(IV) whereas Co is
present in +(II) and +(III) states. The O1s XPS scan reveals the existence of the corresponding
oxides of different metals associated with the LDH surface. The spectra were deconvoluted to
confirm the presence of mixed valency of metals along with the nature of bonding of the
constituent elements in the LDH. The presence of mixed valency of metals indicate the
occurrence of a series of redox reactions on the LDH surface, being assisted by visible light,
which is most likely to degrade the organic dye to colourless simple molecules.
The chemical states of the elements present in the LDH, were identified by comparison of the
obtained photoelectron binding energies (BE) with those taken from literature. The full scan
photoelectron lines indicate the existence of spin states of exhibited by the elements present
within the LDH i.e. Mn3s, Mn2p, Co2p, Ti2p, O1s, C1s, N1s and Cl2p respectively. The XPS
lines were further deconvoluted to investigate the presence of mixed valency as well as to study
the nature of bonding of different groups on the LDH surface.5
The magnitude of peak splitting in Mn 3s peak in the LDH is the diagnostic feature of the
oxidation state of the metal. This peak on further deconvolution indicates the existence of two
multiplet spin splitting, which is likely to have occurred as a result of coupling of nonꢀionized 3s
electron with 3d valenceꢀband electrons. The deconvoluted Mn 3s peak [Fig. 4(B)] shows peak
splitting corresponding to the binding energy difference of ꢁE = 5.5 eV, due to the presence of
+(III) oxidation state of Mn. On the other hand, the Mn 2p line could be related to the existence
of multiplet splitting due to Mn2p3/2 and Mn2p1/2, along with their corresponding shakeꢀup
satellites [Fig. 4(C)] in the binding energy range of 635–660 eV. These results show that Mn
exists in different oxidation states in the LDH. The deconvolution of Mn 2p yielded two peaks
and a satellite. The asymmetric peak at 640.7 eV could be attributed to 2p3/2 state and the shakeꢀ
up satellite at 646.8 eV represents Mn2+ state. On the other hand, the deconvoluted peak, at 652.7
eV represents the 2p1/2 state of Mn. The splitting between the 2p3/2 and 2p1/2 XPS lines was 12
eV. Thus, the detection of the satellite at 646.8 eV in the Mn 2p3/2 region accompanied by peak
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splitting of ꢁE = 5.5 eV in Mn 3s region, proves the existence of Mn2+ and Mn3+ states of the
metal.15
Fig. 4 (A) Full scan XPS survey, (B) Mn 3s XPS line, (C) Mn 2p XPS line, (D) Co 2p XPS line,
(E) Ti 2p XPS line, (F) O1s XPS line, (G) C1s XPS line, (H) N1s XPS line, (I) Cl2p XPS line of
2:1:1 Mn/Co/Ti LDH. High resolution core level XPS lines are represented by color maps and
the deconvoluted peaks are represented by solid lines. Deconvolution was performed with CASA
XPS software (version 2.3.15).
The core level Co 2p XPS line yielded four peaks corresponding to 2p3/2 and 2p1/2 spin states
of Co and their corresponding satellite lines.15, 16 The existence of these XPS lines were further
confirmed by deconvolution of the Co 2p line, giving two sharp peaks at 781 eV and 796 eV
corresponding to 2p3/2 and 2p1/2 spin states of Co respectively with a peak splitting of ꢁE = 15
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eV representing Co2+ and Co3+ states respectively [Fig. 4(D)]. Meanwhile the low intensity
deconvoluted XPS lines at 787 eV and 803 eV represent the associated satellite features of 2p3/2
and 2p1/2 spin states of Co.
The core level Ti 2p XPS line [Fig. 4(E)] indicates the presence of two asymmetric peaks at
binding energies of 456.8 eV and 462.5 eV corresponding to 2p3/2 and 2p1/2 spin states of Ti.
Thus, Mn/Co/Ti LDH surface is likely to contain of two spin states Ti with a peak separation of
~5.7 eV, indicating the presence of Ti4+ species, also confirmed from the XRD data. The
deconvoluted Ti 2p3/2 spin state shows two peaks at 456.5 eV and 457.2 eV indicating the
existence of Ti3+ and Ti4+ respectively. The surface transformation of Ti species also indicates
the occurrence of redox reaction at the surface of the LDH.5, 16
The O1s core level XPS spectrum of the Mn/Co/Ti LDH consists of a peak at 529.9 eV,
accompanied by a valley that extends to 536 eV. The O1s line after deconvolution yields as
many as nine XPS peaks [Fig. 4(F)]. The deconvoluted lines at 529.7 eV and 529.9 eV are likely
to correspond to the existence of Ti2O3 and TiO2 species on the LDH surface. The lines at 530.2
eV and 530.4 eV could be attributed to MnO and Mn2O3 species whereas that observed at 530.7
eV and 531 eV could be assigned to the presence of CoO and Co2O3 species respectively. The
line at 531.3 eV could be due to the existence of physisorbed H2O or ꢀOH groups present on the
2ꢀ
LDH surface. O1s line at 532 eV could be attributed to C=O group of the interlayer CO3
ꢀ
species and the very low intensity line at 535.2 eV could be due to NO3 species, arising from the
use of nitrate salts of Mn and Co, as precursors in the synthesis of Mn/Co/Ti LDH.15, 16
The C1s XPS line [Fig. 4(G)] appears at 288 eV and its corresponding satellite feature is
observed at 289.4 eV. On deconvolution, lines at 288.1 eV and 289.5 eV are obtained, which
could be attributed to C=O and O=CꢀO groups associated with the interlayer CO32ꢀ species.5, 16
The core level N1s XPS line is highly sensitive to the chemical environment of nitrogen and
varies from 396 to 410 eV in the LDH [Fig. 4(H)]. The peak at BE of 402.9 eV associated with a
satellite at 405 eV that extends upto 409 eV is due to N2 species associated with the LDH
surface. Deconvolution of N1s line yields three peaks at BE values of 402.8 eV, 405 eV and
406.8 eV respectively. The first two of these could be attributed to well screened and poorly
screened terminally bonded γꢀN2 species respectively. The low intensity peak at 406.8 eV is
likely to be the satellite of N1s peak. N1s XPS peaks could be attributed to the use of urea as the
basic precipitant in LDH synthesis.15, 16
Cl 2p XPS line is composed of two peaks corresponding to 2p3/2 and 2p1/2 spin states at
binding energies of 198.3 eV and 199.4 eV respectively [Fig. 4(I)]. The peak at 198.3 eV
indicates the presence of Cl 2p3/2 state, and that observed at 199.4 eV is due to 2p1/2 spin state of
Cl. The Cl 2p core level XPS lines in the LDH could have arisen due to the use of TiCl4 as a
precursor of Ti during the LDH synthesis.16
3.3. FTꢀIR, EIS and TG analyses
2ꢀ
The existence of interlayer H2O molecules and CO3 anions in the interlayer galleries of the
LDH could be confirmed further from the FTꢀIR spectra [S3; Fig. S2 (ESI†)]. The absorption
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band at 3440 cm−1 has been attributed to the stretching vibrations associated with the surface and
interlayer H2O molecules and OHꢀ groups, being observed at a lower frequency in Mn/Co/Ti
LDH than that of free water at 3600 cm−1. This may be considered as indicating the existence of
interlayer H2O in the LDH. The shoulders at 3178 cmꢀ1 and 2936 cmꢀ1 could be attributed to the
2ꢀ
Hꢀbonding between interlayer H2O and CO3 anions. The band at 1638 cm−1 is indexed to the
bending vibrations of H2O molecules. Three infrared active vibrations associated with the
interlayer CO32ꢀ species are observed in this LDH, at 1412 cm−1 (υ3), 1128 cmꢀ1 (υ1) and 721 cmꢀ1
(υ2). Apart from these vibrations, the bands at 866 cmꢀ1 and 630 cmꢀ1 could be indexed to MꢀOH
and MꢀO stretching vibrations. The band at 866 cmꢀ1 could be attributed to inꢀplane quadrant
bending and that at 1128 cmꢀ1 to CꢀO stretching. Thus, the IR results further confirm the
existence of CO32ꢀ ions and H2O molecules in the interlayer galleries of the LDH.5, 13
The electrochemical impedance spectroscopy (EIS) of the LDH was done with 2.5 mM
K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) mixture in a 0.5 M aqueous solution of Na2SO4, acting as redox
probe. The narrow arc radius of the Nyquist impedance plot [S4; Fig. S3(A); ESI†] represents a
decrease in solid state interfacial layer resistance, accompanied by a high charge transfer
efficiency of the LDH electrode. Thus, a rapid charge carrier transport could be achieved with an
effective separation, thereby facilitating the semiconduction mechanism. From this, it could be
understood that the electron transporting properties of the LDH is facilitated by suppression of
eꢁ − hꢂ pair recombination.16
The MottꢀSckottky impedance analysis exhibited a reversed sigmoid curve for the LDH
indicating nꢀtype semiconductor properties to the LDH [S4; Fig. S3(B); ESI†]. The flat band
potential, (Vfb), obtained from the Xꢀintercept of the linear region of the Mott Sckottky plot,
showing a negative value indicating the presence of different surface states that could produce
considerable changes in the band positions. Vfb for Mn/Co/Ti LDH electrode was observed at ꢀ
0.77 V vs. SCE; which is equivalent to 0.54 V vs. NHE. This suggests the existence of different
surface states within the LDH which has lead to a considerable change in the band positions. The
occurrence of negative flat band with different surface states is explained on the basis of Fermiꢀ
level pinning.17 In the absence of redox couple, flat band potential (V° ) corresponds to the
ꢃꢄ
energy very close to the conduction band edge (Ec) of a surface state free semiconductor [S4;
Fig. S3(C); ESI†]. In the absence of any specific adsorption, the flat band potential is found to
drop across the Helmholtz layer, ꢁφH; corresponding to the potential attributable to the dipoles
oriented at the electrode interface, Vd; whereas, the potential associated with the spaceꢀcharge
region of the semiconductor electrode, ꢁφSC, is zero.
In order to carry out the impedance measurements, an electrolyte of moderate concentration is
used for neglecting the potential drop across the diffuse double layer associated with the liquidꢀ
LDH electrode interface. On varying the potential difference between the LDH electrode and the
solution, by application of an external potential at Eredox = eVredox in solution, ꢁφH remains
unchanged, while ꢁφSC, remains equal to the applied potential (assuming no specific adsorption
of redox couple and no change in Vd). This forms the basis of the usual semiconduction model,
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wherein, the band energies are most likely to remain fixed [i.e. ꢁ(ꢁφH) ≈ 0], the open circuit
photopotential (ꢁV) is represented by the equationꢀ
ꢁV = ⎸V° − Vꢅꢆꢇꢈꢉ
⎸
(1)
ꢃꢄ
However, no photoresponse is expected for redox couples possessing energies beyond the band
gap energy of the semiconductor electrode. Thus, Equation 1 forms the basis of selection of
redox couples with potential corresponding to the nꢀtype semiconductor, for maximizing the
output voltage of a photochemical cell. On the other hand, the flat band potential is shifted by
adsorption of specific ions, since a drop in potential across the Helmholtz layer, in presence of
surface charge, qs, is found to occur and is represented by Equation (2) asꢀ
ꢋ
ꢋ
ꢌ
ꢌ
( )
2
ꢁφꢊ = ꢍ + Vꢇ = ɛɛ /ꢇ + Vꢇ
ꢎ
ꢏ
where CH represents the Helmholtz layer capacitance, ɛ is the dielectric constant, ɛ0 represents
the permittivity of free space and d is the Helmholtz layer thickness. Thus, adsorption of the
anion (qs < 0) has led to a negative shift in Vfb potential, for the Fe/Ti LDH electrode. The
overall filling or emptying of surface states results in the production of surface charge, which is a
function of the number of states with respective occupancies. According to Equation (1), Vfb will
be shifted, producing a relative location of the conduction and valence band edges with respect to
the energy level of the redox couple of the solution. As the surface state density of the LDH is
very large, a change in potential between the solution and the LDH, will impart an equal change
in ꢁφH, keeping ꢁφSC constant. Under the set of conditions, Fermi level pinning occurs for the
energy level of the surface state (Ess) and the photopotential remain independent of Vredox. As
pinning exists, photoactivity may be observed for redox couples having potentials located
apparently outside the energy gap region (depending on V° value) of the semiconductor [S4;
ꢃꢄ
Fig. S3(D); ESI†]. Moreover, the transport properties of the LDH also occurs due to the negative
flatꢀband potential having higher migration efficiency of charge carriers. Generally, the
conduction band potential (ECB) of nꢀtype semiconductors lies between 0 to ꢀ0.2 V. In this case,
Vfb falls within the range of nꢀtype semiconductors, depending on the effective electronic mass
and carrier concentration. In impedance measurements, the voltage difference between ECB and
Vfb is set at 0.1 V. The negative Fermi level of the LDH makes ECB to lie at ꢀ 0.54 V vs. NHE
(i.e. ꢀ 0.77 V Vs SCE). This imparts a strong reducing power to the LDH involved in the oxygen
•−
reduction reaction. The standard redox potential of O2/O2 couple is ꢀ 0.28 V vs. NHE, a
comparatively high positive value than the observed ECB for the LDH. This suggests that the
•−
electrons photogenerated by the LDH could interact with the adsorbed O2 species forming O2
on the LDH surface. The EIS measurements suggests that photogenerated electrons on the LDH
surface, leaves hole type charge carriers which promotes the photodegradation of the dyes.
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The TG analysis was performed to investigate the thermal stability of the LDH. Three degradation
steps could be observed for the LDH [S4, Fig. S3(E); ESI†]. Weight loss between 70 ꢀ 135 °C with a
peak at 100 °C, indicated by the DTG curve, could be assigned to the loss of physisorbed and interlayer
water constituting 4 % by weight of the material. The second loss, observed from 244 ꢀ 335 °C with a
sharp peak at 310 °C , accounted for 14.5 % weight loss, and was observed due to the dehydroxylation of
the brucite like layers. The third weight loss of 11.5 % from 339 ꢀ 460 °C , associated with a broad peak at
372 °C , was likely to have occurred due to the decomposition of the interlayer CO32ꢀ ions.17, 18 No further
weight loss was observed above 460 °C , which indicated the presence of very negligible amount of
2ꢀ
carbonaceous residues in the LDH. The thermogravimetric also supported the existence of H2O and CO3
ions in the interlayer galleries of the layered material.
3.4. Optical properties
The UVꢀvisible DRS and photoluminescence studies of the LDH provide an insight into
absorption and emission processes on the LDH surface. The life time of the charge carriers could
be evaluated by time resolved photoluminescence emission analysis, as discussed in the
following sectionsꢀ
3.4.1. UVꢀVisible diffuse reflectance spectra (DRS)
UVꢀvis DRS measurements could be utilized to ascertain the nature and coordination states of
Mn, Co and Ti in the layered framework [Fig. 5(A)]. The hump at ~ 230 nm could be attributed
to dꢀd transitions. The strong absorption band in the region of 250 ꢀ 460 nm could be assigned to
2ꢀ
typical M(II) coꢀordinated to CO3 in the interlayer galleries. The broad shoulder observed
between 460 to 700 nm indicates the presence of Ti4+ in the brucite like sheets. The broad valley
signifies a supramolecular guest–guest Hꢀbonding, vander Waals forces or guest–host
interactions. Compared to the commercial catalysts, a band broadening accompanied by a red
shift is observed for the LDH, due to the formation of aggregates. A comparison was done to
evaluate the nature of UVꢀvisible absorption of the LDH over commercial catalysts like MnO2,
CoO, TiO2, and Degussa P25. The DRS data show strong absorbance at 250ꢀ448 nm for all the
commercial catalysts [Fig. 5(A)]. The LDH exhibited stronger absorption between 460ꢀ700 nm,
which was not observed in any of the commercial catalysts. The stronger absorbance of the LDH
in the visible region shows its potential for dye degradation over commercial catalysts.7, 13
3.4.2. Band gap energy
A comparative analysis of band gap energy [Fig. 5(B)] is performed to have a better
understanding of the optical properties of the LDH over the commercial catalysts; following the
classical Tauc equation represented by Equation (3):
ɛhʋ = Kꢐhʋ − Eꢑꢒꢓ
(3)
wherein, Eg represents the optical band gap, hʋ is the photon energy, K is a proportionality
constant, ɛ is the absorption coefficient and n depends on the nature of the transition. The value
associated with the point of intersection of the line tangent to the plotted curve inflection point
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with the horizontal line (hʋ axis) is the band gap Eꢑ value. The best fit of (ɛhʋ)2 versus hʋ plots
was obtained for n = 1/2, suggesting a direct allowed transition across the energy band gap. The
absorption edge energies corresponding to Eg = 2.62 eV was observed for the LDH; this band
transition is in good agreement with the photoluminescence observations of the LDH. The
respective band gap energies of the commercial catalysts are: MnO2 (2.5 eV), CoO (2.68 eV),
TiO2 (3.26 eV) and Degussa P25 (3.17 eV). Excepting MnO2, the LDH has been found to possess
a narrow band gap in comparison to that of the other commercial catalysts. Thus, a higher
photoactivity is associated with the LDH. The red shift observed in the UVꢀVis DRS of the LDH
could be attributed to the narrow band gap of the LDH.16 The band gap in the LDH could have
been reduced due to the presence of localized gap states induced by doping with Ti4+
accompanied by oxygen vacancies within the layered framework.
3.4.3. Photoluminescence (PL) emission measurements
In order to have an insight into the effects of recombination of charge carriers,
photoluminescence (PL) analysis was carried out at room temperature (30 °C). The luminescence
emissions are classified into two categories: band–band PL emission belonging to the electronic
transitions from the conduction band (CB) bottom to the valence band (VB) top and the excitonic
PL emissions that arise from the surface oxygen vacancies and defects associated with the
semiconductor.16, 18, 19 In case of band–band PL emission, lower the luminescence intensity,
higher is the separation rate of photoꢀinduced electron (eCB−)ꢀhole (hVB+) pair; therefore higher is
its photoactivity. In case of excitonic emissions, the relationship between luminescence intensity
and photoactivity is complicated to a considerable extent and depends primarily on the electronic
properties of semiconductors.
In this work, the luminescence emissions of the LDH were performed at excitation
wavelengths of 350 nm, 380 nm and 410 nm respectively [Fig. 5(C)]. The luminescence
intensity was found to decrease on increasing the excitation wavelength, which could be
attributed to higher separation of the charge carriers, consistent with the electrochemical
impedance results. The oxygen vacancies and defects bind the photoꢀinduced electrons to form
excitons, so that the luminescence signal can easily occur. It is known that Ti doped
photocatalytic systems consist of two kinds of trap sites for holes, (i) the deep hole and (ii) the
shallow hole. The deep trapped holes exhibit a lower oxidizing potential and mostly prefer to
interact with physisorbed molecules. The LDH showed several excitonic PL emissions at 448 nm
(2.77 eV), 473 nm (2.62 eV), 531 (2.34 eV), 550 nm (2.25 eV) and 581 nm (2.13 eV)
accompanied by a valley extending upto 700 nm respectively, resulting from the surface oxygen
vacancies and defects present in Mn/Co/Ti LDH. These are suggested to be responsible for the
significant enhancement in the optical properties of the LDH. The UV emission peak at 473 nm
(2.62 eV) could be attributed to the exciton emissions from the conduction band (eCB−) to the
valence band (hVB+). This is consistent with the band gap energy of Mn/Co/Ti LDH. Apart from
the above, other emissions could also be observed and these are mostly associated with excitons
and oxygen defects in the shallow and deep trap centers.
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The emission peak at 448 nm is due to selfꢀtrapped exciton (STE). A STE is generated on the
lattice site on a hole capture by a trapped electron, which is of two types, direct and indirect. A
direct STE occurs due to direct recombination of charge carriers, while in an indirect STE, the
recombination occurs via oxygen vacancies. Since Ti doping generates oxygen vacancy, the STE
peak at 448 nm is most likely to have occurred via oxygen vacancy and is of indirect nature. The
emission peak at 473 nm is due to the Fꢀcenter present within the LDH. The F center is an
oxygen vacancy with two trapped electrons. The emissions associated with oxygen defects occur
as a result of the presence of oxygen vacancy in the shallow or deep levels within the band gap
of the LDH. However, oxygen defects of the grain boundary of the LDH are charged, which are
distributed randomly and acts as shallow trap levels. The observed luminescence peaks at 448
nm, 531 nm, 550 nm and 587 nm could be attributed to the shallow trap levels. However, the
shoulders at 630 nm extending upto 700 nm could be assigned to the deep trap emissions.
The free electrons which are untrapped by the defects, migrate most likely throughout the
lattice and undergo recombination with the holes on the LDH surface. The migrating electrons
are likely to be repelled by the charged oxygen vacancies preventing further recombination with
holes. As defect states arise due to doping, the mobility of the charge carriers is thus slowed
down, which delays the recombination process. Moreover, the dopants present on the LDH
surface obstruct the efficient overlap of charge carriers across the LDH surface. The dꢀstates of
the LDH act as active trap centers and separate the charge carriers. Besides this, doping causes
shifting of the band edges and hence the electrons of the conduction band have to traverse an
indirect path for undergoing recombination with the holes and quench the emission intensity. As
the emission intensity is reduced due to increase in excitation wavelength, the quenching effect is
most likely to play an important part in the reduction of the emission intensity. Doping of ions
increases the interaction between the dopant and defects; thereby forming quenching centers.
The electrons present within the LDH, are most likely to be captured at first by shallow trap
levels from which they jump to Mn, Co or Ti dꢀstates, which may further jump from one dꢀstate
to another and thus forms a number of quenching centers. The electrons, however, after passing
through a channel of dopant and defects, ultimately undergo recombination with the holes and
reduce the emission intensity. Thus, the presence of defects within the microstructures of the
LDH is most likely to play a major role in its high photoactivity under visible light.
3.4.4. Photoluminescence excitation spectra
The photoluminescence excitation spectra of the LDH was monitored at different excitation
wavelengths in order to identify the accuracy of excitation luminescence wavelengths. The
results are illustrated in Fig. 5(D). The excitation spectra showed strong peaks at 350, 380, 410
and 528 nm; on being excited at the emission wavelengths of 450, 475 and 580 nm respectively.
The results validate the excitation wavelengths used for observation of the luminescence
properties. Meanwhile a shift in the luminescence peak from 380 to 384 nm, marked by an
increase in intensity at 580 nm emission wavelength, indicates the existence of multiple
luminescent centres in the LDH.
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3.4.5. Time resolved emission study (TRES)
The dynamics associated with the photoꢀgenerated charge carriers has been evaluated further
using timeꢀresolved photoluminescence emission spectroscopy (TRES), the instrumentation was
based on a streak camera which enables the calculation of the temporal decay time of the
carriers.19 The luminescence data have been fitted on the twoꢀexponential function equationꢀ
t
t
( )
I t = Aꢔ exp( ) + Aꢀ exp ꢕ ꢖ
(4)
τꢔ τꢀ
where τ1, τ2 represents the decay times of the fast and slow components; A1, A2 are the
corresponding luminescence amplitudes. τ2 is associated with indirect formation of selfꢀtrapped
excitons generated by trapped electrons whereas τ1 is due to the direct formation of free electrons
and holes. The typical luminescence lifetime fitting results are shown in Fig. 5(E) and Table 1.
The longer luminescence decay time for the LDH corresponding to excitation at 410 nm could be
attributed to the delay in radiative recombination process of the charge carriers. Chiꢀsquare (χ2)
fitting reveals χ2 values of ~1, for the decay curves, representing the accuracy and validity of the
data fit. The longer luminescence lifetimes of the charge carriers impart high photoactivity to the
LDH for undergoing reactions with the target molecules (dyes). It is observed that the average
lifetime of the photoꢀgenerated charge carriers of 2:1:1 Mn/Co/Ti LDH at 410 nm excitation
wavelength is 5.68 ns, which is 1.24 and 2.07 times longer than that observed at 350 nm and 380
nm excitation wavelengths. The photoꢀexcited electrons or holes with a longer life time, being
generated by photons of wavelength in the visible region on the LDH surface are likely to play
an important role in the decomposition of the dye.
Table 1. Lifetime of photoꢀgenerated charge carriers of Mn/Co/Ti LDH at different wavelengths
λ
ex (nm)
A1
τ1 (ns)
A2
τ2 (ns)
τ (ns)
χ2
370
390
410
7271.3
5582.4
6023.6
6.219
2.258
6.314
3042.3
4378.1
4696.7
4.043
5.718
4.157
2.74
4.58
5.68
0.97
0.98
0.96
3.4.6. Mechanism of photoluminescence
The luminescence in the LDH could be interpreted on the basis of the results obtained from the
luminescence spectra, schematically illustrated in Fig. 5(F). It is observed that the LDH on being
excited at 350, 380 and 410 nm respectively above its band gap energy, a part of the energy is
consumed for promoting the electrons from the valence band (VB) to the conduction band (CB),
leaving holes in the VB, and the remaining part is used to impart kinetic energy to the electrons,
when some of them are likely to jump to the conduction band maximum (CBmax). In this, they
are most likely to undergo nonꢀradiative transition by hopping to the bottom of the conduction
band (Step 1) and may undergo direct UV emission (Step 2). Some of these electrons are likely
to be trapped by shallow and deep traps and emit light (Steps 3 and 4). The electrons are then
likely to hop to Mn, Co or Ti dꢀstates and recombine with the holes, present in the valence band
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(Step 5). This process however delays the recombination process as is evident from the lifetimes
of the carrier species, revealed by TRES analysis. Thus, the semiconductor LDH, possessing
defects and oxygen vacancies within the layered framework, could be a highly efficient
photocatalyst for degradation of dyestuffs.
Fig. 5 (A). UVꢀvisible DRS (B) Tauc plots for calculation of band gap for Mn/Co/Ti LDH with
commercial catalysts (C) Photoluminescence emission spectra at different excitation
wavelengths (D) Photoluminescence excitation spectra at different emission wavelengths (E)
Time resolved emission spectra at different wavelengths (F) Mechanism of photoluminescence
emission of 2:1:1 Mn/Co/Ti LDH
3.5. N2 adsorptionꢀdesorption and dynamic light scattering (DLS) measurements
N2 adsorptionꢀdesorption measurements at 77 K yielded a Type IV isotherm [S5; Fig. S4(A);
ESI†] with a H3ꢀtype hysteresis loop (P/P0 > 0.4 ꢀ 0.8), indicating that the condensation of N2
within the pores of the LDH and its release at reduced pressures followed different paths; a
characteristic of mesoporous materials. The specific surface area of 190 m2g−1 with broad pore
width of 3.86 nm [S5; Fig. S4(B); ESI†] could be determined for the LDH. The large surface
area and pore width could be attributed to the use of urea as a basic precipitant during LDH
synthesis, and the LDH is dominated by both micropores and mesopores having diameters from
1 ꢀ 6 nm. The adsorptionꢀdesorption cumulative pore volume was 0.218 and 0.201 cm3gꢀ1
respectively. Thus the LDH with an appropriate spatial array of pores within its layered
framework, is most likely to assist the electronꢀhole transfer process responsible for its highly
efficient photocatalytic performance.20ꢀ22
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The dynamic light scattering measurements with the LDH were done in aqueous medium in
the pH range of 4ꢀ11 for determining the electrokinetic potential (ξꢀpotential) [S5; Fig. S4(C);
ESI†]. A monotonic decrease of surface charge with pH was observed having a zero point
charge occurring at pH 7.16; which is the isoelectric point of the LDH.16 Thus, the cationic RhB
dye is most likely to adsorb at pH > pHzpc when the surface attains negative charge. The surface
charge was observed to be ꢀ 31.53 at pH 11 suggesting a maximum degradation at this pH.
3.6. Morphology and elemental composition
The morphology, lattice fringe separation and multilayered structure of the LDH were studied
with TEM and AFM measurements. The elemental composition was estimated with EDX.
3.6.1. TEM results
TEM measurements revealed ultraꢀfine hexagonally stacked nanoparticles one over the other
[Fig 6(A, C)]. The SAED pattern [Fig. 6(B)] exhibited a hexagonal arrangement of the
diffraction peaks as expected for the inꢀplane symmetry intrinsic to LDH layers, showing the
houseꢀofꢀcards type of stacking of the heteroꢀlayered LDH crystallites. The distance between the
periodic lattice fringes from (110) plane of the LDH [Fig. 6(D)] was calculated to be 0.313 nm,
matching the value obtained from XRD [S2; Table S1(ESI†)]. The average particle size was
found to be 118 nm, calculated using the histogam plot [Fig. 6(E)]. The cross sectional TEM
image [Fig. 6(G, H)] was utilized to obtain the distance between two consecutive parallel metal
hydroxide layers to be 0.671 nm (= 0.289 + 0.382 nm) from the correlation between the cross
sectional TEM image and the idealized LDH structure [Fig. 6(I)]. This is in agreement with the
c´ lattice parameter, obtained from XRD [S2; Table S2 (ESI†)]. Thus, the existence of parallel
house of cards type stacking of brucite sheets, exhibiting multilayered structure, hexagonal
platelets and periodic lattice fringe separation of diffraction planes, could be confirmed in good
agreement with the previous reports of LDH materials.5, 16, 19, 21, 22
3.6.2. Energy dispersive Xꢀray spectroscopy (EDX) results
The EDX measurements [Fig. 6(F)] showed that the atomic/weight ratio of Mn/Co/Ti in the
LDH was 2.06:1.04:1 matching the stoichiometric Mn/Co/Ti ratio of 2:1:1 in the synthesized
LDH. The presence of Cl, C and N in the EDX spectra is due to the use of TiCl4 as precursor of
Ti and urea as the basic precipitant during the hydrothermal synthesis. The elemental
composition obtained from EDX is presented in Table 2.
3.6.3. Atomic force microscopy (AFM) measurements
The atomic force microscopy analysis provided further insight into the morphology, thickness
and multilayered structure of the LDH [Fig. 7(A)]. The AFM confirms the presence of hexagonal
platelets having internal angles around 120°, consistent with the TEM results.16 The flat
crystallite terrace [Fig. 7(B, C)] of the height profiles provide a strong evidence for the presence
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of parallel brucite sheets, which are aligned parallel to each other in the substrate to maximize
the attractive energy.
Fig. 6. (A) TEM image (B) SAED pattern (C) High resolution TEM image (D) Lattice fringe
separation exhibited by TEM (E) Histogram plot showing particle size distribution (F) EDS
spectrum (G, H) High resolution cross sectional TEM images and its coꢀrelation with
(I) Idealized LDH structure.
The total thickness of Mn/Co/Ti LDH along the cꢀaxis [Fig. 7(C)] is determined to be 24 nm
from the 3D height profile of the AFM image. The thickness of a single brucite sheet is found to
be 0.382 nm from the height profile [Fig. 7(B)] in good agreement with cross sectional TEM
results. On dividing the total thickness with c´ lattice parameter (determined from XRD), it could
be found that the LDH consists of ~ 35ꢀ36 parallel brucite sheets.
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Fig.7. (A) AFM image (B), AFM height profile of a single brucite sheet, (C) 3D AFM total
height profile of Mn/Co/Ti LDH
Table 2. EDX elemental composition of 2:1:1 Mn/Co/Ti LDH
Elements
Weight
%
Atomic
%
Mn/Co/Ti (ratio)
C K
1.82
1.28
N K
2.50
1.84
O K
9.64
2.58
2.06: 1.04: 1
Cl K
Mn K
Co K
Ti K
Total
3.85
2.15
41.35
20.78
20.06
100
46.34
23.31
22.50
100
3.7. Photocatalysis reactions
During the 30 min time allowed for reaching the adsorptionꢀdesorption equilibrium in dark,
aliquots were taken out every 5 min, centrifuged to remove the solid catalyst particles, if any and
the centrifugate was analysed for the dye by UVꢀvis spectrophotometer. No change in the
concentration of the dye was observed illustrating absence of adsorption/degradation on the
surface of the LDH in the dark prior to light irradiation and also reaching adsorptionꢀdesorption
equilibrium [S6; Fig. S5 (ESI†)]. After this period, the reactants were subjected to visible light
irradiation, and at every 10 min intervals, 5 ml aliquots were taken out, centrifuged, and the
centrifugate was analyzed for the dye at λmax of 553 nm for RhB. The decolorization efficiency
of the LDH was evaluated by plotting C/C0 against time (C and C0 correspond to dye
concentration after a time interval and at t = 0, equivalent to the absorbances of the aqueous
solution at a particular interval and at t = 0 respectively). The absorbance due to the dye
decreases to almost zero after 60 min of light exposure indicating completion of the
decolorization process. The solution colour changed to nearly transparent marking the end of the
photocatalytic degradation of the dye [Fig. 8(A, B)].
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Control experiments performed under the same conditions, but without visible light
irradiation, yielded negligible degradation, indicating that visible light plays a dominant role in
the reactions. LDH amount, initial dye concentration and pH were also varied to study their
effects on the degradation of the dye. The photocatalytic efficiency of the LDH was studied
separately by addition of quenchers, 2NaꢀEDTA (h+ scavenger), nꢀbutanol (•OH scavenger),
benzoquinone (Oꢀ• ꢁ scavenger) at the beginning of the reaction in order to establish the role of
ꢁ
•OH, h+ and Oꢀ• on photodegradation.
The presence of hydroxyl radical (•OH), generated by the LDH during photocatalysis was
further confirmed by using a terephthalic acid (TA) fluorescence probe. The superoxide radical
ꢁ
(O•ꢀ ) generated by the LDH during photocatalysis was also investigated by electron
paramagnetic resonance (EPR) spectroscopy, recorded on a Bruker EMX Xꢀband spectrometer
with a field modulation of 100 kHz. The photostability of the LDH was justified by its
comparative FTꢀIR analysis and that recovered after the fifth catalytic cycle. The colourless end
products were compared with the pure dye to evaluate the occurrence of adsorption or
1
degradation during the photocatalytic process. Insights obtained from ex situ HꢀNMR and GCꢀ
MS analyses of the colorless end products of Rhodamine B degradation were used for derivation
of the reaction mechanism.
Fig. 8. (A) UVꢀvisible absorption spectra of Rhodamine B degradation by Mn/Co/Ti LDH at
different time intervals, (B) Snapshot showing aqueous Rhodamine B and the colourless end
products obtained after degradation by Mn/Co/Ti LDH
3.7.1. Effects of LDH amount
LDH amount was varied from 5.0 ꢀ 20.0 mg (in 200 ml of the dye solution of concentration 1 x
10ꢀ5 M) and C/Cₒ vs. irradiation time was plotted as in Fig. 9(A). The decolorization efficiency
increased with LDH amount. RhB degraded ~ 99.8 % at 15.0 mg of LDH. A larger amount
increases the available surface area and consequently, the number of active sites in line with
most observations.7, 11, 19, 21, 22 However, the degradation of the dye decreased with the increase of
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the catalyst load to 20.0 mg (in 200 ml dye solution). In a photocatalytic reaction, the catalyst is
known to have an optimum amount, and an amount more than this causes unfavourable light
scattering and reducces light penetration into the solution. Thus LDH loading of 15.0 mg per 200
ml of the dye solution (1 x 10ꢀ5 M) could be considered as the optimum for RhB degradation.
3.7.2. Effects of pH
The photoassisted degradation of RhB was carried out with 15.0 mg LDH per 200 ml of the dye
solution (1 x 10ꢀ5 M) in the pH range of 4.0 to 11.0. The plots of C/Co vs. irradiation time are
presented in Fig. 9(B). It is seen that in acidic medium, the degradation is low, because at low
pH, H+ ions compete with RhB cations for the available adsorption sites on the LDH surface.
The LDH surface is positive below its iisoelectric point of 7.16 resulting in an electrostatic
repulsion of RhB cations at lower pH, and consequently less adsorption of the dye cations on
LDH surface and less degradation. In alkaline medium, the LDH surface becomes increasingly
negative and degradation increases.5, 16, 22 The maximum degradation efficiency of ~ 99.8 % was
observed at pH 11.
3.7.3. Effects of initial dye concentration
The photoassisted decolorization of Rhodamine B was carried out at the optimum pH (= 11) with
15.0 mg of LDH/200 ml of dye solution at 30 °C [Fig. 9(C)] with three different dye
concentrations of 1 x 10ꢀ3, 1 x 10ꢀ4 and 1 x 10ꢀ5 M. Around 99.8 % degradation of RhB was
achieved with 1 x 10ꢀ5 M solution of the dye. Above this concentration, the degradation
decreased to 77 % and 54 % respectively for 1 x 10ꢀ4 M and 1 x 10ꢀ3 M dye solution. With
increasing dye concentration, more dye molecules are likely to be adsorbed on the LDH surface
and accumulation of the adsorbed dye causes an inhibitive effect on their photocatalytic
degradation due to lack of their direct contact between the photogenerated holes, hydroxyl
radicals, superoxide radicals (generated on the LDH during photocatalysis). Moreover, high dye
concentration most likely acts as an inner filter which shunts the photons away from the LDH
surface. The dye molecules after adsorption blocks the active surface sites on the LDH and hence
the degradation efficiency decreases.5, 16, 19, 22
3.7.4. Characterizations of active species
Photocatalysis by a semiconductor material is driven primarily by active species like •OH, h+ and
O•ꢀꢁ species. The presence of •OH radicals during the photoassisted degradation was measured
using terephthalic acid (TA) fluorescence probe. In this case, 1 x 10ꢀ5 M TA in 4 x 10ꢀ5 M
aqueous NaOH replaced the dye solution in the photocatalytic reactor. TA undergoes reaction
with •OH, generating a highly fluorescent compound 2ꢀhydroxyterephthalic acid; which shows
photoluminescence emission at 426 nm, when subjected to excitation at 315 nm wavelength
[Fig. 9(D)]. However, no photoluminescence signal was observed when the experiment was
carried out without visible light irradiation indicating the generation of •OH radicals by the LDH
under visible light.23 The surface ꢀOH groups present on the LDH are most likely to transform
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themselves to •OH radicals that control the photocatalysis process. The presence of •OH radicals
could be further confirmed by external addition of nꢀbutanol (quencher of •OH) prior to the
photocatalysis reaction under the same set of experimental conditions.16 It is seen that the
photodegradation of the dye was suppressed to a considerable extent, indicating that •OH was
one of the active species that controlled the photodegradation of the dye [Fig. 9(E)].
ꢁ
EPR spectroscopy was used to study the presence of superoxide (O•ꢀ ) species generated on
the LDH surface during photocatalysis. The EPR analysis was done using a Bruker EMX Xꢀband
spectrometer with 100 kHz field modulation. The frequency calibration was done with a
frequency counter microwave bridge (ER 041 XGꢀD) with DPPH being used as the field marker
(g = 2.0036). A HPA 400/30 S lamp (400 W, Philips) was used as irradiating light source for
EPR analysis. The magnitude of magnetic field of the radicals was determined using the formula
hν = gβH. Prior to EPR analysis, the LDH was subjected to thermal treatment at 473 K in
vacuum followed by exposure to an oxygen environment at room temperature. The EPR
spectrum of the LDH [Fig. 9(F)] analysed in O2 environment under visible light (λ > 400 nm)
showed signals at gx = 2.0037, gy = 2.011 and gz = 2.003, which coincide with the gꢀtensor of the
ꢁ
O•ꢀ species. This indicates that O•ꢀꢁ species lead to the generation of visible light sensitive redox
centers, which are primarily responsible for the dye degradation.
Apart from EPR analysis, an indirect radical trapping experiment was also performed with the
ꢁ
test dye by external addition of benzoquinone (BQ; Oꢀ• scavenger) prior to visible light
irradiation, under the same set of photocatalytic conditions. A decrease in degradation of
Rhodamine B [Fig. 9(E)] confirmed that Oꢀ• ꢁ was one of the most active species driving the
photocatalysis.16, 23
The photogenerated holes in a semiconductor material, also play a major role in the
degradation of the dye.24 The generation of holes during the visible light assisted photocatalysis
was investigated by external addition of the quencher, EDTAꢀ2Na to the experimental solution,
prior to visible light irradiation. The suppression of degradation [Fig. 9(E)], suggested that the
photogenerated holes have played an important part in the degradation of the dye.16 Thus, the
ꢁ
photocatalysis by the LDH is assisted by the active species, Oꢀ• , hꢂ and •OH radicals.
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Fig.9. The influence of of (A) LDH amount, (B) pH, (C) Initial dye concentration on
ꢁ
photodegradation of aqueous RhB. (D) EPR spectrum of the LDH for detection of (O•ꢀ ) species
during photocatalysis, (E) Roles of different active species on photodegradation of aqueous RhB,
(F) Photoluminescence spectral changes with visible light irradiation over Mn/Co/Ti LDH in the
basic solution of terephthalic acid (TA) for detection of •OH radicals.
3.7.5. Kinetics
The kinetics of photoassisted degradation was evaluated under optimum conditions (pH 11, LDH
15.0 mg; RhB 1x10ꢀ5 M, dye solution 200 ml) to understand the process catalysed by Mn/Co/Ti
LDH under visible light. The plot of ln (C0/C) vs. irradiation time [S7; Fig. S6 (ESI†)] is close
to linearity, indicating that RhB degradation follows pseudo first order kinetic model represented
by Eqn. (5)
Cꢈ
ln ꢕ ꢖ = kꢗꢘꢘ
t
(5)
C
where C0 is the initial concentration of the dye and C is the concentration at time t. The apparent
rate constant kapp was calculated to be 0.041 min−1 for RhB. The calculated half life was found to
be 16.91 min for RhB using the Eqn. (6)ꢀ
ln2
tꢔ/ꢀ
=
(6)
kꢗꢘꢘ
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The pseudo first order kinetic model suggests that the photocatalytic reaction rate is primarily
dependent on light intensity and the absorption efficiency of the LDH.
3.7.6. Comparative performance of the LDH with commercial catalysts
A comparative analysis of RhB degradation by the synthesized LDH and the commercial
catalysts, MnO2, TiO2, CoO and Degussa P25 under visible light showed a higher photocatalytic
efficiency of the LDH. Control experiments (blank and dark reactions) with the LDH indicated
negligible performance and therefore, the dye degradation is controlled by the intensity of visible
light only. The results are illustrated in Table 3 and Fig. 10, which suggest a superior
performance of the LDH over the commercial catalysts as a photocatalyst for RhB degradation.
Fig.10.Comparative evaluation of Mn/Co/Ti LDH and commercial catalysts for
photodegradation of Rhodamine B by (under visible light and controlled conditions) over
commercial catalysts.
Table 3. Efficiency of degradation of aqueous RhB by LDH and commercial catalysts
(under visible light and controlled conditions)
Catalyst/conditions
Degradation efficiency (%)
2:1:1 Mn/Co/Ti LDH
TiO2
99.8
78
MnO2
75
CoO
50
Degussa P25
Blank reaction
Dark reaction
84
19
25
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3.7.7. Recyclability and photostability of the LDH
The reusability of the LDH photocatalyst is a parameter of vital importance from the industrial
perspective. After each photocatalytic cycle, the LDH was recovered from the reaction mixture
by centrifigation, washed several times with water and dried. The recyclability of the LDH was
tested upto fifth cycle of RhB degradation. The degradation of RhB in the first cycle was 99.8 %,
which decreased by some extents in the next cycles reaching ~ 87 % after the fifth cycle [S8;
Fig.S7(ESI†)]. This illustrates that the LDH maintains good photoactivity for degradation of
RhB even after the fifth cycle. The LDH is a heterogeneous photocatalyst that can be easily
recycled by a simple filtration because of its higher density. The decrease in the photocatalytic
efficiency after its reuse is most likely to be caused by the reduction of a number of active sites
available on the LDH surface after each cycle of photodegradation. However, it was observed
that even after five degradation cycles, the LDH did not exhibit much loss of activity confirming
that the catalyst was not photꢀ corroded during the reactions. The stability of the new
photocatalyst is important with reaspect to its application and it is to be noted that the doped Tiꢀ
37, 38
photocatalysts sometimes have this problem.
Moreover, the linear nature of each curve
suggests that the degradation kinetics follows pseudoꢀfirst order model, in all the degradation
cycles.
A comparative FTꢀIR analysis of the pure LDH and that recovered after the fifth cycle of RhB
degradation [S9; Fig. S8(ESI†)] showed that the material also possess very good photostability.
The IR bands remained at nearly at the same positions with some minor changes being observed
in the fingerprint region of the LDH.
3.7.8. FTꢀIR analysis of the end product of photocatalysis
The occurrence of adsorption or degradation in the decolorization of aqueous RhB is determined
by comparative FTꢀIR analysis of the pure dye with that of the centrifugate of the dark reaction,
blank reaction and the colourless end product of photocatalysis brought about by 2:1:1 Mn/Co/Ti
LDH under visible light irradiation [S10; Fig.S9 (ESI†)]. It is observed that all the characteristic
bands appear at their respective positions for the control experiments (blank and dark reactions)
with some broad peaks in the fingerprint region. The bands appeared to be broad indicating that
adsorption dominated the control experiments. However, the FTꢀIR spectra of the colourless end
products of the reaction showed the absence of most of the characteristic bands of RhB, instead
some new bands appeared at different positions, signifying that the complex dye was degraded to
simple molecules.
3.7.9. Carbon mass balance
Carbon balancing is a unique technique for assessing the accuracy of performance during the
measurements. Good carbon mass balance closure reflects the consistency of the internal data
and provides a reasonable confidence in the accuracy of the underlying data. If carbon mass
balance approaches 100 % or very similar values, the stoichiometric accuracy of the process
could be considered as very reliable.
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In order to investigate the carbon mass balance, small ports were configured within the reactor
top for input of the reactants and an exit port for output of the effluent. Influent and effluent were
typically collected in 5 min intervals. These were quantified within the photocatalytic system
with the help of (a) GCꢀFID (b) Ion chromatograph and (c) TOC analyser. The inlet and outlet
components were monitored throughout the experiments to determine the overall mass balance.
The concentrations were converted to cumulative moles of carbon. Inlet carbon content was
compared to carbon accumulated in the reactor and carbon existing in the effluent to find out the
overall mass balance for the system. Chloride concentrations were below the accurate range of
the ion chromatograph and negligible in comparison to the other constituents, and were not
reported. Fig. 11 presents the overall carbon mass balance of the system.
Fig. 11. Cumulative moles of carbon in separate forms and media
It is seen that the inlet and outlet moles of carbon were nearly equal in the initial stages of
experimentation; however the accumulated and effluent carbon fall to approximately 92 % of the
estimated inlet carbon content. The HPLC/MS used for the quantification of the dye does not
indicate any presence of the parent dye content in the effluent. The carbon mass balance results
confirm mineralization of the dye through a relatively rapid pathway.
3.7.10. Morphology and elemental composition of the spent LDH
The TEM analysis of the recovered catalyst revealed the existence of hexagonal nanoparticles
stacked one over the other like that observed for the pure LDH. No drastic change in the
morphology was observed for the LDH even after the fifth cycle (Fig. 12). The elemental
composition of the LDH recovered after the fifth cycle of degradation was investigated by EDX
measurements (Table 4). Minor changes in the weight and atomic percentages in the LDH could
be observed, but Mn/Co/Ti ratio remained unaltered (= 2.06 : 1.04 : 1.00) even after the fifth
cycle of degradation. This reflected stability of the LDH under the chemical environments of
photodegradation even after the fifth cycle.
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Fig.12. TEM image of the recovered LDH after the fifth photodegradation cycle
Table 4. EDX elemental composition of the spent Mn/Co/Ti LDH after the fifth cycle of
photodegradation
Elements
C K
Weight %
1.98
Atomic %
1.11
Mn/Co/Ti (ratio)
N K
2.95
1.35
O K
9.58
2.38
Cl K
3.85
2.87
2.06: 1.04: 1
Mn K
Co K
Ti K
41.48
20.09
20.07
100
46.85
22.75
22.69
100
Total
3.7.11. Mineralization study
The final products from the photocatalytic conversion of the dye should be some intermediates,
CO2 and H2O. The intermediates are most likely to be aromatic compounds accounting for the
remaining TOC of the reaction mixture. Various aromatic degradation products have been
reported and the complete mineralization of the dye may not have been achieved. The
mineralization was monitored by continuously following the TOC (Total Organic carbon) of the
reaction mixture.36 The percentage reductions in TOC with respect to time for visible light
irradiated dye solution, and that for the dye solution irradiated with visible light as well as treated
with H2O2 (S11; Fig. S10; ESI†) showed that TOC was reduced by 92 % of the original when
the degradation was carried out under visible light alone. Almost 100% reduction could be
achieved within 60 min, whereas 92 % reduction in TOC was observed after 60 min through
photocatalysis by the LDH. Thus it could be inferred that the TOC removal rate is less in the
photocatalytic degradation compared to that of H2O2 assisted decolorization under similar
experimental conditions. This is obviously due to the formation of more stable intermediates in
the photocatalytic process as identified using GCꢀMS and exꢀsitu 1HꢀNMR studies.
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3.7.12. Mechanistic pathways of photodegradation
On the basis of the results obtained from XPS, PL, EIS, DRS, TA fluorescence probe, EPR and
indirect radical and hole trapping experiments, the mechanistic pathways of the degradation of
RhB by Mn/Co/Ti LDH could be suggested. In accordance with the band theory, O1s orbital
could be considered as the valence band and Mn2p, Co2p and Ti2p energy levels as the
conduction band. The photocatalytic degradation is likely to proceed in two different pathways,
through generation of reactive species like •OH and Oꢀ• ꢁ in presence of visible light.
Pathway 1: Electronꢀhole (ꢙꢁ − ꢚꢂ) hopping conduction mechanism
A series of reactions is likely to take place in the sequential pattern (Eqs. 7 to 19) upon visible
light exposure. The electrons are first excited from the valence band to the conduction band,
thereby leaving holes in the valence band. The absorption of photons initiates electronic
excitation with energy equal to or greater than the band gap energy of the semiconductor
Mn/Co/Ti LDH nanomaterial. Thus, 2:1:1 Mn/Co/Ti LDH generates eꢀꢀh+ pairs (Eq. 7) upon
visible light exposure, reducing Mn3+, Co3+ and Ti4+ species on the LDH surface (Eqs. 8ꢀ10). A
strong redox reaction ensues (Eq. 11) initiated by the photoꢀgenerated electrons with intense
lattice vibrations on the LDH surface. h+ hops within the LDH surface and reacts with H2O and
OHꢀ ions yielding OH• radicals (Eqs. 12, 13). The photogenerated electrons generated is likely to
ꢁ
react with adsorbed oxygen species producing superoxide (O•ꢀ ) radicals (Eq. 14). Superoxide
ꢁ
(O•ꢀ ) species undergo further transformation with the adsorbed H+ present on the LDH surface
to produce peroxide radicals (•OOH) (Eq. 15), further yielding H2O2 (Eq. 16) as well as hydroxyl
ꢁ
radicals (•OH) (Eq.17). Hydroxyl (•OH) and superoxide (O•ꢀ ) radicals are highly reactive
species that mineralize the dye to simple molecules (Eq. 18). This follows the recombination
between h+ and eꢀ with dissipation of energy and reduction of quantum efficiency (Eq. 19). The
pathway involves the sequence of reactions as follows:
ꢁ
ꢂ
(
)
( )
7
Mn/Co/Ti LDH + hυ ⟶ Mn/Co/Ti LDH e + h
Mnꢛꢂ + eꢁ ⟶ Mnꢀꢂ
Coꢛꢂ + eꢁ ⟶ Coꢀꢂ
Tiꢜꢂ + eꢁ ⟶ Tiꢛꢂ
Electron trap
(
(
)
( )
8
( )
9
)
Electron trap
(
)
Electron trap
(10)
Migration (11)
Mnꢛꢂ + Coꢛꢂ + Tiꢜꢂ + 3eꢁ + hυ ↔ Mnꢀꢂ + Coꢀꢂ + Tiꢛꢂ
hꢂ + HꢀO ⟶ HO• + Hꢂ
(
)
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
hꢂ + OHꢁ ⟶ HO•
ꢁ
Oꢀ + eꢁ ⟶ O•ꢀ
ꢁ
O•ꢀ + Hꢂ ⟶ HOO•
HOO• + HOO• ⟶ HꢀOꢀ + Oꢀ
ꢁ
HꢀOꢀ + O•ꢀ ⟶ HO• + OHꢁ + Oꢀ
RhB dye + O•ꢀ + HO• ⟶ Degradation products
ꢁ
hꢂ + eꢁ ⟶ Energy
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Pathway 2: By photosensitization of RhB
This pathway proceeds through the photosensitization of RhB under visible light. The dye
molecule acts as a photosensitizer to yield its excited state (Eq. 20), which is likely to transform
further to dye radical with the release of electron (Eq. 21). The dye radical forms the most likely
intermediate products like CO2 and H2O (Eq. 22). The electrons generated are injected into the
conduction band of the semiconductor LDH (Eq. 23) where they are scavenged by O2 forming
O•ꢀꢁ species (Eq. 24), also yielding H2O2 (Eq. 25). H2O2 subsequently generates •OH radicals
(Eq. 26), which further degrades the complex dye to simple molecules (Eq. 27). The sequential
series of reactions are as follows:
RhB dye + hʋ ⟶ dye∗
(20)
(21)
ꢝ
RhB dye∗ ⟶ RhB dye• + eꢁ
ꢝ
RhB dye• ⟶ COꢀ + HꢀO
(22)
(23)
(24)
(25)
(26)
(27)
Mn/Co/Ti LDH + eꢁ ⟶ Mn/Co/Ti LDH•ꢂ
ꢁ
Mn/Co/Ti LDH•ꢂ + Oꢀ ⟶ Mn/Co/Ti LDH + Oꢀ•
ꢁ
O•ꢀ + 2HꢀO + eꢁ ⟶ 2HꢀOꢀ
HꢀOꢀ + eꢁ ⟶ •OH + OHꢁ
•OH + RhB dye ⟶ Degradation products
The mechanistic pathways clearly demonstrate the involvement of holes (h+), hydroxyl (•OH)
ꢁ
and superoxide (O•ꢀ ) species as highly reactive agents for Mn/Co/Ti LDH mediated aqueous
phase photocatalytic degradation of the cationic Rhodamine B (RhB). The XPS and PL
measurements suggest the presence of multiple valencies of Mn, Co and Ti accompanied by
oxygen vacancies within the LDH. The presence of defects and O2/O•ꢀꢁ transformation in the
LDH is confirmed by EIS and EPR analyses. The presence of hydroxyl radical (•OH) is
confirmed by terephthalic acid (TA) fluorescence probe. The presence of holes (h+), hydroxyl
ꢁ
radical (•OH) and superoxide (O•ꢀ ) species are confirmed by indirect radical and hole trapping
experiments by addition of appropriate quenchers at the beginning of the photocatalytic
degradation of RhB. Depending on these observations, the mechanistic pathways have been
presented in the form of eꢀꢀh+ hopping conduction model and by photosensitization of the dye.
The schematic of photodegradation pathway is illustrated in Fig. 13.
29