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Synthesis, characterization and photocatalyticactivity of mixed oxides derived from ZnAlTiternary layered double hydroxides
R.K. Sahu, B.S. Mohanta, N.N. Das
PII: S0022-3697(13)00151-0DOI: http://dx.doi.org/10.1016/j.jpcs.2013.04.002Reference: PCS7051
To appear in: Journal of Physics and Chemistry of Solids
Received date: 1 December 2012Revised date: 28 March 2013Accepted date: 7 April 2013
Cite this article as: R.K. Sahu, B.S. Mohanta, N.N. Das, Synthesis, characteriza-tion and photocatalytic activity of mixed oxides derived from ZnAlTi ternarylayered double hydroxides, Journal of Physics and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2013.04.002
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Synthesis, characterization and photocatalytic activity of mixed oxides derived from ZnAlTi ternary layered double hydroxides
R.K. Sahu, B.S. Mohanta, N.N. Das*
P. G. Department of Chemistry, North Orissa University, Baripada-757 003, Orissa, India *Corresponding author. Tel.: +91 679 225 2088; fax: +91 679 225 3908. Email: [email protected]
Abstract A new series of Ti4+ containing ZnAl-LDHs with varying Zn:Al:Ti (~3:1:0 – 3: 0.5, 0.5) ratio
were prepared by coprecipitation of homogeneous solution metal salts and characterized by
various physicochemical methods. Powder XRD revealed the formation of well crystalline
LDH even at highest Ti4+ content. On thermal treatment at 450 �C, the well crystalline LDH
precursors yielded the mixed oxides with BET surface area in the range 92-118 m2/g. UV–vis
diffuse reflection spectroscopy (DRS) showed a marginal decrease of band gap energy for
calcined ZnAlTi-LDHs in comparison to either ZnO or TiO2-P25. The TEM analyses of a
representative sample (as-synthesised and calcined) indicated more or less uniform
distribution of titanium species. The derived mixed oxides from titanium containing LDH
precursors demonstrated better activity towards photodegradation of methylene blue and
rhodamine B than physical mixture of ZnO and TiO2. Moreover, the present work not only
provided a first hand understanding about semiconductor properties of ZnAlTi-LDHs but
also demonstrated their potential as photocatalysts for degradation of organic pollutants.
Highlights
� Synthesis and characterizations of a new series of ZnAlTi ternary layered double hydroxides.
� Calcination of LDH precursors led to formation of mixed oxides with reduced band gap
energies than ZnO or TiO2-P25.
� The derived mixed oxides are effective photocatalysts under visible light for dyes
degradation.
� Easy separation and possibility of reuse of used catalyst.
2
Keywords A. inorganic compounds; A. oxides, B. chemical synthesis; C. electron microscopy; C. X-ray diffraction
1. Introduction
Layered double hydroxides (LDHs), represent an important class of inorganic layered
materials, have been a subject of numerous investigations during last three decades because
of their potential applications as catalysts, catalyst supports, ion exchangers/adsorbents,
layered hosts for biomolecules, precursors for composite materials etc. [1-9]. The general
formula of LDHs is [M2+(1-x)M3+
x(OH)2]x+ [Am-x/m ]n-.mH2O where M(II) and M(III) include
a variety of bivalent (Mg2+, Zn2+, Co2+, Cu2+, Mn2+) and trivalent (Al3+, Fe3+, Cr3+, V3+, Ga3+,
Ti3+) metal ions, An- is the interlayer anion that may be organic, inorganic, carboxylate,
oxoanion, coordination compounds and polyoxometalates and x generally can have the
values between 0.1 and 0.33 [2]. Off let efforts have been devoted to introduce a tetravalent
ion (e.g. Zr4+, Sn4+ and Ti4+) in the brucite like layer as a partial replacement of M2+ or M3+
ion in order to enhance the anion exchange capacity (AEC) or to tune the acido-basicity of
their resulting mixed oxides [10-18]. Although Zr4+ and Ti4+ containing LDHs have shown
enhanced anion adsorption capacity [10-14] in comparison to those without tetravalent ions,
the recent X-ray absorption spectroscopic studies [19,20] indicated that incorporation of
tetravalent ions in octahedral sheet does not occur, but actually an amorphous M(IV) oxide is
formed and impregnate the LDH crystallites. Even then the LDHs containing Zr4+ and Ti4+
could be useful as catalyst for various organic transformations requiring tailored acido-
basicity and also as adsorbents with enhanced adsorption capacity for remediation of anionic
pollutants from contaminated water.
In recent years there has been a growing interest to develop LDH based
photocatalysts as an alternative to conventional semiconductor materials like TiO2, ZnO and
SnO2 for degradation of variety organic pollutant including dyes [21-26]. In particular, the
mixed oxide derived from ZnAl containing LDHs, without or with Fe, Sn and Ti, are
successful photocatalysts for the degradation of organic compounds like methyl orange,
methylene blue and phenols in aqueous media. In contrast, the use of as synthesized LDHs as
photocatalysts has been sparsely studied because of favourable capture of photoinduced
3
electron by hydroxyl group during photocatalytic process. A recent study of Silva et al. [27]
has reported oxygen generation from photolysis of water using as synthesized ZnM-LDHs
(M = Cr, Ti, Ce) photocatalyst under visible light and the results showed that the LDHs can
be regarded as “doped semiconductor”.
Keeping the above in view and as a sequel to our previous studies [12,13], we report
here in the synthesis of new series of Ti containing ZnAl-LDH. The physicochemical
characterizations and photocatalytic behaviours of derived mixed oxides towards two
commonly occurring dyes, namely methylene blue (MB) and rhodamine B (RhB), under
visible light are also reported.
2. Experimental
2.1 Materials
Zn(NO3)2.6H2O, Al(NO3)3.9H2O, TiCl4, NaOH and Na2CO3 (Merck, GR) were used
for synthesis LDH precursors without further purification. Methylene blue (Qualigens) and
rhodamin B (Merck, GR) were used as received for adsorption experiments. All other
chemicals used in this work were of AR/GR grades. Stock solutions of dyes were prepared
by dissolving required amount of corresponding dye in double distilled water.
2.2 Preparation and calcination of LDH precursors
The LDH precursors with varying Zn:Al:Ti atomic ratios were prepared by
coprecipitation of metal salts solution at constant pH ~ 10 under low supersaturating
conditions. A solution containing the mixture of Zn(NO3)2, Al(NO3)3 and TiCl4 and a
mixture of NaOH (2.0 M) and Na2CO3 (0.20 M) were added drop wise to a well stirred
solution of Na2CO3 (100 ml 0.01M) such that the pH of the resulting slurry was maintained
at ~ 10. Once the addition was completed, the resulting precipitate was aged for 18 h at room
temperature, separated by centrifugation, washed thoroughly with distilled water until the
precipitate was free from chloride and then dried overnight at 90 �C in air-oven. Small
amounts of nitrate/chloride ions retained in the interlayer of LDH precursors were replaced
by carbonate ions by suspending the dried precursors (2 g) in the solutions of Na2CO3 (100
ml, 0.20 M) and stirring for 2 h. The carbonate-exchanged solid was separated by centrifuge,
washed, dried in air oven overnight at 90 oC and stored in air tight bottles for further use. The
4
samples are denoted as ZAT-0 to 4 depending on their Ti contents (Table 1). Based on the
TG-DTA and FT-IR spectral analyses the dried LDH precursors were calcined in air at 450 oC with a heating rate of 5 oC min-1 for 5 h and used for further studies.
2.3 Characterizations
Zn, Al and Ti contents were determined by conventional wet chemical analyses and
also by ICP (Varian Liberty series2) (Das et al., 2010). Powder X-ray diffraction (PXRD) of
carbonate exchanged LDH precursors and their calcined products were recorded on a Rigaku
(Miniflex II) diffractometer at scanning speed 2�(2�) min-1 using Ni filtered CuK� (30 kV, 15
mA) radiation. Thermogravimetric measurements (TG-DTA) were performed on a
Shimadzu DTG 60 Thermal analyser under flowing nitrogen (40 ml min-1) at a heating rate
of 10 �C min-1. The surface area of calcined samples was determined by BET method using
a surface and porosity analyzer (Quantachrome, Novawin) after degassing the samples under
vacuum (10-4 Pa) at 250 �C.
FT-IR spectra in KBr phase were recorded on a Shimadzu IR Affinity-1
spectrophotometer averaging 45 scans with a nominal resolution of 4 cm-1 to improve signal
to noise ratio. The UV-Visible diffuse reflectance spectra (UV-Vis DRS) of solid samples
using BaSO4 as reference and spectral scan of the photocatalytic reaction mixture were
performed with a Shimadzu UV-Visible spectrophotometer. The band gap energies were
estimated from absorption edge using the relationship: E (eV) = h (c/�) nm, where h, c and �
represent the Plank’s constant, velocity of light (meter/s) and is the cutoff wavelength (nm),
respectively. Transmission electron micrographs were recorded using FEI Tecnai 30G2 S-
Twin (Netherlands) operated at 300 kV.
2.2 Photocatalytic activity
The photocatalytic efficiencies of calcined LDH precursors were studied taking two
model dyes viz. methylene blue and rhodamine B. All the photocatalytic experiments were
performed in a 200 ml capacity double walled cylindrical quartz reactor fitted with 125 W
high pressure Hg lamp (� > 420 nm) as source of visible light under magnetic stirring
condition. The temperature of reaction mixture containing dye solution and photocatalyst,
was maintained (30.0 � 0.2) by circulating water through outer walls from Julabo (Germany)
5
F12 water circulator. All the experiments were performed in presence of air at atmospheric
pressure. The reaction was imitated by irradiating the reaction mixture containing 100-150
ml of dye solution at different concentrations (10-45 mg L-1) and amount of catalysts (1.0-5.0
g L-1) with the visible light. The initial pH of the dye solution was adjusted to ~ 6.5 which
was found increase to ~ 8.2 � 0.3 at the end of photocatalytic reactions. At regular intervals
(for kinetic experiments) and at the end of reaction, a definite portion of reaction mixture was
withdrawn, separated the solid catalyst by centrifugation and the residual dye concentrations
were computed by measuring the absorbance at 565 and 545 nm for MB and RhB,
respectively. The measurements of absorbance were carried out on a Systronics 2201 UV-
Visible spectrophotometer using 10 mm matched quartz cell. Parallel experiments under
identical conditions in presence of light without catalyst and in presence of catalyst without
irradiation of light were also carried out to see the individual effect of light and catalyst on
degradation/adsorption of dyes.
3. Results and discussion
3.1 Characterization of as synthesized LDHs
The composition of carbonate exchanged LDH precursors and their crystal lattice
parameters, derived from PXRD patterns, are collected in Table 1. Chemical analyses
indicate that the molar ratios of Zn, Al and Ti are close to those initially taken for preparation
of LDH precursors.
TG-DTA plots of representative uncalcined samples are presented in Fig. 1. All the
samples mainly exhibit two stage weight losses with corresponding endothermic peaks in
DTA profiles. The first stage loss can be tentatively resolved in two overlapped processes
and mainly attribute to the elimination of physically adsorbed and interlayer water molecules.
The second loss, always higher than the first loss, is ascribed to loss of hydroxyl groups from
the brucite-like layer along with interlayer carbonate ions with concomitant destruction of
layered structure [2, 11, 14, 16,17]. A minor weight loss beyond 450 �C is attributed to loss
of oxygen and CO2 through slow decomposition of Zn(Al/Ti) oxycarbonate which are likely
to be formed after decomposition of LDH-like structure. The overall behaviour of LDH
samples are in agreement with those generally reported for ZnAl-LDH samples [14,16]. The
total weight losses at 600 oC ranging from 27-29 % are in good agreement with ZnAl-LDH
precursors with or without Zr. Further, the temperature of weight loss is shifted to lower with
6
incorporation of Ti in the interlayer presumably due to relatively weak electrostatic
interaction of interlayer anions with brucite like layer due to increased interlayer spacing as a
result of higher positive charge in the brucite layer.
The PXRD patterns of the dried carbonate exchanged LDHs are presented in Fig. 2. A
single phase corresponding to hydrotalcite like compounds (LDH: JCPDS File No. 38-487) is
observed for all the samples without any appreciable decrease of crystallinity even at higher
Ti(IV) content (Al/Ti ratio ~ 1.0). No peaks from any other crystalline material could be
detected either due to overlapping with the characteristic LDH peaks or formation of
amorphous hydroxide phases of Ti (e.g. Ti(OH)4 or TiO2.nH2O) and/or ZnTi. Formation of
amorphous TiIV oxide has also been reported in synthesis of Co2AlTi-HT [20]. Assuming a
hexagonal crystal system, the lattice parameters are calculated from (110) and (003)
reflections and presented in Table 1. The marginal increase of both a and c parameters for
ZAT-1, 2, 3 and 4 in comparison to ZAT-0 could be, but not conclusive, due to partial
incorporation of Ti4+ with relatively higher ionic radius (r = 0.072 nm) in place of Al3+ (r =
0.053 nm) in the LDH framework [12,17]. This also leads a higher amount of CO32- as
compensating anion in the interlayer which in turn results an increase of c parameter.
Increase of both a and c lattice parameters along with increase of carbonate content have also
been observed for incorporation of Zr4+ and Ti4+ in the brucite like layer in the cases of ZnAl
and MgAl-LDHs [12,13,17], respectively.
FT-IR spectra of representative carbonate exchanged LDHs, shown in Fig. 3, are very
similar to those generally reported for hydrotalcite like compounds. The broad and strong
adsorption band centred at ~ 3480 cm-1 in case uncalcined samples is attributed to stretching
vibrations of physisorbed water, structural OH group and/or hydrogen bonded hydroxyl
group (OH�OH) [2,28]. A weak shoulder at 3070 cm-1, causing the broadness of this band
may be ascribed to the OH stretching mode of water molecule, hydrogen bonded to the
interlayer carbonate anion. The band close to 1630 cm-1 is originated due to bending mode
(�HOH) of interlayer water molecules. An sharp intense band observed at ~ 1370 cm-1 along
with relatively less intense peak at ~ 1530 cm-1 are assigned to symmetric and antisymmetric
O-C-O stretching vibrations of monodentate carbonate species. The shifts from the normal
position of the free carbonate species, i.e., 1450 cm-1, and the splitting of about 130-154 cm-1
for samples ZAT-1 to ZAT-4 result from a lowering of the symmetry of the species in the
7
interlayer domain. These shifts are higher than in LDHs containing cations with large ionic
sizes such as Y, V and Cr in Mg/Al/Y, Ni/V or Zn/Cr with values of 115, 116, and 126 cm-1,
respectively [16]. As expected, the splitting in case of sample without Ti (ZAT-0) is about
120 cm-1 which is less than those observed for other ZAT samples. These features could
account for a greater distortion of the brucite-like layers and for a heterogeneous distribution
of positive charge in the brucite-like layers containing cations of different charges. On
calcination at 450 �C for 5 h, the characteristic peaks due to CO32- ion are practically absent
in the resulted mixed oxides.
The TEM images of ZAT-2 along with EDX and SAED patterns are presented in Fig.
4. The TEM images of as-synthesised ZAT-2 in dark and bright filed indicate uniform
distribution of titanium species (e.g. as TiO2.nH2O) [29.30]. EDX measurement also
confirmed the presence of Zn, Al and Ti in the same proportions as that of taken for synthesis
of ZAT-2. SAED pattern also reveals more or less uniform distribution of LDH particles.
3.2 Characterization of calcined LDHs
The nature of crystalline phases generated after calcination of LDHs is of interest
from view points of their applications as bifunctional acid-base catalysts/catalyst support,
photocatalysts, ion exchangers etc. On calcination at moderate temperature (450 �C), the
LDH precursors are converted to mixed oxides (Fig. 2, inset) whose lattice ‘a’ parameters are
slightly smaller than that of pure ZnO (a = 2.093 Å) indicating isomorphous substitution of
Al3+ or both Al3+ and small fraction of Ti4+ for Zn2+ in the lattice. Similar observations have
been reported earlier for several calcined LDH samples [2, 11, 14, 16,17]. The overlapping
peaks at 2� values 36.5, 47.8, 55.6, 63.04 (Fig. 2, inset) may also be attributed to the
characteristic peaks of (004), (200), (105), (201) and (204) planes of anatase phase. The
surface areas of calcined Ti-containing samples exhibit lower values (Table 1) than the
sample without Ti and the values are progressively decreased with increase of Ti content in
the samples. A similar trend was also observed earlier for Ti and Zr containing calcined
LDHs [12,16,17].
The DRS spectra of calcined LDHs along with ZnO and TiO2 (Degussa P25) are
presented in Fig. 5 (inset). It is seen that spectral intensity of Ti containing calcined ZAT
samples are relatively higher than that of sample without Ti (ZAT-0). There is a marginal
shift in the absorption band in all ZAT samples in comparison to pure ZnO or TiO2-P25
8
presumably due to presence of intimately mixed ZnO and TiO2 in the oxide samples obtained
on calcination of ZAT-precursors. The band gap energies of the photocatalysts (Fig. 5) were
determined by extrapolating the linear region to the abscissa of Tauc plot, a plot of
(F(R)h�)1/2 against h� where F(R) is the Kubelka–Munk function and the values obtained are
collected in Table 1. All the ZAT samples exhibit relatively lower band gap energy than
those observed in case of pure ZnO (3.29) or TiO2-P25 (3.19)/synthesized TiO2 (~3.22) [29]
and are expected to show better photocatalytic activity compare to either ZnO or TiO2 under
visible light.
The TEM images of calcined ZAT-2 along with EDX and SAED patterns are
presented in Fig. 6. As evident, the average particle size of ZAT-2 increases on calcination.
The EDX analysis also shows the presence of Zn, Al and Ti more or less in the same
proportion as that of uncalcined ZAT-2 sample. HR-TEM images show set of uniform lattice
fringes providing further evidence in favour of crystalline nature of nanoparticles.
Appearance of lattice fringes for anatase and MgAl-LDH has also been observed with d-
values 3.57 and 2.14 Å, respectively in the case of TiO2/MgAl-LDH [30].
3.3 Photocatalytic activity
The photocatalytic activity of calcined LDHs was evaluated towards degradation of
methylene blue and rhodamin B in aqueous medium. The initial pH of the dye solutions was
kept constant at 6.5 � 0.2 in order to avoid any colour change of dye solutions due to acid-
base equilibria with variation of pH (pKa values of MB and RhB are 10.2 and 10.5,
respectively). The pH of reactant solution invariably increased to 8.2 � 0.2 during
photodegradation process. The representative set of results using calcined LDHs for
degradation of MB and RhB as a function of irradiation time are presented in Figs. 7 and 8,
respectively. The activity of physical mixture of ZnO and TiO2-P25 with same weight
percentage of ZnO and TiO2 as that of ZAT-3 is also presented in the figures for comparison.
Blank experiments were also carried out without catalyst to verify the extent of
decolourisation of dyes due to photocatalytic process. It is seen that in the absence of
photocatalyst, about 25 % of MB is degraded in 3 h of irradiation with visible light and is
very similar to that observed earlier [29]. In comparison, the photodegradation of RhB
without catalyst is negligibly small even with irradiation up to 4 h with visible light. As the
9
adsorption of dyes on catalyst is believed to be the primary process in photocatalytic
decolourisation/degradation, it is also essential to assess the amount of dye adsorbed on
catalyst in dark to account the overall activity of ZAT samples. The calcined ZAT samples
can reconstitute to its original LDH structure through rehydration and exhibit positive and
negative surface charges depending on the pH (point of zero charge of calcined ZAT samples
~ 8.0). Also at working pH (initial pH ~ 6.5 and final pH ~ 8.2), MB exists in the cationic
form (pKa > 12) for which the entry of MB in the interlayer is restricted and hence, the
decrease of MB concentration with time in dark is primarily due to adsorption on catalyst
surface. Blank experiment with ZAT-3 under identical conditions in dark shows ~ 12 % MB
(Fig. 7) is adsorbed in 3 h. On the other hand ~ 7.3 % RhB, mostly exist in zwitterionic form
at pH ~ 6.0 due to deprotonation of its carboxyl group (pKa =3.7), is adsorbed on ZAT-3
surface under identical conditions of photocatalytic without visible light irradiation.
The repetitive spectral scan of dye solutions with irradiation time, presented in Figs. 7
and 8 (inset), show progressive decrease of absorbance in the entire range of UV-Vis spectra.
It is clearly seen that the initial peaks of MB at 614 and 664 nm are merged into a relatively
broad peak centred at ~ 650 nm during the reaction. The peak intensities are also reduced
progressively with a shift towards higher wavelengths till the reactant solution turns
colourless. On the other hand, the intensities of MB bands at 292 and 245 nm are reduced
progressively without any shift in their positions. The disappearance of two major
absorbance peaks of MB at 292 and 664 nm (Fig. 7, inset), due to benzene ring and
heteropolyaromatic linkage, indicate their complete destruction at the end of the reaction.
The decolourisation of RhB is also evident from the large decrease in absorbance change
during the photocatalytic reaction without any observable shift in the peak positions (Fig. 8,
inset) indicating the deolourisation of RhB is primarily due to decomposition of conjugated
chromophore structure rather then de-ethylation process [31].
In order to see that effect of catalyst dose for dye degradation, the amount of most
effective catalyst (ZAT-4) was varied in the range 1.0 to 5.0 g L-1 at a constant concentration
of MB (10.9210-5 M) and RhB (7.3110-5 M), initial pH ~ 6.5 and irradiation time of 60 min.
The results obtained are presented in Fig. 9. Although there is a progressive increase of
degradation with increase of catalyst dose, the effect is more pronounced up to 2.0 g L-1 at
least in the case of MB degradation. This increased activity with increasing catalyst dose is
10
obviously to presence more active sites which results in absorption of more number of
photons for formation higher number of active species. The less pronounced activity at
higher dose is presumably due to less availability of dye molecules for degradation. The
concentration of initial dye concentration is another important factor towards overall
degradation of dyes. In the present study, the MB and RhB concentrations were varied from
(3.12-14.04)10-5 M and (2.09-9.4)10-5 M, respectively keeping the initial pH (~ 6.5) and
catalyst dose (ZAT-4, 2.0 g L-1) fixed. As expected, the percentage of degradation decrease
with increase of initial dye concentration. Further, the plots of ln(C/C0) versus irradiation
time are straight line indicating the degradation of dyes follows a first order kinetics. In the
above concentration range, the calculated first order rate constants are found in the range
0.012-0.067 min-1 and 0.0054-0.0191 min-1 for MB and RhB, respectively. The values of MB
degradation are comparable with the reported rate constant (0.0407 min-1) for MB
degradation (initial MB = 1.6 10-5 M, catalyst dose = 2.0 g L-1) with ZnTi sample (Zn/Ti =
3:1) [29]. It is interesting to note that the activity of the physical mixture of ZnO and TiO2 is
lower than all the Ti containing LDH but higher than the activity of sample without Ti (ZAT-
0). Further the degradation of both the dyes is found to increase rather slowly with increase
of Ti content in the catalyst (Fig. 9, inset). This increased activity may be attributed to
resultant effects of decreased band gap energy and BET surface area.
Since the measured band gap of Zn(Al)O/TiO2 is relatively high (> 2.98 eV), the
mechanism operative for dye (D) degradation by semiconductor oxide using UV light is not
feasible in the present case rather a photosensitized pathways should be considered. Unlike
formation of electron-hole (e-/h+) pairs due to absorption of UV light by semiconductor, light
absorption in the present case mainly occurs by the dye molecule [32] adsorbed on the
catalyst surface and transfer the excited electron into Zn(Al)O/TiO2 conduction band. The
electrons on Zn(Al)O/TiO2 can be used further to reduce dissolved oxygen molecule in dye
solution forming O2�� leading to degradation of dye molecule. The overall photocatalytic
process may be delineated as follows and the same is schematically presented in Fig. 10.
D/ZnAl(O)/TiO2 + h� (vis) 1D* or 3D*/ZnAl(O)/TiO2 1D* or 3D*/ZnAl(O)/TiO2 D+� + ZnAl(O)/TiO2(e�
cb)
D+� degradation products
11
ZnAl(O)/TiO2(e�cb) + O2 ZnAl(O)/TiO2 + O2
-�
O2-� + H+ HO2
� HO� and/or O2-� + 2H+ + e- H2O2
D+� + O2-� DO2 degradation products
D + HO� or H2O2 degradation products
The practical utility of a photocatalyst lies in its long time use. A preliminary study was
made using calcined ZAT-4 to see its efficiency in repeated cycle by keeping the catalyst
amount (2.0 g L-1), initial dye concentrations (3.1210-5 M for MB and 2.0910-5 M for
RhB) and initial pH (6.5 � 0.3) fixed in each cycle. For this the catalyst, separated from
residual dye solutions by centrifugation after irradiation for 120 min, was treated with fresh
dye solutions and irradiated again for 120 min. This was repeated for two more cycle and the
degradation activity is found to decrease progressively form 100, 92 and 82 % in first, second
and third cycles, respectively. The decrease in activity is primarily due to partial formation of
parent ZAT-4 LDH precursor through rehydration of calcined ZAT-4 and the resulted
hydroxyl groups of LDH capture some of the photoinduced electrons which in turn reduce
the photocatalytic activity. The XRD pattern of calcined ZAT-4 after third cycle of
photocatalytic reaction is presented in Fig. 11 along with uncalcined ZAT-4 precursor. It is
evident that parent ZAT-4 is gradually formed due to memory effect which results in the
decrease of activity with number of cycle. Further optimization like heating at higher
calcination temperatures to avoid to the formation of parent LDH precursor through
rehydration or variation Al/Ti ratio is required for long time use of LDH based catalyst in
photocatalytic degradation of dyes.
4. Conclusions
Ternary LDH precursors containing Zn, Al and Ti were synthesized by
coprecipitation of metal salts solution at constant pH. PXRD confirmed the formation of
single crystalline LDH like phase even with high Ti content indicating Ti4+ was mostly
present as amorphous TiO2.nH2O. Incorporation of a small fraction Ti4+ in the brucite like
layer could not be neglected. On calcination of LDH precursors at 450 �C, the resulted mixed
oxide showed relatively lower band gap energies compare to ZnO or TiO2-P25. The Ti
12
containing calcined LDHs showed better activity for photodegration of aqueous MB and RhB
than the physical mixture of ZnO-TiO2 and the activity increased with increase of Ti content
in the catalyst. The photodegradation of both the dyes followed a first order kinetics and
under identical conditions, the photodegradation of MB was relatively faster than RhB.
Although the catalyst after reaction was separated easily, the activity of used catalyst
progressively decreased in subsequent cycles. The calcined mixed oxides may be further
exploited for their catalytic activity requiring tailored acid-basicity or as adsorbent for
removal several anionic pollutant from aqueous medium.
Acknowledgement
The financial assistances from University Grants Commission, New Delhi and
Department of Science & Technology, Government of India for infrastructural facilities at
Department of Chemistry, North Orissa University are gratefully acknowledged.
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Captions for Figures Fig. 1: TG-DTA profile of as synthesized LDHs.
Fig. 2: Powder XRD patterns of as synthesized LDHs and their calcined products (inset).
Fig. 3: FT-IR spectra of as synthesized LDH samples.
Fig. 4: TEM analyses of as-synthesised ZAT-2 sample. (a) Bright field and (c) the
corresponding EDX; (b) dark field images and (d) SAED pattern.
Fig. 5: Band gaps from the plots of (F(R)h�)1/2 versus h� using UV-Vis-DR spectra (inset) of
calcined LDHs along with ZnO and TiO2-P25 .
Fig. 6: TEM analyses of calcined ZAT-2 sample. (a) Bright field and (c) the corresponding
EDX pattern; (b) HR-TEM image and (d) SAED pattern.
Fig. 7: Photodegradation of MB as a function of irradiation time under visible-light using the
calcined LDHs and ZnO-P25 mixture. (catalyst, 2.0 g L-1; initial MB, 10.9210-5 M;
pH = 6.5 � 0.2). (Inset) Spectral scans of MB with irradiation time using ZAT-4
under above conditions).
Fig. 8: Photodegradation of RhB as a function of irradiation time under visible-light using
the calcined LDHs and ZnO-P25 mixture. (catalyst, 2.0 g L-1; initial RhB, 7.3110-5
M; pH = 6.5 � 0.2). (Inset) Spectral scans of MB with irradiation time using ZAT-4
under above conditions.
15
Fig. 9: Effect of catalyst dose degradation of MB and RhB using calcined ZAT-4 as the
catalyst (initial MB, 10.9210-5 M; initial RhB, 7.3110-5 M; pH = 6.5 � 0.3). (Inset)
Comparative activity of different calcined LDHs for degradation of MB and RhB
under visible-light irradiation for 60 min.
Fig. 10: Electron transfer process from the excitation of dye in the visible region Fig. 11: Powder XRD patterns of as synthesized ZAT-4 (1) and calcined ZAT-4 after 3rd
cycle of photocatalytic run (2).
16
Graphical abstract
Synthesis, characterization and photocatalytic activity of mixed oxides derived from ZnAlTi ternary layered double hydroxides
R. K. Sahu, B. S. Mohanta, N. N. Das*
P. G. Department of Chemistry, North Orissa University, Baripada-757 003, Orissa, India
TEM image (dark field) of as-synthesised LDH precursor (ZAT-2)
Table 1: Composition and lattice parameters of the LDH samples ______________________________________________________________________ Sample Mg : Al : Ti Lattice parameters Surface area Band gap
---------------------- (m2/g)a energy (eV)a
a, Å c, Å ________________________________________________________________________
ZAT-0 2.96:1.05:0 3.064 22.46 116.5 3.21
ZAT-1 2.99:0.92:0.091 3.077 22.57 109.7 3.04
ZAT-2 2.99:0.81:0.20 3.072 22.57 105.9 3.00
ZAT-3 2.99:0.70:0.31 3.072 22.77 100.3 2.98
ZAT-4 2.99:0.49:0.508 3.077 22.84 94.5 2.96
________________________________________________________________________ a Values of calcined products.
Table 1