Influence of secondary oxide phases in enhancing the photocatalytic properties of alkaline earth elements doped LaFeO3 nanocomposites

Article abstract of DOI:10.1016/j.jpcs.2020.109377

An improvement in the photocatalytic activity of lanthanum ferrite was attempted by doping individual alkaline earth elements (Ba, Ca and Sr) in its La site and Mg in Fe site to get the desired energy band characteristics. All doped compositions of LaFeO<

Full text of DOI:10.1016/j.jpcs.2020.109377

Journal of Physics and Chemistry of Solids 140 (2020) 109377
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Journal of Physics and Chemistry of Solids
journal homepage: http://www.elsevier.com/locate/jpcs
Influence of secondary oxide phases in enhancing the photocatalytic
properties of alkaline earth elements doped LaFeO nanocomposites
3
a
b
c
c
d
f
T. Vijayaraghavan , M. Bradha , Pradeepta Babu , K.M. Parida , G. Ramadoss , S. Vadivel ,
e
a,*
R. Selvakumar , Anuradha Ashok
a
Functional Materials Laboratory, PSG Institute of Advanced Studies, Coimbatore, 641004, India
Department of Physics, Sri Krishna College of Engineering and Technology, Coimbatore, 641008, India
Centre for Nano Science and Nano Technology, SOA University, Bhubaneswar, 751030, Odisha, India
School of Chemical and Biotechnology, SASTRA University, Thanjavur, 613401, India
Nanobiotechnology Laboratory, PSG Institute of Advanced Studies, Coimbatore, 641004, India
Department of Chemistry, PSG College of Technology, Coimbatore, 641004, India
b
c
d
e
f
A B S T R A C T
An improvement in the photocatalytic activity of lanthanum ferrite was attempted by doping individual alkaline earth elements (Ba, Ca and Sr) in its La site and Mg
in Fe site to get the desired energy band characteristics. All doped compositions of LaFeO
3
were synthesised by sol-gel method. Structural studies show the formation
�
�
of secondary oxide phases when calcined at 800 C and phase pure compounds when calcined at 1100 C. Optical studies reveal the capability of these perovskites to
be visible light active and indicate that the bandgaps become narrower after doping alkaline earth elements. Photocatalytic activity was tested through Congo Red
(
CR) dye degradation studies and hydrogen evolution through water splitting. The results indicate that Mg doped LaFeO
3
composite which was obtained after the
�
calcination at 800 C shows higher hydrogen evolution rate and the highest rate of Congo red (CR) dye degradation among all synthesised compositions of undoped
3
and doped LaFeO under natural Xenon lamp irradiation. The possible mechanism of the photocatalytic activity is also proposed in this study.
Fe3 to Fe and formation oxygen vacancies based on the number of
þ
4þ
1
. Introduction
LaFeO is a well-known perovskite photocatalyst due to its narrower
dopant ions [12].
3
3
Recently, several modifications on LaFeO (LFO) were carried out to
bandgap, suitable band edge potentials and adaptability to incorporate
appropriate elements as dopants for property enhancement [1]. It shows
good photocatalytic dye degradation [2–7,42] and hydrogen evolution
properties [8,9,42–45]. Recently, attempts have been carried out to
enhance the solar hydrogen generation by decorating Ni nanoparticles
increase photocatalytic performance by doping different elements and
decorating different nanoparticles as shown in [10,11,13,14,18–23,32]
and Table S1. However, these doped photocatalysts took longer duration
(i. e. 180 mins) for effective dye degradation and nanoparticles deco-
rated LFO photocathodes showed lesser rate of photoelectrochemical
hydrogen evolution than the powder form of LFO photocatalysts. The
formation of pure phase in doped LFO can be achieved only at higher
temperatures. That may leads to uncontrolled grain growth and affects
the bandgap nature. Calcination of doped LFO at lower temperatures
produces small amount secondary oxide phases along with LFO pure
phase. We have shown that the compounds with secondary oxide phases
are more effective in photocatalytic reactions than the phase pure
compounds in our previous work [40]. These secondary oxide phases
boost the photocatalytic performance by creating new active sites for the
creation of necessary ionic species and also reduce the recombination
during photocatalytic reaction.
[
3
45] and Ag nanoparticles [44] on LaFeO photocathode [43]. However,
the photocatalytic efficiency obtained is not as high as some of the other
oxide photocatalysts due to the recombination of electron-hole pairs
during the reaction as well as higher valence band potential. It was
found that point defects in the parent crystal act as recombination
barriers during photocatalytic reactions [1]. In perovskites like LaFeO3,
this can be achieved by substituting suitable alkaline/transition metal
ions in any one or both the cationic sites. Doping aliovalent cation in the
3
La site of LaFeO leads to stabilization of multiple valence state of Fe as
well as formation of structural defects [16]. Similarly, doping at Fe site
with lower valent cation will create charge imbalance. The charge
compensation can be achieved by changing of valence state of Fe from
Secondary phase enhanced photocatalytic properties were reported
*
Corresponding author.
E-mail address: anu@psgias.ac.in (A. Ashok).
https://doi.org/10.1016/j.jpcs.2020.109377
Received 24 October 2019; Received in revised form 27 December 2019; Accepted 25 January 2020
Available online 27 January 2020
0
022-3697/© 2020 Elsevier Ltd. All rights reserved.

T. Vijayaraghavan et al.
Journal of Physics and Chemistry of Solids 140 (2020) 109377
Fig. 1. TG/DSC plots of dried gels obtained during the synthesis of doped LFO (a) LBFO, (b) LCFO, (c) LSFO and (d) LMFO.
previously [38–41]. In particular, with LFO, Zhi-Xian Wei et al., 2009
17] showed that the reason for higher percentage of dye degradation in
Mn doped LFO is presence of impurity phase La CO . Higher rate of
hydrogen evolution was observed in powder form of Cu doped LFO
containing the secondary phase La CuO [12]. In both these studies, the
LBFO, LCFO, LSFO and LMFO respectively and different dopant molar
ratios 0.05, 0.1, 0.2, 0.3 and 0.4 are denoted as P05, P1, P2, P3 and P4
respectively. Phase pure LCFO and LMFO are represented as LCFO-PP
and LMFO-PP.
[
2
O
2
3
2
4
mechanism of enhanced photocatalytic reaction is not explained clearly.
In the present work, enhancement in both photocatalytic efficiency and
rate of reaction of LFO is attempted through doping alkaline earth
metals in La site and Mg in Fe site. We observe that secondary phases
play a crucial role in the property enhancement. The possible mecha-
nism for this behaviour is explained in detail.
2.2. Characterisation
The intermediate powders before calcination were subjected to
thermogravimetry/differential scanning calorimetry (TG/DSC) per-
formed using NETZSCH STA 449F3 simultaneous thermal analyser in
�
static air atmosphere in the temperature range 26–1200 C at a heating
�
rate of 20 C/min. Structural characterisation was carried out by X-ray
2
. Materials and methods
diffraction (XRD) with Cu K radiation using Rigaku D/max 2550 X-ray
α
�
�
diffractometer. X-ray peaks were recorded at 2θ ranging from 20 to 70
�
2
.1. Experimental
at a scan rate of 0.05 per sec. Morphology and crystal structure of the
calcined powders were examined through high resolution transmission
electron microscopy (HRTEM) using JEOL JEM 2100, Japan, at an
accelerating voltage of 200 kV. For HRTEM analysis, each calcined
powder was crushed with ethanol using mortar pestle, sonicated for 3
min and drop casted onto the carbon coated copper grid (200 mesh). The
grid was allowed to dry for 15 min before loaded into HRTEM. The
compositional and oxidation states of the elements in all photocatalysts
were analysed using X-ray Photoelectron Spectroscopy (XPS) in Specs-
Phoibos analyzer with a monochromatized Al K-alpha X-ray source of
1486.74 KeV.
Undoped Lanthanum ferrite and alkaline earth element doped La(1-
x)
A
x
FeO
3
(where A ¼ Ca, Sr, Mg and Ba) were synthesised by wet
chemical method (i.e. sol-gel synthesis). All the chemicals were pro-
cured from MERCK of purity > 99%.
3
Alkaline earth element doped LaFeO by wet chemical method:
Different combinations of molar ratios such as 0.05, 0.1, 0.2, 0.3, 0.4
mol of alkaline earth elements and 0.95, 0.9, 0.8, 0.7, 0.6 mol of La
(
3 2
NO ) respectively were dissolved separately in 100 ml of deionized
water. After the formation of clear solution, 2.7 mM of poly ethylene
glycol and 19 mM of citric acid were added into each precursor-water
mixture. The mixture was stirred vigorously until it formed wet gel.
The bandgaps of the photocatalysts were studied by using JASCO V-
750 UV–Vis spectrophotometer equipped with diffuse reflectance
�
The wet gel was heated at 120 C for 8 h to obtain dry gel which was
accessory in the wavelength region 200–870 nm using BaSO
4
as the
further ground well in agate mortar and pestle to get a homogeneous
mixture.
reference. The absorbance spectra of the dye solution at different time
intervals were obtained by using Shimadzu UV-1800 UV–visible
spectrophotometer.
The dried gel powders were subjected to thermal analysis to identify
the temperature range of the required phase formation. Based on ther-
�
mal analysis, each powder was calcined separately at 800 C for 2 h. All
2.3. Photocatalytic dye degradation studies
calcined samples were ground into fine powders separately in an agate
mortar and pestle. The calculated amount of alkaline earth metal doped
Photocatalytic dye degradation studies were performed for all the
compounds under visible light. The dye degradation experiments were
carried out with 50 ppm of Congo red (CR) dye (2.5 mg of dye dissolved
LaFeO
3
was 15 g and 8 g of same was obtained for each composition. The
were denoted as
alkaline earth element Ba, Ca, Sr and Mg doped LaFeO
3
2

T. Vijayaraghavan et al.
Journal of Physics and Chemistry of Solids 140 (2020) 109377
3

T. Vijayaraghavan et al.
Journal of Physics and Chemistry of Solids 140 (2020) 109377
Table 1
3
Cell parameters and percentages of secondary oxides in undoped and doped LaFeO .
Composites
Cell Parameters (Å)
Percentages of secondary oxides
Grain size (nm)
a
B
C
La
2
O
3
BaO
CaO
SrO
2
2 3
Fe O
LFO
5.5632
5.6079
5.5705
5.6063
5.5570
5.6292
5.5630
5.5565
5.5360
5.5942
5.5932
5.6178
5.5454
5.5392
5.5308
5.4611
5.5191
7.8674
7.8362
7.8850
8.0614
7.8653
7.8413
7.8670
7.8430
7.8553
7.8735
7.8761
7.8086
7.8698
7.8544
7.8570
7.8263
7.8432
5.5532
5.4922
5.5566
5.4955
5.5466
5.5134
5.5530
5.5187
5.4976
5.5211
5.5200
5.4863
5.5544
5.5493
5.5451
5.5526
5.5396
–
–
–
–
–
–
–
–
–
–
–
–
–
38.77
30.68
29.64
43.62
34.93
34.94
26.88
35.88
40.65
31.31
29.24
33.74
35.16
31.94
30.49
24.71
30.76
LBFO-P05
LBFO-P1
LBFO-P2
LCFO-P05
LCFO-P1
LCFO-P2
LCFO-P3
LCFO-P4
LSFO-P05
LSFO-P1
LSFO-P2
LMFO-P05
LMFO-P1
LMFO-P2
LMFO-P3
LMFO-P4
10.22
0.65
6.03
–
2.48
–
12.73
27.82
42.95
–
2.26
–
–
4.94
–
1.26
4.3
4.3
3.36
3.53
14.23
7.86
17.20
–
–
4
–
–
4.01
–
–
4.47
–
–
4.28
–
–
–
–
–
–
–
–
–
–
10.28
14.84
25.43
10.69
0.79
1.09
1.24
1.54
1.51
–
4.21
–
11.68
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Fig. 3. (a) Lower magnification TEM images, (b) Higher magnification images, (c) SAED patterns and (d) Energy dispersive spectra of undoped and doped
Lanthanum ferrites.
4

T. Vijayaraghavan et al.
Journal of Physics and Chemistry of Solids 140 (2020) 109377
Fig. 4. Tauc plots of different of (a) LBFO, (b) LCFO, (c) LSFO and (d) LMFO with different dopant concentrations, (e) band edge potentials of undoped and
doped LFO.
in 50 ml of distilled water) and 50 mg of undoped and doped LaFeO
3
(A
Active species trapping experiment was carried out to understand the
mechanism of photocatalytic dye degradation [37]. The following
scavengers were used for trapping active species involved in experiment,
¼
Ba, Ca and Sr). This mixture was stirred constantly in glass beaker
containing 50 ml of deionized water. The photocatalytic studies have
been carried out in three different conditions: a) under Xenon lamp (35
1) Isopropyl alcohol (IPA) for hydroxyl radicals, 2) Formic acid (CH
2 2
O )
3
Watts) irradiation (light intensity~ 1843 ꢀ 10 lux) (as reported in our
for holes, 3) Tetrachloromethane (CCl ) for electrons and 4) Ascorbic
4
previous work [24]), b) under natural sunlight irradiation between 12 p.
acid (AA) for super oxide anions. The concentrations of scavengers were
fixed as 1 mM. The absorbance spectra of CR dye after adding scavengers
were also taken at uniform intervals for reference. The scavenger which
shows lower percentage of degradation is considered to be the active
species involved in degradation process.
3
m. and 3 p.m. (light intensity~ 638 ꢀ 10 lux) and c) under dark con-
dition. 4 ml of solution in every 15 min interval was drawn from the
beaker and characterized by UV–Vis spectroscopy to check the absor-
bance. The residual dye concentration was estimated through absor-
bance at 510 nm using UV–Vis spectroscopy. The results were expressed
in terms of % degradation.
Gas chromatography-Mass spectroscopy (GC–MS) systems (Perki-
nElmer Clarus SQ8C coupled with Thermo MS DSQ II, USA) with double
focusing mass analyser and DB-5 ms capillary standard non polar col-
umn of internal diameter and dimension of 0.25 mm and 30 m,
Percentage of dye removal was calculated using following equation:
A
0
À A
E ¼
%:
(1)
respectively was used to analyze the mineralization by-products. 1 L of
μ
A
0
sample was used to test the degradation products. The column oven
�
temperature was operated between 70 and 250 C with a progressive
Where A
0
and A represent the initial and final concentrations of dye
�
rise of 6 C/min. High pure helium was used as carrier gas at a flow rate
solution respectively.
of 1 mL/min. The ion source and gas interface temperature of mass
5

T. Vijayaraghavan et al.
Journal of Physics and Chemistry of Solids 140 (2020) 109377
Fig. 5. (a) Full survey spectra of undoped and doped LFO and High resolution spectra of (b) Fe2p, (c) La3d, (d) O1s, (e) Ba3d, (f) Ca2p, (g) Sr3d, (h) Mg 2p.
�
spectrometer detector was held at 250 C and the ions were separated
based on their mass to charge (m/z) ratio in the range of 50–500. The
National Institute of Standards and Technology (NIST) mass spectral
library and a custom library of known compounds were used for the
identification of mineralized compounds.
2.4. Hydrogen and oxygen evolution from photocatalytic water splitting
studies
Photocatalytic hydrogen evolution studies were performed for the
bare LFO and alkaline earth elements doped LFO composites under 125
W medium pressure Hg visible lamp irradiation. The experiments were
Fig. 6. Percentages of CR dye degradation of (a) LBFO-P1, (b) LCFO-P2, (c) LSFO-P1 and (d) LMFO-P3.
6

T. Vijayaraghavan et al.
Journal of Physics and Chemistry of Solids 140 (2020) 109377
Table 2
experiment.
3. Experimental results
Photocatalytic CR dye degradation percentage and grain size of all doped LFO.
Photocatalyst
Dark
Xenon lamp
irradiation
Bandgap
(eV)
Grain size
(nm)
Condition
3
.1. Thermogravimetric analysis of doped and undoped lanthanum
LFO-SG
21.72%
34.15%
21.76%
53.29%
83.94%
57.23%
42.18%
73.28%
38.14%
58.82%
59.24%
23.62%
49.65%
58.70%
74.62%
80.73%
79.61%
73.79%
44.25%
70.12%
58.82%
84.14%
88.04%
89.09%
76.22%
67.38%
70.49%
75.14%
72.28%
61.50%
79.22%
89.88%
96.76%
94.48%
2.52
2.29
2.40
2.16
2.34
2.35
2.36
2.27
2.26
2.25
2.35
2.19
2.17
2.25
2.29
2.51
2.34
38.77
30.68
29.64
43.62
34.94
34.93
26.88
35.88
40.65
31.31
29.24
33.74
35.16
31.94
30.76
24.71
30.49
LBFO-P05
LBFO-P1
LBFO-P2
LCFO-P05
LCFO-P1
LCFO-P2
LCFO-P3
LCFO-P4
LSFO-P05
LSFO-P1
LSFO-P2
LMFO-P05
LMFO-P1
LMFO-P2
LMFO-P3
LMFO-P4
ferrites
Fig. 1 (a), (b), (c) and (d) show the TG curves of the dried gels ob-
tained during the synthesis of alkaline earth element doped LFO with
different molar ratios. TG analyses of doped LFO powders indicate
weight loss in several temperature intervals. Weight loss from room
�
temperature to 300 C in the TG curve observed in all doped LFO might
be due to the removal of adsorbed moisture and organic contents.
�
�
Weight loss from 300 C to 600 C in TG curve in all doped LFO illus-
trates the removal of nitrates. In the case of LCFO and LMFO the TG
�
�
curve after 800 C and 650 C is absolutely horizontal indicating no
weight change and final phase formation. However, weight loss from
�
�
�
�
�
�
6
00 C to 1150 C in LBFO, 600 C–750 C in LCFO, 600 C–950 C in
LSFO can be due to the loss of small amount of oxygen. Formation of the
final phase in this temperature range is not clear since the TG curves
indicate slight variation in weight. Final phase formation temperature
was deduced based on above observations and all dried gel powders
carried out in a 50 ml of batch reactor containing 10 vol % of methanol
in water with 5 mg of each individual photocatalysts for H evolution
and O evolution. This mixture was stirred constantly to ensure proper
2
�
2
were calcined at 800 C for 2 h and subjected to structural analysis.
mixing of the constituents. After mixing, the reactor was closed with
rubber septum and evacuated using a vacuum pump in order to remove
gases present in the reactor. It was then purged with nitrogen gas in
order to create inert atmosphere. Later it was irradiated under 125 W Hg
visible lamp with continuous stirring using a magnetic stirrer. The
evolved gases were collected using water displacement technique and
analysed by gas chromatography (with GC-17A) using 5 Å molecular
sieve column fitted with a thermal conductivity detector (TCD). The
comparison of retention time of the peak that appeared on the chro-
matogram with standards confirmed the evolved gases. For the reus-
ability study the photocatalyst used for water splitting experiment was
3
.2. Structural analysis of undoped and doped LaFeO
3
using XRD
The powder XRD patterns of all ferrites are shown in Fig. 2(a–d). All
the peaks are indexed by comparing their position and intensity with the
crystal structure database JCPDS (Joint Committee on Powder Diffrac-
tion Standards). All doped ferrites exhibit orthorhombic crystal struc-
ture having space group of Pnma (62) with different cell dimensions as
shown in Table 1. After profile fitting with standard LFO structure from
database using X pert High scoreplus software, the refined cell param-
eters and grain sizes are shown in Table 1.
�
centrifuged at 5000 rpm and dried at hot air oven at 120 C for 30 min.
XRD patterns in Fig. 2, indicate the presence of secondary phases in
The dried material was reused for next cycle of water splitting
2 3
the form of Fe O and oxides of the dopants. This also reveals that after
Fig. 7. Absorbance spectra of CR dye by LMFO-P3 under (a) Xenon lamp irradiation, (b) Dark condition, (c) Reusability and (d) active species trapping experiments.
7

T. Vijayaraghavan et al.
Journal of Physics and Chemistry of Solids 140 (2020) 109377
Table 3
Comparison of band edge potentials of CR dye degradation in LCFO-P2 and
LMFO-P3 in phase pure and composite form.
Photocatalysts
Bandgap
eV)
Valence band
potential (eV)
Conduction band
potential (eV)
Grain
(
size (nm)
LFO
2.52
2.36
2.24
2.51
2.23
2.3000
2.2209
2.1609
2.2921
2.1521
À 0.2200
À 0.1381
À 0.0791
À 0.2179
À 0.0779
38.77
26.88
48.26
24.71
40.58
LCFO-P2
LCFO-PP
LMFO-P3
LMFO-PP
Table 4
Photocatalytic CR dye degradation percentage of Ca and Mg doped LFO calcined
�
at 1100 C.
Photocatalyst
Dark Condition
Xenon lamp irradiation
Bandgap (eV)
LFO-SG
21.72%
83.94%
48.47%
80.73%
38.46%
73.79%
84.14%
55.65%
96.76%
63.50%
2.52
2.36
2.23
2.51
2.24
LCFO-P2
LCFO-PP
LMFO-P3
LMFO-PP
Fig. 8. XRD plots of undoped 0.2 M of Ca and 0.3 M of Mg doped LFO calcined
�
at 1100 C.
�
calcining at 800 C the dopants have not been substituted the cations of
according to their corresponding crystallographic data obtained through
XRD. The energy dispersive spectra of all doped LFO are shown in Fig. 3
LFO. In addition to the orthorhombic LaFeO
ing to La (JCPDS No. 83-1344), BaO (JCPDS No. 74-1228), CaO (82-
690), Fe (JCPDS No. 89-8103) and SrO (JCPDS No.73-1740) were
3
phase, peaks correspond-
2 3
O
(
d) reveals that La to Fe ratio is nearly 1:1 and also confirms the presence
1
2
O
3
2
of alkaline earth dopant elements. The surface area of LFO, LCFO-P2 and
LMFO-P3 was measured using BET analysis. It was found that the surface
observed in all doped LFO. The percentages of phases were calculated
using following equation (2) and shown in Table 1.
2
2
area of LFO was increased from 0.9881 m /g to 16.1968 m /g for LMFO
2
Intensity of phase
​
of interest
and 9.4939 m /g for LCFO due to the decreased grain size.
Phase Composition ¼
P
ꢀ 100 %:
(2)
Intensity of all Phases
3
.4. Optical studies of undoped and doped LaFeO
3
The nature and amount of secondary oxides increase with increasing
dopant concentrations which influence the photocatalytic properties as
reported in Parrino, F et al. [12]. Unlike LBFO, LCFO and LSFO, Mg
The bandgap values of the undoped and doped lanthanum ferrites
were calculated using their reflectance spectra. Fig. 4 (a-d) shows Tauc
plots of doped ferrites. It can be observed that the bandgap of LFO in-
creases for smaller dopant concentrations and decreases for higher
dopant concentrations (shown in Table S2). The values also seem to
depend on the portion of secondary oxides. These bandgap values are
suitable for the materials to be active under visible light. The redox
potentials of the photocatalysts were deduced by calculating their band
edge potentials (as mentioned in our previous report [40]). The deduced
band edge potentials of the alkaline earth ferrites are shown in Fig. 4 (e)
and Table S2. It can be observed that due to the presence of secondary
oxides, bandgaps become narrower and values of band edge potentials
become less favourable for photocatalytic behaviour after doping
doped LaFeO
3 2 3
has small amount of only Fe O as secondary phase (i.e.
less than 2%).
3
3
.3. Morphological analysis of undoped and doped LaFeO using HRTEM
Based on their better photocatalytic properties (as shown in section
.6), LBFO-P1, LCFO-P2, LSFO-P1 and LMFO-P3 are chosen for HRTEM
3
analysis and the results are shown in Fig. 3(a). The selected area electron
diffraction (SAED) data (Fig. 3 (c)) of all samples show ring type patterns
revealing their nanocrystalline nature. These patterns can be indexed
based on their respective crystal structures obtained from their corre-
sponding XRD patterns. The diffraction rings corresponding to second-
ary oxides can also be seen in all samples. The interplanar spacings of all
doped LFO in their high resolution images (Fig. 3 (b)) are indexed
3
LaFeO [26,27].
�
Fig. 9. (a) Tauc plot and (b) Percentages of CR dye degradation of 0.2 M of Ca and 0.3 M of Mg doped LFO calcined at 1100 C.
8

T. Vijayaraghavan et al.
Journal of Physics and Chemistry of Solids 140 (2020) 109377
the undoped and alkaline earth elements doped LFO composites in Fig. 5
(
a) shows the presence of La3d, La4d, Fe 2p, Fe3s, O1s, Ba3d, Ca2p,
Ca2s, Sr 3d peaks. The high resolution spectra of La 3d and Fe 2p in all
undoped and doped LaFeO reveals that the La and Fe atoms are in
3
formal oxidation states of 3þ. Presence of Ba 3d, Ca 2p, Sr 3d, Mg 2p
peaks indicate the oxidation states of Ba, Ca, Sr and Mg as 2þ. The high
resolution spectrum of La3d shows two strong La peaks at 834.62 and
8
51.31 eV which corresponds to the spin orbital splitting of 3d5/2 and
3
d
3/2 of La3 ions in oxide form (Fig. 5 (c)). Similarly the peaks observed
þ
3
þ
at 711.47 eV and 724.69 eV correspond to the 2p3/2, 2p1/2 of Fe ions
in oxide form (Fig. 5 (b)). The two oxygen 1s peaks observed at 530.25
eV and 532.58 eV correspond to oxygen lattice species (O
hydroxyl species (O ) (Fig. 6 (d)). The O
peak is related to the La–O and
particles. O is related to the hydroxyl
L
) and oxygen
H
L
Fe–O bonds present in LaFeO
3
H
groups due to the adsorbed moisture from the air [6].
The high resolution spectra of Ba3d (Fig. 5 (e)) show two peaks at
7
80 eV, 795 eV and 785 eV which indicating spin orbital splitting of 3d5/
, 3d3/2 and a satellite peak corresponding to Ba2 [25]. High resolution
þ
2
spectra of Ca 2p (Fig. 5 (f)) show two peaks at 347 eV and 350.2 eV
which indicate spin orbital splitting of 2p3/2 and 2p1/2 respectively
corresponding to Ca2 [26]. High resolution spectra of Sr 3d (Fig. 5 (g))
þ
exhibits spin orbital splitting of 3d5/2, 3d3/2 at 133 eV, 137.5 eV and
2
þ
satellite peak at 135.5 eV corresponding to Sr [27]. The high resolu-
tion spectra of Mg2p (Fig. 5 (h)) shows two peaks at 48.5 eV and 49.8 eV
which corresponds to spin orbital splitting of 2p3/2 and 2p1/2 respec-
tively for Mg2 [28].
þ
3
.6. Photocatalytic CR dye degradationstudies
Fig. 6(a–d) shows dye degradation behaviour of undoped and doped
LFO perovskites synthesised by sol-gel method under visible light irra-
diation from Xenon lamp for 90 min. The data reveals that degradation
of CR is considerably high under Xenon lamp irradiation when
compared dark conditions (shown in Table 2). Among all, LCFO-P2 and
LMFO-P3 showed better CR dye degradation than LBFO-P1 and LSFO-
P1. Initially, a steep increase in degradation percentage indicates the
occurrence of adsorption of CR dye molecules on the surface of the
photocatalysts [54, 55]. Further, slower increase in percentage indicates
the degradation of the adsorbed dye molecules under Xenon lamp irra-
diation. Thus the CR degradation is achieved by combined effect of
adsorption and photocatalysis. The total organic carbon analysis was
carried out to find the organic carbon present after CR dye degradation.
The total organic carbons (TOC) in LCFO-P2 and LMFO-P3 were found to
be 5.916 and 2.943 ppm. This value is lower when compared to initial
concentration of CR (16.17 ppm). The first order kinetics studies of
Fig. 10. (a) GC-MS spectrum and (b) Degradation pathway of CR dye by LMFO-
P3 under Xenon lamp irradiation for 90 min.
doped LFO composites are shown in Fig. S1 (e À h) and plot of C
t 0
/C is
shown in Fig. S1 (a – d). The apparent rate constant of LMFO-P3 is
Table 5
À 1
0
.0626 min . LMFO-P3 exhibited the highest percentage of dye
List of fragmented molecules after CR dye degradation.
degradation among all the studied photocatalysts. Detailed CR dye
degradation studies of this photocatalysts were carried out under
different illumination conditions.
Notation
m/z
Compound Name
(
(
(
(
(
(
(
(
a)
b)
c)
d)
e)
f)
457
227
183
157
128
97
Unknown fragmented compound
1, 2 di amino naphthalene sulfonic acid
Benzidine
Biphenyl
3
.6.1. Detailed photocatalytic CR dye degradation studies by LMFO-P3 at
different conditions
Naphthalene
Phenol
The absorbance spectra of LMFO-P3 under different irradiation
conditions are shown in Fig. 7. LMFO-P3 under Xenon lamp irradiation
exhibits higher photocatalytic activity than dark condition, as evident
from the immediate disappearance of the three active peaks in the
absorbance vs. wavelength plot (Fig. 7 (a and b)). Inset pictures show the
CR dye degradation at different time intervals up to 90 min under
different conditions. Less than 12% difference in degradation revealed
by the reusability test in Fig. 7 (c) affirms the reusability of LMFO-P3 as
photocatalyst. Data from active species trapping experiments shown in
Fig. 7 (d) indicates that super oxide anion radicals are the most active
species in the photocatalytic activity of LMFO-P3.
g)
h)
74
Propionic acid
Acetic acid
58
3
.5. Compositional analysis
The identification of elements and their oxidation states in the
photocatalysts which show better CR dye degradation (details in section
.6) such as LBFO-P1, LCFO-P2, LSFO-P1 and LMFO-P3 were examined
by X-ray Photoelectron Spectroscopy (XPS). The full survey spectra of
3
9

T. Vijayaraghavan et al.
Journal of Physics and Chemistry of Solids 140 (2020) 109377
Fig. 11. (a) Photocatalytic overall water splitting and (b) Reusability test results for LMFO-P3.
Table 6
Based on the results, degradation pathway for CR is proposed as
follows: The number of smaller and higher intensity GC-MS peaks ob-
tained from degradation by-products indicate that the complex CR
structure was disintegrated into smaller molecular weight compounds
such as acetic acid (m/z 60), propionic acid (m/z 74), phenol (m/z 97)
etc as shown in Fig. 10 (a). The CR dye initially gets fragmented into
smaller intermediates with m/z of 457 (unidentified intermediate) fol-
lowed by 227 (due to the reduction of two azo bonds corresponding to 1,
2 di amino naphthalene sulphonic acid). After the removal of amine
group and other cleavages in the benzene ring, it leads to the formation
of molecules with m/z 183 (corresponding to benzidine) and 157
(biphenyl compounds). The cleavage of these intermediates due to the
action of free radicals shows the formation of smaller degraded com-
pounds with the m/z of 126 (naphthalene), 97 and 74. The products and
their molecular structures obtained by the photocatalytic degradation of
CR from this study are shown in Fig. 10 (b) and given in Table 5, which is
well in agreement with the reported literature [35]. The results suggest
that the LMFO-P3 catalyst has achieved effective degradation of CR into
acetic acid and phenol during the photocatalytic irradiation. Hence, this
photocatalyst can be effectively employed to treat the various azo group
containing dyes.
Photocatalytic overall water splitting data of LFO and LMFO.
À 1 À 1
)
Photocatalysts
Bandgap (eV)
Photocatalytic water splitting (
μ
mol.h .g
Hydrogen
Oxygen
LFO
2.52
2.51
89.2
44.5
89.2
LMFO-P3
178.4
3
.6.2. Comparative study on single phase of Ca and Mg doped LFO
.2 M Ca and 0.3 M concentrations of Mg doped LaFeO perovskite
0
3
composites showed better photocatalytic CR dye degradation due to the
presence of small amount of secondary oxides. The CR dye degradation
of phase pure LCFO and LMFO (denoted as LCFO-PP and LMFO-PP)
without secondary oxides were also performed for comparison by
�
calcining the dried gel at 1100 C. The XRD data of these ferrites are
shown in Fig. 8. The diffractograms confirm that these ferrites are phase
pure compounds without any secondary oxides. The shift in higher in-
tensity peaks in LCFO-PP and LMFO-PP are due to the change in cell
parameters because of the substitution of dopants.
Optical studies shown in Fig. 9 (a) and Table 3 reveal that bandgaps
of Ca and Mg doped LFO get decreased due to their phase purity and
higher crystallinity. This effect is also seen in the percentages of CR dye
degradation of doped LFO which is shown in Fig. 9 (b) and Table 4. A
decrease in photocatalytic properties of LCFO and LMFO is observed
when they are in phase pure state. This observation highlights the role of
secondary oxides in increasing the photocatalytic efficiency in both Ca
3
.8. Photocatalytic hydrogen evolution through water splitting
% Mg doped LaFeO (LMFO-P3) which exhibited better photo-
3
3
catalytic activity than all studied compositions in this work was tested
for photocatalytic hydrogen evolution through water splitting. Fig. 11
and Mg doped LaFeO
3
by decreasing electron-hole recombination rate as
reported in previously [15].
(
a) shows the volume of hydrogen and oxygen evolved due to Mg doped
LFO photocatalyst after an exposure time of 4h (also shown in Table 6).
LMFO-P3 exhibited the hydrogen and oxygen evolution rate of 178.4
3
.7. CR dye degradation pathway
À 1
À 1
À 1
À 1
μ
mol h
g
and 89.2
μ
mol h
g
respectively. According to our
The products which are formed after the photocatalytic degradation
previous study, undoped LFO showed hydrogen and oxygen evolution
À 1 À 1
À 1 À 1
g . The reason for higher
of CR using LMFO-P3 photocatalyst were identified using mass spec-
troscopy. Degraded products formed at the end of photocatalytic process
and the possible mechanism of CR degradation under Xenon lamp
irradiation using LMFO-P3 is given in Fig. 10, Table 5 and their detailed
explanation is as follows.
rates of 89.2
μ
mol h
g
and 44.5
μ
mol h
activity in LMFO could be the presence of secondary oxides which act as
active centres to prevent recombination of photo excited charge carriers
at the surface of the composite. The detailed explanation is given in the
mechanism section. Fig. 11 (b) proves the good reusability of LMFO-P3
for overall water spitting over the period of 16 h.
The products which are formed after the photocatalytic degradation
of CR using LMFO-P3 photocatalyst were identified using mass spec-
troscopy. Degraded products formed at the end of photocatalytic process
and the possible mechanism of CR degradation under Xenon lamp
irradiation using LMFO-P3 is given in Fig. 10 (b). The changes in the
degraded products of CR depends on the nature of the catalyst used and
the degradation may occur for the following reasons: (1) possible
3.8.1. Mechanism
3.8.1.1. Mechanism of dye degradation. The mechanism of dye degra-
dation is shown in Fig. 12 (a) and can be explained as follows: Dye
degradation is a downhill photocatalytic reaction, where the energy
involved in the reactants is higher than that of products and thus
spontaneous. XRD studies indicated that all doped compositions of LFO
cleavage of benzene ring (especially decomposition on the sides of
–
–
benzene ring) (2) possible cleavage of azo bonds (À N
NÀ double bond)
(
3) ꢁOH radical attack on C–S bond between the aromatic ring and
contain a small percentage of Fe
phases. Due to the higher valence band edge potential of Fe
holes are created on its surface than LMFO. They will react with water
2
O
3
as secondary phase along with other
sulfonate groups (4) possible cleavage of chromophore group containing
2
O3, more
various C–N and C–C bonds [35].
1
0

T. Vijayaraghavan et al.
Journal of Physics and Chemistry of Solids 140 (2020) 109377
þ
À
Doped LFO composites þ h
þ LFOðCBÞðh Þ:
ν
→ SOðVBÞ ðh Þ þ LFOðCBÞ ðe Þ
þ
(3)
(4)
SOðVBÞðh Þ þ LFOðVBÞðh Þ þ OHÀ → OH :
þ
þ
:
À
À
SO þ LFOðCBÞðe Þ → SOðCBÞðe Þ:
(5)
À
À
SOðCBÞðe Þ þ O
2
→ O :
(6)
2
À
þ OHÀ :
O þ H
2
O → HO
2
(7)
2
:
HO
2
þ H
2
O → H
2
O
2
þ OH :
(8)
:
H
2
O
2
→ OH :
(9)
:
2
OH þ CR Dye→Acetic acid þ Naphthalene þ CO
2
þ H
2
O:
(10)
(11)
þ
2
H
þ CR Dye→Acetic acid þ Naphthalene þ CO
2
þ H
2
O:
As mentioned previously in XRD analysis, the quantity and bandgap
of secondary oxides will affect the photocatalytic property. LBFO com-
pounds are having three secondary oxides, La , BaO and Fe . The
theoretical bandgaps of these secondary oxides are 5.8 eV [30], 4.8 eV
31] and 2.2 eV [29, 32] respectively. Similarly the bandgaps of CaO and
SrO in LCFO, LSFO are 4.17 eV [33] and 3.65 eV [34] respectively. All
secondary oxides have wider bandgap except Fe . Majority of the CR
dye molecules get only adsorbed on the surface rather than undergoing
degradation as reported earlier for La [35], CaO [36] and SrO [34].
2
O
3
2 3
O
[
2
2 3
O
2
O
3
2
Since the percentages of these secondary oxides are lower, CR dye
removal can be achieved by combined effect of adsorption and photo-
catalytic degradation.
3
.8.1.2. Mechanism in water splitting. The mechanism of photocatalytic
water splitting in LMFO composite is explained in Fig. 12 (b). Electrons
and holes are produced on the surface of LMFO under visible light
2 3
irradiation. Since the valence band potential of Fe O is higher than
LMFO, holes will be created and transferred to VB of LMFO. Holes will
react with water molecules and oxidise into oxygen and hydrogen ions.
At the same time, electrons produced at the CB of LMFO get transferred
2 3
to the secondary oxide (Fe O ) due to its lower level of CB than in LMFO.
The hydrogen evolution reaction occurs at the surface of secondary
oxide. The photocatalytically produced charges are actively transferred
2 3
between LMFO and Fe O (as shown in inset of Fig. 12 (c)). The com-
posite facilitated increase in the utilization of electrons and holes for
water splitting reactions by reducing the charge recombination as
observed in enhanced photocatalytic activity for H
2 2
and O production.
This process is explained through chemical reactions as shown below.
þ
À
þ
LMFO þ h
ν
→ Fe
2
O
3ðVBÞðh Þ þ LFOðCBÞðe Þ þ LFOðVBÞðh Þ:
(12)
(13)
(14)
Fig. 12. Mechanism of overall photocatalytic (a) dye degradation and (b) water
splitting and (c) individual mechanism in alkaline earth elements doped LFO.
þ
þ
þ 4Hþ:
Fe
2
O
3ðVBÞðh Þ þ LFOðVBÞðh Þ þ H
2
O → O
2
under visible light irradiation and produce hydroxyl radicals. Simulta-
neously, electrons from doped LFO will get transferred to secondary
oxides due to their lower conduction band potential (as shown in Fig. 12
3ðVBÞðe Þ þ LFOðVBÞðe Þ þ 4Hþ → Fe
À
À
Fe
2
O
2
O
3
þ LFO þ 2H ↑:
2
2 3
The presence of small amount of Fe O separates the electrons from
(
c)). Thus charge transfer will occur based on their band edge potentials.
CB of LMFO thus reducing the recombination. This favours the increased
photocatalytic activity. To the best of our knowledge, so far no reports
are available about the secondary oxide phases in LFO themselves acting
as a co-catalyst for photocatalytic hydrogen evolution reaction. Here the
overall water splitting is due to the collective contribution from all
phases present in doped LFO.
Holes will be transferred from the secondary phase which has higher
valence band potential to compounds with lower potential. Similarly
electrons will transfer from compounds with more negative conduction
band potential to those with lower value. These electrons will further
react with oxygen in water and form super oxide anions. Super oxide
anions will react with organic dye molecules in water and form hydro-
peroxy radical and hydroxyl ion. The hydroxyl ion acts as a strong
oxidizing agent to decompose organic dye into non-toxic by products.
The detailed step by step chemical reaction which occurs on the surface
of doped LFO composites is given in the following chemical equations.
SO represents the secondary oxides of the dopants.
4. Conclusions
Alkaline earth elements doped LaFeO composites were prepared by
3
simple chemical method. The presence of secondary oxides along with
primary phase of LFO was observed through XRD and HRTEM. The
1
1

T. Vijayaraghavan et al.
Journal of Physics and Chemistry of Solids 140 (2020) 109377
bandgap studies confirmed the visible light activity and provided the
band edge potentials to understand photocatalytic mechanism. The
bandgap was found to decrease with the increase in dopant concentra-
tion. The photocatalytic studies were performed through CR dye
degradation and water splitting. The role of specific secondary oxides
present in the nanocomposites in improving the photocatalytic activity
in doped compositions through reduced recombination was realised in
this study. With its smaller grain size as well as smaller percentage of
secondary oxides, Mg doped LFO composite showed better hydrogen
evolution and dye degradation. Trapping experiment showed that
electrons are more active species in the nanocomposites composites than
others. The possible CR dye degradation pathway was proposed. LMFO
composite could be one of the candidate photocatalyst material for en-
ergy and environmental remediations.
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all aware of its content and approve its submission, has not been pub-
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where, no conflict of interest exists, or if such conflict exists, the exact
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CRediT authorship contribution statement
T. Vijayaraghavan: Conceptualization. M. Bradha: Methodology.
Pradeepta Babu: Formal analysis. K.M. Parida: Investigation. G.
Ramadoss: Formal analysis. S. Vadivel: Resources. R. Selvakumar:
Investigation. Anuradha Ashok: Supervision.
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