Phase transition, electronic transitions and visible light driven enhanced photocatalytic activity of Eu–Ni co-doped bismuth ferrite nanoparticles

Article abstract of DOI:10.1016/j.jpcs.2021.110018

This article focusses on the detailed structural and optical properties of ‘Eu’ and ‘Ni’ co-substituted Bismuth Ferrite nanoparticles synthesized using the sol-gel method. Raman analysis have confirmed the substitution induced structural distortion leading to the co-existence of Rhombohedral and Orthorhombic structures supporting the XRD and Rietveld Refinement results. Morphological studies using FESEM and HRTEM have shown irregular and inhomogeneous distribution of particles with presence of mixed phases and the elemental composition is also confirmed through EDAX analysis. The XPS analysis indicated the simultaneous presence of Fe²⁺and Fe³⁺states along with Bi³⁺, Eu³⁺ and oxygen vacancies. Thorough optical investigation using UV–Vis and PL Spectroscopy has shown an increase in absorption intensity in the visible region with a bandgap tuning from 2.17 eV to 2.12 eV in the co-substituted nanoparticles. A possible schematic energy band diagram is proposed on the basis of observed transitions in photoluminescence studies, VB-XPS analysis along with the knowledge of optical transitions in d⁵ systems. Photocatalytic performance of the synthesized nanostructures is studied by degradation of 2-nitrophenol. The enhanced photocatalytic response has been explained on the basis of decreased band-gap, space charge thickness and charge carrier recombination.

Full text of DOI:10.1016/j.jpcs.2021.110018

Journal of Physics and Chemistry of Solids 153 (2021) 110018
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: http://www.elsevier.com/locate/jpcs
Phase transition, electronic transitions and visible light driven enhanced
photocatalytic activity of Eu–Ni co-doped bismuth ferrite nanoparticles
a
b,
*
c
c
c
Subhra S. Brahma , Jyotirmayee Nanda , Naresh K. Sahoo , B. Naik , Anup Anang Das
a
Centre for Nano Science and Nano Technology, ITER, S’O’A Deemed to be University, Khandagiri, Bhubaneswar, 751030, Odisha, India
Department of Physics, ITER, S’O’A Deemed to be University, Khandagiri, Bhubaneswar, 751030, Odisha, India
Department of Chemistry, ITER, S’O’A Deemed to be University, Khandagiri, Bhubaneswar, 751030, Odisha, India
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
This article focusses on the detailed structural and optical properties of ‘Eu’ and ‘Ni’ co-substituted Bismuth
Ferrite nanoparticles synthesized using the sol-gel method. Raman analysis have confirmed the substitution
induced structural distortion leading to the co-existence of Rhombohedral and Orthorhombic structures sup-
porting the XRD and Rietveld Refinement results. Morphological studies using FESEM and HRTEM have shown
irregular and inhomogeneous distribution of particles with presence of mixed phases and the elemental
composition is also confirmed through EDAX analysis. The XPS analysis indicated the simultaneous presence of
Eu–Ni co-Doped bismuth ferrite
Sol-gel synthesis
Structural analysis
Optical analysis
Photocatalysis
2
+
3+
3+
3+
Fe and Fe states along with Bi , Eu and oxygen vacancies. Thorough optical investigation using UV–Vis
and PL Spectroscopy has shown an increase in absorption intensity in the visible region with a bandgap tuning
from 2.17 eV to 2.12 eV in the co-substituted nanoparticles. A possible schematic energy band diagram is pro-
posed on the basis of observed transitions in photoluminescence studies, VB-XPS analysis along with the
knowledge of optical transitions in d5 systems. Photocatalytic performance of the synthesized nanostructures is
studied by degradation of 2-nitrophenol. The enhanced photocatalytic response has been explained on the basis
of decreased band-gap, space charge thickness and charge carrier recombination.
1
. Introduction
of ferroelectric properties of BFO by reducing the volatility of ‘Bi’ ions.
The 4f transitions of rare-earth substituted BFO as catalysts have
In the scientific community, there has been an increase in the de-
mand for multiferroic perovskites due to its diverse practical applica-
tions. Among them, Bismuth Ferrite (BiFeO ) has garnered quite a lot of
improved its optical absorbing ability along with supporting the sepa-
ration of photogenerated charge carriers. The substitution of ‘Fe’ site
with transition metal ions helps in enhancing the multiferroic properties
of BFO by destroying its spin cycloid structure.
3
attention because of its above room temperature co-existence of ferro-
electricity and G-type anti-ferromagnetism, low bandgap, low produc-
tion cost, and high visible light absorption. Due to its interesting
Recently, researchers have adopted co-doping for better tailoring of
magnetoelectric, optical and electrical properties of BFO. The combi-
nation of rare-earth elements with transition metal ions such as Nd–Ni,
Ho–Ni, Ce–Mn, Eu–Mn, Eu–Co and with alkaline earth metal such as
Eu–Ba shave shown exemplary results [12–17]. Magnetic properties of
Ho–Ni doped BFO have shown no structural distortion with enhanced
magnetization values as well as low coercive field [12]. As reported by
authors ‘Ni’ doping plays an important part in the enhancement of
ferromagnetic properties whereas ‘Ho’ favours magnetic exchange in-
teractions. Similarly Neodymium and nickel co-doped BFO exhibit
ferroelectric nature at different frequencies. Photocatalytic activity
studies using degradation of methylene blue dye, shows that the
co-doped samples are good Fenton-like catalysts [14]. With Eu–Mn
3
fundamental physics, BiFeO (BFO) can have potential applications in
various fields, such as data storage, spintronic devices, sensors, photo-
voltaics, and photocatalysis [1–5]. However, certain limitations such as
high leakage current, secondary phase formation, cancellation of
macroscopic magnetization in bulk BFO due to its spiral spin period of
6
2 nm, impedes the practical application of BFO in devices. To overcome
these limitations, there have been several reports on partial substitution
3
+
3+
3+
,
of A (‘Bi’) site with rare-earth lanthanides such as La , Gd ,Nd
3
+
4+
3+
Sm ,Pr , Eu etc [6–8] and B (‘Fe’) site with transition metals such as
3
+
2+
2+
3+
Mn , Cu , Ni etc [9–11]. Doping Bi with rare-earth ions has
resulted in the formation of pure phase compound with an enhancement
*
Corresponding author.
E-mail address: jyotirmayeenanda@soa.ac.in (J. Nanda).
https://doi.org/10.1016/j.jpcs.2021.110018
Received 2 December 2020; Received in revised form 20 February 2021; Accepted 22 February 2021
Available online 27 February 2021
0
022-3697/© 2021 Elsevier Ltd. All rights reserved.

S.S. Brahma et al.
Journal of Physics and Chemistry of Solids 153 (2021) 110018
doping in BFO, Zhu et al. have thoroughly investigated the effect of
rare-earth and transition metal co-doping on its structural, optical and
magnetic properties [16]. The band-gap substantially reduced from
nanoparticles. The co-doping effects on the band-gap tuning, interband
electronic transitions and photocatalytic performance of all the samples
for the breaking down of 2-nitrophenol are extensively studied.
2
.40 eV in pristine BFO to 1.49 eV in Eu–Mn doped samples. The Eu–Mn
co-doped nanostructures show distinct morphologies with defined grain
boundaries and reduction in grain sizes, indicating suppression of grain
growth. Banu et al. have reported about the enhancement of multiferroic
properties upon co-doping BFO with Er–Nb, Mn, Mo. Among them,
Er–Nb doped BFO has shown maximum polarization and highest
magnetoelectric coupling as well as lowest leakage current density [18].
The dopants have also altered the surface morphology of flake-like BFO,
synthesized using sol-gel method. The co-doped samples have agglom-
erated non-uniform structures with some flake-like and cylindrical
structures. Thus, it can be conjectured that the morphology is greatly
affected by the combination of co-dopants. Uman et al. have designed
La–Se doped BFO with lesser band-gap of 1.76 eV compared to 2.04 eV
of BFO and studied its visible-light photocatalytic effect [19]. La–Se
doped samples show sheet type morphology with the size of sheets
increasing as the ‘Se’ concentration increases. Eu, Tb, Ho–Co dopants on
BFO thin films have shown that pores present in pure BFO thin films
have substantially reduced and grain size increased, which maybe due to
the suppression of oxygen vacancies [20].
2. Experimental details
2.1. Synthesis method
The Eu–Ni co-doped bismuth ferrite nanoparticles were synthesized
using wet chemical method, which has proved to be cost-effective and a
better alternative to the typical solid-state method of sample preparation
with certain disadvantages such as high crystallization temperatures and
secondary phase formation. For the synthesis of Bi0.96Eu0.04Fe1-xNi
(x = 0.03,0.05,0.07) (BEFNO) nanoparticles, Bismuth nitrate pentahy-
drate (Bi(NO .5H O), Iron nitrate nonahydrate (Fe(NO .9H O),
Europium oxide (Eu ), and Nickel nitrate hexahydrate (Ni
(NO .6H O) were taken as the starting materials. All the metal nitrate
precursors used were of Merck with analytical grade (99%) purity.
Europium oxide (Eu ) was obtained from HIMEDIA with 99.99%
x 3
O
3
)
3
2
3
)
3
2
2 3
O
3
)
2
2
2 3
O
purity. The precursor solution was prepared using stoichiometric
amounts of all the metal nitrates and the required oxides in diluted Nitric
acid (HNO
ture. Though iron and nickel nitrates are soluble in DI water, diluted
HNO solution is necessarily taken to dissolve bismuth nitrate and
3
(10%)) solution under constant stirring at room tempera-
However, there have been comparatively fewer studies on its optical
properties, interband electronic transitions and its application in pho-
tocatalysis for the breaking down of industrial organic pollutants. With
detailed studies using absorption spectroscopy and spectroscopic
ellipsometry, BFO has shown a low bandgap in the visible range of about
3
europium oxide. Thereafter, tartaric acid with a molar ratio of 1:2 cor-
responding to the metal nitrates was added as the chelating agent. The
solution then turned pale yellow which was stirred continuously for 2 h
◦
2
.2–2.8 eV and high absorption coefficient [13] in comparison to its
at 50 C on the hot plate until all the constituent metal nitrates and
extensively used wide band-gap semiconductors such as TiO (3.3eV),
2
oxides are completely soluble. The solution is then transferred to the hot
◦
ZnS (3.6eV) and ZnO (3.4eV) [21–26]. Despite that, the commerciali-
zation of BFO in this industry has not been very viable because of its low
efficiency. Several techniques such as surface modification, particle size
reduction, doping in a small quantity have been employed to enhance
the efficiency of BFO as a photocatalytic agent [5]. A small amount of
dopant can help to restrict the recombination rate of photo-generated
carriers as it acts as trap sites for electrons and thus improves the pho-
tocatalytic response [27]. There are several reports on the ameliorated
photocatalytic response of BFO on doping, for the degradation of toxic
industrial dyes such as Rhodamine B (RhB), methyl orange, methylene
blue, congo-red, 4-nitrophenol, etc. Sarkar et al. [28] have reported
morphology mediated enhanced photocatalytic activity owing to the
air oven where it was heated at 80 C until it dries completely to form
the brown-coloured amorphous precursor powder. The desired phase
◦
was then achieved through calcination at a temperature of 550 C for 4
h.
2.2. Characterization techniques
The structure of BFO and BEFNO samples were investigated using an
X-ray diffractometer (PANalyticalX’pert PRO) with Cu K
α
radiation.
Rietveld refinement analysis and crystal structure determination were
done with the aid of FULLPROF Suite and VESTA software respectively.
Room temperature Raman and Photoluminescence Spectra were recor-
ded using Horiba LABRam HR800UV Raman Spectrometer. Field
Emission Scanning Electron Microscopy (FESEM) (Carl Zeiss Neon-40
FESEM) and High-Resolution Transmission Electron Microscopy
(HRTEM) (FEI-FP5022/22Tecnai G2 20 S-TWIN) were employed for
morphological analysis. Optical properties were studied using UV–Vis
Spectrophotometer (SCHIMAZE-2450). For photocatalytic measure-
ments a 500 ml reactor (Lelesil Innovative systems, India Model 1170)
was used. The solution was placed in the central quartz jacket illumi-
nated with a 1000 W MPMVL lamp. For temperature control, the reactor
was connected to a water bath (Lauda Thermostat-RA8 Make Germany).
reduction in band-gap of BFO on substitution of ‘Bi3 ’ with ‘Dy
+
3+
.
Degradation of several other industrial dyes such as congo red, methy-
lene blue, and methyl violet were also investigated on doping with
3
+
4+
3+
4+
Gd and Sn [29,30]. La and Se doping have also increased the
photocatalytic efficiency for degradation of acetophenone solution by
tailoring the band-gap from 2.06 eV for pure BFO to 1.97 eV for La–Se
co-doped BFO [27].With ‘Gd’ and ‘Sm’ doping, the authors have re-
ported 2.55 times higher photocatalytic response than pure BFO [31].
Nadeem et al. [32] have recently discussed the enhanced degradation
efficiency of Ni-doped BFO on methylene blue (MB) under visible light
irradiation, due to the lower recombination rates of the charge carriers.
Compared to an increased number of studies of the photocatalytic
response of BFO, for degradation of industrial dyes there are quite scarce
reports on the breaking down of phenol and nitrophenol isomers
3. Results and discussion
3.1. Structural analysis
(
2-nitrophenol, 4-nitrophenol, 3-nitrophenol) using BFO as photo-
catalyst [33–35].
Preliminary structural investigation of all the samples carried out by
X-Ray diffraction (XRD) shows the formation of single-phase with
polycrystalline nature as shown in Fig. 1 (a). All the peaks of pure BFO
are indexed perfectly according to the standard JCPDS card no.
(71–2494). The characteristic doublet reflections from (104) and (110)
A thorough literature survey on the co-doped (lanthanides and
transition metal ions) bismuth ferrite has shown that there is no study on
the structural, optical or photocatalytic response of BFO on co-doping
‘
Eu’ and ‘Ni’ at ‘A’ and ‘B’ sites respectively. This motivated us to
◦
study the interplay between the structural transformation and electronic
transitions (interband), which eventually would make this a good pho-
tocatalyst (visible light responsive) for the degradation of organic pol-
lutants such as 2-nitrophenol. In this paper, we present the structural
at 2θ~32 suggest the formation of a rhombohedrally distorted R3c
structure of BFO. It is clear from the XRD patterns of BEFNO (x = 0.03,
0.05, 0.07) that the most intense peaks at (104) and (110), which are
clearly separated in case of pristine BFO, seem to gradually merge and
form a broad peak. This indicates induced distortion in the lattice due to
properties of single-phase Bi0.96Eu0.04Fe1-xNi
x
O
3
(x = 0.03, 0.05, 0.07)
2

S.S. Brahma et al.
Journal of Physics and Chemistry of Solids 153 (2021) 110018
Fig. 1. (a) XRD patterns of BFO, BEFO (Bi0.96Eu0.04FeO
3
) and Bi0.96Eu0.04Fe1-xNi
x
O
3
(x = 0.03, 0.05, 0.07, 0.09) samples. *sign indicates the presence of impurity
corresponding to Bi25FeO40, (b) Magnified image of doublet peak at about 32⁰, showing peak merging and shifting with the increase of dopant concentration. Rietveld
Refinement of (c) BFO and (d) BEFNO-5 nanostructures.
Table 1
Structural parameters of BFO and the co-substituted samples obtained from Rietveld Refinement and the other relevant formulas.
BFO
BEFNO-3
BEFNO-5
BEFNO-7
Lattice parameters
R3c
R3c
R3c
R3c
�
(
A)
a = b = 5.578
c = 13.848
a = b = 5.567
c = 13.730
Pnma
a = b = 5.592
c = 13.851
Pnma
a = b = 5.579
c = 13.863
Pnma
a = 5.580
b = 7.831
c = 5.582
R3c:368.598
Pnma:243.965
R3c
a = 5.592
b = 7.848
c = 5.581
R3c: 372.986
Pnma:244.512
R3c
a = 5.661
b = 7.940
c = 5.616
R3c:374.190
Pnma:252.507
R3c
�
3
372.732
Volumecell (A )
Bond Length
Fe–O:2.1876
1.8266
�
(
A)
Fe–O: 2.1086
1.8303
Fe–O: 2.1073
1.8345
Fe–O: 2.1098
1.8366
Bi–O:2.3103
Bi–O: 2.4021
Pnma
Bi–O: 2.4110
Pnma
Bi–O: 2.4137
Pnma
Fe–O1: 1.9948
Fe–O2: 2.0114
Bi–O1: 2.5151
Bi–O2: 2.4272
R3c
Fe–O1: 1.9970
Fe–O2: 2.0123
Bi–O1: 2.5188
Bi–O2: 2.4375
R3c
Fe–O1:2.0026
Fe–O2: 2.0128
Bi–O1: 2.5193
Bi–O2: 2.4415
R3c
Bond Angle, θ (deg)
Fe–O–Fe:161.79
Fe–O–Fe:156.02
Pnma
Fe–O–Fe: 156.185
Pnma
Fe–O–Fe:156.187
Pnma
Fe–O1–Fe:157.768
Fe–O2–Fe:157.169
1.46
Fe–O1–Fe:157.651
Fe–O2–Fe:157.152
1.85
Fe–O1–Fe: 157.745
Fe–O2–Fe:157.164
2.66
2
χ
2.6
R
R
wp
15.5
20
14.7
14.8
18.7
p
19.3
18.9
23.3
Crystallite size (nm)
Tolerance factor
Strain
35.52
0.8437
0.016
16.26
15.60
15.29
0.840
0.8399
0.8393
0.00823
0.00966
0.00816
3

S.S. Brahma et al.
Journal of Physics and Chemistry of Solids 153 (2021) 110018
doping. However, a slight trace of impurity for 9% Ni doped sample,
indicates the gradual formation of multiple phases. The impurity is
identified as Bi25FeO40 and is marked as ‘*’ in Fig. 1(a). There are
various reports on the structural transformation of doped BFO from
rhombohedral (R3c) to other phases like orthorhombic, tetragonal, or
cubic phases [32,36]. Considering the magnified view of reflections of
all the samples at 2θ~32◦ in Fig. 1(b), we observe the peak shifts to-
wards higher angle for Eu-doped BFO (BEFO), caused by the doping at
synthesized perovskite nanostructures. The substitution of a larger
2
+
3+
Ni ion in place of Fe , decreases the value of the tolerance factor,
suggesting a higher degree of buckling of the oxygen octahedron. The
values of crystallite size and tolerance factor have been listed in Table 1.
3
.2. Morphological studies
The morphology of BFO and BEFNO-5 nanostructures were studied
3
+
3+
‘
A’ site (Bi = 1.03 Å) with smaller Eu (0.947 Å) ion and change in
using Field Emission Scanning Electron Microscopy (FESEM) and High
Resolution Transmission Electron Microscopy (HRTEM) micrographs
are shown in Fig. 3. The FESEM images clearly show the formation of
inhomogeneous and irregular shape of BFO Fig. 3(a) as well as BEFNO-5
Fig. 3(b). A particular shape for any particle could not be noticed in the
FESEM micrographs. However, BEFNO-5 nanoparticles were observed
to be more porous than pristine BFO. The induced porosity with the
substitution must have increased the surface area of the particles hence
affecting many of its properties. The elemental composition was
confirmed through Energy Dispersive Analysis of X-Ray (EDAX), dis-
played in Fig. 3(c-d). Fig. 3(e-f) shows the HRTEM micrograph of
BEFNO-5 along with its lattice fringes and d-spacing. The lattice fringes
as observed belonged to both the phases present in BEFNO-5 and thus it
verifies the XRD results. The d-spacing corresponding to (104) and (200)
planes belong to the rhombohedral and orthorhombic phase
respectively.
the interplanar spacing as well as grain size effects [37].
However, the co-doped samples exhibit a peak shift towards the
higher angle till 5% of Ni doping. Thereafter, with further increase in
doping, there is a peak shift towards the lower angle which may be
2
+
attributed to the larger mismatch between the ionic radii of Ni (0.69 Å)
3
+
and Fe (0.64 Å) ions [32]. The peak broadening and shifts clearly
indicate the substitution of respective dopant ions at ‘A’ and ‘B’ sites. For
further confirmatory analysis, Rietveld Refinement was carried out for
all the diffraction patterns (up to x = 7%) using the rhombohedral (R3c)
model for pristine BFO and a mixed structural model (R3c + Pnma) for
the co-substituted samples. A reliable Goodness of Fit (GOF) value (<3)
was observed with (R3c + Pnma) structure fitting, hence proving the
co-existence of both the phases in all the substituted samples. The lattice
parameters and structural parameters such as bond lengths, bond angles,
2
cell volume and other refined parameters namely Rwp, R
p
and
χ
are
extracted from Rietveld Refinement analysis, and presented in Table 1.
The representative structure of BEFNO-5, with two phases is drawn with
the help of VESTA software and displayed below in Fig. 2.
3
.3. Raman spectra analysis
The lattice parameters and estimated cell volume of the samples are
found to increase with the gradual increment of ‘Ni’ ion concentration.
The ‘Ni’ doping also affects the bond-lengths between Bi, Fe and O atoms
while maintaining almost same bond angle. The change in the Fe–O
Raman spectra is a sensitive technique employed to notice the atomic
displacements, lattice distortions, and phase transitions and is generally
studied for confirmation of data obtained from XRD and Rietveld anal-
ysis. Fig. 4(a–b) shows the room-temperature Raman spectra of BFO and
6
bond-lengths could be caused by the distortion in the FeO octahedron
ꢀ 1
BEFNO-5 samples recorded in the range of 100–800 cm . The peak
due to ‘Ni’ ion doping which subsequently affects the Bi–O bond length.
The average crystallite size was calculated using the Debye-Scherrer
formula, and found to be 35.52 nm for BFO. The systematic decrease
of crystallite size of co-substituted compounds may be due to the pres-
ꢀ 1
positions below 100 cm cannot be observed due to the rejection of the
notch filter in the spectrometer. Theoretical calculations based on Group
theory have predicted a total of 20 phonon modes for BFO, out of which
3
+
13 (4A
two (A
1
+9E) are both Raman and IR active, 5A
2
modes are silent and
1
modes are polarised
ence of ‘Ni’ ion in the Fe site, which acts as an impurity in the crystal
and induces anisotropicity and suppresses the crystallite growth [28].
The stability of perovskites calculated using Goldschmidt tolerance
factor, was found to be 0.84 for pure BFO, which is pretty close to the
reported value of 0.88 in literature [38]. This confirms the stability of
1
+E) are acoustical modes [39,40]. The A
along the z-axis whereas the E modes in the x-y plane. The Raman
spectra of all the samples are deconvoluted into its Gaussian components
and the peak positions are presented in Table 2. As presented in Table 2,
only 10 Raman active modes (4A
1
+6E) could be seen for pristine BFO
Fig. 2. Structural Representation of BEFNO-5 nanoparticles drawn with the help of VESTA Software.
4

S.S. Brahma et al.
Journal of Physics and Chemistry of Solids 153 (2021) 110018
Fig. 3. (a) FESEM micrograph of BFO and (b) of BEFNO-5 nanostructues. EDAX elemental graphs for (c) BFO and (d) BEFNO-5 respectively. (e) HRTEM image of
BEFNO-5 nanostructure (f) lattice fringes with d-spacings for two planes belonging to the Rhombohedral and Orthorhombic phase respectively.
sample, confirming the R3c structure [11,41]. However, the spectra of
co-substituted samples show a relatively different feature compared to
pristine BFO with a reduction in intensity, peak shifting as well as the
absence of some modes as compared to pure BFO.
the oxygen octahedra, and hence their shifting can be the confirmation
of ‘A’ site substitution with ‘Eu’. This phenomenon is due to the chemical
pressure-induced bond shortening and lattice distortion arising due to
the lattice mismatch between the host and the dopant ion [42].The
hardening of high frequency ‘E’ modes maybe attributed to the oxygen
vacancies created by the substitution of ‘Fe ’ site with ‘Ni ’ ions [12].
Arora et al. have also claimed that the shifting, broadening, and merging
of Raman modes with increasing the dopant concentration is due to the
lattice distortion produced as a result of competition between the
Since the Raman vibrational frequencies are dependent on factors
1
/2
3+
2+
such as force constant and reduced mass by the relation f=(k/M)
,
where k is the force constant, associated with the bond strength and ‘M’
is the reduced mass. The shifting of modes to higher frequencies maybe
caused by the decrease in the average mass at ‘A’ as well ‘B’ site [42].
The observed peak broadening of A
1
-1 and A
1
-2 modes also indicates the
rhombohedral and orthorhombic phase [45]. The disappearance of A
and E mode is reportedly attributed to the structural transition from
rhombohedral to orthorhombic phase [45]. However, the presence of
-1, A -2, and A -3 modes still confirm the existence of the rhombo-
1
-4
change in phonon behaviour due to lattice anharmonicity and distortion
4
produced in the ‘Bi’ site [43]. Nadeem et al. [11] have associated the
ꢀ
1
ꢀ 1
frequencies lower than 162 cm to ‘Bi’ atoms, 162-262 cm to ‘Fe’
A
1
1
1
ꢀ 1
atoms, and above 262 cm to ‘O’ atoms. The change in intensity of
low-frequency modes associated with Bi–O bonds, maybe due to the
increase in dispersion in Bi–O bond lengths [44].The low-frequency
modes are associated with the relative motion of A-site cations against
hedral phase [44]. Hence, Raman spectra analysis supports the XRD and
Rietveld refinement results, which confirm the co-existence of rhom-
bohedral and orthorhombic phase. The appearance of wide bands at
ꢀ 1
ꢀ 1
480 cm and 620 cm for BEFNO-7 could be associated with distortion
5

S.S. Brahma et al.
Journal of Physics and Chemistry of Solids 153 (2021) 110018
Fig.4. (a–b): Room Temperature Raman modes of BFO and BEFNO-5 respectively and its deconvolution into A
1
and E modes.
Table 2
Raman modes with exact values of Raman shifts obtained from deconvolution of acquired Spectra for BFO, BEFNO-3, BEFNO-5, and BEFNO-7 nanoparticles.
1
Raman Shift (cm- )
Sample name/Raman modes
A
1
-1
A
1
-2
A
1
-3
Eꢀ 3
271
270
285
272
Eꢀ 4
340
–
Eꢀ 5
367
368
375
374
A
1
-4
Eꢀ 7
470
479
475
482
Eꢀ 8
521
531
–
Eꢀ 9
BFO
128
139
132
141
166
169
169
166
212
228
201
229
430
–
–
–
610
610
585
616
BEFNO-3
BEFNO-5
BEFNO-7
–
–
539
of FeO
.4. Chemical –state analysis
To investigate the chemical state of various elements present, X-Ray
6
octahedra [46].
nanoparticles. The obtained spectra clearly show the increase in the
absorption intensity in the visible region as well as a slight red-shift of
the absorption edge for the co-substituted compounds. The energy
bandgaps of the samples are estimated using the Tauc’s Relation given
3
2
n
by (
αh
ν
) =A(E
g
-hν)
where ‘
α’ is the Absorption constant, ‘hν’ is the
Photoelectron Spectroscopy (XPS) was carried out and for BEFNO-5
sample (displayed in Fig. 5(a–d)). All of the obtained spectra were
fitted with the aid of the CasaXPS MFC Application. The two main
characteristic peaks of bismuth centered at 164.5 eV and 159.2 eV, as
shown in Fig. 5(a), is ascribed to Bi 4f5/2 and Bi 4f7/2 respectively and is
due to Bi–O bonds [47].
energy of the incident photon, ‘E
g
’ is the bandgap of the material that is
to be determined, ‘A’ is the scaling constant and ‘n’ is factor dependant
on the type of semiconductor material. The exponent ‘n’ is ½ for indirect
bandgap semiconductors and 2 for direct bandgap semiconductors.
Since BFO is a direct bandgap semiconductor, the bandgap is determined
2
by linear extrapolation of the curve between h
ν
and (
αh
ν)
to the ab-
The presence of the two peaks confirm the Bi3 oxidation state in
+
scissa. The inset of Fig. 6 shows the plots of the bandgap energy of BFO
and BEFNO-5 nanoparticles. The table in the inset of Fig. 6 displays the
value of band-gap energy and Urbach energy.
BEFNO-5. The valence state spectra of ‘Eu’ in Fig. 5(b) confirms the
presence of Eu3 oxidation state. Peaks representing Eu 3d3/2 and Eu
+
3
d
5/2 at 1164.34 eV and 1135.27 eV respectively match quite well with
There is a slight decrease in the value of the bandgap until x = 0.05,
previous reports [48]. The deconvoluted spectra of ‘Fe 2p’ in Fig. 5(c)
clearly shows the presence of two doublet peaks centered at 710.12 eV
and 723.4 eV arising due to spin-orbit interaction. Both the peaks can be
assigned to Fe–O bond [47]. Spin-orbit splitting energy as calculated
from the results is ~13.28 eV which is comparable to the theoretical
but with a further increase in the concentration of ‘Ni’ the bandgap is
almost close to that of the pristine BFO. The distortion of the FeO
6
octahedra, change in the bond angle or the rearrangement of the mo-
lecular orbitals upon doping may be one of the causes of the decrease in
the band gap energy [28]. Xu et al. [49] have also explained this phe-
nomenon based on the creation of oxygen vacancies on substitution of
value of 13.6 eV for Fe
2 3
O .The presence of satellite peaks at ~8 eV
3
+
2+
3+
higher than the Fe 2p3/2 peak explains the dominance of Fe state of
with divalent Ni ions in place of trivalent Fe ions. The creation of
oxygen vacancies may have created localized impurity levels within the
bandgap of the material, below the conduction band. The holes formed
as a result of charge conservation, occupy the upper part of valence band
as an acceptor band, and the impurity level locates itself below the
conduction band, hence the band gap decreases with initial concentra-
tions. But as the dopant concentration increases, more holes are formed,
increasing the width of the acceptor band and thus increasing the rate of
recombination of e-h pairs, which ultimately leads to the narrowing of
the oxygen vacancy donor band. A slight increase in the Urbach energy,
iron. The simultaneous presence of Fe2 state along with Fe maybe
due to the charge compensation phenomena. The XPS for ‘Ni’ could not
be found, though its presence in the samples was confirmed through
EDAX analysis in Fig. 3(d). The O1s spectra in (Fig. 5(d)) shows the
presence of 3 peaks at 529.7 eV, 531.3 eV, and 532.4 eV, which may be
ascribed to O-X (X = metal) bonds, oxygen vacancies, and surface
adsorbed oxygen respectively [31].
+
3+
3
.5. Optical elucidation
deduced from the graph between ln
α
vs. h
ν
, corroborates the increase of
2
+
To study the detailed analysis of optical properties of BFO and the co-
oxygen vacancies on increasing the Ni doping concentration. More-
3
+
substituted samples, UV–Vis as well as Pholuminiscence Spectroscopy
PL) studies were carried out. Fig. 6 shows the Energy vs. Absorbance
plot for BFO and Bi0.96Eu0.04Fe1-xNi (x 0.03,0.05,0.07)
over, one transition corresponding to the crystal field of Fe is observed
just below the absorption edge. The absence of another reported tran-
sition is due to the limitations of the instrument. Hence, these transitions
(
x
O
3
=
6

S.S. Brahma et al.
Journal of Physics and Chemistry of Solids 153 (2021) 110018
Fig.5. (a–d): XPS Spectra of Bi 4d, Eu 3d, Fe 2p, and O 1s of BEFNO-5 sample.
1
1
2 2
are studied in detail through PL spectroscopy.
again transform into a e e electronic configuration, but maintain the
Around six d-d transitions are expected between 0 and 3 eV [46,47].
The two intense transition peaks at about 1.3 eV and 2.0 eV seen in Fig. 7
energy difference of Δ = 10 Dq between the highest and the lowest
0
state. With the aid of VB-XPS, the valence band edge was also estimated
3
+
6
4
(
a) can be ascribed to on-site Fe Crystal-field transitions, A1g → T2g
for BEFNO-5 sample (shown in the inset of Fig. 7(a)).
6
4
and A1g → T1g respectively [43,50,51]. These transitions are very much
sensitive to FeO
An electronic band diagram for BEFNO-5 has been proposed in Fig. 8
(b), with the possible electronic transitions elucidated from PL Spec-
6
octahedra distortion, the change in the local environ-
ment of Fe3 ion, Fe–O bond length, and Fe–O–Fe exchange interaction,
and hence a slight shift in the transition energies can be noticed in
BEFNO-5 spectra. The lattice distortion and change in bond-lengths due
to in-complete phase transition are also evident from XRD as well as
Raman spectra analysis, The shoulder centered at ~2.57 eV, the gap
absorption threshold, is driven by Fe1 3d – Fe2 3d interstice electron
transfer.
+
troscopy. With E in the energy scale considered at 0 eV, the fermi en-
v
ergy level is located at 1.17 eV as estimated from VB-XPS data. The other
observed transitions such as 4T2g, 4E(4G) and 4E2(4D) have also been
indicated in the proposed band structure. Besides this, using the two
crystal field transition energies, attempts have been made to estimate
the value of Δ i.e.energy band gap. This process was completed with
0
Tanabe-Sugano diagram for d5 systems [52]. The electron repulsion
The low intensity transitions at 2.65, 2.76 and 3.01 eV can be
parameter Bavg and Δ are found out to be 0.0975 eV and 2.145 eV
0
designated to 6
A
1
( S) → E( G),
6
4
4
6
A
( S) → E
6
4
4
1
2
( D) and oxygen defects
respectively. The calculated band gap for BFO almost matches with the
respectively [37].The presence of oxygen defects has also been evident
from XPS results. The peak around ~3.13 eV is attributed to the charge
transfer (CT) excitations, which is associated with the inter-atomic O 2p
value obtained from UV–Vis studies.
3.6. Photocatalytic activity
–
Fe 3d transition. The occurrence of p-d CT transitions at 3.13 eV is
confirmed by the reported value of ~3.3 eV [51]. A similar kind of
absorption band was also observed for rare-earth substituted BFO
compounds, which indicates that their electronic energy level scheme
looks similar to that of BEFNO [29,31]. Using several correlations and
sub-group analysis, it has been reported that upon symmetry breaking of
The photocatalytic activity of BFO and B0.96Eu0.04Fe1-xNi O (x =
x
3
0.03,0.05,0.07) nanoparticles were studied through the degradation of
2-nitrophenol, which is a very harmful industrial pollutant. Initially, a
blank solution with 50 mg/L of pollutant (2-nitrophenol) in 200 mL of DI
water was irradiated with light for 120 min. To attain the adsorption-
desorption equilibrium, the catalyst was stirred in the pollutant solu-
tion in the dark for 30 min, before being irradiated in the photocatalytic
reactor. As evident from Fig. 9(a), there is almost no degradation seen
high-spin Fe 3 ions from O
+
to C3v (schematically shown in Fig. 8(a)).
h
+
The energy levels of Fe3 free ion (d system) splits into t 23 and e high
5
2
g
g
spin configuration with a crystal field splitting energy seperation of
Δ0 = 10 Dq. In the rhombohedral environment, these energy levels
for the blank solution.With the initial concentration of the pollutant at
′
5
0 mg/L, it was elucidated that the degradation efficiency of all the
7

S.S. Brahma et al.
Journal of Physics and Chemistry of Solids 153 (2021) 110018
Fig. 6. UV–Vis Spectra of BFO and Bi0.96Eu0.04Fe1-xNi
x
O
3
(x = 0.03, 0.05, 0.07) samples. Insets: Band-Gap estimation of BFO and BEFNO-5 samples using Tauc’s Plot,
and Table with the information regarding bandgap and Urbach energy of all the samples.
Fig.7. (a–b): Photoluminescence Spectra and transitions for BFO and BEFNO-5 compounds in the lower and higher energy range respectively. Inset to Fig. 7(a)
Valence band edge calculation from VB-XPS for BEFNO-5 nanoparticles.
substituted nanoparticles is higher as compared to pristine BFO. The
excellent adsorption of the catalyst on all the samples, may be due to the
highly porous nature of the samples, as observed from the FESEM mi-
crographs also. The degradation percentages are calculated for all the
samples and the variation has been shown in the form of a histogram in
Fig. 9(b). The degradation percentage was seen to be increasing on the
substitution of BFO with ‘Eu’ and ‘Ni’ till 5% of Ni doping, to about
The percentage of degradation reduces to 96.78% with 7% of ‘Ni’
doping. So taking the BEFNO-5 sample as the best catalyst, we also
tested its photocatalytic activity with 75 mg/L and 100 mg/L solution.
Fig. 9(c) clearly shows the best results with pollutant concentration of
50 mg/L of solution. With higher pollutant concentration the efficiency
of the catalyst decreases to 89.6% in the case of 75 mg/L and 79.9% in
case of 100 mg/L. The kinetics of the reaction is represented in Fig. 9(d).
The kinetics of reaction fits the Langmuir-Hinshelwood model. It comes
under pseudo-first-order reaction and the rate of the reaction is
9
9.26% in 120 min. Thereafter, on increasing the dopant concentration
there has been a decrease in the photocatalytic activity.
8

S.S. Brahma et al.
Journal of Physics and Chemistry of Solids 153 (2021) 110018
3
+
Fig. 8. (a): Schematic representation of symmetry breaking of Fe ions in BFO from O
h
to C3v phase. (b) Electronic band structure of BEFNO-5 nanoparticles drawn
with the help of VB-XPS and PL Spectroscopy.
Fig. 9. (a) Photocatalytic activity of BFO as well as Bi0.96Eu0.04Fe1-xNi
50 mg/L). (b) Histogram representing the percentage of degradation of BFO as well Bi0.96Eu0.04Fe1-xNi
BEFNO-5 with varying concentration of 2-nitrophenol, and (d) Study of the kinetics of reaction for BEFNO-5 with different concentration of 2-nitrophenol.
x
O
3
(x = 0.03, 0.05, 0.07) as function of irradiation time for the degradation of 2-nitrophenol
(
x 3
O
(x = 0.03, 0.05, 0.07), (c) Degradation efficiency of
calculated using the formula:
lnC /C = kt
Here ‘k’ is the pseudo-first-order rate constant (min 1). The rate
pairs, and (iii) Redox reactions on the surface of the photo-catalyst. As
observed from the graphs, the substituted BFO nanoparticles serve as
better photo-catalyst than the pristine BFO for the degradation of 2-
nitrophenol pollutants. This enhanced property can be understood
based on certain aspects: (i) Decrease in the band-gap and increase in
absorption intensity of the substituted samples which led to the higher
absorption of photon energy, (ii) Decreased recombination of photo-
generated carriers on the addition of dopant ions which act as trap
sites, (iii) Enhancement of surface redox reactions with decreased
crystallite size and increased surface area in the case of substituted
samples, (iv) Better adsorption effects of the pollutants on the surface of
the co-substituted photocatalysts and (v) Narrowing down the surface
charge layer thickness [27,30,53–55].
0
t
(1)
ꢀ
constant calculated for 50 mg/L, 75 mg/L and 100 mg/L are 0.0318
ꢀ 1
ꢀ 1
ꢀ 1
min ,0.0103 min and 0.008 min respectively for BEFNO-5 nano-
particles. So the reaction rate is best in case of 50 mg/L concentration of
the pollutant.
3
.6.1. Photocatalytic response
The photocatalytic oxidation process usually involves three steps: (i)
Absorption of photons with energy greater than the band-gap energy of
ꢀ
+
the photo-catalyst, (ii) Separation or recombination of the e and h
9

S.S. Brahma et al.
Journal of Physics and Chemistry of Solids 153 (2021) 110018
When bismuth ferrite is co-doped with ‘Eu’ and ‘Ni’, the dopant ions
ꢀ
+
act as trapping sites for e and h pairs to facilitate charge separation
and inhibit the recombination process and hence enhance the photo-
catalytic activity of the catalyst. The impurity donor levels of the ma-
terial lying below the conduction band acts as trapping sites for electrons
and when it lies above the valence band edge, electrons can quench the
photogenerated holes [30]. So, the Ni2 dopant ions act as trapping sites
+
for better separation of electrons and holes, those further move towards
⋅‾
•
the surface of the catalyst O to produce reactive O and OH radicals.
2
2
The efficiency of a photocatalyst not only depends on the amount of
photon energy absorbed by the catalyst, but also on the extent of
adsorption of the pollutant on the surface of the photocatalyst. The
energy band-gap of BEFNO-5 being less than pure BFO aids in absorbing
more energy than the parent material. Also the adsorption of pollutants
on the surface of photocatalyst is maximum for BEFNO-5, owing to its
higher porosity as well as increased surface area, hence showing the best
photocatalytic activity. However, on further increasing the dopant
concentration (more than 5%) might have decreased the distance be-
tween the trap sites and thus favoured the recombination process of the
charge carriers rather than the separation. Hence, a dopant amount of
Ni-5% is the optimal concentration that helps to enhance the photo-
catalytic mechanism.
The photo-reactivity and the efficient charge separation and migra-
tion of the charge carriers can also be explained based on ‘space charge
layer thickness’. The thickness of the space charge layer is given by the
formula:
Fig. 10. Schematic representation of the proposed photo-catalytic mechanism
in BEFNO-5.
+
are generated as given in Eq. (3). Holes (h ) in the VB after moving to
[2εV
eN
ε
d
]
1/2
ꢀ
s 0
the surface of the catalyst, can react with water or surface adsorbed OH
W =
(2)
•
to produce OH (Eq. (5) and (6)) or they may directly oxidize the organic
pollutants Eq. (4). Simultaneously, surface adsorbed oxygen can now
Here ‘W’ is the thickness of the space charge layer,
dielectric constants of the medium and air respectively. ‘V
aration potential, ‘e’ is the electronic charge and ‘N ’ is the number of
ε
,
​
and
​
ε
0
are the
•
combine with electrons in the CB to form O
2
-according to Eq. (7). It has
•
s
’ is the sep-
•
‾
been quite broadly recognized that O and OH radicals are extremely
reactive species and actively aid in decomposing the organic pollutants
2
d
dopant atoms So the donor concentration plays a crucial role in opti-
mizing the thickness of the space charge layer. An optimum amount of
dopant concentration increases the surface barrier and narrows down
the space charge layer thickness making it equal to the penetration
depth of the light, subsequently facilitating the separation of charge
carriers for a better photo-catalytic response. From the PL spectra
analysis shown in Fig. 7(a and b), the lower intensity of BEFNO-5 as
compared to pristine BFO also validates lesser chances of charge carriers
recombination and hence better efficiency [21]. However, beyond the
optimum value of donor ions, the space charge layer becomes thinner,
and the penetration depth exceeds the space charge layer, and this
condition favours the recombination of charge carriers resulting in the
decrease of photo-reactivity.
•
•
ꢀ
to produce harmless by-products. Eventually OH and
mineralize the organic pollutants to produce CO , H
harmless in-organic ions as given in Eq. (8) and (9).
O
2
radicals
2
2
O, and other
4
. Conclusion
Europium and Nickel co-doped Bismuth Ferrite nanoparticles were
successfully synthesized using the sol-gel method. Structural studies
using XRD, Rietveld refinement, and Raman spectroscopy have shown
the presence of structural distortion and the presence of rhombohedral
(
R3c) as well as the orthorhombic (Pnma) phase in the doped com-
pounds. A change in bond length and bond-angles and a decrease in
crystallite size was observed on increasing the doping concentration.
The Raman modes of co-substituted samples were found to be with
decreased intensity and shifted, indicating successful substitution and
structural distortion. The microstructural studies using FESEM have
shown agglomerated structures. HRTEM images have well corroborated
the XRD results of mixed phases. The chemical state was understood
using XPS spectroscopy where it has shown the simultaneous presence of
3
.6.2. Proposed mechanism
The schematic representation of the proposed mechanism for the
photocatalytic degradation of 2-nitrophenol using BEFNO-5 as photo-
catalyst is shown in Fig. 10 and the step-by-step reaction mechanism is
as follows:
+
ꢀ
BEFNO + h
ν
→ BEFNO*(h ⋅ VB + e ⋅ CB)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
2+
3+
Fe with Fe ions. It has also given a hint towards the presence of
oxygen defects or vacancies. For detailed optical analysis, we have
carried out the UV–Vis as well as PL spectroscopy. The band-gap of
doped compounds were slightly decreased from 2.17 eV for BFO to 2.12
eV for BEFNO-5 nanoparticles. Thereafter, it slightly increased to 2.16
eV for BEFNO-7. This decrease in band-gap was attributed to the pres-
ence of impurity levels below the conduction band. The PL intensity was
also seen to decrease with doping indicating slowing down of recom-
bination rate. A schematic band structure has been proposed indicating
h+ + 2 ꢀ nitrophenol→oxidised by ꢀ products.
+
O→ OH + H+
⋅
h
+ H
2
ꢀ
+
⋅
OH + h → OH
ads
ꢀ
⋅
ꢀ
e
+ O → O2
2
3
+
⋅OH + 2 ꢀ nitrophenol→CO
the fermi level, E , E and energy levels corresponding to Fe ion in
c v
2
+ H
2
O(by ꢀ products)
O(by ꢀ products)
BEFNO-5 compound. Finally, the efficiency of BEFNO compounds as
photocatalysts were tested through the degradation of 2-nitrophenol. It
was found that efficiency was enhanced after doping with BEFNO-5
giving the best results with 99.26% of degradation efficiency. The
⋅O
ꢀ
+ 2 ꢀ nitrophenol→CO
+ H
2
2
2
When solar light is irradiated on the samples, electron and hole pairs
1
0

S.S. Brahma et al.
Journal of Physics and Chemistry of Solids 153 (2021) 110018
enhancement in photocatalytic activity was maybe attributed to
decreased charge recombination rate (as evident from PL spectroscopy
also) upon the induction of dopants as trap states. On increasing the
pollutant concentration from 50 mg/L to 75 and 100 mg/L, and BEFNO-
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•
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Subhra S. Brahma: Synthesis of Materials.
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Subhra S. Brahma and B. Naik: XPS and HRTEM.
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