Photoelectrochemical properties of the perovskite BaSnO3 synthetized by chemical route. Application to electro-photocatalytic mineralization of ibuprofen

Article abstract of DOI:10.1016/j.jphotochem.2018.06.034

The perovskite BaSnO₃, prepared by chemical route crystallizes in a cubic structure with an average crystallite size of 63 ± 2 nm. The oxide is characterized by scanning electron microscopy, granulometry, optical properties and electrochemistry. The optical properties indicate a direct transition at 3.13 eV. The electrical conductivity follows an exponential law: σ_o exp (-E_a/kT) due to electronic hopping through mixed valences Sn^4+/2+ in agreement with a semi conducting behavior. The intensity potential J(E) curve shows a small hysteresis and exhibits n-type behavior, indicating the presence of oxygen vacancies. The capacitance measurement at pH ～ 5.5 gives a flat band potential of -0.37 V_NHE. As application, the ibuprofen is eliminated by electrocatalysis on BaSnO₃ with a conversion rate of 55% under a current of 150 mA. An increase up to 68% has been reached by electro-photocatalysis under UV illumination; the ibuprofen disappearance obeys to a first order kinetic with a rate constant of 3 × 10^?3 mn^?1 and a reaction mechanism is proposed.

Full text of DOI:10.1016/j.jphotochem.2018.06.034

Accepted Manuscript
Title: Photoelectrochemical Properties of The Perovskite
BaSnO3 Synthetized by Chemical Route. Application to
Electro-photocatalytic Mineralization of Ibuprofen
Authors: A. Sahmi, R. Laib, S. Omeiri, K. Bensadok, M. Trari
PII:
S1010-6030(17)31837-3
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2018.06.034
JPC 11351
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
20-12-2017
17-5-2018
19-6-2018
Please cite this article as: Sahmi A, Laib R, Omeiri S, Bensadok K, Trari
M, Photoelectrochemical Properties of The Perovskite BaSnO3 Synthetized
by Chemical Route. Application to Electro-photocatalytic Mineralization of
Ibuprofen, Journal of Photochemistry and Photobiology, A: Chemistry (2018),
https://doi.org/10.1016/j.jphotochem.2018.06.034
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Photoelectrochemical Properties of The Perovskite BaSnO3 Synthetized by
Chemical Route. Application to Electro-photocatalytic Mineralization of
Ibuprofen
A. Sahmi a1, R. Laib a, S. Omeiri a, K. Bensadok a and M. Trari a
a Laboratory of Storage and Valorization of Renewable Energies, (Faculty of Chemistry, USTHB), BP
32, 16111, Algiers, Algeria
GRAPHICAL ABSTRACT
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(A)
UV light
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(nm)
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 (nm)
The energy diagram of BaSnO3 in ibuprofen solution (pH ~ 5).
The kinetic of ibuprofen mineralization by electrocatalysis ( ), electro-photocatalysis ( )
under UV light in Na2SO4 solution (3.5 g/L), pH  5,  Ibuprofen  = 20 mg/L.
Research highlights
 The photoelectrochemical properties of BaSnO3 prepared by chemical route are
studied.
 The energy diagram shows the feasibility of ibuprofen degradation under UV light.
 The ibuprofen is degraded by photo-electrocatalysis on BaSnO3 with a conversion rate
of 68%.
1 Corresponding author
E-mail address: sahaziz16@gmail.com (A. Sahmi).
Tel. +213 21 24 79 55; Fax. +213 21 24 80 08
1

Abstract
The perovskite BaSnO3, prepared by chemical route crystallizes in a cubic structure
with an average crystallite size of 63 ± 2 nm. The oxide is characterized by scanning electron
microscopy, granulometry, optical properties and electrochemistry. The optical properties
indicate a direct transition at 3.13 eV. The electrical conductivity follows an exponential law:
o exp (-Ea/kT) due to electronic hopping through mixed valences Sn4+/2+ in agreement with a
semi conducting behavior. The intensity potential J(E) curve shows a small hysteresis and
exhibits n-type behavior, indicating the presence of oxygen vacancies. The capacitance
measurement at pH ~ 5.5 gives a flat band potential of -0.37 VNHE. As application, the
ibuprofen is eliminated by electrocatalysis on BaSnO3 with a conversion rate of 55% under a
current of 150 mA. An increase up to 68% has been reached by electro-photocatalysis under
UV illumination; the ibuprofen disappearance obeys to a first order kinetic with a rate
constant of 310-3 mn-1and a reaction mechanism is proposed.
Keywords: perovskite BaSnO3; electrochemistry; ibuprofen; electro-photocatalysis
1. Introduction
There has been a renewed interest in photocatalysis for the stannates ASnO3 where A
is commonly an alkaline earth [1]. They crystallize in the perovskite structure and have found
various applications in optoelectronic [2], dielectric devices [3], gas sensors [4, 5], charge
carrier transporting material in dye sensitized solar cells [6], dielectric response [7],
transparent conducting oxide (TCO) [8] and photocatalysis [9-11]. The electrons are
introduced by oxygen extraction with a decreased carrier effective mass as the impurity band
2

merge with the conduction band leading to the semi conductor absorbing in the visible light
[12, 13].
On the other hand the pharmaceutical industry is responsible of the environmental
pollution and the techniques for the water treatment are continually highlighted. In this regard,
the electro-photocatalysis is an eco-friendly strategy which has not been tested on BaSnO3
and the solar energy is widely deployed for the environmental purification.
BaSnO3 is safe, low cost and chemically stable over the whole pH range. Its synthesis
by chemical route increases the active surface area, resulting in enhanced photo activity
This work covers the physical and photoelectrochemical properties of n-type BaSnO3-
. The ibuprofen (C13H18O2: propionic acid 2-(4-isobutylphenyl)) is a hazardous
pharmaceutical and the oxide is tested with success for the electro-photocatalysis of its
mineralization.
2. Experimental
2.1. Synthesis and characterization of BaSnO3
The electro and photoelectrochemical properties are governed by the preparative
conditions and BaSnO3 was prepared by chemical route. Appropriate amounts of BaCO3
(Merck, 99%) and Sn (purity 99.9% dried under vacuum) were dissolved in HNO3 (65%); the
solution was evaporated in a ventilated place and denitrified until dryness. The powder was
ground, pressed into discs and heat treated at 1040 °C for 6 h (compactness of 95%). The
reduction was realized by heating BaSnO3 discs in glass tube at 600 °C under dynamic
vacuum (< 1 mbar).
X ray diffraction (XRD) was done at a scan rate of 0.05 ° min-1 in the 2 range (20-
80°) with XPERT-PRO diffractometer using Cu k anticathode (= 0.15418 nm). The
morphology of the powder was analyzed by scanning electron microscopy (SEM, Microscope
3

Jeol 6510LV). The zeta potential was determined by measuring the particle velocity induced
under a potential gradient applied across a capillary cell containing BaSnO3 powder
(Zetasizer, Malvern Instruments Ltd., pH 5.5); the granulometry was measured with a particle
by dispersing the powder in distilled water with an ultrasonic bath.
The forbidden band was obtained for the diffuse reflectance using a dual beam UV-
Visible spectrophotometer (Specord 200 Plus). The powder was pressed into pellets and
sintered at 1100 °C. The d.c. conductivity (σ) was measured by the two probe technique using
a homemade equipment. The temperature was recorded by a Cr-Al thermocouple (type K).
The FTIR spectrum was recorded with a Perkin-Elmer spectrometer. The thermo-power (S)
was determined from the ratio (= V/T) V and T are the gradients of voltage and
temperature respectively.
The pellets were encapsulated in glass holders and the electrode surface was polished
with emery papers. The electrochemical properties were studied at ambient temperature in a
standard cell containing a saturated calomel electrode (SCE) and an auxiliary Pt electrode, all
potentials were reported with respect to a normal hydrogen electrode (NHE). The intensity-
potential J(E) curves and electrochemical impedance spectroscopy (EIS) were carried out in
Na2SO4 solution (10-2 M) using a PGZ301 potentiostat (Radiometer analytical); the potential
scan was fixed at 10 mV s-1. The electro oxidation was performed with DAZHENG DC
power supply PS-305D, the current was regulated at 150 mA under a voltage of 25 V. The
photocatalytic oxidation of ibuprofen was realized in an open Pyrex cell under UV light (LED
lamp) with a maximum emission at 470 nm. The degradation of ibuprofen solutions were
analyzed with a UV-Visible spectrophotometer (Optizen 2120 UV–Visible) while the
mineralization was controlled by chemical oxygen demand (COD) using a HACH DR2800
analyzer. The residual COD correspond to the amount of ibuprofen solution, not eliminated
by electrocatalysis:
4

COD (%) = (CODo – CODt) / CODo
(1)
where CODo is the initial COD (= 50 mg/L) and CODt the COD at time t. All solutions were
prepared with distilled water and high purity reagents.
3. Results and discussion
3.1 Characterization
The stannates have gained a great interest in the solar energy conversion [14] and the
preparation by chemical route has enjoyed a continuous attention because of the low energy
consumption. According to the JCPDS N° 15-780, all XRD peaks (Fig. 1) belong to the
perovskite BaSnO3 phase and are indexed in a cubic unit cell with a lattice constant of
0.4117(2) nm, close to that reported elsewhere [ 15, 16]. After reduction, BaSnO3- exhibits a
blue color and the XRD pattern (Fig. 1) is indexed in a cubic unit cell with a tolerance factor t
(1.01: considering Shannon ionic radii). This is due to the formation of Sn2+ (rSn2+ = 0.110
nm) in six fold coordination, generated by a charge compensation mechanism, greater than
Sn4+ (rSn4+ = 0.069 nm) [17].
The oxide was synthesized by chemical route and the peaks are slightly broadened,
indicating smaller crystallites. The crystallite size (49 nm), estimated from the broadening ()
of the intense XRD peak (110) ( = 0.94/  cos), agrees with that determined by Zetasizer
(63 nm). The SEM image of the oxide (Fig. 2) shows a compact structure with a rough surface
and a weak porosity. The morphology consists of regular grains formed by many crystallites
with an average size of ~ 4 µm, a small size distribution (200 – 400 nm) and a zeta potential
of -10 mV (SM 1).
The strong peak at 644 cm-1 in the FTIR spectrum is due to stretching vibration of
SnO6 octahedron [18]. The stretching modes of O–H peak (3145 cm-1) is assigned to
physisorbed water on the oxide surface while the intense band at 1447 cm−1 is due to
5

carbonate while the weak peak at ~1600 is due O-H vibration [19], indicating a low
hygroscopy (SM 2A).
The optical absorption coefficients () and the incident photon energy (h) are related
by the relation:
(αhν)n = Const × (hν – Eg)
(2)
where the exponent n is function of the transition type, 0.5 and 2 respectively for indirect and
direct transitions. The extrapolation of the straight line (h)2 to the abscissa axis gives a
direct gap (n= 2) of 3.13 eV [20] (Fig. 3A) close to that obtained by some of us [21].
Pure BaSnO3 is an insulator with an electrical conductivity smaller than 10-6 -1 cm-1
and the conduction is due to a slight deviation from stoichiometry. The oxide is electrically
neutral and oxygen vacancies are formed with reduction of an equivalent quantity of Sn4+to
Sn2+ to maintain the electro-neutrality [22]:
BaSnO3  BaSnO3- + 2  [e- (CB)] +/2 O2
(3)

The defect of oxygen is expressed by the Kröger and Vink Notation:
Oo 12 O2 + Vö + 2e−
(4)
The electrical conductivity obeys to an exponential law with activation energy of 0.44 eV
(Fig. 3B) which is different from that published elsewhere [23], since the electrical
conductivity depends on the synthesis method. The conduction occurs by electron jump
between mixed valences Sn4+/2+ arranged in octahedra sharing common corners.
BaSnO3 is used in photocatalysis and the detailed photoelectrochemical
characterization has been reported by some of us [24, 25]. The cyclic voltammetry (Fig. 4)
shows an anodic peak at ~ 0.75 V, attributed to the internal oxidation Sn4+/Sn2+ potential. The
irreversibility of the sluggish system is evidenced by the reduction peak (-0.3 V) upon the
reverse scan. The water decomposition voltage is higher than 2.45 V, indicating a high O2-
over potential. Below ~ 0 V, the current increase significantly, due to hydrogen liberation and
6

one can predict that BaSnO3- can be used ad H2-electrode under UV light. The electrical

charge under the anodic peak, measured by integration (~ 33 μC) corresponds to the oxidation
of 2.1×1017 at. Sn2+.
Because of the n type conduction (SM 2B), the photocurrent (Jph) appears when the
electrode is polarized above the flat band potential (Efb). The latter (0.45 V) is accurately
determined from the capacitance (C) of the space charge layer (Fig. 5)
2E − Efb
eεε0ND
−2
C
= ±
(5)
SC
Where ε, εo, ND, e and E are respectively to dielectric constant of vacuum and BaSnO3, the
electrons density, the elementary charge and the applied potential; the sign + or – is for n- or
p- type specimen. The positive slope (C-2 – E) confirms the n-type conduction; the density ND
(8.71018 cm-3) and the potential Efb (-0.85 V) are obtained from the slope (C-2 - E) and
extrapolation with the potential-axis respectively. The Efb value predicted from the relation of
Butler [26] was found to be -0.71 V, such discrepancy is due to the existence of O2-: 2p and
Sn4+: 4s sub band-gap states within the gap region and indicate that the empirical relation
gives rather a rough value. Taking into account the charge neutrality, the electrochemical
reaction can be written Ba2+Sn4+ Sn2+ O3- .
1-



The energy of the conduction band (퐸푐0,푠) is found to be 4.60 ± 0.1 eV (= e  Efb + Ea -
Eg), the conduction band 퐸푐0,푠 derives from Sn4+: 5s wave function with a high reducing power
and corresponds to a potential of 0.1 V [27]. Therefore, the valence band (VB), located at 7.75
eV (= ECB - Eg) is made up of O2-: 2p orbital. Such electrochemical parameters are favourable
for applications of BaSnO3 in the water treatment under visible illumination (Fig. 6). The
small current in the dark and the low conductivity are characteristic of a weak doping density
and a wide depletion width ( ~ 17 nm):
 = {2  o E / e ND}0.5
(6)
where E is the optimal interfacial band bending for the charge separation [28]. The
crystallite is auto biased and the free potential (Uf) of n- BaSnO3 is more positive than the
potential Efb, so the electrons are transferred from BaSnO3-CB to oxygen in solution to

generate radicals O2 .
7

The EIS gives a correlation between the microstructure and interfacial reactions and
quantifies the contributions of the bulk, grains boundaries and diffusion. The arc in the (Zim –
Zre) plane is centered below the real axis, with the existence of a constant phase element
(CPE) indicating a deviation from a pure capacitor defined as ZCPE = [C(j)m]-1, the parameter
m (-1<m < 1) is due to a rough electrode and/or defects states in the gap region (Fig. 7) [29].
As expected, the diameter of the arc decreases from 7.11 (kΩ cm²) in the dark down to
3.52 (kΩ cm²) upon irradiation because of the semi conducting behavior of BaSnO3. The
offset from the origin (400  cm2) is due to the ionic conductivity (∞) of Na+ (50 Ω-1 cm²
2-
mol-1) and SO4 (160 Ω-1 cm² mol-1) and does not change significantly under light (361 Ω-1
cm²). The Bode plots in the dark and under UV light are reported in supplementary material
(SM 3). The straight line at 450 observed at low frequencies is due to a Warburg diffusion of
electro active species.
3.2 Electrocatalysis
Because of the high O2 over potential, the anodic oxidation occurs through radicals on
the perovskite BaSnO3 under high voltage [30, 31]:
M + H2O → M (OH)ads + H+ + e- (2.27 V)
(7)
Ibuprofen is a recalcitrant organic compound stabilized by a mesomeric effect and the
radical M(OH) adsorbed on the surface is a strong oxidant which mineralizes the molecule
the to a high extent, a conversion rate of 55% has been reached (Fig. 8AC). It is helpful to
outline that the 150 mA is the limiting current and BaSnO3 cannot support higher currents.
3.3 Electro-photocatalysis
The electro-photocatalysis is an emerging alternative for the mineralization of
hazardous organic pollutants and is widely used for the water depollution [32, 33]. The oxides
are attractive as photocatalysts because of their semi conductivity and chemical stability.
Tests of stability realized during one year indicated no deterioration in the pH range (1 – 14)
8

and no Ba2+ was detected by chemical analysis. Moreover, the photocatalytic applications
need a medium conductivity with a wide depletion width in order to collect most incident
-

photons [34]. The electrons in the conduction band of BaSnO3 generate O2 and H2O2, the

latter acts as electron acceptor, to form OH used for the oxidation of ibuprofen. Moreover,


the potential of the OH/H2O couple (2.27 V), allows to produce OH radicals (reaction 11)
induced in the mineralization of ibuprofen, covalently bonded to BaSnO3 [35]:
-
-

O2 ads + e  O2
(8)
(9)
2 O2 ads + 2 H3O+ + 2 e-  H2O2 + O2 + 2 H2O
H2O2 + e-  OH + OH-
(10)
(11)
(12)
(13)
H2O + h+ → OH + H+
2 H+ + 2 e- → H2
-

Ibuprofen + (O2 , OH) → Degradation products
An enhanced activity up to 68% is reached by electro-photocatalysis under UV irradiation
(Fig.8BC). The degradation kinetic of ibuprofen obeys to a first order model with a rate
constant of 3 10-3 min-1, about 1.4 fold greater than that obtained by electrocatalysis (2.110-
3
min-1) (Fig. 8D) [36]. It is helpful to outline that hydrogen can compete with the O2

formation because its potential (~ -0.5 V) is less cathodic than BaSnO3-CB. The detailed
investigation by chromatography will be reported in the next future.
4. Conclusion
The transport properties and photo electrochemical characterization of the perovskite
BaSnO3- prepared by chemical route were studied. The phase purity was checked by X-ray
diffraction, revealing a cubic structure with an average crystallite size of 63 nm. The electro
neutrality is assured by reduction of Sn4+ ions into Sn2+. The oxide acquired semi conducting
with a valence compensated system. BaSnO3 exhibits a direct optical transition in the UV
region and n-type conduction, due to oxygen deficiency. The photo-electrochemical
9

properties of BaSnO3 were demonstrated. The intensity potential characteristics showed a
small hysteresis while the Mott-Schottky plot indicated n-type behavior. The electrochemical
impedance spectroscopy shows the predominance of the bulk contribution. The interfacial
reactions were predicted for the energy diagram of the junction BaSnO3/ibuprofen solution,
the latter was established for the optical and electrochemical characterizations. As application,
the ibuprofen was mineralized by electrocatalysis on the perovskite BaSnO3; an enhancement
up to 68% was obtained by electro-photocatalysis under UV light and the elimination follows
obeys to a first order kinetic.
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11

110
16.0k
8.0k
0.0
30
40
50
60
70

Fig. 1. XRD pattern of BaSnO3 prepared by thermal decomposition.
Fig. 2. Low magnification FE-SEM image of rice-like BaSnO3 nanocrystals with EDX
composition.
12

-2
-3
-4
-5
-6
-7
200
150
100
50
(B)
(A)
Ea = 0.44 eV
Eg = 3.13 eV

0
2.5
2.0
2.5
3.0
3.5
3.0
3.5
4.0
1000/T (K-1)
h(eV)
Fig. 3. (A) The direct optical transition of BaSnO3 (n= 2). (B) Variation of Log  as a
function of 1000/T.
13

200
0
-200
-400
-600
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
Potential [V]
Fig. 4. Cyclic voltammetry of BaSnO3 electrode in Na2SO4 solution (3.5 g/L, pH ~ 5,
ibuprofen= 20 mg/L) at a scan rate of 10 mV s−1.
0.15
18000
(A)
0.10
12000
E
= -0.37 V
fb
R2= 0.99
0.05
6000
0
Eon = -0.55 V
R2 = 0.99
0.00
-1.00
-0.8
-0.6
-0.4
-0.2
0.0
-0.75
-0.50
-0.25
0.00
potential (V)
Potential (V)
Fig. 5. (A) The photocurrent Jph2 against the applied potential of BaSnO3 electrode (pH ~ 5)
according to the Gartner model. (B) The Mott-Schottky characteristic.
14

Potential (VNHE
-4.5
)
Energy (eV)
0
-
O2
O2
CB = -0.81 V
e-
H2O/H2 (-0.5 V)
Fig. 6. The energy diagram of BaSnO3 in ibuprofen solution (pH ~ 5).
OH
VB = 2.32 V
H2O
h+
O 2-: 2p
O2/H2O (> 2.3 V)
15

25
20
15
10
5
CPE2
R gb
CPE1
Rel
R Bulk
W
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Under illumination
0
1
2
3
4
5
6
7
2
Z
real(Kcm )
0
10
20
30
40
50
Zreal (k cm2)
Fig. 7. The Nyquist plots in the dark and under illumination of BaSnO3. Inset: the equivalent
electrical circuit.
16

0.8
0.6
0.4
0.2
0.0
(A)
0 min
60
120
180
240
300
360
200
220
240
260
280
(nm)
0.8
(B)
UV light
0.6
0.4
0.2
0.0
0 min
60
120
180
240
300
360
200
220
240
260
(D)
280
(nm)
80
60
40
20
0
1.2
0.9
0.6
0.3
0.0
(C)
2
R
= 0.98
2
R
= 0.91
0
100
200
300
400
0
100
300
400
Time20(0min)
Time (min)
17

Fig. 8. The kinetic of ibuprofen mineralization by electrocatalysis ( ), and electro-
photocatalysis ( ) under UV light in Na2SO4 solution (3.5 g/L), pH5,  Ibuprofen =
20 mg/L.
18