Assessment of divergent functional properties of seed-like strontium molybdate for the photocatalysis and electrocatalysis of the postharvest scald inhibitor diphenylamine

Article abstract of DOI:10.1016/j.jcat.2017.06.001

The bifunctional activity of strontium molybdate (SrMoO₄) in the photodegradation and electrocatalytic determination of diphenylamine (DPAH) was identified and demonstrated. The photocatalytic and electrocatalytic activity of SrMoO₄ were influenced by its structural properties. Those structural properties are scrutinized by various physical spectroscopic and microscopic tools. In this work, we mainly concentrated on two phenomena of SrMoO₄, the photon absorption and electrochemical behavior. UV–visible spectroscopy was used to assess the photon absorption characteristics of SrMoO₄, which mostly generates OH^[rad] radicals for the degradation of DPAH. The rate of photodegradation was demonstrated in terms of irradiation time, pH, catalyst loading, and initial concentration. On the other hand, voltammetry was used to evaluate the electrochemical behavior of SrMoO₄. The overall reaction pathways of DPAH showed that the major contribution of the reaction is generation of radical cations (DPAH^[rad]+). Therefore, we can determine the concentration of DPAH by measuring DPAH^[rad]+. Differential pulse voltammetry is a suitable analytical tool to measure DPAH^[rad]+, which determined the DPAH in the linear range 0.1–35?μM and with a lowest detection limit of 30?nM. One of the greatest challenges of this determination is the selectivity, because DPAH^[rad]+ is highly reactive toward similar functionalities such as anions and cations. Therefore, we carefully investigated and discussed the selectivity in the presence of interfering compounds.

Full text of DOI:10.1016/j.jcat.2017.06.001

Journal of Catalysis 352 (2017) 606–616
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Assessment of divergent functional properties of seed-like strontium
molybdate for the photocatalysis and electrocatalysis of the postharvest
scald inhibitor diphenylamine
a
a
a,
⇑
b
c
Raj Karthik , Natarajan Karikalan , Shen-Ming Chen , Jeyaraj Vinoth Kumar , Chelladurai Karuppiah ,
b
Velluchamy Muthuraj
a
Electroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Section 3,
Chung-Hsiao East Road, Taipei 106, Taiwan, Republic of China
b
Department of Chemistry, VHNSN College, Virudhunagar 626001, Tamil Nadu, India
Battery Research Center of Green Energy, Ming Chi University of Technology, New Taipei City 24301, Taiwan, Republic of China
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The bifunctional activity of strontium molybdate (SrMoO ) in the photodegradation and electrocatalytic
determination of diphenylamine (DPAH) was identified and demonstrated. The photocatalytic and elec-
4
Received 16 April 2017
Revised 31 May 2017
Accepted 1 June 2017
trocatalytic activity of SrMoO
are scrutinized by various physical spectroscopic and microscopic tools. In this work, we mainly concen-
trated on two phenomena of SrMoO , the photon absorption and electrochemical behavior. UV–visible
4
were influenced by its structural properties. Those structural properties
4
spectroscopy was used to assess the photon absorption characteristics of SrMoO
ates OH radicals for the degradation of DPAH. The rate of photodegradation was demonstrated in terms
of irradiation time, pH, catalyst loading, and initial concentration. On the other hand, voltammetry was
4
, which mostly gener-
Keywords:
Carbazole
Å
Radical cations
Interference
Photocatalysis
Electrocatalysis
Strontium molybdate
used to evaluate the electrochemical behavior of SrMoO
4
. The overall reaction pathways of DPAH showed
Å+
that the major contribution of the reaction is generation of radical cations (DPAH ). Therefore, we can
Å+
determine the concentration of DPAH by measuring DPAH . Differential pulse voltammetry is a suitable
Å+
analytical tool to measure DPAH , which determined the DPAH in the linear range 0.1–35 mM and with a
lowest detection limit of 30 nM. One of the greatest challenges of this determination is the selectivity,
Å+
because DPAH is highly reactive toward similar functionalities such as anions and cations. Therefore,
we carefully investigated and discussed the selectivity in the presence of interfering compounds.
Ó 2017 Elsevier Inc. All rights reserved.
1
. Introduction
investigated under various precipitation conditions [9]. In addition,
various synthesis methodologies have been developed to control
Over the past two decades, the alkaline-earth metal molybdates
AMoO ; A @ Mg, Ca, Sr, and Ba) have been extensively studied in
terms of structure and physicochemical properties. Mostly, the
scheelite-type structure (bivalent cations) exhibits excellent
strength, high decomposition temperature, and chemical and ther-
its morphology, such as co-precipitation [10], microwave irradia-
tion [11], hydrothermal methods [12], microwave hydrothermal
preparation [13], and solid-state metathetic synthesis [14]. The
detailed structural studies are well documented but the applica-
(
4
4
tions of SrMoO have not yet been studied. Hence, the present
4
mal stability [1]. In particular, calcium molybdate (CaMoO ) was
study concentrates on the electrochemical and photochemical
focused on due to its admirable photoluminescence properties,
and it has been used as an anode material for Li ion batteries,
solid-state lasers, solar cells, scintillators in medical devices, and
fiber optic communications [2–6]. However, strontium molybdate
applications of SrMoO
are investigated for the [MoO
light energy in the UV region and possibly contributes to
photoluminescence properties [6]. On the other hand, the
4
. The photochemical properties of SrMoO
4
2ꢀ
4
]
tetrahedral unit, which absorbs
(
SrMoO
4
) has received little attention. Past studies documented its
electrochemical properties of SrMoO
4
are inferred from the charge
2
+
2ꢀ
structural evolutions by high-pressure X-ray diffraction and
neutron diffraction patterns [7,8]. Also, its crystal growth has been
distribution between Sr and [MoO
facts, we have applied SrMoO
4
]
units. Inspired by these
4
to the electrochemical determina-
tion and photocatalytic degradation of diphenylamine (DPAH).
DPAH is a derivative of aniline and is mostly used as a posthar-
vest scald inhibitor for apples and pears because of its excellent
⇑
Corresponding author.
E-mail address: smchen78@ms15.hinet.net (S.-M. Chen).
http://dx.doi.org/10.1016/j.jcat.2017.06.001
021-9517/Ó 2017 Elsevier Inc. All rights reserved.
0

R. Karthik et al. / Journal of Catalysis 352 (2017) 606–616
607
antioxidant properties, which protect the skins of apples and pears
by preventing the oxidation of -farnesene in cold storage [15].
photoelectron spectra (XPS) of SrMoO
4
were obtained using a
a
ULVAC-PHI 5000 Versa Prob X-ray photoelectron spectrometer.
The surface morphology was probed by transmission electron
microscopy (TEM- TECNAI G2). Scanning electron microscopy
(SEM) and energy dispersive X-ray spectral studies were carried
out with a Hitachi S-3000 H scanning electron microscope (SEM
Tech Solutions, USA) and HORIBA EMAX X-ACT, respectively. The
electrocatalytic behavior and the detection of DPAH were per-
formed by cyclic voltammetry (CV) and differential pulse voltam-
metry (DPV) using CHI 405a and CHI 900 (CH Instruments, USA).
A conventional three-electrode system was used for electrocat-
alytic studies, where the modified GCE is the working electrode
Industrially, there are several processes using DPAH, despite its
low solubility, because of which it remains on the skins of apples
and pears, and thus persists in fruit juices. Further, wastewater
from storage factories is highly contaminated with DPAH and its
derivatives. DPAH damages red blood cells, which causes erythro-
poiesis and hemosiderosis [16]. Therefore, DPAH is listed as a pri-
ority pollutants by the European Union (EU); EU directive 91/414/
EEC establishes a maximum allowed concentration of DPAH of 5–
ꢀ1
1
0 mg kg [17]. Hence, the determination of DPAH is of consider-
able importance.
2
Hitherto, limited analytical procedures have been developed to
determine DPAH, such as gas chromatography, high performance
liquid chromatography, mass spectrometry, and spectrophotome-
try [18–21]. These techniques are quite costly and need instru-
ments operated by highly skilled technicians. Electrochemistry
offers an alternative method to determine the DPAH. To the best
of our knowledge, there is only one report available for the electro-
chemical determination of DPAH by molecularly imprinted poly-
mers [22]. Herein, we have developed an effective and efficient
(0.07 cm ), platinum wire is the auxiliary electrode, and Ag/AgCl
is used as a reference electrode.
2.3. Preparation of seed-like SrMoO
4
The seed-like SrMoO
method. In a typical synthesis, 40 mL of 0.2 M Na
and stirred at 1000 rpm and 40 mL of 0.1 M SrCl
stirred solution of Na MoO . This mixture was then stirred for 1 h.
The clear solution of Na MoO was completely turned into a white
precipitate while SrCl solution was added. The precipitate was
washed with copious amounts of water and absolute ethanol and
4
was prepared by the simple precipitation
MoO was taken
was added to the
2
4
2
2
4
4
sensing platform to determine DPAH using SrMoO catalyst.
2
4
The photodegradation of DPAH has been investigated in various
pH solutions and for different irradiation times. Here also, only one
report is available for the mechanistic investigation of DPAH degra-
dation by a mixture of acetonitrile and DPAH [23]. As described in
the literature, most of the reaction proceeds through OH radicals
by which the oxidation of DPAH yields radicals on an aromatic ring.
These radicals are further delocalized to produce aniline-type rad-
icals. This reaction is initiated through radical formation on the
aromatic ring or the aromatic amine group. Afterward, these radi-
cals are rearranged to form a covalent bond between two aromatic
rings or to initiate degradation of DPAH to other byproducts. The
2
dried overnight at 80 °C. Finally, the dried SrMoO
at 500 °C for 2 h.
4
was annealed
2.4. Fabrication of modified electrodes
Before modification, the GCE was polished with 0.05 mm alu-
mina slurry and washed with several amounts of DD water to
remove the alumina particles on the GCE surface. The as-
prepared seed-like SrMoO
icated for 20 min. After that, about 8 mL of seed-like SrMoO
4
was redispersed in DD water and son-
sus-
authors monitored direct photolysis by
a liquid chromato-
4
graphic–mass spectrometric technique and analyzed the byprod-
ucts. Finally, they concluded that the major degradation product
is carbazole (CBZ). Following their work, we investigated the pho-
todegradation of DPAH by time-dependent UV–visible spectra and
pension was drop coated on the surface of GCE (GCE working
2
area = 0.07 cm ). The drop-coated electrodes were allowed to dry
at room temperature and then gently rinsed with water to remove
the loosely bound particles. These modified electrodes were fur-
ther used for the electrochemical characterizations.
proposed a photocatalytic mechanism for SrMoO
profile. This heterogeneous photocatalyst generates the OH radi-
cals that are the primary active species targeting the DPAH.
4
by the scavenger
Å
2
.5. Photocatalytic activity of SrMoO
4
2
. Experimental
The photocatalytic activity of the as-synthesized SrMoO for the
4
degradation of DPAH solution under visible light irradiation was
evaluated. For these photocatalysis measurements, our previously
reported procedure was followed with a slight modification
[24,25]. In a typical procedure, 50 mg of the catalysts was dispersed
in 100 mL of DPAH solution (20 mg/L) and stirred for 2 h under dark
conditions to reach adsorption–desorption equilibrium between
the DPAH solution and the catalyst. After that, the suspension was
2
.1. Materials
Strontium chloride (SrCl
2
), sodium molybdate (Na
11N), magnesium chloride (MgCl
sodium chloride (NaCl), calcium chloride (CaCl ), urea (CH
diuron (C O), carbofuran (C12 ), chlorpyrifos
PS), and all other chemicals were purchased from
2
MoO
4
),
),
diphenylamine (DPAH, C12
H
2
2
4 2
N O),
9
H
10
l2
C N
2
H15NO
3
(
C
9
H
11
C
l3NO
3
irradiated with light: a 500 W tungsten incandescent lamp
Sigma-Aldrich and Alfa Aesar companies and used as received
without further purification. A phosphate buffer (PB) electrolyte
solution was prepared by using a mixture of monosodium phos-
(k > 400 nm) was used as the light source. At 5 min time intervals,
4 mL of the suspensions was collected and the concentration change
of DPAH was monitored by a UV–vis spectrophotometer. In a recycle
test, the seed-like SrMoO4 photocatalyst was separated from the
reaction mixture by centrifugation after the photodegradation
experiments and washed with water, and then dried and used again.
2 4 2 4
phate (NaH PO ) and disodium phosphate (Na HPO ). All required
solutions were prepared using double-distilled (DD) water.
2.2. Apparatus and electrochemical measurements
3
. Results and discussion
4
The structural characterization of seed-like SrMoO was carried
out by powder X-ray diffraction (XRD) on a Rigaku MiniFlex II
instrument. Fourier transform infrared spectroscopy (FT-IR) was
performed using an FT/IR-6600 spectrophotometer. The Raman
spectrum was obtained using a Raman spectrometer (Dong Woo
3.1. Characterizations
The structure and crystal lattice parameters of the as-prepared
4
SrMoO were examined by XRD pattern analysis and are shown in
5
00i, Korea) equipped with a charge-coupled detector. The X-ray
Fig. 1A. The obtained discrete peaks at 18.0°, 27.6°, 29.7°, 33.2°,

6
08
R. Karthik et al. / Journal of Catalysis 352 (2017) 606–616
4
Fig. 1. The X-ray diffraction patterns (A), Raman spectrum (B), UV–DRS (C), and Tauc plot (D) of SrMoO .
3
8.1°, 45.1°, 47.6°, 51.5°, 55.9°, 57.1°, 61.7°, 69.7°, 71.8°, and 72.7°
correspond to the (1 0 1), (1 1 2), (0 0 4), (2 0 0), (1 1 4), (2 0 4),
2 2 0), (1 1 6), (3 1 2), (2 2 4), (0 0 8), (4 0 0), (2 0 8), and (3 1 6)
tal field effect, these all degenerative vibrations are split into 26
different types of vibrations (U = 3A + 5A + 5B + 3B + 5E + + E )
g u g u g u
[28].
(
planes, respectively. All the observed peaks are closely matched
with the reference peaks of JCPDS file No: 08-0482, which suggests
In all, 13 vibrations are Raman active. The internal modes
(A ), (E ), (E ), and (B ) are observed at 890, 850, 802,
380, 362, and 320 cm . The free rotation mode and the external
m
1
g
m
2
g
m
3
g
m
4
g
ꢀ1
that SrMoO
and c = 12.01 Å. No additional peaks were observed for the impuri-
ties, which means there is no individual SrO or MoO present in
SrMoO [26]. The complete crystal structure of SrMoO consists
4
crystallizes in tetragonal phase with a = b = 5.406 Å
ꢀ1
modes were detected at 200 and at 162 and 114 cm , respectively.
ꢀ1
3
The peak at 802 cm
indicates doubly bonded oxygen, which
4
4
means the asymmetric stretching vibration of OAMoAO in the
2ꢀ
of Sr, Mo, and O, where the Mo ions coordinate with the O ions
tetrahedrally and the Sr ions coordinate with the O ions in polyhe-
dral arrangements [8,26,27]. The structure was split into two
4
[MoO ]
tetrahedron. There is no peak for the singly bonded oxy-
gen MoAO stretching vibration. However, a peak appears at
ꢀ1
u
380 cm for the bending vibration of MoAO of the A mode. This
2
ꢀ
2+
groups, internal and external; the internal group is [MoO
4
]
and
units [27]. The Raman
spectra clearly distinguish these two groups by active vibration
modes. Fig. 1B shows the Raman spectrum of SrMoO excited by
a 531.8 nm laser. The observed vibrations are associated with the
Raman result shows that the motion of Sr is restricted in the rigid
2
+
2ꢀ
2ꢀ
the external group is Sr in rigid [MoO
4
]
4
[MoO ]
groups [28]. This phenomenon greatly reduces the band
2
+
gap of MoO
of SrMoO
spectra (UV–DRS) spectra using the Tauc plot. Fig. 1C and D show
3
when Sr is introduced into its lattice. The band gap
4
4
is calculated from the UV-visible diffuse reflectance
2
ꢀ
internal (related to the [MoO
4
]
stationary mass center) and
external groups (related to the motion of Sr in the rigid [MoO
4
the UV–DRS spectra and Tauc plot for SrMoO , which shows the
2+
2-
4
]
calculated band gap of SrMoO to be 3.1 eV.
4
ꢀ
2ꢀ
groups). When the free space is considered, [MoO
in tetrahedral (T ) symmetry and composed of four internal modes,
one free rotation mode, and one translation mode. Due to the crys-
4
]
is present
Further, the elements and their oxidation states in SrMoO
examined by X-ray photoelectron spectroscopy (XPS). Fig. 2A
shows the XPS survey spectra of SrMoO , which reveal the respec-
4
were
d
4

R. Karthik et al. / Journal of Catalysis 352 (2017) 606–616
609
4
Fig. 2. The XPS survey spectrum (A) and the high-resolution XPS spectra of Sr3d (B), Mo3d (C) and O1s (D) of SrMoO .
tive signals for Mo, Sr, and O, which agreed closely with EDAX anal-
ysis (Fig. 3F). Fig. 2C shows the high-resolution XPS spectra of
Mo3d, which are further deconvoluted into four peaks for the +5
and +6 oxidation states. The peaks at 232.2 and 235.3 eV corre-
The HBEC contribution suggests that the loosely bound oxygen
can be removed from the SrMoO during annealing. This will create
an oxygen deficiency throughout the molecule that leads to the
lower oxidation state of Mo (Mo ). These oxygen vacancies can
create free carriers such as electrons, which may be the reason
4
5
+
6
+
6+
spond to the Mo 3d5/2 and Mo 3d3/2, suggesting stoichiometric
MoO [29]. However, the peaks at 230.1 and 233.2 eV are attribu-
ted to Mo 3d5/2 and Mo 3d3/2, suggesting the presence of Mo
3
for the semiconducting behavior of SrMoO
surface features of the as-prepared SrMoO
and TEM. Fig. 3A and B show the low- and high-magnification
SEM images of SrMoO , which portray the distinct seed-like mor-
phology at the micrometer scale. This microsize feature is experi-
enced from the high crystallinity of SrMoO with bulk cations (Sr
and Mo). The highly crystalline nature of SrMoO results in excel-
lent photocatalytic activity. Moreover, the TEM images (Fig. 3C)
show that the as-prepared SrMoO has semitransparency to elec-
trons. This semitransparent behavior of SrMoO was achieved
through the annealing process. Fig. 3D shows high-
magnification TEM image of SrMoO , which clearly shows the lat-
4
[31,32]. Finally, the
5
+
5+
4
O
11
4
were probed by SEM
as a dominant phase. In this case, the oxidation state of Mo shuttles
between +6 and +5. Here, the lower oxidation state of Mo is expe-
4
2+
rienced from the dangling bonding site of Sr . The SrO
8
polyhe-
ions are trigonally bonded with rigid tetrahedral
molecular units. This is further inferred from the high-
2
ꢀ
dra’s
MoO
O
4
ꢀ
[
4
]
4
resolution Sr3d spectra (Fig. 2B). The centralized peak at 132.5 eV
contains 3d5/2 and 3d3/2, which suggest a single SrAO bond. Fur-
thermore, the high-resolution O1s spectra have been taken into
account (Fig. 2D).
The O1s spectra were deconvoluted into three peaks, which
consist of three different bonding environments between Sr and
Mo. The peak profile can be fitted as a low-binding-energy compo-
nent (LBEC) and a high-binding-energy component (HBEC) [30].
4
4
a
4
tice patterns for the crystalline planes; the observed d-spacings
between the planes are perfectly matched with the XRD results.
From the TEM images, the length and width of the SrMoO parti-
4

6
10
R. Karthik et al. / Journal of Catalysis 352 (2017) 606–616
Fig. 3. The high-magnification (A) and low-magnification (B) SEM images of seed-like SrMoO
distribution (E) and the EDAX spectrum of as-prepared seed-like SrMoO (F).
4 4
and the HRTEM images (C and D) of SrMoO . The histogram of the particle size
4
cles were calculated as 1.5 and 0.5 mm, respectively. Moreover, a
statistical analysis was performed for the particle distribution
and the data are given in Fig. 3E. The calculated particle sizes are
based on the diameter of the seeds, where the maximum diameters
of the particles are distributed between 0.35 and 0.75 mm.
3.2. Photocatalysis
The photocatalytic degradation of DPAH on SrMoO
gated for various DPAH solutions under visible light irradiation.
The sequential changes in the DPAH concentrations are scrutinized
4
is investi-

R. Karthik et al. / Journal of Catalysis 352 (2017) 606–616
611
by observing the absorption variations in UV–vis spectra at 280 nm
23]. Prior to the study of the photocatalytic activity, blank (with-
different catalyst loadings ranging from 0.25 to 1 mg/mL, while the
concentration of DPAH and light sources remained constant.
Fig. 4C shows the effect of catalyst dosage. The 0.5 mg/mL pho-
tocatalyst gave the best activity with 99% degradation efficiency.
The remaining dosages are suffering from the recombination of
holes and electrons because the effective diffusion of photons is
blocked by a high concentration of the photocatalyst [24,25].
Hence, the generations of holes and photons is insufficient when
the amount of photocatalyst is increased. Meanwhile, at the lowest
dosage, the holes and electrons are completely used by the DPAH;
thus a high concentration of DPAH seeks more photoactive species.
This also leads to decreasing photocatalytic activity; therefore, the
suggested photocatalyst dosage is 0.5 mg/mL for the further exper-
iments. The photodegradation rate directly depends on the initial
concentration of the DPAH. Therefore, the photodegradation of
DPAH is investigated at concentrations of DPAH ranging from 15
to 30 mg/L with a constant catalyst dosage and light source
(Fig. 4D).
[
out catalyst) and dark (in the absence of light) experiments are
evaluated. Fig. 4A shows the photodegradation of DPAH without
catalyst and the absence of light experiment, which shows that
the photocatalytic activity is negligible. Therefore, photolysis of
DPAH in the dark or without catalyst is not beneficial; hence fur-
ther experiments proceed with light and catalyst. Fig. 4B shows
the time-dependent UV–vis spectra of DPAH degradation on
SrMoO for time intervals of 0–45 min. The results show that the
4
absorption was greatly reduced at 45 min, which means that 99%
of the DPAH degradation was achieved in 45 min. In contrast, com-
mercially available TiO
degradation, respectively. This study concluded that the Sr doping
in MoO lattice probably results in enhanced photocatalytic activ-
ity. To ensure the efficiency of SrMoO , the various parameters,
2 3
and MoO exhibited only 45% and 60% of
3
4
such as catalyst loading, initial concentration, and pH, are
evaluated.
The recombination of holes and electrons considerably sup-
It is obvious that the photodegradation rate is decreasing when
the concentration of DPAH increases. Most of the organic mole-
cules are absorbing the photons at different energies; the DPAH
is absorbing the light at 280 nm. As said in the Experimental sec-
tion, our primary light source exhibits different light energies,
presses the photocatalytic activity of SrMoO
alyst loading can play significant role in the control of
recombination properties of SrMoO [24]. Therefore, the photocat-
alytic activity of SrMoO for DPAH degradation was evaluated with
4
; i.e., the effect of cat-
a
4
4
Fig. 4. Comparison of the photodegradation of DPAH on various catalysts with SrMoO
4
in the presence and absence of light and catalyst (A); time-dependent UV–vis
on photodegradation (C) and effect of initial concentration of DPAH (D).
absorption spectrum of DPAH photodegradation on SrMoO (B); effect of catalyst dosage of SrMoO
4
4

6
12
R. Karthik et al. / Journal of Catalysis 352 (2017) 606–616
which are predominately absorbed by DPAH. Therefore, the con-
sumption of photons is higher for DPAH than for SrMoO . As a
4
result, the photodegradation rate is decreased when the concentra-
tion of DPAH is increased. From this study, 20 mg/L of DPAH is the
optimized concentration for degradation efficiency. Furthermore,
the solution pH was also investigated because it plays an impor-
tant role in the generation of active species such as hydroxyl rad-
Å
Åꢀ
2
icals (OH ) and superoxide radicals (O ). These active species are
mainly involved in the degradation of organic molecules. There-
fore, it is necessary to investigate the photodegradation rate in dif-
ferent pH solutions of DPAH. Fig. 5A shows the photodegradation
of DPAH at different pH from 4 to 12; the pH of the DPAH solution
is adjusted by adding HCl (for acidic) and KOH (for basic) [24,25].
The photodegradation rate is increased when the solution pH
increases from 4 to 12. This study certifies that the generation of
active species is directly proportional to the solution pH. At lower
4
Scheme 1. Pictorial representation of photocatalytic activity on SrMoO for the
different reaction pathways. Most of the degradation product from the reaction is
carbazole (CBZ).
+
Åꢀ
Åꢀ
2
pH, when a high concentration of H ions turns the O
2
into HO ,
Åꢀ
Å
Åꢀ
2
this HO
2
consumes the OH and forms H
2
O and O
. This process
Å
obstructs the reaction between DPAH and the reactive OH , which
activity on the surface of SrMoO4 decreased only 8% from the
adsorption of DPAH residues.
results in a decrease in the photodegradation rate. At higher pH,
ꢀ
the OH ions allows the H
2
O to react with the CB (conduction
Å
band) of SrMoO
4
and generate more OH [33]. Sometimes the DPAH
3.3. Electrocatalysis
+
ꢀ
reacts directly with the VB (holes, h ) or CB (electrons, e ) of
SrMoO and produces byproducts (Scheme 1).
4
The electrochemical behavior of DPAH on bare GCE and
To confirm this active species, we tried to find out how DPAH
degrades by investigating the scavenger profile. In this experiment,
acrylamide (AA), ammonium oxalate (AO), and tert-butyl alcohol
SrMoO /GCE was examined in a 0.05 M PB solution containing
200 mM DPAH at a scan rate of 50 mV/s. Fig. 6A shows typical cyclic
voltammograms of DPAH oxidation for the bare GCE and SrMoO4/
4
Åꢀ
+
Å
(
t-BuOH, BA) were used as scavengers for O
2
, h , and OH radicals
GCE; the electrocatalytic performances are higher for SrMoO /GCE.
4
[
33]. For comparison, the photodegradation of DPAH was per-
For the assessment of electrochemical performance, we calculated
formed without scavengers and fixed as the standard. Upon addi-
tion of scavengers, the photodegradation rate was altered for all
the scavengers; however, major changes were observed for the
the electrochemical active surface area for the bare GCE and
SrMoO using the ferricyanide system. For the evaluation, we used
4
the Randles–Sevcik equation. The calculated active surface areas of
Å
2
OH radical scavengers. Fig. 5B shows the rate of photodegradation
bare GCE and SrMoO4 are 0.038 and 0.052 cm , respectively
(
%) in the presence and absence of scavengers. BA suppressed 75%
[34,35]. This higher electroactive surface area is one of the factors
of the degradation; this means more than 75% of the reaction was
that increase the electrocatalytic performance of SrMoO . More-
4
Å
induced by the OH radicals and the remaining 24% of the reaction
over, some other factors influence the electrocatalytic activity: (i)
Åꢀ
+
Å
was initiated by O
2
and h . This study identified OH radicals as the
and contributing
the adsorption of DPAH on the electrode surface, (ii) the oxidation
potentials of the highest occupied molecular orbitals (HOMO) of
the electrode surface being higher than the lowest unoccupied
molecular orbital (LUMO) of DPAH, and (iii) the electrode–elec-
trolyte interfacial properties. These factors promote higher electro-
major active species, generated by SrMoO
4
strongly to the degradation of DPAH. From the observed results
and inspired by previous literature, we have proposed a mecha-
Å
nism of DPAH degradation using OH radicals.
Å
In Scheme 2, we portray the formation of CBZ by quenching OH
chemical performance by the SrMoO . In Fig. 6A, a strong oxidation
4
radicals with DPAH. CBZ is the major degradable product, con-
firmed by the liquid chromatographic-mass spectrometric analysis
peak (peak 1) appears at 0.62 V, corresponding to the one-electron
transfer process [22]. This oxidation generates the unstable radical
Å+
in the previous literature [23]. Finally, the reusability of SrMoO
was investigated by the stability test. Fig. 5C shows the stability
profile of SrMoO for the photodegradation of DPAH. The six con-
secutive cycles of the experiment revealed that photocatalytic
4
cation DPAH , which further oxidizes at higher potential; thus, the
second peak (peak 2) appears at 0.8 V [36,37]. Upon the reverse
scan, no cathodic peaks appeared for the corresponding anodic
peaks 1 and 2. This phenomenon showed that the radical cations,
4
Fig. 5. Effect of pH on the photodegradation of DPAH (A), scavenger profile for assessing the primary active species for the photodegradation of DPAH (B), and reusability test
for seed-like SrMoO (C).
4

R. Karthik et al. / Journal of Catalysis 352 (2017) 606–616
613
Å
Scheme 2. A plausible mechanism of the photodegradation of DPAH by the OH radicals, which produced the aryl radical and led to the formation of carbazole (CBZ).
Fig. 6. The CVs of bare GCE and SrMoO
B); the inset shows the plot of current vs. pH. The effect of various DPAH concentrations for DPAH oxidation on SrMoO
relationship between concentration and current (i ). Effect of scan rates on SrMoO on the oxidation of DPAH from 20 to 50 mV/s (D).
4
/GCE in the presence and absence of 200 mM DPAH (A) at a 50 mV/s sweeping rate. Effect of pH on the oxidation of DPAH from 3 to 11
(
4
from 0 to 120 mM (C); the inset shows the linear
p
4

6
14
R. Karthik et al. / Journal of Catalysis 352 (2017) 606–616
4
Scheme 3. The overall mechanism of the electrochemical oxidation of DPAH on the SrMoO .
Å+
i.e., DPAH , were completely reacted together and formed as the
dimer (diphenylbenzidine, DPB) [36,37]. Conversely, two well-
defined cathodic peaks 3 and 4 were appeared at 0.35 V and
line pHs; the former produced DPAH polymers and the latter pro-
duced DPAH dimers. These results were supported by previous
literature [37,40]. Moreover, the concentration of DPAH is influenc-
ing the electropolymerization process. Fig. 6C shows the CV of the
DPAH oxidation at various concentrations ranging from 0 to
120 mM of DPAH in 0.05 M PB solution at a scan rate of 50 mV/s.
Primary oxidation peak 1 is linearly increased when the concen-
tration of DPAH increases; however, the two cathodic peaks were
shifted to the negative while adding each concentration of DPAH.
This result suggested that when the concentration of DPAH
increases the reaction favors polymerization. Furthermore, scan
rate studies were recommended to investigate the rate of the reac-
tion. Fig. 6D shows the CVs of DPAH oxidation at different scan
rates ranging from 20 to 50 mV/s. The peak current of primary oxi-
dation peak 1 was increasing with increasing scan rate; however,
the peak potential was slightly shifted to the positive side. For
totally irreversible systems, the equilibrium of the reactant and
product is not established rapidly. Hence, the diffusion layer con-
tains the byproducts that are obstructing further oxidation. More-
over, the diffusion layer grows much more than the electrode
surface at lower scan rates and vice versa at higher scan rates. This
diffusion layer creates some internal resistance that further blocks
the movement of DPAH from the bulk solution. Therefore, the rate
of the reaction was minimized for the consecutive cycles. This
result is also in accordance with concentration and pH; hence,
these parameters are effectively participating in the polymeriza-
tion of DPAH. The overall reaction pathways from Scheme 3 indi-
cated that the major contribution of the reaction is the
ꢀ
0.25 V and two anodic peaks 5 and 6 were observed at 0.15 and
.4 V. These anodic peaks 5 and 6 correspond to cathodic peaks 4
and 3, respectively. These redox couples have peak potential sepa-
rations of 60 and 100 mV for 3/6 and 4/5, respectively [37].
0
DE
p
From the observed results, we have proposed a mechanism of
DPAH oxidation (Scheme 3). In Scheme 3, we illustrate the dimer-
ization of DPAH followed by the formation of cation radicals
Å+
(
DPAH ). The two monomers of the radical cation react together
and form a dimer; subsequently, another monomer cation radical
reacts with the parent dimer and forms a polymer. There are ways
to pair up the radical cations in different couplings, i.e., CAC, CAN,
and NAN, that offer different dimerization products [38,39]. The
resulting dimers have different structural and physicochemical
characteristics, which can be examined by their redox couples in
CV. Yang and Bard demonstrated that the CVs of DPB in acetoni-
trile, which confirmed that the dimer formation happened through
CAC coupling only [37]. Guay and Dao demonstrated the CAC cou-
pling by FTIR spectroscopic data, which further confirms that the
0
dimer only forms through the 4,4 C-C phenyl–phenyl coupling
pathway [40]. The repetitive scans in acetonitrile suggest that only
DPB is the final product; however, when adding the extra DPAH, it
undergoes the electropolymerization at higher anodic potential
[
37]. However, in PB solution (pH 7), DPAH initially forms DPB
and finally is converted into polydiphenylamine. The electropoly-
merization can be varied at different pHs; thus, we examined the
electrocatalytic properties at different pHs.
Fig. 6B shows the CVs of DPAH oxidation at various pHs ranging
from 3 to 11. The main oxidation peak (DPAH to DPAH ) shifted to
more negative potential when increasing the pH of the electrolyte.
The peak shifting in pH is obvious; however, the electropolymer-
ization occurs more in acidic environments and dimerization is
favored in alkaline environments. Interestingly, the peak current
of DPAH oxidation at pH 3 and 11, 5 and 9 is almost similar
Å+
generation of radical cations (DPAH ). Therefore, we can deter-
Å+
mine the concentration of DPAH by measuring DPAH .
Å+
The differential pulse voltammogram (DPV) is a suitable analyt-
Å+
ical tool to measure the DPAH ; thus, the DPV was recorded in a
0.05 M PB solution containing various concentrations of DPAH
ranging from 0.1 to 83 mM. Fig. 7A shows the DPV of DPAH oxida-
Å+
tion; the anodic peak for the generation of radical cations (DPAH )
is linearly increased with increasing concentration. The secondary
oxidation peaks 5 and 6 do not significantly contribute to the direct
(
Fig. 6B inset). However, the activity is different at acidic and alka-

R. Karthik et al. / Journal of Catalysis 352 (2017) 606–616
615
Fig. 7. The DPVs of DPAH oxidation at different concentrations ranging from 0.1 to 83 mM (A), the corresponding calibration plot (B), interference of DPAH with other common
ions and pesticides (C), and a comparison of the interfering signal with the standard DPAH signal (D).
measurement of DPAH. Hence, we omitted those peaks for the cal-
ibration data. Fig. 7B shows the calibration curve for the determi-
nation of DPAH , which is directly proportional to the
concentration of DPAH. From the calibration plot, the lowest detec-
tion limit and the linear concentration range were calculated as 30
nM and 0.1–35 mM. This result is highly comparable with previ-
ously reported DPAH sensors, which showed a detection limit
parison (Fig. 7D). These relative 3–7% signals imply that the
aforementioned substances are interfere strongly with DPAH .
Å+
Å+
Particularly, the urea causes maximum interference with the signal
of 107%. This high contribution was ascribed from the involvement
of urea with the oxidation of secondary products (DPB). Predomi-
nantly, the lone pair of nitrogen (from urea) interacts with the rad-
Å+
ical cation (DPAH ) and increases the peak current of oxidation
around 3.9 mM [22]. In contrast, the developed SrMoO
4
/GCE sensor
peak 6. This will further interact with radical generation; thus,
Å+
exhibited a very low detection limit and an acceptable sensitivity
the peak of DPAH deviated from its original response. The same
ꢀ1
ꢀ2
+
ꢀ
of 2.165 mA mM cm . Hence, the reported SrMoO
4
/GCE sensor
thing happened for other interferents such as Na and Cl , here,
also, the cation and anion interfered in similar ways. The accept-
able interference current change was less than 3% for the commer-
cial sensor devices; however, the present work shows more than
3%. However, less than a 7% signal was allowed for the initial lab-
oratory testing [34,41,42]. Further, this compound needs to modify
its structure or surface characteristics to get effective selectivity.
allows low-level electrochemical detection of DPAH. However,
the selectivity is the major problem with detecting the DPAH in
Å+
this study. Therefore, the selectivity of the developed sensor elec-
trode was studied in common ions and some pesticides. Those pes-
ticides are commonly used in agriculture; further, the common
ions usually exist in tap water. Fig. 7C shows the DPV curves of
Å+
DPAH detection in the presence of a 10-fold excess of chlorpyri-
+
fos, carbofuran, diuron, and urea and a 100-fold excess of Na
ꢀ
and Cl . The tested substances interfere strongly with the electro-
4. Conclusions
Å+
chemical activity of DPAH , which influences 103–107% of the
interference signal. The interference signal is plotted for the inter-
fering compounds where the DPAH signal is fixed as 100% for com-
In summary, we have demonstrated the photocatalytic and
4
electrocatalytic properties of SrMoO for the degradation and

6
16
R. Karthik et al. / Journal of Catalysis 352 (2017) 606–616
determination of DPAH. The UV–vis studies showed that the pho-
todegradation of DPAH reached 99% in 45 min. The controlled
experiments showed that the optimal catalyst loading, concentra-
tion, and pH are 50 mg/mL, 20 mg/L, and 12, respectively, for the
effective degradation. The rate of degradation mainly depends on
[12] G. Zhang, S. Yu, Y. Yang, W. Jiang, S. Zhang, B. Huang, J. Cryst. Growth 312
2010) 1866–1874.
(
[
13] J.C. Sczancoski, L.S. Cavalcante, M.R. Joya, J.A. Varela, P.S. Pizani, E. Longo,
Chem. Eng. J. 140 (2008) 632–637.
[14] V. Thangadurai, C. Knittlmayer, W. Weppner, Mater. Sci. Eng., B 106 (2004)
28–233.
2
[
[
15] B.D. Whitaker, Postharvest Biol. Technol. 18 (2000) 91–97.
16] E.F.S. Authority, Conclusion on the peer review of the pesticide risk assessment
of the active substance Pepino mosaic virus strain CH2 isolate 1906, EFSA J. 13
(1) (2015).
Å
the primary active species OH radicals, which is confirmed by
the scavenger profile analysis. Moreover, the electrochemical
determination of DPAH was demonstrated by DPAHÅ+ and indicated
a linear range of 0.1–35 mM a lowest detection limit of 30 nM.
However, the developed sensing tool lacks selectivity: as discussed
[
[
17] European Food Safety Authority, EFSA J. 9 (2011) 2336.
18] J. Dallüge, V.R. Martijn, B. Jan, R.J.J. Vreuls, A. Udo, J. Chromatogr. A 965 (1)
(2002) 207–217.
Å+
[19] B. Saad, H.H. Noor, I.S. Muhammad, H.H. Noor, A. Ahamad, A. Norhayati, Food
Chem. 84 (2) (2004) 313–317.
above, DPAH is highly reactive toward both anions and cations.
The tested substances interfere strongly with the electrochemical
[
[
[
20] Y. Tong, W. Zhuping, Y. Chengdui, Y. Jianyuan, Z. Xinrong, Y. Shijie, D. Xinyuan,
X. Yucai, W. Yuxiu, Analyst 126 (4) (2001) 480–484.
21] B. Hemmateenejad, A. Morteza, S. Fayezeh, Spectrochim. Acta Mol. Biomol.
Spectrosc. A 67 (3) (2007) 958–965.
22] V.L.V. Granado, G.C. Manuel, F.S. César, M. Teresa, S.R. Gomes, R. Alisa, J. Cecilia,
J. Anal. Chim. Acta 809 (2014) 141–147.
Å+
activity of DPAH , which influences 103–107% of the interference
signal. This 7% interfering signal restricts this device for real-time
analysis; however, it is acceptable for the laboratory. Therefore,
the selectivity of the developed sensor should be improved by
introducing some other metals or altering its surface phenomena.
In the future, it is necessary to focus on the design of this com-
pound and its application to DPAH determination.
[23] M. Shamsipur, H. Bahram, J.J. Naser, M.M. Kamran, J. Photochem. Photobiol.:
Chem. 299 (2015) 210–217.
[
24] J. Vinoth Kumar, R. Karthik, S.M. Chen, V. Muthuraj, C. Karuppiah, Sci. Rep. 6
2016) 1–8.
(
[
25] R. Karthik, J. Vinoth Kumar, S.M. Chen, C. Karuppiah, Y.H. Cheng, V. Muthuraj,
ACS Appl. Mater. Interfaces 9 (2017) 6547–6559.
Acknowledgment
[
[
26] D. Chen, K. Tang, F. Li, H. Zheng, Cryst. Growth Des. 6 (2006) 247–252.
27] T.T. Basiev, A.A. Sobol, P.G. Zverev, L.I. Ivleva, V.V. Osiko, R.C. Powell, Opt.
Mater. 11 (1991) 307–314.
This project was supported by the Ministry of Science and Tech-
nology of Taiwan, Republic of China.
[
28] T.T. Basiev, A.A. Sobol, Y.K. Voronko, P.G. Zverev, Opt. Mater. 15 (2000) 205–
216.
[
29] S.I. Woo, J.S. Kim, H.K. Jun, J. Phys. Chem. B 108 (2004) 8941–8946.
References
[30] G. Tyuliev, S. Angelov, Appl. Surf. Sci. 32 (1988) 381–391.
[
[
[
[
[
31] S. Hufner, Photoelectron Spectroscopy, Principles and Applications, Springer,
Berlin, 2003.
32] M. Naeem, S.K. Hasanain, M. Kobayashi, Y. Ishida, A. Fujimori, B. Scott, S. Ismat
Shah, Nanotechnology 17 (2006) 2675.
[
[
1] A.W. Sleight, Acta Crystallogr. Sect. B 28 (1972) 2899–2902.
2] N. Sharma, K.M. Shaju, G.V. Subba Rao, B.V.R. Chowdari, Z.L. Dong, T.J. White,
Chem. Mater. 16 (2004) 504–512.
33] S.J. Liang, S.Y. Zhu, Y. Chen, W.M. Wu, X.C. Wang, L. Wu, J. Mater. Chem. 22
[
[
3] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2–6.
4] E. Cavalli, P. Boutinaud, R. Mahiou, M. Bettinelli, P. Dorenbos, Inorg. Chem. 49
(
2012) 2670–2678.
34] N. Karikalan, R. Karthik, S.M. Chen, A.C. His, Sci. Rep. 7 (2017) 45924, http://dx.
doi.org/10.1038/srep45924.
35] M. Velmurugan, N. Karikalan, S.M. Chen, Y.H. Cheng, C. Karuppiah, J. Colloid
Interface Sci. 500 (2017) 54–62.
36] G. Inzelt, J. Solid State Electrochem. 6 (2002) 265–271.
37] H. Yang, J.B. Allen, J. Electroanal. Chem. Interfacial Electrochem. 306 (1–2)
(
2010) 4916–4921.
[
5] V.S. Marques, L.S. Cavalcante, J.C. Sczancoski, A.F.P. Alcantara, M.O. Orlandi, E.
Moraes, E. Longo, J.A. Varela, M.S. Li, M.R.M.C. Santos, Cryst. Growth Des. 10
(
2010) 4752–4768.
[
[
[
6] B.P. Singh, A.K. Parchur, R.S. Ningthoujam, A.A. Ansari, P. Singh, S.B. Rai, Dalton
Trans. 43 (12) (2014) 4770–4778.
(
1991) 87–109.
[
7] D. Errandonea, R.S. Kumar, M. Xinghua, C. Tu, J. Solid State Chem. 181 (2)
[
[
[
[
38] N. Vettorazzi, J.J. Silber, L. Sereno, J. Electroanal. Chem. 125 (1981) 459–475.
39] D. Serve, Electrochim. Acta 21 (1976) 1171–1181.
40] J. Guay, L.H. Dao, J. Electroanal. Chem. 274 (1989) 135–142.
41] M. Velmurugan, N. Karikalan, S.M. Chen, Z.C. Dai, J. Colloid Interface Sci. 490
(
2008) 355–364.
[
[
8] E. Gürmen, D. Eugene, J.S. King, J. Chem. Phys. 55 (3) (1971) 1093–1097.
9] A. Cameirão, D. René, E. Fabienne, G. Frédéric, J. Cryst. Growth 310 (18) (2008)
4
152–4162.
(
2017) 365–371.
[
10] T. Thongtem, S. Kungwankunakorn, B. Kuntalue, A. Phuruangrat, S. Thongtem,
J. Alloys Compd. 506 (2010) 475–481.
[
42] N. Karikalan, M. Velmurugan, S.M. Chen, C. Karuppiah, K.M. Al-Anazi, M.A. Ali,
B.S. Lou, RSC Adv. 6 (2016) 71507–71516.
[
11] T. Thongtem, A. Phuruangrat, J. Nanopart. Res. 12 (2010) 2287–2294.