Photocatalytic degradation of Rhodamine B and Ibuprofen with upconversion luminescence in Ag-BaMoO4: Er3+/Yb3+/K+ microcrystals

Article abstract of DOI:10.1016/j.jphotochem.2017.02.014

Silver dispersed BaMoO₄ octahedron microcrystals doped with Er³⁺, Yb³⁺, and K⁺ were synthesized by microwave hydrothermal process. The photocatalytic activity of different samples toward the degradation of Rhodamine B (Rh B) dye and Ibuprofen (IBP) drug were carried in solar light and visible light irradiation, respectively. The sample, Ag-BaMoO₄: Er³⁺/Yb³⁺, degraded the Rh B within 90?min about 99.60%, which is nearly 30 times as compared to the host material BaMoO_4. Similarly, Ag-BaMoO₄: Er³⁺/Yb³⁺, degraded the IBP within 90?min about 41.49%, which is nearly 31 times as compared to the host in visible light. IBP photodegradation intermediates were identified by high-resolution quadrupole-time of flight-electrospray ionization-mass spectrometry (HR-QTOF ESI/MS) in negative ion mode and detailed degradation pathway mechanism was proposed. Under 980?nm excitation, two strong green emission peaks (526?nm and 555?nm) and a less intense red emission peak (661?nm) were observed, which are assigned to the ²H_11/2?→?⁴I_15/2, ⁴S_3/2?→?⁴I_15/2, and ⁴F_9/2?→?⁴I_15/2 transitions of the Er³⁺ ions, respectively. Two photons upconversion process for green emission was observed. The upconversion luminescence intensity was increased by two-fold in magnitude by the addition of K⁺ ion in Er³⁺/Yb³⁺ doped BaMoO₄. Moreover, silver particles were found to favor the photocatalytic activity while K⁺ ions significantly enhanced upconversion. The photocatalytic as well as upconversion mechanism was explained. The combination of efficient photocatalytic activity and upconversion emission of the Ag-BaMoO₄: Er³⁺/Yb³⁺/K⁺ make the material suitable for the multifunctional application.

Full text of DOI:10.1016/j.jphotochem.2017.02.014

Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photocatalytic degradation of Rhodamine B and Ibuprofen with
upconversion luminescence in Ag-BaMoO4: Er3+/Yb3+/K+ microcrystals
Schindra Kumar Raya, Yuwaraj K. Kshetrib, Dipesh Dhakalc, Chhabilal Regmia,
Soo Wohn Leea,
*
a
Department of Environmental and Bio-Chemical Engineering, Sun Moon University, Chungnam, 31460, Republic of Korea
Department of Advanced Materials Engineering, Sun Moon University, Chungnam, 31460, Republic of Korea
Department of Life Science and Bio-Chemical Engineering, Sun Moon University, Chungnam 31460, Republic of Korea
b
c
A R T I C L E I N F O
A B S T R A C T S
Article history:
Silver dispersed BaMoO4 octahedron microcrystals doped with Er3+, Yb3+, and K+ were synthesized by
microwave hydrothermal process. The photocatalytic activity of different samples toward the
degradation of Rhodamine B (Rh B) dye and Ibuprofen (IBP) drug were carried in solar light and
visible light irradiation, respectively. The sample, Ag-BaMoO4: Er3+/Yb3+, degraded the Rh B within
90 min about 99.60%, which is nearly 30 times as compared to the host material BaMoO4. Similarly,
Ag-BaMoO4: Er3+/Yb3+, degraded the IBP within 90 min about 41.49%, which is nearly 31 times as
compared to the host in visible light. IBP photodegradation intermediates were identified by high-
resolution quadrupole-time of flight-electrospray ionization-mass spectrometry (HR-QTOF ESI/MS) in
negative ion mode and detailed degradation pathway mechanism was proposed. Under 980 nm
excitation, two strong green emission peaks (526 nm and 555 nm) and a less intense red emission peak
Received 7 November 2016
Received in revised form 11 February 2017
Accepted 16 February 2017
Available online 22 February 2017
Keywords:
Microwave hydrothermal
Upconversion luminescence
Photons
Rhodamine B
Ibuprofen
2
4
4
4
4
(661 nm) were observed, which are assigned to the H11/2 ! I15/2
,
4S3/2 ! I15/2, and F9/2 ! I15/2
transitions of the Er3+ ions, respectively. Two photons upconversion process for green emission was
observed. The upconversion luminescence intensity was increased by two-fold in magnitude by the
addition of K+ ion in Er3+/Yb3+ doped BaMoO4. Moreover, silver particles were found to favor the
photocatalytic activity while K+ ions significantly enhanced upconversion. The photocatalytic as well as
upconversion mechanism was explained. The combination of efficient photocatalytic activity and
upconversion emission of the Ag-BaMoO4: Er3+/Yb3+/K+ make the material suitable for the
multifunctional application.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
pollution that arises from the dyes and pharmaceutical pollutants,
semiconductor photocatalyst can be employed to degrade the
Textile and dyeing industries have a major contribution of
nearly 17–20% to water pollution [1,2]. The majority of dyes applied
in different industries are toxic and carcinogenic that causes a
serious hazard to humans as well as to freshwater and marine
ecosystem [3,4]. Various pharmaceutical wastes are also serious
factors of water pollution. For example, the Non-Steroidal Anti-
Inflammatory Drugs (NSAIDs), Ibuprofen is extensively used to
treat various health problems such as headaches, muscular aches,
rheumatoid arthritis, toothache and fever [5–7]. However, the
release of such drugs in water may cause human health hazards
and toxic effect to the aquatic ecosystem [8]. To solve the water
organic dyes and drugs by utilizing the natural energy of sunlight.
[9,10].
In past decades, the reports of the photocatalytic activity of
BaMoO4 were rarely found, mainly because of its large band gap
with rapid recombination of electron and hole [11]. The Scheelite
6
structures of BaMoO4 photocatalyst have C4h space group with
4.2 eV energy band gap [12]. The wide band gap makes it active
only under UV-light spectral range. To overcome this frustration of
poor photocatalytic activity of BaMoO4, the plasmonic photo-
catalyst is designed that composed mainly of noble metals (silver/
gold) and semiconductors. The plasmonic behavior induces strong
visible light absorption based on localized surface plasmon
resonance (LSPR) [13,14]. An internal electric field and Schottky
junction are produced in the region near the metal–semiconductor
interface, which causes the migration of photogenerated electrons
* Corresponding author.
E-mail address: swlee@sunmoon.ac.kr (S.W. Lee).
http://dx.doi.org/10.1016/j.jphotochem.2017.02.014
1010-6030/© 2017 Elsevier B.V. All rights reserved.

S.K. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
37
and holes in different directions to promote the charge-separation
efficiency for enhancement of photocatalytic activity [13–15].
In addition, the doping of rare earth ions (Er3+/Yb3+) in BaMoO4
host enhances the photocatalytic activity due to the increase in the
optical absorption and increase of surface barrier by 4f electron
transition of the rare earth [16]. The Er3+ and Yb3+ ions have
suitable energy level with equally spatial long-lived metastable 4f
states [17]. Under 980 nm NIR excitation, the Yb3+ has very large
absorption and it transfers energy to Er3+ ions that produce NIR to
visible upconversion. The Er3+ ions transfer the energy to the host
material and make efficient charge separation for dye degradation
[16–18].
pH of the solution was maintained to 9 by using the aqueous
ammonia (30%). The resulting precursor suspension was placed in
a microwave reactor (Eyla MWO-1000 wave Magic) for the
microwave hydrothermal treatment at 150 ꢀC, for 50 min under
200 W microwave power. The obtained precipitate was washed
with deionized water and ethanol several times. Then, it was dried
at 70 ꢀC. It was further calcined at 400 ꢀC for 5 h. The silver nitrate
solutions (1 mol%) were dispersed in the above synthesized-
samples and reduced under UV light. For convenience, samples
were identified hereafter as B (BaMoO4), B1 (BaMoO4: Er3+/Yb3+),
BS (Ag-BaMoO4: Er3+/Yb3+), BK (BaMoO4: Er3+/Yb3+/K+) and BSK
(Ag-BaMoO4: Er3+/Yb3+/K+).
The Er3+/Yb3+ doped BaMoO4 is good upconverting material due
to its moderately low phonon energy [19,20]. The molybdate
structure supports the reduction of the excitation energy and its
low phonon energy improves luminescence efficiency [21]. The Er3
+ ion produces upconversion by sequential absorption of multiple
photons by long-lived ladder like 4f energy levels [22]. In addition
to sensitized luminescence by Yb3+, other strategies such as SPR
has been used for improving upconversion from Er3+ [22,23]. The
SPR of silver nanoparticles has already been utilized in many
applications, which modifies absorption, emission and decay rate
by changing the electric and magnetic fields at the location of an
upconverting emitter [24]. In past decades, a plethora of research
papers has been published in plasmon related upconversion and it
is still challenging field from the view of scientific perspective and
technological stand point [23,24].
2.2. Characterization
The samples were characterized by powder X-ray diffraction
(XRD) using X-ray diffractometer (Rigaku, D/Max 2200HR diffrac-
tometer, Japan). Diffuse reflectance spectra (DRS) were measured
by UV–vis DRS spectrophotometer (V 570, Jasco International Co.
LTD., Japan). Raman spectra were obtained by the Raman
spectrometer (LabRam, Horiba, Scientific Research System,
France). XPS measurement was taken from the Multilab system
with Al Ka source at 15 kV and 200 W. The upconversion spectra
were measured by photoluminescence spectrometer (Shamrock
303i, Andor technology LTD., UK) at 980 nm excitation wavelength.
The morphologies of different samples were analyzed by the field
emission scanning electron microscope (FESEM, JSM-6700F). The
Portable solar simulator (PEC-L01, Pecell, Am 1.5G, 150 mW/cm2)
was used for the photocatalytic performance of the as-synthesized
samples.
The doping of non-lanthanides ion like potassium ion (K+) also
provides an alternative approach to enhance the upconversion
luminescence [25]. The aim of K+ ion doping in BaMoO4 crystal
lattice is to alter the Yb-Er distance that strongly enhances the
intensity of upconversion luminescence [25,26]. The introduction
of K+ also decreases the intrinsic symmetry around Er3+ ions which
increases the electric dipole transition probability leading to
enhanced upconversion [27]. The doping system modifies the free
energy of materials and stabilizes a certain crystal phase and
particular morphology [28]. Moreover, the introduction of K+ in
host lattice can create oxygen vacancies for enhancing the energy
transfer system with better luminescence [29].
Herein, we have synthesized octahedron microcrystals of Ag-
BaMoO4 co-doped with Er3+, Yb3+ and K+ ions by the microwave
hydrothermal process. The photocatalytic degradation of Rh B and
IBP were investigated in detail. HR-QTOF ESI/MS analysis was
carried out for determining the IBP intermediate products to find
out detailed reaction pathway mechanism. The effect of K+ ions and
silver particles on upconversion luminescence intensity was
described.
2.3. Photocatalytic activity
In our photocatalytic experimental procedure, 0.1 g of powder
sample was placed in a Pyrex glass, and 70 mL of aqueous solution
of Rh B dye (20 ppm) was poured into it. The mixture was then
placed in dark for 30 min with constant stirring for obtaining the
absorption-desorption equilibrium. After this procedure, sample
dye suspension was constantly stirred by magnetic stirrer under
the simulated solar light irradiation. The distance between lamp
and Pyrex glass cell containing dye suspension was nearly 12 cm.
The photocatalytic degradation rates of Rh B and IBP were obtained
by using UV–vis spectrophotometer (MECASYS, Optizen2120) at
every 30 min. An ultraviolet cutoff (l< 420 nm) filter in solar
simulator was applied for visible light source for degradation of IBP
and clear liquid after 90 min irradiationwith sample BS was used to
identify the degradation product.
2. Experimental details
2.4. Analytical procedure for IBP degradation
2.1. Preparation of samples
The reverse-phase HPLC-PDA (high performance liquid chro-
matography- photodiode array detector) analysis was performed
with a C18 column (YMC-Pack ODS-AQ; 4.6 mm internal diameter,
150 mm long, and 5 m particle size) connected to a PDA (220 nm)
using isocratic condition of solvent A (acetonitrile:methanol:
phosphate buffer (50:20:30, v/v/v) (pH: 5.6) for 20 min. The sample
irradiated by visible light for 90 min with reasonable amount of
depletion of ibuprofen was used for analysis of transformation
products. The HRQTOF–ESI–MS was used for analysis of the
degradation products. The binary mobile phases were composed of
the solvent A (HPLC-grade water with 0.05% formic acid) and
solvent B (acetonitrile with 0.05% formic acid). Total flow was
The chemicals were of analytical grade and purchased from
Sigma Aldrich. Ag-BaMoO4: Er3+/Yb3+/K+ samples were synthe-
sized by the microwave hydrothermal system. In typical experi-
mental procedure, aqueous solution (40 mL) of the calculated
amount of Barium nitrate (0.005 mol), Erbium chloride (2 mol%)
hexahydrate, Potassium chloride (10 mol%) and Ytterbium chloride
hexahydrate (12 mol%) were prepared in one beakers. Another
beaker contained Molybdic acid (0.005 mol) with Polyethylene
Glycol (PEG). PEG was used as surfactant, which is about 0.1 g. The
mole percentage of Erbium chloride hexahydrate, Potassium
chloride and Ytterbium chloride hexahydrate were taken with
respect to mole of Barium nitrate and Molybdic acid. The different
solutions were mixed each other dropwise by continuous magnetic
stirring for 120 min until the white precipitate was obtained. The
maintained at 0.3 mL/min for the 15 min program. The flow of
acetonitrile was 0–100% from 0 to 9 min and maintained at 100%
until 9–12 min, followed by 100–0% in 12–15 min, and then
stopped at 15 min. For exact mass analysis, HR-QTOF ESI/MS

38
S.K. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
Table 1
analysis was performed in negative ion mode using an ACQUITY
(Billerica, United States of America) column coupled with a
SYNAPT G2-S (Water Corp) column.
Crystallite size of different samples on the basis of Scherrer’s equation.
Samples
Crystallite size (nm)
FWHM (b) (rad)
B
125.6
118.3
76.3
80
0.1550
0.1587
0.1973
0.1913
0.1779
B1
BS
BK
BSK
3. Results and discussion
92.8
3.1. Structural and morphological analysis
XRD patterns of Ag-BaMoO4: Er3+/Yb3+/K+ microcrystals are
shown in Fig.1. In these XRD patterns, all the diffraction peaks were
well-indexed to the Scheelite tetragonal structure of BaMoO4
(JCPDS card no. 29-0193). No additional peaks for associated
impurities were detected in the Ag-BaMoO4: Er3+/Yb3+/K+ samples.
The silver peak was not detected in XRD because of the small
quantity of dispersed Ag that might be below the detection limit of
the instrument. The peak intensities of the doped samples were
found to be decreased in comparison with the undoped sample
which indicates decrease in the degree of cationic ordering [30]. As
shown in Fig. 1, the doping of Er3+/Yb3+/K+ in BaMoO4 causes
decrease in crystallinity as well as the broadening of the (112)
diffraction peak due to lattice shrinkage and steric effect that is
supported by value of full width at half maximum (FWHM) and
crystallite size of Scherrer’s equation in Table 1 [16,26]. According
to the Scherrer’s equation,
maximum intensity (FWHM) and
[31].
u is the scattering angle in radian
Fig. 2 shows the FESEM images of as-prepared samples. Well
defined octahedron-like morphology is evident. The formation
mechanism of micro-octahedron is aggregation and ripening of
small octahedrons [12]. Due to the high surface energy of particles
after doping in the host, the particles are agglomerated [30]. The
particle size ranges from 0.5 to 3.0 mm. Due to the aggregating
nature of octahedron crystals, silver particles could not be
observed on the surface of BaMoO4. Surface defects appear in
the faces of the crystals, which can be explained by the following
reasons: (i) Rapid hydrolysis might occur during addition of
aqueous ammonia into the solution mixture during the synthesis
process, (ii) Formation of supersaturated solution takes place due
to the high concentration of salts and acids dissolved in the
aqueous solution, and (iii) Polyethylene Glycol (PEG), polymeric
surfactant might have been highly adsorbed on the surface of
octahedron due to high collision rate and heating, regulated by
microwave radiation during the microwave hydrothermal process.
D ¼ K
l
=b
cosu
ð1Þ
where, D is the size of crystals,
l
is X-ray wavelength, K is
dimensionless shape factor (0.9),
b
is broadening at half the
Fig. 1. XRD patterns of Er3+/Yb3+/K+ co-doped Ag-BaMoO4 samples annealed at 400 ꢀC for 5 h. Inset shows broadening of diffraction peak (112).

S.K. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
39
Fig. 2. FESEM images of Ag-BaMoO4: Er3+/Yb3+/K+ microcrystals. (a) B; (b) B1; (c) BS; (d) BK; (e) BSK.
Fig. 3. XPS spectra of Ag-BaMoO4: Er3+/Yb3+/K+ sample. (a) Ba 3d; (b) Mo 3d; (c) O 1s; (d) Ag 3d;(e) Er 4d; (f) Yb 4d; (g) Survey spectrum.

40
S.K. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
The effective collision and coalescence between the octahedrons
drive the growth process of microcrystals with a new crystallo-
graphic plane. Actually, the growth rate of crystal face is associated
crystals, external modes and internal modes arise due to the
motion of Ba2+ cation with the rigid molecular unit and vibrations
inside the [MoO4]2ꢁ [34]. Table 2 reveals the Raman active modes
of different samples. The different position of Raman active modes
may be related to defects, the degree of structural order-disorder
into the lattice as well as the interaction force between the clusters.
Sharp and intense Raman active modes inform the presence of a
strong interaction between the ions, which originate from the
stretching and bending vibrations of the metal-oxygen bonds
within the anionic groups. The appearance of additional Raman
peaks at 442.82 cmꢁ1 and 629.24 cmꢁ1 in B1 sample and
433.81 cmꢁ1, 474.52 cmꢁ1 and 620.95 cmꢁ1 in the BK sample, as
shown in Fig. 4, are due to the dopant ions and the bulk vibration
modes [19].
UV–vis diffuse reflectance spectra of different samples are
shown in Fig. 5a. The absorption band ranging from 200 to 320 nm
represents the O-Mo charge transfer transitions of MoO42ꢁ groups
[35]. It is seen that Ag containing samples (BS and BSK) show a
strong absorption band around 325–600 nm which is attributed to
the surface plasmon resonance (SPR) effect caused by the silver
particles [31]. The UV–vis spectra of samples BS and BSK also
suggest that the surface plasmon resonance frequency of Ag
particles overlaps with the green and red emission frequencies of
the upconversion; this overlapping allows the coupling the SPR of
silver with upconversion emission [24]. The coupling of SPR
changes the energy transfer, radiative decay rate, and emission
intensity [36]. In the case of B, B1 and BK samples, the steep
absorption marked by the Urbach’s tail is assigned to the low
energy tails of direct absorption band gap [31]. From the
absorbance spectrum of BSK sample (in Fig. 5a), the absorption
bands at 370 nm, 421 nm, and 520 nm can be assigned to the
transition of Er3+, from 4I15/2 to 4G11/2, 2H9/2 and 2H11/2, respectively
[21].
with the surface energy of each crystal face. Due to the Er3+, Yb3+
,
and K+ dopant ions, faces of octahedrons were found to be slightly
distorted. The possible reasons may be associated with the
insertion of dopant ions into the BaMoO4 host matrix and new
equilibrium positions of surface atoms to balance free energy [32].
The particle size decreased after doping the ions, which is also
evidenced by the results from Scherrer’s equation.
Because each element has a unique set of binding energies, X-
ray photoelectron spectroscopy (XPS) was applied to determine
the presence of specific elements in the sample. Fig. 3 shows the
XPS spectra of BSK (Ag-BaMoO4: Er3+/Yb3+/K+) sample. From the
XPS spectra, it is observed that sample consists of Ba, Mo, O, K, Cl, Er
and Yb element without any impurity peaks. The Ba 3d signal
consists two peaks, Ba 3d3/2 and 3d5/2 at 798.5 eV and 783.2 eV,
respectively [21]. The peaks noted at 370.2 eV and 376.2 eV can be
ascribed to Ag 3d3/2 and Ag 3d5/2 of metallic silver [19]. The
difference (6.0 eV) of binding energy between Ag 3d3/2 and Ag 3d5/2
suggests that the silver is found in metallic form in BSK sample. The
value of binding energy at 235.2 eV, and 238.6 eV is assigned to 3d5/
and 3d3/2 of Mo, respectively, originated from Mo+6 oxidation
2
state [21]. The peak at 533.5 eV is ascribed to O 1s. The binding
energies for Er 4d, Yb 4d3/2, and Yb 4d5/2 are observed at 168.4 eV,
188.1 eV and 180.4 eV respectively [19]. In addition, the peak for K
2p is observed at 286 eV [21]. From the XPS measurements, it was
confirmed that Er3+, Yb3+ and K+ ions have been successfully
incorporated in the BaMoO4 host lattice.
Raman spectra of different samples are shown in Fig. 4. BaMoO4
shows 26 different vibrations in which all the even vibrations (Ag,
Bg and Eg) are Raman active modes and odd vibrations (4Au and
4Eu) are infrared active [33]. The vibrational motion in molecular
The optical band gap energy is related to the absorbance and
electronic transition in the materials [33]. The band gap energies of
different samples were calculated by using Kubelka-Munk
equation, which is shown in Fig. 5b. The band gap energy of
sample B, B1, BK, BS and BSK are 4.34, 4.31, 4.18, 3.95 and 3.71 eV,
respectively. It is noted that the band gap is reduced by addition of
silver particles. The possible reason for the reduction in band gap is
that the metallic clusters of Ag introduces localized energy levels in
BaMoO4 band gap [37]. The small decrease in band gap values for
Er3+/Yb3+/K+ doped BaMoO4 may be related to the quantum
confinement effect [38].
3.2. Photocatalytic activity
The photocatalytic activity of various samples was observed by
photodegradation of Rh B dye (20 ppm) in aqueous solution
dispersions under simulated solar light irradiation. The photo-
catalytic activities were measured under similar conditions with
the help of kinetic plot (C/Co), recycle stability test and degradation
percentage. As shown in Fig. 6a, the negligible absorption of Rh B
dye under dark condition was noted. The presence of rare earth
ions (Er3+/Yb3+) in BaMoO4, sample B1, slightly increases the
degradation process which is nearly two times than the BaMoO4
host. With the silver addition that is for the sample BS the
degradation of Rh B was rapidly increased. The highest photo-
catalytic activity was seen in the sample BS, which degraded the Rh
B within 90 min about 99.60% as shown in Fig. 6a and b. It can be
concluded that silver and rare earth ions enhanced the photo-
catalytic activity dramatically by 30 times than that of the BaMoO4
host. However, the doping of K+ ions in BaMoO4 host slightly
reduces the photocatalytic activity. To quantify the degradation
rate of Rh B, the reaction kinetics was ascribed to a pseudo-first-
Fig. 4. Raman spectra of Ag-BaMoO4: Er3+/Yb3+/K+ samples.
order reaction, ln (Co/C) = kapp t, where kapp, is the apparent first-
ꢂ

S.K. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
41
Table 2
Raman active modes of different samples with extra peaks.
Samples
Bg (cmꢁ1
)
Eg (cmꢁ1
)
Eg (cmꢁ1
)
Bg (cmꢁ1
)
Ag (cmꢁ1
)
Eg (cmꢁ1
)
Bg (cmꢁ1
)
Ag (cmꢁ1
)
B
85.74
85.67
86.92
86.92
82.83
116.83
147.96
196.44
333.43
335.79
326.61
335.72
322.02
800.16
791.18
786.89
779.94
782.03
846.72
839.37
838.12
848.72
833.55
899.44
894.23
889.64
901.11
885.62
B1
BS
BK
BSK
–
–
–
–
–
–
–
–
–
–
–
–
Fig. 5. (a) UV–vis DRS spectra; (b) Kubelka-Munk plot for band gap energy of different samples.
order rate constant while C and Co are concentration at time t and 0
respectively [39]. As displayed in Fig. 6c, the degradation rate
constant of B, B1, BS, BK and BSK samples are 0.000328, 0.00090,
0.06301, 0.00018 and 0.02977 minꢁ1 respectively. This result also
indicates that the sample BS has the best photocatalytic perfor-
mance. Furthermore, the stability and reusability of the sample BS
were investigated by repeating the Rh B degradation experiment
three times. The result presented in Fig. 6d demonstrates the high
photocatalytic stability.
The kinetic plot, degradation percentage of IBP (20 ppm) and
UV–vis spectra by different samples under visible light irradiation
are depicted in Fig. 7. The adsorption of IBP was not seen in dark
condition. Initially, the presence of rare earth ions (Er3+/Yb3+) in
BaMoO4, sample B1, slightly enhances the degradation process
than the BaMoO4 host. The degradation of IBP was rapidly
increased by the addition of silver. The highest photocatalytic
performance was seen in the sample BS, which degraded 41.49% of
IBP within 90 min as shown in Fig. 7a and b. The combined effect of
silver and rare earth ions enhanced the photocatalytic activity
dramatically by 31 times than that of the BaMoO4 host. However,
the doping of K+ ions in BaMoO4 host slightly reduced the
photocatalytic performance. The UV–vis spectra during photo-
degradation of IBP aqueous solution mediated by the sample BS is
depicted in Fig. 7c. The absorption peak of IBP was seen at 221 nm.
The new peak appeared at 257 nm after the degradation in 30 min.
The appearance of the new peak indicates that some intermediated
photoproducts appeared during the degradation process. The
absorbance at 257 nm decreased with an increase of the reaction
time.
Information). The degradation product of IBP was confirmed by
HR-QTOF ESI/MS analysis in Fig. S2 (Supporting Information). The
different IBP fragments were observed on the basis of retention
time (tR), observed molecular mass [MꢁH]ꢁ, and fragment ions (m/
z) value. Table 3 shows the various degradation products and their
skeleton structure, elemental formula, experimental molecular
mass [MꢁH]ꢁ, observed molecular mass [MꢁH]ꢁ, fragments (m/z),
and retention time (tR). The mass spectrum of IBP consists the ion
of m/z, 205.1238. The fragment in the mass spectrum of the product
IBP 1 is the ion of m/z 221.1165, which corresponds to hydroxyl-
ation of the IBP molecule in solution by OHꢃ radical. The OHꢃ radical
may attack at one of the two benzyl positions, tertiary carbon atom
positions, and aromatic ring. A detail mechanism of the IBP
degradation pathway is described in the reaction mechanism
(Supplementary Information) and the corresponding photodegra-
dation pathway for the mineralization has been presented in Fig. 8.
On the basis of various degradation product of IBP, the degradation
pathways followed through the attack by OHꢃ radical with
decarboxylation process, and leading subsequently to further
mineralization [8,40–42].
The photocatalytic activity of Ag-BaMoO4: Er3+/Yb3+ is associ-
ated with the closed junctions and the synergistic plasmon
response of Ag [3]. Under simulated solar light irradiation, both Ag
and BaMoO4 can be simultaneously excited to generate electron-
hole pairs. Afterward, the plasmon resonance excites the electron
in silver, the photogenerated electrons from the conduction band
of BaMoO4 are transferred to the silver and electrons are trapped
by it [37,43]. The silver particles present in the BaMoO4 surface can
also increase the interfaces and heterostructures [44]. Due to these
processes, electrons and holes are separated effectively giving rise
to enhanced photocatalytic activity. The Rh B and IBP react with the
holes in valence band giving rise to the oxidation products whereas
In order to identify the intermediate products of IBP, HR-QTOF
ESI/MS (negative ion mode) was used. The total ion chromatogram
(TIC) of degradation products of IBP is shown in Fig. S1 (Supporting

42
S.K. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
Fig. 6. (a) Kinetic plot for the degradation of Rh B (20 ppm) using solar simulator; (b) degradation percentage; (c) pseudo first order kinetics of different samples; (d) three
recycling runs of BS sample.
Fig. 7. (a) Kinetic plot for the degradation of Ibuprofen (20 ppm); (b) degradation percentage; (c) UV–vis spectra during degradation of IBP aqueous solution mediated by the
sample BS under visible light irradiation.
the electrons in the conduction band give reduction products
[14,45]. However, the doping of K+ ions in BaMoO4 host slightly
reduces the photocatalytic activity because of fast recombination
electron and hole.
The rare earth metals act as stock for photogenerated electrons.
Surface oxygen vacancies and defect states may also arise due to
the rare earth ion doping. The oxygen vacancies and defect states
can capture the photoinduced electrons during photocatalytic
process [46,47]. Also, the presence of the Er3+ ions in the lattice can
help in trapping the photoelectrons by reducing itself to Er2+
,
thereby preventing the recombination chance of electrons and
holes by reaction between the Er2+ species and O2 [48]. The large
cross section area of Yb3+ ions transfers energy to Er3+ ions and Er3+
ions transfer the energy to the host material and make efficient

S.K. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
43
Table 3
Identification of IBP degradation products by HR-QTOF ESI/MS analysis.
Name
Structure
Elemental
formula
[MꢁH]ꢁ
[MꢁH]ꢁ
Fragment ions
(m/z)
Retention time
(tR)
Theoretical
Observed
Ibuprofen (IBP)
C13H18O2
205.1234
205.1238
161,119
5.715
IBP 1a
C13H18O3
221.1193
221.1165
177,159
4.417
2-(4-(1-hydroxy-2-methyl propyl) phenyl)
propanoic acid
IBP 1b
2-hydroxy-2-(4-isobutyl phenyl) propanoic acid
IBP 1c
2-(4-(2-hydroxy-2-methyl propyl) phenyl)
propanoic acid
IBP 1d
2-(3-hydroxy-4-isobutyl phenyl) propanoic acid
IBP 2
C18H18
161.1335
177.1285
161.1306
177.1299
161,119
177,149
5.533
3.538
1-ethyl-4-isobutyl benzene
IBP 3a
C12H18O
1-(4-ethylphenyl)-2-methyl propan-2-ol
IBP 3b
1-(4-ethyl phenyl)-2-methyl propan-1-ol
IBP 3c
1-(4-isobutyl phenyl) ethanol
IBP 3d
1-ethyl-4-isobutyl phenol
IBP 4a
C12H16
159.1183
159.1183
143,117
5.697
1-isobutyl-4-vinyl benzene
IBP 4b
1-ethyl-4-(2-methyl prop-1-en-1-yl) benzene

44
S.K. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
Table 3 (Continued)
Name
Structure
Elemental
formula
[MꢁH]ꢁ
[MꢁH]ꢁ
Fragment ions
(m/z)
Retention time
(tR)
Theoretical
Observed
IBP 4c
1-ethyl-4-(2-methyl allyl) benzene
IBP 5a
C10H14
O
149.0972
149.0914
133,119
5.153
2-methyl-1-phenyl propan-1-ol
IBP 5b
2-methyl-1-phenyl propan-2-ol
Fig. 8. The proposed degradation pathways for the mineralization of IBP.

S.K. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
45
4
555 nm, and 661 nm are assigned to the transitions 2H11/2 ! I15/2
,
4S3/2 ! I15/2 and F9/2 ! I15/2 respectively [21,49]. The intense
green upconversion emission is the result of efficient energy
transfer from Yb3+ to Er3+ [50]. The green upconversion is also
associated with the charge transfer transition with in MoO4
clusters [34].
4
4
4
The upconversion luminescence intensity of BSK and BK
samples (containing K+ ions) is higher as compared to that of B1
and BS samples (without K+ ions). It is evident that the
upconversion emission has been enhanced by K+ ions. It is known
that the emission in Er3+ is originated from electric dipole
transitions among the 4f subshells. These radiative transitions
are in general forbidden by parity considerations [27]. When the
non-luminescent K+ ion occupies the interstitial site in the lattice,
it lowers the local symmetry around the Er3+/Yb3+ ions [22]. As a
result, the intrinsic wave functions of 4f subshell mix with other
wave functions of opposite parity and the parity forbidden nature
of the 4f transition is partially broken enhancing the radiative and
absorption transition probabilities of Er3+/Yb3+ ions [51]. Moreover,
the presence of K+ in the lattice decreases the effective distance
between the Er3+ and Yb3+ and hence increases the energy transfer
probability between the ions [25,52]. Hence, the lowering of local
symmetry around the lanthanide dopants and the effective
decrease of the distance between the Er3+ and Yb3+ ions due to
the presence of K+ ions in the lattice has favored the efficient
upconversion emission [22,25,26].
Fig. 9. Schematic diagram of photocatalytic mechanism.
Although the presence of silver particles enhances the
absorption of visible light by surface plasmon resonance phenom-
enon, a significant reduction in upconversion intensity was
observed in B1 and BS samples. The reason for this reduction in
emission intensity can be correlated with the local field around the
Er3+ ions induced by the Ag, which redistributes the population
level of the emitting excited states of Er3+ [53]. In addition,
plasmon absorption by the aggregation of silver particles,
competed absorption among the silver particles and the degree
of crystallinity also reduce the upconversion intensity [54,55].
charge separation for dye and drug degradation. The model of
photocatalytic degradation of Rh B and IBP by Ag-BaMoO4: Er3
+/Yb3+/K+ photocatalyst is shown in Fig. 9.
3.3. Upconversion properties
The upconversion emission spectra of different samples under
980 nm laser excitation are shown in Fig. 10. Intense green and less
intense red emissions were observed. The emissions at 526 nm
Fig. 10. Upconversion emission spectra upon excitation at 980 nm of Er3+/Yb3+/K+ doped Ag-BaMoO4 samples.

46
S.K. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
Fig. 11. Dependence of emission intensity on incident laser power for sample BK.
Fig. 13. Simplified energy level diagram and the possible upconversion pathways
for Ag-BaMoO4: Er3+/Yb3+/K+ microcrystals. Dashed upward arrows represent
absorption. Solid downward arrows represent radiative emission. Short dotted
downward arrows represent non radiative relaxation.
+/Yb3+/K+ microcrystals has been presented. Under 980 nm laser
excitation, Yb3+ ions are first excited from the ground state 2F7/2 to
2
the excited state F5/2. The excited Yb3+ ion then transfers the
energy (ET-1) to the Er3+ ions exciting it from the 4I15/2 to the 4I11/2
state. Some of the Er3+ ions are further excited from 4I11/2 to the 4F7/
level by absorbing another photon via ET-2. The excited
2
2
4
population non-radiatively relax to the H11/2
,
4S3/2 and F9/2
and subsequently produce green (526 nm and 554 nm) and red
(661 nm) emissions by radiative relaxations to the ground state
4I15/2. In addition, some Er3+ ions in the 4I11/2 level non-radiatively
4
decay to the lower level I13/2 from where they undergo ET-3
process to further contribute to the red emission as shown in the
diagram [19].
The CIE (Commission Internationale de L’Eclairage) diagram of
all the samples under 980 nm excitation has been shown in Fig. 14.
The inset shows the digital photograph of the actual emission from
the representative samples BK and BS against the dark background.
The CIE coordinates as calculated from corresponding upconver-
sion emission spectra are (0.293, 0.687), (0.397, 0.586), (0.264,
0.712) and (0.280, 0.694) for the samples B1, BS, BK and BSK,
respectively. The color coordinates of B1, BS and BK lie in green
region. However, the color coordinates shift to the green-yellow
region for the sample BSK due to enhancement in red emission and
decrease in green emission. This observation shows that the
synthesized materials have a potential application as a green
upconversion optical material.
Fig.12. Double logarithmic plot of incident laser power versus emission intensity of
different samples at 526 nm. (a) B1; (b) BS; (c) BK; (d) BSK.
Furthermore, silver particles can provide additional nonradiative
decay channels, enhancing the nonradiative decay rate, as a result
quenching of upconversion luminescence occurs [55].
Pump power dependence of the upconversion emission
intensity in the sample BK is shown in Fig. 11. The upconversion
photoluminescence intensity has a nonlinear dependence on the
excited light density, which is represented by the following
equation:
IUC¼KPn
ð2Þ
4. Conclusions
Where, I , P, K and n are upconversion photoluminescence
UC
intensity, power of pump laser, material associated coefficient and
the number of the excitation photons participating the upconver-
sion process, respectively [56]. The value of n is given by the slope
of the intensity versus pump power in double logarithmic plot.
From Fig.12, the value of n is found to be 2 indicating a two-photon
absorption process for the green emission. The slop for the red
emission has significant deviation from the two-photon model.
Based on the energy level diagram, as shown in Fig. 13, the
following possible upconversion mechanism in Ag-BaMoO4: Er3
In summary, the photocatalytic and upconversion lumines-
cence of silver dispersed BaMoO4 octahedron microcrystals doped
with Er3+,Yb3+, K+ has been investigated. The individual effect of Ag
and K+ in photocatalysis and upconversion has been evaluated. The
photocatalytic efficiency of Ag-BaMoO4: Er3+/Yb3+ is nearly 30 and
31 times as compared to that of the pure BaMoO4 for Rh B (solar
light irradiation) and IBP (visible light irradiation), respectively.
The HR-QTOF ESI/MS was used to detect IBP photodegradation

S.K. Ray et al. / Journal of Photochemistry and Photobiology A: Chemistry 339 (2017) 36–48
47
Fig. 14. CIE chromaticity diagram of Ag-BaMoO4: Er3+/Yb3+/K+ samples. The inset shows the luminescent photograph of sample BK and BS under 980 nm excitation.
intermediates for finding out detailed degradation pathway
mechanism. The dispersion of silver particles as well as rare earth
ions doping in BaMoO4 played a vital role for photocatalytic
enhancement. Although the addition of K+ reduced the photo-
catalytic efficiency, a two-fold enhancement in upconversion
intensity was observed in BaMoO4: Er3+/Yb3+/K+ as compared to
that of BaMoO4: Er3+/Yb3+. The presence of Ag slightly reduced the
upconversion intensity of BaMoO4: Er3+/Yb3+. Very intense green
and less intense red emissions were observed in Ag-BaMoO4: Er3
+/Yb3+/K+. Two photon absorption process was responsible for the
observed upconversion emission. The presence of Ag and K+ both
in BaMoO4: Er3+/Yb3+ results in the combination of enhanced
photocatalytic activity for removal of organic contaminants (IBP
and Rh B) and efficient upconversion emission which makes the
Ag-BaMoO4: Er3+/Yb3+/K+ microcrystal suitable for the multifunc-
tional application.
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