Cu-α-NiMoO4 photocatalyst for degradation of Methylene blue with pathways and antibacterial performance

Article abstract of DOI:10.1016/j.jphotochem.2017.08.004

Cu doped α-NiMoO₄ photocatalyst has been synthesized by microwave hydrothermal method. The existence of Cu²⁺ ions at lattice position of α-NiMoO₄ was observed on the basis of XRD, HRTEM, SAED, and EDS analysis. The negative zeta potential values indicate the stability of samples. Solar light driven photocatalytic degradation of Methylene blue (MB) dye in water was used to evaluate the photocatalytic performance of Cu doped α-NiMoO₄ photocatalyst. The results revealed that there is an optimum Cu (4 mol%) doping level leads to highly enhanced photocatalytic activity of Cu-α-NiMoO₄, as compared to α-NiMoO₄ host. The experiment also suggested that active species (OH[rad], O₂[rad]^? and h⁺) play a crucial role in the scavenging system. The reduced energy band gap, oxygen vacancy, high BET surface area, and efficient separation of photogenerated electron/hole are responsible for enhancement of photocatalytic performance. MB photodegradation intermediates were identified by high resolution-quadruple time of flight electrospray ionization mass spectrometry (HR-QTOF ESI/MS) in positive ion mode and degradation pathway was proposed. Antibacterial performance was analyzed against Gram-positive (methicillin resistant Staphylococcus aureus and Bacillus Subtilis) and Gram-negative bacteria (Pseudomonas aeruginosa) via well-diffusion method. The formation of larger inhibition zone by small quantity of photocatalyst powder proved the excellent antibacterial performance. The inactivation of microorganism were found in following order: B.Subtilis ? S.aureus ? P.aeruginosa. The result of our study suggested that copper doped α-NiMoO₄ photocatalyst is suitable for degradation of organic contaminates as well as effective for growth inhibition of multidrug-resistant microorganisms.

Full text of DOI:10.1016/j.jphotochem.2017.08.004

Accepted Manuscript
Title: Cu-␣-NiMoO4 photocatalyst for degradation of
Methylene blue with pathways and antibacterial performance
Authors: Schindra Kumar Ray, Dipesh Dhakal, Yuwaraj K.
Kshetri, Soo Wohn Lee
PII:
S1010-6030(17)30646-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2017.08.004
JPC 10777
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
11-5-2017
18-7-2017
1-8-2017
Please cite this article as: Schindra Kumar Ray, Dipesh Dhakal, Yuwaraj K.Kshetri,
Soo Wohn Lee, Cu-␣-NiMoO4 photocatalyst for degradation of Methylene blue with
pathways and antibacterial performance, Journal of Photochemistry and Photobiology
A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.08.004
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<AT>Cu-α-NiMoO4 photocatalyst for degradation of Methylene blue with pathways and
antibacterial performance
<AU>Schindra Kumar Raya, Dipesh Dhakalb, Yuwaraj K. Kshetric, Soo Wohn Leea,*
<AFF>aDepartment of Environmental and Bio-chemical Engineering, Sun Moon University,
Chungnam, 31460, Republic of Korea
<AFF>bDepartment of Life Science and Bio-chemical Engineering, Sun Moon University,
Chungnam 31460, Republic of Korea
<AFF>cResearch Center for Eco-Multifunctional Nanomaterials, Sun Moon University,
Chungnam, 31460, Republic of Korea
<ABS-Head><ABS-HEAD>Graphical abstract
<ABS-P>
<ABS-P><xps:span class="xps_Image">fx1</xps:span>
<ABS-HEAD>Highlights► Fabrication of Cu-α-NiMoO4 rods by microwave hydrothermal
method. ► Photocatalytic enhancement by copper doping in α-NiMoO4 in solar light. ►
Identification of degradation products of Methylene blue by HR-QTOF ESI/MS. ► Propose
a degradation pathway of Methylene blue. ► Strong tendency to inactivate S. aureus, B.
Subtilis, and P. aeruginosa.
<ABS-HEAD>Abstract
<ABS-P>Cu doped α-NiMoO4 photocatalyst has been synthesized by microwave
hydrothermal method. The existence of Cu2+ ions at lattice position of α-NiMoO4 was
observed on the basis of XRD, HRTEM, SAED, and EDS analysis. The negative zeta
potential values indicate the stability of samples. Solar light driven photocatalytic degradation
of Methylene blue (MB) dye in water was used to evaluate the photocatalytic performance of
Cu doped α-NiMoO4 photocatalyst. The results revealed that there is an optimum Cu (4 mole
%) doping level leads to highly enhanced photocatalytic activity of Cu-α-NiMoO4, as
•-
compared to α-NiMoO4 host. The experiment also suggested that active species (OH•, O2
and h+) play a crucial role in the scavenging system. The reduced energy band gap, oxygen
vacancy, high BET surface area, and efficient separation of photogenerated electron/hole are
responsible for enhancement of photocatalytic performance. MB photodegradation
intermediates were identified by high resolution-quadruple time of flight electrospray
ionization mass spectrometry (HR-QTOF ESI/MS) in positive ion mode and degradation
pathway was proposed. Antibacterial performance was analyzed against Gram-positive
(methicillin resistant Staphylococcus aureus and Bacillus Subtilis) and Gram-negative
bacteria (Pseudomonas aeruginosa) via well-diffusion method. The formation of larger
inhibition zone by small quantity of photocatalyst powder proved the excellent antibacterial
performance. The inactivation of microorganism were found in following order: B.
Subtilis˃S. aureus˃P. aeruginosa. The result of our study suggested that copper doped α-
NiMoO4 photocatalyst is suitable for degradation of organic contaminates as well as effective
for growth inhibition of multidrug-resistant microorganisms.
<KWD>Keywords: Microwave hydrothermal; Photocatalytic degradation; Methylene blue;
Inhibition zone; Multidrug-resistant.
<H1>1. Introduction
The textile and dyeing industries are major sources for severe water pollution problem
worldwide [1,2]. The release of dye effluents into the clean water is undesirable because of
their color as well as toxic and carcinogenic degraded products. It causes the serious health

hazards in human, freshwater, and marine organisms [2,3]. Several techniques (chemical,
physical, photocatalysis, and biological) were applied to remove the hazardous organic dyes
from contaminated waste water [4]. Among all techniques, photocatalysis is a potential green
technology that can degrade the toxic organic compounds under the illumination of sunlight
under ambient condition [5,6].
In past decades, Metal molybdate materials can be used as phosphors, laser materials,
scintillators, photocatalyst, antibacterial, optical fibers, humidity sensors, and magnetic
materials [1,7–11]. Among Metal molybdates, NiMoO4 consists of low temperature α-phase
and the high temperature β-phase. α-NiMoO4 has monoclinic structure with C2/m space group
and stable at room temperature [12]. It has an octahedral co-ordination of Mo6+ and Ni2+ ions
with edge shared oxygen atoms [13]. α-NiMoO4 photocatalyst has low cost and
environmentally friendly features with the 2.8 eV energy band gap [14]. However, it has high
electron and hole recombination rate, resulting in lower photocatlytic performance. So, the
photocatalytic activity of Nickel molybdate is hardly found in the literature. To achieve an
excellent photocatalytic performance, doping strategy is a feasible method for preventing the
electron and hole recombination [15]. The introduction of copper doping in photocatalyst
provides the formation of new energy level between the valence band and conduction band
that induces the shift in the light absorption in the longer wavelength [15,16]. The doping
technique can also create the oxygen vacancy in the photocatalyst that can enhances the
photocatalytic activity [17].
Nowadays, the photocatalyic materials have emerged as novel antibacterial agent that can
fight against the multidrug-resistant pathogens (methicillin resistant Staphylococcus aureus,
Bacillus Subtilis and Pseudomonas aeruginosa) [11,18,19]. The multidrug-resistant
pathogens acts as a human health threats. The introduction of metal doping in photocatalyst
enhances the inactivation process of bacteria [20]. During inactivation of bacteria by
photocatalytic particles, the particles may attach the cell wall of bacteria that causes the
disturbance of respiration and permeability as well as enzyme and DNA damage [18]. The
•
•-
generation of reactive oxygen species (OH•, HO2 , H2O2, O2 ) also makes the inactivation of
bacteria [19].
In this research, microwave hydrothermal method was applied for the synthesis of copper
doped α-NiMoO4 rod. The morphologies and optical properties of Cu-α-NiMoO4 were
described. The photocatalytic degradation of Methylene blue (MB) was investigated under
solar light illumination. HR-QTOF ESI/MS analysis was carried out for determination of MB
intermediate products to find out the reaction pathway mechanism. In addition, the role of
active species were investigated during degradation of MB. The well-diffusion method was
performed against methicillin resistant Staphylococcus aureus (S. aureus), Bacillus Subtilis
(B. Subtilis), and Pseudomonas aeruginosa (P. aeruginosa) for antibacterial application.
<H1>2. Materials and methods
<H2>2.1. Synthesis of samples
The chemicals (Sigma Aldrich of analytical grade) were used without any further
purification. In experimental procedure, NiCl2.6H2O (10-2 mole) and CuSO4.5H2O (4 mole %
and 6 mole %) were placed in beaker containing 40 mL water. The aqueous solution of
H2MoO4 (40 ml, 10-2 mole) was prepared in another beaker. The different solutions were
mixed drop wise by the continuous magnetic stirring with maintain the pH 5 by using
aqueous NH3 (30 %). Greenish yellow precipitated was formed by mixing the solutions with
magnetic stirring up to 7 h. The resulting precursor suspension was placed in Microwave
reactor (Eyla MWO-1000 wave Magic) for the microwave hydrothermal treatment (150 0C/1
h/200 W). The obtained precipitate was washed (water/ethanol) several times. Then, it was

dried (70 0C) and calcined in air (600 0C/5 h). For convenience, samples were coded as NM
(α-NiMoO4), NM-Cu 4 (α-NiMoO4/Cu 4 mole %), and NM-Cu 6 (α-NiMoO4/Cu 6 mole %).
<H2>2.2. Characterization
The powder X-ray diffraction (XRD) patterns (Rigaku, D/MAX 2200HR) and UV-Vis
diffuse reflectance spectra (DRS) (V 570, Jasco International Co. Ltd.) of different α-
NiMoO4 samples were observed. The morphologies of samples were analyzed by the Field
emission scanning electron microscope (FESEM, JSM-6700F). The high resolution
transmission electron micrograph (HRTEM) images, selected area electron diffraction
(SAED) patterns, and Energy-dispersive X-ray spectroscopy (EDS) were obtained from FEI
TitanTM 80-300. X-ray photoelectron spectroscopy (XPS) (Multilab system/Al Kα source/15
kV/200 W) and Raman spectra (λex = 530 nm, LabRam, Horiba, Scientific Research System,
France) were measured. The Photocatalytic activity of samples were evaluated by using solar
simulator (portable solar simulator PEC-L01, 150 mW/cm2, Pecell, Am 1.5G) as a light
source.
The Brunauer-Emmett-Teller (BET) surface areas of samples were analyzed by the nitrogen
adsorption-desorption technique (micrometrics, ASAP 2020 automatic analyzer at 77 K). The
zeta potential was measured by Horiba Scientific, SZ-100 (0.1 mg ml-1 photocatalyst
powder/sonication for 2 h). The prepared samples were dispersed in deionized distill water (1
mg/mL) and the suspension was placed in an ultrasonic bath for 2 h. The zeta potential of the
samples in aqueous phase was measured using a nanoparticle analyzer, SZ-100, Horiba
scientific. The electric field was fixed at 3.8 V.
<H2>2.3. Photocatalytic and radical trapping experiment
The powder sample (0.1g) was placed in a Pyrex glass, and 70 mL of aqueous solution of MB
(10 ppm) was poured into it. The mixture was placed in dark (30 min/constant stirring) for
obtaining the absorption-desorption equilibrium. Then, the sample dye suspension was
constantly stirred (magnetic stirrer) under the simulated solar light irradiation. The nearly 12
cm was adjusted between the distance between lamp and Pyrex glass cell containing dye
suspension. The photocatalytic degradation rates of MB (10 ppm) were analyzed by using
UV-Vis spectrophotometer (MECASYS, Optizen2120) at every 60 min and clear liquid after
180 min irradiation with sample NM-Cu 4 was used to identify the degradation products of
MB. Isopropyl alcohol (IPA), triethanolamine (TEOA), and benzoquinone (BQ) were applied
•-
as scavengers to trap the hydroxyl radical (OH•), hole (h+), and superoxide anion radical (O2
), respectively. The scavengers (1 mM) were used during photocatalytic degradation of MB
and analyzed by using a UV-Vis spectrophotometer.
<H2>2.4 Analytical procedure for MB degradation
The high resolution-quantitative time of flight electrospray ionization mass spectrometry
(HR-QTOF ESI/MS) was used for analysis of the degradation products of MB. The reverse-
phase HPLC-PDA analysis was performed with a C18 column (YMC-Pack ODS-AQ; 4.6
mm internal diameter/150 mm long/5 m particle size) using binary conditions (H2O/0.05 %
trifluoroacetic acid buffer/100 % acetonitrile/flow rate (1 mL min-1/30 min). The acetonitrile
concentrations were as follows: 10% (0–2 min), 70% (20–24 min), 100% (24–28 min), and
50% (28–30). UV absorbance (254 nm) was used for each compound to monitor their HPLC
chromatograms. The binary mobile phases were composed of the solvent A (HPLC-grade
H2O /0.05 % trifluoroacetic acid) and solvent B (100 % acetonitrile). Total flow was
maintained (0.3 μL min-1/15 min program). The flow of acetonitrile (0–100 %, 0–9 min/0–
100 %, 9–12 min/ 0–100 %, 12–15 min) was used and stopped (15 min). The exact masses of

the organic compounds were analyzed by HR-QTOF ESI/MS (ACQUITY, UPLC Milford,
MA, USA-SYNAPT, and G2-S Water) in the positive ion mode.
<H2>2.5. Antibacterial activity
The antibacterial activities were evaluated against Pseudomonas aeruginosa KACC10232
(Korean Agricultural Culture Collection), Bacillus Subtilis KACC17047 (Korean
Agricultural Culture Collection), and Staphylococcus aureus ATCC-BAA-1687 (American
type Culture Collection) via well-diffusion method. The pathogens were regenerated in plate
by scraping on LB agar plate with incubating overnight. A single colony of bacteria was
selected using a 10 μL loop and inoculated into a falcon tube (5ꢀmL of LB broth). Then,
bacteria in the centrifuge tube were incubated (37ꢀ0C/200ꢀrpm/16ꢀh). Triplicate plates were
swabbed with the overnight culture (108ꢀcells/mL). The solid medium was gently
punctured with the help of cork borer to make a well in the plates. Finally, the samples (5
mg/mL and 10 mg/mL) were added from the stock into each well and incubated
(24ꢀh/37ꢀ±ꢀ2ꢀ°C) and the inhibition zones were obtained. As all of the plate reveals similar
pattern of inhibition zone, one of the representative plate has been presented.
<H1>3. Results and discussions
<H2>3.1. Characterization of as-prepared samples
Fig. 1 shows the XRD patterns of the pure α-NiMoO4 and Cu2+ doped (4 mole % and 6 mole
%) α-NiMoO4 samples. The diffraction patterns of various samples were all matched with the
monoclinic structure of pure α-NiMoO4 (JCPDS card number 086-0361). In addition, a new
o
peak was observed at 2θ of 26.57 that reveals the existence of trace amount of β-NiMoO4
(JCPDS card number 012-0348) in Cu-α-NiMoO4 samples. The diffraction patterns of Cu-α-
NiMoO4 samples also suggests that copper atoms have entered into the lattice of α-NiMoO4.
The crystallinity of α-NiMoO4 samples were increased by doping of 4 mole % of Cu.
Furthermore, the doping of 6 mole % of Cu, the crystallinity of sample decreased. The
possible reasons may associated with homogenous to heterogeneous nucleation type that
deteriorate the crystallite structure.
The FESEM images of samples are shown in Fig. 2. The sample NM has the rod like
morphology. Most of the rods are broken. The sizes of rods are around 500 nm-4 µm in
length and 100 nm-500 nm diameter. The doping of Cu (4 mole %) in α-NiMoO4 increases
the diameter (100 nm-1.5 µm) and length (1 µm-7 µm) of the rod. Most of the rods are thick.
In addition, broken tube like structure is seen. Furthermore, on increasing the concentration
of Cu2+ in the α-NiMoO4 also produces the broken rod like structure. However, the most of
the rods are thin as compared of NM-Cu 4 sample and thick as compare of NM sample. The
high calcination temperature may cause the formation of broken rod like structure. Fig. 3
reveals the FESEM elemental mapping of NM-Cu 4 that confirms the existence of Ni, Mo, O,
and Cu elements in NM-Cu 4 sample.

TEM micrograph of NM-Cu 4 sample is shown in Figure 4. The rod-like morphology of the
synthesized sample is evident from Fig. 4a and 4b. The HRTEM image in Fig. 4c
corresponds to the rectangular region marked in Fig. 4b. The measured lattice spacing of
0.412 nm, 0.278 nm, and 0.218 nm correspond to the crystal planes (111), (130) and (040)
respectively which are in good agreement with the JCPDS 086-0361 of α-NiMoO4. The edge
of the rod appears to be slightly disordered as can be seen from the HRTEM image of Fig. 4c.
The interplanar spacing of 0.619 nm calculated from Fig. 4f corresponds to the (110) crystal
plane of the α-NiMoO4 which is also the second strongest peak in the XRD spectrum. The
selected area electron diffraction (SAED) patterns in Fig. 4d and 4e belong to of the area
corresponding to the HRTEM images of Fig. 4c and 4f respectively. All the interplanar
spacings calculated from the SAED patterns are consistent with the crystallographic plane of
α-NiMoO4. Moreover, the clear lattice fringes and the diffraction patterns indicate that the
synthesized material is highly crystalline. No other impurity phase could be detected from
HRTEM observation. The EDS 1, 2, and 3 (Fig. 5) belong to the corresponding points in Fig.
4b show the existence of Ni, Mo, Cu, and O in the NM-Cu 4 sample. The quantitative
elemental analysis of corresponding to each point has also been presented in Fig. 5. It is seen
that the elements are uniformly distributed in the crystal lattice. Hence, confirmation of the α-
NiMoO4 phase via XRD, HRTEM, and SAED pattern analysis in conjunction with the result
of EDS analysis of the presence of Cu2+ ions inside the α-NiMoO4 phase indicates that Cu2+
ions exist at the lattice position of α-NiMoO4.
The presence of elements and their oxidation state in α-NiMoO4 and Cu doped α-NiMoO4
was examined by using X-ray photoelectron spectroscopic (XPS) technique and shown in
Fig. 6. The Ni 2p3/2 and Ni 2p1/2 peaks were observed in the sample NM (860.17 eV and
877.50 eV) and NM-Cu (858.75 eV and 876.16 eV). In addition, the satellite peaks of Ni
2p3/2 and Ni 2p1/2 were found in NM (866.33 eV and 884.46 eV) and NM-Cu 4 (864.55 eV
and 876.16 eV). These binding energies of Ni 2p indicates in undoped and doped α-NiMoO4
sample indicates the presence of electron correlation system and interaction between the Ni
and NiO6 octahedral [21]. The Mo 3d5/2 and Mo 3d3/2 spectra were seen in NM (235.55 eV
and 238.83 eV) and NM-Cu 4 (234.06 eV and 237.28 eV) that can be attributed to the Mo6+
oxidation state [1]. The doping of Cu2+ in the α-NiMoO4 causes shifting of low binding
energies of Ni 2p3/2, Ni 2p1/2, 3d5/2, and Mo 3d3/2. Furthermore, O 1s spectrum shows the
three deconvoulated peaks. In sample NM, the binding energy values at 530.24 eV, 533.85
eV, and 534.42 eV are associated to the chemisorbed or dissociated oxygen species, O2-
species in the lattice, and oxygen defects or vacancy, respectively. In addition, copper doped
sample contains peak at 530.04 eV (chemisorbed or dissociated oxygen species), 532.81 eV
(O2- species in the lattice), and 534.22 eV (oxygen defect or vacancy). Copper doped α-
NiMoO4 sample shows the higher amount of oxygen defects or vacancy [22]. Sample NM-Cu
4 exhibited peaks at 936.51 eV and 956.47 eV, which corresponds to binding energy of Cu
2p3/2 and Cu 2p1/2, respectively. The satellite peaks of 2p3/2 and Cu 2p1/2 were seen in 944.86
eV and 956.47 eV, respectively that suggests existence of Cu2+ oxidized state [16]. Hence,
from the XPS analysis, it was confirmed that sample contains Nickel, Molybdenum and
oxygen, and copper element.
The porosities of copper doped/undoped α-NiMoO4 samples were investigated by N2
adsorption-desorption isotherm analysis and the results were presented in Fig. 7(a-c), which
indicates the type-IV with a H2 hysteresis loop as well as mesopores inside the α-NiMoO4
materials. The BET surface area of copper doped α-NiMoO4 sample were found to exhibit
largest surface area as compared to undoped α-NiMoO4. The BET surface area of NM, NM-
Cu 4, NM-Cu 6 were found to be 9.4561 m2g-1, 18.5848 m2g-1, and 14.3080 m2g-1,
respectively. The zeta potential of various samples were shown in Fig. 7(d-f). The zeta
potential of the NM, NM-Cu 4, and NM-Cu 6 were found at -46.3 mV, -43.4 eV, and -19.3

mV, respectively. It is suggested that surface of samples is negatively charged. It also proves
that the samples are stable.
The Raman spectra of copper doped and undoped α-NiMoO4 samples are presented in Fig.
8(a and b). The Raman peaks have been assigned to the symmetric (964.36 cm-1 and 956.01
cm-1) and asymmetric stretching (914.95 cm-1 and 903. 97 cm-1) modes of the terminal Mo=O
bond as well as the distorted MoO6 octahedral present in the undoped and doped α-NiMoO4
samples [13,23]. These Raman spectra suggest the formation of pure α-NiMoO4. The Raman
bands at 708.54 cm-1 is associated with the Ni-O-Mo symmetric stretch. In α-NiMoO4, the
Mo=O bending modes (491.74, 445.38, 420.01, 388.96, 372.12, and 266.62 cm-1), Mo–O–
Mo deformation modes (144.91 and 178.58 cm-1), rotational/transitional modes (197.83 cm-
1), and lattice modes (116.26 cm-1) were observed. In Cu-α-NiMoO4, the Mo=O bending
modes (483.87, 385.24, 365.34, and 245.94 cm-1), Mo–O–Mo deformation modes (170.27
and 137.69 cm-1), and lattice modes (88.48 cm-1) were found [12,13]. The Raman bands at
491.74, 445.38, 420.01, and 483.87 also reveals superposition of MoO6 and NiO6 groups
building α-NiMoO4 [24]. The Raman peaks are shifted to lower wave number by doping
copper in α-NiMoO4. This behavior is probably due to substitution of Ni2+ ions by Cu2+ ions
in the α-NiMoO4 lattice as well as electronegativity variation between the Ni2+ ions and Cu2+
ions towards bonding of oxygen atom. Due to these reasons, the vibrational frequency can
altered between the [MoO4], [NiO8]/[CuO8] clusters, their corresponding [O–Ni–O–Mo–O–
Ni–O], [O–Ni–O–Mo–O–Cu–O], and [O–Cu–O–Mo–O–Cu–O] bond strength [25].
The coordination environments of Ni2+ and Mo6+ in the samples were analyzed by UV-Vis
diffuse reflectance spectroscopy, as shown in Fig. 8c. The absorption peak is observed at 337
nm. The absorption band at 200-415 nm indicates the octahedral (Oh) Mo species and the
charge transfer from O2- ligand to Mo6+ in MoO4 groups in α-NiMoO4[26]. The wavelength
at the region 415-460 nm consists of transfer of electron from the occupied 3d orbitals of the
Ni2+ ions to the unoccupied antibonding Mo-5d states and d-d transitions 3A2g(F) → 3T1g(P)
of the Ni2+ ions in octahedral coordination. Furthermore, the absorption bands at 720 nm
suggests Ni2+ (Oh) species [14,26]. The doping of copper in α-NiMoO4 causes the extending
the absorption spectra towards the longer wavelength or visible absorption. The wide
absorption band were seen at 200-630 nm. From the Fig. 8d, the direct band gap of samples
were calculated from the Kubelka-Munk functions versus the band gap energy plot and
observed to be 2.88, 2.08 and 1.77 eV for NM, NM-Cu 4, and NM-Cu 6, respectively. By
doping of copper in α-NiMoO4 host, the band gap is dramatically reduced.
<H2>3.2. Photocatalytic degradation of Methylene blue
The photocatalytic performance of samples were analyzed by degradation of MB dye (10
ppm) in aqueous solution under simulated solar light irradiation. The photocatalytic
performances were measured under similar condition in terms of UV-Vis degradation spectra,
and kinetic plot (C/Co), in Fig. 9(a-d). In the dark condition, the degradation of MB is 61.46
% in 30 min by NM sample. The photocatalytic degradation of MB is 16.58 % within 180
min (Fig. 9a). This results suggest that α-NiMoO4 has more tendency of absorption and less
tendency of MB photodegradation. However, the absorption decreases and photodegradation
increases by doping of copper in α-NiMoO4. As shown in Fig. 9(b-c), the photocatalytic
degradation of MB is 62 % and 59.39 % by the sample NM-Cu 4 and NM-Cu 6, respectively.
Fig. 9d shows the evolution of degradation rate with the time. The doping of Copper in α-
NiMoO4 makes enhancement of photocatalytic activity. Sample NM-Cu 4 shows best
photocatalytic performance.
To understand the mechanism for photocatalytic degradation of MB, trapping experiments
were carried out by using scavengers. As shown in Fig. 9(e-g), the scavengers decreased UV-
Vis degradation spectra of MB. The isopropanol (IPA), benzoquinone (BQ), and

•-
triethanolamine (TEOA) were used for trapping the OH•, O2 , h+, respectively. Among
scavengers, the addition of IPA (23.58 %) revealed the least degradation percentage of MB as
compared to BQ (33.68 %), and TEOA (42.32 %) in Fig. 9h. Therefore, it was concluded that
hydroxyl radicals were the main active species for degradation of MB in solar light.
In order to identify the intermediate degradation products of MB, HR-QTOF ESI/MS
(positive ion mode) was used. The total ion chromatogram (TIC) of MB degradation products
is shown in Fig 10. The sharp intense peak was appeared in the retention time, 3.68 min of
MB (without degradation) in Fig. 10a. After degradation (180 min) of MB by sample NM-Cu
4, the original peaks (retention time: 3.68) were disappeared and new peaks were formed,
which suggests the degradation of MB in solar light (Fig. 10b). The degradation of MB was
further confirmed by HR-QTOF ESI/MS spectra (Fig. 11). The MB fragments were observed
on the basis of retention time (tR), observed molecular mass [M-H]+, and fragment ions (m/z).
Table 1 shows the degradation products and their skeleton structure, elemental formula,
observed molecular mass, and retention time.
The degradation pathways of MB by photocatalyst has been presented in Fig. 12. The
molecular mass (m/z) of MB (methylthioninium chloride) is 319. The Cl- is ionized and exists
in the detached state as a MB cation [3,7-bis(dimethylamino)phenothiazin-5-ium]. The
molecular mass (m/z) of MB cation is 284, which is observed in the 3.69 min (retention time).
After that, two types of degradation pathway were proposed. In first pathway, there is
formation of high mass peaks at m/z = 301 of 3,7-bis(dimethylamino)-10H-phenothiazine 5-
oxide (retention time: 3.84 min) form the MB cation. The possible reason may be associated
with the formation of hydroxyl radical (OH•) that can attack the C-S+=C functional group of
MB [27,28]. At this stage, the O atom is bonded with Sulphur (S=O) and H atom is bonded
with N atom (N-H). Then, 3,7-bis(dimethylamino)-10H-phenothiazine 5-oxide is further
degraded to form 3, 7-bis (methyl amino)-10H-phenothiazine 5-oxide (m/z: 273 and retention
time: 3.21 min). During this process, the two methyl groups (-CH3) are lost from -N-(CH3)2.
Then, 3,7-bis(methylamino)-10H-phenothiazine 5-oxide is degraded to form 3,7-diamino-
10H-phenothiazine 5-oxide (m/z: 245 and retention time: 2.80 min) due to elimination of
methyl groups. The result also indicates that 30 amine is changed into the 20 amine and 10
amine. These intermediates products were further degraded to form multiple single ring
structures. In second pathways, the 3-(dimethylamino)-7-(methylamino) phenothiazin-5-ium
(m/z: 270 and retention time: 3.49 min) is produced from the MB cation by eliminating the
one methyl group by breaking -N-(CH3)2 bond. Methyl group is further oxidized to methanol
or methanoic acid [29]. Due to the breaking the bond of C-S and N-C bond by OH•, multiple
single ring structures are produced. The single ring structure consists of 2-hydroxy, 5-amino,
benzene sulfonic acid, 5-amino, benzene sulfonic acid, N,N-dimethyl, 5,6-hydroxy
cyclohexa-1,3-diene, 1,2,4-trihydroxy benzene, and phenol) in sodium/potassium mode at
0.93 min retention time. Finally, the small organic species are subsequently mineralized into
−
+
NO3 , SO42−, Cl−, NH4 , C, CO2 and H2O [30].
The mechanisms underlying the observed enhanced photocatalytic performance of Cu doped
α-NiMoO4 photocatalyst can be understood as follows. When absorbing the UV-Vis light
with phonon energy (hν) equal to or exceeding the band gap, the Cu-α-NiMoO4 produces
electron-hole pairs (e-/h+) at the surface (eq. 1).
Cu-α-NiMoO4 + hν → Cu-α-NiMoO4 (h+VB + e-CB) (1)
The photogenerated holes on the surface of Cu-α-NiMoO4 can interact with H2O and OH- to
•-
form OH• (hydroxyl radical). Then, O2 produces O2 (superoxide anion radical) by accepting
•
the electron, which further reacts with HO2 to form H2O2 (hydrogen peroxide) and OH• as
shown in eq. 2 to 6. The OH• has strong capacity to degrade the MB (eq. 7)
Cu-α-NiMoO4 (h+VB) + H2O → OH• + H+ (2)

Cu-α-NiMoO4 (h+VB) + OH- → OH• (3)
•-
Cu-α-NiMoO4 (e-CB) + O2 → O2 (4)
•-
•
O2 + HO2 + H+ → H2O2 + O2 (5) H2O2 → 2OH• (6)
OH• + MB → Intermediate products → CO2 + H2O (7)
It is believed that the photogenerated electron-hole pairs are trapped in the surface of Cu-α-
NiMoO4. In addition, XPS result indicates the presence of Cu2+ ions that acts as trapping
sites. Moreover, the oxygen vacancy also enhances the trapping of electron and holes on the
basis of XPS results. So, the doping can facilitate the formation of intermediate energy levels
that delay the recombination of photogenerated electrons and holes and makes efficient
charge separation [2,5,15,16]. In addition, the high BET surface of Cu doped NiMoO4
photocatalyst not only facilitates the surface absorption of the solar light but also provides
more catalytic sites for photocatalytic enhancement [31]. The schematic diagram of
photocatalytic mechanism of Cu-α-NiMoO4 photocatalyst is shown in Fig 13.
<H2>3.3. Antibacterial properties
The antibacterial activity of α-NiMoO4 and copper doped α-NiMoO4 samples (5 mg/mL and
10 mg/mL) were performed by well-diffusion method against S. aureus, B. Subtilis, and P.
aeruginosa (Fig. 14). The zone of inhibition of various bacteria were found in following
order: B. Subtilis˃S. aureus˃P. aeruginosa. The variation of inhibition zone size is related
with cell wall of pathogen because cell wall plays a major role in bacterial growth with
survival in hostile environments. The cell wall acts as a barrier to the penetration of toxic
molecules. The permeability properties of Gram-positive cell wall depends upon the nature of
peptidoglycan. Moreover, the B. Subtilis has more permeable cell wall than S. aureus. So, the
large inhibition zone is found in B. Subtilis than S. aureus. The outer membrane of Gram-
negative pathogen consists of significant barrier to the penetration of toxic molecules that
accounts the greater resistance of Gram-negative pathogen as compare of Gram-positive
pathogen [32]. Therefore, samples show the larger inhibition zone in S. aureus than P.
aeruginosa. The formation of inhibition zone by various samples against pathogens reveal the
excellent antibacterial performance due to toxicity and high BET surface area, despite
negative surface potential of samples. The inhibition zone is slightly increased with increase
in the concentration of samples due to proper diffusion of particles in medium with strong
interaction with bacterial cell surface [11]. In addition, copper doping α-NiMoO4 slightly
enhances the antibacterial performance due to increase in BET surface area that enhances the
strong interaction with bacteria. Therefore, α-NiMoO4 doped and undoped photocatalyst have
strong biocide effect in reducing the bacterial growth.
The copper doped α-NiMoO4 and undoped α-NiMoO4 particles have strong tendency to
accumulate the surface of bacteria cell that causes penetration of cell membrane. Due to
uptake of particles in the cell, they can interact with phosphorus and sulfur containing
compounds (DNA) and resulting into the cell death. They have capacity to alter the
membrane permeability. Moreover, α-NiMoO4 doped and undoped particles can easily bind
with protein molecules that causes degradation of proteins. They can inactivate the enzymes
as well as ATP production. It is also believed that the generation reactive oxygen species
•
•-
(OH•, HO2 , H2O2, O2 ) makes the oxidative damage to cellular components [11,18–20]. The
schematic diagram of antibacterial mechanism is shown in Fig. 15. The comparison of the
results of this work with other similar reports have been presented in table 2 [14,33–39].
<H1>4. Conclusions

The copper doped/undoped α-NiMoO4 rods were synthesized by microwave hydrothermal
process. The morphological and optical properties of samples were analyzed. XRD, HRTEM,
SAED, and EDS pattern analysis indicate that Cu2+ ions exist at the lattice position of α-
NiMoO4. The negative zeta potential values revealed the stability of samples. The
photocatalytic degradation of MB dye in water increases by doping the copper (4 mole %) in
α-NiMoO4 under solar light irradiation. The reduced band gap, oxygen vacancy, high BET
surface area, and efficient separation of photogenerated charges show contribution for
enhanced photocatalytic performance. The hydroxyl radicals were the main active species for
degradation of MB in solar light on the basis of active species trapping experiments. The HR-
QTOF ESI/MS technique is used to analyze the photodegradation intermediates of MB in
positive ion mode and degradation pathway mechanism is proposed. The formation of large
inhibition zone against pathogens (S. aureus, B. Subtilis, and P. aeruginosa) via well-
diffusion method indicates that small amount of photocatalyst powders have strong tendency
to inactivate the pathogens. The high BET surface area and toxicity of samples are
responsible for antibacterial performance. The inactivation of various bacteria were found in
following order: B. Subtilis˃S. aureus˃P. aeruginosa. So, the copper doped α-NiMoO4
photocatalyst reveals potential application for degradation of organic pollutants and effective
against multidrug resistant pathogens.
Acknowledgments
This Research was supported by the Global Research Laboratory Program of the National
Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and
Technology (MEST) of Korea (Grant Number: 2010-00339).
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<Figure>Fig. 1. XRD patterns of samples.
<Figure>Fig. 2. FESEM images of samples. (a) NM; (b) NM-Cu 4; (c) NM-Cu 6.
<Figure>Fig. 3. FESEM elemental mapping of NM-Cu 4 sample.
<Figure>Fig. 4. (a) TEM micrograph of rod; (b) TEM micrograph of enlarged rod portion; (c
and f) HRTEM images; (d and e) SAED pattern of NM-Cu 4 sample.
<Figure>Fig. 5. EDS analysis of NM-Cu 4 sample. The points 1, 2 and 3 represents the
corresponding point in Fig. 4b.
<Figure>Fig. 6. XPS spectra of samples (NM and NM-Cu 4).
<Figure>Fig. 7. Nitrogen adsorption-desorption isotherm (a-c) and zeta potential graph (d-f)
of various samples.
<Figure>Fig. 8. (a) Raman spectra; (b) Enlarged portion (50-600 cm-1) of Raman spectra; (c)
UV–Vis DRS spectra; (d) Kubelka-Munk plot for band gap energy of samples.

<Figure>Fig. 9. Photocatalytic degradation of MB (10 ppm) dye under solar light irradiation.
(a-c) UV–Vis spectra of samples; (d) Kinetic plot for the degradation of MB; (e-g) UV–Vis
spectra of NM-Cu 4 sample by using IPA, TEOA, and BQ; (h) degradation percentage.
<Figure>Fig. 10. The total ion chromatogram (TIC) of MB degradation products. (a) 0 min;
(b) 180 min.
<Figure>Fig. 11. The HR-QTOF ESI/MS spectra of degraded MB at 180 min by NM-Cu 4
sample.
<Figure>Fig. 12. Suggested Photocatalytic degradation pathways for MB degradation under
solar light irradiation by Cu-α-NiMoO4 photocatalyst.
<Figure>Fig. 13. Schematic illustration of photocatalytic mechanism.
<Figure>Fig. 14. Well-diffusion method of various samples at different concentrations (blue
circle indicates 5mg/mL and red circle indicates 10mg/mL) against pathogens. (a) S. aureus;
(b) P. aeruginosa; (c) B. Subtilis.
<Figure>Fig. 15 Schematic representation of antibacterial mechanism.
<Table>Table 1. Identification of MB degradation products by HR-QTOF ESI/MS analysis.
Name
Structure
Elemental
Molecular
Retention
formula
mass
Time (tR)
(Observed) (min)
3,7-bis(dimethylamino)-10H-
phenothiazine 5-oxide
C16H19N3OS
301.1635
3.84
3,7-bis(dimethylamino)
phenothiazin-5-ium
C16H18N3S+
C15H16N3S+
284.1217
270.1066
3.69
3.49
3-(dimethylamino)-7-
(methylamino) phenothiazin-
5-ium
3,7-bis(methyl amino)-10H-
phenothiazine 5-oxide
C14H15N3OS
273.1674
3.21

3,7-diamino-10H-
phenothiazine 5-oxide
C12H11N3OS
C6H6NO4S
245.1365
211.1437
2.80
0.93
2-hydroxy,5-amino, benzene
sulfonic acid
5-amino,benzene
acid
sulfonic
C6H5NO3S
C8H13NO2
211.9465
177.9960
N,N-dimethyl,5,6-hydroxy
cyclohexa-1,3-diene
1,2,4-trihydroxy benzene
C6H8O3
C6H6O
152.9467
117.9594
Phenol
<Table>Table 2. The comparison of the results of this work with other similar reports.
Synthesis
method
Host
Shape/size
Photocatalytic /Antibacterial Ref.
activity
Chemical
solution
decompositio
n
(NiMoO4)x-
Bi2Ti4O11
Nanoparticles
(30±2 nm)
Degradation of Methyl orange [33]
in 360 min under Hg lamp
Hydrotherma NiMoO4-H2O
Micro
flowers Degradation of Acid fuchsine in [34]
l
(diameter 20 μm)/ 30 min under UV irradiation
nanowires (130 nm
diameter)
Chemical
NiMoO4-CTA lamellar
Degradation of Acid fuchsine [35]
under Xe lamp
precipitation (cetyltrimethyl mesostructured
ammonium)
Pechini
α-NiMoO4
Sphere
nm)
(50-450 80 % degradation of Methylene [14]
blue in 120 min under Visible
irradiation
Co-
NiMoO4
Nanorod (1200 and 60.7 % degradation of Methyl [36]
precipitation
30 nm length)
orange in 90 min under UV
irradiation
Sonochemica NiMoO4
l
Nanoparticles (65– 67 % degradation of Methyl [37]
70 nm)
orange in 60 min under UV
irradiation
Hydrotherma NiMoO4/g-
Nanorod (100 nm Degradation of Rhodamine B in [38]
diameter and 300 120 min under visible
nm length)/ layered irradiation
Nanosheets 98 % degradation of Methylene [39]
l
C3N4
Hydrotherma β-NiMoO4

l
blue in 150 min under diffused
sunlight
Microwave
hydrothermal
Cu-α-NiMoO4 Microrod (100 nm- 62 % degradation of Methylene This
1.5 μm length and blue in 180 min under UV- work
500
nm-7
μm Visible
irradiation/proposed
diameter)
degradation pathway on the
basis of HR-QTOF ESI/MS
analysis/Inactivation of S.
aureus, B. Subtilis, and P.
aeruginosa by well-diffusion
method
TDENDOFDOCTD