Insight Into Malachite Green Degradation, Mechanism and Pathways by Morphology-Tuned α-NiMoO4 Photocatalyst

Article abstract of DOI:10.1111/php.12872

The microwave hydrothermal process was used for the synthesis of various morphologies of α-NiMoO₄ by simply adjusting the pH during experimental conditions. The effect of morphology/size on the photocatalytic performances for degradation of malachite green (MG) has been investigated under UV-Vis/visible light irradiation. Nanorod morphology has strong tendency to degrade (88.18%) the MG as compared to spherical quantum-sized (57.65%) and layered square microsheet (37.98%) under UV-Vis irradiation in 180?min. The active species trapping experiment revealed that active species (OH^?, O₂ ^?? and h⁺) play a crucial role for MG degradation. The high BET surface area, greater amount of oxygen defect and efficient separation of electron–hole pair are responsible for MG degradation. About seventeen types of organic fragments of MG were confirmed by high resolution-quadruple time of flight electrospray ionization mass spectroscopy (HR-QTOF ESI/MS) technique on the basis of retention time and molecular masses. Degradation mechanism and pathways were proposed that follow the demethylation, nitration, decarboxylation, hydrolysis, decarboxylation and oxidation reaction. The reduction of total organic carbon revealed the mineralization of MG during photocatalytic degradation process. Therefore, this article represents the investigation of MG degradation by various morphology of α-NiMoO₄ and detailed degradation mechanism and pathways.

Full text of DOI:10.1111/php.12872

Article type : Research Article
Insight Into Malachite Green Degradation, Mechanism, and
Pathways by Morphology Tuned α-NiMoO4 Photocatalyst
Schindra Kumar Ray1, Dipesh Dhakal2, Soo Wohn Lee*1
1 Department of Environmental and Bio-Chemical Engineering, Sun Moon University, Chungnam,
31460, Republic of Korea,
2 Department of Life Science and Bio-chemical Engineering, Sun Moon University, Chungnam 31460,
Republic of Korea
*Corresponding author e-mail: swlee@sunmoon.ac.kr (Soo Wohn Lee)
ABSTRACT
The microwave hydrothermal process was used for the synthesis of various morphologies of α-
NiMoO4 by simply adjusting the pH during experimental conditions. The effect of morphology/size
on the photocatalytic performances for degradation of malachite green (MG) has been investigated
under UV-Vis/visible light irradiation. Nanorod morphology has strong tendency to degrade (88.18
%) the MG as compared to spherical quantum-sized (57.65 %) and layered square microsheet (37.98
%) under UV-Vis irradiation in 180 min. The active species trapping experiment revealed that active
species (OH•, O2 , and h+) play a crucial role for MG degradation. The high BET surface area, greater
•̶
amount of oxygen defect, and efficient separation of electron-hole pair are responsible for MG
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degradation. About seventeen types of organic fragments of MG were confirmed by high resolution-
quadruple time of flight electrospray ionization mass spectroscopy (HR-QTOF ESI/MS) technique on
the basis of retention time and molecular masses. Degradation mechanism and pathways were
proposed that follow the demethylation, nitration, decarboxylation, hydrolysis, decarboxylation, and
oxidation reaction. The reduction of total organic carbon revealed the mineralization of MG during
photocatalytic degradation process. Therefore, this paper represents the investigation of MG
degradation by various morphology of α-NiMoO4 and detailed degradation mechanism and pathways.
INTRODUCTION
Malachite green (MG) is a category of basic dye that consists of triarylmethane group. It is widely
applied in various industries (textile, aquaculture, biocide, silk, leather, paper, and acrylic) and
inactivation of parasites (bacteria and fungus) for fish. Beside its diverse application, MG is
hazardous and carcinogenesis compounds (1). The severe water pollution is occurred by release of
MG in water that causes serious human health hazards (damage the liver, spleen, kidney as well as
reduction of fertility rate) and toxic effect to the aquatic ecosystem (1,2). To solve the water pollution
problem, semiconductor photocatalysis is potential green technology that can degrade the hazardous
organic dyes by utilizing the natural energy of sunlight via advanced oxidation processes (AOPs) (3-
5).
Metal molybdate has attracted considerable interest because of widespread application in energy
storage materials, phosphor, photocatalyst, antibacterial, laser materials, scintillators, optical fibers,
humidity sensors, and magnetic materials (4-9). Among metal molybdates, NiMoO4 has low
temperature α- phase and high temperature β-phase. The monoclinic structure (C2/m space group) of α-
NiMoO4 has an octahedral co-ordination of Mo6+ and Ni2+ ions with edge shared oxygen atoms. The
low cost, environmentally friendly, 2.8 eV energy band gap, and stable at room temperature features
of α-NiMoO4 make promising application in photocatalysis (5,6). However, little literature was
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available for photocatalytic activity of NiMoO4 due to its high electron-hole recombination rate (5).
Controlling the morphology/size by several strategies (surfactants, pH, solvent, concentration, etc.) of
the photocatalyst played the crucial role for prevention of electron-hole recombination because
photocatalytic activity is surface related phenomena (10). The investigation of organic dyes
degradation by photocatalyst via absorption spectra was found in many literatures that is unable to
give information about the intermediate organic fragments during degradation. So, tuning the various
morphology of photocatalyst, identification of intermediate organic fragments, and complete
degradation pathways are systematic way to investigate the photodegradation of organic dyes.
In this study, various morphologies (layered square microsheet, nanorod, and spherical quantum-
sized) of NiMoO4 were synthesized by microwave hydrothermal method. The optical properties of
NiMoO4 samples were investigated. The photocatalytic performances of samples were evaluated
through the degradation of MG under UV-Vis and visible light irradiation. The active species were
determined by scavenger experiment. The degraded organic fragments were identified with the help
of HR QTOF-ESI/MS analysis and degradation pathways were purposed. Furthermore, the
mineralization of MG was investigated by TOC (total organic carbon) technique.
MATERIALS AND METHODS
Material synthesis and characterization. All the chemicals were used from Sigma-Aldrich. The
aqueous solution of NiCl2.6H2O (2×10-2 mole/40 mL) and H2MoO4 (2×10-2 mole/40 mL) were
prepared in the separate beaker. The greenish-yellow precipitate was formed by mixing the
precursor. The precipitated solutions were maintained to different pH (5, 7, and 8) and continuously
stirred up to 4 h. The pH of precipitated solutions were adjusted by using aqueous NH3 (30%). Then,
it was placed in a microwave reactor (Eyla MWO-1000 wave Magic) for the hydrothermal treatment
0
(150 C/1 h/200 W). The obtained precipitate was washed (water and ethanol) several times. Then,
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0
0
it was dried (70 C) and calcined (500 C/5 h) in air. The samples were coded as NM-M, NM-N, and
NM-Q for pH 5, pH 7, and pH 8, respectively.
The samples were characterized by powder X-ray diffraction (XRD) (Rigaku, D/MAX 2200HR), Field
emission scanning electron microscope (FESEM) (JSM-6700F), Transmission electron microscopy
(TEM) (JEOL-2010), XPS (Multilab system/Al Kα source/15 kV/200 W), Raman spectra (λex = 530 nm,
LabRam, Horiba, France), and UV-Vis diffuse reflectance spectra (DRS) (V 570, Jasco International
Co. Ltd.). The Brunauer-Emmett-Teller (BET) surface areas of NiMoO4 samples were analyzed by the
nitrogen adsorption-desorption technique (micrometrics, ASAP 2020 automatic analyzer at 77 K).
Photocatalytic and radical trapping experiment. The photocatalyst powder (0.1g) was placed in a
Pyrex glass, and 70 mL of aqueous solution of MG (10 ppm) was added into it. The mixture was
placed in dark (40 min/constant stirring) for obtaining the adsorption-desorption equilibrium. Then,
the sample dye suspension was constantly stirred (magnetic stirrer) under the simulated solar and
visible light irradiation (portable solar simulator, PEC-L01, 150 mW/cm2, pecell, Am 1.5G). For
visible light source, an ultraviolet cut-off (λ< 420 nm) filter was used in solar simulator. The distance
between lamp and Pyrex glass cell (containing dye suspension) was nearly 12 cm. The photocatalytic
degradation rates of MG (10 ppm) were evaluated by using UV-Vis spectrophotometer (MECASYS,
Optizen2120) at every 45 min. Then, the clear liquid (after 180 min irradiation with sample NM-N)
was used to identify the degradation products of MG by comparing with fresh MG. In addition, the
total organic carbon (TOC) of the MG was analyzed by a total organic carbon analyzer (TOC-L
CPH/CPN, 00284 Auto sampler, Shimadzu). Prior to the TOC investigation, the solution was filtered
by a syringe (0.2 μm pore size) for removing the photocatalyst from the solution (after 180 min
irradiation in UV-Vis/visible light). All the TOC experiments were carried out in triplicate.
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Isopropyl alcohol (IPA), triethanolamine (TEOA), and benzoquinone (BQ) were used as scavengers
•-
for trapping the hydroxyl radical (OH•), hole (h+), and superoxide anion radical (O2 ), respectively.
The scavengers (1 mM) were used during photocatalytic degradation of MG and analyzed by UV-Vis
spectrophotometer.
Analytical procedure for MG degradation. For analysis of MG degradation, the samples were
analyzed by reverse-phase HPLC-PDA analysis equipped with C18 column (YMC-Pack ODS-AQ;
4.6 mm internal diameter/150 mm long/5 m particle size) that connected with a UV detector (254nm)
using a 30 min binary program. The composition of binary mobile phases consist of the solvent A
(HPLC-grade H2O /0.05 % trifluoroacetic acid) and solvent B (100 % acetonitrile). The acetonitrile
concentrations, 10% (0–2 min), 70% (20–24 min), 100% (24–28 min), and 50% (28–30 min) were
maintained. For identification of degradation products of MG, the high resolution-quantitative time of
flight electrospray ionization mass spectrometry (HR-QTOF ESI/MS) technique was used (5). For the
analysis, ultra-pressure liquid chromatography (UPLC)-photodiode array (PDA) analysis was
performed using a reverse-phase UPLC-PDA with a C18 column (ACQUITY UPLC® BEH, C18, 1.7
μm) connected to a PDA (UPLC LG 500 nm) at absorbance of 254 nm. The binary mobile phases
were composed of same solvents as in HPLC condition explained previously. Total flow (0.3 μL min-
1/15 min program) was maintained. The flow of acetonitrile (0–100 %, 0–9 min/0–100 %, 9–12 min/
0–100 %, 12–15 min) was used. The exact masses of the organic fragments were analyzed by high-
resolution quadruple-time of flight-electrospray ionization-mass spectrometry HR-QTOF ESI/MS in
the positive ion mode using an ACQUITY (Billerica, United States of America) column coupled with
a SYNAPT G2-S (Water Corp) column.
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RESULTS AND DISCUSSION
Characterization of as-prepared samples
Figure 1 reveals the powder XRD patterns of different NiMoO4 samples. The diffraction patterns of
all the samples were well matched with the monoclinic structure of pure α-NiMoO4 (JCPDS card
number 086-0361). In addition, a new peak was appeared at 2θ of 26.35 o that suggests the existence
of trace amount of β-NiMoO4 (JCPDS card number: 012-0348) in NM-N and NM-Q sample. In
sample NM-M, the two new peaks were observed at 2θ of 23.51 o and 27.44 o that show the presence
2-
of β-MoO3 (JCPDS card number: 037-1445). It indicates that MoO4 ions can polymerized into
Mo7O246-during microwave hydrothermal condition at low pH. Then, Mo7O246- is changed into MoO3.
The formation of NiMoO4 and MoO3 phase is represented by following reaction (11,12).
NiCl2.6H2O → Ni2+ + 2Cl- + 6H2O
(1)
(2)
2-
H2MoO4 → 2H+ + MoO4
Ni2+ + 2Cl- + 6H2O +2H+ + MoO42- → NiMoO4 + 2HCl + 6H2O
MoO42- + 8H+ → Mo7O246- + 4 H2O
(3)
(4)
(5)
(6)
(7)
Mo7O246- + 6H+ + 11H2O → 7MoO3.2H2O
MoO3.2H2O → MoO3.H2O + H2O
MoO3.H2O → MoO3 + H2O
The crystallinity of samples were decreased by increasing the pH of the sample. The variation of
XRD peak in different sample indicates the change of morphology and size.
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Figure 2 shows FESEM and TEM images of various samples. Interestingly, the sample, NM-M
presents the layered square microsheet morphology that consists of size around 1µm (Figure 2a and
2b). NM-N reveals the nanorod like morphology (Figure 2c). It consists of diameter about 5-60 nm
with length 10-200 nm. The nanorods are slightly aggregated. The sample, NM-Q shows the spherical
like morphology (Figure 2d). It has quantum size (4-7 nm). The particles are also slightly aggregating
in NM-Q. The results reveal that the particles size were decreased with increased in pH. At low pH,
2-
the H+ ions may attach to the plane of MoO4 ions in the initial stage of crystal growth due to
electrostatic attraction that causes the inhibition of crystals nucleation. Due to this reason, the size of
particle is increased. At high pH, the rate of crystal nucleation is higher than that of crystal growth
that causes the formation of nanoparticles (13).
The elemental composition with chemical states of the elements in the samples were analyzed with
the help of XPS technique. Figure 3 represents the XPS of samples. The binding energy of Ni 2p3/2
and Ni 2p1/2 peaks were observed in sample NM-M (860.53 and 877.73 eV), NM-N (860.23 and
877.61 eV), and NM-Q (859.16 and 877.32 eV). Two satellite peaks of Ni 2p3/2 and Ni 2p1/2 were
found in NM-M (866.53 and 884.73 eV), NM-N (866.23 and 884.76 eV), and NM-Q (865.16 and
883.32 eV). These binding energies of Ni 2p presents the presence of electron correlation system and
interaction between the Ni and NiO6 octahedral in the α-NiMoO4 (5,6). The Mo 3d5/2 and Mo 3d3/2
spectra were seen in sample NM-M (237.72 and 241.24 eV), NM-N (233.10 and 235.81 eV), and
NM-Q (233.88 and 236.88 eV) that can be assigned to the Mo6+ oxidation state (4). In addition, the O
1s peaks could be decovoluted into chemisorbed or dissociated oxygen species (NM-M: 531.14 eV,
NM-N: 529.39 eV, and NM-Q: 529.78 eV), lattice oxygen (NM-M: 535.79 eV, NM-N: 532.48 eV,
and NM-Q: 532.94 eV), and oxygen vacancies or defect (NM-M: 537.14 eV, NM-N: 533.29 eV, and
NM-Q: 533.60 eV). The results suggested that nanorod revealed that higher amount of oxygen
defects or vacancy than other morphologies (5,14). Hence, from the XPS analysis, it was confirmed
that sample consists of nickel, molybdenum and oxygen element.
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The porosities of samples were investigated by N2 adsorption-desorption isotherm analysis (Figure
4a-c) that shows type-IV with a H2 hysteresis loop as well as mesopores inside the materials. The
BET surface area of NM-N (44.50 m2g-1) were found to exhibit largest surface area as compared to
NM-M (26.33 m2g-1) and NM-Q (43.23 m2g-1). The Raman spectra of various samples are shown in
Figure 4d. The strong Raman peak was observed at 818.31 cm-1 that is related to distorted MoO6
octahedral present in sample NM-M. The Raman peak are associated with the symmetric (992.02 and
665.28 cm-1) and asymmetric (957.11 cm-1) stretching modes of the terminal Mo=O bond. The Mo=O
bending modes (286.49, 338.75, 375.02, and 486.53 cm-1) and Mo-O-Mo deformation modes (121.14,
156.06, 196.02, 217.54, and 243.01cm-1) were observed. In remaining samples, the Mo=O bending
modes and Mo-O-Mo deformation modes were depressed. The peaks, 941.52 and 957.11 cm-1 were
observed for sample NM-N and NM-Q, respectively, which are associated with symmetric and
asymmetric stretching modes of the terminal Mo=O bond as well as distorted octahedral. The Raman
band at 704 cm-1 is associated with the Ni-O-Mo symmetric stretch in sample NM-N (5,6,15). The
Raman peaks were slightly varied and shifted due to variation of morphology of samples.
The coordination environments of Ni2+ and Mo6+ in the different morphologies of NiMoO4 samples
were observed by UV-Vis diffuse reflectance spectroscopy (Figure 4e). The octahedral (Oh) Mo
species (200-415 nm) were observed that are associated to the charge transfer from O2- ligand to
Mo6+. The absorption band (415-460 nm) indicates the transfer of electron from the occupied 3d
orbitals of the Ni2+ ions to the unoccupied antibonding Mo-5d states and spin allowed d-d transitions
3
3A2g(F) → T1g(P) of the Ni2+ ions in octahedral coordination. Furthermore, the absorption bands
(723-823 nm) is associated with Ni2+ (Oh) species (5,6,16). From the Figure 4f, the direct band gap of
the different samples were obtained from the Kubelka-Munk functions versus the band gap energy
plot and found to be 3.01, 2.91 and 2.75 eV for NM-M, NM-N, and NM-Q, respectively. It is
observed that the band gap of the samples are reduced with decrease in size. The different values of
optical band gap can also be associated with the morphology and size of the particles. In addition, the
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valence band (VB) and conduction band (CB) potentials of the α-NiMoO4 nanorod at the point of
zero charge can be predicted using the following equations:
EVB = X – Ee + 0.5 Eg
(8)
ECB = EVB – Eg
(9)
Where X, EVB, ECB, Ee, and Eg are the absolute electronegativity, VB potential, CB potential, energy
of electron vs. hydrogen (4.5 eV), and band gap of the semiconductors, respectively. Moreover, the
electronegativity (X) of semiconductor can be calculated by following equation:
X= [x (A)a x (B)b x(C)c]1/(a+b+c)
(10)
Where, a, b, and c represent the number of constituent atoms in the compounds, which is the
arithmetic mean of the electron affinity and the first ionization energy. Based on the above equations,
the value of VB and CB for α-NiMoO4 were found to be 0.195 and 3.105 eV, respectively (17,18).
Photocatalytic degradation of Malachite green
The photocatalytic degradation of samples were examined by degradation of MG (10 ppm) in
aqueous solution under UV-Vis irradiation. All the measurements were carried out under the similar
condition. The characteristic absorption peak of MG at 617 nm was selected to monitor the
photocatalytic degradation of MG. Figure 5a-c represents the UV-Vis degradation spectra of MG
during degradation by different samples. The slightly adsorption of MG under dark condition was
observed. Furthermore, the UV-Vis absorption spectrum of MG under dark condition was monitored
for observing the adsorption of MG by the different samples (Figure 5d). It indicates that adsorption-
desorption equilibrium was established within 40 min. Also, the adsorption rate of MG by samples
was increased with decreased in particle size. Figure 5e shows the variation of C/Co of MG as a
function of time. As shown in Figure 5e, the highest photocatalytic activity was observed in the
sample NM-N, which degraded the MG within 180 min about 88.18 % as compared to NM-M (37.98
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%) and NM-Q (57.65 %) in UV-Vis light. Furthermore, the photocatalytic degradation kinetics curve
of MG by different samples was presented in Figure 5f. The linear curves followed the pseudo-first
order kinetic model, ln (Co/C) = kapp t, where kapp, is the apparent first order rate constant while C and
Co are concentration at time t and 0, respectively (4,5). The degradation rate constant of NM-M, NM-
N, and NM-Q samples are 3.79×10-3, 1.27×10-2, and 6.16×10-3 min-1, respectively. The photocatalytic
degradation kinetics curve reveal that the sample NM-N (nanowire) shows the best photocatalytic
performances than NM-Q (spherical quantum sized) and NM-M (layered square microsheet) in UV-
Vis light.
The kinetic plot (C/Co) of MG by different samples under visible light irradiation was shown in
Figure 5g. This kinetic plot reveals that NM-N degraded MG effectively about 79.43 % as compared
to NM-M (32.24 %) and NM-Q (52.25 %). As shown in Figure 5h, the NM-N sample exhibited the
maximum value of degradation rate constant (1.01×10-2 min-1) than NM-M (3.28×10-3 min-1) and
NM-Q (5.82×10-3 min-1). These photocatalytic degradation graphs (UV-Vis and visible light
irradiation) indicate that NM-N (nanowire) shows the best photocatalytic performances than NM-Q
(spherical quantum sized) and NM-M (layered square microsheet) in UV-Vis and visible light. It also
suggests that the morphology is crucial factor for photocatalytic activity. Figure 5i presents the
degradation percentage of different samples under UV-Vis and visible light irradiation. Figure 5i
suggests that degradation of MG is slightly enhanced in UV-Vis than visible light because UV-Vis
light covers the more solar spectrum than visible light.
The possible reason for enhancement of photocatalytic activity of nanowire may associated with high
BET surface area that provides more catalytic sites as well as enhancement of reactive oxygen
species (ROS) generation. Moreover, nanorod morphology revealed the greater amount of oxygen
defects or vacancy as compared to layered square microsheet and spherical quantum size α-NiMoO4
on the basis of XPS result (5,14). The photocatalytic activity is decreased in quantum size α-NiMoO4
photocatalyst. The possible reason may be related to higher electron/hole recombination rate due to
localization of electron/hole in extremely small size crystals (19).
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In order to find out the role of main active species in the degradation process for photodegradation
mechanism, the trapping experiments were carried out by using scavengers (Figure 6a-c). The
isopropyl alcohol (IPA, 1 mM), benzoquinone (BQ, 1 mM), and triethanolamine (TEOA, 1 mM)
•
̶
were applied for trapping the OH•, O2 , and h+, respectively (5). The adsorption of MG by sample
NM-N with scavenger under dark condition was observed. The adsorption-desorption equilibrium
was obtained within 40 min (Figure 6a). Also, the adsorption-desorption equilibrium was slightly
changed by scavengers. Among different scavenger, IPA revealed the least degradation (21.7 %) of
MG as compared to BQ (73.33 %) and TEOA (56.56 %) in Figure 6b-c. Therefore, the result certified
̶
that hydroxyl radicals (OH•) were the major active species while O2• , and h+ were minor active
species for degradation of MG by sample NM-N under UV-Vis irradiation.
Identification of photodegraded intermediate products
The total ion chromatogram (TIC) of MG degradation products was analyzed by UPLC-PDA (Figure
7). The sharp intense peak was appeared in malachite green, MG (without degradation) in Figure 7a.
After degradation (180 min) of MG by sample NM-N, the original peaks were decreased and various
new peaks were formed that proves the degradation of MG in UV-Vis light.
The photodegraded intermediate products of MG were further analyzed by HR-QTOF ESI/MS
spectra in positive ion mode (Figure 8). The different MG fragments were observed on the basis of
retention time (tR), theoretical molecular mass [M+H]+, observed molecular mass [M+H]+, and
fragment ions (m/z).
The photodegraded products, skeleton structure, elemental formula, theoretical molecular mass,
observed molecular mass, and retention time are shown in Table 1.
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Figure 9 reveals the possible degradation pathways of MG by α-NiMoO4 photocatalyst. There is loss
of Cl- from the MG (fragment 1) by ionization process and organic fragment 2 is formed. Fragment 2
is transferred into fragment 2c and 2d by subsequent removal of methyl group from the unsaturation
bond, =C=N+= attached ring. Due to attack of OH• on the central carbon atom between three rings and
demethylation process, fragment 2 is changed into 2a and 2b fragments. The fragment 2b consists of
3o alcohol. Similarly, the fragments 3, 3d, and 3e are formed by the attack of OH• on center of three
rings (=C=C=) in fragments 2, 2c, and 2d, respectively.
The fragment 3 consists of tertiary alcohol group (3o alcohol) that shows four pathways. At first,
fragment 3 is transformed into the fragment 3d, 3e, and 2b by demethylation process. In this case,
methyl group is removed one by one from –N= of two benzene rings. Fragment 4h, 6g, and 6f are
generated from 2b by eliminating one benzene ring, oxidation, attack of OH•, demethylation, and
nitration processes. Fragment 6f is further changed into the 6b and 6e by reaction of OH• in ketone
(=C=O) center and rings are separated. In this case, oxidation process is also occurred because of
conversion of ketone into carboxylic acid functional group. In addition, fragment 6c is obtained from
6b by removing carboxylic functional group (decarboxylation) via attack of OH•.
Secondly, fragment 3 is converted into the 3b by breaking of bond between the central carbon atom
and benzene ring. In this process, one benzene ring is lost. In addition, fragment 3c is generated from
the 3b by addition OH group in carbon atom (between two benzene ring). Also, fragment 4a is
obtained from the 3b by demethylation process (removal of one methyl group). Fragment 4a can
changed into 4b by attack of OH•. Similarly, fragment 4b gives the products 5a and 5b by attack of
OH•, nitration, and oxidation process, which is further mineralized into CO2 and H2O.
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Thirdly, fragment 3 is changed into 3g and 3h via breaking of benzene ring attached central carbon
bond via attack of OH• and oxidation. In this process, 3o alcohol is oxidized into ketone. Furthermore,
fragment 3g is transformed into the 4c and 4d by oxidation of ketone into carboxylic acid. Fragment
4c is changed into 5c (nitro benzene), which is further transformed into 3h and 6c because of
nitration, hydrolysis, and demethylation processes. Also, fragment 4d is converted into 5d, 6b, and 6c
by demethylation, nitration, decarboxylation, and hydrolysis processes.
At last, fragment 3 gives the product 4e and 4f by interacting with OH•. In this step, oxidation of
tertiary alcohol group into ketone functional group and separation of benzene ring are occurred.
Furthermore, fragment 4f is transformed into the 4g and 4h by removal of methyl group
(demethylation) at each step. Fragment 4g is also generated from 3e by oxidation 3o alcohol into
ketone. Fragment 5e and 6c are obtained from 4e by hydrolysis, demethylation, and nitration
processes. Finally, fragment 6c gives the product 7a. Then, the breaking of the carbon bond in
benzene ring (ring opening process) is occurred and fragment 8a and 8b are formed, which further
gives 1,2-ethanedioic acid. Finally, all the small fragments are mineralized into CO2 and H2O by
attack of OH• (3,5,20).
To confirm the mineralization of MG into CO2 during photocatalytic degradation, TOC method was
used. Table 2 suggests that all samples have capacity to mineralize the MG in UV-Vis and visible
light. Among them, NM-N reveals higher degree of mineralization (UV-Vis: 54.28 % and visible:
42.82 %) than NM-M (UV-Vis: 6.53 % and visible: 2.14 %) and NM-Q (UV-Vis: 33.40 % and
visible: 23.76 %). The reduction of TOC or increase of mineralization percentage by samples
indicates the formation of CO2 and inorganic ions during photocatalytic degradation of MG in UV-
Vis and visible light irradiation (21).
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Upon UV-Vis/visible light irradiation, NiMoO4 absorbs photon of energy that matches or exceeds the
band gap of materials, which provides the driving force for movement and generation of electron-hole
pair. The photogenerated electrons and holes can occupy the conduction band and valance band,
respectively. The interaction of photogenerated holes on the surface of α-NiMoO4 with H2O and OH-
to generate OH• (hydroxyl radical). Then, O2 generates O2•- (superoxide anion radical) via accepting
•
the electron that interacts with HO2 to form H2O2 (hydrogen peroxide) and OH• (4,5). The OH• has
strong tendency to degrade the MG. It is also believed that the Ni2+ acts as an e- and h+ trapper (22).
Also, the high BET surface of NiMoO4 photocatalyst provides better absorption of UV-Vis light in
the surface that creates more catalytic sites for photocatalytic enhancement via high amount of ROS
generation (5,6). The charge transfer transition in MoO42- ions with spin allowed d-d transition of the
Ni2+ in UV-Vis region also causes the high photocatalytic performances of NiMoO4 (12,16). The
photocatalytic degradation mechanism of MG by α-NiMoO4 photocatalyst is shown in Figure 10.
CONCLUSION
Different morphologies of α-NiMoO4 (layered square microsheet, nanorod, and spherical quantum-
sized) were synthesized by microwave hydrothermal method via controlling pH during experimental
condition. The band gap energy of the samples were reduced with decrease in size, which was benefit
for the degradation of MG under the irradiation of UV-Vis and visible light. The nanorod
morphology revealed the strong photocatalytic degradation (UV-Vis: 88.18 % and visible: 79.43 %)
of MG as compared to spherical quantum-sized (UV-Vis: 57.65 % and visible: 52.25 %) and layered
square microsheet (UV-Vis: 37.98 % and visible: 32.24 %) due to high BET surface area, greater
amount of oxygen defect or vacancy, and effective separation of electron-hole pair. The active
species trapping experiment revealed that OH• radical play a major role for MG degradation. The
photodegraded intermediates (17 types) were identified by HR-QTOF ESI/MS technique in positive
ion mode on the basis of retention time. Degradation mechanism and pathways were proposed on the
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basis of degraded organic fragments that follows the demethylation, nitration, decarboxylation,
hydrolysis, decarboxylation, and oxidation reaction. Furthermore, the TOC reduction of MG by
samples proves the mineralization process during photocatalytic degradation. Therefore, this paper
represents the investigation of photocatalytic performances of micro, nano, and quantum sized α-
NiMoO4 towards the MG photocatalytic degradation and detailed investigation of degradation
pathways with mechanism. In addition, various morphologies and sizes of α-NiMoO4 can be applied
for developing future advanced materials and devices.
ACKNOWLEDGEMENTS: 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 CAPTIONS
Figure 1. XRD patterns of different samples.
Figure 2. (a) FESEM image of NM-M, (b) TEM image of NM-M, (c) FESEM image of NM-N, (d)
TEM image of NM-Q.
Figure 3. XPS spectra of samples.
Figure 4. (a-c) Nitrogen adsorption-desorption isotherm, (d) Raman spectra, (e) UV-Vis DRS
spectra, and (f) Kubelka-Munk plot for band gap energy of samples.
Figure 5. Photocatalytic degradation of MG dye under UV-Vis and visible irradiation. (a-c) UV-Vis
spectra of MG during the photocatalytic degradation in UV-Vis light, (d) adsorption-desorption
equilibrium rate of MG under dark condition versus time in the presence of samples, (e)
photocatalytic degradation in UV-Vis light, (f) pseudo-first-order kinetics fitting plot for UV-Vis
light, (g) photocatalytic degradation in visible light, (h) pseudo-first-order kinetics fitting plot for
visible light, and (i) comparison of photocatalytic degradation of samples in UV-Vis and visible light
irradiation.
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Figure 6. (a) Adsorption-desorption equilibrium rate of MG under dark condition versus time in the
presence of scavengers (b) photocatalytic degradation using scavenger, and (c) effect of scavenger on
degradation percentage.
Figure 7. The total ion chromatogram (TIC) of MG degradation products. (a) 0 min and (b) 180
min.
Figure 8. HR-QTOF ESI/MS of degraded MG at 180 min by sample NM-N.
Figure 9. Proposed photocatalytic degradation pathways of MG.
Figure 10. Schematic representation of photocatalytic mechanism.
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Table 1. Identification of MG degradation products by HR-QTOF ESI/MS analysis.
Name
Skeleton structure
Elemental
formula
[M+H]+
[M+H]+ Retentio
n time
theoretic observe
al
(tR)
d
Malachite green
(1)
C23H25ClN2
365.1779 365.174 5.74
5
4,4'-
(phenylmethylen
e)dianiline
C19H18N2
275.1543 275.154 5.32
0
(2a)
N-(4-((4-
C23H25N2
329.2018 329.200 5.02
7
(dimethylamino)
phenyl)(phenyl)
methylene)cycloh
exa-2,5-dien-1-
ylidene)-N-
methylmethanam
inium
(2)
N,N-dimethyl-4-
((Z)-((Z)-4-
C22H22N2
315.1856 315.184 4.96
8
(methylimino)cyc
lohexa-2,5-dien-
1-
ylidene)(phenyl)
methyl)aniline
(2c)
(4-
C14H13NO
212.1070 212.105 4.96
7
(methylamino)ph
enyl)(phenyl)met
hanone
(4g)
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4-((4-
iminocyclohexa-
2,5-dien-1-
C21H20N2
301.1699 301.170 4.69
5
ylidene)(phenyl)
methyl)-N,N-
dimethylaniline
(2d)
bis(4-
(dimethylamino)
phenyl)methanol
C17H22N2O
271.1805 271.181 4.61
2
(3b)
bis(4-
C17H22N2O2
287.1754 287.172 4.34
3
(dimethylamino)
phenyl)methaned
iol
(3c)
(4-
C16H20N2O
257.1576 257.155 3.54
0
(dimethylamino)
phenyl)(4-
(methylamino)ph
enyl)methanol
(4a)
(4-
C16H20N2O2
273.1598 273.156 3.21
6
(dimethylamino)
phenyl)(4-
(methylamino)ph
enyl)methanediol
(4b)
bis(4-
C23H26N2O
347.2118 347.212 2.98
3
(dimethylamino)
phenyl)(phenyl)
methanol
(3)
(4-
C22H24N2O
333.1961 333.190 2.79
2
(dimethylamino)
phenyl)(4-
(methylamino)ph
enyl)(phenyl)met
hanol
(3d)
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(4-
C21H22N2O
319.1805 319.180 2.72
5
aminophenyl)(4-
(dimethylamino)
phenyl)(phenyl)
methanol
(3e)
(4-
C15H15NO
226.1226 226.122 2.49
2
(dimethylamino)
phenyl)(phenyl)
methanone
(4f)
(4-
C20H20N2O
305.1648 305.166 2.43
2
aminophenyl)(4-
(methylamino)ph
enyl)(phenyl)met
hanol
(2b)
Phenol
(3h)
C6H6O
117.0316 117.038 0.960
7
3-
C7H7O3
139.0395 139.037 0.960
0
hydroxybenzoic
acid
(6b)
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Table 2. Total organic carbon (TOC) values, average TOC value, standard deviation, and
mineralization percentage of malachite green (MG) by different samples under UV-Vis and
visible light irradiation.
Light
Sample
TOC (ppm)
Average Standard Mineralization
source
TOC
deviation
%
(ppm)
No light Fresh
MG dye
9.45
9.32
9.26
9.34
0.09
0
UV-Vis
UV-Vis
UV-Vis
Visible
Visible
Visible
NM-M
8.84
4.37
6.34
9.23
5.49
7.12
8.73
4.29
6. 23
9.15
5. 33
7.23
8.63
4.17
6.11
9.06
5.21
7.02
8.73
4.27
6.22
9.14
5.34
7.12
0.10
0.10
0.11
0.08
0.14
0.10
6.53
NM-N
NM-Q
NM-M
NM-N
NM-Q
54.28
33.40
2.14
42.82
23.76
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