Controllable synthesis of flower-like MoSe2 3D microspheres for highly efficient visible-light photocatalytic degradation of nitro-aromatic explosives

Article abstract of DOI:10.1039/c8ta02287a

Nitro-aromatic explosives existing on the surface of the Earth are difficult to degrade, and they greatly harm the ecological environment and human security. Herein, we successfully conducted the large-scale synthesis of novel flower-like MoSe₂ 3D microspheres and nanospheres by a simple hydrothermal method. The two types of MoSe₂ 3D spheres had a high crystal quality with abundant nanosheets, and their diameters were approximately 1.5 μm and 300-400 nm, respectively. The Brunauer-Emmett-Teller (BET) and UV-vis diffuse reflectance spectra (UV-vis DRS) analyses revealed that the specific surface area and the band gap of MoSe₂ microspheres and nanospheres were 33.3 m² g^-1 and 1.68 eV and those of the nanostructures were 13.6 m² g^-1 and 1.52 eV, respectively. Moreover, two different morphologies of MoSe₂ were used for the degradation of nitrobenzene (NB), p-nitrophenol (PNP) and 2,4-dinitrophenol (2,4-DNP) through a photocatalytic process. The results demonstrated that the three nitro-aromatic explosive solutions NB, PNP and 2,4-DNP (40 mg L^-1) could be completely degraded by MoSe₂ 3D microspheres under visible-light irradiation in 3.5 h, 1.5 h and 2.5 h, and the degradation time for MoSe₂ nanospheres was 4.5 h, 2.5 h and 4 h, respectively. The mechanism of the photocatalytic reaction was also investigated in detail, and the photocatalytic degradation process was found to follow the pseudo-first-order kinetics. Our study demonstrated the potential application of MoSe₂ microspheres as a photocatalyst for the degradation of nitro-aromatic explosives and other organic contaminants.

Full text of DOI:10.1039/c8ta02287a

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Controllable synthesis of flower-like MoSe2 3D microspheres for highly efficient
visible light photocatalyst degrading nitro-aromatic explosives
Jingwen Huanga, Bo Jina*, Huiqiang Liub, Xiaojuan Lia, Qingchun Zhanga, Shijin Chua, Rufang
Penga* and Sheng Chub*
a State Key Laboratory of Environmental Friendly Energy Materials & School of Material Science
and Engineering, Southwest University of Science and Technology, Mianyang 621010, Sichuan, P.
R. China.
b State Key Laboratory of Optoelectronic Materials and Technology, Sun YatꢀSen University,
Guangzhou 510275, Guangdong, P. R. China.
* Corresponding author. Tel: +86ꢀ0816ꢀ2419011
E-mail: jinbo0428@163.com; rfpeng2006@163.com and chusheng@mail.sysu.edu.cn
Abstract
Nitroꢀaromatic explosives are difficult to degrade, existing on the surface of the
Earth, and greatly harm the ecological environment and human security. Herein, we
successfully largeꢀscale synthesized novel flowerꢀlike MoSe2 3D microspheres and
nanospheres by using a simple hydrothermal method. The two MoSe2 3D spheres had
a high crystal quality with abundant nanosheets and their diameters are approximately
1.5 ꢁm and 300ꢀ400 nm respectively. The BrunauerꢀEmmettꢀTeller (BET) and UVꢀvis
diffuse reflectance spectra (UVꢀVis DRS) tests revealed that the specific surface area
and band gap of the MoSe2 microspheres and nanospheres were 33.3 m2/g and 1.68
eV, and the nanostructure’s were 13.6 m2/g and 1.52 eV. Moreover, the two different
morphologies of MoSe2 were used for degrading the nitrobenzene (NB),
pꢀnitrophenol (PNP) and 2, 4ꢀdinitrophenol (2, 4ꢀDNP) through a photocatalytic
process. The results demonstrated that the three nitroꢀaromatic explosives solution of
NB, PNP and 2, 4ꢀDNP (40 mg/L) could be completely degraded by MoSe2 3D
microspheres under visible light irradiation for 3.5 h, 1.5 h and 2.5 h, and the
degrading time for MoSe2 nanospheres were 4.5 h, 2.5 h and 4 h, respectively. The

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mechanism of the photocatalytic reaction was also investigated in detail, and the
photocatalytic degradation process was found to follow the pseudoꢀfirstꢀorder kinetics.
Our study demonstrated the potential application of MoSe2 microspheres as a
photocatalyst for nitroꢀaromatic explosives and other organic contaminants.
Keywords: flowerꢀlike MoSe2, 3D microspheres, hydrothermal method,
Photocatalytic degradation, Nitroꢀaromatic explosives
1 Introduction
Nitroꢀaromatic explosives are common pollutant compositions that are ground on
the surface of the Earth. They are synthesized from intermediate products, such as
chemical industry, dyestuff, and medicament products. Nitrophenols are regarded as
undegradable organics because the nitro as a group for absorption electrons can
greatly drop electron cloud density on the benzene ring and hinder the attack of
oxydic electrons 1ꢀ3. Thus, nitrophenols should be urgently degraded to protect the
ecological environment and human security. In recent years, the continuous
innovations for degradation of nitroꢀaromatic explosives have attracted increasing
attention and investigation. TiO2, as a widely used catalyst, has been researched for
the degradation of nitrophenols 4, 5. Owing to the intrinsic wide band gap, the
photocatalytic activity can only be generated under UV light irradiation. The
6
absorption of light quantum is limited, and the catalytic efficiency is poor . To
overcome these issues, numerous composite materials have been designed, such as the
7
9
Ag/ZnO , Ag/C3N4 8, and Fe3O4/TiO2/Au . The improvement in photocatalytic
performance has complicated and introduced new contaminants to the preparation
process of composite materials. Thus, finding a material with a novelꢀinnovative,
highly effective, lowꢀcost and largeꢀscale synthesis for the degradation of nitrophenols
is urgent.
Transition metal dichalcogenides (TMDs) have attracted significant attention due to
their similar structures with twoꢀdimensional (2D) layered graphene and exhibited
unique transformation from an indirect to a direct band gap semiconductor with a

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structure changed to nanoscale from bulk materials 10ꢀ12. TMDs are characterized by a
MX2 formula, where M stands for a transition metal (M = Mo, W, and so on) and X
stands for a chalcogen (X = S, Se, or Te) 13, 14. MoS2 is the excellent representative
one of the TMD group that has attracted extensive research attention as a device for
optoelectronics application and for its potential value in energy sources 15, 16. Apart
from the layered structure of MoS2, MoSe2 also shows special properties in
fieldꢀeffect transistors 17, fast photodetection 18, electrocatalytic hydrogen evolution
reaction 19, energy storage devices 20, and the Terahertz photodetector 21. A series of
synthesized methods has been carried out to prepare MoSe2 nanosheets, such as the
chemical vapor deposition (CVD) 22, mechanical exfoliation 23, and hydrothermal
methods 24. The CVD method can grow the monolayer and fewꢀlayer MoSe2
nanosheets, and mechanical exfoliation can easily acquire different thicknesses for the
MoSe2 nanosheets. However, both mainly obtain a 2D layered MoSe2 material, which
can result in a high tendency of restack and active sites 25, 26. Interestingly, the
hydrothermal method is a highly efficient and simple synthesis method that can form
a 3D MoSe2 structure of porous microspheres and nanospheres through
processꢀoriented aggregation, selfꢀassembly, and Ostwald ripening 27, 28. Formed with
2D layered flakes, the 3D flowerꢀlike structure provides superior performance for
photocatalysis and electrocatalysis owing to the plentiful active sites explored at the
edge of MoSe2 nanosheets 29ꢀ31
.
In this study, flowerꢀlike MoSe2 3D microspheres and nanospheres were
synthesized successfully through a facile hydrothermal method. The BET test showed
the MoSe2 microspheres and nanospheres with a specific surface area of around 33.3
m2/g and 13.6 m2/g, respectively. The UVꢀvis DRS revealed a corresponding direct
band gap of about 1.68 eV(microspheres) and 1.52 eV(nanoshperes). Considering that
the high specific surface area can support abundant active sites and the narrow band
gap is beneficial for absorbing the wavelength of visible light 32, 33, we investigated
the photocatalytic performance of the two different morphologies of MoSe2
nanostructure with the degradation of NB, PNP and 2,4ꢀDNP. The research results

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showed that all the three nitroꢀaromatic explosives could be degraded completely by
MoSe2 3D microspheres under visible light irradiation. The photocatalytic
degradation process followed the pseudoꢀfirstꢀorder kinetics. Finally, the mechanism
of the photocatalytic reaction was explored.
2 Experimental Section
2.1 Preparation of flower-like MoSe2 3D microspheres
Se powders (99.99%), sodium molybdate dehydrate (Na2MoO4·2H2O, 99%),
sodium borohydride (NaBH4, 98%), potassium iodide (KI, 99.99%), and
4ꢀbenzoquinone (BQ, 99%) were purchased from Aladdin Chemistry Co., Ltd.
Acetone, ethyl alcohol, and isopropyl alcohol (IPA) were analytical grade and
obtained from Chengdu Kelong Chemical Reagents Co., Ltd. All the reagents were
used as received without further purification.
In a typical reaction, 5 mmol Na2MoO4·2H2O and 15 mmol Se powders were
dissolved in 60 mL ethyl alcohol and distilled water mixture solvent (volume ratio of
1:1) under ultrasonic stirring (5 min) . Then, 5 mmol NaBH4 was added into the
mixture solution. After continuous stirring, the dark gray hybrid solution turned into
reddish brown. The obtained resulted solution was transferred into a 100 mL Teflon
lined stainless steel autoclave. The autoclave was heated at 200 oC and maintained for
48 h. After cooling to room temperature, the black sediment was acquired and washed
with acetone, ethanol and distilled water for 3 times, respectively. Then, the final
o
sediment was dried at 80 C for 12 h in a vacuum drying and the collecting MoSe2
powder was denoted as sample 1#. When the mixture solvent of ethylene glycol and
distilled water (volume ratio of 1:1) was used instead of ethyl alcohol as a solvent, the
final collected MoSe2 powder was denoted as sample 2#.
2.2 Characterization
Morphological and microstructural properties of the flowerꢀlike MoSe2 3D
microspheres were characterized by Field emission scanning electron microscopy
(FESEM, UItra 55, CARI ZEISS in Germany) equipped with energy dispersive Xꢀray

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spectroscopy and XꢀRay Diffraction (XRD, Philips X’ Pert PRO, Netherlands)
equipped with Cu Ka radiation (k=0.l5418 nm). Composition analysis was
characterized with XPS (Thermo VG 250, USA). Crossꢀsectional samples were
prepared by TEM (Libra 200 PE, operated at 200 kV, Germany) to directly observe
the vertical morphology. Ultraviolet photoemission spectroscopy (UPS)
measurements were performed by the XPS (ESCALAB 250Xi, UK) equipped the
He light source with the photon energy (21.22 eV). The room temperature Raman
spectrum (model; InVia, UK) was recorded on a Raman microscope using a 514.5 nm
He–Cd laser as the excitation source. The UVꢀvis diffused reflectance spectrum was
detected by the UVꢀvis DRS (Uꢀ4100, Hitachi, Japan). UVꢀvis spectrophotometer
(Thermo Scientific Evolution 201, USA) with a double beam light source from 190 to
1100 nm was used. BET surface area was measured by N2 adsorptionꢀdesorption
isotherm (NOVA 3000, Quantachrome, USA).
2.3 Photocatalytic reactions
Photocatalytic activity of the flowerꢀlike 3D MoSe2 microspheres was studied using
NB, PNP, 2, 4ꢀDNP as model pollutants under visible light irradiation. The
photocatalytic reactor (Beijing NBT Technology Co., Ltd, China) composed of quartz
glass tube with a system of circulating water jack, a light source (A 500 W xenon
lamp) and magnetic stirring equipment. The experiments were performed by adding
the photocatalysts (20 mg) into the glass tube fitted with a certain concentration
aqueous solutions of NB, PNP and 2, 4ꢀDNP (40 mg/L), respectively. Then these
reaction mixtures were placed into the photocatalytic reactor and stirred in the dark
reaction at room temperature for 15ꢀ30 min to ensure to reach the
absorption/desorption equilibrium before starting irradiation. Thereafter, the Xenon
lamp was switched on for photodegradation, and after each 15 min, 5 mL of reaction
mixture was centrifuged and analyzed by recording changes of the maximum
absorption spectra using UVꢀvis spectrometer. The total organic carbon (TOC)
removal was detected by the TOC analyzer (HTYꢀCT1000M, Zhejiang Tailin
+
ꢀ
BioEngineering Co., Ltd, China). The ions of NH4 and NO3 which generated from

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the photocatalytic action were examined by the Ion Chromatograph (ICꢀ881,
Metrohm, Switzerland) behind filtration.
For the stability tests, 20 mg MoSe2 1# microspheres were added into 50 mL PNP
(40 mg/L) solution and photocatalytic activity tested as described above for 90 min.
After each test, the photocatalysts were collected carefully, washed several times and
dried at 80 oC for 24 h. The obtained MoSe2 microspheres were used for recycling test
and characterization.
3 Results and discussion
3.1 Characterization of MoSe2 photocatalyst
The representative SEM images of MoSe2 nanostructure are shown in Fig.1
Fig.1a-c showed the SEM images of sample 1# , and the surface of MoSe2 was found
to be hierarchical with a flowerꢀlike shape and a spherical diameter of approximately
1.5 ꢁm. The uniform microspheres were composed of plentiful nanosheets and
possessed a thickness of nearly 15ꢀ20 nm. Therefore, abundant active points might
exist on the edge of these nanosheets by the dangling bonds. The active points might
play an important role in electrocatalysis and photocatalysis. With the changed of
reaction solutions, the corrugations and the size of the flowerꢀlike MoSe2
microspheres begin to reduce. Fig. 1d-f showed the SEM images of the MoSe2
sample 2#. It can be seen that the morphology of sample 2# was flowerꢀlike shape and
tended to be a nanosphere structure. Where the diameter of the spheres was
approximately 300ꢀ400 nm, and the numbers of corrugations composed by single
MoSe2 nanosheet were slightly reduced compared to sample 1#. The results
demonstrated that different solvents have a great influence on the morphology and
size of the product.

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Fig. 1 Morphology characterization of MoSe2 nanostructure: FETSEM images with different
magnifications (aꢀc) sample 1# (dꢀf) sample 2#
The XRD pattern was used to evaluate the crystal structure and the crystallization
quality of the materials. The XRD pattern of the two different morphologies of MoSe2
samples were shown in Fig. 2a with the main peaks at 13.3° (002), 27.1° (004), 31.5°
(100), 37.5° (103), 55.9° (110), 65.4° (200), and 69.5° (203). These peaks
corresponded to the hexagonal phase (P63/mmc space group) of MoSe2. All data
could be indexed from the standard card (JCPDS No.29ꢀ0914) 34. Meanwhile, the
EDS confirmed the stoichiometry of the flowerꢀlike MoSe2 microspheres(sample 1#)
and nanospheres(sample 2#). As shown in Fig. 2b, there only appeared elements Se
and Mo The Mo and Se peaks of the two samples showed that the atom ratio of Mo to
Se were approximately 1: 2.04 and 1: 2.01, which was near the stoichiometry (1:2) of
35
MoSe2
.
Raman spectroscopy was performed on the surface area of the MoSe2 microspheres
(Fig. 2c). Both of the two different morphologies of MoSe2 showed a strong and
identifiable peak at 237.9 cmꢀ1, which was related to the outꢀofꢀplane A1g mode. The
A1g mode showed a red shift compared with the MoSe2 bulk materials (243.7 cmꢀ1).
1
The relative weakness peak located at 283.4 cmꢀ1 was related to the inꢀplane E2g
mode. Furthermore, the Raman peak at around 347.5 cmꢀ1 belonged to the B2g1 mode,

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which was related to the interlayer interaction 17, 35. The presence of the B2g1 mode
suggested that both the flowerꢀlike MoSe2 microspheres and nanospheres had a
multilayer structure with numerous thin layers.
The UVꢀvis DRS of the two MoSe2 samples were taken over the spectral range
from 400 to 800 nm as shown in Fig. 2d. The two MoSe2 samples displayed strong
absorptions at 656.5 nm and 676.5 nm, respectively. Fig. S1 showed the curve of
(Ahυ)2 versus hυ, as an direct band gap of semiconductor, (Ahυ)2 = A(hυꢀEg), where
the absorption coefficient A is a constant. The band gap of the MoSe2 could be
calculated by incising the curve and obtaining the intercept with Xꢀaxis. The
calculated Eg values of flowerꢀlike MoSe2 microspheres (sample 1#) and flowerꢀlike
MoSe2 nanospheres (sample 2#) were approximately 1.68 eV and 1.52 eV respectively,
corresponding to the direct band gaps. Compared with that of the bulk MoSe2
materials, the blue shift of the bands was from 1.1 eV (an indirect band gap), which
indicated the good response of the MoSe2 materials for the range of visible light
photocatalytic degradation activity 36.
UPS experiments were carried out to determine the valence band (EV) information
of MoSe2 samples. As shown in Fig. 2d-e, the valence band (EVB) for flowerꢀlike
MoSe2 microspheres sample 1# and nanospheres sample 2# were located at 0.94 and
1.03 eV, respectively, below EF by linearly extrapolating the leading edge of the
spectrum to the baseline 37. In addition, the conduction band (ECB) minimum can be
calculated using ECB= EVB ꢀ Eg 38. According to UVꢀvis DRS, the band gap of MoSe2
microsphere and nanosphere were 1.68 and 1.52 eV, respectively. Thus, the ECB
minimum should be present at about ꢀ0.74 and ꢀ0.49 eV by calculation.

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Fig. 2 Structure characterization of MoSe2 microspheres(sample 1#) and nanospheres (sample 2#) :
1
1
(a) XRD pattern (b) EDS spectra (c) Raman spectra with the A1g, E2g and B2g modes. (d)
UV−Vis DRS of the asꢀobtained MoSe2, the inset image is the curve of (Ahv)2 ꢀ hv. The
valence band maximum of the UPS spectra (d) MoSe2 microsphere and (e)
nanosphere
The ratio specific surface and porosity analyzer can obtain by testing the nitrogen
adsorption and stripping of the spherical structure materials. The BET test of MoSe2
microspheres and nanospheres were performed by using nitrogen adsorptionꢀ

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desorption. Fig. 3a shows the pore size distribution curves. From desorption branch of
the nitrogen isotherm by BJH method, the sharp peak at 34.9 nm was the maximum
pore volume as a center from 25 to 50 nm. The narrow peak indicated that the major
pores distributed on this area and pore diameter was uniform. Table S1 shows the
multi BET surface areas transform data with a range of relative pressure, where the
corresponding surface area transform curve was showed in Fig. 3a as an inset image.
The intercept and slope of the curve were approximately 3.74 and 150.98, respectively.
According to the BET equation 39, the surface area of the MoSe2 flowers was
calculated to be 33.3 m2/g. The isotherm curves (Fig. 3b) exhibited obvious hysteresis
loops, revealing the mesoporous (2ꢀ50 nm) structure of MoSe2. The adsorbing
capacity was significant increased at the middle and low pressure area, and the
adsorbing capacity curve began to separate and appeared a distinct hysteresis loop,
revealing the type IV of isotherm curve 40, 41. As shown in Fig. S2a, the surface area
of MoSe2 nanoshperes (sample 2#) was calculated to be 13.6 m2/g, which was smaller
than the MoSe2 microspheres (sample 1#). The nanosheets of composing to MoSe2
nanoshperes structure were smaller than the microspheres, which might be the reason
for their small surface area. The large surface area might provide a large number of
active sites, which was beneficial for generating large amounts of photoꢀgenerated
electrons and holes 42.
The XPS spectra for the asꢀsynthesized flowerꢀlike MoSe2 3D microspheres by the
simple hydrothermal method are shown in Fig. 3(c, d). Information on stoichiometry
and binding could be obtained from the 3d spectra of the Mo and Se elements. The
binding energies of Mo at 228.8 and 231.9 eV belonged to the Mo 3d5/2 and Mo 3d3/2
spin orbit peaks of MoSe2, which highlighted the existence of Mo4+. Whereas, the
Mo6+ of the Na2MoO4·2H2O which belonged to the Mo 3d5/2 and Mo 3d3/2 spin orbits
located at 232.5 eV and 235.9 eV. These peaks of Mo 3d5/2 and Mo 3d3/2 shifted,
indicating the reduction with valence of Mo from Mo6+ to Mo4+. Furthermore, the Se
3d5/2 and Se 3d3/2 spin orbit peaks were located at 54.3 and 55.2 eV, respectively. This
finding indicated the decline in the valence of the simple Se substance to Se2ꢀ. The

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XPS spectra of MoSe2 nanospheres(sample 2#) are shown in Fig. S3, which are
similar to the results of sample 1#. These results of XPS were consistent with previous
works for MoSe2 nanosheets 18, 24
.
Fig. 3 (a) Pore size distributions of MoSe2 microspheres (inset) with corresponding surface area
transform curve (b) Nitrogen adsorption−desorption isotherm for MoSe2 3D microspheres (c) XPS
spectrum of Mo 3d and (d) XPS spectrum of Se 3d.
The crystal structure of the flowerꢀlike MoSe2 3D microspheres was examined by
TEM. The results are shown in Fig. 4. Fig. 4(a, b) present different magnifications of
the TEM images. The surface morphology displayed that the synthetic MoSe2
microspheres were made up of abundant nanosheets and had a great crystal quality.
The selected area electron diffraction (SAED) pattern (Fig. 4c) showed several cycle
clear torus tori that demonstrated the obtained MoSe2 microspheres with a
polycrystalline structure. The radius of the first and second cycle torus in the Brillouin
zone corresponded to the lattice distances of 0.28 and 0.165 nm, which were
consistent with the (100) and (110) planes of the MoSe2 hexagonal phase, respectively.

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The highꢀresolution TEM as shown in Fig. 4d, further revealed the periodic arrays
with a 0.28 nm distance assigned to the (100) lattice plane. In addition, an interlayer
distance of nearly 0.7 nm was examined, which matched the thickness of the single
layer of SeꢀMoꢀSe unit and the corresponding (002) plane 20. To observe the surface
atomic distribution clearly, we conducted a simple filtering process on the dashed
lines in Fig. 4e. The results are shown in Fig. 4f. The periodic arrays of the boundless
honeycomb structure showed the MoSe2 nanosheets with a grapheneꢀlike plane
structure18.
Fig. 4 TEM characterization of the obtained MoSe2 sample: (a, b) TEM image of a magnified
view of the flowerꢀlike MoSe2 3D microspheres (c) The SAED pattern taken from the area marked
with a black rectangle in (b). (d, e) The high resolution TEM image of the monolayer. (f) A simple
filtering process to the dashed lines in (e)
3.2 Photocatalytic property of the MoSe2 microspheres

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Fig. 5 Chemical structures of nitroꢀaromatic explosives NB, PNP and 2, 4ꢀ2DNP
Nitroꢀaromatic explosives (Fig. 5) are generally difficult to degrade owing to the
existence nitro as a group for absorption electrons. Thus, significant works have been
conducted to find a good material or composite materials with excellent
photocatalytic activity. Nevertheless, few studies have focused on the TMD materials,
especially their photocatalytic activity for degrading nitroꢀaromatic compounds. The
3D porous flowerꢀlike structures of MoSe2 can provide abundant active sites and
effective transport pathways. The narrow direct band gap of approximately 1.68 and
1.52eV may excite the photoresponse at the range of visible spectrum, thereby
improving the quantum efficiency. In this work, asꢀsynthesized flowerꢀlike MoSe2
microspheres and nanospheres were used to degrade the NB, PNP and 2, 4ꢀDNP
under visible light irradiation.
The results of the photocatalytic activity of MoSe2 microspheres (sample 1#) are
shown in Fig. 6. It can be clearly discovered that the main UVꢀVis absorption peak
(NB: 268 nm, PNP: 318 nm and 2, 4ꢀDNP: 356 nm) gradually decreased, and the
target compound NB, PNP and 2, 4ꢀDNP were completely degraded by the MoSe2
microspheres (20 mg) after about 3.5 h, 1.5 h and 2.5 h, respectively. During the
photocatalytic process, the main absorption peak position did not change, and no other
absorption peak appeared; therefore, photocatalytic degradation mode was dominated
and no other dye sensitization occurred 43. In addition, the photocatalytic performance
of MoSe2 nanospheres (sample 2#) was also investigated under the same conditions.
As shown in Fig. S4, NB, PNP and 2, 4ꢀDNP were completely degraded by MoSe2
nanospheres sample 2# (20 mg) after about 4.5 h, 2.5 h and 4.0 h, respectively, which
may involve free radical degradation mechanisms.

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As a contrast, the removal rate of the same aqueous solution of NB, PNP and 2,
4ꢀDNP were about 13.9%, 12.8% and 11.9%, respectively with the same visible light
irradiation time in the absence of MoSe2 (Fig. S5). The gradual color fading of the
representative PNP reaction solution was recorded by camera as shown in Fig. S6, the
color of the PNP solution changes from pale yellow to nearly colorless with the
prolongation of photodegradation time.
Fig. 6 Photocatalytic property of MoSe2 microspheres (sample 1#): The UVꢀvis absorption spectra
of the 40 mg/L aqueous solution of NB (a), PNP (b) and 2, 4ꢀDNP (c) under visible light
irradiation.
The photocatalytic degradation of compounds NB, PNP and 2, 4ꢀ DNP follows the
pseudoꢀfirstꢀorder kinetics as seen from the linear fitting profiles shown in Fig. 7. The
firstꢀorder rate constant (k) was calculated using Equation 1, as follows:
ꢀln (Ct/C0) = kt
(1)

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where C0 and Ct are the concentrations of nitroꢀaromatic compounds at the irradiation
time of 0 and t, respectively 44. As shown in Table 1, the calculated kinetic rate
constant (k) were 0.009 minꢀ1 (NB), 0.027 minꢀ1 (PNP) and 0.019 minꢀ1 (2,4ꢀDNP).
The kinetic plot for all catalysts is approximately linear, corresponding to the
correlation coefficient (r) of approximately 0.9930, 0.9757 and 0.9223, respectively.
Fig. 7 The kinetics of photocatalytic degradation of the nitroꢀaromatic compounds (a) NB, (b)
PNP, (c) 2, 4ꢀ DNP.
Table 1. The linear fitting results of the reaction kinetics curve with NB, PNP and 2, 4ꢀ DNP.
Kinetic rate constant (k)
Correlation coefficient (r)
NB
0.009 min-1
0.027 min-1
0.9930
0.9757
PNP
2,4-DNP
0.019 min-1
0.9223
To confirm that structure of the flowerꢀlike MoSe2 microspheres were unchanged
after the photocatalytic reaction process, more contrast experimental tests have being

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carried out(Fig. S7) The SEM images (Fig. S7 a, b) were obtained after 4 cycle times
under visible light irradiation. Comparison with Fig. 1a-c, the surfaces of the MoSe2
microspheres were nearly unchanged, and the microspheres still composed tightly by
plentiful nanosheets. XRD pattern (Fig. S7 c) also showed that the crystal structure of
MoSe2 microspheres before and after 4 cycle times photocatalytic action did not
change significantly, which can be indexed the JCPDS card: No.29ꢀ0914 35. The
bonding structure of the asꢀprepared product was confirmed by FTꢀIR measurements.
Fig. S7 d shows the FTꢀ IR spectra in the range of 4000ꢀ500 cmꢀ1 of the MoSe2 before
and after 4 cycle time photocatalytic action. It could clearly be observed that there
were no peaks shifted and disappeared before and after photocatalytic action. The
FTꢀIR spectra showed that the MoSe2 displayed strong signals at 1096 cmꢀ1 and 621
cmꢀ1, which are attributed to the MoꢀSe stretching and SeꢀMoꢀSe stretching vibrations
45
.
Fig. 8 Stability test for MoSe2 microspheres for photocatalytic degradation of PNP under visible
light irradiation.
Good stability is a crucial feature of a photocatalyst in a realistic application for
cyclic utilization. The MoSe2 powders were carefully collected after each common
photocatalytic test and dried for the subsequent photocatalytic action to explore the
stability in photocatalytic activity. Fig. 8 and Fig. S8 present the results of four cycle
tests with PNP, NB and 2, 4ꢀDNP. After four cycles of successive irradiation, the

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removal rate (Ct/C0) of PNP presented a slight reduce but still reached 90%, the NB
was 83.7% and the 2,4ꢀDNP was keep on 95%, respectively. The results showed that
the flowerꢀlike MoSe2 3D microspheres had superior stability for cycle utilization.
Table 2. The rates of photodegradation of nitroꢀaromatic explosives with different catalysts were
compared.
Substrate
W
cat (g)/nsub
Reaction
time
Radiation
source
Catalyst
Ref.
(mmol)
MoSe2 1#
MoSe2 2#
NB
NB
1.23
1.23
1.39
1.39
1.87
1.87
1.33
1.33
1.33
1.33
1.33
1.33
2.0
3.5 h
4.5 h
1.5 h
2.5 h
2.5 h
4 h
Visible light
Visible light
Visible light
Visible light
Visible light
Visible light
UV light
This work
This work
This work
This work
This work
This work
H X, Qi etl
H X, Qi etl
H X, Qi etl
X Y, Wu etl
X Y, Wu etl
X Y, Wu etl
X T, Shen etl
WanꢀKuen
Jo etl
MoSe2 1#
PNP
MoSe2 2#
PNP
MoSe2 1#
2,4ꢀDNP
2,4ꢀDNP
NB
MoSe2 2#
[Au2(dppatc)2]Cl2
[Au2(dppatc)2]Cl2
[Au2(dppatc)2]Cl2
[Ag4(NO3)4(dpppda)]n
[Ag4(NO3)4(dpppda)]n
[Ag4(NO3)4(dpppda)]n
TSAꢀMIPꢀTiO2
3.5 h
3.5 h
4 h
PNP
UV light
2,4ꢀDNP
NB
UV light
5 h
UV light
PNP
5 h
UV light
2,4ꢀDNP
NB
6 h
UV light
1.5 h
UV light
Graphitic CarbonꢀTiO2
MWCNTs/TiO2
NB
0.49
38
4 h
UV light
2,4ꢀDNP
2.5 h
solar light
H Wang etl
Comparison on some experimental parameters of photodegradation catalyzed by
MoSe2 microspheres (sample 1#), MoSe2 nanospheres (sample 2#) and some known
catalysts in the oxidation photodegradation processes are listed in Table 2. The time
of degrading NB catalyzed by TSAꢀMIPꢀTiO2 (2.0 g/mmol (wcat/nsub)) and graphitic
46
47
carbonꢀTiO2 (0.49 g/mmol (wcat/nsub)) under the UV light were 1.5h
and 4h
,
respectively. When the amount of the catalyst was loaded at 1.33g/mmol (wcat/nsub),
the time of degrading NB, PNP and 2,4ꢀDNP catalyzed by [Ag4(NO3)4(dpppda)]n
under the UV light irradiation were 5h, 5h and 6h, respectively 48. Whereas, when
[Au2(dppatc)2]Cl2 was used as the catalyst 49, the degrading time of NB, PNP and
2,4ꢀDNP under the same condition with the former decreased to 3.5 h, 3.5 h and 4 h,
respectively. Obviously, their degradation time on the same amount of the catalyst

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were a bit more than our work by using MoSe2 as catalyst, and the utilization of the
visible sight can broaden the absorption region of the sunlight and increase the
quantum efficiency. For MWCNTS/TiO2 50, although the degradation time for
2,4ꢀDNP (2.5 h) was the same, the amount of the catalyst (38 g/mmol) was much
larger than the use of MoSe2.
+
ꢀ
Table 3. Yields of NH4 and NO3 existed in the NB, PNP and 2, 4ꢀDNP solution after
photocatalyzed degradation
Ion
NB
PNP
2,4ꢀDNP
27.1%
+
ꢀ
NH4
21.6%
20.5%
NO3
33.8%
45.4%
38.1%
+
ꢀ
After photocatalyzed degradation, the yields of NH4 and NO3 existed in the NB,
PNP and 2, 4ꢀDNP solution were also examined by Ion Chromatography (IC). As
+
shown in Table 3, the yields of NH4 in NB, PNP and 2, 4ꢀDNP solution after
photocatalyzed degradation were 21.6%, 20.5% and 27.1%, respectively. The yields
ꢀ
of NO3 in NB, PNP and 2, 4ꢀDNP solution after photocatalyzed degradation were
ꢀ
33.8%, 45.4% and 38.1%, respectively. The yield of NO3 was a little more than the
+
yield of NH4 . Whereas the remaining 34.1%ꢀ44.6% nitro group was probably
released into air as the form of N2 according to the previous reports 49, 51
.
For the purpose of further analysis the photocatalytic action products, the
nitroꢀaromatic explosives solution before and after the visible light irradiation were
filtrated and then analyzed by the TOC test showing in Fig. 9a. Compared with the
undegraded solution, it has a great decline of the TOC content with each
nitroꢀaromatic explosive, and the TOC removal rate of NB, PNP and 2, 4ꢀDNP
solution after photocatalyzed degradation were 62.7%, 65.2% and 61.5% ,
respectively. Obviously, the removal rate of TOC reduction is slower than that of the
photocatalytic degradation, indicating that the nitroꢀaromatic explosives is partially
52, 53
decomposed into H2O and CO2
.

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Fig. 9 (a) The TOC test with the NB, PNP and 2, 4ꢀDNP solution before and after the visible light
ꢀ
irradiation. (b) Trapping tests of the active species (h+,
•OH,
•
O2 ) compared with the no quencher
added to the 4ꢀnirophonel under visible light irradiation.
To explore the mechanism of photocatalytic degradation, holes (h+), hydroxyl
−
radicals (•OH), and superoxide radical (•O2 ) were detected by adding KI (a quencher
−
of h+), IPA (a quencher of •OH), and BQ (a quencher of •O2 ), respectively. The
method showed no difference from the previous photocatalytic degradation test Fig.
9b indicates that the removal rate of the PNP decreased with the injection of KI, IPA,
−
and BQ. The result showed that h+, •OH and •O2 existed during the photocatalytic
activity. Obviously, the photocatalytic degradation rate of the 4ꢀnitrophonel was
strongly restrained by the trapping of h+ (adding KI). Therefore, •OH could not be

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generated successfully when h+ was quenched by KI 54.
Fig. 10 Photocatalytic degradation mechanism of flowerꢀlike MoSe2 microspheres under visible
light irradiation.
In accordance with the aboveꢀmentioned results and analysis, the potential
photocatalytic degradation mechanism for the flowerꢀlike MoSe2 3D microspheres
was proposed Fig. 10. The band gap of MoSe2 was approximately 1.68 eV and could
absorb the visible light. When visible light irradiated on the surface of MoSe2
microspheres, photogenerated electronꢀhole pairs were produced (Equation 2). Then
the photogenerated electrons (ecb−) were excited to the conduction band, while the
photogenerated holes (hvb+) remained in the valence band. The MoSe2 microspheres
were composed of many ultrathin nanosheets, thereby enabling the photoholes to
reach surface bound H2O and generate •OH (Equation 3). By contrast, the
−
photoelectrons combined molecular O2 to •O2 on the microspheres surface
(Equation 4). Nevertheless, the photocatalytic degradation of PNP was heterogeneous
6
according to the previous reports by Paola et al . Initially, the nitro group at the para
position was eliminated by the electrophilic substitution of the powerful oxidizing
agent •OH with the generation of the intermediates as hydroquinone. Then, the

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−
intermediates and •NO2 continued to react with •OH and •O2 to produce CO2, H2O,
N2 and inorganic ions (Equation 5) 4, 55ꢀ57
.
MoSe2 + hv → ec−b + hv+b
hv+b + H2O → •OH + H +
ec−b + O2 → •O2−
(2)
(3)
(4)
(5)
4 Conclusions
Flowerꢀlike MoSe2 3D microspheres were synthesized by using a simple
hydrothermal method. The asꢀobtained MoSe2 samples had a specific surface area of
nearly 33.3 m2/g and a direct band gap of around 1.68 eV. Considering that the
flowerꢀlike structure MoSe2 microspheres can absorb visible light and have abundant
nanosheets, we have investigated its photocatalytic degradation activity with three
nitroꢀaromatic explosives NB, PNP and 2, 4ꢀDNP. The results showed that the NB,
PNP and 2, 4ꢀDNP could be degraded under visible light after 3.5 h, 1.5 h and 2.5 h,
respectively, which showed a high efficient photocatalytic degrading performance
than the flowerꢀlike structur MoSe2 nanostructure. The photocatalytic activity process
followed the pseudoꢀfirstꢀorder kinetics. Trapping tests of active species (h+, •OH,
−
−
•O2 ) were added into the PNP to show that h+, •OH, and •O2 were present during
the photocatalytic activity and that h+ was the most important influencing factor of the
photocatalytic removal rate of PNP. Furthermore, the possible photocatalytic

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mechanism was discussed, and we considered that the activity process of the
photocatalytic degradation of PNP was heterogeneous. Photoelectronꢀholes were
−
firstly generated and produced the active radical •OH and •O2 under visible light
irradiation. Then, the nitro group was eliminated by the powerful oxidizing agent,
•OH, with the generation of intermediates. Finally, the intermediates and •NO2
continued to react with •OH and •O2− to produce CO2, H2O, N2 and inorganic ions.
Acknowledgements
We are grateful for financial support from the Science Challenge Project (project no.
TZ2018004), the National Natural Science Foundation of China (project no.
51327804), China Academy of Engineering Physics Research Institute (18zh0079)
and Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites
and Functional (project no. 14tdfk05).
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