Synergetic effect of TiO2 as co-catalyst for enhanced visible light photocatalytic reduction of Cr(VI) on MoSe2

Article abstract of DOI:10.1016/j.apcata.2016.01.022

MoSe₂-TiO₂ composites were successfully synthesized via a facile solvothermal method. Their morphology, structure and photocatalytic activity in the reduction of Cr(VI) were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV-vis absorption spectroscopy, X-ray photoelectron spectroscopy and electrochemical impedance spectra, respectively. The results show that the introduction of TiO₂ into MoSe₂ boosts the light harvesting efficiency and charge separation and transfer due to their synergistic effect, thus enhancing the visible light photocatalytic activity of MoSe₂ with a maximum Cr(VI) reduction rate of 91% at 120 min compared with pure MoSe₂ (61%) and TiO₂ (5%).

Full text of DOI:10.1016/j.apcata.2016.01.022

Accepted Manuscript
Title: Synergetic effect of TiO2 as co-catalyst for enhanced
visible light photocatalytic reduction of Cr(VI) on MoSe2
Author: Haipeng Chu Wenyan Lei Xinjuan Liu Jinliang Li
Wei Zheng Guang Zhu Can Li Likun Pan Changqing Sun
PII:
S0926-860X(16)30022-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.01.022
APCATA 15737
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Applied Catalysis A: General
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Revised date:
Accepted date:
4-8-2015
28-12-2015
19-1-2016
Please cite this article as: Haipeng Chu, Wenyan Lei, Xinjuan Liu, Jinliang Li, Wei
Zheng, Guang Zhu, Can Li, Likun Pan, Changqing Sun, Synergetic effect of TiO2 as
co-catalyst for enhanced visible light photocatalytic reduction of Cr(VI) on MoSe2,
Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.01.022
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Synergetic effect of TiO2 as co-catalyst for enhanced visible light photocatalytic
reduction of Cr(VI) on MoSe2
Haipeng Chu1a, Wenyan Lei1a, Xinjuan Liua,*, Jinliang Lib, Wei Zhenga, Guang Zhuc,
Can Lia, Likun Panb,* and Changqing Suna
aInstitute of Coordination Bond Metrology and Engineering, College of Materials
Science and Engineering, China Jiliang University, Hangzhou 310018, China
bEngineering Research Center for Nanophotonics & Advanced Instrument, Ministry of
Education, Shanghai Key Laboratory of Magnetic Resonance, Department of Physics,
East China Normal University, Shanghai 200062, China
cAnhui Key Laboratory of Spin Electron and Nanomaterials, Suzhou University,
Suzhou 234000, China
*Corresponding author. Tel: +86 0571 86872475; fax: +86 0571 86872475; E-mail address: lxj669635@126.com
*Corresponding author. Tel.:+86 21 62234132; fax: +86 21 62234321; E-mail address: lkpan@phy.ecnu.edu.cn
1The two authors contributed equally to this work
1

Graphical Abstract
Research Highlights




MoSe2‐TiO2 composites were synthesized via a facile solvothermal method.
TiO2 acts as co‐catalyst to separate the photo‐generated electron‐hole pairs.
MoSe2‐TiO2 composites exhibited enhanced photocatalytic activity.
A high Cr(VI) reduction rate of 91% was achieved under visible light irradiation.
Abstract
MoSe2-TiO2 composites were successfully synthesized via a facile solvothermal
method. Their morphology, structure and photocatalytic activity in the reduction of
Cr(VI) were characterized by scanning electron microscopy, transmission electron
microscopy, X-ray diffraction, UV-Vis absorption spectroscopy, X-ray photoelectron
spectroscopy and electrochemical impedance spectra, respectively. The results show
that the introduction of TiO2 into MoSe2 boosts the light harvesting efficiency and
charge separation and transfer due to their synergistic effect, thus enhancing the
visible light photocatalytic activity of MoSe2 with a maximum Cr(VI) reduction rate
of 91% at 120 min compared with pure MoSe2 (61%) and TiO2 (5%).
Keywords: Transition metal dichalcogenides; MoSe2; TiO2; Cr(VI); photocatalysis
2

1. Introduction
Semiconductor photocatalysis has attracted considerable attention owing to its
wide potential in environmental and energy related applications, such as oxidation of
organic pollutants [1-6], reduction of carbon dioxide [7-9], evolution of hydrogen or
oxygen gas [10-15]. Currently, various semiconductors including metal oxides,
sulphides and nitrides have been explored for photocatalytic reaction. However, these
materials suffer from shortcomings such as the relatively low solar energy conversion
efficiency, instability, high cost, and so on. Therefore, exploring novel visible
light-sensitive photocatalysts with high efficiency and good stability remains a great
challenge [16-22].
Transition metal dichalcogenides (TMDs) have attracted great interest recently
due to their unique structural, electronic, and optical properties [23,24]. As an
important member of TMDs, MoSe2 with a narrow band gap of ~1.4 eV, has been
employed as a good candidate for visible light photocatalysis [25-27]. Nevertheless,
the photocatalytic activity of MoSe2 is limited due to the relatively high
recombination of photo-generated charge carriers. Currently, constructing unique
structures has been suggested to overcome the aforementioned problems. Highly
ordered mesoporous crystalline MoSe2 prepared via a nanocasting strategy using
mesoporous silica SBA-15 as a hard template was reported to exhibit higher visible
light photocatalytic activity in the degradation of Rhodamine B compared to the bulk
MoSe2 due to its larger surface area and pore volume [28]. However, the quick
recombination of photo-generated charge carries still exists for pure MoSe2. Another
3

particularly attractive strategy is to design and develop hybrid composites based on
semiconductor oxides, which could offer the driving forces to separate and transfer
photo-generated charge carriers [29]. TiO2-based composites have been demonstrated
with excellent photocatalytic activity [30]. Recently, the hybrid density functional
calculation shows that in the MX2-TiO2 (M=Mo and W; X=S and Se) composites,
MX2 and TiO2 serve as an efficient photo-sensitizer and co-catalyst, respectively, and
a built-in-potentials in the composites are generated, which is advantageous for the
electron-hole separation from MX2 to TiO2 [31]. Therefore, development of
composite materials based on MoSe2 and TiO2 should be a promising method to
enhance the photocatalytic activity of MoSe2 due to the stepwise structure of
band-edge levels constructed in the MoSe2-TiO2 (MT) composite (Fig. 1). The
conduction band (CB) of TiO2 is -4.2 eV and valence band (VB) -7.4 eV (vs. vacuum)
[32], while the corresponding values of MoSe2 are -3.58 and -5.62 eV (vs. vacuum),
respectively [33]. Such energy levels are beneficial to the transfer of photo-generated
electrons from MoSe2 CB to TiO2 CB, which could efficiently separate the
photo-induced electrons and hinder the charge recombination in the electron-transfer
processes, thus enhance the photocatalytic activity. Unfortunately, up to now, the
exploration on MT composites for photocatalysis has not yet been reported, especially
in terms of photocatalytic reduction of Cr(VI).
Herein, a fast strategy was developed to synthesize the MT composites through a
solvothermal method, and their photocatalytic activity in the reduction of Cr(VI) was
investigated. The MT composites exhibit improved visible light photocatalytic activity
4

compared with pure MoSe2 and TiO2. The possible mechanism that contributes to the
improvement of photocatalytic activity was studied in terms of a series of
characterization and controlled experiments using hole and electron scavengers.
2. Experimental
2.1 Synthesis
A certain amount of TiO2 (P25, Degussa), 18 mmol Na2SO3, 4.5 mmol Se power,
2.25 mmol Na2MoO4 and 4.5 mmol NaBH4 were dissolved into 60 ml aqueous
solution by sonication for 30 min to produce a uniform dispersion. A dilute HCl
solution was dropped in the above solution to adjust the pH value to be 7, and the
mixture was stirred for 30 min. Subsequently, the mixture was transferred into a 100
ml Teflon-lined stainless steel autoclave, and treated at 180 °C for 12 h. The obtained
samples were isolated by filtration, washed three times with distilled water, and
finally dried in a vacuum oven at 80 °C for 24 h. The weight ratios of MoSe2 to TiO2
were 1:0.25, 1:0.5, 1:1, and 1:2 during the preparation, and the corresponding
obtained MT composites were named as MT-0.25, MT-0.5, MT-1 and MT-2,
respectively. Pure MoSe2 were also synthesized by the similar method in the absence
of TiO2. For the electrochemical impedance spectra (EIS) testing, 90 mg sample with
0.2 ml 2.5 wt.% polyvinyl alcohol binder was homogenously mixed in water to form
slurry. Then, the resultant slurries were coated on the graphite flake (2 cm × 2 cm).
Finally, these prepared electrodes were dried in a vacuum oven at 60 °C for 24 h.
5

2.2 Characterization
The morphology and structure of the samples were characterized by
field-emission scanning electron microscopy (FESEM, Hitachi S-4800),
high-resolution transmission electron microscopy (HRTEM, JEOL-2010), X-ray
diffraction (XRD, Holland Panalytical PRO PW3040/60) with Cu Kα radiation (V=30
kV, I=25 mA), and energy dispersive X-ray spectroscopy (EDS, JEM-2100).
Photoluminescence (PL) spectra at room temperature were examined by fluorescence
spectrophotometer (HORIBA Jobin Yvon fluoromax-4). X-ray photoelectron
spectroscopy (XPS) measurements were carried out using a Thermo ESCALAB 250
Xi spectrometer with a monochromatic Al Kα X-ray source. The diffuse absorption
spectra of the samples were recorded using a Hitachi U-3900 UV-Vis
spectrophotometer equipped with an integrated sphere attachment by using BaSO4 as
a reference. EIS measurements were carried out on an electrochemical workstation
(AUTOLAB PGSTAT302N) in dark conditions using a three electrode configuration
with the as-prepared films as working electrode, a Pt foil as counter electrode and a
standard calomel electrode as reference electrode. The electrolyte was 20 mg l-1 Cr(VI)
aqueous solution. EIS were recorded in the frequency range of 0.1 Hz-1 MHz, and the
applied bias voltage and ac amplitude were set at open-circuit voltage and 10 mV,
respectively.
2.3 Photocatalytic experiments
The photocatalytic activity of the as-prepared samples was evaluated through the
experiment of photocatalytic reduction of Cr(VI) under visible light irradiation. The
6

samples (1 g l-1) were dispersed in 80 ml Cr(VI) aqueous solutions (20 mg l-1). The
suspensions were magnetically stirred in the dark for 30 min to reach the
adsorption-desorption equilibrium. Under ambient conditions and stirring, the mixed
suspensions were exposed to visible light irradiation produced by a 400 W metal
halogen lamp (λ > 400 nm) with a cut off filter. At certain time intervals, 2 ml of the
mixed suspensions were extracted and centrifuged to remove the photocatalysts. The
filtrates were analyzed by recording the UV-Vis spectra of Cr(VI) using a Hitachi
U-3900 UV-Vis spectrophotometer. The photocatalytic activity of P25 was also
investigated for comparison. A recycled photocatalytic activity test was carried out
according to the above-mentioned procedure. After each run of photocatalytic reaction,
the fresh Cr(VI) aqueous solution was injected, and the separated photocatalyst was
washed with deionized water carefully and used again. To investigate the
photocatalytic mechanism, trapping experiments were carried out to determine the
main reactive species in the photocatalytic process. The experimental procedure was
similar to the photocatalytic activity measurement except that the hole and electron
scavengers were added into the reaction system.
3. Results and discussion
3.1 Characterizations
Fig. 2(a) and (b) show the FESEM images of MoSe2 and MT-0.5. MoSe2
displays the sheet nanostructure. When TiO2 nanoparticles are introduced, they are
7

well distributed in MoSe2 sheets in the MT composites, as shown in Fig. 2(b). The
morphologies of MT-0.25, MT-1 and MT-2 (not shown here) are similar to that of
MT-0.5. Furthermore, the morphology of MoSe2 in the MT composites (MT-0.5) is
similar to that of pure MoSe2, indicating that the introduction of TiO2 does not affect
the formation of MoSe2. Fig. 2(c) and (d) show the low-magnification and
high-magnification HRTEM images of MT-0.5. It confirms the presence of TiO2
nanoparticles contacting with MoSe2 sheets, which could significantly increase the
separation of photo-generated carriers, and ultimately contribute to the photocatalytic
activity. The lattice spacing measured for the crystalline plane is 0.35 nm,
corresponding to the (101) plane of TiO2 (JCPDS 21-1272) [2]. No typical layer
structure and lattice fringe of MoSe2 can be observed due to the existence of its
amorphous state in the composite. The composition of MT-0.5 was identified by EDS
linked to HRTEM, as shown in Fig. 3. The peaks of Mo, Se, Ti and O in the EDS
spectrum further confirm the existence of MoSe2 and TiO2 in the composite.
The XRD patterns of MoSe2 and MT-0.5 are shown in Fig. 4. Pure MoSe2
exhibits several diffraction peaks at 13.7º, 34.4º and 55.9º, corresponding to (002),
(102) and (110) crystal planes of the hexagonal 2H-MoSe2 phase with space group
P63/mmc (JCPDS: 29-0914) [27]. No other impurity peaks appear for pure MoSe2,
indicating high purity of the as-prepared sample [28]. However, the XRD patterns
show relatively broad diffraction peaks, indicating that the as-prepared products are
somewhat amorphous and have short range structural order [27,34]. The XRD
analysis shows that the main diffraction peaks of the MT-0.5 composites are similar to
8

those of pure MoSe2, indicating that the presence of TiO2 does not result in the
development of new crystal orientations or changes in preferential orientations of
MoSe2. Compared with pure MoSe2, the new peaks corresponding to anatase (JCPDS
21-1272) and rutile (JCPDS 21-1276) phases of TiO2 appear in XRD pattern of
MT-0.5, which further confirms the existence of TiO2 in the composites.
The surface chemical composition of MT-0.5 was characterized by XPS
measurements. Fig. 5(a) and (b) show the high resolution XPS spectra of Mo 3d and
Se 3d. Two characteristic peaks located at 228.7 eV and 232.4 eV can be assigned to
Mo 3d5/2 and Mo 3d3/2, respectively, which is indicative of the reduction of Mo6+ to
Mo4+ and the formation of MoSe2 [35,36]. The peak at 55 eV is attributed to Se 3d3/2,
which is similar to that reported in the literature [37]. Fig. 5(c) shows the high
resolution XPS spectra of Ti 2p, displaying the spin-orbit split lines of Ti 2p3/2 and Ti
2p1/2 at 458.4 and 464.6 eV, respectively, which is in good agreement with that in
TiO2. As shown in Fig. 5(d), the O 1s spectrum of MT-0.5 is fitted to two peaks. The
peak of O1s at 530 eV corresponds to lattice oxygen of TiO2, and a higher binding
energy of 531.7 eV is assigned to mixed contributions from surface hydroxides. All of
these results clearly confirm the formation of MT composites.
Fig. 6 shows the UV-Vis diffuse absorption spectra of MoSe2, P25, MT-0.25,
MT-0.5, MT-1 and MT-2. MoSe2 displays good absorption in visible light range of
400-700 nm. It is observed that the MT composites with an appropriate proportion of
TiO2 exhibit higher visible light absorbance and the intensity increases with the
increase of TiO2 content. Such an enhancement of light absorption is beneficial to the
9

increase of the photo-generated electrons and holes [2,38,39]. However, when the
TiO2 content is further increased (MT-1 and MT-2), the absorption decreases due to
the wide band gap of TiO2, which is similar to those reported in the literatures [40,41].
In addition, compared to pure TiO2, the absorption edges of the MT composites
exhibit a red shift, which is also beneficial to the absorption of visible light.
The charge transfer and recombination behavior of the as-prepared samples was
studied by analyzing the EIS spectra under visible light irradiation. Fig. 7(a) shows
the typical Nyquist plots of MoSe2, MT-0.25, MT-0.5, MT-1 and MT-2 under visible
light irradiation. The semicircle in the EIS spectra is ascribed to the contribution from
the charge transfer resistance (Rct) and constant phase element (CPE) at the
photocatalyst/electrolyte interface. The inclined line, resulting from the Warburg
impedance ZW, corresponds to the ion-diffusion process in the electrolyte. The
corresponding equivalent circuit is shown in the inset of Fig. 7(a). It can be observed
that the Rct decreases with the increase of TiO2 content, which indicates that the
introduction of TiO2 into MoSe2 favors the electron transfer and suppresses the charge
recombination in MoSe2 due to the stepwise energy level structure in the composites.
The result is consistent with the theoretical calculation [31]. However, when the TiO2
content is further increased (MT-1 and MT-2), the Rct increases, which may be
because that excessive TiO2 can act as a recombination center instead of providing an
electron pathway and promote the recombination of electron-hole pairs in TiO2 [42].
Fig. 7(b) shows PL spectra of MoSe2, MT-0.25, MT-0.5, MT-1 and MT-2 with the
excitation wavelength at 365 nm. It can be clearly observed that the introduction of
10

TiO2 markedly decreases the PL intensity, which further confirms that the
recombination of photo-induced electrons and holes in MoSe2 can be effectively
inhibited.
3.2 Photocatalytic properties
Photocatalytic reduction of Cr(VI) by MoSe2, TiO2, MT-0.25, MT-0.5, MT-1
and MT-2 was performed under visible light irradiation, as shown in Fig. 8. The
normalized temporal concentration changes (C/C0) of Cr(VI) during the
photocatalytic process are proportional to the normalized maximum absorbance
(A/A0), which can be derived from the change in the Cr(VI) absorption profile during
the photocatalysis process. It is observed that MT composites with an appropriate
proportion of TiO2 exhibit a better photocatalytic activity than pure MoSe2 and TiO2.
The photocatalytic activity of MT composites is dependent on the proportion of TiO2
in the composites. The reduction rates of Cr(VI) for MoSe2 and TiO2 are 61% and 5%
at 120 min, respectively. When TiO2 is introduced into MoSe2, the reduction rate is
increased to 80% for MT-0.25 and reaches a maximum value of 91% for MT-0.5 at
120 min. It is known that during photocatalysis, the light adsorption, and the charge
transportation and separation are crucial factors [43,44]. The enhancement of the
photocatalytic activity should be mainly ascribed to the increase in light absorption
and the reduction in electron-hole pair recombination in MoSe2 with the introduction
of TiO2 due to the stepwise energy level in the composites, which have been
confirmed by the UV-Vis absorption and EIS measurements. However, when the
TiO2 content is further increased, the reduction rates decreases to 39% and 18% for
11

MT-1 and MT-2, respectively, which may be due to their increased recombination of
photo-generated electron-hole pairs, decreased visible light absorption and high
content of TiO2 [45,46]. The turnover numbers of photocatalytic production are about
0.021, 0.002, 0.009, 0.017, 0.014 and 0.006 for MoSe2, TiO2, MT-0.25, MT-0.5,
MT-1 and MT-2, respectively.
The values of rate constants (k), as shown in Fig. 9, can be obtained directly from
the fitted straight-line plots of ln(C/C0) vs. reaction time t and follow the order:
MT-0.5 (0.0141 min-1) > MT-0.25 (0.0114 min-1) > MoSe2 (0.0078 min-1) > MT-1
(0.0041 min-1) > MT-2 (0.0017 min-1) > P25 (0.0004 min-1), where C0 and C are the
initial concentration and the concentration of Cr(VI) at reaction time t, respectively.
The result shows that MT-0.5 exhibits a best photocatalytic activity under visible light
irradiation.
It is well known that the stability and reusability of photocatalysts are very
important for practical application. The reusability of MT composites (MT-0.5) was
investigated under visible light irradiation with three times of cycling uses, as shown
in Fig. 10. It is noteworthy that only insignificant decrease for photocatalytic activity
is found, which may be due to the loss of photocatalyst during collection process [47].
Furthermore, the morphology and crystal structure of MT-0.5 after the photocatalytic
reaction (labeled as irr-MT-0.5) were characterized by SEM and XRD. The SEM
image of irr-MT-0.5 shows much rougher surface, which may be due to the
photocatalytic reduction of Cr(VI), leading to the deposition of chromium compounds
as Cr2O3 or Cr(OH)3 on surface of MT composites. Similar results were also reported
12

in the literatures [48,49]. The results of XRD indicated that the crystal structure of
irr-MT-0.5 did not show obvious changes before and after the photocatalytic reaction
(Fig. S2). All results confirm that the MT composites can retain their excellent
photocatalytic stability and reusability activity under the studied conditions.
3.3 Possible photocatalytic mechanism
The possible mechanism for the photocatalytic reduction of Cr(VI) over MT
composites is proposed. MoSe2 is excited by the absorption of photons under visible
light irradiation and electron-hole pairs are created. The photo-generated electrons can
transfer from MoSe2 CB to TiO2 CB, leading to the separation of photo-generated
charge carriers. Then the photo-generated charge carriers may migrate to the surface
of MT composites and participate in the photocatalytic reduction and oxidation
reactions. Cr(VI) can be reduced to Cr(III) by the photo-generated electrons.
Meanwhile, the holes can oxidize the water to form oxygen in the photocatalytic
process [50]. The major reaction steps are summarized as follows:
MoSe2  hν  h  e


3
CrO72 14H 6e  2Cr 7HO
2
2


2HO 4h  O2 4H
2
To detect the reductive product, the bonding state of Cr 2p for irr-MT-0.5 was
characterized by XPS. Fig. S3 show the high resolution XPS spectra of Cr 2p. Two
peaks located at 577.3 and 586.9 eV correspond to Cr 2p1/2 and Cr 2p3/2 orbitals,
respectively, which are attributed to Cr(VI) and Cr(III). This result confirms that both
Cr(VI) and Cr(III) exist on the surface of irr-MT-0.5. The appearance of Cr(III) is
13

mainly due to the photocatalytic reduction of Cr(VI) on the surface of photocatalyst
and the Cr(III) species can be precipitated on the surface of irr-MT-0.5 as Cr2O3 or
Cr(OH)3, which is similar to those reported results [48,51]. Furthermore, the weak
peak of Cr 2p indicates that the deposited Cr(III) species can be removed via simply
washing the sample with deionized water or NaOH.
To confirm the role of the photo-generated electrons in the photocatalytic
process, controlled experiments have been carried out with addition of hole (formic
acid, FA) and electron scavengers (K2S2O8). As shown in Fig. 11, the addition of hole
scavengers can enhance the photocatalytic activity of MT-0.5, while the activity
decreases largely with the addition of electron scavengers. FA as a hole scavenger can
capture photo-generated holes in the photocatalytic process, and thus suppress the
recombination of photo-generated carriers, leading to the enhancement of
photocatalytic activity of MT-0.5. However, K2S2O8 as an electron scavenger can
capture photo-generated electrons, and thus decrease the number of photo-generated
electrons to participate in the photocatalytic reaction, which is not beneficial to the
photocatalytic activity. The results confirm that the photo-generated electrons govern
the photocatalytic reduction of Cr(VI), which is consistent with the literatures [24,40].
4. Conclusions
MT composites were successfully synthesized via a simple solvothermal method
and their photocatalytic activities in the reduction of Cr(VI) were investigated. The
14

results indicate that (i) MT composites exhibit an enhanced photocatalytic activity
than pure MoSe2; (ii) the photocatalytic activity of MT composites is dependent on
the proportion of TiO2 in the composites and the MT-0.5 achieves a highest Cr(VI)
reduction rate of 91% at 120 min; (iii) the enhanced photocatalytic activity is mainly
ascribed to the increase in light adsorption and the reduction in photoelectron-hole
pair recombination with the introduction of TiO2.
Acknowledgments
Financial support from the National Natural Science Foundation of China (No.
21401180 and 51402274), Provincial Natural Science Foundation of Anhui
(1508085ME104), and Scientific Research Foundation for the Anhui Provincial
Returned Overseas Chinese Scholars (Electrospun low dimensional TiO2
nano-structures and application in photovoltaic devices) is gratefully acknowledged.
15

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19

Figure captions
Fig. 1. Schematic diagram of energy levels of MoSe2 and TiO2.
Fig. 2. FESEM images of (a) MoSe2 and (b) MT-0.5; (c) low-magnification and (d)
high-magnification HRTEM images of MT-0.5.
Fig. 3. EDS spectrum of MT-0.5.
Fig. 4. XRD patterns of MoSe2 and MT-0.5.
Fig. 5. High-resolution XPS spectra of (a) Mo 3d, (b) Se 3d, (c) Ti 2p and (d) O 1s
spectra for MT-0.5.
Fig. 6. UV-Vis diffuse absorption spectra of MoSe2, P25, MT-0.25, MT-0.5, MT-1
and MT-2.
Fig. 7. (a) Nyquist plots and (b) PL spectra of MoSe2, MT-0.25, MT-0.5, MT-1 and
MT-2. Inset is the corresponding equivalent circuit model.
Fig. 8. Photocatalytic reduction of Cr(VI) by MoSe2, P25, MT-0.25, MT-0.5, MT-1
and MT-2 under visible light irradiation.
Fig. 9. Reaction constant k for Cr(VI) reduction by MoSe2, P25, MT-0.25, MT-0.5,
MT-1 and MT-2 under visible light irradiation.
Fig. 10. Photo-stability of MT-0.5 by investigating its photocatalytic activity with
three times of cycling uses.
Fig. 11. Photocatalytic reduction of Cr(VI) by MT-0.5 with the addition of hole and
electron scavengers.
20

Fig. 1
Fig. 2
21

Fig. 3
Fig. 4
22

Fig. 5
Fig. 6
23

Fig. 7
Fig. 8
24

Fig. 9
Fig. 10
25

Fig. 11
26