Highly effective photocatalytic performance of {001}-TiO2/MoS2/RGO hybrid heterostructures for the reduction of Rh B

Article abstract of DOI:10.1039/c9ra02634g

Effective separation and rapid transfer of photogenerated electron-hole pairs are key features of photocatalytic materials with high catalytic activity, which could be achieved in co-catalysts. It is reported that the two-dimensional (2D) MoS₂ is a promising co-catalyst due to its unique semi-conductive properties and graphene-like layered structure. However, the application of MoS₂ as a co-catalyst is limited by its poor electrical conductivity. On the other hand, it is worth noting that TiO₂ possesses reactive crystal facets, which is one of the dominant mechanisms for the separation of photogenerated electron-hole pairs. In this work, we prepared MoS₂/RGO hybrids as co-catalysts which were doped to TiO₂ with highly reactive {001} planes via the hydrothermal method. It was found that the {001}-TiO₂/MoS₂/RGO photocatalysts with 7 wt% MoS₂/RGO co-catalyst show the highest photodegradation activity for the degradation of Rh B under visible light irradiation (λ > 400 nm), which could result from the synergy of the effective separation of electron-hole pairs by the {001} facets in TiO₂ and the rapid transfer of electron-hole pairs in MoS₂/RGO. The results show that the {001}-TiO₂/MoS₂/RGO hybrid is a low-cost and stable photocatalyst for the effective degradation of Rh B under visible light.

Full text of DOI:10.1039/c9ra02634g

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PAPER
Highly effective photocatalytic performance of
{001}-TiO2/MoS2/RGO hybrid heterostructures for
the reduction of Rh B†
Cite this: RSC Adv., 2019, 9, 15033
a
Ya Gao,a Yongjie Zheng,
Jixing Chai,b Jingzhi Tian,a Tao Jing,a Deqing Zhang,b
*
c
c
*
Junye Cheng,
Huiqing Peng,c Bin Liu
and Guangping Zhengd
*
Effective separation and rapid transfer of photogenerated electron–hole pairs are key features of
photocatalytic materials with high catalytic activity, which could be achieved in co-catalysts. It is reported
that the two-dimensional (2D) MoS2 is a promising co-catalyst due to its unique semi-conductive
properties and graphene-like layered structure. However, the application of MoS2 as a co-catalyst is limited
by its poor electrical conductivity. On the other hand, it is worth noting that TiO2 possesses reactive crystal
facets, which is one of the dominant mechanisms for the separation of photogenerated electron–hole
pairs. In this work, we prepared MoS2/RGO hybrids as co-catalysts which were doped to TiO2 with highly
reactive {001} planes via the hydrothermal method. It was found that the {001}-TiO2/MoS2/RGO
photocatalysts with 7 wt% MoS2/RGO co-catalyst show the highest photodegradation activity for the
degradation of Rh B under visible light irradiation (l > 400 nm), which could result from the synergy of the
effective separation of electron–hole pairs by the {001} facets in TiO2 and the rapid transfer of electron–
hole pairs in MoS2/RGO. The results show that the {001}-TiO2/MoS2/RGO hybrid is a low-cost and stable
photocatalyst for the effective degradation of Rh B under visible light.
Received 8th April 2019
Accepted 8th May 2019
DOI: 10.1039/c9ra02634g
rsc.li/rsc-advances
nanowire lms.17 Another strategy is the loading of co-catalyst,
including graphene (GN),18–20 metallic phases21,22 and nonme-
tallic phases23,24 of materials, as well as polymer-metallic cata-
1. Introduction
Semiconducting photocatalysts exhibit great prospects in the
removal of pollutants from water and air.1–4 Among the pursued
semiconductors, titanium dioxide (TiO2) with a band gap of
3.2 eV between the conduction band (CB) and valence band
(VB), has attracted attention as a photocatalytic material owing
to its nontoxicity, chemical stability, low cost, high chemical
inertness.5–10 The active photocatalytic characteristics of TiO2
are restricted due to the rapid recombination of photo-
generated electron–hole pairs.11,12
There are two strategies that have been carried out to
enhance the photocatalytic performance of TiO2. One of the
strategies is based on the design of unique morphology and
structure of TiO2, such as mesoporous TiO2 hollow shells,13,14
TiO2 nanosheet arrays,15,16 and three-dimensional network TiO2
lysts.25,26 Zhang et al.20 reported a successful strategy to improve
the photocatalytic performance of TiO2 via introducing the GN
without defects to enhance the interfacial contact between GN
and TiO2; Liu et al.22 prepared Ag/Ag(I)–TiO2 photocatalyst by
a facile impregnated method in combination with a calcination
process and the photocatalyst showed a higher visible-light
photocatalytic activity; Qu et al.24 synthesized a S,N co-doping
GQD/TiO2 composite though a facile hydrothermal and its
photocatalytic performance (degradation of Rh B under visible
light) was 10 times higher than that of P25; Jiang et al.26 re-
ported a nanostructured TiO2 thin lm accompanying with the
effects of polymer conjugation and Mo-doping, as a result, the
methylene blue dye degradation rate reached 90.6%. In the
aforementioned methods, heterostructure photocatalysts con-
sisting of two semi-conductive materials with different band
gaps and electronic structures have been proved to be effective
in photocatalysis.27–31
aSchool of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar 161006,
China. E-mail: yongjiezheng@163.com
bSchool of Materials Science and Engineering, Qiqihar University, Qiqihar 161006,
If developed into a co-catalyst material, MoS2 have some
particular advantages due to its graphene-like nanostructures,
relatively high activity and unique semiconducting proper-
ties.32–34 And there are numerous reports about MoS2-based
heterostructure for photocatalytic production of H2. However
the applications of MoS2 in the removal of pollutants are
limited by its poor electrical conductivity.35–37 Generally, there
China
cCenter of Super-Diamond and Advanced Films (COSDAF), Department of Materials
Science and Engineering, City University of Hong Kong, Hong Kong 999077, China.
E-mail: binliu@mail.ipc.ac.cn; jycheng4-c@my.cityu.edu.hk
dDepartment of Mechanical Engineering, The Hong Kong Polytechnic University, Hung
Hom, Kowloon, Hong Kong, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra02634g
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are two synthesis routes in the fabrication of MoS2-based het- previously reported,43 was dispersed in 80 ml of DI. Secondly,
erostructure with effective charge transfer processes. First, 0.99 g of the as-prepared {001}-TiO2 nanosheets were added to
chemical lithium intercalation could induce a phase change in the above-mentioned dispersions. Aer subjected to magnetic
MoS2 from the 2H (showing semiconducting properties) to the stirring for 1 h, the mixture was transfer to a 100 ml Teon-lined
1T (showing metallic properties) phases, resulting in the het- autoclave. Aer reaction for 3 h at 180 ꢀC, the solution was
erostructure. Bai et al.38 rstly reported that compared with (2H) naturally cooled down to the room temperature. Then the {001}-
MoS2–TiO2, the (1T) MoS2–TiO2 exhibits excellent photo- TiO2/MoS2/RGO hybrid was washed withꢀdeionized water and
catalytic performance. Second, the introduction of conductive ethanol for several times and dried at 60 C in a vacuum oven.
materials into MoS2 leads to a heterostructure. Peng et al.39 With the same method described above, the {001}-TiO2/MoS2/
synthesized MoS2/RGO/CdS as an effective photocatalyst which RGO samples with different MoS2/RGO loadings were prepared.
can reduce and detoxify nitroaromatic compounds. The MoS2- The aforementioned facile synthesis route was summarized
based heterostructures prepared by these two methods have in Scheme 1.
played an important catalytic role, while the latter is more
favorable because of its simplicity and efficiency.
2.4 Characterization
Moreover, it is reported that the reactive crystal facets of TiO2
are closely related with its photocatalytic efficiency, and the
{001} facet-dominated single-crystal anatase TiO2 shows higher
photocatalytic activity than P25.40–42 Enlightened by the above-
mentioned ndings, it is suggested that the {001}-TiO2/MoS2/
RGO should be an ideal photocatalyst. In this work, we prepared
a {001}-TiO2/MoS2/RGO hybrid photocatalyst with low cost. The
as-prepared {001}-TiO2/MoS2/RGO exhibited more outstanding
photocatalytic performance for the degradation of Rhodamine
B (Rh B) under visible light irradiation (l > 400 nm), compared
with the pure {001}-TiO2 and {001}-TiO2/MoS2. The enhanced
photocatalytic activity could be ascribed to the effective charge
separation and transfer due to the band gap of MoS2 and high
conductivity of graphene.
The morphology of samples was characterized by the eld
emission scanning electron microscope (SEM, Hitachi S-
3400) and transmission electron microscope (TEM, Hitachi
H7650). The crystallization properties were measured by X-
ray powder diffraction (XRD) patterns (Bruker D8) with
a Cu-Ka radiation. The crystal structure was observed with
the high-resolution electron microscopy (HRTEM, Tecnai
F30). Element information was characterized by X-ray
photoelectron spectrometer (ESCALAB250Xi, Thermosher
Co). Raman spectrum was measured by the equipment Lab
RAM HA Evolution. Ultraviolet-visible (UV-vis) absorption
spectra were performed using a spectrophotometer (Lambda
35, American). The photoluminescence spectra (PL) was
examined with
a uorescence spectrophotometer (RF-
5301PC, Shimadzu Corporation, Japan). The information on
the Brunauer–Emmett–Teller (BET) surface areas of the
sample powders was obtained on an automatic gas adsorp-
tion analyzer (Conta Instruments, USA).
2. Experimental
2.1 Materials
All chemicals were in analytical grade and used without further
purication. Molybdic acid (H2MoO4 $ 85%) was purchased
from Shanghai Aladdin Biochemical Technology Co. Ltd.
Thiourea (CH4N2S) was purchased from Tianjin Fuchen
Chemical Reagent Factory. Graphene oxide dispersion were
purchased from Nanjing Pioneer Nano Material Technology
Co., Ltd. N-Butyl titanate (TBOT) was purchased from Tianjin
Tianli Chemical Reagent Co. Ltd. Hydrouoric acid (HF) was
purchased from Tianjin Fuyu Chemical Reagent Co. Ltd.
2.5 Evaluation of photocatalytic activity
The photocatalytic activities of the catalysts were evaluated
through the degradation of Rh B under a light source of 300 W
Xe arc lamp equipped with a cut-off lter (l > 400 nm) for visible
light. Firstly, 50 mg of photocatalyst was added into 50 ml Rh B
solution (1 mg mlꢁ1). Then, the suspensions were stirred in
dark for 30 minutes to reach the adsorption–desorption equi-
librium. Third, the suspensions were subjected to the illumi-
nation every 10 minutes; in the intervals of illumination 5 ml
suspension was withdrawn and centrifuged to remove the
photocatalyst particles and then was tested by a UV-vis spec-
trophotometer to determine the concentration of Rh B through
measurements on the 554 nm absorption peak. The Rh B
concentration aer adsorption equilibrium is regarded as the
initial concentration (C0).
2.2 Synthesis of {001}-TiO2 nanosheets
The {001}-TiO2 nanosheets were prepared by a simple hydro-
thermal route. Firstly, 25 ml of n-butyl titanate and 3 ml of
hydrouoric acid were added into a 100 ml Teon-lined auto-
clave, under magnetic stirring for 2 h. Aer heating and stirring
ꢀ
for 24 h at 180 C, the solution was naturally cooled to room
temperature and washed with deionized water and ethanol for
several times until its pH value reached 7 and dried at 60 ꢀC in
a vacuum oven to obtain the {001}-TiO2 nanosheets.
3. Results and discussion
3.1 Results of characterization
2.3 Synthesis of {001}-TiO2/MoS2/RGO hybrid
The crystal structures of the prepared samples are charac-
The nanohybrids were also prepared by a hydrothermal terized determined by XRD, and the XRD patterns of pure
method. Firstly, the as-prepared MoS2/RGO (with 7 wt% RGO) {001}-TiO2, MoS2/RGO and {001}-TiO2/MoS2/RGO are shown
which was prepared by a one-step hydrothermal method in Fig. 1a. The diffraction peaks at 2q ¼ 25.28ꢀ, 37.8ꢀ, 48.04ꢀ,
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Scheme 1 Synthetic routes for the {001}-TiO2 nanosheets and the {001}-TiO2/MoS2/RGO composites.
53.89ꢀ, 55.06ꢀ, 62.68ꢀ, 68.76ꢀ, 70.30ꢀ, 75.02ꢀ, 76.01ꢀ and located at 1348.53 and 1588.53 cmꢁ1, respectively, and the
82.65ꢀ can be observed in {001}-TiO2 and {001}-TiO2/MoS2/ I2D/IG ratio of RGO is 0.19 (0.07 < 0.19 < 0.3), suggesting that
RGO samples, which are assigned to (101), (004), (200), (105), the hybrid have triple-layer graphene sheets.43 The results of
(204), (116), (220), (215), (301) and (224) planes of the anatase Raman spectra thus prove the presence of RGO in the
TiO2 (PDF 21-1272), respectively. It is also seen that there are hybrid.
only two extremely weak peaks of the co-catalyst in the
The interactions between the TiO2 and MoS2/RGO in the
pattern for {001}-TiO2/MoS2/RGO (the inset in Fig. 1a). In the {001}-TiO2/MoS2/RGO hybrid are studied by XPS spectra. As
patterns for the MoS2 and MoS2/RGO samples (Fig. S1a†), shown in Fig. 1c, the Mo, S, Ti, O, C and F elements appear in
there are four peaks at 2q ¼ 14.6ꢀ, 32.8ꢀ, 39.8ꢀ and 58.5ꢀ, the survey spectrum. The peak at the binding energy of
which are assigned to (002), (100), (103), and (110) planes of 684.5 eV is assigned to F 1s, which is a typical value for the
MoS2 (JCPDS37-1492), respectively. There are no obvious uorinated TiO2 system such as the surcial Ti–F species.44
diffraction peaks for RGO mostly because of very small Fig. 1d and S2a† shows the high-resolution spectra of Ti 2p for
amount of RGOs in the samples.
TiO2 and {001}-TiO2/MoS2/RGO, respectively. The peaks
To conrm the presence of RGO in the {001}-TiO2/MoS2/ located at the binding energies of 465.31 and 459.57 eV for
RGO hybrid, Raman measurements were carried out. Fig. 1b TiO2 could correspond to Ti 2p1/2 and 4p3/2, respectively,
shows the Raman spectrum of {001}-TiO2/MoS2/RGO hybrid, which can be assigned to a Ti4+ oxidation state. Compared with
it is observed that the weak Eg(1), A1g and Eg(2) peaks for TiO2 pure TiO2, those peaks for {001}-TiO2/MoS2/RGO shi to
are located at 142.97 cmꢁ1, 515.22 cmꢁ1 and 636.34 cmꢁ1
,
465.11 and 459.48 eV, indicating a strong interaction between
respectively. It also can be clearly observed that the E2g and MoS2/RGO and TiO2. As shown in the Mo spectrum of {001}-
A1g peaks for MoS2 are located at 379.74 and 408.88 cmꢁ1
TiO2/MoS2/RGO (Fig. 1e), the bands located at the binding
,
respectively. The peaks of the D- and G-band for RGOs are energies of 232.35 and 229.03 eV are assigned to Mo (3d3/2) and
Fig. 1 (a) XRD patterns for MoS2/RGO, {001}-TiO2 and {001}-TiO2/MoS2/RGO, the inset is at 2q ¼ 5–45ꢀ; (b) Raman spectrum for {001}-TiO2/
MoS2/RGO; (c) XPS survey spectrum for {001}-TiO2/MoS2/RGO; high-resolution spectra of {001}-TiO2/MoS2/RGO (d) Ti 2p; (e) Mo 3d; (f) S 2p.
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Mo (3d5/2) in the normal chemical state of Mo4+, respectively.
Detailed microstructure of the samples can be clearly
The S spectra of {001}-TiO2/MoS2/RGO are observed to have observed in the transmission electron microscopy (TEM) and
two peaks located at 162.82 and 161.71 eV, as shown in Fig. 1f, high-resolution TEM (HRTEM). MoS2 shows an appearance of
which are consistent with those of MoS2. The C 1s spectrum thin nanosheets that look like so and light owers (Fig. 3b), for
can be deconvoluted into three peaks located at 288.75, MoS2/RGO hybrid, it can be clearly seen that the ower-like
286.40, and 284.84 eV (Fig. S2b†), which correspond to the C] MoS2 in the dark region is evenly attached to the bright and
O, C–O, and C–C functionalities, respectively, and indicate the light RGOs (Fig. 3a and c). The {001}-TiO2 has a thickness of
existence of the carboxy, epoxy, and hydroxyl groups in the 11 nm with average dimensions (length ꢂ width) of 64 ꢂ 38 nm
graphene. The results indicate that most of the oxygen- and there is very few small sized {001}-TiO2 (Fig. 3d). Fig. 3e
containing functional groups are successfully removed. The shows the TEM image of {001}-TiO2/MoS2/RGO. The {001}-TiO2
reduction of GO to graphene will dramatically improve the nanosheets and MoS2/RGO can be observed clearly. The
electrical conductivity of the hybrids, and hence signicantly HRTEM image of Fig. 3f shows that the lattice spacing of
enhancing their photocatalytic activity.45,46
0.35 nm and 0.62 nm can be assigned to (101) planes of MoS2
Typical morphology and microstructure of the as-prepared and (101) planes of anatase {001}-TiO2, respectively.
catalyst are characterized using scanning electron microscopy
(SEM). Fig. 2a shows that a large number of stacked MoS2
nanosheets with signicantly curled edges are observed.
3.2 Photocatalytic study
The photocatalytic activity of all the as-prepared samples was
evaluated from measurements on the degradation of Rh B.
Before illumination, the suspensions were prepared via stirring
in dark for 30 minutes, such that they reached the adsorption–
desorption equilibrium.
In this study, rstly, we investigate the optimum ratio of
MoS2 and RGO in a co-catalyst, as shown in Fig. 4a. The weight
percentage of MoS2/RGO in the hybrid is kept at 7 wt%, and the
Fig. 2b shows the microstructure of MoS2/RGO, indicating that
the small MoS2 nanosheets are well loaded onto the surfaces of
RGOs. Fig. 2c shows the morphology of TiO2, which exhibits
a rectangular shape with lengths of about 40–60 nm. Fig. 2d
show the morphology of {001}-TiO2/MoS2/RGO. No obvious
MoS2/RGO structures can be observed, which maybe caused by
the low content of MoS2/RGO. To determine the composition
of {001}-TiO2/MoS2/RGO, the sample is measured by the
energy dispersive spectrometer (EDS) as shown in Fig. S3.†
Fig. S3† shows that there are a large amount of Ti and O
elements, a small amount of Mo and S elements and a few C
elements. Moreover, the distribution of Ti and O elements is
even and consistent, and the Mo and S elements also show
similar features of distribution. Thus, the EDS results
demonstrate that the sample shown in Fig. 3d could be {001}-
TiO2/MoS2/RGO.
{001}-TiO2/MoS2/RGO hybrid with
a MoS2 : RGO ratio of
97 : 3 wt% shows the highest photodegradation rate. Then we
studied the inuence of the species and contents of co-catalyst
on the photodegradation activity. As shown in Fig. 4c, the color
dye without catalyst presents no photodegradation activity
under visible light irradiation. It veries that self-sensitisation
couldn't cause the degradation. The pure {001}-TiO2 exhibits
a low photocatalytic activity which could be caused by the rapid
recombination of the CB electrons and the VB holes. While by
Fig. 2 SEM images of MoS2 (a), MoS2/RGO (b), TiO2 (c) and {001}-TiO2/MoS2/RGO (d).
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Fig. 3 TEM images of RGO (a), MoS2 (b), MoS2/RGO (c), {001}-TiO2 (d), {001}-TiO2/MoS2/RGO (e); HRTEM image of {001}-TiO2/MoS2/RGO (f).
the incorporation of MoS2, the photocatalytic activity of {001}- reaction rate constants (K) are calculated by the pseudo-rst
TiO2 is enhanced signicantly, which is caused by the transfer order model. As shown in Fig. S5a,† the values of K for {001}-
of electrons from {001}-TiO2 to MoS2.43 In fact, the {001}-TiO2/ TiO2, {001}-TiO2/RGO, {001}-TiO2/MoS2 and {001}-TiO2/MoS2/
MoS2 hybrid have exhibited excellent performance for the RGO are 0.0084, 0.0201, 0.01285 and 0.03325, respectively. It
adsorption of Rh B under dark condition (as shown in Fig. S5c– can be seen that the {001}-TiO2/MoS2/RGO hybrid photo-
f†). Through the incorporation of MoS2/RGO as the co-catalyst catalyst
for {001}-TiO2, the photocatalytic activity of the hybrid could photodegradation.
be further improved, as demonstrated in Fig. 4b, the {001}-TiO2/ The stability and reusability of {001}-TiO2/MoS2/RGO were
exhibits
the
highest
rate
(0.03325)
of
0.07(MoS2/0.03RGO) hybrids show the best photocatalytic also evaluated. Fig. S1b† shows the XRD patterns of {001}-
performance. UV-visible investigation shows that aer 60 min TiO2/MoS2/RGO before and aer photocatalysis, and it is
of visible light (l > 400 nm) irradiation, 92.3% of Rh B has been observed that aer photocatalysis the crystal structure has not
degraded by {001}-TiO2/0.07(MoS2/0.03RGO) (as shown in changed. The inset of Fig. S5b† illustrates that the photo-
Fig. 4d). It is suggested that the enhanced separation of pho- catalytic activity of {001}-TiO2/MoS2/RGO displays a slight
togenerated electrons and holes because of the high electron decrease aer running for ve successive cycles, indicating
conductivity of RGO could result in the enhanced photocatalytic that the {001}-TiO2/MoS2/RGO hybrid is stable and efficient
performance. In addition, the photocatalytic performance of photocatalyst.
{001}-TiO2/0.07(MoS2/0.03RGO) catalyst can also be evaluated
by TOC removal. As shown in Fig. S5b,† the{001}-TiO2/
0.07(MoS2/0.03RGO) hybrid catalyst shows the highest TOC
3.3 Mechanisms of the enhanced photocatalytic activity
removal rate of Rh B, which basically coincides with the result of
the degradation rate, illustrating that the Rh B has been thor-
oughly mineralized. The color change of Rh B solution also
reects the efficiency of catalyst. We can clearly observe that the
Rh B solution under the photocatalytic degradation of {001}-
TiO2/MoS2/RGO hybrid is close to white (as shown in Fig. S4†).
The reaction kinetics of Rh B reduction is determined by
plotting the value of ln(C0/C) versus illumination time, and the
It is well known that the photocatalytic activity of semi-
conductive photocatalysts is intrinsically governed by three
processes: (i) photoexcitation of semiconductor to generate
electron–hole pairs, (ii) charge separation and diffusion to the
surface of the catalysts, and (iii) surface activation for redox
reactions.38 In the sections that follow, the mechanisms of the
enhanced photocatalytic performance of {001}-TiO2/MoS2/RGO
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Fig. 4 Photocatalytic degradation of Rh B under visible light irradiation over (l > 400 nm) (a) {001}-TiO2/MoS2/RGO with different RGO
percentages; (b) {001}-TiO2/MoS2/RGO with different contents of MoS2/0.03RGO; (c) blank, {001}-TiO2, {001}-TiO2/0.07GO, {001}-TiO2/
0.07MoS2 and {001}-TiO2/0.07(MoS2/0.03RGO). (d) UV-visible spectra of {001}-TiO2/MoS2/RGO.
are elucidated separately based on the abovementioned graphene can rapidly transfer electrons to hinder the recombi-
processes.
nation of photogenerated electron–hole pairs. We can evaluate
The UV-vis diffuse reectance spectra of TiO2 and {001}-TiO2/ the probability of recombination of electron–hole pairs by PL
MoS2/RGO are shown in Fig. 5a. Compared with pure {001}- spectroscopy, in that the higher uorescence intensity repre-
TiO2, the {001}-TiO2/MoS2/RGO hybrid shows an enhanced sents higher probability of recombination. Fig. 5c shows the PL
visible light absorption. The band gaps of {001}-TiO2 and {001}- spectra under 400 nm excitation wavelengths for pure {001}-
TiO2/MoS2/RGO are determined to be 3.2 and 3.04 eV, respec- TiO2 and {001}-TiO2/MoS2/RGO samples, and a strong emission
tively, according to the onset of the absorption edge. The {001}- peak appears at about 602 nm.48 It is found that the PL intensity
TiO2/MoS2/RGO hybrid exhibits a narrower band gap and of {001}-TiO2/MoS2/RGO sample is lower than that of pure {001}-
higher absorption intensity, meaning that their redox potentials TiO2, which indicates that by the incorporation of MoS2/RGO
as well as the recombination of electrons and holes are reduced co-catalyst, the hybrid can inhibit the recombination of excited
as compared with those of {001}-TiO2.47
electrons and holes in the {001}-TiO2/MoS2/RGO catalysts. The
Fig. 5b shows the nitrogen adsorption/desorption resistance of catalyst is determined by the electrochemical
isotherms of {001}-TiO2 and {001}-TiO2/MoS2/RGO. The impedance spectroscopy (ESI). Fig. 5d shows the ESI spectro-
isotherms for pure {001}-TiO2 reveal a type-IV isotherm with an gram of {001}-TiO2 and {001}-TiO2/MoS2/RGO. Compared with
H3 hysteresis loop, which indicates that the holes are formed pure {001}-TiO2, the {001}-TiO2/MoS2/RGO hybrid presents
by the accumulation of aky particles. The {001}-TiO2/MoS2/ a smaller frequency semicircle, suggesting the resistance of
RGO hybrids display a type-IV isotherm with an H4 hysteresis {001}-TiO2/MoS2/RGO is lower than that of {001}-TiO2. Thus, it
loop, suggesting the slit-shaped pores are produced by the can be concluded that the introduction of MoS2/RGO co-
layered structure. Based on BET calculation, the surface area catalysts into {001}-TiO2 can reduce the recombination of
of {001}-TiO2 is 111.124 m2 gꢁ1 and the surface area of {001}- charge carriers, which is consistent with the results of PL
TiO2/MoS2/RGO is 76.123 m2 gꢁ1, revealing the information on spectrum.
their microstructures consistent with those observed in SEM
images (Fig. 2c and d).
The schematic of photocatalytic enhancement and electron
transfer of {001}-TiO2/MoS2/RGO is shown in Fig. 5e. Under
It is revealed that graphene could improve the photocatalytic visible light irradiation, the VB electrons of MoS2 and {001}-
property of {001}-TiO2/MoS2/RGO hybrid owning to the fact that TiO2 are excited to the CB, leaving holes (h+) in the VB.49,50
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Fig. 5 (a) UV-vis diffusive reflectance spectra of pure {001}-TiO2 and {001}-TiO2/MoS2/RGO; the inset is corresponding plot of the transformed
Kubelka–Munk function versus the energy of light; (b) N2 adsorption/desorption isotherms of pure {001}-TiO2 and {001}-TiO2/MoS2/RGO; (c) PL
spectra of pure {001}-TiO2 and {001}-TiO2/MoS2/RGO under 400 nm excitation wavelength; (d) electrochemical impedance spectroscopy (EIS)
Nyquist plots of the sample electrodes which are made from the pure {001}-TiO2 and {001}-TiO2/MoS2/RGO treated with visible light (l > 400
nm) irradiation; (e) schematic diagram for photocatalytic mechanism of {001}-TiO2/MoS2/RGO.
ꢁ
Meanwhile, the photoinduced electrons of {001}-TiO2 can generate $OH radicals. The powerful oxidizing agents $O2
transfer to MoS2 via the conductive network of RGO. It is worth and $OH could effectively decompose the Rh B into CO2, H2O
noting that the separation of photogenerated electron–hole or other intermediates. As listed in Table 1, we have compared
pairs of {001}-TiO2/MoS2/RGO is more rapid than that of {001}-TiO2/MoS2/RGO with other representative composites
conventional TiO2/MoS2/RGO owing to the especially reactive reported in literature. It can be found that the {001}-TiO2/
{001} facets of anatase TiO2. The photogenerated electrons on MoS2/RGO exhibits desirable catalytic performance, which
the CB of samples can react with O2 to produce superoxide rapid and effective, indicating the promising perspective of
radical anions $O2ꢁ. Meanwhile those photoexcited holes on {001}-TiO2/MoS2/RGO in the removal of pollutants
the VB of samples react with H2O and hydroxide ions to applications.
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Table 1 Catalytic activity of representative composites
Catalyst
Method
Degradation (%) Degradation target Times (min) Ref.
{001}-TiO2/MoS2/RGO Hydrothermal
92.3
92
92
94
95
90
89
82
Rhodamine B
Rhodamine B
Rhodamine B
Rhodamine B
Methylene blue
Methylene blue
Methylene blue
Methylene blue
Methylene blue
Methylene blue
Methylene blue
60
90
180
120
90
75
120
60
This work
BiPO4/MoS2/RGO
Bi2WO6/MoS2/RGO
TiO2/RGO
Hydrothermal
Hydrothermal
Hydrothermal
51
48
52
53
54
55
48
56
57
49
CeO2/SnO2/rGO
rRGO–ZnO–MoS2
NiWO4–ZnO–NRGO
{001}-TiO2/RGO
TiO2/RGO/Ag
Hydrothermal
Hydrothermal
Microwave irradiation method
Hydrothermal
Sol–gel process photo-assisted reduction method 93
120
135
100
rGO–ZnWO4–Fe3O4
TiO2/MoS2/RGO
Microwave irradiation method
Hydrothermal
97
97
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4. Conclusion
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In summary, we have successfully synthesized the {001}-TiO2/
MoS2/RGO hybrids with effective catalytic performance using
a facile hydrothermal method. The {001}-TiO2/MoS2/RGO pho-
tocatalysts with 7 wt% MoS2/RGO co-catalyst show the highest
photodegradation activity for the degradation of Rh B under
visible light irradiation (l > 400 nm). The {001} facets of {001}-
TiO2/MoS2/RGO accelerate the separation of photogenerated
electron–hole pairs, and RGO rapidly transfer the photoinduced
electrons from {001}-TiO2 to MoS2, which further enhances the
charge carrier separation of the {001}-TiO2/MoS2/RGO hybrid. It
is believed that such synergy effect promotes the photocatalytic
performance of {001}-TiO2/MoS2/RGO hybrids. Taking into
account their facile synthesis route and excellent photocatalytic
performance, the MoS2/RGO hybrids are promising co-catalyst
which can be widely used for co-photocatalysts and {001}-
TiO2/MoS2/RGO hybrids are promising candidates for photo-
catalytic degradation of Rh B.
´
ˇ
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Conflicts of interest
There is no conict in the statement.
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Acknowledgements
This work was supported by Province Advantages and Distinc-
tive Subjects Project of Heilongjiang (YSTSXK201810), Hei-
longjiang
Provincial
Education
Department
Project
(LTSW201738), Qiqihar City Science and Technology Project
(SFGG-201601), the Innovation Fund Project for Graduate
Student of Qiqihar University (YJSCX2017-022X).
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