Improved photocatalytic activity of MoS2 nanosheets decorated with SnO2 nanoparticles

Article abstract of DOI:10.1039/c5ra15159g

MoS₂ nanosheets decorated with SnO₂ mesoporous nanoparticles were successfully prepared by a facile two-step method. The MoS₂ nanosheets were pre-synthesized using a solvothermal method and then decorated with the SnO₂ mesoporous nanoparticles through a wet chemical method. The nanocomposite was characterized with powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersed spectrometry (EDX), high-resolution transmission electron microscopy (HRTEM), thermal gravimetric and differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). SnO₂ mesoporous nanoparticles can be selectively formed and attached to the peripheral surface of the layered MoS₂, which was confirmed by FESEM and HRTEM. The photocatalytic activity of the nanocomposite was examined with Rhodamine B (RhB) in aqueous solution under UV light irradiation. The SnO₂ nanoparticles remarkably suppressed the electron-hole recombination effect on the MoS₂ photocatalyst and improved the photocatalytic activity compared to a pristine MoS₂ catalyst. A higher rate of pollutant degradation was accomplished within 50 min that was three times higher than that of the pristine MoS₂ catalyst.

Full text of DOI:10.1039/c5ra15159g

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VATTIKUTI, C. Byon, Ch.Venkata Reddy and R. V. S. S. N. R, RSC Adv., 2015, DOI: 10.1039/C5RA15159G.
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Improved photocatalytic activity of MoS2 nanosheets decorated with SnO2 nanoparticles
S.V. Prabhakar Vattikuti1*, Chan Byon1*, Ch. Venkata Reddy1 and R.V.S.S.N.Ravikumar2
1
School of Mechanical Engineering, Yeungnam University, Gyeongsan, South Korea, 712ꢀ749
2
Department of Physics, Acharya Nagarjuna University, A.P, India, 522510
* Corresponding Authors/ Eꢀmail: vsvprabu@gmail.com, and cbyon@ynu.ac.kr
TEL: +82ꢀ10ꢀ4017ꢀ8527, FAX: +82ꢀ53ꢀ810ꢀ4627
Abstract
MoS2 nanosheet decorated with SnO2 mesoporous nanoparticles were successfully prepared
by a facile twoꢀstep method. MoS2 nanosheets were preꢀsynthesized using a solvothermal
method and then decorated with the SnO2 mesoporous nanoparticles through a wet chemical
method. The nanocomposite was characterized with powder Xꢀray diffraction (XRD), field
emission scanning electron microscopy (FESEM), energy dispersed spectrometry (EDX),
highꢀresolution transmission electron microscopy (HRTEM), thermal gravimetric and
differential thermal analysis (TGꢀDTA), Xꢀray photoelectron spectroscopy (XPS) and
Electrochemcial impedance spectroscopy (EIS). SnO2 mesoporous nanoparticles can be
selectively formed and attached to the peripheral surface of the layered MoS2, which was
confirmed by FESEM and HRTEM. The photocatalytic activity of the nanocomposite was
examined with Rhodamine B (RhB) in aqueous solution under UV light irradiation. The SnO2
nanoparticles remarkably suppressed the electronꢀhole recombination effect on the MoS2
photocatalyst and improved photocatalytic activity compared to pristine MoS2 catalyst. A
higher rate of pollutant degradation was accomplished within 50 min that was three times
higher than that of the pristine MoS2 catalyst.
Keywords: Nanocomposite, Photocatalyst, Solvothermal, Wet chemical Method, MoS2
Introduction
Molybdenum disulfide (MoS2) can be made into a type of layered structure material similar
to graphene. MoS2 has hexagonally packed crystals comprising two of SꢀMoꢀS triꢀlayers that
are weakly bonded by van der Waals interaction [1, 2]. The SꢀMoꢀS triꢀlayer consists of
terrace sites on the basal plane and edge sites at the edges of nanoparticles [3]. Catalytically
active edge sites of MoS2 are difficult to understand and use for advanced catalyst design and
developing improved catalytic materials [3], and effective synthesis of MoS2 with wellꢀ

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defined edge sites has been challenging and rarely reported.
MoS2 is used as a solid lubricant and a catalyst for petroleum purification [4, 5]. It
has been gaining increasing attention in the semiconductor industry due to its unique
properties. In particular, different morphological forms of MoS2 have attracted a recent surge
of attention due to their morphologyꢀdependent properties. Such forms include nanotubes [6],
nanorods [7, 8], nanosheets [9], nanospheres [4, 10], singleꢀlayer structures, and multiꢀlayer
structures [11]. These forms can be suitable for application to solar cells [12], nanoelectronics
[13], optoelectronics [14], tribology [4], and catalysis [5]. Monolayer MoS2 could possibly be
produced reliably by mechanical exfoliation [15] or chemically from the bulk material [16].
The development of heterostructure devices has gained scientific interest for emerging
applications, such as controlled photoluminescence, photocatalysts, plasmonic devices, and
sensors [17ꢀ19]. Nanointerfacing can provide an avenue for controlling the electronic,
electrical, optical, and thermal properties.
The performance characteristics of MoS2 sheets could be improved by incorporation
of semiconductor nanoparticles with good distribution. MoS2 sheets decorated with
nanoparticles might result in particular properties due to synergetic effects [19, 20]. MoS2
nanosheets decorated with semiconductor nanoparticles have recently been reported [19ꢀ21].
Heterostructured catalysts exhibit better performance and have proved more effective
compared to individual catalysts [13, 22]. One of the most difficult challenges in the
development of heterostructured photocatalysts is finding specific materials that have both
high photocatalytic activity and suppressed recombination effects for pollutant degradation.
MoS2 nanosheets with wellꢀdistributed semiconductor nanoparticles can offer more
versatility for chargeꢀcarrier transportation and selective catalysis. This effects result from the
greater specific surface area and porosity. Thus, SnO2ꢀdecorated MoS2 composites may
provide new opportunities to develop new photocatalyst materials.
To the best of our knowledge, there have been no reports about the synthesis of SnO2
incorporated on MoS2 nanosheets as a photocatalyst. We report a twoꢀstep method for
synthesizing such nanosheets and used them as a photocatalyst for the first time. The metalꢀ
interfacing mechanism and its influence on the structural and photocalaytic properties of
SnO2@MoS2 are discussed. The greatest advantage of this method is that mesoporous SnO2
nanoparticles are formed via wet oxidationꢀreduction reaction between metal precursors in

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the presence of CTAB. Photocatalysis experiments prove that this heterogeneously structured
composite has higher photocatalytic activity than pristine MoS2.
Experimental Procedure
All chemicals were analytical grade and used as received without further purification.
Preparation of MoS2 nanosheets
To obtain the MoS2 nanosheets, ammonium hepta molybdate tetrahydrate, citric acid, and
thiophene (C4H4S) were used as the starting materials and sulfur source. 2.05 g of ammonium
heptamolybdate tetra hydrate and 1.56 g of citric acid were dissolved in distilled water and
magnetically stirred for 10 min at 90˚C on a hot plate to form a homogeneous solution. The
white suspension was vigorously stirred and the pH was adjusted to 3.5 with the addition of
ammonia water. Then, 3.15 g of thiophene in water was added dropwise to the solution and
transferred to an autoclave, which was maintained at 200˚C for 10 h. After natural cooling of
the reactor down to 25˚C, the resulting precipitates were collected through centrifugation,
filtered, and washed three times with distilled water and acetone. The final precipitates were
dried under vacuum at 130˚C for 2 h.
Preparation of MoS2 sheets decorated with SnO2 nanoparticles
The MoS2 precipitates (20 mg) were exfoliated in distilled water (25 mL) and heated
at 75˚C under magnetic stirring for 30 min to form a colloidal suspension. Then, SnCl2•2H2O
(2.7 g) was dissolved in the colloidal solution. HCl (1.3 mL) and H2O (30 mL) were added
dropꢀwise to the solution with continuous heating and stirring for 2 h. After several trials, we
optimized the concentration of CTAB to 0.05 mmol. 0.05 mmol of CTAB was added to the
suspension and stirred at 110˚C for 6 h. The final products were collected, filtered, and
washed three times with acetone. The obtained precipitates were dried in a vacuum oven at
120˚C for 12 h.
Characterization of samples
The structural properties of the obtained precipitates were characterized by powder
Xꢀray diffraction (XRD) with a Shimadzu Labx XRD 6100 using CuꢀKα radiation (λ=0.14
nm). The scan range was 10–80̊, and the scan speed was 5 deg/min. The nanoparticles were

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analyzed with a transmission electron microscope (TEM, Hitachi Hꢀ7000) at 110 kV and a
highꢀresolution TEM (HRTEM, Tecnai G2 F 20 SꢀTwin TEM) at an accelerating voltage of
210 kV. The optical properties of the nanoparticles were studied using UVꢀvisible
spectroscopy (Cary 5000 UVꢀVis spectrophotometry). Thermogravimetric (TG) and
differential thermal analysis (DTA) were carried out on a SDT Q600 thermogravimetric
analyzer under N2 flow at a rate of 28 cm3/min. The furnace temperature was increased from
room temperature to 900˚C at a heating rate of 6˚C per minute. The specific BET surface area
and pore size distribution were determined by N2ꢀadsorption using the BJH method with a
Micrometrics ASAP 2000 instrument. The purity of the final product was examined by an
Avatar 370 Fourier transform infrared spectroscope (FTIR) with a spectral range of 4000–400
cmꢀ1 and Xꢀray photoelectron spectroscopy (XPS, Thermo Scientific Kꢀalpha surface analysis
instrument). Electrochemcial impedance spectroscopy (EIS) measurements were carried out
on a CHI 630B workstation. The measurement was conducted in 0.1 M KCl solutions
containing 3 mM K3 [Fe (CN) 6]/K4 [Fe (CN) 6].
Photocatalytic experiments were performed at the natural pH of a solution of
Rhodamine B (RhB), an organic pollutant. The experiments involved a photoreactor that has
a 150ꢀW mercury lamp with a main emission wavelength of 254 nm as an internal light
source, which is surrounded by a quartz vessel. The light source is completely surrounded by
a suspension of the SnO2ꢀdecorated MoS2 nanosheet catalyst and aqueous RhB (100 mL, 10
mg/L). Before irradiation, the suspension was stirred in the dark for 15 min to obtain a good
dispersion and to ensure adsorptionꢀdesorption equilibrium between the organic pollutant
molecules and the catalyst. During light irradiation, the samples of the reaction solution were
collected at particular intervals and examined using an optical spectrophotometer.
Results and discussion
Fig. 1 shows the formation mechanism of the SnO2ꢀdecorated MoS2 nanocomposite and
processing conditions. CTAB cationic surfactant was used to obtain the mesoporous SnO2
nanoparticles. The optimized concentration of CTAB is 0.3 mmol, and the effect of CTAB in
the formation of the mesoporous structured SnO2 particles was elucidated. After addition of a
small account of CTAB, aggregation of CTA+ ions occurs on the surface of MoS2 nanosheets.
A few CTA+ ions form a complex structure on the surface, and excess CTA+ ions go to the
airꢀwater interface to generate mesoporous structured SnO2 particles [23, 24]. This result

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suggests that the CTA+ ions interact with the MoS2 units of the molecules and are involved in
the assembly process to create the mesostructures. Also, CTAB plays an important role in the
assembly process of SnO2 and MoS2 nanosheets.
The powder XRD patterns of the pristine MoS2 and SnO2ꢀdecorated MoS2
nanosheets are shown in Fig. 2. The diffraction peaks of pristine MoS2 (2θ =14.1˚, 28.4˚,
32.9˚, 39.5˚, 49.4˚, 55.4˚, 58.6˚, and 69.4˚) matched with the JCPDS card (No. 75ꢀ1539) and
were confirmed by previous reports [25, 26]. In the case of SnO2ꢀdecorated MoS2, the main
diffraction peaks coincided well with those of pristine MoS2, and SnO2 peaks also appeared
with high intensity. This is attributed to the high dispersion of SnO2 on the surface of MoS2.
The average crystallite size of MoS2 and SnO2ꢀdecorated MoS2 was determined based on the
(102) peak using the DebyeꢀScherrer formula [27]. The crystallite sizes were 23.53 nm and
27.46 nm for pristine MoS2 and SnO2ꢀdecorated MoS2, respectively. The crystallite size of
MoS2 slightly increased after surface modification with SnO2.
FESEM images of pristine MoS2 and SnO2ꢀdecorated MoS2 are shown in Fig. S1 (aꢀ
d). The shape of pristine MoS2 is a sheet form and it is completely agglomerated. The SnO2ꢀ
decorated MoS2 is composed of both spherical (SnO2) and sheet (MoS2) shapes. SnO2
appeared as bright spheres with mesoporous structure, while the MoS2 appeared as gray
sheets. Tiny SnO2 spheres were deposited on the MoS2 sheet surface, which was confirmed
by the XRD results. TEM and HRꢀTEM images of pristine MoS2 and SnO2ꢀdecorated MoS2
are shown in Fig. 3. The TEM images confirm that the SnO2 particles are aggregated on the
MoS2 sheets. For MoS2 nanosheets of a length more than 300 nm, their thickness is around
20–100 nm. It is clear that the SnO2 particles are spread on the MoS2 sheets, which is in good
agreement with the FESEM results. The lattice fringes of the SnO2ꢀdecorated MoS2 have
lattice spacing of 0.61 and 0.33 nm, which can be ascribed to the (002) and (110) planes of
2HꢀMoS2 and SnO2, respectively. The elements present were detected using EDX analysis, as
shown in Fig. S2. Mo, Sn, S, and O elemental peaks can clearly be seen for the SnO2ꢀ
decorated MoS2 sample. This result also confirms that SnO2 spheres are randomly deposited
on the MoS2 surface. No other elements appeared, and the present elements were observed at
their consistent KeV values. The atomic percentage of all the elements was further confirmed
by XPS analysis. HRTEM mapping analysis was done to further understand the distribution
and elements present in the SnO2ꢀdecorated MoS2 nanocomposite, as shown in Fig. S2 (cꢀg).
Mo, Sn, S, and O are predominately distributed within the selected area of the SnO2ꢀ

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decorated MoS2 sample, which is in good agreement with the FESEMꢀEDX results.
TG analysis and DTA of the SnO2ꢀdecorated MoS2 nanocomposite were performed in
nitrogen atmosphere, as shown in Fig. S3. The weight loss of 2.1% between room
temperature and 150˚C is ascribed to residual solvent evaporation, which favors crystal
growth. The weight loss of 9.1% between 150˚C and 390˚C is due to the dehydroxilation
process of the sample. Dehydroxilation promotes the partial breaking of sulfur and hydrogen
when in contact [28]. These phenomena give rise to the small exothermic peak centered at
355˚C in the DTA curve. Sulfur breaking during dehydroxylation favored the formation of
strong MoꢀOꢀSn bonds. The small exothermal peaks between 390˚C and 780˚C in the DTA
curve are attributed to the recrystallization and deformed nanocrystals. Negligible
endothermic peaks appeared above 590˚C. Further sintering above 630˚C caused more
transformations that generated more weight loss and one more exothermic peak. The total
weight loss at 900˚C for the SnO2ꢀdecorated MoS2 sample is 21%, indicating that it was
stable with temperature.
The FTꢀIR spectra of the SnO2ꢀdecorated MoS2 nanocomposite are shown in Fig. S4.
The bands at 3404, 3143, 1647, 1410, and 603 cmꢀ1 are associated with pristine MoS2
nanomaterials [21, 29]. Among these, an absorption band at 3154 cmꢀ1 was generated by the
stretching vibration of hydroxyls [30]. However, when the sample is heated at 400˚C, this
peak disappeared, which is in good agreement with the TGꢀDTA results. The band at 1410
cmꢀ1 corresponds to inꢀplane bending vibration of OꢀH [31]. The absorption bands at about
1647 and 603 cmꢀ1 are ascribed to the inꢀplane OH− group bending vibration and γas MoꢀS
vibration, respectively [32]. New peaks were obtained from the SnO2ꢀdecorated MoS2
nanocomposite. The band at 938 cmꢀ1 is assigned to γas MoꢀO vibration, the one at 720 cmꢀ1 is
attributed to outꢀofꢀplane bending vibration of OH− [29], and the one at 603 cmꢀ1 is attributed
to stretching vibration of SnꢀO [33].
The specific area and pore volume of the SnO2ꢀdecorated MoS2 nanosheets were
measured using nitrogen adsorption. The N2 adsorptionꢀdesorption isotherm in Fig. 4 shows
that the surface area and pore volume (inset) of SnO2ꢀdecorated MoS2 nanosheets were 103.7
m2/g and 0.16 cm3/g respectively. This indicates moderate N2 adsorption capability due to the
SnO2 nanoparticles being mesoporous. A hysteresis loop appears when the relative pressure is
higher than 0.87 and is ascribed to capillary condensation in the mesopores. This is also in

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good agreement with the FESEM results.
The Xꢀray photoelectron survey spectra of pristine MoS2 and the SnO2ꢀdecorated
MoS2 nanocomposite are shown in Fig. S5. Mo 3d, Mo 3p, and S2p peaks were obtained
from pristine MoS2, whereas additional peaks of Sn 3d, Sn 3p, and O1s appeared for the
SnO2ꢀdecorated MoS2 nanocomposite. The XPS analysis confirms that the asꢀobtained SnO2ꢀ
decorated MoS2 nanocomposite is composed of pristine MoS2 and SnO2. Fig. S6 shows the
highꢀresolution spectra of individual elements of the asꢀobtained SnO2ꢀdecorated MoS2
nanocomposite. The main comparison between pristine MoS2 (Mo 3d5/2 = 228.45 eV, Mo
3d3/2 =231.17 eV, S 2p3/2 = 161.31 eV) [34] and SnO2ꢀdecorated MoS2 (Mo 3d5/2 = 228.86 eV,
Mo 3d3/2 = 231.83 eV, S 2p3/2 =161.69 eV, Sn3d5/2 =487.38 eV) [35] reveals that both the Mo
3d5/2 and S 2p3/2 peaks shifted significantly to higher binding energies (BE) by about 0.41 eV
and 0.38 eV, respectively, which is ascribed to the formation of the hybrid structure.
The difference in BE (∆BEsnꢀs) obtained for the SnO2ꢀdecorated MoS2
nanocomposite was 325.69 eV, which varied according to the tin content and preparation
conditions [35]. The main concentrations of S for both pristine MoS2 and the SnO2ꢀdecorated
MoS2 nanocomposite are about 38.1 with negligible reduction. In addition, the O 1s peak
(531.1 eV) of the SnO2ꢀdecorated MoS2 sample shifted to higher binding energy by about
0.24 eV (O 1s = 531.34 eV) due to formation of the heterogeneous compound. The higher
−
energy peak is associated with O3 and the formed SnꢀMoꢀS phase of the sample. The
existence of apical S2− and terminal S2− suggests that an Sꢀrich MoS2 structure in the SnO2ꢀ
decorated MoS2 nanocomposite. It was reported that MoS2 with more active S edge sites is
highly active for catalytic applications [6, 36]. The mesoporous SnO2 has increased active
sites for enhancing the photocatalytic activity.
The UVꢀvis absorption edge of the semiconductor catalyst depends on the electronic
structure feature. The optical absorption properties of pristine MoS2 and SnO2ꢀdecorated
MoS2 nanocomposite were analyzed, and the diffuse absorption spectrum is shown in Fig. S7.
The pristine MoS2 had a weak UV response with an absorption band edge at 220 nm. In the
case of the SnO2ꢀdecorated MoS2 nanocomposite, the light absorption is enhanced with a
slight redꢀshift in the absorbance region, which is ascribed to the high absorption ability of
SnO2 in UV light. This redꢀshift is caused by the presence of SnO2 mesoporous nanoparticles
in the composite, resulting in a possible chargeꢀtransfer transition between SnO2 and MoS2

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[37]. The intensity of optical absorption was enhanced, and the optical absorption edges of
the SnO2ꢀdecorated MoS2 nanocomposite were extended. This result favors the photocatalytic
activity of the SnO2ꢀdecorated MoS2 nanocomposite.
The band gap energy of pristine MoS2 and SnO2ꢀdecorated MoS2 nanocomposite can
be estimated from a plot of (αhʋ) 1/2 versus the photon energy. The intercept of a tangent to
the xꢀaxis was recorded, which gives the band gap energies of the sample, as shown in Fig. 5.
The estimated band gap energies of pristine MoS2 and the SnO2ꢀdecorated MoS2
nanocomposite were approximately 2.31 eV and 2.21 eV, respectively.
The variation in the absorption spectrum of RhB (10 ppm) with respect to irradiation
time using 50 mg/L of SnO2ꢀdecorated MoS2 is shown in Fig. 6. The absorption maximum
obtained at 556 nm decreases gradually when the irradiation time increases. The adsorption
peak nearly disappeared and the red color of the RhB pollutant faded after 50 min of
irradiation, which indicates the complete degradation of RhB. The results obtained with pure
SnO2, MoS2, and SnO2ꢀdecorated MoS2 in the photocatalytic degradation of RhB under UV
light irradiation are shown in Fig. 7. The blank experiment was carried out without any
photocatalyst and result showed that hardly degraded and its degradation percentage is below
4.3% after 50 min of exposure (Fig.7). In contrast, when the photocatalyst is added and
noticeable RhB degradation occurred. No significant concentration decay occurred when
keeping the SnO2ꢀdecorated MoS2 suspension in dark conditions for 50 min. it can be seen
from Fig.7, the pure SnO2 showed lower photocatalytic activity compared to pristine MoS2.
The order of degradation rates of RhB pollutant is SnO2ꢀdecorated MoS2 > pristine MoS2>
pure SnO2. It is clear that the coupled SnO2ꢀMoS2 photocatalyst demonstrated higher
photocatalytic activity than pure SnO2 and MoS2. Hence, the SnO2 content should be a
significant factor affecting the photocatalytic reaction of the SnO2ꢀdecorated MoS2
nanosheets. The degradation of RhB pollutant occurs as a photocatalytic process, and the
SnO2ꢀdecorated MoS2 is more efficient than the pure forms of SnO2 and MoS2, as shown by
the degradation curve in Fig. 7.
The kinetic fitting semilog plot of ꢀln (C/C0) with respect to irradiation time is shown
in Fig. 8. If the concentration of pollutant is proportional to the reaction rate or the pollutant
concentration is in the millimolar range, the effective reaction rate constant (k) of the
photocatalytic reaction is calculated by assuming a pseudoꢀfirstꢀorder kinetic law: C/C0 = exp

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(ꢀkt) [6, 38]. The value of k was determined to be 0.0072 and 0.047 minꢀ1 for the MoS2 and
SnO2ꢀdecorated MoS2 catalyst, respectively. The SnO2ꢀdecorated MoS2 nanocomposite
showed significantly more activity in photocatalyzing the degradation of RhB than the pure
SnO2 and MoS2 nanoparticles, which is ascribed to the specific surface area of the SnO2ꢀ
decorated MoS2 nanocomposite. In addition, the mesoporous structure of SnO2 nanoparticles
may also contribute to suppressing the recombination effect.
The XRD peaks of the MoS2 nanoparticles show somewhat larger widths than that of
the SnO2ꢀdecorated MoS2 nanocomposite. This indicates that MoS2 has a higher degree of
crystallite agglomeration along with a larger number of grain boundaries, which may
accelerate the electronꢀhole recombination. Therefore, the photodegradation of pollutant is
much faster with the SnO2ꢀdecorated MoS2 nanocomposite rather than the MoS2 catalyst. In
addition, the SnO2ꢀdecorated MoS2 heterogeneous structure could facilitate the rectification
of photogenerated charge carriers, as was demonstrated in previous studies. Examples are
MoS2 and WS2 supported by SiO2 and TiO2 [39], MoS2 developed on TiO2 [40], and MoS2 on
CdS [41].
The stability and reusability of a sulfide photocatalyst are significant matters of
concern. Sulfide photocatalysts exhibit activity loss due to corrosion during the
photooxidation reactions. Hence, it is important to verify the reusability of the photocatalyst.
To check this, a fixed portion of SnO2ꢀdecorated MoS2 nanocomposite was recycled four
times in the photocatalytic RhB degradation reaction under UV light irradiation while
restoring the RhB concentration after 50 min by the addition of pristine compound to the
solution. The RhB concentration was measured before and after completion of each cycle.
The results are shown in Fig. 9, which clearly shows that even after the fourth cycle, the
photocatalyst continues to be highly active.
The RhB concentration remaining after the first run (0.3%) is compared with the
value for the fourth run (1.6%) in Fig. 9. After the last of these cycles, the nanocomposite was
tested again for photocatalytic activity, and the apparent constant was k = 0.036 minꢀ1, which
is lower by 11% than that measured for the fresh sample. When the same experiment was
performed using pristine MoS2 nanoparticles, the loss of activity was very high, and more
importantly, the concentration of RhB remaining in the solution increased from 61% in the
first run to 78% in the fourth one.

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In order to observe the relevant changes in composition and redox state of SnO2ꢀ
decorated MoS2 nanocomposite after the four cyclic photodegradation, an XPS analysis was
carried out and results are presented in Fig.10. The used nanocomposite shows a small broad
peak compared to fresh sample at EB= 234.25 eV. EB values above 234.25 eV are rarely
reported in Mo compound, which is attributed to Mo bound to a ligand with lower electron
donating capability [35, 36, 42]. The S2p region a small compound peak appears at EB =
163.4 eV in the 2p3/2 peak, which is ascribed to oxidation of sulphide species. This indicate
that possible reaction and formation of a bond with RhB and to enhance the degradation of
pollutant. However, the consequence of these changes on the photoreaction is moderate. As a
result, SnO2ꢀdecorated MoS2 nanocomposite has higher stability even after cyclic
photoreaction due to their cohesive energies with strong Madelung lattice energies [36, 42]
and structural features, thanks to the higher charge and possible numbers of MꢀS bonds on the
Sn cations. The XPS results of SnO2ꢀdecorated MoS2 nanocomposite before and after
photoreaction do not show the presence of pollutant in it, which reflect the complete
mineralization of the pollutant.
In order to provide further evidence for proposed mechanism, EIS studies were
conducted. It was observed from Fig. 11, the arc radius in the impendence spectrum of the
SnO2ꢀdecorated MoS2 nanocomposite is smaller than the arc radius of the pristine MoS2
sample in the spectrum, which indicates that the SnO2ꢀdecorated MoS2 nanocomposite has a
lower resistance than pristine MoS2, similar results have been reported [43, 44]. This can
attributed to the SnO2@MoS2 heterostructured interface can facilitate the interfacial electronꢀ
hole pairs transfer process [43ꢀ46].
Photocatalytic mechanism
The schematic mechanism of photogenerated electronꢀhole transfer process in SnO2ꢀ
decorated MoS2 nanosheets under UVꢀ light irradiation was proposed and it shown in Fig. 12.
For MoS2 with the band gap energy of 2.31 eV, but it shows little photocatalytic activity due
to the fast recombination effect of the electron/hole pairs in MoS2 under UV irradiation in the
current experimental conditions. The conduction band (CB) of MoS2 is lower than that of
SnO2; hence the former can turn as a sink for the photogenerated electrons [43ꢀ47]. Then, the
photogenerated holes might be trapped within the SnO2 particles due to the holes move in the
opposite direction from the electrons. As a result, the recombination of the electron/hole pairs

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is suppressed and, consequently the SnO2ꢀdecorated MoS2 nanocomposite exhibits higher
photocatalytic activity than that of pure SnO2 and MoS2.
The enhancement in photocatalytic activity of the SnO2ꢀdecorated MoS2
nanocomposite can be explained based on two mechanisms. The first is the adsorption ability
of the MoS2 nanosheets, since the adsorption is a key process in the catalytic destruction of
pollutant. Second is the electron transfer from SnO2 to the MoS2 nanosheets. It is well know
that a metal oxide semiconductor with greater photon energy could promote an electron from
the valence band (VB) to the conduction band (CB) [46ꢀ49]. The promoted electron creates a
vacancy in the VB and a positive hole and resulting, the separation of the photogenerated
electrons and holes was accomplished at the composite interface. This positive hole can react
with hydroxyl groups and form a hydroxyl radical (•OH), a powerful oxidant. The promoted
•−
electrons decrease the dissolved oxygen and yield superoxide anion radicals (O2 ) [47].
The SnO2ꢀdecorated MoS2 heterogeneous structure facilitates the effective electron
transfer from the CB into the MoS2 nanosheets and suppresses the recombination effect. This
proposed mechanism is supported by the higher photocatalytic activity of the SnO2ꢀdecorated
MoS2 nanocomposite. The major proposed reaction steps during the photocatalytic process
are as follows:
SnO2 + hʋ → SnO2 (e− + h+)
SnO2 (e−) + MoS2 → SnO2 + MoS2 (e−)
−
MoS2 (e−) + O2 → + MoS2 + •O2
SnO2 (h+) + OH− → SnO2 + •OH
−
O2 + e− → •O2 → •OH
−
RhB + •O2 /•OH → Products
RhB + h+ → CO2 + H2O + by products
Another reason for the lower photocatalytic activity of the individual MoS2
nanoparticles is that they react with dissolved organic matter and with the surface sulfide ions.
This gives rise to sulfate ions and dissolution of the solid. This phenomenon occurs due to
photocorrosion and results in lower photocatalytic performance.

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Conclusions
We have reported a solvothermal method for effective decoration of MoS2 nanosheets with
SnO2 mesoporous nanoparticles. The characterization results reveal that the SnO2
mesoporous nanoparticles are distributed on the MoS2 nanosheet surface and form
SnO2@MoS2 hetero nanostructures. The SnO2ꢀdecorated MoS2 nanocomposites are formed
by Van der Waals interaction. The morphology and density of the semiconductor
nanoparticles can be tuned by changing the defective sites in MoS2 nanosheets. The result of
the photocatalyst test shows that the nanocomposite has outstanding photocatalytic activity
with excellent stability compared to pristine MoS2 and SnO2 catalyst. The architecture of the
nanocomposite makes a prominent contribution to the excellent photocatalytic performance.
Acknowledgements
This research was supported by the Basic Science Research Program through the National
Research Foundation of Korea (NRF) and funded by the Ministry of Science ICT and Future
Planning (2014R1A2A2A01007081).
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Fig.1 Schematic illustration of the synthesis method of SnO2 decorated MoS2 nanosheets
Fig.2. XRD patterns of: (a) pristine MoS2 and (b) SnO2-decorated MoS2 nanocomposite

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Fig.3. TEM and HRTEM images of (a), (b) pristine MoS2, (c) - (f) SnO2-decorated MoS2
nanocomposite

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Fig.4. N2 adsorption-desorption curve of the SnO2-decorated MoS2 nanocomposite; inset:
pore diameter distribution curve

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1.5
1.4
1.3
1.2
1.1
1.0
2.4
a
b
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
1.2
1.5
1.8
2.1
2.4
2.7
3.0
3.3
3.6
1.2
1.5
1.8
2.1
2.4
2.7
3.0
3.3
Photon energy (eV)
Photon energy (eV)
Fig.5. Tauc plots of (a) pristine MoS2 and (b) SnO2-decorated MoS2 nanocomposite

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Fig.6. Time-dependent UV–vis absorbance spectra of the SnO2-decorated MoS2
nanocomposite
1.0
0.8
0.6
0.4
0.2
0.0
RhB dye
Dark
Pure SnO
2
Pristine MoS
2
SnO ꢀdecorated MoS
2
2
-10
0
10
20
30
40
50
Time (min)
Fig.7. Photodegradation rate of the RhB pollutant under UV light and light irradiation time
(50 min) of the SnO2-decorated MoS2 nanocomposite

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1.2
1.0
0.8
0.6
0.4
0.2
0.0
Pure SnO
2
Pristine MoS
2
SnO ꢀ decorated MoS
2
2
0
10
20
30
40
50
Time (min)
Fig.8. The kinetic plot of photocatalytic degradation of RhB with SnO2-decorated MoS2
nanocomposite under UV light irradiation
100
1st cycle
2nd cycle
3rd cycle
4th cycle
80
60
40
20
0
1.6%
1.2%
0.7%
0.2%
0
50
100
150
200
Time (min)
Fig.9. Recycling photocatalytic degradation of RhB in the presence of SnO2-decorated MoS2
nanocomposite under UV light irradiation

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7000
6000
5000
4000
3000
2000
1000
2600
S2p
3/2
Mo 3d
3/2
S2p
1/2
a
2400
2200
2000
1800
1600
1400
1200
1000
800
b
Mo 3d
5/2
S 2s
168
166
164
162
160
158
238
236
234
232
230
228
226
224
Binding energy (eV)
Binding energy (eV)
18000
16000
14000
12000
10000
8000
Sn 3d
5/2
O 1S
12000
11000
10000
9000
8000
7000
6000
5000
4000
c
d
Sn 3d
3/2
6000
4000
500
498
496
494
492
490
488
486
484
536
534
532
530
528
Binding energy (eV)
Binding energy (eV)
Fig.10 X-ray photoelectron spectra of SnO2-decorated MoS2 nanocomposite sample obtained
after 4 cyclic photoreaction (a) Mo, (b) S, (c) Sn and (d) O elements

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Fig.11 Electrochemical impedance spectra of pristine MoS2 and SnO2-decorated MoS2
nanocomposite samples

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Fig.12. The proposed schematic mechanism of photogenerated electron-hole transfer process
in SnO2-decorated MoS2 nanosheets under UV- light irradiation

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Graphical Abstract