Near-infrared light triggered superior photocatalytic activity from MoS2-NaYF4:Yb3+/Er3+ nanocomposites

Article abstract of DOI:10.1039/c6dt02548j

A near infrared (NIR) responsive photocatalyst, composed of a narrow band gap semiconductor (i.e. MoS₂) and an optical material possessing upconverting ability (i.e. NaYF₄:Yb³⁺/Er³⁺) has been successfully prepar

Full text of DOI:10.1039/c6dt02548j

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Nearꢀinfrared light triggered superior photocatalytic
activity from MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites
Manjunath Chatti, Venkata N. K. B. Adusumalli, Sagar Ganguli and Venkataramanan
Mahalingam*a
a Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER),
Kolkata, Mohanpur, West Bengal, 741246, India.
KEYWORDS: Lanthanides, 2D materials, Nanocrystals, Energy transfer.
ABSTRACT:
A near infrared (NIR) responsive photocatalyst, composed of narrow band gap
semiconductor (i.e. MoS2) and an optical material possessing the upconverting ability
(i.e. NaYF4:Yb3+/Er3+) has been successfully prepared via a simple hydrothermal
method. The latter has the ability to convert the NIR light into visible ones while the
MoS2 uses the light to degrade organic pollutants. Upon near infrared (NIR) excitation of
the MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites, the energy of the strong green and the red
emissions along with the weak violet emissions from the NaYF4:Yb3+/Er3+ nanocrystals
(NCs) are transferred to MoS2. This results in enhanced NIR light triggered
photocatalytic performance as verified by studying the degradation of Rhodamine B
(RhB) dye under 980 nm laser excitation. The strong photocatalytic activity of MoS2ꢀ
NaYF4:Yb3+/Er3+ composites is attributed to the layered nature of photocatalyst which
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leads to efficient separation of photogenerated carriers (electron–hole pairs) and
excellent upconversion properties of NaYF4:Yb3+/Er3+ NCs. The study also shows the
importance of the composite formation as the physical mixture lead to only very low
photocatalytic activity. Our results can be helpful in the structural design and
development of highꢀperformance photocatalysts.
INTRODUCTION
With increased development in science and technology, air and water pollution have
become a major concern to the society. In an effort to control the pollution,
environmentally friendly photocatalysis technique is applied in the degradation of organic
pollutants in air and water. In past, considerable attention has been paid to semiconductor
based photocatalysis for water treatment because of its efficient conversion of organic
pollutants to environmentally friendly by products.1ꢀ4 Generally, the overall photocatalytic
performance of semiconductors are mainly decided by three factors: (1) The adsorption
ability of photocatalyst, (2) the separation and migration rate of the photogenerated
carriers (holes and electrons) to the surface of catalyst, and (3) photoabsorption ability in
the available light energy region.5 Desired photocatalysts are expected to satisfy
aforementioned factors. Considering the above factors, the traditional photocatalysts, such
as TiO2 and ZnO with wide band gap can only exhibit high photocatalytic activity under
ultraviolet light irradiation. This significantly limits their practical applications6ꢀ8 as it is
well known that UV light covers only about 5% of the solar spectrum. On the other hand
the visible light and nearꢀinfrared (NIR) light occupy significant portion of the solar
spectrum i.e. 49% and 46%, respectively.9 Therefore, development of a photocatalyst for
efficient sunlight harvesting is a challenge.10 Furthermore, smaller sphere shaped
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nanoparticles are generally preferred as photocatalysts due to higher specific surface area.
But latter’s photocatalytic activity is significantly affected by higher electronꢀhole
recombination rate.11 Recent studies have demonstrated that the geometry of
photocatalyst has a significant effect on the photocatalytic activity.12
In recent past, photocatalysis using 2D materials has attracted researchers around the world and
has become one of the interesting research topics.13ꢀ15 2D layered materials possess two main
intrinsic advantages which can be utilized to enhance the photocatalytic efficiency. First, 2D
materials maximize the surface area availability per amount of material for photocatalytic
reactions. Second, due to 2D nature the distance that photogenerated carriers (electrons and
holes) have to migrate will be less thereby reducing the possible recombination of electronꢀhole
and potentially enhancing the photocatalytic performance. Among various 2D materials MoS2 is
quite interesting and widely studied. MoS2, being inorganic graphene analogue, consists of
molybdenum atoms sandwiched between two layers of hexagonally closeꢀpacked sulfur atoms.16ꢀ
19
MoS2 with a narrow band gap of 1.8 eV has strong absorption in the visible region of solar
spectrum; therefore it has been explored for photocatalytic applications.20ꢀ24 Although, there are
quite a few reports on visible light photocatalysts, research efforts on NIR driven photocatalytic
activities are still in the early stage.
Recently, there have been few reports on NIR light triggered photocatalysts developed by
integrating semiconductors with upconversion (UC) nanocrystals. The latter possess the ability to
convert NIR photons to visible photons.25ꢀ37 Moreover the long luminescence lifetime of the
lanthanide ions ensures the efficient energy transfer to semiconductors. For example, Weiping et
al developed NIR driven TiO2 and ZnO based photocatalysts by preparing
39
YF3:Yb3+/Tm3+@TiO2,38
NaYF4:Yb3+/Tm3+@TiO2
core/shell
nanoparticles
and
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NaYF4:Yb3+/Tm3+@ZnO40 nanocomposites, respectively. However, because of the larger band
gap of aforementioned semiconductors only a small amount of weak upconverted UV light can
be harvested under NIR irradiation rather than stronger visible emissions. To our knowledge
there are only few reports where researchers have developed composites which can harvest the
upconverted visible light.41ꢀ44 For instance, Li et al. have recently reported the NIR driven
photocatalytic activity of NaYF4:Yb3+,Er3+/CdS composite.41 Similarly, Guo et al have reported
the NIR assisted photocatalysis by NaYF4:Yb3+,Er3+/CdS/TiO2 composite, which is able to
degrade about 90% of MB in 50 hours.42 In another work, enhanced NIR driven photocatalytic
activity was observed from βꢀNaYF4:Yb3+,Tm3+/gꢀC3N4 nanocomposite.44 Our idea is to
integrate UC NCs with layered narrow band gap semiconductor MoS2 to efficiently utilize the
upconverted visible emissions. One of the advantages of using MoS2 is the broad absorption in
the visible region thus can efficiently use the unconverted emissions.
In this work, we report the NIR light triggered photocatalytic activity of MoS2ꢀNaYF4:Yb3+/Er3+
nanocomposites prepared using hydrothermal method. The energy of intense green and red
emissions along with weak upconverted UV emission from UC NCs are efficiently transferred to
the MoS2. This resulted in enhanced NIR light triggered photocatalytic activity of MoS2 which is
verified by monitoring the degradation of Rhodamine B (RhB) dye. The degradation of dye
occurs much faster compared to other reports in the literature. These results pave way to the
structural design of new type of NIR active catalytic materials.
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EXPERIMENTAL SECTION
Chemicals and Materials:
All the chemicals used in this work such as yttrium oxide (Y2O3, 99.99%), ytterbium oxide
(Yb2O3, 99.99%), erbium oxide (Er2O3, 99.99%) were purchased from Sigma Aldrich. Adipic
acid (AA) (90%), ammonium heptamolybdate, thiourea, sodium fluoride (NaF), nitric acid
(HNO3), and absolute ethanol were purchased from Merck. All the materials were used without
further purification. Lanthanide nitrates were prepared by mixing appropriate amount of
corresponding oxides with conc. HNO3 and the residual HNO3 was removed by evaporation.
Synthesis:
Synthesis of adipic acid coated NaYF4:Yb3+/Er3+ nanocrystals: NaYF4:Yb3+/Er3+ NCs were
synthesized via high temperature hydrothermal method. Briefly, Ln(NO3)3, (Ln: 78 mol%Y3+, 20
mol%Yb3+, 2 mol% Er3+) were dissolved in 15 mL distilled water and then 15 mL ethanol was
added to above aqueous solution under magnetic stirring. Then 8 mmol NaF and 4 mmol of
adipic acid were added into the above solution. The mixture was stirred for about 30 minutes
before transferred to a 50 mL autoclave, sealed, and hydrothermally treated at 200°C for 24 h.
The system was cooled to room temperature naturally, and the products were deposited at the
bottom of the vessel. Pure powders could be obtained by purifying the samples with ethanol
several times to remove excess NaF.
Synthesis of MoS2: MoS2 nanosheets were synthesized via hydrothermal method, similar to
those previously reported. Briefly, to a beaker containing 15 mL water appropriate amount of
ammonium heptamolybdate and thiourea was added and stirred for 30 minutes. The clear
transparent solution was subsequently transferred into a teflon lined autoclave and heated at
220°C for 24 h. Subsequently, the reaction system was allowed to cool to room temperature. As
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a final step the product was collected by means of centrifugation, washed twice with ethanol, and
once with deionized water.
Synthesis of MoS2-NaYF4:Yb3+/Er3+ nanocomposites: Hydrothermal synthesis technique was
adopted for the synthesis of MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites. In a typical procedure, 50
mg of MoS2 was dispersed well in 15 mL of water via sonication. To the above dispersion
Ln(NO3)3, (Ln: 78 mol%Y3+, 20 mol%Yb3+, 2 mol% Er3+) were added and stirred for 30 minutes
followed by addition of 15 mL of ethanol. Then 8 mmol NaF and 4 mmol of adipic acid were
added into the above solution. The mixture was stirred for about 1 h, then transferred to a 50 mL
autoclave, sealed, and hydrothermally treated at 200°C for 24 h. The system was cooled to roomꢀ
temperature naturally, and the products deposited at the bottom of the vessel were purified with
ethanol several times to remove excess NaF.
Photocatalytic activity evaluation: The photocatalytic activity of the composites was evaluated
by studying the degradation of Rhodamine B (RhB) under nearꢀinfrared (NIR) light irradiation
by a 980 nm diode laser source from RGB Lase LLC. In each experiment, 20 mg of sample was
dispersed into a quartz vial containing 10 mL RhB solution (25 mg Lꢀ1). Prior to irradiation, the
suspensions were stirred in dark for 1 hour to reach adsorptionꢀdesorption equilibrium.
Subsequently, the colloidal dispersion was exposed to NIR light irradiation inside a closed black
chamber with continuous magnetic stirring. At each irradiation time interval, 1 mL aliquot of the
suspension containing the photocatalyst and RhB solution was collected and centrifuged (6500
rpm, 5 minutes) to remove the photocatalyst particles in order to assess rate of degradation
photometrically. The colloidal dispersion was analyzed by a Hitachi UVꢀ4100 UVꢀvisꢀNIR
spectrophotometer, and the characteristic absorption of RhB at 554 nm was used to monitor the
photocatalytic degradation. All the measurements were carried out at room temperature.
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Photogenerated hydroxyl radical (·OH) detection: 4 mM terephthalic acid (TA) solution was
prepared in 2 mM NaOH solution. To 10 mL TA solution, 20 mg of photocatalyst was added and
dispersed thoroughly via sonication. The resulting mixture was then irradiated with 980 nm laser
and after certain interval, 1.0 mL of suspensions were collected and centrifuged. For the
photoluminescence measurement, 0.5 mL of as collected dispersion was diluted five times. The
generation of 2ꢀhydroxyterephthalic (TAOH) acid was monitored using fluorescence
spectrophotometer with an excitation wavelength of 312 nm.
Characterization techniques:
The crystallinity and phase analysis of the nanomaterials were carried out using powder Xꢀray
diffraction (PXRD) measurements using a RigakuꢀSmart lab diffractometer with CuꢀKα
operating at 70 kV and 35 mA at a scanning rate of 1° per minute in the range of 10ꢀ90°. The
samples were completely powdered and spread evenly on a quartz slide. The morphology of the
nanomaterials was characterized by scanning electron microscopy (SEM). SEM images were
taken using a ZEISS (SUPRA) instrument and for this the samples dispersed in ethanol were
drop casted on a silicon wafer and further coated with a thin film of gold/platinum to avoid
charging effects prior to loading of the samples into the chamber. The FTIR spectra were
recorded using Perkin Elmer RX1 spectrophotometer with KBr disk technique operating in the
range of 4000ꢀ400 cmꢀ1. For recording the FTIR spectra 10 mg of the samples were mixed with
200 mg of KBr to make the pellets. The photoluminescence measurement was performed using
Horiba Jobin Yvon Flurolog spectrophotometer equipped with 450 W Xe lamp. The PL lifetime
measurements were performed with a Horiba Jobin Yvon Fluorolog CP machine equipped with a
pulsed Xe source operating at 25 W. The upconversion emission (UC) measurements were done
by exciting the dispersion of the nanocrystals using a 980 nm diode laser source from RGB Lase
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LLC, which was coupled with a fiber with core diameter of 100 ꢁm. The output signal was
measured with the Horiba Jobin Yvon Flurolog spectrophotometer. BET surface area
measurements were carried out by N2 adsorption at 77.3 K using a Quantachrome Novawin2
instrument. All the measurements were performed at room temperature. Zeta potential
measurements were carried out on a Malvern Zetasizer Nano instrument equipped with a 4.0
mW He–Ne laser operating at λ=633 nm.
RESULTS AND DISCUSSIONS:
Phase analysis: The crystal structure and phase purity of the samples were characterized by
PXRD. Figure 1 (a) shows the PXRD pattern of the MoS2 sample prepared via hydrothermal
route and can be readily indexed to the hexagonal phase of MoS2 which is consistent with the
standard powder diffraction pattern of MoS2 (JCPDS 37ꢀ1492) shown in Figure 1 (b). The
typical XRD patterns of the MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites and NaYF4:Yb3+/Er3+ NCs
are shown in Figure 1 (c and d) respectively. For NaYF4:Yb3+/Er3+ NCs, the diffraction peaks
were wellꢀdefined, the peak positions and intensities agreed well with standard cubic pattern of
NaYF4 (PDF card No. 01ꢀ077ꢀ2042) shown in Figure 1 (e), indicating that the asꢀprepared
NaYF4:Yb3+/Er3+ NCs material is pure and crystallize in cubic phase. Figure 1 (c) shows XRD
pattern of MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites obtained via oneꢀpot hydrothermal route. The
sample showed the characteristic peaks of NaYF4:Yb3+/Er3+ NCs but it is worth to mention that
the appearance of a weak diffraction peak (see Figure c1) at 2θ = 14.4◦. This peak is assigned to
the (002) plane of MoS2 which supports the presence of MoS2 in MoS2ꢀNaYF4:Yb3+/Er3+
nanocomposites. The weak intensity of MoS2 peaks is due to the presence of small amount of
MoS2 in the nanocomposites.
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Morphology analysis: High resolution scanning electron microscopy was used for morphology
and size analysis of the as synthesized samples. FEꢀSEM images shown in Figure 2 (a) illustrate
that the adipic acid capped NaYF4:Yb3+/Er3+ NCs are spherical in morphology. The average
particle size of the NaYF4:Yb3+/Er3+ NCs is approximately 40 nm. The SEM images of MoS2
shown in Figure 2 (b) indicate the formation of cauliflower like morphology. At a closer look it
is clear that MoS2 nanostructure is composed of nanowalls of about several nanometers in
thickness. 45. For the MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites changes in the roughness of MoS2
nanostructures upon deposition of NaYF4 nanocrystals on MoS2 nanostructures can be clearly
observed in the FESEM images shown in Figure 2 (c). We believe during the initial growth of
the nanocomposite, the cations attach on the negatively charged MoS2 nanosheets through
electrostatic interactions. The presumption is supported by the observed zeta potential value of ꢀ
19 mV for MoS2 nanosheets. The nanosheets then act as a nucleation site for the growth of the
NCs. This suggests the successful formation of the MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites. The
EDX spectral results shown in Figure 2 (d) further confirm the presence of elements (such as
Mo, S, Na, Y, F, Yb and Er) in the nanocomposites. The photoluminescence measurements of
the resulting products provide further evidence of formation of MoS2ꢀNaYF4:Yb3+/Er3+
composites. (vide infra)
Surface functionalization-FTIR analysis: The attachment of adipic acid molecules onto the
surface of the nanocrystals is confirmed from Fourier transform infrared (FTIR) analysis. The
FTIR spectra of adipic acid capped NaYF4:Yb3+/Er3+ NCs and MoS2ꢀNaYF4:Yb3+/Er3+
nanocomposites along with pure adipic acid are shown in Figure S1. All the IR absorption bands
of the main functional groups present in the adipic acid are observed. In case of NaYF4
nanocrystals and MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites a weak peak near 1565 cmꢀ1 due to
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carbonyl stretching is observed. This stretching frequency is much lower than the 1708 cmꢀ1
observed for the free adipic acid molecules, implying the binding of the Oꢀatom of the ꢀCOOH
group of adipic acid to the surface of the nanocrystals. The binding of adipic acid molecules to
the nanocrystals makes the nanocrystals surface hydrophilic to disperse them in polar solvents.
UV-vis-NIR absorption properties: Figure 3, shows the UVꢀVis spectrum of MoS2. Three
absorption peaks at 450, 624 and 670 nm are observed for MoS2 which show the semiconducting
nature of MoS2. The two characteristic absorption peaks at 624 nm (1.99 eV) and 670 nm (1.85
eV) arise from direct transition from valance band to conduction band at the Kꢀpoint of the
Brillouin zone, known as the B and A transitions, respectively. The little energy difference
between the two peaks is because of the spinꢀorbital splitting of the valence band. In addition, the
broad absorption band centered at 450 nm (2.75 eV) arising from the complicated C and D
transitions is also observed.46ꢀ50 In the inset of Figure 4, the absorption spectra of (a) MoS2ꢀ
NaYF4:Yb3+/Er3+ nanocomposites and (b) pure NaYF4:Yb3+/Er3+ NCs; the characteristic
absorption peaks between 400ꢀ700 nm corresponding to MoS2 are observed. The observed weak
UVꢀVis absorption of MoS2 in the nanocomposites is due to the low amount of MoS2 loading.
Upconversion (UC) luminescence properties: The UC emission spectrum of adipic acid
encapsulated Yb3+(20%)/Er3+(2%)ꢀdoped NaYF4 NCs upon excitation with a 980 nm continuous
wave (CW) diode laser is shown in Figure 4 (a), where Yb3+ serves as sensitizer and Er3+ acts as
activator. Four major bands are observed around 408, 523 nm, 545 nm, and 655 nm in the UC
emission spectrum. The intense green emissions appearing close to 520 and 545 nm are assigned
2
4
to the H11/2→4I15/2 and S3/2→4I15/2 transitions in Er3+, respectively. The strong emission peak
4
4
appearing in the red region is assigned to transition occurring between the F9/2 and I15/2 levels
of Er3+. The violet emission (408 nm) and UV emission (379 nm) are ascribed to the 2H9/2→4I15/2
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and G11/2→4I15/2 transitions of Er3+ ions, respectively. The involvement of two 980 nm photons
in providing both green and red emission is verified by performing power dependent studies as
the UC emission intensity (I) is directly proportional to nth power of laser power (P), I α Pn. A
plot of ln(I) versus ln(P) for both green and red emissions of the NaYF4:Yb3+/Er3+ NCs along
with the resulting slope values are shown in Figure S2. The calculated values are 1.58 and 1.33
for 545 nm and 650 nm emissions of NaYF4:Yb3+/Er3+ NCs, respectively. The above values
clearly suggest the participation of two 980 nm photons in producing both green and red
emissions. These values are lower than the expected value of 2 due to saturation of the UC
process.51 The possible energy transfer mechanism between Yb3+ and Er3+ is shown in Figure S3,
using the energy level diagram of Yb3+ and Er3+ ions. Figure 4 (c) indicates a drastic decrease in
the observed upconversion emission intensity for MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites. We
believe this strong quenching of the upconverted fluorescence of NaYF4:Yb3+/Er3+ NCs is likely
due to MoS2. This is reasonable because MoS2 has strong absorption over wide wavelength
region (350 to 700 nm) as shown in Figure 3. This covers the fluorescence emission regions of
NaYF4:Yb3+/Er3+ NCs. In order to understand whether the reduction in the UC luminescence
intensity is due to energy transfer or rather due to radiationꢀreabsorption, we performed a control
experiment. A physical mixture of MoS2 and NaYF4:Yb3+/Er3+ NCs was prepared by taking the
same molar ratio as that in the nanocomposite. The mixture was grinded well and their UC
properties were studied. The mixture shows intense UC upon excitation at 980 nm and the extent
of reduction in the upconversion emission intensity is much less compared to that noted for the
nanocomposites. This suggests that the observed decrease in the intensity of the UC emission
intensity for the nanocomposites is likely due to high energy transfer from the excited Er3+ ions
to MoS2 in the nanocomposites. To further understand the quenching mechanism, lifetime
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measurements were performed (as shown in Figure S4). The lifetime curves were obtained by
the direct excitation of the Er3+ ions at an excitation wavelength of 488 nm. The significant
decrease in the decay time of Er3+ ions in nanocomposites (8 µs) compared to that in the
NaYF4:Yb3+/Er3+ NCs alone (40 µs) indicates the energy migration between NaYF4:Yb3+/Er3+
NCs and MoS2 is a resonance energy transfer (RET) process, rather than a
radiation−reabsorption process or charge transfer process. The latter is a difficult process to
occur as the redox potentials of many of the lanthanide ions are high. Especially, it is worth to
mention that the difference in the energy transfer efficiency from Er3+ to MoS2 affects the NIR
photocatalytic activity of MoS2 significantly, which will be discussed in the photocatalytic
activity section. (vide infra)
Photocatalytic study: The photocatalytic performance of the MoS2ꢀNaYF4:Yb3+/Er3+
nanocomposites was evaluated by studying the degradation of Rhodamine B (RhB) dye under
980 nm NIR irradiation. 20 mg of the MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites were dispersed
into a quartz vial containing 10 mL of RhB aqueous solution. In order to know the actual amount
of RhB decomposed via photocatalysis, the dark reaction of RhB in the presence of catalysts was
conducted to establish the adsorption/desorption equilibrium of RhB on the surfaces of the
MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites. As can be seen from Figure 5, the nanocomposite
adsorbs about 37.5% of RhB after 1 hour in dark condition. Thereafter, the solution was
subjected to NIR irradiation and 1 mL of RhB aqueous solution was taken out for UVꢀvis
absorption measurement after regular intervals of 1 hour. This high adsorption of dye molecules
on the composite photocatalyst may be ascribed to the 2D nature of MoS2 nanosheets. In
comparison to nanoparticles, 2D nanosheets are known to possess large surface area, which in
turn is able to adsorb significantly more dye molecules than nanoparticles. Moreover,
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photocatalytic activity of a material largely depends on the extent of adsorption by the
photocatalyst. Hence, both high adsorption and large surface area assist in the photocatalysis
process as well. We believe the nature of adsorption of pollutants on photocatalyst surfaces
follow the Langmuir theory, which postulated that adsorption is monolayer and excess molecules
are reflected back from the surfaces.52 It has been earlier reported that photogenerated oxidizing
species are not able to migrate far from the photocatalysts and the degradation process occurs at
the surface or within a few monolayers around the photocatalytic particles.53
Figure 5 shows the absorbance spectra of RhB loaded MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites
under NIR irradiation as a function of the irradiation time. The observed gradual decrease in the
absorption intensity of RhB at 554 nm with the increase in the irradiation time clearly indicates
the degradation of RhB upon the NIR irradiation. Figure 6 shows the timeꢀdependent profile of
C/C0 (degradation ratio), where C is the concentration of RhB irradiated with a 980 nm laser for
time t and C0 is the original concentration of RhB after attaining adsorption equilibrium with the
photocatalysts well before irradiation. From Figure 6 (d), it is clear that the degradation ratio of
RhB is observed to increase continuously with the irradiation time and reached about 61% after
12 h irradiation in the presence of MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites. The observed
degradation rate is much higher compared to the other upconversion photocatalysts (see Table S1
in supporting information). We strongly believe the observed faster degradation is due to the
layered nature of MoS2 and excellent upconversion properties of NCs. The surface area of a
photocatalyst plays important role behind its activity. Thus, BET surface area measurements
were carried out, which clearly indicate there is not much change in the surface area of MoS2 in
the composite (10.74 m2gꢀ1) upon deposition of NaYF4:Yb3+/Er3+ NCs compared to MoS2 alone
(13.78 m2gꢀ1). Furthermore, to know the difference in the photocatalytic ability of MoS2ꢀ
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NaYF4:Yb3+/Er3+ nanocomposites and the NaYF4ꢀYb/Er/ MoS2 physical mixture, the latter was
mixed with RhB solution and irradiated with NIR light for 12 h. Interestingly, only 18% of the
RhB was found to be decomposed, which is almost over three times lower than that observed for
MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites (61%). We attribute this to the difference in their
energy migration pathways, as has been discussed in the upconversion section. In the
nanocomposites structure, NaYF4:Yb3+/Er3+ NCs and MoS2 are attached closely with each other
and form compact interfaces, this benefits energy transfer processes as the distance between
MoS2 and UC nanoparticles are within energy transfer distance. In contrast, we believe there are
no contact interfaces between NaYF4ꢀYb/Er and MoS2 nanomaterials in the physical mixture,
and thus, MoS2 can be excited mainly via radiationꢀreabsorption. However, in the MoS2ꢀ
NaYF4:Yb3+/Er3+ nanocomposites, more NIR photon energy have been used to activate MoS2
leading to relatively higher photocatalytic activity. These results indicate that energy transfer
plays an important mechanism in the NIR photocatalytic activity. Moreover, NaYF4:Yb3+/Er3+
NCs and MoS2 alone showed limited photocatalytic activities for degradation of RhB. It is also
well established in the literature that the thermal energy generated by NIR irradiation is not
sufficient enough to induce the thermal degradation of RhB.41
It is essential to understand the origin of the degradation of RhB under irradiation of 980 nm NIR
laser in the presence of MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites. In most of the cases, it is the
reactive oxygen species like hydroxyl radicals (·OH) are considered to be responsible for
degradation of organic contaminants because of their strong oxidation ability. To confirm the
generation of ·OH during the photocatalytic process of MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites
under NIR irradiation, we used terephthalic acid (TA) as a fluorescence probe to measure ·OH
generation. The nonfluorescent TA captures ·OH to form hydroxyterephthalic acid (TAOH),
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which shows fluorescence at 424 nm upon excitation at 312 nm UV light. Therefore, the
formation of TAOH as well as the generation of ·OH can be selectively detected by monitoring
the emission intensity at 424 nm. Figure 7 (a) shows photoluminescence spectra for the
supernatant solution of the photocatalyst suspension containing 4 mM TA irradiated with 980 nm
NIR laser for designated time. The collected spectra are characteristic of the fluorescence of
TAOH, which indicates that ·OH is being produced during the NIR photocatalytic reactions.
Moreover, the increased intensity with irradiation time indicates the increase in ·OH generation.
Figure 7 (b), shows the timeꢀdependent evolution of the fluorescence intensity of TAOH in the
presence of different photocatalysts. The rate of generation of ·OH can be evaluated by the slope
of the curve in Figure 7 (b). It is clear from this figure that upon NIR irradiation, almost no ·OH
was generated in the case of only MoS2 and NaYF4:Yb3+/Er3+ NCs suggesting hardly any
degradation of organic species occurs in the presence of these materials. On the other hand, in
the presence of MoS2ꢀNaYF4:Yb3+/Er3+ composites, ·OH radicals were produced upon NIR
irradiation. These results indicate that the energy transfer efficiency between NaYF4:Yb3+/Er3+
NCs and MoS2 is the key to cause the NIR photocatalytic activity.
Photocatalysis Mechanism: Scheme 1 describes the overall processes involved in the
photocatalysis by the MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites under irradiation of 980 nm NIR
laser. It mainly involves three processes i.e. (1) the formation of the excited states of Er3+, (2) the
activation of semiconductor via energy transfer, and (3) the generation of reactive oxygen
species. Upon 980 nm excitation, the higher energy levels of the Er3+ ions (4I11/2, 4F9/2 and 4H9/2)
are populated via a successive energy transfer processes from Yb3+ to Er3+. The semiconductor
MoS2 is excited by the energy transfer from upconverting luminescent NaYF4:Yb3+/Er3+ NCs.
Thus, MoS2 is activated to produce photogenerated carriers, that is, electrons and holes in the
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conduction band (CB) and the valence band (VB), respectively (see Scheme 1). These electronꢀ
hole pairs migrate from the inner region to the surfaces to take part in surface reactions. As
shown in Scheme 1, the excited electrons arriving on the surfaces react with the oxygen adsorbed
·−
on the surfaces of MoS2 to form superoxide radical anion (O2 ), whereas the photogenerated
strongly oxidative holes (h+) react with H2O to produce hydroxyl radicals (·OH), which is
considered as the main contributor for the degradation of RhB dye.
Conclusions
In summary, adipic acid encapsulated NaYF4:Yb3+/Er3+ upconversion nanocrystals were
synthesized successfully by a hydrothermal method. The as synthesized NCs possess strong UC
emission in the visible and UV region of solar spectrum. The potentiality of UC nanocrystals to
absorb the NIR light in solar spectrum is utilized to enhance the photocatalytic performance of
MoS2. The strong NIR driven photocatalytic capability of MoS2ꢀNaYF4ꢀYb3+/Er3+ upconversion
photocatalyst is demonstrated via the degradation of RhB dye. Further, comparative studies
performed between the MoS2 ꢀ NaYF4:Yb3+/Er3+ composites and the physical mixture of MoS2
and NaYF4:Yb3+/Er3+ NCs revealed the presence of different energy transfer pathways in the
nanocomposites and the mixture which resulted in a large difference in the NIR driven
photocatalytic activities. In addition, a series of control experiments revealed that the hydroxyl
radicals are responsible for Rhodamine B (RhB) dye degradation under NIR irradiation. In
conclusion, we have developed a facile method to synthesize MoS2ꢀNaYF4:Yb3+/Er3+
nanocomposites that combine the advantages of both MoS2 and upconversion nanoparticles
which can be used for applications beyond photocatalysis such as for broadband optical limiting
applications, optoelectronic devices, etc.
ASSOCIATED CONTENT
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Supporting Information. FTIR spectra, lifetime, power dependent plot, energy transfer
mechanism, “This material is available free of charge via the Internet at http://pubs.acs.org.”
AUTHOR INFORMATION
Corresponding Author
* Fax: 91ꢀ33ꢀ25873020; Tel: +91(0)9007603474; Eꢀmail: mvenkataramanan@yahoo.com
ACKNOWLEDGMENT
VM thanks the Department of Science and Technology (DST) India, CSIR and IISERꢀKolkata
for the funding. MC thanks KVPY for his scholarship. VNKBA and SG thank UGC and CSIR,
respectively, for their scholarship.
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Figure.1 XRD patterns of (a) MoS2, (c) MoS2ꢀNaYF4:Yb3+/Er3+ (d) NaYF4:Yb3+/Er3+
nanomaterials and (b and e) standard XRD patterns of MoS2 and bulk NaYF4.
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Figure.2 FEꢀSEM images of (a) NaYF4:Yb3+/Er3+ NCs, (b) MoS2 and (c) MoS2ꢀ
NaYF4:Yb3+/Er3+ nanocomposites. EDX spectrum of (d) MoS2ꢀNaYF4:Yb3+/Er3+
nanocomposites.
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Figure. 3. UVꢀVis absorption spectrum of MoS2. Inset shows the UVꢀVis absorption spectra of
(a) MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposite (red curve) and (b) NaYF4:Yb3+/Er3+ (black curve).
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Figure. 4. The UC emission spectra of (a) NaYF4:Yb3+/Er3+ NCs (b) physical mixture of
NaYF4:Yb3+/Er3+ NCs and MoS2 and (c) MoS2ꢀNaYF4:Yb3+/Er3+ composites (magnified
spectrum shown in the inset) under 980 nm excitation.
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Figure. 5 Temporal evolution of the absorbance spectra of RhB degradation under NIR
irradiation in the presence of MoS2ꢀNaYF4:Yb3+/Er3+ nanocomposites.
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Figure. 6 The photocatalytic degradation rates of RhB under NIR irradiation for different
photocatalyst solutions.
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Fig. 7 (a) Emission spectra of terepthalic acid (TA) solution and (b) fluorescence intensity ratio
of the emission peak of TAOH at 420 nm as a function of time upon irradiation with 980 nm
laser in the presence of MoS2ꢀNaYF4ꢀYb/Er nanocomposite.
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Scheme 1. Schematic illustration of nearꢀinfrared activated photocatalysis in MoS2ꢀ
NaYF4:Yb3+/Er3+ nanocomposites.
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