Graphene quantum dots-decorated ZnS nanobelts with highly efficient photocatalytic performances

Article abstract of DOI:10.1039/c5ra28026e

Hybrid nanostructures combining inorganic materials and graphene have shown great potential for the environmentally friendly treatment of effluents. Herein, graphene quantum dots (GQDs)-decorated ZnS nanobelts have been synthesized via a facile hydrothermal method. The electrostatic attraction of two materials and the thermal reduction of graphene are the main driving forces to fabricate well-defined composite nanostructures. GQDs in GQD/ZnS nanocomposites have been found to exist discretely and uniformly on the surfaces of ZnS nanobelts. The photocatalytic activity of GQD/ZnS nanocomposites has been found to be highest at a GQD/ZnS mass ratio of 8 × 10^-4. The photocatalytic rate constant (0.0046 min^-1) of GQD/ZnS nanocomposites having the optimized GQD content in the photodegradation reaction of rhodamine B has been found to be 14 times higher than that of commercially available ZnS powder. Decorated GQDs introduce an additional visible-light response and serve as electron collectors and transporters to block electron-hole recombination efficiently, enhancing the photocatalytic performances of ZnS nanobelts immensely.

Full text of DOI:10.1039/c5ra28026e

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Graphene quantum dots-decorated ZnS nanobelts
with highly efficient photocatalytic performances†
Cite this: RSC Adv., 2016, 6, 24115
Sooho Ham, Yeonho Kim,‡ Myung Jin Park, Byung Hee Hong and Du-Jeon Jang*
Hybrid nanostructures combining inorganic materials and graphene have shown great potential for the
environmentally friendly treatment of effluents. Herein, graphene quantum dots (GQDs)-decorated ZnS
nanobelts have been synthesized via a facile hydrothermal method. The electrostatic attraction of two
materials and the thermal reduction of graphene are the main driving forces to fabricate well-defined
composite nanostructures. GQDs in GQD/ZnS nanocomposites have been found to exist discretely and
uniformly on the surfaces of ZnS nanobelts. The photocatalytic activity of GQD/ZnS nanocomposites has
À4
been found to be highest at a GQD/ZnS mass ratio of 8 Â 10 . The photocatalytic rate constant
À1
(0.0046 min
)
of GQD/ZnS nanocomposites having the optimized GQD content in the
Received 30th December 2015
Accepted 25th February 2016
photodegradation reaction of rhodamine B has been found to be 14 times higher than that of
commercially available ZnS powder. Decorated GQDs introduce an additional visible-light response and
serve as electron collectors and transporters to block electron–hole recombination efficiently,
enhancing the photocatalytic performances of ZnS nanobelts immensely.
DOI: 10.1039/c5ra28026e
www.rsc.org/advances
demonstrated that heterostructures oen exhibit superior prop-
erties or new features compared with their individual constitu-
Introduction
The design and controlled fabrication of nanostructured mate- ents, due to coupling effects that can induce signicant
rials having functional properties have been extensively enhancement in the generation, transportation, and prolonged
1–6
26–28
studied. With growing concern over environmental remedia- lifetime of charge carriers.
tion and energy conversion, nanoscaled semiconductors have
gained considerable interest in photocatalysis,
Graphene quantum dots (GQDs) have received much atten-
especially in tion because they are non-toxic, inexpensive, free of heavy
7–11
the decomposition of organic water pollutants such as colored metals, and environmentally friendly, compared with tradi-
12–16
dyes into CO
2
2
and H O using solar light.
In particular, metal tional quantum dots based on toxic heavy metals which can
suldes have attracted signicant attention in photocatalysis due cause serious health and environmental issues. GQDs are useful
1
5,16
to their suitable band gap and catalytic function.
them, zinc sulde (ZnS), a kind of nontoxic and abundant chal- electron-transfer properties.
cogenide materials, with direct band gaps (3.72 eV and 3.77 eV for of GQDs in common solvents is expected to enable various
Among for photocatalysis applications owing to their photoinduced
30,31
Moreover, the good dispersity
32
cubic zinc blende and hexagonal wurtzite, respectively) is solution-processable applications. Thus, the combination of
a promising candidate for photocatalysts in environmental ZnS and GQDs seems to be an ideal means of improving charge
16–19
remediation
ation of electron–hole pairs by photoexcitation and the highly photocatalytic efficiency.
owing to its unique properties: the rapid gener- separation by hindering charge recombination, thus enhancing
2
0
negative reduction potential of its conduction band. However,
the fast recombination rate of photogenerated electron–hole ZnS) nanobelts with varied mass ratios of GQD/ZnS from 1 Â
Herein, we present the fabrication of GQDs-decorated ZnS (G/
À4
À3
pairs and the narrow absorption bands of its nanostructures 10 to 2 Â 10 via a facile hydrothermal process. ZnS nanobelts
2
1,22
restrict its practical applications as photocatalysts.
To over- have been decorated with GQDs via a spontaneous close-contact
come these issues, ZnS photocatalysts have been modied process through electrostatic attraction to the surface of ZnS
chemically in combination with noble metals, different semi- nanobelts and a subsequent tight-binding process through the
23–25
41
conductors, or carbonaceous materials.
It has been thermal reduction of GQDs onto ZnS nanobelts. The band-gap
energies of G/ZnS nanocomposites have been estimated to be
smaller than those of pristine ZnS nanobelts. As-prepared G/ZnS
photocatalysts have been found to photocatalyze the degradation
Department of Chemistry, Seoul National University, Seoul 08826, Korea. E-mail:
djjang@snu.ac.kr
†
1
‡
Electronic supplementary information (ESI) available. See DOI: of rhodamine B with remarkably enhanced efficiency in
0.1039/c5ra28026e
comparison with individual constituents due to the effective
separation of photoinduced electron–hole pairs (Fig. 1).
Present address: Nano-Bio Electron Microscopy Research Group, Korea Basic
Science Institute, Daejeon 34133, Korea.
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(FTIR) spectra were obtained using a Thermo Scientic Nicolet
6700 spectrometer, extinction spectra were measured using
a Scinco S3100 UV-vis spectrophotometer. Photoluminescence
spectra were measured using a JASCO FP-8300 spectrouo-
rometer, and surface charges were measured using a Malvern
instruments Nano ZS90 zetasizer.
Photocatalytic experiments
The evaluation of the photocatalytic performances of as-prepared
samples was performed at ambient conditions with rhodamine B
Fig. 1 Schematic for a plausible decomposition mechanism of an
organic dye over a G/ZnS nanocomposite under light irradiation.
(RhB) as a representative dye. In a typical procedure, 5 mg of a G/
ZnS catalyst was added to 40 mL of 10 mM RhB(aq) and stirred in
the dark for 30 min to ensure the adsorption–desorption equi-
librium of RhB on the nanocatalyst surface. The reaction mixture
was placed 30 cm away from a 300 W Schoeffel LPS 255 HR xenon
arc lamp, and its aliquots were taken at scheduled intervals
during irradiation and centrifuged to separate the supernatant,
whose UV/vis absorption spectrum was then recorded to monitor
the concentration of remnant RhB.
Experimental
Synthesis of ZnS nanobelts
Single-crystalline wurtzite ZnS nanobelts were prepared via
a reported wet-chemical solution process.
6
Fabrication of GQDs
Carbon bers (300 mg, purchased from Fibre Glast Develop-
ment Corporation) were added into a mixture containing 60 mL
Results and discussion
of 18 M H
2
SO
4
(aq) and 20 mL of 16 M HNO
3
(aq). The mixed Structural characteristics
ꢀ
solution was stirred at 80 C for 24 h, cooled down to room
temperature, and diluted with water (800 mL). The diluted
solution was dialyzed in a dialysis bag retaining molecular
weight of 1000 Da for 3 days to remove the acids.
All the diffraction peaks of both 0.8 G/ZnS nanocomposites and
pristine ZnS nanobelts in Fig. 2 can be indexed as the reference
peaks of wurtzite ZnS (JCPDS card no. 36-1450). The absence of
ꢀ
the typical stacking peak at 26 indicates that the added amount
À4
of GQDs (a GQD/ZnS mass ratio of 8 Â 10 ) was too small to
Fabrication of GQDs–ZnS nanocomposites
show any new diffraction peaks from stacking or conglomera-
43
2
0 mg of as-prepared ZnS nanobelts was dispersed in 30 mL tion of GQDs during thermal reduction. None of the XRD
water and ultrasonicated for 10 min. Subsequently, a calculated peaks have been attributed to impurity, suggesting that our as-
À1
specic volume of 0.1 mg mL GQD in water was dropped prepared G/ZnS nanocomposites maintain the high structural
slowly into the ZnS containing solution. GQD/ZnS mass ratios purity of wurtzite ZnS. The inset of Fig. 2 displays that the
À4
À3
3
were varied from 1 Â 10 to 2 Â 10 ; if the 10 -multiplied diffraction peaks of 0.8 G/ZnS nanocomposites are broader to
weight ratio of GQDs to ZnS nanobelts was x, the nano- some extent than the respective ones of pristine ZnS nanobelts,
composite sample of GQDs-decorated ZnS nanobelts was indicating that the mean crystal diameter of ZnS nanobelts has
designated as x G/ZnS. The mixture solution was then loaded been decreased slightly during hybridization with GQDs. The
into a Teon-lined stainless-steel autoclave of 50 mL capacity, mean crystallite diameters of ZnS nanobelts and 0.8 G/ZnS
ꢀ
placed in a preheated oven at 180 C for 6 h, and cooled to room nanocomposites, estimated from the line widths of the peaks
ꢀ
29
temperature. The color of produced G/ZnS nanocomposites was at the 2q value of 27 using the Scherrer's formula, have been
found to be grayish-white. The precipitated product was
repeatedly centrifuged and washed with ethanol and water, and
ꢀ
dried in a vacuum at 60 C for 6 h to yield the nal powder
product of G/ZnS nanocomposites.
Characterization
High-resolution transmission electron microscopic (HRTEM)
images and energy-dispersive X-ray (EDX) elemental mappings
were recorded using a JEOL JEM-2100F high-resolution micro-
scope. X-ray diffraction (XRD) patterns were obtained with
a Bruker D8 DISCOVER diffractometer using Cu K
a
radiation
(0.15418 nm). X-ray photoelectron spectroscopy (XPS) spectra
were collected with a Thermo Scientic Sigma Probe ESCA
spectrometer using an X-ray source of Al K , and the measured
a
binding energies were calibrated with the C 1s peak at 284.5 eV
Fig. 2 XRD patterns of 0.8 G/ZnS nanocomposites (red) and pristine
of contaminated carbon. While Fourier transform infrared ZnS nanobelts (blue).
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found to be 12.5 and 11.5 nm, respectively, meaning that the hydrothermal reduction, the surface charges of ZnS nanobelts
mean crystal size of ZnS nanobelts has been decreased due to and GQDs were +10 meV and À30 meV, respectively, thus, sug-
surface decoration with GQDs.
gesting that ZnS nanobelts and GQDs are close-contacted via
electrostatic attraction. To understand the binding states of the
constituent elements of GQDs-decorated ZnS nanobelts, we have
measured the XPS spectra of 0.8 G/ZnS nanocomposites (Fig. 4).
The peak energies of 1021.6 and 1044.7 eV in Fig. 4A have been
identied as the binding energies of the Zn 2p3/2 and 2p1/2 of ZnS,
respectively. The XPS spectrum of Fig. 4B has been deconvoluted
into two Gaussian bands, whose peak energies of 160.9 and 162.1
HRTEM images and EDX elemental maps
The HRTEM image of Fig. 3A clearly shows that GQDs with an
average diameter of 20 nm have been decorated successfully
onto ZnS nanobelts via a hydrothermal process. Although the
morphology and structure of ZnS nanobelts have been main-
tained during the hybridization process, the average size of
GQDs has been increased due to aggregation. The initial
average diameter of synthesized GQDs has been estimated as
about 5 nm from the HRTEM image of Fig. S1.† During the
hydrothermal treatment, GQDs have been reduced to decrease
the surface charge of GQDs, thus alleviating electrostatic
repulsion between GQDs. Eventually GQDs have been found to
form aggregates, and the apparently observed average size of
GQDs has been increased. The HRTEM image of Fig. 3B reveals
that interplanar spacings of 0.29, 0.31, and 0.33 nm agree very
well with the respective standard separations of the (101), the
eV have been assigned to the binding energies of the S 2p3 and
/2
34
2
p1/2 of ZnS, respectively. The XPS spectrum of C 1s (Fig. 4C) has
been deconvoluted into three Gaussian bands located at 284.5,
6,29
(002), and the (100) planes of wurtzite ZnS. Hexagonal atomic
lattice with uniform contrast, originating from monolayer gra-
phene, can be also clearly discerned in the enlarged HRTEM
33,42
view of Fig. 3B.
The distribution maps of the constituting
elements, Zn, S, and C in the panels D, E, and F of Fig. 3,
respectively, demonstrate that ZnS have been decorated with
GQDs. Furthermore, Fig. 3F indicates that GQDs in 0.8 G/ZnS
À4
nanocomposites with a GQD/ZnS mass ratio of 8 Â 10 exist
discretely and uniformly on the surfaces of ZnS nanobelts.
Surface properties
Fig. 4 Zn 2p (A), S 2p (B), C 1s (C), and O 1s (D) XPS spectra of 0.8 G/
ZnS nanocomposites. The S 2p and C 1s spectra have been decon-
To conrm electrostatic attraction between ZnS nanobelts and
GQDs, the surface charges have been measured. Before the voluted into two and three Gaussian curves, respectively.
Fig. 3 HRTEM images (A–C) and the elemental maps of Zn, S, and C (D–F) of 0.8 G/ZnS nanocomposites.
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86.0, and 288.1 eV, which can be ascribed to C–C bonds, C–O
Paper
2
bonds, and C]O bonds, respectively. Compared to the C 1s XPS
spectrum of pristine GQDs (Fig. S2†), the relative intensity of C–O
bonds has decreased aer the hydrothermal treatment, meaning
that the thermal treatment has reduced GQDs by decomposing
C–O and C–OH groups to form stable bonds between ZnS
nanobelts and GQDs. However, Fig. 4D shows that only one XPS
peak at 531.4 eV can be attributed to O 1s, probably because the
binding energy of O 1s in the surface hydroxyl bonds of gra-
35–37
phene
is similar to that of O 1s in C]O bonds (see below); Fig. 6 Extinction spectra (A) and Kubelka–Munk plots (B) of 0.8 G/ZnS
note that the band widths (1.95 eV) of O 1s in Fig. 4D is nanocomposites (red) and 0.0 G/ZnS nanobelts (blue) suspended in
ethanol. The green solid lines in panel B show the best linear fits to
estimate the band-gap energy.
substantially broader than that (1.68 eV) of the C 1s of C–C bonds
in Fig. 4C. Nevertheless, our XPS results also support that GQDs
have been successfully incorporated with ZnS nanobelts to form
G/ZnS nanocomposites.
edge by 14 nm due to the chemical interactions of ZnS with
graphene. It is well known that ZnS is a direct semiconductor,
Based on the discussion of the XPS analysis, there are
different carbon functional groups on the surface of 0.8 G/ZnS
whose band gap can be estimated by extrapolating the linear
nanocomposites, which can also be seen in the FTIR spec-
29
portion of the modied Kubelka–Munk function versus the
energy of light (Fig. 6B). The derived band gaps of 0.0 G/ZnS and
0.8 G/ZnS nanocomposites have been estimated as 3.69 and
trum of Fig. 5. The broad absorption band located around 3250
À1
cm can be assigned to the stretching vibration of O–H, while
À1
the characteristic lines at 2940 and 1718 cm correspond to the
3.40 eV, respectively, indicating that coupling with graphene
asymmetric stretching of –CH
2
and the stretching vibration of
38,39
narrows the band gap of semiconductor ZnS in G/ZnS nano-
composites. We suggest that band-gap narrowing as well as
increased visible-light absorption due to graphene decoration
enhances the photocatalytic performances of G/ZnS nano-
composites (see below).
C]O, respectively.
cm have been attributed to the stretching vibration of C]C
and the O–H deformation of C–OH, respectively, and the
The absorption peaks at 1601 and 1413
À1
À1
absorption peaks at 1261 and 1060 cm have been assigned to
the stretching vibrational modes of C–OH and C–OC, respec-
39,40
tively.
Consequently, not only the XPS spectra of Fig. 4 but
also the FTIR spectrum of Fig. 5 support that GQDs have been
decorated to ZnS nanobelts with maintaining the inherent
characteristics of graphene oxide and graphene.
Photocatalytic activities
The photocatalytic degradation of RhB under light irradiation at
ambient conditions has been monitored to evaluate the pho-
tocatalytic activities of G/ZnS nanocatalysts. Fig. 7A indicates
that the concentration of RhB under light irradiation was
decreased most rapidly under the photocatalysis of 0.8 G/ZnS
nanocomposites while that was hardly reduced in the absence
Optical properties
The extinction spectra of Fig. 6A show that both samples of 0.8
G/ZnS nanocomposites and 0.0 G/ZnS nanobelts have the
representative band-gap absorption of ZnS nanobelts in the
ultraviolet region. The enhanced absorption of 0.8 G/ZnS
nanocomposites in the entire visible region can be attributed
to the presence of graphene. However, in comparison with the
extinction spectrum of 0.0 G/ZnS nanobelts, that of G/ZnS
nanocomposites shows the obvious red shi of the absorption
0
of any catalysts. The ln(C/C )-vs.-t kinetic plots of the photo-
catalytic degradation of RhB under light irradiation in the
presence of G/ZnS nanocomposites have been also given in
Fig. 7B; the photodegradation kinetic plots of RhB via G/ZnS
nanocomposites follow pseudo rst-order kinetics. Fig. 7C
shows that the photocatalytic activity of G/ZnS nanocomposites
in the photodegradation reaction of RhB was increased with the
increase of the GQD/ZnS mass ratio until the ratio became 8 Â
À4
10
. This indicates indeed that hybridization with graphene
enhances the photocatalytic activity of ZnS nanoparticles
immensely. However, the photocatalytic activity decreases with
the concentration increase of GQDs above the GQD/ZnS mass
À4
ratio of 8 Â 10 . We consider that excessive GQDs can act as
charge-recombination centers instead of electron pathways.
Table 1 reveals that the observed photocatalytic rate constant of
À1
0
.8 G/ZnS nanocomposites (0.046 min ) is 14 times higher
than that of commercially available ZnS powder (99.99%,
0 mm, purchased from Sigma-Aldrich) and 1.9 times higher
1
than that of 0.0 G/ZnS. As photocorrosion is a major concern for
sulde semiconductors, the stability of as-formed G/ZnS
nanocomposites has been tested via photocatalyst recycling
Fig. 5 FTIR spectrum of 0.8 G/ZnS nanocomposites.
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Fig. 7 Concentration changes (A), first-order kinetic profiles (B), and decay rate constants (C) of RhB via indicated photocatalysts.
tests (Fig. S3†). 70% of the photocatalytic degradation efficiency nanobelts directly without having any linker molecules brings
has remained aer two recycling experiments.
about new visible absorption to our G/ZnS nanocomposites;
The photocatalytic mechanism of G/ZnS nanocomposites newly generated electrons in the Fermi level of graphene with
can be suggested as follows: photogenerated holes from ZnS leaving holes to the valence-band level of ZnS induce band-gap
À
nanobelts can react with OH ions to form cOH radicals for the narrowing to G/ZnS nanocomposites. Because the Fermi level of
further photocatalytic degradation of RhB (Fig. 1). During the ZnS is lower than that of graphene and GQDs attached to ZnS
photocatalytic process, GQDs serve as electron collectors and nanobelts, the energy band of G/ZnS nanocomposites will curve
transporters to block electron–hole recombination efficiently, up to form a metal–semiconductor Schottky junction under
generating long-lived holes on the surfaces of ZnS nanobelts. equilibrium. The excited electrons of graphene are not allowed
The photoluminescence spectra have been measured to test the to transfer to the conduction band of ZnS due to the energy
electron–hole recombination of G/ZnS nanocomposites barrier between graphene and ZnS. This energy barrier also
(Fig. S4†). The band-edge emission of ZnS nanobelts around facilitates charge-separation processes and effectively reduces
324 nm has been drastically quenched by presence of GQDs, recombination chances of newly generated electrons and holes,
supporting that the electron–hole pairs of ZnS nanobelts have signicantly increasing the lifetime of charges in G/ZnS nano-
been separated extensively via the effective transfer of excited composites. Long-lived charges are then suggested to enhance
electrons to GQDs. Longer-lived holes then account for the the photocatalytic performances of G/ZnS nanocomposites on
largely enhanced photocatalytic activity of G/ZnS nano- a large scale.
composites. The thermal reduction of GQDs onto the ZnS
Conclusions
Table 1 Catalytic degradation rate constant of RhB via nanocatalysts
Highly efficient photocatalysts of GQDs-decorated ZnS nano-
belts have been developed via a facile hydrothermal process.
The decoration of ZnS nanobelts with GQDs has been carried
out via two synthetic steps: the spontaneous attachment of
GQDs on the surface of ZnS nanobelts through electrostatic
attraction, followed by the partial thermal reduction of GQDs to
construct well-dened composite nanostructures. Measure-
ments of XRD, HRTEM, XPS, and FTIR have revealed that GQDs
in G/ZnS nanocomposites exist discretely and uniformly on the
surfaces of ZnS nanobelts. It has been found that decorated
GQDs introduce additional visible-light response and serve as
electron collectors and transporters to block electron–hole
under irradiation of a 300 W Xe lamp
À3
À1
Nanocatalyst
GQD/ZnS mass ratio (10
)
k (min )
No catalysts
—
0.002
0.003
0.024
0.039
0.042
0.043
0.046
0.028
0.022
Commercial ZnS
0.0
0.0
0.1
0.2
0.5
0.8
1.0
2.0
0
0
0
0
0
1
2
.0 G/ZnS
.1 G/ZnS
.2 G/ZnS
.5 G/ZnS
.8 G/ZnS
.0 G/ZnS
.0 G/ZnS
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