Bubble template synthesis of CdLa2S4 hollow spheres/reduced graphene oxide nanocomposites as efficient and sustainable visible-light driven photocatalysts

Article abstract of DOI:10.1039/c5ra18014g

A hollow sphere CdLa₂S₄/reduced graphene oxide (rGO) composite was successfully synthesized for the first time via a simple microwave assisted hydrothermal approach using bubbles generated from the decomposition of NH₃H₂O and Na₂S₂O₃ in a domestic microwave oven within several minutes. The large number of catalytic active sites together with the advantages of the hollow structure makes the as-synthesized CdLa₂S₄/rGO composite an efficient catalyst for the degradation of rhodamine B (RhB) and industrial dyes such as Texbrite BA-L (TBA) at high concentration in aqueous solutions. The oxidation reaction of 1,5-diphenyl carbazide (DPCI) by a generated reactive oxygen species (ROS) that has very strong oxidizing power could be attributed to the excellent photoinduced charge separation abilities and observed red shift in these hybrid semiconductor materials. Moreover, the synthesized catalyst can be easily recovered and reused for at least four cycles due to its good stability. The results confirm that as-synthesized CdLa₂S₄/rGO composites are promising potential candidates for high-performance visible light driven photocatalyst materials.

Full text of DOI:10.1039/c5ra18014g

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Page 1 of 30
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Bubble template synthesis of CdLa S hollow spheres/reduced
2
4
graphene oxide nanocomposites as efficient and sustainable visible-
light driven photocatalysts
*
Lei Zhu, and WonꢀChun Oh
Department of Advanced Materials Science & Engineering, Hanseo University, Seosanꢀsi,
Chungnamꢀdo, Korea, 31962
A hollow sphere CdLa
synthesized for the first time via a simple microwave assisted hydrothermal approach using
bubbles generated from the decomposition of NH O and Na in a domestic microwave
oven within several minutes. The large number of catalytic active sites together with the
advantages of the hollow structure makes the asꢀsynthesized CdLa /rGO composite an
2 4
S /reduced graphene oxide (rGO) composite was successfully
3
H
2
2 2 3
S O
2 4
S
efficient catalyst for the degradation of rhodamine B (Rh.B) and industrial dyes such as
Texbrite BAꢀL (TBA) with high concentration in aqueous solutions. The oxidation reaction of
1,5ꢀdiphenyl carbazide (DPCI) by a generated reactive oxygen species (ROS) that has very
strong oxidability could be attributed to the excellent photoinduced charge separation abilities
and observed red shift in these hybrid semiconductor materials. Moreover, the synthesized
catalyst can be easily recovered and reused for at least four cycles due to its good stability.
2 4
The results confirm that asꢀsynthesized CdLa S /rGO composites are promising potential
candidates for highꢀperformance visible light driven photocatalyst materials.
2 4
Keywords: CdLa S , Graphene oxide, Hollow nanosphere, Microwave synthesis, Visible
light
*
Corresponding author.
Eꢀmail: wc_oh@hanseo.ac.kr
Tel: +82ꢀ41ꢀ660ꢀ1337, Fax:+82ꢀ41ꢀ688ꢀ3352

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Introduction
Hollowꢀstructured materials hollow inorganic spheres with low densities, high surface areas,
and unique electronic, magnetic, and chemical properties have become an interesting topic for
1
research. The spheres have promising applications in photonic catalysts, drugꢀdelivery
2
3
4
carriers, electronic devices, controlled release, and optical sensors for highꢀpower laser
5
6
irradiation. In order to prepare hollow structures, soft templates such as surfactants, liquid
7
,8
9
10ꢀ12
crystal template, micelles, or gas bubbles,
have been commonly used due to it is simple
and inexpensive.
Graphene is a new star on applications of condensedꢀmatter physics, electronics, and
13
material science after carbon nanotube and C60
.
Graphene is a singleꢀatom thick sheet
arranged by sp2ꢀbonded carbon atoms in a hexagonal lattice, which shows outstanding
mechanical, thermal, optical, and electrical properties. Therefore, the interesting grapheneꢀ
based hollowꢀstructured materials have been extremely extended to application. Huang et al.
synthesize the rGO wrapped hollow CuS sphere composite by in situformation of hollow CuS
sphere on the surface of rGO nanosheet under simple oneꢀstep solvothermal route assisted by
1
4
ethylene glycol as electrode material for supercapacitor. Mi et al. reported for the first time
CdS hollow spheres supported on the surface of Nꢀgraphene nanosheets (Nꢀgraphene/CdS
1
5
HSNs), and present a simple, templateꢀfree oneꢀpot method for their fabrication. The
prepared samples were found to exhibit higher photocatalytic activity than that of graphene
oxide (GO)/CdS HSNs and sole CdS hollow spheres during photocatalytic degradation
experiments. Ou et al. reported dropletꢀshape hollow Ni S nanoparticles, as well as
3
2
corresponding partially nickelꢀfilled nanoparticles, of narrow diameter distribution and
uniform dispersion were successfully synthesized on twoꢀdimensional graphene templates
1
6
using a facile process with moderate reaction conditions.
As a common semiconductor material, transitionꢀmetal chalcogenides have attracted
17,18
much attention due to their excellent properties and wideꢀranging potential applications.
CdLa , as a ternary semiconductor chalcogenide has a strong absorption in the visible light
2 4
S
region with band gap energy of 2.1ev, therefore it has become a good catalyst for photoꢀ
degradation of organic pollutants and photoproduction of hydrogen from water in the past few

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1
9,20
years.
Kale et al. reported synthesis of selfꢀassembled CdLa2S4 hexagon flowers,
nanoprisms and nanowires via a facile hydrothermal method at 433K, followed application in
1
9
H
2
production in water/methanol solution. Hou et al. found that the coupling of CdS
nanoparticles with CdLa microspheres via a twoꢀstep hydrothermal process led to
significant increase in the photocatalytic H production rate, nevertheless Ptꢀloading was still
2 4
S
2
21
required for H
2
evolution. However, to our knowledge, there is still a need to develop a
more straight forward procedure for fabricating graphene based hollow structured CdLa
2 4
S
in
an effective and reproducible fashion and applications on photoꢀdegradation of organic
pollutants.
It is well known that the synthesis of nanomaterials with uniform size, shape and high
crystallinity is one of the important challenging problems. There are various kinds of
synthesis methods available for the preparation of nanomaterials such as hydrothermal,
microwave synthesis, sol–gel, micro emulsion and the polyol technique. As compared to
common methods, the microwave synthesis technique provides such advantages as a very
short reaction time, small particle size, narrow particle size distribution and it is the high
2
2
purity method suitable for the preparation of polycrystalline products. The microwave
heating is delivered to the surface of the material by radiant and/or convection heating, which
is transferred to the bulk of the material via conduction. So, the microwave energy is
delivered directly to the material through the molecular interactions with electromagnetic
field. Heat can be generated through volumetric heating because microwaves can penetrate
2
3
the material and supply energy.
Herein, this is the first report for the synthesis of graphene nanosheets coated with
CdLa nanometerꢀsized hollow spheres obtained directly via a facile microwave assisted
synthesis method in a domestic microwave oven without using a surfactant. To clarify the
possible formation mechanism of CdLa hollow spheres on the graphene nanosheet, the
microwave assisted synthesis reaction time has been investigated. The asꢀsynthesized
CdLa /rGO nanocomposite show highly enhanced photocatalytic performance for
2 4
S
2
4
2 4
S
2 4
S
degradation of different organic dyes, rhodamine B (Rh.B) and Texbrite BAꢀL (TBA), with
high concentration in aqueous solutions under visible light irradiation, due to the existence of

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the strong interaction between CdLa S hollow spheres and rGO.
2
4
Experimental
Materials
Ethylene glycol and anhydrous ethanol were purchased from Daejung Chemical Co. (Korea).
Sulfuric acid (H SO ), potassium permanganate (KMnO ), H and (CH COO) Cd·2H O,
La(NO ·H O, Na were supplied by Daejung Chemical Co., Ltd, Korea. Texbrite BAꢀL
was purchased from Texchem Co. Ltd, Korea. Rh.B (C28 , 99.99+%) was used as
the model pollutant and was purchased from Samchun Pure Chemical Co., Ltd, Korea.
Ammonium hydroxide (NH O) was purchased from Duksan Pure Chemical Co. (Korea)
2
4
4
2
O
2
3
2
2
3
)
3
2
2 2 3
S O
2 3
H31ClN O
3
H
2
and used as received. All chemicals were used without further purification and all
experiments were carried out using distilled water.
2 4
Synthesis of CdLa S /rGO nanocomposites
In a typical experiment, about 20mg Graphite oxide (GO) prepared from graphite according
25
to the HummersꢀOffeman method in our early reports was dispersed in 80ml ethylene glycol
and then exfoliated to generate graphene oxide nanosheets (GONS) dispersion solution by
26
ultrasonication for 25min using a digital sonifer. Subsequently, calculated amount of
CH COO) Cd.2H O and La(NO .H O were added to the GONS aqueous dispersion,
followed by ultrasonication for 20min. Afterwards, sodium thiosulfate anhydrous (Na S O )
(
3
2
2
3
)
3
2
2
2
3
3 2
and 5mL ammonium hydroxide (NH H O) were added to the mixture transferred into a 120
mL reaction vessel and placed in a conventional microwave oven (Samsung, REꢀ406B 700
W). The solution is then irradiated by microwave at full power for 10 sec on and 10 sec off
for 5min, and cooled at room temperature washed several time with hot water and transferred
into a dry oven. The ammonia promised it to obtain maximal charge so as to prevent the
27
aggregation of graphene oxide caused by salt effect, which was in favor of the uniform
distribution of CdLa S on GONS. The Na S O was taken as 0.5, 0.75 and 1mM and the
2
4
2
2
3
final product labeled as CdG1, CdG2, CdG3 were rinsed with distilled water and ethanol for
several times, and dried at 60 °C for 12 h. Bulk CdLa nanoparticle was also synthesized by
2 4
S
the same procedure except that no GONS was used. The preparation conditions and
schemiatic illustration are shown in Fig.1a.

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Characterization
The crystal structures and phases of the samples were obtained by XRD (Shimata XDꢀD1,
o
Japan) with Cu Kα radiation (λ=1.54056 Å) in the range of 2θ from 10ꢀ80 at a scan speed of
o
ꢀ1
1
.2 m . The morphology of the samples was studied by SEM (JSMꢀ5200 JOEL, Japan).
Energy dispersive Xꢀray spectroscopy (EDX) was also employed for elemental analysis.
Transmission electron microscopy (TEM, JEOL, JEMꢀ2010, Japan) was used to observe the
surface state and structure of the photocatalyst composites at an acceleration voltage of 200
kV. TEM was also used to examine the size and distribution of the CdLa S nanoparticles on
2
4
graphene sheet. Diffuse reflectance spectra were obtained by using a scan UV/Vis
spectrophotometer (Neosysꢀ2000) equipped with an integrating sphere assembly. Raman
spectra of the samples were observed using a spectrometer (Jasco Model Name NRSꢀ3100)
having an excitation laser wavelength of 532.06 nm. FTꢀIR spectroscopy (FTS 3000MX,
Biored Co. Korea) was used to characterize the functional groups of GO and CdG2. The BET
surface areas of the photocatalysts were determined by measuring nitrogen adsorption at 77 K
using a BET analyzer (Monosorb, USA). The decomposition kinetics for the photocatalytic
activity was measured by using spectrometer (Optizen POP, Mecasys, Korea).
Photocatalytic reactions
The photocatalytic activities of the pure CdLa S , CdG1, CdG2, and CdG3 nanocomposites
2
4
were evaluated from the degradation of Rh.B solution under irradiation of visible light (8W,
Fawoo, LumidasꢀH, Korea, λ˃420 nm). In an ordinary photocatalytic test performed at room
ꢀ
5
temperature, 0.03 g photocatalyst was added to 50 mL of 3.0×10 mol/L Rh.B solution,
which was hereafter considered as the initial concentration (c ). To achieve more degradation
0
activity of the CdG2 nanocomposite industrial dye, TBA was also conducted with the same
conditions. The mixture was sonicated for 10 min and stirred for 120 min in the dark in order
to reach adsorption–desorption equilibrium. The first sample was extracted immediately
before the light was turned on in order to determine the dye concentration in solution after
dark adsorption, which was henceforth considered as the initial concentration (cads). The
samples were then withdrawn regularly from the reactor in the order of 30 min, 60 min, 90
min, 120 min, and 150 min, immediately centrifuged to separate any suspended solid. The

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clean transparent solution was analyzed using a UVꢀvis spectrophotometer (Optizen POP) at a
wavelength from 250 nm to 800 nm.
Evaluation of Reactive Oxygen Species (ROS)
In order to detect the type of ROS generated during photocatalytic reactions, the
2
3
experiments were performed according to a previous reported method. Firstly, four 100 mL
transparent volumetric flasks were marked notations (a–d), respectively. Four 10.00 mL DPCI
ꢀ2
stock solutions (1.00 x 10 mol/L) were added to a–d correspondingly followed by the
addition of a 50 mg sample in the order of (1,2,3,4) from a to d. All four solutions were
diluted to 100 mL with double distilled water. The final DPCI concentration and CdG2
−
3
amount were 1.00 × 10 mol/L and 1.00 g/L, respectively. After 120 min of irradiation, 10 ml
of solution was taken from each reactor and extracted with benzene. In the second step, all the
extracted solutions were diluted to 10 mL with benzene solution and their UVꢀVis spetra were
recorded by using a spectrometer.
Results and discussion
Characterization
2
+
3+
Fig. 1a shows the deposition of CdLa
2 4
S on the GONS. In this approach, Cd /La were
captured by hydroxyls (possible epoxies or carboxyls) on the GO by coordination. The
2
+
3+
2
−
Cd /La ions in situ then reacted with S under hydrothermal treatment to form CdLa2S4
nanoparticles with the simultaneous reduction of the graphene oxide (GO). Fig. 1b shows the
XRD pattern of the product obtained from the above procedure. It can be seen that the
diffractogram of graphene exhibits typical peaks at 25.9° and 42.7°, corresponding to the
graphite (002) and (100) reflections (Joint Committee for Powder Diffraction Studies
2
6
(
JCPDS) No. 01ꢀ0646), respectively. In the CdLa
2 4 2 4
S and CdLa S /rGO composites all the
diffraction peaks around at 2θ of 18.07°, 26.8°, 43.8°, and 51.9° could be indexed to CdLa S ,
2
4
indicating that single phase CdLa
2
S
4
was obtained. This result is consistent with those of the
19
previous report. However, no signal for any other phases of GO (001) or graphene (002)
19
2 4
could be detected in GRꢀCdLa S composite. According to Ref. GO can be reduced to
graphene during the hydrothermal reaction and the synthesized graphene sheets can restack to
form poorly ordered graphite along the stacking direction. Earlier studies have shown that if

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the regular stack of GO or graphite is broken, such as by exfoliation, their diffraction peaks
2
7
may also become weak or even disappear.
Further evidence of the deoxygenation of GO to graphene using FTꢀIR analysis is
shown in Fig. 1c. It is well known that some carbon double bonds are oxidized after the acidic
intercalation and thermal treatment, resulting in the presence of oxygenꢀcontaining functional
2
8,29
groups such as ꢀCOOH and ꢀOH on the surface of the graphite oxide nanosheets.
For GO,
−
1
the resonance at 1619 cm can be assigned to the vibrations of the adsorbed water molecules:
−
1
−1
the C=O carbonyl stretching near 1728 cm , the CꢀOH stretching near 1226 cm , and the Cꢀ
−
1
O stretching near 1050 cm .
Meanwhile, the amount and distribution of the carboxylic groups have great influence on
the further positiveꢀselective modification of graphene sheets with nanoparticles. The positive
metal ions in the system would attach to and interact with the carboxylic groups via
30
electrostatic attraction and serve as nucleation precursors. Peaks for C = O functional groups
ꢀ
1
were observed at approximately 1728 cm . Comparison of the spectra reveals lower peak
intensity for functional groups in oxidized graphene. This is because some of the functional
2 4
groups combined with CdLa S particles and removed in the process of reduction of graphene
1
3
oxide to graphene.
The Raman spectra of GO and CdG2 nanocomposite in Fig. 1d displays the presence of
−
1
2
D and G bands at 1350 and 1597 cm , respectively. G band is common to all sp carbon
3
1
2
32
forms and provides information on the inꢀplane vibration of sp bonded carbon atoms. The
3
33
D band suggests the presence of sp defects. The secondꢀorder Raman feature, namely the
−
1
−1
2
D band (secondꢀorder of the D band) at about 2720 cm and D+G band at about 2930 cm
is very sensitive to the stacking order of the graphene sheets along the cꢀaxis as well as to the
number of layers, and shows greater structure (often a doublet) with increasing number of
3
4
graphene layers. Simultaneously, relative intensity of D/G (1.13) was increased compared to
3
5
that in GO (1.07). This further confirmed that GO was reduced to graphene and indicate the
2
considerable decrease in size of the inꢀplane sp domains and and a partially ordered crystal
36
structure of the CdLa S /rGO (CdG2) nanocomposites.
2
4
2 4 2 4
The morphologies of CdLa S , graphene, and CdLa S /rGO (CdG2) nanocomposites
were investigated using SEM. From Fig. 2a, the overall structure can be clearly predicted,

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showing that graphene is a plateꢀlike structure broken off in different direction. A flakeꢀlike
morphology was observed for graphene, reflecting its layered microstructure. The large
interlayer spaces and thin layer edges of graphene can be clearly observed. Fig. 2b shows pure
2 4
CdLa S particles appearing as spherical particles with good dispersion and large size. The
morphology of the CdLa S /rGO composites substantially differs from that of the graphene
2
4
2 4
sheets. Meanwhile, the spherical CdLa S nanoparticles with smaller size compared with pure
CdLa can be observed, showing they were distributed tightly and evenly on the graphene
2 4
S
sheets and formed a grapheneꢀbased nanostructure composite. Because of the presence of
oxygen functionalities on the surface of the graphene sheets, electrostatic force arises among
the nanosheet in the form of van der Waals interaction that tends to force the graphene sheet
to aggregate back to a graphitic structure. The attachment of nanoparticles on the graphene
3
7
sheet is helpful to overcome these interactions. The graphene sheet acts as a support material
to the CdLa particle, which may be advantageous for supplying a path for the generated
2 4
S
electron, and will thus improve the photocatalytic behavior of the nanocomposites.
To obtain information about the changes in the elements and element weight %, the
prepared CdLa
microanalysis and element weight % of the CdLa
such as C, Cd, La, and S are present. The strong C signal should mainly originate from the
graphene nanosheets. The Lα and Lβ1 peaks from the La element appear at 4.65 and 5.04
2
S
4
/rGO composites were examined using EDX. Fig. 2d shows the EDX
2 4
S
/rGO nanocomposite. The main elements
1
2 2 3
KeV, respectively, while the S peak originating from the precursor materials of Na S O
appears at Kα 2.309 KeV.
1
2 4 2 4
Typical TEM images of graphene, pure CdLa S , and CdLa S /rGO (CdG2) are shown in
Fig. 3. The morphology of graphene, consisting of thin stacked flakes with a wellꢀdefined
fewlayer structure at the edge, is clearly shown in Fig. 3a. Fig. 3b shows a TEM image of
pure CdLa S synthesized via microwave assisted hydrothermal method, which reveal
2
4
2 4
spherical structure and defined single crystal of CdLa S with a size of 20ꢀ25nm. In addition,
the CdLa /rGO (CdG2) nanocomposites with different magnifications are clearly shown in
2 4
S
Figs. 3cꢀ3f. The TEM micrograph in Fig. 3c and 3d confirms the formation of hollow
nanospheres, where the distinguished contrasts can be found between the outer shells and the
inner hollow cores. It was found that most hollow nanospheres are composed of a number of

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smaller particles with the size of 15ꢀ20nm. The HRTEM image (Figs. 3e and 3f) suggests that
the fine CdLa particles are singleꢀcrystalline. The spacing of the observed lattice fringe is
identified as 0.36 nm, which is associated with the {211} plane of the cubic phase CdLa
2 4
S
2
0
2 4
S .
The quest for the design of new photocatalyst materials that utilize the entire energy
spectrum is very important. For the purification or removal of environmental pollutants, an
ideal photocatalyst must have maximum efficiency in both the UV and visible range of the
electromagnetic spectrum. The UVꢀvis absorption spectra of the different samples are
presented in Fig. 4a. Due to the carbonated structure of graphene, the unpaired π electron may
cause an interaction with the metal nanoparticles. Such reduction may cause a shift in the
38
band edges and increase the light absorption towards the visible region region. Lee et al also
observed such kind of effect with titanium nanoparticles and observed the optical response
3
9
towards the visible region of electromagnetic spectrum. It can be clearly seen that CdLa
2 4
S
and CdLa /rGO (CdG1, CdG2, CdG3) composites have great absorption in the ultraviolet
2 4
S
region. Moreover, for the CdG1, CdG2, and CdG3 composites, the absorption intensities are
significantly increased in the entire visible region, confirming the effectively improved lightꢀ
harvesting activity. The enhanced lightꢀharvesting intensity of the CdG1, CdG2, and CdG3
composites could become a type of visible light driven photocatalyst.
The diffuse reflectance spectra of CdLa S and CdLa S /rGO were transformed by
2
4
2 4
performing the KubelkaꢀMunk transformation of the measured reflectance according to the
following equation.
2
K=(1ꢀR) /2R=F(R)
①
K is the reflectance transformed according to KubelkaꢀMunk, R is the reflectance (%),
and F(R) is the soꢀcalled remission or KubelkaꢀMunk function. It is well known that the band
gap E and the absorption coefficient α are related, as shown in the following equation.
g
1
/2
αhv = A(hv ꢀ E
g
)
②
where α, v, E , and A are the absorption coefficient, light frequency, band gap, and a
g
constant, respectively. If the compound scatters in a perfectly diffuse manner, K becomes
equal to 2α. In this case, we can use the following expression;
2
[
F(R)hv] = A(hv ꢀ E
g
)
③

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The estimated Eg value was 2.1 eV for the CdLa S sample. This value is red shifted
2
4
from the typical Eg of the CdLa
2.22 eV) of the PGT nanocomposites shown in Fig. 4b.
XPS was carried out for the qualitative analysis of the CdLa
nanocomposite containing CdLa S and graphene. The XPS survey spectrum had peaks
2 4
S /rGO, to obtain CdG3 (2.46 eV)> CdG2 (2.35 eV)> CdG1
(
2 4
S /rGO (CdG2)
2
4
corresponding to Cd, La, S, O, and C consistent with the formation of the CdG2
nanocomposites shown in Fig. 5a. Fig. 5b shows the C1s spectra of the carbon presented in
the composites. The C1s has a strong peak located at around 284.6 eV and a slight peak at
289.1 eV. These binding energies correspond to the CꢀO and C=O functional groups,
respectively. These results reveal that our nanocomposites still contain some partial oxygen
functional groups. The presence of oxygen functional groups may increase the light
4
0
absorption towards the visible region. Fig. 5c shows that the O1s photoelectron kinetic
energies are smaller than those of the C1s, thus the sampling depth is smaller, and therefore
the O1s spectra are slightly more surface specific. We assigned the O1s peak at 532.1 eV to
contributions from the C=O and OꢀC–OH groups; this means that after the heat treatment, the
4
1ꢀ43
conversion to other chemical species may be possible.
The remaining peak at 534.7eV
indicates that the C–OH group is still appearing, it is assumed that the reduction of the
remaining C–OH groups in reduced graphene oxide is very difficult or it may not be possible
44
with our heat treated condition. Fig. 5d shows the La3d peaks which are located at three
different binding energy values. The binding energy positions located at 838.3 eV and
857.6eV were ascribed to La3d 3/2 and La3d 5/2, respectively, which are consistent with the
3
+ 21
binding energy of La . The S species were mainly present in −2 states, corresponding to
4
5
the HSꢀgroups with the binding energy at around 163.5 in the S2p level. The binding energy
value of S2p 3/2 is 161.2 eV and that of S2p 1/2 is 163.2 eV. The Cd 3d spectrum shown in Fig.
5f has a doublet feature due to the spin orbit splitting, whereby the 3d 5/2 and 3d 3/2 peaks
46
agree with those reported in the literature. The Cd 3d core level could be satisfactorily fit to
a single spinꢀorbit pair at 405 eV (Cd3d 5/2) and at 412 eV (Cd3d 3/2).
The nitrogen adsorption isotherms of CdLa S , CdG1, CdG2, CdG3 composites and poreꢀ
2
4
size distribution curves of CdG2 are shown in Fig. 6. The formation of type II adsorption
isotherms confirmed the major presence of micropores and the minor presence of mesopores

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on the surfaces of the CdLa S and the CdG1, CdG2, and CdG3 composites. This indicates
2
4
2 4
that the CdLa S /rGO composites studied were mainly microꢀ and mesoporous in character,
with a minor presence of wider pores where capillary condensation occurred. The type II
isotherms do not exhibit a saturation limit, unlike type I. This type of isotherm indicates
indefinite multiꢀlayer formation after the completion of the monolayer, and is found in
2 4
adsorbents with a wide range of pore sizes. All the isotherm shapes are similar for all CdLa S
series of asꢀprepared samples. Table 1 compares the textural properties of the asꢀprepared
samples and their microꢀstructures. The surface areas and pore volumes of the CdLa S /rGO
2
4
2 4
changed due to the chemical precipitation of the CdLa S compounds via microwave assisted
synthesis for 5 min; this indicated that the synthesis condition could be suitable to prepare
nanomaterials with micropores and mesopores. The pore size distributions for the CdG2
sample are shown in the inset image of Table 1. The sample demonstrated the presence of a
large number of pores in the broad pore distribution range of 0.7–0.9 nm.
Adsorption and photodecolorization process
To evaluate the adsorption ability of the asꢀprepared composite catalysts, the reactor was
placed on the magnetic churn dasher, stirred for 120 min in a dark box to establish an
adsorption–desorption equilibrium. As shown in Fig. 7a, the level of Rh.B adsorption by
CdLa S /rGO was higher than that of the CdLa S powder. The BET surface area of the
2
2
4
4
2 4
CdLa
S
control sample, and the asꢀprepared CdG1, CdG2, and CdG3 nanocomposites was
2
2
2
2
1
7.92 m /g, 43.08 m /g, 57.74 m /g, and 26.44 m /g, respectively. CdG2 exhibits the best
adsorption effect for the Rh.B dye solution. After establishing an adsorption–desorption
equilibrium for the CdG2 and CdG1 catalysts, 41.4% and 28.5% of the Rh.B dye solution was
removed, respectively, while only 11% of the Rh.B solution is removed for pure CdLa
2
S
4
.
The effects of visible light irradiation on RhB degradation due to the pure CdLa
2
S
4
and
CdLa S /rGO (CdG1, CdG2, CdG3) composite catalysts were investigated. From Fig. 7b, we
2
4
can find that after visible light irradiation for 150 min, the degradation effect of CdG2 is
superior (87%), while the degradation ratio of CdG3 and CdG1 for the Rh. B solution is 48%
and 50%, respectively. The overall catalytic efficiency is in the order of CdLa S < CdG3<
2
4
CdG1< CdG2 nanocomposites, which demonstrate all CdLa
2
S
4
/rGO nanocomposites possess
. In addition, it also
improved visibleꢀlight photocatalytic ativity than that of pure CdLa
2 4
S

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reveals that the photocatalytic activity of the asꢀprepared CdLa S /rGO nanocomposites do
2
4
2ꢀ
not improve monotonously with increase of S concentration. It can be seen that when the
2
ꢀ
concentration of S is less than 0.75mM, the photocatalytic activity is improved steadily with
2ꢀ
increasing amount of S precursor. However, further increase in concentration of S (>
.75mM) results in decreasing photocatalytic activity. Thus, it indicated that CdG2 catalyst
exhibit the highest synergetic effect between CdLa and graphene sheet. All calculated ꢀ
ln(C /Cads) values (with the restriction of Cads being the initial concentration in the bulk
0
2 4
S
t
solution after dark adsorption and t being the reaction time) were approximately linear with
the irradiation time represented in Fig. 7c and summarized in Table 1. The Rh.B degradation
ꢀ2
ꢀ1
rate constant for the CdG2composites reached 1.27×10 min under visible light irradiation.
Fig. 8 shows the UVꢀvis spectra of DPCO extract liquors in the presence of CdG1,
CdG2, and CdG3 composites under visible light irradiation. 1,5ꢀdiphenyl carbazide (DPCI)
can be oxidized by oxidizing substances into 1,5ꢀdiphenyl carbazone (DPCO). Under visible
light irradiation, some electrons are transited from the valence band (VB) to the conduction
band (CB). Simultaneously, the electron–hole pairs form on the surface or inside the CdG2
2
samples. The electrons and holes react with the molecular oxygen (O ) dissolved in the
aqueous solution and the water molecules (H
2
2 4
O) absorbed on the surface of CdLa S /rGO,
−
respectively, producing the super oxygen radical anions (•O ) and hydroxyl radicals (•OH).
2
These obtained •OH can oxidize 1,5ꢀdiphenyl carbazide (DPCI) into 1,5ꢀdiphenyl carbazone
(
DPCO). The DPCO can be extracted using a benzene solvent and shows an absorbance at
60 nm wavelength. Sequentially, the production and output of •OH can be easily detected.
5
According to the different irradiation times, the DPCO solution exhibits different absorbances
and shows an obvious increase compared with the corresponding DPCO solution without any
43
irradiation. The above results suggest that the CdG2 composites are much more efficient
than any other composites.
To better understand the formation mechanism and evolution of the hollow
nanostructures, timeꢀdependent experiments of the synthesized CdG2 nanocomposite were
carried out. Deng et al. reported the formation mechanism of hollow nanostructures using a
4
7
gas bubble as a soft template. Based on this study, a plausible mechanism was proposed to
address the templateꢀfree formation of hollow CdLa nanospheres (Fig. 9). This is because
2 4
S

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microwave heating is rapidly delivered to the surface of the material by radiant and/or
convection heating, which is transferred to the bulk of the material via conduction. During the
heating process, the mechanism of the gas bubble formation can be followed:
NH
3
·H
2
O < ꢀꢀ > NH
3
2
↑+H O (1)
2
ꢀ
S2O3 = SO + S ↓+ H O (2)
2
2
As shown in Fig. 9a (step a), the NH
NH ·H O not only inhibited particle agglomeration but also provided a soft gas template for
the formation of the hollow nanosphere. As the reaction time increased, the unstable S O
3
bubble initially generated from the decomposition of
3
2
4
8
2ꢀ
3
2
easily decomposed to generate S and SO
2
gas, thus continuously promoting the formation of a
4
9
hollow nanosphere. HRTEM images of the asꢀprepared CdLa nanostructure immobilizing
2 4
S
on the reduced graphene oxide (CdG2) at different reaction times (3min, 5min, and 7min,
labeled as CdG2ꢀ3, CdG2ꢀ5, and CdG2ꢀ7, respectively) are shown in Fig. 9b. In our work,
2 4
during the initial stage of the reaction, smaller CdLa S nanoparticles aggregated together to
form solid nanospheres with a loose structure. With prolonged microwave assisted
hydrothermal treatment, the inner crystallites of the aggregates underwent mass transfer to the
outer shell by a dissolution–recrystallization process at the cost of the small crystallites,
which have higher surface energies and solubility than the larger crystallites. With the
increasing reaction time, the outward migration of the CdLa S crystal would result in the
2
4
continued expansion of the interior space; thus, the inner space of the spheres is further
increased, which results in the CdLa hollow structures. Furthermore, the effect of
2 4
S
temperature on the CdLa S morphology also supports the fact that Ostwald ripening effect is
2
4
5
0
the essential mechanism. Meanwhile, reproducibility study was performed as irradiating the
solution by microwave under same condition for 3 times, the results were shown in Fig. 9c
2 4
which conforms the reliability of synthesis nanospheres CdLa S by domestic microwave.
The relationship between visible light photocatalytic TBA degradation and hollow
nanostructured CdG2 nanocomposite (microwave treatment time 3min, 5min, and 7min
labeled as CdG2ꢀ3, CdG2ꢀ5, and CdG2ꢀ7, respectively) was conducted. The λmax values of
TBA were found to be 275 nm, which are shown in Fig. 10a. It can be seen that the
decolorization of TBA with high concentration is conspicuous in the presenceof sample
CdG2. From Fig. 10b, it was obvious that the heat teatment time can remarkably influence the

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presence of CdLa S hollow nanospheres, which possess enhanced photocatalytic degradation
2
4
effect of CdG2ꢀ7 is superior (86%), while the degradation ratio of CdG2ꢀ3 and CdG2ꢀ5 for
the TBA solution is 58% and 74%, respectively. The results also imply that microwave
5
1
irradiation plays a vital role, possibly related to crystallinity and structure distribution.
To demonstrate the stability of the cyclic performance of the CdG2ꢀ7 photocatalyst,
consecutive photocatalytic TBA degradation reactions were conducted under visible light
irradiation. As shown in Fig. 10c, CdG2ꢀ7 did not exhibit any significant loss of
photocatalytic activity after four runs of TBA degradation. These results indicate that the
CdG2 photocatalyst has high stability, which can be attributed to the strong interactions
52
between CdLa
2 4
S and rGO, graphene modification improves not only the photocatalytic
performance but also the longꢀterm stability of CdLa S nanocrystals while pure CdLa S is
2
4
2 4
1
9ꢀ21
usually stable in the photocatalytic process.
The XRD pattern of CdG2ꢀ7 after four
53,54
recycling experiments was almost identical to that of the asꢀprepared sample (Fig. 10d).
This result is significant from a practical viewpoint, as the enhanced photocatalytic activity
and stability will lead to a more costꢀeffective operation. These results further confirm that
2 4
CdLa S /rGO nanocomposite can be used as a catalytic material in an environmental
purification process or in waste water treatment in the future.
The scheme of charge transfer process between CdLa S and graphene particles is shown
2
4
2 4
in Fig. 11. When CdLa S was illuminated with visible light, the semiconductor produced
ꢀ
+
electrons (e ) and holes (h ) that gained enough energy or momentum to jump the forbidden
ꢀ
gap or energy barrier denoted as (E ). The electrons (e ) traveled from the valence band (VB)
g
ꢀ
to the conduction band (CB) of CdLa
2
S
4
, producing an increased number of electrons (e ) and
+
holes (h ) pairs. Meanwhile, the graphene attached to the semiconductor material (CdLa
2
4
S )
ꢀ
assisted in the smooth transistion of transferring the electrons (e ) to the CB of CdLa
2
S
4
,
thereby increasing the number of electrons as well as the rate of electronꢀinduced redox
reactions. Based on the above discussion, the addition of graphene in the CdLa
2 4
S
nanocomposite not only improved the visible light absorption intensity but also enhanced the
photocatalytic activity of the nanocomposites because of the high charge seperation induced
ꢀ
by the synergistic effects of graphene on CdLa
2
S
4
. The generated electrons (e ) react with the
ꢀ
dissolved oxygen molecules and produce oxygen peroxide radicals O
2
• . The positive charge

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+
ꢀ
55
hole (h ) can react with OH derived from H
2
O to form hydroxyl radicals OH•. The Rh.B
ꢀ
2
and TBA molecules can then be photocataytically degraded by oxygen peroxide radicals O •
and hydroxyl radicals OH• to CO H O, and other mineralization products. The reactions
2
,
2
involved in the charge mobility and mineralization of the dyes are as follows:
+
+
ꢀ
CdLa S /rGO + Visible light (hυ) →CdLa S /rGO + h + e
(1)
2
4
2 4
+
ꢀ
H
2
O →H + OH (2)
+
ꢀ
h + OH → OH• (3)
-
ꢀ
O
2
+ e → O
2
•
(4)
ꢀ
Rh.B/TBA + OH•+ O
2
2 2
• → CO + H O + mineralized by products (5)
Conclusions
In summary, a hollow sphere CdLa
microwaveꢀassisted synthesis method in a domestic microwave oven. The CdLa
by precipitation reaction uniformly distributed on the graphene nanosheets with hollow sphere
within 7 mins was observed. The strong interaction between CdLa and rGO makes the
hollow sphere CdLa /rGO nanocomposite with an optimum concentration of S (0.75mM)
2
4
S /rGO nanocomposite was prepared vis a fast and facile
2 4
S
triggered
2 4
S
2ꢀ
2 4
S
to show highly enhanced and stable visible light photocatalytic activity for degradation of
organic and industrial dyes. This work demonstrates the important role played by microwave
in the preparations of rGOꢀbased hollow sphere materials and provides new insights into the
photocatalytic degradation of pollutants under visible light irradiation.
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Table 1. BET surface areas, textural properties, and visible light photoꢀdegradation rate (kapp
)
constants of asꢀprepared samples.
Sample
CdLa
2
S
4
CdG1
43.08
CdG2
57.74
CdG3
26.44
BET surface
1
7.92
.048
2
area,m /g
Total pore
0
0.085
0.095
0.049
3
volume,cm /g
Average pore
diameter, nm
1
0.8
7.91
7.28
7.42
ꢀ1
ꢀ3
ꢀ3
ꢀ2
ꢀ3
k
app(min )
1.28×10
4.35×10
1.27×10
3.03×10

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(
a)
(
b)

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(
c)
(
d)
Fig. 1. (a) Schematic illustration of deposition of CdLa
XRD patterns of GO, graphene, CdLa , CdG1, CdG2, and CdG3, (c) FTꢀIR spectra of GO
and CdG2, (d) Raman shift of GO and CdG2.
2 4
S nanoparticles on the GONS, (b)
2 4
S

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Quantitative results
80
60
40
20
0
C
S
Cd
La

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(d)
2 4
Fig. 2. SEM micrographs of asꢀprepared samples: (a) graphene, (b) CdLa S nanoparticle, (c)
CdG2 nanocomposite, (d) EDX elemental microanalysis and element weight % of CdG2
nanocomposite.

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2 4
Fig. 3. TEM micrographs of asꢀprepared samples: (a) graphene, (b) CdLa S nanoparticle, (cꢀ
f) TEM and HRTEM images of CdG2 nanocomposite.
2
Fig. 4. (a) UVꢀvis absorbance spectra (b) Variation of (αhv) versus photon energy
(
hv) of CdLa S , CdG1, CdG2, and CdG3.
2 4

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Fig. 5. XPS results of CdG2 nanocomposite photocatalyst (a) survey scan spectra, (b)
C1s, (c) O1s, (d) La3d, (e) S2p, (f) Cd3d.

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2 2 4
Fig. 6. N adsorption isotherms of CdLa S , CdG1, CdG2, and CdG3, as well as BJH pore
size distributions obtained from the CdG2 composite.

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Fig. 7. (a) UV/Vis spectra of Rh.B concentration against the asꢀprepared samples after
ꢀ5
adsorption–desorption equilibrium. (b) Degradation of Rh.B dye (3×10 mol/L, 50 mL) with
different samples (0.03g). (c) Apparent first order kinetics of Rh.B degradation over CdLa
2 4
S ,
CdG1, CdG2, and CdG3 under visible light irradiation.
Fig. 8. (a) UVꢀvis spectra of DPCO extract liquors in the presence of different samples
under visible light irradiation for 30min. (b) UVꢀvis spectra of DPCO extract liquors in
the presence of CdG2 for different irradiation times.
(
a)
(
b)

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Fig. 9. (a) Schematic illustration of the formation of the CdLa S (0.75mM) nanostructures.
2
4
(
b) HRTEM images of the asꢀprepared CdLa
2 4
S (0.75mM) nanostructure immobilizing on
reduced graphene oxide at different reaction times (3min, 5min, and 7min). (c)
Reproducibility study of synthesis CdLa
.
2 4
S by microwave at 7min for 3 times.

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