Facile synthesis of CeO2 nanoparticle sensitized CdS nanorod photocatalyst with improved visible-light photocatalytic degradation of rhodamine B

Article abstract of DOI:10.1039/c5ra16114b

A heterostructure photocatalyst consisting of one-dimensional (1D) CdS nanorods (NRs) and cerium dioxide (CeO₂) nanoparticles (NPs) was successfully synthesized via a solvothermal method. Different characterization techniques confirmed that CeO₂ NPs were intimately attached on CdS NRs. The visible-light-driven photocatalytic activity was evaluated by the discoloration of rhodamine B (RhB). It was indicated that, under visible light illumination, the photocatalytic rate constant of CdS/CeO₂ was nearly 3.4 times and 28 times higher than that of pure CdS and pure CeO₂, respectively, owing to the high oxygen storage capacity of the CdS/CeO₂ heterostructure, and the inhibition of electron-hole pair recombination benefitting from efficient electron transfer from CdS NRs to CeO₂ NPs. In addition, the degradation mechanism of RhB on CdS/CeO₂ was also discussed. This durable nanocomposite catalyst, with its excellent combination of CdS NRs and CeO₂ NPs, is able to be a promising catalyst for RhB degradation under visible-light irradiation.

Full text of DOI:10.1039/c5ra16114b

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Facile synthesis of CeO2 nanoparticles sensitized CdS nanorods
photocatalyst with improved visible-light photocatalytic degradation
of Rhodamine B
Shansi Gu a, b, Yaoning Chen a, b *,Xingzhong Yuan a, b, c**, Hou Wang a, b, Xiaohong
Chen c, Yang Liu a, b, Qian Jiang a, b, Zhibin Wu a, b, Guangming Zeng a, b
a College of Environmental Science and Engineering, Hunan University, Changsha
410082, PR China
b Key Laboratory of Environment Biology and Pollution Control, Hunan University,
Ministry of Education, Changsha 410082, PR China
c
Collaborative Innovation Center of Resourceꢀconserving & Environmentꢀfriendly
Society and Ecological Civilization, Changsha 410083, PR China
* Corresponding author at: College of Environmental Science and Engineering, Hunan University, Changsha
410082, PR China. Tel.: +86 73188821413; fax: +86 73188821413
Eꢀmail address: cyn@hnu.edu.cn (Y.N. Chen)
** Corresponding author at: College of Environmental Science and Engineering, Hunan University, Changsha
410082, PR China. Tel.: +86 73188821413; fax: +86 731 88823701.
Eꢀmail address: yxz@hnu.edu.cn (X.Z. Yuan)
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Abstract：A heterostructures photocatalyst consisting of oneꢀdimensional (1D) CdS
nanorods (NRs) and cerium dioxide (CeO2) nanoparticles (NPs) were successfully
synthesized via a solvothermal method. Different characterization techniques
confirmed that CeO2 NPs were intimately attached on CdS NRs. The visibleꢀlightꢀ
driven photocatalytic activity was evaluated by the discoloration of Rhodamine B
(RhB). It was indicated that, under visible light illumination, the photocatalytic rate
constant of CdS/CeO2 was nearly 3.4 times and 28 times higher than that of pure CdS
and pure CeO2, respectively, owing to the high oxygen storage capacity of CdS/CeO2
heterostructure, and the inhibition of electron–hole pair recombination benefited from
efficient electron transfer from CdS NRs to CeO2 NPs. In addition, degradation
mechanism of RhB on CdS/CeO2 was also discussed. This durable nanocomposite
catalyst, with its excellent combination of CdS NRs and CeO2 NPs, is able to be a
promising catalyst for RhB degradation under visibleꢀlight irradiation.
Keywords: CdS/CeO2 heterostructures; visibleꢀlight photocatalysis; Rhodamine B;
2

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1. Introduction
Photocatalytic degradation of pollutants through semiconductor compounds has
become a promising approach to solve environmental problems during the last few
decades. Among all semiconducting nanostructures, oneꢀdimensional (1D)
nanostructures have captured particular attention. At first, 1D geometry can result in a
fast and longꢀdistance electron transport. Moreover, their high lengthꢀtoꢀdiameter ratio
can enhance the light absorption and scattering. At last, 1D structures own larger
specific surface area that cannot be obtained in their bulk counterparts1ꢀ4.
7
8
Metal chalcogenides, such as CdS5, ZnS4， CuS6， In2S3 and SnS2 , have been
received growing attention on account of their suitable band gap which belongs to the
visible spectrum. Among these, CdS, with a direct band gap of 2.4 eV, has been
extensively investigated in numerous applications, such as gas sensers9, lasing10 and
waveguide material11, solar cell12, environment purification13 and hydrogen evolution3,
14, owing to its relatively high ability in harvesting visible light, favorable conduction
band potential and inexpensive cost14, 15. However, the quick recombination of photoꢀ
generated charge carriers and the serious photocorrosion property are two major
defects existing in the CdS catalyst14. In order to solve these problems, different
strategies included element doping16, fabricating nanoꢀscaled CdS of different
geometric shapes (i.e. rings10, belts17, tubes18 and rods19) and sensitization with other
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metal oxides14, 19 have been adopted. Particularly, heterostructures photocatalyst
composed of CdS NRs and metal oxides opens up a new window to construct highꢀ
quality and highꢀefficiency visibleꢀlightꢀdriven photocatalysts14, 19, 20. Khan et al.14
reported a hierarchical threeꢀdimensional NiOꢀCdS heteroarchitecture, which showed
an ultraꢀhigh photocatalytic activity for hydrogen generation. The enhanced hydrogen
production rate was mainly ascribed to the enlarged specific surface area and
reduction of the recombination rate of electron–hole pairs. Moreover, 1D CdS/αꢀ
Fe2O3 nanostructures possessed higher activity than CdS NRs for photocatalytic
degradation of Methylene Blue under visibleꢀlight illumination13. On the other hand,
23
earth metal oxides, such as La2O321, Pr6O1122 and CeO2 , are usually chosen as coꢀ
catalysts to decorate other semiconductors for the fabrication of heterstructure
photocatalysts. Among all, CeO2 is considered to be a promising catalyst for catalytic
organics degradation benefiting from its UV absorbing ability, high electrical
conductivity, extensively documented oxygen storage capacity23, multiple redox states
as well as extended stability23ꢀ26. It is well established that CeO2 and its based
composites have been explored in water dissociation27, CO oxidation28 , pollutants
degradation29, hydrodenitrogenation30 and hydrogen production31. Accordingly,
sensitization titanium dioxide (TiO2) nanotube array with CeO2 NPs was conducted.
The combination showed largely enhanced charge storage capacity owing to the
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increased number of surface active sites induced by the CeO2 NPs25. To the best of
our knowledge, there is no report concerning the synthesis of CeO2 nanoparticles
sensitized CdS nanorods heterstructures photocatalyst for the application of dye
degradation.
In this work, we demonstrated the photocatalytic degradation of Rhodamine B
under visible light illumination using a novel CdS nanorods/CeO2 nanoparticles
heteroarchitecture fabricated by a facile hydrothermal route. The degradation
mechanism of RhB dyes on CdS/CeO2 was proposed.
2. Experimental
For the sample preparation, characterization and photocatalytic test have been
shown in the Experimental section in the Supporting Information.
3. Results and discussions
3.1. FE-SEM and TEM images
The detailed morphology and size of the synthesized products were visualized by
SEM and TEM. As shown in Fig. 2a, the highly ordered CdS NRs displayed an
average rod diameter of about 100 nm with a length of 1–2 ꢁm. From Fig. 2b and 2c,
it can be found that the CeO2 NPs with an average diameter around 12 nm were
anchored on the wall of the CdS NRs. Acting as a “spacer”, the CeO2 NPs on CdS
NRs can prevent the CdS NRs from aggregation32. HRTEM measurements were
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further performed to investigate the detailed shape and crystal facets of the CdS/CeO2
heterostructure. As demonstrated in Fig. 2e, the different orientation and lattice fringe
of CdS NRs and CeO2 NPs can be clearly observed. One interplanar distances of 0.34
nm can be indexed to the (002) direction of hexagonal CdS (d = 0.336 nm). The
crystallite attached to the CdS with dꢀspacing of 0.315 nm matched well with the
crystal facet (111) of cubic CeO2. The presence of integration interface between CdS
(002) and CeO2 (111) further confirmed the strong interaction between CdS and CeO2
in CdS/CeO2.
3.2. X-ray diffraction analysis
XRD patterns of the asꢀprepared CdS NRs, CeO2 NPs and CdS/CeO2
nanocomposite were shown in Fig. 3. It was observed that the phase of CdS/CeO2
samples was composited of cubic structure of CeO2 (JCPDS 43ꢀ1002) and hexagonal
structure of CdS (JCPDS 41ꢀ1049). For CdS NRs, the characteristic peaks at 2θ
values of 24.8°, 26.5°, 28.2°, 43.7°, 47.8° and 70.8° can be indexed to the (100), (002),
(101), (110), (103) and (211) planes of hexagonal wurtzite crystal structured CdS NRs,
respectively. The peaks of cubic fluorite structured CeO2 at 33.1°, 56.3° and 69.4°
were originated from the (200), (311) and (400) diffractions, respectively. Though
some crystal plane diffraction peaks of CeO2 may overlapped with the peaks of CdS,
two main peaks {(200) and (311)} of CeO2 appeared in the XRD profiles of
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CdS/CeO2 confirmed the existence of CeO2 NPs on CdS NRs.
3.3. X-ray photoelectron spectroscopy (XPS) analysis
To investigate the chemical and oxidation states for Cd, S, Ce and O, XPS
analysis was carried out. The spin–orbit components (3d5/2 and 3d3/2) of the Cd 3d
peak were well deconvoluted for CdS/CeO2 nanocomposites by two curves at
approximately 411.93 eV and 405.17 eV (Fig. 4a), manifesting that cadmium was in
the Cd2+ state in the samples15. Meanwhile, the determined binding energies of S 2p
were found located at 161.51 eV and 162.7 eV (Fig. 4b), corresponding to sulfide
species (S2−) of CdS/CeO2 composites5. Fig. 4c showed the profile of high resolution
XPS spectra of Ce 3d in CdS/CeO2 composites. It was found that a small amount of
Ce3+ (885.4 and 904.1 e V) were coexisting with Ce4+ (882.1 eV, 888.9 eV, 898 eV,
900.3 eV, 907.6 eV and 916.6 eV) in the nanocomposites28. By fitting the curves and
calculating the Ce3+/ (Ce3+ + Ce4+), a pronouncedly high Ce3+ ratio of 20.5% on
CdS/CeO2 was identified, which was higher than that of CeO2/graphene (18.5%)
reported in earlier study33. Usually, Ce3+ content is proportional to oxygen vacancies,
the utraꢀhigh Ce3+ ratio denote that a certain amount of oxygen vacancies were
introduced in the nanocomposites34. This result was further confirmed by the XPS
result of the O 1s region depicted in Fig. 4d. The O 1s spectrum was matched with
three peaks at 529.12 eV, 531.94 eV and 533.68 eV. The distinct shoulder detected at
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531.94 eV was corresponded to the adsorbed oxygen and weakly bonded oxygen
species; the peak with the highest binding energy belonged to oxygen by hydroxide or
adsorbed water on the catalyst surface. Both of the peaks illustrated above confirmed
the existent of plenty of surface oxygen vacancies in CdS/CeO2. Meanwhile, the
weakest peak, located at 529.12 eV, was found attributed to lattice oxygen in the
anchored CeO2 nanocrystals28, 29, 35. Generally, surface adsorbed oxygen is more
reactive than lattice oxygen in oxidation reactions on account of its higher mobility35.
Hence, the higher surface adsorbed oxygen amount on CdS/CeO2 was helpful for the
photocatalytic pollutant degradation, leading a significantly enhanced photocatalytic
ability of CdS/CeO2 for the degradation of RhB.
3.4. N2 adsorption–desorption
The specific surface areas were recorded from the N2 adsorptionꢀdesorption
isotherms using the BET analysis method. Fig. 5 showed the nitrogen adsorptionꢀ
desorption isotherms for pure CdS, CeO2 and CdS/CeO2 (RC = 20) compound.
Apparent hysteresis loops can be distinguished on the adsorption–desorption
isotherms of CdS nanorods, CeO2 nanoparticles and CdS/CeO2, indicating the
existence of porous structures. The surface area data are summarized in Table 1. As
we can see, the total pore volume of CdS/CeO2 is increased a lot, which may come
from the voids between the aggregated CeO2 particles and CdS rods. Besides, the
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surface area increased by addition of CeO2 to CdS. For the pristine CdS nanorod, a
specific surface area of 13.1 m2/g was obtained, whereas for CdS/CeO2 sample (RC =
20), the specific surface area was 21.8 m2/g. The higher BET surface area for
CdS/CeO2 nanocomposites was presumed to result from the CeO2 NPs (24 m2/g)
linked on CdS NRs depressed the agglomeration of CdS NRs. Consequently, the
increased surface area might be one of the reasons responsible for the enhanced
photocatalytic activity.
3.5. UV–vis diffuse reflectance spectroscopy analysis
The Ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy was employed
to measure the optical properties of the samples. The corresponding absorption
spectra of CdS, CeO2 and CdS/CeO2 (Rc = 20) were displayed in Fig. 6. It was
obvious that CdS/CeO2 nanocomposite exhibited a CdS absorption peak and a feature
band edge of individual CeO2. Compared with pristine CdS and CeO2, CdS/CeO2
appeared spectral response in a broader visibleꢀlight region ascribed to the
photosentisizing effect of CeO2. According to the following equation:
(ɑℎꢀ)ꢁ = A(ℎꢀ − Eꢂ)
(1)
Where α is the absorption coefficient, v is the light frequency, A is a constant, Eg is
the band gap energy, and n is 1/2 and 2 for CdS and CeO2, respectively. The Eg of the
asꢀprepared samples can be calculated from a plot depicting (αhv) n versus (hv) (shown
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in Fig S2). The extrapolated values (the straight line to the x axis) of (hv) at α=0 gives
absorption edge energies consistent with Eg = 2.37 eV and Eg = 2.95 eV for CdS and
CeO2, respectively.
3.6. Photocatalytic activity
3.6.1. Effect of the amount of CeO2 nanoparticles loading on CdS nanorods
Decomposition of RhB (C0 = 40 mg/L) under the identical conditions was carried
out to determine the activities of samples with different molar ratio. The UVꢀvis
spectral change of CdSꢀCeO2/RhB dispersion after 60 min adsorption in the dark was
depicted in Fig. S3. It was clear that no wavelength shift of the band at 554 nm was
observed, indicating RhB was not decomposed during the dark reaction. The
decreased absorbency of CdSꢀCeO2/RhB dispersion was corresponding to the extent
of RhB adsorption on CdS/CeO2. The results of RhB degradation over CdS, CeO2 and
CdS/CeO2 photocatalysts were shown in Fig. 7. It was noticeable that direct visible
light irradiation of the RhB dye solution in the absence of catalysts was
inconsequential toward degrading the dye. The photoactivity of the sample was
strongly dependent on the RC value. With RC increasing from 0 to 20, the
photoactivity of the sample for the RhB photodegradation was enhanced. The
maximum degradation efficiency reached to 96.68% after 48 min for the sample (RC =
20). However, when RC further increased to 30, the photoactivity of the sample
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decreased instead. As expected, all samples showed higher photoactivities for the RhB
degradation than that of pure CdS and CeO2 under visible light (λ ≥ 420 nm). Kinetics
of the photocatalytic degradation of RhB was a pseudoꢀfirstꢀorder36, which can be
explained in the LangmuirꢀHinshelwood model:
ꢀ ln (Ct/C0) = kt
(2)
The apparent rate constant (k) for the degradation of RhB on these samples were
calculated by plotting ln(C0/Ct) versus t (Fig. 8), and the results were shown in Table
2. The excellent photocatalytic performances of CdS/CeO2 heterostructure for RhB
degradation may be due to that (i) high oxygen storage capacity of CeO2 provided
immense sources of active species; (ii) the strong interactions between CdS and CeO2
effectively prevented the photoꢀcorrosion and leaching of CdS; and (iii) effective
charge separation reduced the recombination rates of electronꢀhole pairs. The
influence on RhB degradation was also investigated and displayed in the Supporting
Information.
3.6.2. Analyses of Intermediate Products.
The temporal evolution of the spectral changes during the photodegradation of
RhB over CdS/CeO2 and CdS were given in Fig. 9a and 9b. It was noteworthy that
original RhB depicted a major absorption band at 554 nm, and no shift of the
adsorption band was observed after dark reaction for 1h and visible light illumination
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for 48 min (Fig. S4). In the presence of CdS, the absorption peak of RhB solution
decreased slowly and the absorption band shifted from 554 to 548 nm within 48 min
irradiation. By contrast, absorbance of the absorption peak at 554 nm decreased
rapidly to almost zero and the wavelengths of the characteristic peaks shifted from
554 nm to 500 nm as the photodegradation reaction proceeded in the presence of
CdS/CeO2. Obviously, hypsochromic shifts of the absorption maximum observed with
CdS/CeO2 suspension were more pronounced than those observed with the CdS
system. These hypsochromic shifts were assumed to result from the formation of a
series of Nꢀdeethylated intermediates of RhB in a stepwise manner and the cleavage
of the conjugated chromophore structure21, 37, 38. To comprehend the photocatalytic
degradation process better, intermediates of RhB during the photodegradation were
further confirmed by HPLC monitoring equipped with a UVꢀvisible diode array
detector. The HPLC chromatogram of 0, 8, 16, 24, 32, 40 and 48 min filtrate of
RhB/CdSꢀCeO2 (RC = 20) system were illustrated in Fig. 9c. The maximum
concentration of RhB might appeared at the retention time of 4.42 min (peak a). Four
other peaks (Fig. 9c, peak b–e), which may belong to Nꢀdeꢀethylated intermediates,
namely, N,NꢀdiethylꢀN'ꢀethylrhodamine (DER), N,Nꢀdiethylrhodamine (DR), Nꢀ
ethylꢀ N'ꢀethylrhodamine (EER), Nꢀethylrhodamine (ER) or Rhodamine (R) were also
observed37, 38. With the quick diminishing of RhB, the concentration of other Nꢀ
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deethylated intermediates increased successively and subsequently decreased in
succession with further irradiation, which revealed that the Nꢀdeethylation of RhB
was a stepwise course. It was found that the first Nꢀdeethylated intermediate reached
its maximum concentration after 16 min irradiation, whereas the final Nꢀdeethylated
product got disappeared after 40 min irradiation. Based on these results, it was
concluded that the RhB was effectively decomposed rather than merely bleached
during the CdS/CeO2ꢀassisted photodegradation. The fragmentation pathway of RhB
photocatalytic degradation under visible light irradiation in CdS/CeO2 suspension was
tentatively proposed as depicted in Fig. 10. Moreover, the total organic carbon (TOC)
values of the solution at different time intervals during the RhB degradation was
measured (Fig. S5). After 48 min of irradiation, a relatively high mineralization rate of
86% of TOC removal was achieved, indicating that RhB has been mineralized
efficiently under visible light irradiation.
3.6.3. Recycle test
To probe the practical reusability of the CdS/CeO2 catalyst in the decomposition
of RhB, recycling tests were carried out by repeated reaction cycles under the same
reaction condition. As shown in Fig. 11, the catalytic activity of CdS/CeO2 did not
significantly decrease after six continuous cycles, in which the RhB removal
efficiencies of CdS/CeO2 (Rc = 20) were all above 90%. In sharp contrast, RhB
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removal efficiency of pure CdS for the firstꢀcycle test was about 64%, and then
obvious deactivation can be observed on CdS after four times recycles, indicating that
the CdS/CeO2 owned a superior reusability in RhB degradation. To further
demonstrate the stability of the catalyst, leaching of cadmium from CdS/CeO2 in
solution after six cycle test was measured by via a PerkinꢀElmer Analyst 700 atomic
absorption spectrophotometer. For comparison, Cd content in solution after four cycle
test for CdS catalyst was also detected. Results (inset of Fig. 11) indicated that the Cd
ions released rapidly from CdS solid into the solution, which should be resulted from
CdS photocorrosion. As for CdS/CeO2, rather low Cd species in CdS/CeO2 leached
off even after being used six successive times. Moreover, there is no distinction
between the XRD results of CdS/CeO2 samples before and after the reaction (Fig. S6),
suggesting its structural stability during the photocatalytic oxidation of the pollutant
molecules. The improved stability of CdS/CeO2 may be ascribed to the intimate
interfacial interaction39, 40 between CdS and CeO2 and the introducing of high thermal
stability CeO2.
3.6.4. The proposed mechanism of RhB photodegradation
In order to determine the main active species responsible for the degradation of
RhB over the optimum CdS/CeO2 composite (RC = 20), controlled photoactivity
experiments were performed with adding different radical scavengers (0.1 mmol) at
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the natural pH of the dye. Fig. 12 reflected the photocatalytic activities of CdS/CeO2
(RC = 20) for the degradation of RhB in the presence of different radical scavengers,
i.e., Ethylene Diamine Tetraacetic Acid (EDTA) scavenger for h+41, isopropyl alcohol
(IPA) for •OH and benzoquinone (BQ) for O2•−, respectively. It was apparent that the
order of affecting the rate for degradation of RhB followed that of O2•− > h+ > •OH. It
was shown that significant suppressing effect was obtained after addition of BQ (O2•−
quencher), suggesting the superoxide anion radical was the major oxidative species
responsible for the photooxidative conversion of RhB36, 42. The band edge positions of
the CB and VB of CdS and CeO2 was determined according the following equations:
ꢅ
⁄
Eꢃꢄ = X − E + 1 2Eꢂ
(3)
(4)
Eꢆꢄ = Eꢃꢄ − Eꢂ
Where X is the absolute electronegativity of the semiconductor (X is 5.18 eV and 5.57
eV for CdS and CeO2, respectively), Ee is the energy of free electrons on the hydrogen
scale (4.5 eV) and Eg is the band gap of the semiconductor26. The calculated CB and
VB of CdS were ꢀ0.505 and 1.865 eV, respectively, which were more negative than
those of CeO2 (ꢀ0.405 and 2.545 eV, respectively). The standard redox potential of O2/
O2•− and •OH/OH− is ꢀ0.33 and 2.38 V vs NHE42, respectively. That is, the CB
potentials of CeO2 and CdS were both negative than the standard redox potential of
O2/ O2•−, whereas the standard redox potential of •OH/ OH− was positive to the VB of
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CdS. As a result, the photogenerated electrons on irradiated CdS/CeO2 heterostructure
can reduce O2 to give O2•−, while the photogenerated holes can hardly oxidize OH to
give •OH, therefore O2•− was the protagonist in RhB photodegradation by CdS/CeO2.
Meanwhile, colorless organic molecules which do not absorb in visible regions were
usually used for the evaluation of photocatalytic activity of photocatalyst36,42ꢀ44
.
Therefore, colorless bisphenol A (BPA) and salicylic acid (SA) were chosen as the
target pollutants. Prior to irradiation, the suspension was stirred for 1 h in dark to
reach adsorption–desorption equilibrium. The sample was then irradiated with visible
light with continuous stirring. After 1 h irradiation, the BPA and SA removal ratio of
CdS/CeO2 (Rc = 20) was 31.4% and 46.3%, respectively. While the pure CdS showed
a degradation ratio of 5.2% and 7.7%, for BPA and SA, respectively. Therefore, it
was suggested that the CdS/CeO2 composites is indeed a visibleꢀlight induced
photocatalyst. Besides, considering the redox potential of E0
•+( –1.09 V vs.
RhB*/RhB
SHE) and E0 RhB•+/RhB (1.46 V vs. SHE)42, photoꢀinduced electrons can transfer from
photoexcited dye (RhB*) to the conduction bands of CdS, which has also been
reported by Takuo et al45, 46. Based on the above discussion, a tentative illustration of
possible interparticle electron transfer behavior can be proposed, as illustrated in Fig.
13. Under visibleꢀlight irradiation, photoꢀinduced electrons easily transferred from
photoexcited dye (RhB*) to the CB of CdS to form RhB+•. These transferred electrons
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•−
captured by adsorbed oxidants to generate active species (e.g., O2 ). Afterwards,
RhB+• could be consumed by these generated O2•− radicals. On the other hand, the
electrons could be photoexcited directly from the VB of CdS NRs to the CB of CdS
by adsorbing light, forming electronꢀhole pairs. Immediately, the generated electrons
of CdS transferred to the CB of CeO2. After that, the transferred electrons trapped by
the adsorbed oxygen on the surface of the catalyst to produce reactive oxygen radicals
(O2•−). Finally, RhB dyes adsorbed on the surface of CdS/CeO2 was attacked by the
activated oxygen to decolorize the solution. Simultaneously, holes generated in
CdS/CeO2 can directly react with adsorbed dyes, making contributions to the RhB
degradation process. The overall decomposition of RhB can be expressed by the
following formula:
RhB + hν → RhB*
(5)
RhB* + CdS → RhB•+ + CdS (e–)
CdS + hν → CdS (e– + h+)
(6)
(7)
CdS (e–) + CeO2 → CeO2 (e– )
(8)
•–
e–
) + O2 → O2
(9)
(CB,CeO
2
RhB/ RhB•+ + (O2•– + h+) → mineralized products→CO2+H2O
(10)
4. Conclusions
Nanocomposites made with CdS NRs and CeO2 NPs were successfully
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synthesized using a hydrothermal method. FEꢀSEM and TEM analyses showed that
the CeO2 NPs were anchored on the wall of the CdS NRs strongly. XPS spectra and
XRD result confirmed the formation of CdS/CeO2 heterostructures. The CeO2 played
a vital role in achieving excellent photocatalytic performance of the synthesized
composites toward degradation of RhB due to its large oxygen capacity and
facilitation for the electronꢀhole pair separation. RhB was decomposed around 97%
under visible light over CdS/CeO2, which was much enhanced than original CdS NRs
(64%) and CeO2 NPs (12%). The sample (RC = 20) had the highest kinetic rate
constant (0.07min−1) owing to the effective charge separation and excellent redox
capacity of photocatalysts. Results also demonstrated that the degradation efficiency
increased with decreasing pH values. It is hoped that more ongoing research efforts
could be paid to the earth metal oxides and the fabrication of 1D semiconductor
nanostructures.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (No.
71431006, 21276069), the State Scholarship Fund and the Fundamental Research
Funds for the Central Universities.
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Fig. 1. Chemical structure of Rhodamine B.

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Fig. 2. (a) SEM images of CdS nanorods; (b) SEM image of CdS nanorods decorated with CeO2
nanoparticles; (c) TEM image of 1D CdS/CeO2 nanocomposites; (d) Magnified TEM image of
CdS/CeO2 heterostructure; (e) HRTEM images of the rectangular regions.

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Fig. 3. XRD pattern of CdS/CeO2 nanocomposites (top); CdS nanorods (middle); CeO2 nanoparticles
(bottom).

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Fig. 4. High-resolution XPS spectrum of the (a) Cd 3d, (b) S 2p, (c) Ce 3d and (d) O 1s.

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Fig. 5. N2 adsorption–desorption isotherms for CdS (the magenta line), CeO2 (the gray line) and
CdS/CeO2 (the blue line). (Inset: the corresponding pore size distribution)

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Fig. 6. UV-vis DRS of CdS NRs, CeO2 NPs and CdS/CeO2 (RC = 20) heterostructures.