Advanced Materials Research
ISSN: 1662-8985, Vols. 335-336, pp 460-463
doi:10.4028/www.scientific.net/AMR.335-336.460
© 2011 Trans Tech Publications, Switzerland
Online: 2011-09-02
Hydrothermal synthesis and photocatalytic properties of flower-like CdS
nanostructures
Hongmei Wang1,a*, Dapeng Zhou2,b*, Yuan Lian3,c, Ming Pang4,d, Dan Liu5,e
1,2,3,5
Department of Biological and Chemical Engineering, Jiaxing University, Jiaxing 314001, China
4
Jiaxing Environmental Monitoring Station, Jiaxing 314000, China
a
[email protected], [email protected], [email protected],
d
[email protected], [email protected]
Keywords: CdS; Hydrothermal process; Nanostructures; Photocatalytic properties
Abstract: Hexagonal flower-like CdS nanostructures were successfully synthesized through a facile
hydrothermal method with thiourea as sulfur source. By combining the results of X-ray diffraction
(XRD), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy
(FE-SEM), transmission electron microscopy (TEM), the structural and morphological
characterizations of the products were performed. The photocatalytic activity of CdS nanostructures
had been tested by degradation of Rhodamine B (RB) under UV light compared to commercial CdS
powders, which indicated that the as-syntherized CdS nanostructures exhibited enhanced
photocatalytic activity for degradation of RB. The possible growth mechanism of CdS
nanostructures was proposed in the end.
Introduction
II–IV binary compound semiconductors possess many interesting physical properties and have
potential applications in electronic and optical devices [1]. Of the binary compounds, CdS is the most
typical inorganic semiconductor materials. It plays important roles in both basic science and
application fields [2]. For instances, CdS crystal could be used in photovoltaic cells, radiation
detection devices, infrared windows, phosphor and photoconductor research. Therefore, perfect
single crystals of CdS are expected to be rewarding and are widely studied for many years.
A variety of methods have been developed to prepare the CdS crystals [3], including solid phase
reaction, gas phase reaction, sol-gel process, hydrothermal or solvothermal route, gammairradiation technique. CdS with particular structures like nanowires, nanorods, nanotubes,
microspheres, and so on have been successfully synthesized by various physical and chemical
methods. Here, a convenient hydrothermal process was applied to synthesize the flower-like CdS
nanostructures using thiourea as sulfur source at 180 °C for 24 h. Comparing to other sulfur source,
the action of thiourea on the formation of flower-like CdS nanostructures was revealed.
Experimental
The flower-like CdS nanostructures were synthesized by the hydrothermal process. In brief, 35 ml
0.02 M thiourea was added into the aqueous solution of 35 ml 0.01 M CdCl2 under stirring. After 30
min stirring, the aforementioned solution was transferred into a Teflon lined stainless steel
autoclave with 100mL capacity, sealed and maintained at 180 °C for 20 h. Then, the precipitates
were filtered and washed with distilled water and ethanol several times to remove the impurities.
The products were dried in vacuum at 70 °C for 6 h.
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Advanced Materials Research Vols. 335-336
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The as-prepared sample was characterized by X-ray power diffraction on a D8 Advanced X-ray
diffractometer. XPS measurement was performed on a PHI-5300/ESCA system. The field emission
scan electron microscope (FESEM) image was obtained on Hitachi S-4800. Transmission electron
microscopy (TEM) observation was performed on a JEM-2010 microscope. The photocatalytic
activity of the product was also evaluated by using MB as a model organic compound. The
photocatalytic activity experiments on the obtained CdS nanostructures for the decomposition of
Rhodamine B were performed at ambient temperature. The concentration of RB was monitored by a
Shimadzu UV–vis spectrophotometer (UV-2550).
Results and discussion
Fig.1. X-ray diffraction (XRD) pattern of the as-synthesized materials
Fig. 1 shows a typical XRD pattern of the as-synthesized products. All the diffraction peaks can be
indexed to hexagonal CdS with lattice constants of a = 4.131 Å and c = 6.708 Å, which is consistent
with the literature data of JCPDS 77-2306 [4]. Compared with the standard diffraction pattern, no
peaks of impurities were detected, indicating the high purity of the products.
Fig.2. High-resolution CdS spectra taken for Cd 3d (a) and S 2p (b) of the CdS nanostructures
The composition and purity of the as-synthesized CdS nanostructures are further investigated by
XPS (as shown in Fig.2). The XPS data of the sample indicates the presence of Cd and S. No other
obvious impurities are found on the surface of the samples, indicating that they are relatively pure.
From Fig.2(a) and 2(b), it is obvious that the binding energy of peaks Cd(3d5/2) and S(2p3/2) are
405.0 and 161.7 eV, respectively. The Cd(3d5/2) and S(2p3/2) peak areas were determined for the
quantitative elemental analysis of Cd and S in the products, and an atomic ratio of 1:1 was obtained,
which further confirms that the products are pure CdS.
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Advanced Materials and Structures
Fig. 3(a) shows the FESEM image of the CdS nanostructures by the hydrothermal method using
thiourea as sulfur source. The morphology of flower-like can be observed from the FESEM image.
The typical flower-like CdS nanostructures consisted of CdS nanorods with about several hundred
nanometers in width and several micrometers in length, which protrude from the root of the
flower-like CdS nanostructures. Individual CdS nanorod has a cusp end similar to a sword with the
rough surface, which consists of CdS nanocrystals. The further characterization of abovementioned
sample has been performed by TEM as shown in Fig. 3(b). The morphology of flower-like CdS
nanostructures has also been observed. The CdS nanostructures grow in the two sides along the
sword-like CdS main core just like fish-bone. The selected area electron diffraction (SAED) pattern
performed on the sword-like CdS main core and inserted in Fig. 3(b) shows high light diffraction
spots. The above observation indicates that the flower-like CdS nanostructures are of high
crystallinity and can be indexed as the hexagonal CdS phase, which is in accordance with the XRD
results in Fig. 1.
Fig.3. Morphological characterization of flower-like CdS nanostructures: (a) FESEM image; (b) TEM image
(The upper inset corresponds to the SAED pattern of a rod-like CdS main core)
Fig.4. Photodegradation of Rhodamine B under UV light:
Fig.5. Cycling degradation curves in the presence
(1) without any photocatalyst; (2) with commercially
of CdS nanostructures under UV light irradiation
available CdS (10 mg); (3) with as-synthesized CdS
nanostructures (10 mg).
To demonstrate the potential applicability in photocatalysis of the obtained CdS nanostructures,
we investigated their photocatalytic activity by choosing photocatalytic degradation of Rhodamine
B. As a comparison, we used 10 mg of the commercially available CdS to degrade Rhodamine B at
the same conditions. The results were shown in Fig. 4. Without any catalyst, the concentration of
Rhodamine B solution almost did not change after 60 min (curve 1). The activity increased in turn
for commercially available CdS (curve 2), as-prepared CdS microspheres (curve 3). The Rhodamine
Advanced Materials Research Vols. 335-336
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B solution was decolorized completely by using the CdS nanostructures after UV irradiation more
than 60 min, which was apparently shorter than that of commercial CdS powders. The superiority of
photocatalytic performance of the CdS nanostructures should be attributed to their special structural
features [5]. The advantages of CdS nanostructures are a high surface-to-volume ratio with effective
prevention from further aggregation to maintain the high catalytic activity area arising from the
rough surface structure.
The stability of the photocatalyst is very important for industrial application. Thus, in this
experiment, CdS nanostructures were recycled after bleaching the RB under UV light-irradiation
and were reused four times in the decomposition of RB to test the chemical stability (Fig. 5). After
four recycles for the photodegradation of RB, the catalyst did not exhibit obvious loss of activity
which indicated a high stability of CdS nanostructures in photocatalytic reaction. Our investigation
also indicates that other colorless organic pollutants, such as phenol, are also quickly decomposed
by the as-prepared CdS samples under visible-light irradiation.
The formation of flower-like CdS nanostructures can be explained as follows: During the
hydrothermal process, it is decomposed slowly to release free S2−. Then CdS nuclei are formed in
suitable supersaturation and begin to grow into CdS sheet according to the following reaction:
(NH 2 ) 2 CS + 2H 2 O = CO 2 + 2NH 3 + H 2 S
(1);
Cd 2 + + H 2 S → CdS + 2H +
(2).
Due to hexagonal structure of CdS, the growth rate along c axis is usually the fastest and the
rod-like morphology is frequently obtained. In addition, for the slowly decomposition rate of
thiourea, enough sulfur source can be subsequently provided to proceed secondary growth, which is
also reported previously [6]. Therefore, the rod-like subbranch of CdS is formed on the rod-like main
core.
Conclusions
The flower-like CdS nanostructures had been prepared by the hydrothermal method. The
flower-like CdS nanostructures were single crystalline and hexagonal phase, which consisted of
sword-like CdS nanorods. The obtained CdS nanostructures displayed very high photocatalytic
activity and were much more efficient than that of the commercial CdS powders. The good
photocatalytic performance of the CdS nanostructures should be attributed to their special structures
and implied potential applications in other fields, such as solar cell and so on.
Acknowledgement
The authors acknowledge the support of the jiaxing science and technology projects (No.
2009AY2062).
References
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Advanced Materials and Structures
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Hydrothermal Synthesis and Photocatalytic Properties of Flower-Like CdS Nanostructures
10.4028/www.scientific.net/AMR.335-336.460