CuS Nanoballs

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A facile solution chemical route to self-assembly of CuS ball-flowers andtheir application as an efficient photocatalyst

Zhiguo Cheng, Shaozhen Wang, Qian Wang and Baoyou Geng*

Received 22nd July 2009, Accepted 11th September 2009

First published as an Advance Article on the web 30th September 2009

DOI: 10.1039/b914902c

Ball-flower shaped CuS structures have been synthesized by using mixed copper chloride and thiourea

in a simple hydrothermal process employing poly(vinylpyrrolidone) (PVP) as the surfactant. The

morphological investigations by field emission scanning electron microscope (FE-SEM) reveal that the

ball-flower shaped nanostructures are monodispersed in large quantities. The ball-flower shaped

morphologies are strongly dependent on the different ratios of copper chloride to thiourea, the reaction

temperature and reaction time. The possible growth mechanism of the formation of ball-flower shaped

CuS products is discussed in detail. In addition, the photocatalytic activity of ball-flower shaped CuS

architectures has been tested by the degradation of rhodamine B (RhB) under UV light irradiation,

showing that the as-prepared ball-flower shaped CuS structures exhibit high photocatalytic activity for

the degradation of RhB.

1. Introduction

The architectural controlled synthesis of nanostructured semi-

conductors has received much research attention, due to their

outstanding physical and chemical properties and potential

applications in numerous fields. As an important semiconductor

material, copper sulfide has found many applications due to its

optical properties,1,2 chemical-sensing capability,3 ideal charac-

teristics for solar energy absorption,4 thermoelectric cooling

properties,5 fast-ion conduction at high temperature,6,7 etc.

Recently, various morphologies of copper sulfide have been

reported, such as plate-like,8 rod-like,9 tube-like structures,10

hollow spheres,11,12

flower-like,13

snowflake-like, urchin-likepatterns,14 etc. Many techniques have been established to prepare

copper sulfide in various forms, including hydrothermal or sol-

vothermal methods,15,16 microwave irradiation,17 chemical vapor

deposition,18 in situ template-controlled method.19 Obtaining

new materials via a relative simple route and developing the

morphology-controlled synthesis methodologies are a goal and

evoke great interest in materials chemistry.20 In solution

approaches to copper sulfide, thiourea is a commonly used sulfur

source, for instance, Zhu and co-workers21 have synthesized

hollow spherical copper sulfide in water solution containing

Cu(CH3COO)2, thiourea and 2-HP-b-CD. Qian et al. prepared

CuS crystals with various morphologies using shuttle-like CuO

and thiourea as raw reagents at low temperature.22

Wanget al.proposed a sonochemical approach to single crystalline CuS

nanoplates.23 However, up to now, there have been no reports on

the synthesis of monodispersive ball-flower superstructures

constructed with CuS nanosheets, and the controlled synthesis of

CuS hierarchical nanostructures with different morphologies has

not been performed.

In recent years, considerable attention has been given to the

environmental problem involving organic pollutants in water.

Photocatalysis is a promising technology for the treatment of

contaminants, especially for the removal of organic compounds.

Many investigations have been reported on utilizing metal oxide

nanomaterials as photocatalysts to decompose or destroy the

organic pollutants in water.24 However, there is little research on

the photocatalytic property of CuS superstructures with a high

specific surface area, though other properties of CuS micro-

structure have been widely investigated.

In this work, we synthesized 3D hierarchical CuS ball-flowers

by a newly designed hydrothermal self-assembly process. The

simplest synthetic route to 3D nanostructures is probably a self-assembly in which ordered aggregates are formed in a sponta-

neous process.25 Thiourea was also used as a sulfur source.

However, now, an interesting but challenging question is raised,

namely, what role does thethiourea play in theformation of metal

sulfide nanostructures? To learn about the thiourea-basedprocess

in detail, we investigated the simplest aqueous system containing

only CuCl2 and thiourea. As expected, we found that thiourea

reduces Cu(II) to Cu(I) as it combines to form a complex. The

detailed shape evolution process from the intermediate complex

to the final product was clearly shown, and the mechanism of

formation of the hierarchical structure was studied. The photo-

catalytic properties of the as-obtained 3D hierarchical architec-

tures were investigated under UV light irradiation. The resultsshow that the synthesized CuS structures exhibit the superiority

of photocatalytic performance and have exhibited much better

photocatalytic properties for the photodegradation of RhB than

that of other photocatalysts, such as TiO2nanopowders.

2. Experimental

2.1. Chemicals and synthesis

All reagents were of analytical grade and were used without

further purification. In a typical experiment, an appropriate

College of Chemistry and Materials Science, Anhui Key Laboratory ofFunctional Molecular Solids, Anhui Laboratory of Molecular-BasedMaterials, Anhui Normal University, Wuhu, 241000, P. R. China.E-mail: [email protected]; Fax: +86-553-3869303; Tel: +86-553-3869303

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amount of copper chloride was dissolved in distilled water

(10 mL) and formed a blue solution of a certain concentration

under constant stirring for 10 min. Then, appropriate amounts of

thiourea (Tu) and PVP were dissolved in distilled water (15 mL)

and formed a colorless solution. Then the above two solutions

were mixed slowly, a white suspension was formed in a few

minutes, which was then transferred into a 30 mL Teflon-lined

autoclave and maintained at 120180 C for 624 h, then cooled

to room temperature naturally. The black precipitate was filteredoff, washed with distilled water and absolute ethanol several

times, and then dried in vacuum at 60 C for 4 h. For compar-

ison, the bulk CuS powders were prepared by a liquid state

reaction. Simply, appropriate amounts of CuCl2and Na2S were

added into the beaker under stirring for 15 min and as a result,

black products were obtained.

2.2. Characterization

X-Ray powder diffraction (XRD) was carried out on an

XRD-6000 (Japan) X-ray diffractometer with Cu Ka radiation

(l 1.54060 A) at a scanning rate of 0.05 s1. Scanning electron

microscopy (SEM) micrographs were taken using a HitachiS-4800 Scanning electron microscope coupled with energy

dispersive X-ray spectroscopy (EDS).

2.3. Photocatalytic property

The photocatalytic activity experiments of the obtained CuS for

the decomposition of RhB in air were performed at ambient

temperature. A cylindrical Pyrex flask (capacityca.100 mL) was

used as the photoreactor vessel. CuS microstructues as catalyst

(10 mg) was added to an aqueous RhB solution (2.0 105 M,

50 mL) and was magnetically stirred in the dark for 30 min to

ensure establishing an adsorptiondesorption equilibrium. The

UV light was generated from a 300 W high-pressure mercurylamp. As a comparison, the photocatalytic activity of commer-

cial TiO2 powders (Degussa P25, Degussa Co., the surface area

ca. 45 m2 g1) was also tested in the same experimental condi-

tions. UV-vis absorption spectra were recorded at different

intervals to monitor the reaction using Solid Spec-3700 UV/vis

spectrophotometer.

3. Results and discussion

3.1. Structure and morphology

The typical XRD patterns as shown in Fig. 1 reveal the phase and

purity of the as-obtained CuS ball-flower structures. The

diffraction peaks are indexed to the data of the hexagonal phaseCuS (JCPDS Card File No. 060464). Compared with the

standard diffraction data of the hexagonal phase CuS, we can

find that all peaks shift to the lower angle slightly. The reasons

may be ascribed to the following reasons: on the one hand,

during sample preparation, the surface of the sample was above

the sample platform or the zero point of the instrument was not

accurate. On the other hand, the crystallite size was a non-

uniform distribution after a hydrothermal treatment, which may

also lead the peaks shift to the lower angle.

The morphology of the prepared samples was further investi-

gated with FE-SEM, and their typical images are displayed in

Fig. 2. Fig. 2a displays a representative overview of the ball-

flower CuS architectures (the concentration ratio of thiourea to

Cu2+ (4:2), 24 h, 120 C), which shows that the as-obtained

products are composed of large-scale ball-flower architectures

with diameters of 1.82.4 mm. Fig. 2b and c show high-magnifi-

cation SEM images of CuS ball-flower architectures from

a different angle of view, which vividly demonstrate that the ball-

flower CuS structures are built of two-dimensional nanoplates.

The nanoplates are well-ordered and oriented to form ball-flower

architectures. Fig. 2d shows the EDS pattern of the obtained

ball-flower products, which clearly reveals that the obtained

products mainly consisted of Cu and S elements, the weak O

signal comes from the oxidation of air. The atomic ratio of Cu to

S is 1.08:1, which closes to the stoichiometry of CuS.

3.2. Influencing factors

3.2.1. Concentration ratio of Cu2+ to thiourea. In this work,

we found that CuS ball-flower structures could be selectively

obtained by adjusting the concentration ratio of CuCl2$2H2O to

thiourea and hydrothermal temperature. To determine the effect

Fig. 1 XRD pattern of CuS hierarchical ball-flower structures prepared

at 120 C for 24 h.

Fig. 2 SEM images of (a), (b), (c) different magnifications of the

as-obtained CuS ball-flower structures. (d) EDS spectrum of the obtained

products.

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of the concentration ratio of CuCl2$2H2O to thiourea on the

formation of the products with different shapes, the images of the

products obtained from the solutions with a concentration ratio

of CuCl2$2H2O to thiourea of 1/1.5, 1/2.0, 2/3.0, 2/4.0 and

hydrothermal heat treatment at 120 C for 24 h are shown in

Fig. 3. It is clearly seen that the morphology changed greatly with

the change of the concentration ratio of CuCl2$2H2O to thio-

urea. Fig. 3a and 3b display the images of the product obtained at

a concentration ratio of CuCl2$

2H2O to thiourea of 1/1.5. It canbe seen that trepang-like hierarchical architectures were ach-

ieved. With further increasing the thiourea concentration to

2.0 M, a large number of dandelion-like nanostructures

composed of lots of ultrathin nanosheets were observed (Fig. 3c).

When the concentration ratio of CuCl2$2H2O to thiourea was

2.0/3.0, microsphere-like architectures were obtained, as seen

from Fig. 3e. The as-synthesized product entirely takes on the

ball-flower morphology when the thiourea concentration is

4.0 M (Fig. 3g).

From the above results, it can be deduced that both thiourea

and copper ion concentrations influence the CuS morphologies

critically. When thiourea was added to the copper(II) salt solu-

tion, the color of the solution changed from typically blue to

green, indicating the thioureacopper(II) complex is formed.

Further addition of thiourea or just leaving the green solution

standing for a longer time will cause the solution to become

colorless, indicative of conversion into thioureacopper (I)

complexes.26 During the experiment, when the concentration of

Tu was 1.5 or 2.0, the green color of the thioureacopper(II)

reaction solution was persistently kept until the solution was

transferred to an autoclave. However when the concentration of

Tu was further increased to 3.0 and 4.0, the reaction solutionrapidly became colorless, implying that excess thiourea signifi-

cantly speeds up the redox process. On the basis of the above

analysis, formation of different coordination precursors is

a crucial factor determining the crystal growth process of the

copper sulfide crystals.

3.2.2. Reaction temperature.To investigate the effect of the

reaction temperature on the formation of CuS ball-flower

structures, a series of comparative experiments was carried out

through similar processes. It is found that the reaction temper-

ature has a significant influence on the morphology of the

as-synthesized CuS products. When the reaction temperature is

below 120 C, not only the yield of the products is low, but alsothe morphology of the sample is irregular. When the reaction

temperature is elevated to 120 C, the product is composed of

ball-flower structures, and no other structures are observed any

more (sample S4, Fig. 4a). At a higher reaction temperature of

140 C, (Fig. 4b), the CuS ball-flower structures (sample S5,

Fig. 4b) became thick and rigid. No obvious changes were

observed in the morphology of CuS synthesized from the higher

reaction temperatures 160 (Fig. 4c), however, the surface of the

CuS hierarchical ball-flower structures became compact. When

the reaction temperature increased to 180 C, the sheets assem-

bled in the ball-flower grew thicker than those appearing at 160C and the CuS ball-flower structures assembled by the

compactly packed flakes were observed (Fig. 4d). Therefore, wecan find that the exterior morphology of the final products did

not change obviously with the increase of the reaction tempera-

ture; but the sheets composed ball-flowers thickened with the

Fig. 3 FE-SEM images of the CuS nanostructures obtained by

a hydrothermal treatment for 24 h with thiourea concentration of (a), (b)

1.5 M; (c), (d) 2.0 M; (e), (f) 3.0 M; (g), (h) 4.0 M at 120 C.

Fig. 4 FE-SEM images of the CuS nanostructures obtained by

a hydrothermal treatment for 24 h at (a) 120 C; (b) 140 C; (c) 160 C; (d)

180 C with a copper ion concentration of 0.1 M.


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reaction temperature. It might be related to the reaction rate

increased at high temperature but could not alter the growth

process in the present reaction system. More in-depth studies are

necessary to further understand their growth process.

The detailed experimental parameters together with the

morphological properties of the corresponding products (deno-

ted as S1S7) are listed in Table 1. Typical SEM images of the

samples are shown in Fig. 2a, 2c, 2e and 2g.

3.3. Formation of the CuS structures

In order to understand the evolution process of the ball-flower

structures of CuS material, we carried out time-dependent

experiments during which samples were collected at different

time intervals from the reaction mixture.

As shown in Fig. 5, the CuS self-assembled into hierarchical

ball-flower structures consisting of sheets. In the initial stage

(6 h), only a small amount of the product was obtained. As

shown in Fig. 5a, the morphology of the as-produced samples at

the early stage consisted of very thin flakes with sizes of about

1.21.5 mm, and a small quantity of aggregation. With further

increasing the reaction time to 10 h, these sheets aligned withclearly oriented layers, pointing toward a common center, as

displayed in Fig. 5b. With the increase of the hydrothermal time

to 14 h, no single sheets remained. All of the sheets assembled

into nest-like structures (Fig. 5c). Finally, when the reaction was

further prolonged to 18 h, the cores in the center of the spheres

dissolved completely, resulting in the formation of nest-like

structures with hollow cores, as demonstrated in Fig. 5d. As the

reaction proceeded, the size of the pores among the flakes

decreased obviously. At the elongated reaction time of 24 h, the

morphology of the final product is shown in Fig. 2a. It is clear to

see that much more sheets were assembled into hierarchical

structures, and the typical hierarchical ball-flower structures

formed. From this point, the size and morphology of the productremained the same even at longer reaction times. In addition, the

PVP molecules play an important role in preventing the spheres

from agglomerating as many polymer molecules do in the

formation of other spherical species.27,28 On the other hand, PVP

in this study acts as a directing agent to promote the preferential

3D growth of CuS ball-flower. We also found that other

surfactants, such as CTAB and SDBS etc. did not reach the

equivalent effect as the PVP did. So we can say that the PVP

molecules may be unique for the formation of the ball-flower

structures in the present reaction system.

The XRD patterns of the products prepared at different

reaction times are shown in Fig. 6. As shown in Fig. 6, the

products prepared at different reaction times have similar XRDpatterns, except for relative peak intensity levels which were due

to the random orientation. All diffraction peaks can be indexed

as hexagonal CuS structure with calculated lattice constants of

about a 3.792 A and c 16.344 A. These calculated lattice

constant values are consistent with the reported data for CuS

(JCPDS Card no. 060464).

On the basis of the above analysis, we proposed a reasonable

mechanism for the formation of CuS ball-flower structures. The

whole evolution process is illustrated in Fig. 7. In this formation

process, time was the most important controlling factor. Such

a process is consistent with previous reports of a so-called two-

stage growth process, which involves a fast nucleation of amor-

phous primary particles followed by a slow aggregation andcrystallization of primary particles.2932 In our experiment,

thiourea first coordinated with CuCl2 to produce thiourea

copper (II) complexes, which precipitated to become the nuclei

and quickly grew into the primary particles. In the following

secondary growth stage, the primary particles aggregated into

sheets which became the base of the nest-like structure. The

sheets continued to grow by combining with the remaining

Table 1 Morphologies and structures of CuS samples

Sample T/C T/hThe molar ratio ofCuCl2$2H2O to thiourea Morphologhy

S1 120 24 1/1.5 TreapangS2 120 24 1/2.0 DandelionS3 120 24 2/3.0 MicrosphereS4 120 24 2/4.0 Ball-flowerS5 140 24 2/4.0 Ball-flower

S6 160 24 2/4.0 Ball-flowerS7 180 24 2/4.0 Ball-flower

Fig. 5 SEM images of the CuS nanostructures obtained by hydro-

thermal treatment at 120 C for (a) 6 h; (b) 10 h; (c)14 h; (d) 18 h with

thiourea concentration of 4.0 M.

Fig. 6 The XRD pattern of products at different reaction times: 6 h (a),

10 h (b), 14 h (c) and 18 h.


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primary particles, finally forming the ball-flower structure. Many

forces cause the sheets to self-assemble into the ball-flower

morphology, such as electrostatic and dipolar fields associated

with the aggregate, hydrophobic interactions, hydrogen bonds,

crystal-face attraction, and van der Waals forces.33,34 For

example, Zhong35 described the evolution mechanism of

a flower-like iron oxide precursor in the presence of CTAB,

which followed Ostwald ripening kinetics. Further work is

underway to investigate the details of the self-assembly growth

mechanism.

3.4. Photocatalytic properties of CuS ball-flower

To demonstrate the potential application of as-synthesized

hierarchical ball-flower CuS microstructures in the degradation

of organic contaminants, we have investigated their photo-

catalytic activities by choosing the photocatalytic degradation of

RhB as a model reaction. Fig. 8a shows the optical absorption

spectra of RhB aqueous solution (initial concentration: 2.0

105 M, 50 mL) with 10 mg of the as-prepared CuS powders afterexposure to ultraviolet light (UV) for different durations. The

main absorption peak locates at 553 nm, which corresponds to

the RhB molecules, decreases rapidly with extension of the

exposure time, and completely disappears after about 60 min.

Further exposure leads to no absorption peak in the whole

spectrum, which indicates the total decomposition of RhB. It is

clear that the intense pink color of the starting solution gradually

disappears with increasing exposure time to the UV light.

Further experiments were carried out to compare the catalytic

activity of the as-prepared CuS architectures, bulk CuS powders

and commercial Degussa P25 TiO2 powders. Fig. 8b shows the

curves of the concentration of residual RhB with irradiation

time. Without any catalyst, only a slow decrease in the concen-tration of RhB was detected under UV irradiation. The catalytic

activity of the CuS architectures in the dark was also performed,

which showed that only a little decrease in the concentration of

RhB was detected in the dark, which confirmed the concentra-

tion decrease of RhB solution is mainly due to photodegradation

of the products. The addition of catalysts leads to obvious

degradation of RhB, and the photocatalytic activity depends on

the morphology. For the CuS with a hierarchical micro-

architecture, however, the activity is much higher than that of

bulk CuS powders (line c) and commercial Degussa P25 TiO2powders (line d). The RhB solution is decolorized completely by

using the CuS ball-flowers after UV irradiation for 5060 min

(line e).

It is generally accepted that the catalytic process is mainly

related to the adsorption and desorption of molecules on the

surface of the catalyst. The high specific surface area can provide

more reactive adsorption/desorption sites for photocatalytic

Fig. 7 Formation mechanism of hierarchical ball-flower structures of

CuS.

Fig. 8 (a) Time-dependent absorption spectrum of a solution of RhB

solution (2.0 105 M, 50 mL) in the presence of architectural CuS

(10 mg) under exposure to UV light. (b) The RhB normalization

concentration (from the optical absorbance measurements at 550 nm) in

the solution with different catalysts (10 mg) vs. the exposure time.

Starting RhB concentration C0: 2.0 105 M. Line a: without any

catalyst under UV light. Line b: the architectural CuS in the dark. Line c:

the bulk CuS powders under UV light; Line d: Degussa P25 TiO2powders under UV light; Line e: the architectural CuS under UV light.

Fig. 9 BET measurement of CuS architectures.


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reactions.36 In order to evaluate the surface area of the obtained

CuS architectures, full nitrogen sorption isotherms were

measured (Fig. 9). The specific surface area was thus evaluated to

be 78.6 m2 g1 from data points in this pressure range by the BET

equation. The result showed that the obtained CuS architectures

possess a larger specific surface area than that of Degussa

P25 powders (45 m2 g1).

Therefore, the photocatalytic superiority of the CuS archi-

tectures may be attributed to their special structural features. Thehierarchically microstructured CuS can provide a greater surface

area than the reference samples Degussa P25 powders, which is

obviously beneficial for the enhancement of photocatalytic

performance. In addition, good dispersing and uniformity also

can provide a large active surface area.37,38

4. Conclusions

In summary, nearly monodispersed CuS ball-flower structures

have been successfully synthesized via a facile hydrothermal

process at low-temperature. The morphologies of the architec-

tures can be selectively produced by adjusting the concentration

ratio of CuCl2$

2H2O to thiourea and hydrothermal temperature.The possible growth mechanism is also proposed, and this

method may be extends to the preparation of other metal chal-

cogenide semiconductors. The photocatalytic properties of the

hierarchical CuS architectures was explored; the results revealed

that the obtained products exhibited excellent photocatalytic

activity for degradation of RhB under exposure to UV light

irradiation.

Acknowledgements

This work was supported by the National Natural Science

Foundation of China (20671003, 20971003), the Key Project of

Chinese Ministry of Education (209060), the EducationDepartment of Anhui Province (2006KJ006TD) and the

Program for Innovative Research Team in Anhui Normal

University.

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