Powder Technology 198 (2010) 267–274

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Controllable synthesis, optical and photocatalytic properties of CuS nanomaterialswith hierarchical structures

Fei Li a,b,c,⁎, Jianfang Wu a, Qinghua Qin a, Zhen Li a,c, Xintang Huang b

a Faculty of Materials Science and Chemical Engineering, China University of Geosciences, Wuhan 430074, PR Chinab Center of Nano-Science and Technology, Department of Physics, Central China Normal University, Wuhan 430079, PR Chinac Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China

⁎ Corresponding author. Postal address: Faculty of MEngineering, China University of Geosciences, No. 388, LChina. Tel.: +86 27 678 83737; fax: +86 27 678 83732

E-mail address: alexfly2002@sina.com (F. Li).

0032-5910/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.powtec.2009.11.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 June 2009Received in revised form 19 November 2009Accepted 25 November 2009Available online 29 November 2009

Keywords:CuSNanomaterialsSolvothermalOptical propertiesPhotocatalytic

The controlled synthesis of CuS nanomaterials with hierarchical structures has been realized by chemicalsynthesis between copper nitrate trihydrate and thiourea via a solvothermal route. X-ray diffraction,scanning electron microscopy, and transmission electron microscopy were used to characterize the products.It was shown that CuS nanomaterials with hierarchical structures were composed of numerous nanoplates ornanorods. Experiments demonstrated that the morphologies of CuS nanomaterials were significantlyinfluenced by reaction temperature, growth time and sulfur sources. A growth model was proposed for theselective formation of CuS hierarchical structures. The optical properties of the CuS hierarchical structureswere investigated by ultraviolet–visible spectroscopy and photoluminescence spectroscopy. The ultraviolet–visible spectrum had a broad absorption in the visible range and the photoluminescence spectrum showed astrong green emission. Photocatalytic performance of the CuS hierarchical structures was evaluated bymeasuring the decomposition rate of methylene blue solution under natural light. The CuS hierarchicalstructures showed good photocatalytic activity.

aterials Science and Chemicalumo Road, Wuhan 430074, PR.

l rights reserved.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

As an important p-type semiconductor, copper sulfide (CuS)exhibits many unusual electronic, optical, and other physical andchemical properties [1–4], and has great potential in a versatile rangeof applications such as optical filters and surperionic materials [5],solar controller and solar radiation absorber [6], catalysts [7],nanometer-scale switches [8], high-capacity cathode material inlithium secondary batteries [9], superconductor at low temperature[10], the materials in chemical sensors [11] and thermoelectriccooling material [12].

In the past few years, various attempts have been focused on thesynthesis of CuS with different shapes, including nanoparticles [13],nanoflakes [14], nanotubes [15], microspheres [16], flower-like struc-tures [17], nanowires [18], nanorods [19], urchin-like structures [20],nanoribbons [21], and various preparativemethods such as thermolysisof single-source thiolate-derived precursors, sacrificial templatingmethod, ultrasonic and microwave irradiation, electrodeposition,micelles and microemulsions, chemical vapor reaction process andhydrothermal or solvothermal methods have been reported. Amongthese preparative methods, solution-phase approaches are appealing

due to their low temperature, low cost, high efficiency, and potential forscale-up. It is noted that various surfactants or templates are usuallyused in the reaction systems, and they all play critical roles in themorphological control of CuS nanomaterials. However, the use ofsurfactants or templates will inevitably increase the reaction complex-ity, cause impurity in the products, and is disadvantageous from theviewpoint of green chemistry. Thus, the development of a facile,effective, and surfactant-free approach for the controlled synthesis ofCuS nanomaterials is highly desirable.

In this paper, we reported the synthesis and characterization ofCuS nanomaterials with hierarchical structures by a facile solvother-mal method in ethylene glycol (EG) without the use of any surfactant.The effects of temperature, reaction time and sulfur sources on themorphologies of the CuS nanomaterials were investigated and theformation mechanism of the hierarchical structures was discussed.The optical and photocatalytic properties of the hierarchical CuSstructures were studied in detail. These CuS nanomaterials withhierarchical structures may have great potential application in thefundamental study of nanostructures as well as fabricating nanode-vices based on these nanomaterials.

2. Materials and methods

All chemicals used in this work were of analytical reagent gradeand purchased from Shanghai Chemical Reagents Company, China,and used as received without further purification.

Fig. 1. XRD pattern of CuS nanomaterials synthesized at 150 °C, with reaction for 24 husing thiourea.

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2.1. Synthesis

2.1.1. Solvothermal reactionIn a typical synthesis, 1 mmol Cu(NO3)2.3H2O (0.243 g) was

dissolved in 40 mL ethylene glycol (EG) and a green solution wasformed. Then 2 mmol thiourea (Tu, SC(NH2)2, 0.152 g) was added intothe above-mentioned solution under vigorous stirring for 30 min.Afterwards, the solution was transferred into a 60 mL Teflon-linedstainless steel autoclave, sealed, and maintained at 150 °C for 24 h onan oven. The rate of temperature increasing is 3 °C/min. Then theautoclave was cooled naturally for about 6 h to room temperature.Finally, the products were centrifuged and washed with distilledwater and ethanol three times, respectively, and dried under vacuumat 60 °C for 4 h. The reaction conditions were varied to explore theeffect of different reaction parameters on the size and morphology ofthe products.

2.1.2. Effect of reaction temperatureThe typical procedure was followed at constant molar ratio of Cu:

S (1:2) using thiourea as the sulfur source (0.243 g Cu(NO3)2.3H2Oand 0.152 g thiourea) and constant reaction time of 24 h but withvarying the temperature from 90 °C to 120 °C, 150 °C, and 180 °C.Other reaction conditions were the same as Section 2.1.1.

2.1.3. Effect of reaction timeThe above procedure was followed at constant molar ratio (1:2)

using thiourea as the sulfur source (0.243 g Cu(NO3)2.3H2O and0.152 g thiourea) and constant reaction temperature of 150 °C, but thereaction time was varied at 4 h, 8 h, 12 h, and 24 h. Other reactionconditions were the same as Section 2.1.1.

2.1.4. Effect of sulfur sourcesIn this procedure, Na2S.9H2O (0.48 g), thioacetamide (TAA,

H3CCSNH2, 0.153 g) and thiourea (0.152 g) were used with constantmolar ratio of Cu:S (1:2), constant reaction time (24 h) and constantreaction temperature (150 °C). Other reaction conditions were thesame as Section 2.1.1.

2.2. Characterization

2.2.1. Structural and morphological characterizationThe identity and the phase of the products were characterized by

the X-ray powder diffraction using a Dmax-3β diffractometer withnickel-filtered Cu Kα radiation (λ=1.5417 Å). The size and morphol-ogy of the product were analyzed using the JEOL JSM-6700F fieldemission scanning electron microscopy (SEM). Transmission electronmicrographs were performed on a Tecnai F20 transmission electronmicroscope (TEM) operated at 200 kV.

2.2.2. Optical propertiesAn ultraviolet–visible (UV–Vis) spectrophotometer Perkin-Elmer

Lambda 35was used to carry out the UV–Vis absorptionmeasurementof the CuS nanomaterials. The photoluminescent (PL) properties weremeasured on an F-4500 fluorescence spectrophotometer at roomtemperature using Xe lamp with a wavelength of 220 nm as theexcitation source.

2.2.3. Photocatalytic performanceThe obtained CuS nanomaterials were used as catalyst for the

oxidation and decoloration of the methylene blue (MB) dye with theassistance of hydrogen peroxide (H2O2). The original solution wasprepared by adding 1.3 mL H2O2 (30%, w/w) to 40 mL MB solution(20 mg/L), then 30 mg CuS was added into the solution to form theaqueous dispersion. At once, the dispersion was magnetically stirredin the dark for 30 min to establish an adsorption/desorptionequilibrium condition. Afterwards, the dispersion was put under the

natural light. At given time intervals, the dispersion was sampled(1 mL), diluted (10 mL), and centrifuged to separate the catalyst. Thenthe solution was put into a quartz cell, and the absorption spectrumwas measured with a UV-2401 spectrophotometer.

3. Results and discussion

Solvothermal reaction of copper nitrate trihydrate and thiourea,with a 1:2 molar ratio in EG at 150 °C for 24 h in an autoclave resultedin the formation of the black product. All the peaks in the XRD patternof the sample have been indexed to covellite, the hexagonal phase ofCuS (JCPDS Card No. 06-0464, a=3.792 Å and c=16.34 Å), as shownin Fig. 1. The absence of peaks corresponding to the precursors, copperoxide, or other phases of copper sulfide indicates the purity of theproduct. The strong and sharp diffraction peaks suggest that the as-obtained products are well crystalline.

The typical SEM image in Fig 2 a reveals the formation ofhierarchical structures. This morphology consists of microspheres(∼1 µm), which are constituted through self-assembly of numerousnanoplates and nanorods, as shown in the magnified SEM images inFig. 2 b and c. The microspheres have densely packed arrangement ofnanoplates or nanorods. The mean thickness of the nanoplates is ataround 30–40 nm and the nanorods are about 150 nm in length and30 nm in width.

TEM images (Fig. 3) were used to specify the morphologies andmicrostructures of the CuS nanomaterials with hierarchical structures.The results obtained using TEM are basically in accordance with thoseprovided by SEM. Fig. 3 e and f presents the high-resolution TEM(HRTEM) images of a nanoplate (selected area in Fig. 3 c) and ananorod (selected area in Fig. 3 d), respectively. They both clearlyshow a lattice spacing of 3.0 Å, which is consistent with the distancebetween the {102} lattice planes, which indicates the single-crystalline nature of the CuS nanoplates and nanorods. The effects ofreaction conditions have been studied by varying the reactiontemperature, reaction time, and sulfur source to monitor the growthprocess and the respective change of size and morphology of CuSnanomaterials.

3.1. Effect of temperature

Investigation of the effect of temperature also resulted in theformation of pure hexagonal covellite phase of CuS (Fig. 4 a and b) atrelatively low temperatures, i. e. 90 °C and 120 °C. The improvementin the crystallinity is confirmed by the increase in the intensity of thepeaks and the appearance of the diffraction peak of (108) crystal plane

Fig. 2. SEM images of CuS nanomaterials synthesized at 150 °C, with reaction for 24 h using thiourea. (a) Low-magnification image and (b, c) magnified images.

Fig. 3. TEM images of CuS nanomaterials synthesized at 150 °C, with reaction for 24 h using thiourea. (a) Low-magnification image, (b, c, d) magnified images, (e) HRTEM image of ananoplate selected in (c), and (f) HRTEM image of a nanorod selected in (d).

Fig. 4. XRD patterns of CuS nanomaterials prepared at different temperatures, (a) 90 °C,(b) 120 °C, and (c) 180 °C, with reaction for 24 h using thiourea.

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as the reaction temperature changes from 90 °C to 120 °C. Onincreasing the reaction temperature to 180 °C, covellite CuS iscontaminated with the digenite phase of Cu9S5 as shown in Fig 4 c.It has been proved that EG can act as reducing agents at hightemperatures [22], therefore, CuS can be reduced to Cu9S5 in EG at180 °C.

The SEM images shown in Fig. 5 provide direct information aboutthe size and typical morphologies of the CuS nanomaterials grown atdifferent temperatures. All the CuS nanomaterials exhibit hierarchicalmorphology composed of microspheres (∼1 µm), which are similar tothose prepared at 150 °C. However, the smallest units constituting themicrospheres and their arrangements are different. At 90 °C, allmicrospheres are made up of densely packed nanoplates with verythin thickness in the range from 10 to 20 nm, as shown in Fig. 5 b. At120 °C, all microspheres consist of sparsely packed nanoplates with thethickness in the range from 30 to 40 nm, and the microspheres areincomplete, as shown in Fig. 5 d. It can be concluded that CuS nanorodswere produced as the constituting units only when the temperaturewas increased to 150 °C. Overall, temperature is absolutely an

Fig. 5. SEM images of CuS nanomaterials prepared at different temperatures, (a, b) 90 °C and (c, d) 120 °C, with reaction for 24 h using thiourea.

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important factor in determining not only the phase composition butalso the size and morphology of the CuS nanomaterials.

3.2. Effect of growth time

Fig. 6 presents the morphology development of CuS nanomaterialsgrown for different periods of time, taking place at 150 °C. SEM imagesin Fig. 6 a and b reveal that CuS microspheres composed of nanoplatesor nanorods start to form at the early stage. As time develops (8 h and12 h, Fig. 6 c, d, e and f), CuS microspheres aggregate with each otherto form hierarchical structures. With a further reaction time increaseto 24 h, hierarchical chainlike morphology can be obtained, which canbe seen in Fig. 2 a.

3.3. Effect of sulfur sources

To explore the influence of sulfur sources on the morphologies ofcopper sulfide under otherwise same reaction conditions, Na2S.9H2Oand TAA were used as sulfur sources instead of thiourea. Purehexagonal covellite phase of the CuS crystals (XRD patterns notshown here) was successfully synthesized using these sulfur sources.When Na2S.9H2O was adopted, large amounts of nanoparticles werepresent in Fig. 7 a and b. Large quantities of small nanorods andnanoplates were obtained by employing TAA as the sulfur source,which can be clearly seen in Fig. 7 c and d.

3.4. Yields of the products

Table 1 shows the yields of the products prepared under differentconditions.

When Tu was used as the sulfur source, the yields of the productsprepared at 90 °C, 120 °C and 150 °C for 24 h are almost the same,exceeding 83%. But the yield of the product prepared at 180 °C is aslow as 76.3%, resulting from the partial reduction from CuS to Cu9S5.

From the time-dependent experiments, we find that the yield is veryhigh at the early stage, indicating that the reaction is very rapid. Ontime expansion, the yields become a little higher as the growth time is8 h, 12 h and 24 h.

When different sulfur sourceswere used, the yields of the productsare almost the same, as high as about 84%. This reveals that sulfursources nearly have no effect on the yields of the final products.

3.5. Growth mechanism

It is notable that the copper-thiourea aqueous system has alreadybeen utilized to generate CuS nanomaterials [13], and its chemistry iscomplicated. Under the present solvothermal conditions, the follow-ing reactions may occur:

C u2+ + mTu + nEG→½CuðTuÞmðEGÞn�2+

½CuðTuÞmðEGÞn�2+→solovthermal

CuS↓

Generally, the growth process of crystals includes two stages: aninitial nucleating stage and a subsequent growth stage. The initialfactor responsible for the final morphology of the products is thecrystallographic phase of the seed formed during the nucleationprocess [23], which depends on the nature of the material and on theenvironmental conditions. The formation of hierarchical morphologycan be rationalized on the basis of the above investigations on varyingthe reaction through the stages as shown in Fig. 8. At the initial stage,covellite CuS nuclei are formed in EG. In the subsequent step, thesenuclei preferentially are grown in the same direction and furthertransferred into CuS nanoplates or nanorods owing to the differentsurface energies of the hexagonal crystal structure. On increasing thereaction time, rods or platelike structures self-assemble to formmicrospheres and then the microspheres aggregate to result inhierarchical chainlike morphology. On the whole, a two-tiered

Fig. 6. SEM images of CuS nanomaterials recorded at 150 °C for different times using thiourea: (a, b) 4 h, (c, d) 8 h and (e, f) 12 h.

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organizing scheme for construction of hierarchical CuS nanostructureshas been elucidated: (1) nanoscale formation of building units viaoriented aggregation, i.e. nanorods and nanoplates; (2) macroscopicorganizationof these buildingunits intohierarchical CuSnanostructures.

The reaction temperature is an important factor influencing thegrowth of CuS nanomaterials, which is proved by temperature-dependent experiments. At 90 °C, the nanoplates were very thin dueto the low temperature. When the temperature was increased to120 °C, the nanoplates turned thicker. Due to the slow formation ofCuS, the nanoplates self-assembled to incomplete microspheres. At150 °C, because of the fast formation of CuS, nanoplates and nanorodswere produced simultaneously at the early stage and then self-assembled to hierarchical structures.

It is also worth mentioning here that sulfur sources have greateffects on the final morphologies of CuS nanomaterials. As we know,different sulfur sources have different release rate of S2− ions. OnceNa2S.9H2O was introduced into EG, it dissociated into Na+ and S2−

immediately, and therefore the formation of CuS nuclei occurredrapidly. As a result, CuS nuclei grew in an explosive way to form CuS

nanoparticles because of the high concentration of S2−. TAA canreadily release S2−, but the concentration of S2− is a little lower than S2−

released fromNa2S.When TAAwas introduced into the reaction system,weak anisotropic growth resulted in the small nanoplates. For thioureaas sulfur sources, the strong complex actionbetweenCu2+, thiourea andEG led to the formation of Cu–Tu–EG complexes ([Cu(Tu)m(EG)n]2+).This process decreased the formation speed of CuS greatly and resultedin the anisotropic growth of CuS. Therefore, nanoplates and nanorodswere the constituting units of the hierarchical structures.

We also studied other metal sulfide nanocrystals via a solutionroute [24–27]. Combining these research results, we find that twodeterminant factors have much effect on the morphology of the finalproducts. One is crystal structure and the other is sulfur sources. Indetail, ZnS and PbS are cubic crystals, they are grown in an isotropicmode and demonstrate nanoparticle or microparticle morphology[24–26]. During the growth process, ions or molecules near thesurface are consumed and a concentric diffusion field is producedaround the crystal. This makes the formation of multi-arm crystals,such as multi-arm PbS microcrystals [26]. CdS and CuS are hexagonal

Fig. 7. SEM and TEM images of CuS nanomaterials using different sulfur sources: (a, b) Na2S and (c, d) TAA.

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crystal, and they have the trend towards anisotropic growth.Nanorods or nanoplates are the morphology usually observed.Furthermore, it can be concluded that the release rate of S2− is alsothe determinant factor on the morphology of the metal sulfidenanocrystals. When Na2S.9H2O was used, the products were grown inan isotropic mode and their final morphology was small nanoparti-cles. When TAA and Tu were used, anisotropic growth played a vitalrole in the synthesis of metal sulfide nanocrystals, accordingly, thefinal products were nanorods, nanoplates or their assemblies [27].

3.6. Optical properties

UV–Vis absorption measurement is one of the most importantmethods to reveal the optical properties of semiconductor nanocrys-tals and has been studied extensively, so does PL measurement. Fig. 9a shows the UV–Vis absorption spectrum of the hierarchical CuSstructures synthesized at 150 °C using thiourea. A strong and broadabsorption can be found in the visible range from 400 to 700 nm.Comparedwith early reports [7,18,28], the UV–Vis absorption is muchbroader in the visible range. It can be deduced that the hierarchicalstructure of CuS leads to the broadening of optical absorption.

Table 1Yields of the products prepared under different conditions.

Sulfur source Temperature (°C) Time (h) Yield (%)

Tu 90 24 83.2Tu 120 24 83.6Tu 150 24 84.5Tu 180 24 76.3Tu 150 4 80.9Tu 150 8 82.9Tu 150 12 83.5Na2S.9H2O 150 24 84.8TAA 150 24 83.8

Therefore, the hierarchical CuS structures have great potential in thefield of solar cell.

PL spectrum of the hierarchical CuS structures is present in Fig. 9 b.Although Jiang et al. [29] reported that there was no emission peak forCuS in the range of 400–800 nm, the hierarchical CuS structures had astrong emission band centered at 523 nm. Our result is also differentfrom Refs. [30,31], where ultraviolet emission was reported, but veryconsistent with the PL result reported by Roy and Srivastava[19].Compared to rod-like CuS crystals [19], the small red shift of theemission peak shows that the size and morphology may beresponsible for the PL spectra of CuS. However, the origin of theobserved optical properties is still far fromwell-understood, andmoredetailed investigations are needed.

3.7. Photocatalytic performance

The photocatalytic performance of the hierarchical CuS structureson the oxidation of MB dye has been studied in the presence ofhydrogen peroxide. The catalytic reaction was conducted undernatural light. UV–Vis spectra were applied to demonstrate thephotocatalytic degradation activity of MB. The characteristic absorp-tion peak at 662 nm of MB was used as a monitored parameter duringthe catalytic degradation process. Fig. 10 a shows the absorptionspectra of aqueous solutions of MB tested at different intervals in thepresence of the hierarchical CuS structures. The intensity absorptionpeak at 662 nm of MB decreased gradually with the extension of time,indicating the photocatalytic degradation of MB. Fig. 10 b shows thedegradation rate of MB at different intervals. About 90% of the MBwasdegraded after 90 min. For comparison, we also carried out similartest without the addition of the hierarchical CuS structures. We foundthat the intensity absorption peak at 662 nm of MB almost had nochanges with the time prolonged, which proved the vital role CuSplayed in the degradation of MB.

Fig. 8. Illustration for the growth process of hierarchical CuS structures using thiourea.

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In our work, the hierarchical CuS structures have a lot of unfoldednanoplates that could absorb more photons to produce electron–holepairs under natural light [32], and the deep and capacious poresenable the hierarchical structures to be exposed to the MB solutionsufficiently. Furthermore, the nano-size of plates could reduce theradiationless recombination of electron–hole pairs [33], which is alsoin favour of the photocatalysis of MB. Currently, the studies on thephotocatalytic properties of CuS nanostructures are seldom reported.Ding et al. [34] reported the photocatalytic properties of CuSnanoflowers on Rhodamine B and the degradation rate is close to90% after 2 h halogen lamp irradiation. Our degradation rate is similar

Fig. 9. (a) UV–Vis absorption spectrum of the hierarchical CuS structures and (b) PLspectrum of the hierarchical CuS structures.

to Ding's, but the degradation was carried out without halogen lampirradiation.

4. Conclusions

In summary, CuS nanomaterials with hierarchical structures havebeen successfully prepared using thiourea via a facile and surfactant-free solvothermal method. The results show that the reactiontemperature, growth time and sulfur sources are crucial factors onthe final morphology and size of CuS nanomaterials. On increasing thereaction time, CuS nanorods and nanoplates self-assemble to formmicrospheres and then the microspheres aggregate to result inhierarchical morphology. The advantages of our method for the

Fig. 10. (a) Absorption spectra of MB aqueous solutions in the presence of thehierarchical CuS structures. (b) Degradation rate of MB at different intervals.

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synthesis of hierarchical CuS structures lie in the low temperature andmild reaction conditions, which permit large-scale production at lowcost. The UV–Vis spectrum shows that the hierarchical CuS structureshave a broad absorption band in the visible range and the PL spectrumexhibits a strong emission band centered at 523 nm. The hierarchicalCuS structures have good photocatalytic performance on thedegradation of MB under natural light. Furthermore, a possiblegrowth mechanism has been proposed and this method also opensa new way for shape- and size-controlled synthesis of othersemiconductor metal sulfides.

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