Solvothermal synthesis of CuS semiconductor hollow spheres based on a bubble template route
Jun Liu,Dongfeng Xue ?
Department of Materials Science and Chemical Engineering,School of Chemical Engineering,Dalian University of Technology,Dalian 116012,PR China
a r t i c l e i n f o
Available online 19September 2008PACS:61.43.Gt 62.23.St 64.75.Yz
Keywords:
A1.Crystal morphology A2.Growth from solutions B1.Sul?des
B2.Semiconducting materials
a b s t r a c t
Semiconductor CuS hollow spheres by self-assembly coupled with bubble templating through a facile one-step solvothermal route has been demonstrated.An energy-minimizing-driven self-assembly of CuS nanoparticles on the base of H 2S bubbles is responsible for the formation of the hollow structure.The as-obtained CuS products were characterized by scanning electron microscopy (SEM),X-ray diffraction (XRD),energy-dispersive X-ray (EDX),and UV–vis–NIR spectrophotometer.Due to the unique optical property,these hollow structures are envisaged to be used in applications such as novel building blocks for the advanced materials,catalysis,solar cell devices,and drug delivery system.
&2008Elsevier B.V.All rights reserved.
1.Introduction
In recent years,there has been increasing interest in the controlled synthesis of inorganic nano/microstructures with hollow interiors because of their widespread potential applica-tions [1].The general approach for preparation of hollow structures has involved the use of various removable or sacri?cial templates,including hard ones such as monodisperse silica [2]or polymer latex spheres [3]as well as soft ones,for example,emulsion droplets/micelles [4]and even bubble template [5].However,hollow structures prepared from hard templating routes usually suffer from disadvantages related to high cost and tedious synthetic procedures,which ?nally prevent them from being used in large-scale applications.One-step methods for generating hollow inorganic materials are highly desirable.Recently,the Kirkendall effect has been extensively used as an effective strategy to prepare hollow structures [1,6].We have also most recently developed a facile and general thermal oxidation strategy for the preparation of transition-metal oxide porous hollow architectures [7].This chemical strategy is based on the combination of Kirkendall effect,volume loss,and gas release.
As a p-type semiconductor,copper sul?de (CuS)is one of the most intensively studied materials owing to its technologically important applications in the ?eld of photothermal conversion,solar cell devices,super ionic materials,optical ?lters,gas sensors,and lithium-ion batteries [8–11].To date,various nano/
microstructures of CuS,such as one-dimensional (1D)nanorods [12],two-dimensional (2D)nanodisks [13],and three-dimensional (3D)hierarchical doughnuts [7]have been prepared.Studies in copper-based and other metal oxide structures have been widely undertaken in our laboratory [14–19].Herein,we report a simple one-pot synthesis of CuS hollow spheres on the basis of bubble template mechanism.More importantly,this bubble template strategy is quite versatile and can be applied to other transition-metal sul?des,such as MnS 2.
2.Experimental details
Transition-metal sul?de hollow spheres were prepared via a solvothermal technique in a Te?on-lined stainless steel autoclave.In a typical synthesis of CuS hollow spheres,2mmol of Cu(NO 3)2á3H 2O was dissolved in 25ml of absolute ethanol to form a clear solution,and then 4mmol of thioacetamide (TAA)was added to this solution under vigorous stirring.Afterwards,this solution was transferred into a 30mL Te?on-lined stainless steel autoclave.The autoclave was sealed and maintained at 1201C for 16h.After the solution was cooled to room temperature,the obtained black solid products were collected by centrifuging the mixture,and were then washed with absolute ethanol and deionized water several times and dried at 601C for 6h for further characterization.For hollow MnS 2,Mn(CH 3COO)2á4H 2O was used instead of Cu(NO 3)2á3H 2O while keeping the other reaction conditions unchanged.For solid CuS spheres,2mmol of Cu(NO 3)2á3H 2O and 4mmol of NH 4SCN were dissolved
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Journal of Crystal Growth
0022-0248/$-see front matter &2008Elsevier B.V.All rights reserved.doi:10.1016/j.jcrysgro.2008.09.025
?Corresponding author.
E-mail address:dfxue@https://www.doczj.com/doc/c46872383.html, (D.Xue).
Journal of Crystal Growth 311(2009)500–503
in 25mL deionized water,and then 0.4g poly(vinypyrrolidone)(PVP,M w ?80,000)was added to this solution under vigorous stirring.Afterwards,this solution was transferred into 30mL Te?on-lined stainless steel autoclave and maintained at 2101C for 10h.
The as-prepared samples were characterized by X-ray diffrac-tion (XRD)on a Rigaku-Dmax 2400diffractometer equipped with a graphite monochromatized Cu K a radiation ?ux at a scanning rate of 0.021S à1in the 2y range 5–801.Scanning electron microscopy (SEM)images were taken with a JEOL-5600LV scanning electron microscope,using an accelerating voltage of 20kV.Energy-dispersive X-ray (EDX)microanalysis of the samples was performed during SEM measurements.UV/vis adsorption spectra were recorded on UV–vis–NIR spectrophotometer (JASCO,V-550).
3.Results and discussion
Fig.1a shows the typical XRD pattern of as-synthesized CuS hollow spheres.All peaks in Fig.1a match well with hexagonal
CuS structure (JCPDS no.06-0464)with cell parameters a ?3.792A
?and c ?16.34A ?.No other impurities,such as Cu 1.8S,Cu 7S 4,Cu 1.96S,Cu 2S,oxides or organic compounds related to the reactants,were detected.EDX analysis was also performed for these CuS hollow spheres,as shown in Fig.1b.Only copper and sulfur elements were detected,and their atomic ratio is about 1:1,which is in agreement with the stoichiometric ratio of CuS.
The morphology and microstructure of the as-prepared CuS were studied by SEM.The low-magni?cation SEM images (Fig.2a and b)indicate that the product consists of a wealth of submicrometer spheres with diameter ranging from 500nm to 1m m.Further SEM observation indicates that the obtained CuS submicrometer spheres have hollow interiors.The magni?ed SEM images (Fig.2c and d)show that many broken hollow spheres (which are indicated by arrows)exist in the product.The broken submicrometer spheres presented in Fig.2b–d indicate that the as-prepared CuS spheres are hollow.
A fundamental understanding of the formation mechanism of CuS hollow spheres is critical for achieving control over their properties.To better understand the formation mechanism of CuS hollow spheres,a series of parallel experiments were carried out.
Fig.2.SEM images of hollow CuS submicrospheres:(a and b)low-magni?cation SEM images and (c and d)high-magni?cation SEM images.Arrows are to aid the eye.
102030
405060708005
1015
2θ (degrees)
Energy (keV)
I n t e n s i t y (a .u .)
100
101102
103006
110
103
116JCPDS 6-0464
Cu
S
Cu
Cu
Fig.1.XRD pattern (a)and EDX pattern (b)of the as-synthesized hollow CuS spheres.
J.Liu,D.Xue /Journal of Crystal Growth 311(2009)500–503
501
Under the same experimental conditions,when 0.4g of PVP was present in the solution,uniform CuS hollow spheres with diameter ranging from 300to 500nm were achieved (Fig.3a).Furthermore,when the sulfur source used was varied from TAA to Na 2S,irregular solid microparticles were observed (Fig.3b).As there is no bubble release in the presence of Na 2S,solid particles were obtained.All observations indicate that the sulfur resource used is the prerequisite for the formation of hollow CuS structures.
On the basis of the above observation,the formation of CuS hollow spheres can be explained by a bubble template mechan-ism.The bubble template mechanism has been considered as effective interpretation for the formation of hollow inorganic materials [5].As illustrated in Fig.4,after initial nucleation,the monomers will grow into nanocrystals (step a).These nanocrys-tals have a tendency to aggregate due to their high surface energies,and at the same time,lots of bubbles of H 2S generated in situ (produced by the decomposition of TAA)in the reaction provide the aggregation center (step b).Driven by the minimiza-tion of interfacial energy,small CuS nanocrystals may aggregate around the gas–liquid interface between bubble and solvent (step b)and ?nally hollow CuS spheres form (step c).In comparison,the reaction of Cu(NO 3)2á3H 2O and Na 2S under similar solvother-mal conditions,in which no bubbles were introduced into the system,only led to CuS solid particles.
Compared to the other template methods,this soft template method is simple,convenient and avoids the introduction of impurities,and is suitable for modern nano/micromaterials synthesis.The current chemical strategy is quite general and can be extended to other transition-metal sul?des such as MnS 2.
Fig.3.SEM images of CuS products with the aid of 0.4g PVP (a)and Na 2S used as the sulfur source (b).The other reaction conditions are
unchanged.
Fig.5.SEM images of MnS 2hollow spheres:(a)low-magni?cation SEM image and (b)high-magni?cation SEM image,the inset of Fig.1b shows a typical broken hollow
sphere.
Fig.4.Schematic representation of the formation mechanism of CuS hollow submicrospheres.
J.Liu,D.Xue /Journal of Crystal Growth 311(2009)500–503
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Fig.4shows the representative SEM images of the as-obtained MnS2hollow microspheres.The low-magni?cation SEM image (Fig.5a)indicates that the diameter of MnS2sphere is in the range of1–5m m.The high-magni?cation SEM image(Fig.5b) shows that many spheres are broken and have a hollow interior.
A typical broken MnS2hollow sphere is shown in the inset of Fig.5b.
UV–vis absorption measurement is one of the most important methods to reveal the optical properties of semiconductor nano/ microcrystals and has been studied extensively.Spectra of both hollow and solid CuS spheres with a similar diameter are presented in Fig.6.It can be seen from Fig.6a that the prepared CuS hollow spheres show a distinct blue-shift of50nm compared with that of solid spheres with a similar diameter(the hollow product exhibits a broad peak centered at about635nm,while the solid product exhibit a broad peak centered at about685nm), which is suggested to be the contribution of a quantum size effect in the hexagonal CuS hollow structure.
4.Conclusions
In summary,a novel and facile one-step approach has been successfully developed to synthesize CuS hollow spheres with diameter of500nm–1m m.An energy-minimizing-driven self-assembly of CuS nanoparticles on the base of H2S bubbles is responsible for the formation of hollow structure. These hollow structures show unique optical https://www.doczj.com/doc/c46872383.html,pared with solid CuS sphere with a similar diameter,the hollow product shows a blue-shift of50nm,which is suggested to be the contribution of a quantum size effect in the hexagonal CuS hollow structure.These as-synthesized CuS hollow spheres might?nd applications in sensing,catalysis and lithium-ion batteries.Acknowledgements
We gratefully acknowledge the?nancial support of the program for New Century Excellent Talents in University(Grant no.NCET-05-0278),the National Natural Science Foundation of China(Grant no.20471012),a Foundation for the Author of National Excellent Doctoral Dissertation of PR China(Grant no. 200322),and the Research Fund for the Doctoral Program of Higher Education(Grant no.20040141004).
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Fig.6.(a)UV–vis spectra of as-synthesized hollow and solid CuS spheres and(b)SEM image of CuS solid spheres.The fabrication procedure of the CuS solid spheres is shown in experimental details.
J.Liu,D.Xue/Journal of Crystal Growth311(2009)500–503503