Ionic liquid-assisted synthesis of hierarchical CuS nanostructuresat room temperature
Chao Xu a, Ling Wang a, Dingbing Zou a, Taokai Ying a,b,⁎
a College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, PR Chinab Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, PR China
Received 17 November 2007; accepted 7 February 2008Available online 13 February 2008
Hierarchical nano-/micro-structures with specific morphol-ogy have fascinated scientists all over the world because of theirsophisticated architectures which are expected to provide someunique and exciting properties. To date, many recent efforts havebeen devoted to the synthesis of inorganic materials withhierarchical shapes, including metal [1,2], metal oxide [3],sulfide [4], hydrate [5], and other minerals [6,7].
Copper sulfide has been widely used in many fields, such asoptical filter [8], solar cell [9], and ion conduction [10]. Repor-ted study indicated the covellite CuS had metallic conductivityand could transform into a superconductor at 1.6 K [11]. So far,various strategies have been developed to synthesize CuSnanostructures, including sonochemical methods [12], micro-wave-assisted methods [13], electrosynthesis [14], hydrother-mal methods [15], solid-state reactions [16], and chemical vapor
⁎ Corresponding author. College of Chemistry and Life Sciences, ZhejiangNormal University, Jinhua 321004, PR China. Tel.: +86 579 82282780; fax: +86579 82282269.
deposition (CVD) [17]. However, soft solution route is stillregarded as a convenient, economical, and environmentallyfriendly method [18]. TAA, a very important molecule with twofunctional groups (C=S, NH2), and a commonly used sulfursource in solution approaches to prepare transition metal chal-cogenides, has been exploited in the preparations of nanophasedIn2S3 [19], Bi2S3 nanorods [20], γ-MnS crystallites [21], andCuS microtubes [22]. In these syntheses, there should be acoordination reaction between TAA and metal ions, since TAAhas a strong coordination ability to combine with transitionmetal ions; metal-complex consequently formed in situ and thendissociated to final metal chalcogenides. In recent, Xie et al.[23] used the in situ formed Cu(I)-complex as a self-sacrificedtemplate to synthesize tubular CuS in an aqueous systemat 60 °C. This Cu(I)-complex was determined to be Cu3(TAA)3Cl3, and was too stable to decompose to CuS species at tempe-ratures lower than 50 °C. That is why there have been noavailable soft methods for the synthesis of CuS nanostructuresat room temperature.
The application of ionic liquids in materials communityis attracting more and more attention [24,25]. In the route forinorganic materials formation, ionic liquids as co-solvents
Fig. 1. XRD pattern of the as-prepared hierarchical CuS nanostructures.
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would allow control over the structural properties of the resultantmaterials, particularly with respect to the size, morphology anddispersion, taking advantage of their readily modifiable andcontrollable physical characteristics [26,27]. In addition, theirhigh ionic conductivity and polarizability made them be theexcellent drives to favor the polar reactions for inorganicsyntheses under anhydrous or water-poor conditions [28]. Ourreported study [29] has exhibited ionic liquids could facilitatethe preparation of CuCl nanocrystals in anhydrous system. Doionic liquids play the same role in water-rich system?
Herein, we report for the first time the synthesis of hie-rarchical CuS nanoparticles at room temperature in the presenceof ionic liquid [BMIM]BF4. The method is based on the
Fig. 2. FESEM (a, b) and TEM images (c, d) of th
formation of the sphere-like precursor nanoparticles in anaqueous solution containing CuCl2 and TAA, followed by thedecomposition of this precursor to CuS species. We found theionic liquid promoted this decomposition reaction to get thetarget product at room temperature.
2. Experimental
The ionic liquid [BMIM]BF4 was synthesized by a modifiedprocedure according to the literature [30]. Chloride salt [BMIM]Cl which was obtained from n-chlorobutane and methylimida-zole was stirred with a 1.1-fold excess of NaBF4 in dry acetoneat ambient temperature for 48 h. Sodium chloride was thenfiltered from the solution, and the solvent was removed underreduced pressure. All the other chemicals were purchased fromcommercial sources and used without further purification.
CuS nanomaterials were synthesized via a simple ionicliquid-assisted route. A typical synthesis was as follows. 0.41 gof CuCl2
. 2H2O and 4 mL of [BMIM]BF4 were dissolved indistilled water (40 mL) to form a homogeneous blue solution ina glass jar under constant stirring. 0.18 g of TAA dissolved indistilled water (40 mL), and was subsequently added into the jarwith CuCl2 solution gradually without stirring or vibration. Ayellow suspension formed in a few min. Then the jar was sealedand maintained at room temperature (ca. 30 °C) for 24 h.Subsequently, the resulting black solid product was filtered,washed with distilled water and absolute ethanol in sequence,and then dried in a vacuum at 60 °C for 6 h. Moreover, acomparative experiment was carried out keeping the reactionparameters constant except that ionic liquid was not added.
X-ray diffraction (XRD) patterns were collected on a Philps-PW3040/60 X-ray diffractometer with Kα radiation (λ=0.15418 nm). The morphologies of the products were observed
e as-prepared hierarchical CuS nanostructures.
Fig. 3. XRD pattern (a) and FESEM image (b) of the precursor Cu3(TAA)3Cl3obtained in the presence of ionic liquid. The inset of (a) is the molecularillustration of Cu3(TAA)3Cl3.
Fig. 4. FESEM image of Cu3(TAA)3Cl3 obtained in the absence of ionic liquid.The inset shows SEM image of single Cu3(TAA)3Cl3 hexagonal prism.
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and determined by the scanning electron microscopy (SEM)(HITACHI S-4800). TEM images were captured on a JEOL-2010 transmission electron microscope at an accelerating vol-tage of 200 kV. Element analysis was recorded on a Vario EL-IIIElemental analyzer.
3. Results and discussion
Fig. 1 shows the XRD pattern of as-prepared CuS in the 2θ range of15–70°. All the diffraction peaks could be indexed as hexagonal phaseCuS with lattice parameters of a=3.794 Å and c=16.35 Å, which areconsistent with the standard card (JCPDS Card 03-1090). No charac-teristic XRD peaks arising from impurities such as CuCl2 and Cu2S aredetected, indicating the acquirement of covellite CuS with high purity.
The morphology and structure of the sample are examined byFESEM. Fig. 2(a) and (b) are typical SEM images of as-prepared CuS,clearly showing that the product possesses a spheroidic morphology withdiameters ranging from250 nm to 300 nm.As can be seen in Fig. 2(c), theTEM image reveals the as-prepared CuS powders consist of typicalnanoflakes. The TEM image shown in Fig. 2(d) further confirms the
morphologic aspects about an individual CuS particle. It is found that theCuS nanoparticle is constructed by numerous nanoflakes with a diameterof less than 10 nm. The results indicate that well-difined hierarchical CuSnanostructures assembled by nanoflakes can be obtained under thepresent experimental conditions.
To better understand the role of [BMIM]BF4, the precursor origi-nated from the initial reaction between CuCl2 and TAAwas collected ata reaction time of 30 min. Fig. 3(a) is the XRD pattern of the obtainedintermediate sample. It is obvious that this XRD pattern agrees wellwith the XRD pattern of cyclo-tri-i-thioacetamide-tris(chlorocopper(i))(Cu3(TAA)3Cl3), for which Xie et al. [23] have simulated the standardXRD pattern (there is no standard XRD pattern of this complexin JCPDS). We thus determined the precursor to be Cu3(TAA)3Cl3,which was confirmed by elemental analysis. The values of C: 13.74%,H: 2.91%, N: 8.10%, with weight percentage is in good agreementwith the theoretical data of Cu3(TAA)3Cl3 (C: 13.80%, H: 2.89%,N: 8.04%). SEM image shown in Fig. 3(b) gives us the morphologicinformation of the precursor. A large amount of regular solid sphereswith an average diameter of ca. 200 nm can be found. In addition, thesurface of these spheres obtained at this short reaction time is rathersmooth, and no flakes are observed.
Further exploration experiments were done in aqueous solutionwithout any ionic liquid while keeping other reaction parameters con-stant. As SEM image shown in Fig. 4, the only hexagonal-prism-shaped precursor Cu3(TAA)3Cl3 without sphere-like morphology wasobtained at a reaction time of 30 min if ionic liquid was not used. As thetime increased even up to 24 h, we found that the product still be Cu3(TAA)3Cl3 hexagonal prisms, and no CuS species was detected. Theseindicate the precursor is stable in solution without ionic liquid at roomtemperature, which is consistent with Xie's conclusion: Cu3(TAA)3Cl3is relatively thermally stable at temperatures lower than 50 °C in suchsystem.
Although the addition of ionic liquid [BMIM]BF4 did not changethe composition of the precursor, it is undoubted that ionic liquidplayed a crucial role in the formation of hierarchical CuS nanoparticles.Firstly, the addition of ionic liquid was responsible for the formation ofregular sphere-like morphology of the precursor, which might bebenefit for the final hierarchical structure. Secondly, ionic liquidpromoted the decomposition of precursor Cu3(TAA)3Cl3 to CuSspecies at room temperature. It is easily concluded that ionic liquidsalso facilitate the polar reactions for inorganic syntheses in water-richsystem. The conclusion is an affirmative answer to the question raised
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in the introduction. A possible mechanism is clarified as follows: (i)ionic liquids really had polarity, however, their low interfacial tensionmade the inorganic species have a high nucleation ratio, whichpropelled the formation of the nanocrystal; (ii) A transition in the Cuvalence state from Cu(I)-complex then to Cu(II) in the productindicated that this reaction was an emblematical redox reactioninvolving electron transfer. The ionic liquid here acted as an electricalconductor which drove the precursor decomposition resulted fromaccelerating electron transfer.
4. Conclusion
In summary, a room-temperature method for the synthesis ofhierarchical CuS nanoparticles has been described. An ionicliquid [BMIM]BF4 was used as assisted agent in the formationprocess of the hierarchical CuS nanostructures. Compared withthe traditional preparation method, this route has promoted thereaction to act at as low as room temperature and simplified thereaction procedures. In addition, this study indicated the ionicliquid also promoted the polar reactions under water-rich con-dition. The intriguing property of ionic liquids is expected toextend to prepare other inorganic materials.
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