Fabrication of Micrometer-Scaled Hierarchical Tubular Structures of

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Fabrication of Micrometer-Scaled Hierarchical Tubular Structures of CuS Assembled by Nanoflake-built Microspheres Using an In Situ Formed Cu(I) Complex as a Self-Sacrificed Template Zhenyu Yao, Xi Zhu, Changzheng Wu, Xuanjun Zhang, and Yi Xie*

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 7 1256-1261

Nano-materials and Nano-chemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science & Technology of China, Hefei, Anhui 230026, P. R. China ReceiVed December 14, 2006; ReVised Manuscript ReceiVed April 19, 2007

ABSTRACT: Micrometer-scaled hierarchical tubular structures of CuS assembled by nanoflake-built microspheres were first synthesized in high yield via a one-pot intermediate crystal templating process without surfactant or added templates, in which the intermediate complex Cu3(TAA)3Cl3 formed in situ and subsequently served as a self-sacrificed template. Whereas the intermediate complex and final hierarchical structures were well characterized, the formation mechanism was preliminarily studied based on X-ray diffraction (XRD) studies and scanning electron microscopy (SEM) observations by arresting the growth at a series of intermediate stages in the formation of the hierarchical tubular structures. The benefits for the as-obtained nanostructures arise from their ultrahigh Brunauer-Emmett-Teller (BET) value and the potential capacity advantage for the catalyst industry and hydrogen storage. 1. Introduction Currently, hierarchical structures are of interest to chemists and materials scientists due to their unique functions in the development of advanced devices and systems.1 For instance, Pt nanoshell tubes,2 CdS and CdSe nanotubes and nanowires,3 and hierarchically structured nanowires made of ZnO,4 ZnO/ In2O3,5 SnO,6 SnO2/Fe2O3,7 and V2O5/TiO28 have been synthesized via different routes including electrospinning and hydrothermal methods, and various characteristics of these special hierarchical structures have been studied. In a word, investigation of hierarchical structures of functional materials is very important but challenging. Moreover, in the past decade, owing to their important physical and chemical properties, transition metal chalcogenides with their special microstructure and nanoscaled size have attracted much attention in the field of material science.9-13 For example, as a type of metal chalcogenide, copper sulfide has been found to be an important semiconductive material, which exhibits nearly ideal solar control characteristics14 and fast-ion conduction at high temperature.15,16 Covellite copper sulfide shows metallic conductivity and transforms into a superconductor at 1.6 K.17 Many approaches have been used to synthesize transition metal chalcogenides, including sonochemical methods,18 microwave-assisted methods,19 hydrothermal methods,20,21 thermolysis,22 electrosynthesis,23 solid-state reactions,24 and chemical vapor deposition (CVD).25 However, soft solution processing has been proven to be a convenient, economical, less energy and materials consuming, and environmentally friendly method.26,27 For example, Yu and co-workers27 have synthesized complex CuS microtubes constructed by hexagonal nanoflakes in an acetic acid-assisted solution system including CuCl2 and thioacetamide (TAA); in this system, they found a precursor formed and then dissociated to CuS, and the precursor was determined to be [Cu-(TAA)2]Cl2 based on X-ray fluorescence (XRF) and thermogravimetric analysis (TGA) results. In solution approaches to transition metal chalcogenides, TAA is a commonly used sulfur source, for example, in the sonochemical synthesis of nanophased indium sulfide,28 in the * Corresponding author. Tel: 86-551-3603987. Fax: 86-551-3603987. E-mail: [email protected].

solution-based preparation of bismuth sulfide nanorods,29 and in the hydrothermal synthesis of pure γ-manganese sulfide (MnS) crystallites.30 In these syntheses, TAA was usually simply regarded as a sulfur source. However, there are long pair electrons on nitrogen and sulfur atoms of each TAA molecule that provide the possibility to combine with transition metal ions. It is obvious that there should be a coordination reaction between TAA and metal ions first, but little attention has been paid to this aspect before. In addition, previous research studies implied the reducing effect of the TAA molecule under certain conditions (e.g., excess TAA concentration, at a relatively lower pH value),31 which was often ignored too. Now, an interesting but challenging question is therefore raised: which roles do the TAA play in the formation of metal sulfide nanostructures? To learn about the TAA-based process in detail, we investigated the simplest aqueous system containing only CuCl2 and TAA. As expected, we found that TAA reduces Cu(II) to Cu(I) as it combines to form a complex; however, it is surprising that the complex with Cu(I) formed first, and then transformed to CuS via an oxidation process. This interesting finding has another unexpected gain, i.e., it provides a simple template-free approach for the micrometer-scaled hierarchical tubular structures of CuS assembled by nanoflake-built microspheres. This novel hierarchical tubular structure of CuS exhibited special optical properties and ultrahigh specific surface area. The detailed shapeevolution process from the intermediate complex to the final product was clearly shown, and the mechanism of formation of the hierarchical structure was studied. 2. Experimental Section 2.1. Chemicals and Synthesis. All the reagents were of analytical grade and were used without any further purification. In a typical experiment, CuCl2‚2H2O (2.4 mmol) was dissolved in distilled water (40 mL) and formed a blue solution in a glass jar. Then, thioacetamide (TAA) (2.4 mmol) dissolved in distilled water (30 mL) formed a colorless solution, which was added into the jar with CuCl2 solution gradually without stirring or vibration at room temperature. The mixture gradually turned into a yellow suspension in a few minutes. Then the jar was covered and maintained at 60 °C for about 24 h, and then was allowed to cool to room temperature naturally. The black precipitate that formed at the bottom of the jar was filtered, washed with distilled water and absolute ethanol in sequence, and then dried in a vacuum at 60 °C for 4 h.

10.1021/cg060914i CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007

Hierarchical Tubular Structures of CuS

Figure 1. Powder X-ray diffraction patterns of the as-prepared CuS product. 2.2. Characterization. Transmission electron microscopy (TEM) images and electron diffraction (ED) patterns were taken on a Hitachi model H-800 instrument with a tungsten filament using an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were taken on a KYKY-AMRAY 1000B scanning election microscope. Field emission scanning electron microscopy (FESEM) images were taken on a JEOL JSM-6700F field emission scanning electron microscope or a FEI Sirion 200 field emission scanning electron microscope. X-ray diffraction (XRD) patterns of the sample were collected with a 2θ range from 10° to 70° with a scan speed of about 3°(2θ) /min on a Phillips X Pert (Netherlands) SUPER powder XRD with Cu KR radiation (λ ) 1.5418 Å) using a silicon wafer as the underlay. Element analysis was recorded on a Vario EL-III Elemental analyzer. A Shimadzu spectrophotometer (model SolidSpec-3700) equipped with an integral sphere was used to record the UV-vis diffusion reflection spectra of the samples. Surface areas of the samples were determined by Brunauer-Emmett-Teller (BET) measurements on nitrogen adsorption at 77 K with a Beckman Coulter Surface Area analyzer SA 3100.

3. Results and Discussion 3.1. Phase and Morphology Characterization of the AsObtained CuS Hierarchical Tubular Structures. XRD patterns of the CuS product were shown in Figure 1. All the diffraction peaks could be indexed to the standard diffraction data of the corresponding hexagonal phase CuS with lattice parameters of a ) 3.792 Å and c ) 16.34 Å. (JCPDS Card

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File No. 06-0464). No characteristic peaks of other impurities, such as Cu2S and CuCl2, were observed, indicating that the obtained products consist of pure covellite CuS. The relatively small size of the crystallites of the product makes the diffraction peaks much broader than that of the corresponding standard pattern. Electron microscopy investigations as shown in Figure 2 confirmed the large-scale production of the novel hierarchical tubular structures prepared at 60 °C for 24 h. FESEM provided panoramic images of the product and indicated that the tubes are uniform and straight (Figure 2A). The total length of a single tube was 300-500 µm (Figure 2A), and the extra diameter of the tube was about 5-7 µm with a wall thickness about 1 µm (Figure 2B,C). The open ends of tubes could be distinctly seen. The tube walls were constructed by microspheres with a diameter of about 1 µm (Figure 2C,D), while these microspheres were built by many interleaving but slightly bending nanoflakes (Figure 2D). The yield of hierarchical tubular structures was ultrahigh (>95%) (Figure 2A). Although the entire tube is too large and thick-walled for the TEM observation, we could still get the TEM image of individual microspheres and nanoflakes after ultrasonic dispersion for a long time (30 min to 1 h). Figure 2E confirmed that the sphere with a diameter of 1 µm is composed of numerous nanoflakes and has a diameter of 300 nm and a thickness of about 8 nm (Figure 2E,F), which agrees well with the broad peaks in the XRD pattern. The SAED pattern in Figure 2F showed that the nanoflakes are single crystals with a hexagonal phase corresponding to covellite CuS. 3.2. Understanding the Intermediate Cu3(TAA)3Cl3 Prisms at the Short Reaction Time of 1 h. To better understand the formation mechanism of hierarchical tubular structures, timedependent experiments were carried out while keeping other reaction parameters constant. The intermediate sample could be obtained at a reaction time of 1 h, and it is found that a large amount of hexagonal prisms with an average diameter of 5 µm and average length of 500 µm was found at 1 h as seen from the FESEM images (Figure 3A). In addition, the surface of the prisms obtained at this short reaction time was rather smooth, and no small particles existed on the prism surface (inset in Figure 3A).

Figure 2. FESEM and TEM images of the CuS product prepared at 60 °C for 24 h. (A) FESEM image at low magnification; (B) FESEM image at medium magnification; (C) FESEM image at high magnification; (D) FESEM image of a wall of a broken hierarchical tubular structure; (E) TEM image of an individual CuS sphere composed by nanoflakes; (F) TEM image of a single nanoflake (inset in F shows the SAED pattern).

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was done in the literature33 using the cell parameters of singlecrystal cyclo-tri-µ-thioacetamide-tris(chloro-copper(i))3 from the literature32 (space group No. 161, a ) 19.76 Å, c ) 7.065 Å). After the calculations were performed, simulated powder XRD patterns were produced, which are shown in Figure 4a. From panel a, we can see that the simulated XRD pattern agrees well with the experimental XRD pattern in panel b, which confirms that the yellow intermediate hexagonal prisms obtained at 60 °C for 1 h are Cu3(TAA)3Cl3. Its molecular illustration is shown in Figure 4c. Moreover, the elemental analysis of the yellow prisms also confirms the above conclusion. The values of C: 13.84%, H: 2.84%, N: 8.01%, with weight percentage was in good agreement with the theoretical data of Cu3(TAA)3Cl3 (C: 13.80%, H: 2.89%, N: 8.04%). It is worth noting that the Cu valance in the Cu3(TAA)3Cl3 complex is +1, rather than +2. While the Cu3(TAA)3Cl3 complex was formed in situ without any adding reducer, there is no doubt that a reduction process is involved in this simple CuCl2-TAA aqueous system. It is understandable that S atoms with a valence of -2 in TAA act as the reductant in this aqueous system. When the formed yellow hexagonal prism-shaped Cu3(TAA)3Cl3 intermediate is filtered out at 1 h, SO42- ions could be detected in the filtrate by BaCl2 solution, and the particular odor of NH3 also supported the idea that TAA may have reduced Cu(II) and combined with Cu(I) as follows:

CuCl2 + TAA + H2O f Cu3(TAA)3Cl3 + H+ + NH4+ + Cl- + SO42- + CH3COO- (1)

Figure 3. FESEM images and XRD patterns of products prepared at 60 °C for different reaction times. FESEM images: (A) 1 h; (B) 3 h; (C) 7 h; (D) 10 h; (E) 13 h; (F) 16 h. XRD patterns: (a) 1 h; (b) 3 h; (c) 7 h; (d) 10 h; (e) 13 h; (f) 16 h.

The XRD pattern and the element analysis gave us further information about the as-obtained intermediate sample. The coordination compound is cyclo-tri-µ-thioacetamide-tris(chlorocopper(i)) (Cu3(TAA)3Cl3) from the literature;32 since TAA has a strong ability to combine with transition metal ions, the hexagonal prism-shaped intermediate should be a complex from the coordination reaction between CuCl2 and TAA. Because there is no standard XRD pattern of cyclo-tri-µ-thioacetamidetris(chloro-copper(i)) (Cu3(TAA)3Cl3) in the JCPDS database, we used the Fullprof software package to index the atoms as

3.3. Formation Mechanism of the As-Obtained CuS Hierarchical Tubular Structures. The integration of FESEM images and the corresponding XRD patterns of the different intermediate samples at different reaction stages clearly reveals the growth process of hierarchical tubular structures; the template sacrifice process of Cu3(TAA)3Cl3 prisms could be seen in the time evolution of FESEM images as shown in Figure 3. At a short reaction time of 1 h, we can see that large amounts of prisms with a smooth surface and no small particles existed on the prism surface (inset in Figure 3A). After the sample was kept at 60 °C for 3 h, nanoparticles with a diameter of about 50 nm appeared on the surface of the hexagonal prisms (inset in Figure 3B). The yellow suspension turned brown at this stage suggesting that the black CuS product began to form; the brown suspension is in fact a mixture of yellow hexagonal prisms and black CuS. The XRD peaks for the Cu3(TAA)3Cl3 could be found in the XRD pattern of the samples prepared at 3 and 7 h (Figure 3b,c), indicating that the product was mainly the complex intermediate sample and the proportion of CuS is rather small up to the present. The formation of CuS spherelike structures composed of nanoflakes had been commonly reported before,17,23-26 and it should be a characteristic growth habit for CuS in solution phase approaches. At a reaction time of 7 h, nanoparticles grew to larger microspheres constructed by nanoflakes with an average diameter of 500 nm. The microspheres grew on the surface of hexagonal prisms, then aggregated to cover all the surfaces, like a hexagonal shell consisting of nanoflake-built microspheres (Figure 3C). The brown suspension turned black in 5-7 h after the reaction started, which also suggested that the surface of the hexagonal prisms was covered with CuS nanoflake-built microspheres at this reaction stage. At reaction times of 10 and 13 h as shown in Figure 3, panels D and E, respectively, one can see that the prism’s core keeps dwindling while the covered CuS microspheres grow continuously. Note that the diameter of the Cu3(TAA)3Cl3 core was

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Figure 4. (a) The simulated XRD pattern of Cu3(TAA)3Cl3; (b) the recorded XRD pattern of the intermediate prepared at 60 °C for 1 h; (c) molecular illustration of cyclo-tri-µ-thioacetamide-tris(chloro-copper(i)) (Cu3(TAA)3Cl3).

reduced to about 4 µm after reacting for 10 h, while the average diameter of CuS microspheres increased to 1 µm, revealing that the self-sacrificed role of the intermediate and the growth of CuS took place simultaneously. It is interesting that the hexagonal Cu3(TAA)3Cl3 prism cores gradually dwindled into cylinders with a smaller diameter, perhaps because the ridges of hexagonal prism-shaped intermediate cores may be consumed with higher priority because of the lower distance between the ridges with the CuS shells compared to other parts of the intermediate cores. On the other hand, the growth of more closely packed microspheres made the CuS shell cylindrical too (Figure 3D). As for the intermediate sample collected at 13 h, the diameter of the Cu3(TAA)3Cl3 core was further reduced to about 1 µm (Figure 3E). It is obvious that the intensity of diffraction peaks of hexagonal CuS in XRD patterns became stronger and stronger for the intermediate samples obtained from 7 to 16 h, while the corresponding diffraction peaks of the intermediate sample Cu3(TAA)3Cl3 became gradually weaker (Figure 3c-f), which verifies the increasing content of hexagonal phase CuS in products as the time increased in our reaction system. Eventually, the pure CuS product formed after reacting for 16 h (Figure 3f,F), and hierarchical tubular structures assembled by nanoflake-built microspheres could be achieved. As discussed before, the combination of copper ion and TAA in the mixed solution in situ produced the hexagonal prismshaped Cu3(TAA)3Cl3 complex, which then served as a selfsacrificed template for the final hierarchical tubular structures. Interesting, Cu(I) in Cu3(TAA)3Cl3 was oxidized to Cu(II) in the final CuS product in the decomposition process, which could be described by the following equation:

Cu3(TAA)3Cl3 + O2 + H2O f CuS + NH4+ + H+ + Cl- + CH3COO- (2) In this decomposition process, the in situ formed complex Cu3(TAA)3Cl3 directly acts as both the copper source and the sulfur source. Comparative experiments were carried out to exclude the possible influence of copper or sulfur compound in this mixed solution. Cu3(TAA)3Cl3 hexagonal prisms formed at 60 °C for 1 h were filtered out and washed several times with distilled water, and then put into another jar with 70 mL of distilled water at 60 °C for 23 h. We found that the products obtained here were the same hierarchical tubular structures as the structures shown in Figure 2, which proved that the complex intermediates acted as both the copper source and the sulfur source and confirmed the important role of decomposition of intermediates in the formation of the CuS structure.

Scheme 1. Scheme of Growth Mechanism of the Hierarchical Tubular Structuresa

a (a) An in-situ formed hexagonal prism-shaped Cu (TAA) Cl crystal; 3 3 3 (b) CuS nanoparticles appeared on the surface of intermediate prism; (c) nanoparticles grew up to CuS microspheres composed of nanoflakes; (d) CuS microspheres composed of nanoflakes covered over the intermediate surface to form CuS shells; (e) continuous consuming of intermediate cores and growing of CuS shells made them transformed from hexagonal to nearly cylindrical; (f) a hierarchical tubular structure formed after the exhaustion of the intermediate core.

It is also noticeable that the yellow Cu3(TAA)3Cl3 hexagonal prisms turned brown very quickly when exposed to the atmosphere, which implied that they may be easily oxidized by oxygen in the atmosphere, and then decomposed to CuS. Cl- ions were identified by AgNO3 solution, and the distinct smell of NH3 could also be detected in the comparative experiment above, suggesting that in the decomposition of Cu3(TAA)3Cl3 to CuS there is an oxidation process that most probably is due to oxygen in the atmosphere. While the overall chemical process could be simply expressed as

CuCl2 + TAA + H2O + O2 f CuS + CH3COO- + NH4+ + H+ + Cl- + SO42- (3) the morphology and phase transformation in the system could be described in Scheme 1. 3.4. Reaction Parameters Independent of the As-Obtained CuS Morphology. In the formation of micrometer-scaled hierarchical tubular structures of CuS assembled by nanoflakebuilt microspheres, the reaction temperature plays an important role. Experiments at different reaction temperatures such as 20, 40, 50, 60, 90, and 150 °C were performed. SEM images and

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Figure 5. FESEM images and XRD patterns of product at different reaction temperatures for 24 h. FESEM images: (A) 40 °C; (B) 50 °C; (C) 90 °C; (D) 150 °C. XRD patterns: (a) 40 °C; (b) 50 °C; (c) 90 °C; (d) 150 °C.

Figure 6. FESEM image (a) and XRD pattern (b) of product at 60 °C for 24 h with continuous stirring.

corresponding XRD patterns of products were shown in Figure 5. XRD patterns indicated that the products obtained at lower than 50 °C were only Cu3(TAA)3Cl3, while the final products were totally hexagonal phase CuS at temperatures not lower than 50 °C. The higher the reaction temperature, the better crystallinity of the CuS product (Figure 5a-d). The similar hexagonal prisms with an average diameter of 5-7 µm and a length of 500 µm were found in the sample at 20 and 40 °C for 24 h (Figure 5A), which were also very similar to the hexagonal intermediate prisms prepared at 60 °C for 1 h (Figure 3A). The XRD pattern confirmed its same structure and component (Figures 3a and 5a). These hexagonal prisms were unchanged in temperatures from 20 to 40 °C for 24 h, which indicates that Cu3(TAA)3Cl3 is rather stable in solution at temperatures below 50 °C. The products were hierarchical tubular structures when the temperature was raised to 50 °C or above (Figure 5B-D). Besides hierarchical tubular structures, some microspheres composed of nanoflakes were also found at temperatures higher than 90 °C. XRD investigations supported the above conclusion that the Cu3(TAA)3Cl3 hexagonal prisms formed at temperatures as low as 40 °C and were stable at temperatures lower than 50 °C for even 24 h. XRD results also confirmed that the intermediate prisms would totally transform to CuS in 24 h when the temperature was raised to 50 °C or above. SEM observations indicate that the hierarchical tubular structures of CuS could only be perfectly constructed by the nanoflake-built microspheres at the temperature range of 50-70 °C. The influence of different reaction temperatures could be well understood by our proposed mechanism. As discussed above,

Cu3(TAA)3Cl3 hexagonal prisms are relatively thermally stable at temperatures lower than 50 °C in our reaction system. Thus, the further growing process of CuS structures stopped at stage A in Scheme 1, and only the Cu3(TAA)3Cl3 complex could be collected (Figure 5a). When the temperature was raised to 50 °C, the decomposition of intermediate resulted in the formation of hierarchical tubular structures (Figure 5b). Reactions at higher temperatures, such as 90 or 150 °C (the reaction at 150 °C took place in a sealed Teflon-lined autoclave), which may greatly induce the rapid decomposition of Cu3(TAA)3Cl3 intermediate and formation of CuS microspheres, would also lead to surplus CuS microspheres. These surplus CuS microspheres not only increased the percentage of individual CuS microspheres in final products but also disturbed the assembly of CuS microspheres on the surface of hexagonal prisms intermediates, which resulted in the rough shape of as-prepared structures, shown in Figure 5C,D. In a word, 50-70 °C is the optimal condition for the construction of well-defined hierarchical tubular structures of CuS. The non-stirring condition in our experiments also plays a vital role in the formation of the as-obtained hierarchical tubular structure. It was found that only uniform spheres composed of nanoflakes could be obtained in the reaction for 24 h at 60 °C (Figure 6), where the diameter of the spheres is about 700 nm and the thickness of nanoflakes is about 10 nm. Stirring may destroy the arrangement of CuS microspheres on the surface of intermediate prisms and thus lead to individual CuS microspheres composed of nanoflakes. The non-stirring condition is

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mediate crystal templating process may open the way for new routes to synthesize unconventional hierarchical nanostructures. Further experimental, theoretical, and computational studies are now underway. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20621061) and the State Key Project of Fundamental Research for Nanomaterials and Nanostructures (2005CB623601). References

Figure 7. UV-vis absorption spectra of product prepared at 60 °C for 24 h.

no doubt favorable to the assembly of CuS microspheres on the self-sacrificed template of the intermediate complex. 3.5. Optical Properties and BET Surface Area of the AsObtained CuS Hierarchical Tubular Structures. The optical properties of as-prepared CuS structures were characterized by UV-vis absorption spectra (Figure 7). The wide absorption hump of CuS centered at 654 nm, which indicated that there are distinct red shifts compared with the results in the literature.34,35 The red shift of the maximum, unlike the situation of CuS nanowires,36 may be associated with the formation of aggregated CuS nanoflakes,27 which is similar to the red shift attributed to aggregated silver nanoparticles.37 It is interesting that a broad shoulder at about 350 nm occurred in the spectra of the samples, which is different from the literature.27,35 The unusual phenomena in the UV-vis absorption spectrum may have potential applications in the optical field. Surface areas of the samples were measured by the BET method. The BET surface area of as-prepared CuS hierarchical tubular structures is 262 m2 g-1. It is notable that the BET surface of CuS particles commonly used in the catalytic field is only 0.378 m2 g-1.38 Thus, the as-prepared hierarchical CuS tubes constructed by nanoflake-built microspheres exhibit an ultrahigh surface area and may have potential application in the catalyst industry and hydrogen storage. 4. Conclusions A one-pot in situ formed and self-sacrificed template route to synthesize micrometer-scaled hierarchical tubular structures of CuS assembled by nanoflake-built microspheres of high yield and purity at a rather low temperatures without surfactant or added templates has been developed; the intermediate complex Cu3(TAA)3Cl3 forms in situ and subsequently serves as a selfsacrificed template. While the intermediate complex and final hierarchical structures were well characterized, the formation mechanism was preliminarily studied. An interesting transition in the Cu valence state from Cu(II) in raw material to Cu(I) in the intermediate complex then to Cu(II) in the product was observed and fully studied, which clarified the chemical component, crystal structure, morphology, and template effect of the intermediate complex in this CuCl2-TAA system and may be helpful in understanding of the similar TAA-based system. The UV-vis spectra and BET measurements on nitrogen adsorption indicate special optical properties and the ultra-high specific surface area of the product, which may have potential applications, for example, in the optical industry, the catalysis industry, and hydrogen storage. This one-pot inter-

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