ACTA PHYSICO-CHIMICA SINICA Volume 24, Issue 12, December 2008 Online English edition of the Chinese language journal Cite this article as: Acta Phys. -Chim. Sin., 2008, 24(12): 2257−2262. Received: June 25, 2008; Revised: September 8, 2008. *Corresponding author. Email: wclu@shu.edu.cn; Tel: +8621-66132663. The project was supported by the National Natural Science Foundation of China (20503015) and the Science and Technology Commission of Shanghai, China (0852nm00700). Copyright © 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn ARTICLE Facile Synthesis of Leaf-like Cu(OH) 2 and Its Conversion into CuO with Nanopores Liangmiao Zhang, Wencong Lu*, Yongli Feng, Jipeng Ni, Yong Lü, Xingfu Shang Department of Chemistry, Shanghai University, Shanghai 200444, P. R. China Abstract: Leaf-like Cu(OH) 2 single crystals were synthesized via the controlled emulsion interface method using Span80 (sorbitan monooleate) as the stabilizer of the emulsion system. CuO products with nanopores could be simply obtained by the dehydration of Cu(OH) 2 , while maintaining the strip-shaped architecture. The phase structures and morphologies were measured by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Experimental results showed that Cu(OH) 2 microleaves were single crystals and the growth direction seemed to be in [111] crystal plane of the orthorhombic Cu(OH) 2 . The formation of the nanopores should be attributed to the water loss of the transformation from Cu(OH) 2 to CuO. The formation process of Cu(OH) 2 was investigated by taking TEM images at different stages of the reaction. The formed nanoparticles began to rearrange to form nanorods and microleaves possibly via edge-by-edge and side-by-side oriented-attachments because of the formation of larger crystals greatly reducing the interfacial energy. Besides, CuO microarchitectures exhibit blue shifts in UV-Vis spectra and possess larger band gaps compared with those of bulk crystals. Key Words: Leaf-like Cu(OH) 2 ; Emulsion; Soft template; CuO; Nanopores In the recent years, since the dimensional and structural characteristics of inorganic nanostructures endowed them with potential applications in catalysis, medicine, electronics, cos- metics, etc., synthesis of inorganic nanostructures with spe- cific size and well-defined morphologies has attracted consid- erable attention [1] . Considerable effort has been focused on the assembly of lower dimensional building blocks into two- and three-dimensional (2D and 3D) ordered superstructures, such as snowflakes [2] , nanoribbons [3] , nanodendrites [4] , nanospin- dles [5] , hollow structures [6] , and hierarchical structures [7−10] . The methodology for controllable organization with structural diversity from various nanobuilding blocks is a hot research topic in the recent material research fields. Recently, we suc- ceeded in synthesizing hierarchical cantaloupe-like and hol- low microspherical AlOOH superstructures based on self-as- sembly of one-dimensional (1D) nanorods [11] . To date, several self-assembly processes driven by chemical or physical prin- ciples (such as vapor-liquid-solid growth, laser-assisted cata- lytic growth, template-based liquid-chemistry methods, etc.) have been developed and employed for fabrication of the complex nano- and micro-structures [12−14] . However, process simplification, size, and shape control still remain tremendous challenges in the fabrication of these architectures. Recently, copper-based nanomaterials have received in- creasing attention because of their potential application in op- toelectronic devices, catalysis, and superconductors. The magnetic properties of Cu(OH) 2 are remarkably sensitive to the intercalation of molecular anions [15−17] , making the mate- rial a candidate for sensor applications. More importantly, CuO can be obtained through the dehydration of Cu(OH) 2 , and the original size and morphology of Cu(OH) 2 nanostruc- tures can be retained. As an important transition metal oxide with a narrow band gap (E g =1.85 eV) [18] , CuO has been widely exploited for diverse applications such as heterogene- ous catalysts, gas sensors, superconductors, optical switches, lithium ion electrode materials, and field-emission emitters [19] . In addition, CuO was demonstrated to have a complex mag- netic phase, which formed the basis for several high T c (criti- Liangmiao Zhang et al. / Acta Physico-Chimica Sinica, 2008, 24(12): 2257 − 2262 cal temperature) superconductors and materials with giant magnetoresistance [20] . On the basis of the practical applica- tions of CuO nanomaterials, well-defined CuO nanostructures with various morphologies, such as nanowires [21−23] , nanorib- bons [23] , nanosheets [24] , nanoellipsoids [24,25] , pricky micro- spheres [26] , flowerlike [27,28] , and dendritic assemblies [29] , have been documented. To the best of our knowledge, self-assem- bling preparation of CuO microleaves with highly porous structure has not been reported until now. Herein, the fabrication of Cu(OH) 2 microleaves through wet-chemical method is introduced. This method requires neither high temperature nor long synthesis time, wherein CuO microstrips with porous structure are prepared from the dehydration of Cu(OH) 2 . 1 1 Experimental Analytic grade chemicals of CuSO 4 , ammonia, n-hexane, and surfactant Span80 were purchased from Shanghai Chemi- cal Reagents Company (China) and used as received. A typical synthesis of Cu(OH) 2 microstructure was carried out using the following three solutions: water phase 1 (WP-1): a 1.0 mol·L −1 aqueous solution (36 mL) from ion-exchange water and CuSO 4 solution (18 mmol of Cu); oil phase (OP): n-hexane solution (72 mL) of Span80 (1.50 g) for the stabili- zation of the water-in-oil (W/O) emulsion; water phase 2 (WP-2): 0.2 mol·L −1 ammonia solution (126 mL). The liquid phase system was generated by adding WP-1 solution into the OP, which was mixed using a homogenizer at 10000 r·min −1 . After being emulsified for at least 1 min at room temperature, the mixture was poured into WP-2 in one portion, and the fi- nal pH of the above liquid phase system was adjusted to 4 and further stirred for 2 h for aging. Subsequently, the precipitates were separated by centrifugation, washed with distilled water and absolute alcohol, and finally dried at 60 °C for 12 h. For the dehydration of Cu(OH) 2 microleaves to get CuO, the as- prepared samples were heated at 400−900 °C for 1 h with a heating rate of 5 °C·min −1 . Powder X-ray diffraction (XRD) measurements of the as- prepared samples were carried on a Japan Rigaku D/Max-RB X-ray diffractometer with Cu K α radiation (λ=0.1542 nm). Transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images were captured on JEOL JEM-1200EX II and JEOL JEM-2010F at an acceleration voltage of 200 kV, respectively. The mor- phology of the as-prepared samples was characterized by field emission scanning electron microscopy (FE-SEM, JEOL JSM- 6700F). The UV-Vis absorption spectra of the as-prepared products were recorded by a Shimadzu UV-2051PC photo- spectrometer. Fourier transform infrared (FTIR) spectra were obtained on an AVATAR370 spectrometer. 2 Results and discussion The typical morphologies of the final Cu(OH) 2 architectures were examined by SEM. Fig.1 shows the SEM photographs of the samples. The low magnification image (Fig.1a) shows the panoramic of the product indicating that the Cu(OH) 2 crystal- lites self-organize into assemblies. The assemblies tend to ag- gregate with each other to form large agglomerates. To further examine the surface morphologies of the microarchitecture, a high magnification SEM of a single assembly was recorded, as shown in Fig.1b. The entangled architecture is actually comprised of leaf-like particles with the average thickness of 100 nm, width of 200 nm, and various lengths up to several micrometers. To probe the 3D hierarchical nanoarchitectures in more de- tail, we analyzed the agglomerates by means of TEM. Fig.2a is a TEM image of the microstructures. From Fig.2a, it is evi- dent that these architectures consist of individual leaf- like microstructures that are bundled. These leaves are about 100−300 nm wide in the middle section and connect to each other to form 3D architectures. Furthermore, we found that a prolonged ultrasonication for up to 20 min could absolutely disrupt these assemblies (Fig.2b), implying that the interaction among the constituent 3D microstructures was particularly weak. Fig.2c is a TEM image of an individual leaf; it is esti- mated to be ca 150 nm in width and ca 1.2 μm in length. The leaf-like structure of the products was further examined by high-resolution TEM (HRTEM). Fig.2d shows the magnifica- tion of selected area of the leaf shown in Fig.2c. The fringe spacing measures 0.25 nm, which concurs well with the d value of the orthorhombic Cu(OH) 2 [111] crystal plane [30] . Thus, the growth direction of the microleaves seems to be in the [111] direction. Fig.2e shows the selected-area electron diffraction (SAED), which reveals that Cu(OH) 2 microleaves are single crystals. When the Cu(OH) 2 sample was calcined in air by applying a heating rate of 5 °C·min −1 and holding the calcination temperature at 800 °C for 1 h, porous CuO was obtained, as confirmed by Fig.2f. Compared to the samples without calcination, some newly created pores are observed, while the strip-shaped architecture is still maintained. It should be emphasized here that this leaf-shaped architecture has a high thermal stability and is stable even after calcination at 800 °C. Potentially, this highly thermally stable and porous nanostructure has applications in catalysis. Fig.1 Low magnification (a) and high magnification (b) FE-SEM images of Cu(OH) 2 particles Liangmiao Zhang et al. / Acta Physico-Chimica Sinica, 2008, 24(12): 2257 − 2262 The samples were further examined by IR analysis (Fig.3). As seen in Fig.3a, the IR spectrum indicated the existence of surface hydroxyls and coordinated Span80 molecules on Cu(OH) 2 microleaves. The broad band at 3000−3700 cm −1 is deconvolved to make clear the existence of Span80 in the products and two peaks centered at 3394.6 and 3571.4 cm −1 appeared (Fig.3a), which can be assigned to the stretching mode of hydroxyl of pure Span80 (Fig.3b). The bands at 3488.2 and 1631.1 cm −1 correspond to the stretching and bending modes of the hydroxyls of adsorbed water [31] . The band at 1077.6 cm −1 corresponds to the C−O stretching vibra- tion coordinating to metal cations [32] , which shifts about 10 cm −1 to lower wavenumbers compared to the IR spectrum of pure Span80, suggesting the formation of hydrogen bonds between Span80 and the inorganic components. The band at 424.2 cm −1 can be assigned to Cu−O stretching mode and may prove that Cu(OH) 2 is formed [33] . Generally speaking, anneal- ing can decompose impurity groups in the sample and im- prove the crystal quality. Fig.3c shows the room-temperature infrared absorption spectrum of the annealed CuO products. Except for the absorption peak at around 580 cm −1 owing to Cu−O stretching along [ 0 1 1 ] direction and the mode at 535 cm −1 owing to Cu−O stretching along [101] direction [34,35] , all absorption bands corresponding to the Span80 impurities dis- appear, clearly demonstrating that the impurities have been removed. XRD analysis was used to determine the structure and phase of the samples. Fig.4a shows the XRD pattern of the as-prepared blue products. The prepared material was identi- fied as the orthorhombic Cu(OH) 2 (JCPDS No. 13-0420), con- firming that the inorganic component was Cu(OH) 2 . A con- spicuous feature of the Cu(OH) 2 crystals is their broadness, which indicates the small size of the Cu(OH) 2 crystals. Fig.4(b−f) presents the XRD patterns of the products prepared under the same reaction conditions except different calcination temperatures. At 400 °C, a slow transformation to CuO has already started, but it is amorphous because no peaks can be observed. When the temperature is increased to 600 °C, the Fig.2 (a) Panoramic of Cu(OH) 2 assemblies, (b) with 20 min sonication, (c) an individual leaf-like particle, (d) HRTEM image of the single particle in (c), (e) its corresponding SAED pattern, and (f) CuO microleaves with nanopores Fig.3 Infrared spectra of Cu(OH) 2 (a), Span80 (b), and CuO (c) Fig.4 XRD patterns of the as-prepared Cu(OH) 2 leaf-like particles (a) and particles obtained after calcination at different temperatures for 1 h (b−f) Liangmiao Zhang et al. / Acta Physico-Chimica Sinica, 2008, 24(12): 2257 − 2262 transformation rate increases significantly. Finally, the trans- formation at 800 °C is complete with no Cu(OH) 2 signal re- maining. In a word, slow conversion of Cu(OH) 2 microleaves to CuO microleaves can occur above 400 °C. We observed the surface morphology of the product, which was calcined at 400 °C when the phase transformation occurred. It indicated that there were nanopores on the leaves′ surface (Fig.5), and no pores appeared below this temperature. Combined with the compact morphology and single crystal structure of Cu(OH) 2 by TEM observation, the formation of the nanopores should be attributed to the water loss of the transformation from Cu(OH) 2 to CuO. Other conditions, such as growth temperature and pH val- ues, are also important factors affecting the morphologies of the structures. By control of these aspects, different Cu(OH) 2 nanoarchitectures can be realized. If the same reaction is car- ried out at 60 °C, only randomly packed rod-like structure, rather than a leaf-like pattern, is the dominant morphological configuration (Fig.6a). It is therefore apparent that a relatively higher temperature does not favor the formation of well- defined Cu(OH) 2 crystal leaves. For the systems with increas- ing pH value (pH=10), it is found that several particles sized about 100 nm are obtained (Fig.6b). To investigate the formation process and the growth mechanism of the hierarchical microarchitecture, time-de- pendent experiments were carried out. Fig.7 shows the TEM images of the samples obtained after the reaction has pro- ceeded for 1, 5, 30, and 120 min, respectively. These images clearly exhibit the evolution of Cu(OH) 2 nanostructures from nanoparticles to nanorods and finally to microleaves over the time at 25 °C. Therefore, the formation of Cu(OH) 2 leaf-like microstructures is proposed to be a process composed of the following stages: (i) generation of W/O micelle templates with small, nanometer-sized drops of liquid Cu 2+ , (ii) release of OH – ions from ammonia, which react with the Cu 2+ ions near the surface of the micelles to form Cu(OH) 2 nuclei surround- ing the spherical water pool. In short, the micelles act as soft templates for the formation of Cu(OH) 2 nanoparticles. There- fore, the initial formation of a nanometer-sized micelle tem- plate is of great importance for the initial formation of Cu(OH) 2 nanonuclei. (iii) The formed nanoparticles began to rearrange to form nanorods and microleaves possibly via a side-by-side oriented-attachment, which were energetically favored, because of the formation of larger crystals greatly reducing the interfacial energy. A comparative experiment without Span80 or n-hexane, but with other conditions kept constant, only irregular particles were obtained. Fig.8 shows the morphologies of the products synthesized with the assistance of different amounts of Span80. It seems that leaf-like products can only be synthe- sized under the assistance of enough Span80. We believe that Span80 plays a key role in the formation of the leaf-like shape. In the present study, Span80 was not only used for the prepa- ration of W/O type emulsion, but also served as the structure- directing agent for the formation of superstructures. The first nucleation seeds plus Span80 as an initial nucleus will absorb little particles along the growth direction. In this process, Span80 absorbs the small particles by the hydrophilic sorbitan group acting as a soft template for the formation of nanorods. Self-assembly of nanocrystals is driven by van der Waals forces and hydrogen bonding among the certain organic molecules on the surface of particles [36] . Therefore, the initial Fig.5 TEM image of the products calcined at 400 °C for 1 h Fig.6 TEM images of the products obtained at (a) 60 °C and (b) pH=10 Fig.7 TEM images of the products obtained with the reaction proceeding for different times t/min: (a) 1, (b) 5, (c) 30, (d) 120 Liangmiao Zhang et al. / Acta Physico-Chimica Sinica, 2008, 24(12): 2257 − 2262 formation of a nanometer-sized micelle template and the role of Span80 as structure-directing agent are both important for the formation of Cu(OH) 2 microleaves. Further theoretical and experimental investigations must be done to determine the exact nature of the growth mechanism. UV-Vis absorption measurement is one of the most widely used techniques to reveal the energy structures and optical properties of semiconductor nanocrystals. The optical absorp- tion properties of well-aligned CuO leaf-like particles dis- persed in ethanol solution are investigated at room tempera- ture by UV-Vis spectroscopy. The spectrum is presented in Fig.9a. There is a broad absorption peak centered at 256 nm. Moreover, a classical Tauc approach is further employed to estimate the band gap value of CuO crystals according to the equation αE p =A(E p −E g ) 1/2 (where, α is the absorption coeffi- cient, E p is the discrete photon energy, E g is the band gap en- ergy, and A is a constant) [37] . The plot of (αE p ) 2 −E p for CuO is shown in Fig.9b, exhibiting a linear relationship between 3.28 and 4.00 eV. The extrapolated value (the dot straight line to the x axis) corresponding to the band gap of as-prepared CuO is estimated to be 2.20 eV, which is apparently larger than the reported value for bulk CuO (1.85 eV) [18] . The increase in the band gap of CuO architectures is an indication of quantum confinement effects [38] . 3 3 Conclusions In summary, a sophisticated production of Cu(OH) 2 micro- leaves has been successfully synthesized with micelles acting as soft templates. The addition of Span80 molecules is be- lieved to facilitate the formation of the oriented attachment structures. Furthermore, we also demonstrated that leaf-like CuO products with nanopores can be simply obtained by the dehydration of Cu(OH) 2 . It is expected that the novel CuO ar- chitectures may offer exciting opportunities for potential ap- plications in catalysis, electrochemistry, superconductivity, and superhydrophobic coating. 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Chem., 1995, 99: 5500 . ARTICLE Facile Synthesis of Leaf-like Cu(OH) 2 and Its Conversion into CuO with Nanopores Liangmiao Zhang, Wencong Lu*, Yongli Feng, Jipeng Ni, Yong Lü, Xingfu Shang Department of Chemistry,. CuO microarchitectures exhibit blue shifts in UV-Vis spectra and possess larger band gaps compared with those of bulk crystals. Key Words: Leaf-like Cu(OH) 2 ; Emulsion; Soft template; CuO; . individual leaf-like particle, (d) HRTEM image of the single particle in (c), (e) its corresponding SAED pattern, and (f) CuO microleaves with nanopores Fig.3 Infrared spectra of Cu(OH) 2