Solution-Phase Synthesis of Single-Crystal CuO Nanoribbons and

Apr 7, 2007 - Crystal Growth & Design , 2007, 7 (5), pp 930–934. DOI: 10.1021/ ... and Milan Kanti Naskar. Crystal Growth & Design 2014 14 (6), 2977...
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Solution-Phase Synthesis of Single-Crystal CuO Nanoribbons and Nanorings Xiaoqing Wang, Guangcheng Xi, Shenglin Xiong, Yankuan Liu, Baojuan Xi, Weichao Yu, and Yitai Qian*

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 5 930-934

Hefei National Laboratory for Physical Science at Microscale and Department of Chemistry, UniVersity of Science & Technology of China, Hefei, Anhui 230026, P. R. China ReceiVed NoVember 10, 2006; ReVised Manuscript ReceiVed January 16, 2007

ABSTRACT: A solution-phase route has been developed for the synthesis of single-crystal CuO nanoribbons with widths of 1080 nm, thicknesses of 5-20 nm, and lengths ranging from several hundred nanometers to several micrometers and for nanorings with diameters of 100-300 nm. The CuO nanoribbons and nanorings were fabricated by the reaction of CuCl2 and NaOH solutions with sodium dodecyl benzenesulfonate (SDBS). The as-synthesized products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM), revealing that CuO nanoribbons and nanorings had single-crystal monoclinic structures and nanorings were closed but were not the simple superposition of two ends of the nanoribbons. On the basis of TEM observations, the formation processes of nanoribbons and nanorings can be interpreted by two stages: initial nanoflakes split into nanoribbons due to the Brownian movement of surfactant molecules, and then these nanoribbons that possess polar surfaces coil into nanorings to reduce the electrostatic energy. This ringlike CuO nanomaterial may have some potential value in nanoscale applications. Introduction As an important p-type transition-metal oxide with a narrow band gap (Eg ) 1.2 eV), cupric oxide (CuO) has been extensively studied due to its important properties and applications. CuO has been explored to be used as a new class of electrode material for rechargeable lithium-ion batteries.1-5 With regard to its commercial value, CuO has been used as a heterogeneous catalyst for converting hydrocarbons completely into carbon dioxide and water, degrading nitrous oxide, and improving selective catalytic reduction of nitric oxide with ammonia.6-9 In addition, CuO is used as an optical switch, pigment, fungicide, metallurgy reagent, gas sensor, magnetic storage media, field emission (FE) emitter, and solar cell owing to its photoconductive and photochemical properties.10-24 More and more attention has been paid to ringlike nanomaterials due to their size, special morphology-related properties, and potential nanoscale applications.25-30 Recently, ZnO nanorings have been formed via a spontaneous self-coiling process during the growth of polar nanobelts in a solid-vapor process.25 Furthermore, single-crystal nanorings of β-Ga2O3 have been synthesized using a microwave plasma.31 In addition, closed PbS nanowires can be prepared via a solution synthetical route.32 In recent years, well-defined CuO nanostructures with different morphologies such as nanowires,24 nanorods,33-36 nanoribbons/ nanobelts,37-39 nanotubes,36 nanoplates,40,41 nanoplatelet arrays,42 microspheres43,44 made up of nanoribbons have been obtained by a series of solution-based routes and vapor-phase processes. However, there is no report on the synthesis of CuO singlecrystal nanorings. In this paper, we report the solution synthesis of single-crystal CuO nanorings with diameters of 100-300 nm and nanoribbons with widths of 10-80 nm, thicknesses of 5-20 nm, and lengths ranging from several hundred nanometers to several micrometers. The CuO nanorings and nanoribbons were fabricated by the reaction of CuCl2 and NaOH solutions with sodium dodecyl benzenesulfonate (SDBS). In this synthesis, it was found that the growth process of CuO nanoribbons was * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +86-551-360-1589. Fax: +86-551-360-7402.

different from those in former reports.37-39 A flake-splitting mechanism was proposed. The as-obtained CuO nanorings were characterized by high-resolution transmission electron microscopy (HRTEM), revealing that nanorings had single-crystal monoclinic structures and were closed but were not the simple superposition of two ends of the nanoribbons. The single-crystal nanorings could be formed via a spontaneous self-coiling process during the growth of polar nanoribbons in possession of the alternate stacking of Cu2+ and O2- ions along the [002] axis. In addition, a series of parallel experiments were carried out to study the effect of synthetic parameters on the growth of CuO nanoribbons and nanorings. Experimental Section All the reagents used in the experiments were analytically pure, purchased from Shanghai Chemical Company, and were used without further purification. Synthesis. In a typical procedure, a 25 mL aqueous solution containing 1 mmol of CuCl2‚2H2O and 1 mmol of sodium dodecyl benzenesulfonate (SDBS) was prepared in a beaker, which was stirred with magnetic stirrer for 10 min. Subsequently, 15 mL of 4 M NaOH solutions was added dropwise into the above solution within 5 min. After the solution was further stirred for 10 min, it was transferred to a stainless Teflonlined autoclave of 60 mL capacity, heated at 120 °C for 30 min, and then cooled to room temperature naturally. This mixture was centrifuged to separate the CuO products that were precipitated from the solution. The black precipitates were rinsed with hot distilled water and absolute ethanol several times each to remove impurities such as surfactants. Finally, the products were dried in a vacuum at 55 °C for 6 h. In addition, a series of parallel experiments were carried out to study the effect of synthetic parameters on the growth of CuO nanoribbons and nanorings: varying amounts of SDBS (0, 0.1, and 2 mmoL) or replacing SDBS with sodium dodecylsulfonate (SDS), differing initial concentrations of CuCl2 (0.008 and 0.08 M) and NaOH (1 and 8 M), and changing the reaction temperature (80 and 160 °C).

10.1021/cg060798j CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007

Single-Crystal CuO Nanoribbons and Nanorings

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Figure 1. XRD patterns of the as-obtained products taken from the reaction mixture after heating times of (a) 0 min, (b) 15 min, (c) 30 min, and (d) 2.5 h.

Figure 2. (a) Low-magnification and (b, c) high-magnification TEM images of the obtained CuO nanorings and nanoribbons grown for 30 min in the presence of SDBS.

Characterization. X-ray powder diffraction (XRD) pattern of the products was recorded on a Rigaku (Japan) D/max-γA X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å). The transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were captured on a Hitachi model H-800 instrument at an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) images and SAED patterns were obtained on JEOL-2010 transmission electron microscope at an accelerating voltage of 200 kV. The Raman spectrum was recorded with a LABRAM-HR confocal Laser MicroRaman spectrometer with 514.5 nm radiation from an argon laser at room temperature. X-ray photoelectron spectra (XPS) were obtained from an ESCALAB MK II X-ray photoelectron spectrometer by using nonmonochromatized Mg KR X-ray as the excitation source with an overall energy resolution of 1.0 eV. Results and Discussion Structure and Morphology. Typical XRD patterns of the products are shown in Figure 1. All the peaks can be indexed to monoclinic-phase CuO (space group C2/c). Compared with the standard diffraction peaks from JPCDS Card No. 80-1268, no other peak is observed belonging to the impurities, such as CuCl2, NaOH, SDBS, or other precursor compounds, indicating the high purity of the as-obtained products. Figure 2 shows the TEM images of a representative sample prepared under the same conditions as those presented in Figure 1c, revealing that these

Figure 3. (a, b) TEM images of CuO nanorings. Inset in (a) is the ED pattern of the whole smaller ring; inset in (b) is the corresponding ED of the whole ring. (c) HRTEM image of an individual CuO nanoring in panel b. (d) High-magnification HRTEM image of a quadrate section in panel c. (e) TEM image and SAED pattern (inset) of an individual CuO nanoribbon; (f) corresponding HRTEM image of this nanoribbon.

products are composed of nanoribbons with widths of 10-80 nm, thicknesses of 5-20 nm, and lengths ranging from several hundred nanometers to several micrometers. It is obvious that there are some flexural and ringlike ribbons. The morphology and microstructures of such CuO structures were further studied using HRTEM and SAED. Figure 3a,b shows the TEM images of CuO rings with a radius of about 100-300 nm. The inset of Figure 3a shows the corresponding SAED pattern taken by focusing the whole smaller ring. Another typical CuO ring is shown in Figure 3b, whose top left corner suggests the possible self-coiling of a single nanoribbon into a ring. The ring shown in this image has a radius of ∼100 nm, while the thickness is thinner than 20 nm. Figure 3c shows a typical HRTEM image of the CuO nanoring in Figure 3b. The lattice image (Figure 3d) indicates the single-crystal nature of the CuO nanorings. It is interesting that single-crystal CuO nanorings have the SAED patterns (shown in the insets of Figure 3a,b) similar to polycrystal, which is likely to come from the ringlike morphology and multilayer structure of the nanorings. A lattice spacing of 0.252 nm for the [002] plane of the monoclinic CuO structure can be readily resolved. Figure 3e is a TEM image of an individual nanoribbon with a width around 70 nm and a length of several hundred nanometers. The SAED pattern can be indexed to be the [001] zone axis of monoclinic-phase CuO, demonstrating the single-crystal nature of CuO nanocrystals.

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Figure 4. TEM images of four samples, showing different stages of growth for CuO nanoribbons and nanorings. These samples were taken from the reaction mixture after a heating time of (a) 0 min, (b) 15 min, (c) 30 min, and (d) 2.5 h. Inset in (a) is a typical nanoflake and its ED pattern.

The HRTEM image shows that the regular spacing of the clear lattice planes is ca. 0.229 nm, which corresponds well to (200) planes of monoclinic-phase CuO. On the basis of HRTEM and SAED analysis, it can be demonstrated that the CuO nanoribbons grow along the [010] direction, which is the same as in previous reports.42 Growth Process and Possible Growth Mechanism. Several parallel experiments were carried out to determine the parameters that are important for the formation of nanoribbons and nanorings: (1) No CuO nanoribbons and nanorings were produced in the absence of SDBS or when SDBS was replaced with sodium dodecylsulfonate (SDS). (2) A low concentration of CuCl2, SDBS or a high concentration NaOH solutions did not promote the formation of CuO nanoribbons and nanorings. (3) Temperature did not affect the morphology of the as-obtained products but it can affect the quality of crystallization. The correlative TEM images are shown in Supporting Information. On the basis of our experimental results, we determined the optimized conditions by changing the reaction time to explore the growth process of CuO nanorings; the CuO nanostructures at various stages of the growth process were characterized using TEM. Figure 4a-d shows the TEM images of the samples taken from the reaction mixture at different periods of time (0, 15, 30 min, 2.5 h at 120 °C) (other conditions are the same as those described in the second paragraph of the experimental section). These images clearly show the evolution of CuO nanostructures

Wang et al.

from nanoflakes to nanoribbons and then to nanorings over time at 120 °C. From the XRD patterns (Figure 1a-d), we know that CuO crystallizes easily even at room temperature, and the samples obtained after the above reaction time are all crystals. Figure 4a shows the TEM image of the sample taken from the mixture before heating it. This image demonstrates that the initial products are made of many nanoflakes with widths of 100400 nm and lengths of up to 1-2 µm. Some of these nanoflakes have a tendency to split into nanoribbons, as shown in the inset of Figure 4a. The associated SAED patterns (inset in Figure 4a) reveal that CuO nanoflakes are crystals with a preferential growth direction of [010], which is the same as that of CuO nanoribbons. On the basis of Figure 3f and the inset of Figure 4a, it can be concluded that CuO nanoribbons split from the (200) planes of nanoflakes. After 15 min of heating of the sample, a large amount of nanoribbons and nanorings was formed. Figure 4b is a typical TEM image of the sample taken from this solution at this stage. Most of these nanoribbons had widths of 10-80 nm and lengths ranging from several hundred nanometers to several micrometers. From this image, it is obvious that some ends of the nanoribbons started to curve, and a few nanorings appeared. When we prolonged the time to 30 min, more and more nanorings were obtained. Among them, some nearly perfect rings were formed, as shown in Figure 4c. As the time was increased, the CuO nanorings became less and less prevalent. When the product was heated for 2.5 h, it was found that there were few nanorings but more nanoribbons, which are shown in Figure 4d. For the growth of inorganic ringlike structures in the solution process, several growth mechanisms have been put forward. The restructuring of polymer-inorganic hybrid nanocrystals during mineralization and the dissolution of nanocrystals during crystal growth based on the well-known Ostwald ripening process from the inner side toward the outside is proposed to explain the formation of hexagonal ZnO nanorings.46,47 Murry et al. proposed an oriented attachment mechanism of nanoparticles for the formation of PbSe nanorings.48 Self-assembly of nanocrystals is proposed to explain the formation of semiconductor CdS rings.49 However, we think that the above mechanisms proposed in the solution process are not suitable to explain the formation of the CuO nanorings on the basis of our experimental results. Recently, Shen et al. reported on the first discovery of Ag2V4O11 nanorings and microloops formed by the self-coiling of Ag2V4O11 nanobelts under a hydrothermal process in the absence of any surfactants,45 which to a certain extent are similar to the formation of CuO nanorings, such as self-coiling of nanobelts and the hydrothermal process. However, in our experiments, the surfactant SDBS played an important role in the formation of CuO nanoribbons, which are the building materials for the formation of nanorings. On the basis of the above discussion, a growth mechanism consisting of surfactantassisted splitting of flakes and self-coiling of polar ribbons is proposed. The possible formation processes are shown schematically in Figure 5C, and three distinctive stages are proposed as follows: (1) It is found that SDBS was necessary during the forming process from CuO nanoflakes to nanoribbons, but the exact role of SBDS is still unclear. SBDS might play a role in at least three aspects of the process: preventing the aggregation of CuO nanoparticles in the initial stage of nanoflakes growth; inducing CuO nanoflakes to split into nanoribbons; and kinetically controlling the growth rates of various crystallographic facets of monoclinic CuO through selectively adsorbing on these facets. The hydrophilic head of DBS- could form an anionic surface exposed to water, and thus Cu2+ cations could directly

Single-Crystal CuO Nanoribbons and Nanorings

Crystal Growth & Design, Vol. 7, No. 5, 2007 933

Figure 5. (A) The ac-plane view of the crystal structure of CuO. (B) Simulation image of the initiation and formation of the single-crystal nanoring via self-coiling of a polar nanoribbon. (C) Schematic illustration of a possible process for the formation of CuO nanoribbons and nanorings via splitting of CuO nanoflakes and self-coiling of polar nanoribbons.

attach to the negatively charged DBS- template to initiate the crystal growth of a flakelike structure (eq 1). The produced Cu (DBS)2 then can be transformed into [Cu(OH)4]2- by the reaction with NaOH (eq 2). As the reaction temperature increases, CuO can be formed by the dehydration of [Cu(OH)4]2-, and then the DBS- template can directly interface with the positively charged surfaces of CuO (eq 3).

CuCl2 (aq) + 2SDBS (aq) f Cu(DBS)2 (s) + 2NaCl (aq) (1) Cu(DBS)2 (s) + 4NaOH (aq) f [Cu(OH)4]2- (aq) + 2SDBS (aq) + 2Na+ (aq) (2) [Cu(OH)4]2- (aq) f CuO (s) + H2O + 2OH- (aq) (3) Simultaneously, when the reaction temperature became high gradually, the Brownian movement of surfactant molecules got so intense that they might have destroyed the interaction of CuO crystal facets,52 which meant that CuO flakes first split into small nanoribbons at two ends of the flakes and then subsequently developed into dispersive ribbons. Crystal splitting can be influenced by several factors: solution oversaturation, impurity concentration, and crystal structure.50 The splitting of CuO nanoflakes into nanoribbons may not continue due to the Ostwald ripening process51 and the above-mentioned reasons. (2) The growth of the nanorings structure could be understood on the basis of the polar surface of the CuO nanoribbions. As shown in the Figure 5A, the monoclinic CuO crystal can be described as alternating planes composed of O2- and Cu2+ ions, which are stacked alternately along the [002] axis. The oppositely charged ions form positively charged (002)-Cu and negatively charged (002)-O polar surfaces on the top and bottom surfaces of the nanoribbon (Figure 5B). When the polarizationinduced electrostatic energy is larger than the elasticity energy, to minimize the area of the polar surface, the polar nanoribbons coiling into nanorings may be one possible way to reduce the electrostatic energy.45 (3) As the reaction time increases, the nanoribbons and nanorings may get thicker and less flexible due to the Ostwald ripening process, which leads to the reduction of the CuO nanorings. Composition Analysis of CuO Nanostructures. The room temperature Raman spectrum of CuO nanoribbons and nanorings is shown in Figure 6. CuO has a monoclinic structure with space group symmetry of C62h. There are 12 zone-center opticalphonon modes, 4Au + 5Bu + Ag + 2Bg, three of which Ag + 2Bg are Raman active. In comparison with the Raman vibrational

Figure 6. Raman spectrum of CuO nanocrystals.

spectrum of a CuO single crystal, the bands at 296, 342, and 627 cm-1 correspond to the Ag, Bg, and Bg modes of CuO crystal; the Raman peaks of the CuO nanoribbons and nanorings are broadened and shift to high wavenumbers owing to the size effects of the as-obtained products.54,55 The purity and the composition of the as-obtained products were further investigated by X-ray photoelectron spectroscopy (Figure 7). The binding energies obtained in the XPS analysis were standardized for specimen charging using C1s as the reference at 284.5 eV. As shown in Figure 7a, the peaks at 934 and 954.5 eV are attributed to Cu 2p3/2 and 2p1/3, respectively. As shown in Figure 7b, three Gaussians (marked as R, β, and γ) were resolved by using a curve-fitting procedure. Curve R was identified as O2- binding with Cu at 529.6 eV. Curves β and γ were identified as oxygen absorbed on the surface of CuO nanoribbons and nanorings. These results are in accordance with the previous report.53 By the above XPS analysis, it is further proved that the products are composed of only CuO. The quantitative analysis of the results based on the areas of the peaks gives an atomic ratio of Cu/O as 1:1.16. Conclusion In conclusion, CuO nanoribbons and nanorings, which were fabricated by the reaction of CuCl2 and NaOH solutions with sodium dodecyl benzenesulfonate (SDBS), have been synthesized by a hydrothermal route. The CuO nanoribbons have widths of 10-80 nm, thicknesses of 5-20 nm, and lengths

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Figure 7. XPS spectra of CuO nanocrystals: (a) Cu 2p and (b) O 1s.

ranging from several hundred nanometers to several micrometers; the nanorings have diameters of 100-300 nm. Both XRD and HRTEM measurements indicated that CuO nanoribbons and nanorings were single-crystal, and the nanorings were closed. The formation processes of CuO nanoribbons and nanorings were studied, which involve the primary splitting of nanoflakes into nanoribbons and self-coiling of polar nanoribbons into CuO nanorings. Acknowledgment. This work was supported by National Natural Science Foundation of China and 973 Project of China (2005CB623601). Supporting Information Available: TEM images of CuO nanocrystals obtained by changing synthetic parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

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