Self-Assembled CuO Monocrystalline Nanoarchitectures with

CRYSTAL GROWTH & DESIGN

Self-Assembled CuO Monocrystalline Nanoarchitectures with Controlled Dimensionality and Morphology

2006 VOL. 6, NO. 7 1690-1696

Jinping Liu,† Xintang Huang,*,† Yuanyuan Li,† K. M. Sulieman,† Xiang He,‡ and Fenglou Sun‡ Department of Physics, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, and Plasma Institute, South-Central UniVersity for Nationalities, Wuhan, People’s Republic of China ReceiVed April 6, 2006; ReVised Manuscript ReceiVed May 5, 2006

ABSTRACT: Two-dimensional (2D) CuO layered oval nanosheets and three-dimensional (3D) nanoellipsoids were grown on a large scale at ∼65 °C by a facile template-free method. Shape and dimensionality control of well-defined CuO single crystals could be achieved by simple variations of pH value. At pH 8.5, CuO nanosheets were obtained, whereas at pH 7.5, CuO nanoellipsoids were formed. XRD, SEM, TEM, and HRTEM were used to characterize the products. The growth mechanisms were discussed by monitoring the early growth stages. It was shown that the CuO nanoarchitectures were formed through oriented attachment of tiny single-crystal nanoribbons and nanoparticles. UV-vis spectra were employed to estimate the band gap energies of the nanosized semiconductors. Further control experiments involving changing the growth temperature and alkaline reactant were also carried out to prepare other ultrafine nanoarchitectures. Our work demonstrates the growth of single-crystal CuO architectures built from 0D and 1D nanocrystals through a one-step solution-phase chemical route under controlled conditions. 1. Introduction Over the past few decades, the synthesis of inorganic nanostructures with well-defined morphologies has attracted considerable attention, for the dimensional and structural characteristics of these materials endow them with a wide range of potential applications.1 In particular, the fabrication of hierarchical and complex nano-/microstructures which assemble using nanoparticles, nanorods, nanoribbons, and nanobelts as building blocks at different levels has been proposed and partially realized in recent years.2-5 These novel architectures should facilitate a deeper understanding of the “bottom-up” approaches, offer opportunities in searching for exciting new properties of materials, and be useful for fabricating functional nanodevices etc.5-7 Until now, various kinds of compound materials, such as metal oxide,8 sulfide,7,9 hydrate,10 and other nanostructures,11 have been synthesized with controlled hierarchical/complex morphologies. As an important p-type transition-metal oxide with a narrow band gap (Eg ) 1.2 eV), CuO is a potential field emission material, an important catalyst, and a gas-sensing medium.12-16 It is also a promising material for fabricating solar cells, due to its photoconductive and photochemical properties.13,16b CuObased materials are well-known with regard to their hightemperature superconductivity and huge magnetoresistance.12,16d Many recent efforts have been directed toward the fabrication of nanostructural CuO to enhance its performance in currently existing applications. To date, well-defined CuO nanostructures with different dimensionalities such as nanoparticles, nanoneedles, nanowires, nanowhiskers, nanoshuttles, nanoleaves, nanorods, nanotubes, and nanoribbons14-21 have been obtained successfully by a series of solution-based routes and vaporphase processes. With regard to the self-assembled complex CuO architectures, 3D peanut-like patterns,22 pricky/layered microspheres,13a,b nanodendrites,23 nanoellipsoids,24 dandelionshaped hollow structures,25a and chrysanthemum-like archi* To whom correspondence should be addressed: Fax +86-02767861185. E-mail: [email protected]. † Central China Normal University. ‡ South-Central University for Nationalities.

tectures16d have also been obtained by thermal decomposition of malachite, water baths, hydrothermal methods, natural oxidation processes, etc. In comparison, the self-assembly of CuO 2D uniform complex structures has seldom been reported. Most importantly, studying the relationship between the dimensions of CuO hierarchical structures and reaction conditions (such as solution components, pH value, etc.) is essentially necessary for shape control. Since the shape, crystalline structure, and size of semiconductors are important elements in determining their physical and chemical properties,1,16d it is our goal to realize the rational control synthesis of CuO complex nanomaterials over different dimensionalities. The wet chemical method has been considered to be one of the most promising synthetic routes, due to its low cost, its high efficiency, and its good potential for high-quantity production.8c,16b,21,23,25b,26 Generally, it can be classified into two kinds: the template method, which employs hard templates (AAO, silicon wafer, metallic foils) and soft templates (surfactants, capping reagents), and the template-free method.16b However, the introduction of templates into the synthetic routes is still troublesome and sometimes difficult to handle. Therefore, developing simple methods without using templates or other additives seems to be more promising because of the various expected advantages. In this work, we report a facile template-free wet chemical approach to synthesize well-defined CuO layered oval nanosheets (2D) and nanoellipsoids (3D). Shape and dimensionality control of CuO nanoarchitectures can be achieved by simple variations of pH value. The “oriented attachment” mechanism is responsible for the architecture formation. It is also found that other morphologies of CuO nanostructures could be selected with simple changes of temperatures and alkaline reactants. All of the obtained CuO nanostructures might have potential applications in catalysts, sensors, and lithium ion electrode materials. 2. Experimental Section In a typical synthesis, an aqueous solution composed of 0.04 M hydrated Cu(CH3COO)2‚H2O, of analytical grade, was first prepared at 30 °C with magnetic stirring in a three-neck flask (equipped with a reflux condenser and a Teflon-coated magnetic stirring bar). Subse-

10.1021/cg060198k CCC: $33.50 © 2006 American Chemical Society Published on Web 05/28/2006

CuO Monocrystalline Nanoarchitectures

Crystal Growth & Design, Vol. 6, No. 7, 2006 1691

Table 1. Summarized Reaction Conditions and Corresponding Morphologies of CuO Nanocrystalsa sample

pH

temp, °C

time

product morphology

A B C D E F

8.5 7.5 8.5 7.5 7.5 7.5

65 65 65 65 65 85

24 h 24 h 40 min 5 min 40 min 24 h

G

8.5

85

24 h

H

8.5b

65

24 h

2D nanoribbon-based sheets 3D nanoparticle-based ellipsoids 2D underdeveloped nanosheets 0D small dispersed nanoparticles almost 3D nanoparticle-based ellipsoids 2D nanoparticle-based sheets with larger sizes 2D nanorod-based sheets with more sharp tips laying out irregular 2D “cross-shaped” nanoflakes

a

Experimental conditions and results at other temperatures between 60 and 90 °C are not displayed here. b Using NaOH instead of NH3‚H2O.

quently, 1 M concentrated NH3‚H2O was added dropwise into the above solution. After the addition, the obtained mixture was heated to 65 °C and heating was continued at this temperature for 24 h. Two mixtures (pH 7.5, 8.5) were prepared by adjusting NH3‚H2O. For each experiment, vigorous stirring was maintained throughout the entire process. After the reaction, the dark precipitate was separated by centrifugation, washed with absolute alcohol and distilled water, and finally dried at 60 °C. While the essential conditions were kept the same, some other control experiments were also performed. All of the main reaction parameters and corresponding results are summarized in Table 1. The phase purity of the products was characterized by X-ray powder diffraction (XRD) using a Y-2000 X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å). Scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDS) analyses were obtained on a JSM6700F microscope operated at 5 kV. Transmission electron microscopy (TEM and HRTEM) observations were carried out on a JEOL JEM2010 instrument in bright field and selected area electron diffraction modes and on a HRTEM JEM-2010FEF instrument (operated at 200 kV). Room-temperature UV-vis absorption spectra were recorded on a UV-2550 spectrophotometer in the wavelength range of 200-800 nm.

Figure 1. (a) Low-magnification SEM image of monodispersed welldefined CuO nanosheets obtained at 65 °C and pH 8.5. (b) Enlarged SEM image of CuO nanosheets. The white arrow indicates the sheetlike shape. (c) SEM image of an individual CuO sheet in Figure 1b (indicated by a rectangle), showing the layered structure. (d) XRD result of the CuO sheets. The inset gives the EDS results.

3. Results and Discussion 3.1. Synthesis of CuO Nanosheets and Nanoellipsoids. Figure 1 shows the general morphologies of sample A. From Figure 1a, CuO oval structures with uniform shape and size are obtained on a large scale. The average sizes of the longitudinal axis and horizontal axis are determined to be 250 and 500 nm, respectively. The enlarged image in Figure 1b demonstrates 2D sheetlike patterns (indicated by an arrowhead) with rough surfaces and boundaries of the produced CuO nanostructures. The thicknesses of the nanosheets are in the range of 25-50 nm. An SEM image of an individual CuO nanosheet is shown in Figure 1c. Interestingly, the CuO architecture is composed of many small well-aligned crystal nanoribbons, exhibiting a layered pattern. It is intriguing to note that the nanoribbons can self-assemble into an oval sheetlike hierarchical architecture along the longitudinal axis direction in the absence of any specific additives or templates. The tiny ribbons are attached side by side into an integrated structure, which is similar to the process of forming a ZnO “mother” nanorod from small nanorods.8e It should be pointed out that the general thickness of CuO ribbons is not adequately small compared with their widths, and some of the ribbons could also be considered as nanorods. Figure 1d shows the XRD pattern of sample A. The crystallographic phase is in good agreement with the JCPDS card (Card No. 05-0661, a0 ) 4.684 Å, b0 ) 3.425 Å, c0 ) 5.129 Å) for the monoclinic CuO crystals. Consistent with the XRD result, the EDS result (inset of Figure 1d) also demonstrates only the elements Cu and O contained in the sample and the atomic ratio of Cu to O is equal to 1:1.

Figure 2. (a) Low-magnification and (b) high-magnification SEM images of the CuO nanoellipsoids obtained at 65 °C and pH 7.5. The scale bar in the inserted picture is 100 nm. (c) XRD results for the CuO ellipsoids.

When the experiment was conducted at pH 7.5 (sample B), 3D nanoellipsoids rather than 2D nanosheets were formed. The 3D ellipsoidal shape is confirmed by circular cross-sections observed from the vertically aligned CuO particles (see arrows in Figure 2a and insert of Figure 2b) and the elliptical crosssections detected from the horizontally dispersed particles. As shown in Figure 2a, the products also have uniform morphologies and can be obtained in large quantities. The sizes of the short axis and the long axis of these nanoellipsoids are in the ranges of 60-100 and 200-300 nm, respectively. A magnified SEM image (Figure 2b) indicates that each nanoellipsoid is comprised of numerous 0D nanoparticles, which have an average size of less than 10 nm. The densely packed particles make the ellipsoidal surface rough and corrugated. The composition of sample B is confirmed by XRD results, shown in Figure 2c.

1692 Crystal Growth & Design, Vol. 6, No. 7, 2006

Liu et al.

Figure 4. (a) SEM image of underdeveloped CuO nanosheets obtained at pH 8.5 after 40 min. (b) SEM image of small CuO nanoparticles prepared after reacting for 5 min at pH 7.5. The inset gives an enlarged image, showing that the particles tend to aggregate. (c) SEM image of CuO nanoellipsoids obtained at pH 7.5 after 40 min. (d) XRD results of the CuO products shown in Figure 4a,c.

Figure 3. (a) TEM image of an individual CuO nanosheet. The inset gives the corresponding SAED. (b) HRTEM image of two attached nanoribbons, showing the [010] growth direction. (c) TEM image of an individual CuO ellipsoid. The inset gives the SAED. (d) HRTEM image of the head part of one ellipsoid. From the enlarged image in the inset, the [010] growth direction can be determined. (e) HRTEM image of the central surface of CuO ellipsoids.

All of the diffraction peaks can be indexed as pure monoclinic CuO. It is worth mentioning that these CuO nanoellipsoids are sufficiently stable that they could not be destroyed into dispersed particles even after long periods of ultrasonication. The structures of the nanosheet and nanoellipsoid were further investigated by TEM. Figure 3a shows the bright field TEM image of an individual CuO nanosheet. It can be seen clearly that the oval nanosheet is composed of layered nanoribbons. The widths of the ribbons are in the range of 10-25 nm. The SAED pattern taken from the whole nanosheet is displayed in the inset of Figure 3a. The result indicates that the assembled nanosheet exhibits an almost single-crystal diffraction pattern with its spotlike appearance along the [001] axis of crystalline CuO. This interesting feature gives obvious evidence that CuO nanosheets are formed through “oriented attachment”27-29 of small nanoribbons along the [010] direction. The orientations along the length, width, and thickness of CuO nanosheets could therefore be determined to be [010], [100], and [001], respectively. The HRTEM image in Figure 3b further reveals the single-crystal structure of CuO nanoribbons. The measured spacing of the crystallographic planes is ∼2.7 Å, which corresponds to the [110] lattice fringe of the monoclinic CuO, indicating that the growth direction of the ribbons is [010], as shown in Figure 3b. The parallelism of the lattice fringes indicates that the two attached nanoribbons share the same 3D orientation, forming a structurally uniform single crystal. Also, there are structural defects such as dislocations in some regions, especially in the contacting area between the two ribbons (shown by a white circle). Figure 3c shows a typical TEM image of an isolated CuO nanoellipsoid. The surface of the ellipsoid is rough, further confirming that the structure is assembled from small

nanoparticles. The SAED pattern (insert of Figure 3c) of the whole ellipsoid reveals a single-crystal entity and the preferential [010] growth direction. An HRTEM image taken from the head part of an individual CuO nanoellipsoid is shown in Figure 3d. From the enlarged image of the area labeled by a rectangle (inset of Figure 3d), the lattice interplanar spacing is vertical to the long-axis direction of the CuO ellipsoid and has been determined to be 2.3 Å, corresponding to the (200) plane of monoclinic CuO. This suggests that [010] is the growth direction of the ellipsoid. The fringes in the HRTEM images of the central surface of CuO ellipsoids (Figure 3e and Supporting Information) also show a period of 2.3 Å, which further confirms that the long-axis direction is along [010]. The image also displays well-resolved, continuous fringes with the same orientation, thus implying that the subunits oriented assemble with each other and finally form a single-crystal structure. Since there are unequal sizes and/or uneven surfaces in these 0D subunit particles, the slight mismatching (such as “waved lattice”) cannot be entirely avoided. This explains the origin for the elongation of the diffraction spots observed in the SAED pattern. Since understanding the growth mechanism depends on the revelation of the intermediate steps involved in the growth process, the products obtained at the early stages were studied. Our experimental data proved that nanosized CuO primary ribbons and particles formed immediately when the temperature was above 60 °C, followed by aggregation into larger uniform architectures. Figure 4a shows the SEM image of the products obtained after 40 min of reaction at pH 8.5 (sample C). The sample is composed of many platelet-like architectures ranging from 100 to 150 nm in width. These underdeveloped nanosheets obviously comprise nanoribbons/nanorods with an average diameter and length of 20 and 90 nm, respectively. Some nonattached rodlike/ribbonlike monomers can also be observed. At the early growth stage of CuO nanoellipsoids (sample D), tiny particles which tended to assemble were readily formed after a short reaction time of 5 min (Figure 4b). Close SEM examination (inset of Figure 4b) reveals that the average size of the primary particles is ∼9 nm, which is almost in agreement with the subunit size in the final ellipsoids shown in Figure 2b and Figure 3c. After reaction for 40 min (sample E), 3D


Figure 5. (a) Schematic illustration of a CuO nanoribbon and nanoparticle possessing well-defined crystal planes. (b) Schematic illustration of the growth of CuO nanoarchitectures under different conditions.

ellipsoidal nanoarchitectures, possessing the same size as those obtained after 24 h, were formed, as shown in Figure 4c. XRD results displayed in Figure 4d confirm the formation of monoclinic CuO. The color change from blue to dark brown was also observed to be very quick in our synthetic process, further indicating the formation of CuO nanoarchitectures. 3.2. Growth Mechanisms. In this study, electron microscopy has shown that the final 2D and 3D nanoarchitectures consist of nanosized subunits, which have essentially the same size as the primary crystals obtained at the early growth stage. Thus, it can be concluded that the final products indeed are built from the original small precursors by aggregation. In particular, single-crystal-like structures with slight misorientation can be readily obtained after sufficient organization, indicating an imperfectly “oriented attachment” growth.8e,28 On the basis of our experiment, the evolving process of monocrystalline CuO layered nanosheets and nanoellipsoids is schematically illustrated in Figure 5. CuO nanoribbons/nanorods and nanoparticles should be formed very quickly at the early stage, followed by the arrangement of these building blocks along identical crystal faces. The CuO nanosheets are very different from the reported 2D CuO nanostructures formed on the basis of the normal concept for crystal growth, which is thought to occur via atomto-atom addition to an existing nucleus on the Cu substrate.16d To realize the controlled synthesis of assembled nanoarchitectures, many recent efforts have been directed to the morphological and structural control of primary building blocks. For example, Zeng et al.28 successfully reported a variety of ringlike semiconductors prepared in the aqueous phase by first synthesizing stable primary nanocrystals possessing an intrinsic hexagonal (or cubic) symmetry. Cho et al.29a demonstrated that single-crystal PbSe straight, zigzag, helical, branched, and tapered nanowires as well as single-crystal nanorings can be controllably prepared by the oriented attachment of various PbSe nanocrystals. Their work showed that the preobtained PbSe primary crystals with different shapes or crystal plane reactivities were crucial to the subsequent attachment and can be attained by adjustment of the reaction conditions. In the present work, our main concept is focused on the dimensionality control of CuO primary crystals by changing the solution pH value, thus modulating the morphology and dimensionality of the final assembled structures. Previous work has demonstrated that many


experimental parameters, such as reaction temperature,1f,29b-d reactant concentration,29e and surfactant16d could be manipulated to control the morphology and microstructure of nanocrystals. In addition, the pH value also has a significant influence on the morphology and dimensionality control. As is well known, pH-tunable nanostructures have been extensively studied before;8c,13,14a,16a,17a especially, the aspect ratio of low-dimensional nanostructures is thought to be easily adjusted by changing the pH value. This is on the basis of the fact that the concentration of OH- can significantly affect the nucleation and growth behaviors (such as the number of nuclei and the concentration of “growth units”) of the nanocrystals. Concerning the sole use of NH3‚H2O as the alkaline reactant in our synthetic architecture, two major roles can be identified. First, it could provide basic media and further adjust the solution pH value. Second, NH3‚H2O can coordinate with Cu2+, giving rise to a Cu(NH3)42+ complex which can act as a molecular transporter that transports Cu2+ to the growing seed crystal tips with OHligands attached.16b,21e When CuO nanocrystals were prepared at higher pH value, Cu(NH3)42+ units in high concentrations were formed in solution,21e and thus the quantity of squareplanar Cu(OH)42- units would be large, resulting in the formation of 1D Cu(OH)2 nanoribbons/nanowires, as demonstrated in previous reports.15d,16b Accordingly, CuO 1D primary nanocrystals (nanoribbons or nanorods) could be readily obtained by thermal dehydration of the Cu(OH)2 precursor without obvious morphological change, due to the relatively low reaction temperature, which could maintain the “template” function of Cu(OH)2.15d In comparison, if the experiment was conducted at a low pH value, there were not enough Cu(OH)42- units, due to deficiency in OH- and Cu(NH3)42+. Therefore, the 1D preferential growth of Cu(OH)2 was quenched, resulting in the generation of nanoparticles. CuO nanocrystals obtained subsequently also possessed 0D structures. The described morphological control scheme (also see Figure 5b) is quite similar to that employed for creating ZnO nanostructures in solution.30 It should be stressed that, during the solution synthesis of CuO, the solid precursors Cu(OH)2 existed only momentarily when the temperature was elevated, as observed in our synthetic process. Taking into account of the intrinsic crystallographic structure, it was believed that the generated CuO nanoribbons and nanoparticles possessed well-defined {010},{100}, and {001} facets, as schematically shown in Figure 5a. As evidenced by HRTEM and SAED results, the final assembled CuO nanosheets are elongated along the [010] direction, with their top and lateral surfaces enclosed by {001} and {100} planes, respectively. The three main directions involved in the self-organizing process are [001], [100], and [010]. Due to the difference in aggregation potential and rate, the numbers of primary nanocrystals assembled in the three directions are substantially different and are in the following order: [010] > [100] > [001]. Especially for CuO nanosheets, there is no possibility that the aggregation occurs along the [001] direction, since the assembled structure consists of only one layer of subunit ribbons in this direction. As reported by Zhang et al.,24 among the planes (001), (010), and (100), the most thermodynamically stable surface plane is (001), while the least stable one is (010), due to different copper atom densities on these planes. Therefore, it is understandable that the ordered aggregation preferentially takes place along the [010] direction, which has the highest reactivity. In principle, the attachment of tiny primary crystals at their high-energy surfaces is energetically favored, because the formation of larger crystals can greatly reduce the interfacial energy. Thus, it is reasonable that the final


assembled monocrystals with well-defined morphology and compact structure can be attained by orienting the nanoribbons/ nanorods and nanoparticles along the aforementioned three directions, followed by further reorganization and crystallization (Figure 5b). As we know, a variety of driving forces, such as dipole-dipole interactions,27a partial removal of the stabilizing agents from the nanocrystal surfaces,24,27a and controlled ligand exchange of surface-adsorbed inorganic ions with neutral organic molecules,27b have been utilized in various ordered assembly schemes. The CuO nanoarchitectures obtained in our experiments, however, were produced without using any surfactants, ligands, polymers, or templates. The mechanisms cannot be applied to our synthetic system. One interesting example is that the CuO nanoellipsoids should not be formed through controlled removal of the appropriate ligands such as Cu(HCONH2)42+ from special facets.24 In addition, aggregation through dipolar interactions can also be ruled out with regard to the crystal structure of monoclinic CuO. Although nonprotected and weekly protected nanoparticles were generally thought to be undergoing entropy-driven random assembly,24 single-crystal anisotropic nanostructures obtained by oriented attachment in the absence of surfactants and ligands were also reported in the literature. For instance, Weller et al.31 have demonstrated the formation of ZnO nanorods by the oriented attachment of ZnO nanoparticles along the unique axis of its wurtzite crystal lattice. They further suggested, on the basis of crystal growth kinetics, that oxide nanoparticles were very favorable for oriented attachment, even without organic ligands. Yao et al.20a observed that CuO nanorods could be obtained from the assembly of primary nanoparticles under ambient conditions in the absence of any templates or organic additives. Very recently, Lee et al.32a affirmed that “oriented attachment” was an effective and general mechanism in the formation of anisotropic nanocrystals. They minimized the effects of other experimental aspects on the particle growth and presented strong evidence that nanostructures could readily undergo oriented aggregation, although no ligands were used to promote the attachment of primary particles. Two main mechanisms involved in their experiment are (a) rotation of misaligned adjacent nanoparticles to share an identical crystallographic orientation and thus form configurations of minimum energy and (b) collisions of aligned nanoparticles in suspension.32 Since the growth by oriented collision-induced attachment is statistical and usually leads to products with various shapes, in the present work, the former mechanism seems to be the most probable candidate for directing CuO nanocrystals into morphologically uniform 2D nanosheets and 3D nanoellipsoids. As demonstrated by Moldovan et al.,33 the particle rotation for forming a final singlecrystal structure can be driven by a net torque which results from the misalignment of neighboring particles. Specially, for CuO nanosheets, the oriented organization might be more easily attained due to the geometrical and physical limitation resulting from the 1D shape of subunit nanoribbons/nanorods. We have noticed that the final architecture assembled from CuO nanoribbons is a 2D sheet, while the structure constructed by CuO nanoparticles is a 3D ellipsoid. This difference should be strongly related to the morphology and dimensionality of the primary subunits. Although the aggregation potentials in the [010], [100], and [001] directions have the same order for both 2D nanosheets and 3D nanoellipsoids on the basis of the aforementioned theoretical analyses, the actual organizing rates in these directions might be very different. It is suggested that CuO anisotropic nanoribbons assemble just by using their {010} and {100} crystal planes, leading to one layer in the [001]

Liu et al.

Figure 6. (a) UV-vis absorption spectra of CuO nanosheets and nanoellipsoids. (b) (REphoton)2 vs Ephoton curves of the products.

direction. Comparatively, for CuO nanoparticles, the organizing reactivity in all the 3D directions might be high, due to the small sizes of all crystal planes, resulting in the 3D aggregation process. On the other hand, it is reasonable to assume that a much greater momentum is required to rotate nanoribbons than nanoparticles, mainly because of the relatively stable position and the larger size and mass of nanoribbons. Hence, under our mild growth conditions (65 °C), the rotation of ribbons in 2D planar space to form a single-crystal thermodynamically stable configuration should be more easily realized, and 2D nanosheets rather than 3D structures can be readily formed. 3.3. Optical Properties. UV-vis absorption measurement is one of the most important methods to reveal the energy structures and optical properties of semiconductor nanocrystals and has been studied extensively. In our measurements, the samples were first dispersed in ethanol solvent. The spectra of the CuO nanoarchitectures are presented in Figure 6a. For CuO nanoellipsoids, there is a broad peak centered at ∼335 nm. A similar absorption peak with broader shoulders and lower intensity could also be detected for CuO nanosheets in the wavelength range of 250-500 nm. According to the equation REphoton ) K(Ephoton - Eg)1/2 (where R is the absorption coefficient, Ephoton is the discrete photo energy, K is a constant, and Eg is the band gap energy),34 a classical Tauc approach is further employed to estimate the Eg value of CuO nanocrystals. The plots of (REphoton)2 vs Ephoton based on the direct transition are shown in Figure 6b. The extrapolated value (the straight lines to the x axis) of Ephoton at R ) 0 gives absorption edge energies corresponding to Eg ) 2.05 and 2.47 eV, respectively. These two values are apparently greater than the value for bulk CuO. The increase in the band gap of the CuO nanoarchitectures is indicative of quantum confinement effects17c arising from the tiny particles and ribbons. Even though the subunit sizes in our experiment are much smaller than the sizes of CuO nanoplatelets


Figure 7. (a) Rod-based CuO nanosheets prepared at 85 °C and pH 8.5. (b) Particle-based CuO nanosheets obtained at 85 °C and pH 7.5. (c) HRTEM image of the rodlike CuO in Figure 7a, indicating the [010] direction growth. (d) HRTEM image of particle-based CuO nanosheets.

reported by Zou et al.,35 the calculated band gaps are still smaller than their values. This indicates that the final assembled architectures created herein might have led to crystal enlargement. It should be pointed out that the band gap energy of CuO ellipsoids is larger than that of CuO sheets. We propose that there might be a stronger quantum confinement effect for CuO ellipsoids, because the subunit (tiny particles) size of ellipsoids is smaller in comparison with the size of ribbons/rods in CuO sheets, as indicated by SEM and TEM observations. 3.4. Comparative Experiments. Other conditions such as growth temperature and alkaline reactant are also important factors affecting the morphologies of the nanostructures. By control of these aspects, different CuO nanoarchitectures could be realized. Parts a and b of Figure 7 show the SEM images of CuO products grown at 85 °C at pH 8.5 and 7.5 (samples G and F), respectively. As shown in Figure 7a, while the general morphology and the average size of layered nanosheets are kept unchanged, sharper tips and edges with more pronounced CuO subunits can be observed (indicated by black circles). It is therefore apparent that a relatively higher temperature does not favor the formation of a well-curved elliptical arc. To determine if the subunits still maintain the single-crystal structure, HRTEM results are displayed in Figure 7c. It is clearly observed that the 1D nanostructure is a single crystal, exhibiting a rodlike morphology rather than a ribbonlike morphology. Many other examinations were carried out and gave similar results, indicating that elevated temperature might slightly change the shape of the subunits. The growth direction of CuO nanorod is also along [010], which agrees with that of nanoribbons shown in Figure 3. CuO products prepared at 85 °C with pH 7.5 (shown in Figure 7b) reveal that a sheetlike structure (indicated by arrows), rather than an ellipsoidal pattern, is the dominant morphological configuration. A careful examination further demonstrates the grainy surfaces of the nanosheets, indicative of a nanoparticle-involved, rather than a nanorod/nanoribboninvolved, assembly process in this case. The HRTEM image in Figure 7d indicates that the nanosheets are textured by CuO nanoparticles that are approximately oriented in the same direction of [010], although there are structural defects in the


Figure 8. (a) Low-magnification SEM image of platelike CuO nanostructures obtained using NaOH as alkaline reactant at 65 °C and pH 8.5. (b) Individual image of a CuO nanoplate.

lattice image (shown by arrowheads). Considering the growth process, these dislocations should occur at interfaces between subunit particles. It is worth pointing out that the particle-based CuO nanosheets (average 125 nm in width and 320 nm in length) are larger than the ellipsoids shown in Figure 2, while the sizes of the subunit particles have barely changed. Our indepth experiments further show that the optimal temperature ranges for growing 2D ribbon-based layered nanosheets and 3D particle-based nanoellipsoids are 60-90 and 60-75 °C, respectively. We find that the effect of alkaline reactant on the morphology seems to be more remarkable than that of temperature. For example, CuO nanostructures, obtained by using NaOH instead of NH3‚H2O at 65 °C at pH 8.5 (sample H), are irregular platelike patterns. As shown in Figure 8a, these nanoplates display a cross-shaped configuration and have a large planar diameter distribution of 300-750 nm. Elliptic architectures could not be found in this case at all. The magnified SEM image of an individual CuO nanoplate in Figure 8b clearly demonstrates that each plate should be composed of rodlike subunits and has a rough surface covered with small particles. Moreover, these nanorods pack so densely that we could not observed uniform layered structures. The platelike CuO nanostructures have also been synthesized before: for example, by hydrothermal treatment of aqueous solution containing CuSO4 and NaOH at 200 °C23 and by thermal dehydration of Cu(OH)2 nanostructures under strongly alkaline conditions (KOH/NH3) at 50 °C.16b However, in general, the nanoplates obtained in those studies seemed to have smooth surfaces, which is different from our present results. The final morphology of CuO nanostructures during the solution synthesis should be largely dependent on the synthesis conditions, and the exact mechanism for the nanoplate formation remains to be investigated in the future. 4. Conclusions We have succeeded in constructing well-defined CuO nanoarchitectures by the oriented attachment mechanism based on a simple and mild solution-phase synthesis. At low reaction


temperatures, single-crystal CuO with selected dimensionality and morphology could be prepared by merely changing the solution pH value. The synthetic architecture is first based on morphological control of primary building units and subsequently on arrangement of them along preferential crystal faces. Electron microscopy results indicate that both the 2D layered oval nanosheets and 3D nanoellipsoids are elongated along the [010] direction. The obtained CuO nanoarchitectures exhibit blue shifts in UV-vis spectra and possess larger band gaps compared with those of bulk crystals. Other CuO nanostructures could also be attained by changing the growth temperature and alkaline reactant. Acknowledgment. We gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 50202007). We are appreciative of valuable suggestions from the reviewers for revision of this paper. Supporting Information Available: HRTEM image of the surface of CuO nanoellipsoids prepared at 65 °C and pH 7.5 for 24 h. This material is available free of charge via the Internet at http://pubs.acs.org.

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