Facile Synthesis of Flowerlike Cu2O Nanoarchitectures by a Solution

Facile Synthesis of Flowerlike Cu2O Nanoarchitectures by a Solution Phase Route ... (CTAB) play key roles in the formation of the as-synthesized nanof...
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Facile Synthesis of Flowerlike Cu2O Nanoarchitectures by a Solution Phase Route Luo,*,†,‡

Li,†

Yongsong Suqin Qinfeng Ying Yu,† Zhijie Jia,† and Jialin Li*,†

Ren,†

Jinping

Liu,†

Lanlan

Xing,†

Yan

Wang,†

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 1 87-92

Department of Physics, Central China Normal UniVersity, Wuhan 430079, P. R. China, and Department of Physics Electronic Engineering, XinYang Normal UniVersity, Xinyang 464000, P. R. China ReceiVed July 26, 2006; ReVised Manuscript ReceiVed NoVember 4, 2006

ABSTRACT: Novel flowerlike nanostructures consisting of Cu2O nanopetals were successfully synthesized by a facile wet chemical method for the first time. The synthesized products were systematically studied by X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy. The results showed that the nucleation and growth of the nanoflowers were governed by a nucleation-dissolution-recrystallization growth mechanism. It is noteworthy that the initially formed Cu2O nanoparticles without addition of NaOH were crucial to the growth of the final nanoarchitectures. A UV-vis spectrum was used to estimate the band gap energies of the nanoflowers. Further control experiments were also carried out to investigate the factors that impact the morphology and size of the products. It was demonstrated that the concentrations of NaOH and cetyltrimethylammonium bromide (CTAB) play key roles in the formation of the as-synthesized nanoflowers. By adjusting the concentration of NaOH and CTAB, temperature, and the quantity of water, Cu2O micrograss, nanorods, and pricky microrods can be synthesized accordingly. Our stepwise synthetic method may shed some light on the design of other well-defined complex nanostructures. 1. Introduction In recent years, there has been increasing interest in the controlled synthesis of inorganic micro- and nanostructures with well-defined shapes and sizes because of their widespread potential applications, including photonics, nanoelectronics, catalysis, information storage, and biosensors.1-6 So far, much effort has been employed to fabricate nanomaterials with different shapes, such as nanobelts, nanowires, nanotubes, nanocubes, nanoboxes, hollow spheres, etc.7-11 These novel architectures should be useful for fabricating functional nanodevices and facilitate a deeper understanding of “bottom-up” approaches. Generally, two strategies have been utilized for the “bottom-up” chemical synthesis of nanostructured materials: one is the use of hard templates, which physically confine the size and shape of the growing nanoparticles,12,13 and the other is use of capping agents/surfactants during nanoparticle growth to control its dimension, direction, and morphology.14-18 As a p-type semiconductor (direct band gap ∼ 2.17 eV) with unique optical and magnetic properties, cuprous oxide (Cu2O) is a promising material with potential applications in solar energy conversion, micro-/nanoelectronics, magnetic storage devices, biosensing, and catalysis.19,20 Many recent efforts have been devoted to the shape-controlled synthesis of Cu2O microand nanocrystals. Systematic manipulation of the morphology and architecture of Cu2O microcrystals has been achieved using solution routes21-23 and electrodeposition methods.24 Meanwhile, various approaches have been reported for fabricating Cu2O nanocrystals with varied morphologies, such as wires,25,26 rods,27 cubes,9,28,29 pyramids,28 and octahedra.30,31 These morphologies indicate that surfactant mesophases have proven to be useful and versatile “soft” templates, which may form different conformations by self-assembly and lead to the formation of different Cu2O nanostructures. Moreover, fabrication of complex * To whom correspondence should be addressed. Fax: +86-02767861185. E-mail: [email protected] (Y.L.); [email protected] (J.L.). † Central China Normal University. ‡ XinYang Normal University.

Figure 1. (a) Low-magnification SEM image of the Cu2O nanoflowers. (b) Enlarged SEM image of the Cu2O nanoflowers. (c) SEM image of an individual Cu2O nanoflower. (d) Powder XRD pattern of the assynthesized Cu2O nanoflowers.

architectures with three-dimensional (3D) or highly ordered nanostructures is highly desirable in current materials synthesis, holding the promise of advanced applications in electronics and optoelectronics.32-34 For example, a roselike nanoflower consisting of a two-dimensional (2D) ZnO nanosheet was deposited in a chemical bath using layered basic acetate dehydrate for enhancing light conversion efficiency to electricity in dyesensitized solar cells (DSCs).35 3D flowerlike MoS2 nanostructures were fabricated by a thermal evaporation process, and they exhibited good capability in field emission due to the existence of the open edges in their nanopetals.36 However, it remains a great challenge to develop feasible methods for solution synthesis of well-defined Cu2O flowerlike nanostructures. Herein, we report a facile stepwise solution-phase synthesis of flowerlike Cu2O 3D nanoarchitectures via a nanoparticlemediated process at low temperature (60 °C). Our method involves the use of wet-chemical reduction for preparing Cu2O

10.1021/cg060491k CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006

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Figure 2. (a) TEM image of an individual Cu2O nanoflower. (b) HRTEM image of the head part of one nanopetal. (c) HRTEM image of the tip of another petal. (d) Enlarged image of the area marked by a rectangle in panel c. The [110] growth direction can be determined.

nanoparticles and subsequent conversion to Cu2O nanoflowers at low temperature. The Cu2O nanoparticles are prepared by reducing Cu(CH3COO)2‚H2O with NaBH4 in a H2O/CTAB/N,Ndimethylformamide (DMF) three-phase system in the absence of NaOH. Further, Cu2O nanoflowers are obtained by etching Cu2O nanoparticles with NaOH solution and being recrystallized with the help of CTAB. The obtained Cu2O nanoflowers might have potential applications in catalysts, optics, and sensors. 2. Experimental Section All the chemicals were analytic grade reagents used without further purification. Cu2O nanoflowers were synthesized as follows: 2 mmol of hydrated Cu(CH3COO)2‚H2O was dissolved in 70 mL of DMF-H2O solvent at a volume ratio of DMF/H2O ) 70%/30%, followed by the addition of CTAB (8 mmol, molecular weight ) 364.46). The obtained solution was sonicated in an ultrasonic water bath for 20 min. Then, 1 mmol of NaBH4 was added into the Cu(II)-CTAB solution. The reaction was carried out with magnetic stirring in a three-neck flask (equipped with a reflux condenser and a Teflon-coated magnetic stirring bar) at 60 °C for 5 min. After this, 10 mmol of NaOH was further added to the mixture, and a bright yellow color appeared immediately. The solution was kept at 60 °C for another 10 min and was removed from heat and allowed to cool to room temperature. When the reaction was finished, the precipitation was separated from the solution by centrifugation and washed with alcohol several times. The phase purity of the products was characterized by X-ray powder diffraction (XRD) using a X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å). Scanning electron microscopy (SEM) images were

obtained on a JSM- 6700F microscope operated at 5 kV. Transmission electron microscopy (TEM and HRTEM) observations were carried out on a JEOL JEM-2010 instrument in bright field and on an HRTEM JEM-2010FEF instrument (operated at 200 kV). Room temperature UV-vis absorption spectrum was recorded on a UV-2550 spectrophotometer in the wavelength range of 200-800 nm.

3. Results and Discussion 3.1. Structure and Morphology. Flowerlike Cu2O nanoarchitectures were synthesized by the solution-phase reaction between Cu(CH3COO)2‚H2O and NaBH4 in a H2O/CTAB/DMF three-phase system at the appropriate temperature of 60 °C. In this work, we introduced NaBH4 as the reducing agent, and DMF/H2O as the solvent. The chemical reaction is as follows:

4Cu2+ + BH4- + 5H2O f 2Cu2O + B(OH)3 + 2H2 + 7H+ Figure 1a-c shows the SEM images of the Cu2O nanoflowers at low, medium, and high magnification, respectively. From the SEM observations, it can be seen that the Cu2O product contains numerous flowerlike aggregates, and almost all of them show the same morphology. In addition, each flower is made of many thin nanopetals, which are spokewise, i.e., projected from a common central zone. The petals are beltlike, indicating the significant modified reaction activities of different crystal planes resulted from our specific three-phase chemical conditions. Careful examination reveals that these Cu2O nanopetals are

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Figure 3. Time-dependent evolution of the flowerlike crystal morphology at different growth stages: (a) 0, (b) 3, (c) 6, and (d) 10 min (initial from dropwise NaOH solution).

Figure 4. Schematic illustration of the formation and shape evolution of Cu2O nanostructures under different synthetic conditions.

200-500 nm in length, about 30-80 nm in width in the middle section, and about 7 nm in thickness. This nanostructure cannot be destroyed into discrete petals even under long-time ultrasonication, indicating that the nanoflowers were actually integrated and were not just aggregations of these thin nanopetals. The XRD pattern of the flowerlike Cu2O nanostructures is displayed in Figure 1d. All of the diffraction peaks can be indexed according to the cubic phase of Cu2O with a lattice constant a ) 4.4267 Å, which is consistent with the values in the standard card (PDF file No. 05-0667). Even if fabricated at the low temperature for 15 min, the flowerlike Cu2O nanostructures crystallized well, according to the intensity and half width of the XRD pattern. The relatively broader diffraction peaks suggest the smaller crystallite size for the nanopetals in Cu2O flowers. The structures of the nanoflower were further investigated by TEM. Figure 2a shows the TEM image of an individual Cu2O nanoflower. From this image, it can be seen that the end of the Cu2O nanoflower is pointed like a sword. Figure 2b shows the typical high-resolution transmission electron microscopy (HRTEM) image of an individual Cu2O nanopetal; the image clearly reveals the fringes of (111) planes with a lattice spacing of about 0.25 nm, indicating that the Cu2O nanoflower is single crystal in nature. Accordingly, the growth direction of the Cu2O nanopetals can be determined to be [110]. Figure 2c shows a TEM image of another Cu2O nanopetal. The surfaces of Cu2O

Figure 5. SEM images of the Cu2O nanorods. (a) Low magnification and (b) high magnification view.

nanopetal are rugged, confirming that the structure is assembled from small nanoparticles. The image also displays well-resolved, continuous fringes with the same orientation, thus implying that the subunit particles oriented assemble with each other and finally form a single-crystal structure. The enlarged image of the area labeled by a rectangle (Figure 2d) further reveals the single-crystal structure of Cu2O nanopetal and the preferential [110] growth direction. The fast Fourier transform (FFT) pattern of Figure 2c is displayed in the inset of Figure 2a. The result also indicates that the assembled nanopetal is single crystal and can be indexed as the cubic phase Cu2O, which is in accord with the XRD result in Figure 1d. It is worth mentioning that since there are unequal sizes and/or uneven surfaces in these 0D subunit particles, the slight mismatching cannot be entirely avoided in HRTEM. To shed light on the formation mechanism of these novel flowerlike Cu2O nanostructures, their growth process has been followed by examining the products harvested at different intervals of aging times. The obvious evolutionary stages can be clearly observed and are shown in Figure 3. When there is no NaOH present in the synthetic system, the resultant product Cu2O is nanoparticle as revealed by the SEM image (Figure 3a). This implies that the growth rates of various directions are almost the same when NaOH is absent. However, when NaOH

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Figure 6. SEM images of Cu2O nanostructures synthesized at different conditions: (a) the concentration of NaOH under 0.07 M; (b) the concentration of NaOH greater than 0.25 M; (c) in the absence of CTAB; (d) the ratio of water low 20%; (e) temperature at 50 °C; (f) temperature at 70 °C.

was added to the above solution, after aging for 3 min, surfaces with few plumules of partial Cu2O nanoparticles are produced (Figure 3b). When the aging time is increased to 6 min, Cu2O structures with radial nanoscale strips and sharp tips appear. Figure 3c shows a representative SEM image of the products obtained in this aging time. Upon gradual evolution of the Cu2O nanoscale strips, well-defined nanoflowers are produced after an aging time of 10 min. Most of the obtained products are perfect nanoflowers, and almost no Cu2O nanoparticles can be observed, as shown in Figure 3d. The process of the shape transition from nanoparticles to nanoflowers is summarized in Figure 4. 3.2. Growth Mechanism. On the basis of the results discussed above, we believe that NaOH plays a key role in the formation of the as-synthesized product. The formation of the Cu2O nanoflowers can be rationally expressed as a kinetically controlled nucleation-dissolution-recrystallization mechanism. First, when the reduction reaction was carried out in the solutionphase system at 60 °C, it directly gave Cu2O nanoparticles, which were formed in the solution through a homogeneous nucleation process (Figure 3a). Further, when there was no NaOH present in the reaction solution, the small Cu2O nanoparticles grew onto large nanoparticles via a process known as Ostwald ripening,37 as the aging process continues. Second,

when the NaOH was added to the solution, partial Cu2O nanoparticles started to dissolve into the solution and generated copper(I) ions in the solution with ill-defined intermediates (a yellow metastable “CuOH” is possible38 in the presence of NaBH4, and other Cu2O nanoparticles formed earlier may still exist. Moreover, in this experiment, CTAB is another important factor and could also be considered to influence the growth process of Cu2O nanoflowers. As is well known, CTAB is an ionic compound, which ionizes completely in water. In the synthetic procedure of nanomaterials including Cu2O, CTAB was used as a capping agent and/or a “soft” template. So far, CTAB has been systematically studied in the synthesis of mesostructured materials and may form spherical, cylindrical micelles or even higher-order phases depending on the solution conditions.9,39,40 In our system, CTAB may be used as a “soft” template. We speculate that when an amount of CTAB is added to the reaction solution, many active sites will be produced around the circumference of Cu2O nuclei (the Cu2O nanoparticles formed earlier) in alkaline conditions, as described in Figure 4. In addition, ion-pairs among CTA+, OH-, and Cu+ could form due to electrostatic interactions. These combinations begin to self-assemble and change into a radial aggregated structure; thus, small protuberances in the active sites of the Cu2O nuclei appear (Figure 3b). Furthermore, due to the

Synthesis of Flowerlike Cu2O Nanoarchitectures

Figure 7. (a) UV-vis absorption spectra of Cu2O nanoflowers. (b) The corresponding (REp)2 vs Ep curve.

anisotropic crystal structure, there was an intrinsic tendency for nucleation growth along the 1D direction.41 After a longer aging time, nuclei surrounded by small protuberances gradually grow larger petallike ones (Figure 3d). Finally, flowerlike 3D nanoarchitectures were formed. Herein, we must emphasize that the reductive reaction carried out for 5 min before the addition of NaOH is necessary. When the NaOH (10 mmol) was added directly to the reaction solution (without a 5 min reductive reaction), the resulting products were a lot of disordered nanorods rather than flowerlike structures, as shown in Figure 5. Thus, the fabrication of flowerlike Cu2O nanostructures needs a two-step chemical reaction route. The first step is the formation of Cu2O nanoparticles, and then partial Cu2O nanoparticles start to dissolve into the solution by NaOH in the second step, providing growth units for further growth of Cu2O crystals. Other Cu2O nanoparticles formed earlier may serve as “seeds”. Accordingly, the as-formed growth units centralize around the surface of the “seeds” to produce the small protuberances that are prerequisites for fabricating Cu2O flowers with the help of CTAB. This is a very interesting phenomenon in our stepwise reactions and will give important implications for designing new well-defined complex nanostructures. Control experiment studies also found that the final morphologies of the products are strongly affected by the reaction concentrations of the NaOH. We repeated the synthesis with low concentration of NaOH, (CNaOH < 0.07 M). Figure 6a shows that the surfaces of the nanoparticles only have the small radiant protuberances under this condition. Comparatively, at high concentrations (CNaOH >0.25 M), larger grasslike structures appear (Figure 6b). The grasslike structure is 10 µm in diameter, about 500-700 nm in width. The increase in the size might be due to the larger quantity of Cu2O nanoparticles dissolved in high concentration of alkaline solution. In the supersaturation

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of Cu+ ion solution, it could provide enough Cu+ ions for the growth of leaflike crystals, therefore, the adjusting of an amount of NaOH maybe obtain different morphologies and sizes of the products. When there is no or low concentrations of CTAB (CCTAB < 0.04 M) present in the synthetic system, the resultant product Cu2O are nanorods and their aggregates as revealed by the SEM image (Figure 6c). This result further indicates that CTAB plays an important role in controlling the morphology of the product. Moreover, other factors such as the quantity of water and temperature also affect the morphology of the Cu2O product. For example, under low water content synthetic conditions (the ratio of water and DMF is below 20%), the products have a lot of protuberances on their surface (Figure 6d). At high water concentrations (the ratio of water and DMF go beyond 50%), the products show many disordered nanorod assembles, which is similar to Figure 6c (no CTAB). Figure 6, panels e and f, represent the products obtained at low and high temperatures, respectively. The images reveal that just few nanoflowers can be observed under low temperature (50 °C), and many pricky nanorods can be detected at high temperature (70 °C). Welldefined Cu2O nanoflowers could not be found under all of these conditions. 3.3. Optical Property of the Products. The optical property of Cu2O nanoflowers was investigated by UV-vis absorption spectroscopy. The result is shown in Figure 7a. As can be seen, there is a broad peak in the range of 230-430 nm, centered at 370 nm. According to the equation REp ) K(Ep - Eg)1/2 (where R is the absorption coefficient, K is a constant, Ep is the discrete photo energy, and Eg is the band gap energy),42 a classical Tauc approach is further employed to estimate the Eg value of Cu2O nanoflowers. The plot of (REp)2 versus Ep based on the direct transition is shown in Figure 7b. The extrapolated value (the straight lines to the X axis) of Ep at R ) 0 gives absorption edge energies corresponding to Eg ) 2.23 eV. This value is slightly greater than the value for bulk Cu2O. The increase in the band gap of the Cu2O nanoflowers is indicative of quantum confinement effects arising from the tiny petals. Although the size of the petal tips is smaller than 10 nm as indicated by SEM, the quantum confinement effect is not significant, indicating that the final assembled architectures created herein might have led to crystal enlargement.43 4. Conclusions Novel hierarchical flowerlike Cu2O nanostructures consisting of Cu2O nanopetals have been successfully synthesized by a facile and mild solution-phase route. CTAB and NaOH play important roles in the formation of well-defined nanoflowers. The experimental results indicate that four different growth stages are involved: (i) generation of primary nanoparticles of Cu2O; (ii) the dissolution of the primary Cu2O; (iii) the recrystallization of short Cu2O protuberances; and (iv) crystal aging of Cu2O and formation of flowerlike nanostructures. These nanostructures showed very porous, hierarchical, and unique morphologies and thus can be potentially used for harvesting solar energy, optics, and sensors in the visible range. Moreover, these Cu2O nanostructures may be useful in reinforcing composite materials or in further modifying other nanostructures. Acknowledgment. We gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 20207002).

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