CRYSTAL GROWTH & DESIGN
Hierarchical Growth of Cu2O Double Tower-Tip-like Nanostructures in Water/Oil Microemulsion Hongwei
Zhang,§
Xu
Zhang,*,§
Hongyan
Li,#
Zhikun
Qu,§
Shan
Fan,§
and Mingyan
Ji§
Department of Chemistry, Harbin Normal UniVersity, Harbin 150080, Heilongjiang, People’s Republic of China, and Department of Inspection, Dalian Medical UniVersity, Dalian 116027, Liaoning, People’s Republic of China
2007 VOL. 7, NO. 4 820-824
ReceiVed October 20, 2006; ReVised Manuscript ReceiVed December 4, 2006
ABSTRACT: A novel Cu2O hierarchical double tower-tip-like nanostructure has been successfully synthesized in water/oil (w/o) microemulsion. This kind of nanostructure is made of three-order structures. The primary structure is composed of three stems across and perpendicular to one another, like a hexapod-shaped structure. Four rows of nanorods epitaxially grown on each stem and perpendicular to the stem form the secondary structure. The tertiary structure is two rows of nanorods epitaxially grown on a secondary nanorod if the secondary nanorod is long enough. All the first-order, second-order, and third-order growth take place along orientations of . The obtained samples are characterized by means of X-ray powder diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). It is found that the microemulsion system, water content, ligand, reaction temperature, reaction time, and concentration of NaOH play important roles in the formation of this novel morphology besides the crystal habit of Cu2O. Introduction Cu2O is a p-type semiconductor with a direct band gap of 2 eV,1 which makes it a promising material for superconductors,2 solar cells,3 negative electrode materials,4 magnetic storage,5 biosensing,6 etc. Cu2O has also been found to be a stable photocatalyst for the photochemical decomposition of water into O2 and H2 under visible light irradiation.7 In addition, it not only has a potential application in photocatalytic degradation of organic pollutants under visible light8 but is a prospective candidate for a low-cost photovoltaic power generator for its high optical absorption coefficient.9 In the past few decades, many efforts have been devoted to preparing Cu2O crystals with various morphologies due to the importance of fundamental research and application.10-19 Among them, Chang et al. have prepared a variety of Cu2O multipod frameworks at temperatures over 100 °C.10 Zhang et al. have prepared hexapod-shaped Cu2O via a γ-irradiation reduction route under ambient conditions.11 Chen et al. have obtained hexa-pod-like Cu2O whiskers under hydrothermal conditions.12 It is worth noting that Cu2O crystals in the above examples exhibit the growth habit to be multipod-shaped structures under suitable conditions, although the subunits forming these products are large and the structures of these products are relatively simple. However, it is well-known that the realization of technologically useful nanomaterials depends not only on their morphology and size but also on their spatial orientation and arrangement,20-22 so the controlled synthesis of complex nanostructures made of subunits, especially one-dimensional (1D) subunits (such as nanorods), is significant.23 Recently, selforganized crystal growth of a variety of novel hierarchical structures made of 1D subunits (such as nanorods, nanobelts, and nanotubes) or some simple geometries (such as plates, cubes, and tetrahedrons) have been achieved, such as GaP nanotrees,24 silver nanoinukshuks,25 ZnO nanostructures,26 BaXO4(X ) Mo, W) superstructures,27 hierarchical oxide * To whom correspondence should be
[email protected]. Tel: +86-451-88060867. § Harbin Normal University. # Dalian Medical University.
addressed.
E-mail:
nanostructures,28 hierarchical ZnO nanotubes,29 and hierarchical mesophase crystals.30 However, hierarchical nanostructures of Cu2O crystals made of nanorods have not been reported to be synthesized to date. Inspired by the experimental result reported that fishbone-like BaWO4 nanostructures have been prepared in water/oil (w/o) microemulsion,31 we designed a series of experiments to try to produce a hierarchical nanostructure of Cu2O crystals by using w/o microemulsion. As we know, “water-in-oil” microemulsions contain nanosized water pools that are dispersed in a continuous oil medium and are stabilized by surfactant and cosurfactant molecules localized at the water/ oil interface. The nanoscale water pools can provide ideal microreactors for the formation of nanoparticles.32,33 At last, we successfully prepared complex Cu2O hierarchical double tower-tip-like nanostructures by optimizing reaction conditions, and the growth mechanism is discussed. Experimental Section All reagents were analytically pure and purchased from Beijing Chemical plant and used without further purification. A typical synthesis of Cu2O hierarchical double tower-tip-like nanostructures was as follows: two types of microemulsion solutions were prepared by solubilizing an aqueous CuSO4‚5H2O, EDTA, and NaOH mixed solution or a C6H12O6‚H2O solution into an n-octane/ cetyl trimethyl ammonium bromide (CTAB)/1-butanol system (water content w ) [H2O]/[CTAB] (molar ratio) ) 34).34 After 10 min of vigorous stirring, the above two different microemulsion solutions with equivalent volume were mixed rapidly and stirred for 30 min. The final aqueous solution concentrations were [CuSO4] ) 0.02 mol‚L-1, [EDTA] ) 0.02 mol‚L-1, [NaOH] ) 0.333 mol‚L-1, and [C6H12O6] ) 0.028 mol‚L-1, respectively (the concentrations of these reactants are based on the volume of aqueous solution and not the total volume.). The resulting mixture was then loaded into a 60 mL Teflon-lined autoclave, which was sealed, heated at 60 °C for 12 h, and then cooled to room temperature naturally. The precipitate obtained was separated, washed several times with distilled water and absolute ethanol, and then dried in a vacuum at 25 °C for 5 h. The phase of the products was determined by X-ray powder diffraction (XRD) employed a Y-2000 automated X-ray diffractometer system with Cu-KR radiation (λ ) 1.5405 Å). The scan rate of 0.1° s-1 was applied to record the patterns in the 2θ range of 25-75°. Field emission scanning electron microscopy (FE-SEM) was performed on a HITACHI S-4800 scanning electron microanalyzer.
10.1021/cg0607351 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/03/2007
Growth of Cu2O Double Tower-Tip-Like Nanostructures
Figure 1. A typical XRD pattern of the products prepared with 0.02 mol‚L-1 of CuSO4, 0.02 mol‚L-1 of EDTA, 0.333 mol‚L-1 of NaOH, and 0.028 mol‚L-1 of C6H12O6 at 60 °C for 12 h (w ) 34).
Results and Discussion The obtained samples were characterized by X-ray powder diffraction (XRD). A typical XRD pattern is shown in Figure 1, in which all the peaks can be indexed to the pure cubic phase of Cu2O with the lattice constants a ) 4.2652 Å, compatible with the literature value of a ) 4.2696 Å, (JCPDS No. 050667). No other peaks of impurities are detected. Figure 2 shows the typical FE-SEM images of the Cu2O hierarchical double tower-tip-like nanostructures at different magnifications, respectively. The low magnification (Figure 2a) FE-SEM image shows the abundance and uniformity of the Cu2O hierarchical double tower-tip-like nanostructures. Figure 2b shows a high-magnification FE-SEM image of a single Cu2O microcrystal with a hierarchical double tower-tip-like nanostructure, and from it we can see clearly that the as-produced product has three stems across and perpendicular to one another (the primary structure) whose ends correspond to the six apexes of the double tower-tip-like nanostructure, and four rows of nanorods (the secondary structure) epitaxially grow on each stem and perpendicular to the stem, and two rows of nanorods (the tertiary structure) epitaxially grow on those longer nanorods if the secondary nanorods are long enough. Among them, the primary structures are the longest and the tertiary structures are the shortest. The whole length of the Cu2O stems from one end to the other end can be as long as 2.0 µm as shown in Figure 2b and the diameter is about 100∼150 nm, whereas the length
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of the secondary Cu2O structure grown on the Cu2O stems reduces from 400 nm at the intersection point of the three stems to 150 nm at their ends. These secondary nanorods are evenly spaced on every stem with a regular periodicity of about 50 nm. If the secondary nanorods are long enough and there is no steric hindrance, the tertiary nanorods will grow perpendicular to them. The secondary structure and the tertiary structure have uniform diameters of about 100 nm. It is found that a microemulsion system is a crucial prerequisite for the formation of Cu2O hierarchical double towertip-like nanostructure. If the reaction is proceeded in aqueous solution, Cu2O octahedra and irregular particles are obtained (see Supporting Information Figure S1). Murphy et al. had also reported that Cu2O nano- and microcubes can be synthesized in the presence of CTAB with ascorbic acid as a reducing agent.35 The reason we obtained Cu2O crystals with different morphology may be attributed to the use of a different reducing agent. So the microemulsion system is unique in allowing the preparation of Cu2O hierarchical double tower-tip-like nanostructures. In this study, the effects of the water content, ligand, temperature, time, and concentration of NaOH on the morphology of the products are investigated. Figure 3 shows that some irregular morphology appears when the water content is reduced to w ) 30, rather than the complex Cu2O hierarchical double tower-tip-like nanostructures. This controlled experiment demonstrates that the selected water content (w ) 34) is beneficial to the formation of complex Cu2O hierarchical double tower-tip-like nanostructures. The controlled experiment without using ligand is conducted by keeping the rest of the experimental conditions the same; Cu2O crystals with rod-like morphology are obtained (Figure 4), rather than uniform hierarchical double tower-tip-like nanostructures, which may be attributed to the quick release rate of the Cu2+ due to the absence of the effective coordinated effect of EDTA, so that the stepwise nucleation and growth do not appear. Reaction temperature also affects the morphology of the products. When the reaction is conducted at room temperature, only hexapod-shaped crystals (Figure 5a) are obtained, which means only the primary structures are grown, and the growth of the secondary and tertiary structures do not occur. In addition, some stacking faults on each pod can be found, which may be attributed to heterogeneous crystal growth under static conditions during the growth process. After the reaction temperature is increased to 80 °C, only some irregular short rods are obtained
Figure 2. Typical FE-SEM images of the products prepared with 0.02 mol‚L-1 CuSO4, 0.02 mol‚L-1 EDTA, 0.333 mol‚L-1 NaOH, and 0.028 mol‚L-1 C6H12O6 at 60 °C for 12 h (w ) 34). (a) Low magnification image and (b) high magnification image.
822 Crystal Growth & Design, Vol. 7, No. 4, 2007
Figure 3. FE-SEM image of sample prepared with 0.02 mol‚L-1 CuSO4, 0.02 mol‚L-1 EDTA, 0.333 mol‚L-1 NaOH and 0.028 mol‚L-1 C6H12O6 at 60 °C for 12 h (w ) 30).
Figure 4. FE-SEM image of samples prepared with 0.02 mol‚L-1 CuSO4, 0.333 mol‚L-1 NaOH, and 0.028 mol‚L-1 C6H12O6 at 60 °C for 12 h (w ) 34).
(Figure 5b). Therefore, it can be concluded that the thermodynamics has a prominent effect on the morphology of the product. If the temperature is higher, the microemulsion system is destroyed. Reaction time is also one of the factors affecting growth. When the reaction time is reduced to 6 h, most of the hierarchical double tower-tip-like nanostructures had not grown completely (Figure 6a), which may be attributed to the strong
Zhang et al.
coordination ability of EDTA. Cu2+ ions coordinated by EDTA cannot be released in time, so stepwise nucleation and growth are hindered and the product does not grow thoroughly. When the time is increased to 18 h, some stems become longer (Figure 6b), which demonstrates that the growth of the secondary and tertiary structures will occur even though the growth of the primary structure does not finish under the suitable growth conditions. When the sample is heated for 24 h, the initial hierarchical structure will become disordered (Figure 6c). It is known that the hierarchical structure, by virtue of its extended surface, has a considerably increased surface energy compared with the equilibrium shape of the crystal and is therefore thermodynamically unstable.36 When the sample is heated for a longer time, some secondary and tertiary nanorods will fracture and accumulate disorderly. So the optimum reaction time is 12 h. In addition, it is found that the concentration of NaOH also plays a key role in the formation of hierarchical nanostructures. When the concentration of NaOH is 0.225 mol‚L-1, the products do not show hierarchical double tower-tip-like nanostructures but comblike structures (Figure 7a), and when the concentration of NaOH is 0.635 mol‚L-1, the shape of the products is a joint octahedra (Figure 7b). This emergence of different morphologies at various NaOH concentrations may be attributed to the variation of surfactant adsorption behavior with the change of alkalinity of the reaction system. On the basis of the above experimental results, we think the growth of the Cu2O hierarchical double tower-tip-like nanostructure is controlled not only by kinetics but also by thermodynamics through increasing the reaction temperature and prolonging the reaction time. Its growth mechanism is similar to that of branched ZnO crystallites.37 Usually, the formation of crystals can be divided into two steps: (1) nucleation and (2) growth. There are three times of nucleation in our study, the first time of nucleation is homogeneous because it occurs in the homogeneous microemulsion, and the other two times of nucleation are heterogeneous because they are induced by the site-selective CTAB adsorption on the primary or secondary rods. The growth of the hierarchical nanostructures is divided into three procedures: (1) growth of the primary structure, (2) growth of the secondary structure, and (3) growth of the tertiary structure. Interestingly, when the secondary structure begins to grow, the growth of the former structure does not stop; rather, it still continues. All first-order, second-order, and third-order growth takes place along orientations of . Figure 8 is used to illustrate the schematic representation of the mode for
Figure 5. FE-SEM images of samples prepared with 0.02 mol‚L-1 CuSO4, 0.02 mol‚L-1 EDTA, 0.333 mol‚L-1 NaOH, and 0.028 mol‚L-1 C6H12O6 for 12 h at different reaction temperatures (w ) 34). (a) Room temperature. (b) 80 °C.
Growth of Cu2O Double Tower-Tip-Like Nanostructures
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Figure 6. FE-SEM images of samples prepared with 0.02 mol‚L-1 CuSO4, 0.02 mol‚L-1 EDTA, 0.333 mol‚L-1 NaOH, and 0.028 mol‚L-1 C6H12O6 at 60 °C for different reaction times (w ) 34). (a) 6 h, (b) 18 h, (c) 24 h.
Figure 7. FE-SEM images of samples prepared with 0.02 mol‚L-1 CuSO4, 0.02 mol‚L-1 EDTA, 0.028 mol‚L-1 C6H12O6, and NaOH with various concentrations at 60 °C for 12 h (w ) 34). (a) 0.225 mol‚L-1 and (b) 0.635 mol‚L-1.
Figure 8. The schematic representation of the mode for the growth of Cu2O hierarchical double tower-tip-like nanostructures.
the growth of Cu2O hierarchical nanostructures. The whole growth can be described as follows: The three stems across and perpendicular to one another (the primary structure) are grown first. In the developing process of the primary structure, secondary nucleation takes place due to site-selective CTAB adsorption on the primary rod, which guides the formation of the first few layers of the precipitation on some weak adsorption sites of the surface of the primary rods, thus initiating the sitespecific heterogeneous nucleation process.37 Thereafter, the first few atomic layers of the site-specific precipitation would facilitate the vertical growth of the secondary branches.37 This process continues and the tertiary structure branches off the secondary nanorods in the same way, and the tertiary nanorods only grow on the longer secondary nanorods due to the space limitation. As a result, hierarchical double tower-tip-like nanostructures are formed. It is noteworthy that the dimensions of the as-obtained Cu2O hierarchical double tower-tip-like nanostructures are apparently not commensurate with those of
individual water cores due to the aggregation and coalescence of individual droplets. In our experimental work, after two microemulsions containing different reactants at selected w values were mixed, the interacting micelles fused irreversibly by mutual association, which resulted in a short cylindrical droplet and simultaneously Cu2O nuclei formed rapidly at the collision interface. Then the nucleated cluster sides are blocked by a monolayer of strongly bound CTAB molecules in the process of the further fusion that results in long-range aggregation. With the further progress of the fusion, the abovementioned growth habit of Cu2O takes place, and three stems across and perpendicular to one another grow along the orientations of . In the process of the formation of Cu2O hierarchical double tower-tip-like nanostructures, the effect of site-selective CTAB adsorption on morphology is another important aspect of the growth mechanism. The presence of site-selective CTAB adsorption at the nucleated cluster sides may result in individual droplets slowly self-assembling and
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fusing along the side of the stem by interdigitation of the organic chains. So, as primary growth proceeds, the epitaxial nucleation toward four directions due to the steric effect becomes probable at some weakly bound sites. The linear growth of the stem is faster than the side growth, so Cu2O stems grow longer with an increase in the reaction time. Then the secondary nanorods will grow as the first ones have done, and so will the tertiary nanorods. Therefore, the synergic effect of the microemulsion and the crystal habits of Cu2O induces the growth of Cu2O hierarchical double tower-tip-like nanostructures. Conclusion In summary, novel Cu2O hierarchical double tower-tip-like nanostructures are prepared by optimizing the reaction parameters in w/o microemulsion. It is found that the microemulsion system is a prerequisite for the formation of Cu2O nanostructures with hierarchical double tower-tip-like morphology. The growth of these hierarchical nanostructures includes three procedures, and the next growth procedure starts even though the last one does not end. The whole growth is along the orientations of . In addition, the crystal habit of Cu2O also plays an important role in the formation of Cu2O hierarchical double tower-tip-like nanostructures. This work shows that complex structures and morphologies can be derived from the fundamental microscopic building units. Acknowledgment. This work is supported by the Doctoral Initiation Fund of Harbin Normal University, The Prominent Young Foundation of Harbin Normal University, and Department of Education of Heilongjiang Province (10551100 and YJSCX 2005-45HLJ). We thank Mr. H. C. Mu, Prof. H. Gao, and Mr. G. Wang for supporting the FE-SEM and XRD work. Supporting Information Available: FE-SEM image of Cu2O microcrystals prepared in aqueous solution. This material is available free of charge via the Internet at http://pubs.acs.org.
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