A Modified Electroless Deposition Route to Dendritic Cu Metal

(11) In recent years, there have been many reports on the study of dendritic ... (13) Silver dendrites were also observed by a simple surfactant-free ...
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CRYSTAL GROWTH & DESIGN

A Modified Electroless Deposition Route to Dendritic Cu Metal Nanostructures Chenglin Yan and Dongfeng Xue* State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, Dalian UniVersity of Technology, Dalian 116012, P. R. China

2008 VOL. 8, NO. 6 1849–1854

ReceiVed September 6, 2007; ReVised Manuscript ReceiVed NoVember 22, 2007

ABSTRACT: Metal Cu nanomaterials are highly desirable for being used in many applications and most widely used in electrical conductivity much more than silver and gold because of its low price and stability at high frequencies. In this paper, a modified electroless deposition strategy has been discovered for the first synthesis of novel Cu dendritic nanostructures. We have adopted the diffusion-limited growth and oriented attachment mechanism, which take effect equally during the nucleation and growth process, to account for the formation mechanism of the unique Cu dendritic nanostructures. The obtained Cu dendritic nanostructures can bring wide applications in optics, gas sensors, catalysts, information storage, and other related fields and sheds new insights to understand the formation process of fractal dendritic structures in the natural and synthetic world. Most importantly, the method reported in this work provides a new principle for the designing synthesis of dendritic metal nanomaterials and can be regarded as a general way to fabricate other nanomaterials. 1. Introduction Metal nanomaterials have been an extensively studied theme as driven by their excellent properties and potential applications in microelectronics; in optical, electronic, and magnetic devices; and as catalysts.1,2 The ability to control the synthesis of nanomaterials is important because the properties are often determined by their size, shape, composition, crystallinity, and structure.3–8 Because dendritic nanostructure has many special characteristics, such as its large surface area, good conductivity, etc., and the ability to provide a natural framework for the study of disordered system.9,10 Dendritic metal nanomaterials have become an active research theme as one type of attractive structures, which are generally observed in nonequilibrium growth processes.11 In recent years, there have been many reports on the study of dendritic structures and their formation in both theoretical and experimental aspects.12–14 Gold dendritic nanostructures had been prepared by vapor phase polymerization of pyrrole onto solution-cast films of block copolymer ionomers.12 Palladium dendritic nanostructures have been synthesized using Raney nickel as the template and reducing agent with the assistance of ultrasonic waves.13 Silver dendrites were also observed by a simple surfactant-free method using a suspension of zinc particles as a heterogeneous reducing agent.14 However, dendritic Cu nanostructures as another important metal nanomaterial have not been reported through the electroless deposition method so far. Cu nanomaterials as an important class of metal nanostructures have been increasingly studied because their unusual properties and potential applications in thermal conducting, lubrication, nanofluids, and catalysts.15 Cu is most widely used in electrical conductivity much more than silver and gold because of its low price and stability at high frequencies.16 Therefore, high-purity, uniformly shape, nanostructured Cu is highly desirable for use in the electronic industry.16 Especially, Cu dendritic structures have attracted our attention due to their importance connected to some fractal growth phenomena and crystallography research and because they have wide applica* Corresponding author. Fax: (86) 411-8899-3623. E-mail: dfxue@ chem.dlut.edu.cn.

tions in microdevices as nanojoints. In the present work, a modified electroless deposition strategy has been discovered for the synthesis of unique Cu dendritic nanostructures. This is the first time that Cu dendritic nanostructures are synthesized by such a simple electroless deposition method other than the electrochemical method. We have carefully analyzed the formation processes of Cu dendritic nanostructures on the basis of the diffusion and oriented attachment mechanism. 2. Experimental Procedures Prior to the synthesis, zinc foil was carefully cleaned with absolute alcohol and deionized water, respectively, in an ultrasound bath to remove surface impurities. A typical synthesis of Cu dendritic nanostructures proceeded as follows: 0.12 g of copper chloride (CuCl2 · 2H2O) was dissolved in a deionized water/hydroperoxide solution system. The pH value of the mixture solution was adjusted to around 2.6 using acetic acid (HAc) solution. The resultant mixture solution was then transferred into Teflon-lined stainless steel autoclave, and the previously cleaned zinc foil was then immersed into the mixture solution. The autoclave was then filled with water up to 70% of the total volume. The autoclave was sealed to an electric oven and maintained at 120 °C for 4-18 h. After being cooled to room temperature naturally, the upper pellucid solution was decanted, and the precipitate was collected, filtered off, and washed with deionized water and absolute ethanol several times. Finally, all samples were dried in air at 80 °C for 4 h for further measurements. The as-prepared samples were characterized by an X-ray diffractometer (XRD) on a Rigaku-DMax 2400 diffractometer equipped with the graphite monochromatized Cu KR radiation flux at a scanning rate of 0.02°s-1 in the 2θ range 10-80°. Scanning electron microscopy (SEM) images were taken with a JEOL-5600LV scanning electron microscope, using an accelerating voltage of 20 kV. A JEOL JEM2000EX transmission electron microscope operating at 200 kV accelerating voltage was used for TEM (transmission electron microscopy) analysis. UV/vis diffuse reflectance spectra were obtained using a UV-vis-NIR spectrophotometer (JASCO, V-550).

3. Results and Discussion Cu nanostructures cannot be effectively obtained through the conventional electroless deposition method in a vessel containing Cu2+ solution and zinc foil because of a fast deposition rate under high supersaturation in a short time. In our synthetic system, a modified electroless deposition strategy has been

10.1021/cg700851x CCC: $40.75  2008 American Chemical Society Published on Web 05/06/2008

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Figure 1. Illustrative representation of galvanic displacement on zinc foil in the presence of CuCl2 · 2H2O solution.

Figure 3. (A) XRD pattern of obtained dendritic Cu that was deposited on the surface of the zinc foil; (B) TEM image of Cu dendritic structure (inset is SAED pattern recorded from the branch of Cu dendrite); (C) HRTEM image of Cu dendritic structures.

Figure 2. SEM images of obtained dendritic Cu that was deposited on the surface of the zinc foil in the presence of acetic acidic: (A) lowmagnification SEM image of dendrite showing the high yield and good uniformity; (B and C) high-magnification SEM images of dendrite showing the self-similarity characteristic of fractals.

designed to prepare novel Cu dendritic nanostructures. Specifically, electroless galvanic replacement reaction is realized via the immersion of zinc foil into CuCl2 · 2H2O aqueous solution followed by addition of a small amount of dilute hydroperoxide and HAc solution. The reduction process for these transition metal ions is best described by mixed potential theory, roughly based upon the half-cell reactions. According to the contrast of the cell potentials, E0 Zn2+/Zn) -0.763 V; E0 Cu2+/Cu ) 0.337 V, E0 O2/OH- ) 0.401, Zn metal loses electrons and goes into the solution as Zn2+ ions; Cu2+ ions and O2 in solution then accept the electrons donated by the zinc foil and dendritic Cu metal is thus deposited on the surface of Zn metal foil, which is schematically displayed in Figure 1. Through the introduction of the cathode reaction of O2/OH-, the deposition rate of Cu metal nanostructures can be effectively lowered. Therefore, perfect Cu dendritic nanostructures can be successfully prepared. Experimental results indicate that the deposition rate of Cu metals is very fast in the absence of cathode reaction of O2/ OH- and Cu nanomaterials might agglomerate into larger, irregularly shaped particles. Figure 2 displays SEM images of typical, beautiful Cu dendritic nanostructures with different magnifications prepared through the current modified electroless galvanic method. The panoramic view with a low magnification shown in Figure 2A clearly demonstrates that all samples are homogeneous dendritic

structures with a high yield and good uniformity. Further observation shows that each dendrite consists of a long central backbone and very sharp secondary branches (images B and C in Figure 2), which exhibits good symmetry and self-similarity characteristic of fractals. More complex Cu dendritic structures with an increase in the degree of branching growth are produced in the absence of Ac-. It can be seen from a low magnification of Figure S1A in the Supporting Information that an individual dendrite composed of many complex branches is obtained. A high magnification SEM image of secondary branches shown in Figure S1B indicates that the secondary branches are constructed by many tertiary branches. However, such dendritic structures are in low yield (less than 10%). Therefore, Ac- is crucial for the formation of the homogeneous dendritic structures. It is known that Cu(Ac)2 is a rather weak electrolyte with a low degree of dissociation and Cu2+ ions can coordinate with the Ac- to form complexes in solution. We consider that Cu-Ac complexes are formed before Cu2+ in solution are reduced to form metallic Cu. The formation of Cu-Ac complexes may lead to the controlled release of Cu2+ ions as well as crystallization of Cu dendritic nanostructures. Eventually, perfect Cu dendritic structures in a high yield can be achieved with the assistance of Ac-. The XRD pattern of the as-synthesized Cu dendritic structure is shown in Figure 3A. Two main characteristic peaks for Cu at 2θ ) 43.3 and 50.4°, corresponding to Miller indices (111) and (200), are observed. This confirms that the resultant dendrites are pure face-centered cubic Cu (JCPDS 04-0836). Further structural characterization of Cu dendrites was carried out by TEM measurements. A typical TEM image of Cu dendritic nanostructure is shown in Figure 3B, which reveals a clear and well-defined dendritic fractal structure with branch paralleling each other. Selected area electron diffraction (SAED) pattern (inset of Figure 3B) taken from the branch tip clearly displays only cubic diffraction spots pattern, which indicates that the branch, as the building unit of the Cu dendritic nanostructures, is in a single crystalline state. Figure 3C shows a high-resolution transmission electron microscopy (HRTEM) image recorded from the branch of Cu dendrites. The fringe

Electroless Deposition Route to Dendritic Cu Nanostructures

Figure 4. (A) UV-vis spectrum of Cu dendritic nanostructures showing a wide visible absorption peaks at about 556 nm corresponding to the plasmon band of the Cu nanoparticle surface. (B) UV-vis spectrum of the bulk Cu foil is shown as a reference.

spacing is determined to be 0.205 nm, which is closed to the (111) lattice spacing of the bulk Cu (0.209 nm). The study of the optical properties of Cu dendritic nanostructures is very interesting because very few reports have been published on the formation of metal dendritic nanostructures, especially for Cu dendritic nanostructures. The successful synthesis of Cu dendritic nanostructures allows the experimental confirmation of the theoretical calculation. The UV-vis absorption spectrum of the Cu dendritic structures (Figure 2) and bulk Cu foil are shown in curves A and B in Figure 4, respectively. As seen from Figure 4A, Cu dendritic nanostructures have a wide visible absorption peaks at about 556 nm corresponding to the plasmon band of the Cu nanoparticle surface,17 which is similar to that observed peak for the bulk Cu foil. The UV-vis investigation indicates that Cu dendritic nanostructures show broad absorption in the visible region and possess the same properties as the bulk Cu foil. Therefore, our obtained Cu dendritic nanostructures might find especial applications in microdevices as nanojoints and micro- and nanoelectronic industry. The diffusion-limited aggregation (DLA) model provides much useful insight into fractal growth of dendritic nanostructures controlled by the diffusive processes.18 This model includes a single cluster to which additional particles attach once they reach a site adjacent to the edge of the cluster. The additional particles are launched one at a time from random positions far away from the cluster and move as random walkers until they either attach to the cluster or move out of the finite system. We here propose a possible formation mechanism of Cu dendritic nanostructures based on DLA model. The rate of nucleation and ensuing fractal growth for the formation of Cu dendrites on Zn foil surface from CuCl2 · 2H2O aqueous solution are effectively controlled in the reaction system. Initially, small nanoparticles are formed. As the reaction is allowed to proceed, the aggregation of these nanoparticles then occurs to produce larger particles. Simultaneously, when Cu nanoparticles grow, ions or molecules near the zinc foil surface are consumed by the growing nanoparticles and a concentric diffusion field forms around the particles.19 With prolonging the reaction time, the concentrations of copper salt and the reduction agent (Zn foil) greatly decrease. The supersaturation of the system significantly decreases because of the reactant exhaustion after primary nucleation stage, and nucleation is thus restrained. Being small could make the surface of nanoparticles unstable because of the high surface energy and the large surface curvature. Therefore, an aggregation of Cu nanoparticles to form Cu nanorod occurs subsequently to form the dendritic Cu nanostructures mainly driven by decreasing surface energy.

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Figure 5. Schematic description of the growth process of Cu dendritic nanostructures. The formation process may take place through the oriented attachment of nanoparticles to form nanorods, and eventually results in complex Cu dendritic nanostructures.

This indicates that the formation of Cu dendritic structures is dominated by both diffusion control and oriented attachment processes: the dendritic structure is obtained by adsorption of diffusing Cu nanoparticles once in contact with each other. This process may take place through the oriented attachment of nanoparticles to form nanorods and eventually results in complex Cu dendritic nanostructures. The whole formation process of Cu dendritic nanostructures is schematically shown in Figure 5. To substantially understand the growth mechanism of Cu dendritic nanostructures, we have systematically surveyed the growth process of Cu dendrites by analyzing the samples obtained at different crystallization stages. SEM images shown in Figure 6 display the morphology obtained at different reaction times corresponding to the reaction time of 1, 3, 9, and 18 h, respectively. After about 1 h reaction, a large amount of solid product is generated on the Zn foil surface. The obtained samples mainly consist of two different forms of Cu nanostructures (i.e., nanoparticles and nanorods) shown in Figure 6A (indicated by color circle), which indicates that many nanoparticles have gradually developed into nanorod structures based on an aggregation mechanism. During the reaction, the newly formed Cu nanoparticles joined together through orientation attachment begin to form nanorod structures. Many nanorods are presumed to be the intermediate states of nanoparticles to dendrite transition. When prolonging the reaction time to 3 h, it is observed from Figure 6B that Cu nanorods dominate in the samples and no Cu nanoparticles are found in the samples, which indicates that initially produced Cu particles have completely converted into the rodlike crystals. In addition, some short secondary branches growing out of the rodlike trunk can be occasionally observed (indicated by color circle). When increasing the reaction time to 9 h, as shown in Figure 6C, more secondary branches emerge, these secondary branches grow longer as prolonging the reaction time. Finally, as the reaction time proceeds long enough (18 h), perfect Cu dendritic structures can be obtained (Figure 6D), no Cu nanoparticles are observed. It is believed that oriented attachment process in current reaction system is the prerequisite for the formation of Cu dendritic nanostructures. In the initial induction stage, solutes are formed to yield a supersaturated solution, leading to nucleation. The nuclei then grow through a diffusive mechanism to form crystalline subunits of nanoparticles, which in turn aggregate to form the large polycrystalline assemblages of Cu nanorods. The impetus for aggregation of Cu nanoparticles to form nanorods is to reduce the surface energy from the thermodynamic viewpoint, which is similar to Au and Ag nanowire

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Figure 6. Time-dependent evolution of Cu dendritic structures at different growth stages: (A) 1 h, the coexistence of nanoparticles (indicated by blue circle) and nanorods (indicated by yellow circle) are observed; (B) 3 h, some short secondary branches growing out of the rodlike trunk are indicated by a green circle; (C) 9 h, more secondary branches emerge and the secondary branches grow longer; (D) 18 h, perfect Cu dendritic structures can be obtained at this reaction stage.

Figure 7. (A) TEM image of Cu nanostructures obtained at 3 h, the particles sequentially joined together through aggregation to form a rodlike Cu trunk. (B) TEM image of Cu dendritic nanostructures obtained at 9 h, secondary branches grow out of the rodlike trunk.

formation reported by Maddanimath et al.20 The above phenomenon is in perfect agreement with earlier findings that in an ion solution with concentrations far above the saturation level, amorphous clusters are formed first, which then produce the crystalline crystals at a later stage.21 TEM analysis has provided us further insight into the formation mechanism of dendritic Cu nanostructures through an oriented attachment based on the diffusion-limited growth process. It can be clearly seen from the TEM image shown in Figure 7A that the particles sequentially joined together through aggregation to form a rodlike Cu trunk are found in the sample with an intermediate crystallization time of 3 h. These adjacent nanoparticles are self-assembled by sharing a common crystallographic orientation and docking of these particles at a planar interface, which is believed to be the result of further development of more complex dendritic structure by aggregation based on a diffusion-limited growth process. Previous work has been demonstrated that shape transformation from small nanocrystals to nanorods by oriented attachment mechanisms is an efficient route for the synthesis of nanorods or nanowires. Hyeon and co-workers synthesized anisotropic ZnS nanocrystals through

Figure 8. (A, B) SEM images of Cu dendritic nanostructures obtained in the presence of 0.2 g CTAB. The obtained branches are composed of many small nanoparticles showing a rough surface structure.

shape transformation of ZnS nanospheres to nanorods through oriented attachment mechanisms.22 Similarly, single-crystalline PbSe nanowires were synthesized by Murray and co-workers.23 TEM image of a growing dendrite obtained at 9 h is shown in Figure 7B, from which it is obvious that the secondary branches grow out of the rodlike trunk with increasing the reaction time. TEM observations confirm that the oriented attachment process dominates the growth of Cu dendrites comprised of nanoparticles.

Electroless Deposition Route to Dendritic Cu Nanostructures

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Figure 9. SEM images of Cu dendritic nanostructures obtained at 180 °C with 0.04 mol/L CuCl2 · 2H2O: (A) panoramic morphologies; (B-D) detailed views of dendritic nanostructures showing a groovelike characteristics; the branched nanostructures are parallel to each other and better aligned on the stems.

Figure 10. (A, B) SEM images of Cu dendritic nanostructures obtained in the presence of 0.25 mL of HAc. The obtained Cu samples display dendritic growth with increasing branch structures but are less symmetric.

The ability to control and manipulate the crystallization of crystals as we desire is one of the challenging issues in materials science.24 Modern scientists are now exploring ways to obtain such control at the nanometer and micrometer scale. The

crystallization process of crystals is usually controlled through selecting various surfactants, which can influence on the growth of crystals under nonequilibrium kinetic growth. The surfactant molecules probably acting as the capping agents are absorbed onto the nuclei in the interface, which can affect the diffusion and growth of nuclei in the macroscopic length scale, which is widely and deliberately studied namely DLA theory.25,26 NCetyl-N,N,N-trimethyl-ammonium bromide (CTAB) is employed in the current reaction system to control and modulate the crystal growth behaviors of Cu dendrites during the fractal growth process. When an appropriate amount of CTAB is added into the reaction system for the growth of Cu dendrites. The morphology of the as-prepared Cu dendritic structure has changed a lot (Figure 8A), although the whole framework of dendritic structures is very similar to that shown in Figure 2. A higher magnification SEM image clearly displays their detailed structure characteristics (Figure 8B), the individual Cu dendrite is composed of a long central trunk with the secondary branches paralleling to each other. It should be noted that the secondary branches are composed of many small nanoparticles showing a rough surface structure. It is obvious that many branches are produced through attachment of small nanoparticles to the branches. More interestingly, some tertiary branches develop from the secondary branches are found to construct more complex dendritic structures. Without CTAB, the surface of the secondary branch is very smooth and no tertiary branches are found in the dendritic structures. It is believed that surfactant CTAB prevents the aggregation of Cu nanoparticles in the initial stage and kinetically controlling the growth rates of various crystallographic facets of face-centered cubic Cu through absorbing on specific facets of Cu particles. Because of the adsorption of surfactant CTAB, the primary new born nuclei formed in the initial stage cannot quickly meet together to aggregate and each nucleus adsorbed by CTAB will further preferentially aggregate to form the trunk or branch structure of dendrites. It is generally acceptable that the essential for the growth dendritic pattern may be determined by diffusion kinetics.27 Diffusion kinetics determines the macroscopic approach toward equilibrium, and influence the morphologies created in macroscopic levels. In the macroscopic growth process, surfactant molecules probably acting as the capping

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agents affect the diffusion and growth of nuclei in the macroscopic length scale. As producing Cu nanoparticles in the initial stage, the trunks or branches grow through attachment of small nanoparticles. More complex dendritic structure would be constructed through subsequently connecting more building blocks. Under equilibrium growth conditions, the morphology of a crystal is determined solely by intrinsic factors. The faces comprising the crystal habit correspond directly with the most energetically stable atomic planes in the lattice. In general, faces perpendicular to the fast directions of growth have smaller surface areas and slow growing faces therefore dominate the morphology.28 However, dendrites grow under nonequilibrium conditions and habits are strongly influenced by external conditions such as the reaction temperature and reagent concentration. From the viewpoint of thermodynamics, the dendritic nanostructure with a big surface/volume ratio has a considerably increased surface energy in contrast to the equilibrium shape of crystal, and is thermodynamically unstable. In the current work, our solution growth environment provides unstable and nonequilibrium conditions, which is essential for the growth of dendritic structures. Therefore, it cannot be ignored that the effect of the reaction temperature and reagent concentration on the morphology of Cu dendrites must be considered. At 180 °C with a higher Cu2+ concentration (0.04 mol/L) and the same volume of HAc (0.75 mL), the obtained Cu samples show novel dendritic structures, displayed in SEM images of Figure 9A, whose stems are very long with a length of about 100 µm. It is interesting that the Cu branched nanostructures show a groovelike characteristics, which can be clearly seen in a highermagnification SEM image of Figure 9B. The branched nanostructures are parallel to each other and better aligned on the stems (images C and D in Figure 9). Note that coexistence of the big and small branches are observed, because of the fact that these branch alignment experiences a space-limited growth. When the volume of HAc is decreased to 0.25 mL, the obtained Cu samples also display the dendritic growth with increasing branch structures (Figure 10A). Compared to the dendritic structures shown in Figure 2, this structure is even more dendritic but less symmetric with less straight stems (Figure 10B). As described in DLA model,29,30 random moving nuclei formed in the solution accumulate with each other to form kinetically roughened dendritic structures. Regarding the morphology changes of Cu dendritic samples obtained at different reaction temperature and concentration, the rate of mass transport (i.e., diffusion, migration, and convection) has an important influence on the moving rate of nuclei in the formation process of Cu dendrites. In our reaction system, reaction temperature and concentration can affect the diffusion rate of the nuclei. Therefore, the morphology of Cu dendritic nanostructure can be well-manipulated by controlling the reaction temperature and reagent concentration. 4. Conclusion A modified electroless deposition strategy based on simple reaction with no template and no expensive precision equipment has been successfully developed for the growth of novel Cu dendritic nanostructures, which exhibit greatly uniform shape and size. Diffusion control and oriented attachment mechanism are used to explain the growth process of dendritic nanostructures. UV-vis investigation indicates that Cu dendritic nanostructures show broad absorption in the visible region and might find especial applications in nanodevices as nanojoints and nanoelectronic industry. This facile methodology provides

Yan and Xue

control over surface morphology and deposition rate by careful modulation of plating parameters such as temperature, surfactant, and metal ion concentration. A clear perspective is shown here that more complex dendritic nanostructured materials could be prepared when surfactant molecules are used or changing other reaction parameters. This strategy reported herein is being further explored to create complex forms of nanostructures for new applications and brings new insights into the underlying formation mechanism of dendritic nanostructures in the natural and synthetic world. In addition, dendritic structures of Cu can also be converted at elevated temperature into CuO that could show some unique properties in many reactions of industrial importance. We expect that the current electroless deposition route can be extended to prepare other metals nanomaterials with dendritic structures through designing an appropriate electroless galvanic reaction. Acknowledgment. We gratefully acknowledge the financial support of the program for New Century Excellent Talents in University (Grant NCET-05-0278), the National Natural Science Foundation of China (Grant 20471012), a Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (Grant 200322), and the Research Fund for the Doctoral Program of Higher Education (Grant 20040141004). Supporting Information Available: SEM images of copper dendritic nanostructures prepared in the absence of acetic acid (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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