Article pubs.acs.org/crystal
Morphology Control of Cu Crystals on Modified Conjugated Polymer Surfaces Yu-Fong Huang,† Hung-Shin Shih,† Chi-Wen Lin,∥ Ping Xu,† Darrick J. Williams,‡ Kyle J. Ramos,†,§ Daniel E. Hooks,§ and Hsing-Lin Wang*,† †
C-PCS, Chemistry Division, ‡Center of Integrated Nanotechnology, MPA, and §Shock and Detonation Physics, Weapon Experiments, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ∥ Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Yunlin, Taiwan, R.O.C. S Supporting Information *
ABSTRACT: We report the fabrication of a series of micro-/nanostructured copper particles with various sizes, structures, and morphologies on polyaniline (PANI) membranes via an electrochemical deposition method. Different dopants applied in PANI membranes can lead to the production of Cu particles with various morphologies, including cubic, dendritic, textured spherical, and octahedral structures. On a citric acid (CA)-doped PANI membrane, the deposition of aggregated Cu nanoparticles is observed at an early stage, and these aggregated nanoparticles serve as the template to form larger Cu microspheres through a fill-in process. For a camphorsulfonic acid (CSA)-doped PANI membrane, a morphological transition of Cu metal from octahedral to dendritic structure is observed as the reaction time is prolonged, suggesting a branching growth mechanism. In addition to this unique control of the growth mechanism by varying the dopant, we find certain additives, such as citrate, can alter the growth of copper particles into a two-stage growth process, which results in the formation of copper microspheres decorated by nanowires and jellyfish-like structures for both CSA and CA-doped membranes. To the best of our knowledge, this is the first time where the electrochemical deposition of micro-/nanostructured copper using a two electrode setup with tunability in size, structure, and morphology has been demonstrated. These results offer valuable insights in understanding the underpinning growth mechanisms, imply an efficient method to control size and morphology, and enable designed synthesis of complex copper micro-/nanoparticles.
1. INTRODUCTION Nanoscaled materials possess electronic, optical, and catalytic properties that are influenced by their sizes and shapes, which exhibit large surface area and quantum confinement effects.1−6 Among metals, copper is of great importance due to its high electrical conductivity, catalytic, and surface-enhanced Raman scattering properties.7−10 Therefore, fabrication of copper micro-/nanostructures with various shapes has been extensively investigated.11−18 For instance, Chen et al. prepared nanoporous copper with tunable pore sizes through a dealloying process,16 while Wang et al. synthesized copper nanocubes by adding poly(vinylpyrrolidone) in solutions as a modifier.17 Copper nanowires have also been prepared by using hard templates,12,13 vacuum vapor deposition,18 and reverse micellar methods.11,15 Among the proposed methodologies, fabrication of copper crystals with well-defined morphologies usually requires tilting the balance between two growth mechanisms: habit formation and branching growth.19−21 The surface energy of different crystalline planes determines the habit formation route of a crystal.19,20 In cases where deposition of the growth species is limited through diffusion or growth potential, edges and corners protruding into higher concentration regions of the growth solution may experience faster growth than the natural habit planes. In electrodeposition, the growth of metals usually © 2012 American Chemical Society
follows branch limited growth. Researchers have synthesized a number of copper or copper oxide micro-/nanomaterials with controlled geometric shapes by simply modulating the growth mechanisms.22−25 We have previously demonstrated fabrication of Ag, Au, Pt, and Pd nanostructures through an electroless deposition method by conducting polymers: a metal ion having a higher reduction potential than that of a conducting polymer can be directly reduced by the conducting polymer.26−29 PANI is one of the most promising conjugated polymers for various applications due to its environmental stability and low cost. It has been found that morphology control of the metals can be achieved by varying the dopant, oxidation state, and surface chemistry of PANI.30−34 Moreover, the fabricated metal nanostructures on PANI have been verified to be efficient catalysts for organic synthesis and sensitive detection platforms on the basis of surface enhanced Raman scattering (SERS) techniques.28,30,34 However, metal ions such as Cu2+ or Ni2+, with a lower reduction potential than PANI, cannot be spontaneously reduced by PANI, while electrochemical deposition of Cu Received: September 13, 2011 Revised: February 9, 2012 Published: February 24, 2012 1778
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with different dopants are shown in Figure 1. The well-defined diffraction peaks of all samples can be assigned to the (111), (200), (220), (311), and (222) crystal planes of the face-centered cubic (fcc) Cu phase. Beyond simply validating that the deposits
nanoparticles on a PANI modified glassy carbon electrode (GCE) has been reported, which showed excellent electrocatalytic activity toward the oxidation of ascorbic acid (AA) under weakly basic conditions.35 However, morphology control was not realized in this reported work. Despite the recent success in understanding and controlling the morphology of copper and copper oxide, the controlled synthesis of nanoscaled copper metal crystals with well-defined structures and morphology remains elusive. Herein, we demonstrate an efficient and facile electrochemical deposition method to control the morphology of copper deposits using modified PANI membranes as the electrodes. Copper particles deposited on the PANI membranes exhibit dopant-dependent morphologies ranging from cubic to sphere-like to octahedral to dendritic to layered assemblies. The majority of these structures reveal morphologies that result from either habit formation or branching growth mechanisms of overpotentiallimited character. This study demonstrates the use of polyaniline as a surface modified electrode to fabricate copper metal micro-/nanostructures with a wide range of morphologies by simply varying the dopant of a polymer electrode, reaction time, and solution additives.
2. EXPERIMENTAL SECTION Materials. Polyaniline (PANI) emeraldine base (EB) powder was obtained from Aldrich. N-Methyl-2-pyrrolidone (NMP, 99% Aldrich), heptamethylenimine (HPMI, 98% Acros), CuSO4 (99.6%, J.T. Backer), nitric acid (99%, Aldrich), citric acid (99.9%, Fisher), R-(−)-camphorsulfonic acid (98% Aldrich), R-(−)-mandelic acid (99% Aldrich), nitric acid (>90%, Aldrich), toluenesulfonic acid (98.5%, Aldrich), and hydrochloride acid (37.2%, Fisher) were used as received. Fabrication of PANI Membranes. PANI membranes are prepared by a phase inversion method using water as the coagulation bath.28,30 In a typical experiment, 4.14 g of NMP, 0.747 g of HPMI, and 1.15 g of PANI (EB) powder were mixed in a 12 mL Teflon vial. This mixture was stirred at room temperature until a homogeneous solution was obtained, and it was cast onto a glass substrate and spread with a gardener’s blade (Pompano Beach, FL) to form a wet film with a controlled thickness. The glass substrate, with a wet PANI layer, was then immersed into a water bath for at least 24 h. The resulting membrane was dried at room temperature before being immersed in the 0.25 M organic acid aqueous solution for at least 1 day to ensure complete doping of the PANI membrane. Preparation of Metal Nanostructures on a PANI Membrane. The deposition of copper on PANI membranes was carried out using a two-electrode setup, in which a potential of 6 V was applied using a dc power supply (HP 6209B). In this setup, the PANI membranes serve both as the anode and cathode, and CuSO4 aqueous solution (0.5M) is used as the electrolyte solution. After a reaction period of several minutes, the deposition of copper at the cathode surface can be visually observed. Characterization. The X-ray diffraction (XRD) patterns of the samples were conducted on a Rigaku/Max-3A X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å), and the operation voltage and current were maintained at 40 kV and 40 mA, respectively. Scanning electron microscopic images were obtained with a FEI Inspect SEM.
Figure 1. X-ray diffraction of the copper samples deposited on the surfaces of polyaniline membranes doped by various acids.
are indeed crystalline copper metal, rather than copper salts or oxides, XRD also reveals the preferential growth orientation of the copper particles on PANI membranes with various doping acids. Copper deposits on the HCl-doped PANI membrane show an intense (111) peak and very weak (200) and (220) peaks, with indiscernible (311) and (222) peaks. These particles grown on PANI/HCl reveal a very strong textural anisotropy as compared to the rest of the samples. In contrast, the relative peak intensities between (200) and (220) peaks for copper deposits grown on the phosphoric acid and TSA-doped PANI membranes are exactly the opposite, suggesting a difference not only in the morphology but also in texture. The above results demonstrate the strong impact of dopant on the final structure and morphology of the copper deposits. We believe the capping effect of different dopants on the produced copper seeds may be responsible for the production of larger copper particles with various morphologies. To investigate the dopant-induced morphological evolution, we carried out the experiment with a prolonged electrodeposition time of 60 min. The SEM images of these Cu deposits are shown in Figure 2. It can be seen that Cu deposits on the HCl-doped PANI membrane surface have an extended cubic shape, while Cu deposited on a citric acid-doped PANI membrane has a spherical shape, ∼ 5 μm in diameter. Octahedral Cu structures can be found on a HNO3-doped PANI membrane, while TSA and CSA dopants lead to branched crystals, with four-sided star structures. Cu particles deposited on the phosphoric acid-doped PANI membrane also exhibited spherical shapes; however, upon close examination, we find these particles are comprised of many close-packed Cu nanosheets with a thickness of ∼25 nm, and many submicrometer pores are easily seen from the magnified SEM image. To further elucidate the formation process of the prepared Cu particles, SEM images of Cu structures deposited on different acid-doped PANI membranes at different reaction periods were recorded to investigate the nucleation and growth processes of Cu micro-/nanostructures, as shown in Figure 3.
3. RESULTS AND DISCUSSION Effect of Dopant on the Structure and Morphology of Copper Micro-/Nanostructures. To understand how dopants impact the final structure of copper on the PANI membranes, we doped several PANI membranes (cut from one larger piece) with HCl, HNO3, H3PO4, citric acid (CA), camphorsulfonic acid (CSA), and p-CH3C6H5SO3H (p-TSA). These membranes were then used as the electrodes, where electrodeposition was performed in a 0.5 M CuSO4 solution under a potential of 6 V for 30 min. The XRD patterns of the deposits on PANI membranes 1779
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Figure 2. SEM images of the copper samples deposited on the surfaces of polyaniline membranes doped by (a) phosphoric acid, (b) toluenesulfonic acid, (c) hydrochloric acid, (d) campersulfonic acid, (e) nitric acid, and (f) citric acid. Inset scale bar is 2 μm.
Figure 3. Time resolved SEM of the copper particles on doped polyaniline membrane surfaces revealing evolution of the copper morphology as a function of time.
An octahedral Cu crystal was discovered at an early stage of growth (15 min) on a TSA-doped membrane. As growth progressed, the crystals developed at the edge of the octahedral
shape and became a star-like morphology with a size of about 20 μm. Similar results are observed when using the CSA-doped PANI membrane, where the formed truncated octahedral-like 1780
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et al.37 reported that citrate anions form a passive layer on initially formed silver nanoparticles, and these citrate-bound nanoparticles evolve into larger particles by way of surface growth and aggregation until an optimal size is reached, with the citrate layer preventing further aggregation. Pillai et al.38 also reported a similar result, where citrate ions complex with positively charged Ag2+ dimers to form citrate-capped nanoclusters which continue to grow into larger silver nanocrystals of varying shape and size. While these studies reveal aspects of growth mechanisms at early stages, the citrate-dominated particle morphology at a larger scale is not clearly understood. Cu particles and their aggregates increase in size with time. Passivation may occur by the same mechanism as is observed for cases when citrate is an additive in the growth solution because citrate ions from the membrane surface may diffuse from the membrane into the electrolyte solution. Thus, as soon as copper seeds are formed in the initial reaction step, they may also complex with the citrate ions, and this complex may lead to the aggregation of the initially formed nanospheres. After that, the aggregated particles serve as the template to form microspheres through a filling-in process. Effect of Citric Acid in the Electrolyte Solution on the Morphology of Copper. The growth routes of copper particles deposited on the citric acid-doped PANI membranes are significantly different from those on PANI membranes doped with CSA, TSA, and HCl. Therefore, to discriminate the mechanistic effect of citric acid (or citrate ion) on the morphology of the deposited Cu on PANI membranes, we carried out separate control experiments in which additional citrate ion was purposely introduced in the electrolyte solutions. Figure 4 shows
Cu at an early stage eventually evolves into a dendritic structure through branching growth, as shown in Figure 3 (CSA doped). These results suggest that the use of CSA- or TSA-doped membrane as surface modified electrodes can lead to the formation of octahedral-like copper at the incipient stage, and these octahedral nuclei evolve through branching growth to form the star-like and dendrite-like Cu microstructures. Thus, mass transport limited branching growth is promoted as a crystal, with initially polyhedral shape, grows such that the apexes of the crystal protrude into the region of higher concentration and deplete ions and molecules that feed the growing crystals at the crystal/solution interface. Thus, the apexes can grow faster than the rest of the facets to form branches.36 This growth route is also observed on the surface of HCl-doped PANI membranes, where the cube-like copper at the incipient stages evolves to continuous growth along crystal apexes, as shown in Figure 3 (HCl doped). In contrast to the mass transport limited branching mechanisms observed on TSA-, CSA-, and HCl-doped PANI membranes, Cu metal deposited on the citric acid-doped membrane exhibits a very different growth mechanism, as can be seen in Figure 3 (CA doped). In this case, the formation of aggregated Cu nanospheres with diameters of 20−60 nm is observed and these initially formed nanosphere aggregates may serve as the templates to form Cu microspheres with ∼2 μm in diameter with rough surfaces through Ostwald ripening. As the filling process continues with reaction time, microspheres with an average diameter of ∼6 μm and relatively smooth surface are obtained. In this condition, the citrate anions may be responsible for the observed morphology. Recently, Sergeev
Figure 4. SEM images of the copper deposits (60 min reaction time) on the surfaces of polyaniline membranes doped by (a−c) tolunesulfonic acid, (d−f) campersulfonic acid, and (g−i) citric acid with different amounts of citric acid added in the electrolyte solutions. The inset of Figure 4e shows a magnified SEM image of copper microspheres deposited on the campersulfonic acid-doped polyaniline membranes; the scale bar is 500 nm. 1781
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Figure 5. SEM images of copper samples deposited on the surfaces of polyaniline membranes doped by (a) citric acid, (b) tolunesulfonic acid, and (c) campersulfonic acid at the early reaction stage (15 min) with the addition of 1.0 mL of 0.25 M citric acid in the electrolyte solutions.
the change of morphology of the copper micro-/nanostructures deposited on different acid-doped PANI membranes with the addition of various amounts of citrate ions in the electrolyte solutions. This concentration-dependent particle morphology is consistent with the mass transport limited branching mechanism in which higher concentration allows effective branching of the crystal on the (100) planes. The citrate ions have a dominant effect, as the addition of citrate leads to a change in Cu particle morphology from a truncated octahedral microstructure, to microspheres, and then to coral-like microstructures when using TSA-doped membranes as electrodes (Figure 4a−c). Similar results can also be observed from the CSAdoped membranes surface (Figure 4d−f), where a morphology transition of Cu particles from octahedral microstructures to spheres can be observed with an increase of citric acid concentration in electrolyte solutions. Citrate ions which have preferentially bound the (100) surfaces inhibit the branching mechanism, which is followed by a simple surface growth mechanism yielding micrometer sized particles comprised of many nanoparticles. This similar morphology transition observed for both the CSA- and TSA-doped membranes, upon close examination, reveals significant differences in morphology at the nanoscale. Of particular interest is the Cu particle evolution on the CSAdoped membrane, which has a micrometer sized inner core with many surface-decorated nanoparticles. These nanoparticles have a jellyfish-like morphology with hood (∼300 nm in diameter) features and many tentacle (30 nm in diameter) features (see inset of Figure 4e). In contrast, microspheres consisting of many octahedral nanostructures are observed on TSA-doped PANI membranes. Thus, the morphology is affected by the nature of the dopant and the citrate ions. It is highly possible that the nucleation and incipient stages of growth are dominated by the dopant, while the subsequent growth is dominated by the citrate ions. To further clarify how citrate additive in the electrolyte solution impacts the resulting morphology, we also used citric acid-doped PANI membranes coupled with various amounts of citrate ion in the electrolyte solutions. Figure 4g−i shows the SEM images of the copper micro/nanostructures deposited on the citric acid-doped PANI membranes. We observe copper nanowires on the sphere surface, with the diameters of the nanowires estimated to be less than 50 nm. The diameter of the microspheres formed under this environment is about 500 nm, which is smaller than that obtained without the addition of citrate in solution, suggesting the citrate in the solution also changes the size of the deposited microspheres, likely through similar growth inhibition processes. SEM analysis of particle evolution at early stages of the electrodeposition reveals additional insights about the effects of citric acid in solution on the mechanisms of morphology
evolution. Figure 5 shows the copper particles on the CA-, CSA-, and TSA-doped membrane at an early reaction stage with a diameter of ∼1 μm, with citric acid added in the electrolyte solution. The nucleation process appears to be dominated not only by the presence of citrate molecules but also by the presence of dopant in PANI membranes, as the CSA-doped membrane reveals a very different particle morphology as compared to that of the copper particles on CA- and TSA-doped PANI membranes. Copper particles on a CA-doped PANI membrane are actually formed from an assembly of numerous Cu nanoparticles. Copper particles deposited on a CSA-doped PANI membrane comprised of numerous nanowires capped by a layer of copper, which eventually leads to a jellyfish-like structure (Figure 4e) with a prolonged reaction time (60 min). Proposed Formation Mechanisms. In the experiments reported here, the important parameters for electrodeposition are such that almost all cases are variations on overpotential limited growth. The PANI membranes vary in conductivity. HCl-, CSA-, TSA-, and HNO3-doped PANI have conductivities of 0.47, 0.03, 2.0, and 85 S/cm, respectively. The conductivities for phosphoric acid- or CA-doped PANI are measured to be 0.1 and 0.8 S/cm, respectively. This variation in conductivity is ∼2 orders of magnitude, but compared to metal electrodes, all are actually very low in conductivity (Cu: ∼5.8 × 106 S/cm). Furthermore, the electrolyte solution has no added electrolyte (>45 g/L H2SO4 is typical) and also has low conductivity. Contributing to already poor current density is a likely inefficient anodic reaction, probably oxidation of PANI from emeraldine to pernigraniline base.39,40 A general guideline for ion availability in solution is > ∼0.4 M CuSO4 for good cathodic efficiency. Solution concentration in our case is above this level, so ion availability is adequate. Several contributing factors that severely limit ion mobility and thus current density result in apparent overpotential limited branching growth in all cases, except where additional citric acid was present in the electrode or solution. Longer growth periods led to increased branching, and higher growth potential led to less branching. With all else held constant, the effect of the PANI dopant was to alter the electrode conductivity, enabling very good control of growth conditions in an overpotential-limited branching regime. Metal deposition on the membrane surfaces through reduction requires electron transfer from the PANI to the metal ions. As the metal ions approach the PANI surfaces, they are electrochemically reduced and form nuclei on the PANI membrane surface, which serve as seeds for subsequent growth into larger particles. As the growth takes place on the PANI substrate, factors such as the surface chemistry, hydrophobicity, and surface charge can greatly influence the morphology of the metal particles. It has been reported that additives in solution 1782
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Figure 6. Simplified growth models of copper particles grown on the CA- and CSA-doped polyaniline membranes with and without the citrate additive in the electrolyte solutions.
can impact the nucleation and growth of metals and metal oxides. Recently, Kim et al. reported that the addition of chloride in the reaction solution plays an important role in controlling the formation of seeds and the growth rates of various crystallographic planes to shape the silver nanostructures into nanocubes.41 Siegfried et al. similarly reported the addition of sodium dodecyl sulfate (SDS) as a modifier to decrease the surface energy of the bound plane of copper oxide and thus hindered the growth perpendicular to the plane.23,42 Here, we suspect that some dopant molecules in the PANI membranes may have been redispersed in the electrolyte solution and adsorb on the Cu nuclei surface, leading to a similar effect on the final morphology of the Cu particles. Citric acid, either as electrode dopant or solution additive, has a very pronounced effect on growth, performing its welldocumented mechanistic role of passivation. This passivation has the effect of slowing down crystal growth, prohibiting facet development entirely and limiting particle development severely at the stage of very small nuclei. Thus, the appearance of evolving spheres with varied substructures with citric acid in the system was observed. A schematic representation of simplified growth models is proposed, as shown in Figure 6, which reveals comparative studies with and without the presence of citric acid as additive in the electrolyte solution. These proposed models suggest CA has dominated the mechanisms of copper growth through passivation of the nuclei surface. This effect is particularly evident for the CSAdoped PANI membranes, converting the growth from an
overpotential limited branching mechanism to a two stage growth mechanism that leads to the formation of a Cu particle with a solid core decorated by either Cu nanowires or a jellyfish-like structure. This result is similar to that of our previous studies, which show that chemical deposition of Ag particles on a citric acid-doped PANI membrane leads to a microsphere Ag particle comprised of many densely packed nanosheets27,28,34 and Ag nanowires by increasing the AgNO3 concentrations.32
4. CONCLUSIONS This study demonstrated a simple way to control the morphology of deposited copper metals on the surfaces of polyaniline (PANI) membranes by tailoring their surface chemistry (dopant) and the composition of electrolyte. A wide range of Cu micro-/nanoparticle morphologies ranging from dendritic to octahedral to cubic to spherical structures have been observed. The morphological and structural differences of these deposited metal particles suggest that the doping nature of the PANI membranes dominate the growth mechanism of these copper metals: the overpotential limited branching mechanism resulting from the diffusion of metal ions and the surface growth mechanism typically occurs at lower concentrations. These different growth routes can be overwhelmed by citrate additive, which favors the formation of microspheres decorated with nanowires or jelly-like structures on the citric acid- and camphorsulfonic acid-doped membranes, respectively. A wide range of micro-/nanostructured copper 1783
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particles with controlled size, morphology, and complexity inaccessible in the past have been demonstrated by varying the dopant (tailoring the surface chemistry) and additives. Such a facile synthetic platform represents a means to fabricate metals on polyaniline substrates, and these metal particle/polyaniline composites could have huge potential in low-cost, high sensitivity SERS devices and highly efficient catalytic surfaces.
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ASSOCIATED CONTENT
S Supporting Information *
Additional SEM images. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by BES, biomaterials program. This work was performed in part at the U.S. Department of Energy, Center for Integrated Nanotechnologies, at Los Alamos National Laboratory (DE-AC52-06NA25396) and Sandia National Laboratories (DE-AC04-94AL85000). Partial support from National Nuclear Security Administration Science Campaign 2 and the Department of Energy/Department of Defense Joint Munitions Technology Development Program. P.X. thanks Fundamental Research Funds for the Central Universities (Grant Nos. HIT.NSRIF. 2010065 and 2011017), NSFC (21101041, 91122002) and Director’s Postdoctoral Fellow from LANL for financial support.
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