Electrically Conducting Polymers As Templating Interfaces for

Mar 28, 2012 - Electrical properties were analyzed using FTIR and PXRD was used to ... Of these metals, copper has posed a significant challenge in ...
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Electrically Conducting Polymers As Templating Interfaces for Fabrication of Copper Nanotubes Eliud K. Mushibe,† Dickson Andala,† Steven C. Murphy,† Kate Raiti-Palazzolo,† Jetty L. Duffy-Matzner,‡ and Wayne E. Jones, Jr†,* †

Department of Chemistry, State University of New York at Binghamton, Vestal Parkway East, Binghamton, New York 13902, United States ‡ Department of Chemistry, Augustana College, Sioux Falls, South Dakota 57105, United States S Supporting Information *

ABSTRACT: Submicrometer tubes have been fabricated by a polymerbased template approach using electroless deposition. The copper was deposited on polystyrene fibers functionalized with an interfacial electrically conducting polyaniline thin film layer. Thermal degradation of the functionalized fiber templates resulted in copper tubes of diameter 1600 ± 50 nm with wall thicknesses ranging between 100 and 200 nm. The morphology and elemental analysis of copper coaxial fibers was analyzed using SEM and EDS. Electrical properties were analyzed using FTIR and PXRD was used to study crystal structure of copper nanotubes.

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INTRODUCTION Metal nanostructures have been of increasing interest due to their diverse applications in electronics, medicine, sensors, catalysis, magnetic storage, and optoelectronics stemming from their unique properties compared to bulk materials.1 Since intrinsic properties of these nanostructures can be tailored by controlling their size, shape, composition, crystallinity, and structure, great efforts have been made to control their preparation. Metallic nanostructures such as nanorods, nanowires, nanoplates, nanocubes, nanoprisms, and nanoshells have been successfully prepared with different morphologies.2−4 Among these metallic nanostructures, tubular structures with hollow interior exhibit a range of interesting properties superior to their solid counterparts as demonstrated recently for Pt, Ni, and Au.2,5 Silver and copper are of particular interest due to their high electrical conductivity and the important role they play in the fields of catalysts, microelectrodes, SERS, electronics, resins, and thermal conductivity.6−8 Copper nanotubes have been reported to possess high electrical and thermal conductivity properties relative to their bulk metals. Therefore, they can be invaluable thermal management materials in electronic package where power dissipation demands at device level continue to increase; filler materials for high electric resistance print-circuit board and in electromagnetic interference.9,10 However, there is continuing need to explore the fabrication of tubular hollow structures which have been difficult to prepare in large quantities. Cu nanotubes have been synthesized using surfactant of CTAB in hydrothermal system.11 Synthesis of silver multiwall nanotubes using another kind of nanotubes as a template has also been reported by Liu’s group.12 Although these metal tubular structures were successfully obtained using these synthetic methods, each of their fabrication methods © 2012 American Chemical Society

requires rigorous conditions and relatively complicated processing steps. In addition, they are found to have relatively low aspect ratios. The use of polymer fibers as an exotemplate provides a viable alternative in the synthesis of these metal tubular structures. Electrospun polymer fibers have become extensively utilized as an exotemplate in the synthesis of nanostructured materials.13,14 Polymer nanofibers such as polystyrene, polymethylmethacrylate, polylactide have been utilized as template material in the fabrication of metal tubular nanostructures.15 Jones et al.16 have explored the fabrication of tubular nanostructures of various metals such as gold, silver, nickel, and copper via electroless deposition. Of these metals, copper has posed a significant challenge in obtaining a continuous surface coverage on the fiber surface. This is mainly attributed to the weak interaction of copper with insulating polymers resulting in poor interfacial adhesion and nucleation.17 Overcoming the challenge of forming a continuous thin metal layer on an insulating substrate entails pretreatment of the surface through sensitization and activation to create catalytic active sites. These catalytic sites are usually palladium nuclei chemisorbed from solution. This has been achieved using SnCl2 and PdCl2 pretreatment18−21 where SnCl2 is needed as an initial sensitizer. It was found that the sensitization step can be bypassed if the polymer template surface is first subjected to plasma UV-laser treatment in nitrogen containing atmospheres (N2 or NH3).22 The amine group which is introduced has a strong affinity for Pd, which results in the formation of a continuous layer of the Pd nuclei. Fiber template with amine Received: December 10, 2011 Revised: February 21, 2012 Published: March 28, 2012 6684

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(FT-IR) using Bruker IR model Equinox 55/Digilab FTS-40PRO was also used in the analysis.

functional group can be used to enhance continuous deposition of Cu metal on the fiber surface. In this work, we report external functionalization of insulating polymer fibers using a thin film of polyaniline. Polyaniline provides continuous surface coverage of amine functionality on the nanofibers facilitating the electroless deposition of Cu metal. Polyaniline was selected due to its established ability to form a thin layer on plastics via in situ polymerization.23 Polyaniline also has the ability to undergo spontaneous palladium adsorption through reduction of the metal ions in acid solution to their elemental form.24−26 These combined advantages provide the opportunity to develop a new method for the formation of copper nanotubes using amine containing electrically conducting polymer.

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RESULTS AND DISCUSSION Following successful electrospinning, polystyrene fibers were used as templates in the fabrication of Cu submicrometer tubes.

EXPERIMENTAL SECTION

Polystyrene pellets (Mw 280 000), N,N-dimethylformamide (DMF), palladium chloride, disodium salt of EDTA dehydrate, copper(II) sulfate pentahydrate, triethanolamine, sodium hydroxide, sodium bicarbonate, potassium sodium tartrate, formaldehyde (all from Aldrich), aniline hydrochloride (Fluka), ammonium persulfate (Fischer Chemicals), and hydrochloric acid (HCl) (J.T. Baker) were used as received from the manufacturer. A polystyrene polymer solution, 0.25 mg/mL, was prepared by dissolving commercial pellets in DMF. Electrospinning was achieved by applying a high voltage of 20 kV and collected at a distance of 20 cm between the collector screen and spinneret as described previous.27 The electrospun fibers were peeled off the aluminum foil by soaking in 0.01 M HCl solution for 5 min then rinsed with deionized water. The fibers were then coated with a thin layer of Polyaniline achieved via in situ polymerization. In-situ polymerization of polyaniline was carried out by oxidation of 0.2 M solution of aniline hydroxide in water (50 mL) with 0.25 M solution of ammonium peroxydisulfate in water (50 mL). Ammonium peroxydisulfate was added to equal amount of aniline hydroxide monomer in a beaker to initiate polymerization process. The process was quenched after 10 min by immersing the fibers in aniline monomer solution. The functionalized fibers were finally immersed in deionized water to wash away loosely adsorbed polymer material. Copper deposition on polystyrene fibers was achieved through a wet chemical method by reduction of the metal ion to the zero oxidation state using a modification of a procedure described elsewhere.28 Before electroless deposition, the functionalized fibers were first pretreated in 0.003 M PdCl2 aqueous solution containing 0.01 M HCl. Copper coaxial fibers were obtained by immersion of the pretreated fibers in a plating bath of 0.1 M copper(II) sulfate pentahydrate, 0.5 M potassium sodium tartrate, 1.0 M sodium hydroxide, 0.3 M sodium bicarbonate, 0.06 M disodium salt of EDTA dihydrate, and 0.07 M triethanolamine maintained at pH 9 and 60 °C. Formaldehyde was used as the reducing agent in the solution. Cu nanotubes were obtained by calcining the coaxial fibers at 550 °C under air followed by heating under H2/N2 atmosphere. The surface morphology analysis was carried out by scanning electron microscopy (SEM) and EDS using SUPRA Zeiss 55VP model with accelerating voltage of 15 kV. The surface of the copper coaxial fibers was coated with a thin film of gold/palladium (Au/Pd) to prevent the charging effect of the sample. SEM images of copper nanotubes were obtained without coating the samples. Metal submicrometer tubes were characterized by transimission electron microscopy (TEM) with accelerating voltage of 75 kV. The sample for TEM analysis was dispersed in ethanol by sonication for 30 min. The tubes suspension was then mounted onto lacey carbon Cu grids. Powder X-ray diffraction (PXRD) data was obtained from a Scintag Xray diffractometer with Cu Kα (λ = 1.541 78 Å) as radiation source. Thermogravimetric analysis (TGA) was carried out on Thermogravimetric Analyzer TA 2950, TA Instruments. Thermal degradation analysis of polymer material was performed using Lindberg/Blue Tube Furnace TF 55035A-1. Fourier transmission infrared spectroscopy

Figure 1. SEM image of PS fiber (a), PS fibers functionalized with polyaniline thin layer (b) and cross-sectional view of PS-PANi coaxial fibers (C).

Figure 2. SEM micrograph of copper plated on untreated polystyrene fibers functionalized with polyaniline thin film.

The choice of the polystyrene fiber template was based on their relatively low decomposition temperature of 288 to 325 °C.29 The average diameter of the as spun polystyrene fibers was found to be 700 ± 100 nm. Figure 1(a) shows surface morphology of the polymer fibers as determined by SEM analysis of the fiber mats was uniform and smooth with no bead formation and a random orientation. This was consistent with prior reports of PS nanofibers of similar size.30−32 Polystyrene fibers have been widely used as templates in electroless deposition of metals.16 Despite this extensive background, electroless deposition of copper has been noted to result in a poor adhesion to polymers regardless of pretreatment.33 The nature of the surface has a marked effect on adhesion characteristics of the electrolessly deposited copper metal due to surface free energy. In order to obtain a more hydrophilic surface, we functionalized the as-spun polystyrene 6685

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Figure 3. FT-IR spectra of polyaniline, polystyrene fibers, and polystyrene functionalized with polyaniline (a) and copper−polymer coaxial fibers and copper tubes (b).

Figure 6. EDS spectrum of Cu metal plated on PS fibers functionalized with PANi thin film.

Figure 4. TGA spectra of PS-PANi heated in air.

fibers with polyaniline thin film by insitu polymerization.34,35 Figure 1(b) presents SEM image of polystyrene fibers functionalized with polyaniline thin film. The functionalized coaxial fibers gave an average diameter of 1000 ± 50 nm. The cross-sectional view of the coaxial fibers shows a good interface between fibers and polyaniline thin films as shown in Figure 1(C). Functionalization of the fiber surface increases surface free energy, facilitating the adhesion process. This was indicated from contact angle measurements near 0° on the surface of treated fiber. Functionalization of the fibers together with chemical activation using palladium metal improved wettability with an increased hydrophilicity and simultaneous increase in

Figure 5. SEM image of Cu on polystyrene fibers functionalized with polyaniline thin films (a) at low magnification (b) high magnification. 6686

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higher reactivity toward formation of polyaniline chains than those particles forming in bulk solution. This therefore stimulates the growth of polyaniline chains and provides a nucleus for the growth of thin films. Polyaniline exists in multiple, interconvertible intrinsic oxidation states of imine-like (=N−) and amine-like (−NH−) nitrogen atoms. The existence of these interconvertible oxidation states of polyaniline facilitates a spontaneous reduction of Pd(II) ions in acidic solution to their elemental form.38 This results in a uniform seeding of the surface with palladium metal, which effectively catalyzes the copper plating reaction18,39−41 leading to a continuous nucleation of the metal. Metal deposition on polyaniline functionalized coaxial fibers without pretreatment with Pd(II) ions resulted in formation of noncontinuous metal layer on the fiber surface as shown in SEM image Figure 2. Therefore, the pretreatment of functionalized fibers in Pd(II) solution was essential in achieving a continuous nucleation on the fiber surface. The FTIR spectrum for PS fibers showed peaks at 3060, 3030, 2927, 2848, 1705, 1618, 1494, 1487, 1377, 759, and 699 cm−1 characteristic to polystyrene polymer.42,43 Polyaniline spectrum showed peaks at 1577, 1505, 1480, 1303, and 1150 cm−1, characteristic to a conducting form of polyaniline polymer, Figure 3a.44 Comparing the two spectra to that of polystyrene functionalized with polyaniline, all of the bands observed in respective polymers were reflected in the spectrum of PS-PANi fibers and in the Supporting Information. The characteristic peaks for polyaniline were observed at 1577, 1505 cm−1 (C−C ring stretching), 1303 cm−1 (N−H bending), and 1150 cm−1 (aromatic C−N−C); peaks due to polystyrene were observed at 3060 cm−1 (aromatic C−H stretching), 2927, 2848 cm−1 (C−H stretching), 1494 cm−1 (C−H bending), 1487 cm−1 (phenyl ring C−H in-plane bending) and 759, 699 cm−1 (phenyl ring C−H out of plane bending).43,45 Therefore, functionalizing polystyrene with polyaniline did not result in structural change of polyaniline. This was an indication that during in situ polymerization, there was no bond formation between polyaniline and polystyrene fibers. Upon pretreatment of the functionalized fibers with Pd(II) the color changed from green to purple. The color change was consistent with work done by Lim where Pd(II) solution in HCl fully reduces a polyaniline film resulting in formation of an emaraldine salt form, which is initially green in color.38 The reduced form of the polyaniline film can reduce the Pd(II) ion to its metallic form. The film itself is oxidized in the process to the pernigraniline, purple in color. FTIR of coaxial fibers (Cu on functionalized polystyrene fibers) also suggested full oxidation of polyaniline due to disappearance of broad peaks at 1303 and 1150 reported to be a measure of degree of delocalization of electrons in conducting form of polyaniline.44 However, CuNTs did not show any peaks due to polymer functional groups an indication of complete elimination of polymer template materials, Figure 3b. Thermogravimetric analysis of PS fibers coated with polyaniline thin layers was carried out to determine the thermal decomposition temperature. Thermal degradation of polyaniline under N2 atmosphere resulted in incomplete weight loss. This was attributed to formation of amorphous carbon as described previously.46 Calcination under air atmosphere resulted in more complete weight loss at around 450 °C as seen in thermogram, Figure 4 below, consistent with oxidation of amorphous carbon to carbon dioxide. The weight loss at 300 °C is attributed to loss of polystyrene material which has been

Figure 7. Electron microscopy images of Cu submicrometer tubes by (a) SEM indicating the open ends of the tubes and (b) TEM indicating the hollow core of the tubes.

Figure 8. Histogram showing the size distribution of submicrometer tubes obtained statistically from five SEM images.

Figure 9. SEM images showing extraneous deposition of copper particles in between fibers.

surface free energy. The elimination of the sensitization step is of advantage since presence of Sn inhibits metal deposition which results in skip plating. Functionalization of the as spun polystyrene fibers with polyaniline thin films was achieved via in situ polymerization of aniline monomer. Stejskal et al., proposed a model for the growth of thin films where the oligomeric aniline cation radicals adsorb (physisorb) onto the surface of a substrate during the induction period.36,37 The adsorbed oligomeric chains have a 6687

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Figure 10. PXRD spectra for Cu submicrometer tubes calcined under (a) air and (b) air for 2 h and cooled in 6% H2 in N2.

reported to occur at 288 °C.29 The weight loss commencing just below 300 °C relates to morphological changes in polyaniline. The final sharp weight loss above 400 °C can be attributed to thermal degradation of polyaniline. Generally, the gradual trend of final weight loss is attributed to thermal stability of polyaniline main chain.47 Electroless deposition is a wet chemical technique that involves the use of a chemical reducing agent to drive the heterogeneous reduction of a metal ion or complex. Since electroless deposition is equilibrium driven, optimization of composition and reactivity of the plating system is necessary to obtain optimal results. Some of the key factors that have been previously studied include plating bath concentration, bath temperature, pH stability, additives, and plating time.48−53 In this work, an electroless process was employed in the deposition of copper on pretreated fibers using a modified procedure from the literature.28 An optimized plating bath was developed containing 0.1 M copper(II) sulfate pentahydrate, 0.5 M potassium sodium tartrate, 1.0 M sodium hydroxide, 0.3 M sodium bicarbonate, 0.06 M disodium salt of EDTA tetrahydrate and 0.07 M triethanolamine. Formaldehyde was used as the reducing agent. Use of EDTA (a strong complexing agent) and triethanolamine (an excellent buffer and good complexing agent) resulted in marked improvement in stability of the plating bath at pH 9. In this case, a mixed ligand complex of copper with EDTA-triethanolamine is formed, reducing decomposition in the solution.54 The optimum plating time for the formation of smooth metal coating on the pretreated functionalized fibers was found to be 20 min. In addition to optimization of plating bath, pretreatment of fibers facilitated by functionalization with polyaniline thin film played a great role in formation of smooth, continuous, and uniaxial metal coating, Figure 5a below. The epitaxial growth of copper metal is attributed to uniform distribution of palladium nuclei during seeding. A magnified SEM image, Figure 5b, shows continuous film of metal grains distributed evenly on a well-seeded surface. This is in agreement with the postulated growth mechanism where metal particles grow through aggregation of already formed ones.55 EDX analysis of the Cu coaxial fibers, Figure 6, showed a high percentage of copper on the individual fiber. Copper tubes were obtained by decomposition of the fiber template material in a quartz tube furnace. Thermal decomposition of the polymer material was carried out under air atmosphere at 550 °C at ramp of 10 °C/min. The furnace was held at 550 °C for 2 h before cooling down to room temperature. At this high temperature, polymeric materials are oxidized to carbon dioxide gas which easily escapes through the open end of the tubes formed. During thermal degradation of polymer template, the tubes break at the intersection point of fibers due to interruption in continuous coating. This results in

tubes which are shorter with an average length of ≥8 μm (Figure 7a) compared to long continuous network of fibers (Figure 1a,b). The hollow tubes produced under this atmosphere were found to be in oxides. This was due to the oxidation reaction that takes place upon heating copper metal in presence of oxygen. The PXRD diffraction pattern of the tubes obtained corresponded to that of CuO as identified from JCPD file.56 To obtain pure copper tubes, Figure 7, the coaxial fibers were first calcined at 550 °C for 2 h under air atmosphere followed by another 1 h under 6% H2 in N2 gas. Hydrogen gas was used to facilitate the reduction of CuO to Cu tubes. The SEM image Figure 7a shows tube with hollow opening at the end that are randomly oriented existing as individual tubes. The TEM image, Figure 7b, of the submicrometer tube shows the edge to be thicker and darker than the center consistent with the tubular structure expected for nanotubes. The average diameter of submicrometer tubes obtained statistically as shown in the histogram, Figure 8, was found to be 1600 ± 50 nm with an average length of ≥8 μm as observed in SEM images, Figure 7a. The particulates observed on the sides of the tubes can be attributed to extraneous deposition of copper particles occurring in between fiber template as shown in SEM image of copper coaxial fibers, Figure 9. This phenomenon can be overcome by plating copper for longer periods of time which results in tubes with varying wall thicknesses.59 The PXRD diffraction pattern for the copper tubes Figure 10(b) shows four intense diffraction peaks at 2θ = 42.28, 50.08, 73.66, 89.42. The corresponding d-spacing was calculated to be 2.100, 1.819, 1.284, 1.095 Å which is consistent with the respective d-spacing of (111), (200), (220), and (311) planes. This suggests a face centered cubic structure lattice parameter, a = 3.639 Å, corresponding to that of a pure Copper crystal.57,58 The intensity of the peaks gives us information on growth of particles which is predominantly in 111 direction normal to fiber template.

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CONCLUSIONS In summary, polystyrene fibers coated with thin layer of polyaniline were coated with copper metal by electroless plating technique. Thermal degradation of coaxial fibers in air resulted in complete elimination of polymic material; reduction under H2/N2 atmosphere resulted in copper metal submicrometer tubes. Polyaniline thin films possess protonated amine groups in acidic media of Pd(II) solution and have a high Pd(0) uptake, forming a uniform seeding layer on the fiber surface. The uptake of Palladium from PdCl2 acidic solution by polyaniline was achieved in complete absence of Sn(II) sensitization step. Palladium metal acts as catalyst for the reduction of Cu(II) to Cu(0) on the fiber surface. Formation of a continuous metal layer is a result of the uniform seeding of palladium metal on fibers functionalized with polyaniline thin 6688

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layer. This method provides a convenient, high-throughput approach of fabricating metal tubes of uniform wall thickness. Therefore, this work opens up an avenue to prepare tubes with controlled wall thickness thereby tuning their properties for application in electronics.

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ASSOCIATED CONTENT

S Supporting Information *

TEM image of copper tube with a thick wall thickness achieved by electroless plating for 40 min. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (607) 777-2421; fax: (607) 777-4478; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Binghamton University, research foundation in conjunction with PRF AC5-46117, for research funding and NSF CCLI0942672. Dr. Dae Jung of Institute of Technology Center (ITC) and Dr. Curt Pueschel of the Department of Biological Sciences at SUNY Binghamton for SEM, EDS, and TEM data collection, Mr. Ruigang Zhang for PXRD data collection and Mr. Hui Li for TGA data collection.

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dx.doi.org/10.1021/la204870e | Langmuir 2012, 28, 6684−6690