Gradient nanowires and nanotubes

solidi

status

physica

pss

Phys. Status Solidi B 247, No. 10, 2451–2457 (2010) / DOI 10.1002/pssb.201046240

b

www.pss-b.com

basic solid state physics

Gradient nanowires and nanotubes 1

**,2

2

*,1

Feature Article 3

S. Agarwal , B. Eckhardt , F. Grossmann , A. Greiner , P. Go¨ring , ,3,4 1 R. B. Wehrspohn*** , and J. Wendorff 1

Department of Chemistry, Philipps-Universita¨t Marburg, 35032 Marburg, Germany Fachbereich Physik, Philipps-Universita¨t Marburg, 35032 Marburg, Germany 3 Institute of Physics, Martin Luther University Halle Wittenberg, Heinrich Damerow Str. 4, 06120 Halle, Germany 4 Fraunhofer Institute for Mechanics of Materials, Walter Hu¨lse Str. 1, 06120 Halle, Germany 2

Received 9 May 2010, revised 12 August 2010, accepted 13 August 2010 Published online 10 September 2010 Keywords nanotubes, nanowires, templates, wetting authors: e-mail [email protected], Phone: þ49 6421 28 25573, Fax: þ49 6421 28 25785 þ49 6421 28 21316, Fax: þ49 6421 28 24291 *** Phone: þ49 345 55 28 517, Fax: þ49 345 55 27 391 * Corresponding ** Phone:

Different routes to prepare gradient nanowire and nanotubes are presented including the use of polymeric and metal nanoparticles. In the case of electrospinning, latex particles have been added to electrospinning solution. The properties of the fibres have been studied theoretically and experimentally. In the case of face-to-face-wetting of porous templates, solution as well as melt wetting for different polymeric composition from both sides of the template is carried out. It turned out that solution wetting leads to an intermixing of the compounds whereas as melt wetting leads to sharp compositional interfaces.

The method has been extended to include plasmonic and magnetic nanoparticles for nanophotonic applications.

ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Polymer, metal or semiconductor nanotubes and nanowires, being homogeneous in composition and geometry, are known to display unique properties and functions. They are of interest for a broad range of applications. However, with respect to novel nano-devices, the introduction of compositional and structural gradients is a highly promising approach. Multisegment nanotubes [1–4] and nanorods [5, 6] have been predominantly prepared by means of electrochemical methods. The topic of this project is the preparation, characterization and functionalization of graded nanowires and nanotubes. Based on techniques already developed by us such as electrospinning, tubes by fibre templates (TUFT) and wetting-assisted templating (WASTE) for the preparation of complex nanotubes and nanowires, we will introduce gradients with respect to composition and functionality. Gradients parallel and perpendicular, i.e. radial as well as tangential, to the main axis of the nanowires or nanotubes will be discussed. The target materials are polymers, metals, ceramics and combinations of these materials. We present different routes for the design and preparation of such

gradient structures, the evaluation of the internal structure and finally the characterization of the physical and chemical properties resulting from the gradients in such nano-objects. Finally, we discuss their potential use as photonics, magnetic or magneto-optic building blocks. 2 Theory and experiment of nanowires by electrospinning of colloidal dispersions Electrospinning is a versatile method for the production of polymer nanofibres [7]. Polymer solutions form in strong electrical fields fibres which solidify when solvents evaporate. Watersoluble polymers can be electrospun, but typically the resulting fibres dissolve in water. Water-insoluble polymer nanofibres were prepared by electrospinning colloidal latex spheres from an aequous dispersion [8]. The process results in necklaces of latex spheres (Fig. 1a) which show by transmission electron microscopy (TEM) of crosssectional cuts a hexagonal arrangement (Fig. 1b). The structures are reminiscent of patterns that can be obtained, e.g. in non-equilibrium self-assembly during evaporation [9–11] or at interfaces [12, 13]. ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

solidi

status

physica

pss

b

2452

S. Agarwal et al.: Gradient nanowires and nanotubes

Figure 1 Scanning electron micrograph of electrospun PS latex nanofibres (a) and TEM of a cross-sectional cut (b) [8].

During the electrospinning process the contracting flow pulls the particles towards the centre of the fibre where they form regular and irregular chains. The formation of the chains can be modelled with a phenomenological contracting potential and an approximate inter-particle potential [14]. Mechanical relaxation of random initial conditions then results in configurations that are similar to the experimental findings. The key parameter is r, the number of latex spheres times their diameter by length. The minimum value for a connected necklace of spheres is r ¼ 1. Examples of fibres with higher densities are shown in Fig. 2. Values of r ¼ 2 or 3.5 or 4 form regular arrangements of spheres with several strands of spheres. For intermediate values, the arrangements typically are irregular, because the particles that do not fit have to be attached to the smaller regular chains or give gaps in the bigger regular chains. A comparison between modelled and observed structures is shown in Fig. 3. The comparison confirms the density as the determining parameter for the type of structure that is obtained during electrospinning. Based on the initial experiments and on the outcome of the modelling experiments attempts were undertaken to obtain very thin and smooth latex-based polymer nanofibres [15]. The dense packing of the latex particles was exploited to obtain nanofibres with diameter as low as about 300 nm and very smooth surfaces (Fig. 4). Here, polystyrene (PS) latex dispersion was replaced by a polybutyl acrylate latex dispersion which is known to show a much lower minimum film formation temperature (MFFT). Corresponding to the

Figure 2 (online colour at: www.pss-b.com) Calculated fibre shapes for different densities. The bottom row shows a cross section of the fibre, with the spheres that are cut filled in black [14]. ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 3 (online colour at: www.pss-b.com) Comparison between calculated (left) and observed (right) fibres [14].

lower MFFT also in the electrospun nanofibres the initially spherical colloidal smoothens out as soon as the fibres were formed. Another crucial parameter which has been applied for the formation of thin diameter fibres was the replacement of ionic surfactants by non-ionic surfactants and thereby minimizing electrostatic repulsion of the colloidal particles when the fibres are formed. In order to probe potential applications of the electrospun latex fibres and to investigate the formation of gradients along fibre axis two different concepts were applied. Following these concept, aqueous colloidal latex dispersion were mixed with superparamagnetic Ferrit nanoparticles (FNP, average size 20 nm) or with dye-doped polymer

Figure 4 SEM of electrospun polybutyl acrylate nanofibres [15]. www.pss-b.com

Feature Article Phys. Status Solidi B 247, No. 10 (2010)

2453

Figure 5 (online colour at: www.pss-b.com) TEM of FNP in electrospun polybutyl acrylate latex fibres (a) and squid measurements of the same fibres (b) [16].

microcapsules (not discussed here in detail) [16]. The presence of the FNP was proven by energy dispersive X-ray diffraction as well as by TEM (Fig. 5a). It is clearly visible from the TEM that the distribution of the FNP is nonhomogeneous, which can be explained by the fast fibre formation process. Squid measurements showed that the FNP in the non-oriented fibers revealed no hysteresis or remanescence which is a strong indication for superparamagnetic behaviour (Fig. 5b). In oriented electrospun fibres containing FNP different magnetization was found parallel versus perpendicular to the field which can be explained by inhomogenous gradient type distribution of the FNP and their preferred orientation in the fibres. 3 Gradient nanotubes by template-assisted synthesis Based on the concept of the WASTE process [17] the concept was extended for the preparation of nanotubes from the solution state and melt state with longitudinal gradients and segmented nanotubes [18] and a combination of longitudinal gradient and cross-sectional gradient [19] by face-to-face wetting. According to the concept of bidirectional wetting nanoporous discs with pores open at both ends were simultaneously exposed to different polymeric solutions. As a result wetting occurred under formation of longitudinal gradient nanotubes in composition after solvent nanotubes. PVC and poly(4-bromo-styrene) (PSBr) were used as polymers. Tubular structure was clearly identified by scanning electron microscopy (Fig. 6a and b). The compositional gradient was analysed by two-dimensional mapping of the intensity of the Ka peak of Cl (Fig. 6c) and Br (Fig. 6d). In contrast to wetting with polymeric solutions, in faceto-face melt-wetting experiments the same type of polymers was used in order to ensure that the melts located at both template surfaces move into the pores at a comparable rate. To distinguish between the polymers moving into the pores from the opposite sides of the templates, we synthesized a tailor-made polymer with a chromophore bonded to the polymer backbone by radical copolymerization of methylmethacrylate and anthracenoyl methacrylate (content about 0.2 mol.%). Wetting experiments were carried out with a fluorescent [polymethylmethacrylate (PMMA)/9-vinylanthracen (10%) copolymer] and a non-fluorescent polymer www.pss-b.com

Figure 6 (online colour at: www.pss-b.com) SEM of PVC/PSBr gradient nanotubes showing tubular openings (a), different morphologies along tube main axis (b), two-dimensional mapping of the intensity of the Ka peak of Cl (c) and Br (d) – the color intensity encodes intensities of the elements [18].

(PMMA) using macroporous silicon membranes with a thickness of 270 mm and a pore diameter of 1 mm as porous templates. Macroporous silicon was selected as a template system in order to prepare tubes with a well-defined shape large enough to be characterized by optical microscopy. In the first series of experiments performed at an infiltration temperature of 180 8C, the non-fluorescent PMMA used had a significantly higher molecular weight (Mw ¼ 120,000 g/mol) than the fluorescent PMMA/9-vinylanthracen (Mw ¼ 27,334 g/mol, Mn ¼ 17,048 g/mol). Along the tubes thus obtained, we observed uniform luminescence (Fig. 7). This can be explained by the lower viscosity and therefore faster infiltration of PMMA/9-vinylanthracen compared with the more viscous non-fluorescent PMMA,

Figure 7 (online colour at: www.pss-b.com) Optical (a) and fluorescent microscopy images (b) of PMMA/9-vinylanthracen tubes obtained by face-to-face melt wetting. The infiltration of the fluorescent PMMA/9-vinylanthracen melt occurs at a much higher rate than the infiltration of the second component PMMA with significantly higher molecular weight. All tubes are fluorescent over their entire length [18]. ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

solidi

status

physica

pss

b

2454


Figure 8 (online colour at: www.pss-b.com) (a and b) Fluorescent microscopy images of non-fluorescent PMMA tube segments and fluorescent PMMA/9-vinylanthracen tube segments obtained by face-to-face melt infiltration of PMMA and PMMA/9-vinylanthracen having similar molecular weights. The red circle in (b) indicates the interface between a non-luminescent and a luminescent part of the tube [18].

resulting in the formation of a wetting film on the pore walls exclusively consisting of PMMA/9-vinylanthracen. In order to verify this hypothesis, we selected PMMA (Mw ¼ 23,300 g/mol, Mn ¼ 23,000 g/mol; obtained from Polymer Standard Service, Germany) with a molecular weight comparable to that of the PMMA/9-vinylanthracen. Face-to-face wetting at 180 8C and subsequent release of the tubes thus obtained yielded luminescent tube segments as well as non-luminescent tube segments (Fig. 8). The nonluminescent tube segments are completely optically inactive except, in some cases, at the ends of the tube segments. In contrast, the luminescent tube segments predominantly show homogeneous luminescence along their entire length. Apparently, the two molten polymeric components do not mix when they impinge on each other in the course of the wetting process. Therefore, the tubes preferentially break at the interface between the tube segments thus formed when exerted to mechanical stress in the course of their release. This finding indicates the presence of a sharp, well-defined interface between the two polymeric components. Polymers are commonly immiscible because the combinatorial entropy generated upon mixing is, compared to low molecular mass compounds, negligible. This effect is wellknown from polymer processing. Flow lines resulting from the merging of two different flow fronts, for instance, in injection moulding tend to be weak due to a mismatch of chain orientations and conformations even for compatible systems. Therefore, face-to-face wetting using polymeric melts is complementary to face-to-face wetting using polymeric solutions. In the latter case, the solvents may act as compatibilizer between the polymeric components so that a real gradient zone forms. The method presented here can be easily extended, for example, for the fabrication of metal nanoparticle-containing polymer (metal–polymer nanocomposites) nanotubes with a longitudinal gradient in the metal nanoparticle content, and thus longitudinal gradients in functionality. The embedding of nanoscopic metals into dielectric matrices represents a valid solution to the manipulation and stabilization problems of nano-sized metals. In the functional field, polymers are particularly interesting as an embedding ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 9 (online colour at: www.pss-b.com) Experimental setup used for face-to-face melt wetting.

phase, since they may have a variety of characteristics: They can be a electrical and thermal insulators or conductors, may be of hydrophilic or hydrophobic nature, can be mechanically hard, plastic or rubbery. Finally, polymer-embedding is the easiest and most convenient way for nanostructured metal’s stabilization, handling and use. Polymer-embedded nanostructures are frequently termed nanocomposites because of their biphasic nature. For face-to-face wetting with metal embedded in polymeric melts, macroporous silicon membranes exhibiting a hexagonal pore lattice with an interpore distance of 4.2 mm and a pore radius of about 1.5 mm was used as a template in order to prepare microtubes. The depth of the membrane pores was adjusted to values between 200 and 400 mm. Faceto-face melt wetting was carried out using a experimental setup (Fig. 9), which can work under inert gas or vacuum. Both copper hot plates could be addressed individually by a computer-operated temperature controller. The macroporous silicon template was placed in a specifically designed holder equipped with a poly (tetrafluoroethylene) sealing to minimize heat transfer between the two hot plates. Our first experiments concentrated on the in situ method to prepare different composites which were infiltrated in the porous templates via face-to-face melt wetting in a second step. A large-scale production of polymer-embedded nanosized metals should be necessarily based on the thermal decomposition of metal precursors directly added to the polymer during their hot-processing stage. A number of organic precursors have been studied for this application [20, 21]. www.pss-b.com


Figure 10 SEM picture of different polymer//polymer/metal salt composites prepared by face-to-face melt wetting.

In the face-to-face melt-wetting experiments the same type of polymers was used in order to ensure that the melts located at both template surfaces move into the pores at a comparable rate. Similar to the preparation of metal nanotubes with tailored wall morphologies [22–26] we wetted ordered porous silicon membranes with pure polymer at the one side of open pores and with melt-containing polymer and metal precursor at the opposite side of the membranes (polymer//polymer/metal salt). We used poly (D,L-lactide) (PDLLA), PS and PMMA as polymers and palladium(II) acetate (Pd(OAc)2), platinum(II) acetylacetonate (Pt(acac)2), silver(I) acetylacetonate (Ag(acac)) and nickel(II) acetylacetonate Ni(acac)2 as metal precursor. The polymer/metal precursor composites mixed in a ratio of 3:1, under ambient condition. Annealing at 200 8C led the thermal degradation of the metal precursor within a few tens of seconds and PdII, PtII and AgI were reduced to metal0. In the case of the conversation of NiII to Ni0 the decomposition temperature is 230 8C. In Fig. 10, several examples of this experiment and related scanning electron micrograph (SEM) images are shown. Transmission electron microscopy using a 100 kV JEM 1010 allows us to get a closer look to the microstructural characteristics of the composite microtubes. Figure 11a

Figure 11 (online colour at: www.pss-b.com) (a and c) TEM images of PS//PS/metal composite tubes. (a) PS//PS/Ag tube section (pore diameter DP ¼ 1.5 mm, pore depth ¼ 400 mm), (b) sketch of the experimental situation at the interface polymer//polymer–metal composite and (c) PS//PS/Pt composite microtubes at different position in longitudinal direction accordant to the sketch in (b). www.pss-b.com

2455

shows a TEM image of a PS//PS/Ag tube section (pore diameter DP ¼ 1.5 mm, pore depth ¼ 400 mm) after wetting and annealing for 24 h at 200 8C. The silver nanoparticles are evident in the upper image, but in the lower one no particles are visible. Figure 11b shows a schematic draft of the different parts of the tube in longitudinal direction of the PS//PS/Pt composite microtubes, with corresponding TEM images described in detail in Fig. 11c. Platinum is a metal with interesting photo-catalytic activity. By creating gradients in the local photo-catalytic activity, advanced functions such as movement of the nanotubes is possible. The dispersion of platinum particles in a PS matrix clearly appeared in part (1) of Fig. 11c. In part (2), a complete meniscus is identified. In the PS matrix there are metal particles, too. No particles are observed in Fig. 11c, part (3). This is a pure PS microtube, with an interface between both compartments. No miscibility takes place, therefore they broke up at the interface after resolving the single microtubes. The particles have an average size of about 5 nm (Fig. 12b). A further goal is the study of a novel concept of nanostructured materials formed by the combination of components with plasmonic and magneto-optical (MO) activity. This smart combination will produce ‘magnetoplasmonic’ nanomaterials tailored on the nanoscale. While noble metals exhibiting plasmon resonances have no MO activity and ferromagnetic materials suffer from strong plasmon damping, metallic heterostructures made of noble metals and ferromagnetic materials may sustain surface plasmons and have at the same time MO activity. Therefore, these materials could become key elements in future tunable nano-optical devices and in biosensors with enhanced sensitivity. An additional feature of these unconventional materials is that the optical response can be tuned by means of an external magnetic field. Concerning the preparation of locally varying magnetic properties we made first experiments with nickel-embedded nanoparticles in a PS matrix. We melted the composite opposite to the pure PS. The bright and additionally the dark field TEM images in Fig. 13a and b) show clearly the dispersion of the nickel particles in the polymer matrix. First experiments concentrated on PS//PS/metal composite microtubes. PS microtubes (Mw 100,000, 2 h at

Figure 12 (online colour at: www.pss-b.com) (a) TEM image of PS//PS/Pt composite and (b) the average particle size distribution. ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

solidi

status

physica

pss

b

2456

Figure 13 (a) Bright-field and (b) dark-field TEM images of PS// PS/Ni composite microtubes.

Figure 14 SEM images of bidirectional wetting of different polymer composites.

200 8C) were prepared in a first step and several PS/metal salt (Ni, Pt, Ag, Au and Pd) solutions wetting (24 h at 200 8C) of the pores from the opposite side in a second step subsequently. Representative SEM images of PS//PS with embedded silver, gold and nickel particles are shown in Fig. 14. Further experiments of bidirectional template wetting with the component system polymer/metal salt//polymer/ metal salt is realized experimentally first with the system PDLLA/Ni(acac)2//PDLLA/Ag(acac) – a combination of components with plasmonic and MO activity. The results are shown in Fig. 15. Remaining open issues are miscibility and the influence of the plasmonic properties of the noble metallic nanoparticles to the MO properties of the magnetic metallic


nanoparticles. The surface plasmon resonance of magnetic metallic nanoparticles can enhance MO properties compared with a continuous medium, has not fully considered the effect of the size of those nanoparticles and their interaction. Localized surface plasmons play a key role in the optical properties of metallic structures, particularly nanoscale ones such as nanoparticles embedded in dielectric matrices. The size, shape and concentration of the nanoparticles, as well as the refractive index of the matrix, determine the intensity and spectra of the resulting surface plasmonic resonances. Typically, Au or Ag nanoparticles are used for such systems, but other metals such as Fe, Co and Ni possess spontaneous magnetization. Although these metals give rise to, in some respects, inferior surface plasmonic resonances, the addition of MO properties enables the design of new kinds of plasmonic structures. 4 Conclusion We have developed different routes to fabricate gradient nanowires and nanotubes by either electrospinning or face-to-face wetting. Electrospinning of latex dispersions has been shown to be a powerful method for the preparation of nanowires with controlled morphologies and functionalities. The use of polymeric solutions in the case of face-to-face wetting yields nanotubes with a longitudinal composition gradient dependent on the compatibility of the components. Due to the solvent, intermixing of the two polymeric solutions occurs at the interface. Face-to-face wetting with polymer melts leads to segmented tubes consisting of segments separated by a sharp, well-defined interface. The method presented here can be easily extended, for example, for the fabrication of nanoparticle-containing polymer nanotubes with a longitudinal gradient in the nanoparticle content, and thus longitudinal gradients in functionality. Acknowledgements The authors are indebted to Deutsche Forschungsgemeinschaft for financial support in the frame of the Schwerpunktprogramm SP 1165.

References

Figure 15 Scanning electron microscopy images of PDLLA/Ni(acac)2//PDLLA/Ag(acac) (a) single nanowire, (b) an open tube end, (c) the interface between both composites and (d) enlargement of (c). ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

[1] S. R. Nicewarner-Pen˜a, R. Griffith, B. Freeman, D. Reiss, L. He, D. J. Pen˜a, I. D. Walton, R. Cromer, C. D. Keating, and M. J. Natan, Science 294, 137 (2001). [2] K. Salem, P. C. Searson, and K. W. Leong, Nature Mater. 2, 668 (2003). [3] S. Park, J.-H. Lim, S.-W. Chung, and C. A. Mirkin, Science 303, 348 (2004). [4] S. J. Hurst, E. K. Payne, L. Qin, and C. A. Mirkin, Angew. Chem., Int. Ed. 45, 2672 (2006). [5] M. S. Sander and H. Gao, J. Am. Chem. Soc. 127, 12158 (2005). [6] W. Lee, R. Scholz, K. Nielsch, and U. Go¨sele, Angew. Chem. 117, 6204 (2005). [7] A. Greiner and J. H. Wendorff, Angew. Chem., Int. Ed. 46, 5670 (2007). [8] A. Stoiljkovic, M. Ishaque, U. Justus, L. Hamel, E. Klimov, W. Heckmann, B. Eckhardt, J. H. Wendorff, and A. Greiner, Polymer 48, 3974 (2007). www.pss-b.com


[9] E. Lauga and M. P. Brenner, Phys. Rev. Lett. 93, 238301 (2004). [10] L. V. Govor, G. Reiter, G. H. Bauer, and J. Parisi, Phys. Rev. E 74, 061603 (2006). [11] Y.-S. Cho, G.-R. Yi, S.-H. Kim, M. T. Elsesser, D. R. Breed, and S.-M. Yang, J. Colloid Interface Sci. 318, 124 (2008). [12] L. V. Govor, G. Reiter, J. Parisi, and G. H. Bauer, Phys. Rev. E 69, 061609 (2004). [13] N. Aubry and P. Singh, Phys. Rev. E 77, 056302 (2008). [14] F. Grossmann, PhD Thesis, Philipps-Universita¨t Marburg (2009). [15] A. Stoiljkovic, R. Venkatesh, E. Klimov, V. Raman, J. H. Wendorff, and A. Greiner, Macromolecules 42, 6147 (2009). [16] T. Ro¨cker, PhD Thesis, Philipps-Universita¨t Marburg (2009). [17] M. Steinhart, J. H. Wendorff, A. Greiner, R. B. Wehrspohn, K. Nielsch, J. Schilling, J. Choi, and U. Go¨sele, Science 296, 1997 (2002).

www.pss-b.com

2457

[18] O. Kriha, P. Go¨ring, M. Milbrandt, S. Agarwal, M. Steinhart, R. Wehrspohn, J. H. Wendorff, and A. Greiner, Chem. Mater. 20, 1076 (2008). [19] O. Kriha, L. Zhao, E. Pippel, U. Go¨sele, R. B. Wehrspohn, J. H. Wendorff, M. Steinhart, and A. Greiner, Adv. Funct. Mater. 17, 1327 (2007). [20] C. R. Martin, Science 266, 1961 (1994). [21] C. A. Huber, T. E. Huber, M. Sadoqi, J. A. Lubin, S. Manalis, and C. B. Prater, Science 263, 800 (1994). [22] B. B. Lakshmi, P. K. Dorhout, and C. R. Martin, Chem. Mater. 9, 857 (1997). [23] M. Steinhart, J. H. Wendorff, A. Greiner, R. B. Wehrspohn, K. Nielsch, J. Schilling, and U. Go¨sele, Science 296, 1997 (2002). [24] M. Steinhart, Z. Jia, A. Schaper, R. B. Wehrspohn, U. Go¨sele, and J. H. Wendorff, Adv. Mater. 706, 706 (2003). [25] N. I. Kovtyukhova, T. E. Mallouk, and T. S. Mayer, Adv. Mater. 15, 780 (2003). [26] M. Steinhart, R. B. Wehrspohn, U. Go¨sele, and J. H. Wendorff, Angew. Chem. 116, 1356 (2004); Angew. Chem., Int. Ed. 43, 1334 (2004).

ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim