Appl. Phys. A 70, 365–376 (2000) / Digital Object Identifier (DOI) 10.1007/s003390000440 Applied Physics AMaterialsScience & Processing

Invited paper

Template-based synthesis of nanomaterialsA. Huczko

Department of Chemistry, Warsaw University, 1 Pasteur, 02-093 Warsaw, Poland(Fax: +48-22/822-5996, E-mail: ahuczko@chem.uw.edu.pl)

Received: 27 May 1999/Accepted: 27 October 1999/Published online: 8 March 2000 – Springer-Verlag 2000

Abstract. The large interest in nanostructures results fromtheir numerous potential applications in various areas suchas materials and biomedical sciences, electronics, optics,magnetism, energy storage, and electrochemistry. Ultra-small building blocks have been found to exhibit a broadrange of enhanced mechanical, optical, magnetic, and elec-tronic properties compared to coarser-grained matter of thesame chemical composition. In this paper various templatetechniques suitable for nanotechnology applications withemphasis on characterization of created arrays of tailorednanomaterials have been reviewed. These methods involvethe fabrication of the desired material within the pores orchannels of a nanoporous template. Track-etch membranes,porous alumina, and other nanoporous structures have beencharacterized as templates. They have been used to pre-pare nanometer-sized fibrils, rods, and tubules of conduc-tive polymers, metals, semiconductors, carbons, and othersolid matter. Electrochemical and electroless depositions,chemical polymerization, sol-gel deposition, and chemicalvapour deposition have been presented as major templatesynthetic strategies. In particular, the template-based syn-thesis of carbon nanotubes has been demonstrated as this isthe most promising class of new carbon-based materials forelectronic and optic nanodevices as well as reinforcementnanocomposites.

PACS: 61.46.+w; 61.48.+c; 61.82.Rx

In 1959 Richard Feynman, the future Nobel Laureate, sug-gested in his famous lecture, entitled “There’s Plenty ofRoom at the Bottom”, a variety of tests that might be achievedat very small scales. Three decades later Feynman’s visionhas become the greatest scientific frontier of the century. Itopened a new field of “Nanostructures” having dimensionsof about 10 to1000Å, a size that is small by engineer-ing standards, common by biological standards, and large tochemists.

The central thesis of nanotechnology is that almost anychemically stable structure that can be specified can actu-

ally be built. Nature has plenty of proof that nanotechnologyworks, from the liposomes in cells that manufacture proteinsatom by atom, to the chloroplasts of plants that turn sunlight,carbon dioxide, and water into copies of themselves. Since bi-ological machines exist, and work, and since biology followsthe same laws of physics and chemistry applied by engineers,there’s no question the nanostructural machines will work.Scientists have to figure out, however, how to build them.

Of particular significance is the size dependence of manyproperties in nanomaterials, for example, an enhancement inthe strength and hardness of solids; the possibility of mod-ifications of their electrical properties by control of the ar-rangement within the constituent nanoclusters and of theirassembly; the control of chemical reactivity by the attach-ment of functional side-groups; the control of optical prop-erties by variation of the size and microstructure of thenanoclusters; the possibility of creating nanostructures ofmetastable phases with non-conventional properties, includ-ing superconductivity and magnetism [1]. If a nanocrystal isfabricated such that at least one of its dimensions is smallerthan the length scale of some property, then that property is“confined” and becomes dependent on the size and shape ofwhat is now called a “quantum crystal” [2, 3]. Material struc-tures of reduced size or dimensionality may exhibit quan-tized conductance [4–6], localization phenomena in low-dimensional systems [7], and mechanical properties for thecreation and propagation of dislocations in small metallicsamples [8, 9]. In fact, the technology of nanostructures isdriven largely by the urge to make ever smaller and fasterelectronics [10].

Another example of the “nano-world at work” is thephotoelectric conversion property of nanostructured materi-als [11] which represents an alternative approach to effi-cient and economical solar energy conversion. For nanocrys-talline particles quantum size effects may be manifested inthe photophysical properties. Examples of improved photo-electrochemical cells include nanocrystalline:TiO2 [11, 12],CeSe[13], andZnO [14]. Traditional materials for photoelec-trochemical applications have been single-phase and compos-ite compounds with 3-D structures. More recently, artificially

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structured and low-dimensional materials containing intri-cate, nanometer-scale structures have been developed. Theyfrequently exhibit novel, highly anisotropic, enhanced prop-erties, directly related to the dimensionality and the extradegrees of freedom of the nanostructure.

A recent progress in quantum structure materials, physics,and devices was reviewed by Sakaki [15] and Barbara andWernsdorfer [16] reported on the quantum tunneling effect inmagnetic nanoparticles. The new perspectives in future nano-electronics related to metal nanosized clusters were presentedby Schmid and Hornyak [17].

Obviously, nanotechnology is not a single discipline. Itrequires the collaboration of physicists, chemists, biologists,computer scientists, and engineers. The coming nanome-ter age can, therefore, also be called the age of interdis-ciplinarity [18]. Synthesis, manipulation, modification, andcontrol on the nanometer or even atomic scale have madetremendous progress in the past couple of years [19–22].These works require the use of multiple experimental tech-niques. Among the most “popular” are electron spectro-scopies (XPS, AES), electron microscopies (SEM, TEM,BEEM), ion- and neutral spectroscopies (SIMS, SNMS, ISS),and scanning probe microscopies (SXM) such as atomic forcemicroscopy (AFM) [23].

Many methods for preparing nanomaterials have been de-veloped, ranging from milling techniques to chemical andlithographic methods [24]. Their main weakness is, how-ever, attributed to the poor control of final morphology ofthe produced nanostructures although some specific proper-ties (for example, the enhanced electronic conductivities) areexhibited only with enhanced molecular and supermolecularorder [25, 26].

A method termed “template synthesis” entails the prepar-ation of a variety of micro- and nano-materials of a desiredmorphology and, therefore, provides a route for enhancingnanostructure order. In the broadest sense, a template may bedefined as a central structure within which a network formsin such a way that removal of the template creates a filledcavity with morphological and/or stereochemical features re-lated to those of the template. Various porous “templates” areemployed and the nanostructures are synthesized within thepores. If the templates that are used have cylindrical pores ofuniform diameter, monodisperse nanocylinders of the desiredmaterial are obtained within the voids of the template mate-rial. Depending on the operating parameters, these nanocylin-ders may be solid (a nanorod) or hollow (a nanotubule). Thenanostructures can remain inside the pores of the templatesor they can be freed and collected as an ensemble of freenanoparticles. Alternatively, they can protrude from the sur-face like the bristles of a brush.

Thus, with the template approach, one is able to preparemonodisperse nanorods and nanotubules of almost any de-sired geometry. The method has been used to prepare bothnanotubules and nanofibrils composed of conductive poly-mers, metals, semiconductors, carbon, and other materials.

The object of this paper is to review the template synthesisof nanomaterials. Various nanoporous structures used as thetemplates are described and the methods developed to do thesynthesis within these materials are reviewed. The fundamen-tal features of these novel nanostructures are also discussed.Finally, the applications of template-synthesized nanofibrilsand nanotubules are presented.

One should mention that an entire issue of Chemistry ofMaterials (volume 8, issue 8) has been dedicated to review-ing nanocluster chemistries including the use of templates toprepare nanostructures of various materials. The May 1999issue of Accounts of Chemical Research is devoted to recentadvances in this multidisciplinary field. The template-basedtechnique has also been explored and mastered for years bythe research group at Colorado State University, USA, lead byProf. Charles R. Martin [26–37].

1 Template synthesis of nanomaterials

Nanotubules and nanorods of various materials can besynthesized using a template-based approach. Polymers,metals, semiconductors, carbons, and other materials havebeen deposited within pores of previously characterizedtemplates [31, 32].

1.1 Polymeric nanostructures

The electronically conductive polymers, presented in Fig. 1,polypyrrole [35, 38, 39], poly(3-methylothiophene) [34], andpolyaniline [40] were template-synthesized by electrochem-ical or oxidative polymerization. By controlling the poly-merization time, conductive polymer tubules with thin orthick walls can be obtained. For polypyrrole, the tubules ulti-mately form solid fibrils. In contrast, the polyaniline tubuleswill never close up, even at long polymerization times [31].Following the dissolution of the template polypyrrole nan-otubules were freed. The SEM and TEM observations showedthat the tubules are all10µm long, which is equivalentto the thickness of the template and their diameter beinglarger than the nominal pore diameter [35]. Using the sametechnique, one-dimensionalAu-polypyrrole nanoparticle ar-rays were obtained [38]. Conducting polyaniline filamentswere stabilized in the ordered,3-nm-wide hexagonal channelsof the mesoporous aluminosilicate [40]. The structural data

Fig. 1. Some of the template-synthesized electronically conductive poly-mers, adapted from [31]

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show that about 20 aligned polyaniline chains could fit in thetemplate channels. Nishizawa et al. [39] described templatesynthesis of polypyrrole-coated spinelLiMn2O4 nanotubules.The outer diameter of these tubules agreed with the diam-eter of pores (200 nm) of the template, and the thickness ofthe tubule wall was about50 nm. Nanoscopic polyacetylenefibrils were template-synthesized by polymerizing acetylenewithin 200-nm pores of filtration membrane [27]. The fib-rils show good monodispersity of both length and diameter.Radical polymerization of acrylonitrile within the pores of ze-olites produced polyacrylonitrile (PAN) [41]. On pyrolysis,PAN was transformed into nanometer-size conducting carbonfilaments.

Tang and Xu [42] prepared carbon nanotube-containingpoly(phenylacetylenes) (NT/PPAs) by in situ catalytic poly-merizations of PPAs in the presence of the nanotubes. Theshort nanotubes thickly wrapped in the PPA chains exhibita strong photostabilization effect, protecting the PPA chainsfrom photodegradation. Such a stabilization effect may notonly trigger basic research on the understanding of elec-tronic properties of the nanotubes but also help find techno-logical applications for the nanotubes in electronic and opti-cal systems.

1.2 Nanometals

Through confinement of metals to a nanosized dimension,a variety of changes in their characteristics are induced [29,31, 32, 42–45]. The gold nanoparticles were electrodepositedas shown in Fig. 2. First, a thin layer ofAg is sputtered(Fig. 2a) followed by “plugs” electrochemically grown intothe pores (Fig. 2b). From those ‘nanoposts’Au nanoparti-cles are grown (Fig. 2c), and finally, theAg foundations arechemically removed resulting in an array ofAu nanorods(Fig. 2d). Depending on the pore diameter of the template,Au nanoparticles with different diameter can be obtained.A TEM analysis made possible the determination of as-pect ratios of the nanoparticles. The hollowAu tubulesthat run the complete thickness of the template can alsobe obtained if the deposition is stopped before the pores

Fig. 2. Fabrication procedure forAu nanoparticle-alumina composite,adapted from [32]

are completely filled [31]. Brumlik et al. [29] also preparedAu and Ag microtubules using various procedures. Withthe evaporation/electroplating method the smallest diametertubules had outer diameters of400 nmwith the maximumtubule length about1µm. Using the pore-wall-modificationelectrochemical method, longerAu tubules (around3µm)were grown.400-nm-diameter silver microtubules as long as10µm were obtained via the electroless deposition method.However, tubules with extremely small diameters (30 nm)can also be prepared [29]. The potentiostatic electrochemicaltemplate synthesis yielded different metal nanowires (Ni, Co,Cu, andAu) with nominal pore diameters between 10 and200 nm [42]. The wires produced were true replicas of thepores. Whitney et al. [43] fabricated the arrays of nickel andcobalt nanowires by electrochemical deposition of the metalsinto track-etched-templates. The morphology of the arrayswas studied by using SEM and TEM techniques after re-moval from the template. The nanowires were narrow (30 nm)and continuous, with length equal to that of the matrix thick-ness. Tomassi and Buczko [44] obtained the nanorods of thefollowing metals:Ni, Cu, Sn, Fe, Co, Zn, Cd, Au, and Pt,using the electrodeposition template technique. The diameterof metal particles depended on pore diameter and was foundto be between 5 and30 nm.

Braun et al. [45] proposed a two-step procedure that allowthe application of DNA as a template for the vectorial growthof a 12-µm-long, 100-nm-wide conductive silver nanowire.The study show that the recognition capabilities of DNAcan be exploited for the targeted attachment of functionalnanostructures.

1.3 Carbon nanotubes

Iijima’s [46] discovery of novel carbon tubes of nanometerdimensions has greatly stimulated studies in the field of car-bon fiber growth. The large-scale synthesis of carbon nano-tubes, CNTs, based on an arc-discharge method similar to thesynthesis of fullerenes, was later developed by Ebbesen andAjayan [47]. In fact, this unusual form of carbon was alreadyreported almost50 yearsago as minute vermicular growths(from 10 nmup to about0.2µm in thickness) which can pen-etrate the brickwork of blast furnaces [48].

General methods commonly used to produce carbonnanotubules (both single-walled SWCNTs and multi-walledMWCNTs) – carbon arc and laser plasma techniques [46,47, 49], catalytic pyrolysis of hydrocarbons [50–52], low-pressure condensation of carbon vapors [53–55], and con-densed-phase electrolysis [56, 57] – suffer from the draw-backs that polyhedral carbon particles are also formed, thedimensions of the nanotubes are highly variable and they havea disordered and bundle structure. Thus, growing organizedCNTs on large-scale surfaces is important for their possiblevarious applications.

Templating methods have been successfully applied toproduce ensembles of aligned and monodisperse tubules ofgraphitic carbon with diameters as small as a fewnm. CVDtechnique was commonly applied and hydrocarbons werepyrolyzed in alumina templates yielding graphitic carbonnanofiber and nanotube ensembles (with diameters as smallas 20 nm and lengths around50µm) [33, 36, 37, 58]. Mas-sive arrays of self-oriented monodispersed carbon nanotubes

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were also prepared by CVD decomposition of hydrocarbonson patterned porous silicon [59, 60] or within the pores of ac-tivated silica [61, 62]. Both single- and multi-walled tubuleswere formed with diameters from1–3 nm and lengths up totens ofµm. SWCNTs were also synthesized in1-nmchannelsof porous crystallites [63].

Tang et al. [64] fabricated mono-sized and parallel-aligned SWCNTs in0.73-nm-sized channels of microporousaluminophosphate crystallites by pyrolysis of tripropylaminemolecules in the channels. The electric transport propertiesfor the nanotubes were also studied and their conductiv-ity was shown to monotonically increase with increasingtemperatures, indicating that the SWCN had semiconductorcharacteristics.

The formation of patterned CNTs (having the specific sizeof 30–80 nm) can be also catalyzed by annealedFeNi islands(deposited on theAg film) in the process of CVD of carbonfrom acetylene decomposition [65].

Terrones et al. [66] described a method for generatingaligned CNTs from hydrocarbons over thin films ofCo cat-alyst patterned on a silica substrate by laser etching. Thismethod offered control over length (up to around50µm) andfairly uniform diameters (30–50 nm) of the nanotubules.

1.4 Other nanomaterials

In addition to conductive fibrils, metal and carbon nanorodsand nanotubules, a variety of other nanostructure have beensuccesfully synthesized by using a template technique. Hul-teen and Martin [32] developed chemical strategies forpreparing composite tubular nanostructures for which onecan imagine a host of applications. Thus, a semiconductor–conductor (TiO2–polypyrrole or TiS2–polypyrrole) tubu-lar nanocomposites (60µm long and200 nm diameter) aswell as conductor–insulator–conductor (C–PAN–Au) com-posite structures were fabricated with template synthesis.Also, 15-nm-diameter semiconductor (TiO2) nanotubulesand nanofibers were deposited into the pores of a templatemembrane [32, 67].

A variety of applications for tailored composites in thenanometer-size range from 1 to100 nm, including structuralmaterials, high-performance coatings, catalysts, electronics,photonics, and magnetic and biomedical materials have beenrecently reviewed by Dagani [68].

Near-monodisperse hollow nanotubules ofMoS2 (about30µm long with diameters of50 nm and wall thickness ofabout10 nm) were prepared by thermal decomposition of mo-lecular precursors,(NH4)2MoS4 and(NH4)2Mo3S13, withinthe pores ofAl2O3 membrane template [69]. Template-basedapproaches to the preparation of amorphous, nanoporous sil-icas (using the removable organic templates) were describedby Raman et al. [70].

Pt- or Fe-metal-enclosed carbon tubes were obtained byusing the electrochemical template technique [71, 72]. Thesecomposite nanotubes are uniform and their outer diameterand wall thickness could be estimated to be30 nmand about5 nm, respectively.

Carbon nanotubes have been widely applied as remov-able templates for fabrication oxide nanocomposites (PbO,Bi2O3, V2O5, SiO2, Al2O3, MoO3, MnO2, Co3O4, ZnO,andWO3) [34, 73, 74]. Metal (Ag, Au, AuCl3)-filled carbon

nanotubes were obtained by Chu et al. [75] and individ-ual crystals located within the hollow cavity of nanotubeswere studied by FEGTEM technique with a0.7-nm probefor the EDS data. Galium nitride nanorods (with a diameterof 4 to 50 nm and a length of up to25µm) were preparedthrough a carbon nanotube-confined CVD reaction [76, 77].The GaN nanorods were solid in contrast to the hollow-core structure of CNTs. The nanostructure ofGaN (a singlecrystal with fewer defects) andSiC (a β-SiC crystal withheavy layer sequence faults) nanowires produced by car-bon nanotube-confined reaction has been studied by meansof HRTEM, microanalysis, and microdiffraction [78]. Sili-con nitride nanorods were also fabricated through the high-temperature reaction of carbon nanotubes withSiO under ni-trogen atmosphere. Han et al. [79] prepared boron-doped car-bon nanotubes and crystalline nanorods (with typical diam-eters between 6 and30 nm) through a template substitutionof some carbon atoms of CNTs by boron atoms. Boron ni-tride nanotubes were also successfully synthesized by CNT-substituted reaction method [80]. Solid carbide nanoscalerods ofTiC, NbC, Fe3C, SiC andBCx with typical diametersof between 2 and30 nmwere prepared by the conversion ofcarbon nanotubes to carbide rods by reaction with respectivevolatile oxide and/or halide species [81, 82]. The growth ofthe nanorods involved a template mechanism in which thecarbon nanotubes defined the overall morphology of the pro-duced nanostructures.

The template-based organization of buckminsterfullereneC60 in one-dimensional array was achieved by using vari-ous approaches: fullerene incorporation (by wet chemistry)into the channels of zeolites [83], surface adsorption onthe monolayers of theAu chain structures acting as ad-sorption templates [84], orC60 trapping in situ during car-bon nanotube formation, following pulsed laser vaporizationof graphite [85]. DNA could be also used as a frameworkfor the assembly of fullerene nanomaterials through electro-static interactions with the phosphate groups along the DNAbackbone [86].

Other forms of carbon nanostructures were prepared byusing the template-assisted growth process. Thus, a highlyoriented graphite was synthesized in a two-dimensional open-ing between the lamellae of montmorillonite as the field ofcarbonization [87, 88] – Fig. 3. The carbon was then releasedfrom MONT and its crystallite size (Lc and La were around

Fig. 3. Schematic diagram of graphitization process of the nanocarbon de-rived from intercalated polymer (IPC) and free polymer (FPC), adaptedfrom [87]

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40 nm and> 1µm, respectively) determined by XRD andSEM analyses. Carbon–mineral nanocomposites and micro-porous carbons (about80% of micropore volume is in poressmaller than1.2 nm) were obtained by in situ carbonization oforganic compounds in the inorganic matrixes [89].

2 Template synthetic techniques

Nearly any solid matter can in principle be synthesized withinnanoporous templates, provided a suitable chemical pathwaycan be developed. There are, however, some concerns thatneed to be considered, for example (1) does the deposited ma-terial ‘wet’ the pore; (2) how to avoid the pore blockage; (3)is the template stable with respect to the reaction conditions?There are essentially five representative strategies to carry outtemplate synthesis of nanostructures [31, 32].

2.1 Electrochemical deposition

Electrodeposition of a material within the pores of the ma-trix is preceded by coating one face of the template witha metal film and using this metal film as a cathode for elec-troplating [28, 29, 32, 42–44]. The volume of the pore is con-tinuously filled up beginning from the pore bottom. Thus,the length of a nanostructure can be controlled by varyingthe amount of material deposited. Both metal and conduc-tive polymer nanorods and nanotubules can be sythesizedusing this method. Arrays ofNi and Co nanowires wereelectrodeposited from sulfate solutions as shown in Fig. 4.The process was carried out at constant potential so that thedeposition could be monitored from the current response.Large arrays of uniform and continuousCoandNi nanowires(diameter in the range of30–60 nm) were obtained after dis-solving the polycarbonate template [43]. The electrochem-istry of the template synthesis of nanowires was studied by

Fig. 4. Schematic illustration of the electrode arrangement for electrodepo-sition of Ni andCo nanowires, adapted from [43]

comparing the potentiostatically measured current–time char-acteristics obtained during wire growth for different poredimensions and a pore-size dependence of the diffusion co-efficient for the metal ions was found [42]. Another ex-ample of the vacuum/electroplating method is shown inFig. 5. A gold film is first vacuum evaporated on the sur-face of a membrane yielding extremely thin-walled (about20 nm) gold tubules (Fig. 5a) which can be further strength-ened by electrochemical deposition of silver (Fig. 5b). Fi-nally, the membrane is dissolved away to expose an ensem-ble of free-standing tubules with outer diameters as small as200 nm(Fig. 5c) [29].

2.2 Electroless (i.e., chemical) deposition

Electroless deposition involves the use of a chemical agentto plate a material from the surrounding phase onto a tem-plate surface [29, 39, 40, 73, 77, 82]. This method differs fromthe previous one in that the surface to be coated need not beelectrochemically conductive. The material deposition in thepores starts at the pore wall. Therefore, after short decom-position times, a hollow tubule is obtained within each pore,whereas long deposition times result in solid nanowires. Un-like the electrochemical deposition method where the lengthof the metal nanowire can be controlled at will, electroless de-position yields structures that run the complete thickness ofthe template membrane. The inside diameter of the tubules

Fig. 5. Schematic of the evaporation/electroplating procedure for formingmetal microtubules, adapted from [29]

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Fig. 6. Schematic illustration of the electroless method forLiMn2O4 tubulearray, adapted from [39]

can be controlled by varying the deposition time. Of coursethe outside diameter is determined by the dimensionality ofthe pores in the matrix.

Figure 6 shows a scheme of this method applied fora LiMn2O4 tubule array [39]. First, a porous alumina mem-brane was “plugged” withPtnanoposts (Fig. 6a). An aqueoussolution of Li and Mn nitrates was then applied to fill thepores of the template (Fig. 6b). The membrane was heated at500◦C for 5 h, resulting in formation of tubules ofLiMn2O4within the pores of the template (Fig. 6c). Finally, the aluminamembrane was dissolved usingNaOH solution yielding thetubular arrays of lithium-manganese-oxide (Fig. 6d).

Electroless silver was deposited as shown in Fig. 7 [29].A strip of Scotch-brand tape was attached to the smooth faceof a polycarbonate membrane (Fig. 7a) which was then ac-tivated with a tin (II) chloride solution (Fig. 7b). The tapewas removed (Fig. 7c) and theSn2+ activated membrane wasplaced in a freshly prepared electroless silver plating solu-tion. This causedAg to deposit on the activated pore walls(Fig. 7d) yielding silver nanostructures.

Another approach to ‘chemically’ synthesize nanorods isshown schematically in Fig. 8 [81, 82]. Here, formerly ob-tained carbon nanotubes are ‘consumed’ in the reaction withvolatile metal halide or oxide species to form solid carbidenanorods with diameters between 2 and30 nmand lengths up20µm.

As a matter of fact, carbon tubules have been recentlycommonly used as templates in the synthesis of nanorods andnanotubes [75, 90–95]. Various materials were introduced tothe internal nanotube cavities by a variety of methods. Cook

Fig. 7. Schematic of the electroless deposition for forming metal tubules,adapted from [29]

Fig. 8. Reaction scheme used to prepare carbide nanorods, adaptedfrom [82]

et al. [90] reviewed the arc-evaporation of metal-doped car-bon rods for a number of transition metals and lanthanides.With the exception of cobalt and copper, the encapsulatedmaterials are always transition metals or lanthanide carbides.The arc-evaporation of manganese compounds yielded thenanotubes filled with various manganese carbides [91]. De-moncy et al. [96] showed that the presence of sulfur in cata-lytic quantity is crucial for the production by this techniqueof abundant metal (Cr, Ni, Yb, Dy) containing encapsulatednanowires.

In addition to this in situ method, carbon nanotubes canalso be filled using a one-step chemical method by boilinga sample inHNO3 in the presence of metal nitrates [90, 92].A more versatile two-step method uses pre-opened nanotubeswhich can be filled using standard wet-chemistry techniques.Green et al. [90, 93] used this method to encapsulate a widevariety of inorganic compounds, includingRhCl3, AuCl3,PdCl2, SnO2, pure metals (Fe, Ni), and oxides (Sm2O3).

As was first observed for molten lead and bismuth, thenanotubes can also be filled simply by capillary action [94,95]. Thus, they act as moulds for the fabrication of metalicwires, some of which can be less than2 nm in diameter [95].

2.3 Chemical polymerization

Different conductive polymers (see Fig. 1) can be template-synthesized by the polymerization of the correspondingmonomer to yield tubular nanostructures [27, 31, 33, 35, 38,41, 63, 70, 87–89]. The process can be accomplished by sim-ply immersing the membrane into a solution containing the

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Fig. 9. Scheme of methods for synthesis of enzyme-loaded nanocapsulearrays, adapted from [31]

desired monomer and a polymerization reagent. The polymerpreferentially nucleates and grows on the pore walls, result-ing in tubules at short deposition times and fibers at longtimes. For electronically insulating plastics (i.e., polyacry-lonitrile, PAN) the nanostructures can be further thermallyprocessed to create conducting graphitic carbon tubules andfibrils [32, 97].

Figure 9 shows an interesting example of synthesis of theenzyme-loaded nanocapsule arrays. The surface of the poly-carbonate template membrane is first sputtered with a thinlayer of gold (Fig. 9a) which is followed by the electropoly-merization a polypyrrole film to yield short “plugs” within thepores (Fig. 9b). Polypyrrole tubules are then chemically poly-merized within the pores of the plugged template (Fig. 9c).The capsules are then filled with the desired enzyme (Fig. 9d).Torrseal epoxy is applied to the upper surface of the mem-brane (Fig. 9e), and finally the template is dissolved to yieldthe array of enzyme-loaded nanocapsules (Fig. 9f).

The organic-template approach to prepare nanoporous sil-ica is shown in Fig. 10. Here, the supramolecular templat-ing was used to synthesize inorganic nanomaterials. There isa direct correlation of the templating surfactant array size andshape to the final morphology in the silica mesophase.

2.4 Sol-gel deposition

Sol-gel synthesis within the pores of templates can be con-ducted to create both tubules and fibrils of a variety of ma-terials [32, 34, 38, 74]. The process typically involves prep-aration of a solution of a precursor molecule to obtain firsta suspension of colloid particles (the sol) and then a gelcomposed of aggregated sol particles is thermally treated toyield the desired nanostructure within the pores of the tem-

Fig. 10. Schematic of the organic-template approach to prepare nanoporousamorphous silica, adapted from [70]

Fig. 11. Synthetic protocol for the synthesis of alkyldithiolate-linked Aucolloids, adapted from [38]

plate. Figure 11 presents a scheme for the synthesis of well-organized 1-DAu colloids. TheAg was electroplated insidethe pores of the membrane and the pore side of theAg filmwas derivatized with 1,6-hexanedithiol. The membrane wassubsequently immersed in a solution containingAu colloids.They diffuse down the pore and bind to the free end of thedithiol at the base of the pore. Any unbound colloid was re-moved and the cycle is repeated by alternately soaking themembrane in solutions of the dithiol andAu. Silver coatingand the template were finally dissolved yielding the ‘super-nanostructure’ in which the number of colloids is controlledby the number of immersion cycles.

2.5 Chemical vapor deposition (CVD)

Chemical vapor deposition has long been applied in the com-mercial production of solid thin films. The technique en-tails surface solidification of desired reactants resulting fromtheir gas-phase chemical transformations. The method hassuccessfully been developed for the template synthesis of

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carbon nanotubules [32, 36, 66]. The starting reactants, i.e.ethylene and pyrene [37, 59], acetylene [61, 62, 65], tripropyl-amine [63], methane [60], propylene [58], or 2-amino-4,6-dichloro-s-triazine [66], were thermally activated and decom-posed to solid carbon within porous templates while travers-ing the length of the pore. Thermal decomposition of the gasoccurs throughout the pores, resulting in the deposition ofcarbon films along the length of the pore walls and forma-tion of carbon tubules within the pores. The thickness of the

Fig. 12. Schematic process for the synthesis of oriented carbon nanotubeson porous silicon by catalyst patterning and CVD of ethylene, adaptedfrom [59]

Fig. 13. Schematic drawing of the formation process of carbon tubes bytemplate CVD of propylene, adapted from [58]

walls of tubules is dependent on total reaction time and pre-cursor pressure. The method offers control over length (upto 100µm) and fairly uniform diameters in nanometer range,as well as efficient-producing carbon tubules. Figures 12and 13 illustrate the formation process of carbon tubes by thetemplate thermal decomposition of ethylene (at700◦C) andpropylene (at800◦C), respectively.

The CVD technique can also be utilized to template-synthesize other nanostructures such asAu/TiS2 compos-ites [32, 98].

3 Template materials used

Most of the studies in template synthesis, to date, have en-tailed the use of two types of nanoporous materials, ‘track-etch’ polymeric membranes and porous alumina or silicamembranes. However, there are a variety of other, both nat-ural and synthetic, materials that could be utilized as tem-plates. Formerly synthesized nanostructures can also be usedas templates.

3.1 “Track-etch” membranes

The track-etch method entails bombarding a nonporous sheetof the desired material (standard thickness range from 6to 20µm) with nuclear fission fragments to create damagetracks in the material, and then chemically etching thesetracks into pores [32]. The resulting porous materials containrandomly distributed nanochannels of uniform diameter (assmall as10 nm). Pore densities approaching109 pores cm−2

can be obtained. Such membranes are also often called nu-clear track filters or screen membranes. These commerciallyavailable (Nuclepore, Poretics, Cyclopore, Osmonics, andMillipore) filtration membranes are usually prepared frompolycarbonate or polyester [29–32,38, 43, 99, 100]. Othermaterials (for example mica [101]) are also amenable to thetrack-etch process [32, 102]. The structure of track-etchedmembranes can be modified by a high-frequency dischargeplasma in air [103].

3.2 Anodic aluminum oxide (AAO)

Alumina membranes with uniform and parallel porous struc-ture are obtained by anodic oxidation of aluminum metal insolutions of sulfuric, oxalic, or phosphoric acid [27–34, 36–38, 44, 104]. Dimensions of the pores are tunable in the rangeof 4 to several hundred nanometers, which render it an idealtemplate material for creating arrays of cylindrical nanostruc-tures. Unlike the track-etch membranes, the pores in AAOtemplates have little or no tilt with respect to the surfacenormal resulting in an isolating, non-connecting pore struc-ture [30–32]. Pore densities as high as1011 pores cm−2 canbe obtained, and typical membrane thickness can range from10 to 100µm [30–32, 105]. Li et al. [106] reported recentlyon the growth of highly oriented pores in anodized aluminumoxide.

The AAO membranes have a porosity of40% to65% [29],and are commercially available (Whatman Anapore, AnotechSeparations) [28, 37, 38, 99].

373

Organized nanostructures can be also grown on poroussilicon obtained by electrochemical etching ofSi wafers [59].The resulting porous structure has a thin nanoporous layer(with pore diameters of3 nm) on top of macroporouslayer [107].

3.3 Other nanoporous materials

Various other synthetic and natural nanoporous materialshave been used in the template synthesis. Mesoporous sil-ica were prepared by a sol-gel process from tetraethoxysilane(TEOS) hydrolysis followed by the calcination of a softmaterial called an alcogel [61, 62]. Silica aerogels havea high porosity (90% or higher) and high surface area(800–1000 m2/g), an ideal matrix for the chemical vapor in-filtration of a desired material on an extremely fine scale. Thezeolite-related aluminophosphates VPI-5 [81] andAlPO4-5(AFI) [63], and aluminosilicates MCM-42 [40] and zeo-lite Y [41] were used as microporous inorganic host ma-terials. The intracrystalline space of layered minerals wasalso employed for in situ template formation of nanocom-posites. Thus, Bandosz et al. [89] used the synthetic min-eral, taeniolite TL-0. The lithium form of this mineral hasthe following chemical formula:Li(Mg2Li)(Si4)O10F2×nH2O. Carbon nanostructures were also prepared by usinga two-dimensional opening between the lamellae of mont-morillonite (MONT) [87, 88]. Modification of the porousstructure of alumina ceramics by chemical vapor deposition(CVD) of nanoscaledSiC andSi3N4 led to a new family ofcomposites [108].

Nanotechnological syntheses could also rely upon self-assembling strategies using natural scaffolds as templates forthe construction of synthetic nanostructures. DNA is partic-ulary well suited for such applications due to its structuralregularity, its ability to reversibly assemble through hydro-gen bonding, and the relative ease in obtaining materials ofprecise length within the nanometer regime [45, 86].

Nanoscale templating was also carried out to produce1- and 2-dimensional elongated carbon nanostructures usingAu/Ni (110) [84] and layered cobalt [66] as patternedcatalysts. The interchain separation of obtained nanostruc-tures was dictated by the surface modification of adsorptiontemplates.

Kong et al. [60] described a strategy for making high-quality individual single-walled carbon nanotubes on siliconwafers patterned withµm-scale islands of catalytic mate-rial (Fe). The control of external dimensionality of NTs wasalso achieved by the diameter (about30–80 nm) of the roundalloy particles prepared by annealingFeNi islands depositedon the silica substrate during the catalytic decomposition ofacetylene [65].

A template-based approach to prepare controlled poros-ity materials was demonstrated by Raman et al. [70] in thesynthesis of organic template-derived amorphous silicas. Ra-man’s scheme was inspired by the chemical specificity inher-ent to biological systems observed earlier by Dickey [109].Dickey’s 1949 publication appears to be, in fact, the firstdocumented demostration of molecular “imprinting” or “tem-plating” to control pore size and shape.

The inner dimensionality of produced structures was alsocontrolled usingTa tubes as templates to synthesizeTaCandgraphite microtubules [110].

3.4 Nanostructures as templates

Using carbon nanotubes as templates, various ceramic oxide(silica, vanadium pentoxide, molybdenum oxide, zirconia,and yttria-stabilized zirconia) nanotubes were prepared [111].Multi-walled carbon nanotubes are usually obtained bythe arc-discharge method [73–75] or from metal-catalyzeddecomposition of ethylene in the presence of hydrogen [76,77, 81, 82]. The carbon template can be later removed byheating the coated or filled nanotubes in air at elevated tem-peratures [111]. Thus, the nanoscale rods or tubules withtypical diameters of between 2 and30 nm and lengths ofup to 20µm are separated [81, 82]. The carbon nanotubeacted as a removable template in the synthesis of silicon andboron nitride nanorods [76]. In the course of preparingSi3N4nanorods the silicon/silica mixture was covered with carbonnanotubes and the endothermal reaction

3SiO(g)+3C(s)+2N2(g) = Si3N4(s)+3CO(g)

was carried out at1673 K in Al2O3 crucible under flowingnitrogen. The total transformation of carbon nanotubes intosilicon nitride nanorods (diameter between4–40 nm) wasobserved.

The Pt nanorods and nanoparticles were also preparedusing carbon nanotubes formerly obtained by a templatecarbonization technique and an anodic aluminum oxidefilm [71].

Produced by pulsed laser vaporization of graphite, SWC-NTs contained self-assembled chains ofC60 molecules, re-sembling a nanoscopic peapod [85].

4 Examples of fundamental and applied studies

The reason for research interest in nanowires and nanotubulesof various materials is manifold.

Efforts to create electronic functions and devices based onnanostructures (having superior electronic properties) insteadof bulk materials are inspired by the anticipated enormous in-crease in computing speed and storage density. Thus, the con-ductivity of polymer fibrils, filaments, and tubules has beenextensively investigated [27, 35, 39–41]. The nanoscopic fib-rils of heterocyclic polymers show electronic conductivities(along the fibril axis) that are substantially higher than con-ductivities of bulk films of the analogous polymer [112]. Alsothe template-synthesized polyacetylene fibrils show enhancedsupermolecular order and higher electronic conductivity [27].Wu and Bein [40] demonstrated that the polyaniline fila-ments have significant conductivity while encapsulated innanometer channels as measured by microwave absorptionat 2.6 GHz.

For polypyrrole fibrils the conductivity increases with de-creasing diameter: values higher than one order of magnitudelarger than the bulk value were deduced for nanostructureswith diameters of around30 nm[113].

Intrazeolite polyacrylonitrile can be pyrolyzed to formconducting material consisting of nanometer-size filaments;these systems are promising candidates for low-field conduc-tivity at nanometer-scale dimensions [41].

The investigation of the galvanostatic charge-dischargecharacteristics of the polypyrrole-coatedLiMn2O4 nan-

374

otubule electrodes showed their several times higher ca-pacities than the polypyrrole-coatedLiMn2O4 thin-filmelectrode [39].

Nanometals have interesting optical, electronic, electro-chemical, and magnetic properties [31]. The colloidal sus-pensions of gold differ in color depending on the size ofthe sphericalAu particles. This results from shape-inducedchanges in the plasmon resonance band of theAu nanopar-ticle which corresponds to the wavelength of light that in-duces the largest electric field on the nanoparticles. Thus, theAl2O3 templates with imbedded gold nanocylinders may finddecorative and protective applications [31, 32, 44]. TheAunanodisk electrodes prepared by using the template methodoffer in fundamental and applied electrochemistry the oppor-tunities to study the kinetics of charge-transfer processes thatare too fast to measure at conventional macroscopic elec-trodes [114]. The ultratrace levels of electroactive speciescan be detected with ensembles of nanoscopic electrodes [31]and electrochemistry can be performed in highly resistivemedia [115].

Quantum phenomena are observed in the electron trans-port of thin metal wires when the wire diameter becomescomparable or smaller than some scaling lengths [116–118].Spin-polarized transport leads to interesting phenomena innanostructured materials composed of magnetic transitionmetals [99]. Among these, the so-called giant magnetoresis-tance (GMR) effect discovered in 1988 [119], has attracteda great deal of interest recently. The study of magnetic orderand reversal processes on nanostructured magnets may cre-ate the possibility of synthesis of new devices and high-density data storage materials [99]. Membranes filled withCo, Ni, or Feare magnetic nanocomposites that have a strongperpendicular magnetic anisotropy suitable for perpendicularrecording [120]. Arrays of template-synthesized ferromag-netic Ni andCo nanowires display the preferred magnetiza-tion direction perpendicular to the film plane with enhancedcoercivities as high as 680 oersteds and remnant magnetiza-tion up to90% [43].

DNA-templated synthesis of conducting silver nanowiresallows for the construction of functional circuits since thedifficulties of achieving inter-element wiring and electri-cal interfacing to macroscopic electrodes may be over-come [45]. DNA-templating routes lead to hybrid nanoarchi-tectures in size regimes inaccessible by traditional syntheticmethods [86].

The superconducting properties of lead nanowires exhibitsome differences compared to those of large specimens: theexternal critical field parallel to the nanowire axis is aboutone order of magnitude larger for the sample with diameter120 nmthan for the bulk [99].

Carbon nanotubes represent a very distinctive group ofnanostructures owing to remarkable properties suggestinga range of new technologies. Discussing the likely utilityof carbon nanotubes, nanotube researcher Alex Zettl of theUniversity of California, Berkeley, said: “If I were to writedown all the different applications, I’d have a book for nano-tubes.” [121]. CNTs can be used as catalysts and catalystsupports, selective adsorption agents, composite materials inreinforcement applications, in energy storage [122]. Poly-merized nanotube structures may form new zeolites [123],CNTs can be transferred to diamond by laser irradiation [124]or applied for electrochemical capacitors [125]. They are

well suited for use as nm-sized probe tips in applicationssuch as atomic force microscopy (AFM) [126] and scanningprobe microscopy [127] or “the world’s smallest gas cylin-ders” [128] for storing gases (hydrogen!) [129]. HyperionCatalysis International is already manufacturing MWCNTsin bulk (300 kgevery day) for use in electrically conductingplastics [121].

CNTs constitue a new class of materials with fascinatingelectronical applications. They behave as ideal 1D “quantumwires” [130] and “quantum resistors” [131]. A high-intensityelectron gun based on field emission from a film of alignedcarbon nanotubes has been made [132]. Cathode ray tubes (aslighting elements) equipped with field emitters composed ofMWCNTs have been already manufactured [133].

However, the conventional techniques to produce car-bon nanotubes yield unsorted, web-like products. Obviously,for both fundamental studies and applications the alignmentof the carbon nanotubes is particularly important. Althoughsome progress in this respect has been made already [134–137], the template-based approach to synthesize aligned andtailored carbon nanotubes, as shown above [33, 36, 37, 58, 60,62, 65, 66], seems to be the best technique.

There are several other specific potential applications ofvarious template-synthesized nanostructures. The polycar-bonate template membranes were loaded with enzymes (glu-cose oxidase, catalase, subtilisin, trypsin, and alcohol dehy-drogenase have been tested to date) or drugs to make a newtype of enzymatic bioreactor or in drug delivery [31]. 1DAu-nanoparticle arrays linked by organic conductive polymersand alkyldithiolates may be useful in optimizing electromag-netic enhancements in surface-enhanced Raman spectroscopy(SERS) and in developing functional nanoscale electronic cir-cuit elements [38, 137, 138]. Monodisperse gold nanotubulesdeposited in pores of polymeric membranes, with insidediameter of molecular dimension (less than1 nm), were usedin a simple membrane-permeation test to cleanly separatesmall molecules on the basis of molecular size [139]. Sucha “molecular filter” was used to separate pyridine (molecularweight 79) and quinine (molecular weight 324).

The MoS2 nanoclusters and tubules are known to bea useful catalyst for hydrodesulfurization, an electrode inhigh-energy-density batteries, and an intercalation host toform new materials [140]. The organization ofC60 in 1Dchannels of the microporous zeolites leads to a nanocom-posite which exhibits novel interesting optoelectronic prop-erties [83]. Template-derived amorphous silicas with tailor-made pore sizes and shapes are important in many appli-cations, such as shape-selective catalysis, molecular sieving,chemical sensing, and selective adsorption [70]. Carbide [81,82] and silicon nitride [76] nanorods may find technologicalapplications in solid-state physics and nanostructured com-posite materials.GaN nanorods have promising applicationsfor blue and ultraviolet opto-electronic devices (blue light-emitting diodes) [77].

In addition to carbon nanotubes other new forms of car-bon (for example highly oriented nanographite) are expectedto have various interesting physical properties such as highconductivity, optical anisotropy, and high radiation resistance,and, therefore, can be applied as a new material in the fields ofadvanced technology [87–89].

Semiconductor (TiO2, MnO2, Co3O4, ZnO, WO3, andSiO2) tubules and fibrils were synthesized by a template tech-

375

nique [32, 34].TiO2 in particular is an excellent photocata-lyst for the decomposition of organic pollutants [141]. Therate of decomposition of salicylic acid on an array of immo-bilized TiO2 nanofibers was an order of magnitude higherthan for the thin-filmTiO2 catalyst [32, 34], mostly due tothe increased surface area and corresponding increase of thedecomposition reaction rates.

5 Summary

The general methodology described here allows the designand synthesis of a wide range of nanostructures with specifiedgeometry and surface characteristics, and with a wide rangeof potential applications. For example, such tailored nanoma-terials could be used in inclusion chemistry and electrochem-istry, materials and biomedical sciences, electron microscopy,molecular storage and separation technology, novel opti-cal and electronic devices. The presented approach, termedtemplate-synthesis, entails using the pores and channels inthe nanoporous “template” structures for forming the desirednanomaterials.

Acknowledgements.The work was supported by the Committee for Sci-entific Research (KBN) through the Department of Chemistry, WarsawUniwersity, under Grant No. 3 T09A 058 16.

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