Preparation and modification of carbon nanotubes: Review of recent advances and applications in catalysis and sensing

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Review

Preparation and modification of carbon nanotubes: Review of recent advances and applications in catalysis and sensing Deepa Vairavapandian, Pornnipa Vichchulada, Marcus D. Lay ∗ Department of Chemistry and Nanoscale Science and Engineering Center (NanoSEC), University of Georgia, 30602 Athens, United States

a r t i c l e

i n f o

a b s t r a c t

Article history:

Single-walled carbon nanotubes (SWNTs) have become one of the most intensely stud-

Received 15 May 2008

ied nanostructures because of their unique properties. The inherent physical properties of

Received in revised form

carbon nanotubes make them ideal supports for metal nanoparticles. The use of electrode-

30 July 2008

position to modify SWNTs in order to facilitate applications in areas related to catalysis

Accepted 30 July 2008

and sensing is presented in this manuscript. Preparation of raw SWNT material for electro-

Published on line 13 August 2008

chemical experiments involves various mild or oxidative pretreatments. In this review we focus on progress toward functionalization of SWNTs with metal nanoparticles using elec-

Keywords:

trochemical methods and the applications of metal decorated carbon nanotubes in energy

Single-walled carbon nanotube

related applications.

Sensor

Published by Elsevier B.V.

Aligned networks Electrodeposition Nanoparticle Electrochemistry

Contents 1. 2.

3. 4.

5.

∗

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Purification of SWNT soot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Mild routes to purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Oxidizing routes to purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical properties of carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrodeposition of metal nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Direct electrodeposition of metal nanoparticles on SWNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Modification of electrodes with SWNTs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanoparticle-modified SWNTs in energy applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Corresponding author. Tel.: +1 706 542 1985; fax: +1 706 542 9454. E-mail address: [email protected] (M.D. Lay). 0003-2670/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.aca.2008.07.052

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6.

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5.2. Chemical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Other analytical applications SWNTs modified with metal nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction

A new era of interest within the field of nanotechnology began with the identification of carbon nanotubes (CNTs) in 1991 [1–3]. Iijima reported microtubules of graphitic carbon with concentrically arranged cylinders, today known as multiwalled carbon nanotubes (MWNTs) [4]. In separate reports in 1993, single-walled carbon nanotubes (SWNTs) were reported by Bethune and Iijima [5,6]. The report of this new type of carbon fiber triggered intense research focused on harnessing the unique properties of these nanostructures. SWNTs have many unique physical and chemical characteristics. These nanotubes are the strongest known material [7–16]. SWNTs are 100’s of times stronger than the highest grade high carbon steel commercially available [13,15,17–19]. SWNTs also have a tensile modulus many times higher than steel; they can be stretched over five times their original length with nearly 100% memory and undetectable levels of corresponding structural damage. Furthermore, pristine metallic SWNTs also exhibit enhanced electronic properties, including near ballistic transport [20–26]. Many potential applications have been proposed for carbon nanotubes, including conductive, high-strength composites and paint additives. Recent studies have involved the possible applications of SWNTs as nano test-tubes [27,28], H2 storage media [29,30], batteries [31], field-emission materials [3,29,32–37], tips for scanning probe microscopy [38–44], chromatographic stationary phases [45,46], transistors [47,48], diodes [49], and sensors [1,22,50–62]. SWNTs have interesting physical, morphological and electronic properties and find applications in a variety of areas ranging from biosensors to transistors. The high surface area of SWNTs and the fact that all of the carbon atoms in the SWNTs are surface atoms makes them an ideal candidate as supports for metal and semiconductor nanoparticles, in spite of the low reactivity of sp2 hybridized C atoms. The substrates for supporting metal nanoparticles have a considerable influence on their morphology and properties. Carbon nanotubes functionalized with nanoparticles have shown improved sensor properties and catalytic efficiency. Nanoparticles have chemical, physical and electronic properties that are different from their bulk materials, owing to their small size, and have applications in areas such as sensors, catalysts, etc. [63,64]. Controlling the size and distribution of nanoparticles on a substrate remains one of the main challenges of materials science. The use of carbon nanotubes as conductive supports has received increasing attention recently. Several methods of decorating carbon nanotubes with metal nanoparticles have been proposed [65–74]. Electrodeposition has proven to be a versatile, room temperature method for fabricating metal nanostructures [75–79]. Electrochemical methods offer control of nanoparticle size

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and distribution via electrochemical potential control (and thus the thermodynamics), as well as time a reaction is allowed to occur. This review mainly focuses on the observations and insights of researchers with regard to metal electrodeposition on SWNTs.

2.

Purification of SWNT soot

2.1.

Mild routes to purification

There are three common methods for growing SWNTs: laser ablation [80], electric arc discharge [81,82], and CVD [83–86]. Laser ablation involves the creation of SWNTs during the condensation of laser-vaporized carbon/metal mixtures at ∼1200 ◦ C. For the electric arc discharge method, an electrical arc between carbon cathode and anode electrodes, in the presence of a metal catalyst, provides the energy for SWNT formation. CVD involves the catalytic decomposition of a carbon containing gas at catalytic nanoparticles of a transition metal. Today, variations of the CVD method are most commonly used to form SWNTs [87–93]. Regardless of the growth method, there is always a significant amount of impurities present (graphitic debris, catalyst particles and fullerenes). These impurities often interfere with the desired properties of SWNTs. Therefore, there have been extensive recent investigations into methods of removal of impurities and several highly effective purification techniques: flocculation [94], microfiltration [95,96], chromatographic procedures [97–99], and centrifugation [100,101]. Multiple centrifugation cycles have also proven to be an effective method of purifying SWNT soot, without shortening or oxidizing the SWNTs [102]. Two-dimensional networks formed from SWNTs that were purified by this method showed the electrical response typical of undamaged SWNTs in electrical devices. The low solubility of SWNTs is another major challenge to mass production of SWNT-based devices. This insolubility is due in large part to strong Van der Waals interactions between nanotubes, which results in aggregation. Therefore, after purification, the solubility of SWNT material is improved by use of various surfactants. This method preserves the structure and properties of SWNTs to a much greater extent than covalent modification. Surfactants that show promise in this process include DNA [103,104], polymers [105], and various detergents [106]. Sodium dodecyl sulfate (SDS) is one of the most common surfactants used to form aqueous suspensions of SWNTS. SDS coats SWNTs with micelles, forming a hydrophobic core and hydrophilic surface, thus assisting in homogenization in water. After the addition of the surfactant, sonication (bath or cup-horn) is used to disperse SWNTs in the solution. The micelles can then be removed in subsequent rinsing steps.

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2.2.

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Oxidizing routes to purification

Harsh methods of purification involve oxidation with a strong oxidizing acid, hydrothermal treatment along with extraction and oxidation [107], and annealing at high temperature in the presence of O2 . Many studies show combinations of these protocols are often required for effectively purifying and increasing the solubility of SWNTs from as-produced (AP) SWNT material [11,35,108–112]. In most cases, oxidative purification protocols have the side effect of increasing aqueous solubility due to formation of sidewall defects. Excessive defect formation may limit the use of SWNTs in many applications; introduction of a single defect along the sidewall of a pristine metallic SWNT was found to increase its electrical resistance and causes semiconductive behavior [113]. Acid treatment typically involves the use of a strong oxidizing agent [107]. SWNTs have been refluxed with HNO3 , H2 SO4 , HCl or mixtures of these acids [107,109,110]. Many groups have varied many parameters for this treatment such as duration, concentration, and repeated cycles. It is known that the use of these processes can cause defects and/or shortening of SWNTs. Oxidation usually results in the formation of carboxylate groups on the surface of the tube. Fig. 1 shows the effect of purification on liquid deposited SWNTs. 5 g of SWNT material (Carbolex) was refluxed for 2 h in 5 M HNO3 . The solution was then cooled down to ambient. SWNTs were then filtered and washed with copious amounts of nanopure water (n-H2 O). Next, the filtrate was placed in a drying oven overnight at 70 ◦ C. 1 mg mL−1 of purified SWNTs was sonicated in SDS solution for 30 min. The solution was then centrifuged and decanted multiple times. This process resulted in removal of a significant amount of impurities as determined by AFM micrographs of drop-cast SWNTs.

3. Electrochemical properties of carbon nanotubes Nanoelectrodes offer various interesting properties that can be put to use in a number of electrochemical, biological, chemical applications [114,115]. The fact that carbon nanotubes are nanoscaled conductors makes them an interesting system to study in order to obtain a better understanding of the effect of the various types of diffusion in an electrochemical system. Therefore, understanding the basic electrochemical properties of SWNTs is the key to making improved electrodes for a wide variety of applications. The diameter of SWNTs can range from 0.4 to 2 nm. Studies have shown that SWNTs grown using most methods are comprised of a mixture of two-thirds semiconducting and one-third metallic tubes [20,116–118]. Despite this difference in conductivity, electrochemical studies have shown that semiconducting and metallic SWNTs exhibit similar electrochemical behavior [119]. The electrochemical behavior of graphite yields some insight into SWNTs. In the case of graphite, electrode kinetics are faster at the edge plane than at the basal plane [120]. Electrochemical pretreatment of glassy carbon electrodes pro-

Fig. 1 – Effect of purification of a suspension of SWNT soot on deposits: (a) deposit formed from a suspension of SWNTs that were purified in nitric acid prior to deposition, (b) SWNTs deposited from unprocessed solutions contain many impurities. The carbon nanotubes were aligned by a directional laminar-flow deposition drying procedure.

duces various functional groups such as quinoid, carbonyl and carboxylate sites [121,122]. Other researchers have observed a similar behavior for SWNTs [123]. In SWNTs, such functional groups serve as axial ligands for metal deposition. Further, the presence of oxygenated functionalities at the ends of the SWNTs facilitates electron transfer. Pretreatments done for either purification, or to introduce oxygen functionalities, play a significant role in determining the electrochemical activity of SWNTs. The ends of the SWNTs are more prone to oxidation than the sidewalls due to the presence of greater bond strain and defects. Therefore, oxidatively treated carbon nanotubes have two distinct regions that contribute to their electrochemical behavior: the walls of the nanotubes that are comparable to the basal planes of graphite (largely un-reactive) and the ends are that are comparable to the edge planes of graphite (highly reactive). Both the regions have distinct electrochemical properties due to the difference in chemical bonds present; the defect sites at the ends of SWNTs provide electrochemically active sites for redox reactions.

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Li et al. recently reported an electrochemical study of SWNT paper [124]. They concluded that the pretreatment and packing density of SWNTs define the performance of these electrodes. The Fe(CN)6 3− /Fe(CN)6 4− couple was used to demonstrate that the redox reaction takes place not only at the outer surface of SWNTs but also at the interior of the nanotubes. Hence, they behave as three-dimensional (3D) electrodes. The same group compared two different densities of MWNT arrays [125]. The high-density arrays (∼3 × 109 SWNTs cm−2 ) behaved like a macro-scale electrode, because of the overlapping of diffusion fields between SWNTs, and displayed peaks typical of diffusion at a conventional macroscopic electrode. However, for lower-density samples (∼1 × 108 SWNTs cm−2 ), the electrodes displayed behavior indicative of a combination of radial and laminar diffusion. However, the CV for low-density network also indicated the presence of diffusion limited behavior, as the network exhibited a mixture of micro- and macro-electrode properties. In contrast, ultra low-density (∼10 SWNTs ␮m−2 ) SWNT networks prepared in this group indicate a complete lack of diffusion limited behavior, as radial diffusion dominates (Fig. 2). This is the case with these ultra low-density SWNT networks because at such low densities, the diffusion layer of each individual SWNT does not overlap with its nearest neighbor to a significant extent, and true radial diffusion dominates. Information about the types of diffusion that occur at electrode interfaces is of great importance; mass transport limitations need to be minimized and/or eliminated for applications such as catalysis, sensing and electrodeposition. In the case of catalysis in particular, enhanced diffusion leads to greatly increased reaction rates. Therefore, further studies into the effect of such low-density arrays on diffusion rates are currently underway in this group.

4.

Electrodeposition of metal nanoparticles

4.1. Direct electrodeposition of metal nanoparticles on SWNTs Electrodeposition offers many advantages over hightemperature metal deposition for metal nanoparticle formation on SWNTs. One of the most significant advantages of electrochemical deposition is the ability to control size and distribution of nanoparticles by varying potential, time or solution concentration. Most studies involving metal nanoparticle electrodeposition focus on noble metals such as Ag, Au, Pt, and Pd [126–132], with a few exceptions Ni, Cu [71,133,134], primarily due to the need for components of alternate energy sources. Table 1 shows a list of metal nanoparticles prepared on SWNTs by electrochemical methods. Electrodeposition of metal nanoparticles on carbon nanotubes depends on various parameters, such as pretreatments, method of manufacturing for SWNTs, type of SWNTs, distance of the nanotubes from contact electrode, density of SWNTs in network, etc. As mentioned in the previous section, oxygen functionalities serve as axial ligands for metal nanoparticle precursors to bind to the SWNTs. Therefore, the most common pretreatment methods involve treating them with strong acids or

Fig. 2 – Comparison of CVs obtained for the Fe2+ /Fe3+ redox couple with a macroscopic Au electrode and an ultra low-density SWNT network electrode: (a) the macroscopic gold electrode shows peaks indicative of diffusion limited behavior, (b) the nanoelectrode array shows no contribution from linear diffusion over a wide potential range, indicating that mass transport limitations are avoided.

oxidizing agents such as H2 SO4 /HNO3 , H2 SO4 /H2 O2 , HNO3 , O3 , and KMnO4 . This is essentially a controlled method of damaging the tubes [66,69,74]. An alternative pretreatment method, involving a more gentle electrochemical oxidation, was studied by Guo and Li [123,127]. Oxide functional groups at defect sites on the ends and sidewalls of SWNTs were produced by cycling electrochemical potential in 0.5 M sodium sulfate, following a similar

Table 1 – List of metal nanoparticles prepared on CNTs by electrochemical methods Metal Ag Au Pt Pd Ni Cu

References [135,138,143] [133] [66,69,130,131,133,134,138,139] [132-134,140,142] [136,137,141] [137]

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Fig. 3 – Tapping mode AFM image of Pd electrodeposited on SWNTs from 0.2 mM (NH4)2 PdCl4 + 100 mM KCl, En = −1.5 V vs. Ag/AgCl, deposition time 2.5 s. The tubes in the image are located far from the Ti contact wire perimeter (>10 ␮m) and they are electrically connected to the Ti wire through two or three nanotubes that were sufficiently long. All tubes were equally plated showing there is no significant potential loss at nanotube–nanotube junction [129].

procedure used for activating glassy carbon electrodes [122]. They employed a three-step process to deposit Pd and Pt nanoparticles on the SWNTs: (1) pretreatment by potential cycling, (2) formation of octahedral complexes of Pt(IV) or Pd(IV) and (3) conversion of surface complexes to metal atoms by additional potential cycling. The density of Pt nanoparticles was higher on the thicker bundles than on thinner bundles because of the presence of a higher number of carboxylic acid groups on thicker bundles. Dekker and coworkers have studied the electrodeposition of Au, Pt and Pd on SWNT nanoelectrode arrays deposited on non-conductive supports [129]. Electrical contact to the SWNT network was made through vapor deposited Ti contacts. The average nanoparticle size could be controlled by varying the applied potential and metal salt concentration. Uniform sized nanoparticles were obtained at sufficiently negative potentials. The results also illustrated that even when the SWNTs were far from the Ti contact electrode; they were plated equally well, showing that the nanotube–nanotube junction was not a source of significant potential loss (Fig. 3). However, subsequent reports by other researchers have indicated that there is some loss of potential, due to inter-nanotube junctions, for larger networks [135]. Day et al. have investigated the electrodeposition of Ag and Pt on high-density SWNT networks [135], forming either dispersed nanoparticles, or continuous metal nanowires on these networks, depending on the distance from contact electrode. The driving potential for electrodeposition decreased with increased distance from the metal contact electrode. Therefore, the rate of nanoparticle nucleation and growth increased at close proximity to the contact electrode, resulting in continuous nanowires, rather than separate nanoparticles, near the contact electrode. Fig. 4 shows representative scanning electron microscopy (SEM) images that were obtained for Ag deposition on high-density SWNT networks.

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Day et al. have also investigated the parameters controlling nanoparticle density, size and distribution for the case of Pd and Pt deposition on SWNT networks [130]. Pd and Pt were deposited at various potentials and it was determined that deposition potential and time are important factors in determining nanoparticle size and density. One interesting observation they made was that when Pd deposition was done at very low deposition overpotentials, nucleation took place preferentially at defect sites on SWNTs. However, when higher deposition overpotentials were applied, nanoparticle deposition at pristine regions of the SWNTs was also initiated. Chen et al. proposed changing the size of Pt nanoparticles by changing the viscosity of the electrolyte and adjusting the number of potential pulses used during deposition [136]. Aqueous solutions of H2 PtCl6 were used as electrolyte and the viscosity of the solution was varied by adding different quantities of glycerol. Tuning the viscosity was proposed as a means to control the growth of Pt nanoclusters due to controlling the diffusion of Pt(IV) ions. The shape of a nanoparticle is significant because catalytic and sensing properties depend on the arrangement of surface atoms. Studies to control the shape of the Pd nanoparticles electrodeposited on SWNTs have indicated that smooth and flat facets of Pd nanocubes were electrodeposited on SWNTs on porous anodic alumina templates at sufficiently lower current densities 1.0 mA cm−2 [137]. Arai et al. have reported electrodeposition of Cu and Ni on carbon nanotubes [133,134]. For the case of Ni deposition, they demonstrated selective deposition of Ni on the ends and defect sites of MWNTs. This has been attributed to the high electrical conductivity of MWNTs in the axial direction and easy electron transfer in defect sites (Fig. 5). Ni-coated SWNT nanowires were also made by electrodeposition on SWNTs supported on alumina membranes [138]. This work demonstrated the efficacy of the use of MWNTs in formation of the types of nanoscaled electrical contacts necessary for molecular computing applications. In the application of carbon nanotubes as field-effect transistors, reduction of the contact resistance between metal electrodes and SWNTs is crucial. Electroplating of Au on semiconducting SWNTs was found to reduce the contact resistance between semiconducting SWNTs and Pd electrodes on which the nanotubes were fabricated [139].

4.2.

Modification of electrodes with SWNTs

The examples in the previous section involved direct electrodeposition on bare SWNTs. Yet, SWNTs have also shown great potential in the area of modifying well-known macroscopic electrodes. Qu et al. fabricated a hybrid thin film containing Pt nanoparticles and [tetrakis(N-methylpyridyl) porphyrinato]cobalt (CoTMPyP) modified MWNTs casted onto a glassy carbon electrode [132]. Acid treated MWNTs were cast on a glassy carbon (GC) electrode and immersed in 1 mM CoTMPyP(ClO4 )5 to form GC/MWNTs/CoTMPyP electrode. PtCl6 2− was adsorbed onto this electrode and electrochemically reduced at −0.7 V, to form one layer of MWNTs/CoTMPyP/Pt film on GC electrode. Gao et al. employed a three-step process to electrocrystallize Ag nanoparticles on MWNTs [140]. First an ordered 4-amino benzoic acid (ABA)

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Fig. 4 – FE-SEM images of Ag deposited on SWNTs (from 1 mM AgNO3 in 0.2 M KNO3 : (a) the gold contact electrode on the bottom right, (b–d) Ag nanoparticles on the SWNT network at different distances from contact electrode, (b) very close to contact electrode has more Ag nanoparticles compared to (c) 15 and (d) 30 ␮m from the contact electrode.

monolayer was grafted onto an as-produced MWNT surface by potential cycling and then Ag(NH)3 2+ was adsorbed by immersing in Ag(NH)3 2+ solution. This was followed by potentiostatic reduction to form Ag nanoparticles. They obtained well-dispersed Ag nanoparticles that were anchored to the MWNTs. The electrochemistry of SWNT/polymer composite electrodes has yielded interesting results regarding the catalytic activity of SWNTs. Lim et al. employed electrochemical codeposition to deposit Pd along with glucose oxidase (GOx) onto nafion solubilized MWNTs for making biosensors [141]. Acid purification was done to introduce carboxyl moieties (–COOH) to anchor Pd nanoparticles. At reduction potentials of −0.9 V, Pd–GOx was electrodeposited in the form of roughly spherical nanoparticles sizes ranging from 2 to 4 nm.

SWNTs have also been used to form 3D, high-surface mesoporous composites [142]. Comparison of electrodeposition of Pt on SWNT/nafion composites with that of MWNT/nafion composites showed that higher current was observed for deposition on the SWNT-containing materials because of the higher accessible surface area for SWNTs. Furthermore, the onset potential for Pt reduction was less negative on the SWNT composite and indicated that generation of Pt seeds on SWNTs was easier due to its unique mesoporous structure and high concentration of functional groups on SWNTs. Therefore, these composite materials represent another exciting frontier in SWNT research. This manuscript focuses on electrochemical methods for decorating SWNTs with metal nanoparticles. However, it should be noted that there are numerous other methods of functionalizing SWNTs with metal nanoparticles, including: electron beam evaporation [143], spontaneous deposition [73], covalent coupling after SWNT oxidation [144], microwave irradiation [145], thermal decomposition of metal salts (SWNTs dispersed in water or acetone along with metal salts and heated) [146], laser ablation [147], supercritical fluid technology (in which a metal precursor was dissolved in supercritical carbon dioxide and then reduced by H2 + CO2 to form nanoparticles) [148], gamma irradiation [149], and sonochemical decoration [150].

5. Nanoparticle-modified SWNTs in energy applications 5.1. Fig. 5 – Schematic of the selective deposition of Ni on the MWNTs at defect sites [134].

Catalysis

One of the most significant applications of SWNT/ nanoparticle materials is in the area of catalysis [151,152]. The

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Fig. 6 – Methanol oxidation on Pt supported on OTE/CNT and OTE system. This process is enhanced by the presence of CNTs [153].

nanometer scale dimensions of metal nanoparticles facilitate enhanced diffusion rates and fast electron transfer kinetics; these two properties have the potential to revolutionize the field of catalysis. Therefore, various research groups have done extensive studies on the effect of using carbon nanotube as catalyst. Pt nanoparticles supported on SWNT electrodes have shown improved catalytic properties for both methanol oxidation and oxygen reduction reaction [153]. SWNTs deposited on optically transparent electrodes (OTEs) were used as substrates and Pt nanoparticles were electrodeposited on these SWNTs. Methanol oxidation currents were about 10 times higher when Pt nanoparticles were supported on SWNTs, compared to the same amount of Pt deposited on OTEs (Fig. 6). The current values measured for oxygen reduction reactions were higher by a factor of 1.5 for Pt nanoparticles supported on SWNT compared to unsupported Pt particles (Fig. 7).

Cui et al. studied the electrocatalytic oxygen reduction reaction by Pt nanoparticle electrodeposited on MWNTs [126]. They observed a significant positive shift in oxygen reduction potential and a concurrent increase in oxygen reduction current. Pt deposited on MWNTs also showed enhanced electron transfer rates as well as stability for methanol oxidation [154]. Use of bimetallic catalysts such as Pt-Ru has been commonly accepted in order to improve catalytic efficiency and to protect Pt from CO poisoning. One of the first studies involving electrodeposition of Pt-Ru on SWNTs was done by He et al. [155]. Uniform distributions of catalyst nanoparticles were obtained, with size ranging from 60 to 80 nm, by potentiostatic methods from 0.5 M H2 SO4 aqueous solutions with ruthenium chloride and chloroplatinic acid at −0.25 V. Investigation of the effect of the Pt/Ru ratio on the electrocatalytic activity on methanol oxidation revealed that the maximum current density was obtained at the Pt/Ru ratio of 4:3. The Pt-Ru particles thus showed higher catalytic activity and stability than pure Pt nanoparticles. Therefore, modification of SWNTs with crystalline nanoparticles is a promising route to creation of more efficient catalytic materials. Highly dispersed Pd nanoparticles, prepared by electrodeposition, on SWNT bundles have exhibited high catalytic activity for hydrazine oxidation and indicated that if SWNTs are used as a support, the loading of precious metal catalysts could be minimized [123]. Use of MWNT paper as a Pt catalyst support was investigated by Wang et al. [156]. They concluded that with smaller sized Pt particles (2.5 nm), the performance of fuel cells could be improved. Most of the studies involved the use of random networks, bundles, or SWNT paste electrodes. Aligned networks of SWNTs will enable better control of the density and distribution of nanoparticles. An interesting liquid-phase laminar-flow deposition technique to align carbon nanotubes has been demonstrated by Lay et al. [102,157]. The directional solution drying procedure presently used by this group to make SWNT electrodes for metal NP supports involves depositing SWNT solutions on a prepared surface via nitrogen-propelled drying. SWNTs align parallel to the direction of nitrogen flow as shown in Fig. 1.

5.2.

Fig. 7 – Cyclic voltammetric traces in a 0.5 M H2 SO4 saturated with oxygen: a Pt supported on SWNT shows higher current for oxygen reduction (electrode: 4.3 mg cm−2 SWCNT and 0.04 mg cm−2 Pt). Unsupported Pt with same amount of Pt (0.04 mg cm−2 ) shows lower oxygen reduction current. Scan rate is 20 mV s−1 [153].

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Chemical sensors

One of the most common sensing devices is the chemiresistive type, in which signal transduction occurs through a measured change in resistance upon analyte adsorption or desorption. SWNTs show potential for use in such sensors because of their high sensitivity, small size, high surface area, and high aspect ratio. This enables carbon nanotube sensors to detect smaller concentrations of gas molecules than traditional sensors [51,157–160]. Charge transfer between the gas molecules and semiconducting carbon nanotubes changes the electrical conductance of carbon nanotubes and thus forms the mechanism of chemiresistive sensing. Though SWNTs act as very sensitive chemiresistive sensors, they are not as selective as is necessary for practical sensing applications. Therefore, modifying SWNTs with metal nanoparticles has been investigated in order to achieve greater chemical selectivity (Fig. 8) [161].

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Fig. 8 – Illustration of a carbon nanotube network connecting source and drain electrodes of a field-effect transistor (FET) [166].

There is considerable interest in using hydrogen as an energy source. This presents new demand for enhanced hydrogen sensing capabilities; leak detection is critical for this potentially explosive reactant. SWNTs modified with metal nanoparticles show promising behavior as hydrogen sensors. SWNTs were made more sensitive and selective to hydrogen by deposition of Pd/Pt nanoparticles [143,162–164]. Kong et al. showed the formation of hydrogen sensors from individual semiconducting SWNT modified with Pd nanoparticles [143]. Mubeen et al. have demonstrated fabrication of hydrogen nanosensors by electrodepositing Pd nanoparticles on SWNT networks [162]. Optimized sensors were formed with a baseline resistance of 10 k and a Pd deposition charge of 0.05 C showed excellent properties with a lower detection limit of 100 ppm and a linear response up to 1000 ppm. Low-density networks of SWNTs that were electrochemically modified with Pd showed effective hydrogen sensing for even lower hydrogen concentrations, ∼10 ppm [165]. Fig. 9 shows

Fig. 9 – Sensor response of Pd nanoparticles electrochemically deposited on an SWNT network: the sample was then left in pure nitrogen (4–6 min), followed by purging the cell with dry air (6–10 min) [165].

the sensing behavior of SWNTs that have been modified with Pd nanoparticles. Sensor arrays composed of several functionalized SWNTs were fabricated by site selective electroplating of Pd, Pt, Rh and Au on isolated SWNTs located on a single chip [166]. Electroplated devices were tested for sensitivity to H2 , CO, H2 S, NH3 and NO2 . Sensor response, recorded as change in conductance vs. gate voltage, showed that electrodeposition of different metals on the same device offers reduction in device size and increased number of analyte specific locations; partial least-square analysis of the sensor array output yielded enhanced recognition of the gases. Sun et al. showed the fabrication of flexible, high performance hydrogen sensors by depositing Pd on SWNTs supported on polyethylene terephthalate (PET) substrates [167]. In addition to sensing applications, there is wide interest in the potential use of nanoparticle-loaded SWNTs for H2 storage [168–170]. Hydrogen uptake of up to 2.8% was achieved using Ni nanoparticle-loaded MWNTs [168]. Dag et al. compared hydrogen adsorption on bare SWNTs and Pd or Pt nanoparticle-decorated SWNTs and found that functionalizing SWNTs with transition metals having less than half-filled d-shells have higher efficiency for hydrogen storage [171].

5.3. Other analytical applications SWNTs modified with metal nanoparticles Metal nanoparticle functionalized SWNTs have significant use in biological applications, such as biosensors [172–176]. Male et al. have extensively investigated the use of Cu nanoparticle coated SWNT networks for electrochemical biosensors [174]. They used Cu nanoparticles deposited on SWNTs to modify glassy carbon or Cu electrodes. The presence of these modified SWNT networks increased the sensitivity to glucose by about four-fold compared to that of Cu nanoparticles on glassy carbon or Cu electrodes (this was attributed to an increased electroactive surface area). Hrapovic et al. fabricated sensors with Pt nanoparticle-modified SWNTs that showed improved sensitivity towards hydrogen peroxide [173]. Yang et al. also reported fabrication of an electrochemical enzyme biosensor with Pt nanoparticle-modified MWNTs [175]. Introducing metal nanoparticles onto SWNTs alters their electronic and chemical properties. Hence, control of the size and distribution of nanoparticles provides a route to tuning the electronic properties of SWNTs. Cho et al. showed that decorating SWNTs with transition metal nanoparticles (Co, Ti, Pd, W) significantly altered their electronic work function. Fieldemission experiments indicated that Ti was the most effective coating for enhancing the field-emission properties of SWNTs [177]. Noble metals, like Au have also demonstrated potential for modifying the properties of SWNTs. SWNTs embedded with Au nanoparticles have improved field-emission properties, with a fairly low threshold voltage and high amplification factor [178]. Ag or Au vapor deposited on MWNTs have shown strong optical limiting properties that can be used to make optical limiters to protect sensors from intense laser pulses [179]. And Pt nanoparticle-modified SWNTs have interesting optoelectronic properties which make them one of the promising candidates for electronic applications [180].

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6.

Conclusion

Metal nanoparticles play a significant role in many applications such as sensors, catalysts, field-emitters, etc. As the morphology, density and distribution of the nanoparticles affect their properties, carbon nanotube templates make superior metal nanoparticle supports because of their unique geometry. Electrodeposition method offers simple and versatile methods of decorating SWNTs with metal nanoparticles. Control of size, distribution and density is achievable using electrochemical methods. Understanding the fundamental electrochemical behavior of SWNTS forms the first step in making these nanostructures. The combination of electrodeposition and these nanoscaled ideal electrodes will be an area of great interest in the future.

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