Electrophoretic deposition of carbon nanotube–ceramic nanocomposites

Abstract

The purpose of this paper is to present an up-to-date comprehensive overview of current research progress in the development of carbon nanotube (CNT)–ceramic nanocomposites by electrophoretic deposition (EPD). Micron-sized and nanoscale ceramic particles have been combined with CNTs, both multiwalled and single-walled, using EPD for a variety of functional, structural and biomedical applications. Systems reviewed include SiO₂/CNT, TiO₂/CNT, MnO₂/CNT, Fe₃O₄/CNT, hydroxyapatite (HA)/CNT and bioactive glass/CNT. EPD has been shown to be a very convenient method to manipulate and arrange CNTs from well dispersed suspensions onto conductive substrates. CNT–ceramic composite layers of thickness in the range <1–50 μm have been produced. Sequential EPD of layered nanocomposites as well as electrophoretic co-deposition from diphasic suspensions have been investigated. A critical step for the success of EPD is the prior functionalization of CNTs, usually by their treatment in acid solutions, in order to create functional groups on CNT surfaces so that they can be dispersed uniformly in solvents, for example water or organic media. The preparation and characterisation of stable CNT and CNT/ceramic particle suspensions as well as relevant EPD mechanisms are discussed. Key processing stages, including functionalization of CNTs, tailoring zeta potential of CNTs and ceramic particles in suspension as well as specific EPD parameters, such as deposition voltage and time, are discussed in terms of their influence on the quality of the developed CNT/ceramic nanocomposites. The analysis of the literature confirms that EPD is the technique of choice for the development of complex CNT–ceramic nanocomposite layers and coatings of high structural homogeneity and reproducible properties. Potential and realised applications of the resulting CNT–ceramic composite coatings are highlighted, including fuel cell and supercapacitor electrodes, field emission devices, bioelectrodes, photocatalytic films, sensors as well as a wide range of functional, structural and bioactive coatings.

Introduction

Electrophoretic deposition (EPD) is a well-known colloidal ceramic processing method¹ which is gaining increasing interest as a simple and versatile technique for the production of coatings and films from nanoparticles and carbon nanotubes.² The technique allows the fabrication of coatings, thin and thick films, the shaping of bulk ceramic objects and the infiltration of porous substrates with ceramic particles.1, 2, 3 Comprehensive reviews on the application of EPD in ceramic technology are available.3, 4, 5 EPD is achieved via the motion of charged particles, dispersed in a suitable solvent or aqueous solution, towards an electrode under an applied electric field. Electrophoretic motion of charged particles during EPD results in the accumulation of particles and the formation of a homogeneous and rigid deposit on the relevant electrode. The success of EPD is based on its high versatility which facilitates its use with different materials and combinations of materials. In addition, EPD is a rapid, cost-effective method which requires simple equipment enabling material layers (thin and thick films) to be made in only seconds or minutes. Moreover EPD has a high potential for scaling up to large product volumes and sizes, as well as to a variety of component shapes and complex structures.4, 5 EPD is also considered one of the processing methods of widest application potential in the field of nanomaterials.²

Nanoparticles and other nanoscaled materials such as carbon nanotubes (CNTs) are starting materials for the synthesis of a variety of advanced (nano)structures, including structural and functional coatings, thick and thin films, bioactive materials as well as laminated and functionally graded materials of high microstructural homogeneity. CNTs attract enormous attention due to their extraordinary properties caused by their unique structure, aspect ratio and size.6, 7 The exploitation of these features in a variety of applications, from microelectronics and field emission devices to structural composites and biomedical materials, constitutes a wide and expanding research field.8, 9 In fact many of the remarkable properties of CNTs are now well established10, 11, 12 and current major efforts are devoted to the exploitation of these properties in specific applications.12, 13 In this context, one of the challenges is to tackle the problem of manipulating CNTs, individually or collectively, to produce the particular CNT arrangement needed in each application. Moreover, if the interest is to combine CNTs with other materials to form composites, it is essential to develop processing methods that enable homogeneous dispersion of the CNTs in the appropriate matrices. In particular for the combination with inorganic matrices to form CNT–ceramic or CNT–glass composites, the intrinsic difficult processability of CNTs due to their tendency to agglomerate makes extremely complicated their integration and dispersion into ceramic or glass matrices, this being still a demanding challenge for technologists, as reviewed elsewhere.¹⁴

EPD has been shown to be a very convenient technique for manipulating individual CNTs in liquid suspensions with the aim to produce ordered CNT arrays. A comprehensive overview of the field of EPD of CNTs has been published,¹⁵ where the preparation and characterisation of stable CNT suspensions and the mechanism of EPD of CNTs were discussed. Recent research has investigated the EPD of CNTs for a variety of specific applications such as field emission devices, supercapacitors and photocatalytic coatings, which confirm the excellent capability of EPD to manipulate, arrange and orientate multiwalled and single-walled CNTs.16, 17, 18 In this context, the high aspect ratio and surface charge of functionalized CNTs used for EPD make them also suitable scaffolds or hosts for other nanoparticles via adsorption or nucleation at the acidic sites. For example, metallic and ceramic nanoparticles19, 20, 21 have been homogeneously deposited on the surface of oxidized CNTs; including catalytic²² and rare earth particles.²³ Moreover, stable diphasic suspensions of CNTs and nanoparticles have been shown to be suitable precursors for production of advanced CNT-based ceramic composites.24, 25, 26

The increasing volume of research dealing with the application of EPD to produce carbon nanotube–ceramic composites, as discussed at a recent international conference,²⁷ has motivated the preparation of this review paper. This paper is thus the first review covering comprehensively research work carried out worldwide in the field of EPD of carbon nanotube–ceramic (and carbon nanotube–glass) composites highlighting the different systems developed in the short time since the first results in the field (on SiO₂–CNT composites) were published.²⁸ The experimental characteristics of relevance, such as functionalization of CNTs, design of suspensions for EPD (both single and diphasic suspensions) as well as the specific EPD parameters, such as deposition time, electric field and electrode materials, are discussed for different systems investigated, addressing also the effects of processing conditions on the final quality of CNT–ceramic nanocomposites. The paper highlights also potential applications of the resulting CNT–ceramic nanomaterials produced by EPD.