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Metal Oxide Nanocomposites: State-of-the-Art and New Challenges

Visakh P.M.1* and B. Raneesh2

1Department of Physical Electronics, TUSUR University, Tomsk, Russia

2Department of Physics, Catholicate College, Pathanamthitta, Kerala, India

Abstract

This first chapter discusses about the metal oxide nanocomposites: synthesis and applications with many different topics such as introduction to nanocomposites, graphene-based metal, metal oxide nanocomposites, carbon nanotube-metal oxide nanocomposites, metal oxide-based nanocomposites application towards photocatalysis, metal oxide nanomaterials for sensor applications, metal oxide nanocomposites and its thermal property analysis, semiconducting metal oxides for photocatalytic and gas sensing applications, and other potential applications of metal oxide-based nanocomposites.

Keywords: Metal oxide nanocomposites, carbon nanotube, graphene, photocatalysis, gas sensor

1.1 Introduction to Nanocomposites

The composite materials composed of polymer matrix are easily processible and readily available and therefore widely used in various industries. In composite materials, the fiber acts as load carrier with its strength being greatest along the axis of the fiber. The long continuous fibers aligned in the direction of the load leads to the formation of composite with much enhanced properties than the pure matrix material. Research studies reveal that the spider's web fibers are much stronger than man-made processed fibers. In ancient civilizations across the world, husks or straws mixed with clay have been widely utilized to build houses that last longer for several hundred years [1]. The matrix material in composites serves two important purposes: (a) binding the reinforcement phases in place and (b) uniformly distributing the stresses among the constituent reinforcement materials in the event of an applied force. The matrix offer weight advantages and ease of handling. The inorganic materials, polymers and metals can be utilized as matrix materials in the designing of structural composites [2]. Thermoplastics resins are generally used as molding compounds. The fibers are randomly dispersed in thermoplastics, and so the reinforcement is isotropic but directionality can be achieved using molding processes [3]. Thermosets are retained in a partially cured condition over prolonged periods of time to induce flexibility in them [4]. Generally, the condition of fiber material (chopped, aspect ratio) in epoxy, polymer and phenolic polyamide resins decides the final application of thermosets. Most of the metals and alloys can be used as matrices, however, they often require compatible reinforcement materials which are stable over a range of temperature and also non-reactive [5].

The addition of reinforcements in ceramic overcomes the problems related with high modulus of elasticity and low tensile strain to obtain strength improvement. The addition of reinforcements in adequate amount causes the ceramics to effectively transfer quantum of load to the reinforcement thereby reducing the chances of ceramics rupture at high stress levels. The carbon-carbon composite can be synthesized using compaction of carbon or multiple impregnations of porous frames with liquid carbonizer precursors and subsequent pyrolization or through chemical vapor deposition of pyrolytic carbon [6]. In a 2-D composite, the strength remains only one-third to the strength of a unidirectional fiber-stressed in the direction of fibers. But, in a 3-dimension, less than one-fifth of the strength is obtained. The fiber composites can be either continuous or short fibers. It is generally observed that the continuous fibers exhibit better orientation in matrix. The major proportion (>95%) in reinforced plastics are glass fibers. They are inexpensive, have low density, resistant to chemicals, insulation capacity, easy to process with high strength/stiffness than the plastics with which they are reinforced [7]. However, they are more prone to breakage when subjected to high tensile stress for a long time. Metal fibers when amalgamated with refractory ceramics improve performance by improving their thermal shock and impact resistance properties. The resulting composites possess high strength, light weight and good fatigue resistance.

The properties of boron fibers depend upon their diameter due to the changing ratio of boron to tungsten and the associated surface defects that change according to size. The boron fibers are known for their remarkable stiffness and strength [8]. The uncoated boron-tungsten fibers do not react with molten aluminum and also withstand high temperatures for utilization in hot-press titanium matrices. However, silicon carbide-tungsten fibers are dense and prone to surface damage and require careful, delicate handling, during fabrication of the composite [9]. Quartz fibers can withstand high temperatures, while silica cannot [10]. Quartz fibers are highly elastic and can be stretched to 1% of their length before break point. Laminar composites comprises of layers of materials bonded together and can exists in as many combinations as the number of materials. In laminar composites, several layers of two or more metal materials can occur alternately or in a definite order, and in as many numbers as required for a specific purpose. Both clad and sandwich laminates follow the rule of mixtures from the modulus and strength point of view [11]. Flakes composites have densely packed structures. Metal flakes in polymer matrices can conduct electricity or heat, whereas, mica and glass flakes can resist both. Flakes are much cheaper than fibers. More often, the flakes fall short of expectations while controlling the size, shape and hence produce defects in the end product. The infiltrate can be independent of the matrix which binds the components like powders or fibers, or they could just be used to fill voids [12]. The matrix is not naturally formed in the honeycomb structure, but specifically designed to a predetermined shape.

Reinforcement can be of the square, triangular and round shapes, and the dimensions of all their sides are more or less equal [13]. The dispersion size in particulate composites is in microns range whereas volume concentration is greater than 28%. Their potential properties are based on the relative volumes of the metal and ceramic constituents [14]. Cermets are produced by impregnating the porous ceramic structure with a metallic matrix binder. Cermets can also be used as coating in a powder form where the powder is sprayed through a gas flame and fused to a base material. In a polymer composite, either the constituent matrix material or the fiber is a polymer. The polymer matrix composites (PMCs) compose of a polymer resin as the matrix material and fibers as the reinforcement medium [15]. The techniques to produce carbon fibers are relatively complex. Rayon, polyacrylonitrile (PAN), and pitch are used as organic precursor materials for producing carbon fibers. The processing techniques for composites are different than those for metals processing because composite materials involve two or more different materials [16]. Substantial changes in technology and its requirement in the past three to four decades have created many new needs and opportunities, which has fostered the need of advanced materials in associated manufacturing technology.

1.2 Graphene-Based Metal and Metal Oxide Nanocomposites

Carbon materials exist in all dimensionalities including zero-dimensional (0D) i.e. fullerenes, quantum dots, one-dimensional (1D) carbon nanotubes i.e. CNTs, two-dimensional (2D) i.e. graphene and three-dimensional (3D) i.e. graphite. Graphene is the appellation given to a two-dimensional sheet of sp2-hybridized carbon atoms with exceptionally high crystallinity and electronic property [17]. Lately, the nomenclature ''graphene'' was acclaimed by the commission of IUPAC as a substitute to the older name ''graphite layers'', for the reason that graphite is three-dimensionally (3D) stacked carbon structure. It has arose as a speedily growing wonder material in the field of material science due to its thinnest and the sturdiest structure [18]. In early 2004, it gained high significance after the studies presented by Geim's group, who demonstrated the graphene sheets and stated their unparalleled electronic properties. Later, in 2010, Physics Nobel Prize for pioneering research highlighting the two-dimensional material graphene presented by the Royal Swedish Academy to pioneers namely, Andre Geim and Konstantin Novoselov [19]. The progress in research till date, on graphene is mainly focused on the chemical and physical route of synthesis of pristine graphene, its chemical modification, detailed characterization of its chemical and physical properties and functions, synthesis and characterization of graphene-based polymer composites and metal-oxide nanocomposites, aiming to exemplify the impression of graphene-based nanomaterials on the development of novel analytical developments and its applications. Despite the fact, Geim and coworkers also employed the use of mechanical exfoliation which led to many stimulating investigations on graphene and its electronic and mechanical properties [20]. Recently, a facile and green synthetic stratergy for the large scale production of graphene by means of ball-milling of graphite flakes with carbohydrates namely sucrose, Producing graphite through epitaxial growth under the effect of ultrahigh vacuum (UHV) annealing conditions of SiC surface has attracted many researchers and technologists for...