Functional Nanomaterials Produced By Ball-milling

Nanomaterials and, among them, elongated morphologies such as nanowires, nanotubes, nanoflowers and nanorods attract a dramatic deal of attention in the current materials research. They are capable of extending functionalities of modern devices and are considered to have exciting applications in a variety of fields including sensors, light and electron emitting devices, energy conversion and storage, intelligent switches and self-cleaning materials. The ability to produce large quantities of nanomaterials is needed for testing their properties for a broad scope of applications and for the commercialization of emerging technologies. The development of efficient synthesis methods capable of mass production of nanomaterials is becoming crucial.Ball milling is one of the effective techniques for producing powders with nanocrystalline structures. Ball-milling experiments are usually conducted in cylindrical containers called vials and containing balls. The vials are occasionally equipped with gas inlets in order to vary the grinding atmosphere (Ar, O2, H2, N2or vacuum), and the grinding is done by placing the containers on various types of grinders, vibratory or planetary to obtain powders with highly controlled physical characteristics. The repeated grindings (e.g., welding and fracturing of powders) enable to successively create the defects and new interfaces, therefore leading to a decrease of crystallite size, a greater increase of the surface area of powders, the changes in chemical reactivity and volatility of the materials, and highly homogeneous mixing of components. The obtained powders having the special properties are generally very different from those obtained via other treatment as far as their structure and texture are concerned.In this thesis the applicability of ball milling to the efficient production of nanomaterials is assessed. Several key technological materials, SnO2, ilmenite, TiO2and composits of MoO3/C, are firstly chosen and reported as model systems.1) Ball milling of SnO2is found to increase the evaporation ability of the milled material. The effect is not related to the changes in the surface area but attributed to the structural changes in the material. SnO2nanowires are grown by evaporation of the milled powder. High-resolution transmission electron microscopy (HRTEM) showed that the SnO2nanowires are single crystals in the (101) growth direction. Tree-like SnO2nanowires can be produced via a controlled vapour-solid process during annealing of the ball milled powder. The formation of this unusual morphology is attributed to the presence of stacking faults in nanowires and a high vapor supersaturation. The photoluminescence spectrum of the tree-like nanowires is dominated by a strong emission band centered at548nm.2) Ball milling of ilmenite with a subsequent alkali solution treatment leads to a mass production of ilmenite nanoflowers. Each nanoflower has the size of several micrometers and is composed of thin petals with thickness of5-20nm and width of100-200nm. The morphology, structure and growth mechanism of the FeTiO3architectures are discussed, and the formation of these nanostructures are attributed to a dissolution-precipitation mechanism involving an intermediate sodium-containing phase. Electrochemical properties of the obtained FeTiO3nanostructures are evaluated in aqueous electrolytes, and the nanostructures are found to exhibit pseudocapacitance. This property is pronounced in1M KOH and3M KCl aqueous electrolytes. More specifically, the capacitance of120F/g is measured in1M KOH aqueous electrolyte at the current rate of500mA/g, and approximately50F/g is retained at5A/g. The material has good long term cycling stability. According to our data, FeTiO3nanostructures show functionality as an electrode material for supercapacitors.3) The large quantities of FeTiO3nanostructures produced by ball milling and subsequent alkali leaching can be converted to nanorods of TiO2by simple hydrochloric acid leaching techniques. The average external width, rod lengths, and thickness achieved are5-20nm,50-100nm, and2-5nm, respectively. They grow in the [110] direction. The formation of these nanorods is related to hydrolysis of the dissolved titanium, and the subsequent precipitation of TiO2nanorods can lead to the formation of porous structure. The nanorods obtained after8h of acid leaching have a large specific surface area (96.6m2/g) with a small pore (2-20nm) structure, which exhibit a better photocatalytic activity than that of a commercial rutile TiO2powder and close to of P25in the photodegradation of oxalic acid.4) A method for obtaining porous TiO2from natural ilmenite has been proposed. The method includes four steps:ball milling of a mixture of ilmenite and activated carbon, annealing of the ball milled mixture at1000℃in an inert atmosphere, acid leaching and calcining of the final product in air. The electron microscopy study has demonstrated that the product of this method consists of aggregates of porous nanoparticles. The corresponding pore size distribution is bimodal where the typical range of small (3-20nm) pores corresponds to holes within nanoparticles, and the typical range centered at50-80nm is due to the spaces between nanoparticles in aggregates. We believe that the aggregates of TiO2nanoparticles form via an attack of acid on iron formed by the reduction of FeTiO3into TiO2and Fe, and the development of small mesopores is relevant to the acid leaching of iron directly from ilmenite. The contribution of two types of pores can be adjusted by controlling the time of carbothermal reduction. In addition, it has been demonstrated that the obtained porous TiO2is more active than a commercial rutile powder in the photocatalytic degradation of phenol.5) A MoO3-carbon nanocomposite can be synthesized from a mixture of MoO3and graphite by a controlled ball milling procedure. The as-prepared product consists of nanosized MoO3particles (2-180nm) homogeneously distributed in carbon matrix. The nanocomposite acts as a high capacity anode material for lithium-ion batteries and exhibits good cyclic behavior. Its initial capacity exceeds the theoretical capacity of745mAh/g in a mixture of MoO3and graphite (1:1by weight), and the stable capacity of700mAh/g (94%of the theoretical capacity) is still retained after120cycles. The attractive electrode performance is linked with the unique nanoarchitecture of the composite and is compared with the performance of MoO3-based anode materials reported in the literature previously (nanoparticles, ball milled powders, and carbon-coated nanobelts). The high value of capacity and good cyclic stability of MoO3-carbon nanocomposite are attractive in respect to those of the reported MoO3electrodes.