Fabrication and Characterization of Tin Oxide and Silicon Based Nanostructures for Gas Sensor and Li-ion Battery

Abstract

Nanomaterials have been attracted due to their peculiar and fascinating properties, and applications superior to bulk materials. Various synthesis techniques for nanomaterials have been investigated and it is proven that the nanomaterials show the enhanced physical and chemical properties in various fields such as catalyst, sensors, and energy application. In this research, the three main topics will be discussed: 1) synthesis of SnO2 based hollow sphere and their electrochemical properties, 2) synthesis of one-dimensional SnO2 nanostructures and control growth, 3) preparation of silicon nanostructure by magnesiothermic reduction for Li-ion battery application. In the first chapter, design and synthesis of zero-dimensional SnO2 based nanostructures for anode material of Li ion battery is studied. The demand for high energy and power density lithium ion batteries (LIBs) has increased in hybrid electric vehicles (HEVs) as well as light-weight and portable electronic devices. In commercial LIBs, graphite-based materials are widely used as an anode, but the theoretical capacity is only 372 mAhg. Therefore, intensive researches have been focused on the high capacity electrode materials such as Si, Ge, Sn, and SnO2. Among various candidates, SnO2 is one of the promising materials for anode electrode in LIBs due to its high theoretical capacity (782 mAh g-1). However, a large and uneven volume change, about 300%, occurs upon lithium insertion/extraction, which causes a pulverization and electrical connectivity loss. Hollow sphere is the most promising structure because the interior hollow space can accommodate the volume change during lithiation and delithiation. So, the electrochemical properties of SnO2-based hollow structures have been intensively investigated, and these structures showed an enhanced cycling performance compared to the solid SnO2 nanoparticles. However, most of the studies have focused on the fabrication of unique nanostructures, and their size was limited to 100~200 nm. While the size of hollow spheres appears to be a crucial factor, the synthesis of size-controlled hollow spheres and the size dependence of the electrochemical properties have not been explored. In this study, the size-controlled SnO2 hollow sphere is synthesized by simple sol-gel process at relatively low temperature and the SnO2 hollow sphere electrodes showed the size dependent electrochemical properties, and the smallest SnO2 (25 nm) hollow sphere exhibited a high reversible capacity of 750 mAh/g as well as excellent cyclability. Further improvement of reversible capacity is achieved by adding a Co3O4 on the SnO2 hollow sphere. The SnO2@Co3O4 hollow sphere electrode shows a high reversible capacity of 963 mAh/g after 100 cycles and good rate capability. In the second chapter, synthesis and control growth of one-dimensional SnO2 nanostructures are investigated. Metal oxide nanostructured sensors are the most promising devices among the solid state chemical sensors, because they have many advantages such as a large surface to volume ratio, a Debye length comparable to dimension of nanostructure, and low power consumption. SnO2 is widely applied to semiconductor gas sensor, as well as anode for Li-ion battery. Especially, one-dimensional SnO2 nanostructures, such as nanotube and nanowire, have very thin wall-thickness or diameter so it is favorable to detect a low concentration of gas. A novel method is developed to fabricate a SnO2 nanotube network by utilizing electrospinning and atomic layer deposition (ALD), and the network sensor is proven to exhibit excellent sensitivity to ethanol owing to its hollow, nanostructured character. The electrospun polyacrylonitrile (PAN) nanofibers of 100–200 nm diameter are used as a template after stabilization at 250 oC. An uniform and conformal SnO2 coating on the nanofiber template is achieved by ALD using dibutyltindiacetate (DBTDA) as the Sn source at 100 oC and the wall thickness is precisely controlled by adjusting the number of ALD cycles. The calcination at 700 oC transforms the amorphous nanofibers into SnO2 nanotubes composed of several nanometer-sized crystallites. The SnO2 nanotube network sensor responds to ethanol, H2, CO, NH3 and NO2 gases, but it exhibited an extremely high gas response to ethanol with a short response time (<5 s). Furthermore, single crystalline SnO2 nanowire has been intensively investigated, as well. However, growth control of nanowire is important for fabrication of reliable device and it required that understating the growth behavior of material. However, the growth behavior of SnO2 nanowire is not well-investigated. Generally, aligned nanowire could be induce by understanding of (hetero)-epitaxial relationship between nanowire and substrate. In this study, SnO2 nanowires were synthesized using a thermal evaporation and the epitaxial grown SnO2 nanowires were well-aligned on the r-cut sapphire substrate. Two-types of growth mode, vertical and horizontal growth, are observed on the r-cut sapphire. At first, the vertically aligned SnO2 nanowire shows three growth directions with specific growth angle and it is investigated that this growth behavior is resulted from rutile-tetragonal structure of SnO2. And horizontal growth is induced by control the distance of metal catalyst. The horizontally grown SnO2 nanowire is self-aligned along the one direction and the width of laterally grown nanowire was well-controlled by catalyst size. Most of all, silicon based systems are definitely attractive candidates since silicon has a large theoretical specific capacity at room temperature (Li15Si4 : 3600 mAh g-1) and low operating voltage (around 0.1 V vs. Li/Li+). However, large volume change is occurred during the cycling and it leads to a poor rate capability. To overcome these problems, silicon nanostructures have been applied to anode materials of Li-ion battery and the enhanced electrochemical properties is achieved. But, synthesis of nanostructured silicon is required complicated route with toxic precursor or high cost technique, such as CVD (chemical vapor deposition). Recently, magnesiothermic reduction is suggested as new method for synthesis of pure silicon from SiO2. This is a low energy consumption process due to low processing temperature and short-term heat treatment. In this study, a synthesis of silicon nanosheet is presented by magnesiothermic reduction and commercial sand is used as template and silicon source. As currently-known, the reduced Si is composed of a few nanometers sized silicon grain but the synthesized silicon nanosheet has large sized silicon grain, at least 20 nm. The synthesized silicon nanosheet show relatively batter electrochemical performance than commercial nano-silicon powder. And the reversible capacity and cyclability is improved by applying the graphene wrapping on the silicon nanosheet. But the formation of large sized silicon grain is not explained by current reduction mechanism. So, the reduction mechanism is investigated, additionally. It is revealed that Mg2Si is formed as intermediate phase in the initial stage of reduction at 450 ~ 500 oC, and silicon is generated by consumption of Mg2Si. Furthermore, inverse opal liked silicon nanostructure is prepared on the concept of newly suggested reduction mechanism and the formation mechanism of inverse opal structure is investigated.