Synthesis And Characterization Of One-dimensional Si Nanomaterials And Their Applications As Negative Electrodes In Lithium Ion Batteries

One-dimensional silicon-based nanoscale materials have attracted great interest in the last few years owing to the excellent compatibility with the present silicon technology, and the superior and unique properties of quantum confinement effect and small size effects. Silicon nanowires (SiNWs) and silicon nanotubes (SiNTs) with various morphologies were successfully fabricated by using chemical vapor deposition method. The morphologies, structures and compositions of these nanomaterials were investigated in detail. Furthermore, the optical properties of SiNWs and SiNTs were analyzed though Raman, PL (Photoluminescence) and CL (Cathodoluminescence) spectrum. The application of SiNWs as anode materials in lithium ion batteries was also studied.The main results are summarized as follows:1. Different one-dimensional silicon nanomaterials, i.e., SiNWs and SiNTs, were synthesized by chemical vapor deposition of SiH₄ at 480℃through controlling the flow rate of SiH₄ at 10 sccm and 5 sccm. By increasing the growth temperature from 400℃to 600℃, the transition from gold-encapsulated, bamboo-like SiNTs of smaller diameters (400℃) to bamboo-like SiNTs (480℃), and eventually to completely hollow SiNTs of larger diameters (600℃) was observed. For the samples grown at 400℃, the nanostructures have diameters of around 50～70 nm. As the growth temperature increases, the diameters of the products increase to 70～100 nm at 480℃and 80～150nm at 600℃. All the three samples are amorphous, while the morphology are different from each other.â‘ The products obtained at 480℃are bamboo-like SiNTs, which are composed of a series of periodic dome-shaped hollow compartments. In addition, we found that the encapsulated catalytic particles are located at the tips of the SiNTs, and the nanotubes have a wave-shaped outer surface.â‘¡Compared to the bamboo-like SiNTs with dome-shaped interior grown at 480℃, the products grown at 400℃reveal that gold nanoparticles are encapsulated along the bamboo-like SiNTs, which we called gold-encapsulated SiNTs with dome-shaped interior.â‘¢The hollow interiors along the tubular structures synthesized at 600℃tend to connect with each other, and the products are inclined to form completely hollow SiNTs. No crystal lattice stripes were observed in the HRTEM (High resolution Transmission Electron Microscopy) images, suggesting that all the three samples are amorphous, which were further confirmed by selected area electron diffraction.2. A modified vapor-liquid-solid (VLS) mechanism was proposed to explain the formation of different morphologies of SiNTs. When the liquid droplet becomes supersaturated, silicon growth begins at some nucleation site. At this time, the radius of curvature of one side on the droplet becomes smaller than the other side, which leads to the formation of the additional pressure on the gold surfaces. Thus the liquid droplets are squeezed away by these anisotropic pressures, leaving a dome-shaped (similar to geometry of Au particles) void. If the surface diffusion at 480℃is quick enough to completely wrap the liquid droplet before the additional force to reach a critical point, a solid part will be formed between two dome-shape voids and lead to the obtain of bamboo-like SiNTs. Otherwise, defects will be formed between the two adjacent hollow parts and the formation of silicon nanotubes with completely hollow interior is preferred. At lower temperature (e.g., 400℃), the liquid droplet will be elongated, instead of being squeezed away suddenly. Driven by the decrease of the surface free energy, the two parts of the elongated particle will separate suddenly, subject to the formation of two individual gold nanoparticles and compartment with dome-shaped void.3. Photoluminescence spectroscopy (PL) and cathodoluminescence spectroscopy and imaging (CL) was used to investigate the optical properties of SiNWs and SiNTs. Two major broad bands were found around 455 and 500 nm for SiNTs, and 455 and 530 nm for SiNWs, respectively; however, the PL intensity of SiNTs was much higher than that of SiNWs. As temperature decreased, both the PL intensity of SiNWs and SiNTs increased gradually. When the temperature was lower than 200 K, these materials appeared two blue emission bands which were attributed to the excess Si atoms in Si nanostructures. The CL spectrum of bamboo-like SiNTs, which is the same as that of SiNWs, has two major bands 470 and 630 nm.4. The SiNTs were observed at about 420 and 520 cm^-1 by Raman scattering measurements which were attributed to amorphous SiNTs and excess Si atom in the nanostructures, respectively. Raman spectroscopy of crystalline SiNWs showed a downshift and asymmetric broadening of the Raman first order TO (First-order transverse optical phonon mode, 1TO) phonon peak when compared with the bulk mode, and a shoulder peak at 494 cm^-1 was also observed. As the decrease of the laser power, the shoulder peak became clear, while did not shift with the changes of the laser power. The 1TO phonon peak shifted from 508 cm^-1 to lower wave number as the decreasing laser power.5. SiNWs electrodes were fabricated by two methods. Method I is to spread as-synthesized SiNWs slurry on current collector; Method II is to directly synthesize SiNWs on a substrate of stainless steel. The electrodes prepared by Method I have shown capacity fading and short battery lifetime, while the other one has a stable capacity over many cycles. The SiNWs prepared by Method II also displayed high capacities at higher currents. In addition, the cyclability of the SiNWs at the faster rates was also excellent. X-ray diffraction (XRD) patterns of SiNWs electrodes (Method II) at open circus potential and various cut-off potentials during the initial discharging process reveal the disappearance of the initial crystalline Si and the start of the formation of amorphous Li_xSi. The structural features of SiNWs during the initial Li insertion process were studied by SEM (Scanning electron microscopy) and TEM (Tranmission electron microscopy) to understand the high capacity and good cyclability of our SiNWs electrodes. SiNWs before electrochemical reaction were crystalline. However, after charging with Li, the SiNWs had roughly textured sidewalls, and the average diameter increased. Despite the large volume change, the SiNWs remained intact and did not break into smaller particles which are closely related to the 1D structure. Facile strain relaxation in the SiNWs allows them to increase in diameter and length without breaking. Furthermore, the space between each SiNWs allows for better accommodation of the large volume changes without the initiation of fracture that can occur in bulk or micron-sized materials.