One dimensional nanomaterials : synthesis, characterization and property studies

Abstract

One dimensional nanomaterials have specific properties compared to their high dimensional forms. Carbon nanotubes (CNTs) are one of the most famous one dimensional (1D) nanomaterials with unique chemical and physical properties. The synthesis, characterization and properties of two typical one dimensional material, boron nitride nanotubes (BNNTs) and titanium oxide (TiO{u2082}) nanorods were specifically studied in this thesis. BNNTs have similar nanostructure to CNTs, which can be seen as a hexagonal boron nitride (h-BN) graphene sheet appropriately rolled into a cylinder with a nanometer sized diameter. BNNTs present significant advantages over CNTs for the applications at high temperature in air due to their stronger resistance to oxidation. BNNTs also have different optical properties from CNTs because h-BN has a potential to be a far UV light emitting device. One dimensional TiO{u2082} nonmaterials including nanowires, nanotubes, nanorods and nanobelts have been widely studied recently. Research on the specific physical and chemical properties of ID TiO{u2082} could help to deal with the energy shortage and climate change. These materials have been used in many fields such as in cosmetics as a pigment, sunscreen paints, ointments and toothpaste. Since the early twentieth century, TiO{u2082} materials have also led to many promising applications in areas ranging from photovoltaics and photocatalysis to photo-/electrochromics and as sensors. The synthesis methods and properties of two typical ID nanomaterials, BNNTs and TiO{u2082}, will be introduced in Chapter I. In Chapter II, a few techniques used to synthesize, characterize and investigate properties on both BNNTs and TiO{u2082} nanorods are demonstrated. These include high energy ball milling technique, milling devices and applications, as well as major characterization techniques used in this thesis including Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Powder Diffraction (XRD), X-ray Energy Dispersive Spectrometry (EDS), Fourier Transform Infrared Spectroscopy (FTIR), Ultraviolet-visible Spectroscopy (UV-vis), Thermogravimetric Analysis (TGA) and Vacuum Ultraviolet (VUV) synchrotron light source. Chapter III details a systematic study on electronic structures, luminescence emission and excitation of high-yield and large-quantity BNNTs with different sizes and structures by means of photoluminescence emission (PL) and excitation (PLE) spectroscopy using VUV synchrotron radiation source. Red shifts were measured from the nanotubes compared to h-BN powders and strong excitonic emission was observed from the thin nanotubes (2.5 nm and 5 nrn BNNTs). In addition, the near band edge (deep UV band) features of BNNTs were carefully explored, which revealed that the near band edge features of thick and thin BNNTs are different. Based on the optical absorption of thin BNNTs, it was concluded that the Frenkel-type excitons with large binding energy might dramatically change the band structures of thin BNNTs. The temperature effect on excitation and emission spectra of BNNTs was systematically investigated from 20 to 300 K using a high energy synchrotron radiation and is detailed in Chapter IV. It was found that the thin 2.5 nm BNNT band gap is red-shifted compared to h-BN and thick BNNTs. More importantly, their photoluminescence is temperature dependent, which is different from h-BN and large diameter BNNTs. A featured shoulder peak at ~5.5 eV in the PLE spectra of 2.5 nm thin BNNTs was found, which might be resulted from the strong coupling of electronic transition to thin BNNT lattice vibrations, a case of Frenkel-type excitons. The excitons of thin BNNTs are strongly bound and localized, and self-trapping might occur in the case of thin BNNTs with external diameters around 2.5 nm. Chapter V demonstrates that ammonium oleate surfactants can help the dispersion of multi-walled boron nitride nanotubes (BNNTs) in water to form a BNNT solution which is stable for up to two months due to the non-covalent functionalizations of nanotube surfaces. FTIR and PL analysis with synchrotron radiation source revealed that this BNNT aqueous solution preserves intrinsic optical properties of BNNTs. Chapter VI details a new method for mass production of TiO{u2082} nanorods from mineral ilmenite sands (FeTiO{u2082}). In this process, a mixture of ilmenite and activated carbon was first ball milled; the milled samples were then heated twice at two different temperatures. High-temperature annealing produced metastable titanium oxide phases and subsequent low-temperature annealing in N{u2082}-5%H{u2082} activated the growth of rutile nanorods. This solid state growth process allows large scale production of rutile nanorods. Growth mechanisms of TiO{u2082} nanorods synthesized from mineral ilmenite using ball milling and annealing method are systematically illustrated in Chapter VII. Two annealing processes are used to grow the nanorods. The heating rate and the gas environment in the first annealing step are critical to the formation of intermediate phases; these and the annealing atmosphere in the second heating play very important roles in nanorod formation. One dimensional growth of nanorods induced by low temperature annealing in nitrogen plus hydrogen is possibly driven by atom vacancy diffusion in addition to surface diffusion In Chapter VIII, the optical properties of obtained TiO{u2082} nanorods were investigated. The measurements of optical absorption, photoluminescence and Roman vibration on the ID TiO{u2082} rutile nanorods were conducted. It was found that the E{u0261} mode of produced rutile nanorods had a blue shift, which was caused by the defects formed along (001) direction of rutile nanorod nanocrystal. The threshold of optical absorption of nanorods had a blue shift, and the increasing band energy levels of emission features were attributed to direct transitions in an otherwise indirect band gap TiO{u2082} semiconductor and additional emissions at lower energies must necessarily originate with transitions from intragap energy levels implicating lattice and/or surface defects in the nanorod crystals. Show more