Synthesis of Particulates for Functional Applications: Examples of Nanocomposites and Hematite

Open Access
Author:
Wang, Jun
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 01, 2004
Committee Members:
  • James Hansell Adair, Committee Chair
  • William Blaine White, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Erwin A Vogler, Committee Member
Keywords:
  • Nanocomposite
  • Nanoparticle
  • Optical properties
  • Synthesis
  • Dispersion
  • Hematite
Abstract:
The size and shape dependence of optical properties for particulate materials were investigated in this thesis research. Specific materials of interests include a-Fe2O3 pigments, Ag/SiO2 core-shell structured nanocomposites and tabular SiO2 nanoparticles. The main focus was on the processing parameters related to the particulate morphologies, dispersion and optical properties of each model system and optimization of those conditions so that a general prediction for the optical behavior of the particulate materials was achievable. Iron oxide (hematite) particles with various shapes (platelet, polyhedron, pseudocube and peanut-like) were synthesized by hydrothermal treatment of a Fe(OH)xOy precursor under various conditions. The size and shape of hematite particles could be adjusted by carefully controlling the processing parameters such as holding time, temperature and adsorption ions present in the system. The nearly monosized a-Fe2O3 platelets possessed face diameters of approximately 3 um and a thickness of 0.5 um under scanning electron microscope (SEM). The apparent color of the particles changed as particle size and shape varied. Munsell color notation was employed to compare the color of hematite particles with various size and shape. Diffuse reflectance spectra showed that a ¡°red-shift¡± of 40 nm was observed in platelet, pseudocube and peanut-like particles compared to conventional particles. The band at 850 nm for the 6A1 to 4T1 transition was split in the pseudocubic and peanut-like particles. Raman spectra of the hematite particles also revealed that the vibrational modes of a-Fe2O3 particles diminished as particle size decreased, and dependence of vibrational band intensity to frequency was also observed. The spectral profiles demonstrated significant difference as excitation lines changes from blue (457 nm) to red (647 nm). Unexpectedly, a weak near-infrared luminescence was observed for fine-particle a-Fe2O3 when excited by 1064 nm radiation from a Nd-YAG laser. There were three broad luminescence peaks at 7994, 7225 and 6686 cm-1, with intensities about ten times that of the Raman scattering. The relative intensities varied with particle morphology and with heat treatment. Overall band intensities increased by about a factor of ten for Fe2O3 that has been heated to 500 Celsius and then decreased again for material that has been heated to 1000 Celsius. The emission was assigned to the 4T1g to 6A1g crystal field transition of octahedral Fe3+. A large Stokes shift is associated with these transitions. Possible mechanisms responsible for the optical properties of hematite particles were postulated based on the findings of the experiments. Nanoscale Ag/SiO2 composite particles with a uniform core-shell structure were synthesized by reverse micelle techniques. The silver cluster was about 5 nm and the silica shell thickness was 10 nm when R=2 (R=[water]/[surfactant]), H=100 (H=[water]/[TEOS]) and X=1 (X= [NH4OH]/[TEOS]) was employed for the cyclohexane/Igepal/water tertiary system. A model calculation indicated that the optical properties of the nanocomposites, especially the refractive index for core-shell structured nanoparticles were governed by the ratio of the core to the shell. The spherical nanocomposite particles were washed and concentrated with high performance liquid chromatography (HPLC) in order to remove the surfactant added during synthesis. Spherical SiO2 submicron particles were packed in the HPLC column as a stationary phase for the washing and dispersing of Ag/SiO2 nanocomposite particles. Surface modification of Ag/SiO2 nanocomposite particles with the silane coupling agent 3-aminopropyltriethoxysilane (APS) enhanced the charge of the nanoparticles and improved the efficiency of washing with HPLC. Well-dispersed Ag/SiO2 stable suspensions were successfully attained with ethanol water mixed solvent after HPLC washing based on dynamic light scattering (DLS), transmission electron microscope (TEM) and spin coat/atomic force microscope (AFM) analysis. As a comparison, other conventional methods such as centrifugation, soxhlet extraction and sedimentation were tested and failed to produce well-dispersed Ag/SiO2 nanocomposite suspensions. The states of the Ag/SiO2 dispersion were assessed with pH control and average agglomeration number (AAN) analysis. The hypothesis of nanoscale dispersion was evaluated through the washing of Ag/SiO2 nanocomposite particles with HPLC. Amorphous tabular SiO2 nanoparticles were synthesized from the self-assembled octylamine/water bilayer via a template-directed growth mechanism. Morphology of the tabular SiO2 nanoparticles was analyzed by high-resolution TEM both for room temperature dried and calcined powders. The tabular SiO2 nanoparticles demonstrated well-defined thickness (4-6 nm) with a relative spread face diameter (100-300 nm), which lead to a wide distribution of aspect ratio based on AFM. High surface area up to 1158 m2/g was readily obtained with a very uniform micropore size of about 0.63 nm after thermal treatment at 700 Celsius for 2 hours. Metallic Pd clusters were embedded into the microporous SiO2 matrix via a solution reduction of Pd(NO3)2 by hydrazine hydrate. The infiltration of 33 wt% Pd leads to a 13 % porosity loss of tabular SiO2 nanoparticles. The presence of Pd in the pores was demonstrated by energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The metallic guest species presumably resided in the accessible micropores with an estimated size about 1.3 nm. Hydrogen uptake measurements were performed for the Pd infiltrated nanocomposites.