Roshidah Rusdi

Band Gap Narrowing and Widening of ZnO Nanostructures and Doped Materials

Band gap change in doped ZnO is an observed phenomenon that is very interesting from the fundamental point of view. This work is focused on the preparation of pure and single phase nanostructured ZnO and Cu as well as Mn-doped ZnO for the purpose of understanding the mechanisms of band gap narrowing in the materials. ZnO, Zn0.99Cu0.01O and Zn0.99Mn0.01O materials were prepared using a wet chemistry method, and X-ray diffraction (XRD) results showed that all samples were pure and single phase. UV-visible spectroscopy showed that materials in the nanostructured state exhibit band gap widening with respect to their micron state while for the doped compounds exhibited band gap narrowing both in the nano and micron states with respect to the pure ZnO materials. The degree of band gap change was dependent on the doped elements and crystallite size. X-ray photoelectron spectroscopy (XPS) revealed that there were shifts in the valence bands. From both UV-visible and XPS spectroscopy, it was found that the mechanism for band gap narrowing was due to the shifting of the valance band maximum and conduction band minimum of the materials. The mechanisms were different for different samples depending on the type of dopant and dimensional length scales of the crystallites.

Band Gap Energies of Magnesium Oxide Nanomaterials Synthesized by the Sol-Gel Method

In this work, the band gap energies of magnesium oxide (MgO) were investigated to see if calcination time affects the band gap energies of the MgO. MgO nanomaterials have been prepared by a sol-gel method. MgO precursors produced were calcined at a temperature of 600 °C for 24 hours and 48 hours. The structural characterization of samples is achieved using X-Ray Diffraction (XRD) and the morphology as well as particle size of MgO were examined by Field Emission Scanning Electron Microscopy (FESEM). UV-Vis NIR spectroscopy was used to determine the band gap energies of the materials. From the results, the band gap energy of the MgO with a longer heating time exhibited a higher value.

Optical Band Gap Energies of Magnesium Oxide (MgO) Thin Film and Spherical Nanostructures

Magnesium oxide (MgO) is one of the metal oxides which has unique properties and has great potential applications in industry. In this work, MgO was synthesized by using a sol‐gel and solid state methods. The precursor of MgO was annealed at the temperature of 600 °C for 1 hour and 800 °C for 24 hours for sol‐gel method. For solid state method, magnesium acetate was annealed at the temperature of 800 °C for 24 hours. These samples were characterized by using X‐Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). TEM micrographs show the morphology of the samples of the sol‐gel method are thin film nanostructures whereas sample of the solid state method is spherical shape. The band gap energies of the samples were measured using UV‐Vis NIR Spectrophotometer. The band gap values of the samples were calculated and it was found that there is a difference of band gap energies between samples employing different synthesis route.

Effects of the absorption behaviour of ZnO nanoparticles on cytotoxicity measurements

ZnO absorbs certain wavelengths of light and this behavior is more pronounced for nanoparticles of ZnO. As many toxicity measurements rely on measuring light transmission in cell lines, it is essential to determine how far this light absorption influences experimental toxicity measurements. The main objective was to study the ZnO absorption and how this influenced the cytotoxicity measurements. The cytotoxicity of differently sized ZnO nanoparticles in normal and cancer cell lines derived from lung tissue (Hs888Lu), neuron-phenotypic cells (SH-SY5Y), neuroblastoma (SH-SY5Y), human histiocytic lymphoma (U937), and lung cancer (A549) was investigated. Our results demonstrate that the presence of ZnO affected the cytotoxicity measurements due to the absorption characteristic of ZnO nanoparticles. The data revealed that the ZnO nanoparticles with an average particle size of around 85.7nm and 190nm showed cytotoxicity towards U937, SH-SY5Y, differentiated SH-SY5Y, and Hs888Lu cell lines. No effect on the A549 cells was observed. It was also found that the cytotoxicity of ZnO was particle size, concentration, and time dependent. These studies are the first to quantify the influence of ZnO nanoparticles on cytotoxicity assays. Corrections for absorption effects were carried out which gave an accurate estimation of the concentrations that produce the cytotoxic effects.

Solid Solutions of LiCo1-xNixO2(x=0,0.1,...,0.9) Obtained via a Combustion Synthesis Route and Their Electrochemical Characteristics.

Pure, single-phase and layered materials with good cation ordering are not easy to synthesize. In this work, solid solutions of (x = 0, 0.1, …, 0.9) are synthesized using a self-propagating combustion route and characterized. All the materials are observed to be phase pure giving materials of hexagonal crystal system with R-3m space group. The RIR and R factor values of stoichiometries of (x = 0.1, 0.2, 0.3, 0.4, and 0.5) show good cation ordering. Their electrochemical properties are investigated by a series of charge-discharge cycling in the voltage range of 3.0 to 4.3 V. It is found that some of the stoichiometries exhibit specific capacities comparable or better than those of LiCoO2, but the voltage plateau is slightly more slopping than that for the LiCoO2 reference material.

Mn Substitution with Co in LiCo(1-X)MnxO2 Cathode Materials and their Charge-Discharge Characteristic

LiCoO2 has been used as a cathode material in commercial Li-ion batteries. This is due to advantageous properties of the LiCoO2 like ease of preparation and good electrochemical characteristics. However, the high cost and toxicity of Co has limited its use. Therefore, the substitution of Co in the LiCoO2 by non-toxic and inexpensive transition metallic element is needed. Mn is considered as one of the promising candidates to fulfill all the requirements. Partial substitution of Co by Mn has also been considered to enhance the stability of LiCoO2 lattice, minimize capacity fading and increase cycle life of the Li-ion battery. LiCo(1-x)MnxO2 (x= 0.1, 0.2, 0.3) were prepared by using a self-propagating combustion (SPC) method. X-ray diffraction (XRD) of the samples were carried out for phase analysis and showed that all the materials are pure. The samples were also analyzed using the Field Emission Scanning Electron Microscope (FESEM) to study its morphology and particle size. Finally cathodes were fabricated and assembled in an inert gas-filled fabrication box. Discharge profiles of the materials were carried out in the voltage range of 4.3 V – 3 V. The materials obtained were phase pure and improved the capacity fading of the materials compared to LiCoO2. All of the materials exhibited less than 10% capacity loss even though it does not improve the first cycle discharge capacity.

Electrochemical performance of overlithiated Li1+xNi0.8Co0.2O2: structural and oxidation state studies

Pure, layered compounds of overlithiated Li1+xNi0.8Co0.2O2 (x = 0.05 and 0.1) were successfully prepared by a modified combustion method. XRD studies showed that cell parameters of the material decreased with increasing the lithium content. SEM revealed that the morphology of particles changed from rounded polyhedral-like crystallites to sharp-edged polyhedral crystals with more doped lithium. EDX showed that the stoichiometries of Ni and Co agrees with calculated synthesized values. Electrochemical studies revealed the overlithiated samples have improved capacities as well as cycling behavior. The sample with x = 0.05 shows the best performance with a specific capacity of 113.29 mA·h·g−1 and the best capacity retention of 92.2% over 10 cycles. XPS results showed that the binding energy of Li 1s is decreased for the Li doped samples with the smallest value for the x = 0.05 sample, implying that Li+ ions can be extracted more easily from Li1.05Ni0.8Co0.2O2 than the other stoichiometries accounting for the improved performance of the material. Considerations of core level XPS peaks for transition metals reveal the existence in several oxidation states. However, the percentage of the +3 oxidation state of transition metals for the when x = 0.1 is the highest and the availability for charge transition from the +3 to +4 state of the transition metal during deintercalation is more readily available.

Studies of MEH-PPV and MEH-PPV/MCMB Films and their Light Absorption in the Red Wavelength Region of the Visible Spectrum

Poly [2-methoxy, 5-(2-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV) is a conjugated polymer that exhibit excellent luminescence properties in the visible spectrum. The fundamental absorption edge in the film is formed by the direct allowed transitions. In this work, free-standing films of MEH-PPV and MEH-PPV/MCMB were obtained by a solvent casting method. Mesocarbon Microbead (MCMB), a type of carbon, is added to the polymer producing composite films. The films were characterized by using the UV-Vis-NIR spectrophotometer. The film samples exhibited an absorption band in the red visible region. This is due to the electron transition between the non-localized bands. However, when MCMB is doped into the polymer matrix, the band absorption edge is red-shifted compared to that of the MEH-PPV pure film. Therefore, the optical band gaps of the composite films have decreased due to the presence of MCMB.

Influence of Carbon Additives on Cathode Materials, LiCoO2 and LiMn2O4

Carbon additives are very important components of cathodes in Li-ion batteries. This is because carbon is an electronic conductor whereas cathode materials are ionic conductors. Without the presence of carbon, the electrons will not be able to flow and there will be space charge built-up in the materials. Carbon therefore facilitates the conductivity of charged species in the cathode materials and help to disperse the negative charge accumulation which may otherwise impede Li-ion diffusion within the cathodes. In this work, two types of carbon, namely, activated carbon (micron sized) and Denka Black (nano sized) were used in conjunction with the cathode materials LiCoO2 and LiMn2O4. The amounts of cathode materials were kept constant while the amounts of carbon additives were varied. Galvanostatic charge-discharge was done over a voltage range of 4.2 V to 3.2 V. Results showed that Denka Black gives improved performance for both cathode material. This is believed to be due to the effect of nano sized particles of Denka Black.

Effect of pH on the Crystal Growth of ZnO Nanomaterials and Their Band gap Energies

Zinc oxide nanostructures with vary pH have been successfully synthesize using a sol-gel method. The nanostructured materials were annealed at low temperature of 300 °C for 3 hour. The effect of pH of the synthesis condition of ZnO on the structural and optical properties of the nanostructures were studied using X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and UV-Vis spectrophotometer. XRD results revealed that only sample at very acidic condition was not pure with the presence of impurities. This implies that acidic condition require more energy in order to produce pure and single phase of ZnO nanostructures. FESEM results showed that at very acidic and basic condition, the crystallites tended to agglomerate to form bigger particles. The band gap energy of the ZnO nanostructures at pH 6 and 9 were 3.276 and 3.300 eV respectively. Interestingly, it was found that as the aspect ratio of nanorods increased, the band gap energy of the material also increased.