Performance enhancement of silver-nanoparticle? titania (Ag_NP/TiO2) using chlorophyll as dye sensitizer
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Date
2019
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University of Namibia
Abstract
Titanium dioxide or titania (TiO2) being one of the most promising semiconductors for many photoelectrochemical applications, its utilization remains typically confined to ultraviolet (UV) light absorption because of its wide band gap (3.2 eV for anatase and 3.0 eV for rutile), where by UV only accounts for 4-6% of the solar radiation. There is a need to shift the absorption threshold of the UV-region response of TiO2 into the visible region (accounts for ~45% of the total energy of the solar radiation), which can be achieved by doping TiO2 with dye sensitizers, non-metals, transition metals or through surface plasmon resonance (SPR) using noble metals. To study the absorption spectra of metallic silver nanoparticle/titania/Chlorophyll (Ag-NP/TiO2/Chlorophyll) thin films, sandwich arrangement thin films were fabricated on a quartz glass substrate at 550°C using a spin coater. Respective precursor solutions for Ag-nanoparticles and titania were prepared from Ag salt and a titanium complex using molecular precursor method. The Chlorophyll dye was extracted from the Colophospermum mopane leaves (Mopane tree leaves) and Hyphaene petersiana leaves (Makalani tree leaves) using methanol as a solvent extract. The Ag-nanoparticle/TiO2 (Ag-NP/TiO2) thin films were then immersed in the Chlorophyll dye for 48 hours, to produce Ag-NP/TiO2/Chlorophyll dye composites thin films. A Fourier-transform infrared (FTIR) spectrum was used to confirm whether the dye extract was chlorophyll. The absorption properties of the thin films were studied using Ultraviolet-visible (UV-VIS) Spectrophotometer. The absorption of the entire visible region for the Ag-NP/TiO2 composite thin films was stronger than that for the undoped TiO2 thin film. This is due to the generation of the absorption peak attributed to the surface plasmon resonance (SPR) of metallic silver nanoparticles in the TiO2 matrix, which exhibited a broad band, improving the absorption throughout the entire visible region. Undoped TiO2 thin film showed no absorbance in the visible region. The pure Chlorophyll thin films showed two stronger absorption peaks at the wavelength of 415 nm to 665 nm respectively. Apart from a surface plasmon resonance (SPR) peak around 410 nm, additional wide-range absorption spread in the vis-region at wavelengths greater than 410 nm was observed for Ag-NP/TiO2/Chlorophyll dye thin films. The wide-range absorption is due to the coupling between the dye and plasmon resonance properties of the Ag-nanoparticles. The band gap of the fabricated thin films is in the range of 1.4 and 3.50 eV, with 75%Ag-NP/TiO2/Chlorophyll dye thin film with the lowest band gap of 1.4 eV and the highest band gap of 3.50 eV obtained from an undoped TiO2 thin film. The reduction in bandgap is due to metal – ligand charge transfer (MLCT) transition, through which the photoelectric charge of low-lying empty orbitals such as CO or CN- possessed by chlorophyll injected into the TiO2 band gap. The wide-range absorption observed is therefore clarified by SPR and bandgap narrowing. On the basis of absorption spectra analysis, a plasmonic-band gap narrowing mechanism was proposed.
Description
A research thesis submitted in partial fulfillment of the requirements for the Degree of Master of Science in Chemistry
Keywords
Silver-nanoperticle, Chlorophyll