Development of TiO2 nanostructures with a modified energy band gap for hydrogen extraction
- Authors: Mutubuki, Arnold
- Date: 2024-04
- Subjects: Nanostructures , Nanoscience , Nanochemistry
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10948/64226 , vital:73666
- Description: A rise in fossil fuel depletion has motivated the research towards alternative, cost effective and clean processes for energy production through renewable sources. The scientific community is currently engaged in extensive research to exploit viable, sustainable methods for generating green hydrogen. Titania (TiO2) is historically the most studied photoactive semiconductor material with great potential in photoelectrochemical water splitting (PECWS), following the discovery by Fujishima and Honda in 1972. TiO2 possesses superior physicochemical characteristics and band gap edges, which enables the semiconductor to effectively facilitate the PECWS process. Efforts are still ongoing to explore alternatives for narrowing the optical band gap energy of TiO2, for an efficient photoelectrode. In this research work, open-ended and well-ordered TiO2 nanotubular arrays were synthesised by a three-step anodization process. The third anodization was crucial to detach the TiO2 thin film from an opaque Ti metal substrate. The free-standing thin films were transferred and pasted onto conductive FTO-coated glass substrates transparent to visible light and annealed at 400 ℃ for crystallisation. The multi-step anodization has shown an improved top tube morphology by eliminating an initiation TiO2 mesh formed when a conventional single-step anodization process is used under similar conditions. To widen the absorption range of the samples, CuO nanosheets were deposited onto nanotubular TiO2/FTO films through successive ionic layer adsorption (SILAR), a wet chemical method. The formation of a CuO/TiO2 nanostructure enhances the transfer of photogenerated carriers, suppressing charge recombination. This research focused on investigating the influence of selected SILAR parameters on the formation of CuO nanostructures. The first was the effect of precursor concentration on the structural, morphological and optical properties of the CuO/TiO2/FTO nanostructured photoelectrode. The effect of the precursor concentration on the structure and morphology was evident in the X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) micrographs. Crystallite sizes of deposited CuO increased from 10.6 nm to 15.7 nm when precursor concentration was varied from 0.02 M to 0.10 M. The UV-visible absorbance results show that an increase in precursor concentration leads to a red shift of both the peak absorbance and edge wavelength of the CuO/TiO2/FTO absorbance spectra. This phenomenon is believed to be caused by the presence of CuO, which exhibits active absorption in the visible spectrum. As evidenced by the study, the continued increase in precursor concentration does not result in a further widening of the absorption band. This is demonstrated by the example of a CuO/TiO2/FTO sample decorated with a 0.2 M precursor. The second was the effect of SILAR immersion cycles on the properties of the CuO/TiO2/FTO nanostructure developed. The increase in the number of immersion cycles led to a notable progression in the adsorption cupric oxide on the TiO2/FTO samples. A redshift in the absorbance peak and edge wavelength is observed in the UV-visible spectra of CuO/TiO2/FTO photoelectrode. The efficacy of the SILAR technique in modifying the absorption band of nanotubular TiO2 thin films has been conclusively demonstrated through comprehensive analysis and correlation of the relationships between the structure and optical properties, as evidenced by the XRD patterns, Raman spectra, SEM, TEM micrographs, and UV-visible absorbance spectra. , Thesis (MSc) -- Faculty of Science, School of Computer Science, Mathematics, Physics and Statistics, 2024
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- Date Issued: 2024-04
Photocatalytic reduction of CO2 into green fuels on microspherical nanocomposites comprising of CdS, TiO2, ZnFe2O4 semiconductors and heterojunctions
- Authors: Mudau, Rendani Julian
- Date: 2024-04
- Subjects: Heterojunctions , Chemistry, Inorganic , Nanochemistry
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10948/64214 , vital:73665
- Description: Human anthropogenic activities have led to an increase in CO2 levels resulting in global warming. There is a need to reduce CO2 levels in the atmosphere by capturing the gas and converting the captured CO2 into fuels. Photocatalytic conversion of CO2 into valuable products via photoreduction is a promising technique to reduce carbon dioxide using sunlight and water. Redox reactions between CO2 and water using photocatalysts offer an effective method of lowering CO2, which has detrimental effects on the environment. This dissertation reports the synthesis and characterization of five photochemical catalysts comprising of CdS, TiO2, ZnFe2O4 semiconductors and heterojunctions and their use as photocatalysts for the photocatalytic reduction of CO2 into green fuels. Mott−Schottky plot of TiO2, CdS and CdS/TiO2 confirmed the formation of n-type, n-type, and n-n type material respectively. The band gap of TiO2, CdS, and CdS/TiO2 obtained were 3.21,2.24, 2.04 eV, respectively, the combination of TiO2 and CdS resulted in the lowering of band gap energy which is favourable for photocatalytic reduction studies. XRD of TiO2, CdS and CdS/TiO2 provided relevant information about the various phase reflections of the photocatalysts, it also supported reports confirming the increase in crystallite size of TiO2 anatase phase upon calcination. Chemical environment of the various photocatalysts, such as Ti2p1/2, Ti2p3/2, O1s, Cd3d3/2, Cd3d5/2, S2p1/2 and S2p3/2 were observed in TiO2, CdS and CdS/TiO2. BET surface area of TiO2, CdS and CdS/TiO2 was also observed to be 2.9243, 4.0605, and 9.8450 m2g, respectively. Photocatalytic reduction of CO2 resulted in formic acid, formaldehyde, acetic acid, carbon monoxide, ethanol, and methanol after 12 h. Modification of CdS with TiO2 to form CdS/TiO2 produced high yield of formic acid (217.7 μmol/g) compared to CdS (8.7 μmol/g) and TiO2 (122.5 μmol/g). CdS had high yield of formaldehyde (20.4 μmol/g) and ethanol (57.8 μmol/g) compared to TiO2 (12.9, 3.6 μmol/g of formaldehyde and ethanol, respectively) and CdS/TiO2 (19.7 μmol/g and there was no detection of ethanol). TiO2 shows high yield of methanol (145.2 μmol/g) and is the only catalysts that produced carbon monoxide (52.2 μmol/g) compared to CdS (87.8 μmol/g of methanol) and CdS/TiO2 (50.2 μmol/g of methanol). A detailed elucidation of the various products was reported by means of nuclear magnetic resonance (NMR) spectrometer. ZnFe2O4 and CdS/TiO2/ZnFe2O4 were characterized and employed for photocatalytic reduction of CO2 in water. XRD and XPS confirmed the presence Fe (2+) and Fe (3+) in tetrahedral/octahedral and Zn (2+) in tetrahedral on ZnFe2O4 and CdS/TiO2. CdS/TiO2/ZnFe2O4 presented a lowered band gap of 1.98 eV due to the presence of ZnFe2O4 while the band gap of ZnFe2O4 obtained is 1.95 eV. The Mott-Schottky plots confirmed the conduction (2.81 and 2.59 eV) and valence band (–0.86 and –6.1 eV) of ZnFe2O4 and CdS/TiO2/ZnFe2O4 together with their junctions, respectively. ZnFe2O4 and CdS/TiO2/ZnFe2O4 are p-type and p-n type junctions, respectively. Photocatalytic reduction of CO2 under visible light for 12 h using CdS/TiO2/ZnFe2O4 hollow microspheres nanocomposites produced methanol (182.1 μmol/g), formic acid (1.9 μmol/g), formaldehyde (9.5 μmol/g), ethanol (67.1 μmol/g), and oxalic acid (38.9 μmol/g). Amongst the synthesized catalysts, CdS/TiO2/ZnFe2O4 produced high yield of methanol, ethanol, and oxalic acid. , Thesis (MSc) -- Faculty of Science, School of Computer Science, Mathematics, Physics and Statistics, 2024
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- Date Issued: 2024-04