Al-Shihabi Al-Ani, Athil Khairi J.
(2021)
Novel TiO2 nanotube arrays for photoelectrocatalysis application.
PhD thesis, University of Nottingham.
Abstract
The fast-growing energy and environmental concerns have motivated many researchers to move their efforts towards more sustainable and environmentally friendly renewable energy sources, particularly sunlight. Photoelectrocatalysis is a widely accepted technique that has great potential to utilise sunlight to enable desirable chemical reactions with low environmental impacts. This technique can employ an abundant, affordable, non-toxic, and environmentally friendly stable semiconductor as the photocatalyst, such as TiO2 nanotube arrays (NTAs), to assist water electrolysis in producing a clean fuel under the illumination of solar light.
The present work aims to produce and use TiO2 nanotube arrays (NTAs) as a photocatalyst formed on the Ti foil as a substrate by electrochemical anodization. The morphology and the performance of the NTAs are expected to be influenced by several parameters involving: electrolyte composition, applied voltage, drying method, and anodization duration. In this study, three parameters have been examined: (1) the drying method (2) anodization duration and (3) water purity (which can be reflected by water conductivity). The results showed that increasing the anodization time could affect the morphology of the as-prepared NTAs and invoke chemical dissolution.
This, in turn, caused aggregation of the nanotubes, rising upward and possibly weakening their adhesion on the substrate (Ti) and peeling them off. Some results confirmed that the sample, which was washed in acetonitrile for dehydration, demonstrated photocurrent density higher by about 2.5 times than that dried naturally in the air. The most prominent finding from this study is the effect of water purity on the NTAs morphology. It was noticed that when deionised water (DIW) was replaced by reverse osmosed water (ROW) in the electrolyte, its conductivity increased by about 8.97 %.
In this study, the simultaneous under-potential deposition of multiple elements was applied successfully to broadly cover the TiO2 NTAs arrays to form a sensitiser of kesterite (Cu2ZnSnS4) to enhance the photoconversion efficiency and the transporting of the charge carrier. This enhancement was achieved by using a modified electrochemical atomic layer deposition (EC-ALD) method. The results have shown that it is extremely challenging to deposit multiple elements onto and maintain the well-organised structure of NTAs at the same time. The complexing agent ethylenediaminetetraacetic acid (EDTA) and the pH of precursor solutions were found to be the key factors to assist the successful simultaneous deposition.
The electrodeposition process, surface morphology, crystalline structure, and photocatalytic activity of the as-prepared Cu2ZnSnS4/TiO2 NTAs were examined. The kesterite crystalline structure of Cu2ZnSnS4 was successfully deposited as a single phase.
After the sensitisation with CZTS thin film, an enhancement in photoconversion efficiency from 1.1 % for the pure TiO2 NTAs to 2.0 % was observed, and the bandgap was reduced from 3.10 to 2.43 eV using the sensitiser Cu2ZnSnS4. The thickness of the deposited CZTS layer was estimated to be ca. 5.0 nm. A synergistic effect was observed in the photocatalytic properties of the novel CZTS/TiO2 NTAs composite, and the hydrogen generation rate achieved a maximum of 49.0 mL h-1cm-2.
Applying the EC-ALD to produce the novel CZTS/TiO2 NTAs photoanode from separate precursor solutions offered a significant improvement in the electrical and optical properties. The average inner diameter of the tubes was ca. 95.0 nm with a thickness of ca. 8.0 nm of CZTS. It was also found that the photocurrent density enhanced to ca. 3000.0 µA with 3-fold enhancement in the photoconversion efficiency. The XRD results showed that the intensity peaks of Cu2ZnSnS4 became stronger and sharper than those were observed with the film that deposited from a mixture of the precursors.
The samples that were doped near the inner layer, such as CZTS (50 m) / Ag (10 m) / CZTS (10 m) / TiO2 NTAs and CZTS (50 m) / Ge (10 m) / CZTS (10 m) / TiO2 NTAs exhibited a maximum stable photocurrent density of ca. 7.5 mA cm-2. The metal doping near the inner layer was more effective than near the outer or the middle layer on the performance of the CZTS/TiO2 NTAs composite. The bandgap of the Ag-doped CZTS/TiO2 NTAs and Ge-doped CZTS/TiO2 NTAs composites were drastically narrowed to a very close value of 1.88 and 1.86 eV.
A further thicken in the wall of the tubes was estimated to be ca. 10.0 nm (+/5.0 nm), and the inner diameter was ranging from 85.0 to 90.0 nm after the sensitisation with CZTS followed by the metal doping with Ag and Ge, respectively. A cascade of multiple heterogeneous junctions in a coaxial manner was formed onto the nanotubes after the sensitisation and the doping with CZTS and Ge (the inside and outside surfaces of the walls of the nanotubes were uniformly and homogeneously coated).
Comprehensive advantageous can be merged from the as-synthesised and doped CZTS/TiO2 NTAs photoanode, involving the significant enhancement in light-harvesting ability, a great cohesion between CZTS and the doped metals (Ge and Ag) and the transporting of the charge carrier that arises from the alignment in the stepwise band edge level of the produced photoanode. This approach probably suitable in synthesising multijunction semiconductor materials for the coating of highly structured substrates.
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