Optical and electrochemical properties of the periodically patterned photoanode. (2021)
Type of ContentTheses / Dissertations
Thesis DisciplineChemical Engineering
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
The integration of a photoelectrode into a redox flow battery (RFB) enables direct solar charging of the battery. The efficiency of a solar RFB depends on the number of photons that excite the electrons and lead to electron transfer reaction. The major obstacles during this process are poor light absorption by the photoelectrode, scattering/reflection losses, and rapid recombination of photo-generated charge carriers once they are generated within the semiconductors. This study addresses these limitations by investigation of the effects of patterning the geometry of nano-micro-structures on the photoelectrode as a potential way to modify its optical, electrical and electrochemical properties.
The type of photoelectrode that is considered consists of a layer of TiO₂ on top of fluorine-doped tin oxide (FTO) coated glass. TiO₂ has attracted great interest due to its stability and large band gap (3.2 eV) that covers the redox potential of many species. However, this large band gap requires the absorption of high energy photons with a wavelength below 400 nm which is a small portion of the solar spectrum. Improved light harvesting of ultraviolet photons within the TiO₂ occurs by suppressing the different ways in which light is lost on the photoelectrode. These are 1) reflection from the surface, 2) absorption within the inactive layers, and 3) transmission through the photoactive layer.
Patterning a surface by a nano-sized structure reduces light reflection by causing a gradual change of refractive index from the air to the substrate. In addition, a periodic structure with a periodicity larger than the light wavelength diffracts the light into several beams. Light behaviour was studied on a glass substrate patterned by a periodic pyramidal nanostructure with a periodicity of 670 nm. The periodic structure was fabricated through a combination of laser interference lithography and nanoimprint lithography. The effect of surface patterning on light reflection and diffraction was evaluated using a numerical study based on the Finite-Difference Time-Domain method. The periodic structure reduced the light reflection over the whole wavelength range. At wavelengths smaller than the periodicity of the structure (λ < 670 nm), the light was diffracted into several beams, while at wavelengths higher than the periodicity (λ > 670 nm), the structure acted as a homogenous medium with an equivalent refractive index without causing light diffraction. The effect of the optical behaviour of the structure on the photocatalytic activity of TiO₂ was probed experimentally through photodecomposition of methylene blue at λ=365 nm. The efficiency of methylene blue oxidation was higher on patterned substrate as a result of antireflection behaviour of the periodic structure and longer light path within the semiconductor material caused by diffraction.
The photo-generated charge carriers have a very short time to undergo the electron transfer reaction before charge recombination. Therefore, an effective photon absorption within the TiO₂ layer occurs at the vicinity of the electrode-electrolyte interface. This limits the thickness of the TiO₂ layer and the absorption of photons. This issue can be addressed by patterning the TiO₂ with a structure that provides a long dimension for light absorption, while it requires the charge carriers to pass a short distance to reach the acceptor species. This patterning will also influence the transport phenomena at the electrode-electrolyte interface. To separate the influence of patterning the electrode-electrolyte interface on the mass transport of redox species from its influence on the light/charge carrier behaviour within the photoactive layer, the mass transfer of redox species was investigated using cyclic voltammetry (both numerically and experimentally) on a patterned gold electrode in the presence of Fe²⁺/Fe³⁺ redox species. It was observed that the effect of electrode- electrolyte structure on electrochemical performance depended on the size of the structure relative to the diffusion layer thickness. By reducing the diffusion layer thickness, the redox species concentration profile was affected by the structure. This resulted in a convergent mass transfer of redox species towards the electrode causing a nonuniform current density over the electrode surface, with it being greater at the tip of the structure and lower at the base.
Two approaches were employed to pattern the TiO₂ photoelectrode-electrolyte interface: 1) by direct fabrication of the pattern into the TiO₂ layer using imprint lithography, and 2) by coating the TiO₂ layer on a patterned FTO glass. Direct fabrication by imprint lithography used both a polymer-based and an alcohol-based paste to produce an upright pyramidal microstructure. Imprinting the polymer-based paste resulted in a higher throughput and more control over the final structure. However, annealing caused the polymer-based structure to greatly shrink. In order to coat TiO₂ onto the patterned FTO glass, a pattern containing micro-holes was fabricated into the FTO conductive layer by using photolithography followed by inductive coupled plasma etching. Hydrothermal deposition was then used to grow TiO₂ nanorods on top of the FTO layer. Due to difficulties with reproducibility of the patterning of the TiO₂ layer by imprint lithography, the photoelectrochemical study of the patterned electrode was conducted only on the photoelectrode with a patterned FTO layer. The photoactivity of the patterned photoelectrode was studied during photo-oxidation of V⁴⁺ in a three-electrode setup during linear sweep voltammetry under chopped illumination. Patterning the FTO layer affected the photoelectrode by changing the light behaviour, the ohmic overpotential, the charge recombination rate, and the morphology of the O layer. Overall, etching the FTO layer had a negative effect on the photoactivity of the fabricated electrodes, and an increase in the etched area resulted in a lower photocurrent.
The results showed that patterning technique can be a promising approach to enhance the photoactivity of an RFB photoelectrode. However, the presence of a nano-/micro-structured pattern can affect photoactivity of the photoelectrode in various ways. In order to obtain the maximum photoelectrochemical efficiency the combination of these effects must be considered.
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