Zhang, Xinyue; (2024) Aerosol-assisted Chemical Vapour Deposition of Delafossite CuFeO₂Thin Films and Optimisation of Functional Properties. Doctoral thesis (Ph.D), UCL (University College London).

Text Complete thesis with corrections Xinyue Zhang - Mar 26 2024.pdf - Submitted Version Access restricted to UCL open access staff until 1 June 2025. Download (14MB)

Abstract

Semiconducting metal oxides play a vital role in water splitting and gas sensing, offering a multifunctional approach to sustainable energy production through solar energy utilization and emissions control. The focal point of this endeavour was to synthesize and modify Delafossite-structure CuFeO2 thin films for application as a photocathode for water splitting and in gas sensor devices. Delafossite CuFeO2 is a p-type metal-oxide semiconductor (MOS) with hole conductivity resulting from non-stoichiometric structure due to Cu-deficiency in the Cu-layer and O-interstitials. In this project, high-purity CuFeO2 thin films consisted of a single crystal phase with meticulously controlled thickness and morphology have been synthesised via aerosol-assisted chemical vapour deposition (AACVD) wherein various parameters including precursors, substrates, solvent, deposition temperature, gas flow rate, and post-annealing conditions are optimized. The deposition conditions are gentle and scalable for mass production, boasting remarkable efficiency. As a photocathode, the band gap of the thin film Delafossite CuFeO2 has been measured at ~ 1.8 eV (indirect). The highest photocurrent density of CuFeO2 thin films measured in 1M NaOH electrolyte with 10% 0.1M Na2S2O8 was ~240 μA/cm2 at 0.4 V vs Reversible Hydrogen Electrode (RHE), with an incident photon to charge carrier efficiency (IPCE) of 4.5%. The mechanism of charge carriers transportation and utilization was explained by Transient Absorption Spectroscopy (TAS), which confirmed the relationship between charge carrier recombination and film thickness and morphology. The data revealed the significance of hole thermalization and charge carrier separation in rationalizing the film performance. Delafossite CuFeO2 thin films were engineered for enhanced photoelectrochemical (PEC) water splitting using distinct approaches: post-annealing treatment in air, heterojunction construction with a CuNiOx hole transport layer, and surface decoration with a thin Fe2O3 film. These strategies led to diverse levels of enhancement in PEC performance and offered valuable insights for future research endeavours aimed at realizing the full potential of CuFeO2 films as photocathode materials. Gas-sensing experiments on CuFeO2 thin films fill the gap in research on CuFeO2 gas sensors and extended our knowledge of p-type semiconducting metal oxide gas sensors. In this project, CuFeO2 gas sensors demonstrated diverse responses to oxidized analyte gases across varying temperatures and concentrations. Sensitivity exhibited temperature dependency, with heightened responsiveness notably observed during oxygen and humidity tests. The p-n gas sensing behaviour shifted in response to analyte gases with oxygen (in nitrogen) volumetric ratios ranging from 1% to 20%, highlighting the impact of thermal treatment on the phase and surface oxygen composition of the CuFeO2 gas sensors. Further research should be conducted in this area to thoroughly characterize the sensitivity, selectivity, and reusability of CuFeO2 gas sensors, to gain a deeper understanding of the gas-sensing mechanism, and to enhance the overall gas-sensing performance of CuFeO2.

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