Cui, Fan; (2024) Engineered III-V Semiconductor Nanostructured Materials for Photoelectrochemical Water Splitting. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

One of the most promising techniques for generating sustainable electricity is the photoelectrochemical (PEC) hydrogen creation from water splitting by employing solar energy. For this purpose, semiconductor materials with narrow band gaps are the ideal candidates having broad absorption overlapping major portion of the solar spectrum to harvest large portion of solar spectrum. However, it hasn’t yet been possible to discover appropriate semiconductor materials with opto-electronic properties. The materials having outstanding optical properties are very sensitive and exhibit fast degradation. Hence, the main focus of this thesis is the growth of III-V group nanowires (NWs) i.e. GaAs and GaAsP NWs. GaAs and GaAsP are the key compounds fulfilling major requirements to be used in the solar harvesting applications, however, they demonstrate fast corrosion which is their major drawback. Hence, an effort has been paid to realize the GaAs and GaAsP NWs based photoelectrodes for water splitting as well as the techniques to enhance the stability of the GaAs and GaAsP NWs based photoelectrodes against corrosion. Firstly, narrow-bandgap III-V semiconductor GaAs NWs are developed. Due to their optimal band structure and powerful light-trapping properties, these materials are perfect for creating efficient and affordable solar water-splitting systems. However, compared to their thin-film counterparts, they are more severely eroded by the electrolyte solution because of their nanoscale size. Therefore, the main obstacle to adopting such NWs for useful water splitting applications is their short-term endurance. The study based on the TiO2 protection of NWs was insufficient, thus we grew a thin layer (~7 nm) of compact TiO2 via atomic layer deposition on pre-grown GaAs NWs to give enough protection against degradation. Thus, even after 14 months of storage at room temperature, the photoluminescence intensity of the TiO2 protected GaAs NWs retained 91.4% of initial value, indicating enhanced stability of NWs via TiO2 layer. When applied as a photocathode for water-splitting, the TiO2 coated GaAs NWs based photoelectrode demonstrated a photocurrent density that is about 45% higher than that of unprotected photocathode. Moreover, a Faraday efficiency of about 91% was estimated that is unprecedentedly higher than previously reported narrow-bandgap III-V NWs based photoelectrodes. In contrast to the unprotected NWs, which completely degraded after 35 hours of operation, there were no evident symptoms of corrosion after 67 hours of PEC operation during the stability test in a very acidic electrolyte solution (pH = 1). These findings offer a practical means of enhancing the stability and functionality of III-V NW based photoelectrodes, which is crucial for the real-world applications of solar-energy-based water splitting systems. Secondly, high-quality and high-performance photoelectrode based on GaAsP NWs produced on a silicon substrate via molecular beam epitaxy is presented. To increase the resistance of NWs against corrosion, a thin layer (~7 nm) of compact TiO2 is deposited by the atomic layer deposition (ALD) technique on the surface of the GaAsP NWs. The CoS2 as a co-catalyst is also deposited on the GaAsP/TiO2 heterostructure by electrochemical deposition method. In order to compare the performance of CoS2 as co-catalyst, a commercial co-catalyst Pt is also grown separately on GaAsP/TiO2 heterostructrue via aerosol-assisted chemical vapor deposition (AACVD). It is demonstrated that the deposition of CoS2 significantly improves the photoelectric performance as well as the lifetime of the GaAsP NWs based photoelectrode during water splitting. Under one sun illumination, the GaAsP/TiO2/CoS2 photoelectrode delivered a photocurrent density of ~2.4 mA/cm2 in a 0.5 mol/L sulfuric acid electrolyte. In comparison to this, the GaAsP/TiO2/Pt photoelectrode exhibited over 3 folds higher photocurrent density of ~8 mA/cm2 in a 0.5 mol/L sulfuric acid solution. However, the greater advantage of CoS2 over Pt is its earth abundance. The stability of the photoelectrodes was studied wherein CoS2 based photoelectrode demonstrated higher stability compared to Pt one. For instance GaAsP/TiO2/CoS2 photoelectrode showed outstanding stability of 164 h and retained 60% of initial photocurrent density, whereas, GaAsP/TiO2/Pt photoanode demonstrated slightly lower stability of 141 h under continuous simulated solar light illumination. This shows that there is a trade-off between the low performance and higher stability of CoS2 based photoelectrode. The rate of H2 evolution was observed for these photocathodes where the GaAsP/TiO2/Pt and GaAsP/TiO2/CoS2 based photoelectrode procured 21.3 and 16.3 µmol of H2 gas after 2 hours of continuous operation which are highly comparable. These results infer that GaAsP NWs are very promising candidates for solar derived PEC water splitting and CoS2 being low cost compared to Pt can be employed to fabricate the high efficiency and highly stable PEC photoelectrode.

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