In2Se3-based two-dimensional photocatalysts for water-splitting

Photocatalytic water splitting has been a burgeoning field of study due to its potential to change the world’s fossil-based energy structure; however, an ideal photocatalyst has yet to be identified despite years of research. Recent research has revealed that indium selenide (In₂Se₃) carries great potential in being a promising photocatalytic water splitting catalyst due to its unique intrinsic ferroelectricity, suitability of band structure and ability to be exfoliated into two-dimensional (2D) sheets. It is a relatively less studied material and has not been explored in literature as a water splitting photocatalyst until now. This thesis proposed to explore the photocatalytic water splitting chemistry of 2D-In₂Se₃ experimentally for the first time. In₂Se₃ was exfoliated to 2D-In₂Se₃ by the lithium intercalation method. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) confirmed the thin-sheet like morphology with around 10-50 layers of In₂Se₃. Photocatalytic testing proved its water splitting activity nearly doubled after the exfoliation. In-depth characterisations were conducted to investigate its structure-activity relationship. Time-resolved photoluminescence (TRPL) revealed that the charge migration distance to the active sites was shortened, and electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) proved that Se vacancies were formed after exfoliation. Together, the structural changes led to improved photocatalytic activity. An undecorated semiconductor is e to achieve a high activity due to the harsh requirements for water splitting photocatalysts. Metal doping and heterojunction formation were used as strategies to improve the activity of 2D-In₂Se₃ in this research. 5 wt.% Fe-doped 2D-In₂Se₃ (Fe-In₂Se₃) by the hydrothermal method has shown the highest activity among the screened metal dopants; Fe-In₂Se₃ has a nearly three-fold increase of activity relative to 2D-In₂Se₃. This is believed to be attributed to improved charge separation by the doped Fe³⁺ on 2D-In₂Se₃. Two-dimensional molybdenum disulphide (2D-MoS2) was used to form a heterojunction with 2D-In₂Se₃ due to their similar 2D structure and suitable band structure for water splitting. The successful spontaneous self-assembly construction of MoS2/In₂Se₃ was confirmed by TEM, energy dispersive X-ray spectroscopy (EDX) and zeta potential measurements. 1:1 MoS2/In₂Se₃ (by weight) was the optimal experimental ratio with an improved photocatalytic activity compared to the calculated average value of the two materials. The improved activity is ascribed to the formation of type-II heterojunction as proven by experimental band structure alignment (by Tauc plot and ultraviolet photoelectron spectroscopy (UPS)), which further leads to prolonged exciton lifetime as shown in TRPL analysis. In conclusion, this thesis has investigated experimentally that 2D-In₂Se₃ exhibits water splitting photocatalytic activity and the lithium intercalation method is effective in exfoliating the material. Fe doping and MoS2/In₂Se₃ formation were shown to be effective strategies to further boost its photocatalytic performance. Additional work is needed to further optimise this system and explore other strategies to enhance the water splitting performance of In₂Se₃-based photocatalysts.