Synthesis And Investigation Of Semiconductor Hetero-junction Materials For Photoelectrochemical Cells

The conversion of solar energy into hydrogen by a photoelectrochemical（PEC） cell represents a promising but still challenging way to solve the energy and environmental issues. So far many inorganic semiconductors exhibit poor PEC performance resulting from problems like low light absorption, low quantum efficiency, unsuitable band edge position, severe charge recombination and so on. Using heterojunction structure is a promising solution to overcome the drawbacks above. In addition to combining the qualities of the composers, the heterojunctions can also accelerate the charge separation and transportation, thus enhance the photocatalytic activity. In this dissertation, we fabricated four kinds of heterojunctions based on ZnIn₂S₄ and α-Fe₂O₃. The PEC performance and working mechanism were sdudied in detail.（1） ZnIn₂S₄/TiO₂: The TiO₂ nanorods were synthesized by hydrothermal method. Then Zn In2S4 nanosheets were directly grown on the TiO₂ nanorods by another hydrothermal process. The 2D ZnIn₂S₄/ 1D TiO₂ heterostructure can increase the surface area and the visible light response, meanwhile accelerate the seperation the carriers thus the photocurrent was enhanced 2 times.（2） ZnIn₂S₄/TiO₂/Si: Si nanowires were first fabricated by metal-assisted chemical etching（MACE） method on a Si wafer. Then a TiO₂ protective layer was deposited on the Si nanowires using atomic layer deposition. Finally ZnIn₂S₄ nanosheets were grown on the TiO₂/Si nanowires by a simple hydrothermal reaction. This kind of the multijunction can increase the surface area and accelerate the seperation the carriers thus enhance the PEC performance. The maximum photoconversion efficiency of the heterostructure reached 0.51%, which is 64 times larger than that of the Si nanowires.（3） ZnFe₂O₄/α-Fe₂O₃: α-Fe₂O₃ nanorods were synthesized by ficile hydrothermal reaction, then a ZnO layer was coated on the α-Fe₂O₃ by atomic layer deposition. After a post-annealing process, the ZnFe₂O₄ could be formed on the interface of ZnO and α-Fe₂O₃. Owing to the properties of the typical type-II band alignment, the heterojunction of ZnFe₂O₄/α-Fe₂O₃ can separate the electron-hole pairs effiectively, increasing the photocurrent density from 0.03 mA cm-2 for α-Fe₂O₃ to 0.29 mA cm-2 at 1.23 V.（4） Ni（OH）₂/α-Fe₂O₃: Ultrathin α-Fe₂O₃ films were deposited on FTO glasses, then Ni（OH）2 ultrathin nanosheets were grown on the films by a facile hydrothermal method. Ni（OH）2 serving as the cocatalyst of water oxidation are able to enhance the kinetics of the system, resulting in the reduction of the overpotential. The Ni（OH）2/Fe₂O₃ films demonstrated a cathodical shift by 400 mV, and the photocurrent density at 1.23 V was enhanced from 0.21 to 0.37 mA cm-2 compared with the pristine ultrathin α-Fe₂O₃ films.The works help us further understand the working mechanism of a heterojunction as well as offer us a promising way to achieve highly efficient PEC water splitting.