In Situ Fabrication of Nanoepitaxial TiO2 Protection Layer on Si Substrate: Hole Chemical Conduction Instead of Tunneling Effect

The efficiency of the Si photoanode of photoelectrochemical (PEC) cells in solar water splitting undergoes poor charge separation and transfer process through multiple interfaces. This phenomenon occurs mainly due to a recombination induced by interfacial defects and quantum tunneling effect caused by SiO2 layer insulation. In this study, the in situ fabrication of nanoepitaxial TiO2 on Si substrates in constructing TiO2/Si heterojunction is performed by combining a self-assembly process and a hydrothermal method. The nanoepitaxial growth process is conducted because of the crystal type and lattice matching between anatase-type TiO2 and Si substrate. Studies have shown that a minimal insulator SiO2 layer is left in the heterogeneous interface, which is contacted by a chemical bond. Therefore, a nearly defect-free heterojunction without a SiO2 insulator layer is obtained. After depositing the common catalyst Ni, a large saturated current density of Ni/TiO2/Si photoanode is achieved. This excellent property is caused by fast carrier transfer through the heterogeneous interface, resulting in the efficiency of injection and separation of fast carriers. In comparison with the traditional method, (i.e., the TiO2 protection film deposited by atom layer deposition (ALD)), the Ni/TiO2/Si photoanode fabricated by nanoepitaxial growth has a significantly lower onset potential and remarkably higher current density, which is 11-fold at 1.23 V bias potential. Results confirm that the nanoepitaxy of TiO2 on Si can improve the stability of Si substrates in an alkaline solution for more than 24 h.