Directly Assembled 3D Molybdenum Disulfide on Silicon Wafer for Efficient Photoelectrochemical Water Reduction

DOI: 10.1002/adsu.201700142 chemical fuels became an alternative path for the search of clean energy sources.[1] Therefore, a diverse class of semiconductor photoelectrodes and nonprecious catalytic materials has been investigated for solar water splitting.[2–5] Only a few semiconductor photocathodes and noble metal-free catalysts showed encouraging solar water splitting performances for hydrogen productions.[6–9] Silicon (Si) is an ideal photocathode material with a narrow bandgap (Eg = 1.12 eV) with a wide spectral absorption upon solar radiation[10] and appropriate band-edge positions. However, the poor stability in the liquid electrolytes and the high overpotential for charge transfers at the solid/liquid interfaces are yet to be overcome. Furthermore, the charge generation efficiency of Si photocathodes is limited by the high optical reflectance, i.e., 37% (arithmetic mean) of the incident light is reflected in the entire visible range.[11] Therefore, choosing appropriate catalytic materials to mitigate the overpotentials of Si photocathodes is critical to stabilize the liquid electrolyte for the extended operation time and reduce the substantial reflectance of Si for the higher photoelectrochemical (PEC) performance. Currently, among the noble metal-free catalysts to decrease the overpotential of the p-type Si (p-Si) photocathode for the PEC hydrogen production, molybdenum disulfide (MoS2) has gained considerable attention as a promising hydrogen evolution reaction (HER) catalyst owing to its low hydrogen adsorption free energy and the high photochemical stability at the affordable expenses, compared to conventional catalysts based on precious metals.[12–15] The transition from the indirect bandgap structures in bulk MoS2 to the direct bandgap one in MoS2 monolayers may also improve the charge transport efficiencies.[16,17] There are three main techniques to enhance the catalytic activity of MoS2 layer.[18–21] First, the phase transition of 2H-MoS2 to 1T-MoS2 is effective way to improve the catalytic activity because the 1T-MoS2 has metallic nature. The 1T-MoS2 has many catalytic active sites compared to 2H-MoS2. Second, the introduction of sulfur vacancy and strain in MoS2 is one of promising way to elevate its catalytic activity. By demonstrating the theoretical and experimental results related to MoS2 composed of earth-abundant elements is considered as a promising hydrogen evolution reaction (HER) catalyst for p-type Si photocathode owing to its appropriate hydrogen adsorption free energy for the edge sites and high photochemical stability in acidic electrolytes. However, the direct synthesis of uniform and atomically thin MoS2 on Si by usual chemical vapor deposition techniques remains challenging because of the weak van der Waals interaction between Si and MoS2. Herein, by controlling the gas phase kinetics during metal–organic chemical vapor deposition, wafer-scale direct synthesis of 3D MoS2 films on TiO2-coated p-type Si substrates is demonstrated. The 3D MoS2 layer with a number of edge sites exposed to ambient substantially reduces the HER overpotential of Si photocathode and simultaneously increases the saturation current density due to the antireflection effect. Directly grown 3D MoS2 thin films are stable under extended water reduction duration. The strategy paves the way for efficient assembly of transition metal disulfide HER catalysts on the p-type photocathode.