Vertical MoS₂on SiO₂/Si and Graphene: Effect of Surface Morphology on Photoelectrochemical Properties

Two-dimensional materials have attracted intensive attention recently due to their unique optical and electronic properties and their promising applications in water splitting and solar cells. As a representative layer-structured of transition metal dichalcogenides, MoS₂has attracted considerable devotion owing to its exceptional photo and electro properties. Here, we show that the chemical vapour deposition (CVD) growth of MoS₂on Si photocathode and graphene/Si photocathode can be used to prepare photoelectrocatalysts for water splitting. We explore a bottom-up method to grow vertical heterostructures of MoS₂and graphene by using the two-step CVD. Graphene is first grown through ambient-pressure CVD on a Cu substrate and then transferred onto SiO2/Si substrate by using the chemical wet transfer followed by the second CVD method to grow MoS₂over the graphene/SiO₂/Si. The effect of the growth temperatures of MoS₂is studied, and the optimum temperature is 800 °C. The MoS₂produced at 800 °C has the highest photocurrent density at -0.23 mA cm^-2in 0.5 M Na₂SO₄and -0.51 mA cm^-2in 0.5 M H₂SO₄at -0.8 V vs. Ag/AgCl. The linear sweep voltammetry shows that MoS₂in 0.5 M H₂SO₄has about 55% higher photocurrent density than MoS₂in Na₂SO₄due to the higher protons (H⁺) in the H₂SO₄electrolyte solution, which are sufficiently charged to reduce to H₂and, therefore hydrogen evolves more rapidly where the photocurrent density and hydrogen generation can be enhanced. MoS₂/graphene/SiO₂/Si (MGS) has -0.07 mA cm^-2at -0.8 V vs. Ag/AgCl of photocurrent density, which is 70% lower than that of bare MoS₂because MGS is thicker compared with MoS₂. Thus, MoS₂has potential as a photocatalyst in photoelectrochemical water splitting. The structure and the morphology of MoS₂play an important role in determining the photocurrent performance.