Vertical MoS₂ on 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, MoS₂ has attracted considerable devotion owing to its exceptional photo and electro properties. Here, we show that the chemical vapour deposition (CVD) growth of MoS₂ on 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 MoS₂ and 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 MoS₂ over the graphene/SiO₂/Si. The effect of the growth temperatures of MoS₂ is studied, and the optimum temperature is 800 °C. The MoS₂ produced at 800 °C has the highest photocurrent density at -0.23 mA cm^-2 in 0.5 M Na₂SO₄ and -0.51 mA cm^-2 in 0.5 M H₂SO₄ at -0.8 V versus Ag/AgCl. The linear sweep voltammetry shows that MoS₂ in 0.5 M H₂SO₄ has about 55% higher photocurrent density than MoS₂ in Na₂SO₄ due to the higher concentration of protons (H⁺) in the H₂SO₄ electrolyte solution. Protons are reduced to H₂ at lower overvoltage and hydrogen generation is thus enhanced at higher photocurrent density. MoS₂/graphene/SiO₂/Si (MGS) has -0.07 mA cm^-2 at -0.8 V versus Ag/AgCl of photocurrent density, which is 70% lower than that of bare MoS₂ because MGS is thicker compared with MoS₂. Thus, MoS₂ has potential as a photocatalyst in photoelectrochemical water splitting. The structure and the morphology of MoS₂ play an important role in determining the photocurrent performance.