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.