We have studied the optical properties of two-dimensional (2D) Schottky photodiode heterojunctions made of chemical vapor deposited (CVD) graphene on n- and p-type silicon (Si) substrates. Much better rectification behavior is observed from the diodes fabricated on n-Si substrates in comparison with the devices on p-Si substrates in dark condition. Also, graphene - n-Si photodiodes show a considerable responsivity of 270 mAW (1) within the silicon spectral range in DC reverse bias condition. The present results are furthermore compared with that of a molybdenum disulfide (MoS2) - p-type silicon photodiode. (C) 2015 Elsevier Ltd. All rights reserved.