Among nanoscale semiconductors, nanodiamonds (ND) have not been really considered yet for photoelectrocatalytic reactions in the energy-related field. This originates from the confusion with ideal monocrystalline diamond featuring a wide bandgap (5.5 eV) that requires a deep UV illumination to initiate photoreactivity. At the nanoscale, ND enclose native defects (sp2 carbon, chemical impurities such as nitrogen) that can create energetic states in the diamond’s bandgap decreasing the light energy needed to initiate the charge separation. In addition, the diamond electronic structure can be strongly modified (over several eV) playing on its surface terminations (oxidized, hydrogenated, aminated) which can open the door to optimized band alignments with the species to be reduced or oxidized. Combining these assets, ND becomes competitive with other semiconductors toward photoreactions. The aim of this PhD is to investigate the ability of nanodiamonds in reducing CO2 through photoelectrocatalysis. To achieve this goal, electrodes will be made from nanodiamonds with different surface chemistries (oxidized, hydrogenated and aminated), either using a conventional ink-type approach or a more innovative one that results in a porous material including nanodiamonds and a PVD-deposited matrix. Then, the (photo)electrocatalytic performances under visible illumination of these nanodiamond-based electrodes toward CO2 reduction will be investigated in terms of production rate and selectivity, in presence or not of a transition metal macrocyclic molecular co-catalyst.