The pulsed laser deposition (PLD) technique has been used to decorate TiO2 nanotubes (NTs) with cobalt-nickel (CoNi) nanoparticles (NPs). The TiO2 NTs were produced beforehand through the controlled anodic oxidation of titanium substrates. The effect of the nature of the PLD background gas (Vacuum, O-2 and He) on the microstructure, composition and chemical bondings of the CoNi-NPs deposited onto the TiO2-NTs has been investigated. We found that the PLD CoNi-NPs have a core/shell (oxide/metal) structure when deposited under vacuum, while they are fully oxidized when deposited under O-2. On the other hand, by varying the CoNi-NPs loading of the TiO2-NTs (through the increase of the number of laser ablation pulses (N-LP)), we have systematically studied their photocatalytic effect by means of cyclic-voltammetry (CV) measurements under both AM1.5 simulated solar light and filtered visible light. We show that depositing CoNi-NPs on the substrate under vacuum and He increases the photo-electrochemical conversion effectiveness (PCE) by 600% (at N-LP = 10,000) in the visible light domain, while their overall PCE degrades with N-LP under solar illumination. In contrast, the fully oxidized CoNi-NPs (deposited under O-2) are found to be the most effective catalyst under sunlight with an overall increase of more than 50% of the PCE at the optimum loading around N-LP similar to 1000. Such catalytic enhancement is believed to result from both an enhanced light absorption by CoO (of which bandgap is of similar to 2.4 eV) and the formation of a heterojunction between NiO/CoO nanoparticles and TiO2 nanotubes. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.