The role of CuO nanoparticles decorating TiO2 nanotubes (TNT) thin film photoanodes in the behavior of photo-electrocatalytic (PEC) cells for water splitting reaction is investigated. CuO is present mainly as small nano particles of few nanometer decorating the internal walls of the TiO2 nanotubes. Their presence improves i) the photocurrent behavior, ii) the H-2 generation rate by water splitting in a full PEC device (without application of a bias) and iii) the solar-to-hydrogen (STH) efficiency. The increase is about 20% with respect to parent TNT photoanodes using open spectrum light from a solar simulator and about 50% increase using AM 1.5G filtered light from a solar simulator. An STH efficiency over 2% in the full PEC cell is observed in the best conditions. IPCE (incident photon to current conversion efficiency) measurements clearly evidence that the presence of CuO nanoparticles induce an enhanced IPCE in the 300-340 mn region. The increase in the performances in water splitting is mainly associated to the transient generation of a p-n junction between the CuxO nanoparticles and TNT upon illumination, which enhances photocurrent density by promoting charge separation.