The promotional effects on photocatalytic hydrogen production of CuxO clusters deposited using atomic layer deposition (ALD) on P25 TiO2 are presented. The structural and surface chemistry study of CuxO/TiO2 samples, along with first principles density functional theory simulations, reveal the strong interaction of ALD deposited CuxO with TiO2, leading to the stabilization of CuxO clusters on the surface; it also demonstrated substantial reduction of Ti4+ to Ti3+ on the surface of CuxO/TiO2 samples after CuxO ALD. The CuxO/TiO2 photocatalysts showed remarkable improvement in hydrogen productivity, with 11 times greater hydrogen production for the optimum sample compared to unmodified P25. With the combination of the hydrogen production data and characterization of CuxO/TiO2 photocatalysts, we inferred that ALD deposited CuxO clusters have a dual promotional effect: increased charge carrier separation and improved light absorption, consistent with known copper promoted TiO2 photocatalysts and generation of a substantial amount of surface Ti3+ which results in self-doping of TiO2 and improves its photo-activity for hydrogen production. The obtained data were also employed to modify the previously proposed expanding photocatalytic area and overlap model to describe the effect of cocatalyst size and weight loading on photocatalyst activity. Comparing the trend of surface Ti3+ content increase and the photocatalytically promoted area, calculated with our model, suggests that the depletion zone formed around the heterojunction of CuxO-TiO2 is the main active area for hydrogen production, and the hydrogen productivity of the photocatalyst depends on the surface coverage by this active area. However, the overlap of these areas suppresses the activity of the photocatalyst.@en