The integration of high-purity nano-objects on substrates remains a great challenge for addressing scaling-up issues in nanotechnology. For instance, grafting gold nanoparticles (NPs) on zinc oxide films, a major step process for catalysis or photovoltaic applications, still remains difficult to master. We report a modified photodeposition (P-D) approach that achieves tight control of the NPs size (7.5 ± 3 nm), shape (spherical), purity, and high areal density (3500 ± 10 NPs/μm2) on ZnO films. This deposition method is also compatible with large ZnO surface areas. Combining electronic microscopy and X-ray photoelectron spectroscopy measurements, we demonstrate that growth occurs primarily in confined spaces (between the grains of the ZnO film), resulting in gold NPs embedded within the ZnO surface grains thus establishing a unique NPs/surface arrangement. This modified P-D process offers a powerful method to control nanoparticle morphology and areal density and to achieve strong Au interaction with the metal oxide substrate. This work also highlights the key role of ZnO surface morphology to control the NPs density and their size distribution. Furthermore, we experimentally demonstrate an increase of the ZnO photocatalytic activity due to high densities of Au NPs, opening applications for the decontamination of water or the photoreduction of water for hydrogen production.