Since the deposited material is meso-porous nanostructured Zn, it needs to be annealed to form ZnO. Additionally, annealing is a heat treatment process that often determines the crystal structure, morphology, and chemical properties. Many reports emphasize the importance of annealing and the relationship between the increase in photocatalytic activity and the annealing temperature of the layer, as described by Kim et al. The high activity of nanocomposites obtained after annealing the seed layer is attributed to their high ability to absorb UV radiation, large surface area, and abundance of oxygen defects, which promote the separation of electronhole pairs, reducing electron recombination. Despite the increase in particle size with annealing temperature, the activity of the photocatalyst is enhanced due to the increased number of defects, extended lifetime of charge carriers and higher quantum yield. The post-deposition annealing process is performed in a dry oxygen flow and the main variable responsible for the outcome is the annealing temperature. We know from previous experiments, that the temperature has to be equal or higher than 400 °C, which is why we chose 400, 600 and 800 °C annealing temperatures for samples from each deposition batch, yielding 12 different samples. The annealing time is 20 minutes in each case. Material characterization In order to fully characterize the obtained films, a wide range of studies were carried out. The structural characterization of ZnO thin films were performed by Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray (EDX) on a Zeiss Gemini system. The crystal structure was studied by X-ray diffraction using an Empyrean diffractometer from Malvern Panalytical equipped with a copper anode X-ray tube with a copper anode as a source of Cu Kα radiation. Measurements were made in Bragg-Brentano geometry, in 2θ range: 20 °-80 °with a step of 0.0131 °. The optical characterization was done using room-temperature photoluminescence (PL) with a Kimmon Koha 325 nm He-Cd laser excitation and a Shamrock 500 spectrometer using a 600 lines/mm diffraction grating.