The XRD analysis of the samples before and after annealing between 400 °C and 800 °C is presented in figure . The major diffraction peaks for Zn were seen in the as-deposited samples. These disappeared after annealing and peaks corresponding to the (100), (002), (101), ( ), (110), ( ), ( ), (112), and (201) ZnO planes were detected at different positions for the analyzed samples for all temperatures and morphologies. This confirms that the material is pure ZnO with the wurtzite hexagonal phase (JCPDS no. 36-1451). The sharp peaks on the XRD pattern allow to conclude that obtained porous ZnO thin films are highly crystalline. The peak intensity of the (100), (002), and (101) planes increased with increasing annealing at temperatures of 400 °C, 600 °C, and 800 °C indicating improving crystallinity. High crystallinity promotes the transfer of photo generated electrons and holes from the bulk to the surface of the photocatalyst, enhancing photocatalytic efficiency. The only sample with remaining Zn peaks in the pattern after annealing is Sample A annealed at 400 °C suggesting that not all of the Zn layer was oxidized to ZnO and pure zinc remained in the material. The resulting sizes of the crystallites calculated using Scherrer's formula are shown in Table in the Supporting Information. Annealing with an increase in temperature range 400 °C-600 °C promotes agglomeration and coalescence of the nanowires, as was observed for our samples annealed in an oxygen atmosphere. Consistent with this, increasing the temperature causes the decrease of pore sizes and volume thereby reducing the active surface area of ZnO as a photocatalyst. These results suggest that we can control grain size with annealing temperature. The SEM images (figure ) show that the particle sizes are in the range 50 to 600 nm. The Scherrer equation, however, yields crystallite sizes that are smaller than 44 nm. The SEM gives the particle size but the Scherrer equation gives us the size of the crystallite. In essence, a crystallite is a single crystal. We work with polycrystalline particles, which are made up of many crystallites. As a result, the particle size of a polycrystalline material is larger than its crystallite size. All of the samples presented in this paper appear to be polycrystalline, as evidenced by the many peaks seen in their X-ray diffraction patterns.