oa Morphology-dependent Photocatalytic Activities Of Crystalline Zno And Their Importance For Environmental And Energy Applications

Oxide semiconductors have been widely used in the area of sensors, energy conversion, and environmental cleanup technologies; because of their high multifunctional photo-activities. However, the overall performance of oxide semiconductors is influenced by their photocatalytic activities which are highly dependent on physical properties such as crystallinity, surface area, morphology (e.g., shape and porosity), etc. Thus, understanding the shape-dependent photocatalytic reactions is important for energy or environmental cleanup applications. Herein, two different types of ZnO, rods and plates, were synthesized using solvothermal technique and the shape-dependent photo-activities were evaluated for degradation of methylene blue and phenol and for hydrogen evolution. Experimental results showed that the surface area and bandgap (E=3.26 eV) of rods and plates were found to be nearly identical, but charge transfer varied. X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) analysis revealed that the rods have more pronounced structural oxygen vacancies (Vox, Vo●, and Vo●●) close to the conduction band which lead to electrons trap. On the other hand, the plates relatively have more interstitial oxygen (Oi, Oi', and Oi'') close to the valence band which facilitate hole trapping, reversely increasing the availability of photogenerated electrons and thus resulting in multi-electron transfer reactions. Accordingly, the rods achieved higher degradation efficiency of both methylene blue and phenol than the plates while the opposite was observed for hydrogen evolution. Therefore, ZnO rods can be a relatively good material for production of OH radicals that require degradation of environmental pollutants, whereas ZnO plates can be used for the system that require multiple electron transfer reaction such as energy production via hydrogen evolution.