Photocatalytic H2 generation from aqueous ammonia solution using ZnO photocatalysts prepared by different methods

• ZnO photocatalysts were prepared three different methods.
• ZnO materials tested for photocatalytic hydrogen generation from NH3 decomposition.
• The key role for photocatalysis plays the defects in the lattice of ZnO.
• Oxygen vacancies concentrated at particles surfaces trapped electrons.

ZnO photocatalysts were prepared by thermal annealing of zinc acetate, by precipitation reaction of zinc acetate with sodium hydroxide followed by thermal annealing and by precipitation reaction of zinc acetate with sodium hydroxide in the presence of UV irradiation followed by thermal annealing. The prepared nanoparticles were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), UV–Vis diffuse reflectance (UV–Vis DRS) and photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM) and N2 physisorption measurements. The photocatalytic performance of these materials was investigated for H2 evolution using aqueous ammonia media under 254 nm UV light. The H2 evolution was compared with TiO2 Evonik P25. Beside H2 photocatalytic generation, H2 photochemical generation was also observed. The different used synthetic procedure leading to the formation of different defects (oxygen vacancies (VO) and oxygen excess defects (Oi/OZn)) in the ZnO lattice were factors influencing photocatalytic production of hydrogen. Oxygen vacancy (VO) was recognized as defect significantly decreasing photocatalyst activity. The highest yields of hydrogen were observed in ZnO prepared by thermal annealing of zinc acetate in air. This photocatalyst possesses the lowest concentration of oxygen vacancies causing efficient trapping of electrons. Consequently higher number of electrons in conduction band was available for photocatalytic reaction.