Photoluminescence and photocatalysis of the flower-like nano-ZnO photocatalysts prepared by a facile hydrothermal method with or without ultrasonic assistance

► The flower-like ZnO was successfully prepared with or without ultrasonic assistance. ► A special dual-template directing growth mechanism for such ZnO is proposed. ► The novel photocatalytic activity of such ZnO is highly related to oxygen vacancies. ► Ultrasonic suppresses green emission and decreases the catalytic activity of ZnO. The flower-like ZnO materials were hydrothermally synthesized with or without ultrasonic assistance. When low dosage of citric acid is used, ultrasonic treatment can enhance the distribution and complexation of Zn ions; the formed complicated citrates can serve as soft template directing the formation of seaweed-like ZnO/Zn(OH) 2 precursor; this precursor with citrates attaching on it serves as hard template for the formation of regular flower-like ZnO in the hydrothermal condition. The photoluminescence spectra have identified several kinds of defects in as-synthesized ZnO materials, such as oxygen vacancy ( V o ), zinc vacancy ( V Zn ) and interstitial zinc (Zn i ). The activity evaluation and kinetic calculation show that the samples with higher relative content of oxygen vacancy possess larger reaction rate constant and higher catalytic activity for Rhodamine B photodegradation. The employment of ultrasonic treatment during preparation decreases the relative content of oxygen vacancy, thus suppressing green emission and reducing the photocatalytic activity of such flower-like ZnO.