Crystalline ZnO aerogel microspheres for boosting photocatalytic CO2 reduction

Controlling the crystallinity and microstructure of a semiconductor photocatalyst serves as an effective means to boost its photocatalytic performance through improving the charge transfer and separation, enhancing the light absorption, or maneuvering the surface reactions. Nevertheless, the internal pores in aerogel materials inevitably collapse during the traditional high temperature crystallization process, significantly reducing the porous structure and specific surface area (SSA). Herein, microsphere ZnO aerogels have been successfully fabricated for photoreduction of CO 2 using a cooperative strategy of sol–gel method, solvothermal crystallization and cryodesiccation. The obtained aerogel has a large SSA and high crystallinity. Compared with the commercial ZnO powder, the ZnO aerogel microsphere exhibits about 4-fold increase in specific surface area, leading to an increased contact surface between the photocatalyst and the reactant. At the same time, ZnO aerogel with microsphere morphology possesses high light-harvesting and intrapore light reflecting capabilities, demonstrating enhanced optical utilization. Modulation of the crystallinity of ZnO aerogel facilitated the incorporation of defect engineering (zinc defects (Zn i ) and oxygen vacancies (V o )). As a result, ZnO aerogel microsphere exhibits a 5-fold higher products production rate than ZnO powder for photocatalytic CO 2 reactions due to the synergistic effect of appropriate crystallinity and microsphere appearance. It is hoped that this work may provide some insights to tune the catalyst performance through crystallinity and morphology.