Photo-thermochemical CO₂ Splitting Based on Zinc-germanium Binary Oxide

Using solar energy to split CO2 can realize the conversion and storage of solar energy at the same time, and alleviate the carbon emissions caused by the transitional use of fossil energy. Solar energy based photo-thermochemical reaction is a promising method for the CO2 splitting. To further study the photo-thermochemical reaction mechanism and explore the non-titanium-based catalytic materials, the ZnO/Zn2GeO4 composite material (Z/ZGO) was prepared by solution precipitation method and used for photo-thermochemical CO2 splitting. Composite semiconductor combined the advantages of the two components which made CO production reach 5.55 times that of pure ZnO. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to illustrate the crystal structure and chemical composition of the samples. The XRD pattern found that the samples crystallized well, and no obvious crystal form changes occurred after the reaction. Using SEM to observe the samples before and after the reaction, the particle size did not increase significantly and no obvious sintering phenomenon was found, which indicated that the material has good reaction stability. Photoluminescence (PL), UV-visible diffuse reflectance spectra (UV-visible DRS) and Mott-Schottky plots were used to evaluate the material's light absorption characteristics and energy band position. The band gap of ZnO and Zn2GeO4 samples were 3.27 eV and 4.56 eV, respectively, and the heterojunction was formed in the Z/ZGO sample. The presence of ZnO extended the spectral response range of Zn2GeO4, and due to the migration of photogenerated electron-hole pairs (EHPs) to ZnO, the recombination of EHPs w as reduced. XPS analyses were also used to investigate change of oxygen vacancies during the reaction. The O is XPS spectra of the samples in the three cases (Case A: before light irradiation, Case B: after light irradiation and Case C: after reaction) were analyzed and found that the signal of the oxygen near the vacancies increased after light irradiation and decreased after reaction, which may indicate that oxygen vacancies were formed after light irradiation then consumed by CO2 in the reaction. The Zn2GeO4 sample showed the largest increase in oxygen vacancies signal after light irradiation, indicating that Zn2GeO4 had a strong ability to form oxygen vacancies. Zn2GeO4 improves the capacity of oxygen vacancies formation in the sample. and further improved the yield of photo-thermochemical CO2 splitting reaction. As a result, Z/ZGO combined the advantages of ZnO in light response and Zn2GeO4 in oxygen vacancies formation and improved the CO2 splitting yield. This research has a positive effect on expanding the photo-thermochemical material system and further deepening the photo-thermochemical reaction mechanism.