纳米氧化物光催化剂的缺陷调控研究进展

The ever-growing need for energy prompted for the development of sustainable clean energy sources. One promising direction was the use of photocatalysis for the production of green fuels (H₂, CO, and syngas by splitting the abundant H₂O and CO₂ gases) or the production of methanol and liquid hydrocarbons (from the reaction of CO/CO₂ with H₂). Oxide photocatalysts had been extensively studied in the field of photocatalysis due to their low price, abundant sources, good stability, and less secondary pollution. However, the catalytic efficiency was unsatisfactory as a result of the low utilization rate of light energy of oxides, poor charge transport capacity, and lack of surface effective catalytic active sites. Modulation of the electronic structure, carrier diffusion and surface adsorption properties of the oxides through defect engineering was an effective strategy to improve their photocatalytic activity. This review summarized the research progress on defect chemistry of oxides for photocatalysis applications. First, defect engineering was introduced in terms of defect definition, classification and building strategies. Crystal defects were places where the perfect periodic arrangement of atoms or molecules in a crystalline material were disrupted or destroyed. According to the dimension, defects could be classified into zero-dimensional point defects, one-dimensional line defects, two-dimensional surface defects and three-dimensional bulk defects, and defects of different dimensions could be further subdivided into more types of defects, such as vacancy-type defects, doping, pits, and grain boundaries, etc. In terms of construction methods, the construction methods of vacancy defects (oxygen vacancies, metal vacancies, vacancy associations) and doping were mainly introduced. Then, the proper characterization techniques for defects were summarized and compared, which mainly included electron microscopy, Raman spectroscopy as well as surface-enhanced Raman spectroscopy, synchrotron X-ray absorption fine structure spectroscopy, positron annihilation spectroscopy, electron paramagnetic resonance spectroscopy, time-resolved spectroscopy based on ultrafast spectroscopy, energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, and in situ techniques. In particular, advanced in situ techniques could monitor the dynamic evolution of morphology, morphology, electron transfer behavior and active sites of defective oxides under experimental conditions in real time. The effective characterization of atomic-level defects was a prerequisite for establishing the structure-activity relationship between defect and photocatalytic activity. Furthermore, the important functions of defects in oxide photocatalysts and the relevant applications of defect-based oxide photocatalysts in energy were elucidated in depth. The effects of defects on oxide materials were mainly reflected in: (1) Adjusting the energy band structure of the oxide. One was that the existence of defects will introduce a defect energy level between the valence band and conduction band, which provided an intermediate bridge for the transition of valence band electrons; on the other hand, the existence of defects would change the 2p orbitals of oxygen atoms and the d or p of metal atoms, resulting in a downward shift of the minimum value of the conduction band or an upward shift of the maximum value of the valence band, thereby narrowing the band gap. (2) Affecting the physical properties of oxides such as conductivity and magnetism. For example, defects could cause n-type semiconductors to exhibit p-type conductivity and increase the room temperature ferromagnetism of the semiconductors. (3) Establishing effective catalytic active sites. The atoms of unsaturated coordination at the defects became centers of electron aggregation, facilitating electron transfer between the adsorbed gas and the material, favoring adsorption and activation of the adsorbed gas molecules and improving the surface reaction. The multiple roles of defects made defective oxides exhibited very attractive energy and environmental applications, such as photocatalytic water splitting to produce hydrogen, carbon dioxide (CO₂) reduction, nitrogen fixation, and methane dry reforming. Finally, the key challenges and future opportunities regarding defect engineering in photocatalysis were presented to highlight the development directions of this research field.