Ammonia synthesis via the Haber-Bosch process, which has been heralded as the most important invention of the 20th century, consumes massive amounts of energy, around~1%-2% of the world's annual energy consumption. Developing green and sustainable strategies for NH3 synthesis under ambient conditions, using renewable energy, is strongly desired, by both industrial and scientific researchers. Artificial photosynthesis for ammonia synthesis, which has recently attracted significant attention, directly produces NH3 from sunlight, and N2 and H2O via photocatalysis. This has been regarded as an ideal, energy-saving and environmentally-benign process for NH3 production because it can be performed under normal temperature and atmospheric pressure using renewable solar energy. Although sustainable developments have been achieved since the pioneering work in 1977, many challenging issues (e.g., adsorption and activation of nitrogen molecules on the surface of photocatalysts under mild conditions) have still not been well solved and the photocatalytic activities are generally low. In this miniature review, I summarize the most recent progress of photocatalytic N2 fixation for ammonia synthesis, focusing specifically on two attractive aspects for adsorption and activation of nitrogen molecules:one is engineering of oxygen vacancies, and the other is mimicking natural nitrogenase for constructing artificial systems for N2 fixation. Several representative works focusing on these aspects in artificial systems have been reported recently, and it has been demonstrated that both factors play more significant roles in photocatalytic N2 reduction and fixation under ambient conditions. At the end of the review, I also give some remarks and perspective on the existing challenges and future directions in this field.
