Defect Engineering in Photocatalytic Nitrogen Fixation

Approximately 2% of the energy consumed by humans each year is used to make nitrogen-based fertilizers, with ammonia (NH 3 ) production being the most significant contributor to this energy demand. Currently, NH 3 is synthesized by the Haber–Bosch process, an energy-intensive synthesis requiring both high temperatures (400–500 °C) and pressures (150–250 atm) to achieve meaningful rates of N 2 conversion. As a means of reducing the energy input and carbon footprint of NH 3 synthesis, researchers are now seeking more environmentally friendly approaches for N 2 fixation. Photocatalytic NH 3 synthesis, using sunlight and a semiconductor photocatalyst, represents one of the more promising strategies for reducing N 2 to NH 3 (typically employing water as the reducing agent). Rates of photocatalytic NH 3 synthesis are currently too low to justify serious practical consideration, which can be traced to the sluggish adsorption/activation kinetics of the N 2 molecule on semiconductor catalyst surfaces under ambient temperature and pressure conditions. Recent studies have highlighted the potential of defect engineering for boosting the light-harvesting, charge separation, and adsorption characteristics of semiconductor photocatalysts in reductive processes such as water splitting and CO 2 reduction. Herein, we explore the potential of defect engineering to similarly enhance photocatalytic N 2 fixation. Special emphasis is placed on structure modulation (especially 2D materials and porous structures) and interface engineering (including vacancy creation, metal doping, and strain) for enhancing N 2 activation and conversion. The overarching aim of this Perspective is to provide a snapshot of recent breakthroughs in the rational design of semiconductor photocatalysts for NH 3 synthesis, thus providing a useful scaffold for future research in this very exciting and emerging field.