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 (NH3) production being the most significant contributor to this energy demand. Currently, NH3 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 N2 conversion. As a means of reducing the energy input and carbon footprint of NH3 synthesis, researchers are now seeking more environmentally friendly approaches for N2 fixation. Photocatalytic NH3 synthesis, using sunlight and a semiconductor photocatalyst, represents one of the more promising strategies for reducing N2 to NH3 (typically employing water as the reducing agent). Rates of photocatalytic NH3 synthesis are currently too low to justify serious practical consideration, which can be traced to the sluggish adsorption/activation kinetics of the N2 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 CO2 reduction. Herein, we explore the potential of defect engineering to similarly enhance photocatalytic N2 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 N2 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 NH3 synthesis, thus providing a useful scaffold for future research in this very exciting and emerging field.