Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Strain-stabilized perovskites: The perovskite materials used for solar cells and light-emitting diodes (which are black in color) are generally less stable at room temperature than the electronically inactive nonperovskite phases (which are yellow in color). Steele et al. show that for CsPbI ₃ , strain induced in a thin film after annealing the material to 330°C and then rapidly cooling it to room temperature kinetically trapped the black phase. Grazing-incidence wide-angle x-ray scattering revealed the crystal distortions and texture formation created by interfacial strain. Science , this issue p. 679

The development of green, sustainable and economical chemical processes represents a cornerstone challenge within chemistry today. Semiconductor heterogeneous photocatalysis is currently utilized within a wide variety of societally impactful processes, spanning reactions like hydrogen production and CO2 conversion, to the organic transformation of raw materials, for added-value chemicals. Metal halide perovskites have recently emerged as a new promising class of cheap and easy to make photocatalytic semiconductors, though their unstable ionically bound crystal structure has thus far restricted widespread application. In this review, we examine the issues hampering MHP-based photocatalysis and highlight the general approaches being taken to achieve promising and stable photocatalytic reaction environments. Specifically, we outline the adoption of halogen acid solutions (i.e. HX; X = I or Br) for hydrogen evolution reactions, relatively low-polar solvents for CO2 photoreduction and organic transformations, and the encapsulation of perovskites for CO2 reduction and water splitting. Further, we detail the measures being taken to arrive at intrinsically stable photocatalytic materials, removing the need for atypical environments. With each technology offering unique sets of benefits and challenges, we conclude by outlining potentially promising opportunities and directions for metal halide perovskite-based photocatalysis research moving forward.