Perspective on water-induced colloidal CsPbBr₃ metal halide perovskites and their non-perovskite derivatives

All-inorganic CsPbBr₃ metal halide perovskite nanocrystals have attracted significant attention for their outstanding optoelectronic properties, yet their pronounced sensitivity to water has long been considered a critical limitation. Here, we provide a unifying perspective that reframes water not merely as a degradation factor, but also as an adaptable chemical control that promotes functional diversification in colloidal Cs-Pb-Br systems. We discuss the mechanisms underlying water-induced transformations and the formation of key non-perovskite derivatives that are water-resistant, such as PbBrOH, CsPb₂Br₅, and Cs₄PbBr₆, and highlight their structural relationships with the parent perovskite lattice. Emphasis is placed on their emerging functionalities as wide-bandgap hosts, surface passivation layers, and heterostructure components, as well as on their dynamic, often reversible interconversion with CsPbBr₃ under controlled conditions. We further discuss how to enhance stability and enable tunable optoelectronic performance across applications with the water-resistant non-perovskite derivatives, including photodetectors, light-emitting diodes, photovoltaics, and bioimaging. Finally, we outline key challenges and future directions toward predictive control of moisture-driven phase behaviour, emphasising the need for in situ characterisation and mechanistic understanding. This perspective establishes water as a powerful synthetic and functional tool, offering new design paradigms for next-generation perovskite nanomaterials.