High-entropy
alloy nanoparticles (HEA-NPs) show exceptional properties
and great potential as a new generation of functional materials, yet
a universal and facile synthetic strategy in air toward nonoxidized
and precisely controlled composition remains a huge challenge. Here
we provide a laser scribing method to prepare single-phase solid solution
HEA-NPs libraries in air with tunable composition at the atomic level,
taking advantage of the laser-induced metastable thermodynamics and
substrate-assisted confinement effect. The three-dimensional porous
graphene substrate functions as a microreactor during the fast heating/cooling
process, which is conductive to the generation of the pure alloy phase
by effectively blocking the binding of oxygen and metals, but is also
beneficial for realizing accurate composition control via microstructure
confinement-endowed favorable vapor pressure. Furthermore, by combining
an active learning approach based on an adaptive design strategy,
we discover an optimal composition of quinary HEA-NP catalysts with
an ultralow overpotential for Li-CO2 batteries. This method
provides a simple, fast, and universal in-air route toward the controllable
synthesis of HEA-NPs, potentially integrated with machine learning
to accelerate the research on HEAs.