Two-dimensional hafnium-based semiconductors and their heterostructures with native oxides have been shown unique physical properties and potential electronic and optoelectronic applications. However, the scalable synthesis methods for ultrathin layered hafnium-based semiconductor laterally epitaxy growth and its heterostructure are still restricted, also for the understanding of its formation mechanism. Herein, we report the stable sublimation of alkali halide vapor assisted synthesis strategy for high-quality 2D HfSe₂ nanosheets via chemical vapor deposition. Single-crystalline ultrathin 2D HfSe₂ nanosheets were systematically grown by tuning the growth parameters, reaching the lateral size of 6‒40 μm and the thickness down to 4.5 nm. The scalable amorphous HfO₂ and HfSe₂ heterostructures were achieved by the controllable oxidation, which benefited from the approximate zero Gibbs free energy of unstable 2D HfSe₂ templates. The crystal structure, elemental, and time dependent Raman characterization were carried out to understand surface precipitated Se atoms and the formation of amorphous Hf−O bonds, confirming the slow surface oxidation and lattice incorporation of oxygen atoms. The relatively smooth surface roughness and electrical potential change of HfO₂−HfSe₂ heterostructures indicate the excellent interface quality, which helps obtain the high performance memristor with high on/off ratio of 10⁵ and long retention period over 9000 s. Our work introduces a new vapor catalysts strategy for the synthesis of lateral 2D HfSe₂ nanosheets, also providing the scalable oxidation of the Hf-based heterostructures for 2D electronic devices.