Sluggish separation and migration kinetics of the photogenerated carriers account for the low-efficiency of CO2 photoreduction into CH4.Design and construction two-dimensional (2D) in-plane heterostructures demonstrate to be an appealing approach to address above obstacles.Herein,we fabricate 2D in-plane heterostructured Ag2S-In2S3 atomic layers via an ion-exchange strategy.Photoluminescence spectra,time-resolved photoluminescence spectra,and photoelectrochemical measurements firmly affirm the optimized carrier dynamics of the In2S3 atomic layers after the introduction of in-plane heterostructure.In-situ Fourier transform infrared spectroscopy spectra and density functional theory (DFT) calculations disclose the in-plane heterostructure contributes to CO2 activation and modulates the adsorption strength of CO* intermediates to facilitate the formation of CHO* intermediates,which are further protonated to CH4.In consequence,the in-plane heterostructure achieves the CH4 evolution rate of 20 μmol.g-1.h-1,about 16.7 times higher than that of the In2S3 atomic layers.In short,this work proves construction of in-plane heterostructures as a promising method for obtaining high-efficiency CO2-to-CH4 photoconversion properties.