Revealing the Dominant Role of Built-In Electric Fields at Interfaces: 2D/2D g-C3N4/In4SnS8 Heterojunctions Drive Charge Separation for Photocatalytic H2O2 Production and CO2 Reduction

The efficient photocatalytic production of hydrogen peroxide (H2O2) coupled with the selective reduction of carbon dioxide (CO2) to carbon monoxide (CO) represents a sustainable pathway for solar fuel synthesis and chemical production; however, it remains challenging due to inefficient charge separation and sluggish surface reaction kinetics. Although the two-dimensional (2D) sulfide In4SnS8 exhibits excellent visible-light absorption, its photocatalytic performance is fundamentally limited by rapid charge recombination. To address this limitation, a 2D/2D g-C3N4/In4SnS8 type-II heterojunction has been designed and constructed. This heterostructure achieves remarkable photocatalytic activity through the synergistic integration of interfacial engineering and a built-in electric field (IEF). The optimized g-C3N4/In4SnS8 heterojunction demonstrates production rates of 572 μmol L–1 h–1 for H2O2 and 425.1 μL g–1 h–1 for CO, significantly surpassing the performance of its individual components. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations reveal that *OOH serves as the key intermediate for H2O2 formation, while adsorbed *COOH is identified as a critical surface species during CO2 reduction. Furthermore, ultraviolet photoelectron spectroscopy (UPS) confirms that a spontaneously formed interfacial IEF is the primary driver for the directional separation and migration of photogenerated charges toward active sites. This work not only provides fundamental insights into the rational design of type-II heterojunction photocatalysts but also significantly expands their potential photocatalytic applications.