Epitaxial In Situ Grown Heterojunction with 5s–3p–6p Orbital Coupling for Efficient Photocatalytic Methanol Reforming Toward Hydrogen and Formaldehyde Evolution

Photocatalytic methanol reforming affords a sustainable route for the synergistic production of hydrogen and high‐value formaldehyde; however, the atomic‐scale orbital interactions that fundamentally govern interfacial charge transfer and substrate activation remain a critical challenge. Herein, we overcome this challenge by developing a gradient 5 s –3 p –6 p (Cd 5 s –S 3 p –Bi 6 p ) orbital coupling strategy via in situ epitaxial growth of Bi 2 S 3 on CdS (BS/CdS‐H). This orbital coupling is defined as the gradient hybridization of Cd 5 s , S 3 p , and Bi 6 p orbitals across the epitaxial interface, forming a continuous orbital network that creates a directional electron migration channel and tailors the Cd 5 s orbital occupancy. Consequently, it simultaneously promotes interfacial charge separation and modulates Cd 5 s –O 2 p hybridization, which primarily facilitates O–H dissociation and thereby modulates the kinetic barrier for the subsequent C–H bond cleavage toward HCHO formation. The BS/CdS‐H heterojunction delivers exceptional H 2 and HCHO co‐production rates of 5111.7 and 4630.0 µmol g −1 , representing 47.2‐ and 72.8‐fold enhancements over pristine CdS. This work establishes atomic‐scale orbital coupling as a predictive design principle that transcends conventional band‐alignment paradigms, offering a versatile orbital‐engineering framework for rational development of high‐efficiency solar fuel co‐conversion systems.