Selective loading of spatially separated redox cocatalysts on direct Z-scheme heterojunctions holds great promise for advancing the efficiency of artificial photosynthesis, which however is limited to the photodeposition of noble metal cocatalysts and the fabrication of hollow double-shelled semiconductor heterojunctions. Moreover, the co-exposure of discrete cocatalyst and semiconductor increases the product diversity when both the exposed sites of which participate in CO₂ photoreduction. Herein, we present a facile and versatile protocol to overcome these limitations via surface coating of Z-scheme heterojunctions with bifunctional noble-metal-free cocatalysts. With Cu₂O/Fe₂O₃ (CF) as a model heterojunction and layered Ni(OH)₂ as a model cocatalyst, it is found that Ni(OH)₂ lying on the surfaces of Cu₂O and Fe₂O₃ separately co-catalyzes the CO₂ reduction and H₂O oxidation. Thorough experimental and theoretical investigation reveals that the Ni(OH)₂ outer layer: (i) mitigates the charge recombination in CF and balances their transfer and consumption; (ii) reduces the rate-determining barriers for CO₂-to-CO and H₂O-to-O₂ conversion, (iii) suppresses the side proton reduction occurring on CF, and (iv) protects the CF from component detachment. As expected, the redox reactions stoichiometrically proceed, and significantly enhanced photocatalytic activity, selectivity, and stability in CO generation are achieved by the stacked Cu₂O/Fe₂O₃@Ni(OH)₂ in contrast to CF. This study demonstrates the significance of the synergy between bifunctional cocatalysts and Z-scheme heterojunctions for improving the efficacy of overall redox reactions, opening a fresh avenue for the rational design of artificial photosynthetic systems.

Crystal phase engineering on photocatalytic materials is a subfield of photocatalysis with intensive research, which has been proven as a versatile approach to maneuver their performance for applications in energy- and environment-related fields. In this article, the state-of-the-art progress on phase-engineered photocatalytic materials is reviewed. Firstly, we discuss the phase engineering on pristine semiconductor photocatalysts, in which the phase-dependent light absorption, charge transfer and separation, and surface reaction behaviors in photocatalytic processes are summarized, respectively. Based on the elucidated mechanisms, the implementation of phase junctions in photocatalytic reactions is then presented. As a focus, we highlight the rational design of phase junctions toward steering the charge kinetics for enhanced photocatalytic and photoelectrocatalytic performance. Moreover, the crystal phase engineering on semiconductor-based hybrid photocatalysts is also introduced, which underlines the importance of choosing a suitable phase for semiconductor components and co-catalysts as well as the synergism of different semiconductor phases for improved photocatalytic performance. Finally, the challenges and perspectives in this research field are proposed. In this review, particular emphasis is placed on establishing a linkage between crystal phase and photocatalytic activity to develop a structure-activity guide. Based on the guide, a framework is suggested for future research on the rational phase design of photocatalysts for improved performance in energy and environmental applications.

Photocatalytic reduction of CO₂ into high value-added CH₄ is a promising solution for energy and environmental crises. Integrating semiconductors with cocatalysts can improve the activities for photocatalytic CO₂ reduction; however, most metal cocatalysts mainly produce CO and H₂. Herein, we report a cocatalyst hydridation approach for significantly enhancing the photocatalytic reduction of CO₂ into CH₄. Hydriding Pd cocatalysts into PdH_0.43 played a dual role in performance enhancement. As revealed by our isotopic labeling experiments, the PdH_0.43 hydride cocatalysts reduced H₂ evolution, which suppressed the H₂ production and facilitated the conversion of the CO intermediate into the final product: CH₄. Meanwhile, hydridation promoted the electron trapping on the cocatalysts, improving the charge separation. This approach increased the photocatalytic selectivity in CH₄ production from 3.2% to 63.6% on Pd{100} and from 15.6% to 73.4% on Pd{111}. The results provide insights into photocatalytic mechanism studies and introduce new opportunities for designing materials towards photocatalytic CO₂ conversion.