Cd0.8Mn0.2S/MoO3 composites with an S-scheme heterojunction for efficient photocatalytic hydrogen evolution

Emerging S-Scheme Photocatalyst

Photocatalysis is a green technology to use ubiquitous and intermittent sunlight. The emerging S-scheme heterojunction has demonstrated its superiority in photocatalysis. This article covers the state-of-the-art progress and provides new insights into its general designing criteria. It starts with the challenges confronted by single photocatalyst from the perspective of energy dissipation by borrowing the common behaviors in the dye molecule. Subsequently, other problems faced by single photocatalyst are summarized. Then a viable solution for these problems is the construction of heterojunctions. To overcome the problems and mistakes of type-II and Z-scheme heterojunctions, S-scheme heterojunction is proposed and the underlying reaction mechanism is summarized. Afterward, the design principles for S-scheme heterojunction are proposed and four types of S-scheme heterojunctions are suggested. Following this, direct characterization techniques for testifying the charge transfer in S-scheme heterojunction are presented. Finally, different photocatalytic applications of S-scheme heterojunctions are summarized. Specifically, this work endeavors to clarify the critical understanding on curved Fermi level in S-scheme heterojunction interface, which can help strengthen and advance the fundamental theories of photocatalysis. Moreover, the current challenges and prospects of the S-scheme heterojunction photocatalyst are critically discussed.

Interface engineering: Synergism between S-scheme heterojunctions and Mo-O bonds for promote photocatalytic hydrogen evolution

Simple high-temperature calcination and hydrothermal methods were followed to synthesize CeO₂ and Mo-S, respectively. The efficient photocatalytic hydrogen evolution activity exhibited by the composite catalysts can be attributed to the edge active sites in Mo-S. The Mo-O bonds formed between CeO₂ and Mo-S could further accelerate the processes of separation and migration of electrons between the catalyst interfaces. The hybrid catalyst 10%-CeO₂/Mo-S exhibiting the best hydrogen generation ability (4.3 mmol h^-1g^-1) was obtained by optimizing the content of CeO₂ in CeO₂/Mo-S. Analysis of the PL spectral profile and photocurrent response recorded for the system revealed that 10%-COMS exhibited excellent photogenerated carrier separation ability. Analysis of the LSV and EIS curves revealed that 10%-COMS exhibited the optimal hydrogen production potential. The charge migration resistance provided by the systems was lower than the charge migration resistance provided by CeO₂ and Mo-S. The synergism between the S-scheme heterojunctions and the Mo-O bonds helped accelerate the separation and migration of photo-induced carriers at the catalyst interfaces. The introduction of covalent bonds in the S-scheme heterojunctions and the results presented herein can potentially help develop a new method to realize photocatalytic hydrogen evolution.

Unique ternary Ni-MOF-74/Ni2P/MoSx composite for efficient photocatalytic hydrogen production: Role of Ni2P for accelerating separation of photogenerated carriers

A reasonable introduction of MOFs-derived Ni₂P with high dispersity is a valid way to reduce the recombination rate of photogenerated electron-holes, thus for more effective visible-light-driven water splitting. In this study, Ni-MOF-74/Ni₂P precursor was obtained by low-temperature phosphating method. A ternary heterojunction Ni-MOF-74/Ni₂P/MoS_x with a unique structure is obtained by a solution-based mixing method. The unique structure of Ni-MOF-74/Ni₂P provides advantages for MoS_x load. The UV-visible diffuse reflectance spectroscopy proves that the introduction of Ni₂P improves the utilization of visible light by the composite catalyst 10%-NPMS and promotes more electrons generation, thereby improving photocatalytic hydrogen production activity. It is proved that the introduced Ni₂P can accelerate the separation of photogenerated carriers by characterization (PL, EIS, LSV, etc.) analyses. The composite catalyst 10%-NPMS with the best hydrogen production activity was obtained by adjusting the ratio between Ni-MOF-74/Ni₂P and MoS_x. The photocatalytic hydrogen evolution of the composite catalyst 10%-NPMS (286.16 μmol) is 28.30, 2.78, 3.79 and 2.41 times that of pure Ni-MOF-74, Ni₂P, MoS_x and binary 10%-Ni-MOF-74/MoS_x within 5 h, respectively. And the hybrid 10%-Ni-MOF-74/Ni₂P/MoS_x exhibits excellent photocatalytic hydrogen evolution performance and good stability. This research will provide a new strategy for synthesizing unique ternary composite materials by using metal organic framework materials as precursors.