A review on g-C3N4-based photocatalysts

Photocatalytic water splitting represents a promising strategy for clean, low-cost, and environmental-friendly production of H2 by utilizing solar energy. There are three crucial steps for the photocatalytic water splitting reaction: solar light harvesting, charge separation and transportation, and the catalytic H2 and O2 evolution reactions. While significant achievement has been made in optimizing the first two steps in the photocatalytic process, much less efforts have been put into improving the efficiency of the third step, which demands the utilization of cocatalysts. To date, cocatalysts based on rare and expensive noble metals are still required for achieving reasonable activity in most semiconductor-based photocatalytic systems, which seriously restricts their large-scale application. Therefore, seeking cheap, earth-abundant and high-performance cocatalysts is indispensable to achieve cost-effective and highly efficient photocatalytic water splitting. This review for the first time summarizes all the developed earth-abundant cocatalysts for photocatalytic H2- and O2-production half reactions as well as overall water splitting. The roles and functional mechanism of the cocatalysts are discussed in detail. Finally, this review is concluded with a summary, and remarks on some challenges and perspectives in this emerging area of research.

Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting

Photocatalysis is considered as one of the promising routes to solve the energy and environmental crises by utilizing solar energy. Graphitic carbon nitride (g-C3N4) has attracted worldwide attention due to its visible-light activity, facile synthesis from low-cost materials, chemical stability, and unique layered structure. However, the pure g-C3N4 photocatalyst still suffers from its low separation efficiency of photogenerated charge carriers, which results in unsatisfactory photocatalytic activity. Recently, g-C3N4-based heterostructures have become research hotspots for their greatly enhanced charge carrier separation efficiency and photocatalytic performance. According to the different transfer mechanisms of photogenerated charge carriers between g-C3N4 and the coupled components, the g-C3N4-based heterostructured photocatalysts can be divided into the following categories: g-C3N4-based conventional type II heterojunction, g-C3N4-based Z-scheme heterojunction, g-C3N4-based p–n heterojunction, g-C3N4/metal heterostructure, and g-C3N4/carbon heterostructure. This review summarizes the recent significant progress on the design of g-C3N4-based heterostructured photocatalysts and their special separation/transfer mechanisms of photogenerated charge carriers. Moreover, their applications in environmental and energy fields, e.g., water splitting, carbon dioxide reduction, and degradation of pollutants, are also reviewed. Finally, some concluding remarks and perspectives on the challenges and opportunities for exploring advanced g-C3N4-based heterostructured photocatalysts are presented.

g-C3N4-Based Heterostructured Photocatalysts

S-Scheme Heterojunction Photocatalyst

The appropriate interfacial contact of heterojunction photocatalysts plays a critical role in transfer/separation of interfacial charge carriers. Design of two-dimensional (2D)/2D surface-to-surface heterojunction is an effective method for improving photocatalytic activity since greater contact area can enhance interfacial charge transfer rate. Herein, ultrathin 2D/2D WO3/g-C3N4 step-like composite heterojunction photocatalysts were fabricated by electrostatic self-assembly of ultrathin tungsten trioxide (WO3) and graphitic carbon nitride (g-C3N4) nanosheets. The ultrathin WO3 and g-C3N4 nanosheets were obtained by electrostatic-assisted ultrasonic exfoliation of bulk WO3 and a two-step thermal-etching of bulk g-C3N4, respectively. The thickness of ultrathin WO3 and g-C3N4 nanosheets are 2.5–3.5 nm, which is equivalent to 5–8 atomic or molecular layer thickness. This ultrathin layered heterojunction structure can enhance surface photocatalytic rate because photogenerated electrons and holes at heterogeneous interface more easily transfer to surface of photocatalysts. Therefore, the obtained ultrathin 2D/2D WO3/g-C3N4 step-scheme (S-scheme) heterojunction photocatalysts exhibited better H2-production activity than pure g-C3N4 and WO3 with the same loading amount of Pt as cocatalyst. The mechanism and driving force of charge transfer and separation in S-scheme heterojunction photocatalysts are investigated and discussed. This investigation will provide new insight about designing and constructing novel S-scheme heterojunction photocatalysts.

Ultrathin 2D/2D WO3/g-C3N4 step-scheme H2-production photocatalyst

Semiconductor‐based Z‐scheme heterojunction photocatalysts have received considerable attention for solar energy conversion and environmental purification due to their spatially separated reduction and oxidation sites, effective separation and transportation of photo‐excited charge carriers and strong redox ability. With their wide visible‐light responsive range and high photocatalytic activity, metal sulphide is an important material in developing photocatalysts. This review summarizes and highlights recent research progress in sulphide‐based direct Z‐scheme photocatalysts, followed by analysis on the limitations over all‐solid‐state Z‐scheme photocatalyst. Furthermore, the applications and characterization methods of sulphide‐based direct Z‐scheme photocatalyst are summarized. Finally, the challenges and perspectives of sulphide‐based Z‐scheme photocatalyst are discussed.

Quanlong Xu

Papers

S-Scheme Heterojunction Photocatalyst

Ultrathin 2D/2D WO3/g-C3N4 step-scheme H2-production photocatalyst

Review on Metal Sulphide-based Z-scheme Photocatalysts

Constructing 2D/2D Fe2O3/g-C3N4 Direct Z-Scheme Photocatalysts with Enhanced H2 Generation Performance

Making co-condensed amorphous carbon/g-C3N4 composites with improved visible-light photocatalytic H2-production performance using Pt as cocatalyst