S-Scheme Heterojunction Photocatalyst

There is a growing interest in the conversion of water and solar energy into clean and renewable H2 fuels using earth-abundant materials due to the depletion of fossil fuel and its serious environmental impact. This critical review highlights some key factors influencing the efficiency of heterogeneous semiconductors for solar water splitting (i.e. improved charge separation and transfer, promoted optical absorption, optimized band gap position, lowered cost and toxicity, and enhanced stability and water splitting kinetics). Moreover, different engineering strategies, such as band structure engineering, micro/nano engineering, bionic engineering, co-catalyst engineering, surface/interface engineering of heterogeneous semiconductors are summarized and discussed thoroughly. The synergistic effects of the different engineering strategies, especially for the combination of co-catalyst loading and other strategies seem to be more promising for the development of highly efficient photocatalysts. A thorough understanding of electron and hole transfer thermodynamics and kinetics at the fundamental level is also important for elucidating the key efficiency-limiting step and designing highly efficient solar-to-fuel conversion systems. In this review, we provide not only a summary of the recent progress in the different engineering strategies of heterogeneous semiconductors for solar water splitting, but also some potential opportunities for designing and optimizing solar cells, photocatalysts for the reduction of CO2 and pollutant degradation, and electrocatalysts for water splitting.

Engineering heterogeneous semiconductors for solar water splitting

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Direct Z-scheme photocatalysts: Principles, synthesis, and applications

Recently, great attention has been paid to fabricating direct Z-scheme photocatalysts for solar-energy conversion due to their effectiveness for spatially separating photogenerated electron–hole pairs and optimizing the reduction and oxidation ability of the photocatalytic system. Here, the historical development of the Z-scheme photocatalytic system is summarized, from its first generation (liquid-phase Z-scheme photocatalytic system) to its current third generation (direct Z-scheme photocatalyst). The advantages of direct Z-scheme photocatalysts are also discussed against their predecessors, including conventional heterojunction, liquid-phase Z-scheme, and all-solid-state (ASS) Z-scheme photocatalytic systems. Furthermore, characterization methods and applications of direct Z-scheme photocatalysts are also summarized. Finally, conclusions and perspectives on the challenges of this emerging research direction are presented. Insights and up-to-date information are provided to give the scientific community the ability to fully explore the potential of direct Z-scheme photocatalysts in renewable energy production and environmental remediation.

A Review of Direct Z-Scheme Photocatalysts

Exploring photocatalysts to promote CO2 photoreduction into solar fuels is of great significance. We develop TiO2/perovskite (CsPbBr3) S-scheme heterojunctions synthesized by a facile electrostatic-driven self-assembling approach. Density functional theory calculation combined with experimental studies proves the electron transfer from CsPbBr3 quantum dots (QDs) to TiO2, resulting in the construction of internal electric field (IEF) directing from CsPbBr3 to TiO2 upon hybridization. The IEF drives the photoexcited electrons in TiO2 to CsPbBr3 upon light irradiation as revealed by in-situ X-ray photoelectron spectroscopy analysis, suggesting the formation of an S-scheme heterojunction in the TiO2/CsPbBr3 nanohybrids which greatly promotes the separation of electron-hole pairs to foster efficient CO2 photoreduction. The hybrid nanofibers unveil a higher CO2-reduction rate (9.02 μmol g–1 h–1) comparing with pristine TiO2 nanofibers (4.68 μmol g–1 h–1). Isotope (13CO2) tracer results confirm that the reduction products originate from CO2 source. Rational design and fabrication of high-performance photocatalyst is of great importance for CO2 reduction into solar fuel. Here, the authors demonstrate that S-scheme heterojunction TiO2/CsPbBr3 photocatalyst exhibits enhanced CO2 photoreduction activity.

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Unique S-scheme heterojunctions in self-assembled TiO2/CsPbBr3 hybrids for CO2 photoreduction.

Photocatalytic CO2 reduction is an effective way to simultaneously mitigate the greenhouse effect and the energy crisis. Herein, CdS hollow spheres, on which monolayer nitrogen-doped graphene is in situ grown by chemical vapor deposition, are applied for realizing effective photocatalytic CO2 reduction. The constructed photocatalyst possesses a hollow interior for strengthening light absorption, a thin shell for shortening the electron migration distance, tight adhesion for facilitating separation and transfer of carriers, and a monolayer nitrogen-doped graphene surface for adsorbing and activating CO2 molecules. Achieving seamless contact between a photocatalyst and a cocatalyst, which provides a pollution-free and large-area transport interface for carriers, is an effective strategy for improving the photocatalytic CO2 reduction performance. Therefore, the yield of CO and CH4 , as dominating products, can be increased by four and five times than that of pristine CdS hollow spheres, respectively. This work emphasizes the importance of contact interface regulation between the photocatalyst and the cocatalyst and provides new ideas for the seamless and large-area contact of heterojunctions.

In Situ Grown Monolayer N-Doped Graphene on CdS Hollow Spheres with Seamless Contact for Photocatalytic CO2 Reduction.

Photocatalytic CO2 reduction into solar fuels over photocatalysts has theoretically and practically become a hot research topic. Herein, we fabricated a novel hybrid TiO2 nanofiber coated by CuInS2 nanoplates through a hydrothermal method. The materials were characterized by X-ray diffraction, electron microscopes, UV–vis absorption spectra, nitrogen sorption, X-ray photoelectron spectroscopy and electrochemical impudence spectroscopy. The resulting TiO2/CuInS2 hybrid nanofibers exhibit superior photocatalytic activity for CO2 reduction under irradiation, due to the generation of direct Z-scheme heterojunction between TiO2 and CuInS2. This work may provide an alternate methodology to design and fabricate multicomponent TiO2-based photocatalyst for high-efficiency CO2 photoreduction.

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CuInS2 sensitized TiO2 hybrid nanofibers for improved photocatalytic CO2 reduction

One‐dimensional (1D) nanostructured photocatalyst is a promising candidate for hydrogen (H2) generation, which can be used to deal with the energy crisis. Herein, novel 1D TiO2/CdS well‐hybridized nanofibers (NFs) were synthesized via in situ electrospinning method. These 1D hybrid NFs showed a high H2‐production rate of 2.32 mmol h−1 g−1 with an apparent quantum efficiency of 10.14 %, which was 35‐fold higher than that of pristine TiO2 NFs. X‐ray photoelectron spectroscopy (XPS) analysis and density functional theory calculation implied that the electrons transferred from CdS to TiO2 upon hybridization and created an internal electric field (IEF) pointing from CdS to TiO2. This IEF drove the photoexcited electrons in TiO2 to transfer toward CdS upon light irradiation as revealed by in situ irradiated XPS analysis, suggesting that a step‐scheme (S‐scheme) heterojunction was formed in the TiO2/CdS nanohybrids and greatly promoted the separation of electron‐hole pairs to foster efficient H2 photogeneration. The significant enhancement of photocatalytic activity was also benefited from the easy transfer for electrons in the 1D well‐distributed nanostructure of nanohybrids. This work presents a method for in situ preparing well‐distributed 1D NFs with high photocatalytic activity for H2 production via the S‐scheme pathways.

S‐Scheme Heterojunction TiO2/CdS Nanocomposite Nanofiber as H2‐Production Photocatalyst

Titanium implants have been widely used in bone tissue engineering for decades. However, orthopedic implant-associated infections increase the risk of implant failure and even lead to amputation in severe cases. Although TiO2 has photocatalytic activity to produce reactive oxygen species (ROS), the recombination of generated electrons and holes limits its antibacterial ability. Here, we describe a graphdiyne (GDY) composite TiO2 nanofiber that combats implant infections through enhanced photocatalysis and prolonged antibacterial ability. In addition, GDY-modified TiO2 nanofibers exert superior biocompatibility and osteoinductive abilities for cell adhesion and differentiation, thus contributing to the bone tissue regeneration process in drug-resistant bacteria-induced implant infection.

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Feiyan Xu

Papers

Unique S-scheme heterojunctions in self-assembled TiO2/CsPbBr3 hybrids for CO2 photoreduction.

In Situ Grown Monolayer N-Doped Graphene on CdS Hollow Spheres with Seamless Contact for Photocatalytic CO2 Reduction.

CuInS2 sensitized TiO2 hybrid nanofibers for improved photocatalytic CO2 reduction

S‐Scheme Heterojunction TiO2/CdS Nanocomposite Nanofiber as H2‐Production Photocatalyst

Graphdiyne-modified TiO2 nanofibers with osteoinductive and enhanced photocatalytic antibacterial activities to prevent implant infection.