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g-C3N4-based S-scheme photocatalyst
Zhu B., Zhang L., Yu J.
Graphitic carbon nitride (g-C3N4) is a promising photocatalyst employed in various kinds of photocatalytic reactions. It has many virtues, including facile preparation, visible light response, nontoxicity, and high chemical stability. However, pure g-C3N4 photocatalyst exhibits unsatisfactory activity because of the recombination of photogenerated electrons and holes and the weak oxidation ability of photogenerated holes. To address these issues, numerous efforts are made to fabricate g-C3N4-based S-scheme photocatalysts by coupling g-C3N4 and oxidation photocatalysts (OP). The incorporation of OP promotes charge separation and reinforces the oxidation ability of the S-scheme heterojunction. Themed on g-C3N4-based S-scheme photocatalysts, this chapter begins with the history, basic properties, and preparation methods of g-C3N4. The subsequent content focuses on four representative g-C3N4/metal oxides S-scheme photocatalysts. The synthesis strategy, photocatalytic performance, and charge transfer mechanism are emphasized. Eventually, prospects of g-C3N4-based S-scheme photocatalysts are suggested.
Principles of photocatalysis
Wang L., Yu J.
Photocatalysis is an important ramification of catalysis which generates charge carriers under light irradiation to catalyze chemical reactions. Photocatalysis mimics the natural photosynthesis process, which is driven by the clean and inexhaustible sunlight. Besides, photocatalytic reactions operate at ambient temperatures and pressures, greatly reducing the requirements for reaction facilities as well as the operating cost. Owing to these unique advantages, photocatalysis powered by the renewable solar energy is deemed as an ideal pathway for industrial production, which can greatly decrease the reliance on fossil fuels to alleviate energy crisis and the greenhouse effect. As a result, photocatalysis has been applied in many crucial reactions, such as H2 production via water splitting, CO2 photoreduction to renewable chemical fuels, photodegradation of pollutants, N2 fixation, H2O2 production, sterilization, organic synthesis, etc. However, photocatalysis suffers from low product yields due to the rapid recombination of photogenerated charge carriers and lack of redox abilities. Thus, the design and development of novel photocatalysts are paramount to address these issues and improve the photocatalytic performance. In this chapter, we will systematically introduce the fundamentals of photocatalysis, including the historical evolvement, the principles of a photocatalytic process, different classes of photocatalysts, and the application of photocatalysis in various reactions. We anticipate that readers will grasp basic understanding of photocatalysis and photocatalysts.
Other S-scheme photocatalysts
Bie C., Zhang L., Yu J.
S-scheme heterojunction is widely utilized for designing efficient photocatalysts. S-scheme photocatalysts retain the powerful redox ability by consuming useless photogenerated carriers. The inclusivity of designing S-scheme heterojunction allows a variety of materials to construct S-scheme photocatalysts. In addition to CdS, TiO2, and g-C3N4, other semiconductors, such as spinel, Bi-based compounds, black phosphorus, ZnO, and organic semiconductors, can fabricate S-scheme photocatalysts. Benefiting from the strong redox capability and efficient carrier separation, S-scheme photocatalysts have attracted extensive attention. Given that the conventional-semiconductor-based S-scheme photocatalysts are exposited in the preceding chapters, this chapter mainly introduces other S-scheme photocatalysts, including spinel-based, Bi-based, black phosphorus-based, ZnO-based, and organic-based S-scheme photocatalysts, to promote the progress of S-scheme photocatalysts.
TiO2-based S-scheme photocatalyst
Xu F., Yu J.
Solar-driven photocatalysis with oxide semiconductors shows great potential in solving growing energy and environmental crises. TiO2, as the most popular photocatalyst, has attracted wide attention owing to its stability, nontoxicity, cheapness, etc. Monocomponent TiO2 suffers from the fast recombination of photogenerated charge carriers and thereby shows poor photocatalytic efficiency. TiO2-based type-II photocatalysts achieve charge separation but weaken the redox ability of as-separated charge carriers. TiO2-based S-scheme heterojunctions composed of oxidation and reduction photocatalysts present a solution to the dilemma faced by traditional type-II photocatalysts. In such S-scheme heterojunctions, the useless photogenerated charge carriers are recombined and eliminated, while the useful electrons and holes are reserved for surface redox. The S-scheme pathway facilitates charge separation and meanwhile maintains strong redox capability of survived charge carriers. In this chapter, the phase structure, preparation methods, and band structure of TiO2 photocatalysts are first described. The advantages and disadvantages of typical TiO2-based heterojunction photocatalysts are also summarized. The design principle and photocatalytic mechanism of TiO2-based S-scheme heterojunctions, as well as the driving force for the S-scheme charge transfer and separation pathway, are specially elucidated in detail. Moreover, the applications of TiO2-based S-scheme heterojunctions in the field of H2 production, CO2 reduction, pollutant degradation, etc. are also reviewed. Finally, the challenges and perspectives of S-scheme heterojunctions are underlined, which would deepen a systematic understanding of the design and fabrication of more efficient TiO2-based photocatalysts in the future.
S-scheme photocatalyst
Zhang L., Zhang J., Yu J.
Solar light utilization has garnered the attention of both the scientific and engineering worlds. Solar fuel generation via photocatalysts has great potential. It has been accepted that semiconductor heterojunction photocatalysts are superior to their single component counterpart. This chapter starts with basic semiconductor physics, which aids to clarify the charge transfer mechanism in heterojunction photocatalysts. Then, the intrinsic reasons for constructing heterojunctions are discussed, which aims to overcome the shortcomings of single component photocatalysts via revisiting the Jablonski diagram and simple photochemistry theory. Next, the development of the state-of-the-art S-scheme heterojunction is thoroughly expounded. And, the problems confronted by the type-II, liquid-phase Z-scheme, and all-solid-state Z-scheme photocatalysts that prevailed in the past are elaborated. Besides, the classification of S-scheme heterojunctions is presented. Moreover, the advantages of S-scheme heterojunctions are summarized from the aspects of promoted spatial charge separation and maximized redox ability. Ultimately, a new understanding of the Fermi level alignment considering electron concentrations when forming S-scheme heterojunction is briefly introduced. Hopefully, this chapter will provide directions for the rational design of S-scheme heterojunctions.
Characterization methods of S-scheme photocatalyst
Xia Y., Zhang L., Yu J.
Photocatalysis is considered as an efficient approach for alleviating energy crises and environmental pollution. However, the performance of single photocatalysts is unsatisfactory due to the rapid recombination of photogenerated electrons and holes. Thus, the development of heterojunction photocatalysts is necessary. The emerging S-scheme heterojunction photocatalyst has shown its superiority due to spatial separation and maximized redox abilities of photogenerated electrons and holes. Therefore, numerous researchers have plunged into the design and synthesis of S-scheme heterojunction photocatalysts. In this regard, the development of characterization techniques is imperative to confirm the electron transfer mechanism in S-scheme heterojunctions. Against this backdrop, in this chapter, we summarize the advanced characterization techniques that are used to analyze the charge transfer mechanism in S-scheme heterojunction. For each characterization technique, the fundamental principle is illustrated first. Afterward, the representative examples are presented to demonstrate their applications. This chapter aims to help the researchers in selecting suitable techniques for testifying the S-scheme heterojunction and inspire the development of new S-scheme heterojunction photocatalysts with superior performance.
CdS-based S-scheme photocatalyst
Cheng C., Wang L., Yu J.
Cadmium sulfide (CdS) has gained wide attention in photocatalysis owing to its suitable band structure. However, there are inherent drawbacks of unitary CdS such as the rapid recombination of photogenerated electron–hole pairs and severe photocorrosion, which result in low photocatalytic activity. Constructing S-scheme heterojunctions between CdS and other semiconductors is a promising strategy to suppress carrier recombination and alleviate photocorrosion by separating photogenerated electrons and holes. This chapter aims to provide knowledge on CdS-based S-scheme photocatalysts through case studies. Herein, the basic physical and chemical properties of CdS will be introduced, and its preparation methods will be summarized. Subsequently, three representative CdS-based S-scheme photocatalysts will be discussed with special focuses on their synthesis, photocatalytic performance, and the S-scheme charge carrier transfer mechanism. Finally, a brief perspective on the development of CdS-based S-scheme photocatalysts will be presented.
Principle and surface science of photocatalysis
Li X., Yu J., Jiang C.
Heterogeneous photocatalysis as a sustainable and promising strategy has been intensively investigated for different applications, including solar fuel production and degradation of environmental pollutants. However, it is still challenging to develop highly active, selective, and durable photocatalysts for practical applications. In this chapter, after introducing a brief history of photocatalysis, we first present a comprehensive review on the thermodynamics and dynamics of photocatalysis, which could help the readers deeply understand the fundamental principles of photocatalytic processes. Then, we systematically summarize the basics for the surface/interface science of photocatalysis, including adsorption, surface redox reactions, and interfacial charge separation. Finally, the design principles, modification strategies, and the characterization and evaluation methods of semiconductor photocatalysts are summarized. By addressing these pertinent and important topics in heterogeneous photocatalysis, this chapter is expected to provide a useful reference for researchers in designing and exploring advanced photocatalytic materials for different applications.
Modification of ZnO-based photocatalysts for enhanced photocatalytic activity
Qi K., Yu J.
Photocatalysis is a promising technology to solve the environmental problems caused by organic pollutants, which have become a serious threat to the ecosystems and human health. Zinc oxide (ZnO) has been widely used in the photocatalytic degradation of environmental pollutants, but pure ZnO only works under UV light irradiation and is susceptible to photocorrosion. To improve the photocatalytic activity and stability of ZnO materials, various methods have been developed to enhance the visible light response, accelerate photogenerated charge separation, and promote adsorption of organic pollutants. In this chapter, we briefly summarize the main methods for modification of ZnO materials, including the doping with metal and nonmetal atoms, deposition of noble metals, construction of heterojunctions, and coupling with carbon materials, with examples provided to illustrate the photocatalytic mechanism and enhanced photocatalytic activity and stability of modified ZnO-based photocatalysts.
Design and fabrication of direct Z-scheme photocatalysts
Low J., Yu J., Jiang C.
Direct Z-scheme photocatalysts are composite semiconductor photocatalysts in which the charge transfer between the component semiconductors follows a Z-scheme pathway without the aid of additional electron mediators. Direct Z-scheme photocatalysts have provoked much attention in photocatalysis because of their capability to enhance redox potential and eliminate inactive electron–hole pairs of the photocatalytic system. For this reason, direct Z-scheme photocatalysts have been extensively exploited for various photocatalytic applications, ranging from solar fuel production to environmental remediation. In this chapter, the fundamental principles and development for the direct Z-scheme photocatalysts are described. General characterization methods for differentiating the direct Z-scheme photocatalysts from the conventional type-II photocatalytic system are also summarized. Then, the typical applications of the direct Z-scheme photocatalysts are elaborated. Finally, the challenges and future development directions for the direct Z-scheme photocatalysts are highlighted.
Synthesis and performance enhancement for Bi2WO6 photocatalysts
Li W., Zhu Y.
As a visible light–driven photocatalyst, bismuth tungstate (Bi2WO6) has attracted widespread attention and shown great potential in photocatalysis. To further enhance the photocatalytic performance of Bi2WO6 under visible light irradiation, several methods have been employed to optimize the properties of Bi2WO6, including morphology control, doping, and surface modification. Bi2WO6 nanomaterials of diverse morphologies, such as nanoplates, nanoparticles, hierarchical microspheres, and ultrathin or porous nanostructures, have been synthesized to achieve large surface area or exposure of high activity crystal facets. The doping of ions can narrow the bandgap to improve light absorption at long wavelength, whereas surface modification by π-conjugated materials, quantum dots, and other general semiconductors can promote the separation of photogenerated electron–hole pairs. Moreover, the combination of photochemical and electrochemical technologies exhibited great prospect with fast charge migration rate. Finally, as an example of the various applications of Bi2WO6, the photocatalytic carbon dioxide reduction by Bi2WO6 is systematically reviewed.
Hierarchical porous photocatalysts
Li X., Yu J., Jaroniec M.
Heterogeneous photocatalysis has been long considered to be an appealing and sustainable strategy to simultaneously solve energy-related and environmental problems. Among various kinds of photocatalysts, hierarchical porous photocatalysts are particularly promising due to their significant advantages over bulk structures, including improved light scattering and harvesting ability, larger specific surface areas (therefore, more reaction sites), promoted mass and charge transfer properties, and less severity of nanostructural agglomeration. Thus, we present a comprehensive review on hierarchical porous photocatalysts for their further development. In this chapter, we will systematically summarize the design, fabrication, surface and interfacial engineering, and potential applications (e.g., photocatalytic H2 evolution, CO2 reduction, and degradation of pollutants) of hierarchical porous photocatalysts. We hope this work will help inspire new synthetic methods of these materials for various photocatalytic applications.
Surface and interface modification strategies of CdS-based photocatalysts
Li Q., Li X., Yu J.
Cadmium sulfide (CdS) is an important visible light–responsive semiconductor, but its photocatalytic activity is limited by the rapid recombination of photogenerated charge carriers both in the bulk and on the surface. Moreover, CdS suffers from poor stability owing to photocorrosion, which further hinders its practical applications. The aim of this chapter is to review the major surface and interface modification strategies to improve the photoactivity and photostability of CdS-based photocatalysts. These modification strategies can promote surface reaction kinetics (e.g., by loading cocatalysts and controlling the exposed facets), accelerate charge separation kinetics (e.g., by designing nanostructures with diverse dimensions and morphologies, introducing defects, creating solid solutions, building heterojunction and homojunction), and enhance the photostability (e.g., by utilizing sacrificial agents, fabricating p–n heterojunction or Z-scheme heterojunction, and creating covering layer). Finally, some perspectives are presented, highlighting the main challenges in further optimizing the performance of CdS-based photocatalysts and achieving industrial applications.
Silver-based visible light–responsive photocatalysts
Cheng H., Wang P., Wang Z., Liu Y., Huang B.
Heterogeneous photocatalysis under visible light is of paramount significance to solar energy harvesting and conversion. Among the various photocatalytic materials, silver (Ag)-based photocatalysts are of particular interest in that (i) Ag nanoparticles are responsive to visible light in a wide range due to the localized surface plasmon resonance effect, and (ii) most of the Ag-based semiconductors could absorb visible light with the bandgaps lower than 3 eV, making them outstanding candidates for solar energy utilization. In the past few years, great efforts have been taken to extend visible light absorption, boost photocatalytic efficiency, and improve photochemical stability of Ag-based photocatalysts. In this work, we briefly summarize the recent advances in the Ag-based photocatalysts, including Ag-based plasmonic photocatalysts and Ag-based semiconductor photocatalysts. We hope this will bring some insightful understanding and directions for the development of visible light–driven photocatalysts in future.
Metal–organic frameworks for photocatalysis
Qin Y., Hao M., Li Z.
Metal–organic frameworks (MOFs) are a class of crystalline micro/mesoporous hybrid materials composed of metal ions or metal clusters interconnected by organic linkers. MOFs are recently emerging as a new type of photoactive materials for different photocatalytic applications due to their unique structural characteristics. In this chapter, the fundamentals of MOFs for photocatalysis and the photoexcitation pathways over MOF-based photocatalysts are briefly introduced. Examples of using MOFs for different photocatalytic reactions, including photocatalytic water reduction/oxidation, CO2 reduction, environmental remediation, and organic transformations, are summarized. Strategies for designing and developing MOFs as efficient photocatalysts and for expanding their photocatalytic applications are also demonstrated. Finally, the limitations, challenges, and the future perspective of the application of MOFs for photocatalysis are addressed. The objective of this chapter is to encourage more future efforts to be devoted to design and development of new efficient MOFs with wide applications in photocatalysis.
Surface heterojunction of photocatalysts
Meng A., Yu J.
The efficiency of charge separation at the interface of heterojunction is a key factor that influences the photocatalytic performance of semiconductor photocatalysts. Surface heterojunction, which forms between different facets of a single semiconductor owing to the different atomic arrangements and electronic structures of the facets, is a promising strategy to spatially separate photogenerated charge carriers and promote the photocatalytic activity. In this chapter, progress on the exploration of surface heterojunction photocatalysts is summarized. The discussions mainly focus on the fundamentals of surface heterojunction, including the roles and evidences of surface heterojunction and the synthesis methods of surface heterojunction materials. Additionally, typical examples of surface heterojunction photocatalysts are discussed for a variety of semiconductors, including titanium dioxide, bismuth vanadate, copper(I) oxide, cerium(IV) oxide, bismuth oxychloride, copper tungsten sulfide, and silver-based semiconductors. These discussions are expected to bring deep understanding about the surface heterojunction photocatalysts and provide reference for future research on facet-dependent photocatalysis.
Surface modification of g-C3N4: first-principles study
Zhu B., Zhang L., Yu J.
First-principles calculation is a powerful research tool, which inquires into the properties of materials from the atomic and molecular levels. In terms of photocatalysts, first-principles calculation can provide useful information on the unit cell structure, optical and electronic properties, which are difficult to be disclosed using conventional experimental approaches. In this chapter, progress on the first-principles calculation research of tri-s-triazine-based graphitic carbon nitride (g-C3N4) is summarized. The discussions mainly focus on surface-modified g-C3N4, including g-C3N4 nanotubes, g-C3N4 nanoribbons, element-doped g-C3N4, defective g-C3N4, and g-C3N4-based composite materials. It is hoped that the discussions in this chapter will bring a deep understanding of the influences of surface modification on the photocatalytic performance of g-C3N4-based materials, as well as offer reference for further first-principles study of modified g-C3N4 photocatalytic materials.
Charge carrier transfer in photocatalysis
Liu B., Zhao X., Parkin I.P., Nakata K.
Charge carrier transfer drives photocatalytic reactions. In this chapter, we firstly present a general introduction to the thermodynamics and kinetics of charge carrier transfer in photocatalysis. The Gibbs potential landscape was proposed to elucidate the effect of thermodynamic driving force on photocatalysis and the difference between the roles of photoinduced and heat-induced charge carriers. We mainly discuss the effects of trapping, interparticle connection, and interphasic connection on the charge transfer kinetics of photocatalysis by taking titanium dioxide as a prototype. Thermal activation of charge transfer is also discussed in gas-phase photocatalysis, which shows that decreasing the thermal barrier of charge transfer is a main pathway to increase photocatalytic activity. Theoretical modeling and simulations are good supplements to experimental studies, hence we also summarized our recent works using the phenomenological and Monte-Carlo numerical modeling. Finally, the relation between thermodynamics and kinetics of charge transfer in photocatalysis is discussed.
The surface chemistry of graphene-based materials: functionalization, properties, and applications
Han C., Xu Y.
Isolated graphene, a single atomic layer two-dimensional analog of fullerenes and carbon nanotubes, has sparked enormous excitement because of its fascinating properties and great potential for various applications. While pristine graphene is desirable for applications that require a high electrical conductivity, many other applications require modified or functionalized forms of graphene materials, such as graphene oxide, reduced graphene oxide (rGO), or other functionalized forms. Structurally modifying graphene through chemical functionalization reveals the numerous possibilities for tuning its structure and properties. In this chapter, we focus on basic surface science issues of graphene oxide, rGO, and pristine graphene. The excellent mechanical, electronic, and adsorption properties and photocatalysis applications of the related materials are highlighted. Some challenges and opportunities for future exploration of functionalized graphene–based materials are critically discussed.
Photocatalysts based on polymeric carbon nitride for solar-to-fuel conversion
Cao S., Yu J.
During the past years, polymeric carbon nitrides (CNs) have shown excellent ability toward photocatalytic water reduction and CO2 reduction, as a promising means of efficient solar-to-fuel conversion. This chapter provides a comprehensive overview of strategies for enhancing the performance of CN-based photocatalysts in H2 production and CO2 reduction. The strategies include (1) increasing charge carrier generation by hollow structure creation, elemental doping, and molecular structure modification; (2) boosting charge separation by reducing the thickness of CNs, improving the crystallinity of CNs, and creating heterostructures of metal/CNs, graphene/CNs, type II, and direct Z-scheme heterojunctions; and (3) promoting the surface catalytic process by creation of porous structure and surface defects, construction of hybrid materials with metal–organic frameworks and layered double hydroxides, and facet tuning of metal cocatalysts. Finally, the concluding remarks and the current challenges are highlighted with some perspectives for the future development of CN-based photocatalytic materials.
Bismuth metal and semiconductor-based photocatalysts: structure tuning, activity enhancement, and reaction mechanism
Wang H., Sun Y., Dong F.
Bismuth (Bi) metal and Bi-based semiconductors are promising photocatalysts to harvest solar light for environmental remediation and solar energy conversion. With the development of material characterization techniques and theoretical calculations, research in improving the performance and elucidating the reaction mechanism of Bi-based photocatalytic systems has expanded exponentially in the last decade. Hence, this chapter endeavors to introduce the structure tuning, photocatalytic activity enhancement, and reaction mechanism for Bi metal and Bi-based compounds, including bismuth oxycarbonate ((BiO)2CO3), bismuth oxide (Bi2O3), and bismuth oxyhalides (BiOX, X = Cl, Br, and I). The in-depth understanding and summarization can provide a new perspective and guidance for the rational design of Bi-based photocatalysts and advance the fundamental theories of photocatalysis.
Modeling in environmental interfaces
Paladino O., Hodaifa G., Neviani M., Seyedsalehi M., Malvis A.
This chapter focuses on the use of different modeling approaches for the study of the dynamic behavior of environmental interfaces. It provides an overview of the main recent simulation schemes to deal with multiscale physically based models, with particular reference to both moving interfaces and moving environmental compartments. The chapter also provides a brief review of the most commonly used phenomenological bottom-up modeling approaches, state-space and black-box models, data-based mechanistic models. Two case studies in the field of wastewater treatment/biofuel production and groundwater contamination are presented to illustrate the application of some recent techniques described in the chapter.
Green techniques for wastewaters
Hodaifa G., Paladino O., Malvis A., Seyedsalehi M., Neviani M.
The growth of population and the diversification of its activities have resulted in an exponential increase in the generation of liquid and solid wastes in recent years. In view of the new scenario, the role of novel technologies intended to reduce these wastes, transform them into new sources, and reincorporate them to different processes is gaining a remarkable importance and has resulted in an environmentally concerned society. This chapter includes the main green technology processes applied for wastewater treatment. Conventional or nonconventional methods are specifically selected according to the characteristics of wastewater to be treated. Wastewaters containing high percentage of biodegradable matter are usually treated through conventional techniques. On the contrary, nonconventional techniques are extensively used for the treatment of wastewaters with high nondegradable matter percentages. These technologies constitute an effective response to the current needs of the earth to combat climate change, whose effects are dramatically increasing.
Application of nZVI and its composites into the treatment of toxic/radioactive metal ions
Zhu K., Chen C.
Nano zero-valent iron (nZVI) has received significant attention for its potential applications in the area of removing all kinds of toxic and radioactive metal ions. This chapter summarizes the preparation and use of pristine nZVI, modified nZVI, and supported nZVI. The physicochemical properties as well as the adsorption and reduction behaviors and mechanisms are also discussed in the cleanup of hazardous metal ion wastewater. Moreover, the advantages and limitations of the types of nZVI-based material and their functionality are evaluated. The scopes and limitations of these adsorbents will be addressed while investigating the various types of hazardous metal ions that are harmful.
Removal of toxic/radioactive metal ions by metal-organic framework-based materials
Li J., Ye W., Chen C.
The prominent structural and physical properties of metal-organic frameworks (MOFs) have endowed them as superior scavengers for the extraction of toxic or radioactive metal ions from water. In this chapter, we focus on the capture of toxic or radioactive metal ions from aquatic environments by using recently emerging MOFs and their composites. We highlighted typical examples of MOF materials and their corresponding adsorption properties toward toxic metal ions or radionuclides as well as their interplay mechanisms. In addition, the adsorption properties of various MOFs and their composites were evaluated and compared with those of other widely used adsorbent materials. Finally, we provided the present challenges and future research trends of sequestering toxic or radioactive metal ions by MOFs and MOF-based materials.
