There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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.

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has been the hotspot in the materials science as a metal-free and visible-light-responsive photocatalyst. Pure g-C3N4 suffers from the insufficient sunlight absorption, low surface area and the fast recombination of photo-induced electron-hole pairs, resulting in low photocatalytic activity. Element doping is known to be an efficient method to tune the unique electronic structure and band gap of g-C3N4, which considerably broaden the light responsive range and enhance the charge separation. This review summarizes the recent progress in the development of efficient and low cost doped g-C3N4 systems in various realms such as photocatalytic hydrogen evolution, reduction of carbon dioxide, photocatalytic removal of contaminants in wastewater and gas phase. Typically, metal doping, nonmetal doping, co-doping and heterojunction based on doped g-C3N4 have been explored to simultaneously tune the crystallographic, textural and electronic structures for improving photocatalytic activity by enhancing the light absorption, facilitating the charge separation and transportation and prolonging the charge carrier lifetime. Finally, the current challenges and the crucial issues of element doped g-C3N4 photocatalysts that need to be addressed in future research are presented. This review presented herein can pave a novel avenue and add invaluable knowledge to the family of element doped g-C3N4 for the develop of more effective visible-light-driven photocatalysts.

Doping of graphitic carbon nitride for photocatalysis: A reveiw

Constructing heterojunctions between two semiconductors with matched band structure is an effective strategy to acquire high-efficiency photocatalysts. The S-scheme heterojunction system has shown great potential in facilitating separation and transfer of photogenerated carriers, as well as acquiring strong photoredox ability. Herein, a 0D/2D S-Scheme heterojunction material involving CeO2 quantum dots and polymeric carbon nitride (CeO2 /PCN) is designed and constructed by in situ wet chemistry with subsequent heat treatment. This S-scheme heterojunction material shows high-efficiency photocatalytic sterilization rate (88.1 %) towards Staphylococcus aureus (S. aureus) under visible-light irradiation (λ≥420 nm), which is 2.7 and 8.2 times that of pure CeO2 (32.2 %) and PCN (10.7 %), respectively. Strong evidence of S-scheme charge transfer path is verified by theoretical calculations, in situ irradiated X-ray photoelectron spectroscopy, and electron paramagnetic resonance.

Designing a 0D/2D S-Scheme Heterojunction over Polymeric Carbon Nitride for Visible-Light Photocatalytic Inactivation of Bacteria.

Tri-s-triazine-based crystalline carbon nitride nanosheets (CCNNSs) have been successfully extracted via a conventional and cost-effective sonication-centrifugation process. These CCNNSs possess a highly defined and unambiguous structure with minimal thickness, large aspect ratios, homogeneous tri-s-triazine-based units, and high crystallinity. These tri-s-triazine-based CCNNSs show significantly enhanced photocatalytic hydrogen generation activity under visible light than g-C3 N4 , poly (triazine imide)/Li+ Cl- , and bulk tri-s-triazine-based crystalline carbon nitrides. A highly apparent quantum efficiency of 8.57% at 420 nm for hydrogen production from aqueous methanol feedstock can be achieved from tri-s-triazine-based CCNNSs, exceeding most of the reported carbon nitride nanosheets. Benefiting from the inherent structure of 2D crystals, the ultrathin tri-s-triazine-based CCNNSs provide a broad range of application prospects in the fields of bioimaging, and energy storage and conversion.

Tri-s-triazine-Based Crystalline Carbon Nitride Nanosheets for an Improved Hydrogen Evolution.

The delamination of layered crystals that produces single or few-layered nanosheets while enabling exotic physical and chemical properties, particularly for semiconductor functions in optoelectronic applications, remains a challenge. Here, we report a facile and green approach to prepare few-layered polymeric carbon nitride (PCN) semiconductors by a one-step carbon/nitrogen steam reforming reaction. Bulky PCN, obtained from typical precursors including urea, melamine, dicyandiamide, and thiourea, are exfoliated into few-layered nanosheets, while engineering its surface carbon vacancies. The unique sheet structures with strengthened surface properties endow PCNs with more active sites, and an increased charge separation efficiency with a prolonged charge lifetime, drastically promoting their photoredox performance. After an assay of a H2 evolution reaction, an apparent quantum yield of 11.3 % at 405 nm was recorded for the PCN nanosheets, which is much higher than those of PCN nanosheets. This delamination method is expandable to other carbon-based 2D materials for advanced applications.

A Facile Steam Reforming Strategy to Delaminate Layered Carbon Nitride Semiconductors for Photoredox Catalysis

Conjugated polymers (CPs) are emerging and appealing light harvesters for photocatalytic water splitting owing to their adjustable band gap and facile processing. Herein, we report an advanced mild synthesis of three conjugated triazine-based polymers (CTPs) with different chain lengths by increasing the quantity of electron-donating benzyl units in the backbone. Varying the chain length of the CTPs modulates their electronic, optical, and redox properties, resulting in an enhanced performance for photocatalytic oxygen evolution, which is the more challenging half-reaction of water splitting owing to the sluggish reaction kinetics. Our results could stimulate interest in these functional polymers where a molecular engineering strategy enables the production of suitable semiconductor redox energetics for oxygenic photosynthesis.

Photocatalytic Oxygen Evolution from Functional Triazine‐Based Polymers with Tunable Band Structures

Natural photosynthesis serves as a model for energy and chemical conversions, and motivates the search of artificial systems that mimic nature's energy- and electron-transfer chains. However, bioinspired systems often suffer from the partial or even large loss of the charge separation state, and show moderate activity owing to the fundamentally different features of the multiple compounds. Herein, a selenium and cyanamide-functionalized heptazine-based melon (DA-HM) is designed as a unique bioinspired donor-acceptor (D-A) light harvester. The combination of the photosystem and electron shuttle in a single species, with both n- and p-type conductivities, and extended spectral absorption, endows DA-HM with a high efficiency in the transfer and separation of photoexcited charge carriers, resulting in photochemical activity. This work presents a unique conjugated polymeric system that shows great potential for solar-to-chemical conversion by artificial photosynthesis.

Yuanxing Fang

Papers

Tri-s-triazine-Based Crystalline Carbon Nitride Nanosheets for an Improved Hydrogen Evolution.

A Facile Steam Reforming Strategy to Delaminate Layered Carbon Nitride Semiconductors for Photoredox Catalysis

Photocatalytic Oxygen Evolution from Functional Triazine‐Based Polymers with Tunable Band Structures

Biomimetic Donor–Acceptor Motifs in Conjugated Polymers for Promoting Exciton Splitting and Charge Separation

Metal–organic frameworks for solar energy conversion by photoredox catalysis