Cd3(C3N3S3)2 coordination polymer/graphene nanoarchitectures for enhanced photocatalytic H2O2 production under visible light

Ag Atom Induces Microstrain Environment around Cd Sites to Construct Diatomic Sites for Almost 100% CO2-to-CO Electroreduction

Deeply understanding how local microstrain environment around diatomic sites influences their electronic state and adsorption is crucial for improving electrochemical CO2 reduction (eCO2R) reaction; however, precise engineering of the atomic microstrain environment is challenging. Herein, we fabricate Ag-CdTMT electrocatalysts with AgN2S2-CdN2S2 diatomic sites by anchoring Ag to the nodes of CdTMT (TMT = 2,4,6-trimercaptotriazine anion) coordination polymers. The Ag-CdTMT catalysts achieve approximately 100% Faradaic efficiency for CO reduction with an industrial level current density (∼200 mA cm-2 in H-cell). The embedded Ag atoms induce the formation of Ag-Cd diatomic sites with local microstrain, stretching Cd-N/S bonds, and reinforcing electron localization at Cd sites. The microstrain engineering and adjacent Ag atoms synergistically reduced Cd 4d-C 2p antibonding orbital occupancy for intensifying *COOH adsorption as the rate-determining step. This study provides novel insights into customizing the electronic structure of diatomic sites through strain engineering.

Cascade Self-Generation of Carbon Monoxide Triggered by Photoinduced Holes for Efficient Hypoxic Tumors Therapy

It still remains challenging to design multifunctional therapeutic reagents for effective cancer therapy under a unique tumor microenvironment including insufficient endogenous H2O2 and O2, low pH, and a high concentration of glutathione (GSH). In this work, a CO-based phototherapeutic system triggered by photogenerated holes, which consisted of ionic liquid (IL), the CO prodrug Mn2(CO)10, and iridium(III) porphyrin (IrPor) modified carbonized ZIF-8-doped graphitic carbon nitride nanocomposite (IL/ZCN@Ir(CO)), was designed for cascade hypoxic tumors. Upon light irradiation, the photogenerated holes on IL/ZCN@Ir(CO) oxidize water into H2O2, which subsequently induces Mn2(CO)10 to release CO. Meanwhile, IrPor can convert H2O2 to hydroxyl radical (•OH) and subsequent singlet oxygen (1O2), which further triggers CO release. Moreover, the degraded MnO2 shows activity for glutathione (GSH) depletion and mimics peroxidase, leading to GSH reduction and •OH production in tumors. Thus, this strategy can in situ release high concentrations of CO and reactive oxygen species (ROS) and deplete GSH to efficiently induce cell apoptosis under hypoxic conditions, which has a high inhibiting effect on the growth of tumors, offering an attractive strategy to amplify CO and ROS generation to meet therapeutic requirements in cancer treatment.

Solar-to-H2 O2 Catalyzed by Covalent Organic Frameworks

Benefiting from the excellent structural tunability, robust framework, ultrahigh porosity, and rich active sites, covalent organic frameworks (COFs) are widely recognized as promising photocatalysts in chemical conversions, and emerged in the hydrogen peroxide (H2 O2 ) photosynthesis in 2020. H2 O2 , serving as an environmental-friendly oxidant and a promising liquid fuel, has attracted increasing researchers to explore its potential. Over the past few years, numerous COFs-based photocatalysts are developed with encouraging achievements in H2 O2 production, whereas no comprehensive review articles exist to summarize this specific and significant area. Herein we provide a systematic overview of the advances and challenges of COFs in photocatalytic H2 O2 production. We first introduce the priorities of COFs in H2 O2 photosynthesis. Then, various strategies to improve COFs photocatalytic efficiency are discussed. The perspective and outlook for future advances of COFs in this emerging field are finally offered. This timely review will pave the way for the development of highly efficient COFs photocatalysts for practical production of value-added chemicals not limited to H2 O2 .

Magnetic Polaron States in Photoluminescent Carbon Dots Enable Hydrogen Peroxide Photoproduction

Photoactivation of aspartic acid-based carbon dots (Asp-CDs) induces the generation of spin-separated species, including electron/hole (e- /h+ ) polarons and spin-coupled triplet states, as uniquely confirmed by the light-induced electron paramagnetic resonance spectroscopy. The relative population of the e- /h+ pairs and triplet species depends on the solvent polarity, featuring a substantial stabilization of the triplet state in a non-polar environment (benzene). The electronic properties of the photoexcited Asp-CDs emerge from their spatial organization being interpreted as multi-layer assemblies containing a hydrophobic carbonaceous core and a hydrophilic oxygen and nitrogen functionalized surface. The system properties are dissected theoretically by density functional theory in combination with molecular dynamics simulations on quasi-spherical assemblies of size-variant flakelike model systems, revealing the importance of size dependence and interlayer effects. The formation of the spin-separated states in Asp-CDs enables the photoproduction of hydrogen peroxide (H2 O2 ) from water and water/2-propanol mixture via a water oxidation reaction.

Recent progress in the design of photocatalytic H2O2 synthesis system

Photocatalytic synthesis of hydrogen peroxide under mild reaction conditions is a promising technology. This article will review the recent research progress in the design of photocatalytic H2O2 synthesis systems. A comprehensive discussion of the strategies that could solve two essential issues related to H2O2 synthesis. That is, how to improve the reaction kinetics of H2O2 formation via 2e- oxygen reduction reaction and inhibit the H2O2 decomposition through a variety of surface functionalization methods. The photocatalyst design and the reaction mechanism will be especially stressed in this work which will be concluded with an outlook to show the possible ways for synthesizing high-concentration H2O2 solution in the future.

Influence of calcination temperature on photocatalytic H2O2productivity of hierarchical porous ZnO microspheres

Photocatalytic production of H₂O₂from water and atmospheric oxygen has been recognized as a green and sustainable chemical process, due to the abundance of raw materials and sustainable solar energy. Herein, flower-like hierarchical ZnO microspheres were prepared by hydrothermal method followed by calcination at different temperatures, and their photocatalytic performance in H₂O₂production was examined under simulated sunlight irradiation. The calcination temperature plays a vital role in the structure, morphology, and surface area of the final ZnO products as well as their optical and electrochemical properties, which are determining factors in their photocatalytic activity. The ZnO calcined at 300 °C (Zn-300) exhibits the highest activity and optimal stability, showing productivity of 2793μmol l^-1within 60 min of irradiation, which was 6.5 times higher than that of uncalcined ZnO precursor. The remarkable photocatalytic activity is attributed to enhanced light utilization, large surface area, abundant exposed active sites, improved separation efficiency, and prolonged carrier lifespan. Moreover, the results from cycling experiments indicate the as-prepared ZnO samples exhibit good stability and long-time performance. This work provides useful information for the preparation of hierarchical ZnO photocatalysts.

A metal-free photocatalyst for highly efficient hydrogen peroxide photoproduction in real seawater

Artificial photosynthesis of H2O2 from H2O and O2, as a spotless method, has aroused widespread interest. Up to date, most photocatalysts still suffer from serious salt-deactivated effects with huge consumption of photogenerated charges, which severely limit their wide application. Herein, by using a phenolic condensation approach, carbon dots, organic dye molecule procyanidins and 4-methoxybenzaldehyde are composed into a metal-free photocatalyst for the photosynthetic production of H2O2 in seawater. This catalyst exhibits high photocatalytic ability to produce H2O2 with the yield of 1776 μmol g-1h-1 (λ ≥ 420 nm; 34.8 mW cm-2) in real seawater, about 4.8 times higher than the pure polymer. Combining with in-situ photoelectrochemical and transient photovoltage analysis, the active site and the catalytic mechanism of this composite catalyst in seawater are also clearly clarified. This work opens up an avenue for a highly efficient and practical, available catalyst for H2O2 photoproduction in real seawater.

Recent Progress, Challenges, and Prospects in Two-Dimensional Photo-Catalyst Materials and Environmental Remediation

The successful photo-catalyst library gives significant information on feature that affects photo-catalytic performance and proposes new materials. Competency is considerably significant to form multi-functional photo-catalysts with flexible characteristics. Since recently, two-dimensional materials (2DMs) gained much attention from researchers, due to their unique thickness-dependent uses, mainly for photo-catalytic, outstanding chemical and physical properties. Photo-catalytic water splitting and hydrogen (H2) evolution by plentiful compounds as electron (e-) donors is estimated to participate in constructing clean method for solar H2-formation. Heterogeneous photo-catalysis received much research attention caused by their applications to tackle numerous energy and environmental issues. This broad review explains progress regarding 2DMs, significance in structure, and catalytic results. We will discuss in detail current progresses of approaches for adjusting 2DMs-based photo-catalysts to assess their photo-activity including doping, hetero-structure scheme, and functional formation assembly. Suggested plans, e.g., doping and sensitization of semiconducting 2DMs, increasing electrical conductance, improving catalytic active sites, strengthening interface coupling in semiconductors (SCs) 2DMs, forming nano-structures, building multi-junction nano-composites, increasing photo-stability of SCs, and using combined results of adapted approaches, are summed up. Hence, to further improve 2DMs photo-catalyst properties, hetero-structure design-based 2DMs' photo-catalyst basic mechanism is also reviewed.

Carbon-based materials for photo- and electrocatalytic synthesis of hydrogen peroxide

The high demand for hydrogen peroxide (H₂O₂) has been dominantly supplied by the anthraquinone process for various applications globally, including chemical synthesis and wastewater treatment. However, the centralized manufacturing and intensive energy input and waste output are significant challenges associated with this process. Accordingly, the on-site production of H₂O₂via electro- and photocatalytic water oxidation and oxygen reduction partially is greener and easier to handle and has recently emerged with extensive research aiming to seek active, selective and stable catalysts. Herein, we review the current status and future perspectives in this field focused on carbon-based catalysts and their hybrids, since they are relatively inexpensive, bio-friendly and flexible for structural modulation. We present state-of-the-art progress, typical strategies for catalyst engineering towards selective and active H₂O₂ production, discussion on electro- and photochemical mechanisms and H₂O₂ formation through both reductive and oxidative reaction pathways, and conclude with the key challenges to be overcome. We expect promising developments would be inspired in the near future towards practical decentralized H₂O₂ production and its direct use.

Distorted polymeric carbon nitride via carriers transfer bridges with superior photocatalytic activity for organic pollutants oxidation and hydrogen production under visible light

Polymeric carbon nitride (PCN) has become the most promising metal-free photocatalysts but its activity is low. Molecule doping of PCN has been proved to be an effective strategy to achieve high photocatalytic performance. Herein, we report a bottom-up method to synthesize modified PCN, which includes 2,5-dibromopyrazine doping, thermal-induced exfoliation and condensation/polymerization. The incorporation of electron-deficiency 2,5-dibromopyrazine into the PCN framework can effectively tune the electronic structures and improve the charge-carrier separation. In addition, the incorporation of 2,5-dibromopyrazine induced significant structural changes from planar symmetric to distortion. The optimized pyrazine doped PCN showed a reaction rate enhancement of 4-fold for the degradation of sulfamethazine compared to that of conventional urea-based PCN. Further reactive species and degradation intermediate detection studies, indicated that O₂^- was generated during the photocatalytic process, which could lead to the decomposition, and finally mineralization of sulfamethazine. 2,5-Dibromopyrazine doped PCN also leads to a 6.3-fold improvement in H₂ generation with the visible light. Especially, phytotoxicity experiments showed that the toxicity of sulfamethazine after degradation is greatly reduced, and the as-prepared photocatalyst is environmentally friendly.

Carbon inserted defect-rich MoS2-X nanosheets@CdSnanospheres for efficient photocatalytic hydrogen evolution under visible light irradiation

Carbon -MoS_2-x@CdS (C-MoS_2-x@CdS) core-shell nanostructures with controlled surface sulfur (S) vacancies were prepared via a glucose assisted hydrothermal growth method. The glucose acted as a reducing agent of C-MoS_2-X to partially reduce Mo⁴⁺ ions to Mo³⁺ and served as a carbon source to insert the amorphous carbon into the layered MoS_2-X simultaneously. The presence of Mo³⁺ result in the surface S-vacancies, which can provide more additional active sites and enhance the photocatalytic performance. Moreover, the inserted carbon in layered MoS_2-X enhanced the electron mobility and decreased the resistance electron transfer. Density functional theory (DFT) calculation confirmed that the surface S-vacancies and the amorphous carbon increase the projected density of states at the conduct band edge, which could enhance the photo-absorption and photo-responsibility. The result is consistent with the photocatalytic H₂ production experiment. C2-10%MoS_2-x@CdS presented a high H₂ evolution rate of 61,494 μmol h^-1 g^-1 under visible light irrigation (λ ≥ 420 nm), which is 1.98 times and 158 times higher than that of sample without S-vacancies (10%MoS₂@CdS) and pure CdS, respectively.

Characterization of highly effective plasma-treated g-C3N4 and application to the photocatalytic H2O2 production

Plasma treated g-C₃N₄ (PT-g-C₃N₄) was obtained by a simple and rapid DBD plasma modification process on the pristine g-C₃N₄. Compared with the pristine g-C₃N₄, the grain size of the PT-g-C₃N₄ decreased from 99.2 nm to 57.2 nm, the specific surface area and the pore volume increased by 15% and 33.8%, respectively. Oxygen-containing groups such as -NO₂ and -COOH were observed to form on the surface of PT-g-C₃N₄ so the hydrophilic property of PT-g-C₃N₄ was much higher than that of pristine g-C₃N₄. More importantly, the photocatalytic H₂O₂ production activity of PT-g-C₃N₄ was significantly improved on account of the treatment in plasma atmosphere for only 5 min, the H₂O₂ yield of which was about 13 times that of the pristine g-C₃N₄. Our finding is not only of great significance for effectively promoting the production of H₂O₂ under mild conditions, but also proposes an innovative DBD plasma method to modify the g-C₃N₄ photocatalyst, which effectively promotes the improvement of photocatalytic activity and provides valuable insights for catalyst modification studies.

A comparative perspective of electrochemical and photochemical approaches for catalytic H2O2 production

Recent advances in the design, preparation, and applications of different catalysts for electrochemical and photochemical H₂O₂ production are summarized, and some invigorating perspectives for future developments are also provided.

Photocatalytic Oxygen Evolution from Water Splitting

Photocatalytic water splitting has attracted a lot of attention in recent years, and O2 evolution is the decisive step owing to the complex four-electrons reaction process. Though many studies have been conducted, it is necessary to systematically summarize and introduce the research on photocatalytic O2 evolution, and thus a systematic review is needed. First, the corresponding principles about O2 evolution and some urgently encountered issues based on the fundamentals of photocatalytic water splitting are introduced. Then, several types of classical water oxidation photocatalysts, including TiO2, BiVO4, WO3, α-Fe2O3, and some newly developed ones, such as Sillén-Aurivillius perovskites, porphyrins, metal-organic frameworks, etc., are highlighted in detail, in terms of their crystal structures, synthetic approaches, and morphologies. Third, diverse strategies for O2 evolution activity improvement via enhancing photoabsorption and charge separation are presented, including the cocatalysts loading, heterojunction construction, doping and vacancy formation, and other strategies. Finally, the key challenges and future prospects with regard to photocatalytic O2 evolution are proposed. The purpose of this review is to provide a timely summary and guideline for the future research works for O2 evolution.

Facile synthesis of bimodal macroporous g-C3N4/SnO2 nanohybrids with enhanced photocatalytic activity

It is of vital importance to construct highly interconnected, macroporous photocatalyst to improve its efficiency and applicability in solar energy conversion and environment remediation. Graphitic-like C₃N₄ (g-C₃N₄), as an analogy to two-dimensional (2D) graphene, is highly identified as a visible-light-responsive polymeric semiconductor. Moreover, the feasibility of g-C₃N₄ in making porous structures has been well established. However, the preparation of macroporous g-C₃N₄ with abundant porous networks and exposure surface, still constitutes a difficulty. To solve it, we report a first facile preparation of bimodal macroporous g-C₃N₄ hybrids with abundant in-plane holes, which is simply enabled by in-situ modification through thermally treating the mixture of thiourea and SnCl₄ (pore modifier) after rotary evaporation. For one hand, the formed in-plane macropores endow the g-C₃N₄ system with plentiful active sites and short, cross-plane diffusion channels that can greatly speed up mass transport and transfer. For another, the heterojunctions founded between g-C₃N₄ and SnO₂ consolidate the electron transfer reaction to greatly reduce the recombination probability. As a consequence, the resulted macroporous g-C₃N₄/SnO₂ nanohybrid had a high specific surface area (SSA) of 44.3 m²/g that was quite comparable to most nano/mesoporous g-C₃N₄ reported. The interconnected porous network also rendered a highly intensified light absorption by strengthening the light penetration. Together with the improved mass transport and electron transfer, the macroporous g-C₃N₄/SnO₂ hybrid exhibited about 2.4-fold increment in the photoactivity compared with pure g-C₃N₄. Additionally, the recyclability of such hybrid could be guaranteed after eight successive uses.

The construction of a Fenton system to achieve in situ H2O2 generation and decomposition for enhanced photocatalytic performance

A photo-Fenton system combining Ni_xFe_yO₄–BiOBr with the in situ generation and decomposition of H₂O₂ was constructed for efficient organic compound degradation.

Highly Improved Solar Energy Harvesting for Fuel Production from CO2 by a Newly Designed Graphene Film Photocatalyst

Our growing energy demands must be met by a sustainable supply with reduced carbon intensity. One of the most exciting prospects to realize this goal is the photocatalyst-biocatalyst integrated artificial photosynthesis system which affords solar fuel/chemicals in high selectivity from CO2. Graphene based photocatalysts are highly suitable for the system, but their industrial scale use requires immobilization for improved separation and recovery of the photocatalyst. Therefore for practical purposes, design and fabrication of film type graphene photocatalyst with higher solar energy conversion efficiency is an absolute necessity. As a means to achieve this, we report herein the successful development of a new type of flexible graphene film photocatalyst that leads to >225% rise in visible light harvesting efficiency of the resultant photocatalyst-biocatalyst integrated artificial photosynthesis system for highly selective solar fuel production from CO2 compared to conventional spin coated graphene film photocatalyst. It is an important step towards the design of a new pool of graphene film based photocatalysts for artificial photosynthesis of solar fuels from CO2.