Anthraquinone Redox Relay for Dye-Sensitized Photo-electrochemical H2 O2 Production

Achieving a solar-to-chemical efficiency of 3.6% in ambient conditions by inhibiting interlayer charges transport

Efficiently converting solar energy into chemical energy remains a formidable challenge in artificial photosynthetic systems. To date, rarely has an artificial photosynthetic system operating in the open air surpassed the highest solar-to-biomass conversion efficiency (1%) observed in plants. In this study, we present a three-dimension polymeric photocatalyst achieving a solar-to-H2O2 conversion efficiency of 3.6% under ambient conditions, including real water, open air, and room temperature. The impressive performance is attributed to the efficient storage of electrons inside materials via expeditious intramolecular charge transfer, and the fast extraction of the stored electrons by O2 that can diffuse into the internal pores of the self-supporting three-dimensional material. This construction strategy suppresses the interlayer transfer of excitons, polarizers and carriers, effectively increases the utilization of internal excitons to 82%. This breakthrough provides a perspective to substantially enhance photocatalytic performance and bear substantial implications for sustainable energy generation and environmental remediation.

Comprehensive studies on the biological activities of human metastatic (MDA-MB-231) and non-metastatic (MCF-7) breast cancer cell lines, directly or combinedly treated using non-thermal plasma-based approaches

Progressive incidence and a pessimistic survival rate of breast cancer in women worldwide remains one of the most concerning topics. Progressing research indicates a potentially high effectiveness of use cold atmospheric plasma (CAP) systems. The undoubted advantage seems its simplicity in combination with other anti-cancer modalities. Following observed trend of studies, one inventory CAP system was applied to directly treat human breast cancer cell lines and culturing in two different Plasma Activated Media (PAM) for combined utilization. Proposed CAP treatments on MCF-10 A, MCF-7, and MDA-MB-231 cell lines were studied in terms of impact on cell viability by MTT assay. Disturbances in cell motility following direct and combined CAP application were assessed by scratch test. Finally, the induction of apoptosis and necrosis was verified with annexin V and propidium iodide staining. Reactive species generated during CAP treatment were determined based on optical emission spectrometry analysis along with colorimetric methods to qualitatively assess the NO2-, NO3-, H2O2, and total ROS with free radicals concentration. The most effective approach for CAP utilization was combined treatment, leading to significant disruption in cell viability, motility and mostly apoptosis induction in breast cancer cell lines. Determined CAP dose allows for mild outcome, showing insignificant harm for the non-cancerous MCF-10 A cell line, while the highly aggressive MDA-MB-231 cell line shows the highest sensitivity on proposed CAP treatment. Direct CAP treatment seems to drive the cells into the sensitive state in which the effectiveness of PAM is boosted. Observed anti-cancer response of CAP treatment was mostly triggered by RNS (mostly NO2- ions) and ROS along with free radicals (such as H2O2, OH•, O2-•, 1O2, HO2•). The combined application of one CAP source represent a promising alternative in the development of new and effective modalities for breast cancer treatment.

Ball Milling of La2O3 Tailors the Crystal Structure, Reactive Oxygen Species, and Free Radical and Non-Free Radical Photocatalytic Pathways

Non-free radical photocatalysis with metal oxide catalysts is an important advanced oxidation process that enables the removal of various emerging environmental pollutants, such as tetracycline. Here, four hexagonal La2O3 photocatalysts with different densities of oxygen vacancy and crystalline features are synthesized and then further treated by ball milling. Ball milling of these La2O3 photocatalysts is found to increase the amount of oxygen vacancies on their surfaces and thereby the amount of 1O2 species produced by them. The photocatalytic degradation of TC by these La2O3 photocatalysts depends on the oxygen vacancies present on them. Furthermore, the ones with a strong (101) diffraction peak remove tetracycline from water systems largely with 1O2 and •OH species, whereas those with a weak (101) diffraction peak do so mainly via 1O2 and direct electron transfer (DET) process. Their overall catalytic properties are also studied by density functional theory calculations. Moreover, the organic products produced from tetracycline by La2O3 photocatalysts containing a strong (101) diffraction peak are found to be less toxic than those produced by La2O3 photocatalysts containing a weak (101) diffraction peak. This study also provides convincing evidence that the structures of La2O3 determine the species that is produced by it and that end up mediating photocatalytic reaction pathways (i.e., free radical versus non-free radical) to degrade an emerging environment pollutant.

Integrating Enzymes with Supramolecular Polymers for Recyclable Photobiocatalytic Catalysis

Chemical modifications of enzymes excel in the realm of enzyme engineering due to its directness, robustness, and efficiency; however, challenges persist in devising versatile and effective strategies. In this study, we introduce a supramolecular modification methodology that amalgamates a supramolecular polymer with Candida antarctica lipase B (CalB) to create supramolecular enzymes (SupEnzyme). This approach features the straightforward preparation of a supramolecular amphiphilic polymer (β-CD@SMA), which was subsequently conjugated to the enzyme, resulting in a SupEnzyme capable of self-assembly into supramolecular nanoparticles. The resulting SupEnzyme nanoparticles can form micron-scale supramolecular aggregates through supramolecular and electrostatic interactions with guest entities, thus enhancing catalyst recycling. Remarkably, these aggregates maintain 80 % activity after seven cycles, outperforming Novozym 435. Additionally, they can effectively initiate photobiocatalytic cascade reactions using guest photocatalysts. As a consequence, our SupEnzyme methodology exhibits noteworthy adaptability in enzyme modification, presenting a versatile platform for various polymer, enzyme, and biocompatible catalyst pairings, with potential applications in the fields of chemistry and biology.

Rational Design of Conjugated Acetylenic Polymers Enables a Two-Electron Water Oxidation Pathway for Enhanced Photosynthetic Hydrogen Peroxide Generation

Herein, the design of conjugated acetylenic polymers (CAPs) featuring diverse spatial arrangements and intramolecular spacers of diacetylene moieties (─C≡C─C≡C─) for photocatalytic hydrogen peroxide (H2 O2 ) production from water and O2 , without the need for sacrificial agents, is presented. It is shown that the linear configuration of diacetylene moieties within conjugated acetylenic polymers (CAPs) induces a pronounced polarization of electron distribution, which imparts enhanced charge-carrier mobility when compared to CAPs' networks featuring cross-linked arrangements. Moreover, optimizing the intramolecular spacer between diacetylene moieties within the linear structure leads to the exceptional modulation of the band structures, specifically resulting in a downshifted valence band (VB) and rendering the two-electron water oxidation pathway thermodynamically feasible for H2 O2 production. Consequently, the optimized CAPs with a linear configuration (LCAP-2), featuring spatially separated reduction centers (benzene rings) and oxidation centers (diacetylene moieties), exhibit a remarkable H2 O2 yield rate of 920.1 µmol g-1 h-1 , superior than that of the linear LCAP-1 (593.2 µmol g-1 h-1 ) and the cross-linked CCAP (433.4 µmol g-1 h-1 ). The apparent quantum efficiency (AQE) and solar-to-chemical energy conversion (SCC) efficiency of LCAP-2 are calculated to be 9.1% (λ = 420 nm) and 0.59%, respectively, surpassing the performance of most previously reported conjugated polymers.

Selective Control and Characteristics of Water Oxidation and Dioxygen Reduction in Environmental Photo(electro)catalytic Systems

ConspectusEmploying semiconductor materials is a popular engineering method to harvest solar energy, which is widely investigated for photocatalysis (PC) and photoelectrocatalysis (PEC) that convert solar light to chemical energy. In particular, environmental photo(electro)catalysis has been extensively studied as a sustainable method for water treatment, air purification, and resource recovery. Environmental PC/PEC processes working in ambient conditions are initiated mainly through hole transfer to water (water oxidation) and electron transfer to dioxygen (O₂ reduction) and the subsequent photoredox transformation of water and dioxygen serves as a base of various PC/PEC systems. Through the redox transformations, different products can be generated depending on the number of transferred electrons and holes. The single electron/hole transfer generates radical species and reactive oxygen species (ROS) which initiate the degradation/transformation of various pollutants in water and air, while the multicharge transfer can generate energy-rich chemicals (e.g., H₂, H₂O₂). Therefore, understanding the characteristics of the photoredox reactions of water and dioxygen on the semiconductor surface is critically important in controlling the selectivity and efficiency of photoconversion processes.In this Account, we describe various environmental PC/PEC conversions with a particular focus on how the phototransformation of dioxygen and water is related to the overall processes occurring on diverse semiconductor materials. The activation of water or dioxygen can be controlled by modifying the properties of semiconductors, changing the kind of counterpart half-reaction and the experimental conditions. If water can be used as a ubiquitous reductant under solar irradiation, many kinds of reductive transformations can be carried out under ambient environmental conditions. For example, various toxic oxyanions (or metal ions) can be reductively transformed to harmless or less harmful species or useful chemicals/fuels can be synthesized under ambient conditions if water can provide electrons and protons via solar water oxidation. On the other hand, dioxygen can turn into reactive oxygen species (ROS) as a versatile oxidant or to a chemical like H₂O₂. There should be many more possibilities of utilizing the photoconversion of water and dioxygen for environmentally significant purposes, which are yet to be further developed and demonstrated. In this Account, we highlight the recent strategies and the novel functional materials for effective activation of water and dioxygen in environmental PC/PEC systems. Design of environmentally functional PC/PEC systems should be based on better understanding of water and dioxygen activation.

How to use a rotating ring-disc electrode (RRDE) subtraction method to investigate the electrocatalytic oxygen reduction reaction?

When studying electrochemical oxygen reduction reactions in homogeneous media, special attention must be given to the significant background activity present with conventional electrode materials. The intrinsic electrocatalytic activity of different materials can be investigated using complementary methods, such as the rotating ring-disc electrode (RRDE) technique and chronoamperometric electrolysis with product quantification. This report presents a detailed investigation of the electrocatalytic ability of hydroxy anthraquinone derivatives and riboflavin towards hydrogen peroxide (H₂O₂) production via a novel RRDE subtraction method together with chronoamperometric electrolysis. Qualitative trends linking the two methods were obtained, such as a higher excess current correlating with both higher productivity and selectivity. As such, a valuable tool is provided to increase the understanding of the electrocatalytic ability of homogeneous solutions toward improving the oxygen reduction reaction.

Self-cycled photo-Fenton-like system based on an artificial leaf with a solar-to-H2O2 conversion efficiency of 1.46

Millions of families around the world remain vulnerable to water scarcity and have no access to drinking water. Advanced oxidation processes (AOPs) are an effective way towards water purification with qualified reactive oxygen species (ROSs) while are impeded by the high-cost and tedious process in either input of consumable reagent, production of ROSs, and the pre-treatment of supporting electrolyte. Herein, we couple solar light-assisted H₂O₂ production from water and photo-Fenton-like reactions into a self-cyclable system by using an artificial leaf, achieving an unassisted H₂O₂ production rate of 0.77 μmol/(min·cm²) under 1 Sun AM 1.5 illumination. Furthermore, a large (70 cm²) artificial leaf was used for an unassisted solar-driven bicarbonate-activated hydrogen peroxide (BAP) system with recycled catalysts for real-time wastewater purification with requirements for only water, oxygen and sunlight. This demonstration highlights the feasibility and scalability of photoelectrochemical technology for decentralized environmental governance applications from laboratory benchtops to industry.

Continuous generation of reactive oxygen species is desirable in the advanced oxidation process. Here, the authors report a self-cycled photoFenton-like with a scalable artificial leaf for production of H₂O₂ from water with solar-to-H₂O₂ efficiency of 1.46%.

Piezocatalytic and Photocatalytic Hydrogen Peroxide Evolution of Sulfide Solid Solution Nano-Branches from Pure Water and Air

Hydrogen peroxide (H₂ O₂ ) as a useful chemical has a wide range of applications, and the development of efficient semiconducting materials for H₂ O₂ production is deemed as a promising strategy to realize the energy conversion. In this paper, Cd_x Zn_1-x S (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) nano-branches are fabricated and the piezocatalytic and photocatalytic H₂ O₂ evolution performance are studied. Under ultrasound condition, the H₂ O₂ yield of as-synthesized solid solutions is all higher than those of pristine ZnS and CdS, and optimal evolution rate achieves 21.9 µmol g^-1 h^-1 for Cd_0.5 Zn_0.5 S without any sacrificial agent, while it is increased to 151.6 µmol g^-1 h^-1 under visible light irradiation. The piezo/photoelectrochemical tests, piezoresponse force microscopy (PFM), and computational simulation reveal that the nano-branch structure benefits the mechanical energy conversion more, favoring the H₂ O₂ evolution for Cd_0.5 Zn_0.5 S, and a higher concentration of charge carriers is generated in photocatalysis. The active radical trapping and in situ electron spin resonance (ESR) experiments demonstrate that both of the H₂ O₂ generation pathways are originated from oxygen reduction by the sequential two-step single-electron reaction. This work opens a door for promoting the H₂ O₂ production from nanostructure and solid solution design.

Ultrafast transient absorption spectroelectrochemistry: femtosecond to nanosecond excited-state relaxation dynamics of the individual components of an anthraquinone redox couple

Many photoactivated processes involve a change in oxidation state during the reaction pathway and formation of highly reactive photoactivated species. Isolating these reactive species and studying their early-stage femtosecond to nanosecond (fs-ns) photodynamics can be challenging. Here we introduce a combined ultrafast transient absorption-spectroelectrochemistry (TA-SEC) approach using freestanding boron doped diamond (BDD) mesh electrodes, which also extends the time domain of conventional spectrochemical measurements. The BDD electrodes offer a wide solvent window, low background currents, and a tuneable mesh size which minimises light scattering from the electrode itself. Importantly, reactive intermediates are generated electrochemically, via oxidation/reduction of the starting stable species, enabling their dynamic interrogation using ultrafast TA-SEC, through which the early stages of the photoinduced relaxation mechanisms are elucidated. As a model system, we investigate the ultrafast spectroscopy of both anthraquinone-2-sulfonate (AQS) and its less stable counterpart, anthrahydroquinone-2-sulfonate (AH2QS). This is achieved by generating AH2QS in situ from AQS via electrochemical means, whilst simultaneously probing the associated early-stage photoinduced dynamical processes. Using this approach we unravel the relaxation mechanisms occurring in the first 2.5 ns, following absorption of ultraviolet radiation; for AQS as an extension to previous studies, and for the first time for AH2QS. AQS relaxation occurs via formation of triplet states, with some of these states interacting with the buffered solution to form a transient species within approximately 600 ps. In contrast, all AH2QS undergoes excited-state single proton transfer with the buffered solution, resulting in formation of ground state AHQS- within approximately 150 ps.

Suppressing Water Dissociation via Control of Intrinsic Oxygen Defects for Awakening Solar H2 O-to-H2 O2 Generation

BiVO₄ theoretically has a thermodynamic activity trend toward highly selective water oxidative H₂ O₂ formation, but it is more inclined to generate O₂ in practical. The influence of intrinsic oxygen vacancy (O_vac ), especially, on surface reactivity, has never been considered as a possible activity loss mechanism in the synthetic BiVO₄ . In this work, it is theoretically and experimentally demonstrated that the intrinsic surface O_vac is responsible for lower H₂ O₂ evolution activity via promoting water dissociation to form intermediate. Through an annealing process under a V₂ O₅ rich atmosphere, the surface O_vac can be eliminated that awakens the photoelectrochemical (PEC) water oxidative H₂ O₂ activity in a NaHCO₃ electrolyte, which achieves an average of 58.4%, and increases by up to 4.28 times of the one annealed in air. This work offers a general understanding of catalytic activity loss and may be extended to other photo or electrocatalysts for catalytic selectivity regulation.

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.

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.