Spontaneous exciton dissociation in organic photocatalyst under ambient conditions for highly efficient synthesis of hydrogen peroxide

H2O2 Photosynthesis from H2O and O2 under Weak Light by Carbon Nitrides with the Piezoelectric Effect

Driven by the essential need of a green, safe, and low-cost approach to producing H2O2, a highly valuable multifunctional chemical, artificial photosynthesis emerges as a promising avenue. However, current catalyst systems remain challenging, due to the need of high-density sunlight, poor selectivity and activity, or/and unfavorable thermodynamics. Here, we reported that an indirect 2e- water oxidation reaction (WOR) in photocatalytic H2O2 production was unusually activated by C5N2 with piezoelectric effects. Interestingly, under ultrasonication, C5N2 exhibited an overall H2O2 photosynthesis rate of 918.4 μM/h and an exceptionally high solar-to-chemical conversion efficiency of 2.6% after calibration under weak light (0.1 sun). Mechanism studies showed that the piezoelectric effect of carbon nitride overcame the high uphill thermodynamics of *OH intermediate generation, which enabled a new pathway for 2e- WOR, the kinetic limiting step in the overall H2O2 production from H2O and O2. Benefiting from the outstanding sonication-assisted photocatalytic H2O2 generation under weak light, the concept was further successfully adapted to biomedical applications in efficient sono-photochemodynamic therapy for cancer treatment and water purification.

Oxygen-Centered Organic Radicals-Involved Unified Heterogeneous Self-Fenton Process for Stable Mineralization of Micropollutants in Water

Removing organic micropollutants from water through photocatalysis is hindered by catalyst instability and substantial residuals from incomplete mineralization. Here, a novel water treatment paradigm, the unified heterogeneous self-Fenton process (UHSFP), which achieved an impressive 32% photon utilization efficiency at 470 nm, and a significant 94% mineralization of organic micropollutants-all without the continual addition of oxidants and iron ions is presented. In UHSFP, the active species differs fundamentally from traditional photocatalytic processes. One electron acceptor unit of photocatalyst acquires only one photogenerated electron to convert into oxygen-centered organic radical (OCOR), then spontaneously completing subsequent processes, including pollutant degradation, hydrogen peroxide generation, activation, and mineralization of organic micropollutants. By bolstering electron-transfer capabilities and diminishing catalyst affinity for oxygen in the photocatalytic process, the generation of superoxide radicals is effectively suppressed, preventing detrimental attacks on the catalyst. This study introduces an innovative and cost-effective strategy for the efficient and stable mineralization of organic micropollutants, eliminating the necessity for continuous chemical inputs, providing a new perspective on water treatment technologies.

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.

A photocatalytic redox cycle over a polyimide catalyst drives efficient solar-to-H2O2 conversion

Circumventing the conventional two-electron oxygen reduction pathway remains a great problem in enhancing the efficiency of H2O2 photosynthesis. A promising approach to achieve outstanding photocatalytic activity involves the utilization of redox intermediates. Here, we engineer a polyimide aerogel photocatalyst with photoreductive carbonyl groups for non-sacrificial H2O2 production. Under photoexcitation, carbonyl groups on the photocatalyst surface are reduced, forming an anion radical intermediate. The produced intermediate is oxidized by O2 to produce H2O2 and subsequently restores the carbonyl group. The high catalytic efficiency is ascribed to a photocatalytic redox cycle mediated by the radical anion, which not only promotes oxygen adsorption but also lowers the energy barrier of O2 reduction reaction for H2O2 generation. An apparent quantum yield of 14.28% at 420 ± 10 nm with a solar-to-chemical conversion efficiency of 0.92% is achieved. Moreover, we demonstrate that a mere 0.5 m2 self-supported polyimide aerogel exposed to natural sunlight for 6 h yields significant H2O2 production of 34.3 mmol m-2.

Construction of Conjugated Organic Polymers for Efficient Photocatalytic Hydrogen Peroxide Generation with Adequate Utilization of Water Oxidation

The visible-light-driven photocatalytic production of hydrogen peroxide (H2O2) is currently an emerging approach for transforming solar energy into chemical energy. In general, the photocatalytic process for producing H2O2 includes two pathways: the water oxidation reaction (WOR) and the oxygen reduction reaction (ORR). However, the utilization efficiency of ORR surpasses that of WOR, leading to a discrepancy with the low oxygen levels in natural water and thereby impeding their practical application. Herein, we report a novel donor-bridge-acceptor (D-B-A) organic polymer conjugated by the Sonogashira-Hagihara coupling reaction with tetraphenylethene (TPE) units as the electron donors, acetylene (A) as the connectors and pyrene (P) moieties as the electron acceptors. Notably, the resulting TPE-A-P exhibits a remarkable solar-to-chemical conversion of 1.65% and a high BET-specific surface area (1132 m2·g-1). Furthermore, even under anaerobic conditions, it demonstrates an impressive H2O2 photosynthetic efficiency of 1770 μmol g-1 h-1, exceeding the vast majority of previously reported photosynthetic systems of H2O2. The outstanding performance is attributed to the effective separation of electrons and holes, along with the presence of sufficient reaction sites facilitated by the incorporation of alkynyl electronic bridges. This protocol presents a successful method for generating H2O2 via a water oxidation reaction, signifying a significant advancement towards practical applications in the natural environment.

Efficient Proton Transfer and Charge Separation within Covalent Organic Frameworks via Hydrogen-Bonding Network to Boost H2O2 Photosynthesis

Photocatalytic synthesis based on the oxygen reduction reaction (ORR) has shown great promise for H2O2 production. However, the low activity and selectivity of 2e- ORR result in a fairly low efficiency of H2O2 production. Herein, we propose a strategy to enhance the proton-coupled electron transfer (PCET) process in covalent organic frameworks (COFs), thereby significantly boosting H2O2 photosynthesis. We demonstrated that the construction of a hydrogen-bonding network, achieved by anchoring the H3PO4 molecular network on COF nanochannels, can greatly improve both proton conductivity and photogenerated charge separation efficiency of COFs. Thus, COF@H3PO4 exhibited superior photocatalytic performance in generating H2O2 without sacrificial agents, with a solar-to-chemical conversion efficiency as high as 0.69%. Results indicated that a much more localized spatial distribution of energy band charge density on COF@H3PO4 led to efficient charge separation, and the small energy barrier of the rate-limiting step from *OOH to H2O2 endowed COF@H3PO4 with higher 2e- ORR selectivity.

Optoelectronic Readout of Single Er Adatom's Electronic States Adsorbed on the Si(100) Surface at Low Temperature (9 K)

Integrating nanoscale optoelectronic functions is vital for applications such as optical emitters, detectors, and quantum information. Lanthanide atoms show great potential in this endeavor due to their intrinsic transitions. Here, we investigate Er adatoms on Si(100)-2×1 at 9 K using a scanning tunneling microscope (STM) coupled to a tunable laser. Er adatoms display two main adsorption configurations that are optically excited between 800 and 1200 nm while the STM reads the resulting photocurrents. Our spectroscopic method reveals that various photocurrent signals stem from the bare silicon surface or Er adatoms. Additional photocurrent peaks appear as the signature of the Er adatom relaxation, triggering efficient dissociation of nearby trapped excitons. Calculations using density functional theory with spin-orbit coupling correction highlight the origin of the observed photocurrent peaks as specific 4f→4f or 4f→5d transitions. This spectroscopic technique can facilitate optoelectronic analysis of atomic and molecular assemblies by offering insight into their intrinsic quantum properties.

Keto-anthraquinone covalent organic framework for H2O2 photosynthesis with oxygen and alkaline water

Hydrogen peroxide photosynthesis suffers from insufficient catalytic activity due to the high energy barrier of hydrogen extraction from H2O. Herein, we report that mechanochemically synthesized keto-form anthraquinone covalent organic framework which is able to directly synthesize H2O2 (4784 μmol h-1 g-1 at λ > 400 nm) from oxygen and alkaline water (pH = 13) in the absence of any sacrificial reagents. The strong alkalinity resulted in the formation of OH-(H2O)n clusters in water, which were adsorbed on keto moieties within the framework and then dissociated into O2 and active hydrogen, because the energy barrier of hydrogen extraction was largely lowered. The produced hydrogen reacted with anthraquinone to generate anthrahydroquinone, which was subsequently oxidized by O2 to produce H2O2. This study ultimately sheds light on the importance of hydrogen extraction from H2O for H2O2 photosynthesis and demonstrates that H2O2 synthesis is achievable under alkaline conditions.

Construction of a redox pathway through a polyoxometalate and covalent organic framework for H2O2 photosynthesis

A novel type of electron donor-acceptor system was built from a nitrogen-rich covalent organic framework (PC) and a polyoxometalate (BW12), fabricating a composite material (BW12@PC-250), which shows significantly improved photocatalytic H2O2 yield (56.4 μM h-1) under full spectrum illumination in pure water, being about 30 times higher than that of PC. This is due to the opening of the electron and proton transport pathway between PC and BW12, which paves a new way for POMs to modulate the photocatalytic reactions of COFs.

Efficient Photosynthesis of Hydrogen Peroxide by Cyano-Containing Covalent Organic Frameworks from Water, Air and Sunlight

The insufficient exciton (e- -h+ pair) separation/transfer and sluggish two-electron water oxidation are two main factors limiting the H2 O2 photosynthetic efficiency of covalent organic frameworks (COFs) photocatalysts. Herein, we present an alternative strategy to simultaneously facilitate exciton separation/transfer and reduce the energy barrier of two-electron water oxidation in COFs via a dicyano functionalization. The in situ characterization and theoretical calculations reveal that the dicyano functionalization improves the amount of charge transfer channels between donor and acceptor units from two in COF-0CN without cyano functionalization to three in COF-1CN with mono-cyano functionalization and four in COF-2CN with dicyano functionalization, leading to the highest separation/transfer efficiency in COF-2CN. More importantly, the dicyano group activates the neighbouring C atom to produce the key *OH intermediate for effectively reducing the energy barrier of rate-determining two-electron water oxidation in H2 O2 photosynthesis. The simultaneously enhanced exciton separation/transfer and two-electron water oxidation in COF-2CN result in high H2 O2 yield (1601 μmol g-1  h-1 ) from water and oxygen without using sacrificial reagent under visible-light irradiation. COF-2CN can effectively yield H2 O2 in water with wide pH range, in different real water samples, in scaled-up reactor under natural sunlight irradiation, and in continuous-flow reactor for consecutively producing H2 O2 solution for water decontamination.

Metal-Free Photocatalytic CO2 Reduction to CH4 and H2 O2 under Non-sacrificial Ambient Conditions

Photocatalytic CO2 reduction to CH4 requires photosensitizers and sacrificial agents to provide sufficient electrons and protons through metal-based photocatalysts, and the separation of CH4 from by-product O2 has poor applications. Herein, we successfully synthesize a metal-free photocatalyst of a novel electron-acceptor 4,5,9,10-pyrenetetrone (PT), to our best knowledge, this is the first time that metal-free catalyst achieves non-sacrificial photocatalytic CO2 to CH4 and easily separable H2 O2 . This photocatalyst offers CH4 product of 10.6 μmol ⋅ g-1  ⋅ h-1 under non-sacrificial ambient conditions (room temperature, and only water), which is two orders of magnitude higher than that of the reported metal-free photocatalysts. Comprehensive in situ characterizations and calculations reveal a multi-step reaction mechanism, in which the long-lived oxygen-centered radical in the excited PT provides as a site for CO2 activation, resulting in a stabilized cyclic carbonate intermediate with a lower formation energy. This key intermediate is thermodynamically crucial for the subsequent reduction to CH4 product with the electronic selectivity of up to 90 %. The work provides fresh insights on the economic viability of photocatalytic CO2 reduction to easily separable CH4 in non-sacrificial and metal-free conditions.

An Atomically Dispersed Mn-Photocatalyst for Generating Hydrogen Peroxide from Seawater via the Water Oxidation Reaction (WOR)

In this work, we have fabricated an aryl amino-substituted graphitic carbon nitride (g-C₃N₄) catalyst with atomically dispersed Mn capable of generating hydrogen peroxide (H₂O₂) directly from seawater. This new catalyst exhibited excellent reactivity, obtaining up to 2230 μM H₂O₂ in 7 h from alkaline water and up to 1800 μM from seawater under identical conditions. More importantly, the catalyst was quickly recovered for subsequent reuse without appreciable loss in performance. Interestingly, unlike the usual two-electron oxygen reduction reaction pathway, the generation of H₂O₂ was through a less common two-electron water oxidation reaction (WOR) process in which both the direct and indirect WOR processes occurred; namely, photoinduced h⁺ directly oxidized H₂O to H₂O₂ via a one-step 2e^- WOR, and photoinduced h⁺ first oxidized a hydroxide (OH^-) ion to generate a hydroxy radical (^•OH), and H₂O₂ was formed indirectly by the combination of two ^•OH. We have characterized the material, at the catalytic sites, at the atomic level using electron paramagnetic resonance, X-ray absorption near edge structure, extended X-ray absorption fine structure, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, magic-angle spinning solid-state NMR spectroscopy, and multiscale molecular modeling, combining classical reactive molecular dynamics simulations and quantum chemistry calculations.

Regulating excitonic effects in non-oxide based XPSe3 (X = Cd, Zn) monolayers towards enhanced photocatalysis for overall water splitting

The non-oxide 2D materials have garnered considerable interest due to their potential utilization as photocatalysts, which offer a superior substitute to metal-oxide-based photocatalysts. This study investigates the impact of the dielectric environment on the size and binding energy of excitons in atomically thin, experimentally synthesized semiconducting monolayers [XPSe₃, X = (Cd, Zn)] to address the critical problem of electron-hole recombination, which significantly hinders the efficiency of most photocatalysts. We employ a precise non-hydrogenic model surpassing the hydrogenic-based Mott-Wannier model. Our findings are among the first few demonstrations of an increase in exciton size (and decrease in exciton binding energy) as environmental screening increases. These findings have implications for photocatalytic water splitting and are not limited to metal phosphorus trichalcogenides, but can be applied to other classes of 2D materials as well. This work also compares metal-oxide photocatalysts, which have been the focus of much research over the past five decades, to non-oxide-based metal phosphorus trichalcogenide photocatalysts, which offer a superior alternative due to their ability to address issues such as light-harvesting inability in the visible spectrum and unwanted charge recombination centres. Furthermore, the implications of this study extend beyond photocatalysts and are significant for the design and development of next-generation optoelectronic devices that incorporate excitonic processes, such as solar cells, photodetectors, LEDs, etc.

Synthesis of Silver Nanoparticles-Modified Graphitic Carbon Nitride Nanosheets for Highly Efficient Photocatalytic Hydrogen Peroxide Evolution

As a promising metal-free photocatalyst, graphitic carbon nitride (g-C3N4) is still limited by insufficient visible light absorption and rapid recombination of photogenerated carriers, resulting in low photocatalytic activity. Here, we adjusted the microstructure of the pristine bulk-g-C3N4 (PCN) and further loaded silver (Ag) nanoparticles. Abundant Ag nanoparticles were grown on the thin-layer g-C3N4 nanosheets (CNNS), and the Ag nanoparticles decorated g-C3N4 nanosheets (Ag@CNNS) were successfully synthesized. The thin-layer nanosheet-like structure was not only beneficial for the loading of Ag nanoparticles but also for the adsorption and activation of reactants via exposing more active sites. Moreover, the surface plasmon resonance (SPR) effect induced by Ag nanoparticles enhanced the absorption of visible light by narrowing the band gap of the substrate. Meanwhile, the composite band structure effectively promoted the separation and transfer of carriers. Benefiting from these merits, the Ag@CNNS reached a superior hydrogen peroxide (H2O2) yield of 120.53 μmol/g/h under visible light irradiation in pure water (about 8.0 times higher than that of PCN), significantly surpassing most previous reports. The design method of manipulating the microstructure of the catalyst combined with the modification of metal nanoparticles provides a new idea for the rational development and application of efficient photocatalysts.