Solar-Driven Reduction of Aqueous Protons Coupled to Selective Alcohol Oxidation with a Carbon Nitride-Molecular Ni Catalyst System

Surface properties of carbon nitride materials used in photocatalytic systems for energy and environmental applications

The use of photocatalytic systems involving semiconductor materials for environmental and energy applications, such as water remediation and clean energy production, is highly significant. In line with this, a family of carbon-based polymeric materials known as carbon nitride (CNx) has emerged as a promising candidate for this purpose. Despite CNx's remarkable characteristics of performance, stability, and visible light responsiveness, its chemical inertness and poor surface properties hinder interfacial interactions, which are key to effective catalysis. This highlight reviews the literature focusing on the surface chemistry of CNx, especially its structural formation pathway, reactivity, and solvent interactions. It also explores recent advancements in the use of modified CNx for hydrogen production and arsenic remediation, offering recommendations for future material design improvements.

All-in-One: Plasmonic Janus Heterostructures for Efficient Cooperative Photoredox Catalysis

Janus heterostructures consisting of multiple jointed components with distinct properties have gained growing interest in the photoredox catalytic field. Herein, we have developed a facile low-temperature method to gain anisotropic one-dimensional Au-tipped CdS (Au-CdS) nanorods (NRs), followed by assembling Ru molecular co-catalyst (RuN5) onto the surface of the NRs. The CdS NRs decorated with plasmonic Au nanoparticles and RuN5 complex harness the virtues of metal-semiconductor and inorganic-organic interface, giving directional charge transfer channels, spatially separated reaction sites, and enhanced local electric field distribution. As a result, the Au-CdS-RuN5 can act as an efficient dual-function photocatalyst for simultaneous H2 evolution and valorization of biomass-derived alcohols. Benefiting from the interfacial charge decoupling and selective chemical bond activation, the optimal all-in-one Au-CdS-RuN5 heterostructure shows greatly enhanced photoactivity and selectivity as compared to bare CdS NRs, along with a remarkable apparent quantum yield of 40.2 % at 400 nm. The structural evolution and working mechanism of the heterostructures are systematically analyzed based on experimental and computational results.

Deciphering charge transfer dynamics of a lead halide perovskite-nickel(ii) complex for visible light photoredox C-N coupling

Photoredox catalysis involving perovskite quantum dots (QDs) has gained enormous attention because of their high efficiency and selectivity. In this study, we have demonstrated CsPbBr3 QDs as photocatalysts for the C-N bond formation reaction. The introduction of Ni(dmgH)2 (dmgH = dimethyl glyoximato) as a cocatalyst with CsPbBr3 QDs facilitates photocatalytic C-N coupling to form a wide variety of amides. The optimized interaction between the cocatalyst and photocatalyst enhances charge transfer and mitigates charge recombination, ultimately boosting photocatalytic performance. The photocatalytic activity is notably influenced by the variation in the amount of cocatalyst and 7 wt% Ni(dmgH)2 produces the best yield (92%) of amide. Femtosecond transient absorption spectroscopy reveals that the dynamics of the trap states of QDs are affected by cocatalyst. Further, Ni(dmgH)2 facilitates molecular oxygen activation to form superoxide radicals, which further initiates the radical pathway for the C-N coupling.

Vacancy-Engineered 1D Nanorods with Spatially Segregated Dual Redox Sites for Visible-Light-Driven Cooperative CO2 Reduction

Cooperative CO2 photoreduction with tailored organic synthesis offers a potent avenue for harnessing concurrently generated electrons and holes, facilitating the creation of both solar fuels and specialized chemical compounds. However, controlling the crystallization and morphologies of metal-free molecular nanostructures with exceptional photocatalytic activities toward CO2 reduction remains a significant challenge. These hurdles encompass insufficient CO2 activation potential, sluggish multielectron processes, delayed charge-separation kinetics, inadequate storage of long-lived photoexcitons, unfavorable thermodynamic conditions, and the precise control of product selectivity. Here, melem oligomer 2D nanosheets (MNSs) synthesized through pyrolysis are transformed into 1D nanorods (MNRs) at room temperature with the simultaneous engineering of vacancies and morphology. Transient absorption spectral analysis reveals that vacancies in MNRs trap charges, extending charge carrier lifetimes. Additionally, carbon vacancies enhance CO2 adsorption by increasing amine functional centers. The photocatalytic performance of MNRs for CO2 reduction coupled with benzyl alcohol oxidation is approximately ten times higher (CH3OH and aromatic aldehyde production rate 27 ± 0.5 and 93 ± 0.5 mmol g-1 h-1, respectively) than for the MNSs (CH3OH and aromatic aldehyde production rate 2.9 ± 0.5 and 9 ± 0.5 mmol g-1 h-1, respectively). The CO2 reduction pathway involved the carbon-coordinated formyl pathway through the formation of *COOH and *CHO intermediates, as mapped by in situ Fourier-transform infrared spectroscopy. The superior performance of MNRs is attributed to favorable energy-level alignment, enriched amine surfaces, and unique morphology, enhancing solar-to-chemical conversion.

Increasing Profitability of Ethanol Photoreforming by Simultaneous Production of H2 and Acetal

Often, H2 is produced photocatalytically at the expense of sacrificial agents. When a sacrificial agent is selectively oxidized, this allows coupling of H2 production with synthesis of value-added organic compounds. Herein, it is argued that the conversion of bioethanol into 1,1-diethoxyethane with simultaneous H2 production increases the economic viability of photocatalysis and suggests a semiconductor material that is the most relevant for this purpose.

Electron deficient Bi3+δ serves as N2 absorption sites and inhibits carriers recombination to enhance N2 photo-fixation in BiOBr/TiO2 S-scheme heterojunction

Efficient carriers separation and multiple nitrogen (N2) activation sites are essential for N2 photo-fixation. Here, we found that the BiOBr/TiO2 (BBTO) displayed an attractive reversible photochromism (white → grey) due to the generation of electron deficient Bi3+δ, which was produced by the hole trapping of Bi3+ under light irradiation. Interestingly, more Bi3+δ were detected in the BBTO heterojunction than in pure BiOBr, attributing that the hole trapping was promoted by the built-in electric field in the Step scheme (S-scheme) heterojunction. In the BBTO, the electron deficient Bi3+δ enhanced carriers separation and served as the reactive active site to adsorb more N2. Consequently, the BBTO possessed an excellent N2 photo-fixation activity (191 μmol gcat-1 h-1), which was 7.7 and 18 times higher than that of pure BiOBr (24.8 μmol gcat-1 h-1) and TiO2 (10.6 μmol gcat-1 h-1), respectively. Therefore, this work provides a new perspective for enhancing N2 photo-fixation by the electron deficient photocatalysts with S-scheme heterojunction.

Temperature-dependent excitonic emission characteristics of highly crystallized carbon nitride nanosheets

Highly-crystallized carbon nitride (HCCN) nanosheets exhibit significant potential for advancements in the field of photoelectric conversion. However, to fully exploit their potential, a thorough understanding of the fundamental excitonic photophysical processes is crucial. Here, the temperature-dependent excitonic photoluminescence (PL) of HCCN nanosheets and amorphous polymeric carbon nitride (PCN) is investigated using steady-state and time-resolved PL spectroscopy. The exciton binding energy of HCCN is determined to be 109.26 meV, lower than that of PCN (207.39 meV), which is attributed to the ordered stacking structure of HCCN with a weaker Coulomb interaction between electrons and holes. As the temperature increases, a noticeable reduction in PL lifetime is observed on both the HCCN and PCN, which is ascribed to the thermal activation of carrier trapping by the enhanced electron-phonon coupling effect. The thermal activation energy of HCCN is determined to be 102.9 meV, close to the value of PCN, due to their same band structures. Through wavelength-dependent PL dynamics analysis, we have identified the PL emission of HCCN as deriving from the transitions:σ*-LP,π*-π, andπ*-LP, where theπ*-LP transition dominants the emission because of the high excited state density of the LP state. These results demonstrate the impact of high-crystallinity on the excitonic emission of HCCN materials, thereby expanding their potential applications in the field of photoelectric conversion.

Templated 2D Polymer Heterojunctions for Improved Photocatalytic Hydrogen Production

2D polymers have emerged as one of the most promising classes of organic photocatalysts for solar fuel production due to their tunability, charge-transport properties, and robustness. They are however difficult to process and so there are limited studies into the formation of heterojunction materials incorporating these components. In this work, a novel templating approach is used to combine an imine-based donor polymer and an acceptor polymer formed through Knoevenagel condensation. Heterojunction formation is shown to be highly dependent on the topological match of the donor and acceptor polymers with the most active templated material found to be between three and nine times more active for photocatalysis than its constituent components. Transient absorption spectroscopy reveals that this improvement is due to faster charge separation and more efficient charge extraction in the templated heterojunction. The templated material shows a very high hydrogen evolution rate of >20 mmol h-1 m-2 with an ascorbic acid hole scavenger but also produces hydrogen in the presence of only water and a cobalt-based redox mediator. This suggests the improved charge-separation interface and reduced trapping accessed through this approach could be suitable for Z-scheme formation.

Electrostatic [FeFe]-hydrogenase-carbon nitride assemblies for efficient solar hydrogen production

The assembly of semiconductors as light absorbers and enzymes as redox catalysts offers a promising approach for sustainable chemical synthesis driven by light. However, achieving the rational design of such semi-artificial systems requires a comprehensive understanding of the abiotic-biotic interface, which poses significant challenges. In this study, we demonstrate an electrostatic interaction strategy to interface negatively charged cyanamide modified graphitic carbon nitride (NCNCNX) with an [FeFe]-hydrogenase possessing a positive surface charge around the distal FeS cluster responsible for electron uptake into the enzyme. The strong electrostatic attraction enables efficient solar hydrogen (H2) production via direct interfacial electron transfer (DET), achieving a turnover frequency (TOF) of 18 669 h-1 (4 h) and a turnover number (TON) of 198 125 (24 h). Interfacial characterizations, including quartz crystal microbalance (QCM), photoelectrochemical impedance spectroscopy (PEIS), intensity-modulated photovoltage spectroscopy (IMVS), and transient photocurrent spectroscopy (TPC) have been conducted on the semi-artificial carbon nitride-enzyme system to provide a comprehensive understanding for the future development of photocatalytic hybrid assemblies.

Charge Trapping in Semiconductor Photocatalysts: A Time- and Space-Domain Perspective

Harnessing solar energy to produce value-added fuels and chemicals through photocatalysis techniques holds promise for establishing a sustainable and environmentally friendly energy economy. The intricate dynamics of photogenerated charge carriers lies at the core of the photocatalysis. The balance between charge trapping and band-edge recombination has a crucial influence on the activity of semiconductor photocatalysts. Consequently, the regulation of traps in photocatalysts becomes the key to optimizing their activities. Nevertheless, our comprehension of charge trapping, compared to that of well-studied charge recombination, remains somewhat limited. This limitation stems from the inherently heterogeneous nature of traps at both temporal and spatial scales, which renders the characterization of charge trapping a formidable challenge. Fortunately, recent advancements in both time-resolved spectroscopy and space-resolved microscopy have paved the way for considerable progress in the investigation and manipulation of charge trapping. In this Perspective, we focus on charge trapping in photocatalysts with the aim of establishing a direct link to their photocatalytic activities. To achieve this, we begin by elucidating the principles of advanced time-resolved spectroscopic techniques such as femtosecond time-resolved transient absorption spectroscopy and space-resolved microscopic methods, such as single-molecule fluorescence microscopy and surface photovoltage microscopy. Additionally, we provide an overview of noteworthy research endeavors dedicated to probing charge trapping using time- and space-resolved techniques. Our attention is then directed toward recent achievements in the manipulation of charge trapping in photocatalysts through defect engineering. Finally, we summarize this Perspective and discuss the future challenges and opportunities that lie ahead in the field.

S-vacancy-assisted fast charge transport and oriented ReS2 growth in twin crystal ZnxCd1-xS: an atomic-level heterostructure for dual-functional photocatalytic conversion

The achievement of dual-functional photocatalytic technology requires a photocatalyst with accelerated charge flow and purposeful active-site arrangement. In this study, we developed an oriented embedding strategy to induce ReS2 growth at the S vacancy in twin-crystal Zn0.5Cd0.5S solid solution (Sv-ZCS), obtaining an atomic-level heterostructure (ReS2/Sv-ZCS). The electronic structure calculations demonstrate that the charge density of the Zn atom around the S vacancy is higher than for other Zn atoms and the introduced S vacancy establishes a high-speed channel for electron transport via formed Zn-S-Re bonds at the interface between ReS2 and Sv-ZCS. Photogenerated electrons and holes gathered on Re atoms and Sv-ZCS, respectively, which achieves spatial charge separation and separated arrangement for redox sites. As a result, the optimized ReS2/Sv-ZCS heterostructure possesses high efficiency of electron injection (2.6-fold) and charge separation (8.44-fold), as well as excellent conductivity capability (20.16-fold). The photocatalytic performance of the ReS2/Sv-ZCS composite exhibits highly improved dual-functional activity with simultaneous H2 evolution and selective oxidation of benzyl alcohol. The reaction rate of benzaldehyde and H2 evolution reaches 125 mmol gcat-1 h-1 and 159 mmol gcat-1 h-1, which is the highest efficiency achieved so far for simultaneous coproduction of H2 fuel and organic chemicals on ReS2-based composites. This work enriches the application of ReS2-modified composites in a dual-functional photoredox system and also gives insight into the role of defects in electronic structure modification and activity improvement.

Cobaloxime-Integrated Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution Coupled with Alcohol Oxidation

We report an azide-functionalized cobaloxime proton-reduction catalyst covalently tethered into the Wurster-type covalent organic frameworks (COFs). The cobaloxime-modified COF photocatalysts exhibit enhanced photocatalytic activity for hydrogen evolution reaction (HER) in alcohol-containing solution with no presence of a typical sacrificial agent. The best performing cobaloxime-modified COF hybrid catalyzes hydrogen production with an average HER rate up to 38 μmol h-1 in ethanol/phosphate buffer solution under 4 h illumination. Ultrafast transient optical spectroscopy characterizations and charge carrier analysis reveal that the alcohol contents functioning as hole scavengers could be oxidized by the photogenerated holes of COFs to form aldehydes and protons. The consumption of the photogenerated holes thus suppresses exciton recombination of COFs and improves the ratio of free electrons that were effectively utilized to drive catalytic reaction for HER. This work demonstrates a great potential of COF-catalyzed HER using alcohol solvents as hole scavengers and provides an example toward realizing the accessibility to the scope of reaction conditions and a greener route for energy conversion.

Carbon Vacancies Steer the Activity in Dual Ni Carbon Nitride Photocatalysis

The manipulation of carbon nitride (CN) structures is one main avenue to enhance the activity of CN-based photocatalysts. Increasing the efficiency of photocatalytic heterogeneous materials is a critical step toward the realistic implementation of sustainable schemes for organic synthesis. However, limited knowledge of the structure/activity relationship in relation to subtle structural variations prevents a fully rational design of new photocatalytic materials, limiting practical applications. Here, the CN structure is engineered by means of a microwave treatment, and the structure of the material is shaped around its suitable functionality for Ni dual photocatalysis, with a resulting boosting of the reaction efficiency toward many CX (X = N, S, O) couplings. The combination of advanced characterization techniques and first-principle simulations reveals that this enhanced reactivity is due to the formation of carbon vacancies that evolve into triazole and imine N species able to suitably bind Ni complexes and harness highly efficient dual catalysis. The cost-effective microwave treatment proposed here appears as a versatile and sustainable approach to the design of CN-based photocatalysts for a wide range of industrially relevant organic synthetic reactions.

Ethanol Upgrading to n-Butanol Using Transition-Metal-Incorporated Poly(triazine)imide Frameworks

The upgrading of ethanol to n-butanol was performed using a molecular catalyst integrated into a carbon nitride support, one of the first examples of a supported molecular catalyst performing the Guerbet process. Initial studies using crystalline poly(triazine)imide (PTI) with lithium or transition-metal cations imbedded in the support together with a base as the catalyst system did not produce any significant amounts of n-butanol. However, when using the catalyst material formed by treatment of PTI-LiCl with [(Cp*)IrCl₂]₂ (Cp* = pentamethylcyclopentadienyl) along with sodium hydroxide, a 59% selectivity for butanol (13% yield) was obtained at 145 °C. This PTI-(Cp*)Ir material exhibited distinct UV-vis absorption features and powder X-ray diffractions which differ from those of the parent PTI-LiCl and [(Cp*)IrCl₂]₂. The PTI-(Cp*)Ir material was found to have a metal loading of 27% iridium per empirical unit of the framework. Along with the formation of n-butanol from the Guerbet reaction, the presence of higher chain alcohols was also observed.

Photochemical Hydrogen Atom Transfer Catalysis for Dehydrogenation of Alcohols To Form Carbonyls

Controllable oxidation of alcohols to carbonyls is one of the fundamental transformations in organic chemistry. Herein, we report an unprecedented visible-light-mediated metal-free oxidation of alcohols to carbonyls with hydrogen evolution. By synergistic combination of organophotocatalyst 4CzIPN and a thiol hydrogen atom transfer catalyst, a broad range of alcohols, including primary and secondary benzylic alcohols as well as aliphatic alcohols, were readily oxidized to carbonyls in moderate to excellent yields. A site-selective oxidation has also been achieved by this protocol. Mechanistic investigation indicates that the oxidation proceeds through an oxidative radical-polar crossover process to obtain an α-oxy carbon cation.

Three-Channel Electron Transfer for Estimating Time Constants Correlated with Photocatalytic Photon Utilization

Although the potential barrier for semiconductor-cocatalyst interfacial electron transfer can be reduced by intensifying the irradiation, the photocatalytic reaction still suffers from a low photon utilization. We here propose a trichannel electron transfer model to demonstrate that the photogenerated oxidative intermediates deprive electrons from the photocatalyst and compete with the target reaction. This model can evaluate the time constant for each electronic process involved in the target reaction and predict photocatalytic photon utilization, which is closely related to the evolution of the oxidative intermediates.

An integrated solar battery based on a charge storing 2D carbon nitride

Solar batteries capable of harvesting sunlight and storing solar energy present an attractive vista to transition our energy infrastructure into a sustainable future. Here we present an integrated, fully earth-abundant solar battery based on a bifunctional (light absorbing and charge storing) carbon nitride (K-PHI) photoanode, combined with organic hole transfer and storage materials. An internal ladder-type hole transfer cascade via a transport layer is used to selectively shuttle the photogenerated holes to the PEDOT:PSS cathode. This concept differs from previous designs such as light-assisted battery schemes or photocapacitors and allows charging with light during both electrical charge and discharge, thus substantially increasing the energy output of the cell. Compared to battery operation in the dark, light-assisted (dis)charging increases charge output by 243%, thereby increasing the electric coulombic efficiency from 68.3% in the dark to 231%, leading to energy improvements of 94.1% under illumination. This concept opens new vistas towards compact, highly integrated devices based on multifunctional, carbon-based electrodes and separators, and paves the way to a new generation of earth-abundant solar batteries.

Alternatives to water oxidation in the photocatalytic water splitting reaction for solar hydrogen production

The photocatalytic water splitting process to produce H₂ is an attractive approach to meet energy demands while achieving carbon emission reduction targets. However, none of the current photocatalytic devices meets the criteria for practical sustainable H₂ production due to their insufficient efficiency and the resulting high H₂ cost. Economic viability may be achieved by simultaneously producing more valuable products than O₂ or integrating with reforming processes of real waste streams, such as plastic and food waste. Research over the past decade has begun to investigate the possibility of replacing water oxidation with more kinetically and thermodynamically facile oxidation reactions. We summarize how various alternative photo-oxidation reactions can be combined with proton reduction in photocatalysis to achieve chemical valorization with concurrent H₂ production. By examining the current advantages and challenges of these oxidation reactions, we intend to demonstrate that these technologies would contribute to providing H₂ energy, while also producing high-value chemicals for a sustainable chemical industry and eliminating waste.

Effect of Functional Group Modifications on the Photocatalytic Performance of g-C3 N4

In recent years, photocatalysis has received increasing attention in alleviating energy scarcity and environmental treatment, and graphite carbon nitride (g-C₃ N₄ ) is used as an ideal photocatalyst. However, it still remains numerous challenges to obtain the desirable photocatalytic performance of intrinsic g-C₃ N₄ . Functional group functionalization, formed by introducing functional groups into the bulk structure, is one of the common modification techniques to modulate the carrier dynamics and increases the number of active sites, offering new opportunities to break the limits for structure-to-performance relationship of g-C₃ N₄ . Nevertheless, the general overview of the advance of functional group modification of g-C₃ N₄ is less reported yet. In order to better understand the structure-to-performance relationship at the molecular level, a review of the latest development of functional group modification is urgently needed. In this review, the functional group modification of g-C₃ N₄ in terms of structures, properties, and photocatalytic activity is mainly focused, as well as their mechanism of reaction from the molecular level insights is explained. Second, the recent progress of the application of introducing functional groups in g-C₃ N₄ is introduced and examples are given. Finally, the difficulties and challenges are presented, and based on this, an outlook on the future research development direction is shown.

Improving CO2 photoconversion with ionic liquid and Co single atoms

Photocatalytic CO₂ conversion promises an ideal route to store solar energy into chemical bonds. However, sluggish electron kinetics and unfavorable product selectivity remain unresolved challenges. Here, an ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate, and borate-anchored Co single atoms were separately loaded on ultrathin g-C₃N₄ nanosheets. The optimized nanocomposite photocatalyst produces CO and CH₄ from CO₂ and water under UV-vis light irradiation, exhibiting a 42-fold photoactivity enhancement compared with g-C₃N₄ and nearly 100% selectivity towards CO₂ reduction. Experimental and theoretical results reveal that the ionic liquid extracts electrons and facilitates CO₂ reduction, whereas Co single atoms trap holes and catalyze water oxidation. More importantly, the maximum electron transfer efficiency for CO₂ photoreduction, as measured with in-situ μs-transient absorption spectroscopy, is found to be 35.3%, owing to the combined effect of the ionic liquid and Co single atoms. This work offers a feasible strategy for efficiently converting CO₂ to valuable chemicals.

Process coupling of CO2 reduction and 5-HMF oxidation mediated by defect-enriched layered double hydroxides

Aiming at the comprehensive utilization of waste carbon resources and renewable carbon resources, we put forward the photocatalytic coupling process of CO₂ reduction and 5-hydroxymethylfurfural (5-HMF) oxidation mediated by the anionic compound of layered double hydroxides (LDHs). Specifically, a ZnNiFe-LDH was synthesized by co-precipitation method, during which CO₂ was stored between LDH layers in the form of carbonate. Then, a certain amount of metal vacancies were introduced into LDH nanosheets by selectively etching Zn²⁺ ions. ICP-AES, EPR and XPS showed that the concentration of Zn vacancies gradually increased with the etching time prolonging, which thus optimized the electronic structure of LDH layers. Under the catalysis of the electron-rich metal cations and hydroxyl groups on the layers, the interlayer carbonate was in situ reduced into CO coupled accompanied with the 5-HMF oxidation to 2.5-furandiformaldehyde (DFF). Compared with the unetched ZnNiFe-LDHs, the CO and DFF yields over the LDHs etched for 3 h were increased by 2.84 and 2.82 times under UV-vis irradiation with a density of 500 mW cm^-2. Finally, combined with isotope-labeled ¹³CO₂ experiments and in situ FTIR characterization, we revealed the possible coupling mechanism and defect-induced performance enhancement mechanism.

Engineering β-ketoamine covalent organic frameworks for photocatalytic overall water splitting

Covalent organic frameworks (COFs) are an emerging type of crystalline and porous photocatalysts for hydrogen evolution, however, the overall water splitting activity of COFs is rarely known. In this work, we firstly realized overall water splitting activity of β-ketoamine COFs by systematically engineering N-sites, architecture, and morphology. By in situ incorporating sub-nanometer platinum (Pt) nanoparticles co-catalyst into the pores of COFs nanosheets, both Pt@TpBpy-NS and Pt@TpBpy-2-NS show visible-light-driven overall water splitting activity, with the optimal H2 and O2 evolution activities of 9.9 and 4.8 μmol in 5 h for Pt@TpBpy-NS, respectively, and a maximum solar-to-hydrogen efficiency of 0.23%. The crucial factors affecting the activity including N-sites position, nano morphology, and co-catalyst distribution were systematically explored. Further mechanism investigation reveals the tiny diversity of N sites in COFs that induces great differences in electron transfer as well as reaction potential barriers.

Improved carrier dynamics in nickel/urea-functionalized carbon nitride for ethanol photoreforming

Photoreforming has been shown to accelerate the H₂ evolution rate compared to water splitting due to thermodynamically favorable organic oxidation. In addition, the potential to simultaneously produce solar fuel and value-added chemicals is a significant benefit of photoreforming. To achieve an efficient and economically viable photoreforming process, the selection and design of an appropriate photocatalyst is essential. Carbon nitride is promising as a metal-free photocatalyst with visible light activity, high stability, and low fabrication cost. However, it typically exhibits poor photogenerated charge carrier dynamics, thereby resulting in low photocatalytic performance. Herein, we demonstrate improved carrier dynamics in urea-functionalized carbon nitride with in situ photodeposited Ni cocatalyst (Ni/Urea-CN) for ethanol photoreforming. In the presence of 1 mM Ni²⁺ precursor, an H₂ evolution rate of 760.5 µmol h^-1 g^-1 and an acetaldehyde production rate of 888.2 µmol h^-1 g^-1 were obtained for Ni/Urea-CN. The enhanced activity is ascribed to the significantly improved carrier dynamics in Urea-CN. The ability of oxygen moieties in the urea group to attract electrons and to increase the hole mobility via a positive shift in the valence band promotes an improvement in the overall carrier dynamics. In addition, high crystallinity and specific surface area of the Urea-CN contributed to accelerating charge separation and transfer. As a result, the electrons were efficiently transferred from Urea-CN to the Ni cocatalyst for H₂ evolution while the holes were consumed during ethanol oxidation. The work demonstrates a means by which carrier dynamics can be tuned by engineering carbon nitride via edge functionalization.

Rethinking Electronic Effects in Photochemical Hydrogen Evolution Using CuInS2@ZnS Quantum Dots Sensitizers

Molecular catalysts based on coordination complexes for the generation of hydrogen via photochemical water splitting exhibit a large versatility and tunability of the catalytic properties through chemical functionalization. In the present work, we report on light-driven hydrogen production in an aqueous solution using a series of cobalt polypyridine complexes as hydrogen evolving catalysts (HECs) in combination with CuInS₂@ZnS quantum dots (QDs) as sensitizers, and ascorbate as the electron donor. A peculiar trend in activity has been observed depending on the substituents present on the polypyridine ligand. This trend markedly differs from that previously recorded using [Ru(bpy)₃]²⁺ (where bpy = 2,2'-bipyridine) as the sensitizer and can be ascribed to different kinetically limiting pathways in the photochemical reaction (viz. protonation kinetics with the ruthenium chromophore, catalyst activation via electron transfer from the QDs in the present system). Hence, this work shows how the electronic effects on light-triggered molecular catalysis are not exclusive features of the catalyst unit but depend on the whole photochemical system.

Rational Design of Carbon Nitride Photoelectrodes with High Activity Toward Organic Oxidations

Carbon nitride (CN_x ) is a light-absorber with excellent performance in photocatalytic suspension systems, but the activity of CN_x photoelectrodes has remained low. Here, cyanamide-functionalized CN_x (^NCN CN_x ) was co-deposited with ITO nanoparticles on a 1.8 Å thick alumina-coated FTO electrode. Transient absorption spectroscopy and impedance measurements support that ITO acts as a conductive binder and improves electron extraction from the ^NCN CN_x , whilst the alumina underlayer reduces recombination losses between the ITO and the FTO glass. The Al₂ O₃ |ITO : ^NCN CN_x film displays a benchmark performance for CN_x -based photoanodes with an onset of -0.4 V vs a reversible hydrogen electrode (RHE), and 1.4±0.2 mA cm^-2 at 1.23 V vs RHE during AM1.5G irradiation for the selective oxidation of 4-methylbenzyl alcohol. This assembly strategy will improve the exploration of CN_x in fundamental and applied photoelectrochemical (PEC) studies.

Photocatalyst: To Be Dispersed or To Be Immobilized? The Crucial Role of Electron Transport in Photocatalytic Fixed Bed Reaction

Photocatalytic fixed bed reactors allow a straightforward separation from the process stream and simplify the installation and operation in practical application. However, it is widely believed that the restriction on mass transport and volume activation severely slows the reaction. Here, we demonstrate that photocatalytic fixed bed reactors can deliver a superior reaction rate to the slurry suspension by rationally modulating the electronic process and the most concerning issue of mass transport occurring on a decisecond time scale does not retard the reaction. Although the long-distance transport of photogenerated electrons in porous semiconductor films toward catalytic sites encounters boundary scattering, this electronic process can be far faster than semiconductor-cocatalyst interfacial electron transfer occurring on the decisecond-second time scale. Besides, the fixed bed reaction can be freely amplified without losing photon utilization. Under irradiation provided by a 320 W Hg lamp, we realize a reaction rate of 0.262 mol/h with 65.2% quantum yield for anaerobic dehydrogenation of ethanol.

Photomemristive sensing via charge storage in 2D carbon nitrides

Photomemristive sensors have the potential to innovate current photo-electrochemical sensors by incorporating new sensing capabilities including non-invasive, wireless and time-delayed (memory) readout. Here we report the charge storing 2D carbon nitride potassium poly(heptazine imide), K-PHI, as a direct photomemristive sensing platform by capitalizing on K-PHI's visible light bandgap, large oxidation potential, and intrinsic optoionic charge storage properties. Utilizing the light-induced charge storage function of K-PHI nanosheets, we demonstrate memory sensing via charge accumulation and present potentiometric, impedimetric and coulometric readouts to write/erase this information from the material, with no additional reagents required. Additionally, wireless colorimetric and fluorometric detection of the charging state of K-PHI nanoparticles is demonstrated, enabling the material's use as particle-based autonomous sensing probe in situ. The various readout options of K-PHI's response enable us to adapt the sensitivities and dynamic ranges without modifying the sensing platform, which is demonstrated using glucose as a model analyte over a wide range of concentrations (50 μM to 50 mM). Since K-PHI is earth abundant, biocompatible, chemically robust and responsive to visible light, we anticipate that the photomemristive sensing platform presented herein opens up memristive and neuromorphic functions.

Identification of the Structure of Triethanolamine Oxygenation Products in Carbon Nitride Photocatalysis

Triethanolamine (TEOA) is one of the most commonly used sacrificial agents in photocatalysis. Due to its more complex structure compared to, for example, ethanol, and its sacrificial role in photocatalysis, it gives a mixture of products. The structures of these molecules are not usually analyzed. Herein, we obtain and isolate the products of TEOA and N-tert-butyl diethanolamine oxygenation under photocatalytic conditions with ≈15 % yield, and followingly characterized them by NMR and mass spectroscopy. The reaction is mediated by potassium poly(heptazine imide) (K-PHI) in the presence of O2 and affords formyl esters of β-hydroxyethylene formamides from the corresponding ethanolamines.

An Integrated Carbon Nitride-Nickel Photocatalyst for the Amination of Aryl Halides Using Sodium Azide

The synthesis of primary anilines via sustainable methods remains a challenge in organic synthesis. We report a photocatalytic protocol for the selective synthesis of primary anilines via cross-coupling of a wide range of aryl/heteroaryl halides with sodium azide using a photocatalyst powder consisting of nickel(II) deposited on mesoporous carbon nitride (Ni-mpg-CNx ). This heterogeneous photocatalyst contains a high surface area with a visible light-absorbing and adaptive "built-in" solid-state ligand for the integrated catalytic Ni site. The method displays a high functional group tolerance, requires mild reaction conditions, and benefits from easy recovery and reuse of the photocatalyst powder. Thereby, it overcomes the need of complex ligand scaffolds required in homogeneous catalysis, precious metals and elevated temperatures/pressures in existing protocols of primary anilines synthesis. The reported heterogeneous Ni-mpg-CNx holds potential for applications in the academic and industrial synthesis of anilines and exploration of other photocatalytic transformations.

Finding the Sweet Spot of Photocatalysis─A Case Study Using Bipyridine-Based CTFs

Covalent triazine frameworks (CTFs) are a class of porous organic polymers that continuously attract growing interest because of their outstanding chemical and physical properties. However, the control of extended porous organic framework structures at the molecular scale for a precise adjustment of their properties has hardly been achieved so far. Here, we present a series of bipyridine-based CTFs synthesized through polycondensation, in which the sequence of specific building blocks is well controlled. The reported synthetic strategy allows us to tailor the physicochemical features of the CTF materials, including the nitrogen content, the apparent specific surface area, and optoelectronic properties. Based on a comprehensive analytical investigation, we demonstrate a direct correlation of the CTF bipyridine content with the material features such as the specific surface area, band gap, charge separation, and surface wettability with water. The entirety of these parameters dictates the catalytic activity as demonstrated for the photocatalytic hydrogen evolution reaction (HER). The material with the optimal balance between optoelectronic properties and highest hydrophilicity enables HER production rates of up to 7.2 mmol/(h·g) under visible light irradiation and in the presence of a platinum cocatalyst.

Revealing the Role of Electronic Doping for Developing Cocatalyst-Free Semiconducting Photocatalysts

Developing cocatalyst-free photocatalysts is highly desired because it could avoid the very slow interfacial electron transfer that makes photocatalytic photon utilization a dilemma. However, even in the optimal case, photocatalysts without the use of cocatalysts deliver comparable performance only for conventional construction. We demonstrate here that electronic doping not only provides catalytically active sites in cocatalyst-free photocatalysts but also plays certain additional roles. These electronic states can efficiently channel the trapped electrons to the semiconductor surface without suffering from time-consuming detrapping and can facilitate the extraction of photogenerated holes. These features endow our demonstrated tungsten-doped CdS with evident superiority in photocatalytic performance over conventional counterparts loaded with platinum cocatalysts.

Hole utilization in solar hydrogen production

In photochemical production of hydrogen from water, the hole-mediated oxidation reaction is the rate-determining step. A poor solar-to-hydrogen efficiency is usually related to a mismatch between the internal quantum efficiency of photon-induced hole generation and the apparent quantum yield of hydrogen. This waste of photogenerated holes is unwanted yet unavoidable. Although great progress has been made, we are still far away from the required level of dexterity to deal with the associated challenges of wasted holes and its consequential chemical effects that have placed one of the greatest bottlenecks in attaining high solar-to-hydrogen efficiency. A critical assessment of the hole and its related phenomena in solar hydrogen production would, therefore, pave the way moving forward. In this regard, we focus on the contextual and conceptual understanding of the dynamics and kinetics of photogenerated holes and its critical role in driving redox reactions, with the objective of guiding future research. The main reasons behind and consequences of unused holes are examined and different approaches to improve overall efficiency are outlined. We also highlight yet unsolved research questions related to holes in solar fuel production.

Ultrafast Hot Electron Transfer and Trap-State Mediated Charge Carrier Separation toward Enhanced Photocatalytic Activity in g-C3N4/ZnIn2S4 Heterostructure

Comprehensive understanding of charge carrier dynamics in the heterostructure based photocatalytic materials will strengthen their candidature as future solar energy harvesting resources. Here, in this work, the g-C₃N₄(CN)/ZnIn₂S₄ (ZIS) heterostructure was successfully synthesized and a direct spectroscopic correlation was established between excited-state charge carrier dynamics and enhanced photocatalytic activity using ultrafast transient absorption (TA) spectroscopy. TA analysis demonstrated the dominance of hot electron transfer over the band edge one. The photogenerated hot electrons migrated from the high-energy excitonic states of CN toward ZIS in the subpicosecond time scale. Broad-band (UV to NIR) ultrafast transient pump-probe spectroscopy revealed the collective effect of hot electron transfer as well as trap-state mediated electron delocalization in the enhanced photocatalytic H₂ evolution. This work reveals the role of photogenerated carriers in the photocatalytic performance of the CN/ZIS heterostructure and would create a new avenue toward the advancement of CN based heterostructure in photocatalytic devices.

Photocatalytic C-H Azolation of Arenes Using Heterogeneous Carbon Nitride in Batch and Flow

The functionalization of aryl C(sp2 )-H bonds is a useful strategy for the late-stage modification of biologically active molecules, especially for the regioselective introduction of azole heterocycles to prepare medicinally-relevant compounds. Herein, we describe a practical photocatalytic transformation using a mesoporous carbon nitride (mpg-CNx ) photocatalyst, which enables the efficient azolation of various arenes through direct oxidation. The method exhibits a broad substrate scope and is amenable to the late-stage functionalization of several pharmaceuticals. Due to the heterogeneous nature and high photocatalytic stability of mpg-CNx , the catalyst can be easily recovered and reused leading to greener and more sustainable routes, using either batch or flow processing, to prepare these important compounds of interest in pharmaceutical and agrochemical research.

Multisite PCET with photocharged carbon nitride in dark

A combination of photochemistry and proton coupled electron transfer (PCET) is a primary strategy employed by biochemical systems and synthetic chemistry to enable uphill reactions under mild conditions. Degenerate nanometer-sized n-type semiconductor nanoparticles (SCNPs) with the Fermi level above the bottom of the conduction band are strongly reducing and act more like metals than semiconductors. Application of the degenerate SCNPs is limited to few examples. Herein, we load microporous potassium poly(heptazine imide) (K-PHI) nanoparticles with electrons (e‒) and charge balancing protons (H+) in an illumination phase using sacrificial agents. e‒/H+ in the K-PHI nanoparticles are weakly bound and therefore could be used in a range of PCET reactions in dark, such as generation of aryl radicals from aryl halides, ketyl radicals from ketones, and 6e‒/6H+ reduction of nitrobenzene to aniline. The integration of several features that until now were intrinsic for plants and natural photosynthesis into a transition metal free nanomaterial composed of abundant elements (C, N, and K) offers a powerful tool for synthetic organic chemistry.

Construction of Six-Oxygen-Coordinated Single Ni Sites on g-C3 N4 with Boron-Oxo Species for Photocatalytic Water-Activation-Induced CO2 Reduction

The configuration regulation of single-atom photocatalysts (SAPCs) can significantly influence the interfacial charge transfer and subsequent catalytic process. The construction of conventional SAPCs for aqueous CO₂ reduction is mainly devoted toward favorable activation and photoreduction of CO₂ , however, the role of water is frequently neglected. In this work, single Ni atoms are successfully anchored by boron-oxo species on g-C₃ N₄ nanosheets through a facile ion-exchange method. The dative interaction between the B atom and the sp² N atom of g-C₃ N₄ guarantees the high dispersion of boron-oxo species, where O atoms coordinate with single Ni (II) sites to obtain a unique six-oxygen-coordinated configuration. The optimized single-atom Ni photocatalyst, rivaling Pt-modified g-C₃ N₄ nanosheets, provides excellent CO₂ reduction rate with CO and CH₄ as products. Quasi-in-situ X-ray photoelectron spectra, transient absorption spectra, isotopic labeling, and in situ Fourier transform infrared spectra reveal that as-fabricated six-oxygen-coordinated single Ni (II) sites can effectively capture the photoelectrons of CN along the BO bridges and preferentially activate adsorbed water to produce H atoms to eventually induce a hydrogen-assisted CO₂ reduction. This work diversifies the synthetic strategies for single-atom catalysts and provides insight on correlation between the single-atom configuration and reaction pathway.