Oxophilic gallium single atoms bridged ruthenium clusters for practical anion-exchange membrane electrolyzer

The development of highly efficient and durable alkaline hydrogen evolution reaction (HER) catalysts is crucial for achieving high-performance practical anion exchange membrane water electrolyzer (AEMWE) at ampere-level current density. Herein, we report a design concept by employing Ga single atoms as an electronic bridge to stabilize the Ru clusters for boosting alkaline HER performance in practical AEMWE. Experimental and theoretical results collectively reveal that the bridged Ga sites trigger strong metal-support interaction for the homogeneous distribution of Ru clusters with high density, as well as optimize the Ru-H bond strength due to the electron transfer between Ru and Ga for enhanced intrinsic HER activity. Moreover, the oxophilic Ga sites near the Ru clusters tend to adsorb the hydroxyl species and accelerate the water dissociation for sufficient proton supplement in an alkaline medium. The Ru-GaSA/N-C catalyst exhibits a low overpotential of 4 ± 1 mV (10 mA cm-2) and high mass activity of 9.3 ± 0.5 A mg-1Ru at -0.05 V vs RHE. In particular, the Ru-GaSA/N-C-based AEMWE in 1 M KOH delivers a voltage of only 1.74 V to reach an industrial current density of 1 A cm-2, and can steadily operate at 1 A cm-2 for more than 170 h.

Cocreation of photogenerated electron and hole collectors on polymeric carbon nitride synergistically promotes carrier separation and reaction kinetics towards propelling photocatalytic hydrogen evolution

It is a challenging issue for the creation of photogenerated carrier collectors on the photocatalyst to drive charge separation and promote reaction kinetics in the photocatalytic reaction. Herein, based on one-step dual-modulation strategy, IrO2 nanodots are modified at the edge of polymeric carbon nitride (PCN) nanosheets and atomically dispersed Ir atoms are implanted in the skeleton of PCN to obtain a unique Ir-PCN/IrO2 photocatalyst. IrO2 nanodots and atomically dispersed Ir atoms act as hole and electron collectors to synergistically promote the carrier separation and reaction kinetics, respectively, thereby greatly improving the photocatalytic hydrogen evolution (PHE) performance. As a result, without adding additional cocatalyst, the PHE rate over the optimal Ir-PCN/IrO2-2% sample reaches up to 1564.4 μmol h-1 g-1 under the visible light irradiation, with achieving an apparent quantum yield (AQY) of 15.7% at 420 nm.

Efficient Holes Abstraction by Precisely Decorating Ruthenium Single Atoms and RuOx Clusters on ZnIn2S4 for Photocatalytic Pure Water Splitting

Developing efficient photocatalysts for two-electron water splitting with simultaneous H2O2 and H2 generation shows great promise for practical application. Currently, the efficiency of two-electron water splitting is still restricted by the low utilization of photogenerated charges, especially holes, of which the transfer rate is much slower than that of electrons. Herein, Ru single atoms and RuOx clusters are co-decorated on ZnIn2S4 (RuOx/Ru-ZIS) to employ as multifunctional sites for efficient photocatalytic pure water splitting. Doping of Ru single atoms in the ZIS basal plane enhances holes abstraction from bulk ZIS by regulating the electronic structure, and RuOx clusters offer a strong interfacial electric field to remarkably promote the out-of-plane migration of holes from ZIS. Moreover, Ru single atoms and RuOx clusters also serve as active sites for boosting surface water oxidation. As a result, an excellent H2 and H2O2 evolution rates of 581.9 µmol g-1 h-1 and 464.4 µmol g-1 h-1 is achieved over RuOx/Ru-ZIS under visible light irradiation, respectively, with an apparent quantum efficiency (AQE) of 4.36% at 400 nm. This work paves a new way to increase charge utilization by manipulating photocatalyst using single atom and clusters.

Bi-directional charge transfer channels in highly crystalline carbon nitride enabling superior photocatalytic hydrogen evolution

Introducing a donor-acceptor (D-A) unit is an effective approach to facilitate charge transfer in polymeric carbon nitride (PCN) and enhance photocatalytic performance. However, the introduction of hetero-molecules can lead to a decrease in crystallinity, limiting interlayer charge transfer and inhibiting further improvement. In this study, we constructed a novel D-A type carbon nitride with significantly higher crystallinity and a bi-directional charge transfer channel, which was achieved through 2,5-thiophenedicarboxylic acid (2,5-TDCA)-assisted self-assembly followed by KCl-templated calcination. The thiophene and cyano groups introduced serve as the electron donor and acceptor, respectively, enhancing in-plane electron delocalization. Additionally, introduced potassium ions are intercalated among the adjacent layers of carbon nitride, creating an interlayer charge transfer channel. Moreover, the highly ordered structure and improved crystallinity further facilitate charge transfer. As a result, the as-prepared photocatalyst exhibits superior photocatalytic hydrogen evolution (PHE) activity of 7.449 mmol h-1 g-1, which is 6.03 times higher than that of pure carbon nitride. The strategy of developing crystalline D-A-structured carbon nitride with controlled in-plane and interlayer charge transfer opens new avenues for the design of carbon nitride with enhanced properties for PHE.

Supermolecule Polymer Derived Porous Carbon Nitride Microspheres with Controllable Energy Band Structure for Photocatalytic Hydrogen Evolution Reaction

Porous graphitic carbon nitride microsphere with large specific surface area and controllable energy band structure is synthesized via a simple method with the supermolecule polymer of melamine-cyanuric acid (MCA) as the intermediates. The energy band structure and morphology of carbon nitride are closely correlative to the calcination time. And the CN-20 catalyst fabricated by calcination for 20 h exhibit superior photocatalytic activity of hydrogen evolution reaction (HER) under visible-light (λ ≥ 420 nm) irradiation. The photocatalytic and photoelectrochemical test results indicate that Pt is the optimum cocatalyst candidate compared with Pd, Ru, and Ag. Meanwhile, the time-dependent process of the intermediate pyrolysis to carbon nitride and the internal mechanism of photogenerated charge transfer between semiconductors and cocatalyst is investigated and supplemented by theoretical calculations. This work provides a novel and energy band structure controllable manufacture strategy for porous carbon nitride semiconductor with satisfying visible-light photocatalytic reduction performance.

Noble-Metal-Free Single- and Dual-Atom Catalysts for Artificial Photosynthesis

Artificial photosynthesis enables direct solar-to-chemical energy conversion aimed at mitigating environmental pollution and producing solar fuels and chemicals in a green and sustainable approach, and efficient, robust, and low-cost photocatalysts are the heart of artificial photosynthesis systems. As an emerging new class of cocatalytic materials, single-atom catalysts (SACs) and dual-atom catalysts (DACs) have received a great deal of current attention due to their maximal atom utilization and unique photocatalytic properties, whereas noble-metal-free ones impart abundance, availability, and cost-effectiveness allowing for scalable implementation. This review outlines the fundamental principles and synthetic methods of SACs and DACs and summarizes the most recent advances in SACs (Co, Fe, Cu, Ni, Bi, Al, Sn, Er, La, Ba, etc.) and DACs (CuNi, FeCo, InCu, KNa, CoCo, CuCu, etc.) based on non-noble metals, confined on an arsenal of organic or inorganic substrates (polymeric carbon nitride, metal oxides, metal sulfides, metal-organic frameworks, carbon, etc.) acting as versatile scaffolds in solar-light-driven photocatalytic reactions, including hydrogen evolution, carbon dioxide reduction, methane conversion, organic synthesis, nitrogen fixation, hydrogen peroxide production, and environmental remediation. The review concludes with the challenges, opportunities, and future prospects of noble-metal-free SACs and DACs for artificial photosynthesis.

Theoretical Study of p-Block Metal Single-Atom-Loaded Carbon Nitride Catalyst for Photocatalytic Water Splitting

Graphitic carbon nitride (g-C3N4), recognized for its considerable potential as a heterogeneous photocatalyst in water splitting, has attracted extensive research interest. By using density functional theory (DFT) calculations, the regulatory role of p-block metal (PM) single atoms on the photocatalytic activity of g-C3N4 in overall water splitting was systematically explored. The incorporation of PM atoms (Ge, Sn and Pb) led to a reduction in the overpotentials required for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Combined with the electronic structures analysis via hybrid functional, it was found that the introduction of Ge, Sn or Pb optimizes the positions of the valence band maximum (VBM) and the conduction band minimum (CBM), providing a robust driving force for HER and ensuring substantial driving force for OER. Meanwhile, the presence of these three PMs induces the spatial separation of VBM and CBM, inhibiting the recombination of carriers. These findings have significant implications for the design and preparation of efficient photocatalysts.

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.

Preparation of highly dispersed Ni single-atom doped ultrathin g-C3N4 nanosheets by metal vapor exfoliation for efficient photocatalytic CO2 reduction

Single-atom photocatalysts can modulate the utilization of photons and facilitate the migration of photogenerated carriers. However, the preparation of single-atom uniformly doped photocatalysts is still a challenging topic. Herein, we propose the preparation of Ni single-atom doped g-C3N4 photocatalysts by metal vapor exfoliation. The Ni vapor produced by calcining nickel foam at high temperature accumulates in between g-C3N4 layers and poses a certain vapor pressure to destroy the interlayer van der Waals forces of g-C3N4. Individual metal atoms are doped into the structure while exfoliating g-C3N4 into nanosheets by metal vapor. Upon optimization of Ni content, the Ni single atom doped g-C3N4 nanosheets with 2.81 wt% Ni exhibits the highest CO2 reduction performance in the absence of sacrificial agents. The generation rates of CO and CH4 are 19.85 and 1.73 μmol g-1h-1, respectively. The improved photocatalytic performance is attributed to the anchoring Ni of single atoms on g-C3N4 nanosheets, which increases both carrier separation efficiency and reaction sites. This work provides insight into the design of photocatalysts with highly dispersed single-atom.

Non-Noble-Metal-Doped Carbon Nitride Photocatalysts for Water Splitting Screened Out by Empty Defect States and the d-Band Center

A rational design of water-splitting photocatalysts from the perspective of the electronic structure is highly desirable for optimizing catalytic activities. However, the structure-activity relationship is still unclear, which impedes the development of efficient catalysts. Herein, by comparing systematically the overall water-splitting capability of 20 kinds of metallic elements anchored at three sites (including cavity, carbon vacancy, and nitrogen vacancy) of graphitic carbon nitride (g-C3N4) through density functional theory calculations, we uncover that availability of in-gap empty defect states and the d-band center position are paramount parameters to determine activities of g-C3N4 on photocatalytic water splitting. In-gap empty states play a role in accommodating electrons from H2O to facilitate its splitting. A lower d-band center weakens the interaction between reaction intermediates and g-C3N4, thereby promoting O2 desorption. Metals embedded at carbon vacancies are found to be superior to those at cavities and nitrogen vacancies because the former not only provides ample in-gap empty states but also has a lower d-band center. We also discover a rule that, for a reaction in which the bond order between the metal and intermediate enlarges (reduces), its reaction difficulty increases (decreases) with the increasing atomic number for elements in the same period. After screening, we find that non-noble metals Co, Ni, and Ga anchored at carbon vacancies possess catalytic performances comparable to Pd- and Pt-doped systems, with the rate-determining barriers less than 0.55 eV. Our findings may provide useful information for designing effective photocatalysts.

Artificial Surface Electron Network Prompted Energy Band Structure Tuning: Boosting Solar-to-Hydrogen Evolution Performance

The energy gap and conduction band position of catalysts play crucial roles in solar-to-hydrogen (STH) transformation technology. Unfortunately, although an increase in the conduction band position can effectively promote the photoreduction capacity of the photocatalyst, it will inevitably widen the band gap, thus reducing the light-absorption scale. It seems that there is a contradiction between the reduction of band gap and the improvement of conduction band position, which is that "You can't have your cake and eat it too." Herein, an ultrasimple molecular adsorption strategy was engineered by adsorbing hydrazine hydrate on the surface of TiO2. The theoretical and experimental results indicated that the strong electron-donating effect of amino groups in hydrazine hydrate can promote the redistribution of photogenerated electrons and form surface electron networks on the surface of TiO2 photocatalysts, which can bend the conduction band upward and significantly improve its photoreduction ability. Besides, the adsorption of -NH2 can narrow the band gap width of TiO2 and promote the separation efficiency of photogenerated carriers. More interestingly, it can also effectively enhance the adsorption of H2O and H+, thus greatly elevating the STH efficiency. The STH rate of the as-prepared T-N-3 can be increased by ≈530%. This work sheds light on a new approach for resolving the contradiction between photoreduction and light absorption capabilities to effectively enhance photocatalytic performance.

Large electronegativity differences between adjacent atomic sites activate and stabilize ZnIn2S4 for efficient photocatalytic overall water splitting

Photocatalytic overall water splitting into hydrogen and oxygen is desirable for long-term renewable, sustainable and clean fuel production on earth. Metal sulfides are considered as ideal hydrogen-evolved photocatalysts, but their component homogeneity and typical sulfur instability cause an inert oxygen production, which remains a huge obstacle to overall water-splitting. Here, a distortion-evoked cation-site oxygen doping of ZnIn2S4 (D-O-ZIS) creates significant electronegativity differences between adjacent atomic sites, with S1 sites being electron-rich and S2 sites being electron-deficient in the local structure of S1-S2-O sites. The strong charge redistribution character activates stable oxygen reactions at S2 sites and avoids the common issue of sulfur instability in metal sulfide photocatalysis, while S1 sites favor the adsorption/desorption of hydrogen. Consequently, an overall water-splitting reaction has been realized in D-O-ZIS with a remarkable solar-to-hydrogen conversion efficiency of 0.57%, accompanying a ~ 91% retention rate after 120 h photocatalytic test. In this work, we inspire an universal design from electronegativity differences perspective to activate and stabilize metal sulfide photocatalysts for efficient overall water-splitting.

Bio-Inspired Supramolecular Self-Assembled Carbon Nitride Nanostructures for Photocatalytic Water Splitting

Fast production of hydrogen and oxygen in large amounts at an economic rate is the need of the hour to cater to the needs of the most awaited hydrogen energy, a futuristic renewable energy solution. Production of hydrogen through simple water splitting via visible light photocatalytic approach using sunlight is considered as one of the most promising and sustainable approaches for generating clean fuels. For this purpose, a variety of catalytic techniques and novel catalysts have been investigated. Among these catalysts, carbon nitride is presently deemed as one of the best candidates for the visible light photocatalysis due to its unique molecular structure and adequate visible-range bandgap. Its bandgap can be further engineered by structural and morphological manipulation or by doping/hybridization. Among numerous synthetic approaches for carbon nitrides, supramolecular self-assembly is one of the recently developed elegant bottom-up strategies as it is bio-inspired and provides a facile and eco-friendly route to synthesize high surface area carbon nitride with superior morphological features and other semiconducting and catalytic properties. The current review article broadly covers supramolecular self-assembly synthesis of carbon nitride nanostructures and their photocatalytic water-splitting applications and provides a comprehensive outlook on future directions.

Phosphate ester functionalized fluorene-benzothiadiazole alternating copolymer/hydroxylated g-C3N4 heterojunctions for efficient hydrogen evolution under visible-light irradiation

It is highly desirable to explore functionalized polymer semiconductor/g-C3N4 heterojunction photocatalysts with the tight interfacial connection for promoting the photogenerated electron-hole pair separation, improving the hydrophilicity, extending the visible light response and achieving the efficient visible light-driven H2 evolution. Herein, we synthesized novel poly[9,9-bis(3-ethyl phosphate propyl)fluorene-alt-benzothiadiazole] (PPFBT) with a phosphate ester on every repeating unit by the Suzuki polymerization and then fabricated PPFBT/hydroxylated g-C3N4 (PPFBT/CN-OH) heterojunctions via a surface hydroxyl-induced assembly process. The ratio-optimized 5PPFBT/CN-OH shows the hydrogen evolution activity of 2662.4 μmol·g-1·h-1, an 11.1-time enhancement compared to CN-OH. The improved photocatalytic activity is mainly attributed to the enhanced electron-hole pair separation due to the tight interfacial connection by hydrogen bond (P=O…H-O) and N…S interactions between PPFBT and CN-OH. It is verified that abundant phosphate ester groups of PPFBT improve the hydrophilicity and form coordination bonds with platinum (P=O:Pt) as a cocatalyst to facilitate water splitting for H2 evolution. It is also confirmed that the enhanced electron-hole pair separation is mainly dependent on the excited high-energy level electron transfer from CN-OH to PPFBT. This work provides a rational molecular design strategy for constructing efficient functionalized polymer semiconductor/g-C3N4 heterojunctions for sunlight-driven H2 evolution.

Photosynthesis of Benzonitriles on BiOBr Nanosheets Promoted by Vacancy Associates

Photocatalytic organic functionalization reactions represent a green, cost-effective, and sustainable synthesis route for value-added chemicals. However, heterogeneous photocatalysis is inefficient in directly activating ammonia molecules for the production of high-value-added nitrogenous organic products when compared with oxygen activation in the formation of related oxygenated compounds. In this study, we report the heterogeneous photosynthesis of benzonitriles by the ammoxidation of benzyl alcohols (99 % conversion, 93 % selectivity) promoted using BiOBr nanosheets with surface vacancy associates. In contrast, the main reaction of catalysts with other types of vacancy sites is the oxidation of benzyl alcohol to benzaldehyde or benzoic acid. Experimental measurements and theoretical calculations have demonstrated a specificity of vacancy type with respect to product selectivity, which arises from the adsorption and activation of NH3 and O2 that is required to promote subsequent C-N coupling and oxidation to nitrile. This study provides a better understanding of the role of vacancies as catalytic sites in heterogeneous photocatalysis.

Molecular Heptazine-Triazine Junction over Carbon Nitride Frameworks for Artificial Photosynthesis of Hydrogen Peroxide

Revealing the photocatalytic mechanism between various junctions and catalytic activities has become a hotspot in photocatalytic systems. Herein, an internal molecular heptazine/triazine (H/T) junction in crystalline carbon nitride (HTCN) is constructed and devoted to selective two-electron oxygen reduction reaction (2e- ORR) for efficient hydrogen peroxide (H2 O2 ) production. In-situ X-ray diffraction spectra under various temperatures authenticate the successful formation of molecular H/T junction in HTCN during the calcining process rather than physically mixing. The increased surface photovoltage and transient photovoltage signals, and the decreased exciton binding energy undoubtably elucidate that an obvious increasement of carrier density and diffusion capability of photogenerated electrons are realized over HTCN. Additionally, the analyses of in situ photoirradiated Kelvin probe force microscopy and femto-second transient absorption spectra reveal the successful construction of the strong internal built-in-electric field and the existence of the majority of long-lived shallow trapped electrons associated with molecular H/T junction over HTCN, respectively. Benefiting from these, the photocatalytic results exhibit an incredible improvement (96.5-fold) for H2 O2 production. This novel work provides a comprehensive understanding of the long-lived reactive charges in molecular H/T junctions for strengthening the driving-force for photocatalytic H2 O2 production, which opens potential applications for enhancing PCN-based photocatalytic redox reactions.

Multifunctional carbon nitride nanoarchitectures for catalysis

Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Hydrovoltaic effect-enhanced photocatalysis by polyacrylic acid/cobaltous oxide–nitrogen doped carbon system for efficient photocatalytic water splitting

Severe carrier recombination and the slow kinetics of water splitting for photocatalysts hamper their efficient application. Herein, we propose a hydrovoltaic effect-enhanced photocatalytic system in which polyacrylic acid (PAA) and cobaltous oxide (CoO)–nitrogen doped carbon (NC) achieve an enhanced hydrovoltaic effect and CoO–NC acts as a photocatalyst to generate H2 and H2O2 products simultaneously. In this system, called PAA/CoO–NC, the Schottky barrier height between CoO and the NC interface decreases by 33% due to the hydrovoltaic effect. Moreover, the hydrovoltaic effect induced by H+ carrier diffusion in the system generates a strong interaction between H+ ions and the reaction centers of PAA/CoO–NC, improving the kinetics of water splitting in electron transport and species reaction. PAA/CoO–NC exhibits excellent photocatalytic performance, with H2 and H2O2 production rates of 48.4 and 20.4 mmol g−1 h−1, respectively, paving a new way for efficient photocatalyst system construction.

Oxygenated Triazine-Heptazine Heterostructure Creates an Enormous Ascension to the Visible Light Photocatalytic Hydrogen Evolution Performance of Porous C3 N4 Nanosheets

A highly efficient g-C₃ N₄ photocatalyst is developed by a novel one-pot thermal polymerization method under a salt fog environment generated by heating the aqueous solution of urea and mixed metal salts of NaCl/KCl, namely SF-CN. Thanks to the synergistic effect of the oxygenation and chemical etching of the salt fog, the obtained SF-CN is an oxygenated ultrathin porous carbon nitride with an intermolecular triazine-heptazine heterostructure, meanwhile, shows enlarged specific surface area, greatly enhanced absorption of visible light, narrowed band gap with a lower conduction band, and an increased photocurrent response due to the effective separation of photogenerated holes and electrons, comparing to those of pristine g-C₃ N₄ . The theoretical simulations further reveal that the triazine-heptazine heterostructure possesses better photocatalytic hydrogen evolution (PHE) capability than pure triazine and heptazine carbon nitrides. In turn, SF-CN demonstrates an excellent visible light PHE rate of 18.13 mmol h^-1  g^-1 , up to 259.00 times of that of pristine g-C₃ N₄ .

Metal-Free Carbon Nitride Nanosheet Supported the Pentacoordinated Silicon Intermediates for Photocatalytic Overall Water Splitting

Photocatalytic overall water splitting is a promising approach to overcome the environmental and energy crisis. However, developing effective photocatalysts with well activity, light absorption, and photogenerated carrier lifetime is still a challenge. Herein, combining extensive first-principles and nonadiabatic molecular dynamics calculations, we find that microporous carbon-nitride nanosheets with a pyridinic nitrogen, such as C₂N and C₆N₆, possess the pentacoordinated silicon intermediates' bonding environment. The pentacoordinated silicon as intermediates exhibits good photocatalytic performance for the difficult four-electronic oxygen evolution reaction. The overpotential is only 0.55 V for C₂N, which is significantly lower than that of the tetracoordinated silicon intermediates (2.00 V). Simultaneously, the decoration of the silicon group not only widens the absorption range of visible light but also maintains the lifetime of photogenerated carriers on the nanosecond scale, which enhances the application efficiency of solar energy. Our work paves a new route for advancing photocatalytic overall water splitting.

Mesoporous Carbon Nitride with π-Electron-Rich Domains and Polarizable Hydroxyls Fabricated via Solution Thermal Shock for Visible-Light Photocatalysis

Carbon nitride semiconductors are competitive candidates for visible-light-responsive photocatalysts, but encounter weakened exciton dissociation arising from the elevated Coulomb force of singlet Frenkel excitons with narrowing bandgaps. We overcome this contradiction by co-infusing π-electron-rich domains and polarizable hydroxyl units into mesoporous carbon nitride, realized by solution thermal shock. The embedded delocalized π-conjugated aromatic domains derived from nonconjugated macromolecules downshift the conduction band edge and contribute to spatial separation of photogenerated electrons in the lowest unoccupied molecular orbital and holes in the highest occupied molecular orbital. Meanwhile, polarizable hydroxyls induce distinct electron flow from heptazine-based skeletons to periphery sites and enhance water adsorption as well as proton reduction capacity. Consequently, the polymeric carbon nitride delivers an enhanced hydrogen evolution rate that is 17.5 times larger than thermally treated counterparts derived from urea fabricated via conventional strategies. These results show that our strategy can infuse different functional motifs into carbon nitride and thus improve photocatalytic activity.

Recent Advances of Doping and Surface Modifying Carbon Nitride with Characterization Techniques

As a non-metallic organic semiconductor photocatalyst, graphitic carbon nitride (g–C3N4, CN) has become a research hotspot due to its excellent performance in organic degradation, CO2 reduction and water splitting to produce hydrogen. However, the high recombination rate of electron-hole pairs, low specific surface area and weak light absorption of bulk CN synthesized by the traditional one-step thermal polymerization method seriously restrict its photocatalytic performance and practical application. To enhance the photocatalytic performance of CN, doping and surface modification strategies are usually employed to tune the band gap of carbon nitride and improve the separation of carriers. In this paper, the research progress of different methods to modify CN in recent years is introduced, and the mechanisms of improving the photocatalytic performance are mainly analyzed. Typical modification methods are mainly divided into metal doping, non-metal doping, co-doping and surface-functionalized modification. Some characterization methods that can analyze the doping state and surface modification are also discussed as examples. Finally, the difficulties that need to be addressed through modified CN photocatalysts and the directions for future research are pointed out.

Copper-Bridged Tetrakis(4-ethynylphenyl)ethene Aggregates with Photo-Regulated 1 O2 and O2 .- Generation for Selective Photocatalytic Aerobic Oxidation

Active species regulation is a key scientific issue that essentially determines the selectivity and activity of a photocatalyst. Herein, Cu^I -bridged tetrakis(4-ethynylphenyl)ethene aggregates (T₄ EPE-Cu) with photo-regulated ¹ O₂ and O₂ ^.- generation were demonstrated for selective photocatalytic aerobic oxidation. In this system, transient photovoltage combined with the density functional theory calculations confirmed that Cu-alkynyl was the main oxygen activation site. The adsorbed O₂ tends to produce O₂ ^.- because of the potential well effect of Cu-alkynyl under high-energy light excitation. But under low-energy light, O₂ tends to produce ¹ O₂ via resonance energy transfer with Cu-alkynyl. For α-terpinene oxidation, the ratios of ¹ O₂ products to O₂ ^.- products can be controlled from 1.3 (380 nm) to 10.7 (600 nm). Furthermore, T₄ EPE-Cu exhibited ultrahigh photocatalytic performance for Glaser coupling and benzylamine oxidation, with a conversion and selectivity of over 99 %.

The mesoscale mechanism of P-dopant defects and interface synergy for phenols degradation

The semiconductor based photocatalysis has become a hot spot of current research, and the key challenges are the construction of strong functional heterojunction photocatalysts, and insights on the working mechanism involved. In this work, we constructed a NiFe- LDHs/P-TCN heterojunction with P-dopant defects and interface synergy, and elucidated its mesoscale mechanism among different constituent interfaces. The interface photoelectron transfer was detected by PAS, EPR and other methods, and the enhancing mechanism of the defect sites for interface electron transfer and photocatalytic activity was proposed. The interfacial electrons, photoelectric properties and photocatalytic activity are found to be positively correlated. The result is conducive for a better understanding on working mechanism of heterogeneous photocatalysts, which opened up a broader research space for the rational design and construction of functional interfaces.

Integrating Covalent Organic Framework with Transition Metal Phosphide for Noble-Metal-Free Visible-Light-Driven Photocatalytic H2 Evolution

2D covalent organic frameworks (COFs) are considered as one kind of the most promising crystalline porous materials for solar-driven hydrogen production. However, adding noble metal co-catalysts into the COFs-based photocatalytic system is always indispensable. Herein, through a simple solvothermal synthesis method, TpPa-1-COF, a typical 2D COF, which displays a wide light absorption region, is rationally combined with transition metal phosphides (TMPs) to fabricate three TMPs/TpPa-1-COF hybrid materials, named Ni₁₂ P₅ (Ni₂ P or CoP)/TpPa-1-COF. The incorporated TMPs can be served as electron collectors for accelerating the transfer of charges on TpPa-1-COF, thus the composites are demonstrated to be efficient photocatalysts for promoting water splitting. Benefitting from the richer surface reactive sites and lower H* formation energy barrier, the Ni₁₂ P₅ can most effectively improve the photocatalytic performance of the TpPa-1-COF, and the H₂ evolution rate can reach up to 31.6 µmol h^-1 , approximately 19 times greater than pristine TpPa-1-COF (1.65 µmol h^-1 ), and is comparable to the Pt/TpPa-1-COF (38.8 µmol h^-1 ). This work is the first example of combining COFs with TMPs to construct efficient photocatalysts, which may offer new insight for constructing noble-metal-free COF-based photocatalysts.

Synergetic metal-semiconductor interaction: Single-atomic Pt decorated CdS nano-photocatalyst for highly water-to-hydrogen conversion

Solar driven water-to-hydrogen conversion is a promising technology for the typical sustainable production mode, so increasing efforts are being devoted to exploit high-performance photocatalytic materials. Cadmium sulfide (CdS) is widely used to prepare highly active photocatalysts owing to its merits of broadband-light harvesting and feasible band structure. However, the slow photo-carriers' migration in CdS body structure generally results in high-frequency carriers recombination, which leads to unsatisfied photoactivity. Metallic single-atom surface decoration is an effective method to build the strong metal-support interaction for promotion of photo-carriers' migration. Herein, a simple light-induced reduction procedure was proposed to decorate single-atomic Pt on the surface of CdS nanoparticles for highly photocatalytic HER activity. Research showed that the synergetic metal (Pt)-semiconductor (CdS) interaction significantly promoted the body-to-surface (BTS) photo-carriers' migration of CdS, thereby the high light-to-fuel conversion efficiency (AQY_{500 nm} = 25.70%) and 13.5-fold greater simulated sunlight driven HER rate of bare CdS was achieved by this CdS-Pt nano-photocatalyst. Based on the photo-electrochemical analysis and density functional theory calculations, the remarkably improved HER photoactivity can be attributed to the enhanced light-harvesting, promoted BTS electron migration and reduced reaction energy barriers. This study provides a facile procedure to obtain CdS based photocatalyst with metallic single-atom sites for high-performance HER photocatalysis.

Highly Active and Selective Electroreduction of N2 by the Catalysis of Ga Single Atoms Stabilized on Amorphous TiO2 Nanofibers

The electroreduction of N₂ under ambient conditions has emerged as one of the most promising technologies in chemistry, since it is a greener way to make NH₃ than the traditional Haber-Bosch process. However, it is greatly challenged with a low NH₃ yield and faradaic efficiency (FE) because of the lack of highly active and selective catalysts. Inherently, transition (d-block) metals suffer from inferior selectivity due to fierce competition from H₂ evolution, while post-transition (p-block) metals exhibit poor activity due to insufficient "π back-donation" behavior. Considering their distinct yet complementary electronic structures, here we propose a strategy to tackle the activity and selectivity challenge through the atomic dispersion of p-block metal on an all-amorphous transition-metal matrix. To address the activity issue, lotus-root-like amorphous TiO₂ nanofibers are synthesized which, different from vacancy-engineered TiO₂ nanocrystals reported previously, possess abundant intrinsic oxygen vacancies (V_O) together with under-coordinated dangling bonds in nature, resulting in significantly enhanced N₂ activation and electron transport capacity. To address the selectivity issue, well-isolated single atoms (SAs) of Ga are successfully synthesized through the confinement effect of V_O, resulting in Ga-V_O reactive sites with the maximum availability. It is revealed by density functional theory calculations that Ga SAs are favorable for the selective adsorption of N₂ at the catalyst surface, while V_O can facilitate N₂ activation and reduction subsequently. Benefiting from this coupled activity/selectivity design, high NH₃ yield (24.47 μg h^-1 mg^-1) and FE (48.64%) are achieved at an extremely low overpotential of -0.1 V vs RHE.

Interior and Surface Synergistic Modifications Modulate the SnNb2O6/Ni-Doped ZnIn2S4 S-Scheme Heterojunction for Efficient Photocatalytic H2 Evolution

Interior and surface synergistic modifications can endow the photocatalytic reaction with tuned photogenerated carrier flow at the atomic level. Herein, a new class of 2D/2D SnNb₂O₆/Ni-doped ZnIn₂S₄ (SNO/Ni-ZIS) S-scheme heterojunctions is synthesized by a simple hydrothermal strategy, which was used to evaluate the synergy between interior and surface modifications. Theoretical calculations show that the S-scheme heterojunction boosts the desorption of H atoms for rapid H₂ evolution. As a result, 25% SNO/Ni_0.4-ZIS exhibits significantly improved PHE activity under visible light, roughly 4.49 and 2.00 times stronger than that of single ZIS and Ni_0.4-ZIS, respectively. In addition, 25% SNO/Ni_0.4-ZIS also shows superior structural stability. This work provides advanced insight for developing high-performance S-scheme systems from photocatalyst design to mechanistic insight.

One-Step MOF-Templated Strategy to Fabrication of Ce-Doped ZnIn2 S4 Tetrakaidecahedron Hollow Nanocages as an Efficient Photocatalyst for Hydrogen Evolution

Achieving structure optimizing and component regulation simultaneously in the ZnIn₂ S₄ -based photocatalytic system is an enormous challenge in improving its hydrogen evolution performance. 3D hollow-structured photocatalysts have been intensively studied due to their obvious advantages in solar energy conversion reactions. The synthesis of 3D hollow-structured ZnIn₂ S₄ , however, is limited by the lack of suitable template or synthesis methods, thereby restricting the wide application of ZnIn₂ S₄ in the field of photocatalysis. Herein, Ce-doped ZnIn₂ S₄ photocatalysts with hollow nanocages are obtained via one-step hydrothermal method with an ordered large-pore tetrakaidecahedron cerium-based metal-organic frameworks (Ce-MOFs) as template and Ce ion source. The doping of Ce and the formation of ZnIn₂ S₄ tetrakaidecahedron hollow nanocages with ultrathin nanosheet subunits are simultaneously induced by the Ce-MOFs, making this groundbreaking work. The Ce-doped ZnIn₂ S₄ with a nonspherical 3D hollow nanostructure inherit the tetrakaidecahedron shape of the Ce-MOF templates, and the shell is composed of ultrathin nanosheet subunits. Both theoretical and experimental results indicate that the doping of Ce and the formation of hollow nanocages increase light capture and the separation of photogenerated charge carriers.

Anionic donor-acceptor conjugated polymer dots/g-C3N4 nanosheets heterojunction: High efficiency and excellent stability for co-catalyst-free photocatalytic hydrogen evolution

Herein, we have assembled an anionic donor-acceptor (D-A) conjugated polyelectrolyte dots (Pdots), based on bithiophene units-containing backbone and sulfonate modified side chain (PCP-2F-Li), with porous g-C₃N₄ nanosheets (CNNS) into a new 0D/2D heterojunction (PCP-2F-Li Pdots/CNNS). The well-matched energy levels of PCP-2F-Li and CNNS and the strong electron-donating sulfinates in PCP-2F-Li can significantly accelerate the interfacial electron transfer in heterojunction, while the strong hydrophilicity of PCP-2F-Li can improve the interface wetting and promote the photocatalytic water-splitting. As such, PCP-2F-Li Pdots/CNNS can be used for efficient co-catalyst-free water splitting with a hydrogen evolution rate (HER) of 1932.1 μmol·h^-1·g^-1 over 6 runs, which is 1.85 and 2.29 times of hydrophobic F8T2 Pdots/CNNS and Pt-assisted CNNS, respectively. The apparent quantum yield (AQY) of PCP-2F-Li Pdots/CNNS can reach 7.87 %, 7.73 % and 5.60 % at 420, 450 and 475 nm, respectively. The findings highlight a new type of the Pdots-assisted heterojunctions for high-efficiency and durable co-catalyst-free water splitting.

Constructing porous intramolecular donor–acceptor integrated carbon nitride doped with m-aminophenol for boosting photocatalytic degradation and hydrogen evolution activity

A porous intramolecular D–A integrated carbon nitride with boosted photocatalytic activity was constructed via thermal melting followed by thermal copolymerization of m-aminophenol with urea.

Synergistic Modulation of the Separation of Photo-Generated Carriers via Engineering of Dual Atomic Sites for Promoting Photocatalytic Performance

The separation efficiency of photo-generated carriers is still a great challenge that restricts the practical application of photocatalytic technology. The design of spatial separation path for photo-generated carriers at atomic level provides an innovative approach to address this challenge. Herein, a facile dual atomic sites strategy, consisting of Cu-N₄ and C-S-C active moieties decorated on polymeric carbon nitride (Cu SAs/p-CNS) is reported to simultaneously achieve the highly efficient separation of photo-generated electrons and holes for boosting photocatalytic performance. As a proof of concept, the Cu SAs/p-CNS is successfully applied to the photo-oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF), which exhibits 77.1% HMF conversion and 85.6% DFF selectivity under visible light irradiation. The activity is considerably higher than that of bulk p-CN, S doped p-CN, and p-CN supported Cu single atom catalysts. Theoretical calculations and experimental results suggest that, during photocatalytic reaction, the isolated Cu-N₄ sites directly capture photo-generated electrons, while the surrounding S atoms bear photo-generated holes, which synergistically facilitates the separation of photo-generated carriers and thus results in enhanced photocatalytic activity. This study provides a new perspective for the rational design of high performance photocatalysts at atomic level.

Construction and performance of a simple and efficient g-C3N4 photocatalytic hydrogen production system

Surface and bulk structure modification is an effective strategy to improve the photocatalytic performance of g-C3N4 (CN). In this work, dilute NaOH solution was used in situ to regulate the CN structure for enhanced photocatalytic hydrogen evolution reaction (HER). Characterization results indicate that after treatment with dilute NaOH solution, the surface of CN was hydroxylated, resulting in the change of CN structure and the increase of BET specific surface area. Furthermore, some Na+ ions can be intercalated into the framework of CN, and form the Na-N bond. These modifications boost the HER activity of CN. The test carried out in 7.5 mM NaOH solution shows the highest activity and it is almost 3.7 times higher than that performed in water. Control tests indicate that hydroxides of other alkali and alkali earth metals such as LiOH, KOH, Ca(OH)2, and Ba(OH)2 have similar promotion effects. This work demonstrates a valid and simple way to enhance the HER activity of CN through performing the reaction in a weakly alkaline solution.

Donor Bandgap Engineering without Sacrificing the Reduction Ability of Photogenerated Electrons in Crystalline Carbon Nitride

Crystalline carbon nitride (CCN) with a light response up to 700 nm has been seldom reported but is significant for the artificial photocatalysis. In this study, it is proposed that, unlike acceptors, introducing donors can effectively narrow the bandgap without sacrificing the reduction ability of photogenerated electrons, which is more advantageous to photocatalytic reduction reactions. Hence, a series of heptazine-based K⁺ -implanted CCN (KCN) with a narrow bandgap (2.87-1.86 eV) are constructed by copolymerization of pyrimidine donors. The optimized photocatalysts can extend the light response to 700 nm and account for approximately 122- and 33-fold enhancements in H₂ production (λ>500 nm) in comparison to CN and KCN, respectively. The apparent quantum efficiency (AQE) can reach 8.2 % at 500 nm and is comparable to the top-level CN- and CCN- based materials. Its photoactive wavelength has significant advantages over previously reported CCN-based photocatalysts. This method offers a universal donor bandgap engineering strategy towards photocatalytic reduction reactions.

Synergy of intermolecular Donor-Acceptor and ultrathin structures in crystalline carbon nitride for efficient photocatalytic hydrogen evolution

Crystalline carbon nitride is regarded as the new generation of emerging metal-free photocatalysts as opposed to polymeric carbon nitride (g-C₃N₄) because of its high crystalline structure and ultrahigh photocatalytic water splitting performance. However, further advances in crystalline g-C₃N₄ are significantly restricted by the sluggish separation of charge carriers and limited active sites. In this study, we demonstrate the successful synthesis of heptazine-triazine donor-acceptor-based ultrathin crystalline g-C₃N₄ nanosheets (UCCN) using a combined hot air exfoliation and molten salt (NaCl/KCl) copolymerization approach. The synergy of the donor-acceptor heterojunction and the ultrathin structure greatly accelerated the separation of the charge carriers and enriched the active sites. Accordingly, the superior hydrogen evolution activity and an ultrahigh apparent quantum efficiency of 73.6% at 420 nm under a natural photosynthetic environment were achieved by UCCN, positioning this material at the top among reported conjugated g-C₃N₄ materials. This study provides a novel paradigm for the development of donor-acceptor-based ultrathin crystalline layered materials.

Regulating Local Electron Density of Iron Single Sites by Introducing Nitrogen Vacancies for Efficient Photo-Fenton Process

The activity of heterogeneous photocatalytic H₂ O₂ activation in Fenton-like processes is closely related to the local electron density of reaction centre atoms. However, the recombination of electron-hole pairs arising from random charge transfer greatly restricts the oriented electron delivery to active center. Here we show a defect engineered iron single atom photocatalyst (Fe₁ -N_v /CN, single Fe atoms dispersed on carbon nitride with abundant nitrogen vacancies) for the activation of H₂ O₂ under visible light irradiation. Based on DFT calculations and transient absorption spectroscopy results, the engineered nitrogen vacancies serve as the electron trap sites, which can directionally drive the electrons to concentrate on Fe atoms. The formation of highly concentrated electrons density at Fe sites significantly improves the H₂ O₂ conversion efficiency. Therefore, the optimized single atom catalyst exhibiting a higher ciprofloxacin degradation activity, which was up to 18 times that of pristine CN.

Metal-doped carbon nitrides: synthesis, structure and applications

This perspective provides a comprehensive overview of the latest progress of M–CN, which would promote further development, such as for single-atom catalysis and nanozymatic reactions.

DMAP molecule grafting on a carbon nitride heptazine ring for the better degradation of pollutants – the synergy of electron withdrawing and steric hindrance effects

Graphitic carbon nitride (CN) is a promising candidate for use in photocatalytic pollutant degradation, but it only shows moderate activity because of its sluggish photocarrier transfer and insufficient light absorption.