Enhancing the Activity of a Carbon Nitride Photocatalyst by Constructing a Triazine-Heptazine Homojunction.

Establishing homojunctions at the molecular level between different but physicochemically similar phases belonging to the same family of materials is an effective approach to promoting the photocatalytic activity of polymeric carbon nitride (CN) materials. Here, we prepared a CN material with a uniform distribution of homojunctions by combining two synthetic strategies: supramolecular assemblies as the precursor and molten salt as the medium. We designed porous CN rods with triazine-heptazine homojunctions (THCNs) using a melem supramolecular aggregate (Me) and melamine as the precursors and a KCl/LiBr salt mixture as the liquid reaction medium. The triazine/heptazine ratio is controlled by varying the relative amounts of the chosen precursors, and the molten salt treatment enhances the structural order of the interplanar packing units for the THCN skeleton, leading to rapid charge migration. The resulting built-in electric field induced by the triazine-heptazine homojunction enhances photogenerated charge separation; the optimal THCN catalyst exhibits an excellent H2 evolution rate via photocatalytic water splitting, which is ∼24 times as high as that of reference bulk CN, with long-term stability.

Ligand Isomerization Driven Electrocatalytic Switching.

The prevailing view about molecular catalysts is that the central metal ion is responsible for the reaction mechanism and selectivity, whereas the ligands mainly affect the reaction kinetics. Here, we question this paradigm and show that ligands have a dramatic influence on the selectivity of the product. We show how even a seemingly small change in ligand isomerization sharply alters the selectivity of the well-researched oxygen reduction reaction (ORR) Co phthalocyanine catalyst from an indirect 2e- to a direct 4e- pathway. Detailed analysis reveals that intramolecular hydrogen-bond interactions in the ligand activate the catalytic Co, directing the oxygen binding and thus deciding the final product. The resulting catalyst is the first example of a Co-based molecular catalyst catalyzing a direct 4e- ORR via ligand isomerization, for which it shows an activity close to the benchmark Pt in an actual H2-O2 fuel cell. The effect of the ligand isomerism is demonstrated with different central metal ions, thus highlighting the generalizability of the findings and their potential to open new possibilities in the design of molecular catalysts.

Unconventional Synthesis of Metal (Ni, Co, Ag) Antimony Alloy Particles.

Bimetallic alloy materials attract interest owing to their properties and stability compared to pure metals, especially alloys with nanoscale dimensions. Metal antimony (MSb) alloys, specifically NiSb, are widely used for charge storage applications due to their high stability. Most synthetic approaches to form these materials require drastic conditions (e.g., high temperatures, potent reducing agents, and extended reaction times), limiting control over the final morphology. The other viable approach is a galvanic replacement that uses unstable materials as precursors. In this work, we present a new and facile method to prepare several MSb (M = Ni, Co, Ag) alloys with shape control by reacting Sb2S3 particles with a metal(M)-sulfide single source precursor in trioctylphosphine (TOP) under mild conditions. Furthermore, we explore the role of TOP as a reducing agent and demonstrate how both alloy constituents are crucial for mutual stabilization. Electrochemical studies are also performed on these NiSb particles, showing their ambipolar nature and allowing their utilization as the active ingredient in the demonstrated high-energy-density symmetric supercapacitor.

Boron and Sodium Doping of Polymeric Carbon Nitride Photoanodes for Photoelectrochemical Water Splitting.

Polymeric carbon nitride is a promising photoanode material for water-splitting and organic transformation-based photochemical cells. Despite achieving significant progress in performance, these materials still exhibit low photoactivity compared to inorganic photoanodic materials because of a moderate visible light response, poor charge separation, and slow oxidation kinetics. Here, the synthesis of a sodium- and boron-doped carbon nitride layer with excellent activity as a photoanode in a water-splitting photoelectrochemical cell is reported. The new synthesis consists of the direct growth of carbon nitride (CN) monomers from a hot precursor solution, enabling control over the monomer-to-dopant ratio, thus determining the final CN properties. The introduction of Na and B as dopants results in a dense CN layer with a packed morphology, better charge separation thanks to the in situ formation of an electron density gradient, and an extended visible light response up to 550 nm. The optimized photoanode exhibits state-of-the-art performance: photocurrent densities with and without a hole scavenger of about 1.5 and 0.9 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE), and maximal external quantum efficiencies of 56% and 24%, respectively, alongside an onset potential of 0.3 V.

The Implications of Coupling an Electron Transfer Mediated Oxidation with a Proton Coupled Electron Transfer Reduction in Hybrid Water Electrolysis.

Invited for this month's cover are the groups of Menny Shalom at the Ben-Gurion University of the Negev, Israel and Dr. Biswajit Mondal at Indian Institute of Technology Gandhinagar, India. The image shows the connection between two half-cells: an electron transfer-mediated [(2,2,6,6-tetramethyl-1-piperidin-1-yl)oxyl] (TEMPO)-catalyzed benzylamine oxidation at the anode and a proton-coupled electron transfer at the cathode for hydrogen generation. The difference in pH dependence of the anodic and cathodic processes enables hybrid water electrolysis at low cell potential (∼1.0 V) by adjusting only the pH value of the electrolytic medium. The Research Article itself is available at 10.1002/cssc.202202271.

Developing extended visible light responsive polymeric carbon nitrides for photocatalytic and photoelectrocatalytic applications.

Polymeric carbon nitride (CN) has emerged as an attractive material for photocatalysis and photoelectronic devices. However, the synthesis of porous CNs with controlled structural and optical properties remains a challenge, and processable CN precursors are still highly sought after for fabricating homogenous CN layers strongly bound to a given substrate. Here, we report a general method to synthesize highly dispersed porous CN materials that show excellent photocatalytic activity for the hydrogen evolution reaction and good performance as photoanodes in photoelectrochemical cells (PEC): first, supramolecular assemblies of melem and melamine in ethylene glycol and water are prepared using a hydrothermal process. These precursors are then calcined to yield a water-dispersible CN photocatalyst that exhibits beneficial charge separation under illumination, extended visible-light response attributed to carbon doping, and a large number of free amine groups that act as preferential sites for a Pt cocatalyst. The optimized CN exhibits state-of-the-art HER rates up to 23.1 mmol h-1 g-1, with an AQE of 19.2% at 395 nm. This unique synthetic route enables the formation of a homogeneous precursor paste for substrate casting; consequently, the CN photoanode exhibits a low onset potential, a high photocurrent density and good stability after calcination.

Polymeric carbon nitride as a platform for photoelectrochemical water-splitting cells.

Polymeric carbon nitride (CN) materials are promising low-cost photocatalysts that exhibit a combination of chemical and physical properties suitable for converting light into redox activity on their surface. In this perspective, we describe our experience with this family of materials as light absorbers that serve as an anode in photoelectrochemical cells toward water-splitting. We describe some of the CN deposition techniques and procedures established in our lab. The knowledge gained from powder-based photocatalysis is implemented in photoelectrochemical scenarios and is used to determine the merits and shortcomings of resulting layers. We show how the preparation methods are oriented based on these factors and how high photoelectrochemical water-splitting activity develops in photoanodes we developed where CN(s) act as photoabsorbers. Lastly, we present our view on the future prospects of this field.

The Implications of Coupling an Electron Transfer Mediated Oxidation with a Proton Coupled Electron Transfer Reduction in Hybrid Water Electrolysis.

Electrolysis of water is a sustainable route to produce clean hydrogen. Full water-splitting requires a high applied potential, in part because of the pH-dependency of the H2 and O2 evolution reactions (HER and OER), which are proton-coupled electron transfer (PCET) reactions. Therefore, the minimum required potential will not change at different pHs. TEMPO [(2,2,6,6-tetramethyl-1-piperidin-1-yl)oxyl], a stable free-radical that undergoes fast electro-oxidation by a single-electron transfer (ET) process, is pH-independent. Here, we show that the combination of PCET and ET processes enables hydrogen production from water at low cell potentials below the theoretical value for full water-splitting by simple pH adjustment. As a case study, we combined the HER with the oxidation of benzylamine by anodically oxidized TEMPO. The pH-independent electrocatalytic oxidation of TEMPO permits the operation of a hybrid water-splitting cell that shows promise to perform at a low cell potential (≈1 V) and neutral pH conditions.

Photoelectrochemical alcohols oxidation over polymeric carbon nitride photoanodes with simultaneous H2 production.

The photoelectrochemical oxidation of organic molecules into valuable chemicals is a promising technology, but its development is hampered by the poor stability of photoanodic materials in aqueous solutions, low faradaic efficiency, low product selectivity, and a narrow working pH range. Here, we demonstrate the synthesis of value-added aldehydes and carboxylic acids with clean hydrogen (H2) production in water using a photoelectrochemical cell based solely on polymeric carbon nitride (CN) as the photoanode. Isotope labeling measurements and DFT calculations reveal a preferential adsorption of benzyl alcohol and molecular oxygen to the CN layer, enabling fast proton abstraction and oxygen reduction, which leads to the synthesis of an aldehyde at the first step. Further oxidation affords the corresponding acid. The CN photoanode exhibits excellent stability (>40 h) and activity for the oxidation of a wide range of substituted benzyl alcohols with high yield, selectivity (up to 99%), and faradaic efficiency (>90%).

Dynamics of the nanocrystal structure and composition in growth solutions monitored by in situ lab-scale X-ray diffraction.

In situ characterization of nanoparticle (NP) growth has become the state-of-the-art approach for studying their growth mechanisms; there is broad consensus on the reliability and precision of in situ characterization techniques compared to more traditional ex situ ones. Nonetheless, most of the currently available methods require the use of sophisticated setups such as synchrotron-based X-ray sources or an environmental liquid transmission electron microscopy (TEM) cell, which are expensive and not readily accessible. Herein, we suggest a new approach to study NP growth mechanisms: using a commercially available heating chamber for time-resolved X-ray diffraction (TR-XRD) measurements of NP growth in solution. We demonstrate how this lab-scale in situ XRD can be used to study NP growth mechanisms when complemented by standard ex situ techniques such as TEM and UV-vis spectroscopy. TR-XRD reveals the crystallographic phase and real-time evolution of NP size, shape, and composition. A detailed analysis allows determining the growth mechanism and measuring the alloying kinetics of multinary nanocrystals, demonstrated herein for a colloidal CdxZn1-xS system. This approach proves itself as a promising strategy for NP growth research and could be expanded to related fields that study dynamic changes as the formation and evolution of crystalline materials in solutions.

Supramolecular organization of melem for the synthesis of photoactive porous carbon nitride rods.

Intrinsic defects and structural properties are two main factors influencing the photocatalytic performance of carbon nitride (CN) materials. Here, photoactive porous CN rods are fabricated through the thermal condensation of melem-based hexagonal supramolecular assemblies. To overcome the poor solubility of melem, we exfoliate the bulk melem using hydrochloric acid. The latter allows good dispersibility of the monomer in an aqueous medium, leading to the formation of H-bond bridged supramolecular assembly with good regularity in both size and rod-like morphology. After thermal condensation, a well-ordered structure of porous CN rods with fewer defects due to the high thermal stability of the melem-based supramolecular assembly is obtained. The new CN materials have a high specific surface area, good light-harvesting properties, and enhanced charge separation and migration. The optimal CN material exhibits excellent photocatalytic activity and durability towards hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR, with good selectivity).

Formation of Copper Oxide Nanotextures on Porous Calcium Carbonate Templates for Water Treatment.

The necessity of providing clean water sources increases the demand to develop catalytic systems for water treatment. Good pollutants adsorbers are a key ingredient, and CuO is one of the candidate materials for this task. Among the different approaches for CuO synthesis, precipitation out of aqueous solutions is a leading candidate due to the facile synthesis, high yield, sustainability, and the reported shape control by adjustment of the counter anions. We harness this effect to investigate the formation of copper oxide-based 3D structures. Specifically, the counter anion (chloride, nitrate, and acetate) affects the formation of copper-based hydroxides and the final structure following their conversion into copper oxide nanostructures over porous templates. The formation of a 3D structure is obtained when copper chloride or nitrate reacts with a Sorites scaffold (marine-based calcium carbonate template) without external hydroxide addition. The transformation into copper oxides occurs after calcination or reduction of the obtained Cu2(OH)3X (X = Cl- or NO3-) while preserving the porous morphology. Finally, the formed Sorites@CuO structure is examined for water treatment to remove heavy metal cations and degrade organic contaminant molecules.

Direct growth of uniform carbon nitride layers with extended optical absorption towards efficient water-splitting photoanodes.

A general synthesis of carbon nitride (CN) films with extended optical absorption, excellent charge separation under illumination, and outstanding performance as a photoanode in water-splitting photoelectrochemical cells is reported. To this end, we introduced a universal method to rapidly grow CN monomers directly from a hot saturated solution on various substrates. Upon calcination, a highly uniform carbon nitride layer with tuned structural and photophysical properties and in intimate contact with the substrate is obtained. Detailed photoelectrochemical and structural studies reveal good photoresponse up to 600 nm, excellent hole extraction efficiency (up to 62%) and strong adhesion of the CN layer to the substrate. The best CN photoanode demonstrates a benchmark-setting photocurrent density of 353 µA cm-2 (51% faradaic efficiency for oxygen), and external quantum yield value above 12% at 450 nm at 1.23 V versus RHE in an alkaline solution, as well as low onset potential and good stability.

Highly Efficient Polymeric Carbon Nitride Photoanode with Excellent Electron Diffusion Length and Hole Extraction Properties.

Polymeric carbon nitride (CN) has emerged as a promising semiconductor in photoanodes for photoelectrochemical cells (PECs) owing to its suitable electronic structure, tunable band gap, high stability, and low price. However, the poor electron diffusion within the CN layer and hole extraction to the solution still limit its applicability in PECs. Here, we report the fabrication of a CN photoanode with excellent electron diffusion length and remarkable hole extraction properties by careful design of its electronic interfaces. We combine complementary synthetic approaches to grow tightly packed CN layers forming a type-II heterojunction, which results in a CN photoanode with excellent charge separation, high electronic conductivity, and remarkable hole extraction efficiency. The optimized CN photoanode displays excellent PEC performance, reaching up to 270 µA cm-2 in a 0.1 M KOH solution at 1.23 V vs RHE, extremely low onset potential (∼0.0012 V), and long-term stability up to 18 h.

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures.

Hybrid nanostructures, composed of multi-component crystals of various shapes, sizes and compositions are much sought-after functional materials. Pairing the ability to tune each material separately and controllably combine two (or more) domains with defined spatial orientation results in new properties. In this review, we discuss the various synthetic mechanisms for the formation of hybrid nanostructures of various complexities containing at least one metal/semiconductor interface, with a focus on colloidal chemistry. Different synthetic approaches, alongside the underlying kinetic and thermodynamic principles are discussed, and future advancement prospects are evaluated. Furthermore, the proved unique properties are reviewed with emphasis on the connection between the synthetic method and the resulting physical, chemical and optical properties with applications in fields such as photocatalysis.

Coordination-Directed Growth of Transition-Metal-Crystalline-Carbon Composites with Controllable Metal Composition.

Transition-metal-carbon (CTM) composites show ample activity in many catalytic reactions. However, control of composition, distribution, and properties is challenging. Now, a straightforward path for the synthesis of transition-metal nanoparticles engulfed in crystalline carbon is presented with excellent control over the metal composition, amount, ratio, and catalytic properties. This approach uses molten monomers that coordinate metals ions at high temperature. At high temperatures, strong coordination bonds direct the growth of carbon material with homogeneous metals distribution and with negligible losses, owing to the liquid-like reaction compared to the traditional solid-state reaction. The strength of the approach is demonstrated by the synthesis of mono, binary, and trinary transition-metal-crystalline-carbon composites with tunable and precise elemental composition as well as good electrochemical properties as oxygen evolution reaction electrocatalysts.

Freestanding Hierarchical Carbon Nitride/Carbon-Paper Electrode as a Photoelectrocatalyst for Water Splitting and Dye Degradation.

Freestanding electrodes composed of 2D materials are highly attractive for many applications such as batteries, membranes, actuators, optical devices, and other energy-related devices owing to their low price, unique structure, high specific surface area, and excellent mechanical and electrical properties. Here, we report the facile large-scale fabrication of freestanding hierarchical carbon nitride/carbon electrodes (CN/C) by the in situ crystallization of CN precursors on conductive carbon paper, followed by thermal annealing. The resulting CN exhibits a vertically aligned morphology with a homogeneous layer distribution, improved crystallinity, and excellent contact with the carbon paper. The freestanding electrodes exhibit high electrical conductivity and good photoelectrochemical activity as anodes in water splitting photoelectrochemical cells. Furthermore, we show here as a proof-of-concept that the freestanding CN/C electrodes can be used as photoelectrocatalysts for the oxidative degradation of organic compounds in water, with enhanced activity compared to photocatalytic and electrocatalytic degradation, while the extracted electrons can be used for the simultaneous production of hydrogen at the cathode.

Low-Cost Porous Ruthenium Layer Deposited on Nickel Foam as a Highly Active Universal-pH Electrocatalyst for the Hydrogen Evolution Reaction.

Low-cost and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER) are a key constituent of a low-carbon industrial economy based on intermittent energy production in the near future. A facile wet-chemistry strategy has been developed for the synthesis of a porous Ru layer deposited onto Ni foam (NF) as a competitive candidate for HER over the whole pH range, especially under economical alkaline conditions. The catalyst shows outstanding HER performance, which stems from the porosity of the Ru layer, the electronic structure of the electrode, and the charge transfer between the NF and the Ru layer, which gives rise to the strong activity of the Ru layer in the HER process. Moreover, the Ru loading was as low as approximately 1.1 wt %, representing significant potential for application in cost-effective HER.

Calcareous Foraminiferal Shells as a Template for the Formation of Hierarchal Structures of Inorganic Nanomaterials.

A microorganism template approach has been explored for the fabrication of various well-defined three-dimensional (3D) structures. However, most of these templates suffer from small size (few µm), difficulty to remove the template, or low surface area, which affect their potential use in different applications or makes industrial scale-up difficult. Conversely, foraminifer's microorganisms are large (up to 200 mm), consist of CaCO3 (easy to dissolve in mild acid), and have a relatively high surface area (≈5 m2 g-1). Herein, we demonstrate the formation of hierarchical structures of inorganic materials using calcareous foraminiferal shells such as Sorites, Globigerinella siphonifera, Lox-ostomina amygdaleformis, Calcarina baculatus or hispida, and Peneroplis planatus. Several techniques, such as thermal decomposition of single-source precursors of metal oxides or sulfides, reduction of metal salts directly on the surfaces, and redox reactions, were used for coating of different shell materials and several hybrid compositions, which possess nanofeatures. Finally, we examined the role of the prepared 3D structures on the reduction of 4-nitrophenol (4-NP), ethanol electrooxidation, and water purification. A remarkable performance was achieved in each application. The hierarchical structure leads to the reduction of 4-NP within several minutes, a 27 mA cm-2 current density peak was obtained for ethanol electrooxidation, and more than 95% of the organic dye contaminants were successfully removed. These results show that using foraminiferal shells offers a new way for designing complex hierarchical structures with unique properties.

Carbon Nitride Materials for Water Splitting Photoelectrochemical Cells.

Graphitic carbon nitride materials (CNs) have emerged as suitable photocatalysts and heterogeneous catalysts for various reactions thanks to their tunable band gap, suitable energy-band position, high stability under harsh chemical conditions, and low cost. However, the utilization of CN in photoelectrochemical (PEC) and photoelectronic devices is still at an early stage owing to the difficulties in depositing high-quality and homogenous CN layer on substrates, its wide band gap, poor charge-separation efficiency, and low electronic conductivity. In this Minireview, we discuss the synthetic pathways for the preparation of various structures of CN on substrates and their underlying photophysical properties and current photoelectrochemical performance. The main challenges for CN incorporation into PEC cell are described, together with possible routes to overcome the standing limitations toward the integration of CN materials in PEC and other photoelectronic devices.