Engineering Nitrogen-Doped Carbon Quantum Dots: Tailoring Optical and Chemical Properties through Selection of Nitrogen Precursors.

The process of N-doping is frequently employed to enhance the properties of carbon quantum dots. However, the precise requirements for nitrogen precursors in producing high-quality N-doped carbon quantum dots (NCQDs) remain undefined. This research systematically examines the influence of various nitrogen dopants on the morphology, optical features, and band structure of NCQDs. The dots are synthesized using an efficient, eco- friendly, and rapid continuous hydrothermal flow technique. This method offers unparalleled control over synthesis and doping, while also eliminating convention-related issues. Citric acid is used as the carbon source, and urea, trizma base, beta-alanine, L-arginine, and EDTA are used as nitrogen sources. Notably, urea and trizma produced NCQDs with excitation-independent fluorescence, high quantum yields (up to 40%), and uniform dots with narrow particle size distributions. Density functional theory (DFT) and time-dependent DFT modelling established that defects and substituents within the graphitic structure have a more significant impact on the NCQDs' electronic structure than nitrogen-containing functional groups. Importantly, for the first time, this work demonstrates that the conventional approach of modelling single-layer structures is insufficient, but two layers suffice for replicating experimental data. This study, therefore, provides essential guidance on the selection of nitrogen precursors for NCQD customization for diverse applications.

Electron Ptychographic Phase Imaging of Beam-sensitive All-inorganic Halide Perovskites Using Four-dimensional Scanning Transmission Electron Microscopy.

Halide perovskites (HPs) are promising candidates for optoelectronic devices, such as solar cells or light-emitting diodes. Despite recent progress in performance optimization and low-cost manufacturing, their commercialization remains hindered due to structural instabilities. While essential to the development of the technology, the relation between the microscopic properties of HPs and the relevant degradation mechanisms is still not well understood. The sensitivity of HPs toward electron-beam irradiation poses significant challenges for transmission electron microscopy (TEM) investigations of structure and degradation mechanisms at the atomic scale. However, technological advances and the development of direct electron cameras (DECs) have opened up a completely new field of electron microscopy: four-dimensional scanning TEM (4D-STEM). From a 4D-STEM dataset, it is possible to extract not only the intensity signal for any STEM detector geometry but also the phase information of the specimen. This work aims to show the potential of 4D-STEM, in particular, electron exit-wave phase reconstructions via focused probe ptychography as a low-dose and dose-efficient technique to image the atomic structure of beam-sensitive HPs. The damage mechanism under conventional irradiation is described and atomically resolved almost aberration-free phase images of three all-inorganic HPs, CsPbBr3, CsPbIBr2, and CsPbI3, are presented with a resolution down to the aperture-constrained diffraction limit.

Electrochemical Generation of Catalytically Active Edge Sites in C2 N-Type Carbon Materials for Artificial Nitrogen Fixation.

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH3 ) is a potentially carbon-neutral and decentralized supplement to the established Haber-Bosch process. Catalytic activation of the highly stable dinitrogen molecules remains a great challenge. Especially metal-free nitrogen-doped carbon catalysts do not often reach the desired selectivity and ammonia production rates due to their low concentration of NRR active sites and possible instability of heteroatoms under electrochemical potential, which can even contribute to false positive results. In this context, the electrochemical activation of nitrogen-doped carbon electrocatalysts is an attractive, but not yet established method to create NRR catalytic sites. Herein, a metal-free C2 N material (HAT-700) is electrochemically etched prior to application in NRR to form active edge-sites originating from the removal of terminal nitrile groups. Resulting activated metal-free HAT-700-A shows remarkable catalytic activity in electrochemical nitrogen fixation with a maximum Faradaic efficiency of 11.4% and NH3 yield of 5.86 µg mg-1 cat h-1 . Experimental results and theoretical calculations are combined, and it is proposed that carbon radicals formed during activation together with adjacent pyridinic nitrogen atoms play a crucial role in nitrogen adsorption and activation. The results demonstrate the possibility to create catalytically active sites on purpose by etching labile functional groups prior to NRR.

Boron Carbon Nitride Thin Films: From Disordered to Ordered Conjugated Ternary Materials.

We present an innovative method for the synthesis of boron carbon nitride thin film materials in a simple furnace setup, using commonly available solid precursors and relatively low temperature compared to previous attempts. The as-prepared structural and optical properties of thin films are tuned via the precursor content, leading to a sp2-conjugated boron nitride-carbon nitride mixed material, instead of the commonly reported boron nitride-graphene phase segregation, with tunable optical properties such as band gap and fluorescence.

Electron Deficient Monomers that Optimize Nucleation and Enhance the Photocatalytic Redox Activity of Carbon Nitrides.

Polymeric carbon nitride (PCN) is usually synthesized from nitrogen-rich monomers such as cyanamide, melamine, and urea, but is rather disordered in many cases. Now, a new allotrope of carbon nitride with internal heterostructures was obtained by co-condensation of very electron poor monomers (for example, 5-amino-tetrazole and nucleobases) in the presence of mild molten salts (for example, NaCl/KCl) to mediate the polymerization kinetics and thus modulate the local structure, charge carrier properties, and most importantly the HOMO and LUMO levels. Results reveal that the as-prepared NaK-PHI-A material shows excellent photo-redox activities because of a nanometric hetero-structure which enhances visible light absorption and promotes charge separation in the different domains.

Photo-Driven Ion Transport for a Photodetector Based on an Asymmetric Carbon Nitride Nanotube Membrane.

Conventional photosensing devices work mainly by electron processing and transport, while visual systems in intelligence work by integrative ion/electron signals. To realize smarter photodetectors, some photoionic device or the combination of ionic and electronic devices are necessary. Now, an ion-transport-based self-powered photodetector is presented based on an asymmetric carbon nitride nanotube membrane, which can realize fast, selective, and stable light detection while being self-powered. Local charges are continuously generated at the irradiated side of the membrane, and none (fewer) at the non-irradiated side. The resulting surface charge gradient in carbon nitride nanotube will drive ion transport in the cavity, thus realizing the function of ionic photodetector. With advantages of low cost and easy fabrication process, the concept of ionic photodetectors based on carbon nitride anticipates wide applications for semiconductor biointerfaces.

Semi-heterogeneous Dual Nickel/Photocatalysis using Carbon Nitrides: Esterification of Carboxylic Acids with Aryl Halides.

Cross-coupling reactions mediated by dual nickel/photocatalysis are synthetically attractive but rely mainly on expensive, non-recyclable noble-metal complexes as photocatalysts. Heterogeneous semiconductors, which are commonly used for artificial photosynthesis and wastewater treatment, are a sustainable alternative. Graphitic carbon nitrides, a class of metal-free polymers that can be easily prepared from bulk chemicals, are heterogeneous semiconductors with high potential for photocatalytic organic transformations. Here, we demonstrate that graphitic carbon nitrides in combination with nickel catalysis can induce selective C-O cross-couplings of carboxylic acids with aryl halides, yielding the respective aryl esters in excellent yield and selectivity. The heterogeneous organic photocatalyst exhibits a broad substrate scope, is able to harvest green light, and can be recycled multiple times. In situ FTIR was used to track the reaction progress to study this transformation at different irradiation wavelengths and reaction scales.

Tailoring the Grain Boundary Chemistry of Polymeric Carbon Nitride for Enhanced Solar Hydrogen Production and CO2 Reduction.

Photocatalytic water splitting is a promising and clean way to mimic plant photosynthesis in a sustainable manner. Improvements of the quantum efficiency and optical absorption in the relevant range are necessary steps to approach practicality. Herein, we reported that these issues can be readily addressed when 5-aminotetrazole, a monomer with high nitrogen content, is used for the synthesis of carbon nitride. The molten salt mixture NaCl/KCl is used as a high-temperature solvent to tailor the grain boundary structure and chemistry. Visible light quantum efficiency for H2 production of 0.65 could be obtained in the presence of K2 HPO4 as a double layer modifier. This value is very high, considering that this number depends on light to charge couple conversion, charge localization, as well as a successful oxidation and reduction reaction.

Enhanced Electrocatalytic N2 Reduction via Partial Anion Substitution in Titanium Oxide-Carbon Composites.

The electrochemical conversion of N2 at ambient conditions using renewably generated electricity is an attractive approach for sustainable ammonia (NH3 ) production. Considering the chemical inertness of N2 , rational design of efficient and stable catalysts is required. Therefore, in this work, it is demonstrated that a C-doped TiO2 /C (C-Tix Oy /C) material derived from the metal-organic framework (MOF) MIL-125(Ti) can achieve a high Faradaic efficiency (FE) of 17.8 %, which even surpasses most of the established noble metal-based catalysts. On the basis of the experimental results and theoretical calculations, the remarkable properties of the catalysts can be attributed to the doping of carbon atoms into oxygen vacancies (OVs) and the formation of Ti-C bonds in C-Tix Oy . This binding motive is found to be energetically more favorable for N2 activation compared to the non-substituted OVs in TiO2 . This work elucidates that electrochemical N2 reduction reaction (NRR) performance can be largely improved by creating catalytically active centers through rational substitution of anions into metal oxides.

Morphogenesis of Metal-Organic Mesocrystals Mediated by Double Hydrophilic Block Copolymers.

Mesocrystals-superstructures of crystalline nanoparticles that are aligned in a crystallographic fashion-are of increasing interest for formation of inorganic materials with complex and sophisticated morphologies to tailor properties without changing chemical composition. Here we report morphogenesis of a novel mesocrystal consisting of nanoscale metal-organic frameworks (MOF) by using double hydrophilic block copolymer (DHBC) as a crystal modulator. DHBC selectively prefers the metastable hexagonal kinetic polymorph and promotes anisotropic crystal growth to generate hexagonal rod mesocrystals via oriented attachment and mesoscale assembly. The metastable nature of hexagonal mesocrystals enables further hierarchical morphogenesis by a solvent-mediated polymorphic transformation toward stable tetragonal mesocrystals that retain the outer hexagonal particle morphology. Furthermore, synthesis of hybrid MOFs, where hexagonal mesocrystals are vertically aligned on specific surfaces of cubic MOFs, is demonstrated. The present strategy opens a new avenue to create MOF mesocrystals and their hybrids with controlled size and morphology that can be designed for various potential applications.

Electrostatic Stabilization of Carbon Nitride Colloids in Organic Solvents Enables Stable Dispersions and Transparent Homogeneous CN Films for Optoelectronics.

Covalent modification of phenyl-modified carbon nitride with vinylthiazole groups via visible light induced grafting is reported. Modified structures express negative charge migration to the thiazole edges while the carbon nitride sheet remains positively charged in organic solutions. Such a phenomenon provides electrostatic stabilization of modified carbon nitride particles in organic media leading to highly organodispersible and colloidally stable carbon nitrides. The resulting structures can be homogeneously dispersed in organic solvents and can be cast to transparent films. The usefulness of such a processable colloidal carbon nitride building block is exemplified here by its high luminescence and inkjet printing of films.

Formation and Properties of Poly(Ionic Liquid)-Carbene Nanogels Containing Individually Stabilized Silver Species.

Imidazolium-based ionic liquids have the ability to undergo a variety of chemical reactions through an N-heterocyclic carbene (NHC) intermediate, which has expanded the chemical toolbox for new applications. Despite their uses and exploration, the carbene-forming properties and applications of their polymeric congeners, poly(ionic liquid)s (PILs), is still underdeveloped. Herein, we explore the NHC-forming properties of a theophylline-derived PIL for nanogel synthesis. Using silver oxide as both the carbene-forming reagent and cross-linker, nanogels containing individually stabilized ions can be created with different sizes and morphology, including large "galaxy-like" superstructures. Using high-resolution TEM techniques, we directly observed the sub-nanometer structure of these constructs. These features combined exemplify the unique chemistry of poly-NHCs for single-metal-ion-stabilization nanogel design.

Template- and Metal-Free Synthesis of Nitrogen-Rich Nanoporous "Noble" Carbon Materials by Direct Pyrolysis of a Preorganized Hexaazatriphenylene Precursor.

The targeted thermal condensation of a hexaazatriphenylene-based precursor leads to porous and oxidation-resistant ("noble") carbons. Simple condensation of the pre-aligned molecular precursor produces nitrogen-rich carbons with C2 N-type stoichiometry. Despite the absence of any porogen and metal species involved in the synthesis, the specific surface areas of the molecular carbons reach up to 1000 m2 g-1 due to the significant microporosity of the materials. The content and type of nitrogen species is controllable by the carbonization temperature whilst porosity remains largely unaffected at the same time. The resulting noble carbons are distinguished by a highly polarizable micropore structure and have thus high adsorption affinity towards molecules such as H2 O and CO2 . This molecular precursor approach opens new possibilities for the synthesis of porous noble carbons under molecular control, providing access to the special physical properties of the C2 N structure and extending the known spectrum of classical porous carbons.

Selective Calixarene-Directed Synthesis of MXene Plates, Crumpled Sheets, Spheres, and Scrolls.

Fully exploiting the electronic and mechanical properties of 2D laminar materials not only requires efficient and effective means of their exfoliation into low dimensional layers, but also necessitates a means of changing their morphology so as to explore any enhancement that this may offer. MXenes are a rapidly emerging new class of such laminar materials with unique properties. However, access to other morphologies of MXenes has not yet been fully realised. To this end we have developed the synthesis of MXenes (Ti2 C) as plates, crumpled sheets, spheres and scrolls, which involves selective intercalation of p-phosphonic calix[n]arenes, with control in morphology arising from the choice of the size of the macrocycle, n=4, 5, 6, or 8. This opens up wider avenues of discovery/design for new morphologies from the wider family of MXenes beyond Ti2 C, along with opportunities to exploit any new physico-chemical properties proffered.

Optimizing Optical Absorption, Exciton Dissociation, and Charge Transfer of a Polymeric Carbon Nitride with Ultrahigh Solar Hydrogen Production Activity.

Polymeric or organic semiconductors are promising candidates for photocatalysis but mostly only show moderate activity owing to strongly bound excitons and insufficient optical absorption. Herein, we report a facile bottom-up strategy to improve the activity of a carbon nitride to a level in which a majority of photons are really used to drive photoredox chemistry. Co-condensation of urea and oxamide followed by post-calcination in molten salt is shown to result in highly crystalline species with a maximum π-π layer stacking distance of heptazine units of 0.292 nm, which improves lateral charge transport and interlayer exciton dissociation. The addition of oxamide decreases the optical band gap from 2.74 to 2.56 eV, which enables efficient photochemistry also with green light. The apparent quantum yield (AQY) for H2 evolution of optimal samples reaches 57 % and 10 % at 420 nm and 525 nm, respectively, which is significantly higher than in most previous experiments.

Superconductivity at Interfaces in Cuprate-Manganite Superlattices.

One of the unsolved problems for using high-Tc superconducting cuprates for spintronic applications are the short coherence lengths of Cooper pairs in oxides (a few Å), which requires atomically sharp and defect-free interfaces. This research demonstrates the presence of high-Tc superconducting La1.84 Sr0.16 CuO4 in direct proximity to SrLaMnO4 and provides evidence for the sharpness of interfaces between the cuprate and the manganite layers at the atomic scale. These findings shed light on the impact of the chemical potential at the interface of distinct materials on highly sensitive physical properties, such as superconductivity. Additionally, this results show the high stability of ultrathin layers from the same K2 NiF4 -type family, specifically one unit cell of Sr2- x Lax MnO4 and three unit cells of La1.84 Sr0.16 CuO4 . This work advances both the fundamental understanding of the proximity region between superconducting cuprates and manganite phases and the potential use of oxide-based materials in quantum computing.

Coordinative Stabilization of Single Bismuth Sites in a Carbon-Nitrogen Matrix to Generate Atom-Efficient Catalysts for Electrochemical Nitrate Reduction to Ammonia.

Electrochemical nitrate reduction to ammonia powered by renewable electricity is not only a promising alternative to the established energy-intense and non-ecofriendly Haber-Bosch reaction for ammonia generation but also a future contributor to the ever-more important denitrification schemes. Nevertheless, this reaction is still impeded by the lack of understanding for the underlying reaction mechanism on the molecular scale which is necessary for the rational design of active, selective, and stable electrocatalysts. Herein, a novel single-site bismuth catalyst (Bi-N-C) for nitrate electroreduction is reported to produce ammonia with maximum Faradaic efficiency of 88.7% and at a high rate of 1.38 mg h-1 mgcat -1 at -0.35 V versus reversible hydrogen electrode (RHE). The active center (described as BiN2 C2 ) is uncovered by detailed structural analysis. Coupled density functional theory calculations are applied to analyze the reaction mechanism and potential rate-limiting steps for nitrate reduction based on the BiN2 C2 model. The findings highlight the importance of model catalysts to utilize the potential of nitrate reduction as a new-generation nitrogen-management technology based on the construction of efficient active sites.

Mn(II) sub-nanometric site stabilization in noble, N-doped carbonaceous materials for electrochemical CO2 reduction.

The preparation of stable and efficient electrocatalysts comprising abundant and non-critical row-materials is of paramount importance for their industrial implementation. Herein, we present a simple synthetic route to prepare Mn(II) sub-nanometric active sites over a highly N-doped noble carbonaceous support. This support not only promotes a strong stabilization of the Mn(II) sites, improving its stability against oxidation, but also provides a convenient coordination environment in the Mn(II) sites able to produce CO, HCOOH and CH3COOH from electrochemical CO2 reduction.

Ultrahigh water sorption on highly nitrogen doped carbonaceous materials derived from uric acid.

HYPOTHESIS: Developing materials for thermally driven adsorption chillers and adsorption heat pumps is a growing research field due to the potential of these technologies to address up to 50% of the world's total energy demand. These materials must be abundant, easy to synthesize, hydrophilic, and low in cost. Bare carbon materials are hydrophobic and therefore usually not considered for these applications. However, by introducing heteroatoms and tuning their porosity, the hydrophilicity of carbonaceous networks can be increased significantly. EXPERIMENTAL: Herein, a series of highly nitrogen doped carbonaceous materials (CNs) have been synthesized by submitting uric acid to heat treatment at different temperatures in the presence of an inorganic salt mix as solvent and pore template. The effect of the thermal treatment on the materials composition, pore network, and water sorption capability has been studied. FINDINGS: At 800 °C, a nitrogen depleted carbonaceous material with a maximal water uptake of 1.38gH2O g-1 is obtained. Condensation at 750 °C creates an ultra-hydrophilic CN with a water uptake of 0.8 gH2O g-1 at already much lower partial pressures. While the maximum uptake is mainly ascribed to the mesopore volume of the material, the differences in hydrophilicity can be controlled by functionality.

Chemical Vapor Deposition of Highly Conjugated, Transparent Boron Carbon Nitride Thin Films.

Ternary materials made up only from the lightweight elements boron, carbon, and nitrogen are very attractive due to their tunable properties that can be obtained by changing the relative elemental composition. However, most of the times, the synthesis requires to use up to three different precursor and very high temperatures for the synthesis. Moreover, the low reciprocal solubility of boron nitride and graphene often leads to BN-C composite materials due to phase segregation. Herein, an innovative method is presented to prepare BCN thin films by chemical vapor deposition from a single source precursor, melamine diborate. The deposition occurs homogenously at relatively low temperatures generating very high degree of sp2 conjugation. The as-prepared thin films possess high transparency and refractive index values in the visible range that are of interest for reflective mirrors and lenses. Furthermore, they are wide-bandgap semiconductor with very positive valence band, making these materials very stable against oxidation of interest as protective coating and charge transport layer for solar cells. The simple chemical vapor deposition method that relies on commonly available and low-hazard precursor can open the way for application of BCN thin films in optics, optoelectronics, and beyond.