Nitrogen Defects Research Articles

Optically detected flip-flops between different spin ensembles in diamond

We employ the technique of optical detection of magnetic resonance to study dipolar interaction in diamond between nitrogen-vacancy color centers of different crystallographic orientations and substitutional nitrogen defects. We demonstrate optical measurements of resonant spin flips-flips (second Larmor line), and flip-flops between different spin ensembles in diamond. In addition, the strain coupling between the nitrogen-vacancy color centers and bulk acoustic modes is studied using optical detection. Our findings may help optimizing cross polarization protocols, which, in turn, may allow improving the sensitivity of diamond-based detectors.

Preparation and application of defective graphite phase carbon nitride photocatalysts

With the development of industry and agriculture, the problems of environmental pollution and energy shortage have become increasingly severe. Semiconductor photocatalysis technology is one of the effective ways to solve environmental pollution and energy crisis. The principle of photocatalysis is based on the oxidation-reduction ability of photocatalysts under light conditions, which can achieve the purposes of purification of pollutants, material synthesis and transformation. Graphite phase carbon nitride (g-C3N4), as a new high-efficiency catalyst, has good stability and shows great engineering application potential in photocatalytic technology. However, the unmodified g-C3N4 has a limited visible light response range, and the photo-excited charge carrier recombination rate is high, resulting in low photocatalytic activity. Nitrogen defects are introduced into the g-C3N4 framework. These defects can manipulate the electronic structure, and the interstitial state produced can be used as a band-tail state, which can overlap with the valence band or the conduction band. The mid-gap state of semiconductors can extend the light response and act as an active center for electron-hole excitation. Introducing defects into g-C3N4 can improve the photocatalytic activity of g-C3N4. This paper systematically reviews the physical, chemical and electrochemical properties of g-C3N4 on the basis of experimental and theoretical research progress. The synthesis methods of defect g-C3N4 are summarized, including adjustment before polymerization, adjustment during polymerization, and adjustment after polymerization. The adjustment before polymerization is to introduce defects by changing the precursor, such as adding hydroxide, sodium borohydride and other substances to the precursor. The adjustment during polymerization is to provide a reducing atmosphere during polymerization can prepare g-C3N4 with different nitrogen-vacancy densities, such as hydrogen, ammonia and so on. The adjustment after polymerization is to modify the synthesized defect-free g-C3N4, such as recalcining or acid treatment to achieve the purpose of synthesizing nitrogen vacancies. The effect of defect sites on g-C3N4 is also discussed. The intermediate state induced by nitrogen defects can be transformed into active centers excited by electron holes, and the optical response of defect g-C3N4 is broadened due to the narrowing of the band gap. In the range, nitrogen defects can trap photo-generated carriers and prevent their recombination, thereby increasing the overall quantum efficiency. However, excessive introduction of nitrogen defects will produce deeper interstitial states. These deeper interstitial states can not only capture photo-generated electrons, but also photo-generated h+, which then become the recombination sites of photo-generated carriers. In addition, we separately summarized the application of the defect g-C3N4 in water treatment, such as the degradation of antibiotics and organic pesticides and the reduction of the toxicity of heavy metals, as well as water decomposition, carbon dioxide conversion and photocatalytic denitrification. Defect g-C3N4 has achieved good results in these applications. Although considerable progress has been made in the research of g-C3N4 in recent years, there are still many challenges in preparing g-C3N4 with high-efficiency catalytic performance. Finally, in view of the challenges faced by the application of defective g-C3N4, key discussions and future prospects are proposed from the aspects of mechanism exploration and material development.

Tubular Carbon Nitride with Hierarchical Network: Localized Charge Carrier Generation and Reduced Charge Recombination for High‐Performance Photocatalysis of H2 and H2O2 Production

Polymeric carbon nitride (PCN), as an appealing metal‐free and low‐cost photocatalyst for solar‐to‐fuel conversion, is still suffering from insufficient light‐harvesting efficiency and short excited state lifetime of electron–hole pairs. Herein, a 3D hierarchical network of carbon nitride tubes (NCNT) with intercrossing few‐layer tubes of uniform diameter, synthesized by a synergistic strategy of tailoring the chemical and hydrogen bond, is reported. This hollow network structure facilitates the internal light reflection, thus promoting the light‐induced electron–hole pair generation. Moreover, the few‐layer NCNT with nitrogen defects effectively decreases the undesired electron–hole recombination. These advantages are disclosed by experimental spectra and finite difference time domain (FDTD) simulations and density functional theory (DFT) calculations. Impressively, the optimized NCNT exhibits almost 32 times the photocatalytic hydrogen evolution ability (8.6 mmol h−1 g−1) and 4 times (485.7 μmol h−1 g−1) the hydrogen peroxide production of those of bulk PCN, respectively. This work provides a new strategy to construct a hierarchical nanostructure of a high‐performance carbon nitride photocatalyst through enhancing the solar‐light capture and conversion.

Boosting the Catalytic Performance by Confining Rich Carbon Atoms over Graphite Carbon Nitride Structure

In this study, the porous ultrathin graphitic carbon nitride (CN) nanosheets with rich C and nitrogen defects were prepared by one-step calcining the mixture of melamine and glucose (Glu) in air atmosphere (Glu-CN). Introducing simultaneously rich C atoms and nitrogen defects into CN structures continuously modulates the bandgaps from 2.67 to 1.81 eV of CN photocatalysts. Due to large surface area, more active sites, remarkably longer lifetime of charge carriers and adjustable band gap structure, the prepared ultrathin porous CN nanosheets show the enhanced photocatalytic performance for the degradation of methyl orange (MO) under visible light. The degradation efficiency of optimal CN nanosheet photocatalyst for MO is 5.75 times that of bulk CN. This work provides a facile and universal relevance approach to engineer the band structures of CN by introduction of rich C and porous morphology for high-performance photocatalytic, which can provide informative principles for the design of efficient photocatalysis systems for solar energy conversion.

Nitrogen overgrowth as a catalytic mechanism during diamond chemical vapour deposition

Nitrogen is frequently included in chemical vapour deposition feed gases to accelerate diamond growth. While there is no consensus for an atomistic mechanism of this effect, existing studies have largely focused on the role of sub-surface nitrogen and nitrogen-based adsorbates. In this work, we demonstrate the catalytic effect of surface-embedded nitrogen in nucleating new layers of (100) diamond. To do so we develop a model of nitrogen overgrowth using density functional theory. Nucleation of new layers occurs through C insertion into a C–C surface dimer. However, we find that C insertion into a C–N dimer has substantially reduced energy requirements. In particular, the rate of the key dimer ring-opening and closing mechanism is increased 400-fold in the presence of nitrogen. Full incorporation of the substitutional nitrogen defect is then facilitated through charge transfer of an electron from the nitrogen lone pair to charge acceptors on the surface. This work provides a compelling mechanism for the role of surface-embedded nitrogen in enhancing (100) diamond growth through the nucleation of new layers. Furthermore, it demonstrates a pathway for substitutional nitrogen formation during chemical vapour deposition which can be extended to study the creation of technologically relevant nitrogen-based defects.

Boron-doped nitrogen-deficient carbon nitride-based Z-scheme heterostructures for photocatalytic overall water splitting

Photocatalytic overall water splitting can be achieved using Z-scheme systems that mimic natural photosynthesis by combining dissimilar semiconductors in series. However, coupling well-suited H2- and O2-evolving components remains challenging. Here, we fabricate a Z-scheme system for photocatalytic overall water splitting based on boron-doped, nitrogen-deficient carbon nitride two-dimensional (2D) nanosheets. We prepare ultrathin carbon nitride nanosheets with varying levels of boron dopants and nitrogen defects, which leads to nanosheets that can act as either H2- or O2-evolving photocatalysts. Using an electrostatic self-assembly strategy, the nanosheets are coupled to obtain a 2D/2D polymeric heterostructure. Owing to their ultrathin nanostructures, strong interfacial interaction and staggered band alignment, a Z-scheme route for efficient charge-carrier separation and transfer is realized. The obtained heterostructure achieves stoichiometric H2 and O2 evolution in the presence of Pt and Co(OH)2 co-catalysts, and the solar-to-hydrogen efficiency reaches 1.16% under one-sun illumination. Splitting water using suspensions of particulate carbon nitride-based photocatalysts may be a cheap way to produce hydrogen, but efficiencies have remained low. Now, Shen and colleagues use doped carbon nitride-based Z-scheme heterostructures to split water with a solar-to-hydrogen efficiency of 1.1% in the presence of metal-based co-catalysts.

Bidirectional Progressive Optimization of Carbon and Nitrogen Defects in Solar-Driven Regenerable Adsorbent to Remove UV-Filters from Water

Defect engineering of nanomaterials has emerged as a promising approach to improve their performance for pollutant removal. However, various nanomaterial defects play variant roles in environmental...

Defective Graphitic Carbon Nitride Modified Separators with Efficient Polysulfide Traps and Catalytic Sites for Fast and Reliable Sulfur Electrochemistry

AbstractThe serious shuttle effect of lithium‐sulfur batteries limits the efficient realization of high rate charging and discharging under high sulfur loading in practical applications. Herein, this work reports a strong mitigation toward lithium polysulfide (LiPSs) adsorption/catalysis by introducing defective graphite phase carbon nitride (g‐C3N4) as an effective additive. Without significant weight increase, the nitrogen deficient g‐C3N4, in the form of ultrafine spindle‐like nitrogen deficient g‐C3N4−x (sCN), can be easily combined with commercial poly‐propylene (PP) separators after hydrophilic modification of polydopamine, which corresponds to an ultralow overall weightiness contribution of 0.17 mg cm−2. Physical/electrochemical characterizations and theoretical studies reveal that sCN exhibits strong electrostatic attraction with LiPSs by nitrogen defects and new formation of cyano groups near edges, thereby maintaining rapid and reliable LiS electrochemistry. Of particular importance is the chainmail catalyst design with separators that enable magic polysulfides adsorption effect and desirable thermostability/wettability, which guarantees the sCNPP‐assembled cells with long and stable durability over 500 cycles at 5.0 C (capacity fading rate: 0.05% per cycle), and a high capability of 476 mAh g−1 is obtained.

Alkylamine-Grafted Organic Semiconductors with Plasma-Induced Defects as Electron Promoters of CO-Resistant Pd-Based Nanoparticles for Efficient Light-Driven On-Demand H2 Generation

Tailoring the adsorption characteristics of catalytic substrates and the electronic behavior of catalytically active species in heterogeneous catalysts is beneficial for regulating their catalytic performance. Herein, we report a highly efficient room-temperature photocatalytic formic acid (HCOOH) dehydrogenation system, where plasma-induced carbon nitrides (CNs) grafted by aminosilanes are used as electron promoters of catalytically active AuPdM (M = Ni and Co) nanoparticles (NPs). The various characterizations verify that the plasma-induced CNs have rich nitrogen defects and the efficient separation of photogenerated charge carriers. The visible-light-driven synergism of alkylamine and defective CNs with tunable band structures in the catalysts accounts for their enhanced HCOOH adsorption and the enriched electron density of metal NPs. This indicates that the catalysts have 100% H2 selectivity and higher activities under visible light irradiation than in the dark. The optimal CO-resistant AuPdNi catalyst possesses 66.9% of apparent quantum yield at 420 nm and exhibits the record room-temperature H2 generation activity with the total turnover frequencies of 1075 and 1278 h–1 in the absence and presence of scavenger of photogenerated holes, respectively.

Effect of carbon and nitrogen double vacancies on the improved photocatalytic hydrogen evolution over porous carbon nitride nanosheets

Both carbon and nitrogen defects are formed in the porous carbon nitride after proper PVP addition and heating in air. It exhibits improved photocatalytic hydrogen evolution performance with increasing carbon to nitrogen atomic ratio.

The use of spectroscopy methods for structural analysis of CVD diamond films, polycrystalline and single-crystal diamonds

For the work results correct interpretation, it is important to study initial materials that scientists have to deal with. Currently, there are a large number of different diamond substrates. Comparison of materials among themselves allows you to determine which material you are dealing with. In this work, the methods of infrared (IR) spectrometry, Raman spectroscopy and spectrophotometry are used to study four types of diamond materials: diamond polycrystalline CVD-films; natural single-crystal diamonds; synthetic polycrystalline HPHT-diamonds (such as DSPC – diamond synthetic polycrystal by GOST 9206-80); polycrystalline CVD-diamonds CDM manufactured by E6. In work it was shown that the Raman spectroscopy allows to measure the effect of heat treatment on changes in the diamond structure, even if it is such highly advanced diamond materials as natural diamonds. Heat treatment affects the perfection of diamond crystal structure by reducing stresses and the number of defects in it due to graphitization process. The IR spectrometry method is effective for determining the shape and amount of nitrogen inclusions in diamond structure. To study polycrystalline CVD-films, the spectrophotometry method turned out to be the most effective, because it made possible to determine a small number of nitrogen defects and draw conclusions about the quality of the films. The investigation of polycrystalline diamonds CDM and DSPC demonstrated that, despite their coarse-crystalline structure, diamond crystallites consist of a highly defective diamond phase; in addition, DSPC-diamonds were studied using this method in the first time.

Potassium Doped and Nitrogen Defect Modified Graphitic Carbon Nitride for Boosted Photocatalytic Hydrogen Production

Potassium Doped and Nitrogen Defect Modified Graphitic Carbon Nitride for Boosted Photocatalytic Hydrogen Production

Long range interactions and related carbon-carbon bond reconstruction between interior and surface defects in nanodiamonds.

Interactions between interior substitutional nitrogen defects and surface unsaturated dangling bonds in synthetic nanodiamonds of ∼25 nm size were explored experimentally and theoretically. The experimental results demonstrate the disappearance of the specific paramagnetism of nitrogen centers in the smallest nanoparticles isolated after processing large micron diamonds in a ball mill, accompanied by the formation of unsaturated surface dangling bonds and internal defects. First principles modelling confirms the vanishing of the magnetic moments related with nitrogen centers even for distances from the surface defects greater than 1 nm. To understand this effect, we consider a bond reconstruction scheme with the formation of several carbon-carbon double bonds in the area between the interior and surface point defects. The scheme is in agreement with the changes in electron density through the distance between the two defects. The developed approach can be used to describe the interactions between various defects in carbon-based systems.

Synergistic enhancement of π-electron density in graphitic carbon nitride for significantly improved photocatalytic hydrogen evolution under visible-light irradiation

In this work, a synergistic strategy integrated elemental doping and defect engineering has been developed to modulate the electronic and bandgap structure of g-C3N4 by controlling its conjugated aromatic system. Experimental analysis and theoretical calculations confirm that the generation of imbalanced spatial distributions of charge carriers not only greatly increases the π-electron availability, but also decreases the energy barrier for hydrogen adsorption due to the synergistic effect of the high valence electron of the W dopant and the decreased electronegativity by two-coordinated nitrogen defects, which strengthens the charge transfer efficiency and favors the light capturing capability. The combined benefits of the electronic, optical and textural modification lead to a significant improvement of photocatalytic activity of 3.3 mmol h−1 g−1 for hydrogen evolution under λ ≥ 400 nm light irradiation, about twentyfold enhancement than that of pristine g-C3N4. Such proposed synergistic strategy provides a promising route to promote the performance of g-C3N4 for potential applications in solar energy conversion.

Facile nitrogen and sulfur deficient engineering on sulfur doped g-C3N4 for efficiently photocatalytic H2 evolution

Graphitic carbon nitride (g-C3N4) gets lots of attention as a promising candidate in solar energy conversion. However, the single modification method has limited effectiveness to access highly photocatalytic performance g-C3N4 catalyst. In this work, we present a facile method that can construct nitrogen and sulfur defects in the synthesis process of sulfur-doped g-C3N4. As results, nitrogen and sulfur deficient sulfur-doped g-C3N4 catalyst (SCND-x) integrate the advantages of these two modification methods. SCND-x exhibit a wide light absorption range and an efficient photocarriers separation ability. The optimal sample SCND-3 shows an excellent photocatalytic property with a hydrogen evolution rate of 70.2 μmol h−1 under visible light excitation (λ > 420 nm), which is about 23.4 times better than pristine g-C3N4 (MCN). And the apparent quantum efficiency of SCND-3 is up to 3.3% at 420 nm. In a word, we successfully practice a facile method to prepare high photocatalytic activity catalyst by introducing nitrogen and sulfur defects in sulfur-doped g-C3N4.

Preparation of Defect-Related Luminescent Mesoporous Silica Nanoparticle as Potential Detectable Drug Carrier.

A defect-related luminescent mesoporous silica nanoparticle (DLMSN) with simultaneous excellent luminescence, high drug loading efficiency and release capacity was prepared upon calcination of 3-aminopropyltriethoxysilane (APTES)-functionalized mesoporous silica nanoparticle (AP-MSN) under a relatively moderate temperature. Under ultraviolet excitation at 365 nm, DLMSN exhibited intense white-blue emission with a range of 400-500 nm, which was inferred to originate from the effective carbon or nitrogen defect in the particle causing by APTES calcination. Additionally, the luminescence intensity of DLMSN was significantly affected by APTES concentration and calcination temperature during the preparation procedure. Within all the tested values, the maximum luminescence intensity was achieved when APTES concentration and calcination temperature were 0.851 mmol and 300 °C, respectively. The drug storage and release tests demonstrated that DLMSN had efficient drug storage and good pH-dependent release for ibuprofen (IBU). Interestingly, ibuprofen-loaded DLMSN (IBU@DLMSN) still exhibit an intense luminescence with an emission peak at around 410 nm under 365 nm excitation, which gradually increased with the sustained release of IBU from IBU@DLMSN. These results suggest that the as-prepared DLMSN may have potential as a detectable nanocarrier in the drug delivery field.

One-step synthesis of nitrogen-defective graphitic carbon nitride for improving photocatalytic hydrogen evolution

Defects are significant for graphitic carbon nitride (g-C3N4, CN) in photocatalytic applications. Defective CN not only accelerate charge separation but also extend spectral response. Engineering carbon or nitrogen defects in CN has been achieved by variety of strategies, but it is still a long-term interest to develop a simple and controllable route for engineering defects in CN. Herein, we present tuning the nitrogen defects in CN by either changing the heating rate or prolonging the pyrolysis time during polymerization melamine sulfate. It was found that either lower heating rate or longer pyrolysis time lead to the formation of more N vacancies and suspended terminal amino. As a result, an optimal photocatalytic H2 yield rate (λ > 420 nm) of 905 μmol g−1 h−1 was reached, which was 2 times higher than that of CN prepared with a heating rate of 1 °C/min and pyrolysis at 600 °C for 4 h.

Fe atoms anchored on defective nitrogen doped hollow carbon spheres as efficient electrocatalysts for oxygen reduction reaction

Defective electrocatalysts, especially for intrinsic defective carbon, have aroused a wide concern owing to high spin and charge densities. However, the designated nitrogen species favorable for creating defects by the removal of nitrogen, and the influence of defects for the coordination structure of active site and oxygen reduction reaction (ORR) activity have not been elucidated. Herein, we designed and synthesized a pair of electrocatalysts, denoted as Fe-N/C and Fe-ND/C for coordination sites of atomic iron-nitrogen and iron-nitrogen/defect configuration embedded in hollow carbon spheres, respectively, through direct pyrolysis of their corresponding hollow carbon spheres adsorbed with Fe(acac)3. The nitrogen defects were fabricated via the evaporation of pyrrolic-N on nitrogen doped hollow carbon spheres. Results of comparative experiments between Fe-N/C and Fe-ND/C reveal that Fe-ND/C shows superior ORR activity with an onset potential of 30 mV higher than that of Fe-N/C. Fe-ND sites are more favorable for the enhancement of ORR activity. Density functional theory (DFT) calculation demonstrates that Fe-ND/C with proposed coordination structure of FeN4−x (0<x<4) anchored by OH as axial ligand during ORR, weakens the strong binding of OH* intermediate and promotes the desorption of OH* as rate-determining step for ORR in alkaline electrolyte. Thus, Fe-ND/C electrocatalysts present much better ORR activity compared with that of Fe-N/C with proposed coordination structure of FeN4.

Effects of Eu2+ on the luminescence and afterglow that arise from defects in β-SiAlON:Eu2+

Si6−z Al z O z N8−z (β-SiAlON):Eu2+ is known as a high brightness green phosphor. When β-SiAlON:Eu2+ is excited with UV light (approximately 265 nm), a curved decay afterglow is observed as a result of the trap levels created by the defects in the host crystal. However, the defect signals are hardly detected by electron spin resonance (ESR) and thermoluminescence (TL), which are common defect detection methods. Non-doped (Eu = 0) β-SiAlON emits blue light from a nitrogen defect, and the defect can be detected by time-resolved fluorescence (TR-F) measurement at 15 K. Similarly, upon measuring TR-F at 15 K for Eu-doped β-SiAlON, a blue emission (460 nm) is detected in addition to the green emission of Eu2+ (530 nm). The green emission has an afterglow of several milliseconds that decays with the same decay curve as the blue emission of the defect, and its time constant is 5–6 ms. This blue emission is quenched by the Eu concentration and temperature. The Si dangling bond signal intensity, observed by ESR, and the glow intensity, observed by TL, also decrease with the increment of the Eu concentration. It is difficult to detect the defect as an electron trap owing to the interaction between Eu2+ and the nitrogen defect. However, the afterglow arising from the electrons trapped at the defect level does not decrease with the Eu concentration. The blue emission was quenched at room temperature but the afterglow was not reduced, which also affected the light emission above room temperature. Therefore, it is possible to detect nitrogen defects optically by TR-F at low temperature, as well as the Eu2+ afterglow of several milliseconds.

Electron spin relaxation of P1 centers in synthetic diamonds with potential as B1 standards for DNP enhanced NMR

The microwave magnetic field, B1, in the non-resonant structures typically used for DNP-enhanced NMR is relatively small, so calibration via continuous wave (CW) power saturation requires a sample with longer spin lattice relaxation times than the samples used as CW standards in X-band cavities. HPHT diamonds have strong, easily observed EPR signals from P1 centers (nitrogen defects), and are indefinitely stable. This makes HPHT diamonds attractive as secondary standards for calibration of electron B1 field strength in a variety of experimental arrangements. The concentrations of P1 centers is also typically in the 30–200 ppm range, or equivalently 10–60 mM, and therefore the EPR relaxation observed is relevant to DNP enhanced NMR employing free radical polarizing agents at similar concentrations. Pulsed and CW saturation relaxation measurements T1 and T2 are compared at X-band. Under CW conditions the relevant T1T2 product of time constants in our samples at room temperature is found to be dominated by electron–electron spin diffusion, and the product is large enough that saturation will be possible with the B1 of typical DNP systems. The similarity of T1 and T2 values obtained by pulse measurements at X-band and Q-band suggests that the X-band results can be extrapolated to the higher EPR frequencies used for DNP experiments. The electron spin dynamics observed here in HPHT diamond samples identify them as useful model systems to better delineate the interplay of electron spin relaxation, magic angle spinning, and inhomogeneous microwave irradiation as they affect DNP enhancement.