Carbon Vacancy Defects Research Articles

Dark NO2 Reduction on a Graphene Surface with Implications for Soot Aging and HONO Formation.

Soot, primarily composed of elemental carbon (EC) and organic carbon (OC), is ubiquitous in PM2.5. In the atmosphere, the heterogeneous interaction between NO2 and soot is not only an important pathway driving soot aging but also of central importance to nitrous acid (HONO) formation. It is commonly believed that the surface redox reaction between reductive OC and NO2 dominates the night aging of soot and the conversion of NO2 to HONO. However, completely differing from the currently popular explanation, we find here that the redox reaction between EC and NO2 can also drive the conversion of NO2 to HONO during soot aging. By combining in situ experiments with density functional theory (DFT) calculations, we proposed that the surface carbon vacancy defects on graphite/graphene-like EC should be a type of potential primary adsorption and reactive sites inducing the heterogeneous reduction of NO2. We suggested a new mechanism that NO2 is reduced to form HONO on surface vacancy defects through the splitting of H2O molecules, and the carbon atoms adjacent to surface vacancy are simultaneously oxidized to form hydroxyl-functionalized EC. This novel finding provides insights into the chemical mechanism driving the NO2-to-HONO conversion and rapid soot aging, which expands our knowledge of the heterogeneous chemistry of soot in the atmosphere.

Multi-layers hexagonal hole MXene trap constructed by carbon vacancy defect regulation strategy enables high energy density potassium-ions storage

Due to the slow reaction kinetics of potassium-ions with large size radius in layered materials, the energy density of aqueous potassium-ion hybrid supercapacitors (PIHCs) is severely limited. At the same time, the treatment of defects is a “double-edged sword” in energy storage materials, which requires targeted and precise regulation. In order to solve this problem, multi-layers hexagonal hole MXene trap was constructed by using the carbon vacancy defect regulation strategy, and high specific capacitance and energy density potassium-ion storage was realised in PIHCs. The structure-activity relationship at the atomic level was elucidated in combination with the pseudo-capacitance effect caused by the change of valency of newly exposed titanium atoms in the inner wall of the hexagonal hole MXene trap. By density functional theory, the adsorption energy and kinetic analysis of multi-layers hexagonal hole MXene for potassium-ions were calculated, and the optimal position and quantity of potassium-ions were determined. By quantitative analysis of electron band structure and differential charge density, the internal mechanism of high conductivity and energy density of multi-layers hexagonal hole MXene PIHCs was revealed.

Synthesis of VO‐TiO2/VC‐g‐C3N4 Heterojunction for Improving the Photocatalytic Activity by Introducing Defect Sites

AbstractDefect engineering is a proven strategy for enhancing the performance of photocatalysts. Herein, we present a new method for preparing heterojunction photocatalytic materials, VO‐TiO2/VC‐g‐C3N4, with enhanced activity. The presence of defect sites and a heterojunction structure in the catalyst gives rise to exceptional charge separation efficiency and light absorption capabilities. The synergistic effect of the heterojunction interface and activated defect sites, which act as active sites, significantly enhanced the overall photocatalytic performance of VO‐TiO2/VC‐g‐C3N4. The catalyst VO‐TiO2/VC‐g‐C3N4 exhibits superior photocatalytic activity in the NO degradation reaction: displaying 3.5‐ and 2.2 times higher degradation efficiency than VO‐TiO2 and VC‐g‐C3N4, respectively. Moreover, the nitrogen fixation performance of VO‐TiO2/VC‐g‐C3N4 is 3.8‐ and 3.1 times higher than that of VO‐TiO2 and VC‐g‐C3N4, respectively. Electron paramagnetic resonance (EPR) spectroscopy capture experiments revealed that 1O2 and ⋅ OH are the primary active species in the photocatalytic reaction. The collaboration of carbon vacancy defects, oxygen vacancy defects, and the heterojunction structure created additional active sites for VO‐TiO2/VC‐g‐C3N4. The insights gained from this study offer valuable guidance for the efficient design and synthesis of VO‐TiO2/VC‐g‐C3N4 heterojunction photocatalysts, and lay a solid foundation for further research and development of such systems for large‐scale environmental remediation.

Construction of carbon and nitride double vacancy defects on ultrathin porous g-C3N4 nanosheets assisted by freeze-drying for enhanced photocatalysis

Graphitic carbon nitride based photocatalysts have been found extensive application in the degradation of antibiotics and organic pollutants. However, the insufficient of surface active sites and the low efficiency of photogenerated charge carriers separation hinder the development. Herein, ultrathin porous polymer carbon nitride nanosheets (PCNS-F) with carbon and nitride double vacancy defects were successfully fabricated. The facile freeze-drying step, following solvent-thermal treatment and preceding the thermal etching process, stands as a critical step for triggering vacancy generation and adjusting the carbon/nitride ratio. The optimized PCNS-F photocatalyst exhibits an ultrathin structure featuring a substantial abundance of defects in a porous state, resulting in exceptional structural characteristics (183.3 m² g-1, 0.84 cm³ g-1). Surface defect sites increase, and the efficient separation and transfer of photo-generated charge carriers are promoted with the introduction of a suitable vacancy density. PCNS-F demonstrated outstanding photocatalytic degradation performances for tetracycline (81.7 %) and methylene blue (97.7 %) under visible light irradiation for 2 h, which are 10 and 7.2 times higher than those of bulk g-C3N4. The active species of ∙O2- was confirmed to play a major role in the photodegradation process. This work provides a mild approach for engineering carbon and nitride double vacancy defects on g-C3N4 nanosheets, accompanied by tuning carbon/nitride ratio, to effectively eliminate pollutants and cleanse the environment.’

Selective deposition and confined pyrolysis towards carbon nanorings for highly surface-dominated and durable sodium storage

Implementing a highly surface-dominated Na+ adsorption mechanism over carbon-based anodes is promising for high-rate and durable sodium storage. However, the nanostructure/microenvironment of most existing carbon materials still fails to sufficiently unlock the mechanism. Herein, we initially report selective deposition and confined pyrolysis strategies to engineer double-concave carbon nanorings (CNRs) with a precisely-controlled microenvironment to tackle this issue. The present strategies simultaneously endow the obtained CNRs with ultrahigh N-doping (N-CNRs, up to 18.6 at% even at 800 °C), rich defects, and thin-wall porous nanostructure. Such N-CNRs largely shorten the route distances for ion diffusion and provide enriched active sites, enabling highly surface-dominated sodium storage. Consequently, a high reversible capacity (387 mA h g−1 at 0.1 A g−1), outstanding rate capability (180 mA h g−1 even at 10.0 A g−1), with extra-long cycle life (over 10 000 cycles at 5.0 A g−1, 0.35 ‱ capacity decay per cycle) is achieved. In addition, the density functional theory calculations further illustrate that high N-doping carbon vacancy defects can effectively optimize electron density distribution, thereby significantly enhancing the capacity for sodium adsorption. This work opens a new avenue to microenvironment-engineered novel carbon nanostructures for highly surface-dominated sodium storage.

WITHDRAWN: Multi-layers hexagonal hole MXene trap constructed by carbon vacancy defect regulation strategy enables high energy density potassium-ions storage

Due to the slow reaction kinetics of potassium-ions with large size radius in layered materials, the energy density of aqueous potassium-ion hybrid supercapacitors (PIHCs) is severely limited. At the same time, the treatment of defects is a "double-edged sword" in energy storage materials, which requires targeted and precise regulation. In order to solve this problem, multi-layers hexagonal hole MXene trap was constructed by using the carbon vacancy defect regulation strategy, and high specific capacitance and energy density potassium-ion storage was realized in PIHCs. The structure-activity relationship at the atomic level was elucidated in combination with the pseudo-capacitance effect caused by the change of valency of newly exposed titanium atoms in the inner wall of the hexagonal hole MXene trap. By density functional theory, the adsorption energy and kinetic analysis of multi-layers hexagonal hole MXene for potassium-ions were calculated, and the optimal position and quantity of potassium-ions were determined. By quantitative analysis of electron band structure and differential charge density, the internal mechanism of high conductivity and energy density of multi-layers hexagonal hole MXene PIHCs was revealed.

Coupling effect of vacancy defects and multi-adsorption sites in porous carbon cathode for high-performance aqueous zinc-ion hybrid capacitors

Aqueous zinc-ion hybrid capacitors (AZICs) are considered to be an auspicious electrochemical energy storage device due to their superior cycling stability and high power density, but the inadequate electrolyte ions storage active sites and inconsistent pore sizes of porous carbon materials result in poor energy storage performance. Thus, unveiling the interplay between the heteroatom dopant, carbon vacancy defects, and pore sizes in carbon cathode is essential for promoting the Zn2+ and SO42− ions storage capability. In this work, N, S co-doped multi-adsorption sites porous carbon materials with abundant vacancy defects (HC-0.2) are successfully prepared by a facile direct activating/annealing method, which exhibit strong interaction adsorption ability and boost ion and electron transfer. In terms of zinc storage performance, the HC-0.2 cathode exhibits a high capacity of 245.8mAh/g at 0.2A/g and a long cycling stability with 82.3 % capacity retention over 10,000 cycles at 10A/g. Moreover, the HC-0.2//Zn device has a supreme energy density of 164.1Wh/kg and an ultrahigh power density of 30.1 kW/kg. Meanwhile, through theoretical calculations, the N, S multi-adsorption sites, a suitable pore size, and vacancy defects of the HC-0.2 cathode are crucial for improving the Zn2+ and SO42− ions storage capability. This work can provide a comprehensive understanding of the coupling effect of the multi-adsorption sites and vacancy defects of N, S co-doped porous carbon materials on energy storage capability.

Non-monotonic effect of growth pressure on the responsivity of AlGaN ultraviolet Schottky detectors

Three AlGaN-based Schottky detector samples grown with varying pressure conditions are prepared and their responsivity is investigated. It is found that the responsivity of the three samples first increases and then decreases with the increase of pressure. In addition, the vacancy defect concentration increases and carbon impurities concentration decreases when the reactor pressure increases from 100 mbar to 200 mbar during the i-AlGaN layer growth. It is assumed that carbon impurities and vacancy defects play a negative role in detector’s performance, which increase the recombination of photogenerated carriers and reduce detector responsivity. The relationship between growth pressure and detector responsivity is not linear. It is necessary to select a suitable growth pressure to improve the performance of AlGaN detectors.

High-Level Multireference Investigations on the Electronic States in Single-Vacancy (SV) Graphene Defects Using a Pyrene-SV Model.

The nonplanar character of graphene with a single carbon vacancy (SV) defect is investigated utilizing a pyrene-SV model system by way of complete-active-space self-consistent field theory (CASSCF) and multireference configuration interaction singles and doubles (MR-CISD) calculations. Planar structures were optimized with both methods, showing the 3B1 state to be the ground state with three energetically close states within an energy range of 1 eV. These planar structures constitute saddle points. However, following the out-of-plane imaginary frequency yields more stable (by 0.22 to 0.53 eV) but nonplanar structures of Cs symmetry. Of these, the 1A' structure is the lowest in energy and is strongly deformed into an L shape. Following a further out-of-plane imaginary frequency in the nonplanar structures leads to the most stable but most deformed singlet structure of C1 symmetry. In this structure, a bond is formed between the carbon atom with the dangling bond and a carbon of the cyclopentadienyl ring. This bond stabilizes the structure by more than 3 eV compared to the planar 3B1 structure. Higher excited states were calculated at the MR-CISD level, showing a grouping of four states low in energy and higher states starting around 3 eV.

Positively charged carbon vacancy defect as a near-infrared emitter in 4H-SiC

Positively charged carbon vacancy defect as a near-infrared emitter in 4H-SiC

Insights into the Electrochemical Production of Hydrogen Peroxide over Single-Atom Co–N–C Catalysts with the Introduction of Carbon Vacancy Defect near the Co–N4 Site

With the introduction of carbon divacancy, trivacancy, and tetravacancy defects near the Co-N4 site, we have explored the 2e- ORR activity at the Co-N4 site from the perspective of spatial structure and the atomic orbital by DFT calculations. We demonstrate the hybridization strength between Co 3dyz (3dxz) and O 2py (2px) orbitals is the origin of 2e- ORR activity at the Co-N4 site and the hybridization strength relates to the height of the Co 3d projected orbital in the Z direction. The bond length (LCo-O, LO-O), the charge transfer from the Co site to the *OOH adsorbate (ΔQCo-O), the d-band center of the Co site (εd), and the ICOHP value between Co 3d and O 2p orbitals as descriptors can well predict the 2e- ORR activity at the Co-N4 site. This work provides original insights into the 2e- ORR activity over the single-atom Co-N-C catalysts.

Investigating the Role of Surface Roughness and Defects on EC Breakdown, as a Precursor to SEI Formation in Hard Carbon Sodium-Ion Battery Anodes.

Hard carbon (HC) anodes together with ethylene carbonate (EC)-based electrolytes have shown significant promise for high-performing sodium-ion batteries. However, questions remain in relation to the initial contact between the carbon surface and the EC molecules. The surface of the HC anode is complex and can contain both flat pristine carbon surfaces, curvature, nanoscale roughness, and heteroatom defects. Combining density functional theory and experiments, the effect of different carbon surface motifs and defects on EC adsorption are probed, concluding that EC itself does not block any sodium storage sites. Nevertheless, the EC breakdown products do show strong adsorption on the same carbon surface motifs, indicating that the carbon surface defect sites can become occupied by the EC breakdown products, leading to competition between the sodium and EC fragments. Furthermore, it is shown that the EC fragments can react with a carbon vacancy or oxygen defect to give rise to CO2 formation and further oxygen functionalization of the carbon surface. Experimental characterization of two HC materials with different microstructure and defect concentrations further confirms that a significant concentration of oxygen-containing defects and disorder leads to a thicker solid electrolyte interphase, highlighting the significant effect of atomic-scale carbon structure on EC interaction.

Model for elimination of lifetime-limiting carbon vacancy defects in SiC by thermal treatment

In 4H-SiC semiconductors, carbon vacancies act as traps, which limit the carrier lifetime. During high-temperature treatment of 4H-SiC, the concentration of carbon vacancies can be increased or decreased by several atomistic processes, including the diffusion of carbon vacancies and carbon self-interstitials, and the thermal generation–recombination of defects. In this work, an analytic process model has been developed and calibrated against a collection of measured data. The model describes the concentration of carbon vacancies after thermal processing for a wider range of process conditions than previous works. For inert annealings, bulk recombination, bulk generation, and diffusion of carbon vacancies and carbon interstitials play a critical role. For oxidation processes, carbon interstitials are injected at the oxidizing surface. The injection rate of carbon interstitials at the oxidizing surface and their diffusivity from the surface into the bulk govern the reduction of carbon vacancies via bulk recombination. Basic properties of carbon vacancies and carbon self-interstitials in 4H-SiC, such as the thermal equilibrium concentrations, diffusivities, and bulk recombination rates, are reflected by model parameters and have been determined by model calibration for the temperature range of 1150–1950 °C. High-quality epitaxial films and low-quality substrates are described consistently, when assuming that carbon interstitials can be trapped by defects present only in the substrate.

The structural and electronic properties of Carbon-related point defects on 4H-SiC (0001) surface

Carbon-related point defects on the 4H-SiC surface are essential for understanding the origin of defects at the SiO2/SiC interface and improving the quality of epitaxial materials. In this work, a first principle calculation was carried out to study the structural and electronic properties of carbon antisite (CSi), vacancy (VC) and interstitial defects (Ci1, Ci2, Ci3 and Ci4) on the 4H-SiC (0001) surface. The optimized structures showed that interstitial defects except Ci2 caused the surface reconstruction of 4H-SiC. The variation in formation energies with chemical potentials of the carbon for all defects indicated that the C-rich condition was beneficial to the formation of CSi, Ci1, Ci3 and Ci4, whereas the Si-rich condition was more favorable to VC. We also observed that these defects except Ci4 generated the corresponding defect energy levels in the bandgap of 4H-SiC by calculating the density of states and local charge densities. Furthermore, the effects of defect coverage and lateral lattice strain on structural and electronic properties of these defects were provided.

Rational Design of a Highly Dispersed Fe-N-C Nanosheet with 1,10-Phenanthroline-2,9-Dicarboxylic Acid as a Preorganized Ligand: Boosted Electrochemiluminescence Detection of Tetracycline.

In view of the shortcomings of the current coreactant electrochemiluminescence (ECL) and inspired by natural oxygen (O2) reduction metalloenzymes, a novel ECL amplification strategy was established. A pyrolytic iron- and nitrogen-doped (Fe-N-C) nanosheet rich in singly ionized oxygen vacancy (VO•) defects was rationally designed by destroying the highly saturated coordination with a preorganized ligand 1,10-phenanthroline-2,9-dicarboxylic acid (PDA). Extraordinary catalytic activity for O2 activation was obtained via screening a special pyrolysis temperature using spectroscopic and electrochemical methods. The high-spin ferric centers of highly dispersed FeC nanoclusters and abundant carbon and oxygen vacancy defects fully contributed to the inherent catalytic activity. ECL amplification was achieved by integrating the material with luminol to generate redox-active radicals in situ from dissolved O2 and simultaneously shorten the transferring distance of radicals. Tetracycline (TC), which posed a growing threat to aquatic biodiversity and environmental safety, as a model antibiotic was successfully detected with a detection limit of 3.88 nM (S/N = 3), clarifying a promising application prospect of this new effective ECL amplification strategy in biological analysis and environmental monitoring.

Simulation Study of Carbon Vacancy Trapping Effect on Low Power 4H-SiC MOSFET Performance

The carbon vacancy in 4H-SiC is an important recombination center of the minority carrier and a direct consequence of SiC-based device degradation. In 4H-SiC, this defect acts as the primary carrier-lifetime killer. Whether, low-energy electron radiation exposure or high temperature processing in an inert ambient gas will produce the carbon vacancy defect. Despite, the extensiveness of the studies concerning the defect’s modeling and characterization, numerous essential questions remain. Amongst them, we have the impact of these defects on the performance of 4H-SiC MOSFET. Herein, the influence of intrinsic defect states, namely, Z1/2 and EH6/7 centers, on the 4H-SiC MOSFET electrical outputs is examined via 2D numerical simulation. The obtained results show that the traps act to increase the device on-state resistance (RON), reduce the channel mobility, increase the threshold voltage (Vth). Besides, the increase of the temperature leads to less influence of the traps on the threshold variation. Furthermore, due to their locations in the bandgap, the impact of both Z1/2 and EH6/7 centers at room temperature on the device electrical outputs is extreme. For high temperature the EH6/7 have the severest impact because of the cross section temperature dependency.

Theoretical insights into magnetization in graphene containing single and interacting nanoporous defects

Defect-induced magnetism in two-dimensional materials continues to attract attention due to potential applications in spintronics. Using density-functional theory (DFT) approach, we report on the magnetization in graphene containing carbon vacancy defects, i.e., nanoporous graphene. We consider single nanopores consisting of up to ten vacancies and interacting nanopores consisting in vacancy pairs, divacancy pairs and vacancy-divacancy pairs, separated at varying distances. We found that the interactions between the nanopores weaken as their separation increases such that the formation energy tends to the value for the single nanopore while the magnetic moment tends to the sum of individual magnetic moments. Introducing additional nanopore to a pre-existing nanopore may enhance graphene's magnetic moment, however, the nanopores may also interact to induce zero magnetization. The enhancement, reduction or total annihilation of the magnetic moments may be adduced to interactions between the individual nanopores resulting in the saturation of the dangling bonds and/or the realignment of the electrons responsible for the magnetization. Also, our calculated values of induced magnetic moment of different single nanopores enable us to determine an empirical model for predicting the total magnetic moment as a function of nanopore size. The model is able to produce the magnetic moment of small and large nanopore sizes which are in excellent agreement with the DFT calculated values. Finally, we found our results to be consistent with recent experimental studies on magnetic response of graphene containing vacancies introduced via a synthesis process or ion irradiation, respectively. • An empirical model which captures the variation of magnetic moment with the number of vacancies in graphene is proposed. • Interactions between nanopores weaken as their separation increases. • Introducing additional nanopores to a pre-existing one in graphene may enhance, reduce or annihilate its magnetic moment. • Zero magnetization in porous graphene may be due to strong inter-pore interactions and the saturation of dangling bonds. • Our theoretical results support recent magnetic measurements in synthesized graphene containing vacancies.

Response of 4H-SiC Detectors to Ionizing Particles

We report the response of newly designed 4H-SiC Schottky barrier diode (SBD) detector prototype to alpha and gamma radiation. We studied detectors of three different active area sizes (1 × 1, 2 × 2 and 3 × 3 mm2), while all detectors had the same 4H-SiC epi-layer thickness of approximately µm, sufficient to stop alpha particles up to 6.8 MeV, which have been used in this study. The detector response to the various alpha emitters in the 3.27 MeV to 8.79 MeV energy range clearly demonstrates the excellent linear response to alpha emissions of the detectors with the increasing active area. The detector response in gamma radiation field of Co-60 and Cs-137 sources showed a linear response to air kerma and to different air kerma rates as well, up to 4.49 Gy/h. The detector response is not in saturation for the dose rates lower than 15.3 mGy/min and that its measuring range for gamma radiation with energies of 662 keV, 1.17 MeV and 1.33 MeV is from 0.5 mGy/h–917 mGy/h. No changes to electrical properties of pristine and tested 4H-SiC SBD detectors, supported by a negligible change in carbon vacancy defect density and no creation of other deep levels, demonstrates the radiation hardness of these 4H-SiC detectors.

Density functional theory calculation of the properties of carbon vacancy defects in silicon carbide

As a promising material for quantum technology, silicon carbide (SiC) has attracted great interest in materials science. Carbon vacancy is a dominant defect in 4H-SiC. Thus, understanding the properties of this defect is critical to its application, and the atomic and electronic structures of the defects needs to be identified. In this study, density functional theory was used to characterize the carbon vacancy defects in hexagonal (h) and cubic (k) lattice sites. The zero-phonon line energies, hyperfine tensors, and formation energies of carbon vacancies with different charge states (2−, −, 0,+ and 2+) in different supercells (72, 128, 400 and 576 atoms) were calculated using standard Perdew–Burke–Ernzerhof and Heyd–Scuseria–Ernzerhof methods. Results show that the zero-phonon line energies of carbon vacancy defects are much lower than those of divacancy defects, indicating that the former is more likely to reach the excited state than the latter. The hyperfine tensors of VC+(h) and VC+(k) were calculated. Comparison of the calculated hyperfine tensor with the experimental results indicates the existence of carbon vacancies in SiC lattice. The calculation of formation energy shows that the most stable carbon vacancy defects in the material are VC2+(k), VC+(k), VC(k), VC−(k) and VC2−(k) as the electronic chemical potential increases.

Toward efficient and high rate sodium-ion storage: A new insight from dopant-defect interplay in textured carbon anode materials

Heteroatom dopants and structural defects exist concurrently and share analogous effects in modifying the electrochemical storage performance of doped hard carbon materials. However, the underlying interplay among the dopants and defects has long been ignored, which leads to controversial conclusions on the influence of dopants in defect-rich hard carbon matrix. Herein, through combined theoretical calculations and electrochemical measurements, the interplay among N, S dopants and carbon vacancy defects and its effect on hard carbon materials’ electrochemical performance for sodium-ion storage are studied. The dopant-defect interplay is corroborated to play crucial roles on the capacity and rate capability of carbon anodes, and co-doping of N, S species is demonstrated as an efficient method. Steered by the calculations, a universal in-situ texturing method is developed to synthesize the N, S dual-doped porous hard carbons with preferential features for sodium-ion batteries. The optimized hard carbon material afforded a large reversible capacity (430 ​mA ​h g−1 at 0.05 ​A ​g−1), an unprecedented rate performance (up to 277 ​mA ​h g−1 at 5 ​A ​g−1), and excellent cycling stability in sodium-ion batteries. The results in this work will inspire the rational design and fabrication of carbon materials with high capacity and superb rate capability for sodium-ion storage.