Carbon Vacancies Research Articles

Investigation on bifunctional catalytic performance of Anti-Perovskite Ni3ZnC, Co3ZnC and Ni3FeN for Hydrogen/Oxygen evolution reactions

Low-cost anti-perovskites (X3AB) as efficient bifunctional electrocatalysts for water splitting have been reported recently. However, its microscopic catalytic mechanism is not clear. In this paper, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance of three typical bifunctional anti-perovskite catalysts Ni3ZnC, Co3ZnC and Ni3FeN were calculated for the first time. Comprehensive analysis reveals that HER catalytic performance ranking: Ni3ZnC (100) > Ni3FeN (111) > Co3ZnC (100). Combined with the OER catalytic activity, we concluded that Ni3ZnC (100) may have the best bifunctional performance. We also calculated the transition-metal doping effect and concluded that the doping of Fe for Co3ZnC and Cu for Ni3FeN may further enhance the bifunctional catalytic activity. The effect of experimentally common carbon vacancies was calculated to be detrimental to the HER performance. Our work gave an atomic perspective of HER and OER catalytic mechanisms of anti-perovskites, which is helpful for the further development of water splitting electrocatalysts.

Sea-Urchin carbon nitride with carbon vacancies (C-v) and oxygen substitution (O-s) for photodegradation of Tetracycline: Performance, mechanism insight and pathways

The fast recombination of electron-hole pairs results in low photoactivity, limiting carbon nitride further application in wastewater purification. To address this issue, regulation of electron configuration via optimizing surface atom properties is a promising strategy for improving the separation of electron-hole pairs. Here, sea-urchin carbon nitride (SUCN-6) with C-v and O-s mediated electronic structures were facile prepared, endowing it with a decrease in transportation resistance of charge carriers and increased activation towards oxygen. Under visible light, the SUCN-6 showed a removal efficiency of ∼ 89% towards tetracycline (TC) in 20 min and no obvious decaying after five consecutive cycles. This TC photodegradation follows a first-order kinetic, and its kinetic constant reaches 0.0908 min−1, which is 2.35 folds higher than that of pristine carbon nitride. In addition, several crucial factors of practical wastewater purification (such as pH, TC concentration, water sources, inorganic salts, and organic matters) that might affect photoactivity were investigated. Moreover, the degradation pathways of TC and intermediates were detected by an HPLC-MS and electron spin resonance instrument, and a possible photodegradation mechanism over SUCN-6 was elucidated. This study offers new insights into the photocatalysis kinetics and photodegradation mechanism over carbon nitride for environmental remediation.

Ultrathin porous carbon nitride nanosheets with well-tuned band structures via carbon vacancies and oxygen doping for significantly boosting H2 production

Significant improving g-C3N4’s photocatalytic efficiency still remains a great challenge. In this work, we synthesized ultrathin and porous 2D g-C3N4 nanosheets with a controllable concentration of carbon vacancies and oxygen doping. Water vapor opens the heptazine units, introduces carbon vacancies, and acts as an oxygen source for oxygen doping under high temperatures. The synergistic effect of controllable carbon vacancies and oxygen doping can continuously regulate band structures and significantly improves the separation efficiency of photoexcited charges. As a result, the prepared g-C3N4 with vigoroso reduction potential exhibits a very high photocatalytic H2 evolution rate of 2.414 mmol g−1 h−1 under visible light and 7.414 mmol g−1 h−1 under ultraviolet-visible light, respectively, which outperforms the majority of the previously reported g-C3N4 with well-tuned band structure. This work offers a new design idea for highly active g-C3N4-based photocatalysts with a well-tuned band structure.

Carbon vacancy-mediated exciton dissociation in Ti3C2Tx/g-C3N4 Schottky junctions for efficient photoreduction of CO2

Earth-abundant g-C3N4 is a promising photocatalyst for CO2 reduction, but its practical application is severely limited by the excitonic effect of g-C3N4 derived from strong binding energy and lack of electron-enriched active sites. Herein, we design a novel 2D/2D Schottky junction photocatalysts comprising of Ti3C2Tx-modified defective g-C3N4 nanosheets with carbon vacancy (denoted as Ti3C2Tx/Vc-CN) by a self-assembly method. The carbon vacancies in g-C3N4 promote exciton dissociation into free charge, while the formed Schottky junctions between Ti3C2Tx and Vc-CN further enables a directional charge transfer, thus providing an electron-rich catalytic surface for the CO2 reduction. Thanks to the synergy of promoted exciton dissociation and directional electron transfer, the optimal 20% Ti3C2Tx/Vc-CN display a high CO evolution rate of 20.54 µmol·g−1·h−1 under visible light irradiation, which is 7.4 times higher than that of bare CN. This work highlights the synergy of the promoted exciton dissociation and directional electron transfer in the activity enhancement of photocatalytic CO2 reduction.

Compensation of p-type doping in Al-doped 4H-SiC

One of the major challenges of 4H-silicon carbide (4H-SiC) is that the preparation of low resistivity p-type 4H-SiC single crystals lags seriously behind that of low resistivity n-type 4H-SiC single crystals, hindering the development of important 4H-SiC power devices such as n-channel insulated gate bipolar transistors. In particular, the resistivity of p-type 4H-SiC single crystals prepared through the physical vapor transport technique can only be lowered to around 100 mΩ cm. One of the key causes is the incomplete ionization of the p-type dopant Al with an ionization energy ∼0.23 eV. Another factor is the compensating effect. It cannot simply assume nitrogen (N) is the sole compensatory center, since the number of the compensating center is larger than the concentration of N doping. In this work, we systematically investigate the compensation of native defects and self-compensation in Al-doped 4H-SiC. It is found that the positively charged carbon vacancies (VC2+) are also the dominant compensating centers in Al-doped 4H-SiC. When the Al concentration is in the range of 1016–1019 cm−3, the concentration of holes is lower by one order of magnitude than the Al concentration because of the compensation of VC2+. As the Al concentration exceeds 1020 cm−3, the concentration of holes is only in the order of magnitude of 1019 cm−3 owing to the dominant compensation of VC2+ and supplementary self-compensation of interstitial Al (Ali3+). We propose that the passivation of VC2+ as well as quenching is effective to enhance the hole concentration of Al-doped 4H-SiC.

Surface energies in high‐entropy carbides with variable carbon stoichiometry

AbstractThe carbon vacancy in high‐entropy carbides (HECs) has a significant impact on their physical and chemical properties, yet relevant studies have still been relatively few. In this study, we investigate the surface energies of HECs with variable carbon vacancies through first‐principles calculations. The results show that the surface energy of the (1 0 0) surface of the stoichiometric HECs is significantly lower than that of (1 1 1) surface. With the decrease in carbon stoichiometry, the surface energies of both (1 0 0) and (1 1 1) surfaces increase gradually, which is mainly due to the weakening of covalent bonding and the decrease of metal Hirshfeld‐I (HI) charges. However, the surface energy of (1 0 0) surface increases more quickly than that of (1 1 1) surface and will exceed that of (1 1 1) surface when the carbon stoichiometry decreases to a certain extent, which is primarily attributed to the greater decrease rate of metal HI charges of (1 0 0) surface.

Photochemical production of hydrogen peroxide by digging pro-superoxide radical carbon vacancies in porous carbon nitride

Artificial photosynthesis of H 2 O 2 , an environmentally friendly oxidant and a clean fuel, holds great promise. However, improving its efficiency and stability for industrial implementation remains highly challenging. Here, we report the visible-light H 2 O 2 artificial photosynthesis by digging pro-superoxide radical carbon vacancies in three-dimensional hierarchical porous g-C 3 N 4 through a simple hydrolysis-freeze-drying-thermal treatment. A significant electronic structure change is revealed upon the implantation of carbon vacancies, broadening visible-light absorption and facilitating the photogenerated charge separation. The strong electron affinity of the carbon vacancies promotes superoxide radical ( ⋅ O 2 − ) formation, significantly boosting the H 2 O 2 photocatalytic production. The developed photocatalyst shows an H 2 O 2 evolution rate of 6287.5 μM g −1 h −1 under visible-light irradiation with a long cycling stability being the best-performing photocatalyst among all reported g-C 3 N 4 -based systems. Our work provides fundamental insight into highly active and stable photocatalysts with great potential for safe industrial H 2 O 2 production. • Innovative hydrolysis-freeze-drying-thermal treatment (HFDT) method • 3D hierarchical porous carbon nitride (g-C 3 N 4 ) with carbon vacancies • H 2 O 2 evolution rate of 6287.5 μM g −1 h −1 with an outstanding long cycling stability • Modulating electronic structure and promoting H 2 O 2 evolution through carbon vacancies Ding et al. report an innovative hydrolysis-freeze-drying-thermal treatment method for fabricating a 3D hierarchical porous carbon nitride with carbon vacancies. Owing to the synergistic effects between the carbon vacancies and the unique 3D hierarchical porous structure, the highly efficient photocatalytic H 2 O 2 production is achieved.

G-C3N4 particles with boron and oxygen dopants/carbon vacancies for efficient dehydrogenation in sodium borohydride methanolysis

In the study, metal-free boron and oxygen incorporated graphitic carbon nitride (B and O doped g-C3N4) with carbon vacancy was successfully prepared and applied as a catalyst to the dehydrogenation of sodium borohydride (NaBH4) in methanol for the first time. The hydrogen generation rate (HGR) value was found to be 11,600 mL min−1g−1 by NaBH4 of 2.5%. This is 2.53 times higher than the g-C3N4 catalyst without the addition of B and O. The obtained activation energy was 25.46 kJ mol−1. X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), energy dispersive X-Ray analyser (EDX), Transmission electron microscopy (TEM) and Fourier-transform infrared spectroscopy (FTIR) analyses for characterization were performed. A possible mechanism of H2 production from the reaction using metal-free B and O doped g-C3N4 catalyst with carbon vacancy has been proposed. This study showed that g-C3N4 and its composites with doping atoms can be used effectively in H2 production by NaBH4 methanolysis.

Analysis of Defects and Electrical Characteristics of Variable-Temperature Proton-Irradiated 4H-SiC JBS Diodes

The defects and electrical characteristics of 4H-SiC JBS diodes irradiated by 2 MeV protons under irradiation temperatures of 100–400 K were studied. Forward and reverse current–voltage (I–V), capacitance–voltage (C–V), and deep-level transient spectroscopy (DLTS) measurements were performed to study the changes in the characteristics of the device before and after variable-temperature proton irradiation. As the irradiation temperature increased from 100 to 400 K, the on-resistance decreased from 251 to 204 mΩ, and the carrier concentration gradually increased. The reverse current–voltage experiment results showed that the leakage current increased after proton irradiation at each irradiation temperature compared to before irradiation. The DLTS spectra analyses showed that proton irradiation mainly introduced a carbon vacancy related to the Z1/2 center (E0.68 and E0.72), which may have been the main reason for the changes in the forward and reverse electrical characteristics. The intensity of the DLTS spectrum decreased with the increasing irradiation temperature, indicating that the concentration of defects gradually decreased, due to the increase in the radius of the recombination of a vacancy with a related interstitial atom.

First‐principles study of the thermal properties of Zr2C and Zr2CO

AbstractFirst‐principles calculations of lattice thermal conductivities and thermodynamic properties of Zr2C and Zr2CO were performed using the quasi‐harmonic approximation. Oxygen in the lattice gives Zr2CO higher bonding strength than Zr2C. Thus, the mechanical properties of Zr2C are enhanced when the vacancies in its crystal structure are filled with oxygen. Among the critical parameters that determine the lattice thermal conductivity, Zr2C has significantly higher Grüneisen parameters, thus Zr2C has lower lattice thermal conductivity than Zr2CO. In addition, Zr2CO has a higher heat capacity and thermal expansion coefficient than Zr2C at most temperatures. These results indicate that the addition of oxygen has increased the stiffness and thermal conductivity of zirconium carbide that contains a large fraction of carbon vacancies due to the filling of vacancies in the Zr2C lattice and the formation of Zr–O bonds.

Boosting the Extra Generation of Superoxide Radicals on Graphitic Carbon Nitride with Carbon Vacancies by the Modification of Pollutant Adsorption for High-Performance Photocatalytic Degradation

Boosting the Extra Generation of Superoxide Radicals on Graphitic Carbon Nitride with Carbon Vacancies by the Modification of Pollutant Adsorption for High-Performance Photocatalytic Degradation

Improving the exploration of vacancy evolution in P92 alloy under Fe ion irradiation using positron annihilation

P92 alloy, a candidate structural material for supercritical water reactors, was irradiated with 1 MeV Fe ions at 1, 3, and 7 dpa at room temperature, 300, 450, and 550°C. Slow positron beam Doppler broadening spectroscopy was used to characterize the evolution of the open-volume defects. Irradiation of the Fe9Cr alloy at 300°C induced an unexcepted phenomenon in which the S parameter decreased with an increasing irradiation dose; the phenomenon was ascribed to the pinning effect of Cr on dislocation loops, which promoted the recovery of vacancies. Whereas the reduction in the S parameter was generally inhibited in the P92 alloy. These distinctions are related to the evolution of vacancies. We show that the migration of vacancies hindered by C atoms is mainly related to carbon–vacancy complexes in the P92 alloy at approximately 300°C. As the irradiation temperature increased, the complexes gradually dissociated, and the vacancies were captured and annihilated with interstitial atoms. A better understanding of the evolution of vacancies is helpful for investigating irradiation swelling.

Carbon Vacancy Assisted Contact Resistance Engineering in Graphene FETs

Despite various remarkable properties, the state of the arts of graphene devices are still not up to the mark due to their high contact resistance. The contact resistance milestone has not been achieved yet, probably due to ambiguity in understanding graphene–metal contact properties. In this work, we did a systematic investigation of palladium–graphene contact properties using a density functional theory (DFT) and various process-based experimental approaches. Our study reveals significant interaction of palladium (Pd) with graphene. Their orbitals overlap leads to potential barrier lowering at the interface, which can be reduced further by bringing graphene closer to the bulk Pd using carbon vacancy engineering at the contacts. Thus, the carbon vacancy-assisted barrier modulation reduces contact resistance by increasing carrier transmission probabilities at the interface. The theoretical findings have been emulated experimentally by carbon vacancy engineering at the graphene field-effect transistors (FETs). Different contact-engineered graphene devices with Pd contacts show significant contact resistance reduction, measuring as low as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 78~\Omega ~\cdot ~\mu \text{m}$ </tex-math></inline-formula> at room temperature. The contact resistance shows a “V” shape curve as a function of defect density. Also, the optimum contact resistance achieved is significantly lower than their pristine counterpart, as predicted by the theoretical estimates. Due to contact engineering, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I_{{ \mathrm{\scriptscriptstyle ON}}}}$ </tex-math></inline-formula> improves by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 6\times $ </tex-math></inline-formula> , transconductance by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 8\times $ </tex-math></inline-formula> , and device mobilities by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 6\times $ </tex-math></inline-formula> in the device FETs. These investigations and understanding can help to boost the performance of graphene FETs, especially for high-frequency RF applications.

Theoretical investigation of vacancy related defects at 4H-SiC(0001̅)/SiO2 interface after wet oxidation

The stability and formation mechanism of the defects relevant to silicon and carbon vacancies at the 4H-SiC()/SiO2 interface after wet oxidation are investigated by first-principles calculation based on the density functional theory. The difference in the total energy of the defects agrees with the experimental results concerning the density of defects. We found that the characteristic behaviors of the generation of defects are explained by the positions of vacancies and antisites in the SiC() substrate and that the formation of silicon and carbon vacancies is relevant to the generation mechanism of defects. The generation of silicon and carbon vacancies is attributed to the termination of dangling bonds by H atoms introduced by wet oxidation, resulting in the generation of carbon-antisite–carbon-vacancy and divacancies defects in wet oxidation.

Vacancy ordering in substoichiometric zirconium carbide: A review

AbstractZirconium carbide has a wide range of substoichiometry facilitated by varying numbers of carbon vacancies. Most experimental studies consider a solid solution of carbon and vacancies without long‐range ordering of vacancies. However, theoretical studies predict several superstructural long‐range ordered phases to be stable at low temperatures, and these predictions have been validated by experimental fabrication in some cases. The thermophysical properties of zirconium carbide are, therefore, affected not only by the number of carbon vacancies, but their arrangement, which increases the potential of its use as a tuneable ceramic in various applications in aerospace and nuclear industries. This review summarizes the experimental and theoretical studies exploring the long‐range ordered zirconium carbides including the known crystal structures, the mechanism for vacancy ordering, and the presently available information on the thermophysical properties. Explanations for the infrequent experimental observations of ordered zirconium carbides are also discussed considering fabrication temperatures, vacancy diffusion, and the effects of impurities, which may be helpful for future synthesis. While our understanding of vacancy ordering in zirconium carbide has vastly improved in recent years, there remain significant knowledge gaps. Areas where more experimental and theoretical studies are needed for further understanding are highlighted.

Three‐In‐One Alkylamine‐Tuned MoOx for Lab‐Scale to Real‐Life Aqueous Supercapacitors

AbstractAqueous supercapacitors show advantages of high safely, prolonged lifespan, and low cost, etc. but there have never been commercial market products, nor quantitative investigation of practical pouch devices of aqueous supercapacitors. Herein, to achieve their lab‐scale to real‐life manufacture, a unique MoOx for supercapacitor use is constructed by a hydrothermal and annealing strategy suitable for industrialization, which plays three key functions, including precisely adjusted interlayer spacing, conductive flexible graphite carbon and abundant oxygen vacancies. As a result, the as‐synthesized electrode yields an ultra‐high specific capacitance of 717 F g−1 at 1 A g−1 and ultra‐long cycling durability with no obvious capacitive loss even after 100 000 cycles. Realistically, the assembled asymmetric supercapacitor (MoOx‐HDA‐3//MnO2) exhibits extraordinary energy density of 78.2 Wh kg−1, superior to many advanced supercapacitors reported to date. We have fabricated pouch devices, which can successfully run 3C products such as tablets and smartphones, and maintain stable electrochemical performance even after heavy strikes, fires, and pressures. Quantitative investigation results confirm that the pouch device delivers an excellent specific capacitance of 74.7 F g−1 and a high energy density of 41.5 Wh kg−1. This work enhances the confidence of pushing aqueous supercapacitors to realistic energy storage market.

Leveraging doping and defect engineering to modulate exciton dissociation in graphitic carbon nitride for photocatalytic elimination of marine oil spill

Limited light absorption ability and rapid photogenerated electron-hole recombination severely impedes applications of g-C3N4. Herein, highly efficient point-defect engineering including dual dopants as well as vacancy was adopted to modulate the photo-induced exciton dissociation kinetics. Specifically, P, O dopants and carbon vacancy modified 3D honeycomb-like POCVN was fabricated through facile one-pot polymerization with (NH4)3PO4 as P, O precursors. The obtained POCVN catalyst exhibited superior photocatalytic degradation performance for n-tetradecane under visible light illumination (38.1 %), which was 4.6 and 1.8 times higher than that of bulk g-C3N4 (8.2 %) and tubular g-C3N4 (21.1 %), respectively. The suppressed recombination of electrons and holes contributed to the superior catalytic performance compared to pristine g-C3N4, single P and single O doping g-C3N4. Structural analysis demonstrated P atoms may replace C atoms of N-C = N, O atom located at P-O-C and carbon vacancies located at N = C-N2 position at heptazine framework. Based on experimental and theoretical analysis, it was found P, O and Cv defects prefer to accumulate together in 3-trizine ring, which is conductive to the formation of localized defect state in the band gap region. It resulted in high exciton dissociation efficiency, thus, more reactive radicals were generated. Based on this, degradation path, interference parameter, reactive radicals and toxicity evaluation were investigated deeply and systematically. This work shed light on non-metal green ecological remediation material for marine oil spill.

Tuning the electronic and optical properties of two-dimensional hydrogenated c-BN nanosheets by Be and C dopants and vacancy defects

• Dopants and defects of UHBNs could tailor the electronic properties with narrow band gaps. • Antisite defects will be more likely to occur in experimental process. • Faster speed of spread in UHBNs than bulk due to low refractive index. • The visible light protection of new 2D structures have the potential to photosensitive devices. Substitutional and interstitial beryllium (Be) and carbon (C) dopants, vacancies and anti-vacancy defects in two-dimensional (2D) ultrathin hydrogenated cubic BN (c-BN) nanosheets (UHBNs) were studied using a first-principles method. Charge redistribution provides opportunities to use appropriate dopants and defects to redistribute charge and tailor the electronic properties of UHBNs to obtain p -/ n -type semiconductors with narrow band gaps. Anti-vacancy defects have lower energy than the vacancy defects, which means that in the experimental synthetic preparation process, dislocation occupancy between atoms will be more likely to occur. The refractive index is lower, the speed of spread in these materials is expected to be faster than that in bulk BN, and the region of absorption shifts from ultraviolet to visible light. Our results present an exciting opportunity for making ultrathin BN nanosheets for next-generation optoelectronic nanomaterials.

Photocatalytic degradation of pharmaceuticals by pore-structured graphitic carbon nitride with carbon vacancy in water: Identification of intermediate degradants and effects of active species

Pharmaceuticals are increasingly used in daily life and have been massively discharged to the aquatic environment. The removal of pharmaceuticals from water by various nanomaterials including graphitic carbon nitride (g-C3N4) has received extensive attention. Herein, we synthesized a carbon-defective carbon nitride with pore structure through a simple thermal polymerization method for photodegradation of lidocaine, mepivacaine and ropivacaine (typical amide local anesthetics). The results showed that the degradation process conformed to the pseudo-first-order reaction kinetics, and the degradation rate constant of organic pollutants using CCN-600 (i.e., g-C3N4 synthesized at 600 °C) reached 5.05 × 10−2 min−1, about 2.5 times higher than that of the prototype g-C3N4 (2.09 × 10−2 min−1). The capture experiment of active species and the electron paramagnetic resonance (EPR) test demonstrated that superoxide radical (O2−) played a major role in the degradation process. Based on the possible photodegraded intermediate products identified, the degradation pathways were deduced. This study provides not only a new strategy for fabrication of pore-structured g-C3N4 with carbon vacancy, but also a reference method for the treatment of pharmaceuticals in water bodies.

Plasmonic Nanozyme of Graphdiyne Nanowalls Wrapped Hollow Copper Sulfide Nanocubes for Rapid Bacteria‐Killing

AbstractPlasmon stimulation represents an appealing way to modulate enzyme mimic functions, but utilization efficiency of plasmon excitation remains relatively low. To overcome this drawback, a heterojunction composite based on graphdiyne nanowalls wrapped hollow copper sulfide nanocubes (CuS@GDY) with strong localized surface plasmon resonance (LSPR) response in the near‐infrared (NIR) region is developed. This nanozyme can concurrently harvest LSPR induced hot carriers and produce photothermal effects, resulting in dramatically increased peroxidase‐like activity when exposed to 808 nm light. Both experimental results and theoretical calculations show that the remarkable catalytic performance of CuS@GDY is due to the unique hierarchical structure, narrow bandgap of GDY nanowalls, LSPR effect of CuS nanocages, fast interfacial electron transfer dynamics, and carbon vacancies on CuS@GDY. This plasmonic nanozyme exhibits rapid, efficient, broad‐spectrum antibacterial activity (&gt;99.999%) against diverse pathogens (methicillin‐resistant Staphylococcus aureus, Staphylococcus aureus, and Escherichia coli). This study not only sheds light on the mechanism of the nanozyme‐/photocatalysis coupling process, but also opens up a new avenue for engineering plasmonic NIR light driven nanozymes for rapid synergistic photothermal and photo‐enhanced nanozyme therapy.