Carbon Vacancies Research Articles

Elastic properties of solid-solution refractory metal carbides with vacancy from virtual crystal approximation and supercell method

The virtual crystal approximation (VCA), as an efficient approach, has been used to study the alloys composed of refractory metals. In this work, we estimate the VCA in the application of solid-solution refractory metal carbides with point defects within the framework of ab initio calculations. Results indicate the VCA predicted Cij are different from those from supercell method with the same stress-stain approach. The elastic constants C44 from VCA are close to the average values derived from the refractory metal carbides. For the solid-solution refractory metal carbides with the severe lattice distortion, Cij are very sensitive to the atomic displacement and their moduli are softened. The carbon vacancies have a large effect on the elastic constants, whereas product a small effect on the anisotropy and hardness of solid-solution refractory metal carbides.

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.

Densification, microstructures, and mechanical properties of (Zr, Ti)(C, N) ceramics fabricated by spark plasma sintering

Dense (Zr, Ti) (C, N) ceramics were fabricated by spark plasma sintering (SPS) at 1900–2000 °C using ZrC, TiCN and ZrH2 powders as raw materials. A single Zr-rich (Zr, Ti)(C, N) solid solution was formed in Zr0.95Ti0.05C0.975N0.025 and Zr0.80Ti0.20C0.90N0.10 ceramics (nominal composition). A Ti-rich solid solution appears in Zr0.50Ti0.50C0.75N0.25 ceramics. The coaddition of TiCN and ZrH2 promoted the densification of (Zr, Ti) (C, N) ceramics by forming solid solutions and carbon vacancies, which could reduce critical resolved shear stress (CRSS) and promote carbon and metal atom diffusion. ZrC-45 mol% TiCN-10 mol% ZrH2 (raw powder composition) possesses good comprehensive mechanical properties (Vickers hardness of 24.5 ± 0.9 GPa, flexural strength of 503 ± 51 MPa, and fracture toughness of 4.3 ± 0.2 MPa·m1/2), which reach or exceed most ZrC-based (Zr, Ti) C and (Zr, Ti) (C, N) ceramics in previous reports.

Non-stoichiometry of (TiZrHfVNbTa)Cx and its significance to the microstructure and mechanical properties

(TiZrHfVNbTa)Cx with variable stoichiometry are fabricated by pressureless sintering utilizing self-synthesized carbide powders via carbothermal reduction reaction. The densification behavior and microstructure evolution coupled with corresponding adjustable mechanical properties are investigated. The single-phase rock-salt crystal structure is retained despite the carbon stoichiometry approaching 0.6, indicating (TiZrHfVNbTa)Cx can maintain structural stability even containing high carbon vacancy. The carbon vacancy is beneficial for promoting densification procedure. The relative density of (TiZrHfVNbTa)C0.6 sintered at 2150 °C can reach 97.9 %, while the similar value for (TiZrHfVNbTa)C1.0 is obtained even at 2400 °C. While, remarkable grain growth accompanied by decline in relative density also occurs for lower carbon stoichiometry. With the variation of carbon content, the concentration of carbon-metal bonds changes gradually, leading to the adjustable mechanical properties. This work provides a potential approach to synthesize non-stoichiometric high-entropy carbides with high carbon vacancy via low-temperature pressureless sintering.

NH2-MIL-125(Ti) modified graphitic carbon nitride with carbon vacancy for efficient photocatalytic NO removal

This paper reports the application of NH2-MIL-125(Ti) modified carbon nitride Cv-C3N4 with carbon vacancies in the removal of NO. We performed a series of characterizations of the complex and compared various ratios with the individual components. The results of UV spectrum analysis show that the composite's range of light absorption expanded due to the modification of Ti metal–organic framework. Furthermore, the results of photocurrent and electrical impedance indicate the compound has a better ability to generate and transfer electrons. The increase in the compound's NO removal efficiency (up to 63%) indicates that MOF has a positive effect on Cv-C3N4 modification—a good method for photocatalysis. Moreover, the compound can promote photocatalysis in a favorable direction.

Surface Properties of Graphite for Electrocatalysis of Vanadium Redox Reactions

The interfacial processes controlling charge transfer in vanadium redox reactions at graphite electrodes are still poorly understood. Accurate knowledge of the underlying mechanism is mandatory to improve the efficiency of conversion from electrical to chemical energy and vice versa. Various surface modifications are known to improve the catalytic activity of graphite by introducing active sites. Differentiating between influences on structural properties by activation processes is difficult due to the heterogeneity of the material and the interplay of microstructural changes and surface chemistry. This requires the application of multiple analytical tools and the systematic correlation with electrochemical results. In this way, our group has recently demonstrated that the 30-year-old mechanism involving surface oxygen does not stand up to thorough investigation. Instead, we primarily identified lattice defects such as edges and carbon vacancies as the actual origin of enhanced electron transfer kinetics. We have studied these defects on various scales and introduced an onion-like model to explain why thermal treatment is effective but care must be taken to remove the material from the oxidizing atmosphere at the right time. Otherwise abundant oxygen groups and dull pore edges will counteract the intended activation. Special attention was subsequently paid to the interaction of microstructure and electronic states, establishing a relationship of catalytic activity and the schematic band structure. Whereas edges decrease the work function of graphite, vacancies increase the density of states close to the Fermi level. Using real and model electrodes, we distinguished between the architecture of structural disorder and found zigzag edges to provide superior kinetics over armchair edges. Our experimental studies were complemented by theoretical density functional theory calculations to learn about the adsorption properties of electrolyte species with regard to the orientation of graphite. The potential-dependent structural changes and coordination spheres of the electrode and the near-electrode electrolyte were investigated by operando Raman spectroscopy to elucidate the interaction between vanadium and carbon.

Novel Way to Turn Spent Li-Ion Battery Graphite into Valuable and Active Catalyst for Electrochemical Oxygen Reduction

Battery technology has been recognized as one of the main driving forces and key technologies required to create a carbon-neutral economy and, therefore, batteries are currently one of the most rapidly growing technology sectors due to the rapid rise in the number of electric vehicles (EV´s). Additionally, the employment of Li-ion batteries (LIBs) in portable electronics has become a necessity in the modern world. This produces spend Li-ion batteries (SLIBs) in substantial volumes, it was estimated that in 2019 188,000 tonnes of SLIBs were available for recycling and it is expected that this number will rise to 125 million in 2030.1,2 To obtain the full benefits of electrification the value chain associated with batteries needs to become more sustainable, which strongly depends on the ability of SLIBs recycling to maximize the components return back to materials circulation. However, current SLIBs recycling strategies has been focused on cathode materials alone while largely overlooking the anode of the battery. In turn, anode recycling has so far concentrated mostly on the metal in the electrode and only very recently has the focus shifted towards the recycling of graphite powders.In this work, a novel strategy for recycling SLIB graphite and reforming it as a valuable catalyst material for electrochemical oxygen reduction reaction (ORR) is proposed. A modified Hummers method was applied to fabricate graphene oxide (GO) from SLIB graphite powder. The GO reduction and doping with nitrogen was achieved by pyrolyzing sample at 800 °C in the presence of DCDA. The prepared nitrogen-doped graphene (NG-Bat) was characterised by various physicochemical (SEM, TEM, XRD, Raman and XPS) and electrochemical characterization methods (LSV, EIS, chronoamperometry). For electrochemical characterisation RDE setup with the glassy carbon working electrode, modified with studied catalyst, in 0.1 M KOH solution was used. The physical and electrochemical characteristics of NG-Bat were compared with commercially available nitrogen doped graphene (NG-Com) sample.Based on XRD and Raman data, SLIB-derived graphite maintains its high quality, even after intense exploitation in the batteries and is therefore a valuable material that is worth recycling.3 Homogeneous distribution of carbon, nitrogen, and oxygen in NG-Bat sample was achieved and the overall nitrogen content was found to be 18.4 at%, based on XPS analysis. The onset potential of the ORR for the NG-Bat catalyst material is 0.867 V, and for the NG-Com catalyst 0.797 V. The NG-Bat shows a larger oxygen reduction current and higher electrocatalytic activity than the catalyst based on NG-Com (Figure 1). Based on EIS fitting very different charge transfer resistance and double-layer capacitance values are found, probably caused by different conductivities of the catalyst materials. NG-Bat showed much high electrocatalytic activity towards the ORR than commercial NG-Com based-catalyst material, caused probably by a higher content of active nitrogen species and the presence of carbon vacancies on the surface of graphene, as confirmed by XRD, Raman and XPS experiments.The findings demonstrate that SLIB graphite is still a valuable material and a promising precursor for GO fabrication. One of the potential applications of the SLIB-derived graphene could be the production of catalyst material for the oxygen reduction reaction (ORR), thereby open new avenues to the fuel cell, metal-air battery, and sensor industries.

Insight into Point Defects and Complex Defects in β-Mo2C and Carbide Evolution from First Principles.

In this paper, first principles method was adopted to investigate the point defects, Vanadium-related defects and defect combinations (vacancy (V), substitutional (S) and/or interstitial (I)) in molybdenum β-Mo2C and explore the use of first principles calculation data in analysing the link between different carbides and the effects of doping elements. Supercell models with different defect types were established and optimized, and the formation energy data of defects was developed. The structure evolution during the optimization process is analysed in detail to establish the main characteristics of changes and the relevant electronic properties. The data for different types of intrinsic defects and combined defects complexes was developed and key results is analysed. The results show that carbon vacancy (VC) is stable but does not inevitably exist in pure β-Mo2C. Interstitial site II is a very unstable position for any type of atoms (Mo, V and C), and analysis of the structure evolution shows that the atom always moves to the interface area near the interstitial site I between two layers. In particular, a C atom can expand the lattice structure when it exists between the layer interfaces. One type of the defects studied, the substitution of Mo with V (designated as ‘SV-Mo’), is the most stable defect among all single point defects. The data for defect complexes shows that the combination of multiple SV-Mo defects in the super cell being more stable than the combination of other defects (e.g., ‘VMo+IC’, ‘SV-Mo+VC’). The data with increasing SV-Mo in (Mo, V)2C system is developed, and typical data (e.g., formation energy) for Mo-rich carbides and V carbides are correlated and the potential of the data in analysing transition of different carbides is highlighted. The relevance of using first principles calculation data in the studying of V-doping and the complex carbides (V- and Mo-rich carbides) evolution in different materials systems and future focus of continuous work is also discussed.

Efficiently and selectively photocatalytic cleavage of C[sbnd]C bond by C3N4 nanosheets: Defect-enhanced engineering and rational reaction route

Photocatalytic cleavage of CC bond in lignin under mild conditions transforms solar energy into high value-added aromatic compounds. However, the limited understanding of cleavage mechanisms and the catalyst structure-performance relation obstruct the rational design of efficient photocatalysts. Reported herein is the construction of porous carbon nitride (C3N4) with carbon vacancies as heterogeneous photocatalysts via a sequential self-assembly and diol-induced thermal exfoliation. In the photocatalytic cleavage of βO4 ether bonds, carbon vacancies in C3N4 can serve as the trapping site to suppress the recombination of photogenerated charge, and endow the C3N4 with high level of N and thus make it easier for the oxidation of Cβ–H, contributing to the enhancement of performance. Furthermore, the experimental result and in situ ATR-SEIRAS spectra unveil that O2 could facilitate the cleavage of CC bond but is not indispensable in this process, which brings new fundamental understanding of βO4 bonds cleavage.

Kick-out diffusion of Al in 4H-SiC: an ab initio study

As a semiconductor with a wide bandgap, 4H silicon carbide (4H-SiC) has considerable potential for high-temperature and high-power devices. It is widely established that p-type 4H-SiC is formed predominantly by doping Al. Although Al diffusion in 4H-SiC is often negligible at low temperatures due to the tight bonding of Al in 4H-SiC, the diffusion coefficient of Al dramatically rises when the temperature is rather high. While diffusion is the most fundamental physical processes, the diffusion mechanism of Al in 4H-SiC remains unknown. Due to the large atomic radius of Al relative to the host Si/C atoms and the fact that Al occupies the Si lattice in 4H-SiC, the diffusion of Al is typically mediated by point defects such as vacancies and self-interstitials. We now investigate the diffusion of Al in 4H-SiC using first-principles calculations and compare the activation energy of Al diffusion mediated by carbon vacancies (VC) to that of Al diffusion mediated by Si interstitials (Sii). It is found that Al diffusion is actually a Sii-mediated process, in which a nearby Sii first kicks a substitutional Al atom to an interstitial site. The kicked-out Al then spreads via interstitial sites. The diffusion coefficient is calculated, which is comparable to experimental results.

Plasma-treatment of polymeric carbon nitride for efficient NO abatement under visible light

Photocatalysis is considered to be efficient in combatting emission nitrogen oxide (NO x ), which is one of the atmospheric pollutants affecting human health. Polymeric carbon nitride (PCN) is a low-cost polymeric photocatalyst with a two-dimensional structure that is sensitive to the visible sunlight in the solar spectrum, but its photocatalytic efficiency needs to be enhanced for the purpose of pollutant abatement. In this study, PCN was treated using a facile ambient pressure dielectric barrier discharge (DBD) plasma in air, Ar and Ar-5% H2 flow. According to the spectroscopic characterization and NO removal tests, the DBD plasma did not destroy the crystal structure of PCN, but improved the separation efficiency of photogenerated charges and enhanced the capacity of NO abatement. The plasma treatment in Ar-5% H2 showed an optimal removal efficiency of 69.19% and a selectivity for nitrate of 90.51% under visible light irradiation. The hydrogen plasma etched the PCN surface, resulting in more defects (carbon vacancies) and carbonyl group on the surface, while the air plasma was found to increase the suspending –NO x bonding on the surface for the increased NO x emission under illumination. The generation of high-energy electron and reactive radicals in the electrical discharges could cause the surface modification of PCN for efficient photocatalysis.

Mechanical properties and their sensitivity to point defects: (HfNbTaTiZr)C high-entropy carbide

High-entropy carbides (HECs) show excellent mechanical properties, but the strengthening mechanism remains unclear. In this work, using first-principles calculations, we comparatively investigated the ideal mechanical properties of $(\mathrm{HfNbTaTiZr})\mathrm{C}$ and its constituent transition metal carbides (TMCs), as well as their sensitivity to point defects. It was found that the properties near the equilibrium point follow the rule of mixture, while those far from the equilibrium state are governed by the weakest link. Most importantly, the introduction of carbon vacancy significantly weakens the mechanical properties of the TMCs, while it less affects and even improves the mechanical properties of the HEC, indicating that the insensitivity to point defects may be one of the significant features of HECs. The results showed that the interaction between lattice distortion and defects would play an important role in improving the mechanical properties of HECs, which may provide guidance for the composition design of HECs.

Experimental and theoretical studies on atomic structures of the interface states at SiO2/4H-SiC(0001) interface

We investigated the atomic structures of the interface states (gap states) at the SiO2/4H-SiC(0001) interface using hard x-ray photoelectron spectroscopy (HAXPES), operando hard x-ray photoelectron spectroscopy, extended x-ray absorption fine structure, and first principles molecular dynamics (FPMD) calculations. For the interface states, two types were observed: continuous interface states in the whole gap and interface states with discrete energy levels near the conduction band minimum (CBM). From HAXPES, we found that carbon clusters and carbon vacancies were formed at the SiO2/4H-SiC(0001) interface. FPMD calculations on the SiO2/4H-SiC(0001) interface showed that the interface states in the whole gap were attributed to the various atomic geometries of the CßSi3 species and the carbon clusters with various sizes and surrounding atoms. For the interface states with a discrete energy level near the CBM, we could not find their atomic structure using our current calculations. We calculated the carbon vacancies prepared on the side of an SiC substrate at the SiO2/4H-SiC(0001) interface, indicating the formation of a discrete energy level in the midgap. It is likely that carbon vacancies formed at the step of the interface may modulate the midgap energy level to energy below the CBM. Therefore, we propose that the interface states with discrete energy levels near the CBM could be attributed to the carbon vacancies formed on the steps at the SiO2/4H-SiC(0001) interface.

Hydrogen dynamics on defective monolayer graphene

The hydrogen dynamics on a graphene sheet is studied in the presence of carbon vacancies. We analyze the motion of atomic H by means of molecular dynamics (MD) simulations, using a tight-binding Hamiltonian fitted to density-functional calculations. Hydrogen passivates the dangling bonds of C atoms close to a vacancy, forming C–H bonds with H located at one or the other side of the layer plane. The hydrogen dynamics has been studied from statistical analysis of MD trajectories, along with the autocorrelation function of the atomic coordinates. For a single H atom, we find an effective barrier of 0.40 eV for crossing the graphene layer, with a jump rate ν=2×106 s−1 at 300 K. The atomic jumps behave as stochastic events, and their number for a given temperature and time interval follows a Poisson probability distribution. For two H atoms close to a vacancy, strong correlations in the atomic dynamics are found, with a lower jump frequency ν=7×102 s−1 at room temperature. These results provide insight into the diffusion mechanisms of hydrogen on graphene, paving the way for a complete understanding of its motion through defective crystalline membranes.

Interaction of primary precipitates in reduced -activation ferritic/martensitic steel F82H with hydrogen atoms: Atomistic calculation based on the density functional theory

• We used DFT to perform first-principles calculations to understand the fundamental aspects of the interactions between hydrogen isotopes and VC and Cr 23 C 6 crystals in F82H steel. • The single H atom in the carbon vacancy in VC crystal is situated at the center of the vacancy, and the trapping energy is 1.99 eV. • Each H atom and C atom are bound to each other with strong covalent forces, and up to six H atoms can be trapped in the vanadium vacancy in VC crystal. • The trapping energy of H atom in the Cr(8c) vacancy is the strongest among the five types of vacancies in Cr 23 C 6 crystal. To understand the interaction of hydrogen isotopes with VC and Cr 23 C 6 precipitates, in reduced-activation ferritic/martensitic steel (F82H), we have performed first-principles calculations based on density functional theory. Energy calculations and electron density analysis were performed with a focus on the hydrogen retention by vacancies in precipitates. The H atoms in the C vacancy in the VC crystal are bound to the surrounding Cr atoms by relatively weak covalent forces and Coulombic attraction, and up to four H atoms are trapped. In the case of a Cr vacancy, H atoms are strongly covalently bonded to neighboring C atoms as well as to an interstitial H atom, capturing up to six H atoms. H atoms in vacancies in Cr 23 C 6 have a weak bonding force with the Cr atom but a strong repulsive force with the C atom. As a result, H atom is not trapped in the Cr(4a) vacancy, and H atoms are trapped only at a distance from the C atom in the Cr(48H) and Cr(32f) vacancies. The Cr(8c) and C vacancies are relatively far from the C atoms and have higher hydrogen trapping energies. The Cr 23 C 6 precipitate containing vacancies may be a dominant trapping site in the temperature range when H atoms immediately dissociate from vacancies. In addition, the VC precipitate may be the only trapping site for hydrogen isotopes at higher temperatures when Cr 23 C 6 precipitates cannot trap them.

Synergistic Effect of Electrical Stress and Neutron Irradiation on Silicon Carbide Power MOSFETs

The synergistic effect of radiation and bias voltage for the commercial silicon carbide (SiC) power MOSFETs was investigated by 14-MeV neutron irradiation in this article. The types of C3M0065090D and C2M0080120D commercial SiC power MOSFETs from CREE studied in this article show consistent changes after the 14-MeV neutron irradiation. The results show that the devices biased during irradiation experienced more degradation than that only irradiated and biased devices. Compared with the devices only irradiated and biased, the devices biased during irradiation experienced an additional decrease in saturation current and a further decrease in peak transconductance. The defects in the devices were characterized by the low-frequency noise (LFN) and deep-level transient spectrum (DLTS) methods. The results show that the synergistic effect increased the defect concentration of the device, resulting in further degradation of the electrical performance of the device compared to the control group without a bias voltage. The defect characterization shows that the drain–source voltage leads to the accumulation of carbon and silicon vacancies induced by irradiation at the interface, which results in intensified radiation damage and significant changes in the performance of the device at low neutron fluence.

Optical Characterization of 4H-SiC Thick Epitaxial Layer for Particle Detection

In the present work a deep characterization of 4H-SiC epi-layer was done. A thick layer was epitaxially grown through chemical vapor deposition (CVD) process in a horizontal hot-wall reactor in order to obtain a 250 microns thick epi-layer. This sample will be used as particle detector in hostile environments such as neutron detection in a nuclear fusion reactor. Raman and Photoluminescence (PL) spectroscopy have been used in order to evaluate the general status of epitaxy and, with the support of the Time Resolved Photoluminescence, also important properties such as carrier lifetime and diffusion length have been evaluated. Carrier lifetime evaluation before and after a thermal oxidation process at 1400° C for 48h was estimated, by considering a lifetime increment after oxidation process, due to the decrease of carbon vacancies. Finally, the influence of stacking fault (SF) defects on carrier lifetime was evaluated observing a decrease of the lifetime for the defects at 430 nm (2.88 eV) for both oxidated and non-oxidated samples.

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.