B and cyano groups co-doped g-C3N4 with multiple defects for photocatalytic nitrogen fixation in ultrapure water without hole scavengers

Cyano-Rich g-C3 N4 in Photochemistry: Design, Applications, and Prospects

Cyano-rich g-C3 N4 materials are widely used in various fields of photochemistry due to the very powerful electron-absorbing ability and electron storage function of cyano, as well as its advantages in improving light absorption, adjusting the energy band structure, increasing the polarization rate and electron density in the structure, active site concentration, and promoting oxygen activation ability. Notwithstanding, there is yet a huge knowledge break in the design, preparation, detection, application, and prospect of cyano-rich g-C3 N4 . Accordingly, an overall review is arranged to substantially comprehend the research progress and position of cyano-rich g-C3 N4 materials. An overall overview of the current research position in the synthesis, characterization (determination of their location and quantity), application, and reaction mechanism analysis of cyano-rich g-C3 N4 materials to provide a quantity of novel suggestions for cyano-modified carbon nitride materials' construction is provided. In view of the prevailing challenges and outlooks of cyano-rich g-C3 N4 materials, this paper will purify the growth direction of cyano-rich g-C3 N4 , to achieve a more in-depth exploration and broaden the applications of cyano-rich g-C3 N4 .

Facile synthesis of N vacancy g-C3N4 using Mg-induced defect on the amine groups for enhanced photocatalytic •OH generation

Photocatalysis offers opportunities to degrade recalcitrant organic pollutants without adding treatment chemicals. Nitrogen (N) vacancy is an effective point-defect engineering strategy to mitigate electron-hole recombination and facilitate hydroxyl radical (•OH) production via superoxide radical (O₂^•-) generation during photocatalytic application of graphitic carbon nitride (g-C₃N₄). Here, we report a novel strategy for fabrication of N-vacancy-rich g-C₃N₄ (NvrCN) via post-solvothermal treatment of Mg-doped g-C₃N₄. The addition of the Mg precursor during the polycondensation of urea created abundant amine sites in the g-C₃N₄ framework, which facilitates formation of N vacancies during post-solvothermal treatment. Elemental analysis and electron paramagnetic resonance spectra confirmed a higher abundance of N vacancies in the resultant NvrCN. Further optical and electronic analyses revealed the beneficial role of N vacancies in light-harvesting capacity, electron-hole separation, and charge transfer. N vacancies also provide specific reaction centers for O₂ molecules, promoting oxygen reduction reaction (ORR). Therefore, •OH generation increased via enhanced formation of H₂O₂ under visible light irradiation, and NvrCN photocatalytically degraded oxytetracycline 4-fold faster with degradation rate constant of 1.85 × 10^-2 min^-1 (light intensity = 1.03 mW/cm², catalyst concentration = 0.6 g/L, oxytetracycline concentration = 20 mg/L) than pristine g-C₃N₄. Overall, this study provides a facile method for synthesizing N-vacancy-rich g-C₃N₄ and elucidates the role of the defect structure in enhancing the photocatalytic activity of g-C₃N₄.

Exceptional visible-light photoelectrocatalytic activity of dual Z-scheme Bi@BiOI-Bi2O3/C3N4 heterojunction for simultaneous remediation of Cr(VI) and phenol

Developing highly efficient and stable photocatalysts remains a major challenge for the remediation of environmental pollutants. In this work, the Bi⁰ decorated BiOI-Bi₂O₃/C₃N₄ heterojunction (Bi@BiOI-Bi₂O₃/C₃N₄) film was fabricated through ultrasonic stripping, I^- etching and in situ UV-reduction processes and then characterized thoroughly by various analytical techniques. The characteristics of simultaneous mitigation of phenol and Cr(VI) were evaluated over Bi@BiOI-Bi₂O₃/C₃N₄ photoanode under visible light. The results exhibited that both phenol and Cr(VI) were removed completely by the photoanode at 2.5 V within 1.5 h, superior to our previous report. The synergy of the surface plasmon resonance (SPR) effect of Bi⁰ and ternary heterojunction accelerated the separation and transfer of photo-induced charge carrier and thus heavily promoted the removal efficiency. Moreover, the excellent stability of this photoanode was hold with no considerably activity attenuation after 4 cycles. Finally, a dual Z-scheme charge transfer process was presented. This work offers an attractive pathway to construct highly active photoelectrode with promising application for simultaneous remediation of organics and heavy metals in wastewater.

Interlayer Charge Transfer Over Graphitized Carbon Nitride Enabling Highly-Efficient Photocatalytic Nitrogen Fixation

Exploiting cost-effective, high-efficiency, and contamination-free semiconductors for photocatalytic nitrogen reduction reaction (N₂ RR) is still a great challenge, especially in sacrificial-free system. On basis of the electron "acceptance-donation" concept, a boron-doped and carbon-deficient g-C₃ N₄ (B_x CvN) is herein developed through precise dopant and defect engineering. The optimized B₁₅ CvN exhibisted an NH₃ production rate of 135.3 µmol h^-1  g^-1 in pure water with nine-fold enhancement to the pristine graphitic carbon nitride (g-C₃ N₄ ), on account of the markedly elevated visible-light harvesting, N₂ activation, and multi-directional photoinduced carriers transfer. The decorated B atoms with coexistent occupied and empty sp³ hybridized orbitals are theoretically proved to be in charge of the increase of N₂ adsorption energy from -0.08 to -0.26 eV and the change in N₂ adsorption model from one-way to two-way end-on pattern. Noticeably, the elaborate coordination of doped B atoms and carbon vacancies greatly facilitated the interlayer interaction and vertical charge migration of B_x CvN, which is distinctly revealed through the charge density difference calculations. The current study provides an alternative groundbreaking perspective for advancing photocatalytic N₂ RR through the targeted configuration of the defect and dopant sites.

Facile Chemical Vapor Modification Strategy to Construct Surface Cyano-Rich Polymer Carbon Nitrides for Highly Efficient Photocatalytic H2 Evolution

The surface grafting of electro-negative cyano groups on polymer carbon nitrides (PCNs) is an effective way to tail their electronic structure. Despite the significant progress in the synthesis of cyano group-enriched PCN, developing a simple and efficient method remains challenging. Here, a facile strategy was developed for fabricating surface cyano-rich PCN (PCN-DM) with a porous structure via chemical vapor modification using diaminomaleonitrile. The cyano groups of diaminomaleonitrile substituted the amino groups on PCN surface via a deamination. The hydrogen production rate of the PCN-DM was approximately 17 times higher than that of pristine PCN. This significant increase in photocatalytic performance could be assigned to the fusion of cyano groups in the surface of PCN, forming new gap states that broadened the visible-light harvesting and accelerated charge separation for photoredox reactions. This study unveils a promising approach for incorporating functional units in the design of novel photocatalysts for efficient hydrogen production.

Formic acid assisted fabrication of Oxygen-doped Rod-like carbon nitride with improved photocatalytic hydrogen evolution

Rod-like carbon nitrides synthesized by calcinating supramolecular precursors prepared from acid (or alkali) and melamine have attracted great attention because they have large surface area and abundant accessible active sites. However, they are highly inefficient in separating charges, which limits their photocatalytic activity. Here, we prepared porous, rod-shaped carbon nitrides doped with oxygen by calcinating the precursors prepared from melamine and formic acid. The porous O-doped g-C₃N₄ nanorods have a large surface area of 81.4 m² g^-1. In particular, the oxygen doped into the catalyst enables it to have high efficiency in utilizing light in a range of 420-600 nm, and significantly improves its ability to separate photogenerated carriers. Under light irradiation (λ ≥ 420 nm), the prepared catalyst exhibits high photocatalytic activity with a hydrogen production rate of 12,766 μmol g^-1h^-1, which is 18.3 times that of pure carbon nitride. This research provides a novel way of preparing highly active non-metallic photocatalysts.

Morphology-engineered carbon quantum dots embedded on octahedral CdIn2S4 for enhanced photocatalytic activity towards pollutant degradation and hydrogen evolution

In recent years, carbon quantum dots (CQDs) and CdIn₂S₄ have considered as the representatives of the most potential photocatalysts applied in the field of photocatalysis for efficiently solving energy shortage and environmental pollution. In this work, a novel CQDs hybridized CdIn₂S₄ (CQDs/CIS) heterostructure with 2D nanosheet/3D nanooctahedra morphology was successfully fabricated by a simple in-situ solvothermal method. Most interestingly, the morphology of hybrid gradually evolved from 3D octahedron to 2D nanosheet with the increase of CQDs. This unique 2D/3D structure and synergistic effect between CQDs and CdIn₂S₄ increased the multi-dimensional active reaction sites and enhanced the quantum yield and the separation efficiency of photogenerated electron pairs. Therefore, CQDs/CIS hybrids showed excellent photocatalytic activities of H₂ generation, RhB and TCH degradation. Especially, CQDs/CIS-3 heterostructure presented the highest photocatalytic efficiency and its hydrogen generation activity (956.79 μmol g^-1 h^-1) was 7.57-fold improvement by contrast with pure CdIn₂S₄ (126.35 μmol g^-1 h^-1). Moreover, RhB and TCH degradation rate constants of CQDs/CIS-3 were about 8.14 and 2.32 times higher than those of CdIn₂S_4, respectively. Furthermore, the effect of CQDs on the evolution of heterostructure morphology and photocatalytic mechanism were also proposed. This research work would offer useful enlightenment for elucidating the affect of CQDs on the morphology evolution and construction of CQDs-based hybrid with excellent performances for H₂ production and pollutant removal.

Co-Doped, Tri-Doped, and Rare-Earth-Doped g-C3N4 for Photocatalytic Applications: State-of-the-Art

Rapid industrialization and overpopulation have led to energy shortages and environmental pollution, accelerating research to solve the issues. Currently, metal-free photocatalysts have gained the intensive attention of scientists due to their environmental-friendly nature and ease of preparation. It was noticed that g-C3N4 (GCN) consists of a few outstanding properties that could be used for various applications such as water treatment and clean energy production. Nonetheless, bare GCN contains several drawbacks such as high charge recombination, limited surface area, and low light sensitivity. Several solutions have been applied to overcome GCN limitations. Co-doping, tri-doping, and rare-earth-doping can be effective solutions to modify the GCN structure and improve its performance toward photocatalysis. This review highlights the function of multi-elemental and rare-earth dopants in GCN structure, mechanisms, and performance for photocatalytic applications as well as the advantages of co-doping, tri-doping, and rare-earth-doping of GCN. This review summarizes the different roles of dopants in addressing the limitations of GCN. Therefore, this article critically reviewed how multi-elemental and rare-earth-doping affect GCN properties and enhanced photoactivity for various applications.

Flexible construct of N vacancies and hydrophobic sites on g-C3N4 by F doping and their contribution to PFOA degradation in photocatalytic ozonation

N vacancies, hydrophobic sites and electron rich zone were simply regulated by doping F into g-C₃N₄ (CN) to accelerate photocatalytic ozonation of PFOA. Activity of F-CN was superior to that of CN, with 74.3% PFOA removal by F-CN/Vis/O₃ but only 57.1% by CN/Vis/O₃. Experimental results and theory simulations suggested that the photogenerated hole (h_vb⁺) oxidation with the help of N vacancies was vital for PFOA degradation. N vacancies on both CN and F-CN would trap O atom of PFOA and seize electron from α -CF₂ group, which made PFOA more easily to be oxidized. Doping of F narrowed band gap, lowered the valence band position and enhanced the oxidation potential of h_vb⁺. The hydrophobic sites would accelerate the mass transfer of O₃ and PFOA, enhance O₃'s single electron reduction with e_cb^- to generate hydroxyl radicals (•OH) and reduce the recombination of h_vb⁺-e_cb^-. Under the joint function of h_vb⁺, N vacancies and •OH, PFOA degradation in F-CN/Vis/O₃ proceeded through the gradually shortening of perfluoroalky chain and loss of CF₂ unit. The acute and chronic toxicity of generated short-chain perfluorocarboxylic acid toward fish, green algae daphnid were predicted by ECOSAR. And the toxicity change of solutions was examined by luminescent bacteria.

Enhanced photocatalytic nitrogen fixation in BiVO4: constructing oxygen vacancies and promoting electron transfer through Ohmic contact

The Ag nanoparticles deposited on the surface of BiVO4 containing oxygen vacancies are employed in photocatalytic N2 fixation. The NH3 generation rate is enhanced by constructing abundant oxygen vacancies and promoting electron transfer by Ohmic contact.