Biomass-Induced Diphasic Carbon Decoration for Carbon Nitride: Band and Electronic Engineering Targeting Efficient N2 Photofixation

Modulation of π-Electron Density in Ultrathin 2D Layers of Graphite Carbon Nitride for Efficient Photocatalytic Hydrogen Production

The rational design and synthesis of novel semiconductor nano-/quantum materials have been ambitiously pursued in the field of photocatalysis as the technology is promising and critical for attaining future energy and environmental sustainability. Herein, the integrity of aromatic carbon into graphitic carbon nitride (CN) at the same molecular plane with a few 2D layers is achieved by using modulated precursors of CN, forming carbon regulated ultrathin CN (CUCN) with improved charge transfer kinetics and photocatalytic hydrogen production. The grafted graphite rings adjacent to carbon nitride frameworks induce a significant rearrangement and relocalization of the overall framework, and form conjugated sp2 hybridized interfaces and internal electric fields that drive the separation and directional transfer of photogenerated electrons from CN sheets towards intralayer graphite regions, where the photocatalytic hydrogen evolution reaction occurs extensively, yielding largely increased HER rate of 2231.8 µmol g-1 h-1 by 8.2 times relative to CN, as well as a remarkable apparent quantum yield of 2.93% under monochromatic light at 420 nm. The high physicochemical stability and low synthesis cost of CUCN make it a potential benchmark photocatalyst that can be readily modified via element doping, heterojunction introduction, defect engineering, and so on, to further enhance its HER performance.

Electron repulsion tuned electronic structure of TiO2 by fluorination for efficient and selective photocatalytic ammonia generation

The photocatalytic conversion of nitrogen into high-value ammonia products holds tremendous potential in the global nitrogen cycle. However, the activation of N2 and competition of hydrogen evolution limit the improvement of nitrogen fixation performance. In this study, we developed a fluorinated TiO2 (F-TiO2) using a hydrothermal-annealing method. The incorporation of F dopants not only enhances the adsorption and activation of N2 through electronic structure regulation, but also facilitates an in situ increase in active sites via the electron repulsion effect between F and Ti atoms. In addition, the presence of F on the surface effectively improved the nitrogen supply problem and optimized the nitrogen fixation selectivity for its hydrophobic modulation. The NH3 yield of the F-TiO2 photocatalyst reached 63.8 μmol h-1 g-1, which was 8.5 times higher than that of pure TiO2. And the selectivity experiment showed that the electronic ratio of NH3 to H2 production reached 0.890. This research offers valuable insights for the design of highly efficient and selective nitrogen-fixing photocatalysts.

Unraveling the Synergistic Mechanism of Boosted Photocatalytic H2O2 Production over Cyano-g-C3N4/In2S3/Ppy Heterostructure and Enhanced Photocatalysis-Self-Fenton Degradation Performance

In this work, cyano contained g-C3N4 comodified by In2S3 and polypyrrole (C≡N─CN/IS/Ppy) materials are synthesized for the photocatalytic production of H2O2 and photocatalysis-self-Fenton reaction for highly efficient degradation of metronidazole. The results from UV-vis spectrophotometry, surface photovoltage, and Kelvin probe measurements reveal the promoted transport and separation efficiency of photoinduced charges after the introduction of In2S3 and Ppy in the heterojunction. The existence of a built-in electric field accelerates the photoinduced charge separation and preserves the stronger oxidation ability of holes at the valence band of C≡N─CN. Linear sweep voltammetry measurements, zeta potential analyzations, nitroblue tetrazolium determination, and other measurements show that Ppy improves the conversion ratio of •O2 - to H2O2 and the utilization ratio of •O2 -, as well as suppresses decomposition of H2O2. Accordingly, the H2O2 evolution rate produced via a two-step single-electron reduction reaction reaches almost 895 µmol L-1 h-1, a value 80% and 7.2-fold higher than those obtained with C≡N─CN/IS and C≡N─CN, respectively. The metronidazole removal rate obtained via photocatalysis-self-Fenton reaction attains 83.7% within 120 minutes, a value much higher than that recorded by the traditional Fenton method. Overall, the proposed synthesis materials and route look promising for the H2O2 production and organic pollutants degradation.

Preparation of Structure Vacancy Defect Modified Diatomic-Layered g-C3 N4 Nanosheet with Enhanced Photocatalytic Performance

Structure self-modification of graphitic carbon nitride (g-C3 N4 ) without the assistance of other species has attracted considerable attention. In this study, the structure vacancy defect modified diatomic-layered g-C3 N4 nanosheet (VCN) is synthesized by thermal treatment of bulk g-C3 N4 in a quartz tube with vacuum atmosphere that will generate a pressure-thermal dual driving force to boost the exfoliation and formation of structure vacancy for g-C3 N4 . The as-prepared VCN possesses a large specific surface area with a rich pore structure to provide more active centers for catalytic reactions. Furthermore, the as-formed special defect level in VCN sample can generate a higher exciton density at photoexcitation stage. Meanwhile, the photogenerated charges will rapidly transfer to VCN surface due to the greatly shortened transfer path resulting from the ultrathin structure (≈1.5 nm), which corresponds to two graphite carbon nitride atomic layers. In addition, the defect level alleviates the drawback of enlarged bandgap caused by the quantum size effect of nano-scaled g-C3 N4 , resulting in a well visible-light utilization. As a result, the VCN sample exhibits an excellent photocatalytic performance both in hydrogen production and photodegradation of typical antibiotics.

Photocatalytic Synthesis of Ammonia from Pinecone Graphite-Phase Carbon Nitride Loaded with MoS2 Nanosheets as Co-catalysts

Photocatalytic nitrogen fixation is a promising alternative to the Haber-Bosch process to alleviate the energy and environmental crises. Here, we designed a pinecone-shaped graphite-phase carbon nitride (PCN) catalyst supported with MoS₂ nanosheets by a supramolecular self-assembly method. The catalyst shows an excellent photocatalytic nitrogen reduction reaction (PNRR) due to the larger specific surface area and the enhancement of visible light owing to the reduced band gap. Under simulated sunlight, the sample of PCN loaded with 5 wt % MoS₂ nanosheets (MS5%/PCN) shows a PNRR efficiency of 279.41 μmol g^-1 h^-1, which is 14.9 times that of bulk graphite-phase carbon nitride (g-C₃N₄), 4.6 times that of PCN, and 5.4 times that of MoS₂, respectively. The unique pinecone-like structure of MS5%/PCN not only improves the ability of light absorption but also assists in the uniform loading of MoS₂ nanosheets. Likewise, the existence of MoS₂ nanosheets improves the light absorption ability of the catalyst and reduces the impedance of the catalyst. Furthermore, as a co-catalyst, MoS₂ nanosheets can efficiently adsorb nitrogen (N₂) and serve as active N₂ reduction sites. From the perspective of structural design, this work can offer novel solutions for the creation of effective N₂-fixing photocatalysts.

Surface-assisted synthesis of biomass carbon-decorated polymer carbon nitride for efficient visible light photocatalytic hydrogen evolution

Template is frequently studied as a structure-directing agent to tune the nanomorphology of photocatalysts. However, the influences of template on the polymerization of precursors and compositions of the resulting samples are rarely considered. Herein, a biomass carbon-modified graphitic carbon nitride (CCNx) with a thin-layer morphology is synthesized via one-pot surface-assisted polymerization of melamine precursor on organic yeast. The formation of the hydrogen bond between melamine and yeast induces a strong interfacial confinement, giving rise to small-sized CCNx. In addition, the carbon materials derived from yeast dramatically broaden n → π* visible light harvesting, improve electron delocalization, and greatly enhance charge carrier separation. The optimized CCNx presents a much higher photocatalytic hydrogen production rate of 2704 μmol g^-1h^-1 under visible light irradiation (λ ≥ 420 nm), which is nearly 11-fold that of its pristine counterpart. This work realizes the synergistic effect between morphology tunning and composition tailoring by using biomass template, which shows a great potential in developing efficient metal-free photocatalysts.

Interspersed Bi Promoting Hot Electron Transfer of Covalent Organic Frameworks Boosts Nitrogen Reduction to ammonia

Seeking highly-efficient, non-pollutant, and chemically robust photocatalysts for visible-light-driven ammonia production still remained challenging, especially in pure water. The key bottle-necks closely correlate to the nitrogen activation, water oxidization, and hydrogen evolution reaction (HER) processes. In this study, a novel Bi decorated imine-linked COF-TaTp (Bi/COF-TaTp) through N-Bi-O coordination is reasonably designed to achieve a boosting solar-to-ammonia conversion of 61 µmol^-1  g^-1  h^-1 in the sacrificial-free system. On basis of serial characterizations and DFT calculations, the incorporated Bi is conducive to the acceleration of charge carriers transfer and N₂ activation through the donation and back-donation mode. The N₂ adsorption energy of 5% Bi/COF-TaTp is calculated to be -0.19 eV in comparison with -0.09 eV of the pure COF-TaTp and the electron exchange between N₂ and the modified catalyst is much more intensive. Moreover, the accompanied hydrogen production process is effectively inhibited by Bi modification, demonstrated by the higher energy barrier for HER over Bi/COF-TaTp (2.62 eV) than the pure COF-TaTp (2.31 eV) when using H binding free energy (ΔG_H* ) as a descriptor. This work supplies novel insights for the design of photocatalysts for N₂ reduction and intensifies the understanding of N₂ adsorption and activation over covalent organic frameworks-based materials.

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.

Recent Advances in Application of Graphitic Carbon Nitride-Based Catalysts for Photocatalytic Nitrogen Fixation

Ammonia, the second most-produced chemical, is widely used in agricultural and industrial applications. However, traditional industrial ammonia production dominated by the Haber-Bosch process presents huge resource and environment issues due to the massive energy consumption and CO₂ emission. The newly emerged nitrogen fixation technology, photocatalytic N₂ reduction reaction (p-NRR), uses clean solar energy with zero-emission, holding great prospect to achieve sustainable ammonia synthesis. Although great efforts are made, the p-NRR catalysts still suffer from poor N₂ adsorption and activation, inferior light absorption, and fast recombination of photocarriers. Due to the tunable electronic structure of the metal-free polymeric graphitic carbon nitride (g-C₃ N₄ ), the above-mentioned issues can be significantly alleviated, making it the most promising p-NRR photocatalyst. This review summarizes the recent development of g-C₃ N₄ -based catalysts for p-NRR, including the working principle of p-NRR catalysts, the challenges of developing p-NRR catalysts, and corresponding solutions. Particularly, the roles of defect engineering and heterojunction construction on g-C₃ N₄ to the enhancement of photocatalytic performances are emphasized. In addition, computational studies are introduced to deepen the understanding of reaction pathways. At last, perspectives are provided on the development of p-NRR catalysts.