Graphitic carbon nitride with thermally-induced nitrogen defects: an efficient process to enhance photocatalytic H2 production performance

A Novel Non-Metallic Photocatalyst: Phosphorus-Doped Sulfur Quantum Dots

In this paper, a novel phosphorus-doped sulfur quantum dots (P-SQDs) material was prepared using a simple hydrothermal method. P-SQDs have a narrow particle size distribution as well as an excellent electron transfer rate and optical properties. Compositing P-SQDs with graphitic carbon nitride (g-C₃N₄) can be used for photocatalytic degradation of organic dyes under visible light. More active sites, a narrower band gap, and stronger photocurrent are obtained after introducing P-SQDs into g-C₃N₄, thus promoting its photocatalytic efficiency by as much as 3.9 times. The excellent photocatalytic activity and reusability of P-SQDs/g-C₃N₄ are prospective signs of its photocatalytic application under visible light.

Generation Mechanism of the Defects in g-C3N4 Synthesized in N2 Atmosphere and the Method for Improving Photocatalysis Activity

One of the most important methods for modifying semiconductors is defect engineering, but only the right quantity of defects in the right chemical environment can produce desirable results. Heat treatment processes associated with g-C3N4 are occasionally carried out in N2 atmosphere, however, the catalytic performance of g-C3N4 produced by direct condensation of only nitrogen-rich precursors in N2 atmosphere is often unsatisfactory. This is typically attributed to the introduction of numerous defects, but the actual relationship between the formation of defects and the N2 atmosphere is rarely explained, and the resulting quantity of defects is difficult to control. We propose that the melam to melem transition is restricted due to the lack of O2 during the heat treatment of the nitrogen-rich precursor of g-C3N4 in N2 atmosphere, which leads to a substantial quantity of defects in the synthesized g-C3N4. To enhance its photocatalytic property, we propose a method to reduce the quantity of defects due to calcinating in N2 atmosphere by protonating the precursor in a way that increases the polymerization of the product. The test analysis indicated that only a moderate quantity of defects that contribute to electron excitation and enhance the separation efficiency and density of photogenerated carriers were retained, and the hydrogen evolution performance of the prepared catalyst was significantly improved.

The effect of N-vacancy on the photocatalytic activity of graphitic carbon nitride in the oxidative Mannich reaction

N-vacancy g-CN was used in Mannich oxidative reaction as a photocatalyst, having mid-gap states that enhance reaction kinetics. This facile photocatalyst enabled successful formation of challenging THIQ with EWG and chemo-selectivity on C–C bond.

Soluble carbon nitride nanosheets as an alternate precursor for hard-templated morphological control

Carbon nitride (CN_x) is an organic semiconductor with promising applications in solar energy conversion via photocatalytic water splitting. Current efforts are being devoted to improve the photocatalytic activity of CN_x as it is limited by charge recombination which may be exacerbated by its low surface area. Hard-templated synthesis of CN_x by filling mesoporous silica with small molecule precursors is one of the conventional approaches to improving the efficiency of CN_x. However the relationship between activity and surface area has yet to be fully established. Herein we develop a new approach using soluble acidified CN_x nanosheets as the precursor in an effort to decrease the potential for unintended chemical modifications and better understand the complex relationship between morphology and activity. Deprotonation of acidified CN_x occurs at moderate temperatures to restore the π-π interactions. We prepared three modified morphologies with this synthetic route and completed a thorough study of the structural, optical, and photocatalytic properties. Supported by charge carrier dynamics studies, we found that the silica-templated CN_x with modified morphologies suffered from higher trap state densities and resulted in lower photocatalytic activity compared to CN_x prepared without a template. Characterization techniques showed that the chemical structure of the templated CN_x obtained is not sensitive to changes in the silica template shape. Our observations highlight the complex relationship between structure, photophysics, and activity, and demonstrate that hard templating can modify more than the intended surface area.

Synthesis of Vitamin B3 through a Heterogeneous Photocatalytic Approach Using Metal-Free Carbon Nitride-Based Catalysts

Vitamin B3 (nicotinic acid, VB3) was synthesized through the photocatalytic oxidation of 3-pyridinemethanol (3PM) under visible-light-emitting diode (LED) irradiation using metal-free graphitic carbon nitride (GCN) - based materials. A bulk (GCN) material was prepared by a simple thermal treatment using dicyandiamide as the precursor. A post-thermal treatment under static air and nitrogen flow was employed to obtain the GCN-T and GCN-T-N materials, respectively. The conditions adopted during the post-treatment revealed differences in the resulting materials' morphological, electronic, and optical properties. The post-treated photocatalysts revealed an enhanced efficiency in the oxidation of 3PM into VB3, with the GCN-T-N photocatalyst being the best-performing material. The defective surface, reduced crystallinity, and superior photoabsorption of GCN-T-N account for this material's improved performance in the production of VB3. Nevertheless, the presence of nitrogen vacancies in the carbon nitride structure and, consequently, the creation of mid-gap states also accounts to its highly oxidative ability. The immobilization of GCN-T-N in sodium alginate hydrogel was revealed as a promising strategy to produce VB3, avoiding the need for the photocatalyst separation step. Concerning the mechanism of synthesis of VB3 through the photocatalytic oxidation of 3PM, it was possible to identify the presence of 3-pyridinecarboxaldehyde (3PC) as the intermediary product.

Catalytic conversion of seawater to fuels: Eliminating N vacancies in g-C3N4 to promote photocatalytic hydrogen production

The use of solar energy to decompose seawater and produce hydrogen is of great significance in solving the energy crisis. Numerous studies have shown that vacancies can significantly improve photocatalytic activity due to their electron-rich nature. However, our recent research has shown that materials with vacancies are not suitable for photocatalytic reactions in seawater. In this study, g-C₃N₄ with rich N vacancies was selected as the research object, and urea was used as the precursor; in this system, the N vacancies in g-C₃N₄ could be effectively reduced by the addition of ZIF-8 (ZCNQx). The activity of ZCNQ40 was 5.6 times higher than that of g-C₃N₄ in fresh seawater, but only 3.1 times higher in freshwater. Based on the analysis of the experimental results, we believe that g-C₃N₄ has a limiting relationship between H+ adsorption catalysis and H₂ product desorption. In addition, seawater contains many heteroatoms that will also compete with proton (H+) reduction. The results of our study show that catalysts with vacancies are not necessarily suitable for catalytic reactions in seawater media. This research will stimulate new ideas for research into the conversion of solar energy to chemical energy in seawater media.

Floating Photocatalysts for Effluent Refinement Based on Stable Pickering Cellulose Foams and Graphitic Carbon Nitride (g-C3N4)

The transfer of heterogeneous photocatalysis applications from the laboratory to real-life aqueous systems is challenging due to the higher density of photocatalysts compared to water, light attenuation effects in water, complicated recovery protocols, and metal pollution from metal-based photocatalysts. In this work, we overcome these obstacles by developing a buoyant Pickering photocatalyst carrier based on green cellulose nanofibers (CNFs) derived from wood. The air bubbles in the carrier were stable because the particle surfactants provided thermodynamic stability and the derived photocatalytic foams floated on water throughout the test period (4 weeks). A metal-free semiconductor photocatalyst, g-C3N4, was facilely embedded inside the foam by mixing the photocatalyst with the air-bubble suspension followed by casting and drying to produce solid foams. When tested under mild irradiation conditions (visible light, low energy LEDs) and no agitation, almost three times more dye was removed after 6 h for the floating g-C3N4-CNF nanocomposite foam, compared to the pure g-C3N4 powder residing on the bottom of a ca. 2 cm-high water pillar. The buoyancy and physicochemical properties of the carrier material were imperative to render escalated oxygenation, high photon utilization, and faster dye degradation. The reported assembly protocol is facile, general, and provides a new strategy for assembling green floating foams that can potentially carry a number of different photocatalysts.