A simple synthesis route of sodium-doped g-C3N4 nanotubes with enhanced photocatalytic performance

Progress on enhancing the charge separation efficiency of carbon nitride for robust photocatalytic H2 production

Solar-driven H2 production from water splitting with efficient photocatalysts is a sustainable strategy to meet the clean energy demand and alleviate the approaching environmental issues caused by fossil fuel consumption. Among various semiconductor-based photocatalysts, graphitic carbon nitride (g-C3N4) has attracted much attention due to its advantages of long term-stability, visible light response, low cost, and easy preparation. However, the intrinsic Coulombic attraction between charge carriers and the interlayer electrostatic barrier of bulk g-C3N4 result in severe charge recombination and low charge separation efficiency. This perspective summarizes the recent progress in the development of g-C3N4 photocatalytic systems, and focuses on three main modification strategies for promoting charge transfer and minimizing charge recombination, including structural modulation, heterojunction construction, and cocatalyst loading. Based on this progress, we provide conclusions regarding the current challenges of further improving photocatalytic efficiency to fulfill commercial requirements, and propose some recommendations for the design of novel and satisfactory g-C3N4 photocatalysts, which is expected to progress the solar-to-hydrogen conversion.

Decade Milestone Advancement of Defect-Engineered g-C3N4 for Solar Catalytic Applications

Over the past decade, graphitic carbon nitride (g-C3N4) has emerged as a universal photocatalyst toward various sustainable carbo-neutral technologies. Despite solar applications discrepancy, g-C3N4 is still confronted with a general fatal issue of insufficient supply of thermodynamically active photocarriers due to its inferior solar harvesting ability and sluggish charge transfer dynamics. Fortunately, this could be significantly alleviated by the "all-in-one" defect engineering strategy, which enables a simultaneous amelioration of both textural uniqueness and intrinsic electronic band structures. To this end, we have summarized an unprecedently comprehensive discussion on defect controls including the vacancy/non-metallic dopant creation with optimized electronic band structure and electronic density, metallic doping with ultra-active coordinated environment (M-Nx, M-C2N2, M-O bonding), functional group grafting with optimized band structure, and promoted crystallinity with extended conjugation π system with weakened interlayered van der Waals interaction. Among them, the defect states induced by various defect types such as N vacancy, P/S/halogen dopants, and cyano group in boosting solar harvesting and accelerating photocarrier transfer have also been emphasized. More importantly, the shallow defect traps identified by femtosecond transient absorption spectra (fs-TAS) have also been highlighted. It is believed that this review would pave the way for future readers with a unique insight into a more precise defective g-C3N4 "customization", motivating more profound thinking and flourishing research outputs on g-C3N4-based photocatalysis.

Self-assembly synthesis of hollow phosphorus-doped graphitic carbon nitride microboxes for the photodegradation of organic pollutants

The rational design of photocatalysts with efficiency and stability is highly desirable but remains challenging. Here, we report a supramolecular self-assembly strategy to construct hollow phosphorus-doped g-C3N4 microboxes (PCNMs). Considering the effects of multiple parameters on the structure and activity of samples, the orthogonal design is innovatively introduced to optimize technology parameters for screening high-performance g-C3N4. Under visible light irradiation (λ ≥ 420 nm), rhodamine B (RhB, 4 mg L-1) is completely degraded in just 80 seconds in the presence of the optimal PCNM. The kinetic rate constant of RhB degradation with the PCNM is 3.4633 min-1, demonstrating unprecedented activity that is about 112 times higher than that of bulk g-C3N4 (0.0309 min-1) synthesized by direct polycondensation of melamine. Additionally, the optimal PCNM also shows enhanced degradation efficiency for tetracycline. The outstanding properties are primarily attributed to the hollow architecture, high specific surface area, and phosphorus doping. This work advances the design of photocatalysts correlating various factors, opening an avenue for optimizing photocatalytic synthesis and activity.

Fabrication of a Nanosized g-C3N4-Loaded Cellulose Microfiber Bundle to Induce Highly Efficient Water Treatment via Photodegradation

Graphite carbon nitride (g-C3N4) with a suitable structure and strong amine activity is designed and prepared to serve as a hydrogen bond donor for the microfibrilization of corncob cellulose to form a cellulose microfiber (CMF) bundle. Simultaneously, well-dispersed nanosized g-C3N4 is loaded into the bundle to form a photocatalyst for efficient photodegradation of rhodamine B (Rh B) in water. Under the optimal preparation conditions at 165 °C, 10 min, and 0.08 mol/L H2SO4, the yield of g-C3N4-functionalized cellulose microfibers (CMF-g-C3N4) reaches to the highest over 70%. The catalytic rate of CMF-g-C3N4 is 3.3 times larger than that of pure g-C3N4. The degradation rate of Rh B is maintained at over 90% in 10 cycles of photocatalytic degradation. The obtained CMF-g-C3N4 also has good thermal stability and mechanical properties. This research suggests a particularly simple way to transform cellulose into a highly efficient photocatalyst for water treatment.

Boron and Sodium Doping of Polymeric Carbon Nitride Photoanodes for Photoelectrochemical Water Splitting

Polymeric carbon nitride is a promising photoanode material for water-splitting and organic transformation-based photochemical cells. Despite achieving significant progress in performance, these materials still exhibit low photoactivity compared to inorganic photoanodic materials because of a moderate visible light response, poor charge separation, and slow oxidation kinetics. Here, the synthesis of a sodium- and boron-doped carbon nitride layer with excellent activity as a photoanode in a water-splitting photoelectrochemical cell is reported. The new synthesis consists of the direct growth of carbon nitride (CN) monomers from a hot precursor solution, enabling control over the monomer-to-dopant ratio, thus determining the final CN properties. The introduction of Na and B as dopants results in a dense CN layer with a packed morphology, better charge separation thanks to the in situ formation of an electron density gradient, and an extended visible light response up to 550 nm. The optimized photoanode exhibits state-of-the-art performance: photocurrent densities with and without a hole scavenger of about 1.5 and 0.9 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE), and maximal external quantum efficiencies of 56% and 24%, respectively, alongside an onset potential of 0.3 V.

Preparation of Ag3PO4/CoWO4 S-scheme heterojunction and study on sonocatalytic degradation of tetracycline

In this study, 0.6Ag3PO4/CoWO4 composites were synthesized by hydrothermal method. The prepared materials were systematically characterized by techniques of scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), N2 adsorption/desorption, and UV-vis diffuse reflectance spectrum (DRS). Furthermore, the sonocatalytic degradation performance of 0.6Ag3PO4/CoWO4 composites towards tetracycline (TC) was investigated under ultrasonic radiation. The results showed that, combined with potassium persulfate (K2S2O8), the 0.6Ag3PO4/CoWO4 composites achieved a high sonocatalytic degradation efficiency of 97.89 % within 10 min, which was much better than bare Ag3PO4 or CoWO4. By measuring the electrochemical properties, it was proposed that the degradation mechanism of 0.6Ag3PO4/CoWO4 is the formation of S-scheme heterojunction, which increases the separation efficiency of electron-hole pairs (e--h+) and generates more electrons and holes, thereby enhancing the degradation activity. The scavenger experiments confirmed that hole (h+) was the primary active substance in degrading TC, and free radicals (OH) and superoxide anion radical (O2-) were auxiliary active substances. The results indicated that 0.6Ag3PO4/CoWO4 nanocomposites could be used as an efficient and reliable sonocatalyst for wastewater treatment.

Construction and performance of a simple and efficient g-C3N4 photocatalytic hydrogen production system

Surface and bulk structure modification is an effective strategy to improve the photocatalytic performance of g-C3N4 (CN). In this work, dilute NaOH solution was used in situ to regulate the CN structure for enhanced photocatalytic hydrogen evolution reaction (HER). Characterization results indicate that after treatment with dilute NaOH solution, the surface of CN was hydroxylated, resulting in the change of CN structure and the increase of BET specific surface area. Furthermore, some Na+ ions can be intercalated into the framework of CN, and form the Na-N bond. These modifications boost the HER activity of CN. The test carried out in 7.5 mM NaOH solution shows the highest activity and it is almost 3.7 times higher than that performed in water. Control tests indicate that hydroxides of other alkali and alkali earth metals such as LiOH, KOH, Ca(OH)2, and Ba(OH)2 have similar promotion effects. This work demonstrates a valid and simple way to enhance the HER activity of CN through performing the reaction in a weakly alkaline solution.