In site acid template induced facile synthesis of porous graphitic carbon nitride with enhanced visible-light photocatalytic activity

The synergy of adsorption and photosensitization of platinum-doped graphitic carbon nitride for improved removal of rhodamine B

Graphitic carbon nitride (g-C₃N₄) has attracted growing attention recently for photodegradation of pollutants. However, the photosensitization performance of g-C₃N₄ was limited by insufficient generation efficiency of reactive oxygen species (ROS) and weak light absorption. In this study, platinum (Pt)-doped g-C₃N₄ photocatalyst was synthesized by thermal polycondensation using dicyandiamide and chloroplatinic acid. The structure and composition of Pt-doped g-C₃N₄ were tested by scanning electron microscope (SEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma-mass spectrometry (ICP-MS), which indicated that the Pt-doped g-C₃N₄ was successfully prepared. Compared with bare g-C₃N₄, Pt²⁺-doped g-C₃N₄ has wider light absorption range, lower band gap, and higher photon-generated carrier migration efficiency, which significantly improved the light absorption range and photosensitization efficiency of Pt²⁺-doped g-C₃N₄, while photodegradation efficiency for Rhodamine B (RhB) increased from 50 to 90%. The effecting factors of adsorption and photocatalytic degradation performance of Pt²⁺-doped g-C₃N₄ for RhB were investigated in detail. The adsorption is a monolayer adsorption process that fits the Langmuir model, as well as being a spontaneous endothermic process. Using a white LED as an excitation source, electrons and holes in Pt²⁺-doped g-C₃N₄ were generated. The electrons reacting with dissolved oxygen produce active oxygen species such as •OH and ¹O₂, which can degrade RhB on the surface of Pt²⁺-doped g-C₃N₄. The photocatalytic method has the advantages of simple operation, low cost, and high efficiency, and has the potential to directly remove dyes in wastewater utilizing sunlight.

Insight into the role of charge carrier mediation zone for singlet oxygen production over rod-shape graphitic carbon nitride: Batch and continuous-flow reactor

As a new approach of creating the photo-exited electron (e^-) and hole (h⁺) mediation zone for highly selective singlet oxygen (¹O₂) production, the rod-type graphitic carbon nitride (NCN) has been synthesized from the nitric acid-modified melamine followed by the calcination. The NCN exhibited a higher surface area and surface oxygen adsorption ability than bulk graphitic carbon nitride (BCN). The increment of CO and NH_x groups on NCN corresponded to e^- and h⁺ mediation groups, respectively, resulting in higher production of ¹O₂ than BCN. Moreover, those mediation groups on NCN result in higher recombination efficiency and longer e^- decay time. As a result, the optimized NCN-0.5 (derived from 0.5 M of nitric acid-modified melamine) displayed 5.8 times higher kinetic rate constant of atrazine (ATZ) removal under UVA-LED irradiation compared to BCN. This study also evaluated the ATZ degradation pathways and toxicity effect of by-products. In addition, continuous flow experiments using NCN-0.5 showed superior ATZ removal performance with a hybrid concept between a slurry photocatalysis and a continuous stirred tank reactor system using actual effluent obtained from a wastewater treatment plant. Thus, this work provides an insight into the strategy for highly selective ¹O₂ production and the potential for water purification application.

Graphitic carbon nitride nanosheets via acid pretreatments for promoted photocatalysis toward degradation of organic pollutants

Acid treatment serves as an effective engineering strategy to modify the structure of graphitic carbon nitride (g-C₃N₄) for enhanced metal-free photocatalysis, while their lacks a comprehensive understanding about the impacts of different acid species and acid treatment approaches on the intrinsic structure and properties of g-C₃N₄ and structure-activity relationships are ambiguous. Employing inorganic/organic acids including hydrochloric acid (HCl), nitric acid (HNO₃), acetic acid (HAc), sulphuric acid (H₂SO₄), or oxalic acid (H₂C₂O₄) as treatment acids, herein, we compare the impacts of different acid pretreatment approaches on the structure and properties of g-C₃N₄. Due to different acid-melamine interaction modes and the activation roles of various acids, the obtained g-C₃N₄ samples exhibit varied structures, physiochemical properties and photocatalytic activities. Compared with bulk graphitic carbon nitride (BCN), g-C₃N₄ prepared by acid pretreatment show enhanced photocatalytic performance on bisphenol A (BPA) degradation. The photocatalytic degradation rates of BPA by g-C₃N₄ prepared by HNO₃, HAc, H₂SO₄, H₂C₂O₄, or HCl pretreatment are about 2.2, 2.7, 2.8, 3.2 and 3.8 folds faster than that by BCN. HCl pretreatment proves to be the optimal approach, with the derived g-C₃N₄ (HTCN) showing more intact heptazine structural units, and increased specific surface area, which promote the exposure of more active sites, accelerate charge transfer, and give rise to a notable improvement in photocatalysis, eventually. Mechanistic investigations through quenching experiments and electron paramagnetic resonance (EPR) characterization unveil that superoxide ion radical (O₂^-) and photo-induced holes (h⁺) worked principally in the photodegradation reaction. This work provides new insights for the rational selection of acid types and treatment methods to synthesize metal-free carbon nitrides with improved activity for photocatalytic applications.

Porous graphitic carbon nitride for solar photocatalytic applications

Photocatalysis is attracting increased attention in solving the energy crisis and environmental pollution. Graphitic carbon nitride (g-C3N4), a non-metal photocatalyst, has been regarded as an ideal photocatalyst to solve these problems because of its chemical stability and unique optical properties. However, traditional g-C3N4 exhibits moderate photocatalytic activity due to its low specific surface area and fast recombination rate of photogenerated electrons. Among the many modified g-C3N4 materials, porous carbon nitride (PCN) can solve the shortcomings of traditional g-C3N4 because of PCN's increased number of surface-active sites, specific surface area, light harvesting, diffusion and adsorption/activation. However, a frontier, comprehensive summary of the development of PCN is less reported. Thus, a review on recent developments in PCN research is urgently needed to further promote its advancement. In this review, the synthesis methods, structures and properties and photocatalytic applications of PCN photocatalysts are described in detail. The current challenges and future development of PCN/PCN-based photocatalysts are discussed. This review may present an up-to-date view of the PCN development to provide an in-depth understanding of PCN-based photocatalysts.

Enhancement of photocatalytic activity of g-C3N4 by hydrochloric acid treatment of melamine

Modified g-C₃N₄ samples (g-X, where X corresponds to the number of hours of acid treatment of the melamine) with outstanding photocatalytic performance were prepared by using hydrochloric acid-treated melamine as a precursor and calcining at 550 °C for 2 h. An x-ray diffractometer, field-emission scanning electron microscope, infrared spectrometer, N₂ adsorption-desorption test, x-ray photoelectron spectroscopy, and ultraviolet-visible diffuse-reflectance spectroscopy analysis were carried out to characterize the phase composition, microstructure, chemical structure, specific surface area (SSA), chemical states, elemental composition and optical properties of the samples, respectively. The photocatalytic performance of the samples was evaluated by degrading the Rhodamine B (RhB) aqueous solution. The results showed that the crystal structure and vibration bands of melamine changed due to the reaction with hydrochloric acid. The crystallinity and grain size of g-C₃N₄ in g-X (X = 1, 2, 4, 6, 8, 10) reduced, and the SSA values of g-X increased compared to that of the g-0 sample, which was synthesized from pristine melamine. The g-X samples exhibited excellent photocatalytic activity towards degradation of RhB compared to g-0. The photocatalytic activity of the g-X samples increased gradually as the acid treatment time of the melamine increased from 1 h to 2 h, and then decreased gradually with the extension of the acid treatment time. The rate constant (k) values of g-X are higher than that of g-0. g-2 presented the highest rate constant (k = 0.052 min^-1), which was 5.5 times higher than that of g-0. The improved photocatalytic activity of the g-X samples was attributed to the higher SSA value, the appearance of surface defects, the outstanding photo-carrier separation efficiency and stronger light harvesting ability of g-X, with the last two factors being more significant. Acid treatment of melamine is helpful in the preparation of high performance g-C₃N₄ photocatalyst, and the microstructure and photocatalytic performance of g-C₃N₄ were affected significantly by the acid treatment time.

The facile synthesis and enhanced photocatalytic activity of a graphitic carbon nitride isotype heterojunction with ordered mesopores

In order to improve the photocatalytic activity of graphitic carbon nitride, we prepared a g-C₃N₄ isotype heterojunction with ordered mesopores through simple one-step calcination.

Graphitic carbon nitride based nanocomposites for the photocatalysis of organic contaminants under visible irradiation: Progress, limitations and future directions

Graphitic carbon nitride (g-C₃N₄) has drawn great attention recently because of its visible light response, suitable energy band gap, good redox ability, and metal-free nature. g-C₃N₄ can absorb visible light directly, therefore has better photocatalytic ability under solar irradiation and is more energy-efficient than TiO₂. However, pure g-C₃N₄ still has the drawbacks of insufficient light absorption, small surface area and fast recombination of photogenerated electron and hole pairs. This review summarizes the recent progress in the development of g-C₃N₄ nanocomposites to photodegrade organic contaminants in water. Element doping especially by potassium has been reported to be an efficient method to promote the degradation efficacy. In addition, compound doping improves photodegradation performance of g-C₃N₄, especially Ag₃PO₄-g-C₃N₄ which can completely degrade 10mgL^-1 of methyl orange under visible light irradiation in 5min, with the rate constant (k) as high as 0.236min^-1. Moreover, co-doping enhances the photodegradation rate of multiple contaminants while immobilization significantly improves catalyst stability. Most of g-C₃N₄ composites possess high reusability enabling their practical applications in wastewater treatment. Furthermore, environmental conditions such as solution pH, reaction temperature, dissolved oxygen, and dissolved organic matter all have important effects on the photocatalytic ability of g-C₃N₄ photocatalyst. Future work should focus on the synthesis of innovative g-C₃N₄ nanocomposites for the efficient removal of organic contaminants in water and wastewater.

Fabrication of Ag3PO4/GO/NiFe2O4 composites with highly efficient and stable visible-light-driven photocatalytic degradation of rhodamine B

Effective visible-light-driven Ag3PO4/GO/NiFe2O4 Z-scheme magnetic composites were successfully fabricated by a simple ion-exchange deposition method. The Ag3PO4/GO/NiFe2O4 (8%) composite exhibited excellent photocatalytic activity (degradation efficiency was ∼96% within 15 min and kinetic constant reached 0.1956 min-1) and stability when compared to Ag3PO4, NiFe2O4, and Ag3PO4/NiFe2O4 for rhodamine B (RhB) degradation. Furthermore, by electrochemical and fluorescence measurements, the Ag3PO4/GO/NiFe2O4 (8%) material also showed larger transient photocurrent, lower impedance, and longer fluorescence lifetime (7.82 ns). Comparing the activity result dependence with characterization results, it was indicated that photocatalytic activity depended on fast charge transfer from Ag3PO4 to NiFe2O4 through GO sheet. The h+ and ·O2 - species played important roles in RhB degradation under visible-light. A possible Z-scheme mechanism is proposed over the Ag3PO4/GO/NiFe2O4 (8%) composite. This study might provide a promising visible light responsive photocatalyst for the photocatalytic degradation of organic dyes in wastewater.

Study on the Visible-Light Photocatalytic Performance and Degradation Mechanism of Diclofenac Sodium under the System of Hetero-Structural CuBi₂O₄/Ag₃PO₄ with H₂O₂

Two kinds of CuBi₂O₄/Ag₃PO₄ with different heterojunction structures were prepared based on the combination of hydrothermal and in-situ precipitation methods with surfactant additives (sodium citrate and sodium stearate), and their characteristics were systematically resolved by X-ray Diffraction (XRD), Brunauer–Emmett–Teller (BET), X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscope (SEM)/ High-resolution Transmission Electron Microscopy (HRTEM), UV-vis Diffuse Reflectance Spectra (DRS) and Photoluminescence (PL). Meanwhile, the photocatalytic properties of the catalysts were determined for diclofenac sodium (DS) degradation and the photocatalytic mechanism was also explored. The results indicate that both of the two kinds of CuBi₂O₄/Ag₃PO₄ exhibit higher photocatalytic efficiency, mineralization rate, and stability than that of pure CuBi₂O₄ or Ag₃PO₄. Moreover, the catalytic activity of CuBi₂O₄/Ag₃PO₄ can be further enhanced by adding H₂O₂. The free radical capture experiments show that in the pure CuBi₂O₄/Ag₃PO₄ photocatalytic system, the OH^• and O₂^•− are the main species participating in DS degradation; however, in the CuBi₂O₄/Ag₃PO₄ photocatalytic system with H₂O₂, all OH^•, h⁺, and O₂^•− take part in the DS degradation, and the contribution order is OH^• > h⁺ > O₂^•−. Accordingly, the photocatalytic mechanism of CuBi₂O₄/Ag₃PO₄ could be explained by the Z-Scheme theory, while the catalysis of CuBi₂O₄/Ag₃PO₄ with H₂O₂ follows the heterojunction energy band theory.