Metal-free g-C3N4 Photocatalysts Research Articles

Recent Advances in g-C3N4-Based Photocatalysts for NOx Removal

Nitrogen oxides (NOx) pollutants can cause a series of environmental issues, such as acid rain, ground-level ozone pollution, photochemical smog and global warming. Photocatalysis is supposed to be a promising technology to solve NOx pollution. Graphitic carbon nitride (g-C3N4) as a metal-free photocatalyst has attracted much attention since 2009. However, the pristine g-C3N4 suffers from poor response to visible light, rapid charge carrier recombination, small specific surface areas and few active sites, which results in deficient solar light efficiency and unsatisfactory photocatalytic performance. In this review, we summarize and highlight the recent advances in g-C3N4-based photocatalysts for photocatalytic NOx removal. Firstly, we attempt to elucidate the mechanism of the photocatalytic NOx removal process and introduce the metal-free g-C3N4 photocatalyst. Then, different kinds of modification strategies to enhance the photocatalytic NOx removal performance of g-C3N4-based photocatalysts are summarized and discussed in detail. Finally, we propose the significant challenges and future research topics on g-C3N4-based photocatalysts for photocatalytic NOx removal, which should be further investigated and resolved in this interesting research field.

Hydrophilic bi-functional B-doped g-C3N4 hierarchical architecture for excellent photocatalytic H2O2 production and photoelectrochemical water splitting

Graphitic carbon nitride (g-C3N4) has attracted great interest in photocatalysis and photoelectrocatalysis. However, their poor hydrophilicity poses a great challenge for their applications in aqueous environment. Here, we demonstrate synthesis of a hydrophilic bi-functional hierarchical architecture by the assembly of B-doped g-C3N4 nanoplatelets. Such hierarchical B-doped g-C3N4 material enables full utilization of their highly enhanced visible light absorption and photogenerated carrier separation in aqueous medium, leading to an excellent photocatalytic H2O2 production rate of 4240.3 μM g−1 h−1, 2.84, 2.64 and 2.13 times higher than that of the bulk g-C3N4, g-C3N4 nanoplatelets and bulk B doped g-C3N4, respectively. Photoanodes based on these hierarchical architectures can generate an unprecedented photocurrent density of 1.72 mA cm−2 at 1.23 V under AM 1.5 G illumination for photoelectrochemical water splitting. This work makes a fundamental improvement towards large-scale exploitation of highly active, hydrophilic and stable metal-free g-C3N4 photocatalysts for various practical applications.

Fabrication, characterization of O doped g-C3N4 materials via a green ascorbic acid-assisted calcination route

Using melamine as raw material, metal-free g-C3N4 photocatalyst was synthesized by thermal polymerization method. After grinding the produced g-C3N4 and ascorbic acid (Vitamin C, Vc), the mixture was calcined at 300 °C to obtain rich O doped g-C3N4 (O-g-C3N4) materials. Some physical chemistry technologies, e.g. XRD, TG-DSC, FT-IR, Raman, XPS, SEM, TEM, BET, UV–vis DRS, PL, and photoelectrochemical test were used to characterize the catalyst. The results show that O-g-C3N4 possesses high O content (~10 wt%) and displays no obvious morphological change compared to g-C3N4. However, after O doping, its band gap energy was significantly reduced and its visible light absorption ability was largely boosted. In addition, the CO bond generated by O doping can act as a strong electron trap centers and effectively promote the separation of photo-generated electrons and holes pair. The prepared O-g-C3N4 displayed high performance for aqueous Cr(VI) removal. Under xenon lamp illumination for 150 min, over 90%Cr(VI) can be reduced over O-g-C3N4. Moreover, in Cr(VI) removal, O-g-C3N4 appeared high photocatalytic stability and high compatibility with common anions (SO42−, CO32−, Cl−) and cations (Ca2+, Na+, K+) in water.

Low-temperature in situ fabrication of porous S-doped g-C3N4 nanosheets using gaseous-bubble template for enhanced visible-light photocatalysis

Developing one-step modification strategy for highly efficient metal-free g-C3N4 photocatalyst has always been challenging and yet demanding. In this study, we have demonstrated an in-situ low-temperature production of porous S-doped g-C3N4 nanosheets, using ammonium chloride (NH4Cl) as a gaseous-bubble template (GBT) to boost the visible-light photocatalytic degradation of organic pollutant dyes. The photocatalytic activities of GBT-induced porous S-doped g-C3N4 nanosheets (so-called p–SCN–x, x = mass ratio of NH4Cl/thiourea) were examined towards the degradation of Rhodamine B (Rh.B) under visible light. Importantly, both surface area and photocatalytic activities of the p-SCN–xsample were optimized by adjusting the mass fraction of thiourea/NH4Cl. The optimized p–SCN–2 exhibited a maximum photocatalytic degradation of 97.50%, which is 7.75-fold higher than that of GBT-free analogue (so-called bulk SCN-0). Moreover, the p–SCN–2 was tested for colorless tetracycline hydrochloride as an emerging pollutant, demonstrating 87% degradation under visible light irradiation. The enhanced performance of p-SCN–2photocatalyst was mainly attributed to an improved surface area, boosted light-harvesting range, and efficient electron-hole separation, and facilitated transfer of charge carriers by dye-sensitization mechanism. Moreover, the scavengers quenching study proved that the superoxide anion (•O2−) radicals and holes (h+) were the leading active sorts. In situ fabrication of p-SCN–x nanosheets at low temperature provided an inexpensive and green synthetic route for highly active g-C3N4–based photocatalysts that can resolve environmental pollution and clean energy issues.

Removal of Methylene Blue Dye Using Metal-Free g-C<sub>3</sub>N<sub>4</sub> Photocatalyst over Natural Sunlight Irradiation

This work presented the high activity of metal-free g-C3N4 photocatalyst for methylene blue (MB) removing over natural sunlight irradiation. These g-C3N4 photocatalysts materials were synthesized by a conventional thermal condensation method using melamine as a precursor under treated at the various annealing temperatures (450 °C, 500 °C, 550 °C, 600 °C and 650 °C). All as-synthesized samples were characterized and confirmed by a several techniques, such as, X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), UV-vis diffuse reflectance spectrometer (DRS), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), and Brunauer-Emmett-Teller (BET) specific surface area. XRD and FTIR results confirmed that the as-synthesized g-C3N4 samples were completely synthesized at annealing temperature of 500 °C. SEM images showed the morphologies of the g-C3N4 samples had more flake-like structures upon the increasing of annealing temperatures. While DRS results indicated that the absorption edges of as-synthesized g-C3N4 samples were shifted to visible-light region, except the sample as-synthesized at 650 °C (g-C3N4-650 °C). Moreover, the photocatalytic properties of metal-free g-C3N4 photocatalyst materials were evaluated by degrading of MB dye solution under natural sunlight irradiation for 100 min. The results revealed that the highest photocatalytic activity was exhibited by the sample synthesized at 600 °C, which the apparent rate constant (kapp.) was 0.0291 min-1. The orders of activities as: g-C3N4-600 °C > g-C3N4-650 °C > g-C3N4-550 °C > g-C3N4-500 °C > g-C3N4-450 °C. Hence, the metal-free g-C3N4 photocatalyst appears to be an attractive-material for water or wastewater purification applications over activated by sunlight irradiation.

Multistage Polymerization Design for g-C3N4 Nanosheets with Enhanced Photocatalytic Activity by Modifying the Polymerization Process of Melamine.

Graphene-like g-C3N4 nanosheets (NSs) have been successfully synthesized with a modified polymerization process of melamine by cocondensation with volatile salts. Volatile ammonium salts such as urea–NH4Cl/(NH4)2SO4/(NH4)3PO4 were added with melamine to modulate the thermodynamic process during polymerization and optimize the structure formation in situ. The surface area, surface structure, and surface charge state of the obtained g-C3N4 NSs could be controlled by simply adjusting the mass ratio of the melamine/volatile ammonium salt. As a consequence, the g-C3N4 NSs exhibited much higher activity than bulk g-C3N4 for the photocatalytic degradation of target pollutants (rhodamine B, methylene blue, and methyl orange), and it also exhibited greater hydrogen evolution under visible light irradiation with an optimal melamine/volatile ammonium salt ratio. The as-prepared g-C3N4 NSs with melamine–urea–NH4Cl showed the highest visible light photocatalytic H2 production activity of 1853.8 μmol·h–1·g–1, which is 9.4 times higher than that of bulk g-C3N4 from melamine. The present study reveals that the synergistic effect of the enhanced surface area, surface structure, and surface charge state is the key for the enhancement of photocatalytic degradation and hydrogen evolution, which could be controlled by the proposed strategy. The result is a good explanation for the hypothesis that adding properly selected monomers can truly regulate the polymerization process of melamine, which is beneficial for obtaining g-C3N4 NSs without molecular self-assembly. Considering the inexpensive feedstocks used, a simple synthetic controlling method provides an opportunity for the rational design and synthesis, making it decidedly appealing for large-scale production of highly photocatalytic, visible-sensitizable, metal-free g-C3N4 photocatalysts.

Simultaneously enhanced photon absorption and charge transport on a distorted graphitic carbon nitride toward visible light photocatalytic activity

The graphitic carbon nitride (g-C3N4) has emerged as one of the most promising candidates to replace the metal oxide-based catalysts for highly efficient photocatalytic materials. However, the intrinsic drawbacks of weak visible-light adsorption and poor charge separation efficiency seriously limit its practical applications. Thus, struggling controls over their structure parameters to optimize the photoelectrical properties on molecular-level for realizing highly active metal-free g-C3N4 photocatalysts have attracted a lot of focuses. Herein, a novel isopropanol assisted solvothermal-copolymerization strategy is rationally designed to synthesize a compact O, S-co-doping g-C3N4 (CNUS) with markedly reinforced π-π* and n-π* electron transitions. The meliorative structure and energy level configuration result in elevated effects for both visible-light (photon) adsorption and photo-induced carries transfer, and the CNUS exhibits outstanding photocatalytic hydrogen evolution and rhodamine B degradation performance under visible light. In addition, after continuously testing, the CNUS still shows superior stability with nearly negligible decay for both photocatalytic reactions. The characterization results indicate that the incorporated oxygen and sulfur engineer the charge, and the layered-stacking distance decreases from 0.328 to 0.322 nm, compared its counterpart (CNU prepared by direct pyrolysis of urea). Importantly, the enriched charge facilitates the rate-limiting separation of photogenerated carriers, and hence improved the visible light photocatalytic efficiency.

Porous-C3N4 with High Ability for Selective Adsorption and Photodegradation of Dyes Under Visible-Light

Porous-C3N4 with high photocatalytic activity of anionic dyes was successfully synthesized by a SiO2-template method. Its photocatalytic selectivity could easily be switched to cationic dye via a simple post-treatment in NaOH solution. Through careful investigation of influencing parameters (the photocatalytic performances, the structure and surface characteristics of the catalysts), the selective degradation of various dyes is closely related to the adsorption selectivity of the porous g-C3N4 catalysts, which originated from the different surface charge. This study provides a new insight into developing metal-free g-C3N4 photocatalyst with high efficiency and selectivity in environmental purification.

Thylakoid-Inspired Multishell g-C3N4 Nanocapsules with Enhanced Visible-Light Harvesting and Electron Transfer Properties for High-Efficiency Photocatalysis.

Inspired by the orderly stacked nanostructure and highly integrated function of thylakoids in a natural photosynthesis system, multishell g-C3N4 (MSCN) nanocapsule photocatalysts have been prepared by SiO2 hard template with different shell layers. The resultant triple-shell g-C3N4 (TSCN) nanocapsules display superior photocatalysis performance to single-shell and double-shell counterparts owing to excellent visible-light harvesting and electron transfer properties. Specially, with the increase of the shell layer number, light harvesting is greatly enhanced. There is an increase of the entire visible range absorption arising from the multiple scattering and reflection of the incident light within multishell nanoarchitectures as well as the light transmission within the porous thin shells, and an increase of absorption edge arising from the decreased quantum size effect. The electron transfer is greatly accelerated by the mesopores in the thin shells as nanoconduits and the high specific surface area of TSCN (310.7 m2 g-1). With the tailored hierarchical nanostructure features, TSCN exhibits a superior visible-light H2-generation activity of 630 μmol h-1 g-1 (λ > 420 nm), which is among one of the most efficient metal-free g-C3N4 photocatalysts. This study demonstrates a bioinspired approach to the rational design of high-performance nanostructured visible-light photocatalysts.

Enhanced Photocatalytic Performance of Luminescent g-C3N4 Photocatalyst in Darkroom.

Graphitic-C3N4(g-C3N4), a low-cost visible-light-driven photocatalyst, was used for the photocatalytic oxidation of aqueous methylene blue (MB) in the dark with Sr4Al14O25:(Eu,Dy) assistance. The Sr4Al14O25:(Eu,Dy)/g-C3N4 photocatalysts were fabricated through the ultrasonic dispersion method. The commercial Sr4Al14O25:(Eu,Dy) phosphor was used as a long afterglow supplier for exciting g-C3N4 in the dark. The results demonstrated that the metal-free g-C3N4 photocatalyst could use the eye-visible long afterglow to photocatalytically decompose MB dyes in the dark. This work may expand the appealing application of g-C3N4 for the environmental cleanup.

Metal-free g-C3N4 photocatalyst by sulfuric acid activation for selective aerobic oxidation of benzyl alcohol under visible light

In this work, modification of graphitic carbon nitride photocatalyst with acid was accomplished with a facile method through reflux in different acidic substances. The g-C3N4-based material was found to be a metal-free photocatalyst useful for the selective oxidation of benzyl alcohol with dioxygen as the oxidant under visible light irradiation. Acid modification had a significant influence on the photocatalytic performance of g-C3N4. Among all acid tested, sulfuric acid-modified g-C3N4 showed the highest catalytic activity and gave benzaldehyde in 23% yield for 4h under visible light irradiation, which was about 2.5 times higher than that of g-C3N4. The acid modification effectively improved surface area, reduced structural size, enlarged band gap, enhanced surface chemical state, and facilitated photoinduced charge separation, contributing to the enhanced photocatalytic activity. It is hoped that our work can open promising prospects for the utilization of metal free g-C3N4-based semiconductor as visible-light photocatalyst for selective organic transformation.