A Metal-Free Carbon-Based Catalyst: An Overview and Directions for Future Research

Metal-free carbon porous materials (CPMs) have gained the intensive attention of scientists and technologists because of their potential applications, ranging from catalysis to energy storage. Various simple and facile strategies are proposed for the preparation of CPMs with well-controlled sizes, shapes, and modifications on the surface. The extraordinary tenability of the pore structure, the environmental acceptability, the unique surface and the corrosion resistance properties allow them to be suitable materials for a large panel of catalysis applications. This review briefly outlines the different signs of progresses made towards synthesizing CPMs, and their properties, including catalytic efficiency, stability, and recyclability. Finally, we make a comparison of their catalytic performances with other nanocomposites, and we provide an outlook on the expected developments in the relevant research works.

Boron- and phenyl-codoped graphitic carbon nitride with greatly enhanced light responsive range for photocatalytic disinfection

The development of metal-free photocatalyst to make maximum use of the solar energy for photocatalytic disinfection is highly desired. Herein, boron-and phenyl-codoped graphitic carbon nitride was prepared by thermal polycondensation of cyanamide with 3-aminobenzeneboronic acid and applied as photocatalyst to inactivate Escherichia coli (E. coli). The photocatalysts exhibited the enhanced light responsive range over ultraviolet to near infrared light and 99.9% bacteria could be inactivated within 3 h with a low concentration of photocatalyst under the irradiation of simulated solar light. The disinfection mechanism was studied by scavenger experiments, indicating H₂O₂ was the main reactive species for the inactivation of bacteria. Finally, the photocatalyst was deposited on the surface of solid material and also exhibited strong disinfection performance. Taking advantage of excellent disinfection activity and low cytotoxicity, the photocatalyst showed a promising application in solar-driven photocatalytic disinfection in public place.

A "waiting" carbon nitride radical anion: a charge storage material and key intermediate in direct C-H thiolation of methylarenes using elemental sulfur as the "S"-source

Potassium poly(heptazine imide), a carbon nitride semiconductor, in the presence of hole scavengers and visible light gives stable radical anion with the specific density of unpaired electrons reaching 112 mmol g^–1.

Potassium poly(heptazine imide), a carbon nitride based semiconductor with high structural order and a valence band potential of +2.2 V vs. NHE, in the presence of hole scavengers and under visible light irradiation gives the corresponding polymeric radical anion, in which the specific density of unpaired electrons reaches 112 μmol g^–1. The obtained polymeric radical anion is stable under anaerobic conditions for several hours. It was characterized using UV-vis absorption, time resolved and steady state photoluminescence spectra. The electronic structure of the polymeric radical anion was confirmed by DFT cluster modelling. The unique properties of potassium poly(heptazine imide) for storing charges were employed in visible light photocatalysis. A series of substituted dibenzyldisulfanes was synthesized in 41–67% yield from the corresponding methylarenes via cleavage of the methyl C–H bond under visible light irradiation and metal-free conditions.

Boron and nitrogen co-doped ordered microporous carbons with high surface areas

Boron and nitrogen co-doped ordered microporous carbons with high surface areas are obtained by using NaY zeolite as a hard template and an ionic liquid, 1-ethyl-3-methylimidazolium tetracyanoborate (EMIT), as a BN source. An acetylene-gas supply during a pyrolysis is effective to avoid the unfavourable reaction of zeolite and EMIT.

Ultra-highly fluorescent N doped carbon dots-CdTe QDs nanohybrids with excitation-independent emission in the blue-violet region

Luminescent carbon dots (CDs) are of significant practical application interest such as in optoelectronic devices and sensitive probing in the life science and environment fields. In this study, N doped CDs-CdTe quantum dots (QDs) nanohybrids (CdTe/N-CDs) were synthesized by a plasma heating process using silk fibroin and CdTe QDs as precursors. The synthesis, doping, hybridizing and passivation of the CdTe/N-CDs were carried out in a single-step process. The as-synthesized CdTe/N-CDs dispersed in ethanol exhibited blue-violet photoluminescence with excitation-independent emission characteristics (strong emissions at 405 nm and 429 nm, and a weak emission at 456 nm). Additionally, the optimal excitation wavelength for the CdTe/N-CDs was found at 360–380 nm, which very closely matches the intrinsic wavelength of GaN-based LEDs. Furthermore, the obtained CdTe/N-CDs exhibited a very high quantum yield of ∼84%, showing great potential in developing chip-based high performance optoelectronics devices. The emission mechanism and emission enhancement by related factors including N-bonded configurations in the carbon base and the transfer of photo-excited electrons from the CdTe QDs to the N doped CDs were studied, as well.

The photoluminescence quantum yield of N-doped carbon dots was brought up to 84% by hybridizing with CdTe QDs.

Facile synthesis of NiS2 nanoparticles ingrained in a sulfur-doped carbon nitride framework with enhanced visible light photocatalytic activity: two functional roles of thiourea

A one-step, simple and low-cost strategy was successfully used to produce novel NiS₂/S-doped g-C₃N₄ photocatalysts.

Boosted visible light photodegradation activity of boron doped rGO/g-C3N4 nanocomposites: the role of C–O–C bonds

Boron doping is an effective way to promote the chemical interaction between rGO and g-C₃N₄.

Molecular engineering of polymeric carbon nitride: advancing applications from photocatalysis to biosensing and more

Different designs and constructions of molecular structures of carbon nitride for emerging applications, such as biosensing, are discussed.

Controllable Synthesis of Mesoporous Sulfur-Doped Carbon Nitride Materials for Enhanced Visible Light Photocatalytic Degradation

Mesoporous sulfur-doped graphitic carbon nitride (MCNS) materials were successfully synthesized using thiourea as a low-cost precursor and SiO₂ gel solution as a template through a simple thermal condensation method. The effects of three synthesis key factors, namely, the reaction temperature, the reaction time, and the weight ratio of SiO₂/thiourea, and also their interactions on the removal rate of methyl orange (MO) were investigated using response surface methodology, and the samples were subjected to several characterization techniques. Results showed that the optimized physicochemical properties could be achieved for the MCNS samples by controlling the synthesis key factors, and it was found that the reaction temperature and the reaction time had significant influences on the MO photocatalytic removal. Among bulk graphitic carbon nitride (g-C₃N₄), CN (undoped g-C₃N₄), CNS (sulfur-doped g-C₃N₄ without template), and TiO₂ (Degussa P25) samples, the optimized MCNS-4 illustrated the highest photocatalytic activity toward the removal of MO under visible light irradiation. The enhanced performance originated from the synergistic effects of high surface area, mesoporous texture, sulfur doping, and high visible light absorption, which were helpful for the separation and transportation of the photogenerated electron-hole pairs. Furthermore, MCNS-4 revealed high reusability and stability without any significant decrease in its efficiency. Our findings not only confirm the importance of simultaneous sulfur doping and mesoporous structure to synthesize highly active photocatalysts but also might provide a new insight into textural engineering of carbon nitride materials only by the optimization of the synthesis key variables, considering their interactions without relying on extra metal oxides.

A Nanojunction Polymer Photoelectrode for Efficient Charge Transport and Separation

A metal-free photoanode nanojunction architecture, composed of B-doped carbon nitride nanolayer and bulk carbon nitride, was fabricated by a one-step approach. This type of nanojunction (s-BCN) overcomes a few intrinsic drawbacks of carbon nitride film (severe bulk charge recombination and slow charge transfer). The top layer of the nanojunction has a depth of ca. 100 nm and the bottom layer is ca. 900 nm. The nanojunction photoanode results into a 10-fold higher photocurrent than bulk graphitic carbon nitride (G-CN) photoanode, with a record photocurrent density of 103.2 μA cm-2 at 1.23 V vs. RHE under one sun irradiation and an extremely high incident photon-to-current efficiency (IPCE) of ca. 10 % at 400 nm. Electrochemical impedance spectroscopy, Mott-Schottky plots, and intensity-modulated photocurrent spectroscopy show that such enhancement is mainly due to the mitigated deep trap states, a more than 10 times faster charge transfer rate and nearly three times higher conductivity due to the nanojunction architecture.

Sulfur-Modified Graphitic Carbon Nitride Nanostructures as an Efficient Electrocatalyst for Water Oxidation

There is an urgent need to develop metal-free, low cost, durable, and highly efficient catalysts for industrially important oxygen evolution reactions. Inspired by natural geodes, unique melamine nanogeodes are successfully synthesized using hydrothermal process. Sulfur-modified graphitic carbon nitride (S-modified g-CN _x ) electrocatalysts are obtained by annealing these melamine nanogeodes in situ with sulfur. The sulfur modification in the g-CN _x structure leads to excellent oxygen evolution reaction activity by lowering the overpotential. Compared with the previously reported nonmetallic systems and well-established metallic catalysts, the S-modified g-CN _x nanostructures show superior performance, requiring a lower overpotential (290 mV) to achieve a current density of 10 mA cm^-2 and a Tafel slope of 120 mV dec^-1 with long-term durability of 91.2% retention for 18 h. These inexpensive, environmentally friendly, and easy-to-synthesize catalysts with extraordinary performance will have a high impact in the field of oxygen evolution reaction electrocatalysis.

Alkali-Assisted Synthesis of Nitrogen Deficient Graphitic Carbon Nitride with Tunable Band Structures for Efficient Visible-Light-Driven Hydrogen Evolution

A facile synthetic strategy for nitrogen-deficient graphitic carbon nitride (g-C₃ N_x ) is established, involving a simple alkali-assisted thermal polymerization of urea, melamine, or thiourea. In situ introduced nitrogen vacancies significantly redshift the absorption edge of g-C₃ N_x , with the defect concentration depending on the alkali to nitrogen precursor ratio. The g-C₃ N_x products show superior visible-light photocatalytic performance compared to pristine g-C₃ N₄ .

Enhancing visible light photocatalytic activity of nitrogen-deficient g-C3N4 via thermal polymerization of acetic acid-treated melamine

Nitrogen-deficient graphitic carbon nitride (CN-HAc) was synthesized by thermal condensation of acetic acid-treated melamine as a precursor. The nitrogen vacancies play a remarkable role on controlling the electronic structure of g-C₃N₄, such as extending the optical absorption and enhancing the separation efficiency of photogenerated charge carriers, resulting in the improvement of photocatalytic activity. The photocatalytic activity of the catalysts was evaluated by splitting water and degradation of rhodamine B (RhB) under visible light irradiation (λ>420nm). The average H₂ evolution rate on CN-HAc is 24μmolh^-1, which is about 5 times of that on pristine g-C₃N₄. Meanwhile, CN-HAc exhibits superior photocatalytic mineralization of RhB. The possible formation mechanism of nitrogen-deficient in the framework of g-C₃N₄ is proposed.

Boron-doped graphitic carbon nitride nanosheets for enhanced visible light photocatalytic water splitting

A new type of boron-doped graphitic carbon nitride (B-g-C₃N₄) nanosheets was prepared by a benign one-pot thermal polycondensation process.

Highly active g-C3N4 as a solid base catalyst for knoevenagel condensation reaction under phase transfer conditions

g-C₃N₄ with phase transfer catalyst (PTC) advantages for the Knoevenagel condensation reaction.

Homogenous Boron-doping in Self-sensitized Carbon Nitride for Enhanced Visible-light Photocatalytic Activity

We report a solvothermal approach for the preparation of homogeneously B-doped self-sensitized carbon nitride (B-SSCN) composed of a core of B-doped carbon nitride microspheres and a covalently linked shell of s-triazine oligomers. Compared to the undoped structure, the obtained B-SSCN photocatalyst exhibits an enhanced visible-light activity, excellent stability for photocatalytic hydrogen generation due to a reduced band-gap, enhanced charge-separation efficiency, and better surface reactivity of B-SSCN. This work provides a new strategy to uniformly insert heteroatoms into the polymeric carbon nitride framework for the development of metal-free photocatalysts towards efficient production of solar fuels.

Self-assembly graphitic carbon nitride quantum dots anchored on TiO2 nanotube arrays: An efficient heterojunction for pollutants degradation under solar light

In this study, an efficient heterojunction was constructed by anchoring graphitic carbon nitride quantum dots onto TiO2 nanotube arrays through hydrothermal reaction strategy. The prepared graphitic carbon nitride quantum dots, which were prepared by solid-thermal reaction and sequential dialysis process, act as a sensitizer to enhance light absorption. Furthermore, it was demonstrated that the charge transfer and separation in the formed heterojunction were significantly improved compared with pristine TiO2. The prepared heterojunction was used as a photoanode, exhibiting much improved photoelectrochemical capability and excellent photo-stability under solar light illumination. The photoelectrocatalytic activities of prepared heterojunction were demonstrated by degradation of RhB and phenol in aqueous solution. The kinetic constants of RhB and phenol degradation using prepared photoelectrode are 2.4 times and 4.9 times higher than those of pristine TiO2, respectively. Moreover, hydroxyl radicals are demonstrated to be dominant active radicals during the pollutants degradation.

Cadmium sulfide/graphitic carbon nitride heterostructure nanowire loading with a nickel hydroxide cocatalyst for highly efficient photocatalytic hydrogen production in water under visible light

Photocatalytic hydrogen production from water in a noble-metal-free system has attracted much attention in recent years. Herein we report on the use of core/shell cadmium sulfide/graphitic carbon nitride (CdS/g-C3N4) heterojunction nanorods modified by nickel hydroxide (Ni(OH)2) as a highly efficient photocatalyst for visible light-driven hydrogen production from water. Due to efficient separation of the photoexcited charge carriers in the CdS/g-C3N4 core/shell nanorods and the synergistic effect of Ni(OH)2, the optimal hydrogen evolution rate over Ni(OH)2-CdS/g-C3N4 is 115.18 μmol h(-1) mg(-1) under visible light irradiation (λ > 420 nm), which is ∼26 times higher than the CdS/g-C3N4 nanorod composite without Ni(OH)2 and ∼7 times better than the 0.5 wt% Pt-CdS/g-C3N4 nanorod composite. The apparent quantum efficiency is ∼16.7% at an excitation of 450 nm. During photocatalysis, no degradation of Ni(OH)2 was observed based on the XPS data, indicating that it is a robust cocatalyst. Moreover, the present photocatalyst showed excellent photocatalytic stability for hydrogen production and the turnover number (TON) reached ∼24,600 over 90 hours.

How does the B,F-monodoping and B/F-codoping affect the photocatalytic water-splitting performance of g-C3N4?

B/F codoping would be a promising strategy to promote the photocatalytic overall water-splitting performance of g-C₃N₄ under visible-light irradiation.

Efficient degradation of methylene blue over boron-doped g-C3N4/Zn0.8Cd0.2S photocatalysts under simulated solar irradiation

Boron doping and heterostructures are closely related to the enhanced photocatalytic function of the hybrids.

Preparation of CoNC catalysts via heating a mixture of cobaltporphyrin and casein for ethylbenzene oxidation

Biomass-derived cobalt-coordinated N-doped carbon (CoNC) for C–H bond oxidation is synthesized by a facile procedure based on pyrolysis of cobaltporphyrin with natural, amino acid-rich biomass casein as a supplementary nitrogen source.

Boron and nitrogen co-doped carbon dots as a metal-free catalyst for hydrogen generation from sodium borohydride

The high catalytic activity of B,N co-doped CDs is due to the synergistic effect of nitrogen and boron dopants and the intrinsic properties of carbon dots.

Highly selective oxidation of cyclohexene to 2-cyclohexene-1-one in water using molecular oxygen over Fe–Co–g-C3N4

The Fe–Co–g-C₃N₄ catalyst is highly effective for the selective oxidation of cyclohexene to 2-cyclohexene-1-one using oxygen in water.

A “pillared” process to construct graphitic carbon nitride based functionalized mesoporous materials

Graphitic carbon nitride based mesoporous materials were constructed via a novel “pillared” way, which can expand the “tool box” available to synthesize multi-functional porous materials.

An overview of the structural, textural and morphological modulations of g-C3N4 towards photocatalytic hydrogen production

This study highlights the recent trends in the structural, textural and morphological variations of g-C₃N₄ for visible-light-induced hydrogen evolution.

Cobalt-doped graphitic carbon nitride with enhanced peroxidase-like activity for wastewater treatment

The peroxidase-like activity of graphitic carbon nitride (g-C₃N₄) is dramatically increased by a small cobalt doping. The cobalt-doped g-C₃N₄ was used for wastewater treatment, exhibiting much higher degradation rate than that of pure g-C₃N₄.