Artificial photosynthesis of hydrocarbon fuels by utilizing solar energy and CO2 is considered as a potential route for solving ever-increasing energy crisis and greenhouse effect. Herein, hierarchical porous O-doped graphitic carbon nitride (g-C3 N4 ) nanotubes (OCN-Tube) are prepared via successive thermal oxidation exfoliation and curling-condensation of bulk g-C3 N4 . The as-prepared OCN-Tube exhibits hierarchically porous structures, which consist of interconnected multiwalled nanotubes with uniform diameters of 20-30 nm. The hierarchical OCN-Tube shows excellent photocatalytic CO2 reduction performance under visible light, with methanol evolution rate of 0.88 µmol g-1 h-1 , which is five times higher than bulk g-C3 N4 (0.17 µmol g-1 h-1 ). The enhanced photocatalytic activity of OCN-Tube is ascribed to the hierarchical nanotube structure and O-doping effect. The hierarchical nanotube structure endows OCN-Tube with higher specific surface area, greater light utilization efficiency, and improved molecular diffusion kinetics, due to the more exposed active edges and multiple light reflection/scattering channels. The O-doping optimizes the band structure of g-C3 N4 , resulting in narrower bandgap, greater CO2 affinity, and uptake capacity as well as higher separation efficiency of photogenerated charge carriers. This work provides a novel strategy to design hierarchical g-C3 N4 nanostructures, which can be used as promising photocatalyst for solar energy conversion.

Hierarchical Porous O-doped g-C3N4 with Enhanced Photocatalytic CO2 Reduction Activity

In recent years, heterogeneous photocatalysis has received much research interest because of its powerful potential applications in tackling many important energy and environmental challenges at a global level in an economically sustainable manner. Due to their unique optical, electrical, and physicochemical properties, various 2D graphene nanosheets-supported semiconductor composite photocatalysts have been widely constructed and applied in different photocatalytic fields. In this review, fundamental mechanisms of heterogeneous photocatalysis, including thermodynamic and kinetics requirements, are first systematically summarized. Then, the photocatalysis-related properties of graphene and its derivatives, and design rules and synthesis methods of graphene-based composites are highlighted. Importantly, different design strategies, including doping and sensitization of semiconductors by graphene, improving electrical conductivity of graphene, increasing eloectrocatalytic active sites on graphene, strengthening interface coupling between semiconductors and graphene, fabricating micro/nano architectures, constructing multi-junction nanocomposites, enhancing photostability of semiconductors, and utilizing the synergistic effect of various modification strategies, are thoroughly summarized. The important applications including photocatalytic pollutant degradation, H2 production, and CO2 reduction are also addressed. Through reviewing the significant advances on this topic, it may provide new opportunities for designing highly efficient 2D graphene-based photocatalysts for various applications in photocatalysis and other fields, such as solar cells, thermal catalysis, separation, and purification.

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Graphene in Photocatalysis: A Review

Element doping is an efficient strategy for tuning the electronic structure and improving the photocatalytic activity of graphitic carbon nitride (g-C3N4). Employing the density functional theory computation performed by CASTEP module, we investigated the band structures, electronic and optical properties of monolayer g-C3N4 doped with halogens (F, Cl, Br or I). First, the halogen atoms occupying the interstitial space enclosed by three tri-s-triazine units in the monolayer g-C3N4 unit cell was demonstrated to be the most stable configuration in terms of adsorption energy. On the basis of these interstitial-doped monolayer g-C3N4 systems, it is found that the introduction of halogen atoms leads to various density of states (DOS) and redistribution of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). The F atom tends to occupy the valance band and HOMO due to its extremely high electronegativity. By contrast, the Cl, Br and I atoms are involved in the conduction band and LUMO. In sum, the calculation results show that the halogen-doped monolayer g-C3N4 systems have narrowed band gap, increased light absorption and reduced work function, which are conducive to high photocatalytic activity. The conclusions presented in this work indicate the availability of halogen-doped monolayer g-C3N4 with considerable photocatalytic performance.

First principle investigation of halogen-doped monolayer g-C3N4 photocatalyst

Graphitic carbon nitride (g-C3N4) is an attractive photocatalyst which has appealing visible light absorption, outstanding layered porous structure, high stability and nontoxicity. Many experimental methods have been developed to modify the pristine g-C3N4 and enhanced photocatalytic activities have been witnessed. First-principle calculation based on density functional theory is an effective approach to investigate the structural, electronic, optical and thermodynamic properties of molecules and crystals, which provides important information to elucidate the improved photocatalytic activity of modified g-C3N4 at atomic or unit-cell levels, or even further, to predict the property and photocatalytic activity of experimentally un-synthesized g-C3N4-based photocatalysts. This review is dedicated to this important material, i.e. tri-s-triazine-based g-C3N4 and summarized a panorama of the major advances in the first-principle investigation. The existing challenges and future directions at the forefront of this emerging research hotpot have also been discussed.

First-principle calculation study of tri- s -triazine-based g-C 3 N 4 : A review

Recent advances in non-metal modification of graphitic carbon nitride for photocatalysis: a historic review

First principle modeling of oxygen-doped monolayer graphitic carbon nitride

The interfacial effect on the structural, electronic and optical properties of a hybrid triazine-based graphitic carbon nitride and graphene nanocomposite is calculated using the first-principles method. It reveals the favorable stacking pattern utilizing the ab initio thermodynamics approach. The electronic band structure presents that the high carrier mobility is maintained in a hybrid g-CN/G nanocomposite, and a moderate band gap is opened by the interactions between g-C3N4 and graphene. Moreover, the opened band gap can be tuned regularly with the interfacial distance. Based on the analysis of the imaginary part of dielectric function of the graphene, the g-C3N4 monolayer and the hybrid g-CN/G nanocomposite, it is found that the hybrid g-CN/G nanocomposite displays enhanced and extended optical absorption compared to simplex graphene and the g-C3N4 monolayer.

Structural, electronic and optical properties of a hybrid triazine-based graphitic carbon nitride and graphene nanocomposite

Using first-principles calculations, we present a multifunctional material of g-C4N3/graphene bilayer with great potentials in the field of spintronics and photocatalysis. In g-C4N3/graphene bilayer, N atoms create localized spin polarization and p-doped graphene shows high charge carrier density, which makes this nanocomposite a perfect candidate for spintronic applications. Meanwhile, the charge redistribution occurred between the two layers also facilitates the separation of photogenerated electron–hole pairs. Moreover, the inclusion of C atoms into g-C3N4 closes the band gap of g-C3N4/graphene completely and induces more levels near the Fermi energy, and thus the g-C4N3/graphene bilayer displays enhanced visible light absorption compared to the g-C3N4/graphene bilayer.

A multifunctional material of two-dimensional g-C4N3/graphene bilayer

In order to clarify the W diffusion behavior at the WCu/Ni interface, a WCu/Ni diffusion couple was prepared, and the phase structure and constituents of the diffusion layer were characterized by a scanning electron microscopy and transmission electron microscopy. Based on the experimental data, the interdiffusion coefficient was calculated. The diffusion behavior was also verified by the simulation according to the first principle method. The results show that a diffusion layer with the thickness of 1 μm, which is composed of Cu rich solid solution and W rich solid solution, is formed on the surface of W particles. The interdiffusion coefficient of W in the transitional layer is 3.381 × 10 −15  cm 2 /s, which is much larger than that of W in Ni. This is in good agreement with the simulation result.

Diffusion behavior of W in the WCu/Ni interface

The structural and electronic properties of neutral oxygen vacancies in (Mo + C)-doped anatase TiO2 were investigated using frozen-core projector-augmented wave (PAW) method within GGA +U approximation. Six possible oxygen vacancy sites were considered in the present work. The results show that the octahedral vertex adjacent to Mo and opposite from C is the most stable position for oxygen vacancy based on the results of the formation energy. The Fermi level is located at above the bottom of the conduction band and a typical n-type metallic behavior occurs as a result of the oxygen vacancy appeared in (Mo + C) doped TiO2.

Jie Cui

Papers

First principle modeling of oxygen-doped monolayer graphitic carbon nitride

Structural, electronic and optical properties of a hybrid triazine-based graphitic carbon nitride and graphene nanocomposite

A multifunctional material of two-dimensional g-C4N3/graphene bilayer

Diffusion behavior of W in the WCu/Ni interface

First principles study of oxygen vacancies in (Mo + C)-doped anatase TiO2