As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Polydopamine and Its Derivative Materials: Synthesis and Promising Applications in Energy, Environmental, and Biomedical Fields

Graphitic (g)-C3N4 with a layered structure has the potential of forming graphene-like nanosheets with unusual physicochemical properties due to weak van der Waals forces between layers. Herein is shown that g-C3N4 nanosheets with a thickness of around 2 nm can be easily obtained by a simple top-down strategy, namely, thermal oxidation etching of bulk g-C3N4 in air. Compared to the bulk g-C3N4, the highly anisotropic 2D-nanosheets possess a high specific surface area of 306 m2 g-1, a larger bandgap (by 0.2 eV), improved electron transport ability along the in-plane direction, and increased lifetime of photoexcited charge carriers because of the quantum confinement effect. As a consequence, the photocatalytic activities of g-C3N4 nanosheets have been remarkably improved in terms of OH radical generation and photocatalytic hydrogen evolution.

Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities

Semiconductor-based photocatalysis is considered to be an attractive way for solving the worldwide energy shortage and environmental pollution issues. Since the pioneering work in 2009 on graphitic carbon nitride (g-C3N4) for visible-light photocatalytic water splitting, g-C3N4 -based photocatalysis has become a very hot research topic. This review summarizes the recent progress regarding the design and preparation of g-C3N4 -based photocatalysts, including the fabrication and nanostructure design of pristine g-C3N4 , bandgap engineering through atomic-level doping and molecular-level modification, and the preparation of g-C3N4 -based semiconductor composites. Also, the photo-catalytic applications of g-C3N4 -based photocatalysts in the fields of water splitting, CO2 reduction, pollutant degradation, organic syntheses, and bacterial disinfection are reviewed, with emphasis on photocatalysis promoted by carbon materials, non-noble-metal cocatalysts, and Z-scheme heterojunctions. Finally, the concluding remarks are presented and some perspectives regarding the future development of g-C3N4 -based photocatalysts are highlighted.

Polymeric Photocatalysts Based on Graphitic Carbon Nitride

A novel reduced melon photocatalyst with a bandgap of 2.03 eV developed here has a widened visible light absorption range and suppressed radiative recombination of photo-excited charge carriers due to the homogeneous self-modification with nitrogen vacancies. As a consequence, the reduced melon shows a much superior photocatalytic activity compared to the pristine melon in generating •OH radicals and degrading the organic pollutant Rhodamine B.

Increasing the visible light absorption of graphitic carbon nitride (melon) photocatalysts by homogeneous self-modification with nitrogen vacancies.

Vacancy defects can play an important role in modifying the electronic structure and the properties of photoexcited charge carriers and consequently the photocatalytic activity of semiconductor photocatalysts. By controlling the polycondensation temperature of a dicyandiamide precursor in the preparation of graphitic carbon nitride (g-C3N4), we introduced nitrogen vacancies in the framework of g-C3N4. These vacancies exert remarkable effects on modifying the electronic structure of g-C3N4 as shown in UV-visible absorption spectra and valence band spectra. Steady and time-resolved fluorescence emission spectra show that, due to the existence of abundant nitrogen vacancies, the intrinsic radiative recombination of electrons and holes in g-C3N4 is greatly restrained, and the population of short-lived and long-lived charge carriers is decreased and increased, respectively. As a consequence, the overall photocatalytic activity of the g-C3N4, characterized by the ability to generate center dot OH radicals, photodecomposition of Rhodamine B, and photocatalytic hydrogen evolution under both UV-visible and visible light, was remarkably improved.

Nitrogen Vacancy-Promoted Photocatalytic Activity of Graphitic Carbon Nitride

We show that in contrast to conventional compound photocatalysts, α-sulfur crystals of cyclooctasulfur (S8) are a visible-light-active elemental photocatalyst. The α-S crystals were found to have the ability not only to generate ·OH radicals but also to split water in a photoelectrochemical process under both UV–vis and visible-light irradiation. Although the absolute activity obtained was low because of the large particle size and poor hydrophilicity of the α-S crystals studied, there is great potential for increasing the activity with the assistance of known strategies such as surface modification, nanoscaling, doping, and coupling with other photocatalysts.

α-Sulfur Crystals as a Visible-Light-Active Photocatalyst

Narrowing the bandgap of wide-bandgap semiconductor photocatalysts (for instance, anatase TiO2) by introducing suitable heteroatoms has been actively pursued for increasing solar absorption, but usually suffers from a limited thermodynamic/kinetic solubility of substitutional dopants in bulk and/or dopant-induced recombination centres. Here we report a red anatase TiO2 microsphere with a bandgap gradient varying from 1.94 eV on its surface to 3.22 eV in its core by a conceptually different doping approach for harvesting the full spectrum of visible light. This approach uses a pre-doped interstitial boron gradient to weaken nearby Ti-O bonds for the easy substitution of oxygen by nitrogen, and consequently it substantially improves the nitrogen solubility. Furthermore, no nitrogen-related Ti3+ was formed in the red TiO2 due to a charge compensation effect by boron, which inevitably occurs in common nitrogen doped TiO2. The red anatase TiO2 exhibits photoelectrochemical water splitting activity under visible light irradiation. The results obtained may shed light on how to increase high visible light absorbance of wide-bandgap photocatalysts.

Ping Niu

Papers

Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities

Increasing the visible light absorption of graphitic carbon nitride (melon) photocatalysts by homogeneous self-modification with nitrogen vacancies.

Nitrogen Vacancy-Promoted Photocatalytic Activity of Graphitic Carbon Nitride

α-Sulfur Crystals as a Visible-Light-Active Photocatalyst

A red anatase TiO2 photocatalyst for solar energy conversion