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?

Nanoparticles: Properties, applications and toxicities

A review on g-C3N4-based photocatalysts

Liming Dai,*,†,‡ Yuhua Xue,†,‡ Liangti Qu,* Hyun-Jung Choi, and Jong-Beom Baek* †Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, School of Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China School of Energy and Chemical Engineering/Center for Dimension-Controllable Covalent Organic Frameworks, Ulsan National Institute of Science and Technology (UNIST), 100 Banyeon, Ulsan, 689-798, South Korea

Metal-free catalysts for oxygen reduction reaction.

This Review focuses on noncovalent functionalization of graphene and graphene oxide with various species involving biomolecules, polymers, drugs, metals and metal oxide-based nanoparticles, quantum dots, magnetic nanostructures, other carbon allotropes (fullerenes, nanodiamonds, and carbon nanotubes), and graphene analogues (MoS2, WS2). A brief description of π–π interactions, van der Waals forces, ionic interactions, and hydrogen bonding allowing noncovalent modification of graphene and graphene oxide is first given. The main part of this Review is devoted to tailored functionalization for applications in drug delivery, energy materials, solar cells, water splitting, biosensing, bioimaging, environmental, catalytic, photocatalytic, and biomedical technologies. A significant part of this Review explores the possibilities of graphene/graphene oxide-based 3D superstructures and their use in lithium-ion batteries. This Review ends with a look at challenges and future prospects of noncovalently modified graph...

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Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications

The use of solar energy to produce molecular hydrogen and oxygen (H2 and O2) from overall water splitting is a promising means of renewable energy storage. In the past 40 years, various inorganic and organic systems have been developed as photocatalysts for water splitting driven by visible light. These photocatalysts, however, still suffer from low quantum efficiency and/or poor stability. We report the design and fabrication of a metal-free carbon nanodot-carbon nitride (C3N4) nanocomposite and demonstrate its impressive performance for photocatalytic solar water splitting. We measured quantum efficiencies of 16% for wavelength λ = 420 ± 20 nanometers, 6.29% for λ = 580 ± 15 nanometers, and 4.42% for λ = 600 ± 10 nanometers, and determined an overall solar energy conversion efficiency of 2.0%. The catalyst comprises low-cost, Earth-abundant, environmentally friendly materials and shows excellent stability.

http://science.sciencemag.org/content/sci/347/6225/970.full.pdf

Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway

Nanocomposites of carbon nanodots embedded in a C3N4 matrix are prepared by heating a mixture of ammonia-treated carbon nanodots and urea powder at 550 °C for 3 h in an alumina crucible.

Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-Electron Pathway.

As an interesting layered material, molybdenum disulfide (MoS2) has been extensively studied in recent years due to its exciting properties. However, the applications of MoS2 in optoelectronic devices are impeded by the lack of high-quality p–n junction, low light absorption for mono-/multilayers, and the difficulty for large-scale monolayer growth. Here, it is demonstrated that MoS2 films with vertically standing layered structure can be deposited on silicon substrate with a scalable sputtering method, forming the heterojunction-type photodetectors. Molecular layers of the MoS2 films are perpendicular to the substrate, offering high-speed paths for the separation and transportation of photo-generated carriers. Owing to the strong light absorption of the relatively thick MoS2 film and the unique vertically standing layered structure, MoS2/Si heterojunction photodetectors with unprecedented performance are actualized. The self-driven MoS2/Si heterojunction photodetector is sensitive to a broadband wavelength from visible light to near-infrared light, showing an extremely high detectivity up to ≈1013 Jones (Jones = cm Hz1/2 W−1), and an ultrafast response speed of ≈3 μs. The performance is significantly better than the photodetectors based on mono-/multilayer MoS2 nanosheets. Additionally, the MoS2/Si photodetectors exhibit excellent stability in air for a month. This work unveils the great potential of MoS2/Si heterojunction for optoelectronic applications.

MoS2/Si Heterojunction with Vertically Standing Layered Structure for Ultrafast, High‐Detectivity, Self‐Driven Visible–Near Infrared Photodetectors

Fast-response and high-sensitivity deep-ultraviolet (DUV) photodetectors with detection wavelength shorter than 320 nm are in high demand due to their potential applications in diverse fields. However, the fabrication processes of DUV detectors based on traditional semiconductor thin films are complicated and costly. Here we report a high-performance DUV photodetector based on graphene quantum dots (GQDs) fabricated via a facile solution process. The devices are capable of detecting DUV light with wavelength as short as 254 nm. With the aid of an asymmetric electrode structure, the device performance could be significantly improved. An on/off ratio of ∼6000 under 254 nm illumination at a relatively weak light intensity of 42 μW cm–2 is achieved. The devices also exhibit excellent stability and reproducibility with a fast response speed. Given the solution-processing capability of the devices and extraordinary properties of GQDs, the use of GQDs will open up unique opportunities for future high-performance...

Solution-Processed Graphene Quantum Dot Deep-UV Photodetectors

A high doping technique has been widely used for record-efficiency crystalline silicon (Si) solar cells to minimize the series resistance losses and to form a back surface field. However, it requires high temperatures (up to 1000 °C) and involves toxic gases, which may not be compatible for hybrid organic–silicon solar cells. Here, we report that a high power conversion efficiency (PCE) of 13.7% with a device area of 0.8 cm2 has been achieved for organic-nanostructured Si hybrid solar cells by inserting a cesium carbonate (Cs2CO3) layer between Si and the rear electrode aluminium (Al), which is realized by a solution process under low-temperature annealing (<150 °C). Transient and constant current–voltage, capacitance–voltage, and scanning Kelvin probe microscope measurements are used to characterize the effect of the Cs2CO3 layer on the device performance. The insertion of Cs2CO3 not only decreased the contact resistance, but also generated a built-in electric field on the rear electrode. The recombination rates are suppressed at the back surface due to the deflection of minority carriers. These findings show a promising strategy to achieve high performance organic–silicon solar cells with a simple, low temperature and cost effective process.

Shuit-Tong Lee

Papers

Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway

Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-Electron Pathway.

MoS2/Si Heterojunction with Vertically Standing Layered Structure for Ultrafast, High‐Detectivity, Self‐Driven Visible–Near Infrared Photodetectors

Solution-Processed Graphene Quantum Dot Deep-UV Photodetectors

High efficiency hybrid PEDOT:PSS/nanostructured silicon Schottky junction solar cells by doping-free rear contact