2.3. Evaluation of Photocatalytic Water Splitting 6507 2.3.1. Photocatalytic Activity 6507 2.3.2. Photocatalytic Stability 6507 3. UV-Active Photocatalysts for Water Splitting 6507 3.1. d0 Metal Oxide Photocatalyts 6507 3.1.1. Ti-, Zr-Based Oxides 6507 3.1.2. Nb-, Ta-Based Oxides 6514 3.1.3. W-, Mo-Based Oxides 6517 3.1.4. Other d0 Metal Oxides 6518 3.2. d10 Metal Oxide Photocatalyts 6518 3.3. f0 Metal Oxide Photocatalysts 6518 3.4. Nonoxide Photocatalysts 6518 4. Approaches to Modifying the Electronic Band Structure for Visible-Light Harvesting 6519

Semiconductor-based Photocatalytic Hydrogen Generation

Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in...

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Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

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

Graphene has attracted tremendous research interest in recent years, owing to its exceptional properties. The scaled-up and reliable production of graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO), offers a wide range of possibilities to synthesize graphene-based functional materials for various applications. This critical review presents and discusses the current development of graphene-based composites. After introduction of the synthesis methods for graphene and its derivatives as well as their properties, we focus on the description of various methods to synthesize graphene-based composites, especially those with functional polymers and inorganic nanostructures. Particular emphasis is placed on strategies for the optimization of composite properties. Lastly, the advantages of graphene-based composites in applications such as the Li-ion batteries, supercapacitors, fuel cells, photovoltaic devices, photocatalysis, as well as Raman enhancement are described (279 references).

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Graphene-based composites

The increasing human need for clean and renewable energy has stimulated research in artificial photosynthesis, and in particular water photoelectrolysis as a pathway to hydrogen fuel. Nanostructured devices are widely regarded as an opportunity to improve efficiency and lower costs, but as a detailed analysis shows, they also have considerably disadvantages. This article reviews the current state of research on nanoscale-enhanced photoelectrodes and photocatalysts for the water splitting reaction. The focus is on transition metal oxides with special emphasis of Fe2O3, but nitrides and chalcogenides, and main group element compounds, including carbon nitride and silicon, are also covered. The effects of nanostructuring on carrier generation and collection, multiple exciton generation, and quantum confinement are also discussed, as well as implications of particle size on surface recombination, on the size of space charge layers and on the possibility of controlling nanostructure energetics via potential determining ions. After a summary of electrocatalytic and plasmonic nanostructures, the review concludes with an outlook on the challenges in solar fuel generation with nanoscale inorganic materials.

Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting

We measured the proton conductivity of bulk graphite oxide (GO'), a graphene oxide/proton hybrid (GO-H), and a graphene oxide (GO) nanosheet for the first time. GO is a well-known electronic insulator, but for proton conduction we observed the reverse trend, as it exhibited superionic conductivity. The hydrophilic sites present in GO as -O-, -OH, and -COOH functional groups attract the protons, which propagate through hydrogen-bonding networks along the adsorbed water film. The proton conductivities of GO' and GO-H at 100% humidity were ∼10(-4) and ∼10(-5) S cm(-1), respectively, whereas that for GO was amazingly high, nearly 10(-2) S cm(-1). This finding indicates the possibility of GO-based perfect two-dimensional proton-conductive materials for applications in fuel cells, sensors, and so on.

Graphene Oxide Nanosheet with High Proton Conductivity

Graphene oxide (GO) nanosheets have many semiconducting π-conjugated sp2 domains of various sizes within an oxygenated sp3 domain. The chemical and physical properties, therefore, can be widely tuned by adjusting the degree of oxidation through photoreaction, because the semiconducting domains act as a semiconducting photocatalyst when irradiated with light of energy exceeding the band gap of the domains. In this paper, we report new photoreactions similar to photocatalytic reactions in which H2 and CO2 are evolved from an aqueous suspension of GO nanosheets under UV irradiation; these reactions are based on the photoreactions of oxygen-containing functional groups and carbon. As a result, reduced GO (rGO) nanosheets with many holes and defects were produced. We conjecture that some carbons in the holes belong to zigzag edges, CH bonds, or both and that the rGO nanosheets therefore have ferromagnetic properties. In photoelectrochemical measurements of a GO nanosheet electrode, a large anodic photocurrent ...

Photoreaction of Graphene Oxide Nanosheets in Water

Graphene oxide (GO) nanosheets were reduced by UV irradiation in H2 or N2 under mild conditions (at room temperature) without a photocatalyst. Photoreduction proceeded even in an aqueous suspension of nanosheets. The GO nanosheets reduced by this method were analyzed by X-ray photoelectron spectroscopy and Raman spectroscopy. It was found that epoxy groups attached to the interiors of aromatic domains of the GO nanosheet were destroyed during UV irradiation to form relatively large sp2 islands resulting in a high conductivity. I−V curves were measured by conductive atomic force microscopy (AFM; perpendicular to a single nanosheet) and a two-electrode system (parallel to the nanosheet). They revealed that photoreduced GO nanosheets have high conductivities, whereas nonreduced GO nanosheets are nearly insulating. Ag+ adsorbed on GO nanosheets promoted the photoreduction. This photoreduction method was very useful for photopatterning a conducting section of micrometer size on insulating GO. The developed pho...

Simple Photoreduction of Graphene Oxide Nanosheet under Mild Conditions

One-nanometer-thick nickel hydroxide nanosheets were prepared by exfoliation of layered nickel hydroxides intercalated with dodecyl sulfate (DS) ions. The shape of the nanosheets was hexagonal, as was that of the layered nickel hydroxides intercalated with DS ions. The nickel hydroxide nanosheets exhibited charge-discharge properties in strong alkaline electrolyte. The morphology of the nanosheet changed during the electrochemical reaction.

Synthesis of Hexagonal Nickel Hydroxide Nanosheets by Exfoliation of Layered Nickel Hydroxide Intercalated with Dodecyl Sulfate Ions

X-ray photoelectron spectroscopy (XPS) is among the most powerful techniques to analyze defective structures of carbon materials such as graphene and activated carbon. However, reported assignments of defects, especially sp3C and sp2C, are questionable. Most reports assign sp3C peaks to be higher than sp2C peaks, whereas a few reports assign sp3C peaks to be lower than sp2C peaks. Our group previously reported that calculated binding energies of sp3C were basically lower than those of sp2C. This work clarified that one of the reasons for the prevailing ambiguous assignments of sp3C peaks is charging effects of diamond.

Michio Koinuma

Papers

Graphene Oxide Nanosheet with High Proton Conductivity

Photoreaction of Graphene Oxide Nanosheets in Water

Simple Photoreduction of Graphene Oxide Nanosheet under Mild Conditions

Synthesis of Hexagonal Nickel Hydroxide Nanosheets by Exfoliation of Layered Nickel Hydroxide Intercalated with Dodecyl Sulfate Ions

Origins of sp(3)C peaks in C1s X-ray Photoelectron Spectra of Carbon Materials.