This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references)

Heterogeneous photocatalyst materials for water splitting

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

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

Semiconductor-mediated photocatalysis has received tremendous attention as it holds great promise to address the worldwide energy and environmental issues. To overcome the serious drawbacks of fast charge recombination and the limited visible-light absorption of semiconductor photocatalysts, many strategies have been developed in the past few decades and the most widely used one is to develop photocatalytic heterojunctions. This review attempts to summarize the recent progress in the rational design and fabrication of heterojunction photocatalysts, such as the semiconductor–semiconductor heterojunction, the semiconductor–metal heterojunction, the semiconductor–carbon heterojunction and the multicomponent heterojunction. The photocatalytic properties of the four junction systems are also discussed in relation to the environmental and energy applications, such as degradation of pollutants, hydrogen generation and photocatalytic disinfection. This tutorial review ends with a summary and some perspectives on the challenges and new directions in this exciting and still emerging area of research.

Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances

Hollow micro-/nanostructures are of great interest in many current and emerging areas of technology. Perhaps the best-known example of the former is the use of fly-ash hollow particles generated from coal power plants as partial replacement for Portland cement, to produce concrete with enhanced strength and durability. This review is devoted to the progress made in the last decade in synthesis and applications of hollow micro-/nanostructures. We present a comprehensive overview of synthetic strategies for hollow structures. These strategies are broadly categorized into four themes, which include well-established approaches, such as conventional hard-templating and soft-templating methods, as well as newly emerging methods based on sacrificial templating and template-free synthesis. Success in each has inspired multiple variations that continue to drive the rapid evolution of the field. The Review therefore focuses on the fundamentals of each process, pointing out advantages and disadvantages where appropriate. Strategies for generating more complex hollow structures, such as rattle-type and nonspherical hollow structures, are also discussed. Applications of hollow structures in lithium batteries, catalysis and sensing, and biomedical applications are reviewed.

Hollow Micro-/Nanostructures: Synthesis and Applications**

The shape-controlled synthesis of nano- and microstructured materials has opened up new possibilities to improve their physical and chemical properties. In this work, new types of Bi(2)WO(6) with complex morphologies, namely, flowerlike, tyre- and helixlike, and platelike shapes, have been controllably synthesized by a facile hydrothermal process. The benefits of the present work also stem from the first report on the transformation of Bi(2)WO(6) from three-dimensional (3D) flowerlike superstructures to 2D platelike structures, and on the formation of tyre- and helixlike Bi(2)WO(6) superstructures. UV/Vis absorption spectra show that the optical properties of Bi(2)WO(6) samples are relevant to their size and shape. More importantly, the photocatalytic activities of Bi(2)WO(6) nano- and microstructures are strongly dependent on their shape, size, and structure for the degradation of Rhodamine B (RhB) under visible-light irradiation. The reasons for the differences in the photocatalytic activities of these Bi(2)WO(6) nano- and microstructures are further investigated.

Bi2WO6 nano- and microstructures: shape control and associated visible-light-driven photocatalytic activities.

Octahedral Cu2O crystals with tunable edge length were synthesized by reducing copper hydroxide with hydrazine without using any surfactant. Systematic experiments were carried out to investigate the factors which impact on the morphology and size of the products. The molar ratios of the reagents (NH3:Cu2+ and OH-:Cu2+) determined the morphology and size of the corresponding products via affecting the coordination between NH3 and Cu2+. It is demonstrated that the ratio of growth rate along 〈111〉 versus 〈100〉 was varied by adjusting the molar ratio of NH3 to Cu2+, thus Cu2O crystals with different morphologies such as spheres, cubelike, and octahedra were obtained. The edge lengths of octahedra can be easily tuned from 130 to 600 nm by adjusting the molar ratio of OH- to Cu2+. It is an effective and facile method for the controlled synthesis of octahedral Cu2O. The obtained octahedral Cu2O particles show improved ability on adsorption and photodegradation of methyl orange compared with cubic Cu2O particles.

Shape Evolution and Size-Controllable Synthesis of Cu2O Octahedra and Their Morphology-Dependent Photocatalytic Properties

Nanomaterials are attracting increasing attention due to their potential applications and enhanced properties. It is well known, for instance, that the size, morphology, and structure of nanomaterials significantly influence their physical and chemical properties and, therefore, their applications. Extensive studies have been devoted to the controlled synthesis of nanocrystals in the past few decades, and hollow nanostructures are of particular interest owing to potential applications such as drug delivery, artificial cells, lightweight fillers, catalysis, and chemical storage. 3] Various methodologies have been developed to achieve this special nanostructure. Template synthesis, for example, is a typical and effective route, and hard or soft templates, such as polymeric core supports, silica spheres, vesicles, and liquid droplets, have been applied. However, the resulting hollow spheres are generally single-shelled and polycrystalline. Recently, polycrystalline double-shelled hollow spheres have been synthesized by using hollow latex and polymer spheres as templates. The synthesis of hollow structured materials with a single-crystalline shell wall, designable inner space, and a complex structure (such as multishelled) will further extend their applications, although it remains a technological challenge. The self-assembly of surfactant molecules in aqueous solution leads to the formation of micelles and closed bilayer aggregates such as vesicles (also referred to as organized molecular assemblies) or dynamic nanostructures of surfactant molecules. Much effort has been devoted to the study and preparation of vesicles because they can be widely applied as delivery vehicles in the pharmaceutical and cosmetic industries and also as soft templates for the transcriptive synthesis of organic and inorganic hollow spheres. Since the bilayer structures separate an aqueous interior from an aqueous exterior, the interface between the surfactant groups and the solution presents a specific site for the growth of materials from solution. Crystal growth and solidification at the interface leads to transcriptive imprinting of the template morphology and the formation of hollow structured materials. Some vesicles such as dioctadecyldimethylammonium bromide (DODAB), poly(diallyldimethylammonium) chloride (PDADMAC), and mixtures of dodecyltrimethylammonium bromide (DTAB) and sodium dodecylbenzenesulfonate (SDBS), have been successfully used to synthesize inorganic single-shelled hollow spheres. Scheme 1a,b illustrates the

Template Synthesis of Multishelled Cu2O Hollow Spheres with a Single-Crystalline Shell Wall†

A novel flower-like Bi2WO6 superstructure was successfully realized by a facile hydrothermal process without any surfactant or template. Based on the evolution of this morphology as a function of hydrothermal time, the formation mechanism was proposed to be as follows: (1) self-aggregation of nanoparticles; (2) formation of crystalline nanoplates by Ostwald ripening; and (3) organization of the in situ-formed nanoplates into spherical superstructures. The pretty flower-like superstructure of Bi2WO6 was retained after calcination at 550 °C for 4 h. Both the uncalcined and calcined Bi2WO6 exhibited excellent visible-light-driven photocatalytic efficiencies for the degradation of Rhodamine B (RhB), up to 84 and 97% within 60 minutes, respectively, which were much higher than those of TiO2 (P-25) and Bi2WO6 sample prepared by solid-state reaction (SSR-Bi2WO6). Close investigation indicated that plenty of pores with different sizes existed in the Bi2WO6 superstructures, which could serve as hierarchical transport paths for small molecules and might greatly improve their photocatalytic activities.

Fabrication of flower-like Bi2WO6 superstructures as high performance visible-light driven photocatalysts

By introducing VO(3)(-) into the reaction system, uniform hierarchical nanostructures of Bi(2)O(3) have been successfully synthesized by a template-free aqueous method at 60-80 degrees C for 6 h. The as-prepared hierarchitectures are composed of 2D nanosheets, which intercross with each other. Based on the electron microscope observations, the growth of such hierarchitectures has been proposed as an Ostwald ripening process followed by self-assembly. The nucleation, growth, and self-assembly of Bi(2)O(3) nanosheets could be readily tuned, which brought different morphologies and microstructures to the final products. Pore-size distribution analysis revealed that both mesopores and macropores existed in the product. UV-vis spectroscopy was employed to estimate the band gap energies of the hierarchical nanostructures. The photocatalytic activities of as-prepared Bi(2)O(3) hierarchitectures were 6-10 times higher than that of the commercial sample, which was evaluated by the degradation of RhB dye under visible light irradiation (lambda>420 nm).

Haolan Xu

Papers

Bi2WO6 nano- and microstructures: shape control and associated visible-light-driven photocatalytic activities.

Shape Evolution and Size-Controllable Synthesis of Cu2O Octahedra and Their Morphology-Dependent Photocatalytic Properties

Template Synthesis of Multishelled Cu2O Hollow Spheres with a Single-Crystalline Shell Wall†

Fabrication of flower-like Bi2WO6 superstructures as high performance visible-light driven photocatalysts

Bi2O3 Hierarchical Nanostructures: Controllable Synthesis, Growth Mechanism, and their Application in Photocatalysis