This feature article summarizes recent research on and development of semiconductor-based photocatalyst materials that are applicable to environmental remediation and/or chemical synthesis purposes. A wide variety of TiO2 particles and/or films have been studied during the past 30 years because they are the most stable and powerful photocatalysts leading to the degradation of various organic pollutants. The photocatalytic performance of other semiconductor materials such as ZnO, SnO2, WO3, Fe2O3 and CdS has also been intensively investigated. A general limitation in the efficiency of any photocatalytic process is the recombination of the photogenerated charge carriers, i.e., of electrons and holes, following bandgap illumination. Considerable efforts have been made to suppress this recombination and hence to enhance the charge carrier separation and the overall efficiency by means of coupling of different semiconductors with desirable matching of their electronic band structures, or incorporation of noble metal nanoclusters onto the surface of semiconductor photocatalyst particles. Modification of the physicochemical properties, such as particle size, surface area, porosity and/or crystallinity of the semiconductor materials, and optimization of the experimental conditions, such as pH, illumination conditions and/or catalyst loading, during photocatalytic reactions have also been carefully addressed to achieve high reaction rates or yields. To utilize solar energy more efficiently, i.e., to extend the optical absorption of the mostly UV-sensitive photocatalysts into the visible light range, numerous research groups have contributed to the development of novel visible light active photocatalysts. With the application of semiconductors with narrower bandgaps such as CdS, Fe2O3 and WO3 being straightforward choices, doping of wide bandgap semiconductors like TiO2 has been the most popular technique to enhance the catalysts' optical absorption abilities. Research on mixed-oxide-based semiconductor photocatalysts with deliberately modulated band structures has also attracted tremendous attention in the past decade, concentrating on, for example, the generation of H2 and/or O2 from H2O splitting, and the degradation of organic pollutants under visible light irradiation. Both theoretical calculations and experimental results have convincingly shown that the developed materials can serve as highly efficient photocatalysts that are both environmentally and economically significant.

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Photoelectrocatalytic materials for environmental applications

Janus particles, colloid-sized particles with two regions of different surface chemical composition, possess energetic interactions that depend not only on their separation but also on their orientation. Research on Janus and colloidal particles that are chemically patchy in even more complicated fashion has opened a new chapter in the colloid research field. This article highlights recent progress in both experiment and theory regarding synthesis and self-assembly of Janus particles, and tentatively outlines some areas of future opportunity.

Janus Particle Synthesis and Assembly

Although the concept of Janus particles was raised in the early 1990s, the related research has not attracted considerable interest until recently due to the special properties and applications of these colloidal particles as well as the advances in new fabrications. Janus particles can be divided into three categories: polymeric, inorganic, and polymeric–inorganic, and each kind of Janus particles can be spherical, dumbbell-like, half raspberry-like, cylindrical, disk-like, or any of a variety of other shapes. Different Janus particles may share common preparation principles or require specific fabrication processes, and may have different assembly behaviours and properties. This critical review discusses the main fabrication methods of the three kinds of Janus particles, and then highlights the important properties and applications of these Janus particles developed in recent years, and finally proposes some perspectives on the future of Janus particle research and development.

Fabrication, properties and applications of Janus particles

The site-specific engineering of colloidal surfaces has provided a powerful approach to pushing the boundaries of today's materials research. The resulting surface-anisotropic and patchy particles have become the center of vital research areas, ranging from the need for large-scale fabrication techniques to exploring new applications of these materials. This Review summarizes patchy particle fabrication techniques, including but not limited to particle and nanosphere lithography and glancing-angle deposition. The variety of existing patchy particle fabrication techniques is revealed and the need for a scalable approach to high-volume patchy particle production is identified. Ongoing modeling efforts describing patchy particle interactions and properties are reviewed as potential predictive tools. Research endeavors that deal with the directed assembly of patchy particles in electric and magnetic fields, as well as with supraparticular assembly through chemical interactions, are discussed. The Review is concluded with a note on the future application of patchy particles as phoretic motors.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/marc.200900614

Fabrication, assembly, and application of patchy particles.

A new approach has been developed for the fabrication of visible light photocatalysts. Nanoclusters of MoS2 and WS2 are coupled to TiO2 by an in situ photoreduction deposition method taking advantage of the reducing power of the photogenerated electrons from TiO2 particles. The photocatalytic degradation of methylene blue and 4-chlorophenol in aqueous suspension has been employed to evaluate the visible light photocatalytic activity of the powders. The blue shift in the absorption onset confirms the size quantization of MS2 nanoclusters, which act as effective and stable sensitizers, making it possible to utilize visible light in photocatalysis. Quantum size effects alter the energy levels of the conduction and valence band edges in the coupled semiconductor systems, which favors the interparticle electron transfer. In addition, the coupled systems are believed to act in a cooperative manner by increasing the degree of charge carrier separation, which effectively reduces recombination.

Preparation and photocatalytic behavior of MoS2 and WS2 nanocluster sensitized TiO2.

Spherical silica particles that are able to assemble at a phase boundary of a dual-phase mixture of water and an immiscible organic solvent were prepared by a partial modification of their surface hydroxyl groups with an alkylsilylation agent. Scanning electron microscopic observation of these particles in which their remaining surface hydroxyl groups had been selectively modified with colloidal gold particles revealed that each particle has an asymmetric surface structure:  one side of the surface is hydrophilic and the other is hydrophobic. We found that these particles could form a micellar structure in water in the presence of an organic solution of a toluene/polystyrene mixture. The micellar structure was evidenced by formation of golf-ball-like polystyrene particles with dimples imprinting morphologies of the hydrophobic part of modified silica particles.

Asymmetrically modified silica particles : A simple particulate surfactant for stabilization of oil droplets in water

The photocatalytic activity of CdS nanocrystallites (n-CdS) supported by TiO2 (P-25) nanocrystallites was higher for reductive dechlorination of 1,2,3,4-tetrachlorobenzene than that of n-CdS alone under visible light irradiation. This synergistic effect of TiO2 could be explained as being due to the adsorptive interaction of triethylamine (TEA) at the CdS/TiO2 interface, which contributed to the effective quenching of the holes formed in n-CdS by TEA.

Enhanced Photocatalytic Dechlorination of 1,2,3,4-Tetrachlorobenzene Using Nanosized CdS/TiO2 Hybrid Photocatalyst under Visible Light Irradiation

PROBLEM TO BE SOLVED: To provide a method for easily and reproducibly manufacturing ultra-fine particles of noble metals such as silver, gold and platinum, which have a nano-size (an average particle diameter of 30 nm or smaller) and a small particle-size distribution. SOLUTION: The method for manufacturing the ultra-fine particles comprises dissolving or dispersing a metal salt expressed by a general formula (1) in an organic solvent, and applying a microwave to the solvent in the state to activate it. Thereby formed ultra-fine particles are constituted of the above metal in the metal salt. COPYRIGHT: (C)2005,JPO&NCIPI

Takao Sakata

Papers

Asymmetrically modified silica particles : A simple particulate surfactant for stabilization of oil droplets in water

Enhanced Photocatalytic Dechlorination of 1,2,3,4-Tetrachlorobenzene Using Nanosized CdS/TiO2 Hybrid Photocatalyst under Visible Light Irradiation

Method for manufacturing ultra-fine particles

Enhanced Photocatalytic Dechlorination of 1,2,3,4‐Tetrachlorobenzene Using Nanosized CdS/TiO2 Hybrid Photocatalyst under Visible Light Irradiation.