Nano Materials (Plasmonic Photocatalysts)

Heterogeneous photocatalysis is a promising technology especially for environmental remediation. Despite more than a decade of worldwide research in developing photocatalytic efficiency improving techniques, many questions regarding the large scale application of photocatalytic reactors still remain unanswered. Recently, improving the photocatalytic efficiency has gained scientific attention because it might lead to more economical and robust photocatalytic operation for environmental remediation. In this review, fundamental and comprehensive assessments of the photocatalytic concepts and their applications for environmental remediation are reviewed. The existing challenges and strategies to improve the photocatalytic efficiency are discussed. Further, recent developments and future research prospects on photocatalytic systems for environmental applications are also addressed.

This book details the chemistry of visible light-induced photocatalysis using different classes of nanocomposites. Starting with a general introduction and explanation of basic principles and mechanisms of (visible) light-induced photocatalysis in the first two chapters, the following chapters detail the different types and classes of nanocomposites currently used in light-induced photocatalytic applications, including e.g. metal and mixed metal-oxide nanoparticles and –composites, nanoporous materials, polymeric and carbon-based nanocomposites. They explain the characteristics and importance of the different types of nanocomposites, as well as their synthesis and fabrication.In the end of the book an outlook on the unique applications of novel nanocomposites is offered, for example in water treatment and disinfection and removal of pollutants from wastewater, self-cleaning window panes based on photoactive materials, and many more. The book also addresses the challenges in present photocatalytic research, and therefore is a must-read for everybody interested in the developing field of nanocomposites and visible light-induced photocatalysis.

We report the synthesis of silver (Ag) nano-spheres (NS) supported on zinc oxide (ZnO) nanorods through two step mechanism, using open vessel microwave reactor. Direct reduction of ZnO from zinc nitrates was followed by deposition precipitation of the silver on the ZnO nanorods. The supported Ag/ZnO nanoparticles were then characterized by electron microscopy, X-ray diffraction, FTIR, photoluminescence and UV–vis spectroscopy. The visible light photocatalytic activity of Ag/ZnO system was investigated using a test contaminant, methylene blue (MB). Almost complete removal of MB in about 60 min for doses higher than 0.5 g/L of the Ag/ZnO photocatalyst was achieved. This significant improvement in the photocatalytic efficiency of Ag/ZnO photocatalyst under visible light irradiation can be attributed to the presence of Ag nanoparticles on the ZnO nanoparticles which greatly enhances absorption in the visible range of solar spectrum enabled by surface plasmon resonance effect from Ag nanoparticles.

Ag and CuO nanoparticles (NPs) synthesized on the surface of commercial TiO 2 (P25) by radiolytic reduction were characterized by diffuse reflectance spectroscopy (DRS), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). In the case of modification with silver and copper, results from HAADF-STEM, EDS, XPS, and XAS show that Ag@CuO nanoparticles (large silver cores decorated with small clusters of CuO) were obtained on TiO 2 −P25. The photocatalytic properties of bare and modified TiO 2 −P25 were studied for phenol photodegradation and for acetic acid oxidation under UV and visible irradiation. The mechanisms involved in photocatalysis were studied by time-resolved microwave conductivity (TRMC) and action spectra (AS). The electronic properties of the surface-modified TiO 2 −P25 were studied by TRMC to follow the charge-carrier dynamics. The modification with Ag nanoparticles or CuO nanoclusters induces an increase in the photocatalytic activity under both UV and visible light. The photocatalytic activity of Ag@CuO/P25 is higher under UV light but lower under visible light compared to the activity of CuO/P25 and Ag/P25. TRMC measurements show that surface modification of TiO 2 −P25 with Ag, CuO, and Ag@CuO nanoparticles plays a role in charge-carrier separation, increasing the activity under UV-light, and that Ag@CuO NPs are more efficient electron scavengers than Ag NPs and CuO nanoclusters. The localized surface plasmon resonance (LSPR) of Ag NPs and the narrow band gap of CuO induce an activity under visible light. The TRMC shows also responses under visible-light irradiation at different fixed wavelengths indicating that electrons are injected from Ag NPs in the conduction band (CB) of TiO 2 −P25. Moreover, under visible light, the photocatalytic activity of CuO/P25 is higher than that of plasmonic Ag/P25. CuO is able to activate TiO 2 −P25 in a wider range of wavelengths under visible-light irradiation, compared to the activation achieved by the presence of silver. The action spectra correlate with the absorption spectra for irradiation wavelengths in the range of 350−470 nm proving that decomposition of acetic acid is carried out by a photocatalytic mechanism.

Solar Energy is an everlasting source of energy with minimal carbon footprint. However, due to lack of reliability and consistency it needs to be converted into more reliable and effective means that can be used to provide energy on demand. Hydrogen is a promising carrier and is thus an efficient mean of energy to be converted in, stored and transported. A more direct approach towards harnessing Solar Energy is by photo-aided generation of hydrogen via splitting of water using photolysis. Photo-catalytic water splitting is therefore a promising method for future energy security. On the other hand, strain sensing is a useful technique to measure medium range loads in trusses or tension rods & can easily replace the existing fragile & expensive semiconductor based sensors. This was done by using a composite of TiO 2 /Reduced Graphene Oxide (RGO); TiO 2 (anatase) was synthesized via sol-gel process and the main precursor was titanium tetrapropoxide (TTIP). Titania (anatase) characterized by XRD and photo spectrometry while Graphene oxide was synthesized via modified Hummer's Method. The obtained Reduce Graphene Oxide was dispersed using Sodium Dodecyl Benzene (SDB) and Hydrazine Hydrate. A drastic synergetic effect was found by simply mixing RGO with TiO 2 Solution. This not only broadened the photo-activity spectrum of TiO 2 from UV region to the more available visible light radiation but also exhibit strain sensing properties and considerable tunable gauge factor. The photocatalytic effect of our composite was tested by coating it over Polycarbonate & then analyzing emitted gas bubbles in a UV radiation chamber while strain sensing was done by coating it on an elastic substrate & applying loads against values of resistance which were measured. This study will also include the reduction of recombination and band gap of TiO 2 in order to synchronize it with the available Solar Spectrum thus maximizing solar-to-hydrogen efficiency.

Titanium dioxide (TiO2) nanoparticles were decorated with different amounts of silver nanoparticles (AgNPs) using an electrochemically active biofilm (EAB), which is a biogenic approach that leads to the formation of Ag@TiO2 nanocomposites. UV-vis spectroscopy, photoluminescence, X-ray diffraction and electron microscopy showed AgNPs, 2–5 nm in size, well-dispersed and anchored to the TiO2 surface and overall synthesis of Ag@TiO2 nanocomposites. The photocatalytic performance of the as-synthesized Ag@TiO2 nanocomposites was evaluated in terms of their efficiency for the photodecomposition of methylene blue (MB) in an aqueous solution under visible light irradiation. The nanocomposites showed exceptionally high photodecomposition efficiency (>7 times) compared to commercial TiO2 (Sigma). The enhanced photocatalytic activity was attributed to the synergistic contribution of both a delayed charge recombination rate caused by the high electronic mobility of the AgNPs and the increased surface area originating from the nanometer sized AgNPs on TiO2. The nanocomposites also showed exceptionally high stability and reusability under similar experimental conditions.

Assemblies of closely separated gold nanoparticles exhibit a strong collective plasmonic response due to coupling of the plasmon modes of the individual nanostructures. In the context of self-assembly of nanoparticles, close-packed two-dimensional (2D) clusters of spherical nanoparticles present an important composite system that promises numerous applications. The present study probes the collective plasmonic characteristics and resulting photothermal behavior of close-packed 2D Au nanoparticle clusters to delineate the effects of the cluster size, interparticle distance, and particle size. Smaller nanoparticles (20 and 40 nm in diameter) that exhibit low individual scattering and high absorption were considered for their relevance to photothermal applications. In contrast to typical literature studies, the present study compares the optical response of clusters of different sizes ranging from a single nanoparticle up to large assemblies of 61 nanoparticles. Increasing the cluster size induces significant changes to the spectral position and optophysical characteristics. Based on the model outcome, an optimal cluster size for maximum absorption per nanoparticle is also determined for enhanced photothermal effects. The effect of the particle size and interparticle distance is investigated to elucidate the nature of interaction in terms of near-field and far-field coupling. The photothermal effect resulting from absorption is compared for different cluster sizes and interparticle distances considering a homogeneous water medium. A strong dependence of the steady-state temperature of the nanoparticles on the cluster size, particle position in the cluster, incident light polarization, and interparticle distance provides new physical insight into the local temperature control of plasmonic nanostructures.

Titanium dioxide exists in a large number of polymorphs, the most common ones being, in order of abundance, rutile, anatase and brookite. Recently, there has been an increasing degree of attention on TiO2 , due to its photocatalytic and antibacterial properties. In this work, titania nanopowders synthesized via an aqueous sol–gel method were modified with silver and/or tungsten, with potential application as photocatalysts and antibacterial agents. The dried gels were thermally treated at two temperatures, and the occurrence of amorphous phase—in both dried and calcined gels—was obtained using the Rietveld method. In powders calcined at 600 °C, the crystal structure of rutile was determined with the Rietveld method on the full profile fitting of the X-ray powder diffraction patterns, while microstructural information was achieved by means of Williamson–Hall plots and line broadening analysis methods. The photochromic property of the Ag and W/Ag modified samples—when subjected to both UV-and visible-light irradiation and for different time expo-sures—was also investigated via UV–Vis spectroscopy. Powders were shown to possess a tuneable photochromism due to the surface plasmon resonance of the added silver, depending on the firing temperature, light (and time) exposure, and on whether or not tungsten was present in the nanopowders.

Metal oxides are of great technological importance in environmental remediation and electronics because of their capability to generate charge carriers when stimulated with required amount of energy. The promising arrangement of electronic structure, light absorption properties, and charge transport characteristics of most of the metal oxides has made possible its application as photocatalyst. In this article definition of metal oxides as photocatalyst, structural characteristics, requirements of the photocatalyst, classification of photocatalysts and the mechanism of the photocatalytic process are discussed.

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Describes different types of nanocomposites, such as metal nanocomposites and mixed metal-oxide nanoparticles, nanoporous nanocomposite materials, polymer and carbon-based nanocomposites
Offers a concisely structured and systematic overview for newcomers to a developing field
Provides the fundamentals as well as an outlook on applications, future perspectives and challenges

Chalcogenides are narrow-band gap semiconductors that have been widely used as photocatalysts. These narrow-band gap materials allow more efficient absorption of over 40% of solar energy in the visible light range, which will eventually improve its photocatalytic properties. Under visible light irradiation, these materials generate electron and hole (e/h+) pairs. Photo-generated e/h+ pairs have been utilized to split water into hydrogen and oxygen and to remove and degrade industrial, pharmaceutical and agricultural organic/inorganic/biological pollutants that have been accumulated in the environment. In this perspective review, different types of chalcogenides, namely, binary, multinary (ternary and quaternary) and chalcogenide-based heterostructures are presented briefly. This perspective review also highlights the mechanisms involved and remarkable photocatalytic activity enhancement under visible light irradiation that has been widely researched such as the photocatalytic degradation of industrial pollutants and photocatalytic inactivation of bacteria. Lastly, future prospects for the use of chalcogenides as photocatalysts and chalcogenide-based heterostructures were discussed.

Bimetallic plasmonic nanoparticles enable tuning of the optical response and chemical stability by variation of the composition. The present numerical simulation study compares Ag−Au alloy, Ag@Au core−shell, and Au@Ag core−shell bimetallic plasmonic nanoparticles of both spherical and anisotropic (nanotriangle and nanorods) shapes. By studying both spherical and anisotropic (with LSPR in the near-infrared region) shapes, cases with and without interband transitions of Au can be decoupled. Explicit comparisons are facilitated by numerical models supported by careful validation and examination of optical constants of Au−Ag alloys reported in the literature. Although both Au−Ag core−shell and alloy nanoparticles exhibit an intermediary optical response between that of pure Ag and Au nanoparticles, there are noticeable differences in the spectral characteristics. Also, the effect of the bimetallic constitution in anisotropic nanoparticles is starkly different from that in spherical nanoparticles due to the absence of Au interband transitions in the former case. In general, the improved chemical stability of Ag nanoparticles by incorporation of Au comes with a cost of reduction in plasmonic enhancement, also applicable to anisotropic nanoparticles with a weaker effect. A photothermal heat transfer study confirms that increased absorption by the incorporation of Au in spherical Ag nanoparticles also results in an increased steady-state temperature. On the other hand, anisotropic nanoparticles are inherently better absorbers and hence better photothermal sources, and their photothermal properties are apparently not strongly affected by the incorporation of one metal in the other. This study of the optical/spectral and photothermal characteristics of bimetallic Au−Ag alloy versus core−shell nanoparticles provides detailed physical insight for development of new taylor-made plasmonic nanostructures.

Chalcogenides are narrow-band gap semiconductors that have been widely used as photocatalysts. These narrow-band gap materials allow more efficient absorption of over 40% of solar energy in the visible light range, which will eventually improve its photocatalytic properties. Under visible light irradiation, these materials generate electron and hole (e À /h +) pairs. Photo-generated e À /h + pairs have been utilized to split water into hydrogen and oxygen and to remove and degrade industrial, pharmaceutical and agricultural organic/inorganic/biological pollutants that have been accumulated in the environment. In this perspective review, different types of chalcogenides, namely, binary, multinary (ternary and quaternary) and chalcogenide-based heterostructures are presented briefly. This perspective review also highlights the mechanisms involved and remarkable photocatalytic activity enhancement under visible light irradiation that has been widely researched such as the photocatalytic degradation of industrial pollutants and photocatalytic inactivation of bacteria. Lastly, future prospects for the use of chalcogenides as photocatalysts and chalcogenide-based heterostructures were discussed.

Chalcogenides are narrow-band gap semiconductors that have been widely used as photocatalysts. These narrow-band gap materials allow more efficient absorption of over 40% of solar energy in the visible light range, which will eventually improve its photocatalytic properties. Under visible light irradiation, these materials generate electron and hole (e/h +) pairs. Photo-generated e/h+ pairs have been utilized to split water into hydrogen and oxygen and to remove and degrade industrial, pharmaceutical, and agricultural organic/inorganic/biological pollutants that have been accumulated in the environment. In this perspective review, different types of chalcogenides, namely, binary, multinary (ternary and quaternary), and chalcogenide-based heterostructures are presented briefly. This perspective review also highlights the mechanisms involved and remarkable photocatalytic activity enhancement under visible light irradiation that has been widely researched such as the photocatalytic degradation of industrial pollutants and photocatalytic inactivation of bacteria. Lastly, future prospects for the use of chalcogenides as photocatalysts and chalcogenide-based heterostructures were discussed.

About this Research Topic:

Photocatalysis is a chemical reaction that takes place in presence of a photocatalyst and a suitable light. In other words, it is a reaction that uses light to generate a pair of excited electron and positive hole to induce redox reactions, as the first step, with both positive and negative Gibbs-energy change.

A photocatalyst is a material that harvests light to bring it to higher energy level and provides such energy to a reacting substance to make a chemical reaction to take place. Multi-anion photocatalysts contain two or more anion-forming elements and exhibit unprecedented photocatalytic, optoelectronic and physicochemical properties compared with homo-anion counterparts. For example, N,S co-doped TiO2 has been reported for enhanced visible light induced photocatalytic activities for energy and environment related applications.

Photocatalysis promises a solution to challenges associated with the intermittent nature of sunlight that is considered as renewable and ultimate energy source to power and energy related activities on the Earth. Other examples are water splitting, hydrogen production, water treatment and disinfection, removal of pollutants from contaminated water and air, photocatalytic tiles, photocatalytic paints, self-cleaning window panels based on photoactive materials and many more. Hence, there is a need to understand the fundamental concepts of photocatalysis in a simple way of using multi-anion photocatalysts.

This research topic will provide a complete overview to photocatalysis combining the theoretical and fundamental concepts of photocatalysis from synthesis of novel photocatalysts, reaction engineering, and exploring several important applications of photocatalysis. Most importantly, this research topic will mainly focus on multi-anion photocatalysts for environmental remediation, energy production, conversion etc. The objectives include: 1. All the latest information about multi-anion photocatalyst materials will be compiled and readers will be benefited. 2. Fundamental and advanced understanding of multi-anion photocatalysts materials will be collected. 3. Use of multi-anion photocatalyst materials in energy conversion and environmental remediation will be focused.

This research topic will provide vast and extensive information about the multi-anion compounds as novel photocatalysts. All the latest developments in this field will be invited, evaluated and compiled. This research topic will be a valuable collection that will stimulate the interest of target audience as researchers and students in designing advanced nanostructured materials for wastewater treatment, energy production and conversion, environmental remediation etc. using multi-anion photocatalysts. We welcome submissions of Original Research, Review, Mini-review and Prospective from the following aspects:

• Recently developed multi-anion photocatalyst materials that assist in energy production and conversion, environmental remediation, etc.
• Fundamental understanding of controlled synthesis of multi-anion photocatalyst materials.
• Basics and mechanism of energy production and conversion, environmental remediation, etc.

Keywords: Multi-Anion Photocatalysts; Photocatalysts; Photocatalysis; Energy Conversion; Environmental Remediation

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