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Summary: The advent of graphene and, more recently, two-dimensional materials has opened new perspectives in electronics, optoelectronics, energy harvesting, and sensing applications. This book, based on a Special Issue published in Nanomaterials - MDPI covers experimental, simulation, and theoretical research on 2D materials and their van der Waals heterojunctions. The emphasis is the physical properties and the applications of 2D materials in state-of-the-art sensors and electronic or optoelectronic devices.

Summary: Given that the threat of water shortage is expanding across the globe, the evolution of advanced technologies that enable water purification and, thus, water re-use in an energy and resource efficient manner are of great importance. In this regard, nanomaterials have been playing a crucial role and offering new opportunities for the construction of permeable and selective membranes and adsorbents. Such features are of paramount importance, particularly given the limited available energy resources. In this book, several recent studies are introduced that deal with water treatment via nanomaterial-based technologies. Such state-of-the-art technologies have employed nanomaterials that are made of polymer, composite, ceramic, and carbon, etc., and are shaped in various dimensionalities and forms such as particle (0D), fiber (1D), and film (2D-3D). The nanostructured membranes and adsorbents as well as photocatalytic nanosystems capable of active photodecomposition of organic pollutants, e.g., dyes, are the main focal points of discussion.

Summary: The papers selected for publication in the Special Issue "Urban and industrial wastewater disinfection and decontamination by Advanced Oxidation Processes (AOPs): current issues and future trends" and published in this book, include topics related with (waste)water treatment and its reuse, disinfection, and pollutant degradation, advancing the know-how of the topics or assessing their integration into the new age of the circular economy of water. Likewise, aspects such as modeling of degradation processes and new materials synthesis were published, aiming to improve, optimize and predict the efficacy of the existing or the novel treatment processes. Moreover, pilot plant operation and large-scale processes were featured, assessing the feasibility of these new treatment methods in real world applications. Overall, the Special Issue and this book present innovative solutions on the field of water and wastewater treatment, with a view on the future technologies that will form the next advances of the field.

Summary: This Special Issue highlights the last decade's progress regarding new nanostructured materials. In this regard, the development of nanoscale syntheses and innovative characterization tools resulted in the tailored design of nanostructured materials with versatile abilities in many applications were investigated. Various types of engineered nanostructures, usually metal nanoparticles or nanoporous metal oxides, have been synthesized for various applications. This Special Issue covers the state-of-the-art of advanced nanoparticles in many disciplines (chemistry, pharmacy, nano-medicine, agriculture, catalysis, and environmental science). The Special Issue entitled, Synthesis, Characterization and Application of Sustainable Advanced Nanomaterials, including 18 original research works, focuses on highlighting the progress, challenges, and future directions in the area of the synthesis and characterization of nanomaterials and nanostructures with multiple applications in chemistry, physics, biology, and medicine [1-3].

Summary: With increasing attention on the reduction of CO2 emissions, the sustainability of construction materials has become one of the most growing issues in concrete science and engineering research fields. New sustainable construction materials, such as alkali-activated materials, calcium sulfoaluminate (CSA) cement, and limestone calcined clay cement (LC3), have been suggested and high-performance and/or highly durable construction materials using special admixtures, such as nanomaterials, have been studied to reduce the environmental impact during the life cycle of construction materials.This Special Issue aims to highlight and share recent findings in developing new sustainable cementitious materials, modeling their hydration kinetics, investigating their microstructure, improving the performance and durability of cementitious materials using functional supplementary ingredients, suggesting novel test methods for new construction materials, etc. In this Special Issue are presented the following areas:Sustainable concrete and alternative binders;Hydration kinetics of sustainable cementitious materials;Characterization of sustainable cementitious materials;Re-utilization of industrial by-products for construction materials;High strength and durable cementitious materials;Property enhancement by functional additives;Energy storage through construction materials.

Summary: At present, more and more procedures and technologies used to discover and characterize new materials are available, including advanced characterization techniques.This Special Issue covers a wide range of topics about obtaining and characterizing new materials, from the nano to macro scales, including for new alloys, ceramics, composites, biomaterials, and polymers and the procedures and technologies used to enhance their structure, properties, and functions. To select new materials for future use, we must first understand their structure and their characteristics using modern techniques such as microscopy (SEM, TEM, AFM, STM, etc.), spectroscopy (EDX, XRD, XRF, FTIR, XPS, etc.), and mechanical tests (tensile, hardness, elastic modulus, toughness, etc.) and their behaviors (in vitro and in vivo; corrosion; and thermal-DSC, STA, DMA, magnetic properties, and biocompatibility), among many others.

Summary: This reprint focuses on new trends in photo-electrocatalysis, specifically addressed to the remediation of wastewater and energy production. The remediation of wastewater, up to a level that is acceptable for discharge into receiving waterbodies, involves an ever-growing demand of energy, so effective and low-energy treatment processes are highly desirable. Among the other treatments, photo- and photo-electrochemical treatment processes may be considered as advanced oxidation processes (AOP), which are based on the generation of OH radicals, strong oxidizing agents able to indiscriminately degrade even the most persistent organic compounds. Photocatalysis and photo-electrocatalysis can be considered as effective methods for organic degradation, especially when the semiconductor is active in the range of visible light. Several results are presented on new morphologies and structures, which allow more photoactive, visibly responsive, and stable materials, as well as studies on combined processes in which photo- or photo-electrochemistry contribute to an increase in the sustainability of the whole process, lowering costs and achieving the most valuable final products. In view of the circular economy concept, microbial fuel cell systems are also considered as possible way to recover energy from organic pollutants contained in wastewater.

Summary: This Special Issue on "New Trends in Catalysis for Sustainable CO2 Conversion", released in the Catalysts open access journal, shows new research about the development of catalysts and catalytic routes for CO2 valorization, in addition to the optimization of the reaction conditions for the process. This issue includes ten articles and three reviews about different innovative processes for CO2 conversion.Carbon capture and storage (CCS) is a physical process consisting of the separation the CO2 (emitted by industry and the combustion processes for energy generation) and its transportation to geological storage isolates it from the atmosphere in the long term. However, the most promising routes for CO2 mitigation are those pursuing its catalytic valorization. By applying specific catalysts and suitable operating conditions, CO2 molecules react with other components to form longer chains (i.e., hydrocarbons). Accordingly, effort should be made to catalytically valorize CO2 (alone or co-fed with syngas) as an alternative way of reducing greenhouse gas emissions and obtaining high-value fuels and chemicals. Carbon capture and utilization (CCU) is a developing field with significant demand for research in the following aspects:The development of new catalysts, catalytic routes, and technologies for CO2 conversion;The study of new processes for obtaining fuels and chemicals from CO2;Optimization of the catalysts and the reaction conditions for these processes;Further steps in advanced processes using CO2-rich feeds (H2+CO2 or CO2 mixed with syngas), increasing product yields.

Summary: The conversion and storage of renewable energy sources is key to the transition from a fossil-fuel-based economy to a low-carbon society. Many new game-changing materials have already impacted our lives and contributed to a reduction in carbon dioxide emissions, such as high-efficiency photovoltaic cells, blue light-emitting diodes, and cathodes for Li-ion batteries. However, new breakthroughs in materials science and technology are required to boost the clean energy transition. All success stories in materials science are built upon a tailored control of the interconnected processes that take place at the nanoscale, such as charge excitation, charge transport and recombination, ionic diffusion, intercalation, and the interfacial transfer of matter and charge. Nanostructured materials, thanks to their ultra-small building blocks and the high interface-to-volume ratio, offer a rich toolbox to scientists that aspire to improve the energy conversion efficiency or the power and energy density of a material. Furthermore, new phenomena arise in nanoparticles, such as surface plasmon resonance, superparamegntism, and exciton confinement. The ten articles published in this Special Issue showcase the different applications of nanomaterials in the field of energy storage and conversion, including electrodes for Li-ion batteries and beyond, photovoltaic materials, pyroelectric energy harvesting, and (photo)catalytic processes.

Summary: Society is currently confronted with the continuing environmental problems of global warming and ocean acidification related to increasing CO2 emission from anthropogenic sources. These environmental issues are also connected to the inevitable energy supply shortage due to the eventual depletion of fossil fuel sources. As a solution, the technology of recycling CO2 into useful organic materials continues to attract attention. This methodology can be categorized into two main parts: CO2 fixation and CO2 reduction. For both reactions, molecular catalysts based on transition metal coordination complexes and organometallic compounds have been developed and examined. Molecular catalysts can be characterized and iteratively improved at the molecular level through spectroscopic experiments and the isolation of intermediate species, which is particularly advantageous in comparison to heterogeneous catalysts. The fixation of CO2 into organic compounds to form a carbon-carbon bond by using organometallic catalysts is a direct methodology for CO2 utilization and represents the potential reversible storage of electrochemical energy in chemical bonds. The resultant carboxylic acid-containing compounds formed as the initial products can be subsequently converted into other organic materials, even products with new chiral centers. The reduction of CO2 by two electrons (often with a proton donor as a co-substrate) yields carbon monoxide (CO) and formic acid (HCOOH), which can be further converted to useful chemicals. Reduction reactions involving more than two electrons and two protons can produce formaldehyde (HCHO), methanol (CH3OH), and methane (CH4), which are also desirable as chemicals and fuels. For molecular electrocatalysts, more negative potentials than the equilibrium ones for CO2 reduction are generally required; the difficulty is that the equilibrium potentials for CO2 reduction are generally negative of the equilibrium potential for proton reduction to produce H2, representing a competing thermodynamically favored process. A complementary approach to an electrochemical one is to mediate CO2 reduction with photo-induced electron transfer reactions. Photo- and electrocatalytic CO2 reduction can be used to achieve artificial photosynthesis, or the production of commodity chemicals and fuels with renewable energy inputs originating from solar sources. This Research Topic covers the molecular catalysts based on coordination and organometallic compounds for CO2 fixation/reduction. It includes chemical, electrochemical, and photochemical reactions. It also covers systematic studies of reaction mechanisms and the spectroscopic characterization of catalytic intermediates. Molecular catalysts for CO2 fixation/reduction used as co-catalysts with heterogeneous catalytic systems are also included. Non-precious and abundant transition metal catalysts for CO2 fixation/reduction are important for future industrial applications as core components of the next generation of energy technologies.

Summary: This reprint is the compilation of the articles submitted in the Special Issue entitled "Metallic or Metallic Oxide (Photo)catalysts for Environmental Applications" from the journal Catalysts, which gives an overview of the latest advances in the development of metallic or metallic oxide (photo)catalytic materials, with environmental applications for the elimination of organic pollutants or the valorization of biomass.

Summary: Water pollution from biorefractory pollutants and emerging contaminants is still a very relevant problem worldwide. Examples of these pollutants include disinfection by-products, pharmaceutical and personal care products, persistent organic chemicals, as well as their degradation products. The occurrence of these contaminants in water has raised increasing concern due to their accumulation and persistence in the environment and the threat they pose to the ecological system and human health. In this Special Issue, papers regarding the advancements in materials and processes for use in the electro- and photoelectrochemical removal of different pollutants from water are collected. The synthesis, characterization and application of materials for use in electrochemical or photoelectrochemical techniques are presented, as well as studies concerning catalytic processes and reaction kinetics.

Summary: This Special Issue reprint covers the synthesis of luminescent nanomaterials, applications for in vitro and in vivo imaging, detection based on fluorescence resonance energy, and the applications of luminescent nanoparticles for photodynamic activation and solid-state displays, as well as new materials and structures, such as perovskite quantum dots, and novel phenomena, such as aggregation-induced emissions. In total, there are 15 papers (2 reviews and 13 research articles) in this Special Issue consisting of the major areas of upconversion of luminescent nanomaterials; luminescent nanomaterials for solid-state lighting, displays, and anti-counterfeiting; particle-based sensing technology; particle-based therapeutics; and new materials and structures. This Issue represents new developments in luminescence nanotechnology.

Summary: Nanomaterials are a large area of research at present. These materials, which have at least one of their dimensions in the nanoscale (i.e., in a length range from 1 nm to 100 nm), have remarkable or unconventional properties, unlike bulk materials. These materials are currently used in many applications; however, new potential uses are being investigated. In this sense, there is large interest in their use in medicine, electronic devices, the production and storage of energy, composite materials, etc. The production of nanomaterials is addressed through physical and/or chemical methods; however, most of these methods exhibit low reproducibility or a low production rate or make use of toxic chemicals. In order to avoid most of these drawbacks, the laser-based synthesis of nanomaterials has emerged as an alternative to overcome these limitations. This family of methods use a laser beam to produce different nanomaterials (e.g., nanoparticles, nanowires or 2D materials) using diverse approaches. Techniques such as those based on laser ablation, laser vaporization, pulsed laser deposition (PLD), laser-chemical vapor deposition (LCVD), etc., are being explored at present to fabricate these nanoscale materials with a controlled size and shape. In this context, here we present research papers addressing the most recent developments in this field to summarize the current state of the art in the synthesis of nanomaterials using laser techniques.

Summary: Population growth and climate change are leading to global water scarcity. Water shortages are thus hindering rural, urban and industrial development. These days, approximately half of the world's population is affected temporarily by water scarcity. To enable a secure water supply, alternative water sources must be generated to tackle the challenge of water scarcity. An important alternative resource is the reuse of treated wastewater. Water reuse processes are rarely considered and implemented. In contrast to the storage and use of rainwater, treated wastewater is a valuable resource, as it is available daily. Certain wastewater treatment processes are required to produce the new resource "reused water". The treatment processes depend on the quality of the wastewater since industrial and municipal wastewater flows are characterized by different concentrations. Moreover, water reuse methods must be developed in order to use the treated wastewater as efficiently as possible. Ideally, the reused water can be provided according to the "fit for purpose" principle and applied directly in areas such as irrigation, street cleaning, toilet flushing or make-up water for cooling systems.The Special Issue brings together new wastewater treatment technologies and water reuse concepts to tackle the challenges of climate change with the aim of bringing the resource "reused water" according to the "fit for purpose" principle to the user. This issue aims to draw on global experiences, approaches and solutions.

Summary: This reprint is a compilation of the articles submitted in the Special Issue entitled, "Heterogeneous Photocatalysis: A Solution for a Greener Earth", from the journal Catalysts, which presents an overview of the latest advances in the development of innovative photocatalytic processes.

Summary: Catalysts are widely used in a great variety of technologies, providing remarkable efficiency in order to address sustainable energy production, climate change challenges, and to reduce industrial emissions. In the framework of the Environmental Catalysis section promoted by the Catalysts Editorial Office, this Special Issue, entitled "Environmental Friendly Catalysts for Energy and Pollution Control Applications", comprises novel studies representing the state-of-the-art research for efficient energy generation and industrial emission control based on new environmentally friendly catalyst materials (EFCs). In particular, in this Special Issue (SI), different kinds of catalysts are presented for catalytic solutions, including the reduction of NOx emissions (new zeolite catalyst modified with Pt), the elimination of volatile organic compounds (Co3O4@SiO2 and acidic surface transformed natural zeolite) and the removal of SO2 emissions (through adsorption processes with sodium citrate). Moreover, novel biocatalysts for bioanodes and new functional nanostructured catalysts based on metal-organic framework (MOFs) for different applications are also included. Additionally, articles compiled in this SI are also focused on the improvement of catalytic processes. Thus, selected processes based on activated carbons (modified with titanium dioxide) and optimized Fenton processes for the removal of aqueous organic pollutants or for the inactivation of bacteria are also presented.