Nanomaterials for Eco-friendly Applications

The synthesis of 3D architectures composed of carbon nanotubes (CNT) is one of the most exciting and challenging research domains in nanotechnology. These systems have great potential for supercapacitors, catalytic electrodes, artificial muscles and in environmental applications. In this chapter, we present an overview of the CNT sponges, which are characterized by high hydrophobicity, high oil absorption capacity, high-performance in mechanical test and high porosity making it an attractive candidate for environmental applications.

Different technologies are being developed to reduce emissions, and capture or store CO₂ emitted into the atmosphere. However, despite efforts, CO₂ capture methodologies continue to be a challenge to transform it into a stable, non-polluting product. In this context, the electrolytic conversion of CO₂ to carbon in molten salts has excellent possibilities for the production of large numbers of carbonaceous materials. Carbon nanostructures can be easily obtained by changing process parameters such as electrolytes, electrodes, and atmosphere. The final materials (carbon nanotubes, carbon spheres, carbon fibers) can have exceptional performance in energy conversion and storage, as well as electrocatalysis and merit future research.

The need for replacing fossil fuels by renewable energy sources has been growing over the last years due to the increasing demand for energy worldwide and environmental issues. One of the main compounds studied for future application as fuel is hydrogen gas (H₂). Extensive research has been performed on H₂ production via water splitting using solar light since Honda and Fujishima report in 1972. One of the most utilized materials for water splitting applications is titanium dioxide (TiO₂). TiO₂ is abundant, inexpensive, non-corrosive, stable and environment-friendly. However, its large band gap and rapid electron-hole recombination hinders the hydrogen production efficiency. In order to improve the efficiency in photoconversion, TiO₂ has been modified with several materials, including carbon nanotubes (CNT). A variety of different structural forms of CNT/TiO₂ composites have been studied. In this chapter, we review morphologies and methods of synthesis of CNT/TiO₂ nanocomposites, parameters employed at electrochemical tests and results of H₂ evolution.

The use of semiconductors as catalysts in photocatalytic reactions is an alternative for the synthesis of esters with the potential to simplify the production process by enabling the use of ambient temperature and pressure. Photocatalysis is a process involving the use of a light source to trigger oxidation and reduction reactions through the formation of electrons and holes by excitation of the semiconductor. The use of photocatalysts with a high surface area such as TiO₂ nanotubes (TNTs) can offer better photocatalytic performance. They have a higher surface area and a high number of hydroxyl groups versus oxide precursors (TiO₂). The hydrothermal mechanism is a simple and effective method to synthesize TNTs, wherein the TiO₂ reacts with a concentrated NaOH solution at high pressure. In the related esterification experiments with TNTs as a photocatalyst, the lowest band gap energy sample exhibited the highest rate of oleic acid esterification among all the investigated TNTs. In addition to the band gap, other factors such as the TNT crystalline phase and surface area were important in photocatalytic performance.

TiO₂ materials, especially nanostructures, must not only be cost-effective, but they must also meet many other requirements: high photocatalytic activity, large active superficial area, chemical resistance, ease of manufacture, and fast synthesis route. However, it is commonly recurrent that TiO₂ nanostructures, nanoparticles or nanotubes, still have a high deficiency to collect a large part of the light spectrum. Nevertheless, anatase/rutile superficial defects, which increases considerably charge carrier recombination, can be circumvented by the addition of transition/noble metals, to intentionally increase the material photocatalytic properties and extend applications, in the field of H₂ generation.

The increasing efficiency of the organic-inorganic hybrid perovskite solar cells (PSCs), together with a series of advantages such as long carrier diffusion lengths, tunable bandgap, great light absorption potential and low-cost fabrication processes, have made this material the focus of attention of the solar cell researchers, despite the drawbacks such as device instability, J-V hysteresis and lead toxicity. This chapter will discuss the origin of perovskites and its application in solar cells, the structure of PSCs, how they are assembled, their obstacles and future perspectives.

Solar energy is a renewable power source that is harnessable nearly everywhere in the world. The investments in solar devices increase each year. Silicon is the dominant material in the production of commercial solar cells. More than 80% of world production is based on monocrystalline and polycrystalline silicon. Efficiency record commercial silicon solar panels convert about 25% of the sunlight into energy while the vast majority of conventional panels convert between 15 and 16%. The main factors of energy loss are the loss by light reflection on the cell surface and the loss by the energy emitted in the ultraviolet (UV) and infrared (IR) ranges which is directly transmitted and converted to heat without being harnessed by the cell. To overcome these losses, usually anti-reflective materials are applied on solar devices. In this chapter, the use of forsterite (Mg₂SiO₄) as an anti-reflective coating (ARC) is explored. Besides the antireflection property, forsterite is easily doped with several rare-earths (REs). Some elements of these group are capable of upconverting energy from de IR to the visible (Vis) spectral range. The theory behind the upconversion (UC) phenomenon is also presented here. Studies indicate that the use of forsterite ARC doped with UC REs on commercial silicon solar cells might be a low-cost solution to increase the efficiency of commercial devices.

This chapter presents studies on the use of chitin and its chitosan derivative as adsorbents in effluent treatment. Initially the origin of chitin is addressed, which is predominantly obtained from fishing waste. The objective is to present an overview of the main results obtained during the effluent treatment using chitin, chitosan and its derivatives in the removal of different classes of dyes, metallic ions, that is, several pollutants.

This chapter describes aspects of the application of spinel ferrite nanoparticles (SFNPs) in the purification of water bodies and wastewater. The structural and magnetic properties of these ferrimagnetic systems are described. Several synthesis methods are illustrated, with special focus on green processes. Furthermore, the adsorption of inorganic and organic contaminants is discussed, and several recently published adsorbent/adsorbate systems are presented. The utilization of spinel ferrites in the photodegradation of organic materials is also examined. Finally, the recovery and recycling aspects of the utilization of ferrite nanoparticles in the purification of wastewater are also presented.

In recent years, environmental issues have become increasingly significant, mainly due to the contamination of water resources by chemicals and pharmaceuticals, such as medicines, disinfectants, contrast media, detergents, pesticides, dyes, paints, preservatives. Advanced techniques, such as advanced oxidative processes (AOPs), are being studied and improved for the removal of these contaminants in wastewater. Among some AOPs we can mention the use of heterogeneous photocatalysis process using materials based on perovskite (e.g. KNbO₃) due to their different properties, such as ferroelectricity, optics and applications involving photocatalysis. The use of these materials has provided an improvement in photocatalytic activity through permanent internal polarization that can effectively separate the photoexcited charge carriers and effectively increase the efficiency of the degradation process.

Nano magnetic glass-ceramics can be a great alternative to the traditional use of nanoparticles in the various fields of engineering and biomedicine. The ability to generate nanoparticles in a glassy matrix, protected from oxidative processes, and choose the appropriate original glass, as well as the control of the crystallization process through thermal treatments and manufacturing processes, all these elements make the glass-ceramic extremely attractive for development of new strategies for science. In this chapter, was made an overview regarding the production of magnetic glass-ceramics from wastes, as well as the formation of nanocrystal phases in glass-ceramics and the importance of magnetite and alternative ferrites to enhance the magnetic proprieties. Several points were addressed to bring a general conception to the reader, and it dealt with possible future applications for these materials. In this way, magnetic glass-ceramics have great opportunities: magnetic devices, contrast agents for magnetic resonance imaging, magnetic hyperthermia, drug delivery, waste sorbents and microwave absorption devices.