Special Issue: Synthesis and Applications of Nanomaterials

1. Introduction

The synthesis and application of new nanomaterials has been one of the most active fields of science since interest in these materials started growing about 30 years ago, when researchers discovered that these new materials with nanoscale dimensions or features could impart entirely unique or improved properties. The range of materials prepared and investigated has expanded to include nanomaterials of metals, alloys, semiconductors, metal oxides, various carbon nanostructures, layered structures, polymers, and anything else that the imaginative scientists in this field can create. All dimensionalities are being explored, from nanoparticles with two-dimensional structures to three-dimensional nanoporous or composite materials [1]. The range of synthesis methods introduced is broad and falls generally into top-down or bottom-up approaches [2]. Nanoparticles especially have been developed with many compositions [3], and the possibilities of gold nanoparticles, among the first introduced and investigated, continue to grow [4]. Every area of technology can potentially be transformed by the introduction of nanomaterials. The 10 papers in this Special Issue report on the preparation of nanomaterials, including nanoparticles, layered nanomaterials, and nanostructured coatings, with applications including water treatment, wear and resistance, drug release, and chemical analysis.

2. Overview of Contributions

In the article by Thiam et al., the synthesis of Zinc oxide nanoparticles was achieved using Licania tomentosa leaf extract, serving as an example of a green approach to nanoparticle synthesis [5]. In these methods based on plant extracts, phytochemicals serve as both reducing and stabilizing agents. Zinc oxide, a II–V semiconductor, is a highly useful type of nanoparticle that has photocatalytic properties and is nontoxic. In this study, the authors demonstrate the antimicrobial activity of ZnO nanoparticles against a wide range of bacteria. The photocatalytic properties of these nanoparticles are exploited to carry out the photodegradation of two dye molecules often found as pollutants in water. Both the antimicrobial and photocatalytic properties of these particles are largely due to the generation of reactive oxygen species upon irradiation of the nanoparticles in water, in addition to other contributing mechanisms. This paper uses a green method to make nontoxic nanoparticles useful for two aspects of water treatment. In its nanoparticle form, ZnO has a bandgap reported as 3.0 eV, which makes it useful as a photocatalyst. The authors also emphasize how the structural details of nanoparticles depend on the synthesis method used, and how their behavior depends on subtle structural details, a key point to remember about many types of nanoparticles. There is thematic overlap with the paper by Castro et al., who report a green chemistry synthesis of alumina nanoparticles.

In the paper by Batakliev et al., a green nanotechnology theme is also found in their effort to make a biodegradable nanocomposite material and study its micromechanical properties and self-healing of scratches by heating [6]. The authors use 3D printing and melt extrusion to make flat nanocomposites of graphene-reinforced polylactic acid (PLA)/polycaprolactone (PCL). The material showed resistance to scratching, and its heterogeneous structure of hard and soft phase-separated domains allowed for the healing of scratches by heating, as polymer chains could return to their original conformations. In this study, graphene nanoplatelets of superior mechanical strength and high surface area impart valuable new mechanical properties to a biodegradable polymer composite.

The paper by Kim et al. focuses on a significant problem related to the ecological impact of the semiconductor industry [7]. Chemical mechanical planarization (CMP) is required for the production of ever smaller semiconductor devices. The traditional SiO₂ nanoparticle slurries used for CMP are difficult to recover and result in a significant amount of waste. In this study, the authors optimize the SiO₂ shell thickness of γ-Fe₂O₃@SiO₂ core–shell nanoparticles and apply them to the CMP of tungsten films, achieving a low surface roughness and a high magnetic recovery rate. The study again emphasizes the positive impact of nanomaterials on the environment, a theme that has emerged from more papers in the Special Issue than anticipated.

The removal of toxic metals and of organic pollutants by adsorption onto nanomaterials is an emerging strategy for environmental remediation. Hence, improved models for understanding adsorption behavior onto particles surfaces is the topic of the paper by de Sousa et al. [8]. In this paper, the authors improve the well-known Langmuir adsorption isotherm model by incorporating a temperature-dependent term in order to predict adsorption isotherms at different temperatures. The model is validated using literature data and experiments on the adsorption of copper ion by a zeolite.

The paper by Castro et al. focuses on a green synthesis method for aluminum oxide nanoparticles using lemongrass extract as a reducing agent [9]. The resulting particle size was found to depend on the choice of precursor. The plant extract contains many components and, while some act as reducing agents, others act as chelating ions, while some stabilize particles against agglomeration. Al₂O₃ nanoparticles can be applied as antimicrobial agents, as additives in coating, as active elements in gas sensors, and in other applications. The studies in this Special Issue emphasize how careful attention must be given to preparation conditions to tune the size and morphology of the resulting nanoparticles. Once a nanoparticle of a given material is synthesized, it can possibly be used in many different applications that make use of the different properties unique to its nanoscale dimensions.

Spherical nanoparticles of Bi(C₆N₇O₃) were formed from potassium cyanurate—K₃[C₆N₇O₃]—nanorods through ion exchange in an autoclave in the paper by Jia et al. for use in the photocatalytic degradation of an antibiotic in water [10]. The spherical nanoparticles provided a greatly enhanced efficiency in the photodegradation of tetracycline, an antibiotic that is polluting natural waters, as compared to potassium cyanurate or graphitic carbon nitride. Emerging contaminants, of which antibiotics are a major class, and their detection using nanomaterials is also an active field [11]. The sensitivity of nanoparticle activity to composition and shape is effectively revealed by this study.

Fisher and Liu incorporate cellulose nanocrystals to stabilize oil-in-water nanoemulsions where the oil is lemongrass essential oil [12]. These natural essential oils are eco-friendly and also have antifungal and antimicrobial properties, while cellulose nanocrystals are a green stabilizer. Compared to standard emulsions, nanoemulsions have droplet sizes around 100 nm. The effects of freeze drying and storage on the nanoemulsion and its activity are studied in detail in this contribution.

Carbajal Arizaga and coworkers study the use of layered double hydroxides for the delivery of the anticancer drug doxorubicin (DOX) [13]. Three different methods of loading the drug into LDH nanoparticles are compared: incubation with DOX, co-precipitation, and mechanochemical reaction. Magnesium/aluminum double hydroxide was stable under all three DOX loading processes, and the mechanochemical reaction loaded the most DOX. X-ray diffraction (XRD) and FTIR are used to provide characterization data for the particles.

In the contribution by Muniyappan et al., laser surface texturing is applied to the titanium alloy Ti₆Al₄V, which is then coated with a layer of TiN through physical vapor deposition [14]. The wear properties of the coating were studied, and a laser-textured sample with TiN coating was found to have a lower wear rate. These titanium alloys are important in medical implants. In this study, the authors reveal that the nanostructuring of surfaces can have a large impact on their mechanical behavior.

The review article by Adeniji et al. examines the growing application of nanomaterials in solid phase microextraction (SPME) through the modification of SPME fibers by nanomaterials to improve their specificity and capacity for the adsorption of volatile organic compounds, many of which are emerging pollutants [15]. The introduction of nanomaterials into SPME technology has the potential to advance environmental monitoring.

3. Future Prospects

One theme that has emerged from this Special Issue is that nanomaterials can be produced using methods that are environmentally friendly and can be applied to technologies for monitoring and remediating the environment. This suggests a strong motivation for future work along these directions. Another theme that emerges is that nanomaterials and nanostructured surfaces are complex, with morphologies and nanoscale features that depend strongly on preparation, compositional details, and processing conditions. It is clear from these contributions that applying detailed characterization using multiple methods after the preparation of a nanomaterial is essential, including, as applicable, diffraction, nanoscale microscopy, and thermal and spectroscopic methods. Furthermore, the performance characteristics of nanomaterials and nanostructured surfaces are critically dependent on these details, and much remains to be understood through further investigation. The possibilities for new nanomaterials and new applications are limited only by the imagination.